Activation of C–H bonds of thiosemicarbazones by transition metals: synthesis, structures and importance of cyclometallated compounds

Abstract

Transition metals, namely, palladium(II), platinum(II), ruthenium(II), rhodium(III) and iridium(III) have induced activation of C–H bonds of thiosemicarbazones and yielded mono-, di-, tri- and tetra-nuclear complexes with or without tertiary phosphines as co-ligands. Mono-thiosemicarbazones (R¹R²C²=N³–N²H–C¹(=S)–N¹R³R⁴) undergo loss of C–H (R¹ ring) and N²–H protons and formed cyclometallated derivatives. In mono- and di-nuclear complexes, the thio-ligands coordinate as terdentate (C,N,S) dianions, while in tri- and tetra-nuclear complexes, these ligands act as tetradentate (C,N,μ-S) dianions. The mono-thiosemicarbazones generally involve μ-S bridging in oligomers. Only one bis-thiosemicarbazone is reported to form mononuclear or mixed metal di-nuclear complexes. This review describes synthetic aspects, molecular structures, electronic absorption spectroscopy, fluorescence, cyclic voltammetry, NMR(¹H, ¹³C, ³¹P) spectroscopy and applications (biological, catalysis) of complexes. The factors for metallation and conclusions with future scope of investigations are also mentioned. Literature coverage is upto date, to the best of my knowledge, though a few omissions cannot be ruled out.

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Tarlok S. Lobana

Tarlok Singh Lobana born in 1950 in Gado Majra Village (Rajpura) in Punjab graduated in 1969 from S A College Ambala, post-graduated from Panjabi University, Patiala and received his Ph.D. degree from Guru Nanak Dev University (GNDU) in 1977. From the position of Research Assistant (1975), he rose to the level of Professor of Chemistry (1991) and retired in December 2010 and was reemployed until December 2012 and is Emeritus Scientist at GNDU with effect from January 2013 sponsored under Council of Scientific and Industrial Research New Delhi. His research interest have been coordination chemistry, pre-designed synthesis of metal derivatives of N,S-donors, metal mediated C–S rupture, organometallic chemistry. He has been Chairman of Department (May 1997 November 1999) and later Dean Sciences 2001 to 2002. He has been Fellow of National Academy of Sciences in 1999. He was the recipient of the Bronze Medal of the Chemical Research Society of India (2002). He has been INSA-Royal Society Fellow (UK, 1989), EPFL Visiting Professor Laussane (Suisse, 1991), DAAD Fellow (Munich, 1992), Spain Govt Visiting Fellow (1996–1997), JSPS Visiting Fellow (2002–2003), Visiting Professor at National Christian University Chungli Taiwan (2005). He has been complimentary e-member of Royal Society of Chemistry 2012 to 2014. He has published 215 research papers/reviews in international journals of repute.

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1. Introduction

The intramolecular activation of aromatic C–H bonds of coordinated ligands such as Schiff bases, azines etc. by transition metals represents an active area of research because cyclometallated compounds show significant applications, such as their use in regiospecific organic and organometallic synthesis, in insertion reactions, in the synthesis of new metal mesogenic compounds, reactions with nucleophiles and in catalytic materials, as liquid crystals, as analytical tools and for the design of metal complexes with promising anticancer or photochemical properties.^1–23 The activation of C–H bonds of thiosemicarbazones with transition metals has also received some attention and the current review is intended to describe this area.

The coordination chemistry of thiosemicarbazones describing bonding, structural aspects, biological, catalytical and analytical applications has received attention of many investigators.^24–26 This review begins with brief general introductory comments about thiosemicarbazones/their complexes highlighting their importance. The intramolecular activation of C–H bonds of aromatic, aliphatic or heterocyclic rings of coordinated thiosemicarbazones by the transition metals is surveyed and described in this review. The synthetic aspects, molecular structures, electronic absorption spectroscopy, fluorescence, cyclic voltammetry, NMR (¹H, ¹³C, ³¹P) spectroscopy and applications (biological, catalysis) of cyclometallated complexes form the main focus of review. The factors for metallation and conclusions with future scope of investigations are also discussed.

2. Thiosemicarbazones – introductory comments

Thiosemicarbazones are normally N,S-donor ligands and represent a class of Schiff bases which can be obtained by the condensation of an aldehyde, or a ketone with a thiosemicarbazide (eqn (1)). Broadly speaking these thiosemicarbazones are either mono-thiosemicarbazones (R¹R²C=N–NH–C(=S)–NR³R⁴, Structure I) or bis-thiosemicarbazones (Structure II). In mono-thiosemicarbazones, the substituents R¹, R², R³ and R⁴ could be different, and this provides a great possibility of variation of this class of thio-ligands. Likewise in bis-thiosemicarbazones two arms are connected via a ring, or a C–C bond.²⁶

R₂CO + H₂N–NH–C(=S)–NH₂ → R₂C=N–NH–C(=S)–NH₂ (1)

Mono-thiosemicarbazones exist as thione–thiol tautomers (IIIa–IIIb) and can bind to a metal center in the neutral (IIIa), or the anionic forms (IIIc). The form IIIc is generated after loss of –N²H or –SH hydrogen ions of a thiosemicarbazone and a large number of bonding modes have been observed in their metal complexes wherein a thio-ligand is coordinating in neutral or anionic forms.^27–43 Thiosemicarbazones are potential chemotherapeutics and are noted for their pharmacological properties, particularly as antiparasitic,^44–49 as antibacterial,^50–52 and as antitumoral agents.^53–56 Further, metal complexes of thiosemicarbazones have shown better biological activity than the parent thiosemicarbazones.^26,57–68 Significantly, among various metals, palladium(II) and copper(II) complexes with thiosemicarbazones have been particularly interesting because of their antitumor, antibacterial, antifungal, catalytic activities and ion sensors.^26,69–89

3. Metallation of thiosemicarbazones

In this section, only such reaction systems are a focus of discussion which involve formation of metal–carbon bonds via activation of C–H bonds of rings (R¹) of thiosemicarbazones (R¹R²C=N–NH–C(=S)–NR³R⁴, Structure I). Various complexes formed via activation of C–H bonds, their structures, mechanistic pathways, factors promoting metallation, biological, catalytical and analytical aspects will be covered. Complexes having metal–carbon bonds which do not originate from thiosemicarbazones are excluded from this review. However, there are reviews reported in the literature, which cover M–C bonds formed by auxiliary alkyl or aryl groups with no activation of C–H bonds of thiosemicarbazones in forming M–C bonds.^26,90–94 Reaction systems where activation was expected but did not occur may also find mention in the review where relevant. It is the mono-thiosemicarbazone ligands which have predominantly shown cyclometallation. There is only one report where a bis-thiosemicarbaone showed C–H activation. The subject matter under review is discussed under different sub-heads. The general structure of mono-thiosemicarbazones exhibiting metallation (Structure I) shall be used for defining various thiosemicarbazones with different substituents at C², N², and N¹ atoms in the forthcoming discussion. Thiosmeicarbazones lose a ring proton (C–H) and –N²H proton and coordinate to the metal ions as terdentate (C,N,S) dianions involving μ-S bridging only in oligomers (Structure IVb), excepting a few cases where they coordinate as monoanion as shown in structure Vb. Bis-thiosemicarbazones have coordination pattern similar to IVb (mononuclear complexes) but use both arms (Structure II) in the same way for diuclear complexes. In that case they act as hexadenate tetra-anions. The subject matter of review is discussed under different headings: synthetic aspects, molecular structures, electronic absorption spectroscopy and fluorescence, cyclic voltammetry, NMR spectroscopy, factors promoting metallation, applications – biological and catalysis and finally conclusion.

Several complexes have co-ligands and thus various co-ligands which appear in this review are summarized here.

PPh₃: triphenylphosphine;

MePh₂P: methyldiphenylphosphine;

dppm: bis(diphenylphopshino)methane, Ph₂P–CH₂–PPh₂

dppe: 1,2-bis(diphenylphophino)ethane, Ph₂P–(CH₂)₂–PPh₂

dppp: 1,3-bis(diphenylphophino)propane, Ph₂P–(CH₂)₃–PPh₂

dppb: 1,4-bis(diphenylphophino)butane, Ph₂P–(CH₂)₄–PPh₂

dppn: 1,5-bis(diphenylphophino)pentane, Ph₂P–(CH₂)₅–PPh₂

dpph: 1,6-bis(diphenylphophino)hexane, Ph₂P–(CH₂)₆–PPh₂

vdpp: 1,1-bis(diphenylphosphino)vinylene, CH₂=C(PPh₃)₂

dppen: cis- and trans-1,2-bis(diphenylphophino)ethene, Ph₂P–CH=CH–PPh₂

bdpb: ortho-bis(diphenylphosphino)benzene, o-C₆H₄(PPh₂)₂

bdcp: bis(diphenylphosphino)cyclopropane, (CH₂)₂C(PPh₃)₂

Fc: 1,1′-bis(diphenylphosphino)ferrocene, PPh₂–C₅H₄–Fe–C₅H₄–PPh₂

3.1 Synthetic aspects

Palladium. Palladium(II) is the one single metal ion which has shown the largest efficiency in activation of C–H bonds of ring substituents of coordinated thiosemicarbazones. The details of complexes displaying activation of C–H bonds are described in this section.

Variation of R¹ groups at C² carbon with R² substituent as hydrogen. The presence of p-methoxyphenyl (R¹) substituent at C², R² = H and N¹R³R⁴ = N¹HR³ gives rise to p-methoxyphenyl thiosemicarbazones with structure L^1–3. Reactions of L¹ and L² with lithium tetrachloropalladate, in presence of sodium acetate in methanol solvent and that of L³ with potassium tetrachloropalladate in ethanol (molar ratio M : L: 1 : 2) yielded tetranuclear complexes, {Pd₄(C,N,μ-S-L^1–3-2H)₄} (1–3).⁹⁵ There is activation of C⁸–H hydrogen (ortho to C³) of p-methoxyphenyl moiety as well as that of imino–N²(H)–hydrogen and the thio-ligands coordinate to the metal as di-anions through terdentate C,N,S group. Interestingly, the shift in the position of methoxy group from para-position (C⁶) to meta-position (C⁵) when reacted with lithium tetrachloropalladate in methanol did not involve C–H activation and only N³,S-chelated cis-square planar complexes, cis-[Pd(N³,S-L)₂] after deprotonation of N²–H hydrogen were obtained.⁹⁵ Another series of thio-ligands, L^4–8 with palladium(II) acetate in acetic acid also yielded tetranuclear complexes, {Pd(C,N,μ-S-L)}₄ (L = L^4–8) 4–8.^96,97

Equimolar reactions of N-acetyl-3-indolecarbaldehyde thiosemicarbazones (L^9–11) with potassium tetrachloropalladate in water–ethanol have yielded cyclometallated tetranuclear complexes, {Pd₄(C,N,μ-S-L^9–11-2H)₄} (9–11),⁹⁸ which are stoichiometrically similar to 1–3. There is a six-membered metallacyclic ring unlike five-membered ring usually observed as for example in 1–8. Oligomers 6 (L⁹) and 10 (L¹⁰) suspended in acetone on reaction with a ditertiary phosphine, Ph₂P–CH₂–CH₂–PPh₂ (M : L ratio 1 : 2) formed P,P-bridged dinuclear complexes 12 (L⁹) and 13 (L¹⁰).⁹⁸ The oligomerization occurred through coordinated sulfur in tetranuclear complexes 1–11.

Thiosemicarbazone L¹¹ with two methyl groups at ortho- and meta-positions of phenyl ring on reaction with palladium acetate in acetic acid yielded a trinuclear cyclometallated palladium(II) compound {Pd₃(C,N,μ-S-L¹¹-2H)₃} 14.⁹⁹ Each thiosemicarbazone ligand is tridentate with the metal bonded to the carbon atom from the o-methyl group (C⁴–Me), to the azomethine nitrogen and to the sulfur atom, which bridges to an adjacent palladium center. The central core Pd₃S₃ of the trinuclear complex confirms the presence of a non-planar six-membered metallacylic ring.⁹⁹ Metallation of the aliphatic methyl group (sp³ hybrid carbon) is rare occurrence in metal-thiosemicarbazone chemistry.

Reactions of thio-ligands L^12–13 (methyl substituent at N² atom instead of hydrogen) with potassium tetrachloropalladate in ethanol–water yielded mononuclear complexes, [Pd(C,N,S-L^12–13-H)Cl] 15–16, with the ligand acting as mono-anion, C,N,S-terdentate.¹⁰⁰ This behavior is in contrast to that shown by the ligands L^1–3 which gave polynuclear complexes (1–3). Reactions of thio-ligands L^14–18 with Na₂PdCl₄ in presence of 2-picolinic acid in presence of Et₃N in hot ethanol formed two types of products, (a) cis-square planar complexes with Pd(N³,S-L)₂ core and (b) brown polymeric complexes, [{Pd(CNS-L^14–18-2H)}_n].¹⁰¹ When these polymeric complexes were reacted with PPh₃ in ethanol, mononuclear complexes 17–21 were obtained in which thio-ligands coordinate as terdentate C,N,S donors.¹⁰¹

Variation of R¹ groups at C² carbon with R² substituent as other than hydrogen. In this section, palladium(II) chemistry of thio-ligands with different groups (R¹, R²) at C² and at N¹ (R³, R⁴) atoms is described. Specifically, R² group at C² is other than hydrogen. Reactions of thiosemicarbazones L^19–25 (with methyl/methoxy substituents in the phenyl ring and with Me/Et substituents at C²) with K₂PdCl₄ in water–ethanol {or Pd(OAc)₂ in glacial acetic acid} gave tetranuclear complexes, {Pd₄(C,N,μ-S-L-2H)₄} (L = L^19–24), 22–27; ^100,102L = L²⁵, 28 (ref. 95) in which thio-ligands acted as C,N,S-donor groups and oligomerization occurred through coordinated S donor atom. The oligomers 22–27 were suspended in acetone followed by the addition of a tertiary phosphine, Ph₃P/PMePh₂, which converted oligomers into mononuclear, [Pd(C,N,S-L)(PPh₃)] complexes (29–34).^100,102 The phosphine complexes 30–33 suspended in ethanol when treated with conc. HCl involved protonation of N² nitrogen and the thio-ligands though remain C,N,S-terdentate but bear one negative charge and Cl⁻ ion is the counter ion to balance the cationic species in ionic complexes, [Pd(C,N,S-L)(PPh₃)]⁺Cl⁻ (35–37,¹⁰⁰ 38 (ref. 102).

Further, tetranuclear complexes 22, 26 and 27 suspended in acetone with ditertiary phosphines (dppm, vdpp, dppb, and Fc) yielded dinuclear complexes (39–44) as well as mononuclear complexes with one pendant PPh₂ group in each case (45–48).¹⁰² Reaction of tetranuclear complex 28 with Ph₂P–CH₂–PPh₂, in acetone also formed P,P-bridged dimer, {Pd(C,N,S-L²⁵)}₂(μ-Ph₂P–CH₂–PPh₂) and attempt to crystallize in chloroform led to the formation of a N³, S-chelated complex, cis-[Pd(N³,S-L²⁵)₂] with no metallation retained.⁹⁵

Ligands L^26–34, having halogen substituents at 4-position of phenyl ring (R¹) and with R² substituents being Me, when reacted with Li₂[PdCl₄] in presence of sodium acetate in methanol solvent {or Pd(AcO)₂} in acetic acid; K₂PdCl₄ in ethanol–water} also formed tetra-nuclear complexes,{Pd₄(C,N,μ-S-L-2H)₄}(L = L^26–34; 49–57).^103–108 Reactions of tetranuclear complexes 49–57 with diphosphines in acetone in 1 : 2 molar ratio formed phosphine bridged dinuclear complexes, {Pd(C,N,S-L)}₂(μ-P,P-ligand) (P,P-ligand is a diphosphine) (58–82),^103–108 and in 1 : 4 molar ratio in acetone yielded mononuclear complexes, 83–89 (dppm) and 90 (dppen). Complexes 83–86 when treated with aq. HCl in acetone formed ionic complexes, 91–94 with dppm as P,P-chelator. One –PPh₂ group is pendant in each case. Complexes 83, 84, 88 and 89 were reacted with PdCl₂(PhCN)₂ in acetone which yielded hetero dinuclear complexes 95–98.¹⁰⁴

Ligands L^35–38, having halogen substituents at 5- or 6-position of phenyl ring (R¹) and with R² substituents being Me, when reacted with Li₂[PdCl₄] in presence of sodium acetate in methanol solvent {or Pd(AcO)₂} in acetic acid; K₂PdCl₄ in ethanol–water} also formed tetra-nuclear complexes,{Pd₄(C,N,μ-S-L-2H)₄} (L = L^35–38; 99–102).^105,106 Reactions of tetranuclear complexes 99–102 with diphosphines in acetone in 1 : 2 molar ratio formed P,P-bridged dinuclear complexes, {Pd(C,N,S-L)}₂(μ-P,P-ligand) (P,P-ligand is a diphosphine) (103–120),^105,106 and ligands, L^36–38 with dppm in 1 : 4 molar ratio in acetone yielded mononuclear 121–122 complexes.¹⁰⁶

The presence of two halogens in the phenyl ring (R¹) and with R² being methyl/ethyl gave ligands L^39–40 which on reaction with K₂PdCl₄ in ethanol–water again formed tetranuclear complexes 123 and 124 with C,N,S-terdentate thio-ligands.¹⁰⁹ Tetranuclear complexes 123 and 124 with dppm in acetone formed mononuclear complexes 125 and 126 with one pendant –PPh₂ group in each case. Compounds 123 and 124 with PPh₃ in acetone yielded mononuclear complexes, 127 and 128 (ref. 109) Likewise 123 and 124 when reacted with ditertiary phosphines, namely, 1,1′-bis(diphenylphosphino)ferrocene (Fc), trans-bis(diphenylphosphino)ethylene (trans-dppen) and bis(diphenylphosphino)benzene (bdpb) formed dinuclear complexes, 129–134.¹⁰⁹

A series of ligands L⁴¹ to L⁴⁸ having single or double methoxy substitution of phenyl ring also yielded dinuclear complexes, [Pd₂Cl₂(C,N,μ-S-L^41–48) (μ-P,P-diphosphine)] 135 to 142,¹¹⁰ similar to complexes 95 to 98 as discussed earlier. Molecular structures of [Pd₂Cl₂(μ-C,N,S-L⁴⁶) (μ-P,P-bdcp)] 141, [Pd₂(μ-C,N,S-L⁴¹)(μ-P,P-dppm)(κ²-P,P-dppm)] 143 and [Pd₂(μ-C,N,S-L⁴⁴)(μ-P,P-bdcp)(κ²-P,P-dppm)] 147 {bdcp – bis(diphenylphosphino)cyclopropane},¹¹⁰ are determined. The dinuclear complexes with silver perchlorate in acetone gave ionic complexes 143 to 152.

When there is no substituent in the phenyl ring (R¹), but an aliphatic or aromatic substituent (R²) is present at C² carbon and this gave rise to the thio-ligands L^49–53 which on reaction with PdCl₂(PPh₃)₂ in toluene (or acetonitrile) in presence of triethyl amine base has yielded only mononuclear square planar complexes, [Pd(C,N,S-L-2H)(PPh₃)] (152–156)^111,112 due to the presence of coordinated PPh₃ which prevented oligomerization. Reactions were carried out in 1 : 2 (M : L) molar ratio (152),¹¹¹ or 1 : 1 molar ratio (153–156),¹¹² the latter ratio appears to work satisfactorily. The ortho-metallated complexes 152, 157 as well N³,S-chelated 158 were obtained from the reaction of PdCl₂(PPh₃)₂ with the ligands L^49,54,55 in ethanol in the presence of the base Et₃N.^111,113 When R¹ group was thiopheneyl and with R² = Me, using the same reaction method metallation similar to complexes 153–156 occurred and formed complexes 159–160 (L^56–57).¹¹² Pyrrole group (R²) at C² did not form Pd–C bonds, rather pyrrole –NH moiety showed its deprotonation, and formed N,N,S-coordination in [Pd(N,N,S-L^56–57) (PPh₃)]¹¹⁴ unlike C,N,S-coordination shown in 159–160 (L^56–57). The ligand L⁵⁸ with a ethyl substituent at C² has also formed dimeric complexes 161–163,¹¹⁰ similar to complexes 135 or 143.

Platinum. Platinum(II) has also shown cyclometallation of coordinated thiosemicarbazones in a manner similar to that displayed by palladium(II), though number of reactions reported in comparison are much less. In a typical example, reaction of K₂PtCl₄ with the thio-ligand, L⁵⁹ with R² as Me substituent at C² in water–ethanol mixture gave an orange tetranuclear complex, [Pt(C,N,S-L⁵⁹-2H)]₄ 164.¹¹⁵ Similarly, with other ligands, L^59–63 having different R² substituents at C², tetra-nuclear complexes, 165–168 were obtained.¹¹⁵ Treatment of oligomers 164–168 with PPh₃ in 1 : 4 molar ratio in acetone followed by crystallization from acetone-n-hexane gave mononuclear complexes, [Pt(C,N,S-L-2H)(PPh₃)] (169–173). With tri(p-methoxyphenyl)-phosphine, it formed a similar monomer, 170. Dinuclear complexes 174–181 were formed from tetranuclear complex 164 using various ditertiary phosphines. A series of thio-ligands, L^4–6,8 with [Pt(μ-Cl)(η-C₄H₇)]₂ in acetone or K₂PtCl₄ in methanol yielded tetranuclear complexes, {Pt(C,N,μ-S-L)}₄ (L = L^4–6,8) 182–185.^96,97

Reactions of each of thio-ligands L^64–68 with PtCl₂(PPh₃)₂ in ethanol in presence of triethyl amine (1 : 1 molar ratio) for a period of 6 h gave cyclometallated complexes, [Pt(C,N,S-L-2H)(PPh₃)] (186–190) in which the thio-ligands as dianions are C,N,S-donors.¹¹⁶ Interestingly, when these ligands were reacted with K₂PtCl₄ in methanol in presence of triethyl amine (2 : 1: L : M molar ratio) for 24 h, no metallation occurred, rather cis-square planar N³,S-chelated complexes cis-[Pt(N³,S-L)₂] were obtained. Here the thio-ligands are coordinating as mono anions (losing N²–H proton).¹¹⁶

The thio-ligands L^69–70 were refluxed with [cis-PtMe₂(cod)] (1 : 1 molar ratio) in n-octane under nitrogen atmosphere which gave orange tetra nuclear complexes, [Pt(C,N,S-L-2H)]₄ 191 and 192.¹¹⁷ To a suspension of 191/192 in chloroform was added PPh₃ (1 : 4 molar ratio) followed by heating for a short time yielded mononuclear complexes 193 and 194. Similar reactions with ditertiary phosphines having longer P,P bite, namely, dppe, dppp and dppb formed dinuclear complexes, 195–200. With dppm and vdpp, mononuclear complexes, 201–204, with one pendant –PPh₂ group were obtained. To a suspension of 191/192 in chloroform was added [W(CO)₅(Ph₂P–CH₂–PPh₂)] (1 : 4 molar ratio) followed by heating for a short time which yielded hetero dinuclear complexes, 205 and 206. Treatment of 201 and 203 with [W(CO)₅(THF)] also gave same products 205 and 206.

Ruthenium. Ruthenium is another metal which has shown cyclometallation. Reactions of thio-ligands L^71–75 with [Ru(PPh₃)₃(CO)HCl] in refluxing methanol in presence of triethylamine afforded two types of complexes which were separated by thin layer chromatography.¹¹⁸ One class of complexes were having C,N,S-donors, [Ru(C,N³,S-L^71–75-2H) (PPh₃) (CO)] 207–211 and second class of complexes have N²,S-chelation with no cyclometallation, namely, [Ru(N²,S-L-H)(PPh₃)₂(H)CO]. Both PPh₃ ligands are trans in both category of complexes. Hydrogen is trans to sulfur in second category of complexes and CO is trans to N²/N³ atoms. Reactions of thio-ligands L^71–75 with [Ru(PPh₃)₂(CO)₂Cl₂] in refluxing toluene yielded dinuclear complexes [Ru₂ (C,N,S-L)₂(PPh₃)₂(CO)₂] (212–216).¹¹⁹ When these reactions were carried out in refluxing ethanol, only complexes [Ru(N²,S-L-H)(PPh₃)₂(H)CO] were formed in good yield with no cyclometallation. Here the source of hydride bonded to Ru^II is believed to be ethanol.¹¹⁹ Reactions of [RuHCl(CO)(AsPh₃)₃] with the thio-ligands L^76–86 in presence of LiBr/NaOAc in methanol yielded a series of Ru^II complexes [Ru(C,N,S-L^76–86)(CO(ASPh₃)₂)] 217–227.¹²⁰

Equimolar reaction of [Ru(PPh₃)₂(CO)₂Cl₂] with 2,6-diacetylpyridine thiosemicarbazone (L⁸⁷) {only example of a bis(thiosemicarbazone) involved in metallation} in refluxing ethanol followed by purification of orange solid using thin layer chromatography gave complex, [Ru(CO)(PPh₃)₂(C,N,S-L⁸⁷-2H)] 228 in which pyridine ring is metallated and the ligand is dianion coordinating through C,N and S donor atoms.¹²¹ Further, reaction of organo ruthenium complex 228 with M(PPh₃)₂Cl₂ (M = Pd, Pt) in ethanol followed by purification with TLC gave hetero dinuclear complexes, [RuM{(CNS)₂-L⁸⁷-4H}(CO)(PPh₃)₃] 229 (M = Pd), 230 (M = Pt) in which the second arm of the thio-ligands metallates Pd/Pt. Thus both 4 and 6 positions of the pyridine ring are metallated. Molecular structures of 228 and 230 are confirmed by X-ray crystallography and that of complex 229 with the help of DFT calculations.

Thiophene-2-carbaldehyde thiosemicarbazone (L⁸⁸) with trans-Ru(dppm)₂Cl₂ (dppm = Ph₂P–CH₂–PPh₂) in 2 : 1 molar ratio (L : M) in toluene in presence of Et₃N amine base yielded a complex [Ru(C,N,S-L⁸⁸-2H)(P,P-dppm) (P-dppm)] 231 – first case of cyclometallation in Ru^II – thiosemicarbazone chemistry.¹²² Benzaldehyde thiosemicarbazone (L⁸⁹) showed similar behavior and formed complex, [Ru(C,N,S-L⁸⁹-2H)(P,P-dppm)(P-dppm)] 232. Here cyclometallation has been attributed to the short bite of chelating dppm, providing a chance to thio-ligand to come in close proximity of the metal leading to cyclometallation and sixth site is occupied by one end of dppm ligand. No cyclometallation occurred when triphenyl phosphine or tri-p-tolyl phosphine were used in place of dppm in the starting precursor.

Rhodium and iridium. From the reactions of Rh(PPh₃)₃Cl with the thio-ligands L^90–94 in ethanol in presence of triethyl amine, cyclometallated products 233–237 have been isolated.¹²³ Complexes 237 and 238 were two products from the same ligand L⁹⁴. Here in 233–237, the coordinated S is oxidized. In all these complexes the ligands are C,N,S donor atoms coordinating to the metal center, the metal is oxidized to Rh^III, two positions are occupied by PPh₃ ligands and 6^th site is occupied by hydride. Cyclometallated complexes 239–243 were obtained similarly but no Et₃N base was used in the formation of these complexes.¹²³ In these complexes one site is occupied by chloride ion. Molecular structures of 236, 238 and 242 have been obtained. Reactions of the thio-ligands L^90–94 with Ir(PPh₃)₃Cl (1 : 1 molar ratio) in ethanol in presence of triethyl amine under dinitrogen atmosphere yielded a series of cylcometallated complexes, [Ir(C,N,S-L)(PPh₃)₂(H)] 244–248.¹²⁴ When same reactions were carried out in toluene, two types of cyclometallated complexes 244–248 and [Ir(C,N,S-L)(PPh₃)₂(Cl)] 249–253 were formed.

3.2 Molecular structures

The molecular structures of several cyclometallated compounds of thiosemicarbazones have been reported and a list of compounds with some important bond parameters is placed in Table 1. Broadly speaking there are four types of cyclometallated compounds of thiosemicarbazones: tetranuclear, trinucelar, dinuclear and mononuclear. Palladium is the only metal which has yielded all the four types of compounds, while platinum gave tetranuclear, dinuclear and mononuclear complexes. Finally, other metals (Ru, Rh, Ir) gave dinuclear (Ru) or mononuclear (Ru, Rh, Ir) compounds. Generally tertiary phosphines appear as the preferred choice (or suitable one) of co-ligands, though a few other co-ligands such as, CO, H⁻ do also appear in the compounds. Only selected original molecular structures are given in this section and briefly commented about their bonding patterns. Further, the bond parameters (Table 1) for all the compounds are also briefly commented in this section.

Table 1 Important bond lengths (Å) and bond angles (°) of complexes

Palladium(II)
a Bridging.b tsc = thiosemicarbazone ligand.
Tri- and tetra-nuclear complexes
{Pd3(C,N,μ-S-L^11-2H)3} 14 (ref. 99)
Pd–C	2.038(11)	N^3–Pd–S (trans)	176.5(3)
Pd–N^3	2.001(11)	C–Pd–S (trans)	165.9(4)
Pd–S^a	2.403(3), 2.322(4)	C–Pd–N^3	86.4(5)
S–C	1.782(13)	S–Pd–N^3	82.7(3)
{Pd4(C,N,μ-S-L^10-2H)4} 10 (ref. 98)
Pd–C	2.021(5)	N^3–Pd–S (trans)	173.23(12)
Pd–N^3	2.064(4)	C–Pd–S (trans)	176.29(14)
Pd–S^a	2.3208(13), 2.3380(14)	C–Pd–N^3	94.27(18)
S–C	1.749(6)	S–Pd–N^3	83.45(12)
{Pd4(C,N,μ-S-L^4-2H)4} 4 (ref. 96)
Pd–C	2.01(2)	N^3–Pd–S (trans)	—
Pd–N^3	1.98(2)	C–Pd–S (trans)	—
Pd–S^a	2.382(6), 2.316(5)	C–Pd–N^3	80.4(8)
S–C	1.78(2)	S–Pd–N^3	81.4(5)
{Pd4(C,N,μ-S-L^24-2H)4} 27 (ref. 102)
Pd–C	2.015(8)	N^3–Pd–S (trans)	176.35(18)
Pd–N^3	1.994(7)	C–Pd–S (trans)	162.6(3)
Pd–S^a	2.319(2), 2.3611(19)	C–Pd–N^3	81.0(3)
S–C	1.790(8)	S–Pd–N^3	82.66(18)
{Pd4(C,N,μ-S-L^27-2H)4} 50 (ref. 103)
Pd–C	2.007(5)	N^3–Pd–S (trans)	177.11(13)
Pd–N^3	1.989(4)	C–Pd–S (trans)	164.66(17)
Pd–S^a	2.3186(16), 2.3551(16)	C–Pd–N^3	80.9(2)
S–C	1.792(6)	S–Pd–N^3	84.10(13)
{Pd4(C,N,μ-S-L^30-2H)4} 53 (ref. 103)
Pd–C	1.990(7)	N^3–Pd–S (trans)	177.48(17)
Pd–N^3	1.988(6)	C–Pd–S (trans)	165.0(2)
Pd–S^a	2.315(2), 2.355(2)	C–Pd–N^3	81.4(3)
S–C	1.794(7)	S–Pd–N^3	84.01(19)
{Pd4(C,N,μ-S-L^32-2H)4} 55 (ref. 108)
Pd–C	2.023(11)	N^3–Pd–S (trans)	177.0(3)
Pd–N^3	1.985(10)	C–Pd–S (trans)	163.6(4)
Pd–S	2.325(4); 2.369(3)	C–Pd–N^3	80.6(5)
S–C	1.747(17)	S–Pd–N^3	83.9(3)
{Pd4(C,N,μ-S-L^36-2H)4} 100 (ref. 106)
Pd–C	2.006(5)	N^3–Pd–S (trans)	175.78(14)
Pd–N^3	2.003(5)	C–Pd–S (trans)	162.97(18)
Pd–S	2.3664(16); 2.2997(16)	C–Pd–N^3	81.3(2)
S–C	1.821(6)	S–Pd–N^3	82.79(14)
{Pd4(C,N,μ-S-L^38-2H)4} 102 (ref. 106)
Pd–C	1.979(9)	N^3–Pd–S (trans)	176.6(2)
Pd–N^3	1.993(7)	C–Pd–S (trans)	160.3(3)
Pd–S	2.353(2); 2.317(2)	C–Pd–N^3	80.9(4)
S–C	1.773(9)	S–Pd–N^3	81.9(2)
Mono- and di-nuclear complexes
Pd(C,N,S-L^15-2H)(PPh3)} 18 (ref. 101)
Pd–C	2.051(3)	N^3–Pd–P (trans)	176.76(9)
Pd–N^3	2.029(2)	C–Pd–S (trans)	162.87(7)
Pd–S	2.3560(10)	C–Pd–N^3	81.57(10)
Pd–P	2.2498(6)	S–Pd–N^3	81.36(9)
S–C	1.754(3)
[Pd(C,N,S-L^20-2H)(PPh3)] 30 (ref. 100)
Pd–C	2.044(4)	N^3–Pd–P (trans)	176.93(9)
Pd–N^3	2.024(3)	C–Pd–S (trans)	163.74(10)
Pd–S	2.3419(11)	C–Pd–N^3	81.03(13)
Pd–P	2.2519(9)	S–Pd–N^3	82.77(9)
S–C	1.757(4)
[Pd(C,N,S-L^24-2H)(PPh3)] 34 (ref. 102)
Pd–C	2.033(5)	N^3–Pd–P (trans)	178.75(11)
Pd–N^3	2.029(4)	C–Pd–S (trans)	164.5(2)
Pd–S	2.3270(14)	C–Pd–N^3	81.3(2)
Pd–P	2.2527(13)	S–Pd–N^3	83.19(11)
S–C	1.750(5)
[Pd(C,N,S-L^29-2H)(η^1-dppm)] 85 (ref. 103)
Pd–C	2.026(3)	N^3–Pd–P (trans)	177.29(8)
Pd–N^3	2.018(2)	C–Pd–S (trans)	163.95(9)
Pd–S	2.3192(9)	C–Pd–N^3	80.77(7)
Pd–P	2.2626(8)	S–Pd–N^3	83.23(7)
S–C	1.754(3)
[Pd(C,N,S-L^30-2H)(η^1-dppm)] 86 (ref. 103)
Pd–C	2.022(4)	N^3–Pd–P (trans)	177.26(9)
Pd–N^3	2.015(3)	C–Pd–S (trans)	164.05(12)
Pd–S	2.3231(11)	C–Pd–N^3	80.54(14)
Pd–P	2.2659(10)	S–Pd–N^3	83.51(9)
S–C	1.746(4)
[Pd(C,N,S-L^32-2H)(η^1-P-dppm)] 88 (ref. 108)
Pd–C	2.035(3)	N^3–Pd–P (trans)	177.75(8)
Pd–N^3	2.034(3)	C–Pd–S (trans)	163.81(10)
Pd–S	2.3204(10)	C–Pd–N^3	81.21(12)
Pd–P	2.2592(8)	S–Pd–N^3	82.75(8)
S–C	1.753(4)
[Pd(C,N,S-L^33-2H)(η^1-P-dppm)] 89 (ref. 108)
Pd–C	2.038(5)	N^3–Pd–P (trans)	177.45(12)
Pd–N^3	2.031(4)	C–Pd–S (trans)	163.80(15)
Pd–S	2.3323(15)	C–Pd–N^3	81.30(19)
Pd–P	2.2616(13)	S–Pd–N^3	82.56(12)
S–C	1.758(6)
[Pd(C,N,S-L^32-2H)(η^1-P-dppen)] 90 (ref. 108)
Pd–C	2.088(2)	N^3–Pd–P (trans)	179.07(8)
Pd–N^3	2.019(2)	C–Pd–S (trans)	163.47(9)
Pd–S	2.3430(13)	C–Pd–N^3	80.88(12)
Pd–P	2.2536(10)	S–Pd–N^3	82.78(8)
S–C	1.742(3)
[Pd(C,N,S-L^40-2H)(Ph3P)] 128 (ref. 109)
Pd–C	2.038(4)	N^3–Pd–P (trans)	n/a
Pd–N^3	2.036(3)	C–Pd–S (trans)	n/a
Pd–S	2.3324(9)	C–Pd–N^3	81.30(12)
Pd–P	2.2614(9)	S–Pd–N^3	82.48(8)
S–C	1.762(4)
[Pd(C,N,S-L^49-2H)(PPh3)] 152 (ref. 111)
Pd–C	2.0411(19)	N^3–Pd–P (trans)	177.05(5)
Pd–N^3	2.0326(15)	C–Pd–S (trans)	163.45(6)
Pd–S	2.3464(6)	C–Pd–N^3	81.47(7)
Pd–P	2.2513(5)	S–Pd–N^3	82.26(5)
S–C	1.758(2)
[Pd(C,N,S-L^49-2H)(PPh3)] 152 (ref. 113)
Pd–C	2.047(3)	N^3–Pd–P (trans)	176.75(7)
Pd–N^3	2.030(2)	C–Pd–S (trans)	163.50(7)
Pd–S	2.349(10)	C–Pd–N^3	81.26(9)
Pd–P	2.254(8)	S–Pd–N^3	82.49(7)
S–C	1.758(3)
[Pd(C,N,S-L^50-2H)(PPh3)] 153 (ref. 112)
Pd–C	2.0390(19)	N^3–Pd–P (trans)	173.54(5)
Pd–N^3	2.0283(16)	C–Pd–S (trans)	164.27(5)
Pd–S	2.3347(5)	C–Pd–N^3	81.15(7)
Pd–P	2.2489(5)	S–Pd–N^3	83.18(5)
[Pd(C,N,S-L^51-2H)(PPh3)] 154 (ref. 112)
Pd–C	2.036(4), 2.103(2)	N^3–Pd–P (trans)	176.69(9)
Pd–N^3	2.038(3), 2.058(3)	C–Pd–S (trans)	164.02(11)
Pd–S	2.3225(9), 2.3014(10)	C–Pd–N^3	81.41(13)
Pd–P	2.2592(9), 2.2554(10)	S–Pd–N^3	82.80(9)
[Pd(C,N,S-L^52-2H)(PPh3)] 155 (ref. 112)
Pd–C	2.3335(7)	N^3–Pd–P (trans)	178.44(5)
Pd–N^3	2.0285(17)	C–Pd–S (trans)	164.35(6)
Pd–S	2.3335(7)	C–Pd–N^3	81.39(8)
Pd–P	2.2466(6)	S–Pd–N^3	83.41(5)
[Pd(C,N,S-L^54-2H)(PPh3)] 157 (ref. 113)
Pd–C	2.027(3)	N^3–Pd–P (trans)	177.00(8)
Pd–N^3	2.034(3)	C–Pd–S (trans)	163.53(8)
Pd–S	2.345(9)	C–Pd–N^3	81.37(11)
Pd–P	2.260(8)	S–Pd–N^3	82.26(8)
S–C	1.750(3)
[Pd(C,N,S-L^56-2H)(PPh3)] 159 (ref. 112)
Pd–C	2.026(5)	N^3–Pd–P (trans)	173.85(13)
Pd–N^3	2.034(4)	C–Pd–S (trans)	163.78(16)
Pd–S	2.3237(15)	C–Pd–N^3	81.5(2)
Pd–P	2.2417(14)	S–Pd–N^3	82.34(14)
[Pd(C,N,S-L^57-2H)(PPh3)] 160 (ref. 112)
Pd–C	2.050(14), 2.036(12)	N^3–Pd–P (trans)	173.7(3), 176.1(4)
Pd–N^3	2.045(13), 2.029(14)	C–Pd–S (trans)	165.4(4), 161.5(4)
Pd–S	2.333(4), 2.320(4)	C–Pd–N^3	82.1(5), 80.4(5)
Pd–P	2.239(4), 2.241(4)	S–Pd–N^3	83.4(4), 81.3(4)
{Pd2(C,N,S-L^24-2H)2}(μ-Ph2P–CH2–PPh2) 44 (ref. 102)
Pd–C	2.046(7)	N^3–Pd–P (trans)	178.31(18)
Pd–N^3	2.028(7)	C–Pd–S (trans)	163.3(3)
Pd–S	2.348(2)	C–Pd–N^3	81.5(3)
Pd–P	2.253(2)	S–Pd–N^3	82.27(19)
S–C	1.766(9)
{Pd2(C,N,S-L^28-2H)2}(μ-Ph2P–CH2–CH2–CH2–PPh2) 62 (ref. 105)
Pd–C	2.040(10)	N^3–Pd–P (trans)	177.4(2)
Pd–N^3	2.020(7)	C–Pd–S (trans)	163.7(2)
Pd–S	2.322(3)	C–Pd–N^3	80.6(3)
Pd–P	2.274(2)	S–Pd–N^3	83.7(2)
S–C	1.743(10)
{Pd(C,N,S-L^31-2H)}2(μ-P,P-dppe) 67 (ref. 106)
Pd–C	2.047(7)	N^3–Pd–P (trans)	175.97(16)
Pd–N^3	2.008(5)	C–Pd–S (trans)	163.53(18)
Pd–S	2.3209(19)	C–Pd–N^3	80.2(2)
Pd–P	2.2637(18)	S–Pd–N^3	83.32(16)
S–C	1.743(7)
{Pd(C,N,S-L^30-2H)}2(μ-P,P-dppe) 72 (ref. 108)
Pd–C	2.042(6)	N^3–Pd–P (trans)	176.03(16)
Pd–N^3	2.025(5)	C–Pd–S (trans)	163.78(17)
Pd–S	2.3225(19)	C–Pd–N^3	80.4(2)
Pd–P	2.2544(16)	S–Pd–N^3	83.69(16)
S–C	1.766(8)
{Pd(C,N,S-L^26-2H)}2(μ-P,P-dppen) 75 (ref. 108)
Pd–C	2.023(12)	N^3–Pd–P (trans)	178.7(3)
Pd–N^3	2.012(9)	C–Pd–S (trans)	164.7 (3)
Pd–S	2.327(4)	C–Pd–N^3	81.0(4)
Pd–P	2.259(3)	S–Pd–N^3	83.6(3)
S–C	1.764(12)
{Pd(C,N,S-L^32-2H)}2(μ-P,P-dppen) 76 (ref. 108)
Pd–C	2.036(2)	N^3–Pd–P (trans)	178.77(6)
Pd–N^3	2.022(2)	C–Pd–S (trans)	164.51(8)
Pd–S	2.3208(8)	C–Pd–N^3	81.26(9)
Pd–P	2.2421(7)	S–Pd–N^3	83.25(6)
S–C	1.759(3)
{Pd(C,N,S-L^34-2H)}2(μ-P,P-dppe) 79 (ref. 107)
Pd–C	2.0324(2)	N^3–Pd–P (trans)	176.18(8)
Pd–N^3	2.024(3)	C–Pd–S (trans)	163.42(3)
Pd–S	2.3333(9)	C–Pd–N^3	80.55(8)
Pd–P	2.2663(8)	S–Pd–N^3	82.88(8)
S–C	1.7704(11)
{Pd(C,N,S-L^35-2H)}2(μ-P,P-dppp) 105 (ref. 105)
Pd–C	2.044(8)	N^3–Pd–P (trans)	177.67(18)
Pd–N^3	2.039(6)	C–Pd–S (trans)	162.8(8)
Pd–S	2.337(2)	C–Pd–N^3	80.6(3)
Pd–P	2.261(2)	S–Pd–N^3	82.22(18)
S–C	1.769(8)
[Pd2Cl2(C,N,μ-S-L^32-2H)(μ-P,P-dppm)] 97 (ref. 104)
Pd1–C	2.012(4)	N^3–Pd(1)–P (trans)	176.65(10)
Pd1–N^3	2.020(3)	C–Pd(1)–S (trans)	165.15(12)
Pd1–S	2.3214(11)	C–Pd(1)–N^3	81.16(16)
Pd–P	2.2465(11)	S–Pd(1)–N^3	84.00(10)
S–C	n/a
Pd2–S (bridging)	2.3074(11)	S(1)–Pd(2)–Cl(1)	170.41(4)
Pd2–P	2.2644(11)	P(2)–Pd(2)–Cl(2)	179.20(4)
Pd2–Cl	2.3160(11), 2.3448(11)
[Pd2Cl2(C,N,μ-S-L^46-2H)(μ-P,P-bdcp)] 141 (ref. 110)
Pd1–C	2.025(4)	N^3–Pd–P (trans)	—
Pd1–N^3	2.023(3)	C–Pd–S (trans)	—
Pd1–S	2.3276(11)	C–Pd–N^3	80.64(15)
Pd–P	2.2579(12)	S–Pd–N^3	83.42(10)
S–C	1.810(4)
Pd2–S (bridging)	2.2943(12)	Cl–Pd(2)–Cl	88.14(4)
Pd2–P	2.2624(12)	S–Pd–P(2)	90.01(4)
Pd2–Cl	2.3415(13); 2.3288(12)
[Pd2(C,N,μ-S-L^41-2H)(μ-P,P-dppm)(κ^2-P,P-dppm)](ClO4)2 143 (ref. 110)
Pd1–C	2.032(3)	N^3–Pd–P (trans)	—
Pd1–N^3	2.029(2)	C–Pd–S (trans)	—
Pd1–S	2.3361(11)	C–Pd(1)–N^3	81.13(11)
Pd–P	2.2553(8)	S–Pd(1)–N^3	82.31(8)
S–C	1.807(3)	S(1)–Pd(2)–P(2)	83.65(3)
Pd2–S (bridging)	2.3627(8)	P(3)–Pd(2)–P(4)	70.56(3)
Pd2–P	2.3452(8)
[Pd2(C,N,μ-S-L^44-2H)(μ-P,P-bdcp) (κ^2-P,P-dppm)](ClO4)2 147 (ref. 110)
Pd1–C	2.038(6)	N^3–Pd–P (trans)	—
Pd1–N^3	2.034(5)	C–Pd–S (trans)	—
Pd1–S	2.3214(16)	C–Pd(1)–N^3	80.5(2)
Pd1–P	2.2520(17)	S–Pd(1)–N^3	83.24(14)
S–C	1.810(6)	S(1)–Pd(2)–P(2) 8	87.30(6)
Pd2–S (bridging)	2.3402(18)	P(3)–Pd(2)–P(4)	69.87(6)
Pd2–P	2.3515(17)
Platinum(II) and ruthenium(II) complexes
{Pt4(C,N,μ-S-L^4-2H)4} 182 (ref. 96)
Pt–C	2.020(5)	N^3–Pt–S (trans)	—
Pt–N^3	1.996(5)	C–Pt–S (trans)	—
Pt–S^a	2.3491(16), 2.3036(15)	C–Pt–N^3	83.57(15)
S–C	1.798(5)	S–Pt–N^3	80.8(2)
[Pt(C,N,S-L^60-2H)(Ph3P)] 170 (ref. 115)
Pt–C	2.02(2)	N^3–Pt–P (trans)	176.0(4)
Pt–N^3	2.03(2)	C–Pt–S (trans)	161.4(5)
Pt–S	2.335(5)	C–Pt–N^3	77.9(6)
Pt–P	2.235(5)	S–Pt–N^3	83.7(4)
S–C	1.78(2)
[Pt(C,N,S-L^65-2H)(Ph3P)] 187 (ref. 116)
Pt–C	2.045(4)	N^3–Pt–P (trans)	176.87(12)
Pt–N^3	2.032(4)	C–Pt–S (trans)	162.52(13)
Pt–S	2.340(13)	C–Pt–N^3	80.85(17)
Pt–P	2.230(10)	S–Pt–N^3	81.70(13)
S–C	1.755(5)
{Pt2(C,N,S-L^70-2H)2}(Ph2P–CH2–CH2–PPh2) 198 (ref. 117)
Pt–C	2.022(5)	N^3–Pt–P (trans)	n/a
Pt–N^3	2.050(4)	C–Pt–S (trans)	n/a
Pt–S	2.317(2)	C–Pt–N^3	80.42(2)
Pt–P	2.220(1)	S–Pt–N^3	82.00(12)
S–C	1.773(5)
[Ru(C,N^3,S-L^72-2H)(PPh3)2(CO)] 208 (ref. 118)
Ru–C(tsc)/Ru–C(CO)^b	2.077(7)/1.796(8)	P–Ru–P (trans)	173.41(7)
Ru–N^3	2.109(6)	C(tsc)–Ru–S (trans)^b	156.22(19)
Ru–S	2.4614(19)	C(CO)–Ru–N^3 (trans)	172.5(3)
Ru–P	2.366(2), 2.350(2)	C–Ru–N^3	78.9(2)
S–C	1.731(7)	S–Ru–N^3	77.35(16)
[Ru(C,N,S-L^79-2H)(CO)(AsPh3)2] 220 (ref. 120)
Ru–C(tsc)/Ru–C(CO)^b	2.061(3)/1.838(3)	As–Ru–As (trans)	175.180(13)
Ru–N^3	2.0732(19)	C(tsc)–Ru–S (trans)^b	157.42(7)
Ru–S	2.4641(7)	C(CO)–Ru–N^3 (trans)	170.64(11)
Ru–As	2.4695(3), 2.4379(3)	C–Ru–N^3	78.41(9)
S–C	1.755(2)	S–Ru–N^3	79.05(6)
[Ru(C,N,S-L^88-2H)(P,P-dppm)(P-dppm)] 231 (ref. 122)
Ru–C	2.076(3)	P–Ru–P (trans)	174.36(3)
Ru–N^3	2.067(3)	C–Ru–S (trans)	156.92(11)
Ru–S	2.4369(11)	P–Ru–N^3	161.92(9)
Ru–P	2.2918(11), 2.3138(11), 2.3684(11)	C–Ru–N^3	78.95(13)
P–Ru–P (chelating)	71.33(4)	S–Ru–N^3	79.21(8)
[Ru(C,N,S-L^89-2H)(P,P-dppm)(P-dppm)] 232 (ref. 122)
Ru–C	2.086(3)	P–Ru–P (trans)	169.38(3)
Ru–N^3	2.058(2)	C–Ru–S (trans)	157.58(8)
Ru–S	2.4385(8)	P–Ru–N^3	162.58(7)
Ru–P	2.3384(8), 2.3045(7), 2.3392(8)	C–Ru–N^3	78.90(10)
P–Ru–P (chelating)	69.99(3)	S–Ru–N^3	79.37(7)
[Ru (C,N,S-L^87-2H)(CO)(PPh3)2] 228 (ref. 121)
Ru–C(tsc)/Ru–C(CO)^b	2.072(8)/1.831(8)	P–Ru–P (trans)	176.81(7)
Ru–N^3	2.096(6)	C(tsc)–Ru–S (trans)^b	158.0(2)
Ru–S	2.437(3)	C(CO)–Ru–N^3 (trans)	176.7(3)
Ru–P	2.379(3), 2.387(3)	C–Ru–N^3	78.3(3)
S–C	1.735(9)	S–Ru–N^3	79.83(17)
[Ru2(C,N,S-L^74-2H)2(PPh3)2(CO)2] 215 (ref. 119)
Ru–C(tsc)/Ru–C(CO)^b	2.050(3)/1.857(3)	P–Ru–S (trans)	172.34(3)
Ru–N^3	2.092(2)	C(tsc)–Pt–S (trans)^b	157.19(8)
Ru–S	2.4786(7), 2.4804(7)	C(CO)–Ru–N^3 (trans)	173.29(11)
Ru–P	2.3071(7)	C–Ru–N^3	n/a
S–C	1.781(3)	S–Ru–N^3	n/a
[RuPt{(C,N,S)2-L^87-4H}(CO)(PPh3)3] 230 (ref. 121)
Ru–C(tsc)/Ru–C(CO)^b	2.093(8)/1.830(10)	P–Ru–S (trans)	171.88(9)
Ru–N^3	2.099(7)	C(tsc)–Ru–S (trans)^b	157.9(2)
Ru–S	2.462(3)	C(CO)–Ru–N^3 (trans)	177.2(3)
Ru–P	2.360(3), 2.423(3)	C–Ru–N^3	79.0(3)
S–C	1.734(10)	S–Ru–N^3	78.98(19)
Pt–C	2.048(9)	N^3–Pt–P (trans)	174.2(2)
Pt–N^3	2.034(7)	C–Pt–S (trans)	162.6(3)
Pt–S	2.339(3)	C–Pt–N^3	80.9(3)
Pt–P	2.236(3)	S–Pt–N^3	82.0(2)
S–C	1.758(10)
Rhodium(III) and iridium(iii): mononuclear complexes
[Rh {C,N,S(=O)2-L^93-2H}(H)(PPh3)2] 236 (ref. 123)
Rh–C	2.039(3)	S=O	1.461(2), 1.465(2)
Rh–N^3	2.096(2)	P–Rh–P (trans)	165.40(3)
Rh–S	2.3366(8)	C–Rh–S (trans)	160.14(9)
Rh–P	2.3380(8), 2.3483(8)	H–Rh–N^3(trans)	172.61(7)
Rh–H	0.437(4)	S–Rh–N^3	80.46(7)
S–C	1.855(3)	C–Rh–N^3	79.73(10)
[Rh(C,N,S-L^94-2H)(H)(PPh3)2] 238 (ref. 106)
Rh–C	2.038(5)	P–Rh–P (trans)	169.23(6)
Rh–N^3	2.074(4)	C–Rh–S (trans)	159.99(17)
Rh–S	2.4116(15)	H–Rh–N^3 (trans)	176(2)
Rh–P	2.3218(16), 2.3041(15)	S–Rh–N^3	79.91(12)
Rh–H	1.57(6)	C–Rh–N^3	80.2(2)
S–C	1.743(6)
[Rh (C,N,S-L^93-2H)(Cl)(PPh3)2] 242 (ref. 123)
Rh–C	2.044(11)	P–Rh–P (trans)	179.56(10)
Rh–N^3	1.990(10)	C–Rh–S (trans)	162.2(3)
Rh–S	2.450(3)	Cl–Rh–N^3 (trans)	176.1(3)
Rh–P	2.379(3), 2.371(3)	S–Rh–N^3	81.1(3)
Rh–Cl	2.385(3)	C–Rh–N^3	81.1(4)
S–C	1.708(14)
[Ir(C,N,S-L^94-2H)(H)(PPh3)2] 248 (ref. 124)
Ir–C	2.042(8)	P–Ir–P (trans)	165.43(8)
Ir–N^3	2.073(7)	C–Ir–S (trans)	159.7(2)
Ir–S	2.424(2)	H–Ir–N^3 (trans)	175(4)
Ir–P	2.302(2), 2.305(2)	S–Ir–N^3	79.5(2)
Ir–H	1.41(2)	C–Ir–N^3	80.3(3)
S–C	1.738(9)
[Ir(C,N,S-L^91-2H)(Cl)(PPh3)2] 250 (ref. 124)
Ir–C	2.013(9)	P–Ir–P (trans)	169.53(8), 169.53(8)
Ir–N^3	2.006(7)	C–Ir–S (trans)	160.4(2)
Ir–S	2.433(3)	Cl–Ir–N^3 (trans)	173.7(2)
Ir–P	2.346(3), 2.367(3)	S–Ir–N^3	81.1(2)
Ir–Cl	2.389(3)	C–Ir–N^3	79.4(3)
S–C	1.745(9)

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Palladium. There are eight tetra-nuclear compounds (Pd: 4, 10, 27, 50, 53, 55, 100 and 102)^{96,98,99,102,103,106,108} for which molecular structures are determined (Table 1). All these tetra-nuclear compounds are identical in bonding (C,N,μ-S). Usually there is one substituent in the phenyl ring (ligand types: A, B), though there are cases with more than one substituent as described in Section 3.1. Molecular structures of four representative tetranuclear oligomers 27 (L²⁴), 4 (L⁴), 100 (L³⁶) and 10 (L¹⁰) are shown in Fig. 1–4 respectively. Ligand L²⁴ has no substituent in the phenyl ring (R = H) with R² = Et at C² carbon (27, Fig. 1),¹⁰² ligand L⁴ has one substituent in the phenyl ring (R = –CHMe₂) with R² = H at C² carbon (4, Fig. 2),⁹⁶ and L³⁶ has substituents in phenyl ring (R = F) and also at C² (R² = Me) carbon atoms (100, Fig. 3).¹⁰⁶ In these three cases, tetranuclear complexes were formed involving phenyl ring metallation at C⁸ carbon [ortho to ipso (C³) carbon]. Compounds 4, 27 and 100 belong to category A ligands. The thio-ligand L¹⁰ which belongs to category C has also formed tetranuclear compound 10 (Fig. 4). In each case, a thio-ligand as dianion chelates to one metal center and oligomerization occurs through sulfur; each sulfur bridges two metal centers. The central core, Pd₄S₄ of oligomers forms an eight membered ring. Molecular structure of tri-nuclear complex 14(L¹¹) is shown in Fig. 5.⁹⁹ Here the thio-ligand L¹¹ has R² = H, R³ = Et with phenyl ring having two methyl substituents at 4- and 7-positions. It is the methyl group at 4-position which is metallated and not the phenyl ring owing to possibility of its closer proximity to the metal. Thiosemicarbazone (L¹¹) as dianion is coordinating as (C,N,μ-S) and the central core Pd₃S₃ of the trinuclear complex adopts a non-planar six-membered ring.⁹⁹

Fig. 1 Molecular structure of {Pd₄(C,N,μ-S-L²⁴-2H)₄} 27 (L²⁴: R² = Et, R³ = H, R = H) [reproduced with permission from ref. 102. Copyright (1999) Royal Society of Chemistry].

Fig. 2 Molecular structure of {Pd₄(C,N,μ-S-L⁴-2H)₄} 4 (L⁴: R², R³ = H, H; R = –CHMe₂ at 6-position) [reproduced with permission from ref. 96. Copyright (1998). American Chemical Society].

Fig. 3 Molecular structure of {Pd₄(C,N,μ-S-L³⁶-2H)₄} 100 (L³⁶: R² = Me, R³ = Et, R = F at 6-position) [reprinted with permission from ref. 106. Copyright (2012) Elsevier].

Fig. 4 Molecular structure of {Pd₄(C,N,μ-S-L¹⁰-2H)₄} 10 (L¹⁰: R = Me) [reproduced with permission from ref. 98 Copyright (2006) Wiley-VCH Verlag GmbH & Co. KGaA].

Fig. 5 Molecular structure of {Pd₃(C,N,μ-S-L¹¹-2H)₃} 14 [reprinted with from ref. 99. Copyright (2009) Elsevier].

There are seventeen mononuclear (18, 30, 34, 85, 86, 88–90, 128, 152–155, 157, 159, 160)^{100–103,108,109,111–113} and twelve dinuclear (44, 62, 67, 72, 75, 76, 79, 97, 105, 141, 143, 147)^{102,104–108,110} cyclopalladated compounds whose molecular structures have been reported (Table 1). These mono- and di-nuclear compounds have terdentate (C,N,S) thio-ligands with PPh₃ and bis(tertiaryphosphines) as co-ligands. Bis(tertiaryphosphines) are either mono-dentate (dppm, dppen) or bridging bidentate (dppe, dppen, etc.). Representative molecular structures are depicted below. The thio-ligands L¹⁵ and L⁵⁶ with PPh₃ as co-ligand have formed mononuclear four coordinated square planar complexes, [Pd(C,N,S-L¹⁵-2H)(PPh₃)] 18 (L¹⁵: R² = H, R³ = H, R = Me at 6-position) (Fig. 6) and [Pd(C,N,S-L⁵⁶-2H)(PPh₃)] 159 (L⁵⁶: R² = Me, R³ = Me, see ligand type D above) (Fig. 7). In both cases either methyl-substituted phenyl (18) or thiopheneyl) (159) rings are metallated. Bis(tertiaryphosphines), dppm and dppen are mono-denate in four coordinated square planar complexes, [Pd(C,N,S-L²⁹-2H)(η¹-P-dppm)] 85 (L²⁹: R² = Me, R³ = Me, R = Br at 4-position) (Fig. 8) and [Pd(C,N,S-L³²)(η¹-P-dppen)] 90 (L³²: R² = Me, R³ = Me, R = F at 4-position) (Fig. 9) with one functional group, –PPh₂ pendant in both cases.

Fig. 6 Molecular structure of Pd(C,N,S-L¹⁵-2H)(PPh₃)} 18 (L¹⁵: R² = H, R³ = H, R = Me at 6-position) [reproduced with permission from ref. 101. Copyright (2013) Royal Societyof Chemistry].

Fig. 7 Molecular structure of [Pd(C,N,S-L⁵⁶-2H)(PPh₃) ]159 (L⁵⁶: R² = Me, R³ = Me, see ligand type D above) [reprinted with permission from ref. 112. Copyright (2012) Elsevier].

Fig. 8 Molecular structure of [Pd(C,N,S-L²⁹-2H)(η¹-P-dppm)] 85 (L²⁹: R² = Me, R³ = Me, R = Br at 4-position) [reprinted with permission from ref. 103. Copyright (2006) Elsevier].

Fig. 9 Molecular structure of [Pd(C,N,S-L³²)(η¹-P-dppen)] 90 (L³²: R² = Me, R³ = Me, R = F at 4-position) [reprinted with permission from ref. 108. Copyright (2006) Elsevier].

The co-ligands, dppm and dppen are bridging bidentate in dinuclear complexes, {Pd(C,N,S-L²⁴)}₂(μ-P,P-dppm) 44 (Fig. 10) and {Pd(C,N,S-L²⁶)}₂(μ-P,P-dppen) 75 (Fig. 11). Bis(tertiaryphosphines), dppe and dppp are only bridging bidentate in dinuclear complexes, {Pd(C,N,S-L³¹)}₂(μ-P,P-dppe) 67 (Fig. 12) and {Pd(C,N,S-L²⁸)}₂(μ-P,P-dppp) 62 (Fig. 13). In another category of dinuclear complexes, [Pd₂Cl₂(C,N,μ-S-L³²)(μ-P,P-dppm)] 97 (Fig. 14), [Pd₂Cl₂(C,N,μ-S-L⁴⁶)(μ-P,P-bdcp)] 141 (Fig. 15) and [Pd₂(C,N,μ-S-L⁴¹)(μ-P,P-dppm)(κ²-P,P-dppm)](ClO₄)₂ 143 (Fig. 16), the co-ligands dppm and bdcp are bridging two Pd metal centers which are differently coordinated. One metal center has Pd(CNSP) (97, 141, 143) core and second metal center has Pd(PSCl₂) (97, 141) or Pd(P₃S) (143) core.

Fig. 10 Molecular structure of dinuclear{Pd(C,N,S-L²⁴)}₂(μ-P,P-dppm) 44 (L²⁴: R² = Et, R³ = H, R = H) [reproduced with permission from ref. 102. Copyright (1999) Royal Society of Chemistry].

Fig. 11 Molecular structure of {Pd(C,N,S-L²⁶)}₂(μ-P,P-dppen) 75 (L²⁶: R² = Me, R³ = Me, R = Cl at 4-position of ring) [reprinted with permission from ref. 108 Copyright (2006) Elsevier].

Fig. 12 Molecular structure of {Pd(C,N,S-L³¹)}₂(μ-P,P-dppe) 67 (L³¹: R² = Me, R³ = Ph, R = Br at 4-position of ring) [reprinted with permission from ref. 106. Copyright (2012) Elsevier].

Fig. 13 Molecular structure of {Pd(C,N,S-L²⁸)}₂(μ-P,P-dppp) 62 (L²⁸: R² = Me, R³ = Ph, R = Cl at 4-position of ring) [reprinted with permission from ref. 105. Copyright (2006) Elsevier].

Fig. 14 Molecular structure of [Pd₂Cl₂(C,N,μ-S-L³²) (μ-P,P-dppm)] 97 (L³²: R² = Me, R³ = Me, R = F at 4-position of ring) [reprinted with permission from ref. 104. Copyright (2003) American Chemical Society].

Fig. 15 Molecular structure of [Pd₂Cl₂(C,N,μ-S-L⁴⁶) (μ-P,P-bdcp)] 141 (L⁴⁶: R² = Me, R³ = Me, R = MeO at 4-position of ring) [reprinted with permission from ref. 110. Copyright (2013) Elsevier].

Fig. 16 Molecular structure of [Pd₂(C,N,μ-S-L⁴¹)(μ-P,P-dppm)(κ²-P,P-dppm)](ClO₄)₂ 143 (L⁴¹: R² = Me, R³ = Me, R = MeO at 6-position of ring) (perchlorates not shown) [reprinted with permission from ref. 110. Copyright (2013) Elsevier].

Platinum. There is one tetranuclear (182),⁹⁶ one dinuclear (198)¹¹⁷ and two mononuclear (170, 187)^115,116 platinum(II) compounds whose crystal structures have been determined. The coordination pattern of thio-ligands in tetranuclear Pt^II compound, {Pt₄(C,N,μ-S-L⁴-2H)₄} 182 (Fig. 17), mononuclear [Pt(C,N,S-L⁶⁰-2H)(Ph₃P)] 170 (Fig. 18) and [Pt(C,N,S-L⁶⁵-2H)(Ph₃P)] 187 (Fig. 19) as well as di-nuclear {Pt₂(C,N,S-L⁷⁰-2H)₂}(dppe) 198 (Fig. 20) is similar to the corresponding Pd^II analogous compounds discussed in the preceding section.

Fig. 17 Molecular structure of {Pt₄(C,N,μ-S-L⁴-2H)₄} 182 (L⁴: R² = H, R³ = H; R = –CHMe₂ at 6-position [reproduced with permission from ref. 96. Copyright (1998) American Chemical Society].

Fig. 18 Molecular structure of [Pt(C,N,S-L⁶⁰-2H)(Ph₃P)] 170 (L⁶⁰: R² = Me, R³ = H, R = Me at 6-position of ring) [reprinted with permission from ref. 115. Copyright (2000) Elsevier].

Fig. 19 Molecular structure of [Pt(C,N,S-L⁶⁵-2H)(Ph₃P)] 187 (L⁶⁵: R² = H, R³ = H, R = Me at 6-position of ring) [reprinted with permission from ref. 116. Copyright (2012) Elsevier].

Fig. 20 Molecular structure of {Pt₂(C,N,S-L⁷⁰-2H)₂}(dppe) 198 (L⁷⁰: R² = Me, R³ = Et) [reproduced with permission from ref. 117 Copyright (2007) Wiley-VCH Verlag GmbH & Co. KGaA].

Ruthenium, rhodium and iridium. For ruthenium(II), molecular structures of five mononuclear (208, 220, 228, 231, 232)^{118,120–122} and two dinuclear (215, 230)^119,121 compounds have been reported. Finally molecular structures of three mononuclear rhodium(III) (236, 238, 242)¹²³ and two mononuclear iridium(III) (248, 250)¹²⁴ have been determined. The co-ligands PPh₃ and AsPh₃ are trans in mononuclear complexes [Ru(C,N³,S-L⁷²-2H)(CO)(PPh₃)₂] 208 (Fig. 21) and [Ru(C,N,S-L⁷⁹-2H)(CO)(AsPh₃)₂] 220 (Fig. 22) with terdentate thio-ligands (C,N,S) coordinating in square plane with fourth site occupied by CO ligand. Thiophene based thio-ligand L⁸⁸ (D type) as well as thio-ligand L⁸⁹ (A type) coordinated as terdentate (C,N,S) to square planes of octahedral [Ru(C,N,S-L⁸⁸-2H)(P,P-dppm)(P-dppm)] 231 (Fig. 23) and [Ru(C,N,S-L⁸⁹-2H)(P,P-dppm)(P-dppm) 232 (Fig. 24) respectively. Here one dppm chelates Ru^II with one P donor atom occupying fourth site of square plane and second P donor atom binding to metal center along axial side. The sixth site along axial side is occupied by one P donor end of second dppm molecule leaving second donor end, –PPh₂ pendant. Dinuclear Ru^II complex, [Ru₂(C,N,μ-S-L⁷⁴)₂(PPh₃)₂(CO)₂] 215 has one CO group and C,N,S-donor set of one thio-ligand coordinates the square plane and two such square planes are bridged through the coordinated sulfur and two PPh₃ molecules coordinate along the terminal axial sides (Fig. 25). In mononuclear octahedral Rh^III/Ir^III compounds (236, 238, 242, 248 and 250),^123,124 two PPh₃ ligands in each case are trans along the axial side, while terdenate (C,N,S) thio-ligands occupy the square plane of the metal centers and fourth side in the plane is occupied by H⁻ or Cl⁻ donor groups (Fig. 26–30).

Fig. 21 Molecular structure of [Ru(C,N³,S-L⁷²-2H)(CO)(PPh₃)₂] 208 (L⁷²: R² = H, R³ = H, R = Me at 6-position of ring) [reprinted with permission from ref. 118. Copyright (2011) Elsevier].

Fig. 22 Molecular structure of [Ru(C,N,S-L⁷⁹-2H)(CO)(AsPh₃)₂] 220 (L⁷⁹: R², R³ = Me, Ph; R = Cl at 6-position) [reprinted with permission from ref. 120. Copyright 2009 Elsevier].

Fig. 23 Molecular structure [Ru(C,N,S-L⁸⁸-2H)(P,P-dppm)(P-dppm)] 231 (L⁸⁸: R², R³ = H, H; see ligand type D above) [reprinted with permission from ref. 122. Copyright (2008) American Chemical Society].

Fig. 24 Molecular structure of [Ru(C,N,S-L⁸⁹-2H)(P,P-dppm)(P-dppm) 232 (L⁸⁹: R², R³, R = H, H, H) [reprinted with permission from ref. 122. Copyright (2008) American Chemical Society].

Fig. 25 Molecular structure of [Ru₂(C,N,μ-S-L⁷⁴-2H)₂(PPh₃)₂(CO)₂] 215 (L⁷⁴: R², R³ = H, H; R Cl at 6-position) [reproduced with permission from ref. 119. Copyright (2008) Wiley-VCH Verlag GmbH & Co. KGaA].

Fig. 26 Molecular structure of [Rh(C,N,S(=O)₂-L⁹³-2H)(H)(PPh₃)₂] 236 (L⁹³: R², R³ = H, H; R = Cl at 6-position) [reprinted with permission from ref. 123. Copyright (2006) American Chemical Society].

Fig. 27 Molecular structure of [Rh(C,N,S-L⁹⁴-2H)(H)(PPh₃)₂] 238 (L⁹⁴: R², R³ = H, H; R = NO₂ at 6-position) [reprinted with permission from ref. 123. Copyright (2006) American Chemical Society].

Fig. 28 Molecular structure of [Rh(C,N,S-L⁹³-2H)(H)(PPh₃)₂] 242 (L⁹³: R², R³ = H, H; R = Cl at 6-position) [reprinted with permission from ref. 123. Copyright (2006) American Chemical Society].

Fig. 29 Molecular structure of [IrH(C,N,S-L⁹⁴-2H)(PPh₃)₂] 248 (L⁹⁴: R², R³ = H, H; R = NO₂) at 6-position [reprinted with permission from ref. 124. Copyright 2010 Elsevier].

Fig. 30 Molecular structure of [Ir(C,N,S-L⁹¹-2H)(PPh₃)₂(Cl)] 250 (L⁹¹: R², R³ = H, H; R = Me at 6-position) [reprinted with permission from ref. 124. Copyright 2010 Elsevier].

The basic coordination pattern of thio-ligands around a metal center of tetranuclear Pd^II complexes (Table 1) belongs to E type core, except in complex 10 which has F type core and trinuclear complex 14 which has G type core (Chart 1). In E type core, metal forms 5-membered metallacyclic ring, and in F and G type cores, there are 6-membered rings. Due to the difference in nature of rings, the angles around a metal center are different. For example, in complex 27 (E core), the trans bond angles, N³–Pd–S (bridging) and C–Pd–S are 176.35(18) and 162.6(3)° respectively. These angles change to respective 173.23(12) and 176.29(14)° in 10 (F core) and 176.5(30) and 165.29(14)° in 14 (G core). Similarly, other angles C–Pd–N³ and N³–Pd–S are affected. The Pd–C, Pd–N and Pd–S bond distances differ to a small extent. The bond parameters of tetranuclear compounds 50, 53, 55, 100, 102 and 182 are similar to that of 27 (Table 1). The geometry around Pd/Pt metal centers in oligomers can be labeled as distorted square planar.

Chart 1 Basic coordination cores of complexes of different nuclearity; b refers to bridging sulfur.

It may be pertinent to note that compounds 9–13 have F type coordination pattern for thio-ligands, only 14 belongs to G type core. All other compounds 1–8, 15–50, 51–100, 101–150, 151–200, 201–253 described in Section 3.1 (Table 1 structurally characterized) have E type coordination pattern (S^b-bridging sulfur is replaced by P donor in phosphine complexes). A brief commentary on bond parameters of mono- and di-nuclear Pd/Pt complexes with phosphines as co-ligands is provided here. The coordination patterns of phosphines in mono- and di-nuclear Pd/Pt complexes are given in Chart 2. In case of Pd^II complexes, all the five patterns (H–L, Chart 2) have been observed, while in case of Pt^II only two patterns (H, J) were observed. The trans bond angles, N³–M–P and C–M–S fall in the ranges, 176–179 and 162–165° respectively while C–M–N³ and S–M–N³ angles lie in the range, 80–83°. Various M–C, M–N, M–S and M–P bond distances are normal and are only marginally different. The C–S bond distances suggest the presence of partial double bond character. Ruthenium(II) complexes have shown four types (M, N, O, P) of coordination patterns as shown in Chart 3. Rhodium(III) and iridium(III) complexes belong to only M type coordination pattern. The trans bond angles suggest distorted octahedral geometries of complexes of these metals (see Table 1).

Chart 2 Different coordination patterns of phospines in Pd/Pt complexes.

Chart 3 Coordination patterns of Ru, Rh and Ir complexes.

3.3 Electronic absorption spectroscopy, fluorescence and cyclic voltammetry

The reported electronic absorption spectral data of metal complexes are placed in Table 2.^{101,113,118–121,123,124} The assignments have been supported by DFT calculations in several cases using standard model procedures. In Pd^II compounds, the transitions occur from π → π* (Region I), n → π* orbitals in Regions II and III. In case of ruthenium compounds, π → π* and n → π* transitions are assigned in high energy regions I and II and in low energy visible region (III), the MLCT transitions (metal to ligand charge transfer) involve movement of metal t_2g electron to π* orbitals on –C=N-(azomethine imine) moiety.^118–121 The behavior of Rh/Ir compounds is similar to that of Ru compounds.^123,124 Mononuclear and dinuclear complexes did not differ much in transitions. Only one report deals with fluorescence spectroscopy. Complexes 217–227 (Ru) showed weak emission in the region, 590–596 nm, corresponding to excitation wave length of 450 nm.¹²⁰

Table 2 Electronic absorption spectral data of complexes^a

Compounds	Region III nm	Region II nm	Region I nm	Ref.
a M-mononuclear; D-dinuclear.
Pd: 17–21 (M)	452 to 494 L → L	326–364 L → L	272–262 L → L	101
Pd: 157–158 (M)	429–447 L → L	336–368 L → L	266–272 L → L	113
Ru: 207–211 (M)	462–547 L → L	342–400; 300–338 L → L	269–272 L → L	118
Ru: 212–216 (D)	436–562 MLCT	302–396 L → L	270–352 L → L	119
Ru: 217–227 (M)	462–580 MLCT	307–398 L → L	242–250 L → L	120
Ru: 228 (M) Ru–Pd/Pt: 229–230 (D)	492, 480–488 MLCT	334–400 L → L	276–304 L → L	121
Rh: 233–243 (M)	400–510 MLCT	290–360 L → L	234–286 L → L	123
Ir: 244–253 (M)	436–510 MLCT	—	232–382 L → L	124

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Only complexes of Ru, Rh and Ir are studied for their cyclic voltammetric studies. The CV study of Ru^II complexes 207 to 211 in CH₂Cl₂–CH₃CN mixture (tetrabutylammonium hexafluro phosphate, TBAP, as supporting electrolyte) showed two oxidative responses in the region 0.46 to 1.33 V and one reductive response in the region, −1.00 to −1.41 V relative to SCE. These are attributed to the oxidation (former) and reduction (latter) of the coordinated ligand.¹¹⁸ The CV of dinuclear complexes 212 to 216 in CH₂Cl₂–CH₃CN mixture using tetrabutylammonium perchlorate as supporting electrolyte have shown reversible Ru^II–III redox behavior {E_1/2 – 0.48 to 0.70 V; ΔE_p (E_pa − E_pc) = 70 to 80 mV}. Additionally dinuclear complexes have shown an irreversible oxidative response in the range, 1.09 to 1.47 V and this is assigned to Ru^III–Ru^IV oxidation.¹¹⁹ Mononuclear Ru^II complexes 217 to 227 exhibit a quasi reversible one electron reduction Ru^II/Ru^I) in the range −0.83 to −0.86 V (SCE, TBAP) and it is noted that the formal potential of all the couples appear to correlate linearly with the Hammett constant of the p`phenyl of the acetophenone thiosemicarbazone ligands.¹²⁰

The CV study of Rh^III complexes 233 to 243 showed two oxidative responses in the regions, 0.64 to 1.07 V (reversible in some cases), 0.93 to 1.73 V and one reductive response in the region −1.06 to −1.32 V relative to SCE. The first oxidative response is assigned to Rh^III–Rh^IV oxidation, second irreversible oxidative response to the coordinated thiosemicarbazone and finally the reductive response is attributed to reduction of the coordinated thiosemicarbazone.¹²³ Iridium(III) complexes 244 to 253 has also shown two oxidative responses in the regions, 0.47 to 0.92 V (reversible), 1.07 to 1.36 V and one reductive response in the region −0.74 to −1.47 V relative to SCE. The first oxidative response is assigned to Ir^III–Ir^IV oxidation, second irreversible oxidative response to the coordinated thiosemicarbazone and finally the reductive response is attributed to reduction of the coordinated thiosemicarbazone.¹²⁴

3.4 NMR spectroscopy

The ortho-metallated compounds have also been studied using ¹H, ¹³C and ³¹P NMR spectroscopy (see structure E for numbering scheme of a thio-ligand in general). The ¹H NMR spectral data are given only for representative examples and are placed in Table 3. The reported ¹³C and ³¹P NMR data are given for significant or diagnostic signals for nearly all the compounds which are listed in Table 4 and 5 respectively. The ¹H NMR spectral signals are first commented briefly here. For example, tetra-nuclear oligomer 1 (ref. 95) showed –C²–H, –N¹–H, –N¹–Me and –OMe (R-ring) proton signals at δ = 6.78(s), 5.1(br), 2.3(d) and 3.87(s) ppm respectively which were mostly up-field relative to the free ligand L¹ in this case. The –N²–H proton as expected disappeared in oligomer 1 which was at 9.66 ppm in the free ligand (Table 3). Several selected complexes listed in Table 3 are shown to demonstrate this trend and is true of all other complexes reported in literature. The –NH₂ protons in the free ligands usually appear as a pair or a broad single signals, and generally as a single signal in complexes. Metal-hydride signals of Rh/Ir complexes appeared as multiplets (coupling from Rh/P nuclei, I = 1/2 in each case) in the high field region (Table 3).

Table 3 ¹H NMR data of representative complexes (δ in ppm)

Compd. no.	–C^2–H	–N^2–H	–N^1–H	–N^1–R	R-ring^a	(Ref.)
a Refers to the substituent of ring at C^2 carbon.b Metallated Me group of ring.c n.a. – not applicable, n.o. – not observed.d Metal hydride, Rh–H; and Ir–H; dt = doublet of triplet; td = triplet of doublet.e Obscured by PPh3.
L^1	7.80 (s)	9.66 (s)	7.56 br	3.26 (d, Me)	3.84 (s, OMe)	95
1	6.78 (s)	—	5.1 (br)	2.3 (d, Me)	3.87 (s, OMe)	95
L^9	8.30 (s)	11.48 (s)	8.24, 7.61 (br)	—	2.68 (s, OMe)	98
12	8.79 (d) JH–P=2.7	—	n.o.^c	n.o.^c	2.66 (s, OMe)	98
L^11	8.23 (s)	10.31 (s)	7.42 (s)	1.31 (m), 3.76 (m) (Et)	2.34, 2.40 (2 s, Me)	99
14	7.91 (s)	—	4.99 (m)	1.29 (m), 3.47 (m) (Et)	2.23 s (Me), 3.10 (s, CH2)^b	99
L^12	7.84(s)	3.74 s (N^2–Me)	8.22 s, 8.27 s	n.a.^c	3.78 s (MeO)	100
15	8.14 s	3.46 (N^2–Me)	n.o.^c	n.a.^c	3.72 s (MeO)	100
L^13	7.85 (s)	3.76 s (N^2–Me)	8.31 s, 8.38 s	n.a.^c	3.79 (MeO)	100
16	8.18 s	3.47 (N^2–Me)	9.47 s, 8.74 s	n.a.^c	3.68 s (MeO)	100
17	8.18 s	—	4.79 s (NH2)	n.a.^c	3.84 s (MeO)	101
L^63	2.28 s (C^2–Me)	8.70 br	7.60 br	3.25d (Me)	2.38 s (Me)	115
168	1.72 s (C^2–Me)	—	5.20 br	3.06d (Me)	2.36 s (Me)	115
173	1.75 s (C^2–Me)	—	4.7 br	2.96d (Me)	2.44 s (Me)	115
175	2.31 s	—	5.20 br	n.a.^c	0.89 t (Me); 3.70 t (CH2O–)	115
208	6.96 s	—	5.34 s	n.a.^c	1.85 s(Me)	118
213	7.78 s	—	4.04 s	n.a.^c	1.94 s(Me)	119
234	6.70 s	–; −11.5 td	4.69 s	n.a.^c	1.91 s(Me)	123
240	6.93 s	—	4.39 s	n.a.^c	1.90 s(Me)	123
245	7.29–7.66^e	–, −19.33 dt^d −15.27 dt^d	4.49 s	n.a.^c	1.99 s(Me)	124
250	6.90 s	—	4.60 s	n.a.^c	2.00 s(Me)	124

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Table 4 The ¹³C NMR data of complexes (δ in ppm)

^13C NMR^a
Compd. no.	δ(C^8)	δ(C^2)	δ(C^1(S))	δ(C^3)	Solvent	(Ref.)
a Reference TMS.
L^1	129.2	142.0	177.9	127.2	CDCl3	95
1	148.0	156.2	167.9	132.1	CDCl3	95
2	141.9	158.8	168.2	132.6	CDCl3	95
4	165.7	158.7	167.8	147.41	dmso-d6	96
5	164.9	160.0	167.1	146.4	CDCl3	97
6	166.4	158.6	169.1	146.9	CDCl3	97
7	168.5	158.7	171.1	145.6	CDCl3	97
8	166.7	158.3	168.4	147.6	CDCl3	97
15	160.1	157.6	189.1	139.6	CDCl3	100
16	160.1	157.8	174.6	139.6	CDCl3	100
22	147.5	166.8	167.3	165.1	dmso-d6	102
23	147.2	165.9	169.6	165.1	CDCl3	100
24	147.5	165.8	169.1	165.0	CDCl3	100
25	147.0	166.5	172.1	161.0	CDCl3	100
27	148.5	167.2	172.0	165.5	dmso-d6	102
28	142.2	159.5	168.0	132.0	CDCl3	95
29	151.1	166.2	174.1	158.5	dmso-d6	102
33	149.0	165.6	175.2	163.7	dmso-d6	102
123	165.1	166.1	167.7	150.7	dmso-d6	109
124	165.5	168.1	170.8	149.4	dmso-d6	109
127	152.7	163.6	176.6	136.1	dmso-d6	109
128	164.1	168.2	176.9	151.5	dmso-d6	109
129	152.7	163.8	178.6	136.1	dmso-d6	109
130	163.3	164.0	176.6	152.6	dmso-d6	109
131	153.2	163.9	165.1	136.8	dmso-d6	109
132	151.4	168.3	176.8	136.2	dmso-d6	109
133	163.7	168.4	176.6	151.3	dmso-d6	109
134	164.7	169.1	177.4	152.1	dmso-d6	109
182	154.2	162.1	165.0	145.1	dmso-d6	96
183	154.0	162.0	165.0	145.6	CDCl3	97
184	158.9	160.8	167.3	146.2	CDCl3	97
185	156.9	161.2	Not obsd.	146.7	CDCl3	97

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Table 5 The ³¹P NMR data of complexes (δ in ppm; J in Hz)^abc

δP			δP			(Ref.)
a 85% H3PO4.b (MeO)3P reference markers.c {M-mononuclear; D-dinuclear, M-P, mononuclear with pendant PPh2 group; M-C, mononuclear with chelating dppm donors facing different donors bonded to metal; D-UM, dinuclear with phosphines bridging unequal metals; D-UUM-dinuclear with unsymmetrical phosphines bonded to unequal metals; M-UP, mononuclear with unequal phosphines}.
12^D	70.4	dmso-d6	13^D	70.3	dmso-d6	98
30^M	36.7	CDCl3	31^M	36.8	CDCl3	100
32^M	36.7	CDCl3	35^M	38.4	CDCl3	100
36^M	38.5	CDCl3	37^M	38.6	CDCl3	100
38^M	38.3	CDCl3	39^D	34.4	CDCl3	102
40^D	27.0	CDCl3	41^D	25.4	CDCl3	102
42^D	34.2	dmso-d6	43^D	27.4	CDCl3	102
44^D	25.4	CDCl3			CDCl3	102
45^M-P	27.6 (d, Pa; ^2JPP = 71.0), −23.7 (d, Pb; ^2JPP = 71.0)				CDCl3	102
46^M-P	22.3 (d, Pa; ^2JPP = 71.0), −30.2 (d,Pa; ^2JPP = 71.0)				CDCl3	102
47^M-P	42.7 (d, Pa; ^2JPP = 65.7.0), −16.4 (d, Pa; ^2JPP = 65.7)				CDCl3	102
48^M-P	27.2 (d, Pa; ^2JPP = 73.0), −24.0 (d, Pb; ^2JPP = 73.0)				CDCl3	102
58^D	23.7	CDCl3	59^D	22.9	CDCl3	103
60^D	31.5	CDCl3	61^D	32.5	CDCl3	105
62^D	26.8	CDCl3	63^D	28.7	CDCl3	105
64^D	23.6	CDCl3	65^D	23.7	CDCl3	103
66^D	23.7	CDCl3	67^D	32.5	CDCl3	106
68^D	26.8	CDCl3	69^D	28.7	CDCl3	106
70^D	32.5	CDCl3				106
71^D	23.8	CDCl3	72^D	31.8	CDCl3	108
73^D	32.0	CDCl3	74^D	31.4	CDCl3	108
75^D	32.4	CDCl3	76^D	32.4	CDCl3	108
77^D	26.9	CDCl3	78^D	26.9	CDCl3	108
79^D	31.6	CDCl3	80^D	32.3	CDCl3	107
81^D	26.9	CDCl3	82^D	28.7	CDCl3	107
83^M-P	24.1 (d, Pa; ^2JPP = 79.8), −26.7 (d, Pb; ^2JPP = 79.8)				CDCl3	103
84^M-P	23.7 (d, Pa; ^2JPP = 79.8), −26.7 (d, Pb; ^2JPP = 79.8)				CDCl3	103
85^M-P	23.9 (d, Pa; ^2JPP = 79.8), −26.8 (d, Pb; ^2JPP = 79.8)				CDCl3	103
86^M-P	23.6 (d,; Pa ^2JPP = 75.1), −26.7 (d, Pb; ^2JPP = 75.1)				CDCl3	103
87^M-P	23.7 (d, Pa; ^2JPP = 75.1), −26.7 (d, Pb; ^2JPP = 75.1)				CDCl3	106
88^M-P	24.4 (d, Pa; ^2JPP = 75.1), −26.3 (d, Pb; ^2JPP = 75.1)				CDCl3	108
89^M-P	24.4 (d, Pa; ^2JPP = 75.1), −26.3 (d, Pb; ^2JPP = 75.1)				CDCl3	108
90^M-P	22.3 (d, Pa; ^2JPP = 75.3), −25.2 (d, Pb; ^2JPP = 75.3)				CDCl3	108
91^M-C	24.4 (d, Pa; ^2JPP = 13.1); 22.8 (d, Pb; ^2JPP = 13.3)				CDCl3	103
92^M-C	24.3 (d, Pa; ^2JPP = 13.1); 22.7 (d, Pb; ^2JPP = 13.1)				CDCl3	103
93^M-C	24.1 (d, Pa; ^2JPP = 13.1), 22.7 (d, Pb; ^2JPP = 13.1)				CDCl3	103
94^M-C	24.2 (d, Pa; ^2JPP = 13.7); 22.8 (d, Pb; ^2JPP = 14.5)				CDCl3	103
95^D-UM	22.5 (d, Pa; ^2JPP = 26.0); 16.7 (d, Pb)				CDCl3	104
96^D-UM	22.9 (d, Pa; ^2JPP = 26.0); 16.8 (d, Pb)				CDCl3	104
97^D-UM	22.7 (d, Pa; ^2JPP = 26.7); 16.8 (d, Pb)				CDCl3	104
98^D-UM	22.9 (d, Pa; ^2JPP = 28.5); 16.8 (d, Pb)				CDCl3	104
103^D	32.1	CDCl3	104^D	31.9	CDCl3	105
105^D	27.1	CDCl3	106^D	28.7	CDCl3	105
107^D	23.6	CDCl3	108^D	32.0	CDCl3	106
109^D	27.1	CDCl3	110^D	8.7	CDCl3	106
111^D	32.7	CDCl3				106
112^D	31.5	CDCl3	113^D	32.5	CDCl3	105
114^D	26.8	CDCl3	115^D	28.7	CDCl3	105
116^D	24.2	CDCl3	117^D	32.4	CDCl3	106
118^D	27.3	CDCl3	119^D	29.2	CDCl3	106
120^D	32.9	CDCl3				106
121^M-P	24.4 (d, Pa; ^2JPP = 75.1), −26.5 (d, Pb; ^2JPP = 75.1)				CDCl3	106
122^M-P	25.0 (d, Pa; ^2JPP = 75.1), −26.4 (d, Pb; ^2JPP = 75.1)				CDCl3	106
127^M	38.8	dmso-d6	128^M	38.6	dmso-d6	109
129^D	29.2	dmso-d6	130^D	34.2	dmso-d6	109
131^D	39.0	dmso-d6	132^D	29.1	dmso-d6	109
133^D	33.7	dmso-d6	134^D	38.8	dmso-d6	109
135^D-UM	23.2 (d, Pa; ^2JPP = 30.2), 17.1 (d, Pb; ^2JPP = 30.2)				CDCl3	110
136^D-UM	21.4 (d, Pa; ^2JPP = 30.5), 17.7 (d, Pb; ^2JPP = 30.5)				CDCl3	110
137^D-UM	38.1 (d, Pa; ^2JPP = 73.8), 29.3 (d, Pb; ^2JPP = 73.8)				CDCl3	110
138^D-UM	38.1 (d, Pa; ^2JPP = 71.1), 29.8 (d, Pb; ^2JPP = 71.1)				CDCl3	110
139^D-UM	37.6 (d, Pa; ^2JPP = 77.0), 29.4 (d, Pb; ^2JPP = 77.0)				CDCl3	110
140^D-UM	43.8 (d, Pa; ^2JPP = 61.0), 31.8 (d, Pb; ^2JPP = 61.0)				CDCl3	110
141^D-UM	42.8 (d, Pa; ^2JPP = 61.0), 31.8 (d, Pb; ^2JPP = 61.0)				CDCl3	110
142^D-UM	44.4 (d, Pa; ^2JPP = 63.6), 31.1 (d, Pb; ^2JPP = 63.6)				CDCl3	110
143^D-UUM	28.5 (d, Pa, ^2Jab, 32.6), 17.3 (ddd, Pb, ^2Jbd, 398.8, ^2Jab, 32.6, ^2Jbc,14.2), −23.0 (dd, Pc, ^2Jcd, 69.2, ^2Jbc, 12.2), −29.4 (dd, Pd, ^2Jbd, 398.8, ^2Jcd, 69.2)				CDCl3	110
144^D-UUM	39.6 (d, Pa, ^2Jab, 86.5), 22.6 (ddd, Pb, ^2Jbd, 399.7, ^2Jab, 86.5, ^2Jbc, 20.3), −8.6 (d, Pd, ^2Jbd, 399.7), −10.2 (d, Pc, ^2Jbc, 20.3)				CDCl3	110
145^D-UUM	26.1 (d, Pa, ^2Jab, 33.6), 13.1 (ddd, Pb, ^2Jbd, 394.2, ^2Jab, 33.6, ^2Jbc, 22.9), −7.5 (dd, Pd, ^2Jbd, 394.2, ^2Jcd, 5.1), −8.6 (dd, Pc, ^2Jbc, 22.9, ^2Jcd, 5.1)				CDCl3	110
146^D-UUM	24.8 (d, Pa,^2Jab, 33.1), 12.5 (ddd, Pb, ^2Jbd, 394.2, ^2Jab, 33.1, ^2Jbc, 10.2), −13.7 (dd, Pc, ^2Jcd, 38.1, ^2Jbc, 10.2), −16.6 (dd, Pd, ^2Jbd, 394.2, ^2Jcd, 38.1)				CDCl3	110
147^D-UUM	45.7 (d, Pa, ^2Jab, 68.7), 24.4 (ddd, Pb, ^2Jbd, 389.1, ^2Jab, 68.7, ^2Jbc, 7.6), −29.2 (dd, Pd,^2Jbd, 389.1, ^2Jcd, 73.8), −33.0 (dd, Pc,^2Jcd, 73.6, ^2Jbc, 7.6)				CDCl3	110
148^D-UUM	66.3 (d, Pd, ^2Jbd, 369.7), 62.0 (d, Pc, ^2Jbc, 22.9), 40.3 (d, Pa, ^2Jab, 86.5), 22.8 (ddd, Pb ^2Jbd, 396.7, ^2Jab, 86.5, ^2Jbc, 22.9)				CDCl3	110
149^D-UUM	61.0 (d, Pd, ^2Jbd, 368.8), 58.7 (d, Pc, ^2Jbc, 17.8), 42.8 (d, Pa, ^2Jab, 61.0), 26.9 (ddd, Pb, ^2Jbd, 368.8, ^2Jab, 61.0, ^2Jbc, 17.8)				CDCl3	110
150^D-UUM	39.2 (d, Pa, ^2Jab, 83.9), 22.7 (ddd, Pb, ^2Jbd, 399.3, ^2Jab, 83.9, ^2Jbc, 20.3), −8.88 (d, Pd, ^2Jbd, 399.3), −10.4 (d, Pc, ^2Jbc, 20.3)				CDCl3	110
151^D-UUM	50.8 (d, Pa, ^2Jab, 66.1), 30.4 (ddd, Pb, ^2Jbd, 394.2, ^2Jab, 66.1, ^2Jbc, 22.9), −8.2 (d, Pd, ^2Jbd, 394.2), −13.1 (d, Pc, ^2Jbc, 22.9)				CDCl3	110
153^M	39.1	CDCl3	154	38.2^M	CDCl3	112
155^M	39.3	CDCl3	156	39.1^M	CDCl3	112
159^M	40.4	CDCl3	160	40.3^M	CDCl3	112
Platinum(II)
169^M	22.7 (JP–Pt = 3880)	CDCl3	170^M	23.0 (JP–Pt = 3854)	CDCl3	115
171^M	22.8 (JP–Pt = 3857)	CDCl3	172^M	21.7 (JP–Pt = 4071)	CDCl3	115
173^M	21.7 (JP–Pt = 4107)	CDCl3	174^M	17.6 (JP–Pt = 3869)	CDCl3	115
175^D	11.6 (JP–Pt = 3943)	CDCl3	176^D	17.7 (JP–Pt = 3843)	CDCl3	115
177^D	13.3 (JP–Pt = 3941)	CDCl3	178^D	15.2 (JP–Pt = 3813)	CDCl3	115
179^D	15.4 (JP–Pt = 3819)	CDCl3	180^D	15.2 (JP–Pt = 3815)	CDCl3	115
181^D	13.4 (JP–Pt = 3870)	CDCl3			CDCl3	115
193^M	16.60 (JP–Pt = 3743)	CDCl3	194^M	16.65 (JP–Pt = 3743)	CDCl3	117
195^D	12.56 (JP–Pt = 3693)	CDCl3	196^D	8.11 (JP–Pt = 3672)	CDCl3	117
197^D	7.33 (JP–Pt = 3693)	CDCl3	198^D	12.56 (JP–Pt = 3743)	CDCl3	117
199^D	8.14 (JP–Pt = 3662)	CDCl3	200^D	7.28 (JP–Pt = 3680.4 Hz)	CDCl3	117
201^M-P	6.15d (^2JPP = 71.2, JP–Pt = 3723); −27.9d (^2JPP = 71.2)				CDCl3	117
202^M-P	23.0d (^2JPP = 76.3, JP–Pt = 3784); −12.55d (^2JPP = 76.3)				CDCl3	117
203^M-P	3.70d (^2JPP = 73.2, JP–Pt = 3717); −30.40d (^2JPP = 73.2)				CDCl3	117
204^M-P	23.1d (^2JPP = 76.2, JP–Pt = 3684); −12.5d, ^2JPP = 76.2)				CDCl3	117
205^D-UM	8.75d (^2JPP = 25.4, JP–w = 245.7); 2.87 (d, ^2JPP = 25.4; JP–Pt = 3784)				CDCl3	117
206^D-UM	6.27d (^2JPP = 24.9, JP–w = 246), 0.40d (^2JPP = 24.9, JP–Pt = 3772)				CDCl3	117
231^M-UP	−115.5, −82.8, −55.0^b		232^M-UP	−133.8, −116.5, −99.5, −72.9^b	CDCl3	122

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¹³C NMR spectroscopy. The ¹³C NMR data for diagnostic carbon atoms (C⁸, C², C¹ and C³) are listed in Table 4. Here C⁸ carbon is bonded to metal center, C² is azomethine carbon bonded to coordinating N³ (–C²=N³–), C¹ carbon is again close to coordinating sulfur atom and finally C³ is ipso carbon of the ring. These carbons undergo shifts to high or low field relative to the uncoordinated ligand and is demonstrated with one example. The C⁸, C², and C³ signals of oligomer 1 at 148.0, 156.2 and 132.1 ppm are significantly down field relative to the free ligand at (129.2, 142.0 and 127.2 ppm respectively) except C¹ signal at 167.9 ppm which is upfield relative to the free ligand signal at 177.9 ppm. This upfield shift implies greater shielding of C¹ carbon when sulfur binds to metal. These trends are general and deviation from the trend can not be ruled out and the reader is advised to look to the original literature for rigorous comparison. The values of the chemical shifts for each category of these carbons fall in a fairly wide range owing to the divergence in the nature of thio-ligands as well as co-ligands forming the complexes listed in Table 4.

³¹P NMR spectroscopy. The reported ³¹P NMR data of phosphine containing complexes is placed in Table 5. For more comprehensive understanding of the data given in this Table, compounds are grouped into various categories. Mononuclear complexes containing one monodenate phosphine each (M: 30–32, 35–38, 127,128,153–160,169–174, 193,194) showed one ³¹P NMR signal. Likewise dinuclear complexes having equivalent P,P-bridging (D: 12, 13, 39–44, 58–82, 103–120, 129–134, 175–181, 195–200) also showed one signal each. Mononuclear complexes with pendant PPh₂ group (M-P: 45–48, 83–90, 121, 122, 201–204) have two non-equivalent P nuclei, P_a, P_b and thus showed two doublets involving P_a–P_b coupling. The coordinated P_a gave doublet in the low field region while the pendant P_b showed doublet in the high field region. The mononuclear complexes (M-C: 91–94) with chelating dppm donors but facing different thio-ligand donors showed two doublets in the low field region with small P_a–P_b coupling. The dinuclear complexes (D-UM: 95–98, 135–142, 205, 206) with diphosphines bridging unequal metals, again showed a pair of doublets with pattern similar to M-C category of complexes. The dinuclear complexes with unsymmetrical phosphines bonded to unequal metals (D-UUM: 143–151) have four types of P nuclei and four signals involving various couplings between P_a, P_b, P_c, P_d nuclei were observed. Finally mononuclear complexes with unequal phosphines (M-UP: 231, 232) showed different 3 or 4 ³¹P NMR signals (Chart 4).

Chart 4 Bonding modes of phopshines in complexes. M-mononuclear, D -dinuclear, M-P: mononuclear with pendant PPh₂ group, M-C: mononuclear with chelating dppm donors facing different donors bonded to metal, D-UM: dinuclear with phosphines bridging unequal metals, D-UUM: dinuclear with unsymmetrical phosphines bonded to unequal metals, M-UP: mononuclear with unequal phosphines.

3.5 Factors controlling cylometallation

This survey reveals that thiosemicarbazones (R¹R²C=N–NH–C(=S)–NR³R⁴) have cyclometallated palladium(II), platinum(II), ruthenium(II), rhodium(III) and iridium(III) only – a group of soft Lewis acids. There is no other metal ion which has formed similar direct M–C (thiosemicarbazone) bond. The R¹ groups at C² carbon undergoing cyclometallation are phenyl rings (with or without a substituent, e.g. MeO, F, etc.), furan, thiopheneyl and pyridyl rings. The R² group is H, Me or Et as the most common ones and NR³R⁴ group may be –NH₂, NHR³ or NR₂ type. Only R¹ group has formed M–C bonds and among R², R³ and R⁴ groups, only R² promoted cyclometallation. Among several cyclometallated compounds, there are only a few examples in which cases R¹ was phenyl group with no substitution and R² was hydrogen (Pd:¹⁰¹ 19; Pt:¹¹⁶ 188; Ru:^118,119,122 209, 214, 232; Rh:¹²³ 235, 241; Ir:¹²⁴ 246, 251). The cyclometallation by phenyl ring (R¹) can be categorized, in relation to thiosemicarbazones, into four different types when: (i) R¹ has one or two substituents with R² being hydrogen, (ii) R¹ has no substituent but R² is methyl, ethyl or phenyl, (iii) R¹ has substituents and R² is methyl, ethyl or phenyl and (iv) R and R² are hydrogens. Chart 5 shows, in summary, metallation process with change of R and R² substituents.

Chart 5 Metallation with change of R and R² groups.

As regards metallation by furan (R¹) and thiopheneyl (R¹) rings, there is no report where rings are having substituents, but R² group must be other than hydrogen for cyclometallation of these heterocyclic rings-based on available information. However, with this category of thio-ligands there are limited examples with Pd^II (R¹ = thiopheneyl group: 159, 160),¹¹² Pt^II (R¹ = furan group, 191–206)¹¹⁷ and Ru^II (R¹ = thiopheneyl, 231, 232)¹²² and none with Rh^III and Ir^III. Ruthenium(II) complexes 228–230 have single and double metallation by the pyridyl ring of bis-thiosemicarbazones (L⁸⁷),¹²¹ but there is methyl substitution (R²) at C² carbon which appears necessary for metallation. It may be noted that there is only one example (ligand L¹¹, complex 14)⁹⁹ where methyl group present at a position ortho (C⁴) to ipso (C³) underwent cyclometallation and not the phenyl ring.

It is pertinent to bring forward here some factors supporting cyclometallation. The presence of R substituents in phenyl ring (R¹) alter the electron density on aromatic ring via their electromeric or inductive effects which enhance the possibility of activation of C–H bond leading to cyclometallation. This pertains to cases (i) and (iii) as listed in Chart 5. When phenyl ring (R¹) has R substituent as hydrogen and R² is also hydrogen (case iv), there are only a few examples where metallation has occurred (Pd:¹⁰¹ 19; Pt:¹¹⁶ 188; Ru:^118,119,122 209, 214, 232; Rh:¹²³ 235, 241; Ir:¹²⁴ 246, 251). Thus when phenyl ring (R¹) has R substituent as hydrogen, for causing metallation R² must be non-hydrogen (case ii) and this has led to formation of several cyclometallated complexes. This is true also in case of thiosemicarbzones where R¹ groups are thiopheneyl, furan or pyridyl, requiring R² as non-hydrogen moiety excepting Ru^II complex 231 (R² = H) (vide infra). The effect of methyl substituent at C² carbon in making thiopheneyl group to metallate is shown in Chart 6 using an example of Pd metal complex. Thiophen-2-formaldehyde thiosemicarbazone (R² = H) yielded complexes A and B with no metallation,¹¹² but the presence of methyl group at C² carbon assisted metallation (159–160) by way of enhancing Lewis basicity of N³ nitrogen and also posing steric effect as shown in Chart 6. Chart 7 displays metallation shown by dppm in Ru^II chemistry forming complexes 231 and 232.¹²² The smaller bite of dppm assisted metallation by thiopheneyl and phenyl rings in unusual way, as in contrast, dppe merely formed non-metallated complexes.¹²²

Chart 6

Chart 7 Metallation caused by dppm in unusual way.

4. Applications – biological and catalysis

4.1 Biological applications

Among thio-ligands L³⁹ and L⁴⁰ and their complexes 123, 124, 127–134, only mononuclear Pd^II complexes 127 and 128 were most effective antimalarial agents against two Plasmodium falciparum strains, 3D7 (chloroquine sensitive) and K1 (chloroquine and pyrimethamine resistant).¹⁰⁹ Tetranuclear complexes 4(Pd) and 182(Pt) showed higher cytotoxic activity in tumor cell lines resistant to cis-platin and etoposide. Other complexes (Pd: 5–8; Pt: 183–185) had a lower cytotoxic values, but are more active than cis-platin and other clinically used drugs.^25a,96,97 Complexes 186 to 190 showed cytotoxicity against two human tumor cell lines (viz. HL-60 and U-937), and some are more active than cis-platin.¹¹⁶

4.2 Catalysis

Complex [Pd(C,N,S-L¹⁶-2H)(PPh₃)] 19 has been found to be excellent catalyst for Suzuki type C–C coupling which involved reactions of aryl halides with phenylboronic acids (eqn (2)).¹⁰¹ The R₁ ring substituents of aryl halides used were –C(O)CH₃, –CHO and –CN with R₂ substituents of phenylboronic acids being H, OCH₃ or Cl. The C–X bonds activated were C–Cl, C–Br and C–I. Complexes [Pd(C,N,S-L¹⁴-2H)(PPh₃)] 17 and [Pd(C,N,S-L¹⁸-2H)(PPh₃)] 21 showed similar activity as tested for C–I activation with R₁ = –C(O)CH₃, R₂ = H.

Complex 19 has also been investigated for catalytic activity towards Buchwald type C–N coupling reactions between aryl halides (X = I, Br tested) (eqn (3)) and primary/secondary amines (Chart 8). It was found that this complex exhibited ten times higher TON for X = I over X = Br. This catalyst showed slightly better catalytic efficiency than bis-complex [Pd(N³,S-L¹⁶-H)₂].¹⁰⁶ The TON was low for C–Cl bond activation as tested for bis-complex for all the amines used (Chart 8). Both types of reactions investigated have either better or comparable catalytic activity to that reported in literature. A special feature is efficient activation of C–Cl bonds which is rather scarce.^125–129

Chart 8 Amines/secondary amines.

Complexes 152 and 157 have been tested as catalyst for the Suzuki coupling of phenylboronic acid with p-haloacetophenones.¹¹³ Here activation of C–I, C–Br, C–Cl, and C–F bonds was performed (X = I, Br, Cl, F, eqn (4)). It may be added here that complex, [PdCl(N³,S-btsc)(PPh₃)] 158 (btsc is anion of benzaldehyde thiosemicarbazone chelating to Pd center via its N,S-donor atoms) also showed similar activity but efficiency was somewhat low relative to organometallic complexes 152 and 157, especially in terms of time taken to complete the reaction.¹¹³ The activation of C–Cl bonds are industrially significant due to easy availability of the relatively inexpensive arylchlorides and also the activation of C–F bonds though less efficient is scarce.¹¹³ In addition to C–C coupling, C–N coupling is also observed using complexes 152, 157 and [PdCl(N³,S-btsc)(PPh₃)] 158 (eqn (5) and (6)). However there was no activation of C–F bond. The trend in C–N bond formation was similar to that with primary amines, however turn over numbers were lower in case of secondary amines (eqn (6)) relative to the primary amines (eqn (5)).¹¹³

5. Conclusion

An interesting feature of reactions of thiosemicarbazones with Pd^II/Pt^II, in the absence of co-ligands such as tertiary phosphines, has yielded self-assembled cyclometallated tetranuclear oligomers. These oligomers on further reactions with tertiary phosphines yielded mono- and di-nuclear complexes which have seven different types of bonding modes displayed by these phosphines (Chart 4). The M–C bonds formed by thiosemicarbazones appear to be inert and remain intact even when phosphines react with oligomers. However the facile aerial oxidation of the thiolate-sulfur of the coordinated thiosemicarbazone to sulfinate formed rhodium complexes (233 to 237). Here it was shown by theoretical studies this oxidation is attributed to the presence of the hydride ligand in the complexes, which makes the sulfur atom electron rich and susceptible to oxidation by molecular oxygen. The presence of electronegative chloride instead of hydride (239–243) reduces electron density on the sulfur atom making its spontaneous aerial oxidation impossible.¹³⁰ A few complexes (Pd/Pt) tested for anti-malarial and anti-cancer activity have shown a big scope for developing this area for antimicrobial, anti-malarial and anti-cancer activity. Further, a few Pd complexes tested for catalysis have emerged excellent catalyst for Suzuki type C–C coupling which involved reactions of aryl halides/p-haloacetophenones with phenylboronic acids via activation of C–F, C–Cl, C–Cl and C–I bonds. Also a few complexes have been investigated for catalytic activity towards Buchwald type C–N coupling reactions between aryl halides and primary/secondary amines. The activation of C–Cl bonds are industrially significant due to easy availability of the relatively inexpensive arylchlorides and also the activation of C–F bonds though less efficient is scarce. Thus catalysis provides a useful area of investigations. This review provides a summary of coordination modes of phosphines in complexes and is rich in NMR spectroscopy, especially ³¹P NMR. The cyclometallation area encompassing thiosemicarbazones provides a promising area of research specially with respect to biological applications and catalysis.

Acknowledgements

This author thanks RSC advances for invitation to write review on metallation by thiosemi-carbazones and the Council of Scientific and Industrial Research (CSIR), New Delhi for Emeritus Scientist position vide Scheme no. [21(0904)/12-EMR-II].

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