Synthesis and structural characterisation of Pd( II ) and Pt( II ) complexes with a ﬂ exible, ferrocene-based P,S-donor amidophosphine ligand †

1 ’ -Diphenylphosphino-1-{[(2-(methylthio)ethyl)amino]carbonyl}ferrocene ( 1 ), accessible via amidation of 1 ’ -(diphenylphosphino)ferrocene-1-carboxylic acid (Hdpf) with 2-(methylthio)ethylamine, reacts with [PdCl 2 (cod)] (cod = cycloocta-1,5-diene) at a 1 : 1 metal-to-ligand ratio to give trans -[PdCl 2 ( 1 - κ 2 P , S )] ( trans - 2 ) as the sole product. A similar reaction with [PtCl 2 (cod)] a ﬀ ords a mixture of cis - and trans - [PtCl 2 ( 1 - κ 2 P , S )] ( cis - and trans - 3 ), which can be separated by fractional crystallisation. Complexation reactions performed with 2 equiv. of the ligand are less selective, yielding mixtures of the expected bis-phos-phine complexes ( i.e. , trans -[PdCl 2 ( 1 - κ P ) 2 ], or a mixture of cis - and trans -[PtCl 2 ( 1 - κ P ) 2 ]) with the respective monophosphine complexes. The structures of 1 , trans - 2 , cis - 3 and trans - 3 determined by X-ray di ﬀ raction demonstrate the ability of the title ligand to act as a ﬂ exible cis - or trans -P,S-chelate donor (the ligand bite angles are 174.03(2)/173.05(2)° for trans - 2 / 3 and 92.86(2)° for cis -


Introduction
Ligands capable of acting as trans-chelate donors are attractive research targets.They not only represent structurally interesting molecules but are also useful tools for the elucidation of structural effects, reaction mechanisms, etc. 1 The most often studied trans-chelating ligands still remain diphosphines, whose donor moieties are brought into appropriate positions with the aid of a rigid organic backbone. 1,2Symmetrical ligands featuring other donor atoms (e.g., N,N-donors 3 and bis-carbenes 4 ) and, particularly, their donor-asymmetric counterparts are much less common.
NMR spectra of 1 and 1O show signals of the terminal SMe groups (δ H /δ C : 2.16/14.97for 1, and 2.06/14.96for 1O) and the ethane-1,2-diyl linkers.Signals due to the PPh 2 -substituted ferrocene units are observed at the expected positions and with the characteristic J PC values (cf.ref. 8-10).The 31 P NMR shifts are −16.9 and 31.6 for 1 and 1O, respectively.The compounds display typical amide bands (namely ν NH and amide I/II bands) in their IR spectra and give rise to pseudomolecular ions in the ESI mass spectra.
The molecular structures of 1 and 1O determined by singlecrystal X-ray diffraction analysis are depicted in Fig. 1 and 2. Selected geometric data are given in Table 1.The ferrocene moieties in the molecules of 1 and 1O are regular showing tilt angles below ca.5°and, accordingly, similar individual Fe-C distances (2.030(2)-2.059(2)Å for 1, and 2.024(2)-2.062(2)Å for 1O).However, their substituents assume different mutual orientations (see τ angles in Table 1).While the conformation of 1 is anticlinal eclipsed, the substituents in 1O are moved closer to a synclinal eclipsed conformation by the intramolecular N-H⋯OvP hydrogen bond.In both cases, the amide planes and the planes of their parent cyclopentadienyl ring are twisted (see φ in Table 1) so that the amide nitrogen is directed towards the side of the ferrocene unit.The amide substituents in 1 and 1O adopt similar orientations pointing toward the other Cp ring and have the substituents at the C24-C25 bond at ca. 60°or in gauche conformation.

Preparation of Group 10 metal complexes
Divalent Group 10 metal ions were chosen for an evaluation of the coordination properties of compound 1 mainly because they include the borderline Ni(II) as well as the typical soft Pd(II)/Pt(II) metal ions showing different kinetic properties. 15,16nfortunately, repeated attempts at obtaining some Ni(II)-1 complexes failed.The reactions of [NiCl 2 (dme)] (dme = 1,2-dimethoxyethane) or NiCl 2 •6H 2 O with 1 equiv. of 1 afforded only rapidly decomposing mixtures, from which no defined product could be isolated.This may reflect a mismatch situation   resulting from a relatively smaller size of the Ni(II) ion and the soft character of the donor atoms present in 1.
In contrast, the reaction of 1 with [PdCl 2 (cod)] (cod = cycloocta-1,5-diene; Pd : 1 = 1 : 1) proceeded rapidly and cleanly to provide a single product showing its 31 P NMR signal at a lower field (δ P 23.6, Δ P = 40.5 ppm) compared with the free ligand.This shift, together with a low-field shift of the SMe resonance (Δ H = 0.13 ppm) and its splitting with the 31 P nucleus ( 4 J PH = 4.7 Hz), already suggested chelate coordination of ligand 1.Indeed, this was corroborated by X-ray crystallography showing the product to be the trans-chelate complex trans-[PdCl 2 (1-κ 2 S,P)] (trans-2 in Scheme 3).
Compound trans-2 shows broad 1 H NMR signals for protons at the ferrocene unit and in the ethane-1,2-diyl bridge at room temperature. 17Nonetheless, this broadening is in line with the formulation as it reflects a limited molecular mobility resulting from spatial constraints imposed by a tight chelate coordination.The amide vibrations of trans-2 are observed shifted vs. free 1 (amide I and II by ca.+30 and −10 cm −1 , respectively) whilst the amide 13 C NMR resonance (CvO) remains nearly unaffected.The complex displays a pseudomolecular ion ([M + Na] + ) and fragment ions attributable to [M − Cl − HCl] + and [Pd(Ph 2 Pfc)] + (fc = ferrocene-1,1′-diyl) in its ESI MS spectrum.
An analogous reaction of 1 with [PtCl 2 (cod)] (Scheme 3; CHCl 3 /refluxing for 3 h) afforded a mixture of two compounds showing different 31 P NMR shifts and 1 J PtP coupling constants.Considering the fact that even the signals of the SMe protons were flanked with 195 Pt satellites, the products were formulated as isomeric P,S-chelate complexes cis-3 (δ P 5.3, 1 J PtP = 3695 Hz; ca.35%) and trans-3 (δ P 6.6, 1 J PtP = 3495 Hz; ca.65%), 18 being distinguished through the 1 J PtP coupling constants. 19The isomers differ also in their 1 H NMR spectra displaying four degenerate signals and eight anisochronic (diastereotopic) signals for the ferrocene protons in trans-3 and cis-3, respectively.
Complexation reactions performed at 1 : 2 metal-to-ligand ratios proved more complicated.The crude reaction mixture obtained after mixing [PdCl 2 (cod)] with two molar equivalents of 1 (90 min/room temperature) contained three different compounds, which were identified by 31 P NMR spectroscopy as trans-2, 1O (both minor components) and the expected bisphosphine complex, trans-[PdCl 2 (1-κP) 2 ] (trans-4).Repeated attempts to isolate this major product (ca.85% in the reaction mixture) by crystallisation failed and the compound could therefore only be characterised by NMR spectroscopy in a solution.
A similar reaction between [PtCl 2 (cod)] and two equivalents of 1 (at room temperature for 90 min) afforded a mixture of five different compounds. 31P NMR monitoring revealed the presence of 1O, cis-and trans-3 (all minor components) and two isomeric bis-phosphine complexes, cis-and trans-[PtCl 2 (1-κP) 2 ] (5).Likewise cis-and trans-3, the isomers of complex 5, can be clearly differentiated by means of 31 P NMR spectroscopy 19a,b (cis-5: δ P 10.2, 1 J PtP = 3790 Hz; trans-5: δ P 10.9, 1 J PtP = 2615 Hz).In accordance with the geometry of the leaving ligand and kinetic inertness of Pt(II) species, the bis-phosphine complexes were formed in cis : trans ratio of ca. 2 : 1 which, however, changed in favour of the thermodynamically preferred trans-isomer after refluxing for 18 h (cis : trans = 1 : 2).When the Pt-precursor was replaced with K[PtCl 3 (η 2 -C 2 H 4 )], whose alkene ligand destabilises the chloride in mutual trans position owing to its large trans-influence, 20 the product ratio determined initially (room temperature/90 min) was inverted (cis : trans = 1 : 2) and did not change upon refluxing for 18 h.Even in this case, however, minor amounts of 1O, cis-and trans-3 were detected in the reaction mixture.

The molecular structures of the Pd(II) and Pt(II) complexes
As indicated above, the structures of trans-2, trans-3 and cis-3 were determined by X-ray diffraction analysis.Only the latter compound was isolated in an unsolvated form while the transcomplexes accommodated molecules of adventitious water (trans-2) or the crystallisation solvent (trans-3) in their structures (for a discussion of the crystal packing, see ESI †).The molecular structures of cis-and trans-3 are presented in Fig. 3.A structural diagram for trans-2 is available as ESI (Fig. S2 †).Relevant geometric data are summarised in Table 2.
The complexes adopt the expected square-planar geometry so that the central atoms and their four ligating atoms are coplanar within less than 0.001 Å in trans-2, ca.0.03 Å in trans-3 and ca.0.16 Å in cis-3.Rather surprisingly, the largest deviation from a coplanar arrangement detected for the latter complex is associated with the smallest departure of the individual inter-ligand angles from the ideal 90°(ca.88-93°).Inter-ligand angles in the trans-complexes range ca.83-94°, with the extreme values pertaining to the S-M-Cl1/2 angles.
The M-donor distances are found within the usual ranges, 21 yet they nicely demonstrate the different trans-influence of the ligands (PR 3 > SR 2 ≫ Cl − ) 20 and the fact that two soft donor moieties (with a high trans-influence) bonding to a soft metal ion in mutually trans positions destabilise each other. 22,23hus, the Pt-P and, particularly, the Pt-S bonds are longer in trans-3 than in cis-3, 24 while for cis-3, the Pt-Cl distance trans to P is significantly longer than that trans to S. It is also noteworthy that because of practically identical radii of Pd(II) and Pt(II), 25 and identical coordination environments, the overall structures of trans-2 and trans-3 are very similar.The M-donor (M = Pd, Pt) bond distances and inter-ligand angles in these complexes differ by no more than ca.0.03 Å and 1°, respectively.
In order to comply with the demands of the coordinated metal ions, the ligand molecule undergoes a pronounced conformational reorganisation.The major changes are seen at the ferrocene unit, whose substituents are rotated to positions more proximal than in the structure of uncoordinated 1.In trans-2 and 3, the ferrocene units are synclinal eclipsed, and in cis-3 they assume an even closer position near to staggered synclinal.The amide units are also rotated from positions observed in free 1 (by ca.40°) so that the N atoms are forced away from the ferrocene unit, slightly more in cis-3 than in the trans-complexes.On the other hand, the tilt angles remain rather low, the largest tilting being 5.5(1)°for cis-3.The orientation of the substituents at the C24-C25 are virtually the same as that in the free ligand, but the bond is differently orientated towards the plane of the amide-substituted cyclopentadienyl ring. 26The C-P-C angles and C25-S-C26 angles change only marginally.

Electrochemistry
The electrochemical behaviour of ligand 1, phosphine oxide 1O and Pd(II) complex trans-2 was studied by cyclic and differential pulse voltammetry at a glassy carbon disc electrode in 1,2-dichloroethane containing 0.1 M Bu 4 N[PF 6 ] as the supporting electrolyte.
Compound 1 showed three consecutive oxidations within the accessible potential range (Fig. 4).The first oxidation   observed at E pa = 0.28 V vs. ferrocene/ferrocenium 27 was quasireversible.At relatively lower scan rates or when the switching potential was set to more positive values, it appeared electrochemically irreversible.However, when scanned separately and with higher scan rates, a reductive counter-wave could be detected (see inset in Fig. 4). 28he subsequent oxidations seen at E pa = 0.60 and 0.80 V were also irreversible and probably composite in nature.Their complex nature was manifested in the difference pulse voltammograms (Fig. 5).It is also noteworthy that another wave, partly replacing the first oxidation wave, emerged upon repeated scanning (see Fig. 4, signal marked with an asterisk).Based on a comparison of the redox potentials and in view of our previous work, 11a this wave could be tentatively assigned to the oxidation of 1O resulting primary, ferrocene-based oxidation and the associated follow-up chemical reactions of the unstable ferrocenium intermediate. 29nlike 1, the redox behaviour of the corresponding phosphine oxide 1O was simple (Fig. 6).The compound, lacking the lone pair of phosphorus as a reactive site, underwent a single, one-electron reversible oxidation at E°′ = 0.40 V.The separation of the peaks in cyclic voltammograms was ca.80 mV at a scan rate of 0.1 V s −1 . 30The anodic and cathodic peak currents increased linearly with the square root of the scan rate (i p ∝ ν 1/2 ) and their ratio remained close to unity over the range 0.02-1.0V s −1 .This oxidation, attributed to the ferrocene/ ferrocenium couple, appeared shifted to more positive potentials than the ferrocene reference, in accordance with the electronwithdrawing nature of the substituents attached to the ferrocene unit (cf. the Hammett's σ p constants: 31 CONHMe 0.35, PPh 2 0.19, P(O)Ph 2 0.53) that render the oxidation more difficult.
Similarly to 1O, complex trans-2 (Fig. 7) displayed a oneelectron, reversible oxidation at E°′ = 0.46 V. 32 This wave was Fig. 4 Cyclic voltammogram of 1 recorded in 1,2-dichloroethane on a glassy carbon electrode (first scan in red, second scan in blue).The signal marked with an asterisk is presumably due to 1O.The inset shows partial cyclic voltammograms recorded at different scan rates (yellow 0.02 V s −1 , red 0.05 V s −1 , green 0.10 V s −1 , blue 0.20 V s −1 , and black 0.50 V s −1 ).shifted with respect to the free ligand, most likely owing to an electron density transfer from the ligand (ferrocene unit) upon coordination.An additional irreversible, multi-electron reduction was observed at E pc = −1.34V 27 vs.ferrocene/ferrocenium.Together with an associated, small oxidation wave at E pc ca.−0.61 V, this probably reflects an irreversible reductive removal of one Pd-bound chloride as described previously for [PdCl 2 (PPh 3 ) 2 ]. 33

Conclusions
Functional ferrocene-based phosphino-amide 1 is readily accommodated as a P,S-donor in both the cis-and trans-MCl 2 square-planar fragments of the heavier Group 10 metal ions.The latter coordination mode appears particularly remarkable because the ligand is structurally flexible and donor-asymmetric, unlike the majority of trans-spanning donors known to date.Furthermore, the results presented earlier for the structurally related ligands I-IV (see Introduction) and herein for compound 1, demonstrate that the formation of trans-chelate complex donors does not necessarily require the prospective trans-spanning donor to comprise a rigid organic backbone if this is replaced with appropriately selected structural fragments.Further work focusing on other metal ions and a comparison of the structurally related donors 1, III and IV is currently underway.

Materials and methods
All manipulations were performed under an argon atmosphere and with the exclusion of direct daylight.Chloroform and dichloromethane were dried by standing over K 2 CO 3 and distilled under argon.Methanol was distilled from MeONa.Solvents (from Lachner) for work-up and crystallisations were used without any additional purification.Hdpf, 11a [MCl 2 (cod)] (M = Pd, Pt), 34 and K[PtCl 3 (C 2 H 4 )] 35 were prepared according to the literature procedures.All other chemicals were obtained from Sigma-Aldrich.NMR spectra were recorded at 25 °C using a Varian UNITY Inova 400 MHz spectrometer operating at 399.95 MHz for 1 H, at 100.58 MHz for 13 C, and at 161.90 for 31 P, if not specified otherwise.Chemical shifts (δ/ppm) are given relative to internal tetramethylsilane ( 1 H and 13 C) and to external 85% H 3 PO 4 ( 31 P).Infrared spectra were recorded with an FT IR Nicolet Magna 760 spectrometer in the range 400-4000 cm −1 .Electrospray ionisation (ESI) mass spectra were obtained with an Esquire 3000 (Bruker; low resolution) or an LTQ Orbitrap XL instrument (Thermo Fisher Scientific; high resolution).
A chloroform solution (ca. 1 mL) of the crude product was layered with hexane.Subsequent crystallisation at room temperature for several days afforded orange crystals of cis-3, which were filtered off, washed with hexane (3 mL) and pentane (3 × 3 mL) and dried under vacuum.Yield after two crystallisations: 25 mg (51%), orange crystals.The mother liquor was evaporated and the residue was dissolved in ethyl acetate.The solution was layered with pentane and allowed to crystallise by liquid-phase diffusion for several days.The yellow-orange crystals of trans-3 that formed were filtered off, washed with pentane and dried under vacuum.Yield of trans-3: 12 mg, 26%.
Characterisation data for cis-[PtCl 2 (1-κ 2 S,P)] (cis-3).] with two equivalents of 1.A solution of 1 (20 mg, 0.04 mmol) in chloroform (1 mL) was added to a solution of K[PtCl 3 (C 2 H 4 )] (8 mg, 0.02 mmol) in methanol (1 mL).The resulting orange-red mixture was stirred at room temperature for 90 min and evaporated. 1 H NMR and 31 P{ 1 H} NMR spectra again indicated the presence of 1O, cis-3, and trans-3 in trace amounts, and the bis( phosphine) complexes cis-and trans-5 in the ratio of 1 : 2 as the major products.The ratio did not change during refluxing for 18 h.
The structures were solved by direct methods (SHELXS97 36 ) and refined by full-matrix least-squares based on F 2 (SHELXL97 36 ).The non-hydrogen atoms were refined with anisotropic displacement parameters and the CH hydrogens were included at their calculated positions and refined as riding atoms.The OH and NH hydrogens were identified on the difference electron density maps and refined as riding atoms with U iso (H) set to 1.2U eq (O/N).Relevant crystallographic data and structure refinement parameters are summarised in Table S1 (see ESI †).
Geometric data and all structural drawings were obtained with a recent version of the PLATON program. 37The numerical values were rounded with respect to their estimated deviations (ESDs) given to one decimal place.Parameters pertaining to atoms in constrained positions are given without ESDs.

Fig. 5
Fig. 5 Differential pulse voltammograms of 1 recorded in 1,2-dichloroethane on a glassy carbon electrode and with different modulation amplitudes (given in the figure).

Fig. 6
Fig. 6 Representative cyclic (left; scan rate 0.1 V s −1 ) and differential pulse (right) voltammograms of 1O recorded in 1,2-dichloroethane on a glassy carbon electrode.The voltammograms are presented with the same scale; modulation amplitude for the differential pulse voltammograms is given in the figure.

Fig. 7
Fig. 7 Cyclic voltammogram of trans-2 recorded in 1,2-dichloroethane on a glassy carbon electrode (first scan in red, second scan in blue).

Table 1
Selected intramolecular distances and angles for 1 and 1O (in