Oxidative addition of disul fi de / diselenide to group 10 metal ( 0 ) and in situ functionalization to form neutral thiasalen / selenasalen group 10 metal ( II ) complexes †

Three components, one pot synthesis of thiasalen/selenasalen Ni(II), Pd(II) and Pt(II) complexes, 14–19, by the oxidative addition of S–S/Se–Se bond of bis(o-formylphenyl)disulfide/-diselenide to Ni(0), Pd(0) and Pt(0) followed by in situ Schiff base formation with ethylenediamine is reported. S–S or Se–Se bonds were cleaved and coordinated to the metal center as thiolate (ArS) or selenolate (ArSe) while the formal oxidation state of metal centers was changed from ‘0’ to ‘+2’. The disulfide/diselenide reacted with zerovalent metals at room temperature to give only the monometallic complexes. All complexes (except Pd– thiolate complex 15) were studied by single crystal X-ray crystallography and revealed the square planar geometry around metal centers.


Introduction
Schiff base synthesized by the reaction of two equivalents of salicylaldehyde with one equivalent of ethylenediamine is known as 'salen' which provides potentially tetradentate chelating systems with a N2O2 donor set.Metal complexes based on salen and its derivatives have drawn considerable attention in last few decades 1 due to their applications in catalysis, biological studies, material science, molecular magnetism and sensory materials. 2As catalysts, such complexes have been successfully applied in many reactions such as oxidation, olefin epoxidation, cycloaddition and in asymmetric synthesis. 3ecently, salen-complexes were utilized to develop metal organic frameworks (MOFs) for their novel applications such as hydrogen gas storage and highly efficient catalysis. 4Sulfur and selenium analogues of salen (thiasalen and selenasalen with N2S2 and N2Se2 ligating sites) have attracted scant attention due to instability of thiol and selenol groups compared to the hydroxyl group of salen.
Oxidative addition of E-E bonds (E = S and Se) of diorganodisulfides and -diselenides to low-valent transition metal complexes is a mild and efficient method to synthesize chalcogenolato-metal complexes. 5The oxidative addition of diorganyl disulfides/diselenides to Pd(0) complexes, such as [Pd(PPh 3 ) 4 ], was investigated for their addition to alkynes, which is an efficient single step method for the formation of two C-S/Se bonds in a stereoselective manner. 6Mechanistic studies on these reactions have been carried out using various types of disulfides/diselenides and alkynes from both the experimental and theoretical point of views. 7Yamamoto and Sekine have reported the oxidative addition of diaryl disulfides, ArS-SAr, with [Ni(COD) 2 ] (COD = 1,5-cyclooctadiene) in the presence of basic ligands such as 2,2′-bipyridine, triethylphosphine. 8Oxidative addition of diaryl disulfides (ArS) 2 with [Pd(PPh 3 ) 4 ] and [Pt(PPh 3 ) 4 ] formed dimeric and monomeric complexes, respectively. 9The tendency of formation of dimeric complexes can be minimized by using electron withdrawing substituents on the aromatic rings of diaryl disulfide.This study also revealed that Pt(0) complexes have less tendency to form dimeric complexes compared to Pd(0) precursors.The oxidative addition of S-S bond of the ring systems to Ni(0), Pd(0) and Pt(0) to form S,S-dithiolate chelate complexes were also studied. 10orley et al. 11 have shown the formation of selenolate complexes via oxidative addition of Ph 2 Se 2 and Fc 2 Se 2 [Fc = ferrocenyl] to Pd(0) and Pt(0).Laitinen et al. reported the oxidative addition of Th 2 Se 2 and Ph 2 Se 2 to Pd(0) and Pt(0). 12Reversible oxidative addition of Se-Se bonds to Pt(0) and Pt(II) precursors was observed by changing the ligand environment. 13However, oxidative addition of Se-Se bond of diorganodiselenide to Ni(0) is not reported till the date.In addition to oxidative addition of E-E bonds to group 10 M(0) precursors, the cleavage of E-E bonds also have also been reported with Group 10 metals in higher oxidation states. 14ynthesis of ligands and complexes simultaneously by the formation of both carbon-heteroatom and heteroatom-metal bonds is a powerful technique in the preparation of metalloorganic self assemblies. 15The introduction of thiolates/selenolates into metal complexes directly by the oxidative addition of dichalcogenides are often complicated by the presence of dimeric/polynuclear and non-stoichiometric compounds, however, it can be avoided by using the chelating diselenide.5a Here we have used this approach to synthesize thiasalen and selenasalen complexes.
Earlier we reported the Pd(II) and Pt(II) complexes (5-11) which were prepared by the reactions of bis(alkyl)thiasalen (1) and bis(alkyl)selenasalen (2-4) ligands with Pd(II) and Pt(II) metal precursors. 16We have observed dealkylative complexation via nucleophilic substitution at C-E bond (E = S, Se) where monocationic complexes act as a leaving groups.However, the attempts to prepare the analogous Ni(II) complexes were unsuccessful.In the synthesis of complexes 5-11, the two step process was applied; (i) synthesis and isolation of the ligand and then (ii) complexation with the metal precursors.Here we report the complete series of six neutral group 10 d 8 metal complexes of thia/selenasalen ligands via oxidative addition of E-E bond of bis(orthoformyl)phenyl disulfide (12)  and bis(orthoformyl)phenyl diselenide (13) to Ni(0), Pd(0) and Pt(0) and subsequent in situ reaction of ethylene diamine.Thus it is a three component one pot reaction for facile synthesis of sulfur and selenium analogues of salen group 10 metal complexes.It is the first report on the synthesis of neutral thiasalen complexes of Pd and Pt and selenosalen complexes of Ni, Pd and Pt.

Synthesis of ligands
The compounds 12 17 and 13 18 were synthesized according to the reported procedures. 1 H NMR spectrum of the ligand 12 showed a singlet at δ 10.23 ppm for aldehyde protons whereas for ligand 13 showed a singlet at δ 10.17 ppm.
Complexation of compound 12 and 13 with Ni(0), Pd(0) and Pt(0) Reactions of ligands 12 and 13 with Ni(0) ([Ni(COD) 2 ]), Pd(0) ([Pd(PPh 3 ) 4 ]) and Pt(0) ([Pt(PPh 3 ) 4 ]) afforded six neutral thiolate and selenolate complexes 14-19 via oxidative addition of E-E bond (E = S, Se) to Ni(0), Pd(0) and Pt(0) at room temperature (Scheme 1).As a result, S-S or Se-Se bonds were cleaved and coordinated to the metal center as thiolate (ArS − ) or selenolate (ArSe − ) while the formal oxidation state of metal center was changed from '0' to '+2'.The intermediate aldehyde complexes were used in situ as template for the preparation of desired Schiff base derivatives.Thus, it provides a one pot three component method for synthesis of the thiasalen and selenasalen d 8 metal complexes by template synthesis using oxidative addition of bis(o-formylphenyl)disulfide/diselenide to M(0) (M = Ni, Pd, Pt) precursors and imine bond formation by reaction with ethylenediamine.Template synthesis is a well established method for the one pot synthesis of Schiff-base complexes.In the template synthesis method a metal ion first coordinates with aldehyde to create the template for the reaction with amine to form metal ion coordinated Schiff-base. 19hus, the concept of simultaneous use of oxidative addition and template synthesis is used here for one pot synthesis of thiasalen/selenasalen based metal complexes.
When one equivalent of ligand 12 was treated with one equivalent of [Ni(COD) 2 ], [Pd(PPh 3 ) 4 ] and [Pt(PPh 3 ) 4 ] and one equivalent of 1,2-ethelenediamine, complexes 14, 15 and 16 were formed, respectively.All the three complexes are red in colour and stable in solid as well as in solution phase at ambient conditions.Complex 14 is soluble in DCM, CHCl 3 , DMF and DMSO while complexes 15 and 16 are soluble in DMF and DMSO.Previously, Yamamura et al. 20 reported the synthesis of complex 14 by the reaction of bis(2-(tert-butylthio)benzylidene)ethylenediamine with NiCl 2 •6H 2 O with in situ cleavage of tert-butyl group.Goswami and Eichhorn 15 reported the synthesis of 14 by the cleavage of disulfide bond in bis-(o-formylphenyl)disulfide using [Ni(en) 3 ]Cl 2 .In both the cases, Ni(II) ion was taken as metal source.Elemental analysis data of the compounds 14, 15 and 16 are consistent with the proposed formula and structures. 1 H/ 13 C NMR spectra of the complexes 14, 15 and 16 are in good agreement with the proposed structures. 1 H NMR spectra of the complexes 14, 15 and 16 showed a singlet at δ 8.59, 8.72 and 9.02 ppm for azomethine protons, respectively.Complexes 14, 15 and 16 showed eight signals in the 13 C NMR spectra for eight different types of carbons indicating symmetrical nature of the complexes in solution.FT-IR spectra of the complexes 14, 15 and 16 displayed the characteristic ν (CvN) stretching frequencies at 1610, 1626 and 1617 cm −1 , respectively, which were shifted to lower energy values compared to the ν (CvO) stretching frequency (1691 cm −1 ) of precursor 12.
The selenium analogues 17, 18 and 19 were prepared by similar procedure using precursor bis(orthoformylphenyl) diselenide 13.Formulations of complexes 17-19 were supported by elemental analyses, however, elemental analysis for 18 was found to be slightly deviated from the calculated value of percentage of carbon.Color, stability and solubility of these complexes are quite similar to that of their sulfur analogues. 1H NMR and 13 C NMR spectra of 17, 18 and 19 showed symmetrical nature of these complexes.The characteristic imine protons were observed at δ 8.66, 8.76 and 9.09 ppm, respectively, in 1 H NMR spectra of 17, 18 and 19. 77 Se NMR of complexes 17, 18, 19 showed the single resonance at δ 354.9, 393.7, 318.9 ppm, respectively.In the FT-IR spectra of complexes 17, 18 and 19, peaks at 1609, 1622 and 1611 cm −1 , respectively, were assigned to ν (CvN) stretching frequency.ESI-MS spectra of complexes 14 and 17 displayed the molecular ion peak at 356.37 and 452.1934, respectively as [M + H] + moieties, while the Pd(II) and Pt(II) complexes did not ionize in ESI mass spectrometer.

Crystal structure study
The molecular structures and supramolecular assemblies of 14, 16, 17, 18 and 19 have been determined by X-ray crystallography.
Crystal structure of Ni-thiolate complex 14 Complex 14 crystallizes in orthorhombic space group Pna2 1 with the square planar geometry around metal center (Fig. 1).Space group, unit cell and bond lengths/angles are quite similar to those reported by Yamamura et al., 20 while Goswami and Eichhorn 15 reported the monoclinic space group P2 1 /c with asymmetric unit containing two independent molecules and a dichloromethane molecule.

Crystal structure of Ni-selenolate complex 17
The square planar geometry around Ni(II) center in complex 17 was confirmed by single crystal X-ray crystallography.Complex 17 crystallizes in triclinic space group P1 ˉwith two asymmetric molecules in the unit cell (Fig. 3).The four Ni-Se bonds are nearly equal (average 2.263 Å) and comparable to those reported for selenocarbamoyl benzamidine base Ni(II) complex (2.278 and 2.293 Å) 21 and 2-aminophenyl diselenolate based Ni(II) complex (2.295 Å) 22 which also have square planar geometry around the Ni(II) center with N2Se2 donor set.The average Ni-N bond distance is 1.894 Å.The various intermolecular nonbonding interactions including CH⋯π (aromatic) interaction between the two molecules led to the formation of 1-D chain along b axis (Fig. S3 †).Selected bond lengths and bond angles are shown in Table 2.
Crystal structure of Pd-selenolate complex 18 Similar to complex 17, the crystal structure of complex 18 crystallizes in the triclinic space group P1 ˉwith two molecules in the asymmetric unit (Z = 4, Z′ = 2) and a square planar    geometry around the Pd(II) centers (Fig. 4).The average distance of four Pd-Se bonds is 2.366 Å which is slightly shorter than Pd-Se bonds of nicotinoyl selenide based square planar Pd(II) complex (2.444 and 2.428 Å). 23 The average of four Pd-N bond distances is 2.036 Å. Selected bond lengths and bond angles are shown in Table 3. Crystal packing of 18 shows the intermolecular C-H⋯π, π⋯π and Se⋯H short contacts (Fig. S4 †).
Crystal structure of Pt-selenolate complex 19 Complex 19 crystallizes in the triclinic space group P1 ˉwith two molecules in the asymmetric unit (Z = 4, Z′ = 2) and a square planar geometry around the Pt(II) centre (Fig. 5).The four Pt-Se bonds are nearly equal and comparable to those in selenoether-selenolate complex 8 (selenoether Pt-Se, 2.3636(6) Å and selenolate Pt-Se, 2.3583(6) Å). 16a The average Pt-N bond distance is 2.019 Å. Selected bond lengths and bond angles are shown in Table 4. Molecular packing of the complex is similar to that of nickel and palladium analogues.Complete crystallographic data for complex 14, 16-19 are provided in Table 5.

Experimental section
All reagents were purchased from Aldrich/Merck and used without further purification.Acetonitrile was distilled from P 2 O 5 and kept over molecular sieves. 1 H and 13 C NMR spectra were recorded either on Bruker 500 MHz or on JEOL-FT NMR-AL 400 MHz spectrometer using DMSO-d 6 as solvent and tetramethylsilane (SiMe 4 ) as internal standards. 77Se NMR spectra were recorded on Bruker 500 MHz spectrometer using DMSO-d 6 as solvent. 77Se NMR chemical shifts are reported using Ph 2 Se 2 as external standard with chemical shifts of 470 ppm with respect to Me 2 Se, thus the values are reported with respect to Me 2 Se.IR spectra of the compounds have been recorded on a Perkin-Elmer spectrophotometer as KBr pellets.
The mass spectra were recorded on a MICROMAX Q-TOF-MICRO instrument.Melting points were measured using a digital melting point apparatus, SECOR INDIA.

Crystal structure determination
Single crystals of the compounds suitable for X-ray diffraction were grown from dimethyl formamide/dimethyl sulfoxide solution by diffusing diethyl ether vapors in a closed beaker.The crystals were carefully chosen using a stereomicroscope supported by a rotatable polarizing stage.The data were collected on Bruker's KAPPA APEX II CCD Duo with graphite monochromated Mo-Kα radiation (0.71073 Å).The crystals were glued to a thin glass fibre using FOMBLIN immersion oil and mounted on the diffractometer.The intensity data were processed using Bruker's suite of data processing programs (SAINT), and absorption corrections were applied using SADABS. 24The crystal structure was solved by direct methods using SHELXS-97 and the data were refined by full matrix leastsquares refinement on F 2 with anisotropic displacement parameters for non-H atoms, using SHELXL-97. 25ORTEP diagrams are drawn from X-seed version 2.0. 26