PAPER www.rsc.org/dalton | Dalton Transactions Aryl, methyl-diplatinum complexes each with a metal–metal donor–acceptor bond and bridging 2-diphenylphosphinopyridine (PN) ligands: general synthetic approach and mechanism of isomerization† Bita Shafaatian, a Alireza Akbari, a S. Masoud Nabavizadeh, a Frank W. Heinemann b and Mehdi Rashidi* a Received 11th June 2007, Accepted 2nd August 2007 First published as an Advance Article on the web 23rd August 2007 DOI: 10.1039/b708769a A general synthetic method has been designed to prepare a series of unsymmetrical cationic organo-diplatinum complexes each containing two bridging 2-diphenylphosphinopyridine (PN), PPh 2 py, ligands and a platinum–platinum donor–acceptor bond. Thus, reaction of cis-[PtR 2 (SMe 2 ) 2 ] (R = Ph, p-MeC 6 H 4 or p-FC 6 H 4 ), 1, or cis,cis-[R 2 Pt(l-SMe 2 ) 2 PtR 2 ](R = Me) with 2 equiv. or 4 equiv., respectively, of PN in CH 2 Cl 2 gave cis-[PtR 2 (PN-j 1 P) 2 ], 2. When complex 2 was reacted with 1 equiv. of HX (X = CF 3 COO) in CH 2 Cl 2 , an approximately 2 : 1 mixture of trans-[PtRX(PN-j 1 P) 2 ], 3, and [PtR(g 2 -PN)(PN-j 1 P)]X, 4, was obtained. The reaction of one equiv. of the latter monomeric mixture with 0.5 equiv. of cis,cis-[R ′ 2 Pt(l-SMe 2 ) 2 PtR ′ 2 ] (R ′ = Me) or one equiv. of cis-[PtR ′ 2 (SMe 2 ) 2 ] (R ′ = p-MeC 6 H 4 ) in CH 2 Cl 2 immediately gave the head-to-head (HH) stereoisomer of the diplatinum complex hh-[RPt(l-PN) 2 PtR ′ 2 ]X, 6. However, the same reaction in benzene gave the corresponding head-to-tail (HT) stereoisomer ht-[RPt(l-PN)(l-NP)PtR ′ 2 ]X, 9, in pure form after a few hours. The conversion of the HH isomer 6 to the HT isomer 9 in CH 2 Cl 2 took place very slowly during 10 d, while the conversion in C 6 H 6 was much faster and took place over 5 h. Based on the observations, a mechanism for the conversion of the kinetic HH stereoisomer to the thermodynamic HT stereoisomer is suggested which involves association of X − with the N 2 PtR ′ 2 center following by one-arm dissociation of one of the PN bridging ligands from the nitrogen terminal in the HH isomer, and subsequent exchange of the ligating atom and reformation of the HT arrangement. The methyl-di p-tolyl dimer ht-[MePt(l-PN)(l-NP)Pt(p-MeC 6 H 4 ) 2 ]X, 9e, in solution gradually isomerizes to ht-[(p-MeC 6 H 4 )Pt(l-PN)(l-NP)PtMe(p-MeC 6 H 4 )]X, 11, by an aryl ligand transfer. All the complexes were fully characterized using multinuclear ( 1 H, 31 P and 195 Pt) NMR spectroscopy and the complexes ht-[PhPt(l-PN)(l-NP)PtMe 2 ]X, 9a, and ht-[(p-MeC 6 H 4 )Pt(l-PN)(l-NP)PtMe(p-MeC 6 H 4 )]X, 11, were further characterized by single crystal X-ray crystallography. 1. Introduction The rich reactive features of complexes with heterodonor lig- ands, such as the hemilabile pyridyl phosphine ligands, 1–4 un- der moderate conditions have attracted considerable attention because of their possible applications in catalyst design. 5–7 The electronic differentiation associated with the hard nitrogen and soft phosphorus donors in these hemilabile ligands directs their reactivities and coordination behaviors, 1 and could render novel catalytic properties to their complexes. For example activation of C–Cl and Si–Cl bonds, 6 and C–Cl/Si–H exchange reac- tions, have been possible using platinum(II) complexes contain- ing some nitrogen–phosphorus donor ligands, while the related bis(phosphine) complexes are unable to catalyze the reactions. Ever since Balch and coworkers introduced the chemistry of a Department of Chemistry, Faculty of Sciences, Shiraz University, Shiraz 71454, Iran. E-mail: rashidi@chem.susc.ac.ir; Fax: +98 711 2286008; Tel: +98 711 2284822 b Institut fuer Anorganische Chemie, Universitaet Erlangen-Nuernberg, Egerlandstrasse 1, D-91058 Erlangen, Germany † CCDC reference numbers 650140 and 650141. For crystallographic data in CIF or other electronic format see DOI: 10.1039/b708769a the P,N ligand 2-diphenylphosphinopyridine (PN), PPh 2 py, in the early 1980s, 8,9 many dinuclear and polynuclear transition metal complexes containing this PN ligand acting in a bridging mode have been synthesized and the catalytic properties of some of them have also been investigated. 1–7,10–14 However, the rigidity of the PCN bite usually forces this ligand to favor short metal–metal separations, 1 and this, as compared to the usual flexibility observed for the corresponding bis(phosphine) donor dppm [bis(diphenylphosphino)methane], 15,16 limits the range of compounds that it can form. The complexes containing simple monodentate coordination of this PN ligand via the phosphorus atom are known, 2,17,18 and there are also reports of chelation to the same metal via the nitrogen atom. 2,18,19 In this study, using suitable monomeric precursors, we have been able to design a general approach to synthesize a series of cationic diplatinum complexes with M to M donor–acceptor bonds 4,20 with the general formula [RPt(l-PN) 2 PtMe 2 ][CF 3 COO], in which R = Me, Ph, p-MeC 6 H 4 or p-FC 6 H 4 , or [MePt(l- PN) 2 Pt(p-MeC 6 H 4 ) 2 ][CF 3 COO], as two stereoisomers that differ in the relative orientations of the bridging PN ligands, the kinetic head-to-head (HH) isomer and the thermodynamically stable head-to-tail (HT) isomer. In the latter type, an intramolecular This journal is © The Royal Society of Chemistry 2007 Dalton Trans., 2007, 4715–4725 | 4715