Ring Closure Kinetics of Bidentate Hemilabile P,N and P,S Ligands on a Platinum(II) Complex

Abstract

Complexes of the type cis-[PtPh2(CO)(η1-P−N)] and cis-[PtPh2(CO)(η1-P−S)], where bidentate phosphorus−nitrogen and phosphorus−sulfur ligands are bound to the metal centre in a monodentate fashion [P−N = Ph2PC5H4N (Ph2PPy), Ph2P(CH2)2C5H4N (ppye), Ph2P(o-C6H4)NMe2 (PNMe2), Ph2P(CH2)nNMe2 (n = 2, 3, i.e., peNMe2 and ppNMe2) and P−S = Ph2P(CH2)2SEt (P−SEt), Ph2P(CH2)nSPh (n = 1, 2, i.e., P−CH2SPh and P−SPh)], were prepared in situ by reaction of the hybrid ligands with cis-[PtPh2(CO)(SEt2)]. In each case, the first observed process was the fast substitution of diethyl sulfide by the phosphanyl group leading to the monosubstituted ring-open cis-[PtPh2(CO)(η1-P−X)] (X = N or S) complexes, which were characterised in solution by 1H and 31P{1H} NMR spectroscopy. These initially formed species undergo a slow ring closure process with extrusion of carbon monoxide and formation of the chelate [PtPh2(P−X)] products, except in the case of the short-bite Ph2PPy and P−CH2SPh ligands and of ppNMe2, where ring closure was not observed. The chelate complexes were isolated as solids from the reaction of the ligands with cis-[PtPh2(Me2SO)2]. A single-crystal X-ray diffractometric study of cis-[PtPh2(P−SEt)] (18) was performed. The crystal packing showed linear chains originated by weak intermolecular Pt···H−C hydrogen bonding interactions. The chelation kinetics of P−X in the cis-[PtPh2(CO)(η1-P−X)] complexes have been monitored in [D]chloroform by 1H and 31P{1H} NMR. The rates of ring closure were found to be strongly dependent on the nature (S or N) and steric hindrance of the chelating end of the monocoordinated bidentate P−X ligand, and on the size of the ring formed. In contrast, ring size plays a negligible role, if any, in the dechelation reactions of cis-[PtPh2(S−S)] [S−S = 1,2-bis(phenylthio)ethane, dpte and 1,3-bis(phenylthio)propane, dptp] using diphosphanes (dppm and dppp) as reagents. These kinetic data, together with those of previous work, give useful insight into the factors controlling cyclisation reactions and the stability of the rings in square planar platinum(II) complexes.