Hydridodimethylplatinum(IV) Complexes with Bis(pyridine) Ligands:  Effect of Chelate Ring Size on Reactivity

The effect of changing the bite angle of the dipyridyl ligand LL on the reactivity of the dimethylplatinum(II) complexes [PtMe2(LL)] has been studied, by comparing complexes with the ligands LL = di-2-pyridylamine (DPA) or di-2-pyridyl ketone (DPK), which form a six-membered chelate ring, to compounds with 2,2‘-bipyridyl derivatives, which form a five-membered chelate ring. The complex [PtMe2(DPA)] undergoes easy oxidative addition of methyl iodide to give the corresponding platinum(IV) complex [PtIMe3(DPA)]. Both [PtMe2(DPA)] and [PtMe2(DPK)] are protonated by acids HX at low temperature to give the hydridodimethylplatinum(IV) complexes [PtHXMe2(NN)] and [PtH(S)Me2(NN)]X (S = solvent), which can exist in two isomeric forms with H trans to X or N. The structure of the complex [PtHClMe2(DPK)] was determined crystallographically. In a solution containing excess CD3OD, extensive hydrogen/deuterium exchange occurs into the methylplatinum groups and methane product at low temperature, indicating very easy reversibility of the exchange between hydridomethylplatinum(IV) and (methane)platinum(II) complexes. The hydridomethylplatinum(IV) complexes reductively eliminate methane at room temperature in solution but have significantly higher thermal stability and undergo more extensive H−D exchange than when NN = 2,2‘-bipyridyl. The reaction of [PtMe2(DPA)] with excess HCl gave [PtCl2(DPA)], and the reaction of [PtMe2(DPK)] with excess CF3SO3H gave the aqua complex [Pt(OH2)2(DPK)](CF3SO3)2 or the binuclear hydroxo complex [Pt2(μ-OH)2(DPK)2](CF3SO3)2, depending on the experimental conditions.