Synthesis and electrochemistry of platinum complexes of hydroquinon-2-ylmethyl- and p-benzoquinon-2-ylmethyl-diphenylphosphine

Platinum(II) complexes of new (hydroquinon-2-ylmethyl)diphenylphosphine (PPh(2)thqH(2)) and diphenyl(quinon-2-ylmethyl)phosphine (PPh(2)tq) ligands have been studied. Reaction of (2,5-dimethoxybenzyl)diphenylphosphine (PPh(2)dmb) with [PtCl2(PhCN)(2)] afforded [PtCl2(PPh(2)dmb)(2)] 1a. Metathesis reactions gave the bromo (Ib) and iodo (Ic) congeners. Deprotection of the hydroquinone groups in la using boron tribromide followed by treatment with base produced the O,P-chelated hydroquinonate phosphine complex, cis-[Pt(O,P-PPh(2)thqH)(2)] 2. Reaction of 2 with hydrobromic acid afforded cis-[PtBr2(PPh(2)thqH)(2)] 3 which can be oxidised to give the quinone phosphine complex cis-[PtBr2(PPh(2)tq)(2)] 4. Unlike previously reported quinone phosphine complexes, 4 is robust and stable to hydrolysis; its reduction with excess of zinc and dilute hydrobromic acid produced cis-[Pt(O,P-PPh(2)thqH)(2)(ZnBr2)] 5. Crystal structure analyses of 2.2dmf, 4.0.5dcm and 5.2dmf were performed. The electrochemistries of Ib, 3, and 4 have been characterised by cyclic voltammetry and controlled potential electrolyses. Cyclic voltammograms of 4 in the presence of dilute hydrobromic acid exhibit a four-electron cathodic process, attributed to reduction to 3; those of 3 show an anodic process attributed to oxidation to 4. The electrochemistry of 4 under aprotic conditions is extraordinary. Although there are two well separated, pendant quinone substituents only a single one-electron reduction process is observed. The reduction affords a radical species (6(.-)) which has been characterised by cyclic voltammetry, and by EPR and UV/Vs/NIR spectroscopy. It is argued from the available data that 6(.-) is a novel platinum(IV) complex with bound hydroquinonate and semiquinonate (sq) groups, namely [PtBr2(O,P-PPh(2)thq)(O,P-PPh(2)tsq(.))](-), and a possible mechanism for its remarkable formation is discussed.