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

Abstract

Platinum(II) complexes of new (hydroquinon-2-ylmethyl)diphenylphosphine (PPh₂thqH₂) and diphenyl(quinon-2-ylmethyl)phosphine (PPh₂tq) ligands have been studied. Reaction of (2,5-dimethoxybenzyl)diphenylphosphine (PPh₂dmb) with [PtCl₂(PhCN)₂] afforded [PtCl₂(PPh₂dmb)₂] 1a. Metathesis reactions gave the bromo (1b) and iodo (1c) congeners. Deprotection of the hydroquinone groups in 1a using boron tribromide followed by treatment with base produced the O,P-chelated hydroquinonate phosphine complex, cis-[Pt(O,P-PPh₂thqH)₂] 2. Reaction of 2 with hydrobromic acid afforded cis-[PtBr₂(PPh₂thqH)₂] 3 which can be oxidised to give the quinone phosphine complex cis-[PtBr₂(PPh₂tq)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-PPh₂thqH)₂(ZnBr₂)] 5. Crystal structure analyses of 2·2dmf, 4·0.5dcm and 5·2dmf were performed. The electrochemistries of 1b, 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/Vis/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-PPh₂thq)(O,P-PPh₂tsq˙)]^–, and a possible mechanism for its remarkable formation is discussed.