Co-ordinative properties of a hybrid phosphine–bipyridine ligand

Abstract

6-(2-Diphenylphosphinoethyl)-2,2′-bipyridine, L, has been prepared in three steps from 6-methyl-2,2′-bipyridine: (i) metallation followed by reaction with paraformaldehyde; (ii) bromination with HBr–MeCO₂H; (iii) phosphination using Li(PPh₂). Reaction of L with [{RuCl₂(CO)₂}_n] in NEt₃–MeOH gave a mixture of trans-Cl₂-[RuCl₂L(CO)] 1 and cis-Cl₂-[RuCl₂L(CO)] 2, the L ligand displaying in both complexes a meridional tridentate binding mode. This behaviour was confirmed, in the case of 1, by an X-ray structural study. Electrochemical oxidation of 1 and 2 is reversible and occurs at E_½ = 0.80 V and 1.04 V vs. Ag–Ag⁺ (0.01 M), respectively. The cyclic voltammogram of 1 shows one irreversible system at E_pc = –1.71 V and two reversible systems at E_½ = –1.86 and –1.91 V. In the presence of CO₂ the latter redox system exhibits a pronounced electrocatalytic current, resulting in the formation of carbon monoxide. Reaction of L with [{RuCl₂(CO)₂}_n] in the presence of small amounts of H⁺ afforded the monodentate L complex cis-Cl₂-trans-P,P-cis-(CO)₂-[RuCl₂L₂(CO)₂] 3 (yield 80%). Copper(I)-induced assembly of two molecules of 3 results in the selective and quantitative formation of a 36-membered metallomacrocycle 4. This complex exhibits a quasi-reversible oxidation at E_½ = 0.29 V and an electrodeposition–redissolution redox system at E_pc = –0.21 and E_pa = –0.10 V due to formation of Cu⁰ on the electrode surface. Reaction of L with [{RhCl(C₂H₄)₂}₂] led to the (chloromethyl)rhodium(III) complex [RhCl₂(CH₂Cl)L] 5 (90%). Treatment of 5 in ethanol in the presence of silver(I) resulted in the formation of [RhCl₂(CH₂OEt)L] 6 (63% yield), where the meridional arrangement of the phosphine was maintained.