COORDINATIVE PROPERTIES OF A HYBRID PHOSPHINE-BIPYRIDINE LIGAND

6-(2-Diphenylphosphinoethyl)-2,2'-bipyridine L, has been prepared in t hree 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-Cl-2-[RuCl2L(CO)] 1 and cis-Cl-2 -[RuCl2L(CO)] 2, the L ligand displaying in both complexes a meridiona l tridentate binding mode. This behaviour was confirmed, in the case o f 1, by an X-ray structural study. Electrochemical oxidation of 1 and 2 is reversible and occurs at E-1/2 = 0.80 V and 1.04 V vs. Ag-Ag+ (0. 01 M), respectively. The cyclic voltammogram of 1 shows one irreversib le system at E-pc = -1.71 V and two reversible systems at E-1/2 = -1.8 6 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 presenc e of small amounts of H+ afforded the monodentate L complex cis-Cl-2-t rans-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 quantitati ve formation of a 36-membered metallomacrocycle 4. This complex exhibi ts a quasi-reversible oxidation at E-1/2 = 0.29 V and an electrodeposi tion-redissolution redox system at E-pc = -0.21 and E-pa = -0.10 V due to formation of Cu-0 on the electrode surface; Reaction of L with [{R hCl(C2H4)(2)}(2)] led to the (chloromethyl)-rhodium(III) complex [RhCl 2(CH2Cl)L] 5 (90%). Treatment of 5 in ethanol in the presence of silve r(I) resulted in the formation of [RhCl2(CH2OEt)L] 6 (63% yield), wher e the meridional arrangement of the phosphine was maintained.