Selective formation of positional isomers of substituted µ3-benzynes in triosmium clusters by degrading aryldimethylarsines

Abstract

The cluster [Os₃(CO)₁₁{AsMe₂(C₆H₄Me-2)}] in refluxing octane gave the unexpected positional isomer of the methylbenzyne cluster [Os₃H(CO)₉(AsMe₂)(µ₃-C₆H₃Me-4)]. Similarly the isomeric clusters [Os₃(CO)₁₁L][L =(2- or 4-methoxyphenyl)dimethylarsine] both gave the 3-methoxybenzyne cluster [Os₃H(CO)₉(AsMe₂)(µ₃-C₆H₃OMe-3)]. A mechanism involving hydrogen-atom transfer between metal and µ₃-benzyne with phenyl intermediates is proposed to account for these migration reactions. The complexes [Os₃H(CO)₉(AsMe₂)(µ₃-C₆H₃R)](R = 4-Me or 3-MeO) exist in solution as rapidly interconverting mixtures of isomers at room temperature which give separate ¹H n.m.r. signals at low temperatures. The rapidly interconverting isomers differ in the relative arrangements of the C₆H₃R and the hydride ligands and not by positional isomerism of the benzyne ligands.