Pentamethylcyclopentadienyl-rhodium and -iridium complexes. Part 36. Reaction of [Rh2(C5Me5)2Cl4] with phenylacetylene to give the η4-cyclobutadiene complex [Rh(C5Me5)(C4Ph2HCPhC5Ph2H2)] and the η4-benzocyclobutadiene complex [Rh(C5Me5)(C8Ph4H2)], identified by X-ray structure determinations

Abstract

Reaction of [Rh₂(C₅Me₅)₂Cl₄] with PhCCH in acetonitrile in the presence of Na₂CO₃ gives two complexes (3) and (4) as well as some isomers of triphenylbenzene and acetophenone. Complexes (3) and (4) were characterised by ¹³C n.m.r. spectroscopy and by single-crystal X-ray structure determinations. Complex (3) has the rhodium π-bonded η⁵ to a C₅Me₅ ring and η⁴ to the C₄ ring of a tetraphenylbenzocyclobutadiene. The benzocyclobutadiene is very close to planar and only a little distorted upon co-ordination; the structure of the ligand is best understood in terms of a ‘1,2-divinylcyclobutadiene’ type of bonding. Complex (4) has the rhodium π-bonded η⁵ to a C₅Me₅ ring and η⁴ to a cyclobutadiene; this cyclobutadiene carries two phenyl substituents (1,3-) and an (unco-ordinated) 6-(1,3,6-triphenylfulvenyl) substituent. Possible routes by which (3) and (4) could be formed are discussed.