Skeletal rearrangements of arylborane complexes mediated by redox reactions: thermal and photochemical oxidation by metal ions

Abstract

A variety of metal salts have been found to undergo reduction by thermal and photochemical interaction with tetraarylborate salts and with neutral alkyl- and aryl-borane complexes. In the cases of Cu 2+, Cu +, Ni 2+, Co 2+, Pd 2+, Pt 2+, Ag +, Zn 2+, Hg 2+, Sn 2+, Pb 2+ and Rh 3+ salts, such photochemical reductions with NaBPh 4 led to the deposition of the free metal, while a number of binary mixtures of metal salts led to the deposition of both metals, sometimes as true alloys, under such photoreductions. In the course of these reductions the arylborate reductants underwent oxidative coupling of the aryl groups to form biaryls in a strictly intra-ionic (for BAr 4 −) or intramolecular (Ar 3B) manner respectively. Individual studies of the photochemistry of the tetraarylborate anion itself, of cuprous tetraphenylborate and of the triphenylborane—pyridine complex have adduced evidence for a gamut of reactive intermediates capable of serving as the photoreductant for metal ions, such as triarylborane radical anions, diarylborate(I) anions or arylborenes, 7-borabicycloheptadiene anions or neutral complexes and finally arylborohydride anions or arylboronhydrides. The role of these intermediates both in the photoinduced skeletal rearrangements of arylboranes and in the concomitant reduction of metal ions is discussed in critical detail.