Spectroscopic Characterization of (diphos)Rh(h6-PhBPh3) in Solution and Study on its Reactivity as a Total Phenylating Agent. Formation of C-C and C-C-C Bonds by Reaction with Aldehydes

(Diphos)Rh(η6-PhBPh3) (1) has been fully characterized in solution by 1H, 13C, 31P and 11B NMR spectroscopies. The NMR spectra show that 1 is a fluxional system in solution at room temperature, as the Rh(diphos) moiety rotates freely around its Rh-(η6-Ph) bond vector. In coordinating solvents, easy decoordination of the tetraphenylborate anion occurs to afford [Rh(diphos)(S)2]BPh4 (S is the solvent). The thermal behaviour of 1 has also been studied. By heating in acetone under controlled conditions, 1 is easily converted into Rh(diphos)Ph, prepared in low yield by reacting Rh(diphos)Cl(S) with PhLi. Prolonged heating, at 348 K, of solutions of 1 produces good yields of benzene and biphenyl, together with traces of terphenyl. The reactivity of 1 towards paraformaldehyde has been investigated and compared with that of NaBPh4. The study has been extended to benzaldehyde. We have established that rhodium mediates the transfer of phenyl groups from boron to RCHO (R = H, Ph), thus promoting the formation of C-C and C-C-C bonds