Access to Unsaturated Ruthenium Complexes via Phosphine Complexation with Triphenylborane:  Synthesis and Structure of a Zwitterionic Arene Complex, (η6-Ph-BPh2H)Ru(PMe3)2(SiMe3)

Triphenylborane, BPh3, serves as a phosphine “sponge”, scavenging free PMe3 from alkane solutions of the 18e- complex (PMe3)4Ru(SiMe3)H to form sparingly soluble Ph3B−PMe3 and the 16e- (PMe3)3Ru(SiMe3)H. Under nitrogen atmosphere the 16e- (PMe3)3Ru(SiMe3)H forms a dimeric N2 adduct, [(PMe3)3Ru(SiMe3)H]2N2. Both the 16e- complex and its N2 adduct exist in equilibrium with the 18e- silene complex, (PMe3)3Ru(CH2SiMe2)H2. However, long reaction times in the presence of excess borane leads to removal of another phosphine ligand as Ph3B−PMe3 and formation of a new zwitterionic complex, (η6-PhBPh2H)Ru(PMe3)2(SiMe3) (5), in which a molecule of borane has abstracted a ruthenium hydride ligand and also coordinates as an η6-arene. The merits and limitations of BPh3 as a phosphine removal agent are discussed. Compound 5 has been characterized by single-crystal X-ray analysis and exhibits unusually long Ru−Carene bonds.