Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution-phase substitution chemistry of [Ru(η⁵-indenyl)(NCPh)(PPh₃)₂]⁺ (1) by both electrospray ionization mass spectrometry and ³¹P{¹H} NMR are essentially identical, despite an enormous difference in sample concentrations for these complementary techniques. These studies demonstrate dissociative substitution of the NCPh ligand in 1. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt₂H > PPh₂H ≫ PCy₂H. The impact of steric congestion at Ru is evident also in reactions of 1 with tertiary phosphines; initial substitution products [Ru(η⁵-indenyl)(PR₃)(PPh₃)₂]⁺ rapidly lose PPh₃, enabling competitive re-coordination of NCPh. Further solution experiments, relevant to the use of 1 in catalytic hydrophosphination, show that PPh₂H out-competes PPh₂CH₂CH₂CO₂Bu ^t (the product of hydrophosphination of tert-butyl acrylate by PPh₂H) for coordination to Ru, even in the presence of a 10-fold excess of the tertiary phosphine. Additional information on relative phosphine binding strengths was obtained from gas-phase MS/MS experiments, including collision-induced dissociation experiments on the mixed phosphine complexes [Ru(η⁵-indenyl)PP'P″]⁺, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η⁵-indenyl)(PR₂H)(PPh₃)]⁺, generated in the gas-phase, undergo preferential loss of PR₂H. We propose that competing orthometallation of PPh₃ is responsible for the surprising stability of the [Ru(η⁵-indenyl)(PPh₃)]⁺ fragment under these conditions.