Competitive Ligand Exchange and Dissociation in Ru Indenyl Complexes

Kinetic profiles obtained from monitoring the solution phase substitution chemistry of [Ru(η<sup>5</sup>-indenyl)(NCPh)(PPh<sub>3</sub>)<sub>2</sub>]<sup>+</sup> (<b>1</b>) by both ESI-MS and <sup>31</sup>P{<sup>1</sup>H} 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 <b>1</b>. Competition experiments using different secondary phosphine reagents provide a ranking of phosphine donor abilities at this relatively crowded half-sandwich complex: PEt<sub>2</sub>H > PPh<sub>2</sub>H >> PCy<sub>2</sub>H. The impact of steric congestion at Ru is evident also in reactions of <b>1</b> with tertiary phosphines; initial substitution products [Ru(η<sup>5</sup>-indenyl)(PR<sub>3</sub>)(PPh<sub>3</sub>)<sub>2</sub>]<sup>+</sup> rapidly lose PPh<sub>3</sub>, enabling competitive recoordination of NCPh. Further solution experiments, relevant to the use of <b>1</b> in catalytic hydrophosphination, show that PPh<sub>2</sub>H out-competes PPh<sub>2</sub>CH<sub>2</sub>CH<sub>2</sub>CO<sub>2</sub>Bu<i><sup>t</sup></i> (the product of hydrophosphination of <i>tert</i>-butyl acrylate by PPh<sub>2</sub>H) for coordination to Ru, even in the presence of a ten-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 (CID) experiments on the mixed phosphine complexes [Ru(η<sup>5</sup>-indenyl)PP’P’’]<sup>+</sup>, which ultimately appear in solution during the secondary phosphine competition experiments. Unexpectedly, unsaturated complexes [Ru(η<sup>5</sup>-indenyl)(PR<sub>2</sub>H)(PPh<sub>3</sub>)]<sup>+</sup>, generated in the gas-phase, undergo preferential loss of PR<sub>2</sub>H. We propose competing orthometallation of PPh<sub>3</sub> is responsible for the surprising stability of the [Ru(η<sup>5</sup>-indenyl)(PPh<sub>3</sub>)]<sup>+</sup> fragment under these conditions.