Heterometallic Trinuclear Polyhydrido Complexes Containing Ruthenium and a Group 9 Metal, [Cp*₃Ru₂M(μ₃-H)(μ-H)₃] (M = Ir or Rh; Cp* = η⁵-C₅Me₅): Synthesis, Structure, and Site Selectivity in Reactions with Phosphines

A heterometallic trinuclear polyhydrido complex, [Cp*3Ru2Ir(μ3-H)(μ-H)3] (Cp* = η5-C5Me5) (1), was synthesized by dehydrogenative coupling between a dinuclear ruthenium tetrahydrido complex, [Cp*Ru(μ-H)4RuCp*], and an equimolar amount of a mononuclear iridium tetrahydrido complex, Cp*IrH4. The Ru−Rh analogue, [Cp*3Ru2Rh(μ3-H)(μ-H)3] (2), was synthesized by reacting a 2:1 mixture of the dichlorido dimers (Cp*RuCl2)2 and (Cp*RhCl2)2 with NaBH4. The molecular structure of [Cp*2(C5Me4Et)Ru2Ir(μ3-H)(μ-H)3] (1′′) was determined through single-crystal X-ray diffraction. The mutual exchange of the hydrido ligands in 1 among the coordination sites was analyzed using variable-temperature (VT)-1H NMR spectroscopy. Complex 1 undergoes an H/D exchange between the hydrido ligands and C6D6 via C−H bond cleavage/formation. The reactivity of 1 toward tertiary and secondary phosphines was studied to elucidate the roles of each metal atom in the substrate activation step. The reactions of 1 with tertiary phosphines afford the phosphine complexes [Cp*3Ru2Ir(PR3)(μ-H)4] (R = Me, 3a; Et, 3b; Ph, 3c; OMe, 3d), in which the phosphine is bound to the one of the Ru atoms in a terminal mode. Treatment of 1 with PPh2H exclusively produces a μ-phosphido complex, [Cp*3Ru2Ir(μ-PPh2)(μ-H)3] (5), in which the phosphido ligand bridges the Ru−Ir edge through the intermediary μ-phosphido complex [Cp*3Ru2Ir(μ-PPh2)(μ-H)5] (6), in which the phosphido ligand bridges the Ru−Ru edge. The fluxional behavior of the μ-phosphido ligand in 5 was elucidated by VT-1H NMR and selective pulse irradiation. Thermolysis of 5 generates a μ3-phosphinidene complex, [Cp*3Ru2Ir(μ3-PPh)(μ-H)2] (7), via P−C(ipso) bond cleavage and the reductive elimination of benzene.