Synthesis and Reactivity of a Coordinatively Unsaturated Ruthenium(II) Parent Amido Complex:  Studies of X−H Activation (X = H or C)

Abstract

The five-coordinate parent amido complex (PCP)Ru(CO)(NH 2 ) ( 2 ) (PCP = 2,6-(CH 2 P t Bu 2 ) 2 C 6 H 3 ) has been prepared by two independent routes that involve deprotonation of Ru(II) ammine complexes. Complex 2 reacts with phenylacetylene to yield the Ru(II) acetylide complex (PCP)Ru(CO)(C⋮CPh) ( 5 ) and ammonia. In addition, complex 2 rapidly activates dihydrogen at room temperature to yield ammonia and the previously reported hydride complex (PCP)Ru(CO)(H). The ability of the amido complex 2 to cleave the H−H bond is attributed to the combination of a vacant coordination site for binding/activation of dihydrogen and a basic amido ligand. Complex 2 also undergoes an intramolecular C−H activation of a methyl group on the PCP ligand to yield ammonia and a cyclometalated complex. The reaction of (PCP)Ru(CO)(Cl) with MeLi allows the isolation of (PCP)Ru(CO)(Me) ( 8 ), and complex 8 undergoes an intramolecular C−H activation analogous to the amido complex 2 to produce methane and the cyclometalated complex. Determination of activation parameters for the intramolecular C−H activation transformations of 2 and 8 reveal identical Δ H ⧧ {18(1) kcal/mol} with Δ S ⧧ = −23(4) eu and −18(4) eu, respectively. Density functional theory has been applied to the study of intermolecular activation of methane and dihydrogen by (PCP‘)Ru(CO)(NH 2 ) to yield (PCP‘)Ru(CO)(NH 3 )(X) (X = Me or H; PCP‘ = 2,6-(CH 2 PH 2 ) 2 C 6 H 3 ). The results indicate that the activation of dihydrogen is both exoergic and exothermic. In contrast, the addition of a C−H bond of methane across the Ru−NH 2 bond has been calculated to be endoergic and endothermic. The surprising endoergic nature of the methane C−H activation has been attributed to a large and unfavorable change in Ru−N bond dissociation energy upon conversion from Ru-amido to Ru-ammine.