Organic chemistry of dinuclear metal centres. Part 4. µ-Carbene and µ-vinyl complexes of ruthenium from allenes

Abstract

Heating [Ru2(CO)(µ-CO){µ-C(O)C2Ph2}(η-C5H5)2] with an allene R1CHCCHR2(R1= R2= H or Me; R1= H, R2= Me) in toluene at 100 °C displaces diphenylacetylene and produces allyl complexes [Ru(CO){η3-C3H4–nMen[2-Ru(CO)2(η-C5H5)]}(η-C5H5)](2, n= 0, yield 90%; 3, n= 1, 70%; 4, n= 2, 10%). A Ru–Ru bond is broken in this process but is regenerated upon protonation of (2) with HBF4, which yields the µ-vinyl species [Ru2(CO)2(µ-CO){µ-C(Me)CH2}(η-C5H5)2][BF4. Treatment of this with NaBH4 effects hydride addition to the µ-vinyl to form the µ-dimethylcarbene complex [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2] in 77% yield. Similar sequential addition of H+ and H– to (3) gives [Ru2(CO)2(µ-CO){µ-C(Me)Et}(η-C5H5)2] in 60% yield, but (4) does not transform to a µ-CEt2 complex. The molecular structure of [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2] was established by X-ray diffraction. The two ruthenium atoms are 2.712(1)A apart and are bridged symmetrically by the carbonyl and the dimethylcarbene ligands. The interplanar angle between the two bridge systems is 153°. Each Ru atom carries one terminal carbonyl group and one cyclopentadienyl ligand. These cyclopentadienyl ligands lie in an eclipsed cis orientation, skew to the Ru ⋯ Ru axis, on the convex side of the bridge system, giving the molecule as a whole Cs symmetry. The mean Ru–C (carbene) separation is 2.113(4)A and the plane of the carbene ligand coincides with the molecular mirror plane. The structure is triclinic and has been refined to R 0.018 for 2 001 intensities measured on a four-circle diffractometer. In solution the complex exists as both cis and trans isomers which interconvert rapidly on the n.m.r. time-scale above room temperature. This, and related behaviour of the complex [Ru2(CO)2(µ-CO)(µ-CMe2)(PMe2Ph)(η-C5H5)2], is interpreted in terms of a bridge ⇌ terminal carbene exchange. Minor products of the synthesis of [Ru2(CO)2(µ-CO)(µ-CMe2)(η-C5H5)2] are the isomers [Ru2(CO)2(µ-H){µ-C(Me)CH2}(η-C5H5)2] and [Ru2(CO)2(µ-H){µ-CHC(H)Me}(η-C5H5)2]; the former is dominant in solution but an X-ray diffraction study revealed the latter as the solid-state structure. The molecule comprises two Ru(CO)(η-C5H5) fragments joined by a Ru–Ru bond [2.857(2)A] which is bridged by hydrido- and 2-methylvinyl ligands. The vinyl ligand spans the Ru–Ru bond in a σ, η2 mode, with a Ru–C(σ) separation of 2.013(4)A and Ru–C(π) separations of 2.189(4) and 2.275(4)A. The hydrido-ligand bridges with a small degree of asymmetry, but lies coplanar with the Ru (1)Ru(2)(µ-C) metallacycle. The terminal ligands on each ruthenium atom are arranged to give the molecule an overall cis configuration. However, the carbonyl ligands are not perfectly eclipsed, the torsion angle (O)C–Ru–Ru–C(O) being 15.7(2)°. The structure is monoclinic, and has been refined to R 0.028 for 2 041 intensities measured on a four-circle diffractometer at 200 K.