Synthesis and X-ray structural studies on the cluster compounds [RuRh3(µ3-CO)2(CO)3(η-C5Me5)3] and [RuRh2(µ-CO)(µ3-CO)(CO)2(η4-C8H10)(η-C5Me5)2

Abstract

The compounds [Ru(η6-C8H10)(cod)](C8H10= cycle-octa-1,3,5-triene, cod = cycle-octa-1,5-diene) and [Rh(CO)2(η-C5Me5)] react in toluene at 60 °C to give the cluster compounds [RuRh3(µ3-CO)2(CO)3(η-C5Me5)3](1) and [RuRh2(µ-CO)(µ3-CO)(CO)2(η4-C8H10)(η-C5Me5)2](2). The structures of these compounds were established by single-crystal X-ray diffraction studies. Compound (1) has a distorted tetrahedral RuRh3 metal atom core [Rh–Rh 2.684(1)–2.739(1), Ru–Rh 2.620(1)–2.715(1)A]. The rhodium atoms are each ligated by an η-C5Me5 group, the ruthenium atom is terminally bonded to three carbonyl groups, while the remaining two CO ligands triply and asymmetrically bridge the Rh3 face and one RuRh2 face of the metal polyhedron. In compound (2) the C8H10 ligand attached to the ruthenium is bicyclo[4.2.0]octa-2,4-diene. The molecule comprises a metal atom triangle [Ru–Rh 2.766(1) and 2.815(1), Rh–Rh 2.672(1)A] and an η-C5Me5 group is co-ordinated to each rhodium atom. The four CO ligands show a remarkable variety of bonding modes. One carbonyl group attached to the ruthenium atom is essentially terminally bound to that centre [Ru–C–O 167.5(4)°], while a second carbonyl group attached to the ruthenium semi-bridges an edge of the metal triangle [Ru–C–O 158.1(4)°; Ru–CO 1.930(4), Rh ⋯ CO 2.412(5)A]. The Rh–Rh vector is symmetrically bridged by another CO ligand [Rh–C–O 137.3(3) and 138.2(3)°, Rh–CO 1.989(3) and 2.000(4)A]. The remaining carbonyl group asymmetrically bridges the face of the triangle [Ru–CO 2.247(4), Rh–CO 2.038(4) and 2.069(4)A]. Spectroscopic studies (i.r. and n.m.r.) on (1) and (2) are reported. Both compounds show site exchange of CO groups in solution (13C-{lH} n.m.r.) but the process in (1) is limited to the Ru(CO)3 group, with the dynamic behaviour persisting at –80 °C. With (2), at room temperature, all four CO ligands exchange, but at –80 °C separate signals for the µ-CO and µ3-CO groups are seen, although only one resonance is observed for the two CO groups attached to the ruthenium atom. These and other data are discussed, and a mechanism proposed to account for the formation of (1) and (2).