Synthesis and fluxional behavior of mixed metal trinuclear clusters of the cobalt group containing carbene, carbyne and alkyne ligands

The addition of diazoalkanes and alkynes to a series of unsaturated heteronuclear metal duster complexes of the general formula L-M(CpCo)₂(μ₃- CO)₂ has been described. One focus of the investigation was to study the kinetics and thermodynamics of ligand reorientation about the metal triangle. The nature of the metal fragment L-M was found to influence both the rate of reorientation as well as the preference for a certain positional isomer. The differences in energy for different orientations of the ligand with respect to the different metalmetal edges were found to be relatively small, and in many cases two or more isomers with ligands bridging different edges were observed in equilibrium with each other. Reaction of the trinuclear complexes diazoalkanes can result in two types of adduct, one containing a carbene and one with a hydrazone ligand. The carbene ligands are mobile about the different edges of the metal triangle. In the Cp*Rh complex 7, an intermediate involving a triply bridging carbene ligand was observed for this process, a coordination mode which was not known for carbene ligands so far. In the case of the Cp*Ir complex 8 refluxing in toluene resulted in loss of a CO ligand resulting in the carbyne hydride complex 18. The hydride ligand exchanges with the hydrogen of the carbyne ligand at a rate observable by magnetization transfer techniques. The addition of alkynes R-C=C-R' to the unsaturated complexes L-M(CpCo)₂(μ₃- CO)₂ results in the formation of the alkyne adducts L-M(CpCo)₂(μ₃3-CO)(η²,μ₂-R-C=C-R') (R = R' = H, Tol, Ph; R = Tol, R' = H). In all complexes, the alkyne coordinates in a μ₃-II fashion with the C=C bond parallel to a metal edge. Fast interconversion of the different positional isomers takes place at room temperature, but at -100 °C the rate of interchange is frozen out on the NMR time scale. In addition, the rotation of the tolyl substituents of the alkynes is locked at low temperature due to steric interaction with the Cp* and Cp ligands. Furthermore, ¹³C NMR relaxation times were used to study the rotational reorientation of phenyl substituents in several single and linked bis-carbyne cluster complexes of the general form (CpCo)₃(μ₃₂. The rate of phenyl group rotation was found to be determined predominantly by steric interaction with the Cp ligands of the (CpCo)₃ units.