Synthesis and characterisation of two novel Rh(I) carbene complexes: Crystal structure of [Rh(acac)(CO)(L1)]

The Rh(acac)(CO)(L)] (acac = acetylacetonato; L₁ = 1,3-bis-(2,6-diisopropylphenyl)imidazolinylidene and L₂ = 1,3-bis-(2,4,6-trimethylphenyl)imidazolinylidene) complexes were prepared by the action of the parent carbene on Rh(acac)(CO)₂] in THF. The crystal structure characterisation of Rh(acac)(CO)(L₁)] revealed a slightly distorted square planar geometry with the carbene ligand orientated almost perpendicular to the equatorial plane; an elongated trans Rh-O bond of 2.0806(18) Å reflecting the considerable trans-influence of the carbene ligand. By measuring the CO stretching frequencies in a range of Rh(acac)(CO)(L)] complexes (L = CO, L₁, L₂, PPh₃, PⁿBu₃, P(O-2,4-^tBu₂-Ph)₃) the following electron donating ability series was established: L₁ ∼ L₂ ∼ PⁿBu₃ > PPh₃ > P(O-2,4-^tBu₂-Ph)₃ > CO; indicating the carbenes investigated in this study to have a similar electronic cis-influence as trialkyl phosphines. Both complexes do not display hydroformylation activity towards 1-hexene in the absence of added phosphine or phosphite ligands under the conditions investigated (P = 60; T = 85 °C). In the presence of a phosphine or phosphite ligand the resulting hydroformylation catalysis was identical to that observed for Rh(acac)(CO)₂] and the corresponding ligand and subsequent high-pressure ³¹P NMR studies confirmed substitution of the carbene ligand under these conditions.