Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part V. Polysubstituted derivatives of [Ir4(CO)12] |
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| Authors: | Andrs Strawczynski Chris Hall Giacomo Bondietti Renzo Ros Raymond Roulet |
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| Institution: | Andrés Strawczynski,Chris Hall,Giacomo Bondietti,Renzo Ros,Raymond Roulet |
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| Abstract: | The dynamic behaviour of twelve polysubstituted derivatives of Ir4(CO)12] has been investigated in solution, using 2D-EXSY, and VT-31P- and 13C-NMR. Ir4(CO)6(μ2-CO)3(η4-diarsine) PPh3] and Ir4(CO)6(μ2-CO)3(η4-nor-bornadiene)(PMePh2)] exhibit two isomeric forms in solution, which interconvert through an intramolecular change of basal face. The related cluster Ir4(CO)6(μ2-CO)3(η4-norbornadiene)PPh3] exists as a single isomer in solution. It displays rotation of CO ligands about the apical Ir-atom, followed by two consecutive changes of basal face. The tetrasubstituted clusters with two chelating ligands Ir4(CO)5(μ2-CO)3(η4-diolefin)2] also exhibit rotation of apical CO's, the activation energy increases with greater steric hindrance of the radical ligands. A quantitative analysis of the 31P- and 13C-2D-EXSY spectra followed by simulation of the corresponding VT-NMR spectra of IR4(CO)5(μ2-CO)3(μ2-L)2] (L = bis(diphenylphosphino)methane and 1,3-bis(diphenylphosphino)propane) revealed a pairwise averaging of the P-atoms, caused by two parallel changes of basal face averaging all CO ligands. In addition, the restricted rotation of ligands about the apical Ir-atom was identified at higher temperatures. The remaining clusters are either rigid on the NMR time scale, or display CO-scrambling about a single Ir-atom. |
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