Intramolecular Dynamics of Tetranuclear Iridium Carbonyl Cluster Compounds. Part V. Polysubstituted derivatives of [Ir₄(CO)₁₂]

Abstract

Abstract The dynamic behaviour of twelve polysubstituted derivatives of [Ir 4 (CO) 12 ] has been investigated in solution, using 2D‐EXSY, and VT‐ 31 P‐ and 13 C‐NMR. [Ir 4 (CO) 6 (μ 2 ‐CO) 3 (η 4 ‐diarsine) PPh 3 ] and [Ir 4 (CO) 6 (μ 2 ‐CO) 3 (η 4 ‐nor‐bornadiene)(PMePh 2 )] exhibit two isomeric forms in solution, which interconvert through an intramolecular change of basal face. The related cluster [Ir 4 (CO) 6 (μ 2 ‐CO) 3 (η 4 ‐norbornadiene)PPh 3 ] 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 [Ir 4 (CO) 5 (μ 2 ‐CO) 3 (η 4 ‐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 31 P‐ and 13 C‐2D‐EXSY spectra followed by simulation of the corresponding VT‐NMR spectra of [IR 4 (CO) 5 (μ 2 ‐CO) 3 (μ 2 ‐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.