Solution Equilibria in Trialkyl-Phosphite Derivatives of [Ir4(CO)12]. Crystal Structure of [Ir4(CO)11{P(OCH2)3CEt}]

The monosubstituted [Ir4(CO)11L] clusters (L = P(OPh)3, 1; L = P(OMe)3, 2; L = P(OCH2)3CEt, 3) were obtained in good yields by the reaction of [Ir4(CO)11I]− with the corresponding phosphite. In the solid state, cluster 3 has a Cs geometry with all terminal ligands as shown by an X-ray analysis. Three isomers are present in solution: one with terminal ligands (A) and two with three edge-bridging CO's and with L in axial (B) or radial (C) position (see Scheme). The thermodynamic and kinetic parameters of isomerisations B ⇌ A and A ⇌ C were determined by simulation of the variable-temperature 31P-NMR spectra. The three isomers correspond to three minima on the kinetic pathway of CO scrambling, whose relative energies vary independently within a small range (1–9 kJ mol−1 at 298 K). At low temperature, isomer C is always the least stable and is not observed for 1 which bears the most bulky phosphite ligand. The isomerisations are due to two intramolecular merry-go-rounds of CO groups about two unequivalent faces of the unbridged species A.