A penta-nuclear iridium(III) hydride cluster: aggregation of an iridium(I) precatalyst

The crystal structure of a unique penta-nuclear Ir cluster, tetra-kis-[1,3-bis-(2,4,6-tri-methyl-phen-yl)-1,3-di-hydro-2H-imidazol-2-yl-idene-κC ²]carbonyldi-μ₃-hydrido-tetra-μ₂-hydrido-nona-hydridopenta-iridium(III), [Ir₅(μ₃-H)₂(μ₂-H)₄H₉(C₂₁H₂₄N₂)₄(CO)], has been refined from X-ray data and supported by density functional theory (DFT) calculations. The five iridium(III) sites of the cluster form a trigonal-bipyramidal structure: three are located in the equatorial triangular plane and are capped by axial metal sites above and below the centre of this plane. Four of these iridium atoms are associated with an N-heterocyclic carbene ligand, and a fifth, which is located in the equatorial plane, is bonded to a CO ligand, which must come from the methanol solvent. The 15 hydride ligands in the cluster could not be located in electron-density difference maps and their locations were optimized by using DFT approaches to calculate the lowest energy structure. These methods revealed the presence of nine terminal, four μ₂-, and two μ₃-bridging hydrides, which unusually cap faces of three metal atoms. The cluster formed from reaction of an Ir^I precursor with H₂ and NaOMe base in methanol, and it likely reflects an example of a catalytic deactivation product when active Ir^III hydrogenation, isotope exchange, or signal amplification by reversible exchange (SABRE) catalysts aggregate in solution to form crystalline or other solid-state products.