Unsaturation in homoleptic tetranuclear iridium carbonyls: a comparison of density functional theory with the MP2 method in metal cluster structures

Abstract

The lowest energy Ir4(CO)12 structure is predicted by density functional theory to be a triply bridged structure analogous to the experimental structures for its lighter congeners M4(CO)9(μ-CO)3 (M=Co, Rh). The experimental unbridged structure for Ir4(CO)12 is predicted to lie ~6 kcal/mol above the triply bridged structure. However, the MP2 method predicts the unbridged structure for Ir4(CO)12 to be the lowest energy structure by ~9 kcal/mol over the triply bridged structure. The lowest energy Ir4(CO)11 structure is predicted to be a doubly bridged structure with a central tetrahedral Ir4 unit. A higher energy Ir4(CO)11 structure at ~18 kcal/mol above this global minimum is found with an unusual μ4-CO group bridging all four atoms of a central Ir4 butterfly. This Ir4(CO)8(μ-CO)2(μ4-CO) structure is analogous to the lowest energy Co4(CO)11 structure found in a previous theoretical study, as well as Rh4(CO)4(μ-CO)4(PBu 3 t )2(PtPBu 3 t )(μ4-CO), which has been synthesized by Adams and coworkers. The Ir4 tetrahedron is remarkably persistent in the more highly unsaturated Ir4(CO) n (n = 10, 9, 8) structures with relatively little changes in the Ir–Ir distances as carbonyl groups are removed. This appears to be related to the spherical aromaticity in the tetrahedral Ir4 structures.