An Unusual Carbonyl Chemical Shift in a Carbonylhexairidium Cluster: A Combined Solid‐State NMR and DFT Approach

Abstract

AbstractThe 13C NMR spectrum of TMBA2[Ir6(CO)15] [TMBA =(CH3)3N(CH2C6H5)] shows, at low temperatures, an unprecedented μ2‐bridging carbonyl low‐frequency shift, with the resonances of the terminal μ1‐carbonyl ligands placed at higher frequencies. The chemical shift tensors and the shielding anisotropies of the carbonyl ligands, obtained from solid‐state NMR analysis, allow us to determine the nature of the M–CO interaction. The results have been compared with the 13C MAS data of Ir6(CO)16 where μ3‐CO ligands are present. Further evidence for the assignment and for the peculiar chemical shift value of bridging carbonyl ligands in TMBA2[Ir6(CO)15] has been obtained by the DFT calculation of the NMR parameters. The scalar and spin‐orbit (SO) relativistic two‐component zero‐order regular approximation (ZORA) methods were employed in the geometry optimization and NMR chemical shift calculations, respectively. The large SO contribution (δ = 26.6 ppm) to the μ2‐bridging CO ligand 13C chemical shifts accounts for the position of the experimentally observed resonance.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)