An assessment of the intercarbon stretching phenomenon in C-substituted “pseudocloso” {3,1,2-RuC2B9} metalladicarbaboranes

Abstract

DFT calculation of cluster shapes and their related boron-vertex 11B nuclear shieldings for a range of substituted 12-vertex metalladicarbaborane cluster compounds [3,1,2-R2-3-(η6-C6H6)-3,1,2-RuC2H9H9] confirms that the array of experimentally observed C–C distances in the group of progressively more open closo, ‘semipseudocloso’ and ‘pseudocloso’ clusters results from the inherent propensity of the [(η6-C6H6)RuC2H9H9R2] cluster to occupy, in the absence of steric drivers, either closo-structured or so-called ‘pseudocloso’-structured fundamental local minima on the overall potential energy surface. The shallow profiles of the potential wells, together with the electronic and steric influences of substituents on the cluster carbon vertices, alter the relative energies of the closo and ‘pseudocloso’ minima, and alter the energy profile between them, thereby accounting for the observed variations in the distances in the crystallographically measured C–C separations. We propose that these differ fundamentally due to either the closo or ‘pseudocloso’ configuration being electronically preferred, with some instances of the ‘pseudocloso’ configuration being compressed towards closo by packing forces, or stretched further beyond ‘pseudocloso’ by steric effects of bulky substituents on carbon. This contrasts with one view postulated in the literature which holds that steric repulsion between the C-bound substituents is the sole driver that forces an inherently fundamental closo cluster to open into an anomalous ‘pseudocloso’ configuration. In two reported cases, the cluster flexibility is evident in solution and, in these cases, GIAO calculation of the 11B chemical shieldings explains the measured 11B chemical shifts, which can be interpreted in terms of the dynamic interchange of the closo and ‘pseudocloso’ species. A currently held view is that the ‘weighted average’ of 11B chemical shifts reflects the inherent degree of electronic ‘pseudocloso’ character in a particular cluster can therefore often equally be rationalised in terms of lability between similarly stable fundamentally closo and ‘pseudocloso’ forms.