Solution, Solid-State, and Computational Analysis of Agostic Interactions in a Coherent Set of Low-Coordinate Rhodium(III) and Iridium(III) Complexes

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR3)(2)][BAr4F] (M = Rh, Ir; R = Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT H-1 and (PNMR)-P-31 spectroscopy) and solid-state (single crystal X-ray diffraction) data, and analysis in silico (DFT-based NBO and QTAIM analysis), the structural features of the constituent agostic interactions have been systematically interrogated. The combined data substantiates the adoption of stronger agostic interactions for the Ir-III compared to Rh-III complexes and, with respect to the phosphine ligands, in the order PiBu(3) > PCy3 > PiPr(3) > PPh3. In addition to these structure-property relationships, the effect of crystal packing on the agostic interactions was investigated in the tricyclohexylphosphine complexes. Compression of the associated cations, through inclusion of a more bulky solvent molecule (1,2-difluorobenzene vs. CH2Cl2) in the lattice or collection of data at very low temperature (25 vs. 150K), lead to small but statistically significant shortening of the (M) under bar -H-(C) under bar distances.