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

Abstract

A homologous family of low-coordinate complexes of the formulation trans-[M(2,2'-biphenyl)(PR₃ )₂ ][BAr^F₄ ] (M=Rh, Ir; R=Ph, Cy, iPr, iBu) has been prepared and extensively structurally characterised. Enabled through a comprehensive set of solution phase (VT ¹ H and ³¹ P NMR 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 PiBu₃ >PCy₃ >PiPr₃ >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. CH₂ Cl₂ ) in the lattice or collection of data at very low temperature (25 vs. 150 K), lead to small but statistically significant shortening of the M-H-C distances.