A Comparison of Non-Covalent Interactions in the Crystal Structures of two σ-Alkane Complexes of Rh Exhibiting Contrasting Stabilities in the Solid State.

Abstract

Non-covalent interactions surrounding the cationic Rh σ-alkane complexes within the crystal structures of [(Cy₂PCH₂CH₂PCy₂)Rh(NBA)][BAr^F₄], [1-NBA][BAr^F₄] (NBA = norbornane, C₇H₁₂; Ar^F = 3,5-(CF₃)₂-C₆H₃), and [1-propane][BAr^F₄] are analysed using Quantum Theory of Atoms in Molecules (QTAIM) and Independent Gradient Model approaches, the latter under a Hirshfeld partitioning scheme (IGMH). In both structures the cations reside in an octahedral array of [BAr^F₄]^- anions within which the [1-NBA]⁺ cation system exhibits a greater number of C–H…F contacts to the anions. QTAIM and IGMH analyses indicate these include the strongest individual atom-atom non-covalent interactions between the cation and the anion in these systems. The IGMH approach highlights the directionality of these C–H…F contacts that contrasts with the more diffuse C–H…π interactions. The accumulative effects of the latter lead to a more significant stabilizing contribution. IGMH %δG^atom plots provide a particularly useful visual tool to identify key interactions and highlight the importance of a -{C₃H₆}- propylene moiety that is present within both the propane and NBA ligands (the latter as a truncated -{C₃H₄}- unit) and the cyclohexyl rings of the phosphine substituents. The potential for this to act as a privileged motif that confers stability on the crystal structures of σ-alkane complexes in the solid-state is discussed. The greater number of C–H…F inter-ion interactions in the [1-NBA][BAr^F₄] system, coupled with more significant C–H…π interactions are all consistent with greater non-covalent stabilisation around the [1-NBA]⁺ cation. This is also supported by larger computed δG^atom indices as a measure of cation-anion non-covalent interaction energy.