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)₂C₆H₃), 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.