A comparison of non-covalent interactions in the crystal structures of two σ-alkane complexes of Rh exhibiting contrasting stabilities in the solid state

Non-covalent interactions surrounding the cationic Rh σ-alkane complexes within the crystal structures of [(Cy2PCH2CH2PCy2)Rh(NBA)][BArF4], [1-NBA][BArF4] (NBA = norbornane, C7H12; ArF = 3,5-(CF3)2C6H3), and [1-propane][BArF4] 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 [BArF4]− 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 %δGatom 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][BArF4] 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 δGatom indices as a measure of cation-anion non-covalent interaction energy.