C-H/O interactions are among the most abundant weak hydrogen bonds due to the omnipresence of C-H groups and oxygen atoms. Previous studies have shown that coordination with metals can cause the strengthening of noncovalent interactions, such as hydrogen bonds, stacking interactions, cation–π, and anion–π interactions. In this work, we studied the influence of transition metal coordination on C-H/O interactions of aromatic ligands in organometallic complexes.
Crystal structures of high quality, deposited in the Cambridge Structural Database (CSD, version 5.43, November 2022), were studied using the ConQuest program (version 2022.3.0) to find C-H/O interactions between the η6-coordinated benzene and the oxygen atom of a Z1-O-Z2 fragment, where Z1 or Z2 could be any atom. The contact was accepted as a C-H/O interaction if the O∙∙∙H distance was shorter than 2.9 Å and the C-H∙∙∙O angle was bigger than 110°. The density functional theory was employed to calculate the energies of C-H/O interactions.
Our CSD search resulted in 152 C-H/O interactions of coordinated benzene, mostly in organometallic half-sandwich compounds. The analysis of geometrical parameters shows that preferred O∙∙∙H distances in these interactions are between 2.3 Å and 2.5 Å. These O∙∙∙H distances are shorter for coordinated than uncoordinated aromatic rings, indicating that coordinated aromatic rings form stronger C-H/O hydrogen bonds than uncoordinated ones. These findings are confirmed by our preliminary B3LYP-D3/aug-cc-pVDZ calculations, which showed that in the organometallic half-sandwich model system Cr(C6H6)(CO)3∙∙∙H2O the C-H/O interaction reaches the energy of -3.88 kcal/mol, which is considerably stronger than the C-H/O interaction between (uncoordinated) benzene and water (-1.64 kcal/mol).
Our joint crystallographic and computational study points towards the enhanced ability of coordinated aromatic rings to form substantial C-H/O interactions. This study provides further insights into the strengthening of noncovalent interactions via transition metal coordination.
