C–H⋯C hydrogen bond and alkali metal C⋯Z⋯C (Z = Li, Na, K) analogues – N-heterocyclic carbenes in coordination spheres of protons and alkali metal cations: crystal structures and theoretical analysis

Abstract

A Cambridge Structural Database, CSD, search was performed to find structures where a proton or an alkali metal cation is a coordination centre that interacts with N-heterocyclic carbene, NHC, ligands. The search was fixed to include at least one NHC ligand in a coordination sphere of the proton or the alkali metal cation. 21 high precision structures that fulfil the search conditions were found. It was found that in such structures, the proton forms a C–H covalent bond which may be involved in a few different types of hydrogen bonds while the alkali metal cations form other interactions which are analogues to the hydrogen bond. Among these interactions, C⋯Z⋯C bonds (Z = Li, Na, K) were found where the C-centres are those of two NHC species. The proton or the alkali metal cation is a centre where linear, trigonal or tetrahedral coordination occurs. DFT BP86-D4/TZ2P calculations were performed on species similar to those found in the CSD, i.e. complexes of one and two imidazol-2-ylidene molecules with protons or with alkali metal cations. The interactions in these complexes were also analysed with the use of other theoretical approaches: the Quantum Theory of Atoms in Molecules, QTAIM, the Natural Bond Orbital, NBO, analysis and the energy decomposition analysis, EDA. The results show that these are often very strong interactions possessing properties typical for covalent bonds.