A hydrogen bond of the type C−H⋅⋅⋅X (X=O or N) is known to influence the structure and function of chemical and biological systems in solution. C−H⋅⋅⋅O hydrogen bonding in solution has been extensively studied, both experimentally and computationally, whereas the equivalent thermodynamic parameters have not been enumerated experimentally for C−H⋅⋅⋅N hydrogen bonds. This is, in part, due to the lack of systems that exhibit persistent C−H⋅⋅⋅N hydrogen bonds in solution. Herein, a class of molecule based on a biologically active norharman motif that exhibits unsupported intermolecular C−H⋅⋅⋅N hydrogen bonds in solution has been described. A pairwise interaction leads to dimerisation to give bond strengths of about 7 kJ mol⁻¹ per hydrogen bond, which is similar to chemically and biologically relevant C−H⋅⋅⋅O hydrogen bonding. The experimental data is supported by computational work, which provides additional insight into the hydrogen bonding by consideration of electrostatic and orbital interactions and allowed a comparison between calculated and extrapolated NMR chemical shifts.

Deprotonation of the 1-isopropyl-3-(phenylamino)pyridin-1-ium iodide gives the corresponding neutral betaine, which is formalised as a pyridinium-amido ligand when coordinated to a metal. Spectroscopic, structural and theoretical methods have been used to investigate the metal–ligand bonding, ligand dynamics and electron distribution. Collectively, the data show that the ligand can be characterised as a pseudo-amide and is a strong donor akin to alkyl phosphines and N-heterocyclic carbenes. Furthermore, rotation about both N substituent CN bonds occurs, which is in contrast to the two alternative pyridinium positional isomers that exhibit neutral resonance structures. For comparison, compounds and complexes derived from norharman were prepared, which contain an additional CC bond supporting conjugation and the accessibility of a neutral resonance structure. Notwithstanding the formal neutral structure, norharman-derived ligands are comparably strong donors, and have the additional advantage of exhibiting stability to dioxygen and water.