In silico screening of pyridine coordination and thermodynamic stability in zinc–pincer complexes

Abstract

This study is an in silico screening of zinc PNP and NNN pincer complexes with variation of the R groups in the ligands for the homogeneous catalysis of the hydrogenation of pyridine. This was done by investigating geometry, hemilability, and binding energies. The scope lies in identifying which complexes are thermodynamically capable of binding pyridine, which is researched using the Gibbs free energy of the binding reaction. Energies and optimized geometries were obtained using Density Functional Theory with ORCA and the supercomputer Snellius. The screening revealed that PNP-R complexes favor tetrahedral geometries after optimization, but three and five coordinated structures are also possible. Hemilability of the phosphorus and/or nitrogen arm were observed. All PNP-complexes showed thermodynamically unfavored binding of pyridine. Additionally, NNN-R complexes did not bind to pyridine. Only when phenyl was used as the R group in the NNN backbone did pyridine bind. This is believed to be due to a hydride migrating to one of the phenyl rings, but even then, the binding energy was not thermodynamically favorable. Moreover, research on ligand substitution by pyridine showed that in most complexes this reaction is unfavorable. Only in three coordinated structures of NNN was this reaction thermodynamically feasible. These findings contribute to understanding the pincer ligand dynamics of PNP and NNN complexes and suggest directions for future research.