In silico screening of Ruthenium pincer complexes for Liquid Organic Hydrogen Carrier (LOHC) applications

Abstract

This work presents a high-throughput in silico screening of ruthenium(II) pincer complexes as potential catalysts for pyridine hydrogenation To explore how backbone architecture, substituent sterics, and hemilability govern substrate binding energetics, this study screened 24 Ru(II) pincer complexes comprising four distinct lutidine-based pincer backbones (PNpyP, PONpyOP, NONpyON, NNpyN) and six substituents (Me, iPr, tBu, Bu, Ph, Cy). Additionally, 18 complexes representing three specific coordination modes from the PNpyP ligand family were screened using pyridine as a model substrate.

MACE was employed to generate stereoisomer and conformer libraries without conformational bias. Structures were screened with the Universal Force Field, refined via DFT (PBE0-D3(BJ)/def2-SVP), and analysed using ensemble-averaged steric descriptors (percent buried volume and bite angle) and pyridine binding free energies across three coordination pathways: (i) carbonyl auxiliary substitution, (ii) central nitrogen donor dissociation, and (iii) phosphorus side-arm dissociation.

The results reveal that pyridine binding to form RuH₂(py)(𝜅³-PNP), RuH₂(CO)(𝜅²-P,N-PNP)(py), or RuH₂(CO)(𝜅²-P,P-PNP)(py) complexes is generally unfavorable. However, binding becomes more accessible via the hemilabile dissociation of either the nitrogen or phosphorus pincer donor. Dissociation of the nitrogen donor emerged as the most energetically accessible pathway (Δ𝐺 = 30–105 kJ/mol), particularly with phenyl substituents where 𝜋–𝜋 stacking stabilises binding (Δ𝐺 = 29.9 kJ/mol). Conversely, dissociation of the phosphorus pincer donor (Δ𝐺 = 60–105 kJ/mol) presents the most favorable binding energetics in the presence of bulky substituents, as the enhanced flexibility of the phosphine side-arm accommodates steric bulk more effectively. Substitution of the carbonyl remains largely inaccessible (Δ𝐺 = 100–150 kJ/mol), as it requires the coordination of pyridine to a fully occupied complex.

Analysis of the ensemble-averaged steric descriptors revealed that an optimal balance between ligand hemilability, steric constraints, and geometric flexibility governs substrate accessibility. A percent buried volume in the range of 40–50% and P–Ru–P bite angles near 95–115^◦ showed significantly improved pyridine binding energetics. This was exemplified by the RuH₂(CO)(𝜅²-P,P-PNP)(py) complex, where structural reorganisation upon nitrogen dissociation allows the complex to exploit conformational flexibility and stabilise substrate binding through non-covalent interactions. This creates a confined,welldefined pocket adjacent to the metal center that effectively accommodates the substrate. These findings warrant further investigation to bridge computational insights with improved catalytic performance.