Axial Ligand Effects on the Mechanism of Ru-CO Bond Photodissociation and Photophysical Properties of Ru(II)-Salen PhotoCORMs/Theranostics: A Density Functional Theory Study.

Density functional theory (DFT) calculations were employed to study a series of complexes of general formula [Ru(salen)(X)(CO)] (X = Cl, F, SCN, DMSO, Phosphabenzene, Phosphole, TPH, CN, N, NO, CNH, NHC, P(OH), PF, PH). The effect of ligands X on the Ru-CO bond was quantified by the -philicity, ΔC NMR parameter. The potential of ΔC to be used as a probe of the CO photodissociation by Ru(II) transition metal complexes is established upon comparing it with other -effect parameters. An excellent linear correlation is found between the energy barrier for the Ru-CO photodissociation and the ΔC parameter, paving the way for studying photoCORMs with the C NMR method. The strongest -effect on the Ru-CO bond in the [Ru(salen)(X)(CO)] complexes are found when X = CNH, NHC, and P(OH), while the weakest for X = Cl, NO and DMSO -axial ligands. The Ru-CO bonding properties were scrutinized using Natural Bond Orbital (NBO), Natural Energy Decomposition Analysis (NEDA) and Natural Orbital of Chemical Valence (NOCV) methods. The nature of the Ru-CO bond is composite, i.e., electrostatic, covalent and charge transfer. Both donation and backdonation between CO ligand and Ru metal centre equally stabilize the Ru(II) complexes. Ru-CO photodissociation proceeds via a MC triplet excited state, exhibiting a conical intersection with the TMLCT excited state. Calculations show that these complexes show bands within visible while they are expected to be red emitters. Therefore, the [Ru(salen)(X)(CO)] complexes under study could potentially be used for dual action, photoCORMs and theranostics compounds.