A Relative Stability Scale of Nitrogen Donor Ligand Ruthenium Complexes Based on their Stability Constants calculated from First Principles by DFT Methods.

Using DFT calculations (M06/def2-TZVP/SMD solvent model) a relative stability scale is established from first principles for a series of iron and ruthenium complexes with ligand frameworks based on chelating pyridine- and pyrrole-type (porphyrin and phthalocyanine) ligands by determining the equilibrium constants for the complex formation reactions [M(H 2 O) 6 ] 2+ + nL → [M(L) n (H 2 O) m ] 2+ + (6 − m) H 2 O; M=Fe, Ru in aqueous solution through Hess's law and the van't Hoff equation. For a subset of the iron complexes, comparison against the available experimental data systematically underestimates the calculated constants by multiple orders of magnitude for the high-spin hexaaqua complex as the starting material, but shows a linear correlation trend between the experimental and calculated values. A similar correlation exists between the calculated values for the iron and ruthenium complexes. For the latter, for which no experimental data are available, the values are very high. On the relative scale calculated, the stability constants for the macrocyclic porphyrin and phthalocyanine complexes are unexpectedly low when calculated from the neutral ligand, but very high when calculated for reactions starting from the deprotonated ligand in its dianionic state. The limitations of the model are discussed, in particular with regards to the likely very large influence of solvation energies and hydrogen-bond networks not explicitly formulated in the SMD solvent model employed and Brønsted acid–base reactions between complexes and ligands, and its merits critically evaluated.