Amine adducts of porphinato carbonylruthenium(II): studies of conformational effects and dynamics

The ¹H chemical shifts for 4-methylpyridine, imidazole, 2,5-dimethylimidazole, 2-undecylimidazole, pyrazole, 3,5-dimethylpyrazole, benzylamine, and α-methylbenzylamine bound to carbonylruthenium mesoporphyrin-IX-dimethyl ester or carbonylruthenium tetra-p-isopropylphenylporphyrin have been determined. The upfield shifts induced in the bound-nitrogen base resonances have been shown to arise principally from the magnetic anisotropy of the porphinato ligand. On this basis, models for calculating magnetic anisotropy in porphyrin systems have been tested and then applied to the determination of adduct geometry. These studies show that steric bulk of substituents ortho to the nitrogen binding determine the stability of various modes and control conformation in ligands. Furthermore, the results suggest adduct geometry and ligand-to-ring plane distances are relatively independent of porphyrin. Correlations between base structure and adduct stability were developed by measuring relative dissociation constants and first-order rate constants for dissociation. These measurements show a direct correlation between thermodynamic and kinetic stability and an inverse correlation between stability and increased steric bulk ortho to the donor atom.