The effect of substituents on the reactivity of dichloridotriphenylphosphinoruthenium(II) complexes: kinetic and mechanistic study

Abstract

The rates of substitution of chloro ligands from a series of ruthenium(II) complexes, [Ru(κ3-L)(PPh3)Cl2] (L = 2,2′:6′,2′′-terpyridine, 1; 4′-(4-methylphenyl)-2,2′:6′,2′′-terpyridine, 2; 4,4′4″-tri-tert-butyl-2,2′:6′,2′′-terpyridine, 3; 4′-(4-chlorophenyl)-2,2′:6′,2′′-terpyridine, 4; 4-chloro-2,2′:6′,2′′-terpyridine, 5 and 2,6-bis(2-pyrazolyl)pyridine, 6), by thiourea nucleophiles was investigated under pseudo-first-order conditions in methanol as a function of nucleophile concentration and temperature. The chloro ligands were substituted in two steps and the reactivity trend was 4 > 5 > 2 > 1 > 6. Complexes 2 and 3 having donor substituents on the 2,2′:6′,2′′-terpyridine backbone experience a trans-effect making them more reactive than 1. Complexes 4 and 5 are more reactive than 1 due to enhanced π-back-bonding brought about by electron-withdrawing substituents on their 2,2′:6′,2′′-terpyridine backbones. The reactivity of 4 is higher than 5 due to greater electron acceptor-ability of the chlorophenyl substituent than the chloro substituent in 5. The 2,6-bis(pyrazolyl)pyridine ligand in 6 retards the reactivity of the complex compared to 1 due to the cis-donor effect of the pyrazole. The reactivity of the complexes is associative for all nucleophiles in step one and only thiourea in step two. The substitution reactions proceed by a steady changeover from an associative interchange mechanism (I a) to a dissociative interchange (I d) mechanism on increasing steric hindrance.