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.

Graphical Abstract

Keywords:

• Cis-effect
• kinetics
• ruthenium(II)
• substitution
• thiourea
• trans-effect

Acknowledgments

We thank Mr. Craig Grimmer for his support with the NMR analysis, Mrs. Caryl Janse Van Rensburg for her help with mass spectra and elemental analyses and Shaun Ball with management of the deliveries.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

This research was funded by the University of KwaZulu Natal South Africa.