Deprotonation/protonation of coordinated secondary thioamide units of pincer ruthenium complexes: Modulation of voltammetric and spectroscopic characterization of the pincer complexes?

Dalton Transactions Pub Date: 2011-03-22 DOI: 10.1039/C0DT01283A

Abstract

New pincer ruthenium complexes, [Ru(SCS)(tpy)]PF6 (1) (SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl), tpy = 2,2′:6′,2′'-terpyridyl) and [Ru(SNS)(tpy)]PF6 (2) (SNS = 2,5-bis(benzylaminothiocarbonyl)pyrrolyl), having κ3SCS and κ3SNS pincer ligands with two secondary thioamide units were synthesized by the reactions of [RuCl3(tpy)] with N,N'-dibenzyl-1,3-benzenedicarbothioamide (L1) and N,N'-dibenzyl-2,5-1H-pyrroledicarbothioamide (L2), respectively, and their chemical and electrochemical properties were elucidated. The structure of 1 was determined by X-ray crystallography. The complexes 1 and 2 showed a two-step deprotonation reaction by treatment with 1,8-diazabicyclo[5,4,0]undec-7-ene (DBU), and the addition of DBU led to a shift of the metal-centered redox couples to a lower potential by 720 and 550 mV, respectively. The di-deprotonated complexes were also studied by 1H-NMR and UV-vis spectroscopy. The addition of methanesulfonic acid (MSA) to the di-deprotonated complexes enabled the recovery of 1 and 2, indicating that the thioamide moiety underwent a reversible deprotonation-protonation process, which resulted in regulating the redox potentials of the metal center. The Pourbaix diagram of 1 revealed that 1 underwent a one-proton/one-electron transfer process in the pH range of 5.83–10.35, and a two-proton/one-electron process at a pH of over 10.35, indicating that the deprotonation/protonation process of the complexes is related to proton-coupled electron transfer (PCET).

Graphical abstract: Deprotonation/protonation of coordinated secondary thioamide units of pincer ruthenium complexes: Modulation of voltammetric and spectroscopic characterization of the pincer complexes
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