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

New pincer ruthenium complexes, [Ru(SCS)(tpy)]PF(6) (1) (SCS = 2,6-bis(benzylaminothiocarbonyl)phenyl), tpy = 2,2':6',2''-terpyridyl) and [Ru(SNS)(tpy)]PF(6) (2) (SNS = 2,5-bis(benzylaminothiocarbonyl)pyrrolyl), having κ(3)SCS and κ(3)SNS pincer ligands with two secondary thioamide units were synthesized by the reactions of [RuCl(3)(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 (1)H-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).

Regioregulated Syntheses of Poly(aminopyridine)s by Pd-catalyzed Amination Reaction.

Regioregulated poly(aminopyridine)s were synthesized by a Pd-catalyzed CN coupling reaction. The polymerization using Pd(0) and a bulky monodentate phosphine ligand distinctively produced the para-linked and meta-linked poly(aminopyridine)s, without the need for a protection process. The regioregularity of the polymer was confirmed by (1) H NMR spectroscopy. Model reactions were studied to evaluate the possibility of crosslinkage in the polymer. A large difference in reactivity was observed between 5-amino-2-bromopyridine and 2-amino-5-bromopyridine, which should have afforded same product. Density functional theory (DFT) calculations indicated that electron densities of the Br-bound carbon atom and the pyridine-nitrogen atom determine the reactivity of the monomers.