Electrochemical evidence that pyranopterin redox chemistry controls the catalysis of YedY, a mononuclear Mo enzyme

New Rh2(II,II) Architecture for the Catalytic Reduction of H+

Travis A. White†, Suzanne E. Witt†, Zhanyong Li‡, Kim R. Dunbar*‡, and Claudia Turro*†

† Department of Chemistry and Biochemistry, The Ohio State University, Columbus, Ohio 43210, United States

‡ Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States

Inorg. Chem., Article ASAP

DOI: 10.1021/acs.inorgchem.5b01823

Publication Date (Web): September 25, 2015

Copyright © 2015 American Chemical Society

*(K.R.D.) E-mail: dunbar@mail.chem.tamu.edu., *(C.T.) E-mail: turro.1@osu.edu.

http://pubs.acs.org/doi/10.1021/acs.inorgchem.5b01823

Synopsis

Formamidinate-bridged Rh2II,II complexes with coordinated bidentate diimine ligands electrocatalytically reduce H+ to H2 in CH3COOH/DMF solutions using a glassy carbon electrode. The choice of diimine ligand is shown to impact the rate of H2 production, with phenanthroline ligands functioning at a faster rate than ligands possessing pyrazine subunits.

Abstract

Formamidinate-bridged Rh2II,II complexes containing diimine ligands of the formula cis-[Rh2II,II(μ-DTolF)2(NN)2]2+ (Rh2-NN2), where DTolF = p-ditolylformamidinate and NN = dppn (benzo[i]dipyrido[3,2-a:2′,3′-h]quinoxaline), dppz (dipyrido[3,2-a:2′,3′-c]phenazine), and phen (1,10-phenanthroline), electrocatalytically reduce H+ to H2 in DMF solutions containing CH3COOH at a glassy carbon electrode. Cathodic scans in the absence of acid display a RhIII,II/II,II reduction at −0.90 V vs Fc+/Fc followed by NN0/– reduction at −1.13, −1.36, and −1.65 V for Rh2-dppn2, Rh2-dppz2, and Rh2-phen2, respectively. Upon the addition of acid, Rh2-dppn2 and Rh2-dppz2 undergo reduction–protonation–reduction at each pyrazine-containing NN ligand prior to the Rh2II,II/II,I reduction. The Rh2II,I species is then protonated at one of the metal centers, resulting in the formation of the corresponding Rh2II,III-hydride. In the case ofRh2-phen2, the reduction of the phen ligand is followed by intramolecular electron transfer to the Rh2II,II core in the presence of protons to form a Rh2II,III-hydride species. Further reduction and protonation at the Rh2 core for all three complexes rapidly catalyzes H2 formation with varied calculated turnover frequencies (TOF) and overpotential values (η): 2.6 × 104 s–1 and 0.56 V for Rh2-dppn, 2.8 × 104 s–1 and 0.50 V for Rh2-dppz2, and 5.9 × 104 s–1 and 0.64 V forRh2-phen2. Bulk electrolysis confirmed H2 formation, and further CH3COOH addition regenerates H2 production, attesting to the robust nature of the architecture. The cis-[Rh2II,II(μ-DTolF)2(NN)2]2+ architecture benefits by combining electron-rich formamidinate bridges, a redox-active Rh2II,II core, and electron-accepting NN diimine ligands to allow for the electrocatalysis of H+ substrate to H2 fuel.