Relevance of Chemical vs. Electrochemical Oxidation of Tunable Carbene Iridium Complexes for Catalytic Water Oxidation

Based on previous work that identified iridium(III) Cp* complexes containing a C,N-bidentate chelating triazolylidene-pyridyl ligand (Cp* = pentamethylcyclopentadienyl, C5Me5-) as efficient molecular water oxidation catalysts, a series of new complexes based on this motif has been designed and synthesized in order to improve catalytic activity. Modifications include specifically the introduction of electron-donating substituents into the pyridyl unit of the chelating ligand (H, a; 5-OMe, b; 4-OMe, c; 4-tBu, d; 4-NMe2, e), as well as electronically active substituents on the triazolylidene C4 position (H, 8; COOEt, 9; OEt, 10; OH, 11; COOH, 12). Chemical oxidation using cerium ammonium nitrate (CAN) indicates a clear structure-activity relationship with electron-donating groups enhancing catalytic turnover frequency, especially when the donor substituent is positioned on the triazolylidene ligand fragment (TOFmax = 2500 h(-)(1) for complex 10 with a MeO group on pyr and a OEt-substituted triazolylidene, compared to 700 h(-)(1) for the parent benchmark complex without substituents). Electrochemical water oxidation does not follow the same trend, and reveals that complex 8b without a substituent on the triazolylidene fragment outperforms complex 10 by a factor of 5, while in CAN-mediated chemical water oxidation, complex 10 is twice more active than 8b. This discrepancy in catalytic activity is remarkable and indicates that caution is needed when benchmarking iridium water oxidation catalysts with chemical oxidants, especially when considering that application in a potential device will most likely involve electrocatalytic water oxidation.Catalysis and Surface Chemistr

CCDC 1909356: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

CCDC 1909362: Experimental Crystal Structure Determination

Related Article: Marta Olivares, Cornelis J. M. van der Ham, Velabo Mdluli, Markus Schmidtendorf, Helge Müller-Bunz, Tiny W.G.M Verhoeven, Mo Li, Hans J. W. Niemantsverdriet, Dennis G. H. Hetterscheid, Stefan Bernhard, Martin Albrecht, J. W. Hans Niemantsverdriet|2020|Eur.J.Inorg.Chem.|2020|801|doi:10.1002/ejic.20200009

Fischer-Type Carbene Complexes of Tris(1,4-phenylene)amines and Tri(2-furyl)phosphine

Novel chromium and tungsten mono- and multiethoxycarbene complexes were synthesized from tris(4- bromophenyl)amine and tri(2-furyl)phosphine substrates. A comparative study between the amine and phosphine multicarbene complexes revealed small differences between the respective compounds. In solution, an equilibrium situation was observed between the mono- and bis-carbene complexes of tri(2-furyl)phosphine. The monocarbene amine ligand differs from the analogous phosphine ligand in the way the heteroatom lone pair electrons are able to interact with the carbene functionality. Unlike for the phosphine, the greater electron delocalization of the amine affected the coordination ability of the ligand when reacted with [Pt(COD)Cl2]. These properties of the monocarbene-carrying ligands were confirmed and supported by solid state structural studies, spectroscopic data, and density functional theory (DFT) calculations.National Research Foundation (NRF) of South Africa under grant number 73679, the Spanish MINECO grant CTQ2013-44303-P, and European FEDER.http://pubs.acs.org/journal/orgnd72016-02-28hb201