Oana R. Luca

Redox-active ligands in catalysis

Natures use of redox-active moieties combined with 3d transition-metal ions is a powerful strategy to promote multi-electron catalytic reactions. The ability of these moieties to store redox equivalents aids metalloenzymes in promoting multi-electron reactions, avoiding high-energy intermediates. In a biomimetic spirit, chemists have recently developed approaches relying on redox-active moieties in the vicinity of metal centers to catalyze challenging transformations. This approach enables chemists to impart noble-metal character to less toxic, and cost effective 3d transitional metals, such as Fe or Cu, in multi-electron catalytic reactions.

Influence of the Ligand Field on Slow Magnetization Relaxation versus Spin Crossover in Mononuclear Cobalt Complexes

The electronic and magnetic properties of the complexes [Co(terpy)Cl2 ] (1), [Co(terpy)(NCS)2 ] (2), and [Co(terpy)2 ](NCS)2 (3) were investigated. The coordination environment around Co(II) in 1 and 2 leads to a high-spin complex at low temperature and single-molecule magnet properties with multiple relaxation pathways. Changing the ligand field and geometry with an additional terpy ligand leads to spin-crossover behavior in 3 with a gradual transition from high spin to low spin.

Organometallic Ni Pincer Complexes for the Electrocatalytic Production of Hydrogen

Nonplatinum metals are needed to perform cost-effective water reduction electrocatalysis to enable technological implementation of a proposed hydrogen economy. We describe electrocatalytic proton reduction and H(2) production by two organometallic nickel complexes with tridentate pincer ligands. The kinetics of H(2) production from voltammetry is consistent with an overall third order rate law: the reaction is second order in acid and first order in catalyst. Hydrogen production with 90-95% Faradaic yields was confirmed by gas analysis, and UV-vis spectroscopy suggests that the ligand remains bound to the catalyst over the course of the reaction. A computational study provides mechanistic insights into the proposed catalytic cycle. Furthermore, two proposed intermediates in the proton reduction cycle were isolated in a representative system and show a catalytic response akin to the parent compound.

A tridentate Ni pincer for aqueous electrocatalytic hydrogen production

A NiII complex with a redox-active pincer ligand reduces protons at a low overpotential in aqueous acidic conditions. A combined experimental and computational study provides mechanistic insights into a putative catalytic cycle.

Redox-active cyclopentadienyl Ni complexes with quinoid N-heterocyclic carbene ligands for the electrocatalytic hydrogen release from chemical fuels

We now report the electrocatalytic dehydrogenation of tetrahydroquinaldine by an electron-rich CpNi N-heterocyclic carbene (NHC) with quinoid ligand motifs and explore the effects of quinone additives on CpNi compounds without quinoid NHC ligands. Our CpNi(NHC) catalyst exhibits dehydrogenative electrocatalytic activity and demonstrates that a molecular catalyst precursor can be viable in the electrode-driven (H+ + e−) release step of “virtual hydrogen storage”.

Oxidative functionalization of benzylic C–H bonds by DDQ

C–H activation of the methyl group of toluene and related ArCH3 derivatives by 2,3-dichloro-4,5-dicyano-1,4-benzoquinone (DDQ) gives insertion products, ArCH2O[C6Cl2(CN)2]OH via a rate-determining hydride abstraction by DDQ. The resulting benzylic ether can undergo reactions with phosphines to give benzylic phosphonium salts (Wittig reagents) and with phosphites to give phosphonate esters (Horner–Wadsworth–Emmons reagents).

DDQ as an electrocatalyst for amine dehydrogenation, a model system for virtual hydrogen storage

2,3-Dichloro-5,6-dicyanobenzoquinone (DDQ) is an electrochemical oxidation catalyst for a secondary amine, a model system for virtual hydrogen storage by removal of a hydrogen equivalent from an amine; a computational study provides mechanistic information.

Catalysis by electrons and holes: formal potential scales and preparative organic electrochemistry

The present review surveys current chemical understanding of catalysis by addition and removal of an electron. As an overarching theme of this type of catalysis, we introduce the role of redox scales in oxidation and reduction reactions as a direct analogue of pK_a scales in acid/base catalysis. Each scale is helpful in determining the type of reactivity to be expected. In addition, we describe several means of generating electrons and holes via chemical reactions, plasmonic resonance, radiolytic, photochemical and electrochemical methods. We specifically draw parallels between the now well-established fields of photoredox catalysis and chemical opportunities made available by electrochemical methods. We highlight accessible potential ranges for a series of electrochemical solvents and provide a discussion on experimental design, pitfalls and some remaining challenges in preparative organic electrochemistry.

Electrocatalytic Reduction of Nitrogen and Carbon Dioxide to Chemical Fuels: Challenges and Opportunities for a Solar Fuel Device

Aspects of the electrochemical reduction of nitrogen and carbon dioxide at molecular and heterogeneous catalysts are discussed. We focus on recent advances in the field and touch on some of the remaining challenges in the production of solar fuels from N2 and CO2 with a direct, integrated solar fuel device. As such, we now propose metrics of catalyst assessment for non-H2 solar fuels.

Organic reactions for the electrochemical and photochemical production of chemical fuels from CO2 – The reduction chemistry of carboxylic acids and derivatives as bent CO2 surrogates

The present review covers organic transformations involved in the reduction of CO2 to chemical fuels. In particular, we focus on reactions of CO2 with organic molecules to yield carboxylic acid derivatives as a first step in CO2 reduction reaction sequences. These biomimetic initial steps create opportunities for tandem electrochemical/chemical reductions. We draw parallels between long-standing knowledge of CO2 reactivity from organic chemistry, organocatalysis, surface science and electrocatalysis. We point out some possible non-faradaic chemical reactions that may contribute to product distributions in the production of solar fuels from CO2. These reactions may be accelerated by thermal effects such as resistive heating and illumination.