Spin-flip luminescence

In molecular photochemistry, charge-transfer emission is well understood and widely exploited. In contrast, luminescent metal-centered transitions only came into focus in recent years. This gave rise to strongly phosphorescent Cr^III complexes with a d³ electronic configuration featuring luminescent metal-centered excited states which are characterized by the flip of a single spin. These so-called spin-flip emitters possess unique properties and require different design strategies than traditional charge-transfer phosphors. In this review, we give a brief introduction to ligand field theory as a framework to understand this phenomenon and outline prerequisites for efficient spin-flip emission including ligand field strength, symmetry, intersystem crossing and common deactivation pathways using Cr^III complexes as instructive examples. The recent progress and associated challenges of tuning the energies of emissive excited states and of emerging applications of the unique photophysical properties of spin-flip emitters are discussed. Finally, we summarize the current state-of-the-art and challenges of spin-flip emitters beyond Cr^III with d², d³, d⁴ and d⁸ electronic configuration, where we mainly cover pseudooctahedral molecular complexes of V, Mo, W, Mn, Re and Ni, and highlight possible future research opportunities.

Water-Soluble α-Amino Acid Complexes of Molybdenum as Potential Antidotes for Cyanide Poisoning: Synthesis and Catalytic Studies of Threonine, Methionine, Serine, and Leucine Complexes

Water-soluble complexes are desirable for the aqueous detoxification of cyanide. Molybdenum complexes with α-amino acid and disulfide ligands with the formula K[(L)Mo₂O₂(μ-S)₂(S₂)] (L = leu (1), met (2), thr (3), and ser (4)) were synthesized in a reaction of [(DMF)₃MoO(μ-S)₂(S₂)] with deprotonated α-amino acids; leu, met, thr, and ser are the carboxylate anions of l-leucine, l-methionine, l-threonine, and l-serine, respectively. Potassium salts of α-amino acids (leu (1a), met (2a), thr (3a), and ser (4a)) were prepared as precursors for complexes 1-4, respectively, by employing a nonaqueous synthesis route. The ligand exchange reaction of [Mo₂O₂(μ-S)₂(DMF)₆](I)₂ with deprotonated α-amino acids afforded bis-α-amino acid complexes, [(L)₂Mo₂O₂(μ-S)₂] (6-8). A tris-α-amino acid complex, [(leu)₂Mo₂O₂(μ-S)₂(μ-leu + H)] (5; leu + H is the carboxylate anion of l-leucine with the amine protonated), formed in the reaction with leucine. 5 crystallized from methanol with a third weakly bonded leucine as a bridging bidentate carboxylate. An adduct of 8 with SCN^- coordinated, 9, crystallized and was structurally characterized. Complexes 1-4 are air stable and highly water-soluble chiral molecules. Cytotoxicity studies in the A549 cell line gave IC₅₀ values that range from 80 to 400 μM. Cyclic voltammetry traces of 1-8 show solvent-dependent irreversible electrochemical behavior. Complexes 1-4 demonstrated the ability to catalyze the reaction of thiosulfate and cyanide in vitro to exhaustively transform cyanide to thiocyanate in less than 1 h.

Reaction Chemistry of the syn-[Mo2O2(μ-S)2(S2)(DMF)3] Complex with Cyanide and Catalytic Thiocyanate Formation

Removal of cyanide as nontoxic thiocyanate under physiological conditions may serve as a catalytic detoxification route in vivo. Aqueous catalytic reaction conditions were explored where at the conditions employed the reaction proceeded to exhaustion in 1 h. The complex, syn-[Mo₂O₂(μ-S)₂(S₂)(DMF)₃] 1, participates in a ligand exchange reaction of the dimethylformamide ligands and cyanide. Simultaneous sulfur abstraction reaction from the terminal disulfide group forms thiocyanate and terminal sulfido ligand. Respective reaction rates for the two reactions appear competitive where different products were isolated solely based on change of reaction temperature. The approach to determine the number of cyanide ligands participating in the ligand exchange reaction by varying the stoichiometry and reaction temperature led to identification and isolation of tetranuclear complexes 2 and 5 and dinuclear complexes 3, 4, and 6. A synthesized and fully characterized thiocyanate analog of 6 (7) supports spectroscopic characterization of 6. The tetranuclear anion, [Mo₄O₄(μ-S)₆(CN)₄]^4-, 2, was crystallized from a reaction at ambient temperature. The dinuclear anion, [Mo₂O₂(μ-S)₂(S)(CN)₃]^3-, 3, was crystallized from similar reaction conditions at lower temperature. The reaction yield of thiocyanate obtained at pH of 7.4 and at 9.2 as a function of time, for several ratios of cyanide, favors the sulfur abstraction reaction at elevated pH. The sulfur abstraction reaction is the first step in a proposed mechanism of the reaction of cyanide and thiosulfate to form thiocyanate and sulfite by 1.

Molybdenum- and tungsten-containing formate dehydrogenases and formylmethanofuran dehydrogenases: Structure, mechanism, and cofactor insertion

An overview is provided of the molybdenum- and tungsten-containing enzymes that catalyze the interconversion of formate and CO2 , focusing on common structural and mechanistic themes, as well as a consideration of the manner in which the mature Mo- or W-containing cofactor is inserted into apoprotein.

Reduction of Carbon Dioxide by a Molybdenum-Containing Formate Dehydrogenase: A Kinetic and Mechanistic Study

Carbon dioxide accumulation is a major concern for the ecosystems, but its abundance and low cost make it an interesting source for the production of chemical feedstocks and fuels. However, the thermodynamic and kinetic stability of the carbon dioxide molecule makes its activation a challenging task. Studying the chemistry used by nature to functionalize carbon dioxide should be helpful for the development of new efficient (bio)catalysts for atmospheric carbon dioxide utilization. In this work, the ability of Desulfovibrio desulfuricans formate dehydrogenase (Dd FDH) to reduce carbon dioxide was kinetically and mechanistically characterized. The Dd FDH is suggested to be purified in an inactive form that has to be activated through a reduction-dependent mechanism. A kinetic model of a hysteretic enzyme is proposed to interpret and predict the progress curves of the Dd FDH-catalyzed reactions (initial lag phase and subsequent faster phase). Once activated, Dd FDH is able to efficiently catalyze, not only the formate oxidation (kcat of 543 s(-1), Km of 57.1 μM), but also the carbon dioxide reduction (kcat of 46.6 s(-1), Km of 15.7 μM), in an overall reaction that is thermodynamically and kinetically reversible. Noteworthy, both Dd FDH-catalyzed formate oxidation and carbon dioxide reduction are completely inactivated by cyanide. Current FDH reaction mechanistic proposals are discussed and a different mechanism is here suggested: formate oxidation and carbon dioxide reduction are proposed to proceed through hydride transfer and the sulfo group of the oxidized and reduced molybdenum center, Mo(6+)═S and Mo(4+)-SH, are suggested to be the direct hydride acceptor and donor, respectively.

Structural and functional models in molybdenum and tungsten bioinorganic chemistry: description of selected model complexes, present scenario and possible future scopes

A brief description about some selected model complexes in molybdenum and tungsten bioinorganic chemistry is provided. The synthetic strategies involved and their limitations are discussed. Current status of molybdenum and tungsten bioinorganic modeling chemistry is presented briefly and synthetic problems associated therein are analyzed. Possible future directions which may expand the scope of modeling chemistry are suggested.

Recent developments in the study of molybdoenzyme models

Over the past two decades, a plethora of crystal structures of molybdenum enzymes has appeared in the literature providing a clearer picture of the enzymatic active sites and increasing the challenge to chemists to develop accurate models for those sites. In this minireview we discuss the most recent model studies aimed to reproduce detailed features of the pterin-dithiolene ligand, both as the uncoordinated form and as a chelate coordinated to molybdenum.