Syntheses, Structures, and Reactions of Binary and Tertiary Thiomolybdate Complexes Containing the (O)Mo(Sx) AND (S)Mo(Sx) Functional Groups (x = 1, 2, 4). Elsevier; 1998. pp. 1-73. [DOI: 10.1016/s0898-8838(08)60024-0]

Synthesis and Dynamics of Ferrous Polychalcogenides [Fe(Ex)(CN)2(CO)2]2- (E = S, Se, or Te)

Elemental chalcogens react with [Fe(CN)₂(CO)₃]^2- to give the following ferrous derivatives: [K(18-crown-6)]₂[Fe(S₅)(CN)₂(CO)₂], [K(18-crown-6)]₂[Fe(S₂)(CN)₂(CO)₂], [K(18-crown-6)]₂[Fe(Se₄)(CN)₂(CO)₂], [K(18-crown-6)]₂[Fe(Te₂)(CN)₂(CO)₂], and (NEt₄)₂[Fe(Te₂)(CN)₂(CO)₂]. While these complex anions crystallized in a single stereochemistry (i.e., trans dicyanides or cis dicyanides), they isomerize in solution upon irradiation. The results are benchmarked by the corresponding studies on benzyl thiolate [K(18-crown-6)]₂[Fe(SBn)₂(CN)₂(CO)₂].

Coordination Properties of Non-Rigid Phosphinoyldithioformate Complexes of the [Mo2O2(µ-S)2]2+ Cation in Catalytic Sulfur Transfer Reactions with Thiiranes

Two “Mo2O2S2”-based complexes with phosphinoyldithioformate ligands were synthesized from the metathesis reaction of [R2P(O)CS2]− with (Me4N)2[Mo2O2(µ-S)2(Cl)4] to give [Mo2O2(µ-S)2{R2P(O)CS2}2] (1; R = Ph, 2; R = Bn). The complexes were fully characterized, including the X-ray crystal structure for 1. Variable temperatures 31P NMR of 1 and 2 exhibit non-rigid behavior in solution where three and two coordination isomers were present, respectively. The organic substituent on the P atom greatly impacts the complex non-rigid properties and behavior. The catalytic activity of 1 and 2 towards sulfur atom transfer (SAT) using propylene sulfide and cyclohexene sulfide was explored, employing homogeneous reaction conditions at an ambient temperature on the NMR scale. The complexes showed distinctly different properties along with high conversions in short reaction times. A catalytic cycle consistent with the results is proposed.

A W/Cu Synthetic Model for the Mo/Cu Cofactor of Aerobic CODH Indicates That Biochemical CO Oxidation Requires a Frustrated Lewis Acid/Base Pair

Constructing synthetic models of the Mo/Cu active site of aerobic carbon monoxide dehydrogenase (CODH) has been a long-standing synthetic challenge thought to be crucial for understanding how atmospheric concentrations of CO and CO₂ are regulated in the global carbon cycle by chemolithoautotrophic bacteria and archaea. Here we report a W/Cu complex that is among the closest synthetic mimics constructed to date, enabled by a silyl protection/deprotection strategy that provided access to a kinetically stabilized complex with mixed O^2-/S^2- ligation between (bdt)(O)W^VI and Cu^I(NHC) (bdt = benzene dithiolate, NHC = N-heterocyclic carbene) sites. Differences between the inorganic core's structural and electronic features outside the protein environment relative to the native CODH cofactor point to a biochemical CO oxidation mechanism that requires a strained active site geometry, with Lewis acid/base frustration enforced by the protein secondary structure. This new mechanistic insight has the potential to inform synthetic design strategies for multimetallic energy storage catalysts.

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.

High-resolution mass spectrometry for molybdenum speciation in sulfidic waters

High resolution electrospray ionization mass spectrometry (ESI-HRMS) was used to study the speciation of molybdenum in interaction with sulfide and chloride. While reactions between molybdate and sulfide lead to creation of four conventional thiomolybdate species (MoO₃S^2-, MoO₂S₂^2-, MoOS₃^2-, MoS₄^2-), chemical formula assignment of recorded mass spectra confirmed the presence of intermediate thiomolybdate compounds (MoO₃S₂^2-, MoO₂S₃^2-, MoOS₄^2-, MoS₅^2-, MoS₆^2-) and thio-chloro-molybdate compounds (MoO₃Cl^-, MoO₂SCl^-, MoOS₂Cl^-, MoOS₃Cl^-). Two of the intermediate thiomolybdate compounds were previously suggested theoretically, and this study provides analytical support for the existence of these compounds. Fast ESI-HRMS analysis has allowed us to conduct a highly-resolved short-term kinetic study of these reactions, and we suggest that the reactivity between molybdate and sulfide is more complex than previously thought, particularly during the first 24 h of interaction. Also, the solution composition will impact reaction pathways, and different outcomes found in the literature may arise from choices of ionic strength and pH adjusting agents in previous studies . The occurrence of the thio-chloro-molybdate species detected in this study should be implemented in future Mo speciation models to better assess Mo reactivity in sulfidic waters and reducing environments.

[MoO(S2)2L]1- (L = picolinate or pyrimidine-2-carboxylate) Complexes as MoSx-Inspired Electrocatalysts for Hydrogen Production in Aqueous Solution

Crystalline and amorphous molybdenum sulfide (Mo-S) catalysts are leaders as earth-abundant materials for electrocatalytic hydrogen production. The development of a molecular motif inspired by the Mo-S catalytic materials and their active sites is of interest, as molecular species possess a great degree of tunable electronic properties. Furthermore, these molecular mimics may be important for providing mechanistic insights toward the hydrogen evolution reaction (HER) with Mo-S electrocatalysts. Herein is presented two water-soluble Mo-S complexes based around the [MoO(S₂)₂L₂]^1- motif. We present ¹H NMR spectra that reveal (NEt₄)[MoO(S₂)₂picolinate] (Mo-pic) is stable in a d₆-DMSO solution after heating at 100 °C, in air, revealing unprecedented thermal and aerobic stability of the homogeneous electrocatalyst. Both Mo-pic and (NEt₄)[MoO(S₂)₂pyrimidine-2-carboxylate] (Mo-pym) are shown to be homogeneous electrocatalysts for the HER. The TOF of 27-34 s^-1 and 42-48 s^-1 for Mo-pic and Mo-pym and onset potentials of 240 mV and 175 mV for Mo-pic and Mo-pym, respectively, reveal these complexes as promising electrocatalysts for the HER.

Cytotoxicity studies of water soluble coordination compounds with a [Mo2O2S2](2+) core

Selected molybdenum sulfur compounds with the formulas (M)[Mo2O2S4L] where (Et4N)2(1), L=S4(2-), (Et4N)(2), L=Cp, (3), L=DMF, K(5), L=serine, M=Et4N(+), K(+), Na(+) and [Mo2O2S2L2] where Na2(4), L=cysteine, and (6), L=threonine, were prepared and subjected to cytotoxicity studies in vitro. The results were analyzed to rank the compounds according to their relative cytotoxicity and to correlate the observed toxicity to specific composition. The results guide future efforts to synthesize highly water soluble, non-toxic, compounds. Strong correlation was observed between toxicity and cation selection, as well as selection of biocompatible ligands combined with alkali metal salts. The most toxic compound analyzed showed about 50 times less cytotoxicity than the cisplatin reference compound in HT-29 cells. Preliminary results from in vivo data agree with the ranking obtained in vitro.

Nitrate and periplasmic nitrate reductases

The nitrate anion is a simple, abundant and relatively stable species, yet plays a significant role in global cycling of nitrogen, global climate change, and human health. Although it has been known for quite some time that nitrate is an important species environmentally, recent studies have identified potential medical applications. In this respect the nitrate anion remains an enigmatic species that promises to offer exciting science in years to come. Many bacteria readily reduce nitrate to nitrite via nitrate reductases. Classified into three distinct types--periplasmic nitrate reductase (Nap), respiratory nitrate reductase (Nar) and assimilatory nitrate reductase (Nas), they are defined by their cellular location, operon organization and active site structure. Of these, Nap proteins are the focus of this review. Despite similarities in the catalytic and spectroscopic properties Nap from different Proteobacteria are phylogenetically distinct. This review has two major sections: in the first section, nitrate in the nitrogen cycle and human health, taxonomy of nitrate reductases, assimilatory and dissimilatory nitrate reduction, cellular locations of nitrate reductases, structural and redox chemistry are discussed. The second section focuses on the features of periplasmic nitrate reductase where the catalytic subunit of the Nap and its kinetic properties, auxiliary Nap proteins, operon structure and phylogenetic relationships are discussed.

Turnstiles and bifurcators: the disequilibrium converting engines that put metabolism on the road

The Submarine Hydrothermal Alkaline Spring Theory for the emergence of life holds that it is the ordered delivery of hydrogen and methane in alkaline hydrothermal solutions at a spontaneously precipitated inorganic osmotic and catalytic membrane to the carbon dioxide and other electron acceptors in the earliest acidulous cool ocean that, through these gradients, drove life into being. That such interactions between hydrothermal fuels and potential oxidants have so far not been accomplished in the lab is because some steps along the necessary metabolic pathways are endergonic and must therefore be driven by being coupled to thermodynamically larger exergonic processes. But coupling of this kind is far from automatic and it is not enough to merely sum the ΔGs of two supposedly coupled reactions and show their combined thermodynamic viability. An exergonic reaction will not drive an endergonic one unless 'forced' to do so by being tied to it mechanistically via an organized "engine" of "Free Energy Conversion" (FEC). Here we discuss the thermodynamics of FEC and advance proposals regarding the nature and roles of the FEC devices that could, in principle, have arisen spontaneously in the alkaline hydrothermal context and have forced the onset of a protometabolism. The key challenge is to divine what these initial engines of life were in physicochemical terms and as part of that, what structures provided the first "turnstile-like" mechanisms needed to couple the partner processes in free energy conversion; in particular to couple the dissipation of geochemically given gradients to, say, the reduction of CO(2) to formate and the generation of a pyrophosphate disequilibrium. This article is part of a Special Issue entitled: The evolutionary aspects of bioenergetic systems.

Structural chemistry of (PPh4)2M(WS4)2 materials

In this paper we discuss the structural chemistry of (PPh(4))(2)M(WS(4))(2) (M = Co, Ni, Zn) materials. For M = Ni we and others have been unable to grow single crystals and we report the structure determination from powder diffraction. The material is triclinic (P1, a = 9.3730(2), b = 12.4951(3), c = 12.5189(3) A, alpha = 65.814(1), beta = 83.751(1), gamma = 69.571(1) degrees at T = 293 K). It contains square-planar coordination around Ni. For M = Zn we have isolated two polymorphs. We describe new analysis of the complex superstructure and diffuse scattering observed in the tetragonal polymorph (I4, a = 18.723(4), c = 13.563(4) A at T = 120 K) and the bulk preparation of a monoclinic (P2(1)/c, a = 18.6397(4), b = 15.3693(5), c = 18.9822(5) A, beta = 109.239(2) degrees at T = 293 K) polymorph isostructural with the M = Co material.

Facets of early transition metal–sulfur chemistry: Metal–sulfur ligand redox, induced internal electron transfer, and the reactions of metal–sulfur complexes with alkynes

Metal-sulfur ligand redox interplay, induced internal electron transfer reactions, and the generation of dithiolene and organosulfur ligands in the reactions of metal-sulfur compounds with alkynes are important and useful facets of early transition metal-sulfur chemistry. This review focuses on developments in these areas over the past 30 years.

Assembly of Heterometallic Clusters and Coordination Polymers by Combining Mo−S-Based Clusters with Mn2+

Addition of [Mo(V)2O2S2(edt)2]2- (edt =1,2-ethanedithiolate) to acetonitrile and/or methanol solutions of MnII containing bipyridines [4,4'-trimethylenedipyridine (TDP), 4,4'-bipyridine (4,4'-bpy), 2,2'-bipyridine (2,2'-bpy)] or 15-crown-5 produces three new heterometallic cluster coordination polymers, [Mn2[Mo2O2S2(edt)2]2(TDP)3(CH3OH)2(NCMe)2].3CH3OH.0.25MeCN (1), [Mn(TDP)2(H2O)2]2+[Mn[Mo2O2S2(edt)2)2(TDP)2]]2-.6CH3OH (2), [Mn[Mo2O2S2(edt)2](TDP)2(CH3OH)(H2O)].CH3OH (3), and three new multinuclear clusters, [Mn[Mo2O2S2(edt)2](4,4'-bpy)(CH3OH)4].0.5(4,4'-bpy) (4), [Mn[Mo2O2S2(edt)2](2,2'-bpy)2].2CH3OH (5), and (NEt4)2[Mn(15-crown-5)[Mo2O2S2(edt)2]2] (6). All compounds were characterized by X-ray crystallography. The coordination mode of Mn in these compounds depends on the ligands and the crystallization conditions. Compound 2 readily converts to 1 or 3 depending on the reaction and solvent conditions. Compounds 1 and 2 were analyzed using thermogravimetric analysis combined with mass spectroscopy (TG-MS) in the temperature range 25-500 degrees C. The room-temperature magnetic moments for compounds 1-6 were determined.

Mononuclear Five-Coordinate Molybdenum(IV) and -(V) Monosulfide Complexes Coordinated with Dithiolene Ligands: Reversible Redox of Mo(V)/Mo(IV) and Irreversible Dimerization of [MoVS]- Cores to a Dinuclear [MoV2(μ-S)2]2- Core

A mononuclear five-coordinate molybdenum(IV) monosulfide complex, (Et4N)2[MoS(L)2] (L = cyclohexene-1,2-dithiolate) (1), was obtained and characterized by IR, UV-vis spectroscopic methods, and X-ray crystallography. 1 was oxidized by an equivalent ferrocenium cation to give the corresponding mononuclear molybdenum(V) complex, (Et4N)[MoS(L)2] (2), which was stable for a few minutes under a lower concentration than 0.3 mM and then further dimerized to (Et4N)2[Mo(L)2]2(mu-S)2 (3).

Car-Parrinello Molecular Dynamics Study of the Initial Dinitrogen Reduction Step in Sellmann-Type Nitrogenase Model Complexes

We have studied reduction reactions for nitrogen fixation at Sellmann-type model complexes with Car-Parrinello simulation techniques. These dinuclear complexes are especially designed to emulate the so-called open-side FeMoco model. The main result of this work shows that in order to obtain the reduced species several side reactions have to be suppressed. These involve partial dissociation of the chelate ligands and hydrogen atom transfer to the metal center. Working at low temperature turns out to be one necessary pre-requisite in carrying out successful events. The successful events cannot be described by simple reaction coordinates. Complicated processes are involved during the initiation of the reaction. Our theoretical study emphasizes two experimental strategies which are likely to inhibit the side reactions. Clamping of the two metal fragments by a chelating phosphane ligand should prevent dissociation of the complex. Furthermore, introduction of tert-butyl substituents could improve the solubility and should thus allow usage of a wider range of (mild) acids, reductants, and reaction conditions.