Models for aerobic carbon monoxide dehydrogenase: synthesis, characterization and reactivity of paramagnetic MoVO(μ-S)CuI complexes

Molybdenum-Copper Antagonism In Metalloenzymes And Anti-Copper Therapy

The connection between 3d (Cu) and 4d (Mo) via the "Mo-S-Cu" unit is called Mo-Cu antagonism. Biology offers case studies of such interactions in metalloproteins such as Mo/Cu-CO Dehydrogenases (Mo/Cu-CODH), and Mo/Cu Orange Protein (Mo/Cu-ORP). The CODH significantly maintains the CO level in the atmosphere below the toxic level by converting it to non-toxic CO2 for respiring organisms. Several models were synthesized to understand the structure-function relationship of these native enzymes. However, this interaction was first observed in ruminants, and they convert molybdate (MoO4 2- ) into tetrathiomolybdate (MoS4 2- ; TTM), reacting with cellular Cu to yield biological unavailable Mo/S/Cu cluster, then developing Cu-deficiency diseases. These findings inspire the use of TTM as a Cu-sequester drug, especially for treating Cu-dependent human diseases such as Wilson diseases (WD) and cancer. It is well known that a balanced Cu homeostasis is essential for a wide range of biological processes, but negative consequence leads to cell toxicity. Therefore, this review aims to connect the Mo-Cu antagonism in metalloproteins and anti-copper therapy.

Combining [MoVIO3] and [M0(CO)3] (M = Mo, Cr) Fragments within the Same Complex: Synthesis and Reactivity of the Single Oxo-Bridged Heterobimetallics Supported by Xanthene-Based Heterodinucleating Ligands

A functional model of Mo-Cu carbon monoxide dehydrogenase (CODH) enzyme requires the presence of an oxidant (metal-oxo) and a metal-bound carbonyl in close proximity. In this work, we report the synthesis, characterization, and reactivity of a heterobimetallic complex combining Mo(VI) trioxo with Mo(0) tricarbonyl. The formation of the heterobimetallic complex is facilitated by the xanthene-bridged heterodinucleating ligand containing a hard catecholate chelate and a soft iminopyridine chelate. A catechol-coordinated square-pyramidal [MoVIO3] fragment interacts directly with the iminopyridine-bound [Mo0(CO)3] fragment via a single (oxo) bridge, with the overall disposition being related to the proposed first step in the CODH mechanism, where square-pyramidal [MoVIO2S] interacts with the [Cu-CO] via a single sulfido bridge. Our attempt to obtain a sulfido-bridged analogue (using [MoO3S]2- precursor) led to a mixture of products possibly containing different (oxo and sulfido) bridges. Despite a direct interaction between Mo(VI) and Mo(0) segments, no internal redox is observed, with the high lying occupied MOs being mostly d-π orbitals at Mo0(CO)3 and the low lying unoccupied MOs being d-π orbitals at MoVIO3. Due to the overall rigid structure, the heterobimetallic complex was found to be stable up to 100 °C in DMF-d7 (based on 1H NMR). The decomposition of the complex above this temperature does not produce CO2 (based on gas chromatography), dissociating stable Mo(CO)3(DMF)3 instead (based on IR). We also synthesized and studied the reactivity of the Mo(VI)/Cr(0) analogue. While this complex demonstrated more facile decomposition, no CO2 production was observed. Density functional theory calculations suggest that the formation of [CO2]2- and its subsequent reductive elimination is endergonic in the present system, likely due to the stability of fac-Mo0(CO)3 and the relative nucleophilic character of the carbonyl carbon engendered by back donation from Mo(0). The calculations also indicate that the replacement of one oxo by sulfido (both terminal and bridging), replacement of catechol with dithiolene, and replacement of Mo(0) with Cr(0) does not affect significantly the energetics of the process, likely requiring the use a less stable and less π-basic CO anchor.

Studies Relevant to the Functional Model of Mo-Cu CODH: In Situ Reactions of Cu(I)-L Complexes with Mo(VI) and Synthesis of Stable Structurally Characterized Heterotetranuclear MoVI2CuI2 Complex

In this study, we report the synthesis, characterization, and reactions of Cu(I) complexes of the general form Cu(L)(LigH₂) (LigH₂ = xanthene-based heterodinucleating ligand (E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol); L = PMe₃, PPh₃, CN(2,6-Me₂C₆H₃)). New complexes [Cu(PMe₃)(LigH₂)] and [CuCN(2,6-Me₂C₆H₃)(LigH₂)] were synthesized by treating [Cu(LigH₂)](PF₆) with trimethylphosphine and 2,6-dimethylphenyl isocyanide, respectively. These complexes were characterized by multinuclear NMR spectroscopy, IR spectroscopy, high-resolution mass spectrometry (HRMS), and X-ray crystallography. In contrast, attempted reactions of [Cu(LigH₂)](PF₆) with cyanide or styrene failed to produce isolable crystalline products. Next, the reactivity of these and previously synthesized Cu(I) phosphine and isocyanide complexes with molybdate was interrogated. IR (for isocyanide) and ³¹P NMR (for PPh₃/PMe₃) spectroscopy demonstrates the lack of oxidation reactivity. We also describe herein the first example of a structurally characterized multinuclear complex combining both Mo(VI) and Cu(I) metal ions within the same system. The heterobimetallic tetranuclear complex [Cu₂Mo₂O₄(μ₂-O)(Lig)₂]·HOSiPh₃ was obtained by the reaction of the silylated Mo(VI) precursor (Et₄N)(MoO₃(OSiPh₃)) with LigH₂, followed by the addition of [Cu(NCMe)₄](PF₆). This complex was characterized by NMR spectroscopy, high-resolution mass spectrometry, and X-ray crystallography.

Inspired by Nature-Functional Analogues of Molybdenum and Tungsten-Dependent Oxidoreductases

Throughout the previous ten years many scientists took inspiration from natural molybdenum and tungsten-dependent oxidoreductases to build functional active site analogues. These studies not only led to an ever more detailed mechanistic understanding of the biological template, but also paved the way to atypical selectivity and activity, such as catalytic hydrogen evolution. This review is aimed at representing the last decade’s progress in the research of and with molybdenum and tungsten functional model compounds. The portrayed systems, organized according to their ability to facilitate typical and artificial enzyme reactions, comprise complexes with non-innocent dithiolene ligands, resembling molybdopterin, as well as entirely non-natural nitrogen, oxygen, and/or sulfur bearing chelating donor ligands. All model compounds receive individual attention, highlighting the specific novelty that each provides for our understanding of the enzymatic mechanisms, such as oxygen atom transfer and proton-coupled electron transfer, or that each presents for exploiting new and useful catalytic capability. Overall, a shift in the application of these model compounds towards uncommon reactions is noted, the latter are comprehensively discussed.

Lineage-specific energy and carbon metabolism of sponge symbionts and contributions to the host carbon pool

Marine sponges host a wide diversity of microorganisms, which have versatile modes of carbon and energy metabolism. In this study we describe the major lithoheterotrophic and autotrophic processes in 21 microbial sponge-associated phyla using novel and existing genomic and transcriptomic datasets. We show that the main microbial carbon fixation pathways in sponges are the Calvin–Benson–Bassham cycle (energized by light in Cyanobacteria, by sulfur compounds in two orders of Gammaproteobacteria, and by a wide range of compounds in filamentous Tectomicrobia), the reductive tricarboxylic acid cycle (used by Nitrospirota), and the 3-hydroxypropionate/4-hydroxybutyrate cycle (active in Thaumarchaeota). Further, we observed that some sponge symbionts, in particular Acidobacteria, are capable of assimilating carbon through anaplerotic processes. The lithoheterotrophic lifestyle was widespread and CO oxidation is the main energy source for sponge lithoheterotrophs. We also suggest that the molybdenum-binding subunit of dehydrogenase (encoded by coxL) likely evolved to benefit also organoheterotrophs that utilize various organic substrates. Genomic potential does not necessarily inform on actual contribution of autotrophs to light and dark carbon budgets. Radioisotope assays highlight variability in the relative contributions of photo- and chemoautotrophs to the total carbon pool across different sponge species, emphasizing the importance of validating genomic potential with physiology experimentation.

Carbon Dioxide Reduction: A Bioinspired Catalysis Approach

While developed in a number of directions, bioinspired catalysis has been explored only very recently for CO₂ reduction, a challenging reaction of prime importance in the context of the energetic transition to be built up. This approach is particularly relevant because nature teaches us that CO₂ reduction is possible, with low overpotentials, high rates, and large selectivity, and gives us unique clues to design and discover new interesting molecular catalysts. Indeed, on the basis of our relatively advanced understanding of the structures and mechanisms of the active sites of fascinating metalloenzymes such as formate dehydrogenases (FDHs) and CO dehydrogenases (CODHs), it is possible to design original, active, selective, and stable molecular catalysts using the bioinspired approach. These metalloenzymes use fascinating metal centers: in FDHs, a Mo(W) mononuclear ion is coordinated by four sulfur atoms provided by a specific organic ligand, molybdopterin (MPT), containing a pyranopterin heterocycle (composed of a pyran ring fused with a pterin unit) and two sulfhydryl groups for metal chelation; in CODHs, catalytic activity depends on either a unique nickel-iron-sulfur cluster or a dinuclear Mo-Cu complex in which the Mo ion is chelated by an MPT ligand. As a consequence, the novel class of catalysts, designed by bioinspiration, consists of mononuclear Mo, W, and Ni and as well as dinuclear Mo-Cu and Ni-Fe complexes in which the metal ions are coordinated by sulfur ligands, more specifically, dithiolene chelates mimicking the natural MPT cofactor. In general, their activity is evaluated in electrochemical systems (cyclic voltammetry and bulk electrolysis) or in photochemical systems (in the presence of a photosensitizer and a sacrificial electron donor) in solution. This research is multidisciplinary because it implies detailed biochemical, functional, and structural characterization of the inspiring enzymes together with synthetic organic and organometallic chemistry and molecular catalysis studies. The most important achievements in this direction, starting from the first report of a catalytically active biomimetic bis-dithiolene-Mo complex in 2015, are discussed in this Account, highlighting the challenging issues associated with synthesis of such sophisticated ligands and molecular catalysts as well as the complexity of reaction mechanisms. While the very first active biomimetic catalysts require further improvement, in terms of performance, they set the stage in which molecular chemistry and enzymology can synergistically cooperate for a better understanding of why nature has selected these sites and for developing highly active catalysts.

Synthesis and Cu(I)/Mo(VI) Reactivity of a Bifunctional Heterodinucleating Ligand on a Xanthene Platform

In an effort to probe the feasibility of a model of Mo-Cu CODH (CODH = carbon monoxide dehydrogenase) lacking a bridging sulfido group, the new heterodinucleating ligand LH₂ was designed and its Cu(I)/Mo(VI) reactivity was investigated. LH₂ ((E)-3-(((5-(bis(pyridin-2-ylmethyl)amino)-2,7-di-tert-butyl-9,9-dimethyl-9H-xanthen-4-yl)imino)methyl)benzene-1,2-diol) features two different chelating positions bridged by a xanthene linker: bis(pyridyl)amine for Cu(I) and catecholate for Mo(VI). LH₂ was synthesized via the initial protection of one of the amine positions, followed by two consecutive alkylations of the second position, deprotection, and condensation to attach the catechol functionality. LH₂ was found to exhibit dynamic cooperativity between two reactive sites mediated by H-bonding of the catechol protons. In the free ligand, catechol protons exhibit H-bonding with imine (intramolecular) and with pyridine (intermolecular in the solid state). The reaction of LH₂ with [Cu(NCMe)₄]⁺ led to the tetradentate coordination of Cu(I) via all nitrogen donors of the ligand, including the imine. Cu(I) complexes were characterized by multinuclear NMR spectroscopy, high-resolution mass spectrometry (HRMS), X-ray crystallography, and DFT calculations. Cu(I) coordination to the imine disrupted H-bonding and caused rotation away from the catechol arm. The reaction of the Cu(I) complex [Cu(LH₂)]⁺ with a variety of monodentate ligands X (PPh₃, Cl^-, SCN^-, CN^-) released the metal from coordination to the imine, thereby restoring imine H-bonding with the catechol proton. The second catechol proton engages in H-bonding with Cu-X (X = Cl, CN, SCN), which can be intermolecular (XRD) or intramolecular (DFT). The reaction of LH₂ with molybdate [MoO₄]^2- led to incorporation of [Mo^VIO₃] at the catecholate position, producing [MoO₃(L)]^2-. Similarly, the reaction of [Cu(LH₂)]⁺ with [MoO₄]^2- formed the heterodinuclear complex [CuMoO₃(L)]^-. Both complexes were characterized by multinuclear NMR, UV-vis, and HRMS. HRMS in both cases confirmed the constitution of the complexes, containing molecular ions with the expected isotopic distribution.

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.

A bioinspired molybdenum-copper molecular catalyst for CO2 electroreduction

A bimetallic Mo–Cu complex inspired by the active site of the carbon monoxide dehydrogenase enzyme mediates the electroreduction of carbon dioxide to formic acid.

Non-noble metal molecular catalysts mediating the electrocatalytic reduction of carbon dioxide are still scarce. This work reports the electrochemical reduction of CO₂ to formate catalyzed by the bimetallic complex [(bdt)Mo^VI(O)S₂Cu^ICN]^2– (bdt = benzenedithiolate), a mimic of the active site of the Mo–Cu carbon monoxide dehydrogenase enzyme (CODH2). Infrared spectroelectrochemical (IR-SEC) studies coupled with density functional theory (DFT) computations revealed that the complex is only a pre-catalyst, the active catalyst being generated upon reduction in the presence of CO₂. We found that the two-electron reduction of [(bdt)Mo^VI(O)S₂Cu^ICN]^2– triggers the transfer of the oxo moiety to CO₂ forming CO₃^2– and the complex [(bdt)Mo^IVS₂Cu^ICN]^2– and that a further one-electron reduction is needed to generate the active catalyst. Its protonation yields a reactive Mo^VH hydride intermediate which reacts with CO₂ to produce formate. These findings are particularly relevant to the design of catalysts from metal oxo precursors.

The Challenging in silico Description of Carbon Monoxide Oxidation as Catalyzed by Molybdenum-Copper CO Dehydrogenase

Carbon monoxide (CO) is a highly toxic gas to many living organisms. However, some microorganisms are able to use this molecule as the sole source of carbon and energy. Soil bacteria such as the aerobic Oligotropha carboxidovorans are responsible for the annual removal of about 2x108 tons of CO from the atmosphere. Detoxification through oxidation of CO to CO2 is enabled by the MoCu-dependent CO-dehydrogenase enzyme (MoCu-CODH) which-differently from other enzyme classes with similar function-retains its catalytic activity in the presence of atmospheric O2. In the last few years, targeted advancements have been described in the field of bioengineering and biomimetics, which is functional for future technological exploitation of the catalytic properties of MoCu-CODH and for the reproduction of its reactivity in synthetic complexes. Notably, a growing interest for the quantum chemical investigation of this enzyme has recently also emerged. This mini-review compiles the current knowledge of the MoCu-CODH catalytic cycle, with a specific focus on the outcomes of theoretical studies on this enzyme class. Rather controversial aspects from different theoretical studies will be highlighted, thus illustrating the challenges posed by this system as far as the application of density functional theory and hybrid quantum-classical methods are concerned.

Rectangle and [2]catenane from cluster modular construction

A cluster rectangle and a [2]catenane, respectively, featuring P^N-type and cuboidal cluster subunits are synthesized from cluster modular constructions.

Cooperative bimetallic reactivity of a heterodinuclear molybdenum–copper model of Mo–Cu CODH

Modeling the reactivity of Mo–Cu CODH: Cu(i) brings the substrate close to Mo–oxo and develops electrophilic character in CO carbon.