Electronic Desymmetrization of Cu(μ-E) Clusters (E = S, Se) Induced by Edge-to-Face π-Stacking Interactions in the Second Coordination Sphere.

A pair of cyclophane-encapsulated [Cu(μ-E)] complexes (E = S and Se) were characterized by resonant X-ray diffraction anomalous fine structure (DAFS), revealing unexpected polarization among the three Cu sites attributed to long-range effects of π-stacking interactions with cocrystallized benzene molecules. The resonant K-edge energies of individual Cu sites within the cluster molecules were found to vary as a function of distance from the cocrystallized benzene. This pattern was interpreted in the context of T-shaped, edge-to-face π-stacking with the assistance of theoretical charge density difference calculations.

Synthetic Challenges toward Modeling Formate Dehydrogenases Using [W≡S] Complexes Supported by a Tetradentate [NS] Ligand.

This report documents our attempts at synthesizing a terminal [W≡S] complex supported by a tetradentate, diamido/dithiolate ligand ([NS]). The target compound was selected because it would serve as a synthetic model for the active sites of formate dehydrogenase (FDH) enzymes. Although the desired [NS]W≡S species was observed as an NEt adduct by mass spectrometry in one case, generally unwanted side reactions prevented isolation and definitive characterization of the target compound.

Frustrated Al/M Heterobimetallic Complexes (M = Cr, Mo, W) That Exhibit Both Lewis and Radical Pair Behavior.

Exploration of new heterobinuclear Al/M combinations is relevant to contemporary strategies for cooperative bond activation. Here, we report the synthesis and characterization of six new Al/M heterobimetallic complexes (M = Cr, Mo, W) that exhibit end-on "isocarbonyl"-type Al─O═C═M bridges with metalloketene character rather than featuring Al─M─C≡O motifs with metal-metal bonding. The new compounds were characterized experimentally by nuclear magnetic resonance and infrared spectroscopies and theoretically using density functional theory, natural bond orbital, and quantum theory of atoms in molecules calculations.

Quantifying effects of second-sphere cationic groups on redox properties of dimolybdenum quadruple bonds.

A series of four dimolybdenum paddlewheel complexes supported by anionic ,-dimethylglycinate (DMG) or zwitterionic ,,-trimethylglycine (TMG) ligands was synthesised to examine the effects of charged groups in the second coordination sphere on redox properties of MoMo bonds. An average shift in reduction potential of +35 mV per cationically charged group was measured, which is approximately half of what would be expected for an analogous mononuclear complex.

Cu site differentiation in tetracopper(i) sulfide clusters enables biomimetic NO reduction.

Copper clusters feature prominently in both metalloenzymes and synthetic nanoclusters that mediate catalytic redox transformations of gaseous small molecules. Such reactions are critical to biological energy conversion and are expected to be crucial parts of renewable energy economies. However, the precise roles of individual metal atoms within clusters are difficult to elucidate, particularly for cluster systems that are dynamic under operating conditions.

Lessons from recent theoretical treatments of Al-M bonds (M = Fe, Cu, Ag, Au) that capture CO.

Complexes with Al-M bonds (M = transition metal) have emerged as platforms for discovering new reaction chemistry either through cooperative bond activation behaviour of the heterobinuclear unit or by modifying the properties of the M site through its interaction with the Al centre. Therefore, elucidating the nature of Al-M bonding is critical to advancing this research area and typically involves careful theoretical modelling. This Frontier article reviews selected recent case studies that included theoretical treatments of Al-M bonds, specifically highlighting complexes capable of cooperative CO activation and focusing on extracting lessons particular to the Al-M sub-field that will inform future studies with theoretical/computational components.

Photochemical Synthesis of Acyl Fluorides Using Copper-Catalyzed Fluorocarbonylation of Alkyl Iodides.

Acyl fluorides are important reagents due to their unique balance between reactivity and stability. Here, we report a copper-catalyzed carbonylative coupling strategy for synthesizing acyl fluorides under photoirradiation. Alkyl iodides were transformed in high yields into acyl fluorides by using a commercially available copper precatalyst (CuBr·SMe) and a readily available fluoride salt (KF) at ambient temperature and mild CO pressure (6 atm) under blue light irradiation.

Coordination-induced O-H/N-H bond weakening by a redox non-innocent, aluminum-containing radical.

Several renewable energy schemes aim to use the chemical bonds in abundant molecules like water and ammonia as energy reservoirs. Because the O-H and N-H bonds are quite strong (>100 kcal/mol), it is necessary to identify substances that dramatically weaken these bonds to facilitate proton-coupled electron transfer processes required for energy conversion. Usually this is accomplished through coordination-induced bond weakening by redox-active metals.

Triazenide-supported [CuS] structural mimics of Cu that mediate NO disproportionation rather than reduction.

As part of the nitrogen cycle, environmental nitrous oxide (NO) undergoes the NO reduction reaction (NORR) catalyzed by nitrous oxide reductase, a metalloenzyme whose catalytic active site is a tetranuclear copper-sulfide cluster (Cu). On the other hand, heterogeneous Cu catalysts on oxide supports are known to mediate decomposition of NO (deNO) by disproportionation. In this study, a Cu model system supported by triazenide ligands is characterized by X-ray crystallography, NMR and EPR spectroscopies, and electronic structure calculations.

Uncovering a CF Effect on X-ray Absorption Energies of [Cu(CF ) ] and Related Copper Compounds by Using Resonant Diffraction Anomalous Fine Structure (DAFS) Measurements.

Understanding the electronic structures of high-valent metal complexes aids the advancement of metal-catalyzed cross coupling methodologies. A prototypical complex with formally high valency is [Cu(CF ) ] (1), which has a formal Cu(III) oxidation state but whose physical analysis has led some to a Cu(I) assignment in an inverted ligand field model. Recent examinations of 1 by X-ray spectroscopies have led previous authors to contradictory conclusions, motivating the re-examination of its X-ray absorption profile here by a complementary method, resonant diffraction anomalous fine structure (DAFS).

Activation of robust bonds by carbonyl complexes of Mn, Fe and Co.

Metal carbonyl complexes possess among the most storied histories of any compound class in organometallic chemistry. Nonetheless, these old dogs continue to be taught new tricks. In this Feature, we review the historic discoveries and recent advances in cleaving robust bonds (, C-H, C-O, C-F) using carbonyl complexes of three metals: Mn, Fe, and Co.

Metal Site-Specific Electrostatic Field Effects on a Tricopper(I) Cluster Probed by Resonant Diffraction Anomalous Fine Structure (DAFS).

Studies of multinuclear metal complexes are greatly enhanced by resonant diffraction measurements, which probe X-ray absorption profiles of crystallographically independent metal sites within a cluster. In particular, X-ray diffraction anomalous fine structure (DAFS) analysis provides data that can be interpreted akin to site-specific XANES, allowing for differences in metal K-edge resonances to be deconvoluted even for different metal sites within a homometallic system. Despite the prevalence of Cu-containing clusters in biology and energy science, DAFS has yet to be used to analyze multicopper complexes of any type until now.

Light-Mediated Synthesis of Aliphatic Anhydrides by Cu-Catalyzed Carbonylation of Alkyl Halides.

Acid anhydrides are valuable in the chemical industry for their role in synthesizing polymers, pharmaceuticals, and other commodities, but their syntheses often involve multiple steps with precious metal catalysts. The simplest anhydride, acetic anhydride, is currently produced by two Rh-catalyzed carbonylation reactions on a bulk scale for its use in synthesizing products ranging from aspirin to cellulose acetate. Here, we report a light-mediated, Cu-catalyzed process for producing aliphatic, symmetric acid anhydrides directly by carbonylation of alkyl (pseudo)halides in a single step without any precious metal additives.

Reactivity of a Dithiocarbamate-Ligated [W≡S] Complex with Hydride Donors: Toward a Synthetic Mimic of Formate Dehydrogenase.

Formate dehydrogenase (FDH) enzymes catalyze redox interconversion of CO and HCO, with a key mechanistic step being the transfer of H from HCO to an oxidized active site featuring a [M≡S] group in a sulfur-rich environment (M = Mo or W). Here, we report reactivity studies with HCO and other reducing agents of a synthetic [W≡S] model complex ligated by dithiocarbamate (dtc) ligands. Reactions of [WS(dtc)][BF] () conducted in MeOH solvent generated [WS(S)(dtc)] () and [WS(μ-S)(dtc)] () products by a solvolysis pathway that was accelerated by the presence of [MeN][HCO] but did not require it.

Recent advances in cooperative activation of CO and NO by bimetallic coordination complexes or binuclear reaction pathways.

The gaseous small molecules, CO and NO, play important roles in climate change and ozone layer depletion, and they hold promise as underutilized reagents and chemical feedstocks. However, productive transformations of these heteroallenes are difficult to achieve because of their inertness. In nature, these gases are cycled through ecological systems by metalloenzymes featuring multimetallic active sites that employ cooperative mechanisms.

Cooperative Activation of CO and Epoxide by a Heterobinuclear Al-Fe Complex via Radical Pair Mechanisms.

Activation of inert molecules like CO is often mediated by cooperative chemistry between two reactive sites within a catalytic assembly, the most common form of which is Lewis acid/base bifunctionality observed in both natural metalloenzymes and synthetic systems. Here, we disclose a heterobinuclear complex with an Al-Fe bond that instead activates CO and other substrates through cooperative behavior of two radical intermediates. The complex L(Me)AlFp (, L = HC{(CMe)(2,6-PrCHN)}, Fp = FeCp(CO), Cp = η-CH) was found to insert CO and cyclohexene oxide, producing LAl(Me)(μ:κ-OC)Fp () and LAl(Me)(μ-OCH)Fp (), respectively.

Preparation of Potassium Acyltrifluoroborates (KATs) from Carboxylic Acids by Copper-Catalyzed Borylation of Mixed Anhydrides.

We report the preparation of potassium acyltrifluoroborates (KATs) from widely available carboxylic acids. Mixed anhydrides of carboxylic acids were prepared using isobutyl chloroformate and transformed to the corresponding KATs using a commercial copper catalyst, B (pin) , and aqueous KHF . This method allows for the facile preparation of aliphatic, aromatic, and amino acid-derived KATs and is compatible with a variety of functional groups including alkenes, esters, halides, nitriles, and protected amines.

Synthesis and characterization of heteromultinuclear Ni/M clusters (M = Fe, Ru, W) including a paramagnetic (NHC)Ni-WCp*(CO) heterobinuclear complex.

A diverse range of heteromultinuclear Ni/[M] clusters (M = CpFe(CO), CpRu(CO), Cp*W(CO)) supported by a N-heterocyclic carbene ligand have been synthesized by reacting the Ni precursor, [IPrNi(μ-Cl)], with [M] reagents under various conditions. Clusters with NiFe, NiFe, NiRu, NiRu, NiRu, and NiW, and NiW cores were all characterized using NMR and IR spectroscopies and X-ray crystallography. The Ni-containing paramagnetic heterobinuclear species, IPrNi-Wp* (), was further characterized by EPR spectroscopy and DFT calculations.

Copper-Catalyzed Carbonylative Coupling of Alkyl Halides.

Transition metal-catalyzed carbonylation reactions represent a direct and atom-economical approach to introduce oxygen functionality into organic compounds, with CO acting as an inexpensive and readily available C1 feedstock. Despite the long history of carbonylation catalysis, including many processes that have been industrialized at bulk scale, there remain several challenges to tackle. For example, noble metals such as Pd, Rh, and Ir are typically used as catalysts for carbonylation reactions, rather than earth-abundant alternatives.

Cobalt-Catalyzed ()-β-Selective Hydrogermylation of Terminal Alkynes.

A cobalt-catalyzed method for the hydrogermylation of alkynes is reported, providing a selective and accessible route to ()-β-vinyl(trialkyl)germanes from terminal alkynes and HGeBu. As shown in multiple examples, the developed method demonstrates a broad functional group tolerance an practical utility for late-stage hydrogermylation of natural products. The method is compatible with alkynes bearing both aryl and alkyl substituents, providing unrivaled selectivity for previously challenging 1° alkyl-substituted alkynes.

Coordination chemistry of the Cu site in nitrous oxide reductase and its synthetic mimics.

Atmospheric nitrous oxide (NO) has garnered significant attention recently due to its dual roles as an ozone depletion agent and a potent greenhouse gas. Anthropogenic NO emissions occur primarily through agricultural disruption of nitrogen homeostasis causing NO to build up in the atmosphere. The enzyme responsible for NO fixation within the geochemical nitrogen cycle is nitrous oxide reductase (NOR), which catalyzes 2H/2e reduction of NO to N and HO at a tetranuclear active site, Cu.

One-Step Synthesis of Acylboron Compounds via Copper-Catalyzed Carbonylative Borylation of Alkyl Halides*.

A copper-catalyzed carbonylative borylation of unactivated alkyl halides has been developed, enabling efficient synthesis of aliphatic potassium acyltrifluoroborates (KATs) in high yields by treating the in situ formed tetracoordinated acylboron intermediates with aqueous KHF . A variety of functional groups are tolerated under the mild reaction conditions, and primary, secondary, and tertiary alkyl halides are all applicable. In addition, this method also provides facile access to N-methyliminodiacetyl (MIDA) acylboronates as well as α-methylated potassium acyltrifluoroborates in a one-pot manner.

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 CO 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/S ligation between (bdt)(O)W and Cu(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.

C-C and C-X coupling reactions of unactivated alkyl electrophiles using copper catalysis.

Transition metal-catalysed cross-coupling reactions are widely used for construction of carbon-carbon and carbon-heteroatom bonds. However, compared to aryl or alkenyl electrophiles, the cross-coupling of unactivated alkyl electrophiles containing β hydrogens remains a challenge. Over the past few years, the use of suitable ligands such as bulky phosphines or N-heterocyclic carbenes (NHCs) has enabled reactions of unactivated alkyl electrophiles not only limited to the traditional cross-coupling with Grignard reagents, but also including a diverse range of organic transformations via either S2 or radical pathways.

Impact of Electronic and Steric Changes of Ligands on the Assembly, Stability, and Redox Activity of Cu(μ-S) Model Compounds of the Cu Active Site of Nitrous Oxide Reductase (NOR).

Model compounds have been widely utilized in understanding the structure and function of the unusual Cu(μ-S) active site (Cu) of nitrous oxide reductase (NOR). However, only a limited number of model compounds that mimic both structural and functional features of Cu are available, limiting insights about Cu that can be gained from model studies. Our aim has been to construct Cu(μ-S) clusters with tailored redox activity and chemical reactivity via modulating the ligand environment.