Nitric oxide activation facilitated by cooperative multimetallic electron transfer within an iron-functionalized polyoxovanadate-alkoxide cluster

Switching of the redox centers of a tris-2-mercaptophenolato chromium(III) metalloligand by a guest metal ion

This work reports that the redox-active metalloligand (ML) fac-[Cr^III(mp)₃]^3- (mp: 2-mercaptophenolato) coordinates with a Co(III) ion to afford the trianionic complex [Co^III{fac-Cr^III(mp)₃}₂]^3-. The free ML shows ligand-centered redox processes, whereas the guest-metal-bound trinuclear structure exhibited a guest-metal-centered Co(II)/Co(III) redox couple, demonstrating redox switching through guest-metal binding to the MLs.

The Elusive Mononitrosylated [Fe4 S4 ] Cluster in Three Redox States

Iron-sulfur clusters are well-established targets in biological nitric oxide (NO) chemistry, but the key intermediate in these processes-a mononitrosylated [Fe4 S4 ] cluster-has not been fully characterized in a protein or a synthetic model thereof. Here, we report the synthesis of a three-member redox series of isostructural mononitrosylated [Fe4 S4 ] clusters. Mononitrosylation was achieved by binding NO to a 3 : 1 site-differentiated [Fe4 S4 ]+ cluster; subsequent oxidation and reduction afforded the other members of the series. All three clusters feature a local high-spin Fe3+ center antiferromagnetically coupled to 3 [NO]- . The observation of an anionic NO ligand suggests that NO binding is accompanied by formal electron transfer from the cluster to NO. Preliminary reactivity studies with the monocationic cluster demonstrate that exposure to excess NO degrades the cluster, supporting the intermediacy of mononitrosylated intermediates in NO sensing/signaling.

Modelling local structural and electronic consequences of proton and hydrogen-atom uptake in VO2 with polyoxovanadate clusters

We report the synthesis and characterisation of a series of siloxide-functionalised polyoxovanadate-alkoxide (POV-alkoxide) clusters, [V6O6(OSiMe3)(OMe)12] n (n = 1-, 2-), that serve as molecular models for proton and hydrogen-atom uptake in vanadium dioxide, respectively. Installation of a siloxide moiety on the surface of the Lindqvist core was accomplished via addition of trimethylsilyl trifluoromethylsulfonate to the fully-oxygenated cluster [V6O7(OMe)12]2-. Characterisation of [V6O6(OSiMe3)(OMe)12]1- by X-ray photoelectron spectroscopy reveals that the incorporation of the siloxide group does not result in charge separation within the hexavanadate assembly, an observation that contrasts directly with the behavior of clusters bearing substitutional dopants. The reduced assembly, [V6O6(OSiMe3)(OMe)12]2-, provides an isoelectronic model for H-doped VO2, with a vanadium(iii) ion embedded within the cluster core. Notably, structural analysis of [V6O6(OSiMe3)(OMe)12]2- reveals bond perturbations at the siloxide-functionalised vanadium centre that resemble those invoked upon H-atom uptake in VO2 through ab initio calculations. Our results offer atomically precise insight into the local structural and electronic consequences of the installation of hydrogen-atom-like dopants in VO2, and challenge current perspectives of the operative mechanism of electron-proton co-doping in these materials.

Inorganic clusters as metalloligands: ligand effects on the synthesis and properties of ternary nanopropeller clusters

Redox-active multimetallic platforms with synthetically addressable and hemilabile active sites are attractive synthetic targets for mimicking the reactivity of enzymatic co-factors toward multielectron transformations. To this end, a family of ternary clusters featuring three edge metal sites anchored on a [Co6Se8] multimetallic support via amidophosphine ligands are a promising platform. In this report, we explore how small changes in the stereoelectronic properties of these ligands alter [Co6Se8] metalloligand formation, but also substrate binding affinity and strength of the edge/support interaction in two new ternary clusters, M3Co6Se8L6 (M = Zn, Fe; L(-) = Ph2PN(-)iPr). These clusters are characterized extensively using a range of methods, including single crystal X-ray diffraction, electronic absorption spectroscopy and cyclic voltammetry. Substrate binding studies reveal that Fe3Co6Se8L6 resists coordination of larger ligands like pyridine or tetrahydrofuran, but binds the smaller ligand CNtBu. Additionally, investigations into the synthesis of new [Co6Se8] metalloligands using two aminophosphines, Ph2PN(H)iPr (LH) and iPr2PN(H)iPr, led to the synthesis and characterization of Co6Se8LH6, as well as the smaller clusters Co4Se2(CO)6LH4, Co3Se(μ2-PPh2)(CO)4LH3, and [Co(CO)3(iPr2PN(H)iPr)]2. Cumulatively, this study expands our understanding on the effect of the stereoelectronic properties of aminophosphine ligands in the synthesis of cobalt chalcogenide clusters, and, importantly on modulating the push-pull dynamic between the [Co6Se8] support, the edge metals and incoming coordinating ligands in ternary M3Co6Se8L6 clusters.

Heavy chalcogenide-transition metal clusters as coordination polymer nodes

While metal-oxygen clusters are widely used as secondary building units in the construction of coordination polymers or metal-organic frameworks, multimetallic nodes with heavier chalcogenide atoms (S, Se, and Te) are comparatively untapped. The lower electronegativity of heavy chalcogenides means that transition metal clusters of these elements generally exhibit enhanced coupling, delocalization, and redox-flexibility. Leveraging these features in coordination polymers provides these materials with extraordinary properties in catalysis, conductivity, magnetism, and photoactivity. In this perspective, we summarize common transition metal heavy chalcogenide building blocks including polynuclear metal nodes with organothiolate/selenolate or anionic heavy chalcogenide atoms. Based on recent discoveries, we also outline potential challenges and opportunities for applications in this field.

Site-selective halogenation of mixed-valent vanadium oxide clusters

Here, we expand on the synthesis and characterization of chloride-functionalized polyoxovanadate-alkoxide (POV-alkoxide) clusters, to include the halogenation of mixed-valent vanadium oxide assemblies.

Mononuclear, Nonheme, High-Spin {FeNO}7/8 Complexes Supported by a Sterically Encumbered N4S-Thioether Ligand

The synthesis of a new nonheme iron NO binding complex, [Fe^II(CH₃CN)(N3Py^2PhSEtCN)](BF₄)₂ (1), is reported. Complex 1, which contains two sterically encumbering phenyl substituents, exhibits a high-spin (hs) Fe^II (S = 2) ground state in contrast to the S = 0 ground state for unsubstituted [Fe^II(CH₃CN)(N3PySEtCN)(BF₄)₂. Reaction of 1 with NO_(g) in CH₃CN yields an {FeNO}⁷ (S = ³/₂) complex 2, which slowly decays at 25 °C with loss of NO^• to regenerate 1. One-electron reduction of 2 with Cr(C₆H₆)₂ at -40 °C yields the metastable, S = 1 {FeNO}⁸ complex 3. The nitrosyl moieties in thioether-ligated 2 and 3 are significantly less activated than in thiolate-ligated [Fe(NO)(N3PyS)]^+/0, a structurally analogous pair of hs {FeNO}^7/8 complexes. Calculations reveal that reduction of 2 is iron-centered, which may be a general property of hs {FeNO}^7/8 complexes.

A Mononuclear, Nonheme FeII-Piloty's Acid (PhSO2NHOH) Adduct: An Intermediate in the Production of {FeNO}7/8 Complexes from Piloty's Acid

Reaction of the mononuclear nonheme complex [Fe^II(CH₃CN)(N3PyS)]BF₄ (1) with an HNO donor, Piloty's acid (PhSO₂NHOH, P.A.), at low temperature affords a high-spin ( S = 2) Fe^II-P.A. intermediate (2), characterized by ⁵⁷Fe Mössbauer and Fe K-edge X-ray absorption (XAS) spectroscopies, with interpretation of both supported by DFT calculations. The combined methods indicate that P.A. anion binds as the N-deprotonated tautomer (PhSO₂NOH^-) to [Fe^II(N3PyS)]⁺, leading to 2. Complex 2 is the first spectroscopically characterized example, to our knowledge, of P.A. anion bound to a redox-active metal center. Warming of 2 above -60 °C yields the stable {FeNO}⁷ complex [Fe(NO)(N3PyS)]BF₄ (4), as evidenced by ¹H NMR, ATR-IR, and Mössbauer spectroscopies. Isotope labeling experiments with ¹⁵N-labeled P.A. confirm that the nitrosyl ligand in 4 derives from P.A. In contrast, addition of a second equivalent of a strong base leads to S-N cleavage and production of an {FeNO}⁸ species, the deprotonated analog of an Fe-HNO complex. This work has implications for the targeted delivery of HNO/NO^-/NO· to nonheme Fe centers in biological and synthetic applications, and suggests a new role for nonheme Fe^II complexes in the assisted degradation of HNO donor molecules.