Marcello Gennari

A mononuclear Cu complex bearing a pentadentate polypyridylalkylamine ligand mediates four-electron O ₂ reduction in both organic and aqueous media, revealing the key role of ligand fine-tuning in catalytic performance.

Nonheme iron catalysts are attractive for bioinspired aerobic oxidations and oxygenations, as well as for the oxygen reduction reaction (ORR), although their potential in ORR remains largely underexploited. This study presents a rare example of such a system, displaying O₂ activation activity and ORR catalytic selectivity toward H₂O generation. The triiron(II) complex [Fe^IIL]₃(BF₄)₃, where L^- is a thiolate-containing N3S donor bis(2-pyridylmethyl)amine derivative, was fully characterized in the solid state and in MeCN solution, where it dissociates into the mononuclear solvent adduct [FeL(MeCN)]⁺. Upon exposure to O₂ in an acetonitrile solution, [FeL(MeCN)]⁺ generates a μ-oxo diiron(III), that further evolves to an adduct featuring an original {Fe₄OF₅}³⁺ core. In the presence of 2,6-lutidinium tetrafluoroborate (proton source), and octamethylferrocene (reducing agent), [FeL(MeCN)]⁺ catalyzes the ORR with exclusive production of H₂O. This system is finally discussed with respect to a few other reported nonheme Fe-based ORR catalysts operating under chemical conditions.

The bis­(borane) o-{(C₆F₅)₂B}₂C₆F₄ (1) reacts with dioxygen in the presence of decamethylferrocene (FeCp*₂, Cp* = pentamethylcyclopentadienyl) to deliver the salt [(μ–OH)-o-{B­(C₆F₅)₂}₂C₆F₄]­[FeCp*₂] (2) featuring a hydroxide sequestered by the two adjacent boron atoms. Mechanistic investigations of this formal 4-electron reduction of O₂ suggest that it goes through the formation of a superoxide adduct [1-O₂]⁻, which evolves through disproportionation into O₂ and a peroxo-adduct [1-O₂]^2–. Upon coordination of another equivalent of 1, the thus-generated 4-fold boron-sequestered peroxide {[1]₂-O₂}^2– undergoes homolytic O–O bond cleavage to yield a pair of oxyl radicals [1-O^•]⁻. These highly reactive intermediates subsequently perform hydrogen atom abstraction to lead to the hydroxide salt 2. Our observations point to FeCp*₂ being the H atom purveyor in this final step.

The bio‐inspired dinuclear iron(II) complex [Fe^II(L)]₂ (L²⁻=2,2’‐(2,2’‐bipyridine‐6,6’‐diyl)bis(1,1’‐diphenylethanethiolate)), featuring alkyl thiolate ligands, was found to react with hydrazine (N₂H₄) to form an unprecedented alkyl thiolate‐supported Fe^II−N₂H₄ adduct, [LFe^II(N₂H₄)]. This complex was characterized structurally by X‐ray crystallography, spectroscopically (¹H NMR, UV‐vis, IR), and electrochemically. Unexpectedly, in CH₃CN solution, [LFe^II(N₂H₄)] gradually evolved to yield an Fe^II‐acetohydrazonohydrazide complex, [LFe^II(H₂N₂C(CH₃)N₂H₃)], the C−N bond formation resulting from the reaction between N₂H₄ and CH₃CN mediated by the Fe^II ion. Its structure, confirmed by X‐ray diffraction on single crystals, reveals a η¹‐coordination of the acetohydrazonohydrazide ligand. The proposed mechanism is based on the electrophilic activation of a CH₃CN molecule by Fe^II, followed by the nucleophilic attack of a free hydrazine. This work expands the reactivity landscape of iron‐hydrazine complexes and provides new insights into potential pathways for N−N bond functionalization in nitrogen fixation processes.

The cleavage of C-S bonds represents a crucial step in fossil fuel refinement to remove organosulfur impurities. Efforts are required to identify alternatives that can replace the energy-intensive hydrodesulfurization process currently in use. In this context, we have developed a series of bis-thiolato-ligated Cr-III complexes supported by the L2- ligand (L2- = 2,2 '-bipyridine-6,6 '-diyl(bis(1,1-diphenylethanethiolate), one of them displaying desulfurization of one thiolate of the ligand under reducing and acidic conditions at ambient temperature and atmospheric pressure. While only 5-coordinated complexes were previously isolated by reaction of L2- with 3d metal M-III ions, both 5- and 6-coordinated mononuclear complexes have been obtained in the case of Cr-III, viz., [(CrLCl)-L-III], [(CrLCl2)-L-III](-), and [(CrLCl)-L-III(CH3CN)]. The investigation of the reactivity of [(CrLCl)-L-III(CH3CN)] under reducing conditions led to a dinuclear [(Cr2L2)-L-III(mu-Cl)(mu-OH)] compound and, in the presence of protons, to the mononuclear Cr-III complex [Cr-III(L-N2S)(2)](+), where LN2S- is the partially desulfurized form of L2-. A desulfurization mechanism has been proposed involving the release of H2S, as evidenced experimentally.

Immobilization of well‐defined homogenous (electro)catalysts onto conductive supports offers an attractive strategy for designing advanced functional materials for energy conversion. In this context, this study reports (i) the introduction of a pyrene anchoring group on a PNP−pincer Ir^I complex previously described as a selective catalyst for the electrodriven CO₂ reduction (CO₂RR) into CO in DMF/water mixtures, (ii) the comparison of its CO₂RR activity in DMF/water mixtures with the ones of two pyrene‐free reference complexes, and (iii) its activity in pure water after immobilization onto carbon nanotubes (CNTs). Surprisingly, in homogeneous conditions we find HCOO⁻, instead of CO, as the main CO₂ reduction product for the three catalysts. After immobilization on CNTs, even if non‐negligible competitive proton reduction reaction is observed in fully aqueous media, the complex is still able to drive CO₂RR and produce HCOO⁻ with a significantly lower overpotential with respect to solution studies.

Suitably functionalized porous matrices represent versatile platforms to support well-dispersed catalytic centers. In the present study, porous organic polymers (POPs) containing phosphine oxide groups were fabricated to bind transition metals and to be investigated for potential electrocatalytic applications. Cross-linking of mono- and di-phosphine monomers with multiple phenyl substituents was subject to the Friedel–Crafts (F-C) reaction and the oxidation process, which generated phosphine oxide porous polymers with pore capacity up to 0.92 cm³/g and a surface area of about 990 m²/g. The formation of the R₃P·BH₃ borohydride adduct during synthesis allows to extend the library of phosphine-based monomeric entities when using FeCl₃. The porous polymers were loaded with 0.8–4.2 w/w % of cobalt(II) and behaved as hydrogen evolution reaction (HER) catalysts with a Faradaic efficiency of up to 95% (5.81 × 10^–5 mol H₂ per 11.76 C) and a stable current density during repeated controlled potential experiments (CPE), even though with high overpotentials (0.53–0.68 V to reach a current density of 1 mA·cm^–2). These studies open the way to the effectiveness of tailored phosphine oxide POPs produced through an inexpensive and ecofriendly iron-based catalyst and for the insertion of transition metals in a porous architecture, enabling electrochemically driven activation of small molecules.

Sulfur-rich metalloproteins and metalloenzymes, containing strongly covalent metal–thiolate (cysteinate) or metal–sulfide bonds in their active site, are ubiquitous in nature. The metal–sulfur motif is a highly versatile tool involved in various biological processes: (i) metal storage, transport, and detoxification; (ii) electron transfer; (iii) activation of the sulfur atom to promote different types of S-based reactions including S-alkylation, S-oxygenation, S-nitrosylation, or disulfide or thiyl radicals formation; (iv) activation of small earth-abundant molecules (such as water, dioxygen, superoxide radical anion, carbon oxides, nitrous oxide, and dinitrogen). This Account describes our investigations carried out during the past 10 years on bio-inspired and biomimetic low-nuclearity complexes containing metal–thiolate bonds. The general objective of these structural, spectroscopic, electrochemical, and catalytic studies was to determine structure–properties–function correlations useful to (i) understanding the peculiar features or the mechanism of the mimicked natural systems and/or (ii) reproducing enzymatic reactivities for specific catalytic applications. By employing a unique highly preorganized N₂S₂-donor ligand with two thiolate functions, in combination with different first-row transition metals (Mn, Fe, Co, Ni, Cu, Zn, or V), we got access to a series of bio-inspired sulfur-rich complexes displaying a widespread spectrum of structures, properties, and functions. We isolated a dicopper(I) complex that, for the first time, mimicked concomitantly the key structural, spectroscopic, and redox features of the biological Cu_A center, a highly efficient electron transfer agent involved in the respiratory enzyme cytochrome c oxidase. In the field of sulfur activation, we explored (i) sulfur methylation promoted by a Zn–dithiolate complex that mimics Zn-dependent thiolate alkylation proteins and shows different selectivity compared to the Ni and Co congeners and (ii) a series of Co, Fe, and Mn complexes as the first copper-free systems able to promote thiolate/disulfide interconversion mediated by (de)coordination of halides. Concerning metal-centered reactivity, we investigated two families of metal–thiolate catalysts for small-molecule activation, especially relevant in the fields of sustainable fuel production and energy conversion: (i) two isostructural Mn and Fe dinuclear complexes that activate and reduce dioxygen selectively, either to hydrogen peroxide or water as a function of the experimental conditions; (ii) a family of dinuclear MFe (M = Ni or Fe) hydrogenase mimics active for catalytic H₂ evolution both in organic solution and on modified electrodes in water. This Account thus illustrates how the versatility of thiolate ligation can support selected functions for transition metal complexes, depending on the nature of the metal, the nuclearity of the complex, the presence and type of co-ligands, the second coordination sphere effects, and the experimental conditions.

Iron centers featuring thiolates in their metal coordination sphere (as ligands or substrates) are well-known to activate dioxygen. Both heme and non-heme centers that contain iron-thiolate bonds are found in nature. Investigating the ability of iron–thiolate model complexes to activate O₂ is expected to improve the understanding of the key factors that direct reactivity to either iron or sulfur. We report here the structural and redox properties of a thiolate-based dinuclear Fe complex, [Fe^II₂(LS)₂] (LS^2– = 2,2′-(2,2′-bipyridine-6,6′-iyl)bis(1,1-diphenylethanethiolate)), and its reactivity with dioxygen, in comparison with its previously reported protonated counterpart, [Fe^II₂(LS)(LSH)]⁺. When reaction with O₂ occurs in the absence of protons or in the presence of 1 equiv of proton (i.e., from [Fe^II₂(LS)(LSH)]⁺), unsupported μ-oxo or μ-hydroxo Fe^III dinuclear complexes ([Fe^III₂(LS)₂O] and [Fe^III₂(LS)₂(OH)]⁺, respectively) are generated. [Fe^III₂(LS)₂O], reported previously but isolated here for the first time from O₂ activation, is characterized by single crystal X-ray diffraction and Mössbauer, resonance Raman, and NMR spectroscopies. The addition of protons leads to the release of water and the generation of a mixture of two Fe-based “oxygen-free” species. Density functional theory calculations provide insight into the formation of the μ-oxo or μ-hydroxo Fe^III dimers, suggesting that a dinuclear μ-peroxo Fe^III intermediate is key to reactivity, and the structure of which changes as a function of protonation state. Compared to previously reported Mn–thiolate analogues, the evolution of the peroxo intermediates to the final products is different and involves a comproportionation vs a dismutation process for the Mn and Fe derivate, respectively.