Metallodithiolates as ligands in coordination, bioinorganic, and organometallic chemistry

Site specific redox properties in ligand differentiated di-nickel complexes inspired by the acetyl CoA synthase active site

Bimetallic transition metal complexes with site-specific redox properties offer a versatile platform for understanding electron polarization, intramolecular electron transfer processes, and customizing electronic and magnetic properties that might impact reactivity and catalyst design. Inspired by the dissymmetric nickel sites in the Acetyl CoA Synthase (ACS) Active Site, three new bimetallic Ni(N2S2)-Ni(S2C2R2) complexes based on Ni(N2S2) metalloligand donor synthons, Nid, in mimicry of the nickel site distal to the redox-active iron sulfur cluster of ACS, and nickel dithiolene receiver units, designated as Nip, the nickel proximal to the 4Fe4S cluster, were combined to explore the influence of ligand environment on electronic structure and redox properties of each unit. The combination of synthons gave a matrix of three S-bridged dinickel complexes, characterized by X-ray crystallography, and appropriate spectroscopies. Computational modeling is connected to the electronic characteristics of the nickel donor and receiver units. This study demonstrated the intricacies of identifying sites of electrochemical redox processes, within multi-metallic systems containing non-innocent ligands.

Development of (NO)Fe(N2S2) as a Metallodithiolate Spin Probe Ligand: A Case Study Approach

ConspectusThe ubiquity of sulfur-metal connections in nature inspires the design of bi- and multimetallic systems in synthetic inorganic chemistry. Common motifs for biocatalysts developed in evolutionary biology include the placement of metals in close proximity with flexible sulfur bridges as well as the presence of π-acidic/delocalizing ligands. This Account will delve into the development of a (NO)Fe(N2S2) metallodithiolate ligand that harnesses these principles. The Fe(NO) unit is the centroid of a N2S2 donor field, which as a whole is capable of serving as a redox-active, bidentate S-donor ligand. Its paramagnetism as well as the ν(NO) vibrational monitor can be exploited in the development of new classes of heterobimetallic complexes. We offer four examples in which the unpaired electron on the {Fe(NO)}7 unit is spin-paired with adjacent paramagnets in proximal and distal positions.First, the exceptional stability of the (NO)Fe(N2S2)-Fe(NO)2 platform, which permits its isolation and structural characterization at three distinct redox levels, is linked to the charge delocalization occurring on both the Fe(NO) and the Fe(NO)2 supports. This accommodates the formation of a rare nonheme {Fe(NO)}8 triplet state, with a linear configuration. A subsequent FeNi complex, featuring redox-active ligands on both metals (NO on iron and dithiolene on nickel), displayed unexpected physical properties. Our research showed good reversibility in two redox processes, allowing isolation in reduced and oxidized forms. Various spectroscopic and crystallographic analyses confirmed these states, and Mössbauer data supported the redox change at the iron site upon reduction. Oxidation of the complex produced a dimeric dication, revealing an intriguing magnetic behavior. The monomer appears as a spin-coupled diradical between {Fe(NO)}7 and the nickel dithiolene monoradical, while dimerization couples the latter radical units via a Ni2S2 rhomb. Magnetic data (SQUID) on the dimer dication found a singlet ground state with a thermally accessible triplet state that is responsible for magnetism. A theoretical model built on an H4 chain explains this unexpected ferromagnetic low-energy triplet state arising from the antiferromagnetic coupling of a four-radical molecular conglomerate. For comparison, two (NO)Fe(N2S2) were connected through diamagnetic group 10 cations producing diradical trimetallic complexes. Antiferromagnetic coupling is observed between {Fe(NO)}7 units, with exchange coupling constants (J) of -3, -23, and -124 cm-1 for NiII, PdII, and PtII, respectively. This trend is explained by the enhanced covalency and polarizability of sulfur-dense metallodithiolate ligands. A central paramagnetic trans-Cr(NO)(MeCN) receiver unit core results in a cissoid structural topology, influenced by the stereoactivity of the lone pair(s) on the sulfur donors. This {Cr(NO)}5 radical bridge, unlike all previous cases, finds the coupling between the distal Fe(NO) radicals to be ferromagnetic (J = 24 cm-1).The stability and predictability of this S = 1/2 moiety and the steric/electronic properties of the bridging thiolate sulfurs suggest it to be a likely candidate for the development of novel molecular (magnetic) compounds and possibly materials. The role of synthetic inorganic chemistry in designing synthons that permit connections of the (NO)Fe(N2S2) metalloligand is highlighted as well as the properties of the heterobi- and polymetallic complexes derived therefrom.

Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR νNO 1775 s, 1716 vs, 1668 vs cm-1 (THF)] between two adjacent [Fe(NO)2] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)2(μ-ON2Me)Fe(NO)2] (1). Electronic interplay between [Fe(NO)2] units and [ON2Me]- ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)2] unit(s) to enforce a "constrained" π-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2- electron configuration (Stotal = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)2]10 unit, while preserving the {Fe(NO)2}9 core in DNICs {Fe(NO)2}9-[·ON2Me]2- [K-18-crown-6-ether)][(ON2Me)Fe(NO)2] (2) and {Fe(NO)2}9-[·ON2Me] [(ON2Me)Fe(NO)2][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)2}9-[·ON2Me]2-] DNIC 3 ↔ [{Fe(NO)2}9-[ON2Me]-] ↔ [{Fe(NO)2}9-[·ON2Me]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(μ-ONMe)2Fe2(NO)4] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)2] (5). In parallel with this finding, the electronic structures of [{Fe(NO)2}9-{Fe(NO)2}9-[·ON2Me]2-] DNIC 1, [{Fe(NO)2}9-[·ON2Me]2-] DNIC 2, [{Fe(NO)2}9-[·ON2Me]] DNIC 3, [{Fe(NO)2}9-[ONMe]-]2 DNIC 4, and [{Fe(NO)2}9-[·ONMe]2-] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.

Bond Trading: Intramolecular Metal and Ligand Exchange within a NO/Ni/Co Complex

With the goal of generating hetero-redox levels on metals as well as on nitric oxide (NO), metallodithiolate (N2 S2 )CoIII (NO- ), N2 S2 = N,N- dibenzyl-3,7-diazanonane-1,9-dithiolate, is introduced as ligand to a well-characterized labile [Ni0 (NO)+ ] synthon. The reaction between [Ni0 (NO+ )] and [CoIII (NO- )] has led to a remarkable electronic and ligand redistribution to form a heterobimetallic dinitrosyl cobalt [(N2 S2 )NiII ∙Co(NO)2 ]+ complex with formal two electron oxidation state switches concomitant with the nickel extraction or transfer as NiII into the N2 S2 ligand binding site. To date, this is the first reported heterobimetallic cobalt dinitrosyl complex.

A single carbon atom controls the geometry and reactivity of CoII(N2S2) complexes

Three- vs. two-carbon N-to-N connectors give rise to monomeric, tetrahedral N2S2Co(II) (μeff = 4.24 BM) or dimeric [(N2S2)Co(II)]2 (diamagnetic) complexes, respectively. Differences in the derivative products of the Lewis acid receivers, W0(CO)3 and W0(CO)4, illustrate nucleophilicity of the thiolate sulfur lone pairs in each case, as well as their structural control.

Sulfur Lone Pairs Control Topology in Heterotrimetallic Complexes: An Experimental and Theoretical Study

Heterotrimetallic complexes with (N2S2)M metallodithiolates, M = Ni2+, [Fe(NO)]2+, and [Co(NO)]2+, as bidentate chelating ligands to a central trans-Cr(NO)(MeCN) unit were characterized as the first members of a new class, NiCrNi, FeCrFe, CoCrCo. The complexes exhibit a cisoid structural topology, ascribed to the stereoactivity of the available lone pair(s) on the sulfur donors, resulting in a dispersed, electropositive pocket from the N/N and N/S hydrocarbon linkers wherein the Cr-NO site is housed. Computational studies explored alternative isomers (transoid and inverted cisoid) that suggest a combination of electronic and steric effects govern the geometrical selectivity. Electrostatic potential maps readily display the dominant electronegative potential from the sulfurs which force the NO to the electropositive pocket. The available S lone pairs work in synergy with the π-withdrawing ability of NO to lift Cr out of the S4 plane toward the NO and stabilize the geometry. The metallodithiolate ligands bound to Cr(NO) thus find structural consistency across the three congeners. Although the dinitrosyl [(bme-dach)Co(NO)-Mo(NO)(MeCN)-(bme-dach)Co(MeCN)][PF6]2 (CoMoCo') analogue displays chemical noninnocence and a partial Mo-Co bond toward (N2S2)Co'(NCCH3) in an "asymmetric butterfly" topology [Guerrero-Almaraz P.Inorg. Chem.2021, 60(21 (21), ), 15975-15979], the stability of the {Cr(NO)}5 unit prohibits such bond rearrangement. Magnetism and EPR studies illustrate spin coupling across the sulfur thiolate sulfur bridges.

Ligand Control of Dinitrosyl Iron Complexes for Selective Superoxide-Mediated Nitric Oxide Monooxygenation and Superoxide-Dioxygen Interconversion

Through nitrosylation of [Fe-S] proteins, or the chelatable iron pool, a dinitrosyl iron unit (DNIU) [Fe(NO)2] embedded in the form of low-molecular-weight/protein-bound dinitrosyl iron complexes (DNICs) was discovered as a metallocofactor assembled under inflammatory conditions with elevated levels of nitric oxide (NO) and superoxide (O2-). In an attempt to gain biomimetic insights into the unexplored transformations of the DNIU under inflammation, we investigated the reactivity toward O2- by a series of DNICs [(NO)2Fe(μ-MePyr)2Fe(NO)2] (1) and [(NO)2Fe(μ-SEt)2Fe(NO)2] (3). During the superoxide-induced conversion of DNIC 1 into DNIC [(K-18-crown-6-ether)2(NO2)][Fe(μ-MePyr)4(μ-O)2(Fe(NO)2)4] (2-K-crown) and a [Fe3+(MePyr)x(NO2)y(O)z]n adduct, stoichiometric NO monooxygenation yielding NO2- occurs without the transient formation of peroxynitrite-derived •OH/•NO2 species. To study the isoelectronic reaction of O2(g) and one-electron-reduced DNIC 1, a DNIC featuring an electronically localized {Fe(NO)2}9-{Fe(NO)2}10 electronic structure, [K-18-crown-6-ether][(NO)2Fe(μ-MePyr)2Fe(NO)2] (1-red), was successfully synthesized and characterized. Oxygenation of DNIC 1-red leads to the similar assembly of DNIC 2-K-crown, of which the electronic structure is best described as paramagnetic with weak antiferromagnetic coupling among the four S = 1/2 {FeIII(NO-)2}9 units and S = 5/2 Fe3+ center. In contrast to DNICs 1 and 1-red, DNICs 3 and [K-18-crown-6-ether][(NO)2Fe(μ-SEt)2Fe(NO)2] (3-red) display a reversible equilibrium of "3 + O2- ⇋ 3-red + O2(g)", which is ascribed to the covalent [Fe(μ-SEt)2Fe] core and redox-active [Fe(NO)2] unit. Based on this study, the supporting/bridging ligands in dinuclear DNIC 1/3 (or 1-red/3-red) control the selective monooxygenation of NO and redox interconversion between O2- and O2 during reaction with O2- (or O2).

Magnetic coupling between Fe(NO) spin probe ligands through diamagnetic NiII, PdII and PtII tetrathiolate bridges

Reaction of the nitrosylated-iron metallodithiolate ligand, paramagnetic (NO)Fe(N2S2), with [M(CH3CN)n][BF4]2 salts (M = NiII, PdII, and PtII; n = 4 or 6) affords di-radical tri-metallic complexes in a stairstep type arrangement ([FeMFe]2+, M = Ni, Pd, and Pt), with the central group 10 metal held in a MS4 square plane. These isostructural compounds have nearly identical ν(NO) stretching values, isomer shifts, and electrochemical properties, but vary in their magnetic properties. Despite the intramolecular Fe⋯Fe distances of ca. 6 Å, antiferromagnetic coupling is observed between {Fe(NO)}7 units as established by magnetic susceptibility, EPR, and DFT studies. The superexchange interaction through the thiolate sulfur and central metal atoms is on the order of NiII < PdII ≪ PtII with exchange coupling constants (J) of -3, -23, and -124 cm-1, consistent with increased covalency of the M-S bonds (3d < 4d < 5d). This trend is reproduced by DFT calculations with molecular orbital analysis providing insight into the origin of the enhancement in the exchange interaction. Specifically, the magnitude of the exchange interaction correlates surprisingly well with the energy difference between the HOMO and HOMO-1 orbitals of the triplet states, which is reflected in the central metal's contribution to these orbitals. These results demonstrate the ability of sulfur-dense metallodithiolate ligands to engender strong magnetic communication by virtue of their enhanced covalency and polarizability.

Hybrids of [FeFe]- and [NiFe]-H2ase Active Site Models

Complexes of the type (diphosphine)Ni(μ-SR)2Fe(CO)3 are investigated with azadithiolate (adt, HN(CH2S-)2) as the dithiolate. The resulting complexes are hybrid models for the active sites of the [NiFe]- and [FeFe]-hydrogenases. The key complex (dppv)Ni(μ-adt)Fe(CO)3 (3) was prepared from the complex Ni[(SCH2)2NCbz](dppv), which contains a Cbz-protected adt ligand (Cbz = C(O)OCH2Ph, dppv = cis-1,2-(Ph2P)2C2H2). This complex combines with Fe2(CO)9 to give (dppv)Ni[(μ-SCH2)2NCbz]Fe(CO)3, which is readily deprotected to give 3. Complex 3 undergoes protonation at both Fe and N to give successively [(dppv)Ni(μ-adt)FeH(CO)3]+ ([H3]+) and [(dppv)Ni(μ-adtH)FeH(CO)3]2+ ([H3H]2+). The redox properties and dynamics of these complexes resemble previously reported analogues with propanedithiolate. Solutions of [H3]+ readily degrade to [(dppv)Ni[(μ-SCH2)2NCH2]Fe(CO)3]+ ([4]+), which features a methylene group linking N and Fe. Complex [4]+ can be made in high yield by reaction of [H3]+ with CH2O, and this conversion was also demonstrated with 13CH2O. Complex [4]+ undergoes hydrogenolysis by photochemical reaction with H2 to give [(dppv)Ni[(μ-SCH2)2NMe]FeH(CO)3]+, the N-methylated analogue of [H3]+. Upon treatment ith Me3O+, [4]+ undergoes quaternization, giving [(dppv)Ni[(μ-SCH2)2N(Me)CH2]Fe(CO)3]2+. In contrast with the lability of [H3]+, the phosphine-substituted derivative [(dppv)Ni(μ-adt)FeH(CO)2(PPh3)]+ did not degrade. Most complexes were characterized by X-ray crystallography.

Through the Looking Glass: Using the Lens of [SNS]-Pincer Ligands to Examine First-Row Metal Bifunctional Catalysts

ConspectusHomogeneous catalysis is at the forefront of global efforts to innovate the synthesis of fine chemicals and achieve carbon-neutrality in energy applications. For decades, the push toward sustainable catalysis has focused on the development of first-row transition metal catalysts to supplant widespread use of precious metals. Metal-ligand cooperativity is an effective strategy to yield high-performing first-row metal molecular catalysts. Despite remarkable progress, state of the art catalysts often employ phosphorus-based ligands which are air-sensitive, potentially toxic, and on occasion offset the cost-savings of the metal. Thus, the development of simple and economical ligands composed of biomimetic donors should be a key focus that cannot be overlooked in the pursuit of sustainable catalyst candidates. This is an Account of our group's efforts to develop first-row transition metal complexes which use [SNS]-pincer ligands for bifunctional catalysis. We have synthesized two potentially tridentate ligands, one bearing an amido and two thioether donors [(S^MeNS^Me), L1] and one which includes thiolate, imine, and thioether donors [(SNS^Me), L2], and used them as platforms upon which to explore the reaction pathways of first-row metals. The [SNS] ligand, L1, leads to formation of high-spin paramagnetic metal complexes of the type M(L1)₂ in which the 6-membered ring thioether donor is hemilabile (M = Mn, Fe, Co). This allows Mn(L1)₂ to function as a carbonyl hydroboration catalyst that operates by a novel hydride-free, inner-sphere reaction pathway. Exploring the reactivity of L2 with Fe and Ni revealed unique coordination chemistry and a variety of mono-, di-, tri-, and tetranuclear complexes enabled by bridging thiolates. Further studies showed L2 undergoes selective C_aryl-S bond cleavage upon coordination to a metal with electron-rich phosphine donors, yielding a new (CNS)^2- pincer ligand. The analogous reaction with L1 afforded a new (CNS^Me)^- pincer ligand via both C_aryl-S and benzylic C-H bond cleavage. In an attempt to prepare Fe(L2)₂, we obtained instead an Fe(N₂S₃) complex in which imine C-C bond formation affords a potentially hexadentate redox-active ligand. The Fe(N₂S₃) complex is a selective catalyst for hydroboration of aldehydes and appears to operate through a complicated mechanism. In contrast, a mechanistic study of Mn(L2)(CO)₃-photocatalyzed dihydroboration of nitriles indicated that both the flexibility of the κ³-SNS^Me ligand (fac- vs mer-coordination) and ability of Mn to undergo a spin-state change are required to access low energy barriers for this transformation. To effectively compare the reactivity of the thiolate vs amido donor, we prepared two Cu complexes, Cu(L1)(IPr) and Cu(L2)(IPr) [IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene], showing that, while both served as carbonyl hydroboration catalysts, only the amido complex was an effective catalyst for carbonyl hydrosilylation. In addition, complexes of the type Zn(L1)₂, Zn(L2)₂, and Zn(L1)(L2), were also effective for catalytic carbonyl hydroboration. While Zn(L1)(L2) was most active, catalyst speciation studies showed that each undergoes bifunctional catalyst activation to form a Zn bis(alkoxide) catalyst. Overall, our observations using [SNS] ligands with first-row transition metals show how the absence of traditional phosphine donors leads to different fundamental reactivity. Furthermore, this Account demonstrates the gap of knowledge which exists in understanding the reactivity of sulfur-based ligands to promote more widespread adoption of sustainable ligands.

Bioinspired and biomolecular catalysts for energy conversion and storage

Metalloenzymes are remarkable for facilitating challenging redox transformations with high efficiency and selectivity. In the area of alternative energy, scientists aim to capture these properties in bioinspired and engineered biomolecular catalysts for the efficient and fast production of fuels from low-energy feedstocks such as water and carbon dioxide. In this short review, efforts to mimic biological catalysts for proton reduction and carbon dioxide reduction are highlighted. Two important recurring themes are the importance of the microenvironment of the catalyst active site and the key role of proton delivery to the active site in achieving desired reactivity. Perspectives on ongoing and future challenges are also provided.

Cooperative redox and spin activity from three redox congeners of sulfur-bridged iron nitrosyl and nickel dithiolene complexes

The synthesis of sulfur-bridged Fe-Ni heterobimetallics was inspired by Nature's strategies to "trick" abundant first row transition metals into enabling 2-electron processes: redox-active ligands (including pendant iron-sulfur clusters) and proximal metals. Our design to have redox-active ligands on each metal, NO on iron and dithiolene on nickel, resulted in the observation of unexpectedly intricate physical properties. The metallodithiolate, (NO)Fe(N2S2), reacts with a labile ligand derivative of [NiII(S2C2Ph2)]0, NiDT, yielding the expected S-bridged neutral adduct, FeNi, containing a doublet {Fe(NO)}7. Good reversibility of two redox events of FeNi led to isolation of reduced and oxidized congeners. Characterization by various spectroscopies and single-crystal X-ray diffraction concluded that reduction of the FeNi parent yielded [FeNi]-, a rare example of a high-spin {Fe(NO)}8, described as linear FeII(NO-). Mössbauer data is diagnostic for the redox change at the {Fe(NO)}7/8 site. Oxidation of FeNi generated the 2[FeNi]+⇌[Fe2Ni2]2+ equilibrium in solution; crystallization yields only the [Fe2Ni2]2+ dimer, isolated as PF6- and BArF- salts. The monomer is a spin-coupled diradical between {Fe(NO)}7 and NiDT+, while dimerization couples the two NiDT+ via a Ni2S2 rhomb. Magnetic susceptibility studies on the dimer found a singlet ground state with a thermally accessible triplet excited state responsible for the magnetism at 300 K (χMT = 0.67 emu·K·mol-1, µeff = 2.31 µB), and detectable by parallel-mode EPR spectroscopy at 20 to 50 K. A theoretical model built on an H4 chain explains this unexpected low energy triplet state arising from a combination of anti- and ferromagnetic coupling of a four-radical molecular conglomerate.

Water electrolysis: from textbook knowledge to the latest scientific strategies and industrial developments

Replacing fossil fuels with energy sources and carriers that are sustainable, environmentally benign, and affordable is amongst the most pressing challenges for future socio-economic development. To that goal, hydrogen is presumed to be the most promising energy carrier. Electrocatalytic water splitting, if driven by green electricity, would provide hydrogen with minimal CO2 footprint. The viability of water electrolysis still hinges on the availability of durable earth-abundant electrocatalyst materials and the overall process efficiency. This review spans from the fundamentals of electrocatalytically initiated water splitting to the very latest scientific findings from university and institutional research, also covering specifications and special features of the current industrial processes and those processes currently being tested in large-scale applications. Recently developed strategies are described for the optimisation and discovery of active and durable materials for electrodes that ever-increasingly harness first-principles calculations and machine learning. In addition, a technoeconomic analysis of water electrolysis is included that allows an assessment of the extent to which a large-scale implementation of water splitting can help to combat climate change. This review article is intended to cross-pollinate and strengthen efforts from fundamental understanding to technical implementation and to improve the 'junctions' between the field's physical chemists, materials scientists and engineers, as well as stimulate much-needed exchange among these groups on challenges encountered in the different domains.

Challenges in the Synthesis of Active Site Mimics for [NiFe]-Hydrogenases

One of the more active areas in bioorganometallic chemistry is the preparation and reactivity studies of active site mimics of the [NiFe]-hydrogenases. One area of particular recent progress involves reactions that interconvert Ni(μ-X)Fe centers for X = OH, H, CO, as described by Song et al. Such reactions illustrate new ways to access intermediates related to the Ni-R and Ni-SI states of the enzyme. Most models are derivatives of the type (diphosphine)Ni(SR)2Fe(CO)3-n(PR'3)n. In recent work, the methodology has been generalized to include FeII(diphosphine) derivatives of Ni(N2S2), where N2S22- is the tetradentate diamine-dithiolate (CH2N(CH3)CH2CH2S-)2. Indeed, models based on Ni(N2S2) have proven valuable, but these studies also highlight challenges in working with heterobimetallic complexes, specifically the tendency of some such Ni-Fe complexes to convert to homometalliic Ni-Ni derivatives. This kind of problem is not readily detected by X-ray crystallography. With this caution in mind, we argue that one series of complexes recently described in this journal are almost certainly misassigned.

Nickel(II)-SNS Thiolate Complexes: Reactivity and Solution Dynamics

Nickel coordination chemistry with a biomimetic thiolate-imine-thioether SNS^Me ligand is accompanied by diverse reactivity and multidentate ligand dynamics. Reaction of Ni(acac)₂ with 2 equiv of 2-(methylthio)-phenyl-benzothiazolidine (MPB) affords the bis(arylimino-phenylene-thiolate) complex Ni(κ²-SNS^Me)₂ (1; acac = acetylacetonate). Thermolysis of 1 in refluxing toluene is accompanied by imine C-C bond formation, yielding [Ni(N₂S₂)] (2) with a redox-active ligand. Protonation of 1 with NHTf₂ at a low temperature released 1 equiv of MPB, yielding crystals of the dimeric dication {[Ni(μ-κ³-SNS^Me)]₂}(NTf₂)₂ (3; Tf = SO₂CF₃) in high yield. In contrast, the same reaction at room temperature gave also paramagnetic complexes {Ni[μ-Ni(κ³-SNS^Me)₂]₂}(NTf₂)₂ (4) and {Ni[μ-Ni(κ³-SNS^Me)₂]₃}(NTf₂)₂ (5) that feature coordination of two or three pseudo-octahedral, paramagnetic Ni(κ³-SNS^Me)₂ units to a central Ni(II) dication via thiolate bridges. Remarkably, dissolution of 3 in a variety of solvents, including weakly coordinating CH₂Cl₂, rapidly generates a mixture of 4 and Ni(NTf)₂. Treatment of this mixture with Lewis bases L gave high yields of dimers {[Ni(μ-κ³-SNS^Me)L]₂}(NTf₂)₂ for L = CNXylyl (6a) and {[Ni(μ-κ³-SNS^Me)]₂(μ-dmpm)}(NTf₂)₂ (6b; dmpm = bis(dimethylphosphino)methane) or monomers [Ni(κ³-SNS^Me)L](NTf₂) for L = PMe₃ (7a) and P(OMe)₃ (7b). Addition of 2 equiv of the strong donor N-heterocyclic carbene ligand, IPr, to 3, however, led to thioether demethylation, affording neutral dithiolate complex Ni(κ³-SNS)(IPr) (8). Reaction products were characterized by NMR and mass spectrometry and complexes 1-5, 6a, 6b, 7a, and 8 by single-crystal X-ray diffraction.

Linear and Bent Nitric Oxide Ligand Binding in an Asymmetric Butterfly Complex:

Two synthetic approaches to install metallodithiolate ligands on molybdenum centers using the synthons [Mo₂(CH₃CN)₁₀]⁴⁺ and (N₂S₂)Co(NO) [N₂S₂ = N,N-bis(2-mercaptoethyl)-1,4-diazacycloheptane and NO = nitric oxide], or [Mo(NO)₂(CH₃CN)₄]²⁺ (CH₃CN = acetonitrile) and [(N₂S₂)Co]₂ lead to a bis-nitrosylated, trimetallic dication, CoMoCo'. This unique asymmetric butterfly complex, with S = 1, has a bent NO within the small {Co(NO)}⁸ wing (denoted as Co), reflecting Co^III(NO^-), and is S-bridged to a linear {Mo(NO)}⁶ diamagnetic unit. The latter is further S-bridged to a pentacoordinate (N₂S₂)Co^III(CH₃CN) donor in the larger wing and is the origin of the two unpaired electrons, denoted as Co'. The asymmetry in Mo-Co distances, 3.33 Å in the Co wing and 2.73 Å in the Co' wing, indicated a Mo-Co' bonding interaction. The transfer of NO from (N₂S₂)Co(NO) in the former path is needed to cleave the strong quadruple bond in [Mo≣Mo]⁴⁺, with the energetic cost compensated for via a one-electron bond between Mo and Co', as indicated by natural bonding orbital analysis.

Synthesis of a Series of a Few Hydrosulfide Complexes of Cu(I). A μ3-SH-Bridged Rare Cubane-like Tetramer Showing Efficient Catalytic Activity toward Azide-Alkyne Cycloaddition

A cubane-like tetranuclear hydrosulfido complex of Cu(I), [Cu₄(SH)₄(PPh₃)₄] (1), has been synthesized by the reaction of Cu(NO₃)₂·3H₂O, NaSCOPh, and Cu(PPh₃)₂NO₃ and characterized structurally. Complex 1 represents the first example of crystallographically characterized μ₃-SH-bridged cubanoid hydrosulfide. By direct reactions of [(PPh₃)₂Cu(NO₃)] and NaSH, neutral hydrosulfide complexes [Cu(SH)(PPh₃)₂]·C₆H₆ (2), [Cu₂(SH)₂(PPh₃)₃] (3), and [Cu₂(SH)₂(PPh₃)₄] (4) have also been synthesized and structurally characterized. Complex 2 is monomeric with a terminal hydrosulfide ligand. The other two, 3 and 4, are μ₂-SH-bridged unsymmetrical and symmetrical dinuclear complexes, respectively. In the symmetric one (4), both Cu(I) ions are tetrahedrally coordinated while in the unsymmetric one (3), one Cu(I) ion is tetrahedral and the other one has a trigonal-planar coordination geometry. The catalytic activity of a hydrosulfido complex in a "click" azide-alkyne cycloaddition reaction has been explored for the first time, and complex 1 is found to be an efficient catalyst for the regioselective synthesis of glycoconjugate triazoles.

Metalloligands based on Robson-type amino-thiophenolato macrocycles for assembly of heterotrimetallic complexes

The ability of macrocyclic Co and Ni aminothiolate complexes to act as metalloligands towards cuprate ions was established. Adduct formation is enabled by a thiolate-to-Cu⁺ charge transfer (CT) interaction giving stable heterotrimetallics with magnetic properties.

Mononuclear Mn complexes featuring N,S-/N,N-donor and 1,3,5-triaza-7-phosphaadamantane ligands: synthesis and electrocatalytic properties

Mononuclear Mn(i) carbonyl complexes incorporating 2-mercaptobenzothiazole or 2-mercaptobenzimidazole and phosphaadamantane ligands were evaluated as electrocatalysts for the HER both in acetonitrile and acetonitrile/water.

Biosynthetic Approaches towards the Design of Artificial Hydrogen-Evolution Catalysts

Hydrogen is a clean and sustainable form of fuel that can minimize our heavy dependence on fossil fuels as the primary energy source. The need of finding greener ways to generate H₂ gas has ignited interest in the research community to synthesize catalysts that can produce H₂ by the reduction of H⁺ . The natural H₂ producing enzymes hydrogenases have served as an inspiration to produce catalytic metal centers akin to these native enzymes. In this article we describe recent advances in the design of a unique class of artificial hydrogen evolving catalysts that combine the features of the active site metal(s) surrounded by a polypeptide component. The examples of these biosynthetic catalysts discussed here include i) assemblies of synthetic cofactors with native proteins; ii) peptide-appended synthetic complexes; iii) substitution of native cofactors with non-native cofactors; iv) metal substitution from rubredoxin; and v) a reengineered Cu storage protein into a Ni binding protein. Aspects of key design considerations in the construction of these artificial biocatalysts and insights gained into their chemical reactivity are discussed.

Vibrational characterization of a diiron bridging hydride complex - a model for hydrogen catalysis

A diiron complex containing a bridging hydride and a protonated terminal thiolate of the form [(μ,κ2-bdtH)(μ-PPh2)(μ-H)Fe2(CO)5]+ has been investigated through 57Fe nuclear resonance vibrational spectroscopy (NRVS) and interpreted using density functional theory (DFT) calculations. We report the Fe-μH-Fe wagging mode, and indications for Fe-μD stretching vibrations in the D-isotopologue, observed by 57Fe-NRVS. Our combined approach demonstrates an asymmetric sharing of the hydride between the two iron sites that yields two nondegenerate Fe-μH/D stretching vibrations. The studied complex provides an important model relevant to biological hydrogen catalysis intermediates. The complex mimics proposals for the binuclear metal sites in [FeFe] and [NiFe] hydrogenases. It is also an appealing prototype for the 'Janus intermediate' of nitrogenase, which has been proposed to contain two bridging Fe-H-Fe hydrides and two protonated sulfurs at the FeMo-cofactor. The significance of observing indirect effects of the bridging hydride, as well as obstacles in its direct observation, is discussed in the context of biological hydrogen intermediates.

Synthetic Metallodithiolato Ligands as Pendant Bases in [FeIFeI], [FeI[Fe(NO)]II], and [(μ-H)FeIIFeII] Complexes

The development of ligands with specific stereo- and electrochemical requirements that are necessary for catalyst design challenges synthetic chemists in academia and industry. The crucial aza-dithiolate linker in the active site of [FeFe]-H₂ase has inspired the development of synthetic analogues that utilize ligands which serve as conventional σ donors with pendant base features for H⁺ binding and delivery. Several MN₂S₂ complexes (M = Ni²⁺, [Fe(NO)]²⁺, [Co(NO)]²⁺, etc.) utilize these cis-dithiolates to bind low valent metals and also demonstrate the useful property of hemilability, i.e., alternate between bi- and monodentate ligation. Herein, synthetic efforts have led to the isolation and characterization of three heterotrimetallics that employ metallodithiolato ligand binding to di-iron scaffolds in three redox levels, (μ-pdt)[Fe(CO)₃]₂, (μ-pdt)[Fe(CO)₃][(Fe(NO))^II(IMe)(CO)]⁺, and (μ-pdt)(μ-H)[Fe^II(CO)₂(PMe₃)]₂⁺ to generate (μ-pdt)[(Fe^I(CO)₃][Fe^I(CO)₂·NiN₂S₂] (1), (μ-pdt)[Fe^I(CO)₃][(Fe(NO))^II(IMe)(CO)]⁺ (2), and (μ-pdt)(μ-H)[Fe^II(CO)₂(PMe₃)][Fe^II(CO)(PMe₃)·NiN₂S₂]⁺ (3) complexes (pdt = 1,3-propanedithiolate, IMe = 1,3-dimethylimidazole-2-ylidene, NiN₂S₂ = [N,N'-bis(2-mercaptidoethyl)-1,4-diazacycloheptane] nickel(II)). These complexes display efficient metallodithiolato binding to the di-iron scaffold with one thiolate-S, which allows the free unbound thiolate to potentially serve as a built-in pendant base to direct proton binding, promoting a possible Fe-H^-···⁺H-S coupling mechanism for the electrocatalytic hydrogen evolution reaction (HER) in the presence of acids. Ligand substitution studies on 1 indicate an associative/dissociative type reaction mechanism for the replacement of the NiN₂S₂ ligand, providing insight into the Fe-S bond strength.

A bioinspired thiolate-bridged dinickel complex with a pendant amine: synthesis, structure and electrocatalytic properties

By employing X(CH₂CH₂S^-)₂ (X = S, tpdt; X = O, opdt; X = NPh, npdt) as bridging ligands, four thiolate-bridged dinickel complexes supported by phosphine ligands, [(dppe)Ni(μ-1SSS':2SS-tpdt)Ni(dppe)][PF₆]₂ (1[PF₆]₂, dppe = Ph₂P(CH₂)₂PPh₂), [(dppe)Ni(μ-1SSN:2SS-npdt)Ni(dppe)][PF₆]₂ (2[PF₆]₂) and [(dppe)Ni(t-Cl)(μ-1SSX:2SS-C₄H₈S₂X)Ni(dppe)][PF₆] (3[PF₆], X = S; 4[PF₆], X = O) were facilely obtained by the salt metathesis reaction. These four thiolate-bridged dinickel complexes have all been fully characterized by spectroscopic methods and X-ray crystallography. In 2[PF₆]₂, elongation of the Ni-N bond distance, possibly caused by steric hindrance, indicates that the pendant nitrogen group shuttles between the two nickel centers in solution, which is evidenced by ³¹P{¹H} NMR spectroscopic results. Furthermore, these thiolate-bridged dinickel complexes have all been proved to be electrocatalysts for proton reduction to hydrogen. Notably, complex 2[PF₆]₂ featuring a pendant amine group in the secondary coordination sphere exhibits the best catalytic activity at a relatively low overpotential.

Metal-Templated, Tight Loop Conformation of a Cys-X-Cys Biomimetic Assembles a Dimanganese Complex

With the goal of generating anionic analogues to MN₂ S₂ ⋅Mn(CO)₃ Br we introduced metallodithiolate ligands, MN₂ S₂ ^2- prepared from the Cys-X-Cys biomimetic, ema^4- ligand (ema=N,N'-ethylenebis(mercaptoacetamide); M=Ni^II , [V^IV ≡O]²⁺ and Fe^III ) to Mn(CO)₅ Br. An unexpected, remarkably stable dimanganese product, (H₂ N₂ (CH₂ C=O(μ-S))₂ )[Mn(CO)₃ ]₂ resulted from loss of M originally residing in the N₂ S₂ ^4- pocket, replaced by protonation at the amido nitrogens, generating H₂ ema^2- . Accordingly, the ema ligand has switched its coordination mode from an N₂ S₂ ^4- cavity holding a single metal, to a binucleating H₂ ema^2- with bridging sulfurs and carboxamide oxygens within Mn-μ-S-CH₂ -C-O, 5-membered rings. In situ metal-templating by zinc ions gives quantitative yields of the Mn₂ product. By computational studies we compared the conformations of "linear" ema^4- to ema^4- frozen in the "tight-loop" around single metals, and to the "looser" fold possible for H₂ ema^2- that is the optimal arrangement for binucleation. XRD molecular structures show extensive H-bonding at the amido-nitrogen protons in the solid state.

Recent advances in the mechanisms of the hydrogen evolution reaction by non-innocent sulfur-coordinating metal complexes

This review comments on the homogeneous HER mechanisms for catalysts carrying S-non-innocent ligands in the light of experimental and computational data.

Biomimetic heterobimetallic architecture of Ni(ii) and Fe(ii) for CO2 hydrogenation in aqueous media. A DFT study

In this work, density functional theory has been employed to design a heterobimetallic catalyst of Ni(ii) and Fe(ii) for the effective CO2 hydrogenation to HCOOH. Based on computational results, our newly designed catalyst is found to be effective for such conversion reactions with free energy as low as 14.13 kcal mol-1 for the rate determining step. Such a low value of free energy indicates that the NiFe heterobimetallic catalyst can prove to be very efficient for the above said conversion. Moreover, the effects of ligand substitutions at the active metal center and the effects due to various spin states are also explored, and can serve as a great tool for the rational design of NiFe catalyst for CO2 hydrogenation.

Connecting Main-Group Metals (Al, Ga, In) and Tungsten(0) Carbonyls via the N2S2 Metallo-Ligand Strategy

Tetradentate N2S2 ligands (such as bismercaptoethanediazacycloheptane in this study) have seen extensive use in combination with transition metals. Well-oriented N2S2 binding sites are ideal for d8 transition metals with square planar preferences, especially NiII, but also as a square pyramidal base for those metals with pentacoordinate preferences, such as [V≡O]2+, [Fe(NO)]2+, and [Co(NO)]2+. Further reactivity at the thiolate sulfurs generates diverse bi, tri, and tetra/heterometallic compounds. Few N2S2 ligands have been explored to investigate the possibility of binding to main group metals, especially group III (MIII) metals, and their utility as synthons for main group/transition metal bimetallic complexes. To open up this area of chemistry, we synthesized three new five-coordinate main group XMN2S2 complexes with methyl as the fifth binding ligand for M = Al, and chloride for M = Ga and In. The seven-membered diazacycle, dach, was engaged as a rigid stabilized connector between the terminal thiolate sulfurs. The pentacoordinate XMN2S2 complexes were characterized by 1H-NMR, 13C-NMR, +ESI-Mass spectra, and X-ray diffraction. Their stabilities and reactivities were probed by adding NiII sources and W(CO)5(THF). The former replaces the main group metals in all cases in the N2S2 coordination environment, demonstrating the weak coordinate bonds of MIII–N/S. The reaction of XMN2S2 (XM = ClGaIII or ClInIII) with the labile ligand W(0) complex W(CO)5(THF) resulted in Ga/In–W bimetallic complexes with a thiolate S-bridge. The synthesis of XMN2S2 complexes provide examples of MIII–S coordination, especially Al–S, which is relatively rare. The bimetallic Ga/In–S–W complex formation indicates that the nucleophilic ability of sulfur is retained in MIII–S–R, resulting in the ability of main group MIII–N2S2 complexes to serve as metalloligands.

Redesign of a Copper Storage Protein into an Artificial Hydrogenase

We report the construction of an artificial hydrogenase (ArH) by reengineering a Cu storage protein (Cspl) into a Ni-binding protein (NBP) employing rational metalloprotein design. The hypothesis driven design approach involved deleting existing Cu sites of Csp1 and identification of a target tetrathiolate Ni binding site within the protein scaffold followed by repacking the hydrophobic core. Guided by modeling, the NBP was expressed and purified in high purity. NBP is a well-folded and stable construct displaying native-like unfolding behavior. Spectroscopic and computational studies indicated that the NBP bound nickel in a distorted square planar geometry that validated the design. Ni(II)-NBP is active for photo-induced H₂ evolution following a reductive quenching mechanism. Ni(II)-NBP catalyzed H⁺ reduction to H₂ gas electrochemically as well. Analysis of the catalytic voltammograms established a proton-coupled electron transfer (PCET) mechanism. Electrolysis studies confirmed H₂ evolution with quantitative Faradaic yields. Our studies demonstrate an important scope of rational metalloprotein design that allows imparting functions into protein scaffolds that have natively not evolved to possess the same function of the target metalloprotein constructs.

pH Dependent Reversible Formation of a Binuclear Ni2 Metal-Center Within a Peptide Scaffold

A disulfide-bridged peptide containing two Ni2+ binding sites based on the nickel superoxide dismutase protein, {Ni2(SODmds)}, has been prepared. At physiological pH (7.4) it was found that the metal sites are mononuclear with a square planar NOS2 coordination environment with the two sulfur-based ligands derived from cysteinate residues, the nitrogen ligand derived from the amide backbone and a water ligand. Furthermore, S K-edge X-ray absorption spectroscopy indicated that the two cysteinate sulfur atoms ligated to nickel are each protonated. Elevation of the pH to 9.6 results in the deprotonation of the cysteinate sulfur atoms, and yields a binuclear, cysteinate bridged Ni22+ center with each nickel contained in a distorted square planar geometry. At both pH = 7.4 and 9.6 the nickel sites are moderately air sensitive, yielding intractable oxidation products. However, at pH = 9.6 {Ni2(SODmds)} reacts with O2 at an ~3.5-fold faster rate than at pH = 7.4. Electronic structure calculations indicate the reduced reactivity at pH = 7.4 is a result of a reduction in S(3p) character and deactivation of the nucleophilic frontier molecular orbitals upon cysteinate sulfur protonation.

Proton affinity studies of nickel N2S2 complexes and control of aggregation

The thiolate ligands of [NiFe]-H₂ase enzymes have been implicated as proton-binding sites for the reduction/oxidation of H⁺/H₂. This study examines the ligand effect on reactivity of NiN₂S₂ complexes with an array of acids in methanol solution. UV-Vis absorption spectroscopy is utilized to observe the transformation from the monomeric species to a trimetallic complex that is formed after proton-induced ligand dissociation. Nickel complexes with a flexible (propyl and ethyl) N to N linker were found to readily form the trimetallic complex with acids as weak as ammonium (pK_a = 10.9 in methanol). A more constrained nickel complex with a diazacycloheptane N to N linker required stronger acids such as 2,2-dichloroacetic acid (pK_a = 6.38 in methanol) to form the trimetallic complex and featured the formation of an NiN₂S₂H⁺ complex with acetic acid (pK_a = 9.63 in methanol). The most strained ligand, which featured a diazacyclohexane backbone, readily dissociated from the nickel center upon mixture with acids with pK_a ≤ 9.63 and showed no evidence of a trimetallic species with any acid. This research highlights the dramatic differences in reactivity with proton sources that can be imparted by minor alterations to ligand geometry and strain.

Synthetic Designs and Structural Investigations of Biomimetic Ni-Fe Thiolates

Described are the syntheses of several Ni(μ-SR)₂Fe complexes, including hydride derivatives, in a search for improved models for the active site of [NiFe]-hydrogenases. The nickel(II) precursors include (i) nickel with tripodal ligands: Ni(PS₃)^- and Ni(NS₃)^- (PS₃^3- = tris(phenyl-2-thiolato)phosphine, NS₃^3- = tris(benzyl-2-thiolato)amine), (ii) traditional diphosphine-dithiolates, including chiral diphosphine R,R-DIPAMP, (iii) cationic Ni(phosphine-imine/amine) complexes, and (iv) organonickel precursors Ni( o-tolyl)Cl(tmeda) and Ni(C₆F₅)₂. The following new nickel precursor complexes were characterized: PPh₄[Ni(NS₃)] and the dimeric imino/amino-phosphine complexes [NiCl₂(PCH═N^An)]₂ and [NiCl₂(PCH₂NH^An)]₂ (P = Ph₂PC₆H₄-2-). The iron(II) reagents include [CpFe(CO)₂(thf)]BF₄, [Cp*Fe(CO)(MeCN)₂]BF₄, FeI₂(CO)₄, FeCl₂(diphos)(CO)₂, and Fe(pdt)(CO)₂(diphos) (diphos = chelating diphosphines). Reactions of the nickel and iron complexes gave the following new Ni-Fe compounds: Cp*Fe(CO)Ni(NS₃), [Cp(CO)Fe(μ-pdt)Ni(dppbz)]BF₄, [( R,R-DIPAMP)Ni(μ-pdt)(H)Fe(CO)₃]BAr^F₄, [(PCH═N^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH₂NH^An)Ni(μ-pdt)(Cl)Fe(dppbz)(CO)]BF₄, [(PCH═N^An)Ni(μ-pdt)(H)Fe(dppbz)(CO)]BF₄, [(dppv)(CO)Fe(μ-pdt)]₂Ni, {H[(dppv)(CO)Fe(μ-pdt)]₂Ni]}BF₄, and (C₆F₅)₂Ni(μ-pdt)Fe(CO)₂(dppv) (DIPAMP = (CH₂P(C₆H₄-2-OMe)₂)₂; BAr^F₄^- = [B(C₆H₃-3,5-(CF₃)₂]₄^-)) Within the context of Ni-(SR)₂-Fe complexes, these new complexes feature new microenvironments for the nickel center: tetrahedral Ni, chirality, imine, and amine coligands, and Ni-C bonds. In the case of {H[(dppv)(CO)Fe(μ-pdt)]₂Ni}⁺, four low-energy isomers are separated by ≤3 kcal/mol, one of which features a biomimetic HNi(SR)₄ site, as supported by density functional theory calculations.

Bimetallic nickel-cobalt hydrides in H2 activation and catalytic proton reduction

The synergism of the electronic properties of nickel and cobalt enables bimetallic NiCo complexes to process H2. The nickel-cobalt hydride [(dppe)Ni(pdt)(H)CoCp*]+ ([1H]+ ) arising from protonation of the reduced state 1 was found to be an efficient electrocatalyst for H2 evolution with Cl2CHCOOH, and the oxidized [Ni(ii)Co(iii)]2+ form is capable of activating H2 to produce [1H]+ . The features of stereodynamics, acid-base properties, redox chemistry and reactivity of these bimetallic NiCo complexes in processing H2 are potentially related to the active site of [NiFe]-H2ases.