Molecular and Electronic Structures of One-Electron Oxidized NiII–(Dithiosalicylidenediamine) Complexes: NiIII–Thiolate versus NiII–Thiyl Radical States

Introducing the penicillamine moiety into a metallopeptide mimicking the NiSOD enzyme: electronic and kinetic effects

The novel NiSOD related metallopeptide incorporates penicillamine moiety in the active center which alters both the electronic and kinetic features.

Stable thiolate adducts of Rh2(OAc)4 - assembly of hexametallic Ni4Rh2 complexes

Thiolate adducts of dirhodium(II) tetraacetate have proven difficult to prepare. We isolated a stable, paramagnetic Ni₄Rh₂ adduct containing Ni-based metallothiolates bound in axial positions of the Rh₂⁴⁺ core. The adduct formation is accompanied by a change of the magnetic exchange interaction in the dinuclear Ni₂ subunits.

Uranium Going the Soft Way: Low-Valent Uranium(III) Coordinated to an Arene-Anchored Tris-Thiophenolate Ligand

The synthesis of a tripodal, S-based ligand, namely the mesitylene-anchored, tris-thiophenolate-functionalized (mes(^Me,AdArS)₃)^3- (1)^3-, and its coordination chemistry with low-valent uranium to form [U^III((SAr^Ad,Me)₃mes)] (1-U) are reported. Single-crystal X-ray diffraction analysis reveals a C₃-symmetric molecular structure. Full characterization of 1-U was performed using nuclear magnetic resonance, UV-vis-NIR electronic absorption, and electron paramagnetic resonance spectroscopies as well as SQUID magnetometry, thus confirming the U(III) oxidation state. Alternating current magnetic studies show that 1-U exhibits single-molecule magnet behavior at low temperatures in a non-zero external field. Comparison of these results to those of the previously reported mesitylene-anchored complexes, [U^III((OAr^Ad,Me)₃mes)] and [U^III((OAr^tBu,tBu)₃mes)], indicates a drastic change in the electronic structure when moving from phenolate-based ligands to thiophenolate-based 1, which is further discussed by means of computational analysis (NBO, DFT, and QTAIM). Despite the U-O bonds being stronger, a much higher covalency was found for the U-S analogue.

Tuning of Thiyl/Thiolate Complex Near-Infrared Chromophores of Platinum through Geometrical Constraints

The chemistry of radical-ligand complexes of the transition metals has developed into a vibrant field of research that spans from fundamental studies on the relationship between the chemical and electronic structures to applications in catalysis and functional materials chemistry. In general, fine-tuning of the relevant properties relies on an increasingly diversifying pool of radical-proligand structures. Surprisingly, the variability of the conformational freedom and the number of distinct bonding modes supported by many radical proligands is limited. This work reports on the angular constraints and relative geometric alignment of metal and ligand orbitals as key parameters that render a series of chemically similar thiyl/thiolate complexes of platinum(II) electronically and spectroscopically distinct. The use of conformational flexible thiophenols as primary ligand scaffolds is essential to establishing a defined radical-ligand [(^areneS)₂Pt^II]^•+ core whose electronic structure is modulated by a series of auxiliary coligands at platinum.

Heterobimetallic [NiFe] Complexes Containing Mixed CO/CN- Ligands: Analogs of the Active Site of the [NiFe] Hydrogenases

The development of synthetic analogs of the active sites of [NiFe] hydrogenases remains challenging, and, in spite of the number of complexes featuring a [NiFe] center, those featuring CO and CN^- ligands at the Fe center are under-represented. We report herein the synthesis of three bimetallic [NiFe] complexes [Ni( N₂ S₂)Fe(CO)₂(CN)₂], [Ni( S₄)Fe(CO)₂(CN)₂], and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] that each contain a Ni center that bridges through two thiolato S donors to a {Fe(CO)₂(CN)₂} unit. X-ray crystallographic studies on [Ni( N₂ S₃)Fe(CO)₂(CN)₂], supported by DFT calculations, are consistent with a solid-state structure containing distinct molecules in the singlet ( S = 0) and triplet ( S = 1) states. Each cluster exhibits irreversible reduction processes between -1.45 and -1.67 V vs Fc⁺/Fc and [Ni( N₂ S₃)Fe(CO)₂(CN)₂] possesses a reversible oxidation process at 0.17 V vs Fc⁺/Fc. Spectroelectrochemical infrared (IR) and electron paramagnetic resonance (EPR) studies, supported by density functional theory (DFT) calculations, are consistent with a Ni^IIIFe^II formulation for [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺. The singly occupied molecular orbital (SOMO) in [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ is based on Ni 3d_z² and 3p S with the S contributions deriving principally from the apical S-donor. The nature of the SOMO corresponds to that proposed for the Ni-C state of the [NiFe] hydrogenases for which a Ni^IIIFe^II formulation has also been proposed. A comparison of the experimental structures, and the electrochemical and spectroscopic properties of [Ni( N₂ S₃)Fe(CO)₂(CN)₂] and its [Ni( N₂ S₃)] precursor, together with calculations on the oxidized [Ni( N₂ S₃)Fe(CO)₂(CN)₂]⁺ and [Ni( N₂ S₃)]⁺ forms suggests that the binding of the {Fe(CO)(CN)₂} unit to the {Ni(CysS)₄} center at the active site of the [NiFe] hydrogenases suppresses thiolate-based oxidative chemistry involving the bridging thiolate S donors. This is in addition to the role of the Fe center in modulating the redox potential and geometry and supporting a bridging hydride species between the Ni and Fe centers in the Ni-C state.

Synthesis and Speciation-Dependent Properties of a Multimetallic Model Complex of NiSOD That Exhibits Unique Hydrogen-Bonding

The complex Na₃[{Ni^II(nmp)}₃S₃BTA^alk)] (1) (nmp^2- = deprotonated form of N-(2-mercaptoethyl)picolinamide; H₃S₃BTA^alk = N¹,N³,N⁵-tris(2-mercaptoethyl)benzene-1,3,5-tricarboxamide, where H = dissociable protons), supported by the thiolate-benzenetricarboxamide scaffold (S₃BTA^alk), has been synthesized as a trimetallic model of nickel-containing superoxide dismutase (NiSOD). X-ray absorption spectroscopy (XAS) and ¹H NMR measurements on 1 indicate that the Ni^II centers are square-planar with N₂S₂ coordination, and Ni-N and Ni-S distances of 1.95 and 2.16 Å, respectively. Additional evidence from IR indicates the presence of H-bonds in 1 from the approximately -200 cm^-1 shift in ν_NH from free ligand. The presence of H-bonds allows for speciation that is temperature-, concentration-, and solvent-dependent. In unbuffered water and at low temperature, a dimeric complex (1^A; λ = 410 nm) that aggregates through intermolecular NH···O═C bonds of BTA units is observed. Dissolution of 1 in pH 7.4 buffer or in unbuffered water at temperatures above 50 °C results in monomeric complex (1^M; λ = 367 nm) linked through intramolecular NH···S bonds. DFT computations indicate a low energy barrier between 1^A and 1^M with nearly identical frontier MOs and Ni-ligand metrics. Notably, 1^A and 1^M exhibit remarkable stability in protic solvents such as MeOH and H₂O, in stark contrast to monometallic [Ni^II(nmp)(SR)]^- complexes. The reactivity of 1 with excess O₂, H₂O₂, and O₂^•- is species-dependent. IR and UV-vis reveal that 1^A in MeOH reacts with excess O₂ to yield an S-bound sulfinate, but does not react with O₂^•-. In contrast, 1^M is stable to O₂ in pH 7.4 buffer, but reacts with O₂^•- to yield a putative [Ni^II(nmp)(O₂)]^- complex from release of the BTA-thiolate based on EPR.

Ligand-centred oxidative chemistry in sterically hindered salen complexes: an interesting case with nickel

Salen ligands are ubiquitous chelators, whose nickel complexes readily undergo a ligand-centred redox chemistry in non-coordinating solvents.

Accessing Ni(III)-thiolate versus Ni(II)-thiyl bonding in a family of Ni-N2S2 synthetic models of NiSOD

Superoxide dismutase (SOD) catalyzes the disproportionation of superoxide (O2(• -)) into H2O2 and O2(g) by toggling through different oxidation states of a first-row transition metal ion at its active site. Ni-containing SODs (NiSODs) are a distinct class of this family of metalloenzymes due to the unusual coordination sphere that is comprised of mixed N/S-ligands from peptide-N and cysteine-S donor atoms. A central goal of our research is to understand the factors that govern reactive oxygen species (ROS) stability of the Ni-S(Cys) bond in NiSOD utilizing a synthetic model approach. In light of the reactivity of metal-coordinated thiolates to ROS, several hypotheses have been proffered and include the coordination of His1-Nδ to the Ni(II) and Ni(III) forms of NiSOD, as well as hydrogen bonding or full protonation of a coordinated S(Cys). In this work, we present NiSOD analogues of the general formula [Ni(N2S)(SR')](-), providing a variable location (SR' = aryl thiolate) in the N2S2 basal plane coordination sphere where we have introduced o-amino and/or electron-withdrawing groups to intercept an oxidized Ni species. The synthesis, structure, and properties of the NiSOD model complexes (Et4N)[Ni(nmp)(SPh-o-NH2)] (2), (Et4N)[Ni(nmp)(SPh-o-NH2-p-CF3)] (3), (Et4N)[Ni(nmp)(SPh-p-NH2)] (4), and (Et4N)[Ni(nmp)(SPh-p-CF3)] (5) (nmp(2-) = dianion of N-(2-mercaptoethyl)picolinamide) are reported. NiSOD model complexes with amino groups positioned ortho to the aryl-S in SR' (2 and 3) afford oxidized species (2(ox) and 3(ox)) that are best described as a resonance hybrid between Ni(III)-SR and Ni(II)-(•)SR based on ultraviolet-visible (UV-vis), magnetic circular dichroism (MCD), and electron paramagnetic resonance (EPR) spectroscopies, as well as density functional theory (DFT) calculations. The results presented here, demonstrating the high percentage of S(3p) character in the highest occupied molecular orbital (HOMO) of the four-coordinate reduced form of NiSOD (NiSODred), suggest that the transition from NiSODred to the five-coordinate oxidized form of NiSOD (NiSODox) may go through a four-coordinate Ni-(•)S(Cys) (NiSODox-Hisoff) that is stabilized by coordination to Ni(II).

Iridium-mediated C–S bond activation and transformation: organoiridium(iii) thioether, thiolato, sulfinato and thiyl radical compounds. Synthesis, mechanistic, spectral, electrochemical and theoretical aspects

The iridium-mediated C–S bond scission by an uncommon SET reductive process: exploration of S-centered reactivity of iridium(iii) thiolato complex.

Synthesis and characterization of mononuclear Zn(II), Co(II) and Ni(II) complexes containing a sterically demanding silanethiolate ligand derived from tris(2,6-diisopropylphenoxy)silanethiol

Four heteroleptic complexes of nickel(ii), cobalt(ii) and zinc(ii), containing a monodentate silanethiolate ligand derived from tris(2,6-diisopropylphenoxy)silanethiol (TDST), were prepared and characterized. Nickel(ii) and cobalt(ii) complexes of the formula M(NH3)2(TDST)2 (M = Ni(ii) complex , M = Co(ii) complex ) were obtained from the respective chlorides. Zinc complexes of the general formula Zn(acac)(TDST)(L), where L = EtOH (complex ) or H2O (complex ), were obtained from zinc acetylacetonate. A single-crystal X-ray structural analysis revealed that all crystalline products are solvent adducts. The geometries of ligands in the complexes are typical: distorted tetrahedral in zinc and cobalt(ii) complexes and square planar in nickel(ii) compounds. Magnetic studies performed for Ni(ii) and Co(ii) compounds confirmed the diamagnetic character of the first complex and high-spin paramagnetic configuration of the latter. Nickel(ii) and cobalt(ii) complexes were additionally characterized by UV-Vis and IR spectroscopy. IR bands for ligands in the complexes were assigned with the help of the DFT vibrational frequency calculations.