Synthesis and Sulfur Oxygenation of a (N3S)Ni Complex Related to Nickel-Containing Superoxide Dismutase

A Nonheme Iron(III) Superoxide Complex Leads to Sulfur Oxygenation

A new alkylthiolate-ligated nonheme iron complex, FeII(BNPAMe2S)Br (1), is reported. Reaction of 1 with O2 at -40 °C, or reaction of the ferric form with O2•- at -80 °C, gives a rare iron(III)-superoxide intermediate, [FeIII(O2)(BNPAMe2S)]+ (2), characterized by UV-vis, 57Fe Mössbauer, ATR-FTIR, EPR, and CSIMS. Metastable 2 then converts to an S-oxygenated FeII(sulfinate) product via a sequential O atom transfer mechanism involving an iron-sulfenate intermediate. These results provide evidence for the feasibility of proposed intermediates in thiol dioxygenases.

Pharmaceutical manifestation of Knoevenagel condensed metal (II) complexes through virtual, in vitro and in vivo assessments

Sulphur containing compounds possess a great deal of interest due to wide range of beneficial activities towards biotic species. This work also deals with the study of biological examination of newly synthesized sulphur containing Cu(II) and Zn(II) complexes derived from (E)-4-(phenylimino)-3-((E)-1-(phenylimino)ethyl)pent-2-ene-1-thiol Schiff bases. Moreover, the DNA nuclease efficiency of the synthesized metal complexes is studied by UV absorption studies, Fluorescence studies, Viscosity and CV titrations which confirm the intercalative mode of binding. Pharmacokinetic studies and drug like activity of these compounds are screened with the help of SWISS ADME online freeware. Their morphological nature is corroborated by various spectral techniques. Optimized geometry and biologically accessible nature of the synthesized compounds are investigated by Gaussian 09 W software. Interestingly, molecular docking studies are carried out against cancer DNA and 6J10 cancer cell. Anti-inflammatory and in vitro antioxidant activities have been studied to validate the theoretical prediction. Based on these preliminary pharmacological activities, the in vitro cytotoxicity and in vivo antitumor activities are examined using MCF-7, HeLa, Hep-2, HepG-2 and Ehrlich ascites carcinoma (EAC) cell lines. All the above examinations reveal that the nitro substituted transition metal complexes possess higher biological bustle.Communicated by Ramaswamy H. Sarma.

A Series of Ni Complexes Based on a Versatile ATCUN-Like Tripeptide Scaffold to Decipher Key Parameters for Superoxide Dismutase Activity

The cellular level of reactive oxygen species (ROS) has to be controlled to avoid some pathologies, especially those linked to oxidative stress. One strategy for designing antioxidants consists of modeling natural enzymes involved in ROS degradation. Among them, nickel superoxide dismutase (NiSOD) catalyzes the dismutation of the superoxide radical anion, O₂^•-, into O₂ and H₂O₂. We report here Ni complexes with tripeptides derived from the amino-terminal Cu^II- and Ni^II-binding (ATCUN) motif that mimics some structural features found in the active site of the NiSOD. A series of six mononuclear Ni^II complexes were investigated in water at physiological pH with different first coordination spheres, from compounds with a N3S to N2S2 set, and also complexes that are in equilibrium between the N-coordination (N3S) and S-coordination (N2S2). They were fully characterized by a combination of spectroscopic techniques, including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations and their redox properties studied by cyclic voltammetry. They all display SOD-like activity, with a k_cat ranging between 0.5 and 2.0 × 10⁶ M^-1 s^-1. The complexes in which the two coordination modes are in equilibrium are the most efficient, suggesting a beneficial effect of a nearby proton relay.

Ethereal Hydroperoxides: Powerful Reagents for S-Oxygenation of Bridging Thiophenolate Functions

S-Oxygenation of thiophenolate bridges by ethereal hydroperoxides was studied. [Ni^II₂L^S(PhCO₂)]⁺ (1), where L^S = macrocyclic aminethiolate supporting ligand, is S-oxygenated readily in a mixed methanol/acetonitrile solution with ether/dioxygen at room temperature in the presence of daylight. The reactions were found to depend strongly on the choice of the ether. Uptake of two O atoms occurs in dioxane to give a mixed thiolate/sulfinate complex [Ni^II₂L^SO₂(PhCO₂)]⁺ (2) containing the rare five-membered Ni(μ_1,1-S)(μ_1,2-OS)Ni core. In tetrahydrofuran, four O atoms are taken up by 1 to generate the bis(sulfinate) species [Ni^II₂L^SO₄(PhCO₂)]⁺ (3). A mono-S-oxygenated sulfenate intermediate can be detected by electrospray ionization mass spectrometry. The oxygenation reactions proceed in high yields without complex disintegration and invariably provide μ_1,2-bridging sulfinates as established by spectroscopy (IR and UV/vis), X-ray crystallography, and accompanying density functional theory calculations. The oxygenation of the S atoms has a strong impact on the electronic structures of the nickel complexes. The monosulfinate complex 2 has an S = 2 ground state resulting from moderate ferromagnetic exchange coupling interactions (J = +15.7 cm^-1; H = -2JS₁S₂), while an antiferromagnetic exchange interaction in 3 shows the presence of a ground state with spin S = 0 (J = -0.56 cm^-1).

Photooxidation Reactions of Cyclometalated Palladium(II) and Platinum(II) Complexes

New C,N,S-cyclometalated palladium(II) and platinum(II) complexes have been synthesized and their structural, electrochemical, and photochemical properties examined. The blue color of these complexes in solution changed to yellow under visible-light irradiation. By measurement of the absorption spectra for quantifying changes in color, isosbestic points for each complex clearly indicated the presence of only two species responsible for the change of color. X-ray analysis revealed that the visible-light-induced yellow species were S-oxygenated sulfinato complexes. Photosensitized generation of singlet oxygen (¹O₂) was confirmed by the direct detection of singlet oxygen luminescence at 1275 nm. The present cyclometalated palladium(II) and platinum(II) complexes are efficient photosensitizers of singlet oxygen, which rapidly reacts with coordinating sulfur atoms.

Mononuclear Ni(II) Complexes with a S3O Coordination Sphere Based on a Tripodal Cysteine-Rich Ligand: pH Tuning of the Superoxide Dismutase Activity

The superoxide dismutase (SOD) activity of mononuclear Ni^II complexes, whose structures are inspired by the NiSOD, has been investigated. They have been designed with a sulfur-rich pseudopeptide ligand, derived from nitrilotriacetic acid (NTA), where the three acid functions are grafted with cysteines (L^3S). Two mononuclear complexes, which exist in pH-dependent proportions, have been fully characterized by a combination of spectroscopic techniques including ¹H NMR, UV-vis, circular dichroism, and X-ray absorption spectroscopy, together with theoretical calculations. They display similar square-planar S3O coordination, with the three thiolates of the three cysteine moieties from L^3S coordinated to the Ni^II ion, together with either a water molecule at physiological pH, as [NiL^3S(OH₂)]^-, or a hydroxo ion in more basic conditions, as [NiL^3S(OH)]^2-. The ¹H NMR study has revealed that contrary to the hydroxo ligand, the bound water molecule is labile. The cyclic voltammogram of both complexes displays an irreversible one-electron oxidation process assigned to the Ni^II/Ni^III redox system with E_pa = 0.48 and 0.31 V versus SCE for NiL^3S(OH₂) and NiL^3S(OH), respectively. The SOD activity of both complexes has been tested. On the basis of the xanthine oxidase assay, an IC₅₀ of about 1 μM has been measured at pH 7.4, where NiL^3S(OH₂) is mainly present (93% of the Ni^II species), while the IC₅₀ is larger than 100 μM at pH 9.6, where NiL^3S(OH) is the major species (92% of the Ni^II species). Interestingly, only NiL^3S(OH₂) displays SOD activity, suggesting that the presence of a labile ligand is required. The SOD activity has been also evaluated under catalytic conditions at pH 7.75, where the ratio between NiL^3S(OH₂)/ NiL^3S(OH) is about (86:14), and a rate constant, k_cat = 1.8 × 10⁵ M^-1 s^-1, has been measured. NiL^3S(OH₂) is thus the first low-molecular weight, synthetic, bioinspired Ni complex that displays catalytic SOD activity in water at physiological pH, although it does not contain any N-donor ligand in its first coordination sphere, as in the NiSOD. Overall, the data show that a key structural feature is the presence of a labile ligand in the coordination sphere of the Ni^II ion.

The Role of Mixed Amine/Amide Ligation in Nickel Superoxide Dismutase

Superoxide dismutases (SODs) utilize a ping-pong mechanism in which a redox-active metal cycles between oxidized and reduced forms that differ by one electron to catalyze the disproportionation of superoxide to dioxygen and hydrogen peroxide. Nickel-dependent SOD (NiSOD) is a unique biological solution for controlling superoxide levels. This enzyme relies on the use of cysteinate ligands to bring the Ni(III/II) redox couple into the range required for catalysis (∼300 mV vs. NHE). The use of cysteine thiolates, which are not found in any other SOD, is a curious choice because of their well-known oxidation by peroxide and dioxygen. The NiSOD active site cysteinate ligands are resistant to oxidation, and prior studies of synthetic and computational models point to the backbone N-donors in the active site (the N-terminal amine and the amide N atom of Cys2) as being involved in stabilizing the cysteines to oxidation. To test the role of the backbone N-donors, we have constructed a variant of NiSOD wherein an alanine residue was added to the N-terminus (Ala0-NiSOD), effectively altering the amine ligand to an amide. X-ray absorption, electronic absorption, and magnetic circular dichroism (MCD) spectroscopic analyses of as-isolated Ala0-NiSOD coupled with density functional theory (DFT) geometry optimized models that were evaluated on the basis of the spectroscopic data within the framework of DFT and time-dependent DFT computations are consistent with a diamagnetic Ni(II) site with two cysteinate, one His1 amide, and one Cys2 amidate ligands. The variant protein is catalytically inactive, has an altered electronic absorption spectrum associated with the nickel site, and is sensitive to oxidation. Mass spectrometric analysis of the protein exposed to air shows the presence of a mixture of oxidation products, the principal ones being a disulfide, a bis-sulfenate, and a bis-sulfinate derived from the active site cysteine ligands. Details of the electronic structure of the Ni(III) site available from the DFT calculations point to subtle changes in the unpaired spin density on the S-donors as being responsible for the altered sensitivity of Ala0-NiSOD to O₂.

Streams, cascades, and pools: various water cluster motifs in structurally similar Ni(ii) complexes

Hydrogen bonding (HB) interactions are well known to impact the properties of water in the bulk and within hydrated materials.

Steric Enforcement about One Thiolate Donor Leads to New Oxidation Chemistry in a NiSOD Model Complex

Ni-containing superoxide dismutase (NiSOD) represents an unusual member of the SOD family due to the presence of oxygen-sensitive Ni-SCys bonds at its active site. Reported in this account is the synthesis and properties of the Ni^II complex of the N₃S₂ ligand [N₃S₂^Me2]^3- ([N₃S₂^Me2]^3- = deprotonated form of 2-((2-mercapto-2-methylpropyl)(pyridin-2-ylmethyl)amino)-N-(2-mercaptoethyl)acetamide), namely Na[Ni(N₃S₂^Me2)] (2), as a NiSOD model that features sterically robust gem-(CH₃)₂ groups on the thiolate α-C positioned trans to the carboxamide. The crystal structure of 2, coupled with spectroscopic measurements from ¹H NMR, X-ray absorption, IR, UV-vis, and mass spectrometry (MS), reveal a planar Ni^II (S = 0) ion coordinated by only the N₂S₂ basal donors of the N₃S₂ ligand. While the structure and spectroscopic properties of 2 resemble those of NiSOD_red and other models, the asymmetric S ligands open up new reaction paths upon chemical oxidation. One unusual oxidation product is the planar Ni^II-N₃S complex [Ni(L^ox)] (5; L^ox = 2-(5,5-dimethyl-2-(pyridin-2-yl)thiazolidin-3-yl)-N-(2-mercaptoethyl)acetamide), where two-electron oxidation takes place at the substituted thiolate and py-CH₂ carbon to generate a thiazolidine heterocycle. Electrochemical measurements of 2 reveal irreversible events wholly consistent with thiolate redox, which were identified by comparison to the Zn^II complex Na[Zn(N₃S₂^Me2)] (3). Although no reaction is observed between 2 and azide, reaction of 2 with superoxide produces multiple products on the basis of UV-vis and MS data, one of which is 5. Density functional theory (DFT) computations suggest that the HOMO in 2 is π* with primary contributions from Ni-dπ/S-pπ orbitals. These contributions can be modulated and biased toward Ni when electron-withdrawing groups are placed on the thiolate α-C. Analysis of the oxidized five-coordinate species 2^ox* by DFT reveal a singly occupied spin-up (α) MO that is largely thiolate based, which supports the proposed Ni^III-thiolate/Ni^II-thiyl radical intermediates that ultimately yield 5 and other products.

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.

Copper coordinated ligand thioether-S and NO2(-) oxidation: relevance to the CuM site of hydroxylases

In order to gain insight into the coordination site and oxidative activity of the CuM site of hydroxylases such as peptidylglycine α-hydroxylating monooxygenase (PHM), dopamine β-monooxygenase (DβM), and tyramine β-monooxygenase (TβM), we have synthesized, characterized and studied the oxidation chemistry of copper complexes chelated by tridentate N2Sthioether, N2Osulfoxide or N2Osulfone donor sets. The ligands are those of N-2-methylthiophenyl-2'-pyridinecarboxamide (HL1), and the oxidized variants, N-2-methylsulfenatophenyl-2'-pyridinecarboxamide (HL1(SO)), and N-2-methylsulfinatophenyl-2'-pyridinecarboxamide (HL1(SO2)). Our studies afforded the complexes [(L1)Cu(II)(H2O)](ClO4)·H2O (1·H2O), {[(L1(SO))Cu(II)(CH3CN)](ClO4)}n (2), [(L1)Cu(II)(ONO)] (3), [(L1(SO))Cu(II)(ONO)]n (4), [(L1)Cu(II)(NO3)]n (5), [(L1(SO))Cu(II)(NO3)]n (6) and [(L1(SO2))Cu(II)(NO3)] (7). Complexes 1 and 3 were described in a previous publication (Inorg. Chem., 2013, 52, 11084). The X-ray crystal structures revealed either distorted octahedral (in 2, 4-6) or square-pyramidal (in 1, 3) coordination geometry around Cu(II) ions of the complexes. In the presence of H2O2, conversion of 1→2, 3-5→6 and 6→7 occurs quantitatively via oxidation of thioether-S and/or Cu(ii) coordinated NO2(-) ions. Thioether-S oxidation of L1 also occurs when [L1](-) is reacted with [Cu(I)(CH3CN)4](ClO4) in DMF under O2, albeit low in yield (20%). Oxidations of thioether-S and NO2(-) were monitored by UV-Vis spectroscopy. Recovery of the sulfur oxidized ligands from their metal complexes allowed for their characterization by elemental analysis, (1)H NMR, FTIR and mass spectrometry.

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).

Steric and electronic effects on arylthiolate coordination in the pseudotetrahedral complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate)

Synthesis and characterization of several new pseudotetrahedral arylthiolate complexes [(Tp(Ph,Me))Ni-SAr] (Tp(Ph,Me) = hydrotris{3-phenyl-5-methyl-1-pyrazolyl}borate; Ar = Ph, 2,4,6-(i)Pr3C6H2, C6H4-4-Cl, C6H4-4-Me, C6H4-4-OMe) are reported, including X-ray crystal structures of the first two complexes. With prior results, two series of complexes are spanned, [(Tp(Ph,Me))Ni-S-2,4,6-RC6H2] (R'' = H, Me, (i)Pr) plus the xylyl analogue [(Tp(Ph,Me))Ni-S-2,6-Me2C6H3], as well as [(Tp(Ph,Me))Ni-S-C6H4-4-Y] (Y = Cl, H, Me, OMe), intended to elucidate steric and/or electronic effects on arylthiolate coordination. In contrast to [(Tp(Me,Me))Ni-SAr] analogues that adopt a sawhorse conformation, the ortho-disubstituted complexes show enhanced trigonal and Ni-S-Ar bending, reflecting the size of the 3-pyrazole substituents. Moreover, weakened scorpionate ligation is implied by spectroscopic data. Little spectroscopic effect is observed in the series of para-substituted complexes, suggesting the observed effects are primarily steric in origin. The relatively electron-rich and encumbered complex [(Tp(Ph,Me))Ni-S-2,4,6-(i)Pr3C6H2] behaves uniquely when dissolved in CH3CN, forming a square planar solvent adduct with a bidentate scorpionate ligand, [(κ(2)-Tp(Ph,Me))Ni(NCMe)(S-2,4,6-(i)Pr3C6H2)]. This adduct was isolated and characterized by X-ray crystallography. Single-point DFT and TD-DFT calculations on a simplified [(κ(2)-Tp)Ni(NCMe)(SPh)] model were used to clarify the electronic spectrum of the adduct, and to elucidate differences between Ni-SAr bonding and spectroscopy between pseudotetrahedral and square planar geometries.

Hexacoordinate nickel(II)/(III) complexes that mimic the catalytic cycle of nickel superoxide dismutase

A functional model complex of nickel superoxide dismutase (NiSOD) with a non-peptide ligand which mimics the full catalytic cycle of NiSOD is unknown. Similarly, it has not been fully elucidated whether NiSOD activity is a result of an outer- or inner-sphere electron-transfer mechanism. With this in mind, two octahedral nickel(II)/(III) complexes of a bis-tridentate N2 S donor carboxamide ligand, N-2-phenylthiophenyl-2'-pyridinecarboxamide (HL(Ph)), have been synthesized, structurally characterized, and their SOD activities examined. These complexes mimic the full catalytic cycle of NiSOD. Electrochemical experiments support an outer-sphere electron-transfer mechanism for their SOD activity.

Thioether-ligated iron(II) and iron(III)-hydroperoxo/alkylperoxo complexes with an H-bond donor in the second coordination sphere

The non-heme iron complexes, [Fe(II)(N3PySR)(CH3CN)](BF4)2 () and [Fe(II)(N3Py(amide)SR)](BF4)2 (), afford rare examples of metastable Fe(iii)-OOH and Fe(iii)-OOtBu complexes containing equatorial thioether ligands and a single H-bond donor in the second coordination sphere. These peroxo complexes were characterized by a range of spectroscopic methods and density functional theory studies. The influence of a thioether ligand and of one H-bond donor on the stability and spectroscopic properties of these complexes was investigated.

Synthesis and ligand non-innocence of thiolate-ligated (N4S) Iron(II) and nickel(II) bis(imino)pyridine complexes

The known iron(II) complex [Fe(II)(LN3S)(OTf)] (1) was used as starting material to prepare the new biomimetic (N4S(thiolate)) iron(II) complexes [Fe(II)(LN3S)(py)](OTf) (2) and [Fe(II)(LN3S)(DMAP)](OTf) (3), where LN3S is a tetradentate bis(imino)pyridine (BIP) derivative with a covalently tethered phenylthiolate donor. These complexes were characterized by X-ray crystallography, ultraviolet-visible (UV-vis) spectroscopic analysis, (1)H nuclear magnetic resonance (NMR), and Mössbauer spectroscopy, as well as electrochemistry. A nickel(II) analogue, [Ni(II)(LN3S)](BF4) (5), was also synthesized and characterized by structural and spectroscopic methods. Cyclic voltammetric studies showed 1-3 and 5 undergo a single reduction process with E(1/2) between -0.9 V to -1.2 V versus Fc(+)/Fc. Treatment of 3 with 0.5% Na/Hg amalgam gave the monoreduced complex [Fe(LN3S)(DMAP)](0) (4), which was characterized by X-ray crystallography, UV-vis spectroscopic analysis, electron paramagnetic resonance (EPR) spectroscopy (g = [2.155, 2.057, 2.038]), and Mössbauer (δ = 0.33 mm s(-1); ΔE(Q) = 2.04 mm s(-1)) spectroscopy. Computational methods (DFT) were employed to model complexes 3-5. The combined experimental and computational studies show that 1-3 are 5-coordinate, high-spin (S = 2) Fe(II) complexes, whereas 4 is best described as a 5-coordinate, intermediate-spin (S = 1) Fe(II) complex antiferromagnetically coupled to a ligand radical. This unique electronic configuration leads to an overall doublet spin (S(total) = 1/2) ground state. Complexes 2 and 3 are shown to react with O2 to give S-oxygenated products, as previously reported for 1. In contrast, the monoreduced 4 appears to react with O2 to give a mixture of sulfur oxygenates and iron oxygenates. The nickel(II) complex 5 does not react with O2, and even when the monoreduced nickel complex is produced, it appears to undergo only outer-sphere oxidation with O2.

Addition of dioxygen to an N4S(thiolate) iron(II) cysteine dioxygenase model gives a structurally characterized sulfinato-iron(II) complex

The non-heme iron enzyme cysteine dioxygenase (CDO) catalyzes the S-oxygenation of cysteine by O(2) to give cysteine sulfinic acid. The synthesis of a new structural and functional model of the cysteine-bound CDO active site, [Fe(II)(N3PyS)(CH(3)CN)]BF(4) (1) is reported. This complex was prepared with a new facially chelating 4N/1S(thiolate) pentadentate ligand. The reaction of 1 with O(2) resulted in oxygenation of the thiolate donor to afford the doubly oxygenated sulfinate product [Fe(II)(N3PySO(2))(NCS)] (2), which was crystallographically characterized. The thiolate donor provided by the new N3PyS ligand has a dramatic influence on the redox potential and O(2) reactivity of this Fe(II) model complex.

In vivo and in vitro evaluation of highly specific thiolate carrier group copper(II) and zinc(II) complexes on Ehrlich ascites carcinoma tumor model

A new series of copper(II) and zinc(II) complexes have been designed and synthesized using a new type of Schiff bases derived from the reaction of 3-(3-phenyl-allylidene)-pentane-2,4-dione with para substituted aniline and benzene-1,2-dithiol. Their structures have been established by analytical and spectral data. The higher ɛ and low A(‖) values together with positive reduction potentials for these copper complexes suggest that they can mimic the functional properties of naturally occurring proteins. In vivo and in vitro antitumor functions of the complexes against Ehrlich ascites carcinoma tumor model have been investigated. The minimum inhibitory concentration of the complexes has also been investigated against Mycobacterium tuberculosis strain H37Rv. These complexes exhibit significant antitumor, cytotoxic and antituberculosis activity.

Bisamidate and mixed amine/amidate NiN2S2 complexes as models for nickel-containing acetyl coenzyme A synthase and superoxide dismutase: an experimental and computational study

The distal nickel site of acetyl-CoA synthase (Ni(d)-ACS) and reduced nickel superoxide dismutase (Ni-SOD) display similar square-planar Ni(II)N(2)S(2) coordination environments. One difference between these two sites, however, is that the nickel ion in Ni-SOD contains a mixed amine/amidate coordination motif while the Ni(d) site in Ni-ACS contains a bisamidate coordination motif. To provide insight into the consequences of the different coordination environments on the properties of the Ni ions, we systematically examined two square-planar Ni(II)N(2)S(2) complexes, one with bisthiolate-bisamidate ligation (Et(4)N)(2)(Ni(L1)).2H(2)O (2) [H(4)L1 = N-(2-mercaptoacetyl)-N'-(2-mercaptoethyl)glycinamide] and another with bisthiolate-amine/amidate ligation K(Ni(HL2)) (3) [H(4)L2 = N-(2''-mercaptoethyl)-2-((2'-mercaptoethyl)amino)acetamide]. Although these two complexes differ only by a single amine versus amidate ligand, their chemical properties are quite different. The stronger in-plane ligand field in the bisamidate complex (Ni(II)(L1))(2-) (2) results in an increase in the energies of the d --> d transitions and a considerably more negative oxidation potential. Furthermore, while the bisamidate complex (Ni(II)(L1))(2-) (2) readily forms a trinuclear species (Et(4)N)(2)({Ni(L1)}(2)Ni).H(2)O (1) and reacts rapidly with O(2), presumably via sulfoxidation, the mixed amine/amidate complex (Ni(II)(HL2))(-) (3) remains monomeric and is stable for days in air. Interestingly, the Ni(III) species of the bisamidate complex formed by chemical oxidation with I(2) can be detected by electron paramagnetic resonance (EPR) spectroscopy while the mixed amine/amidate complex immediately decomposes upon oxidation. To explain these experimentally observed properties, we performed S K-edge X-ray absorption spectroscopy and low-temperature (77 K) electronic absorption measurements as well as both hybrid density functional theory (hybrid-DFT) and spectroscopy oriented configuration interaction (SORCI) calculations. These studies demonstrate that the highest occupied molecular orbital (HOMO) of the bisamidate complex (Ni(II)(L1))(2-) (2) has more Ni character and is significantly destabilized relative to the mixed amine/amidate complex (Ni(II)(HL2))(-) (3) by approximately 6.2 kcal mol(-1). The consequence of this destabilization is manifested in the nucleophilic activation of the doubly filled HOMO, which makes (Ni(II)(L1))(2-) (2) significantly more reactive toward electrophiles such as O(2).