Heterobimetallics of nickel-iron dinitrosyl: electronic control by chelate and diatomic ligands

Stable Bimetallic FeII/{Fe(NO)2}9 Moiety Derived from Reductive Transformations of a Diferrous-dinitrosyl Species

A dimeric dithiolate-bridged species, [Fe(NO)(PS2)]₂ (1) containing two {FeNO}⁷ units, can be isolated by treating [Fe(CO)₂(NO)₂] with PS2H₂ (PS2H₂ = bis(2-dimercaptophenyl)phenylphosphine). Crystallographic studies reveal the syn-configuration of NO units and the bridging thiolates in the butterfly shape of the 2Fe2S core. Addition of PPh₃ to the solution of dinuclear 1 leads to the formation of mononuclear {FeNO}⁷ [Fe(NO)(PS2)(PPh₃)] (2) that shows electrochemical responses similar to those of 1. One-electron reduction of 1 with Cp*₂Co or KC₈ results in the isolation of thiolate-bridged bimetallic DNIC, [(PS2)Fe(μ-PS2)Fe(NO)₂]^- ([3]^-), confirmed by several spectroscopies including single-crystal X-ray diffraction studies. The bimetallic DNIC [3]^- is a rare example obtained from the one-electron reduction of a dinuclear Fe-NO {FeNO}⁷ model complex. With the assistance of redox behaviors of 2, electrochemical studies imply that the reduction of 1 leads to the formation of a mononuclear {FeNO}⁸ [Fe(NO)(PS2)(THF)]^- intermediate, which involves disproportionation or NO^- transfer to yield [3]^-. Based on IR data and magnetic properties, the electronic structure of [3]^- can be described as a Fe^II/{Fe(NO)₂}⁹ state. Isolation of the {Fe(NO)₂}⁹ moiety coordinated by the Fe ancillary complex lends strong support to the NO scrambling behavior in the effectiveness of the activity of flavodiiron nitric oxide reductases (FNORs).

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

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.

Crystal structure of NiFe(CO)5[tris(pyridyl-meth-yl)aza-phosphatrane]: a synthetic mimic of the NiFe hydrogenase active site incorporating a pendant pyridine base

The synthesis and the crystal structure of a NiFe complex containing an aza­phosphatrane ligand is discussed. The NiFe(TPAP)(CO)₅ complex contains a pendant pyridine base in close proximity to the NiFe metal center.

The reaction of Ni(TPAP)(COD) {where TPAP = [(NC₅H₄)CH₂]₃P(NC₂H₄)₃N} with Fe(CO)₅ resulted in the isolation of the title heterobimetallic NiFe(TPAP)(CO)₅ complex di-μ-carbonyl-tricarbon­yl[2,8,9-tris­(pyridin-2-yl­meth­yl)-2,5,8,9-tetra­aza-1-phosphabi­cyclo­[3.3.3]undeca­ne]ironnickel, [FeNi(C₂₄H₃₀N₇P)(CO)₅]. Characterization of the complex by ¹H and ³¹P NMR as well as IR spectroscopy are presented. The structure of NiFe(TPAP)(CO)₅ reveals three terminally bound CO mol­ecules on Fe⁰, two bridging CO mol­ecules between Ni⁰ and Fe⁰, and TPAP coordinated to Ni⁰. The Ni—Fe bond length is 2.4828 (4) Å, similar to that of the reduced form of the active site of NiFe hydrogenase (∼2.5 Å). Additionally, a proximal pendant base from one of the non-coordinating pyridine groups of TPAP is also present. Although involvement of a pendant base has been cited in the mechanism of NiFe hydrogenase, this moiety has yet to be incorporated in a structurally characterized synthetic mimic with key structural motifs (terminally bound CO or CN ligands on Fe). Thus, the title complex NiFe(TPAP)(CO)₅ is an unique synthetic model for NiFe hydrogenase. In the crystal, the complex mol­ecules are linked by C—H⋯O hydrogen bonds, forming undulating layers parallel to (100). Within the layers, there are offset π–π [inter­centroid distance = 3.2739 (5) Å] and C—H⋯π inter­actions present. The layers are linked by further C—H⋯π inter­actions, forming a supra­molecular framework.

Dioxygen controls the nitrosylation reactions of a protein-bound [4Fe4S] cluster

Iron–sulfur clusters are exceptionally tuneable protein cofactors, and as one of their many roles they are involved in biological responses to nitrosative stress.

Simultaneous nitrosylation and N-nitrosation of a Ni-thiolate model complex of Ni-containing SOD

Nitric oxide (NO) is used as a substrate analogue/spectroscopic probe of metal sites that bind and activate oxygen and its derivatives. To assess the interaction of superoxide with the Ni center in Ni-containing superoxide dismutase (NiSOD), we studied the reaction of NO⁺ and NO with the model complex, Et₄N[Ni(nmp)(SPh-o-NH₂-p-CF₃)] (1; nmp^2- = dianion of N-(2-mercaptoethyl)picolinamide; ^-SPh-o-NH₂-p-CF₃ = 2-amino-4-(trifluoromethyl)benzenethiolate) and its oxidized analogue 1^ox , respectively. The ultimate products of these reactions are the disulfide of ^-SPh-o-NH₂-p-CF₃ and the S,S-bridged tetrameric complex [Ni₄(nmp)₄], a result of S-based redox activity. However, introduction of NO to 1 affords the green dimeric {NiNO}¹⁰ complex (Et₄N)₂[{Ni(κ²-SPh-o-NNO-p-CF₃)(NO)}₂] (2) via NO-induced loss of nmp^2- as the disulfide and N-nitrosation of the aromatic thiolate. Complex 2 was characterized by X-ray crystallography and several spectroscopies. These measurements are in-line with other tetrahedral complexes in the {NiNO}¹⁰ classification. In contrast to the established stability of this metal-nitrosyl class, the Ni-NO bond of 2 is labile and release of NO from this unit was quantified by trapping the NO with a Co^II-porphyrin (70-80% yield). In the process, the Ni ends up coordinated by two o-nitrosaminobenzenethiolato ligands to result in the structurally characterized trans-(Et₄N)₂[Ni(SPh-o-NNO-p-CF₃)₂] (3), likely by a disproportionation mechanism. The isolation and characterization of 2 and 3 suggest that: (i) the strongly donating thiolates dominate the electronic structure of Ni-nitrosyls that result in less covalent Ni-NO bonds, and (ii) superoxide undergoes disproportionation via an outer-sphere mechanism in NiSOD as complexes in the {NiNO}^9/8 state have yet to be isolated.

A matrix of heterobimetallic complexes for interrogation of hydrogen evolution reaction electrocatalysts

Experimental and computational studies address key questions in a structure-function analysis of bioinspired electrocatalysts for the HER. Combinations of NiN2S2 or [(NO)Fe]N2S2 as donors to (η5-C5H5)Fe(CO)+ or [Fe(NO)2]+/0 generate a series of four bimetallics, gradually "softened" by increasing nitrosylation, from 0 to 3, by the non-innocent NO ligands. The nitrosylated NiFe complexes are isolated and structurally characterized in two redox levels, demonstrating required features of electrocatalysis. Computational modeling of experimental structures and likely transient intermediates that connect the electrochemical events find roles for electron delocalization by NO, as well as Fe-S bond dissociation that produce a terminal thiolate as pendant base well positioned to facilitate proton uptake and transfer. Dihydrogen formation is via proton/hydride coupling by internal S-H+···-H-Fe units of the "harder" bimetallic arrangements with more localized electron density, while softer units convert H-···H-via reductive elimination from two Fe-H deriving from the highly delocalized, doubly reduced [Fe2(NO)3]- derivative. Computational studies also account for the inactivity of a Ni2Fe complex resulting from entanglement of added H+ in a pinched -S δ-···H+··· δ-S- arrangement.

Synthetic advances inspired by the bioactive dinitrosyl iron unit

Resulting from biochemical iron-NO interactions, dinitrosyl iron complexes (DNICs) are small organometallic-like molecules, considered to serve as vehicles for NO transport and storage in vivo. Formed by the interaction of NO with cellular iron sulfur clusters or with the cellular labile iron pool, DNICs have been documented to be the largest NO-derived adduct in cells, even surpassing the well-known nitrosothiols (RSNOs). Continuing efforts in biological chemistry are aimed at understanding the movement of DNICs in and out of cells, and their important role in NO-induced iron efflux leading to apoptosis in cells. Intrigued by the integrity of the unique dinitrosyl iron unit (DNIU) and the possibility of roles for it in human physiology or medicinal applications, the understanding of fundamental properties such as ligand effects on its ability to switch between two redox levels has been pursued through biomimetic complexes. Using metallodithiolates and N-heterocyclic carbenes (NHCs) as ligands to Fe(NO)2, the synthesis of a library of novel DNICs, in both the oxidized, {Fe(NO)2}(9), and reduced, {Fe(NO)2}(10), forms (Enemark-Feltham notation), offers opportunity to examine structural, reactivity, and spectroscopic features. The raison d'etre for the MN2S2·Fe(NO)2 synthesis development is for the potential to exploit the ease of accessing two redox levels on two different metal sites, a property presumably required for achieving two electron redox processes in base metals. Hence such molecules may be viewed as synthetic analogues of [NiFe]- or [FeFe]-hydrogenase active sites in nature, both of which use bridging thiolates for connection of the two centers. A particular success was the development of an Fe(NO)N2S2·Fe(NO)2(+/0) redox pair for proton reduction electrocatalysis. Monomeric, reduced NHC-DNICs of the L2Fe(NO)2 type are synthesized via the Fe(CO)2(NO)2 precursor, and oxidized thiolate-containing forms are derived from the dimeric (μ-RS)2[Fe(NO)2]2. Monomeric NHC-DNICs are four coordinate, pseudotetrahedral compounds with planar Fe(NO)2 units in which the slightly bent Fe-NO groups are directed symmetrically inward at both redox levels. They serve as stable analogues of biological histidine binding sites. In agreement with IR data, Mössbauer spectroscopic parameters, and DFT computations, the prototypic NHC-DNICs indicate extensive delocalization of the electron density of iron via π-backbonding. Such π-delocalization presents an unusual reaction path for the one electron process of RS(-)/RSSR interconversion. Comparisons with imidazole-DNICs find NHCs to be the "better" ligands to Fe(NO)2 and prompted investigations in (a) possible relationships between such imidazole- and NHC-containing DNICs, (b) systems that might mimic the reactivity of DNICs with the endogenous gaseotransmitter CO, and (c) mechanistic details of such processes. In a broader context, these studies aim to further describe the behavior of the {Fe(NO)2} unit as a single molecular entity when subjected to various ligand environments and reaction conditions.

Recent Advances in Multinuclear Metal Nitrosyl Complexes

The coordination chemistry of metal nitrosyls has expanded rapidly in the past decades due to major advances of nitric oxide and its metal compounds in biology. This review article highlights advances made in the area of multinuclear metal nitrosyl complexes, including Roussin's salts and their ester derivatives from 2003 to present. The review article focuses on isolated multinuclear metal nitrosyl complexes and is organized into different sections by the number of metal centers and bridging ligands.

Stereochemical diversity of {MNO}(10) complexes: molecular orbital analyses of nickel and copper nitrosyls

The great majority of {NiNO}(10) complexes are characterized by short Ni-N(O) distances of 1.60-1.65 Å and linear NO units. Against this backdrop, the {CuNO}(10) unit in the recently reported [Cu(CH3NO2)5(NO)](2+) cation (1) has a CuNO angle of about 120° and a very long 1.96 Å Cu-N(O) bond. According to DFT calculations, metal-NO bonding in 1 consists of a single Cu(dz(2))-NO(π*) σ-interaction and essentially no metal(dπ)-NO(π*) π-bonding, which explains both the bent CuNO geometry and the long, weak Cu-N(O) bond. This σ-interaction is strongly favored by a ligand trans to the NO; indeed such a trans ligand may be critical for the existence and stability of a {CuNO}(10) unit. By contrast, {NiNO}(10) complexes exhibit a strong avoidance of such trans ligands. Thus, a five-coordinate {NiNO}(10) complex appears to favor a trigonal-bipyramidal structure with the NO in an equatorial position, as in the case of [Ni(bipy)2(NO)](+) (6). An unusual set of Ni(d)-NO(π*) orbital interactions accounts for the strongly bent NiNO geometry for this complex.

The Preparation, Structural Characteristics, and Physical Chemical Properties of Metal-Nitrosyl Complexes

The preparation and characterization of a representative group of novel non-heme metal nitrosyl complexes that have been synthesized over the last decade are discussed here. Their structures are examined and classified based on metal type, the number of metal centers present, and the type of ligand that is coordinated with the metal. The ligands can be phosphorus, nitrogen, or sulfur based (with a few exceptions) and can vary depending on the presence of chelation, intermolecular forces, or the presence of other ligands. Structural and bonding characteristics are summarized and examples of reactivity regarding nitrosyl ligands are given. Some of the relevant physical chemical properties of these complexes, including IR, EPR, NMR, UV-vis, cyclic voltammetry, and X-ray crystallography are examined.

The useful properties of H2O as a ligand of a hydrogenase mimic

This paper investigates the required properties of Ru-coordinated ligands of a Ni-Ru based hydrogenase mimic. A series of ligands, including MeCN, pyridine, H(2)O and OH(-) were coordinated to Ru, with H(2)O being the only ligand to promote H(2)-activation. In addition, a tethered pyridyl moiety was synthesised and found to completely inhibit H(2)-activation. We conclude, therefore, that H(2)O is the ideal ligand for this mimic as a result of both its mild basicity and the availability of two lone pairs for simultaneous binding to Ru and H(2).

A model for the CO-inhibited form of [NiFe] hydrogenase: synthesis of CO3Fe(micro-StBu)3Ni{SC6H3-2,6-(mesityl)2} and reversible CO addition at the Ni site

A [NiFe] hydrogenase model compound having a distorted trigonal-pyramidal nickel center, (CO)(3)Fe(micro-S(t)Bu)(3)Ni(SDmp), 1 (Dmp = C(6)H(3)-2,6-(mesityl)(2)), was synthesized from the reaction of the tetranuclear Fe-Ni-Ni-Fe complex [(CO)(3)Fe(micro-S(t)Bu)(3)Ni](2)(micro-Br)(2), 2 with NaSDmp at -40 degrees C. The nickel site of complex 1 was found to add CO or CN(t)Bu at -40 degrees C to give (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CO)(SDmp), 3, or (CO)(3)Fe(S(t)Bu)(micro-S(t)Bu)(2)Ni(CN(t)Bu)(SDmp), 4, respectively. One of the CO bands of 3, appearing at 2055 cm(-1) in the infrared spectrum, was assigned as the Ni-CO band, and this frequency is comparable to those observed for the CO-inhibited forms of [NiFe] hydrogenase. Like the CO-inhibited forms of [NiFe] hydrogenase, the coordination of CO at the nickel site of 1 is reversible, while the CN(t)Bu adduct 4 is more robust.

Synthesis, structures, spectroscopic and electrochemical properties of dinitrosyl iron complexes with bipyridine, terpyridine, and 1,10-phenathroline

Three new dinitrosyl iron complexes LFe(NO)(2) (L = 2,2'-bipyridine (bipy) (1), 2,2',2''-terpyridine (terpy) (2) and 1,10-phenathroline (phen) (3)) were synthesized by the reaction of Fe(NO)(2)(CO)(2) with corresponding ligands in tetrahydrofuran. Complexes 1-3 were studied using IR, UV-vis, MS, NMR, and electrochemical techniques. Complexes 1 and 2 were also characterized using single crystal X-ray diffraction analysis. IR spectra of complexes 1-3 display two strong characteristic NO stretching frequencies (nu(NO)) in the region reflecting donor properties of the ligands. Cyclic voltammetry studies show two quasi-reversible one-electron reductions for all complexes. Electrochemical investigations using different concentrations show that an irreversible one-electron reduction at -1.85 V for complex 2 and -1.80 V for complex 3 are from solvated species. Single-crystal X-ray structural analysis reveals that complex 1 crystallizes in the triclinic P1 space group and the asymmetric unit consists of one Fe(NO)(2)(bipy) molecule with the two NO groups located on two sides of Fe(bipy) plane. Complex 2 crystallizes in monoclinic P21/n space group, and the asymmetric unit contains one Fe(NO)(2)(terpy) molecule, in which the NO groups are located on two sides of the plane consisted of Fe and two coordinated pyridyl rings, but almost parallel to the uncoordinated pyridyl ring. The crystal packings of both complexes 1 and 2 show intermolecular H-bonding and strong pi-pi stacking interactions.

Dithiolate-bridged Fe-Ni-Fe trinuclear complexes consisting of Fe(CO)(3-n)(CN)(n) (n = 0, 1) components relevant to the active site of [NiFe] hydrogenase

A dithiolate-bridged Fe-Ni-Fe trinuclear carbonyl complex [(CO)(3)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(3)] (1, ndt = norbornane-exo-2,3-dithiolate) has been synthesized from the reaction of [Fe(CO)(4)I(2)] and Li(2)[Ni(ndt)(2)]. This reaction was found to occur with concomitant formation of a tetranuclear cluster [Ni(3)(mu-ndt)(4)FeI] (2). Treatment of 1 with Na[N(SiMe(3))(2)] transforms some of the CO ligands into CN(-), and the monocyanide complex (PPh(4))[(CO)(2)(CN)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(3)] (3) and the dicyanide complex (PPh(4))(2)[(CO)(2)(CN)Fe(mu-ndt)Ni(mu-ndt)Fe(CO)(2)(CN)] (4) were isolated. X-ray structural analyses of the trinuclear complexes revealed a Fe-Ni-Fe array in which the metal centers are connected by the ndt sulfur bridges and direct Fe-Ni bonds. Hydrogen bonding between the CN ligand in 3 and cocrystallized ethanol was found in the solid-state structure. The monocyanide complex 3 and dicyanide complex 4 reacted with acids such as HOTf or HCl generating insoluble materials, whereas complex 1 did not react.

Probing intermediates in the activation cycle of [NiFe] hydrogenase by infrared spectroscopy: the Ni-SIr state and its light sensitivity

The [NiFe] hydrogenase from the sulphate-reducing bacterium Desulfovibrio vulgaris Miyazaki F is reversibly inhibited in the presence of molecular oxygen. A key intermediate in the reactivation process, Ni-SI_r, provides the link between fully oxidized (Ni-A, Ni-B) and active (Ni-SI_a, Ni-C and Ni-R) forms of hydrogenase. In this work Ni-SI_r was found to be light-sensitive (T ≤ 110 K), similar to the active Ni-C and the CO-inhibited states. Transition to the final photoproduct state (Ni-SL) was shown to involve an additional transient light-induced state (Ni-SI₁₉₆₁). Rapid scan kinetic infrared measurements provided activation energies for the transition from Ni-SL to Ni-SI_r in protonated as well as in deuterated samples. The inhibitor CO was found not to react with the active site of the Ni-SL state. The wavelength dependence of the Ni-SI_r photoconversion was examined in the range between 410 and 680 nm. Light-induced effects were associated with a nickel-centred electronic transition, possibly involving a change in the spin state of nickel (Ni²⁺). In addition, at T ≤ 40 K the CN⁻ stretching vibrations of Ni-SL were found to be dependent on the colour of the monochromatic light used to irradiate the species, suggesting a change in the interaction of the hydrogen-bonding network of the surrounding amino acids. A possible mechanism for the photochemical process, involving displacement of the oxygen-based ligand, is discussed.

Thiolate-bridged dinuclear iron(tris-carbonyl)-nickel complexes relevant to the active site of [NiFe] hydrogenase

The reaction of NiBr(2)(EtOH)(4) with a 1:2-3 mixture of FeBr(2)(CO)(4) and Na(SPh) generated a linear trinuclear Fe-Ni-Fe cluster (CO)(3)Fe(mu-SPh)(3)Ni(mu-SPh)(3)Fe(CO)(3), 1, whereas the analogous reaction system FeBr(2)(CO)(4)/Na(S(t)Bu)/NiBr(2)(EtOH)(4) (1:2-3:1) gave rise to a linear tetranuclear Fe-Ni-Ni-Fe cluster [(CO)(3)Fe(mu-S(t)Bu)(3)Ni(mu-Br)](2), 2. By using this tetranuclear cluster 2 as the precursor, we have developed a new synthetic route to a series of thiolate-bridged dinuclear Fe(CO)(3)-Ni complexes, the structures of which mimic [NiFe] hydrogenase active sites. The reactions of 2 with SC(NMe(2))(2) (tmtu), Na{S(CH(2))(2)SMe} and ortho-NaS(C(6)H(4))SR (R = Me, (t)Bu) led to isolation of (CO)(3)Fe(mu-S(t)Bu)(3)NiBr(tmtu), 3, (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(CH(2))(2)SMe}, 4, and (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{S(C(6)H(4))SR}, 5a (R = Me) and 5b (R = (t)Bu), respectively. On the other hand, treatment of 2 with 2-methylthio-phenolate (ortho-O(C(6)H(4))SMe) in methanol resulted in (CO)(3)Fe(mu-S(t)Bu)(3)Ni(MeOH){O(C(6)H(4))SMe}, 6a. The methanol molecule bound to Ni is labile and is readily released under reduced pressure to afford (CO)(3)Fe(S(t)Bu)(mu-S(t)Bu)(2)Ni{O(C(6)H(4))SMe}, 6b, and the coordination geometry of nickel changes from octahedral to square planar. Likewise, the reaction of 2 with NaOAc in methanol followed by crystallization from THF gave (CO)(3)Fe(mu-S(t)Bu)(3)Ni(THF)(OAc), 7. The dinuclear complexes, 3-7, are thermally unstable, and a key to their successful isolation is to carry out the reactions and manipulations at -40 degrees C.

Detection and determination of the {Fe(NO)(2)} core vibrational features in dinitrosyl-iron complexes from experiment, normal coordinate analysis, and density functional theory: an avenue for probing the nitric oxide oxidation state

As it is now well-established that nitric oxide plays an important role in many physiological processes, there is a renewed interest in dinitrosyl-iron complexes (DNICs). The question concerning the electronic structure of DNICs circles around the formal oxidation states of the iron and nitric oxide of the Fe(NO)2 core. Previous infrared measurements of nu(NO) alone point out inconsistencies in assigning electron configurations and charges on metals, inherent from the measurement of one parameter external to the metal. This work represents the first experimental and theoretical attempt to assign vibrational modes for the {Fe(NO)2}9 core of DNICs. The following complexes are investigated, [PPN][S5Fe(NO)2] (1), [PPN][Se5Fe(NO)2] (2), [PPN][(SPh)2Fe(NO)2] (3), and [PPN][(SePh)2Fe(NO)2] (4). The analysis of isotopically edited Raman data together with normal coordinate calculation permitted assignment of nu(NO) and nu(Fe-NO) stretching and delta(Fe-N-O) bending modes in these complexes. The assignments proposed are the first ever reported for the DNICs; a comparison of nu(NO) and nu(Fe-NO) stretching frequencies in DNICs is now feasible. The Fe(NO)2 core electronic configuration in these complexes is described as {Fe1+(*NO)2}. Results from 1 and 3 have been complemented by density functional theory (DFT) frequency calculations. In addition to providing a reasonably correct account of the observed frequencies, DFT calculations also give a good account of the frequency shifts upon 15NO substitution providing the first link between DFT and Raman spectroscopies for DNICs. Through the use of a combination of NO intraligand and metal-ligand vibrational data for the Fe(NO)2 core, normal coordinate analysis gives a NO stretching force constant, which compared to molecular NO gas, is significantly reduced for all four complexes. The hybrid U-B3LYP/6-311++G(3d,2p) density functional method has been employed to analyze the molecular orbital compositions of predominantly NO orbitals based on the crystal structure of complex 1. The molecular orbital not only revealed the bonding nature of the {Fe(NO)2}9 core but also provided a qualitative correct account of the observed low NO vibrational frequencies. The calculation shows that the NO is involved in a strong donor bonding interaction with the Fe1+. This donor bonding interaction involves the 5sigma molecular orbital of the NO, which is sigma-bonding with respect to the intramolecular NO bond, and removal of electron density from this orbital destabilizes the NO bond. Though it is too ambiguous to extrapolate a nu(Fe-NO)/nu(NO) correlation line for {Fe(NO)2}9 DNICs based only on the data reported here, the feasibility of using a vibrational systematics diagram to extract the electron configurations and charges on metals is demonstrated based on the vibrational data available in the literature for iron-nitrosyl complexes. The data provided here can be used as a model for the determination of effective charges on iron and the bonding of nitric oxides to metals in DNICs.

Density functional study of the catalytic cycle of nickel–iron [NiFe] hydrogenases and the involvement of high-spin nickel(II)

In light of recent experiments suggesting high-spin (HS) Ni(II) species in the catalytic cycle of [NiFe] hydrogenase, a series of models of the Ni(II) forms Ni-SI(I,II), SI-CO and Ni-R(I,II,III) were examined in their high-spin states via density functional calculations. Because of its importance in the catalytic cycle, the Ni-C form was also included in this study. Unlike the Ni(II) forms in previous studies, in which a low-spin (LS) state was assumed and a square-planar structure found, the optimized geometries of these HS Ni(II) forms resemble those observed in the crystal structures: a distorted tetrahedral to distorted pyramidal coordination for the NiS4. This resemblance is particularly significant because the LS state is 20-30 kcal/mol less stable than the HS state for the geometry of the crystal structure. If these Ni(II) forms in the enzyme are not high spin, a large change in geometry at the active site is required during the catalytic cycle. Furthermore, only the HS state for the CO-inhibited form SI-CO has CO stretching frequencies that match the experimental results. As in the previous work, these new results show that the heterolytic cleavage reaction of dihydrogen (where H2 is cleaved with the metal acting as a hydride acceptor and a cysteine as the proton acceptor) has a lower energy barrier and is more exothermic when the active site is oxidized to Ni(III). The enzyme models described here are supported by a calibrated correlation of the calculated and measured CO stretching frequencies of the forms of the enzyme. The correlation coefficient for the final set of models of the forms of [NiFe] hydrogenase is 0.8.

Modulation of the electronic structure and the Ni-Fe distance in heterobimetallic models for the active site in [NiFe]hydrogenase

Reaction of the mononuclear Ni(II) thiolate complexes [Ni(L)] [L, L(1), H2L(1), bis(2-mercaptoethyl)-1,2-dimercaptoethane; L(2), H2L(2), N,N'-dimethyl-N,N'-bis(2-mercaptoethyl)-bis(aminoethyl)sulfide] with [FeCp(CO)2I] gives the dithiolate-bridged heterobimetallic species, [Ni(L(1))FeCp(CO)]PF6, 1, and [Ni(L(2))FeCp]I, 2, respectively. Binding of a Fe(CO)3 fragment via reaction of square-planar [Ni(pdt)(dppe)] (dppe, 1,2-diphenylphosphinoethane; pdt(2-), 1,3-propanedithiolate) with Fe3(CO)12 or [Fe(CO)3(BDA)] (BDA, benzylidene acetone) affords diamagnetic [(dppe)Ni(mu-pdt)Fe(CO)3], 3, in which the Ni(II) center is bound tetrahedrally to two thiolate S-donors and to two P-donors. The complex [(dppe)Ni(mu-pdt)Fe(CO)3], 3, reacts in solution via rearrangement to afford [(OC)Ni(mu-dppe)(mu-pdt)Fe(CO)2], 4, in which one P-donor of dppe is bound to Ni and the other to Fe, and a CO ligand has transferred from Fe to Ni. Additionally, the syntheses of 3 and 4 afford the side products [(dppe)Ni(CO)2] and [(OC)3Fe(pdt)Fe(CO)3] together with the trinuclear species [(dppe)(CO)Fe(mu-CO)(mu-pdt)Fe(mu-pdt)Fe(CO)3], 5. Reaction of [Ni(pdt)(dppe)] with [FeCp(CO)2I] in CH2Cl2 affords two products [(dppe)Ni(mu-pdt)FeCp(CO)]PF6, 6, and [(dppe)Ni(pdt)(mu-I)Ni(dppe)]PF6, 7. The complexes 2, 3, and 4 show Ni-Fe distances of 2.539(4), 2.4666(6), and 2.4777(7) A, respectively, with relatively acute dihedral angles of 79.5-81.8 degrees for the Ni-S2-Fe bridge, thus mimicking the shortened Ni...Fe distance (2.5 A) and the acute dihedral angle of the Ni-S2-Fe moiety observed in certain active forms of [NiFe]hydrogenase. The role of direct Ni-Fe bonding in these complexes is discussed and linked to electronic structure calculations on [(dppe)Ni(pdt)Fe(CO)3], 3, which confirm the presence of a bent Ni(d(z2))-Fe(d(z2)) sigma-bond in a singlet ground state.

Dithiolate Complexes of Manganese and Rhenium: X-ray Structure and Properties of an Unusual Mixed Valence Cluster Mn3(CO)6(μ-η2-SCH2CH2CH2S)3

Treatment of Mn(2)(CO)(10) with 3,4-toluenedithiol and 1,2-ethanedithiol in the presence of Me(3)NO.2H(2)O in CH(2)Cl(2) at room temperature afforded the dinuclear complexes Mn(2)(CO)(6)(mu-eta(4)-SC(6)H(3)(CH(3))S-SC(6)H(3)(CH(3))S) (1), and Mn(2)(CO)(6)(mu-eta(4)-SCH(2)CH(2)S-SCH(2)CH(2)S) (2), respectively. Similar reactions of Re(2)(CO)(10) with 3,4-toluenedithiol, 1,2-benzenedithiol, and 1,2-ethanedithiol yielded the dirhenium complexes Re(2)(CO)(6)(mu-eta(4)-SC(6)H(3)(CH(3))S-SC(6)H(3)(CH(3))S) (3), Re(2)(CO)(6)(mu-eta(4)-SC(6)H(4)S-SC(6)H(4)S) (4), and Re(2)(CO)(6)(SCH(2)CH(2)S-SCH(2)CH(2)S) (5), respectively. In contrast, treatment of Mn(2)(CO)(10) with 1,3-propanedithiol afforded the trimanganese compound Mn(3)(CO)(6)(mu-eta(2)-SCH(2)CH(2)CH(2)S)(3) (6), whereas Re(2)(CO)(10) gave only intractable materials. The molecular structures of 1, 3, and 6 have been determined by single-crystal X-ray diffraction studies. The dimanganese and dirhenium carbonyl compounds 1-5contain a binucleating disulfide ligand, formed by interligand disulfide bond formation between two dithiolate ligands identical in structure to that of the previously reported dimanganese complex Mn(2)(CO)(6)(mu-eta(4)-SC(6)H(4)S-SC(6)H(4)S). Complex 6, on the other hand, forms a unique example of a mixed-valence trimangenese carbonyl compound containing three bridging 1,3-propanedithiolate ligands. The solution properties of 6 have been investigated by UV-vis and EPR spectroscopies as well as electrochemical techniques.

Homodinuclear iron thiolate nitrosyl compounds [(ON)Fe(S,S-C6H4)2 Fe(NO)2]- and [(ON)Fe(SO2,S-C6H4)(S,S-C6H4)Fe(NO)2]- with {Fe(NO)}7-{Fe(NO)2}9 electronic coupling: new members of a class of dinitrosyl iron complexes

Reaction of Fe(CO)2(NO)2 and [(ON)Fe(S,S-C6H3R)2]- (R = H (1), CH3 (1-Me))/[(ON)Fe(SO2,S-C6H4)(S,S-C6H4)]- (4) in THF afforded the diiron thiolate/sulfinate nitrosyl complexes [(ON)Fe(S,S-C6H3R)2 Fe(NO)2]- (R = H (2), CH3 (2-Me)) and [(ON)Fe(S,SO2-C6H4)(S,S-C6H4)Fe(NO)2]- (3), respectively. The average N-O bond lengths ([Fe(NO)2] unit) of 1.167(3) and 1.162(4) A in complexes 2 and 3 are consistent with the average N-O bond length of 1.165 A observed in the other structurally characterized dinitrosyl iron complexes with an {Fe(NO)2}9 core. The lower nu(15NO) value (1682 cm(-1) (KBr)) of the [(15NO)FeS4] fragment of [(15NO)Fe(S,S-C6H3CH3)2 Fe(NO)2]- (2-Me-15N), compared to that of [(15NO)Fe(S,S-C6H3CH3)2]- (1-Me-15N) (1727 cm(-1) (KBr)), implicates the electron transfer from {Fe(NO)2}10 Fe(CO)2(NO)2 to complex 1-Me/1 may occur in the process of formation of complex 2-Me/2. Then, the electronic structures of the [(NO)FeS4] and [S2Fe(NO)2] cores of complexes 2, 2-Me, and 3 were best assigned according to the Feltham-Enemark notation as the {Fe(NO)}7-{Fe(NO)2}9 coupling (antiferromagnetic interaction with a J value of -182 cm(-1) for complex 2) to account for the absence of paramagnetism (SQUID) and the EPR signal. On the basis of Fe-N(O) and N-O bond distances, the dinitrosyliron {L2Fe(NO)2} derivatives having an Fe-N(O) distance of approximately 1.670 A and a N-O distance of approximately 1.165 A are best assigned as {Fe(NO)2}9 electronic structures, whereas the Fe-N(O) distance of approximately 1.650 A and N-O distance of approximately 1.190 A probably imply an {Fe(NO)2}10 electronic structure.

Direct interaction of coenzyme M with the active-site Fe-S cluster of heterodisulfide reductase

Heterodisulfide reductase (HDR) catalyzes the formation of coenzyme M (CoM-SH) and coenzyme B (CoB-SH) by the reversible reduction of the heterodisulfide, CoM-S-S-CoB. This reaction recycles the two thiol coenzymes involved in the final step of microbial methanogenesis. Electron paramagnetic resonance (EPR) and variable-temperature magnetic circular dichroism spectroscopic experiments on oxidized HDR incubated with CoM-SH revealed a S=1/2 [4Fe-4S]3) cluster, the EPR spectrum of which is broadened in the presence of CoM-33SH [Duin, E.C., Madadi-Kahkesh, S., Hedderich, R., Clay, M.D. and Johnson, M.K. (2002) Heterodisulfide reductase from Methanothermobacter marburgensis contains an active-site [4Fe-4S] cluster that is directly involved in mediating heterodisulfide reduction. FEBS Lett. 512, 263-268; Duin, E.C., Bauer, C., Jaun, B. and Hedderich, R. (2003) Coenzyme M binds to a [4Fe-4S] cluster in the active site of heterodisulfide reductase as deduced from EPR studies with the [33S]coenzyme M-treated enzyme. FEBS Lett. 538, 81-84]. These results provide indirect evidence that the disulfide binds to the iron-sulfur cluster during reduction. We report here direct structural evidence for this interaction from Se X-ray absorption spectroscopic investigation of HDR treated with the selenium analog of coenzyme M (CoM-SeH). Se K edge extended X-ray absorption fine structure confirms a direct interaction of the Se in CoM-SeH-treated HDR with an Fe atom of the Fe-S cluster at an Fe-Se distance of 2.4A.