Synthesis and Characterization of {Ni(NO)}10and {Co(NO)2}10Complexes Supported by Thiolate Ligands

Phosphine/thiolate-containing dinitrosyl cobalt complexes (DNCCs): synthesis, characterization, interconversion, X-ray diffraction identification and NO release

Cobalt carbonyl/nitrosyl complexes, (PPh3)(CO)2Co(NO) (1) and (PPh3)2(CO)Co(NO) (2), were obtained by reacting (CO)3Co(NO) with one equiv. and two equiv. of PPh3, respectively. The process of isoelectronic replacement of CO with NO+ resulted in the formation of a cationic complex {Co(NO)2}10 [(PPh3)2Co(NO)2][BF4] (3). Complex (PPh3)(SPh)Co(NO)2 (4), which contains a thiophenolate ligand, was synthesized by ligand exchange of complex 3 with [PPh4][SPh] in a 1 : 1 molar ratio in THF solution. The addition of one equiv. of [PPh4][SPh] to complex 4 led to the formation of complex [PPh4][(SPh)2Co(NO)2] (5). The interconversions among complexes 1-5 were substantiated with the application of IR spectroscopy and X-ray single-crystal diffraction techniques. Notably, complex 4 exhibited commendable NOs (nitric oxide species: NO+/˙NO/NO-) transfer capabilities in the presence of [Fe(TPP)Cl] (5,10,15,20-tetraphenyl-21H,23H-porphine iron(III) chloride).

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.

An Integrated View of Nitrogen Oxyanion Deoxygenation in Solution Chemistry and Electrospray Ion Production

There has been an increasing interest in chemistry involving nitrogen oxyanions, largely due to the environmental hazards associated with increased concentrations of these anions leading to eutrophication and aquatic "dead zones". Herein, we report the synthesis and characterization of a suite of MNO_x complexes (M = Co, Zn: x = 2, 3). Reductive deoxygenation of cobalt bis(nitrite) complexes with bis(boryl)pyrazine is faster for cobalt than previously reported nickel, and pendant O-bound nitrito ligand is still readily deoxygenated, despite potential implication of an isonitrosyl primary product. Deoxygenation of zinc oxyanion complexes is also facile, despite zinc being unable to stabilize a nitrosyl ligand, with liberation of nitric oxide and nitrous oxide, indicating N-N bond formation. X-ray photoelectron spectroscopy is effective for discriminating the types of nitrogen in these molecules. ESI mass spectrometry of a suite of M(NO_x)_y (x = 2, 3 and y = 1, 2) shows that the primary form of ionization is loss of an oxyanion ligand, which can be alleviated via the addition of tetrabutylammonium (TBA) as a nonintuitive cation pair for the neutral oxyanion complexes. We have shown these complexes to be subject to deoxygenation, and there is evidence for nitrogen oxyanion reduction in several cases in the ESI plume. The attractive force between cation and neutral is explored experimentally and computationally and attributed to hydrogen bonding of the nitrogen oxyanion ligands with ammonium α-CH₂ protons. One example of ESI-induced reductive dimerization is mimicked by bulk solution synthesis, and that product is characterized by X-ray diffraction to contain two Co(NO)₂⁺ groups linked by a highly conjugated diazapolyene.

Structures and Properties of Dinitrosyl Iron and Cobalt Complexes Ligated by Bis(3,5-diisopropyl-1-pyrazolyl)methane

Two dinitrosyl iron and cobalt complexes [Fe(NO)2(L1”)](BF4) and [Co(NO)2(L1”)](BF4) are synthesized and characterized, supported by a less hindered bidentate nitrogen ligand bis(3,5-diisopropyl-1-pyrazolyl)methane (denoted as L1”), are surprisingly stable under argon atmosphere. X-ray structural analysis shows a distorted tetrahedral geometry. Spectroscopic and structural parameters of the dinitrosyl iron and cobalt complexes are consistent with the previous reported {Fe(NO)2}9 and {Co(NO)2}10. Two N–O and M–N(O) stretching frequencies and their magnetic properties are also consistent with the above electronic structural assignments. We explored the dioxygen reactivities of the obtained dinitrosyl complexes. Moreover, the related [FeCl2(L1”)], [Co(NO3)2(L1”)], and [Co(NO2)2(L1”)] complexes are also characterized in detail.

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.

Reductive nitrosylation of nickel(ii) complex by nitric oxide followed by nitrous oxide release

Ni(ii) complex of ligand ( = bis(2-ethyl-4-methylimidazol-5-yl)methane) in methanol solution reacts with an equivalent amount of NO resulting in a corresponding Ni(i) complex. Adding further NO equivalent affords a Ni(i)-nitrosyl intermediate with the {NiNO}(10) configuration. This nitrosyl intermediate upon subsequent reaction with additional NO results in the release of N2O and formation of a Ni(ii)-nitrito complex. Crystallographic characterization of the nitrito complex revealed a symmetric η(2)-O,O-nitrito bonding to the metal ion. This study demonstrates the reductive nitrosylation of a Ni(ii) center followed by N2O release in the presence of excess NO.

N2O Formation via Reductive Disproportionation of NO by Mononuclear Copper Complexes: A Mechanistic DFT Study

The mechanism of the copper(I)-mediated reductive disproportionation reaction of NO to form N₂O was investigated for five different 3,5-substituted tris(pyrazolyl)borate copper complexes (CuTp^R₁,R₂) by means of DFT calculations. A thorough search of the potential surface was performed, using the B3LYP functional with the def2-SVP basis set for optimization purposes and def2-TZVP single-point calculations for constructing the potential energy surface for two of these complexes. The results can be condensed into six competing reaction mechanisms, two of which were more closely investigated using full def2-TZVP optimized potential and free energies. The results consistently predict the same mechanism to have the lowest overall barrier. For all five different complexes, this is found to be in good agreement with the experimental reaction barriers. The key intermediate for the transition from the N-bound reactant to the O-bound product contains a stable (NO)₃ unit with one N-Cu and one O-Cu bond, which was not included in the mechanistic considerations reported in the literature. Further analysis of the charge distribution and the spin density demonstrates the formation of a Cu(II)-(N₂O₂^-) intermediate and the electronic influence of the different ligands.

A cobalt–nitrosyl complex with a hindered hydrotris(pyrazolyl)borate coligand: detailed electronic structure, and reactivity towards dioxygen

The cobalt–nitrosyl complex [Co(NO)(L3)] is supported by a highly hindered tridentate nitrogen ligand, hydrotris(3-tertiary butyl-5-isopropyl-1-pyrazolyl)borate (denoted as L3⁻), and shows a linear Co–N–O unit.

Polymeric cobalt(ii) thiolato complexes – syntheses, structures and properties of 1∞[Co(SMes)2] and 1∞[Co(SPh)2NH3]

Structural and magnetic characterization of the first examples of polymeric cobalt chalcogenolato complexes namely 1∞[Co(SMes)₂] and 1∞[Co(SPh)₂NH₃].

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.

Nitric oxide release from a nickel nitrosyl complex induced by one-electron oxidation

Reaction of [Ni(NO)(bipy)][PF6] (2) with AgPF6 or [NO][PF6] in MeCN results in formation of [Ni(bipy)x(MeCN)y](2+) and release of NO gas in moderate yields. In contrast, the addition of the inner sphere oxidant Ph2S2 to 2 does not result in denitrosylation. Instead, the diphenyldisulfide adduct [{(bipy)(NO)Ni}2(μ-S2Ph2)][PF6]2 (3) is formed in good yield. However, oxidation of 2 with 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) does results in cleavage of the Ni-NO bond and generation of NO. The metal-containing product, [(bipy)Ni(η(2)-TEMPO)][PF6] (4), can be isolated as an orange-brown solid in excellent yields. In the solid state, complex 4 contains a side-on bound TEMPO(-) ligand, which is characterized by a long N-O bond length [1.383(2) Å]. The contrasting reactivity of Ph2S2 and TEMPO likely relates to their different redox potentials, as Ph2S2 is a relatively weak oxidant. Finally, the addition of pyridine-N-oxide to 2 results in the formation of the adduct, [(bipy)Ni(NO)(ONC5H5)][PF6] (5). No evidence of NO release is observed in this reaction, probably because of the low one-electron (1e(-)) reduction potential of pyridine-N-oxide.

Zinc thiolate reactivity toward nitrogen oxides: insights into the interaction of Zn2+ with S-nitrosothiols and implications for nitric oxide synthase

Zinc thiolate complexes containing N(2)S tridentate ligands were prepared to investigate their reactivity toward reactive nitrogen species, chemistry proposed to occur at the zinc tetracysteine thiolate site of nitric oxide synthase (NOS). The complexes are unreactive toward nitric oxide (NO) in the absence of dioxygen, strongly indicating that NO cannot be the species directly responsible for S-nitrosothiol formation and loss of Zn(2+) at the NOS dimer interface in vivo. S-Nitrosothiol formation does occur upon exposure of zinc thiolate solutions to NO in the presence of air, however, or to NO(2) or NOBF(4), indicating that these reactive nitrogen/oxygen species are capable of liberating zinc from the enzyme, possibly through generation of the S-nitrosothiol. Interaction between simple Zn(2+) salts and preformed S-nitrosothiols leads to decomposition of the -SNO moiety, resulting in release of gaseous NO and N(2)O. The potential biological relevance of this chemistry is discussed.

Late-metal nitrosyl cations: synthesis and reactivity of [Ni(NO)(MeNO2)3][PF6]

The reaction of [NO][PF(6)] with excess Ni powder in CH(3)NO(2), in the presence of 2 mol % NiI(2), results in the formation of [Ni(NO)(CH(3)NO(2))(3)][PF(6)] (1), which can be isolated in modest yield as a blue crystalline solid. Also formed in the reaction is [Ni(CH(3)NO(2))(6)][PF(6)](2) (2), which can be isolated in comparable yield as a pale-green solid. In the solid state, 1 exhibits tetrahedral geometry about the Ni center with a linear nitrosyl ligand [Ni1-N1-O1 = 174.1(8)°] and a short Ni-N bond distance [1.626(6) Å]. As anticipated, the weakly coordinating nitromethane ligands in 1 are easily displaced by a variety of donors, including Et(2)O, MeCN, and piperidine (NC(5)H(11)). More surprisingly, the addition of mesitylene to 1 results in the formation of an η(6)-coordinated nickel arene complex, [Ni(η(6)-1,3,5-Me(3)C(6)H(3))(NO)][PF(6)] (6). In the solid state, complex 6 exhibits a long Ni-C(cent) distance [1.682(2) Å], suggesting a relatively weak Ni-arene interaction, a consequence of the strong π-back-donation to the nitrosyl ligand. The addition of anisole to 1 also results in the formation of a η(6) nickel arene complex, [Ni(η(6)-MeOC(6)H(5))(NO)][PF(6)] (7). This complex also exhibits a long Ni-C(cent) distance [1.684(1) Å].

Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Several nitrosyl complexes of Fe and Co have been prepared using the sterically hindered Ar-nacnac ligand (Ar-nacnac = anion of [(2,6-diisopropylphenyl)NC(Me)](2)CH). The dinitrosyliron complexes [Fe(NO)(2)(Ar-nacnac)] (1) and (Bu(4)N)[Fe(NO)(2)(Ar-nacnac)] (2) react with [Fe(III)(TPP)Cl] (TPP = tetraphenylporphine dianion) to generate [Fe(II)(NO)(TPP)] and the corresponding mononitrosyliron complexes. The factors governing NO transfer with dinitrosyliron complexes (DNICs) 1 and 2 are evaluated, together with the chemistry of the related mononitrosyliron complex, [Fe(NO)Br(Ar-nacnac)] (4). The synthesis and properties of the related cobalt dinitrosyl [Co(NO)(2)(Ar-nacnac)] (3) is also discussed for comparison to DNICs 1 and 2. The solid-state structures of several of these compounds as determined by X-ray crystallography are reported.

Reactions of synthetic [2Fe-2S] and [4Fe-4S] clusters with nitric oxide and nitrosothiols

The interaction of nitric oxide (NO) with iron-sulfur cluster proteins results in degradation and breakdown of the cluster to generate dinitrosyl iron complexes (DNICs). In some cases the formation of DNICs from such cluster systems can lead to activation of a regulatory pathway or the loss of enzyme activity. In order to understand the basic chemistry underlying these processes, we have investigated the reactions of NO with synthetic [2Fe-2S] and [4Fe-4S] clusters. Reaction of excess NO(g) with solutions of [Fe2S2(SR)4](2-) (R = Ph, p-tolyl (4-MeC6H4), or 1/2 (CH2)2-o-C6H4) cleanly affords the respective DNIC, [Fe(NO)2(SR)2](-), with concomitant reductive elimination of the bridging sulfide ligands as elemental sulfur. The structure of (Et4N)[Fe(NO)2(S-p-tolyl)2] was verified by X-ray crystallography. Reactions of the [4Fe-4S] clusters, [Fe4S4(SR)4](2-) (R = Ph, CH2Ph, (t)Bu, or 1/2 (CH2)-m-C6H4) proceed in the absence of added thiolate to yield Roussin's black salt, [Fe4S3(NO)7](-). In contrast, (Et4N)2[Fe4S4(SPh)4] reacts with NO(g) in the presence of 4 equiv of (Et4N)(SPh) to yield the expected DNIC. For all reactions, we could reproduce the chemistry effected by NO(g) with the use of trityl-S-nitrosothiol (Ph3CSNO) as the nitric oxide source. These results demonstrate possible pathways for the reaction of iron-sulfur clusters with nitric oxide in biological systems and highlight the importance of thiolate-to-iron ratios in stabilizing DNICs.