Chemistry of nitrosyliron complexes supported by a β-diketiminate ligand

Iron(iv) alkyl complexes: electronic structure contributions to Fe-C bond homolysis and migration reactions that form N-C bonds from N2

High-valent iron alkyl complexes are rare, as they are prone to Fe-C bond homolysis. Here, we describe an unusual way to access formally iron(iv) alkyl complexes through double silylation of iron(i) alkyl dinitrogen complexes to form an NNSi2 group. Spectroscopically validated computations show that the disilylehydrazido(2-) ligand stabilizes the formal iron(iv) oxidation state through a strongly covalent Fe-N π-interaction, in which one π-bond fits an "inverted field" description. This means that the two bonding electrons are localized more on the metal than the ligand, and thus an iron(ii) resonance structure is a significant contributor, similar to the previously-reported phenyl analogue. However, in contrast to the phenyl complex which has an S = 1 ground state, the ground state of the alkyl complex is S = 2, which places one electron in the π* orbital, leading to longer and weaker Fe-N bonds. The reactivity of these hydrazido(2-) complexes is dependent on the steric and electronic properties of the specific alkyl group. When the alkyl group is the bulky trimethylsilylmethyl, the formally iron(iv) species is stable at room temperature and no migration of the alkyl ligand is observed. However, the analogous complex with the smaller methyl ligand does indeed undergo migration of the carbon-based ligand to the NNSi2 group to form a new N-C bond. This migration is followed by isomerization of the hydrazido ligand, and the product exists as two isomers that have distinct η1 and η2 binding of the hydrazido group. Lastly, when the alkyl group is benzyl, the Fe-C bond homolyzes to give a three-coordinate hydrazido(2-) complex which is likely due to the greater stability of a benzyl radical compared to that for methyl or trimethylsilylmethyl. These studies demonstrate the availability of a hydrocarbyl migration pathway at formally iron(iv) centers to form new N-C bonds directly to N2, though product selectivity is highly dependent on the identity of the migrating group.

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

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

Mössbauer and Nuclear Resonance Vibrational Spectroscopy Studies of Iron Species Involved in N-N Bond Cleavage

Diketiminate-supported iron complexes are capable of cleaving the strong triple bond of N2 to give a tetra-iron complex with two nitrides (Rodriguez et al., Science, 2011, 334, 780-783). The mechanism of this reaction has been difficult to determine, but a transient green species was observed during the reaction that corresponds to a potential intermediate. Here, we describe studies aiming to identify the characteristics of this intermediate, using a range of spectroscopic techniques, including Mössbauer spectroscopy, electronic absorption spectroscopy, Raman spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, and nuclear resonance vibrational spectroscopy (NRVS) complemented by density functional theory (DFT) calculations. We successfully elucidated the nature of the starting iron(II) species and the bis(nitride) species in THF solution, and in each case, THF breaks up the multiiron species. Various observations on the green intermediate species indicate that it has one N2 per two Fe atoms, has THF associated with it, and has NRVS features indicative of bridging N2. Computational models with a formally diiron(0)-N2 core are most consistent with the accumulated data, and on this basis, a mechanism for N2 splitting is suggested. This work shows the power of combining NRVS, Mössbauer, NMR, and vibrational spectroscopies with computations for revealing the nature of transient iron species during N2 cleavage.

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).

Photolytic C-Diazeniumdiolate Disassembly in the β-Diketiminate Complexes [MeLM(O2N2CPh3)] (M = Fe, Co, Cu)

The reaction of [K(18-crown-6)][O2N2CPh3] with [MeLCo(μ-Br)2Li(OEt2)] (MeL = {(2,6-iPr2C6H3)NC(Me)}2CH) generates the trityl diazeniumdiolate complex, [MeLCo(O2N2CPh3)] (1), in moderate yield. Similar metathesis reactions result in the formation of the Fe and Cu analogues, [MeLM(O2N2CPh3)] (Fe, 2; Cu, 3), which can also be isolated in moderate yields. Complexes 1-3 were characterized by ultraviolet-visible (UV-vis) spectroscopy, and their solid-state structures were determined by X-ray crystallography. These complexes were further characterized via 1H NMR spectroscopy (in the case of 1 and 2) or EPR spectroscopy (in the case of 3). Irradiation of complexes 1 and 2 with 371 nm light generates the known dinitrosyl complexes, [MeLM(NO)2] (M = Co, 4; Fe, 5), along with Ph3CH and 9-phenylfluorene. We propose that 4 and 5 are formed via the putative hyponitrite intermediates, [MeLM(κ2-O,O-ONNO)], which are formed by photoinduced homolysis of the C-N bond of the [O2N2CPh3] ligand. In contrast, irradiation of complex 3 with 371 nm light, in the presence of 1 equiv of PPh3, led to the formation of the Cu(I) complexes, [MeLCu(PPh3)], [(ArNCMeC(NO)CMeNAr)Cu(PPh3)] (6), and [(ArNCMeC(NO)CMeNAr)Cu]2 (7), of which the latter two are products of γ-nitrosation of the β-diketiminiate ligand. Also formed in this transformation are Ph3CN(H)OCPh3, Ph3PO, and N2O, along with trace amounts of NO.

Bacterial nitric oxide reductase (NorBC) models employing click chemistry

Bacterial NO Reductase (NorBC or cNOR) is a membrane-bound enzyme found in denitrifying bacteria that catalyzes the two-electron reduction of NO to N₂O and water. The mechanism by which NorBC operates is highly debated, due to the fact that this enzyme is difficult to work with, and no intermediates of the NO reduction reaction could have been identified so far. The unique active site of NorBC consists of a heme b₃/non-heme Fe_B diiron center. Synthetic model complexes provide the opportunity to obtain insight into possible mechanistic alternatives for this enzyme. In this paper, we present three new synthetic model systems for NorBC, consisting of a tetraphenylporphyrin-derivative clicked to modified BMPA-based ligands (BMPA = bis(methylpyridyl)amine) that model the non-heme site in the enzyme. These complexes have been characterized by EPR, IR and UV-Vis spectroscopy. The reactivity with NO was then investigated, and it was found that the complex with the BMPA-carboxylate ligand as the non-heme component has a very low affinity for NO at the non-heme iron site. If the carboxylate functional group is replaced with a phenolate or pyridine group, reactivity is restored and formation of a diiron dinitrosyl complex was observed. Upon one-electron reduction of the nitrosylated complexes, following the semireduced pathway for NO reduction, formation of dinitrosyl iron complexes (DNICs) was observed in all three cases, but no N₂O could be detected.

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).

Distortion of the [FeNO]2 Core in Flavodiiron Nitric Oxide Reductase Models Inhibits N-N Bond Formation and Promotes Formation of Unusual Dinitrosyl Iron Complexes: Implications for Catalysis and Reactivity

Flavodiiron nitric oxide reductases (FNORs) carry out the reduction of nitric oxide (NO) to nitrous oxide (N₂O), allowing infectious pathogens to mitigate toxic levels of NO generated in the human immune response. We previously reported the model complex [Fe₂(BPMP)(OPr)(NO)₂](OTf)₂ (1, OPr^- = propionate) that contains two coplanar NO ligands and that is capable of quantitative NO reduction to N₂O [White et al. J. Am. Chem. Soc. 2018, 140, 2562-2574]. Here we investigate, for the first time, how a distortion of the active site affects the ability of the diiron core to mediate N₂O formation. For this purpose, we prepared several analogues of 1 that contain two monodentate ligands in place of the bridging carboxylate, [Fe₂(BPMP)(X)₂(NO)₂]^3+/1+ (2-X; X = triflate, 1-methylimidazole, or methanol). Structural data of 2-X show that without the bridging carboxylate, the diiron core expands, leading to elongated (O)N-N(O) distances (from 2.80 Å in 1 to 3.00-3.96 Å in 2-X) and distorted (O)N-Fe-Fe-N(O) dihedral angles (from coplanarity (5.9°) in 1 to 52.9-85.1° in 2-X). Whereas 1 produces quantitative amounts of N₂O upon one-electron reduction, N₂O production is substantially impeded in 2-X, to an initial 5-10% N₂O yield. The main products after reduction are unprecedented hs-Fe^II/{Fe(NO)₂}^9/10 dinitrosyl iron complexes (DNICs). Even though mononuclear DNICs are stable and do not show N-N coupling (since it is a spin-forbidden process), the hs-Fe^II/{Fe(NO)₂}^9/10 DNICs obtained from 2-X show unexpected reactivity and produce up to quantitative N₂O yields after 2 h. The implications of these results for the active site structure of FNORs are discussed.

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.

The Fe2 (NO)2 Diamond Core: A Unique Structural Motif In Non-Heme Iron-NO Chemistry

Non-heme high-spin (hs) {FeNO}⁸ complexes have been proposed as important intermediates towards N₂ O formation in flavodiiron NO reductases (FNORs). Many hs-{FeNO}⁸ complexes disproportionate by forming dinitrosyl iron complexes (DNICs), but the mechanism of this reaction is not understood. While investigating this process, we isolated a new type of non-heme iron nitrosyl complex that is stabilized by an unexpected spin-state change. Upon reduction of the hs-{FeNO}⁷ complex, [Fe(TPA)(NO)(OTf)](OTf) (1), the N-O stretching band vanishes, but no sign of DNIC or N₂ O formation is observed. Instead, the dimer, [Fe₂ (TPA)₂ (NO)₂ ](OTf)₂ (2) could be isolated and structurally characterized. We propose that 2 is formed from dimerization of the hs-{FeNO}⁸ intermediate, followed by a spin state change of the iron centers to low-spin (ls), and speculate that 2 models intermediates in hs-{FeNO}⁸ complexes that precede the disproportionation reaction.

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.

Transformation of the hydride-containing dinitrosyl iron complex [(NO)2Fe(η2-BH4)]− into [(NO)2Fe(η3-HCS2)]−via reaction with CS2

Hydride-insertion reactivity of DNIC [(NO)₂Fe(η²-BH₄)]⁻ promotes the reductive transformation of CS₂ into DNIC [(NO)₂Fe(η³-HCS₂)]⁻ featuring Fe 3d_z2-to-HCS₂ π* backbonding interaction.

Fe in biosynthesis, translocation, and signal transduction of NO: toward bioinorganic engineering of dinitrosyl iron complexes into NO-delivery scaffolds for tissue engineering

The ubiquitous physiology of nitric oxide enables the bioinorganic engineering of [Fe(NO)₂]-containing and NO-delivery scaffolds for tissue engineering.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

Ferrate(ii) complexes with redox-active formazanate ligands

The synthesis of mono(formazanate) iron complexes is described. In the presence of tetrabutylammonium halides, salt metathesis reactions afford the ferrate(ii) complexes [Bu4N][LFeX2] (L = PhNNC(p-tol)NNPh; X = Cl, Br) in good yield, and the products are characterized in detail. The high-spin ferrate(ii) complexes show cyclic voltammograms that are consistent with reversible, ligand-based one-electron reduction. The halides in these ferrate(ii) compounds are labile, and are displaced by 4-methoxyphenyl isocyanide (4 equiv.) as evidenced by formation of the low-spin, cationic octahedral complex [LFe(CNC6H4(p-OMe))4][Br]. Thus, a straightforward route to mono(formazanate) iron(ii) complexes is established.

Bite-Angle-Regulated Coordination Geometries: Tetrahedral and Trigonal Bipyramidal in Ni(II) with Biphenyl-Appended (2-Pyridyl)alkylamine N,N'-Bidentate Ligands

Two simple biphenyl-appended (2-pyridyl)alkylamine N-bidentate ligands, L^e and L^m, having ethylene and methylene spacers between donor groups, with bite angles L^e ≈ 100° and L^m ≈ 80°, dictate pseudotetrahedral and trigonal-bipyramidal geometries in six high-spin Ni(II)-halide complexes, [Ni(L^e)X₂] and [Ni(L^m)₂X](ClO₄) (where X = Cl^-, Br^-, I^-), respectively. The structures in the solid state, determined using X-ray crystallography, and in solution, determined using spectroscopic methods (UV-vis-NIR and paramagnetic ¹H NMR), which complement each other, are described.

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.

Synthesis, characterization and catalytic activity of rare-earth metal amides incorporating cyclohexyl bridged bis(β-diketiminato) ligands

The bridged bis(β-diketiminato) ligands supported rare-earth amides exhibited high catalytic activity towards the hydrophosphination of β-nitroalkene and α,β-unsaturated carbonyl derivatives with an excellent regioselectivity.

Alkali Metal Variation and Twisting of the FeNNFe Core in Bridging Diiron Dinitrogen Complexes

Alkali metal cations can interact with Fe-N2 complexes, potentially enhancing back-bonding or influencing the geometry of the iron atom. These influences are relevant to large-scale N2 reduction by iron, such as in the FeMoco of nitrogenase and the alkali-promoted Haber-Bosch process. However, to our knowledge there have been no systematic studies of a large range of alkali metals regarding their influence on transition metal-dinitrogen complexes. In this work, we varied the alkali metal in [alkali cation]2[LFeNNFeL] complexes (L = bulky β-diketiminate ligand) through the size range from Na(+) to K(+), Rb(+), and Cs(+). The FeNNFe cores have similar Fe-N and N-N distances and N-N stretching frequencies despite the drastic change in alkali metal cation size. The two diketiminates twist relative to one another, with larger dihedral angles accommodating the larger cations. In order to explain why the twisting has so little influence on the core, we performed density functional theory calculations on a simplified LFeNNFeL model, which show that the two metals surprisingly do not compete for back-bonding to the same π* orbital of N2, even when the ligand planes are parallel. This diiron system can tolerate distortion of the ligand planes through compensating orbital energy changes, and thus, a range of ligand orientations can give very similar energies.

To Transfer or Not to Transfer? Development of a Dinitrosyl Iron Complex as a Nitroxyl Donor for the Nitroxylation of an Fe(III) -Porphyrin Center

A positive myocardial inotropic effect achieved using HNO/NO(-) , compared with NO⋅, triggered attempts to explore novel nitroxyl donors for use in clinical applications in vascular and myocardial pharmacology. To develop M-NO complexes for nitroxyl chemistry and biology, modulation of direct nitroxyl-transfer reactivity of dinitrosyl iron complexes (DNICs) is investigated in this study using a Fe(III) -porphyrin complex and proteins as a specific probe. Stable dinuclear {Fe(NO)2 }(9) DNIC [Fe(μ-(Me) Pyr)(NO)2 ]2 was discovered as a potent nitroxyl donor for nitroxylation of Fe(III) -heme centers through an associative mechanism. Beyond the efficient nitroxyl transfer, transformation of DNICs into a chemical biology probe for nitroxyl and for pharmaceutical applications demands further efforts using in vitro/in vivo studies.

Iron and chromium complexes containing tridentate chelates based on nacnac and imino- and methyl-pyridine components: triggering C-X bond formation

Nacnac-based tridentate ligands containing a pyridyl-methyl and a 2,6-dialkyl-phenylamine (i.e., (2,6-R2-C6H3N═C(Me)CH═C(Me)NH(CH2py); R = Et, {Et(nn)PM}H; R = (i)Pr, {(i)Pr(nn)PM}H) were synthesized by condensation routes. Treatment of M{N(TMS)2}THFn (M = Cr, n = 2; M = Fe, Co, n = 1; TMS = trimethylsilane; THF = tetrahydrofuran) with {(i)Pr(nn)PM}H) afforded {(i)Pr(nn)PM}MN(TMS)2 (1-M(iPr); M = Cr, Fe); {Et(nn)PM}MN(TMS)2 (1-M(Et); M = Fe, Co) was similarly obtained. {R(nn)PM}FeBr (R = (i)Pr, Et; 2-Fe(R)) were prepared from FeBr2 and {R(nn)PM}Li, and alkylated to generate {R(nn)PM}Fe(neo)Pe (R = (i)Pr, Et; 3-Fe(R)). Carbonylation of 3-Fe(R) provided {(i)Pr(nn)PM}Fe(CO(neo)Pe)CO (4-Fe(iPr)), and carbonylations of 1-Fe(R) (R = Et, (i)Pr) and 1-Cr(iPr) induced deamination to afford {R(nn)PI}Fe(CO)2 (R = (i)Pr, 5-Fe(iPr); Et, 5-Fe(Et)), where PI is pyridine-imine, and {κ(2)-N,N-pyrim-pyr}Cr(CO)4 (6-Cr(iPr)), in which the aryl-amide side of the nacnac attacked the incipient PI group. Carbon-carbon bonds were formed at the imine carbon of the {R(nn)PI} ligand. Addition of [{(i)Pr(nn)PI}(2-)](K(+)(THF)x)2 to FeCl3 generated {(i)Pr(nn)CHpy}2Fe2Cl2 (7-Fe(iPr)), and TMSN3 induced the deamination of 1-Fe(Et), but with disproportionation to provide {[Et(nn)CHpy]2}Fe (8-Fe(Et)). Ph2CN2 induced C-C bond formation with 1-Fe(iPr) via its thermal degradation to ultimately afford {(i)Pr(nn)CHpy}2(FeN═CPh2)2 (9-Fe(iPr)). The compounds were examined by X-ray crystallography (1-M(iPr), M = Cr, Fe; 1-Co(Et); 2-Fe(iPr); 4-Fe(iPr); 5-Fe(iPr); 6-Cr(iPr); 7-Fe(iPr); 8-Fe(Et); 9-Fe(iPr)), Mössbauer spectroscopy, and NMR spectroscopy. Structural parameters assessing redox noninnocence are discussed, as are structural and mechanistic consequences of the various electronic environments.

Iron complexes derived from {nacnac-(CH2py)2}- and {nacnac-(CH2py)(CHpy)}n ligands: stabilization of iron(II) via redox noninnocence

Nacnac-based tetradentate chelates, {nacnac-(CH2py)2}(-) ({nn(PM)2}(-)) and {nacnac-(CH2py)(CHpy)}(n) ({nn(PM)(PI)}(n)) have been investigated in iron complexes. Treatment of Fe{N(TMS)2}2(THF) with {nn(PM)2}H afforded {nn(PM)2}FeN(TMS)2 [1-N(TMS)2], which led to {nn(PM)2}FeCl (1-Cl) from HCl and to {nn(PM)2}FeN3 (1-N3) upon salt metathesis. Dehydroamination of 1-N(TMS)2 was induced by L (L = PMe3, CO) to afford {nn(PM)(PI)}Fe(PMe3)2 [2-(PMe3)2] and {nn(PM)(PI)}FeCO (3-CO). Substitution of 2-(PMe3)2 led to {nn(PM)(PI)}Fe(PMe3)CO [2-(PMe3)CO], and exposure to a vacuum provided {nn(PM)(PI)}Fe(PMe3) (3-PMe3). Metathesis routes to {nn(PM)(PI)}FeL2 (2-L2; L = PMe3, PMe2Ph) and {nn(PM)(PI)}FeL (3-L; L = PMePh2, PPh3) from [{nn(PM)(PI)}(2-)]Li2 and FeBr2(THF)2 in the presence of L proved feasible, and 1e(-) and 2e(-) oxidation of 2-(PMe3)2 afforded 2(+)-(PMe3)2 and 2(2+)-(PMe3)2 salts. Mössbauer spectroscopy, structural studies, and calculational assessments revealed the dominance of iron(II) in both high-spin (1-X) and low-spin (2-L2 and 3-L) environments, and the redox noninnocence (RNI) of {nn(PM)(PI)}(n) [2-L2, 3-L, n = 2-; 2(+)-(PMe3)2, n = 1-; 2(2+)-(PMe3)2, n = 0]. A discussion regarding the utility of RNI in chemical reactivity is proffered.

Heme versus non-heme iron-nitroxyl {FeN(H)O}⁸ complexes: electronic structure and biologically relevant reactivity

Researchers have completed extensive studies on heme and non-heme iron-nitrosyl complexes, which are labeled {FeNO}(7) in the Enemark-Feltham notation, but they have had very limited success in producing corresponding, one-electron reduced, {FeNO}(8) complexes where a nitroxyl anion (NO(-)) is formally bound to an iron(II) center. These complexes, and their protonated iron(II)-NHO analogues, are proposed key intermediates in nitrite (NO2(-)) and nitric oxide (NO) reducing enzymes in bacteria and fungi. In addition, HNO is known to have a variety of physiological effects, most notably in the cardiovascular system. HNO may also serve as a signaling molecule in mammals. For these functions, iron-containing proteins may mediate the production of HNO and serve as receptors for HNO in vivo. In this Account, we highlight recent key advances in the preparation, spectroscopic characterization, and reactivity of ferrous heme and non-heme nitroxyl (NO(-)/HNO) complexes that have greatly enhanced our understanding of the potential biological roles of these species. Low-spin (ls) heme {FeNO}(7) complexes (S = 1/2) can be reversibly reduced to the corresponding {FeNO}(8) species, which are stable, diamagnetic compounds. Because the reduction is ligand (NO) centered in these cases, it occurs at extremely negative redox potentials that are at the edge of the biologically feasible range. Interestingly, the electronic structures of ls-{FeNO}(7) and ls-{FeNO}(8) species are strongly correlated with very similar frontier molecular orbitals (FMOs) and thermodynamically strong Fe-NO bonds. In contrast, high-spin (hs) non-heme {FeNO}(7) complexes (S = 3/2) can be reduced at relatively mild redox potentials. Here, the reduction is metal-centered and leads to a paramagnetic (S = 1) {FeNO}(8) complex. The increased electron density at the iron center in these species significantly decreases the covalency of the Fe-NO bond, making the reduced complexes highly reactive. In the absence of steric bulk, monomeric high-spin {FeNO}(8) complexes decompose rapidly. Notably, in a recently prepared, dimeric [{FeNO}(7)]2 species, we observed that reduction leads to rapid N-N bond formation and N2O generation, which directly models the reactivity of flavodiiron NO reductases (FNORs). We have also made key progress in the preparation and stabilization of corresponding HNO complexes, {FeNHO}(8), using both heme and non-heme ligand sets. In both cases, we have taken advantage of sterically bulky coligands to stabilize these species. ls-{FeNO}(8) complexes are basic and easily form corresponding ls-{FeNHO}(8) species, which, however, decompose rapidly via disproportionation and H2 release. Importantly, we recently showed that we can suppress this reaction via steric protection of the bound HNO ligand. As a result, we have demonstrated that ls-{FeNHO}(8) model complexes are stable and amenable to spectroscopic characterization. Neither ls-{FeNO}(8) nor ls-{FeNHO}(8) model complexes are active for N-N coupling, and hence, seem unsuitable as reactive intermediates in nitric oxide reductases (NORs). Hs-{FeNO}(8) complexes are more basic than their hs-{FeNO}(7) precursors, but their electronic structure and reactivity is not as well characterized.

A combined experimental and theoretical study of dinitrosyl iron complexes containing chelating bis(diphenyl)phosphinoX (X = benzene, propane and ethylene): X-ray crystal structures and properties influenced by the presence or absence of π-bonds in chelating ligands

Recent discoveries involving the roles of nitric oxide in humans have stimulated intense interest in transition metal nitrosyl complexes. A series of dinitrosyl iron complexes with the formula [(DPPX)Fe(NO)2], {DPPX = 1,2-bis(diphenylphosphino)benzene (1), 1,3-bis(diphenylphosphino)propane (2), and cis-1,2-bis(diphenylphosphino)ethylene (3)} has been prepared and characterized through a combination of FT-IR, NMR, UV-vis, X-ray crystallography, and electrochemical techniques. Infrared spectroscopy showed NO shifts to the region of 1723 and 1674 cm-1 for complexes 1 and 3, and 1708 and 1660 cm-1 for 2, indicating that ligand 2 acts as a stronger σ-donor. The X-ray crystallographic data showed that 1 and 3 possess the rare repulso conformation while 2 has the attracto conformation. CV studies on compounds 1, 2 and 3 display two quasi-reversible oxidations with the E°1/2 values at 0.101 and 0.186 V, 0.121 and 0.184 V, and 0.019 and 0.342 V, respectively. The larger ΔE value for compound 2 compared with that of 1 and 3 is attributed to the lack of π-bonds between the two phosphorus atoms. Theoretical calculations using density functional theory were carried out on the synthesized compounds and model compounds and the results are consistent with the experimental data. The calculated HOMO-LUMO gaps for compounds 1, 2 and 3 are 3.736, 4.060, and 3.669 eV, respectively, which supports the stronger back-donation for compound 2 than that of compounds 1 and 3.

Synthesis, spectroscopy, and hydrogen/deuterium exchange in high-spin iron(II) hydride complexes

Very few hydride complexes are known in which the metals have a high-spin electronic configuration. We describe the characterization of several high-spin iron(II) hydride/deuteride isotopologues and their exchange reactions with one another and with H2/D2. Though the hydride/deuteride signal is not observable in NMR spectra, the choice of isotope has an influence on the chemical shifts of distant protons in the dimers through the paramagnetic isotope effect on chemical shift. This provides the first way to monitor the exchange of H and D in the bridging positions of these hydride complexes. The rate of exchange depends on the size of the supporting ligand, and this is consistent with the idea that H2/D2 exchange into the hydrides occurs through the dimeric complexes rather than through a transient monomer. The understanding of H/D exchange mechanisms in these high-spin iron hydride complexes may be relevant to postulated nitrogenase mechanisms.

Coordination-triggered NO release from a dinitrosyl iron complex leads to anti-inflammatory activity

The appropriate control of redox and coordination chemistry of dinitrosyl iron complexes enables them to become anti-inflammatory agents.

Cobalt-Magnesium and Iron-Magnesium Complexes with Weakened Dinitrogen Bridges

The cooperative binding of N₂ by late transition metals and main-group metals is a promising strategy for N-N bond weakening and activation. We report the use of activated Rieke magnesium for reduction of iron and cobalt complexes supported by bulky β-diketiminate ligands. Binding of N₂ is accompanied by assembly of a linear M-NN-Mg-NN-M (M = Co, Fe) core with N-N bonds that are weakened, as judged by infrared spectroscopy. Both the cobalt and iron complexes require THF solvent, because of Mg-THF binding. The cobalt complex can be isolated as a pure solid, but the iron complex is stable only in solution. These results demonstrate the correlation between the binding mode and N-N weakening in heterobimetallic N₂ complexes.

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.

Ligand effects on hydrogen atom transfer from hydrocarbons to three-coordinate iron imides

A new β-diketiminate ligand with 2,4,6-tri(phenyl)phenyl N-substituents provides protective bulk around the metal without exposing any weak C-H bonds. This ligand improves the stability of reactive iron(III) imido complexes with Fe═NAd and Fe═NMes functional groups (Ad = 1-adamantyl; Mes = mesityl). The new ligand gives iron(III) imido complexes that are significantly more reactive toward 1,4-cyclohexadiene than the previously reported 2,6-diisopropylphenyl diketiminate variants. Analysis of X-ray crystal structures implicates Fe═N-C bending, a longer Fe═N bond, and greater access to the metal as potential reasons for the increase in C-H bond activation rates.