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.     

The vibrational group frequency of the N-O* stretching band of nitroxide stable free radicals

The group frequency of the N-O radical stretching vibration has received scant attention in the literature. The few existing treatments of the vibrational spectroscopy of nitroxides are incomplete at best and potentially misleading to workers in the field. To close this gap in the available knowledge, the existing literature on the vibrational spectra of nitroxide stable free radicals is critically reviewed with particular reference to the wavenumber position of the N-O stretching vibration, nu(N-O). Poor evidentiary bases for the assignment nu(N-O) were found in many instances. Ab initio Density Field Theory calculations using a model chemistry of UB3LYP at the 6-311++G(d,p) level were performed to obtain a theoretical band position of nu(N-O) for comparison with the published data. Large discrepancies between the theoretical and experimental values were found for the radical 3-carbamoyl-2,2,5,5-tetramethyl-3-pyrrolin-1-yloxyl, which currently sets the lower limit of the accepted wavenumber range of nu(N-O), as well as for the nitronyl and iminyl nitroxides. The wavenumber position of nu(N-O) was found to occur in the range 1450-1420cm(-1) for 5-membered cyclic nitroxides and 1395-1340cm(-1) for 6-membered cyclic and acyclic nitroxides. In nitronyl nitroxides, the symmetric stretching vibration occurs in the region 1470cm(-1), but coupling to other modes makes specific band assignments problematic for the nitronyl nitroxide group.     

Studies on electron spin resonance spectroscopy of biradical molecules containing (14)N-O and (15)N-O moieties

The biradicals with (14)N-Oxide and (15)N-Oxide at the both ends of a molecule are synthesized for the molecular ruler of protein structure, and a potential device for quantum computing. We also establish a general synthetic method for reliable biradical formation. ESR spectra are recorded for the biradicals containing (15)N-Oxide and (14)N-Oxide with various interdistance separations. We find that two types of biradicals yielded different ESR spectra depending upon the distance between the (15)N-O and (14)N-O moieties in a molecule. This is due to electron spin dipole-dipole interaction occurring between the radicals. We also find that there is an indication of isotopic nuclear effects in the dipole-dipole interactions. The present study implies feasibility of the distance measurement between two different N-Oxides containing (14)N and (15)N isotopes. We conclude that quantum entanglement effects are observed through the dipolar interactions, which enable application of quantum computing devices operating in the liquid state.     

Samarium(II) iodide reduction of isoxazolidines

SmI₂ was used as reducing agent for the N-O bond cleavage in isoxazolidines. The procedure revealed is general and particularly useful for the transformation of 5-spirocyclopropane isoxazolidines to the corresponding β-aminocyclopropanols, a troublesome transformation with other known reagents.     

Electronic structure of six-coordinate iron(III)-porphyrin NO adducts: the elusive iron(III)-NO(radical) state and its influence on the properties of these complexes

This paper investigates the interaction between five-coordinate ferric hemes with bound axial imidazole ligands and nitric oxide (NO). The corresponding model complex, [Fe(TPP)(MI)(NO)](BF4) (MI = 1-methylimidazole), is studied using vibrational spectroscopy coupled to normal coordinate analysis and density functional theory (DFT) calculations. In particular, nuclear resonance vibrational spectroscopy is used to identify the Fe-N(O) stretching vibration. The results reveal the usual Fe(II)-NO(+) ground state for this complex, which is characterized by strong Fe-NO and N-O bonds, with Fe-NO and N-O force constants of 3.92 and 15.18 mdyn/A, respectively. This is related to two strong pi back-bonds between Fe(II) and NO(+). The alternative ground state, low-spin Fe(III)-NO(radical) (S = 0), is then investigated. DFT calculations show that this state exists as a stable minimum at a surprisingly low energy of only approximately 1-3 kcal/mol above the Fe(II)-NO(+) ground state. In addition, the Fe(II)-NO(+) potential energy surface (PES) crosses the low-spin Fe(III)-NO(radical) energy surface at a very small elongation (only 0.05-0.1 A) of the Fe-NO bond from the equilibrium distance. This implies that ferric heme nitrosyls with the latter ground state might exist, particularly with axial thiolate (cysteinate) coordination as observed in P450-type enzymes. Importantly, the low-spin Fe(III)-NO(radical) state has very different properties than the Fe(II)-NO(+) state. Specifically, the Fe-NO and N-O bonds are distinctively weaker, showing Fe-NO and N-O force constants of only 2.26 and 13.72 mdyn/A, respectively. The PES calculations further reveal that the thermodynamic weakness of the Fe-NO bond in ferric heme nitrosyls is an intrinsic feature that relates to the properties of the high-spin Fe(III)-NO(radical) (S = 2) state that appears at low energy and is dissociative with respect to the Fe-NO bond. Altogether, release of NO from a six-coordinate ferric heme nitrosyl requires the system to pass through at least three different electronic states, a process that is remarkably complex and also unprecedented for transition-metal nitrosyls. These findings have implications not only for heme nitrosyls but also for group-8 transition-metal(III) nitrosyls in general.     

Structural and spectroscopic characterization of mononuclear copper(I) nitrosyl complexes: end-on versus side-on coordination of NO to copper(I)

Two crystal structures of the mononuclear copper(I)-nitrosyl complexes [Cu(L3)(NO)] (1) and [Cu(L3')(NO)](ClO4) (2) with the related coligands L3- (hydrotris(3-tert-butyl-5-isopropyl-1-pyrazolyl)borate) and L3' (tris(3-tert-butyl-5-isopropyl-1-pyrazolyl)methane) are presented. These compounds are then investigated in detail using a variety of spectroscopic methods. Vibrational spectra show nu(N-O) at 1698 cm(-1) and nu(Cu-NO) split at 365/338 cm(-1) for 1, which translates to force constants of 12.53 (N-O) and 1.31 mdyn/A (Cu-NO), respectively. The weak Cu-NO force constant is in agreement with the observed instability of the Cu-NO bond. Interestingly, complex 2 with the neutral coligand L3' shows a stronger N-O bond, evident from nu(N-O) at 1742 cm(-1). This difference is attributed to a true second coordination sphere effect, where the covalency of the Cu(I)-NO bond is not altered. The EPR spectrum of 1 is in agreement with the Cu(I)-NO(radical) electronic structure of the complexes, as obtained from density functional theory (DFT) calculations. In addition, an interesting trend between g parallel(gz) and the Cu-N-O angle is established. Finally, high-quality MCD spectra of 1 are presented and assigned using TD-DFT calculations. Based on the in-depth spectroscopic characterization of end-on bound NO to copper(I) presented in this work, it is possible to determine the binding mode of the Cu-NO intermediate of Cu nitrite reductase studied by Scholes and co-workers (Usov, O. M.; Sun, Y.; Grigoryants, V. M.; Shapleigh, J. P.; Scholes, C. P., J. Am. Chem. Soc. 2006, 128, 13102-13111) in solution as strongly bent (approximately 135 degrees) but likely not side-on.     

The first A2B2-heterometal ferrimagnetic chain. Structures and magnetic properties of polymeric [Gd2Cu2]n and the corresponding monomer

One-dimensional polymeric complexes consisting of alternating dicopper(II) and digadolinium(III) units exhibited ferrimagnetic behavior which was ascribable to antiferromagnetic coupling across the oximate N-O bridges between the high-spin homodinuclear units.     

Synthesis and spectroscopic characterization of copper(II)-nitrito complexes with hydrotris(pyrazolyl)borate and related coligands

This study focuses on the geometric (molecular) structures, spectroscopic properties, and electronic structures of copper(II)-nitrito complexes as a function of second coordination sphere effects using a set of closely related coligands. With anionic hydrotris(pyrazolyl)borate ligands, one nitrite is bound to copper(II). Depending on the steric demand of the coligand, the coordination mode is either symmetric or asymmetric bidentate, which leads to different ground states of the resulting complexes as evident from EPR spectroscopy. The vibrational spectra of these compounds are assigned using isotope substitution and DFT calculations. The results demonstrate that nu sym(N-O) occurs at higher energy than nu asym(N-O), which is different from the literature assignments for related compounds. UV-vis absorption and MCD spectra are presented and analyzed with the help of TD-DFT calculations. The principal binding modes of nitrite to Cu(II) and Cu(I) are also investigated applying DFT. Using a neutral tris(pyrazolyl)methane ligand, two nitrite ligands are bound to copper. In this case, a very unusual binding mode is observed where one nitrite is eta1-O and the other one is eta1-N bound. This allows to study the properties of coordinated nitrite as a function of binding mode in one complex. The N-coordination mode is easily identified from vibrational spectroscopy, where N-bound nitrite shows a large shift of nu asym(N-O) to >1400 cm-1, which is a unique spectroscopic feature. The optical spectra of this compound exhibit an intense band around 300 nm, which might be attributable to a nitrite to Cu(II) CT transition. Finally, using a bidentate neutral bis(pyrazolyl)methane ligand, two eta1-O coordinated nitrite ligands are observed. The vibrational and optical (UV-vis and MCD) spectra of this compound are presented and analyzed.     

Titanium alkoxyimido (Ti=N-OR) complexes: reductive N-O bond cleavage at the boundary between hydrazide and peroxide ligands

The first Group 4 alkoxyimido compounds are reported. The Ti=N-O(t)Bu group in Ti(N(2)N(Me))(NO(t)Bu)(py) undergoes facile 2-electron N-O bond cleavage with PhCCMe as the reductant to form a 1,2-diamidoalkene group via two highly selective N-C bond forming events.     

N-氧化吡啶-2-甲醛衍生物的配合物研究 III: N-氧化吡啶 -2-甲醛缩氨基硫脲的双核铜配合物的晶体结构和电子结构

合成了N-氧化吡啶 -2-甲醛缩氨基硫脲的双核铜配合物。晶体结构的测定表明两Cu原子之间是通过两个单原子氧桥相联, 每个桥联氧原子既处于一个Cu原子为中心的四方锥底面, 又是另一个Cu原子四方锥的锥顶。晶体属单斜晶系, 空间群为Ce, 晶体结构参数为a=16.445,b=13.889,c=12.770A,β=122.82°。根据半经验MO法的计算结果, 指出了红外谱中N-O键和C=N 键特征峰朝不同方向位移的原因, 并对磁偶合常数作了估计。     

Properties of chloroformyl peroxynitrate, ClC(O)OONO(2)

The synthesis of ClC(O)OONO(2) is accomplished by photolysis of a mixture of Cl(2), NO(2), and CO in large excess of O(2) at about -70 degrees C. The product is isolated after repeated trap-to-trap condensation. The solid compound melts at -84 degrees C, and the extrapolated boiling point is 80 degrees C. ClC(O)OONO(2) is characterized by IR, Raman, (13)C NMR, and UV spectroscopy. According to the IR matrix spectra, the compound exists at room temperature only as a single conformer. The molecular structure of ClC(O)OONO(2) is determined by gas electron diffraction. The molecule possesses a gauche structure with a dihedral angle of phi(COON) = 86.7(19) degrees , and the C=O bond is oriented syn with respect to the O-O bond. The short O-O bond (1.418(6) A) and the long N-O bond (1.511(8) A) are consistent with the facile dissociation of ClC(O)OONO(2) into the radicals ClC(O)OO and NO(2). The experimental geometry of ClC(O)OONO(2) is reproduced reasonably well by B3LYP/6-311+G(2df) calculations, whereas the MP2 approximation predicts the N-O bond considerably too long and the dihedral angle too small.     

New half sandwich-type Ru(II) coordination compounds characterized by the fac-Ru(dmso-S)3 fragment: influence of the face-capping group on the chemical behavior and in vitro anticancer activity

The Ru(II) complex fac-[RuCl(dmso-S)(3)(dmso-O)(2)][PF(6)] (P2) was found to be an excellent precursor for the facile preparation in high yield of half sandwich-type compounds of the general formula fac-[RuCl(dmso-S)(3)(N)(2)][PF(6)] (e.g. (N)(2) = 1,2-diaminoethane (en, 4), trans-1,2-diaminocyclohexane (dach, 5), or 2 NH(3) (6)). Neutral half sandwich-type compounds of the general formula fac-[RuCl(dmso-S)(3)(N-O)] where N-O is an anionic chelating ligand (e.g. N-O = picolinate (pic, 7)) are best prepared from the universal Ru(II)-dmso precursor cis-[RuCl(2)(dmso)(4)] (P1). These complexes, that were fully characterized in solution and in the solid state, are structurally similar to the anticancer organometallic compounds [Ru(η(6)-arene)(chel)Cl][PF(6)](n) but, in place of a face-capping arene, have the fac-Ru(dmso-S)(3) fragment. In contrast to what observed for the corresponding arene compounds, that rapidly hydrolyze the Cl ligand upon dissolution in water, compounds 4-6 are very stable and inert in aqueous solution. Probably their inertness is the reason why they showed no significant cytotoxicity against the MDA-MB-231 cancer cell line.     

Infrared spectra of the M(NO)n (M = Sn, Pb; n = 1, 2) and PbNO- molecules

Reactions of laser-ablated tin and lead atoms with nitric oxide molecules in solid argon and neon have been investigated using matrix-isolation infrared spectroscopy. In the argon experiments, absorptions at 1560.1, 1625.8, and 1486.7 cm(-1) are assigned to the N-O stretching vibrations of the SnNO and Sn(NO)2 molecules, and absorptions at 1541.9, 1630.0, 1481.8, and 1457.5 cm(-1) are assigned to the N-O stretching vibrations of the PbNO, Pb(NO)2, and PbNO- molecules on the basis of isotopic shifts and splitting patterns. The present neon experiments only produce neutral tin and lead mononitrosyls. Density functional theory calculations have been performed on these tin and lead nitrosyls. The good agreement between the experimental and calculated vibrational frequencies, relative absorption intensities, and isotopic shifts substantiates the identification of these nitrosyls from the matrix infrared spectra.     

beta(2,2)-Aminoxy acids: a new building block for turns and helices

The conformational properties of peptides 1-4 built from 3-aminoxy-2,2-dimethyl-propionic acid, a beta2,2-aminoxy acid, were investigated by NMR spectroscopy and X-ray crystallography. A novel beta N-O turn involving a nine-membered-ring intramolecular hydrogen bond between NHi+2 and COi was formed in diamides 1 and 2, which was further stabilized by another six-membered-ring intramolecular hydrogen bond between NHi+2 and NOi+1. Triamides 3 and 4 displayed a well-defined helical structure featuring two consecutive beta N-O turns. The X-ray structure of 4 revealed that the amide carbonyl group at position i+2 was twisted +65.9 degrees from that at i position, suggesting a novel 1.79 helix. Therefore, beta2,2-aminoxy acid can be used as a new building block for turns and helices.     

Comparative density functional theory and post-Hartree-Fock (CCSD, CASSCF) studies on the electronic structure of halogen nitrites ClONO and BrONO using quantum chemical topology

In this paper, the electronic structures of cis- and trans-ClONO and BrONO are studied at the CCSD∕aug-cc-pVTZ, CASSCF(14,12)/aug-cc-pVTZ, and B3LYP/aug-cc-pVTZ computational levels. For the Cl-O bond, topological analysis of the electron density field, ρ(r), shows the prevalence of the shared-electron type bond (?(2)ρ((3,-1)) < 0). The Br-O bond, however, represents the closed-shell interaction (?(2)ρ((3,-1)) > 0). Topological analysis of the electron localization function, η(r), and electron localizability indicator (ELI-D), (D) (σ)(r), shows that the electronic structure of the central N-O bond is very sensitive to both electron correlation improvements (coupled-cluster single double (CCSD), CASSCF, density functional theory (DFT)) and bond length alteration. Depending on the method used, the N-O bond can be characterized as a "normal" N-O bond with a disynaptic V(N,O) basin (DFT); a protocovalent N-O bond with two monosynaptic, V(N) and V(O), basins (CCSD, CASSCF); or a new type, first discovered for FONO, characterized by a single monosynaptic, V(N) basin (CCSD, DFT). The total basin population oscillates between 0.46-0.96 e (CCSD) and 0.86-1.02 e (CASSCF). The X-O bond is described by the single disynaptic basin, V(X,O), with a basin population between 0.76 and 0.81 e (CCSD) or 0.77 and 0.85 e (CASSCF). Analysis of the localized electron detector distribution for the cis-Cl-O1-N=O2 shows a manifold in the Cl···O2 region, associated with decreased electron density.     

Nitroxide/substrate weak hydrogen bonding: attitude and dynamics of collisions in solution

The study of intermolecular collisions and bonding interactions in solutions is of critical importance in understanding and predicting solute/solvent properties. Previous work has established that stable paramagnetic nitroxide molecules are excellent probes of intermolecular interactions for hydrogen bonding in polar solvents. In this study, 1H, 2H, 13C, 15N NMR and liquid/liquid intermolecular transfer dynamic nuclear polarization (L2IT DNP) results are obtained for the paramagnetic probe molecule, TEMPO, interacting with the common aprotic and protic polar solvents, CH3CN and CH3CONH2, yielding a profile of both dipolar and scalar interactions. A significant scalar contact hyperfine is observed for the N-O...H-C interaction (13CH3 hyperfine, a/h=0.66 MHz) in the CH3CN/TEMPO system, whereas the N-O...H-C and N-O...H-N interactions for the TEMPO/CH3CONH2 system yield 13CH3 and 15N hyperfine couplings of a/h=0.16 and -0.50 MHz, respectively. The distance and attitude of the scalar interaction for the nitroxide hydrogen bonding at the methyl group in CH3CN and the amino group in CH3CONH2 are computed using density functional theory (DFT), yielding good agreement with the experimental results. These results show that the hyperfine coupling provides a sensitive probe of weak hydrogen-bonding interactions in solution.     

A density-functional-theory study of atomic nitrogen abstraction from Si(100)-(2 x 1) by a gaseous O(3P) atom

We have employed density-functional theory (DFT) to investigate the abstraction of a nitrogen atom from the Si(100)-(2 x 1) surface by a gas-phase O(3P) atom for different initial bonding configurations of nitrogen at the surface. For the N-Si(100) structures investigated, nitrogen abstraction by an O(3P) atom is predicted to be exothermic by at least 1.9 eV. Abstraction in a single elementary step is found only for the interaction of an O(3P) atom with nitrogen bound in a coordinatively saturated configuration, and an energy barrier of 0.20 eV is computed for this reaction. For nitrogen bound in coordinatively unsaturated configurations, abstraction is predicted to occur by precursor-mediated pathways in which the initial O-surface collision results in the formation of a N-O bond and the concomitant release of between 2.7 and 4.8 eV of energy into the surface, depending on the initial N-Si(100) structure. This initial step produces different surface structures containing an adsorbed NO species, which can then undergo a series of elementary steps leading to NO desorption. Since the barriers for these steps are found to be less than 1 eV in all cases, a significant excess of energy is available from initial N-O bond formation that could activate NO desorption within no more than a few vibrational periods after the initial gas-surface collision. Nitrogen abstraction by such a pathway is essentially an Eley-Rideal process since NO desorption occurs rapidly after the initial gas-surface collision, without the reactants thermally accommodating with the surface. These computational results indicate that nitrogen abstraction by gaseous O(3P) atoms should be facile, even at low surface temperatures, if nitrogen is bound to the Si(100) surface in coordinatively unsaturated configurations.     

Theoretical studies on N-O or N-N bond formation from aryl azide catalyzed by iron(II) bromide complex

DFT calculations have been carried out to study the reaction mechanism on N-O or N-N bond formation from aryl azide catalyzed by iron(II) bromide complex. A favorable reaction pathway is proposed to account for the construction of the core structure of 2H-indazoles or 2,1-benzisoxazoles.     

Novel thermolytic transformations of n-benzoyl 2-aza 3-oxa bicyclo(2.2.1)hept-5-ene and n-benzoyl 2-aza 3-oxa bicyclo(2.2.1)heptane

Novel thermolytic pathways were encountered in careful studies with N-benzoyl 2-aza 3-oxa bicyclo (2.2.1)heptene (1) and N-benzoyl 2-aza 3-oxa bicyclo(2.2.1)heptane (2). Compound 1 thermally fragments by four major pathways, namely, (3,3)-sigmatropic shift, (4 + 2) cycloreversion, N-O rupture and C-N homolysis. The (3,3)-sigmatropic shift provides a novel route to unusual, bicyclic heterocycles. Electron withdrawing aryl groupings tend to promote the (3,3)-sigmatropic shift pathway over others. The (4 + 2)-cycloreversion of 1 leads to nitroso carbonyl benzene and cyclopentadiene. The weak σ bonds of PhCONO undergo ready homolysis. The intermediate arising from N-O rupture leads to a cyclopentenone radical similar to that involved in the PG endoperoxide → PGE change. This radical either combines with benzoyl radical leading to 4-dibenzoylamino cyclopentenone or is transformed to an enamide by loss of hydrogen, which, in turn, undergoes (4 + 2)-cycloaddition with cyclopentadiene leading to a novel spiro adduct. The C-N rupture leads to the formation of benzanilide.Nitrosocarbonyl benzene is a powerful electrophile. With cyclohexene it forms a hydroximic ester, initiated by nucleophile acceptance at the CO oxygen. In contrast, it reacts with P(OMe)₃ leading to diphenylurea via nucleophile acceptance at NO oxygen. Thermolysis of 2 proceeds largely by N-O rupture, similar to that normally observed in the PG endoperoxide → PGE change.     

Copper-mediated chelation-assisted ortho nitration of (hetero)arenes

A novel copper-mediated chelation-assisted ortho C-H nitration of (hetero)arenes has been developed for the first time, which used dioxygen as terminal oxidant and 1,2,3-TCP as solvent, leading to the synthesis of nitroaromatics with excellent regioselectivity and in good yields. Mechanistic investigations indicate a mechanism involving a four-centered transition state, with simultaneous cleavage of an ortho C-H bond and a N-O bond of the nitrate anion on the 2-arylpyridine-coordinated copper(II) complex.