氮分子与过渡金属中心钼、铁、钒相互作用的理论研究

通过密度泛函理论计算研究了过渡金属模型体系的二氮复合物(N2)M(NH2)3(NH3)和(N2)M(SH)3(NH3)(-OCH2COO-)(M=Mo,V,Fe),预测了质子-电子还原二氮过程的能量变化.计算结果表明,对于单过渡金属中心体系,钼与N2存在强的相互作用,钒与N2相互作用较弱,铁不能有效地结合N2.通过质子化过渡金属模型体系中高柠檬酸二齿配体的羧基氧,并改变金属中心的价态和体系的自旋态,可以调控羧基氧与金属配位键的生成与断裂.两类模型体系的计算结果显示,对于单过渡金属中心的钼、钒、铁体系,钼是结合N2最有效的活性位.     

Insights into the mechanistic dichotomy of the protein farnesyltransferase peptide substrates CVIM and CVLS

Protein farnesyltransferase (FTase) catalyzes farnesylation of a variety of peptide substrates. (3)H α-secondary kinetic isotope effect (α-SKIE) measurements of two peptide substrates, CVIM and CVLS, are significantly different and have been proposed to reflect a rate-limiting S(N)2-like transition state with dissociative characteristics for CVIM, while, due to the absence of an isotope effect, CVLS was proposed to have a rate-limiting peptide conformational change. Potential of mean force quantum mechanical/molecular mechanical studies coupled with umbrella sampling techniques were performed to further probe this mechanistic dichotomy. We observe the experimentally proposed transition state (TS) for CVIM but find that CVLS has a symmetric S(N)2 TS, which is also consistent with the absence of a (3)H α-SKIE. These calculations demonstrate facile substrate-dependent alterations in the transition state structure catalyzed by FTase.     

Intramolecular hydroboration of unsaturated phosphine boranes

Homoallylic phosphine boranes undergo intramolecular hydroboration upon activation by triflic acid. The reaction occurs via an intermediate B-trifluorosulfonyloxyborane complex such as 15, followed by S(N)1-like or S(N)2-like displacement of the triflate leaving group, apparently leading to the formation of a four-center transition state. In the case of trisubstituted double bonds, as in the substrates 29 and 32, ionic hydrogenation of the alkene competes with internal hydroboration.     

N2 hydrogenation from activated end-on bis(indenyl) zirconium dinitrogen complexes

Sodium amalgam reduction of the bis(indenyl)zirconium dihalide complexes, (eta5-C9H5-1-iPr-3-Me)2ZrX2 (X = Cl, Br, I), yielded the corresponding end-on dinitrogen complexes, [(eta5-C9H5-1-iPr-3-Me)2Zr(NaX)]2(mu2, eta1, eta1-N2), with inclusion of 1 equiv of salt per zirconocene. The solid state structures of the chloro and iodo congeners establish short Zr N and elongated N N bonds, consistent with modest to strong activation of the coordinated dinitrogen molecule. Exposure of the N2 compounds to 1 atm of dihydrogen resulted in rapid N H bond formation to yield a hydrido zirconocene hydrazido compound concomitant with salt elimination. These studies establish a new structural type of zirconocene dinitrogen complex and demonstrate that side-on coordination of the N2 ligand in the ground state is not a prerequisite for dinitrogen hydrogenation.     

Gas phase SN2 reactions of halide ions with trifluoromethyl halides: front- and back-side attack vs. complex formation

Density functional theory computations and pulsed-ionization high-pressure mass spectrometry experiments have been used to explore the potential energy surfaces for gas-phase S(N)2 reactions between halide ions and trifluoromethyl halides, X(-) + CF(3)Y --> Y(-) + CF(3)X. Structures of neutrals, ion-molecule complexes, and transition states show the possibility of two mechanisms: back- and front-side attack. From pulsed-ionization high-pressure mass spectrometry, enthalpy and entropy changes for the equilibrium clustering reactions for the formation of Cl(-)(BrCF(3)) (-16.5 +/- 0.2 kcal mol(-1) and -24.5 +/- 1 cal mol(-1) K(-1)), Cl(-)(ICF(3)) (-23.6 +/- 0.2 kcal mol(-1)), and Br(-)(BrCF(3)) (-13.9 +/- 0.2 kcal mol(-1) and -22.2 +/- 1 cal mol(-1) K(-1)) have been determined. These are in good to excellent agreement with computations at the B3LYP/6-311+G(3df)//B3LYP/6-311+G(d) level of theory. It is shown that complex formation takes place by a front-side attack complex, while the lowest energy S(N)2 reaction proceeds through a back-side attack transition state. This latter mechanism involves a potential energy profile which closely resembles a condensed phase S(N)2 reaction energy profile. It is also shown that the Cl(-) + CF(3)Br --> Br(-) + CF(3)Cl S(N)2 reaction can be interpreted using Marcus theory, in which case the reaction is described as being initiated by electron transfer. A potential energy surface at the B3LYP/6-311+G(d) level of theory confirms that the F(-) + CF(3)Br --> Br(-) + CF(4) S(N)2 reaction proceeds through a Walden inversion transition state.     

Kinetic evidence for remote pi-aryl participation in the BF3-catalyzed rearrangement of [2 + 2] photocycloadducts of diarylhomobenzoquinones with diphenylacetylene

[reaction: see text] The BF(3)-catalyzed rearrangement of cyclobutene-fused m- and p-substituted diarylhomobenzoquinones exclusively gave the keto-alcohols via a Wagner-Meerwein vinyl-anion migration followed by the annulation of a delta-located endo-aryl group. The Hammett treatments for the endo/exo substituent effects, as well as the kinetic solvent effects, indicated that this reaction proceeds through the concerted S(N)2-like mechanism involving a rate-determining endo-aryl-assisted transition state.     

Probing the Primary Photochemical Processes of Octahedral Iron(V) Formation with Femtosecond Mid‐infrared Spectroscopy

Species containing iron at an oxidation state higher than +III are often termed “high‐valent iron” and are considered to be key catalytic intermediates in biochemistry. Here, we report the direct time‐domain probing of the photochemical formation of an octahedral nitrido iron(V) complex through dinitrogen cleavage from an diazido iron(III) precursor by using femtosecond mid‐infrared (MIR) spectroscopy. From the time‐resolved vibrational spectra, a mechanism is suggested for the photooxidation of the metal within 10 ps. This mechanism involves an initial ultrafast non‐adiabatic transition, followed by a quasithermal N?N bond rupture on the ground‐state surface.     

Theoretical investigation on the electronic and geometric structure of GaN2+ and GaN4+

The electronic and geometric structures of gallium dinitride cation, GaN2+ and gallium tetranitride cation, GaN4+ were systematically studied by employing density functional theory (DFT-B3LYP) and perturbation theory (MP2, MP4) in conjunction with large basis sets, (aug-)cc-pVxZ, x = T, Q. A total of 7 structures for GaN2+ and 24 for GaN4+ were identified, corresponding to minima, transition states, and saddle points. We report geometries and dissociation energies for all the above structures as well as potential energy profiles, potential energy surfaces, and bonding mechanisms for some low-lying electronic states. The calculated dissociation energy (De) of the ground state of GaN2+, X1Sigma+, is 5.6 kcal/mol with respect to Ga+(1S) + N2(X1Sigmag+) and that of the excited state, ?3Pi, is 24.8 kcal/mol with respect to Ga+(3P) + N2(X1Sigmag+). The ground state and the first excited minimum of GaN4+ are of 1A1(C2v) and 3B1(C2v) symmetry with corresponding De of 11.0 and 43.7 kcal/mol with respect to Ga+(1S) + 2N2(X1Sigmag+) for X1A1 and Ga+(3P) + 2N2(X1Sigmag+) for 3B1.     

氮化锇配合物离子[OsN(mnt)2]-的电子结构和光谱性质的理论研究

理论研究了离子型配合物[OsN(mnt)2]-[mnt=S2C2(CN)2)]的电子结构和光谱性质, 考察不同配体三价N、二硫氰烯S2C2(CN)2和金属Os的相互作用对光化学性质的影响. 分别在B3LYP/LANL2DZ和CIS/LANL2DZ水平上优化了配合物的基态和激发态结构. 与基态(1A1)相比, 激发态(3A2)的Os≡N 的键长缩短了0.0066 nm, 这与计算得到的频率增大一致, 使用TD-DFT方法计算得到了配合物的吸收和发射光谱. 计算得到的位于300 nm(f=0.1497)和262 nm(f=0.2890)的强吸收都来自1A1→1B1跃迁, 分别指认为SC→Os≡N+CN 和N+SC→Os≡N+CN的电子跃迁. 最低能量的吸收位于446 nm(f=0.0206) 处, 来自1A1→1B2的电子跃迁, 指认为N→Os和 N+SC→CN. 计算得到配合物在气态的磷光发射位于678 nm(A3A2→X1A1)处, 而在丙酮溶液中则蓝移到了625 nm处, 跃迁属性不变, 都是N→Os和S→Os的跃迁.     

Nitride formation by thermolysis of a kinetically stable niobium dinitrogen complex

The reduction of [P(2)N(2)]NbCl (where [P(2)N(2)] = PhP(CH(2)SiMe(2)NSiMe(2)CH(2))(2)PPh) with KC(8) under a dinitrogen atmosphere generates the paramagnetic dinuclear dinitrogen complex ([P(2)N(2)]Nb)(2)(mu-N(2)) (2). Complex 2 has been characterized crystallographically and by EPR spectroscopy. Variable-temperature magnetic susceptibility measurements indicate that 2 displays antiferromagnetic coupling between two Nb(IV) (d(1)) centers. A density functional theory calculation on the model complex [(PH(3))(2)(NH(2))(2)Nb](2)(mu-N(2)) was performed. Thermolysis of ([P(2)N(2)]Nb)(2)(mu-N(2)) in toluene generates the paramagnetic bridging nitride species where one N atom of the dinitrogen ligand inserts into the macrocycle backbone to form [P(2)N(2)]Nb(mu-N)Nb[PN(3)] (3) (where [PN(3)] = PhPMe(CHSiMe(2)NSiMe(2)CH(2)P(Ph)CH(2)SiMe(2)NSiMe(2)N)). Complex 3 has been characterized in the solid state as well as by variable-temperature magnetic susceptibility measurements. The reaction of ([P(2)N(2)]Nb)(2)(mu-N(2)) with phenylacetylene displaces the dinitrogen fragment to generate a paramagnetic eta(2)-alkyne complex, [P(2)N(2)]Nb(eta(2)-HCCPh) (4).     

Formation of phosphorus-nitrogen bonds by reduction of a titanium phosphine complex under molecular nitrogen

The reduction of high oxidation state metal complexes in the presence of molecular nitrogen is one of the most common methods to synthesize a dinitrogen complex. However, the presence of strong reducing agents combined with the poor binding ability of N2 can lead to unanticipated outcomes. For example, the reduction of [NPN]ZrCl2(THF) (where NPN = PhP(CH2SiMe2NPh)2) with KC8 under N2 leads to the formation of the side-on bridged dinuclear dinitrogen complex ([NPN]Zr(THF))2(mu-eta2:eta2-N2) with an N-N bond distance of 1.503(3) A; however, reduction of the corresponding titanium precursor, [NPN]TiCl2, under N2 does not generate a dinitrogen complex, rather the bis(phosphinimide) derivative, ([N(PN)N]Ti)2, is isolated in which the added N2 is incorporated between the titanium and phosphine centers. Performing the reaction under 15N2 results in the 15N label being incorporated in the phosphinimide unit. A suggested mechanism for this process involves an initially formed dinitrogen complex being over reduced to generate a species with bridging nitrides that undergoes nucleophilic attack by the coordinated phosphine ligands and formation of the P=N bond of the phosphinimide.     

Predicting Dinitrogen Activation via Transition-Metal-Involved [4+2] Cycloaddition Reaction

As the strongest triple bond in nature, the N≡N triple bond activation has always been a challenging project in chemistry. On the other hand, since the award of the Nobel Prize in Chemistry in 1950, the Diels-Alder reaction has served as a powerful and widely applied tool in the synthesis of natural products and new materials. However, the application of the Diels-Alder reaction to dinitrogen activation remains less developed. Here we first demonstrate that a transition-metal-involved [4+2] Diels-Alder cycloaddition reaction could be used to activate dinitrogen without an additional reductant by density functional theory calculations. Further study reveals that such a dinitrogen activation by 1-metalla-1,3-dienes screened out from a series of transition metal complexes (38 species) according to the effects of metal center, ligand, and substituents can become favorable both thermodynamically (with an exergonicity of 28.2 kcal mol⁻¹) and kinetically (with an activation energy as low as 13.8 kcal mol⁻¹). Our findings highlight an important application of the Diels-Alder reaction in dinitrogen activation, inviting experimental chemists’ verification.     

Kinetic effects in heterometallic dinitrogen cleavage

The rhenium(I) dinitrogen complex (PhMe2P)4ClRe(N2) reacts with [Mo2(S2CNEt2)6](OTf)2 (6) to give the N(2)-bridged complex [(PhMe2P)4ClRe(mu-N2)Mo(S2CNEt2)3]OTf ([7]OTf). Spectroscopic (nu(NN) = 1818 cm(-1)) and structural data [d(NN) = 1.167(6) A] indicate that the bridging N(2) moiety in 7+ is slightly activated relative to free N2 or to the mononuclear Re complex. However, the complex is stable with respect to N2 cleavage. The putative products of such a cleavage, the known (Et2NCS2)3Mo(N) (5) and the newly prepared [(PhMe2P)(4)ClRe(N)]OTf ([9]OTf), are stable compounds that do not react with each other to give products of nitride coupling. Thus, the failure of 7+ to interconvert with 5 and 9+ is due not to the thermodynamic stability of the NN bond but rather to kinetic factors that disfavor N2 cleavage and nitride coupling. Implications of this result for using polar effects to facilitate N2 cleavage to nitrides as a strategy for nitrogen fixation are discussed.     

Dinitrogen binding at vanadium in a tris(alkoxide) ligand environment

We report the first tris(alkoxide)V(III) complex to bind dinitrogen. Removal of THF from V(OR)(3)THF furnishes the highly reactive V(OR)(3) fragment, which binds dinitrogen to form [V(OR)(3)](2)(μ-N(2)) in the solid state. Dinitrogen is readily released upon dissolution of the complex. Structural and DFT studies are consistent with significant activation of N(2) when bound by the vanadium tris(alkoxide) platform.     

A photochemical activation scheme of inert dinitrogen by dinuclear Ru(II) and Fe(II) complexes

A general photochemical activation process of inert dinitrogen coordinated to two metal centers is presented on the basis of high-level DFT and ab initio calculations. The central feature of this activation process is the occupation of an antibonding pi* orbital upon electronic excitation from the singlet ground state S0 to the first excited singlet state S1. Populating the antibonding LUMO weakens the triple bond of dinitrogen. After a vertical excitation, the excited complex may structurally relax in the S1 state and approaches its minimum structure in the S1 state. This excited-state minimum structure features the dinitrogen bound in a diazenoid form, which exhibits a double bond and two lone pairs localized at the two nitrogen atoms, ready to be protonated. Reduction and de-excitation then yield the corresponding diazene complex; its generation represents the essential step in a nitrogen fixation and reduction protocol. The consecutive process of excitation, protonation, and reduction may be rearranged in any experimentally appropriate order. The protons needed for the reaction from dinitrogen to diazene can be provided by the ligand sphere of the complexes, which contains sulfur atoms acting as proton acceptors. These protonated thiolate functionalities bring protons close to the dinitrogen moiety. Because protonation does not change the pi*-antibonding character of the LUMO, the universal and well-directed character of the photochemical activation process makes it possible to protonate the dinitrogen complex before it is irradiated. The pi*-antibonding LUMO plays the central role in the activation process, since the diazenoid structure was obtained by excitation from various occupied orbitals as well as by a direct two-electron reduction (without photochemical activation) of the complex; that is, the important bending of N2 towards a diazenoid conformation can be achieved by populating the pi*-antibonding LUMO.     

DFT study on chemical N2 fixation by using a cubane-type RuIr3S4 cluster: energy profile for binding and reduction of N2 to ammonia via Ru-N-NHx (x = 1-3) intermediates with unique structures

The N-N bond activation of the dinitrogen ligand in the cubane-type mixed-metal sulfido cluster, [(Cp*Ir) 3{Ru(tmeda)(N 2)}(mu 3-S) 4] (tmeda = Me 2NCH 2CH 2NMe 2), is investigated by using DFT calculations at the B3LYP level of theory. The elongated N-N bond distance, red-shifted N-N stretching, and negatively charged N 2 ligand indicate that the dinitrogen is reductively activated by complexation. The degree of the N-N bond activation is classified into the "moderately activated" category, [ Studt, F. ; Tuczek, F. J. Comput. Chem. 2006, 27, 1278 ] as in the Mo-triamidoamine complex that can catalyze N 2 reduction [ Yandulov, D. V. ; Schrock, R. R. Science 2003, 301, 76 ]. Availability of the RuIr 3S 4 cluster as a catalyst for N 2 reduction is discussed by optimizing possible intermediates in a catalytic cycle analogous to that proposed by Yandulov and Schrock. A calculated energy profile of the catalytic cycle demonstrates that the RuIr 3S 4 cluster can transform dinitrogen into ammonia in the presence of lutidinium cation and Cp* 2Co as proton and electron sources, respectively. The RuIr 3S 4 clusters with an NNH x ( x = 1-3) ligand, which are intermediates in the catalytic cycle, have a significantly bent Ru-N-N linkage, although precedent NNH x complexes generally adopt a linear M-N-N array. The unique structures of the nitrogenous ligands in these intermediates are interpreted in terms of the bonding interaction between the hydrogen atom bonded to the N 2 ligand and the adjacent iridium atom in the cuboidal RuIr 3S 4 framework.     

Model studies on p-hydroxybenzoate hydroxylase. The catalytic role of Arg-214 and Tyr-201 in the hydroxylation step

A model C-(4a)-flavinhydroperoxide (FlHOOH) is described that contains the tricyclic isoalloxazine moiety, the C-4a-hydroperoxide functionality, and a beta-hydroxyethyl group to model the effect of the 2'-OH group of the ribityl side chain of native FADHOOH. The electronic structures of this reduced flavin (H(3)()Fl(red)()), its N1 anion (H(2)()Fl(red)()(-)()), oxidized flavin (HFl(ox)()), and FlHOOH have been fully optimized at the B3LYP/ 6-31+G(d,p) level of theory. This model C-4a-flavinhydroperoxide is used to describe the transition state for the key step in the paradigm aromatic hydroxylase, p-hydroxybenzoate hydroxylase (PHBH): the oxidation of p-hydroxybenzoate (p-OHB). The Tyrosine-201 residue in PHBH is modeled by phenol, and Arginine-214 is modeled by guanidine. Electrophilic aromatic substitution proceeds by an S(N)2-like attack of the aromatic sextet of p-OHB phenolate anion on the distal oxygen of FlHOOH 3. The transition structure for oxygen atom transfer is fully optimized [B3LYP/6-31+G(d,p)] and has a classical activation barrier of 24.9 kcal/mol. These data suggest that the role of the Tyr-201 is to orient the p-OHB substrate and to properly align it for the oxygen transfer step. Although the negatively charged phenolate oxygen does activate the C-3 carbon of p-OHB phenolate anion toward oxidation relative to ortho oxidation of the carboxylate anion, it appears that H-bonding the Tyr-201 residue to this phenolic oxygen stabilizes both the ground state (GS) and the transition state (TS) approximately equally and therefore plays only a minor role, if any, in lowering the activation barrier. Complexation of p-OHB with guanidine has only a modest effect upon the oxidation barriers. When the complex is in the form of a salt-bridge (10a), the barrier is only slightly reduced. When the TSs are placed in THF solvent (COSMO) with full geometry optimization, salt-bridge TS-A is slightly favored (DeltaDeltaE() = 2.3 kcal/mol).     

Nucleophilic participation in the transition state for human thymidine phosphorylase

Recombinant human thymidine phosphorylase catalyzes the reaction of arsenate with thymidine to form thymine and 2-deoxyribose 1-arsenate, which rapidly decomposes to 2-deoxyribose and inorganic arsenate. The transition-state structure of this reaction was determined using kinetic isotope effect analysis followed by computer modeling. Experimental kinetic isotope effects were determined at physiological pH and 37 degrees C. The extent of forward commitment to catalysis was determined by pulse-chase experiments to be 0.70%. The intrinsic kinetic isotope effects for [1'-(3)H]-, [2'R-(3)H]-, [2'S-(3)H]-, [4'-(3)H]-, [5'-(3)H]-, [1'-(14)C]-, and [1-(15)N]-thymidines were determined to be 0.989 +/- 0.002, 0.974 +/- 0.002, 1.036 +/- 0.002, 1.020 +/- 0.003, 1.061 +/- 0.003, 1.139 +/- 0.005, and 1.022 +/- 0.005, respectively. A computer-generated model, based on density functional electronic structure calculations, was fit to the experimental isotope effect. The structure of the transition state confirms that human thymidine phosphorylase proceeds through an S(N)2-like transition state with bond orders of 0.50 to the thymine leaving group and 0.33 to the attacking oxygen nucleophile. The reaction differs from the dissociative transition states previously reported for N-ribosyl transferases and is the first demonstration of a nucleophilic transition state for an N-ribosyl transferase. The large primary (14)C isotope effect of 1.139 can occur only in nucleophilic displacements and is the largest (14)C primary isotope effect reported for an enzymatic reaction. A transition state structure with substantial bond order to the attacking nucleophile and leaving group is confirmed by the slightly inverse 1'-(3)H isotope effect, demonstrating that the transition state is compressed by the impinging steric bulk of the nucleophile and leaving group.