Selectivity and mechanism of hydrogen atom transfer by an isolable imidoiron(III) complex

In the literature, iron-oxo complexes have been isolated and their hydrogen atom transfer (HAT) reactions have been studied in detail. Iron-imido complexes have been isolated more recently, and the community needs experimental evaluations of the mechanism of HAT from late-metal imido species. We report a mechanistic study of HAT by an isolable iron(III) imido complex, L(Me)FeNAd (L(Me) = bulky β-diketiminate ligand, 2,4-bis(2,6-diisopropylphenylimido)pentyl; Ad = 1-adamantyl). HAT is preceded by binding of tert-butylpyridine ((t)Bupy) to form a reactive four-coordinate intermediate L(Me)Fe(NAd)((t)Bupy), as shown by equilibrium and kinetic studies. In the HAT step, very large substrate H/D kinetic isotope effects around 100 are consistent with C-H bond cleavage. The elementary HAT rate constant is increased by electron-donating groups on the pyridine additive, and by a more polar medium. When combined with the faster rate of HAT from indene versus cyclohexadiene, this trend is consistent with H(+) transfer character in the HAT transition state. The increase in HAT rate in the presence of (t)Bupy may be explained by a combination of electronic (weaker Fe=N π-bonding) and thermodynamic (more exothermic HAT) effects. Most importantly, HAT by these imido complexes has a strong dependence on the size of the hydrocarbon substrate. This selectivity comes from steric hindrance by the spectator ligands, a strategy that has promise for controlling the regioselectivity of these C-H bond activation reactions.     

Thermodynamics of hydrogen atom transfer to a high-valent iron imido complex

Thermodynamic investigations relevant to hydrogen atom transfer by the high-valent iron imido complex [LMesFe[triple bond]NAd]OTf have been undertaken. The complex is found to be weakly oxidizing by cyclic voltammetry (E1/2 = -0.98 V vs Cp2Fe+/Cp2Fe in MeCN). A combination of experimental and computational studies has been used to determine the acidity of LMesFe-N(H)Ad+ (pKa = 37 in MeCN), allowing the N-H BDFE (88(5) kcal/mol) to be calculated from a thermodynamic cycle. Consistent with this value, [LMesFe[triple bond]NAd]OTf reacts with 9,10-dihydroanthracene (C-H BDE = 78(1) kcal/mol) to form anthracene.     

Theoretical investigation of the hydrogen atom transfer in the hydrated A-T base pair

The hydrated A-T base pair has been studied in order to understand the structural modifications and their electronic rearrangements induced by the movement of the hydrogen atoms in the H-bonds. The comparison of these results with that of the nonhydrated system can explain the role of the H-bonds of the water molecules in this system. Two naïve schemes have been considered, one where the hydrogen bonds of the water molecules are only indirectly involved in the hydrogen atoms transfer between the bases and another where the water molecules are directly involved in this transfer. The results support the idea that the real mechanisms are more complexes than these schemes. Some new stable structures of the A-T(H₂O)₂ and the A-T(H₂O)₄ systems have been found and the mechanisms of their generations have been analysed.     

Quantum-chemical investigation of the Diels-Alder reaction in condensed isoindoles with a nodal nitrogen atom

On the basis of calculated data on the electron structures of condensed isoindoles with a nodel nitrogen atom it is hypothesized that these compounds may act as dienes in the Diels-Alder reaction. Conclusions regarding the relative activities and the peculiarities of cycloaddition in the investigated structures were drawn from the static reactivity indexes obtained within the Pariser-Parr-Pople approximation.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 216–222, February, 1988.     

An investigation of the mechanism of single and double hydrogen atom transfer reactions in alkyl benzoates by the ortho effect

Single and double hydrogen atom transfers in reactions (1) and (2) in the mass spectra of ethyl benzoate, isopropyl benzoate, and isobutyl benzoate have been investigated with reference to the ortho effect: (1) [C₆H₅CO₂R]⁺? → [C₆H₅CO₂H]⁺? (m/z 122) + (R-H); (2) [C₆H₅CO₂R]⁺? → [C₆H₅CO₂H₂]⁺ (m/z 123) + · (R-2H). It is demonstrated that the intermediate ion [C₆H₅CO₂H₂]⁺ has the protonated benzoic acid structure with the hydrogen atom on the carbonyl group.     

Ab initio investigation of the NH(X)-N2 van der Waals complex

The NH-N(2) van der Waals complex has been examined at the CCSD(T) level of theory using aug-cc-pVDZ and aug-cc-pVTZ basis sets. The full basis set superposition error correction was applied. Two minimum energy structures were located for the electronic ground state. The global minimum corresponds to a linear geometry of the complex (NH-N-N), with D(e)=236 cm(-1) and R(c.m.)=4.22 A. The secondary minimum corresponds to a T-shaped geometry of C(2v) symmetry, where the nitrogen atom of the H-N moiety points toward the center of mass of the N(2) unit, aligned with the a-inertial axis of the complex. The binding energy and R(c.m.) value for the secondary minimum were 144 cm(-1) and 3.63 A, respectively. This potential energy surface is consistent with the properties of matrix-isolated NH-N(2), and it is predicted that linear NH-N(2) will be a stable complex in the gas phase at low temperatures.     

Quantum-chemical investigation of the potential-energy surface of the T+CH3NH2 reaction

Conclusions According to the data from the calculations by the semiempirical SCF method in the MINDO/3 all-valence electron approximation and by a nonempirical method in the STO-4-31G basis, the reaction of hot T atoms in the T+CH₃NH₂ system most probably takes place according to a mechanism involving the abstraction of an H atom. In the case of hot T atoms there is also a possibility of the replacement of the NH₂ group according to a Waiden inversion mechanism or its direct abstraction.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 1958–1961, September, 1982.     

Quantum-chemical investigation of the protonated forms of 2-(2-furyl)pyrrole

The electron and conformational structures, as well as the internal rotation, of 2-(2-furyl)pyrrole and its -protonated forms were studied by the MNDO method with complete optimization of the geometry. In conformity with the experiments (PMR), the two delocalized cations with a cis orientation of the heteroatoms that are formed as a result of protonation of the pyrrole or furan ring have the greatest and virtually equal stabilities (H=738.7 and 740.6 kJ/mole).Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 746–753, June, 1991.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Quantum-chemical investigation of the mechanism of ionic polymerization in crystals

The results of quantum-chemical investigations of radiation-induced polymerization in molecular crystals are presented. The detailed calculations of the potentials energy curves characterizing the chain generation and propagation reactions, with explicit account of the reacting system interaction with the crystalline environment, provide a reasonable interpretation of the experimental observations. The basic conclusion is that the addition of the growing polymer ion to the monomer molecule placed at a lattice point needs no activation energy. The polymer chain is spontaneously moving in the interlayer space of the crystalline lattice at every propagation step, the overall displacement reaching a macroscopic value. Several termination mechanisms are briefly discussed. The possibility of molecular tunneling cannot be completely eliminated, however, it is expected to occur at the nonchemical preliminary stage of the reaction.     

Quantum-chemical investigation of nature of neutral intermolecular hydrogen bond in complex H3PO...HF

Results are presented from modeling the complex H₃PO...HF by the ab initio Hartree-Fock-Roothaan method. A generalized method is proposed for the quantitative fragmentary analysis of molecular orbitals (MOs). It is shown that when the complex is formed, the chemical bond between the phosphine oxide and the HF molecule is formed by means of a shift of the electron pair of the -bond of the HF to a 2p orbital of the O atom and the formation of a bonding three-center MO localized on the O, H, and F atoms, and also through a shift of an electron pair from the O atom to a 2p orbital of the F atom and the formation of a nonbonding MO localized on the O and F atoms.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 28, No. 1, pp. 38–41, January–February, 1992.     

Quantum-chemical investigation of the complex of Mg-porphin with tetracyanoquinodimethane

The potential hypersurface for the interaction of the molecules of Mg-porphin (P) and tetracyanoquinodimethane (TCQ) was investigated by the CNDO/2 method. The calculated energy minimum amounts to –8.95 eV and corresponds to a distance of 2.1 Å between the planes of the molecules with some displacement in relation to the central mutual arrangement. The results are regarded as tentative in view of the fact that the CNDO/2 method exaggerates the interaction. The electronic spectra of the P·TCQ complex are calculated by the CNDO/S-CI method for a series of arrangements of P and TCQ. Satisfactory agreement with experiment is obtained with a distance of about 3 Å between the planes of the molecules. It was shown that there are two transitions with charge transfer in the near IR region of the spectrum and also several transitions of the same type in the visible and near UV regions.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 26, No. 4, pp. 431–437, July–August, 1990.     

Metal atom dynamics in a charge transfer complex

Temperature-dependent ⁵⁷Fe Mössbauer spectroscopy over the interval 89 < T < 335 K has been used to study the detailed metal atom dynamics in the charge transfer complex (CT) decamethlferrocene-acenaphthenequinone. The quadrupole splitting, area ratio and recoil-free fraction parameters clearly reflect the phase transition (T_pt) at 257 K. The root-mean-square-amplitude of vibrations of the metal atom in the CT complex have been compared to that determined earlier for decamethyl ferrocene. The vibrational amplitudes are isotropic below T_pt but anisotropic above this temperature.     

Proton-promoted oxygen atom transfer vs proton-coupled electron transfer of a non-heme iron(IV)-oxo complex

Sulfoxidation of thioanisoles by a non-heme iron(IV)-oxo complex, [(N4Py)Fe(IV)(O)](2+) (N4Py = N,N-bis(2-pyridylmethyl)-N-bis(2-pyridyl)methylamine), was remarkably enhanced by perchloric acid (70% HClO(4)). The observed second-order rate constant (k(obs)) of sulfoxidation of thioaniosoles by [(N4Py)Fe(IV)(O)](2+) increases linearly with increasing concentration of HClO(4) (70%) in acetonitrile (MeCN)at 298 K. In contrast to sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+), the observed second-order rate constant (k(et)) of electron transfer from one-electron reductants such as [Fe(II)(Me(2)bpy)(3)](2+) (Me(2)bpy = 4,4-dimehtyl-2,2'-bipyridine) to [(N4Py)Fe(IV)(O)](2+) increases with increasing concentration of HClO(4), exhibiting second-order dependence on HClO(4) concentration. This indicates that the proton-coupled electron transfer (PCET) involves two protons associated with electron transfer from [Fe(II)(Me(2)bpy)(3)](2+) to [(N4Py)Fe(IV)(O)](2+) to yield [Fe(III)(Me(2)bpy)(3)](3+) and [(N4Py)Fe(III)(OH(2))](3+). The one-electron reduction potential (E(red)) of [(N4Py)Fe(IV)(O)](2+) in the presence of 10 mM HClO(4) (70%) in MeCN is determined to be 1.43 V vs SCE. A plot of E(red) vs log[HClO(4)] also indicates involvement of two protons in the PCET reduction of [(N4Py)Fe(IV)(O)](2+). The PCET driving force dependence of log k(et) is fitted in light of the Marcus theory of outer-sphere electron transfer to afford the reorganization of PCET (λ = 2.74 eV). The comparison of the k(obs) values of acid-promoted sulfoxidation of thioanisoles by [(N4Py)Fe(IV)(O)](2+) with the k(et) values of PCET from one-electron reductants to [(N4Py)Fe(IV)(O)](2+) at the same PCET driving force reveals that the acid-promoted sulfoxidation proceeds by one-step oxygen atom transfer from [(N4Py)Fe(IV)(O)](2+) to thioanisoles rather than outer-sphere PCET.     

Kinetics and mechanism of oxygen atom abstraction from a dioxo-rhenium(VII) complex

The kinetics of reaction between triarylphosphanes and two newly prepared dioxorhenium(VII) compounds has been evaluated. The compounds are MeRe(VII)(O)(2)("O,S") in which "O,S" represents an alkoxo, thiolato chelating ligand. With MeReO(3), ligands derived from 1-mercaptoethanol and 1-mercapto-2-propanol form MeRe(O)(2)(met), 2, and MeRe(O)(2)(m2p), 3. These compounds persist in chloroform solution for several hours at room temperature and for 2-3 weeks at -22 degrees C, particularly when water is carefully excluded. They were obtained as red oils with clean (1)H NMR spectra, but attempts to obtain pure, crystalline products were not successful because one decomposition pathway shows a kinetic order >1. The fastest reaction occurs between P(p-MeOC(6)H(4))(3) and 2; k(298) = 215(7) L mol(-1) s(-1) in chloroform at 25(1) degrees C. The other rate constants follow a Hammett correlation against 3sigma, with rho = -0.69(7). This study relates to oxygen atom transfer reactions catalyzed by MeReO(mtp)PPh(3), 1, in which MeRe(O)(2)(mtp), 4, is a postulated intermediate that does not build up to a measurable concentration during the catalytic cycle. Compound 2 does not react with MeSTol, but MeS(O)Tol was formed when tert-butyl hydroperoxide was added. This suggests that equilibrium lies to the left in this reaction, 2 + MeSTol + L = MeReO(met)L + MeS(O)Tol, and is drawn to the right by a reaction between MeReO(met)L and the hydroperoxide. Triphenyl arsane does not react with 2, but thermodynamic versus kinetic barriers were not resolved.     

Sustainable radical reduction through catalyzed hydrogen atom transfer reactions (CHAT-reactions)

A system with coupled catalytic cycles is described that allows radical reduction by catalyzed hydrogen atom transfer (CHAT) from transition metal hydrides. These intermediates are generated through H₂ activation. Radical generation is carried out by titanocene catalyzed electron transfer to epoxides. The reaction provides a novel entry into the atom-economical reduction of radicals that has long been considered as a critical issue for the industrial application of radical chemistry.