Fluorination of anionic σ-complexes by cesium fluoroxysulfate

Anionic -complexes and related structures react with cesium fluoroxysulfate to give predominantly the C-fluoro derivatives: fluoronitrocyclohexadienes or substituted fluoronitrobenzenes. Oxidation or degradation of a complex to form substituted nitrobenzenes is also possible. The balance between all these processes is determined by the structure of the original complex.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 7, pp. 1615–1619, July, 1991.     

Transition metal-alkane σ-complexes with oxygen donor co-ligands

A new family of long-lived alkane σ-complexes of the type (L(OEt))Re(CO)(2)(alkane) [alkane = cyclopentane, cyclohexane, pentane; L(OEt) = cyclopentadienyltris(diethylphosphito)cobaltate(III)] has been observed using both IR and NMR spectroscopies and computationally interrogated with DFT methods. The oxygen-rich coordination spheres makes these complexes perhaps more relevant as models for intermediates in metal oxide mediated hydrocarbon transformations than other known alkane σ-complexes.     

Anionic σ-complexes of 1,3,5-tris(fluorosulfonyl)benzene

We have found that symmetrical benzene trisulfonic-acid trifluoride 1 is able to undergo nucleophilic addition at free positions on the aromatic ring. Its reactions with sodium sulfite, morpholine and carbanions of malonic and acetoacetic esters, dimedone and nitromethane lead to the formation of comparatively stable anionic σ-complexes 3-8.     

Reactions of activated organonickel σ-complexes with elemental (white) phosphorus

The reactivity of organonickel σ-complexes of the type [NiBr(Ar)(bpy)] (Ar is 2,4,6-tri-methylphenyl (Mes) or 2,4,6-triisopropylphenyl (Tipp); bpy is 2,2′-bipyridine) toward elemental (white) phosphorus was studied. For the reaction to occur, the complexes must be activated by removal of the bromide anion from the coordination sphere of nickel. This can be achieved either in the presence of halogen scavengers or by electrochemical reduction. The arylphosphinic acids ArP(O)(OH)H formed by hydrolysis of organic nickel phosphides are the major reaction products of the overall process.     

Pathways for C-H bond cleavage of propane σ-complexes on PdO(101)

We used dispersion-corrected density functional theory (DFT-D3) calculations to investigate the initial C-H bond cleavage of propane σ-complexes adsorbed on the PdO(101) surface. The calculations predict that propane molecules adsorbed in η(1) configurations can undergo facile C-H bond cleavage on PdO(101), where the energy barrier for C-H bond activation is lower than that for desorption for each molecular complex. The preferred pathway for propane dissociation on PdO(101) corresponds to cleavage of a primary C-H bond of a so-called staggered p-2η(1) complex which initially coordinates with the surface by forming two H-Pd dative bonds, one at each CH(3) group. Among all of the adsorbed propane complexes, the staggered p-2η(1) complex has the highest binding energy and must overcome the lowest energy barrier for C-H bond scission. Analysis of the atomic charges reveals that propane C-H bond cleavage occurs heterolytically on PdO(101), and suggests that primary C-H bond activation is favored because a more stabilizing charge distribution develops within the 1-propyl transition state structures. Lastly, we conducted kinetic simulations using microkinetic models derived from the DFT-D3 structures, and find that the models reproduce the apparent activation energy for propane dissociation on PdO(101) to within 14% of that determined experimentally. We show that the entropic contributions of the adsorbed transition structures greatly exceed those predicted by the harmonic oscillator model, and that quantitative agreement with the apparent dissociation pre-factor may be obtained by approximating two of the frustrated adsorbate motions as free motions while treating the remaining modes as harmonic vibrations.     

Bond energies, reaction volumes, and kinetics for σ- and π-complexes of MoCO5L

The photosubstitution reactions of molybdenum hexacarbonyl with σ and π donor ligands were investigated using photoacoustic calorimetry and computational methods in a series of linear alkane solvents (pentane, hexane, heptane, octane, decane, and dodecane). The results show that reaction volumes make a significant contribution to the photoacoustic signal and must be considered during thermodynamic calculations based on photoacoustic measurements. The enthalpies of CO substitution by an alkane solvent and subsequent substitution by each Lewis base were determined. Corresponding Mo-L bond energies (kcal mol(-1)) were calculated: L = linear alkanes (13), triethylsilane (26), 1-hexyne (27), 1-hexene (27), and benzene (17). The relative energies are in agreement with computational results. The experimental reaction volume for CO substitution by alkane was positive (15 mL mol(-1)) and negative or close to zero for alkane substitution by a Lewis base (for example, -11 mL mol(-1) for triethylsilane and 3.6 mL mol(-1) for benzene). The errors in the experimental and computational reaction volumes are large and often comparable to the reaction volumes. An improved calibration of the methods as well as a better understanding of the underlying physics involved is needed. For the Lewis bases reported in this study, the second-order rate constants for the displacement of a coordinated alkane are less than diffusion control (5 × 10(6)-4 × 10(7) M(-1) s(-1)) and decrease monotonically with the alkane chain length. The rate constants correlate better with steric effects than with bond energies. An interchange mechanism is consistent with the results.     

High selectivity for primary C-H bond cleavage of propane σ-complexes on the PdO(101) surface

We investigated regioselectivity in the initial C-H bond activation of propane σ-complexes on the PdO(101) surface using temperature programmed reaction spectroscopy (TPRS) experiments. We observe a significant kinetic isotope effect (KIE) in the initial C-H(D) bond cleavage of propane on PdO(101) such that the dissociation yield of C(3)H(8) is 2.7 times higher than that of C(3)D(8) at temperatures between 150 and 200 K. Measurements of the reactivity of (CH(3))(2)CD(2) and (CD(3))(2)CH(2) show that deuteration of the methyl groups is primarily responsible for the lower reactivity of C(3)D(8) relative to C(3)H(8), and thus that 1° C-H bond cleavage is the preferred pathway for propane activation on PdO(101). By analyzing the rate data within the context of a kinetic model for precursor-mediated dissociation, we estimate that 90% of the propane σ-complexes which dissociate on PdO(101) during TPRS do so by 1° C-H bond cleavage.     

Nitroazines 24. stable anionic σ-adducts of condensed nitrotriazines

Stable anionic -adducts of azolo-ring condensed 1, 2, 4-traizines with water were synthesized, and their x-ray structural analysis was carried out.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 1, pp. 52–57, January, 1994.     

Relativistic DFT calculations of the NMR properties and reactivity of transition metal methane σ-complexes: insights on C-H bond activation

Relativistic ZORA DFT methods have been employed to predict the NMR properties of methane and methyl hydride complexes of rhodium and iridium. Two of these compounds, the rhodium methane and the iridium methyl hydride complexes, have been recently characterized by NMR spectroscopy. Calculations reveal that relativistic effects are largely responsible of the high shielding observed for the proton and carbon resonances of the methane moiety. The key steps for the reaction mechanism of C-H cleavage catalyzed by both compounds have been investigated at the relativistic level. Although the structure of the intermediates and TSs for the Rh and Ir complexes is rather similar, subtle differences in the energetics are responsible of the different catalytic activity of the two complexes.     

First example of the formation of bipolar spiro σ-complexes from 6-bromo-5,7-dinitrobenzo[e]-1,2,3,4-tetrazine 1,3-di-N-oxide

The reactions of 3,5,7-trimethyltropolone and 2-(N-benzylamino)tropone with 6-bromo-5,7-dinitrobenzo[e]-1,2,3,4-tetrazine 1,3-di-N-oxide afforded novel bipolar spiro -complexes. The kinetic and activation parameters of the R,S-enantiotopomerization of a chiral spiro -complex of 2-(N-benzylamino)tropone were determined using dynamic ¹H NMR spectroscopy.     

Intramolecular donor-acceptor interaction in stilbene and styrene π-complexes with tricarbonylchromium. Observation in the IR spectra

The IR spectra of complexes derived from conjugated arylalkenes and tricarbonylchromium, namely (stilbene)tricarbonylchromium and (styrene)tricarbonylchromium, displayed three absorption bands instead of two expected in the region of carbonyl stretching vibrations (1800–2000 cm⁻¹). Additional absorption bands also appeared in the region corresponding to metal-π-fragment stretching vibrations (250–400 cm⁻¹). These findings indicated additional interaction involving the central metal atom, carbonyl ligands, and aromatic π-system. Such interaction increases mobility of the tricarbonylchromium fragment which may become capable of readily migrating from one π-fragment to another under certain conditions.     

Effect of medium on the equilibrium of cationic σ- and π-complexes: interconversion of 1-pentafluorophenylthio-1,2-dimethyl-acenaphthylenium and 1,2-dimethylacenaphtylene-1,2-S-pentafluorophenylepisulphonium ions

The equilibrium of 1-pentafluorophenylthio-1,2-dimethylacenaphthylenium and 1,2-dimethylacenaphthylene-1, 2-S-pentafluorophenylepisulphonium ions in different media has been investigated by¹H and¹³C NMR spectroscopy using isotope perturbation. It has been found that this equilibrium is weakly sensitive to variation in the medium.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1057–1060, June, 1993.     

On the possibility of existence of η5-π-complexes of C60 fullerene: π-complexes with silicon

Qualitative estimates of possibility of the formation of fullerene ⁵--complexes have been carried out by the MNDO/PM3 method. It was shown, as exemplified by the C₆₀ cluster, that the introduction of five univalent functional groups R (R = H, Cl, Br) to -positions of a separate pentagon of C₆₀ with the formation of [R₅C₆₀]^– anions results in a pronounced increase in the -electron density on the atoms of this five-membered cycle and more favorable conditions for the formation of -bonds with the ⁵-ligand. The nature of the interaction between the atoms of the separate cycle in [R₅C₆₀]^– anion and ⁵-ligand was analyzed by the example of hypothetical sandwich systems R₅C₆₀SiCp. Half-sandwich complexes R₅C₆₀SiX (X = H, Cl) were also investigated. The local energy minima were found on the potential energy surfaces (PES) of systems R₅C₆₀SiCp and R₅C₆₀SiX with C_5p symmetry. These systems transform barrirlessly into q5-7E-complexes with the angular structure if the symmetry restrictions are removed. The most favorable conditions for ⁵--complexes of R₅C₆₀ to form are realized for R = H. The results obtained were compared to those of semiempirical and nonempirical calculations of bis (cyclopentadienyl) silicon.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 10, pp. 2422–2429, October, 1996.     

High electrophilicity at silicon in η3-silane σ-complexes: Lewis base adducts of a silane ligand, featuring octahedral silicon and three Ru-H-Si interactions

New η(3)-silane σ-complexes [PhBP(Ph)(3)]RuH(η(3)-H(2)SiRR') (RR' = PhMe, Ph(2)) were synthesized. Lewis bases [THF, 4-(dimethylamino)pyridine, and PMe(3)] coordinate to the silicon centers of these complexes to form stable adducts. The base adducts, [PhBP(Ph)(3)]Ru(μ-H)(3)SiRR'(base), feature three nonclassical Ru-H-Si interactions and hexacoordinate silicon centers, as determined by multinuclear NMR spectroscopy, X-ray crystallography, and computational investigations.     

Aryne insertion into I-I σ-bonds

A new protocol for the efficient synthesis of o-diiodoarenes has been developed. This method allows the synthesis of substituted and polycyclic o-diiodoarenes, which are difficult to obtain by classical methods. This diiodination process involves the formal insertion of arynes into the I-I σ-bond.     

Exohedral η5-π-complexes of fullerene C20

The problem of stabilization of polyhedral carbon clusters, which do not obey the isolated pentagon rule, is discussed taking a dodecahedral fullerene C₂₀ as an example. Ab initio MO LCAO calculations in the HF/3-21G approximation showed that fullerene C₂₀ as well as its C₂₀H₅ hydrides with C _5v symmetry can form stable 2⁵--complexes of the CpFeC₂₀FeCp and H₅C₂₀FeC₂₀H₅ types. The energies of the ⁵-Fe--C₂₀ -bonds in these complexes were compared with those of the Fe--Cp bond in ferrocene and the Fe--C₆₀ bond in the ⁵--C₆₀H₅FeCp complex.     

New synthetic routes to cationic arene π-complexes of transition metals

A review is given of the authors' studies on the development of new methods for the synthesis of cationic arene complexes of transition metals. The synthesis in strong protic acids facilitated obtaining -arene cations of Mn, Re, Ru, Os, Co, Rh and Ir. New nonacidic methods allowed the set of available cationic arene -complexes of Mn, Ru, Rh and Ir to be increased considerably. Based on the developed methods a number of previously unknown binuclear organometallic cations are obtained.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1186–1195, July, 1993.     

Tautomerism, Hammett σ, and QSAR

A consideration of equilibrium model-based equations suggests that tautomeric equilibria do not markedly affect observed potency if the tautomer bound represents at least 50% of the compound in solution. Tautomeric equilibria can enhance or attenuate the correlation of potency with Hammett σ. Additionally, tautomeric equilibria can lead to a correlation of potency with σ even in the absence of a correlation of binding with σ.     

Peculiarities of dissociative ionization of alkylcyclopentadienyl nitrosyl π-complexes of nickel

The decomposition of alkylcyclopentadienyl nitrosyl -complexes of nickel, (C₅H₄R)(NO)Ni (R=H, Et,i-Pr, CH₂Ph), under the action of electron impact has been studied. The nature of the nitrosyl ligand has been shown to be the factor determining the main fragmentation pathway which involves the abstraction of an NO molecule. The effect of the nature of the ligand on the ability of the molecular ion (C₅H₄R)LNi⁺ (L=C₅H₄R, C₅H₅, C₃H₅, NO) to rearrange with hydrogen atom migration from one ligand to another has been considered. The structure of the alkyl group R determines a competing fragmentation pathway involving cleavage of the -C-C bond with respect to the cyclopentadienyl ring in the substituent.Translated fromIzyestiya Akademii Nauk. Seriya Khimicheskaya, No. 11, pp. 1985–1988, November, 1993.