Catalytic C-H bond amination from high-spin iron imido complexes

Dipyrromethene ligand scaffolds were synthesized bearing large aryl (2,4,6-Ph(3)C(6)H(2), abbreviated Ar) or alkyl ((t)Bu, adamantyl) flanking groups to afford three new disubstituted ligands ((R)L, 1,9-R(2)-5-mesityldipyrromethene, R=aryl, alkyl). While high-spin (S=2), four-coordinate iron complexes of the type ((R)L)FeCl(solv) were obtained with the alkyl-substituted ligand varieties (for R=(t)Bu, Ad and solv=THF, OEt(2)), use of the sterically encumbered aryl-substituted ligand precluded binding of solvent and cleanly afforded a high-spin (S=2), three-coordinate complex of the type ((Ar)L)FeCl. Reaction of ((Ad)L)FeCl(OEt(2)) with alkyl azides resulted in the catalytic amination of C-H bonds or olefin aziridination at room temperature. Using a 5% catalyst loading, 12 turnovers were obtained for the amination of toluene as a substrate, while greater than 85% of alkyl azide was converted to the corresponding aziridine employing styrene as a substrate. A primary kinetic isotope effect of 12.8(5) was obtained for the reaction of ((Ad)L)FeCl(OEt(2)) with adamantyl azide in an equimolar toluene/toluene-d(8) mixture, consistent with the amination proceeding through a hydrogen atom abstraction, radical rebound type mechanism. Reaction of p-(t)BuC(6)H(4)N(3) with ((Ar)L)FeCl permitted isolation of a high-spin (S=2) iron complex featuring a terminal imido ligand, ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))), as determined by (1)H NMR, X-ray crystallography, and (57)Fe Mo?ssbauer spectroscopy. The measured Fe-N(imide) bond distance (1.768(2) ?) is the longest reported for Fe(imido) complexes in any geometry or spin state, and the disruption of the bond metrics within the imido aryl substituent suggests delocalization of a radical throughout the aryl ring. Zero-field (57)Fe Mo?ssbauer parameters obtained for ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a Fe(III) formulation and are nearly identical with those observed for a structurally similar, high-spin Fe(III) complex bearing the same dipyrromethene framework. Theoretical analyses of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) suggest a formulation for this reactive species to be a high-spin Fe(III) center antiferromagnetically coupled to an imido-based radical (J = -673 cm(-1)). The terminal imido complex was effective for delivering the nitrene moiety to both C-H bond substrates (42% yield) as well as styrene (76% yield). Furthermore, a primary kinetic isotope effect of 24(3) was obtained for the reaction of ((Ar)L)FeCl(N(p-(t)BuC(6)H(4))) with an equimolar toluene/toluene-d(8) mixture, consistent with the values obtained in the catalytic reaction. This commonality suggests the isolated high-spin Fe(III) imido radical is a viable intermediate in the catalytic reaction pathway. Given the breadth of iron imido complexes spanning several oxidation states (Fe(II)-Fe(V)) and several spin states (S=0→(3)/(2)), we propose the unusual electronic structure of the described high-spin iron imido complexes contributes to the observed catalytic reactivity.     

Imido transfer from bis(imido)ruthenium(VI) porphyrins to hydrocarbons: effect of imido substituents, C-H bond dissociation energies, and Ru(VI/V) reduction potentials

[Ru(VI)(TMP)(NSO2R)2] (SO2R = Ms, Ts, Bs, Cs, Ns; R = p-C6H4OMe, p-C6H4Me, C6H5, p-C6H4Cl, p-C6H4NO2, respectively) and [Ru(VI)(Por)(NTs)2] (Por = 2,6-Cl2TPP, F20-TPP) were prepared by the reactions of [Ru(II)(Por)(CO)] with PhI=NSO2R in CH2Cl2. These complexes exhibit reversible Ru(VI/V) couple with E(1/2) = -0.41 to -0.12 V vs Cp2Fe(+/0) and undergo imido transfer reactions with styrenes, norbornene, cis-cyclooctene, indene, ethylbenzenes, cumene, 9,10-dihydroanthracene, xanthene, cyclohexene, toluene, and tetrahydrofuran to afford aziridines or amides in up to 85% yields. The second-order rate constants (k2) of the aziridination/amidation reactions at 298 K were determined to be (2.6 +/- 0.1) x 10(-5) to 14.4 +/- 0.6 dm3 mol(-1) s(-1), which generally increase with increasing Ru(VI/V) reduction potential of the imido complexes and decreasing C-H bond dissociation energy (BDE) of the hydrocarbons. A linear correlation was observed between log k' (k' is the k2 value divided by the number of reactive hydrogens) and BDE and between log k2 and E(1/2)(Ru(VI/V)); the linearity in the former case supports a H-atom abstraction mechanism. The amidation by [Ru(VI)(TMP)(NNs)2] reverses the thermodynamic reactivity order cumene > ethylbenzene/toluene, with k'(tertiary C-H)/k'(secondary C-H) = 0.2 and k'(tertiary C-H)/k'(primary C-H) = 0.8.     

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.     

A high-valent iron-oxo corrolazine activates C-H bonds via hydrogen-atom transfer

Oxidation of the Fe(III) complex (TBP(8)Cz)Fe(III) [TBP(8)Cz = octakis(4-tert-butylphenyl)corrolazinate] with O-atom transfer oxidants under a variety of conditions gives the reactive high-valent Fe(O) complex (TBP(8)Cz(+?))Fe(IV)(O) (2). The solution state structure of 2 was characterized by XAS [d(Fe-O) = 1.64 ?]. This complex is competent to oxidize a range of C-H substrates. Product analyses and kinetic data show that these reactions occur via rate-determining hydrogen-atom transfer (HAT), with a linear correlation for log k versus BDE(C-H), and the following activation parameters for xanthene (Xn) substrate: ΔH(++) = 12.7 ± 0.8 kcal mol(-1), ΔS(++) = -9 ± 3 cal K(-1) mol(-1), and KIE = 5.7. Rebound hydroxylation versus radical dimerization for Xn is favored by lowering the reaction temperature. These findings provide insights into the factors that control the intrinsic reactivity of Compound I heme analogues.     

High-valent iron and manganese complexes of corrole and porphyrin in atom transfer and dioxygen evolving catalysis

Manganese(V) imido complexes of 5,10,15-tris(pentafluorophenyl)corrole (H(3)tpfc) can be prepared by the reaction of Mn(III)(tpfc) and organic nitrene generated from either photolytic or thermal activation of organic azides. The terminal imido complexes of manganese(V) were among the first structurally characterized examples of Mn(V) terminal imido complexes in the literature. They feature a short Mn≡N triple bond and a nearly linear M[triple bond, length as m-dash]N-C angle. The ground state of (tpfc)Mn(V)(NAr) is singlet. Contrary to expectations, arylimido complexes of manganese(V) were stable to moisture and did not undergo [NR] group transfer to olefins. Manganese(V) imido corrole with an activated tosyl imido ligand was prepared from iodoimine (ArINTs) and manganese(III) corrole. The resulting complex (tpfc)Mn(NTs) is paramagnetic (S = 1), hydrolyzes to (tpfc)Mn(O) in the presence of water, abstracts hydrogen atoms from benzylic C-H bonds, and catalyzes aziridination of alkenes. Mechanistic studies on the aziridination and hydrogen atom transfer reactions are reviewed. This perspective also describes the reaction chemistry of the heme enzyme chlorite dismutase, the mechanism by which dioxygen is formed on a single-metal site, and recent advances in functional modelling of this enzyme. We also compare the reactivity of water-soluble iron versus manganese porphyrins towards the chlorite anion.     

用NMR法研究几种不对称双烃基硫桥六羰基二铁的异构化动力学

本文用常规核磁共振方法研究了(苄基硫)(烃基硫)六羰基二铁(烃基为甲基、乙基)和动态核磁共振方法研究了(苄基硫)(叔丁基硫)六羰基二铁的异构化反应动力学, 提出了可能的反应机理, 讨论了影响反应速率的因素.     

Oxidation of tertiary silanes by osmium tetroxide

In the presence of an excess of pyridine ligand L, osmium tetroxide oxidizes tertiary silanes (Et(3)SiH, (i)Pr(3)SiH, Ph(3)SiH, or PhMe(2)SiH) to the corresponding silanols. With L = 4-tert-butylpyridine ((t)Bupy), OsO(4)((t)Bupy) oxidizes Et(3)SiH and PhMe(2)SiH to yield 100 +/- 2% of silanol and the structurally characterized osmium(VI) mu-oxo dimer [OsO(2)((t)Bupy)(2)](2)(mu-O)(2) (1a). With L = pyridine (py), only 40-60% yields of R(3)SiOH are obtained, apparently because of coprecipitation of osmium(VIII) with [Os(O)(2)py(2)](2)(mu-O)(2) (1b). Excess silane in these reactions causes further reduction of the OsVI products, and similar osmium "over-reduction" is observed with PhSiH(3), Bu(3)SnH, and boranes. The pathway for OsO(4)(L) + R(3)SiH involves an intermediate, which forms rapidly at 200 K and decays more slowly to products. NMR and IR spectra indicate that the intermediate is a monomeric Os(VI)-hydroxo-siloxo complex, trans-cis-cis-Os(O)(2)L(2)(OH)(OSiR(3)). Mechanistic studies and density functional theory calculations indicate that the intermediate is formed by the [3 + 2] addition of an Si-H bond across an O=Os=O fragment. This is the first direct observation of a [3 + 2] intermediate in a sigma-bond oxidation, though such species have previously been implicated in reactions of H-H and C-H bonds with OsO(4)(L) and RuO(4).     

Synthesis and reactivity of bis(imido) uranium(VI) cyclopentadienyl complexes

The bis(imido) uranium(VI)-C(5)H(5) and -C(5)Me(5) complexes (C(5)H(5))(2)U(N(t)Bu)(2), (C(5)Me(5))(2)U(N(t)Bu)(2), (C(5)H(5))U(N(t)Bu)(2)(I)(dmpe), and (C(5)H(5))(2)U(N(t)Bu)(2)(dmpe) can be synthesized from reactions between U(N(t)Bu)(2)(I)(2)(L)(x) (L=THF, x=2; L=dmpe, x=1) and Na(C(5)R(5)) (R=H, Me); these complexes represent the first structurally characterized C(5)H(5)-compounds of uranium(VI) and they further highlight the differences between UO(2)(2+) and the bis(imido) fragment.     

Modeling the cis-oxo-labile binding site motif of non-heme iron oxygenases: water exchange and oxidation reactivity of a non-heme iron(IV)-oxo compound bearing a tripodal tetradentate ligand

The spectroscopic and chemical characterization of a new synthetic non-heme iron(IV)-oxo species [Fe(IV)(O)((Me,H) Pytacn)(S)](2+) (2, (Me,H)Pytacn=1-(2'-pyridylmethyl)-4,7-dimethyl-1,4,7-triazacyclononane, S=CH(3)CN or H(2)O) is described. Complex 2 was prepared by reaction of [Fe(II)(CF(3)SO(3))(2)((Me,H) Pytacn)] (1) with peracetic acid. Complex 2 bears a tetradentate N(4) ligand that leaves two cis sites available for binding an oxo group and a second external ligand but, unlike the related iron(IV)-oxo species with tetradentate ligands, it is remarkably stable at room temperature (t(1/2)>2 h at 288 K). Its ability to exchange the oxygen atom of the oxo ligand with water has been analyzed in detail by means of kinetic studies, and a mechanism is proposed on the basis of DFT calculations. Hydrogen-atom abstraction from C-H bonds and oxygen-atom transfer to sulfides by 2 have also been studied. Despite its thermal stability, 2 proves to be a very powerful oxidant that is capable of breaking the strong C-H bond of cyclohexane (bond dissociation energy=99.3 kcal mol(-1)).     

Synthesis of cationic rhenium(VII) oxo imido complexes and their tunability towards oxygen atom transfer

A facile method is described for the synthesis of cationic Re(VII) cis oxo imido complexes of the form [Re(O)(NAr)(salpd)+] (salpd = N,N'-propane-1,3-diylbis(salicylideneimine)), 4, [Re(O)(NAr)(saldach)+] (saldach = N,N'-cyclohexane-1,3-diylbis(salicylideneimine)), 5, and [Re(O)(NAr)(hoz)2+] (hoz = 2-(2'-hydroxyphenyl)-2-oxazoline) (Ar = 2,4,6,-(Me)C(6)H(2); 4-(OMe)C(6)H(4); 4-(Me)C(6)H(4); 4-(CF3)C6H4; 4-MeC(6)H(4)SO(2)), 6, from the reaction of oxorhenium(V) [(L)Re(O)(Solv)+] (1-3) and aryl azides under ambient conditions. Unlike previously reported cationic Re(VII) dioxo complexes, these cationic oxo imido complexes can be obtained on a preparative scale, and an X-ray crystal structure of [Re(O)(NMes)(saldach)+], 5a, has been obtained. Despite the multiple stereoisomers that could arise from tetradentate ligation of salen ligands to rhenium, one major isomer is observed and isolated in each instant. The electronic rationalization for stereoselectivity is discussed. Investigation of the mechanism suggests that the reactions of Re(V) with aryl azides proceed through an azido adduct similar to the group 5 complexes of Bergman and Cummins. Treatment of the cationic oxo imido complexes with a reductant (PAr(3), PhSMe, or PhSH) results in oxygen atom transfer (OAT) and the formation of cationic Re(V) imido complexes. [(salpd)Re(NMes)(PPh(3))(+)] (7) and [(hoz)2Re(NAr)(PPh(3))(+)] (Ar = m-OMe phenyl) (9) have been isolated on a preparative scale and fully characterized including an X-ray single-crystal structure of 7. The kinetics of OAT, monitored by stopped-flow spectroscopy, has revealed rate saturation for substrate dependences. The different plateau values for different oxygen acceptors (Y) provide direct support for a previously suggested mechanism in which the reductant forms a prior-equilibrium adduct with the rhenium oxo (ReVII = O<--Y). The second-order rate constants of OAT, which span more than 3 orders of magnitude for a given substrate, are significantly affected by the electronics of the imido ancillary ligand with electron-withdrawing imidos being most effective. However, the rate constant for the most active oxo imido rhenium(VII) is 2 orders of magnitude slower than that observed for the known cationic dioxo Re(VII) [(hoz)2Re(O)(2)(+)].     

Synthesis and reactivity of low-coordinate iron(II) fluoride complexes and their use in the catalytic hydrodefluorination of fluorocarbons

Transition metal fluoride complexes are of interest because they are potentially useful in a multitude of catalytic applications, including C-F bond activation and fluorocarbon functionalization. We report the first crystallographically characterized examples of molecular iron(II) fluorides: [L(Me)Fe(mu-F)]2 (1(2)) and L(tBu)FeF (2) (L = bulky beta-diketiminate). These complexes react with donor molecules (L'), yielding trigonal-pyramidal complexes L(R)FeF(L'). The fluoride ligand is activated by the Lewis acid Et2O.BF3, forming L(tBu)Fe(OEt2)(eta1-BF4) (3), and is also silaphilic, reacting with silyl compounds such as Me3SiSSiMe3, Me3SiCCSiMe3, and Et3SiH to give new thiolate L(tBu)FeSSiMe3 (4), acetylide L(tBu)FeCCSiMe3 (5), and hydride [L(Me)Fe(mu-H)]2 (6(2)) complexes. The hydrodefluorination (HDF) of perfluorinated aromatic compounds (hexafluorobenzene, pentafluoropyridine, and octafluorotoluene) with a silane R3SiH (R3 = (EtO)3, Et3, Ph3, (3,5-(CF3)2C6H3)Me2) is catalyzed by addition of an iron(II) fluoride complex, giving mainly the singly hydrodefluorinated products (pentafluorobenzene, 2,3,5,6-tetrafluoropyridine, and alpha,alpha,alpha,2,3,5,6-heptafluorotoluene, respectively) in up to five turnovers. These catalytic perfluoroarene HDF reactions proceed with activation of the C-F bond para to the most electron-withdrawing group and are dependent on the degree of fluorination and solvent polarity. Kinetic studies suggest that hydride generation is the rate-limiting step in the HDF of octafluorotoluene, but the active intermediate is unknown. Mechanistic considerations argue against oxidative addition and outer-sphere electron transfer pathways for perfluoroarene HDF. Fluorinated olefins are also hydrodefluorinated (up to 10 turnovers for hexafluoropropene), most likely through a hydride insertion/beta-fluoride elimination mechanism. Complexes 1(2) and 2 thus provide a rare example of a homogeneous system that activates C-F bonds without competitive C-H activation and use an inexpensive 3d transition metal.     

A terminal Ni(III)-imide with diverse reactivity pathways

The synthesis and structure of the beta-diketiminato Ni(I) lutidine adducts [MexNN]Ni(2,4-lutidine) (x = 2 (2); x = 3 (3)) are described which serve as synthons to the "naked" 13-electron [MexNN]Ni fragments in reactions with N3Ad to give Ni-imido complexes. The singly bridged imide {[Me2NN]Ni}2(mu-NAd) (4) possesses short Ni-Ni (2.506(1) A) and Ni-N(imido) distances (1.732(4)-1.752(4) A). Steric modification of the beta-diketiminate ligand to include an additional methyl group in the N-aryl 4-position affords the Ni(III) terminal imide [Me3NN]Ni=NAd (8) isolated in 52% yield. The X-ray structure of terminal imide 8 reveals a contracted Ni-N(imido) bond distance (1.662(2) A) and an only somewhat bent imido linkage (Ni-N-C = 164.5(2) degrees ) consistent with a significant degree of multiple bond character. Frozen glass EPR studies of 5 indicate a rhombic environment in which one of the signals exhibits strong hyperfine coupling (A = 22 G) to the imido 14N (I = 1) nucleus. The terminal imide 5 undergoes complete imido group transfer to CO and CNBut to give AdNCO and AdNCNBut, respectively, as well as with PMe3 to afford AdN=PMe3. Exemplifying the radical character at the imido N atom, 5 adds to cobaltocene and abstracts a H atom from 1,4-cyclohexadiene to give the Ni(II)-amides [Me3NN]Ni-NAd(eta4-C5H5)CoCp (7) and [Me3NN]Ni-NHAd (8).     

DFT calculations of d0 M(NR)(CHtBu)(X)(Y) (M = Mo, W; R = CPh3, 2,6-iPr-C6H3; X and Y = CH2tBu, OtBu, OSi(OtBu)3) olefin metathesis catalysts: structural, spectroscopic and electronic properties

DFT(B3PW91) calculations have been carried out to rationalise the structural, electronic and spectroscopic properties of Mo and W imido M(NR1)(CHR2)(X)(Y) olefin metathesis catalysts by using either simplified or actual ligands of the experimental complexes. The calculated structures, energetics (preference for the syn isomer and alkylidene rotational barrier for the syn/anti interconversion), and spectroscopic properties (NMR J(C-H) coupling constants) are in good agreement with available experimental data. Additionally, the alkylidene nu(C-H) stretching frequencies, not available experimentally, have been calculated. These quasi-tetrahedral complexes have a linear imido group and a C-H alkylidene agostic interaction, which stabilizes the syn isomer. Whether looking at M(NR1)(CHR2)(X)(Y), M = Mo, W, or the isolobal Re complexes, Re(CR1)(CHR2)(X)(Y), a linear correlation is obtained between both the alkylidene nu(C-H) stretching frequencies and J(C-H) coupling constants with the calculated alkylidene C-H bond lengths. These correlations show that the strength of the alpha-C-H agostic interaction increases from alkylidyne Re to imido group 6 complexes and from Mo to W. The NBO and AIM Bader analyses show firstly that the imido and alkylidyne groups are both triply bonded to the metal, but that the triply bonded imido ligand is a weaker electron donor than the alkylidyne, hence the stronger alpha-C-H agostic interaction for group 6 imido complexes. Secondly, one of the pi bonds of the triply bonded ligand is weakened at the transition state of the alkylidene rotation: while no lone pair is formed, the metal-ligand triple bond is polarized. This is more favourable for an imido than for an alkylidyne ligand, hence the lower alkylidene rotational barrier for the former complexes. Conversely, the aryl imido is even less of an electron donor than the alkyl imido group, which in turn strengthens the alpha-C-H agostic interaction and lowers the alkylidene rotational barrier even more.     

Molybdenum(VI) dioxo and oxo-imido complexes of fluorinated β-ketiminato ligands and their use in OAT reactions

Substitution of a methyl by a trifluoromethyl moiety in well-known β-ketimines afforded the ligands (Ar)NC(Me)CH(2)CO(CF(3)) (HL(H), Ar = C(6)H(5); HL(Me), A r= 2,6-Me(2)C(6)H(3); HL(iPr), Ar = 2,6-(i)Pr(2)C(6)H(3)). Subsequent complexation to the [MoO(2)](2+) core leads to the formation of novel complexes of general formula [MoO(2)(L(R))(2)] (R = H, 1; R = Me, 2; R = iPr, 3). For reasons of comparison the oxo-imido complex [MoO(N(t)Bu)(L(Me))(2)] (4) has also been synthesized. Complexes 1-4 were investigated in oxygen atom transfer (OAT) reactions using the substrate trimethylphosphine. The respective products after OAT, the reduced Mo(IV) complexes [MoO(PMe(3))(L(R))(2)] (R = H, 5; R = Me, 6; R = iPr, 7) and [Mo(N(t)Bu)(PMe(3))(L(Me))(2)] (8), were isolated. All complexes have been characterized by NMR spectroscopy, and 1-4 also by cyclic voltammetry. A positive shift of the Mo(VI)-Mo(V) reduction wave upon fluorination was observed. Furthermore, molecular structures of complexes 2, 4, 5, and 8 have been determined via single crystal X-ray diffraction analysis. Complex 8 represents a rare example of a Mo(IV) phosphino-imido complex. Kinetic measurements by UV-vis spectroscopy of the OAT reactions from complexes 1-4 to PMe(3) showed them to be more efficient than previously reported nonfluorinated ones, with ligand L' = (Ar)NC(Me)CH(2)CO(CH(3)) [MoO(2)(L')(2)] (9) and [MoO(N(t)Bu)(L')(2)] (10), respectively. Thermodynamic activation parameters ΔH(?) and ΔS(?) of the OAT reactions for complexes 2 and 4 have been determined. The activation enthalpy for the reaction employing 2 is significantly smaller (12.3 kJ/mol) compared to the reaction with the nonfluorinated complex 9 (60.8 kJ/mol). The change of the entropic term ΔS(?) is small. The reaction of the oxo-imido complex 4 to 8 revealed a significant electron-donating contribution of the imido substituent.     

Activation of C-H / H-H bonds by rhodium(II) porphyrin bimetalloradicals

Reactivity, kinetic, and thermodynamic studies are reported for reactions of a rhodium(II) bimetalloradical with H(2), and with the methyl C-H bonds for a series of substrates CH(3)R (R = H, CH(3), OH, C(6)H(5)) using a m-xylyl diether tethered diporphyrin ligand. Bimolecular substrate reactions involving the intramolecular use of two metalloradical centers and preorganization of the four-centered transition state (M*...X...Y*...M) result in large rate enhancements as compared to termolecular reactions of monometalloradicals. Activation parameters and deuterium kinetic isotope effects for the substrate reactions are reported. The C-H bond reactions become less thermodynamically favorable as the substrate steric requirements increase, and the activation free energy (DeltaG++) decreases regularly as DeltaG degrees becomes more favorable. An absolute Rh-H bond dissociation enthalpy of 61.1 +/- 0.4 kcal mol(-1) is directly determined, and the derived Rh-CH(2)R BDE values increase regularly with the increase in the C-H BDE.     

Electron microscopic imaging of a single Group 8 metal atom catalyzing C-C bond reorganization of fullerenes

Heating a bulk sample of [60]fullerene complexes, (η(5)-C(5)H(5))MC(60)R(5) (M = Fe, Ru, R = Me, Ph), produces small hydrocarbons because of coupling of R and C(5)H(5) via C-C and C-H bond activation. Upon observation by transmission electron microscopy, these complexes, encapsulated in single-walled carbon nanotubes, underwent C-C bond reorganization reactions to form new C-C bond networks, including a structure reminiscent of [70]fullerene. Quantitative comparison of the electron dose required to effect the C-C bond reorganization of fullerenes and organofullerenes in the presence of a single atom of Ru, Fe, or Ln and in the the absence of metal atoms indicated high catalytic activity of Ru and Fe atoms, as opposed to no catalytic activity of Ln. Organic molecules such as hydrocarbons and amides as well as pristine [60]fullerene maintain their structural integrity upon irradiation by ca. 100 times higher electron dose compared to the Ru and Fe organometallics. The results not only represent a rare example of direct observation of a single-metal catalysis but also have implications for the use of single metal atom catalysis in Group 8 metal heterogeneous catalysis.     

Oxo transfer reactions mediated by bis(dithiolene)tungsten analogues of the active sites of molybdoenzymes in the DMSO reductase family: comparative reactivity of tungsten and molybdenum.

The discovery of tungsten enzymes and molybdenum/tungsten isoenzymes, in which the mononuclear catalytic sites contain a metal chelated by one or two pterin-dithiolene cofactor ligands, has lent new significance to tungsten-dithiolene chemistry. Reaction of [W(CO)(2)(S(2)C(2)Me(2))(2)] with RO(-) affords a series of square pyramidal desoxo complexes [W(IV)(OR')(S(2)C(2)Me(2))(2)](1)(-), including R' = Ph (1) and Pr(i)() (3). Reaction of 1 and 3 with Me(3)NO gives the cis-octahedral complexes [W(VI)O(OR')(S(2)C(2)Me(2))(2)](1)(-), including R' = Ph (6) and Pr(i)() (8). These W(IV,VI) complexes are considered unconstrained versions of protein-bound sites of DMSOR and TMAOR (DMSOR = dimethylsulfoxide reductase, TMAOR = trimethylamine N-oxide reductase) members of the title enzyme family. The structure of 6 and the catalytic center of one DMSO reductase isoenzyme have similar overall stereochemistry and comparable bond lengths. The minimal oxo transfer reaction paradigm thought to apply to enzymes, W(IV) + XO --> W(VI)O + X, has been investigated. Direct oxo transfer was demonstrated by isotope transfer from Ph(2)Se(18)O. Complex 1 reacts cleanly and completely with various substrates XO to afford 6 and product X in second-order reactions with associative transition states. The substrate reactivity order with 1 is Me(3)NO > Ph(3)AsO > pyO (pyridine N-oxide) > R(2)SO > Ph(3)PO. For reaction of 3 with Me(3)NO, k(2) = 0.93 M(-)(1) s(-)(1), and for 1 with Me(2)SO, k(2) = 3.9 x 10(-)(5) M(-)(1) s(-)(1); other rate constants and activation parameters are reported. These results demonstrate that bis(dithiolene)W(IV) complexes are competent to reduce both N-oxides and S-oxides; DMSORs reduce both substrate types, but TMAORs are reported to reduce only N-oxides. Comparison of k(cat)/K(M) data for isoenzymes and k(2) values for isostructural analogue complexes reveals that catalytic and stoichiometric oxo transfer, respectively, from substrate to metal is faster with tungsten and from metal to substrate is faster with molybdenum. These results constitute a kinetic metal effect in direct oxo transfer reactions for analogue complexes and for isoenzymes provided the catalytic sites are isostructural. The nature of the transition state in oxo transfer reactions of analogues is tentatively considered. This research presents the first kinetics study of substrate reduction via oxo transfer mediated by bis(dithiolene)tungsten complexes.     

Organometallic dithiolene complexes of the group 8-10 metals: reactivities, structures and electrochemical behavior

Research progress in the organometallic dithiolene complexes such as [Cp(or Cp*)M(dithiolene)] (M = Co, Rh, Ir, Ni), [(C(6)R(6))Ru(dithiolene)] and [(C(4)R(4))Pt(dithiolene)] complexes during the past decade is described and the reactivities, structures and electrochemical behavior are summarized in this paper. The five-membered metalladithiolene ring (MS(2)C(2)) undergoes addition reactions to the M[double bond, length as m-dash]S bond to form 18-electron adducts by an imido, alkylidene, alkene or norbornene group and also undergoes dimerizations on the basis of the unsaturation in the ring. The aromaticity of the ring causes substitution reactions on the dithiolene carbon by a C-centered radical, S-centered radical or succinimide group when the ring has a C-H bond. Furthermore a dithiolene-dithiolene homo-coupling reaction by an acid or dithiolene-aryl cross-coupling occurs based on the aromaticity in the ring. Dissociations of the 18-electron adducts are observed by those thermolyses, photolyses, electrochemical redox reactions and other chemical reactions with tertiary phosphorus compounds. One representative example of them is the imido adduct dissociation with PR(3) under heating toward the intramolecular imido migration to a Cp ligand. Since all products are rearomatized by those adduct dissociations, it is concluded that the 'coexistence of aromaticity and unsaturation' in the metallacycle mediates the diverse chemical reactions.     

Electron-transfer oxidation of coenzyme B12 model compounds and facile cleavage of the cobalt(IV)-carbon bond via charge-transfer complexes with bases. A negative temperature dependence of the rates

The electron-transfer oxidation and subsequent cobalt-carbon bond cleavage of vitamin B12 model complexes were investigated using cobaloximes, (DH)2Co(III)(R)(L), where DH- = the anion of dimethylglyoxime, R = Me, Et, Ph, PhCH2, and PhCH(CH3), and L = a substituted pyridine, as coenzyme B12 model complexes and [Fe(bpy)3](PF6)3 or [Ru(bpy)3](PF6)3 (bpy = 2,2'-bipyridine) as a one-electron oxidant. The rapid one-electron oxidation of (DH)2Co(III)(Me)(py) (py = pyridine) with the oxidant gives the corresponding Co(IV) complexes, [(DH)2Co(IV)(Me)(py)]+, which were well identified by the ESR spectra. The reorganization energy (lambda) for the electron-transfer oxidation of (DH)2Co(Me)(py) was determined from the ESR line broadening of [(DH)2Co(Me)(py)]+ caused by the electron exchange with (DH)2Co(Me)(py). The lambda value is applied to evaluate the rate constants of photoinduced electron transfer from (DH)2Co(Me)(py) to photosensitizers in light of the Marcus theory of electron transfer. The Co(IV)-C bond cleavage of [(DH)2Co(Me)(py)]+ is accelerated significantly by the reaction with a base. The overall activation energy for the second-order rate constants of Co(IV)-C bond cleavage of [(DH)2Co(IV)(Me)(py)]+ in the presence of a base is decreased by charge-transfer complex formation with a base, which leads to a negative activation energy for the Co(IV)-C cleavage when either 2-methoxypyridine or 2,6-dimethoxypyridine is used as the base.     

Study of Ligand Substitution Reactions at Pentacyanoferrates(II) in Aqueous Salt and Micellar Solutions

The ligand substitution reactions Fe(CN)(5)(4-(t)bupy)(3-) + 4-CNpy and Fe(CN)(5)(4-(t)bupy)(3-) + pzCO(2)(-) (4-(t)Bupy = 4-tert-butylpyridine; 4-CNpy = 4-cyanopyridine; pzCO(2)(-) = pyrazinecarboxylate) were studied in several aqueous salt and micellar solutions. Kinetic data in aqueous solutions showed that the two processes follow a dissociative mechanism, D, and the dependence of the first-order rate constants on [salt] on electrolyte aqueous solutions allow the estimation of the activation volumes corresponding to both reactions. Under true first-order conditions no kinetic micellar effects were found in anionic (SDS) and nonionic (Triton X-100) aqueous micellar solutions. In cationic micellar solutions (CTAB, CTAC, and TTAB) small kinetic micellar effects were found. These were related to the different ionic concentrations and the different polarity and structure of the Stern layer surrounding the cationic micellar aggregates, where the reactions take place, with respect to pure water. Copyright 2000 Academic Press.