Oxo-molybdenum(VI,V,IV) complexes of the facially coordinating tris(mercaptoimidazolyl)borate ligand: synthesis, characterization, and oxygen atom transfer reactivity

A series of monooxo-Mo(IV,V) and dioxo-Mo(VI) complexes of the "soft" tripodal ligand, sodium tris(mercaptoimidazolyl)borate (NaTm(Me)), have been synthesized as potential oxygen atom transfer (OAT) models for sulfite oxidase. Complexes have been characterized by X-ray crystallography, cyclic voltammetry, and EPR, where appropriate. Oxygen atom transfer kinetics of Tm(Me)MoO(2)Cl, both stoichiometric and catalytic, have been studied by a combination of UV-vis and (31)P NMR spectroscopies under a variety of conditions. OAT rates are consistent with previously established relationships between redox potential/reactivity and mechanistic studies. The analysis of these complexes as potential structural and functional analogues of relevance to molybdoenzymes is further discussed.     

Multielectron atom transfer reactions of perchlorate and other substrates catalyzed by rhenium oxazoline and thiazoline complexes: reaction kinetics, mechanisms, and density functional theory calculations

The title complexes, the Re(O)L(2)(Solv)(+) complexes (L = hoz, 2-(2'-hydroxyphenyl)-2-oxazoline(-) or thoz, 2-(2'-hydroxyphenyl)-2-thiazoline(-); Solv = H(2)O or CH(3)CN), are effective catalysts for the following fundamental oxo transfer reaction between closed shell molecules: XO + Y --> X + YO. Among suitable oxygen acceptors (Y's) are organic thioethers and phosphines, and among suitable oxo donors (XO's) are pyridine N-oxide (PyO), t-BuOOH, and inorganic oxyanions. One of the remarkable features of these catalysts is their high kinetic competency in effecting perchlorate reduction by pure atom transfer. Oxo transfer to rhenium(V) proceeds cleanly to afford the cationic dioxorhenium(VII) complex Re(O)(2)L(2)(+) in a two-step mechanism, rapid substrate (XO) coordination to give the precursor adduct cis-Re(V)(O)(OX)L(2)(+) followed by oxygen atom transfer (OAT) as the rate determining step. Electronic variations with PyO derivatives demonstrated that electron-withdrawing substituents accelerate the rate of Re(VII)(O)(2)L(2)(+) formation from the precursor adduct cis-Re(V)(O)(OX)L(2)(+). The activation parameters for OAT with picoline N-oxide and chlorate have been measured; the entropic barrier to oxo transfer is essentially zero. The potential energy surface for the reaction of Re(O)(hoz)(2)(OH(2))(+) with PyO was defined, and all pertinent intermediates and transition states along the reaction pathway were located by density functional theory (DFT) calculations (B3LYP/6-31G). In the second half of the catalytic cycle, Re(O)(2)L(2)(+) reacts with oxygen acceptors (Y's) in second-order reactions with associative transition states. The rate of OAT to substrates spans a remarkable range of 0.1-10(6) L mol(-)(1) s(-)(1), and the substrate reactivity order is Ph(3)P > dialkyl sulfides > alkyl aryl sulfides > Ph(2)S approximately DMSO, which demonstrates electrophilic oxo transfer. Competing deactivation and inhibitory pathways as well as their relevant kinetics are also reported.     

Oxygen atom transfer reactivity from a dioxo-Mo(VI) complex to tertiary phosphines: synthesis, characterization, and structure of phosphoryl intermediate complexes

The oxygen atom transfer (OAT) reactivity of TpMoO2Cl with PMe3, PEt3, and PPhMe2 (where Tp = hydrotris(3,5-dimethylpyrazol-1-yl)borate) has been investigated. The OAT reactions proceed through a diamagnetic Mo(IV) phosphoryl intermediate complex of general formula TpMoOCl(OPR3) (OPR3 = OPMe3, OPEt3, OPPhMe2), which have been isolated and characterized by 1H and 31P NMR, UV-visible, and infrared spectroscopies and electrospray ionization mass spectrometry. Solid-state crystal structures of TpMoOCl(OPMe3) and TpMoOCl(OPPhMe2) are also reported, the oxygen-to-phosphorus distances agree with a double-bond formulation and a single bond between the metal and the phosphoryl oxygen atom. The stability of the phosphoryl intermediate complexes depends on the steric properties of the coordinated phosphine-oxides. These intermediate complexes have been converted to solvent-coordinated species, TpMoOCl(solv) (solv = acetonitrile or dmf), and the coordinated solvents exchange with the bulk solvent.     

Active-site stereochemical control of oxygen atom transfer reactivity in sulfite oxidase

A number of both experimental and computational studies have recently been reported for symmetric, six-coordinate dioxomolybdenum(VI) complexes as models of the fully oxidized form of the molybdopterin enzyme sulfite oxidase (SO). Such studies have suggested that the two terminal oxo donors in SO are electronically equivalent. However, the consensus structure of the catalytically competent Mo(VI) active site in SO is five-coordinate square pyramidal, possessing two terminal oxo donors, an ene-1,2-dithiolate chelate and a cysteine sulfur donor ligand. Computational studies at the density functional level of theory have been performed on a minimal model of the SO active site, [Mo(VI)O2(S2C2Me2)(SCH3)]-, in C1 symmetry to evaluate the composition of the LUMO, which is the putative electron acceptor orbital in the oxygen atom transfer (OAT) reaction with the sulfite substrate. The LUMO in this model is principally composed of a Mo dxy - ppi* interaction between the Mo and the equatorial oxygen (Oeq), while the axial oxygen (Oax) possesses no contribution to this orbital. In fact, the LUMO+1 orbital which possesses a substantial amount of Oax character lies nearly 1 eV higher in energy than the LUMO. It has also been suggested that changes in the Oax-Mo-Sthiolate-C torsion angle during the course of enzyme catalysis may aid in selection of Oeq for OAT. Calculations were performed in which this torsion angle was varied by 20 degrees through 360 degrees . These calculations demonstrate that the Mo dxy -Oeq ppi* interaction, and therefore the Oeq atom character, always dominates the LUMO. The results presented here suggest that oxygen atom selection and activation are a direct function of the low-symmetry structure of the oxidized SO active site and provide a role for the ene-1,2-dithiolate in promoting OAT reactivity through its kinetic trans effect on the equatorial oxo donor.     

Thermodynamic, kinetic, and mechanistic study of oxygen atom transfer from mesityl nitrile oxide to phosphines and to a terminal metal phosphido complex

The enthalpies of oxygen atom transfer (OAT) from mesityl nitrile oxide (MesCNO) to Me(3)P, Cy(3)P, Ph(3)P, and the complex (Ar[(t)Bu]N)(3)MoP (Ar = 3,5-C(6)H(3)Me(2)) have been measured by solution calorimetry yielding the following P-O bond dissociation enthalpy estimates in toluene solution (±3 kcal mol(-1)): Me(3)PO [138.5], Cy(3)PO [137.6], Ph(3)PO [132.2], (Ar[(t)Bu]N)(3)MoPO [108.9]. The data for (Ar[(t)Bu]N)(3)MoPO yield an estimate of 60.2 kcal mol(-1) for dissociation of PO from (Ar[(t)Bu]N)(3)MoPO. The mechanism of OAT from MesCNO to R(3)P and (Ar[(t)Bu]N)(3)MoP has been investigated by UV-vis and FTIR kinetic studies as well as computationally. Reactivity of R(3)P and (Ar[(t)Bu]N)(3)MoP with MesCNO is proposed to occur by nucleophilic attack by the lone pair of electrons on the phosphine or phosphide to the electrophilic C atom of MesCNO forming an adduct rather than direct attack at the terminal O. This mechanism is supported by computational studies. In addition, reaction of the N-heterocyclic carbene SIPr (SIPr = 1,3-bis(diisopropyl)phenylimidazolin-2-ylidene) with MesCNO results in formation of a stable adduct in which the lone pair of the carbene attacks the C atom of MesCNO. The crystal structure of the blue SIPr·MesCNO adduct is reported, and resembles one of the computed structures for attack of the lone pair of electrons of Me(3)P on the C atom of MesCNO. Furthermore, this adduct in which the electrophilic C atom of MesCNO is blocked by coordination to the NHC does not undergo OAT with R(3)P. However, it does undergo rapid OAT with coordinatively unsaturated metal complexes such as (Ar[(t)Bu]N)(3)V since these proceed by attack of the unblocked terminal O site of the SIPr·MesCNO adduct rather than at the blocked C site. OAT from MesCNO to pyridine, tetrahydrothiophene, and (Ar[(t)Bu]N)(3)MoN was found not to proceed in spite of thermochemical favorability.     

Repurposing Iron- and 2-Oxoglutarate-Dependent Oxygenases to Catalyze Olefin Hydration

Mononuclear nonheme iron(II) and 2-oxoglutarate (Fe/2OG)-dependent oxygenases and halogenases are known to catalyze a diverse set of oxidative reactions, including hydroxylation, halogenation, epoxidation, and desaturation in primary metabolism and natural product maturation. However, their use in abiotic transformations has mainly been limited to C−H oxidation. Herein, we show that various enzymes of this family, when reconstituted with Fe(II) or Fe(III), can catalyze Mukaiyama hydration—a redox neutral transformation. Distinct from the native reactions of the Fe/2OG enzymes, wherein oxygen atom transfer (OAT) catalyzed by an iron-oxo species is involved, this nonnative transformation proceeds through a hydrogen atom transfer (HAT) pathway in a 2OG-independent manner. Additionally, in contrast to conventional inorganic catalysts, wherein a dinuclear iron species is responsible for HAT, the Fe/2OG enzymes exploit a mononuclear iron center to support this reaction. Collectively, our work demonstrates that Fe/2OG enzymes have utility in catalysis beyond the current scope of catalytic oxidation.     

Cross-linked poly(4-vinylpyridine-N-oxide) as a polymer-supported oxygen atom transfer reagent

Oxygen atom transfer (OAT) reagents are common in biological and industrial oxidation reactions. While many heterogeneous catalysts have been utilized in OAT reactions, heterogeneous OAT reagents have not been explored. Here, cross-linked poly(4-vinylpyridine-N-oxide), called x-PVP-N-oxide, was tested as a heterogeneous OAT reagent and its oxidation chemistry compared to its molecular counterpart, pyridine-N-oxide. The insoluble oxidant x-PVP-N-oxide demonstrated comparable reactivity to pyridine-N-oxide in direct oxidation reactions of phosphines and phosphites in acetonitrile, but x-PVP-N-oxide did not react in other solvents. The polymer backbone of x-PVP-N-oxide, however, allowed for easy filtering and recycling in sequential oxidation reactions. In addition, x-PVP-N-oxide was tested as the stoichiometric oxidant in a copper-catalyzed OAT reaction to α-diazo-benzeneacetic acid methyl ester. The heterogeneous oxidant was much less reactive than pyridine-N-oxide, indicating that interaction with the metal catalyst was challenging. These results demonstrated a proof-of-concept that recyclable, polymer-supported OAT reagents could be a viable OAT reagents in direct oxidation reactions without metal catalysts.     

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)(+)].     

Oxygen atom transfer reactions from isolated (oxo)manganese(V) corroles to sulfides

A series of five free-base corroles were metalated and brominated to form 10 manganese(III) corroles. Two of the free-base corroles and six manganese(III) corroles were analyzed by X-ray crystallography, including one complex that may be considered a transition-state analogue of oxygen atom transfer (OAT) from (oxo)manganese(V) to thioansisole. Oxidation by ozone allowed for isolation of the 10 corresponding (oxo)manganese(V) corroles, whose characterization by (1)H and (19)F NMR spectroscopy and electrochemistry revealed a low-spin and triply bound manganese-oxygen moiety. Mechanistic insight was obtained by investigating their reactivity regarding stoichiometric OAT to a series of p-thioanisoles, revealing a magnitude difference on the order of 5 between the β-pyrrole brominated (oxo)manganese(V) corroles relative to the nonbrominated analogues. The main conclusion is that the (oxo)manganese(V) corroles are legitimate OAT agents under conditions where proposed oxidant-coordinated reaction intermediates are irrelevant. Large negative Hammett ρ constants are obtained for the more reactive (oxo)manganese(V) corroles, consistent with expectation for such electrophilic species. The least reactive complexes display very little selectivity to the electron-richness of the sulfides, as well as a non-first-order dependence on the concentration of (oxo)manganese(V) corrole. This suggests that disproportionation of the original (oxo)manganese(V) corrole to (oxo)manganese(IV) and (oxo)manganese(VI) corroles, followed by substrate oxidation by the latter complex, gains importance when the direct OAT process becomes progressively less favorable.     

Oxidation-state and metal-ion dependent stereoisomerization in oxo molybdenum and tungsten complexes of a bulky alkoxy heteroscorpionate ligand

Monooxo Mo(V) complexes of a N2O heteroscorpionate ligand designated (L10O) are found to exist as isolable cis and trans isomers. We have been able to trap the kinetically labile cis isomer and follow its isomerization to the thermodynamically more stable trans form. We have also followed the kinetics of isomerization between the cis and trans isomers of the corresponding dioxo Mo(VI) and W(VI) species. Here the trans is the labile isomer that spontaneously converts to the thermodynamically more stable cis. It is observed that at 60 degrees C in DMSO the Mo(VI) complex isomerizes approximately 6.5 times faster than the Mo(V) and nearly 5 times faster than the corresponding W(VI) analogs. The temperature dependence to the kinetics of the Mo(V) and Mo(VI) isomerizations give activation parameters that are similar for both oxidation states and consistent with those previously observed in [(L1O)MoOCl2] suggesting a similar twist mechanism is operating in all cases. Thus there are oxidation state, metal ion and donor atom dependent differences in isomeric stability that could have significant implications for understanding the mechanisms of both enzymatic and nonenzymatic oxo atom transfer reactions catalyzed by complexes of Mo, W and Re.     

Faster oxygen atom transfer catalysis with a tungsten dioxo complex than with its molybdenum analog

The synthesis and characterization of a series of molybdenum ([MoO(2)Cl(L(n))]; L(1) (1), L(2) (3)) and tungsten ([WO(2)Cl(L(n))]; L(1) (2), L(2) (4)) dioxo complexes (L(1) = 1-methyl-4-(2-hydroxybenzyl)-1,4-diazepane and L(2) = 1-methyl-4-(2-hydroxy-3,5-di-tert-butylbenzyl)-1,4-diazepane) of tridentate aminomonophenolate ligands HL(1) and HL(2) are reported. The ligands were obtained by reductive amination of 1-methyl-1,4-diazepane with the corresponding aldehyde. Complexes 3 and 4 were obtained by the reaction of [MO(2)Cl(2)(dme)(n)] (M = Mo, n = 0; W, n = 1) with the corresponding ligand in presence of a base, whereas for the preparation of 1 and 2 the ligands were deprotonated by KH prior to the addition to the metal. They were characterized by NMR and IR spectroscopy, by cyclic voltammetry, mass spectrometry, elemental analysis and by single-crystal X-ray diffraction analysis. Solid-state structures of the molybdenum and tungsten cis-dioxo complexes reveal hexa-coordinate metal centers surrounded by two oxo groups, a chloride ligand and by the tridentate monophenolate ligand which coordinates meridionally through its [ONN] donor set. In the series of compounds 1-4, complexes 3 and 4 have been used as catalysts for the oxygen atom transfer reaction between dimethyl sulfoxide (DMSO) and trimethyl phosphine (PMe(3)). Surprisingly, faster oxygen atom transfer (OAT) reactivity has been observed for the tungsten complex [WO(2)Cl(L(2))] (4) in comparison to its molybdenum analog [MoO(2)Cl(L(2))] (3) at room temperature. The kinetic results are discussed and compared in terms of their reactivity.     

The nature and function of the catalytic centres of the DMSO reductases

Density functional theory calculations have been performed to probe aspects of the function of the reaction centres of the DMSO reductase enzymes, in respect of catalysis of oxygen atom transfer (OAT). The first comparison between Mo and W at the active site of these enzymes has been accomplished by a consideration of the reaction profile for OAT from DMSO to [MoIV(OMe)(S2C2H2)2]1- versus that for the corresponding reaction with [WIV(OMe)(S2C2H2)2]1-. Both reaction profiles involve two transition states separated by a well-defined intermediate; however, whilst the second transition state (TS2) is clearly rate-limiting for the Mo system, the two transition states have a similar energy for the W system. The activation energy for OAT from DMSO to [WIV(OMe)(S2C2H2)2]1- is ca. 23 kJ mol-1 lower for the corresponding reaction with Mo, consistent with the significantly faster rate of reduction of DMSO by Rhodobacter capsulatus W-DMSO reductase than by its Mo counterpart. Consistent with the principle of the entatic state, the geometrical constraints imposed by the protein on the metal centre of the Mo- and W-DMSO reductases facilitate OAT by favouring a trigonal prismatic geometry for the transition state TS2 that is close to that observed for the metal in the oxidised form of each of these enzymes. The effects of different tautomers of a simplified form of the pyran ring-opened, dihydropterin state of the molybdopterin cofactor on the reaction profile for OAT have been considered. The major effect, a significant lowering of the activation barrier associated with TS2, is observed for a protonated form of a tautomer that involves conjugation between the pyrazine and metallodithiolene rings.     

The structure and reactivity of iron nitride complexes

The structure and reactivity of discrete iron nitride complexes is described. Six-coordinate, four-fold symmetric nitrides are thermally unstable, and have been characterized at cryogenic temperatures by an arsenal of spectroscopic methods. By contrast, four-coordinate, three-fold symmetric iron nitrides can be prepared at room temperature. A range of diamagnetic iron(IV) nitrides have been reported and in some cases, isolated. Among these are the isolable, yet reactive, tris(carbene)borate iron(IV) nitrides. These complexes can effect two-electron nitrogen atom transfer to a range of substrates, in some cases with complete atom transfer occuring through Fe-N bond cleavage. These nitrides are also active in single electron pathways, including the synthesis of ammonia by a mechanism involving hydrogen atom transfer to the nitride ligand. One-electron oxidation of a tris(carbene)borate iron(IV) nitride leads to an isolable iron(V) complex that is unusually reactive for a metal nitride.     

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.     

Stereochemical consequences of oxygen atom transfer and electron transfer in imido/oxido molybdenum(IV, V, VI) complexes with two unsymmetric bidentate ligands

Two equivalents of the unsymmetrical Schiff base ligand (L(tBu))(-) (4-tert-butyl phenyl(pyrrolato-2-ylmethylene)amine) and MoCl(2)(NtBu)O(dme) (dme = 1,2-dimethoxyethane) gave a single stereoisomer of a mixed imido/oxido Mo(VI) complex 2(tBu). The stereochemistry of 2(tBu) was elucidated using X-ray diffraction, NMR spectroscopy, and DFT calculations. The complex is active in an oxygen atom transfer (OAT) reaction to trimethyl phosphane. The putative intermediate five-coordinate Mo(IV) imido complex coordinates a PMe(3) ligand, giving the six-coordinate imido phosphane Mo(IV) complex 5(tBu). The stereochemistry of 5(tBu) is different from that of 2(tBu) as shown by NMR spectroscopy, DFT calculations, and X-ray diffraction. Single-electron oxidation of 5(tBu) with ferrocenium hexafluorophosphate gave the stable cationic imido phosphane Mo(V) complex [5(tBu)](+) as the PF(6)(-) salt. EPR spectra of [5(tBu)](PF(6)) confirmed the presence of PMe(3) in the coordination sphere. Single-crystal X-ray diffraction analysis of [5(tBu)](PF(6)) revealed that electron transfer occurred under retention of the stereochemical configuration. The rate of OAT, the outcome of the electron transfer reaction, and the stabilities of the imido complexes presented here differ dramatically from those of analogous oxido complexes.     

A manganese(V)-oxo π-cation radical complex: influence of one-electron oxidation on oxygen-atom transfer

One-electron oxidation of Mn(V)-oxo corrolazine 2 affords 2(+), the first example of a Mn(V)(O) π-cation radical porphyrinoid complex, which was characterized by UV-vis, EPR, LDI-MS, and DFT methods. Access to 2 and 2(+) allowed for a direct comparison of their reactivities in oxygen-atom transfer (OAT) reactions. Both complexes are capable of OAT to PPh(3) and RSR substrates, and 2(+) was found to be a more potent oxidant than 2. Analysis of rate constants and activation parameters, together with DFT calculations, points to a concerted OAT mechanism for 2(+) and 2 and indicates that the greater electrophilicity of 2(+) likely plays a dominant role in enhancing its reactivity. These results are relevant to comparisons between Compound I and Compound II in heme enzymes.     

Ab initio study of the complexes of halogen-containing molecules RX (X=Cl, Br, and I) and NH3: towards understanding the nature of halogen bonding and the electron-accepting propensities of covalently bonded halogen atoms

Ab initio calculations have been performed on a series of complexes formed between halogen-containing molecules and ammonia to gain a deeper insight into the nature of halogen bonding. It appears that the dihalogen molecules form the strongest halogen-bonded complexes with ammonia, followed by HOX; the charge-transfer-type contribution has been demonstrated to dominate the halogen bonding in these complexes. For the complexes involving carbon-bound halogen molecules, our calculations clearly indicate that electrostatic interactions are mainly responsible for their binding energies. Whereas the halogen-bond strength is significantly enhanced by progressive fluorine substitution, the substitution of a hydrogen atom by a methyl group in the CH(3)X...NH(3) complex weakened the halogen bonding. Moreover, remote substituent effects have also been noted in the complexes of halobenzenes with different para substituents. The influence of the hybridization state of the carbon atom bonded to the halogen atom has also been examined and the results reveal that halogen-bond strengths decrease in the order HC triple bond CX > H(2)C=CHX approximately O=CHX approximately C(6)H(5)X > CH(3)X. In addition, several excellent linear correlations have been established between the interaction energies and both the amount of charge transfer and the electrostatic potentials corresponding to an electron density of 0.002 au along the R-X axis; these correlations provide good models with which to evaluate the electron-accepting abilities of the covalently bonded halogen atoms. Finally, some positively charged halogen-bonded systems have been investigated and the effect of the charge has been discussed.     

Mechanistic insight into the reactivity of oxotransferases by novel asymmetric dioxomolybdenum(VI) model complexes

The asymmetric molybdenum(VI) dioxo complexes of the bis(phenolate) ligands 1,4‐bis(2‐hydroxybenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐methylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐dimethylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐3,5‐di‐tert‐butylbenzyl)‐1,4‐diazepane, 1,4‐bis(2‐hydroxy‐4‐flurobenzyl)‐1,4‐diazepane, and 1,4‐bis(2‐hydroxy‐4‐chlorobenzyl)‐1,4‐diazepane (H₂(L1)–H₂(L6), respectively) have been isolated and studied as functional models for molybdenum oxotransferase enzymes. These complexes have been characterized as asymmetric complexes of type [MoO₂(L)] 1–6 by using NMR spectroscopy, mass spectrometry, elemental analysis, and electrochemical methods. The molecular structures of [MoO₂(L)] 1–4 have been successfully determined by single‐crystal X‐ray diffraction analyses, which show them to exhibit a distorted octahedral coordination geometry around molybdenum(VI) in an asymmetrical cis‐β configuration. The Mo? O_oxo bond lengths differ only by ≈0.01 Å. Complexes 1 , 2 , 5 , and 6 exhibit two successive Mo^VI/Mo^V (E_1/2, ?1.141 to ?1.848 V) and Mo^V/Mo^IV (E_1/2, ?1.531 to ?2.114 V) redox processes. However, only the Mo^VI/Mo^V redox couple was observed for 3 and 4 , suggesting that the subsequent reduction of the molybdenum(V) species is difficult. Complexes 1 , 2 , 5 , and 6 elicit efficient catalytic oxygen‐atom transfer (OAT) from dimethylsulfoxide (DMSO) to PMe₃ at 65 °C at a significantly faster rate than the symmetric molybdenum(VI) complexes of the analogous linear bis(phenolate) ligands known so far to exhibit OAT reactions at a higher temperature (130 °C). However, complexes 3 and 4 fail to perform the OAT reaction from DMSO to PMe₃ at 65 °C. DFT/B3LYP calculations on the OAT mechanism reveal a strong trans effect.     

Identifying similarities and differences between analogous bisdithiolene and bisdiselenolene complexes: A computational study

Due to ligand non‐innocence and reversible one‐electron‐transfer processes dithiolene complexes have been intensively studied both experimentally and computationally. While the substitution of the ligating sulfur atoms by selenium provides a means to delicately tune the behavior of dithiolene compounds, diselenolene complexes have not been as thoroughly examined. Yet, the search for such ligands has been ongoing since the 1970s. Thus, we have looked at several metal‐bisdiselenolene complexes and have compared key properties of these complexes with their bisdithiolene analogues to determine the effect of substituting the chalcogen atom. The results herein show that substitution of the sulfur atoms by selenium within these complexes only subtly changes the thermodynamics and kinetic reactivity of bisdithiolene complexes while not significantly affecting the geometries of the complexes. The significance being that the relatively minor structural changes that occur upon redox is a key feature of dithiolene complexes. Due to ligand non‐innocence and reversible one‐electron‐transfer processes dithiolene complexes have been intensively studied, however, diselenolene complexes have not. First‐principles calculations show that substitution of the sulfur atoms by selenium within the investigated complexes does offer the ability to subtly tune the thermodynamics and kinetic reactivity of bisdithiolene complexes, while not significantly affecting the geometries of the complexes. © 2015 Wiley Periodicals, Inc.     

Kinetics of formation of charge transfer complexes of poly(vinyl pyridines) with iodine, 7,7′8,8′-tetracyanoquinodimethane and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone by electronic spectroscopic studies

The kinetics of formation of charge transfer complexes of poly(4-vinyl pyridine), poly(2-vinyl pyridine), and poly(2-vinyl pyridine-co-styrene) with iodine, 7,7′,8,8′-tetracyanoquinodimethane and 2,3-dichloro-5,6-dicyano-1,4-benzoquinone has been studied using electronic absorption spectroscopy. The charge transfer complexes of analogous low molecular weight donors, namely 2- and 4-picolines with the same set of acceptors have also been investigated for comparison. The composition and the equilibrium constants of the charge transfer complexes have been obtained. The equilibrium constant of the polymeric complexes is found to be higher than that of analogous complexes of the low molecular weight donors. A new method for determining the rate constants of the association and dissociation of the equilibrium involving the charge transfer complex formation has been proposed. The rate constants vary with the concentration of the acceptor. It indicates that the charge transfer complexes undergo a further reaction and hence the observed rate constants are not true but apparent rate constants. The charge transfer complexes have also been investigated by electron paramagnetic resonance spectroscopy.