Deciphering the effect of amine versus imine ligands of copper(II) complexes in 2-aminophenol oxidation.

A series of amine (1-6) and imine (5',6') based copper(II) complexes with tridentate (NNO) ligand donors were synthesized and characterized using modern analytical techniques. All the complexes were subjected to 2-aminophenol (OAP) oxidation to form 2-aminophenoxazin-3-one, as a functional analogue of an enzyme, phenoxazinone synthase. In addition, a critical comparison of the reactivity using the amine-based complexes with their respective imine counterparts was achieved in both experimental as well as theoretical studies. For instance, the kinetic measurement revealed that the imine-based copper(II) complexes (kcat, 2.4 × 105-6.2 × 106 h-1) are better than amine-based (kcat, 6.3 × 104-3.9 × 105 h-1) complexes. The complex-substrate adducts [Cu(L3)(OAP)] (7) and [Cu(L3')(OAP)] (7') were characterized for both systems by mass spectrometry. Further, the DFT study was performed with amine- (3) and imine- (3') based copper(II) complexes, to compare their efficacy in the oxidation of OAP. The mechanistic investigations reveal that the key elementary step to determine the reactivity of 3 and 3' is the proton-coupled electron transfer (PCET) step occurring from the intermediates 7/7'. Further, the computed HOMO-LUMO energy gap of 7' was smaller than 7 by 0.8 eV, which indicates the facile PCET compared to that of 7. Moreover, the coupling of the OAP moiety using imine-complexes (ΔGR.E = -5.8 kcal/mol) was found to be thermodynamically more favorable than amine complexes (ΔGR.E = +3.3 kcal/mol). Overall, the theoretical findings are in good agreement with the experimental results.

N-2,2,2-Trifluoroethylisatin ketimine as a 1,2-dipolarophile for [3 + 2]-addition to access optically pure spirothiazolidine oxindoles.

The first report on using N-2,2,2-trifluoroethylisatin ketimine as a 1,2-dipolarophile for [3 + 2]-addition and the first asymmetric synthesis of N-2,2,2-trifluoroethylspirothiazolidine oxindoles is described. The organocatalyzed asymmetric [3 + 2]-addition reaction of N-2,2,2-trifluoroethylisatin ketimine with 1,4-dithiane-2,5-diol provided an array of N-2,2,2-trifluoroethylspirothiazolidine oxindoles (up to 25 examples) in excellent yield, enantioselectivity, and diastereoselectivity (up to 96% yield, 99% ee, and 99 : 1 dr). In addition, the synthetic utility of the developed methodology has been demonstrated by transforming optically pure spirothiazolidine into medicinally important spirothiazolidinone and spirothiazolidinone-S-oxide.

Density Functional Theory Study on [Ni0(1,10-Phenanthroline)]-Catalyzed Reductive Carboxylation of Alkyl and Aryl Halides with CO2: Effect of the Lewis Acid and ß-H Elimination Side Reaction in the Crucial CO2 Insertion Step.

The [Ni0(phen)] (1)-catalyzed reductive carboxylation of propyl and phenyl chloride with a CO2 molecule has been compared using the density functional theory method. The reactivity of 1 in the initial oxidative addition with the propyl and phenyl chloride and in the subsequent single electron transfer step to form the NiI intermediates, [NiI(phen)(R'-CH2)], 4, (R' = CH3-CH2-), and [NiI(phen)(C6H5)], 4', is almost the same. However, an apparent reactivity difference is interpreted in the CO2 insertion step, which involves the NiI and the NiII intermediates. In both propyl and phenyl chloride, the NiI-mediated CO2 insertion is kinetically more preferred than that mediated by the analogues NiII intermediates (3 and 3') by +5.0 and +33.4 kcal/mol, respectively. This trend in energetics clearly shows that the CO2 insertion of phenyl chloride exclusively occurs via the NiI-mediated pathway, whereas in propyl chloride, it follows both NiI- and NiII-mediated pathways. Overall, the catalytic efficacy of 1 is found to be higher in phenyl chloride (by +11.3 kcal/mol) than that in propyl chloride. Furthermore, the effect of a plausible ß-H elimination side reaction in the CO2 insertion step is modeled for propyl chloride. Herein, the ß-H elimination of the NiII propyl species (3) is kinetically more feasible than its CO2 insertion, while the ß-H elimination of NiI propyl (4) is rather difficult compared to its CO2 insertion by +16.3 kcal/mol. This strongly supports the suitability of the NiI intermediate in the CO2 insertion step. In addition, the role of the Lewis acid (X) in the CO2 insertion step is tested by incorporating various Lewis acids (X = MgCl2, ZnCl2, AlCl3, and LiCl) in the NiII propyl (7) and NiI propyl (5) intermediates. The Lewis acids effectively facilitate the CO2 insertion step, and the effect due to MgCl2 is found to be more evident. MgCl2 enhances the CO2 insertion of 5 and 7 by 89 and 84%, respectively, and hence, the NiI-mediated CO2 insertion of propyl halide (ΔG⧧ = +1.4 kcal/mol) is now comparable with that of phenyl halide (ΔG⧧ = +0.9 kcal/mol). This suggests that in the presence of Lewis acids, the catalytic efficacy of 1 is enhanced for the reductive carboxylation of propyl halide and exhibits similar reactivity to that of phenyl halide.

In-situ interfacial compatibilization via edge-sulfurated few layer graphene during the formation of crosslinked graphene-rubber nanocomposites.

Herein, we report various physico-chemical approaches to probe the nature of the interface between few layers graphene (FLG) and carboxylated nitrile rubber (XNBR) nanocomposites prepared through efficient blending of XNBR latex with an aqueous dispersion of FLG. The extent of physical interaction between FLG and XNBR was investigated using Lorentz-Park and Cunneen-Russell models. The chemical interface between FLG and sulfur crosslinked XNBR was studied using model reactions between sulfur and graphene in presence of zinc 2-mercaptobenzothiazole (ZMBT). We propose that an edge sulfurated FLG is formed, which could chemically bond with XNBR during the vulcanization process. Density Functional Theory (DFT) was employed to unravel the mechanistic insights, which support this hypothesis and suggest a kinetically favorable sulfuration of both XNBR and FLG. The formation of a chemical bond between edge-FLG and XNBR through the proposed intermediacy of sulfurated FLG leads to the observed improvement in mechanical properties of the nanocomposites.

A Density Functional Theory Study on Comparing the Reactivity of [Mn(13-TMC)(OOH)]2+ and [Mn(13-TMC)(O2)]+ for the Sulfoxidation of Thioanisole: Elucidation of Substrate and Non-Redox Metal Ion Effects.

The reactivities of [Mn(13-TMC)(OOH)]2+ (1) and [Mn(13-TMC)(O2)]+ (2) in the sulfoxidation of thioanisole have been compared using density functional theory methods. The orientation of the 13-TMC ligand and substrate and non-redox metal ion effects have been considered to improve the oxidation efficiency of 1 and 2. In 1, the syn- and anti-orientation of the 13-TMC ligand do not change the coordination of the Mn ion. In contrast, the orientation of the 13-TMC ligand regulates the geometry of 2, wherein the syn-13-TMC ligand exhibits the MnIII-peroxo (2hs and 2ls) species, while the anti-13-TMC shows the MnII-superoxo (2'hs and 2'ls) species. However, the MnII-superoxo species are found to be less stable than the MnIII-peroxo complexes by around +26.6 kcal/mol. The ground state geometries of 1 and 2 with the syn-13-TMC ligand are found to be more stable in the high- (S = 2) spin states (1hs and 2hs) than the low- (S = 1) spin complexes (1ls and 2ls), by +15.6 and +25.5 kcal/mol, respectively. The computed mechanistic pathways clearly indicate that the sulfoxidation of thioanisole by 1hs is kinetically (by +16.6 to +46.1 kcal/mol) and thermodynamically (+14.4 to +56.1 kcal/mol) more preferred than 1ls, 2hs, and 2ls species. This is mainly due to the feasible heterolytic O1-O2 bond cleavage followed by the proton transfer step. In addition, the molecular electrostatic potential analysis indicates that the higher oxidation efficacy of 1hs than 2hs is due to the -OOH moiety. The reactivity of 1hs is further enhanced by incorporating electron donating substituents in thioanisole, wherein the p-NH2 thioanisole decreases the ΔG‡ of 1hs by 28%. Interestingly, the incorporation of non-redox metal ions (Mn+ = Sc3+, Y3+, Mg2+, and Zn2+) improves the reactivity of 2hs, wherein the non-redox metal ions tend to bind with the oxygen atoms of 2hs and subsequently shift the one-electron reduction potential (E0(red) vs SCE) toward the positive side. The positive shift in the E0(red) is more evident in 2hs-Y3+ that significantly decreases the ΔG‡ of 2hs by 58.7%, which is in fact lower than the ΔG‡ of 1hs by +2.0 kcal/mol. Hence, in the presence of Y3+, the reactivity of 2hs is comparable with 1hs in the sulfoxidation of thioanisole.

Facile, environmentally benign and scalable approach to produce pristine few layers graphene suitable for preparing biocompatible polymer nanocomposites.

The success of developing graphene based biomaterials depends on its ease of synthesis, use of environmentally benign methods and low toxicity of the chemicals involved as well as biocompatibility of the final products/devices. We report, herein, a simple, scalable and safe method to produce defect free few layers graphene using naturally available phenolics i.e. curcumin/tetrahydrocurcumin/quercetin, as solid-phase exfoliating agents with a productivity of ∼45 g/batch (D/G ≤ 0.54 and D/D' ≤ 1.23). The production method can also be employed in liquid-phase using a ball mill (20 g/batch, D/G ≤ 0.23 and D/D' ≤ 1.12) and a sand grinder (10 g/batch, D/G ≤ 0.11 and D/D∼ ≤ 0.78). The combined effect of π-π interaction and charge transfer (from curcumin to graphene) is postulated to be the driving force for efficient exfoliation of graphite. The yielded graphene was mixed with the natural rubber (NR) latex to produce thin film nanocomposites, which show superior tensile strength with low modulus and no loss of % elongation at break. In-vitro and in-vivo investigations demonstrate that the prepared nanocomposite is biocompatible. This approach could be useful for the production of materials suitable in products (gloves/condoms/catheters), which come in contact with body parts/body fluids.

Novel nickel(ii) complexes of sterically modified linear N4 ligands: effect of ligand stereoelectronic factors and solvent of coordination on nickel(ii) spin-state and catalytic alkane hydroxylation.

A series of Ni(ii) complexes of the types [Ni(L)(CH3CN)2](BPh4)21-3, 5 and [Ni(L4)](BPh4)24, where L = N,N'-bis(2-pyrid-2-ylmethyl)-1,4-diazepane (L1), N-(6-methylpyrid-2-ylmethyl)-N'-(pyrid-2-ylmethyl)-1,4-diazepane (L2), N,N'-bis(6-methyl-2-pyridylmethyl)-1,4-diazepane (L3), N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethylenediamine (L5) and L4 = N,N'-bis((1-methyl-1H-imidazole-2-yl)methyl)-1,4-diazepane, have been isolated and characterized. The complex cations of 1 and 4 possess, respectively, distorted octahedral and low-spin square planar coordination geometries in which nickel(ii) is meridionally coordinated to all four nitrogen atoms of L1 and L4. DFT studies reveal that L5 with the ethylenediamine backbone coordinates in the cis-ß mode in [Ni(L5)(CH3CN)2]2+5, but in the cis-α mode in [Ni(L5)(H2O)2]2+. Also, they illustrate the role of ligand donor atom type, diazacyclo backbone and steric hindrance to coordination of pyridyl nitrogen in conferring novel coordination geometries on Ni(ii). All these complexes catalyse the oxidation of cyclohexane in the presence of m-CPBA as the oxidant up to 600 turnover numbers (TON) with relatively good alcohol selectivity (A/K, 5.6-7.2). Adamantane is oxidized to 1-adamantanol, 2-adamantanol and 2-adamantanone with high bond selectivity (3°/2°, 8.7-11.7). The incorporation of methyl substituent(s) on one (2) or both (3) of the pyridyl rings and the replacement of both the pyridylmethyl arms in 1 by imidazolylmethyl arms to give 4 decrease the catalytic efficiency. Interestingly, 5 with the cis-ß mode of coordination provides two labile cis coordination sites for oxidant binding, leading to higher total TON and product/bond selectivity.

Computational prediction of cyclometalated Ir(III), Rh(III) and Co(III) amido complexes to capture up to three CO2 molecules.

An organometallic complex showing the ability to capture up to three CO2 molecules is unprecedented in chemistry while with convincing theoretical evidence we show that [(CH3)2PCH2NH2] (pcn) based cyclometalated Ir(iii), Rh(iii) and Co(iii) amido complexes can perform such a reaction under mild conditions. The CO2 capture in these complexes occurs via three successive CO2 insertion steps into the M-N(amido) bonds which are driven by modest activation barriers (14.5 to 23.1 kcal mol(-1)) and highly exothermic products (-18.4 to -47.4 kcal mol(-1)). The Rh(iii) is more effective than Co(iii) and Ir(iii) and solvation by acetonitrile further enhances the CO2 insertion ability of all three complexes. The feasibility of the successive CO2 insertion reactions has been explained on the basis of the synergistic effect induced by the pcn ligand, viz. the nucleophilic affinity of the nitrogen atom, the ring strain of the [η(2)-NHCH2P(CH3)2] moiety and the electronic effect of the phosphorous atom.

Metal- and ligand-assisted CO2 insertion into Ru-C, Ru-N, and Ru-O bonds of ruthenium(II) phosphine complexes: a density functional theory study.

The CO2 insertion reactions of [L4Ru(η(2)-CH2C6H4)] (1), [L4Ru(η(2)-OC6H3Me)] (2), and [L4Ru(η(2)-NHC6H4)] (3), where L = PH3 and PMe3, are modeled using density functional theory methods. In 1 and 2, the metal-assisted CO2 insertion occurs because of the favorable initial axial phosphine dissociation mechanism, whereas in 3, the ligand (NHC6H4)-assisted mechanism operates (ΔG(⧧) = +19.0 kcal/mol), wherein the nucleophilic affinity of the -NHC6H4 moiety aids the CO2 insertion process. The modeled mechanisms are consistent with the experimental findings by Hartwig et al. (J. Am. Chem. Soc, 1991, 113, 6499), in which the rate of the reactions of 1 and 2 depends on the added phosphine concentration, whereas the rate of the reaction of 3 is independent of the added phosphine concentration. In 1 and 2, CO2 is preferably inserted into the Ru-Caryl bond rather than the competitive Ru-CH2 and Ru-O bonds, respectively. In 1, the π-type orbital interaction of the aryl ring with the metal center is found to stabilize the transition state for Ru-Caryl bond insertion (ΔG(⧧) = +25.7 kcal/mol). In 2, the Ru-Caryl insertion (ΔG(⧧) = +23.0 kcal/mol) is thermodynamically preferred, while the kinetically preferred Ru-O bond insertion (ΔG(⧧) = +17.4 kcal/mol) is highly reversible. The more electron-donating and sterically bulky PMe3 facilitates the CO2 insertion of 1 and 2 because the initial dissociation of axial PMe3 is easier than that of PH3 by ca. +11.0 kcal/mol, whereas in the case of 3, the effect of PMe3 slightly increases the ΔG(⧧) value of 3. The increase in the nucleophilic affinity of amido nitrogen in 3 and the increase in the polarity of the solvent decrease the ΔG(⧧) value of 3 by 48%. The inclusion of the chelating dimethylphosphinoethane ligand in 3 along with the electron-donating substituent at the -NHC6H4 moiety and the polar solvent further reduces the ΔG(⧧) value of 3 by 62%, which demonstrates the role of the chelating ligand, electron-donating substituent, and polar solvent in the ligand-assisted CO2 insertion reactions.

Nickel(II) complexes of pentadentate N5 ligands as catalysts for alkane hydroxylation by using m-CPBA as oxidant: a combined experimental and computational study.

A new family of nickel(II) complexes of the type [Ni(L)(CH(3)CN)](BPh(4))(2), where L=N-methyl-N,N',N'-tris(pyrid-2-ylmethyl)-ethylenediamine (L1, 1), N-benzyl-N,N',N'-tris(pyrid-2-yl-methyl)-ethylenediamine (L2, 2), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(6-methyl-pyrid-2-yl-methyl)-ethylenediamine (L3, 3), N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-(quinolin-2-ylmethyl)-ethylenediamine (L4, 4), and N-methyl-N,N'-bis(pyrid-2-ylmethyl)-N'-imidazole-2-ylmethyl)-ethylenediamine (L5, 5), has been isolated and characterized by means of elemental analysis, mass spectrometry, UV/Vis spectroscopy, and electrochemistry. The single-crystal X-ray structure of [Ni(L(3))(CH(3)CN)](BPh(4))(2) reveals that the nickel(II) center is located in a distorted octahedral coordination geometry constituted by all the five nitrogen atoms of the pentadentate ligand and an acetonitrile molecule. In a dichloromethane/acetonitrile solvent mixture, all the complexes show ligand field bands in the visible region characteristic of an octahedral coordination geometry. They exhibit a one-electron oxidation corresponding to the Ni(II) /Ni(III) redox couple the potential of which depends upon the ligand donor functionalities. The new complexes catalyze the oxidation of cyclohexane in the presence of m-CPBA as oxidant up to a turnover number of 530 with good alcohol selectivity (A/K, 7.1-10.6, A=alcohol, K=ketone). Upon replacing the pyridylmethyl arm in [Ni(L1)(CH(3)CN)](BPh(4))(2) by the strongly σ-bonding but weakly π-bonding imidazolylmethyl arm as in [Ni(L5)(CH(3)CN)](BPh(4))(2) or the sterically demanding 6-methylpyridylmethyl ([Ni(L3)(CH(3)CN)](BPh(4))(2) and the quinolylmethyl arms ([Ni(L4)(CH(3)CN)](BPh(4))(2), both the catalytic activity and the selectivity decrease. DFT studies performed on cyclohexane oxidation by complexes 1 and 5 demonstrate the two spin-state reactivity for the high-spin [(N5)Ni(II)-O(.)] intermediate (ts1(hs), ts2(doublet)), which has a low-spin state located closely in energy to the high-spin state. The lower catalytic activity of complex 5 is mainly due to the formation of thermodynamically less accessible m-CPBA-coordinated precursor of [Ni(II) (L5)(OOCOC(6)H(4)Cl)](+) (5 a). Adamantane is oxidized to 1-adamantanol, 2-adamantanol, and 2-adamantanone (3°/2°, 10.6-11.5), and cumene is selectively oxidized to 2-phenyl-2-propanol. The incorporation of sterically hindering pyridylmethyl and quinolylmethyl donor ligands around the Ni(II) leads to a high 3°/2° bond selectivity for adamantane oxidation, which is in contrast to the lower cyclohexane oxidation activities of the complexes.

Iron(III) complexes of tripodal tetradentate 4N ligands as functional models for catechol dioxygenases: the electronic vs. steric effect on extradiol cleavage.

A few mononuclear iron(iii) complexes of the type [Fe(L)Cl2]Cl , where L is a tetradentate tripodal 4N ligand such as N,N-dimethyl-N',N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (), N,N-diethyl-N',N'-bis(pyrid-2-ylmethyl)ethane-1,2-diamine (), N,N-dimethyl-N',N'-bis-(6-methylpyrid-2-ylmethyl)ethane-1,2-diamine (), N,N-dimethyl-N'-(pyrid-2-ylmethyl)-N'-(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine (), N,N-dimethyl-N',N'-bis(1-methyl-1H-imidazol-2-ylmethyl)ethane-1,2-diamine () and N,N-dimethyl-N',N'-bis(quinolin-2-ylmethyl)ethane-1,2-diamine (), have been isolated and characterized by CHN analysis, UV-Visible spectroscopy and electrochemical methods. The complex cation [Fe(H)Cl3](+) possesses a distorted octahedral geometry in which iron is coordinated by the monoprotonated 4N ligand in a tridentate fashion and the remaining three sites of the octahedron are occupied by chloride ions. The DFT optimized octahedral geometries of , and contain iron(iii) with a high-spin (S = 5/2) ground state. The catecholate adducts [Fe(L)(DBC)](+), where H2DBC is 3,5-di-tert-butylcatechol, of all the complexes have been generated in situ in acetonitrile solution and their spectral and redox properties and dioxygenase activities have been studied. The DFT optimized geometries of the catecholate adducts [Fe()(DBC)](+), [Fe()(DBC)](+) and [Fe()(DBC)](+) have also been generated to illustrate the ability of the complexes to cleave H2DBC in the presence of molecular oxygen to afford varying amounts of intra- (I) and extradiol (E) cleavage products. The extradiol to intradiol product selectivity (E/I, 0.1-2.0) depends upon the asymmetry in bidentate coordination of catecholate, as determined by the stereoelectronic properties of the ligand donor functionalities. While the higher E/I value obtained for [Fe()(DBC)](+) is on account of the steric hindrance of the quinolyl moiety to coordination the lower value observed for [Fe()(DBC)](+) and [Fe()(DBC)](+) is on account of the electron-releasing effect of the N-methylimidazolyl moiety. Based on the data obtained it is proposed that the detachment of the -NMe2 group from the coordination sphere in the semiquinone intermediate is followed for dioxygen binding and activation to yield the extradiol cleavage product.

Cu(II) coordination chemistry of patellamide derivatives: possible biological functions of cyclic pseudopeptides.

Two synthetic derivatives of the naturally occurring cyclic pseudooctapeptides patellamide  A-F and ascidiacyclamide, that is, H(4)pat(2), H(4)pat(3), as well as their Cu(II) complexes are described. These cyclic peptide derivatives differ from the naturally occurring macrocycles by the variation of the incorporated heterocyclic donor groups and the configuration of the amino acids connecting the heterocycles. The exchange of the oxazoline and thiazole groups by dimethylimidazoles or methyloxazoles leads to more rigid macrocycles, and the changes in the configuration of the side chains leads to significant differences in the folding of the cyclic peptides. These variations allow a detailed study of the various possible structural changes on the chemistry of the Cu(II) complexes formed. The coordination of Cu(II) with these macrocyclic species was monitored by high-resolution electrospray mass spectrometry (ESI-MS), spectrophotometric (UV/Vis) and circular dichroic (CD) titrations, and electron paramagnetic resonance (EPR) spectroscopy. Density functional theory (DFT) calculations and molecular mechanics (MM) simulations have been used to model the structures of the Cu(II) complexes and provide a detailed understanding of their geometric preferences and conformational flexibility. This is related to the Cu(II) coordination chemistry and the reactivity of the dinuclear Cu(II) complexes towards CO(2) fixation. The variation observed between the natural and various synthetic peptide systems enables conclusions about structure-reactivity correlations, and our results also provide information on why nature might have chosen oxazolines and thiazoles as incorporated heterocycles.

A combined experimental and computational study on the sulfoxidation by high-valent iron bispidine complexes.

Iron-bispidine complexes are efficient catalysts for the oxidation of thioanisole to phenylmethylsulfoxide with iodosylbenzene as oxidant. With the tetradentate bispidine ligand L(1) (L(1) = 2,4-pyridyl-3,7-diazabicyclo[3.3.1]nonane)) the catalytic efficiency is smaller than with the pentadentate bispidine ligand L(2) (L(2) = 2,4-pyridyl-7-(pyridine-2-ylmethyl)-3,7-diazabicyclo[3.3.1]nonane)). Based on the redox potentials (iron complexes with L(1) are stronger oxidants than with L(2)) and known efficiencies in catalytic olefin oxidation and C-H activation reactions, the expectations were different. A DFT-based analysis is used to explain the apparent contradiction, and this is based on differences in the electronic ground states of the ferryl complexes as well as in the oxygen transfer transition states.

Modification of the magnetic properties of a heterometallic wheel by inclusion of a Jahn-Teller distorted Cu(II) ion.

An investigation into the physical consequences of including a Jahn-Teller distorted Cu(II) ion within an antiferromagnetically coupled ring, [R(2)NH(2)][Cr(7)CuF(8)((O(2)C(t)Bu)(16))] is reported. Inelastic neutron scattering (INS) and electron paramagnetic resonance (EPR) spectroscopic data are simulated using a microscopic spin Hamiltonian, and show that the two Cr-Cu exchange interactions must be inequivalent. One Cr-Cu exchange is found to be antiferromagnetic and the other ferromagnetic. The geometry of the Jahn-Teller elongation is deduced from these results, and shows that a Jahn-Teller elongation axis must lie in the plane of the Cr(7)Cu wheel; the elongation is not observed by X-ray crystallography, due to positional disorder of the Cu site within the wheel. An electronic structure calculation confirms the structural distortion of the Cu site.

The Ru(IV)=O-catalyzed sulfoxidation: a gated mechanism where O to S linkage isomerization switches between different efficiencies.

Two Ru(IV)=O catalysts with either a pentadentate bispidine ligand L(1) or a bidentate pyrazolate L(2)/terpy L(3) combination of ligands have very different efficiencies as oxygen transfer catalysts for the selective oxidation of sulfides to sulfoxides: the [Ru(II)(L(1))(solvent)](2+)/iodosyl benzene system has an initial TOF of approx. 40 h(-1) and quantitative yield, with [Ru(II)(L(2))(L(3))(solvent)](+) the initial TOF is approx. 12 h(-1) with a maximum yield of approx. 60%. By experiment (cyclovoltametry) it is shown that there is S- to O-linkage isomerization of the Ru(II) sulfoxide product complex, and this may partially switch off the catalytic cycle for the L(2)/L(3)-based catalyst. It emerges that the reasons for the reduced efficiency in the case of the L(2)/L(3), in comparison with the L(1)-based catalyst, are a more efficient linkage isomerization, a more stable S-bonded, in comparison with the O-bonded, Ru(II)-based isomer, and inefficient ligand exchange in the product (hydrolysis produces the free sulfoxide and the Ru(II) precatalyst). These interpretations are qualitatively in good agreement with preliminary DFT-based data.

Oxidation of cyclohexane by high-valent iron bispidine complexes: tetradentate versus pentadentate ligands.

The iron-bispidine-catalyzed oxidation of cyclohexane with H(2)O(2), where either a tetradentate or a pentadentate bispidine ligand is coordinated to the iron center, yields up to 35% cyclohexanol and cyclohexanone (alcohol/ketone ratio of up to 4). Product distribution (including (18)O labeling studies), kinetic isotope effects, and the ratio of tertiary/secondary alcohols with adamantane as a substrate (tertiary/secondary) suggest that (i) H abstraction by a ferryl complex is the rate-determining step and that the emerging cyclohexyl radical is short-lived, (ii) there is a parallel reaction involving oxidation by OH radicals, and (iii) there are considerable differences in the reaction pathways between the tetradentate and pentadentate ligand catalyst. These interpretations are fully supported by a DFT-based computational analysis.

Iron vs. ruthenium--a comparison of the stereoselectivity in catalytic olefin epoxidation.

The synthesis and the full characterization of a new ruthenium(II) complex with the pentadentate bispidine ligand L(1) is reported and shown to be a very active catalyst for olefin epoxidation. The selectivity in the epoxidation of cis- and trans-beta-methylstyrene with the formation of cis and trans products, each, was determined and compared with that of the iron bispidine complex of L(1). There is a significant difference in selectivity between the two catalysts in the epoxidation of cis-beta-methylstyrene but the epoxidation of trans-beta-methylstyrene is highly stereoselective with both catalysts. Based on these results, electrochemical and labeling studies, a radical pathway for the epoxidation and isomerization is proposed, and this is supported by computational data. DFT indicates that, with both catalysts, the epoxidation is based on a stepwise mechanism, which, in the first step leads to a radical-based intermediate. This exists in two configurations, which interconvert with a relatively low energy barrier. The product ratio depends on the relative energies of the two configurations of the radical intermediate and the height of the energy barriers to the cis- and trans-epoxide products. For the Fe-based system there is, as expected, the additional complication of the availability of various spin levels, and multi-state reactivity is observed. The computed structures and energies are in agreement with the observed data.

Oxidation of cyclohexane by a high-valent iron bispidine complex: a combined experimental and computational mechanistic study.

Experimental data suggest that there are various competing pathways for the catalytic and stoichiometric oxygenation of cyclohexane, assisted by iron-bispidine complexes and using various oxidants (H(2)O(2), O(2), PhIO). Density functional theory calculations indicate that both Fe(IV)=O and Fe(V)=O species are accessible and efficiently transfer their oxygen atoms to cyclohexane. The reactivities of the two isomers each and the two possible spin states for the Fe(IV)=O and Fe(V)=O species are sufficiently different to allow an interpretation of the experimental data.

Intramolecular alkyl phosphine dehydrogenation in cationic rhodium complexes of tris(cyclopentylphosphine).

[Rh(nbd)(PCyp(3))(2)][BAr(F) (4)] (1) [nbd = norbornadiene, Ar(F) = C(6)H(3)(CF(3))(2), PCyp(3) = tris(cyclopentylphosphine)] spontaneously undergoes dehydrogenation of each PCyp(3) ligand in CH(2)Cl(2) solution to form an equilibrium mixture of cis-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 a) and trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(2)][BAr(F) (4)] (2 b), which have hybrid phosphine-alkene ligands. In this reaction nbd acts as a sequential acceptor of hydrogen to eventually give norbornane. Complex 2 b is distorted in the solid-state away from square planar. DFT calculations have been used to rationalise this distortion. Addition of H(2) to 2 a/b hydrogenates the phosphine-alkene ligand and forms the bisdihydrogen/dihydride complex [Rh(PCyp(3))(2)(H)(2)(eta(2)-H(2))(2)][BAr(F) (4)] (5) which has been identified spectroscopically. Addition of the hydrogen acceptor tert-butylethene (tbe) to 5 eventually regenerates 2 a/b, passing through an intermediate which has undergone dehydrogenation of only one PCyp(3) ligand, which can be trapped by addition of MeCN to form trans-[Rh{PCyp(2)(eta(2)-C(5)H(7))}(PCyp(3))(NCMe)][BAr(F) (4)] (6). Dehydrogenation of a PCyp(3) ligand also occurs on addition of Na[BAr(F) (4)] to [RhCl(nbd)(PCyp(3))] in presence of arene (benzene, fluorobenzene) to give [Rh(eta(6)-C(6)H(5)X){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (7: X = F, 8: X = H). The related complex [Rh(nbd){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] 9 is also reported. Rapid ( approximately 5 minutes) acceptorless dehydrogenation occurs on treatment of [RhCl(dppe)(PCyp(3))] with Na[BAr(F) (4)] to give [Rh(dppe){PCyp(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (10), which reacts with H(2) to afford the dihydride/dihydrogen complex [Rh(dppe)(PCyp(3))(H)(2)(eta(2)-H(2))][BAr(F) (4)] (11). Competition experiments using the new mixed alkyl phosphine ligand PCy(2)(Cyp) show that [RhCl(nbd){PCy(2)(Cyp)}] undergoes dehydrogenation exclusively at the cyclopentyl group to give [Rh(eta(6)-C(6)H(5)X){PCy(2)(eta(2)-C(5)H(7))}][BAr(F) (4)] (17: X = F, 18: X = H). The underlying reasons behind this preference have been probed using DFT calculations. All the complexes have been characterised by multinuclear NMR spectroscopy, and for 2 a/b, 4, 6, 7, 8, 9 and 17 also by single crystal X-ray diffraction.