Spectroscopic and theoretical study of a mononuclear manganese(III) complex exhibiting a tetragonally compressed geometry

Inelastic neutron scattering and high-field electron paramagnetic resonance data are presented for [Mn(bpia)(OAc)(OCH(3))](PF(6)), where bpia is bis(picolyl)(N-methylimidazole-2-yl)amine. Modeling of the data to the conventional fourth-order spin-Hamiltonian yielded the following parameters: D = 3.526(3) cm(-1), E = 0.588(6) cm(-1), B(0)(4) = -0.00084(7) cm(-1), B(2)( 4)= -0.002(2) cm(-1), (4)(4) = -0.0082(5) cm(-1), g(x) = 1.98(1), g(y) = 1.952(6), and g(z) = 1.978(5). The complex is of particular interest given the biochemical activity and the unusual stereochemistry distinguished by a rare example of a tetragonally compressed octahedron and a pronounced angular distortion imposed by the tetradentate tripodal bpia ligand. Ligand field, density functional theory, and complete active space self-consistent field ab initio calculations are presented that aim to relate the spectroscopic data to the molecular geometry.     

Preparation of highly efficient manganese catalase mimics

The series of compounds [Mn(bpia)(mu-OAc)](2)(ClO(4))(2) (1), [Mn(2)(bpia)(2)(muO)(mu-OAc)](ClO(4))(3).CH(3)CN (2), [Mn(bpia)(mu-O)](2)(ClO(4))(2)(PF(6)).2CH(3)CN (3), [Mn(bpia)(Cl)(2)](ClO)(4) (4), and [(Mn(bpia)(Cl))(2)(mu-O)](ClO(4))(2).2CH(3)CN (5) (bpia = bis(picolyl)(N-methylimidazol-2-yl)amine) represents a structural, spectroscopic, and functional model system for manganese catalases. Compounds 3 and 5 have been synthesized from 2 via bulk electrolysis and ligand exchange, respectively. All complexes have been structurally characterized by X-ray crystallography and by UV-vis and EPR spectroscopies. The different bridging ligands including the rare mono-mu-oxo and mono-mu-oxo-mono-mu-carboxylato motifs lead to a variation of the Mn-Mn separation across the four binuclear compounds of 1.50 A (Mn(2)(II,II) = 4.128 A, Mn(2)(III,III) = 3.5326 and 3.2533 A, Mn(2)(III,IV) = 2.624 A). Complexes 1, 2, and 3 are mimics for the Mn(2)(II,II), the Mn(2)(III,III), and the Mn(2)(III,IV) oxidation states of the native enzyme. UV-vis spectra of these compounds show similarities to those of the corresponding oxidation states of manganese catalase from Thermus thermophilus and Lactobacillus plantarum. Compound 2 exhibits a rare example of a Jahn-Teller compression. While complexes 1 and 3 are efficient catalysts for the disproportionation of hydrogen peroxide and contain an N(4)O(2) donor set, 4 and 5 show no catalase activity. These complexes have an N(4)Cl(2) and N(4)OCl donor set, respectively, and serve as mimics for halide inhibited manganese catalases. Cyclovoltammetric data show that the substitution of oxygen donor atoms with chloride causes a shift of redox potentials to more positive values. To our knowledge, complex 1 is the most efficient binuclear functional manganese catalase mimic exhibiting saturation kinetics to date.     

Reevaluation of the kinetics of polynuclear mimics for manganese catalases

Considerable effort has been expended in order to understand the mechanism of manganese catalases and to develop functional mimics for these enzymes. For many years, the most efficient reactivity mimic was [MnIVsalpn(mu-O)]2 [H2salpn = 1,3-bis(salicylideneiminato)propane], a compound that cycles between the MnIV2 and MnIII2 oxidation levels instead of the MnII2 and MnIII2 oxidation states used by the enzyme, with kcat = 250 s(-1) and kcat/KM = 1000 M(-1) s(-1). Recently, a truly exceptional high value of kcat was reported for the complex [Mn(bpia)(mu-OAc)]22+ [bpia = bis(picolyl)(N-methylimidazol-2-yl)amine]. On the basis of a calculated kcat value of 1100 s(-1) and an efficiency kcat/KM of 34 000 M(-1) s(-1), this complex has been suggested to represent a significant breakthrough in catalytic efficiencies of manganese catalase mimics. However, a plot of ri/[cat]T vs [H2O2]0, where the saturation value approaches 1.5 s(-1), is inconsistent with the 1100 s(-1) value tabulated for kcat. Similar discrepancies are observed for two other families of manganese complexes containing either a Mn2(mu-OPh)22+ core and different substituted tripodal ligands or complexes of methyl and ethyl salicylimidate, with an Mn2(mu-OPh)24+ core. Reevaluation of the kinetic parameters for these three systems reveals that the originally reported values were overestimated by a factor of approximately 1000 for both kcat and kcat/KM. We discuss the origin of the discrepancy between the previously published kinetic parameters and the newly derived values. Furthermore, we provide a short analysis of the existing manganese catalase mimics in an effort to provide sound directions for future investigations in this field.     

Metal complexes formed by metal-assisted solvolysis of di-pyridylketone azine: structures and magnetic properties

A series of metal complexes were achieved from the metal-assisted solvolysis reaction of di-pyridylketone azine (dpka). The tetranuclear nickel cluster , [Ni(2)[dpk(O)(OH)][dpk(O)(OCH(3))](N(3))(2)](2), is centrosymmetric with a central core described as an edge-shared triangle core. Neighboring Ni(II) ions are alternately bridged by (micro(2)-N(3), micro(3)-O) and (micro(2)-O, micro(3)-O) double bridges. Complex , [Cu(4)[dpk(O)(OCH(3))](4)(N(3))(2)](CuCl(2))(2) contains a tetranuclear cluster and two identical [CuCl(2)]M(-) anions. The tetranuclear structure has two crystallographically imposed twofold axes, in which the four copper ions are arranged to be rhombic shape. The neighboring copper(ii) ions along the lateral are bridged by single micro(2)-O from the ligand dpk(O)(OCH(3)) and the short diagonal copper ions are bridged by two symmetric end-on azides. In dinuclear Cu(ii) complex [Cu(2)[dpka(OCH(2)CH(3))]Cl(2)](ClO(4)) (3), the metal centers are coordinated in a planar configuration and bridged by a -N-N- bridge. It is also observed that the Cl atom coordinated to one Cu(II) center is also weakly coordinated to another inversion related Cu(II) to generate a centrosymmetric dimer. The metal centers in one-dimensional polymeric Cu(ii) complex [Cu(2)[dpka(OCH(3))](N(3))(2)(ClO(4))](n) (4), however, are bridged by a -N-N- bridge and an end-to-end azide bridge, alternately. Magnetic susceptibility measurements indicate that shows ferromagnetic interaction within the tetranuclear cluster, and that displays moderately strong antiferromagnetic interaction (J = -56.7 cm(-1)) for the bis(micro-N(3)) bridge. For compound , it shows strong antiferromagnetic coupling (J = -286 cm(-1)) between the intradinuclear Cu(II) ions mediated by the single N-N bridge and negligible magnetic interactions between the adjacent dinuclear Cu(II) ions mediated by the single end-to-end azide bridge. The mechanism of the metal-assisted solvolysis reaction was also discussed.     

Synthesis, structures, and properties of ruthenium(II) complexes of N-(1,10-phenanthrolin-2-yl)imidazolylidenes

Mononuclear ruthenium complexes [RuCl(L1)(CH(3)CN)(2)](PF(6)) (2a), [RuCl(L2)(CH(3)CN)(2)](PF(6)) (2b), [Ru(L1)(CH(3)CN)(3)](PF(6))(2) (4a), [Ru(L2)(CH(3)CN)(3)](PF(6))(2) (4b), [Ru(L2)(2)](PF(6))(2) (5), [RuCl(L1)(CH(3)CN)(PPh(3))](PF(6)) (6), [RuCl(L1)(CO)(2)](PF(6)) (7), and [RuCl(L1)(CO)(PPh(3))](PF(6)) (8), and a tetranuclear complex [Ru(2)Ag(2)Cl(2)(L1)(2)(CH(3)CN)(6)](PF(6))(4) (3) containing 3-(1,10-phenanthrolin-2-yl)-1-(pyridin-2-ylmethyl)imidazolylidene (L1) and 3-butyl-1-(1,10-phenanthrolin-2-yl)imidazolylidene (L2) have been prepared and fully characterized by NMR, ESI-MS, UV-vis spectroscopy, and X-ray crystallography. Both L1 and L2 act as pincer NNC donors coordinated to ruthenium (II) ion. In 3, the Ru(II) and Ag(I) ions are linked by two bridging Cl(-) through a rhomboid Ag(2)Cl(2) ring with two Ru(II) extending to above and down the plane. Complexes 2-8 show absorption maximum over the 354-428 nm blueshifted compared to Ru(bpy)(3)(2+) due to strong σ-donating and weak π-acceptor properties of NHC ligands. Electrochemical studies show Ru(II)/Ru(III) couples over 0.578-1.274 V.     

Unique example of flexible phenol coordination in mononuclear manganese compounds

The synthesis and characterization of six novel mononuclear Mn(II) and Mn(III) complexes are presented. The tripodal ligands 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (HL1), 2-[[((6-methylpyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino]methyl]-4-nitrophenol (HL2), (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)(2-hydroxybenzyl)amine (HL3) and 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-bromophenol were used. All ligands provide an N3O donor set. The compounds [Mn(II)(HL1)Cl2].CH3OH (1), [Mn(III)(L1)Cl2] (2), [Mn(II)(HL2)(EtOH)Cl2] (3), [Mn(II)(HL3)Cl2].CH3OH (4), [Mn(III)(HL4)Br2] (5) and [Mn(III)(L1)(tcc)] (6), with tcc = tetrachlorocatecholate dianion, were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR and UV-vis spectroscopy, cyclic voltammetry, and elemental analysis. Compound 1 crystallizes in the triclinic space group P1, compounds 2, 3 and 4 were solved in the monoclinic space group P2(1)/c, whereas the structure determination of and succeeded in the orthorhombic space groups Pbca and P2(1)2(1)2(1), respectively. Notably, the crystal structures of 1 and 3 are the first Mn(II) complexes featuring a non-coordinating phenol moiety. Compound 2 oxidizes 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations with a turnover number of kcat = 173 h(-1). The electronic influence of different substituents in para position of the phenol group is lined out.     

Synthesis and characterization of di- and trivalent pyrazolylborate β-diketonates and cyanometalates

The syntheses, structures, and magnetic properties of a series of di- and trivalent hydridotris(3,5-dimethylpyrazol-1-yl)borate (Tp*) cyanomanganates are described. Treatment of tris(acetylacetonate)manganese(III) [Mn(acac)(3)] with KTp* and tetra(ethyl)ammonium cyanide affords [NEt(4)][(Tp*)Mn(II)(κ(2)-acac)(CN)] (1), as the first monocyanomanganate(II) complex; attempted oxidation of 1 with iodine affords {(Tp*)Mn(II)(κ(2)-acac(3-CN))}(n) (2) as a one-dimensional chain and bimetallic {[NEt(4)][(Tp*)Mn(II)(κ(2)-acac(3-CN))](2)(μ-CN) (3) as the major and minor products, respectively. A fourth complex, [NEt(4)][(Tp*)Mn(II)(η(2)-acac(3-CN))(η(1)-NC-acac)] (4), is obtained via treatment of Mn(acac(3-CN))(3) with KTp* and [NEt(4)]CN, while [NEt(4)](2)[Mn(II)(CN)(4)] (5) was prepared from manganese(II) trifluoromethanesulfonate and excess [NEt(4)]CN. Tricyanomanganate(III) complexes, [cat][(Tp*)Mn(III)(CN)(3)] [cat = NEt(4)(+), 7; PPN(+), 8], are prepared via sequential treatment of Mn(acac(3-CN))(3) with KTp*, followed by [NEt(4)]CN, or [cat](3)[Mn(III)(CN)(6)] with (Tp*)SnBu(2)Cl. Magnetic measurements indicate that 1, 2, and 4 contain isotropic Mn(II) (S = (5)/(2); g = 2.00) centers, and no long-range magnetic ordering is found above 1.8 K. Compounds 7 and 8 contain S = 1 Mn(III) centers that adopt singly degenerate spin ground states without orbital contributions to their magnetic moments.     

Synthesis, Structure, and Reactivity of Model Complexes of Copper Nitrite Reductase

The copper(I) and copper(II) complexes with the nitrogen donor ligands bis[(1-methylbenzimidazol-2-yl)methyl]amine (1-BB), bis[2-(1-methylbenzimidazol-2-yl)ethyl]amine (2-BB), N-acetyl-2-BB (AcBB), and tris[2-(1-methylbenzimidazol-2-yl)ethyl]nitromethane (TB) have been studied as models for copper nitrite reductase. The copper(II) complexes form adducts with nitrite and azide that have been isolated and characterized. The Cu(II)-(1-BB) and Cu(II)-AcBB complexes are basically four-coordinated with weak axial interaction by solvent or counterion molecules, whereas the Cu(II)-(2-BB) and Cu(II)-TB complexes prefer to assume five-coordinate structures. A series of solid state structures of Cu(II)-(1-BB) and -(2-BB) complexes have been determined. [Cu(1-BB)(DMSO-O)(2)](ClO(4))(2): triclinic, P&onemacr; (No. 2), a = 9.400(1) ?, b = 10.494(2) ?, c = 16.760(2) ?, alpha = 96.67(1) degrees, beta = 97.10(1) degrees, gamma = 108.45(1) degrees, V = 1534.8(5) ?(3), Z = 2, number of unique data [I >/= 3sigma(I)] = 4438, number of refined parameters = 388, R = 0.058. [Cu(1-BB)(DMSO-O)(2)](BF(4))(2): triclinic, P&onemacr; (No. 2), a = 9.304(5) ?, b = 10.428(4) ?, c = 16.834(8) ?, alpha = 96.85(3) degrees, beta = 97.25(3) degrees, gamma = 108.21(2) degrees, V = 1517(1) ?(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3388, number of refined parameters = 397, R = 0.075. [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)): triclinic, P&onemacr; (No. 2), a = 7.533(2) ?, b = 8.936(1) ?, c = 19.168(2) ?, alpha = 97.66(1) degrees, beta = 98.62(1) degrees, gamma = 101.06(1) degrees, V = 1234.4(7) ?(3), Z = 2, number of unique data [I >/= 2sigma(I)] = 3426, number of refined parameters = 325, R = 0.081. [Cu(2-BB)(MeOH)(ClO(4))](ClO(4)): triclinic, P&onemacr; (No. 2), a = 8.493(3) ?, b = 10.846(7) ?, c = 14.484(5) ?, alpha = 93.71(4) degrees, beta = 103.13(3) degrees, gamma = 100.61(4) degrees, V = 1270(1) ?(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2612, number of refined parameters = 352, R = 0.073. [Cu(2-BB)(N(3))](ClO(4)): monoclinic, P2(1)/n (No. 14), a = 12.024(3) ?, b = 12.588(5) ?, c = 15.408(2) ?, beta = 101,90(2) degrees, V = 2282(1) ?(3), Z = 4, number of unique data [I >/= 2sigma(I)] = 2620, number of refined parameters = 311, R = 0.075. [Cu(2-BB)(NO(2))](ClO(4))(MeCN): triclinic, P&onemacr; (No. 2), a = 7.402(2) ?, b = 12.500(1) ?, c = 14.660(2) ?, alpha = 68.14(1) degrees, beta = 88.02(2) degrees, gamma = 78.61(1) degrees, V = 1233.0(4) ?(3), Z = 2, number of unique data [I>/= 2sigma(I)] = 2088, number of refined parameters = 319, R = 0.070. In all the complexes the 1-BB or 2-BB ligands coordinate the Cu(II) cations through their three donor atoms. The complexes with 2-BB appear to be more flexible than those with 1-BB. The nitrito ligand is bidentate in [Cu(2-BB)(NO(2))](ClO(4))(MeCN) and essentially monodentate in [Cu(1-BB)(DMSO-O)(NO(2))](ClO(4)). The copper(I) complexes exhibit nitrite reductase activity and react rapidly with NO(2)(-) in the presence of stoichiometric amounts of acid to give NO and the corresponding copper(II) complexes. Under the same conditions the reactions between the copper(I) complexes and NO(+) yield the same amount of NO, indicating that protonation and dehydration of bound nitrite are faster than its reduction. The NO evolved from the solution was detected and quantitated as the [Fe(EDTA)(NO)] complex. The order of reactivity of the Cu(I) complexes in the nitrite reduction process is [Cu(2-BB)](+) > [Cu(1-BB)](+) > [Cu(TB)](+) > [Cu(AcBB)](+).     

Copper(II) complexes of novel N-alkylated derivatives of cis,cis-1,3,5-triaminocyclohexane. 2. Metal-promoted phosphate diester hydrolysis

Aqueous copper(II) N,N',N' '-trimethyl-cis,cis-1,3,5-triaminocyclohexane (Cu(tach-Me(3))(2+)(aq)) promotes the hydrolysis of activated phosphate diesters in aqueous medium at pH 7.2. This complex is selective for cleavage of the phosphate diester sodium bis(p-nitrophenyl) phosphate (BNPP), the rate of hydrolysis of the monoester disodium p-nitrophenyl phosphate being 1000 times slower. The observed rate acceleration of BNPP hydrolysis is slightly greater than that observed for other Cu(II) complexes, such as [Cu([9]aneN(3))Cl(2)] ([9]aneN(3) identical with 1,4,7-triazacyclononane). The rate of hydrolysis is first-order in phosphate ester at low ester concentration and second-order in [Cu(tach-Me(3))](2+)(aq), suggesting the involvement of two metal complexes in the mechanism of substrate hydrolysis. The reaction exhibits saturation kinetics with respect to BNPP concentration according to a modified Michaelis-Menten mechanism: 2CuL + S <==> LCu-S-CuL --> 2CuL + products (K(M) = 12.3 +/- 1.8 mM(2), k(cat) = (4.0 +/- 0.4) x 10(-)(4) s(-1), 50 degrees C) where CuL (triple bond) [Cu(tach-Me(3))](2+), S (triple bond) BNPP, and LCu-S-CuL is a substrate-bridged dinuclear complex. EPR data indicate that the dicopper complex is formed only in the presence of BNPP; the active LCu-S-CuL intermediate species then slowly decays to products, regenerating monomeric CuL.     

Catalytic transesterification of dialkyl phosphates by a bioinspired dicopper(II) macrocyclic complex

For a number of phosphoryltransfer enzymes, including the exonuclease subunit of DNA polymerase I, a mechanism involving two-metal ions and double Lewis-acid activation of the substrate, combined with leaving group stabilization, has been proposed. Inspired by the active site structure of this enzyme, we have designed as a synthetic phosphoryl transfer catalyst the dicopper(II) macrocyclic complex LCu(2). Crystal structures of complexes [(L)Cu(2)(mu-NO(3))(NO(3))](NO(3))(2) (1), [(L)Cu(2)(mu-CO(3))(CH(3)OH)](BF(4))(2) (2), and [(L)Cu(2)(mu-O(2)P(OCH(3))(2))(NO(3))](NO(3))(2) (3) illustrate various possibilities for the interaction of oxoanions with the dicopper(II) site. 1 efficiently promotes the transesterification of dimethyl phosphate (DMP) in CD(3)OD, k(cat) = 2 x 10(-)(4) s(-)(1) at 55 degrees C. 1 is the only available catalyst for the smooth transesterification of highly inert simple dialkyl phosphates. From photometric titrations and the pH dependence of reactivity, we conclude that a complex [(L)Cu(2)(DMP)(OCH(3))](2+) is the reactive species. Steric bulk at the -OR substituents of phosphodiester substrates O(2)P(OR)(2)(-) drastically reduces the reactivity of 1. This is explained with -OR leaving group stabilization by Cu coordination, an interaction which is sensitive to steric crowding at the alpha-C-atom of substituent R. A proposed reaction mechanism related to that of the exonuclease unit of DNA polymerase I is supported by DFT calculations on reaction intermediates. The complex [(L)Cu(3)(mu(3)-OH)(mu-CH(3)O)(2)(CH(3)CN)(2)](ClO(4))(3) (4) incorporates a [Cu(OH)(OCH(3))(2)(CH(3)CN)(2)](-) complex anion, which might be considered as an analogue of the [PO(2)(OCH(3))(2)(OCD(3))](2)(-) transition state (or intermediate) of DMP transesterification catalyzed by LCu(2).     

Characterization and dioxygen reactivity of a new series of coordinatively unsaturated thiolate-ligated manganese(II) complexes

The synthesis, structural, and spectroscopic characterization of four new coordinatively unsaturated mononuclear thiolate-ligated manganese(II) complexes ([Mn(II)(S(Me2)N(4)(6-Me-DPEN))](BF(4)) (1), [Mn(II)(S(Me2)N(4)(6-Me-DPPN))](BPh(4))·MeCN (3), [Mn(II)(S(Me2)N(4)(2-QuinoPN))](PF(6))·MeCN·Et(2)O (4), and [Mn(II)(S(Me2)N(4)(6-H-DPEN)(MeOH)](BPh(4)) (5)) is described, along with their magnetic, redox, and reactivity properties. These complexes are structurally related to recently reported [Mn(II)(S(Me2)N(4)(2-QuinoEN))](PF(6)) (2) (Coggins, M. K.; Kovacs, J. A. J. Am. Chem. Soc.2011, 133, 12470). Dioxygen addition to complexes 1-5 is shown to result in the formation of five new rare examples of Mn(III) dimers containing a single, unsupported oxo bridge: [Mn(III)(S(Me2)N(4)(6-Me-DPEN)](2)-(μ-O)(BF(4))(2)·2MeOH (6), [Mn(III)(S(Me2)N(4)(QuinoEN)](2)-(μ-O)(PF(6))(2)·Et(2)O (7), [Mn(III)(S(Me2)N(4)(6-Me-DPPN)](2)-(μ-O)(BPh(4))(2) (8), [Mn(III)(S(Me2)N(4)(QuinoPN)](2)-(μ-O)(BPh(4))(2) (9), and [Mn(III)(S(Me2)N(4)(6-H-DPEN)](2)-(μ-O)(PF(6))(2)·2MeCN (10). Labeling studies show that the oxo atom is derived from (18)O(2). Ligand modifications, involving either the insertion of a methylene into the backbone or the placement of an ortho substituent on the N-heterocyclic amine, are shown to noticeably modulate the magnetic and reactivity properties. Fits to solid-state magnetic susceptibility data show that the Mn(III) ions of μ-oxo dimers 6-10 are moderately antiferromagnetically coupled, with coupling constants (2J) that fall within the expected range. Metastable intermediates, which ultimately convert to μ-oxo bridged 6 and 7, are observed in low-temperature reactions between 1 and 2 and dioxygen. Complexes 3-5, on the other hand, do not form observable intermediates, thus illustrating the effect that relatively minor ligand modifications have upon the stability of metastable dioxygen-derived species.     

New chloro,mu-oxo,and alkyl derivatives of dioxomolybdenum(VI) and -tungsten(VI) complexes chelated with N(2)O tridentate ligands: synthesis and catalytic activities toward olefin epoxidation

A new series of cis-dioxomolybdenum(VI) complexes MoO(2)(L(n))Cl (n = 1-5) were prepared by the reaction of MoO(2)Cl(2)(DME) (DME = 1,2-dimethoxyethane) with 2-N-(2-pyridylmethyl)aminophenol (HL(1)) or its N-alkyl derivatives (HL(n)) (n = 2-5) in the presence of triethylamine. The new mu-oxo dimolybdenum compounds [MoO(2)(L(n))](2)O (n = 1, 4, 5, 7) were also prepared by treating the corresponding ligand HL(n) with MoO(2)(acac)(2) (acac = acetylacetonate) in warm methanolic solutions or (NH(4))(6)[Mo(7)O(24)].4H(2)O in the presence of dilute HCl. Treatment of MoO(2)(L(1))Cl or [MoO(2)(L(1))](2)O with the Grignard reagent Me(3)SiCH(2)MgCl gave the alkyl compound MoO(2)(L(1))(CH(2)SiMe(3)), which represents the first example of dioxomolybdenum(VI) alkyl complex supported by a N(2)O-type ancillary ligand. The analogous chloro and mu-oxo tungsten derivatives WO(2)(L(n))Cl (n = 6, 7) and [WO(2)(L(n))](2)O (n = 1, 4, 6, 7) were prepared by the reaction of WO(2)Cl(2)(DME) with HL(n) in the presence of triethylamine. Similar to their molybdenum analogues, the tungsten alkyl complexes WO(2)(L(n))(R) (n = 6, 7; R = Me, Et, CH(2)SiMe(3), C(6)H(4)(t)Bu-4) were synthesized by treating WO(2)(L(n))Cl or [WO(2)(L(n))](2)O (n = 6, 7) with the appropriate Grignard reagents. The catalytic properties of selected dioxo-Mo(VI) and -W(VI) chloro and mu-oxo complexes toward epoxidation of styrene by tert-butyl hydroperoxide (TBHP) were also investigated.     

Synthesis, structure, and electrochemical properties of copper(I) complexes with S/N homoscorpionate and heteroscorpionate ligands

Dinuclear Cu(I) complexes with bifunctionalized homoscorpionate ligands, hydrotris(thioxotriazolyl)borato [Li(Tr(Me,o)(-)(Py)) (1) and Li(Tr(Mes,Me)) (2)], and the heteroscorpionate ligand hydro[bis(thioxotriazolyl)-3-(2-pyridyl)pyrazolyl]borato [K(Br(Mes)pz(o)(-)(Py))] (3) were synthesized and crystallographically characterized. The complexes [Cu(Tr(Mes,Me))](2) (4) and [Cu(Tr(Me,o)(-)(Py))](2) (5) exhibit a similar coordination geometry where every metal is surrounded by three thioxo groups in a trigonal arrangement. The presence of a [B-H...Cu] three-center-two-electron interaction in both compounds causes the overall coordination to become tetrahedrally distorted (S(3)H coordination for each metal). The complex [Cu(Br(Mes)pz(o)(-)(Py))](2) (6) presents a trigonal geometry in which the metals interact with two thioxo groups and a bridging pyrazolyl nitrogen atom. A weak contact with a pyridine nitrogen atom completes the coordination of the metals (S(2)N,N' coordination for each metal). [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit fluxional behavior in solution as evidenced by variable-temperature NMR spectroscopy, and for 5 and 6 two species in equilibrium [in the ratio 2/1 for 5 (CDCl(3)) and 3/2 for 6 (CD(2)Cl(2))] are distinguishable in the (1)H NMR spectra at 270 K. 2D-NOESY spectra recorded at 270 K assisted in the attribution of solution molecular geometries for each isomer of 5 and 6. The free energy of activation (DeltaG()(Tc)) was determined for both equilibria from the evaluation of the coalescence temperature. DFT calculations were performed to describe plausible molecular geometry for the minor isomer of 5 and 6 and to propose a possible mechanism of interconversion between major and minor isomers. Cyclic voltammograms were recorded in CH(2)Cl(2) (3 and 6) or CH(2)Cl(2)/CH(3)CN (1/1, v/v) (2, 4, and 5) solutions using 0.1 M TBAHFP or TBAOTf as supporting electrolytes. [Cu(Tr(Mes,Me))](2), [Cu(Tr(Me,o)(-)(Py))](2), and [Cu(Br(Mes)pz(o)(-)(Py))](2) exhibit a quasi-reversible Cu(I)/Cu(II) redox behavior with E(pa) = +719 mV and E(pc) = +538 mV for 4, E(pa) = +636 mV and E(pc) = -316 mV for 5, and E(pa) = +418 mV and E(pc) = -319 mV for 6.     

Copper(I) complex O(2)-reactivity with a N(3)S thioether ligand: a copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues

In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.     

Ring-opening reactions induced by gold(I) of five- and four-coordinate palladium(II) and platinum(II) complexes containing tripodal or linear polyphosphines

The tripodal ligands NP(3)(tris[2-(diphenylphosphino)ethyl]amine) and PP(3)(tris[2-(diphenylphosphino)ethyl]phosphine), form five-coordinate [Pd(NP(3))X]X [X = Cl (1), Br (2)], [M(PP(3))X]X [M = Pd: X = Cl (4), Br (5), I (6); M = Pt, X = Cl (7), Br (8), I (9)] and four-coordinate[Pd(NP(3))I]I (3) complexes containing three fused rings around the metal. The interaction between Au(tdg)X (tdg = thiodiglycol; X = Cl, Br) or AuI and the respective ionic halo complexes 1-9 in a 1:1 stoichiometric ratio occurs via a ring-opening reaction with formation of the heterobimetallic systems PdAu(NP(3))X(3)[X = Cl (11), Br (12), I (13)], [MAu(PP(3))X(2)]X [M = Pd: X = Cl (14), Br (15), I (16); M = Pt: X = Cl (17), Br (18), I (19)]. The cations of complexes 17 and 18 were shown, by X-ray diffraction, to contain a distorted square-planar Pt(II) arrangement (Pt(P(2)P)X) where PP(3) is acting as tridentate chelating ligand and an almost linear PAuX moiety bearing the dangling phosphorus formed in the ring-opening process. PPh(3) coordinates to Au(I) and not to M(II) when added in excess to 14 and 17. Complexes 14-17 and [Pt(P(4))](BPh(4))(2) (10) (P4=linear tetraphosphine) also react with A(I), via chelate ring-openings to give MAu(2)(PP(3))X(4) [M = Pd: X = Cl (20), Br (21), I (22); M = Pt: X = Cl (23)] and [Pt(2)Au(2)(mu-Cl)(2)(mu-P(4))(2)](BPh(4))(4) (24), respectively.     

Amine olefin rhodium(I) complexes: pKa and NH bond strength

A combination of the rigid bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine (trop(2)NH) and 5-amino-5H-dibenzo[a,d]cycloheptene (tropNH(2)) ligand allowed the synthesis of the stable pentacoordinated 18 electron amine olefin rhodium(i) complex [Rh(trop(2)NH)(tropNH(2))]O(3)SCF(3)(); this complex can be cleanly deprotonated [pK(a)(DMSO) = 20.6(1)] to the corresponding amide [Rh(trop(2)N)(tropNH(2))](6) which is reversibly oxidised at -0.466 V (vs. Fc/Fc(+)). The coordinated NH bond strength in is estimated to be 379 +/- 10 kJ mol(-1).     

Substituent effect on formation of heterometallic molecular wheels: synthesis, crystal structure, and magnetic properties

The reaction of manganese(III) Schiff bases of the type salen(2-) (N,N'-ethylenebis(salicylideneaminato)) with X-substituted (X = CH(3), Cl) pyridinecarboxamide dicyanoferrite(III) [Fe(X-bpb)(CN)(2)](-) gave rise to a series of cyanide-bridged Mn(6)Fe(6) molecular wheels, [Mn(III)(salen)](6)[Fe(III)(bpmb)(CN)(2)](6) x 7H(2)O (1), [Mn(salen)](6)[Fe(bpClb)(CN)(2)](6) x 4H(2)O x 2CH(3)OH (2), [Mn(salen)](6)[Fe(bpdmb)(CN)(2)](6) x 10H(2)O x 5CH(3)OH (3), [Mn(5-Br(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 24H(2)O x 8CH(3)CN (4), and [Mn(5-Cl(salpn))](6)[Fe(bpmb)(CN)(2)](6) x 25H(2)O x 5CH(3)CN (5). Compared with [Fe(bpb)(CN)(2)](-), which always gives rise to 1D or polynuclear species when reacting with Mn(III) Schiff bases, the introduction of substituents (X) to the bpb(2-) ligand has a driving force in formation of the novel wheel structure. Magnetic studies reveal that high-spin ground state S = 15 is present in the wheel compounds originated from the ferromagnetic Mn(III)-Fe(III) coupling. For the first time, the quantum Monte Carlo study has been used to modulate the magnetic susceptibility of the huge Mn(6)Fe(6) metallomacrocycles, showing that the magnetic coupling constants J range from 3.0 to 8.0 K on the basis of the spin Hamiltonian [Formula: see text]. Hysteresis loops for 1 have been observed below 0.8 K, indicative of a single-molecule magnet with a blocking temperature (TB) of 0.8 K. Molecular wheels 2-5 exhibit frequency dependence of alternating-current magnetic susceptibility under zero direct-current magnetic field, signifying the slow magnetization relaxation similar to that of 1. Significantly, an unprecedented archlike Mn(2)Fe(2) cluster, [Mn(5-Cl(salpn))](2)[Fe(bpmb)(CN)(2)](2) x 3H(2)O x CH(3)CN (6), has been isolated as an intermediate of the Mn(6)Fe(6) wheel 5. Ferromagnetic Mn(III)-Fe(III) coupling results in a high-spin S = 5 ground state. Combination of the high-spin state and a negative magnetic anisotropy (D) results in the observation of slow magnetization relaxation in 6.     

Synthesis, structure, and characterization of new mononuclear Mn(II) complexes. Electrochemical conversion into new oxo-bridged Mn(2)(III,IV) complexes. Role of chloride ions

Two Mn(II) complexes are isolated and X-ray characterized, namely, cis-[(L(2))Mn(II)(Cl)(2)] (1) and [(L(3))Mn(II)Cl(OH(2))](ClO(4)) (2(ClO(4))), where L(2) and L(3) are the well-known tetradentate N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)ethane-1,2-diamine and N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)propane-1,3-diamine ligands, respectively. The crystal structure reveals that whereas the ligand L(2) is in the cis-alpha conformation in complex 1, the ligand L(3) is in the more unusual cis-beta conformation in 2. EPR spectra are recorded on frozen solutions for both complexes and are characteristic of Mn(II) species. Electrochemical behaviors are investigated on acetonitrile solution for both complexes and show that cation 2 exists as closely related Mn(II) species in equilibrium. For both complexes exhaustive bulk electrolyses of acetonitrile solution are performed at oxidative potential in various experimental conditions. In the presence of 2,6-lutidine and after elimination of chloride ligands, the formation of the di-mu-oxo mixed-valent complexes [(L(2))Mn(III)(mu-O)(2)Mn(IV)(L(2))](3+) (3a) and [(L(3))Mn(III)(mu-O)(2)Mn(IV)(L(3))](3+) (4) is confirmed by UV-vis and EPR spectroscopies and cyclic voltammetry. In addition crystals of 4(ClO(4))(3) were isolated, and the X-ray structure reveals the cis-alphaconformation of L(3). In the absence of 2,6-lutidine and without elimination of the exogenous chloride ions, the electrochemical oxidation of 1 leads to the formation of the mononuclear Mn(III) complex, namely, [(L(2))Mn(III)(Cl)(2)](+) (5), as confirmed by UV-vis as well as parallel mode EPR spectroscopy and cyclic voltammetry. In the same conditions, the electrochemical oxidation of complex 2 is more intricate, and a thorough analysis of EPR spectra establishes the formation of the binuclear mono-mu-oxo mixed-valent [(L(3))ClMn(III)(mu-O)Mn(IV)Cl(L(3))](3+) (6) complexes. Electrochemical conversion of Mn(II) complexes into mixed-valent Mn(2)(III,IV) oxo-bridged complexes in the presence of 2,6-lutidine is discussed. The role of the chloride ligands as well as that of L(3) in the building of oxo bridges is discussed. Differences in behavior between L(2) and L(3) are commented on.     

Laser-spectroscopic laboratory studies of atmospheric aqueous phase free radical chemistry

Nitrate radical (NO(3)) reactions with benzene (R-1), toluene (R-2), p-xylene (R-3), p-cresol (R-4) and mesitylene (R-5) have been studied by laser photolysis/long path laser absorption (LP-LPLA) in aqueous solution. Rate constants of k(1)=(4.0+/-0.6). 10(8), k(2)=(1.2+/-0.3). 10(9), k(3)=(1.6+/-0.1). 10(9), k(4)= (8.4+/-2.3). 10(8) and k(5)=(1.3+/-0.3). 10(9) lmol(-1)s(-1) were obtained at T=298 K. In addition, reaction rate coefficients for SO(-)(5)+Fe(2+)-->prod. (R-6) and SO(-)(5)+Mn(2+)-->prod. (R-7) of k(6)=(4.3+/-2.4). 10(7) lmol(-1)s(-1) and k(7)=(4.6+/-1.0). 10(6) lmol(-1)s(-1) (T=298 K, I-->0) have been obtained by the application of laser photolysis/UV-VIS broadband diode array spectroscopy. A new laser photolysis/UV-long path laser absorption experiment has been applied to study the reaction of the Cl(-)(2) radical anion with dissolved sulfur(IV). For the reactions Cl(-)(2)+HSO(-)(3)-->2Cl(-)+H(+)+SO(-)(3) (R-8) and Cl(-)(2)+SO(2-)(3)-->2Cl(-)+SO(-)(3) (R-9) rate coefficients of k(8)=(1.7+/-0.2). 10(8) lmol(-1)s(-1) (T=298 K, I-->0) and of k(9)=(6.2+/-0.3). 10(7) lmol(-1)s(-1) (T=279 K, I-->0) were obtained.     

Reactive intermediates relevant to the carbonylation of CH(3)Mn(CO)(5). Activation parameters for key dynamic processes

Reported is a time-resolved infrared and optical kinetics investigation of the transient species CH(3)C(O)Mn(CO)(4) (I(Mn)) generated by flash photolysis of the acetyl manganese pentacarbonyl complex CH(3)C(O)Mn(CO)(5) (A(Mn)) in cyclohexane and in tetrahydrofuran. Activation parameters were determined for CO trapping of I(Mn) to regenerate A(Mn) (rate = k(CO) [CO][I(Mn)]) as well as the methyl migration pathway to form methylmanganese pentacarbonyl CH(3)Mn(CO)(5) (M(Mn)) (rate = k(M)[I(Mn)]). These values were Delta H(++)(CO) = 31 +/- 1 kJ mol(-1), Delta S(++)(CO) = -64 +/- 3 J mol(-1) K(-1), Delta H(++)(M) = 35 +/- 1 kJ mol(-1), and Delta S(++)(M) = -111 +/- 3 J mol(-1) K(-1). Substantially different activation parameters were found for the methyl migration kinetics of I(Mn) in THF solutions where Delta H(++)(M) = 68 +/- 4 kJ mol(-1) and Delta S(++)(M) = 10 +/- 10 J mol(-1) K(-1), consistent with the earlier conclusion (Boese, W. T.; Ford, P. C. J. Am. Chem. Soc. 1995, 117, 8381-8391) that the composition of I(Mn) is different in these two media. The possible isotope effect on k(M) was also evaluated by studying the intermediates generated from flash photolysis of CD(3)C(O)Mn(CO)(5) in cyclohexane, but this was found to be nearly negligible (k(M)(h)/k(M)(d) (298 K) = 0.97 +/- 0.05, Delta H(++)(M)(d) = 37 +/- 4 kJ mol(-1), and Delta S(++)(M)(d) = -104 +/- 12 J mol(-1) K(-1)). The relevance to the migratory insertion mechanism of CH(3)Mn(CO)(5), a model for catalytic carbonylations, is discussed.