Structures and redox reactivities of copper complexes of (2-pyridyl)alkylamine ligands. Effects of the alkyl linker chain length

Ligand effects on the structures and redox reactivities of copper complexes have been examined using (2-pyridyl)alkylamine derivatives as the supporting ligands, where particular attention has been focused on the effects of the alkyl linker chain length connecting the tertiary amine nitrogen atom and the pyridine nucleus: N[bond]CH(2)[bond]Py (Pym) vs N[bond]CH(2)CH(2)[bond]Py (Pye). X-ray crystallographic analysis of the copper(I) complex of tridentate ligand (Phe)L(Pym2) [N,N-di(2-pyridylmethyl)-2-phenylethylamine] (complex 1) has demonstrated that it possesses a trigonal pyramidal geometry in which a d[bond]pi interaction with an eta(1)-binding mode exists between the metal ion and one of the ortho carbons of the phenyl ring of the ligand side arm (phenethyl). The result shows sharp contrast to the d[bond]pi interaction with an eta(2)-binding mode existing in the copper(I) complex of (Phe)L(Pye2) [N,N-di[2-(2-pyridyl)ethyl]-2-phenethylamine] (complex 2). Such a d-pi interaction has been shown to affect the stability of the copper(I) complex in CH(2)Cl(2). Oxygenation of copper(I) complex 1 supported by (Phe)L(Pym2) produces a bis(mu-oxo)dicopper(III) complex, also being in sharp contrast to the case of the copper(I) complex 2 with ligand (Phe)L(Pye2), which preferentially affords a (micro-eta(2):eta(2)-peroxo)dicopper(II) complex in the reaction with O(2). Such an effect of the alkyl linker chain length of the metal binding site has also been found to operate in the RSSR (disulfide)/2RS(-) (thiolate) redox system. Namely, ligand (S2,R)L(Pym1) (di[2-[(alkyl)(2-pyridinylmethyl)amino]ethyl] disulfide) with the methylene linker group (Pym) induced the reductive disulfide bond cleavage in the reaction with copper(I) ion to give a bis(micro-thiolato)dicopper(II) complex, while the ligand with the ethylene linker group (Pye), (S2,Bn)L(Pye1) (di[2-[(benzyl)(2-(2-pyridinyl)ethyl)amino]ethyl] disulfide), gave a disulfide-dicopper(I) complex. These ligand effects in the Cu(2)[bond]O(2) and Cu(2)[bond]S(2) systems have been discussed by taking into account the difference in electron-donor ability of the pyridine nucleus between the Pym and Pye ligand systems.     

Carbon-halogen bond activation mechanism by copper(I) complexes of (2-pyridyl)alkylamine ligands

The reaction of p-substituted benzyl halides ((Y)BnX; X = Cl, Br, and I; Y = p-substituent, OMe, t-Bu, Me, H, F, Cl, and NO(2)) and copper(I) complexes supported by a series of (2-pyridyl)alkylamine ligands has been investigated to shed light on the mechanism of copper(I) complex mediated carbon-halogen bond activation, including ligand effects on the redox reactivity of copper(I) complexes which are relevant to the chemistry. For both the tridentate ligand (Phe)L(Pym2) [N,N-bis(2-pyridylmethyl)-2-phenylethylamine] and tetradentate ligand TMPA [tris(2-pyridylmethyl)amine] complexes, the C-C coupling reaction of benzyl halides proceeded smoothly to give corresponding 1,2-diphenylethane derivatives and copper(II)-halide complex products. Kinetic analysis revealed that the reaction obeys second-order kinetics both on the copper complex and the substrate; rate = k[Cu](2)[(Y)BnX](2). A reaction mechanism involving a dinuclear copper(III)-halide organometallic intermediate is proposed, on the basis of the kinetic results, including observed electronic effects of p-substituents (Hammett plot) and the rate dependence on the BDE (bond dissociation energy) of the C-X bond, as well as the ligand effects.     

Equilibrium of low- and high-spin states of Ni(II) complexes controlled by the donor ability of the bidentate ligands

Low-spin nickel(II) complexes containing bidentate ligands with modulated nitrogen donor ability, Py(Bz)2 or MePy(Bz)2 (Py(Bz)2 = N,N-bis(benzyl)-N-[(2-pyridyl)methyl]amine, MePy(Bz)2 = N,N-bis(benzyl)-N-[(6-methyl-2-pyridyl)methyl]amine), and a beta-diketonate derivative, tBuacacH (tBuacacH = 2,2,6,6-tetramethyl-3,5-heptanedione), represented as [Ni(Py(Bz)2)(tBuacac)](PF6) (1) and [Ni(MePy(Bz)2)(tBuacac)](PF6) (2) have been synthesized. In addition, the corresponding high-spin nickel(II) complexes having a nitrate ion, [Ni(Py(Bz)2)(tBuacac)(NO3)] (3) and [Ni(MePy(Bz)2)(tBuacac)(NO3)] (4), have also been synthesized for comparison. Complexes 1 and 2 have tetracoordinate low-spin square-planar structures, whereas the coordination environment of the nickel ion in 4 is a hexacoordinate high-spin octahedral geometry. The absorption spectra of low-spin complexes 1 and 2 in a noncoordinating solvent, dichloromethane (CH2Cl2), display the characteristic absorption bands at 500 and 540 nm, respectively. On the other hand, the spectra of a CH2Cl2 solution of high-spin complexes 3 and 4 exhibit the absorption bands centered at 610 and 620 nm, respectively. The absorption spectra of 1 and 2 in N,N-dimethylformamide (DMF), being a coordinating solvent, are quite different from those in CH2Cl2, which are nearly the same as those of 3 and 4 in CH2Cl2. This result indicates that the structures of 1 and 2 are converted from a low-spin square-planar to a high-spin octahedral configuration by the coordination of two DMF molecules to the nickel ion. Moreover, complex 1 shows thermochromic behavior resulting from the equilibrium between low-spin square-planar and high-spin octahedral structures in acetone, while complex 2 exists only as a high-spin octahedral configuration in acetone at any temperature. Such drastic differences in the binding constants and thermochromic properties can be ascribed to the enhancement of the acidity of the nickel ion of 2 by the steric effect of the o-methyl group in the MePy(Bz)2 ligand in 2, which weakens the Ni-N(pyridine) bond length compared with that of the nonsubstituted Py(Bz)2 ligand in 1.     

Modeling the mononuclear, dinuclear, and trinuclear copper(I) reaction centers of copper proteins using pyridylalkylamine ligands connected to 1,3,5-triethylbenzene spacer

The structure and O2-reactivity of copper(I) complexes supported by novel ligands, Pye2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(2-(pyridin-2-yl)ethyl)-)aminomethyl)benzene), Pye3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(2-(pyridin-2-yl)ethyl))aminomethyl)benzene), MePym2 (1,3,5-triethyl-2,4-bis((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)benzene), and MePym3 (1,3,5-triethyl-2,4,6-tris((N-benzyl-N-(6-methylpyridin-2-ylmethyl))aminomethyl)benzene) have been examined. The ligands are designed to construct mono-, di-, and trinuclear copper(I) complexes by connecting two or three pyridylalkylamine metal-binding sites to a 1,3,5-triethylbenzene spacer. Thus, the reaction of the ligands with [CuI(CH3CN)4]X (X = PF6, CF3SO3) or CuICl gave the expected mononuclear copper(I) complexes [CuI(Pye2)(CF3SO3)] (1) and [CuI(Pye3)](CF3SO3) (2), dinuclear copper(I) complex [CuI2(MePym2)(Cl)]CuICl2 (3), and trinuclear copper(I) complex [CuI3(MePym3)(CH3CN)3](CF3SO3)3 (4), the structures of which were determined by X-ray crystallographic analysis. The mononuclear copper(I) complexes, 1 and 2, exhibit a distorted three-coordinate T-shape structure and a trigonal planar structure, respectively, which are very close to the coordination geometry of the CuA site of PHM (peptidylglycine alpha-hydroxylating monooxygenase) and the CuB site of CcO (cytochrome c oxidase). Notably, 1 and 2 showed a significantly high oxidation potential (990 mV vs SCE), thus showing virtually no reactivity toward O2. On the other hand, the metal centers of the dinuclear and trinuclear copper(I) complexes, 3 and 4, exhibit a distorted trigonal planar geometry and a trigonal pyramidal geometry, respectively. In contrast to the mononuclear copper(I) complexes, these dinuclear and trinuclear copper(I) complexes reacted with O2 to induce an aromatic ligand hydroxylation reaction involving an NIH-shift of one of the ethyl substituents on the benzene spacer. The NIH-shift of the alkyl substituent on the aromatic ring is strong evidence of the electrophilic aromatic substitution mechanism, although the active oxygen intermediate could not be directly detected during the course of the reaction. The biological relevance of the copper(I) complexes is also discussed on the basis of structure and O2-reactivity.     

Thioester hydrolysis reactivity of zinc hydroxide complexes: investigating reactivity relevant to glyoxalase II enzymes

A recently reported binuclear zinc hydroxide complex [(L(1)Zn(2))(mu-OH)](ClO(4))(2) (, L(1) = 2,6-bis[(bis(2-pyridylmethyl)amino)methyl]-4-methylphenolate monoanion) containing a single bridging hydroxide was examined for thioester hydrolysis reactivity. Treatment of it with hydroxyphenylthioacetic acid S-methyl ester in dry CD(3)CN results in no reaction after approximately 65 h at 45(1) degrees C. Binuclear zinc hydroxide complexes of the N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine (L(2)) and N-methyl-N-((6-neopentylamino-2-pyridyl)methyl)-N-((2-pyridyl)ethyl)amine (L(3)) chelate ligands were prepared by treatment of each ligand with molar equivalent amounts of Zn(ClO(4))(2).6H(2)O and KOH in methanol. These complexes, [(L(2)Zn)(2)(mu-OH)(2)](ClO(4))(2) and [(L(3)Zn)(2)(mu-OH)(2)](ClO(4))(2) (), which have been structurally characterized by X-ray crystallography, behave as 1 : 1 electrolytes in acetonitrile, indicating that the binuclear cations dissociate into monomeric zinc hydroxide species in solution. Treatment of them with one equivalent of hydroxyphenylthioacetic acid S-methyl ester per zinc center in acetonitrile results in the formation of a zinc alpha-hydroxycarboxylate complex, [(L(2))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O or [(L(3))Zn(O(2)CCH(OH)Ph)]ClO(4).1.5H(2)O, and CH(3)SH. These reactions, to our knowledge, are the first reported examples of thioester hydrolysis mediated by zinc hydroxide complexes. The results of this study suggest that a terminal Zn-OH moiety may be required for hydrolysis reactivity with a thioester substrate.     

Influence of the chelate ligand structure on the amide methanolysis reactivity of mononuclear zinc complexes

Zinc complexes of three new amide-appended ligands have been prepared and isolated. These complexes, [(dpppa)Zn](ClO4)2 (4(ClO4)2; dpppa = N-((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), [(bdppa)Zn](ClO4)2 (6(ClO4)2; bdppa = N,N-bis((N,N-diethylamino)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)amine), and [(epppa)Zn](ClO4)2 (8(ClO4)2; epppa = N-((2-ethylthio)ethyl)-N-((6-pivaloylamido-2-pyridyl)methyl)-N-((2-pyridyl)methyl)amine), have been characterized by X-ray crystallography (4(ClO4)2 and 8(ClO4)2), 1H and 13C NMR, IR, and elemental analysis. Treatment of 4(ClO4)2 or 8(ClO4)2 with 1 equiv of Me4NOH.5H2O in methanol-acetonitrile (5:3) results in amide methanolysis, as determined by the recovery of primary amine-appended forms of the chelate ligand following removal of the zinc ion. These reactions proceed via the initial formation of a deprotonated amide intermediate ([(dpppa-)Zn]ClO4 (5) and [(epppa-)Zn]ClO4 (9)) which in each case has been isolated and characterized (1H and 13C NMR, IR, elemental analysis). Treatment of 6(ClO4)2 with Me4NOH.5H2O in methanol-acetonitrile results in the formation of a deprotonated amide complex, [(bdppa-)Zn]ClO4 (7), which was isolated and characterized. This complex does not undergo amide methanolysis after prolonged heating in a methanol-acetonitrile mixture. Kinetic studies and construction of Eyring plots for the amide methanolysis reactions of 4(ClO4)2 and 8(ClO4)2 yielded thermodynamic parameters that provide a rationale for the relative rates of the amide methanolysis reactions. Overall, we propose that the mechanistic pathway for these amide methanolysis reactions involves reaction of the deprotonated amide complex with methanol to produce a zinc methoxide species, the reactivity of which depends, at least in part, on the steric hindrance imparted by the supporting chelate ligand. Amide methanolysis involving a zinc complex supported by a N2S2 donor chelate ligand (3(ClO4)2) is more complicated, as in addition to the formation of a deprotonated amide intermediate free chelate ligand is present in the reaction mixture.     

Solvent effect on the thermodynamics of Ag(I) coordination to tripodal polypyridine ligands

An investigation on the thermodynamics of complex formation between Ag(I) ion and two tripodal ligands tris[(2-pyridyl)methyl]amine (TPA) and 6,6′-bis-[bis-(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (BTPA) has been carried out in the aprotic solvents dimethylsulfoxide (DMSO) and dimethylformamide (DMF) by means of potentiometry and titration calorimetry. The results for TPA are compared with those already obtained for other aliphatic tripodal polyamines. In general, the TPA ligand forms complexes less stable than 2,2′,2″-triaminotriethylamine (TREN) and tris(2-(methylamino)ethyl)amine (Me₃TREN) as a result of the combination of higher structural rigidity of TPA and lower σ-donor ability of pyridinic moieties with respect to primary and secondary amines. The same trend is found if the stability of Ag(I) complex with TPA is compared with that of tris(2-(dimethylamino)ethyl)amine (ME₆TREN), despite the pyridinic nitrogen is formally a tertiary one. Theoretical calculations run to explain the reasons of this weaker interaction indicate that this difference is due to solvation, rather than to steric or σ-donor effects. The ligand BTPA is able to form bimetallic species whose relative stability is largely influenced by the different solvation of Ag(I) ion in DMSO and DMF rather than by the difference in the dielectric constants of these two media.     

Chelate ring size variations and their effects on coordination chemistry and catechol dioxygenase reactivity of iron(III) complexes

The catechol dioxygenase reactivity of iron(III) complexes using tripodal ligands was investigated. Increasing, as well as decreasing, chelate ring sizes in the highly active complex [Fe(tmpa)(dbc)]B(C6H5)4 (tmpa = tris[(2-pyridyl)methyl]amine; dbc = 3,5-di-tert-butylcatecholate dianion), using related ligands, only resulted in decreased reactivity of the investigated compounds. A detailed low-temperature stopped-flow investigation of the reaction of dioxygen with [Fe(tmpa)(dbc)]B(C6H5)4 was performed, and activation parameters of DeltaH++ = 23 +/- 1 kJ mol(-1) and DeltaS++ = -199 +/- 4 J mol(-1) K(-1) were obtained. Crystal structures of bromo-(tetrachlorocatecholato-O,O')(bis((2-pyridyl)methyl)-2-pyridylamine-N,N',N')-iron(III), (mu-oxo)-bis(bromo)(bis((2-pyridyl)methyl)-2-pyridylamine-N,N',N' ',N')-diiron(III), dichloro-((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N',N' ',N')-iron(III) and (tetrachlorocatecholato-O,O')((2-(2-pyridyl)ethyl)bis((2-pyridyl)methyl)amine-N,N',N' ',N')-iron(III) are reported.     

Rhodium, palladium and platinum complexes of tris(pyridylalkyl)amine and tris(benzimidazolylmethyl)amine N4-tripodal ligands

To investigate the influence of a potentially N4-tripodal amine ligand on the structure and internal exchange processes of its complexes with late transition metals, five rhodium, six palladium and two platinum complexes have been prepared from seven alkyl-bridged N-heterocyclic amine tripodal ligands: tris(2-pyridylmethyl)amine, (2-(2-pyridylethyl))bis(2-pyridylmethyl)amine, bis(2-(2-pyridylethyl))-2-pyridylmethylamine, bis(2-(2-pyridylethyl))amine, ((6-(hydroxymethyl)-2-pyridyl)methyl)bis(2-pyridylmethyl)amine, tris(2-benzimidazolylmethyl)amine (tbima) and tris(3-ethyl-2-benzimidazolylmethyl)amine. Single-crystal X-ray diffraction studies were completed for ten complexes: the d6-rhodium(III) complexes are octahedral with kappa 4 N-bound ligands, whereas the d8-palladium(II) and d8-platinum(II) complexes are square planar, kappa 3 N-bound by the tripodal ligand with a dangling N-donor leg, except for the unusual [Pd2(tbima)2Cl2]Cl2 dimer in which each palladium(II) ion is square planar and bound by two benzimidazole legs from one tbima ligand, one leg from the other tbima ligand and a chloride ancillary ligand. Cation bilayers are a common structural motif in the crystal structures. Variable-temperature 1H NMR studies reveal exchange occurs between the coordinated and dangling N-donor legs in the palladium and platinum complexes. Exchange free energy (Delta G++ c) values have been calculated and some general rules governing the favoured complex structures and exchange pathways elucidated. The palladium(II) and platinum(II) complexes of a ligand with an pyridylethyl leg are unstable with respect to elimination of vinylpyridine.     

Nonheme FeIVO complexes that can oxidize the C-H bonds of cyclohexane at room temperature

Nonheme oxoiron(IV) complexes of two pentadentate ligands, N4Py (N,N-bis(2-pyridylmethyl)-bis(2-pyridyl)methylamine) and Bn-tpen (N-benzyl-N,N',N'-tris(2-pyridylmethyl)-1,2-diaminoethane), have been generated and found to have spectroscopic properties similar to the closely related tetradentate TPA (tris(2-pyridylmethyl)amine) complex reported earlier. However, unlike the TPA complex, the pentadentate complexes have a considerable lifetime at room temperature. This greater thermal stability has allowed the hydroxylation of alkanes with C-H bonds as strong as 99.3 kcal/mol to be observed at room temperature. Furthermore, a large deuterium KIE value is found in the oxidation of ethylbenzene. These observations lend strong credence to postulated mechanisms of mononuclear nonheme iron enzymes that invoke the intermediacy of oxoiron(IV) species.     

Trigonal bipyramidal geometry and tridentate coordination mode of the tripod in FeCl(2) complexes with tris(2-pyridylmethyl)amine derivatives bis-alpha-substituted with bulky groups. structures and spectroscopic comparative studies

A series of dichloroferrous complexes with ligands derived from the tris(2-pyridylmethyl)amine tripod has been prepared and characterized. The X-ray crystal structures of the complexes [bis(2-bromo-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2) ((Br(2)TPA)Fe(II)Cl(2)) and [bis(2-phenyl-6-pyridylmethyl)(2-pyridylmethyl)amine]Fe(II)Cl(2), ((Ph(2)TPA)Fe(II)Cl(2)) are reported. In these complexes, the tripod coordinates in the tridentate mode, with a substituted pyridyl arm dangling away from the metal. Both complexes have a trigonal bipyramidal iron center with two equatorial chloride ions. Their crystal structures are compared with those of the [tris(2-pyridylmethyl)amine]Fe(II)Cl(2) and [(2-bromo-6-pyridylmethyl)bis(2-pyridylmethyl)amine]Fe(II)Cl(2) complexes ((TPA)Fe(II)Cl(2) and (BrTPA)Fe(II)Cl(2), respectively) in which the ligand coordinates in the tetradentate mode. For all complexes, the metal to ligand distances are systematically above the value of 2.0 A, and (1)H NMR displays paramagnetically shifted resonances with short relaxation times. This indicates that the iron is in a high-spin state. Electric conductivity measurements show that, for all complexes, the measured values lie within the same range, significantly below those expected for ionic complexes. Together with the analysis of the UV-visible and NMR data, this strongly suggests that the coordination mode of the tripod is retained in solution.     

Two dinuclear iron(II) complexes K[Fe2(L1)(SCN)4]·2(C3H8O) and [Fe2(L1)(SeCN)3(C5H5N)]·H2O are stabilised in the '[HS-LS]' state by a bis-tetradentate pyrazolate-based ligand

Two air-sensitive dinuclear iron(II) complexes, K[Fe(II)(2)(L(1))(SCN)(4)]·2(C(3)H(8)O) (1) and [Fe(II)(2)(L(1))(SeCN)(3)(C(5)H(5)N)]·H(2)O (2), of 3,5-bis[N,N-bis(2-pyridylmethyl)aminomethyl]-1H-pyrazolate [(L(1))(-)] have been prepared. Interestingly, complex 1 is anionic, featuring four coordinated SCN(-) anions and a potassium counterion whereas complex 2 is neutral, containing a coordinated pyridine molecule and only three coordinated SeCN(-) anions. These are the first iron complexes reported for this type of ligand. Magnetic measurements and M?ssbauer spectra show that both 1 and 2 are in a '[HS-LS]' mixed spin state between 300 and 2 K.     

Cobalt(III) carbonate and bicarbonate chelate complexes of tripodal tetraamine ligands containing pyridyl donors: the steric basis for the stability of chelated bicarbonate complexes

The synthesis and characterization (X-ray crystallography, UV/vis spectroscopy, electrochemistry, ESI-MS, and (1)H, (13)C, and (59)Co NMR) of the complexes [Co(L)(O(2)CO)]ClO(4)xH(2)O (L = tpa (tpa = tris(2-pyridylmethyl)amine) (x = 1), pmea (pmea = bis((2-pyridyl)methyl)-2-((2-pyridyl)ethyl)amine) (x = 0), pmap (pmap = bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine) (x = 0), tepa (tepa = tris(2-(2-pyridyl)ethyl)amine) (x = 0)) which contain tripodal tetradentate pyridyl ligands and chelated carbonate ligands are reported. The complexes display different colors in both the solid state and solution, which can be rationalized in terms of the different ligand fields exerted by the tripodal ligands. Electrochemical data show that [Co(tepa)(O(2)CO)](+) is the easiest of the four complexes to reduce, and the variation in E(red.) values across the series of complexes can also be explained in terms of the different ligand fields exerted by the tripodal ligands, as can the (59)Co NMR data which show a chemical shift range of over 2000 ppm for the four complexes. [Co(pmea)(O(2)CO)](+) is fluxional in aqueous solution, and VT NMR spectroscopy ((1)H and (13)C) in DMF-d(7) (DMF = dimethylformamide) over the temperature range -25.0 to 75.0 degrees C are consistent with inversion of the unique six-membered chelate ring. This process shows a substantial activation barrier (DeltaG(#) = 58 kJ mol(-1)). The crystal structures of [Co(tpa)(O(2)CO)]ClO(4)xH(2)O, [Co(pmea)(O(2)CO)]ClO(4).3H(2)O, [Co(pmap)(O(2)CO)]ClO(4), and [Co(tepa)(O(2)CO)]ClO(4) are reported, and the complexes containing the asymmetric tripodal ligands pmea and pmap both crystallize as the 6-isomer. The carbonate complexes all show remarkable stability in 6 M HCl solution, with [Co(pmap)(O(2)CO)](+) showing essentially no change in its UV/vis spectrum over 4 h in this medium. The chelated bicarbonate complexes [Co(pmea)(O(2)COH)]ZnCl(4), [Co(pmap)(O(2)COH)][Co(pmap)(O(2)CO)](ClO(4))(3), [Co(pmap)(O(2)COH)]ZnCl(4)xH(2)O, and [Co(pmap(O(2)COH)]ZnBr(4)x2H(2)O can be isolated from acidic aqueous solution, and the crystal structure of [Co(pmap)(O(2)COH)]ZnCl(4)x3H(2)O is reported. The stability of the carbonate complexes in acid is explained by analysis of the crystallographic data for these, and other slow to hydrolyze chelated carbonate complexes, which show that the endo (coordinated) oxygen atoms are significantly hindered by atoms on the ancillary ligands, in contrast to complexes such as [Co(L)(O(2)CO)](+) (L = (NH(3))(4), (en)(2), tren, and nta), which undergo rapid acid hydrolysis and which show no such steric hindrance.     

Thermodynamic study of Cd(II) complex formation with tripodal N-donor ligands in DMSO

The complex formation of Cd(II) with N-donor ligands in dimethylsulfoxide (DMSO) is investigated by means of potentiometry and titration calorimetry. The ligands considered in this work are tripodal polyamines and polypyridines: 2,2′,2″-triaminotriethylamine (TREN), tris(2-(methylamino)ethyl)amine (Me₃TREN), tris(2-(dimethylamino)ethyl)amine (Me₆TREN), tris[(2-pyridyl)methyl]amine (TPA) and 6,6′-bis-[bis-(2-pyridylmethyl)aminomethyl]-2,2′-bipyridine (BTPA). These ligands are characterized by a systematic modification of the donor groups to relate their structure to the thermodynamics of the complexes formed. The TREN and Me₃TREN ligands form highly stable species. The stability of the complex formed with the fully methylated Me₆TREN is much lower than with other polyamines and the enthalpic and entropic terms suggest an incomplete coordination to the metal ion. In general, the TPA ligand forms complexes less stable than TREN and Me₃TREN as a result of the combination of higher structural rigidity of TPA and lower basicity of pyridine moiety with respect to primary and secondary amines. Pyridine-containing ligands display, in general, a less unfavorable formation entropy than tripodal polyamines here considered. In particular, TPA forms a more stable 1:1 species with respect to Me₆TREN due to the entropic term, being the enthalpy less negative. The ligand BTPA is able to form only a monometallic complex, where the metal ion is likely to be encapsulated as indicated by the obtained thermodynamic parameters.     

The versatile binding mode of transition-state analogue inhibitors of tyrosinase towards dicopper(II) model complexes: experimental and theoretical investigations

We describe 2-mercaptopyridine-N-oxide (HSPNO) as a new and efficient competitive inhibitor of mushroom tyrosinase (K(IC) =3.7 μM). Binding studies of HSPNO and 2-hydroxypyridine-N-oxide (HOPNO) on dinuclear copper(II) complexes [Cu(2)(BPMP)(μ-OH)](ClO(4))(2) (1; HBPMP=2,6-bis[bis(2-pyridylmethyl)aminomethyl]-4-methylphenol) and [Cu(2)(BPEP)(μ-OH)](ClO(4))(2)) (2; HBPEP=2,6-bis{bis[2-(2-pyridyl)ethyl]aminomethyl}-4-methylphenol), known to be functional models for the tyrosinase diphenolase activity, have been performed. A combination of structural data, spectroscopic studies, and DFT calculations evidenced the adaptable binding mode (bridging versus chelating) of HOPNO in relation to the geometry and chelate size of the dicopper center. For comparison, binding studies of HSPNO and kojic acid (5-hydroxy-2-(hydroxymethyl)-4-pyrone) on dinuclear complexes were performed. A theoretical approach has been developed and validated on HOPNO adducts to compare the binding mode on the model complexes. It has been applied for HSPNO and kojic acid. Although results for HSPNO were in line with those obtained with HOPNO, thus reflecting their chemical similarity, we showed that the bridging mode was the most preferential binding mode for kojic acid on both complexes.     

Quantitative evaluation of d-pi interaction in copper(I) complexes and control of copper(I)-dioxygen reactivity

Crystal structures of the copper(I) complexes 1(X), 2, and 3 of a series of tridentate ligands L1(X), L2, and L3, respectively (L1(X): p-substituted derivatives of N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X=H, Me, OMe, Cl, NO(2); L2: N,N-bis[2-(2-pyridyl)ethyl]-2-methyl-2-phenylethylamine; L3: N,N-bis[2-(2-pyridyl)ethyl]-2,2-diphenylethylamine) were solved to demonstrate that all the copper(I) complexes involve an eta(2) copper-arene interaction with the phenyl ring of the ligand sidearm. The Cu(I) ion in each complex has a distorted tetrahedral geometry consisting of the three nitrogen atoms (one tertiary amine nitrogen atom and two pyridine nitrogen atoms) and C(1)-C(2) of the phenyl ring of ligand sidearm, whereby the Cu-C distances of the copper-arene interaction significantly depend on the para substituents. The existence of the copper-arene interaction in a nonpolar organic solvent (CH(2)Cl(2)) was demonstrated by the observation of an intense MLCT band around 290 nm, and the magnitude of the interaction was evaluated by detailed analysis of the (1)H and (13)C NMR spectra and the redox potentials E(1/2) of the copper ion, as well as by means of the ligand-exchange reaction between the phenyl ring and CH(3)CN as an external ligand. The thermodynamic parameters DeltaH(o) and DeltaS(o) for the ligand-exchange reaction with CH(3)CN afforded a quantitative measure for the energy difference of the copper-arene interaction in the series of copper(I) complexes. Density functional studies indicated that the copper(I)-arene interaction mainly consists of the interaction between the d(z(2) ) orbital of Cu(I) and a pi orbital of the phenyl ring. The copper(I) complexes 1(X) reacted with O(2) at -80 degrees C in CH(2)Cl(2) to give the corresponding (micro-eta(2):eta(2)-peroxo)dicopper(II) complexes 4, the formation rates k(obs) of which were significantly retarded by stronger d-pi interaction, while complexes 2 and 3, which exhibit the strongest d-pi interaction showed significantly lower reactivity toward O(2) under the same experimental conditions. Thus, the d-pi interaction has been demonstrated for the first time to affect the copper(I)-dioxygen reactivity, and represents a new aspect of ligand effects in copper(I)-dioxygen chemistry.     

Dioxo-bridged dinuclear manganese(III) and -(IV) complexes of pyridyl donor tripod ligands: combined effects of steric substitution and chelate ring size variations on structural,spectroscopic, and electrochemical properties

The syntheses and structural, spectral, and electrochemical characterization of the dioxo-bridged dinuclear Mn(III) complexes [LMn(mo-O)(2)MnL](ClO(4))(2), of the tripodal ligands tris(6-methyl-2-pyridylmethyl)amine (L(1)) and bis(6-methyl-2-pyridylmethyl)(2-(2-pyridyl)ethyl)amine (L(2)), and the Mn(II) complex of bis(2-(2-pyridyl)ethyl)(6-methyl-2-pyridylmethyl)amine (L(3)) are described. Addition of aqueous H(2)O(2) to methanol solutions of the Mn(II) complexes of L(1) and L(2) produced green solutions in a fast reaction from which subsequently precipitated brown solids of the dioxo-bridged dinuclear complexes 1 and 2, respectively, which have the general formula [LMn(III)(mu-O)(2)Mn(III)L](ClO(4))(2). Addition of 30% aqueous H(2)O(2) to the methanol solution of the Mn(II) complex of L(3) ([Mn(II)L(3)(CH(3)CN)(H(2)O)](ClO(4))(2) (3)) showed a very sluggish change gradually precipitating an insoluble black gummy solid, but no dioxo-bridged manganese complex is produced. By contrast, the Mn(II) complex of the ligand bis(2-(2-pyridyl)ethyl)(2-pyridylmethyl)amine (L(3a)) has been reported to react with aqueous H(2)O(2) to form the dioxo-bridged Mn(III)Mn(IV) complex. In cyclic voltammetric experiments in acetonitrile solution, complex 1 shows two reversible peaks at E(1/2) = 0.87 and 1.70 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and the Mn(III)Mn(IV) <--> Mn(IV)(2) processes, respectively. Complex 2 also shows two reversible peaks, one at E(1/2) = 0.78 V and a second peak at E(1/2) = 1.58 V (vs Ag/AgCl) assigned to the Mn(III)(2) <--> Mn(III)Mn(IV) and Mn(III)Mn(IV) <--> Mn(IV)(2) redox processes, respectively. These potentials are the highest so far observed for the dioxo-bridged dinuclear manganese complexes of the type of tripodal ligands used here. The bulk electrolytic oxidation of complexes 1 and 2, at a controlled anodic potential of 1.98 V (vs Ag/AgCl), produced the green Mn(IV)(2) complexes that have been spectrally characterized. The Mn(II) complex of L(3) shows a quasi reversible peak at an anodic potential of E(p,a) of 1.96 V (vs Ag/AgCl) assigned to the oxidation Mn(II) to Mn(III) complex. It is about 0.17 V higher than the E(p,a) of the Mn(II) complex of L(3a). The higher oxidation potential is attributable to the steric effect of the methyl substituent at the 6-position of the pyridyl donor of L(3).     

Hydrogen-atom transfer in open-shell organometallic chemistry: the reactivity of Rh(II)(cod) and Ir(II)(cod) radicals

A series of new metalloradical rhodium and iridium complexes [M(II)(cod)(N-ligand)](2+) in the uncommon oxidation state +II were synthesized by one-electron oxidation of their [M(I)(cod)(N-ligand)](+) precursors (M=Rh, Ir; cod=(Z,Z)-1,5-cyclooctadiene; and N-ligand is a podal bis(pyridyl)amine ligand: N,N-bis(2-pyridylmethyl)amine (dpa), N-(2-pyridylmethyl)-N-(6-methyl-2-pyridylmethyl)amine (pla), or N-benzyl-N,N-bis(6-methyl-2-pyridylmethyl)amine (Bn-dla). EPR spectroscopy, X-ray diffraction, and DFT calculations reveal that each of these [M(II)(cod)(N-ligand)](2+) species adopts a square-pyramidal geometry with the two cod double bonds and the two pyridine fragments in the basal plane and the N(amine) donor at the apical position. The unpaired electron of these species mainly resides at the metal center, but the apical N(amine) donor also carries a considerable fraction of the total spin density (15-18 %). Density functional calculations proved a valuable tool for the analysis and simulation of the experimental EPR spectra. Whereas the M(II)(olefin) complexes are quite stable as solids, in solution they spontaneously transform into a 1:1 mixture of M(III)(allyl) species and protonated M(I)(olefin) complexes (in the forms [M(I)(olefin)(protonated N-ligand)](2+) for M=Rh and [M(III)(H)(olefin)(N-ligand)](2+) for M=Ir). Similar reactions were observed for the related propene complex [M(II)(propene)(Me(2)tpa)](2+) (Me(2)tpa=N,N,N-tris(6-methyl-2-pyridylmethyl)amine). The decomposition rate of the [M(II)(cod)(N-ligand)](2+) species decreases with increasing N-ligand bulk in the following order: dpa>pla>Bn-dla. Decomposition of the most hindered [M(II)(cod)(Bn-dla)](2+) complexes proceeds by a second-order process. The kinetic rate expression v=k(obs)[M(II)](2) in acetone with k(obs)=k'[H(+)][S], where [S] is the concentration of additional coordinating reagents (MeCN), is in agreement with ligand-assisted dissociation of one of the pyridine donors. Solvent coordination results in formation of more open, reactive species. Protonation of the noncoordinating pyridyl group increases the concentration of this species, and thus [H(+)] appears in the kinetic rate expression. The kinetic data are in agreement with bimolecular hydrogen-atom transfer from M(II)(cod) to another M(II) species (DeltaH( not equal)=11.5+/-2 kcal mol(-1), DeltaS( not equal)=-27+/-10 cal K(-1) mol(-1), and DeltaG( not equal)(298 K)=19.5+/-5 kcal mol(-1)).     

Characterization of imidazolate-bridged dinuclear and mononuclear hydroperoxo complexes

Dinucleating ligands having two metal-binding sites bridged by an imidazolate moiety, Hbdpi, HMe(2)bdpi, and HMe(4)bdpi (Hbdpi = 4,5-bis(di(2-pyridylmethyl)aminomethyl)imidazole, HMe(2)bdpi = 4,5-bis((6-methyl-2-pyridylmethyl)(2-pyridylmethyl)aminomethyl)imidazole, HMe(4)bdpi = 4,5-bis(di(6-methyl-2-pyridylmethyl)aminomethyl)imidazole), have been designed and synthesized as model ligands for copper-zinc superoxide dismutase (Cu,Zn-SOD). The corresponding mononucleating ligands, MeIm(Py)(2), MeIm(Me)(1), and MeIm(Me)(2) (MeIm(Py)(2) = (1-methyl-4-imidazolylmethyl)bis(2-pyridylmethyl)amine, MeIm(Me)(1) = (1-methyl-4-imidazolylmethyl)(6-methyl-2-pyridylmethyl)(2-pyridylmethyl)amine, MeIm(Me)(2) = (1-methyl-4-imidazolyl-methyl)bis(6-methyl-2-pyridylmethyl)amine), have also been synthesized for comparison. The imidazolate-bridged Cu(II)-Cu(II) homodinuclear complexes represented as [Cu(2)(bdpi)(CH(3)CN)(2)](ClO(4))(3).CH(3)CN.3H(2)O (1), [Cu(2)(Me(2)bdpi)(CH(3)CN)(2)](ClO(4))(3) (2), [Cu(2)(Me(4)bdpi)(H(2)O)(2)](ClO(4))(3).4H(2)O (3), a Cu(II)-Zn(II) heterodinuclear complex of the type of [CuZn(bdpi)(CH(3)CN)(2)](ClO(4))(3).2CH(3)CN (4), Cu(II) mononuclear complexes of [Cu(MeIm(Py)(2))(CH(3)CN)](ClO(4))(2).CH(3)CN (5), [Cu(MeIm(Me)(1))(CH(3)CN)](ClO(4))(2)( )()(6), and [Cu(MeIm(Me)(2))(CH(3)CN)](ClO(4))(2)( )()(7) have been synthesized and the structures of complexes 5-7 determined by X-ray crystallography. The complexes 1-7 have a pentacoordinate structure at each metal ion with the imidazolate or 1-methylimidazole nitrogen, two pyridine nitrogens, the tertiary amine nitrogen, and a solvent (CH(3)CN or H(2)O) which can be readily replaced by a substrate. The reactions between complexes 1-7 and hydrogen peroxide (H(2)O(2)) in the presence of a base at -80 degrees C yield green solutions which exhibit intense bands at 360-380 nm, consistent with the generation of hydroperoxo Cu(II) species in all cases. The resonance Raman spectra of all hydroperoxo intermediates at -80 degrees C exhibit a strong resonance-enhanced Raman band at 834-851 cm(-1), which shifts to 788-803 cm(-1) (Deltanu = 46 cm(-1)) when (18)O-labeled H(2)O(2) was used, which are assigned to the O-O stretching frequency of a hydroperoxo ion. The resonance Raman spectra of hydroperoxo adducts of complexes 2 and 6 show two Raman bands at 848 (802) and 834 (788), 851 (805), and 835 (789) cm(-1) (in the case of H(2)(18)O(2), Deltanu = 46 cm(-1)), respectively. The ESR spectra of all hydroperoxo complexes are quite close to those of the parent Cu(II) complexes except 6. The spectrum of 6 exhibits a mixture signal of trigonal-bipyramid and square-pyramid which is consistent with the results of resonance Raman spectrum.     

Formation, characterization, and reactivity of bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands.

Bis(mu-oxo)dinickel(III) complexes supported by a series of bis[2-(2-pyridyl)ethyl]amine ligands have been successfully generated by treating the corresponding bis(mu-hydroxo)dinickel(II) complexes or bis(mu-methoxo)dinickel(II) complex with an equimolar amount of H(2)O(2) in acetone at low temperature. The bis(mu-oxo)dinickel(III) complexes exhibit a characteristic UV-vis absorption band at approximately 410 nm and a resonance Raman band at 600-610 cm(-1) that shifted to 570-580 cm(-1) upon (18)O-substitution. Kinetic studies and isotope labeling experiments using (18)O(2) imply the existence of intermediate(s) such as peroxo dinickel(II) in the course of formation of the bis(mu-oxo)dinickel(III) complex. The bis(mu-oxo)dinickel(III) complexes supported by the mononucleating ligands (L1(X) = para-substituted N,N-bis[2-(2-pyridyl)ethyl]-2-phenylethylamine; X = OMe, Me, H, Cl) gradually decompose, leading to benzylic hydroxylation of the ligand side arm (phenethyl group). The kinetics of the ligand hydroxylation process including kinetic deuterium isotope effects (KIE), p-substituent effects (Hammett plot), and activation parameters (Delta H(H)(*) and Delta S(H)(*)) indicate that the bis(muxo)dinickel(III) complex exhibits an ability of hydrogen atom abstraction from the substrate moiety as in the case of the bis(mu-oxo)dicopper(III) complex. Such a reactivity of bis(mu-oxo)dinickel(III) complexes has also been suggested by the observed reactivity toward external substrates such as phenol derivatives and 1,4-cyclohexadiene. The thermal stability of the bis(mu-oxo)dinickel(III) complex is significantly enhanced when the dinucleating ligand with a longer alkyl strap is adopted instead of the mononucleating ligand. In the m-xylyl ligand system, no aromatic ligand hydroxylation occurred, showing a sharp contrast with the reactivity of the (mu-eta(2):eta(2)-peroxo)dicopper(II) complex with the same ligand which induces aromatic ligand hydroxylation via an electrophilic aromatic substitution mechanism. Differences in the structure and reactivity of the active oxygen complexes between the nickel and the copper systems are discussed on the basis of the detailed comparison of these two systems with the same ligand.