Synthesis, characterization, and photoresponsive properties of a series of Mo(IV)-Cu(II) complexes

Six Mo(IV)-Cu(II) complexes, [Cu(tpa)](2)[Mo(CN)(8)]·15H(2)O (1, tpa = tris(2-pyridylmethyl)amine), [Cu(tren)](2)[Mo(CN)(8)]·5.25H(2)O (2, tren = tris(2-aminoethyl)amine), [Cu(en)(2)][Cu(0.5)(en)][Cu(0.5)(en)(H(2)O)][Mo(CN)(8)]·4H(2)O (3, en = ethylenediamine), [Cu(bapa)](3)[Mo(CN)(8)](1.5)·12.5H(2)O (4, bapa = bis(3-aminopropyl)amine), [Cu(bapen)](2)[Mo(CN)(8)]·4H(2)O (5, bapen = N,N'-bis(3-aminopropyl)ethylenediamine), and [Cu(pn)(2)][Cu(pn)][Mo(CN)(8)]·3.5H(2)O (6, pn = 1,3-diaminopropane), were synthesized and characterized. Single-crystal X-ray diffraction analyses show that 1-6 have different structures varying from trinuclear clusters (1-2), a one-dimensional belt (3), two-dimensional grids (4-5), to a three-dimensional structure (6). Magnetic and ESR measurements suggest that 1-6 exhibit thermally reversible photoresponsive properties on UV light irradiation through a Mo(IV)-to-Cu(II) charge transfer mechanism. A trinuclear compound [Cu(II)(tpa)](2)[Mo(V)(CN)(8)](ClO(4)) (7) was synthesized as a model of the photoinduced intermediate.     

Synthesis, characterization, and structures of copper(II)-thiosulfate complexes incorporating tripodal tetraamine ligands

The reaction of [Cu(L)(H(2)O)](2+) with an excess of thiosulfate in aqueous solution produces a blue to green color change indicative of thiosulfate coordination to Cu(II) [L = tren, Bz(3)tren, Me(6)tren, and Me(3)tren; tren = tris(2-aminoethyl)amine, Bz(3)tren = tris(2-benzylaminoethyl)amine, Me(6)tren = tris(2,2-dimethylaminoethyl)amine, and Me(3)tren = tris(2-methylaminoethyl)amine]. In excess thiosulfate, only [Cu(Me(6)tren)(H(2)O)](2+) promotes the oxidation of thiosulfate to polythionates. Products suitable for single-crystal X-ray diffraction analyses were obtained for three thiosulfate complexes, namely, [Cu(tren)(S(2)O(3))].H(2)O, [Cu(Bz(3)tren)(S(2)O(3))].MeOH, and (H(3)Me(3)tren)[Cu(Me(3)tren)(S(2)O(3))](2)(ClO(4))(3). Isolation of [Cu(Me(6)tren)(S(2)O(3))] was prevented by its reactivity. In each complex, the copper(II) center is found in a trigonal bipyramidal (TBP) geometry consisting of four amine nitrogen atoms, with the bridgehead nitrogen in an axial position and an S-bound thiosulfate in the other axial site. Each structure exhibits H bonding (involving the amine ligand, thiosulfate, and solvent molecule, if present), forming either 2D sheets or 1D chains. The structure of [Cu(Me(3)tren)(MeCN)](ClO(4))(2) was also determined for comparison since no structures of mononuclear Cu(II)-Me(3)tren complexes have been reported. The thiosulfate binding constant was determined spectrophotometrically for each Cu(II)-amine complex. Three complexes yielded the highest values reported to date [K(f) = (1.82 +/- 0.09) x 10(3) M(-1) for tren, (4.30 +/- 0.21) x 10(4) M(-1) for Bz(3)tren, and (2.13 +/- 0.05) x 10(3) M(-1) for Me(3)tren], while for Me(6)tren, the binding constant was much smaller (40 +/- 10 M(-1)).     

Factors that control catalytic two- versus four-electron reduction of dioxygen by copper complexes

The selective two-electron reduction of O(2) by one-electron reductants such as decamethylferrocene (Fc*) and octamethylferrocene (Me(8)Fc) is efficiently catalyzed by a binuclear Cu(II) complex [Cu(II)(2)(LO)(OH)](2+) (D1) {LO is a binucleating ligand with copper-bridging phenolate moiety} in the presence of trifluoroacetic acid (HOTF) in acetone. The protonation of the hydroxide group of [Cu(II)(2)(LO)(OH)](2+) with HOTF to produce [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF) makes it possible for this to be reduced by 2 equiv of Fc* via a two-step electron-transfer sequence. Reactions of the fully reduced complex [Cu(I)(2)(LO)](+) (D3) with O(2) in the presence of HOTF led to the low-temperature detection of the absorption spectra due to the peroxo complex [Cu(II)(2)(LO)(OO)] (D) and the protonated hydroperoxo complex [Cu(II)(2)(LO)(OOH)](2+) (D4). No further Fc* reduction of D4 occurs, and it is instead further protonated by HOTF to yield H(2)O(2) accompanied by regeneration of [Cu(II)(2)(LO)(OTF)](2+) (D1-OTF), thus completing the catalytic cycle for the two-electron reduction of O(2) by Fc*. Kinetic studies on the formation of Fc*(+) under catalytic conditions as well as for separate examination of the electron transfer from Fc* to D1-OTF reveal there are two important reaction pathways operating. One is a rate-determining second reduction of D1-OTF, thus electron transfer from Fc* to a mixed-valent intermediate [Cu(II)Cu(I)(LO)](2+) (D2), which leads to [Cu(I)(2)(LO)](+) that is coupled with O(2) binding to produce [Cu(II)(2)(LO)(OO)](+) (D). The other involves direct reaction of O(2) with the mixed-valent compound D2 followed by rapid Fc* reduction of a putative superoxo-dicopper(II) species thus formed, producing D.     

Efficient and specific strand scission of DNA by a dinuclear copper complex: comparative reactivity of complexes with linked tris(2-pyridylmethyl)amine moieties

The compound [Cu(II)(2)(D(1))(H(2)O)(2)](ClO(4))(4) (D(1) = dinucleating ligand with two tris(2-pyridylmethyl)amine units covalently linked in their 5-pyridyl positions by a -CH(2)CH(2)- bridge) selectively promotes cleavage of DNA on oligonucleotide strands that extend from the 3' side of frayed duplex structures at a site two residues displaced from the junction. The minimal requirements for reaction include a guanine in the n (i.e. first unpaired) position of the 3' overhang adjacent to the cleavage site and an adenine in the n position on the 5' overhang. Recognition and strand scission are independent of the nucleobase at the cleavage site. The necessary presence of both a reductant and dioxygen indicates that the intermediate responsible for cleavage is produced by the activation of dioxygen by a copper(I) form of the dinuclear complex. The lack of sensitivity to radical quenching agents and the high level of site selectivity in scission suggest a mechanism that does not involve a diffusible radical species. The multiple metal center exhibits a synergy to promote efficient cleavage as compared to the action of a mononuclear analogue [Cu(II)(TMPA)(H(2)O)](ClO(4))(2) (TMPA = tris(2-pyridylmethyl)amine) and [Cu(OP)(2)](2+) (OP = 1,10-phenanthroline) at equivalent copper ion concentrations. The dinuclear complex, [Cu(II)(2)(D(1))(H(2)O)(2)](ClO(4))(4), is even capable of mediating efficient specific strand scission at concentrations where [Cu(OP)(2)](2+) does not detectably modify DNA. The unique coordination and reactivity properties of [Cu(II)(2)(D(1))(H(2)O)(2)](ClO(4))(4) are critical for its efficiency and site selectivity since an analogue, [Cu(II)(2)(DO)(Cl(2))](ClO(4))(2), where DO is a dinucleating ligand very similar to D(1), but with a -CH(2)OCH(2)- bridge, exhibits only nonselective cleavage of DNA. The differences in the reactivity of these two complexes with DNA and their previously established interaction with dioxygen suggest that specific strand scission is a function of the orientation of a reactive intermediate.     

Equilibrium and Redox Kinetics of Copper(II)-Thiourea Complexes

Stopped-flow spectrophotometric measurements identify and determine equilibrium data for thiourea (tu) complexes of copper(II) formed in aqueous solution. In excess Cu(II), the complex ion [Cu(tu)](2+) has a stability constant beta(1) = 2.3 +/- 0.1 M(-)(1) and molar absorptivity at 340 nm of epsilon(1) = (4.0 +/- 0.2) x 10(3) M(-)(1) cm(-)(1) at 25.0 degrees C, 2.48 mM HClO(4), and &mgr; = 464 mM (NaClO(4)). The fast reduction of Cu(II) by excess tu obeys the rate law -d[Cu(II)]/dt = k'[Cu(II)](2)[tu](7) with a value for the ninth-order rate constant k' = (1.60 +/- 0.18) x 10(14) M(-)(8) s(-)(1), which derives from a rate-determining step involving the bimolecular decomposition of two complexed Cu(II) species. Copper(II) catalyzes the reduction of hexachloroiridate(IV) by tu according to the rate law -d[IrCl(6)(2)(-)]/dt = (k(2,unc)[tu](2) + k(1,cat) [tu](5)[Cu(II)])[IrCl(6)(2)(-)]. Least-squares analysis yields values of k(2,unc) and k(1,cat) equaling 385 +/- 4 M(-)(2) s(-)(1) and (3.7 +/- 0.1) x 10(13) M(-)(6) s(-)(1), respectively, at &mgr; = 115 mM (NaClO(4)). The corresponding mechanism has a rate-determining step that involves the oxidation of [Cu(II)(tu)(5)](2+) by [IrCl(6)](2)(-) rather than the bimolecular reaction of two cupric-tu complexes.     

Biomimetic aryl hydroxylation derived from alkyl hydroperoxide at a nonheme iron center. Evidence for an Fe(IV)=O oxidant

Many nonheme iron-dependent enzymes activate dioxygen to catalyze hydroxylations of arene substrates. Key features of this chemistry have been developed from complexes of a family of tetradentate tripodal ligands obtained by modification of tris(2-pyridylmethyl)amine (TPA) with single alpha-arene substituents. These included the following: -C(6)H(5) (i.e., 6-PhTPA), L(1); -o-C(6)H(4)D, o-d(1)-L(1); -C(6)D(5), d(5)-L(1); -m-C(6)H(4)NO(2), L(2); -m-C(6)H(4)CF(3), L(3); -m-C(6)H(4)Cl, L(4); -m-C(6)H(4)CH(3), L(5); -m-C(6)H(4)OCH(3), L(6); -p-C(6)H(4)OCH(3), L(7). Additionally, the corresponding ligand with one alpha-phenyl and two alpha-methyl substituents (6,6-Me(2)-6-PhTPA, L(8)) was also synthesized. Complexes of the formulas [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(n)())Fe(II)(OTf)(2)] (n = 1-7, OTf = (-)O(3)SCF(3)), and [(L(8))Fe(II)(OTf)(2)](2) were obtained and characterized by (1)H NMR and UV-visible spectroscopies and by X-ray diffraction in the cases of [(L(1))Fe(II)(NCCH(3))(2)](ClO(4))(2), [(L(6))Fe(II)(OTf)(2)], and [(L(8))Fe(II)(OTf)(2)](2). The complexes react with tert-butyl hydroperoxide ((t)()BuOOH) in CH(3)CN solutions to give iron(III) complexes of ortho-hydroxylated ligands. The product complex derived from L(1) was identified as the solvated monomeric complex [(L(1)O(-))Fe(III)](2+) in equilibrium with its oxo-bridged dimer [(L(1)O(-))(2)Fe(III)(2)(mu(2)-O)](2+), which was characterized by X-ray crystallography as the BPh(4)(-) salt. The L(8) product was also an oxo-bridged dimer, [(L(8)O(-))(2)Fe(III)(2)(mu(2)-O)](2+). Transient intermediates were observed at low temperature by UV-visible spectroscopy, and these were characterized as iron(III) alkylperoxo complexes by resonance Raman and EPR spectroscopies for L(1) and L(8). [(L(1))Fe(II)(OTf)(2)] gave rise to a mixture of high-spin (S = 5/2) and low-spin (S = 1/2) Fe(III)-OOR isomers in acetonitrile, whereas both [(L(1))Fe(OTf)(2)] in CH(2)Cl(2) and [(L(8))Fe(OTf)(2)](2) in acetonitrile afforded only high-spin intermediates. The L(1) and L(8) intermediates both decomposed to form respective phenolate complexes, but their reaction times differed by 3 orders of magnitude. In the case of L(1), (18)O isotope labeling indicated that the phenolate oxygen is derived from the terminal peroxide oxygen via a species that can undergo partial exchange with exogenous water. The iron(III) alkylperoxo intermediate is proposed to undergo homolytic O-O bond cleavage to yield an oxoiron(IV) species as an unobserved reactive intermediate in the hydroxylation of the pendant alpha-aryl substituents. The putative homolytic chemistry was confirmed by using 2-methyl-1-phenyl-2-propyl hydroperoxide (MPPH) as a probe, and the products obtained in the presence and in the absence of air were consistent with formation of alkoxy radical (RO(*)). Moreover, when one ortho position was labeled with deuterium, no selectivity was observed between hydroxylation of the deuterated and normal isotopomeric ortho sites, but a significant 1,2-deuterium shift ("NIH shift") occurred. These results provide strong mechanistic evidence for a metal-centered electrophilic oxidant, presumably an oxoiron(IV) complex, in these arene hydroxylations and support participation of such a species in the mechanisms of the nonheme iron- and pterin-dependent aryl amino acid hydroxylases.     

H2O2-reactivity of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine ligands with 6-phenyl substituents

The structure and H(2)O(2)-reactivity of a series of copper(II) complexes supported by tris[(pyridin-2-yl)methyl]amine (TPA) derivatives having a phenyl group at the 6-position of pyridine donor group(s) [(6-phenylpyridin-2-yl)methyl]bis[(pyridin-2-yl)methyl]amine (Ph(1)TPA), bis[(6-phenylpyridin-2-yl)methyl][(pyridin-2-yl)methyl]amine (Ph(2)TPA), and tris[(6-phenylpyridin-2-yl)methyl]amine (Ph(3)TPA) have systematically been examined to get insights into the aromatic substituent (6-Ph) effects on the coordination chemistry of TPA ligand system. The X-ray crystallographic analyses have revealed that [Cu(II)(TPA)(CH(3)CN)](ClO(4))(2) (CuTPA) and [Cu(II)(Ph(3)TPA)(CH(3)CN)](ClO(4))(2) (3) exhibit a trigonal bipyramidal structure, whereas [Cu(II)(Ph(1)TPA)(CH(3)CN)](ClO(4))(2) (1) shows a slightly distorted square pyramidal structure and [Cu(II)(Ph(2)TPA)(CH(3)CN)](ClO(4))(2) (2) has an intermediate structure between trigonal bipyramidal and square pyramidal. On the other hand, the UV-vis and ESR data have suggested that all the copper(II) complexes have a similar trigonal bipyramidal structure in solution. The redox potentials of CuTPA, 1, 2, and 3 have been determined as E(1/2) = -0.34, -0.28, -0.16, and -0.04 mV vs Ag/AgNO(3), respectively, demonstrating that introduction of each 6-Ph group causes positive shift of E(1/2) about 0.1 V. Notable difference in H(2)O(2)-reactivity has been found among the copper(II) complexes. Namely, CuTPA and 1 afforded mononuclear copper(II)-hydroperoxo complexes CuTPA-OOH and 1-OOH, respectively, whereas complex 2 provided bis(mu-oxo)dicopper(III) complex 2-oxo. On the other hand, copper(II) complex 3 was reduced to the corresponding copper(I) complex 3(red). On the basis of the H(2)O(2)-reactivity together with the X-ray structures and the redox potentials of the copper(II) complexes, the substituent effects of 6-Ph are discussed in detail.     

Inner-sphere mechanism for molecular oxygen reduction catalyzed by copper amine oxidases

Copper and topaquinone (TPQ) containing amine oxidases utilize O2 for the metabolism of biogenic amines while concomitantly generating H2O2 for use by the cell. The mechanism of O2 reduction has been the subject of long-standing debate due to the obscuring influence of a proton-coupled electron transfer between the tyrosine-derived TPQ and copper, a rapidly established equilibrium precluding assignment of the enzyme in its reactive form. Here, we show that substrate-reduced pea seedling amine oxidase (PSAO) exists predominantly in the Cu(I), TPQ semiquinone state. A new mechanistic proposal for O2 reduction is advanced on the basis of thermodynamic considerations together with kinetic studies (at varying pH, temperature, and viscosity), the identification of steady-state intermediates, and the analysis of competitive oxygen kinetic isotope effects, (18)O KIEs, [kcat/KM((16,16)O2)]/[kcat/KM((16,18)O2)]. The (18)O KIE = 1.0136 +/- 0.0013 at pH 7.2 is independent of temperature from 5 degrees C to 47 degrees C and insignificantly changed to 1.0122 +/- 0.0020 upon raising the pH to 9, thus indicating the absence of kinetic complexity. Using density functional methods, the effect is found to be precisely in the range expected for reversible O2 binding to Cu(I) to afford a superoxide, [Cu(II)(eta(1)-O2)(-I)](+), intermediate. Electron transfer from the TPQ semiquinone follows in the first irreversible step to form a peroxide, Cu(II)(eta(1)-O2)(-II), intermediate driving the reduction of O2. The similar (18)O KIEs reported for copper amine oxidases from other sources raise the possibility that all enzymes react by related inner-sphere mechanisms although additional experiments are needed to test this proposal.     

Synthesis, characterization, crystal structure and preliminary reactivity behaviour of new heteropolytopic ligands based on the 1,3,5-triazine spacer and pyrazolyl, tris-pyrazolylmethyl and tris-pyrazolylethoxy bonding fragments

Four new potentially polytopic nitrogen donor ligands based on the 1,3,5-triazine fragment, L(1)-L(4) (L(1) = 2-chloro-4,6-di(1H-pyrazol-1-yl)-1,3,5-triazine, L(2) = N,N'-bis(4,6-di(1H-pyrazol-1-yl)-1,3,5-triazin-2-yl)ethane-1,2-diamine, L(3) = 2,4,6-tris(tri(1H-pyrazol-1-yl)methyl)-1,3,5-triazine, and L(4) = 2,4,6-tris(2,2,2-tri(1H-pyrazol-1-yl)ethoxy)-1,3,5-triazine) have been synthesized and characterized. The X-ray crystal structure of L(3) confirms that its molecular nature consists of a 1,3,5-triazine ring bearing three tripodal tris(pyrazolyl) arms. L(1), L(2), and L(4) react with Cu(I), Cu(II), Pd(II) and Ag(I) salts yielding mono-, di-, and oligonuclear derivatives: [Cu(L(1))(Cy(3)P)]ClO(4), [{Ag(2)(L(2))}(CF(3)SO(3))(2)]·H(2)O, [Cu(2)(L(2))(NO(3))(2)](NO(3))(2)·H(2)O, [Cu(2)(L(2))(CH(3)COO)(2)](CH(3)COO)(2)·3H(2)O, [Pd(2)(L(2))(Cl)(4)]·2H(2)O, [Ru(L(2))(Cl)(OH)]·CH(3)OH, [Ag(3)(L(4))(2)](CF(3)SO(3))(3) and [Ag(3)(L(4))(2)](BF(4))(3). The interaction of L(3) with Ag(I), Cu(II), Zn(II) and Ru(II) complexes unexpectedly produced the hydrolysis of the ligand with formation, in all cases, of tris(pyrazolyl)methane (TPM) derivatives. In detail, the already known [Ag(TPM)(2)](CF(3)SO(3)) and [Cu(TPM)(2)](NO(3))(2), as well as the new [Zn(TPM)(2)](CF(3)SO(3))(2) and [Ru(TMP)(p-cymene)]Cl(OH)·2H(2)O complexes have been isolated. Single-crystal XRD determinations on the latter derivatives confirm their formulation, evidencing, for the Ru(II) complex, an interesting supramolecular arrangement of the anions and crystallization water molecules.     

Characterization of the chloranilate(*3-) pi radical as a strong spin-coupling bridging ligand

Dinuclear [(TPA)Co(II)(CA2-)Co(II)(TPA)](BF4)2.2MeOH (1) [TPA = tris(2-pyridylmethyl)amine] and [(TPA)CoII(CA*3-)CoII(TPA)](BF4).2Et2O (2) with a bridging chloranilate radical ligand formed by reduction of 1 are crystallographically and magnetically characterized. 1 has shown a weak antiferromagnetic coupling within the Co(II) dimer [J/kB = -0.65 K (-0.45 cm(-1))], while 2 has a 2 orders of magnitude stronger antiferromagnetic interaction between the Co(II) ion and a radical [J/kB = -75 K (52 cm(-1))].     

Supramolecular spin-crossover iron complexes based on imidazole-imidazolate hydrogen bonds

The [Fe(II)(H(3)L)](BF(4))(2).3H(2)O (1) complex was synthesized, where H(3)L (tris[[2-[(imidazole-4-yl)methylidene]amino]ethyl]amine) is a tripodal ligand obtained by condensation of tris(2-aminoethyl)amine and 4-formylimidazole (fim) in a 1:3 molar ratio. Starting from 1, a series of complexes, [Fe(II)(H(1.5)L)](BF(4))(0.5) (2) (=[Fe(II)(H(3)L)][Fe(II)(L)]BF(4)), [Fe(H(1.5)L)]BF(4) (3) (=[Fe(II)(H(3)L)][Fe(III)(L)](BF(4))(2)), [Fe(III)(H(3)L)](BF(4))(3).fim.H(2)O (4), and [Fe(III)(L)].2.5H(2)O (5), has been synthesized and characterized. The single-crystal X-ray structure of each complex has been determined. The Fe(II) compound, 2, and a mixed valence Fe(II)-Fe(III) compound, 3, involve formally hemi-deprotonated ligands, H(1.5)L. The structure of 3 consists of a homochiral two-dimensional assembled sheet, arising from the intermolecular hydrogen bonds between [Fe(II)(H(3)L)](2+) and [Fe(III)(L)](0) (3). All but 5 exhibit spin crossover between low-spin (LS) and high-spin (HS) states. This is a rare case where both Fe(II) and Fe(III) complexes containing the same ligand exhibit spin-crossover behavior. Magnetic susceptibility and M?ssbauer studies showed that 3 has three accessible electronic states: LS Fe(II)-LS Fe(III), HS Fe(II)-LS Fe(III), and HS Fe(II)-HS Fe(III). Compounds 1-3 show the light-induced excited spin-state trapping effect at the Fe(II) sites upon irradiation with green light. The solution magnetic properties, electronic spectra, and electrochemical properties of 1, 4, and 5 were also studied.     

Supramolecular copper hydroxide tennis balls: self-assembly, structures, and magnetic properties of octanuclear [Cu(8)L(8)(OH)(4)](4+) clusters (HL = N-(2-pyridylmethyl)acetamide)

The self-assembly of supramolecular copper "tennis balls" that possess unusual magnetic properties using a small pyridyl amide ligand is described. Copper(II) complexes of N-(2-pyridylmethyl)acetamide (HL) were synthesized in methanol. In the absence of base, the mononuclear complex [Cu(HL)(2)](ClO(4))(2) (1) was prepared. The structure of 1, determined by X-ray crystallography, contains a copper(II) ion surrounded by bidentate HL ligands coordinated via the pyridyl N atom and the carbonyl O atom in a trans, square planar arrangement. Reactions carried out in the presence of triethylamine resulted in cluster complexes [Cu(8)L(8)(OH)(4)](ClO(4))(4) and [Cu(8)L(8)(OH)(4)](CF(3)SO(3))(4) [2(ClO(4))(4) and 2(OTf)(4), respectively]. The cationic portions of 2(ClO(4))(4) and 2(OTf)(4) are isostructural, containing eight copper(II) ions, eight deprotonated ligands (L(-)), and four mu(3)-hydroxide ligands. The top and bottom halves of the cluster are related by a pseudo-S(4) symmetry operation and are held together by bridging L(-) ligands. Solutions of 2(ClO(4))(4) and 2(OTf)(4), which were shown to contain the full [Cu(8)L(8)(OH)(4)](4+) fragment by electrospray mass spectrometry and conductance experiments, are EPR silent. Magnetic susceptibility measurements for 2(ClO(4))(4) as a function of temperature and magnetic field showed the Cu ions all to exhibit magnetic moments in the range expected for the d(9) configuration. At low temperatures, the magnetization was reduced due to predominantly antiferromagnetic interactions between ions. Analysis showed that partially frustrated interactions among the four Cu ions making up each half of the cluster gave good agreement with the data once a large molecular anisotropy was taken into account, with J(c) = 106 cm(-1), D = 27 cm(-1), and g = 2.17.     

Modeling features of the non-heme diiron cores in O2-activating enzymes through the synthesis, characterization, and oxidation of 1,8-naphthyridine-based complexes

Multidentate naphthyridine-based ligands were used to prepare a series of diiron(II) complexes. The compound [Fe(2)(BPMAN)(mu-O(2)CPh)(2)](OTf)(2) (1), where BPMAN = 2,7-bis[bis(2-pyridylmethyl)aminomethyl]-1,8-naphthyridine, exhibits two reversible oxidation waves with E(1/2) values at +310 and +733 mV vs Cp(2)Fe(+)/Cp(2)Fe, as revealed by cyclic voltammetry. Reaction with O(2) or H(2)O(2) affords a product with optical and M?ssbauer properties that are characteristic of a (mu-oxo)diiron(III) species. The complexes [Fe(2)(BPMAN)(mu-OH)(mu-O(2)CAr(Tol))](OTf)(2) (2) and [Fe(2)(BPMAN)(mu-OMe)(mu-O(2)CAr(Tol))](OTf)(2) (3) were synthesized, where Ar(Tol)CO(2)(-) is the sterically hindered ligand 2,6-di(p-tolyl)benzoate. Compound 2 has a reversible redox wave at +11 mV, and both 2 and 3 react with O(2), via a mixed-valent Fe(II)Fe(III) intermediate, to give final products that are also consistent with (mu-oxo)diiron(III) species. The paddle-wheel compound [Fe(2)(BBAN)(mu-O(2)CAr(Tol))(3)](OTf) (4), where BBAN = 2,7-bis(N,N-dibenzylaminomethyl)-1,8-naphthyridine, reacts with dioxygen to yield benzaldehyde via oxidative N-dealkylation of a benzyl group on BBAN, an internal substrate. In the presence of bis(4-methylbenzyl)amine, the reaction also produces p-tolualdehyde, revealing oxidation of an external substrate. A structurally related compound, [Fe(2)(BEAN)(mu-O(2)CAr(Tol))(3)](OTf) (5), where BEAN = 2,7-bis(N,N-diethylaminomethyl)-1,8-naphthyridine, does not undergo N-dealkylation, nor does it facilitate the oxidation of bis(4-methylbenzyl)amine. The contrast in reactivity of 4 and 5 is attributed to a difference in accessibility of the substrate to the diiron centers of the two compounds. The M?ssbauer spectroscopic properties of the diiron(II) complexes were also investigated.     

Syntheses, emission properties and intramolecular ligand exchange of zinc complexes with ligands belonging to the tmpa family

The zinc complexes [(L1)(2)Zn(MeOH)(2)](OTf)(2), [(L1)ZnCl(2)], [(L2)ZnCl(2)], [(L2)Zn(OTf)(H(2)O)]OTf and [(Me-bispic)ZnCl(2)] of the ligands N-[(2-pyridyl)methyl]-2,2'-dipyridylamine (L1), N-[bis(2-pyridyl)methyl]-2-pyridylamine (L2) and N-methyl-[bis(2-pyridyl)methyl]amine (Me-bispic) were synthesised and characterised. The first copper(I) complexes of the ligands L1 and L2 were also synthesised and structurally characterised. [(L1)ZnCl(2)] showed unexpected fluxional behaviour in solution and revealed an interesting intramolecular ligand exchange mechanism in the coordination sphere of the zinc ion. Furthermore, strong blue emission was observed under UV-light excitation.     

Redox-adaptable copper hosts. Pyridazine-linked cryptands accommodate copper in a range of redox States

A series of structurally characterized copper complexes of two pyridazine-spaced cryptands in redox states + (I,I), (II,I), (II), (II,II) are reported. The hexaimine cryptand L(I) [formed by the 2 + 3 condensation of 3,6-diformylpyridazine with tris(2-aminoethyl)amine (tren)] is able to accommodate two non-stereochemically demanding copper(I) ions, resulting in [Cu(I)(2)L(I)](BF(4))(2) 1, or one stereochemically demanding copper(II) ion, resulting in [Cu(II)L(I)()](BF(4))(2) 3. Complex 3 crystallizes in two forms, 3a and 3b, with differing copper(II) ion coordination geometries. Addition of copper(I) to the monometallic complex 3 results in the mixed-valence complex [Cu(I)Cu(II)L(I)](X)(3) (X = PF(6)(-), 2a; X = BF(4)(-), 2b) which is well stabilized within this cryptand as indicated by electrochemical studies (K(com) = 2.1 x 10(11)). The structurally characterized, octaamine cryptand L(A), prepared by sodium borohydride reduction of L(I), is more flexible than L(I) and can accommodate two stereochemically demanding copper(II) ions, generating the dicopper(II) cryptate [Cu(II)(2)L(A)](BF(4))(4) 4. Electrochemical studies indicate that L(A) stabilizes the copper(II) oxidation state more effectively than L(I); no copper redox state lower than II,II has been isolated in the solid state using this ligand.     

Synthesis, structural and magnetic properties of a series of copper(II) complexes containing a monocarboxylated perchlorotriphenylmethyl radical as a coordinating open-shell ligand

A series of complexes of copper(II)-containing a perchlorotriphenylmethyl radical functionalized with a carboxylic group as a new ligand is reported. The compounds [Cu(PTMMC)(2)(L)(3)](PTMMC = (tetradecachloro-4-carboxytriphenyl)methyl radical; L =(1) H(2)O, (2) pyrimidine and ethanol or (3) pyridine), [Cu(2)(PTMMC)(2)(MeCOO)(2)(H(2)O)(2)](4) and [Cu(HPTMMC)(2)(L)(3)](HPTMMC =alpha-H-(tetradecachlorotriphenyl)methane-4-carboxylic acid; L = pyridine)(5) were structurally characterized. In complexes 1, 2, 3, and 5, the copper(II) ion is coordinated to two PTMMC (or HPTMMC) units in a slightly distorted square planar surrounding, while 4 shows a paddle-wheel copper(II) dimer structure, where each Cu metal ion has four O atoms of different carboxylate groups, two of them belonging to two PTMMC radicals. The copper(II)-radical exchange couplings are antiferromagnetic for complexes 1, 2 and 3. A linear three-spin model was applied to complexes 1, 2 and 3 to give J/k(B)=-24.9, -15.0 and -20.7 K, respectively. Magnetic properties of 4 show that it is one of the scarce examples of a spin-frustrated system composed of organic radicals and metal ions. In this case, experimental data were fitted to a magnetic model based on a symmetrical butterfly arrangement to give a copper(II)-copper(II) exchange coupling of J/k(B)=-350.0 K and a copper(II)-radical exchange coupling of J/k(B)=-21.3 K, similar to that observed for the copper(II)-radical interactions in complexes, and.     

A double arene hydroxylation mediated by dicopper(II)-hydroperoxide species

The dicopper(II) complex [Cu(2)(L)](4+) (L = alpha,alpha'-bis[bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino]-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)-hydroquinone complex in which the xylyl ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated ligand 2,6-bis([bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)benzene-1,4-diol was isolated by decomposition of the product complex. The incorporation of two oxygen atoms from H(2)O(2) into the ligand was confirmed by isotope labeling studies using H(2)(18)O(2). The pathway of the unusual double hydroxylation was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (6) and 2,6-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging micro-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6 and containing a protonated phenol is rapidly hydroxylated by H(2)O(2) and represents the first product formed in the hydroxylation of [Cu(2)(L)](4+). Kinetic studies performed on the reactions of [Cu(2)(L)](4+) and 3 with H(2)O(2) show that the second hydroxylation is faster than the first one at room temperature (0.13 +/- 0.05 s(-1) vs 5.0(+/-0.1) x 10(-3) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (Delta H++ = 39.1 +/- 0.9 kJ mol(-1) and Delta S++ = -115.7 +/- 2.4 J K(-1) mol(-1) for the first hydroxylation; Delta H++ = 77.8 +/- 1.6 kJ mol(-1) and Delta S++ = -14.0 +/- 0.4 J K(-1) mol(-1) for the second hydroxylation). By studying the reaction between [Cu(2)(L)](4+) and H(2)O(2) at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu(2)(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12,000), 444 (epsilon 1200), and 610 nm (epsilon 800 M(-1)cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].     

Imidazole and imidazolate iron complexes: on the way for tuning 3D-structural characteristics and reactivity. Redox interconversions controlled by protonation state

X-ray structures for six Fe(II) and Fe(III) complexes from two closely heptadentate N-tripodal ligands, L1H(3) = tris[(imidazol-4-yl)-3-aza-3-butenyl]amine and L2H(3) = tris[(imidazol-2-yl)-3-aza-3-butenyl]amine, are described: three complexes in the L1 series (namely, [Fe(II)(L1H(3))](2+) and [Fe(III)(L1H(3))](3+) at low pH and [Fe(III)(L1)](0) at high pH) and three complexes in the L2 series (namely, [Fe(II)(L2H(3))](2+) at low pH and [Fe(II)(L2H)](0) and [Fe(III)(L2)](0) at high pH). Most of these complexes are stable in both Fe(II) and Fe(III) redox states and with the ligand in various protonation states. In the solid state, hydrogen bonds networks were obtained. Structural differences induced by 2- or 4-imidazole substitution are described and discussed. In solution, interconversions between different forms, with regard to oxidation and protonation states, were investigated by UV-visible spectroscopy, cyclic voltammetry, and potentiometry. The deprotonation pattern of these polyimidazole iron(II) and iron(III) complexes is described in detail. pK(a)s of the imidazolate/imidazole moieties in MeOH/H(2)O are reported. Two new species, namely, [Fe(II)(L1)](-) and [Fe(II)(L2)](-), were shown to be obtained in DMSO upon strong base addition and characterized by UV-vis spectroscopy and cyclic voltammetry. Half-wave potentials of Fe(III)/Fe(II) complexes with ligand moieties in several protonation states are reported, both in DMSO and in MeOH/H(2)O. Because of the presence of free imidazole groups coordinated to the iron, the potential of the iron(III)/iron(II) couples can be tuned by pH. A shift of DeltaE = E(deprot) - E(prot) ranging from -270 to -320 mV per exchanged proton in DMSO was measured. This study shows moreover that interconversions (with regard to both redox and protonation states) can be reversed several times. As the complexes have been isolated in order to be tested as superoxide dismutase mimics, preliminary reactions with dioxygen and with superoxide, considered as oxidant and reducer of biological importance, are reported. In these two series, O(2)(-) behaves either as a base or as a reducer and no adducts have been observed.     

Mechanistic study of hydrogen transfer to imines from a hydroxycyclopentadienyl ruthenium hydride. Experimental support for a mechanism involving coordination of imine to ruthenium prior to hydrogen transfer

Reaction of [2,3,4,5-Ph(4)(eta(5)-C(4)COH)Ru(CO)(2)H] (2) with different imines afforded ruthenium amine complexes at low temperatures. At higher temperatures in the presence of 2, the complexes decomposed to give [Ru(2)(CO)(4)(mu-H)(C(4)Ph(4)COHOCC(4)Ph(4))] (1) and free amine. Electron-rich imines gave ruthenium amine complexes with 2 at a lower temperature than did electron-deficient imines. The negligible deuterium isotope effect (k(RuHOH)/k(RuDOD) = 1.05) observed in the reaction of 2 with N-phenyl[1-(4-methoxyphenyl)ethylidene]amine (12) shows that neither hydride (RuH) nor proton (OH) is transferred to the imine in the rate-determining step. In the dehydrogenation of N-phenyl-1-phenylethylamine (4) to the corresponding imine 8 by [2,3,4,5-Ph(4)(eta(4)-C(4)CO)Ru(CO)(2)] (A), the kinetic isotope effects observed support a stepwise hydrogen transfer where the isotope effect for C-H cleavage (k(CHNH)/k(CDNH) = 3.24) is equal to the combined (C-H, N-H) isotope effect (k(CHNH)/k(CDND) = 3.26). Hydrogenation of N-methyl(1-phenylethylidene)amine (14) by 2 in the presence of the external amine trap N-methyl-1-(4-methoxyphenyl)ethylamine (16) afforded 90-100% of complex [2,3,4,5-Ph(4)(eta(4)-C(4)CO)]Ru(CO)(2)NH(CH(3))(CHPhCH(3)) (15), which is the complex between ruthenium and the amine newly generated from the imine. At -80 degrees C the reaction of hydride 2 with 4-BnNH-C(6)H(9)=NPh (18), with an internal amine trap, only afforded [2,3,4,5-Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Ph)(C(6)H(10)-4-NHBn) (19), where the ruthenium binds to the amine originating from the imine, showing that neither complex A nor the diamine is formed. Above -8 degrees C complex 19 rearranged to the thermodynamically more stable [Ph(4)(eta(4)-C(4)CO)](CO)(2)RuNH(Bn)(C(6)H(10)-4-NHPh) (20). These results are consistent with an inner sphere mechanism in which the substrate coordinates to ruthenium prior to hydrogen transfer and are difficult to explain with the outer sphere pathway previously proposed.     

Multinuclear platinum(II)-amine complexes containing bis(aminopropyl)dicarba-closo-dodecaborane(12) ligands

Treatment of the bridging bidentate 1,Z-bis(aminopropyl)-1,Z-dicarba-closo-dodecaborane(12)(1,Z-bis(aminopropyl)-1,Z-carborane) ligands of the type 1,Z-[H(2)N(CH(2))(3)](2)-1,Z-C(2)B(10)H(10)(L(1), Z= 7, 5) or (L(2), Z= 12, 6) with two equivalents of trans-[PtClI(2)(NH(3))](-), followed by halogen ligand metathesis with AgOTf and HCl((aq)) afforded the novel diplatinum(II)-amine species cis-[[PtCl(2)(NH(3))](2)L(n)](7(n= 1) or 8(n= 2), respectively). Similarly, the reaction of L(1) or L(2) with the labile trans-[PtCl(dmf)(NH(3))(2)](+) afforded trans-[[PtCl(NH(3))(2)](2)L(n)](OTf)(2)(9(n= 1) or 10(n= 2), respectively) in good yield and purity. However, isolation of the analogous 1,2-carborane complexes was not possible owing to decomposition reactions that led to extensive degradation of the carborane cage and reduction of the metal centre. The mixed dinuclear complex [cis-[PtCl(2)(NH(3))]-L(1)-trans-[PtCl(NH(3))(2)]]OTf (19) was prepared by treatment of the Boc-protected amine ligand 1-[(Boc)(2)N(CH(2))(3)]-7-[H(2)N(CH(2))(3)]-1,7-C(2)B(10)H(10)(L(3), 15) with trans-[PtCl(dmf)(NH(3))(2)](+) to yield trans-[PtCl(NH(3))(2)L(3)]OTf (16), followed by acid deprotection of the pendant amine group, complexation with trans-[PtClI(2)(NH(3))](-), and halogen ligand metathesis using AgOTf and HCl((aq)). A novel trinuclear species containing 5 was prepared by the addition of two equivalents of 15 to the labile precursor cis-[Pt(dmf)(2)(NH(3))(2)](2+) followed by acid deprotection of the pendant amine groups. Further complexation with two equivalents of trans-[PtClI(2)(NH(3))](-) followed by halogen ligand metathesis using AgOTf and HCl((aq)) afforded the triplatinum(II)-amine species [cis-[Pt(NH(3))(2)(L(1))(2)]-cis-[PtCl(2)(NH(3))](2)](OTf)(2)(23). Complexes 7-10, 19 and 23 represent the first examples of multinuclear platinum(ii)-amine derivatives containing carborane cages. Preliminary in vitro cytotoxicity studies for selected complexes are also reported.