Electron transfer. 141. Reactions of indium(I) with transition metal center oxidants

Aqueous solutions of the hypovalent state In(I) reduce a series of complexes of Fe(III) and selected derivatives of Ir(IV), Cr(V), Pt(IV), Ag(III), and Ni(IV). All reactions yield In(III). Ions derived from Fe(III) and Ir(IV) undergo net le- conversions, whereas the other oxidants change by two units. Reductions of Fe(III) are strongly promoted by increasing pH or by adding Cl-, Br-, NCS-, or N3-. Reaction appears to proceed through monoligated complexes, FeIII(Lig-)2+, and the kinetic response to alteration of added ligand indicates initiation mainly via a bridged transition state FeIII-Lig-InI. Oxidations by Fe(CN)6(3-) are exceptionally rapid, those by FeIII(EDTA) are unusually slow, and redox is blocked by addition of excess F-. Reduction of IrCl6(2-) proceeds somewhat more slowly than predicted by the marcus model for outer-sphere reactions. Conversions of the 2e- oxidants are rapid. For these, multistep routes initiated by le- changes are reasonable, but direct 2e-paths involving oxygen transfer (from CrVO) or Cl+ transfer (from PtCl6(2-)) cannot be ruled out. Whether inner-sphere 2e- transactions without transfer of atomic species can be competitive remains an open question.     

Reductions by titanium(II) as catalyzed by titanium(IV)

The cobalt(III) complexes, [(NH3)5CoBr]2+ and [(NH3)5CoI]2+ are reduced by Ti(II) solutions containing Ti(IV), generating nearly linear (zero-order) profiles that become curved only during the last few percent of reaction. Other Co(III)-Ti(II) systems exhibit the usual exponential traces with rates proportional to [Co(III)]. Observed kinetics of the biphasic catalyzed Ti(II)-Co(III)Br and Ti(II)-Co(III)I reactions support the reaction sequence: [Ti(II)(H20)n]2+ + [Ti(IV)F5]- (k1)<==>(k -1) [Ti(II)(H2O)(n-1)]2+ + [(H2O)Ti(IV)F5]-, [Ti(II)(H2O)(n-1)]2+ + Co(III) (k2)--> Ti(III) + Co(II) with rates determined mainly by the slow Ti(IV)-Ti(II) ligand exchange (k1 = 9 x 10(-3) M(-1) s(-1) at 22 degrees C). Computer simulations of the catalyzed Ti(II)-Co(III) reaction in perchlorate-triflate media yield relative rates for reduction by the proposed active [Ti(II)(H2O)(n-1)]2+ intermediate; k(Br)/k(I) = 8.     

Carboxylate and alkyl carbonate coordination at the hydrophobic binding site of redox-active dicobalt amine thiophenolate complexes

A series of new dicobalt complexes of the permethylated macrocyclic hexaamine dithiophenolate ligand H(2)L(Me) have been prepared and investigated in the context of ligand binding and oxidation state changes. The octadentate ligand is an effective dinucleating ligand that supports the formation of bioctahedral complexes with a central N(3)Co(mu-SR)(2)(mu-X)CoN(3) core structure, leaving a free bridging position X for the coordination of the substrates. The acetato- and cinnamato-bridged complexes [(L(Me))Co(II)(2)(mu-O(2)CMe)](+) (2) and [(L(Me))Co(II)(2)(mu-O(2)CCH=CHPh)](+) (5) were prepared by reaction of the mu-Cl complex [(L(Me))Co(II)(2)(mu-Cl)](+) (1) with the corresponding sodium carboxylates in methanol. The electrochemical properties of these and of the methyl carbonate complex [(L(Me))Co(II)(2)(mu-O(2)COMe)](+) (8) were also investigated. All complexes undergo two stepwise oxidations at ca. E(1)(1/2) = +0.22 and at E(2)(1/2) = ca. +0.60 V vs SCE, affording the mixed-valent complexes [(L(Me))Co(II)Co(III)(mu-O(2)CR)](2+) (3, 6, 9) and the fully oxidized Co(III)Co(III) forms [(L(Me))Co(III)(2)(mu-O(2)CR)](3+) (4, 7, 10), respectively. Compounds 3, 6, 9 and 4, 7, 10 refer to acetato-, cinnamato-, and methylcarbonato species, respectively. The Co(II)Co(III) compounds were prepared by comproportionation of the respective Co(II)(2) and Co(III)(2) compounds. The Co(III)Co(III) species were prepared by bromine oxidation of the Co(II)Co(II) forms. The crystal structures of complexes 2.BPh(4).MeCN, 3.(I(3))(2), 5.BPh(4).2MeCN, 6.(ClO(4))(2).EtOH, 7.(ClO(4))(3).MeCN.(H(2)O)(3), and 9.(ClO(4))(2).(MeOH)(2).H(2)O were determined by single-crystal X-ray crystallography at 210 K. The oxidations occur without gross structural changes of the parent complexes. The Co(II)Co(III) complexes are composed of high-spin Co(II) (d(7)) and low-spin Co(III) (d(6)) ions. The Co(III)Co(III) complexes are diamagnetic. The oxidation reactions affect the binding mode of the substrates. In the Co(II)(2) and Co(II)Co(III) forms the carboxylates bridge the two Co(2+) ions in a symmetric mu-1,3 fashion with uniform C-O bond distances, whereas asymmetric bridging modes, with one short C=O and one long C-O distance, are adopted in the fully oxidized species. This is consistent with the observed shifts in vibrational frequencies for nu(as)(C-O) and nu(s)(C-O) across the series.     

Switching on oxygen activation by cobalt complexes of pentadentate ligands

The monoanionic N(4)O ligand N-methyl-N,N'-bis(2-pyridylmethyl)ethylenediamine-N'-acetate (mebpena(-)) undergoes oxidative C-N bond cleavage in the presence of Co(II) and O(2). The two resultant fragments are coordinated to the metal ion in the product [Co(III)(2-pyridylformate)(mepena)]ClO(4) (mepena(-) = N-methyl-N'-(2-pyridylmethyl)ethylenediamine-N'-acetato). Bond cleavage does not occur in the presence of chloride ions and [Co(III)(mebpena)Cl](+), containing intact mebpena(-), can be isolated. The oxidative instability of the mebpena(-) in the presence of Co(II) and air stands in contrast to the oxidative stability of the family of very closely related penta- and hexa-dentate ligands in their cobalt complexes. Cyclic voltammetry on the matched pair [Co(III)Cl(mebpena)](+) and [Co(II)Cl(bztpen)](+), bztpen = N-benzyl-N,N',N'-tris(2-pyridylmethyl)ethylenediamine, shows that substitution of a pyridine donor for a carboxylato donor results in a relatively small cathodic shift of 150 mV in the E°(Co(II)/Co(III)) oxidation potential, presumably this is enough to determine the contrasting metal oxidation state in the complexes isolated under ambient conditions. DFT calculations support a proposal that [Co(II)(mebpena)](+) reacts with O(2) to form a Co(III)-superoxide complex which can abstract an H atom from a ligand methylene C atom as the initial step towards the observed oxidative C-N bond cleavage.     

Electron transfer. 123. Competitive redox reactions involving a Cobalt(I) intermediate

he hypophosphito derivative of Co^III, (NH₃)₅CoO₂PH₂ ²⁺, decomposes in basic media, yielding Co(II) quantitatively, along with a 1:1 mixture of hypophosphite and phosphite. Earlier studies point to a cobalt (I) intermediate, which rapidly reduces a second molecule of Co(III) reactant to Co(II). In the presence of an external cobalt (III) oxidant (Co³X), the latter competes with the hypophosphito complex for Co(I), lowering the yield of free hypophosphite. From the ratio of phosphorus products (P³/P¹), evaluated by proton decoupled ³¹P NMR, the relative reactivities (k_t/k_c) of the external Co(III) “traps” and the hypophosphito complex may be determined. A log-log plot comparing values of k_t/k_c with rates for reductions of the same series of Co(III) oxidants with Ru(NH₃)₆ ²⁺ (k_Ru values) is badly scattered with a slope of only 0.23, well below the value of unity stipulated by the Marcus model, indicating that an outer-sphere mechanism cannot operate for all of the Co(III)-Co(I) reactions and may not operate for any, except for that with Co(NH₃)₅(py)₃₊. Among the carboxylato-substituted oxidants, there are no rate enhancements by neighbouring pyridine, -SR,-OH, -CHO, or -SO₃H functions, in contrast to the accelerations previously observed for reductions by Cr(II), Eu(II) and Ti(III), which are attributed to intermediacy of chelate-stabilized precursor complexes. It is suggested that the Co(III)-Co(I) reactions, which determine the ratio of phosphorus products, are much more rapid than the ligand substitution at the Co(I) center which must precede chelate formation, and that the latter therefore does not intervene significantly in the redox processes.     

Reductions by aquatitanium(II)

Solutions of titanium(II), prepared by dissolving titanium wire in mixtures of hydrofluoric and triflic acids, reduce quinones, nitrosodisulfonate anion, and complexes of cobalt(III). When the oxidant is taken in excess, these reactions yield Ti(IV), whereas with excess reductant, the principal product is Ti(III). These reactions are compared with those by Ti(III). Despite differences in rate laws, it is clear that rate ratios for the two reductants (kTiII/kTiIII) fall well below 10(4), the minimum selectivity corresponding to estimated differences in formal potentials, and in some instances, Ti(II), the stronger reductant, reacts more slowly. For both Ti(III) and Ti(II), reductions within the series [Co(NH3)5X]2+(where X=F, Cl, Br, and I), the fluoro complex reacts much more rapidly than its congeners, and the bromo and iodo complexes are slowest, an order similar to that for Eu2+ reductions, but opposite to that for Cr(II) and Cu(I). The [Co(NH3)5Br]2+ reaction with excess Ti(II) proceeds at rates very nearly independent of [oxidant] during the first 80-90% reaction, implying that initiation occurs via unimolecular conversion of Ti(II) to an activated cationic reducing species, in the same manner as the earlier described reduction of I3- by Ge(II) in aqueous HCl.     

Electron transfer. 157. Reactions of hypervalent manganese species with S(2) metal-ion reductants

Solutions of the complexes of hypervalent manganese, [Mn(III)(C(2)O(4))(3)](3)(-) (in oxalate buffers), [Mn(IV)(bigH)(3)](4+) (in biguanide buffers), and [(bipy)(2)Mn(III)(O)(2)Mn(IV)(bipy)(2)](3+) (in bipyridyl buffers) may be reduced by s(2) center reductants In(I), Sn(II), and Ge(II), yielding Mn(II) quantitatively. In all cases, rates are determined by the initial act of electron transfer, giving an s(1) transient (In(II), Sn(III), or Ge(III)); subsequent steps are rapid and kinetically silent. The In(I)-Mn(III) and Ge(II)-Mn(III) reactions are inhibited by added oxalate, whereas the Sn(II)-(Mn(III)Mn(IV)) reaction is strongly accelerated by Cl(-). The In(I)-Mn(IV) reaction is complicated by formation of a 1:1 addition compound In(I).Mn(IV). We find no evidence for two-unit steps in any of these systems.     

Separation of Intrinsic and Electrostrictive Volume Effects in Redox Reaction Volumes of Metal Complexes Measured Using High-Pressure Cyclic Staircase Voltammetry

Redox reaction volumes, obtained by high-pressure cyclic voltammetry, are reported for a selection tris(diimine), tris(diamine), hexaammine, and hexaaqua couples of Fe(III/II), Cr(III/II), Ru(III/II), and Co(III/II). Separation of the intrinsic and electrostrictive volume contributions for these couples has been achieved, some in both aqueous and acetonitrile solutions. For the Co(phen)(3)(3+/2+) system, the intrinsic volume change is estimated to be +15.3 +/- 2.1 cm(3) mol(-)(1) (based on measurements in water) and +16.5 +/- 2.0 cm(3) mol(-)(1) (in acetonitrile). For the Co(bipy)(3)(3+/2+) system, values are +12.7 +/- 1.4 cm(3) mol(-)(1) (in water) and +15.5 +/- 2.5 cm(3) mol(-)(1) (in acetonitrile). Using these experimentally determined intrinsic contributions, a simple structural model suggests that the intrinsic volume change for these reactions can be described using the change in effective volume of a sphere with radius close to that of the coordinating-atom-metal bond length. Electrostrictive volume changes for the 3+/2+ complex-ion couples are a function of solute size and coordinated ligands. For Ru(H(2)O)(6)(3+) and Fe(H(2)O)(6)(3+) reduction, volume behavior is significantly different from that of the other systems studied and can be rationalized in terms of possible H-bonding interactions with surrounding solvent which affect the electrostrictive volume changes but which are not available for the ammine and other complexes studied.     

Tetra-2,3-pyrazinoporphyrazines with externally appended pyridine rings. 4. UV-visible spectral and electrochemical evidence of the remarkable electron-deficient properties of the new tetrakis-2,3-[5,6-di{2-(N-methyl)pyridiniumyl}pyrazino]porphyrazinatometal octacations, [(2-Mepy)8TPyzPzM]8+ (M = MgII(H2O), CoII, CuII, ZnII)

Metal derivatives of the octacationic tetrakis-2,3-[5,6-di{2-(N-methyl)pyridiniumyl}pyrazino]porphyrazine macrocycle [(2-Mepy)(8)TPyzPzH(2)](8+) (2-Mepy = 2-(N-methyl)pyridiniumyl ring) isolated as water-soluble hydrated iodide salts of the general formula [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O, (M = Mg(II)(H(2)O), Co(II), Cu(II), Zn(II); x = 2-5) were prepared from the corresponding neutral complexes [Py(8)TPyzPzM].xH(2)O previously reported. Reaction of these complexes with CH(3)I in N,N-dimethylformamide under mild conditions led to full quaternization of all eight pyridine N atoms and formation of the octacations [(2-Mepy)(8)TPyzPzM](8+). Clathrated water molecules could be eliminated from the species [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O by mild heating ( Co(I) process, but the site of electron transfer is reversed and the final product upon a further one-electron reduction is formulated as a Co(II) dianion as opposed to a Co(I) pi-anion radical. This sequence is similar to what was earlier reported for reduction of the same compound in pyridine. Reversible one-electron oxidations are also observed for the unmethylated species [Py(8)TPyzPzM].xH(2)O where M = Co(II) and Mn(II) in DMSO. Remarkably, the octacationic macrocycles [(2-Mepy)(8)TPyzPzM](I(8)).xH(2)O, (M = Mg(II)(H(2)O), Co(II), Cu(II), and Zn(II); x = 2-5) are more easily reduced at any step of the reduction than the corresponding unquaternized species with the same metal ion. This indicates a higher tendency to stepwise electron uptake after the quaternization process, which enhances the charge redistribution capability within the species formed by the electroreduction.     

Investigating the solid state hosting abilities of homo- and hetero-valent [Co7] metallocalix[6]arenes

A family of homo-valent [Co(II)(7)(OH)(6)(L(1))(6)](NO(3))(2) (1), [(MeOH)(2) is a subset of Co(II)(7)(OH)(6)(L(1))(6)](NO(3))(2) (2) (where L(1)H = 2-iminomethyl-6-methoxyphenol) and hetero-valent [(NO(3))(2) is a subset of Co(III)Co(II)(6)(OH)(6)(L(2))(6)](NO(3))·3MeCN (4) (where L(2)H = 2-iminophenyl-6-methoxyphenol) complexes possess metallic skeletons describing planar hexagonal discs. Their organic exteriors form double-bowl shaped topologies, and coupled with their 3-D connectivity, this results in the formation of molecular cavities in the solid state. These confined spaces are shown to behave as host units in the solid state for guests including solvent molecules and charge balancing counter anions. Magnetic susceptibility measurements on 2 and 4 reveal weak ferro- and ferrimagnetism, respectively. The utilisation of other Co(II) salt precursors gives rise to entirely different species including the mononuclear and trinuclear complexes [Co(II)(L(2))(2)] (5) and [Co(III)(2)Na(I)(1)(L(3))(6)](BF(4)) (6) (where L(3)H = 2-iminomethyl-4-bromo-6-methoxyphenol).     

Cobalt(II), nickel(II) and copper(II) complexes of a hexadentate pyridine amide ligand. Effect of donor atom (ether vs. thioether) on coordination geometry, spin-state of cobalt and M(III)-M(II) redox potential

Using an acyclic hexadentate pyridine amide ligand, containing a -OCH(2)CH(2)O- spacer between two pyridine-2-carboxamide units (1,4-bis[o-(pyrydine-2-carboxamidophenyl)]-1,4-dioxabutane (H(2)L(9)), in its deprotonated form), four new complexes, [Co(II)(L(9))] (1) and its one-electron oxidized counterpart [Co(III)(L(9))][NO(3)]·2H(2)O (2), [Ni(II)(L(9))] (3) and [Cu(II)(L(9))] (4), have been synthesized. Structural analyses revealed that the Co(II) centre in 1 and the Ni(II) centre in 3 are six-coordinate, utilizing all the available donor sites and the Cu(II) centre in 4 is effectively five-coordinated (one of the ether O atoms does not participate in coordination). The structural parameters associated with the change in the metal coordination environment have been compared with corresponding complexes of thioether-containing hexadentate ligands. The μ(eff) values at 298 K of 1-4 correspond to S = 3/2, S = 0, S = 1 and S = 1/2, respectively. Absorption spectra for all the complexes have been investigated. EPR spectral properties of the copper(II) complex 4 have been investigated, simulated and analyzed. Cyclic voltammetric experiments in CH(2)Cl(2) reveal quasireversible Co(III)-Co(II), Ni(III)-Ni(II) and Cu(II)-Cu(I) redox processes. In going from ether O to thioether S coordination, the effect of the metal coordination environment on the redox potential values of Co(III)-Co(II) (here the effect of spin-state as well), Ni(III)-Ni(II) and Cu(II)-Cu(I) processes have been systematically analyzed.     

Electron transfer. 147. Reductions with gallium(I)

Solutions 0.03-0.05 M in gallium(I) can be generated by treatment of the "mixed" halide Ga(I)Ga(III)Cl(4) with cold water under argon and then removing the precipitated metallic gallium and Ga(OH)(3) by centrifugation. Ga(I) is lost from such preparations with a half-life of about 3 h at 0 degrees C. These solutions, which may be handled by conventional techniques, readily reduce I(3)(-), IrCl(6)(2)(-), Fe(bipy)(3)(3+), Fe(NCS)(2+), aquacob(III)alamin, and a group of ring-substituted derivatives of Ru(NH(3))(5)(py)(3+) but are inert to (NH(3))(5)CoCl(2+) and (NH(3))(5)CoBr(2+). All reactions give Ga(III). Reduction of HCrO(4)(-) in 2-ethyl-2-hydroxybutanoate buffers (pH 3.6) yields a Cr(IV) chelate of the buffering anion but forms Cr(III) when carried out in 0.01 M H(+). Reactions of le(-) oxidants proceed via successive single changes with the conversion Ga(II) --> Ga(III) much more rapid than Ga(I) --> Ga(II). Only for the reactions of I(3)(-) and Fe(NCS)(2+) is there evidence for redox bridging.     

Polynuclear complexes of the pendent-arm ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane

The ligand 1,4,7-tris(acetophenoneoxime)-1,4,7-triazacyclononane (H(3)L) has been synthesized and its coordination properties toward Cu(II), Ni(II), Co(II), and Mn(II) in the presence of air have been investigated. Copper(II) yields a mononuclear complex, [Cu(H(2)L)](ClO(4)) (1), cobalt(II) and manganese(II) ions yield mixed-valence Co(III)(2)Co(II) (2a) and Mn(II)(2)Mn(III) (4) complexes, whereas nickel(II) produces a tetranuclear [Ni(4)(HL)(3)](2+) (3) complex. The complexes have been structurally, magnetochemically, and spectroscopically characterized. Complex 3, a planar trigonal-shaped tetranuclear Ni(II) species, exhibits irregular spin-ladder. Variable-temperature (2-290 K) magnetic susceptibility analysis of 3 demonstrates antiferromagnetic exchange interactions (J = -13.4 cm(-1)) between the neighboring Ni(II) ions, which lead to the ground-state S(t) = 2.0 owing to the topology of the spin-carriers in 3. A bulk ferromaganetic interaction (J = +2 cm(-1)) is prevailing between the neighboring high-spin Mn(II) and high-spin Mn(III) ions leading to a ground state of S(t) = 7.0 for 4. The large ground-state spin value of S(t) = 7.0 has been confirmed by magnetization measurements at applied magnetic fields of 1, 4 and 7 T. A bridging monomethyl carbonato ligand formation occurs through an efficient CO(2) uptake from air in methanolic solutions containing a base in the case of complex 4.     

Tunability of the M(II)M(III)/M(II)(2) and M(III)(2)/M(II)M(III) (M=Mn, Co) couples in bis-μ-O,O'-carboxylato-μ-OR bridged complexes

A comparison of the electrochemical properties of a series of dinuclear complexes [M(2)(L)(RCO(2))(2)](+) with M = Mn or Co, L = 2,6-bis(N,N-bis-(2-pyridylmethyl)-sulfonamido)-4-methylphenolato (bpsmp(-)) or 2,6-bis(N,N-bis(2-pyridylmethyl)aminomethyl)-4-tert-butylphenolato (bpbp(-)) and R = H, CH(3), CF(3) or 3,4-dimethoxybenzoate demonstrates: (i) The electron-withdrawing sulfonyl groups in the backbone of bpsmp(-) stabilize the [M(2)(bpsmp)(RCO(2))(2)](+) complexes in their M(II)(2) oxidation state compared to their [M(2)(bpbp)(RCO(2))(2)](+) analogues. Manganese complexes are stabilised by approximately 550 mV and cobalt complexes by 650 mV. (ii) The auxiliary bridging carboxylato ligands further attenuate the metal-based redox chemistry. Substitution of two acetato for two trifluoroacetato ligands shifts redox couples by 300-400 mV. Within the working potential window, reversible or quasi-reversible M(II)M(III)? M(II)(2) processes range from 0.31 to 1.41 V for the [Co(2)(L)(RCO(2))(2)](+/2+) complexes and from 0.54 to 1.41 V for the [Mn(2)(L)(RCO(2))(2)](+/2+) complexes versus Ag/AgCl for E(M(II)M(III)/M(II)(2)). The extreme limits are defined by the complexes [M(2)(bpbp)(CH(3)CO(2))(2)](+) and [M(2)(bpsmp)(CF(3)CO(2))(2)](+) for both metal ions. Thus, tuning the ligand field in these dinuclear complexes makes possible a range of around 0.9 V and 1.49 V for the one-electron E(M(II)M(III)/M(II)(2)) couple of the Mn and Co complexes, respectively. The second one-electron process, M(II)M(III)? M(III)(2) was also observed in some cases. The lowest potential recorded for the E°(M(III)(2)/M(II)M(III)) couple was 0.63 V for [Co(2)(bpbp)(CH(3)CO(2))(2)](2+) and the highest measurable potential was 2.23 V versus Ag/AgCl for [Co(2)(bpsmp)(CF(3)CO(2))(2)](2+).     

Ion atmosphere relaxation and percolative electron transfer in Co bipyridine DNA molten salts

Polypyridyl complexes of Co decorated with 350-Da polyether chains (Co(350)(2+)) form molten phases of nucleic acids when paired with DNA counterions (Co(350)DNA) or 25-mer oligonucleotides. Analysis of voltammetry and chronoamperometry of mixtures of these phases with complexes having ClO(4)(-) counterions (Co(350)(ClO(4))(2)) and no other diluent provides charge transport rates from the oxidation and reduction currents for the complexes. As the mole fraction of the Co(350)(ClO(4))(2) complex in the mixture is varied from ca. 0.25 to 1, the physical diffusion constants derived from the Co(III/II) wave increase from 1 x 10(-11) cm(2)/s to 5 x 10(-10) cm(2)/s, and apparent diffusion constants dominated by the Co(II/I) electron self-exchange increase from 1 x 10(-10) cm(2)/s to 2 x 10(-8) cm(2)/s. Pure Co(350)DNA melts, containing no Co(350)(ClO(4))(2) complex, do not exhibit recognizable voltammetric waves; DNA suppresses the Co(II/I) electron transfer reactions of Co complexes for which it is the counterion. There are therefore two microscopically distinct kinds of Co(350) complexes, those with DNA and those with ClO(4)(-) counterions, with respect to their Co(II/I) electron-transfer dynamics, leading to percolative behavior in their mixtures. The electron-transfer rates of the Co(II/I) couple are controlled by the diffusive relaxation of the ionic atmosphere around the reaction pair, and the inactivity of the bound Co complexes can be attributed to the very low mobility of the anionic phosphate groups in the DNA counterion. Substitution of sulfonated polystyrene for DNA produced similar results, suggesting that this phenomenon is general to other polymer counterions of low mobility. We conclude that the measured Co(II/I) charge transport and electron-transfer rate constants reflect more the diffusive mobility of the perchlorate counterion than the intrinsic Co(II/I) electron hopping rate.     

Observation of an Organic Acid Mediated Spin State Transition in a Co(II)-Schiff Base Complex: An EPR, HYSCORE, and DFT Study

The interactions of a weak organic acid (acetic acid, HOAc) with a toluene solution of the Co(II)-Schiff base type complex, (R,R')-N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-diamino Co(II) (labeled [Co(1)]), was investigated using EPR, HYSCORE, and DFT computations. This activated [Co(II)(1)] system is extremely important within the context of asymmetric catalysts (notably the hydrolytic kinetic resolution of epoxides) despite the lack of detailed structural information about the nature of the paramagnetic species present. Under anaerobic conditions, the LS [Co(II)(1)] complex with a |yz, (2)A(2)? ground state is converted into a low-spin (LS) and a high-spin (HS) complex in the presence of the acid. The newly formed LS state is assigned to the coordinated [Co(II)(1)]-(HOAc) complex, possessing a |z(2), (2)A(1)? ground state (species A; g(x) = 2.42, g(y) = 2.28, g(z) = 2.02, A(x) = 100, A(y) = 120, A(z) = 310 MHz). The newly formed HS state is assigned to an acetate coordinated [Co(II)(1)]-(OAc(-)) complex, possessing an S = (3)/(2) spin ground state (species B, responsible for a broad EPR signal with g ≈ 4.6). These spin ground states were confirmed with DFT calculations using the hybrid BP86 and B3LYP functionals. Under aerobic conditions, the LS and HS complexes (species A and B) are not observed; instead, a new HS complex (species C) is formed. This complex is tentatively assigned to a paramagnetic superoxo bridged dimer (AcO(-))[Co(II)(1)···O(2)(-)Co(III)(1)](HOAc), as distinct from the more common diamagnetic peroxo bridged dimers. Species C is characterized by a very broad HS EPR signal (g(x) = 5.1, g(y) = 3.9, g(z) = 2.1) and is reversibly formed by oxygenation of the LS [Co(II)(1)]-(HOAc) complex to the superoxo complex [Co(III)(1)O(2)(-)](HOAc), which subsequently forms the association complex C by interaction with the HS [Co(II)(1)](OAc(-)) species. The LS and HS complexes were also identified using other organic acids (benzoic and propanoic acid). Thermal annealing-quenching experiments revealed the additional presence of [Co(III)(1)O(2)(-)](HOAc) adducts, corroborating the presence of species C and the presence of diamagnetic dimer complexes in the solution, such as the EPR silent (HOAc)[Co(III)(1)(O(2)(2-))Co(III)(1)](HOAc). Overall, it appears that a facile interconversion of the [Co(1)] complex, possessing a LS ground state, occurs in the presence of acetic acid, producing both HS and LS Co(II) states, prior to formation of the oxidized active form of the catalyst, [Co(III)(1)](OAc(-)).     

Synthesis and characterization of cobalt(II) complexes with tripodal polypyridine ligand bearing pivalamide groups. Selective formation of six- and seven-coordinate cobalt(II) complexes

The reactions of CoX(2) (X = Cl(-), Br(-), I(-) and ClO(4)(-)) with the tripodal polypyridine N(4)O(2)-type ligand bearing pivalamide groups, bis(6-(pivalamide-2-pyridyl)methyl)(2-pyridylmethyl)amine ligand (H(2)BPPA), afforded two types of Co(II) complexes as follows. One type is purple-coloured Co(II) complexes, [CoCl(2)(H(2)BPPA)] (1(Cl)) and [CoBr(2)(H(2)BPPA)] (1(Br)) which were prepared when X = Cl(-) and Br(-), respectively. The other type is pale pink-coloured Co(II) complexes, [Co(MeOH)(H(2)BPPA)](ClO(4)(-))(2) (2·(ClO(4)(-))(2)) and [Co(MeCN)(H(2)BPPA)](I(-))(2) (2·(I(-))(2)), which were obtained when X = I(-) and ClO(4)(-), respectively. From the reaction of 1(Cl) and NaN(3), a purple-coloured complex, [Co(N(3))(2)(H(2)BPPA)] (1(azide)), was obtained. These Co(II) complexes were characterized by X-ray structural analysis, IR and reflectance spectroscopies, and magnetic susceptibility measurements. All these Co(II) complexes were shown to be in a d(7) high-spin state based on magnetic susceptibility measurements. The former Co(II) complexes revealed a six-coordinate octahedron with one amine nitrogen, three pyridyl nitrogens, and two counter anions, and one coordinated anion, Cl(-), Br(-) and N(3)(-), forming intramolecular hydrogen bonds with two pivalamide N-H groups. On the other hand, the latter Co(II) complexes showed a seven-coordinate face-capped octahedron with one amine nitrogen, three pyridyl nitrogens, two pivalamide carbonyl oxygens and MeCN or MeOH. In these structures, intramolecular hydrogen bonding interaction was not observed, and the metal ion was coordinated by the pivalamide carbonyl oxygens and solvent molecule instead of the counter anions. The difference in coordination geometries might be attributable to the coordination ability and ionic radii of the counteranions; smaller strongly binding anions such as Cl(-), Br(-) and N(3)(-) gave the former complexes, whereas bulky weakly binding anions such as I(-) and ClO(4)(-) afforded the latter ones. In order to demonstrate this hypothesis, the small stronger coordinating ligand, azide, was added to complexes 2·(ClO(4)(-))(2) to obtain the dinuclear cobalt(II) complex in which two six-coordinate octahedral cobalt(II) species were bridged with azide, 3·(ClO(4)(-)). Also, the abstraction reaction of halogen anions from complexes 1(Cl) by AgSbF(6) gave a pale pink Co(II) complex assignable to 2·(SbF(6)(-))(2).     

Synthesis, crystal structures, and magnetic properties of a new family of heterometallic cyanide-bridged Fe(III)2M(II)2 (M=Mn, Ni, and Co) square complexes

New heterobimetallic tetranuclear complexes of formula [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Mn(II)(bpy)(2)](2)(ClO(4))(2)·CH(3)CN (1), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2a), [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Ni(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (2b), [Fe(III){HB(pz)(3)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3a), and [Fe(III){B(pz)(4)}(CN)(2)(μ-CN)Co(II)(dmphen)(2)](2)(ClO(4))(2)·2CH(3)OH (3b), [HB(pz)(3)(-) = hydrotris(1-pyrazolyl)borate, B(Pz)(4)(-) = tetrakis(1-pyrazolyl)borate, dmphen = 2,9-dimethyl-1,10-phenanthroline, bpy = 2,2'-bipyridine] have been synthesized and structurally and magnetically characterized. Complexes 1-3b have been prepared by following a rational route based on the self-assembly of the tricyanometalate precursor fac-[Fe(III)(L)(CN)(3)](-) (L = tridentate anionic ligand) and cationic preformed complexes [M(II)(L')(2)(H(2)O)(2)](2+) (L' = bidentate α-diimine type ligand), this last species having four blocked coordination sites and two labile ones located in cis positions. The structures of 1-3b consist of cationic tetranuclear Fe(III)(2)M(II)(2) square complexes [M = Mn (1), Ni (2a and 2b), Co (3a and 3b)] where corners are defined by the metal ions and the edges by the Fe-CN-M units. The charge is balanced by free perchlorate anions. The [Fe(L)(CN)(3)](-) complex in 1-3b acts as a ligand through two cyanide groups toward two divalent metal complexes. The magnetic properties of 1-3b have been investigated in the temperature range 2-300 K. A moderately strong antiferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Mn(II) (S = 5/2) ions has been found for 1 leading to an S = 4 ground state (J(1) = -6.2 and J(2) = -2.7 cm(-1)), whereas a moderately strong ferromagnetic interaction between the low-spin Fe(III) (S = 1/2) and high-spin Ni(II) (S = 1) and Co(II) (S = 3/2) ions has been found for complexes 2a-3b with S = 3 (2a and 2b) and S = 4 (3a and 3b) ground spin states [J(1) = +21.4 cm(-1) and J(2) = +19.4 cm(-1) (2a); J(1) = +17.0 cm(-1) and J(2) = +12.5 cm(-1) (2b); J(1) = +5.4 cm(-1) and J(2) = +11.1 cm(-1) (3a); J(1) = +8.1 cm(-1) and J(2) = +11.0 cm(-1) (3b)] [the exchange Hamiltonian being of the type H? = -J(S?(i)·S?(j))]. Density functional theory (DFT) calculations have been used to substantiate the nature and magnitude of the exchange magnetic coupling observed in 1-3b and also to analyze the dependence of the exchange magnetic coupling on the structural parameters of the Fe-C-N-M skeleton.     

Time-resolved spectroscopy of the metal-to-metal charge transfer excited state in dinuclear cyano-bridged mixed-valence complexes

Visible pump-probe spectroscopy has been used to identify and characterize short-lived metal-to-metal charge transfer (MMCT) excited states in a group of cyano-bridged mixed-valence complexes of the formula [LCo(III)NCM(II)(CN)(5)](-), where L is a pentadentate macrocyclic pentaamine (L(14)) or triamine-dithiaether (L(14S)) and M is Fe or Ru. Nanosecond pump-probe spectroscopy on frozen solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at 11 K enabled the construction of difference transient absorption spectra that featured a rise in absorbance in the region of 350-400 nm consistent with the generation of the ferricyanide chromophore of the photoexcited complex. The MMCT excited state of the Ru analogue [L(14)Co(III)NCRu(II)(CN)(5)](-) was too short-lived to allow its detection. Femtosecond pump-probe spectroscopy on aqueous solutions of [L(14)Co(III)NCFe(II)(CN)(5)](-) and [L(14S)Co(III)NCFe(II)(CN)(5)](-) at room temperature enabled the lifetimes of their Co(II)-Fe(III) MMCT excited states to be determined as 0.8 and 1.3 ps, respectively.     

The Interaction of C60, C70, and C60(CN)2 radical anions with cobalt(II) tetraphenylporphyrin in solid multicomponent complexes

A method for the synthesis of the multicomponent ionic complexes: [Cr(I)(C(6)H(6))(2) (.+)][Co(II)(tpp)(fullerene)(-)].C(6)H(4)Cl(2), comprising bis(benzene)chromium (Cr(C(6)H(6))(2)), cobalt(II) tetraphenylporphyrin (Co(II)(tpp)), fullerenes (C(60), C(60)(CN)(2), and C(70)), and o-dichlorobenzene (C(6)H(4)Cl(2)) has been developed. The monoanionic state of the fullerenes has been proved by optical absorption spectra in the UV/vis/NIR and IR ranges. The crystal structures of the ionic [[Cr(I)(C(6)H(6))(2)](.+)](1.7)[[Co(II)(tpp)(C(60))](2)](1.7-). 3.3 C(6)H(4)Cl(2) and [[Cr(I)(C(6)H(6))(2)] (.+)](2)[Co(II)(tpp)[C(60)(CN)(2)]](-)[C(60)(CN)(2) (.-)]).3 C(6)H(4)Cl(2) are presented. The essentially shortened Co.C(fullerene) bond lengths of 2.28-2.32 A in these complexes indicate the formation of sigma-bonded [Co(II)(tpp)][fullerene](-) anions, which are diamagnetic. All the ionic complexes are semiconductors with room temperature conductivity of 2 x 10(-3)-4 x 10(-6) S cm(-1), and their magnetic susceptibilities show Curie-Weiss behavior. The neutral complexes of Co(II)(tpp) with C(60), C(60)(CN)(2), C(70), and Cr(0)(C(6)H(6))(2), as well as the crystal structures of [Co(II)(tpp)](C(60)).2.5 C(6)H(4)Cl(2), [Co(II)(tpp)](C(70)). 1.3 CHCl(3).0.2 C(6)H(6), and [Cr(0)(C(6)H(6))(2)][Co(II)(tpp)] are discussed. In contrast to the ionic complexes, the neutral ones have essentially longer Co.C(fullerene) bond lengths of 2.69-2.75 A.