Correlations between Spectroscopic, Electrochemical, and Kinetic Properties of Cyano-Bridged Binuclear Complexes. Analyses of Temperature, Pressure, and Solvent Effects

Homogeneous solvation controlled photoreduction of cobalt(III) complexes in aqueous 2-methyl-2-propanol solutions linear solvation energy relationship and cyclic voltammetric analyses

The effect of solvent participation on the ligand-to-metal charge transfer (LMCT, L-->Co(III)) reduction of the of Co(III)(en)(2)Br(RC(6)H(4)NH(2))(2+) where R=m-OCH(3), p-F, H, m-CH(3), p-CH(3,)p-OC(2)H(5) and p-OCH(3) were examined in aqueous 2-methyl-2-propanol (Bu(t)OH) solutions. The change in the reduction behavior of Co(III) centre was also examined through cyclic voltammetric studies. The observed reduction in quantum yield due to LMCT excitation can mainly be accounted using linear solvation energy relationship (LSER) comprising model correlation equations. These consist of empirical parameters such as Grunwald-Winstein's solvent ionizing power, Y, Dimroth-Richardt's solvent micro-polarity parameter, E(T)(N), Gutmann's donor number, DN(N), along with Kamlet-Taft's solvatochromic parameters (hydrogen bond acceptor acidity/basicity alpha/beta and solvent dipolarity/polarizability, pi*). The origin of solvent effect is found to be due to microscopic interaction between the solvent donor and the nitrogen-bound hydrogen of the ligand. Cyclic voltammograms show an irreversible reduction of Co(III) in DMF using Glassy Carbon Electrode, GCE, the redox peaks for the aniline complexes appear at -0.20 and 0.525V. Irradiation of the complexes with UV light (lambda=254nm) in binary mixtures produce Co(II)(aq) and the concentration of this species are highly dependent on x(alc) (x(alc)=mole fraction of alcohol). The observed quantum yield (logPhi(Co(II))) is found to be linearly related to mole fraction of organic co-solvent added in the mixture, therefore, logPhi(Co(II))=26.41 x 10(-2) when x(2)=0.0094 and 43.75 x 10(-2) when x(2)=0.076 for a typical complex Co(III)(en)(2)Br(p-OCH(3)C(6)H(4)NH(2))(2+) in aqueous 2-methyl-2-propanol at 300K. Cyclic voltammetry and LSER analyses illustrate the variation of reduction property of Co(III) by the aryl ligand and homogeneous solvation of the excited state of the complex Co(III)(en)(2)Br(RC(6)H(4)NH(2))(2+) in H(2)O/Bu(t)OH mixtures.

Pressure and temperature effects on metal-to-metal charge transfer in cyano-bridged CoIII–FeIIcomplexes

The effects of pressure and temperature on the energy (Eop) of the metal-to-metal charge transfer (MMCT, FeII-->CoIII) transition of the cyano-bridged complexes trans-[L14CoNCFe(CN)5]- and cis-[L14CoNCFe(CN)5]- (where L14=6-methyl-1,4,8,11-tetraazacyclotetradecan-6-amine) were examined. The changes in the redox potentials of the cobalt and iron metal centres with pressure and temperature were also examined and the results interpreted with Marcus-Hush theory. The observed redox reaction volumes can mainly be accounted for in terms of localised electrostriction effects. The shifts in Eop due to both pressure and temperature were found to be less than the shifts in the energy difference (DeltaE degrees]) between the CoIII-FeII and CoII-FeIII redox isomers. The pressure and temperature dependence of the reorganisational energy, as well as contributions arising from the different spin states of CoII, are discussed in order to account for this trend. To study the effect of pressure on CoIII electronic absorption bands, a new cyano-bridged complex, trans-[L14CoNCCo(CN)5], was prepared and characterised spectroscopically and structurally. X-Ray crystallography revealed this complex to be isostructural with trans-[L14CoNCFe(CN)5].5H2O.

Oxidation of Mixed-Valence CoIII/FeII Complexes Reversed at High pH: A Kinetico-Mechanistic Study of Water Oxidation

The outer-sphere oxidation of Fe(II) in the mixed-valence complex trans-[L(14S)Co(III)NCFe(II)(CN)(6)](-), being L(14S) an N(3)S(2) macrocylic donor set on the cobalt(III) center, has been studied. The comparison with the known processes of N(5) macrocycle complexes has been carried out in view of the important differences occurring on the redox potential of the cobalt center. The results indicate that the outer-sphere oxidation reactions with S(2)O(8)(2-) and [Co(ox)(3)](3-) involve a great amount of solvent-assisted hydrogen bonding that, as a consequence from the change from two amines to sulfur donors, are more restricted. This is shown by the more positive values found for DeltaS(#) and DeltaV(#). The X-ray structure of the oxidized complex has been determined, and it is clearly indicative of the above-mentioned solvent-assisted hydrogen bonding between nitrogen and cyanide donors on the cobalt and iron centers, respectively. trans-[L(14S)Co(III)NCFe(III)(CN)(6)], as well as the analogous N(5) systems trans-[L(14)Co(III)NCFe(III)(CN)(6)], trans-[L(15)Co(III)NCFe(III)(CN)(6)], and cis-[L(13)Co(III)NCFe(III)(CN)(6)], oxidize water to hydrogen peroxide at pH > 10 with a rather simple stoichiometry, i.e., [L(n)()Co(III)NCFe(III)(CN)(5)] + OH(-) --> [L(n)()Co(III)NCFe(II)(CN)(5)](-) + (1)/(2)H(2)O(2). In this way, the reversibility of the iron oxidation process is achieved. The determination of kinetic and thermal and pressure activation parameters for this water to hydrogen peroxide oxidation leads to the kinetic determination of a cyanide based OH(-) adduct of the complex. A second-order dependence on the base concentration is associated with deprotonation of this adduct to produce the final inner-sphere reduction process. The activation enthalpies are found to be extremely low (15 to 35 kJ mol(-1)) and responsible for the very fast reaction observed. The values of DeltaS(#) and DeltaV(#) (-76 to -113 J K(-1) mol(-1) and -5.5 to -8.9 cm(3) mol(-1), respectively) indicate a highly organized but not very compressed transition state in agreement with the inner-sphere one-electron transfer from O(2-) to Fe(III).