Claudio Tavagnacco

Electrochemical reduction of dioxygen in the presence of 4,6-dimethyl-2-thiopyrimidine in DMF

Abstract The electrochemistry of dioxygen reduction in the presence of a model molecule of biological significance, 4,6-dimethyl-2-thiopyrimidine (LH) was studied in dimethylformamide at a glassy carbon (GC) electrode. The first, mono-electronic, reduction step took place under kinetic control of charge transfer or of the following chemical reactions, or both, depending on the time scale of the experiment, leading to a complicated pattern of events. A father–son reaction step and other cross protonation reactions could explain the detailed results obtained from the electrochemical investigation. The involvement of LH in the dioxygen reduction path was indicated by the presence of a new cathodic peak in cyclic voltammetry, which did not pertain to the voltammetric behavior of either species, and was shifted by ca. 70 mV from that due to the O 2 /O 2 − reduction in the absence of LH. The experimental results could be consistently interpreted by assigning the new peak to the O 2 /O 2 − coupled with a fast protonation of O 2 − by LH, and explaining the amount of the displacement by the rate of the protonation reaction.

Synthesis and characterization of a novel tetranuclear bimetallic complex containing rhodium(II) and zinc(II) as metal centres

Rh(Hdmg)2(PPh3)Cl (Hdmg=monoanion of dimethylglyoxime) undergoes reduction with zinc amalgam to give the tetranuclear bimetallic compound [Rh(Hdmg)(ClZndmg)(PPh3)]2, characterized by elemental analysis, electronic, IR and NMR spectroscopy and X-ray crystallography. It crystallizes in the triclinic space group P1 with: a=14.262(4), b=14.675(4), c=14.698(4) A, α=82.15(4), β=87.57(4), γ=65.89(3)°, Z=2. The structure was refined up to R=0.13 for 4704 reflections. The molecule consists of two [Rh(dmg)(Hdmg)(PPh3)] subunits linked by an [Rh(II)(II)] bond of length equal to 2.852(3) A and by two Zn ions bridging the oxygens of the equatorial ligands. The binding with zinc relieves the equivalence among the atoms of each subunit.

Rhodium complexes with diacetyl monoxime ligands; crystal structure of [Rhcis- (C4H6NO)2cis-(PPh3)2]ClO4·CHCl3

Abstract The reaction of RhCl 3 ·3H 2 O with Hdamo and PPh 3 (Hdamo = diacetyl monoxime = C 4 H 7 NO), both in the absence ( 1 ) and in the presence of HClO 4 ( 2 ) afforded the complex Rh(damo)(PPh 3 ) 2 Cl 2 . The reaction of the acidic water/ethanolic solution of [Rh(H 2 O) 6 ] 3+ with Hdamo and PPh 3 ( 3 ) gave the mixture of the isomers of the compound [Rh(damo) 2 (PPh 3 ) 2 ]ClO 4 ( 3A and 3B ). A similar mixture of isomers was formed when [Rh(H 2 O) 6 ] 3+ reacted with H 2 dopn (H 2 dopn = 3,9-dimethyl-4,8-diazaundeca-3,8-diene-2,10-dione dioxime) and PPh 3 ( 4 ). Two isomers ( 4A = 3B ) and [Rh cis -(damo) 2 cis -(PPh 3 ) 2 ]ClO 4 ·CHCl 3 ( 4B ) have been isolated. All the compounds were characterized by elemental analysis NMR and IR spectroscopy. Single-crystal X-ray diffraction studies were carried out on the complex 4B . In this compound the coordination environment around rhodium(III) is pseudooctahedral composed of two cis -oxime nitrogen atoms [av. RhN = 2.031(3) A], two trans -oxime oxygen atoms [av. RhO = 2.026(2) A] and two cis -phosphine phosphorus atoms [av. RhP = 2.403(1) A].

Electrocatalytic dioxygen reduction in the presence of a rhodoxime

Polarographic and voltammetric evidences are reported for the production of intermediates superoxo [Rh]OO and hydroperoxo [Rh]OOH derivatives at the electrode surface, in the reduction of dioxygen in the presence of rhodium bis(dimethylglyoximato) chloro triphenylphosphine, [ClRh(III)(DMG)2(PPh3)] (I), in the absence and in the presence of protons in non-aqueous solvents. In the latter case the electrocatalytic reduction of dioxygen is observed. From the trends of the concentrations of dioxygen vs the rhodium complexes, a mechanism for the electrochemical process is proposed. The molar ratio of dioxygen to the rhodium complex when the latter is completely consumed for the formation of the adduct, as compared with the 1:1 stoichiometric ratio determined by the Hill plot, together with the influence of the electrode material, suggests that the reduction of dioxygen involves adsorbed species.

Para-substituted diphenylborylated organocobaloximes: effects of substituents on conformation and redox properties.

Abstract Some new derivatives of organocobaloximes containing para -substituted diphenylboryl groups, RCo(DH) 2− n (DB( p- XPh) 2 ) n L (R=alkyl or aryl group, L= N -MeIm, Py or H 2 O, X=OCH 3 , CH 3 or Cl, n =1 or 2) have been synthesized. The X-ray structures and the 1 H-NMR spectra are compared with those of the corresponding RCo(DH) 2− n (DBPh 2 ) n L and RCo(DBF 2 ) 2 L complexes. The insertion of X groups in the phenyl rings does not significantly affect the equatorial CoN distances, whereas the CoPy distances increase slightly in the order (DB( p- OCH 3 Ph) 2 ) 2 p- ClPh) 2 ) 2 2 ) 2 . 1 H-NMR spectra suggest that the conformational distribution in solution is similar to that observed in the corresponding BPh 2 derivatives. Electrochemical studies on the corresponding MeCo(DB( p- XPh) 2 ) 2 H 2 O compounds show a mono-electron Co(III)/Co(II) transfer reaction followed by two parallel reactions: (a) mono-electron Co(II)/Co(I) transfer; (b) homolytic dissociation of the CoC bond with the formation of Co(I) species, the relative rates of the two processes being dependent on X. As the electron-withdrawing power of the equatorial ligand increases, the reduction potentials associated with both Co(III)/Co(II) and Co(II)/Co(I) processes shift towards more positive values, indicating a decrease of electron density on the Co atom. The effects are comparable with those observed by changing the axial ligands.

CATIONIC RHODIUM(III) BISDIMETHYLGLYOXIMATES WITH WATER AND TRIPHENYLPHOSPHINE AXIAL LIGANDS

Preparations of [Rh(Hdmg) 2 (H 2 O) 2 ]ClO 4 ( 1 ) (Hdmg=dimethylgyoximate) from [Rh(H 2 O) 6 ](ClO 4 ) 3 and dimethylglyoxime and of [Rh(Hdmg) 2 (PPh 3 ) 2 ]ClO 4 ( 2 ) from 1 and PPh 3 are described. 1 crystallizes in the space group with a =14.910(3), b =6.058(1), c =18.617(4), A, β =107.649(8)°, Z =4. The structure was refined up to R =0.021 for 1849 reflections. The IR and NMR spectra and electrochemical behavior of 1 and 2 are discussed and compared with other rhodoximes. Three polarographic and CV processed can be detected for 2 , the first two, respectively to Rh(II) and Rh(I), being followed by fast reactions.

Thermodynamic and kinetic effects of the organic group in the electrochemical reduction of organocobaloximes

Abstract The electrochemical reduction of alkylcobaloximes at the dropping mercury electrode involves an initial one electron transfer process to give a radical anion with the cobalt in the CoII formal oxidation state. The CoC bond cleavage in the radical anion is an homolytic process yielding the CoI cobaloxime and competing with a second electron transfer. The nature of the organic group and of the solvent coordinated in the trans position to the Co atom influences the thermodynamics of the electron transfer and the stability of the radical anion. Electronic and steric effects on the thermodynamics of the first electron transfer can be distinguished. These effects have only a small influence on the kinetics of the first electron transfer.

Redox thermodynamics of cytochrome c in mixed water–organic solvent solutions

Abstract Bovine heart cytochrome c was studied through cyclic voltammetry in mixed water–organic solvent solutions under different conditions of temperature and the thermodynamic properties Δ S ° rc and Δ H ° rc calculated by the dependence of E ° by temperature. The effect of the organic fraction of the solvent on the E ° values of the native cyt c was found to be determined mainly by the decrease in dielectric constant of the medium. Specific interactions on the protein surface do not seem to play a remarkable role. The thermodynamic properties changes induced by the organic fraction have been interpreted tentatively in terms of solvation properties of cytochrome c and structural features of the protein environment.

Electrochemistry of 4,6-dimethyl-2-thiopyrimidine and 4,6-dimethyl-1-phenyl-2-thiopyrimidine in dimethylformamide

Abstract The electrochemical behaviour of 4,6-dimethyl-2-thiopyrimidine (LH) and 4,6-dimethyl-1-phenyl-2-thiopyrimidine (LΦ) and their protonated forms was investigated in dimethylformamide on Hg electrodes. Adsorption processes affect the oxidation mechanism of both the compounds and the formation of Hg(I) complexes was observed. The electrochemical reduction of LH provides an example of the so-called “father—son reaction” but this kind of mechanism cannot occur with LΦ. Only in the species LH is a proton present which can be abstracted by the product of the first electron transfer LH · − while LΦ can be protonated by the solvent. However, in the presence of strong acids, for both molecules LH and LΦ the formation of the dimer from the radical anion obtained in the first electron transfer is kinetically preferred to the father—son reaction.

Electrochemistry of cobalt mixed Schiff base/oxime chelates☆

Abstract Diaquocobaloxime and three other chelating ligands obtained by substitution of the O ⋯ H ⋯ O bridge with (CH2)3 or BF2 groups have been examined in order to compare the effects of changes in the in-plane ligand on the relative stability of different oxidation states of the cobalt atom and on the cis-effect of the equatorial structure with those observed in the corresponding organometallic derivatives. The cis-effect of the in-plane ligand was evaluated from the electrochemistry of the CoIII/CoII electron transfer which is dependent upon the Co-solvent bond strength changes in the axial positions. The axial interaction depends mainly on the nature of the axial ligand. The introduction of either one (CH2)3 group or two BF2 groups or both leads to a displacement of the stability range of the CoII oxidation state towards more anodic redox potentials. The (CH2)3 group appears to be more effective both in the displacement of the redox potential and in teh bathochromic shift of the visible band at 450–500 nm of the CoII chelates.