Patrizia Siega

Synthesis, molecular structure, and characterization in solution of a new series of inorganic and organometallic Co(III) Schiff base complexes

Abstract A new series of inorganic and organometallic cobalt Schiff base complexes containing the tetradentate ligand tmsalen (4,4′,7,7′, tetramethylsalen) have been synthesized. They have been characterized by 1H NMR and UV–Vis spectroscopy and by ESI MS spectrometry, both in coordinating and non-coordinating solvents. Two inorganic complexes [Co(tmsalen)(L)2]ClO4, where L=py (2) and N-MeIm (3), two dimeric organometallic derivatives [RCo(tmsalen)]2 (R=CH3 (4) and CH2CF3 (7)) and one organometallic penta-coordinated species (R=i-Pr (6)) have been structurally characterized. It is noteworthy that the formation of organometallic dimeric and penta-coordinated species occurs even if the synthesis is carried out in presence of py. Solution data confirm the strong tendency to the penta-coordination of the complexes containing the most electron-donating alkyl groups, which maintain penta-coordination even in methanol.

Self-Assembly of Cobaloximes and Rhodoximes with 3-Aminophenylboronic Acid: A Molecular Triangle and a Polymer

The reactions of methylaquacobaloxime and ethylaquacobaloxime with 3-aminophenylboronic acid in water produce supramolecular triangles 3a and 3b respectively, through the reaction of boron with the oxime oxygens of the alkylcobaloxime and the coordination of pyridine to cobalt. The reaction is reversible and strongly pH dependent. The complexes 3a and 3b precipitate immediately from the solution at pH 7, but dissolve quickly at both acidic (below 4) and basic (above 9) pH values. The aggregates are re-formed by readjusting the pH to neutrality and the precipitation-dissolution cycle may be repeated several times without an apparent decrease in yield. The complexes were fully characterised by 1H and 13C NMR spectroscopy. The X-ray structure shows that the neutral chiral supramolecule 3a is a triangle of cobalt atoms with an approximate C3 symmetry. The side of the triangle is about 9.5 A. The ring closure is ensured by the torsion angles around the B−C and C−NH2 bonds, which vary in the range 25−30° and 97−109°, respectively. The reaction of methylaquarhodoxime with 3-aminophenylboronic acid in water at pH 7 results in the displacement of H2O by 3-aminophenylboronic acid only. The polymeric form 4 precipitates from CD3OD in the NMR tube with time. The X-ray structure shows that 4 is a polymeric species with a zig-zag chain arrangement, with Rh···Rh distances of 9.2 A. The chain arrangement is characterized by torsion angles around the B−C and C-NH2 bonds of 61° and 92°, respectively. (© Wiley-VCH Verlag GmbH, 69451 Weinheim, Germany, 2002)

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.

Cobalt(III) complexes with tridentate hydrazone ligands: protonation state and hydrogen bond competition†

Cobalt(III) complexes of the type [Co(HL)(L)] were synthesized under solvothermal conditions staring from [Co(C5H7O2)3] and the corresponding ligand H2L (salicylaldehyde 4-hydroxybenzhydrazone, 3-methoxysalicylaldehyde 4-hydroxybenzhydrazone, 4-methoxysalicylaldehyde 4-hydroxybenzhydrazone, salicylaldehyde benzhydrazone, 3-methoxysalicylaldehyde benzhydrazone, 4-methoxysalicylaldehyde benzhydrazone). The presence of differently protonated forms of the same ligand in the complexes was supported by IR and NMR spectroscopy as well as by the single crystal X-ray diffraction method. The effects of weak interactions on the supramolecular architecture and their role on the ligand form stabilization have been analysed. Molecular interactions within the unit cells were investigated and quantified by extensive quantum chemical analysis on models built from crystal structures using density functional theory and empirical dispersion. Ligands used in this study were prepared under environmentally friendly conditions by mechanochemical synthesis. Their thermal behaviour and phase transitions were investigated using TG and DSC analysis and the powder X-ray diffraction method.

Trans and Cis Effects of Axial Fluoroalkyl Ligands in Vitamin B12 Analogues: Relationship between Alkyl- and Fluoroalkyl-Cobalamins

CF2HCbl, CF3Cbl , and CF3CH2Cbl have been synthesized and characterized in solution by (1)H NMR and UV-vis spectroscopy, and their X-ray crystal structures have been determined using synchrotron radiation. The structure of CF3CH2Cbl is reported for the first time, whereas those of CF2HCbl and CF3Cbl are re-examined to obtain more precise structural data. Comparison of the structural data obtained with the alkylcobalamin analogues, MeCbl and EtCbl, indicates that the Co-C and Co-NB3 bond lengths are shorter in the fluoroalkylcobalamins. The structural data of the fluoroalkylcobalamins previously reported in the literature had been conflicting in this regard. Thus, a much less dramatic shortening of the two axial bonds was found for CF3Cbl, whereas in the case of CF2HCbl, the Co-NB3 bond length is shorter than in MeCbl. Direct comparison of the structures of CF3CH2Cbl and EtCbl indicates a large distortion of the axial fragment in the former case that can be attributed to steric effects. A number of previously reported correlations of the effect of the β-ligand on the structure and properties of cobalamins are re-examined in light of the present results. Particular emphasis is placed on the axial fragment. This analysis substantially confirms and, with the new data reported here, adjusts and expands the data set for correlations between trans and cis influences of the β-ligand of cobalamins and their structure (Co-X and Co-NB3 distances and corrin fold angle) and properties (UV-vis spectra, NMR spectra, and pK(base-off)).