Copper (II) arylcarboxylates : Substituent effects on structure, thermal properties, magnetism, redox and carbon-carbon bond-forming reaction of carbonyls / Mohammad Isa Mohamadin

This research focused on the synthesis, structural elucidation, and physical characterisations of copper(II) arylcarboxylates of general formula [Cu2(p-XC6H4COO)4L2], where X = H, NO2, OH, NH2, F, Cl, Br and I, and a complex with the chemical formula [Cu2(3,5-(NO2)2C6H3COO)4L2], and L = CH3CH2OH and/or H2O. A total of nine binuclear copper(II) arylcarboxylates were successfully prepared, and their magnetic, thermal and redox properties were determined. In addition, suitably selected complexes were also subjected to reaction with selected carbonyls namely acetone, and a mixture of benzaldehyde and acetophenone, in order to determine their role in the carbon-carbon bond formation reaction. The instrumental analyses used to characterize these complexes were CHN elemental analyses, atomic absorption spectroscopy (AAS), FTIR and UV-vis spectroscopies, X-ray crystallography (for crystalline complexes), magnetic susceptibility by the Gouy method, thermogravimetry, differential scanning calorimetry, and cyclic voltammetry. In addition, NMR (1H- and 13C-) and GCMS were used to elucidate the structure of the product(s) obtained from the reaction with carbonyls. All complexes were dimeric with square pyramidal geometry at copper(II) centres with syn-syn coordination mode of carboxylate ligand in the solid states. However, complexes having strong electron-withdrawing group, namely X = NO2, 3,5-(NO2)2 and F, have a more distorted geometry around copper(II). The structures were stable in CH3OH-CH3COOH solvents, but formed monomeric structures in strong coordinating ligands, namely DMSO and DMF. Especially were the copper(II) complexes with NO2- and I substituted arylcarboxylates ligands, which dissociated in DMSO-CHCl3. The complexes showed antiferromagnetic interaction of variable strengths. The values of µeff and 2J range from 1.80 B.M. to 2.54 B.M, and from -382 cm-1 to -133 cm-1 , respectively. However, no direct correlation between the type of substituent and the strength of the magnetic interaction could be established. The complexes were thermally stable up to 240oC. The decomposition pathway involved the decarboxylation of the ligands, leaving residues assumed to be mainly copper(II) oxide. Complexes with electron-donating group were less thermally stable compared to those with electron-withdrawing groups. All complexes were redox-active. Facile reduction depended on the type of substituent as well as on the geometrical distortion at copper(II) centres. Complexes with electron-donating group were reduced at more negative potential (harder to be reduced) compared to those with electron withdrawing group. The redox reactions were quasi-reversible involving extensive structural reorganization and chemical reactions (the EC mechanism). The reaction of CuB with CH3COCH3 in acidic condition formed a black mixed-valence complex, [Cu(II)Cu(I)(R)3(RH)2L4].CH3COCH3, where R = C6H5COO; L = CH3COCH2C(OH)(CH3)2. The ligand L was a product of the carbon-carbon bond-forming reaction between CH3COCH3 molecules. The mixed-valence complex was thermally stable (Tdec = 126oC), strongly antiferromagnetic (µeff = 1.33 B.M.; 2J = - 556 cm-1 ), and has a highly delocalized electron with surprisingly low band gap energy of 1.2 eV. Under similar conditions, CuNO2B, CuOHB and CuClB reacted with CH3COCH3 to form the mixed-valence complexes [Cu(II)Cu(I)(R)3(RH)2L10].3CH3COCH3, [Cu(II)Cu(I)(R)3(RH)5L5]. CH3COCH3 and [Cu(II)Cu(I)(R)3(RH)2L15].CH3COCH3 (where R = 4- NO2C6H4COO, 4-HOC6H4COO and 4-ClC6H4COO respectively, L = CH3COCH2C(OH)(CH3)2). The copper(II) complex substituted with a strong electron-withdrawing group (NO2) formed the mixed-valence product in higher yield compared to that with electron-donating group (OH). Some of the findings of this project were published in six (6) ISI journals, one (1) non-ISI journal, and presented at three (3) national and five (5) international conferences (Appendix 1)