| In this work we have synthesized and structurally characterized polynuclear nickel (II) complexes. During this work, we also unexpectedly synthesized mono-nuclear Schiff base nickel(II) complexes (1,2,3,4), octahedral mono-nuclear nickel complexes with mixed ligands (5,6,7), mu-sodium di-nuclear Schiff base (including amino acid Schiff base) nickel(II) complexes (8,9), tetra and bis-tetra-nuclear nickel salen (salen = tetradentate Schiff base ligand formed from salicylaldehyde and substituted salicylaldehydes with ethylene diamine, salphen = salicylaldehyde with o-phenylenediamine) complexes (10,11,12). We successfully characterized all these structurally. Some of them serve as models for the binuclear active site of urease as they contain Ni in two different coordination environments. Beside the polynuclear nickel(II) clusters complexes (13,14,15), manganese(III) salen complexes (16,17) were also synthesized in the attempt to obtain poly-nuclear manganese Schiff base salen complexes. Finally, two octahedral nickel complexes (18,19) were synthesized and crystallized. These complexes were synthesized using metal salts such as NiCl2.6H 2O, Ni(ClO4)2.4H2O, Ni(NO3) 2.6H2O, Ni(CH3COO)2.4H2O, NiBr2, Mn(ClO4)2.6H2O, Mn(CH 3COO)2.4H2O. Four Schiff base ligands, N,N'-bis(3-methoxysalicylaldehyde)ethylenediimine( L1), N,N'-bis(salicylaldehyde-orthophenylenediimine (L2 ), 5-nitro salicylaldehydene-2,2-dimethyl ethylimine-1-ol (L3 ) and 4-hydroxysalicyladene methylimine (L4) were synthesized and characterized by FT-IR, NMR, GC-MS and melting point (MP). Other compounds used as ligands or mixed ligands are L-tryptophan and 2-benzylaminopyridine. Complexes 1-4 were found to be diamagnetic at room temperature with magnetic moments values of 1.09, 1.08, 1.58, and 1.17BM respectively which are higher than zero but within the diamagnetic range of 0-1.6 BM expected for square planar nickel (II) complexes. These values differ from 0.0 BM for square planar nickel complexes can be due to spin-orbit interaction while the octahedral mono-nuclear complexes 5-7 are all paramagnetic (3.34, 3.01, 2.11BM values) although 7 is on the weak side which may indicate an antiferromagnetic interaction between nickel centers mediated by hydrogen bonding interactions. All octahedral nickel complexes were found to be paramagnetic, namely hepta-nuclear 14 (2.34 BM/Ni) and 15 (2.96 BM/Ni). The FT-IR spectra have shifts to low wavenumbers of about 16 to 25 cm-1 for the functional groups (such as C=N). The UV-vis and NIR spectroscopic data of high (ca 30 mMol/L) and low (ca 0.1 mMol/L) concentrations using appropriate solvents revealed metal ion d-d transition 3A2g ---> 3T2g (8500-12,000 cm -1), 3A2g ---> 3T1g (F) (13,000--19,000 cm-1), and 3A 2g ---> 3T1g (P) (23,000 -- 35,000 cm -1) for most octahedral complexes with low extinction coefficients ranging from 7 to 70 Mol-1.cm-1 , however, pi --> pi*, n--> pi* and ligand to metal charge transfer (LMCT) such as pi (N=C) --> pNi absorbance coefficients between 220-400 nm (45455-25000 cm-1) are expectedly high (1.31 x 104 for 15). Cyclic voltammetry analysis of these complexes in dry acetonitrile revealed that 1, 2 and 3 exhibit quasi-reversible with redox processes with potential of DeltaEp = Epa-Ep c of ca 0.494V and 0.472V (vs Ag/AgCl) respectively. Complexes 4, 8, 10, 11 and 13 revealed quasi-reversible redox process while 5, 6, 7, 9 showed irreversible processes suggesting that the (+2) oxidation state of the nickel ion is stabilized by the ligands. X-ray crystallographic data showed that in most complexes the Nickel centers adopt an octahedral environment except the salens where they are square planar. Complex 13 is a cubane-like with Ni4O4 core while 14 and 15 contain a heptanuclear moiety made up of six nickel subunits surrounding a central nickel center. The difference lies in the magnetic moment as reported earlier. From this work, we have demonstrated that some of the complexes synthesized, namely the di-, tetra and poly-nuclear ones are good candidates as models of enzyme active sites while the clusters are good candidates for single molecule magnets. Further magnetic studies such as variable temperature magnetism are necessary in support of this. (Abstract shortened by UMI.)... |
