An investigation of nickel(II), copper(II) and cobalt(II) complexes employing bidentate ligands derived from 2(β-ketoalkyl) heteroaromatic compounds

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Virginia Tech


Ni(L)₂ complexes, where L equals 2-(trifluoroacetyl heteroaromatic anion were prepared and characterized. 2-(Trifluoroacetyl)picolinato anion (TFAP) gives a Ni(L)₂ complex that appears to be a 5-coordinate square pyramid dimer. The complexes Ni(4MeTFAP)₂ and Ni(5MeTFAP)₂ are square planar complexes while Ni(6MeTFAP)₂ is tetrahedral due to steric interference. Quinoline derivatives are also tetrahedral while quinoxaline and pyrazine derivatives are octahedral polymers with the 5th and 6th donor atoms coming from the nitrogen atom in the 4-position. The Ni(L)₂ complexes containing ketoalkyl groups could not be isolated.

Nickel(II) complexes were also isolated as base adducts. Solid (Ni(TFAP)₂(H₂0))₂ appears to have bridging water molecules that could not be removed without decomposition while solid Ni(TFAP)₂(H₂0)₁.₅ appears to have terminal and/or lattice water that is easily removed. Octahedral pyridine adducts were formed when L equals TFAP, 4MeTFAP and 5MeTFAP. Ni(6MeTFAP)₂ did not form any base adducts because of steric interference.

In non-donor solvents, the complexes Ni(TFAP)₂, Ni(4MeTFAP)₂, and Ni(5MeTFAP)₂ exhibit properties that indicate a polymer/monomer equilibrium. Solution properties are a function of concentration and temperature. At fixed concentrations the molecular weights and magnetic moments decrease as the temperature increases. The visible spectra changes are understood in terms of polymer/monomer equilibria.

Ni(L)₂ complexes containing the CF₃ group undergo a discrete gas-solid reaction with H₂0, NH₃ and CH₃NH₂. The nickel atom is in an octahedral environment due to the addition of the base. Gaseous NH₃ and H0₂ while CH₃NH₂ does not dissociate. The proton is believed to be transferred to the ligand so that it coordinates in the keto form. (Ni(TFAP)₂)₂ reacts with one mole of H₂0 per nickel atom to give Ni₂(TFAP)₃(HTFAP)(H₂0)(OH) while NH₃ gives Ni(TFAP)(HTFAP)(NH₃)(NH₂). Other complexes do not react with H₂O and Ni(6MeTFAP)₂ reacts reversibly with NH₃ at ambient temperatures.

Cu(L)₂ complexes formed when the keto group was CF₃, phenyl or an alkyl group. The complexes appear to be square planar or distorted octahedral except where steric interference forces the pseudotetrahedral structure. All complexes are monomeric in the gas phase, The Cu(L)₂ complexes do not form base adducts from solution and do not react the gaseous H₂0 or NH₃.

The complexes Co(L)₂ are tetrahedral except for Co(TFAP)₂ which exist as an associated species, The complex is 5-coordinate when prepared at 100° and an octahedral polymer when prepared at 150°. The complexes Co(L)₂(Py)₂ are discussed for L equal TFAP, 4MeTFAP and 5MeTFAP. The complexes are octahedral and decompose to Co(L)₂ on heating at 100°. The Co(L)₂ complexes do not react with gaseous H₂0 or NH₃.