title: Cobalt(II), nickel(II) and copper(II) complexes of a hexadentate pyridine amide ligand. Effect of donor atom (ether vs. thioether) on coordination geometry, spin-state of cobalt and M(III)-M(II) redox potential.
abstract: Using an acyclic hexadentate pyridine amide ligand, containing a -OCH(2)CH(2)O- spacer between two pyridine-2-carboxamide units (1,4-bis[o-(pyrydine-2-carboxamidophenyl)]-1,4-dioxabutane (H(2)L(9)), in its deprotonated form), four new complexes, [Co(II)(L(9))] (1) and its one-electron oxidized counterpart [Co(III)(L(9))][NO(3)]·2H(2)O (2), [Ni(II)(L(9))] (3) and [Cu(II)(L(9))] (4), have been synthesized. Structural analyses revealed that the Co(II) centre in 1 and the Ni(II) centre in 3 are six-coordinate, utilizing all the available donor sites and the Cu(II) centre in 4 is effectively five-coordinated (one of the ether O atoms does not participate in coordination). The structural parameters associated with the change in the metal coordination environment have been compared with corresponding complexes of thioether-containing hexadentate ligands. The μ(eff) values at 298 K of 1-4 correspond to S = 3/2, S = 0, S = 1 and S = 1/2, respectively. Absorption spectra for all the complexes have been investigated. EPR spectral properties of the copper(II) complex 4 have been investigated, simulated and analyzed. Cyclic voltammetric experiments in CH(2)Cl(2) reveal quasireversible Co(III)-Co(II), Ni(III)-Ni(II) and Cu(II)-Cu(I) redox processes. In going from ether O to thioether S coordination, the effect of the metal coordination environment on the redox potential values of Co(III)-Co(II) (here the effect of spin-state as well), Ni(III)-Ni(II) and Cu(II)-Cu(I) processes have been systematically analyzed.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Kanpur, Kanpur 208 016, India.
authors.forename: Sharmila
authors.initials: S
authors.lastname: Pandey
authors.forename: Partha Pratim
authors.initials: PP
authors.lastname: Das
authors.forename: Akhilesh Kumar
authors.initials: AK
authors.lastname: Singh
authors.forename: Rabindranath
authors.initials: R
authors.lastname: Mukherjee
mesh_terms: D000577:Amides / Q000737:chemistry*; D003035:Cobalt / Q000737:chemistry*; D056831:Coordination Complexes / Q000138:chemical synthesis / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D018360:Crystallography, X-Ray; D055664:Electrochemical Techniques; D008024:Ligands; D008968:Molecular Conformation; D009532:Nickel / Q000737:chemistry*; D010084:Oxidation-Reduction; D011725:Pyridines / Q000737:chemistry; D013113:Spin Labels
chemical_list: D000577:Amides; D056831:Coordination Complexes; D008024:Ligands; D011725:Pyridines; D013113:Spin Labels; D003035:Cobalt; D003300:Copper; D009532:Nickel; C023666:pyridine