title: Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(II) complexes with tetradentate o-phenylenedioxamidates and related ligands.
abstract: A series of four-coordinate square-planar nickel(II) complexes of o-phenylenebis(N'-methyloxamidate)(L1) and related o-phenylene(N'-methyloxamidate)oxamate (L2) and o-phenylenebis(oxamate)(L3) tetradentate ligands have been synthesized and characterized structurally, spectroscopically and electrochemically. The parent nickel(II)-L1 complex presents an intense MLCT band in the UV region (lambda max = 357 nm) and a distinctive 1 s --> 4p CT satellite in the Ni K-edge XANES spectrum (E = 8339.2 eV). These features together with the short Ni-N(amidate) bond lengths (1.85-1.93 A) as revealed by the analysis of the Ni K-edge EXAFS spectrum and confirmed by single-crystal X-ray diffraction are typical of square-planar low spin (S = 0) Ni(II) ions. The dianionic nickel(II) complexes, [Ni(II)L(i)](2-)(i = -3), experience two redox processes in acetonitrile at 25 degrees C. The first redox process, at moderately low potentials (E1 = 0.12-0.52 V vs. SCE), is a reversible one-electron metal-centered oxidation to the corresponding monoanionic nickel(III) complexes, [Ni(III)L(i)]-. The second redox process, at relatively high potentials (E2 = 0.86-1.04 V vs. SCE), is a quasireversible to irreversible one-electron oxidation largely centered on the o-benzenediamidate fragment of the non-innocent ligand, yielding the corresponding neutral nickel(iii) complexes with a o-benzosemiquinonediimine pi-cation radical ligand, [Ni(III)(L(i))*+]. The singly and doubly oxidized species of the parent nickel(II)-L1 complex have been prepared by chemical oxidation and characterized spectroscopically in acetonitrile at -40 degrees C. The stable singly oxidized nickel(III)-L1 species presents an intense LMCT band in the NIR region (lambda max = 910 nm) and a rhombic X-band EPR spectrum (g1 = 2.193, g2 = 2.080 and g3 = 2.006) characteristic of square-planar low spin (S = 1/2) Ni(III) ions. The unstable double oxidized nickel(III)-L1 pi-cation radical species exhibits a rather intense visible band (lambda max = 645 nm) that is tentatively assigned as a MLCT transition from the Ni(III)-benzosemiquinone type ground state to the Ni(IV) excited state.
authors: Laboratoire de Chimie Inorganique, UMR 8613, Université Paris-Sud, 91405, Orsay, France. jour@icmo.u-psud.fr
authors: Xavier
authors: X
authors: Ottenwaelder
authors: Ally
authors: A
authors: Aukauloo
authors: Yves
authors: Y
authors: Journaux
authors: Rosa
authors: R
authors: Carrasco
authors: Joan
authors: J
authors: Cano
authors: Beatriz
authors: B
authors: Cervera
authors: Isabel
authors: I
authors: Castro
authors: Simona
authors: S
authors: Curreli
authors: M Carmen
authors: MC
authors: Muñoz
authors: Antonio L
authors: AL
authors: Roselló
authors: Bernardino
authors: B
authors: Soto
authors: Rafael
authors: R
authors: Ruiz-García
mesh_terms: D018360:Crystallography, X-Ray; D004563:Electrochemistry; D004578:Electron Spin Resonance Spectroscopy / Q000379:methods; D008024:Ligands; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008958:Models, Molecular; D015394:Molecular Structure; D009532:Nickel / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry; D010070:Oxalates / Q000737:chemistry*; D010084:Oxidation-Reduction; D010655:Phenylenediamines / Q000737:chemistry*; D012680:Sensitivity and Specificity; D013052:Spectrometry, X-Ray Emission / Q000379:methods; D013055:Spectrophotometry, Infrared / Q000379:methods; D013056:Spectrophotometry, Ultraviolet / Q000379:methods; D013237:Stereoisomerism
chemical_list: C516302:2-phenylenebis(N'-methyloxamidate); D008024:Ligands; D009942:Organometallic Compounds; D010070:Oxalates; D010655:Phenylenediamines; D009532:Nickel