Pressure-induced spin-state crossovers in six-coordinate cobalt(II) complexes: a near-edge x-ray absorption study

Pressure induced separation of phase-transition-triggered-abrupt vs. gradual components of spin crossover

The application of pressure on [Co(II)(dpzca)2], which at ambient pressure undergoes abrupt spin crossover (SCO) with thermal hysteresis, gives unique insights into SCO. It reversibly separates the crystallographic phase transition (I41/a↔P21/c) and associated abrupt SCO from the underlying gradual SCO, as shown by detailed room temperature (RT) X-ray crystallography and temperature dependent magnetic susceptibility studies, both under a range of 10 different pressures. The pressure effects are shown to be reversible. The crystal structure of the pressure-induced low-spin state is determined at RT at 0.42(2) and 1.78(9) GPa. At the highest pressure [1.78(9) GPa] the Co-N bond lengths are consistent with the complex being fully LS, and the conjugated terdentate ligands are significantly distorted out of plane. The abrupt SCO event can be shifted up to RT by application of a hydrostatic pressure of ∼0.4 GPa. These magnetic susceptibility (vs. temperature) and X-ray crystallography (at RT) studies, under a range of pressures, show that the SCO can be tuned over a wide range of temperature and pressure space, including RT SCO.

Synthesis, structure, spectroscopy and redox chemistry of square-planar nickel(ii) complexes with tetradentate o-phenylenedioxamidates and related ligands

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.

Pressure- and temperature-induced valence tautomeric interconversion in a o-dioxolene adduct of a cobalt-tetraazamacrocycle complex

An electronic switch at the molecular level has been realized by using a class of ionic compounds of the formula [Co(L)(diox)]Y (L = tetraazamacrocyclic ligand, Y = mononegative anion). Such compounds undergo temperature- and pressure-induced intramolecular one-electron transfer equilibria. The transition temperature of interconversion varies with the nature of the counterions Y (Y = PF6, BPh4, I). Surprisingly the effect of the anion on the transition temperature is not only governed by its volume but also by its coulombic interaction.