Bulky countercation effects on the crystal packing of anionic dithiooxalato-containing Ni(II), Pd(II) and Pt(II) complexes: spectroscopic-redox correlations

Herein, we report the synthesis, X-ray structures, thermal stability, spectroscopic characterization and redox properties of compounds of formulas (BzPh(3)P)(2)(Ni(dto)(2)] (1), (BzPh(3)P)(2)[Pd(dto)(2)]center dot nCH(2)Cl(2) [n = 0 (2) and 3 (2 center dot 3CH(2)Cl(2))] (BzPh(3)P)(2)[Pt(dto)(2)] (3), (Ph4As)(2)[Ni(dto)(2)]center dot n(CH3)(2)CO [n = 0 (4) and 1 (4 center dot(CH3)(2)CO)], (Ph-4-As)(2)[Pd(dto)(2)] (5) and (Ph4As)(2)[Pt(dto)(2)] (6) (dto(2-) = dithiooxalate, BzPh(3)(P+) = benzyltriphenylphosphonium and Ph4As+ = tetraphenylarsonium), together with the synthesis and X-ray structure of the corresponding diphenyl dithioxalate proligand (Ph(2)dto). The crystal structures of 1-6 and those of the solvated compounds 2 center dot 3CH(2)Cl(2) and 4 center dot Me2CO have in common the presence of discrete planar [M(dto)(2)](2-) complex anions where the divalent metal ions exhibit rectangular (M = Ni) or quasi square planar MS4 (M = Pd and Pt) environments. The M-S bond lengths are typical of nd(8) M-II ions [n = 3 (Ni), 4 (Pd) and 5 (Pt)] in a low-spin (LS) electronic configuration, varying in the order 2.17 (Ni) < 2.30 (Pd) approximate to 2.29 angstrom (Pt). The electroneutrality is achieved by bulky BzPh(3)P(+) (1-3 and 2 center dot 3CH(2)Cl(2)) and Ph4As+ [4-6 and 4 center dot(CH3)(2)CO] countercations. Segregated (1-3 and 4-6) or mixed [2 center dot 3CH(2)Cl(2) and 4 center dot(CH3)(2)CO] layers of complex anions and cations linked by electrostatic forces and Van der Waals interactions together with very weak S center dot center dot center dot O = C and C-H center dot center dot center dot O = C type contacts occur in them. The dichloromethane and acetone molecules of crystallization occupy the interlayer space or are embedded within the mixed layers, respectively. The redox chemistry of the [M(dto)(2)](2-) complex anions of 4-6 was investigated by cyclic voltammetry in acetonitrile solution. The one-electron metal centred irreversible oxidations were evident in the cyclic voltammograms with oxidation potentials of +0.18 (Ni), +0.72 (Pd) and 10.45 V (Pt) vs. Fc(+)/Fc. The UV-vis spectra of acetonitrile solutions of the square planar [M(dto)(2)](2-)species of 4-6 are similar, and they show a broad, intense metalto-ligand charge transfer (MLCT) transition centred at 498 (Ni), 376 (Pd) and 422 nm (Pt). A linear correlation between the values of energy of the MLCT transition and those of the formal redox potential is found along this [M-II(dto)(2)](2-) family pointing out an important contribution from a nd-nd (n = 3-5) transition.