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

Abstract

Herein, we report the synthesis, X-ray structures, thermal stability, spectroscopic characterization and redox properties of compounds of formulas (BzPh₃P)₂[Ni(dto)₂] (1), (BzPh₃P)₂[Pd(dto)₂]·nCH₂Cl₂ [n = 0 (2) and 3 (2·3CH₂Cl₂)], (BzPh₃P)₂[Pt(dto)₂] (3), (Ph₄As)₂[Ni(dto)₂]·n(CH₃)₂CO [n = 0 (4) and 1 (4·(CH₃)₂CO)], (Ph₄As)₂[Pd(dto)₂] (5) and (Ph₄As)₂[Pt(dto)₂] (6) (dto²⁻ = dithiooxalate, BzPh₃P⁺ = benzyltriphenylphosphonium and Ph₄As⁺ = tetraphenylarsonium), together with the synthesis and X-ray structure of the corresponding diphenyl dithioxalate proligand (Ph₂dto). The crystal structures of 1–6 and those of the solvated compounds 2·3CH₂Cl₂ and 4·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 nd⁸ 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) ≈ 2.29 Å (Pt). The electroneutrality is achieved by bulky BzPh₃P⁺ (1–3 and 2·3CH₂Cl₂) and Ph₄As⁺ [4–6 and 4·(CH₃)₂CO] countercations. Segregated (1–3 and 4–6) or mixed [2·3CH₂Cl₂ and 4·(CH₃)₂CO] layers of complex anions and cations linked by electrostatic forces and Van der Waals interactions together with very weak S⋯OC and C–H⋯OC 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 +0.45 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 metal-to-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.