Spin-state equilibria in non-aqueous solution and quantum-mechanical investigations of iron(II) and nickel(II) complexes with 4-substituted 2,6-bis(benzimidazol-2-yl)pyridines

Abstract

Cationic complexes with a series of tridentate ligands, L = 4X-substituted 2,6-bis(benzimidazol-2-yl)pyridines, [ML₂][ClO₄]₂(M = Fe or Ni; X = H, OH or Cl), were isolated and characterized, together with the free pyridines, by elemental analysis, Fourier-transform IR, ¹H NMR and UV/VIS spectroscopy. The syntheses were performed via condensation of o-phenylenediamine with 4-substituted pyridine-2,6-dicarboxylic acids. Ligand-field parameters were estimated for the nickel complexes. The [FeL₂]²⁺ species show thermally induced spin-crossover behaviour (¹A₁→⁵T_2g) which has been investigated in methanol, nitromethane and 20%(v/v) dimethylformamide in MeOH. The behaviour is complicated by two complex dissociation equilibria, for which equilibrium constants have been evaluated. Ligand substitution is reflected in a change of the spin state in solution (µ_exptl= 2.50, X = H; 4.19, OH; and 4.49 µ_B, Cl at 295 K, in MeOH) and in the metal-to-ligand charge-transfer band (500–557 nm); when M = Fe and X = H there is a pronounced spin-crossover equilibrium in methanolic solution (µ_exptl= 1.31–3.45 µ_B for 213–328 K). A small variation of the magnetic moments when M = Fe and X = OH (µ_exptl= 3.77–4.73 µ_B at 220–332 K) might indicate a temperature-variable population of the ⁵E_g sublevel or variation in hydrogen bonding. The results are compared with quasi-relativistic quantum-mechanical calculations, and the spin-crossover behaviour of the new ligands, L, with substituents X = CHO, NH₂, CN, Me, NO₂, OH, CONH₂, COCl, SH, F, Cl, Br or I has been estimated. The differences in the calculated heats of formation between the high-and low-spin forms of [FeL₂]²⁺ when plotted against Δδ(=¹H NMR para increment for substituents X in benzene) show a turning point in the region around X = H and in this region spin-crossover behaviour is observed. Outside this region there is very little or no such behaviour and it is therefore possible to predict the spin-crossover behaviour for other substituents X from the Δδ value.