SPIN-CROSSOVER COMPLEXES IN SOLUTION .3. SUBSTITUENT AND SOLVENT EFFECTS ON THE SPIN-EQUILIBRIUM OF 4-SUBSTITUTED IRON(II)-2,6-BIS-(BENZIMIDAZOL-2'-YL)-PYRIDINE SYSTEMS

Transition metal complexes of the composition [Fe(4-X-bzimpy)2](ClO4)2 [bzimpy = 2,6-bis-(benzimidazol-2'-yl)pyridine and X = H, OH, Cl] sho w thermally accessible spin-crossover behaviour in solution that depen ds on both the ligand and the solvent. H-1-NMR spectroscopy and UV-vis ible spectroscopy measurements suggest that ligand substituent effects , solvent donor-acceptor properties and hydrogen-bonding may be employ ed to ''fine-tune'' the ligand field strength and hence to affect the spin-crossover behaviour. The ligand substitution changes in solution are reflected by the magnetic data (X = H:mu(exp) = 2.50 mu(B); X = OH :mu(exp) = 4.20 mu(B) and X = Cl:mu(exp) = 4.30 mu(B) at 294 K in MeOH ), and by the shift of metal-to-ligand charge-transfer band (X = H, la mbda = 557 nm; X = OH, lambda = 520 nm; X = Cl, lambda = 500 nm). [Fe( bzimpy)2](ClO4)2 exhibits a pronounced spin-crossover equilibrium (1A1 --> 5T2) in solution (K(sc) = 0.26 at 293 K; mu(exp) = 1.30 --> 3.40 mu(B) for 213 --> 328 K in MeOH). A small variation of magnetic moment s of [Fe(4-OH-bzimpy)2](ClO4)2 (mu(exp) = 3.77 --> 4.73 mu(B) at 220 - -> 332 K) might indicate either the existence of (temperature dependen t) hydrogen bonding between the ligand and solvent molecules or a temp erature dependent variation in the population of the 5E(g) sublevel. T he presence of strong donor solvents (DN greater-than-or-equal-to 30) shifts the spin-state of the complexes.