Spin-transition in [Fe(OH-BZIMPY)(2)](ClO4)(2)center dot 3H(2)O induced by deprotonation

Coordination of 4-OH-2,6-bis-(benzimidazol-2'-yl)pyridine (OH-bzimpy = H(3)L) with ironI(II) increases the acidity of the iminohydrogen atoms of the benzimidazole groups. The first three protonation constants (logK(n)) both for the free ligand, (OH-bzimpy) and the complex, [Fe(OH-bzimpy)(2)](ClO4)(2) . 3H(2)O, were evaluated in 30:70 (v/v) H2O:EtOH. Deprotonation of [Fe(OH-bzimpy)(2)](ClO4)(2) . 3H(2)O leads a change in spin-state to the low-spin (LS) state associated with a decrease of the spin-crossover equilibrium constant (K-SC) and a shift of absorbtion maxima. An opposite shift of spin-state to the high-spin (HS) state is observed with addition of acid to the complex solution. Values of spin-equilibra constants (K-SC) are given for different pH values. The magnetic moment (mu/mu B), as well as the spin-equilibrium constant (K-SC) decreases during titration of the complex of N,N-triethylamine (Et(3)N) in MeOH, resulting in a change in the spin-state to the low-spin (LS) state.