Synthesis, reactivities, and magnetostructural properties of Fe^III, Fe^III-O-Fe^III, and Zn^IIFe^III-O-Fe^IIIZn^II complexes of a tetraiminodiphenolate macrocycle

Abstract

The mononuclear iron(III) complexes [Fe(LH₂)(H₂O)Cl](ClO₄)₂·2H₂O (1) and [Fe(LH₂)(H₂O)₂](ClO₄)₃·H₂O (2) have been prepared by reacting [Pb(LH₂)](ClO₄)₂ with FeCl₃·6H₂O and Fe(ClO₄)3·6H₂O, respectively. Complex 2 upon treatment with 1 equiv of alkali produces the oxo-bridged dimer [{Fe(LH₂)(H₂O)}₂(μ-O)](ClO₄)₄·2H₂O (3). In these compounds, LH₂ refers to the tetraiminodiphenol macrocycle in the zwitterionic form whose two uncoordinated imine nitrogens are protonated and hydrogen-bonded to the metal-bound phenolate oxygens. The aqua ligands of complexes 1-3 get exchanged in acetonitrile. Reaction equilibria involving binding and exchange of the terminal ligands (Cl^-/H₂O/CH₃CN) in these complexes have been studied spectrophotometrically. The equilibrium constant for the aquation reaction (K_aq) [1]²⁺ + H₂O ⇔^K_aq [2]³⁺ + Cl^- in acetonitrile is 8.65(5) M, and the binding constant (K_Cl^-) for the reaction [1]²⁺ + Cl^-⇔^K_aq[1Cl]⁺ + CH₃CN is 4.75(5) M. The pK_D value for the dimerization reaction 2[2]³⁺ + 2OH^-⇔K_aq [3]⁴⁺ + 3H₂O in 1:1 acetonitrile-water is 9.38(10). Complexes 1-3 upon reaction with Zn(ClO₄)₂·6H₂O and sodium acetate (OAc), pivalate (OPiv), or bis(4-nitrophenyl)phosphate (BNPP) produce the heterobimetallic complexes [{FeLZn(μ-X)}₂(μ-O)](ClO₄)₂, where X = OAc (4), OPiv (5), and BNPP (6). The pseudo-first-order rate constant (k_obs) for the formation of 4 at 25 °C from either 1 or 3 with an excess of Zn(OAc)₂·2H₂O in 1:1 acetonitrile-water at pH 6.6 is found to be the same with k_obs = 1.6(2) × 10^-4 s^-1. The X-ray crystal structures of 3, 4, and 6 have been determined, although the structure determination of 3 was severely affected because of heavy disordering. In 3, the Fe-O-Fe angle is 168.6(6)°, while it is exactly 180.0° in 4 and 6. Cyclic and square-wave voltammetric (CV and SWV) measurements have been carried out for complexes 1-4 in acetonitrile. The variation of the solvent composition (acetonitrile-water) has a profound effect on the E_½ and ΔE_p values. The binding of an additional chloride ion to an iron(III) center in 1-3 is accompanied by a remarkable shift of E_½ to more negative values. The observation of quasi-reversible CV for complexes containing a Fe^III -O-Fe^III unit (3 and 4) indicates that in the electrochemical time scale unusual Fe^III-O-Fe^II is produced. The ¹H NMR spectra of complexes 3-6 exhibit hyperfine-shifted signals in the range 0-90 ppm with similar features. The metal-hydrogen distances obtained from T₁ measurements are in good agreement with the crystallographic data. Variable-temperature (2-300 K) magnetic susceptibility measurements carried out for 3 and 4 indicate strong antiferromagnetic exchange interaction (H = -2JS₁·S₂) between the high-spin iron(III) centers in the Fe-O-Fe unit with J =-114 cm^-1 (3) and -107 cm^-1 (4).