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

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 (Kaq) [1]2+ + H2O [2]3+ + Cl- in acetonitrile is 8.65(5) M, and the binding constant (KCl−) for the reaction [1]2+ + Cl- [1Cl]+ + CH3CN is 4.75(5) M. The pKD value for the dimerization reaction 2[2]3+ + 2OH- [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 (kobs) 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 kobs = 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 E1/2 and ΔEp values. The binding of an additional chloride ion to an iron(III) center in 1−3 is accompanied by a remarkable shift of E1/2 to more negative values. The observation of quasi-reversible CV for complexes containing a FeIII −O−FeIII unit (3 and 4) indicates that in the electrochemical time scale unusual FeIII−O−FeII 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).