898226-54-5Relevant articles and documents
Synthesis, reactivities, and magnetostructural properties of Fe III, FeIII-O-FeIII, and ZnIIFe IIIO-FeIIIZnII complexes of a tetraiminodiphenolate macrocycle
Biswas, Papu,Ghosh, Meenakshi,Dutta, Sujit K.,Floerke, Ulrich,Nag, Kamalaksha
, p. 4830 - 4844 (2008/10/09)
The mononuclear iron(III) complexes [Fe(LH2)(H 2O)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 ?Kaq [2]3+ + Cl- in acetonitrile is 8.65(5) M, and the binding constant (KCl-) for the reaction [1] 2+ + Cl- ?KCl- [1Cl]+ + CH3CN is 4.75(5) M. The pKD value for the dimerization reaction 2[2]3+ + 2OH- ?KCl- [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)2·2H 2O 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·S 2) between the high-spin iron(III) centers in the Fe-O-Fe unit with J = -114 cm-1 (3) and -107 cm-1 (4).
