DIFFERENT GROUND SPIN STATES IN IRON(III) COMPLEXES WITH QUADRIDENTATE SCHIFF-BASES - SYNTHESIS, CRYSTAL-STRUCTURES, AND MAGNETIC-PROPERTIES

The synthesis, structure, and magnetic characterization of [Fe(L-1)(HI m)(2)] Y (Y = ClO4- (1a), PF6- (1b), and BPh4- (1c)), [Fe(L-2)(HIm)(2) ]ClO4 (2a), [Fe(L-2)(HIm)(2)]ClO4 . H2O (2b), [NaFe(L-2)(HIm)(2)(ClO4) (2)] (2c), [Fe(L-3)(HIm)(2)]ClO4 (3). [Fe(L-4)(HIm)(2)]ClO4 (4a), and [Fe(L-4)(HIm)(2)] BPh4. H2O (4b) are reported (L-1 = N,N'-3,4-tolueneb is(3-ethoxysalicylideneimine), L-2 = chloro-o-phenylenebis(3-methoxysa licylideneimine), L-3 = -chloro-o-phenylenebis(3-ethoxysalicylideneimi ne), L-4 = N,N'-1,2-propylenebis(3-methoxysalicylideneimine), and HIm = imidazole). Compound la crystallizes in the triclinic system, space group P (1) over bar, Z = 2, with a = 10.609(3) Angstrom. b = 10.762(3 ) Angstrom. c =15.043(3) Angstrom, alpha = 105.13(2)degrees. beta = 90 .67(2)degrees, and gamma = 102.09(2)degrees. Compound 2c crystallizes in the monoclinic system, space group C2/c. Z = 4, with a = 14.969(6) Angstrom, b = 17.324(6) Angstrom, c = 13.050(10) Angstrom, and beta = 100.85(6)degrees. Compound 4a crystallizes in the orthorhombic system, space group P2(1)2(1)2(1), Z = 4. with a 12.402(2) Angstrom, b = 14.3 54(2) Angstrom, c = 15.773(2) Angstrom. The structures of 1a and 4a ar e made up of discrete [Fe(L-1)(HIm)(2)](+) and [Fe(L-4)(HIm)(2)](+) ca tionic units, while 2c consists of dinuclear neutral entities [NaFe(L- 2)(HIm)(2)(ClO4)(2)]. The iron(III) ion is six-coordinated, the equato rial plane being occupied by the cis tetradentate Schiff base and the axial positions by the imidazole Ligands. The metal-ligand bond distan ces as well as the relative orientation of the imidazole Ligands are d iscussed in relation with the spin state of the title compounds. The m agnetic properties reveal that 1a-c, 2a, 3, and 4b are high-spin speci es while 2c is a low spin complex, and 2b and 4a display spin-crossove r behavior. The thermodynamic model of Slichter and Drickamer was appl ied to account for the temperature variable high-spin molar fraction d educed from the magnetic behavior. The intermolecular interaction para meter, Gamma, the enthalpy Delta H, and the entropy, Delta S, changes associated with the spin transition of 2b were estimated as Gamma = 1. 6 kJ mol(-1). Delta H = 12 kJ mol(-1). and Delta S = 60 J mol(-1) K-1.