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)(HIm)(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-toluenebis(3-ethoxysalicylideneimine), L-2 = N,N'-4-chloro-o-phenylenebis(3-methoxysalicylideneimine), L-3 = N,N'-4-chloro-o-phenylenebis(3-ethoxysalicylideneimine), 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 = 14.354(2) Angstrom, c = 15.773(2) Angstrom. The structures of 1a and 4a are made up of discrete [Fe(L-1)(HIm)(2)](+) and [Fe(L-4)(HIm)(2)](+) cationic units, while 2c consists of dinuclear neutral entities [NaFe(L-2)(HIm)(2)(ClO4)(2)]. The iron(III) ion is six-coordinated, the equatorial plane being occupied by the cis tetradentate Schiff base and the axial positions by the imidazole Ligands. The metal-ligand bond distances as well as the relative orientation of the imidazole Ligands are discussed in relation with the spin state of the title compounds. The magnetic properties reveal that 1a-c, 2a, 3, and 4b are high-spin species while 2c is a low spin complex, and 2b and 4a display spin-crossover behavior. The thermodynamic model of Slichter and Drickamer was applied to account for the temperature variable high-spin molar fraction deduced from the magnetic behavior. The intermolecular interaction parameter, 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.