SPIN CONVERSION VERSUS ANTIFERROMAGNETIC INTERACTION IN IRON(III) TRINUCLEAR SPECIES - CRYSTAL-STRUCTURES AND MAGNETIC-PROPERTIES OF [FE-3(P-MEOPTRZ)(8)(H2O)(4)](BF4)(6) AND [FE-3(P-MEOPRTZ)(6)(H2O)(6)](TOS)(6) [P-MEOPTRZ=4-(P-METHOXYPHENYL)-1,2,4-TRIAZOLE, TOS=TOSYLATE]

By reaction of iron(II) salts with 4-(p-methoxyphenyl)-1,2,4-triazole, hereafter abbreviated as p-MeOprtz, two compounds have been synthesiz ed, namely [Fe-3(p-MeOptrz)(8)(H2O)4](BF4)(6) (1) and [Fe-3(p-MeOptrz) (6)(H2O)(6)](tos)(6) (tos = tosylate) (2a). The crystal structures hav e been solved at room temperature. Both compounds crystallize in the t riclinic system, space group P (1) over bar. The lattice parameters ar e as follows: for 1.2H(2)O, a = 15.107(7) Angstrom, b = 14.678(7), c = 12.392(6) Angstrom, alpha = 112.14(6)degrees, beta = 95.85(5)degrees, gamma 104.40(6)degrees, Z = 1; for 2a.2CH(3)OH.8H2O, a = 12.983(5) An gstrom, b = 16.324(7) Angstrom, c = 17.362(8) Angstrom, alpha = 105.28 (3)degrees, beta = 109.34(4)degrees, gamma = 106.94(4)degrees, Z = 1. Both structures consist of linear trinuclear cations with a +6 charge, and noncoordinated anions and solvent molecules. The central iron ato m lies on an inversion center and is triply bridged to each of the ext ernal iron atoms by the p-MeOptrz ligands through the nitrogen atoms i n the 1,2-positions. Each external iron atom completes its octahedral surroundings with one p-MeOptrz ligand and two water molecules in 1 an d with three water molecules in 2a. The average value of the dihedral angle between triazole and phenyl rings of the bridging ligands is equ al to 71.4 degrees in 1 and 33.2 degrees in 2a. Heating 2a at 100 degr ees C affords a compound 2b of formula [Fe-3(p-MeOptrz)6(H2O)3]-(tos)6 The magnetic properties of 1, 2a.4H(2)O, and 2b have been investigate d. For 1, the three iron(II) ions are high-spin in the whole temperatu re range. The temperature dependence of the molar magnetic susceptibil ity has beenquantitatively interpreted by full diagonalization of the spin Hamiltonian, taking into account the isotropic interaction betwee n nearest neighbor spin carriers, and the local anisotropies. The inte raction parameter between central and external iron(II) ions has been found at J = -5.7 cm(-1) (H = -JS(A).S-B). For 2a.4H(2)O and 2b the ce ntral iron(II) ion undergoes a gradual spin conversion centered at T-1 /2 = 245 and 330 K, respectively. This spin conversion has also been s tudied by Mossbauer spectroscopy. The switching between high-spin and spinconversion situations when passing from 1 to 2a has been attribute d to the variation of the dihedral angle within the p-MeOptrz bridging ligands, caused in this instance by the nature of the anion.