Multinuclear Fe(III) Complexes with Polydentate Ligands of the Family of Dicarboxyimidazoles: Nuclearity- and Topology-Controlled Syntheses and Magneto-Structural Correlations

Abstract

Two polydentate ligands of the family of dicarboxyimidazoles, H(2)MeDCBI (= 4,5-dicarboxy-1-methyl-1H-imidazole) and H(3)DCBI (= 4,5-dicarboxyimidazole), have been used in reactions with the [Fe(3,5-(t)()Bu(2)salpn)](+) species {3,5-(t)Bu(2)salpn = the dianion of 1,3-bis-[(3,5-di-tert-butylsalicylidene)amino]propane} to synthesize selectively complexes of different nuclearities. Four complexes have been synthesized: the mononuclear complex [Fe(3,5-(t)Bu(2)salpn)(HMeDCBI)] (1), the two binuclear but topologically different complexes [Fe(3,5-(t)Bu(2)salpn)(MeDCBI)Fe(3,5-(t)Bu(2)salpn)] (2) and {[Fe(3,5-(t)Bu(2)salpn)](2)(HDCBI)} (3), and the trinuclear complex {[Fe(3,5-(t)Bu(2)salpn)](2)(DCBI)Fe(3,5-(t)Bu(2)salpn)} (4). The structures of these complexes have been determined by X-ray crystallography. Variable-temperature direct-current magnetic susceptibility measurements were conducted for all compounds to obtain information about their electronic structure and to investigate the extent of magnetic communication among the Fe(III) centers. The results of these measurements allowed us to correlate the different structural motifs with the possible magnetic interactions that arise in multinuclear complexes of dicarboxyimidazoles. For 1, the room-temperature chi(M)T value reveals an S = (5)/(2) ground state. The data for the binuclear but topologically different complexes 2 and 3, and the trinuclear complex 4 suggest that weak intramolecular antiferromagnetic interactions are present, with interaction parameters ranging from -3.6 to -5.1 cm(-1). Differences in the extent of the magnetic communication between the metal centers through the two different interaction pathways of the ligands MeDCBI and DCBI (either through the imidazole ring or through the carboxylate groups) have been observed in complexes 2-4 that can be explained by the structural differences observed in the crystal structures of these compounds (the separation of the metal centers and the coplanarity of the metal ion orbitals with the pi system of the ligands). Cyclic voltammetry measurements for the mononuclear compound 1 show an irreversible reduction wave that is attributed to Fe(3+) + e(-) --> Fe(2+). The electrochemical behavior of the multinuclear complexes 2-4 is more complicated; however, it indicates that there is a degree of electronic communication between the Fe(III) centers.