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

Two polydentate ligands of the family of dicarboxyimidazoles, H2MeDCBI (= 4,5-dicarboxy-1-methyl-1H-imidazole)
and H3DCBI (= 4,5-dicarboxyimidazole), have been used in reactions with the [Fe(3,5-tBu2salpn)]+ species {3,5-tBu2salpn = 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-tBu2salpn)(HMeDCBI)] (1), the two binuclear but topologically different complexes [Fe(3,5-tBu2salpn)(MeDCBI)Fe(3,5-tBu2salpn)] (2) and {[Fe(3,5-tBu2salpn)]2(HDCBI)} (3), and the trinuclear complex {[Fe(3,5-tBu2salpn)]2(DCBI)Fe(3,5-tBu2salpn)} (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 χMT 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 π system of the ligands).
Cyclic voltammetry measurements for the mononuclear compound 1 show an irreversible reduction wave that is
attributed to Fe3+ + e- → Fe2+. 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.