Synthesis, structure and magnetism of a tetranuclear Fe(III) complex containing an [Fe 4(μ 3-O) 2] 8+ core

Abstract

The tetranuclear Fe(III) complex [L 2Fe 2(μ 3-O) 2(μ 2-CH 3COO) 3(Sao) 2Fe 2]X (X=ClO 4, ( 1), PF 6 ( 2)), where L is the cyclic amine 1,4,7-trimethyl-1,4,7-triazacyclononane and Sao is the dianion of salicylaldoxime (H 2Sao), has been synthesized and characterized on the basis of elemental analysis, IR, electronic, Mössbauer spectroscopy and variable-temperature (4–285 K) magnetic susceptibility measurements. The molecular structure has been established by X-ray diffraction of 2. The structure consists of an [Fe 2 wing(μ 3-O) 2Fe 2 body] 8+-butterfly core. Each Fe(III) ion is in a distorted octahedral environment. The two Fe(Sao) + units function as bridges between two terminal ‘wing’ iron ions through their deprotonated oximate oxygen and represent the ‘body’ iron ions in the tetranuclear core. Three acetate groups are singly bridging between the iron ions. Mössbauer spectra display two quadrupole doublets, consistent with two different high-spin ferric sites. Magnetic susceptibility measurements reveal a diamagnetic ground state with antiferromagnetic exchange interactions among the four high-spin ferric centers. The exchange coupling constant J 12 (‘body-body’ interaction) could not be determined due to prevailing spin frustration, but the ‘wing-body’ antiferromagnetic interaction, J, was evaluated to be −46 cm −1 [ H̆=−2 J( S̆ 1· S̆ 3+ S̆ 2· S̆ 3+ S̆ 1· S̆ 4+ S̆ 2· S̆ 4)−2 J 12 S̆ 1· S̆ 2].