STRUCTURE AND PROPERTIES OF A TETRANUCLEAR IRON(III) CAGE COMPLEX - A MODEL FOR HEMERYTHRIN

The reaction of glutaric acid with LFe2CI6 (L = 1,4-bis(1,4,7-triaza-1-cyclononyl) butane) in the presence of base and KPF6 affords the novel tetranuclear cage complex [LFe2O(C5H604)]2(PF6)4 (2). Complex 2 forms dichroic (orange and green) crystals in the triclinic space group P1BAR (No. 2), with the following unit cell parameters: a = 11.141 (2) angstrom, b = 11.2344 (14) angstrom, c = 17.278 (2) angstrom, alpha = 72.648 (9)-degrees, beta = 74.477 (10)-degrees, gamma = 70.702 (10)-degrees, V = 1913.5 (4) angstrom3, and Z = 1. The structure of 2 contains two hemerythrin-like(mu-oxo)bis(mu-carboxylato)diiron(III) cores separated from one another by 7.748 (1) angstrom. Spectroscopic, magnetic, and electrochemical studies of 2 are presented and compared with those of the related tetranuclear complex [LFe2O(OAc)2]2(PF6)4(l). Both complexes 1 and 2 are shown to contain two magnetically-independent, antiferromagnetically-coupled diiron cores (1, J = -113 (2) cm-1; 2, J = -110 (2) cm-1), which are retained in solution. The redox behavior of 2 differs substantially from that of l as determined by cyclic voltammetry. Reduction of l results in a cathodic peak at -620 mV versus Ag/AgCl for which there is no corresponding anodic peak. In 2, however, there are two reduction peaks at -500 and -635 mV versus Ag/AgCl, respectively, which correspond, as seen in the reversed scan, to two oxidation peaks at -360 and -580 mV. This result indicates that the cage complex 2 is more stable to electrochemical reduction than complex 1.