Effects of the μ4-bridging atom in a tetranuclear iron cluster on the activation of nitric oxide

Abstract

A series of tetrairon μ_4-fluoride clusters, [LFe_3F(PhPz)_3Fe][OTf]_n (n=1-3), was synthesized using the previously reported 1,3,5- triarylbenzene framework, a multinucleating ligand contg. pyridine and alkoxide donors, which bind three of the four metal centers. These three metal sites contain six-coordinate iron atoms. 3-phenyl-pyrazolate ligands (PhPz) bridge the triiron core to a four-coordinate, site-differentiated iron center. These clusters have notably different redox properties than the closely related μ_4-oxo analogs, [LFe_3O(PhPz)_3Fe][OTf]_n (n=0-3). While for [LFe_3O(PhPz)_3Fe][OTf]_2 three reversible redox events are obsd. at -1.73 V (vs. Fc /Fc+), -0.72 V, and +0.02 V (Fe^(II)_4 Fe^(II)Fe^(III)_3), only two redox events are obsd. for the fluoride-bridged cluster, [LFe_3F(PhPz)_3Fe][OTf], at -0.52 V (Fe^(II)_4 /Fe^(II)_3Fe^(III)) and +0.17 V, (Fe^(II)_3Fe^(III)/Fe^(II)_2 Fe^(III)_2). Each redox state of the fluoridebridged clusters, [LFe_3F(PhPz)_3Fe][OTf]_n (n=1-3), was isolated and further investigated through spectroscopy and X-ray diffraction. These studies reveal that all of the oxidn. state changes occur exclusively at one of the six- coordinate iron sites in the triiron core. Treatment of the above complexes with nitric oxide (NO) results in clusters with ν_(N-O) (1823 and 1842 cm^(-1)) higher than the corresponding oxo-bridged clusters of the same oxidn. states. The effects of the interstitial atom on small mol. activation will be discussed.