New Members of a Class of Iron−Thiolate−Nitrosyl Compounds:  Trinuclear Iron−Thiolate−Nitrosyl Complexes Containing Fe₃S₆ Core

The neutral trinuclear iron−thiolate−nitrosyl, [(ON)Fe(μ-S,S-C₆H₄)]₃ (1), and its oxidation product, [(ON)Fe(μ-S,S-C₆H₄)]₃[PF₆] (2), were synthesized and characterized by IR, X-ray diffraction, X-ray absorption, electron paramagnetic resonance (EPR), and magnetic measurement. The five-coordinated, square pyramidal geometry around each iron atom in complex 1 remains intact when complex 1 is oxidized to yield complex 2. Magnetic measurements and EPR results show that there is only one unpaired electron in complex 1 (S_total = ¹/₂) and no unpaired electron (S_total = 0) in 2. The detailed geometric comparisons between complexes 1 and 2 provide understanding of the role that the unpaired electron plays in the chemical bonding of this trinuclear complex. Significant shortening of the Fe−Fe, Fe−N, and Fe−S distances around Fe(1) is observed when complex 1 is oxidized to 2. This result implicates that the removal of the unpaired electron does induce the strengthening of the Fe−Fe, Fe−N, and Fe−S bonds in the Fe(1) fragment. A significant shift of the ν_NO stretching frequency from 1751 cm^-1 (1) to 1821, 1857 cm^-1 (2) (KBr) also indicates the strengthening of the N−O bonds in complex 2. The EPR, X-ray absorption, magnetic measurements, and molecular orbital calculations lead to the conclusion that the unpaired electron in complex 1 is mainly allocated in the Fe(1) fragment and is best described as {Fe(1)NO}⁷, so that the unpaired electron is delocalized between Fe and NO via d−π^* orbital interaction; some contributions from [Fe(2)NO] and [Fe(3)NO] as well as the thiolates associated with Fe (1) are also realized. According to MO calculations, the spin density of complex 1 is predominately located at the Fe atoms with 0.60, −0.15, and 0.25 at Fe(1), Fe(2), and Fe(3), respectively.