Stable Bimetallic Fe^II/{Fe(NO)₂}⁹ Moiety Derived from Reductive Transformations of a Diferrous-dinitrosyl Species

A dimeric dithiolate-bridged species, [Fe(NO)(PS2)]2 (1) containing two {FeNO}7 units, can be isolated by treating [Fe(CO)2(NO)2] with PS2H2 (PS2H2 = bis(2-dimercaptophenyl)phenylphosphine). Crystallographic studies reveal the syn-configuration of NO units and the bridging thiolates in the butterfly shape of the 2Fe2S core. Addition of PPh3 to the solution of dinuclear 1 leads to the formation of mononuclear {FeNO}7 [Fe(NO)(PS2)(PPh3)] (2) that shows electrochemical responses similar to those of 1. One-electron reduction of 1 with Cp*2Co or KC8 results in the isolation of thiolate-bridged bimetallic DNIC, [(PS2)Fe(μ-PS2)Fe(NO)2]- ([3]-), confirmed by several spectroscopies including single-crystal X-ray diffraction studies. The bimetallic DNIC [3]- is a rare example obtained from the one-electron reduction of a dinuclear Fe-NO {FeNO}7 model complex. With the assistance of redox behaviors of 2, electrochemical studies imply that the reduction of 1 leads to the formation of a mononuclear {FeNO}8 [Fe(NO)(PS2)(THF)]- intermediate, which involves disproportionation or NO- transfer to yield [3]-. Based on IR data and magnetic properties, the electronic structure of [3]- can be described as a FeII/{Fe(NO)2}9 state. Isolation of the {Fe(NO)2}9 moiety coordinated by the Fe ancillary complex lends strong support to the NO scrambling behavior in the effectiveness of the activity of flavodiiron nitric oxide reductases (FNORs).