Electronic Structure and Transformation of Dinitrosyl Iron Complexes (DNICs) Regulated by Redox Non-Innocent Imino-Substituted Phenoxide Ligand

The coupled NO-vibrational peaks [IR nu(NO) 1775 s, 1716 vs, 1668 vs cm(-1) (THF)] between two adjacent [Fe(NO)(2)] groups implicate the electron delocalization nature of the singly O-phenoxide-bridged dinuclear dinitrosyliron complex (DNIC) [Fe(NO)(2)(mu-ON2Me)Fe(NO)(2)] (1). Electronic interplay between [Fe(NO)(2)] units and [ON2Me](-) ligand in DNIC 1 rationalizes that "hard" O-phenoxide moiety polarizes iron center(s) of [Fe(NO)(2)] unit(s) to enforce a "constrained" pi-conjugation system acting as an electron reservoir to bestow the spin-frustrated {Fe(NO)(2)}(9)-{Fe(NO)(2)}(9)-[(ON2Me)-O-](2-) electron configuration (S-total = 1/2). This system plays a crucial role in facilitating the ligand-based redox interconversion, working in harmony to control the storage and redox-triggered transport of the [Fe(NO)(2)](10) unit, while preserving the {Fe(NO)(2)}(9) core in DNICs {Fe(NO)(2)}(9)-[(ON2Me)-O-](2-) [K-18-crown-6-ether)][(ON2Me)Fe(NO)(2)] (2) and {Fe(NO)(2)}(9)-[(ON2Me)-O-] [(ON2Me)Fe(NO)(2)][PF6] (3). Electrochemical studies suggest that the redox interconversion among [{Fe(NO)(2)}(9)-[(ON2Me)-O-](2-)] DNIC 3 <-> [{Fe(NO)(2)}(9)-[ON2Me](-)] <-> [{Fe(NO)(2)}(9)-[(ON2Me)-O-]] DNIC 2 are kinetically feasible, corroborated by the redox shuttle between O-bridged dimerized [(mu-ONMe)(2)Fe-2(NO)(4)] (4) and [K-18-crown-6-ether)][(ONMe)Fe(NO)(2)] (5). In parallel with this finding, the electronic structures of [{Fe(NO)(2)}(9)-{Fe(NO)(2)}(9)-[(ON2Me)-O-](2-)] DNIC 1, [{Fe(NO)(2)}(9)-[(ON2Me)-O-](2-)] DNIC 2, [{Fe(NO)(2)}(9)-[(ON2Me)-O-]] DNIC 3, [{Fe(NO)(2)}(9)-[ONMe](-)](2) DNIC 4, and [{Fe(NO)(2)}(9)-[(ONMe)-O-](2-)] DNIC 5 are evidenced by EPR, SQUID, and Fe K-edge pre-edge analyses, respectively.