Highest Recorded N-O Stretching Frequency for 6-Coordinate {Fe-NO}7 Complexes: An Iron Nitrosyl Model for His3 Active Sites

We report the synthesis, structure, and reactivity of [Fe(T1Et4iPrIP)(OTf)(2)] [1; T1Et4iPrIP = tris(1-ethyl-4-isopropylimidazolyl)phosphine]. Compound 1 reacts reversibly with nitric oxide to afford [Fe-(T1Et4iPrIP)(NO)(THF)(OTf)](OTf) (2), which is the first example of a 6-coordinate {FeNO}(7) S = 3/2 complex containing a linear Fe-N-O group. 2 exhibits the highest nu(NO) for compounds in this class. Density functional theory studies reveal an enhanced degree of beta-electron transfer from pi*(NO) to the Fe d orbitals accounting for the large stretching frequency.