{Fe(NO)2}9 Dinitrosyl Iron Complex Acting as a Vehicle for the NO Radical

To carry and deliver nitric oxide with a controlled redox state and rate is crucial for its pharmaceutical/medicinal applications. In this study, the capability of cationic {Fe­(NO)2}9 dinitrosyl iron complexes (DNICs) [(RDDB)­Fe­(NO)2]+ (R = Me, Et, Iso; RDDB = N,N′-bis­(2,6-dialkylphenyl)-1,4-diaza-2,3-dimethyl-1,3-butadiene) carrying nearly unperturbed nitric oxide radical to form [(RDDB)­Fe­(NO)2(•NO)]+ was demonstrated and characterized by IR, UV–vis, EPR, NMR, and single-crystal X-ray diffractions. The unique triplet ground state of [(RDDB)­Fe­(NO)2(•NO)]+ results from the ferromagnetic coupling between two strictly orthogonal orbitals, one from Fe dz2 and the other a π*op orbital of a unique bent axial NO ligand, which is responsible for the growth of a half-field transition (ΔMS = 2) from 70 to 4 K in variable-temperature EPR measurements. Consistent with the NO radical character of coordinated axial NO ligand in complex [(MeDDB)­Fe­(NO)2(•NO)]+, the simple addition of MeCN/H2O into CH2Cl2 solution of complexes [(RDDB)­Fe­(NO)2(•NO)]+ at 25 °C released NO as a neutral radical, as demonstrated by the formation of [S5Fe­(NO)2]− from [S5Fe­(μ-S)2FeS5]2–.