New members of a class of dinitrosyliron complexes (DNICs): the characteristic EPR signal of the six-coordinate and five-coordinate {Fe(NO)2}9 DNICs

Abstract

Compared to the tetrahedral {Fe(NO)(2)}(9) dinitrosyliron complexes (DNICs) [(L)(2)Fe(NO)(2)](-) (L=SR, imidazolate) displaying EPR signal g=2.03, the newly synthesized six-/five-coordinate {Fe(NO)(2)}(9) DNICs [(TPA)Fe(NO)(2)][BF(4)] (1-TPA) (TPA=2-[CH(2)-C(5)H(4)N](3)N), [((iPr)PDI)Fe(NO)(2)][BF(4)] (2-(iPr)PDI) ((iPr)PDI=2,6-[2,6-(i)Pr(2)-C(6)H(3)N=CMe](2)C(5)H(3)N) and [(PyImiS)Fe(NO)(2)] (4-PyImiS) (PyImiS=2-[2-(C(5)H(4)N)CMe=N]C(6)H(4)S) exhibit the distinct EPR signal g=2.015-2.018. The Fe K-edge pre-edge energy (7113.4-7113.6eV) derived from the 1s→3d transition in the octahedral and square-pyramidal environment of the Fe center, falling within the range of 7113.4-7113.8eV for the tetrahedral {Fe(NO)(2)}(9) DNICs, implicates that the iron cores of DNICs 1-TPA, 2-(iPr)PDI and 4-PyImiS are tailored to minimize the electronic changes accompanying changes in coordination geometry. In contrast to the thermally stable 1-TPA, 2-(iPr)PDI and 4-PyImiS, the spontaneous transformation of the proposed thermally unstable five-coordinate {Fe(NO)(2)}(9) DNIC [(PyPepS-H)Fe(NO)(2)](-) (6-PyPepS) (PyPepS-H=[SC(6)H(4)-o-NC(O)(C(5)H(4)N)]) into the {Fe(NO)}(7)-{Fe(NO)}(7) [(μ-PyPepS-H)Fe(NO)](2) (7) along with release of nitroxyl demonstrates that the distinct electron-donating ability of the coordinated ligands ([PyPepS-H]>[PyImiS]~[TPA]>[(iPr)PDI]) regulates the stability and geometric structure of {Fe(NO)(2)}(9) DNICs. This study also shows the aspect of how the geometric structure of {Fe(NO)(2)}(9) DNICs imposed by the electron-donating ability and conformation of the coordinated ligands (tridentate [(iPr)PDI] vs tridentate [PyImiS] vs tetradentate [TPA] vs tridentate [PyPepS-H] vs bidentate [SC(6)H(4)-o-NC(O)Ph](2-)) regulates the Fe-NO bonding of {Fe(NO)(2)}(9) DNICs and presumably the release of nitroxyl from DNICs.