Self-Assembly of Dinitrosyl Iron Units into Imidazolate-Edge-Bridged Molecular Squares: Characterization Including Mössbauer Spectroscopy

Imidazolate-containing {Fe(NO)2}9 molecular squares have been synthesized by oxidative CO displacement from the reduced Fe(CO)2(NO)2 precursor. The structures of complex 1 [(imidazole)Fe(NO)2]4, (Ford, Li, et al.; Chem. Commun. 2005, 477–479), 2 [(2-isopropylimidazole)Fe(NO)2]4, and 3 [(benzimidazole)Fe(NO)2]4, as determined by X-ray diffraction analysis, find precise square planes of irons with imidazolates bridging the edges and nitrosyl ligands capping the irons at the corners. The orientation of the imidazolate ligands in each of the complexes results in variations of the overall structures, and molecular recognition features in the available cavities of 1 and 3. Computational studies show multiple low energy structural isomers and confirm that the isomers found in the crystallographic structures arise from intermolecular interactions. EPR and IR spectroscopic studies and electrochemical results suggest that the tetramers remain intact in solution in the presence of weakly coordinating (THF) and noncoordinating (CH2Cl2) solvents. Mössbauer spectroscopic data for a set of reference dinitrosyl iron complexes, reduced {Fe(NO)2}10 compounds A ((NHC-iPr)2Fe(NO)2), and C ((NHC-iPr)(CO)Fe(NO)2), and oxidized {Fe(NO)2}9 compounds B ([(NHC-iPr)2Fe(NO)2][BF4]), and D ((NHC-iPr)(SPh)Fe(NO)2) (NHC-iPr = 1,3-diisopropylimidazol-2-ylidene) demonstrate distinct differences of the isomer shifts and quadrupole splittings between the oxidized and reduced forms. The reduced compounds have smaller positive isomer shifts as compared to the oxidized compounds ascribed to the greater π-backbonding to the NO ligands. Mössbauer data for the tetrameric complexes 1–3 demonstrate larger isomer shifts, most comparable to compound D; all four complexes contain cationic {Fe(NO)2}9 units bound to one anionic ligand and one neutral ligand. At room temperature, the paramagnetic, S = 1/2 per iron, centers are not coupled.