Interactions of Iron Dinitrosyl Compounds with Imidazole and its Derivatives

Abstract

<p> Nitric oxide has been implicated in a number of biological processes, the majority of them involving iron nitrosyl complexes. The urgency then is to further study and characterize these complexes to further the understanding of biological mechanisms. However, the chemical sensitivity of these species precludes the purification and isolation of these compounds which, unfortunately, has directed the trend to merely detecting the presence of these compounds rather than isolating them. To this day, a large number of Electron Paramagnetic Resonance (EPR) detectable, biological compounds have not been isolated.</p> <p> To this end, the series of biologically relevant compounds of the form Fe(NO)2(L)2 [ L = imidazole 1, 1-methylimidazole (1-MeIm) 2, 4-methylimidazole (4-MeIm) 3, benzimidazole (benzim) 4, and 5,6-dimethylbenzimidazole (56benzim) 5 ] have been synthesized by direct reaction of the appropriate imidazole ligand with Fe(NO)2(CO)2. The compounds were extremely air sensitive, both in solution and as a dry solid. This hindered attempts to purify these compounds and so, infra red (IR), nuclear magnetic resonance (NMR) and EPR spectroscopic studies were undertaken of 2:1 reaction mixtures of the appropriate imidazole ligand and Fe(NO)2(CO)2. These studies revealed that the rapid substitution of the carbonyl ligands is facilitated by a catalytic, 17-electron, electron transfer chain mechanism (ETC), where the imidazole ligand acts to oxidize the 18-electron complex into the active 17-electron Fe(NO)2(CO)2+ species.</p> <p> In the course of the EPR study of 2, crystals formed that were suitable for single-crystal, X-ray diffraction. The compound crystallizes with a monoclinic unit cell, in the C2/c space group with unit cell dimensions: a= 13.985(5) Å, b = 11.529 (5) Å, c = 15.471(4) Å, α= 90°, β= 91.72(2)°, γ = 90°, V = 2493(2) Å^3, Z = 8. During the course of study of 6, crystals suitable for single crystal X-ray diffraction were obtained. The compound crystalizes with a monoclinic unit cell, in the P2/c space group and unit cell dimensions: a= 11.707(9) Å, b = 8.1783 (5) Å, c = 17.2489 (13) Å, α= 90°, β= 106.562 (1)°, γ = 90°, V = 1583.0 (2) Å^3 A, Z = 2.</p> <p> A relatively new mass spectrometry (MS) procedure was utilized for the reaction mixtures of 1 - 5, which involved a combination of electrochemical oxidation and electrospray. The method proved very useful, yielding data that could not be obtained by other MS techniques. Oligomeric species of the form L-[(Fe(NO)2L]x (x = 2,3,4,5 or 6), were also detected by MS for each compound reaction mixture. The oligomers involved linear chains of iron dinitrosyl fragments linked via the imidazole nitrogens. However, it is believed that these oligomers are produced as a result of the conditions met by the mass spectrometer.</p> <p> A reaction of Fe(NO)2(PPh3)(CO) with 1-MeIm was also performed in hopes of producing a more stable mono-substituted complex. However, the reaction also proceeded via an Electron Transfer Chain (ETC) pathway to produce Fe(NO)2(PPh3)2 6.</p>