This dissertation describes the preparation, characterization, and reactivity of several critical unstable intermediates of relevance to the global N-cycle. Their spectroscopic signatures and chemical reactivities were used to correlate and delineate their observed biological effects and functions in nature. Chapter 1 introduces the importance of nitric oxide (NO) in physiology and its involvement in the N-cycle. The chapter briefly overviews the complexity of metal-mediated NOx activation in physiology, agriculture, and the environment. Also described in this chapter are significant areas of N-chemistry that are currently underexplored, but important to study if we are to move the field forward.
Chapter 2 highlights the reactions of organic C-NONOate compounds with iron porphyrins. NONOates (diazeniumdiolates) containing the [X{N2O2}]– functional group are frequently employed as nitric oxide (NO) donors in biology, and some NONOates have been shown to bind to metalloenzymes. In this chapter, the preparation, crystal structures, detailed magnetic behavior, redox properties, and reactivities of two alkyl C-NONOate complexes of heme models, namely (OEP)Fe(2-ON(t-Bu)NO) (1) and (TPP)Fe(2-ON(t-Bu)NO) (2) (OEP = octaethylporphyrinato dianion, TPP = tetraphenylporphyrinato dianion) are presented. The compounds display the unusual NONOate O,O-bidentate binding mode for porphyrins, resulting in significant apical Fe displacements (+0.60 Å for 1, and +0.69 Å for 2) towards the axial ligands. Magnetic susceptibility and magnetization measurements recorded in the 1.8–300 K temperature range at magnetic fields from 0.02 to 5T, yielded magnetic moments of 5.976 and 5.974 Bohr magnetons for 1 and 2, respectively, clearly identifying them as high-spin (S = 5/2) ferric compounds. Variable-frequency (9.4 GHz and 34.5 GHz) EPR measurements, coupled with computer simulations, confirmed the magnetization results and yielded more precise values for the spin Hamiltonian parameters : gavg = 2.00 ± 0.03, |D| = 3.89 ± 0.09 cm-1, and E/D = 0.07 ± 0.01 for both compounds, where D and E are the axial and rhombic zero field splittings. IR spectroelectrochemistry studies reveal that the first oxidations of these compounds occur at the porphyrin macrocyles and not at the Fe-NONOate moieties. Reactions of 1 and 2 with a histidine mimic (1-MeIm) generates RNO and NO, both of which may bind to the metal center if sterics allow, as shown by a comparative study with the Cupferron complex (T(p-OMe)PP)Fe(2-ON(Ph)NO). Protonation of 1 and 2 yields N2O as a gaseous product, presumably from the initial generation of HNO that dimerizes to the observed N2O product.
Chapter 3 focuses on probing the previously unknown key reaction steps and intermediates in the NO to N2O reduction by fungal cyt P450 nitric oxide reductase (NOR) using heme models. Low temperature IR and 1H NMR spectroscopic characterizations of (por)Fe(HNO)(L) (por = OEP, PPDME, TTP; L = 5-MeIm, 1-MeIm, Im) derivatives from the reactions of hydride with ferric−NO species are described. The NO bands of the heme model Fe−HNO products are in the range of 1381 cm-1 to 1389 cm-1. 1H NMR spectroscopy, a more sensitive technique for the bound HNO, shows mild cis and trans effects in the chemical shifts observed in the 13.65−14.26 ppm range. The NO bands, 1H NMR chemical shifts, and coupling constants (J15N-H ~77 Hz) of the ferrous Fe−HNO derivatives are close to those from literature values. Results from the DFT calculations are consistent with the direct attack of H− at the nitrosyl N-atom to form the Fe−HNO products. Interestingly, these Fe−HNO species exhibit varied decomposition pathways, one involving H2 formation from Fe(N−H)O bond cleavage and the other involving Fe−N(H)O bond cleavage to generate N2O gas. Importantly, these Fe−HNO species react with external NO to form N2O in which the central N-atom and O-atom originate from the external NO reagent, and the terminal N-atom derives from the bound HNO. This is the first experimental evidence of the N−N coupling step in heme models supporting Fe−HNO as an active intermediate in NO reduction to N2O catalyzed by cyt P450nor. Differential reactivity of six- and five-coordinate ferric heme−NO models with hydride is also described in this chapter. The formation of Fe−HNO from hydride attack at the bound six-coordinate ferric nitrosyl was shown to be thermodynamically and kinetically favorable. However, for the five-coordinate case, although Fe−HNO formation is thermodynamically favored, (NO)Fe−H formation is the kinetically favored and experimentally observed outcome. The last chapter (Chapter 4) of my dissertation centers on the transformation of metal−NO to metal−RNO derivatives via attack of C-based nucleophiles to form new carbon−nitrogen bonds. C−based nucleophiles react at two sites of ferric−NO compounds, (i) at the nitrosyl N-atom to afford a low yield ferrous−RNO derivative, and (ii) at the Fe center to form a high yield organometallic Fe−phenyl product. In the case of the Ru-analogues, the nucleophilic reactions of {RuNO}6 species with phenyl anions result in a reasonable yield of the Ru-PhNO product. The (OEP)Ru(PhNO)(5-MeIm) product from the reaction of [(OEP)Ru(NO)(5-MeIm)]BF4 with the phenyl nucleophile was characterized by IR spectroscopy, and its molecular structure confirmed by X-ray crystallography. The formation of organic RNO compounds from the inorganic metal−NO precursors via nucleophilic attack by C−based nucleophiles represents the first experimental evidence of the conversion of inorganic-NOx to organo-NO derivatives mediated by heme models. This chemistry was extended to nitrogen−nitrogen bond formation in these systems using an N−based nucleophile (NaN3). Nucleophilic attack of azide (N3−) at a bound NO+ in {MNO}6 (M = Fe, Ru) species generates N2O gas as detected by gas phase IR spectroscopy.