Syntheses, molecular structures, and fiber-optic infrared spectroelectrochemistry of nitrosyl metalloporphyrins containing axially bound alkoxide, thiolate, imidazole, and imidazolate ligands.

Abstract

This dissertation describes the syntheses, molecular structures, and infrared spectroelectrochemistry of nitrosyl ruthenium and osmium porphyrins containing axially bound alkoxide, thiolate, imidazole, and imidazolate ligands.

Chapter 3 describes the syntheses of the neutral and cationic (por)Ru(NO)(RIm) (por = TPP, TTP, T(p-OMe)PP), OEP; RIm = imidazolate (Im), imidazole (HIm), 1-methylimidazole (1-MeIm), 4-methylimidazole (4-MeHIm), 5-methylimidazolate (5-MeIm), and 5-methylimidazole (5-MeHIm)) compounds. The neutral imidazolate (por)Ru(NO)(RIm) (R = nothing, 4-Me) complexes were prepared from the addition of the corresponding RHIm to (por)Ru(NO)(O- i-C5H11). The cationic imidazole [(por)Ru(NO)(RIm)] + complexes were prepared from the solvent-free addition of RHIm (R = nothing, 1-Me, 4-Me) to the precursor [(por)Ru(NO)]+ complex. Both the neutral imidazolate and cationic imidazole complexes were characterized by IR and 1H NMR spectroscopy, and by FAB or ESI mass spectrometry. The IR spectra (in CH2Cl2) of the neutral imidazolate complexes displayed nuNO bands in the 1845--1855 cm-1 range, while the cationic imidazole complexes displayed similar bands in the 1854--1877 cm-1 range. Bands in these ranges are suggestive of linear NO ligands in these types of complexes. The 1H NMR spectra of the product that resulted from addition of 4-methylimidazole to [(OEP)Ru(NO)]+ resulted in two sets of peaks that were assigned to the structurally isomeric [(OEP)Ru(NO)(4-MeHIm)]+ and [(OEP)Ru(NO)(5-MeHIm)] + complexes. Further, we were able to determine that the less sterically stable 4-MeHIm containing isomer undergoes a first order dissociation of the 4-MeHIm ligand from the Ru center with the rate constant (k) of 1.44 x 10-5 s-1 with a half-life (t1/2) of 4.81 x 104 s. This was followed by the rebinding of the ligand to the metal center to form the [(OEP)Ru(NO)(5-MeHIm)]+ isomer. The solid-state crystal structures of the imidazolate and imidazole adducts of the (OEP)Ru(NO) complexes were determined by single-crystal X-ray crystallography.

Chapter 2 describes the syntheses of (OEP)Ru(NO)(XR) (XR = OEt, SEt, S-i-C5H11, SPh) complexes and the redox behavior of the osmium and ruthenium compounds (OEP)M(NO)(OEt) and (OEP)M(NO)(SEt) (M = Os, Ru), as determined by cyclic voltammetry and infrared spectroelectrochemistry. The (OEP)Ru(NO)(XR) complexes were prepared in 61--85% yields through the formal trans addition of RSNO to (OEP)Ru(CO) with loss of CO. These nitrosyl alkoxide and thiolate complexes were characterized by IR and 1H NMR spectroscopy, and by ESI mass spectrometry. Infrared spectroelectrochemical studies revealed that the (OEP)Os(NO)(OEt) compound undergoes a single reversible oxidation process in dichloromethane. In contrast, the thiolate compound (OEP)Os(NO)(SEt) undergoes a net irreversible oxidation resulting in formal loss of the SEt ligand. Extended Huckel calculations on crystal structures of these two compounds provide insight into the nature of their HOMOs. In the case of the alkoxide compound, the HOMO is largely metalloporphyrin centered. However, the HOMO of the thiolate compound consists of a pi bonding interaction between the metal dxz orbital and the px orbital on the sulfur, and a pi antibonding interaction between the metal d orbital and a pi* orbital on NO. The redox behavior of the Ru analogues have been determined, and are compared with those of the Os compounds.

Chapter 4 describes the electrochemistry and infrared spectroelectrochemistry of (OEP)Ru(NO)(Im) and [(OEP)Ru(NO)(RIm)]+ (R = H, 1-Me). Electrochemical oxidation of the (OEP)Ru(NO)(Im) complex in CH2Cl2 displayed a partially reversible single electron transfer centered on the porphyrin followed by an apparent radical hydrogen extraction from the solvent by the nitrogen atom of the imidazolate ring to form the cationic [(OEP)Ru(NO)(HIm)] + derivative. The [(OEP)Ru(NO)(HIm)]+ and [(OEP)Ru(NO)(1-MeIm)] + complexes, on the other-hand, exhibit porphyrin based reversible single electron transfers upon oxidation. However, the reduction of the [(OEP)Ru(NO)(HIm)] + complex proceeds through a partially reversible single electron transfer followed by an apparent H· extraction from the nitrogen atom of the imidazole by the solvent system to form the neutral (OEP)Ru(NO)(Im) derivative. The reduction of the 1-MeIm complexes proceeds through a partially reversible single electron transfer that was followed by the loss of the bond between 1-MeIm ligand the ruthenium center of the porphyrin. Extended Huckel calculations on crystal structures of the (OEP)Ru(NO)(Im) and [(OEP)Ru(NO)(HIm)]+ complexes provides insight on their redox properties. The calculated HOMOs for the isoelectronic structures show charge centered on the porphyrin, providing supporting evidence for the generation of porphyrin-centered pi-radical cations upon oxidation. The calculated LUMO of the cationic complex, however, suggests that the reduction first occurs on the metal-nitrosyl fragment of the [(OEP)Ru(NO)(HIm)] + complex, increasing the antibonding interaction between the metal dxz and pi*. orbital on NO. Presumably, the resulting unfavorable bent Ru-NO- like geometry is relieved by an electronic rearrangement that leads to the extraction of H· from the nitrogen of the imidazole by the solvent system.

Chapter 1 introduces the fundamental issues involved in the chemistry of Group 8 Fe, Ru, and Os nitrosyl porphyrin complexes with trans and O- and S-bound ligands, imidazole, and pyridine and their derivatives. Additionally, this chapter introduces important aspects of the biochemistry of the relevant nitrosyl adducts of heme proteins.