PREPARATION AND REACTIVITY OF METAL NITROSYL COMPLEXES OF BOTH PORPHYRIN AND EQUATORIAL DICARBOXAMIDE N,N,N,N-CHELATES

Abstract

This dissertation explored the synthesis and characterization of metal–NO complexes using porphyrins and N,N,N,N-chelates as equatorial ligands. The metals employed in this study include the first-row transition metals (Fe, Cr, Mn) and Ru (which stabilizes low-spin states). Reactivity studies included electrophilic and nucleophilic attacks, as well as NO coupling. A combination of experimental techniques including IR, UV-vis and NMR spectroscopy, X-ray crystallographic analysis, cyclic voltammetry and IR spectroelectrochemistry were employed. DFT calculations were also employed to complement the experimental results and elucidate reaction mechanisms.Chapter 1 briefly describes the importance of nitric oxide (NO) and its metal complexes (M-NO). The binding mode of NO is dependent on the metal and other ligands coordinated to that metal. This affects the molecular and electronic structures of these M-NO complexes which in turn influences the reactivity and function of the complexes. Chapter 2 describes the synthesis of new ruthenium compounds based on the N,N,N,N-chelate Me2bpbMe2 (bpb = 1,2-bis(pyridine-2-carboximido)benzene). These compounds were characterized by spectroscopic methods (1H NMR and IR spectroscopy) and X-ray crystallographic analysis. The carbonyl compound (Me2bpbMe2)Ru(CO)(H2O) was generated from the reaction of the free base Me2bpbMe2H2 with Ru3(CO)12 in refluxing DMF. Isoamyl nitrite reacts with this compound to yield the trans-addition nitrosyl alkoxide (Me2bpbMe2)Ru(NO)(O-i-C5H11). Nitrosothiols similarly add in a formal trans-addition manner to yield (Me2bpbMe2)Ru(NO)(SR/Ar) (SR/Ar = S-i-C5H11, SPh, SC6F4H, SC(Me)2CHNHC(O)Me) derivatives. The (Me2bpbMe2)Ru(NO)(O-i-C5H11) compound undergoes alkoxide exchange reactions with PhOH and HOC6F4H to generate (Me2bpbMe2)Ru(NO)(OPh) and (Me2bpbMe2)Ru(NO)(OC6F4H), respectively. The neutral alkoxide/aryloxide nitrosyl compounds exhibit higher NO bands (1809–1842 cm-1) relative to their thiolate analogues (1755–1823 cm-1). The X-ray crystal structures of (Me2bpbMe2)Ru(NO)(OPh), (Me2bpbMe2)Ru(NO)(OC6F4H), and (Me2bpbMe2)Ru(NO)(SPh), have been determined, and reveal near-linear Ru–N–O moieties (164–174°) consistent with these complexes being formulated as {RuNO}6 species. The electrooxidation behavior of (Me2bpbMe2)Ru(NO)(OC6F4H), (Me2bpbMe2)Ru(NO)(SC6F4H), and (Me2bpbMe2)Ru(NO)(SPh) were examined by cyclic voltammetry and IR spectroelectrochemistry in CH2Cl2. (Me2bpbMe2)Ru(NO)(OC6F4H) and (Me2bpbMe2)Ru(NO)(SC6F4H) display reversible first oxidations, whereas (Me2bpbMe2)Ru(NO)(SPh) displays an irreversible first oxidation with likely loss of the thiolate ligand. Chemical reactivity of (Me2bpbMe2)Ru(NO)(SPh) with H+ and Me+ results in the generation of the free thiol PhSH and thioether PhSMe, respectively. Interestingly, attempts to synthesize (Me2bpbMe2)Ru(NO)Cl from the precursor Ru(NO)Cl3 in ethanol revealed that this reaction was dependent on the ligand:Ru ratio. The (Me2bpbMe2)Ru(NO)Cl was only isolated using a ligand:Ru ratio of 1:1 and a ligand:Ru ratio of 1:2 resulted in the isolation of a unique bimetallic compound [Ru(NO)Cl3]2(-Me2HbpbHMe2) with an (N,O)2-dichelate binding mode. Chapter 3 of this dissertation highlights the use of (OEP)Cr(NO) in the Lewis-acid assisted N-N coupling reaction to generate N2O. DFT calculations were employed to elucidate the mechanism of this reaction. In this chapter, the synthesis of a series of new (por)Cr(NO) (por = TMP, OEP, TTP, TPP, T(p-OMe)PP) compounds using two synthetic approaches is described. In the first approach NO gas was introduced into a CH2Cl2 solution of the reduced (por)CrII and in the second approach an alkyl thionitrite was utilized as an NO transfer agent. These compounds are paramagnetic as confirmed in solution by the Evans method and they display strong NO bands in the range 1663-1682 cm-1. Single-crystal X-ray diffraction of the compounds (OEP)Cr(NO) and (TMP)Cr(NO)(MeOH) revealed essentially linear Cr-N-O moieties with bond angles ranging from 176-180°. A bis-NO crystal of (TPP)Cr(NO)2 (which represents the first structural characterization of a porphyrin chromium bis-NO complex) was obtained in a crystallization attempt to crystallize (TPP)Cr(NO). The structure of the bis-NO compound comprised of one linear and one bent NO ligand. It was shown that (OEP)Cr(NO) couples with an external NO molecule in the presence of the Lewis acid BF3 to generate N2O in yields ranging from 4-8%. It was shown, computationally, that the N-N bond formation between (OEP)Cr(NO) and external NO is promoted by BF3 coordination to the bound NO group. An N-bound hyponitrite intermediate is generated which dissociates and rebinds through the oxygen before eventually releasing N2O. This work was extended to (por)Mn(NO) and it was shown that (OEP)Mn(NO) also couples with an external NO molecule in the presence of BF3 to generate N2O (~19% yield). Chapter 4 details the nucleophilic attack reactions of C-based nucleophiles on the cationic ruthenium nitrosyl compounds [(por)Ru(NO)(1-MeIm)]OTf (por = OEP, T(p-OMe)PP). The aryl C-based nucleophile Ph- reacts with [(T(p-OMe)PP)Ru(NO)(1-MeIm)]OTf at the metal center to generate the organometallic compound (T(p-OMe)PP)Ru(NO)(Ph) as characterized by IR spectroscopy. Interestingly, the reactivity of the alkyl C-based nucleophile iPrMgCl was influenced by the age of the reagent. For example, a reagent which had been in the refrigerator for more than a year reacted with [(T(p-OMe)PP)Ru(NO)(1-MeIm)]OTf at both the Ru center and the nitrosyl N-atom to generate (T(p-OMe)PP)Ru(NO)(iPr) and (T(p-OMe)PP)Ru(iPrNO)(1-MeIm), respectively. Reactions of both [(T(p-OMe)PP)Ru(NO)(1-MeIm)]OTf and [(OEP)Ru(NO)(1-MeIm)]OTf with a freshly opened bottle of iPrMgCl resulted in the generation and isolation of only the (por)Ru(iPrNO)(1-MeIm) product. The formation of these products was confirmed by 1H NMR and IR spectroscopy. This chapter also explored the structural and reactivity relationship between the nitrosyl (NO+) and the valence isoelectronic aryldiazonium (ArN2+) moieties. New aryldiazonium ruthenium porphyrin compounds [(por)Ru(PhN2)(L)]X (por = T(p-OMe)PP, OEP; L = H2O, py, 1-MeIm; X = BF4, PF6, OTf) were synthesized and characterized with 1H NMR and IR spectroscopy, and by X-ray crystallographic analysis. Structural characterization of [(T(p-OMe)PP)Ru(PhN2)(H2O)]BF4, [(OEP)Ru(PhN2)(py)]BF4, and [(OEP)Ru(PhN2)(1-MeIm)]PF6 reveal essentially linear Ru-N-N (