Tunneling in hydrogen atom transfer reactions of ruthenium complexes, nitroxyl radicals, and hydroxylamines.

摘要

This thesis focuses on the development of a novel ruthenium hydrogen atom transfer (HAT) system. Chapter 1 describes syntheses and characterizations of both ruthenium acetylacetonato and hexafluoroacetylacetonato systems with protonated and deprotonated pyridine-imidazole ligands, including thermodynamic measurements and determinations of the bond dissociation free energies (BDFEs). RuII(acac)2(py-imH) (RuIIimH ), [RuIII(acac)2(py-imH)]OTf, RuIII (acac)2(py-im) (RuIIIim), Ru II(hfac)2(py-imH), and [DBU-H] [RuII(hfac) 2(py-im)] have been synthesized and fully characterized (acac = 2,4-pentanedionato, hfac = 1,1,1,5,5,5-hexafluoro-2,4pentanedionato, py-imH = 2-(2'-pyridyl)imidazole, DBU = 1,8diazabicyclo[5.4.0]undec-7-ene). From the E1/2 and p Ka values, the BDFEs of the N--H bonds in RuIIimH and RuII(hfac)2(py-imH) are calculated to be 62.0 and 79.6 kcal mol-1 in MeCN---a remarkable difference of 17.6 kcal mol-1 for such structurally similar compounds. Consistent with these values, there is facile net hydrogen atom transfer from RuIIimH to TEMPO· (2,2,6,6-tetramethylpiperidine-l-oxyl radical) in MeCN to give RuIIIim and the hydroxylamine TEMPO-H (OG° = --4.5 kcal mol-1). RuII(hfac)2(py-imH) is not oxidized by TEMPO· (DeltaG° = +13.1 kcal mol-1), but in the reverse direction TEMPO-H readily reduces in situ generated RuIII(hfac)2(py-im).;The HAT reaction of RuIIimH with TEMPO · in MeCN has a surprisingly large hydrogen/deuterium kinetic isotope effect (KIE) of kH/k D = 23 +/- 3 at 298 K, which indicates tunneling. Based on the Marcus cross relation, self-exchange reactions were examined, and the pseudo self-exchange reaction of 4-oxo-TEMPO· (2,2,6,6-tetramethyl-4-oxo-piperidine-l-oxyl radical) and TEMPO-H also exhibits a large KIE. Chapter 2 describes temperature dependent thermodynamic and kinetic measurements of pseudo self-exchange reactions of nitroxyl radicals and hydroxylamines, and investigates large KIEs and tunneling mechanisms in these reactions. Chapter 3 presents the kinetics and tunneling for the HAT reactions of RuIIimH(D) with TEMPO · and the self-exchange reactions between RuII imH(D) and RuIIIim, and application of the Marcus cross relation for both hydrogen and deuterium atom transfers.;A separate project involving high-valent Tp-OsVI nitrido complexes and their reactions with the nucleophiles hydroxide and thiosulfate are reported in Chapter 4 [Tp = hydrotris(1-pyrazolyl)borate].