Yes / UV/Vis absorption and NMR spectroscopy titrations have been used to investigate the formation of complexes between cations and neutral H-bond acceptors in organic solvents. Complexes formed by two different H-bond acceptors with fifteen different cations were studied in acetone and in acetonitrile. The effects of water and ion pairing with the counter-anion were found to be negligible in the two polar solvents employed for this study. The data were used to determine self-consistent H-bond donor parameters (α) for a series of organic and inorganic cations; guanidinium, primary, tertiary and quaternary ammonium, imidazolium, methylpyridinium, lithium, sodium, potassium, rubidium and caesium. The results demonstrate the transferability of α parameters for cations between different solvents and different H-bond acceptor partners, allowing reliable prediction of cation recognition properties in different environments. Lithium and protonated nitrogen cations form the most stable complexes, but the α parameter is only 5.0, which is similar to the neutral H-bond donor 3-trifluoromethyl, 4-nitrophenol (α = 5.1). Quaternary ammonium is the weakest H-bond donor investigated with an α value of 2.7, which is comparable to an alcohol. The α parameters for alkali metal cations decrease down the group from 5.0 (Li+) to 3.5 (Cs+). / Financial support from the Engineering and Physical Sciences Research Council (EP/K025627/2) and Unilever

Interpretation of crystallographic and mutational studies of antifreeze proteins (AFPs) requires molecular modeling of AFPs with ice. Most models proposed so far suggested H-bonds as the major driving force of AFP-ice association. However, the bulk water offers optimal network of H-bonds and van der Waals contacts to the isolated AFP and ice suggesting that corresponding components of free energy would not decrease upon AFP-ice association. In an attempt to resolve this controversy, we Monte Carlominimized complexes of several AFPs with taking into account, in addition to nonbonded interactions and H-bonds, the hydration potential for proteins (Augspurger and Scheraga, 1996). Parameters of the hydration potential for ice were developed basing on an assumption that at the melting temperature the free energy of water-ice association is small. Simulations demonstrate that desolvation of hydrophobic groups in the AFPs upon their fitting to the grooves at the ice surface presents the major stabilizing contributions to the free energy of AFP-ice binding. Our results explain available data on structure of AFPs and their mutational analyses, in particular, a paradoxical fact that substitution of Thr residues to Val does not affect potency of Winter Flounder AFP. / Thesis / Master of Science (MS)

Group 9 transition metal pincer complexes have shown tremendous utility in a variety of E-H (E = main group element) bond activation reactions. In an effort to access new types of highly reactive pincer-like transition metal complexes this research focuses on the development of new late metal complexes supported by tridentate bis(phosphino)silyl ligands of the type [?3-(2-R2PC6H4)2SiMe]- ([R-PSiP]; R = Cy, iPr). The incorporation of a strongly electron donating and highly trans-labilizing silyl group at the central anionic position may promote the formation of new coordinatively unsaturated compounds capable of enhanced reactivity. In this regard, the synthesis of coordinatively unsaturated Rh and Ir complexes supported by R-PSiP ligation and their ability to activate E-H bonds will be detailed. The synthesis of Cy-PSiP ligated Rh and Ir species and the ability to access the products of N–H bond oxidative addition with these species was investigated. Both [Cy-PSiP]Rh and [Cy-PSiP]Ir complexes were shown to form isolable complexes of the type [Cy-PSiP]M(H)(NHR) (M = Rh, R = aryl; M = Ir, R = H, aryl). However, attempts to generate such amido hydride complexes by N-H activation of the corresponding amine led to divergent reactivity, where adducts of the type [Cy-PSiP]Rh(NH2R) were obtained for Rh, while N-H bond oxidative addition was observed for Ir to form the targeted amido hydride complexes, including a rare example of ammonia N-H bond oxidative addition to form a monomeric, terminal parent amido complex that was crystallographically characterized. Due to the scarcity of transition metal complexes that are capable of N-H bond oxidative addition, a thorough investigation of the N-H bond activation mediated by [Cy-PSiP]Rh and Ir with various N-H containing substrates, including alkyl amines, hydrazine derivatives, and benzamides was initiated. Extension of this reactivity to the related diisopropylphosphino derivative [iPr-PSiP]IrI was also probed, as the resulting complexes were envisioned to be less susceptible to potential cyclometalation processes. Indeed, oxidative addition of primary alkyl amines, hydrazines, and benzamides was observed for [R-PSiP]Ir. These results comprise an unprecedented example of a metal complex that is capable of facile N-H bond activation in such a wide range of substrates, including challenging substrates such as ammonia and alkyl amines. A rare example of Rh-mediated N-H oxidative addition was also observed for the reaction of [Cy-PSiP]RhI with benzophenone hydrazone. The potential for these [R-PSiP]Ir(H)(NHR) complexes to insert unsaturated substrates was investigated, as the development of new pathways for the formation of C-N bonds via transition metal catalyzed N-H bond oxidative addition to a metal center followed by insertion of an alkene or alkyne into the M-N or M-H bond may provide a new pathway for accessing intermolecular amination reactions. Insertion chemistry attempts with various alkenes, alkynes, allenes, C=O and C?N containing compounds is described. Lastly, the synthesis of IrIII complexes of the type {[R-PSiP]IrR'}+X? (R = Cy, iPr; R’ = H, Me; X = OTf, BF4, B(C6F5)4) and their interactions with the C-H bonds of arenes and aldehydes, as well as, the Si-H bonds of hydrosilanes is detailed. The Si-H bond activation chemistry observed was typically influenced by the counter anion X. Thus, the more coordinating anions OTF and BF4 were shown to coordinate to and stabilize the highly electrophilic Si in transiently generated Ir silylene species.

The transfer of a hydrogen atom is a crucial step in a wide variety of chemical and biological processes and modus operandi of many metalloenzymes. While several factors that govern the reactivity and selectivity were already clarified in the past century, a growing body of experimental and theoretical studies also revealed numerous gaps in our unified understanding. As a consequence, the direct functionalization of non-activated C-H bonds by synthetic catalysts is still very limited. In the thesis, the hydrogen-atom-abstraction (HAA) reactions are broken down into the elementary proton- and electron-transfer steps and the reactivity/selectivity of oxidants is analyzed with respect to their physico-chemical properties, acidity constants and reduction potentials. First, a quantum chemical (QM)-based computational protocol for calculation of reduction potentials of iron complexes is introduced and validated over a large series of experimental data, including a set of challenging mononuclear FeIV O species that provide direct connection to biomimetic non-heme iron catalysis. Next, the methodology is extended to deal with reduction potentials of transition-metal complexes possessing higher total molecular charges, experimentally measured in polar solvents. In such cases, the accurate description of solvation...

Recognizing the dual ability of the nitride ligand to react as a nucleophile or an electrophile – depending on the metal and other supporting ligands – is a key to their broad-range reactivity; thus, three DFT studies were initiated to investigate these two factors effects (the metal and supporting ligands) for tuning nitride ligand reactivity for C-H and N-H bond activation/functionalization. We focused on studying these factors effects from both a kinetic and thermodynamic perspective in order to delineate new principles that explain the outcomes of TMN reactions. Chapter 2 reports a kinetic study of C–H amination of toluene to produce a new Csp3–N (benzylamine) or Csp2–N (para-toluidine) bond activated by diruthenium nitride intermediate. Studying three different mechanisms highlighted the excellent ability of diruthenium nitride to transform a C-H bond to a new C-N bond. These results also revealed that nitride basicity played an important role in determining C–H bond activating ability. Chapter 3 thus reports a thermodynamic study to map basicity trends of more than a one hundred TMN complexes of the 3d and 4d metals. TMN pKb(N) values were calculated in acetonitrile. Basicity trends decreased from left to right across the 3d and 4d rows and increases from 3d metals to their 4d congeners. Metal and supporting ligands effects were evaluated to determine their impacts on TMNs basicity. In Chapter 4 we sought correlations among basicity, nucleophilicity and enhanced reactivity for N–H bond activation. Three different mechanisms for ammonia decomposition reaction (ADR) were tested: 1,2-addition, nitridyl insertion and hydrogen atom transfer (HAT). Evaluating nitride reactivity for the aforementioned mechanisms revealed factors related to the metal and its attached ligands on TMNs for tuning nitride basicity and ammonia N–H activation barriers.