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The photoaquation mechanisms for a series of Cr(III) am(m)ine complexes have been investigated using laser flash photolysis with conductivity detection. The observation of transient increases in solution conductivity at pH > 4 and a conductivity decay lifetime longer than the doublet emission decay lifetime at pH < 3, have confirmed an intermediate in the photoaquation of cis-Cr(cyclam)(NH₃)₂³⁺. Transient increases in solution conductivity characteristic of an intermediate species were also observed for Cr(en)₃³⁺, Cr(tn)₃³⁺ and Cr(sen)³⁺ at pH > 4. The conductivity changes occurring in solution have been modelled for possible photoaquation mechanisms for the am(m)ine complexes, based on numerical integration of the rate expressions for all relevant mechanistic species. The comparison of these results with experimental data indicates that the intermediates observed for Cr(en)₃³⁺, Cr(tn)₃³⁺ and Cr(sen)³⁺ are the initially formed photoproducts, Cr(NN)₂(N-N)(OH₂)³⁺, where NN = a bidentate ligand or a sen arm. These species can undergo two processes, protonation of the dangling amine arm, or deprotonation of the aquo group. At low pH the first process dominates and conductivity decays due to proton uptake are observed. As the pH of the solution increases, the second process becomes competitive, and transient increases in solution conductivity are observed when this becomes the faster process. The modelled results for cis-Cr(cyclam)(NH₃)₂³⁺ indicate that the photoaquation occurs via two modes; (i) direct loss of ammonia, and (ii) loss and recoordination of a cyclam to displace ammonia, with both modes generating the observed photoproduct, cis-Cr(cyclam)(NH₃)(OH₂)³⁺. The modelling indicates that 0.67 of the overall photochemistry occurs via the cyclam loss mode. The intermediate has been identified as the initial product of the cyclam loss mode. Cr(cyc-N)(NH₃)₂(OH₂)³⁺. The rate of reaction observed via this mode is limited by the recoordination of the cyclam amine. As this is slower than the rate of doublet decay, the conductivity lifetimes observed at pH < 3 are longer than the doublet lifetime. The slow rate of recoordination also delays the release and subsequent protonation of ammonia, allowing for competitive deprotonation of the aquo group at pH > 4, and generating the observed transient increases in solution conductivity. The relevance of these results to Cr(III) chemistry in general, including possible ²E state reaction mechanisms is discussed. The photoaquation of Cr(CN)₆³⁻ was investigated using laser flash photolysis with conductivity detection. Theory predicts that the signal magnitudes observed for this complex should be constant throughout the pH range 2.7 - 5.3. Experimental results showed that the observed signals dropped from a maximum of 120 mV at pH 2.75 to 45 mV at pH 5.25. Possible explanations for this pH dependence are presented. The stereochemistry of the thermal and photoaquation products of rac- & Λ-Cr(sen)³⁺ has been investigated using capillary electrophoresis. Two products were found in the photoaquation reactions, trans-Cr(sen-NH)(OH₂)⁴⁺ and a product resulting from loss of a secondary amine. The thermal reaction produced trans-Cr(sen-NH)(OH₂)⁴⁺ as the major product with virtually no cis-Cr(sen-NH)(OH₂)⁴⁺ enantiomers being observed. Efficient racemization of Λ-Cr(sen)³⁺ to Δ-Cr(sen)³⁺was also observed in the thermal reaction, consistent with racemization occurring via bond rupture and recoordination. The photoaquation results are discussed in terms of VC theory and a reinterpretation of conflicting literature results for the thermal and photochemical aquation of Cr(sen)³⁺ is presented. / Graduate

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This thesis describes the investigations of leaving group effects on enzymatic catalysis by Methylyoxal synthase (MGS). MGS is a glycolytic enzyme involved m bacterial metabolism which catalyzes the irreversible elimination of dihydroxyacetone phosphate (DHAP). In order to probe the effect of various leaving groups to MGS catalysts, a range of' ’mutant’ substrates have been synthesised where the phosphate group of natural substrate DHAP has been replaced by sulfate, thiosulfate and bromo groups yielding the substtates dthydroxyacetone sulfate (DHAS), dihydroxyacetone thiosulfate (DHATS) and bromohydroxyacetone (BHA). To quantify an overall enzymatic rate acceleration detailed kinetic analysis of the solution, non-enzymatic reactions of these substrates have been made. Rates for elimination and enolization via C-1 and C-3 deprotonation respectively have been quantified for each substrate using high resolution (^1)H NMR spectroscopy for a range of pD values. From the second order rate constants for buffer catalyzed deprotonation, k(_B) (M(^-1)s(^-1)) comparisons can be made with k(_car)/K(_M) (M(^-1)s(^-1)) for the enzymatic reactions at the same pD values. The non-enzymatic reactions of DHAS were monitored in the pD range of 7.3-9.2 using phosphate and quinuclidinone buffers. Second order rate constants, k(_B) (M(^-1)s(^-1)) of 5.60 x 10(^-4), 5.28 X 10(^-3), 5.82 x l0(_-3) and 7.40 x l0(^-3) M(^-1)s(^-1) for C-1 deprotonation and 1.41 x l0(^-3), 2.24 x l0(^-3) and 3.30 x 10(^-3) for C-3 deprotonation show that increasing the pD results m an increase in the rate of buffer catalyzed deprotonation (phosphate catalyzed C-3 deprotonation was not observed at this pD). This is indicative of a general base catalyzed deprotonation mechanism. First order rate constants, k(_M) (s(^-1)) for the buffer independent catalyzed deptotonation reactions of 1.39 x 10' (^5), 1.22 X 10(^-4), 1.26 X 10(^4) and 2.04 x l0(^-4) for C-1 deprotonation and 9.66 x l0(^-5), 1.10 x 10(^-4) and 9.30 X 10(^-5) for C-3 deprotonation show that an increase 0.7 pD units leads to only a 67 % and 12 respective tacrease in the mtes of deprotonation at the C-l md C-3 positions. This is reflected in the relatively level pD rate profile of the reaction. The background, non-enzymatic reactions of DHATS were carried out in acetic acid buffers and DCl solutions, In DCl solution an average K(_int) (s(^-1)) value of 1.33 x l0(^-5) s(^-1) was obtained. For the acetate catalyzed C-1 deprotonation reactions it was found that moving from pD 3.75 to 4.38 leads to an overall 4.5 fold increase in the value of k(_int) (s(^-1)) for the buffer independent deprotonation reaction of the C-1 protons of the molecule. Values of 9.03 x l0(^-5), 1.03 x 10(^-4) and 4.07 x 10(^-4) s(^-1) were obtained for the reactions in acetate 5, 10 and 20 % f(_B) respectively. Accordingly, in the same pD range the values for the average of first order rate constants, k(_av) (s(^-1)) increase by a factor of 4.9 fold. This is believed to represent a hydroxide dependent reaction. An average k(_B) (M(^-1)s(^-1)) of 5 x 10(^-2) M(^-1)s(^-1) was obtained for the acetate catalyzed reaction at the C-1 position of the molecule. Reactions of the C-3 protons of this molecule were not quantifiable by (^-1)H NMR and thus were followed by mass spectrometry which suggests that a potential dimerisation reaction occurs following deprotonation at this position. Reactions of the C-3 protons of BHA were monitored in quinuclidinone buffet, with values of k(_B)(M(^-1)s(^-1)) = 3.35 x 10(^-3)M(^-1)s(^-1) and k(_int) (s(^-1)) = 9.35 x l0(^-5) s(^-1) obtained for the buffer catalyzed reaction and buffer-independent reaction respectively. The C-1 deprotonation reactions of BHA were monitored in acetate buffers (70-90 % f(_B)). An average k(_B) (M(^-1)s(^-1)) of 9.17 X 10(^-5) M(^-1)s(^-1) was obtained for the buffer catalyzed reaction in this pD range (5.18-6.09). In the same pD range values of k(_int) (s') - 3.7 x 10(^-6). 4.7 x l0(^-6) and 1.19 x l0(^-5) s (^1)were obtained. The reactions of DHA and DHAS were monitored m the presence of MGS (19.6 and 78.4 µg/mL). While no reaction was observable for DHA in the presence of enzyme. DHAS was found to undergo C-1 and C-3 deprotonation leading to average k(_cat)/K(_M) values of 56.5 M(^-1)s(^-1) and 43.5 M(^-1)s(^-1) for exchange and elimination respectively. The rate for elimination is approximately 90,000 fold slower than that observed for natural substrate DEHAP, k(_cat)/k(_M) = 5.2 x 10(^-6) M(^-1)s(^-1) indicating that the phosphodianior group, of DHAP is for optimal cataysis.

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Historically, research projects have originated from the literature group meetings and it was at one of those late night Wednesday meetings, nascent in my graduate career, that I presented a theoretical paper over the mechanism of the Morita Baylis-Hillman reaction. Something about it caught my attention and a project was born. The Morita Baylis-Hillman reaction had been heavily studied in the literature in recent years by both experimental and computational means. Some of these computational studies had even defined a complete theoretical mechanistic energy profile for these reactions. This dissertation describes a combination of experimental and theoretical mechanistic probes, including the observation of intermediates, the independent generation and partitioning of intermediates, thermodynamic and kinetic measurements for both the main reaction and interrelated side reactions, isotopic incorporation from solvent, and kinetic isotope effects, to fully define a more realistic picture of the free-energy profile for a Morita Baylis-Hillman reaction in methanol. Although the majority of this dissertation will be about the Morita Baylis-Hillman reaction in methanol, it could not have been fully accomplished without having to study the Morita Baylis-Hillman in DMSO and the Morita Baylis-Hillman utilizing acrylonitrile as well. All of these observations will be discussed.

Described here are detailed kinetic studies probing the structural elements which are crucial for the catalytic activity of dinuclear Zn(II) complexes towards phosphate diester cleavage. First, two sets of dinuclear Zn(II) complexes (a member with and without a bridging oxyanion linker group) were synthesized and their ability to promote the cyclization of 2-hydroxypropyl-p-nitrophenyl phosphate, a common model for RNA, was compared. Kinetic studies indicated that the complexes without the oxyanion linker were more active in promoting the cyclization in methanol under pH controlled conditions at 25 degrees. Quantitative energetics analysis shows that the rate reduction is attributable to a decrease in the second-order rate constant for the cyclization reaction, which adds 3.7 and 6.5 kcal/mol of activation energy to the respective reactions mediated by the complex with the oxyanion linker. Secondly, we have investigated a series of dinuclear Zn(II) complexes that incorporate various substituents including hydrophobic and hydrogen-bonding ones. Analysis of the data at the pH optimum for each reaction indicates that the presence of the H-bonding groups and alkyl groups provides similar increases (at least an order of magnitude) of the kcat terms over the unfunctionalized complex. There is also no clear trend that H-bonding groups or the alkyl groups provide stronger binding to the substrate than the parent complex. We also describe here the preparation and kinetic analysis of a series of solid supported transition metal catalysts for the cleavage of P=O chemical warfare simulants and P=S pesticides. We report a kinetic study of a 1,10-phenanthroline:Zn(II) complex immobilized on macroporous polystyrene which is capable of accelerating the cleavage of G-agent and V-agent simulants in methanol at neutral and ambient temperature by up to one hundred thousand-fold. The materials are recoverable and can be recycled at least ten times. We have also devised a methodology for simple immobilization of an ortho-palladated dimethylbenzylamine complex on macroporous polystyrene and amorphous silica gel. We report the catalyst preparation and a detailed kinetic study of their catalysis of the methanolysis of five P=S pesticides at neutral and ambient temperature. The polymeric catalysts give over billion-fold acceleration compared to the uncatalyzed background reaction at the same pH. / Thesis (Ph.D, Chemistry) -- Queen's University, 2010-11-23 12:18:46.936