• A. Discovery of novel reactivity under the Sonogashira reaction conditions B. Synthesis of functionalized BODIPYs and BODIPY-sugar conjugates

      Yalagala, Ravi Shekar; Department of Chemistry
      A. During our attempts to synthesize substituted enediynes, coupling reactions between terminal alkynes and 1,2-cis-dihaloalkenes under the Sonogashira reaction conditions failed to give the corresponding substituted enediynes. Under these conditions, terminal alkynes underwent self-trimerization or tetramerization. In an alternative approach to access substituted enediynes, treatment of alkynes with trisubstituted (Z)-bromoalkenyl-pinacolboronates under Sonogashira coupling conditions was found to give 1,2,4,6-tetrasubstituted benzenes instead of Sonogashira coupled product. The reaction conditions and substrate scopes for these two new reactions were investigated. B. BODIPY core was functionalized with various functional groups such as nitromethyl, nitro, hydroxymethyl, carboxaldehyde by treating 4,4-difluoro-1,3,5,7,8-pentamethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene with copper (II) nitrate trihydrate under different conditions. Further, BODIPY derivatives with alkyne and azido functional groups were synthesized and conjugated to various glycosides by the Click reaction under the microwave conditions. One of the BODIPY–glycan conjugate was found to form liposome upon rehydration. The photochemical properties of BODIPY in these liposomes were characterized by fluorescent confocal microscopy.
    • Applications of dihydroarenediols to chemoenzymatic synthesis : approaches to total synthesis of morphine alkaloids

      Finn, Kevin J.; Department of Chemistry (Brock University, 2006-05-28)
      The present studies describe, as a primary goal, our recent progess toward the synthesis of morphine alkaloids from aromatic precursors. Model substrates were synthesized which allowed investigation into Diels-Alder, radical cascade, and palladium-catalyzed bond-forming reactions as possible routes to the morphine alkaloid skeleton. As a secondary objective, three separate series of aromatic substrates were subjected to whole-cell oxidation with Escherichia coli JM 109 (pDTG601), a recombinant organism over-expressing the enzyme toluene dioxygenase. Included in this study were bromothioanisoles, dibromobenzenes, and cyclopropylbenzene derivatives. The products of oxidation were characterized by chemical conversion to known intermediates. The synthetic utility of one of these bacterial metabolites, derived from oxidation of o-dibromobenezene, was demonstrated by chemical conversion to (-)conduritol E.
    • Bis(dialkylamino)cyclopropeneimine Substituted Proton Sponge Derivatives: Synthesis, Theory, and Application

      Belding, Lee; Department of Chemistry
      The work presented herein describes the synthesis, as well as the experimental and theoretical investigation of hitherto unknown cyclopropenimine containing compounds, mostly within the proton sponge backbone. The properties of these molecules are discussed in the context of other proton sponge derivatives reported in the literature. The superbasicity, catalytic activity, and fluorescent nature of these cyclopropenimine derivatives are also investigated and discussed.
    • Chemoenzymatic Formal Total Syntheses of Tetrodotoxin and an Approach to Daphenylline

      Baidilov, Daler; Department of Chemistry
      This thesis describes chemoenzymatic formal total syntheses of tetrodotoxin and a concise synthetic approach to daphenylline. Advanced intermediates for the syntheses of tetrodotoxin reported by the groups of Fukuyama, Alonso, and Sato were prepared. Key steps included toluene dioxygenase-mediated dihydroxylation of either iodobenzene or benzyl acetate and a [4+2] hetero-Diels-Alder cycloaddition/Kornblum–DeLaMare rearrangement sequence to construct a common enone intermediate. The resulting key enone was transformed into Fukuyama's intermediate in four steps, into Alonso's intermediate in six steps, and into Sato's intermediate in seven steps. Fukuyama’s route employed a highly stereoselective allyl cyanate-to-isocyanate rearrangement to install the nitrogen atom at C8a. This protocol was also successfully applied in designing a synthetic avenue to daphenylline. The ABC tricyclic skeleton of daphenylline was successfully constructed in just eight steps starting from readily available (S)-carvone.
    • Chemoenzymatic synthesis of amaryllidaceae alkaloids and their C-1 analogues : symmetry based approach to total synthesis of thebaine

      Collins, Jonathan A.; Department of Chemistry (Brock University, 2010-10-26)
      Described herein is the chemoenzymatic total synthesis of several Amaryllidaceae constituents and their unnatural C-I analogues. A new approach to pancratistatin and related compounds will be discussed along with the completed total synthesis of 7 -deoxypancratistatin and trans-dihydrolycoricidine. Evaluation of all new C-l analogues as cancer cell growth inhibitory agents is described. The enzymatic oxidation of dibromobenzenes by Escherichia coli 1M 109 (pDTG60 1) is presented along with conversion of their metabolites to (-)-conduritol E. Investigation into the steric and functional factors governing the enzymatic dihydroxylation of various benzoates by the same organism is also discussed. The synthetic utility of these metabolites is demonstrated through their conversion to pseudo-sugars, aminocyclitols, and complex bicyclic ring systems. The current work on the total synthesis of some morphine alkaloids is also presented. Highlighted will be the synthesis of several model systems related to the efficient total synthesis of thebaine.
    • Chemoenzymatic Total Synthesis of ent-Oxycodone

      Makarova, Mariia; Department of Chemistry
      This thesis describes the approach towards chemoenzymatic total synthesis of ent-dihydroisocodeine and chemoenzymatic total synthesis of ent-oxycodone as well as the development of a new method for the preparation of rearranged allylic isocyanates. The synthesis of ent-dihydroisocodeine started from phenethyl acetate and included a microbial oxidation of phenethyl acetate by E. coli JM109 (pDTG601A), a Mitsunobu reaction to the couple A- and C-rings, Heck cyclization to construct the E-ring and Henry reaction to introduce the nitrogen functionality as key steps. The construction of the B-ring proved to be challenging and neither radical cyclization nor attempts to perform photochemistry or nucleophilic opening of an epoxide gave any positive results. The chemoenzymatic total synthesis of ent-oxycodone was accomplished starting from phenethyl acetate in 23 steps. The tricyclic intermediate was furnished in the same manner as described above. The olefin to ketone conversion and a double Henry reaction allowed the construction of the B-ring. Unfortunately, it was established that the resulting hydroxyl group at C14 and amino group at C9 were of the undesired trans stereochemistry. To complete the morphine skeleton the transformation of the side chain at C13 to an N-methyl-p-toluenesufonamide via Mitsunobu reaction as well as the elimination of the amino group at C9 via formation of an N-oxide were performed. Subsequent radical cyclization of the side chain at the C9 position formed the last D-ring. The silyl ether deprotection followed by oxidation provided ent-oxycodone. The other approach to construct the D-ring was based on the formation of a lactone and the elimination of the amino group via an N-oxide. The nitrogen functionality was reinstalled using sodium azide and was accompanied by the introduction of the C10 hydroxyl group. Reduction of the azide and subsequent formation of the amide allowed access to the core skeleton of the target compound. The removal of the C10 hydroxyl group accomplished the synthesis. The last project involved the development of a new method for the preparation of rearranged allylic isocyanates from allylic alcohols using 1-cyano-4-dimethylaminopyridinium as the source of electrophilic cyanide. Experimental and spectral data are provided for all the compounds.
    • Chemoenzymatic Total Synthesis of Morphine alkaloids: Synthesis of Dihydrocodeine and Hydrocodone via a Double Claisen Strategy and ent-Hydromorphone via an Oxidative Dearomatization/intramolecular [4+2] Cycloaddition

      Varghese, Vimal; Department of Chemistry (Brock University, 2015-01-15)
      This thesis describes the chemoenzymatic synthesis of three morphine alkaloids. The total synthesis of dihydrocodeine and hydrocodone was accomplished starting from bromobenzene in 16 and 17 steps, respectively. The key steps included a microbial oxidation of bromobenzene by E. coli JM109 (pDTG601A), a Kazmaier-Claisen rearrangement of glycinate ester to generate C-9 and C-14 stereo centers, a Johnson-Claisen rearrangement to set the C-13 quaternary center, and a C-10/C-11 ring closure via a Friedel-Crafts reaction. In addition, the total synthesis of ent-hydromorphone starting from β-bromoethylbenzene in 12 steps is also described. The key reactions included the enzymatic dihydroxylation of β-bromoethylbenzene to the corresponding cis-cyclohexadienediol, a Mitsunobu reaction, and an oxidative dearomatization followed by an intramolecular [4+2] cycloaddition.
    • A DFT Guided/Experimental Approach to Asymmetric Allylation and Phase-Transfer Catalysis

      Mirabdolbaghi, Roya; Department of Chemistry (Brock University, 2014-09-10)
      The Dudding group is interested in the application of Density Functional Theory (DFT) in developing asymmetric methodologies, and thus the focus of this dissertation will be on the integration of these approaches. Several interrelated subsets of computer aided design and implementation in catalysis have been addressed during the course of these studies. The first of the aims rested upon the advancement of methodologies for the synthesis of biological active C(1)-chiral 3-methylene-indan-1-ols, which in practice lead to the use of a sequential asymmetric Yamamoto-Sakurai-Hosomi allylation/Mizoroki Heck reaction sequence. An important aspect of this work was the utilization of ortho-substituted arylaldehyde reagents which are known to be a problematic class of substrates for existing asymmetric allylation approaches. The second phase of my research program lead to the further development of asymmetric allylation methods using o-arylaldehyde substrates for synthesis of chiral C(3)-substituted phthalides. Apart from the de novo design of these chemistries in silico, which notably utilized water-tolerant, inexpensive, and relatively environmental benign indium metal, this work represented the first computational study of a stereoselective indium-mediated process. Following from these discoveries was the advent of a related, yet catalytic, Ag(I)-catalyzed approach for preparing C(3)-substituted phthalides that from a practical standpoint was complementary in many ways. Not only did this new methodology build upon my earlier work with the integrated (experimental/computational) use of the Ag(I)-catalyzed asymmetric methods in synthesis, it provided fundamental insight arrived at through DFT calculations, regarding the Yamamoto-Sakurai-Hosomi allylation. The development of ligands for unprecedented asymmetric Lewis base catalysis, especially asymmetric allylations using silver and indium metals, followed as a natural extension from these earlier discoveries. To this end, forthcoming as well was the advancement of a family of disubstituted (N-cyclopropenium guanidine/N-imidazoliumyl substituted cyclopropenylimine) nitrogen adducts that has provided fundamental insight into chemical bonding and offered an unprecedented class of phase transfer catalysts (PTC) having far-reaching potential. Salient features of these disubstituted nitrogen species is unprecedented finding of a cyclopropenium based C-H•••πaryl interaction, as well, the presence of a highly dissociated anion projected them to serve as a catalyst promoting fluorination reactions. Attracted by the timely development of these disubstituted nitrogen adducts my last studies as a PhD scholar has addressed the utility of one of the synthesized disubstituted nitrogen adducts as a valuable catalyst for benzylation of the Schiff base N-diphenyl methylene glycine ethyl ester. Additionally, the catalyst was applied for benzylic fluorination, emerging from this exploration was successful fluorination of benzyl bromide and its derivatives in high yields. A notable feature of this protocol is column-free purification of the product and recovery of the catalyst to use in a further reaction sequence.
    • Diastereoselective Synthesis of Planar Chiral N-Substituted Ferrocenes Derived from Epimeric Imidazolones and their Application to Asymmetric Hydrogenation of Quinolines

      John, Joshni; Department of Chemistry (Brock University, 2015-03-02)
      This thesis describes the synthesis and use of an N-substituted ferrocene bearing a proline-derived chiral directing group and diastereoselective lithiation-electrophile quench of the pro-Sp hydrogen of the ferrocene to give planar chiral products in >95:5 dr. The auxiliary group is found to be stable to lithium bases of types RLi and R2NLi giving the same diastereoselectivity. The anti- epimer of the previously mentioned syn auxiliary induces lithiation of pro Rp rather than pro Sp hydrogen in >95:5 dr. Upon electrophile quench and elimination, the enantiomer of the syn-derived planar chiral imidazolone is obtained. Hence, this method provides a practical way to prepare planar chiral enantiomers in this series without the use of a more expensive D-proline derived starting material. The syn and anti epimers have β, γ-stereogenic centers and the origin of stereoselectivity in lithiation appears to be driven by the conformational bias exerted by the β-silyloxy moiety in each chiral auxiliary. In the thesis, this conclusion is supported using insensitivity of lithiation selectivity to the bulkiness of the base, comparison of enantiomers, deuteration experiments, nOe difference studies and computational modeling of the ground states and lithiation transition states for both substrates. The products are then converted to ligand precursors to make iridium and rhodium complexes. Among them, one of the cationic iridium complex is found to be effective in the asymmetric hydrogenation of 2-substituted quinolines with enantioselectivities up to 80% at pressures as low as 5 atm.
    • Electron Transfer Involving the Phylloquinone (A1) Cofactor of Photosystem I Examined with Time Resolved Absorbance and Electron Paramagnetic Resonance Spectroscopy

      Mula, Samuel Jr.; Department of Chemistry (Brock University, 2015-01-23)
      The dependence of the electron transfer (ET) rate on the Photosystem I (PSI) cofactor phylloquinone (A1) is studied by time-resolved absorbance and electron paramagnetic resonance (EPR) spectroscopy. Two active branches (A and B) of electron transfer converge to the FX cofactor from the A1A and A1B quinone. The work described in Chapter 5 investigates the single hydrogen bond from the amino acid residue PsaA-L722 backbone nitrogen to A1A for its effect on the electron transfer rate to FX. Room temperature transient EPR measurements show an increase in the rate for the A1A- to FX for the PsaA-L722T mutant and an increased hyperfine coupling to the 2-methyl group of A1A when compared to wild type. The Arrhenius plot of the A1A- to FX ET in the PsaA-L722T mutant suggests that the increased rate is probably the result of a slight change in the electronic coupling between A1A- and FX. The reasons for the non-Arrhenius behavior are discussed. The work discussed in Chapter 6 investigates the directionality of ET at low temperature by blocking ET to the iron-sulfur clusters FX, FA and FB in the menB deletion mutant strain of Synechocyctis sp. PCC 6803, which is unable to synthesize phylloquinone, by incorporating the high midpoint potential (49 mV vs SHE) 2,3-dichloro-1,4-naphthoquinone (Cl2NQ) into the A1A and A1B binding sites. Various EPR spectroscopic techniques were implemented to differentiate between the spectral features created from A and B- branch electron transfer. The implications of this result for the directionality of electron transfer in PS I are discussed. The work discussed in Chapter 7 was done to study the dependence of the heterogeneous ET at low temperature on A1 midpoint potential. The menB PSI mutant contains plastiquinone-9 in the A1 binding site. The solution midpoint potential of the quinone measures 100 mV more positive then wild-type phylloquinone. The irreversible ET to the terminal acceptors FA and FB at low temperature is not controlled by the forward step from A1 to FX as expected due to the thermodynamic differences of the A1 cofactor in the two active branches A and B. Alternatives for the ET heterogeneity are discussed.
    • Half-sandwich Complexes of Ruthenium Supported by N-Heterocyclic Carbene Ligands: Synthesis and Application to Catalysis

      Mai, Van Hung; Department of Chemistry
      This thesis presents the preparation and catalytic reactivity of novel half-sandwich ruthenium complexes supported by N-Heterocyclic Carbene (NHC) ligands. The cationic half-sandwich ruthenium complexes [Cp(IPr)Ru(CH3CN)2]+ show interesting reactivities toward the transfer hydrogenation of different unsaturated substrates, such as ketones, olefins, N-heterocycles, and nitriles. Kinetic studies disclose that a neutral trishydride ruthenium complex is actually involved in the catalytic cycle, playing the role as a resting state. Further investigations on the sub-class of trishydride ruthenium complexes bearing NHC ligands (Cp'(NHC)RuH3) reveal that these complexes have an unusual and great catalytic performance toward the hydrodefluorination (HDF) of fluorinated aromatic and aliphatic compounds. The combined kinetic studies, cross-over experiments and rate law analysis suggest an unusual mechanistic pathway for the Cp*(IPr)RuH3 catalyzed HDF. This study is one of the rare examples where isopropanol is employed as a reducing agent for the metal-mediated HDF reaction. A class of silyl dihydride ruthenium complexes, derived from Cp(IPr)RuH3 are prepared. These silyl hydrido derivatives are great compounds for the study of the inter ligand hypervalent interaction (IHI), an interesting phenomenon for many non-classical silane complexes. This study also suggests that the replacement of phosphines by their isolobally analogous NHC ligands result in stronger IHI interactions in the corresponding compounds. Another type of non-classical interaction was systematically scrutinized in a ii series of new cationic and neutral silane sigma complexes of ruthenium bearing different silyl moieties. These new NHC-supported ruthenium complexes allow for direct comparation with the known phosphine analogues, which reveals interplay of steric and electronic factors on the extent of Si-H complexation to metal and the extent of additional interligand interactions between Ru-Cl and chlorosilane ligand. Finally, new trishydride ruthenium complexes bearing NHC ligands (Cp'(NHC)RuH3) catalyze the H/D exchange reaction of various N-heterocycle substrates; their catalytic performance can be considered as one of the mildest, and most efficient approaches.
    • Half-sandwich Complexes of Ruthenium; Synthesis and Application to Catalysis

      Lee, Sun Hwa; Department of Chemistry (Brock University, 2014-09-15)
      This thesis describes syntheses and catalytic reactivity of several half-sandwich complexes of ruthenium. The neutral ruthenium trihydride complex, Cp(PPri3)RuH3(1), can efficiently catalyse the H/D exchange reaction between various organic substrates and deuterium sources, such as benzene-d6. Moreover, the H/D exchange reactions of polar substrates were also observed in D2O, which is the most attractive deuterium source due to its low cost and low toxicity. Importantly, the H/D exchange under catalytic conditions was achieved not only in aromatic compounds but also in substituted liphatic compounds. Interestingly, in the case of alkanes and alkyl chains, highly selective deuterium incorporation in the terminal methyl positions was observed. It was discovered that the methylene units are engaged in exchange only if the molecule contains a donating functional group, such as O-and N-donors, C=C double bonds, arenes and CH3. The cationic half-sandwich ruthenium complex [Cp(PPri3)Ru(CH3CN)2]+(2) catalyses the chemoselective mono-addition of HSiMe2Ph to pyridine derivatives to selectively give the 1,4-regiospecific, N-silylated products. An ionic hydrosilylation mechanismis suggested based on the experiments. To support this mechanistic proposal, kinetic studies under catalytic conditions were performed. Also, the 1,4-regioselective mono-hydrosilylation of nitrogen containing compounds such as phenanthroline, quinoline and acridine can be achieved with the related Cp*complex [Cp*(phen)Ru(CH3CN)]+(3) (phen = 1,10-phenanthroline) and HSiMe2Ph under mild conditions. The cationic ruthenium complex 2 can also be used as an efficient catalyst for transfer hydrogenation of various organic substrates including carbonyls, imines, nitriles and esters. Secondary alcohols, amines, N-isopropylidene amines and ether compounds can be obtained in moderate to high yields. In addition, other ruthenium complexes, 1,3 and [Cp*(PPri3)Ru(CH3CN)2]+(4), can catalyse transfer hydrogenation of carbonyls although the reactions were sluggish compared to the ones of 2. The possible intermediate, Cp(PPri3)Ru(CH3CN)(H), was characterized by NMR at low temperature and the kinetic studies for the transfer hydrogenation of acetophenone were performed. Recently, chemoselective reduction of acid chlorides to aldehydes catalysed by the complex 2 was reported. To extend the catalytic reactivity of 2, reduction of iminoyl chlorides, which can be readily obtained from secondary amides, to the corresponding imines and aldehydes was investigated. Various substituted iminoyl chlorides were converted into the imines and aldehydes under mild conditions and several products were isolated with moderate yields.
    • Half-sandwich silane complexes of ruthenium and iron : synthesis, structure and application to catalysis

      Gutsulyak, Dmitry V.; Department of Chemistry (2012-04-04)
      The present thesis describes syntheses, structural studies, and catalytic reactivity of new non-classical silane complexes of ruthenium and iron. The ruthenium complexes CpRu(PPri3)CI(T]2-HSiR3) (1) (SiR3 = SiCh (a), SiClzMe (b), SiCIMe2 (c), SiH2Ph (d), SiMe2Ph (e» were prepared by reactions of the new unsaturated complex CpRu(PPri3)CI with silanes. According to NMR studies and X-ray analyses, the complexes la-c exhibit unusual simultaneous Si··· H and Si··· CI-Ru interactions. The complex CpRu(PPri3)CI was also used for the preparation of the first examples of late transition metal agostic silylamido complexes CpRu(PPri3)(N(T]2-HSiMe2)R) (2) (R= Ar or But), which were characterized by NMR spectroscopy. The iron complexes CpFe(PMePri2)H2(SiR3) (3) (SiR3 = SiCh (a), SiClzMe (b), SiCIMe2 (c), SiH2Ph (d), SiMe2Ph (e» were synthesized by the reaction of the new borohydride iron complex CpFe(PMePri2)(B~) with silanes in the presence NEt3. The complexes 3 exhibit unprecedented two simultaneous and equivalent Si··· H interactions, which was confirmed by X-ray analyses and DFT calculations. A series of cationic ruthenium complexes [CpRu(PR3)(CH3CN)(112-HSiR'3)]BAF (PR3 = PPri 3 (4), PPh3 (5); SiR'3 = SiCh (a), SiClzMe (b), SiClMe2 (c), SiH2Ph (d), SiMe2Ph (e» was obtained by substitution of one of the labile acetonitrile ligands in [CpRu(PR3)(CH3CNh]BAF with sHanes. Analogous complexes [TpRu(PR3)(CH3CN)(T]2 -HSiR' 3)]BAF (5) were obtained by the reaction of TpRu(PR3)(CH3CN)CI with LiBAF in the presence of silanes. The complexes 4-5 were characterized by NMR spectroscopy, and the observed coupling constants J(Si-H) allowed us to estimate the extent of Si-H bond activation in these compounds. The catalytic activity in hydrosilylation reactions of all of the above complexes was examined. The most promising results were achieved with the cationic ruthenium precatalyst [CpRu(PPri3)(CH3CN)2t (6). Complex 6 shows good to excellent catalytic activity in the hydrosilylation of carbonyls, dehydrogenative coupling of silanes with alcohols, amines, acids, and reduction of acid chlorides. We also discovered very selective reduction of nitriles and pyridines into the corresponding N-silyl imines and l,4-dihydropyridines, respectively, at room temperature with the possibility of catalyst recycling. These chemoselective catalytic methods have no analogues in the literature. The reactions were proposed to proceed via an ionic mechanism with intermediate formation of the silane a-complexes 4.
    • High-Nuclearity Lanthanide(III) Complexes as Single-Molecule Magnets and Luminescent Materials

      Mazarakioti, Eleni; Department of Chemistry
      The employment of the bridging/chelating Schiff base ligands, N-salicylidene-o-aminophenol (saphH2), N-salicylidene-o-aminocyclohexanol (sachH2) and N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH2), in lanthanide (LnIII) cluster chemistry has afforded four families of polynuclear and dinuclear complexes with new structural motifs, and interesting magnetic and optical properties. Chapter 1 deals with most of the fundamental aspects within the areas of polynuclear metal complexes, molecular magnetism and optics as these are applied to 4f-metal based systems, while the research results are reported in Chapters 2, 3 and 4. In the first project (Chapter 2), the coordination chemistry of the organic chelating/bridging ligand, N-salicylidene-o-aminophenol (saphH2) in lanthanide cluster chemistry was investigated. The general LnIII/X-/saphH2/base reaction system has led to a family of (NHEt3)[Ln7(OH)2(saph)10(Me2CO)2] (Ln = Gd (1); Tb (2); Dy (3)) clusters with a new core topology that comprises two {Ln4} butterflies sharing a common metal vertex. The {DyIII7} analogue exhibits slow magnetization relaxation, whereas all heptanuclear compounds show ligand-centered blue-green emissions. The second project of this thesis, which is discussed in Chapter 3, comprises the first use of the Schiff base ligand N-salicylidene-2-aminocyclohexanol (sachH2; mixture of cis- and trans-analogue) in metal cluster chemistry which has afforded a new family of [Ln7(OH)6(CO3)3(sach)3(sachH)3(MeOH)6] (Ln = Gd (4); Tb (5); Dy (6)) clusters with ideal D3h point group symmetry and metal-centered trigonal prismatic topology. Solid-state and solution studies revealed single-molecule magnetism (SMM) and photoluminescence behaviors. Moreover, in order to investigate the steric and stereoisomerism effects of the ligand on the chemical and structural identity of the {Ln7} clusters, the pure trans-analogue of the sachH2 ligand was utilized. As a result, a new family of octanuclear [Ln8(OH)4(CO3)2(trans-sach)8(EtOH)4] (Ln = Gd (7); Tb (8); Dy (9); Eu (10)) clusters were obtained, while the solid-state studies revealed SMM behavior and lanthanide-centered emissions. In the last chapter of this thesis (Chapter 4), the Schiff base ligand N-salicylidene-2-amino-5-chlorobenzoic acid (sacbH2) was introduced for a first time in lanthanide cluster chemistry. This has afforded a family of dinuclear [Ln2(NO3)4(sacbH)2(H2O)2(MeCN)2] compounds (Ln = Gd (11); Tb (12); Dy (13)) with the Dy-analogue exhibiting SMM behaviour with a high-energy barrier for the magnetization reversal and interesting magnetization dynamics. All research-based Chapters (Chapters 2-4) are divided into subsections in order to facilitate the understanding of the research concepts by the familiar and non-familiar readers and contextualize the messages, goals and conclusions of each individual project. I felt it prudent to always begin with a short preface of the work that summarizes the most important aspects of the specific project, followed by the complete experimental part and discussion of the results, and finishing up with conclusions and some future perspectives.
    • High-Nuclearity Metal Complexes and Single-Molecule Magnets from the Employment of Oximato- and Alkoxido-based Ligands

      Giannopoulos, Dimosthenis; Department of Chemistry
      The employment of 2-pyrrolyloximes, pyridine-2,6-diketones and 3-hydroxy-2-naphthohydroxamic acid in homometallic 3d- and heterometallic 3d/4f-metal cluster chemistry has yielded new families of Fe, Mn and Mn/Dy clusters. These complexes were shown to possess interesting structural motifs and single-molecule magnetism (SMM) behaviour. The introductory chapter discusses the fundamentals of molecular magnetism, polynuclear metal complexes, as well as the approaches used for the synthesis of new polynuclear metal complexes and the selection criteria for the chelating/bridging ligands. Chapters 2, 3 and 4 report the results of the current thesis. In Chapter 2, the synthesis and characterization of a family of complexes resulting from the employment of 2-pyrrolyloximes in high-nuclearity transition metal cluster chemistry is reported. Complexes {Fe10} (1) and {Fe12} (2) are two of the highest nuclearity iron clusters containing an oximate ligand, while complex 3 is a barrel-like {Mn25Na} complex that exhibits SMM behaviour. Although there are previously reported examples of discrete {Mn25} barrel-like SMMs, complex 3 is the highest nuclearity Mn cluster organized into a 1D polymer through chelation with diamagnetic metal centers. Chapter 3 includes the synthesis and characterization of new Mn complexes featuring ligands that result from the metal-assisted reactivity of pyridyl- and pyrazine-based diketones. Complexes {Mn6} (4) and {Mn10} (5) are the highest nuclearity Mn clusters containing any form of the ligand 2,6-di-(2-pyridylcarbonyl)pyridine [(py)CO(py)CO(py)]. Despite the large number of {Mn6} and {Mn10} complexes reported in the literature, both complexes 4 and 5 possess unique topologies in their respective oxidation state levels. Complex {Mn3Na2} (6) possesses a iii unique metal stoichiometry and is the only compound containing any form of the ligand pyridine-2,6-diylbis(pyrazine-2-ylmethanone) [(pz)CO(py)CO(pz)]. More interestingly, complex 6 contains the first {MnIII3(μ3-O2−)}7+ triangular core where the Mn centers are solely bridged by an oxido group, essentially being a unique ‘edge-naked’ equilateral triangle. In Chapter 4, the synthesis and characterization of complexes bearing the ligand 3-hydroxy-2-naphthohydroxamic acid are presented. The {Mn10} complexes 7 and 8 are the highest nuclearity 3d-metal and the first homometallic Mn clusters containing the hydroxime form of the ligand. Both compounds possess unique metal topologies, which are affected by the nature of the carboxylate ligand present in the reaction mixture, and they behave as SMMs. The use of 3-hydroxy-2-naphthohydroxamic acid in Mn/Dy cluster chemistry has afforded the {Mn4Dy} complexes 9 and 10, as well as a family of {Mn8Dy2} complexes (11 and 12). These compounds are the first Mn/Dy complexes containing this particular hydroxime ligand and they also possess unique metal stoichiometries and topologies. The reported heterometallic products resulted from our efforts to deliberately replace the divalent Mn atoms located in 7 and 8 with DyIII as a means of enhancing the magnetic properties of the former. Complexes 11 and 12 were found to be single-molecule magnets.
    • Hydrosilylation and hydroboration catalyzed by imido-hydride complexes of molybdenum (IV)

      Shirobokov, Oleg G.; Department of Chemistry (2012-04-04)
      This thesis describes the synthesis, structural studies, stoichiometric and catalytic reactivity of novel Mo(IV) imido hydride complexes (Cp)(ArN)Mo(H)(PMe3) (1) and (Tp )(ArN)Mo(H)(PMe3) (2). Both 1 and 2 catalyze hydrosilylation of a variety of carbonyls. Detailed kinetic and DFT studies found that 1 reacts by an unexpected associative mechanism, which does not involve Si-H addition either to the imido group or the metal. Despite 1 being a d2 complex, its reaction with PhSiH3 proceeds via a a-bond metathesis mechanism giving the silyl derivative (Cp )(ArN)Mo(SiH2Ph)(PMe3). In the presence of BPh3 reaction of 1 with PhSiH3 results in formation of (Cp)(ArN)Mo(SiH2Ph)(H)2 and (Cp)(ArN)Mo(SiH2Ph)2(H), the first examples ofMo(VI) silyl hydrides. AI: 1 : 1 reaction between 2, PhSiD3 and carbonyl substrate established that hydrosilylation is not accompanied by deuterium incorporation into the hydride position of the catalyst, thus ruling out the conventional mechanism based on carbonyl insertion carbonyl. As 2 is nomeactive to both the silane and ketone, the only mechanistic alternative we are left with is that the metal center activates the carbonyl as a Lewis acid. The analogous nonhydride mechanism was observed for the catalysis by (ArN)Mo(H)(CI)(PMe3), (Ph3P)2(I)(O)Re(H)(OSiMe2Ph) and (PPh3CuH)6. Complex 2 also catalyzes hydroboration of carbonyls and nitriles. We report the first case of metal-catalyzed hydroboration of nitriles as well as hydroboration of carbonyls at very mild conditions. Conversion of carbonyl functions can be performed with high selectivities in the presence of nitrile groups. This thesis also reports the first case of the HlH exchange between H2 and Si-H of silanes mediated by Lewis acids such as Mo(IV) , Re(V) , Cu(I) , Zn(II) complexes, B(C6Fs)3 and BPh3.
    • Investigating the Cluster Chemistry of α-Methyl-2-pyridine methanol (mpmH) with Select 3d Ions

      Abbasi, Parisa; Department of Chemistry
      This thesis describes an investigation of the coordination chemistry of the potentially chiral bridging, chelating ligand, α-methyl-2-pyridinemethanol (mpmH) with select 3d ions for the discovery of polynuclear clusters with single molecule magnet (SMM) properties. Chapter 1 introduces the theory of molecular magnetism, SMMs and the concepts of chiral SMMs, magnetochiral dichroism and multiferroics. In Chapter 2, two NiII clusters, {Ni8} and {Ni18} prepared from rac-mpmH are reported. The {Ni8} cluster crystallizes in a trapezoidal prismatic topology and contains tetrazolate ligands that are formed via a metal-assisted click reaction. The molecular structure of the second {Ni18} cluster is highly disordered comprising of eight edge-sharing cubane subunits. Dc magnetic susceptibility measurements reveal dominant ferromagnetic interactions down to ~18 K, stabilizing spin states with large values, whereas at T < 18 K the antiferromagnetic contribution results in the population of smaller, but appreciable non-zero spin states. Ac magnetic susceptibility measurements confirm the presence of two relaxation processes at two temperature regimes that is extremely rare for a 3d-metal based SMM. The first at low temperature (5 K) is attributed to conventional SMM behavior with τ0 = 3.26 × 10-10 s and Ueff = 11 K. The origin of high temperature (15 K) relaxation process with a large Ueff = 381 K and τ0 = 2.7 × 10-15 s is less clear, but tentatively assigned to spin-glass properties. In Chapter 3, the synthesis and structure of a large mixed-valence [MnII2MnIII28MnIV] polynuclear cluster with a closed cage-like conformation is presented. Ac magnetic susceptibility measurements show the compound is an SMM with Ueff of 58 K, that is large for a 3d cluster, and a τ0 = 3 × 10−8 s. Chapter 4 describes the coordination chemistry of racemic and chiral-mpmH with CuII and FeIII, where the synthesis and magnetostructural properties of a chiral {Cu4} tetramer, a non-chiral 1-D chain, as well as a chiral {Fe6} and a non-chiral{Fe8} cluster are reported. Dc magnetic susceptibility measurements on all four complexes reveal the presence of dominant antiferromagnetic exchange interactions affording S = 0 spin ground states at low temperature that precludes the observation of any SMM behavior.
    • Ligand design for molecule-based magnetic and/or conducting materials

      ACHA, ROLAND; Department of Chemistry (Brock University, 2013-06-04)
      Work in the area of molecule-based magnetic and/or conducting materials is presented in two projects. The first project describes the use of 4,4’-bipyridine as a scaffold for the preparation of a new family of tetracarboxamide ligands. Four new ligands I-III have been prepared and characterized and the coordination chemistry of these ligands is presented. This project was then extended to exploit 4,4’-bipyridine as a covalent linker between two N3O2 macrocyles. In this respect, three dimeric macrocycles have been prepared IV-VI. Substitution of the labile axial ligands of the Co(II) complex IV by [Fe(CN)6]4- afforded the self-assembly of the 1-D polymeric chain {[Co(N3O2)H2O]2Fe(CN)6}n•3H2O that has been structurally and magnetically characterized. Magnetic studies on the Fe(II) complexes V and VI indicate that they undergo incomplete spin crossover transitions in the solid state. Strategies for the preparation of chiral spin crossover N3O2 macrocycles are discussed and the synthesis of the novel chiral Fe(II) macrocyclic complex VII is reported. Magnetic susceptibility and Mössbauer studies reveal that this complex undergoes a gradual spin crossover in the solid state with no thermal hysteresis. Variable temperature X-ray diffraction studies on single crystals of VII reveal interesting structural changes in the coordination geometry of the macrocycle accompanying its SCO transition. The second project reports the synthesis and characterization of a new family of tetrathiafulvalene derivatives VIII – XII, where a heterocyclic chelating ligand is appended to a TTF donor via an imine linker. The coordination chemistries of these ligands with M(hfac)2.H2O (M( = Co, Ni, Mn, Cu) have been explored and the structural and magnetic properties of these complexes are described.
    • Magnetically Interesting Coordination Complexes Based on Macrocyclic Ligands

      Ras Ali, Zineb; Department of Chemistry
      The synthesis and study of select 3d and/or 4f coordination complexes prepared from crown ether and Schiff-base dual compartmental macrocycles are described herein, working towards the discovery and study of new families of macrocyclic-based single molecule magnets (SMMs). Chapter 1 introduces the general theory of magnetism, molecular magnetism and SMMs and provides the reader with a brief overview of the relevant coordination chemistry of the two families of macrocycles. In Chapter 2, two 15-crown-5 complexes [Ln(NO3)3(OH2)2(MeOH)], (where Ln(III) = Tb (I) and Dy (II)) have been prepared and characterized. X-ray diffraction studies reveal the two complexes crystallize as 1-D chains. Variable temperature ac magnetic susceptibility studies reveal that (II) is an SMM with two effective energy barriers, Ueff = 26 K (18 cm−1); τ0 = 4.10 × 10−7 s and Ueff = 41 K (29 cm−1); τ0 = 1.35 × 10−8 s, whereas ab initio studies suggest that the observation of slow relaxation of magnetization in the Tb complex (I) is hindered by the presence of rapid quantum tunneling mechanisms (QTM). Solid state photoluminescence measurements reveal the two complexes have well-resolved f–f transitions, where a Gaussian fit of the fine structure of the highest-energy emission band for the Dy(III) complex allows the Stark splitting of the ground state to be determined. In Chapter 3, select Ln(III) complexes with benzo and dibenzo 15-crown-5 macrocycles were synthesized and characterized. Reaction of Dy(III) together with benzo 15-crown-5 afforded a unique [Dy(OH2)8]3+ complex (III), where the hydrated Dy(III) cation is fully encapsulated within a supramolecular cage formed by three benzo 15-crown-5 macrocycles. Interestingly, the close to perfect square antiprismatic geometry of the 4f ion enhances its axial anisotropy, which suppresses quantum tunnelling mechanisms (QTM) in the ground and first excited states, resulting in the observation of SMM behavior in zero dc field. For this system the magnetic data were further supported by solid-state photoluminescence and ab initio studies, The introduction of a second benzene ring into the organic framework of the macrocycle increases its rigidity, where on coordination to Dy(III), affords the partially encapsulated complex (IV), which displays slow relaxation of magnetisation, consistent with SMM properties. In Chapter 4, the coordination chemistry of a dual compartmental Schiff-base macrocycle H2L3 containing O3O2 and N3O2 cavities was explored together with select 3d and 4f ions. In the first part of this chapter, the coordination chemistry of H2L3 with 3d metal ions is presented, where in the presence of NaOH, the Na(I) ions reside in the O3O2 cavity and the 3d ions occupy the second N3O2 cavity. Three coordination complexes containing Cu(II), Zn(II), and Mn(II) ions were prepared and characterized. The Cu(II), and Zn(II) complexes are monomeric with molecular formulae [CuNa(L3b)ClCH3OH]‧6H2O (V) and [ZnNa(L3b)(CH3COO)(CH3OH)]‧H2O (VI) respectively, while the Mn(II) complex crystallizes as a trimer with stoichiometry [Mn3Na2(L3)2(CH3COO)4]·5.75CH3OH·0.5H2O (VII). For complexes (V) and (VI), nucleophilic addition of the NH of the N3O2 cavity to the carbon atom of the adjacent imine results in a contraction of the N3O2 cavity and the formation of a five-membered imidazoline ring to afford the modified ligand L3b.The magnetic properties of (V) and (VII) are also reported. In the second part of this chapter, coordination of the macrocycle to select 4f ions in the absence of any base afforded the mononuclear complexes [Dy(H2L3)(H2O)2(CH3OH)2]Cl3·CH3OH, (VIII), and [Ln(H2L3)(H2O)3(CH3OH)] Cl3, where Ln(III) = Tb (IX), Er (X), and Gd (XI), in which the Ln(III) ion is coordinated in the O3O2 cavity. Magneto-structural studies on these complexes reveal that the Dy complex has a slightly different structure than the other three complexes, however all four 4f ions crystallize with square antiprismatic geometries, where only the Dy(III) complex (VIII) displays SMM properties.