Thesis (Ph. D.)--University of Washington, 1997. / Vita. Includes bibliographical references (leaves [158]-172).

Thesis (Ph. D.)--University of Hong Kong, 2004. / Also available in print.

M.Sc. (Chemistry) / Please refer to full text to view abstract

Ph.D. / This study comprises the preparation and characterization of new carbene complexes of iron, molybdenum, gold, copper and silver from azolyl and thienyl precursors. In addition, the syntheses of aminoacyl chelate and metallacyclic iron compounds as well as unique 6-membered dimeric and 18-membered trimeric thienyl gold compounds are reported. Furthermore, di(vinyl)carbene complexes, which are not yet known for gold and which are very rare for most other metals, are also described. In contrast to most other carbene complexes that result from precursors in which the heteroatom is situated a or y to the coordinated carbon atom, the new amino(organo)-, organo(thio)- and di(organo) carbene complexes are unique in that they have been prepared from azolyl or thienyl precursors in which the nucleophilic heteroatom is located outside the coordinated ring system and is separated from the coordinated carbon by several bonds. The complex [CpFe(CO)2C1] reacts with lithiated pyrazole, (thienyl)oxazoline or (thienyl)pyridine to form precursor pyrazolyl and thienyl iron(II) compounds which upon alkylation or protonation with CF3SO3Me or CF3SO3H afford amino(organo)-, organo(thio)- or heterometallacyclic alkoxy(amino)- and hydroxy(amino)carbene complexes as well as compounds which show pyridinium character. The heterometallacyclic complexes incorporate unusual ferropyridine or ferropyrole rings. ' 3C-{'}I} NAIR data of the above compounds show that the coordinated carbons of the azolyl and thienyl ligands are significantly affected upon carbene formation, although an X-ray crystallographic investigation indicates that carbene formation has little if any effect on bond distances in the azolyl ligand when it becomes a coordinated azolinylidene. The molecular structures of the pyrazolyl complex [CpFe(C0) 2{C=CHCH=NNPh}], the pyrazolinylidene complex [CPFe(C0) 2{CCHHNHI4P11}1[CF3S03], the 2-(2'-oxazolinyl)thien- 5-ylidene complex [CpFe(C0)2{CCHHC(1\1HCMe2CH26)}1[CF3S03] and the heterometallacyclic complex [CpFe(C0){CC(=&CMe 2CH2(5)SCH---9-1)] indicate small variations in iron-carbon distances of 1.981(2), 1.969(5), 1.99(1) and 1.959(7) A. Four equivalents of 2-(4',5'-dihydro-4',4'-dimethy1-2'-oxazolinyl)thien-3-yllithium reacts with the acetate [Mo2(O2CMe)4] to form the stable, neutral, quadruply bonded dimolybdenum complex [Mo2{C=C(6=NCMe2CH26)SCH2CH2} 2(02CMe)2], while 4-methylthiazoly1 lithium reacts with the same acetate to form the unstable, thiazolyl molybdate [Mo 2{6=NCMe—Cfg} 8]4". Reaction of [AuCI(tht)] (tetrahydrothiophene) with 2-(4',5'-dihydro-4',4'-dimethy1-2'-oxazoliny1)- thien-3-yllithium or 2-(4',5'-dihydro-4',4'-dimethy1-2'-oxazolinyl)thien-5-yllithium afforded dimeric and trimeric thienyl oligomers of gold(I). The molecular structure of the six-membered dimeric compound [Au{C=C(=NCMe 2CH2O)SCH=CH}li shows a Au...Au separation of 2.8450(6)A, while such interactions are absent in the trimeric compound [Au{C=C(C=NCMe2CH2O)SCH=CH}] 3 . Protonation of the former compound as well as the stable monomers obtained from [Au(tht)C 6F5] or [Au(Cl)PPh 3J afforded unique di(vinyl)carbene as well as imine complexes. Reaction of 2-(2'-pyridyl)thien-5-yllithium with [Au(COPPh 3] and subsequent alkylation yields an organo(thio)carbene compound. Finally, a series of cationic copper(I) bis(carbene) complexes were formed upon sequential treatment of copper(I) trifluoromethanesulfonate with thiazolyl- or pyrazolyllithium and CF3SO3Me. A similar reaction with 4-methylthiazol-2-yllithium and silver triflate produced the first amino(thio)carbene complex of silver.

M.Sc. / Please refer to full text to view abstract.

The present study involves the use of main group organometallics: organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are well stabilized by the π‐donor substituents such as alkoxy and amino groups and low oxidation state metals such as Group 6 (Chromium, Molybdenum or Tungsten). Carbenes are an important intermediate in the synthesis of a range of compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and successful characterisation of three α‐alkoxystannanes was achieved via nucleophilic addition of tributylstannyllithium to the respective aldehydes, followed by an immediate MOM protection of the resulting alcohol. Six α‐aminostanannes were synthesised, consisting of N‐BOC, N‐acetyl and N‐ethyl derivatives of pyrrolidine and piperidine, via α‐lithiation and subsequent tinlithium transmetallation in the presence of TMEDA. The ¹³C NMR analysis highlighted an interesting phenomenon of tin‐carbon coupling that revealed unique structural information of both types of stannanes. DFT analysis was completed on the series of stannanes; a predicted frequency analysis was obtained which complemented the experimental Infra‐red data in elucidation of the compounds. The α‐alkoxy and α‐aminostannanes provided stable precursors to the organolithiums required for the synthesis of the novel Fischer chromium carbenes. The organolithiums were obtained via tinlithium exchange at low temperatures, followed by the addition of chromium hexacarbonyl to form the acylpentacarbonyl‐chromate salt. Alkylation of this intermediate using a Meerwein salt, Me₃OBF₄, gave rise to the novel Fischer chromium carbene complexes. Fischer chromium carbenes derived from the two isomeric butyl and isobutyl stannanes and the two N‐ethyl α‐aminostannanes were successfully synthesised. The difficulty encountered in the purification of the Fischer carbene complexes hindered the full characterisation, due to the presence of a by‐product, tetrabutyltin.

This study has focused on the design and preparation of bi– and tridentate N–Heterocyclic Carbene (NHC) ligands in order to investigate the effect of a multidentate approach to the formation, stability and catalytic activity of coordination complexes. Chapters 1 – 3 provide background information of relevant catalysis, carbene and coordination chemistry, followed by previous work performed within our research group. In Chapter 4 attention is given to the synthetic aspects of the research conducted, comprising two distinct approaches to the preparation of unsymmetrical saturated and unsaturated NHCs. Firstly, an investigation of the saturated NHC ligands yielded three novel, unsymmetrical pro–ligands, viz., two halopropyl imidazolinium salts and a bidentate hydroxypropyl imidazolinium salt. Secondly, eight imidazolium salts have been generated, including a hydroxypropyl analogue and novel decyl and tridentate malonyl derivatives. These compounds were prepared using microwave–assisted methodology for the alkylation of N– mesitylimidazole – an approach that drastically reduced reaction times (from 8 hours – 7 days to ca. 0.5 – 2 hours) and facilitated isolation of the imidazolium salts. Many of the compounds prepared in this study are novel and were fully characterized using HRMS and 1– and 2–D NMR analysis. Coordination studies using a selection of the prepared pro–ligands afforded an alkoxy–NHC silver derivative and four novel Ru–complexes, viz., Grubbs II–type Ru–complexes containing:– chloropropyl imidazolinylidene; propenyl imidazolylidene; and bidentate alkoxypropyl imidazolylidene ligands. Furthermore, a well–defined benzyl mesitylimidazolylidene Ru–complex has been isolated, which exhibited good stability in air. DFT–level geometry–optimization studies, using the Accelrys DMol3 package have given valuable insights into the likely geometries of the prepared and putative catalysts.

This organometallic study involves the use organostannanes and organolithiums as precursors to chromium Fischer carbene complexes. Fischer carbenes are typically electrophilic and are stabilized by a single π-donor substituent, and contain low oxidation state metals (often but not always from Group 6). They are highly reactive and can give access to a range of biologically active compounds through cyclopropanations, insertions, coupling and photochemical reactions. Synthesis and characterization of three MOM-protected α-alkoxy organostannanes was successfully carried out via a nucleophilic addition of tributylstannyllithium to suitable aldehydes, and immediate protection of the alcohol with MOM. Two N-BOC protected α-amino organostannanes were successfully synthesized and characterized via α-lithiation and tin-lithium exchange in the presence of TMEDA. Tin-lithium transmetallation of the organostannanes allowed access to the organolithiums required for the synthesis of novel Fischer carbenes. Addition of chromium hexacarbonyl to the organolithiums formed the acylpentacarbonyl chromate salt which was alkylated with Meerwein salt, resulting in the Fischer carbene and a by-product, tetrabutyltin, which proved difficult to remove. Several Fischer carbenes were synthesized and characterized, some simple and known and some novel. In silico work explored the reaction coordinate of the [2+2] cycloaddition towards the formation of β-lactams, and the photoactivation cycle that precedes this process. Computational work also showed the effect of the ligand on the stability and reactivity of the carbene. It was found that in some cases the oxygen on the ligand could negatively influence the stability of the carbene (when compared to a simple methyl carbene). A link between bond orders and back donation in Fischer carbenes was explored in an attempt to theoretically predict the stability of a range of carbenes. / Microsoft� Office Word 2007

A computational chemistry analysis combined with a crystallographic database study of the bonding in late transition metal N-heterocyclic carbene (NHC) complexes is reported. The results illustrate a metal-carbon bond for these complexes, approximately 4% shorter than that of a M-C single bond found in metal alkyl complexes. As a consequence of this result, two hypotheses are investigated. The first hypothesis explores the possibility of multiple-bond character in the metal-carbon linkage of the NHC complex, and the second, considers the change in the hybridization of the carbenoid carbon to incorporate more p character. The latter hypothesis is supported by the results. Analysis of these complexes using the natural bond orbital method evinces NHC ligands possessing trans influence.

This dissertation summarizes my research in the Nuckolls group on two projects, with a central theme of achieving control of electronic coupling in various nanoscale systems. The two studies of interest aim at the study of emerging properties from alkali-doping of polyaromatic hydrocarbons (PAH), and the synthesis of novel metal chalcogen molecular clusters. Chapter 1 is divided into two parts. Part one provides a brief history of the forces we associate with bond formation. We will learn that although defining a “chemical bond” is helpful, it is limited to our incomplete understanding of what forces contribute to its existence. The behavior of an electron in externally applied magnetic fields will be discussed, where the collective behavior of electrons in a material can be measured, showing a myriad of emerging properties. The known superconducting alkali-doped PAHs are introduced, followed by the unresolved problems of reproducibility and lack of structural data to accompany superconducting samples. Finally, the proximity of AFM to superconductivity is discussed, which could give us insights on further exploration of hight temperature organic superconductors. Part two introduces atomically precise clusters of atoms, also knows as superatoms. Various synthetic approaches to create metal chalcogenide superatoms are introduced. Next, a closer look into the cobalt selenide core, [Co6Se8], is presented. The ability to selectively substitute the ligands on this superatom, achieves dimensional control. The subunit can be seen as a 0-dimensional subunit, where it readily gives away its electrons. Furthermore, assembly of the clusters into 1-, 2-, and 3-dimensional structures is described. Chapter 2 introduces a novel approach to acquire phase pure alkali-doped PAHs, p-terphenyl specifically. Previous reports of solution-processed doping of PAH have inspired highly reliable synthesis of these salts, by employing a chelating agent to stabilize the alkali metal. The first half of chapter 2 analyzes one such crystal in detail, describing emerging AFM fluctuations. The AFM coupling between nearest neighboring p-terphenyls occurs in all three crystallographic directions. Interestingly, this coupling can be seen as an unconventional bond between two terphenyl units along the hard axis, and resembles resonance structures seen in polyacetylene. The second half of the chapter further investigates the novel method, obtaining a library of alkali-doped p-terphenyls. This approach allows for selective variation of either the alkali-metal, the chelating agent, or the electronic structure of p-terphenyl. Obtaining nearly a dozen structures allows for a study of trends of doping level and accompanied magnetic properties. Lastly, Chapter 3 proves a new mechanism for ligand substitution of cobalt selenide superatomic clusters, using an easily removable carbene as the ligand. This approach grants access to new surface ligands and core shapes to expand the properties of these superatoms. Through this approach, larger atomically precise materials can be targeted, giving rise to new types of electronic properties.