Abstract of the Dissertation

The Use of Monocarba-closo-dodecaborate Carborane Anion in Catalyst Design

by

Allen L. Chan

Doctor of Philosophy, Graduate Program in Chemistry

University of California, Riverside, December 2016

Dr. Vincent Lavallo, Chairperson

Carboranes are polyhedral boron-carbon molecular clusters stabilized by electron-delocalized covalent bonding throughout the skeletal framework. Once thought of as novel curiosities due to their structure and bonding, carboranes have now been applied in many diverse fields, from medicine to nanoscale engineering. Of particular interest is the icosahedral monocarba-closo-dodecaborate(-) (HCB11H11 anion) carborane and its derivatives due to their many unique properties. This type of carborane is well known for its extremely low nucleophilicity and considered the most weakly coordinating of all known anions. Also, these carboranes are extremely difficult to oxidize and most are stable to the most aggressive reagents. Because of these unique properties, we explore the use of functionalized carboranes as ligand R-groups in transition metal complexes for their eventual application in catalysis.

Organic azides are invaluable building blocks for synthetic chemists. Herein we report the synthesis of various carboranyl azides and some unusual reactivity during the carboranyl azide synthesis. The synthesis and characterization of the parent carboranyl azide (Li+N3CB11H11-) hexabrominated carboranyl azide (Li+N3CB11Br6-), the perbrominated carboranyl azide (Li+N3CB11Br11-), and the perchlorinated carboranyl azide (Li+N3CB11Cl11-) are reported. We also report unique reactivity involving these new compounds, such as room temperature B-Cl activation of the perchlorinated carba-closo-dodecaborate anion.

Though the formation of Schiff bases from amines have long been used in organic chemistry, the condensation of amines to form imines with the carboranyl amine had not yet been reported at the time. Here, we report examples of imine formation with carboranyl amine as well as metal complex formation with carboranyl imines. Also, we report the synthesis of an N-hetereocyclic carbene with the hexabrominated carborane.

Phosphine ligands have been used extensively as ligands in metal catalyzed cross-coupling reactions. Metal catalyzed cross-coupling reactions are a mainstay of synthetic chemistry. Here, we report the synthesis of various carboranyl phosphine ligands and their use as R-ligands in Pd catalyzed coupling reactions. We also disclose on new and previously unreported activity with carboranyl Pd complexes with different substrates.

The beginnings of boron rich nanoclusters chemistry are in part due to the pioneering work of Alfred Stock. It was Stock’s air sensitive syntheses that paved the road to many pyrophoric borohydrides. Surprisingly some of these pyrophoric borohydrides were found to be useful starting materials for synthesizing the extremely stable closo-carborane anions. Once assembled these polyhedral clusters are composed of mostly boron atoms with at least one carbon atom and are about a nanometer in length. Due to their great stability these anionic nanoclusters have become well known as weakly coordinating anions. This is largely due to the fact that the negative charge is delocalized over the entire polyhedral cluster and that chemist have been able to isolate extremely reactive cations using these anions. However, the applications of these boron rich nanoclusters is not limited to their ability to act as weakly

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coordinating anions but has extended into the fields of medicine and material sciences. One field of study is using these materials as anions tethered to ligand scaffolds, with these anionic scafolds chemist have been able to create metal complexes and, in some cases, even produce state of the art catalyst. The primary focus of this thesis is to explore the use of an anionic boron rich nanoclusters as ligand a substituent/s in N-heterocyclic carbenes (NHC) ligands. The story begins with the synthesis of an unsymmetrical NHC bearing one N-bound anionic boron rich nanocluster and one N-bound aryl group. This synthesis proved to be modular accommodating multiple boron rich nanoclusters and aryl groups. To prove that the anionic NHC salts were viable ligands for coordination chemistry a set of anionic and zwitterionic Au(I) complex were generated. These results showed that these ligands are viable and that the Au(I) complexes are catalytically active for hydroamination chemistry. Envisioning a bimetallic system where the anionic Au(I) complex was coulombically bound to a metal cation lead to the formation of a bimetallic Au(I)/Ag(I) complex. This bimetallic complex was tested as a halide abstracting reagent. Indeed this halide abstracting reagent is capable of chloride abstraction from transition metal complexes. As a proof of principle crystal structures were obtained for Au-/Rh+ and Au-/Ru+ systems. Lastly anionic Au(I) complexes with main group cations is reported. These cations include Ph3C+, R3Si+ and a borenium cation.

The discovery of polyhedral boranes in the early 1900’s by Alfred Stock and coworkers opened the door to an entirely new realm of structural chemistry. Boron’s rather unique property of catenation, the ability to build molecules of unlimited size by covalently bonding to itself, has been exploited to develop the field of polyhedral borane chemistry. In 1960 the first icosahedral carborane was synthesized, B12H122- dianion, by Pitochelli and Hawthorne. Aside from the interesting structure and bonding of the cluster it was found to be incredibly thermally stable, withstanding heats above 800 °C, and chemically inert to almost all reagents. These properties have led to the B12H122- being called the most stable molecule known. The isoelectronic analogues of this structure where one or more BH has been replaced by a CH+ are known as carboranes. The first of this family was prepared in the industrial setting in the 1950’s but went unreported until 1963. The post-World War II government programs (ZIP and HEF) were purposed with developing borane-based rocket fuel that would utilize the higher energies generated by the combustion of boron hydrides in contrast to hydrocarbons. In 1957 ortho-carborane, 1,2-C2B10H12 was isolated by Reaction Motors Inc, however this ground-breaking work on the o-carborane wasn’t published until 1963 when the groups at Thiokol and Olin-Mathieson released a series of papers. Years later, the monoanionic cluster HCB11H111- was synthesized by Knoth in 1967. The related closed 10-vertex anion HCB9H91- was also prepared by Knoth in 1971 and displays many of the same characteristics as the 12-vertex mono-anion. These monoanionic closed cluster carboranes have found application most notably as weakly coordinating anions but are also used in medicine, material science, electrochemistry, and transition metal catalysis. It is these monoanionic carborane clusters that will be the primary focus of this thesis.

A major part of this work is directed at functionalizing these carborane clusters and incorporating them into imidazolium salts. The imidazoliums bearing functionalized carboranes can then converted into the corresponding N-heterocyclic carbenes (NHC) to be used as ligands for transition metal complexes. This thesis will also contain work in the area of electrolyte development exploiting the weakly coordinating nature of the carborane anion. Both the 10- and 12-vertex carborane clusters can be paired with various cations for their implementation in secondary batteries. Magnesium electrolytes were developed and tested in collaboration with the Guo lab at UC Riverside. Solid-state electrolytes were also developed for various cations and tested by our collaborators at Sandia National Laboratory, Argonne National Laboratory, and the Jet Propulsion Laboratory.

ABSTRACT OF THE DISSERTATION

Neutral and Anionic Carboranes as Electronically and Sterically Diverse Phosphine Substituents

by

Christopher Alexander Lugo

Doctor of Philosophy, Graduate Program in Chemistry

University of California, Riverside, June 2017

Dr. Vincent Lavallo, Chairperson

The chemistry of carboranes has expanded greatly since their discovery in the 1960’s. Displaying 3D aromaticity, superior thermal and chemical stability, and facile tunability, these molecules have seen a wide range of applications but little has been said for their potential as transition metal catalysts. We sought to remedy this rift by 1) constructing a library of ligands that integrate carboranes into phosphines as R-group substituents, 2) ligating the ligands to an array of transition metal centers, and 3) surveying the reactivity of the isolated metal complexes.

By substituting the inherently bulky ortho-carborane with a terphenyl moiety at the C-2 position, and a phosphine at the C-1 position, a massive ligand was created. The donor and steric properties were analyzed via a rhodium carbonyl complex. A nido version of the above terphenyl carboranyl phosphine was also synthesized and its behavior with iridium was investigated. Through preparation of phosphines of the anionic 12 and 10-vertex carboranes, rhodium carbonyl complexes were synthesized to quantitatively assess their donor properties. Through utilization of a carboranylphosphino iron complex, the ability to cyclically trimerize methylacrylate has been demonstrated through the isolation a phosphonium enolate species. Amongst the vast library of monoanionic carboranyl phosphines there are none containing a dianionic phosphine. Using PCl3 as a launch pad, a variety of monoanionic and dianionic ligands were systematically prepared with ranging electronic and steric profiles.

Carboranes are polyhedral borane clusters containing one or more carbon atoms in the polyhedral boron framework. Since their discovery in the 1960’s and 1970’s, carborane anions have been utilized to study reactive compounds due to their weakly coordinating properties and remarkable stability arising from their unique structure and bonding. Other applications of carboranes include ligand design, medicine, energy storage, and materials chemistry. One branch of the Lavallo laboratory focuses on the application of carborane anions in ligand and catalyst design, specifically with N-heterocyclic carbenes (NHCs) and phosphine ligands. This work reports the incorporation of the anionic 10-vertex closo-carborane (HCB9H9-) and the anionic 10-vertex perchlorinated closo-carborane (HCB9Cl9-) into ligand motifs as alkyl and aryl surrogates. Several novel anionic imidazolium salts and dianionic carbenes were synthesized using methods developed by our lab, including a perchlorinated water stable carbene. A zwitterionic iridium (I) complex and several iridium (III) dihydrido solvento complexes containing a phosphine appended with the anionic 10-vertex perchlorinated closo-carborane were also prepared and their reactivity explored.

1-carba-closo-dodecaborates, more commonly referred to as carborane anions, are icosahedral CB11 cages with substituents at each vertex. These substituents are highly variable allowing for a large range of properties. Such properties include being weakly basic, chemically inert, non-nucleophilic, and resistant to reduction or oxidation. Due to these properties, carborane anions have been given the term "weakly coordinating" to describe their interactions with cations. The isolation of numerous reactive species, superacids, and coordinatively unsaturated cations are testaments to the weak coordinating ability of carboranes.

Recently carborane anions have begun being transition from being used for fundamental chemistry to applied chemistry with their utility in Lewis acid catalysis being explored by a number of research groups. This dissertation follows this transition as the exploration and discovery of new cations is explored in Chapter 2. Next by careful substitution at the C-vertex the synthesis of carborane anions that can coordinate to transition metals was attempted. By limiting the coordination of the transition metal to the substituent off the C-vertex

the carborane ligand can stabilize traditional transition metal catalysts. The two specific substituents targeted in this research are phosphines and azides.

Phosphines are an obvious choice as their use in transition metal catalysis is ubiquitous. Chapter 3 reports the synthesis and coordination to a gold (I) center of the diisopropyl undecachlorocarborane phosphine anion, iPr2P(CB11Cl11)-.Upon coordination to the gold, zwitterionic and anionic complexes were isolated. These compounds success in hydroamination of alkynes with amines serve as a proof of principle in the use of carboranes in ligand design.

Azides arethe next target for the synthesis of new carborane anion ligands, since the azide can be a versitile synthon. Chapter 4 discusses the attempted synthesis of the undecachlroinated carboranyl azide, N3CB11Cl11-, and the competing side reaction discovery. This cycloaddition results it the cleavage of a typically inert B-Cl bond.

The discovery of a new reaction involving both a carbon and a boron vertex being substituted resulted in the expansion of possible carborane fused triazole anions. Before these can be used as ligands their fundamental chemistry is studied in Chapters 5 and 6. The study of the carborane fused heterocycle resulted in the synthesis of a triazolium carborane zwitterion and a triazolium carborane radical anion.

Carbenes are neutral, divalent, six electron carbon species. Although initially perceived as reaction intermediates, pioneering work from Bertrand showed that they can be isolated in the laboratory. Further, Arduengo isolated N-Heterocyclic Carbenes (NHCs), a stable variety of persistent carbenes usually flanked by bulky aryl groups. These compounds have become popular as strongly coordinating ligands for transition metals producing some of the most active catalysts so far. Since then, there has been a growing interest in the development of NHCs for transition metal catalysis. Over the past few years, our lab has been developing NHCs featuring weakly coordinating Carborane anions. Due to the inherent charge on these molecules, the resulting NHCs are overall mono or dianionic depending on the number of carboranes in the molecule.To impart more robust nature for reactivity investigations, the dianionic 12-vertex closo carboranyl NHCs were hexahalogenated. The monoanionic Au(I) complex of this NHC is itself a weakly coordinating anion allowing us to pair it with a variety of main-group and organometallic cations. The synthetic utility of these ion-pairs was demonstrated by a tandem hydroamination of alkynes followed by the hydrosilylation of the imines using a Au(I)/N-methyl benzothiazolium system. The scope of these carboranyl NHCs was then expanded to the 10-vertex closo carborane where stable NHCs bearing this carborane were synthesized using novel synthetic methods. Perchlorination of this carbene, including the backbone bearing the imidazolylidene ring rendered an unprecedented water stable carbene. Moreover, we show the 10-vertex carboranyl NHC is a strong ligand through synthesis of the corresponding Cu(I) and Au(I) complexes. The dicarbollide ligand, derived from the o-carborane was then functionalized to produce the bis(dicarbollide)iron appended NHCs, the isolobal equivalents of ferrocene. Despite the long-established analogy with ferrocenes, these complexes were seen to behave in a promiscuous way. An unusual transmetalation reaction with other transition metals was discovered to produce the first examples of metallacarborane appended NHCs. Coordination of these NHCs with Cu(I) produced bimetallic complexes with a rare coordination environment. The promiscuous nature of the dicarbollide-NHC ligands was further illustrated by their diverse reactivity with a Pd(II) precursor.

A global reliance on renewable energy sources necessarily requires the progression, and extensive adoption, of energy storage technologies in parallel. Among these, electrochemical energy storage is proving itself the most efficient means of banking energy for portable electronic devices, transportation, and grid-scale applications. Unfortunately, continued development of secondary batteries such as the lithium-ion battery have not kept pace with current energy demands. At present, much attention is directed towards deriving greater energy efficiency from lithium-based architectures while simultaneously moving towards advanced cell chemistries “beyond lithium-ion”. These research thrusts are heavily dependent on the development of novel cell components that can either directly improve battery performance, mediate lesser-developed electrochemical systems, or provide valuable insight into their complex, dynamic chemistry.Herein, the synthesis and implementation of novel electrolyte solutions are investigated for their influence in reversible electrochemical systems with the goal of advancing secondary battery technologies. The electrolytes studied here employ the exotic chemistry of carborane anions: weakly coordinating carbon and boron-containing cluster molecules renowned for their extreme thermal, chemical, and electrochemical stability. As a testament to these properties, presented are multiple systems in which carboranyl electrolytes are favorably paired with pure Li, Na, and Mg metal anodes. Central to this work is chemical modification of the anions which we explore in an effort to unlock favorable chemical and electrochemical properties in the resulting electrolytes. In one instance, we present a “sparingly solvated ionic liquid” species which is applied towards high capacity lithium-sulfur cell chemistries. In another, we expand the scope of carborane electrolytes to sodium metal anodes whose high reversibility is enabled by a fluorine-free solid-electrolyte interphase. Lastly, we incorporate similar anion functionalization strategies to achieve improved ionic conductivity and oxidative stability in an electrolyte applied to magnesium-ion batteries. Here, The carborane anion is demonstrated as an ideal platform for precision engineering strategies which represents a unique approach to electrolyte design.

Carborane anions have been increasingly utilized as a vital tool in the study of reactive compounds as a result of their high stability and weakly coordinating properties. These unique properties has anchored them into the toolbox of any chemist pursing unique and novel chemistry. Though they have been mainly used as counter anions to stabilize reactive cations, their application in organometallic chemistry is still in its infancy. In this report, we provide an in depth study on their impact towards organometallic chemistry. Specifically, we studied their reactivity with the trityl cation and found that it does not react the way it has historically been explained. Moreover, we sought to use them as phosphine ligand substituents. Their use in the hydroamination of alkynes with gold was explored. Furthermore, their incorporation into a series of zwitterionic Ru complexes were examined. We also investigated how carboranyl phosphines affect the activity of Drent-type palladium polymerization of ethylene. Lastly, we probed an exotic palladium dimer with its reactivity with ethylene.

Since the discovery of polyhedral boranes, these molecules have expanded from items of molecular curiosity to having a wide scope of applications in contemporary chemistry. Borane clusters range in size and shape and can possess various heteroatoms. The term ‘carborane’ denotes a class of borane clusters containing one or more carbon atom in the polyhedral boron framework. Carboranes are structurally and electronically unique; while the carbon and boron atoms appear to exceed normal bonding configurations, these atoms are actually participating in delocalized bonding throughout the cluster. During the Cold War, the American government invested in rocket propulsion fuels featuring clusters primarily composed of boron, but the fuels were ultimately determined to be impractical. As the rush for boron fuel came to an end, research performed on borane clusters was published in the early 1960’s, revealing the first carboranes. The work presented here utilizes two different anionic carboranes: the 12-vertex cluster, carba-closododecaborate anion (HCB11H11-), and the smaller 10-vertex cluster, carba-closo-decaborate anion (HCB9H9-). Each anionic cluster possesses an overall charge of -1 delocalized throughout the 3-dimensional cluster, contributing to the tremendous stability of these structures. Substitution of the clusters’ hydride substituents with halides renders the carboranes even more weakly coordinating and much less susceptible toward chemical decomposition.

A large component of this work addresses the incorporation of both the 10 and 12- vertex carborane anions into ligand motifs. Carborane clusters are not typically included in catalyst design, and since the 1960’s there have been only a few investigations exploring the applications of carboranes in ligand development. Carboranes are excellent synthons for constructing covalently bonded, architecturally novel molecules whose electronic and steric properties can be tailored to specific purposes. Beyond their use in ligand design, this thesis also contains work developing carborane-functionalized materials. Due to the charged nature of the carboranes, they are ideal building blocks for ionic liquids. Carboranes possess an array of favorable properties and the goal of the work presented in this thesis is to contribute to a more comprehensive understanding of these unique molecules.