Solution Phase Route towards Titanium Carbide

Thesis

Presented in Partial Fulfillment of the Requirements for the Degree Master of Science

in the Graduate School of The Ohio State University

By

Jingtao Zhang

Graduate Program in Chemistry

The Ohio State University

2019

Master’s examination Committee:

Joshua E. Goldberger, Advisor

Shiyu Zhang, Committee Copyright by

Jingtao Zhang

2019

i Abstract

Carbide materials has shown that they have broad applications in the industry because they are refractory, chemically robust, and electronically conducting ceramic materials. Additionally, some carbide materials, such as WC, have unique catalytic potentials in hydrogen evolution reactions, oxygen evolution reactions and fuel cell reactions. However, the traditional way of synthesizing carbide materials requires ultra-high temperatures (>1000 oC), on account of the slow diffusion constants of carbon through most materials. The need high temperatures are not only resource intensive and costly; they make it virtually impossible to control the morphology.

Furthermore, in many carbide materials, such as SiC, these high temperatures often result in the formation graphitic side products either on the surface, or as inclusions, due to the evaporation of the metals. Thus, establishing lower-temperature scalable, solution-phase routes for solid-state carbide materials would enable the creation of carbide materials in a variety of highly controlled morphological form factors, as well as potential reduce the formation of graphitic carbon.

Our overall strategy is to explore the solution-phase synthesis of carbide materials through the transmetalation reaction between the electron rich carbon center in tetraborylmethanes and metal halides. In exploring the possibility of this kind of reaction, we first explored the feasibility of forming metal-carbon bonds via this transmetalation pathway via molecular reactivity studies. First, tetrakis

ii (1,3-propanediolatoboryl)methane (compound 1) was chosen as our carbanion precursor for its stability, solubility and reactivity as our carbide synthon. We studied the conditions and limits of trimethylgermyl substitution. We established that organolithium-mediated deborylation-metalation methodologies could substitute up to two trimethyl groups on the central methane. (TMG)CB(pg)3 and (TMG)2CB(pg)2 (pg

= 1,3-propanediolate, TMG = trimethylgermyl) were successfully synthesized and characterized. However, we found that additional attempts to replace additional boryl groups on the central carbon center always resulted in protodeboronation over a broad range of temperatures. While these proof of concept studies do indicate that we can readily form two C-Ge bonds from these precursors, these studies indicate higher temperatures are likely necessary for complete substitutional reactivity to the carbide.

After confirming the desired molecular reactivity, we subsequently established a sol-gel process for creating crystalline titanium carbide using tetraborylmethane precursors. First, the reaction of compound 1 with TiBr4 always resulted in the formation of TiO2 at high temperature. Thus, an oxygen-free derivative, tetrakis

(1,3-propanediaminoboryl)methane (compound 5) was synthesized and characterized.

o We established that the solvothermal reaction of compound 5 and TiBr4 at 210 C followed by a 300oC annealing process produced an amorphous titanium carbide framework. Then, crystalline titanium carbide with >100 nm domain sizes is obtained after annealing at 650-750o C. Overall, these initial results are a pivotal first step of a solution-phase route towards transition metal carbides, enabling future explorations on the creation of different metal carbide materials and morphological control.

iii Acknowledgments

I would like to thank Dr. Joshua Goldberger for his tremendous support and help as my advisor. He helped me with research, presentation, and thesis preparation. This master thesis would not have been possible without his help.

Next, I would like to thank everyone in the group especially Dr. Zachary Baum, and Dr. Daniel Weber, who supported me a lot experimentally.

In addition, I really appreciate Dr. Shiyu Zhang as my committee member, who also helped me with coursework and research. My deepest thanks and gratitude to all others that contributed and helped in this work.

iv Vita

2013 ...... Zhejiang University

2017 to present ...... Graduate Research Associate,

Department of Chemistry,

The Ohio State University

Fields of Study

Major Field: Chemistry

v Table of Contents

Abstract...... ii

Acknowledgments...... iv

Vita...... v

Fields of Study...... v

Table of Contents...... vi

List of Figures...... viii

List of Tables...... xi

Chapter 1: Introduction...... 1

Structure of main group and transition metal carbides...... 2

Traditional synthesis of carbide materials...... 4

Molecular carbides...... 6

Summary...... 9

Chapter 2: Tetraborylmethane Synthesis and Stepwise Trimethylgermyl Substitution

...... 10

Overview...... 10

Introduction...... 11

vi Results and Discussion...... 15

Experimental Section...... 21

Summary...... 34

Chapter 3: Solution Phase Route Towards Titanium Carbide...... 35

Overview...... 35

Introduction...... 36

Results and Discussion...... 40

Experimental Section...... 49

Summary...... 51

Chapter 4: Conclusions and Future Outlook...... 52

References...... 53

vii List of Figures

Figure 1.1 Crystal structures of binary a) group 4 and b) group 6 carbides and the c)

AWC2 structure type. (C = black, Group 4 = red, Group 6 = blue, A = yellow)...... 3

n- Figure 1.2 Molecular Carbide Examples. Left: Crystal structure of W6CCl18 , blue: W,

2+ 30 black: C, Green: Cl; Right: Molecular carbide dication [Au6C(PPh3)6] ...... 7

Figure 1.3 A potential metal-carbon condensation pathway towards metal carbides via the transmetalation of tetraborylmethane precursors with metal halides...... 8

Figure 2.1 Metalation products starting from tetraborylmethane. Left: Ref. 34, Top:

Ref. 35, Bot: Ref. 30, Right: Ref. 36...... 12

Figure 2.2 a) synthesis of C(Bpg)4; b) the step wise trimethylgermyl substitution; c) the reaction intermediates for the first substitution...... 14

Figure 2.3 General chelation replacement scheme...... 20

Figure 2.4 Nuclear Magnetic Resonance spectroscopy of compound 1. Top: 1H NMR;

Bot: 13C NMR...... 25

Figure 2.5 Nuclear Magnetic Resonance spectroscopy of compound 2. Top: 1H NMR;

Bot: 13C NMR...... 26

Figure 2.6 Nuclear Magnetic Resonance spectroscopy of compound 3. Top: 1H NMR;

Bot: 13C NMR...... 27

viii Figure 2.7 Nuclear Magnetic Resonance spectroscopy of compound 4. Top: 1H NMR;

Bot: 13C NMR...... 28

Figure 2.8 Nuclear Magnetic Resonance spectroscopy of compound 5. Top: 1H NMR;

Bot: 13C NMR...... 29

Figure 2.9 Nuclear Magnetic Resonance spectroscopy of compound 5 11B NMR..... 30

Figure 2.10 ESI-Mass spectroscopy of compound 5. Left: protodeboronation derivative; Right: compound 5- 345.21 found, 345.32 calculated for [M+H]+...... 31

Figure 2.11 Single Crystal Diffraction structure of protodeboronation version of compound 1 (HC(Bpg)3). Grey: carbon; Blue: boron; Red: oxygen. Hydrogen atoms are left out for clarity34...... 32

Figure 2.12 Single Crystal Diffraction structure of compound 4. Grey: carbon; Blue: boron; Red: oxygen; Green: germanium. Hydrogen atoms are left out for clarity34....33

Figure 3.1 Powder XRD pattern of titanium oxide formed upon the room temperature

o reaction of TiBr4+C(Bpg)4, in chlorobenzene, followed by a 3 hr 800 C anneal...... 44

Figure 3.2 Thermal gravimetric analysis of as-made amorphous titanium carbide framework...... 45

Figure 3.3 Fourier-transform infrared spectroscopy of red ) as-made amorphous titanium carbide framework, black ) annealed TiC...... 45

ix Figure 3.4 Powder XRD pattern of different temperatures annealing of titanium carbide using flat plat mode. Red) 350oC, blue) 450oC, pink) 550oC, green) 650oC, and navy) 750oC...... 46

Figure 3.5 Raman spectroscopy of different temperatures annealing of titanium carbide. Red) 350oC, blue) 450oC, pink) 550oC, green) 650oC, navy) 750oC, and black) commercial titanium carbide...... 46

Figure 3.6 Scanning electron microscopy images of titanium carbide after 750oC annealing...... 47

Figure 3.7 Transmission electron microscopy images of titanium carbide. a-c ) TEM image of 350oC annealed, 550oC annealed, and 750oC annealed TiC, respectively; d-e)

Selected area electron diffraction pattern of 350oC annealed, 550oC annealed, and

750oC annealed TiC, respectively...... 47

Figure 3.8 Energy-dispersive X-ray spectroscopy of titanium carbide after 750oC annealing...... 48

Figure 3.9 X-ray Photoelectron Spectroscopy images of titanium carbide after 750oC annealing. Left: Full Spectrum, Right: Ti 2p region...... 48

x List of Tables

Table 2.1 Attempts for a third substitution with TMG group using compound 5...... 18

Table 2.2 Major product formed with reacting C[B(OMe)2]4 with different diamino or dithiol chelating ligands. LDA: [(CH3)2CH]2NLi; DBU: 1,8-Diazabicyclo

[5.4.0]undec -7-ene; DIEA: N,N-Diisopropylethylamine...... 20

Table 2.3 Crystallographic data for protodeboronation product of compound 2...... 32

Table 2.4 Crystallographic data of compound 4...... 33

Table 3.1 Reported synthetic conditions for titanium carbide. OFu = furfuryl alcohol group; PAN = polyacrylonitrile; Bipy = 2,2’-Bipyridine; PEI = polyethyleneimine...39

xi Chapter 1: Introduction

The refractory metal carbides occupy a unique, but important niche in solid-state materials. The high hardness of early transition metal carbides like TiC and WC make them among the most prevalent materials for machining tools1–3. HfC has the highest melting points of all materials (~3950 oC and 3940oC, respectively) making it of great interest for applications in extreme environments4. SiC is a wide band gap semiconductor, with a melting point of 2830oC and is extremely refractory. It has broad range applications in high voltage power electronics and as a substrate for GaN thin films3.

Other carbide materials, such as WC, have unique catalytic potentials in hydrogen evolution reactions, oxygen evolution reactions and fuel cell reactions 5–7. Since 2011, a novel class of 2D carbide called MXenes have been developed8. They combine the features of transition metal carbide and hydrophilic nature of their surface terminated ligand. These features dramatically increase the applications of carbide materials in the field of energy storage, neural electrodes, water purification, and electromagnetic shielding8–10.

While it has been centuries after the discovery of many carbide materials, it is still synthetically challenging to synthesize many solid-state carbide materials, thus preventing many industrial applications of these materials. Unlike many other anions in solid-state materials, such as halides, oxides, and chalcogenides, most current carbide materials

1 synthesis often requires extreme conditions such as high temperatures well over 1000oC and most often between 1500-2500 oC1. These extreme temperatures needed for synthesis are a consequence of the incredibly small diffusion constants of carbon through most metals. Clearly, synthesizing carbide materials at high temperatures is not energy efficient, and is often complicated by the lack of suitable reaction vessels and heating methods.

Furthermore, these extreme conditions during synthesis make it virtually impossible to utilize templates to control the morphologies at the nanoscale. Additionally, many carbide materials which have a large mismatch in the vapor pressures of their constituent elements, such as SiC, often end up forming graphitic inclusions and coatings at high temperature11. The formation of graphitic species during high-temperature carbide synthesis will further reduce the overall crystallinity.

Structure of main group and transition metal carbides

The carbon anions in solid-state materials can range from methides C4-, acetylides,

2- 4- C2 , and sesquicarbides C3 . The majority of early transition metal carbides typically form methides12. These early transition metal carbides, especially binary carbides, crystallize either into a rock salt lattice (such as TiC, ZrC, HfC) (Figure 1.1a) or a hexagonal inverse lattice (such as WC and MoC)(Figure 1.1b). The latter one features trigonal prismatic coordination around the metal center. The oxidation states in these binary carbide materials can be formally assigned as M4+ and C4- using an ionic model, however, many of their properties, such as their metallic nature, and their cleavage

2 strength, reflect a significant amount of metal-carbon covalency and back electron transfer from the carbon to the metal. Due to the high temperatures of synthesis, these early transition metal carbides often exhibit a high degree of carbon vacancies (MC1-x, x <

0.2 for TiC), and occasionally these vacancies can order13. Additionally, apart from binary carbide materials, families of ternary carbides have been reported using arc melting. Most of these are methides. A large class of these ternary carbide materials have the AMC2 stoichiometry (A and M is a trivalent metal and pentavalent metal, respectively)

14–16 . These AMC2 structures can be seen as a network of ribbons of face sharing MC5 trigonal bipyramids of MC5 connected by axial corner-sharing carbon atoms (Figure

1.1c). The A cation sits between the ribbons counterbalancing the charge. We expect there to be a much richer diversity of carbide structures, if synthesis could be accomplished at lower temperatures.

Figure 1.1 Crystal structures of binary a) group 4 and b) group 6 carbides and the c)

AWC2 structure type. (C = black, Group 4 element = red, Group 6 = blue, A = yellow)

3 Traditional synthesis of carbide materials

Many synthetic routes towards bulk and thin film carbides have been developed. First, the most intuitive method to synthesize these materials would be the direct reaction of the constituent elements. The direct reaction of Si and C to form SiC can be accomplished at

2800oC17. Another common method to create metal carbide materials at the industrial scale is the carbothermal reduction of metal oxides. A typical synthetic route to create silicon carbide at lower temperatures uses SiO2 sand and carbon in a furnace and requires temperatures over 1600oC. It is important to emphasize that both the direct reaction and carbothermal reduction routes require high synthetic temperatures. Another common synthetic route is the solid-state metathesis reaction between metal halides and CaC2 or

Al4C3. These incredibly exothermic reactions are often initiated at lower temperatures but increase to over 1200oC throughout the process18–21. While this method can produce carbide materials relatively quickly, it is difficult to create phase pure products, and commonly graphite byproducts are formed. One of the most powerful approaches towards synthesizing SiC in various form factors is the decomposition of precursor polymers under inert atmosphere. For instance, the most dominant method uses a silicon-backbone polymer, typically poly(methylsilyne), which can be extruded into filaments or shaped, and then repeatedly annealed heated to give high quality, crack-free SiC22. This technology has been commercialized by Starfire Systems only for SiC. Next, in recent decades, flux-based synthetic approaches which use liquid metal as solvent at high temperature were shown to produce high melting point carbide materials. For instance, a cobalt metal flux was used to synthesize VC, by heating the constituent elements at 2000

4 oC to fully dissolve the elements with subsequent cooling to 1200oC. 23–25. One advantage of this flux-based approach is that it can provide an excellent route to complex quaternary carbide materials such as Tb1.8Si8C2(B12)3 that cannot be formed using high-temperature techniques such as arc melting26. This strategy, however, still suffers from requirement of extremely high melting points for many metals. Finally, the electrochemical reduction of metal oxides in salt fluxes has been shown to be a viable route for the creation of metal carbide powders. For example, HfC powder was readily formed from the electrochemical reduction of HfO2 that was mixed with graphite, pelletized, and sintered, using molten

CaCl2 chloride bath at 1173 K. Sintering at 1973 K yielded HfC with an average grain size of about 2 μm27. Again, this typically requires high temperatures, in order to melt the ionic salt.

Approaches towards creating thin film carbide materials include chemical vapor deposition (CVD), and physical vapor deposition (PVD). First, CVD is one of the most powerful thin film processing technologies. In this route gas phase precursors are directly reacted on the surface of a substrate to grow a film. For instance, the growth of SiC thin films has been achieved using SiH4, H2, and small hydrocarbons such as propane or ethylene as the carbon source, and typically occurs at 1500-1600oC28. In 2006, Hu et al. demonstrated the formation of β-SiC nanowires using a single-source CH3SiCl3 precursor

o 29 and H2 at 1050 C . Second, PVD has been shown to be a viable route towards growing carbide films. PVD typically entails the evaporation of a metal using a thermal or e-beam evaporator on a carbon substrate followed by direct reaction, or the evaporation of a metal carbide onto a substrate using sputtering. One example is that polycrystalline WC thin

5 films were prepared on a glassy carbon substrate at a substrate temperature of 1000K6. It is important to point out that very often only a substrate temperature is reported, but these processes typically require a large amount of energy to evaporate the precursors.

Overall, while there are many ways to synthesize carbide materials, they all need high temperatures and suffer from different drawbacks. None of these methods enable the synthesis of nanoscale carbides with controlled morphologies. The development of a lower temperature scalable, solution-phase route towards the synthesis of amorphous and crystalline carbide material would not only be less expensive, but also enable future explorations on the creation of novel metastable metal carbide materials and with morphological control.

Molecular carbides

The creation of a solution-phase route towards carbide materials would also facilitate the synthesis of molecular metal-carbide clusters. Due to the paucity of carbido precursors, there are very few molecular carbides that exist. Probably the most famous

2+ 4- example is [Au6C(PPh3)6] cluster from Schmidbaur, which consists of a C anion in the

30 center of an Au octahedron . This cluster was synthesized via the reaction of Ph3PAuCl with a tetraboryl methane and CsF. Additionally, Corbett et al. (1980) synthesized carbide clusters with carbon metal bonding motifs for materials such as M6CX14 whereas M can be Zr and Hf, and X is halogen atoms31. These carbide clusters typically have a central endohedral C4- anion, in octahedral coordination with six M3+ cations, which are

6 terminated with bridging and terminal halogens. The synthesis of these materials typically

o entails the high temperature (700-900 C) reaction of MCl4, M, and graphite. Similarly,

32 Hinz et al. produced [Ti6C]Cll4 . It was first obtained by metallothermic reduction TiCl3 with Na in the presence of paraffin as a carbon source at relatively low temperatures (300 oC) with approximately 30-50% yield. Next, after the discovery of fullerenes, Castleman et al. (1990) showed that many novel magic ion clusters featuring a carbon element and

+ early transition metals, such as M8C12 (where M can be Ti, Zr, Hf, V, Cr and Nb), can form via the laser ablation of metal carbide films33. The stoichiometry of these magic cluster was determined using mass spectrometry. However, these clusters are incredibly reactive preventing further experimental characterization of their structure and properties.

n- Figure 1.2 Molecular Carbide Examples. Left: Crystal structure of W6CCl18 , blue:

2+ 30 W, black: C, Green: Cl; Right: Molecular carbide dication [Au6C(PPh3)6] .

The key innovation in creating a solution-based route towards carbide materials is the development of a carbido precursor that can be fully transmetalated. Such a precursor could serve as a carbon source for this synthesis of molecular metal carbides. Here we 7 propose the use of tetraborylmethanes as a carbido precursor that can react with metal halides. Each transmetalation form a carbon metal bond, that later constructs the metal carbide framework (Figure 1.3) We will first establish the reaction conditions necessary to maximize M-C bond via the synthesis of metal carbide clusters. In contrast to solid-state materials, which need highly crystalline networks for structural characterization via powder diffraction, the synthesis of small molecule model systems can be readily analyzed via a wide array of solution-based spectroscopy techniques including NMR, Mass spectrometry, FTIR, and Raman. This will allow the determination of the reaction conditions necessary to facilitate M-C bond formation.

O O O O B B M M O O O X B B B M X M MC X O B O O B O O O O

Figure 1.3 A potential metal-carbon condensation pathway towards metal carbides via the transmetalation of tetraborylmethane precursors with metal halides.

8 Summary

As the application of carbide materials continue to deepen, the demand for finding a synthetic way with reduced costs and minimum equipment requirements increases. The main focus of this thesis is to establish a solution route towards carbide materials. Chapter

2 focuses on the synthesis of our tetraboryl methane C(Bpg)4 precursor, and the optimization of C-Ge bond formation via transmetalation with TMG. Furthermore, we developed an oxygen-free tetraboryl methane precursor, to eliminate the possibility of metal-oxygen bond formation. In chapter 3, we successfully developed a sol-gel route towards the synthesis of amorphous TiC at 210 oC and crystalline TiC upon annealing at

650 oC. This represents the first solution phase route for transition metal carbide synthesis and will be further used in developing other carbide materials.

9 Chapter 2: Tetraborylmethane Synthesis and Stepwise Trimethylgermyl

Substitution

Overview

As discussed in chapter 1, the solution phase study of a molecular metal-carbide clusters would help understand the mechanism in metal-carbon bond formation and facilitate the synthesis of bulk carbide materials. Due to the paucity of C4- synthons, only few works have been reported30,34–36. In exploring the suitability of tetraborylmethane precursors as C4- synthons, we have studied the limits of stepwise trimethylgermyl substitutions on tetrakis (1,3-propanediolatoboryl) methane. We have shown that organolithium-mediated deborylation-metalation methodologies readily proceed for up to two substitutions. This enables the synthesis of new borylmethane derivatives including

(TMG)CB(pg)3 and (TMG)2CB(pg)2 (pg = 1,3-propanediolate, TMG = trimethylgermyl).

While further base-activated deborylation attempts lead to protodeboronation caused by the limited reactivity, this work helps us understand the mechanism of metal-carbon bond formation, and also enable future explorations of polyborylmethanes as a solution phase route precursor for bulk carbide materials synthesis.

Furthermore, our previous publication has suggested the likelihood of deesterification of the boronate ester with existence of other metal or main group element(such as Si)34.

We delveloped a novel oxygen-free tetraboryl methane precursor--Tetrakis

(1,3-propanediaminoboryl)methane for carbide materials synthesis.

Note that this chapter contains some information from our previous publication34.

10 Introduction

Tetraborylmethane Review

Completely metalated C4- centers have proven to be unique and important case-studies in molecular chemistry, bioinorganic chemistry, and the solid-state sciences.

For example, the interstitial C4- in nitrogenase iron-molybdenum cofactor plays a central role in enabling the fixation of nitrogen to produce ammonia37,38. Furthermore, solid-state carbides are important structural39, electronic40, and catalytic41 materials. Despite its relevance in bioinorganic chemistry, solid-state carbide materials, organometallics, and carbometalloclusters, incorporating electron rich carbon centers is synthetically challenging. Potential organic precursors that contain carbon-hydrogen bonds are likely unsuitable, due to the notoriously high barriers for activation. In solid-state chemistry, extremely high temperatures are needed for complete metalation around carbon. For instance, the synthesis of endohedral W6CCln requires annealing WCl4, graphite, and W metal at 650oC42. Most solid-state carbide syntheses are achieved with over 1000oC annealing, and such processes often result in graphitic impurities43. Improving our understanding of the metal-carbon bond formation and reactivity of molecular C4- moieties is therefore needed to enable the low temperature solution-phase pathways for materials, clusters, and as use as single-carbon synthons for organometallic chemistry.

Alkylboranes (RBR’3) and alkylboranates (R (B(OR’)2) are excellent candidates for exploration as precursors to achieve metalated C4- centers. With diverse metal-carbon bond-forming chemistries, including applications in Suzuki-Miyaura coupling44 and other

11 late-transition metal catalytic cycles45–52, and even with early transition metal complexes

53, they are a versatile platform for directed metal-carbon bond formation across the periodic table. In particular, tetraborylmethanes (C(B(OR)2)4), which feature 4 carbon-boron bonds as potential reactive sites, have been explored54,55. A complete

36 2+ 30 metalation was already achieved in C(HgOAc)4, C(HgI)4 , and [(Ph3PAu)6C] , as well

35 as three C-B substitutions in (Ph3Sn)3C(Bpg) (Figure 2.1).

Figure 2.1 Metalation products starting from tetraborylmethane. Left: Ref. 34, Top:

Ref. 35, Bot: Ref. 30, Right: Ref. 36.

12 Germanium Carbide & Diboryldigermyl-methanes

While silicon carbide is widely used in industrial applications because of its hardness, conductivity, and thermal robustness, crystalline stoichiometric GeC does not exist in the binary phase diagram and cannot be prepared by high temperature solid-state synthesis 56.

There are only few studies on the growth of germanium carbide films (Ge1−xCx) and

o researchers have found that annealing amorphous Ge1-xCx films above 500 C results in the irreversible phase separation into Ge and graphite57. The only feasible route towards preparing this metastable phase would be a low temperature solution phase pathway.

Therefore, understanding the extent to which a C4- synthon can be metalated with germyl substitutions may allow future explorations of these species as solution phase route carbide synthesis precursors. It is also important to point out that the synthesis of the tetrakis(trimethylgermyl) methane has yet to be reported.

Herein, we establish the stepwise substitution of trimethylgermyl substituents on the quaternary carbon of C(Bpg)4. We show that base-activated substitution proceeds readily for up to two substituents. In so doing, we report the first synthesis of methyldiboronate esters (TMG)2C(B(OR)2)2 bearing two TMG groups. This germyl-substituted quaternary methane could be seen as a molecular carbide and is our first step in understanding how to prepare bulk binary carbide materials(Figure 2.2).

13 Figure 2.2 a) synthesis of C(Bpg)4; b) the step wise trimethylgermyl substitution; c) the reaction intermediates for the first substitution.

Oxygen-free Tetraborylmethane Precursor

Finally, in the course of exploring the transmetalation of C(Bpg)4 with trimethylgermyl, we found that there is a propensity for boronate ester cleavage after two substitutions when boron activation occurs with strong bases34. This is because of the large bonding energy between metal and oxygen. To date, there have been no oxygen-free

C4- synthons developed. However, there is an emerging literature of protected boryl groups such as 1,3,2-diazaborinane (protected with 1,3-diaminopropane)58,

14 1,3,2-dithiaborinane (protected with 1,3-propanedithiol)59. These protected boryl groups have extended application in transmetalation reaction, especially Suzuki coupling reaction60–62. Here, we successfully synthesized a novel oxygen-free tetraboryl methane precursor--Tetrakis (1,3-propanediaminoboryl)methane for the purpose of carbide materials synthesis.

Results and Discussion

Optimized Synthesis of Tetraborylmethanes

We began our investigation by first reoptimizing the reported synthesis of the tetraborylmethane C(B(OMe)2)4 (compound 1) and its transesterification to C(Bpg)4

(compound 2)54. First, boron trichloride and trimethyl borate are reacted together neat in a

1:5 ratio to yield B(OMe)2Cl, with the additional equivalents of trimethyl boratepreventing disproportionation. C(B(OMe)2)4 is then prepared via the reduction of CCl4 with Li metal in the presence of this B(OMe)2Cl (Figure 2.1a).

Specifically, a mixture of CCl4 and B(OMe)2Cl is added dropwise over the span of 2 h to

Li metal in THF solvent. We have found that several modifications to the initially reported method54 are necessary to reproducibly achieve large yields of ~80% for 1 at the

100 mmol scale. First, since the small particle size Li dispersions in hydrocarbon solvent originally used in the synthesis of 1 are no longer commercially available, an alternative high-surface-area source of metallic Li was essential to speed up the kinetics of the reduction of CCl4. Specifically, we utilized 10 stoichiometric equivalents of 170

15 mm-thick battery-grade Li foil relative to CCl4. When high surface area Li is not used, the unreacted B(OMe)2Cl reacts with THF to form ether cleavage products, a well-known phenomenon in the chemistry of boron halides63. Using pure Li rather than the previously reported 50% hydrocarbon dispersion has the added benefit of allowing a completely ethereal solvent, which is crucial for promoting Li activity. Second, reaction yield was maximized by rigorously controlling the addition temperature between -30 and -34oC over the 2 h addition. By controlling these variables, we were able to avoid the destructive sublimation step, which was originally prescribed for purification of 1, and could reproducibly produce >95% pure powder of this intermediate in 30 g batches without purification. We have found that this sublimation step can result in >50% yield due to thermal degradation, consistent with the original process. Transesterification to 2 with 1,3-propanediol also required adjustments from the literature35. While catalytic quantities of BF3OEt2 were originally prescribed as a Lewis acid, we found that the reaction only proceeds smoothly and reproducibly using stoichiometric amounts of this reagent (e.g. 1 equivalent), with completion occurring after just 1 h. Extending the reaction time overnight causes the precipitated 2 to redissolve and react with the CH3OH byproduct, to yield the protodeboronated HC(Bpg)3. The crystal structure of this product is also retrieved. This protodeboronated product is readily formed from 2 in the presence of protic species.

16 Optimized Transmetalation via Trimethylgermyl Substitutions

With large quantities of the methide precursor 2 in hand, we were able to readily synthesize the (TMG)C(Bpg)3 (compound 3) and (TMG)2C(Bpg)2 (compound 4) (Figure

2.1b) by first deborylating with organolithium reagents. These reactions proceed via the organolithium (n-BuLi) first attacking the B to form a 4-coordinate boronate intermediate, followed by cleavage of the original carbon-boron bond to generate a carbanion64(Figure

2.1c). The TMG-Br is then introduced into the reaction mixture and goes through a transmetalation reaction with the organolithium intermediate. This approach is limited to alkylboronates which generate long-lived organolithium intermediates, since strongly

Lewis acidic substrates can react with the base. As a consequence, the boronate must first be deborylated in the stepwise fashion described.

In the hope of synthesizing germyl-substituted quaternary methanes, we ended up forming diboryldigermyl methanes. Base-activated transmetalation methodologies with organolithium reagents worked efficiently with the first two substitutions. Attempts to form a third substitution, either via different solvents, different temperatures, or different activation bases, led to either protodeboronation products using alkoxide bases (Table

2.1), or boronate deesterification using MeLi. Bis(Trimethylgermyl) propylene glycolate was also observed as a product. In addition, no reaction was observed using nBuLi as a base. The lack of desired reactivity and greater propensity for protodeboronation is partly due to the decrease in stability of carbanion intermediate that is generated with the reduced number of π-accepting boryl groups. The bulkiness of the TMG group is another reason preventing substitution from occurring. While a carbon center with four C-Ge 17 bonds was not achievable using this strategy, these germylated borylmethanes can still be a good starting point for more complex carbon-germanium molecular or polymeric materials.

Table 2.1 Attempts for a third substitution with TMG group using compound 5.

Base Solvent Temperature/oC Conversion/% Major Product nBuLi THF 0 - -

MeLi Et2O 25 >95 (TMG)2C(BMe2)2

KOtBu THF 25 >95 (TMG)2CH(Bpg)

KOtBu THF 25 >95 (TMG)2CH2 CsF THF 66 <5 -

Synthesis of Oxygen-free Tetraborylmethane

In Chapter 3, we will show that attempts to react compound 2 with metal halides always resulted in the formation of metal oxides, we also pursued the development of an oxygen-free tetraboryl C4- synthon. Our strategy was to replace the oxygen with a nitrogen or sulfur ligand. The most straightforward synthetic route to form these compounds would be to react compound 1 in the presence of various diamine or dithiol chelating ligands in order to replace the methoxy groups. In particular we focused on replacing the methoxy ligands with 1,3-diaminopropane, 1,2-diaminoethyl,

1,8-diaminonapthalene, and 1,3-propanedithiol. Multiple attempts were made under different conditions, as well as with various Lewis acid or base catalysts(Table 2.2). We were able to successfully replace all methoxy groups with 1,3-propylene diamine to form compound 5 C(Bpn)4. Other than this product, most other chelation reactions were 18 inhibited by the low stability of 1, for which protodeboronation was observed the major reaction. Attempts to chelate 1 with ethylene diamine to produce tetrakis(1,2-ethanediaminoboryl)methane resulted in incomplete methoxy substitution as

1 evidenced by the presence of residual –OCH3 peaks in the H NMR.

Considerable effort was spent optimizing the reaction conditions for C(Bpn)4. Pure

o C(Bpn)4 was prepared in 60% yield after multiple 4-day reaction treatments at 5 C with

~4 eq. of 1,3-diaminopropane. Residual solvent was pumped down between each treatment. Higher reaction temperatures resulted in more protodeboronation. The synthesis of C(Bpn)4 still requires further explorations to maximize the yield and shorten reaction times. Meanwhile multiple Lewis base catalysts were attempted to speed up the reaction including BF3OEt2, AlCl3, AlClEt2, AlCl2Et. Separation of these catalysts using aprotic methods proved challenging. The yield of compound 5 can be increased when using greater equivalents of 1,3-diaminopropane. However, the excess

1,3-diaminopropane was difficult to remove due to its high boiling point and its strong intermolecular interactions with compound 5. Finally, due to its low stability, crystal growth of compound 5 was not achieved.

19 Figure 2.3 General chelation replacement scheme

Table 2.2 Major product formed with reacting C[B(OMe)2]4 with different diamino or dithiol chelating ligands. LDA: [(CH3)2CH]2NLi; DBU: 1,8-Diazabicyclo[5.4.0]undec

-7-ene; DIEA: N,N-Diisopropylethylamine.

Chelating Solvent Temperature/oC Catalyst Major Product reactant

THF 25 BF3OEt2 N.R.

PhMe 25 BF3OEt2 N.R. 1,8-Diamino PhMe 25 - N.R. naphthalene PhMe 110 - Protodeboronation THF 25 - N.R.

THF 25 BF3OEt2 N.R. THF 25 LDA N.R. 1,3-Propane THF 25 KOtBu Protodeboronation dithiol THF 25 NaOEt Protodeboronation THF 25 DBU N.R. THF 25 DIEA N.R. 1,3-Propane THF 5 - Compound 5 diamine

1,2-Ethylene Incomplete THF 25 - diamine Substitution

20 Experimental Section

Synthesis

1. Synthesis of Tetrakis(dimethoxyboryl)methane (C(B(OMe)2)4) (compound 1)

By adding 5.5 equivalents of distilled trimethyl borate(56.11g, 540mmol) into boron trichloride(11.51g, 99mmol) liquid under dry ice-acetone bath, dimethoxyboron chloride was prepared. Carbon tetrachloride(11.29g, 74mmol) was added to the mixture and later added to lithium thin film THF suspension(4.20g, 600mmol) dropwise at around -31±2oC.

The overall mixture was heated to the refluxing temperature for 30 minutes and cooled down to room immediately. Final pure product was then separated by filtration and recrystallization from anhydrous hexane and Et2O mixture. Yield: 18.80g (84.06%)

2. Synthesis of Tetrakis(1,3-propanediolatoboryl)methane (C(Bpg)4) (compound 2)

Starting with a 200ml THF solution of compound 1(18.80g, 61.8mmol), 1 equivalent of boron trifluoride diethyl etherate was added dropwise as well as excessive amount of

1,3-propanediol (23.59g, 310mmol). The solution was stirred for another hour and reaction was stopped immediately. White precipitated was retrieved by filtration and dried under vacuum. Yield: 16.88g (79.40%).

21 3. Synthesis of Trimethylgermyltris (1,3-propanediolatoboryl) methane

(TMG)C(Bpg)3 (compound 3)

Starting with a 100ml THF solution of compound 2(2.63g, 7.46mmol), 1.6M n-BuLi hexane solution(8.17ml 13.1mmol) was added under dry ice-acetone bath condition. The temperature was kept and the solution was stirred for another 15 minutes until it was heated to room temperature slowly. Trimethylgermanium bromide(TMG-Br)(2.95,

14.9mmol) was added dropwise through a syringe. The solution was stirred overnight at room temperature and pumped down under vacuum to remove volatile solvents.

Remaining solid was rinsed with 25ml hexane and final pure product was prepared from recrystallization of anhydrous hexane and Et2O mixture. Yield: 2.59g (90.2%).

4. Synthesis of Bis(trimethylgermyl)bis(1,3-propanediolatoboryl)methane

((TMG)2C(Bpg)2) (compound 4)

For one typical batch, starting with a 30ml Et2O solution of compound 2(154mg,

0.40mmol), 1.6M n-BuLi hexane solution(0.44ml, 0.70mmol) was added under dry ice-acetone bath condition. The temperature was kept and the solution was stirred for another 30 minutes until it was heated to room temperature slowly. Trimethylgermanium bromide (TMG-Br) (158mg, 0.8mmol) was added dropwise through a syringe. The solution was stirred for additional two days at room temperature and pumped down under vacuum to remove volatile solvents. Yield: 120mg (71.72%)

22 Remaining solid was rinsed with 10ml hexane and final pure product was prepared from flash column chromatography on silica gel using DCM as an eluent (Rf = 0.75).

5. Attempts to make tri(trimethylgermyl)(1,3-propanediolatoboryl)methane

((TMG)3C(Bpg))

Similar procedures were used to prepare compound 4 as well as trying different organolithium bases, different solvents, different temperature and reaction time. In a typical attempt reaction, compound 4 in THF solution was added one kind of activation base and added TMG-Br later. Under most circumstances, protodeboronation was observed with little or no TMG substitution.

6. Synthesis of Tetrakis(1,3-propanediaminoboryl)methane (C(Bpn)4) (compound 5)

Starting with a 10ml THF solution of compound 1(304mg, 1mmol), 4.5 equivalents of 1,3-propanediamine(333mg, 4.5mmol) was added to the solution dropwise under ice bath condition. Reaction was stirred for two days and no other Lewis base was used.

Product mixture was pumped down under vacuum after the reaction finished and collected. Above procedures were repeated at least twice to make sure there is no methoxyl group left. Final product is rinsed with 10 ml hexane. Yield: 206mg (59.9%)

23 Nuclear Magnetic Resonance spectroscopy

Nuclear Magnetic Resonance spectroscopy was done for each compound. NMR samples were prepared using 0.75 mL ampoules of CDCl3 (99.9%D) or MeOD (99.8%D) purchased from Sigma-Aldrich. NMR spectroscopy were recorded using Bruker AVIII

400 (1H: 400 MHz, 13C: 100 MHz) or Bruker AVIII HD 600 (1H: 600 MHz, 13C: 151

1 13 MHz). H and C chemical shifts were aligned to residual NMR signal of CHCl3 (d 7.26 ppm), MeOD (d 3.31ppm) and CDCl3 (d 77.0 ppm), respectively.

24 Figure 2.4 Nuclear Magnetic Resonance spectroscopy of compound 1. Top: 1H NMR;

Bot: 13C NMR.

25 Figure 2.5 Nuclear Magnetic Resonance spectroscopy of compound 2. Top: 1H NMR;

Bot: 13C NMR.

26 Figure 2.6 Nuclear Magnetic Resonance spectroscopy of compound 3. Top: 1H NMR;

Bot: 13C NMR.

27 Figure 2.7 Nuclear Magnetic Resonance spectroscopy of compound 4. Top: 1H NMR;

Bot: 13C NMR.

28 →

Figure 2.8 Nuclear Magnetic Resonance spectroscopy of compound 5. Top: 1H NMR;

Bot: 13C NMR.

29 Figure 2.9 Nuclear Magnetic Resonance spectroscopy of compound 5. 11B NMR.

Mass Spectrometry Analysis

Mass spectra were collected on a Bruker MicroTOF ESI-MS coupled to HPLC using positive ion mode. Methanol was used as solvent for every compound.

Compound 1: ESI-MS (m/z, MeOH, positive ion mode): 327.21 found, 327.17 calculated for [M+Na]+.

Compound 2: ESI-MS (m/z, MeOH, positive ion mode): 375.16 found, 375.17 calculated for [M+H]+.

Compound 3: ESI-MS (m/z, MeOH, positive ion mode): 425.12 found, 425.09 calculated for [M+K]+.

30 Compound 4: ESI-MS (m/z, MeOH, positive ion mode): 441.11 found, 441.07 calculated for [M+Na]+.

Compound 5: ESI-MS (m/z, MeOH, positive ion mode): 345.21 found, 345.32 calculated for [M+H]+.

Figure 2.10 ESI-Mass spectroscopy of compound 5. Left: protodeboronation derivative; Right: compound 5- 345.21 found, 345.32 calculated for [M+H]+.

31 Single Crystal Diffraction

Singal crystal diffraction was carried out on a Bruker APEX3diffractometer equipped with a CCD detector.

Figure 2.11 Single Crystal Diffraction structure of protodeboronation version of compound 1 (HC(Bpg)3). Grey: carbon; Blue: boron; Red: oxygen. Hydrogen atoms are left out for clarity34.

Table 2.3 Crystallographic data for protodeboronation product of compound 2.

32 Figure 2.12 Single Crystal Diffraction structure of compound 4. Grey: carbon; Blue: boron; Red: oxygen; Green: germanium. Hydrogen atoms are left out for clarity34.

Table 2.4 Crystallographic data of compound 4.

33 Summary

In this chapter, we established the synthesis and reactivity of new C4- synthons. We have demonstrated that two C-B bonds of a tetraborylmethane can be readily replaced by

TMG groups using base-activated transmetalation methodologies. A novel diboryldigermyl methane product was synthesized with high yield. The overall stability of the digermylated 4 is higher than that of initial tetraborylmethane 2. Attempts to substitute a third carbon-boron bond elucidated a dramatic decrease in reactivity towards base-activated metalation, partly due to the reduced stabilization of the carbanion intermediate, and instead favoring either protodeboronation or boronate ester cleavage.

Simultaneously, a novel, symmetric oxygen-free tetraborylmethane precursor – tetrakis(1,3-propanediaminoboryl)methane – was developed for the first time. The tetrasubstituted compound 1, 2, and 5 have the potential to serve as the precursors for the subsequent low temperature solution phase route towards carbide materials. Together, this work will enable future explorations of polyborylmethanes as synthons for organometallic and solid-state chemistry.

34 Chapter 3: Solution Phase Route Towards Titanium Carbide

Overview

In this chapter, we explore the feasibility of using these tetraboryl methanes precursors as a solution-phase methide precursor for metal carbide materials. Initial attempts of synthesizing silicon carbide and titanium carbide starting with transmetalation reactions of the oxygen-containing compound 1, compound 2 or any other tetraboryl methane precursors always resulted in metal oxide upon annealing at higher temperature.

We discovered that utilizing the oxygen-free tetrakis(1,3-propanediaminoboryl)methane

(compound 5) enabled the formation of titanium carbide at elevated temperatures using a sol-gel process. The solvothermal reaction between compound 5 and titanium tetrabromide at 210 oC enabled the formation of an amorphous branched Ti-C with a mass that is indicative of tri-substitution. Annealing this tri-substituted product above 300 oC resulted in complete substitution and the formation of amorphous TiC, which began to crystallize at 650 oC. This represents one of the lowest synthetic temperatures to form bulk quantities of crystalline TiC. TGA, Raman, XRD, SEM, TEM, EDX, and XPS were used to characterize titanium carbide and to understand the synthetic mechanism.

35 Introduction

As discussed in chapter 1 and chapter 2, the refractory metal carbides occupy a unique but important niche in solid-state materials. This especially the case for titanium carbide, one of the early transition metal carbides. TiC is an extremely hard refractory ceramic material with Mohs hardness close to 9.5. It has an elastic modulus of 400 GPa with a shear modulus of 188 GPa65. TiC is widely used asa wear- and-abrasion resistant surface coating on metal parts for machining tools66. It can enhance the precision and smoothness of steel materials at high cutting speeds, as component of sintered ceramic-metal composites (i.e. cermets)67. Additionally, TiC serves as an excellent thermal barrier coating material for spacecraft atmospheric reentry68.

While TiC has many industrial applications, it only occurs in nature as a very rare mineral called khamrabaevite. Researchers have developed many strategies for synthesizing this material in both in bulk and thin film form (Table 3.1). First, the most direct route for the synthesis of bulk TiC is the direct reaction of titanium metal with graphite powder, for which the combustion synthesis can be initiated at 1600oC17. By far, the most common synthetic route is the carbothermal reduction of Titanium oxide with graphite69–75. Typically, this process forms highly crystalline TiC at >1200oC. Crystalline

TiC can be observed at temperatures >800oC, although typically 1100oC is required to reduce all the TiO2. This carbothermal reduction can also be accomplished with titanium alkoxide single source precursors, as well as with polymeric titanium alkoxides which enable the formation of titanium carbide fibers76,77. The formation of bulk titanium carbide powders via the decomposition of titanium metallocenes requires 1450oC for 36 crystalline TiC formation. Some solid-state metathesis reactions have been found to successfully form crystalline TiC at lower temperatures. The lowest reported TiC formation is the high reduction of TiCl4 and CCl4 with Na liquid in sealed autoclaves when initiated at 450oC78. Another similar route used Mg liquid at 900-1100 oC to reduce

79 TiCl4 and CCl4 . These processes are extremely dangerous and not industrially scalable.

o Li et al. reported the metathesis TiCl4 and CaC2 at 500 C to form TiC with 3 stoichiometric equivalents of graphite80. Finally, a patented process for bulk crystalline

TiC formation reacts TiCl2 with polyacrylonitrile in dimethylacetamide solvent, at

900oC81.

CVD and PVD methods have also been reported to form both amorphous and crystalline TiC thin films at lower temperatures than bulk. The CVD reaction of TiCl4,

CH4, and H2 is the most intuitive route for thin film TiC growth, however, temperatures of 1200 oC is required for crystal growth82. Other studies have explored TiC formation via the thermal decomposition of single source precursors. For example, the decomposition

o 83,84 of Ti(bipy)3 (bipy = 2,2’-bipyridine) forms amorphous TiC thin films at 500 C . The conversion temperature to crystalline TiC using this route was not reported. As another example, Tetrakis(neopentyl)titanium was reported to form TiC at 150 oC85. Although crystalline TiC was claimed to form at pressures <10-5 Torr, no diffraction data was shown in this or subsequent publications. This precursor was also utilized in atomic layer

o deposition of TiC, in which a substrate was treated with H2 plasma at 300 C after every cycle86. This process forms amorphous TiC with 4-7 nm diameter nanocrystalline grains.

A solution-based deposition approach in which H2TiF6 films mixed with

37 polyethyleneimine (PEI) were spun coat onto substrates, and then thermally treated at

1000oC, also forms crystalline TiC thin films87.

Alternatively, PVD has been reported to be a possible route for reducing reaction temperature. It is also important to point out that very often only a substrate temperature is reported, yet these processes typically require a large amount of energy to evaporate the precursors and with extremely low vacuum. For instance, TiC films can be formed via the reaction of atomized Ti formed during sputtering with acetylene at a substrate temperature of 450oC88. Similarly, crystalline TiC films have been reported with thermally evaporated Ti in an ethylene atmosphere, at a substrate temperature of 35oC89.

Finally, thin films of crystalline TiC have been reported by sputtering Ti metal onto a C substrate, followed by annealing at 700oC or greater90.

In conclusion, most titanium carbide synthetic routes require either high temperature, high activation energy or produced graphite side product. Some CVD and PVD processes can form TiC at lower substrate temperatures, however, these only produce thin film titanium carbide. The development of a solution-phase/sol-gel route would be a scalable approach for bulk TiC in numerous morphologies.

Here we developed a solution-phase route for the synthesis of crystalline TiC using tetraborylmethane precursors as the carbon source. First, we show that reactions between

TiCl4 and the oxygen-containing C(Bpg)4 always result in the formation of TiO2 and graphite carbon upon annealing(Figure 3.1). Meanwhile, the oxygen-free C(Bpn)4

o precursor will react with TiBr4 at 210 C, to form a trisubstituted framework. Further

38 annealing at 650oC gives nanocrystalline TiC. This approach shows the potential for utilizing oxygen-free tetraborylated methane as a carbido source for the low-temperature synthesis of transition metal carbide materials.

Table 3.1 Reported synthetic conditions for titanium carbide. OFu = furfuryl alcohol group; PAN = polyacrylonitrile; Bipy = 2,2’-Bipyridine; PEI = polyethyleneimine.

Reactants Temp/oC Form Comments Ref Bulk TiC Synthesis-Direct reaction Ti + C 1600 Bulk - 17 Bulk TiC Synthesis-Carbothermal reduction 69–75 TiO2 + C 1200-1250 Bulk - min 800 Nanofibers Bulk TiC Synthesis-Carbothermal with different precursors Ti[OCH(CH₃)₂]₄ 800 Nanofibers TiC starts forming at 800 oC, 77 Thin film 1100 oC is necessary for 76 [O1.5Ti(OFu)1-x(O-n-Bu)x]n 1150 Bulk complete carbothermal reduction. 91 (C5H5)2Ti(C≡CC6H5) 1000-1450 Bulk Graphite formation Bulk TiC Synthesis-Metal thermal reduction 79 TiCl4+CCl4+Mg 900 Bulk Not scalable 78 TiCl4+CCl4+Na 450 Bulk Not scalable Bulk TiC Synthesis-Metathesis 80 TiCl4+CaC2 500 Bulk Graphite formation 81 PAN+TiCl2 900 Bulk Graphite formation Thin Film TiC Synthesis-CVD 82 TiCl4+CH4+H2 1200-1350 Thin film Crystalline 83,84 Ti(bipy)3 500-550 Thin film Amorphous no XRD 85 Ti(neopentyl)4 150-250 Thin film no XRD reported 86 Ti(neopentyl)4+H2 300 Thin film Amorphous + occasional 4-7 nm nanocrystalline grains 87 H2TiF6+PEI 1000 Thin film Crystalline formation Thin Film TiC Synthesis-PVD 89 Ti+C2H4 35 Thin film Electron beam evaporator 88 Ti+C2H2 450 Thin film cylindrical titanium cathode Ti+C 750 Thin film 700 nm Sputtered Ti film 90

Thin Film TiC Synthesis-Other 92 Ti+CH4 * - Plasma Flame synthesis, Temperature not reported

39 Results and Discussion

TiO2 Formation Via Reaction of Oxygenated Tetraborylmethane Precursors

We first explored the feasibility of using C(B(OMe)2)4 and C(Bpg)4 to synthesize TiC under different conditions. At room temperature, a spontaneous reaction occurs between

TiBr4 and these precursors in different solvents (chlorobenzene, toluene, hexane, benzene) in the absence of an activating base, to form a polymeric gel. This gel was subsequently washed with chlorobenzene to remove the intermediate byproducts that would be expected such as Br-Bpg. Further annealing to 300-800 oC produces rutile titanium dioxide(Figure 3.1). There are two likely routes for TiO2 formation. First, TiBr4 can promote the deesterification of the boronate ester groups to form an insoluble titanium alkoxide intermediate, which thermally decomposes into titanium dioxide. Alternatively, the oxygen-containing ligands were not entirely substituted, and any residual propylene glycolate or methoxy groups would promote oxide formation upon annealing. This is especially the case when the tetraborylmethane is not completely substituted. Other experiments in our group had reported that similars reaction of these oxygenated tetraboryl methane precursors with SiCl4 and Nb(NMe2)5 produced SiO2 and Nb2O5, respectively93. Thus, to prevent the possibility of any metal-oxide formation, we explored the reactivity with oxygen-free compound 5.

Solvothermal Synthesis of TiC

The solvothermal reaction of TiBr4 with C(Bpn)4 followed by annealing was found to be a viable route for forming amorphous TiC (350-550 oC) and crystalline TiC

40 (650-750oC). A typical reaction is accomplished via the addition of a ~1.1:1 molar ratio of

o TiBr4 and 5 in chlorobenzene, followed by a solvothermal reaction for 4 days at 210 C.

Upon completion, a grey black gel is formed. The grey black gel is purified by multiple washes 1,3-propanediamine and chlorobenzene in succession which removes the Br-Bpn byproduct. The residual solvent was pumped down and the black solid was isolated. This powder was considered to be an amorphous titanium carbide polymer.

To further characterize the stoichiometry of this polymer, we performed Thermal gravimetric analysis of the as-made product (Figure 3.2) A major weight loss started at

~250oC and is completed by 350 oC. Overall, a 70% mass loss observed. The mass loss can be used to estimate the degree of Ti-C bond formation from the solvothermal reaction, considering the annealed product is TiC. If the amorphous product is a disubstituted framework, such that each Ti would have two residual Br ligands, and two Ti-C bonds, and each C has two Bpn ligands, an 85% mass loss is expected. In contrast, a 73% mass loss is expected assuming the amorphous titanium carbide polymer is trisubstituted, such that there would be one Br ligand on each Ti, and one Bpn ligand on each C, along with three Ti-C bonds. Fourier Transform Infrared Spectroscopy (FTIR) further confirms the presence of the propylene diamino group in the unannealed titanium carbide polymer(Figure 3.3). The intense N-H stretch at 3200 cm-1, and C-H stretch region ranging from 3000-2600 cm-1 along with the C-C fingerprints at 1600-1000 cm-1 in the unannealed titanium carbide polymer(Figure 3.3) are very similar to that of the reported spectrum of tris(dimethylamino)borane in the SDBS. Upon annealing above 400oC, there are no noticeable infrared modes in the FTIR spectra. This further confirms the complete

41 elimination of the organic substitutents corresponding to the major mass loss in the

TGA(Figure 3.2).

Powder X-ray Diffraction analysis of the amorphous Titanium carbide polymer after annealing at different temperatures confirms the formation of crystalline TiC at temperatures above 650 oC (Figure 3.4). When the as-grown product is sealed in a quartz tube and annealed at temperatures ranging from 350-550 oC the resulting diffraction pattern is indicative of amorphous TiC. After annealing above 650 oC diffraction peaks corresponds to nanocrystalline TiC start to emerge. The five observed peaks correspond to the 111, 200, 220, 311, and 222 reflections in the TiC rock salt lattice. Upon annealing to 750 oC, these reflections become much sharper, indicative of larger crystalline domain sizes. A Le Bail refinement of the 750 oC pattern using GSAS software, confirmed that the lattice had an Fm-3m space group with a 4.3214±0.0053Å lattice constant. This is in close agreement with the accepted literature values of TiC (a = 4.3268 ± 0.0006Å)94. In addition, the average crystal domain sizes can be estimated using the Debye-Scherrer equation95. The mean crystal domain size was calculated to be 4.4±0.2nm at 650oC, and

101.4±5.7 nm at 750oC. Still in the 750 oC, there is a broad peak at the same position as each major reflection, which is indicative of the presence of residual small domain nanocrystalline TiC.

Next, Raman spectroscopy was collected to confirm the formation of titanium carbide(Figure 3.5). Titanium carbide has the NaCl rock salt structure. Here, every atom has inversion symmetry, which would theoretically give no first-order Raman scattering.

However, when there is a deficiency of carbon atoms, the overall inversion symmetry of 42 nearby atoms will be destroyed, thereby making titanium carbide Raman active96. As shown in the above Raman figure, the corresponding modes after annealing above 550oC are an excellent match to that of the commercial titanium carbide.

SEM and TEM further confirm the morphology of the TiC material. SEM micrographs of the powder when annealed above 750 oC show a nanoparticulate morphology with grain sizes that are less than 200 nm (Figure 3.6). TEM also confirms the evolution of crystallinity upon annealing(Figure 3.7). The 350 oC product showed a nanoparticulate morphology with grain sizes less than 50 nm, and an electron diffraction pattern that is completely amorphous. The 550 oC annealed product still has a very similar nanoparticulate morphology while very broad rings corresponding to diffraction from the

200 and 400 peaks could be observed. Upon annealing to 750 oC, the morphology the grain size increases in some regions, and grain sizes > 200 nm can be observed embedded in a matrix of smaller nanocrystalline TiC materials. This is consistent with our XRD results. Electron diffraction analysis further confirms the formation of TiC, with a calculated lattice constant of a = 4.32Å, which is a match to that of XRD. Also, the

Energy Dispersive X-ray Spectroscopy confirms the presence of Ti in product(Figure

3.8). While oxygen and carbon are also observed, their presence is inconclusive as a consequence of using a formvar grid. No other elements including N, B, Br were observed in the EDX spectra.

Finally, the X-ray photoelectron spectrum of the 750 oC annealed sample is in close agreement with the literature value(Figure 3.9). In TiC, the Ti 2p3/2 and the Ti 2p1/2 binding energies are reported to be at 454.9-455.1 eV and 460.9-461.1 eV97–99. Indeed, we 43 observe two major 2p3/2 and 2p1/2 peak sets. The most intense peaks exist at 454.98eV and

460.90eV, corresponding to TiC. In addition, a much weaker shoulder at higher binding energies 459.30eV and 465.27eV are present. These likely correspond to a partially oxidized surface.

Figure 3.1 Powder XRD pattern of titanium oxide formed upon the room

o temperature reaction of TiBr4+C(Bpg)4, in chlorobenzene, followed by a 3 hr 800 C anneal.

44 Figure 3.2 Thermal gravimetric analysis of as-made amorphous titanium carbide framework.

Figure 3.3 Fourier-transform infrared spectroscopy of red ) as-made amorphous titanium carbide framework, black ) annealed TiC

45 Figure 3.4 Powder XRD pattern of different temperatures annealing of titanium carbide using flat plat mode. Red) 350oC, blue) 450oC, pink) 550oC, green) 650oC, and navy) 750oC

Figure 3.5 Raman spectroscopy of different temperatures annealing of titanium carbide. Red) 350oC, blue) 450oC, pink) 550oC, green) 650oC, navy) 750oC, and black) commercial titanium carbide.

46 Figure 3.6 Scanning electron microscopy images of titanium carbide after 750oC annealing.

Figure 3.7 Transmission electron microscopy images of titanium carbide. a-c ) TEM image of 350oC annealed, 550oC annealed, and 750oC annealed TiC, respectively; d-e)

Selected area electron diffraction pattern of 350oC annealed, 550oC annealed, and 750oC annealed TiC, respectively.

47 Figure 3.8 Energy-dispersive X-ray spectroscopy of titanium carbide after 750oC annealing.

Figure 3.9 X-ray Photoelectron Spectroscopy images of titanium carbide after 750oC annealing. Left: Full Spectrum, Right: Ti 2p region.

48 Experimental Section

Synthesis

1. Synthesis of amorphous titanium carbide using compound 5

A suspension of compound 5(172mg, 0.5mmol) in chlorobenzene(12ml) was added with TiBr4 solid(184mg, 0.5mmol). Reaction mixture was sealed in a parr bomb and heated up to 210oC for 4 days. Black precipitation was retrieved after cooling down to room temperature. The precipitation was washed with 20ml chlorobenzene and 10ml

1,3-propanediamine twice. The as-made amorphous titanium carbide was then pumped down to remove solvent.

2. Attempt synthesis of amorphous titanium carbide using compound 1

A suspension of compound 1(176mg, 0.5mmol) in chlorobenzene(12ml) was added with TiBr4 solid(184mg, 0.5mmol). Reaction mixture was sealed in a parr bomb and heated up to 210oC for 4 days. The precipitation was washed with 20ml chlorobenzene twice. The as-made amorphous mixture was then pumped down to remove solvent.

Further annealing of this mixture always led to titanium oxide.

3. Synthesis of crystallinity titanium carbide

The as-made amorphous titanium carbide from procedure 1 was placed in a crucible and sealed in a vacuum quartz tube with pressure below 0.080mmHg. The tubes were

49 heated up to 350oC, 450oC, 550oC, 650oC, and 750oC, respectively. Crystallinity titanium carbide was retrieved after annealing over a low temperature 650oC and collected for future characterization.

Instrumental Details

XRD was performed on a Bruker D8 Advance Powder XRD using a polymeric sample holder. Raman spectroscopy was performed on Renishaw Raman IR microprobe using 633 nm laser source and 1% power intensity. XPS was performed in an air-free holder and retrieved after Argon sputter etching for 8minutes with Kratos XPS. FTIR was performed using a PerkinElmer Frontier Dual-Range FTIR spectrometer inside a glovebox. TEM/EDX was performed using a Tecnai G2 30 TWIN with a 300 kV

Transmission Electron Microscope. SEM was performed using a FEI Helios Nanolab 600

Dual Beam Focused Ion Beam/Scanning Electron Microscope

50 Summary

In summary, we have developed a solution phase solvothermal route towards the titanium carbide using an oxygen-free tetraborylmethane precursor(compound 5). TGA indicates that our as-made amorphous titanium carbide has tri-substituted Ti-C framework with major weight loss finished at 350oC. XRD suggests the formation of nanocrystalline titanium carbide at 650oC, and an average crystal domain size increased to 101.4±5.7 nm when heating to 750oC. This represents one of the lowest for bulk titanium carbide synthesis reported to date. Raman peak positions match that of commercial titanium carbide. TEM confirms its polycrystalline nature and the observed lattice constant is the same as ICSD database. Starting with compound 5, many other oxygen free C4- synthons can be made for the use of synthesizing carbide materials. Low temperature requirement also opens the possibility to synthesize carbide (GeC) that are impossible to make at higher temperature.

51 Chapter 4: Conclusions and Future Outlook

In conclusion, a solution phase route towards titanium carbide has been developed.

On the molecular level, we have proved the feasibility of “transmetalation” reaction using organolithium base activation method. By replacing C-B bonds with TMG groups,

(TMG)2C(Bpg)2 (compound 4) was synthesized. Attempts for a third substitution failed for a decrease in the stability of carbanion intermediate and led to mostly protodeboronation product. Simultaneously, an oxygen-free tetraborylmethanes C(Bpn)4

(compound 5)was synthesized for the first time and characterized. These works can enable future exploration of tetraborylmethanes in organometallic chemistry and solid-state chemistry.

As for solid-state chemistry, amorphous titanium carbide was achieved using

o C(Bpn)4 and TiBr4 under solvothermal conditions at 210 C. Low-temperature annealing of as-made amorphous titanium carbide led to crystallinity carbide with average crystal domain size of 101.4±5.7 nm.

Together, this work developed a novel solution phase route towards carbide materials.

This enables future explorations of using tetraborylmethanes as C4- synthons for different kinds of organometallic and solid-state reactions. This also can be adapted for the reparation of existing (HfC), and novel (GeC) carbide materials synthesis.

52 References

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