Synthetic Studies of Sodium Aminodiboranate Salts with Electron- Withdrawing Substituents

SYNTHETIC STUDIES OF SODIUM AMINODIBORANATE SALTS WITH ELECTRON- WITHDRAWING SUBSTITUENTS

CONNOR I. DALY

THESIS

Submitted in partial fulfillment of the requirements for the degree of Master of Science in Chemistry in the Graduate College of the University of Illinois at Urbana-Champaign, 2020

Urbana, Illinois

Adviser:

Professor Gregory S. Girolami

ABSTRACT

Magnesium diboride, MgB2, has the highest superconducting critical temperature of all traditional superconductors and is being considered for the fabrication of superconducting integrated circuits that can operate at higher temperatures than analogous niobium alloy circuits.

A significant barrier to preparing superconducting MgB2 thin films, however, is that the films must be deposited below 300 °C to prevent Mg from subliming out of the film and causing loss of the superconducting properties. One approach to solve this problem is to prepare lowtemperature chemical vapor deposition (CVD) precursors which would enable deposition of

MgB2 at temperatures low enough to prevent loss of Mg from the thin film.

In chapter 1, we describe the synthesis of three new sodium aminodiboranate salts,

Na(BH3)2NRR′, with trimethylsilyl groups on nitrogen. These compounds – sodium N,N-bis-

(trimethylsilyl)aminodiboranate, sodium N-trimethylsilylaminodiboranate, and sodium N- methyl-N-trimethylsilylaminodiboranate – were prepared by the reaction of μ-aminodiborane(6) species with sodium hydride. IR, 1H NMR, 13C NMR, and 11B NMR spectra are reported for all of these compounds, along with the crystal structures of N-trimethylsilylaminodiboranate and N- methyl-N-trimethylsilylaminodiboranate. Comparisons of 11B NMR chemical shifts suggest that steric, inductive, and hyperconjugative effects of the substituents on nitrogen are all in competition in these systems.

In chapter 2 we describe the preparation of fluorinated aminodiborantes, although here the longer reaction times required for their synthesis lead to undesirable production of butoxyborate species through the ring-opening reaction of thf with BH3. To circumvent this problem, we employed non-cyclic ethers such as diglyme and cyclopentyl(methyl)ether to investigate the preparation of μ-aminodiborane(6) species from the fluorinated amines N,N-

ii bis(2,2,2-trifluoroethyl)amine, N-(2,2,2-trifluoroethyl)amine, and N-methyl-N-(2,2,2-trifluoroethyl)amine. We find that there is a correlation between the pKa of the ammonium form of the amine with the relative stabilities of the μ-aminodiborane(6) and aminoborane: N,N-bis(2,2,2- trifluoroethyl)amine forms the aminoborane preferentially, whereas both N-(2,2,2- trifluoroethyl)amine and N-methyl-N-(2,2,2-trifluoroethyl)amine form the µ-aminodiborane(6) in good yields. Sodium N-methyl-N-(2,2,2-trifluoroethyl)aminodiboranate was prepared and characterized by 11B NMR spectroscopy, although efforts to isolate it were unsuccessful.

iii

ACKNOWLEDGMENTS

It becomes all the more apparent at the end of this work, that its completion was something that I would never have been capable of on my own. If I were to choose one lesson that completing my thesis work has taught me, outside of numerous chemical lessons, it would be that collaboration with other skilled individual is crucial for any successful endeavor both inside and outside the sciences, and I would like to take this moment to thank them.

First and foremost, I would thank my advisor, Professor Gregory Girolami, for serving as my mentor over the course of this degree, and for taking a chance on me and accepting me into his group after I left the Makri group halfway through my graduate career. I hold his wisdom, patience, compassion, and gift to foster interest in chemistry to the highest of standards and, moving forward, I aspire one day in the future to become an educator of similar skill to serve my future students. Thank you for allowing me to work with you and the talented members of your group, and I wish for you to find success in many interesting scientific problems in the future.

I would like to thank all Girolami group members who have worked with me during my tenure here. I would especially like to thank both Brian Trinh and Sumeng Liu for mentoring me and helping me refine almost everything I know about synthetic air-free chemistry, Nels

Anderson for his help orienting myself to the group and for our many discussions about more rudimentary inorganic chemistry, as well as the use of his former fume hood where I finished the last of my experiments for this thesis, Chris Caroff for his incredible depth of knowledge on boron chemistry and his initiative which saw me learn more than I ever would have on my own,

Nathan Edmonsond his enduring nature, constructive conversations and criticisms, and his patience as I learned how to properly spell his last name, and Joe Wright for his patience in having me as his student mentor.

I thank our collaborators in the materials department, Dr. John Abelson and his lab members for all the help on CVD, I especially thank Xiaoqing Chu for characterizing thin films grown in their lab and helping me learn more about CVD then I ever could have on my own.

I would also like to thank Nancy Makri, my mentor and research advisor before joining the Girolami group, for her patience and continued support when it became apparent that her research was not a good fit for my interests.

I thank the help of the SCS facilities staff, specifically Beth Eves and Kiran Subedi from the Microanalysis Laboratory and Dean Olsen, who not only provided excellent conversation during those few times where I was waiting on a particularly long data collection and he was free, but also answered all of my questions regarding the NMR instruments and methodologies, and was always free regardless of the time of day to address issues with their upkeep or functionality. I would also like thank Danielle Grey and especially Toby Woods of the X-ray laboratory for being quick enough to collect crystal structure data on my air-sensitive crystals, helping me install olex2, and giving me ideas on how to crystallize my compounds.

I would like to thank all of the members of the IMP office, past and present, who were always willing to work with me, bring my attention to free food, and provide interesting conversations while waiting for things to print. I would like to especially thank Katie Trabaris for her numerous talks about our shared interests in fiction, and for her ability to untangle any of my scheduling conflicts, Lisa Johnson for introducing me to new kinds of music, Theresa Struss for her sarcastic quips and Raegan Smith for her positive attitude and helping me schedule meetings.

Finally, I thank my friends and family who helped me cope with the emotional burdens of graduate school. Without them, I would not be here writing this. I thank David Eury, David

Craig, and Nick McGarrell for being three of the best friends anyone could ever wish for. I thank

Amartya Bose, Sohang Kundu, and Reshmi Dani for their support when I changed groups and their continued friendship afterwards. And last, but certainly not least, I thank my parents Kevin

Daly and Leah Mann-Daly for the unconditional love and support they have consistently given to me over the years, and for encouraging me to always work hard, challenge myself, and pursue excellence. I give special thanks to my younger sister, Fionuala (Nuala) Daly for her enduring belief in me, and her endless attempts at sarcasm; I look forward to supporting you in the future, and hope to have the honor of receiving your acknowledgment one day when you reach the heights only you know that you can reach.

TABLE OF CONTENTS

CHAPTER 1: SYNTHESIS AND CHARACTERIZATION OF SODIUM

AMINODIBORANATE SALTS PREPARED FROM SILYLAMINES ...... 1

1.1 Introduction ...... 1

1.2 Results and Discussion ...... 5

1.3 Experimental ...... 15

1.4 References ...... 22

CHAPTER 2: SYNTHETIC AND 11B NMR STUDIES OF AMINODIBORANATE SALTS

PREPARED FROM PRIMARY AND SECONDARY FLUOROAMINES ...... 29

2.1 Introduction ...... 29

2.2 Results and Discussion ...... 31

2.3 Experimental ...... 37

2.4 References ...... 40

APPENDIX A: SYNTHESIS OF Mo[H3BNMe2BH3]2 WITH TIN AS A

CO-REDUCTANT ...... 45

A.1 Introduction ...... 45

A.2 Experimental ...... 46

A.3 References ...... 49

vii

CHAPTER 1: SYNTHESIS AND CHARACTERIZATION OF SODIUM

AMINODIBORANATE SALTS PREPARED FROM SILYLAMINES

1.1 Introduction

Metal diboride (MB2) thin films are used for a number of applications in microelectronics,1-3 and also as components of refractory linings,4-7 electrodes,8 optical fibers,9 and cutting tools.10 In microelectronics, they are potential new materials for gate dielectrics and diffusion barriers in the middle-of-line stage of integrated circuit (IC) fabrication.11-12 These applications stem from the unusual properties exhibited by transition metal diborides such as

TiB2, ZrB2 and HfB2: they have melting points exceeding 3000 °C, electrical resistivities as low as 15 μΩ-cm, and hardnesses approaching 30 GPa.13 In addition, many metal diborides are chemically and mechanically robust.

Of particular interest is magnesium diboride (MgB2), which has a superconducting critical temperature (TC) of 39 K, the highest of all known conventional, phonon mediated,

14-16 superconductors; MgB2 is a material of interest for the construction of MRI magnets that do not require liquid helium to operate17-18 and for the fabrication of superconducting integrated circuits that can operate at higher temperatures than analogous niobium alloy circuits.19-20 To ensure device functionality and integrity, the MgB2 thin films must be crystalline and stoichiometric. A significant obstacle in the deposition of superconducting MgB2 films, however, is that Mg sublimes out of the bulk if the temperature exceeds about 300 °C, leaving behind magnesium-depleted, boron-rich films.21

Physical vapor deposition (PVD),22-23 chemical vapor deposition (CVD),24-25 and atomic layer deposition (ALD)26-27 methods can all be used to prepare thin films of metal diborides. In

PVD, thin films of the desired composition are deposited by generating highly reactive particles

1 from pure bulk targets by evaporation28 or sputtering,29 followed by transporting them to the substrate on which the films grow. In CVD, a molecular precursor, which contains some or all of the elements in the desired thin film, is vaporized and flowed over a hot substrate, sometimes in combination with other precursors. Subsequent chemical reactions afford thin films of the desired material on the substrate.30 Unlike PVD methods, which tend to be “line of sight” owing to its use of highly reactive particles, CVD methods can produce uniform coatings on high aspect-ratio trenches and holes, and even on hidden surfaces. ALD, which is a variant of CVD, is an iterative growth process in which the substrate is exposed alternately to two or more molecular precursors. During each step, the exposure saturates the surface of the substrate with precursor; the excess precursor is then pumped away before exposure to the other precursor.

Each precursor reacts selectively with functional groups left behind in the previous exposure step and generates a new set of surface functional groups for the next exposure step.

To date, no purely CVD method has been able to deposit superconducting films of MgB2.

The reason is that all of the CVD precursors investigated to date, such as Lewis base adducts of magnesium borohydride or octahydrotriborate,19 are too thermally stable, and do not decompose on surfaces below about 300 °C. The only successful deposition of MgB2 films by CVD has been by means of a hybrid physical-chemical vapor deposition method (HPCVD),31-33 in which magnesium vapor is co-flowed with a precursor containing magnesium and boron.

A significant challenge is to devise a method to grow MgB2 films by a purely CVD route.

Such an advance would require the development of new classes of CVD precursors, which are able to transform to MgB2 at temperatures low enough to disfavor the loss of Mg from the deposited thin film by sublimation.

Of the MgB2 precursors available, magnesium borohydride, Mg(BH4)2, magnesium octatrihydridoborate, Mg(B3H8)2, and magnesium nonahydridohexaborate, Mg(B6H9)2, and

Lewis base adducts thereof, are often unsuitable for CVD applications due to their low volatility.

One exception to this rule is Mg(B3H8)2(Et2O)2, which has the required volatility but results in boron-rich films.19 More recently, however, the Girolami group has demonstrated that many metal salts of the aminodiboranates, M(H3BNR1R2BH3)x, are useful CVD precursors for a

34-36 variety of MBx thin films. These ligands have been the subject of extensive study, and in particular the N,N-dimethylaminodiboranate (DMADB) anion has been used to synthesize a large body of volatile homoleptic and heteroleptic metal complexes through salt metathesis with alkaline earth, d-block, and f-block halides. For example, bis(N,N-dimethylaminodiboranato) magnesium, Mg[H3BNMe2BH3], has a vapor pressure of 800 mTorr at 25 °C that is the highest of all known magnesium compounds at room temperature.37

In unpublished work, the Girolami and Abelson groups have found that

Mg(H3BNMe2BH3)2 is too thermally stable: no film is deposited at temperatures below about

300 °C, and above this temperature the deposits are depleted in magnesium.38 The goal of the present work is to prepare magnesium compounds of other aminodiboranate anion to try to lower the onset temperature for deposition. If successful, such precursors could provide a way to deposit thin films of superconducting MgB2.

Aminodiboranate anions may be thought of as two borane groups joined by an amido

(NR1R2) linker. Mallek focused efforts on tuning the properties of this linker by preparing analogous compounds with different substituents on nitrogen, hypothesizing that the addition of either sterically bulky or electron-withdrawing substituents would weaken the B-N bonds and enable deposition take place at lower temperatures. In his investigations into increasing the steric

i -1 bulk, Mallek prepared the diisopropyl substituted aminodiboranate [H3BN Pr2BH3] (DIPADB),

3939 both as a sodium salt and as the magnesium complex Mg(DIPADB)2. He further and showed that the B-N bonds in Mg(DIPADB)2 are readily cleaved upon mild heating; the products of the

i thermolysis are Mg(BH4)2 and two equiv. of the aminoborane Pr2N=BH2 (Figure 1.1). In this reaction, one B-N bond and one B-H bond in each DIPADB ligand is broken; the aminoborane is

liberated and the resulting BH3 and H groups combine to form a borohydride ligand.

Mg(DIPADB)2 Mg(BH4)2 + 2 N BH2 2

Figure 1.1. Thermolysis of bis(N,N-diisopropylaminodiboranato)magnesium, Mg(DIPADB)2.

Reproduced from reference 39.

In his investigations of electron-withdrawing substituents, Mallek attempted to prepare and isolate aminodiboranate salts with fluoroalkyl substituents. In general, these efforts were unsuccessful or inconclusive: the aminodiboranate salts either could not be generated or decomposed before they could be isolated.

We now describe the preparation of aminodiboranates in which the nitrogen atom bears trimethylsilyl substituents. The nitrogen atoms in silylamines are poorly basic due to negative hyperconjugation (i.e., back-bonding interactions) between the nitrogen lone pair and empty σ*

40-47 orbitals involving the SiMe3 groups. This interaction removes electron density from the amine. As a result, the N-B bond in aminodiboranates with SiMe3 substituents should be relatively weak. Our expectation is that this precursor design will lead to lower onset temperatures for CVD, thus opening up the possibility of growing thin films of superconducting

MgB2.

Here we demonstrate the successful synthesis and characterization of three new sodium aminodiboranate complexes with trimethylsilyl substituents.

1.2 Results and Discussion

Synthesis of Sodium Aminodiboranate Salts from Primary and Secondary

Silylamines. There are two general methods to prepare sodium aminodiboranate salts,

Na[(BH3)NR2BH3]: Keller’s method, which involves the reaction of a µ-aminodiborane(6)

48 (B2H5NR2) with sodium hydride, and Nöth’s method, which involves the sodium reduction of a

49 borane-amine (BH3∙NR2H). Mallek has shown that Keller’s method may be used to prepare aminodiboranates with sterically-bulky and electron-withdrawing substituents on nitrogen.39 The entire reaction sequence is shown below:

R1 R2 N HNR1R2 + BH3(thf) HNR1R2(BH3) + BH3(thf) + NaH Na[H3BNR1R2BH3] 1-3 d H2B BH2 H

We felt that Keller’s method offered the best opportunity to prepare the N-trimethylsilyl substituted aminodiboranates Na[H3BNH(SiMe3)BH3], 1, and Na[H3BN(SiMe3)2BH3], 2. The reaction of HN(SiMe3)2 with BH3∙thf at elevated temperature is known to generate two µ- aminodiborane(6) species: bis(N,N-trimethylsilyl)-µ-aminodiborane(6) and N-trimethylsilyl-µ- aminodiborane(6).50 A 11B NMR spectrum of a mixture of these two species consists of two overlapping triplets of doublets near δ -26 (Figure 1.2).

H Si(Me)3 (Me)3Si Si(Me)3 N N

H2B BH2 H2B BH2 H H

δ -24.3 td δ -26.0 td JBH = 33 Hz (µ-H), JBH = 34 Hz (µ-H), 128 Hz (t-H) 128 Hz (t-H)

3 1 -1 -3 -5 -7 -9 -11 -13 -15 -17 -19 -21 -23 -25 -27 -29 -31 -33 -35 f1 (ppm)

Figure 1.2. 11B NMR spectrum of the mixture of µ-aminodiborane species generated by heating

hexamethyldisilizane with borane-thf. The rightmost triplet is broadened, probably due to

exchange between the terminal and bridging hydrogen atoms.51

These two products are generated because the initially formed borane-amine adduct

HN(SiMe3)2∙BH3 can thermolyze by means of two pathways (Figure 1.3). In one pathway, H2 is lost to give the aminoborane intermediate (Me3Si)2N=BH2; loss of H2 is the usual outcome when boranes react with non-tertiary amines at elevated temperatures. Subsequent addition of a borane group across the formal double bond of the aminoborane gives the bis(N,N-trimethylsilyl)-µ- aminodiborane(6) product. Due to the lability of the N-SiMe3 bond in the presence of

52 electrophiles, HN(SiMe3)2∙BH3 can also react by a second pathway: loss of HSiMe3 to give the aminoborane intermediate (Me3Si)NH=BH2; this species then reacts with a second equivalent of borane to give the N-trimethylsilyl-µ-aminodiborane(6) product. The two products can be separated by distillation at reduced pressures.

Me3Si SiMe3 Me3Si N +BH3 N BH2 H2B BH2 -H2 Me3Si H

HN(Si(Me)HN(SiMe33))2BH3 H SiMe3 H N -HSi(Me)-HSiMe3 +BH3 N BH2 H2B BH2 Me3Si H

Figure 1.3. Reaction mechanism for the formation of both bridging µ-aminodiborane.

Treatment of the N-trimethysilyl-µ-aminodiborane(6) with NaH in thf affords the new aminodiboranate salt Na[H3BNH(SiMe3)BH3] (1). The yield is poor, however, because

53-54 significant amounts of NaBH4 are formed, along with small amounts of NaB2H7, as judged from the 11B NMR spectrum (Figure 1.4). Similar treatment of bis(N,N-trimethylsilyl)-µ- aminodiborane(6) with NaH gives the closely related aminodiboranate salt

Na[H3BN(SiMe3)2BH3] (2) in much higher yield (over 75%).

H SiMe3 NaBH4 N δ -42 (p)

BH3 BH3 JBH = 81.1 Hz

δ -19 (q)

JBH = 90.3 Hz NaB2H7 δ -25 (q)

JBH = 84 Hz

12 8 4 0 -4 -8 -12 -16 -20 -24 -28 -32 -36 -40 -44 -48 f1 (ppm)

Figure 1.4. 11B NMR spectra of the reaction mixture for the treatment of N-trimethysilyl-µ- aminodiborane(6) with NaH in thf.

The methyl-trimethylsilyl analog Na[H3BNMeSiMe3BH3] (3) can be prepared in a similar manner. The starting material for this synthesis, the amine-borane adduct

H2NMe(SiMe3)∙BH3, is prepared as shown below:

Et2O Filter Distill THF HNMe(SiMe ) + BH HNMe(SiMe3)BH3 2.5 H2NMe + ClSiMe3 3 3 4 hrs 0℃

NMR and IR Studies of New Sodium Aminodiboranate Salts. The 11B NMR spectra

1 of the new aminodiboranates all consist of binomial quartets with JBH ≈ 91 Hz (Figure 1.5); the chemical shifts are given in Table 1.1 along with those of some related compounds. It has been shown previously that replacing an NH group in an aminodiboranate with an NMe group causes the 11B NMR chemical shift to change by +4.2 ppm, and that the effect is additive within a few tenths of a ppm: carrying out two such substitutions changes the shift by +8.4 ppm.55 The new

11 compounds show that replacing one NH group with an NSiMe3 group causes the B NMR chemical shift to change by +0.6 ppm if the other substituent is H, but -0.9 ppm if the other substituent is Me. In addition, the effect seems to be non-additive: replacing two NH groups with two NSiMe3 groups causes the shift to change by +2.4 ppm.

11 It is clear, however, that replacing NH with SiMe3 has a smaller effect on the B NMR chemical shifts than replacing NH with NMe. This smaller effect may be correlated with the inductive properties of the substituents as quantified by their Hammett parameters (σp: H, 0.00;

56 SiMe3, -0.07; Me, -0.17). These parameters indicate that the electron donating abilities of these

11 groups increase in the order H < SiMe3 < Me; the substituent effects on the B NMR chemical shifts in Table 1.1 are in this same order.

H SiMe3

δ -19.3 N

BH3 BH3 a)

Me3Si SiMe3 b) δ -17.7 N

H3B BH3

Me SiMe3 Me Si(Me)3 δ -14.8 N c) N

H3BH BHBH33

-11.0 -11.5 -12.0 -12.5 -13.0 -13.5 -14.0 -14.5 -15.0 -15.5 -16.0 -16.5 -17.0 -17.5 -18.0 -18.5 -19.0 -19.5 -20.0 -20.5 -21.0 -21.5 f1 (ppm)

11 Figure 1.5. Comparison of B NMR spectra of Na[H3BNH(SiMe3)BH3], 1, (a),

Na[H3BN(SiMe3)2BH3], 2, (b), and Na[H3BNMe(SiMe3)BH3], 3, (c) all in thf-d8 at 20 °C.

11 Table 1.1. B NMR chemical shifts for the new aminodiboranates Na[(BH3)NR2BH3],

compared with those of some related compounds.

11 NR2 B NMR shift ref

55 NH2 -19.9

NHSiMe3, 1 -19.3 this work

N(SiMe3)2, 2 -17.5 this work

NMe(SiMe3), 3 -14.8 this work

NHMe -15.7 55

55 NMe2 -11.5

11 If the non-linear dependence of the B NMR shifts on the number of SiMe3 groups is real, it may reflect competition between the electron-withdrawing effects expected for SiMe3 groups due to negative hyperconjugation competing with their electron-donating properties.

Inductive effects, however, are not the only factor the affect 11B chemical shifts: it is known, for example, that increasing steric bulk generally leads to shielding of the 11B NMR

39 chemical shifts in aminodiboranate salts such as in the series NEt2 < N(CH2)5 < N(C4H4) <

39, 55, 57 NMe2. For compounds 1-3, the chemical shift order NMe(SiMe3) < N(SiMe3)2 <

NH(SiMe3) (from most shielded to most deshielded) cannot be explained in terms of steric factors alone. We conclude that inductive, hyperconjugative, and steric effects probably all play a role in determining the 11B NMR chemical shifts of these aminodiboranates.

The 1H NMR spectra of the new aminodiboranates all contain a single broad 1:1:1:1

1 11 quartet with JBH = 90 Hz for the BH3 hydrogen atoms, due to coupling with the B (I = 3/2)

1 nucleus. The H NMR chemical shifts of the BH3 protons are δ 1.23, 1.21, and 1.20 for 1-3,

13 1 respectively. The C{ H} NMR spectra in thf-d8 of the new aminodiboranates all contain one resonance near δ 2.00 for the NSiMe3 group, and 3 also has a resonance at δ 45.82 for the NMe group.

The IR spectra of compounds 1-3 each contain a sharp band near 1250 cm-1 for the

-1 symmetric Si-C-H bending mode and a weaker band near 840 cm for the antisymmetric Si-C-H bending mode; for 1 the IR frequencies are 1247 and 840 cm-1; for 2 they are 1252 and 848 cm-1; for 3 they are 1264 and 843 cm-1. The IR spectra also show broad bands between 2000 and 2500 cm-1 that are due to the B-H stretching modes. Typically, higher frequencies are seen for terminal

B-H bonds and lower frequencies are seen for B-H bonds that bridge to a metal center; in addition, the frequencies can be affected by mixing into symmetric and asymmetric modes. Here,

10 the ionic nature of the salts leads to a small range of frequencies, which makes it more difficult to assign the bands seen.49 The frequencies are 2250 and 2333 cm-1 for 1; 2234 and 2299 cm-1 for

2; and 2332 and 2236 cm-1 for 3. In the IR spectrum of 2 there is a very sharp band at 3231 cm-1 that is not present in 2 or 3; this band is assigned to the N-H stretch.

Crystallographic Results. Crystals of both compounds were grown by slow diffusion of pentane diffusion into Et2O solutions, and the molecular structures of both 1 and 3 were determined by X-ray crystallography (see Tables 1.2-1.4 and Experimental for details). In

Na[H3BNH(SiMe3)BH3], 1 (Figure 1.7), the N(1)–B(1) and N(1)–B(2) bond distances are

1.5889(11) Å and 1.5926(11) Å, respectively, whereas in Na[H3BNMe(SiMe3)BH3], 3 (Figure

1.8), the N(1)–B(1) and N(1)–B(2) bond distances are 1.5916(12) Å and 1.5924(12) Å, respectively. It is interesting to note that the B–N bond distances in both compounds are 1.59 Å, whereas the B-N bond distances for sodium salts of electron rich aminodiboranates, such as

Na[H3NHMeBH3]·(O2C4H8)0.5 are ~1.57 Å. This small difference supports the hypothesis that the trimethylsilyl substituents weaken the B–N bond.

Both 1 and 3 are polymeric in the solid-state: the cations and anions are linked into two- dimensional sheets by means of Na···H–B bridges through the BH3 centers. For 1 the closest neighbors to the sodium ion are six hydrogen atoms best described as forming a distorted trigonal antiprismatic arrangement; within each BH3 unit, two of the hydrogen atoms are bound in a κ2H fashion to one sodium ion, and the third hydrogen is bound to a neighboring Na center.

The Na(1)···H(11) and Na(1)···H(12) bond distances are 2.334(12) Å and 2.302(12) Å respectively. In 3, the local environment of the sodium cations consists of seven hydrogen atoms arranged in a distorted pentagonal bipyramidal geometry; two BH3 groups are bound to sodium

2 in a κ H fashion in the pentagonal plane, and three other BH3 groups (one equatorial and two

11 axial) are κ1H. The Na(1)···H(11), Na(1)···H(21), and Na(1)···H(22) bond lengths are of

2.373(12) Å, 2.407(12) Å, and 2.403(12) Å respectively. These features are within the expected range of previously reported sodium aminodiboranate salts.55

Table 1.2. Crystallographic data for 1 and 3 at 100 K.

1 3

formula C3H16B2NNaSi C4H18B2NNaSi FW (g mol-1) 138.87 152.89 λ (Å) 0.71073 0.71073 crystal system monoclinic tetragonal

space group P21/c I41/a a (Å) 12.0600(3) 13.2904(3) b (Å) 6.7804(2) 13.2904(3) c (Å) 12.1801(3) 23.4837(5) V(Å3) 925.18(4) 4148.0(2) Z 4 16

−3 ρcalc (g cm ) 0.997 0.979 µ (mm-1) 0.218 0.199 R (int) 0.0188 0.0296

absorption correction face-indexed multi-scan max, min transm factors 0.9203, 0.9844 0.6629, 0.7461 data/restraints/parameters 2299/0/104 3171/0/110 goodness-of-fit on F2 1.086 1.057

a R1[I > 2s(I)] 0.0210 0.0263

b wR2 (all data) 0.0588 0.0677

Figure 1.6. Structure of Na[H3BNH(SiMe3)BH3], 1. Ellipsoids show the 50% probability density surfaces.

Table 1.3. Selected bond lengths and angles for Na[H3BNH(SiMe3)BH3], 1.

Bond Lengths (Å)

Na1-B1 2.8258(10) Si1-C1 1.8569(9) Na1-B2 2.7490(10) Si1-C2 1.8597(9) B1-N1 1.5889(11) Si1-C3 1.8515(9) B2-N1 1.5926(11) Na1-H11 2.334(1) N1-H1 0.843(10) Na1-H12 2.302(1)

Bond Angles (deg) B1-Na1-Na1 97.39(2) B1-N1-Si1 114.64(6) Na1-B1-Na1 92.61(3) B2-N1-Si1 112.59(5) N1-B1-Na1 93.23(5) N1-Si1-C1 109.77(4) N1-B2-Na1 103.61(5) N1-Si1-C2 107.05(4) H21-Na1-H12 47.30(6) N1-Si1-C3 108.74(4)

Figure 1.7. Structure of Na[H3BNMe(SiMe3)BH3], 3. Ellipsoids show the 50% probability density surfaces.

Table 1.4. Selected bond lengths and angles for Na[H3BNMe(SiMe3)BH3], 3.

Bond Lengths (Å)

Na1-B1 2.8573(10) Si1-C2 1.8569(9) Na1-B2 2.8174(11) Si1-C3 1.8597(9) B1-N1 1.5916(11) Si1-C4 1.8515(9) B2-N1 1.5924(12) Na1-H12 2.373(12)

N1-C1 1.4924(11) Na1-H21 2.407(12) Na1-H22 2.403(12)

Bond Angles (deg) B1-Na1-Na1 41.09(2) B1-N1-Si1 109.55(7) Na1-B1-Na1 97.20(3) B2-N1-Si1 110.84(5) N1-B1-Na1 116.44(5) N1-Si1-C2 109.15(4) N1-B2-Na1 118.77(5) N1-Si1-C3 108.90(4) H11-Na1-H12 47.30(6) N1-Si1-C4 109.72(4)

Interestingly, the Na···H distances in 1 and 3 fall between 2.3 and 2.4 Å, as opposed to distances of 2.4-2.6 Å seen for electron-rich sodium aminodiboranates salts; it is important to note, however, that this difference may reflect the presence of 1,4-dioxane molecules that are bound to the sodium ions.

Concluding Remarks. We have demonstrated the synthesis and characterization of three sodium salts of new aminodiboranate anions with trimethylsilyl substituents. Future experiments should aim at isolating and purifying magnesium salts of the ligands; specifically starting from 3 which has both an electron-withdrawing NSiMe3 group and an electron donating NMe group.

Studies of the thermolysis of these magnesium salts and the resulting film compositions, deposition profiles, and conductivities will test the hypothesis that electron-withdrawing aminodiboranates can serve as MgB2 precursors.

1.3 Experimental

All operations were carried out in vacuum or under argon using standard Schlenk techniques. All glassware was dried in an oven at 150 °C, assembled hot, and allowed to cool under vacuum (5 mTorr) before being back filled and evacuated three times with argon to remove oxygen before use. Tetrahydrofuran (thf), diethyl ether (Et2O), and pentane were distilled under nitrogen from sodium/benzophenone, and toluene was distilled under nitrogen from sodium metal. Krytox grease was used to seal the joints of all reactions involving the reflux of boranes in tetrahydrofuran for more than 24 h. Borane-tetrahydrofuran solution, 1 M in borane, (Sigma-Aldrich) was vacuum transferred under argon prior to use. Hexamethyldisilizane

(Sigma-Aldrich), chlorotrimethylsilane (Sigma-Aldrich) and 2 M methylamine in thf (Sigma-

Aldrich) were stored under argon and used as received. 60% sodium hydride dispersion in

15 mineral oil (Sigma-Aldrich) was used as received. The compound N-methyl-N- trimethylsilylamine was prepared by means of a literature procedure,58 modified by using a 2 M solution of methylamine in thf instead of gaseous methylamine.

Elemental analyses were carried out by the University of Illinois Microanalytical

Laboratory. The IR spectra were recorded on a Nicolet 200 infrared spectrometer as Nujol mulls between KBr plates. The 1H NMR data were obtained on either a Varian Unity Inova 400 instrument at 400 MHz or a Varian Unity U500 instrument at 500 MHz. The 13C NMR data were obtained on a Carver-Bruker 500 CryoProbe instrument at 500 MHz. 11B NMR data were obtained on a Varian Unity Inova 400 instrument at 400 MHz. Chemical shifts are reported in δ

1 11 units (positive shifts to high frequency) relative to tetramethylsilane ( H NMR) or BF3·Et2O ( B

NMR).

µ-Bis(trimethylsilyl)aminodiborane and µ-Trimethylsilylaminodiborane.50 To

BH3·thf (100 mL of a 1 M solution in thf, 100 mmol) was added a solution of HN(SiMe3)2 (10.4 mL, 50 mmol) in thf (10 mL). The mixture was stirred at room temperature until the conversion to the µ-bis(trimethylsilyl)aminodiborane was complete, as judged from a 11B NMR spectrum of an aliquot. Typically, this step required 6 days. The mixture was separated by distillation at reduced pressure. The first fraction consisted of µ-trimethylsilylaminodiborane (yield: 2.62 g;

46%). The second fraction (50 mTorr/45 °C) consisting of µ-bis(trimethylsilyl)aminodiborane

11 1 (yield: 4.54 g, 49%). B NMR (thf, 20 °C): (SiMe3)2NB2H5: δ -26.03 (td, JBH = 128, 33 Hz).

1 (SiMe3)HNB2H5: δ -24.31 (td, JBH = 128, 34 Hz).

Sodium N-Trimethylsilylaminodiboranate (1). NaH (1 g, 50 mmol) was washed with pentane (3 × 50 mL) and dried under vacuum for 30 min. The µ-trimethylsilylaminodiborane solution was transferred onto the NaH and allowed to stir for 21 h. The solution was filtered from

16 the remaining NaH and the filtrate was taken to dryness in vacuum. The residue was extracted with Et2O (2 × 20 mL), and the extract was filtered. The filtrate was taken to dryness in vacuum, and the residue and washed with toluene (30 mL) and pentane (3 × 30 mL) and dried under vacuum at room temperature overnight to afford a white solid. Yield: 0.33 g (4%). Anal. Calcd

1 for NaNSiB2C3H16: C, 26.0; H, 11.6; N, 10.1. Found: C, 28.0; H, 12.7; N, 13.3. H NMR (thf-d8,

1 20 °C): δ 1.73 (br s, 1H, NH), 1.23 (1:1:1:1 q, JBH = 89 Hz, 6H, BH3), 0.12 (s, 9H, SiMe3).

13 1 11 1 C{ H} NMR (thf-d8, 20°C): δ 2.07 (s, SiMe3). B NMR (thf, 20°C): δ -17.7 (q, JBH = 91 Hz).

IR (cm-1): 3231 m, 2299 br, 2234 br, 1458 m, 1377 w, 1252 m, 1199 w, 1152 m, 1026 w, 995 w,

973 w, 848 s, 779 vw, 756 vw, 691 vw, 660 vw.

Sodium N,N-Bis(trimethylsilyl)aminodiboranate (2). Method A. NaH (2 g, 0.1 mol) was washed with pentane (3 × 50 mL) and dried in vacuum for 30 min. The solution of µ-bis-

(trimethylsilyl)aminodiborane was transferred onto the NaH and the mixture was stirred for 23 h.

The solution was filtered from the remaining NaH and the filtrate was taken to dryness in vacuum. The residue was washed with toluene (30 mL) and pentane (3 × 30 mL) and dried under vacuum at room temperature overnight to afford the product as a white solid. Yield: 8.23 g

(78%). Anal. Calcd for NaNSi2B2C6H24∙0.9thf C, 41.7; H, 11.4; N, 5.70. Found: C, 39.5; H, 12.5;

1 1 N, 4.76. H NMR (thf-d8, 20 °C): δ 1.21 (1:1:1:1 q, JBH = 89 Hz, 6H, BH3), 0.20 (s, 18H,

13 1 11 1 SiMe3). C{ H} NMR (thf-d8, 20°C): δ 1.87 (s, SiMe3). B NMR (thf, 20°C): δ -19.3 (q, JBH =

91 Hz). IR (cm-1): 2333 br, 2250 br, 1458 s, 1376 m, 1247 s, 1054 s, 949 m, 840 br, 785 w, 762 w, 726 w, 681 m, 651 w, 501 w.

Method B. To BH3·thf (100 mL of a 1 M solution in thf, 100 mmol) was added

HN(SiMe3)2 (10.5 mL, 50 mmol). The mixture was heated to reflux until the conversion to both the µ-bis(trimethylsilyl)aminodiborane and µ-trimethylsilylaminodiborane was complete, as

17 judged from a 11B NMR spectrum of an aliquot. Typically, this step required 3 d. The mixture was separated by distillation at reduced pressures (30 mTorr/32 °C). In a separate flask, NaH (1 g, 50 mmol) was washed with pentane (3 × 50 mL) and dried under dynamic vacuum for thirty minutes. The µ-bis(trimethylsilyl)aminodiborane solution was transferred onto the NaH and allowed to stir for 21 h. The solution was filtered from the remaining NaH and the filtrate was taken to dryness in vacuum. The product, a white solid, was washed with toluene (1 × 30 mL) and pentane (3 × 30 mL) and dried under vacuum at room temperature overnight. Yield: 4.96 g

11 (47%). The B NMR data match that of Na[H3BN(SiMe3)2BH3] prepared by method A.

N-Methyl-N-trimethylsilylamine Borane. A solution of chlorotrimethylsilane (10 mL,

79 mmol) in Et2O (200 mL) was frozen in liquid nitrogen and then H2NMe (100 mL of a 2 M solution in thf, 200 mmol) was transferred to the frozen solution. The solution was allowed to warm to room temperature in a water bath and then was stirred for 4 h, during which time a white powder precipitated from the solution. The solution was filtered from the precipitate at 0

°C. To this solution was added BH3·thf (80 mL of a 1 M solution in thf, 80 mmol) with stirring.

After 30 min the solvent was removed under reduced pressure to provide a white solid. Yield:

11 1 4.4 g (48%). B NMR (thf, 20°C): δ -17.01 (q, JBH = 98 Hz).

Sodium N-Methyl-N-trimethylsilylaminodiboranate (3). The crude amine-borane adduct from the previous step was dissolved in thf (20 mL) and to this solution was added

BH3·thf (40 ml, 40 mmol). The mixture was refluxed for 3 d. In a separate flask, NaH (1.6 g, 40 mmol) was washed with pentane (3 × 50 mL) and dried under vacuum for 30 m. The solution was transferred to the NaH and allowed to stir overnight for 21 h. The solution was filtered, and the filtrate was taken to dryness in vacuum. The product, a white solid, was washed with toluene

(1 × 30 mL) and pentane (3 × 30 mL). Yield: 3.1 g (51%). Anal. Calcd for NaNSiB2C4H18: C,

1 31.4; H, 11.9; N, 9.16. Found: C, 31.0; H, 7.65; N, 12.8. H NMR (thf-d8, 20 °C): δ 1.72 (s, 3H,

13 1 NMe), 1.20 (1:1:1:1 q, JBH = 90 Hz, 6H, BH3), 0.15 (s, 9H, SiMe3). C{ H}NMR (thf-d8, 20°C):

11 1 -1 δ 45.82 (s, NMe2), 3.92 (s, SiMe3). B NMR (thf, 20°C): δ -14.4 (q, JBH = 91 Hz). IR (cm ):

2724 vw, 2332 br, 2236 br, 1457 br, 1416 vw, 1377 m, 1254 w, 1216 w, 1249 w, 1164 w, 1080 m, 1021 m, 965 m, 843 br, 750 w, 772 w, 687 w, 648 m.

General Crystallographic Details. Single crystals of 1 and 3 were mounted on glass fibers with Paratone-N oil (Exxon) and immediately cooled to -100 °C in a cold nitrogen gas stream on the diffractometer. Standard peak search and indexing procedures gave rough cell dimensions, and least squares refinement yielded the cell dimensions given in Table 1.4.

Data were collected using the measurement parameters given in Table 1.4. The measured intensities were reduced to structure factor amplitudes and their estimated standard deviations by correction for background, scan speed, and Lorentz and polarization effects. Systematically absent reflections were deleted and symmetry equivalent reflections were averaged to yield the set of unique data. Unless otherwise noted, all unique data were used in the least squares refinement.

For all crystals, the analytical approximations to the scattering factors were used, and all structure factors were corrected for both real and imaginary components of anomalous dispersion. In the final cycle of least squares, independent anisotropic displacement factors were refined for the non-hydrogen atoms. Hydrogen atoms attached to boron and nitrogen were located in the difference maps, and their positions were refined with independent isotropic displacement parameters. Hydrogen atoms attached to carbon were placed in idealized positions; the methyl groups were allowed to rotate about the C-C axis to find the best least-squares positions. The displacement parameters for methyl hydrogens were set to 1.5 times Ueq. Final

19 refinement parameters are given in Table 1.4. A final analysis of variance between observed and calculated structure factors showed no apparent errors.

Na[H3BNH(SiMe3)BH3], 1: The 2/m Laue symmetry and the systematic absences 0k0 with k ≠ 2n and h0l with l ≠ 2n were uniquely consistent with the space group P21/c, and this choice was confirmed by successful refinement of the proposed model. No correction for crystal decay was necessary, but a face-indexed absorption correction was applied, the minimum and maximum transmission factors being 0.9203 and 0.9844. The structure was solved by direct methods (SHELXTL); subsequent least-squares refinement and difference Fourier calculations revealed the positions of the all the non-hydrogen atoms. The quantity minimized by the least-

2 2 2 2 2 2 -1 2 squares program was Σw(Fo - Fc ) , where w = {[σ(Fo )] + (0.03P) + 0.22P} and P = (Fo +

2 2Fc )/3. No correction for isotropic extinction was necessary. Successful convergence was indicated by the maximum shift/error of 0.001 for the last cycle. The largest peak in the final

Fourier difference map (0.39 eÅ-3) was located 0.86 Å from C3.

Na[H3BNMe(SiMe3)BH3], 3: The 4/m Laue symmetry and the systematic absences hkl with h + k + l ≠ 2n, 00l with l ≠ 4n, and hk0 with h or k ≠ 2n were uniquely consistent with the space group I41/a, and this choice was confirmed by successful refinement of the proposed model. No correction for crystal decay was necessary, but a multi-scan absorption correction was applied (SADABS), the minimum and maximum transmission factors being 0.6629 and 0.7461.

The structure was solved by direct methods (SHELXTL); subsequent least-squares refinement and difference Fourier calculations revealed the positions of the all the non-hydrogen atoms.

2 2 2 2 2 The quantity minimized by the least-squares program was Σw(Fo - Fc ) , where w = {[σ(Fo )] +

2 -1 2 2 (0.028P) + 2.6P} and P = (Fo + 2Fc )/3. The largest peak in the final Fourier difference map

(0.33 eÅ-3) was located 0.71 Å from N1. No correction for isotropic extinction was necessary.

Successful convergence was indicated by the maximum shift/error of 0.001 for the last cycle.

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CHAPTER 2: SYNTHETIC AND 11B NMR STUDIES OF AMINODIBORANATE SALTS

PREPARED FROM PRIMARY AND SECONDARY FLUOROAMINES

2.1 Introduction

In the development of new CVD precursors for the deposition of metal or metal- containing thin films, an important design consideration is that the precursor should be as volatile as possible.1-9 In addition, liquids are particularly attractive as CVD precursors because, unlike solid precursors, their surface areas and thus their volatilization rates are nearly constant with time, and the flux of precursor to the substrate is easier to control. Common strategies to increase the volatilities of CVD precursors include disfavoring crystal packing by using ligands and substituents that lower the molecular symmetry or increase the conformational flexibility, preventing oligomerization through the use of polydentate or sterically bulky ligands, or weakening intermolecular interactions by reducing the molecular surface area and employing peripheral substituents with low polarizabilities.9

Many compounds containing fluorine or fluorinated ligands have been used as CVD precursors for metals and metal-containing thin films, owing to their tendency to have high vapor pressures relative to non-fluorinated analogs. For example, one fluorinated inorganic complex that is widely used commercially for deposition of tungsten is the hexafluoride, WF6, which exhibits a vapor pressure of ~1000 torr at room temperature.10 Similarly, a popular precursor for the deposition of copper is the hexafluoroacetylacetonate complex Cu(hfac)(vtms) where vtms is vinyltrimethylsilane.11-12 In general, the replacement of methyl groups with trifluoromethyl groups, CF3, often leads to an increase volatility with negligible impact on the film quality unless the temperature is high enough to break the C-F bonds. This heightened volatility arises from the

29 small size, high electronegativity, and low polarizability of fluorine atoms; these factors weaken the intermolecular attractive forces.

A large volume of research demonstrates the utility of fluorinated species for the deposition of thin films of metals,3-4, 13 metal oxides,14-15 metal nitrides,16 and metal carbides.17 In contrast, the literature of fluorinated precursors for the deposition of boron-containing thin films is sparse. Hybrid CVD methods using boron trifluoride (BF3) as one of the precursors have been reported for the growth of boron nitride.18 No fluorinated single-source CVD precursors for the deposition of metal diborides have been reported.

As described in chapter 1, earlier efforts in our group to develop a CVD precursor for the growth of superconducting MgB2 focused initially on the aminodiboranate Mg(BH3NMe2BH3)2.

This compound proved to be too thermally robust: at low temperatures the compound gave no deposit, whereas at higher temperatures (ca. 300 °C), the films were boron-rich owing to the loss of Mg by vaporization. One strategy to weaken the B-N bonds in this class of precursors is to employ substituents on nitrogen that are electron-withdrawing such as fluoroalkyl groups: inductive effects should remove electron density from the nitrogen center, and thus weaken the

N-B bond.

The fluorinated sodium aminodiboranate Na[H3BNH(CH2CF2H)BH3] has been previously characterized by Mallek,19 but his efforts to prepare aminodiboranates with fluorinated substituents on nitrogen were largely unsuccessful: the compounds either could not be generated or decomposed before isolation. In addition, he did not explore the preparation of magnesium derivatives of any fluorinated aminodiboranates.

Reported herein is an extension of Mallek’s work on the preparation of aminodiboranates with electron-withdrawing fluoroalkyl substituents.

2.2 Results and Discussion

Synthesis µ-Aminodiboranes with Fluorinated Substituents. The two main routes to the synthesis of sodium aminodiboranate salts20-21 both start with a amine-borane adduct. Two amine-borane adducts are known in which the amine bears fluoroalkyl groups:

19 22 (CF2HCH2)NH2∙BH3 and (CF3CH2)NH2∙BH3. For the first of these adducts, N-(2,2- difluoroethyl)amine-borane, Mallek19 observed that Nöth’s route,21 reduction the amine-borane adduct with sodium metal in thf, afforded a number of unidentifiable byproducts, as judged by

11B NMR spectroscopy, which could not be separated. In an effort to follow Keller’s route,20 he added BH3·THF the amine-borane, and treated the resulting μ-aminodiborane with NaH. The latter route afforded the desired sodium aminodiboranate, as judged by the 11B NMR spectra, but the compound decomposed slowly at room temperature, and he was unable to obtain analytically pure material.

In line with this approach, we first attempted to prepare the µ-aminodiborane(6)

B2H5N(CF3CH2)2 by treating N,N-bis(2,2,2-trifluoroethyl)amine-borane, (CF3CH2)2NH∙BH3, with one equivalent of borane-thf in refluxing thf. After 3 d, the 11B NMR spectra indicated that the amine-borane adduct had reacted completely, but only negligible amounts of the desired [µ-

N,N-bis(2,2,2-trifluoroethyl)amino]diborane(6), B2H5N(CF3CH2)2, had formed as judged by the intensity of the triplet at δ -17.27 (Figure 2.1). The major products were N,N-(2,2,2- trifluoroethyl)aminoborane, (CF3CH2)2N=BH2, and the diaminoborane bis(N,N-bis(2,2,2-

23-24 11 trifluoroethyl)borane, HB[N(CH2CF3)2]2. These species had B NMR chemical shifts of δ

44.7 and +31.4, respectively.

δ -17.2 (td) JBH = 131 Hz (tH), N 21 Hz (µH) H2B BH2

(b)

F3C CF3

F3C N BH 2 HB[N(CHHB(N(CH??? 2CF2CF3)2]32)2 F C N 3 δ 31.4 (d) BH3 δ 44.7 (t) JBH = 145 Hz H BH J = 124 Hz 3 BH δ -10.2 (q) JBH = 102 Hz (a)

50 45 40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 f1 (ppm)

11 Figure 2.1. B NMR spectra of the products from the reaction of (CF3CH2)2NH∙BH3 with

BH3∙thf in refluxing thf after (a) 1 day, (b) 2 days, and (c) 3 days.

The results suggested that the two trifluoroethyl substituents in (CF3CH2)2NH are sufficiently electron-withdrawing,25-27 and therefore the nitrogen atom is so weakly basic, that the corresponding aminodiboranate is too unstable. Accordingly, we decided to investigate the

N-methyl-N-(2,2,2-trifluoroethyl) analog: the presence of the electron-donating methyl group and only one fluorinated substituent should increase the basicity of the nitrogen center, but it should still be less basic than dimethylamine (pKa for the ammonium salt = 10.73).

The desired amine (CF3CH2)MeNH, is commercially available as the hydrochloride salt, but only at a cost of $200/g. We therefore prepared it a modified literature procedure28 by treating N-methyltrifluoroacetamide (CF3CONHMe) with two equiv of lithium aluminum

32 hydride at 0 °C, followed by addition of H2O, co-distillation of the amine (bp 6 °C) and the ether

(bp 35 °C), and precipitation as the hydrochloride salt, [(CF3CH2)MeNH2]Cl, 1, by addition of an ether solution of anhydrous HCl. Due to the gaseous nature of the amine, conversion to the hydrochloride salt is convenient for isolation and storage. The 1H NMR spectrum of 1 in MeCN- d3 at 20 °C consists of a broad singlet peak at δ 10.19 for the NH2 protons, a quartet at d 3.71 for the CH2 protons of the trifluoroethyl group, and a singlet at d 2.69 for the N-CH3 group.

O Et O H O Distill 2 2 F3C N HCl/Et2O H2NMe(CH2CF3)Cl C + 2 LiAlH4 H 2 hr. 0℃ F3C N b.p. 47℃ 68% Yield H 40% Yield

We then investigated the synthesis of the amine-borane adduct N-methyl-N-(2,2,2- trifluoroethyl)amine-borane, (CF3CH2)MeNH∙BH3 by treatment of 1 with sodium borohydride

29 (NaBH4). This step is traditionally performed in thf or 1,2-dimethoxyethane (dme), but the high volatility expected for the amine-borane adduct prompted us to explore the higher-boiling ether solvent, diglyme. Although the hydrochloride salt 1 is poorly soluble in diglyme, we find that it reacts with 1 equiv of NaBH4 to evolve H2 gas and generate of the amine-borane adduct

11 3 HNMe(CH2CF3)∙BH3, 2, quantitatively as shown by B NMR spectroscopy (Figure 2.2). We decided to use this solution directly without isolating or purifying the amine-borane.

2-4 h H2NMe(CH2CF3)Cl + NaBH4 HNMe(CH2CF3)BH3 + NaCl + H2 2 diglyme or thf

Me CH2CF3 δ – 11.5 (q) N JBH = 100 Hz 1 3 H BH3

5 0 -5 -10 -15 -20 -25 -30 f1 (ppm) 2 11 Figure 2.2. B NMR spectrum of HNMe(CH2CF3)∙BH3, 2, in diglyme at 20 °C.

5 0 -5 -10 -15 -20 -25 -30 f1 (ppm) We next investigated the synthesis of the corresponding aminodiborane. Treatment of the solution of the amine-borane adduct 2 with BH3∙thf (a reagent that comes as a solution in thf) does give (CF3CH2)MeNB2H5, 3, which appears as a triplet at δ -17.0 (Figure 2.3). Also formed in this reaction are very small amounts of the aminoborane (CF3CH2)2N=BH2 (δ 41.7) and the thf ring-opening products24 tributyl borate (δ 18.8) and dibutyl borate (δ 27.5), as well as bis(N- methyl-N-(2,2,2-trifluoroethyl)boranamine, HB[NMe(CH2CF3)]2 (δ 30.4). The formation of these thf-derived products is most-likely a consequence of the long reaction time needed for the amine-borane to react (9 days).

δ – 16.99 (q) HB[NMe(CH CF )] n 2 3 2 B(O Bu)3 δ 30.43 (d) JBH = 135 Hz δ 18.81 (s) CF3 JBH = 144 Hz n Me CF3 HB(O Bu)2 N δ 27.49 (d) Me δ 41.75 (t) BH3 JBH = 170 Hz N JBH = 131 Hz BH 3 δ – 11.76 (q) H2B BH2 Me JBH = 99 Hz H N BH2

F3C

44 42 40 38 36 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14 -16 -18 -20 -22 -24 f1 (ppm)

11 Figure 2.3. B NMR spectrum of the products from the reaction of (CF3CH2)MeNH∙BH3 with

BH3∙thf at 20 °C after 9 days.

Although the reaction above gives the desired aminodiborane, the formation of butyl borates by thf ring opening complicates the synthesis because the latter reaction consumes a necessary reagent, borane, and as a result controlling the reaction stoichiometry is difficult. We therefore investigated a reaction protocol that did not include thf. A convenient alternative borane source that fulfills these requirements is borane-dimethyl sulfide, BH3∙SMe2, which comes as a solution in dimethyl sulfide, and is used in organic synthesis as a reductant, and in

34 hydroboration reactions.29 We also explored two different non-heterocyclic ethers as solvents, diglyme and cyclopentyl methyl ether (cpme), which because of their relatively high boiling points would facilitate separation of the volatile aminodiborane by distillation.

Treatment of the amine-borane 2 with one equivalent of BH3∙SMe2 in diglyme or cpme and heating at 70 °C for 3 d gives the desired µ-aminodiborane(6), (CF3CH2)MeNB2H5, 3, as

11 observed by B NMR spectroscopy. A similar reaction of (CF3CH2)NH2∙BH3 with BH3∙SMe2

22 under the same conditions gives the analogous aminodiborane (CF3CH2)HNB2H5 (Figure 2.4).

The 11B NMR resonance for the latter compound consists of a well-resolved triplet of doublets at

δ -22.6. This reaction pathway is very clean, and provides a reliable route to two new fluorinated

µ-aminodiborane(6) species. Efforts to isolate both aminoborane adducts by distillation at reduced pressures (5 mTorr) resulted in borane-dimethylsulfide co-distilling with R2NB2H5.

R1 R2

3 d, 70 ℃ N H B BH HNR1R2(BH3) + BH3-SMe2 2 2 diglyme or cpme H

CF3

H N δ -22.6 (td)

H2B BH2 JBH = 130 Hz (t-H), 31 Hz (µ-H) H

CF3 CF3

Me Me

N N

δ -11.7 (q) H BH3 H2B BH2 J = 100 Hz δ -17.0 (td) BH H JBH = 130 Hz (t-H), 28 Hz (µ-H)

BH3-SMe2 ẟ -19.5 (q)

-9 -10 -11 -12 -13 -14 -15 -16 -17 -18 -19 -20 -21 -22 -23 -24 -25 -26 -27 f1 (ppm)

11 Figure 2.4. B NMR spectra of the products of the reactions of BH3∙SMe2 with (a)

(CF3CH2)NH2∙BH3 and (b) (CF3CH2)MeNH∙BH3, both in cpme after 2 days at 20 °C.

11 The B NMR chemical shift of (CF3CH2)MeNB2H5 is deshielded by 5.6 ppm with respect to that of (CF3CH2)HNB2H5, and this difference is probably an inductive effect: electron donors evidently deshield 11B NMR resonances of aminodiboranes, other factors being equal.

11 Observation of Na[H3BNMe(CH2CF3)BH3] by B NMR Spectroscopy. Given the successful reaction scheme to prepare µ-aminodiborane(6) analogues from primary and secondary fluoroamines, we investigated whether reaction with NaH would allow for the preparation of a new aminodiboranates. Reaction of the aminodiborane (CF3CH2)MeNB2H5, 3,

11 with NaH in cpme results in the formation of significant amounts of NaBH4, as observed by B

NMR spectroscopy (Figure 2.5). The 11B NMR spectrum also contains a quartet at δ -9.79 whose

1 JBH coupling constant of 91 Hz is essentially identical to those of NaDMADB and its numerous

19 30 analogs. The small nonet at δ -29.6 is due to NaB3H8. To date, we have been unable to isolate the aminodiboranate salt from the reaction mixture.

- BH4 ! -42.2 (p) JBH = 81 Hz

δ – 9.8 (q) JBH = 91 Hz

Me CH2CF3 N

H3B BH3 NaB3H8 ! -29.6 (m) JBH = 33Hz

40 35 30 25 20 15 10 5 0 -5 -10 -15 -20 -25 -30 -35 -40 -45 f1 (ppm)

11 Figure 2.5. B NMR spectrum of the reaction of (CF3CH2)MeNB2H5 with NaH for 2 hours in diglyme at 20 °C.

Concluding Remarks. Two new amine-borane adducts with fluorinated substituents on the nitrogen have been prepared and used in an improved synthetic route for the preparation of

µ-aminodiborane(6) species, (CF3CH2)MeNB2H5 and (CF3CH2)HNB2H5. In contrast, we were able to prepare only negligible amounts of the µ-aminodiborane(6) species (CH2CF3)2NB2H5; instead, the primary product was the aminoborane (CH2CF3)2NBH2, which is (formally) generated from the aminodiborane by loss of BH3.

These results allow us to correlate the pKa of the ammonium form of an amine with the relative stability of the aminodiborane (R2NB2H5) vs. the aminoborane (R2N=BH2). The two

31 fluoroalkyl groups in (CF3CH2)2NH (calculated pKa = 1.2) cause this amine to be too weakly basic to form an aminodiborane, and instead it forms an aminoborane. In contrast, both

32 33 (CF3CH2)NH2 (calculated pKa = 5.5, experimental pKa = 5.7) and (CF3CH2)MeNH

34 (calculated pKa = 5.8) are able to form aminodiboranes.

Finally, sodium N-methyl-N-(2,2,2-trifluoroethyl)aminodiboranate was observed but not isolated. Future experiments should be directed toward isolating and characterizing

Na[H3B(CF3CH2)MeNBH3] and utilizing it to prepare the corresponding magnesium salt. If the electron withdrawing fluoroalkyl substituents weaken the N-B bond relative to that in the

DMADB analog, as expected, then Mg[H3B(CF3CH2)MeNBH3]2 may serve as a precursor for the deposition of thin films of superconducting MgB2.

2.3 Experimental

All operations were carried out in vacuum or under argon using standard Schlenk techniques. Krytox grease was used to seal the joints of all reactions involving the reflux of boranes in thf or diglyme for more than 24 h. Solvents were distilled under nitrogen from

37 sodium/benzophenone (pentane, diethyl ether, tetrahydrofuran, cyclopentyl methyl ether and diglyme), or sodium (toluene). Borane-tetrahydrofuran solution, 1 M in borane, (Sigma-Aldrich) was distilled under vacuum before use. Borane-dimethyl sulfide (10 M solution in dimethyl sulfide; Sigma-Aldrich), N,N-bis(2,2,2-trifluoroethyl)amine (Oakwood Chemicals), N-methyl-N-

(2,2,2-trifluoroethyl)amine (Oakwood Chemicals), and N-(2,2,2-trifluoroethyl)amine

(H2N(CH2CF3), Oakwood) were under argon before use. Hydrogen chloride diethyl ether solution, 2 M in hydrogen chloride (Alfa-Aesar), anhydrous sodium sulfate (Sigma-Aldrich), N- methyltrifluoroacetamide, 98%, (Sigma-Aldrich), 60% sodium hydride dispersion in mineral oil

(Sigma-Aldrich), and lithium aluminum hydride (Sigma-Aldrich) were used as received. The compound N-methyl-N-(2,2,2-trifluoroethyl)amine hydrochloride was prepared using a modified literature procedure which substituted HCl gas with hydrogen chloride diethyl ether solution.35

The 1H NMR data were obtained on a Varian Unity Inova 400 instrument at 400 MHz.

11B NMR data were obtained on a Varian Unity Inova 400 instrument at 400 MHz. Chemical shifts are reported in � units (positive shifts to high frequency) relative to tetramethylsilane (1H

11 NMR) or BF3·Et2O ( B NMR).

N-Methyl-N-(2,2,2-trifluoroethyl)amine Hydrochloride, 1. To LiAlH4 (7.7 g, 200 mmol) suspended in Et2O (100 mL) at 0°C was added a solution of CF3CONHCH3 (12.6 g, 100 mmol) in Et2O (50 mL) dropwise over 2 h. Upon complete addition of the amide, the solution was stirred for 1 h at 0 °C and then for 20 h at room temperature. The solution was recooled to 0

°C then quenched by addition of H2O (100 mL, 5.5 mol) dropwise. The resulting mixture was stirred for 1 h at room temperature. The volatile portion was distilled off (30 °C) from the aqueous/organic solvent mixture, and the distillate was dried over anhydrous Na2SO4. The solution was filtered and treated with HCl (200 mL of a 1 M solution in Et2O, 200 mmol), which

38 caused a white precipitate to form. The precipitate was collected by filtering and dried in vacuum

1 1 for 12 h. Yield: 5.97 g (40%). H NMR (MeCN-d3, 20 °C): d 10.21 (br s, 2H, NH), 3.71 (q, JHH

= 9 Hz, 2H, CH2CF3), 2.69 (s, 3H, NCH3).

N-Methyl-N-(2,2,2-trifluoroethyl)amine-borane, 2. A suspension of

HNMe(CH2CF3)∙HCl (1.95 g, 13 mmol) and NaBH4 (0.51 g, 13 mmol) in diglyme (40 mL) was stirred at room temperature for 4 h. The resulting colorless solution was filtered, and the filtrate

11 1 containing the product can be used as prepared. B NMR (diglyme, 20 °C): d -11.76 (q, JBH =

99 Hz).

N,N-Bis(2,2,2-trifluoroethyl)amine-borane. To a stirred solution of BH3∙thf (30 mL, 30 mmol) at room temperature was added HN(CH2CF3)2 (4.9 mL, 30 mmol). The resulting colorless

11 1 solution can be used as prepared. Yield: 100%. B NMR (thf, 20 °C): d -11.51 (q, JBH = 100

Hz).

11 Comment on B NMR Spectra of μ-Aminodiboranes (μ-NR2)B2H5. µ-

Aminodiborane(6) compounds are derivatives of diborane, B2H6, in which one of the two

11 bridging µ-H groups is replaced with a primary or secondary NR2 group. The B NMR spectra of µ-aminodiborane(6) species often appear as triplets of doublets; such spectra are in the slow exchange limit, in which each boron nucleus is coupled to the one bridging and two terminal hydrogen atoms. Sometimes, however, if the exchange between the terminal and bridging hydrides is in the slow exchange regime (i.e., faster than in the slow exchange limit but still well below the fast exchange limit), the resonance appears as a broad triplet. In the limit of fast exchange, the 11B NMR spectrum appears as a binomial sextet.

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APPENDIX A: SYNTHESIS OF Mo[H3BNMe2BH3]2 WITH TIN AS A CO-REDUCTANT

A.1 Introduction

Current integrated circuits use Ta/TaN bilayers as diffusion barriers in Cu interconnects.

Ta and TaN have electrical resistivities of 14 and 200 µΩ cm, respectively, which are too high for future technology nodes. Molybdenum metal and molybdenum diboride, MoB2, which have electrical resistivities are 5.3 and 12.9 µΩ cm, respectively, are under consideration as potential replacements for Ta/TaN. No CVD methods currently exist to deposit these two materials at low temperatures, however, and therefore we became interested in the possibility that the

1 molybdenum aminodiboranate Mo[H3BNMe2BH3]2, previously prepared in our laboratories, could be used for this purpose.

The following is an improved preparation of Mo[H3BNMe2BH3]2, which employs tin as a reductant to assist in the conversion of the MoIII starting material to the desired MoII product. A

11 B NMR spectrum of the crude reaction products shows that Mo[H3BNMe2BH3]2 is the only molybdenum-containing product formed (Figure A.1); the only other species present are the aminodiborane B2H5(NMe2) and the aminoborane dimer (NMe2-BH2)2.

A themogravimetric study of Mo[H3BNMe2BH3]2, conducted under N2 at a ramp rate of

10 °C/min, showed that the onset temperature for thermolysis is 136 °C and thermolysis is complete by 180 °C (Figure A.2). The final mass is 43% of the initial mass; if Mo(DMADB)2 generates Mo, the expected final mass would be 40%, whereas if it generates MoB2, the expected final mass would be 49%. Preliminary CVD studies conducted by Xiaoqing Chu in the Abelson laboratories showed that, at Treservoir = 95 C and Tsubstrate = 400-450 °C, Mo[H3BNMe2BH3]2 gives

45 smooth amorphous films with compositions of 50 at. % Mo, 30 at. % B and 20 at. % C. Under these conditions, films up to 40 nm thick were obtained at growth rate of about 4 nm/min.

A.2 Experimental

All operations were carried out in vacuum or under argon using standard Schlenk techniques. All was dried in an oven at 150 °C, assembled hot, and allowed to cool under vacuum (5 mTorr) before being back filled with argon. Tetrahydrofuran (thf) and diethyl ether

(Et2O) were distilled under nitrogen from sodium/benzophenone. Tin metal (Sigma-Aldrich) was used as received. Trichlorotris(tetrahydrofuran)molybdenum(III)2 and sodium N,N- dimethylaminodiboranate3 were prepared by literature procedures. The 11B NMR data were obtained on a Varian Unity Inova 400 instrument at 400 MHz. 11B NMR chemical shifts are

reported in δ units (positive shifts to high frequency) relative to BF3∙Et2O.

Bis(N,N-dimethylaminodiboranato)molybdenum(II). To a suspension of Sn (0.51 g,

4.30 mmol) and MoCl3(thf)3 (1.54 g, 3.69 mmol) in Et2O (30 mL) at 0 °C was added a solution of Na[H3BNMe2BH3] (1.10 g, 11.6 mmol) in Et2O (30 mL) at 0 °C. The reaction mixture was stirred at 0 °C for 1 h and allowed to warm to room temperature. Gas slowly evolved. The mixture was stirred at room temperature overnight for 13 h to give a dark green solution and a dark gray precipitate. The solution was filtered at room temperature from the precipitate and any excess Sn and was taken to dryness in vacuum. The dark green solid was purified by sublimation

(70 °C, 5 mTorr) overnight under static vacuum to afford a green crystalline sublimate. Yield:

11 1 0.31 g (31%). B NMR (thf, 20 °C): δ 25.75 (dt, JBH = 126, 73 Hz, major isomer); δ 24.85 (dt,

1 JBH = 126, 73 Hz, minor isomer). The population ratio of the major to minor isomer was approximately 3:1. The 11B NMR data (Figure A.3) match literature values.4

11 Figure A.1. B NMR spectrum of the products of the reaction of MoCl3(thf)3 with Sn and

Na[H3BNMe2BH3] in diethyl ether.

Figure A.2. Thermogravimetic analysis of Mo[H3BNMe2BH3]2 under N2 at a ramp rate of 10

°C/min.

b) A2

28.6 28.2 27.8 27.4 27.0 26.6 26.2 25.8 25.4 25.0 24.6 24.2 23.8 23.4 23.0 22.6 f1 (ppm)

11 Figure A.3. (a) The observed B NMR spectrum of the two isomers of Mo[H3BNMe2BH3]2 in thf at 20 °C and (b) the simulated 11B NMR spectrum with parameters given in the text.

A.3 References

1. Kim, D. Y. Part I. Synthesis of Metal Hydroborates as Potential Chemical Vapor

Deposition Precursors. Part II. Chemical Vapor Deposition of Titanium-Doped Magnesium

Diboride Thin Films. Ph. D. Thesis, University of Illinois at Urbana-Champaign, 2007.

2. Maria, S.; Poli, R.; Gallagher, K. J.; Hock, A. S.; Johnson, M. J. A., Ether Complexes of

Molybdenum(III) and Molybdenum(IV) Chlorides. Inorg. Synth. 2014, 36, 15-18.

3. Nöth, H.; Thomas, S., Metal Tetrahydroborates and Tetrahydroboratometalates. Part 24.

Solvates of Sodium Bis(borane)dimethylamide. Eur. J. Inorg. Chem. 1999, 1373-1379.

4. Mallek, J. Volatile Metal Borohydride Complexes: Synthesis and Characterization of

New Chemical Vapor Deposition Precursors. Ph. D. Thesis, University of Illinois at Urbana-

Champaign, 2014.