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Preparation of ytterbium and europium borides from Yb(II) and Eu(II) boron hydride precursors
Salupo, Terese Ann, Ph.D.
The Ohio State University, 1993
UMI 300 N. ZeebRd. Ann Arbor, MI 48106
Preparation of Ytterbium and Europium Borides
from Yb(ll) and Eu(ll) Boron Hydride Precursors
Dissertation
Presented in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy in the Graduate School of
The Ohio State University
by
Terese A. Salupo, B.S.
**********
The Ohio State University
1993
Dissertation Committee: Approved by:
Dr. Bruce E. Bursten
Dr. James A. Cowan
Dr. Sheldon G. Shore Advisor Department of Chemistry To Mom and Dad ACKNOWLEDGEMENTS
I would like to thank Professor Sheldon G. Shore for his guidance and support during my years in the Shore Group. I especially appreciate his sense of humor which made for many moments of laughter. I thank my mentor, Professor
Howard C. Knachel, for his neverending enthusiasm for chemistry and encouragement throughout both my undergraduate and graduate studies. I thank
Ms. Marilyn Frohwerk for a challenging and enjoyable first exposure to chemistry which greatly influenced me to pursue a career in the field.
A special thank you goes to the Shore Group members past and present for being my family away from home. Never a day has gone by that I did not hear laughter on the second floor of Evans Lab. Although the names are too numerous to mention, I would especially like to acknowledge Drs. Tim Coffy, Deborah
McCarthy, Tim Shay, Jim White, and Jung Oh and also Bob Godfroid and
Janghoon Chung. Words cannot express my gratitude to Debbi McCarthy, Tim
Shay and Jung Oh for making me laugh during the toughest year of my life.
Thanks to Fr. Vinny McKiernan and my movie companions, Anne Petratis and Dr. Nick Pontikos, for reminding me that there is more to life than just chemistry. I thank David Chang and Dick Weisenberg for their assistance with gas
mass spectrometry and Clare McDonald and John Mitchell for their assistance with
Scanning Electron Microscopy.
The most important thank you goes to Mom and Dad and Grandma and
Grandpa Kulchar. This degree is just as much yours as it is mine because of the
lifetime I have had of your love and sacrifice which have shaped me into what I am today. You are the best teachers I have ever had. Thank you also to the rest of the Salupo clan, Judy, Pat, Nick, Patti, Anthony, Vince, Stacy, and Nicholas, for always being my stability when things get rough. I love you more than words can
say.
Last and certainly not least, I thank the Big Guy for His many, many
blessings and for the strength and wisdom to get through all the graduate school obstacles. VITA
September 11, 1964 Born: Garfield Heights, Ohio
1986 B.S. Chemistry, University of Dayton, Dayton, Ohio.
1989-1991 Graduate Research Assistant Department of Chemistry The Ohio State University Columbus, Ohio.
1987-1989 Graduate Teaching Assistant 1991 -1993 Department of Chemistry, The Ohio State University Columbus, Ohio.
PUBLICATIONS
"Clusters Derived from [OSgCO^ ]2'. Crystal Structures of [Et4N] [HOs3(CO) 11 ] and H2Os4(CO)13h, J.A. Krause, U. Siriwardane, T.A. Salupo, J.R. Wermer, D. Knoeppel, and S.G. Shore, J.Organometallic Chemistry, in press.
"Synthesis of Dinuclear Carbonylates of the Iron Subgroup: [M2(CO)8]2' (M *= Ru, Os) and [FeRu(CO)g]2". Structures of [PPh4]2[FeRu(CO)8] CH3CN and [PPh4]2[Fe2(CO)8]'2CH3CN", N.K. Bhattacharyya, T.J. Coffy, W. Quintana, T.A. Salupo, J.C. Bricker, T.B. Shay, M. Payne, and S.G. Shore, Organometallics 9 (1990) 2368-2374.
"Symmetrical Ring Cleavage of the Gold (I) Phosphorus Ylide Dimer [Au(CH2)2PPh2]2 by Hydrogen Halides in Toluene", H.C. Knachel, H.J. Galaska, T.A. Salupo, J.P. Fackler, Jr., and H.H. Murray, Inorganica Chimica Acta, 126 (1987) 7-10.
v "Reactions of HCI(g), DCI(g), and HBr(g) with the Adduct Pyrazine-Phosphorus (V) Chloride", H.C. Knachel, S.D. Owens, S.H. Lawrence, M.E. Dolan, M.C. Kerby, T.A. Salupo, Inorg. Chem. 25 (1986) 4606-4608.
FIELDS OF STUDY
Major Field: Chemistry
Minor Field: Inorganic Chemistry
Studies in Bioinorganic and Coordination Chemistry: Professors Daryle H. Busch and Daniel L. Leussing.
Studies in Organometallics, Cluster Chemistry, Group Theory and Catalysis: Professors Bruce E. Bursten, Devon W. Meek, Eugene P. Schram, Sheldon G. Shore, and Andrew Wojcicki.
Studies in Ceramic Engineering: Professors Charles H. Drummond III and Sheikh A. Akbar. TABLE OF CONTENTS
DEDICATION ...... ii
ACKNOWLEDGEMENTS...... iii
VITA...... v
UST OF TABLES ...... ix
UST OF FIGURES ...... x
UST OF ABBREVIATIONS ...... xiii
CHAPTER
I. IN TR O D U C TIO N ...... 1
II. STATEMENT OF THE PROBLEM ...... 24
III. RESULTS AND DISCUSSION ...... 27
A. Metathesis Reactions with L^YbClg...... 28 1. Reaction with KB3H8 ...... 29 2. Reaction with NaBHgCN ...... 34
B. Liquid Ammonia Reduction Reactions ...... 41 1. Reactions with NH4B3H8 ...... 42 2. Reactions with [HNMe3]2B12H12 ...... 48 3. Reactions with [H N E tg J g B ^ H ^ ...... 57 4. Reactions with NH3BH3 ...... 66 5. Reaction with [HNMe3]BgH1 4...... 72 6. Reaction with B 10H 1 4 ...... 78
C. Thermal Decomposition Studies of Precursor Complexes 79 1. Pyrolysis of (C H g C N ^ Y b fB H ^ ...... 79
2. Pyrolysis of (NH 3 )xYbB12H1 2 ...... 81 3. Pyrolysis of (NH3)xEu(B3H8)2 ...... 82
vii D. Thin Film Studies ...... 86 1. YbB2 thin film studies ...... 86 2. Eu B6 thin film studies ...... 91
IV. EXPERIMENTAL...... 98
A. A pparatus ...... 98
B. Solvents and Reagents...... 108
C. Preparation of Starting Materials ...... 114
D. R eactions ...... 117 1. Preparation of YbB12H1 2 ...... 117 2. Preparation of EuB12H-|2 ...... 118 3. Preparation of YbB10H1 0 ...... 119 4. Preparation of EuB10H1 0 ...... 120 5. Preparation of (CH3CN)xYb(BH3CN)2 ...... 121 6. Preparation of (C5H5N)xYb(BH3CN)2 ...... 122 7. Preparation of (NH3)xYb(NH2BH3)2 ...... 123 8. Preparation of (NH3)xEu(NH2BH3)2 ...... 124 9. Preparation of (NH^ixYb(B3l-y 2 from NH4B3H8 and Yb in NH3 ...... 125 10. Preparation of (NH3)xEu(B3H8)2 from NH4B3H8 and Eu in NH3 ...... 126 11. Preparation of (CH3CN)xYb(B3Ho)2 by metathesis ...... 127 12. Preparation of (C5H5N)xYb(B3H j2 by metathesis 128 13. Reaction of B10H14 with Yb in NH3 ...... 128 14. Reaction of [HNMeg]BgH14 with Yb in NH3 ...... 129 15. Pyrolysis of (CH3CN)xYb(BH4)2 ...... 130 16. Pyrolysis of (CH3 CN) xEu (Bh J 2 ...... 131
17. Pyrolysis of (NH 3 )xVbB12H1 2 ...... 132 18. Pyrolysis of (NH3)xEu (B3H8)2 ...... 133 19. Pyrolysis of (C5H5N)xYb(BH3CN)2 ...... 134
20. Pyrolysis of (C5H5N)xYb(NH2BH 2 J2 ...... 135 21. Thin film studies of YbB2 ...... 136 22. Thin film studies of EuB6 ...... 137
UST OF REFERENCES ...... 138
viii UST OF TABLES
Table 1. Some rare earth ores...... 2
Table 2. Valence electron configurations of the lanthanide elements...... 4
Table 3. Physical Data for the Divalent Lanthanides ...... 11
Table 4. Colors of Divalent Lanthanide Complexes...... 14
Table 5. Infrared Absorptions for MBH4 Complexes...... 21
Table 6. Characteristics of Lanthanide Borides ...... 23
Table 7. NMR Data for BH3CN' Complexes ...... 37
Table 8. Infrared Data for LnB10H10 Complexes (cm'1) ...... 58
Table 9. NMR Data for B10H102' Complexes ...... 63
Table 10. NMR Data for NH2BH3' Complexes and NH3BH3 ...... 70
Table 11. NMR Data of BgH14'Complexes ...... 75
ix UST OF FIGURES
Figure 1. Valence orbitals of the Lanthanides...... 5
Figure 2. Periodic variations in crystal radii ...... 7
Figure 3. Crystal structures of some trivalent lanthanide complexes...... 9
Figure 4. X-ray structure of Snr^CgMeg^...... 17
Figure 5. X-ray structure of (CH3CN)4Yb(BH4)2 ancJ (CH3CN)6Yb{B10H 14) ...... 19
Figure 6. Infrared spectra of B3H8* complexes in the i/(B-H) region ...... 30
Figure 7. 11B and 1H NMR spectra of (CH3CN)xYb(B3Hg)2 in CD3CN ...... 33
Figure 8. Infrared spectra of BH3CN' complexes ...... 36
Figure 9. 11B NMR spectra of BH3CN'complexes...... 38
Figure 10. 1H NMR spectra of BH3CN' complexes ...... 39
Figure 11. Infrared spectra of B3H8* complexes ...... 43
Figure 12. 11B NMR spectra of B3H8' complexes ...... 46
Figure 13. 1H NMR spectra of B3H8' complexes ...... 47
Figure 14. Structures of Be(BH4)2 and Eu(B3H8)2 ...... 49
Figure 15. Infrared spectra of YbB12H12 complexes in in the i/(B-H) region ...... 51
x Figure 16. Infrared spectra of EuB12H12 complexes in the t/(B-H) region ...... 52
Figure 17. 11B NMR spectra of B12H122' complexes in CD3CN ...... 54
Figure 18. 1H NMR spectra of B12H122‘ complexes in CD3CN ...... 55
Figure 19. Infrared spectra of YbB10H10 complexes in the i/(B-H) region ...... 60
Figure 20. Infrared spectra of EuB10H10 complexes in the i/(B-H) region...... 61
Figure 21. 11B NMR spectra of B10H102'compounds...... 62
Figure 22. 1H NMR spectra of B10H102' compounds ...... 65
Figure 23. 1H NMR spectra of (C5H5N)xYb(NH2BH3)2 and NH3BH3 in d5-pyridine ...... 69
Figure 24. 11B NMR spectra of (C5H5N)xYb(NH2BH3)2 andNH3BH3 in d5-pyridine ...... 71
Figure 25. Infrared spectra of Lr^NHgBH^g complexes ...... 73
Figure 26. 11B NMR spectrum of [(CD3CN)xYb][BgH14]2 ...... 76
Figure 27. 1H NMR spectra of BgH14‘ compounds ...... 77
Figure 28. 11B NMR spectrum of product of Yb with two equivalents of decaborane in ammonia ...... 80
Figure 29. 11B NMR spectrum of volatile products of (NH^xEufBaHg^ decomposition ...... 84
Figure 30. Summary of lanthanide borides prepared from lanthanide boron hydride precursors ...... 85
Figure 31. Scanning electron micrographs of ytterbium diboride films on tantalum ...... 87
Figure 32. Scanning electron micrographs of ytterbium diboride on tantalum, (a) 1-coating edge at X200. (b) 3-coating edge at X18 0 ...... 88
xi Figure 33. Scanning electron micrographs of 1-coating of ytterbium diboride on tantalum, (a) X300. (b) X6000 ...... 90
Figure 34. Scanning electron micrographs of 8 coatings of ytterbium diboride on tantalum, (a) X1500 (b) X10000 ...... 92
Figure 35. Scanning electron micrographs of ytterbium diboride on tungsten, (a) X2500.(b) X10000 ...... 93
Figure 36. Scanning electron micrographs of 20 coatings of ytterbium diboride on tantalum, (a) X1500. (b) X10000 ...... 94
Figure 37. EDS spectrum of YbB2 film...... 95
Figure 38. Scanning electron micrograph of two coatings of europium hexaboride on tantalum ...... 97
Figure 39. Secondary vacuum line manifold...... 100
Figure 40. Toepler pump apparatus ...... 101
Figure 41. Vacuum line extractor ap p aratu s ...... 104
Figure 42. Tube furnace set up for pyrolysis experiments...... 109
xii UST OR ABBREVIATIONS
NMR Nuclear Magnetic Resonance
IR Infrared
Ln Lanthanide element
L Lewis base ligand
DIME 2-methoxyethyl ether (diglyme)
DME 1,2-dimethoxyethane (glyme) py pyridine
THF tetrahydrofuran
Cp cyclopentadienyl ligand
SEM Scanning Electron Microscopy
EDS Energy Dispersive Spectrometry CHAPTER I
INTRODUCTION
A. Background
The mineral containing lanthanide elements, now known as gadolinite, was first discovered by Carl Axel Arrhenius in 1787 in a quarry in Ytterby, Sweden. In
1794, Johan Gadolin extracted 'yttria" from a sample of the black material with unusual properties. "Yttria", thought to be a single rare earth element was actually a mixture of the oxides of yttrium, erbium and terbium. By 1907, with the exception of radioactive promethium, all of the rare earths had been discovered.1
The challenge at this point was the isolation of the lanthanides in pure form. Their very similar chemical and physical properties caused tremendous difficulties in separating mixtures of the oxides typically found in nature (Table 1).2 Because of the similarities between the chemistry of scandium, yttrium, and lanthanum and the
14 lanthanide elements, the former are often incorporated into discussions of the lanthanides.3
The atomic numbers of the lanthanides range from 57 to 71. The properties of the lanthanides are dictated largely by the presence of the f orbitals in the valence shell. In the lanthanide series (Ce to Lu) electrons occupy 4f and 6s orbitals. The 5d orbitals, close in energy to the 4f orbitals, are occupied in cerium, Table 1. Some rare earth ores.
Monazite CeP0 4
Xenotime YP04
Gadolinite FeBe2Y 2Si201 q
Allanite (Ce,Ca)2FeAI0(Si2O7)(Si04)(OH)
Cerite Ce3 CaSi 3 0 1 3 H3
Euxenite Y3(NbfTa}3Ti2015
Bastnasite CeFC03
Polycrase (Ce>Y,Th,U)(T/Nb,Ta)20 6
Fluorocerite CeF3
Ce represents a mixture of the lower lanthanides (e.g. La, Ce, Nd); Y represents a mixture o f later lanthanides. gadolinium, and lutetium. The valence electron configurations are shown in
Table 2.4
The chemistry and properties of the lanthanides more closely resemble those of the alkali and alkaline earth metals than the transition metal elements.
The reason for this phenomenon is the low radial extension of f orbitals which, in effect, prevent substantial overlap with the orbitals of incoming ligands (Figure
1).2,5 The d orbitals are readily accessible in transition metals, but the f orbitals around the lanthanide lie within the 6s and 5d orbitals and behave more like core orbitals than valence orbitals. Comparison of crystal-field splitting energies of d- block elements to f-block elements supports the observation of the lower covalency of lanthanide-ligand bonds versus transition metal-ligand bonds.
Crystal-field stabilization energies for lanthanide complexes are approximately 1 kcal/mole while transition metal complexes have values approximating 100 kcal/mole.1,6 This suggests greater ionic character in bonds involving lanthanide elements and greater covalent character in bonds involving transition metal elements.
The lanthanide contraction results from the poor shielding ability of f electrons. Consequently, the 5s and 5p electrons are attracted to a greater extent by the nuclear charge. With increasing atomic number across the lanthanide series, the poorly shielded 4f electrons experience more positive effective nuclear charge causing contraction of the 5s and 5p orbitals. As is expected, the atomic radii of the lanthanide elements decrease with increasing atomic number. Table 2. Valence electron configurations of the lanthanide elements.
Atomic Name no. Symbol Atom M1* M3+ M4+
Lanthanum 57 La Sd6s* 5d' [Xe] Cerium 58 Ce ApSd16s1 AP 4 / [Xe] Praseodymium 59 Pr Apes1 Ap Ap AP Neodymium 60 Nd Ap 6s1 Ap AP AP Promethium 61 Pm Apes1 AP AP - Samarium 62 Sm Ap 6sJ APAP —
Europium 63 Eu Apes1 AP Ap -
Gadolinium 64 Gd ApSdSs1 ApSd' AP - Terbium 65 Tb Ap Ss1 ' Ap AP AP Dysprosium 66 Dy 4 / ,0 6r1 Ap° AP Ap Holmium 67 Ho 4 /" 6 s J A p 1 Ap° — Erbium 68 Er A p H s 1 A p 1 Ap> —
Thulium 69 Tm A p i 6s1 4 / ' 3 A p 1 — Ytterbium 70 Yb A P 4 6s1 A P 4 A p 1 —
Lutetium 71 Lu Ap S5d6s1 — A p 4 — b. a. iue . aec obtl o te ataie, a Tre of Three (a) lanthanides, the of orbitals Valence 1. Figure
probability fxyz h svn -rias () ail xeso of extension Radial (b) f-orbitals. seven the ataie aec orbitals. valence lanthanide fz(5z2-3r2) r fz(x2-y2)
Figure 2 illustrates the decreasing radii from lanthanum to lutetium.7 The difficulty in separating the lanthanides is mostly due to small size differences between them, a 0.2 A range, rather than electronic considerations.7 Transition metals beyond the lanthanide series also experience the lanthanide contraction so that the higher atomic number elements of a given family (e.g. Hf and Zr, W and Mo) exhibit similar physical properties.
The lanthanides are considered hard acids in the Pearson sense and, therefore, have preference for hard donor atoms (e.g. O and F).8 The trivalent lanthanides are the most stable ions but both high and low valent ions also exist for certain lanthanide elements, Ce4+, Sm2+, Eu2+, and Yb2+.9 The stability of the divalent and tetravalent ions is due to the greater thermodynamic stability of f°, f7 and f14 configurations. The d-orbitals are preferentially filled in atoms (e.g.
Gd, La) where, by doing so, the stable f orbital configurations can be preserved.
This is not the case, however, for the trivalent ions. Tb3+, for example, maintains an f8 configuration rather than an f7d1 configuration.1 The large ionic and atomic radii of the lanthanides allow coordination numbers between 6 and 12 but are typically 8 or 9. Larger coordination numbers are generally accommodated by the larger elements early in the lanthanide series. The geometries and coordination numbers of lanthanide complexes are more highly influenced by steric factors than by electrostatic considerations. 0 10 0 30 40 50 60 70 60 00 Atomic number, Z
Figure 2 Periodic variation in crystal radii. B. Complexes of the Lanthanides
The majority of lanthanide research has been done in the area of organometallic lanthanide chemistry. Numerous reviews have been published in this area, most of which employ trivalent lanthanide ions.3,4,8'12 The first examples of organolanthanide complexes were reported by Wilkinson and Birmingham in
1954.13 Reactions between lanthanide trichlorides (Ln = Sc, Y, La, Ce, Pr, Nd,
Sm, and Gd) and sodium cyclopentadienide in THF produced the corresponding lanthanide(lll) tricyclopentadienyl complexes, LnfCgHgJg.
LnClg + 3NaC5H5 - * (CgH^gLn + 3NaCI (1)
By 1969, the cyclopentadienyl complexes of the complete lanthanide series had been published. Figure 3 illustrates two of the many cyclopentadienyl complexes,
ScfCgHgJg and Gd^gHgJg THF, that have been structurally characterized.14,15 It is no coincidence that most of the isolated organolanthanide complexes involve n-bonded cyclopentadienyl or substituted cyclopentadienyl ligands. Their steric bulk gives them the ability to satisfy the large coordination sphere of the lanthanide center thus stabilizing the complex. The electropositive nature and coordinative unsaturation of the lanthanides prompt further stabilization by basic coordinating ligands such as THF and DME.10 The bonding in these complexes is highly ionic as evidenced by similarities between spectroscopic data of the coordinated and
"naked" ions.12 Neutral homoleptic alkyl lanthanide complexes are much less common than the rather ubiquitous cyclopentadienyl complexes. The first stable CIO
C12
C1 8
(a) (b)
Figure 3. Crystal structures of some trivalent lanthanide complexes, (a) Gd(C 5 H5 )3 -THF and (b) Sc(CsH5)3. CO 10
complex containing a o lanthanide-ligand bond was triphenyl scandium prepared
by Hart and Saran in 1968.® The simplest formula for trivalent lanthanide alkyls is
LnR3; however, although the complex may be electronically satisfied, the
coordinative unsaturation of the complexes, particularly for simple alkyls (CH3,
C2 H5, etc.), makes them highly reactive and, therefore, precludes their isolation and characterization. Characteristic of all organolanthanide complexes is their air and moisture sensitivity.
C. Divalent Lanthanides
Due to the greater stability of the trivalent lanthanides, the majority of lanthanide complexes published involve metals in the +3 oxidation state, organolanthanides as well as coordination complexes in aqueous media. The importance of divalent lanthanides, however, must not be understated. All of the
rare earths can be found in the + 2 oxidation state in a CaF 2 matrix, but only
Sm2+, Eu2 + , Yb2+ are stable enough for their chemistries to be explored under normal reaction conditions. Their stabilities can be rationalized by their valence
electron configurations shown in Table 3. 1 6 The 4f shell is exactly half-filled in
Eu2+ and completely filled in Yb2+, both very energetically favorable configurations. The 4f® configuration in Sm2+ is not quite as stable but sufficiently so to permit the study of its chemistry. The relative stabilities of the divalent lanthanides are Eu2+ > Yb2+ > Sm2+ , Sm2+ being the most reactive as one might expect from the above argument. The relative stabilities are also reflected 11
Table 3. Physical Data for the Divalent Lanthanides
Sm 2 + Eu2 + Yb 2 +
valence electron [Xe]4f6 [Xe]4f7 [Xe]4f14 configuration
Ionic radius 1.27 1.25 1 .1 4 (CN = 8, A)
Reduction Potential -1.50 -0.51 - 1 . 1 0 + 3 /+ 2, Volts
Heff (B.M) 3 .4 -3 .8 7.9 diamag. in their reduction potentials; Eu3 +/Eu2+ = -0.35 Volts, Yb 3 +/Yb2+ = -1.15 Volts,
and Sm3+/ Sm2+ = -1.55 Volts . 1 7 These values show their powerful abilities as reducing agents. The ionic radii of the divalent lanthanides decrease with increasing atomic number due to the lanthanide contraction. Samarium(H) with an
atomic number of 62 is the largest of the three (rSm2 +, CN = 8 ; 1.27A) and Yb2+
with an atomic number of 70 has the smallest ( ^ 2 +, CN = 8 : 1.14A.18 The smaller radius of Yb2+ makes it more stable toward coordinative unsaturation than
Sm2+ or Eu2 + . A major distinction between the di- and trivalent ions is the smaller charge to size ratio of the divalent ions. Consequently, the trivalent ions are more oxophilic and tend to form bonds with more ionic character. More "covalent" interactions are possible with the divalent lanthanides. The similarities in size of the
divalent lanthanides with the Group IIA ions (rSf 2 +(CN= 6 ): 1.16A; rCa 2 +(CN = 6 ):
1 .0 0 A) account for the similarities in the chemistries of the two. 1 , 1 9
Of the divalent lanthanides, only Yb2+ is diamagnetic. The magnetic
moments of Sm2+ and Eu2+ are 3.4-3.8/zB and 7.9/xB, respectively. 9 Despite the paramagnetism of Sm2+, its magnetic moment is small enough to permit analysis by NMR spectroscopy. The paramagnetism of Europium(ll) causes large peak shifts and line broadening in the NMR spectra of Eu2+ complexes.
The complexes of the divalent lanthanides are typically very highly colored.
The color is attributed to electronic transitions from the 4f to the 5d levels which absorb in the visible region of the spectrum. These transitions in many of the trivalent ions cause absorptions predominantly in the ultraviolet region giving 13 colorless or weakly colored solutions. Although Sm2+ and Eu2+ are isoelectronic,
Eu3+ is colorless but Sm2+ is red-brown. Likewise, Eu2+ is straw-yellow but
Gd3+ is colorless . 1 The colors of divalent lanthanide complexes depend upon the coordination environment of the metal ion (Table 4). Ytterbium(ll) complexes can be red, orange, yellow, green, blue or purple. Samarium(ll) complexes can be green, red or purple. The ligand effects for Eu2+ complexes are not as
pronounced as shown by isolation of only yellow, orange or red solutions. 9
Divalent lanthanides have been prepared by a variety of methods.
Ammonia solutions of europium and ytterbium metals produce solutions of the
lanthanide (I I) solvate and solvated electrons . 2 0 , 2 1
Ln + NH3 -» [(NH3 >xLn]2+ + 2[(NH3)y e l (2)
Ln = Yb, Eu
Upon standing, the lanthanide amides form. The first organometallic divalent lanthanide complex was prepared by reacting europium metal with
cyclopentadiene in liquid ammonia. 2 2
nh3 Eu + 3CgHg Cp^Eu + CgHg (3)
This method also proved successful for ytterbium2 3 Samarium(ll) and ytterbium(ll) halides have been prepared by reaction of the lanthanide metal with
1,2-diiodoethane in THF or by reaction of lanthanide metal with NH4X in liquid Table 4. Colors of Divalent Lanthanide Complexes.
Sm2+ Eu2 + Yb2+ c h 3c n green bright orange yellow
C5 H 5 N green red-orange deep violet
h 2o red colorless pale green 15 . 24 25 ammonia. ’
THF
Ln + ICH2 CH2I -» (THF)xLnl2 + C2 H4 (4)
Ln = Sm, Yb
n h 3
Ln + 2NH4X -> (NHg^LnXg + H2 (5)
Ln = Yb, Eu; X = Cl, Br, I
A third method for conversion of the metals to the +2 oxidation state is with a lanthanide/mercury amalgam in the presence of an oxidizing agent. The same
result can be obtained using Cp2 Hg. 2 6
THF
Cp2Hg + Ln Cp2 Yb(THF)x + Hg ( 6 )
Divalent lanthanides have also been prepared by reduction of trivalent lanthanide complexes. Both cyclopentadienyl and halide complexes of trivalent lanthanides
have been reduced to the divalent ions by alkali metals. 2 7 , 2 8
THF
SmCp3 + KC1 0 H8 -> SmCp2’THF + KC5 H5 + C 1 0 Hg (7)
THF
YbCp2CI + Na - * Cp2Yb + NaCI ( 8 ) 16
Figure 4 shows the x-ray structure of SmCp2, the first dicyclopentadienyl
lanthanide without coordinating solvent to be structurally characterized . 2 9 This complex is the lanthanide equivalent of transition metal metallocene complexes.
The predicted geometry of the l_nCp2 complexes is one in which the planes of the cyclopentadienyl rings are parallel to one another with the lanthanide ion between the two. Although a linear arrangement seems to be the geometry of lowest energy, the x-ray structures reveal that the rings are actually bent toward one
another. 3 0 The origin of this deviation is still a subject of debate but is not surprising considering the non-directional behavior of the valence f orbitals.
Perhaps steric effects are overridden by crystal packing effects.
There are several drawbacks to these preparative techniques. Samarium metal is insoluble in ammonia and cannot be oxidized to Sm2+ by methods employing liquid ammonia as the solvent. The complexes of Sm2+ are often very highly reactive and insoluble in common solvents making their isolation and
characterization extremely difficult. 9 Finally, the oxophilic nature of the lanthanides promotes solvent separated behavior in oxygen-containing solvents such as THF or DME. Solvating ligands can stabilize divalent lanthanides to the exclusion of less nucleophilic ligands in solution.
D. Lanthanide Boron Hydride Chemistry
Although a great deal of research has been published about organometallic trivalent lanthanide borohydride complexes, until recently, the chemistry of divalent 17
Figure 4. X-ray structure of Sm(C5Mes)2. 18 lanthanide complexes with boron hydrides has been virtually unexplored. Almost all of the systems studied have incorporated stabilizing cyclopentadienyl type
ligands or other 7r-systems into the lanthanide coordination sphere. Divalent lanthanide boron hydride complexes have been shown to be precursors to lanthanide borides. Cyclopentadienyl ligands were not desirable because of the greater difficulty in removing these ligands under pyrolytic conditions. Concerns
about the introduction of carbon-containing impurities have also been expressed. 6
The structures of [Sc{/7 -C 5 H3 (SiMe3 )2 } 2 (/i-H)2 BH2] and (THF) 3 Y[BH4]3have
been reported. 3 1 , 3 2 Divalent samarium and ytterbium decarbollide complexes,
Ln(C2 BgH 1 1 )(THF) 4 (Ln = Sm, Yb), have been prepared and structurally
characterized. 3 3 The bonding behavior of the dicarbollide cage, isolobal to C5 H5', mirrors the ionic bonding within the cyclopentadienyl lanthanides rather than the hydrogen-bridge bonding found in the borohydride complexes.
Boron hydride complexes of divalent ytterbium and europium,
(CH3 CN)4 Yb[Oi-H)3 BH]2, (C5 H5 N)4 Yb[BH4 ]2 2C 5 H5 N, (CH3 CN)2 Eu[BH4]2,
( C g H ^ 8 Eu(BH4)2, (CH3 CN)6 Yb[B1 0 H14] and (CH 3 CN)xEu[B 1 0 H14], have been isolated and characterized (Figure 5). Only the ytterbium(ll) complexes have been
confirmed by x-ray crystallography . 6 , 3 4 , 3 5 Acetonitrile and pyridine were ideal ligands because of their greater lability than oxygen-containing or cyclopentadienyl ligands and their sufficient stability at room temperature. These ligands were easily removed under thermal treatment yet formed complexes that were stable enough to be isolated at room temperature. (CH3CN)4Yb(BH4)2 (CH3CN)6Yb(BioHi4)
Figure 5. X-ray structures of (CH3CN)4Yb(BH4)2 and (CH3CN)6Yb(BtoHl4). 20
Characterization and determination of bonding modes in most of the lanthanide borohydride complexes has been done by infrared spectroscopy. Four possible bonding modes (Table 5) exist for mononuclear metal borohydride complexes. The bi- and tridentate configurations are most common among lanthanide borohydride complexes. Infrared assignments have been tabulated for
each of the bonding modes. 3 6 Due to the large radii of the lanthanides, coordinative unsaturation often promotes polymeric structures. Marks and
Grynkewich determined from vibrational data that desolvated Cp2 LnBH4 (Ln = Sm,
Er, Yb) complexes adopted polymeric structures similar to Be(BH4 ) 2 and
CHgZnfBH^ . 3 7 _ 3 9 The chelation of the borohydride unit in the solvated species was highly influenced by the size of the lanthanide. Infrared data suggested that
Cp2 Sm(BH4)THF preferred tridentate coordination while 2 CpYb(BH4)THF
preferred bidentate coordination. For Cp2 ErBH4 THF, the structure was bidentate in the solid state but tridentate in solution. Similarly for
[Ln{r7 -C 5 H3 (SiMe3 )2 } 2 (BH4 )(THF)n] complexes the smaller metals, Sc, Y, and Yb adopted bidentate ligation of borohydride while La, Pr, Nd, and Sm were large
enough to accommodate tridentate ligation. 3 1
E. Lanthanide Borides
In recent years, lanthanide borides have been used in several commercial applications. LaB6, for example, is used in electron gun sources for electron
microscopes because of its thermionic emission capabilities. 4 0 Thin films 21
Table 5. Infrared Absorptions for MBH 4 Complexes.
Structure Frequency (cm-1) Absorption Type
1 (monodentate) 2300-2450 B-Ht stretch
2 0 0 0 B-Hb stretch 2000-1700 M-Hb stretch
1000-1150 BH3 deformation M — H — B -H
n h
II (bidentate) 2400-2600 B-Ht stretch 1650-2150 B-Hb stretch 1300-1500 Bridge stretch
1 1 0 0 - 1 2 0 0 BH2 deformation H H
III (tridentate) 2450-2600 B-Ht stretch
2 1 0 0 - 2 2 0 0 B-Hb stretch 1150-1250 Bridge deformation
H
IV (ionic) 2200-2300 B-Ht stretch
1050-1150 BH2 deformation M +B H 4- 22 oflanthanide borides have also found applications as protective coatings (the dodecaborides are solids with remarkable hardness and stiffness) and as reflective
and interference films in the optics industry. 4 1 , 4 2 Studies of the unique
intermediate valency of some of the rare earth borides (e.g. 1 YbB 2 and SmBg)
have also been pursued4 1 The stable phases of ytterbium and europium borides
are shown in Table 6 .
Rare earth borides are typically prepared by borothermic reduction involving rare earth oxides and elemental boron or direct synthesis from elemental lanthanide and boron. Thermolysis temperatures for these traditional methods range from 1000°C to 1800°C.43,44 An alternate method for the formation of
lanthanide borides utilized lanthanide boron hydride precursor complexes. 3 4 , 3 5
Both YbB4 and EuB 6 formed when the corresponding lanthanide borohydride complexes were thermally treated at 1000°C.
The chemistry of the lanthanides is becoming increasingly important in
catalysis and materials science . 1 0 , 4 2 The chemistry of the divalent lanthanides, in particular, offers great promise for these applications yet remains relatively unexplored. Table 6. Characteristics of Lanthanide Borides
Structure Space Group Ln valency Structure Type Description YbB2434S aib2 P6/mmm trivalent hep metal atoms
Y b B ^ 46-48 ThB4 P4/mbm mixed tetragonal Bg units connected by B2 units
Bg octahedra in comers YbBg49-51’43 CaBg Pm3m divalent of simple cube, metal in body center
YbB1241'48'50 UBi 2 Fm3m mixed B12 cube-octahedron
2 inter-penetrating fee YbB6648S253 yb66 Fm3c not reported lattices of B12(B12)2 units,6 Yb per unit cell
Bg octahedra in comers E uB 649-51,54 CaB6 Pm3m divalent of simple cube, metal in body center
ro w CHAPTER II
STATEMENT OF THE PROBLEM
The graduate research agenda consists of the preparation and characterization of divalent ytterbium and europium complexes with boron hydride cluster anions as well as simple boron containing anions. The nature of bonding between the boron hydride moiety and the lanthanide metal will be determined.
The main goal of the research is utilization of the lanthanide boron hydride complexes as precursors to lanthanide borides. The precursor complexes will be thermally treated in a vacuum system to isolate the boride phases corresponding to the lanthanide to boron ratio in the precursor complex. Ultimately, coatings of dilute solutions of the precursor complexes will be applied to refractory metal substrates and heated under optimum pyrolysis conditions to form thin deposits of ytterbium or europium borides.
The systems that will be studied are very delicate in the sense that boron hydride anions partake in weak bonding interactions with the oxophilic divalent lanthanide cation. The nature of the valence f orbitals precludes extensive covalent bonding in complexes involving the lanthanides. The choice of solvent will, therefore, be crucial in order to minimize the competition between solvent and boron hydride for a coordination site on the lanthanide. For this reason, oxygen
24 25 containing ligands such as THF or DME will be avoided. Rather, nitrogen containing ligands such as acetonitrile or pyridine will be employed to reduce solvent-separated behavior in the products. Although the majority of isolable lanthanide complexes contain cyclopentadienyl-type ligands, Ti-bonding ligands will also be avoided in favor of acetonitrile and pyridine which are more labile under pyrolytic conditions yet are stable enough at room temperature to minimize decomposition.
Divalent lanthanides are preferred over trivalent lanthanides due to their lower charge to size ratio. Greater "covalent" interactions between the metal and boron hydride would be expected in a divalent complex compared with a trivalent complex. This factor may also inhibit solvent-separated behavior in the complexes.
The complexes of ytterbium and europium will be prepared by either metathesis or ammonia reduction reactions. Because both techniques require solubility of the lanthanide metal in ammonia, samarium complexes will not be prepared.
Since most of the lanthanide boron hydride complexes reported in the literature incorporate the BH4' ligand, the intent of this research is to prepare and characterize complexes containing higher nuclearity boron clusters bound to the lanthanide via hydrogen-bridge bonds. Apart from the four hydrogen bridge bonding modes, polymerization and solvent-separated interactions are also possible. These complexes, having a higher boron to ytterbium ratio than the borohydride complexes, would serve as precursors to the more boron rich lanthanide boride phases. 26
Preparation of lanthanide borides using precursor complexes is
preferable to direct synthesis from the elements for several reasons. The
precursor complexes have the advantage of bringing the lanthanide and boron in
closer proximity to one another so that boride formation becomes more
thermodynamically favorable. The pyrolysis temperatures are expected to be
lower than those reported for preparations from the elements or oxides. Secondly, the choice of precursor complex allows more control over the metal to boron ratio
which is not as predictable in the solid state reactions. Precursor complexes will
be designed to provide the same lanthanide to boron ratio as in the desired
lanthanide boride. Thirdly, dilute solutions of precursor complexes provide an
excellent media by which lanthanide borides can be coated onto substrates of
varying sizes and shapes. Preparation of lanthanide boride coated tungsten
filaments by this method would have great potential in the cathodic assembly of
electron guns. CHAPTER III
RESULTS AND DISCUSSION
The design of divalent lanthanide boron hydride complexes in this work was based upon the known lanthanide borides which could be produced from potential precursors. Several criteria were required for suitable precursor complexes.
Firstly, complexes containing the same lanthanide to boron ratio as the target boride phase were preferred. Borides could be formed from other stoichiometries, but this would result in an excess of either boron or lanthanide metal. The stable ytterbium and europium borides, YbB2, YbB4, YbB6, YbB12, and EuB6, could be potentially produced from the corresponding ytterbium and europium boron hydrides:
L^Yb(BH4 ) 2 YbB2 (9)
LxYb(B 2 H7 ) 2 - > YbB4 (1 (|
LxYb(B 3 H8 ) 2 YbB6 (1 1 )
L*YbB1 2 H 1 2 - > YbB1 2 (12)
LxEu(B 3 H8 ) 2 EuBg (12)
27 28
The second criterion was the use of solvating ligands that form stable complexes at room temperature yet are labile enough at higher temperatures to be cleanly removed from the lanthanide coordination sphere. Nitrogen containing ligands
(e.g. NH3, CH3 CN, C5 H5 N) had a lesser tendency to produce solvent separated complexes as has been seen in complexes solvated by oxygen-containing
ligands . 6 Cyclopentadienyl ligands were also avoided because the complexes would be too stable under pyrolytic conditions. Thirdly, precursor studies were done with the divalent metal ions because of their smaller charge to size ratios and oxophilicity than the trivalent ions and because of their greater ability to form more
"covalent" interactions with boron hydride anions. Samarium complexes were not studied because of the high reactivity of Sm2+ and the insolubility of samarium metal in ammonia.
For metathesis reactions, Yb2+ and Eu2+ were generated by standard
procedures in liquid ammonia . 2 5 For the more direct one-step liquid ammonia reductions, ytterbium or europium metal were oxidized by ammonium boron hydride salts.
A. Metathesis Reactions with L^YbC^
Metathesis proved to be a successful reaction route in the preparation of
l_xYb(BH 4 ) 2 (L=CH3CN or C5 H5 N) . 3 5 Both L^YbClg and L^EuCI 2 were reacted
with two equivalents of NaBH4 with displacement of two chloride ions by
borohydride anion: 29
L
L^LnClg + 2 NaBH 4 - * L^LnfBFg,, + 2NaCI (14)
Ln = Eu, Yb; L = CH3 CN, C5 H5N
In acetonitrile, the solution changes from colorless to deep orange as
(CH3 CN)xYb(BH 4 ) 2 forms and bright yellow as (CH 3 CN)xEu(BH 4 ) 2 forms.
In this study, analogous metathesis reactions were done replacing NaBH 4
with other simple boron hydride anions.
1. Reaction with KB3H8
The B3 H8' anion was chosen for this experiment not only because of its
stability but also because coordination with ytterbium or europium would produce
a complex with a 6 : 1 boron to ytterbium ratio that is desired for lanthanide
hexaboride precursors. The potassium salt was employed because of the ease
of preparing and purifying it. 5 5 The sodium salt could not be isolated in pure form
by this method.
The ammonia solvated lanthanide dichloride starting materials exhibited
limited solubility. Washing the solid with acetonitrile or pyridine increased its
solubility sufficiently to permit metathesis to proceed. Solutions of the ytterbium
dichlorides in acetonitrile and pyridine were colorless and deep purple,
respectively.
The reactions were run with either acetonitrile or pyridine. For the acetonitrile adduct, infrared data was obtained in nujol and compared with the
nujol spectrum of KB3 H8 (Figure 6 ). The two spectra were virtually identical with 2600 2400 2200 2000 1800 Wavenumbers
Figure 6. Infrared spectra of B3 H8" complexes in thev(B-H) region. (a) (CH3CN)xYb(B3H8)2 and (b) KB3 H8 in nujol. COo (CH3 CN)xYb(B 3 H8 ) 2 peaks at 2475(m), 2429(s), 2366(m,sh), 2336(s), 2129(m),
2096(m), and 1653(w) cm * 1 and KB 3 H8 peaks at 2474(m), 2425(s), 2374(m,sh),
2330(s), 2210(w,sh), 2129(m) and 2096(m) cm*1. The presence of the ytterbium(ll) coordinated species is observed as the solution changes from clear to bright orange which is typical of Yb2+ complexes in acetonitrile. The similarities between the spectra of the potassium and ytterbium compounds indicates that the
ytterbium complex must be solvent separated. The symmetry of the3 H 8B moiety is maintained in the infrared spectrum. A very weak peak appears in the infrared
spectrum at 1653 cm * 1 but is sharper than the Yb-H-B peak found at 1600 cm * 1
in the infrared spectrum of (CH3 CN)xYb(BH 4 ) 2 . 6 Other than the peak at 1653 cm' 1
in the Yb(B3 H8 ) 2 spectrum and the peak at 2210 cm" 1 in the KB3 H8 spectrum, the positions and shapes of the other peaks are the same.
Most of the ytterbium dichloride remained as an undissolved green solid throughout the reaction but must have been slightly soluble to react to form the orange solution. A color change was not evident for the pyridine adduct. The dichloride was much more soluble in pyridine than acetonitrile and immediately formed the deep purple solution immediately upon exposure to pyridine vapors.
Only a small amount of the dichloride remained undissolved. The solution
remained deep purple after stirring with KB3 H8 for several hours. The solid that was filtered from the solution in either solvent was washed with fresh solvent and evaluated for KCI. Upon dissolving in water, bubbling occurred and the solids turned grey in a gelatinous solution probably from formation of Yb(OH)3. 32
Apparently not all of the ytterbium dichloride had reacted because of its insolubility.
The actual yield of KCI was not measured because of problems with complete separation from solution. A non-hydrolyzed sample of the solid metathesis product exhibited KCI lines in the x-ray powder pattern, but some remained dissolved in acetonitrile or pyridine. The metathesis of ytterbium dichloride with
two equivalents of RbB3 H8 yielded only 56% RbCI.
The 11B and 1H NMR signals for the alkali metal B 3 H8' salts appeared at
-30.1 ppm (nonet,JB.H = 34Hz) and 0.1 ppm (JB_H = 33Hz), respectively. In
comparison, the Yb(B 3 H8 ) 2 11B NMR resonance in CD3CN (Figure 7) falls slightly downfield at -27.8 ppm (JB_H = 33Hz). In d5-pyridine the shift remains unchanged.
The 1H NMR data for (CH 3 CN)xYb(B 3 H8 ) 2 show similar results. The presence of
ytterbium(II) shifts the B3 H8' peak from 0.15 ppm to a position slightly downfield
at 0.21 ppm (JB_H = 29Hz). A more noticeable effect is the broadening of the
peak such that the coupling is almost unresolved. The borohydride peak in the
Yb(BH4 ) 2 complexes shifted from -40 ppm, reported for alkali metal salts, to
-32 ppm without the broadening of peaks. 3 4 The 1H NMR signals, however, do
broaden and in the case of (C5 H5 N)xYb(BH4)2, the quartet broadens into a singlet.
The broadening suggests more than just solvent separated interaction with
ytterbium, but the coupling is maintained suggesting the fluxionality of the
octahydrotriborate unit.
The peak at 1653 cm ' 1 does not coincide with the data for borohydride
complexes in Table 5, but is consistent with data reported for (CH3 CN)xYb(BH4)2. 11B NMR 1H NMR
T ~ —T~ — i------1------1------1— -26 -27 -28 -29 -30 l.S 1.0 .5 0.0 -.5 PPM PPM
Figure 7. 11B and 1H NMR spectra of (CH3CN)xYb(B3H8)2 in CD 3 CN.
CO CO 34
The color change does suggest reaction has occurred, but whether or not one or two chlorides are displaced from the lanthanide dichloride is still questionable because of the solubility of KCI in both acetonitrile and pyridine. The differences between the infrared and NMR spectra of the alkali metal and lanthanide octahydrotriborates were not significant enough to confirm the existence of hydrogen-bridge bonding. The broadening and shift of NMR peaks, however, does suggest that the boron hydride is in close enough proximity to ytterbium to
alter the B3 H8‘ resonances.
2. Reaction with NaBH3CN
Since acetonitrile had been shown to coordinate to lanthanides through nitrogen, and borohydride through a hydrogen-bridge bond, NaBH3CN was
reacted with L^YbClg (L = CH3CN or C5 H5 N) to determine whether both types of bonding could occur simultaneously producing a polymeric structure. Two equivalents of NaBH3CN were reacted with ytterbium dichloride in either pyridine
or acetonitrile. In acetonitrile, the colorless (CH3 CN)xYbCI 2 solution changes to a deep red-orange solution after stirring for several hours.
L
L^YbO, + 2NaBH3CN -> LxYb(BH 3 CN) 2 + 2NaCI (15)
L = CH3 CN, C5 H5N
Removal of acetonitrile leaves a deep red-orange oil in the reaction flask which may be due to polymer formation. The product could not be precipitated, 35 therefore, was not examined by elemental analysis. The v(B-H) absorptions for
(CH3 CN)xYb(BH 3 CN) 2 appeared at 2336(s), 2305(m,sh), 2256(w), 2219(w),
2000(vw,br) and 1123(m) cm - 1 in acetonitrile. The i/(B-H) absorptions for
(C5 H5 N)xYb(BH 3 CN ) 2 in nujol varied slightly from the acetonitrile solvated complex
with peaks at 2355(s,sh), 2338(s), 2286(m,sh), 2210(m), and 2020(vw) cm ' 1 respectively. The corresponding NaBH3CN peaks lie at 2354(m), 2332(s),
2301 (m.sh), 2286(m,sh), 2221 (w), 2000(vw), 1195(w), and 1127(m) cm'1. The i/(C=N) absorptions for both sodium and ytterbium compounds all appeared
between 2168 and 2181 cm'1. The three spectra (Figure8 ) are very similar except for the acetonitrile peaks that appear in the solution spectra and the pyridine peak
at 1597 cm ' 1 in the spectrum of the pyridine solvated powder. An additional weak
peak at 1684 cm ' 1 in the nujol spectrum of (C5 H5 N)xYb(BH 3 CN) 2 is attributed to
a Yb-H-B stretch absorption. Also, a strong shoulder at 2355 cm ' 1 appears in
both NaBH3CN and (C 5 H5 N)xYb(BH 3 CN) 2 spectra but not in the solution spectrum in acetonitrile. Apparently, the fine structure is probably lost in solution due to loss of lattice vibrations, the integrity of the Yb-H-B bond is no longer maintained and acetonitrile displaces the hydrogen bridge bond to the lanthanide center. Also, the i/(C=N) peak in the pyridine solvated solid shifts to a slightly lower frequency probably as a consequence of pyridine’s basicity. Because pyridine is more basic than acetonitrile, it can contribute more electron density into the ytterbium center, and, therefore, the C=N bond of the BH3CN unit, causing a lower frequency of vibration. 36
2600 2400 2200 2000 1800 1600 Wavenumbers
Figure 8. Infrared spectra of BH3CN~ complexes. (a) (C 5 H5 N)xYb(BH 3 CN)2 in nujol. (b) NaBH 3 CN in CH3 CN. (c) (CH3CN)xYb(BH 3CN)2 in CH3 CN. 37
The NMR data for the BH3CN' complexes are reported in Table 7. The major difference between the NaBH3CN and Yb(BH3CN)2 NMR spectra (Figures
9 and 10) is the broadening of both 1H and 11B NMR peaks in the ytterbium coordinated complexes. In pyridine, the outer peaks of the BH3 quartet in the 11B spectrum are no longer resolved. In the 1H NMR spectrum, the highly resolved
Table 7. NMR Data for BH3CN' Complexes
11B NMR 1H NMR
NaBHgCN -39.1 ppm (q,89Hz) 0.25 ppm (q,89Hz)
(C5H5N)xYb(BH3CN)2 -43.9 ppm (q,br,81 Hz) 1.85 ppm (s),1.75(vbr)
(CH3CN)xYb(BH3CN)2 -39.6 ppm (q,89Hz) -0.20 ppm (q,91Hz)
quartet in the NaBH3CN spectrum merges into a broad unresolved resonance at
1.75 ppm. A sharp singlet emerges from the hump at 1.8 ppm which is most likely a solvent impurity. In acetonitrile, the quartet appears as two resolvable peaks with two shoulders. The 11B NMR shifts are only slightly higher than the resonance for the sodium salt. The BH3 resonances in transition metal carbonyl
BH3CN' complexes reported in the literature also show shifts to slightly higher frequencies relative to the NMe4+ salt when the nitrogen coordinates to Cr, Mo or
W.56 One would expect BH3CN' to be more basic than CH3CN considering they -36 -40 -44 -48
Figure 9. "*"*B NMR spectra of BH3CN'complexes. (a) (C 5 H5 N)xYb(BH 3 CN)2 in d5-py, (b) (CH3CN)xYb(BH3CN)2 in CD 3 CN, (c) NaBH 3 CN in CD3 CN. 39
3.0 2.Q -i.O
PPM
Figure 10. 1H NMR spectra of BH3CN- complexes. (a) (C 5H5N)xYb(BH 3CN)2 in d5-pyridine. (b) (CH3CN)xYb(BH 3CN)2 in CD3CN. (c) NaBH3CN in CD 3CN. 40 are both isoelectronic but BH3CN' is negatively charged. Acetonitrile has been shown to solvate Yb2+ complexes. The cyanotrihydroborate ion is expected to behave similarly.
The i/(B-H) stretches are also reported for the Cr, Mo, and W complexes.
Weak absorptions appear from 2335-2337 and 2290-2312 cm'1 compared with absorptions at 2332(s), and 2301 (m.sh) in NaBH3CN; at 2336(s) and 2305(m,sh) in (CH3CN)xYb(BH3CN)2; and at2338(s) and 2286(m,sh) in (C5H5N)xYb(BH3CN)2.
No indications are reported for M-H-B bonding in the transition metal complexes.
The degree of solvation could not be determined for the acetonitrile adduct which is an oil. The pyridine, adduct, however, could be isolated as a solvated solid and submitted to heat treatment to remove pyridine. The pyridine of solvation was calculated to be 3.78 from the weight of the pyrolysis residue
(0.0840 g) and the weight loss of pyridine (0.0995 g), hence, the formula
(C 5H 5N )3. 78Yb(BH3CN)2. Attempts to obtain a suitable sample for elemental analysis were unsuccessful. Washing of the wet residue with hexanes caused a weakening of the 11B NMR signal.
The existence of hydrogen bridge bonding could only be conjectured from the nujol infrared spectrum and the polymeric behavior of the acetonitrile adduct. The color change of the reaction mixture from colorless to orange, characteristic of
Yb2+, for the metathesis in acetonitrile shows definite conversion to the divalent state. 41 B. Liquid Ammonia Reduction Reactions
Problems with separation of the lanthanide complexes from the alkali metal salt from the metathesis reactions prompted the need for an alternate procedure.
The use of liquid ammonia to generate the divalent lanthanide was still desired rather than employing Ln/Hg amalgams which have a tendency to form solvent- separated complexes.6 With Howell and Pytlewski’s preparation of ytterbium dichloride in mind, a similar procedure was employed whereby the chloride was replaced by the desired boron hydride anion.25
n h 3 Ln + 2NH4CI - * (NH3)xLnCI2 + H2 + 2NH3 (16)
NH3 Ln + x[HNR3]yBH - * (NH3)zLn(BH)x + H2 + 2NR3 (17)
Ln = Eu, Yb; BH = boron hydride anion R = H, Me or Et; x=1,y=2 or x=2,y=1
Unlike the two-step metathesis route, this procedure produces the lanthanide boron hydride complex in one-step. Separation of the products is facilitated by the volatility of hydrogen and amine. In addition, the extent of reaction can be determined from measurement of hydrogen evolved in the reaction. 42 1. Reactions with NH4B3H8
The alternative to the metathesis reaction with alkali metal octahydrotriborate ion was the liquid ammonia reaction of NH4B3H8 with ytterbium or europium metal.
nh3 Ln + 2NH4B3H8 - * (NH3)xLn(B3H8)2 + H2 + 2NH3 (18)
Ln = Eu, Yb
When two equivalents of NH4B3H8 were added to europium or ytterbium metal, evolution of hydrogen commenced. Gas evolution was also accompanied by a color change in the solution. The ytterbium-containing mixture changed from the characteristic deep blue (NH3)xLn2+/2e' solution to reddish-yellow. The europium- containing mixture was initially deep blue but eventually turned green. Both products were soluble in ammonia. In both cases, the slow removal of ammonia left a green residue in the flask. Continued exposure to dynamic vacuum converted the products to oils. The hydrogen yield for ytterbium and europium reactions were 95% and 91 %, respectively. The discrepancy from theoretical yield may be due to small amounts of Yb(NH2)2 and Eu(NH2)2 formed as a side product. The ytterbium octahydrotriborate oil turned orange when washed with acetonitrile but returned to an oil when solvent was removed again. The europium analogue was isolated as a solvated solid by pumping on the sample long enough to remove most of the ammonia but before it became an oil.
Infrared data (Figure 11) were obtained in acetonitrile for the ytterbium complex and in fluorolube for the europium complex then compared to the 2600
1800 1600
F'9Ure « • In,rare* s _ e e ,
" “ S U S . Vb^H8)23rn8c® e^-^ 44 potassium salt. In the Yb(B3H8)2 spectrum, there is a loss of fine structure and a narrowing of the outer envelope of the i/(B-Ht) absorption. The peak at 2425 cm'1 in KBgHg appears as a doublet at 2442 and 2403 cm'1 in the Yb(B3H8)2 spectrum.
In the Eu (B3H8)2 spectrum, the major y(B-Ht) peak shifts to 2387 cm'1 with a shoulder at 2453 cm'1. The i/(B-Hb) absorptions shift slightly to lower frequencies in both Yb(B3H8)2 (2125 and 2071 cm'1) and Eu(B3H8)2 (2120 and 2075 cm'1) compared to the bridging absorptions in KB3H8 at 2129 and 2096 cm'1. The unresolved bridging peaks in Eu(B3H8)2 may be, in part, a consequence of an overlapping fluorolube impurity. The shift caused by lanthanide coordination results from a donation of electron density from the B3H8' framework to the eiectrophilic lanthanide center. A shift in this direction weakens the two electron three center B-H-B bonds and decreases its frequency of vibration. The differences in the spectral data suggest that the interaction between the lanthanide and boron hydride is more than just solvent-separated in nature. Moreover, an additional peak appears in the fluorolube mull spectrum of Eu(B3H8)2 at 1731 cm'1 that is attributed to a Yb-H-B stretch absorption. This peak, however, does not appear in the solution spectrum of Yb(B3H8)2. It is not surprising that this peak would appear in only the solid state spectrum. Even under ideal conditions, hydrogen-bridge bonds are weak because of limited overlap with the valence f-orbitals. In the solution spectrum, solvent molecules in high concentration can more readily displace the hydrogen-bridge bonds to the lanthanide center. Only after sufficient solvent has been removed can the boron hydride anion compete 45 for a coordination site on the metal. The infrared data supports the solvent- separated behavior of Yb(B3H8)2 in acetonitrile and hydrogen-bridge bonding behavior of Eu(B3H8)2 in the solid state. The data for Eu(B3H8)2 most closely resembles the absorptions for monodentate ligation predicted by Marks for borohydride complexes.36
The 1H and 11B NMR spectra (Figures 12 and 13) were compared to the spectra of RbB3H8 in CD3CN. The 11B NMR spectrum of RbB3H8 shows a nonet at -30.0 ppm (JB_H = 33Hz). The fluxionality of the anion makes all borons equivalent. In the (CD3CN)xYb(B3H8)2 spectrum, the characteristic B3H8" nonet appears, but is shifted downfield to -25.1 ppm. The (CD3CN)xEu(B3H8)2 spectrum shows a broad singlet shifted upfield at -36.8 ppm over a 6 ppm range. The 1H
NMR resonance of RbB3H8 in CD3CN is a highly resolved decet at 0.15 ppm (JB_H
= 34Hz). The 1H NMR signal of the ytterbium complex, however, broadens such that only the two most intense peaks of the decet are resolvable. The remaining peaks appear as shoulders. The peak shifts slightly downfield to 0.21 ppm. The
1H NMR signal of the europium analogue shifts 3 ppm upfield to -3.0 ppm relative to RbB3H8. The broad singlet extends across a 5 ppm range.
These data suggest that in solution, solvent molecules hinder hydrogen- bridge bonding. Shifts do appear in the NMR spectra, but paramagnetic europium(ll) should have a much larger shift than 3 ppm. The B3H8' moiety in the ytterbium(ll) complex maintains its fluxionality. In the solid state the europium complex exhibits hydrogen bridge bonding. The oily nature of both ytterbium and Y b (B 3 H 8)2 -25.1 ppm, 33Hz
r T T -18 -22 -26 -30 -30 -34 -38 -42 PPM PPM
Figure 12. 11B NMR spectra of B3H8" complexes. a. Yb(B3H8>2 0.21 ppm, 29Hz b. RbB3H8 0.15ppm, 33Hz
T “T" T — T“ — i 0.5 0.0 -0.5 0.0 - 2.0 -4.0 - 6.0 - 8.0 PPM PPM
Figure 13. 1H NMR spectra of B3H8“ complexes. 48 europium products suggests that they may assume a polymeric configuration in the absence of solvent similar to that found for Be(BH4)2 (Figure 14).37 Once solvent is removed, the coordinatively unsaturated lanthanide center becomes open to hydrogens of the B3H8‘ unit. A polymeric structure permits the coordination sphere to be filled to a greater extent than in the monomeric structure.
2. Reactions with [HNMe3]2B12H12
n h 3 Ln + [HNMe3]2B12H12 -> (NH3)x(NMe3)yLnB12H12 + H2 + (2-y)NMe3 (19)
Ln = Eu, Yb
When one equivalent of [HNMe3]2B12H12 reacts with ytterbium metal, the solution turns from deep blue to a yellowish-green slurry. Reaction with europium metal changes the solution to a grey slurry. The yields of hydrogen evolved for the ytterbium and europium reactions are 96% and 92%, respectively, indicating that the reactions produce relatively pure lanthanide dodecahydrododecaborates in high yield. Trimethylamine coordinates to the lanthanide as it is produced with a lower degree of solvation than ammonia. The dried products are pale green
(NH3)xYbBi2H12 and white (NH3)xEuB12H12 powders. Elemental analyses of the desolvated solids confirmed the molecular formulas.
Typically, infrared absorptions in the y(B-H) region of the solid state infrared spectra of B12H122' salts show fine structure assigned to coupling with lattice H H Be B H H H H \/ B H H 7 7 a.
Figure 14. Structures of Be(BH4)2 vibrations. The spectrum of [HNMe3]2B12H12 confirms this with absorptions at
2525(m,sh), 2491 (s), 2468(vs), 2458(vs), 2432(s), and 2414(s) cm'1. When the
B12^122 moietycoordinates to Yb2+ or Eu2+ these absorptions merge intatwo broad peaks. Ammonia-solvated ytterbium dodecahydrododecaborate exhibits i/(B-H) absorptions at 2514(s,sh), 2496(s), and 2424(m) cm'1 (Figure 15). The corresponding peaks in the (NH3)xEuB12H12 spectrum (Figure 16) appear at
2516(s,sh), 2489(s), and 2428(m,sh) cm'1. An additional weak peak emerges from the baseline of the spectrum of the europium complex at 1733 cm'1 which is attributed to a Eu-H-B stretch vibration. Broadening of the i/(B-H) absorptions was also reported for Ag+ , Cu+ , and Hg2+ salts of B12H122'.57 No changes occurred in the B12H122' absorptions of hydrated Ln3+ dodecahydrododecaborate complexes (Ln = La, Sm, Gd, Ho, Er, Tm, and Yb).58 From the infrared data it was concluded that the complexes were solvent separated. In this research, however, the absence of water precludes solvent-separated behavior as well as conversion to the trivalent metal ions. Europium appears to affect the boron cage to a greater extent than ytterbium as is seen by the presence of the Eu-H-B stretch at 1733 cm'1. The products were desolvated by heating at 120-130°C under dynamic vacuum then analyzed by infrared spectroscopy. In both desolvated powders, the original i/(B-H) peaks broaden into one peak. The peaks at 2420 and 2428 cm'1 are no longer resolved. Loss of coordinated solvent on ytterbium results in the appearance of a sharp absorption at 1731 cm'1. The corresponding peak in the europium complex remains unchanged. 51
a. b.
2600 2400 2200 2000 1800 1600 Wavenumbers
Figure 15. Infrared spectra of YbB i2H i2 complexes in the v(B-H) region, (a) (NH3)xYbBi2Hi2 in nujol and (b) desoivated YbBi2H-|2 in fluorolube. The peaks between 2200 and 2400 cm"1 are fluorolube impurities. a. b.
2600 2400 2200 2000 1800 1600 Wavenumbers
Figure 16. Infrared spectra of EUB12H12 complexes in the v(B-H) region. (a) (NH 3)xE u B i 2 H i 2 in nujol and (b) desolvated EUB12H12 in nujol. The 1H and 11B NMR spectra of the products in CD3CN were compared to
NMR data for the sodium salt. The paramagnetism of Eu2+ broadens the
Bi 2Hi 22‘ peak into the baseline. No peaks appeared in the 11B NMR spectrum from 100 ppm to -280 ppm. In the 1H NMR spectrum from +50 to -50 ppm, a very weak and broad peak appears at -25 ppm. The YbB12H12 11B NMR signal
is almost identical to the signal for Na2B12H12 (Figure 17). The fluxionality of the
B12 cage makes all borons equivalent and results in a doublet. The doublet at
-14.5 ppm in YbB12H12 is very slightly shifted downfield from the Na2B12H12 peak
at -15.1 ppm. Ytterbium has little effect on the B12 cage and most likely forms a solvent-separated complex with B12H122'. In the EuB12H12 spectrum, the B12H122' signal is no longer intact. In Eu(B3H8)2, the paramagnetism of europium shifted the boron peak only slightly upfield indicative of a solvent-separated interaction.
Although the absence of a 11B resonance is not conclusive evidence for the
existence of direct bonding between europium and B12H122‘, this observation is
consistent with such an interaction. The 1H NMR spectrum of YbB12H12 is quite
different from the B12H122' signal of [HNMe3]2B12H12 which is a symmetrical nonet
centered at 1.1 ppm with the most intense peaks of the multiplet being the
outermost peaks (Figure 18). The 1H NMR spectrum of YbB12H12 is dominated
by a broad peak at 1.0 ppm within which is buried a series of smaller peaks. The
acetonitrile resonance protrudes from the multiplet at its expected shift of 1.94
ppm. The peak at 1.0 ppm is attributed to coordinated acetonitrile. A solvent shift
upfield was also found in the spectrum of (C5H5N)xYbB10H10 although acidic Yb2+ T T" -10 -14 -18 -10 ■14 -18
PPM PPM
Figure 17. NMR spectra of B12H122" complexes in CD3CN. (a) Na 2 B i 2 H*i2 and (b) YbBi 2 H i 2 -
cn 55
a. Na 2B i2H i2 b. YbBi2H i2
CD3 CN
I— “I 3.0 2.0 1.0 0.0 PPM
Figure 18. 1H NMR spectra of B-|2Hi22" compounds. 56 would be expected to cause a downfield shift in the acetonitrile signal.35 The signal for the B12H122' protons is buried within the solvent peaks. Only the outer peak of the B12H122' multiplet at 2.2 ppm is resolvable. This same peak appeared at 1.9 ppm in the [HNMe3]2B12H12 spectrum, a shift of 0.3 ppm.
The stability of and charge on the B12 cage make B12H122' ideal as a counterion for the divalent lanthanides. The infrared and NMR data are consistant with a hydrogen-bridge bonding interaction between the boron hydride and Eu2+ or Yb2+ in the desolvated ytterbium and europium complexes as well as the solvated europium complex. The solvated ytterbium complex exhibited only solvent-separated behavior. The ability for europium to undergo direct interaction with the boron cage may be a direct consequence of the greater stability and the smaller charge to size ratio of Eu2+ relative to Yb2+. The same type of interaction is possible for YbB12H12 when solvent is no longer available to displace the relatively weak Yb-H-B bonds. The same result was found in the Ln(B3H8)2 complexes. The steric bulk of the B12 cage prevents polymerization from occurring as in the oily Ln(B3H8)2 products, hence LnB12H12 can be isolated as fine powders. Whether bonding occurs at an axial or equatorial position of the boron cage could not be determined from the spectroscopic data. 57
3. Reactions with [HNEt 3]2B10H10
n h 3 Ln + [HNMe3]2B10H10 - * (NH3)x(NEt3)yLnB10H10 + H2 + (2-y)NEt3
Ln = Eu.Yb (20)
When one equivalent of [HNEt3]2B10H10 reacts with ytterbium metal, the solution changes from deep blue to an olive green slurry. In the analogous europium reaction, the mixture changes to slate grey and finally to a pale green slurry as the reaction proceeds. The yield of hydrogen, confirmed by gas mass spectral analysis, which evolves in both ytterbium and europium reactions is 93%.
The reactions produce pure lanthanide decahydrodecaborates in high yield. As
NEt3 is produced in the reaction, a small fraction coordinates to the lanthanide.
The dried products are pale green (NH3)xYbB10H10 and greenish-white
(NH3)xEu B10H10 powders as in the ammonia-solvated B12H122' complexes. The molecular formulas of the desolvated powders are confirmed by the elemental analyses.
Pea green lanthanide B10H102' powders have previously been prepared by thermal treatment of (NH3)xLnB10H14 complexes (Ln = Yb.Eu) at 190°C.35 In
Table 8 the infrared spectroscopic data for the ammonia-solvated and desolvated powders are compared to the data for the LnB10H10 powders prepared by decomposition of (NH3)xLnB10H14. The ammonia-solvated powders exhibit very broad i/(B-H) absorptions in the 2460 cm'1 region of the spectrum which extend over a 150 cm'1 range. The fine structure becomes resolved when solvent is Table 8. Infrared Data for LnB-ioHio Complexes (cnr1).
(N H a^Y bB ^H jo YbB10H10 (NH3) xEuBi 0H io EuB10H10 YbB10H1027c EuB10H1027 c
V(N-H) 3346(w) 3355(w) ------— V(B-H) 2471(s,br) 243 5(w) 2455(s,br) 2540(w,sh) 2525(sh) 2503 (s) 247l(s) 2529(w,sh) 2456(vs,br) 2456(vs,br) 2447(s) 2504(s) 2474(s,sh) 245 5(s) 243 8(s) v(Ln-H-B) 1594(vw) 1734(m) 1728(w) 1588(vw) 1602(vw) V(B-B) 1076(w) 1076(w) 1077(w) 1077(w) 1076(w) 1086(w) 1021(w) 1023(w) 1020(w) 1020(w) 1016(mw) 1019(mw) 974(w) 974(w)
U1 CD 59 driven from the sample and is accompanied by the appearance of the v(Ln-H-B) stretch absorptions at 1728 cm'1 for EuB10H10 and at 1734 cm'1 for YbB10H10
(Figures 19 and 20). The ammonia-solvated YbB10H10 spectrum shows a very weak peak at 1594 cm'1 which is similar to the Ln-H-B stretch vibrations of
YbB10H10 and EuB10H10 prepared from pyrolysis of (NH3)xLnB10H14. The peak shifts to a higher frequency and becomes more intense in the desolvated spectrum. No i/(Eu-H-B) peak appears in the solvated (NH3)xEuB10H10 powder spectrum. The peak shapes and positions agree well with those previously reported.6 As expected, the spectra of the ammonia-solvated powders contain additional peaks in the v(N-H) region due to coordinated ammonia. The spectra for all of the samples contain the characteristic B-B cage vibrations at 1076 and
1020 cm'1. Muetterties, et al. reported that the cage absorptions disappear when
Bi0Hio2- is coordinated to Cu(l) as a consequence of polarization of the cage.57
This does not occur in the divalent lanthanide complexes even though the charge to size ratios of Yb2+ (1.25), and Eu2+ (1.60) are of the same magnitude as that for Cu+ (1.67).18
Table 9 summarizes the 1H and 11B NMR data for several B10H102' complexes. No boron or proton signals were found in the spectrum for B10H102' coordinated to paramagnetic europium. The ytterbium complex exhibited two resonances in the 11B NMR spectrum assigned to axial and equatorial borons of the B10 cage (Figure 21). The axial borons are found farther downfield than the equatorial borons. Compared to the NMR data for YbB10H10 reported by Shore a.
b.
2600 2400 2200 2000 1600 1600 Wavenumbers
Figure 19. Infrared spectra of YbB-ioH-io complexes in the v(B-H) region. (a) desolvated YbBigHio and (b) (NH3)xYbBioHio in nujol.
I 2500 2400 2300 2200 2600 2100 2000 1900 1800 1700 1600 1500 CM-1
Figure 20. Infrared spectra of Eu Bkj Hio complexes in the v(B-H) region. (a) (NH3 )x Eu Bi o Hio and (b) desolvated Eu Bi q Hio in nujol. o YbB -ioH io 2.0ppm,130Hz -26 .2 ppm
d5-py
K 2 B 1 0 H 1 0 0.2ppm,146Hz -28.0ppm,114Hz
d5-py
— r ~ —T~ —r~ —r~ 0.0 -10 -20 -30 PPM
Figure 21. 11B NMR spectra of Biq Hio 2" compounds. 63
Table 9. NMR Data for B 10H102' Complexes.
11B NMR 1H NMR
YbB1 0 H 1 0 in d5-py 1.9 ppm(d,130Hz) 1 . 8 ppm(mult,broad)
-26.2 ppm(d,broad) 5.1 ppm(q,141Hz)
YbB1 0 H 1 0 in d 5 -py6 3.1 ppm(d,143Hz) 1.77 ppm(s,broad)
-24.9 ppm(d,118Hz) 4.90 ppm(q,142Hz)
0 . 6 K2 B1 0 H 1 0 0.2 ppm(d,146Hz) - ppm(mult)
-28.0 ppm(d,114Hz) 3.1 ppm(q,141Hz)
and White, the values lie1 ppm upfield. Relative to KgB^H^.the experimental
values for YbB 1 0 H 1 0 lie 2 ppm downfield. The weak peaks at -15 ppm are due to
impurities and are also found in the NMR of YbB 1 0 H 1 0 prepared from decomposition. Both axial and equatorial boron resonances broaden as a result of ytterbium coordination, but the equatorial borons are more greatly affected. The coupling is no longer resolved and the signal becomes a broad singlet. The axial doublet does not broaden as drastically as the equatorial doublet. The spectrum reported in the literature is similar, but contains an equatorial doublet that is broad yet still resolved. The data suggest that the lanthanide center has a greater effect on and must, therefore, be in closer proximity to the equatorial borons of the cage.
At the same time, the fluxionality of the cage is maintained. It can be envisioned that the boron cage "walks" across the metal through fluxional hydrogen-bridge 64 bonds involving the cage hydrogens in equatorial positions. An opposite effect
occurs in the Cu2 B1 0 H 1 0 spectrum. 5 7 The axial peaks of the 11B NMR spectrum
broaden and shift upfield relative to (NH4 ) 2 B1 0 H10. The equatorial borons broaden to the point where they can no longer be resolved yet maintain the same chemical
shift as in the (NH 4 )2 B1 0 H1 0 spectrum. This data suggests an axial approach of
Cu+ toward the cage.
The 1H NMR spectrum of K ^qH-jq consists of a symmetrical multiplet at
-0 . 6 ppm due to the equatorial hydrogens as well as a highly resolved quartet at
3.1 ppm (JB_H = 141 Hz) due to the axial hydrogens. Weak peaks due to 10B coupling are also observed. Coordination of the boron framework to Yb2+ causes the equatorial multiplet to broaden and lose its symmetry (Figure 22). A quartet
at 3.40 ppm and a triplet at 1.28 ppm from the protons in NEt 3 produced in the ammonia reaction protrude from the multiplet. A sharp singlet at 2.00 ppm also
appears in the spectrum of YbB1 0 H 1 0 prepared from (NH3 )xYbB 1 0 H 1 4 and is
attributed to coordinated pyridine. 6 The multiplet shifts approximately 2.4 ppm downfield from the corresponding peak in the K^qH-jq spectrum. The signal
from the axial protons broadens substantially and shifts 2 ppm downfield from the axial peak in the spectrum for the potassium salt. In addition, the four peaks are no longer of equal intensity and are more poorly resolved such that 10B coupling can no longer be observed. The ratio of axial:equatorial protons is approximately
1:4. YbBioHio K2B10H10 5.1ppm,141Hz 3.1ppm,141Hz 1.8ppm -0.6ppm
(d5-py) (CD3CN)
***** I 6.0 4.0 2.0 0.0 - 2.0 4.0 2.0 PPM
Figure 2 2 . 1H NMR spectra of B1 0 H1 0 2" compounds. 66
Although no B1 0 H102' peaks appear in the EuB1 0 H 1 0 spectra a very weak
peak appears at -16.1 ppm from the impurity that appears in the ^ B 1 0 H 1 0 and
YbB1 0 H 1 0 11B NMR spectra. The peak broadens and shifts slightly upfield yet the
Bi 0 Hio2- peaks are no longer present. This suggest that the Eu2+ ion is close
enough to the B1 0 cage to cause paramagnetic line broadening of the NMR signal.
If the compound is solvent-separated, the peaks should appear along with the
impurity peak. This solvent-separated effect was seen in the Eu(B3 H8 ) 2 spectrum where line broadening and peak shift occurred minimally. No further conclusions can be made regarding the position of Eu2+ on the cage by spectroscopic evidence. The behavior can be surmised from the ytterbium spectra. The two distinct axial and equatorial peaks suggest that the complex is either fluxional or solvent-separated. The former is the more probable choice because of the shift and broadening of both 1H and 11B NMR peaks brought about by the presence of ytterbium. The downfield shifts are reminiscent of the coordination found in
YbB1 QH 1 4 and Yb(BH 4 ) 2 .3 4 , 3 5 This argument is further substantiated by the presence of Ln-H-B absorptions in the infrared spectra.
4. Reactions with NH 3BH3
The reactions of alkali metals have been useful in formulating analogous reactions with the lanthanide elements. Similarities between the physical and chemical properties of these elements have lent themselves to predicting the outcomes of divalent lanthanide reactions. Sodium aminoborate can be prepared 67
by reacting sodium metal with ammoniaborane in ammonia .5 9
n h 3
2 NH3 BH3 + 2Na 2NaNH 2 BH3 + H2 (21)
The product was not isolated in pure form and decomposed in most solvents.
Both sodium and potassium aminoborates were previously prepared by reaction
of NH3 BH3 with NaH or KH in THF .6 0
THF
NH3 BH3 + MH -* MNH2 BH3 + H2 (22)
M = Na,K
An 87% yield was reported for the reaction with KH. The sodium salt was soluble in ammonia but decomposed in acetonitrile and THF. The potassium salt was insoluble in most solvents.
The lanthanide(ll) aminoborates can be prepared according to the original
procedure for NaNH2 BH3. When two equivalents of NH3 BH3 react with ytterbium
n h 3 Ln + 2 NH3BH3 -> (NH3)xLn(NH 2BH3)2 + H2 (23)
Ln = Yb.Eu or europium metal, hydrogen gas evolves. The ytterbium solution changes from the characteristic blue solvated ytterbium solution to green and finally to yellow within 10min. The europium solution proceeds more slowly and changes from 68
blue to green. The yields of hydrogen, identified by gas mass spectral analysis,
which evolve in the ytterbium and europium reactions are 91% and8 8 %,
respectively. The reaction in ammonia produces (NH3 )xLn(NH 2 BH3 ) 2 in relatively
high yield and purity. The dried products are coarse brownish-green solids.
Both Eu(NH2 BH3 ) 2 and Yb(NH 2 BH3 ) 2 decompose to an opaque pale orange gel
in acetonitrile with concomitant gas evolution. Both products are readily soluble
in ammonia and pyridine. The formula for the pyridine solvated powders are
(C5 H5 N)2 Yb(NH2 BH3 ) 2 and (C 5 H5 N) 1 goEu(NH2 BH3 ) 2 as determined by the
elemental analysis data. The pyridine solutions decompose to a white suspension
within 1 day at 0°C but are stable indefinitely at -30°C.
The 1H and 11B NMR data of Yb(NH 2 BH3 ) 2 are compared to the data for
NaNH 2 BH3 and NH 3 BH3 in Table 10. As expected, paramagnetic Eu2+ broadens
the 1H and 11B NMR signals such that they coalesce into the baseline and cannot
be observed. The spectra of the ytterbium and sodium compounds are similar
enough to indicate similar structures. The triplet from the nitrogen-bound
hydrogens found in the 1H NMR spectrum of NH3 BH3 at 5.8 ppm disappears in
the Yb(NH2 BH3 ) 2 and NaNH 2 BH3 spectra but is replaced by a broad singlet at
approximately 0.8 ppm (Figure 23). This peak is assigned to the -NH 2 resonance
which is significantly shifted upfield due to the additional electron density around
nitrogen brought about in the reduction of NH3 BH3 by ytterbium metal. The
sodium salt would be expected to be more ionic than the ytterbium salt and,
therefore, should have more highly shielded 2-NH protons that resonate at a lower 69
i:o
9.0 0 7.00 6 .0 0 9 .0 0 3.00 2.00 1.00 0 .0 PPM
Figure 23. 1H NMR spectra of (a) (C5HsN)xYb(NH2BH3)2 and (b) NH3BH3 in d5-pyridine. frequency. The reverse is actually observed. The broad 2 -NH singlet in the
NaNH 2 BH3 spectrum is 0.14 ppm downfield from the corresponding Yb(NH2 BH3 ) 2
singlet. The shifts of the -BH3 quartets favor the above argument more
dramatically. This peak in the ytterbium coordinated aminoborate appears 1.32
ppm downfield from the NaNH2 BH3 quartet.
Table 10. NMR Data for NH 2BH3~ Complexes in NH3BH3
11B NMR 1H NMR
Yb(NH2 BH3 ) 2 in d5-py -2 0 . 1 ppm(q,81Hz) 3.00 ppm(q,87Hz)
0.70 ppm(s,broad)
NaNH 2 BH3 -21.2 ppm(q,8 6 Hz) 1.68 ppm(q, 8 6 Hz)
in d 8 -THF6 0 0.84 ppm(s.broad)
NH3 BH3 in d5-py -22.4 ppm(q,93Hz) 5.83 ppm(t.broad)
2.68 ppm(q,98Hz)
The -BH3 quartet in the 11B NMR spectrum (Figure 24) appears 1.1 ppm
downfield from the corresponding NaNH2 BH3 quartet. The NH3 BH3 quartet shifts
upfield to -22.4 ppm from both aminoborate salts. The quartets in NaNH 2 BH3 and
NH3 BH3 are highly resolved, but the Yb(NH2 BH3 ) 2 quartet is broad and only the
innermost peaks can be resolved. The B-H coupling of the NH2 BH3' salts
decreases relative to the 93Hz coupling observed in 3 NHBH3. The coupling is -15.0 -20.0 -25.0 -30.0 PPM PPM
Figure 24.,11B NMR spectra of (a) (C5 H5 N)xYb(NH 2 BH3 )2 and (b) NH 3 BH3 in d5 -pyridine. 72 even lower in the ytterbium aminoborate because of peak broadening.
Very broad i/(B-H) absorptions appear in the infrared spectra of
(C5 H5 N)xYb(NH 2 BH3 ) 2 a n d ( N H3) x E u ( N H 2BH 3)2 between 2000 cm' 1 and
2450 cm' 1 (Figure 25). The peaks of the ytterbium complex are slightly sharper and more resolved. Broad weak peaks appear in the y(Ln-H-B) region near 1700
cm' 1 in both spectra. The infrared spectra of (C5 H5 N)xLn(BH 4 ) 2 (Ln = Eu.Yb)
complexes contain broad i/(B-Ht) absorptions between 2100 cm' 1 and 2450 cm ' 1
and weak Ln-H-B deformation peaks between 1800 cm' 1 and 2000 cm ' 1 . 6
Corresponding low frequency y(B-H) absorptions are also found in the spectrum
of NaBH 4 and are indicative of four-coordinate boron . 6 1 The y(Ln-H-B) peaks in
the aminoborate complexes suggest that hydrogen-bridging between -BH3 and
Ln2+ of an adjacent molecule is extremely weak at best. Polymeric structures are not likely for these compounds because of the ease in which they can be isolated as highly soluble powders.
5. Reaction with [HNMe3]BgH14
nh3
Yb + 2[HNMe3 ][B 9 H14] -> [(NH3 )xYb][BgH 1 4 ] 2 + 2NMe3 + H2 (24)
The reaction of ytterbium metal with two equivalents of [HNMe3 ][BgH14] produces a mustard yellow slurry and non-condensable gas. The gas was identified as hydrogen by gas mass spectral analysis and isolated in 47% yield.
Drying the sample leaves an oily green residue which dissolves in acetonitrile 2400 2300 22002100 2000 1900 1800 1700 1600 1500 2500
Figure 25. Infrared spectra of (a) (C5 HsN)xYb(NH 2 BH3 )2 and (b) (NH3 )xEu(NH 2 BH3 )2 in nujol. CO 74 forming the clear orange color characteristic of acetonitrile solvated Yb2+ complexes.
Appearance of impurities amidst BgH14' peaks in the 11B NMR in CD3CN
confirms that the low hydrogen yield is due to side reactions occurring. Table1 1
summarized the 11B and 1H NMR data for (CD 3 CN)xYb[B 9 H1 4 ] 2 and KBgH14. The
BgH14' doublets remain unchanged at -8.1 ppm, -20.5 ppm, and -23.7 ppm in the presence of ytterbium (Figure 26). New peaks from the formation of BgH12‘,
B3 H8-, B1 0 H14, and other side products are minor peaks. The similarities between the spectra of the potassium and ytterbium BgH14' complexes suggest that in both cases the boron hydride cage associates itself with the metal as a solvent separated ion. Likewise, the 1H NMR peaks are virtually the same except for the peaks assigned to acetonitrile (Figure 27). In the spectrum of the potassium salt, the acetonitrile multiplet appears at 1.93 ppm with a solvent impurity at 2.1 ppm.
In the 1H NMR spectrum of [(CD3 CN)xYb][BgH14]2, a weak peak at 1.93 ppm is buried within the BgH14' peaks, but a major peak appears at 0.9 ppm. Although the BgH14' moiety is unaffected, the peak due to coordinated acetonitrile experiences broadening and a significant shift of 1 ppm upfield. This same
behavior was also observed in the proton spectrum of YbB1 2 H12. Except for the triplet at 3.6 ppm, the boron-containing impurity peaks are too weak and are buried within the BgH14‘ peaks.
The hydrogen-bridge bonding found in the1 0 BH142' complexes was not
observed in the BgH14‘ reaction with ytterbium.3 5 This may be a consequence of 75
Table 11. NMR Data of BgH^- Complexes
[(CD3CN)xYb][B 9H14] 2 kb 9h14
1 0 . 0 ppm(d,br)
-0 . 2 ppm(d,br) -8.1 ppm(d,144Hz) -8.4 ppm(d,142Hz) -9.9 ppm(d, 140Hz) -12.5 ppm(s) -14.7 ppm(d, 140Hz) 11B NMR -20.5 ppm(d,135Hz) -20.6 ppm(d,132Hz) -22.4 ppm(s) -23.7 ppm(d, 139Hz) -23.8 ppm(d,139Hz) -26.9 ppm(d,137Hz) -29.4 ppm(m=9,33Hz) -53.0 ppm(d, 127Hz)
3.6 ppm(t,br, 45Hz) 2.9 ppm(s) 2.9 ppm(s) 2.3 ppm(s) 2.3 ppm(s)
1 . 8 ppm(s) 1 . 8 ppm(s)
1H NMR 1 .2 ppm(s,br) 1 .2 ppm(s,br) 0.9 ppm(s,br) 0.7 ppm(s,br)
0 . 2 ppm(s,br) 0 . 2 ppm(s,br) -1.5 ppm(s.br) -1.5 ppm(s,br) « 110H 11* * I «*** *
10T0 5*0 -5.015.0 -35.0 PPM
Figure 26. 11B NMR spectrum of [(CD3 CN)xYb][B 9 H i4 ]2 in CD3 CN.
“>4 o> 4 .0 a;s 3 .0 2 .5 2 .0 1.0 0 .0 - 1 .0 - 1.5 - 2 .0 - 2 .5 PPM
4.5 3.5 3.0 2.5 2.0 0.0 - 1.0 - 2.5
Figure 27. 1H NMR spectra of BgH-u- compounds. (a) [(CD3CN)xYb][BgHi4]2. (b) KB 9 H 1 4 in CD3 CN. 78 either the lower basicity of the monoanion or the steric restrictions of two Bg cages around the metal center.
This reaction was designed to produce a ytterbium boron hydride with two
B9 H 1 4 cages bound to divalent ytterbium via hydrogen-bridge bonds. This complex could then be utilized as a precursor to the stable YbB12 phase. The solvent-separated complex that is actually isolated does not hold ytterbium and boron in close proximity to one another to favor formation of a pure dodecaboride.
6 . Reaction with B10H14
Previously, ytterbium metal reacted with one equivalent of decaborane producing the ytterbium(ll) decaborate complex.35
n h 3 Yb + B10H14 - * (NH3)xYbB10H14 (25)
This reaction follows a similar reaction involving sodium/mercury amalgam.62
2Na/Hg + B1QH14 Na2B10H14 (26)
Varying the stoichiometry of this reaction alters the product of the reaction.
2Na/Hg + 2B10H14 -> 2NaB10H13 + H2 (27)
Similarly, changing the stoichiometry of the ytterbium reaction should produce the
B10H13' species. n h 3
Yb + 2B 1 0 H 1 4 -> (NH3 )xYb(B 1 0 H1 3 ) 2 + H2 (28)
When two equivalents of decaborane react with ytterbium metal in ammonia, and oily brown solid settles to the bottom of a light green solution. The dried product is a flaky, shiny, purplish-brown solid. Hydrogen evolves in only 45% yield. The incompleteness of the reaction is also evidenced by a mixture of species in the 11B NMR spectrum (Figure 28). The spectrum is identical to that
obtained for the 1:1 Yb:B1 0 H 1 4 reaction and contains B 1 0 H13' peaks as well as
B10^15 and YbBioH^ peaks . 6 Apparently hydrogen-bridge bonding occurs with
Bi0 H i42' but not with the monoanions. Their signals resemble the signals for the alkali metal salts, whereas the peak of the boron involved in the Yb-H-B bridge in
YbB1 0 H 1 4 shifts downfield from the corresponding signal in the alkali metal salt . 6
C. Thermal Decomposition Studies of Precursor Complexes
1. Pyrolysis of (CH3CN)xYb(BH 4)2
Previous decomposition studies of ytterbium (II) borohydride led to the
formation of the tetraboride even though the B:Yb ratio favored the YbB2 phase 6
-h 2,-c h 3c n
2(CHoCNLYb(BHJp ------> YbB4 + Yb (29) ° 1000*C,vac.
Heating at higher temperatures under dynamic vacuum results in sublimation of some of the metal creating a more boron rich boride phase. The diboride phase has been prepared by heating elemental ytterbium and boron in a tantalum 1 0.0 5.0 -5.0 - 1 0.0 -15.0 - 20.0 -25.0 -30.0 PPM
Figure 28. 11B NMR spectrum of product of Yb with two equivalents of decaborane in ammonia.
00 o 81
crucible at lOOO’ C. 4 3 By changing the conditions of pyrolysis, the YbB2 can be
prepared from (CH3 CN)xYb(BH4)2. a pure sample of the diboride is isolated as a black powder by lowering the furnace temperature to 550°C and maintaining
1 atm of argon gas in the reaction tube.
-h 2,-c h 3c n
(CHoCN) Yb(BH4)P ------> YbB2 (30) ° H * 550°C,1atm Ar
Hydrogen and acetonitrile can be removed cleanly from the metal at temperatures below 250° C. The lines in the x-ray powder pattern are consistent with the
reported values for YbB2 . 6 3
2. Pyrolysis of (NH3 )xYbB 1 2 H 1 2
With a 12:1 boron to ytterbium ratio, (NH 3 )xYbB 1 2 H 1 2 was predicted to produce the dodecaboride as the stable pyrolysis product which has not been previously isolated from ytterbium boron hydride precursors. Ammonia as well as residual trimethylamine produced in the ammonia reduction reaction can be driven off cleanly at temperatures below 196°C. Hydrogen evolves from decomposition of the cage until the temperature of the pyrolysis tube reaches 600° C. Separation and measurement of ammonia, trimethylamine, and hydrogen collected in the
decomposition renders the formula (NMe3 ) 0 3 0 (NH3 ) 2 7 5 YbB1 2 H 1 2 for the solvated powder before heating. Most of the acetonitrile-, pyridine-, DIME-, and ammonia- solvated powders of the lanthanide boron hydride complexes have degrees if
solvation ranging from 1.5 to 3 solvent molecules per metal center. 6 The solvation of the YbB1 2 H 1 2 powder is consistant with this data. Further heating of the sample
to 1 000°C under dynamic vacuum results in sublimation of ytterbium from the tube
leaving greyish-black solid identified as YbB6 by x-ray powder diffraction.
-n h 3
2(NHqLYbB 1 pH1P ------> YbBfi + Yb + 18B (31) ' 3'X 12 12 i000°c,vac. 6 V '
Heating under dynamic vacuum promotes sublimation of ytterbium, therefore,
lowering the boron to ytterbium ratio favoring formation of the less boron-rich
hexaboride. Heating at 1000°C under an atmosphere of argon does inhibit
sublimation of ytterbium, but still produces YbB6 and excess amorphous boron.
-h 2, -n h 3
(NHo)yYbB1 2 H1P ------> YbBfi + 6 B (32) 1000°C,1atm Ar b '
The dodecaboride has previously been prepared by borothermic reduction of
Yb2 0 3 by elemental boron under vacuum at 1200°C then at 1800#C under an
atmosphere of argon gas . 6 4 Perhaps higher temperatures are necessary to form
YbB12, however, comparable temperatures exceed the limits of the furnace.
3. Pyrolysis of (NH3 )xEu(B 3 H 8 ) 2
A solid sample of (NH 3 )xEu(B 3 H8 ) 2 can be isolated only under limited drying time under dynamic vacuum, otherwise, it becomes an oil. Pyrolysis of this
precursor complex is expected to produce the hexaboride because of the6 : 1 83
boron to ytterbium ratio. Even though EuB6 is the only stable boride phase of europium, the boron to ytterbium ratio in this precursor should prevent simultaneous generation of amorphous boron. This complex is much less stable
at lower temperatures than the LnB1 2 H1 2 and LnB 1 0 H1 0 powders and begins decomposing with hydrogen and ammonia evolution at 60°C. At 90#C, the pressure of the condensable gas decreases substantially. Examination of the volatile thermal decomposition products by 11B NMR (Figure 29) reveals that the
major products are NH 3 BH3 and B 3 H8' with some B(OH) 3 produced by exposure
of the sample to air. The presence of NH3 BH3 explains the sudden drop in ammonia pressure as ammonia reacts with the volatile boron containing products.
The weak bonding between3 H8' B and Eu2+ is evidenced by the relative ease in which the boron hydride sublimes from the reaction tube. Heating the remainder of the solid at 950°C produces only a small amount of EuB6. Amorphous boron is probably a product as well. The ytterbium analogue was not subjected to pyrolytic treatment because it could only be isolated as an oil.
(NH3 )xEu(B 3 H8 ) 2 — — > EuB6 + B3 H8‘ + NH3 BH3 + H2 + B (33)
A summary of europium and ytterbium borides prepared from corresponding boron hydride precursors is shown in Figure 30. All of the stable boride phases can be prepared with the exception of YbB12. The only boride formed in high yield and purity is YbB2. All others also produce excess I I 1 I f ( 20 18 14 I I 16 12 10 - 2 -4 -6 -e -To0 -1 -T 2 -14 -16 -IB -20 -22-224 -26 -28 -i0 -32 PPM
Figure 29. 11B NMR spectrum of volatile products of (NH3)xEu(B3Hs)2 decomposition.
00 -t* (CH3CN)4Yb(BH4)2 ^ YbB2
2(CH3CN)4Yb(BH4)2 ^ 000^ 3^ ^ YbB* + Yb
H2,-NH3 ^ BsH8. + BHsNH3 + EuBg (NH3)xEu(B 3H8)2 - A, vac
-h 2,-n h 3 (NH3)xYbBi 2H i2 YbB6 + 6 B 1000°C,1atm
-H2j-NH3 2(NH3)xYbB 12Hi2 YbB6 + Yb + 18B 1000°C,vac
-H2,-NH3 YbB6 + 4B (NH3)xYbBi0Hio 1000°C,1atm *
-H2,-NH3 E11B0 + 4B (NH3)xEuB 10Hio 1000°C,1 atm *
Figure 30. Summary of lanthanide borides prepared from lanthanide boron hydride precursors. 86 amorphous boron and/or lanthanide metal.
D. Thin Film Studies
1. YbB2 thin film studies
-H2, -CH3CN 1atm Ar (CH~CNLYb(BFLWSub ------> ------> YbBJSub (34) ° x H £ 330°C 550°C * Sub = Substrate = Ta or W
The diboride system was chosen for thin film studies because of the lower temperatures at which it is formed and the purity in which it can be isolated. All
other lanthanide boron hydride precursors produce elemental boron. Lanthanide
metal sublimes from precursors heated under dynamic vacuum. Further
contamination can be avoided by cleaning the tantalum and tungsten substrates
prior to deposition of the precursor solution.
When tantalum foil substrates are dipped into a 1% solution of
(CH3 CN)xYb(BH4)2, the surface tension of the acetonitrile prevents uniform
coverage. A one-coating sample of the borohydride converts to YbB2 when
hydrogen and acetonitrile are removed under vacuum below 300°C followed by
heating of amorphous YbB 2 at 550#C under 1atm argon.
Scanning electron micrographs taken of various regions of the foil piece
show greater coverage of YbB2, represented by the dark areas in
Figures 31 and 32, near the edges and minimal coverage in the center. The
thickness of the boride deposits is approximately 1 /xm based upon the s ^ s. CvV Figure 31. Scanning electron micrographs of ytterbium diboride films on tantalum. (a)X800 (b) X1500. Figure 32. Scanning electron micrographs of ytterbium diboride on tantalum, (a) 1-coating edge at X200. (b) 3-coating edge at X180. 89 micrograph taken at a magnification of X1500. A three-coating film on tantalum was prepared by heating each coating to 330°C under vacuum to remove volatiles prior to a final heating at 550°C under 1 atm argon. Once again, coverage is much greater along the edges of the substrate and in areas of deformity in the foil that act as pockets for boride formation. Figure 32b is a micrograph of such an area along the edge of the sample. The diboride forms a dark strip in the center of the micrograph that completely covers the substrate material. Directly adjacent to this area, black deposits are very sparse. These areas are mostly exposed tantalum. The coverage increases towards the bottom of the micrograph closer to the edge of the substrate. The uneven distribution of boride results from the initial uneven application of the borohydride solution. These findings warranted modification of the deposition procedure to minimize the surface tension of the precursor solution. An air brush was employed to atomize the precursor solution in order to deposit equal amounts of borohydride on all regions of the substrate. At low magnification, regions of higher concentration of boride appear in the micrograph (Figure 33) but these deposits are now spread more evenly across the substrate surface. The tantalum surface can still be seen beneath the dark area. Closer examination of the boride regions at higher magnification (Figure 33b) reveals that the dark regions are not continuous films of YbB2, but are composed of very small, random spots on the substrate. For a one-coating sample, complete coverage of the surface is not expected. Eight-coating films were deposited on tungsten and tantalum Figure 33. Scanning electron micrographs of 1-coating of ytterbium diboride on tantalum, (a) X300. (b) X6000. 91 substrates. In the micrograph of the tantalum sample (Figure 34), the surface of the metal can no longer be seen. At lower magnification, a less uniform layer grows from a continuous layer beneath it. Closer inspection of this same region at higher magnification shows that the particles of the bottom layer are fused together and provide a foundation from which the conglomerates above it can grow. The micrographs of boride-coated tungsten look similar (Figure 35). The choice of substrate does not seem to affect the outcome of the thin films in these studies. The quality of the boride films are more dependent upon the number of coatings. In the micrographs of the twenty-coating sample on tantalum (Figure 36), the thin film is one continuous layer absent of the clusters seen in the eight-coating samples. Cracks in the boride layer accentuate the dimensionality of the film. The energy dispersive spectrometer detected high concentrations of ytterbium and boron in the dark regions of the surfaces and negligible concentrations in the lighter areas which produced predominantly substrate peaks in the EDS spectrum (Figure 37). 2. EuB6 thin film studies -h 2,-c h 3c n 3(CH3CN)xEu(BH4)2/Sub ------> EuBg/Sub + 2Eu (35) Sub = Tantalum foil The instability of (NH3)xEu(B3H8)2 precluded its use as a stoichiometric Figure 34. Scanning electron micrographs of 8-coatings of ytterbium diboride on tantalum, (a) X1500. (b) X10000. Figure 35. Scanning electron micrograph of 8-coatings of ytterbium diboride on tungsten. (a)X2500. (b) X10000. Figure 36. Scanning electron micrograph of 20 coatings of ytterbium diboride on tantalum. (a)X1500 (b)X10000. Counts (XIQ3 ) 4 3 2 1 V .b T a 0 1 2 Range (keV) Figure 37. EDS spectrum of YbB2 film. 96 precursor to EuB6. The borohydride complex was shown to produce the hexaboride while excess elemental europium sublimed from the reaction tube.6 When a pre-treated piece of tantalum foil is dipped into a 3% solution of (CH3CN) xEu (BH4)2 and heated at 1000°C for 12h under dynamic vacuum, europium metal sublimes from the reaction tube as in the bulk sample. The scanning electron micrograph (Figure 38) shows the coverage of two coatings of the hexaboride. The black areas represent the EuB6 deposits. The coverage of the boride is non-continuous over the entire surface of the tantalum which is not surprising for a two-coating sample. The striations in the foil are still evident even in the areas where the boride is deposited. The non-uniformity of the boride deposits is most likely due to uneven application of the precursor solution brought about by the surface tension of the solvent on the foil piece. Figure 38. Scanning electron micrograph of europium hexaboride on tantalum. CHAPTER IV EXPERIMENTAL A. Apparatus 1. Vacuum System A high vacuum system was employed for all reactions due to the sensitivity of starting materials and products to air and moisture. The main and secondary manifolds were made from 25 mm Pyrex tubing. Three secondary manifolds, a distillation train, Toepler pump system, and McCleod gauge were connected to the main manifold by way of ground glass stopcocks and 8 mm Pyrex tubing. A vacuum of 10'5 torr was attained using a Welch Duo-Seal rotary pump in series with a mercury diffusion pump. A solvent trap cooled with liquid nitrogen was positioned between the main manifold and diffusion pump and prevented condensable materials from entering the rotary pump. A second trap maintained at -78°C with an isopropanol/C02 slush was positioned between the mercury diffusion pump and the rotary pump to prevent mercury vapors from escaping from the vacuum system. Ground glass joints were well greased with a 50/50 mixture of Apiezon N and T greases. Reaction ports were greased with Dow Corning silicone high vacuum grease. 98 99 A diagram of the secondary manifold is shown in Figure 39. The manifold was assembled using Kontes 4 mm stopcocks and closed off on either end using 24/40 ground glass stoppers. A Pyrex partition divided the manifold into two parts. On either side of the partition 9 mm Solv-Seal ports were attached which were compatible with a solv-seal U-trap for collection of condensable materials under dynamic vacuum. Two horizontal 14/35 ground glass joints were employed as vacuum extractor ports. Four vertical 14/35 ground glass ports were used for routine manipulations of reaction materials. A mercury blowout extended from each port to ensure the pressure within the vacuum system does not exceed an atmosphere. The distillation train was constructed with four U-tubes connected in series and attached on either end to the main manifold through 8mm Pyrex tubing. Each U-tube could be isolated by standard taper ground glass stopcocks. A port on one end of the distillation train was made from a 4 mm Kontes stopcock attached to a 14/35 ground glass joint and mercury blowout to directly introduce volatile materials into the train. Pressure measurements were made on a mercury manometer attached to one of the U-tubes. A Toepler pump system (Figure 40) permitted measurement of non- condensable gases. The pump system was attached to the main manifold through a ground glass stopcock and to the secondary manifold through a 4 mm Kontes stopcock with 8 mm Pyrex tubing. A 1L mercury reservoir was attached to a mercury manometer and four storage volumes with 8 mm Pyrex tubing. The four J& j © ® (r M f iL r iir & Figure 39. Secondary vacuum line manifold. to main manifold to secondary manifold to rough pump and air bleed Figure 40. Toepler pump apparatus. 102 storage volumes were separated by either ground glass or Kontes stopcocks to control the total volume for expansion of the gas. The Toepler pump system accurately measured gas quantities ranging from 0.02 to 5.0 mmol. A three-way Kontes stopcock with a 14/35 ground glass port was inserted between the manometer volume and the main manifold. Such an arrangement allowed flasks to be directly attached to the Toepler system and either evacuated by way of the main manifold or opened to the manometer storage volume for collection of gases for gas mass spectral analysis. The electrical leads were connected to a switch box in series with a solenoid valve which could be opened to either an air bleed or a rotary pump. The McCleod gauge operated from the same rotary vacuum pump. 2. Inert Atmosphere Box Air and moisture sensitive materials were handled in either of two Vacuum Atmospheres controlled atmosphere boxes purged with prepurified nitrogen (Matheson) continuously circulated over Ridox oxygen scavenger and Linde 3A molecular sieves. Oxygen and moisture levels were obtained at levels below 10 ppm and monitored using a titanocene/THF mixture which changes from blue or green to yellow or orange in the presence of residual oxygen or moisture impurities. One box was equipped with a refrigerator maintained at -30°C. Welch Duo-Seal and Leybold-Heraus mechanical pumps evacuated the entry/exit ports of both dry boxes. 103 3. Glassware Pyrex or Kimax round bottom flasks with 9 mm or 15 mm Solv-Seal joints were used as reaction vessels. Reaction flasks were connected to vacuum line ports with adaptors constructed from a 14/35 ground glass joint, Kontes 4 mm stopcock, and 9 mm or 15 mm solv-seal joint. Filtration of reaction components was accomplished with a vacuum line extractor (Figure 41) attached to the horizontal 14/35 ground glass ports of the vacuum line. Reactants were introduced into flask A in an inert atmosphere. Dried solvents were then vacuum transferred to the evacuated extractor while cooling the reaction flask with either isopropanol/C02 or liquid nitrogen. Kontes valves C and D were closed and reaction mixtures stirred at room temperature with a teflon stir bar. To filter the extractor was inverted 180° after which solvent could pass through a medium or fine glass frit into collection flask B. Solids were rewashed by opening valve C and recondensing solvent into the reaction flask cooled with a liquid nitrogen swab or dry ice. For many of the reactions proceeding in liquid ammonia, a 5-inch 15 mm solv-seal tube was used to facilitate complete recovery of finely divided powder products. Pyrolysis reaction tubes were constructed of 13 inches of 15 mm o.d. quartz tubing, 1 1/2 inches of a 13 mm o.d. quartz graded seal and a 15 mm Pyrex solv- seal joint. The tube was closed off with a 9 mm solv-seal joint, Kontes 4 mm stopcock, and 15 mm solv-seal joint adaptor. Figure 41 Vacuum line extractor apparatus. 105 NMR tubes and elemental analysis vials for air-sensitive samples were glassblown to 9 mm solv-seal joints. All glassblown tubes and vials were checked under vacuum for pinhole leaks with an Electro-Technic tesla coil before proceeding. Solid samples were introduced in an inert atmosphere and closed off with vacuum adaptors. NMR solvents were vacuum transferred into evacuated NMR tubes cooled with isopropanol/C02 then flame sealed with a torch. All seals were coated with Apiezon vacuum wax to prevent pinhole leaks. Glassware was initially washed with water or dilute HCI and then immersed into a NoChromix acid bath after which they were rinsed with distilled water and dried in a >100°C oven. In addition, vacuum extractor frits were rinsed with acetone and CH2CI2. 4. Air Brush Thin films of precursor complexes were applied to substrates with a Model Master paint sprayer (#50623) using an ultra-high purity lecture bottle of nitrogen gas as propellant. 5. X-ray diffraction Single crystal x-ray diffraction data were collected on an Enraf-Nonius CAD4 diffractometer using graphite monochromatic molybdenum Ka radiation. A low temperature unit with a dry nitrogen refrigeration unit was fitted onto the diffractometer. 106 X-ray powder diffraction data of air sensitive samples were collected using the Debye-Scherrer camera technique. An Enraf-Nonius-Delft Diffractis 582 x-ray generator with a Cu radiation source was employed. X-ray powder samples were sealed in 0.5 mm x-ray capillaries. Diffraction data of air stable samples were obtained on a Rigaku Geigerflex powder diffractometer with Cu target. Samples were spread uniformly onto a glass sample holder with a small amount of water for sufficient adhesion. Sample patterns were identified using the JCPDS Powder Diffraction File for Inorganic Phases.63 6. Nuclear Magnetic Resonance Spectra 11B and 1H Fourier Transform Nuclear Magnetic Resonance (FT-NMR) spectra were obtained on a Bruker AM-250 with operating frequencies of 250.13 MHz and 80.25 MHz for 1H and 11B, respectively. Hydrogen containing samples were scanned under deuterium lock. Proton peaks in deuterated solvents served at internal references for chemical shifts based on 6 TMS = 0.00 ppm. 11B spectra were run unlocked using an external BCI3 standard reference in CD2CI2 (6 = 46.0 ppm relative to BF3 OEt2). 7. Infrared Spectra Fourier Transform Infrared Spectral data were collected on either a Mattson Polaris or a Perkin-Elmer 16PC spectrometer. Most samples were prepared as mulls in nujol or fluorolube. The sample holder was constructed from two circular 107 NaCI plates (33 mm x 4 mm) surrounded by o-rings and sandwiched between a 20 mm solv-seal coupler with teflon supports. Air-sensitive samples were prepared in the glove box. 8. Elemental Analyses Samples were sent to Analytische Laboratorien, Postfach 13 15, D-5250 Engelskirchen, Germany or Mikroanalytisches Labor Pascher, Postfach 2129, D- 53416 Remagen, Germany for confirmation of chemical composition. 9. Mass Spectra Volatile reaction products were identified on a Balzers 112 Quadrupole Mass Spectrometer. 10. Scanning Electron Microscopy Boride deposits were studied on a JOEL 840 field emission scanning electron microscope or a JOEL 820 SEM with a Link Analytical Oxford Instruments eXL Energy Dispersive Spectrometer. Substrates were affixed to the sample holder with a graphite adhesive emulsion and degassed overnight in a desiccator attached to a rough pump. Micrographs were taken with an accelerating voltage of 10kV. Energy dispersive spectra were obtained at an accelerating voltage of 3kV. 108 11. Pyrolysis Reactions Samples were loaded into the quartz pyrolysis tube and attached to the vacuum line as shown in Figure 42. The sample was situated in the center of a Lindberg 1330 Watt tube furnace with maximum temperature of 1200°C. Samples of boride precursor complexes were introduced into the quartz furnace tube inside the dry box, closed off and attached to the vacuum line with an extension tube made from 10 mm Pyrex tubing, a 9 mm solv-seal joint and a ground glass 14/35 joint. B. Solvents and Reagents 1. Acetonitrile CHgCN (Fisher Scientific) was stirred over P20 5 under vacuum for 1 week or until an orange-yellow polymeric material formed at the bottom of the flask. The contents of the flask were transferred on the vacuum line to a clean 500 ml solvent storage bulb with a Kontes stopcock and 14/35 ground glass joint. Residual non- condensable gas was degassed from the storage bulb at -78°C. Deuterated CHgCN, 99.8%, was purchased from Isotec,Inc. was dried in the same manner. 2. Ammonia Anhydrous NH3 (Matheson) was transferred on the vacuum line to a degassed Pyrex storage tube cooled to -196°C containing sodium. Liquid ammonia/Na mixtures were copper colored with high concentrations of sodium 109 To vacuum line Quartz Sample Tube Kontes Stopcocks 9 mm Solv-Seal Tube Furnace joints and couplers Figure 42. Tube furnace set up for pyrolysis experiments. 110 and deep blue for lower concentrations. The storage tube was cooled to -196°C and degassed before each use. 3. Ammoniaborane NH3 BH3> 90%, was purchased from Aldrich Chemical Co., sublimed at 60°C under vacuum and handled in the dry box. 4. Ammonium chloride NH4CI (Baker) was sublimed and stored in the glove box. 5. Argon An Ar cylinder purchased from Liquid Carbonic was attached directly to the vacuum line using rubber hose with a 9 mm Solv-Seal outlet. 6. Borane-tetrahydrofuran BHgTHF (Aldrich) was received as a 1.0M solution in a sure-seal bottle. The bottle was stored in the dry box refrigerator at -30°C. 7. Decaborane B1 0 H 1 4 (suPPlied by D-R Gaines group) was vacuum sublimed at a temperature < 90°C to obtain a crystalline white solid. 111 8. Disodium dodecahydrododecaborate Na 2 B1 2 H12xTHF (Boulder Scientific Co.) was passed through an ion- exchange resin to determine the degree of THF solvation. A weighed sample of Na 2 B1 2 H12xTHF was passed through a Dowex-50W 100-200 mesh hydrogen form cation exchange resin (Sigma). The filtrate was then titrated with a standardized NaOH solution and x calculated to be 3.76. 9. Europium Eu ingot purchased from Strem Chemicals was washed with hexanes under vacuum to remove residual mineral oil from storage. The ingots were vacuum dried, cut into 70-100 mg pieces and stored in the dry box. 10. Mercury Hg for the vacuum line and amalgam reactions triple distilled (Bethlehem Instruments) was used as received. 11. Nitrogen N2 gas, 99.99%, (Liquid Carbonic) for thin film applications was received in a lecture bottle and used as received. 12. Phosphorous Pentoxide P2 0 5 was purchased from Baker Chemicals and used as received. 13. Potassium K (Aldrich) was washed with hexanes to remove mineral oil and stored in the dry box. 14. Pyridine C5 H5 N (Fisher Scientific) was dried over sodium for several days. The solution changes from colorless to black when all water had reacted. The dried solvent was then transferred at -78°C to a solvent bulb with a Kontes stopcock/ground glass attachment containing sodium pieces. 15. Sodium Na (Aldrich) was washed with hexanes to remove mineral oil and stored in the dry box. 16. Sodium borohydride NaBH 4 (Fisher Scientific) was used as received and stored in the dry box. 17. Sodium cyanotrihydridoborate NaBH3CN (Alfa) was used as received 113 18. Tantalum foil Ta foil, 0.127 mm thick, (Johnson Matthey Aesar) was cut into 8 mm x 10 mm pieces and pre-treated with a 3% HF solution for several minutes. The substrate pieces were rinsed with distilled water and heated under vacuum to 900°C in a quartz furnace tube to remove water, adsorbed gases, and other impurities. 19. Tetrahydrofuran (THF) C4 H80 (Mallinckrodt) was refluxed under vacuum in a flask containing sodium benzophenone until the solution changed from colorless to deep purple. The solution was then distilled into a clean storage bulb with a Kontes stopcock/ground glass attachment containing sodium benzophenone. 20. Tetramethylammonium octahydrotriborane [N(CH3 )4 ]B3 H8 (Strem) was used as received and kept refrigerated. 21. Thallium acetate ThC2H30 2 (Alfa) was used as received. 22. Triethylammonium decahydrodecaborate [HN(C2 H5 )3 ] 2 B1 0 H10, (supplied by G. Kodama), was used as received. 114 23. Trimethylamine hydrochloride N(CH3 ) 3 HCI (Aldrich) was used as received and stored in the dry box. 24. Tungsten foil W foil, 0.25 mm thick, 99.9+% (Aldrich) was sheared into 8 mm x 10 mm pieces and pre-treated in a 3% HF solution for several minutes. The substrate pieces were rinsed with distilled water and heated to 900°C under dynamic vacuum to remove impurities. 25. Ytterbium powder Yb powder, 3N (REO), (Strem) was used as received and stored in the dry box. C. Preparation of Starting Materials 1. Ammonium octahydrotriborane NH4 B3 H8 was prepared according to the procedure published by Amberger and Gut . 6 5 Thallium octahydrotriborane was prepared by reaction of thallium acetate with N(CH3 )4 B3 H8 in water. The metathesis reaction with NH4I was modified from the literature procedure which proceeded in air in aqueous medium resulting in an explosion due to formation of diborane. Freshly prepared TIB3 H8 and NH4I were added to a 150 ml round bottom flask with 24/40 ground glass outlet wrapped in foil to prevent decomposition of NH4 I. The flask was attached 115 to a vacuum extractor equipped with a cooling jacket for ammonia filtrations. Liquid ammonia (10 ml) was condensed into the evacuated flask at -196°C and warmed to its boiling point with stirring. After 30 min. the foil was removed revealing a yellow-orange solid characteristic of Til. The reaction flask was opened to a mercury blowout and quickly inverted while filling the cooling jacket with isopropanol/C02. The receiving flask was cooled to -196°C to induce filtration of the clear solution. Ammonia was removed and the yellow solid left on the frit was washed several times with CH3 CN. The dried NH3 and CH3CN soluble product was a white solid with an x-ray powder diffraction pattern distinct from the x-ray patterns of the TIB3 H8 and NH4I starting materials. 2. Acetonitrile solvated europium dichloride (NH3 )xEuCI 2 was prepared according to Howell and Pytlewski’s published procedure using Eu and NH4CI in liquid ammonia .2 5 The white solid was washed several times with dry acetonitrile and dried under vacuum. 3. Acetonitrile solvated ytterbium dichloride (NH3 )xYbCI 2 was prepared from reaction of Yb metal with NH4CI in liquid ammonia according to Howell and Pytlewski’s published procedure 2 5 The finely divided pale green powder was washed several times with dry acetonitrile then dried under vacuum. 116 4. Potassium octahydrotriborane KB3Hg was prepared according to the published procedure. 5 5 The white solid was washed several times with ammonia followed by acetonitrile or pyridine to remove residual THF. 5. Rubidium octahydrotriborane RbBgHg was prepared using the same procedure as for KB3 H8 above. 6. Trimethylammonium dodecahydrododecaborate [HN(CH 3 )3 ] 2 B1 2 H 1 2 was prepared by metathesis of Na 2 B1 2 H 1 2 with two equivalents of HN(CH3)3CI in water. The product precipitated as a white solid and was then washed several times with water and dried. The x-ray powder pattern of the product was distinct from the patterns of the starting materials. 7. Potassium tetradecahydrononaborate KBgH 1 4 was prepared from B 1 0 H 1 4 and KOH in water according to the published procedure. 6 6 8. Trimethylammonium tetradecahydrononaborate [HNMe3 ]BgH 1 4 was prepared by metathesis of KBgH1 4 with HNMe3CI in distilled deoxygenated water. The white precipitate which formed immediately upon mixing was filtered and washed several times with fresh water before drying 117 under vacuum. The product was washed in liquid ammonia to displace residual water. D. Reactions 1. Preparation of YbB12H12 A 15 mm Solv-Seal reaction tube containing a Teflon coated stir bar was charged with ytterbium metal (0.1178 g, 0.6808 mmol) and [HNMe3 ]2 B1 2 H 1 2 (0.1764 g, 0.63731 mmol) in the dry box. Ammonia (~ 4 ml) was condensed into the tube and warmed to the boiling point with stirring producing a yellowish-green slurry. Non-condensable gases were toeplerized from the reaction mixture at -196°C and measured. The cycle was repeated until gas evolution ceased (H2 yield = 0.6430 mmol, 96%). Ammonia was transferred to a solvent trap. The ammonia-solvated iight green powder was dried under vacuum for 2 h then introduced into a Ta capsule that had been pre-treated with a 3% HF solution, rinsed with water, and heated in a quartz tube at 650°C for 24h under dynamic vacuum. Ammonia-solvated YbB 1 2 H 1 2 (0.0316 g) was inserted into the quartz tube, evacuated and heated slowly to while trapping condensables at -196°C and toeplerizing non-condensable gases. Condensable gases were completely removed at 140°C and identified as NH3 and NMeg by gas mass spectral analysis. Ammonia (0.2293 mmol) was separated from NMe 3 (0.0254 mmol) by passing the mixture through a -97°C trap. Hydrogen gas evolution ceased at 600°C. Yield H 2 = 0.1558 mmol. These measurements corresponded to the formula 118 (NMe3 ) 0 3 0 (NH3 ) 2 7 5 ^bB 1 2 H12fbr the solvated powder. Anal. Calc’d for YbB1 2 H12: Yb, 54.96%; B, 41.20%; H, 3.84%. Found: Yb, 54.45%; B, 40.60%; H, 3.97%. Infrared spectrum (fluorolube, KBr plates): i/(N-H) 3356(m), 3237(m)cm'1; i/(B-H) 2496(s), 2424(m) cm'1. Infrared spectrum desolvated powder at 120°C: v(B-H) 2509(m,sh), 2467(s) cm'1; y(Yb-H-B) 1731 (m) cm*1. 11B NMR spectrum in CDgCN (303 K, 6 BFgOEt, = 0.00 ppm): 6 = -14.5 ppm (d, JB_H = 123 Hz). 1H NMR spectrum in CDgCN (303 K, TMS 6 = 0.00 ppm): 6 = 1.00 ppm (multiplet, v. broad). 2. Preparation of EuB12H12 A 15 mm Solv-Seal reaction tube with a Kontes/ground glass vacuum adaptor and Teflon coated stir bar was charged with [HNMe 3 ]2 B1 2 H 1 2 (0.1783 g, 0.6804 mmol) and Eu ingot (0.0.1051 g, 0.6944 mmol) in the dry box. The reaction tube was evacuated and ammonia (~ 4 ml) was condensed into it at -196°C. Non- condensable gases were collected and measured in the Toepler pump system. The mixture was warmed to the boiling point, stirred, and degassed several times until hydrogen evolution ceased and a gray slurry was produced. Yield H2 = 0.6280 mmol, 92%. After the ammonia was removed the white ammonia-solvated powder remaining in the reaction tube was dried under dynamic vacuum for several hours. The solvated powder (0.0713 g) was then introduced into a tantalum capsule that had been pre-treated with a 3% HF solution, rinsed with distilled water, and heated overnight in a quartz tube at 600°C under dynamic 119 vacuum. The pyrolysis profile of (NH3 )xEuB 1 2 H1 2 was obtained from 50°C to 850°C. Condensable gases were collected in a solvent trap at -196°C and non- condensable gases were collected and measured in the Toepler pump system. Coordinated solvent was driven from the sample tube between 50°C and 150°C. A negligible amount of non-condensable gas evolved between 50°C and 130°C but was too small to measure on the Toepler system. Hydrogen (1.350 mmol) evolved between 150°C and 625°C. Anal. Calc’d for EuB 1 2 H12: Eu, 51.72%; B, 44.16%; H, 4.12%. Found: Eu, 51.40%; B, 43.84%; H, 4.19%. Infrared spectrum (nujol, KBr plates): i/(N-H) 3362 cm'1; y(B-H) 2516(s,broad), 2489(s), 2428(m) cm'1; i/(Eu-H-B) 1733(w) cm'1; i/(B-B) 1197(w), 1070(w), 972(w) cm'1. Infrared spectrum desolvated powder at 130°C: i/(B-H) 2502(s,sh), 2477(s,broad), 2420(m,sh) cm'1; v(Eu-H-B) 1731 (w) cm'1; i/(B-B) 1197(w), 1070(w), 972(w) cm'1. 3. Preparation of YbB10H10 Ytterbium (0.0990 g, 0.572.1 mmol), [HNEt3 ] 2 B1 0 H 1 0 (0.1812 g, 0.5617 mmol), and a Teflon coated stir bar were introduced into a 15 mm Solv- Seal reaction tube with a vacuum adaptor in the dry box. The reaction tube was evacuated on the vacuum line and ammonia (~ 3 ml) was condensed into the flask at -196°C. The reaction mixture was warmed to the boiling point, stirred, and degassed several times until hydrogen evolution ceased. Within 10 min. of stirring the color of the solution changed from dark blue to an olive green slurry and most of the non-condensable gas had evolved. Yield 2 H = 0.5224 mmol, 93%) 120 Ammonia was transferred to the solvent trap and the light green powder dried under vacuum. Coordinated ammonia was removed at 150°C. Anal. Calc’d for YbB1 0 H10: Yb, 59.42%; B, 37.12%; H, 3.46%. Found: Yb, 59.15%; B, 35.73%; H, 3.40%. Infrared spectrum of (NH3 )xYbB 1 0 H 1 0 (nujol, KBr plates): y(N-H) 3346(w) cm'1; i/(B-H) 2471 (s,broad) cm'1; i/(Yb-H-B) 1594(vw) cm'1; i/(B-B) 1076(w), 1021 (w) cm'1. Infrared spectrum of desolvated powder: i/(B-H) 2535(w), 2471 (s), 2447(s) cm'1; i/(Yb-H-B) 1734(m) cm'1; i/(B-B) 1076(w), 1023(w) cm'1. 11B NMR spectrum in d5-py (303 K, 5 BF3 OEt2 = 0.00 ppm): 6 = 1.9 ppm (d, JB_H = 130 Hz), -26.2 ppm (d, broad). 1H NMR spectrum in d5-py (303 K, 6 TMS = 0.00 ppm): 6 = 5.1 ppm (quartet, JB_H = 141 Hz), 1.8 ppm (br. multiplet). 4. Preparation of EuB10H10 In the dry box a 15 mm Solv-Seal reaction tube equipped with a Teflon coated stir bar and Kontes/ground glass vacuum adaptor was charged with Eu ingot (0.0785 g, 0.5166 mmol) and [HNEt 3 ]2 B1 0 H 1 0 (0.1633 g, 0.5062 mmol). Ammonia (~ 3 ml) was condensed into the evacuated reaction tube at -196°C. Upon warming to the boiling point the mixture became slate gray but after 10 min., the solution turned deep blue. Eventually the solution became a pale green slurry after degassing the reaction tube. The product was shaken, frozen at -196°C, and degassed several times until all non-condensable gases had evolved. The non- condensable gas collected in the Toepler system was confirmed as 2 H by gas mass spectral analysis. Yield H 2 = 0.4717 mmol, 93%. Ammonia was removed 121 on the vacuum line leaving a greenish-white powder. Coordinated NH3 and NEt 3 were removed by heating the powder to 120°C under vacuum. Anal. Calc’d for EuB1 0 H10: Eu, 56.25%; B, 40.02%; H, 3.73%. Found: Eu, 55.55%; B, 38.21%; H, 3.46%. Infrared spectrum (nujol, KBr plates): v(N-H) 3355(w) cm'1; i/(B-H) 2455(s,broad) cm*1; i/(B-B) 1077(w), 1020(w) cm'1. Infrared spectrum of desolvated powder: v(B-H) 2450(w,sh), 2529(w,sh), 2504(s), 2474(s,sh), 2455(s), 2438(s) cm'1; v(Eu-H-B) 1728(w) cm*1; v(B-B) 1077(w), 1020(w) cm'1. 5. Preparation of (CH 3 CN)xYb(BH 3 CN ) 2 Ytterbium dichloride was prepared from Yb (0.1947 g, 1.125 mmol) and NH4CI (0.1203 g, 2.249 mmol) in liquid ammonia. The light green ammonia- solvated solid product was washed with acetonitrile to remove ammonia from the insoluble (NH3 )xYbCI2. A 50 ml round bottomed 9 mm Solv-Seal flask containing (CH3 CN)xYbCI 2 was charged with NaBH3CN (0.1357 g, 2.159 mmol) and attached to a vacuum extractor. Acetonitrile (10 ml) was condensed into the evacuated reaction flask at -78°C, warmed to room temperature and stirred for 2h. The solution immediately changed from colorless to a bright orange color and eventually became reddish-orange. The mixture was filtered to separate the green insoluble solid from the red-orange solution. Removal of acetonitrile from the filtrate left only a deep red-orange oily residue in the collection flask. Orange crystals were obtained from a saturated solution stored at -30°C in an inert atmosphere but were only weakly diffracting. X-ray quality single crystals were not 122 isolated. Infrared spectrum (CH3 CN, KBr plates): v(B-Hstr) 2336(s), 2305(m,sh), 2272(m), 2256(w), 2219(w) cm'1; i/(C=N) 2181 (s) cm'1; j/(B-Hbend) 2000(vw,br), 1123(m)cm'1. 11B NMR spectrum in CD3CN (303 K, 6 BF3 OEt2 = 0.00 ppm): 6 = -39.9 ppm (quartet, JB_H = 8 8 Hz). 1H NMR spectrum in CD3CN (303 K, 6 TMS = 0.00 ppm): 6 = -0.55 ppm (quartet, broad). 6 . Preparation of (C 5 H 5 N)xYb(BH 3 CN ) 2 Ytterbium dichloride was prepared from Yb (0.3026 g, 1.749 mmol) and NH4CI (0.1841 g, 3.442 mmol) in liquid ammonia. The light green ammonia- solvated powder immediately turned purple when washed with pyridine. A 50 ml round bottomed 9 mm Solv-Seal flask containing (C 5 H5 N)xYbCI 2 was charged with NaBH3CN (0.2112 g, 3.360 mmol) and attached to a vacuum extractor. Pyridine was condensed into the evacuated flask at -78°C. The pale purple insoluble solid was filtered from the deep purple solution after stirring the reaction mixture for 3h at room temperature. Pyridine was removed from the filtrate leaving a pyridine- solvated purple residue in the collection flask. (C5 H5 N)xYb(BH 3 CN) 2 (0.1835 g) was heated to 130°C under dynamic vacuum for 1 h to displace pyridine from the sample. The red pyrolysis residue weighed 0.0840 g (0.3324 mmol Yb(BH3 CN)2) with a loss of 0.0995 g (1.258 mmol) pyridine. The pyridine of solvation, x, was calculated to be 3.78, hence, the overall formula (C5 H5 N) 3 7 8 Yb(BH3 CN)2. Several attempts to isolate x-ray quality single crystals were unsuccessful. Infrared spectrum (nujol, KBr plates): v(B-Hstr) 2338(s), 2283(w,sh) cm'1; 123 i/(C=N) 2171 (s) cm'1; i/(B-Hbend) 1217(m), 1117(m) cm'1. 11B NMR spectrum in d5-py (303 K, 6 BF3 OEt2 = 0.00 ppm); 6 = -43.9 ppm (quartet, JB_H = 81 Hz). 1H NMR spectrum in d5-py (303 K, 6 TMS = 0.00 ppm): 1.85 ppm (s), 1.60 ppm (v.broad). 7. Preparation of (NH3)xYb(NH2BH3)2 In the dry box ytterbium (0.1690 g, 0.977 mmol) and NH 3 BH3 (0.0597 g, 1.935 mmol) were introduced into a 15 mm Solv-Seal reaction tube equipped with a Teflon stir bar and Kontes/ground glass vacuum adaptor. Ammonia (~ 5 ml) was condensed into the evacuated tube at -196°C then warmed to the boiling point and stirred. The dark blue solution became yellow-green after stirring and degassing the flask several times. Most non-condensable gas evolution occurred within the first 15 min. of stirring and ceased after warming and stirring the solution the fourth time. The non-condensable gas was identified as H2 by gas mass spectral analysis. Yield H 2 = 0.8580 mmol, 89%. Removal of ammonia resulted in a tarry yellow-brown residue. The product formed a blue-green solution in pyridine but readily decomposed with gas evolution to a pale orange gel in acetonitrile. X-ray quality single crystals could not be isolated from a saturated solution of the product in pyridine. A sample of pyridine solvated product (0.1815 g) was heated to 230°C in a pre-treated tantalum capsule in a quartz pyrolysis tube. Hydrogen produced (2.422 mmol) was measured in the Toepler pump system and pyridine displaced (0.0914 g) was determined from the weight 124 of the final product (0.0852 g). The formula for this sample, (C5 H5 N)2. 3 9 Yb(NH2 BH3)2, was calculated from the amount of H 2 and pyridine displaced during pyrolysis. Anal. Calc’d for YbB 2 C 1 0 H2 0 N4: Yb, 44.26%; B, 5.53%; C, 30.72%; H, 5.16%; N, 14.32%. Found: Yb, 44.3%; B, 5.52%; C, 30.64%; H, 5.08%; N, 14.3%. Infrared spectrum of (C5 H5 N) 2 3 gYb(NH 2 BH3 ) 2 (nujol, KBr plates): v(N-H) 3372(vw), 813(w) cm*1; v(B-H) 2334(w,br), 2235(m,sh), 2189(s), 2136(s), 2031 (w.sh); i/(C 5 H5 N) 1594(m), 1213(w), 1167(m), 1146(m,sh), 1064(m), 1033(m), 1000(m), 753(m), 723(s), 702(m), 617(w) cm*1. 11B NMR spectrum in d5- py (303 K, 6 BF3 OEt2 = 0.00 ppm): -19.9 ppm (quartet, JB H = 8 6 Hz). 1H NMR spectrum in d5-py (303 K,6 TMS = 0.00 ppm): 3.00 ppm (quartet, JB_H = 87 Hz), 0.7 ppm (s). 8. Preparation of (NH3)xEu(NH2BH3)2 A 15 mm Solv-Seal reaction tube with Teflon stir bar and Kontes/ground glass vacuum adaptor was charged with europium ingot (0.1194 g, 0.7857 mmol) and NH 3 BH3 (0.0484 g, 1.568 mmol). Liquid ammonia (5 ml) was condensed into the reaction tube at -196°C then warmed to the boiling point. The deep blue solution was stirred for 1 0 min. before freezing it and degassing non-condensable reaction products. Only a small amount of gas had evolved. The mixture was warmed, stirred and degassed fifteen times over a 3h period before all gas had been produced. Slowly the solution changed from deep blue to blue-green and each time the flask was degassed more yellow residue deposited on the side of 125 the reaction tube. Yield 2H = 0.6876 mmol, 8 8 %. Ammonia was removed leaving a brownish-green solid. An elemental analysis sample of (C 5 H5 N) 1 g0 Eu(NH2 BH3 ) 2 was prepared by washing the product in pyridine, filtering out insoluble solids, and drying under vacuum. Anal. Calc’d for EuB 2 Cg 5 0 Hig 5 0 N3g0: Eu, 41.98%; B, 5.97%; C, 31.53%; H, 5.43%; N, 15.08%. Found: Eu, 41.45%; B, 6.03%; C, 31.19%; H, 5.17%; N, 14.84%. Infrared spectrum (nujol, KBr plates): y(N-H) 3366(w), 3317(w) cm'1; i/(B-H) 2281 (m, broad), 2184(s, broad), 2036(m, broad) cm'1; i/(Eu-H-B) 1733(s) cm'1. 9. Preparation of (NH3)xYb(B3H8)2 from NH4B3H8 and Yb in NH3 A 15 mm Solv-Seal reaction tube equipped with a Teflon stir bar and Kontes/ground glass vacuum adaptor was charged with NH 4 B3 H8 (0.0921 g, 1.574 mmol) and ytterbium metal (0.1383 g, 7.992 mmol). Liquid ammonia ( 6 ml) was condensed into the evacuated reaction vessel at -196°C. Upon warming to the boiling point the mixture changed from deep blue to a clear reddish-yellow solution. The product was stirred, cooled to -196°C and degassed three times until non-condensable gas was no longer produced. The non-condensable gas was identified as H 2 by gas mass spectral analysis. Yield H 2 = 0.7496 mmol, 95%. A yellowish-green oil remained in the reaction tube after the ammonia was removed. The green oil turned orange after dissolving in acetonitrile but returned to the green oil again when it was dried under vacuum for 30 min. Infrared spectrum (CD3 CN): i/(B-Ht) 2444(s) cm'1; i/(B-Hb) 2120(m), 2082(m) cm'1. 126 11B NMR spectrum in CDgCN (303 K, 6 BFg OEtg = 0.00 ppm): -25.1 ppm (nonet, Jb_h = 33 Hz). 1H NMR spectrum in CDgCN (303 K, 6 TMS = 0.00 ppm): 0.2 ppm (decet, v.broad, JB_H = 29 Hz). 10. Preparation of (NH3)xEu(B3H8)2 from NH4B3H8 and Eu in NH3 In the dry box NH4 B3 H8 (0.1025 g, 1.751 mmol) and europium metal (0.1345 g, 0.8851 mmol) were introduced into a 15 mm Solv-Seal reaction tube equipped with a Teflon stir bar and Kontes/ground glass vacuum adaptor. Liquid ammonia (~ 5 ml) was condensed into the flask at -196°C and warmed to the boiling point. The deep blue solution turned green with stirring for 10 min. The mixture was stirred, frozen and degassed three times until all non-condensable gas had evolved. Hydrogen was confirmed by gas mass spectral analysis. Yield H2= 0.7999 mmol, 91%. Transfer of ammonia to the solvent trap left a green solid in the flask which became an oil after 30 min. of vacuum drying. The solvated solid, however, is stable under static vacuum at -78°C for 1 week. Infrared spectrum of (NH3 )xEu(B 3 H8 ) 2 (fluorolube, KBr plates): i/(N-H) 3368(m), 3282(w,sh); y(B-H) 2453(s,sh), 2387(s), 2303(s,br), 2120(m); j/(Eu-H-B) 1727(m) cm'1. 11B NMR spectrum in CD3CN (303 K, 6 BF3 OEt2 = 0.00 ppm): -35.7 ppm (s,v.broad). 1H NMR spectrum in CDgCN (303 K, 6 TMS = 0.00 ppm): -3.03 ppm (s,v.broad). 127 11. Preparation of (CH3CN)xYb(B3H8)2 by metathesis A sample of (CH 3 CN)xYbCI 2 was prepared in a 50 ml Solv-Seal reaction flask equipped with a Teflon stir bar and Kontes/ground glass vacuum adaptor. Ytterbium metal (0.1505 g, 0.8697 mmol) and NH4CI (0.0913 g, 1.707 mmol) were introduced into the reaction flask and stirred in liquid ammonia (5 ml). Over a 1h period, the mixture was stirred at the boiling point, frozen at -196°C and degassed three times during which the solution changed from deep blue to a mustard yellow slurry. Ammonia was removed leaving a pale green powder. The product was washed three times with CH3CN then dried under vacuum. The reaction flask was charged with KB3 H8 (0.0347 g, 0.4359 mmol) in the dry box and attached to a vacuum line extractor. Acetonitrile was condensed into the flask at -78°C and warmed to room temperature. The solution immediately changed from colorless to orange, but a large portion of the green solid was insoluble. The reaction proceeded for2 h before filtering the green solid from the bright orange solution. Most of the potassium chloride metathesis product remained soluble in the acetonitrile. Many attempts to isolate single crystals from a concentrated solution of (CH3 CN)xYb(B 3 H8 ) 2 were unsuccessful. Infrared spectrum of desolvated product (nujol, KBr plates): i/(B-Ht) 2475(m), 2429(s), 2344(m,sh), 2331 (s) cm*1; v(B-Hb) 2131 (m), 2096(m) cm*1. 11B NMR spectrum in CD3CN (303 K, 6 BF3 OEt2 = 0.00 ppm): -27.8 ppm (nonet, JB_H = 33 Hz). 1H NMR spectrum in CD3CN (303 K, 5 TMS = 0.00 ppm): 0.00 ppm (decet, JBH = 33 Hz). 128 12. Preparation of (C5H5N)xYb(B3H8)2 by metathesis A sample of (NH 3 )xYbCI 2 was prepared as above using ytterbium metal (0.0554 g, 0.3202 mmol) and NH4CI (0.0337 g, 0.6300 mmol) in liquid ammonia. The light green powder immediately turned purple when it was washed two times with pyridine then dried under vacuum. In the dry box RbB3 H8 (0.0760 g, 0.6033 mmol) was introduced into the reaction flask and attached to a vacuum extractor. Pyridine (10 ml) was condensed into the flask at -78°C and dissolved most of the ytterbium dichloride. The mixture was stirred for 2h at room temperature then filtered under vacuum to separate the purple solid from the deep purple solution. Most of the RbCI was soluble in pyridine and could not be cleanly separated from the ytterbium complex. Pyridine was removed from the sample. The desolvated solid turned from purple to yellow under dynamic vacuum. A small sample of solid was transferred to an NMR tube and dissolved in fresh pyridine regenerating the purple pyridine-solvated ytterbium complex. The remainder of the sample was heated in a sand bath at 200°C under vacuum for 3h. 11B NMR spectrum of (C5 H5 N)xYb(B 3 H 8 ) 2 in d5-py (303 K, 6 BF3 OEt2 = 0.00 ppm): -30.0 ppm (nonet, JB_H = 33 Hz). 11B NMR (C5 H5 N)xYb(B 3 H8 ) 2 heated to 200°C in d 5 -py: -13.2 ppm (quartet, JB H = 97 Hz), -30.4 ppm (nonet, JB_H = 33 Hz). 13. Reaction of B10H14 with Yb in NH3 A 15 mm Solv-Seal reaction tube equipped with a Teflon stir bar and Kontes/ground glass vacuum adaptor was charged with B 1 0 H 1 4 (0.1164 g, 129 0.9524 mmol) and ytterbium metal (0.0831 g, 0.4802 mmol) in the dry box. Liquid ammonia (3 ml) was condensed into the flask at -196°C and warmed to the boiling point. A light murky light green solution formed and a brownish oily solid settled to the bottom of the tube. Three stir/freeze/degas cycles were required to remove all non-condensable gases. Yield H2 = 0.2147 mmol, 45% (based on complete conversion to (NH3 )xYb(B 1 0 H13)2. Ammonia was transferred to a solvent trap and the brown oily product became a flaky, shiny purplish-brown solid. Additional non- condensable gas evolved as the solid dried under dynamic vacuum. 11B NMR spectrum in CD3CN (303 K, 6 BF3 OEt2 = 0.00 ppm): 7.0 ppm (d, JBH = 133 Hz), 2.5 ppm (w, broad), -4.7 ppm (d, JB_H = 131 Hz), -14.3 ppm (d, JB H = 161 Hz), -19.7 ppm (d, broad), -22.3 ppm (s), -25.7 ppm (d, JB_H = 119 Hz), -31.1 ppm (t, JB_H = 106 Hz), -35.0 ppm (d, JB_H = 134 Hz), -39.8 ppm (d, JB_H = 132 Hz). 14. Reaction of [HNMe3]BgH14 with Yb in NH3 In the dry box ytterbium metal (0.0805 g, 0.4652 mmol), [HNMe3 ]2 BgH 1 4 (0.1624 g, 0.9468 mmol), and a Teflon coated stir bar were introduced into a 15 mm Solv-Seal reaction tube. Liquid ammonia (3 ml) was condensed into the flask at -196°C. The mixture was warmed and stirred until the ammonia boiled then cooled to -78°C to prevent a build-up of pressure in the reaction vessel. After stirring for 10 min., the mixture was cooled to -196°C and degassed. Non- condensable gases were collected in the Toepler system. The cycle was repeated 130 two additional times until all reaction gases had been collected. Yield H2 = 0.4652 mmol, 47%. The color of the solution changed from yellow-green to mustard yellow as the reaction progressed. Removal of ammonia produced an oily green residue.A small sample of the oil transferred to an NMR tube dissolved in CDgCN producing a clear orange solution. Some brownish-green solid deposited on the bottom of the tube. 11BNMR spectrum in CD3CN (303 K, 6 BF3 OEt2 = 0.00 ppm): 10.0 ppm (d, broad), -0.2 ppm (d, broad), -8.1 ppm (d, JB H = 144 Hz), -9.9 ppm (d, JB_H = 140 Hz), -12.5 ppm (s), -14.7 ppm (d, JB H = 140 Hz), -20.5 ppm (d, JB H = 135 Hz), -22.4 ppm (s), -23.7 ppm (d, JB_H = 139 Hz), -26.9 ppm (d, JB_H = 137 Hz), -29.4 ppm (nonet, JB_H = 33 Hz), -53.0 ppm (d, JB_H = 127 Hz). 15. Pyrolysis of (CH3CN)xYb(BH4)2 Acetonitrile solvated ytterbium dichloride was prepared from ytterbium metal (0.1313 g, 0.7588 mmol) and NH4CI (0.0803 g, 1.501 mmol) in liquid ammonia followed by washing with dry acetonitrile. The metathesis reaction with NaBH4 (0.0548 g, 1.448 mmol) in acetonitrile (10 ml) produced a clear orange solution which when filtered and dried became a bright yellow solid. A sample of (CH 3 CN)xYb(BH 4 ) 2 was loaded into a pre-treated tantalum capsule and introduced into a quartz reaction tube in the dry box. The tube was evacuated, heated slowly to 250°C to remove solvent and hydrogen, and maintained at this temperature for 2 h. Non-condensAble gas was isolated in the 131 Toepler system and condensable products were trapped at -196°C. The temperature was raised to 550°C while maintaining 1 atm argon pressure in the pyrolysis tube. The sample was heated at this temperature for6 h before cooling. X-ray powder diffraction of the black pyrolysis product confirmed the presence of pure YbB2. X-ray powder data (d-spacings): 3.77(m), 2.84(s), 2.26(s), 1.88(w), 1.63(m), 1.56(w), 1.50(w), 1.42(w), 1.32(w), 1.23(w), 1.14(w), 1.13(w), 1.06(w), 1.02(w), 0.99(w). Powder Data File d-spacings for YbB 2 (l/l0): 3.74(50), 2.82(80), 2.25(100), 1.86(10), 1.62(20), 1.55(30), 1.49(10), 1.32(20), 1.23(20), 1.14(10), 1.12(10), 1.06(10), 1.02(20), 0.99(6). 16. Pyrolysis of (CH3CN)xEu(BH4)2 Acetonitrile solvated europium dichloride was prepared from europium ingot (0.1449 g, 0.9535 mmol) and NH4CI (0.1009 g, 1.886 mmol) in liquid ammonia. The product was washed with acetonitrile and dried under vacuum. Sodium borohydride (0.0688 g, 1.819 mmol) was introduced into the reaction flask in the dry box and stirred with (NH 3 )xEuCI 2 in acetonitrile for2 h. Europium borohydride was isolated by filtering the reaction mixture and removing solvent from the greenish-yellow solution leaving a bright yellow solid. A sample of (CH 3 CN)xEu(BH 4 ) 2 powder (0.9788 g) was introduced into a pre-treated tantalum capsule and inserted into a quartz furnace tube. The evacuated tube was slowly heated while trapping out condensable products at -196°C and non-condensable gases in the Toepler system. Non-condensable gas 132 began to evolve in small quantities at 100°C with a drastic increase in evolution rate between 250-260°C. All volatile products were removed at 400°C. Yield H2 = 1.918 mmol. The formula for the solvated powder, (CH3 CN) 0 i8 5 Eu(BH4)2, was calculated from the weight of the original sample and the amount of hydrogen lost during decomposition. Europium boride was only slightly crystalline after heating for 1 2 h at 700°C but distinctly more crystalline after heating at 1000°C for an additional 12h. The x-ray powder diffraction pattern was identical to data reported for EuB6 in the Powder Data File.6 3 X-ray powder data (d-spacings): 4.15(m), 2.93(s), 2.39(m), 2.07(w), 1.86(m), 1.70(w), 1.47(w), 1.44(m), 1.32(w), 1.26(w). Powder Data File d-spacings for EuB 6 (l/l0): 4.21(50), 2.98(100), 2.42(80), 2.08(80), 1.87(100), 1.71(80), 1.48(50), 1.39(100), 1.32(80), 1.25(80). 17. Pyrolysis of (NH3)xYbB12H12 A sample of (NH 3 )xYbB 1 2 H 1 2 (0.0887 g) was introduced into a pre-treated tantalum capsule in the glove box and inserted into a quartz furnace tube. The tube was evacuated and slowly heated while monitoring production of volatile reaction products. Trimethylamine and ammonia collected in a solvent trap at -196°C while hydrogen was isolated in the Toepler system. Gas mass spectra confirmed the identity of gaseous products. Heating the light green solid to 125°C removed most of the solvent from the sample. Hydrogen evolution began at 170°C and continued until 665°C. The tube was heated to 1000°C and maintained at this temperature under dynamic vacuum for 24h. Ytterbium metal sublimed 133 from the sample and deposited on the walls of the reaction tube at the furnace opening. The grayish-black solid, YbB6, was confirmed by x-ray powder diffraction. X-ray powder data (d-spacings): 4.11 (s), 2.91 (vs), 2.38(m), 2.07(mw), 1.85(m), 1.69(mw), 1.46(w), 1.38(mw), 1.31 (w), 1.25(w). Powder Data File d-spacings for YbB 6 (l/l0): 4.14(60), 2.92(100), 2.39(35), 2.07(20), 1.85(45), 1.69(25), 1.46(8), 1.38(20), 1.31(14), 1.25(10). A second sample (0.0316 g) was heated as above. Ammonia and NMe 3 were removed before expanding 1 atm argon gas into the tube at 500°C. The reaction tube was left opened to the mercury blowout while heating the sample slowly to 1000°C to accommodate excess pressure build-up. The sample was heated at 1000°C for 17h under 1 atm argon in a closed reaction tube. X-ray powder diffraction confirmed YbB6 as the only stable phase formed from pyrolysis. Excess boron was amorphous. 18. Pyrolysis of (NH 3 )xEu(B 3 H8 ) 2 A pre-treated tantalum capsule was charged with a sample of (NH3 )xEu(B 3 H8 ) 2 (0.0926 g) and inserted into a quartz pyrolysis tube. The reaction tube was evacuated and heated slowly while trapping out condensable gases at -196°C and toeplerizing non-condensable gas. Decomposition began at 60°C with evolution of both hydrogen and ammonia. At 90°C the ammonia pressure in the mercury blowout had decreased substantially. At 115°C a white solid sublimed onto the walls of the solvent trap. All ammonia had been trapped 134 by 140°C but hydrogen continued to evolve until 800°C. The temperature of the furnace was maintained at 950°C for 12h. A yellowish-brown residue deposited on the tube walls just outside the tantalum capsule. The remainder of the sample in the capsule, a clumpy grayish-black solid, was analyzed by x-ray powder diffraction. A very weak pattern for EuB6 was found. Gas mass spectral analysis confirmed hydrogen and ammonia as the non-condensable and condensable fractions, respectively. The solvent trap was warmed to room temperature, removed from the vacuum line, and washed with 10 ml of acetonitrile. The white solid was somewhat soluble in acetonitrile. The acetonitrile soluble and insoluble portions were analyzed by NMR spectroscopy. The 11B NMR spectrum of the acetonitrile insoluble solid in THF contained no boron resonances. 11B NMR spectrum of white acetonitrile soluble solid in CH3CN (303 K, 5 BF3 OEt2 = 0.00 ppm): 20.2 ppm (s), -12.8 ppm (multiplet, broad), -19.9 ppm (d, JBH = 99 Hz), -22.4 ppm (quartet, JB_H = 92 Hz), -29.7 ppm (nonet, JB_H = 33 Hz). 19. Pyrolysis of (C5H5N)xYb(BH3CN)2 A sample of (C 5 H5 N)xYb(BH 3 CN ) 2 (0.1835 g) was introduced into a 15 mm Solv-Seal reaction tube in the dry box. The flask was evacuated and slowly heated to 130°C in a sand bath for 1 h. Pyridine was collected in the solvent trap and a negligible quantity of non-condensable gas was isolated in the Toepler system. The formula of the solvated solid, (C5 H5 N) 3 7 8 Yb(BH3 CN)2, was calculated from the weight of the original sample and the weight loss of coordinated pyridine 135 (0.0995 g, 1.258 mmol). 20. Pyrolysis of (C5H5N)xYb(NH2BH3)2 A sample of the purple solid (0.1815 g), (C5 H5 N)xYb(NH 2 BH3)2l was introduced into a pre-treated tantalum capsule and inserted into a quartz furnace tube in the dry box. The tube was evacuated and heated slowly while freezing condensable gases in the solvent trap at -196°C and isolating non-condensable gases in the Toepler system. At 60°C the first non-condensable gas evolved from the sample at a slow rate. The rate of gas production increased between 100°C and 150°C then declined until all had been removed by the time the furnace temperature reached 230°C. Yield2 H = 2.422 mmol, 0.0049 g. The sample was heated for 1 2 h at 950°C. The amount of solvating pyridine in the original sample was determined by subtracting the weight of hydrogen from the weight loss of the sample (0.0963 g). Yield C5 H5N = 0.0914 g, 1.155 mmol. The formula for the solvated purple solid was calculated from this data, (C 5 H5 N)2 3 gYb(NH2 BH3)2. During pyrolysis the purple powder turned to a black solid. A small amount of opaque yellow residue deposited on the walls of the quartz tube outside the tantalum capsule and a thin brown ring deposited on the walls of the tube near the opening of the furnace. The x-ray powder diffraction pattern of the black pyrolysis product agreed well with data for Yb2 0 3 from the literature. 136 21. Thin film studies of YbB2 Initially, 8 mm x 10mm pieces of pre-treated tantalum foil were dipped into a 1% solution of (CH 3 CN)xYb(BH 4 ) 2 in the dry box. Samples were introduced into a quartz furnace tube and evacuated. The temperature was slowly raised to 330#C while pumping away volatiles. The one-coating sample was heated at 550#C under 1atm of argon for 12h. The three-coating sample was heated to 330° C, cooled and recoated. When all coatings had been applied was the sample heated to 5509 C. The scanning electron micrographs and x-ray microanalysis were obtained. The coating procedure was modified by using an air brush to apply the borohydride solution. A lecture bottle of ultra-high purity nitrogen gas was attached to the air brush via a gas gauge with an adaptor compatible with the air brush hose. The air brush apparatus was assembled in the dry box. The 1% solution was poured into the glass chamber of the air brush and sprayed onto tantalum and tungsten foil pieces using the nitrogen in the lecture bottle as the propellant. One-, eight-, and twenty-coating samples on tantalum were heated to 3309 C after each application and then to 5509 C when the last coating had been applied. An eight-coating film on tungsten was also prepared. Scanning electron micrographs were obtained at a 10kV accelerating voltage. The x-ray microanalysis plot of the black YbB 2 deposits at a 3kV accelerating voltage contained boron and ytterbium peaks. Background oxygen in the microscope was unavoidable as well as residual carbon. Tantalum and tungsten peaks were not 137 detected in these areas. 22. Thin film studies of EuBg An 8mm x 10mm piece of pre-treated tantalum foil was dipped twice in a 3% solution of (CH3CN)xEu(BH4)2- The sample was introduced into a quartz furnace tube and evacuated. 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