Synthesis of LiBC4N4, BC3N3, and related C-N compounds of boron: New precursors to light element ceramics

Article abstract of DOI:10.1021/ja0006752

The synthesis of a novel framework cyanide with composition LiBC₄N₄ (1) is described. X-ray powder diffraction as well as ⁷Li, ¹¹B, and ¹³C NMR of 1 reveal an ordered structure which consists of two interpenetrating diamond-like networks of BC₄ and LiN₄ tetrahedra linked by C-N bonds. The structure is simple cubic, symmetry P43m, a = 5476(3) A. In solution the presence of the B(CN)₄ anion has been established for the first time by spectroscopic methods. Molecular B(CN)₃ is isolated for the first time as a Lewis acid-base complex with bases such as NMe₃ and SiMe₃CN, and a single-crystal X-ray structure of B(CN)₃NMe₃ (3) has been obtained. The novel solid-state cyanide BC₃N₃ (2) is prepared from the thermal elimination of NCSiMe₃ from B(CN)₃NCSiMe₃ (4). In route to 4, compounds B(CN)₂(SMe)NCSiMe₃ (5) and B(CN)₂(SMe)-NMe₃ (6) have also been isolated. A single-crystal X-ray structure of 6 reveals that the compound crystallizes in the monoclinic space group P2₁/n. The new crystalline solids 2 and 1 (isoelectronic to C) are promising precursors for high-pressure and high-temperature syntheses of novel phases with structures related to Si₃N₄, diamond, and graphite.

Full text of DOI:10.1021/ja0006752

J. Am. Chem. Soc. 2000, 122, 7735-7741
7735
Synthesis of LiBC4N4, BC3N3, and Related C-N Compounds of
Boron: New Precursors to Light Element Ceramics
Darrick Williams,† Brett Pleune,† J. Kouvetakis,*,† M. D. Williams,† and R. A. Andersen‡
Contribution from the Departments of Chemistry, Arizona State UniVersity, Tempe Arizona 85287, and
UniVersity of CaliforniasBerkeley, Berkeley, California 94720
ReceiVed February 24, 2000
Abstract: The synthesis of a novel framework cyanide with composition LiBC4N4 (1) is described. X-ray
7
powder diffraction as well as Li, 11B, and 13C NMR of 1 reveal an ordered structure which consists of two
interpenetrating diamond-like networks of BC4 and LiN4 tetrahedra linked by C-N bonds. The structure is
simple cubic, symmetry P43hm, a ) 5476(3) Å. In solution the presence of the B(CN)4 anion has been established
for the first time by spectroscopic methods. Molecular B(CN)3 is isolated for the first time as a Lewis acid-
base complex with bases such as NMe3 and SiMe3CN, and a single-crystal X-ray structure of B(CN)3NMe3
(3) has been obtained. The novel solid-state cyanide BC3N3 (2) is prepared from the thermal elimination of
NCSiMe3 from B(CN)3NCSiMe3 (4). In route to 4, compounds B(CN)2(SMe)NCSiMe3 (5) and B(CN)2(SMe)-
NMe3 (6) have also been isolated. A single-crystal X-ray structure of 6 reveals that the compound crystallizes
in the monoclinic space group P21/n. The new crystalline solids 2 and 1 (isoelectronic to C) are promising
precursors for high-pressure and high-temperature syntheses of novel phases with structures related to Si3N4,
diamond, and graphite.
Introduction
stoichiometric cyanides of boron, B(CN)3 and LiB(CN)4, present
attractive targets since they are the molecular analogues of the
extended stoichiometric solids C3N3B and LiBC4N4. The long-
term goal is to use high pressure to transform these compounds
into denser, stronger, and harder forms while maintaining their
stoichiometry. In this study we present for the first time the
synthesis of a novel solid-state cyanide material with composi-
tion C3N3B (2) and a crystalline derivative with the exact
stoichiometry LiBC4N4 (1). These compounds are fundamentally
interesting (they are typical examples of classical inorganic
compounds with unique solid-state structures), but more im-
portantly preliminary results indicated that they are also suitable
as precursors for the proposed high-pressure studies.
Surprisingly, the existence of pure and crystalline LiBC4N4
(1) has not been established to date; previous attempts have
only produced uncharacterized materials with a formula purport-
edly close to that of the desired product.9 A molecular form of
B(CN)3 has also been known for a long time; however, the
synthetic procedure is impractical and has not been reproduced.
The reported preparation involved the reaction of AgCN with
BCl3 which was allowed to take place, unintentionally, over
the course of 40 years. This experiment has not been since
duplicated, to our knowledge.10
Recent research on the development of new superhard
materials has stimulated interest in the design and synthesis of
first-row element (Li, Be, B, C, N, O) compounds that are
isoelectronic to diamond or have compositions related to that
of Si3N4.1,2 Typical examples of such systems include metastable
forms of planar (sp2) and tetrahedral (sp3) carbon nitride with
C3N4 stoichiometry3-5 and a phosphorus-substituted derivative
with C3N3P composition.6 The discovery of a related material
with composition C3N3B and structure similar to that of the
hypothetical â-C3N4 model is of interest because it is expected
to be more stable and stronger than C3N4. A three-dimensional
C3N3B structure would incorporate C in the tetrahedral sites,
while sp2-hybridized B replaces one-fourth of the trigonal N
sites. There are several advantages to such an arrangement: (a)
elimination of the destabilizing N-N lone pair interaction that
exists in the hypothetical â-C3N4 model 7,8 and (b) an increase
in cohesive energy because of the stronger B-C bonds.
Furthermore, the development of the LiBC4N4 derivative is of
particular interest because it is isoelectronic to carbon (i.e., four
valence electrons per atom), and it might provide a unique
synthetic route to new hard materials with diamond-like or
graphite-like structures by high-pressure methods.
Our immediate objective is to develop pure and well-
characterized materials with the desired elemental compositions
C3N3B (2) and LiBC4N4 (1) by soft chemistry methods. The
Results and Discussion
The synthetic methodology used for the synthesis of 1 utilizes
a common reagent, SiMe3CN, which undergoes metathesis with
LiBF4 to produce the desired composition at low temperatures
in nonaqueous solvents (eq 1). The product is initially an
(1) Sung, C. M.; Sung, M. Mater. Chem. Phys. 1996, 1, 47.
(2) Badding, J. V. AdV. Mater. 1977, 9, 877.
(3) Liu, A.; Cohen, M. L. Phys. ReV. Lett. 1990, 245, 841.
(4) Teter, D. M.; Hemley, R. J. Science 1996, 271, 53.
(5) Kouvetakis, J.; Bandari, A.; Todd, M.; Wilkens, B.Chem. Mater.
1994, 6, 811.
4SiMe3CN + LiBF4 f LiBC4N4 + 4SiMe3F
(1)
(6) McMurran, J.; Kouvetakis, J.; Nesting, D. C.; Hubbard, J. L. Chem.
Mater. 1998, 10, 590.
(7) Hughbanks, T.; Tian, Y. Solid State Commun. 1995, 96, 321.
(8) Nesting, D. C.; Badding, J. V. Chem. Mater. 1996, 8, 1535.
amorphous brown solid that crystallizes upon heating at
temperatures in excess of 250 °C. The amorphous and crystalline
solid is readily purified by extraction with dry CH3CN. The
10.1021/ja0006752 CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/01/2000

7736 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Williams et al.
Figure 1. (Top) Structure of LiBC4N4 shown as interconnected LiN4
and BC4 tetrahedra. Black, gray, and white spheres are B, C, and N,
respectively. Striped spheres with dangling bonds at the corners are
the Li atoms. (Bottom) Unit cell of LiBC4N4.
Figure 2. NMR spectra of LiB(CN)4. (Top) 7Li solid state, and solution
(in the offset); (middle) 11B solid state, and solution (in the offset);
(bottom) 13C solution spectra.
pure material is a colorless solid, stable in air, and slightly
soluble in polar organic solvents. Combustion analysis for C,
H, and N is consistent with the proposed LiBC4N4 composition.
The X-ray diffraction pattern revealed a simple cubic tetrahedral
structure with a lattice parameter a ) 5.476(3) Å. Rietveld
refinement indicated an ordered structure in which each Li is
bonded to four N atoms and each B is bonded to four C atoms
to form LiN4 and BC4 tetrahedra. These metal-centered tetra-
hedra are joined at the vertexes by C-N bonds to form two
identical and interpenetrating networks (Figure 1). The observed
and calculated diffraction patterns for this topology of P43hm
symmetry are in good agreement. The C-N bond length was
found to be 1.204(5) Å, which is within the range for metal
cyanides.11,12 The Li-N and B-C bond lengths are 1.982(5)
and 1.555(4) Å, respectively. The structure is similar to that
proposed for LiGa(CN)4 (a ) 5.847), in which LiN4 and GaC4
tetrahedra are also linked by C-N bonds.11
The infrared spectrum shows ν CtN at 2270 cm-1 and a set
of intense absorptions at 995-960, 530, and 400 cm-1 that are
assigned to ν(10B-C-11B-C), ν(Li-N), and B-C (bending),
respectively. The solid-state magic angle spinning (MAS) NMR
spectra confirmed the presence of the CN ligands and revealed
7
single resonances for both 11B and Li at δ -43 and δ -2.83,
respectively, indicating single environments for these atoms in
the solid state. The most plausible interpretation of the NMR
and IR data (only B-C and Li-N absorptions) is that the B
atom is in a single coordination environment in which it is
bonded to only carbon, and the Li is in another environment in
which it is bonded exclusively to nitrogen. These results are
consistent with the ordered framework structure of LiN4 and
BC4 tetrahedra found by X-ray diffraction.
(9) Wittig, G.; Raff, G. Z. Naturforsch. 1951, 6B, 225.
(10) Chaigneau, M. C. R. Hebd. Seances Acad. Sci. 1954, 239, 220. Guy,
J.; Chaigneau, M. Bull. Soc. Chim. Fr. 1956, 257. Evers, E. C.; Freitag, W.
O.; Kriner, W. A.; MacDiarmid, A. G.; Sujishi, S. J. Inorg. Nucl. Chem.
1960, 13, 239.
(11) Brousseau, L. C.; Williams, D.; Kouvetakis, J.; O’Keeffe, M. J.
Am. Chem. Soc. 1997, 119, 6292.
(12) Williams, D. J.; Partin, D. E.; Lincoln, F. J.; Kouvetakis, J.;
O’Keeffe, M. J. Solid State Chem. 1997, 134, 164. Williams, D. J.;
Kouvetakis J.; O’Keeffe, M. Inorg. Chem. 1998, 37, 4617.
The solution NMR spectra of LiBC4N4 (1) demonstrated for
-
the first time the existence of the tetrahedral anion, B(CN)4
with B-C bonds (Figure 2). The 13C spectrum in CD3CN

Synthesis of LiBC4N4, BC3N3, and Related Compounds
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7737
Table 1. Crystal Data for B(CN)2‚(SMe)NMe3 (6) at -130 °C
Table 2. Selected Distances (Å) and Angles (deg) for 6
space group: P21/n
a ) 6.3488(1) Å
b ) 10.7251(6) Å
c ) 13.5604(8) Å
V ) 923.32(8) Å3
mol wt ) 196.06
Distances
R ) 90.00°
F
calcd ) 1.216 g cm-1
S(1)-C(6)
B(1)-C(1)
B(1)-N(3)
C(2)-N(2)
N(3)-C(3)
1.824 (5)
1.594 (9)
1.620 (7)
1.151 (6)
1.497 (6)
S(1)-B(1)
B(1)-C(2)
C(1)-N(1)
N(3)-C(5)
N(3)-C(4)
1.894 (6)
1.600 (9)
1.147 (6)
1.484 (6)
1.512 (6)
â ) 90.45° µcalcd ) 0.292 mm-1
γ ) 90.00° size ) 0.15 × 0.06 × 0.04 mm
Z ) 4
GOF ) 0.970
diffractometer: SMART CCD
radiation: Mo KR ) 0.17069 Å
monochromator: graphite crystal
scan: ω (0.90°/min)
Angles
C(6)-S(1)-B(1)
C(1)-B(1)-N(3)
C(1)-B(1)-S(1)
N(3)-B(1)-S(1)
N(2)-C(2)-B(1)
C(5)-N(3)-C(4)
C(5)-N(3)-B(1)
C(4)-N(3)-B(1)
105.8 (3)
107.7 (4)
111.2 (4)
114.2 (4)
176.8 (5)
108.1 (4)
110.5 (4)
110.6 (4)
C(1)-B(1)-C(2)
C(2)-B(1)-N(3)
C(2)-B(1)-S(1)
N(1)-C(1)-B(1)
C(5)-N(3)-C(3)
C(3)-N(3)-C(4)
C(3)-N(3)-B(1)
110.2 (5)
107.8 (4)
105.7 (4)
177.9 (5)
109.1 (4)
108.3 (4)
110.1 (4)
θ range: 1.5-23.25°
reflns: 3863 total, 1315 independent, 826 observed
Rint: 0.0906
residuals:a R ) 0.0589; wR ) 0.01261
all data: R ) 0.1127
a R ) ∑(||Fo| - |Fc||)/∑|Fo|. wR ) [∑w(|Fo| - |Fc| )2/∑w(Fo )2]1/2
.
2
2
2
showed a single resonance at δ 123 which is split into 1:1:1:1
pattern due to coupling to 11B (S ) 3/2). The observed 13C-11B
coupling constant (71 Hz) is typical of those found in NMR
studies of classical boron alkyl compounds such as BMe3 and
7
3
MeB(OMe)2.13 Coupling of 13C to Li (also S ) /2) is not
observed in this study at 30-60 °C (the reported values of 13C-
7Li are less than 15 Hz13,14). The solution 11B and Li NMR
7
spectra are sharp single resonances at δ -8.5 and δ 0.112,
respectively (Figure 2). The NMR data are consistent with the
presence of only B-C bonds and support the presence of a the
-
anion B(CN)4 and the implication of a solvated cation Li
(NCMe)x in solution.
The successful preparation of 1 encouraged us to develop a
suitable synthetic method for the related B(CN)3. Our initial
attempts to synthesize B(CN)3 involved metathetical exchange
of NMe2 in B(NMe2)3 by cyanide using SiMe3CN as the cyanide
source. Partial displacement is readily obtained at 22 °C, and a
mixture of the B(CN)(NMe2)2 and B(CN)2(NMe2) was formed.
However, complete CN substitution on a single B center could
not be accomplished using this methodology even when high
temperatures and long reactions times were used. These extreme
conditions lead to intractable polymeric materials rather than
molecular solids. Alternatively, the reaction of the related
thiomethoxyborane, B(SMe)3, with SiMe3CN results in complete
displacement of a SMe group by cyanide at 60 °C in solution
to yield B(CN)3NCSiMe3 (4). This is the first example of a
molecular compound that incorporates the elusive B(CN)3 core
(eq 2). At 22 °C the reaction of B(SMe)3 with SiMe3CN yields
the disubstituted derivative complex B(CN)2(SMe)NCSiMe3 (5)
as the major product (eq 3). Compound 5 was characterized by
spectroscopic methods and elemental analysis; see the Experi-
mental Section for details. The B(CN)2(SMe)NMe3 (6) deriva-
tive of 5 was also prepared as a crystalline solid by reaction of
B(CN)2(SMe)NCSiMe3 with NMe3 (eq 4) and was characterized
by single-crystal X-ray diffraction (Tables 1 and 2). An ORTEP
diagram displaying the B(CN)2(SMe) core of 5 and 6 is shown
in Figure 3.
Figure 3. Molecular structure of B(CN)2(SMe)NMe3. The atoms are
50% probability ellipsoids. The hydrogen atoms are omitted for clarity.
proposed empirical formula, and by IR and NMR spectroscopies.
The IR spectra revealed two sharp absorption bands at 2310
and 2232 cm-1 which are assigned to the coordinated ligand,
SiMe3CN, and the terminal cyanides of the B(CN)3 fragment,
respectively (Figure 4). Previous studies have considered the
stretching frequencies of the cyanide ligand in many metal
cyanide compounds and have demonstrated that bridging
cyanide groups exhibit higher frequencies than do terminal
cyanide groups, as we have also observed in this case.15 The
1H NMR spectrum reveals a single resonance corresponding to
the Si-Me3 protons at δ 0.04. In addition, the 13C spectrum
shows a resonance at δ 123 split into a 1:1:1:1 pattern by
coupling to 11B (S ) 3/2), and the 11B spectrum shows a single
resonance at δ 143.
Heating of 4 at 200-250 °C results in complete loss of SiMe3-
CN from the adduct, affording a nonvolatile air-stable solid at
a virtually quantitative yield based on the decomposition reaction
depicted by eq 5. Dissolution of this material with an excess of
SiMe3CN at 80 °C regenerated (SiMe3CN)B(CN)3 in nearly
quantitative yield, indicating that the reaction is reversible.
B(SMe)3 + 4Me3SiCN f
B(CN)3NCSiMe3 {250 °C} BC3N3 + SiMe3CN
(5)
B(CN)3NCSiMe3 + 3Me3SiSMe (2)
80 °C
B(SMe)3 + 3Me3SiCN f
Elemental analysis on BC3N3 showed an insignificant amount
of H, consistent with compete elimination of SiMe3CN, and a
lower than expected N content (see Experimental Section for
B(CN)2(SMe)NCSiMe3 + 2Me3SiSMe (3)
B(CN)2(SMe)‚CNSiMe3 + NMe3 f
B(CN)2(SMe)(NMe3) + CNSiMe3 (4)
(13) Mason, J. Multinuclear NMR; Mason, J., Ed.; Plenum Press: New
York, 1987; p 231.
(14) McKeever, L. D.; Waack, R.; Doran, M. A.; Baker, E. B. J. Am.
Chem. Soc. 1961, 91, 1057.
(15) Kawaguchi, M. AdV. Mater. 1997, 9, 615
The identity of 4 was established by elemental analysis for
C, H, and N, the results of which are consistent with the

7738 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Williams et al.
Figure 6. Schematic of model structures for BC3N3. The structure
shown as a square array of B atoms bridged by CN ligands is also
adopted by tetranuclear clusters such as [R2MCN]4, where R ) allyl
or alkyl and M ) Au, Pd.16
C,N orientational disorder between bridging cyanides in the
structure. The 13C MAS NMR spectrum, on the other hand,
revealed a weak and rather broad C-N resonance centered at
δ 118, but the line width of the signal did not allow determi-
nation of the exact number of C sites with certainty. The
spectroscopic evidence suggests that B is four-coordinated with
two shared and two terminal CN groups associated with each
B atom, an arrangement which is consistent with the idealized
BC3N3 formula. The stability of this material toward hydrolysis
and oxidation is also consistent with boron using its s and p
orbitals in the coordination polymer. Figure 6 displays possible
structures of BC3N3 which are consistent with the stoichiometry
of the compound as well as the IR and NMR data.
Figure 4. Infrared spectra of B(CN)3NCSiMe3, BC3N3, and B(CN)3-
NMe3, showing C-N stretching bands. The higher frequencies bands
for B(CN)3NCSiMe3 and BC3N3 correspond to the bridged CN ligands.
Further evidence for the existence of BC3N3 is provided by
the treatment of the material with trimethylamine (NMe3), which
leads quantitatively to the formation of a volatile and air-stable
molecular adduct with composition B(CN)3NMe3. A correct
1
elemental analysis for this compound was obtained and H
NMR, 13C NMR, and IR spectra showed the presence of the
coordinated NMe3 molecule and the B(CN)3 core. The mass
spectra displayed isotopic envelopes for the molecular ion M+
[M ) B(CN)3NMe3] as the highest mass peak at 148 amu as
well as mass peaks for M+ - Me, M+ - CN, M+ - (CN)2,
Figure 5. EELS spectrum for C3N3B featuring the K-shell ionization
edges for B, C, and N. Inset shows the spectrum after background
subtraction.
+
NMe3 (the strongest peak), B(CN)2+, and (CN)2+. The
experimental isotopic patterns are in excellent agreement with
the calculated patterns for the compound. A single-crystal X-ray
diffraction structure of B(CN)3NMe3 (Tables 3 and 4) reveals
that it is a Lewis acid-base complex between Me3N and
B(CN)3. The compound crystallizes in space group Pnma with
four molecules in the unit cell. Each molecule lies on a
crystallographic mirror plane, and the Me and the CN groups
are staggered. An ORTEP diagram showing the molecular
conformation and the atom numbering scheme is presented in
Figure 7. The B-N bond distance [1.596(3) Å] and the B-C
bond distance [1.594(4) Å] are identical. The C-N bond length
of the cyanide ligand [1.143(3) Å] is within the range found
for similar cyanides,16 and it is slightly shorter than the C-N
bond length in LiB(CN)4 [1.204(5) Å]. The geometry at the B
atom is very close to tetrahedral [109.6(12)-109.03(13)°], and
the BsCtN branches of units are nearly linear [178.90(17)°].
details). This is presumably due to formation of refractory
nitrides during combustion analysis. The material is polycrys-
talline, and all the reflections in the X-ray diffraction pattern
could be indexed using a tetragonal cell with a ) 8.214 Å and
c) 13.832 Å. Electron diffraction confirmed that C3N3B is
polycrystalline. Examination of individual crystallites by electron
energy loss spectroscopy (EELS) revealed only B, C, and N
and verified the absence of oxygen or other impurities. The IR
spectrum (Figure 5) shows sharp absorptions at 2376 and 2250
cm-1 that are assigned to bridging (B-NC-B) and terminal
cyanide groups, respectively, and a strong absorption at 1015-
984 cm-1 corresponding to [(10B-11B)-CN,NC]. The spectrum
is different from that of the black residue10 generated from
decomposition of BF2CN. The solid-state 11B NMR spectrum
revealed two closely spaced and narrow peaks, at δ -35.13,
-40.64, in the ratio of approximately 1:1. This indicates the
presence of at least two possible types of B sites and suggests
(16) Dunbar K. R.; Heinz, R. A. Prog. Inorg. Chem. 1997, 45, 283.

Synthesis of LiBC4N4, BC3N3, and Related Compounds
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7739
Table 3. Crystal Data for B(CN) ‚NMe3 (3) at -130 °C
3
space group: Pnma
a ) 12.0366(15) Å
b ) 10.2805(13) Å
c ) 6.9085(9) Å
V ) 854.0(4) Å3
Z ) 4
mol wt ) 147.97
F
calcd ) 1.151 g cm-3
µcalcd ) 13.81 cm-1
size ) 0.12 × 0.15 × 0.21 mm
GOF ) 1.83
diffractometer: SMART CCD
radiation: Mo KR ) 0.17069 Å
monochromator: graphite crystal
scan: ω (0.3° per frame)
2θmax: 49.4o
reflns: 4274 total, 1026 unique
Rint: 0.039
residuals:a R ) 0.036; wR ) 0.043
all data: R ) 0.052
2
2
2
a R ) ∑(||Fo| - |Fc||)/∑|Fo|. wR ) [∑w(|Fo| - |Fc| )2/∑w(Fo )2]1/2
.
Table 4. Selected Distances (Å) and Angles (deg) for 3
Distances
N(1)-C(1)
N(3)-C(3)
N(3)-C(4)
C(1)-B(1)
C(3)-H(1)
C(3)-H(2)
C(4)-H(4)
1.143(3)
1.501(3)
1.505(2)
1.594(4)
0.95(2)
N(2)-C(2)
N(3)-C(4)
N(3)-B(1)
C(2)-B(1)
C(3)-H(2)
C(4)-H(3)
C(4)-H(5)
1.146(2)
1.505(2)
1.596(3)
1.586(2)
0.980(16)
1.036(18)
0.947(17)
Figure 8. Typical EELS spectra of B-C-N phase and Li phase (inset)
featuring the K-shell ionization edges for B, C, and N, and Li,
respectively. The presence of π* and σ* features indicates that the
B-C-N material is graphitic.
0.980(16)
0.873(12)
at higher temperatures loss of (CN)2 is observed in the mass
spectrum. The thermal dissociation of C3N3B via elimination
of (CN)2 is likely to yield a stoichiometric material with
composition BCN (eq 6).
Angles
C(3)-N(3)-C(4) 108.37(12) C(3)-N(3)-C(4) 108.37(12)
C(3)-N(3)-B(1) 110.18(17) C(4)-N(3)-C(4) 108.2(2)
C(4)-N(3)-B(1) 110.81(12) C(4)-N(3)-B(1) 110.81(12)
N(1)-C(1)-B(1) 178.0(2)
N(3)-C(3)-H(1) 107.6(14)
N(3)-C(3)-H(2) 108.1(9)
H(1)-C(3)-H(2) 112.3(11)
N(3)-C(4)-H(3) 107.7(10)
N(3)-C(4)-H(5) 107.2(10)
H(3)-C(4)-H(5) 111.2(13)
N(2)-C(2)-B(1) 178.90(17)
N(3)-C(3)-H(2) 108.1(9)
H(1)-C(3)-H(2) 112.3(11)
H(2)-C(3)-H(2) 108.2(18)
C3N3B f BCN + (CN)2
(6)
N(3)-C(4)-H(4)
99.8(8)
Heating C3N3B in an atmosphere of nitrogen did not result
in elimination of (CN)2. However, it transformed the crystalline
cyanide into a polymeric network, perhaps via cyclization of
the (CtN) groups to form (C3N3) triazene-type rings. The
presence of C3N3 moieties is indicated by the appearance of
the characteristic C-N ring modes in the IR and the strong
π-to-π* transitions in the EELS spectrum, indicating substantial
sp2 hybridization for both carbon and nitrogen. A possible
structure of a stoichiometric C3N3B polymer incorporates C3N3
rings linked together at the carbon sites by trigonal planar B to
form a two-dimensional network similar to that of graphitic
C3N4. Such a structure should be an ideal precursor for the high-
pressure synthesis experiments proposed in the Introduction. Our
preliminary results are certainly consistent with formation of a
planar C3N3B. The vibrational and EELS spectra are consistent
with a layered model structure, and the X-ray diffraction pattern
is reminiscent of highly turbostatic amorphous carbon. High-
pressure studies are currently in progress.
H(3)-C(4)-H(4) 112.1(15)
H(4)-C(4)-H(5) 117.9(16)
N(3)-B(7)-C(4) 109.93(18) N(3)-B(1)-C(2) 109.63(12)
N(3)-B(1)-C(2) 109.63(12) C(1)-B(1)-C(2)
C(1)-B(1)-C(2) 109.03(13) C(2)-B(1)-C(2)
109.03(13)
109.6(2)
A brief survey of preliminary high-pressure experiments on
LiB(CN)4 in the multianvil have shown that a transparent glassy
material, which is extremely hard but amorphous and has a
composition close to that of the starting material, is produced
at 100 kbar and 400 °C. Further reactions at 900 °C and 100
kbar resulted in a crystalline black solid with platelet-like
morphology. High-resolution electron diffraction and EELS
examinations at the nanometer scale revealed that most crystals
in the sample have a graphite-like layered structure and consist
of sp2-hybridized B, C, and N atoms (Figure 8), indicating that
the development of a planar B-C-N network has been
achieved. EELS was also used to identify a different set of
particles which are extremely beam sensitive (thus difficult to
analyze) and are found to incorporate primarily Li and a small
amount of C. The formation of separate B-C-N and Li phases
from the compression of LiB4CN4 at 100 kbar and 900 °C
Figure 7. Molecular structure of B(CN)3‚NMe3. The heavy atoms are
50% probability ellipsoids, and the hydrogen atoms are of arbitrary
size.
All angles around the N atom of the NMe3 group are within
109.5 ( 1°. Selected bond lengths and angles are listed in Table
4.
Thermolysis of BC3N3 has been performed under a variety
of conditions. Heating in situ in the inlet of the mass
spectrometer and under high vacuum in a flask revealed that
the compound is thermally stable up to about 450 °C. However,

7740 J. Am. Chem. Soc., Vol. 122, No. 32, 2000
Williams et al.
(CN)4: IR (Nujol, cm-1) 2271 (w), 995 (s), 961 (s), 530 (s), 398 (s);
suggests that the compound is disproportionating under these
conditions, presumably by elimination of LiCN. A systematic
search and characterization for high-pressure phases in the Li-
B-C-N system is currently in progress. The primary objective
remains the determination of the synthetic conditions that favor
formation of single-phase LiBC4N4 with diamond-like tetrahe-
dral structure.
1
13C NMR (CD3CN) δ -8.63 (1:1:1:1 quartet, JC ) 70.9 Hz); 11B
11
B
NMR (CD3CN) δ -8.5 (s); 7Li NMR (CD3CN) δ 0.112 (s); 13C MAS
7
NMR δ 120.5 (s, br); 11B MAS NMR δ -43.7 (s); Li MAS NMR δ
-2.83 (s).
C3N3B (2). A sample of pure B(CN)3‚CNSiMe3 was heated in a
Pyrex tube at 250 °C for 1.5 h under dynamic vacuum. A nonvolatile
brown solid (C3N3B) formed near the bottom of the tube, and modest
amounts of starting material sublimed to the top of the tube. The only
byproduct was SiMe3CN. Anal. Calcd for C3BN3: C, 40.4; H, 0.0; N,
47.2. Found: C, 38.0; H, 0.9; N, 37.6. IR (Nujol, cm-1): 2376 (m),
2250 (w), 1015 (s, br), 984 (s, br). 11B MAS NMR: δ 35.13, -40.64.
13C MAS NMR: δ 117.9 (br). The X-ray diffraction pattern gave peaks
with the following diameter values and percent relative intensity: [d
(Å), (I/Io)] 5.79 (100), 4.44 (77.9), 3.46 (39.1), 3.29 (36.5), 2.91 (39.7),
2.84 (47.9), 2.77 (39.2), 2.68 (48.5), 2.55 (28.7), 2.31 (21.9), 1.96 (19.2).
The tetragonal unit cell parameters are a ) 8.214 Å and c ) 13.832 Å
B(CN)3‚NMe3 (3). An excess of dry NMe3 (0.25 g, 4.0 mmol) was
combined at -196 °C with B(CN)3‚NCSiMe3 (0.25 g, 1.0 mmol). The
mixture was stirred at 22 °C for 1 h, and then the volatile materials
were removed in a vacuum to yield a light orange solid. The solid was
purified by extraction with hexane (2 × 20 mL) followed by sublimation
at 90 °C, 0.03 Torr, to afford a colorless crystalline solid. Anal. Calcd
for C6H9BN4: C, 48.64; H, 6.08; N, 37.83. Found: C, 48.20; H, 6.16;
N, 37.17. IR (Nujol, cm-1): 2225 (w), 1460 (vs), 1270 (s), 977 (m),
933 (s), 865 (s), 815 (s), 651 (m), 579 (w), 499 (w). 1H NMR
(CDCl3): δ 3.03 (s). 13C NMR (CDCl3): δ 50.9 (1:1:1:1 quartet,
Concluding Remarks
The preparation of a novel framework cyanide with composi-
tion LiBC4N4, the BC3N3 derivative, and related molecular
cyanides of boron such as M‚B(CN)3 and M‚B(CN)2(SMe) (M
) SiMe3CN, NMe3) has been achieved. In solution, the
existence of the B(CN)4- moiety has also been established for
the first time.17 The synthesis of new families of compounds
based on this species should be possible and awaits exploration.
It is of considerable interest in materials science to discover
synthetic pathways leading to new semiconductors and new host
materials based on the graphite network as well as superhard
diamond-like systems that incorporate boron, carbon, and
nitrogen. Previously synthesized B-C-N materials suffer from
poor crystallinity and often incorporate impurities which impede
accurate characterization and reliable determination of their
physical properties.15 Preliminary studies show that LiBC4N4
can be used as a starting material for high-pressure synthesis
of pure and crystalline B-C-N with graphite-like structure.
Stoichiometric LiBC4N4 and BC3N3 might also be highly
suitable as precursors to other novel structures such as cubic
and hexagonal B/C/N (as large crystals), B/C/N clusters and
nanotubes, and C-N-rich B-C-N phases with structures
related to C3N4.
1JC ) 70.9 Hz), 123. 11B NMR (CDCl3): δ 8.543. EIMS (m/e):
11
B
isotopic envelopes centered at 148 (M+), 133 (M+ - Me), 122 (M+
-
CN), 106 (M+ - CN-CH3), 95 [M+ - (CN)2], 81 [M+ - Me-(CN)2],
63 [B(CN)2]+, 59 (NMe3)+, 52 [(CN)2]+.
B(CN)3NCSiMe3 (4). A toluene solution (15 mL) of Me3SiCN (1.8
g, 18.0 mmol) was added to a toluene solution (15 mL) of B(SMe)3
(0.87 g, 6.0 mmol) at 22 °C. The reaction was heated at 60 °C for 18
h, during which time a red-brown crystalline precipitate developed.
After filtration the solid was extracted with hexane (20 mL), the hexane
was evaporated, and the solid was dried in vacuo. Sublimation at (160
°C/0.03 mmHg) yielded 222 mg of the product (20% yield). Anal. Calcd
for C7H9BN3Si: C, 43.5; H, 3.7; N, 30.2. Found: C, 41.8; H, 3.7; N,
29.1. IR (Nujol, cm-1): 2310 (s), 2232 (m), 1272 (s), 985 (m), 951
(s), 926 (m), 859 (s), 781 (m), 640 (w), 597 (w), 529 (w), 495 (w). 1H
NMR (CD3COCD3): δ 0.04 (s). 13C NMR (CD3COCD3): δ 122.9 (1:
Experimental Section
General Procedures. Reactions were performed under prepurified
nitrogen using standard Schlenk and drybox techniques. Dry, air-free
solvents were distilled prior to use. 1H NMR (300 MHz) and 13C NMR
(125.7 MHz) spectra were recorded on a Varian Gemini 300 and a
Varian Unity 500 spectrometer, respectively. FTIR spectra were
recorded on a Nicolet Magna-IR 550 spectrometer. Elemental analyses
were performed by Desert Analytics (Tucson, AZ). Electron impact
mass spectra were obtained on a Finnigan-MAT model 312 mass
spectrometer (IE ) 70 eV) in the Arizona State University departmental
mass spectrometry facility. LiBF4 and NaSMe (Aldrich) were used as
received, and NMe3 was dried over P2O5. SiMe3CN (Aldrich 98%) was
purified by distillation, and its purity was checked by NMR and gas-
phase IR. The CD3CN NMR spectra for 13C, 11B, and 7Li were
referenced to TMS, BF3 (Et2O), and LiCl, respectively. The (MAS)
NMR spectra for 13C, 11B, and 7Li were referenced to hexamethylben-
zene, B2O3, and LiCl, respectively. The PEELS spectra (Figure 5) were
collected on a Philips CM200FEG 200 kV high-resolution and analytical
TEM with information limit to 1.4 Å, focused probe size down to ∼0.5
nm, and equipped with an EDX detector, PEELS, and facilities for
electron holography. PEELS spectra (Figure 8) were collected on a
Vacuum Generators HB501 dedicated STEM equipped with Gatan
dixiPEELS, focused probe size down to ∼0.5 nm.
1:1:1 quartet, (1JC B ) 70.9 Hz), 1.9 (s).11B NMR (CD3CN): δ -7.527
11
(s).
B(CN)2(SMe)NCSiMe3 (5). A toluene solution (15 mL) of Me3SiCN
(1.8 g, 17.0 mmol) was added to a toluene solution (15 mL) of B(SMe)3
(0.75 g, 5.0 mmol) at 22 °C. The product precipitated as colorless
crystals in about 10 min. The solid was collected by filtration to yield
570 mg (58%) of B(CN)2(SMe)NCSiMe3. Mp: 82 °C dec. Anal. Calcd
for C7H12BN3SSi: C, 40.0; H, 5.2; N, 20.1. Found: C, 39.5; H, 4.8;
N, 20.9. IR (Nujol, cm-1): 2276 (s), 2223 (w), 1261 (s), 1019 (m),
995 (m), 971 (m), 951 (s), 873 (s), 800 (s), 781 (m), 636 (w), 597 (w),
533 (w), 495 (w), 422 (w). 1H NMR (CDCl3): δ 2.03 (s, 3H, -SCH3),
0.59 (s, 9H, -Si(CH3)3). 13C NMR (CDCl3): δ 126.3, 12.1 (s), -0.8
(s). 11B NMR (CDCl3): δ 0.341 (s). EIMS (m/e): isotopic envelopes
centered at 207 (M+), 181 (M+ - CN), 147 (MeSSiMe3+), 84 (Me2-
SiCN+), 73 (Me3Si+), and 63 (M+ - MeSSiMe3).
B(CN)2(SMe).NMe3 (6). An excess of NMe3 (0.20 g, 3.4 mmol)
was condensed directly onto solid B(CN)2(SMe)NNSiMe3 (0.20 g, 0.95
mmol) at -196 °C. The mixture was warmed slowly to room
temperature, and the volatiles were removed in vacuo, leaving behind
a pale yellow solid. The solid was washed with hexane (2 × 20 mL)
and dried in vacuo. Yield: 0.15 g (93%). Colorless single crystals for
X-ray diffraction were grown by slow sublimation of the crude material
in a sealed tube under vacuum. IR (Nujol, cm-1): 2211 (w), 1473 (vs),
1264 (s), 999 (m), 972 (s), 932 (m), 907 (m), 864 (s), 807 (s), 648
(m), 576 (w), 362 (w). 1H NMR (CDCl3): δ 2.87 (s, 9H, NMe3), 2.14
(s, 3H, SMe). 13C NMR (CDCl3): δ 126.9 (1:1:1:1 quartet, JCB ) 72
Hz), 49.7 (s), 13.1 (s).
LiBC4N4 (1). Me3SiCN (4.8 g, 0.048 mol) was added dropwise to a
stirred mixture of LiBF4 (1.0 g, 0.011 mol) in 50 mL of Bu2O at -78
°C. The mixture was stirred at 22 °C for 18 h and then refluxed for 4
h. After filtration the resulting white solid residue was heated at 250
°C for 18 h in vacuo and then extracted with warm CH3CN. The
solution was evaporated to dryness to yield a colorless solid. Anal.
Calcd for C4BliN4: C, 39.34; H, 0.00; N, 45.50. Found: C, 39.25; H,
<0.05; N, 43.10. The low value obtained for N is attributed to the
formation of refractory nitrides during combustion analysis. LiB-
(17) The synthesis of LiBC4N4 was described by Darrick Williams in a
dissertation entitled “The Synthesis and Characterization of Binary and
Ternary group 11-12 and 13 Cyanides”, Arizona State University, Tempe
AZ, 1998; Diss. Abstr., Int., B 1999, 59, 10.
Structure of B(CN)3‚NMe3 (3) and B(CN)2(SMe)NCSiMe3 (6). A
colorless polyhedral crystal of N4C6H9B (0.12 × 0.15 × 0.21 mm)

Synthesis of LiBC4N4, BC3N3, and Related Compounds
J. Am. Chem. Soc., Vol. 122, No. 32, 2000 7741
was mounted on a glass fiber using Paratone N hydrocarbon oil. All
measurements were made on a SMART CCD area detector with
graphite-monochromated Mo KR radiation. Cell constants were obtained
from a least-squares refinement using the measured positions of 1690
reflections in the range 3.00° < 2θ < 45.00°, corresponding to a
primitive orthorhombic cell. Based on the systematic absences of 0kl,
k + l * 2n and hk0, h * 2n, packing considerations, a statistical analysis
of intensity distribution, and the successful solution and refinement of
the structure, the space group was determined to be Pnma (No. 62).
The data were collected at -133 ( 1 °C. Frames corresponding to an
arbitrary hemisphere of data were collected using ω scans of 0.3°
counted for a total of 20.0 s per frame. The data were integrated to a
maximum 2θ value of 49.4° and were corrected for Lorentz and
polarization effects and analyzed for agreement and possible absorption.
An empirical absorption correction based on comparison of redundant
and equivalent reflections is applied (Tmax ) 0.96, Tmin ) 0.69). The
structure was solved by direct methods and expanded using Fourier
techniques. The non-hydrogen atoms were refined anisotropically, and
the hydrogen atoms were refined with isotropic thermal parameters.
The final cycle of full-matrix least-squares refinement was based on
574 observed reflections [I > 3.00σ(I)] and 76 variable parameters and
the data collection. The cell constants were obtained using 741
reflections taken from the data set with I > 10σ(I). The data were
corrected for absorption using SADABS following the removal of eight
σ outliers (Tmax ) 0.98, Tmin ) 0.61). The crystal was twinned with
twinning law a,b,-c. The minority twin component refined to be 15%
of the structure. The structure was solved using Patterson synthesis
and expanded using Fourier techniques. The non-hydrogen atoms were
refined anisotropically, and the hydrogen atoms were refined isotro-
pically using a riding model. The final cycle of full-matrix least-squares
refinement was based on 826 observed reflections [I > 2.00σ(I)] and
105 variable parameters and converged with unweighted and weighted
agreement factors of R ) ∑||Fo| - |Fc||/∑|Fo| ) 0.0589, Rw ) [∑w(|Fo|
2
- |Fc|)2/∑wFo )]1/2 ) 0.1261. The maximum and minimum peaks on
the final difference Fourier map corresponded to 0.342 and -0.270
e-/Å3, respectively. All calculations were performed using the SHELX-
TL version 5.03 of Siemens Analytical X-ray Instruments.
Acknowledgment. We thank the National Science Founda-
tion and the Army Research Office (ARO) (DAAH04-96-1-
0264) for financial support. We thank Dr. F. Hollander from
the Chemistry Department at the University of Californias
Berkeley for his work on solving the crystal structures of
B(CN)3(NMe3) and B(CN)2SMe (NMe3). We thank Dr. Crozier
and Dr. McCartney for their help with the EELS spectra, and
Dr. D. Walker for the preliminary high-pressure experiments.
converged with unweighted and weighted agreement factors of R )
∑||Fo| - |Fc||/∑|Fo| ) 0.036, Rw ) [∑w(|Fo| - |Fc|)2/∑wFo )]1/2
)
2
0.043. The maximum and minimum peaks on the final difference
Fourier map corresponded to 0.16 and -0.13 e-/Å3, respectively. All
calculations were performed using the teXsan crystallographic software
package of Molecular Structure Corp.
The data for B(CN)2SMe‚CNSiMe3 (6) were collected at -124 °C
using a SMART CCD system and a colorless crystal with dimensions
0.15, 0.06, 0.04 mm. Frames were collected by ω scans and of 0.3°
counted for 20 s per frame. The data were integrated using Siemens
SAINT with box parameters of 1.6 × 0.6, which yielded 3863
reflections to a resolution of 0.9 Å. The sample did not degrade during
Supporting Information Available: Full tables of bond
lengths and angles, atomic coordinates, and displacement
parameters for 3 and 6 (PDF). This material is available free of
charge via the Internet at http://pubs.acs.org.
JA0006752