Ca2LiC3H: A New Carbide Hydride Phase
A R T I C L E S
carried out with differing amounts of CaH2 showed none of the
variation in unit cell parameters which is typically associated
with Vegard’s law behavior. Instead, the unit cell parameters
remained constant, but the yield and product quality of the title
phase improved as the amount of CaH2 reactant was increased.
This indicates that this phase forms in the completely hydrided
state, which is also properly charge-balanced.
The title compound is one of a very small number of inorganic
4-
compounds containing the C3 anion. This chain of three
carbons, which can be viewed as deprotonated propadiene (C3H4,
also known as allene), is also found in Mg2C3, Sc3C4, R4C7 (R
) Y, Ho, Er, Tm, Lu), R5Re2C7 (R ) Sc, Er, Tm, Lu), and
Ca3C3Cl2.6,8,24-26 In Ca2LiC3H, this anion is perfectly linear,
with the central carbon located on a center of inversion. The
C-C bond length of 1.3254(9) Å is similar to that in propadiene
(∼1.31 Å), indicating its double bond nature; the other reported
structures containing the C34- unit also feature a C-C bond of
similar length (range: 1.32-1.35 Å). The distribution of cations
around this unit is shown in Figure 2c. While Mg2C3 and Sc3C4
both feature clustering of cations around the terminal carbons
Figure 3. 7Li MAS NMR spectra of Ca2LiC3H samples synthesized using
Ca/Li/C/CaH2 mmol ratios of 10:10:6:x. Spinning sidebands are marked
with asterisks.
valuable method to convert carbon into useful multiple bonded
reactants for organic syntheses or to produce isotopically
enriched precursors.28 Mg2C3 is also of interest for this.
However, synthesis of this compound involves reaction of Mg
powder with flowing pentane gas at high temperatures, or
lengthy (85 h) ball milling of the elements followed by
annealing; both syntheses are somewhat inconvenient.8,28
4-
of the C3 anion (in agreement with the location of formal
charges), Ca2LiC3H, R4C7, and Ca3C3Cl2 have short metal-carbon
bonds to the central carbon as well. This may be indicative of
the relative ionicity of the interactions between the metal cations
4-
With the incorporation and full occupancy of the H- and C3
anions, the title phase has a charge-balanced composition,
(Ca2+)2Li+(C3)4-(H-). This appears to be the only reported
structure with ordered coexisting carbide and hydride anions.
A number of studies have been carried out on rare earth carbide
hydrides such as YbC0.5H and La2C3H1.5, but these are highly
disordered (and in the latter case amorphous) phases with
varying amounts of interstitial carbon and hydrogen.29,30 The
most comparable compound is La2C2H2, which forms from the
reaction of La2C3 with hydrogen at 1070K. It is theorized to
contain both interstitial H- (in tetrahedral sites) and C24- units
(in octahedral sites), but was characterized by powder diffrac-
tion, and actual refinement of the structure from the powder
data was not possible (XRD data were dominated by La
scattering; the approximate structure model is based on
La2C2O2).31
4-
and the C3 unit, with the smaller and more polarizing Mg2+
and Sc3+ inducing a more covalent interaction with the terminal
carbon atoms of the carbide anion, as compared to the more
ionic interactions with larger cations Ca2+ and R3+. This is also
indicated by electron localization function analysis (vide infra).
Reaction of a compound containing the C34- unit with a protic
reagent should yield gaseous C3H4, as was reported for Mg2C3
and R4C7.8,24 Ca2LiC3H reacts rapidly and violently with water.
A more controllable protolysis reaction was carried out using
ammonium chloride as the source of acidic protons via its
thermal decomposition to gaseous HCl and ammonia, following
the procedure of a similar study on Mg2C3.8 The reaction with
Ca2LiC3H is shown below:
7
NMR Spectroscopy. Li MAS NMR data were collected at
Ca2LiC3H (s) + 5NH4Cl (s) f 2CaCl2 (s) + LiCl (s) +
5NH3 (g) + C3H4 (g) + H2 (g)
room temperature for products of reactions of Ca/Li/C/CaH2 in
mmol ratios of 10:10:6:x, with x ) 0, 0.25, 0.5, 0.75, and 1
mmol. All spectra in Figure 3 exhibit the same three lithium
resonances: two in the Knight-shifted metallic lithium region
(286 and 270 ppm) and one in the ionic lithium region (4 ppm,
referenced to LiCl). As the amount of CaH2 reactant increases,
the intensity of the ionic lithium peak in the product spectrum
increases. This is in accordance with the increasing yield of
Ca2LiC3H, indicating that this peak at 4 ppm corresponds to
the lithium in this phase. The two metallic lithium peaks at 286
and 270 ppm always occur at a 3:1 ratio. They are likely due
to CaLi2; this intermetallic will be formed in the solidified
residual traces of Ca/Li flux adhering to the solid products after
centrifugation. Weak peaks in the powder XRD patterns for
these samples indicate the presence of this phase. CaLi2 has
the C14 Laves phase structure (hexagonal space group P63/
mmc), with lithium atoms in 2a and 6h Wyckoff sites.32 The
The mass spectra of aliquots of gaseous products feature a
strong signal at 39 m/z, confirming production of C3H4. This
could be in the form of propadiene or propyne or a mixture of
these isomers (this was seen for the protolysis of Mg2C3),
although the envelope of isotopes from m/z ) 35-40 better
matches that of propadiene.27 Both gases are of interest for
industrial uses (as gas welding fuels, either pure or in a methyl
acetylene-propadiene-propane mixture known as MAPP gas),
and they are also convenient building blocks for organic
synthesis. The reaction of elemental carbon to form Ca2LiC3H
and the ready protolysis of this phase to form C3H4 may be a
(24) (a) Mattausch, H.; Gulden, T.; Kremer, R. K.; Horakh, J.; Simon, A.
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654.
(25) (a) Po¨ttgen, R.; Jeitschko, W. Z. Naturforsch., B 1992, 47, 358–364.
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Z. Naturforsch., B 1997, 52, 231–236.
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(26) (a) Hoffmann, R.; Meyer, H. J. Z. Anorg. Allg. Chem. 1992, 607, 57–
71. (b) Meyer, H. J. Z. Anorg. Allg. Chem. 1991, 593, 185–192.
(27) Stein, S. E. Mass Spectra. In NIST Chemistry WebBook, NIST Standard
Reference Database Number 69; Linstrom, P. J., Mallard, W. G., Eds.;
National Institute of Standards and Technology: Gaithersburg, MD,
(29) Haschke, J. M. Inorg. Chem. 1975, 14, 779–783.
(30) Kienle, L.; Garcia Garcia, F. J.; Duppel, V.; Simon, A. J. Solid State
Chem. 2006, 179, 993–1002.
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