www.elsevier.nl/locate/ica
Inorganica Chimica Acta 298 (2000) 216 – 220
Some interesting structural chemistry of lithium
F. Albert Cotton *, Hong-Cai Zhou
Department of Chemistry, Laboratory for Molecular Structure and Bonding, Texas A&M Uni6ersity, PO Box 30012,
College Station, TX 77842 -3012, USA
Received 7 July 1999; accepted 14 September 1999
Abstract
The lithium salt of a-carboline, Li carb, (for which the systematic name is 1H-pyrido[2,3-b]indole), has been prepared and
crystallized from both tetrahydrofuran and toluene. In each case the molecule is tetranuclear, with a distorted tetrahedron of
lithium atoms, and in each case, the ligands are arranged in a Li4(carb)4 assembly having S4 point symmetry. In the compound
crystallized from tetrahydrofuran, each lithium atom is coordinated by three nitrogen atoms and an oxygen atom, whereas in the
compound obtained from toluene each lithium atom is only three-coordinate. The structures are topologically the same, but differ
in the spatial orientations of the carboline planes. © 2000 Elsevier Science S.A. All rights reserved.
Keywords: Crystal structures; Lithium compounds
1. Introduction
The existence of the element lithium has been known
since 1817 and the metal was isolated in 1855. Its
chemistry [1] has been relatively slow to develop, especially the structural aspect, but in recent decades, X-ray
crystallography has made a huge contribution. Nevertheless, there is still a great deal to be learned about the
basic principles and possibilities in the structural chemistry of lithium. We present here two compounds that
have very remarkable structures; moreover, these two
structures differ from each other for reasons that relate
directly to the different methods of preparation.
2. Experimental
2.1. General procedures
All manipulations were carried out under nitrogen
using standard Schlenk and glove box techniques. Sol-
* Corresponding author. Tel.: + 1-409-845 4432; fax: + 1-409-845
9351.
E-mail address: cotton@tamu.edu (F.A. Cotton)
vents were purified using conventional methods, and
were freshly distilled under nitrogen prior to use. The
compound a-carboline (Hcarb) was synthesized following a literature method [2], and sublimed before use;
MeLi (1.0 M in THF–cumene) and BuLi (1.6 M in
hexanes) were purchased from Aldrich and used as
received.
2.2. Preparation of Li4(carb)4 ·(THF)4 (1)
The compound Hcarb (0.34 g, 2.0 mmol) was dispersed in THF (10 ml), and cooled to − 78°C. Methyl
lithium in THF–cumene (1.0 M, 2.0 ml) was added
dropwise. There was immediate effervescence, and a
pale yellow solution was obtained. The solution was
then transferred to a Schlenk tube, and layered with
hexanes (80 ml). Colorless, block-shaped crystals were
obtained in quantitative yield.
2.3. Preparation of Li4(carb)4 ·toluene (2 ·toluene)
The preparative procedure is essentially the same as
that of compound 1, but the reaction was carried out in
toluene instead of in THF, and BuLi in hexanes was
used in order to avoid introducing coordinating solvent
molecules.
0020-1693/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved.
PII: S 0 0 2 0 - 1 6 9 3 ( 9 9 ) 0 0 4 5 3 - 3
F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216–220
2.4. Crystallographic studies
Data collection for compound 1 was performed on a
Nonius FAST area detector diffractometer at −60°C.
The data set for 2·toluene was obtained using an
Enraf–Nonius CAD4 diffractometer at room temperature. Crystal and structure refinement data for 1 and
2·toluene are listed in Table 1.
217
respectively. In 2·toluene, the disordered interstitial
molecule of toluene was modeled in two orientations,
and refined at 64% occupancy for the major
orientation.
3. Results and discussion
Our interest in the anion of a-carboline (I), began
several years ago [3] for the purpose of having a ligand
very much like the anion of 7-azaindole (II), but
modified so that the tendency of II to be disordered in
crystals of M2(azindolate)4 (III), would be thwarted.
Utilization of I ordinarily requires the treatment of
a-carboline with lithium to give the reagent lithium
carbolinate, Li carb. It was in the course of examining
this reagent that we obtained the crystals whose structures are reported here. The crystals obtained from
THF and toluene are designated as 1 and 2·toluene,
Both molecules 1 and 2 have a distorted tetrahedral
array of lithium atoms, but in each case the arrangement of the carbolinate ligands reduces the symmetry
to S4. In both molecules the carbolinate ligands form
bridges across tetrahedral faces in the way shown schematically as A.
Table 1
Crystal data and structure refinement
Compound
1
2·toluene
Empirical formula
Formula weight
Space group
a (A, )
b (A, )
c (A, )
a (°)
b (°)
g (°)
V (A, 3)
Z
Temperature (K)
Radiation l, (A, )
Dcalc (g cm−3)
Absorption coefficient (mm−1)
Data collection instrument
C60H60Li4N8O4
984.92
P1(
12.823(3)
13.939(2)
18.221(6)
69.82(4)
69.81(3)
64.15(2)
2677(1)
2
213(2)
0.71073
1.222
0.076
Nonius FAST
Reflection collected
Independent reflections
13702
6776
[Rint = 0.0600]
6771/0/685
1.095
0.31(5)
0.075/0.104
0.163/0.189
C51H36Li4N8
788.64
Pna21
21.569(4)
13.975(3)
13.519(3)
90
90
90
4075(2)
4
293(2)
0.71073
1.285
0.076
Enraf–Nonius
CAD4
3057
3050
[Rint = 0.0122]
3050/17/526
1.207
0.64(7)
0.075/0.147
0.193/0.227
Data/restrains/parameters
Goodness-of-fit on F 2
Largest peak (e A, −3)
a
R1 c/R1 d
b
wR2 c/wR2 d
R1 =(Fo−Fc)/Fo.
wR2 = w[(F 2o −F 2c )2/[w(F 2o )2]]1/2; w= 1/[s 2(F 2o )+(a×P)2+b×
P], P= [max(F 2o or 0)+2(F 2c )]/3.
c
Denotes value of the residual considering only the reflections with
I\2s(I).
d
Denotes value of the residual considering all the reflections.
a
b
The major difference between 1 and 2 arises because
of the solvent from which each is crystallized. From
THF, one obtains molecule 1, with each lithium atom
coordinated by a THF molecule to complete a very
distorted tetrahedral arrangement, LiN3O, about it. For
2, which is crystallized from toluene, with no additional
potential ligand available, the lithium atoms remain
three-coordinate. However, 2 can be recrystallized from
THF to give 1. Thus, 1 could be formulated as
2·(THF)4. Presumably, 2 could be recrystallized from
many other coordinating solvents, S, to give 2·(S)4
products with structures akin to that of 1.
We now examine and compare these molecules in
more detail. For 1 the molecule is depicted in Fig. 1,
and the principal dimensions are listed in Table 2. The
molecule of 2 is shown in Fig. 2 and its principal
dimensions are listed in Table 3. Fig. 3 shows a
molecule of 1 stripped of its four THF ligands side by
side with a molecule of 2, each viewed from the same
direction. A numbering scheme common to both
molecules has been used in all figures. The main features of the two molecules may now be compared
quantitatively.
F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216–220
218
Table 2
Selected bond lengths (A, ) and angles (°) for 1
Fig. 1. The Li4(carb)4(THF)4 molecule. Atoms in the Li4N8O4 core
are represented by thermal displacement ellipsoids at the 50% probability level. Carbon atoms are drawn at an arbitrary scale, and
hydrogen atoms are omitted for clarity.
Bond lengths
Li(1)N(101)
Li(1)N(402)
Li(2)N(102)
Li(2)N(301)
Li(3)N(202)
Li(3)N(401)
Li(4)N(101)
Li(4)N(401)
2.122(7)
2.096(7)
2.059(8)
2.091(8)
2.075(7)
2.122(7)
2.088(7)
2.110(7)
Li(1)N(201)
Li(1)O(1)
Li(2)N(201)
Li(2)O(2)
Li(3)N(301)
Li(3)O(3)
Li(4)N(302)
Li(4)O(4)
2.101(7)
2.021(7)
2.110(8)
2.003(7)
2.106(7)
2.020(7)
2.058(7)
2.005(7)
Bond angles
O(1)Li(1)N(402)
N(402)Li(1)N(201)
N(402)Li(1)N(101)
O(2)Li(2)N(102)
N(102)Li(2)N(301)
N(102)Li(2)N(201)
O(3)Li(3)N(202)
N(202)Li(3)N(301)
N(202)Li(3)N(401)
O(4)Li(4)N(302)
N(302)Li(4)N(101)
N(302)Li(4)N(401)
101.2(3)
128.3(3)
103.8(3)
102.8(3)
122.6(3)
102.8(3)
99.4(3)
104.3(3)
126.0(3)
100.9(3)
124.6(3)
103.8(3)
O(1)Li(1)N(201)
O(1)Li(1)N(101)
N(201)Li(1)N(101)
O(2)Li(2)N(301)
O(2)Li(2)N(201)
N(301)Li(2)N(201)
O(3)Li(3)N(301)
O(3)Li(3)N(401)
N(301)Li(3)N(401)
O(4)Li(4)N(101)
O(4)Li(4)N(401)
N(101)Li(4)N(401)
100.2(3)
114.5(3)
109.0(3)
103.1(3)
111.7(3)
113.4(3)
113.9(3)
103.0(3)
110.0(3)
99.7(3)
112.4(3)
114.5(3)
each lithium atom has only a small effect upon the
LiN bonds.
3.1. Li···Li distances
3.3. Dihedral angles between ligand planes
In each case the Li(1)···Li(3) and Li(2)···Li(4) distances are significantly longer than the other four, but
the difference is more exaggerated in the case of 2, as
shown by the following ranges:
Li···Li
1···3, 2···4:
Other four:
1
3.460–3.464
3.147–3.242
2
3.43–3.60
2.93–3.06
As can be seen in Fig. 3, the only major difference
between the two structures is in the orientations of the
planar ligands. The situation is reminiscent of the way
the pitch of an airplane propeller can be varied. In 2,
each ligand plane is approximately perpendicular to the
On the whole, the Li tetrahedron is a little more
distorted but is also more compact in 2 than in 1.
3.2. NLi distances and NLiN angles
It might be expected that for 1, where the coordination number of each lithium atom is four, the LiN
distances might be longer than in 2, where each Li atom
is only three-coordinate. This is true, as the following
figures show:
1
2
Average LiN (A, )
2.10
2.04
Range of LiN (A, )
2.06–2.12
1.95–2.17
As for the NLiN angles, there is no significant difference, with the average values and ranges being 114° and
102 – 125° in 1 compared to 114° and 103 – 128° in 2. On
the whole, the presence or absence of a fourth bond to
Fig. 2. Drawing of the Li4(carb)4 molecule in 2·toluene, with displacement ellipsoids drawn at the 50% probability level. All hydrogen
atoms are omitted for clarity.
F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216–220
219
Table 3
Selected bond lengths (A, ) and angles (°) for 2
Bond lengths
Li(1)N(101)
Li(1)N(402)
Li(2)N(102)
Li(2)N(301)
Li(3)N(202)
Li(3)N(401)
Li(4)N(101)
Li(4)N(401)
1.95(3)
2.05(4)
2.02(3)
2.17(3)
1.98(3)
2.10(4)
2.17(4)
1.97(3)
Li(1)N(201)
Li(1)C(210)
Li(2)N(201)
Li(2)C(310)
Li(3)N(301)
Li(3)C(410)
Li(4)N(302)
Li(4)C(110)
2.09(4)
2.68(4)
1.99(3)
2.62(4)
1.97(3)
2.65(4)
2.03(4)
2.64(4)
Bond angles
N(402)Li(1)N(201)
N(201)Li(1)N(101)
N(102)Li(2)N(201)
N(202)Li(3)N(301)
N(301)Li(3)N(401)
N(302)Li(4)N(401)
109(2)
117(2)
125(2)
119(2)
115(2)
119(2)
N(402)Li(1)N(101)
N(102)Li(2)N(301)
N(301)Li(2)N(201)
N(202)Li(3)N(401)
N(302)Li(4)N(101)
N(101)Li(4)N(401)
118(2)
102(2)
112(2)
108(2)
107(2)
114(2)
planes of those adjacent, these angles being: 91.2, 91.5,
95.0, and 89.3°. However, in 1 the ligands are raked
back and the corresponding set of angles is: 115, 113,
112, and 124°. As a consequence, the closest nonbonded Li···C contacts are somewhat different in the
two cases, viz., an average of 2.99 A, in 1 and 2.65 A, in
2.
To our knowledge, structures of precisely this kind
have not been previously reported for lithium compounds. We are aware of several compounds in which
there is a tetrahedron of lithium atoms with each face
bridged by a m3-carbon atom and an appended donor
atom is also coordinated. In several of these, the molecular symmetry is S4 and in all cases the lithium atoms
are four-coordinate without the presence of any further
ligands [4]. In these compounds the Li···Li distances are
much shorter (ca. 2.5 A, ) than those in 1 and 2 and the
key feature is the presence of m3-carbon atoms. We have
found no previous example in which there are three-coordinate lithium atoms as in 2. There are also
analogous structures for certain compounds of copper(I) and silver(I), where the bridging ligands are
dithiocarbamates or dithiophosphates [5].
A final observation on these structures is that they
have the somewhat unusual point symmetry S4. This
symmetry is of interest because it provides the only
known exception among real molecules (1,3,5,7tetramethylcyclooctatetraene being another S4 case)
to the commonly used (but false) criterion for chirality,
namely that a molecule having neither a plane nor
a center of symmetry will be chiral. It is evident that 1
and 2 fulfill that (false) criterion, but they are superimposable on their mirror images. The correct criterion
[6] is, of course, that if a molecule lacks any improper
axis of rotation (s and i being S1 and S2, respectively)
it will be chiral. In practice the only point group of
Fig. 3. Views of 1 and 2 along the same direction, with the THF
molecules removed from the former. Atoms in the Li4N8 cores are
represented by thermal displacement ellipsoids at the 50% probability
level. Carbon atoms are shown as arbitrarily scaled small circles, and
hydrogen atoms are omitted.
Sn type and lacking an inversion center ever observed
is S4. Note that with an S6 axis (which is known to
occur [7,8]), there is necessarily a center of inversion
(S 36 = i ).
4. Supplementary material
Full listings of atomic coordinates, bond distances,
bond angles, anisotropic displacement parameters, hydrogen atom coordinates (27 pages) are available from
author F.A. Cotton.
220
F.A. Cotton, H.-C. Zhou / Inorganica Chimica Acta 298 (2000) 216–220
Acknowledgements
We thank the National Science Foundation for support. We also thank Dr X. Wang for crystallographic
assistance.
[5]
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F.A. Cotton, L.M. Daniels, C.A. Murillo, H.-C. Zhou, C.R.
Acad. Sci. (1999) in press.