Crystal structure of a butyllithium 1,2-dipiperidinoethane dimer complex

Article abstract of DOI:10.1007/s10870-007-9188-7

The crystal structure of butyllithium solvated by 1,2-dipiperidinoethane, BuLi?DPE-6, which has been used as an initiator for a number of important commercial anionic polymerization reactions, is reported. The complex crystallizes from pentane as a centrosymmetric dimer in the monoclinic space group P2₁/n with a∈=∈9.8619(12), b∈=∈17.963(2), c∈=∈10.3655(12) A, β∈=∈114.090(2)° and Z∈=∈2. The dimer is located on a crystallographic inversion center. The crystal under investigation was found to be non-merohedrally twinned. In contrast to the two other dimeric BuLi complexes previously structurally characterized in the solid state, BuLi?DPE-6 has a planar central Li ₂C₂ core. Semi-empirical (PM3) calculations were used to determine the lowest energy conformations of the dimer and also identified three structural motifs that affect Li₂C₂ dimer ring planarity. Springer Science+Business Media, LLC 2007.

Full text of DOI:10.1007/s10870-007-9188-7

Journal of Chemical Crystallography, Vol. 37, No. 6, June 2007 (ꢀC 2007)
DOI: 10.1007/s10870-007-9188-7
Crystal structure of a butyllithium 1,2-dipiperidinoethane
dimer complex
(1)∗
(1)
(2)
(2)
Michael A. Nichols, Rachel M. Sobinsky, Allen D. Hunter, and Matthias Zeller
Received December 4, 2006; accepted February 7, 2007
Published Online February 27, 2007
The crystal structure of butyllithium solvated by 1,2-dipiperidinoethane, BuLi·DPE-6,
which has been used as an initiator for a number of important commercial anionic poly-
merization reactions, is reported. The complex crystallizes from pentane as a centrosym-
metric dimer in the monoclinic space group P21/n with a = 9.8619(12), b = 17.963(2),
◦
c = 10.3655(12) Å, β = 114.090(2) and Z = 2. The dimer is located on a crystallographic
inversion center. The crystal under investigation was found to be non-merohedrally twinned.
In contrast to the two other dimeric BuLi complexes previously structurally characterized
in the solid state, BuLi ·DPE-6 has a planar central Li2C2 core. Semi-empirical (PM3)
calculations were used to determine the lowest energy conformations of the dimer and also
identified three structural motifs that affect Li2C2 dimer ring planarity.
KEY WORDS: crystal structure; butyllithium; 1,2-dipiperidinoethane; PM3 semi-empirical calculations;
BuLi·DPE-6 dimer complex.
1
4
15
Introduction
Butyllithium (BuLi) continues to be one
polyisobutylene, polyethylene and poly-1,3-
16
6
13
cyclohexadiene. Using Li/ C NMR, Collum
and co-workers found BuLi to be dimeric in
◦
of the most popular and important alkyllithium
reagents used in organic synthesis and an-
toluene-d solutions at − 70 C in the presence
8
1
7,18
of 1 to 4 equivalents of DPE-6.
In this paper,
1
,2
ionic polymerization chemistry.
Remarkably,
the crystal structure of the dimeric aggregate of
BuLi complexed with DPE-6 is reported.
only 32 crystal structures of complexes con-
3
taining BuLi have been reported and of those,
only 6 contain BuLi as the sole organolithium
4
–7
compound.
complexing agent 1,2-dipiperidinoethane (DPE-
) have been used in a variety of anionic
polymerization reactions to produce polybu-
BuLi solutions mixed with the
Experimental section
6
Materials
8
–11
12,13
Butyllithium (Aldrich, 2.5 M in hexanes)
was used without further purification and pen-
tane (Fisher, HPLC grade) was dried over 4 Å
molecular sieves and degassed with nitrogen prior
to use. DPE-6 was synthesized using a modifica-
tadiene rubber,
polyisoprene rubber,
(1)
Department of Chemistry, John Carroll University, 20700 North
Park Blvd., University Heights, OH 44118.
Department of Chemistry, Youngstown State University Youngs-
town, OH 44555.
(2)
19
∗
tion of a previously-reported method. Piperi-
dine (5.82 mL, 59 mmol), 1,2-dichloroethane
To whom correspondence should be addressed; e-mail: mnichols
@
jcu.edu
4
33
1
074-1542/07/0600-0433/0 ꢀC 2007 Springer Science+Business Media, LLC

4
34
Nichols, Sobinsky, Hunter, and Zeller
(
2.14 mL, 27 mmol), and NaOH (2.28 g, 57 mmol)
in TWINABS is incomplete, thus resulting in
too many independent reflections and making
it impossible to give a correct value for Rint.
(See Table I). The twin operation involves a
were all added to approximately 50 mL of water
and refluxed for 48 h. The solution was saturated
with KOH and extracted with diethyl ether. After
removal of the solvent by rotary evaporation, the
crude DPE-6 was vacuum distilled from calcium
hydride and stored under nitrogen. An average
yield is 60%. The purity of DPE-6 was determined
using GC-MS (HP 5890 GC/HP5972B MS) and
◦
180 rotation, and the twin matrix was found
to be 0.07898 −0.00125 −0.91777, 0.00573
−0.99997 0.00654, −1.08092 −0.00107
−0.07874.
23
Using SHELXTL the structure was solved
with only the reflections of component 1 by di-
rect methods and was refined with reflections
containing at least one contribution of compo-
nent 1 above d = 0.75 using the hklf 5 method.
The BASF factor refined to 0.136(1). Refine-
ment of an extinction coefficient was found to be
insignificant. Non-hydrogen atoms were refined
anisotropically. All hydrogen atoms were found
in the difference density Fourier map and were
1
H NMR (Varian Gemini-300).
Preparation of-BuLi·DPE-6 crystals
BuLi (3 mL of 2.5 M solution in hexane),
0 mL of pentane and 1.5 g of DPE-6 were mixed
1
in a 20 mL GC vial under a nitrogen atmosphere
and sealed with a septum. The solution was stored
◦
at − 4 C; large colorless crystals were obtained
after several weeks. The crystals were quickly sus-
pended in mineral oil at ambient temperature, a
suitable crystal was selected, cut to an appropriate
size and mounted with the help of mineral oil on
a 50 µm MicroMesh MiTeGen Micromount.
Table I. Crystallographic Data and Structure Refinement Summary
for n-BuLi·DPE-6
CCDC deposit no.
Empirical Formula
Formula weight
Temperature (K)
Crystal system
Space group
Unit cell dimensions
a (Å)
626775
C32H66Li2N4
520.77
100(2)
Monoclinic
P21/n
Crystallographic study
9.8619(12)
17.963(2)
10.3655(12)
90
114.090(2)
90
1676.3(3)
2
1.032
Diffraction data were collected using a
Bruker AXS SMART APEX CCD diffractometer
at 100(2) K using monochromatic Mo Kα
radiation with the omega scan technique. The
crystal under investigation was found to be
non-merohedrally twinned. The two components
b (Å)
c (Å)
◦
α ( )
◦
β ( )
◦
γ ( )
Volume (Å3)
Z
3
20
Calculated density (mg/m )
Absorption coefficient (mm 1) 0.059
were identified using the program Gemini,
−
21
were integrated using SAINT +
and were
F (0 0 0)
584
3
merged and corrected for absorption using
Crystal size (mm )
Reflections collected/unique
Maximum and minimum
transmission
0.57 × 0.46 × 0.33
21
5111/5111
TWINABS. 12620 data (3831 unique) involved
component 1 only (mean I/sigma 10.6), 12445
data (3766 unique) involved component 2 only
0.981 and 0.866
Data/restraints/parameters
R (int)
5111/0/272
not defined (see crystallographic
(
mean I/sigma 3.6), and 7149 data (2682 unique)
study)
1.004
involved both components (mean I/sigma 10.6).
Goodness-of-fit on F2
22
Due to “twin pairing errors” inherent to the
integration software (equivalent reflections being
counted as overlapping for one reflection, but as
not overlapped for an equivalent one) the merging
Final R indices [I > 2σ(I)]
R indices (all data)
R1 = 0.0509, wR = 0.1377
2
R1 = 0.0659, wR2 = 0.1461
Largest diffraction peak and
0.505 and − 0.177
3
hole (e/Å )

Crystal structure of a butyllithium 1,2-dipiperidinoethane dimer complex
435
refined with an isotropic displacement parameter
.5 times (methyl) or 1.2 times (all others) that of
the adjacent carbon atom. Crystal data and exper-
imental details are listed in Table I.
semi-empirical (PM3) calculations. Of 484 ini-
tially generated and minimized conformations,
100 having final energies within 10 kcal/mol of
the global minimum and best representing a di-
verse set were kept and analyzed.
1
Molecular modeling calculations
Results and discussion
Molecular modeling calculations were per-
An ORTEP view of the BuLi ·DPE-6 com-
plex including the atomic numbering scheme
is shown in Fig. 1. There are several notable
structural features. The complex crystallizes as
a centrosymmetric dimer that is located on a crys-
tallographic inversion center. The DPE-6 ligand’s
piperidine rings adopt chair conformations and the
24
formed using Spartan ’04. The initial geom-
etry was constructed using the coordinates of
the atoms obtained from the crystal structure.
The global minimum was obtained using con-
formational searching (Monte-Carlo method) fol-
lowed by optimization of each conformer using
Fig. 1. ORTEP diagram of the BuLi·DPE-6 complex including the atomic numbering scheme.
Thermal ellipsoids (50%) are shown for all non-hydrogen atoms. Dashed lines indicate agostic
Li–H interactions.

4
36
Nichols, Sobinsky, Hunter, and Zeller
Fig. 2. A partial packing diagram of the BuLi·DPE-6 dimer complex. Hydrogen
atoms have been omitted for clarity. Crystal packing forces the butyl groups to adopt
a gauche conformation.
lithium cations are coordinated to the axial posi-
tions of the nitrogen atoms. This solid-state struc-
ture is presumably similar to the complex’s solu-
or anti (δ-CH relative to α-CH ) conformations.
3 2
These vary from both gauche, one gauche/one
anti, to both anti in the DPE-6-, TMEDA- and
( − )-sparteine-solvated dimers, respectively. The
anti conformations in the BuLi ·DPE-6 complex
most likely result from the crystal packing, as
shown in Fig. 2.
17,18
tion structure observed by Collum.
Agostic
interactions between one of the α-CH hydrogen
2
atoms of the butyl group and the lithium cation
are also evident in the structure (2.009(13) Å)
and have also been observed in other BuLi so-
The DPE-6-, TMEDA-, and ( − )-sparteine-
solvated BuLi dimers do have significant differ-
ences in the degree of planarity of their central
Li C cores. The BuLi ·DPE-6 complex reported
6
1
25
lutions using Li / H HOESY NMR. The Li-C
and Li-N bond distances range from 2.212(2) to
2.235(2) Å and 2.212(2) to 2.428(2) Å, respec-
2
2
tively. The C–Li–C, and Li–C–Li bond angles in
here is planar because the dimer is centrosymmet-
ric and lies on a crystallographic inversion center;
◦
the dimer ring are 66.70(9) and 113.30(9) and the
◦
◦
N–Li–N bond angle is 82.13(7) . These parame-
the other two are non-planar by 14–17 . In an at-
ters are similar to those of the only two other pub-
lished crystal structures of BuLi dimers; those sol-
vated by (−)-sparteine and TMEDA. The three
dimer complexes differ in one minor structural
feature: whether the butyl groups adopt gauche
tempt to understand whether the most stable con-
formation of the BuLi·DPE-6 complex is planar
and to identify the steric factors that may influ-
ence BuLi dimer ring planarity, molecular model-
ing studies were performed. These consisted of a
4
6

Crystal structure of a butyllithium 1,2-dipiperidinoethane dimer complex
437
Fig. 3. Examples of three structural motifs that affect the Li
2 2
C dimer ring planarity. These were
identified from the molecular modeling studies. Only the α-CH hydrogen atoms of the BuLi groups and
2
the relevant piperidine ring hydrogens are shown. In Structures A, B and C, the Li
2
C
2
dimer rings are
◦
non-planar by 28, 16 and 0 , respectively. Structure B is the global minimum (Erel = 0.00 kcal/mol);
Structure A has the 2nd lowest energy (Erel = + 0.37 kcal/mol); Structure C has the 5th lowest energy
(
E
rel = + 1.21 kcal/mol).
conformational search followed by semi-empi-
rical (PM3) optimization of each structure. The
application of PM3 calculations to organolithium
compounds has proven reliable26 and is the most
viable option given this large system.
The two most stable conformations are
shown in Fig. 3: Structure B is the global min-
imum (Erel = 0.00 kcal/mol) and Structure A is
the next most stable (E = +0.37 kcal/mol) con-
rel
formation. Structures B and A are non-planar by
◦
16 and 28 , respectively. These calculations in-
dicate the four lowest energy conformations (Erel
0.00 to + 0.80 kcal/mol) are non-planar and by
◦
amounts (16–29 ) comparable those observed in
the crystal structures of the TMEDA-, and ( − )-
◦
sparteine-solvated BuLi dimers (14–17 ).

4
38
Nichols, Sobinsky, Hunter, and Zeller
Examination of the ten lowest energy con-
Grant CAP-491, and by Youngstown State Uni-
versity.
formations (having energies within 2.74 kcal/mol
of the global minimum) also yields three distinct
structural motifs: three conformations have nearly
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