A 'lithium-bonded' cyclopropyl edge: The X-ray crystal structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6 and computational studies

Article abstract of DOI:10.1021/ja9618661

The short Li-C distances (Li₁-C₂ = 2.615(3) ?, Li₁-C₃ = 2.644(3) ?) in the X-ray crystal structure of [Li-O-C(Me)-(c-CHCH₂CH₂)₂]₆ (7)₆ characterize Li-cyclopropane edge coordinations. The Li-cyclopropane interactions increase the C₂-C₃ distances (1.519(3) ?) relative to those of the free cyclopropyl edges (C₂-C₄ = C₆-C₇ = 1.499(2) ?) by 0.02 ?. The bent bonds of cyclopropane give rise to an electrostatic potential pattern, which strongly favors edge coordination as is observed experimentally in (7)₆, but also permits a metastable Li⁺ face complex. The cyclopropane edge also is the favored site for hydrogen bonding, but not for protonation. The C-C bond length elongations, the coordination energies E(coord), and the charge redistributions upon metal cation edge interactions all are related to the distances between the cyclopropane C-C bond centers and the cations. This is evaluated for the alkali metal cation-cyclopropane complexes (cation = Li⁺ to Cs⁺). More generally, the cyclopropane C-C bond length variations can be employed as a structural measure for the magnitudes of electrostatic interactions.

Full text of DOI:10.1021/ja9618661

J. Am. Chem. Soc. 1996, 118, 12183-12189
12183
A “Lithium-Bonded” Cyclopropyl Edge: The X-ray Crystal
Structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6 and
Computational Studies
Bernd Goldfuss, Paul von Rague´ Schleyer,* and Frank Hampel
Contribution from the Institut fu¨r Organische Chemie I der UniVersita¨t Erlangen-Nu¨rnberg,
Henkestrasse 42, D-91054 Erlangen, Germany
ReceiVed June 3, 1996. ReVised Manuscript ReceiVed August 19, 1996X
Abstract: The short Li-C distances (Li1-C2 ) 2.615(3) Å, Li1-C3 ) 2.644(3) Å) in the X-ray crystal structure of
[Li-O-C(Me)-(c-CHCH2CH2)2]6 (7)6 characterize Li-cyclopropane edge coordinations. The Li-cyclopropane
interactions increase the C2-C3 distances (1.519(3) Å) relative to those of the free cyclopropyl edges (C2-C4 )
C6-C7 ) 1.499(2) Å) by 0.02 Å. The bent bonds of cyclopropane give rise to an electrostatic potential pattern,
which strongly favors edge coordination as is observed experimentally in (7)6, but also permits a metastable Li+
face complex. The cyclopropane edge also is the favored site for hydrogen bonding, but not for protonation. The
C-C bond length elongations, the coordination energies Ecoord, and the charge redistributions upon metal cation
edge interactions all are related to the distances between the cyclopropane C-C bond centers and the cations. This
is evaluated for the alkali metal cation-cyclopropane complexes (cation ) Li+ to Cs+). More generally, the
cyclopropane C-C bond length variations can be employed as a structural measure for the magnitudes of electrostatic
interactions.
Introduction
molecules by cyclopropane (1) is said to be responsible for the
anaesthetic activity of 1.8 However, in weaker van der Waals
complexes, NH3,9 HNMe2,10 and NMe310 are located above the
cyclopropane ring plane (face coordination mode 4).
Complexes of cyclopropane (1) with various electrophiles and
nucleophiles exhibit unusual molecular structures (2-4).1-10
Computations on corner- (2a-asym Cs, 2a-sym Cs) and edge-
protonated cyclopropane (3a, C2V) reveal an extremely flat
potential energy surface.1 The ability of cyclopropanes to
participate as hydrogen-bonding proton acceptors, first demon-
strated inter- and intramolecularly by Schleyer et al. using IR
spectroscopy,2 has been confirmed recently by MW
spectroscopy.3-6 Like the π-systems in unsaturated hydrocar-
bons, the cyclopropane “bent bonds”7 act (edge coordination
mode 3) as proton acceptors for HOR,2 H2O,3 HF,4 HCl,5 and
HCN.6 The breaking of hydrogen bonds between water
X Abstract published in AdVance ACS Abstracts, October 15, 1996.
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Acta 1992, 65, 655. (b) Sieber, S.; Buzek, P.; Schleyer, P. v. R.; Koch,
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As the closest congener of hydrogen, lithium can form
“lithium bonds” in analogy to hydrogen bonds.11 Similar to
hydrogen, free ions, ion pairs, and atoms of lithium and heavier
alkali metals all coordinate to hydrocarbon π-systems.12 These
alkali metal π-interactions have been studied extensively, in
particular because of their role in biochemistry,13 e.g. in ion
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S0002-7863(96)01866-5 CCC: $12.00 © 1996 American Chemical Society

12184 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Goldfuss et al.
Scheme 1. Electrostatic Interactions in Hexameric Lithium
Alkoxide Clusters (Table 1)
channels, and in electrostatic catalyses of pericyclic reactions.14
The π-contacts appear with phenyl groups in aryl-,15 alkyl-,16
and amidolithiums,17 as well as in vinyl-18 and ethynyllithiums.19
Similar π-interactions are found in the lithium alkoxides [Li-
O-C(t-Bu)dCH2)]620 (5)6 and [Li-O-C(Me)2-CtCH]619 (6)6
(Scheme 1). Short Li H-C distances (“agostic”21 interactions)
are apparent in the structures of [i-PrLi]6,22 [n-BuLi]6,23
[t-BuLi]6,23 and [c-(CHCMe2CMe2)CH2Li]6.24 Despite the early
discovery of hydrogen-bonded cyclopropanes,2 a “lithium-
bonded” cyclopropane edge has not been observed hitherto.
Figure 1. The X-ray crystal structure of [Li-O-C(Me)-(c-CHCH2-
CH2)2]6 (7)6. Hydrogen atoms are omitted. See Table 1 for Li-C and
Table 2 for C-C distances.
Interactions of (transition) metals with neutral hydrocarbon
fragments are involved in catalytic C-H and C-C bond
activations.25 Hence, studies of complexes of cyclopropane and
alkali metals provide insights into the electrostatic component
of metal C-C interactions.
The present combined experimental and theoretical study on
alkali metal ion interactions with cyclopropyl groups focuses
on the X-ray crystal structure of [Li-O-C(Me)-(c-CHCH2-
CH2)2]6 (7)6. This reveals edge-coordinated cyclopropyllithium
arrangements. High-level computations on the molecular and
electronic structures as well as the energies of alkali metal
cation-cyclopropane complexes help interpret the experimental
results and reveal details of electrostatic metal cation C-C
interactions.
Figure 2. Asymmetric unit in the X-ray crystal structure of [Li-O-
C(Me)-(c-CHCH2CH2)2]6 (7)6. See Table 1 for Li-C and Table 2
for C-C distances.
Results and Discussion
The X-ray Crystal Structure of [Li-O-C(Me)-(c-CHCH2-
CH2)2]6 (7)6. Despite the widespread use of alkali metal
alkoxides in syntheses as well as in catalytic and ceramic
processes, surprisingly little structural information is avail-
able.12,26 The lithiums in the X-ray structures of 520 and 619
interact electrostatically with organic π-systems (Scheme 1).
To probe the possible electrostatic interactions between the
positively charged lithium centers in a (LiO)x cluster and the
C-C bonds of a cyclopropyl group, we synthesized and
crystallized Li-O-C(Me)-(c-CHCH2CH2)2 (7) from the non-
polar solvent hexane.27 The unsubstituted cyclopropyl rings in
7 facilitate coordination with the lithiums.24 The 2:1 cyclo-
propyl:lithium ratio results in both coordinated and free cyclo-
propyl groups. The latter serve as “internal standards” to assess
the structural effects of Li coordination.
Single-crystal X-ray analysis of Li-O-C(Me)-(c-CHCH2-
CH2)2 (7) revealed a hexameric aggregate (crystallographic S6
symmetry), as in (5)620 and (6)6.19 Electrostatic interactions in
(7)6 between the Li centers and the cyclopropyl groups are
shown clearly in Figures 1 and 2. The Li atoms are coordinated
5-fold by three O atoms in the (LiO)6 core and by two C atoms
(Li1-C2 ) 2.615(3) Å, Li1-C3 ) 2.644(3) Å) of the cyclo-
(13) (a) Mecozzi, S.; West, A. P., Jr.; Dougherty, D. A. J. Am. Chem.
Soc. 1996, 118, 2307. (b) Dougherty, D. A. Science 1996, 271, 163. (c)
Caldwell, J. W.; Kollman, P. A. J. Am. Chem. Soc. 1995, 117, 4177. (d)
Kumpf, R. A.; Dougherty, D. A. Science 1993, 261, 1708.
(14) (a) Jiao, H.; Schleyer, P. v. R. J. Chem. Soc., Faraday Trans. 1994,
90, 1559. (b) Jiao, H.; Schleyer, P. v. R. J. Am. Chem. Soc. 1995, 117,
11529. (c) Jiao, H.; Schleyer, P. v. R. Angew. Chem. 1993, 105, 1830;
Angew. Chem., Int. Ed. Engl. 1993, 32, 1760.
(15) Ruhlandt-Senge, K.; Ellison, J. J.; Wehmschulte, R. J.; Pauer, F.;
Power, P. P. J. Am. Chem. Soc. 1993, 115, 11353.
(16) Eaborn, C.; Hitchcock, P. B.; Smith, J. D.; Sullivan, A. C. J. Chem.
Soc., Chem. Commun. 1983, 1390.
(17) Barr, D.; Clegg, W.; Mulvey, R.; Snaith, R. J. Chem. Soc., Chem.
Commun. 1984, 287.
(18) (a) Ro¨lle, T.; Hoffmann, R. W. J. Chem. Soc., Perkin Trans. 2 1995,
1953. (b) Sorger, K.; Bauer, W.; Schleyer, P. v. R.; Stalke, D. Angew.
Chem. 1995, 107, 1766; Angew. Chem., Int. Ed. Engl. 34, 1594.
(19) Goldfuss, B.; Schleyer, P. v. R.; Hampel, F. Alkali Metal Cation
π-Interactions in Metalated and Nonmetalated Acetylenes. J. Am. Chem.
Soc., in press.
(20) Williard, P. G.; Carpenter, G. B. J. Am. Chem. Soc. 1985, 107, 3345.
(21) Brookhart, M.; Green, M. L. H. J. Organomet. Chem. 1983, 250,
395.
(22) Siemeling, U.; Redecker, T.; Neumann, B.; Stammler, H.-G. J. Am.
Chem. Soc. 1994, 116, 5507.
(23) Kottke, T.; Stalke, D. Angew. Chem. 1993, 105, 619; Angew. Chem.,
Int. Ed. Engl. 1993, 32, 580.
(24) Maercker, A.; Bsata, M.; Buchmeier, W.; Engelen, B. Chem. Ber.
1984, 117, 2547. No interactions between the cyclopropyl C-C bonds
and lithiums are apparent in [c-CHCMe2CMe2)CH2Li]6.
(25) (a) Schneider, J. J. Angew. Chem. 1996, 108, 1132; Angew. Chem.,
Int. Ed. Engl. 1996, 35, 1068 and references therein. (b) Holthausen, M.
C.; Fiedler, A.; Schwarz, H.; Koch, W. Angew. Chem. 1995, 107, 2430;
Angew. Chem., Int. Ed. Engl. 1995, 34, 2282. (c) Weisshaar, J. C. Acc.
Chem. Res. 1993, 26, 213. (d) Eller, K.; Schwarz, H. Chem. ReV. 1991,
91, 1121.
(26) (a) Herrmann, W. A.; Huber, N. W.; Runte, O. Angew. Chem. 1995,
107, 2371; Angew. Chem., Int. Ed. Engl. 1995, 34, 2187, (b) Caulton, K.
G.; Hubert-Pfalzgraf, L. G. Chem. ReV. 1990, 90, 969. (c) Veith, M. Chem.
ReV. 1990, 90, 3. (d) Bradley, D. C. Chem. ReV. 1989, 89, 1317.
(27) The methyl group in 7 is essential for good hydrocarbon solubility.
Attempts to crystallize the parent compound Li-O-C(H)-(c-CHCH2CH2)2
from hexane solutions failed due to its insolubility: Goldfuss, B.; Schleyer,
P. v. R. Unpublished results.

Structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12185
Table 1. Selected X-ray Crystal Data for [Li-O-C(t-Bu)dCH2]6
(5)6,a [Li-O-C(Me)2-CtC-H]6 (6)6,b and
[Li-O-C(Me)-(c-CHCH2CH2)2]6 (7)6 (Scheme 1)
c
(5)6
(6)6
(7)6
Li1-O1 (Å)
Li1a-O1 (Å)
Li1b-O1 (Å)
1.976 (9)
1.869 (9)
1.954 (9)
1.955 (5)
1.877 (5)
1.923 (5)
1.937 (3)
1.881 (3)
1.926 (3)
Li1-O1-C1 (deg)
88.0 (9)
Li1a-O1-C1 (deg) 140.0 (4)
Li1b-O1-C1 (deg) 132.9 (4)
105.3 (2)
130.8 (2)
134.2 (2)
105.6 (1)
132.6 (1)
135.1 (1)
Li1-C1 (Å)
Li1-C2 (Å)
Li1-C3 (Å)
2.349 (9)
2.420 (8), 2.53d
2.687 (5)
2.443 (5)
2.749 (6)
2.680 (3)
2.615 (3)
2.644 (3)
a Reference 20. b Reference 19. c One of two asymmetric units in
the unit cell with approximate S6 symmetry. d Average value of the
two asymmetric units.
Table 2. C-C Distances (Å) in the Li-Coordinated and the Free
[Li-O-C(Me)-(c-CHCH2CH2)2]6 (7)6 Cyclopropyl Groups
c-CHCH2CH2 (Li-coord)
c-CHCH2CH2 (free)
C2-C3
C2-C4
C3-C4
1.519 (3)
1.499 (2)
1.508 (3)
C6-C7
C6-C8
C7-C8
1.499 (2)
1.493 (2)
1.498 (3)
propyl groups. Other cyclopropyl moieties are free from such
interactions. The Li1-C2-C3-C4 dihedral angle 176.0° docu-
ments the nearly coplanar arrangement of the Li1-C2-C3 and
C2-C3-C4 faces and the cyclopropane edge coordination
(Figure 2). As in [Li-O-C(t-Bu)dCH2]6 (5)620 and [Li-O-
Figure 3. Cyclopropane 1 D3h and Li+ and LiOH cyclopropane
complexes 2b C2V, 3b C2V, 4b C3V, and 3b-OH C2V; RB3LYP/6-
311+G** (C, H, O), /6-31G* (Li) optimized geometries. The bond
distances are given in angstroms.
19
cyclopropane has been investigated extensively,1 but studies of
Li+-cyclopropane interactions are rare.31
C(Me)2-CtCH]6 (6)6 (Scheme 1), this coordination in (7)6
results in a tilt of the organic fragment O1-C1 toward Li1 (Li1-
O1-C1 ) 105.6(1)°, Li1a-O1-C1 ) 132.6(1)°, Li1b-O1-C1
) 135.1(1)°) and leads to a differentiation in the Li-O1
distances (Li1-O1 ) 1.937(3) Å, Li1a-O1 ) 1.881(3) Å, Li1b-
O1 ) 1.926(3) Å, Table 1).19 This “lithium bonding”11 to the
cyclopropane edge in (7)6 results in increased C-C bond lengths
of the cyclopropyl rings: The coordinated cyclopropyl edges
(C2-C3 ) 1.519(3) Å) are 0.02 Å longer than the equivalent
free edges C2-C4 (1.499(2) Å) and C6-C7 ) (1.499(2) Å) and
are 0.026 Å longer than C6-C8 (1.493(2) Å) (Table 2); C3-C4
(1.508(3) Å) is slightly elongated by 0.01 Å relative to C7-C8
(1.498(3) Å).28
The geometries of the lithium-bonded cyclopropyl groups in
the X-ray crystal structure of 7 (Figure 2) are confirmed
computationally: Consistent with results on hydrogen-bonded
cyclopropane species,2-6 the edge complex 3b (Ecoord ) 22.9
kcal/mol) is the most stable minimum on the Li+-cyclopropane
potential surface; the corner transition structure (2b) (Ecoord
)
13.2 kcal/mol) and the face isomer (4b) (Ecoord ) 11.6 kcal/
mol) are 9.7 and 11.3 kcal/mol less stable (Figure 3, Tables 3
and 4).
Comparisons between 3b and 4b and the analogous H+
species (3a, 4a, Figure 4) bring out significant differences
(Tables 3 and 4): 3a is a transition structure and close in energy
to the corner-protonated minimum 2a-asym. In contrast to 4b,
face-protonated 4a is a high-energy second-order saddle point
(Tables 3 and 4). Whereas the protonated hydrocarbons exhibit
strong covalent or multicenter bonding (see the H+ natural
population analysis (NPA) charges in Table 3),29b,30c electrostatic
interactions provide the basis for the stable Li+ locations in 3b
and 4b. The C-C bent bonds7 of 1 result in areas of negative
electrostatic potential outside the ring (Figures 5 and 6a), which
favor the Li+ edge coordination of 3b over the Li+ corner
position in 2b. The positively charged H atoms result in three
electrostatic potential maxima above the ring plane of 1,
surrounding a lower but still positive area (Figure 6b). This
explains the “meta”-stable Li+ position in 4b. As in both the
Li+ (3b) and the LiOH complex (3b-OH), electrostatics
dominate the Li-cyclopropane interactions (see the Li NPA
charges in Table 3); Li+ is a valid model for lithium-bonded
species.
The Cyclopropane Edge as a Structural Probe for
Electrostatic Interactions. A variety of complexes between
alkali metal ions12 and saturated as well as unsaturated
hydrocarbons have been studied both experimentally29 and
computationally.30 The metal cation binding energies (Ecoord
,
as in eq 1) in these species decrease from Li+ to Cs+ 29a and
are large for aromatics (Ecoord C6H6-Li+ ) 37 kcal/mol),29b
but they are substantial even for saturated hydrocarbons such
as cyclohexane (Ecoord C6H12-Li+ ) 24 kcal/mol).29b Methyl
substitution of unsaturated hydrocarbons increases the Li+
coordination energy.29b Whereas protonated species are strongly
covalent, the analogous Li+ complexes are bound largely
ionically.14,29b,30c The potential energy surface of protonated
(28) These CR-Câ elongations by CR-CR coordination are not repro-
duced computationally (see below) and may arise from substituent effects
and asymmetric edge coordination of the cyclopropyl groups in (7)6. No
significant interactions between the (7)6 clusters are apparent in the crystal.
(29) (a) Hodges, R. V.; Beauchamp, J. L. Anal. Chem. 1976, 48, 825.
(b) Staley, R. H.; Beauchamp, J. L. J. Am. Chem. Soc. 1975, 97, 5920. (c)
Wieting, R. D.; Staley, R. H.; Beauchamp, J. L. J. Am. Chem. Soc. 1975,
97, 924.
(30) (a) Fujii, T.; Tokiwa, H.; Ichikawa, H.; Shinoda, H. J. Mol. Struct.
(THEOCHEM) 1992, 277, 251. (b) Guo, B. C.; Purnell, J. W.; Castleman,
A. W., Jr. Chem. Phys. Lett. 1990, 168, 155. (c) Bene, J. E. D.; Frisch, M.
J.; Raghavachari, K.; Pople, J. A.; Schleyer, P. v. R. J. Phys. Chem. 1983,
87, 73.
(31) (a) At the RHF/6-31G level, edge-coordinated Li+-C3H6 was found
to be 53.0 kcal/mol more stable than the corner isomer: Davidson, E. R.;
Shiner, V. J., Jr. J. Am. Chem. Soc. 1986, 108, 3135. (b) In studies on
ring-opening reactions of cyclopropane, an edge Li+-C3H6 with a strongly
expanded C-C bond was computed, but the nature of this species was not
clarified: Yamabe, S.; Minato, T.; Seki, M.; Inagaki, S. J. Am. Chem. Soc.
1988, 110, 6047.

12186 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Goldfuss et al.
Table 3. Coordination Energies and Natural Charges of Protonated
and Lithium-Complexed Hydrocarbonsa
PG (NIMAG)b Ecoord (kcal/mol)c q H+ or q Li+ (au)d
1
D3h (0)
Cs (0)
Cs (1)
2a-asym
2a-sym
2b
3a
3b
3b-OH
4a
4b
179.83
179.80
13.18
177.07
22.91
8.86
+0.321
+0.311
+0.985
+0.336
+0.982
+0.953
+0.660
+0.974
C
C
C
C
C
C
D
C
D
C
2V (1)
2V (1)
2V (0)
2V (0)
3V (2)
3V (0)
3h (1)
2V (1)
4h (1)
2V (1)
95.85
11.63
8
8-Li+
9
15.01
19.40
23.33
19.62
+0.981
+0.979
+0.969
+0.967
9-Li+
10
Cs (0)
C1 (0)
10-Li+
11
D
2h (0)
11-Li+
C2V (0)
a B3LYP/6-311+G** (C, H, O), /6-31G* (Li) optimized geometries.
b Point groups and (in parentheses) number of imaginary frequencies,
obtained from B3LYP frequency calculations. c H+ or Li+ coordination
energies Ecoord (ZPE corrected, eq 1) of the protonated or Li+-complexed
species. d Natural charges (ref 42) of coordinated H+ or Li+.
Table 4. Relative Energies (kcal/mol) and Numbers of Imaginary
Frequencies of Protonated and Li+-Complexed Cyclopropanesa
corner
edge
face
H+
2a-asym
2b
0.00 (0)
9.73 (1)
3a
3b
2.76 (1)
0.00 (0)
4a
4b
83.98 (2)
11.28 (0)
Li+
a See Table 3.
Figure 5. Electrostatic potential maps (RHF/6-31+G**//B3LYP/6-
311+G**) of the C-C bonds in cyclopropane 1 D3h, eclipsed ethane
8 D3h, planar cyclobutane 9 D4h, and ethene 11 D2h. Energies in kcal/
mol.
lobes, caused by the C-C bent bonds,7 resemble those of the
π-electrons in ethene 11 (Figure 5).
The other alkali metal cation-cyclopropane complexes 3c-f
document further the energetic, electronic, and structural effects
of the edge coordination mode (3). All alkali metal cations in
3b-f have nearly unit charges (Table 5) and hence serve as
good “point charge models”.12 The computed coordination
energies Ecoord (eq 1) decrease in 3b-f with increasing distances
r between the cyclopropane edge and the cations M+ (Table
6). The best correlation is obtained between Ecoord and 1/r2.5
for 3b-f (Figure 8). Ion/quadrupole interactions (the potential
energy V of interacting n- and m-poles varies with the distance;
see eq 2)32 are discussed as dominant contributions in cation
benzene π-bondings.13,33 For these π-interactions, however, a
1/rx (x < 2) dependence is found.13b To estimate the electrostatic
contribution of the metal cation binding in 3b-f, the metal
centers are replaced by dummy charges and the electrostatic
Figure 4. Protonated cyclopropanes 2a-asym Cs, 2a-sym Cs, 3a C2V,
4a C3V; RB3LYP/6-311+G** (C, H) optimized geometries. The bond
distances are given in angstroms.
The special bent bond7 character in 1 is also apparent from
a comparison with eclipsed ethane (8) and planar cyclobutane
(9). The -H2C-CH2- topologies in 1, 8, and 9 are very similar
(Figure 7). Due to their higher polarizability, larger (and
unsaturated) hydrocarbons exhibit usually higher Li+ Ecoord than
smaller ones.29b However, the Li+ Ecoord of 3b (22.91 kcal/
mol) is higher than the Ecoord of 8-Li+ (15.0 kcal/mol), of
9-Li+ (19.4 kcal/mol), and of the Li+-ethene complex 11-
Li+ (19.6 kcal/mol) and even approaches the Ecoord of the Li+-
propene complex 10-Li+ (23.3 kcal/mol, Table 3, Figure 7).
Consistent with the lower Li+ affinity of 8 and 9, no negative
electrostatic potential areas of the C-C bonds are apparent in
8 and 9 (Figure 5). In 1, the negative electrostatic potential
potentials (EP) at these points are computed (Table 6).13a
A
(32) Atkins, P. W. Physical Chemistry; Oxford Press: Oxford, 1992.
(33) (a) Luhmer, M.; Bartik, K.; Dejaegere, A.; Bovy, P.; Reisse, J. Bull.
Soc. Chim. Fr. 1994, 131, 603. (b) Williams, J. H. Acc. Chem. Res. 1993,
26, 593.

Structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12187
Figure 7. Hydrocarbons and their Li+ complexes: eclipsed ethane 8
D3h, 8-Li+ C2V, planar cyclobutane 9 D4h, 9-Li+ C2V, propene 10 Cs,
10-Li+ C1, ethene 11 D2h, 11-Li+ C2V; RB3LYP/6-311+G**
(C, H), /6-31G* (Li) optimized geometries. The bond distances are
given in angstroms.
Table 5. Natural Charges q (au) of Alkali Metal
Cation-Cyclopropane Edge Complexesa
q M
q CR
q H(CR)
q Câ
q H(Câ)
1 (D3h)
-0.404
-0.529
-0.501
-0.492
-0.468
-0.454
-0.446
+0.202
+0.235
+0.208
+0.222
+0.212
+0.208
+0.206
-0.404
-0.340
-0.501
-0.356
-0.363
-0.369
-0.373
+0.202
+0.240
+0.302
+0.233
+0.226
+0.223
+0.221
3b (C2V)
4b (C3V)
3c (C2V)
3d (C2V)
3e (C2V)
3f (C2V)
+0.980
+0.973
+0.988
+0.997
+0.999
+0.999
Figure 6. (a) Electrostatic potential surface and contour map (RHF/
6-31+G**//B3LYP/6-311+G**) of a C-C bond in cyclopropane (1,
D3h) in a distance of 2 Å from the center of the CCC ring. Energies
in kcal/mol. (b) Electrostatic potential surface and contour map (RHF/
6-31+G**//B3LYP/6-311+G**) of the cyclopropane (1, D3h) face in
a distance of 2 Å above the CCC ring plane. Energies in kcal/mol.
a RB3LYP/6-31+G** (C, H), /6-31G(2d) (Li, Na), /LanL2DZ(2d)
ECP (K, Rb, Cs) optimized geometries. The B3LYP/6-31++G** (C,
H), /6-31G(2d) (Li, Na), /LanL2DZ(2d) ECP (K, Rb, Cs) wave
functions are used for the natural population analysis, ref 42.
correlation between Ecoord and EP reveals a slope of 1.5 (Figure
9), which accounts for the increasing electrostatic contributions
with increasing metal cation sizes (compare the NPA charges
in Table 5).
HC + X+ f HC-X+ + Ecoord
r (Figure 10a). The best correlation is found between the CR,
Câ charges and 1/r2 (Figure 10b), reflecting the distance
dependence of the electric fields Fel of the cations (eq 3).32
In accord with our experimental observations on (7)6, Li+
cation-cyclopropane edge coordination increases the CR-CR
bond lengths in 3b-f relative to the length in 1 (Table 6, Figure
11a). This bond elongation (∆CR-CR) decreases linearly with
increasing distance r in 3b-f (Figure 11b). Hence, the structural
change in the coordinated cyclopropane C-C edge is related
to the coordination energy Ecoord and the changes in the charge
(HC ) hydrocarbon; X ) H+, Li+ to Cs+)
1/r(n+m-1)
Fel ) q/(4πꢀ0)r2 1/r2
(1)
(2)
(3)
V
As is apparent from CR, Câ charges (Table 5), the polarization
of negative charge decreases in 3b-f with increasing distance

12188 J. Am. Chem. Soc., Vol. 118, No. 48, 1996
Goldfuss et al.
Table 6. Energies (kcal/mol) and Bond Distances (Å) of Alkali Metal Cation-Cyclopropane Edge Complexesa
b
Ecoord
EPc
M-CR
M-(C-C)d
CR-CR
∆(CR-CR)
CR-Câ
1 (D3h)
1.510
1.560
1.513
1.551
1.535
1.530
1.526
1.510
1.503
1.513
1.503
1.506
1.506
1.507
3b (C2V)
4b (C3V)
3c (C2V)
3d (C2V)
3e (C2V)
3f (C2V)
-22.48
-10.56
-12.95
-6.26
-4.07
-2.84
-16.51
+4.99
-10.78
-6.31
-4.63
-3.59
2.188
2.286
2.579
3.065
3.374
3.636
2.044
2.112e
2.460
2.967
3.286
3.555
0.050
0.003
0.041
0.025
0.020
0.016
a RB3LYP/6-31+G** (C, H), /6-31G(2d) (Li, Na), /LanL2DZ(2d) ECP (K, Rb, Cs) optimized geometries. b Coordination energies Ecoord (ZPE
corrected, eq 1) of the M+-complexed species. c Electrostatic potential energies at the metal centers, which were replaced by dummy charges.
d Distances r from the centers of the CR-CR bonds to the cations. e Distance from the center of the ring to the cation.
a
b
Figure 8. Correlation between the metal cation coordination energies
Ecoord (eq 1) and the metal cation edge distances r of 3b-f.
Figure 10. (a) Natural charges of carbon atoms, reflecting the charge
polarizations by the metal cations of 3b-f. (b) Correlation between
carbon charges and the metal cation edge distances r of 3b-f.
Li+ edge coordination energy of 1 (Ecoord ) 22.9 kcal/mol) in
comparison to Ecoord of eclipsed ethane (8, 15.0 kcal/mol) and
planar cyclobutane (9, Ecoord ) 19.4 kcal/mol). Unlike the
protonated cyclopropanes (see comparison in Table 4), the Li+-
cyclopropane face complex 4b is predicted to be a 11.3 kcal/
mol higher energy minimum, but the corner-lithiated 2b is a
transition structure (9.7 kcal/mol less stable than 3b). The extent
of C-C bond length elongations, the coordination energies
Ecoord, and the cyclopropyl CR, Câ charge polarizations all are
related to the distances between the C-C edge and the alkali
metal cations in 3b-f. Hence, the cyclopropane C-C bond
length elongations, as observed experimentally in the X-ray
crystal structure (7)6, provide a measure for the degree of the
electrostatic interaction with edge-coordinating metal cations.
Figure 9. Correlation between the metal cation coordination energies
Ecoord (eq 1) and the electrostatic potentials EP at the replaced metal
centers of 3b-f.
distribution of the cyclopropyl group due to counterion com-
plexation.
Conclusions
The edge interactions of Li ions of the (LiO)6 cluster with
the cyclopropyl groups result in ca. 0.02 Å bond length
elongations in the X-ray structure of (7)6. This finding
documents the analogy between hydrogen-bridged and lithium-
bonded cyclopropyl groups and emphasizes the “electrostatic
component” in C-C bond metal cation coordination and
activation. The electrostatic potential pattern of the C-C bonds
in cyclopropane (1) resembles organic π-systems, such as ethene
(11), and explains the favorable Li+ edge coordination mode
in the X-ray crystal structure (7)6 and in 3b. Furthermore, the
electrostatic potential provides the basis for the unusually high
Experimental Section
The experiments were carried out under an argon atmosphere by
using standard Schlenk as well as needle/septum techniques. The
solvents were freshly distilled from sodium/benzophenone. Dicyclo-
propyl ketone (Aldrich) was distilled prior to use. The NMR spectra
were recorded on JEOL GX and JEOL Alpha 500 (CP-MAS)

Structure of [Li-O-C(Me)-(c-CHCH2CH2)2]6
J. Am. Chem. Soc., Vol. 118, No. 48, 1996 12189
C8H13O1Li1: C, 72.7; H, 9.9. Found: C, 71.9; H, 10.1 Colorless single
crystals of 7 were obtained by cooling hexane solutions.
a
X-ray crystal data for (7)6: Mr ) 132.12; rhombohedral; space group
R3h; a ) b ) 12.217(3) Å, c ) 26.562(6) Å; V ) 3433.1(13) Å3; Dcalc
) 1.150 Mg m-3; Z ) 18; F(000) ) 1296; Mo KR (λ ) 0.71 073 Å);
T ) 173(2) K; data were collected with a Enraf Nonius CAD4-Mach3
diffractometer on a crystal with the dimensions 0.40 × 0.30 × 0.30
mm using the ω-scan method (3.0° < 2θ < 54.0°). Of the total of
1773 collected reflections, 1556 were unique and 1212 with I > 2σ(I)
were observed. The structure was solved by direct methods using
SHELXS 86; 143 parameters with all data were refined by full matrix
least squares on F2 using SHELXL93 (G. M. Sheldrick, Go¨ttingen,
1993). All non-hydrogen atoms were refined anisotropically; the
hydrogen atoms were refined independently and isotropically. The final
R values were R1 ) 0.0492 (I > 2σ(I)) and R2w ) 0.1385 (all data)
2
2
with R1 ) ∑|Fo-Fc|/∑Fo and R2w ) ∑w|(Fo - Fc2)2|/∑(w(Fo )2)0.5
;
GOF ) 1.110; largest peak (0.287 e Å-3) and hole (-0.208 e Å-3).
Further details are available on request from the Director of the
Cambridge Crystallographic Data Center, Lensfield Road, GB-
Cambridge CB2 1 EW, by quoting the journal citation.
b
Theoretical Section
All theoretical structures were optimized using the gradient
techniques implemented in GAUSSIAN 9438 with Becke’s three-
parameter hybrid functional incorporating the Lee-Yang-Parr
correlation functional (Becke3LYP).39 The 6-311+G** and
6-31+G** (C, H, O) as well as 6-31G(d) (Li) and 6-31G(2d)
(Li, Na) all-electron basis sets were used. For K, Rb, and Cs
9-valence electron effective core potentials40 and the LanL2DZ
basis sets, K (341/311), Rb (341/321), Cs (341/321), each
augmented with two polarization functions,41 were used. The
characters of the stationary points, the zero-point energy
corrections, and the harmonic vibration frequencies were
obtained from analytical and, for pseudopotential computations
of the K, Rb, and Cs systems, from numerical frequency
calculations. All partial charges are based on the natural
population analysis42 of the Becke3LYP electron density. The
electrostatic potentials were evaluated with RHF/6-31+G**
wave functions on optimized B3LYP geometries.
Figure 11. (a) Cyclopropane CR-CR edge elongations by metal cation
coordinations of 3b-f. (b) Correlation between the cyclopropane edge
elongations ∆(CR-CR) and the metal cation edge distances r of 3b-f.
spectrometers (1H, 400 MHz; 13C, 100.6 MHz) and referenced to TMS
or to adamantane (CP-MAS). IR spectra were determined neat or as
Nujol mulls between NaCl disks on a Perkin-Elmer 1420 spectrometer.
Mass spectral data were obtained on a Varian MAT 311A spectrometer
and the elemental analyses (C, H) on a Heraeus micro automaton.
[Li-O-C(Me)-(c-CHCH2CH2)2] (7). Dicyclopropylmethyl-
carbinol34 was synthesized by a procedure analogous to that for
dicyclopropylcarbinol:35 A solution of methylmagnesium iodide in 100
mL of diethyl ether was prepared from 6.1 g (0.25 mol) of Mg and
35.5 g (0.25 mol) of methyl iodide.36 To this Grignard mixture was
slowly added 27.5 g (0.25 mol) of dicyclopropyl ketone diluted in 100
mL of diethyl ether. After 1 h of reflux, hydrolysis with H2O/NH4Cl,
extraction with diethyl ether, and drying over Na2SO4, the distillation
afforded 26.5 g (0.21 mol) of dicyclopropylmethylcarbinol HO-
Acknowledgment. This paper is dedicated to Prof. Rolf
Gleiter on the occasion of his 60th birthday. This work was
supported by the Fonds der Chemischen Industrie (also through
a scholarship to B.G.), the Stiftung Volkswagenwerk, the
Convex Computer Corp., and the Deutsche Forschungsgemein-
schaft. We thank Priv.-Doz. Dr. J. J. Schneider (Essen) for
fruitful suggestions.
Supporting Information Available: Tables giving the X-ray
crystal data and the zero-point energies of the computed systems
(9 pages). See any current masthead page for ordering and
Internet access instructions.
1
C(Me)-(c-CHCH2CH2)2 (84% yield): bp 35 °C/1.5 mbar; H NMR
(CDCl3) δ 1.41 (s, OH), 1.10 (s, CH3), 0.89 (m, CH), 0.37 (m, CH2);
13C{1H} NMR/DEPT (CDCl3) δ 69.32 (C), 25.25 (CH3), 19.88 (CH),
-0.46 (CH2); IR (neat, cm-1) 3460 (ν OH), 3080, 3000, 2960, 2920
(ν CH), 1365, 1155, 1100, 1040, 1010 (δ CCC).37
JA9618661
To a stirred solution of 0.24 g (1.9 mmol) of dicyclopropylmethyl-
carbinol in 1.0 mL of hexane at 0 °C was added 1.17 mL of n-BuLi/
hexane (1.6 M, Acros). After the solution was stirred for 5 min at
room temperature, the white precipitate was separated from the solvent
and dried in vacuum: 1H NMR (CDCl3) δ 0.76 (s, CH3), 0.36 (m,
CH), 0.29 (m, CH2); 13C{1H} NMR/DEPT (CDCl3) δ 69.01 (C), 24.77
(CH3), 23.44 (CH), -0.20 (CH2); 13C CP-MAS δ 70.35 (C), 28.42
(CH3), 20.85 (CH), 2.74 (CH2); IR (Nujol mull, cm-1) 3070 (ν CH),
1155, 1130, 1010 (δ CCC); MS (EI, 70 eV, 120 °C) m/e 403 ([M]3 -
Li+), 271 ([M]2 - Li+), 139 ([M]1 - Li+), 111 (HOC-(c-CHCH2-
CH2)2+), 98 (O(c-CHCH2CH2)2+), 43 (C3H7+); Anal. Calcd for
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V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski, J.; Stefanov, B. B.;
Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala, P. Y.; Chen, W.;
Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.; Martin, R. L.;
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(34) Hanack, M.; Eggensperger, H. Angew. Chem. 1962, 74, 116. No
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(35) Hart, H.; Curtis, O. J. Am. Chem. Soc. 1956, 78, 112.
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