Alkali metal cation π-interactions in metalated and nonmetalated acetylenes: π-Bonded lithiums in the X-ray crystal structures of [Li-C≡C-SiMe2-C6H4-OMe]6 and [Li-O-CMe2-C≡C-H]6 and computa

Article abstract of DOI:10.1021/ja9622517

The X-ray crystal structure of [Li-C≡C-SiMe₂-C₆H₄-OMe]₆ (14)₆ features nearly symmetric π-interactions between the lithium ions and the acetylide anions (Li₁-C(β) = 2.353(9) A?, Li₁

Full text of DOI:10.1021/ja9622517

1072
J. Am. Chem. Soc. 1997, 119, 1072-1080
Alkali Metal Cation π-Interactions in Metalated and
Nonmetalated Acetylenes: π-Bonded Lithiums in the X-ray
Crystal Structures of [Li-CtC-SiMe2-C6H4-OMe]6 and
[Li-O-CMe2-CtC-H]6 and Computational Studies
Bernd Goldfuss, Paul von Rague´ Schleyer,* and Frank Hampel
Contribution from the Institut fu¨r Organische Chemie der UniVersita¨t Erlangen-Nu¨rnberg,
Henkestrasse 42, D-91054 Erlangen, Germany
ReceiVed July 2, 1996. ReVised Manuscript ReceiVed NoVember 4, 1996X
Abstract: The X-ray crystal structure of [Li-CtC-SiMe2-C6H4-OMe]6 (14)6 features nearly symmetric
π-interactions between the lithium ions and the acetylide anions (Li1-Câ ) 2.353(9) Å, Li1-CR ) 2.292(9) Å).
These π-contacts are facilitated by the chelating o-anisyl methoxy groups (Li1-CR-Câ ) 77.6(4)°, Li1-O1 ) 2.169(9)
Å). The Li-CR distances in the (LiCR)6 core of (14)6 differ significantly (Li1R-CR ) 2.132(9) Å, Li1b-CR
)
2.205(11) Å). This Li-CR distance differentiation is unique in organolithium hexamers, and is due to Li(CtC-R)
“side-on-π” and “end-on-σ” contacts, as is shown computationally in H-CtC-Li(LiH)2 (20). A second X-ray
crystal structure, [Li-O-CMe2-CtC-H]6 (22)6, reveals electrostatic π-interactions between the lithiums in the
(LiO)6 core and the nonmetalated acetylene groups (Li1-C2 ) 2.443(5) Å, Li1-C3 ) 2.749(6) Å). These Li-C
π-contacts shorten upon acetylene lithiation, as is shown computationally in Li-O-CH2-CtC-(H/Li) (24-H/Li).
Additional computations reveal that the π-interactions in (HCtC)M2H (26-Li-Cs) complexes (modelling oligo- and
polymeric M-CtC-R) are weak (only 0.7 kcal/mol for Li), but substantial in M+(H-CtC-H) (27-Li-Cs) species
(20.2 kcal/mol for Li+). In 26-Li-Cs, the π-contacts increase the CtC bond lengths slightly (0.005 Å for Li) and
lower the CtC stretching frequencies (33 cm-1 for Li), they polarize charge density from CR toward Câ and hence
result in counterion-induced charge delocalizations. The degrees of π-interactions both in (26-Li-Cs) and in (27-
Li-Cs) decrease with increasing size of the alkali cations.
Introduction
Scheme 1. Polymer Sheet Structure of the Alkali Metal
Acetylides 1-Na-Rb and 2-Na-Csa
In 1976, Apeloig, Schleyer, Binkley, Pople, and Jorgensen
discovered computationally that the dilithioacetylene monomer
(Li2C2) prefers a double π-bridged over a linear structure:1a
M )
M-CtC-H
M-CtC-Me
M-CtC-Ph
Electrostatic interactions are mainly responsible.1,2a This paper
is concerned with similar π-interactions both in polar alkali
metal acetylides (π-M [M-CtC-R]) and in nonmetalated
acetylenes (π-M [H-CtC-R]). Such π-interactions are clearly
evident in the structures of alkali metal2 acetylides when the
H
Li
Na
K
Rb
Cs
anion
1
2
13
1-Li
1-Na
1-K
1-Rb
1-Cs
1-anion
2-Li
2-Na
2-K
2-Rb
2-Cs
13-Li
13-Na
13-K
13-Rb
13-Cs
X Abstract published in AdVance ACS Abstracts, January 1, 1997.
(1) (a) Apeloig, Y.; Schleyer, P. v. R.; Binkley, J. S.; Pople, J. A.;
Jorgensen, W. L. Tetrahedron Lett. 1976, 3923. (b) Schleyer, P. v. R. J.
Phys. Chem. 1990, 94, 5560. (c) Ritchie, J. P.; Bachrach, S. M. J. Am.
Chem. Soc. 1987, 109, 5909. (d) Jaworski, A.; Person, W. B.; Adamowicz,
L.; Bartlett, R. J. Int. J. Quantum Chem. Symp. 1987, 21, 613.
(2) For reviews on structures of alkali metal organic compounds, see:
(a) Sapse, A.-M.; Schleyer, P. v. R., Eds. Lithium Chemistry, Wiley: New
York, 1995. (b) Lambert, C.; Schleyer, P. v. R. Angew. Chem. 1994, 106,
1187; Angew. Chem., Int. Ed. Engl. 1994, 33, 1129. (c) Lambert, C.;
Schleyer, P. v. R. Methoden Org. Chem. (Houben-Weyl), 4th ed.; 1952-,
Bd. E19d, 1993, p 1. (d) Weiss, E. Angew. Chem. 1993, 105, 1565; Angew.
Chem., Int. Ed. Engl. 1993, 32, 1501. (e) Gregory, K.; Schleyer, P. v. R.;
Snaith, R. AdV. Inorg. Chem. 1991, 37, 47. (f) Schade, C.; Schleyer, P. v.
R. AdV. Organomet. Chem. 1987, 27, 169. (g) Setzer, W. N.; Schleyer, P.
v. R. AdV. Organomet. Chem. 1985, 24, 353. (h) Bock, H.; Ruppert, K.;
Na¨ther, C.; Havlas, Z.; Herrmann, H.-F.; Arad, C.; Go¨bel, I.; John, A.;
Meuert, J.; Nick, S.; Rauschenbach, A.; Seitz, W.; Vaupel, T.; Solouki, B.
Angew. Chem. 1992, 104, 564; Angew. Chem., Int. Ed. Engl. 1992, 31,
550.
a The increasing penetration of M-CtC-R layers with increasing
size of M is shown.
M-CR and M-Câ distances are similar,3 e.g., in the polymer
sheet arrangements of M-CtC-H (1-Na-Rb)4 and of
M-CtC-Me (2-Na-Cs)4-6 (Scheme 1, Table 1).
However, such metal π-contacts are not always significant;
e.g., note the large M-Câ separations (>3 Å) in the oligomeric
lithium acetylides: [(t-Bu-CtC-Li)4(THF)4] (3),7 [(t-Bu-
(3) For a discussion of π-interactions in alkaline earth metal acetylides
see: Chang, C.-C.; Srinivas, B.; Wu, M.-L.; Chiang, W.-H.; Chiang, M.
Y.; Hsiung, C.-S. Organometallics 1995, 14, 5150.
(4) Weiss, E.; Plass, H. Chem. Ber. 1968, 101, 2947.
(5) Pulham, R. J.; Weston, D. P. J. Chem. Res. (S) 1995, 406.
(6) Pulham, R. J.; Weston, D. P.; Salvesen, T. A.; Thatcher, J. J. J. Chem.
Res. (S) 1995, 254.
S0002-7863(96)02251-2 CCC: $14.00 © 1997 American Chemical Society

Alkali Metal Cation π-Interactions in Acetylenes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1073
CtC-Li)12(THF)4],7 [(Ph-CtC-Li)tmpda]2 (4),8 and [(Ph-
CtC-Li)4(tmhda)4/2] (5)9 (Table 1). In contrast, the short Be-
Câ distances indicate π-interactions in [(Me-CtC)2BeNMe3]2
(6) (Table 1).10
Li-Cs decrease with increasing size of the alkali metal cations
(Table 2).15 Negative charge delocalization from CR to Câ is
indicated both by the increased metal cation/acetylide interac-
tions upon increased M-CtC-R layer penetration (see Scheme
1)4 and by the lower ν-CtC frequencies.15 However, an
increase in ion size also gives rise to lower ν-CtC frequencies
(see below).15 How can the effect of π-coordination be
differentiated?
Alkali metal π-bonding to benzene ligands has been inves-
tigated extensively owing to its important role in biological ion
channels.16 As “lithium-bonded”17 cyclopropanes emphasize
the analogy to hydrogen-bonded cyclopropanes,17d,18 π-“lithium-
bonded” acetylenes stress the analogy to π-hydrogen-bonded
acetylenes.18a,19 The Li+20 and LiH21 π-association energies of
acetylene are appreciable and are even larger when the
acetylenes are metalated.22 Notwithstanding, Li+ π-bonding has
not been observed experimentally in X-ray crystal structures
of homogeneous lithium acetylides or in compounds with
nonmetalated acetylene groups.2
For an assessment of electrostatic metal acetylene π-interac-
tions,23 we now report the X-ray crystal structures of lithiated
(Li-CtC-SiMe2-C6H4-OMe) and of nonlithiated (Li-O-
CMe2-CtC-H) acetylene moieties. Both exhibit π-bonded
Li ions. High level computations reveal the structural, the
energetic, and the ω-CtC vibrational consequences of alkali
cation π-interactions in related metal acetylene models and
assess the electronic effects of π-coordination.
3, R = t-Bu; L = THF
4, M = Li; R = Ph; L = tmpda
1
–
5, R = Ph; L = tmhda
–
)
6, M = Be; R = Me; L = (C C-Me)(NMe
3
/
–
2
Moreover, the alkali cations in the heterometallic magnesiates
Li2[(Ph-CtC)3Mg(tmeda)]2 (7),11 Na2[(t-Bu-CtC)3Mg(tmeda)]2
(8),12 and Na2[(t-Bu-CtC)3Mg(pmdta)]2 (9)12 connect the
acetylene moieties of the [Mg(CtC-R)3]- fragments through
π-contacts (Table 1). Analogous structures as in 7 to 9 result
from replacement of M+ by EtMg+ in (Et)(Ph-CtC)3(Mg)2
(tmeda)]2(C6H6) (10).13 Similarly, the lithiums in [Me3SiC
(CtC-t-Bu)2Li]2 (11)2g as well as the magnesium ion in
[(C5HMe4)2Ti(CtC-SiMe3)2][Mg(THF)Cl] (12)14 are located
between the arms of “tweezers” formed by the acetylene groups
(Table 1).
Results and Discussion
Syntheses and X-Ray Crystal Structures of Homo-Lithium
Acetylenes Featuring Electrostatic π-Interactions. Why is
π-coordination not apparent in the structures of the oligomeric
lithium acetylides 3, 4, and 5? This may be due to (a)
insufficient energy gain upon π-bridging (see below), (b) the
lower tendency of the smaller alkali metals to undergo multi-
hapto coordination,2b,c,f and (c) the competition between the
substituents on the acetylene moieties and the lithium coordinat-
ing solvent (Scheme 2a).
M1 )
M2 )
Li
Na
Na
R )
L )
7
8
9
Mg
Mg
Mg
Mg
Ph
tmeda
tmeda
pmdta
tmeda
(15) Nast, R.; Gremm, J. Z. Anorg. Allgem. Chem. 1963, 325, 62.
(16) (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.
t-Bu
t-Bu
Ph
10
Mg (-Et)
(17) (a) Sannigrahi, A. B.; Kar, T.; Niyogi, B. G.; Hobza, P.; Schleyer,
P. v. R. Chem. ReV. 1990, 90, 1061. (b) Scheiner, S. In Lithium Chemistry;
Sapse, A.-M., Schleyer, P. v. R., Eds.; Wiley: New York, 1995; p 67. (c)
Kollman, P. A.; Liebman, J. F.; Allen, L. C. J. Am. Chem. Soc. 1970, 92,
1142. (d) Goldfuss, B.; Schleyer, P. v. R.; Hampel, F. J. Am. Chem. Soc.
1996, 118, 12183.
(18) (a) Schleyer, P. v. R.; Trifan, D. S.; Bacskai, R. J. Am. Chem. Soc.
1958, 80, 6691. (b) Joris, L.; Schleyer, P. v. R.; Gleiter, R. J. Am. Chem.
Soc. 1968, 90, 327. (c) Andrews, A. M.; Hillig, K. W., II; Kuczkowski, R.
L. J. Am. Chem. Soc. 1992, 114, 6765. (d) Buxton, L. W.; Aldrich, P. D.;
Shea, J. A.; Legon, A. C.; Flygare, W. H. J. Chem. Phys. 1981, 75, 2681.
(e) Legon, A. C.; Aldrich, P. D.; Flygare, W. H. J. Am. Chem. Soc. 1982,
104, 1486. (f) Kukolich, S. G. J. Chem. Phys. 1983, 78, 4832.
(19) (a) Steiner, T.; Tamm, M.; Lutz, B.; Mass, J. v. d. Chem. Commun.
1996, 1127. (b) Allen, F. H.; Howard, J. A. K.; Hoy, V. J.; Desiraju, G.
R.; Reddy, D. S.; Wilson, C. C. J. Am. Chem. Soc. 1996, 118, 4081.
(20) (a) Bene, J. E. D.; Frisch, M. J.; Raghavachari, K.; Pople, J. A.;
Schleyer, P. v. R. J. Phys. Chem. 1983, 87, 73. (b) Dykstra, C. E.; Schaefer,
H. F., III J. Am. Chem. Soc. 1978, 100, 1378.
(21) (a) Houk, K. N.; Rondan, N. G.; Schleyer, P. v. R.; Kaufmann, E.;
Clark, T. J. Am. Chem. Soc. 1985, 107, 2821. (b) Plattner, D. A.; Li, Y.;
Houk, K. N. In Modern Acetylene Chemistry; Stang, P. J., Diederich, F.,
Eds.; VCH: Weinheim, 1995; p 1.
(22) Klusener, P. A. A.; Hanekamp, J. C.; Brandsma, L.; Schleyer, P. v.
R. J. Org. Chem. 1990, 55, 1311.
11
12
How do electrostatic π-interactions affect the electronic
structures in metal acetylides? The penetration of the alkali
cations into the acetylide layers in 1-Na-Rb and 2-Na-Cs
increases as the counterions become larger (Scheme 1).4 The
IR ν-CtC stretching frequencies of 1-Na-Cs, 2-Li-Cs, and 13-
(7) Geissler, M.; Kopf, J.; Schubert, B.; Weiss, E.; Neugebauer, W.;
Schleyer, P. v. R. Angew. Chem. 1987, 99, 569; Angew. Chem., Int. Ed.
Engl. 1987, 26, 587.
(8) Schubert, B.; Weiss, E. Chem. Ber. 1983, 116, 3212.
(9) Schubert, B.; Weiss, E. Angew. Chem. 1983, 95, 499; Angew. Chem.,
Int. Ed. Engl. 1983, 22, 496.
(10) Bell, N. A.; Nowell, I. W.; Shearer, H. M. M. J. Chem. Soc., Chem.
Commun. 1982, 147.
(11) Schubert, B.; Weiss, E. Chem. Ber. 1984, 117, 366.
(12) Geissler, W.; Kopf, J.; Weiss, E.; Chem. Ber. 1989, 122, 1395.
(13) Viebrock, H.; Abeln, D.; Weiss, E. Z. Naturforsch. 1994, B49, 89.
(14) Troyanov, S.; Varga, V.; Mach, K. Organometallics 1993, 12, 2820.
(23) More covalent contributions are apparent in transition metal
π-acetylene complexes, e.g.: (a) Chi, K.-M.; Lin, C.-T.; Peng, S.-M.; Lee,
G.-H. Organometallics 1996, 15, 2660. (b) Mingos, D. M. P.; Yau, J.;
Menzer, S.; Williams, D. J. Angew. Chem. 1995, 107, 2045; Angew. Chem.,
Int. Ed. Engl. 1995, 34, 1894.

1074 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Goldfuss et al.
Table 1. M-CR, M-Câ, and CR-Câ Distances (Å) in Alkali and Alkaline Earth Metal Acetylenes
a
M-CR
M-Câ
CR-Câ
M-CtC-Hb
M ) Na, 1-Na
2.49(5)/2.7
2.87/3.0
2.98/3.2
3.0
3.3
3.4
1.17(6)
1.2
1.2
M ) K, 1-K
M ) Rb, 1-Rb
M-CtC-Meb
M ) Na, 2-Na
2.37(15)/2.7
2.55(5)/3.0
2.19
2.13/2.16
2.20
1.763/2.042
2.32
2.571(3)
2.590(5)
2.265(4)
2.11
2.8
3.1
>3.1
3.08
>3.1
2.538
2.48
2.974(3)
2.900(5)
2.678(4)
2.34
2.456(9)
2.353(9)
1.09(20)
1.19(6)
1.20
1.24
2.20
M ) K, 2-K
(t-Bu-CtC-Li)4(THF)4, 3c
[(Ph-CtC-Li)tmpda]2, 4d
[(Ph-CtC-Li)4(tmhda)2], 5e
[(MeCtC)2BeNMe3]2, 6f
Li2[(PhCtC)3Mg(tmeda)]2, 7g
Na2[(t-Bu-CtC)3Mg(tmeda)]2, 8h,i
Na2[(t-Bu-CtC)3Mg(pmdta)]2, 9h,i
[(Et)(PhCtC)3(Mg)2(tmeda)]2(C6H6), 10i,j
[Me3SiC(CtC-t-Bu)2Li]2, 11k
[(C5HMe4)2Ti(CtC-Si]Me3)2][Mg(THF)Cl], 12l
1.22
1.200(4)
118.5(7)
1.220(6)
2.269(8)
2.132(9)/2.205(11)/
2.292(9)
1.22(1)
1.217(6)
m
[Li-CtC-SiMe2-C6H4OMe]6, (14)6
a The shorter σ-(M-CR) and the longer π-(M-CR) distances are given. b Reference 4. c Reference 7. d Reference 8. e Reference 9. f Reference
10. g Reference 11. h Reference 12. i Selected bond distances. j Reference 13. k Reference 2g. l Reference 14. m See Figure 1.
Table 2. Experimental ν-CtC Frequencies (cm-1) of Alkali
Metal Acetylides (see Scheme 1 for the Structures)
M-CtC-Ha 1, M-CtC-Mea 2, M-CtC-Pha 13,
M
1-Li-Cs
2-Li-Cs
13-Li-Cs
14
H
1974
2124
2111
2020b
1980c
Li
Na
K
Rb
Cs
2053c
2032c
2023c
2020c
2012c
2036d
2018c
2000c
1990c
1990d
1867e
1858c
1851c
1838c
a Reference 15. b Neat. c Nujol mull. d KBr disk. e Reference 46.
Scheme 2
Figure 1. X-ray crystal structure of [Li-CtC-SiMe2-C6H4-OMe]6
(14)6. Hydrogen atoms are omitted. Selected distances (Å) and angles
(deg): CR-Câ, 1.217(6); Li1-CR, 2.292(9); Li1-Câ, 2.353(9); Li1-
O1, 2.169(9); Li1a-CR, 2.132(9); Li1b-CR, 2.205(11); Li1-CR-Câ,
77.6(4); Li1a-CR-Câ, 152.0(5); Li1b-CR-Câ, 127.8(5).
14
a: Steric competition
between the substituent
R and the solvent S
Lithium-acetylene
π-interactions,
facilitated by chelation
b:
The lithium ions (Li1) in (14)6 are coordinated 5-fold by three
CR carbon atoms, by the oxygen atoms of the o-anisyl methoxy
groups (Li1-O1: 2.169(9) Å), and by the Câ atoms of the
acetylene moieties (Li1-Câ: 2.353(9) Å, Table 1). The CR-
Câ-Si1 arrangements (177.8(5)°) are nearly linear. As the
CRtCâ-R fragments tilt toward Li1, the Li(1,1a,1b)-CR-Câ
angles differ strongly; Li1-CR-Câ (77.6(4)°) is much smaller
than Li1a-CR-Câ (152.0(5)°) and the Li1b-CR-Câ angle is
intermediate (127.8(5)°). The CRtCâ distances (1.217(6) Å)
in (14)6 are increased relative to acetylene 1 (exp, 1.20 Å;25
calc, 1.199 Å; see below). The methyl groups (C5) on O1 bend
15° out of the plane of the aryl rings (torsion angle C5-O1-
C66-C62 ) -165°).
Recent computations show that solvation by H2O molecules
has sufficient energy to overcome the π-interactions between
lithiums and the triple bonds in (1-Li)2.24
(1-Li)
4H O
2
(1-Li) •
2
2
The goal of this research, to realize a homogenous lithium
acetylide exhibiting Li-(CRtCâ) π-interactions, as in (1-Li)2,
without competing external solvent interactions, led us to
examine lithium (o-anisyl)dimethylsilylacetylide, Li-CtC-
SiMe2-C6H4-OMe (14). The o-anisyl methoxy group chela-
tion in 14 should result in short Li-Câ contacts (Scheme 2b).
Indeed, the X-ray crystal structure of hexameric 14 (crystal-
lographic S6 symmetry) shows such nearly symmetric π-interac-
tions; note the short Li-Câ distances in Figure 1 and Table 1.
Comparisons of (14)6 and the organolithum hexamers [(c-
{C6H11}Li)6 (15)6‚(C6H6)2],26 [c-{(Me2C)2CH}CH2Li]6 (16)6,27
30
[Me3SiCH2Li]6 (17)6,28 (n-BuLi)6 (18)6,29 and (i-PrLi)6 (19)6
are instructive. The (LiCR)6 cores in all these hexameric
(25) March, J. AdVanced Organic Chemistry, Wiley: New York, 1985
and references therein.
(26) Zerger, R.; Rhine, W.; Stucky, G. J. Am. Chem. Soc. 1974, 96, 6048.
(27) Maercker, A.; Basata, M.; Buchmeier, W.; Engelen, B. Chem. Ber.
1984, 117, 2547.
(28) Tecle, B.; Rahman, A. F. M. M.; Oliver, J. P. J. Organomet. Chem.
1986, 317, 267.
(24) Gareyev, R.; Streitwieser, A. J. Org. Chem. 1996, 61, 1742.

Alkali Metal Cation π-Interactions in Acetylenes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1075
Table 3. Bond Distances (Å) and Angles (deg) of the (LiCR)6 Cores in Hexameric Organolithium Compounds (Scheme 3)
Li1-Li1a
Li1a-Li1b
Li1-Li1b
Li1-Li1b-Li1a
Rg
Li1-CR
Li1a-CR
Li1b-CR
[(c-{C6H11}Li)6 (15)6‚(C6H6)2]a
2.968
2.976
3.18
2.939
2.959
3.700
2.397
2.462
2.45
2.429
2.395
2.794
2.397
2.462
2.45
2.429
2.395
2.794
74.4
76.5
80.9
74.5
76.3
82.9
70.3
72.2
79.5
70.3
72.5
80.5
2.184h
2.159
2.20h
2.184h
2.123
2.20h
2.300
2.297
2.28
2.270
2.308
2.205
b
[c-{(Me2C)2CH}CH2Li]6, (16)6
c
[Me3SiCH2Li]6, (17)6
d
(n-BuLi)6, (18)6
2.159h
2.180h
2.292
2.159h
2.180h
2.132
e
(i-PrLi)6, (19)6
f
(14)6
a Reference 26. b Reference 27. c Reference 28. d Reference 29. e Reference 30. f See Figure 1. g Back to seat angle R of the Li6 chair, Scheme
3b. h Average values of similar distances, which do not differ in more than 0.04 Å; see reference.
15
16
organolithium compounds are formed by two stacked (LiCR)3
rings (Scheme 3a). Folded Li6 chairs are apparent in the (LiCR)6
units (Scheme 3b). The CR atoms cap the Li3 faces, with one
long (Li1-Li1a) and two short (Li1-Li1b and Li1a-Li1b) distances
(Scheme 3c, Table 3).
Relative to (15)6, (16)6, (17)6, (18)6, and (19)6, the lithium
acetylide (14)6 exhibits a Li3 triangle with unusual long Li-Li
distances and a rather flat Li6 chair (large “back-to-seat” angle
R, Scheme 3b, Table 3). In (15)6 to (19)6, the distances of the
CR caps to Li1 and Li1a are short (Scheme 3c, Table 3). Longer
Li1b-CR distances connect the two stacked (LiCR)3 rings
(Scheme 3a, Table 3). In (14)6, however, the Li1-CR distances
are significantly longer than the Li1b-CR bonds between the
(LiCR)3 subunits (Table 3).
a
Our computational model for (14)6, H-CtC-Li(LiH)2 (20)
(Figure 2), reveals that energy gain upon bending of the
CRtCâ-H fragment is low (1.36 kcal/mol in 20, Figure 2) but
that the π-coordination, which results from the tilt of the
CRtCâ-R units, gives rise to the unusual Li-CR distance
differentiations in (14)6: Shorter σ-Li-CR (2.027 Å) and longer
π-Li-CR (2.083 Å) distances are apparent in 20-Cs relative to
20-C2W Li-CR (2.040 Å, Figure 2). This is due to short “end-
on-σ” and long “side-on-π” CR contacts of the Li ions, which
coordinate to the σ- and the π-regions of the acetylide ions
(Scheme 4, Figure 3).2c,b
Deprotonation or metalation increases the affinity of acetylene
groups toward metal ion π-coordination (Figure 4),22 but the
Li+ and LiH π-interaction energies of nonmetalated acetylene
are relatively large (22.2 kcal/mol for π-Li+(H-CtC-H); see
below).22,31 The short contacts between lithiums and the carbon
atoms in lithium pinacolone enolate [Li-O-C(t-Bu)dCH2]6,
(21)6 document π-interactions with Li ions in the (LiO)6 cluster
(Scheme 5).32 Are analogous electrostatic π-interactions33 with
(nonmetalated) acetylene moieties possible? To provide an
answer, we synthesized and crystallized the lithium acetylene
alkoxide Li-O-CMe2CtC-H (22). The X-ray crystal analy-
sis reveals a hexameric aggregate [Li-O-CMe2-CtC-H]6
(22)6 with crystallographic S6 symmetry (Figure 5).
b
Figure 2. (a) H-CtC-Li(LiH)2 (C2V, 20-C2v): B3LYP/6-311+G**
optimized geometry, total energy -100.613 35 au; B3LYP/6-31G* zero-
point energy 20.13 kcal/mol (NIMAG ) 1). (b) H-CtC-Li(LiH)2
(Cs, 20-Cs): B3LYP/6-311+G** optimized geometry, total energy
-100.61590 au; B3LYP/6-31G* zero-point energy 20.37 kcal/mol
(NIMAG ) 0); π-coordination energy relative to 20-C2v ) 1.36 kcal/
mol.
Scheme 3
The lithium centers Li1 in (22)6 exhibit short contacts to the
organic moieties (Li1-C1 ) 2.687(5) Å, Li1-C2 ) 2.443(5)
Å, and Li1-C3 ) 2.749(6) Å (Scheme 5, Table 4). This
suggests similar electrostatic interactions as in (21)6 and in [Li-
O-C(Me)-(c-CHCH2CH2)2]6 (23)6 (Scheme 5).17d This in-
terpretation also is supported by the tilt of the O1-C1 moieties
toward Li1 (Li1-O1-C1 ) 105.3(2)°, Li1a-O1-C1 ) 130.8(2)°,
Li1a-O1-C1 ) 134.2(2)°; Table 4) and the coplanarity of the
O1-Li1 and C1-C2 bonds (Li1-O1-C1-C2 dihedral angle )
10.4°, Figure 5).34 The differences in the Li(1,1a,1b)-O1 distances
are remarkable: Li1-O1 (1.955(5) Å) are longer than Li1a-O1
(29) Kottke, T.; Stalke, D. Angew. Chem. 1993, 105, 619; Angew. Chem.,
Int. Ed. Engl. 1993, 32, 580.
(30) Siemeling, U.; Redecker, T.; Neumann, B.; Stammler, H.-G. J. Am.
Chem. Soc. 1994, 116, 5507.
(31) For electrostatic potential computations of H-CtC-H and F-CtC-
H, see: Clark, D. T.; Adams, D. B. Tetrahedron 1973, 29, 1887.
(32) (a) Williard, P. G.; Carpenter, G. P. J. Am. Chem. Soc. 1985, 107,
3345. (b) Williard, P. G.; Carpenter, G. P. J. Am. Chem. Soc. 1986, 108,
462.
(33) For a discussion of electrostatic Li-C π-interactions in lithium aryls
see: Ruhlandt-Senge, K.; Ellison, J. J.; Wehmschulte, R. J.; Pauer, F.;
Power, P. P. J. Am. Chem. Soc. 1993, 115, 11353
(34) Probably due to steric hindrance, ideal syn-periplanar arrangements
(Li1-O1-C1-C2 ) 0°) are avoided.

1076 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Goldfuss et al.
Scheme 4. Differentiation of Li1-CR and Li1a-CR Bonds in
and results in shorter Li(CtC) π-contacts (2.115 and 2.290 Å,
Figure 6c). Moreover, no minimum corresponding to 24-H
(with X ) Li) could be optimized at B3LYP/6-31G*; only 24-
Li-coord resulted.
a
the X-ray Crystal Structure of (14)6
Structural, Energetic, and Vibrational Effects of π-Inter-
actions in Alkali Metal Acetylides. Similar CR-M and Câ-M
distances (see above) and lower CtC stretching frequencies
are important indicators of π-bonding in polar metal acetylides.
Are energetic, structural, and vibrational π-interaction criteria
related with the charge distributions in alkali metal acetylides
M-CtC-H (1-Li-Cs, Table 5)? The (HCtC)M2H complexes
correspond to the X-ray crystal structures of 4 and of 6 without
(25-Li-Cs, C2V) and with (26-Li-Cs, Cs) π-interactions (Table
6). Alkali metal cation π-coordinations are apparent in the
cationic acetylene complexes 27-Li-Cs (Table 7).
a The R-CtC tilt results in “side-on-π” (Li1) in addition to the “end-
to-σ” (Li1a) coordination. See also Figure 3.
All Cs structures 26-Li-Cs with π-interactions are slightly
more stable (0.73 kcal/mol (Li) to 0.07 kcal/mol (Cs)) than their
C2V counterparts without π-contacts 25-Li-Cs (Table 6). The
Cs-C2V energy differences, ∆E, and hence the degree of
π-interaction, decrease with increasing ion sizes (increasing
distances r between M+ and the CtC centers). This also is
apparent with the π-coordination energies Ecoord of 27-Li-Cs
(Table 7); these correlate with 1/r3 (Figure 7).35 Although 25-
Li and 26-Li have the largest energy difference, 25-Li is the
only minimum among the C2V species, 25-Li-Cs (Table 6).36
M )
Li
Na
K
25-Li
25-Na
25-K
26-Li
26-Na
26-K
27-Li
27-Na
27-K
Rb
Cs
25-Rb
25-Cs
26-Rb
26-Cs
27-Rb
27-Cs
The CtC distances elongate as the metal ions become larger
in 25-Li to 25-Cs, e.g., from 1.225 Å to 1.234 Å, as well as
from 1.224 Å in 1-Li to 1.231 Å ion 1-Cs, reflecting the
σ-effects of the cations in these π-bonding free structures (Tables
5 and 6, Figure 8). The π-interactions in 27-Li to 27-Cs (CtC
) 1.205 and 1.201 Å) result only in small CtC lengthenings
relative to 1 (1.199 Å) (Table 7, Figure 8). Similarly, the
π-interactions in 26-Li to 26-Cs (CtC ) 1.230 and 1.234 Å)
elongate the CtC distances only slightly relative to the
corresponding 25-Li-Cs structures. The CtC lengthenings due
to π-interactions (25-Li-Cs vs 26-Li-Cs) are the greatest the
smaller the cations (Figure 8). That the CRtCâ distance is a
poor criterion for π-interactions has been noted.3,37
The σ-coordinated cations shift the harmonic ω-CtC stretch-
ing frequencies to lower values as the metals become larger:
from 25-Li (2007 cm-1) to 25-Cs (1941 cm-1) and from 1-Li
(2014 cm-1) to 1-Cs (1955 cm-1, Tables 5 and 6, Figure 9).
The π-interactions in 27-Li to 27-Cs (ω-CtC ) 2034 and 2053
cm-1) lower the ω-CtC frequencies relative to 1 (ω-CtC )
Figure 3. (a) MO contour plots (RHF/6-31+G*) of H-CtC-Li-
(LiH)2 (C2V, 20-C2v) reflecting σ- (“in-plane” HOMO-1) and π- (“in-
plane” HOMO) components of the Li-C bonds. (b) MO contour plots
(RHF/6-31+G*) of H-CtC-Li(LiH)2 (Cs, 20-Cs) reflecting σ- (“in-
plane” HOMO-1) and π- (“in-plane” HOMO) components of the Li-C
bonds.
(1.877(5) Å) or Li1b-O1 (1.923(5) Å). Like the long Li1-CR
bonds in (14)6, the long Li1-O1 distances in (22)6 are obviously
due to the “side on p”-coordinated O1 lone pairs.
To assess the energetics of Li π-bonding in the X-ray crystal
structure of (22)6, monomeric Li-O-CH2-CtC-H models
were computed without (24-H, Figure 6a) and with (24-H-
coord, Figure 6b) Li (H-CtC) π-contacts (Scheme 6). Both
structures are minima. The Li (H-CtC) π-interaction is rather
weak, and 24-H-coord is only 0.64 kcal/mol more stable than
24-H-coord (Figure 6, a and b). As in the X-ray crystal
structure (22)6, the π-interactions in 24-H-coord are clearly
apparent from the short Li(CtC) distances (2.241 and 2.478
Å, Figure 6b). Lithiation of the acetylene moiety in 24-H-coord
increases the Li(CtC) π-interaction in 24-Li-coord (Figure 4)
(35) Whereas in alkali metal cation cyclopropane edge complexes Ecoord
depends on 1/r2.5 (ref 17d), for benzene a 1/rn (n < 2) correlation was found
(ref 16b).
(36) An earlier computational study using the CCSD/DZP level without
diffuse functions describes (HCC)Li2H as a C2V transition structure: Bolton,
E. E.; Laidig, W. D.; Schleyer, P. v. R.; Schaefer, H. F., III J. Am. Chem.
Soc. 1994, 116, 9602.
(37) Tecle, B.; Ilsley, H.; Oliver, J. P. Inorg. Chem. 1981, 20, 2335.

Alkali Metal Cation π-Interactions in Acetylenes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1077
Figure 4. (a-c) Electrostatic potential maps (RHF/6-31+G*) of H-CtC-H (D∞h, 1), H-CtC- (C∞V, 1-anion), and Li-CtC-H (C∞V, 1-Li).
The negative values (kcal/mol) reflect the cation coordination affinities of the systems.
Table 4. Selected Bond Distances (Å) and Angles (deg) for
b
[Li-O-C(t-Bu)dCH2]6, (21)6,a [Li-O-CMe2-CtC-H)]6 (22)6
c
and [Li-O-C(Me)-(c-CHCH2CH2)2]6 (23)6 (Scheme 5)
a,d
b
c
(21)6
(22)6
(23)6
Li1-O1
1.976(9)
1.869(9)
1.954(9)
88.0(9)
1.955(5)
1.877(5)
1.923(5)
105.3(2)
130.8(2)
132.4(2)
2.687(5)
2.443(5)
2.749(5)
1.937(3)
1.881(3)
1.926(3)
105.6(1)
132.6(1)
135.1(1)
2.680(3)
2.615(3)
2.644(3)
Li1a-O1
Li1b-O1
Li1-O1-C1
Li1a-O1-C1
Li1b-O1-C1
Li1-C1
140.0(4)
132.9(4)
2.349(9)
Li1-C2
Li1-C3
2.420(8), 2.53e
a Reference 32. b Figure 5. c Reference 17d. d One of two similar
asymmetric units in the unit cell with approximate S6 symmetry.
e Average value of the two asymmetric units.
cations and appears as a useful indicator for π-interactions,
especially for the smaller cations (Figure 9).
The σ-effects of the cations give rise to increased (Câ f CR)
charge polarizations with smaller cation sizes in 1-Li-Cs and
in 25-Li-Cs relative to 1 (see the CR, Câ charges in Tables 5
and 6 and in Figure 10). The π-interactions in 26-Li to 26-Cs
compensate for these counterion induced CR, Câ charge separa-
tions and hence give rise to CR f Câ charge delocalizations
relative to the corresponding 25-Li-Cs structures. These cation
induced charge delocalizations increase with decreasing cation
sizes and hence with increasing degrees of the π-interactions
(Table 6, Figure 10).
Figure 5. X-ray crystal structure of [Li-O-CMe2-CtC-H]6 (22)6.
The methyl groups C5 are disordered. Hydrogen atoms are omitted
except the acetylenic hydrogen atom H3. Selected distances (Å) and
angles (deg): C1-C2, 1.486(4); C2-C3, 1.172(5); Li1-C1, 2.687(5);
Li1-C2, 2.443(5); Li1-C3, 2.749(6); Li1-O1, 1.955(5); Li1a-O1,
1.878(5); Li1b-O1, 1.932(5); Li1-O1-C1, 105.3(2); Li1a-O1-C1,
130.8(2); Li1b-O1-C1, 134.2(2).
Conclusions
Intramolecular coordination of the o-anisyl methoxy groups
in [Li-CtC-SiMe2-C6H4-OMe]6 (14)6 eliminates external
solvent effects and facilitates lithium π-interactions with the
acetylide moieties (Li1-Câ ) 2.353(9) Å), as is evident in the
X-ray crystal structure analysis of (14)6. The strong tilt of the
CtC-R fragments in (14)6 gives rise to clearly different Li-
CR bond lengths in the (LiCR) core (Li1-CR ) 2.292(9) Å, Li1a-
CR ) 2.132(9) Å, Li1b-CR ) 2.205(11) Å), due to “end-on-σ”
and “side-on-π” coordinated acetylide moieties. Such distinct
differences in Li-CR bond distances are unprecedented in the
X-ray crystal structures of hexameric organolithium compounds.
Similar π-interactions are evident in the X-ray crystal structure
of [Li-O-CMe2-CtC-H]6 (22)6 from short distances be-
tween the lithium ions in the (LiO)6 cluster and the nonmetalated
acetylene moieties (Li1-C2 ) 2.443(5) Å, Li1-C3 ) 2.749(6)
Å). Although the π-contacts are clearly evident structurally,
the computational models for the X-ray crystal structures (14)6,
as well as (22)6, H-CtC-Li(LiH)2 (20), and Li-O-CH2-
CtC-H (24-H), point to the weak nature of these π-interactions
(1.36 and 0.64 kcal/mol, respectively). Further computations
Scheme 5. Electrostatic Interactions in Hexameric Lithium
Alkoxide Clusters
2062 cm-1; Table 7, Figure 9). Similarly, the π-interactions in
26-Li (1974 cm-1) to 26-Cs (1937 cm-1) result in lower ω-CtC
stretching frequencies than in the corresponding 25-Li-Cs
structures. The relative reduction of ω-CtC stretching frequen-
cies (25-Li-Cs vs 26-Li-Cs) is the stronger the smaller the

1078 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Goldfuss et al.
lengths (up to 0.005 Å for Li), to lowered ω-CtC frequencies
(up to 33 cm-1 for Li) and to cation-induced charge delocal-
ization, which increase with decreasing cation size (Cs to Li).
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. Anisole
and 2-methyl-3-butyn-1-ol (Aldrich) were distilled prior to use. Sodium
acetylide as a toluene/mineral oil suspension and n-BuLi were purchased
from Acros. A hexane solution of 6Li-enriched n-Bu6Li was prepared
as described by Seebach et al.38 The NMR spectra were measured on
a JEOL GX spectrometer and referenced to TMS or THF: 1H, 400
a
6
MHz; 13C, 100.6 MHz; 29Si, 79.4 MHz; Li, 58.9 MHz. IR spectra
were determined neat or as Nujol mulls between NaCl discs on a Perkin-
Elmer 1420 spectrometer. Mass spectral data were obtained on a Varian
MAT 311A spectrometer and the elemental analyses (C, H) on Heraeus
micro automaton. The X-ray crystal data were collected with an Enraf
Nonius CAD4-Mach3 diffractometer using the ω-scan method (3.0°
< 2Θ < 54.0°). The structures were solved by direct methods using
SHELXS 86. The parameters were refined with all data by full-matrix
least-squares on F2 using SHELXL93 (G. M. Sheldrick, Go¨ttingen,
1993). All nonhydrogen atoms were refined anisotropically; the
hydrogen atoms were fixed in idealized positions using a riding model.
b
R1 ) ∑|Fo - Fc|/∑Fo and wR2 ) ∑w|(Fo2 - Fc2)2|/∑(w(Fo2)2)0.5
.
Further details are available on request from the Director of the
Cambridge Crystallographic Data Center, Lensfield Rd, GB-Cambridge
CB2 1 EW, by quoting the journal citation.
Li-CtC-SiMe2-C6H4-OMe (14). A solution of ca. 6.8 g (0.06
mol) o-lithioanisole in THF, TMEDA, and hexane was prepared from
37.5 mL (0.06 mol) of BuLi (1.6 M) in hexane, 7.0 g (0.06 mol) of
TMEDA, 6.5 g (0.06 mol) of anisole and subsequent solvation of the
precipitate with ca. 10 mL of THF.39 This solution was added dropwise
at 0 °C to 7.7 g (0.06 mol) of dichlorodimethylsilane in 150 mL of
diethyl ether. The resulting mixture was stirred at room temperature
for 6 h and filtered; the volatile components were removed by
distillation. The residue was taken up in 150 mL of diethyl ether and
cooled to 0 °C; a suspension of 2.9 g (0.06 mol) of sodium acetylide
in 20 mL of diethyl ether was added. The mixture was stirred for 6 h
at room temperature. Hydrolysis with H2O/NH4Cl, extraction with
diethyl ether, drying over Na2SO4, and distillation yielded (9.8 g, 52
mmol, 87%) o-anisyldimethylsilylacetylene, HCtC-SiMe2-C6H4-
OMe: bp 60 °C/1.6 mbar; 1H NMR (CDCl3) δ 7.69 (d, C6H4), 7.35 (t,
C6H4), 6.97 (t, C6H4), 6.79 (t, C6H4), 3.76 (s, OCH3), 2.49 (s, CCH),
0.44 (s, Si(CH3)2); 13C {1H} NMR (CDCl3) δ 165.14, 136.88, 132.40,
124.62, 121.53, 110.48 (C6H4), 95.30 (CR), 89.80 (Câ), 55.92 (O-CH3),
c
Figure 6. (a) Li-O-CH2-CtC-H (Cs 24-H) without Li (CtC)
π-contacts: B3LYP/6-311+G** optimized geometry, total energy
-198.888 99 au; B3LYP/6-31G* zero-point energy 31.91 kcal/mol
(NIMAG ) 0). (b) Li-O-CH2-CtC-H (Cs 24-H-coord) with Li
(CtC) π-contact; B3LYP/6-311+G** optimized geometry, total energy
-198.889 92 au; B3LYP/6-31G* zero-point energy 31.85 kcal/mol
(NIMAG ) 0); π-coordination energy relative to 24-H ) 0.64 kcal/
mol. (c) Li-O-CH2-CtC-Li (Cs 24-Li-coord) with Li (CtC)
π-contact: B3LYP/6-311+G** optimized geometry, total energy
-205.829 44 au; B3LYP/6-31G* zero-point energy 26.40 kcal/mol
(NIMAG ) 0).
Scheme 6. The Li (CtC) π-Interaction Model for the
X-Ray Crystal Structure (22)6 (Figure 6)
0.84 (Si(CH3)2); 29Si {1H} NMR (CDCl3) δ -21.02; IR (neat, cm-1
)
3280 (ν CtC-H); 3070, 3002 (ν C-H arene), 2960, 2900, 2840 (ν
C-H aliphatic), 2020 (ν CtC).
Lithiation of 0.28 g (1.5 mmol) of HCtC-SiMe2-C6H4-OMe
with 0.9 mL (1.5 mmol) of n-BuLi (1.6 M) in THF or hexane solution
(-20 °C, then 5 min. RT) afforded Li-CtC-SiMe2-C6H4-OMe
(14) (0.27 g, 1.4 mmol, 93% yield in hexane): NMR40 of 6Li-14 δ 1H
(THF-d8, +25 °C) 8.13 (d, C6H4), 7.24 (t, C6H4), 6.88 (t, C6H4), 6.77
(d, C6H4), 3.74 (s, OCH3), 0.31 (s, Si(CH3)2); δ 13C{1H} (THF-d8, -90
°C) 171.62 (pentuplet, CR), 164.91 (C6H4-OMe), 138.65 (C6H4), 131.27
(C6H4), 127.04 (C6H4-SiMe2), 120.25 (C6H4), 114.23 (s, Câ), 109.11
(C6H4), 55.00 (OCH3), 0.97 (Si(CH3)2); δ 6Li (THF-d8, -95 °C) -0.26
(s); δ 29Si {1H} (THF-d8, 31 °C) -30.55; IR (Nujol, cm-1) 3050 (ν
C-H arene), 1980 (ν CtC); MS (EI, 70 eV, 120 °C) m/e 354 [Me-
O-C6H4-SiMe2-CtC-SiMe2-C6H4-OMe]+, 339 [Me-O-C6H4-
SiMe2-CtC-SiMe-C6H4-OMe]+, 324 [Me-O-C6H4-SiMe-
CtC-SiMe-C6H4-OMe]+, 309 [Me-O-C6H4-Si-CtC-SiMe-
C6H4-OMe]+, 294 [Me-O-C6H4-Si-CtC-Si-C6H4-OMe]+, 279
X = H 24-H (C )
(C )
(C )
s
X = H 24-H-coord
X = Li 24-Li-coord
s
s
Table 5. Bond Distances (Å),a Harmonic Vibrational Frequencies
ω (cm-1),b and Natural Charges q (aq)c of Alkali Metal Acetylides
M-CtC-H
M-CR CRtCâ ω-CtC q M
q CR
q Câ
q H
1 (D∞h
1-Li (C∞V
1-Na (C∞V
1-K (C∞V
1-Rb (C∞V
1-Cs (C∞V
1-anion (C∞V
)
1.063 1.199 2062
-0.224 -0.224 +0.223
)
)
)
)
)
1.919 1.224 2014 +0.937 -0.746 -0.386 +0.195
2.222 1.225 2002 +0.909 -0.668 -0.436 +0.195
2.666 1.229 1970 +0.950 -0.643 -0.493 +0.185
2.848 1.230 1964 +0.948 -0.626 -0.506 +0.184
3.057 1.231 1955 +0.961 -0.621 -0.521 +0.182
)
1.243 1882
-0.461 -0.694 +0.155
a B3LYP/6-311+G** (C, H), 6-31G (Li, Na), LanL2DZ, ECP (K,
Rb, Cs) optimized structures. b Unscaled B3LYP frequencies. c Natural
Population Analysis of the B3LYP electron densities, ref 45.
(38) Seebach, D.; Ha¨ssig, R.; Gabriel, J. HelV. Chim. Acta 1983, 66,
308.
(39) Brandsma, L.; Verkruijsse, H. PreparatiVe Polar Organometallic
show the π-interactions in alkali metal (HCtC)M2H (26-Li-
Cs) complexes to be weakly stabilizing (0.73 kcal/mol for Li)
and to decrease with increasing cation sizes (0.07 kcal/mol for
Cs). The π-contacts give rise to slightly increased CtC bond
Chemistry; Springer: Berlin 1987.
(40) For NMR [6Li 13C] coupling studies on lithium acetylides see: (a)
Fraenkel, G.; Pramanik, P. J. Chem. Soc., Chem. Commun. 1983, 1527. (b)
Ha¨ssig, R.; Seebach, D. HeV. Chim. Acta 1983, 66, 2269.

Alkali Metal Cation π-Interactions in Acetylenes
J. Am. Chem. Soc., Vol. 119, No. 5, 1997 1079
Table 6. Energies,a Bond Distances (Å),a Harmonic Vibrational Frequencies ω (cm-1),b and Natural Charges q (au)c of (HCtC)M2H
Complexes without (C2V) and with (Cs) π-Interactions
E, ZPE (NIMAG)d
∆Ee M1-CR M1a-CR M-Câ CRtCâ ω-CtC
q Mf
q CR
q Câ
q H
25-Li (C2V)
26-Li (Cs)
25-Na (C2V) -401.966 01, 14.03 (1, -79)
26-Na (Cs)
25-K (C2V)
26-K (Cs)
25-Rb (C2V) -125.102 71, 12.60 (1, -45)
26-Rb (Cs) -125.103 08, 12.67 (0)
25-Cs (C2V) -117.125 51, 12.35 (1, -41)
26-Cs (Cs) -117.125 74, 12.42 (0)
-92.445 65, 15.79 (0)
-92.446 81, 15.79 (0)
2.091
2.126
2.415
2.428
2.834
2.838
3.025
3.031
3.229
3.232
2.091
2.051
2.415
2.389
2.834
2.836
3.005
3.033
3.229
3.243
3.183
2.446
3.484
2.929
3.860
3.416
4.035
3.595
4.217
3.835
1.225
1.230
1.227
1.230
1.231
1.233
1.232
1.232
1.234
1.234
2007
1974
1983
1968
1956
1950
1948
1942
1941
1937
+0.873 -0.775 -0.337 +0.204
+0.874 -0.652 -0.478 +0.213
+0.878 -0.703 -0.409 +0.199
+0.879 -0.642 -0.477 +0.202
+0.923 -0.658 -0.487 +0.192
+0.923 -0.621 -0.523 +0.193
+0.924 -0.637 -0.508 +0.190
+0.923 -0.607 -0.537 +0.191
+0.937 -0.626 -0.529 +0.188
+0.936 -0.605 -0.549 +0.188
0.73
0.65
0.21
0.16
0.07
-401.967 15, 14.10 (0)
-133.658 25, 12.95 (1, -44)
-133.658 69, 13.02 (0)
a B3LYP/6-311+G** (C, H), 6-31G (Li, Na), LanL2DZ, ECP (K, Rb, Cs) optimized structures. b Unscaled B3LYP frequencies. c Natural Population
Analysis of the B3LYP electron densities, ref 45. d Total energies E (au), unscaled zero-point energies ZPE (kcal/mol), numbers and values (cm-1
of imaginary frequencies in parentheses. e Relative Cs-C2V energies ∆E (kcal/mol). f Average value for M1 and M1a.
)
Table 7. The π- Coordination Energies Ecoord (kcal/mol),a Bond Distances (Å),a Harmonic Vibrational Frequencies ω (cm-1),b and Natural
Charges q (au)c of the Alkali Cation Acetylene Complexes M+(H-CtC-H)
Ecoord
M+ π-(CtC)d
CRtCâ
ω-CtC
q M
q C
q H
1 (D∞h
)
1.199
1.205
1.204
1.202
1.201
1.201
2062
2034
2039
2048
2051
2053
-0.223
-0.261
-0.252
-0.245
-0.241
-0.238
+0.223
+0.272
+0.259
+0.246
+0.242
+0.239
27-Li (C2V)
27-Na (C2V)
27-K (C2V)
27-Rb (C2V)
27-Cs (C2V)
20.22
13.29
7.50
5.73
4.53
2.254
2.633
3.152
3.454
3.739
+0.978
+0.986
+0.997
+0.998
+0.999
a B3LYP/6-311+G** (C, H), 6-31G (Li, Na), LanL2DZ, ECP (K, Rb, Cs) optimized structures. b Unscaled B3LYP frequencies. c Natural Population
Analysis of the B3LYP electron densities, ref 45. d Distance between the metal and the center of the CtC bond.
Figure 9. The harmonic ω-CtC frequencies of the alkali metal
acetylene compounds.
Figure 7. The 1/r3 dependence of the coordination energy Ecoord in
M+ (H-CtC-H) complexes (27-Li-Cs).
Figure 10. The natural charges on CR and Câ in the alkali metal
acetylides.
Figure 8. The CtC distances of the alkali metal acetylene compounds.
0.71073 Å); T ) 293 (2) K; crystal dimensions: 0.30 × 0.20 × 0.20
[Me-O-C6H4-Si-CtC-Si-C6H4-O]+. Anal. (C11H13O1Li1Si1).
Calcd: C, 67.3; H, 6.6. Found: C, 66.8; H, 6.8. Single crystals of 14
were obtained from cooled hexane solutions.
X-ray crystal data for (14)6: Mr ) 196.24; rhombohedric; space
group R-3; a ) b ) 22.577(3) Å, c ) 12.774(2) Å; V ) 5638.8(14)
Å3; Dcalc ) 1.040 Mgm-3; Z ) 18; F(000) ) 1872; Mo KR (λ )
mm; total reflections 2137; unique 1966; I > 2σ(I), 1009; parameters,
128. Final R values: R1 ) 0.0917 (I > 2σ(I)) and wR2 ) 0.1759 (all
data). GOF ) 1.148; largest peak (0.171 e Å-3) and hole (-0.150 e
Å-3).
Li-O-CMe2-CtC-H (22). A solution of 0.223 g (2.66 mmol)
of 2-methyl-3-butyn-2-ol (H-CtC-CMe2-OH) in 10 mL of hexane

1080 J. Am. Chem. Soc., Vol. 119, No. 5, 1997
Goldfuss et al.
was cooled to 0 °C and 1.66 mL (2.66 mmol) of 1.6 M BuLi in hexane
were added. The white suspension (isolated: 0.23 g, 2.56 mmol, 96%
yield), stirred at room temperature for 5 min, dissolved on warming.
Slowly cooling to room temperature afforded colorless crystals. NMR
electron effective core potentials and the LanL2DZ basis sets [K, (341/
311); Rb, (341/321); Cs, (341/321)] were employed.44 The character
of the stationary points, the zero-point energy correction, and the
harmonic vibration frequencies were obtained from analytical and, for
pseudo-potential computations of the K, Rb, and Cs systems, from
numerical frequency calculations. All partial charges are based on
Natural Population Analysis (NPA)45 of the Becke3LYP electron
density. The electrostatic potentials were evaluated with RHF/6-31+G*
wave functions on optimized B3LYP geometries.
Acknowledgment. This work was supported by the Fonds der
Chemischen Industrie (also through a scholarship to B.G.), the Stiftung
Volkswagenwerk, the Convex Computer Corporation, and the Deutsche
Forschungsgemeinschaft.
1
of 22: δ H (CDCl3, +25 °C) 2.35 (s, H-CtC), 1.44 (s, CH3); δ
13C{1H} (CDCl3, +25 °C) 94.57 (C1), 67.67 (C3), 64.51 (C2), 35.06
(CH3); IR (Nujol, cm-1) 3270 (ν C-H); MS (EI, 70 eV, 80 °C) m/e
457 [M5-Li+], 399 [457+ - COMe2], 341 [399+ - COMe2]. Anal.
(C5H7Li1O1). Calcd: C, 66.7; H, 7.8. Found: C, 66.4; H, 7.9. Single
crystals of 22 were obtained from cooled hexane solutions.
X-ray crystal data for (22)6: Mr ) 90.05; rhombohedral, obv.; space
group R-3; a ) b ) 10.767(2) Å, c ) 26.933(3) Å, V ) 2704.0(9) Å3;
Dcalc ) 0.995 Mgm-3; Z ) 18; F(000) ) 864; Mo KR (λ ) 0.71073
Å); T ) 293 (2) K; crystal dimensions: 0.30 × 0.30 × 0.20 mm; total
reflections 1438; unique 1234; I > 2σ(I), 700. H3 was refined
independently and anisotropically; the other hydrogen atoms were fixed
in idealized positions using a riding model. Final R values: R1 )
0.0781 (I > 2σ(I)) and wR2 ) 0.1882 (all data). GOF ) 1.185; largest
peak (0.192 e Å-3) and hole (-0.155 e Å-3).
Supporting Information Available: Tables giving crystal
data and structure refinement details, atomic coordinates, bond
distances and angles, and thermal parameters for 14 and 22 (12
pages). See any current masthead page for ordering and Internet
access instructions.
JA9622517
(43) GAUSSIAN94, Revision C.3: Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman,
J. R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.;
Al-Laham, M. A.; Zakrzewski, 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.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.;
Baker, J.; Stewart, J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.
Gaussian, Inc.: Pittsburgh, PA, 1995.
(44) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299.
(45) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899. (b) Reed, A. E.; Schleyer, P. v. R. J. Am. Chem. Soc. 1990, 112,
1434.
Theoretical Section
All computed structures were optimized with Becke’s three-
parameter hybrid functional41 incorporating the Lee-Yang-Parr cor-
relation functional42 (Becke3LYP) using the gradient techniques
implemented in GAUSSIAN94.43 The 6-311+G** (C, H, Li) and
6-31G (Li, Na) basis sets were used. For K, Rb, and Cs 9-valence
(41) Becke, A. D. J. Chem. Phys. 1993, 98, 5648.
(42) Lee, C.; Yang, W.; Parr, R. G. Phys. ReV. B 1988, 37, 785.
(46) Goubeau, J.; Breuer, O. Z. Anorg. Allg. Chem. 1961, 310, 110.