5196 which then undergoes an insertion reaction to give the observed q3-crotyl species. In agreement with this suggestion reaction
-n
1+ BF,-
The Curious Structure of the Lithiocarbon' C3Li4 Sir: C3Li4, prepared by lithiation of propyne by n-butyllithi ~ m , is~ a ~readily , ~ available lithiocarbon.' Only the most general information is available concerning the structure of this species. C3Li4 reacts to give either allene- or propyne-type products depending on the reagents and the conditions.2dThis suggests an open rather than a cyclic arrangement of the carbon atoms. The IR absorption band of C3Li4 in the 1700-cm-' region has been interpreted in terms of formulation I or
2,L = P(OMe),,
11'
Q
I 3, L = P(OMe),
1
LI
,c-c-c-L,
1L1
+l
J
~Il-c-c-c-LI
t
*LI "
Bu-t 4, L = P(OMe)3 Ll
To
8
Ll . \ c - c r - L l
1'
.c;? ,
L'
m
.-C-C3-Li Ll
H
5,L=P(OMe),
o'm
6. L = P ( O M e ) ,
Ll-c-c-
with NaBD4 proceeds regioselectively to give 6 with deuterium incorporated in the illustrated position adjacent to the methyl group. A related 0-allylic elimination reaction forming a q3-crotyl complex has been observed' on heating the E isomer of Ir[C(Me)=CHMe]COL2; however, in contrast to the molybdenum system the Z isomer undergoes a cis-@-vinyl H elimination to form the corresponding hydride and but-2yne.
h'
Ix
x
-Ll
6
W e have used a b initio molecular orbital calculations4 to investigate possible C3Li4 structures. Complete geometry optimizations within each assumed symmetry were carried out using the minimal STO-3G basis set.4b Single point calculations using these optimized geometries followed, employing the split valence 4-31G (for carbon)4c and 5-21'3 (for lithAcknowledgment. W e thank the S.R.C. for support. i ~ m bases. ) ~ Our ~ earlier work has shown that polylithio derivatives adopt highly unconventional g e ~ m e t r i e s the ; ~ same References and Notes trend was expected in the present search. Nevertheless, we Reaction (room temperature, CH2C12) of (Mo(CO)3($-C5H5)]2 or [Mobegan with the lithiated analogue of allene, 111. Although I11 (CO)3(v5-CgH7)]2 with AgBF4 in the presence of an excess of an acetylene to be a local minimum when optimized within the ~ - C ~ Hproved ~ affords a silver mirror, and the cations [ M O ( C O ) ( ~ ~ - R C ~ R ' ) ~ ( Por q5-CsH7)]+BF4-(R = R 1 = H; R = R' = Me; R = Bu-t, R 1 = H; R = Me, R 1 constraint of D2d symmetry, the energy compared unfavorably = H; R = Ph, R' = H; R = R 1 = Ph), which on treatment (room temperature, with that of other structures. The planar D2h allene (IV) (with CH2C12)with P(OMe)3 (excess) gives the highly colored cations 1and 2 in good yield. The 'H NMR spectrum(20 OC,CD3N02)of, forexample, [Mo($four x electrons), which had nearly the same bond lengths and M ~ C ~ M ~ ) [ P ( O M ~ ) ~ ] ~ ( D ~ - C ~ Hshows , ) ] + BaF sharp ~ - triplet ( 4 J ~ p= 1.O Hz.) angles, was more stable by 0.5 kcal/mol at the 4-31G/5-21G at i7.7 (MeC=C) collapsing reversibly (coalescence temperature, in level (Table I). The theoretical estimates for the rotational (CD&CO at -40 OC) to two resonances ( T 7.3 (3 H, brs), 8.4 (3 H, br s)) on cooling, which suggests that acetylene propellor rotation occurs; the obbarrier in allene are between 27 and 92 kcal/mol$ experiservation of 31Pcoupling excludes acetylene dissociation. The coalescence mental values of -46 kcal/mol have been reported for 1,3temperature of the corresponding $-C5H5 substituted cation is even lower (>-go OC), which is interesting in view of the report' that the 'H NMR dialkylallene~.~ Lithiation is thus able not only to reduce the 'PFe- collapses spectrum ((CD&CO) of [Mo(q*-MeC2Me~dipho~~~~-!%H~)] D2d-D~h energy differences dramatically, but also to reverse to a single MeCEC signal only at 110 OC. the normal order of stability. Similarly, 1 , l -dilithioethylene One example of this type of cation has been recently reported (J. A. Segal, M. L. H. Green, J. C. Daren, and K. Prout, J. Chem. SOC.Chem. Commun., has been found to prefer the perpendicular, rather than the 766 (1976)) the synthesis following a less accessible route via bisnormal planar c0nformation.5~ (arene)molybdenumchemistry. Satisfactory elemental analyses, NMR spectra, and mass spectra have been Encouraged by the tendency of lithium to bridge,5c we exobtained for all the complexes described. amined the tetrabridged D2d-constrained structure, V, which H. C. Clark and M. H. Chisholm, Acc. Chem. Res.. 202 (1973). proved to be more stable than I11 by 19.5 kcal/mol at STO-3G E. 0. Fischer and U. Schubert, J. Organomet. Chem., 100,59 (1975). J. W. Faller, and A. M. Rosan, J. Am. Chem. SOC.,98,3388 (1976). but 13.3 kcal/mol less stable a t 4-31G/5-21G. Since both I11 J. Schwartz, D. W. Hart, and B. McGiffert, J. Am. Chem. SOC., 96, 5613 and V are D2d it is evident that a barrier (within this symme(1974). try) exists between them. On the other hand, the conventional Martin Bottrill, Michael Green* acetylenic structure, VI, is not a local minimum, but collapses Department of Inorganic Chemistry to a triply bridged form, VII, upon optimization under C J ~ University of Bristol, Bristol BS8 ITS. England symmetry. VI1 is more stable than 111-V a t both basis set levels. The effect of the three bridging lithiums is seen in the Received April 6 , 1977 Journal of the American Chemical Society
/
99:17
/
August 17,1977
5797 Table I. Structure and Relative Energies of Various C3Li4 Isomers
Relative energy. kcal/ mol
Molecule
Point group Parameter
STO-3G
4-316/521G
ValuesU
opt.
1.323 1791 122 3 1.322 1.807 122.6 1.371 1.902 1.928 1.409 1.279 1.774 1.913 69.6 1.316 1.741 1.830 1.326 1.794 1.869 149 3 155.7 93.2
35.6
19.0
34.2
18.5
16.1
32.3
14 3
12.4
22.6 O.Oh
0.0'
S T O - 3 G optimized geometries, bond lengths in Angstroms and bond angles in degrees. Corresponding total energy. -141.44166 au.
