Lipkowitz
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Face Bonding in Cyclobutadiene
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papa Interactions. 2. 7r Face Bonding in Cyclobutadiene Kenney B. Lipkowitz Contribution from the Department of Chemistry, Indiana-Purdue Uniuersity at Indianapolis, Indianapolis, Indiana 46205. Receiced April 20, I978
Abstract: The T face bonding of rectangular singlet cyclobutadiene with carbon monoxide, hydrogen cyanide, and benzene is investigated. C N D O / 2 total energies for several possible intermolecular geometries are found to be unreliable when compared to minimal basis a b initio results. A long-range attractive interaction between cyclobutadiene and carbon monoxide is found for geometry 111 but all other complexes are unstable. Rotational interconversion from one geometry to another is considered for several complexes.
Introduction While metal-complexed and sterically congested cyclobutadienes are stable and quite well characterized, the parent hydrocarbon is not. In fact, its geometry and electronic configuration may be considered a chemical conundrum rivaled only by the classical-nonclassical carbonium ion controversy. One thing for certain about this cyclic member of the C,H, family is that, although it has a very short lifetime, it can be studied in a matrix a t low temperature.' Typically a suitable precursor is trapped in an inert matrix. Photofragmentation yields cyclobutadiene that may then be leisurely studied. Unfortunately, though, several byproducts are extruded into the matrix cavity that tend to obfuscate assignment of IR absorption bands. Until recently these side products have been completely ignored, but Maier2 has published a communication in which it was suggested that earlier interpretations of IR data needed modification as a consequence of cyclobutadiene R face interactions with carbon dioxide. This was independently confirmed by Krantz3 and there is reason to believe that any spectral assignments made in a matrix be taken cum grano salis in light of the potential complexing ability of cyclobutadiene. Only a few photolabile precursors of cyclobutadiene are known. a-Pyrone (1) produces the desired hydrocarbon along
oo-
C4H4 + C 0 ,
-
C,H,
R
a R
+
(6)
R
R=CO,Me 10
8
-
C,H, + 7
0
1I
R=-C-O-C--
(7)
0
II
9
with carbon dioxide (eq Photofragmentation of anhydride 2 produces C4H4, carbon dioxide, and carbon monoxide (eq 2).5Photolysis of pyridine produces in low yield cyclobutadiene and hydrogen cyanide (eq 3).4d The more complex parent structures, 4,6,8, and 9, are likewise converted to cyclobutadiene and the corresponding byproducts (eq 4-7): all of which contain an aromatic chromophore. Other cyclobutadiene complexes have been proposed as high-energy transients during molecular rearrangement. For example, the reaction of phenylthiophenes in glow discharges show that 2-phenylthiophene will isomerize to 3-phenylthiophene and vice versa. One postulated intermediate is structure
(1)
1
4:-
11
C,H,
+ COL + CO
11.' This was also suggested as a reasonable intermediate to (2)
0 2
0-
C,H,
+ HCN
(3)
N
explain I3C labeling patterns during electron impact.8 The automerizations of Dewar thiophene 12 and the corresponding exo S-oxide, 13, have been studied. The exact mechanism of site exchange is unknown but a concerted, pseudopericyclic process involving peripheral migration of sulfur was tentatively favored over a transannular migration that would result in 14 and 15 as transient species9 Since then
3
4
5
12
14
0
0
6
7
0002-7863/78/1500-7534$01 .OO/O
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0 1978 American Chemical Society
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/ November 22, 1978
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Journal o f t h e American Chemical Society
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H- CZN xs y +V I y
v
VI1
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1.2
I
1.6
/
1.8
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2.0
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2.2
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2.4
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2.8
Intermolecular Distance
xPy
The CO bond i s p a r a l l e l w i t h t h e c y c l o b u t a d i e n e p l a n e and p a r a l l e l w i t h i t s long axis.
Ill.
The CO bond i s p a r a l l e l w i t h t h e c y c l o b u t a d i e n e p l a n e and p e r p e n d i c u l a r t o i t s long a x i s .
V.
VI.
VII. VIIl.’
I
3.4
l
3.6
l 01
6)
The CO b o n d i s p e r p e n d i c u l a r t o t h e c y c l o b u t a d i e n e p l a n e
11.
:V.
/
3.2
Figure 2. Total energy of a benzene-cyclobutadiene complex (geometry VIII) calculated by C N D 0 / 2 .
VIII’
VI11
I.
I
3.0
HCN i s p e r p e n d i c u l a r t o :he
c y c l o b u t a d i e n e p l a n e and d i r e c t l y on t h e z a x i s .
nCk i s p e r p e n d i c u l a r t o t h e c y c l o b u t a d i e n e p l a n d and on :he
z axis.
HCN i s p a r a l l e l t o t h e c y c l o b u t a d i e n e p l a n e a n d p a r a l l e l w i t h i t s l o n g a x i s . HCN i s p a r a l l e l t o t h e c y c l o b u t a d i e n e p l a n e and p e r p e n d i c u l a r t o i t s l o n g a x i s
C H is
a r a l l e l t o t h e c y c l o b u t a d i e n e p l a n e and r o t a t e d a l o n g t h e z a x i s as
stab in”V I I I ’ .
Figure 1. The intermolecular geometries of rectangular cyclobutadiene with carbon monoxide, hydrogen cyanide, and benzene.
an enlightening study of thia allylic rearrangements suggested that square pyramidal structures 14.and 15 are indeed quite acceptable rearrangement intermediates.I0 Finally the tremendous amount of work directed toward understanding the C 1 O H1 0 thermal and photochemical hypersurfaces has shown that photolysis of 16 results (along with other products) in a substantial quantity of benzene (17) and cyclobutadiene dimer 19. One suggestion was that diradical 18 serves as a likely precursor to these products (eq 8).” An
0ED +
16
17
19
18
alternative source of 17 and 19 is the symmetry-allowed excited state [2 21 cycloreversion reaction as depicted in eq 9.l’
+
(9)
20
Other cyclobutadiene and substituted cyclobutadiene complexes have been mentioned by Maier.] Our interest in pepo bonding led us to investigate the K face bonding of the parent cyclobutadiene and carbon dioxide.12 In that study we suggested that a ir complex between C4H4 and C 0 2 is not feasible and the “complex” observed by Maier was a result of C02 being artificially constrained to lie over the cyclobutadiene by the matrix. In fact, a rather severe repulsive interaction appears to exert itself at an intermolecular distance of 2.4-2.6 A regardless of how C02 approaches the ir plane. Furthermore, it was indicated that the geometry of the complex in the matrix cavity is a minimum energy complex that may
be represented by the structure in which C02 is parallel to the C4H4 molecular plane and perpendicular to the long axis of rectangular cyclobutadiene. In this paper we wish to discuss the 7r bonding abilities of cyclobutadiene with other molecules, particularly carbon monoxide, hydrogen cyanide, and benzene.
Results The method of investigation is to study the total energy of cyclobutadiene and its artificial ligand as a function of intermolecular distance using standard quantum mechanical techniques. Two approaches to this problem are tried: a semiempirical technique (CNDO/2) and a b initio calculations. In this paper, cyclobutadiene is treated as a rectangular singlet species with a geometry previously described.I2 Bond lengths for carbon monoxide, hydrogen cyanide, and benzene are experimentally determined.I3 The original C N D 0 / 2 parameters were used unchanged.I4 For the a b initio calculations, a minimal STO-3G basis set was implementedI5 and in all cases at least one, and, where appropriate, two symmetry planes were utilized to reduce computational times. All species calculated individually (infinite intermolecular separation) quickly converged using the program PHANTOM.]^ Evaluation of the complexes tended to oscillate in the S C F calculation but could be made to converge in less than 50 iterations by applying a density matrix averaging of 0.25. As in the case of carbon dioxide several geometries were used.I2 These are labeled I-VI11 in Figure 1. In all cases the center of cyclobutadiene is on the origin. For all geometries where the “ligand” is perpendicular to the cyclobutadiene plane the intermolecular distance is the distance from the origin of the Cartesian coordinate system to the atom closest to C4H4. The plot of total energy vs. intermolecular distance for benzene-cyclobutadiene calculated with the semiempirical program is shown in Figure 2. The same system calculated a b initio is shown in Figure 3. Energies for CO-cyclobutadiene computed with C N D 0 / 2 and a b initio are presented in Figures 4 and 5 , respectively. Ab initio values for HCN-cyclobutadiene are in Figure 6. Discussion I. Semiempirical Results. The semiempirical evaluation of D6h benzene with D2h cyclobutadiene shows a tremendous stabilization a t an intermolecular distance of 1.60 k (Figure 2). Electron transfer from one ring to the other is not evident, but rather, a total electron reorganization within each ring has taken place. Perusal of the density matrix a t the minimal energy geometry indicates a structure closely resembling biradical 21. This intermediate is known and irreversibly rear-
Lipkowitz
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a Face Bonding in Cyclobutadiene I
-
GEOMETRY I GEOMETRY I 1 - - - - - GEOMETRY : I I . . * * * . *
1
Intermolecular Distance ( A )
,
2.2
Figure 3. STO-3G evaluation of total energy of complex V I l l as a function of intermolecular distance.
2.4
2.6
8
1
2.8
3.0
3 2
1
1
3 4
3.6
3.8
4'0
4.4
4.2
I
4.6
1
4.8
=
Intermolecular Distance f A )
Figure 5. STO-3G evaluation of total energy for several cyclobutadienecarbon monoxide complexes. 25
.
20
15
'
--
I
:
GEOMETRY :Y GEOMETRY V GE3METRY V : GEOMETRY $ 1 1
'
10
_L
I
I
1 1.8
1
I
2.0
2 2
2.6
2 6
2.8
3.0
3.2
*,
*
-- --I
__ ~
1
I
I
I
'
3 4
3 6
3.8
4 0
4 2
'
l
4 4
intermolecular D i s t a n c e ( A )
Figure 6. STO-3G total energies for several cyclobutadiene-hydrogen cyanide complexes.
1.00 1 . 2 0 1 . 4 0
1.60
2 0 0 2 . 2 3 2.40 2 . 6 0 2.80 Intervolecular d i s t a n c e (a)
l.'80
3.00
3.i0
=
Figure 4. Total energy of several cyclobutadiene-carbon monoxide complexes calculated with C N D 0 / 2 . a
21
22
ranges to hypostrophene (22).17While the thought of benzene and cyclobutadiene giving rise to hypostrophene is tempting, this suggestion is not tenable in light of the unreasonably large calculated interaction energy. Indeed, C N D 0 / 2 has been known to overestimate certain interactions, particularly those involving a complexes.18
The semiempirical calculations of cyclobutadiene and carbon monoxide give energy minima between 1.20 and 1.40 A depending on geometry (Figure 4). These minima are several orders of magnitude larger than might be expected. Since we had originally intended to investigate relatively large organic molecules interacting with cyclobutadiene we initiated this study with approximate molecular orbital methods. It appears, though, that C N D 0 / 2 is unreliable for this study and we have abandoned this approach. 11. Ab Initio Results. The poor C N D 0 / 2 results are, in part, a consequence of neglecting too many repulsive interactions (thus overestimating the attractive ones). This can be rectified with a minimal basis a b initio approach. Contrary to the C N D O results, all cases studied indicate a repulsive interaction that generally becomes severe at an intermolecular separation of 3.20-3.00 A. S C F theory never predicts van der Waals binding since the latter is due to correlation effects. Semiempirical schemes such as C N D O do not give this effect either. Hence the calculation presented here eliminates only chemically bound complexes, and not van der Waals complexes. In a previous communicationI2 we investigated the interactions of cyclobutadiene and carbon dioxide. Three distinct geometries were studied, all of which displayed repulsive interactions. We concluded that the cyclobutadiene complex should exist as a minimal energy complex and, for reasons discussed by K r a n t ~ suggested ,~ the geometry where the car-
1538
Journal of the American Chemical Society
bon dioxide bond axis is parallel with the hydrocarbon plane and perpendicular to the long axis of the rectangle. In the case of carbon monoxide-cyclobutadiene, the energies of configurations I1 and 111are virtually indistinguishable and both show less of an adverse interaction than geometry I between 2.60 and 3.40 A (Figure 5 ) . It would seem that one of these two (I1 and 111) and possibly both geometries may exist in a matrix with a rectangular cyclobutadiene. Alternatively, these two geometries may be interconverting via carbon monoxide free rotation about the z axis. Indeed, the calculated change in ener y upon 1 5 O incremental rotation from geometry 111to I1 at 3.2 intermolecular separation reaches a maximum a t 60” rotation. This maximum is less than 0.5 kcal mol-’, which indicates virtually free carbon monoxide rotation. The intermolecular separation of 3.2 was arbitrarily chosen but similar calculations a t other distances qualitatively demonstrate the same behavior of showing a n energy maximum for diagonal geometries. Interconversion from I1 to 111 via geometry I is not reasonable since a barrier of 2-4 kcal mol-’ exists between either I1 and I or I11 and I . Of course if the matrix is very tight, forcing the carbon monoxide and cyclobutadiene within 2.40 A of one another, this interconversion appears more reasonable. At long range there is a slight attractive interaction (
