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Molecular Structure of a Benzocyclobutadiene.

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solvent is then removed under vacuum and the residue taken
up in benzene, filtered and crystallized from pentane/toluene.
0.68g ZnC12 affords 2.7g (69%) ( 3 a ) [m.p. 77"C], while
0.92g CdC12 affords 2.848 (65%) ( 3 b ) [m.p. 94"C], and
1.41 g (Me3P)2NiC12affords 3.0g (78 %) ( 3 c ) [m.p. 90°C
(dec.)]. (3 a) and ( 3 b ) should preferably be sublimed. Spectroscopic data for C12H36B2CdP4 ( 3 b ) : IR (Nujol): vBH2 2390
and 2350cm-'. 'H-NMR (C6H.5, TMS): 6CH3=1.29, "d",
6H, J(HCP)=9.75 Hz; 6CH2=0.05, "d", 2H, J(HCP)=13.5,
J(HCCd) 3 3 . 8 H ~ ' ~"B-NMR
'.
(C6H6, BF3.Et20): 6= -31.8,
J(PB)=J(BH)=92Hz"l.
l3C('H}-NMR (C6D6, TMS):
6CH3=20.1,AXX',J(PC)=43.9Hz; SCH2= 1.18, broad signal
with satellites,J(CCd)=280.8 Hz. 31P{'H}-NMR (C6H6,85 %
H3P04): 6= -2.34, q, J(PB)= 100.7Hz.
P
[4] G. Miiffer,Diplomarbeit, Technische Universitat Miinchen 1977.
[ S ] 1227 structure factors ( I 2 3.3 o,Syntex P21/XTL, o-Scan, 2" s 2 8 ~ 4 8 " ,
h=71.069pm), R 1 =0.05: monoclinic, space group P2,/c, 2 = 2 ,
b = 2036.7(2),
c=960.5(2)pm,
p = 119.21(8)",
a=629.5( I),
v= 1075 x 1 0 6 ~ ~ 3 .
[6] A6XX'A6- and AzXXA; splitting.
[7] Cf. N . E. Miller, E . L. Muetterties, J. Am. Chem. SOC.86, 1033 (1964).
We are grateful to Dr. B. Wruckmayer and L. Wufdmann, Universitat
Miinchen, for these spectra.
Molecular Structure of a Benzocyclobutadiene
By Werner Winter and Henner Straub"]
In connection with the cyclobutadiene problem['], cyclobutadienoid compounds such as the monobenzo-annelated derivative of [4]annulene are also of theoretical and preparative
interestr2].Besides the matrix isolation of the benzocyclobutadiene ( I ) , R' = R2= H, at 8 KC3],substituted benzocyclobutadienes isolable at room temperature have recently also been
synthesized[41.Such kinetically stabilized derivatives should
permit the first experimental determination of the molecular
geometry of a free benzocyclobutadiene.The derivative ( I )[4b1
was used for an X-ray structure analysis, since this yellow
crystalline hydrocarbon is storable under nitrogen for months
and the electronic system of such a derivative substituted
only with alkyl groups can hardly be falsified compared to
that of the parent compound (R' =R2=H). The bicyclic 8n
system can be described by the three KekulC structures A,
B, and C (KekulC indices: A 0.905, B 0.941, C 0.832)f51.
R2@R1
R2
Fig. 1. Structure of crystalline nickel complex ( 3 c ) (H atoms of methyl
and methylene groups are omitted for clarity). Averaged bond lengths [pm]
:
and angles
r]
~~~
R2
R
\
R2
R'
R2
2R 2 W
'
R2
"'aR
'
R2
R2
R'
'
R2
-
R'
~
Ni-C
P-B
P-C (CHI)
P-C (CH,)
199.4(9)
190 (1)
174.8(8)
181 (1)
C-Ni-C
P-B-P
B-P-C
B-P-C
90.0(3)
110.3(5)
112.3(4)
108.9(4)
(CHz)
(CH3)
A mixture of ( I ) (2.1 g, 8.6mmol) and LiA1Me4 (0.8g,
8.5mmol) is carefully warmed at a pressure of lo-' torr.
At 115°C evolution of gas is observed and a colorless liquid
distils over which rapidly crystallizes. M.p. 70-71 "C; dec.
>20O0C. CsHZ4BAlP2 ( 4 ) : IR (Nujol): vBH2 2410 and
2360cm-'. 'H-NMR (C6H6, TMS): SCH3=1.35, "d", 6H,
J(HCP)= 10.5 Hz; SCHz ~ 0 . 3 1 ,"d", 2H, J(HCP)= 18.OHz;
G(AlCH3)= -0.29, S, 3H. "B-NMR (C6H6, BF3.Et20): 6 =
- 32.5, J(PB)=J(BH)=97.5 Hzr7]. 13C('H}-NMR (C6D6,
TMS): 6CH3= 72.5, AXX', J(PC)=21.3 Hz; 6CH2= 67.7, s (br).
31P{'H}-NMR (C6H6, 85 % H3Po4): 6= -4.65, q,
J(PB)=94.6 Hz.
Received: November 23, 1977 [Z 885 b IE]
German version: Angew. Chem. 90, 126 (1978)
CAS Registry numbers:
( l ) , 65293-66-5: ( 3 a ) , 65293-65-4: ( 3 b ) , 65293-64-3; ( 3 c ) , 65293-63-2; ( 4 ) ,
65293-62-1: I3C, 14762-74-4: LiAI(CH,).,, 14281-94-8
[l] G. H! Parshaff in E. L. Muetterties: The Chemistry of Boron and Its
Compounds, Chap. 9. Wiley, New York 1967.
[2] S . G. Shore, G . E. Ryschkewitsch in E . L. Muetterties: Boron Hydride
Chemistry. Academic Press, New York 1975, Chap. 3, 6.
[3] H . Schmidbaur, 0. Gasser, Angew. Chem. 88, 542 (1976): Angew. Chem.
Int. Ed. Engl. IS, 502 (1976): H . Schmidbaur, H.-J. Fiiffer,ibid. 88, 541
(1976) and 15, 501 (1976), resp.
Angew. Chem. Int. Ed. Engf. 17 (1978) N o . 2
W
W
Fig. 1. ORTEP plot of ( 1 ) . The mean standard deviation for the bond
lengths is 0.006 A, for the bond angles 0.4".
The crystal structure analysisr6]of (1) shows, within the
limit of experimental error, that the benzocyclobutadiene
moiety is planar. Further, since the C atoms bound directly
to the bicycle deviate maximally by only about +0.04A from
this plane the whole system may be :egarded as nearly planar.
The shortest C-C bond (1.36 A) in the four-membered
ring of ( I ) is to be interpreted as an isolated double bond;
[*I Dr. W. Winter, Dr. H. Straub
Institut fur Organische Chemie der Universitat
Auf der Morgenstelle 18, D-7400 Tiibingen (Germany)
127
it is hardly influenced by the other 6n electrons of the six-membered ring. The longest bonds of the four-membered ring
(1.53 are somewhat longer than the usual single bonds
of sp’-hybridized C atoms. Both bond lengths are in good
agreement with those of other cyclobutadiene systemsr7Jwhere,
as also in ( I ) , the steric interaction of the bulky substituents
could be responsible for stretching of the corresponding bonds
of the four-membered ring. The shortest distance of only
2.11 between an H atom of a methyl group in the 3,6
position and the H atom of a methyl group of the tert-butyl
moiety clearly shows that the substituents R’ and R 2 in (I)
approach each other to within the van der Waals distance.
The measured bond lengths in ( 1 ) rule out any noticeable
participation of the resonance structure ( 1 C). The benzocyclobutadiene ground state is accordingly best described in terms
of the two resonance structures (1 A ) and ( 1 B ) , the bond
lengths in the six-membered ring suggesting greater participation of structure ( 1 B). Comparison of the experimentally
determined bond lengths and angles of ( 1 ) with the calculated
values of benzocyclobutadiene (R’ = RZ= H)[5381shows surprisingly good agreement. A large deviation is found only
in the lengths of four-membered ring single bonds (calculated
1.49A), which can be attributed to the repulsive effect of
the bulky substituents.
A)
Heating of the violet Rh’ cyclobutadiene chelate complex
(1
in inert organic solvents (e.g . boiling xylene) leads,
via cleavage of PPh3, to an ocher-colored complex (2) which
is insoluble in all the usual solvents:
A
+*
J
(2)
- PPhg
PPh3
L
I
L
Ph
XQ
(3)
Received: November 8, 1977 [Z 886a IE]
German version: Angew. Chem. 90, 142 (1978)
Review: G . Maier, Angew. Chem. 86, 491 (1974); Angew. Chem. Int.
Ed. Engl. 13,425 (1974); S. Masamune, Pure Appl. Chem. 44, 861 (1975).
Reviews: M . P. Caua, M . J . Mitchell: Cyclobutadiene and Related Compounds. Academic Press, New York 1967; K . P. C . Vollhardt, Fortschr.
Chem. Forsch. 59, 113 (1975).
0 . L. Chapman, C . C . Chang, N . R . Rosenquist, J. Am. Chem. SOC.
98, 261 (1976).
a) H . Straub, Angew. Chem. 86, 412 (1974); Angew. Chem. Int. Ed.
Engl. 13, 405 (1974); b) Tetrahedron Lett. 1976, 3513; Justus Liebigs
Ann. Chem., in press; c) F . Toda, N . Dan, J. Chem. SOC.Chem. Commun.
1976, 30; F . Toda, K . Tanaka, ibid. 1976, 1010; K . P. C . Vollhardt, L.
S. Yee, J. Am. Chem. SOC.99, 2010 (1977).
M . Milun, N . Trinajstif, Z. Naturforsch. B 2 8 , 478 (1973); and references
cited therein.
Orthorhombic, space group P21212121; a = 10.042, b= 10.885,
c=16.146A; 2 = 4 ; 1613 symmetry independent reflections [2>20(1)],
Nonius CAD-4, graphite monochromator, MoK,,; solution: non-centrosymmetric direct methods (SHELX-75, G. M . Sheldrick); refinement:
R = 0.092, R , = 0.084.
H . Irngartinger, H . Rodewald, Angew. Chem. 86, 783 (1974); Angew.
Chem. Int. Ed. Engl. 13, 740 (1974); L. T J . Delbaere, M . N . G . James,
N . Nakamura, S. Masamune, J. Am. Chem. SOC.97, 1973 (1975); cf.
also P. J . Garratt, Pure Appl. Chem. 44, 783 (1975).
M . J . S. Dewar, G . J . Gleicher, Tetrahedron 21, 1817 (1965).
Valence Isomeric q 2- and q4-Cyclobutadiene-Rh’Complexes
By Werner Winter and Joachim Striihle“]
Transition metal complexes with q4-cyclobutadiene ligands
are well known. The isolation of iron complexes with q2-coordinated cyclobutadiene was first described in 1974“’; these
unusual coordination compounds were characterized on the
basis of spectroscopic data and their reaction products.
We now report the first synthesis of isomeric rhodium(1)
complexes bearing variously coordinated cyclobutadiene.
[*] Dr. W. Winter [‘I
Institut f i r Organische Chemie der Universitat
Auf der Morgenstelle 18, D-7400 Tubingen (Germany)
Prof. Dr. J. Strahle
Institut fur Anorganische Chemie der Universitat Tiihingen
[ ‘1 To whom correspondence should be addressed.
128
L = P( O C H 3 ) s
L’ = Cyclobuta- p h o s p h e p i n
x@= c10,@
Apart from an elemental analysis, the insolubility of (2)
precluded investigation of the complete structure or possible
structural changes of the cyclobutadiene ligands. An attempt
was therefore made to transform this product into a “soluble
form” by cleavage of the Rh-CI bridges. Treatment of (2)
with an excess of trimethyl p h ~ s p h i t e [ ~in’ ethanol in the
presence of ClO: led within a few minutes to complete dissolution of the solid. Crystallization with CH2C12/ether gives mixture of red (3) and bright-yellow crystalls ( 4 ) (total yield
> 80%). Apart from differences in the usual analytical data
(conductivity, IR, UV, NMR)r41, (3) and ( 4 ) differ characteristically in the 31P{1H}-NMR spectrum: while (3) shows only
signals of the Rh-coordinated P atoms, the spectrum of ( 4 )
also displays a singlet at 6= -0.7 (H3P04 ext; 36.43MHz).
This signal can be unequivocally assigned to a non-coordinated
phosphepin P atom. The “inert gas rule” thus leads to formulation of a q2- [for ( 3 ) ] and a q4-cyclobutadiene complex
for ( 4 ) .
For confirmation of the structure we have carried out an
X-ray structure analysis on (3)[?
Figure 1 shows the essential structural features of (3):
the ligand arrangement at the rhodium approximately corresponds to a trigonal bipyramid, in which the phosphepin P
atom occupies an apical position and the cyclobutadiene bond
an equatorial position in the seven-membered ring. Strong
back-bonding into n* orbitals of the seven-membered ring
cyclobutadiene double bond leads to elongation of the C+C
bond to 1.51 A and pronounced angulation of the four-membered ring at the coordinated C atoms. The non-coordinated
double bond of the four-membered ring, with a length of
1.34A, retains its double bond character.
Angew. Chem. Int. Ed. Engl. 17 (1978) N o . 2
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