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Insertion of Ethylene into a Platinum(II)-Hydrido-Complex Containing a Bidentate Ligand Spanning trans-Positions.

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Table I . Ratc constants k I for the topornerization of the a n ~ o n( I ) in the
form of its potassium salt.
-
__
T
["CI
Method
Solvent
k1
[s -
'1
~~
Equilibration
after D-labeling
Saturation
transfer
Line-shape
analysis
~
4.5
10-4
1.4.x
10-3
61
THF
THF
DMSO
62
DMSO
1.7 x
89
99.5
108
115
I20
DMSO
DMSO
DMSO
DMSO
DMSO
1
27
33
1.1 x 10-1
lo-'
4
7
I1
20
O n KO
D
Conrotatory ring opening leads to formation of a monodeuterio-cis,cis,cis,trans-[9]annulene anion in which deuterium
is found predominantly at an Hb position. The 'H-NMR
signal at 6 = -3.52 showed mainly a doublet broadened by
D-H coupling along with a small superimposed triplet.
Warming to 30°C (3 h) led to a trip1et:doublet ratio = 3: 1,
the value expected for complete scrambling of the deuterium
over all ring positions.
Solution of the complex kinetics for equilibration of the
ninedeuterated isomers of ( I ) [ ' ] provided further information.
The rate constant k l for a single isomerization step (equivalent
to the topomerization rate constant, k l , for the undeuterated
species) could be determined (Table 1) from the time-dependence of the deuterium equilibration. This was accessible
experimentally via measurement of the change in the tripletdoublet ratio at 27 and 33°C.
The Eyring plot of the rate constants kl, measured by
three methods over a temperature range of 93"C, showed
a straight line. This finding provides good evidence that the
measurements between 61 and 120°C also involved the topomerization directly demonstrated at 27 and 33°C. The activation entropy, near zero (AS* = 6 k 8 e. u.), is in accord with
the intramolecular character of the topomerization. The activation enthalpy is AH * = 23.6 k 1.5 kcal/mol. Hence the rotation
of the trans bond in the cis,cis,cis,trans-[9]annuleneanion
requires a considerably higher activation energy than the corresponding process in the uncharged larger annulenes and
in the x-isoelectronic cis,cis,cis,cis,tran.s-[ 1O]ann~lene[~].
This fact can be best explained in terms of the energetically
unfavorable localization of charge required in intramolecular
reactions of ( 1 ) (and other charged annulenes). A comparison
of the topomerization and isomerization of ( I ) to the thermodynamically more stableall-cis-[9]annulene anion (,)[lo] confirms this interpretation. Thus, preliminary measurements
show that isomerization of ( 1 ) to (2) at 65°C is > lo6 times
slower than the topomerization. The transition state of the
isomerization necessitates a much more pronounced charge
localization than that of the topomerization'"].
[2] J . F . M.0th. Pure Appl. Chem. 25, 573 (1971): F . Sondhrimrr, Accounts
Chem. Res. 5. 81 (1972).
[3] G. Schriider. G. Plinhe, D. M . Smith, and J . F . iM. Orh, Angew. Chem.
85, 350 (1973);Angew. Chem. internat. Edit. 1.325 (1973): G. Srhriirler,
private communication.
[4] J . F . M . 0 t h . G. Anrhoine. and J . M . Gillrs. Tetrahedron Lett. 1968,
6265.
[5] G. Borhr. D.Morrens. and M! Danirr. Angew. Chem. 81, 1003 (1969);
Angew. Chem. internat. Edit. 8, 984 (1969).
161 We thank Prof. G. Binsrh, Universitat Munchen. for valuable information and the Program D N M R 11.
[7] S. ForsPn and R. Hoffmuiin. J . Chem. Phys. 39, 2892 (1963): F . A.
L. Anrr and A . J . R . Bourn, J. Amer. Chem. SOC.89, 760 (1967).
[8] A . A. Frosr and R. C. Pearson: Kinetik und Mechanismen homogener
chemischer Reaktionen. Verlag Chemie, Weinheim 1964, pp. 161 N.:
we thank Prof. L. Birberbach for help with a few mathematical problems.
[9] S . Masamune, K . Hojo. K . H q o . G. Bigam, and D. L. Rabensrrin.
J . Amer. Chem. SOC.93, 4966 (1971).
[lo] 7: J . K a t z and P . J . Garralr, J. Amer. Chem. SOC. 85, 2852 (1963);
E. A. Lulancrrrr and R. E . Bensoii, ihid. 85. 2853 ( I 963).
[ I I ] C. Boche and A. Biebrrhach, unpublished results.
Insertion of Ethylene into a Platinum(n)-HydridoComplex Containing a Bidentate Ligand Spanning
rvans-Positions[**l
By G . Brachrr, P. S. Pregosin, and L. M . Venand']
The insertion of ethylene into Pt-H bonds has been discussed in a number of recent papers"]. The two mechanisms
which have been put forward for reactions of this type are
summarized below :
Scheme 1
Scheme 2
Received: April 30, 1975 [Z 250 IE]
German version: Angew. Chem. 87. 550 (1975)
As pointed out"] the kinetic data reported are compatible
with either scheme.
We report here the occurrence of ethylene insertion into
a complex which cannot react by Scheme 1 .
It has recently been
that 2,l I-bis(dipheny1phosphinomethyl)benzo[c]phenanthrene ( 1 ) forms square planar
complexes of type (2) in which the bidentate ligand spans
trans-positions. As ligand ( I ) cannot form mononuclear complexes of the type cis-[PtX,(/j] because of severe overlap
of the phenyl groups['], we have prepared acetone solutions
of trans-[PtH(acetone)(I )][BFd] (3) and studied their reactions with ethylene.
55954-94-4: C,H,K,
[*] Prof. Dr. L. M. Venanzi. Dr. P. S. Pregosin, and G. Brdcher
CAS Registry numbers:
( 3 ) , 55820-91-2; C,H,DK.
~~~~
55740-60-8
-
[I] Cf., e.8.. the rotation of rrans bonds in the [18]annulene requires an
activation enthalpy ofAH* = 16.1 kcal/mol [2], while the [17]annulene
anion [3] and the [16]annulenedianion d o not show any line broadening
in the 'H-NMR spectrum, even on warming t o 140°C [4].
A n y w . Chem. internut. Edit. I Vol. I 4 ( 1 9 7 5 )
No. 8
Laboratorium fur Anorganische Chemie der Eidgenossischen
Technischen Hochschule
Universitiitstrasse 6, CH-8006 Zurich (Switzerland)
[**] This work was supported by the Schweizerischer Nationalfonds zur
Forderung der wissenschaftlichen Forschnng.
563
isomerism by insertion of a CH,, 0, or NH insulator into the
bonds common to two rings (bridging principle)u21.
I_\
(1)
(21
trans-[PtHCI(l)] ( 4 ) , prepared by addition of ligand ( I )
to a benzene solution of trans-[PtHC1(Ph3P)2][31,was suspended in acetoneand treated with one equivalent of Ag[BF4].
After filtering off the precipitated AgC1, the solution, containing complex (3), was treated with ethylene (ca. 1.2 atm) for
12h.
The organometallic species present in the resulting solution
was characterized as done for the corresponding complexes
with monodentate phosphanes"], i. e., either by addition of
chloride and isolation of trans-[Pt(C2H5)C1(l)] ( 5 ) or by
bubbling carbon monoxide through the solution and isolating
trans-[Pt(C2H5)(CO)(l)][BF4] (6). Complexes (4) to (6)
gave satisfactory elemental analyses. The 'H- and 31P-NMRand IR spectra of complexes ( 3 ) to (6) are in agreement
with their respective formulations. Thus, our experiments show
that ethylene insertion occurs under relatively mild conditions
even in complex (3).
The implications of our results as to whether Scheme 1
or Scheme 2 provides a better description of the olefin insertion
reaction in complexes trans-[PtH(acetone)(R3P)2] (7) are
not clear. On the one hand they seem to provide evidence
in favor of Scheme 2. On the other, it could be argued that
the significant differences in rates of ethylene insertion between
(3) and (7)"l indicate that Scheme 1 does indeed correspond
to the lower energy pathway and that Scheme 2 becomes
operative only when the cis-intermediates required by Scheme
1 cannot be formed[41.
The significance of our experiments, however, rests in the
demonstration that ethylene insertion occurs even when the
hydrido and olefin ligands cannot adopt mutually cis-orientations in a square-planar complex.
+
(1)
arene
An approximately planar arrangement of the peripheral
annulene ring of ( 1 ) can be achieved only at the expense
of a certain degree of steric compression of the two inner
bridge hydrogen atoms. The effective radii of these hydrogen
atoms should thus determine whether ( I ) possesses a delocalized or a fluxional n-electron system, i.e., whether the hydrocarbon (1) is an arene or an olefin.
r
1
All attempts to prepare (1) by conventional methods from
1,4,5,8,9,10-hexahydroanthracenehave so far failed. A new
synthetic concept based on (a) the observation that the conformational equilibrium between syn- and anti-bicyclo[5.4. lldodeca-2,5,7,9,1I-pentaene [ ( 3 ) and ( 4 ) , respectively] lies practically entirely on the side of the syn conformer (3)[31, and
(b) the course of bromination of the recently discovered cyclopropa-arene 1H-3,8-methanocyclopropa[ lO]annulene (5)14]
provided the desired breakthrough.
6- & -
Received: May 13, 1975 [Z 256 IE]
German version: Angew. Chem. 87,547 (1975)
CAS Registry numbers:
(3). 55758-62-8; ( 4 ) . 55758-63-9; i s ) , 55758-64-0;
( 6 ) , 55758-66-2: C,H,, 74-85-1 ; rrans-[PtHCI(PPh,),],
olefin
Br
Br
16841-99-9
[l] H. C . Clark, C. JablonskL, J. Halpern, A. Manrovani, and 7: A. Wed,
Inorg. Chem. 13, 1541 (1974); H. C . Clark and C . R . Jablonski, ibid.
13, 2213 (1974).
[2] N. J . DeStefano, D. K. Johnson, and L. M. Venanzi, Angew. Chem.
86, 133 (1974); Angew. Chem. internat. Edit. 13, 133 (1974).
[3] J . C. Bailar and H. Iratani, Inorg. Chem. 4, 1618 (1965).
[4] It is doubtful whether direct comparison of rates of ethylene insertion
in (3) and ( 7 ) is really meaningful as activation energies for reactions
of complexes of type (2) are expected to be higher than for those
of the rrans-[PtX2(R3P)2]type.
syn -1,6 :8,13-Bismethano[ 14]annulene['I
By Emunuel Vogel, Johannes Sombroek, and Wolfgang Wagemann[*l
The synthesis of syn-1,6: 8,13-bismethano[l4]annulene (1)
has been the object of intense effort in this laboratory since
our recognition that anthracene is formally convertible into
bridged [l4]annulenes exhibiting syn-anti configurational
['I
564
s&[
-
&s]-
Prof. Dr. E. Vogel, Dr. J. Sombroek, and DipLChem. W. Wagemann
lnstitut fur Organische Chemie der Universitat
5 Koln 41, Greinstrasse 4 (Germany)
Angen,. Chem. inrernar. Edit. J Vol. 14 (1975) J N o . 8
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platinum, complex, containing, spanning, ethylene, insertion, hydride, transp, ligand, bidentate, positional
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