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High-Pressure NMR Studies of [(3-C5H5)(5-C5H5)Cr(CO)2]. Evidence for Concerted Ring Interchange

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High-pressure NMR Studies of
I 1
I(93-CsHs)(95-C,Hs)Cr(CO), I
Evidence for Concerted Ring Interchange
By John M . Millar, Rodney V: Kastrup, Suzanne Harris,*
and Istvan I: Horvath*
1.5 atm CO
Dedicated to Professor Cyorgy Bor on the occasion
of his 65th birfhday
1 1+2
Reversible ring slippage of coordinatively saturated sandwich and half-sandwich complexes is a possible path to accommodate ligand activation in homogeneous catalytic reactions.['] The reversible reaction of [(q5-C5H5)zCr(CO)J(1)
with CO at elevated pressure has recently been studied
[Eq. (a)] .['I On the basis of high-pressure IR spectral data,
the formation of the ring-slipped species [(q3-C,H,)(q5C,H,)Cr(CO),] (2), analogous to the structurally characterized complex [(q3-C,H,)(q5-C5Hs)W(C0)2],[31
was suggested. We now wish to report that 2 undergoes concerted ring
interchange, with an activation barrier of 13.5 f
1.0 kcal mol- which does not involve CO-eliminationinduced q 3 + q5 ring slippage or CO-addition-induced
q 5 + q 3 ring slippage.
130 atm CO
-70 O C
1 1
Dr. 1. T. Horvath, Dr. R. V. Kastrup, Dr. J. M. Milbar, Dr. S. Harris
Corporate Research Science Laboratories
Exxon Research and Engineering Company
Annandale. NJ 08801 (USA)
mhH, 0-6940 Wernherm. 1990
Fig 1 I3C NMR spectra of [(C,H,),Cr] under 1 5 atm of ' T O at 30°C and
under 130 atm of "CO at 30 "C and - 70 "C The extremely weak signals at
d = 256 and 245 are due to [(qs-C,H,),Cr,(CO),I, which IS formed above 0°C
under CO pressure [2]
Reaction of [(q5-C,H,),Cr] with CO at 1.5 atm resulted in
the formation of l.[41
The I3C NMR spectrum of 1 at room
temperature shows a resonance at 6 = 75.9 for the C,H,
ligands and another at 6 = 263.1 for the carbonyl ligand
(Fig. 1, top), while the 'H NMR spectrum shows a singlet at
6 = 4 for the C,H, ligands. When the CO pressure was increased to 130 atm, the intensity of the peak at 6 = 75.9 in
the 13CNMR spectrum decreased and a new singlet appeared at 6 = 96.9 (Fig. 1, middle). Similarly, the 'H NMR
spectrum showed a new singlet at 6 = 5.2, while the intensity
of the peak at 6 = 4 decreased. The new resonances are due
to the formation of 2, which at room temperature undergoes
ring interchange. Surprisingly, there was only one resonance
in the carbonyl region at 6 = 259.4, slightly shifted from the
value observed for 1. The ratio of its intensity to the combined intensity of the signals for the C,H, ligands in 1 and
2, however, clearly indicates that the former resonance is due
to the carbonyls of both 1 and 2.
On cooling to 0 "C, the peak at 6 = 75.9 disappeared, as
expected from the temperature dependence of equilibrium
and only 2 is present in the solution. Upon further
cooling, the peak at 6 = 96.9 broadened and coalesced at
- 15 "C. Two singlets of equal intensity at 6 = 89.0 and 102.7
were resolved on cooling to -70°C (Fig. 1, bottom). Although this proves the presence of two different C,H, ligands in 2, rotation of the q3- and qs-C,H, ligands renders
the ring carbon atoms equivalent even at - 140 "C in a 1 : 1
mixture of [DJtoluene and CD,CI,. Simulations of the
13CNMR spectrum at various temperatures give 76000 sas the rate of exchange of the two C,H, ligands at
room temperature and an energy of activation of
13.5 1 kcal mol-' (Fig. 2). As expected from the variabletemperature I3C NMR results, two different rings are resolved in the low-temperature 'H NMR spectrum as well.
[msl I
0.1 -
103T-' [K-'1
Fig. 2. Arrhenius plot for the concerted q3/q5 ring interchange in
[(r13-C,H,)(r15-C,H,)Cr(CO),I (2).
Saturation-transferexperiments do not show any evidence
for exchange between the coordinated CO ligands in 2 and
free CO. Our measurements indicate an upper limit of 10 sfor this process at room temperature. Mechanistically, it is
clear that 2 undergoes concerted ring interchange, which
does not involve CO-elimination-induced q 3 -+ q5 ring slip-
Angew. Chem In1 Ed. Engl. 29 (1990) N o 2
page followed by CO-addition-induced q 5 -+ q3 ring slippage. An examination and comparison of the electronic
structures of [(q5-C,H,),M(CO),] and [(q3-C,H,(q5C,H,)M(CO),] complexes suggest why and how a ring interconversion not requiring loss of a CO ligand occurs in 2.
Strong M-to-CO 71 back-donation is generally observed in
18-electron [(q5-C,H,),M(C0),] complexes,[6* and we find
that this K back-donation becomes even more important in
a complex such as 2.
Recent calculations for [(C,H,),M(CO),], M = Ti, Zr,
show that the ordering of the lower-energy metal-based orbitals is as shown in Figure 3a.[’I The HOMO in the 18-electron (d2) Ti and Zr complexes is an a, orbital which facilitates back-donation from the metal to the CO x* orbitals
lying in the plane of the CO ligands. The two lowest-energy
unoccupied orbitals are the b, and a2orbitals. These orbitals
are antibonding between the metal and C,H, orbitals (this
interaction is not shown in Fig. 3), but are stabilized by
bonding interactions with the CO K* orbitals which are perpendicular to the plane of the two CO ligands. The addition
of two moreelectrons in [(C,H,),M(CO),], M = Cr, Mo, W,
leads to a formally 20-electron complex, and the unusual
bent, rather than merely slipped, q3-C,H, configuration observed for one of the C,H, rings in [(q3-C,H,)(q’C,H,)W(CO),] is usually associated with the attainment of
a formally 18-electron complex. In order to better understand the effects of this bent C,H, ring, Fenske-Hall[81 MO
calculations were carried out for [(q3-C5H5)(q5C,H,)W(CO),]. The results of these calculations show that
the bending of the C,H, ring and the resulting C , symmetry
lead to a loss of most of the M-C,H, antibonding character
in the b, (now a’) orbital (Fig. 3 b). As a result, this orbital is
with the similar 13CNMR chemical shifts for the carbonyl
ligands in 1 and 2.
In light of the strong M-CO back-donation observed in
the dicarbonyl complex 2, it is perhaps not surprising that
the concerted ring interchange does not involve elimination
or addition of a CO ligand. Interconversion of the two rings
(the bent q 3 and the flat qs rings) presumably occurs
through a simultaneous straightening and bending of the
two rings. The intermediates involved in such a conversion
would continue to be stabilized by K back-donation, since
even though the a’ (b,) orbital would be somewhat destabilized during the process, this orbital would remain occupied
and serve to remove charge from the metal throughout the
whole process.
Received: September 12.1989 [Z 3548 JE]
German version: Angew. Chem. 1/12 (1990) 216
CAS Registry number’
2, 123892-90-0.
[I] J. M. O’Connor. C. P. Casey, Chem. Rev.87 (1987) 307.
[2] E. U. van Raaij. H. H. Brintzinger, J Orgunomet. Chem. 356 (1988) 315.
[3] G. Huttner, H. H. Brintzinger, L. G. Bell, P. Friedrich. V. Bejenke, D.
Neugebauer, J Orgunomet. Chem. 145 (1978) 329.
[4] Compound 1 was prepared in situ in a high-pressure sapphire NMR tube [ 5 ]
by allowing 0.1 g of chromocene (Strem) in 3 mL of [D,]toluene/CD,CI,
(Ill) (or in 3 mL of [D,]toluene) to react with ”CO (99.9% isotopic purity,
Isotech Inc.) at 1.5 atm pressure. After the formation of 1 was confirmed,
the tube was charged with 130 atm ‘ T O at room temperature. Variabletemperature NMR spectra were obtained on Bruker MSL 300 and Varian
XL 300 instruments operating at 75 MHz for carbon.
[5] D. C. Roe, J Mugn. Reson. 63 (1985) 388.
[6] J. W. Lauher, R. Hoffmann, J. Am. Chem. SOC.98 (1976) 1729.
[7] M. Casarin, E. Ciliberto, A. Gulino, I. Fragala, Orgunometdrcs 8 (1989)
[8] M. B. Hall, R. F. Fenske, Inorg. Chem. 11 (1972) 768.
Mixed Kolbe Electrolyses
with Sugar Carboxylic Acids **
By Andreas Weiper and Hans J. Schaifer *
Dedicated to Professor Christoph Riichardt on the occasion
of his 60th birthday
M = Ti, Zr
M = Cr,Mo,W
Fig. 3 . Lowest-energy metal-based orbitals for [(C,H,),M(CO),] complexes in
which (a) both C,H, rings are bound to the metal via a normal q5-coordination
mode and (b) one of the C,H, rings binds to the metal via the unusual bent
q3-coordination mode.
significantly stabilized in the W complex (and presumably in
the Mo and Cr complexes) and becomes the HOMO. The
occupation of this orbital in the W, Mo, or Cr complex leads
to increased back-donation to the CO R* orbitals, which in
turn stabilizes the complex. Both sets of CO x* orbitals now
accept charge from the metal. The significant degree of back
donation is consistent both with the CO vibrational frequencies in 2 (the average of the two CO stretching frequencies in
2 is close to the single CO stretching frequency in 1)[’] and
Angt’u. Chem. Int. Ed. Engl. 29 (1990) No. 2
Carboxylic acids can be decarboxylated to give radicals
and/or carbocations by anodic oxidation. Dimers or the
products of addition to double bonds are accessible via the
radical pathway (Kolbe electrolysis).111
The cationic pathway
(non-Kolbe electrolysis) can give esters, ethers, acetals, and
olefins as well as fragmentation and rearrangement products.[1C.21
The pathway taken is determined not only by the
reaction conditions (pH of the electrolyte, kind of electrode
material, added salts, current density) but also, to a large
extent, by the structure of the carboxylic acid. For example,
carboxylic acids bearing hydrogen atoms or electron-withdrawing substituents on the a-carbon atom preferentially
afford dimers, whereas carboxylic acids bearing alkyl or aryl
groups or electron-donating substituents on the a-carbon
atom give primarily or exclusively products arising from
[*] Prof. Dr. H. J. Schifer, Dipl.-Chem. A. Weiper
Organisch-chemisches Insitut der Universitat
Corrensstrasse 40. D-4400 Munster (FRG)
[**I Electroorganic Synthesis, Part 46. This work was supported by the Bundesministerium fur Forschung und Technologie and by the Fonds der
Chemischen Industrie. Part 45: R. Schnelder, H. J. Schafer, S.ynthe.7i.v1989.
fJ VCH Veriugsgeseilschuft mbH, D-6940 Weinheim, 1990
8 02.50j0
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nmr, evidence, high, pressure, ring, studies, concerted, interchange, c5h5
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