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Determination of the Activation Volume V of Simple Molecular Rearrangements by High-Resolution 1H-NMR Spectroscopy at High Pressures.

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the first synthesis of derivatives of the C5BH,' ion with a
pentagonal-pyramidal structure.
Reaction of pentamethylcyclopentadienyltrimethylgermane
( 1 )['I with two equivalents of boron triiodide proceeds via
extrusion of trimethyliodogermane to form the salt (3 ) having
a nido-carbaborane structure in the cation. Although the intermediate pentamethylcyclopentadienyldiiodoborane (2)
cannot be isolated, it can be detected by NMR spectro~copy['~.
[3] Compounds analogous t o ( 3 ) with B-Br
and B-CI bonds in the
cluster can be prepared from the corresponding cyclopentadienyldihaloboranes and the halides BX3 or AIX3: W Schmitt, Diplomarbeit, Universitat Wiirrburg 1977.
[4] 1. Y Ahmed, C. D . Schmulhach, Inorg. Chem. 8, 1411 (1969).
[S] R . W Rudolph, Acc. Chem. Res. 9, 446 (1976); R. E . Williams, Inorg.
Chem. 10, 210 (1971).
[6] Compare the structures of (C5HS)>Be[ A . Haaiund, Acta Chem. Scand
23, 2921 (1969)] and of (CCH,);' [ H . Hogreren, P. W Kwanr, J. Am.
Chem. Soc. 96, 2208 (1974)l.
Determination of the Activation Volume A V * of Simple Molecular Rearrangements by High-Resolution
H-NMR Spectroscopy at High Pressures[**]
By Hans-Dietrich Liidemann, RudolfRauchschwalbe, and Elmar
High resolution NMR has become the most prominent
method for the determination of the activation parameters
AG+, AH*, and AS* in simple intra- and intermolecular
rearrangements. In modern monographs separate chapters
are devoted to the calculation of these parameters from the
temperature dependence of the proton signals[']. An additional
study of the pressure dependence of the spectra allows the
determination of the activation volume from the relation :
Compound ( 3 ) proves to be fairly air-stable and is readily
soluble in polar aprotic solvents such as methylene chloride
and acetonitrile. Its constitution and structure are confirmed
by analytical and spectroscopic data.
'H-NMR (in CH2CI2): 6CH3=2.32 ppm (quartet,
3JHB=2.2Hz). "B-NMR (in CH2CI2 ES. B F 3 . 0 E t z ) :
6(CH3)5C5BI+= 50.8 ppm (8 of 16 lines expected theoretically, J = 2.2 Hz); 6B1; = 127.6 ppm. I3C-NMR (in
CH2CI2/C2D2Br4; {'H)): 6CH3= -10.11,
115.2ppm. Mass spectrum: m/e=273 CloHl,B1+(32%
int.). Conductivity (in acetonitrile, 25°C): 173.3Q cm2 mol '
( 1 . 9 3 3 . 1 0 - 3 ~ )[cf. Et4N+BBr:: 176.9 R-I cm2 mol-'
(1.753. lo-' M)[41].
Compared to cyclopentadienylboranes of type ( 2 ) , the 'B
resonance of the cluster (3) shows a drastic upfield shift
(z8Oppm). Hence it is concluded that the positive charge
in (3) is not localized on the boron atom but distributed
over the entire cluster. Interestingly, coupling of the CH3
protons with the "B isotope is observed with ( 3 ) , but not
with (2).
According to M O considerations and the resulting rules[',
for orbital and electron requirements, the cluster (3) corresponds to a minimum on the energy surface. The bonding
found in (3) should also apply to isoelectronic systems containing neighboring elements of
Thus an alternative
mode of bonding is available for q5-cyclopentadienyl compounds of Main Group elements instead of the hitherto discussed "centrosymmetric n-bonding".
dlnk - - A V +
In this communication we report the effect of pressure on
two simple intramolecular rearrangements. The spectra were
recorded in a recently developed high pressure NMR cell[']
fitting into the standard probe of a Varian XL 100 spectrometer. This cell can be operated at pressures up to 2000
bar and variable temperature with a resolution of 1 Hz.
I ) Rotation of the dimethylamino group of N,N-dimethylacetamide around the C-N bond:
Figure 1 gives the signals of the two N-methyl groups
in the vicinity of the coalescence temperature (7J.
A solution of ( I ) (4.0g, 16mmol) in CH2Cl2 (10ml) is
added slowly to boron triiodide (12.5g, 32mmol) in CH2C12
(20 ml) at - 20°C. After stirring for 1 h, all volatile components
are drawn off at 0°C. Repeated recrystallization of the residue
from CH2CI2 affords the product (3) as colorless needles,
m.p. 160°C (dec.); yield 7.8g (62 %).
Received: March I , 1977 [Z 685 IE]
German version: Angew. Chem. 89. 339 (1977)
[I] K . Wade, Chem. Br. 11, 177 (1975).
[2] A . Du~rson,P E . Rakita, Inorg. Chem. 9, 289 (1970).
Augew. Chem. l n r . Ed. Engl. 16 (1977) No. 5
Fig. I . a) Pressure dependence of the 'H-NMR signals of the N-methyl
groups ofdimethylacetarnide. b) Signal of the C-methyl group for comparison:
the half-width of this line is hardly changed by pressure.
[*] Priv.-Doz. Dr. H.-D. Liidemann, Dr. R. Rauchschwalbe, E. Lang
Institut fur Biophysik und Phystkalische Biochernie der Universitit
Universitatsstrasse 3 I , D-8400 Regensburg (Germany)
[**I This work was supported by the Fonds der Chemischen Industrie.
Increasing pressure shifts the coalescence of the signals
to higher temperatures with AT,/Ap=4.5 K.kbar-'. AG*
and k, were calculated at the various pressures by insertion
of T,, the low temperature values of the frequency difference,
and the half-width of the signals into the standard equations['.']. The inversion rates between 74 and 83°C were
calculated as a function of pressure assuming Arrhenius-type
behavior. Insertion of these rates into eq. (1) yields
A T/+ = 10.3 1.O cm3.mol- '. The inversion of the dimethylamino group is thus connected with a considerable volume
increase in the immediate vicinity of the molecule.
2) Ring int'ersion of cyclohexane:
An increase of pressure shifts T, to lower temperatures
(AT,/Ap= - 1.0 K.kbar-'); within the limits of experimental
error the difference Av between the chemical shifts is not
affected by pressure. Also, the half-width bE is only slightly
changed. By application of the methods described in the first
example, the inversion rates k were calculated as function
of pressure. AV* was found to be - 1 . 9 ~ 0 . 5 c m 3 ~ m o l ~ ' .
The data used are given in Table 1.
Table 1. Pressure dependence of the proton resonance spectra of cyclohexane
in the coalescence region.
Pressure [bar]
Coalescence temperature [K]
Frequency dilference [Hz]
Half-width [Hz]
Inversion rate (217K) k Z I 7 [s-'1
An Organometallic Five-Membered Ring with HeteroMetal-Metal Bonding[**]['I
By Erika Rottinger, Ruiner Muller, and Heinrich Vahrenkamp[']
As Lewis bases, organometalloarsanes such as (l)"] are
capable of substituting C O ligands in metal complexes, and
thus of forming As-bridged dinuclear complexes. Mild reaction
conditions are necessary for the preparation of hetero dinuclear
complexes, whereas at high temperatures reaction of the
organometalloarsanes with themselves is predominant"]. We
have now observed the first combination of these two reaction
possibilities with formation of the trinuclear metal-arsenic
five-membered ring (2).
Reaction of ( 1 ) with excess C5H5Co(C0)2at elevated temperature affords dark-green ( 2 ) . The expected simple Asbridged dinuclear complex (3) is not formed, even under
milder conditions. It must be assumed, therefore, that ( I )
initially reacts with itself to give the non-isolable dinuclear
organornetalloarsane ( 4 ) , which then reacts with the cobalt
complex C 5 H 5 C ~ ( C 0 )This
2 . assumption is supported by the
fact that the dinuclear complex [C5H5(CO)Fe-As(CH3)2]2,
which can be formed via (41, is obtained as main product
of the reaction.
The unexpected result is that increasing pressure significantly accelerates the ring inversion. This can be qualitatively
explained by assuming that the twist or boat conformations
adopted by the molecule during ring inversion have slightly
smaller molar volumes than the energetically more favorable
chair forms. Further studies on substituted cyclohexanes and
in other solvents will be necessary for a better description
of this effect.
Measuring frequency: 100.1 MHz, FT technique, 16k
memory, spectral width: lo00 Hz.-Example 1:Solution (v01.%) of dimethylacetamide (60), tetramethylsilane (20), and
[D6]-acetone (20).-Example 2: Solution (v01.- %) of cyclohexane (40), tetramethylsilane (30), [Dz]-dichloromethane (1 5),
and [D,4]-methylcyclohexane (15).
Received: January 24, 1977 [Z 683 IE]
German version: Angew. Chem. 89,340 (1977)
CAS Registry numbers:
Dimethylacetamide. 127-19-5; cyclohexane, 110-82-7: [D,,]-methylcyclohexane, 101 20-28-2
The heterocycle (2) manifests itself in the IR spectrum
(cyclohexane, cm-') by three C O stretching modes (1942s,
1928 vs, 1758 vs) and in the 'H-NMR spectrum (benzene, int.
TMS, ppm) by three C5H5 signals (3.95, 4.35, 4.69) and four
CH3 signals (1.26,1.34,1.43,1.50). Its structure was determined
by X-ray cry~tallography[~].
Figure 1 shows the approximate
planarity of the novel five-membered ring system and the
CO-bridged hetero metal-metal bond. Since the IR spectrum
of (2) in solution shows only one bridging C O group, while
the solid state structure shows two such groups, (2) must
be a fluxional moleculer41.
In the case of the two previously known five-membered
heterocycles with carbonylmetal moieties[51the synthetic route
precluded formation of heterobimetallic species. Complexes
[*I Prof.
H . Giiiither: NMR-Spektroskopie. Georg Thieme Verlag, Stuttgart 1973,
p. 239.
[2] A . Jueschkr, H . Miiiisch, H . C . Schmiil, H . Frieboliii, A . Manrischwrk.
J Mol. Spectrosc. 31. 14 (1969).
[3] U . Caurz, H:D. Liidrviunrl, Ber. Bunsenges. Phys. Chem. 6'0, 607 (1976).
Dr. H. Vahrenkamp, Dlpl.-Chem. E. Rottinger, Dipl.-Chem. R.
Chemisches Labordtorium der Universitat
Albertstr. 21, D-7800 Freiburg (Germany)
I**]This work was supported by the Deutsche Forschungsgemeinschaft
and the Rechenzentrurn der Universitir Freiburg.
Angrw. Chem. l i i t , E d . Engl. 16 ( 1 9 7 7 ) N o . 5
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simple, volume, spectroscopy, resolution, molecular, nmr, rearrangements, high, activation, determination, pressure
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