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High-Field Conditioned NMR Spin Decoupling in Organomercury Compounds.

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Relaxation Studies on
the Stabilization of Trityl Cations by Acids**
By Franz L. Dickert* and Helmut Fackler
Superacids enable the preparation of extremely reactive
carbenium ions"]. The well stabilized trityl cations Trta, on
the other hand, are already obtained from triphenylmethano1 by acids of the strength of usual mineral acids'']. Thus,
in the case of ionogenic trityl trifluoroacetate Trt(TFA) the
conductivity in acetonitrile increases considerably on addition of trifluoroacetic acid H(TFA)[3"1.This is not due to
the conductivity of the acid, which is practically undissociated in a~etonitrile['~],
but to formation of trityl cations:
+ TFA'
To explain this increase in conductivity we carried out a
series of relaxation studiesr4](pressure-jump method with
conductivity detection and optical observation). The findings in Figure 1'3'1 indicate that ion-pair formation can be
The rate constants and activation parameters are listed
in Table 1. kZIwas determined by I9F-NMR measurements
independently of the relaxation investigation^'^^]. The shift
of the dissociation equilibrium ( 1 ) in favor of the formation of trityl cations on addition of acid is due to a drastic
reduction in k 1 2 .Trifluoroacetic acid, having an acceptor
number A N = 105.3 on the Gutmann scale[51,solvates anions extremely effectively via hydrogen bonding; hence,
the nucleophilicity of the trifluoroacetate ions is reduced
more and more with increasing acid concentration. Addition of acid has little effect o n k 2 , . As already observed in
previous e~perirnentd'~],
trityl trifluoroacetate and trifluoroacetic acid form a 1 : 1 complex. This leads to a slight increase in k Z 1(Table I).
Accordingly, eq. (1) must be modified by two additional
solvation equilibria:
+ n
. .H ( T F A ) 11+ (n-1)H ( T F A )
T r t @ + (TFA)?.
. [H(TFA)J,
Hence, rapid desolvation of the anions precedes ion recombination. In agreement with this hypothesis is the finding that A H ; increases on addition of acid (Table I). The
experimentally determined activation enthalpy AH
then made u p of the positive anion-desolvation enthalpy
AHy2 and the activation enthalpy AH;(O) in absence of
From eq. (1) and the results in Table 1, a value of 16 kJ/
mol is obtained for AHy2 in 1 M trifluoroacetic acid solution. This enthalpic effect outweighs the increase in activation entropy in the temperature range investigated and
leads to strongly reduced ion-recombination rate constants
klZ.--In this way the formation of carbenium ions is favored when solutions of ionogens are treated with acid.
%G IM"2 I
Received: August 5, 1981 [Z 57 IE]
German version: Angew. Chem. 94 (1982) 303
Fig. I. Reciprocal relaxation times I/T as a function of the total concentrations of trityl trifluoroacetate in acetonitrile (273 K) at variable concentrations of added trifluoroacetic acid.
neglected as preliminary step during ion recombination according to eq. (I).
[I] G. A. Olah, G. K. S. Prakash, J. Sommer, Science 206 (1979) 13.
[2] M. Walper, H. D. Brauer, H. Kelm, Z . Narurforsch. 8 3 0 (1975) 561.
[3] a) H. Blumenstock, Dissertation, Universitat Erlangen-Nurnberg 198 I ; b)
H.Blumenstock, F. L. Dickert, A. Reichenbacher, Z. Phys. Chem. (Frankfurt am Main) 113 (1979) 199; c) H. Fackler, Diplomarbeit, Universitat
Erlangen-Nurnberg 1981.
[4] H. Strehlow, W. Knoche: Fundamentals of Chemical Relaxation. Verlag
Chemie, Weinheim 1977.
[5] V. Gutmann, Electrochim. Acta 21 (1976) 661.
Table I. Rate constants of ion recombination k i l , activation parameters, and
rate constants of dissociation kllfor trityl trifluoroacetate in trifluoroacetic
High-Field Conditioned NMR Spin Decoupling
in Organomercury Compounds
I .o
( 6 . 0 k 2 . 0 ) ~10'
( 8 . 2 2 0 . 8 ) ~10'
(2.1&0.2)x 10'
( 1 . 7 k 0 . 2 ) ~lo5
- 17f4
- 9f4
I 1.O f0.6
I 1.Ok 0.9
I1.0f 1.2
11.Of 1.8
Prof. Dr. F. L. Dickert, H.Fackler
Institut fur Physikalische und Theoretische Chemie
der Universitat Erlangen-Niirnberg
Egerlandstr. 3, D-8520 Erlangen (Germany)
This work was supported by the Deutsche Forschungsgemeinschaft and
the Fonds der Chemischen Industrie.
Angew. Chem. In!. Ed. Engl. 21 (1982) No. 4
By Reinhard Benn, Harald GiintheP, Adalbert Maercker,
Volkmar Menger, and Peter Schmitt
Of the mechanisms which influence spin-lattice relaxation rates R , (= l/TJ in nuclear magnetic resonance,
the contribution from the chemical shift anisotropy
[*I Prof. Dr. H.Gunther, Prof. Dr. A. Maercker, V. Menger, P. Schmitt
FB 8, Organische Chemie, Universitat-Gesamthochschule
Postfach 21 0209, D-5900 Siegen 21 (Germany)
Dr. R. Benn
Max-Planck-lnstitut fur Kohlenforschung
Kaiser-Wilhelm-Platz I, D-4330 Mulheim an der Ruhr (Germany)
0 Verlag Chemie GmbH, 6940 Weinheim, 1982
0570-0833/82/0404-0295 $02.50/0
( R Y A )not only depends on the correlation time z, but also
on the magnetic flux density Bo[']:
R 7"
r: Bi [Ao*]T,
If R Y A is the most important parameter for a given magnetic nucleus, the high magnetic fields available today
from cryomagnets, may lead to very rapid relaxation. In
the case of spin-spin coupling this can cause line broadening for neighboring nuclei or even spin, spin-decoupling.
Heavy metal nuclei such as 199Hgin linear molecules of
the type R-Hg-R
exhibit relatively high chemical shift
anisotropies, whose significance for the observed relaxation behavior has recently been stressedf2].We show here
that with silylmercury compounds, high-field NMR spectroscopy can lead to the disappearance of 199Hg, 'Hspin,spin coupling and, in consequence, to a loss of information.
measured at 9.4 T (vo('H) = 400 MHz) selective line broadening occurs, which in the 'H-NMR spectrum practically
leads to loss of the coupling. The same effect was observed
with bis(allyldimethylsilyl)mercury 2, and even with di[err-butylmercury 3.
That enhanced spin-lattice relaxation of the 199Hgnucleus is indeed involved is shown by the fact that the satellite lines produced by 'J('3C,'H)- and 'J( I3C,"Si) coupling
remain sharp (Fig.
it also becomes apparent from the
T I data. As shown in Table 1 the TI values measured at
three different field strengths closely follow the ratio
(B:)/Bt')2, so that even at 1.88T, R Y must provide the
predominant contribution to the relaxation of the 199Hg
Table 1. '"Hg spin-lattice relaxation times T , [ms] for 1, 2, and 3 at various
magnetic flux densities &[TI [a].
(H,C ),Si-Hg-Si(CH,),
3 10
29. I
[a] Measured using the inversion recovery method [I] with Bruker WP 80,
WH 270, and WH 400 spectrometers; v<,('"'Hg)= 14.3, 48.2 and 71.7 MHz,
respectively: solvent C,D,/C,H,, (9: I); conc.: 30 wt.-%
The 'H- and I3C-NMR spectra of bis(trimethylsilyl)mercury 1, respectively, at I.88T(vo('H)=80 MHz), show the
expected satellite lines (Fig. I) due to 3J('99Hg,'H) and
2J('99Hg,13C)spin-spin coupling. When the same sample is
Since R Y A is also of significance with other metal nucleif4I, the possibility of enhanced spin-lattice relaxation
must be considered in high-field NMR studies of organometallic compounds in order to prevent erroneous assignments, particularly since broad residual signals can
disappear in the noise.
Received: December 28, 1981 [Z 58 1Ej
German version: Angew. Chem. 94 (1982) 314
@ 1.88 T
[I] F. W. Wehrli, T. Wirthlin: Interpretation of Carbon-13 NMR-Specfra.
Heyden, London 1976.
[2] D. G. Gillies, L. P. Blaauw, G. R. Hays, R. Huis, A. D. H. Claque, J.
Magn. Reson. 42 (1981) 420.
[3] a) In small molecules "C and '9Si usually relax by the field-independent
dipolar mechanism. b) Apparently because of the nonlinear structure of
2, the field-strength dependence is observed to a lesser extent.
[4] G. R. Hays, D. G. Gillies, L. P. Blaauw, A. D. H. Claque, J. Magn. Reson.
45 (1981) 102.
[5] Cf. also T. F. Schaaf, J. P. Oliver, J. Am. Chem. SOC.91 (1969) 4327; T. N.
Mitchell, H. C. Marsmann, J. Orgonomef. Chem. 150 (1978) 171.
@ 9.L T
@ 1.88 T
(p-H)3[C5H5RhP(iPr)31 :
A Dinuclear (Rh-Rh) Complex
with Three Bridging Hydrido Ligands
By Helmut Werner* and Justin Wolf
Hydrido(phosphane)rhodium compounds play an important role as catalysts or as intermediates in catalytic hydrogenations (e. g. with Rh(PPh,),CI)['I. Recently, we have
been particularly interested in the behavior of the Lewis
basic alkyne(phosphane)complexes C5H5Rh(C2R,)P(iPr)3
with Brernsted acidsf2', and in this connection have attempted preparation of the dihydrido(phosphane)complex
3 from RhC1H2[P(iPr)3]2f31and NaC5H5. However, only
small amounts of 3 are formed in this reaction.
/-----Fig. I. IH- and "C-NMR spectra of bis(trimethylsilyl)mercury 1 [5] at 1.88
and 9.41; the "C-NMR spectrum is broadband decoupled. The following
coupling constants are obtained from the satellite spectra:
'J( '>C,IH) = 1 19.6; 2J(2'Si;'H)= 6.6, 3J("'"Hg,'H) = 40.7; 'J( 1'C,29Si)= 40. I ,
2J(1'C,'y9Hg)=92.1 Hz.
0 Verlag Chemie GmbH, 6940 Weinheim. 1982
[*I Prof. Dr. H. Werner, J. Wolf
lnstitut fur Anorganische Chemie der Universitat
Am Hubland, D-8700 Wurzburg (Germany)
0570-0833/82/0404-0296 $02.50/0
Angew. Chem. int. Ed. Engl. 21 (1982) No. 4
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spina, nmr, compounds, high, organomercury, field, decoupling, conditions
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