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ENDOR Spectroscopy in Water Reductions and Oxidations of Organic Compounds.

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framework and outside of the xco plane-is in agreement
with expectations based on ESR/ENDOR measurements[31
in lo-' M solutions (cf. 1), in which partially fluxional""
radical ion pairs are present as monomers in their respective solvent cage.
Received: March 30, 1988;
revised: May 3, 1988 [ Z 2685 1EI
German version: Angew. Chem. 100 (1988) 1125
CAS Registry number: [ f l u o r e n ~ n e ~ ~ N a @ ( d m115512-56-6.
H. Bock, B. Hierholzer, P. Schmalz, Angew. Chem. 99 (1987) 811; Angew.
Chem. Int. Ed. Engl. 26 (1987) 791, and references cited therein.
Cf. e.g.: a) M. Szwarc (Ed.): Ions and Ion Pairs in Organic Reactions,
Yol. I , Val. 2, Wiley-Interscience, New York 1972, 1974; b) Y. Marcus:
/on Soluation. Wiley, Chichester 1985, p. 218f.: c) J:M. Lehn, Angew.
Chem. 100 (1988) 91; Angew. Chem. Int. Ed. Engl. 27 (1988) 89; cf. references cited therein as well as in [I, 31.
W. Lubitz, M. Plato, K. Mobius, R. Biehl, J. Phys. Chem. 83 (1979) 3402,
and references cited therein.
Experimental Procedure (cf. H:F. Herrmann, Dissertation, Frankfurt am
Main 1988): Na (76 mg, 3.3 mmol) and sublimed fluorenone (595 mg,
3.3 mmol) were weighed into a Schlenck trap under argon at 260 K and,
after evacuation (high vacuum), 30 mL of DME (dried over Na/K) was
added by condensation. Following activation with ultrasound (five 5-s
treatments at short intervals) and reaction for 3 d, n-hexane (2 mL) was
condensed into the reaction mixture, which was then cooled in a cryostat to 230 K at a rate of 5"/h. The air-sensitive dark red needles of 2
(dec. >413 K), which grew within 20 h, were characterized as follows:
Titration of the sodium hydroxide solution formed after hydrolysis of a
weighed sample indicates a formula weight of 366 (calcd for nuorenone. Na.2DME: 383). Quenching of dried crystals (which thereby lose
DME) with 02-saturated CDCI, and integration of the NMR signal intensities gave a ratio fluorenone :DME = 1 : > 1.3. A solid-state ESR
spectrum (g= 2.0034, compared with g = 2.00343 reported in the literature for solutions 131) indicates that the crystal contains fluorenone radical anions. This is further supported by the decreased 1R frequency
(S(CO)= 1540 crn-') relative to fluorenone (1600 cm-') and by the absorption spectrum in the visible (Cmmax=19000 cm-').
Extensive literature search for the structures of other radical-anion salts
yielded the following: In [TCNQoQRb"] (A. Hoekstra, T. Spoelder,
A. Vos, Acta Crystallogr. Sect. B28 (1972) 14) the tetracyanoquinodimethane radical anions are stacked on top of one another. In
[biphenyl]oQK%l,lor [biphenyloeRb~,,](J. 3. Mooij, A. A. K. Klaassen,
E. de Boer, H. M. L. Degens, T. E. M. van den Hark, J. H. Noordik, J .
Am. Chem. SOC.98 (1976) 680; J. H. Noordik, J. Schreurs, R. 0. Gouid,
J J . Mooij, E. de Boer, J . Phys. Chem. 82 (1978) 1105) the countercations, which are embedded in polyethers, are separated from the radical
anions. Therefore, the structure presented here (Fig. I ) is presumably the
first one in which interactions between the radical anion and its countercation are visible.
[6] 2, X-ray structure analysis (180 K): space group PI,Z = I , a=962.4(15),
b = 1071.3(IS), c = 1114.8(13) pm, a = 93.4(1), /J=96.6( I), y= 1 l2.4( I)',
p(MoKR)=0.68cm-'. Siemens AED 2,20552". 4299 reflections, 3470
with I> 2 a ( / ) .Direct methods, Na/C/O positions refined anisotropically, H positions isotropically. R , =0.047, R2=0.050. Further details of
the crystal structure investigation may be obtained from the Fachinformationszentrurn Energie, Physik, Mathernatik GmbH, D-7514 Eggenstein-Leopoldshafen 2 (FRG), on quoting the depository number CSD53 118, the names of the authors, and the journal citation. Further details
on distances [pm] and angles ["I (cf. Fig. 1): Na-02 252.4, Na-03 245.1,
Na-04 245.3, Na-05 247.1, N a . . .C9 320.2, Na.. .ClO 371.9, N a . . ,C13
436.3, C-C in DME 148.9 to 150.8, C - 0 in DME 141.5 to 143.1, C9-CIO
145.1, CIO-CI 1 143.1, CII-CI2 145.2, other distances in six-membered
rings 137.6 to 140.8; 01-Na-03 108.5, 01'-Na-03 93.4, 01-Na-04 94.5,
OI'-Na-04 115.8, ClO-C9-C13 106.0, OI-C9-C10 126.4, OI-C9-C13
127.5, other angles in five-membered ring 107.4 to 109.2. Angles in sixmembered ring 119.1 to 120.9.
171 a) H. R. Luss, D. G. Smith, Acta Crystallogr. Sect. 8 2 8 (1972) 884. Other
differences involve the rive-membered ring, which is equalized under
constant angles fd,,(fluorenone)= 139.0 to 148.6 pm; d,,(fluorenoneoQ)= 142.5 to 147.5 prn); b) B. Bogdanovic, C. Kriiger, B. Wermeckes, Angew. Chem. 92 (1980) 844; Angew. Chem. Int. Ed. Engl. 19
(1980) 817. The salt
above quoted and from the crystal structure in Figure I has been postulated by H. van Willigen, C. F. Mulks, J . Chem. Phys. 76 (1981) 2135.
191 Cf., e.g., J. March: Advanced Organic Chemistry. Wiley, New York 1985,
p. 111Of. The reaction of fluorenone with Na affords the pinacoi in 95%
yield (W. E. Bachrnann, J . Am. Chem. Sac. 55 (1933) 1179).
[lo] For example, the 2,5-bis(trimethylsilyl)-p-benzosemiquinone/KQ contact ion pair, according to temperature-dependent ESR measurements
attains the slow-exchange region at 223 K (P. Hanel, Dissertation, Universitat Frankfurt am Main 1987; cf. H. Bock, B. Solouki, P. Rosmus, R.
Dammel, P. Hanel, B. Hierholzer, U. Lechner-Knoblauch, H.-P. Wolf in
H. Sakurai (Ed.): Organosilicon and Bioorganosi~icon Chemistry. Ellis
Horwood, Chichester 1985, p. 590. A MNDO study (R. Dammel, ibid..
p. 67) yields an activation enthalpy of 30 kJ/mol and a value of 400 pm
for the distance of K" above the radical anion molecular plane at the
saddle point of the fluxional motion.
ENDOR Spectroscopy in Water: Reductions and
Oxidations of Organic Compounds**
By Hans Bock* and Bernhard Hierholzer
Dedicated to Dr. Wolf-Dieter Luz
on the occasion of his 60th birthday
Although water-free solutions are frequently advantageous for the generation, detection, and isolation"] of radical ions and their salts, biological redox processes mostly
take place in water."] An extension of high-resolution
ENDOR spectroscopy,[31the best method currently available for the structural elucidation of paramagnetic com-
[ R - N2M N - C H 3 ] " "
pounds in liquid phase, to the medium water is therefore
desirable, despite the disadvantageous temperature-dependent properties of
By choosing selective oneelectron-transfer reagents and working under Oz-free conditions, we have succeeded in detecting and characterizing
the organic radical ions 1 to 4 in aqueous solutions by
their ENDOR spectra (Fig. 1).
The experimental prerequisites for recording ENDOR
spectra exhibiting an acceptable signal-to-noise ratio
also shows a planar, compressed Li202four-membered ring, although
with identical LiO bond lengths.
[8] a) N. Hirota, J . Am. Chem. SOC.89 (1967) 32; b) S. W. Mao, K. Nakamura, N. Hirota, ibid. 96 (1974) 5341; c) M. Guerra, P. Palrnieri, G . F.
Pedulli, Chem. Phys. 33 (1978) 45. Cf. the literature cited in these references as well as in [2, 31. A DZdgeometry that deviates both from the
Angew Chem. In!.
Ed. Engl. 27 (1988) No. 8
[*] Prof. Dr. H. Bock, Dr. B. Hierholzer
Institut fur Anorganische Chernie der Universitat
Niederurseler Hang, D-6000 Frankfurt am Main 50 (FRG)
Electron Transfer and Ion-Pair Formation, Part 6. This work was supported by the Deutsche Forschungsgemeinschaft, the Fonds der Chemischen Industrie, and the Land Hessen -Part 5 : [I].
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(number of scans > lo2) in aqueous solutions are capillary
tubes with walls of constant thickness and diameters
< 1 mm as well as optimized concentrations of the redox
components of
mol L-’.[’] Concerning both the
reactions in conductivity-pure water under argon and the
assignment of the ENDOR spectra (Fig. I), the following
comments are added:
l M H Z
1. The ENDOR spectrum of the radical anion 1 (Fig. 1)
does not show the 23Na coupling observed under
aprotic conditions,’61 indicating the formation of a ‘‘solvent-separated’’ ion pair. Because the ‘H couplings of
the five ring hydrogen atoms are comparable to those of
the CsO “contact ion pair” generated in aprotic medium, the countercations Na:q (r=270 pm[61)and Cs@
( r = 170 pmC6]),surrounded by the flexible monoaza
crown, cause similar x spin populations in the naphthosemiquinone radical anion. It should be further noted
that the frequently used oxidation of hydroquinones
with O2 in alkaline solution, which is disadvantageous
for ENDOR measurements, has been replaced by a
one-electron reduction with NaBH,. Generated in this
way, the radical anions of biologically important quinones should also be characterizable by ENDOR spectroscopy.
2. Reduction of the dications with dithionite affords the
radical cations of N,N‘-dimethyl- (2a) and N-dodecylN’-methylviologen (2b). Their ENDOR coupling constants, which were determined in aqueous solution and
agree with ESR literature values,[’I are identical, even
though sodium dodecyl sulfate in 100-fold excess was
added during the measurement of the dodecyl derivative 2b (Fig. 1). Nonetheless, the demonstrated possibility to record ENDOR spectra of aqueous micellar solutions could be the starting point for obtaining information on aggregation and permeability of these dynamic
systems by the addition of appropriate salts.
3. The electron-rich tetrakis(dimethy1amino)ethene exhibits a negative oxidation potential < -0.6 VL8] and,
therefore, is oxidized even by 0,-free water to the radical cation 3. The ENDOR coupling constants of 3[*]are
in agreement with ESR values, which were measured
after reduction of the dication with zinc in acetonitrile,[*]and can be interpreted in terms of a planar C,N,
molecular framework in which rotation around the C N
axis is sterically hindered. According to our ENDOR
measurements (Fig. l), the oxidative hydrolysis of
(R2N)2C=C(NR2)2,which has been the subject of contr~versy,[’~
clearly begins with a one-electron transfer resulting in formation of the radical cation.
4. Upon addition of AgBF, to an aqueous solution of the
ethene radical cation 3, the ENDOR spectrum of the
corresponding hydrazine radical cation 4 (Fig. 1 and
Ref. [lo]) is obtained, which can also be recorded after
oxidation of tetramethylhydrazine with AgBF, in water.
In addition, workup of a preparative run results in the
isolation of tetramethylurea and dimethylf~rmamide.[~~
These findings suggest that the reaction proceeds by
AgO oxidation of the ethene radical cation 3 to the dication and its nucleophilic hydrolysis to an unstable intermediate, 5 , which must be common to all the subseFig. 1. ENDOR spectra in water: radical anion 1 of the crown ether derivaat
tive 2-(1,4,7,10-tetraoxa- 13-aza-l3-cyclopentadecyl)-I,4-naphthoquinone
270 K, radical cation 2b (R = Ci2HZ5)
of N-dodecyl-W-methylviologen(with
addition of sodium dodecyl sulfate in 100-fold excess) at 282 K, radical cation 3 of tetrakis(dimethy1amino)ethene at 271 K, and radical cation 4 of
tetramethylhydrazine at 270 K (for explanation see text).
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0.570-0833/88/0808-1070 ?
Angew. Chem. l n f . Ed. Engl. 27 (1988) No. 8
The cation radii given were taken from: HoNeman- Wibergr Lehrbuch
der Anorganischen Chemie, 91st-100th ed., d e Gruyter, Berlin 1985.
The E N D O R coupling constants of 2a and 2b agree with the ESR
values: H. Fischer (Ed.): Landolt-Bornstein, Gruppe 2, Magnetische
Eigenschafen freier Radikale, Bd. I , Springer, Berlin 1965):
For ESR measurements on micelles in frozen solutions, see, e.g.1 E.
Szajdzinska-Pietek, R. Maldonado, L. Kevan, R. M. Jones, J . Am.
Chem SOC.107 (1985) 6467, and references cited therein.
Cf., e.g., cyclovoltammetric measurements in D M F (H. Bock, D. Jaculi, Angew. Chem. 96 (1984) 298; Angew. Chem lnt. Ed. Engl. 23
(1984) 305). The coupling constants of 3 are (ESR values: K. Kuwata, D. H. Geske, J . Am. Chem. SOC.86 (1964) 2197):
Cf., e.g., S. Patai (Ed.): l 2 e Chemistry ofthe Amino Group, Wiley,
London 1968; N. Wiberg, J. W. Buchler, Chem. Ber. 96 (1963) 3000,
ENDOR coupling constants of 4 (ESR values: S. F. Nelson, G. R.
Weismann, P. J. Hintz, D. Olp, M. R. Fahay, J. Am. Chem. SOC.96
(1974) 2916):
quent products. Both the dimerization of dimethylaminyl radicalsI8I and their sole formation upon y-irradiation of tetramethylurea in a 4 K matrix[”] have been reported.
In summary, the ENDOR experiments in water described here establish the utility of this physiologically important medium for the high-resolution electron/nucleus
double-resonance measurement technique and the value of
the accessible information on electron-transfer reactions.
Concerning the structure, see H. Bock, W. Kaim. Acc. Chem. Res. I S
(1982) 9, and references cited therein.
M. C. R. Symons, Chem. Phys. Lett. 117 (1985) 381; J. Chem. SOC.
Chem. Commun. 1987, 866.
Received: April 18, 1988 [Z 2706 IE]
German version: Angew. Chem. 100 (1988) 1127
CAS Registry numbers:
1, 88288-46-4; 1 ( N a complex), 88288-48-6; 2 (R=CH3), 25239-55-8; 2
(R=CI2H>S), 115483-93-7; 3, 34525-41-2; 4, 34504-32-0.
H. Bock, H.-F. Herrmann, D. Fenske, H. Goesrnann, Angew. Chem. 100
(1988) 1125; Angew. Chem. Int. Ed. Engl. 27 (1988) 1067, and references cited therein.
Cf., e.g., B. Alberts, D. Bray, J. Lewis, M. Raff, K. Roberts, J. D.
Watson: Molekularbiologie der Zelle, VCH, Weinheim 1986; Molecular
Biology o f t h e Cell, Garland Publishing, New York 1983.
Cf. the review of H. Kurreck, B. Kirste, W. Lubitz, Angew. Chem. 96
(1984) 171; Angew. Chem. I n f . Ed. Engl. 23 (1984) 173, and references
cited therein. Difficulties in carrying out ENDOR measurements in
HzO are discussed o n p. 189 and p. 190, respectively.
Thus, the high melting point of water restricts the ENDOR “window of
measurement” to 0 to 50”C, its high dipole moment is responsible for
considerable dielectric losses, i.e., demands drastically reduced sample
cross sections, a n d its small, only weakly temperature-dependent viscosity interferes with the establishment of optimal rotation correlation
times [3]
Cf. B. Hierholzer, Dissertation. Universitat Frankfurt am Main 1988.
The water used, which was distilled in a BIDEST quartz apparatus,
degassed repeatedly at lo-’ rnbar, and stored under argon, exhibited a
conductivity of < lo-’ Siemens cm I.
H. Bock, B. Hierholzer, F. Vogtle, G. Hollmann, Angew. Chem. 96
(1984) 74; Angew. Chem. Int. Ed. Engl. 23 (1984) 57. The ‘H coupling
constants of 1 are:
T H F 151
T H F [5]
Angem. Chem. Int. Ed. Engl. 27 (1988) No. 8
Dications Based on a Hydrotris(phosphonio)borate
By Hubert Schmidbaur,* Thomas Wimmer,
Gabriele Reber, and Gerhard Miiller
Phosphane-boranes, containing a P-B bond, are a little
investigated class of compounds, despite their noteworthy
properties.”] The presence of one or more formal positive
charges in immediate proximity to the boron atom leads to
a reduction o r even reversal (“umpolung”) in the polarity
of the B-H bonds. As a result, prototypes having the stoichiometries A and B are sensitive neither to oxidation nor
to hydrolysis. These substances are often stable even
toward aqueous acids and alkaline solutions; extremely
strong bases attack the alkyl
These properties
should be more pronounced in the hydrotris(phosph0nio)borate dications C, the boron atom of which is sur[*I
Prof. Dr. H. Schmidbaur, DipLChem. T. Wimmer,
DiplLChem. G. Reber [‘I, Dr. G. Miiller [‘I
Anorganisch-chemisches Institut der Technischen Universitat Miinchen
Lichtenbergstrasse 4, D-8046 Garching (FRG)
X-ray structure analyses.
This work was supported by the Deutsche Forschungsgemeinschaft
(Leibniz-Programm), the Fonds der Chemischen Industrie, and Hoechst
AG. We thank J. Riede for preparing the crystallographic data sets.
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water, spectroscopy, oxidation, compounds, organiz, reduction, endo
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