Cation-anion CH?O- interactions in the metal-free phenolate, tetra-n-butylammonium phenol-phenolate

Article abstract of DOI:10.1016/S0040-4020(02)00900-6

Deprotonation of phenol by tetra-n-butylammonium hydroxide and subsequent removal of water by P₂O₅ results in quantitative formation of tetra-n-butylammonium phenol-phenolate. The compound crystallizes as an ion pair, with the solute phenol molecule intimately attached to the phenolate anion by an O-H?O^- hydrogen bond (O?O 2.471(5)?). A short contact between one α-C atom of the tetra-n-butylammonium cation and the phenolate O atom indicates that the ion pair interaction is strengthened by an additional C-H?O^- hydrogen-bond, despite the attached phenol.

Full text of DOI:10.1016/S0040-4020(02)00900-6

Tetrahedron 58 (2002) 7847–7850
2
Cation–anion CH· · ·O interactions in the metal-free phenolate,
tetra-n-butylammonium phenol-phenolate
*
Richard Goddard, H. Martin Herzog and Manfred T. Reetz
Max-Planck-Institut f u¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 M u¨ lheim an der Ruhr, Germany
Received 21 May 2002; revised 25 July 2002; accepted 26 July 2002
Abstract—Deprotonation of phenol by tetra-n-butylammonium hydroxide and subsequent removal of water by P O results in quantitative
5
2
formation of tetra-n-butylammonium phenol-phenolate. The compound crystallizes as an ion pair, with the solute phenol molecule intimately
2
attached to the phenolate anion by an O–H· · ·O hydrogen bond (O· · ·O 2.471(5) A). A short contact between one a-C atom of the tetra-n-
˚
2
butylammonium cation and the phenolate O atom indicates that the ion pair interaction is strengthened by an additional C–H· · ·O
hydrogen-bond, despite the attached phenol. q 2002 Elsevier Science Ltd. All rights reserved.
1
. Introduction
In contrast, the interaction between the phenolate anion and
the tetra-n-butylammonium cation appears to be completely
different. Calculations show that in the same solvent the
cation–anion interaction energy is much weaker with the
The vast majority of structures of alkali metal and alkaline
earth phenoxides are characterized by extended aggregates
in which the phenolate anion is intimately bound up with the
metal. Often the presence of sterically bulky ligands is
necessary for the preparation of distinct molecular species,
and in many cases the strong ionic interactions between the
negatively charged O atom of the phenoxide and the
positively charged metal ions force the phenolate anions to
2
1
tetra-n-butylammonium cation (2.4 kcal mol ) than with
þ
21 7
Na (4.5 kcal mol ). Kinetic investigations on the
nucleophilic attack of the phenolate anion on butyl halides
in aprotic solvents have shown that the rate of the reaction
depends strongly on the counterion, with the reactions
involving the tetra-n-butylammonium cation proceeding
1
2
þ
adopt m -bridging, m -face-bridging, and in the case of
2
much faster than those with K . Moreover, whereas the rate
3
3
þ
of the solvent, with the tetra-butylammonium cation they do
NaOPh·THF, m -sites. So strong is the attraction beween
4
constants with K depend greatly on the dielectric constant
the phenolate anion and the alkali metals that a terminally
bonded phenolate has only been observed in a complex
containing a sodium cation otherwise bonded to an
8
not. Indeed, tetra-n-butylammonium bromide has been
shown to have catalytic effect on the O-alkylation of phenol
by benzyl halides by KF in DMF under solid–liquid phase
4
1
8-crown-6-ether ligand.
9
transfer conditions.
Unless mediated by solvents or ligands the formation of the
aggregates is so uncontrolled that crystals with a definite
stoichiometry suitable for X-ray analysis are often difficult
to prepare and in some cases the structures of the
compounds have only been determined from powder
Here we describe the preparation of the metal-free
phenolate, tetra-n-butyl-ammonium phenol-phenolate, and
the determination of its crystal structure. We undertook the
work in order to establish whether the salt would contain the
free phenolate anion, or whether it would in fact undergo
self-association, similar to that observed for tetra-n-
butylammonium-1-cyano-1-phenylethanide, in which the
N atoms of two cyanophenylethanide ions take up bridging
positions between two tetra-n-butylammonium cations to
5
,6
diffraction data. In C H OM (M¼Li, Na, K, Rb, Cs),
6
5
for example, the electropositive metals have such an affinity
for the phenolate anion that there are indications of
additional bonding interaction between the metal cation
5
and the C atoms of the phenyl ring. Such interactions have
been suggested as the cause of the activation of the phenyl
ring in the Kolbe–Schmitt solid–gas direct carboxylation of
1
0
form a supramolecular ion pair.
3
,6
alkalimetal phenolates by CO to give salicylic acid.
2
2. Results and discussion
Our interest in the structure of the metal-free phenolate,
tetra-n-butylammonium phenol-phenolate, stemmed from
the observation that the carbanions formed by the
deprotonation by tetra-n-butylammonium hydroxide of
Keywords: tetra-n-butylammonium phenol-phenolate; ion pair interaction;
metal-free phenolate.
*
Corresponding author. Tel.: þ49-208-306-2000; fax: þ49-208-306-2985;
e-mail: reetz@mpi-muelheim.mpg.de
0040–4020/02/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4020(02)00900-6

7848
R. Goddard et al. / Tetrahedron 58 (2002) 7847–7850
C–H and N–H acid compounds, such as malonates or
with the assignment, since the C–O distance in the
1
1
12
˚
phenolate anion O1–C17 at 1.316(5) A is significantly
shorter than that in the phenol molecule (O2–C23
nitriles, are not free carbanions, as first thought, but
rather supramolecular ion pairs involving O· · ·H–C or
N· · ·H–C hydrogen bonds between the electronegative
atoms in the anions and the acidic C–H bonds of the
˚
1.342(6) A). This latter bond is shorter than that oberved
˚
17
in pure phenol at 123 K (C–Oaverage¼1.372(6) A), pre-
sumably as a consequence of the weaker O–H bond due to
the stronger O–H· · ·O2 bond to the phenolate anion.
1
3–15
a-C-atoms of the tetra-butylammonium cations.
Since
phenol is more acidic than these compounds, we expected
the anion–cation interaction to be weaker and possibly
completely absent.
The O· · ·O distance in the phenol-phenolate moiety at
˚
2
.471(5) A is also significantly shorter than the average
˚
17
Tetra-n-butylammonium phenol-phenolate was prepared by
reaction of two equivalents of phenol with one of tetra-n-
butylammonium hydroxide in a method similar to that
originally described by Nielsen and Hammerich, except that
the water was removed by phosphorus pentoxide, instead of
azeotropic distillation with cyclohexane under reduced
O· · ·O distance in phenol, which is 2.68(2) A. The torsion
angle C17–O1· · ·O2–C23 is not 1808 as one might expect
for an isolated system but 154.68. This is likely to be the
result of an additional C–H· · ·O2 hydrogen bonding
interaction between the the phenolate O atom and one a-C
atom of the tetra-butylammonium cation. The correspond-
ing O· · ·C distance of 3.340(5) A (O1· · ·C13) is significantly
longer than that in 1,3-dimesitylimidazolium 2,6-di-tert-
˚
butyl-4-methylphenolate, which at 2.801(4) A is thought to
1
6
˚
pressure (Scheme 1).
1
8
be the shortest observed to date, but it lies in the middle of
the range for similar X· · ·H–C interactions found in the
structures of various supramolecular ion pairs (3.26–
˚
13,14
3
.44 A).
The angles around C13 also support
Scheme 1.
the formulation of a C–H· · ·O interaction, since the
N–C13· · ·O1 and C14–C13· · ·O1 angles (average 1108)
are close to those one would expect for a linear C–H· · ·O2
bond. It appears therefore that the phenol-phenolate anion
and the tetra-n-butyl-ammonium cation form an intimate ion
pair held together by a C–H· · ·O2 hydrogen bond.
Care was taken to ensure that the compound was kept free
from protic solvents, and crystals were grown from
dimethylformamide.
The results of the X-ray analysis are summarized in Fig. 1,
which shows that the salt contains the phenol-phenolate
anion. Attempts to prepare the phenol-free salt were
unsuccessful, and in all cases the phenol-phenolate anion
was obtained. Although the anion initially appears
symmetrical, with PhO groups at both ends of the fragment,
the bridging H atom, H, is clearly attached to O2, and thus
there is no ambiguity about the location of the negative
charge. H was located in a difference Fourier synthesis that
was calculated with all other atoms, including H atoms,
present in the structure, and it refined well with a final
At first sight the situation is mirrored in the structure of
the Pd phenolate, 1-aqua-1-(hydrogen-tris(pyrazol-1-yl)-
borate)-1-pallada(IV)cyclopentane phenol phenolate,
which is also a three component system comprising a
cation, a phenolate anion and a solvate phenol molecule in
1
9
close contact with one another. However, in the latter case
2
it is the O · · ·H–O hydrogen bond to the water molecule on
˚
the cation (2.465 A) that is significantly shorter than the
2
O · · ·H–O hydrogen bond to the solvate phenol molecule
˚
(2.645 A), in accord with the expected affinity of the
phenolate anion for the cation.
˚
2
displacement parameter of U ¼0.07(2) A . This is some-
H
what larger than that of the atom to which is attached
(
˚
2
U (iso)¼0.0479(8) A ), but that is to be expected for a
An analogous arrangement is found in the structure of
benzyl dimethyl-(2-hydroxy-3-phenoxypropyl)ammonium
phenolate monohydrate solvate, another three component
O2
˚
terminal atom. The O2–H distance of 0.90(7) A is never-
theless normal, and the O1· · ·H–O2 hydrogen bond is
almost linear (175(2)8). The C–O distances from the phenyl
ring to the O atom in the two PhO moieties are also in accord
2
system, in which the O · · ·O distance between the
phenolate O atom and the b-hydroxy group of
˚
the substituted ammonium cation at 2.514 A is shorter
2
than the O · · ·O distance to the solvate molecule (in this
20
˚
case H O, 2.698 A).
2
In both these cases, hydrogen bonding to the cation appears
to take preference over hydrogen bonding to a solvate
molecule. In tetra-n-butylammonium phenol-phenolate,
however, the contrary is true. Clearly, the absence of a
suitable hydrogen bond donor on the tetrabutylammonium
cation prevents it from functioning as the major hydrogen
bond donor.
Despite the interaction between C13 and O1, the alkyl
groups of the tetra-n-butylammonium cations adopt the
all-trans conformation, and the C atoms of the trans n-butyl
groups are essentially coplanar with the N atom (rms
Figure 1. Molecular structure of tetra-n-butylammonium phenol-phenolate,
showing C–H· · ·O (dotted) and O–H· · ·O (dashed) H-bonding
interactions.
2
2

R. Goddard et al. / Tetrahedron 58 (2002) 7847–7850
7849
˚
deviation: 0.05 A (N, C1–C8) and 0.03 A (N, C9–C16)).
˚
presence of phosporus pentoxide, tetra-n-butylammonium
phenol-phenolate was obtained in quantitative yield.
Suitable crystals for an X-ray analysis were obtained by
dissolving the residue in dimethyl formamide at 408C and
cooling the saturated solution from 20 to 2208C at a rate of
0.58C/h. 73% isolated yield; IR (KBr): no N–H absorption,
The n-butyl groups pack together in the crystal so that the
tetra-butylammonium cations and phenol-phenolate anions
occupy alternating aprotic and protic layers. Apart from the
interactions between the cation and the anion, and between
the anion and the solvate phenol molecule, there appears to
be no other significant short contacts in the structure. This
is in stark contrast to the structure of sodium phenolate·
broad H-bond absorption between 2080–2800 cm2 ; H
1
1
NMR (200 MHz, DMSO-d ): d (ppm) 1.05 (t, CH , 12H),
6
3
2
(phenol), revealed to be the only compound in the phase
1.48 (tq, CH , 8H), 1.76 (m, CH , 8H), 3.32 (m, CH , 8H),
2 2 2
1
3
diagram of the sodium phenoxide/phenol system, in which
the phenolate and two phenol molecules are strongly bound
to sodium atoms and one another. Interestingly, among
the sodium phenolates it is only in the structure of
C H ONa·3H O that the O atom of the phenolate ion does
6.31 (t, 1H), 6.42 (d, 2H), 7.0 (t, 2H); C NMR (50 MHz,
DMSO-d ): d (ppm) 13.3, 19.0, 22.9, 57.3, 117.0, 128.4.
6
2
1
Anal. Calcd for C H NO (429.7): C, 78.27; H, 11.02; N,
2
2
8
47
3.26. Found: C, 56.7; H, 10.54; N, 4.00.
6
5
2
not participate in the coordination to the sodium atom, and
2
this has been explained in terms of solvation enthalpies.
4.2. X-Ray crystallographic structure determination
The crystal used for the X-ray analysis was mounted on a
glass rod and cooled by a cold N gas stream. The structure
2
2
2
4,25
3. Conclusions
was solved by Direct Methods,
matrix least-squares, where the quantity minimized was
and refined by full-
P
[ w(F
2
o
2 2 25
Deprotonation of phenol with tetra-n-butylammonium
hydroxide over P O results in the formation of crystals of
2F
tropically, and apart from H attached to O2 of the solvate
c
) ].
Non-H atoms were refined aniso-
2
5
tetra-n-butyl ammonium phenol-phenolate, in which a
phenol molecule is intimately bound to the phenolate
anion. All attempts to prepare the phenol-free salt were
unsuccessful. In spite of the presence of the H-bond to a
phenol molecule, the phenolate anion is C–H· · ·O2
hydrogen bonded to one of the a-C atoms of the
tetrabutylammonium cation, indicating that the cation–
anion C–H· · ·O2 interaction is also possible in protic
solvents. The present structural results and previous
crystallographic data concerning tetraalkylammonium
phenol molecule, which was refined with an isotropic
2
˚
atomic displacement parameter (U
were included in the refinement using a riding model.
Refinement of all the H atoms converged at C13–
¼0.07(2) A ), H atoms
H
˚
H13b· · ·O1 167(2)8 (C13–H13b 0.91(4) A; H13b· · ·O1
˚
2.45(4) A) but at the expense of an unacceptably low data/
parameter ratio. The maximum residual electron density is
3
˚
˚
0.25 e/A , 0.75 A from H11B.
þ
2
Crystal data. C16
H
36N ·C
H
O ·C
H
6
3
O, M
¼429.67,
6
5
6
w
10,11,13,14
salts
help in designing chiral phase transfer catalysts.
of enolates and other ‘carbanions’ may be of
2
colourless prism 0.49£0.42£0.39 mm , monoclinic Cc
3
˚
(No. 9), a¼13.721(5), b¼13.447(3), c¼16.160(5) A, b¼
˚
3
1
1
14.60(3)8, U¼2711(1) A , T¼100 K, Z¼4, D ¼
x
.053 g cm2 , l¼0.71069 A, m¼0.064 mm , Enraf-
3
˚
21
2
6
4
. Experimental
Nonius CAD4 diffractometer, 2.23,u,27.448, 8873
measured reflections, 3211 independent, 2964 with
4
.1. General
I.2s(I). Structure solved by direct methods and refined
2
by least-squares using Chebyshev weights on F to
o
FT-IR spectra were recorded on a Perlin–Elmer 1600
R
1
¼0.061 [I.2s(I)], wR
0.172 (all data), 284 parameters,
2
1
13
˚
3
spectrophotometer. H and C NMR specta were recorded
on a Bruker WH 400 spectrometer at 400 MHz and a Bruker
AC 300 spectrometer at 300 MHz.
S¼1.13, residual electron density þ0.251/20.241 e/A .
Crystallograpic data (excluding structure factors) have been
deposited with the Cambridge Crystallographic Data Centre
as supplementary publication no. CCDC-184571. Copies
of the data can be obtained free of charge on application to
CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: (inter-
nat.) þ44-1223/336-033; e-mail: deposit@ccdc.cam. ac.uk).
Chemical shifts are expressed in ppm relative to trimethyl-
silane as internal standard and J values are expressed in Hz.
All reactions were undertaken in glass apparatus that had
been thrice evacuated by oil pump and flushed with argon.
Commercial reagents with a purity of greater than 97% were
dried before use, otherwise distilled, recrystallized or
sublimed. Tetrahydrofuran (THF) was dried over KOH
and CaH , then refluxed and distilled over LiAlH
Acknowledgements
2
4
and triphenylmethane. Dimethylformamide (DMF) was
refluxed at 608C and 25 mbar for 3 days with Desmodurw
and dibutyl-tin dilaurate, and subsequently distilled using a
The authors thank the Deutsche Forschungsgemeinschaft
(Leibniz Program) for support.
3
0 cm silvered condenser.
4
1
.1.1. Tetra-n-butylammonium phenol-phenolate. To
0 mmol (0.94 g) phenol dissolved in 10 ml THF were
References
added 5 ml of a 1 M tetra-n-butylammonium hydroxide
solution in methanol and the solvent mixture removed by
vacuum. After 60 h at rt under oil-pump vacuum in the
1. Caulton, K. G.; Chisholm, M. H.; Drake, S. R.; Folting, K.;
Huffman, J. C.; Streib, W. E. Inorg. Chem. 1993, 32,
1970–1976.

7850
R. Goddard et al. / Tetrahedron 58 (2002) 7847–7850
2
. Raithby, P. R.; Reed, D.; Snaith, R.; Wright, D. S. Angew.
Chem. 1991, 103, 1040–1042, Angew. Chem., Int. Ed. Engl.
991, 30, 1011–1013.
. Kunert, M.; Dinjus, E.; Nauck, M.; Sieler, J. Chem. Ber./Recl.
997, 130, 1461–1465.
18. Cowan, J. A.; Clyburne, J. A. C.; Davidson, M. G.; Harris,
R. L. W.; Howard, J. A. K.; K u¨ pper, P.; Leech, M. A.;
Richards, S. P. Angew. Chem. 2002, 114, 1490–1492, Angew.
Chem., Int. Ed. 2002, 41, 1432–1434.
1
3
4
5
6
1
19. Canty, A. J.; Jin, H.; Roberts, A. S.; Skelton, B. W.; White,
A. H. Organometallics 1996, 15, 5713–5722.
. Fraser, M. E.; Fortier, S.; Markiewicz, M. K.; Rodrigue, A.;
Bovenkamp, J. W. Can. J. Chem. 1987, 65, 2558–2563.
. Dinnebier, R. E.; Pink, M.; Sieler, J.; Stephens, P. W. Inorg.
Chem. 1997, 36, 3398–3401.
20. Pankratov, V. A.; Frenkel, T. M.; Shvorak, A. E.; Lindeman,
S. V.; Struchkov, Y. T. Russ. Chem. Bull. 1993, 42, 81–87.
21. J o¨ rchel, P.; Sieler, J. Z. Anorg. Allg. Chem. 1995, 621,
1058–1062.
. Dinnebier, R. E.; Pink, M.; Sieler, J.; Norby, P.; Stephens,
P. W. Inorg. Chem. 1998, 37, 4996–5000.
. Viout, P. J. Mol. Catal. 1981, 10, 231–240.
22. Sieler, J.; Pink, M.; Zahn, G. Z. Anorg. Allg. Chem. 1994, 620,
743–748.
7
8
. Ugelstad, J.; Ellingsen, T.; Berge, A. Acta Chem. Scand. 1996,
23. See for example: (a) Corey, E. J.; Xu, F.; Noe, M. C. J. Am.
Chem. Soc. 1997, 119, 12414–12415. (b) O’Donnell, M. J.;
Delgado, F.; Hostettler, C.; Schwesinger, R. Tetrahedron Lett.
1998, 39, 8775–8778. (c) Dehmlow, E. V.; Wagner, S.;
M u¨ ller, A. Tetrahedron 1999, 55, 6335–6346. (d) Ducry, L.;
Diederich, F. Helv. Chim. Acta 1999, 82, 981–1004. (e) Lygo,
B.; Crosby, J.; Lowdon, T. R.; Wainwright, P. G. Tetrahedron
2001, 57, 2391–2402. (f) Perrard, T.; Plaquevent, J.-C.;
Desmurs, J.-R.; H e´ brault, D. Org. Lett. 2000, 2, 2959–2962.
(g) Corey, E. J.; Zhang, F.-Y. Org. Lett. 2000, 2, 4257–4259.
(h) Belokon, Y. N.; Kochetkov, K. A.; Churkina, T. D.;
Ikonnikov, N. S.; Vyskocil, S.; Kagan, H. B. Tetrahedron:
Asymmetry 1999, 19, 1723–1728. (i) Ooi, T.; Doda, K.;
Maruoka, K. Org. Lett. 2001, 3, 1273–1276. (j) Okino, T.;
Takemoto, Y. Org. Lett. 2001, 3, 1515–1517. (k) Adam, W.;
Rao, P. B.; Degen, H.-G.; Levai, A.; Patonay, T.; Saha-M o¨ ller,
C. R. J. Org. Chem. 2002, 67, 259–264. (l) Solladi e´ -Cavallo,
A.; Crescenzi, B. Synlett 2000, 3, 327–330.
2
0, 1593–1598.
. Alauddin, M. M.; Miller, J. M.; Clark, J. H. Can. J. Chem.
984, 62, 263–265.
0. Reetz, M. T.; H u¨ tte, S.; Goddard, R. J. Prakt. Chem. 1999,
41, 297–301.
1. Reetz, M. T.; Knauf, T.; Minet, U.; Bingel, C. Angew. Chem.
9
1
1
1
3
1988, 100, 1422–1424, Angew. Chem., Int. Ed. Engl. 1988,
27, 1373–1374.
1
1
1
1
1
1
2. March, J. Advanced Organic Chemistry: Reactions, Mecha-
nisms, and Structure; 4th ed. Wiley: New York, 1992.
3. Reetz, M. T.; H u¨ tte, S.; Goddard, R. J. Am. Chem. Soc. 1993,
115, 9339–9340.
4. Reetz, M. T.; H u¨ tte, S.; Herzog, H. M.; Goddard, R.
Macromol. Symp. 1996, 107, 209–217.
5. Goddard, R.; H u¨ tte, S.; Reetz, M. T. Acta Crystallogr., Sect.
C: Cryst. Struct. Commun. 2000, 56, 878–880.
6. Nielsen, M. F.; Hammerich, O. Acta Chem. Scand. 1989, 43,
269–274.
24. Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, 46,
467–473.
7. (a) Zavodnik, V. E.; Bel’skii, V. K.; Zorkii, P. M. Zh. Strukt.
Khim. 1987, 28, 175–177. (b) Scheringer, C. Z. Kristallogr.,
Kristallgeom., Kristallphys., Kristallchem. 1963, 119,
25. Sheldrick, G. M. SHELXL-97: Program for Crystal Structure
Refinement; University of G o¨ ttingen, 1997.
2
73–283. (c) Gillier-Pandraud, H. Bull. Soc. Chim. Fr.
26. Enraf-Nonius. EXPRESS/CAD-4 Software, Version 5.1.
Enraf-Nonius: Delft, 1995.
1967, 1988–1995.