The protiotetramethylammonium dication (CH3)3NCH42+: Hydrogen/deuterium exchange and calculational studies. Search for the parent protioammonium dication NH52+

Article abstract of DOI:10.1021/ja962572i

The protonation of ammonium ions NR₄⁺ (R = H, CH₃) to form the corresponding protioammonium dications HNR₄²⁺ (R = H, CH₃) was investigated by hydrogen/deuterium exchange experiments and by ab initio molecular orbital calculations. Hydrogen/deuterium exchange was observed by ²H NMR spectroscopy at the CH₃ groups of tetramethylammonium ion N(CH₃)₄⁺ when reacted with excess DF:SbF₅ superacid. On the basis of calculational results, the intermediacy of the corresponding C-H-protonated protiotetramethylammonium dication (CH₃)₃NCH₄²⁺ is suggested to account for the observed exchange. Under similar conditions, no exchange was observed in the parent ammonium ion NH₄⁺ in FSO₃D:SbF₅ superacid, although the NH₅²⁺ dication was calculated to be kinetically stable with a considerable barrier to deprotonation. The pentafluoroammonium dication NF₅²⁺ was also investigated theoretically, and the results were compared to those of neutral NF₅.

Full text of DOI:10.1021/ja962572i

4594
J. Am. Chem. Soc. 1997, 119, 4594-4598
2+
The Protiotetramethylammonium Dication (CH3)3NCH4 :
Hydrogen/Deuterium Exchange and Calculational Studies.
2+ 1
Search for the Parent Protioammonium Dication NH5
George A. Olah,* Arwed Burrichter, Golam Rasul, and G. K. Surya Prakash*
Contribution from the Loker Hydrocarbon Research Institute and Department of Chemistry,
UniVersity of Southern California, UniVersity Park, Los Angeles, California 90089-1661
ReceiVed July 25, 1996. ReVised Manuscript ReceiVed March 14, 1997X
+
Abstract: The protonation of ammonium ions NR4 (R ) H, CH3) to form the corresponding protioammonium
2+
dications HNR4 (R ) H, CH3) was investigated by hydrogen/deuterium exchange experiments and by ab initio
2
molecular orbital calculations. Hydrogen/deuterium exchange was observed by H NMR spectroscopy at the CH3
+
groups of tetramethylammonium ion N(CH3)4 when reacted with excess DF:SbF5 superacid. On the basis of
calculational results, the intermediacy of the corresponding C-H-protonated protiotetramethylammonium dication
2+
(CH3)3NCH4 is suggested to account for the observed exchange. Under similar conditions, no exchange was
observed in the parent ammonium ion NH4+ in FSO3D:SbF5 superacid, although the NH52+ dication was calculated
to be kinetically stable with a considerable barrier to deprotonation. The pentafluoroammonium dication NF52+ was
also investigated theoretically, and the results were compared to those of neutral NF5.
Introduction
H+, ion 5 represents a gold analogue of the elusive protioam-
2+
monium dication NH5
.
A major step in the development of structural chemistry a
century ago was the realization of the ionic rather than covalent
nature of ammonium compounds.2 Ammonium compounds, as
well recognized, are tetravalent ionic (1) and not pentacovalent
nitrogen compounds (2). Considerable interest and controversy
have centered around hypervalency, hypercoordination, and
formal expansion of the valence octet for first-row elements in
the past years.3-5 Hypercoordinated boron6 (e.g., pentahydri-
doboron BH5 3) and carbon7 (e.g., the methonium ion, the parent
carbonium ion CH5+ 4) species are well-known and character-
ized. The isoelectronic pentacoordinated nitrogen compound
NH52+ is so far unknown. Studies on pentavalent neutral NH5
have also been reported.8
It is generally not possible to directly observe small gitonic
dications12 in solution by methods such as NMR or IR-Raman
spectroscopy. There is evidence for the formation of NO2H2+
and CH3COH2+ in highly acidic media. 17O NMR spectroscopic
studies of nitronium ion in superacidic HSO3F:SbF5 medium
have indicated the involvement of the protonitronium dication
NO2H2+ 13
This dication has also been observed in the gas
.
phase.14 IR studies of the acetyl cation show significant shifts
of the CO frequencies depending on the acidity of the system,
indicating protosolvation.15 Hydrogen/deuterium exchange
experiments in superacids have clearly indicated the involvement
Schmidbaur et al. have recently reported9 the preparation and
subsequent X-ray analysis of the aurated azonium dication
((C6H5)3PAu)5N2+ 5, in which the central nitrogen atom is
coordinated by five gold(I) ligands. Considering the isolobal
relationship10,11 (i.e., similarity in bonding) between LAu+ and
of such dications as H4O2+ and H4S2+ 16,17
In addition, a
.
number of other small dications have been observed in the gas
phase by charge-stripping mass spectrometry, including the
2+ 18
smallest two-electron diatomic dication He2
.
In these studies
it was found that many dications, although thermodynamically
extremely unstable, are kinetically stabilized by high barriers
X Abstract published in AdVance ACS Abstracts, May 1, 1997.
(1) Chemistry in Superacids. Part 26. For Part 25, see: Rasul, G.; Olah,
G. A. Inorg. Chem. 1997, 36, 1278.
(2) Werner, A. Ber. 1903, 36, 147.
(3) Cooper, D. L.; Cunningham, T. P.; Gerratt, J.; Karadakow, P. B.;
Raimondi, M. J. Am. Chem. Soc. 1994, 116, 4414.
(4) Dewar, M. J. S.; Healy, E. Organometallics 1982, 1, 1705.
(5) Mitchell, K. A. R. Chem. ReV. 1969, 69, 157.
(6) Mesmer, R. E.; Jolly, W. L. Inorg. Chem. 1962, 1, 608.
(7) Olah, G. A.; Prakash, G. K. S.; Williams, R. E.; Field, L. D.; Wade,
K. Hypercarbon Chemistry; Wiley-Interscience: New York, 1987.
(8) Olah, G. A.; Donovan, D. J.; Shen, J.; Klopman, G. J. Am. Chem.
Soc. 1975, 97, 3559.
(9) Grohmann, A.; Riede, J.; Schmidbaur, H. Nature 1990, 345, 140.
(10) Hoffmann, R. Angew. Chem., Int. Ed. Engl. 1982, 21, 711.
(11) Stone, F. G. A. Angew. Chem., Int. Ed. Engl. 1984, 23, 85.
(12) Olah, G. A. Angew. Chem., Int. Ed. Engl. 1993, 32, 767.
(13) Prakash, G. K. S.; Rasul, G.; Burrichter, A.; Olah, G. A. In Nitration:
Recent Laboratory and Industrial DeVelopments; Albright, L. F., Carr, R.
V. C., Schmitt, R. J., Eds.; American Chemical Society: Washington, DC,
1996; pp 10-18.
(14) Weiske, T.; Koch, W.; Schwarz, H. J. Am. Chem. Soc. 1993, 115,
6312.
(15) Vol’pin, M.; Akhrem, I.; Orlinkov, A. New J. Chem. 1989, 13, 771.
(16) Olah, G. A.; Prakash, G. K. S.; Barzaghi, M.; Lammertsma, K.;
Schleyer, P. v. R.; Pople, J. A. J. Am. Chem. Soc. 1986, 108, 1032.
(17) Olah, G. A.; Prakash, G. K. S.; Marcelli, M.; Lammertsma, K. J.
Phys. Chem. 1988, 92, 878.
(18) Lammertsma, K.; Schleyer, P. v. R.; Schwarz, H. Angew. Chem.,
Int. Ed. Engl. 1989, 28, 1321.
S0002-7863(96)02572-3 CCC: $14.00 © 1997 American Chemical Society

2+
Protiotetramethylammonium Dication [(CH3)3NCH4
]
J. Am. Chem. Soc., Vol. 119, No. 20, 1997 4595
to decomposition.19 Theoretical calculations are a particularly
powerful tool for probing these dications. In the experimentally
studied superacid solutions, however, further stabilization is
possible due to clustering (protosolvation) and thus charge
dispersion.
Table 1. Total Energies (-au) and Zero-Point Vibrational
Energies (kcal/mol)
MP2(fu)/
6-31G**//
MP2(fu)/
QCISD/
6-311G**//
QCISD/
structure
9 (C4V)
11 (C3V)
12 (C2V)
6-31G** (ZPE)
G2
6-311G** NIMAGa
Several experimental and theoretical studies have previously
been reported on the possible intermediacy of pentacoordinated
nitrogen compounds. On the basis of hydrogen/deuterium
56.57883 (29.4b) 56.62512 56.61391
56.53648 (28.2b) 56.58392
0
1
1
0
0
0
0
0
0
0
0
56.57660 (29.0b) 56.62225
+
NH32+ (D3h) 55.17674 (17.0b) 55.23075 55.21603
exchange experiments between molten NH4 salts and LiD or
NH4+ (Td)
H2
13 (Td)
14 (CS)
15 (CS)
16 (D3h)
F2
56.73765 (29.7b) 56.78348 56.77186
+
ND4 salts and LiH, Olah et al. postulated the possible
1.15766 (6.1b)
213.49363 (98.6c)
213.50781 (101.5c)
551.11298 (10.7c)
352.02840 (8.9c)
199.03883 (1.6c)
1.16643
1.16834
intermediacy of pentacoordinated NH4D or ND4H.8 It was,
however, subsequently shown that the observed H2 to D2 ratios
greatly exceeded those expected solely for the formation of an
intermediate NH4D or ND4H, and therefore, a catalytic isotope
exchange reaction of the formed HD must be also involved.20,21
Theoretical calculations on NH5 by Simonetta et al.22 indicated
that of the various geometries possible for NH5, the trigonal
bipyramidal D3h structure is the most favorable. In addition,
the study indicated that pentacoordinated NH5 is unstable with
respect to decomposition to NH3 + H2 and is less stable than
the corresponding ion pair NH4+H-. Further, it is to be
considered that since no free H- exists in the condensed phase,
a NIMAG (number of imaginary frequencies) at the MP2(fu)/6-
31G**//MP2(fu)/6-31G** level. b ZPE at MP2(fu)/6-31G**//MP2(fu)/
6-31G** scaled by a factor of 0.93. c ZPE at HF/6-31G*//HF/6-31G*
level scaled by a factor of 0.8929.
Results and Discussion
the observed hydrogen/deuterium exchange could be accounted
In order to investigate small gitonic azonium dications, high-
level ab initio calculations were carried out on the parent
monocations NH4+ and N(CH3)4+ and their protonated dications,
+
for by an intermediate in which LiD is complexed with NH4
.
Recent ab initio calculations suggested that covalent NF5 is
a vibrationally stable molecule.23,24 So far, however, all
experimental attempts to prepare and characterize NF5, either
as a covalent molecule or as an ionic NF4+F- salt,25 have failed.
Christe, Schrobilgen, and Olah et al. have studied26 the thermal
2+
+
as well as on dication NF5 and its parent monocation NF4
.
Geometry optimizations were performed at MP2(fu)/6-31G**
(fu ) full) and/or QCISD/6-311G** level. Frequency calcula-
tions were carried out to characterize the structures as minimum,
saddle point, or transition structure and to evaluate zero-point
vibrational energies (ZPE). Improved energies were obtained
with the Gaussian-2 (G2) method. The G2 theory is a composite
method based on MP2(fu)/6-31G* geometry which is treated
in single-point calculations with a variety of basis sets at the
post-SCF level.27 Atomic charges were obtained using the
natural bond orbital analysis (NBO)28 method. Unless otherwise
stated, MP2(fu)/6-31G** structures and charges and G2 energies
will be discussed in the text. Similar to previous29 calculations
on the parent carbonium ion CH5+, we have considered a
number of symmetry-constrained structures for the geometry
optimization of pentacoordinated nitrogen compounds, including
the Cs forms 6 and 7, the C2V form 8, the square pyramidal C4V
form 9, and the trigonal bipyramidal D3h form 10. Total
energies (-au) and zero-point vibrational energies (kcal/mol)
of the ions are summarized in Table 1. Optimized geometries
are given in Figures 1-3.
-
decomposition of NF4+HF2 by using 18F-labeled HF2-. In
these studies it was found that the attack of HF2- on NF4+ occurs
exclusively on fluorine and not on nitrogen, contrary to the
predictions based on bond polarities. It was thus argued that
nitrogen is sterically not able to accommodate five fluorine
atoms in its first coordination sphere.26
In the present study, we have investigated by hydrogen/
deuterium exchange experiments and by ab initio molecular
+
orbital calculations, the protonation of ammonium ions NR4
(R ) H, CH3) to form the corresponding protioammonium
2+
dications HNR4 (R ) H, CH3). Hydrogen/deuterium ex-
2
change was observed by H NMR spectroscopy at the CH3
+
groups of tetramethylammonium ion N(CH3)4 when reacted
with excess DF:SbF5 superacid. On the basis of calculational
results, the intermediacy of the corresponding C-H-protonated
2+
gitonic12 protiotetramethylammonium dication CH4N(CH3)3
is suggested to account for the observed exchange. The parent
pentacoordinated NH52+ dication was calculated to be kinetically
stable with a considerable barrier to deprotonation of 25.9 kcal/
mol. No hydrogen/deuterium exchange was observed, however,
+
when NH4 was reacted with FSO3D:SbF5. The theoretical
results were compared to those of the isoelectronic carbon
+
analogue CH5 as well as the neutral NH5 molecule. The
2+
pentafluoroammonium dication NF5 was also investigated
theoretically, and the results were compared with its neutral
analogue NF5.
(19) Koch, W.; Schwarz, H. In Structure/ReactiVity and Thermochemistry
of Ions; Ausloos, P., Lias, S. G., Eds.; D. Reidel: Boston, MA, 1987; p
413.
(20) Johnson, R. W.; Holm, E. R. J. Am. Chem. Soc. 1977, 99, 8077.
(21) Gruendler, W.; Schaedler, H. D. Z. Chem. 1980, 20, 111.
(22) Morosi, G.; Simonetta, M. Chem. Phys. Lett. 1977, 47, 396.
(23) Michels, H. H.; Montgomery, J. A., Jr. J. Chem. Phys. 1990, 93,
1805.
(24) Ewig, C. S.; Van Wazer, J. R. J. Am. Chem. Soc. 1989, 111, 4172.
(25) Christe, K. O.; Wilson, W. W. J. Am. Chem. Soc. 1992, 114, 9934.
(26) Christe, K. O.; Wilson, W. W.; Schrobilgen, G. J.; Chirakal, R. V.;
Olah, G. A. Inorg. Chem. 1988, 27, 789.
(27) Curtiss, L. A.; Raghavachari, K.; Trucks, G. W.; Pople, J. A. J.
Chem. Phys. 1991, 94, 7221.
(28) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899.
(29) Schleyer, P. v. R.; Walkimar, J.; Carneiro, M. J. Comput. Chem.
1992, 13, 99.

4596 J. Am. Chem. Soc., Vol. 119, No. 20, 1997
Olah et al.
state 11 for the dissociation reaction which lies 25.9 kcal/mol
above minimum structure 9. Thus, dication 9 has a considerable
kinetic barrier to dissociation. However, it should be taken into
consideration that these data refer to the isolated idealized gas
phase and that in the condensed phases electrophilic (protolytic)
solvation or clustering effects may have great influence.
Particularly in small dications such interactions should diminish
the effect of charge-charge repulsion and thus could bring the
cations into a thermodynamically more feasable range. On the
other hand, it may also lower the kinetic barrier to deprotonation.
Counterions may also play a considerable role in the protonation
equilibria. In superacid systems, however, exceedingly delo-
Figure 1. MP2(fu)/6-31G**-optimized geometries of 9-12 (QCSID/
6-311G**).
-
-
calized and weak nucleophiles like SbF6 or related Sb2F11
anions might not compete efficiently with the rather localized
lone pairs of the mono- or dications. Transition state 11 has
C3V symmetry and is characterized by one long (1.942 Å) and
one short (1.070 Å) axial C-H bond. We have also identified
the transition state 12 for hydrogen scrambling in dication 9.
The barrier for hydrogen scrambling between two C4V structures
through 12 is calculated to be only 1.0 kcal/mol. Thus,
hydrogen scrambling in NH52+ should be very facile. This type
of facile hydrogen scrambling has also been calculated for
2+
Figure 2. MP2(fu)/6-31G**-optimized geometries of CH4N(CH3)3
and N(CH3)4 ions 13 and 14, respectively.
2+ 7
methonium dications CH42+, CH5+, and CH6
.
+
Significantly, we also found that the gas phase reaction of
NH32+ dication with H2 forming NH52+ 9 is highly exothermic
by 143.0 kcal/mol.
2+
The NH3 dication has previously been observed in the gas
phase by charge-transfer mass spectrometry (CTMS) and has
also been studied theoretically.31 Our calculations indicate that
it might be possible to observe the NH52+ dication 9 in the gas
2+
Figure 3. MP2(fu)/6-31G**-optimized geometries of NF52+ and NF3
dications 15 and 16, respectively.
2+
phase by reacting NH3 and H2.
As mentioned, Olah et al. have observed H/D exchange when
reacting NH4+ with LiD or ND4+ with LiH.8 The intermediacy
of neutral NH5 was suggested to explain the exchange reactions.
It was, however, subsequently pointed out that exchange reaction
with HD formed in deprotonation can explain much of the
exchange. Simonetta et al. have previously22 calculated the
reaction of NH4+ salts with LiH forming NH5. In these studies,
pentacoordinated NH5 was found to be a stable structure with
D3h symmetry at the 4-31G level. We were able to reproduce
Simonetta’s calculations at the 4-31G level, but found that the
reported D3h structure is a transition state rather than a minimum
structure on the NH5 potential energy surface. Furthermore,
we found that at correlated MP2(fu)/6-31G** level NH5
+
In experimental studies, we have reacted NH4+ and N(CH3)4
ions with excess deuterated superacids, such as FSO3D:SbF5
and DF:SbF5, in order to probe hydrogen/deuterium exchange
or its absence. Under these extremely acidic conditions,
ammonium ions do not show any deprotonation equilibria with
their neutral amines. In fact, 1:1 molar mixtures of FSO3D:
SbF5 (H0 ) -21) and DF:SbF5 (H0 ) -25) are among the
strongest superacid systems known, in which no free amine can
be present.30 Thus, if observed, hydrogen/deuterium exchange
+
+
in NH4 and N(CH3)4 ions cannot proceed via a deprotona-
tion-reprotonation mechanism, but would involve the corre-
sponding protonated gitonic dications as intermediates.
Protioammonium Dication NH52+. At both MP2(fu)/6-
31G** and QCISD/6-311G** levels, structure 9 (C4V symmetry)
was found to be the single minimum on the potential energy
surface of NH52+. Structures 6-8 and 10 are not minimum
structures at these levels of theory. The optimized structure of
NH52+ 4 was characterized by a slightly longer axial (1.106 Å)
and slightly longer equatorial (1.055 Å) N-H bonds compared
+
spontaneously dissociates into NH4 and H- ions. Similar to
its isoelectronic carbon analogue CH5-, neutral NH5 has not
-
been observed experimentally. CH5 has been calculated to
be thermodynamically and kinetically unstable,32,33 in accord
that 10 electrons in the carbon valence shell cannot give a stable
species (in contrast to CH5+, which is an 8 electron species).
Attempts to observe hydrogen/deuterium exchange in am-
monium ion NH4+ when treated with excess deuterated super-
acid were unsuccessful. Ammonium chloride NH4+Cl- was
reacted with DSO3F:SbF5 solution at 100 °C. No hydrogen/
+
to the N-H bonds of the NH4 ion (1.022 Å). NBO charges
of the ion 9 were calculated to be -1.19 au on the central
nitrogen atom, 0.57 au on the axial hydrogen atom, and 0.65
au on the equatorial hydrogen atoms. Similar NBO charges
were obtained at QCISD/6-311G** level (-1.39, 0.62, and 0.69
au, respectively).
2
deuterium exchange was observed by H NMR even after 14
days.
The dissociation of 9 into NH4+ and H+ is expectedly highly
exothermic by 99.1 kcal/mol. We have identified the transition
(31) Proctor, C. J.; Porter, C. J.; Ast, T.; Bolton, P. D.; Beynon, J. H.
Org. Mass Spectrom. 1981, 16, 454.
(32) Sini, G.; Hiberty, P. C.; Shaik, S. S. J. Chem. Soc., Chem. Commun.
1989, 772.
(30) Olah, G. A.; Prakash, G. K. S.; Sommer, J. Superacids; Wiley-
Interscience: New York, 1985.
(33) Cremer, D.; Kraka, E. J. Phys. Chem. 1986, 90, 33.

2+
Protiotetramethylammonium Dication [(CH3)3NCH4
]
J. Am. Chem. Soc., Vol. 119, No. 20, 1997 4597
Similarly, ammonium-d4 chloride ND4+Cl- was also reacted
with FSO3H:SbF5 solution at 100 °C. Again, no deuterium/
2
hydrogen exchange was observed by H NMR after 2 weeks.
The lack of hydrogen/deuterium exchange in NH4+ suggests
that the protioammonium dication NH52+ 9 does not form under
the experimental conditions in the studied superacid medium.
However, the use of methods such as charge-stripping mass
2+
spectrometry18 allowing the reaction of NH3 and H2 might
2+
be successful to observe the elusive NH5 dication in the gas
phase.
Protiotetramethylammonium Dication. Tetramethylam-
monium tetrafluoroborate N(CH3)4+BF4- was reacted with DF:
Figure 4. 2H NMR Spectrum of(CH3)4N+ in DF:SbF5: (a) after 12 h
and (b) after 14 days.
1
2
SbF5 at 20 °C, and the solution was analyzed by H and H
NMR spectroscopy (δ(1H) 2.26 s). At 20 °C, no decomposition
of the ion was observed. We observed hydrogen/deuterium
exchange in N(CH3)4+ of the methyl protons (δ(1H) 2.26 s) with
the simultaneous appearance of a peak at δ(2H) 2.28 whose
intensity increased with time (Figure 4). Up to 2% deuterium
incorporation at the CH3 groups in 14 was observed during 14
days at 20 °C. No evidence was found for any protonation of
the central nitrogen atom in N(CH3)4+. The intermediacy of
the C-H-deuterated tetramethylammonium dication is consid-
ered to account for the observed exchange at the methyl protons
+
in N(CH3)4 14:
No minimum structure could be located for pentacoordinated
2+
nitrogen dication HN(CH3)4
because of its spontaneous
dissociation upon optimization. The C-H-protonated form 13,
however, was found to be a stable minimum at MP2(fu)/6-
31G** level with a three-center two-electron bond at the CH3
carbon (Figure 2). Protonation of tetramethylammonium ion
+
2+
N(CH3)4 14 forming CH4N(CH3)3 13 dication was further
calculated to be slightly exothermic by 6.0 kcal/mol at the MP2-
(fu)/6-31G** + ZPE level. These results are in line with
previous studies in which alkyl cations, such as tert-butyl34 and
2-propyl cations,35 as well as acylium ions36 were found to
undergo C-H protonation to form highly electron deficient,
superelectrophilic12 protioalkyl and protioacyl dications, re-
spectively.
An alternative mechanism involving deprotonation of (CH3)4N+
to trimethylammonium methylide is unlikely under these highly
acidic conditions. It has been shown previously that even
deprotonation equilibria of the tert-butyl cation to isobutylene
is not possible under these conditions.34,35 Yet another possible
mechanism involving free methyl cation (CH3+) transfer to
fluoride ion F- leading to CH3F also seems less probable
because of the exceedingly high acidity of HF:SbF5 medium
(H0 ) -25 to -30). Furthermore, the bulk of the hydrogen
fluoride in this medium exists in the form of H2F+.30 The
We have also attempted to observe CH3/13CH3 exchange
+
in N(CH3)4 ion. Tetramethylammonium tetrafluoroborate
N(CH3)4+BF4- was reacted with 13CH3SO3CF3:SbF5 (1:1 molar
solution) at 50 °C. Periodic monitoring by 13C NMR over a
period of 1 week, however, did not indicate any CH3/13CH3
exchange in the N(CH3)4+ cation. In addition, attempts to find
a stable minimum structure at MP2(fu)/6-31G** level for the
N(CH3)52+ dication failed, because of its spontaneous dissocia-
-
residual BF4 anion will also be extremely non-nucleophilic
due to its complexation with SbF5. Furthermore, we have not
detected any CH3F:SbF5 complexes or its facile proton/
deuterium exchange by NMR spectroscopy. If there were such
a cleavage, we should have observed the formation of trim-
ethylammonium ion (CH3)3NH+. However, no such cleavage
was ever detected.
+
+
tion into N(CH3)4 and CH3 cations upon optimization.
To probe the mechanism of the observed hydrogen/deuterium
exchange in tetramethylammonium ion under superacid condi-
tions, we have carried out ab initio calculations. Protonation
+
of the tetramethylammonium ion N(CH3)4 may result in two
Both our experimental and theoretical results suggest that the
2+
different dications: N-protonated gitonic12 dication HN(CH3)4
+
nitrogen atom in N(CH)4 ion is sterically unable to accom-
2+
and C-H-protonated dication (CH3)3NCH4 13.
modate any further ligand (H+ or CH3+) to form pentacoordi-
2+
nated azonium dications HN(CH3)4 or N(CH3)52+, respec-
(34) Olah, G. A.; Hartz, N.; Rasul, G.; Prakash, G. K. S. J. Am. Chem.
Soc. 1993, 115, 6985.
(35) Olah, G. A.; Hartz, N.; Rasul, G.; Prakash, G. K. S.; Burkhart, M.;
Lammertsma, K. J. Am. Chem. Soc. 1994, 116, 3187.
(36) Olah, G. A.; Burrichter, A.; Rasul, G.; Prakash, G. K. S.; Hachoumy,
M.; Sommer, J. J. Am. Chem. Soc. 1996, 118, 10421.

4598 J. Am. Chem. Soc., Vol. 119, No. 20, 1997
Olah et al.
fluoride and D2O as reported.37 Benzoyl fluoride was prepared from
benzoyl chloride and anhydrous HF as described in the literature.38
1H and 2H NMR spectra were obtained on a Bruker AM 400
spectrometer equipped with a variable temperature probe at 400 and
61.4 MHz, respectively. 13C NMR spectra were taken on a Varian
Associates Model Unity-300 spectrometer at 75.4 MHz. Quartz NMR
tubes (5 mm) were used for samples containing DF. NMR spectra
were obtained with respect to TMS by using an acetone-d6 capillary as
external standard.
+
tively. Instead, protonation of N(CH3)4 leads to the
thermodynamically more favorable C-H-protonated dication
2+
CH4N(CH3)3 13.
Pentafluoroammonium Dication NF52+. Geometry opti-
mization of NF5 at MP2(fu)/6-31G** level gave a single
2+
minimum structure 15 (CS symmetry), resembling a complex
between NF32+ 16 and F2 with a long N-F bond (1.905 Å, see
Figure 3). Previous26 calculations on the neutral NF5 molecule
have also resulted in a complex of NF3 and F2 rather than a
All calculations were carried out using the Gaussian 9439 package
of programs.
+
pentacoordinated species. NF4 when reacted with 18F- gave
only 19F18F, but no fluorine exchange of NF4+. We have also
Preparation of FSO3D. Doubly distilled FSO3H (14.8 mL, 0.26
mol) were placed into a flask equipped with a magnetic stirrer under
nitrogen atmosphere. The solution was cooled to 0 °C, and 14 mL
(0.26 mol) of D2SO4 (98 wt %) were added. The ensuing mixture was
stirred at room temperature for a period of 24 h. Distillation yielded
16.6 g (0.16 mol) of FSO3D (bp ) 165 °C). 2H NMR of the distillate
indicated a deuterium enrichment of approximately 90 atom % D (δ-
(2H) 9.59 s).
optimized NF32+ dication 16, and its geometry is given in Figure
2+
2+
3. The reaction of NF3 dication with F2 forming NF5
dication was calculated to be exothermic by 28.5 kcal/mol at
the MP2(fu)/6-31G** + ZPE level.
There have apparently been no prior experimental studies of
2+
NF3
dication 16 in the gas phase, although previous24
2+
calculations have shown that NF3 is a structurally stable
molecule with a trigonal planar geometry. Our calculations
indicate that it might be possible to observe NF52+ dication 16
in the gas phase. Similar to neutral NF5, however, NF52+ does
not contain a pentacoordinated central nitrogen atom but can
be visualized as a complex of NF3 and F2. This further
suggests that the nitrogen atom is sterically unable to accom-
modate five fluorine atoms in its first coordination sphere.
+
-
H/D Exchange in N(CH3)4 with DF:SbF5. N(CH3)4+BF4 (150
mg) was placed into a Kel-F tube and cooled to -78 °C in a dry ice/
acetone bath. Approximately 2 mL of DF:SbF5 (2.5:1 molar solution)
was added to the solution at -78 °C. The ensuing mixture was
vigorously stirred (Vortex stirrer) under periodic cooling. After the
+
1
2
sample was characterized by H and H NMR spectroscopy at -15
°C, it was kept at room temperature for 14 days and periodically
1
2
monitored by H and H NMR spectroscopy.
Attempted H/D Exchange in NH4 (ND4+) with FSO3D:SbF5
+
(FSO3H:SbF5). NH4+Cl- (ND4+Cl-) (30 mg) was placed into an NMR
tube and cooled to -78 °C in a dry ice/acetone bath. Approximately
0.5 mL of a 1:1 molar solution of FSO3D:SbF5 (FSO3H:SbF5) was added
to the solution at -78 °C. The ensuing mixture was vigorously stirred
(Vortex stirrer) under periodic cooling until the NH4+Cl- (salt was
Conclusions
+
The protonation of ammonium ions NR4 (R ) H, CH3) to
the corresponding protioammonium dications HNR42+ (R ) H,
CH3) was investigated by hydrogen/deuterium exchange experi-
ments and by ab initio molecular orbital calculations. Hydrogen/
1
2
completely dissolved. The sample was characterized by H and H
2
NMR spectroscopy and subsequently heated in an oil bath to 100 °C.
deuterium exchange was observed by H NMR spectroscopy
1
2
at the CH3 groups of tetramethylammonium ion N(CH3)4+ when
reacted with excess DF:SbF5 superacid. On the basis of the
calculated results, the intermediacy of the corresponding C-H-
protonated gitonic protiotetramethylammonium dication (CH3)3-
NCH42+ is suggested to account for the observed exchange. Lack
Periodic monitoring by H and H NMR over a period of 14 days did
+
not indicate any H/D exchange in NH4 (ND4+) cation.
Attempted CH3/13CH3 Exchange in N(CH3)4+ with 13CH3SO3CF3:
SbF5. N(CH3)4+BF4- (50 mg) was dissolved in approximately 0.5 mL
of CH3SO3CF3:SbF5 (1:1 molar solution) in an NMR tube and cooled
to -78 °C in a dry ice/acetone bath. Subsequently, 2 drops of 13C-
enriched 13CH3SO3CF3 were added to the solution, and the ensuing
mixture was vigorously stirred (Vortex Stirrer). After the sample was
characterized by 13C NMR spectroscopy, it was heated to 50 °C in an
oil bath. Periodic monitoring by 13C NMR over a period of 1 week
+
of similar exchange in parent NH4 ion indicates the inability
to form the NH52+ dication in superacids. Protonation of NH4
+
2+
is highly endothermic by some 99.8 kcal/mol. However, NH5
was calculated to be kinetically stable with a considerable barrier
to deprotonation (25.9 kcal/mol). The theoretical results were
also compared to those of the isoelectronic carbon analogue
+
did not indicate any CH3/13CH3 exchange in N(CH3)4 cation.
+
2+
Acknowledgments. Support of our work by the National
Science Foundation is gratefully appreciated. A.B. wishes to
thank Professor Jean Sommer for his hospitality and help during
a stay at Strasbourg, France, as well as the Konrad-Adenauer-
Foundation for a scholarship. We thank Dr. M. Hachoumy and
Mr. J. D. Sauer at Strasbourg for their help related to NMR
spectroscopy.
CH5 as well as to neutral NH5. NF5 dication was also
investigated theoretically and found to be a stable molecule at
MP2(fu)/6-31G** level. Similar to neutral NF5, however, its
structure does not resemble a pentacoordinated nitrogen com-
2+
pound, but rather a complex between NF3 and F2. These
results are in line with previous26 studies, which suggested that
nitrogen is sterically unable to accommodate five fluorine atoms
in its first coordination sphere (although it can accommodate
five hydrogens).
JA962572I
(37) Olah, G. A.; Kuhn, S. J. Z. Anorg. Allg. Chem. 1956, 287, 282.
(38) Olah, G. A.; Kuhn, S. J. Org. Synth. 1965, 45, 3.
Experimental Section
(39) Gaussian 94, Revision A.1. Frisch, M. J.; Trucks, G. W.; Schlegel,
H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J. R.;
Keith, T. A.; Peterson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-
Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski,
J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts,
R.; Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart,
J. J. P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A.; Gaussian, Inc.:
Pittsburgh, PA, 1995.
Ammonium chloride, ammonium-d4 chloride (99.9 atom % D),
tetramethylammonium tetrafluoroborate, CH3SO3CF3, 13CH3SO3CF3 (99
atom % 13C), D2SO4 (98 wt %, 99.5 atom % D; Acros), anhydrous NF
(Setic Labo), D2O (99.9 atom %; Acros), and benzoyl chloride
(Lancaster) were purchased from Aldrich if not indicated otherwise
and were used as received. SbF5 (Allied-Chemical) and FSO3H (3M)
were doubly distilled prior to use. DF was prepared from benzoyl