Proton-transfer and H2-elimination reactions of trimethylamine alane: Role of dihydrogen bonding and lewis acid-base interactions

Article abstract of DOI:10.1021/ic802262h

Proton-transfer and H₂-elimination reactions of aluminum hydride AlH3(NMe3) (TMAA) with XH acids were studied by means of IR and NMR spectroscopy and DFT calculations. The dihydrogen-bonded (DHB) intermediates in the interaction of the TMAA wit

Full text of DOI:10.1021/ic802262h

Inorg. Chem. 2009, 48, 3667-3678
Proton-Transfer and H2-Elimination Reactions of Trimethylamine Alane:
Role of Dihydrogen Bonding and Lewis Acid-Base Interactions
Oleg A. Filippov,† Victoria N. Tsupreva,† Lyudmila M. Golubinskaya,† Antonina I. Krylova,†
Vladimir I. Bregadze,† Agusti Lledos,*,‡ Lina M. Epstein,† and Elena S. Shubina*,†
A. N. NesmeyanoV Institute of Organoelement Compounds, Russian Academy of Sciences,
VaViloV Street 28, 119991 Moscow, Russia, and Departament de Qu´ımica, Edifici Cn, UniVersitat
Auto´noma de Barcelona, 08193 Bellaterra, Spain
Received November 25, 2008
Proton-transfer and H2-elimination reactions of aluminum hydride AlH3(NMe3) (TMAA) with XH acids were studied
by means of IR and NMR spectroscopy and DFT calculations. The dihydrogen-bonded (DHB) intermediates in the
i
interaction of the TMAA with XH acids (CH3OH, PrOH, CF3CH2OH, adamantyl acetylene, indole, 2,3,4,5,6-
pentafluoroaniline, and 2,3,5,6-tetrachloroaniline) were examined experimentally at low temperatures, and the
spectroscopic characteristics, dihydrogen bond strength and structures, and the electronic and energetic characteristics
of these complexes were determined by combining experimental and theoretical approaches. The possibility of two
different types of DHB complexes with polydentate proton donors (typical monodentate and bidentate coordination
with the formation of a symmetrical chelate structure) was shown by DFT calculations and was experimentally
proven in solution. The DHB complexes are intermediates of proton-transfer and H2-elimination reactions. The
extent of this reaction is very dependent on the acid strength and temperature. With temperature increases the
elimination of H2 was observed for OH and NH acids, yielding the reaction products with Al-O and Al-N bonds.
The reaction mechanism was computationally studied. Besides the DHB pathway for proton transfer, another pathway
starting from a Lewis complex was discovered. Preference for one of the pathways is related to the acid strength
and the nucleophilicity of the proton donor. As a consequence of the dual Lewis acid-base nature of neutral
aluminum hydride, participation of a second ROH molecule acting as a bifunctional catalyst forming a six-member
cycle connecting aluminum and hydride sites notably reduces the reaction barrier. This mechanism could operate
for proton transfer from weak OH acids to TMAA in the presence of an excess of proton donor.
Introduction
drogen complexes.2 Hydrogen can also be produced from
main-group hydrides. Borohydrides of alkaline metals, such
as LiBH4, readily evolve hydrogen by hydrolysis,3,4 and
hydrogen elimination from LiBH4 complexes with less-
reactive proton donors, such as alcohols, has been reported.5
However, in contrast to transition-metal hydrides,6,7 the
In recent years, the interest in main-group metal hydride
chemistry has significantly enlarged. Some of these hydrides
hold promise as the means of storing hydrogen, for electro-
chemical applications, e.g., in fuel cells or for electroplating
of metals, as possible fuels, and as reducing agents for a
wide range of substrates.1 Protonation of transition-metal
hydrides has been widely employed as one of the most
common and convenient methods of preparation of dihy-
(2) (a) Kubas, G. J. Metal Dihydrogen and σ-Bond Complexes; Kluwer
Academic/Plenum Publishers: New York, 2001. (b) Kubas, G. J. Chem.
ReV. 2007, 107, 4152–4205.
(3) Kojima, Y.; Kawai, Y.; Kimbara, M.; Nakanishi, H.; Matsumoto, S.
Int. J. Hydrogen Energy 2004, 29, 1213–1217.
(4) Filinchuk, Y.; Hagemann, H. Eur. J. Inorg. Chem. 2008, 3127–3133.
(5) (a) Custelcean, R.; Jackson, J. E. J. Am. Chem. Soc. 1998, 120, 12935–
12941. (b) Custelcean, R.; Jackson, J. E. J. Am. Chem. Soc. 2000,
122, 5251–5257. (c) Custelcean, R.; Vlassa, M.; Jackson, J. E. Angew.
Chem., Int. Ed. 2000, 39, 3299–3302.
(6) (a) Belkova, N. V.; Shubina, E. S.; Epstein, L. M. Acc. Chem. Res.
2005, 38, 624–631. (b) Belkova, N. V.; Epstein, L. M.; Shubina, E. S.
ARKIVOC 2008, (iv), 120–138.
* To whom correspondence should be addressed: E-mail: shu@ineos.ac.ru
(E.S.S.); agusti@klingon.uab.es (A.L.). Fax: +7 495 1355085 (E.S.S.); +34
93 5812920 (A.L.).
†
Russian Academy of Sciences.
Universitat Auto´noma de Barcelona.
‡
(1) (a) Aldridge, S.; Downs, S. Chem. ReV. 2001, 101, 3305–3365. (b)
Downs, A. J.; Greene, T. M. AdV. Inorg. Chem. 1999, 46, 101–171.
10.1021/ic802262h CCC: $40.75  2009 American Chemical Society
Inorganic Chemistry, Vol. 48, No. 8, 2009 3667
Published on Web 03/12/2009

Filippov et al.
mechanism of proton-transfer reactions to main-group hy-
drides has been much less explored. Discovery of dihydro-
gen-bonded (DHB) complexes8,9 formed by main-group
hydrides led to a new perspective on H2-elimination pro-
cesses.10 The ability of main-group hydrides to form dihy-
drogen bonds was initially studied by experimental and
theoretical methods for boron hydrides.11,12 Theoretical
investigations showed the formation of weak dihydrogen
bonds by neutral BH3NEt3 and BH3OEt3 hydrides and the
strong base behavior of ionic BH4 hydride. Theoretical
studies of the dihydrogen bond between simple main-group
hydrides and proton donors (for example, LiH, BeH2, MgH2,
BH3, and AlH3 with HF, H2O, and NH3) as models to
elucidate the H · · · H interaction nature have also been
published.13-15
containing materials.18 Despite the wide use of aluminum
hydrides, little is known about the mechanism by which they
react. The structures and decomposition paths of DHB
-
complexes between AlH4 and three proton donors (H2O,
HF, and HCl) were studied by ab initio methods.19 However,
the large reaction activity of aluminum tetrahydride makes
it difficult to study the DHB formation and proton transfer
experimentally in solution. The tetramethylpiperidine adduct
of alane features a weak Al-H · · · H-N dihydrogen bond in
the solid state, and it was suggested that this interaction
represents a transition state for dihydrogen elimination.20 The
selective reaction at the bridging hydride of pyrazolate- and
hydride-bridged dialuminum complexes with protic acids21
was theoretically explained as resulting from Al-H· · ·H-O
dihydrogen bonding.21,22 We performed a computational
study of proton-transfer and H2-elimination reactions of group
13 hydrides EH4- (E ) B, Al, and Ga) with alcohols.23 The
general features of the reaction mechanism were deduced
from this study: the DHB adduct (EH · · ·HO) initially formed
leads, after overcoming the activation barrier, to the concerted
step of H2-elimination and alkoxo product formation. A
comparison was made with the mechanism of the proton-
transfer reaction to transition-metal hydrides. The energy
barrier for AlH4- alcoholysis was found to be the lowest in
the group, in agreement with the high reactivity of alumo-
hydrides in protic media.
12b
-
Experimental detection of EH · · · HX (E ) Al and Ga)
interactions in solution is still scarce. Recently variable-
temperature IR investigations of the DHB adduct formed
-
between GaH4 and weak XH acids in combination with
theoretical study were reported by our group.16 The chemistry
of aluminum hydrides has been the subject of numerous
experimental and theoretical studies in diverse areas ranging
from synthetic chemistry17 to film growth of aluminum-
(7) (a) Bakhmutov, V. I. Eur. J. Inorg. Chem. 2005, 245–255. (b) Besora,
M.; Lledo´s, A.; Maseras, F. Chem. Soc. ReV., 2009, DOI:10.1039/
B608404B.
(8) Crabtree, R. H.; Siegbahn, P. E. M.; Eisenstein, O.; Rheingold, A. L.;
Koetzle, T. F. Acc. Chem. Res. 1996, 29, 348–354.
(9) Bakhmutov, V. I. Dihydrogen Bonds: Principles, Experiments, and
Applications; John Wiley & Sons, Inc.: Hoboken, NJ, 2008.
(10) Kenward, A. L.; Piers, W. E. Angew. Chem., Int. Ed. 2008, 47, 38–
41.
(11) Richardson, T. B.; de Gala, S.; Crabtree, R. H.; Siegbahn, P. E. M.
J. Am. Chem. Soc. 1995, 117, 12875–12876.
(12) (a) Shubina, E. S.; Bakhmutova, E. V.; Saitkulova, L. N.; Epstein, L
M. MendeleeV Commun. 1997, 83–84. (b) Epstein, L. M.; Shubina,
E. S.; Bakhmutova, E. V.; Saitkulova, L. N.; Bakhmutov, V. I.;
Gambaryan, N. P.; Chistyakov, A. L.; Stankevich, I. V. Inorg. Chem.
1998, 37, 3013–3017. (c) Shubina, E. S.; Belkova, N. V.; Bakhmutova,
E. V.; Saitkulova, L. N.; Ionidis, A. V.; Epstein, L. M. Russ. Chem.
Bull. 1998, 47, 846–851.
(13) Cybulski, H.; Pecul, M.; Sadlej, J. J. Chem. Phys. 2003, 119, 5094–
5104.
(14) (a) Kulkarni, S. A. J. Phys. Chem. A 1998, 102, 7704–7711. (b)
Kulkarni, S. A.; Srivastava, A. K. J. Phys. Chem. A 1999, 103, 2836–
2842. (c) Kulkarni, S. A. J. Phys. Chem. A 1999, 103, 9330–9335.
(d) Cramer, C. J.; Gladfelter, W. L. Inorg. Chem. 1997, 36, 5358–
5362.
We anticipate the decreased reactivity of a neutral
aluminum hydride should allow a deeper experimental study
of dihydrogen bond formation with proton donors in solution.
Here we will present the results of the spectroscopic and
theoretical investigation of the neutral trimethylamine alane
(AlH3(NMe3), TMAA, and 1). We have the aim to demon-
strate the interaction of 1 with a large series of XH (X ) O,
N, and C) acids and to elucidate the peculiarities of the
mechanism of proton-transfer and H2- elimination reactions
of 1 with different mono- and polydentate acids.
Experimental Section
Experimental Details. Trimethylamine alane, Me3N·AlH3, was
obtained according to ref 24 by a preliminary preparation of an
ether-toluene solution of aluminum hydride. AlCl3 (3.33 g, 25
mmol) in ether was added to a suspension of NaAlH4 (4.6 g, 85
mmol) in 700 mL of ether-toluene mixture (1:3) in 30 min, and
the mixture was stirred for 4 h. NaAlH4 was additionally milled in
a vibration mill filled with steel balls. NaCl precipitate was filtrated,
(15) (a) Alkorta, I.; Elguero, J.; Mo, O.; Yan˜ez, M.; Del Bene, J. J. Phys.
Chem. A 2002, 106, 9325–9330. (b) Grabowski, S. J.; Sokalski, W. A.;
Leszczynski, J. J. Phys. Chem. A 2004, 108, 5823–5830. (c)
Grabowski, S. J.; Sokalski, W. A.; Leszczynski, J. J. Phys. Chem. A
2005, 109, 4331–4341.
(16) Belkova, N. V.; Filippov, O. A.; Filin, A. M.; Teplitskaya, L. N.;
Shmyrova, Y. V.; Gavrilenko, V. V.; Golubinskaya, L. M.; Bregadze,
V. I.; Epstein, L. M.; Shubina, E. S. Eur. J. Inorg. Chem. 2004, 3453–
3461.
(18) (a) Jang, T. W.; Rhee, H. S.; Ahn, B. T. J. Vac. Sci. Technol., A 1999,
17, 1031–1035. (b) Jang, T. W.; Moon, W.; Baek, J. T.; Ahn, B. T.
Thin Solid Films 1998, 333, 137–141. (c) Choi, H.; Hwang, S. Chem.
Mater. 1998, 10, 2323–2325. (d) Yun, J. H.; Kim, B. Y.; Rhee, S. W.
Thin Solid Films 1998, 312, 259–263.
(17) (a) Paquette, L. A. In Encyclopedia of Reagents for Organic Synthesis;
Paquette, L. A., Ed.; Wiley: Chichester, U.K., 1995, vol 5, pp
3009-3014. (b) Galatsis, P. In Encyclopedia of Reagents for Organic
Synthesis; Paquette, L. A., Ed.; Wiley: Chichester, U.K., 1995, vol 3,
pp 1908-1912. (c) Seyden-Penne, J. Reductions by the Alumino- and
Borohydrides in Organic Synthesis; VCH: New York, 1991. (d) Eisch,
J. J. In ComprehensiVe Organic Synthesis; Trost, B. M., Fleming, I.,
Eds.; Pergamon: Oxford, U.K., 1991, vol 8, pp 733-761. (e) Zweifel,
G.; Miller, J. A. In Organic Reactions; Dauben, W. G., Ed.; Wiley:
New York, 1984, vol 32, pp 375-517. (f) Hajos, A. Complex Hydrides
and Related Reducing Agents in Organic Synthesis; Elsevier: New
York, 1979.
(19) Marincean, S.; Jackson, J. E. J. Phys. Chem. A 2004, 108, 5521–
5526.
(20) Atwood, J. L.; Koutsantonis, G. A.; Lee, F.-C.; Raston, C. L. J. Chem.
Soc., Chem. Commun. 1994, 91–92.
(21) Yu, Z.; Wittbrodt, J. M.; Xia, A.; Heeg, J.; Schlegel, H. B.; Winter,
C. H. Organometallics 2001, 20, 4301–4303.
(22) Shi, F.-Q.; Song, B.-A. Int. J. Quantum Chem. 2008, 108, 1107–1113.
(23) Filippov, O. A.; Filin, A. M.; Belkova, N. V.; Tsupeva, V. N.; Lledo´s,
A.; Ujaque, G.; Epstein, L. M.; Shubina, E. S. Inorg. Chem. 2006,
45, 3086–3096.
(24) Zakharkin, L. I.; Gavrilenko, V. V.; Maslin, D. N. Russ. J. Gen. Chem.
1971, 41, 577.
3668 Inorganic Chemistry, Vol. 48, No. 8, 2009

Proton-Transfer and H2-Elimination in AlH3(NMe3)
Table 1. Spectroscopic Characteristics of DHB Complexes of
and the solution was cooled to 0 °C. Me3N (5.9 g, 100 mmol) was
added to an ether-toluene solution of AlH3. After removing the
solvent, a powder of trimethylamine alane (8.2 g, yield 92%, mp
74-75 °C) was obtained.
Monodentate Proton Donors with AlH3(NMe3) in Hexane and MCP
HX
ν(XH)init, cm-1
ν(XH)bond, cm-1
∆(XH),a cm-1
indole
3496
3312
3604
3278
3249
3392
214
63
212
AdCCH
Fluorinated alcohols were provided by P&M (Moscow, Russia).
The solutions for IR studies were prepared under argon by standard
Schlenk technique. Hexane and methylcyclopentane (Sigma-Ald-
rich) were purified by distillation from CaH2 before use. The
anhydrous solvents were thoroughly degassed prior to use. The IR
spectra of hexane and methylcyclopentane solutions were measured
by an Infralum FT-801 (Lumex) FTIR spectrometer. Low-temper-
iPrOH
a ∆(XH) ) ν(XH)init - ν(XH)bond
.
F(r) was performed using the AIMPAC program package29 based
on the wave function obtained by the B3LYP calculations.
Results and Discussion
ature IR studies were carried out in the ν(OH) (3700-3100 cm-1
)
and ν(AlH) (1950-1600 cm-1) regions by use of a home-modified
cryostat (Carl Zeiss Jena) in the 140-300 K temperature range.
The cryostat modification allows the transfer of the reagents
(premixed either at low or room temperature) directly into the cell
under an inert atmosphere and at the desired temperature. The
accuracy of the temperature adjustment was (0.5 K. For the
measurements in the ν(XH) range, the XH acid concentrations were
10-2 to 10-3 M to avoid self-association, whereas TMAA was taken
in excess (from 1.2- to 3-fold). For the measurements in the ν(AlH)
range, the 1.5- to 4-fold excess of XH acids was used.
Room-temperature NMR spectra were measured on a Bruker
AVANCE 600 spectrometer. The 1H chemical shifts were calculated
from the resonances of toluene-d8 as the internal standard. The 27Al
NMR spectra were measured using a 1 M solution of AlCl3 in water
as the external standard. The studied solutions were prepared in
cold toluene-d8 and were transferred into NMR tubes. The
concentrations of both compounds were taken the same as in the
case of the IR investigations.
Computational Details. We have employed the same methodol-
ogy we successfully applied earlier for the study of DHB complexes
of anionic group 13 tetrahydrides.23 Full geometry optimizations
were carried out with the GAUSSIAN0325 package at DFT level
using the hybrid B3LYP functional.26 The 6-311++G(d,p) basis
set was used for all the atoms. The nature of all the stationary points
on the potential energy surfaces was confirmed by a vibrational
analysis. IRC calculations were carried out in both directions starting
from the located transition states. Natural atomic charges and
Wiberg bond indices27 were calculated using the natural-bond
orbital (NBO) analysis28 option as incorporated in GAUSSIAN03.
Topological analysis of the electron-density distribution function
Characterization of Dihydrogen-Bonded Intermediates.
The interaction of AlH3(NMe3) 1 with monodentate XH acids
(adamantyl acetylene (AdCCH), indole, CH3OH, (CH3)2-
CHOH (iPrOH), and CF3CH2OH (TFE)) and polydentate
XH2 acids (2,3,4,5,6-pentafluoroaniline (PFA) and 2,3,5,6-
tetrachloroaniline (TeCA)) was investigated by IR and NMR
spectroscopy in solvents of low polarity (hexane and meth-
ylcyclopentane) in the temperature range of 140-300 K.
DFT calculations were performed to analyze the Al-H· · ·HX
interaction.
IR Spectroscopic Investigation with Monodentate
CH, NH, and OH Acids. The IR spectra of indole were
measured in hexane and methylcyclopentane (MCP) with an
excess of aluminum hydride. The intensity of the initial
ν(NH)init ) 3496 cm-1 band decreases in the presence of
TMAA, and a new low-frequency broad band at 3278 cm-1
appears (Table 1). This illustrates the formation of a DHB
complex between the proton donor and aluminum hydride.
The intensities of the ν(NH)bond bands increase as the
temperature decreases. In the IR spectra of AdCCH in MCP,
the initial band is observed at 3312 cm-1 in the range of CH
group stretching vibrations. Addition of 4 equiv of TMAA
in the temperature range of 140-220 K decreases the
intensity of ν(CH)init, and a new broad band at 3249 cm-1
appears, which can be attributed to the formation of
AlH· · ·HC dihydrogen bond. The intensity of this new band
lowers upon warming, and the band completely disappears
already at 240 K.
In the range of the Al-H stretching vibrations of TMAA
(ν(AlH)init ) 1704 cm-1), the new low-frequency band
ν(AlH)bond ) 1660 cm-1 (as overlapping broad shoulder,
∆ν(AlH) ) 44 cm-1) appears at 140 K in the presence of
AdCCH (Figure 1). The intensity of ν(AlH)bond band lowers
upon warming, disappearing completely at 240 K. It was
shown earlier that dihydrogen bond formation results in a
low-frequency shift of the M-H or B-H and Ga-H
stretching frequencies for the groups participating in
H-bonding.12b,30 Thus, the spectral changes observed for
the TMAA-AdCCH system correspond to formation of
AlH · · · HC DHB complex.
(25) Frisch, M. J. ; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb,
M. A.; Cheeseman, J. R.; Montgomery, Jr., J. A.; Vreven, T.; Kudin,
K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone,
V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.;
Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa,
J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene,
M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.;
Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev,
O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala,
P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.;
Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas,
O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.;
Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.;
Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.;
Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen,
W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision
C.02; Gaussian, Inc.: Wallingford, CT, 2004.
In the case of indole, when the temperature increases, high-
frequency shoulders (1754 and 1784 cm-1) appear in the
(26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648–5652.
(27) Wiberg, K. Tetrahedron 1968, 24, 1083–1096.
(28) (a) Reed, A. E.; Curtiss, L. A.; Weinhold, F. Chem. ReV. 1988, 88,
899–926. (b) Glendening, E. D.; Badenhoop, J. K.; Reed, A. E.;
Carpenter, J. E.; Bohman, J. A.; Morales, C.; Weindhold, F. NBO
5.0, Theoretical Chemistry Institute, University of Wisconsin: Madison,
WI, 2001.
(29) Cheeseman, J.; Keith, T. A.; Bader, R. W. F. AIMPAC program
package, McMaster University: Hamilton, Ontario (Canada), 1992.
(30) Epstein, L. M.; Belkova, N. V.; Shubina, E. S. In Recent AdVances in
Hydride Chemistry; Peruzzini, M., Poli, R., Eds.; Elsevier: Amsterdam,
Netherlands, 2001, pp 351-374.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3669

Filippov et al.
complexes of anionic 13 group tetrahydrides.23 Pyrrole and
phenylacetylene were used as models of NH and CH acids,
correspondingly, and CH3OH, CF3CH2OH, and CF3OH were
used as models of OH acids of different strength.
Minima corresponding to DHB complexes with CH and
NH proton donors were found (Figure 3 and Table 2).
Phenylacetylene forms with TMAA a weak DHB complex
1·PHA with H · · ·H distance of 2.200 Å, only slightly shorter
than the sum of hydrogen van der Waals radii (2.4 Å). The
interaction is stronger for pyrrole as the H· · · H distance in
1 · Pyrr (1.870 Å) shows. The changes in the X-H and
Al-H bonds after adduct formation are small but typical
for dihydrogen bonds, confirming the dihydrogen bond
formation.23 Dihydrogen bond formation entails low-
frequency shifts of the ν(XH) and ν(AlH) bands. For
example, calculated frequency shifts are ∆νcalc(NH) ) 143
cm-1 for the TMAA-pyrrole DHB complex and ∆νcalc(CH)
) 61 cm-1 for the TMAA-PhCCH complex. These changes
are in good agreement with the experimental data.
Figure 1. IR spectra in the ν(AlH) range of AlH3(NMe3) (c ) 0.03M) in
the presence of 0.09 M AdCCH at 140 K (solid line) and at 160 K (dashed
line) with band decomposition (dotted lines), MCP, d ) 0.22 cm.
Minima on the potential energy surface corresponding to
DHB complexes were also found for the TMAA adducts with
OH acids: TMAA-CH3OH 1 · M_a, TMAA-CF3CH2OH
1 · TFE_a, and TMAA-CF3OH 1 · TFM_a. The optimized
geometry of a DHB complex with methanol is depicted in
Figure 4a. The O-H · · · H-Al interactions are stronger than
those involving C-H or N-H bonds and lead to more
pronounced changes in geometries of partners. The structural
characteristics obtained (Table 2) are comparable to those
observed for related DHB complexes of main-group neutral
trihydrides.12b,14 Increasing of the ROH proton-donating
ability in the series CH3OH < CF3CH2OH < CF3OH results
in shortening of the H · · · H distance and larger elongations
of O-H and Al-H bonds involved in the interaction.
Comparing DHB complexes of anionic AlH4 -ROH23 with
-
Figure 2. IR spectra in the ν(AlH) range of AlH3(NMe3) (c ) 0.03M) in
the presence of 0.06 M indole at 140 K (solid line), at 260 K (dashed line),
and at 280 K (dash-dot line), MCP, d ) 0.22 cm.
neutral TMAA-ROH, we can observe that H · · ·H bond
length is shorter and elongations of O-H and Al-H bonds
are larger for the anionic tetrahydride. These structural
characteristics of dihydrogen bonds usually are in strong
correlation with the proton-accepting ability of hydrides; thus,
the lower basicity in the H-bond of TMAA, comparing to
Al-H region (Figure 2). It was shown earlier that when one
-
hydrogen of a GaH4 is substituted as a result of reaction,
the IR spectra of the compounds derived feature three high-
frequency bands in the range of the Ga-H stretching
vibration,16 corresponding to GaH3X- product. Based on this
work, we can assign these two bands to the formation of the
monosubstituted product:
-
the AlH4 anion, is similar to what happens in the
BH3NEt3-BH4 pair.12b
-
A second minimum was found for the TMAA-CH3OH
(1 · M_b) and TMAA-TFE (1 · TFE_b) complexes. In both
systems, it corresponds to the coordination of an oxygen lone
pair to the aluminum atom with formation of a Lewis
acid-base complex in which aluminum is five-coordinated
with a bipyramidal trigonal structure (Figure 4b). The amine
and alcohol ligands are occupying the axial positions of the
BPT (N-Al-O angle: 176.5° in 1 · M_b and 178.5° in
1 · TFE_b). Formation of the Lewis complex leads to an
Al-N bond elongation of 0.063 Å for 1 · TFE_b and 0.106
Å for the stronger 1 · M_b complex, in contrast to the DHB
complexes where Al-N bond shortens by 0.005-0.009 Å.
A weak H · · · H interaction is also at work in these Lewis
complexes. Despite the fact the H · · · H distances are larger
than the sum of vdW radii of hydrogens and absence of
electronic density (Wiberg bond index (WBI), critical points),
(Me3N)AlH3 + HX a (Me3N)H2AlH· · ·HX f
(Me3N)H2AlX + H2 (1)
When XH ) indole increasing the temperature to 260 K
shifts the equilibrium to proton-transfer products. At 290 K,
we observe only the reaction product bands (1754 and 1784
cm-1). The interaction between TMAA and AdCCH is
weaker and does not result in any further reaction, the
equilibrium being displaced toward free reagents upon
warming to room temperature.
Structural Analysis. The geometries of TMAA complexes
with XH acids were optimized with the DFT method using
the B3LYP functional and 6-311++G(d,p) basis set. This
methodology was successfully applied earlier by us for DHB
3670 Inorganic Chemistry, Vol. 48, No. 8, 2009

Proton-Transfer and H2-Elimination in AlH3(NMe3)
Figure 3. Optimized geometries of complexes of TMAA with phenylacetylene 1·PHA (left) and pyrrole 1·Pyrr (right) (hydrogen atoms of NMe3 removed
for clarity).
-
Table 2. Calculated Selected Structural Parameters (Å and degrees) of TMAA and AlH4 Complexes with OH, NH, and CH Proton Donors
a
a
complex
rH· · ·H (rAl· · ·O)
∠XH· · ·H
∆r(XH)a
∆r(AlH)bond
∆r(AlH)free
TMAA-PhCCH 1·PHA
TMAA-Pyrrole 1·Pyrr
TMAA- CH3OH 1·M_a
AlH4--CH3OHb
TMAA-CH3OH 1 ·M_b
TMAA-TFE 1·TFE_a
AlH4--TFEb
TMAA-TFE 1 ·TFE_b
TMAA-CF3OH 1·TFM_a
AlH4--CF3OHb
a ∆r ) r(initial) - r(complex). b From ref 23.
2.200
1.870
152.4
157.9
160.5
164.6
81.3
172.6
174.8
76.6
0.005
0.008
0.011
0.023
0.002
0.014
0.027
0.002
0.028
0.086
0.002
0.007
0.009
0.012
-0.002
-0.004
-0.004
-0.009
0.010
-0.007
-0.013
0.005
1.702(3.853)
1.622(4.005)
2.581(2.212)
1.599(3.892)
1.513(3.806)
2.775(2.367)
1.478(3.804)
1.258(3.789)
0.014
0.017
167.2
179.2
0.016
0.030
-0.010
-0.019
Table 3. Changes of NPA Charges (∆q), WBI, Electron Densities (Fc)
at the H · · ·H or Al· · ·O Bond Critical Points, and Complexation
Energies for the TMAA and AlH4-Complexes with CH, NH, and OH
Proton Donors
∆E,d
complex
∆q(Al) ∆q(X)a WBI
Fc kcal·mol-1
TMAA-PhCCH
0.023 -0.009 0.005 0.008
0.045 -0.014 0.015 0.015
0.053 -0.048 0.025 0.022
0.059 -0.055 0.047 0.026
-1.4
-3.4
-5.6
-10.1
-4.8
-5.7
-16.0
-2.8
-8.2
-22.2
TMAA-Pyrrole
TMAA-CH3OH 1·M_a
AlH4--CH3OHb
TMAA-CH3OH 1 ·M_bc -0.028 0.010 0.172 0.029
TMAA-TFE 1·TFE_a
0.065 -0.048 0.041 0.027
0.072 -0.074 0.076 0.035
AlH4--TFEb
TMAA-TFE 1 ·TFE_bc -0.034 0.015 0.132 0.021
TMAA-CF3OH 1·TFM_a 0.087 -0.061 0.071 0.035
AlH4--CF3OHb
0.138 -0.049 0.191 0.063
a X ) C, N for CH, NH, and OH acids. b From ref 23. c WBI, Fc are for
Figure 4. Optimized geometries of TMAA adducts with methanol: DHB
complex 1·M_a (left) and Lewis complex 1·M_b (right) (hydrogen atoms
of NMe3 removed for clarity).
d
Al-O bond. ∆E + ZPVE in kcal ·mol-1
.
considerably increases to 2.9 kcal · mol-1 for the trifluoro-
ethanol complexes until disappearance of the Lewis complex
for trifluoromethanol.
Electron-Density Analysis. Formation of DHB and Lewis
complexes entails transfer of electron density between the
two units constituting the complex and rearrangements of
density within the confines of the interacting molecules. The
electron density was analyzed using different approaches,
namely changes of natural population analysis (NPA)
charges, Wiberg bond indexes (WBI), and Bader’s “atoms
in molecule” (AIM) theory (Table 3).
Changes of NPA charges on Al and O atoms appear to be
sensitive to the type of complex formed: in DHB complexes
the positive charge on Al and negative charge on X become
larger, ∆q increases with increasing the proton donor
strength. In Lewis complexes (1 · M_b and 1 · TFE_b)
charges on Al and O decrease due to the electron donation
from the oxygen lone pair, resulting in the Al-O bond
formation. Herein, there is a noteworthy difference between
this weak AlH · · · HO contact results in the very small
elongation of the OH bond (∼0.002 Å) in 1 · M_b and
1 · TFE_b. This elongation leads to small low-frequency
shifts of ν(OH) (17 and 20 cm-1 for 1 · M_b and 1 · TFE_b,
respectively) without changes in intensity.
The three ν(AlH) vibrations shift to a low frequency range,
but the intensity of these vibrations does not increase. The
high-frequency band of ν(AlH)free is absent, in contrast to
DHB complexes. Therefore, trends in νAlH vibrations allow
distinguishing Lewis and DHB complexes by spectra.
Notably CF3OH, which is a stronger proton donor (Brøn-
sted acid) but a weaker electron donor (Lewis base), does
not form the Lewis complex in a gas phase. Optimizations
starting from a Lewis adduct structure always end up in the
DHB complex. The evolution of the relative energies of DHB
and Lewis complexes agrees with the competition between
Brønsted acidity and Lewis basicity of the alcohols. For
methanol both complexes (1·M_a and 1·M_b) have similar
stabilities, the DHB complex being only 0.8 kcal ·mol-1 more
stable than the Lewis complex (Table 3). This difference
Inorganic Chemistry, Vol. 48, No. 8, 2009 3671

Filippov et al.
Figure 5. Optimized geometries of DHB complexes of TMAA with perfluoroaniline: monodentate 1·PFA_a (left) and bidentate 1·PFA_b (right) (hydrogen
atoms of NMe3 removed for clarity).
Table 4. Calculated Structural, Spectroscopical, and Electronic
DHB and Lewis complexes because upon DHB complex
formation the charges on the interacting sites (hydridic
hydrogen and XH proton) are always increasing.
Characteristics of PFA-TMAA Complexes
1·PFA_b
H· · ·H1
H· · ·H2
1·PFA_aa
1.835
170.0
0.008
0.007
-0.005
The increase of positive charge on the Al atom directly
depends on the H · · · H interaction strength, ranging from
0.023 for PhCCH to 0.087 Å for CF3OH. The increase of
negative charge on the proton donor X atom follows the same
trend. The values of electron density on dihydrogen bonds
expressed by WBI are also in direct correlation with the
H· · ·H bond strength. Bond critical points (3,-1) were found
for H · · · H and Al · · · O interactions of DHB and Lewis
complexes, respectively. The electron densities on critical
points (Fc) correlate with WBI values. The energies for DHB
interactions gathered in Table 3, calculated as the differences
between the energy of the complex and the energies of the
isolated reactants, also follow the same trend. The energy
values range between -1.4 kcal · mol-1 for phenylacetylene
and -8.2 kcal · mol-1 for CF3OH. WBIs and complexation
energies of investigated DHB complexes with neutral
aluminum hydrides are significantly smaller than those for
related DHB complexes with the anionic aluminum tetrahy-
dride23 (Table 3). Methanol DHB and Lewis complexation
energies are in the same range (-5.6 and -4.8 kcal · mol-1,
respectively). On the contrary with TFE, the DHB complex
is notably more stable than the Lewis complex (complexation
energies of -5.7 and -2.8 kcal · mol-1, respectively). On
the whole, computational results confirm the formation of
both H · · ·H (DHB) and Lewis (Al-O) complexes of TMAA
with alcohols.
rH· · ·H
2.251
137.4
0.003
2.449
129.1
0.002
∠ NH· · ·H
∆r(NH)
∆r(AlH)bond
∆r(AlH)free
0.004
0.003
-0.007
∆q(Al)
∆q(N)b
0.013
0.050
-0.009
-0.005
WBI
Fc
∆EZPVE
0.003
0.008
-2.95
0.001
0.006
0.016
0.045
-4.05
∆(NH), cm-1
(∆A(NH), km ·mol-1
-38 (-12)
-31 (+144)
-34 (+42)
)
-110 (+499)
a Data for bonded NH group. b N atom in perfluoroaniline molecule.
different proton donors or acceptors, we have found that
TMAA is able to form both monodentate and bidentate
chelate complexes with perfluoroaniline (PFA) (DFT calcu-
lations). Optimization of TMAA-PFA adducts led to two
different minima (Figure 5).
The monodentate complex 1 · PFA_a is characterized by
geometric parameters similar to those found for other NH
acids, such as pyrrole, with practically linear NH · · ·H moiety
(Table 4). The NH · · · H angles in the bidentate complex
1 · PFA_b (137° and 129°) are unfavorable to DHB com-
plexes and smaller than those obtained for symmetrical
bidentate GaH4 complexes (141-148°).16 The complex
-
IR Spectral and Theoretical Investigations with
Bidentate NH2 Acids. The interaction of TMAA with
potentially bidentate NH2 acids has also been investigated.
These polydentate proton donors offer the possibility of two
types of coordination when interacting with polyhydrides:
the typical monodentate with only one H· · · H dihydrogen
bond and bidentate, entailing interaction of two hydrides with
both hydrogen atoms of the XH2 group and formation of a
symmetrical chelated structure, as shown by us previously.16
Previous calculations of EH4- · H2XR (E ) B and Ga)
complexes have shown that the preference for monodentate
or bidentate structures was dependent on the central atom.
1 · PFA_a possesses two different weak H · · · H contacts of
2.251 and 2.449 Å. The last is formally larger than the sum
of hydrogens’ van der Waals radii, but the elongations of
N-H and Al-H bonds suggest the presence of a second
H· · ·H interaction. Bidentate coordination was confirmed by
electron distribution analysis. For both H· · · H bonds in the
bidentate complex (3,-1) critical points exist, and the
presence of an additional ring (3,-3) critical point confirms
the formation of the chelate structure. The molecular graph
of TMAA-PFA bidentate cycle complex is given in the
Supporting Information. Changes in the charges of the Al
atom and the proton donor N atom in both complexes follow
the same trend as for complexes of monodentate acids (Table
3). WBIs also show the presence of the bond between
-
This last type of dihydrogen bond was specific to the GaH4
-
anion and was not found for BH4 .
-
In contrast to our previous researches showing that the
different coordination types can be attained only by using
hydrogen atoms in NH2 acid and TMAA hydrides in
1 · PFA_b.
3672 Inorganic Chemistry, Vol. 48, No. 8, 2009

Proton-Transfer and H2-Elimination in AlH3(NMe3)
Table 5. Spectroscopic Characteristics of DHB Complexes of Bidentate
NH Acids with (Me3N)AlH3 in Hexane
NH
ν(NH)init, cm-1
ν(NH)complex, cm-1
∆ν(NH), cm-1
PFA
3500(as)
3407(s)
3516(as)
3412(s)
3473free
19.5a,a
-162.5a,a
-29b
3291bond
TeCA
3487bond(as)
3395bond(s)
-17b
a ∆ν(NH) ) ν(NH)complex - [ν(NH)as + ν(NH)s]/2. b ∆ν(NH) )
ν(NH)complex - ν(NH)init
.
Figure 7. IR spectra in the ν(NH) range of C6Cl4HNH2 (c ) 0.01 M, dotted
line) in the presence of 0.025 M AlH3(NMe3) (dashed line) with bands of
decomposition (solid lines), 230 K, hexane, d ) 0.22 cm.
relative to their initial positions (3500 and 3407 cm-1) is 27
and 116 cm-1, respectively. These facts are in agreement
with DFT results for the monodentate complex and with the
16
-
spectral pattern for the GaH4 -p-nitroaniline complex,
which was characterized by one H · · · H bond.
Thus, the new band at 3473 cm-1 is attributed to the free
NH vibration and the band at 3291 cm-1 to the NH · · ·H
bonded band. The correct shifts of these bands can be
calculated by taking the mean frequency of free PFA
[ν(NH)as +ν(NH)s]/2 as a reference (∆ν(NH)free ) 19.5 cm-1,
∆ν(NH)bond ) -162.5 cm-1).16,32
Figure 6. IR spectra in the ν(NH) range of C6F5NH2 (c ) 0.01M) (dotted
line) and in the presence of 0.03 M AlH3(NMe3) at 190 K (solid line) and
at 210 K (dash-dot line) and hexane, d ) 0.22 cm.
Relative energies of both TMAA-PFA DHB complexes
are similar (complexation energies of -4.1 and -3.0
kcal · mol-1 for the monodentate and bidentate complexes,
respectively), hampering a clear prediction of the experi-
mentally favored complex in solution. In order to benchmark
the spectroscopic traces able to discriminate between the two
types of dihydrogen bonds, we have followed the same
procedure than we used recently in the study of intermo-
lecular vibrations of H3EH- · · ·HOR complexes,31 performing
initially a computational study of the vibration spectra.
Changes in the N-H frequencies after complexation (∆ν(NH))
are collected in Table 5 and will be compared with the
experimental frequencies.
Two substituted anilines (perfluoroaniline and 2,3,5,6-
tetrachloroaniline) were used experimentally as polydentate
acids interacting with TMAA. In the range of the stretching
vibrations of NH2 group of C6F5NH2, two bands are observed
and assigned to symmetrical (νs )3407 cm-1) and asym-
metrical (νas )3500 cm-1) stretching vibrations. In the
presence of TMAA, three new bands (3473, 3368, and 3291
cm-1) appear (Figure 6, Table 5). The intensities of two
bands (3473and 3291 cm-1) are growing upon cooling while
the intensity of the band at 3368 cm-1 is not.
In the Al-H stretching region, the dihydrogen bond
formation leads to the appearance of the low-frequency
shoulder (ν(AlH)bond ) 1678 cm-1 and ∆ν(AlH) ) -26
cm-1) at 190-230 K.
The third band in the ν(NH) region at 3368 cm-1 could
be assigned to the stretching vibration of one NH group in
the PFA where the second NH proton is substituted by
AlH2(NMe3) (product of one H2 molecule evolution, see
below). The intensity of this band grows upon heating, which
is accompanied by a visible gas evolution. At the same time,
new high-frequency weak bands at 1815 and 1777 cm-1
appear in the Al-H stretching vibration region. These new
stretches (one N-H and two Al-H) can be clearly assigned
to the product of the aminolysis reaction (eq 2). We suppose
a formation of monosubstituted product by analogy with the
TMAA-indole system (see above).
(Me3N)AlH3+H2NC6F5 a (Me3N)H2AlH· · ·H2NC6F5 f
(Me3N)H2AlNHC6F5+H2 (2)
Analysis of the data in Table 4 shows that it is possible to
recognize the type of coordination of the polydentate proton
donor by means of IR spectroscopy. Analysis of the relative
width and shifts of these bands helps to determine their
character and the type of coordination.
To support this hypothesis we used NMR spectroscopy.
The room-temperature H NMR spectrum of C6F5NH2 in
toluene-d8 shows a singlet of the NH2 group at 2.68 ppm.
The 27Al NMR spectrum of TMAA in toluene features a
broad singlet at 111.6 ppm. The 27Al NMR spectrum of the
1
The first new band (ν(NH) ) 3473 cm-1) is narrow, but
the second band (ν(NH) ) 3291 cm-1) is broad, so we
suggest that they could be assigned to the free NH and
bonded NH · · · H bonds, respectively. The shift of the bands
(31) Filippov, O. A.; Tsupreva, V. N.; Epstein, L.; Lledo´s, A.; Shubina, E.
J. Phys. Chem. A 2008, 112, 8198–8204.
(32) Patwari, G. N.; Ebata, T.; Mikami, N. Chem. Phys. 2002, 283, 193–
207.
Inorganic Chemistry, Vol. 48, No. 8, 2009 3673

Filippov et al.
Table 6. Spectroscopic and Thermodynamic Characteristics of DHB
As seen from the -∆H° values, the AlH-HX hydrogen
bonding is of medium strength (4.2-1.5 kcal · mol-1).
Calculated dihydrogen bond enthalpies (∆H°calc) in model
systems follow the same trend and have values similar to
those estimated from eq 3 (Table 6). The energy of the
H · · · H interaction was estimated using the correlation
between the energy of the contact (EHH) and the value of
the potential energy density function V(r). The corresponding
Complexes with (Me3N)AlH3
c
∆H°calc
,
HX
Pi ∆ν(XH),a cm-1 ∆H°,b kcal·mol-1
kcal·mol-1
indole
iPrOH
C6F5NH2 0.55
AdCCH 0.26
0.75
0.63
214
212
163
63
-4.2
-4.1
-3.3
-1.5
-3.4 (pyrrole)
-5.6 (methanol)
-3.6d
-0.5
(phenylacetylene)
a ∆ν(XH) ) ν(XH)init - ν(XH)bond b From eq 3. c DFT calculations.
.
bond critical point36 (EHH ) 1/2V(r)) for CH acid (EHH
)
d For the monodentate TMAA-PFA complex.
-1.5 kcal· mol-1) and NH2 acid (EHH ) -4.0 kcal· mol-1)
was in a good agreement with the experiment.
equimolar TMAA-C6F5NH2 solution measured immediately
after the mixing shows two singlets at 113.4 and 97.4 ppm
of a 1:1 integral ratio. The first singlet corresponds to
AlH3NMe3, and the second singlet corresponds to the reaction
product. Correspondingly in the 1H NMR spectrum, the NH2
resonance shifts to 2.95 ppm (as the result of its involvement
in hydrogen bond formation6a,7a) and the new singlet appears
at 3.05 ppm. The integral intensities ratio of these signals is
2:1. Hence, the changes in IR and NMR spectra allow
assigning the new signals to NMe3AlH2NHC6F5.
Dependence between -∆H° and Pi (proton-donor ability
of XH acids) allows one to estimate the proton-accepting
ability of the hydride ligand in 1 using the Iogansen’s “rule
of factors”33 (eq 4), where Pi is the proton-donating ability
for XH acids and -∆H11) 5.7 kcal · mol-1 for the standard
phenol-diethylether pair in corresponding solvent (hexane)
with Pi ) Ej ) 1.0.
-∆Η° ) -∆Η11PiEj
(4)
IR spectra of 2,3,5,6-tetrachloroaniline (TeCA) reveal two
bands of NH stretching vibrations ν(NH)as ) 3516 cm-1 and
ν(NH)s ) 3412 cm-1, accompanied by weak shoulders at
3499 and 3402 cm-1 (Figure 7), which can be assigned to
the weak NH-Cl hydrogen bonds of TeCA self-associates.
Addition of excess TMAA to the TeCA solution results in
the appearance of two new shifted narrow bands at 3487
and 3395 cm-1 (Figure 7). The small shifts of these bands
(∆ν(NH)as ) -29 cm-1 and ∆ν(NH)s ) -17 cm-1) along
with their narrowness are in agreement with the DFT
calculation data for a cyclic complex (see Table 4). Thus,
we can state the formation of both monodentate and cyclic
bidentate complexes by TMAA with different substituted
anilines. The cyclic bidentate TMAA-TeCA complex slowly
decomposes at room temperature similarly to other com-
plexes studied with formation of a NMe3AlH2NHC6Cl4H
aminolysis product.
The basicity factor (Ej) is independent of the proton donor
and solvent and characterizes the proton-accepting ability
of the H-bonding site. Thus, it is possible to compare the
proton-accepting ability of TMAA (Ej ) 1.0 ( 0.1) with
that of other main-group hydrides in dihydrogen bond
formation. TMAA’s Ej is significantly greater than that of
neutral Et3NBH3’s hydride (Ej ) 0.53), but smaller than that
of anionic BH4 hydride (Ej ) 1.25).12b In spite of this
-
-
difference between Ej values of TMAA and BH4 , the
proton-transfer reactions are much more favorable for TMAA
-
in comparison with the BH4 anion. Such irregularity was
found for protonation of transition-metal hydrides, and this
is not rare for such bases.6
Proton Transfer Reaction. For most of the alcohols, the
experimental detection of reaction intermediates in solution
was not possible even at 140 K solely the products were
detected. Only in case of the weakest acidic alcohol iPrOH,
the DHB complex can be detected at 190 K by the shift on
the ν(AlH) (ν(AlH)bond ) 1658 cm-1, ∆ν(AlH) ) 46 cm-1)
and νOH bands (∆ν(OH) ) 212 cm-1). We observe the
instant disappearance of initial TMAA and dihydrogen bond
ν(AlH) bands when raising the temperature to 200 K. At
the same time, the high-frequency bands (1760 and 1790
cm-1) appear, testifying the formation of the monosubstituted
product (Me3N)AlH2OCH(CH3)2. Band assignment was made
on the basis of frequency calculations. Similar spectral
changes were observed studying the interaction between
GaH4- and OH proton donors.16 No DHB complex but only
protonation product was observed in the IR spectra in the
interaction with a stronger alcohol (TFE).
Strength of AlH· · · HX Interaction and TMAA
Basicity. The enthalpy of AlH · · ·HX formation (-∆H°) was
determined from the correlation equation:
18∆ν
720 + ∆ν
-∆Η° )
(3)
where ∆ν is the shift on the νXH stretching frequency due
to dihydrogen bond formation. This equation was proposed
by Iogansen for classical H-bonded systems33-35 and also
proved to be applicable for DHB systems. It was shown for
numerous DHB complexes of transition-metal,30 boron, and
gallium hydrides16,12b with organic acids that the enthalpy
values from the eq 3 are in a good agreement with enthalpy
values calculated by Vant-Hoff’s method. The data in Table
6 show that as in other systems -∆H° values of DHB
complexes increase with the proton-donor ability of alcohols.
In whole, proton transfer to TMAA from OH acids was
observed at a temperature of > 200 K and from NH acids at
> 250 K, and only DHB complexes of TMAA with CH acids
exist at room temperature without proton transfer (it will be
(33) Iogansen, A. V. Theor. Experim. Khim. 1971, 7, 302-311; Chem.
Abstr. 1971, 75, 101848.
(34) Iogansen, A. V. The Hydrogen Bond; Nauka: Moscow, Russia, 1981,
p 134.
(35) Iogansen, A. V. Spectrochim. Acta, Part A 1999, 55, 1585–1612.
(36) (a) Espinosa, E.; Molins, E.; Lecomte, C. Chem. Phys. Lett. 1998,
285, 170–173. (b) Espinosa, E.; Alkorta, I.; Rozas, I.; Elguero, J.;
Molins, E. Chem. Phys. Lett. 2001, 336, 457–461.
3674 Inorganic Chemistry, Vol. 48, No. 8, 2009

Proton-Transfer and H2-Elimination in AlH3(NMe3)
Figure 8. Transition states for proton transfer to TMAA from DHB complex with CH3OH (TS-M_a, left) from the Lewis complex with CH3OH (TS-M_b,
center), and the optimized structure of [Me3NAlH4]+ (right) (hydrogens of NMe3 removed for clarity).
observed only in the case of the additionally heated solution).
The reaction that is taking place involves nucleophilic attack
from the X lone pair and Al-X bond formation accompanied
with H2 formation and release:
difference being in the orientation of the η2-H2 ligand: while
in the pure Me3NAlH4+ moiety (Figure 8c) both hydrogens
are equidistant to the other hydride ligands (rotation angle
is 0°), in TSs the H2 moiety is rotated (85° in TS-M_a and
45° in TS-TFM). The shortening of O · · · Al distances with
respect to the intermediate complexes indicates that nucleo-
philic attack is concomitant with proton transfer. Thus, proton
transfer, H2-elimination, and formation of alkoxo derivatives
take place in a single step as it was shown for anionic
(Me3N)AlH3 + HX f (Me3N)H2AlX + H2
(5)
This reaction may play a key role in the formation of stable
hydrogen containers, which would be stable at ambient
conditions with H2 evolution occurring only after applying
additional actions.
AlH4 .23 The dual nature of aluminum hydrides having both
-
Lewis acid and basic sites promotes such behavior.
Proton transfer from CF3OH goes only by pathway a
because no Lewis complex was found in this system. The
increase of the proton donor strength leads to the transition
state (TS-TFM), showing a more advanced H-H formation
(shorter H-H distance) and a more incipient O-Al attack
(longer Al-O distance) (Figure 9). The transition state for
proton transfer from NH acid (pyrrole) to TMAA is similar
to theTS-M_a except there is a weaker N-Al interaction
and less advanced H2 formation.
The mechanism of the proton-transfer reaction has been
investigated by DFT calculations. The interaction of TMAA
with methanol and TFE gives rise to two different adducts
(DHB and Lewis complexes, Figure 4), which can be taken
as initial intermediates for reaction 5. Two pathways, one
(pathway a) starting from the DHB complex (1 · M_a or
1 · TFE_a) and the second (pathway b) starting from the
Lewis complex (1 · M_b or 1 · TFE_b), have been explored.
Two transition states, one for the dihydrogen bond pathway
and the other for the Lewis complex pathway, have been
found for the proton transfer with methanol and TFE.
Transition state geometries for the reaction with methanol
(TS-M_a and TS-M_b) are depicted in Figure 8. TS
structures for the reaction with TFE are similar and can be
found in the Supporting Information. Despite the different
interaction governing the formation of dihydrogen bond and
Lewis intermediates, transition states in pathway b are similar
to those in pathway a, and both can be described as
dihydrogen complexes. The H2 and OCH3 ligands are located
in the same manner in both TS-M_a and TS-M_b, except
their relative positions with respect to the NMe3 ligand: OR
is located opposite to this group in TS-M_a and adjacent in
TS-M_b, and the reverse happens for H2. Obviously this is
the result of initial attack position of the alcohol in the
intermediates (see Figure 4).
The relative energies of all the species associated with the
reaction profile are presented in Table 7 with the energy of
the separated Me3NAlH3 + XH taken as a zero of energies.
The reaction is strongly exothermic for all the systems with
the great energy of stabilization of corresponding organyl
oxo- or azo- derivatives playing a main role as a driving
23
-
force in the proton transfer, as it was for anionic AlH4 .
-
Comparing the reaction of CH3OH with TMAA and AlH4 ,
it can be seen that despite the lower proton-accepting ability
and weaker DHB complex of TMAA (∆E complex ) -5.6
-
and -10.7 kcal ·mol-1 for TMAA and AlH4 , respectively),
the energy barrier is considerably lower for TMAA (17.4
-
kcal · mol-1) than for AlH4 (27.3 kcal · mol-1). Comparing
the geometries of both transition states, this decrease can be
attributed to the stronger Al-O interaction in the TMAA
system. Although most of the distances are similar in both
TSs, the Al-O is considerably shorter for TMAA (2.017
The short H-H distances in the transition states allow
describing them as dihydrogen complexes (Figure 8) similar
Å) than for AlH4 (2.232 Å).23 Whereas proton transfer is
23
-
-
to TSs of proton transfer to AlH4 . In order to verify the
η2-H2 nature of these structures, we have optimized the
isolated cation [(Me3N)AlH4]+. As for [AlH5] theoretically
characterized as AlH3(η2-H2),23 the optimized geometry of
this cation can be described as [(Me3N)AlH2(η2-H2)]+ with
a H2 unit weakly interacting with [(Me3N)AlH2]+ (Figure
8c). The same arrangement is found in the TSs, the main
easy for the anionic hydride, nucleophilic attack is favored
in the neutral hydride, resulting in an energy barrier decrease.
The TMAA alkoxo product is also more stable (-27.5 and
-24.0 kcal/mol, respectively, Table 7). Pathway b from the
Lewis complex has a somewhat lower barrier (14.3
kcal ·mol-1) and leads to the same product. The same trends
Inorganic Chemistry, Vol. 48, No. 8, 2009 3675

Filippov et al.
Figure 9. Transition states for proton transfer to TMAA from CF3OH (TS-TFM) and from pyrrole (TS-Pyrr) (hydrogen atoms of NMe3 removed for
clarity).
Table 7. Relative Energies (in kcal·mol-1) for Complexes, TSs, and
interacting also with the hydride. The second possibility
arises from computational evidence of solvent assistance in
Products of the Reaction of Aluminum Hydrides with Proton Donorsa
∆E
∆E
∆E
proton-transfer reactions in systems containing both donor
and acceptor sites with solvent molecules acting as bifunc-
tional catalysts.39 In particular, it has been reported that
ethanol molecules can play such a role.40
Optimization of the TMAA-(CH3OH)2 systems affords
the two complexes depicted in Figure 10. When the second
CH3OH molecule is only interacting with the first one, as in
a normal methanol dimer situation (1 · 2M_a, Figure 10a),
strengthening of the DHB complex occurs (H · · ·H distance
shortens by 0.071 Å and interaction energy increases by 7.4
kcal ·mol-1). Although the increase in the interaction energy
is mainly due to the additional R-OH · · · OHR hydrogen
bond, there is also a cooperative effect. Energetically the
c
type of complex
complexb TSb productb ∆E#
TMAA-CH3OH (pathway a)
AlH4--CH3OHd
TMAA-CH3OH (pathway b)
TMAA-TFE (pathway a)
AlH4--TFEd
-5.6
-10.7
-4.8
11.8 -27.5 17.4
16.6 -24.0 27.3
9.5 -27.5 14.3
12.9 -30.8 18.6
5.5 -33.8 22.5
9.0 -30.8 11.8
7.6 -38.7 15.8
-5.7
-17.0
-2.8
-8.2
-21.8
-3.4
-13.0
-14.3
TMAA-TFE (pathway b)
TMAA-CF3OH
AlH4--CF3OHd
-16.5 -48.1
5.3
TMAA-Pyrrole
26.6 -19.2 30.0
6.6 -35.6 19.6
-2.0 -35.6 12.3
TMAA-(CH3OH)2 (via 1·2M_a)
TMAA-(CH3OH)2 (via 1·2M_b)
a ∆E are the electronic energies, including zero-point vibrational
correction. b The energy of separated R3AlH + XH is taken as a zero of
energies. c Energy barriers (energy of TS respective to the corresponding
complex). d From ref 23.
cooperative effect can be expressed as ∆Ecoop
)
are found for the reaction with TFE: pathway b barrier (11.8
{∆E(1·2M_a) - [∆E(1·M_a) + ∆E(CH3OH · · ·OHCH3)]},
and its value ∆Ecoop ) - 1.6 kcal · mol-1 is lower than that
obtained for the stronger ruthenium dihydride (PP3)RuH2 (Ej
) 1.33).41 In addition to the traditional structure of a H-bond
complex with a methanol dimer, a second minimum was
found with the formation of a six-membered ring via three
new bonds (classical H-bonding between methanol mol-
ecules, dihydrogen bond OH· · · HAl and Lewis interaction
Al-O) (1 · 2M_b, Figure 10b). Formation of this ring
complex is energetically more favorable than 1 · 2M_a by
1.3 kcal · mol-1, cooperative effect doubling from 1 · 2M_a
to 1 · 2M_b (Table 7). Namely both types of complexes can
be intermediates of proton transfer from weak OH acids to
TMAA in the presence of an alcohol excess. Alcoholysis
reaction starting from both intermediates has been explored.
kcal ·mol-1) is lower than pathway a (18.6 kcal ·mol-1), and
-
both are lower than that for AlH4 (22.5 kcal · mol-1). On
-
the contrary for CF3OH, the barrier for AlH4 (5.3
kcal ·mol-1) is considerably lower than that for TMAA (15.8
kcal· mol-1), proving that this reaction is governed by the
basic strength of the hydride proton acceptor. Calculated
energetic characteristics of the proton transfer from pyrrole
to TMAA explain different behavior of OH and NH acids
in the experiment. Namely, the higher transition-state energy
in the case of pyrrole can explain the observation of the
proton-transfer process only at higher temperatures in
comparison to OH proton donors.
It has been shown that proton-transfer reactions with weak
Brønsted acids as alcohols are favored by the presence of a
considerable excess of alcohol.37 A key factor for explaining
this fact is the stabilization of the alkoxide anion product by
strong H-bonding with the excess of alcohol, forming
[RO · · · HOR]- homoconjugate pairs.7b,31 Although in our
systems the alkoxide anion is bonded to the aluminum in
the products, we have also studied the possibility of
cooperative effects by including in the calculations a second
alcohol molecule.38 Two possibilities have been considered
for the initial placement of this additional CH3OH molecule:
(a) solvating only the first methanol molecule and (b)
(38) (a) Jeffrey, G. A.; Saender, W. Hydrogen Bonding in Biological
Structures; Springer-Verlag: Berlin, Germany, 1991, chapter 2.6. (b)
Karpfen, A. AdV. Chem. Phys. 2002, 123, 469–510.
(39) (a) Lledo´s, A.; Bertra´n, J. Tetrahedron Lett. 1981, 22, 775–778. (b)
Bergquist, C.; Bridgewater, B. M.; Harlan, C. J.; Norton, J. R.; Friesner,
R. A.; Parkin, G. J. Am. Chem. Soc. 2000, 122, 10581. (c) Prabhakar,
R.; Blomberg, M. R. A.; Siegbahn, P. E. M. Theor. Chem. Acc. 2000,
104, 461–464. (d) Jee, J.-E.; Comas-Vives, A.; Dinoi, C.; Ujaque,
G.; van Eldik, R.; Lledo´s, A.; Poli, R. Inorg. Chem. 2007, 46, 4103–
4113.
(40) Comas-Vives, A.; Gonza´lez-Arellano, C.; Corma, A.; Iglesias, M.;
Sa´nchez, F.; Ujaque, G. J. Am. Chem. Soc. 2006, 128, 4756–4765.
(41) Gutsul, E. I.; Belkova, N. V.; Sverdlov, M. S.; Epstein, L. M.; Shubina,
E. S.; Bakhmutov, V. I.; Gribanova, T. N.; Minyaev, R. M.; Bianchini,
C.; Peruzzini, M.; Zanobini, F. Chem. Eur. J. 2003, 9, 2219–2228.
(37) Belkova, N. V.; Besora, M.; Epstein, L. M.; Lledo´s, A.; Maseras, F.;
Shubina, E. S. J. Am. Chem. Soc. 2003, 125, 7715–7725.
3676 Inorganic Chemistry, Vol. 48, No. 8, 2009

Proton-Transfer and H2-Elimination in AlH3(NMe3)
Figure 10. Complexes of TMAA with two methanol molecules: cooperative DHB complex 1·2M_a (left) and ring complex 1·2M_b (right) (hydrogens
of NMe3 removed for clarity).
Figure 11. Transitions state for proton transfer to TMAA involving two CH3OH molecules: TS-2M_a from cooperative DHB complex (left) and TS-2M_b
from ring complex (right) (hydrogens of NMe3 removed for clarity).
While proton transfer from methanol in 1·2M_a proceeds
(through transition state TS-2M_a, Figure 11a) in the same
way as to 1 · M_a, starting from the ring complex 1 · 2M_b
the proton is transferred by a different way: the new Al-O
bond is formed with the ROH molecule initially interacting
with the Al atom, while a proton from the initially dihydrogen
bond CH3OH molecule goes to the hydride to form the
dihydrogen molecule. The second methanol molecule is
acting as a bifunctional catalyst, transferring a proton to the
hydride and accepting a proton from the Al-bonded methanol
molecule. All of these phenomena take place in one step
through a six-membered transition state (TS_2M_b, Figure
11b). Whereas the energy barrier associated with the first
type of solvent assistance in TS-2M_a is slightly higher than
in TS-1_a (19.6 and 17.4 kcal · mol-1, respectively), the
barrier in the six-membered ring mechanism is notably
reduced (12.3 kcal · mol-1). Participation of the second
CH3OH molecule also favors the exothermicity of the
reaction, product stabilization energy increasing by 8.1
kcal· mol-1.
Conclusions
It has been shown from variable-temperature IR investiga-
tion that TMAA forms DHB AlH-HX complexes (X ) C,
N, and O) in solution with a number of XH acids. The
spectroscopic features, strength and TMAA basicity factor
(Ej), structural parameters, as well as electronic and energetic
characteristics of these complexes were determined by
combining experimental and theoretical approaches. The
existence of both monodentate and bidentate chelate com-
plexes of TMAA with bidentate NH2 acids was disclosed
the first time in solution. The complex with typical mono-
dentate coordination was experimentally found in the system
TMAA-PFA, whereas in the system TMAA-TeCA the
complex with bidentate coordination was spectroscopically
characterized. DHB complexes are intermediates in the
proton-transfer reaction, which entails formation of an Al-X
bond and H2-elimination. The extent of this reaction is very
dependent on the acid strength: proton transfer to TMAA
from OH acids was observed at temperature >200 K and
from NH acids >250 K, and only DHB complexes of TMAA
Inorganic Chemistry, Vol. 48, No. 8, 2009 3677

Filippov et al.
with CH acids exist at room temperature without proton
transfer. The reaction that is taking place involves, in addition
to the proton transfer, nucleophilic attack from the X lone
pair to the Al center. We have found that the dual acid-base
nature of neutral aluminum hydrides having both a Lewis
acid site (Al) and a basic site (hydrides) allows, besides the
dihydrogen bond pathway for proton transfer, another
pathway starting from a Lewis complex to be at work.
Preference for one of the pathways is related to the acid
strength and nucleophilicity of the proton donor. Increasing
of OH acid proton-donating ability results in proton transfer
via the dihydrogen bond intermediate. Calculations have
shown that increasing the proton-donor concentration allows
the reaction to take place with a new mechanism, starting
from a six-membered intermediate with a second R-OH
molecule interacting with both a hydride and the first R-OH
molecule. The second molecule acts as bifunctional catalyst
accepting and transferring a proton and decreasing the energy
barrier. This result points out that these types of complexes,
involving two proton donor molecules, could be intermedi-
ates of a proton transfer from weak OH acids to the TMAA
in the presence of an excess of proton donor. In conclusion,
the combined experimental and theoretical study of TMAA
complexes with weak proton donors allows understanding
the mechanism of proton-transfer and H2-elimination reac-
tions of aluminum hydride and gives some hints on the
factors that could control proton transfer to neutral aluminum
hydrides in solution.
Acknowledgment. This work was supported by the
Russian Foundation for Basic Research (No. 07-03-00739),
Russian Federation President Grant (MK-380.2008.3), IN-
TAS YSF (06-1000014-5809), and Spanish MICINN (Project
CTQ2008-06866-C02-01/BQU). The use of the computa-
tional facilities of the Centre de Supercomputacio´ de Cata-
lunya is greatly appreciated.
Supporting Information Available: Tables of optimized ge-
ometries (Cartesian coordinates) for all the calculated species and
the molecular graph of TMMA-PFA bidentate cycle complex. This
material is available free of charge via the Internet at http://pubs.
acs.org.
IC802262H
3678 Inorganic Chemistry, Vol. 48, No. 8, 2009