Aluminum-ligand cooperative N-H bond activation and an example of dehydrogenative coupling

Article abstract of DOI:10.1021/ja4032874

Activation of N-H bonds by a molecular aluminum complex via metal-ligand cooperation is described. (^PhI₂P^2-)AlH (1b), in which ^PhI₂P^2- is a tridentate bis(imino)pyridine ligand, reacts with anilines to give the N-H-activated products (^PhHI₂P^-)AlH(NHAr) (2). When heated, 2 releases H₂ and affords (^PhI₂P ^-)Al(NHAr) (3). Complex 1b catalyzes the dehydrogenative coupling of benzylamine to afford H₂, NH₃, and N-(phenylmethylene) benzenemethanamine.

Full text of DOI:10.1021/ja4032874

Communication
pubs.acs.org/JACS
Aluminum−Ligand Cooperative N−H Bond Activation and an
Example of Dehydrogenative Coupling
Thomas W. Myers and Louise A. Berben*
Department of Chemistry, University of California, Davis, California 95616, United States
S
* Supporting Information
Scheme 1
ABSTRACT: Activation of N−H bonds by a molecular
aluminum complex via metal−ligand cooperation is
described. (PhI2P2−)AlH (1b), in which PhI2P2− is a
tridentate bis(imino)pyridine ligand, reacts with anilines
to give the N−H-activated products (PhHI2P−)AlH-
(NHAr) (2). When heated, 2 releases H2 and affords
(PhI2P−)Al(NHAr) (3). Complex 1b catalyzes the
dehydrogenative coupling of benzylamine to afford H2,
NH3, and N-(phenylmethylene)benzenemethanamine.
(PhI2P2−)Al(NHAr) [Ar = Dipp (3a), Ph (3b)]. The syntheses
of the aluminum hydride complexes followed a procedure
adapted from our previous work on the related iminopyridine
ligand system (Scheme 1).9 The PhI2P ligand and 2 equiv of
sodium metal were stirred together in THF for 24 h, during
which time the initial green solution became dark purple. After
addition of an “AlCl2H” solution10 and subsequent workup, 1a
was isolated as a burgundy powder in 71% yield. Complex 1a
was found to be diamagnetic, and the proton NMR spectrum
was informative (Figure S1 in the Supporting Information). A
broad feature at 4.60 ppm corresponded to the Al−H proton,
and the resonances for the THF ligand were observed as broad
singlets at 3.59 and 1.37 ppm. The IR spectrum displayed an
Al−H stretching band at 1966 cm−1 (Figure S2 and Table S1),
which is within the range for other known terminal aluminum
hydride complexes.11
o develop sustainable catalytic transformations using
Tabundant metals such as aluminum, an understanding of
bond activation reactions is a necessary first step. Introducing
functionality into unactivated bonds continues to be of
significant interest, and in particular, the activation of N−H
bonds often relies on the oxidative addition of N−H bonds to a
transition-metal center.1 However, the activation of N−H
bonds via oxidative addition to transition metals can be
impeded by the tendency of the Lewis basic nitrogen atom to
bind open coordination sites.2 In contrast, heterolytic activation
of N−H bonds is more likely to remain feasible even when
amine coordination occurs as a first step.2
A heterolytic reaction pathway has led to limited but fruitful
examples of N−H activation in both transition-metal and main-
group chemistry. Milstein and co-workers reported the
heterolytic activation of amines by Ru pincer complexes via a
metal−ligand cooperative pathway.3 Subsequently the com-
plexes were shown to effect catalytic amine dehydrogenation.4
Heterolytic activation of N−H bonds has also been
accomplished via main-group frustrated Lewis pairs to achieve
catalytic hydrogenation of unsaturated systems.5 To date, main-
group-mediated reactions have afforded stoichiometric N−H
activation of aniline and ammonia6 and both stoichiometric and
catalytic dehydrogenation of amine boranes.7 Heterolytic
activation of alkynes has been observed at gallium.8
Single crystals of 1a were grown via slow diffusion of pentane
into a concentrated THF solution of 1a, and the structure was
determined by single-crystal X-ray diffraction (Figure 1). The
geometry at the Al center is very close to square-pyramidal,
We previously explored the redox chemistry of iminopyridine
complexes of aluminum.9 Herein we report aluminum
complexes of a tridentate bis(imino)pyridine ligand that is
henceforth denoted by PhI2P (see Scheme 1 for its structure).
We isolated the tetrahydrofuran (THF)-adduct and base-free
aluminum hydride complexes (PhI2P2−)AlH(THF) (1a) and
(PhI2P2−)AlH (1b) and found that 1b effects activation of the
N−H bonds in anilines via a metal−ligand cooperative
pathway. These reactions afford (PhHI2P−)AlH(NHAr) [Ar =
2,6-diisopropylphenyl (Dipp) (2a), Ph (2b)]. When these
complexes are heated, they release H2, forming four-coordinate
Figure 1. Solid-state structure of (PhI2P2−)AlH(THF) (1a). Pink, red,
blue, gray, and white ellipsoids represent Al, O, N, C, and H atoms,
respectively. The thermal ellipsoids have been set at 50% probability,
and H atoms (except hydride) have been omitted.
Received: April 2, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja4032874 | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
with τ = 0.117.12 The bond lengths in 1a are consistent with a
twice-reduced oxidation state of the ligand, where the two-
electron reduction event is formally localized at the pyridine
nitrogen and one of the imine nitrogens (Tables S2 and S3).9,13
Thus, the ligand is asymmetric with one formally imine N-
demand of the NHDipp− group, the reaction of 1b with 1 equiv
of aniline was investigated, and this led to formation of an
analogous dark-blue product identified as (PhHI2P−)AlH-
1
(NHPh) (2b) by H NMR spectroscopy. In both 2a and 2b,
the amido nitrogen atom of PhI2P2− is protonated and the
resulting anilido ligand is coordinated to the Al center.
Complexes 2a and 2b were found to be diamagnetic, and
donor [Al−Nim = 2.077(2) Å], an anionic pyridine [Al−Npy
1.851(2) Å], and one anionic amido N-donor [Al−Nam
=
=
1
1.954(2) Å]. The C−C bond distances in the pyridine ring
alternate between long [1.405(3) Å] and short [1.380(3) Å],
consistent with a loss of aromaticity. These results are also in
agreement with the diamagnetism observed by NMR spectros-
copy.
their H NMR spectra clearly showed that protonation of the
amido nitrogen in PhI2P2− induced a higher degree of
asymmetry to the ligand than was observed in 1a or 1b
(Figures S4 and S5). For instance, all of the protons on the
pyridine ring became inequivalent. The single-crystal X-ray
structures for 2a and 2b confirmed the higher degree of
asymmetry of the protonated PhHI2P− ligand in comparison
with PhI2P2− in 1a. For example, the alternating single and
double bonds in the dearomatized pyridine ring of PhHI2P−
were even more distinct than they were in 1a (Tables S2 and
S3).
Extraction of 1a into benzene followed by evaporation of the
solvent in vacuo led to removal of THF from the Al
coordination sphere and the formation of the tan-colored
four-coordinate complex 1b, as identified by 1H NMR
spectroscopy. The 1H NMR spectrum indicated that the
oxidation state of the ligand remained unchanged at 2−, that
the Al−hydride resonance had shifted downfield to 4.74 ppm,
and that the THF ligand resonances were no longer apparent
(Figure S3). The IR absorption band for Al−H also shifted by
185 cm−1 to 1781 cm−1 (Figure S2). We could not obtain
single crystals of 1b, but all of the data we did acquire, including
the results of combustion analysis, were consistent with the
assignment of 1b as four-coordinate (PhI2P2−)AlH. The
When either 2a or 2b was heated at 70 °C for several hours
in toluene, 1 equiv of H2 was released, as measured by GC-
TCD analysis. The formation of the corresponding new red-
brown four-coordinate complexes (PhI2P2−)Al(NHAr) [Ar =
Dipp (3a), Ph (3b)] was concomitantly observed. Both 3a and
3b are diamagnetic, and their proton NMR spectra were similar
to that of four-coordinate 1b (Figures S6 and S7). To probe the
mechanism of formation of 2b and its conversion to 3b, aniline-
1
electronic structure of the PhI2P2− ligand, according to the H
NMR spectrum, is consistent with that of 3a (vide infra). When
1b was dissolved in THF, 1a was observed in quantitative NMR
yield.
1
d7 was employed. The H NMR spectrum of 2b-d7 showed
diminished intensity for the two N−H resonances and no
change in the Al−H resonance (Figure S8). When 2b-d7 was
heated, HD gas was observed (Figure S9), and the spectrum of
3b-d6 showed a decrease in the intensity of the N−H resonance
(Figure S10). These results are in accord with Scheme 2.
Single crystals of 3a were grown by chilling a concentrated
hexane solution of 3a at −25 °C for 3 days, and the X-ray
crystal structure was determined (Figure 2 right). Complex 3a
is four-coordinate and best described as distorted tetrahedral,
with a tetrahedrality of 51.6°.14 Notably, Npy no longer lies in
plane with the rest of PhI2P2−: the Nim−Cim−Cpy(o)−Npy torsion
angles are 5.2° and 8.1°, as compared to 1.5° and 1.87° in 1a.
This deviation presumably stems from the preference of the Al
center for a tetrahedral geometry, which conflicts with the
preference of PhI2P2− to be planar. The displacement of Npy
from the plane removes it from the conjugated π system of the
rest of the ligand, affording a symmetric conjugated system
involving the Nim, Cim, and Cpy atoms (Scheme 2 and Figure 2
right). The two Cim−Nim bond lengths are very similar at
1.374(3) and 1.370(3) Å, and this distance falls between that
commonly observed for true Cim−Nim single and double bonds,
as in the asymmetric closed-shell form of PhI2P2−. The two
Cim−Cpy(o) bond lengths are also very similar at 1.424(3) and
1.426(3) Å, and the C−C bonds in the pyridine ring [1.394(3),
1.397(3), 1.399(3), and 1.383(3) Å] are equivalent and longer
than the bonds in neutral pyridine. The symmetry of this
geometric structure suggests an electronic structure similar to
the biradical triplet state commonly observed with bis(imino)-
pyridine ligands, in which a radical resides on each of the Cim
atoms.13 However, the bond lengths and angles in 3a are more
consistent with a structure in which these radicals are somewhat
delocalized and, as indicated by NMR spectroscopy, anti-
ferromagnetically coupled. Antiferromagnetic coupling in a
two-electron-reduced bis(imino)pyridine ligand was observed
previously in a nickel(II) complex.15 Notably, in that case the
Addition of H2NDipp to a tan benzene solution of 1b led to
the immediate formation of a dark-blue solution (Scheme 2).
Scheme 2
X-ray diffraction analysis performed on a single crystal grown
from the product revealed it to be (PhHI2P−)AlH(NHDipp)
(2a), in which the N−H bond of H2NDipp was added across
the aluminum−amido bond in 1b (Figure 2 left). To probe
whether 2a might have been the result of the large steric
Figure 2. Solid-state structures of (PhHI2P−)AlH(NHDipp) (2a) and
(PhI2P2−)Al(NHDipp) (3a). Pink, blue, gray, and white ellipsoids
represent Al, N, C, and H atoms, respectively. The thermal ellipsoids
have been set at 50% probability, and H atoms (except the hydride and
N-bound protons) have been omitted.
B
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Journal of the American Chemical Society
Communication
M. S.; Melen, R. L.; Rawson, J. M.; Wright, D. S. Chem. Commun.
2011, 47, 2682.
I2P ligand was also distorted from planarity with Nim−Cim−
Cpy(o)−Npy torsion angles of 11.87° and 11.45°.
(8) (a) Fedushkin, I. L.; Khvoinova, N. M.; Skatova, A. A.; Fukin, G.
K. Angew. Chem., Int. Ed. 2003, 42, 5223. (b) Fedushkin, I. L.;
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Schumann, H. Eur. J. Inorg. Chem. 2005, 2332.
(9) (a) Myers, T. W.; Kazem, N.; Stoll, S.; Britt, R. D.; Shanmugam,
M.; Berben, L. A. J. Am. Chem. Soc. 2011, 133, 8662. (b) Myers, T. W.;
Berben, L. A. J. Am. Chem. Soc. 2011, 133, 11865. (c) Myers, T. W.;
Berben, L. A. Inorg. Chem. 2012, 51, 8997.
The ability of 1b to activate N−H bonds via a metal−ligand
cooperative mechanism prompted us to investigate its potential
as an amine dehydrogenation catalyst. Benzylamine and 20 mol
% 1b was heated at reflux in toluene at 70 °C for 24 h, and the
formation of N-(phenylmethylene)benzenemethanamine was
observed with 75% conversion and 3.6 turnovers, as measured
by GC−MS (Figure S11).16 The formation of stoichiometric
H2 and NH3 was observed by GC-TCD (Figure S12).
In conclusion, we have shown that a bis(imino)pyridine
complex of aluminum activates N−H bonds via a metal−ligand
cooperative mechanism and that this N−H bond activation
event can initiate amine dehydrogenation catalysis and convert
benzylamine into the homocoupled imine. Efforts to expand
the scope of the bond activation and catalysis observed here are
underway.
(10) Formed in situ from LiAlH4 + 3AlCl3 in THF.
(11) For example, see: (a) Kuo, P. C.; Chem, I. C.; Chang, J. C.; Lee,
M. T.; Hu, C. H.; Hung, C. H.; Lee, H. M.; Huang, J. H. Eur. J. Inorg.
Chem. 2004, 4898. (b) Alexander, S. G.; Cole, M. L.; Forsyth, C. M.;
Furfari, S. K.; Konstas, K. Dalton Trans. 2009, 2326.
(12) Addison, A. W.; Rao, T. N.; Van Rijn, J. J.; Verschoor, G. C. J.
Chem. Soc., Dalton Trans. 1984, 1349.
(13) Bowman, A. C.; Milsmann, C.; Atienza, C. C. H.; Lobkovsky, E.;
Wieghardt, K.; Chirik, P. J. J. Am. Chem. Soc. 2010, 132, 1676.
(14) Battaglia, L. P.; Corradi, A. B.; Marcotrigiano, G.; Menabue, L.;
Pellacani, G. C. Inorg. Chem. 1979, 18, 148.
ASSOCIATED CONTENT
■
S
* Supporting Information
(15) Ghosh, M.; Weyhermuller, T.; Wieghardt, K. Dalton Trans.
̈
Preparation of complexes; X-ray data for 1a, 2a, and 3a; CIF
files; IR and NMR spectra; and GC-TCD and GC−MS data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
2010, 39, 1996.
(16) This is comparable to turnover numbers achieved using main-
group amine borane dehydrogenation catalysts.7
AUTHOR INFORMATION
■
Corresponding Author
laberben@ucdavis.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Alfred P. Sloan Foundation for support of this
work.
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