Yttrium-catalysed dehydrocoupling of alanes with amines

Article abstract of DOI:10.1039/c4cc04484c

We report [Y{N(SiMe₃)₂}₃] as a precatalyst for the dehydrocoupling of sterically demanding amines with β-diketiminate stabilised aluminium dihydrides. While simple fluorinated anilines readily undergo Al-H/N-H dehydrocoupling under thermal conditions, catalytic methods are required to achieve reasonable rates of reaction for ortho-substituted anilines or hindered aliphatic amines. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c4cc04484c

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Yttrium-catalysed dehydrocoupling of alanes
with amines†
Cite this: Chem. Commun., 2014,
50, 9536
Adi E. Nako, Sarah J. Gates, Nicole Schadel, Andrew J. P. White and
¨
Mark R. Crimmin*
Received 12th June 2014,
Accepted 5th July 2014
DOI: 10.1039/c4cc04484c
www.rsc.org/chemcomm
We report [Y{N(SiMe3)2}3] as a precatalyst for the dehydrocoupling of
sterically demanding amines with b-diketiminate stabilised aluminium
dihydrides. While simple fluorinated anilines readily undergo Al–H/
N–H dehydrocoupling under thermal conditions, catalytic methods
are required to achieve reasonable rates of reaction for ortho-
substituted anilines or hindered aliphatic amines.
Fig. 1 The dehydrogenation of amineboranes and the dehydrocoupling
of amines with silanes.
The drive to control the rate of hydrogen release from amine–
borane has given rise to a number of mechanistically diverse
catalyst systems capable of the dehydrocoupling of protic and
hydridic substrates.1 While new catalysts for amine–borane
(or phosphine–borane) dehydrogenation continue to attract
attention,2 in the past few years related methods for heteroatom–
heteroatom bond formation by dehydrocoupling have emerged
(Fig. 1).3 For example, the catalytic reaction of amines with silanes
provides an atom-efficient route to form nitrogen–silicon bonds
and incorporate silyl-protecting groups into organic molecules.4
Although a number of precatalysts have been developed that are
capable of catalysing the release of multiple equivalents of H2 from
H3NꢀBH3,1 the formation of unsaturated products or intermediates,
such as silylimines [R2SiQNR], from the dehydrocoupling of
R2SiH2 and H2NR are rare.5
to address the issue of selectivity while vastly increasing reaction
rates, to the best of our knowledge catalysts for this reaction
remain unreported. Herein we report our initial findings on the
thermal and catalytic dehydrocoupling of amines with sterically
demanding alanes.
Thermal dehydrocoupling of anilines with alanes: The reaction
of 4-fluoroaniline with 1a–b in toluene gave the corresponding
mono-substitution products in good yield within 2 h at 25 1C.
Variation of the substrate to 2,4,6-trifluoroaniline had little effect
on the efficiency of the reaction with 1a. The corresponding
aluminium amides 2a–c were isolated in 62–68% yield following
preparative scale experiments. The variation in the isolated yields
is more likely a consequence of the conditions of crystallisation
rather than the efficiency of thermal dehydrocoupling (Table 1).
Amine-adducts of AlH3 have been proposed as hydrogen
storage materials, and aluminium amides themselves may act
as single source precursors for the chemical vapour deposition
of aluminium nitride.6,7 Although the dehydrocoupling (or
protonolysis) of aluminium hydrides with amines is a common
method to make Al–N bonds, these reactions can suffer from slow
reaction rates and often salt-metathesis methods are preferred.8
For substrates containing more than one Al–H bond achieving a
selective reaction is also a potential problem. Catalytic approaches
to the dehydrocoupling of amines and alanes offer an opportunity
Table 1 The thermal dehydrocoupling of sterically demanding alanes
with anilines
Department of Chemistry, Imperial College London, South Kensington,
London SW7 2AZ, UK. E-mail: m.crimmin@imperial.ac.uk; Tel: +44 (0)2075942846
† Electronic supplementary information (ESI) available: Including full experi-
mental procedures, X-ray crystallography data and details of calculations. CCDC
1007407–1007409. For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/c4cc04484c
a
Isolated yields.
9536 | Chem. Commun., 2014, 50, 9536--9538
This journal is ©The Royal Society of Chemistry 2014

View Article Online
Communication
ChemComm
Catalytic dehydrocoupling: Attempts to extend the scope of
the thermal reaction to hindered aliphatic amines (pKa B 40–45)
or ortho-substituted anilines (pKa B 30) failed to proceed selectively
at useful reaction rates. For example, the reaction of an excess of
tert-butylamine with 1a in C6D6 (0.1–0.2 M in 1a) only reached
completion after 12 d at 80 1C.
Scheme 1 The thermal dehydrocoupling of a 5-coordinate alane with
p-fluoroaniline.
In contrast, at 25 1C with addition of 5 mol% [Y{N(SiMe3)2}3]
vigorous gas evolution was observed. Monitoring the latter reaction
1
by H NMR spectroscopy revealed not only the formation of H2
(d = 4.49 ppm) but complete and selective formation of 4d within
30 minutes at 25 1C. The catalyst loading could be lowered to
0.5 mol% without a detrimental effect on the yield or reaction time.
These data allow a coarse estimation of a minimum catalyst TOF
of 500 hꢁ1 for this reaction; a value that compares favourably to
that of 41000 hꢁ1 at 50 1C for boratabenzene yttrium alkyl
catalysts employed for the dehydrogenation of dimethylamine–
borane.10c Scale up of this reaction in toluene gave the product in
a 64% isolated yield.
The catalytic dehydrocoupling of sterically demanding alanes can
be generalised to a number of ortho-substituted anilines (Table 2,
4a–c, 4g) primary amines (Table 2, 4d–e) and a secondary amine
(Table 2, 4f).‡ It is noteworthy that rare-earth bis(trimethylsilyl)amides
have proven efficient catalysts for amine–silane dehydrocoupling,
amine–borane dehydrogenation, and the dehydrocoupling of tri-
phenylphosphonium methylide with phenylsilane.4,10,11 Further-
more, aluminium hydrides themselves have recently emerged as
efficient catalysts for amine–borane dehydrocoupling.12
In all cases, thermal control reactions show either trace or no
conversion under the same conditions and require both elevated
temperatures and extended reaction times to reach similar yields as
the catalytic experiments (Table 2). For example, complex 4g has
been previously reported by H. Roesky and co-workers and may be
synthesized by the thermolysis of 1b in neat 2,6-di-iso-propylaniline
at 150 1C for 1 h. Conducting this reaction in a flame-sealed NMR
tube in toluene-d8 gave only trace conversion after 1 day at 150 1C.13
With 5 mol% [Y{N(SiMe3)2}3] the reaction proceeds at 25 1C giving
4g in 87% isolated yield, albeit with a longer reaction time of 15 h
(Table 2, 4g). Under these catalytic conditions, hindered 21 amines
The reaction scope was extended to include the 5-coordinate
aluminium dihydride, 1d (Scheme 1).9 Reaction with 2 equiv. of
4-fluoroaniline proceeds rapidly at 25 1C to yield 3. The reaction is
not selective and attempts to modify the conditions to isolate the
hydrido/amide intermediate failed. Although we have previously
demonstrated that 1d remains 5-coordinate in solution,9 in the
current case, we propose that the dissociation of the pendant amine
becomes favourable following the first Al–N bond formation. As a
result the rate of the second dehydrocoupling becomes comparable
with the first and the reaction is no longer selective.
1
In order to test this hypothesis, H–15N HMBC experiments
were conducted on C6D6 solutions of 1d, 3 and a reaction
mixture containing the hydrido/amide intermediate 30 at
298 K. 15N chemical shifts of d = 28.1, 29.8 and 22.4 ppm were
recorded for the NMe2 group of 1d, 30 and 3 respectively. While
these data are consistent with amine dissociation in 3, the
difference between the chemical shifts of the b-diketiminate
nitrogens Dd = 67.8, 59.2 and 4.2 ppm of 1d, 30 and 3
respectively, provides direct evidence for a dramatic change
in the electronic structure of the ligand. Hence, 1d and 30
possess a trigonal bipyramidal structure with the equatorial
and axial positions being magnetically and chemically distinct
while 3 possess a four-coordinate geometry. In combination
with an X-ray crystallography study (Fig. 2) these data show that
the pendant amine of 3 not coordinated to Al either in solution
or the solid-state. While 30 appears to remain 5-coordinate in
solution at 298 K, it can be expected that amine dissociation
will more facile than that of 1d and open up the coordination at Al,
favouring a second dehydrocoupling.
Fig. 2 The crystal structure of (a) 2a, (b) 4b (disordered toluene molecule omitted for clarity), and (c) 3. Selected bond angles (1) and bond lengths (Å):
2a, Al–N1 1.8893(11), Al–N13 1.81.32(16), N1–Al–N10 96.52; 4b, Al–N1 1.8951(16), Al–N3 1.9021(15), Al–N30 1.8182(16), N1–Al–N3 95.55(7);
3, Al–N1 1.9084(17), Al–N3 1.905(2), Al–N31 1.7998(19), Al–N41 1.805(19), N1–Al–N3 94.17(8).
This journal is ©The Royal Society of Chemistry 2014
Chem. Commun., 2014, 50, 9536--9538 | 9537

View Article Online
ChemComm
Communication
Table 2 [Y{N(SiMe3)2}3] catalysed dehydrocoupling of sterically demanding
alanes with ortho-substituted anilines and amines
catalysis, if a further catalytic dehydrogenation of the aluminium
hydrido/amide complexes reported herein could be achieved it may
generate a reactive 3-coordinate aluminium imido, BDIAl = NR.8b
This latter moiety has been proposed as an intermediate in intra-
molecular C–H activation and [2+2] cycloaddition reactions.14,15
Notes and references
‡ General experimental procedure for catalytic dehydrocoupling: In a
glovebox, the alane (0.33 mmol) and [Y{N(SiMe3)2}3] (0.016 mmol) were
weighed out into a 20 mL glass scintillation vial. Dry toluene or diethyl
ether (2 mL) was added followed by addition of the amine (0.33 mmol).
Vigorous effervescence was observed and the vial was sealed. The reaction
mixture was left for 0.5–15 h at 25 1C before the mixture was passed
through glass fibre filter paper and the solvent removed under reduced
pressure. The crude reaction mixture was then either recrystallized or
washed with cold hexane to give the pure product (48–89% yield).
1 (a) T. J. Clark, K. Lee and I. Manners, Chem. – Eur. J., 2006, 12, 8634;
(b) F. Gauvin, J. F. Harrod and H. G. Woo, Adv. Organomet. Chem.,
1998, 42, 363.
2 (a) M. C. Denney, V. Pons, T. J. Hebden, D. M. Heinekey and
K. I. Goldberg, J. Am. Chem. Soc., 2006, 128, 12048; (b) C. A. Jaska
and I. Manners, J. Am. Chem. Soc., 2004, 126, 9776.
3 (a) E. M. Leitao, T. Jurca and I. Manners, Nat. Chem., 2013, 5, 817;
(b) R. J. Less, R. L. Melen and D. S. Wright, RSC Adv., 2012, 2, 2191.
4 (a) W. Xie, H. Hu and C. Cui, Angew. Chem., Int. Ed., 2012, 51, 11141;
(b) M. S. Hill, D. J. Liptrot, D. J. MacDougall, M. F. Mahon and
T. P. Robinson, Chem. Sci., 2013, 4, 4212; (c) F. Buch and S. Harder,
Organometallics, 2007, 26, 5132; (d) J. F. Dunne, S. R. Neal,
J. Engelkemier, A. Ellern and A. Sadow, J. Am. Chem. Soc., 2011,
133, 16782.
5 Y. Chen, H. Song and C. Cui, Angew. Chem., Int. Ed., 2010, 49, 8958.
6 (a) D. J. Grant and D. A. Dixon, J. Phys. Chem. A, 2005, 109, 10138;
(b) B. M. Wong, D. Lacina, I. M. B. Nielsen, J. Graetz and
M. D. Allendorf, J. Phys. Chem. C, 2011, 115, 7778.
7 (a) F. C. Sauls and L. V. Interrante, Coord. Chem. Rev., 1993, 128, 193;
(b) Y. Saito, Y. Sugahara and K. Kuroda, J. Am. Ceram. Soc., 2000,
83, 2436; (c) J. Khanderi, D. Rische, H. W. Becker and R. A. Fischer,
J. Mater. Chem., 2004, 14, 3210; (d) U. Werner, Struct. Bonding, 2003,
105, 41.
a
b
Isolated yields. Data in parentheses are results from thermal back-
c
d
ground reactions. NMR scale yields. 2.5–5 equiv. of amine used.
e
4 : 1 mixture of mono- and bis-dehydrocoupling.
such as di-iso-propylamine and hexamethyldisilazane gave no
reaction with 1a despite prolonged heating at 80 1C.
The reaction products have been characterised by multi-
nuclear NMR and infrared spectroscopy and CHN analysis.
Complexes 2a–d and 4a–h display Al–H stretches in the region
of 1788–1875 cmꢁ1, while in some cases N–H stretches were
8 (a) A. Y. Timoshkin, Coord. Chem. Rev., 2005, 249, 2094–2131;
(b) J. Koller and R. G. Bergman, Organometallics, 2010, 29, 3350;
(c) A. Heine and D. Stalke, Angew. Chem., Int. Ed. Engl., 1992, 31, 854;
´
(d) M. L. Montero, H. Wessel, H. W. Roesky, M. Teichert and I. Uson,
Angew. Chem., Int. Ed. Engl., 1997, 36, 629.
9 A. E. Nako, S. J. Gates, A. J. P. White and M. R. Crimmin, Dalton
Trans., 2013, 42, 15199.
observed between 3344–3449 cmꢁ1 these bands were often weak 10 For amine-borane dehydrogenation see: (a) M. S. Hill, G. Kociok-
¨
Kohn and T. P. Robinson, Chem. Commun., 2010, 46, 7587; (b) P. Cui,
and broad. Multinuclear NMR data supports the assignment and
while the Al–H resonances are observed as extremely broad peaks
T. P. Spaniol, L. Maron and J. Okuda, Chem. – Eur. J., 2013, 19, 13437;
(c) E. Lu, Y. Yuan, Y. Chen and W. Xia, ACS Catal., 2013, 3, 521.
in the region of d = 3.0–5.0 ppm, the aliphatic N–H protons of 4d 11 A. E. Nako, A. J. P. White and M. R. Crimmin, Chem. Sci., 2013,
and 4e demonstrate a 3JH–H coupling to the Al–H and resonate at
4, 691.
12 (a) H. J. Cowley, M. S. Holt, R. L. Melen, J. M. Rawson and D. S. Wright,
d = 0.56 (d, J = 3.6 Hz) and 0.36 (d, J = 3.2 Hz) ppm respectively.
Chem. Commun., 2011, 47, 2682; (b) M. M. Hansmann, R. L. Melen and
Complexes 2a, 3 and 4b have been characterised by single crystal
X-ray diffraction and while the metrical parameters do not
warrant discussion it is noteworthy that the pendant amine of 3
is not coordinated to aluminium in the solid state.
In summary, we have reported [Y{N(SiMe3)2}3] as a precatalyst for
the dehydrocoupling of amines with sterically demanding alanes.
Catalysis proceeds rapidly at 25 1C and is significantly more efficient
than thermal methods. While these findings may open up new
possibilities for the application of aluminium amides as materials
precursors, as ligands for transition metals, or as intermediates in
D. S. Wright, Chem. Sci., 2011, 2, 1554; (c) R. J. Less, H. R. Simmonds and
D. S. Wright, Dalton Trans., 2014, 43, 5785.
13 H. Zhu, Z. Yang, J. Magull, H. W. Roesky, H.-G. Schmidt and
M. Noltemeyer, Organometallics, 2005, 24, 6420.
14 For putative three-coordinate LnAl = NAr see: (a) N. J. Hardman, C. Cui,
H. W. Roesky, W. H. Fink and P. P. Power, Angew. Chem., Int. Ed., 2001,
40, 2172; (b) H. Zhu, J. Chai, V. Chandrasekhar, H. W. Roesky, J. Magull,
D. Vidovic, H.-G. Schmidt, M. Noltemeyer, P. P. Power and W. A. Merrill,
J. Am. Chem. Soc., 2004, 126, 9472; (c) H. Zhu, J. Chai, V. Jancik,
H. W. Roesky, W. A. Merrill and P. P. Power, J. Am. Chem. Soc., 2005,
127, 10170.
15 For an isolated four-coordinate LnAl = NAr see: J. Li, X. Li, W. Huang,
H. Hu, J. Zhang and C. Cui, Chem. – Eur. J., 2012, 18, 15263.
9538 | Chem. Commun., 2014, 50, 9536--9538
This journal is ©The Royal Society of Chemistry 2014