Lewis base complexes of AlH3: Prediction of preferred structure and stoichiometry

Article abstract of DOI:10.1039/c3dt00047h

The structures adopted by a range of complexes AlH₃·nL, (n = 1 or 2), have been explored in detail to identify the factors that determine the value of n, and whether a monomeric or dimeric arrangement is preferred for the 1 : 1 complexes. Single-crystal X-ray diffraction, vibrational and NMR spectroscopies, and thermal analysis data have been collected, DFT calculations have been performed for AlH₃·nL species, and pK_a values have been collated for a series of amine and phosphine ligands L. The pK_a of the ligand L exerts an important influence on the type of complex formed: as the basicity of L increases, a monomeric structure is favoured over a dimeric arrangement. Dimeric amine complexes form if pK_a < 9.76, while monomeric complexes are preferred when pK_a > 9.99. The steric requirements of L also influence the structural preference: bulky ligands with large cone angles (>163°) tend to favour formation of monomers, while smaller cone angles (<125°) encourage the formation of dimeric or 1 : 2 adducts. The steric bulk of the ligand appears to be more important for phosphine complexes, with smaller phosphines being unable to stabilise the complex at ambient temperatures even through dimerisation. Raman spectroscopy and DFT calculations have been particularly helpful in elucidating the stoichiometric preferences of complexes that have been contentious; these include AlH₃·NMe ₂Et, AlH₃·NMe₃ and AlH ₃·nEt₂O. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3dt00047h

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Lewis base complexes of AlH3: prediction of preferred
structure and stoichiometry†
Cite this: Dalton Trans., 2013, 42, 6965
Terry D. Humphries,* Keelie T. Munroe, Andreas Decken and G. Sean McGrady*
The structures adopted by a range of complexes AlH3·nL, (n = 1 or 2), have been explored in detail to
identify the factors that determine the value of n, and whether a monomeric or dimeric arrangement is
preferred for the 1 : 1 complexes. Single-crystal X-ray diffraction, vibrational and NMR spectroscopies, and
thermal analysis data have been collected, DFT calculations have been performed for AlH3·nL species, and
pKa values have been collated for a series of amine and phosphine ligands L. The pKa of the ligand L
exerts an important influence on the type of complex formed: as the basicity of L increases, a monomeric
structure is favoured over a dimeric arrangement. Dimeric amine complexes form if pKa < 9.76, while
monomeric complexes are preferred when pKa > 9.99. The steric requirements of L also influence the
structural preference: bulky ligands with large cone angles (>163°) tend to favour formation of mono-
mers, while smaller cone angles (<125°) encourage the formation of dimeric or 1 : 2 adducts. The steric
bulk of the ligand appears to be more important for phosphine complexes, with smaller phosphines
being unable to stabilise the complex at ambient temperatures even through dimerisation. Raman
spectroscopy and DFT calculations have been particularly helpful in elucidating the stoichiometric prefer-
ences of complexes that have been contentious; these include AlH3·NMe2Et, AlH3·NMe3 and AlH3·nEt2O.
Received 5th January 2013,
Accepted 6th March 2013
DOI: 10.1039/c3dt00047h
www.rsc.org/dalton
and ethers10,21,22 and other oxygen-donor ligands.23 Binary
AlH3 has been isolated only from its AlH3·nEt2O precursor and
Introduction
Crystalline aluminium hydride or alane, AlH3, is a uniquely its synthesis relies on the formation and desolvation of this
complicated hydride that has frustrated and fascinated its pur- complex (eqn (1) and (2)).10 The volatile nature of the amine
suers in equal measure since its discovery six decades ago.1 A derivatives and their ability to sublime intact has been
remarkable hydrogen content of 10.1% by mass has promoted exploited in the production of high purity thin films of alu-
extensive studies of alane as a material for the storage and minium metal by various types of chemical vapour deposition
transportation of hydrogen,2–4 as well as investigations of its (CVD).24–26
potential as a rocket fuel or explosive.5–8 AlH3 has also been
Et2O
explored in detail as a chemical reducing agent for a range of
3LiAlH4 þ AlCl3 ꢀ! 4AlH3 ꢁ nEt2O
ð1Þ
ð2Þ
ꢀLiCl
functional groups.9
In addition to its multifarious chemical applications, alane
is blessed with a rich and varied structural chemistry. Apart
from the unique polymorphism displayed by the binary
hydride – with at least seven structural polymorphs being
known10 – AlH3 forms complexes with a variety of Lewis
donors including carbenes,11–13 phosphines,14–16 amines,17–20
Δ
AlH3 ꢁ nEt2O ꢀ! AlH3
ꢀEt2O
Amine complexes of AlH3 are also precursors to other
useful materials, as they take part in important transform-
ations such as metallation, hydrometallation and displace-
ment reactions. Some examples of metallation involve the
reaction of the trimethylamine complex of AlH3 with alcohols
and thiols to produce alkoxy- or mercapto-alanes,27,28 while
the reaction with p-tert-butyl(calix[4]arene) results in trimetal-
Department of Chemistry, University of New Brunswick, P.O. Box 4400, Fredericton,
NB E3B 5A3, Canada. E-mail: smcgrady@unb.ca; Fax: +1 506 453-4981;
Tel: +1 506 452-6340
†Electronic supplementary information (ESI) available: DFT-calculated sym- lation and deprotonation by the amine–alane complex, yield-
ing a bis(calix[4]arene).29 Hydrometallation or reduction by
tertiary amine adducts of AlH3 is highly selective for a variety
metries for AlH3 complexes; Cartesian coordinates and geometrical parameters
for AlH3·2TMA, AlH3·TMA, AlH3·2BDMA, AlH3·2TMEDA, AlH3·2TMPDA,
AlH3·2DIOX, AlH3·TEA; FT-IR and Raman spectra of AlH3·2TMA, AlH3·TMA,
of functional groups, and results in high yields of products.
AlH3·TEA, AlH3·2BDMA, AlH3·2TMEDA, AlH3·2TMPDA, AlH3·2DIOX, AlH3·TEA,
For example, AlH3·NEt2Me reduces carboxylic acids to alcohols
and nitriles and amides to amines, but will not attack halides
AlH3·nEt2O; crystallographic data; cone angle diagrams for all molecules. See
DOI: 10.1039/c3dt00047h
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or nitro groups.30 AlH3·NMe3 has also been reported to with any amine donor can now be predicted accurately and
undergo displacement reactions: LiH will react with the ter- with confidence for the first time.
tiary amine complex to form LiAlH4,31 while lithium alkyls will
displace the hydride moieties to produce amine adducts of
alkylalanes.32,33
Experimental details
General considerations
AlH3·nL complexes are known to adopt a variety of struc-
tural arrangements. The 1 : 1 species can be either monomeric
or dimeric, and their 1 : 2 counterparts are either monomeric All materials handled in this study are extremely air- and
or polymeric, depending on the nature of L. 1 : 1 complexes moisture-sensitive, requiring all manipulations to be per-
(AlH3·L) generally adopt a pseudo-tetrahedral configuration at formed using standard Schlenk or glove-box techniques
the metal centre. Examples of monomeric systems include in vacuo or under an atmosphere of purified N2 or Ar (H2O and
complexes with tertiary amines and phosphines, where L is O2 < 1 ppm). All samples were stored in a freezer at −40 °C
quinuclidene (QUIN),16 tri-n-butylamine (n-TBA),34 tricyclo- until required. Et2O was dried and deoxygenated using a
hexylphosphine (PCy3)15 and tert-tributylphosphine (PBut3).14 Grubbs-type solvent dispensing system, and pentane and
Closely related to these 1 : 1 adducts are the dimeric systems, DMEA (≥99%; Sigma-Aldrich) were used as received. All sol-
in which bridging hydride moieties span two Al centres. vents were degassed prior to use, placed over 4 Å molecular
Examples in this class include the complexes where L is tri- sieves and stored under Ar. C6D6 and C6D5CD3 solvents for
methylamine (TMA), benzyldimethylamine (BDMA), 1-methyl- NMR spectroscopy were transferred directly from sealed
tetrahydropyridine (MTHP), 4-dimethylaminopyridine (DMAP), ampoules (1 mL) in a glove-box into a 5 mm NMR tube
tris(allyl)amine (TAA), THF and NMe2(CH2)3Cl.23,35–38
equipped with a Teflon valve, while CDCl3 and CD2Cl2 were
The five-coordinate 1 : 2 adducts of alane (AlH3·2L) adopt a stored over 4 Å molecular sieves under Ar in a glass ampoule
trigonal bipyramidal (TBP) arrangement at the Al centre, with sealed with a Teflon valve. The AlCl3 used in this study was
three equatorial hydrides and two axial donor ligands. This ReagentPlus®, anhydrous, granular, ≥99.9% (Sigma-Aldrich),
type of adduct is the least common, with only QUIN,20 THF23 and LiAlH4 was reagent grade, 95%, powder (Sigma-Aldrich);
and TMA39–41 versions known. Polymeric alane compounds are which was purified by dissolution in, and recrystallisation
more abundant: these also adopt a TBP array at the metal from, Et2O.
centre; examples include complexes with N,N,N′,N′-tetramethyl-
The syntheses and characterisation of AlH3·TMA and
ethylenediamine (TMEDA),42 N,N,N′,N′-tetramethyl-1,3-propane- AlH3·2TMA have been reported previously,40 and the syntheses
diamine (TMPDA)43 and 1,2-bis(di-isopropylphosphino)- and structures of AlH3·TMEDA, AlH3·TMPDA, AlH3·DIOX,
ethane (DIPE).15 Several mixed-donor alane complexes have AlH3·TEA and AlH3·2BDMA are reported by us elsewhere.45
also been reported, including those with TMA and 1,2-bis(di-
NMR spectra were measured using Varian NMR spectro-
methylphosphino)ethane (DMPE),16 or TMA/PBut3 ligands,14 meters equipped with Oxford Instruments magnets; the
and the polymeric complex formed with N-ethylmorpholine magnetic field used for observation of individual nuclei is
(EMORPH), in which each Al centre is coordinated by both an reported in the appropriate Experimental sections. 13C NMR
N and an O donor atom in the chain.44
spectra were referenced to the C6D5CD3 singlet at 137.86 ppm,
While a large number of reports exist describing the syn- C6D6 peak at 128.36 ppm or CD2Cl2 peak at 53.8 ppm; accord-
thesis and characterisation of these AlH3·nL complexes, there ing to the solvent used. Likewise, 1H spectra were referenced to
does not appear to have been any systematic investigation the C6D5CD3 sharp singlet peak at 7.09 ppm, sharp C6D6
carried out into the factors that favour monomeric over singlet at 7.16 ppm, or the sharp CD2Cl2 peak at 5.32 ppm.
dimeric arrangements for the 1 : 1 complexes, or monomeric 27Al NMR spectra were referenced using Al(NO3)3 in D2O
versus polymeric arrangements for their 1 : 2 counterparts. (1.1 mol L−1) as an external standard. 14N NMR spectra were
Here we report the second part of a detailed study of the struc- measured using a 10 mm sample probe and referenced to
tural preferences of AlH3·nL species, and we identify and CH3NO2 (0.0 ppm). Chemical shifts are reported in ppm (parts
quantify the factors that contribute to or determine these per million) and coupling constants in Hz. The following
preferences.45 In this endeavour we have drawn on techniques abbreviations are used: s (singlet), d (doublet), t (triplet) and
as varied as single-crystal X-ray diffraction, vibrational and m (multiplet). FT-IR spectra were measured using a Thermo
multinuclear NMR spectroscopies, DFT calculations and Nicolet Nexus 470 FT-IR spectrometer with a resolution of
thermal analysis, and we have collated the results with 2 cm−1 over the spectral range 225 to 4000 cm−1, typically aver-
measured pKa values for the ligands in the AlH3·nL species. aging 32 scans. Solid and liquid samples were measured as
Accordingly, we reveal here how the size and basicity of L Nujol mulls contained between NaCl plates. Gas phase
determines the value of n; and when n = 1, whether AlH3·L is samples were measured in a cell equipped with KBr windows.
likely to be monomeric or dimeric in the solid state. Hence the A background scan was recorded through the evacuated IR cell
dispute over the structure of AlH3·TMA has been settled, and before admission of the sample. FT-Raman spectra were
the previously unknown structure of AlH3·DMEA (DMEA = recorded using
a Thermo Nicolet NXR 9610 FT-Raman
NMe2Et), has been elucidated. As a result of this study, the pre- spectrometer with a resolution of 4 cm−1 over the spectral range
ferred stoichiometry and structure of the AlH3 complex formed 98 to 3703 cm−1. Samples were excited with a semiconductor
6966 | Dalton Trans., 2013, 42, 6965–6978
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laser operating at 976 nm. All samples were measured at room Cone angle calculations
temperature and spectra were collected using a liquid N2-
Tolman first introduced the concept of the ligand cone angle
(also known as the Tolman cone angle or θ) in 1970, to assess
the effective size of a ligand in a series of nickel phosphine
complexes. It is defined as the solid angle formed with the
metal at the vertex and the hydrogen atoms at the perimeter of
the cone. Tolman originally crafted his measuring apparatus
using CPK atomic models attached to a pivot and a block of
wood.55 The cone angles reported here were measured in an
analogous manner to that described by Burchell et al.,
whereby molecules were geometrically optimised by DFT calcu-
lations.56 This method gives a comparable angle to that calcu-
lated for phosphines by the ‘solid-angle concept’ developed by
Immirzi, which was derived from Tolman’s original work.
For example, in a nickel tri(cyclohexyl)phosphine complex,
Immirzi calculated a cone angle range of 145.0–149.1°,
Tolman calculated 170°, while in this work the cone angle of
PCy3 was measured to be 152°.55,57
cooled Ge detector. Solid samples were contained in sealed
glass capillaries, while liquid samples were measured through
a small glass vial. All Raman and FT-IR spectra are presented
in the ESI.† Assignment of the features in both of the
FT-Raman and FT-IR spectra was assisted using vibrational fre-
quencies generated from the DFT calculations and viewed in
ChemCraft.46 Vibrational modes were identified by reference
to the displacement vectors for each of the vibrations.
Mass spectra were recorded using a CEC 21-110 mass
spectrometer in EI mode. Elemental analysis was performed
in-house utilising a LECO CHNS-932 micro-analyser, with the
furnace temperature maintained at 1000 °C. The quoted accu-
racy of the instrument is 1.00%, and samples were run in tri-
plicate. Experimental elemental percentages for the AlH3
complexes reported here are generally lower than the theoreti-
cal values on account of the thermal frailty and air-sensitive
nature of the compounds, as well as their volatility. This com-
plication has been reported previously elsewhere.36
DSC and TGA measurements were performed using a TA
Instruments DSC Q20 and TGA Q50, respectively. Samples
were loaded and transferred to the instrument in T-zero pans.
In TGA experiments, the top pan was pierced at the com-
mencement of analysis to permit the egress of gases upon
decomposition of the analyte. Both techniques employed N2 as
the purge gas: DSC used a flow rate of 190 mL min−1 and TGA
employed 40 mL min−1 for the balance and 60 mL min−1 for
the sample. Samples were typically measured at a heating rate
of 2 °C min−1. Data were collected at 0.5 s per point, and were
analysed using TA Instruments Thermal Analysis V.2.0 soft-
ware, and processed using TA Instruments Universal Analysis
2000 V.3.9A. Melting points and decomposition temperatures
were determined using DSC methods.
DFT calculations were performed using the GAUSSIAN 03
program suite,47 employing the B3LYP density functional48–50
along with the implemented 6-311G(d,p) basis set.51–54 The
structures reported here were found to be true local minima
on the potential energy surface by calculating analytical fre-
quencies. All structures were optimised in the highest possible
symmetry, as described in Table 1.
Cone angles were measured as follows: in ChemCraft,46 geo-
metrically optimised DFT structures (or ones created from the
CIF file for AlH3·n-TBA or AlH3·BDMA) were orientated to place
the Al–N bond in a vertical position and the molecule was
rotated horizontally to form the largest cone angle before
viewing the molecule in ‘isometry’ mode and by selecting
‘show van der Waals spheres’ in the view tab. Cone angles
were then calculated by measuring the angle from the centre
of the Al atom to the edge of the closest van der Waals sphere
on the nearest H atom.
Synthesis and characterisation
Synthesis of AlH3·DMEA. This was carried out as described
by Frigo et al.58 AlCl3 (3.77 g; 28.3 mmol) and LiAlH4 (3.60 g;
94.8 mmol) were introduced into a Schlenk tube which was
cooled in an ice bath, and pentane (100 mL) was added. DMEA
(12.2 mL; 112 mmol) was syringed slowly onto the slurry,
which was then allowed to warm to room temperature and
stirred overnight. The resulting solution was filtered from the
light grey precipitate into another vessel and was stored at
−40 °C for 24 h, affording a white crystalline solid. The
Schlenk tube was then placed in a cold ethanol bath at −30 °C
Table 1 DFT-calculated symmetries and ΔHca (kJ mol−1) for AlH3·nL adducts
1 : 1 Monomer
1 : 1 Dimer
Highest symmetry
1 : 2
Highest symmetry
Ligand
Highest symmetry
ΔHc
ΔHc
−90.15
ΔHc
BDMA
DIOX
Et2O
TEA
DMEA
TMA
QUIN
TMEDA
TMPDA
—
Cs
—
C1
—
C2
C2
C2
C2h
—
—
—
C2
C2v
C2
D3
C2
C2v
C1
C1
C2
−123.47
−116.82
−77.76
−84.44
—
C1
C3
C1
C3v
C3v
C1
Ci
−71.52
−89.05
−52.14
−62.25
−96.28
−112.58
—
−90.29
−102.2
−128.10
−145.90
−155.71
−125.57
−136.47
−108.13
−117.68
−101.15
−104.67
—
—
a ΔHc = (−1)[((E(AlH ) + E(Lig)) − (E(comp))) × 627.5095 × 4.184].
3
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and the pentane was removed by filtration. The residue was in vacuo to leave AlH3·nEt2O as a white solid. IR (see ESI,†
allowed to warm to room temperature, at which point the solid nujol); νmax/cm−1 1793sbr, 1652mbr (νAl–H); 722w (δsH–Al–H).
melted to afford a clear colourless liquid. Pentane was added Raman (see ESI,† liquid, 1028 scans, 0.0303 W); νmax/cm−1
to the Schlenk tube to remove any unreacted DMEA. The vessel 2973s, 2933s, 2894m, 2869m and 2762w (νC–H); 1827sbr,
was then cooled to −30 °C and the pentane was removed by fil- 1719mbr (νAl–H); 1451s and 1327m (δC–H); 1071sbr, 399w,
tration once again. Any remaining liquid was removed 327w (νC–O); 770w (δasH–Al–H); 666w (νsH–Al–H); 550w
in vacuo. The residue (yield 4.31 g, 37% based upon AlCl3) was (νAl–O): 197w (νAl–O). Decomp: (DSC) 61–91 °C (lit.59 >60 °C).
allowed to warm to room temperature, filtered into an
1
ampoule under Ar and stored in a freezer at −40 °C. H NMR
(300 MHz, C6D6); δ (ppm) = 4.13 (br, 3H, AlH3, FWHM =
Results and discussion
60.174), 2.20 (q, J = 7.3 Hz, 2H, N–CH2), 1.87 (s, 3H, N–CH3),
0.72 (t, J = 7.3 Hz, 6H, CH3). 27Al NMR (76 MHz, C6H6); δ
It is well established in the literature that alane adducts of the
form AlH3·nL can adopt a variety of structural arrangements,
(ppm) = 136.001 (FWHM = 1065.18 Hz). 13C NMR (75 MHz,
C6D6); δ (ppm) = 54.647 (N–CH2), 44.931 (CH2CH3), 9.860
(N–CH3). 14N NMR (41 MHz, C6H6); δ (ppm) = −354.215
(FWHM = 628.978 Hz). IR (see ESI,† gas-phase); νmax/cm−1
2979s, 2942m and 2834w (νC–H); 1771sbr (νAl–H); 1464w and
1388s (δC–H); 1237w and 1189w (δsC–H); 1097w, 1028w, 930w
and 878w (νC–N); 751m (νasH–Al–H); 659w (νsH–Al–H). Raman
(see ESI,† liquid, 1024 scans, 0.0303 W); νmax/cm−1 2980s,
2944s, 2914s, 2850m, 2802w and 2726w (νC–H); 1805sbr
(νAl–H); 1447m and 1320w (δC–H); 1186w (δsC–H); 1100w and
1083w (νasC–N); 1066w, 1029m, 1008m and 927w (νsC–N);
781w (δasN–Al–H); 744w (νsH–Al–H); 456w (νAl–N); 302w
(δsAl–H). λmax(Hexane)/cm−1 288(s), 214(w). Mp (DSC) 3.16 °C
(lit.58 5 °C). Found: C, 44.10; H, 13.09; N, 12.50%. Calc. for
C5H16NAl: C, 46.58; H, 13.68; N, 13.58%).
Synthesis of AlH3·nEt2O. This was carried out as described
by Brower et al.,10 with appropriate adjustments. AlCl3 (6.65 g;
50 mmol) was dissolved in Et2O (100 mL), and this solution
was added to a solution of LiAlH4 in Et2O (3.04 g; 200 mmol in
600 mL). The resulting mixture was stirred for several min. A
light grey precipitate of LiCl started to appear soon after
mixing the two solutions. This was removed by filtration,
along with any remaining LiAlH4. The Et2O was then removed
which include both monomeric and dimeric forms for 1 : 1
complexes, and monomeric and polymeric structures for 1 : 2
species.12,14–16,23,34–44,60 In this study we have considered
several structural and electronic features of the ligand L, in an
attempt to define correlations with the type of complex formed
and hence to predict the structural preferences of a given
adduct. The Lewis base properties of a ligand L can be reliably
estimated from its pKa. Although this is formally a measure of
the Brønsted basicity of L, the correlation is valid in most
cases.61 The tertiary amines and phosphines discussed here
act as Lewis bases towards Al, but their Brønsted basicity
allows their pKa to be measured directly.62 According to Pear-
son’s HSAB Theory, amines are hard bases and Al3+ is a hard
acid.63 Even though the hardness of H+ is defined as infinite,
that of Al3+ is estimated as 45.8 eV, (cf. Li+; 35.1 eV), and hence
we consider the amine pKa values to represent a meaningful
measure of their donor behaviour towards the AlH3 centres in
the complexes under consideration.64 Less justified is the use
of pKa values for the softer phosphine ligands to predict their
Lewis base behaviour, but these are included in the study for
comparison. Table 2 reports the pKa values of tertiary amines
and phosphines whose AlH3 adducts have been characterised
Table 2 pKa values, decomposition temperatures and melting points for AlH3 complexes characterised by X-ray crystallography
L
pKa of L
Adduct type
Cone angleb (°)
Melting point (°C)
Decomposition temperature (°C)
PBut3
QUIN
11.467
11.068
1 : 1 (monomeric)14
1 : 1 (monomer)16
1 : 220
182
120
116
—
103–10514
10869
>18014
—
—
17569
TMEDA
TMPDA
TEA
n-TBA
DMEA
TMA
10.762
10.570
10.6562
10.5962
9.9962
9.7662
Polymerica,42
Polymeric45,71
1 : 1 (monomer)a,72
1 : 1 (monomer)34
1 : 1 (monomer)c
1 : 1 (dimer)a,73
1 : 2a,39,73
139.545
137.345, 13571
18.045
142.7–167.745
140–16945
122.4–140.345
—
—
163
164
142
122
116
—
170
117
142
—
—
3.2a
131.4–150.8a
140.2–142.2a
115.6–139.0a
—
68.6a, 7672
94.0a, 9017
—
DMAP
PCy3
BDMA
9.737
1 : 1 (dimer)37
1 : 1 (monomeric)15
1 : 1 (dimer)60
1 : 2a
9.6567
8.9362
161–16315
75–7660
67.4445
11038
161–16315
—
102.6–148.545
—
TAA
EMORPH
DIOX
8.3162
7.7062
—
1 : 1 (dimer)38
Polymeric44
—
—
66–6744
82.445
13744
Polymerica
125.2–150.5a
a This work. b Cone angles derived from optimised DFT geometries apart from PBut3,57 PCy3,57 BDMA60 and n-TBA34 which are calculated from
the respective CIF files. c Stoichiometry derived from analysis in this work; not crystallography.
6968 | Dalton Trans., 2013, 42, 6965–6978
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