An unsolvated lithium trihydroaluminate and the correponding trialkynylaluminates supported by an anionic triazacyclononane ligand

Article abstract of DOI:10.1039/b202238a

Reaction of the anionic tacn ligand ((tacn)H = 1,4-diisopropyl-1,4,7-triazacyclononane) with LiAlH₄ in THF afforded a dimeric, unsolvated lithium trihydroaluminate, which upon treatment with terminal acetylenes HCCR (R = Ph, SiMe₃) y

Full text of DOI:10.1039/b202238a

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An unsolvated lithium trihydroaluminate and the correponding
trialkynylaluminates supported by an anionic triazacyclononane
ligand
Chunming Cui, Joseph A. R. Schmidt and John Arnold*
Department of Chemistry, University of California at Berkeley and Chemical Sciences Division,
Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460, USA
Received 13th February 2002, Accepted 24th May 2002
First published as an Advance Article on the web 21st June 2002
Reaction of the anionic tacn ligand ((tacn)H = 1,4-diisopropyl-1,4,7-triazacyclononane) with LiAlH4 in THF
afforded a dimeric, unsolvated lithium trihydroaluminate, which upon treatment with terminal acetylenes HCCR
(R = Ph, SiMe3) yielded the corresponding monomeric trialkynylaluminates in which one acetylide ligand bridges
the Li and Al atom at the Cα atom while the two terminal acetylide ligands are coordinated to the Al atom.
Hydroaluminates are well known reducing agents in organic
chemistry. In recent years, there has been increased attention on
the structural characterization of well-defined molecular tri-
hydroaluminates incorporating bulky organic ligands due to
their interesting structural features and their potential use for
selective reductions.1 By comparison, the related acetylide
derivatives have received relatively little attention; nonetheless,
their importance in the selective alkynylation of carbonyl com-
pounds2a,b and other catalytic transformations has been noted
recently.2c,d They have been generated in situ for the appli-
cations, but very little is known regarding their structures,
probably due to their intrinsic tendency to form large aggre-
gates in the solid state as well as their low solubility in common
organic solvents.3
Our studies of transition metal complexes incorporating the
anionic tacn ligand ((tacn)H = 1,4-diisopropyl-1,4,7-triaza-
cyclononane) have shown that this ligand can function as either
a tridentate, six-electron donor or may partially dissociate thus
freeing coordination sites for further reactivity. In the latter
mode, the ligand is also able to stabilize unusual bimetallic
Scheme 1
complexes via coordination of a second metal to the uncom-
plexed nitrogen donor atoms.4,5 As both hydride and acetylide
are very strong bridging ligands, we reasoned that this anionic
macrocyclic ligand system could stabilize some interesting
unsolvated and low molecularity hydro- and alkynyl-aluminates
with unusual coordination geometries. Here we report the first
isolation of an unsolvated lithium trihydroaluminate 1 and the
related trialkynylaluminates 2 and 3 supported by the tacnϪ
ligand.
indication of the molecular structure of the molecules, so we
turned to diffraction methods to address this issue.
The molecular structure (Fig. 1) was determined by single
crystal X-ray analysis. The final model shows a weakly bound
dimeric structure in which one hydride ligand bridges between
aluminium and lithium. The molecular structure of 1 is unique
in comparison to several known solvated lithium aluminates
1a
such as [(SiMe3)2NAlH(µ-H)2Li(OEt2)]2
and (PhMe2Si)3-
CAlH(µ-H)2Li(THF)2]2,1d all of which feature Al2Li2H4 rings in
the solid state. The aluminium atom in 1 is coordinated to the
amido nitrogen atom of the tacnϪ ligand, and three hydride
ligands, one of which bridges to a lithium atom. The three
Al–H distances (both terminal and bridging) are essentially
the same (average 1.57 Å), and are in good agreement with
literature values.6 The Li–H bond length (1.91(3) Å) may be
compared to those found in solid LiH (2.040 Å)7 and LiAlH4
(1.88–2.16 Å),8 but is significantly longer than those in
[(SiMe3)2NAlH(µ-H)2Li(OEt2)]2 (1.777 Å).1a
Results and discussion
The reaction of 1,4-diisopropyl-1,4,7-triazacyclononane
((tacn)H) with LiAlH4 in THF at room temperature afforded a
solvent free lithium trihydroaluminate 1 in excellent yield
(Scheme 1). Complex 1 has been characterized by 1H, 7Li,
and 13C NMR spectroscopy, IR spectroscopy, and elemental
1
analysis. The most notable features of the H NMR spectrum
are the absence of resonances attributable to incorporated sol-
vent or the Al–H protons; evidence for the latter is, nevertheless,
provided by the IR spectrum, which shows a broad Al–H
absorption centered at 1707 cmϪ1. The 7Li NMR is not
especially informative and serves only to confirm the results
of a positive lithium flame test. These data alone give little
Addition of three equivalents of terminal acetylenes,
HCCPh or HCCSiMe3, to 1 at room temperature leads to the
elimination of H2 and the formation of trialkynylaluminates 2
and 3 in high yield (Scheme 1). Compounds 2 and 3 were char-
1
7
acterized by H, Li, and 13C NMR spectroscopy, IR spectra,
and elemental analysis. All three alkyne ligands are equivalent
2992
J. Chem. Soc., Dalton Trans., 2002, 2992–2994
This journal is © The Royal Society of Chemistry 2002
DOI: 10.1039/b202238a

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Å).11 The Li1–C2 distance (2.718(5) Å) is long, inferring only
weak electrostatic interaction between the lithium atom and
the acetylide π electrons.11 Interestingly, DFT calculations12
(B3LYP method; 6-31G** basis set) predicted an optimized
geometry almost identical to that found experimentally with
Li–C1 = 2.248 Å and the same minor elongation of the
Li-coordinated acetylene (1.206 Å vs. 1.200 Å for the non-
coordinated acetylenes).
Compounds 1, 2 and 3 represent the first known Group 13
compounds supported by anionic tridentate macrocyclic
tacn ligands,13 and the latter two species are the first well-
characterized alkynylaluminates. Efforts to exchange the Li ion
in these complexes for transition metals in attempts to prepare
µ-acetylides have been unsuccessful to date, although further
studies on related systems are in progress.
Experimental
Standard Schlenk-line and glove box techniques were used
throughout. Pentane, diethyl ether, and toluene were passed
through a column of activated alumina and degassed with
argon. LiAlH4 was crystallized from Et2O before use. C6D6
was vacuum transferred from sodium/benzophenone ketyl.
Unless otherwise specified, 1H and 13C{1H} NMR spectra were
recorded in C6D6 at ambient temperature on a Bruker DRX-
500 spectrometer. 1H NMR chemical shifts are given relative to
C6D5H (δ 7.15). 13C NMR chemical shifts are relative to C6D6
(δ 128.39). 7Li NMR chemical shifts were referenced to
an external LiCl (3 M in D2O) standard at 0 ppm. IR samples
were prepared as Nujol mulls and taken between KBr
plates. Elemental analyses were determined by the Micro-
analytical Laboratory of the College of Chemistry, University
of California, Berkeley. Single crystal X-ray structure
determinations were performed at CHEXRAY, University of
California, Berkeley.
Fig. 1 ORTEP17 drawing of 1. Selected bond lengths [Å] and angles
[Њ]: Al1–N1 1.877(2), Al1–H1 1.56(3), Al1–H2 1.58(3), Al1–H3 1.57(3),
Li1–H1 1.91(3), Li1–N1 2.028(4), Li1–N2 2.095(4), Li1–N3 2.094(4);
H1–Al1–H2 108(1), H2–Al1–H3 113(1), N1–Al1–H2 109(1), N1–Li1–
H1* 120.3(8).
Synthesis of 1
To a solution of LiAlH4 (0.38 g, 10.00 mmol) in THF (20 mL)
was added 1,4-diisopropyl-1,4,7-triazacyclononane ((tacn)H)
(2.12 g, 10.00 mmol) at 0 ЊC. The mixture was allowed to warm
to room temperature and stirred for 15 h. The solvent was
removed under vacuum. The resulting oily residue was dis-
solved in diethyl ether (50 mL), and stored at Ϫ40 ЊC overnight
1
to give colorless crystals of 1 (2.4 g, 92%). H NMR (C6D6):
Fig. 2 ORTEP drawing of 2. Selected bond lengths [Å] and angles [Њ]:
Al1–N1 1.843(2), Al1–C1 1.981(3), Al1–C9 1.948(3), Al1–C17 1.950(3),
Li1–N1 2.083(4), Li1–N2 2.032(4), Li1–N3 2.027(5), Li1–C1 2.230(5),
Li1–C(2) 2.718, C1–C2 1.212(3), C9–C10 1.212(3), C17–C18 1.207(4);
Al1–C1–C2 178.1(2), Al1–C9–C10 174.1(2), Al1–C17–C18 169.2(2),
C1–C2–C3 172.6(3), C9–C10–C11 176.9(3), C17–C18–C19 177.3(3),
Al1–N1–Li1 88.9(1), Al1–C1–Li1 81.5(1).
δ 0.86 (d, J = 6.7 Hz, 6 H, Me), 0.98 (d, J = 6.8 Hz, 6 H, Me),
3.03, 2.49, 2.10, 1.82 (m, 12 H, CH2CH2) 3.16 (sept, J = 6.6 Hz,
2 H, CH). 13C NMR (C6D6): δ 18.8, 19.1 (CH3), 48.9 (CH),
50.5, 53.7 (CH2CH2). 7Li NMR (C6D6): δ 2.44. IR (Nujol,
KBr): ν = 1707 cmϪ1. Anal. calc. for C12H29N3LiAl (249.30):
C, 57.82; H, 11.73; N, 16.85. Found: C, 57.82; H, 11.69; N,
16.75%.
in solution at room temperature according to 1H and 13C NMR
spectroscopy and again, the 7Li data renders no structural
information.
Synthesis of 2 and 3
To a solution of 1 (0.50 g, 2.01 mmol) in toluene (15 mL) was
added phenylacetylene (0.66 g, 6.50 mmol) at room temper-
ature. Gas evolution was observed immediately upon addition.
The mixture was stirred for 15 h at room temperature. The
solvent was removed under vacuum to afford a white powder,
which was subsequently crystallized from toluene at Ϫ10 ЊC to
give colorless crystals of 2 (1.0 g, 85%). 3 was prepared simi-
larly, and crystallized from pentane–toluene (10 : 1) to give 3 as
a colorless crystalline solid (90%).
The molecular structure of 2 (Fig. 2) represents the first
structurally characterized alkynylaluminate. The compound is
monomeric in the solid state, with a four-coordinate aluminium
atom bound to two terminal acetylides, one novel bridging
acetylide group, and the tacn amido nitrogen. The Al1–C1
bond length (1.981(3) Å) is slightly longer than the Al–C
(terminal) distances (average 1.949 Å), and also longer than
those in the neutral aluminium acetylide compound {tBu2-
PzAl(CCPh)2}2 (tBu2Pz = 3,5 di-tert-butylpyrazolate) (1.92 Å).9
The Al1–C1–C2 (178.1(2)Њ) angle is nearly linear, whereas the
angles between the aluminium atom and the two terminal
acetylides (174.1(2)Њ and 169.2(2)Њ) deviate slightly from linear-
ity. The Li1–C1 distance (2.230(5) Å) is only slightly longer
than those found in (tBuCCLi)4(THF)4 (2.19 Å),10 and is in the
range reported for those in (LiCCSiMe2C6H4OMe)6 (2.13–2.92
2: 1H NMR (C6D6): δ 0.67 (d, J = 6.8 Hz, 6 H, Me), 0.80 (d,
J = 6.8 Hz, 6 H, Me), 1.84, 1.92, 2.13, 2.51, 2.63 (m, 12 H,
CH2CH2), 3.63 (sept, J = 6.8 Hz, 2 H, CH), 6.92–7.05 (m, 9 H,
Ar–H), 7.53 (m, 6 H, Ar–H). 13C NMR (C6D6): δ 17.0, 18.4
(CH3), 47.1 (CH), 50.1, 53.1 (CH2CH2), 106.1 (Al–C), 124.8
(CPh), 125.2, 126.1, 130.9, 131.0 (Ph). 7Li NMR (C6D6): δ 2.74.
IR (Nujol, KBr): ν = 1520, 1592 (Ph), 2108, 2122 (CCPh) cmϪ1
.
J. Chem. Soc., Dalton Trans., 2002, 2992–2994
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Montero, H. Wessel, H. W. Roesky, M. Teichert and I. Uson,
Angew. Chem., 1997, 109, 644; M. L. Montero, H. Wessel,
H. W. Roesky, M. Teichert and I. Uson, Angew. Chem., Int. Ed.
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4 J. A. R. Schmidt and J. Arnold, J. Am. Chem. Soc., 2001, 123, 8424.
5 (a) G. R. Giesbrecht, A. Shafir and J. Arnold, Chem. Commun.,
2000, 2135; (b) J. A. R. Schmidt, S. A. Chmura and J. Arnold,
Organometallics, 2001, 20, 1062.
Anal. calc. for C36H41N3AlLi (549.66): C, 78.67; H, 7.52; N,
7.64. Found: C, 78.14; H, 7.85; N, 7.95%.
3: H NMR (C6D6): δ 0.07 (s, 27 H, SiMe3), 0.78 (m, 6 H,
1
CH3), 0.89 (m, 6 H, CH3), 2.30–2.62 (m, 6 H, CH2CH2), 1.96
(m, 6 H, CH2CH2), 3.30 (m, 2 H, CH). 13C NMR (C6D6): δ 0.4
(SiMe3), 18.2, 18.4, 19.4, 19.6 (CH3), 48.0 (CH), 49.5, 49.7, 54.1,
7
54.3 (CH2CH2), 93.7, 94.1 (CSiMe3), 113.5 (Al–C). Li NMR
(C6D6, 194 MHz): δ 2.33. IR (Nujol, KBr): ν = 2060, 1948 cmϪ1
.
Anal. calc. for C27H53N3AlLiSi3: C, 60.29; H, 9.93; N, 7.81.
Found: C, 59.42; H, 10.30; N, 8.05%.
X-Ray structural analyses for 1 and 2
A fragment of a colorless block of 1 or 2 was mounted in a
glass capillary. Data were collected on a Siemens Smart dif-
fractometer. Data were integrated by the program SAINT14 to
a maximum 2θ value of 49.4Њ. The structure was solved by
direct methods15 and expanded using Fourier techniques.16 The
nonhydrogen atoms were refined anisotropically. The three
hydrogen atoms bound to the aluminium ion were refined
isotropically, while the rest were included in fixed positions.
6 D. J. Linton, P. Schooler and A. E. H. Wheatley, Coord. Chem. Rev.,
2001, 223, 53.
7 E. Zintl and A. Harder, Z. Phys. Chem., 1935, 28, 478.
8 A. Sklar and B. Post, Inorg. Chem., 1967, 6, 669.
9 W. Zheng, N. C. Mösch-Zanetti, H. W. Roesky, M. Hewitt,
F. Cimpoesu, T. R. Schneider, A. Stasch and J. Prust, Angew. Chem.,
2000, 112, 3229; W. Zheng, N. C. Mösch-Zanetti, H. W. Roesky,
M. Hewitt, F. Cimpoesu, T. R. Schneider, A. Stasch and J. Prust,
Angew. Chem., Int. Ed., 2000, 39, 3099.
10 M. Geissler, J. Kopf, B. Schubert, E. Weiss, E. Eeugebauer and
P. V. Schleyer, Angew. Chem., 1987, 99, 569; M. Geissler, J. Kopf,
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Chem., Int. Ed. Engl., 1987, 26, 587.
11 B. Goldfuss, P. V. Schleyer and F. Hampel, J. Am. Chem. Soc., 1997,
119, 1072.
12 DFT calculations were carried out using the TITAN software
package (Wavefunction Inc., Irvine, CA) using Becke’s 3 Parameter/
HF ϩ Slater ϩ Becke88 ϩ VWN ϩ LYP (B3LYP) method and the
Gaussian basis set 6-31G**.
13 Aluminium complexes incorporating pendant neutral tacn ligands
were described recently, see (a) D. A. Robson, L. H. Rees,
P. Mountford and M. Schroder, Chem. Commun., 2000, 1269;
(b) D. A. Robson, S. Y. Bylikin, M. Cantuel, N. A. H. Male,
L. H. Rees, P. Mountford and M. Schroder, J. Chem. Soc.,
Dalton Trans., 2001, 157.
Crystal data for 1. C12H29N3LiAl, M = 249.30, crystal dimen-
sions 0.32 × 0.28 × 0.13 mm, monoclinic, space group P21/n,
a = 9.1950(1), b = 13.4305(3), c = 13.0959(3) Å, β = 106.265(1)Њ,
V = 1552.53(5) Å3, Z = 4, dcalc = 1.067 g cmϪ3; F(000) = 552.00,
λ = 0.71069 Å, T = 139 K, µ(Mo-Kα) = 1.15 cmϪ1, R1 = 0.0590,
wR2 = 0.0559. Of the 6935 reflections that were collected, 2678
were unique (Rint = 0.042).
Crystal data for 2. C36H41N3AlLi, M = 549.66, crystal dimen-
sions 0.34 × 0.22 × 0.18 mm, monoclinic, space group P21/c,
a = 8.3268(4), b = 18.0244(9), c = 22.140(1) Å, β = 99.666(1)Њ,
V = 3275.7(2) Å3, Z = 4, dcalc = 1.114 g cmϪ3, F(000) = 1176.00,
µ(Mo-Kα) = 0.89 cmϪ1, T = 130 K, 2θmax = 49.4Њ, R1 = 0.0672,
wR2 = 0.0534. Of the 14627 reflections collected, 5617 were
unique (Rint = 0.048).
CCDC reference numbers 173850 (1) and 173851 (2).
See http://www.rsc.org/suppdata/dt/b2/b202238a/ for crystal-
lographic data in CIF or other electronic format.
14 Saint: SAX Area-detector integration program, V4.024, Siemens
Industrial Automation, Inc., Madison, WI, 1995.
15 SIR92: A. Altomare, M. C. Burla, M. Camalli, M. Cascarano,
C. Giacovazzo, A. Guagliardi and G. Polidori, J. Appl. Crystallogr.,
1993, 26, 343.
Acknowledgements
Financial support of this work by the NSF is gratefully
acknowledged.
16 DIRDIF92: P. T. Beurskens, G. Admiraal, G. Beurskens, W. P.
Bosman, S. Garcia-Granda, R. O. Gould, J. M. M. Smits,
C. Smykalla, The DIRECT program system, Technique report of
the Crystallography Laboratory, University of Nijmegen, The
Netherlands, 1992.
References and notes
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J. Chem. Soc., Dalton Trans., 2002, 2992–2994