Synthesis, structure, and properties of a stable 1,2-dibromodialumane(4) bearing a bulky aryl substituent

Article abstract of DOI:10.1021/om300237q

The stable 1,2-dibromodialumane(4) Bbp(Br)Al-Al(Br)Bbp (1; Bbp = 2,6-bis[bis(trimethylsilyl)methyl]phenyl) was synthesized by the reduction of the corresponding dibromoalumane etherate BbpAlBr₂?OEt ₂ (2). The structure and properties

Full text of DOI:10.1021/om300237q

Note
pubs.acs.org/Organometallics
Synthesis, Structure, and Properties of a Stable 1,2-
Dibromodialumane(4) Bearing a Bulky Aryl Substituent
Tomohiro Agou,†,‡ Koichi Nagata,† Heisuke Sakai,§ Yukio Furukawa,§ and Norihiro Tokitoh*,†
†Institute for Chemical Research and ‡Kyoto University Pioneering Research Unit for Next Generation, Kyoto University, Gokasho,
Uji, Kyoto 611-0011, Japan
§Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, 3-4-1 Okubo,
Shinjuku-ku, Tokyo 169-8555, Japan
S
* Supporting Information
ABSTRACT: The stable 1,2-dibromodialumane(4) Bbp(Br)Al−Al-
(Br)Bbp (1; Bbp = 2,6-bis[bis(trimethylsilyl)methyl]phenyl) was
synthesized by the reduction of the corresponding dibromoalumane
etherate BbpAlBr2·OEt2 (2). The structure and properties of 1 have
been elucidated on the basis of the analyses of spectroscopic and
crystallographic data and are supported by DFT calculations.
espite their potential importance as reactive intermediates
and promising building blocks for unique Al clusters,
hence stabilize the dialumane(4) form. Since then, only a few
examples of dialumanes(4) with the general formula R2Al−AlR2
(R = bulky substituents) free of Lewis base coordination have
been synthesized and isolated (Chart 1b).6,7 Recently,
tetraorganyldialumanes(4) 7 and 8, bearing only two bulky
substituents on the Al−Al moiety, could be synthesized by
trapping reactions of the transient dialumene ArAlAlAr.8
Although 1,2-dihalodialumanes(4) are expected to be potential
building blocks for novel low-coordinated aluminum specie-
s,8a,9−11 dialumanes(4) bearing halogen groups, as in the
prototypical 1,2-dihalodialumane(4) R(X)Al−Al(X)R (X =
halogen, R = bulky substituent), have not yet been systemati-
cally investigated. The lack of extensive studies is probably due
to the difficulties arising from (i) the stabilization of the highly
reactive X−Al−Al−X moiety and (ii) the necessary suppression
of halogen atom migration and intermolecular reactions with
external nucleophiles, resepctively.12 Except for 1,2-diiododia-
lumane(4) 6,8a all halodialumanes(4) reported to date have
been confined to those thermodynamically stabilized by
D
dialumanes(4) (>Al−Al<)1 were long considered to be an
elusive class of compounds.2,3 Their fleetingness was explained
on the basis of theoretical calculations for the structure of Al2H4
(Chart 1a), according to which initial isomerizations should
Chart 1. (a) Relative Energies of Al2H4 Isomers Calculated
at the MP4/6-31G(d,p) Level of Theory and (b) Structurally
Characterized Dialumanes(4) without Lewis Base
Coordination
coordination of Lewis bases to the Al−Al moiety.11,13
A
systematic investigation into halodialumanes(4) free of
coordinated Lewis bases should significantly contribute to the
understanding of the nature of functionalized alumane(4)
derivatives. Here, we describe the synthesis, structure, and
properties of the 1,2-dibromodialumane(4) Bbp(Br)Al−Al-
(Br)Bbp (1), which is the first example of a 1,2-
dibromodialumane(4) without coordination of a Lewis base.
Dibromoalumane etherate 2 was prepared by the reaction of
BbpLi14 with AlBr3 in Et2O. The subsequent reduction with 1.3
equiv of potassium graphite (KC8) in hexane at ambient
temperature afforded 1,2-dibromodialumane(4) 1 as an air- and
moisture-sensitive colorless solid in 57% yield (Scheme 1). In a
vacuum-sealed tube, 1 sublimes above 232 °C without any sign
trigger readly available thermal decomposition mechanisms.
These in turn probably result in the formation of highly reactive
mixed-valent forms.4 Then, in 1988, Uhl demonstrated that it is
possible to kinetically stabilize and isolate dialumane(4) 3 by
the introduction of bulky substituents around the Al−Al
moiety.5 The bulky substituents suppress the aforementioned
isomerizations as well as the subsequent decomposition and
Received: March 21, 2012
Published: April 19, 2012
© 2012 American Chemical Society
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Organometallics
Note
Scheme 1. Synthesis of 1,2-Dibromodialumane(4) 1
of decomposition, showing its impressive thermal stability. The
27Al NMR spectrum of 1 in C6D6 did not exhibit any apparent
signal, even after prolonged measurements (several days). We
tentatively attribute the absence of any 27Al NMR signals to the
quadrupolar effects of the tricoordinated aluminum atoms.
X-ray diffraction analysis on single crystals of 1, obtained by
recrystallization from toluene/hexane at −40 °C, revealed the
molecular structure in the solid state (Figure 1). Dialumane(4)
Figure 2. Raman spectra of 1: (top) solid-state Raman spectrum of 1
(λex 532 nm); (bottom) calculated Raman spectrum of 1 (HWHM = 4
cm−1, without scaling).
0.7071(3s3p0.51)Al), which is in accordance with the Al−Al
bond contraction and the higher Al−Al stretching frequency.
The NBO analysis furthermore suggested that the 4p(Br) →
3p*(Al) π-donations should both decrease the positive charge
located on the Al atom (Al, +1.05 e; Br, −0.50 e) and enhance
the stability of 1, as is evident from the donor−acceptor
stabilization energy, which was obtained from a second-order
perturbation theory analysis (−29.63 kcal mol−1 for each Al−Br
moiety).
The UV−vis spectrum of 1 in toluene showed one broad
absorption band at λmax 302 (ε = 5400), 330 (sh, ε = 3000) and
380 nm (sh, ε = 220 M−1 cm−1) (Figure 3a). TD-DFT
Figure 1. Molecular structure of 1: (a) top view; (b) side view
showing the dihedral angle between the least-squares plane of the aryl
ring and the Al−Al plane (Dis groups are omitted). Thermal
displacement ellipsoids are set at the 50% probability level, and
hydrogen atoms are omitted. (c) Selected bond lengths (Å) and angles
(deg). Structural parameters obtained from the DFT-optimized
geometry of 1 are shown in parentheses (B3PW91/6-31G+G(2df)-
[Al,Br]:6-31G(d)[Si,C,H]).
1 has a crystallographic center of symmetry in the middle of the
Al−Al bond, and the Al atoms exhibit a planar geometry (sum
of the bond angles around the Al atom 360°). Therefore, the
skeletal atoms of the Al−Al moiety in 1 (two C(ipso-Bbp), two
Br, and two Al atoms) lie in the same plane, to which the aryl
rings of the Bbp groups are orientated almost perpendicularly.
The Al−Al bond length in 1 (2.592(3) Å) is shorter than those
of the previously reported dialumane(4) derivatives bearing
four carbon or silicon substituents (cf. 3, 2.660(1) Å;5a 4,
2.647(3) Å;6a 5, 2.751(2) Å6b) and comparable to that of 6
(2.609(2) Å).8a,15,16 The relatively short Al−Al bond length in
1,2-dihalodialumanes(4) 1 and 6 can be explained by the
combination of two electronic effects of the halogen groups: (i)
a relaxation of the electrostatic repulsion between the Al atoms
due to n(X)→3p*(Al) electron donation17 and (ii) an increase
in the s character of the Al−Al bond as a result of the negative
inductive effect of the halogen groups (Bent’s rule).18 The Al−
C and Al−Br bond lengths in 1 are comparable to those in the
dibromoalumane 2,4,6-(t-Bu)3C6H2AlBr2 (Al−C = 1.953(8) Å,
Al−Br = 2.281(2) Å).15,19 The experimentally obtained solid-
state structure of 1 was reproduced in good agreement by the
use of DFT calculations, as shown in Figure 1.20
Figure 3. (a) UV−vis spectra of 1 in toluene and THF. (b) Frontier
Kohn−Sham orbitals of 1 and the lowest energy transition (M062X/6-
311+G(2df)[Al,Br]:6-31+G(d)[Si,C,H]). Hydrogen atoms are omit-
ted for clarity.
calculations for 1 suggested a correlation of the weak
absorption at 380 nm to the σ(Al−Al) → π(Al−Al) electron
transition (Figure 3b) and indicated the absorptions at shorter
wavelengths (λmax 302 and 330 nm) to be the result of a
combination of several electron transitions from π(Bbp)
orbitals to π*(Bbp) and σ*(Al−Br) orbitals.23 The longest
absorption wavelength of 1 is hypsochromically shifted with
respect to those of 4 (λmax 420 nm)6a and 5 (λmax 525 nm).6c,24
The UV−vis spectrum of 1 in THF (Figure 3a) was different
from that in toluene. Only one weak absorption was observed
at λmax 301 nm, indicating the formation of THF adducts
[1(thf)] and [1(thf)2] (Scheme 2). The coordination of THF
molecules to 1 should raise the π(Al−Al) orbital (LUMO) and
increase the HOMO−LUMO energy gap, resulting in the
hypsochromic shifts of the absorption. TD-DFT calculations on
Me2O adducts of the model dialumane Me(Br)Al−Al(Br)Me
The solid-state Raman spectrum of 1 (Figure 2) exhibited a
signal at 453 cm−1, which can be assigned to the Al−Al
stretching vibration (cf. νcalcd 451 cm−1).21 The higher Al−Al
stretching frequency of 1 relative to that of 3 (373 cm−1)5a is
indicative of an increase of the Al−Al bond strength, hence
corroborating the contraction of the Al−Al bond. The natural
bond orbital (NBO) analysis22 for 1 revealed a high s character
of the Al−Al bond (σ(Al−Al) = 0.7071(3s3p0.51)Al +
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Organometallics
Note
Scheme 2. Plausible Coordination Equilibrium between 1
and THF
AUTHOR INFORMATION
Corresponding Author
*Tel: +81-774-38-3200. Fax: +81-774-38-3209. E-mail:
tokitoh@boc.kuicr.kyoto-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was financially supported by Grants-in-Aid for
Scientific Research (B) (No. 22350017), Young Scientist (B)
(No. 21750037), and the Global COE Program B09 from the
Ministry of Education, Culture, Sports, Science and Technol-
ogy of Japan. Computational time was provided by the Super
Computer System, Institute for Chemical Research, Kyoto
University. This study was carried out with the FT-ICR-MS and
the NMR spectrometer in the JURC at the Institute for
Chemical Research, Kyoto University. The synchrotron
radiation experiments were performed at the BL38B1 of the
SPring-8 with the approval of the Japan Synchrotron Radiation
Research Institute (JASRI) (Proposal No. 2011B1296).
(1Me; λcalcd 387 nm, f = 0.0103), [1Me(OMe2)] (λcalcd 331 nm, f
= 0.0268), and [1Me(OMe2)2] (λcalcd 291 nm, f = 0.0526) were
able to reproduce the experimentally observed hypsochromic
shift of the absorption maxima of 1 in THF in good agreement.
Titration of 1 with THF in toluene indicated the stepwise
formation of [1(thf)] and [1(thf)2], and the 1H NMR spectrum
of a mixture of 1 and THF in C6D6 suggested a fast exchange of
the coordinated THF molecules in solution (see the Supporting
Information for details). Removal of all volatiles from this
sample resulted in the quantitative recovery of 1, indicating a
complete reversibility of the THF coordination to the Al−Al
moiety in 1.
We have shown here the synthesis of the first 1,2-
dibromodialumane(4) 1, which is free of Lewis base
coordination. Compound 1 is a thermally stable solid, and its
molecular structure has been unambiguously assigned on the
basis of its spectroscopic and crystallographic properties. The
Al−Al bond in 1 is contracted relative to those of previously
reported tetraorganyldialumane(4) derivatives and exhibits
higher s character, which can be explained in terms of a
combination of σ-withdrawing and π-donating effects of the
bromine substituents. The reactivity of 1, including substitution
and reduction of the Al−Br moiety, is currently the subject of
investigations in our laboratory, and results will be reported in
due time.
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ASSOCIATED CONTENT
* Supporting Information
Text, figures, tables, and CIF files giving experimental
procedures, analytical data for new compounds, computational
results, the complete ref 20, and X-ray crystallographic data.
This material is available free of charge via the Internet at
http://pubs.acs.org.
■
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dx.doi.org/10.1021/om300237q | Organometallics 2012, 31, 3806−3809