Janus-faced aluminum: A demonstration of unique lewis acid and lewis base behavior of the aluminum atom in [LAlB(C6F5)3]

Article abstract of DOI:10.1002/anie.200502251

(Chemical Equation Presented) Some elements go both ways: The reaction of [LAl] (L=HC(CMeNAr)₂, Ar = 2,6-iPr₂C₆H ₃) with BAr^F₃ (Ar^F = C ₆F₅) yields [LAlBAr^F₃] (see scheme). Crystallographic studies and ab initio calculations show the formation of an Al-F bond, which shows the aluminum center having both Lewis base and Lewis acid character.

Full text of DOI:10.1002/anie.200502251

Communications
Aluminum Complexes
charge depletion close to the aluminum atom in the semiplane
of the six-membered ringindicates that 1 is Lewis acidic.
Herein, we report the reaction of [LAl] (1) with B(C6F5)3 to
yield [LAlB(C6F5)3] (2), the first aluminum compound
displayingboth Lewis base and Lewis acid character at the
metal center.
The reaction of a 1:1 molar ratio of [LAl] (1) and B(C6F5)3
in toluene between ꢀ788C and room temperature resulted in
the formation of 2 (Scheme 1). Compound 2 was character-
DOI: 10.1002/anie.200502251
Janus-Faced Aluminum: A Demonstration of
Unique Lewis Acid and Lewis Base Behavior of
the Aluminum Atom in [LAlB(C6F5)3]**
Zhi Yang, Xiaoli Ma, Rainer B. Oswald,
Herbert W. Roesky,* Hongping Zhu, Carola Schulzke,
Kerstin Starke, Marc Baldus, Hans-Georg Schmidt, and
Mathias Noltemeyer
Compounds of aluminum with a formal + 3 oxidation state
on aluminum, such as trihalides, trialkyls, and triaryls, show
classical behavior of Lewis acids.[1] In recent years, another
Scheme 1. Formation of 2. Ar=2,6-iPr2C6H3, ArF =C6F5.
class of compounds containingaluminum with the
+ 1
oxidation state has attracted great interest.[2] These com-
pounds, which have a nonbondinglone pair of electrons at the
aluminum center, are proposed to have singlet, carbene-like
character and they exhibit the potential for Lewis base
behavior. In 2000, Cowley and co-workers reported the first
example of an aluminum(i)–boron donor–acceptor bond, in
ized by 1H, 13C, 11B, 19F, and 27Al NMR spectroscopy, as well as
EI mass spectrometry and elemental analysis. 1H, 13C, 11B, and
19F NMR spectroscopic analysis was carried out at room
temperature in [D6]benzene or [D8]toluene. No resonance
signals were observed in the 27Al NMR spectra of 2 in C6D6 or
C7D8; consequently, the measurement was carried out in the
solid state. The 19F NMR spectrum of 2 exhibits nine partly
overlappingresonances, and therefore an unambiugous
assignment is not possible. However, this pattern indicates a
distorted B(C6F5)3 group caused by an Al–F interaction. The
EI mass spectrum shows the molecular ion of 2 (m/z 956).
Single crystals of 2 suitable for X-ray crystallographic analysis
were obtained by keepingthe hexane solution at room
temperature for two weeks.[8] The solid-state structure con-
sists of individual molecules of the Lewis acid/base adduct
(Figure 1). An Al–F interaction arises from close intramolec-
ular contact between one of the ortho fluorine atoms and the
Al atom with the formation of an AlBC2F five-membered ring
(Figure 2). There is a distorted tetrahedral geometry around
[Cp*Al!B(C6F5)3] (Cp* = C6Me5),[3] and one year later a
[4]
ꢀ
correspondingAl Al bond in [Cp*Al!Al(C6F5)3]. In
neither of these systems did an aluminum center show both
Lewis acid and Lewis base behavior.
The reduction of [I2AlL] (L = HC(CMeNAr)2; Ar= 2,6-
iPr2C6H3) with potassium resulted in the formation of
monomeric [LAl] (1).[5] This was the first stable two-
coordinate aluminum(i) compound to be prepared and
structurally characterized in the solid state. The fascinating
aspect of 1 is its dual Lewis acid and Lewis base character.
Ab initio calculations[6] with analysis of the Laplacian of the
electron density[7] within the plane show a geometrically
active lone pair of electrons on the aluminum atom with a
probable quasi-trigonal-planar orientation of orbitals. This
observation clearly indicates that 1 is Lewis basic. Moreover,
ꢀ
the aluminum atom, with an average Al N bond length of
ꢀ
1.892(6) Š. This distance is considerably shorter than the Al
[*] Z. Yang, X. Ma, Prof. Dr. H. W. Roesky, H. Zhu, Dr. C. Schulzke,
K. Starke, H.-G. Schmidt, M. Noltemeyer
Institut für Anorganische Chemie
N bonds in 1 (av 1.957(6) Š). This observation is consistent
with the partial transfer of the lone pair of electrons on the
aluminum center upon formation of the donor–acceptor
Universität Göttingen
Tammannstrasse 4, 37077 Göttingen (Germany)
Fax: (+49)551-39-3373
ꢀ
bond. The Al B bond in 2 (2.183(3) Š) is slightly longer than
ꢀ
that in [Cp*Al B(C6F5)3] (2.169(2) Š). Also, the geometry of
the B(C6F5)3 group changes from trigonal planar to distorted
tetrahedral in 2. The extent of the geometrical change has
been taken as an indication of the strength of the donor–
acceptor interaction.[9] The sum of the C-B-C angles around
the boron atom in 2 (330.3(2)8) is the smallest of those
(333.5(2)–342.2(2)8) reported for B(C6F5)3 compounds.[3,10,11]
Thus, 1 appears to be a stronger base than {Cp*Al}. However
it must to be considered that the relatively close Al–F contact
in 2 (2.156(2) Š) changes the electron density on the
aluminum center. The noticeable Al–F interaction is indi-
E-mail: hroesky@gwdg.de
Dr. R. B. Oswald
Institut für Physikalische Chemie
Universität Göttingen
Tammannstrasse 6, 37077 Göttingen (Germany)
Dr. M. Baldus
Max-Planck-Institut für Biophysikalische Chemie
Solid-State NMR
Am Fassberg 11, 37077 Göttingen (Germany)
[**] This work was supported by the Göttinger Akademie der Wissen-
schaften and the Fonds der Chemischen Industrie. L=HC-
(CMeNAr)2, Ar=2,6-iPr2C6H3. The Roman god Janus has two faces
looking in opposite directions. Herein, the aluminum is showing its
two “faces” of Lewis acid and Lewis base properties.
ꢀ
cated by the lengthening of the C F bond (1.414(6) Š)
relative to the remaining14 C F bonds (av 1.355 Š). In
addition, the C(41)-B(1)-Al(1) angle is clearly smaller
ꢀ
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Angew. Chem. Int. Ed. 2005, 44, 7072 –7074

Angewandte
Chemie
the fluorine and aluminum centers. The consequence of this
ꢀ
interaction is the elongation of the C(42) F(42) bond by
ꢀ
0.075 Š (calcd 1.4385 Š) relative to the other C F bonds in
the same ring(calcd range 1.3633–1.3636 Š). From natural
bond order (NBO) analysis,[12] the bond between aluminum
and fluorine can be described as the overlap of two hybrid
orbitals of spn type with one located at Al (16.24% s and
83.76% p) and the other at F(42) (11.56% s and 88.44% p).
2.72
The bondingorbital located on C(42) has sp
character,
2.22
whereas the remainingcarbon atoms in this ringhave sp
character. This situation is also clearly visible in the corre-
spondingorbital picture. Figure 3 shows the contour plots of
ꢀ
two orbitals contributingto the formation of the Al F bond.
Figure 1. X-ray crystal structure of 2. Selected bond lengths [Š] and
ꢀ
ꢀ
ꢀ
angles [8]: Al(1) B(1) 2.183(5), Al(1) F(42) 2.156(3), C(42) F(42)
ꢀ
ꢀ
ꢀ
1.414(4), Al(1) N(2) 1.885(4), Al(1) N(1) 1.900(3), C(33) F(33)
1.371(5); C(41)-B(1)-Al(1) 100.4(3), C(51)-B(1)-Al(1) 115.1(3), C(31)-
B(1)-Al(1) 110.1(3), F(42)-Al(1)-B(1) 85.99(15); ꢀaCBC 330.3(2)8.
Figure 2. Depiction of Al-B-C(41)-C(42)-F(42) five-membered ring.
(100.4(3)8) than the Cipso-B-Al angles (115.1(3)8 and
110.1(3)8) of the two non-interactingperfluorophenyl rings.
ꢀ
These data indicate that the lengthening of the C F bond and
the narrowingof the Al-B-C angle are due to the F !Al
interaction and are consistent with a F!Al donor–acceptor
behavior.
It is evident from the crystallographic data that there is a
weak interaction between the aluminum and fluorine F(42)
centers. To gain a better understanding of the bonding
situation, 2 was examined by means of ab initio calculations.
The first and crucial step in these calculations is to reproduce
the crystallographic data with a reliable quantum-chemical
method. Startingfrom this structure, the analysis of the
molecular orbitals and bond order gives the most accurate
picture of the electronic structure.
Figure 3. Schematic representation of the Al–F linkage resulting from
the overlap of wave functions centered on aluminum and fluorine. The
two relevant orbitals are shown here as contour plots. The clearly
visible deformation in the second plot is due to the fact that there is a
ꢀ
The calculated structural parameters (Al F(42) 2.1626 Š,
ꢀ
strong interaction with orbitals forming the Al N bonds.
ꢀ
F(42) C(42) 1.4384 Š, and F(42)-Al-B 85.2368) are in good
agreement with the crystallographic data (Figure 1). The
bond-order analysis reveals that the electron density of the
fluorine atom is distributed between the carbon and alumi-
In conclusion, we have prepared [LAlB(C6F5)3], a unique
compound of aluminum showingLewis base and Lewis acid
character at the aluminum center. There are no known
precedents of this type of bondingin the literature.
ꢀ
ꢀ
num centers with a (Al F)/(F C) ratio of 0.2930/0.7148,
which indicates that there is a significant interaction between
Angew. Chem. Int. Ed. 2005, 44, 7072 –7074
ꢀ 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
7073

Communications
[7] R. F. W. Bader, Chem. Rev. 1991, 91, 893 – 928.
Experimental Section
[8] Crystal data for 2: C47H41AlBF15N2, Mr = 956.61, triclinic, space
All manipulations were performed under a dry, oxygen-free atmos-
¯
group P1, a = 1168.97(12), b = 1265.34(13), c = 1607.85(17) pm,
phere (N2 or Ar) by usingSchlenk and glove-box techniques.
a = 74.641(8), b = 73.606(8), g = 76.347(8)8, V= 2.1664(4) nm3,
Z = 2, 1calcd = 1.466 Mgmꢀ3, F(000) = 980, 1.69 ꢁ q ꢁ 24.668; of
21853 reflections collected, 7306 were independent. The R val-
ues are R1 = 0.0557 and wR2 = 0.0750 (I > 2s(I)); min./max.
residual electron density: 0.297/ꢀ0.305 eŠꢀ3. All non-hydrogen
atoms were located by difference Fourier synthesis and refined
anisotropically. All hydrogen atoms were included at geometri-
cally calculated positions and refined by usinga ridingmodel.
CCDC-273349 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.
cam.ac.uk/data_request/cif.
2: Toluene (20 mL) was added to a mixture of 1 (0.223 g,
0.5 mmol) and B(C6F5)3 (0.256 g, 0.5 mmol) at ꢀ788C. The mixture
was stirred and slowly warmed to room temperature. After stirring
the mixture for an additional 15 h, the solvent was removed under
reduced pressure, and the solution was treated with hexane (30 mL).
The solution was filtered and allowed to stand for two weeks at room
temperature to afford colorless crystals of 2. Yield: 0.09 g(19%); m.p.
208–2098C; EI-MS: m/z (%) 956 (10) [M+], 403 (100) [LꢀMe];
1H NMR (300.13 MHz, C6D6): d = 6.80–6.75 (m, 6H, Ar-H), 4.91 (s,
3
1H, g-H), 2.80 (sept, JH,H = 6.8 Hz, 4H, CHMe2), 1.61 (s, 6H, Me),
1.15 (d, 3JH,H = 6.8 Hz, 12H, CHMe2), 0.87 ppm (d, 3JH,H = 6.8 Hz,
12H, CHMe2); 13C NMR (75.48 MHz, C6D6): d = 173.33 (CN), 142.75,
139.49, 129.27, 124.76 (Ar), 150.25, 147.13, 140.75, 138.63, 137.38,
134.95 (br, C6F5), 102.07 (g-C), 24.53, 25.09(CHMe2), 22.68 (CHMe2),
20.74 ppm (Me). 11B NMR (95.29 MHz, C6D6): d = ꢀ26.52 ppm;
19F NMR (188.28 MHz, C7D8) d = ꢀ124.28 (brm), ꢀ128.26 (brm),
ꢀ129.97 (d), ꢀ154.41 (t), ꢀ156.49 (brm), ꢀ157.27 (t), ꢀ158.86 (brm),
ꢀ160.24 (t), ꢀ160.99 ppm (t); 27Al NMR (400 MHz, 16 KHz, MAS,
AlCl3): d = 0–50 ppm; elemental analysis (%) calcd for
C47H41AlBF15N2 (Mr = 956.61): C 59.01, H 4.32, N 2.93; found: C
58.66, H 4.67, N 2.70.
[9] H. Jacobsen, H. Berke, S. Dꢀring, G. Kehr, G. Erker, H. Frꢀhlich,
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Details of ab initio calculations: The well-established B3LYP[13, 14]
method was employed for all the ab initio calculations because of the
size of the system. Two different basis sets were used for the
computations: a small one as the 3-21G basis set, and an extended one
in which the aluminum atoms are described with functions taken from
[15,16]
the 631-G basis set includingdouble-diffuse functions.
The
Gaussian G03[17] program suite was used to optimize the structure
with the 3-21G basis first, and this structure was used as the starting
geometry for a further optimization with the larger basis set to give an
appropriate description of the aluminum atom and its binding
situation. The resultingstructure was used for visualization of the
orbitals. The nature of the quantum-chemical method results in a
wave function that produces molecular orbitals involvingnearly every
atom. Therefore, this method leads to a picture that, despite being
mathematically correct, is difficult to interpret. A more descriptive
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and F(42) was obtained by analyzingthe bond order followinga
proposal of I. Mayer.[19]
[16] G. A. Petersson, A. Bennett, T. G. Tensfeldt, M. A. Al-Laham,
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[18] S. F. Boys, Rev. Mod. Phys. 1960, 32, 296 – 299.
[19] I. Mayer, Chem. Phys. Lett. 1983, 97, 270 – 274.
Received: June 27, 2005
Published online: October 11, 2005
Keywords: aluminum · boron · fluorine · Lewis acids ·
.
Lewis bases
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Angew. Chem. Int. Ed. 2005, 44, 7072 –7074