A Lewis acid adduct of an alanediyl: An aluminum(I)-boron donor-acceptor bond [1]

Full text of DOI:10.1021/ja993537p

950
J. Am. Chem. Soc. 2000, 122, 950-951
Communications to the Editor
A Lewis Acid Adduct of an Alanediyl: An
Aluminum(I)-Boron Donor-Acceptor Bond
John D. Gorden, Andreas Voigt, Charles L. B. Macdonald,
Joel S. Silverman, and Alan H. Cowley*
Department of Chemistry and Biochemistry
The UniVersity of Texas at Austin
Austin, Texas 78712
ReceiVed October 1, 1999
Despite a recent surge of interest in the lower oxidation state
chemistry of the group 13 elements,1 much less is known about
monomeric species of the type RM(I) (M ) B, Al, Ga, In) than
the more familiar carbenes, nitrenes, and their heavier congeners.
Theoretical studies2 indicate that, regardless of the nature of the
substituent R, the ground state of each four-valence-electron RM-
(I) species is a singlet. In the particular case of (η5-C5Me5)Al,
the DFT-calculated singlet-triplet energy gap is between 67.6
and 70.9 kcal/mol, depending on the basis set employed.3
Moreover, the a1-symmetry HOMO of this alanediyl possesses
distinctly lone pair character suggestive of potential Lewis base
behavior. We report the synthesis and X-ray crystal structure of
(η5-C5Me5)Al f B(C6F5)3 (1), the first example of an aluminum
(I)-boron donor-acceptor bond.
Figure 1. Molecular structure of (η5-C5Me5)Al f B(C6F5)3 (1) showing
the atom numbering scheme. Important distances (Å) and angles (deg):
Al-B 2.169(3), Al-C(1) 2.164(3), Al-C(2) 2.185(3), Al-C(3) 2.179-
(3), Al-C(4) 2.160(2), Al-C(5) 2.166(2), Al-(ring centroid) 1.802(3),
B-C(11) 1.633(3), B-C(17) 1.634(3), B-C(23) 1.637(3), B-Al-X (ring
centroid) 172.9(1), C(11)-B-C(17) 114.7(2), C(11)-B-C(23) 111.3-
(2), C(17)-B-C(23) 113.8(2).
group to aluminum.5 For comparison, the 27Al chemical shifts
for uncoordinated monomeric Al(η5-C5Me5) and tetrameric [Al-
(η5-C5Me5)]4 are δ ) -80 and -150, respectively.1c The
foregoing spectroscopic conclusions were confirmed by X-ray
crystallography.8 Compound 1 crystallizes in the P1h space group
with Z ) 2; the solid state consists of individual molecules of
the Lewis acid-base adduct (Figure 1) and there are no unusually
short intermolecular contacts. The C5Me5 group is attached in an
η5 fashion and ring centroid-Al-B moiety is essentially linear
(172.9(1)°). The average Al-C distance of 2.171(3) Å is
considerably shorter than those determined for Al(η5-C5Me5)
(2.388(7) Å)9 and [Al(η5-C5Me5)]4 (2.344 Å).10 Such shortening
The addition of toluene (30 mL) to a mixture of [Al(η5-C5-
Me5)]44 (0.15 g, 0.93 mmol of Al(η5-C5Me5) units) and B(C6F5)3
(0.47 g, 0.92 mmol) resulted in a yellow-colored solution. After
being stirred for 16 h at room temperature, the reaction mixture
was filtered, and the solvent and volatiles were removed from
the filtrate to afford a purple oil from which a 40% yield of
colorless crystals of 1 (mp 126-129 °C dec) deposited over a
period of days. Mass spectral data5 were consistent with the
proposed Lewis acid-base adduct formulation. Moreover, the 11
B
NMR chemical shift for 15 fell in the tetracoordinate boron region
and the 19F chemical shifts of the (equivalent) C6F5 groups5 were
similar to those observed for other Lewis base complexes of
B(C6F5)3.6 The 27Al NMR chemical shift of the broad singlet
resonance of 1 (δ -59.4) was reasonably close to the value of δ
-71.5 computed by the GAIO method,7 and the equivalence of
the methyl protons was suggestive of η5-attachment of the Me5C5
(6) For a selection of structurally characterized donor adducts of B(C6F5)3,
see: (a) Bradley, D. C.; Hursthouse, M. B.; Motevalli, M.; Zheng, D. H. J.
Chem. Soc., Chem. Commun. 1991, 7. (b) Ro¨ttger, D.; Erker, G.; Fro¨hlich,
R.; Kotila, S. J. Organomet. Chem. 1996, 518, 17. (c) Bradley, D. C.; Harding,
I. S.; Keefe, A. D.; Motevalli, M.; Zheng, D. H. J. Chem. Soc., Dalton Trans.
1996, 3931. (d) Parks, D. J.; Piers, W. J. Am. Chem. Soc. 1996, 118, 9440.
(e) Parks, D. J.; Piers, W.; Parvez, M.; Atencio, R.; Zaworotko, M. J.
Organometallics 1998, 17, 1369. (f) Jacobsen, H.; Berke, H.; Do¨ring, S.; Kehr,
G.; Erker, G.; Fro¨hlich, R.; Meyer, O. Organometallics 1999, 18, 1724.
(7) Ditchfield, R. Mol. Phys. 1974, 27, 789; Wolinski, K.; Hinton, J. F.;
Pulay, P. J. Am. Chem. Soc. 1990, 122, 8251. This single-point calculation
employed the X-ray crystal structure parameters for 1.
(1) See, for example: (a) Brothers, P. J.; Power, P. P. AdV. Organomet.
Chem. 1996, 39, 1. (b) Uhl, W. Angew. Chem., Int. Ed. Engl. 1993, 32, 1386.
(c) Dohmeier, C.; Loos, D.; Schno¨ckel, H. Angew. Chem., Int. Ed. Engl. 1996,
35, 129.
(2) For alanediyls, see (a) Ahlrichs, R.; Ehrig, M.; Horn, H. Chem. Phys.
Lett. 1991, 183, 227. (b) Schneider, U.; Ahlrichs, R.; Horn, H.; Scha¨fer, A.
Angew. Chem., Int. Ed. Engl. 1992, 31, 353. (c) Gauss, J.; Schneider, U.;
Ahlrichs, R.; Dohmeier, C.; Schno¨ckel, H. J. Am. Chem. Soc. 1993, 115, 2402.
(d) Purath, A.; Dohmeier, C.; Ecker, A.; Schno¨ckel, H. Organometallics 1998,
17, 1894.
(8) Crystal data for 1: C28H15AlBF15, triclinic, P1h, a ) 9.534(2) Å, b )
9.902(2) Å, c ) 15.658(3) Å, R ) 91.04(3), â ) 104.10(3), γ ) 105.93(3)°,
V ) 1372.9(5) Å3, Z ) 2, Dcalcd ) 1.631 g cm-3, µ(Mo KR) 0.195 mm-1. A
suitable single of 1 was covered with mineral oil and mounted on a Nonius-
Kappa CCD diffractometer at 153 K. A total of 11 088 independent reflections
were collected in the range 5.9 < 2θ < 55.0° using Mo KR radiation (λ )
0.71073 Å). Of these, 6252 were considered observed (I > 2.0 σ(I)) and were
used to solve (direct methods) and refine (full-matrix, least-squares on F2)
the structure of 1; wR2 ) 0.1372, R ) 0.0549. Crystal data for 2: C22H15-
AlF10, orthorhombic, Pnma, a ) 9.049(2) Å, b ) 19.160(4) Å, c ) 11.902(2)
Å, V ) 2063.6(7) Å3, Z ) 4, Dcalcd ) 1.598 g cm-3, µ(Mo KR) 0.195 mm1.
A suitable single crystal of 2 was covered with mineral oil and mounted on
a Nonius-Kappa CCD diffractometer at 153 K. A total of 4469 independent
reflections were collected in the range 6.04 < 2θ < 73.32° using Mo KR
radiation (λ ) 0.71073 Å). Of these, 2435 were considered observed (I > 2.0
σ(I)) and were used to solve (direct methods) and refine (full-matrix, least-
squares on F2) the structure of 2; wR2 ) 0.1948, R ) 0.0684.
(3) Macdonald, C. L. B.; Cowley, A. H. J. Am. Chem. Soc. 2000, in press.
(4) Prepared by the method of Schulz, S.; Roesky, H. W.; Koch, H. J.;
Sheldrick, G. M.; Stalke, D.; Kuhn, A. Angew. Chem., Int. Ed. Engl. 1993,
32, 1729.
(5) 1: MS (CI, CH4) m/z 675 (0.93%) [M + H]+; 512 (66.98%) [B(C6F5)3]+;
164 (2.02%) [(C5Me5)AlH2]+. HRMS (CI, CH4) calcd for C28H16AlBF14,
1
655.0859; found 655.0884. H NMR (300.00 MHz, 295 K, C6D6) δ 1.39 (s,
15H, C5Me5). 19F NMR (282.72 MHz, 295 K, C6D6) δ -127.2 (s, m-C6F5),
δ -154.9 (s, p-C6F5), δ -159.8 (s, o-C6F5). 11B NMR (96.28 MHz, 295 K,
C6D6) δ -32.9 (s). 27Al NMR (78.21 MHz, 295 K, C6D6) δ -59.4 (br,
w1/2 ) 1564 Hz). 2: MS (CI, CH4) m/z 496 (17.95%) (M+); 477 (36.71%)
[M - F]+ 329 (100%) [M - C6F5)]+. HRMS (CI, CH4 calcd for C22H15AlF10,
496.0829; found 496.0817. 1H NMR (300.00 MHz), 295 K, C6D6) δ 1.63 (s,
15H, C5Me5). 19F NMR (282.78 MHz, 295K, C6D6) δ -119.0 (s, m-C6F5),
δ -149.0 (s, p-C6F5), -155.8 (s, o-C6F5). 27Al NMR (78.21 MHz, 295 K,
C6D6) δ 57.6 (br, w1/2 ) 4505 Hz).
(9) Haaland, A.; Kjell-Gunnar, M.; Shlykov, S. A.; Volden, H. V.;
Dohmeier, C.; Schno¨ckel, H. Organometallics 1995, 14, 3116.
(10) Yu, Q.; Purath, A.; Donchev, A.; Schno¨ckel, H. J. Organomet. Chem.
1999, 584, 94.
10.1021/ja993537p CCC: $19.00 © 2000 American Chemical Society
Published on Web 01/12/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 5, 2000 951
is anticipated as the aluminum lone pair is transformed into the
donor-acceptor bond with the concomitant development of partial
positive and negative charges on aluminum and boron, respec-
tively. There is a very little information in the literature with which
to compare the Al-B bond distance of 1 (2.169(3) Å). In the
hydride-bridged complexes Me3NAl(η2-H2BH2)311 and [η5-C5H5)-
Ti(µ2-H)2]2Al(η2-H2BH2)12 the average Al-B separations are 2.18-
(2) to 2.27(3) Å, respectively, while in a variety of aluminum-
substituted carboranes, these distances range from ∼2.13 to 2.24
Å.13 A DFT calculation14 on the model compound (η5-C5Me5)-
AlBH3 revealed that the global minimum possesses a “staggered”
Cs geometry similar to that observed for 1 with a computed Al-B
bond distance of 2.127 Å. As a consequence of donor action on
the part of the alanediyl, the geometry of B(C6F5)3 changes from
trigonal planar to distorted tetrahedral. The sum of bond angles
at boron is 339.8(2)°, and to the extent that this geometrical
change is a measure of the strength of the donor-acceptor
interactions, it is interesting to note an almost identical sum of
bond angles in (C6H5)3PB(C6F5)3.6f
Figure 2. Molecular structure of (C6F5)2Al(η3-C5Me5) (2) showing the
atom numbering scheme. Important distances (Å) and angles (deg): Al-
C(1) 2.018(3), Al-C(11) 1.672(3), Al-C(12) 2.067(3), C(1)-Al-C(1)*
103.5(2), C(11)-Al-C(12) 46.09(13).
2.183(2) Å),10 suggesting the existence of the same donor-
acceptor bonding mode in both cases.
Interestingly, when [Al(η5-C5Me5)]4 was treated with In(C6F5)3
using the same procedure as that described above for the B(C6F5)3
reaction, the product was colorless, crystalline (C6F5)2Al(η3-C5-
Me5) (2) (mp 158 °C). The proposed formulation for 2 was
consistent with mass spectral data5 and the presence of C6F5 and
The present results have a bearing on the current debate16
concerning the nature of the bonding between group 13 univalent
ligands, RM, and transition metal carbonyl fragments, M′(CO)n.
Much of the discussion has centered on whether the bonding is
of the donor-acceptor type, viz. RM f M′(CO)n, or whether
M′-to-M back-bonding is important as reflected by the canonical
r
1
C5Me5 groups was evident from 19F and H NMR spectroscopic
data;5 however, to establish for example the hapticity of the
cyclopentadienyl ring it was necessary to perform an X-ray crystal
structure.8 Individual molecules of 2 crystallize in the ortho-
rhombic space group Pnma with Z ) 4; there are no unusually
short intermolecular contacts (Figure 2). The C5Me5 group is
attached to aluminum in an η3 fashion, a coordination mode that
has been seen previously only in the case of the dimers [(η3-C5-
Me5)(R)Al-η-Cl]2 (R ) Me, i-Pr).18 The Al-C(11) and Al-C(12)
distances are 1.672(3) and 2.067(3) Å, respectively while the Al-
(1)...C(13) distance is 2.687 Å. The Al-C(1) distance of 2.018-
(3) Å in 2 is slightly longer than those in the THF (1.995(3) Å),19
benzene (1.979(7) Å),20 and toluene (1.984(2) Å)20 complexes of
Al(C6F5)3. It is possible that 2 was produced via C6F5 transfer
from the adduct (η5-C5Me5)Al f In(C6F5)3. Such a view would
be consistent with the modest In-C bond energy and the relative
stability of the In(I) oxidation state.
forms RM a M′(CO)n and RM
M′(CO) . The isolation of 1
n
a
proves that an alanediyl can function as a pure donor ligand
because there is no question of back-bonding in this particular
case. Moreover, the experimental structural parameters and the
DFT computed charge distribution and orbital occupancy for the
alanediyl fragment of 13 are very similar to those of the terminal
alanediyl transition metal complexes (η5-C5Me5)AlFe(CO)4 (av
Al-C ) 2.147(8) Å)17 and (η5-C5Me5)AlCr(CO)5 (av Al-C )
(11) Bailey, N. A.; Bird, P. H.; Wallbridge, M. G. H. Inorg. Chem. 1968,
7, 1575.
(12) Sizov, A. I.; Molodnitskaya, I. V.; Bulychev, B. M. Bel’skii, V. K.;
Soloveichik, G. L. J. Organomet. Chem. 1988, 344, 185.
(13) See, for example, Schubert, D. M.; Bandman, M. A.; Rees, W. S.,
Jr.; Knobler, C. B.; Lu, P.; Nam, W.; Hawthorne, M. F. Inorg. Chem. 1990,
9, 2046.
(14) All BP8615 calculations were performed using the Gaussian 94 (revision
B2) suite of programs. All-electron basis sets were used for C, H, F. (6-31G
(d)) and the group 13 elements (6-31+G(d)). Frisch, M. J.; Trucks, G. W.;
Schlegel, H. B.; Gill, P. M. W.; Johnson, B. G.; Robb, M. A.; Cheeseman, J.
R.; Keith, T.; Petersson, G. A.; Montgomery, J. A.; Raghavachari, K.; Al-
Laham, M. A.; Zakrzewski, V. G.; Ortiz, J. V.; Foresman, J. B.; Cioslowski,
J.; Stefanov, B. B.; Nanayakkara, A.; Challacombe, M.; Peng, C. Y.; Ayala,
P. Y.; Chen, W.; Wong, M. W.; Andres, J. L.; Replogle, E. S.; Gomperts, R.;
Martin, R. L.; Fox, D. J.; Binkley, J. S.; Defrees, D. J.; Baker, J.; Stewart, J.
P.; Head-Gordon, M.; Gonzalez, C.; Pople, J. A. Gaussian 94, revision B2;
Gaussian, Inc.: Pittsburgh, PA, 1995.
Acknowledgment. We are grateful to the National Science Foundation
and the Robert A. Welch Foundation for support.
Supporting Information Available: X-ray experimental details with
positional parameters and full bond distances and angles for 1 and 2 and
a summary of the DFT calculations (PDF). An X-ray crystallographic
file, in CIF format is also available. This material is available free of
charge via the Internet at http://pubs.acs.org.
JA993537P
(15) Becke, A. D. Phys. ReV. 1988, 38, 3098; Perdew, J. P. Phys. ReV.
1986, 33, 8822.
(18) Shonberg, P. R.; Paine, R. T.; Campana, C. F. J. Am. Chem. Soc.
1979, 101, 7726; Schonberg, P. R.; Paine, R. T.; Campana, C. F.; Duesler, E.
N. Organometallics 1982, 1, 799.
(19) Belgardt, T.; Storre, J.; Roesky, H. W.; Noltemeyer, M.; Schmidt,
H.-G. Inorg. Chem. 1995, 34, 3821.
(20) Hair, G. S.; Cowley, A. H.; Jones, R. A.; McBurnett, B. G.; Voigt, A.
J. Am. Chem. Soc. 1999, 121, 4922.
(16) (a) Su, J.; Li, X.-W.; Crittendon, C.; Campana, C. F.; Robinson, G.
H. Organometallics 1997, 16, 4511. (b) Cotton, F. A.; Feng, X. Organome-
tallics 1998, 17, 128. (c) Boehme, C.; Frenking, G. Chem. Eur. J. 1999, 5,
2184. (d) Chem. Eng. News March 16, 1998, p 31 and August 2, 1999, p 23.
(17) Weiss, J.; Stetzkamp, D.; Nuber, B.; Fischer, R. A.; Boehme, C.;
Frenking, G. Angew. Chem., Int. Ed. Engl. 1997, 36, 70.