Three-coordinate cationic aluminum alkyl complexes incorporating β- diketiminate ligands [15]

Full text of DOI:10.1021/ja991759h

J. Am. Chem. Soc. 1999, 121, 8673-8674
Three-Coordinate Cationic Aluminum Alkyl
8673
Complexes Incorporating â-Diketiminate Ligands
Catherine E. Radzewich,† Ilia A. Guzei,‡ and
Richard F. Jordan*,†
Department of Chemistry, The UniVersity of Iowa,
Iowa City, Iowa 52242 and Department of Chemistry,
Iowa State UniVersity, Ames, Iowa 50011
ReceiVed May 27, 1999
Low-coordinate cationic aluminum complexes are expected to
be highly electrophilic and therefore are of interest for Lewis acid
catalysis, olefin polymerization, and other potential applications.1
i
Bochmann has reported that transient “R2Al+” (R ) Me, Bu)
-
species can be generated in toluene-d8 but abstract C6F5 from
B(C6F5)4 to form neutral Al and B complexes.2 The bis-
-
(cyclopentadienyl) aluminum cation in [(C5H5)2Al][MeB(C6F5)3]
+
is more stable than AlR2 alkyl species and has been exploited
as an initiator for the cationic polymerization of isobutylene.3 No¨th
recently described the synthesis of [Al(NR2)2(L)][AlX4] salts (NR2
) 2,2,6,6-tetramethylpiperidide; L ) pyridine bases; X ) Br, I)
and concluded on the basis of 27Al NMR, conductivity, and
computational results that the Al cations have three-coordinate
structures.4 Several classes of four-coordinate Al cations have also
been reported.5 Here we describe three-coordinate, base-free
aluminum alkyl cations that incorporate â-diketiminate ligands.
The reaction of {HC(CMeNAr)2}AlMe2 (1, Ar ) 2,6-iPr2-
phenyl) with [Ph3C][B(C6F5)4] in C6D6 or C6D5Cl proceeds by
methyl abstraction and yields [{HC(CMeNAr)2}AlMe][B(C6F5)4]
(2) and Ph3CMe (eq 1). Complex 2 is soluble in C6D5Cl, separates
as a liquid clathrate (oil) from benzene, and was isolated as an
off-white solid by the addition of hexanes to a liquid clathrate in
benzene.6 The addition of benzene/hexanes (1:10 by volume) to
the isolated powder of 2, gently heating to 50 °C for 2 days, and
slowly cooling the mixture yielded 2‚benzene as colorless crystals.
Complex 2 crystallizes as an ion pair in which the B(C6F5)4- anion
binds weakly to the {HC(CMeNAr)2}AlMe+ cation through a
meta fluorine (Figure 1).7a The Al-Fmeta contact (Al-F(33),
2.151(1) Å) is significantly longer than typical terminal Al-F
(∼1.65 Å) and bridging Al-F-Al (∼1.80 Å) bond distances.8
Nevertheless, the Al-F(33) interaction results in lengthening of
the C(33)-F(33) bond (1.394(2) Å) by 0.04 Å compared to the
Figure 1. Molecular structure of 2. The hydrogen atoms have been
omitted. Key bond distances (Å) and angles (deg) not given in text: Al-
C(6) 1.905(2), N(1)-Al-N(2) 101.97(6), N(1)-Al-C(6) 125.97(8),
N(2)-Al-C(6) 125.04(7).
average C-F bond length of the anion (1.350(6) Å). The geometry
at Al is slightly distorted from planar to pyramidal: the sum of
angles around Al is 353° and the Al is displaced from the N-N-
1
CH3 plane by 0.28 Å. The -85 °C H NMR spectrum of 2 in
i
CD2Cl2 contains two doublets for the Pr-Me groups, which is
consistent with a C2V-symmetric structure and slow rotation around
the N-aryl bonds. The -85 °C 19F NMR spectrum (CD2Cl2) is
not perturbed from that of free B(C6F5)4-.9 These results indicate
that B(C6F5)4- or CD2Cl2 coordination to the {HC(CMeNAr)2}-
AlMe+ cation, if present, is weak and labile under these
conditions. The 1H and 19F NMR spectra of 2 are unchanged up
to room temperature.
(1)
* Address correspondence to this author at Department of Chemistry, The
University of Chicago, 5735 S. Ellis Ave., Chicago, IL 60637.
† The University of Iowa.
The reaction of 1 with B(C6F5)3 proceeds by methyl abstraction
and yields [{HC(CMeNAr)2}AlMe][B(C6F5)3Me] (3, eq 2).
‡ Iowa State University.
(1) (a) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125. (b)
Ihara, E.; Young, V. G., Jr.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 8277.
(c) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 9384.
(2) Bochmann, M.; Sarsfield, M. J. Organometallics 1998, 17, 5908.
(3) (a) Bochmann, M.; Dawson, D. M. Angew. Chem., Int. Ed. Engl. 1996,
35, 2226. (b) Dohmeier, C.; Schno¨ckel, H.; Robl, C.; Schneider, U.; Ahlrichs,
R. Angew. Chem., Int. Ed. Engl. 1993, 32, 1655.
(4) Krossing, I.; No¨th, H.; Schwenk-Kircher, H. Eur. J. Inorg. Chem. 1998,
927, 7.
(5) (a) Cosle´dan, F.; Hitchcock, P. B.; Lappert, M. F. Chem. Commun.
1999, 705. (b) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White,
A. J. P.; Williams, D. J. Chem. Commun. 1998, 2523. (c) Atwood, D. A.;
Jegier, J. Inorg. Chem. 1996, 35, 4277. (d) Emig, N.; Nguyen, H.; Krautscheid,
H.; Re´au, R.; Cazaux, J.-B.; Bertrand, G. Organometallics 1998, 17, 3599.
(e) Atwood, D.; Jegier, J. Chem. Commun. 1996, 1507. (f) Uhl, W.; Wagner,
J.; Fenske, D.; Baum, G. Z. Anorg. Allg. Chem. 1992, 612, 25. (g) Self, M.
F.; Pennington, W. T.; Laske, J. A.; Robinson, G. H. Organometallics 1991,
10, 36. (h) Kynast, U.; Kelton, B. W.; White, A. H.; Henderson, M. J.; Raston,
C. L. J. Organomet. Chem. 1990, 384, C1. (i) Engelhardt, L. M.; Kynast, U.;
Raston, C. L.; White, A. H. Angew. Chem., Int. Ed. Engl. 1987, 26, 681. (j)
Review: Atwood, D. A. Coord. Chem. ReV. 1998, 176, 407.
(6) Leading references to liquid clathrate phenomena; (a) Atwood, J. L. In
Coordination Chemistry of Aluminum; Robinson, G. H., Ed.; VCH: New York,
1993; pp 197-232. (b) Lambert, J. B.; Zhao, Y.; Wu, H.; Tse, W. C.;
Kuhlmann, B. J. Am. Chem. Soc. 1999, 121, 5001.
(7) (a) X-ray data for 2 benzene: triclinic, P1h, a ) 13.6005(7) Å, b )
14.6761(7) Å, c ) 15.9556(8) Å, R ) 68.142(1)°, â ) 82.499(1)°, γ ) 75.838-
(1)°, V ) 2863.2(2) Å3, Z ) 2, T ) 173(2) K, Dcalc ) 1.411 g/cm3, R1 )
0.0374, wR2 ) 0.0936 (I g 2σ(I)). (b) X-ray data for 3: triclinic, P1h, a )
14.0700(7) Å, b ) 16.6044(9) Å, c ) 20.36(1) Å, R ) 99.262(1)°, â ) 92.093-
(1)°, γ ) 98.103(1)°, V ) 4639.3(4) Å3, Z ) 4, T ) 173(2) K, Dcalc ) 1.413
g/cm3, R1 ) 0.0368, wR2 ) 0.0898 (I g 2σ(I)). All nonhydrogen atoms were
refined with anisotropic displacement coefficients. All hydrogen atoms were
treated as idealized contributions. Software and sources of the scattering factors
are contained in the SHELXTL (version 5.1) program library (G. Sheldrick,
Bruker Analytical X-ray Systems, Madison, WI). Absorption corrections were
applied using the program SADABS (Blessing, R. H. Acta Crystallogr., Sect.
A 1995, 51, 33-38). Further details concerning the synthesis, characterization,
and crystallographic analysis of 2‚benzene and 3 are given in the Supporting
Information.
(8) (a) [((C5Me5)AlF)2SiPh2]2 contains Al-F-Al bridges (Al-F, 1.85 Å
ave). Schulz, S.; Schoop, T.; Roesky, H. W.; Ha¨ming, L.; Steiner, A.; Irmer,
R. H. Angew. Chem., Int. Ed. Engl. 1995, 34, 919. (b) [((Me3Si)3C)AlF2]3
contains terminal Al-F bonds (Al-F, 1.67 Å ave) and bridging Al-F-Al
interactions (Al-F, 1.80 Å ave). Schnitter, C.; Klimek, K.; Roesky, H. W.;
Albers, T.; Schmidt, H.-G.; Ro¨pken, C.; Parisini, E. Organometallics 1998,
17, 2249. (c) For a review of fluorocarbon coordination chemistry see: Plenio,
H. Chem. ReV. 1997, 97, 3363.
(9) The 19F NMR spectrum of 2 is identical to that of [Ph3C][B(C6F5)4];
see ref 1b.
10.1021/ja991759h CCC: $18.00 © 1999 American Chemical Society
Published on Web 09/03/1999

8674 J. Am. Chem. Soc., Vol. 121, No. 37, 1999
Communications to the Editor
The -35 °C NMR spectra of 3 in CD2Cl2 contain {HC-
(CMeNAr)2}AlMe+ resonances that are identical to those of 2
and B(C6F5)3Me- resonances that are very similar to those of
[NBu3Bz][B(C6F5)3Me]. These results indicate that 3 exists as
free or weakly ion-paired or solvated ions in CD2Cl2 solution
under those conditions. However, the behavior of 3 in toluene-d8
is more complex. The -13 °C 1H NMR spectrum of 3 in toluene-
d8 contains separate Al-Me (δ -0.83) and B-Me (δ 1.36)
resonances. The latter resonance is slightly shifted from that of
[NBu3Bz][B(C6F5)3Me] (δ 1.09), probably due to ion-pairing
effects. The Al-Me and B-Me resonances broaden and coalesce
(23 °C) as the temperature is raised, and at 57 °C, a single Al-
Me/B-Me resonance is observed at δ -0.52, close to the position
expected for 1. Additionally, the 19F NMR spectrum changes
significantly as the temperature is raised; in particular the p-F
resonance broadens and shifts from δ -163.2 at -3 °C to δ
-147.2 at 37 °C. At 62 °C, the 19F NMR spectrum of 3 (δ -129.6,
-147.2, -162.0) is very similar to that of B(C6F5)3 (δ -129.2,
-142.9, -160.7) although the p-F resonance is still broadened.
Concurrent with these changes, the 11B NMR resonance of 3 (δ
-15.0 at -58 °C) broadens into the baseline as the temperature
is raised, as expected for conversion of four-coordinate B to three-
coordinate B. These spectral changes are reversible and show that
3 undergoes substantial reversion to 1 and B(C6F5)3 at higher
temperatures, as indicated in eq 2. The observation of Me- rather
than C6F5- transfer from B(C6F5)3Me- to {HC(CMeNAr)2}AlMe+
is surprising given that Al-MeB interactions are not observed
in the solid-state structure of 3 and similar compounds decompose
Figure 2. Structure of the “triple ion” of 3. The hydrogen atoms have
been omitted. Key bond distances (Å) and angles (deg) not given in text:
Al(1)-C(6) 1.902(2), Al(2)-C(6A) 1.907(2), N(1)-Al(1)-N(2) 103.18(6),
N(1)-Al(1)-C(6) 126.62(7), N(2)-Al(1)-C(6) 125.20(7), N(1A)-
Al(2)-N(2A) 102.86(6), N(1A)-Al(2)-C(6A) 128.37(7), N(2A)-Al(2)-
C(6A) 128.46(7).
Compound 3 is soluble in C6D5Cl, benzene, and toluene and was
isolated as a powder by the addition of hexanes to a solution of
3 in benzene, followed by filtration. Crystals of 3 were obtained
by crystallization from toluene-d8.7b The asymmetric unit contains
two cations and two anions which crystallize as a [{HC-
(CMeNAr)2}AlMe]2[B(C6F5)3Me]+ “triple ion” (Figure 2) and a
free B(C6F5)3Me- anion. One cation/anion pair (Al(1)/B(1)) within
the triple ion interacts through a Al-Fmeta contact (Al(1)-F(33)
) 2.275(1) Å) that is similar to that observed for 2, and the
C(33)-F(33) bond is correspondingly lengthened to 1.383(2) Å.
The sum of the angles around Al(1) is 355°, and the displacement
of Al(1) from the N-N-CH3 plane is 0.24 Å. The Al(2) cation
interacts with the B(1) anion by two very long Al-F contacts to
ortho and meta fluorines of the same C6F5 ring that interacts with
Al(1). These Al-F distances are extremely long (Al(2)-F(35),
3.129(4) Å; Al(2)-F(36), 3.411(5) Å), and Al(2) is best regarded
as a base-free, three-coordinate cation. The geometry at Al(2) is
planar: the sum of the angles around Al(2) is 360° and the
displacement of Al(2) from the N-N-CH3 plane is only 0.06
Å. The closest Al-F contacts involving the B(2) borate anion
are 4.97 Å (Al(1)) and 5.55 Å (Al(2)), and there are no Al-
MeB contacts.
by irreversible C6F5 transfer.10
-
This work shows that three-coordinate cationic aluminum alkyl
complexes can be generated by alkyl abstraction from neutral
dialkyl precursors by [Ph3C][B(C6F5)4] or B(C6F5)3. The electronic
properties and steric bulk of the â-diketiminate ligand contribute
to the stability of the {HC(CMeNAr)2}AlMe+ cation. The Al-N
bond lengths of 2 (Al-N(1) 1.824(1), Al-N(2) 1.828(1) Å) and
3 (Al(1)-N(1) 1.822(1), Al(1)-N(2) 1.827(1), Al(2)-N(1A)
1.822(1), Al(2)-N(2A) 1.812(1) Å) are ∼0.1 Å shorter than those
of 1 (Al(1)-N(1) 1.920(2), Al(1)-N(5) 1.942(2) Å),1c which is
indicative of increased ionic interactions or N-Al π-bonding in
the cationic species. The sterically demanding 2,6-iPr2-phenyl
groups disfavor the formation of dinuclear species and hinder
exchange reactions with the anion. Similar strategies based on
bulky alkoxide, amide, or alkyl ligands have been exploited in
the synthesis of neutral three-coordinate aluminum complexes.11
The electrophilic character of the {HC(CMeNAr)2}AlMe+ cation
is manifested by ion-pairing with the weakly nucleophilic
B(C6F5)4- and B(C6F5)3Me- anions in the solid state. These Al-
F-C interactions perturb the Al coordination geometry and
lengthen the C-F bonds but appear to be weaker than the M-F
interactions observed in transition metal salts of these anions.12
For example, ion pairing in [(C5Me5)2ZrH][B(C6F5)3H] (4) occurs
through Zr-F contacts to meta and ortho flourines of the anion
(Zr-Fortho ) 2.416(3), Zr-Fmeta ) 2.534(3) Å).13 These Zr-F
contacts are ∼10% shorter than the Al-F contacts in 2 when
corrected for the difference in ionic radii between Zr(IV) and
Al(III).14 Furthermore, the Zr-F interactions in 4 can be detected
in toluene solution by 19F NMR below -30 °C, whereas Al-F
interactions are not detected for 2 and 3 in solution by 19F NMR.
The difference in the solution behavior of 2 and 3 shows that,
as for transition metal systems, the anion plays a key role in
stabilizing cationic main group species. The reactivity of 2, 3,
and related complexes will be discussed in a future report.1c
(2)
(10) Qian, B.; Ward, D. L.; Smith, M. R., III Organometallics 1998, 17,
3070.
(11) (a) Rahman, A. F. M. M.; Siddiqui, K. F.; Oliver, J. P. Organometallics
1982, 1, 881. (b) Healy, M. D.; Barron, A. R. Angew. Chem., Int. Ed. Engl.
1992, 31, 921. (c) Barron, A. R.; Dobbs, K. D.; Franci, M. M. J. Am. Chem.
Soc. 1991, 113, 39. (d) Wehmschulte, R. J.; Power, P. P. J. Am. Chem. Soc.
1997, 119, 8387-8388. (e) Petrie, M. A.; Olmstead, M. M.; Power, P. P. J.
Am. Chem. Soc. 1991, 113, 8704. (f) Ruhlandt-Senge, K.; Power, P. P. Inorg.
Chem. 1991, 30, 2633. (g) Wehmschulte, R. J.; Ruhlandt-Senge, K.; Power,
P. P. Inorg. Chem. 1994, 33, 3205. (h) Sheldrick, G. M.; Sheldrick, W. S. J.
Chem. Soc. (A) 1969, 2279. (i) Waggoner, K. M.; Hope, H.; Power, P. P.
Angew. Chem., Int. Ed. Engl. 1988, 27, 1699. (j) Petrie, M. A.; Ruhlandt-
Senge, K.; Power, P. P. Inorg. Chem. 1993, 32, 1135. (k) Wehmschulte, R.
J.; Power, P. P. J. Am. Chem. Soc. 1996, 118, 791. (l) Waggoner, K. M.;
Ruhlandt-Senge, K.; Wehmschulte, R. J.; He, X.; Olmstead, M. M.; Power,
P. P. Inorg. Chem. 1993, 32, 2557. (m) Healy, M. D.; Power, M. B.; Barron,
A. R. Coord. Chem. ReV. 1994, 94, 63.
Acknowledgment. This work was supported by DOE Grant DE-
FG02-88ER13935 and the Eastman Chemical Company.
(12) (a) Horton, A. D.; Orpen, A. G. Organometallics 1991, 10, 3910. (b)
Siedle, A. R.; Newmark, R. A.; Lamanna, W. M.; Huffman, J. C. Organo-
metallics 1993, 12, 1491. (c) Yang, X.; Stern, C. L.; Marks, T. J. Organo-
metallics 1991, 10, 840.
(13) Yang, X.; Stern, C. L.; Marks, T. J. J. Am. Chem. Soc. 1994, 116,
10015.
(14) Shannon, R. D. Acta Crystallogr., Sect. A 1976, 32, 751.
Supporting Information Available: Synthetic procedures, charac-
terization data for new compounds, and details of the X-ray structure
determinations of 2 and 3 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA991759H