Reactivity of cationic organoaluminum aminotroponiminate compounds with unsaturated substrates. Formation of dinuclear dicationic aluminum complexes [20]

Full text of DOI:10.1021/ja992599c

J. Am. Chem. Soc. 1999, 121, 11605-11606
11605
Scheme 1
Reactivity of Cationic Organoaluminum
Aminotroponiminate Compounds with Unsaturated
Substrates. Formation of Dinuclear Dicationic
Aluminum Complexes
Andrey V. Korolev,† Ilia A. Guzei,‡ and Richard F. Jordan*,†
Department of Chemistry, The UniVersity of Iowa
Iowa City, Iowa 52242
Department of Chemistry, Iowa State UniVersity
Ames, Iowa, 50011
ReceiVed July 22, 1999
Cationic organoaluminum compounds have drawn attention
because of their potential applications as Lewis acids and as
catalysts for the coordination polymerization of olefins.1 Cationic
aluminum alkyls containing bi- or tridentate N-chelating ligands
including amidinates,2 aminotroponiminates,3 â-diketiminates,4
pyridyliminoamides,5 and diethylenetriamides6 have been syn-
thesized recently, and several exhibit catalytic activity for ethylene
polymerization. However, little is known about the reactivity
properties of these species. Here we report that base-free
Unlike 1a-c, 3 is insoluble in C6H5Cl and only sparingly
soluble in CH2Cl2. An X-ray crystallographic analysis estab-
lished that 3 crystallizes from CH2Cl2 as the complex salt
[{(iPr2-ATI)Al(µ-OiPr)}2][B(C6F5)4]2‚2CH2Cl2 (3‚2CH2Cl2). The
structure of the dinuclear cation of 3 is shown in Figure 1. The
planar Al2O2 ring features an acute O(1)-Al(1)-O(1A) angle
(80.2(1)°) and an obtuse Al(1)-O(1)-Al(1A) angle (99.8(1)°),
and is nearly perpendicular to the ATI rings (angle between planes
) 93.2°). The Al-N bond distances (Al(1)-N(1) 1.820(3) Å,
Al(1)-N(2) 1.832(3) Å) are ca. 0.05 Å shorter than those in the
dinuclear monocation {(iPr2-ATI)AlMe}2(µ-Me)+,3 and ca. 0.09
Å shorter than those in neutral (iPr2-ATI)AlMe2.9 The Al-O bond
distances (Al(1)-O(1) 1.809(2) Å, Al(1)-O(1A) 1.817(3) Å) are
similar to those in neutral aluminum µ-OR compounds.10 The
geometry at the oxygens is pyramidal (sum of angles around O(1)
) 336.0°) rather than planar as normally observed in alkoxy-
bridged aluminum compounds due to steric crowding between
the µ-OiPr and NiPr groups.10b-e The dinuclear structure of the
cation explains the low solubility of 3.
(iPr2-ATI)AlR+ cations (B(C6F5)4 salts; R ) Et (1a), Bu (1b),
Pr (1c); iPr2-ATI ) N,N′-diisopropylaminotroponiminate) undergo
â-H transfer, insertion, and σ-bond metathesis reactions with
acetone and tert-butyl acetylene, which in some cases lead to
unique dinuclear dicationic Al species.
-
i
Compound 1a reacts with 1 equiv of acetone to yield the
1
acetone adduct 2a (Scheme 1). The H NMR spectrum of 2a in
C6D5Cl (23 °C) in the absence of excess acetone contains two
doublets at ca. δ 1.0 for the isopropyl methyl groups consistent
with Cs symmetry at Al, and a resonance for coordinated acetone
(δ 2.02). However, in the presence of even a slight (4%) excess
of acetone, the two isopropyl methyl doublets collapse to a single
doublet indicative of time-averaged C2V symmetry at Al due to
intermolecular acetone exchange by an associative mechanism.
Compound 2a is slowly (3 d, 23 °C, CD2Cl2) converted to the
cationic isopropoxide complex 3 in quantitative yield by net â-H
transfer with release of ethylene. Complexes 1b and 1c react with
acetone in a similar manner; however, the corresponding inter-
mediates 2b and 2c are less stable than 2a, and are completely
converted to 3 within 5 h at 23 °C in C6D5Cl. For comparison,
AlEt3 reacts with diethyl ketone by competitive ketone insertion
into the Al-Et bond, â-H transfer, and enolization; the product
ratio depends on the AlEt3/ketone ratio.7 The monomeric Al alkyls
(BHT)xAlEt3-x (BHT ) 2,6-di-tert-butyl-4-methylphenoxide)
react with enolizable ketones by enolization and subsequent aldol
condensation.8 No aldol condensation products are observed in
the reactions of 1a-c with 1 equiv of acetone.
Compounds 1a-c catalytically dimerize tert-butyl acetylene
to the head-to-tail dimer 2-tert-butyl-5,5-dimethyl-1-hexen-3-yne
(4, C6D5Cl, 23 °C, ca. 4 t.o./h, >90% selectivity for 4) as shown
in Scheme 2.11 Support for the mechanism in Scheme 2 is
provided by the following observations from NMR and GC-MS
studies of stepwise reactions. (i) 1a reacts with 1 equiv of tBuCt
CH by â-H transfer to yield cationic vinyl compound 5 and
ethylene quantitatively. 5 is stable in C6D5Cl solution at 23 °C in
the absence of tBuCtCH. The trans stereochemistry is established
3
by a vinyl JHH value of 21 Hz.12 (ii) 5 reacts with additional
* Address correspondence to this author. Present address: Department of
Chemistry, The University of Chicago, 5735 South Ellis Ave., Chicago, IL
60637.
tBuCtCH by σ-bond metathesis to yield alkynyl complex 6 and
† The University of Iowa.
‡ Iowa State University.
(8) (a) Power, M. B.; Bott, S. G.; Clark, D. L.; Atwood, J. L.; Barron, A.
R. Organometallics 1990, 9, 3086. (b) Power, M. B.; Apblett, A. W.; Bott, S.
G.; Atwood, J. L.; Barron, A. R. Organometallics 1990, 9, 2529.
(9) Dias, H. V. R.; Jin, W.; Ratcliff, R. E. Inorg. Chem. 1995, 34, 6100.
(10) (a) Holloway, C. E.; Melnik, M. J. Organomet. Chem. 1997, 543, 1.
(b) Turova, N. Ya.; Kozunov, V. A.; Yanovskii, A. I.; Bokii, N. G.; Struchkov,
Yu. T.; Tarnopol’skii, B. L. J. Inorg. Nucl. Chem. 1979, 41, 5. (c) Wengrovius,
J. H.; Garbauskas, M. F.; Williams, E. A.; Going, R. C.; Donahue, P. E.;
Smith, J. F. J. Am. Chem. Soc. 1986, 108, 982. (d) Cayton, R. H.; Chisholm,
M. H.; Davidson, E. R.; DiStasi, V. F.; Du, P.; Huffman, J. C. Inorg. Chem.
1991, 30, 1020. (e) Kunicki, A.; Sadowski, R.; Zachara, J. J. Organomet.
Chem. 1996, 508, 249.
(1) Atwood, D. A. Coord. Chem. ReV. 1998, 176, 407.
(2) Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1997, 119, 8125.
(3) Ihara, E.; Young, V. G., Jr.; Jordan, R. F. J. Am. Chem. Soc. 1998,
120, 8277.
(4) (a) Qian, B.; Ward, D. L.; Smith, M. R., III Organometallics 1998, 17,
3070. (b) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc.
1998, 120, 9384. (c) Radzewich, C. E.; Guzei, I. A.; Jordan, R. F. J. Am.
Chem. Soc. 1999, 121, 8673.
(5) Bruce, M.; Gibson, V. C.; Redshaw, C.; Solan, G. A.; White, A. J. P.;
Williams, D. J. J. Chem. Soc., Chem. Commun. 1998, 2523.
(6) Emig, N.; Nguyen, H.; Krautscheid, H.; Reau, R.; Cazaux, J.-B.;
Bertrand, G. Organometallics 1998, 17, 3599.
(11) Small amounts of trimer and tetramer products were detected by
GC-MS.
(12) Silverstein, R. M.; Webster, F. X. Spectrometric Identification of
Organic Compounds, 6th ed.; Wiley: New York, 1998.
(7) (a) Pasynkiewicz, S.; Dluzniewski, T.; Sonnek, G. J. Organomet. Chem.
1987, 321, 1. (b) Pasynkiewicz, S.; Sliwa, E. J. Organomet. Chem. 1965, 3,
121.
10.1021/ja992599c CCC: $18.00 © 1999 American Chemical Society
Published on Web 11/25/1999

11606 J. Am. Chem. Soc., Vol. 121, No. 49, 1999
Communications to the Editor
Figure 1. Structure of the {(iPr2-ATI)Al(µ-OiPr)}22+ dication. Thermal
ellipsoids are drawn at the 30% probability level. Selected bond distances
(Å) and angles (deg) not given in the text: Al(1)-Al(1A) 2.774(2), O(1)-
C(1) 1.508(5); N(1)-Al(1)-N(2) 89.0(1), O(1)-Al(1)-O(1A) 80.2(1).
Hydrogen atoms are omitted.
2+
Figure 2. Structure of the {(iPr2-ATI)Al(µ-CtCtBu)}2 dication.
Thermal ellipsoids are drawn at the 30% probability level. Selected bond
distances (Å) and angles (deg) not given in the text: Al(1)-N(1) 1.834-
(2), Al(1)-N(2) 1.837(2); N(1)-Al(1)-N(2) 88.86(7), C(14)-Al(1)-
C(14A) 88.49(7), Al(1)-C(14)-Al(1A) 91.51(7). Hydrogen atoms are
omitted.
Scheme 2
CtC unit (Al(1)-C(14)-C(15) 163.6(2)°). The long Al-C bonds
(Al(1)-C(14A) 2.151(2)°) are formed by donation of alkynyl
π-electrons to an empty Al p orbital (Al(1A)-C(14)-C(15)
103.9(1)°).15 The C(14)-C(15) bond (1.217(3) Å) retains triple
bond character. The [Al2C2] plane is almost perpendicular to the
ATI rings (angle between planes ) 93.6°). Similar structures have
been observed for neutral dinuclear aluminum compounds.15,16 It
is not known whether 6 observed by NMR in solution before
crystallization is mononuclear or dinuclear.
These results show that (i) (iPr2-ATI)AlR+ complexes that
contain â-hydrogens react with unsaturated substrates by â-H
transfer, (ii) (iPr2-ATI)Al(CtCR)+ species catalyze terminal
alkyne dimerization by an insertion/σ-bond metathesis process,
and (iii) {(iPr2-ATI)AlX}2 species form unique dinuclear
2+
dications when X is a potential bridging group such as -OR or
-CtCR.17,18 The latter result suggests that (iPr2-ATI)Al(alkenyl)+
and even (iPr2-ATI)Al(alkyl)+ cations might adopt dinuclear
structures.19
Acknowledgment. This work was supported by Department of Energy
Grant DE-FG02-88ER13935 and Eastman Chemical Company.
Supporting Information Available: Synthetic procedures, charac-
terization data for new compounds, and details of the X-ray structure
determinations of 3 and 6 (PDF). This material is available free of charge
via the Internet at http://pubs.acs.org.
JA992599C
t
tert-butyl ethylene, followed by BuCtCH insertion to yield 7.
(13) (a) Duchateau, R.; Meetsma, A.; Teuben, J. H. J. Chem. Soc., Chem.
Commun. 1996, 223. (b) Yoshida, M.; Jordan, R. F. Organometallics 1997,
16, 4508. (c) Horton, A. D. J. Chem. Soc., Chem. Commun. 1992, 185 and
references therein.
(14) (a) Eisch, J. J. In ComprehensiVe Organometallic Chemistry, 2nd ed.;
McKillop, A., Ed.; Pergamon: New York, 1995; Vol. 11, pp 277-311. (b)
Zietz, J. R.; Robinson, G. C.; Lindsay, K. L. In ComprehensiVe Organometallic
Chemistry, 1st ed.; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.;
Pergamon: New York, 1982; Vol. 7, p 365.
(15) Erker, G.; Albrecht, M.; Kruger, C.; Nolte, M.; Werner, S. Organo-
metallics 1993, 12, 4979 and references therein.
(16) (a) Almenningen, A.; Fernholt, L.; Haaland, A. J. Organomet. Chem.
1978, 155, 245. (b) Stucky, G. D.; McPherson, A. M.; Rhine, W. E.; Eisch,
J. J.; Considine, J. L. J. Am. Chem. Soc. 1974, 96, 1941.
(17) Several dinuclear dicationic Al compounds incorporating 5-coordinate
Al centers have been reported: (a) [(EtAl)2‚diaza-18-crown-6][EtAlCl3]2: Self,
M. F.; Pennington, W. T.; Laske, J. A.; Robinson, G. H. Organometallics
1991, 10, 36. (b) [{Salpen(tBu)4Al}2][GaCl4]2 and [{Salomphen(tBu)3Al}2]-
[GaCl4]2: Munoz-Hernandez, M.-A.; Sannigrahi, B.; Atwood, D. A. J. Am.
Chem. Soc. 1999, 121, 6747.
(18) Dinuclear dicationic Zr compounds have been characterized or
proposed: (a) Martin, A.; Uhrhammer, R.; Gardner, T. G.; Jordan, R. F.;
Rogers, R. D. Organometallics 1998, 17, 382. (b) Cuenca, T.; Royo, P. J.
Organomet. Chem. 1985, 293, 61. (c) Horton, A. D.; de With, J. Organome-
tallics 1997, 16, 5424.
(19) For {R2Al(µ-alkenyl)}2 compounds see: Albright, M. J.; Butler, W.
M.; Anderson, T. J.; Glick, M. D.; Oliver, J. P. J. Am. Chem. Soc. 1976, 98,
3995.
When 5 is reacted with 1 equiv of tBuCtCH, a 4:1:1 mixture of
6, 7, and unreacted 5 is formed, indicating that the two reactions
occur at similar rates. Removal of the volatiles from this reaction
mixture followed by addition of C6H5Cl affords yellow crystalline
6 in 55% isolated yield. (iii) The reaction of 5 with excess
tBuCtCH results in catalytic formation of 4. (iv) Isolated 6 is
insoluble in C6D5Cl, but dissolves in the presence of excess
tBuCtCH yielding 7 with catalytic formation of 4. (v) NMR
spectra under catalytic conditions contain only signals for 7,
indicating that this species is the resting state of the catalytic cycle.
The neutral bis-amidinate compound {PhC(NSiMe3)2}2AlH and
a variety of early transition metal species catalytically dimerize
terminal alkynes by analogous mechanisms.13 For comparison,
AlR3 compounds normally react with terminal alkynes by σ-bond
metathesis to give R2Al-CtCR′ and RH, while insertion and
â-H transfer reactions are less common.14
6 crystallizes from C6H5Cl as the complex salt [{(iPr2-ATI)-
Al(µ-CtCtBu)}2][B(C6F5)4]2‚5C6H5Cl. The dinuclear dication of
6 (Figure 2) consists of two (iPr2-ATI)Al units linked by two
unsymmetrical σ,π-alkynyl bridges. The short Al-C σ-bonds (Al-
(1)-C(14) 1.971(2) Å) are associated with a slightly bent Al-