Complexes of strong bidentate Lewis acids derived from 2,2′-(1,3-butadiyne-1,4-diyl)bis[phenol]

Article abstract of DOI:10.1021/om970901p

Treatment of 2,2′-(1,3-butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol] (1) with 2 equiv of Al(i-Bu)₃ converts the two hydroxyl groups into OAl(i-Bu)₂ groups, thereby transforming diphenol 1 into the corresponding bis(di-i-butylaluminum phenoxide) 8, which holds two strongly Lewis acidic atoms of aluminum in a potentially convergent orientation. Bidentate Lewis acid 8 forms a crystalline 1:1 adduct with 1,2-dimethoxyethane (DME). An X-ray crystallographic study revealed that the adduct is a linear oligomer in which the two Lewis acidic sites of reagent 8 each bind a basic oxygen atom from different molecules of DME. However, low-temperature NMR studies indicated that in solution a discrete 1:1 adduct 9 is favored, in which the two Lewis acidic sites of reagent 8 each bind one of the two basic sites in a single molecule of DME. Formation of adduct 9 provides an example of the recognition and binding of a multidentate Lewis base by a complementary multidentate Lewis acid. Addition of 2 equiv of TiCl₄ to diphenol 1 converts the hydroxyl groups into OTiCl₃ groups and produces the corresponding bis(trichlorotitanium phenoxide) 11, which forms an unusual 1:2 complex with CH₃COOC₂H₅. An X-ray crystallographic study of this complex established that each Lewis acidic atom of titanium binds only 1 equiv of CH₃-COOC₂H₅ to form an unprecedented pentacoordinate adduct with a square-pyramidal geometry. Formation of diverse reagents 8 and 11 from the same precursor demonstrates that the strategy of converting organic compounds with suitably oriented hydroxyl groups into the corresponding metal alkoxides is a versatile and effective way to make multidentate Lewis acids.

Full text of DOI:10.1021/om970901p

1128
Organometallics 1998, 17, 1128-1133
Com p lexes of Str on g Bid en ta te Lew is Acid s Der ived
fr om 2,2′-(1,3-Bu ta d iyn e-1,4-d iyl)bis[p h en ol]
Okba Saied, Michel Simard,1 and J ames D. Wuest*
De´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec, H3C 3J 7 Canada
Received October 16, 1997
Treatment of 2,2′-(1,3-butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol] (1) with
2 equiv of Al(i-Bu)3 converts the two hydroxyl groups into OAl(i-Bu)2 groups, thereby
transforming diphenol 1 into the corresponding bis(di-i-butylaluminum phenoxide) 8, which
holds two strongly Lewis acidic atoms of aluminum in a potentially convergent orientation.
Bidentate Lewis acid 8 forms a crystalline 1:1 adduct with 1,2-dimethoxyethane (DME).
An X-ray crystallographic study revealed that the adduct is a linear oligomer in which the
two Lewis acidic sites of reagent 8 each bind a basic oxygen atom from different molecules
of DME. However, low-temperature NMR studies indicated that in solution a discrete 1:1
adduct 9 is favored, in which the two Lewis acidic sites of reagent 8 each bind one of the
two basic sites in a single molecule of DME. Formation of adduct 9 provides an example of
the recognition and binding of a multidentate Lewis base by a complementary multidentate
Lewis acid. Addition of 2 equiv of TiCl4 to diphenol 1 converts the hydroxyl groups into
OTiCl3 groups and produces the corresponding bis(trichlorotitanium phenoxide) 11, which
forms an unusual 1:2 complex with CH3COOC2H5. An X-ray crystallographic study of this
complex established that each Lewis acidic atom of titanium binds only 1 equiv of CH3-
COOC2H5 to form an unprecedented pentacoordinate adduct with a square-pyramidal
geometry. Formation of diverse reagents 8 and 11 from the same precursor demonstrates
that the strategy of converting organic compounds with suitably oriented hydroxyl groups
into the corresponding metal alkoxides is a versatile and effective way to make multidentate
Lewis acids.
In tr od u ction
sufficiently close together that the resulting sites of
Lewis acidity can operate in conjunction to recognize
and bind basic substrates; however, the sites cannot be
so close together that they interact directly in an
The study of multidentate Lewis acids, which are
complex reagents with multiple sites of Lewis acidity,
is an increasingly active area of research.2-5 Valuable
applications of multidentate Lewis acids are likely to
emerge as special features of their coordination chem-
istry become more fully understood and as effective
methods for making them are developed. We have
shown that a particularly convenient way to make
multidentate Lewis acids is to convert organic com-
pounds with suitably oriented hydroxyl groups or simi-
lar sites into metal alkoxides or related derivatives with
strong Lewis acidity.3,4 For example, appropriate diols
can be treated with 2 equiv of TiCl4 to produce the
corresponding bis(trichlorotitanium alkoxides). By simple
procedures of this type, hydroxyl groups can be trans-
formed into various sites of strong Lewis acidity.
To allow useful multidentate Lewis acids to be created
by this strategy, the hydroxyl groups must be held
(5) For other recent studies of multidentate Lewis acids, see:
Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res 1997, 30, 267. Ooi, T.;
Takahashi, M.; Maruoka, K. J . Am. Chem. Soc. 1996, 118, 11307.
Holman, K. T.; Halihan, M. M.; J urisson, S. S.; Atwood, J . L.;
Burkhalter, R. S.; Mitchell, A. R.; Steed, J . W. J . Am. Chem. Soc. 1996,
118, 9567. Gabba¨ı, F. P.; Schier, A.; Riede, J .; Schichl, D. Organome-
tallics 1996, 15, 4119. Scheerder, J .; Engbersen, J . F. J .; Reinhoudt,
D. N. Recl. Trav. Chim. Pays-Bas 1996, 115, 307. Nozaki, K.; Yoshida,
M.; Takaya, H. Bull. Chem. Soc. J pn. 1996, 69, 2043. Grdenic´, D.;
Korpar-Cˇ olig, B.; Matkovic´-Cˇ alogovic´, D. J . Organomet. Chem. 1996,
522, 297. Tikhonova, I. A.; Dolgushin, F. M.; Yanovsky, A. I.; Struchkov,
Yu. T.; Gavrilova, A. N.; Saitkulova, L. N.; Shubina, E. S.; Epstein, L.
M.; Furin, G. G.; Shur, V. B. J . Organomet. Chem. 1996, 508, 271.
Tamao, K.; Hayashi, T.; Ito, Y. J . Organomet. Chem. 1996, 506, 85.
Atwood, J . L.; Holman, K. T.; Steed, J . W. J . Chem. Soc., Chem.
Commun. 1996, 1401. Chapman, W. H., J r.; Breslow, R. J . Am. Chem.
Soc. 1995, 117, 5462. Spaether, W.; Erker, G.; Rump, M.; Kru¨ger, C.;
Kuhnigk, J . Organometallics 1995, 14, 2621. Dakternieks, D.; J urk-
schat, K.; Zhu, H.; Tiekink, E. R. T. Organometallics 1995, 14, 2512.
Stemmler, A. J .; Kampf, J . W.; Pecoraro, V. L. Inorg. Chem. 1995, 34,
2271. Reilly, M.; Oh, T. Tetrahedron Lett. 1995, 36, 217. Nguyen, P.;
Dai, C.; Taylor, N. J .; Power, W. P.; Marder, T. B.; Pickett, N. L.;
Norman, N. C. Inorg. Chem. 1995, 34, 4290. Anderson, H. L.; Bashall,
A.; Henrick, K.; McPartlin, M.; Sanders, J . K. M. Angew. Chem., Int.
Ed. Engl. 1994, 33, 429. Zheng, B.; Srebnik, M. J . Organomet. Chem.
1994, 474, 49. Balch, A. L. Prog. Inorg. Chem. 1994, 41, 239.
Rothmaier, M.; Simon, W. Anal. Chim. Acta 1993, 271, 135. Ko¨ster,
R.; Seidel, G.; Wagner, K.; Wrackmeyer, B. Chem. Ber. 1993, 126, 305.
J acobson, S.; Pizer, R. J . Am. Chem. Soc. 1993, 115, 11216. Fields, L.
B.; J acobsen, E. N. Tetrahedron: Asymmetry 1993, 4, 2229. Krishna-
murti, R.; Kuivila, H. G.; Shaik, N. S.; Zubieta, J . Organometallics
1991, 10, 423. Danks, I. P.; Sutherland, I. O.; Yap, C. H. J . Chem.
Soc., Perkin Trans. 1 1990, 421. Kaufmann, D.; Boese, R. Angew.
Chem., Int. Ed. Engl. 1990, 29, 545. Narasaka, K.; Sakurai, H.; Kato,
T.; Iwasawa, N. Chem. Lett. 1990, 1271.
(1) Laboratoire de Diffraction des Rayons-X, De´partement de
Chimie, Universite´ de Montre´al.
(2) Vaugeois, J .; Simard, M.; Wuest, J . D. Coord. Chem. Rev. 1995,
145, 55. Simard, M.; Vaugeois, J .; Wuest, J . D. J . Am. Chem. Soc. 1993,
115, 370. Sharma, V.; Simard, M.; Wuest, J . D. J . Am. Chem. Soc.
1992, 114, 7931.
(3) Bachand, B.; Wuest, J . D. Organometallics 1991, 10, 2015.
Be´langer-Garie´py, F.; Hoogsteen, K.; Sharma, V.; Wuest, J . D. Inorg.
Chem. 1991, 30, 4140. Phan Viet, M. T.; Sharma, V.; Wuest, J . D. Inorg.
Chem. 1991, 30, 3026. Galeffi, B.; Simard, M.; Wuest, J . D. Inorg.
Chem. 1990, 29, 955.
(4) Saied, O.; Simard, M.; Wuest, J . D. Organometallics 1996, 15,
2345.
S0276-7333(97)00901-1 CCC: $15.00 © 1998 American Chemical Society
Publication on Web 02/24/1998

Complexes of Strong Bidentate Lewis Acids
Organometallics, Vol. 17, No. 6, 1998 1129
unproductive way. In this paper, we show that our
strategy can be used successfully to convert 2,2′-(1,3-
butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphe-
nol] (1) into reagents with two sites of strong Lewis
acidity held in a potentially convergent orientation.6
is estimated to require at least 31.4 kcal mol-1 8
We
.
suggest that exchange in compound 4 occurs by an
analogous mechanism of single-bond cleavage and that
opening of the ring is accelerated because Al-O bonds
in the Al2O2 rings of dimeric dialkylaluminum phenox-
ides are intrinsically weaker than those of dimeric
aluminum trialkoxides. In addition, relief of strain in
the Al2O2 ring of compound 4 may help lower ∆Gq.
However, the observed value of ∆Gq is still too large to
imply that the desired bidentate Lewis acid 3 is readily
accessible from isomer 4.
To prevent loss of full bidentate Lewis acidity by
inadvertent intramolecular association, the two OAlR2
groups must be attached to a framework that holds
them farther apart. Such a framework is provided by
diphenol 1, which was prepared in 80% overall yield by
standard oxidative coupling9 of the known acetylene 6,4
followed by mild reductive deprotection of the resulting
diester 7. Treatment of diphenol 1 in pentane with 2
Resu lts a n d Discu ssion
In earlier work,4 we demonstrated that treatment of
the closely analogous diphenol 2 with 2 equiv of Al(i-
Bu)3 did not yield the desired product 3, with two OAl-
(i-Bu)2 groups containing potentially convergent Lewis
acidic atoms of aluminum. Instead, we obtained an
isomer 4 in which the OAl(i-Bu)2 groups associate
intramolecularly to form a four-membered Al2O2 ring
characteristic of dimers of similar dialkylaluminum
alkoxides and aryloxides.4 The sites of Lewis acidity
in hypothetical structure 3 are therefore too close
together to prevent their direct interaction. However,
an X-ray crystallographic study showed that isomer 4
incorporates significant strain, which is revealed by
abnormal bending of the triple bond and notable defor-
mations of the Al2O2 ring.4 These observations encour-
aged us to hope that structure 4 might be in rapid
equilibrium in solution with significant amounts of the
desired bidentate Lewis acid 3.
To study this possibility, we recorded variable-tem-
perature NMR spectra of compound 4. At 25 °C, its 13C
NMR spectrum (100 MHz, toluene-d8) showed separate
peaks for all three different carbon atoms in the two
pairs of nonequivalent i-Bu groups. In particular,
signals assigned to the CH3 groups appeared at δ 28.39
and 28.53. As the temperature was increased, these
signals broadened and finally coalesced at Tc ) 55 °C,
enabling us to estimate that ∆Gq ) 17.0 ( 0.2 kcal mol-1
for exchange of the i-Bu groups.7 In principle, this
exchange can occur by two distinct processes: (1)
Breaking two Al-O bonds in compound 4 and cleaving
the Al2O2 ring, thereby producing the desired bidentate
Lewis acid 3, or (2) breaking only one Al-O bond and
opening the Al2O2 ring to generate intermediate 5,
followed by rapid rotation of the uncoordinated Al(i-Bu)2
group around its single remaining Al-O bond. Break-
ing a single Al-O bond in the Al2O2 ring of [Al(i-OPr)3]2
equiv of Al(i-Bu)3 produced a homogeneous solution, and
partial evaporation then gave crystals, presumably of
the desired bidentate Lewis acid 8. When the treatment
of diphenol 1 with Al(i-Bu)3 was followed by the addition
of 1 equiv of 1,2-dimethoxyethane (DME), we obtained
an 86% yield of crystals of a 1:1 complex of bidentate
Lewis acid 8 with DME. Similarly, 1:1 complexes were
also formed when bidentate Lewis acid 8 was treated
with other bidentate bases such as dioxane and py-
ridazine; in contrast, a representative monodentate
base, tetrahydrofuran, gave a 1:2 complex. The struc-
ture of the 1:1 complex of bidentate Lewis acid 8 with
DME was determined by an X-ray crystallographic
study. The results of this study, summarized in Figure
1 and in Tables 1 and 2, established that in the solid
state the two OAl(i-Bu)2 groups of compound 8 do not
associate intramolecularly and are therefore fully avail-
able for binding Lewis bases. However, the two Lewis
acidic atoms of aluminum do not bind the two comple-
mentary basic atoms of oxygen in a single molecule of
DME, thereby forming the discrete 1:1 adduct 9; in-
stead, the Lewis acidic sites each bind a basic site in
two different molecules of DME, creating an oligomeric
complex of 1:1 stoichiometry. The observed bond lengths
and angles are similar to those found in related struc-
tures.4,10 In particular, normal values are observed for
(6) For discussions of convergent functional groups, see: Rebek, J .,
J r.; Askew, B.; Killoran, M.; Nemeth, D.; Lin, F.-T. J . Am. Chem. Soc.
1987, 109, 2426. Rebek, J ., J r.; Marshall, L.; Wolak, R.; Parris, K.;
Killoran, M.; Askew, B.; Nemeth, D.; Islam, N. J . Am. Chem. Soc. 1985,
107, 7476.
(8) Oliver, J . P.; Kumar, R. Polyhedron 1990, 9, 409. Wilson, J . W.
J . Chem. Soc. A 1971, 981.
(7) Sandstro¨m, J . Dynamic NMR Spectroscopy; Academic Press:
London, 1982; p 96.
(9) J ones, G. E.; Kendrick, D. A.; Holmes, A. B. Org. Synth. 1987,
65, 52.

1130 Organometallics, Vol. 17, No. 6, 1998
Saied et al.
sponding to the CH3 groups appeared as a sharp singlet
at δ 3.66 (s, 6H), while the signal corresponding to the
CH2 groups had split into two widely separated and
incompletely resolved multiplets (ω1/2 ) 15 Hz) at δ 3.76
(m, 2H) and 5.02 (m, 2H). These observations support
the hypothesis that the predominant species in solution
is not a linear oligomer but rather the discrete 1:1
adduct 9, in which both Lewis acidic sites in bidentate
Lewis acid 8 operate conjointly to bind the two basic
sites in a single molecule of DME.11 In particular, if
DME is bound in its extended conformation in the
manner suggested by structure 10, then the CH3 groups
are equivalent by symmetry but the protons of each CH2
group are diastereotopic and likely to have markedly
different chemical shifts, as observed. In principle,
exchange of the diastereotopic methylene protons can
occur by (1) breaking a single dative Al‚‚‚O bond or (2)
interconverting doubly bonded structure 10 and its
enantiomer by simple rotations, without cleaving dative
Al‚‚‚O bonds.
Our work establishes that diphenol 1 can be easily
converted into bis(aluminum phenoxide) 8, a strong
multidentate Lewis acid; moreover, such reagents can
recognize and bind complementary multidentate Lewis
bases.11 By similarly simple procedures, compound 1
can also be transformed into other strong bidentate
Lewis acids with distinctly different properties. An
important advantage of our basic strategy for making
multidentate Lewis acids is that a wide variety can be
made from a single organic precursor. For example,
treatment of diphenol 1 with 2 equiv of TiCl4, followed
by the addition of 2 equiv of CH3COOC2H5, produced
red crystals of a 1:2 complex of bis(trichlorotitanium
phenoxide) 11 with CH3COOC2H5 in 63% yield. Be-
cause TiCl4 and titanium(IV) chloroalkoxides normally
form hexacoordinate octahedral 1:2 complexes with
simple basic ligands,12,13 the observed 1:2 stoichiometry
of the complex of bidentate reagent 11 suggested that
one Lewis acidic atom of titanium binds both equiva-
lents of CH3COOC2H5 while the other atom of titanium
remains free, or that the two sites of Lewis acidity each
bind 1 equiv of CH3COOC2H5, and hexacoordination is
then achieved by using chloride or CH3COOC2H5 itself
to doubly bridge the two atoms of titanium.
F igu r e 1. ORTEP view of the structure of the 1:1 complex
of bidentate Lewis acid 8 with DME. Non-hydrogen atoms
are represented by ellipsoids corresponding to 40% prob-
ability, hydrogen atoms are shown as spheres of arbitrary
size, and bonds to aluminum are drawn using solid lines.
the lengths of the covalent Al-O(1) bond (1.734(1) Å)
and the dative Al‚‚‚O(22) bond (1.941(2) Å), as well as
for the Al-O(1)-C(1) bond angle (156.6(1)°). Aluminum
shows distorted tetrahedral coordination, and the bond
angles at aluminum range from 99.0(1)° to 120.6(1)°.
In addition, coordination of the bound oxygen atoms of
DME is approximately trigonal planar, and the sum of
the bond angles at O(22) is 359.3(1)°.
The structure of the complex was determined by X-ray
crystallography, and the results are summarized in
Figure 2 and in Tables 3 and 4. These data reveal that
the complex incorporates several unusual features. In
particular, the structure is symmetric, and the two
atoms of titanium each bind a single molecule of CH3-
COOC2H5 to form pentacoordinate adducts with ap-
Even though the 1:1 adduct of bidentate Lewis acid
8 and DME proved to be a linear oligomer in the solid
state, it is readily soluble in CH2Cl2. At 25 °C, the H
(11) For previous studies of the complexation of multidentate Lewis
bases by multidentate Lewis acids, see: Nadeau, F.; Simard, M.;
Wuest, J . D. Organometallics 1990, 9, 1311.
1
(12) Branchadell, V.; Oliva, A. Inorg. Chem. 1995, 34, 3433. Bran-
chadell, V.; Oliva, A. J . Am. Chem. Soc. 1992, 114, 4357.
(13) For previous structural studies of complexes of esters with TiCl4
and titanium trichloroalkoxides, see: Gau, H.-M.; Lee, C.-S.; Lin, C.-
C.; J iang, M.-K.; Ho, Y.-C.; Kuo, C.-N. J . Am. Chem. Soc. 1996, 118,
2936. Sobota, P.; Szafert, S.; Głowiak, T. J . Organomet. Chem. 1996,
526, 329. Cozzi, P. G.; Floriani, C.; Chiesi-Villa, A.; Rizzoli, C.
Organometallics 1994, 13, 2131. Kakkonen, H. J .; Pursiainen, J .;
Pakkanen, T. A.; Ahlgre´n, M.; Iiskola, E. J . Organomet. Chem. 1993,
453, 175. Poll, T.; Metter, J . O.; Helmchen, G. Angew. Chem., Int. Ed.
Engl. 1985, 24, 112. Bassi, I. W.; Calcaterra, M.; Intrito, R. J .
Organomet. Chem. 1977, 127, 305. Brun, L. Acta Crystallogr. 1966,
20, 739.
NMR spectrum of a 0.063 M solution prepared by
dissolving the crystalline 1:1 adduct in CD2Cl2 showed
two broad signals assigned to the CH3 and CH2 groups
of DME at δ 3.75 (bs, 6H) and 4.28 (bs, 4H), respectively.
The observed broadening indicates that DME must
remain at least partly bound in solution. At the lowest
accessible temperature (-100 °C), the signal corre-
(10) Healy, M. D.; Power, M. B.; Barron, A. R. Coord. Chem. Rev.
1994, 130, 63.

Complexes of Strong Bidentate Lewis Acids
Organometallics, Vol. 17, No. 6, 1998 1131
Ta ble 3. Selected Bon d Len gth s a n d An gles for
th e 1:2 Com p lex of Bid en ta te Lew is Acid 11 w ith
CH3COOC2H5
Bond Lengths (Å)
Ti-O(1)
Ti-O(21)
Ti-Cl(1)
Ti-Cl(2)
1.782(2)
2.078(2)
2.273(1)
2.264(1)
Ti-Cl(3)
2.204(1)
1.348(3)
1.222(4)
1.198(4)
O(1)-C(1)
O(21)-C(22)
C(7)-C(8)
Bond Angles (deg)
O(1)-Ti-O(21)
O(1)-Ti-Cl(1)
O(1)-Ti-Cl(2)
O(1)-Ti-Cl(3)
O(21)-Ti-Cl(1)
O(21)-Ti-Cl(2)
O(21)-Ti-Cl(3)
86.2(1)
Cl(1)-Ti-Cl(2)
Cl(1)-Ti-Cl(3)
Cl(2)-Ti-Cl(3)
Ti-O(1)-C(1)
Ti-O(21)-C(22)
C(6)-C(7)-C(8)
90.7(1)
105.6(1)
102.0(1)
152.6(2)
149.0(2)
178.1(3)
150.3(1)
91.2(1)
103.0(1)
82.8(1)
161.5(1)
96.3(1)
Ta ble 4. Cr ysta llogr a p h ic Da ta for th e 1:2
Com p lex of Bid en ta te Lew is Acid 11 w ith
CH3COOC2H5
F igu r e 2. ORTEP view of the structure of the 1:2 complex
of bidentate Lewis acid 11 with CH3COOC2H5. Non-
hydrogen atoms are represented by ellipsoids correspond-
ing to 40% probability, hydrogen atoms are shown as
spheres of arbitrary size, and bonds to titanium are drawn
using solid lines.
formula
fw
C34H44Cl6O6Ti2
857.19
cryst syst
space group
cell constants
a, Å
b, Å
c, Å
triclinic
P1h
Ta ble 1. Selected Bon d Len gth s a n d An gles
for th e 1:1 Com p lex of Bid en ta te Lew is Acid 8
w ith DME
8.691(2)
9.644(4)
12.540(4)
90.29(3)
98.83(2)
105.68(3)
998.7(6)
1
R, deg
Bond Lengths (Å)
â, deg
Al-O(1)
Al-O(22)
Al-C(31)
1.734(1)
1.941(2)
1.971(2)
Al-C(41)
O(1)-C(1)
C(7)-C(8)
1.967(2)
1.336(2)
1.197(3)
γ, deg
cell volume, Å3
Z
Bond Angles (deg)
T, K
210
1.425
4.281
calcd, g cm-3
O(1)-Al-O(22)
O(1)-Al-C(31)
O(1)-Al-C(41)
O(22)-Al-C(31)
O(22)-Al-C(41)
C(31)-Al-C(41)
99.0(1)
107.7(1)
120.6(1)
104.0(1)
104.8(1)
117.6(1)
C(1)-O(1)-Al
156.6(1)
D
µ
calcd, mm-1
C(6)-C(7)-C(8)
Al-O(22)-C(21)
Al-O(22)-C(23)
C(21)-O(22)-C(23)
175.9(2)
127.0(1)
118.3(1)
114.0(2)
radiation (λ, Å)
graphite-monochromated
Cu KR (1.540 56)
0.40 × 0.30 × 0.06
(h, (k, (l
22 669
3710
0.986
0.0473
cryst dimens, mm
data collcn range
no. of reflns collcd
no. of reflns refined
goodness-of-fit
R1
Ta ble 2. Cr ysta llogr a p h ic Da ta for th e 1:1
Com p lex of Bid en ta te Lew is Acid 8 w ith DME
formula
fw
C46H74Al2O4
745.01
wR2
0.1204
0.724
-0.762
∆Fmax, e Å-3
∆Fmin, e Å-3
cryst syst
space group
cell constants
a, Å
b, Å
c, Å
triclinic
P1h
to 105.6(1)°. The observed structure is noteworthy
because (1) no other pentacoordinate 1:1 adducts of
TiCl4 or titanium(IV) chloroalkoxides with basic organic
ligands are known and (2) calculations have predicted
that the favored geometry of the hypothetical 1:1
complex of TiCl4 with formaldehyde is trigonal bipyra-
midal, not square pyramidal.12 The unusual preference
of bis(trichlorotitanium phenoxide) 11 for pentacoordi-
nation is presumably due to steric hindrance created
by substituents adjacent to the sites of Lewis acidity.
The C(7)-C(8) triple bond in the 1:2 complex of bis-
(trichlorotitanium phenoxide) 11 with CH3COOC2H5 is
oriented in such a way that its interaction with Ti trans
to Cl(3) would produce a normal hexacoordinate complex
with approximate octahedral geometry. However, com-
plexes of Ti(IV) with olefins or acetylenes are very
rare,14 and the observed Ti‚‚‚C(7) and Ti‚‚‚C(8) distances
(3.231(3) and 3.416(3) Å, respectively) are extremely
long. We conclude that bis(trichlorotitanium phenoxide)
11 forms true pentacoordinate complexes.
8.967(3)
11.816(4)
12.551(4)
67.11(3)
73.45(2)
78.23(2)
1167.8(7)
1
R, deg
â, deg
γ, deg
cell volume, Å3
Z
T, K
210
1.059
0.843
D
µ
calcd, g cm-3
calcd, mm-1
radiation (λ, Å)
graphite-monochromated
Cu KR (1.540 56)
0.38 × 0.19 × 0.17
(h, (k, (l
29 061
4418
0.982
0.0495
cryst dimens, mm
data collcn range
no. of reflns collcd
no. of reflns refined
goodness-of-fit
R1
wR2
0.1364
0.296
-0.481
∆Fmax, e Å-3
∆Fmin, e Å-3
proximate square-pyramidal geometries. Cl(3) is ori-
ented apically, while Cl(1), Cl(2), O(1), and O(21) define
the basal plane. Bond angles at Ti in the basal plane
range from 82.8(1)° to 91.2(1)°, while those defined by
apical Cl(3), Ti, and the basal ligands vary from 96.3(1)°
When TiCl4 reacts with 2 equiv of a simple carbonyl
compound in a stepwise manner, binding of the first
(14) For an example, see: Balaich, G. J .; Hill, J . E.; Waratuke, S.
A.; Fanwick, P. E.; Rothwell, I. P. Organometallics 1995, 14, 656.

1132 Organometallics, Vol. 17, No. 6, 1998
Saied et al.
products that were used without further purification. Flash
chromatography was performed in the normal way.19
equivalent to form a pentacoordinate 1:1 intermediate
is normally calculated to be less exothermic than
binding of the second equivalent to form the final
hexacoordinate 1:2 adduct.12 If so, binding of CH3-
COOC2H5 in the novel pentacoordinate complex of
bidentate Lewis acid 11 should be somewhat weaker
than in related hexacoordinate complexes, and the
dative Ti‚‚‚O(21) bond length should be abnormally long.
In fact, the observed distance (2.078(2) Å) is similar to
those previously found in hexacoordinate complexes of
esters with TiCl4 and titanium trichloroalkoxides.13 In
addition, the average length of the two basal Ti-Cl
bonds (2.268(1) Å) is normal, and the Ti-O(1) bond
length (1.782(2) Å) is similar to those found in other
titanium(IV) chlorophenoxides and their complexes,15
which exhibit strong oxygen-p titanium-d π-bonding.16
Further evidence of strong Ti-O(1) π-bonding in the 1:2
complex of bidentate Lewis acid 11 with CH3COOC2H5
is the observed opening of the Ti-O(1)-C(1) bond angle
to 152.6(2)°. Curiously, the Ti-O(1) bond shows normal
strength even though O(1) is trans to Cl(1) rather than
to the more weakly π-donating ester ligand. The only
evidence that pentacoordination has any special effects
on bond lengths is provided by unusual shortening of
the apical Ti-Cl(3) bond (2.204(1) Å). Such shortening
is typical of bonds to apical ligands in square-pyramidal
complexes.17 Although titanium has an unusual geom-
etry and stoichiometry of coordination, CH3COOC2H5
is bound in the normal η1(σ) manner,18 with a Ti‚‚‚
O(21)-C(22) bond angle of 149.0(2)° and a Ti‚‚‚O(21)-
C(22)-C(21) dihedral angle of 1.9(6)°.
2,2′-(1,3-Bu t a d iyn e-1,4-d iyl)bis[6-(1,1-d im et h ylet h yl)-
4-m eth ylp h en ol] Dia ceta te (7). A mixture of CuCl (0.845
g, 8.54 mmol) and N,N,N′,N′-tetramethylethylenediamine
(0.373 g, 3.21 mmol) in acetone (15 mL) was stirred at 25 °C
for 1 h. The suspension was filtered, and the filtrate was
added to a solution of 2-(1,1-dimethylethyl)-6-ethynyl-4-meth-
ylphenol acetate (6; 0.740 g, 3.21 mmol)4 in acetone (30 mL).
The resulting mixture was stirred at 60 °C for 2 h under an
atmosphere of O2. Then 2 N aqueous HCl was added, the
mixture was extracted with CHCl3, and the combined extracts
were washed with H2O and dried with anhydrous MgSO4.
Volatiles were removed by evaporation under reduced pres-
sure, and the residue was purified by flash chromatography
(silica, hexane (95%)/ethyl acetate (5%)) to give 2,2′-(1,3-
butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol]
diacetate (7; 0.717 g, 1.56 mmol, 97%) as a colorless solid. An
analytically pure sample was prepared by recrystallization
1
from CHCl3: mp 126-128 °C; IR (melt) 1769 cm-1; H NMR
(300 MHz, CDCl3) δ 1.38 (s, 18H), 2.34 (s, 6H), 2.44 (s, 6H),
7.26 (s, 4H); 13C NMR (75.4 MHz, CDCl3) δ 20.7, 21.1, 30.0,
34.3, 77.2, 78.2, 116.7, 129.4, 131.6, 134.9, 141.6, 149.5, 168.7;
HRMS (FAB) calcd for C30H34O4 + H m/e 459.2535, found m/e
459.2553.
2,2′-(1,3-Bu t a d iyn e-1,4-d iyl)b is[6-(1,1-d im et h ylet h yl)-
4-m eth ylp h en ol] (1). A solution of 2,2′-(1,3-butadiyne-1,4-
diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol] diacetate (7;
4.51 g, 9.83 mmol) in toluene (60 mL) was stirred at -78 °C
under dry Ar and was treated dropwise with a solution of (i-
Bu)2AlH (52 mL, 1.5 M in toluene, 78 mmol). The cold mixture
was treated with H2O (80 mL), warmed to 25 °C, and filtered
through Celite. The filtrate was extracted with CHCl3, and
the combined extracts were dried with anhydrous MgSO4.
Volatiles were removed by evaporation under reduced pres-
sure, and the residue was purified by flash chromatography
(silica, hexane (90%)/ethyl acetate (10%)) to give 2,2′-(1,3-
butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-4-methylphenol]
(1; 3.31 g, 8.84 mmol, 90%) as a colorless solid. An analytically
pure sample was prepared by recrystallization from CHCl3:
Con clu sion s
Our results show that the conversion of hydroxyl
groups into metal alkoxides can serve as the basis of
an effective strategy for constructing strong multiden-
tate Lewis acids from simple organic precursors. Mul-
tidentate Lewis acids made by this strategy can be
designed to have useful properties, such as the ability
to recognize and bind complementary multidentate
Lewis bases. Moreover, this strategy allows multiden-
tate Lewis acids with markedly different properties to
be made from the same precursor. We expect that our
strategy for making multidentate Lewis acids will make
them more readily available and will help promote
further study of their unique coordination chemistry.
mp 142-144 °C; IR (melt) 3517, 2128 cm-1 1H NMR (300
;
MHz, CDCl3) δ 1.40 (s, 18H), 2.26 (s, 6H), 5.91 (s, 2H), 7.09 (s,
2H), 7.11 (s, 2H); 13C NMR (75.4 MHz, CDCl3) δ 20.3, 29.0,
34.4, 78.1, 79.7, 107.9, 128.8, 129.7, 129.9, 135.6, 154.6; HRMS
(FAB) calcd for C26H30O2 m/e 374.2246, found m/e 374.2230.
1:1 Ad d u ct of Bid en ta te Lew is Acid 8 a n d DME.
A
solution of 2,2′-(1,3-butadiyne-1,4-diyl)bis[6-(1,1-dimethylethyl)-
4-methylphenol] (1; 20.6 mg, 0.0550 mmol) in pentane (0.8 mL)
was stirred at 25 °C under dry Ar and treated dropwise with
neat Al(i-Bu)3 (21.8 mg, 0.110 mmol). After 10 min, neat DME
(5.0 mg, 0.055 mmol) was added. Partial evaporation of the
solvent yielded colorless crystals of the 1:1 adduct of bidentate
Lewis acid 8 and DME (35.4 mg, 0.0475 mmol, 86%): IR
1
(Nujol) 2130 cm-1; H NMR (300 MHz, CDCl3) δ 0.20 (d, 8H,
3J ) 6.4 Hz), 0.90 (d, 24H, 3J ) 6.5 Hz), 1.38 (s, 18H), 1.81
(m, 4H), 2.23 (s, 6H), 3.75 (bs, 6H), 4.28 (bs, 4H), 7.06 (s, 4H);
13C NMR (75.4 MHz, CDCl3) δ 20.6, 22.8, 25.7, 28.2, 29.6, 34.8,
59.7 (b), 69.0 (b), 78.0, 82.9, 112.6, 126.6, 129.5, 131.4, 139.1,
158.7.
Exp er im en ta l Section
Toluene and pentane were dried by distillation from sodium,
and CH2Cl2 was dried by distillation from P2O5. TiCl4 was
purified by distillation; all other reagents were commercial
1:2 Com p lex of Bid en ta te Lew is Acid 11 w ith CH3-
COOC2H5. A solution of 2,2′-(1,3-butadiyne-1,4-diyl)bis[6-(1,1-
dimethylethyl)-4-methylphenol] (1; 15.0 mg, 0.0400 mmol) in
pentane (0.8 mL) was stirred at 25 °C under dry Ar and treated
dropwise with neat TiCl4 (15.6 mg, 0.0822 mmol). The
resulting red suspension was then treated with CH3COOC2H5
(7.2 mg, 0.082 mmol) and CH2Cl2 (0.4 mL). Partial evapora-
tion of the solvent yielded red crystals of the 1:2 complex of
bidentate Lewis acid 11 with CH3COOC2H5 (21.6 mg, 0.0252
mmol, 63%): IR (Nujol) 1644 cm-1; 1H NMR (300 MHz, CDCl3)
(15) For previous structural studies of titanium trichlorophenoxides,
see: Matilainen, L.; Klinga, M.; Leskela¨, M. Polyhedron 1996, 15, 153.
Troyanov, S.; Pisarevsky, A.; Struchkov, Yu. T. J . Organomet. Chem.
1995, 494, C4.
(16) For a discussion of metal-oxygen π-bonding, see: Chisholm,
M. H.; Rothwell, I. P. In Comprehensive Coordination Chemistry;
Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon
Press: New York, 1987; Vol. 2, Chapter 15.3.
(17) Kepert, D. L. In Comprehensive Coordination Chemistry;
Wilkinson, G., Gillard, R. D., McCleverty, J . A., Eds.; Pergamon
Press: New York, 1987; Vol. 1, Chapter 2.
(18) Shambayati, S.; Crowe, W. E.; Schreiber, S. L. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 256.
(19) Still, W. C.; Kahn, M.; Mitra, A. J . Org. Chem. 1978, 43, 2923.

Complexes of Strong Bidentate Lewis Acids
Organometallics, Vol. 17, No. 6, 1998 1133
3
δ 1.31 (t, 6H, J ) 7.2 Hz), 1.52 (s, 18H), 2.34 (s, 6H), 2.41 (s,
Su p p or tin g In for m a tion Ava ila ble: Tables of atomic
coordinates and isotropic thermal parameters, all bond lengths
and angles, anisotropic thermal parameters, hydrogen atom
coordinates and thermal parameters, and distances to weighted
least-squares planes for the 1:1 complex of bidentate Lewis
acid 8 with DME and the 1:2 complex of bidentate Lewis acid
11 with CH3COOC2H5 (27 pages). Ordering information is
given on any current masthead page.
6H), 4.37 (q, 4H, J ) 7.2 Hz), 7.13 (m, 2H), 7.22 (m, 2H); 13
NMR (75.4 MHz, CD2Cl2) δ 14.3, 21.3, 21.5, 30.3, 35.6, 63.4,
83.3, 83.4, 119.1, 129.6, 132.7, 135.7, 138.4, 170.7, 176.0.
C
3
Ack n ow led gm en t. We are grateful to the Natural
Sciences and Engineering Research Council of Canada,
the Ministe`re de l’EÄ ducation du Que´bec, and Merck
Frosst for financial support. In addition, acknowledg-
ment is made to the donors of the Petroleum Research
Fund, administered by the American Chemical Society,
for support of this research.
OM970901P