Complexes of Strong Bidentate Lewis Acids Derived from 2,7-Bis(1,1-dimethylethyl)fluorene-1,8-diol

Article abstract of DOI:10.1021/ic9715411

Treatment of 2,7-bis(1,1-dimethylethyl)fluorene-1,8-diol (3) with 2 equiv of TiCl₄ converts the two OH groups into OTiCl₃ groups and thereby yields bis(trichlorotitanium phenoxide) 7, a structurally well-defined reagent that holds two sites of strong Lewis acidity in close proximity. Bidentate Lewis acid 7 forms a crystalline 1:2 complex with 4,4′-dimethylbenzophenone. An X-ray crystallographic study revealed that each atom of titanium binds only one molecule of ketone to form an unusual pentacoordinate adduct with an approximately trigonal bipyramidal geometry. Treatment of fluorenediol 3 with 2 equiv of Al(CH₂CH₃)₃ does not yield the expected bis(diethylaluminum phenoxide) 8, but rather its dimer 9. Formation of diverse reagents 7 and 9 from the same precursor demonstrates that the strategy of converting organic compounds with suitably oriented hydroxyl groups into the corresponding metal alkoxides is a particularly versatile and effective way to make strong, structurally well-defined multidentate Lewis acids.

Full text of DOI:10.1021/ic9715411

2620
Inorg. Chem. 1998, 37, 2620-2625
Ar ticles
Complexes of Strong Bidentate Lewis Acids Derived from
2,7-Bis(1,1-dimethylethyl)fluorene-1,8-diol
Okba Saied,* Michel Simard,1 and James D. Wuest
De´partement de Chimie, Universite´ de Montre´al, Montre´al, Que´bec, H3C 3J7 Canada
ReceiVed December 9, 1997
Treatment of 2,7-bis(1,1-dimethylethyl)fluorene-1,8-diol (3) with 2 equiv of TiCl4 converts the two OH groups
into OTiCl3 groups and thereby yields bis(trichlorotitanium phenoxide) 7, a structurally well-defined reagent that
holds two sites of strong Lewis acidity in close proximity. Bidentate Lewis acid 7 forms a crystalline 1:2 complex
with 4,4′-dimethylbenzophenone. An X-ray crystallographic study revealed that each atom of titanium binds
only one molecule of ketone to form an unusual pentacoordinate adduct with an approximately trigonal bipyramidal
geometry. Treatment of fluorenediol 3 with 2 equiv of Al(CH2CH3)3 does not yield the expected bis-
(diethylaluminum phenoxide) 8, but rather its dimer 9. Formation of diverse reagents 7 and 9 from the same
precursor demonstrates that the strategy of converting organic compounds with suitably oriented hydroxyl groups
into the corresponding metal alkoxides is a particularly versatile and effective way to make strong, structurally
well-defined multidentate Lewis acids.
Introduction
An increasingly active area of research in coordination
chemistry is the study of multidentate Lewis acids, which are
reagents with multiple sites of Lewis acidity available for the
complexation of complementary Lewis bases.2-7 Complexes
of multidentate Lewis acids are chemically important because
they promise to be more highly associated than analogous
Lewis acid.5 Unfortunately, rotation of the aryl groups around
complexes of simple monodentate Lewis acids.7 In addition,
the diyne linker in compound 2 is expected to occur readily,
the recognition of bases by multidentate Lewis acids should be
and no conformational constraints ensure that the two sites of
more selective, and the reactivity of bound substrates should
Lewis acidity are held in the closest possible proximity in a
be modified more dramatically.7 As a result, multidentate Lewis
potentially convergent manner.8 As a result, they are free to
acids are likely to become increasingly useful to chemists as
operate independently.
the special features of their coordination chemistry are revealed,
and as simple ways to make them are devised.
(6) For other recent studies of multidentate Lewis acids, see: (a)
Our earlier work has shown that a particularly convenient
strategy for making strong multidentate Lewis acids is to convert
organic compounds with multiple hydroxyl groups or similar
sites into the corresponding metal alkoxides or related
derivatives.3-5 For example, treatment of diphenol 1 with 2
equiv of TiCl4 can be used to make complexes of bis-
(trichlorotitanium phenoxide) 2, which is a strong bidentate
Hawthorne, M. F.; Zheng, Z. Acc. Chem. Res. 1997, 30, 267. (b) Ooi,
T.; Takahashi, M.; Maruoka, K. J. Am. Chem. Soc. 1996, 118, 11307.
(c) Holman, K. T.; Halihan, M. M.; Jurisson, S. S.; Atwood, J. L.;
Burkhalter, R. S.; Mitchell, A. R.; Steed, J. W. J. Am. Chem. Soc.
1996, 118, 8, 9567. (d) Gabba¨ı, F. P.; Schier, A.; Riede, J.; Schichl,
D. Organometallics 1996, 15, 4119. (e) Scheerder, J.; Engbersen, J.
F. J.; Reinhoudt, D. N. Recl. TraV. Chim. Pays-Bas 1996, 115, 307.
(f) Nozaki, K.; Yoshida, M.; Takaya, H. Bull. Chem. Soc. Jpn. 1996,
69, 2043. (g) Grdenic´, D.; Korpar-Cˇ olig, B.; Matkovic´-Cˇ alogovic´, D.
J. Organomet. Chem. 1996, 522, 297. (h) 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. (i) Tamao, K.; Hayashi,
T.; Ito, Y. J. Organomet. Chem. 1996, 506, 85. (j) Chapman, W. H.,
Jr.; Breslow, R. J. Am. Chem. Soc. 1995, 117, 5462. (k) Spaether,
W.; Erker, G.; Rump, M.; Kru¨ger, C.; Kuhnigk, J. Organometallics
1995, 14, 2621. (l) Dakternieks, D.; Jurkschat, K.; Zhu, H.; Tiekink,
E. R. T. Organometallics 1995, 14, 2512. (m) Stemmler, A. J.; Kampf,
J. W.; Pecoraro, V. L. Inorg. Chem. 1995, 34, 2271. (n) Reilly, M.;
Oh, T. Tetrahedron Lett. 1995, 36, 217. (o) Nguyen, P.; Dai, C.;
Taylor, N. J.; Power, W. P.; Marder, T. B.; Pickett, N. L.; Norman,
N. C. Inorg. Chem. 1995, 34, 4290. (p) Anderson, H. L.; Bashall, A.;
Henrick, K.; McPartlin, M.; Sanders, J. K. M. Angew. Chem., Int. Ed.
Engl. 1994, 33, 429. (q) Zheng, B.; Srebnik, M. J. Organomet. Chem.
1994, 474, 49. (r) Balch, A. L. Prog. Inorg. Chem. 1994, 41, 239. (s)
(1) Laboratoire de Diffraction des Rayons-X, De´partement de Chimie,
Universite´ de Montre´al.
(2) Vaugeois, J.; Simard, M.; Wuest, J. D. Organometallics 1998, 17,
1215. (b) Vaugeois, J.; Simard, M.; Wuest, J. D. Coord. Chem. ReV.
1995, 145, 55. (c) Simard, M.; Vaugeois, J.; Wuest, J. D. J. Am. Chem.
Soc. 1993, 115, 370. (d) Nadeau, F.; Simard, M.; Wuest, J. D.
Organometallics 1990, 9, 1311.
(3) Saied, O.; Simard, M.; Wuest, J. D. Organometallics 1996, 15, 2345.
(4) (a) Sharma, V.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1992,
114, 7931. (b) Bachand, B.; Wuest, J. D. Organometallics 1991, 10,
2015. (c) Be´langer-Garie´py, F.; Hoogsteen, K.; Sharma, V.; Wuest,
J. D. Inorg. Chem. 1991, 30, 4140. (d) Phan Viet, M. T.; Sharma, V.;
Wuest, J. D. Inorg. Chem. 1991, 30, 3026. (e) Galeffi, B.; Simard,
M.; Wuest, J. D. Inorg. Chem. 1990, 29, 955.
(5) Saied, O.; Simard, M.; Wuest, J. D. Organometallics 1998, 17, 1128.
S0020-1669(97)01541-3 CCC: $15.00 © 1998 American Chemical Society
Published on Web 05/13/1998

Complexes of Strong Bidentate Lewis Acids
Inorganic Chemistry, Vol. 37, No. 11, 1998 2621
To allow structurally well-defined multidentate Lewis acids
to be created by our strategy, the hydroxyl groups should be
oriented more rigidly, in positions close enough together to
ensure that the resulting sites of Lewis acidity can act in
conjunction but far enough apart to prevent the sites from
interacting directly in unproductive ways. In this paper, we
show that our strategy can be used successfully to convert 2,7-
bis(1,1-dimethylethyl)fluorene-1,8-diol (3) into reagents with
well-defined structures that hold two sites of strong Lewis
acidity in close proximity.
Figure 1. ORTEP view of the structure of bis(trichlorotitanium phen-
oxide) 7. Non-hydrogen atoms are represented by ellipsoids corre-
sponding to 40% probability, and hydrogen atoms are shown as spheres
of arbitrary size.
ides.10 In addition, the average Ti-O-C bond angle has a large
value (163.6(3)°) that is close to those found in related
compounds with strong oxygen-p titanium-d π-bonding.10,11 As
expected, each atom of titanium adopts an approximately
tetrahedral geometry, and the bond angles at titanium vary only
from 107.9(1) to 111.0(1)°.
Treatment of a solution of bis(trichlorotitanium phenoxide)
7 in CH2Cl2 with 2 equiv of 4,4′-dimethylbenzophenone,
followed by partial evaporation of the solvent, provided red
crystals of a 1:2 complex, which was isolated as a CH2Cl2
solvate in 81% yield. Because TiCl4 and titanium(IV) chloro-
alkoxides normally form hexacoordinate octahedral 1:2 com-
plexes with simple basic ligands,12 the observed 1:2 stoichi-
ometry of the complex of bidentate reagent 7 was unexpected.
It suggested that one Lewis acidic atom of titanium binds both
equivalents of ketone while the other atom of titanium remains
free, or that the two sites of Lewis acidity each bind 1 equiv of
ketone, and hexacoordination is then achieved by using chloride
or the carbonyl oxygen atoms of the ketones to doubly bridge
the two atoms of titanium.
The structure of the solvated 1:2 complex of bis(trichloro-
titanium phenoxide) 7 with 4,4′-dimethylbenzophenone was
determined by X-ray crystallography, and the results are
summarized in Figure 2 and in Tables 1 and 3. These data
reveal that the complex incorporates several unusual features.
In particular, the two atoms of titanium each bind a single
molecule of 4,4′-dimethylbenzophenone to form pentacoordinate
adducts with approximately trigonal bipyramidal geometries.
All Ti-O bonds are oriented apically, and all Ti-Cl bonds are
equatorial. The Cl-Ti-Cl bond angles range from 111.4(1)
to 133.8(1)° and average 119.5(1)°, the Cl-Ti-O bond angles
involving the phenoxide oxygen atoms range from 87.8(1) to
99.9(1)° and average 94.4(1)°, the Cl-Ti-O bond angles
involving the carbonyl oxygen atoms of the bound ketones range
from 82.6(1) to 91.2(1)° and average 86.1(1)°, and the O-Ti-O
bond angles average 170.8(1)°. The observed structure is
noteworthy because the 1:2 adduct of bis(trichlorotitanium
Results and Discussion
Fluorenediol 3 was prepared in 86% yield from the known
dimethyl ether 49 by adding BBr3 and then water. Dimethyl
ether 4 was synthesized in four steps by a modification of the
published method, which requires six steps.9 The modification
involves direct tert-butylation of 3-bromophenol, condensation
of the resulting 5-bromo-2-(1,1-dimethylethyl)phenol9 with
formaldehyde to produce diphenol 5,9 methylation to give
protected intermediate 6,9 and subsequent Ni(0)-catalyzed
intramolecular aryl-aryl coupling. Our modifications increased
the overall yield of dimethyl ether 4 from 3-bromophenol from
11% to 28%. Moreover, we found that fluorenediol 3 could
be made from 3-bromophenol in only three steps by direct aryl-
aryl coupling of unprotected diphenol 5, but the overall yield
was somewhat lower.
To test our general strategy for making multidentate Lewis
acids from organic compounds with suitably oriented hydroxyl
groups, we attempted to convert fluorenediol 3 into the
corresponding bis(trichlorotitanium phenoxide) 7, a strong
bidentate Lewis acid, by adding 2 equivalents of TiCl4 to a
solution of compound 3 in CH2Cl2. Slow evaporation then
provided a 90% yield of red crystals of a compound assigned
structure 7. This assignment was confirmed by X-ray crystal-
lography, which provided the structural data summarized in
Figure 1 and in Tables 1and 2. These data reveal that compound
7 is a true bidentate Lewis acid, with two free OTiCl3 groups
held in close proximity. The average lengths of the Ti-Cl
bonds (2.174(2) Å) and the Ti-O bonds (1.727(3) Å) are similar
to those found in other uncomplexed titanium trichlorophenox-
Rothmaier, M.; Simon, W. Anal. Chim. Acta 1993, 271, 135. (t) Ko¨ster,
R.; Seidel, G.; Wagner, K.; Wrackmeyer, B. Chem. Ber. 1993, 126,
305. (u) Jacobson, S.; Pizer, R. J. Am. Chem. Soc. 1993, 115, 11216.
(v) Fields, L. B.; Jacobsen, E. N. Tetrahedron: Asymmetry 1993, 4,
2229.
(7) For a discussion of the special thermodynamic and kinetic effects of
the multiple binding of bases by multidentate Lewis acids, see:
Vaugeois, J.; Wuest, J. D. J. Am. Chem. Soc. 1998, submittted for
publication.
(8) For discussions of convergent functional groups, see: (a) Rebek, J.,
Jr.; Askew, B.; Killoran, M.; Nemeth, D.; Lin, F.-T. J. Am. Chem.
Soc. 1987, 109, 2426. (b) Rebek, J., Jr.; Marshall, L.; Wolak, R.; Parris,
K.; Killoran, M.; Askew, B.; Nemeth, D.; Islam, N. J. Am. Chem.
Soc. 1985, 107, 7476.
(10) For previous structural studies of titanium trichlorophenoxides, see:
(a) Matilainen, L.; Klinga, M.; Leskela¨, M. Polyhedron 1996, 15, 153.
(b) Troyanov, S.; Pisarevsky, A.; Struchkov, Yu. T. J. Organomet.
Chem. 1995, 494, C4.
(11) 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.
(9) Sharma, V.; Bachand, B.; Simard, M.; Wuest, J. D. J. Org. Chem.
1994, 59, 7785.
(12) (a) Branchadell, V.; Oliva, A. Inorg. Chem. 1995, 34, 3433. (b)
Branchadell, V.; Oliva, A. J. Am. Chem. Soc. 1992, 114, 4357.

2622 Inorganic Chemistry, Vol. 37, No. 11, 1998
Saied et al.
Table 1. Crystallographic Data for Bis(trichlorotitanium phenoxide) 7 (Column A), the CH2Cl2 Solvate of the 1:2 Complex of
Bis(trichlorotitanium phenoxide) 7 with 4,4′-Dimethylbenzophenone (Column B), and the Dimer 9 of Bis(diethylaluminum phenoxide) 8
(Column C)a
column A
column B
column C
empirical formula
fw
crystal system
space group
cell constants
a, Å
C21H24Cl6O2Ti2
616.90
monoclinic
P21/c
C51H52Cl6O4Ti2‚CH2Cl2
1122.35
monoclinic
P21/c
C58H88Al4O4
957.264
triclinic
P1h
15.551(6)
9.988(5)
19.842(10)
90
113.96(4)
90
12.756(3)
21.458(13)
20.047(7)
90
101.28(2)
90
12.330(3)
14.588(3)
16.638(9)
73.95(3)
84.88(4)
79.97(2)
2829(2)
2
b, Å
c, Å
R, deg
â, deg
γ, deg
cell volume, Å3
Z
2816(2)
4
5381(4)
4
T, K
225
1.455
10.39
210
1.385
4.181
225
1.135
1.186
D
calcd, g cm-3
µ
calcd, mm-1
cryst dimens, mm
data collcn range
no. of reflcns collcd
no. of reflcns refined
goodness-of-fit
R1
0.64 × 0.28 × 0.18
(h, +k, +l
19488
5338
0.920
0.0608
0.1512
0.974
0.70 × 0.43 × 0.11
(h, (k, (l
38277
10208
0.981
0.0598
0.1715
0.639
0.58 × 0.46 × 0.38
(h, (k, (l
45681
10709
1.031
0.0581
0.1662
0.686
-0.407
wR2
∆Fmax, e Å-3
∆Fmin, e Å-3
-0.743
-1.164
a Radiation used was graphite-monochromated Cu KR (λ ) 1.540 56 Å).
Table 2. Selected Bond Lengths (Å) and Angles (deg) for
Bis(trichlorotitanium phenoxide) 7
Ti(1)-O(1)
Ti(1′)-O(1′)
Ti(1)-Cl(1)
Ti(1′)-Cl(1′)
1.723(3)
1.731(3)
2.157(2)
2.192(2)
Ti(1)-Cl(2)
Ti(1′)-Cl(2′)
Ti(1)-Cl(3)
Ti(1′)-Cl(3′)
2.179(2)
2.159(2)
2.185(2)
2.170(2)
Cl(1)-Ti(1)-O(1)
109.6(1) Cl(1′)-Ti(1′)-Cl(2′) 108.6(1)
109.9(1)
107.9(1) Cl(1′)-Ti(1′)-Cl(3′) 109.6(1)
Cl(2′)-Ti(1′)-O(1′) 109.7(1) Cl(2)-Ti(1)-Cl(3) 109.8(1)
Cl(3)-Ti(1)-O(1)
Cl(3′)-Ti(1′)-O(1′) 111.0(1) C(1)-O(1)-Ti(1)
Cl(1)-Ti(1)-Cl(2) 109.5(1) C(1′)-O(1′)-Ti(1′)
Cl(1′)-Ti(1′)-O(1′) 108.4(1) Cl(1)-Ti(1)-Cl(3)
Cl(2)-Ti(1)-O(1)
110.1(1) Cl(2′)-Ti(1′)-Cl(3′) 109.6(1)
162.7(3)
164.6(3)
phenoxide) 2 with CH3COOCH2CH3 is the only other related
complex known to incorporate pentacoordinate Ti(IV), and in
this case an approximately square pyramidal geometry is
favored.5 Calculations have predicted that the preferred geom-
etry of the hypothetical 1:1 complex of TiCl4 with formaldehyde
is trigonal bipyramidal, not square pyramidal,12 but our observa-
tions suggest that both structures are accessible. The unusual
preferences of bis(trichlorotitanium phenoxides) 2 and 7 for
pentacoordination are presumably due to steric hindrance created
by substituents adjacent to the sites of Lewis acidity.
Figure 2. ORTEP view of the structure of the CH2Cl2 solvate of the
1:2 complex of bis(trichlorotitanium phenoxide) 7 with 4,4′-dimeth-
ylbenzophenone. Non-hydrogen atoms are represented by ellipsoids
corresponding to 40% probability, hydrogen atoms are shown as spheres
of arbitrary size, bonds to titanium are drawn using solid lines, and
CH2Cl2 is omitted for clarity.
Other geometric parameters in the structure of the 1:2
complex of bis(trichlorotitanium phenoxide) 7 with 4,4′-
dimethylbenzophenone have normal values. For example, the
average dative Ti‚‚‚O bond length involving the carbonyl oxygen
atoms (2.062(3) Å) and the average Ti-O bond length involving
the phenoxide oxygen atoms (1.770(3) Å) are closely similar
to those found in the related 1:2 complex of bis(trichlorotitanium
phenoxide) 2 with CH3COOCH2CH3.5 The Ti-Cl bond lengths
also have a normal average value (2.248(2) Å), and the average
phenoxy Ti-O-C bond angles are expectedly large (155.3-
(2)°), which confirms the presence of strong Ti-O π-bonding.
Although CH3COOCH2CH3 is bound by bis(trichlorotitanium
phenoxide) 2 in the normal η1(σ) manner,13 with Ti close to
the carbonyl plane, 4,4′-dimethylbenzophenone is bound by bis-
(trichlorotitanium phenoxide) 7 with an average Ti‚‚‚OdC bond
angle of 161.7(3)° and an average Ti‚‚‚OdC-C dihedral angle
of 33.0(10)°. This reveals that Ti lies far from the carbonyl
plane and that the interaction must therefore have important π
character.
(13) For a recent discussion of the σ and π complexation of carbonyl
compounds by TiCl4, see: Singh, D. K.; Springer, J. B.; Goodson, P.
A.; Corcoran, R. C. J. Org. Chem. 1996, 61, 1436.

Complexes of Strong Bidentate Lewis Acids
Inorganic Chemistry, Vol. 37, No. 11, 1998 2623
Table 3. Selected Bond Lengths (Å) and Angles (deg) for the
CH2Cl2 Solvate of the 1:2 Complex of Bis(trichlorotitanium
phenoxide) 7 with 4,4′-Dimethylbenzophenone
Ti(1)-O(1)
1.764(3)
1.776(3)
2.066(3)
2.058(3)
2.257(2)
2.236(2)
Ti(1)-Cl(2)
Ti(1′)-Cl(2′)
Ti(1)-Cl(3)
Ti(1′)-Cl(3′)
O(20)-C(20)
O(20′)-C(20′)
2.265(2)
2.271(2)
2.223(1)
2.234(1)
1.248(5)
1.269(4)
Ti(1′)-O(1′)
Ti(1)-O(20)
Ti(1′)-O(20′)
Ti(1)-Cl(1)
Ti(1′)-Cl(1′)
O(1)-Ti(1)-O(20)
168.8(1) O(20′)-Ti(1′)-Cl(2′)
85.6(1)
O(1′)-Ti(1′)-O(20′) 172.8(1) O(20′)-Ti(1′)-Cl(3′)
86.4(1)
133.8(1)
112.6(1)
111.4(1)
121.3(1)
112.4(1)
125.2(1)
162.9(2)
147.6(2)
165.0(3)
O(1)-Ti(1)-Cl(1)
O(1)-Ti(1)-Cl(2)
O(1)-Ti(1)-Cl(3)
O(1′)-Ti(1′)-Cl(1′)
O(1′)-Ti(1′)-Cl(2′)
O(1′)-Ti(1′)-Cl(3′)
O(20)-Ti(1)-Cl(1)
O(20)-Ti(1)-Cl(2)
O(20)-Ti(1)-Cl(3)
O(20′)-Ti(1′)-Cl(1′)
94.1(1) Cl(1)-Ti(1)-Cl(2)
91.6(1) Cl(1)-Ti(1)-Cl(3)
99.9(1) Cl(2)-Ti(1)-Cl(3)
98.1(1) Cl(1′)-Ti(1′)-Cl(2′)
87.8(1) Cl(1′)-Ti(1′)-Cl(3′)
95.1(1) Cl(2′)-Ti(1′)-Cl(3′)
82.6(1) Ti(1)-O(1)-C(1)
83.0(1) Ti(1′)-O(1′)-C(1′)
91.2(1) Ti(1)-O(20)-C(20)
87.8(1) Ti(1′)-O(20′)-C(20′) 158.4(3)
Ti(1)-O(20)-C(20)-C(21)
-158.5(10)
21.8(15)
-44.9(9)
136.4(6)
Figure 3. ORTEP view of the structure of the dimer 9 of bis-
(diethylaluminum phenoxide) 8. Non-hydrogen atoms are represented
by ellipsoids corresponding to 40% probability, hydrogen atoms are
shown as spheres of arbitrary size, and bonds to aluminum are drawn
using solid lines.
Ti(1)-O(20)-C(20)-C(28)
Ti(1′)-O(20′)-C(20′)-C(21′)
Ti(1′)-O(20′)-C(20′)-C(28′)
Our work establishes that fluorenediol 3 can easily be
converted into bis(trichlorotitanium phenoxide) 7, a bidentate
Lewis acid that is strong and structurally well-defined. In
principle, equally simple procedures should transform compound
3 into many other bidentate Lewis acids. This versatility is an
important advantage of our basic strategy for making multi-
dentate Lewis acids because it allows a wide variety of reagents
to be made from a single organic precursor. To test the gen-
erality of our strategy, we attempted to make a new bidentate
Lewis acid 8 by treating a solution of fluorenediol 3 in toluene
with 2 equivalents of Al(CH2CH3)3. Partial evaporation of
solvent yielded colorless crystals of a bis(diethylaluminum
phenoxide) in 62% yield. The aromatic region of its 1H NMR
spectrum, recorded at 25 °C, showed that the fluorene ring was
symmetrically substituted but that two different ethyl groups
were present, thereby excluding the expected structure 8.
exchange of the ethyl groups.14 In principle, this exchange can
occur in hypothetical dimer 9 by breaking individual Al-O
bonds and opening the Al2O2 rings, or simply by rotating the
rings around axes defined by their O‚‚‚O diagonals. Exchange
can occur in hypothetical ethyl-bridged structure 10 by breaking
at least one three-center two-electron bond and opening the
Al2C2 ring. The observed ∆G q is similar to those measured
for processes in which analogous Al2O2 rings are opened,5 and
it is much larger than the ∆G q observed for exchange of the
terminal and bridging ethyl groups in [Al(CH2CH3)3]2, which
is only 10.2 ( 0.3 kcal mol-1 in toluene at 25 °C.15 As a result,
we conclude that the structure favored in solution must be dimer
9 or a higher cyclic oligomer, and we suggest that exchange
occurs by breaking individual Al-O bonds and opening the
Al2O2 rings.
An X-ray crystallographic study subsequently confirmed that
dimer 9 is formed in the solid state. The results of this study,
summarized in Figure 3 and in Tables 1 and 4, show that dimer
9 incorporates several interesting features. Normal Al2O2 rings
are rigorously or approximately planar,3 but those of dimer 9
are distinctly puckered, and the average of the absolute values
of the Al-O-Al-O dihedral angles is 12.8(1)°. However, the
oxygen atoms are nearly trigonal planar, and the average value
of the sums of their bond angles is 358.8(1)°, so the amount of
strain is small. This is confirmed by the average Al-O bond
length (1.884(2) Å), which is rather long but still within the
range of distances normally observed in the Al2O2 rings of
dimeric alkylaluminum alkoxides and aryloxides (1.840-1.895
Å).3 In addition, the average values of the Al-O-Al angles
(99.1(1)°), O-Al-O angles (79.6(1)°), Al-C bond lengths
(1.956(3) Å), nonbonded Al‚‚‚Al distances (2.866(2) Å), non-
bonded O‚‚‚O distances (2.411(2) Å), and C-Al-C angles
(112.0(1)°) are similar to those found in other Al2O2 rings.3
The Al2O2 rings of dimer 9 are approximately orthogonal to
the aryl groups to which they are connected, and the average
Two closely related alternative structures that are consistent
with the observed NMR spectrum are the dimer 9 and the ethyl-
bridged monomer 10. To distinguish these alternatives, we
recorded variable-temperature 1H NMR spectra (400 MHz,
toluene-d8) and analyzed the exchange of the nonequivalent ethyl
groups. At 25 °C, the spectrum showed separate ethyl quartets
centered at δ 0.51 and 0.62. As the temperature was increased,
these signals broadened and finally coalesced at Tc ) 92 °C,
enabling us to estimate that ∆G q ) 18.2 ( 0.2 kcal mol-1 for
(14) Sandstro¨m, J. Dynamic NMR Spectroscopy; Academic Press: London,
1982; p 96.
(15) Yamamoto, O.; Hayamizu, K.; Yanagisawa, M. J. Organomet. Chem.
1974, 73, 17.

2624 Inorganic Chemistry, Vol. 37, No. 11, 1998
Saied et al.
Table 4. Selected Bond Lengths (Å) and Angles (deg) for the
Dimer 9 of Bis(diethylaluminum phenoxide) 8
Al(1)-O(11)
Al(1)-O(31)
Al(1)-C(112)
Al(1)-C(114)
Al(3)-O(11)
Al(3)-O(31)
Al(3)-C(312)
Al(3)-C(314)
Al(2)-O(21)
Al(2)-O(41)
Al(2)-C(212)
Al(2)-C(214)
1.894(2)
1.865(2)
1.963(2)
1.943(3)
1.882(2)
1.904(2)
1.960(3)
1.951(3)
1.868(2)
1.903(2)
1.966(3)
1.953(2)
Al(4)-O(21)
Al(4)-O(41)
Al(4)-C(412)
Al(4)-C(414)
Al(1)‚‚‚Al(3)
Al(2)‚‚‚Al(4)
O(11)‚‚‚O(31)
O(21)‚‚‚O(41)
O(11)-C(11)
O(31)-C(31)
O(21)-C(21)
O(41)-C(41)
1.889(2)
1.870(2)
1.969(3)
1.947(3)
2.862(1)
2.871(2)
2.424(2)
2.398(2)
1.415(2)
1.417(2)
1.409(2)
1.410(2)
simple organic precursors. Precursors with rigidly oriented
hydroxyl groups give rise to multidentate Lewis acids that are
structurally well-defined, in which the sites of Lewis acidity
are held close enough together to be able to act in conjunction
but far enough apart to avoid interacting directly in unproductive
ways. In principle, multidentate Lewis acids made by this
strategy can be designed to have useful properties, such as the
ability to recognize and bind complementary Lewis bases.
Moreover, this strategy allows multidentate Lewis acids with
markedly different properties to be made from the same
precursor. We expect that our strategy for synthesizing structur-
ally well-defined multidentate Lewis acids will make them more
readily available and will help promote further study of their
unique coordination chemistry.
O(11)-Al(1)-O(31)
80.3(1) O(21)-Al(4)-O(41)
79.3(1)
O(11)-Al(1)-C(112) 109.3(1) O(21)-Al(4)-C(412) 107.6(1)
O(11)-Al(1)-C(114) 119.0(1) O(21)-Al(4)-C(414) 121.8(1)
C(112)-Al(1)-C(114) 113.7(1) C(412)-Al(4)-C(414) 112.1(1)
O(31)-Al(1)-C(112) 118.9(1) O(41)-Al(4)-C(412) 119.0(1)
O(31)-Al(1)-C(114) 111.8(1) O(41)-Al(4)-C(414) 113.7(1)
O(11)-Al(3)-O(31)
79.6(1) C(11)-O(11)-Al(1)
124.8(1)
135.4(1)
98.6(1)
134.1(1)
125.4(1)
98.8(1)
136.9(1)
121.7(1)
99.7(1)
119.0(1)
141.5(1)
99.1(1)
O(11)-Al(3)-C(312) 120.6(1) C(11)-O(11)-Al(3)
O(11)-Al(3)-C(314) 110.8(1) Al(1)-O(11)-Al(3)
C(312)-Al(3)-C(314) 112.6(1) C(31)-O(31)-Al(1)
O(31)-Al(3)-C(312) 108.1(1) C(31)-O(31)-Al(3)
O(31)-Al(3)-C(314) 121.9(1) Al(1)-O(31)-Al(3)
O(21)-Al(2)-O(41)
79.0(1) C(21)-O(21)-Al(2)
Experimental Section
O(21)-Al(2)-C(212) 126.2(1) C(21)-O(21)-Al(4)
O(21)-Al(2)-C(214) 108.0(1) Al(2)-O(21)-Al(4)
C(212)-Al(2)-C(214) 109.8(1) C(41)-O(41)-Al(2)
O(41)-Al(2)-C(212) 104.6(1) C(41)-O(41)-Al(4)
O(41)-Al(2)-C(214) 128.4(1) Al(2)-O(41)-Al(4)
Pentane was dried by distillation from sodium, CH2Cl2 was dried
by distillation from P2O5, and TiCl4 was purified by distillation. All
other reagents were commercial products that were used without further
purification. Flash chromatography was performed in the normal way.16
5-Bromo-2-(1,1-dimethylethyl)phenol.9,17 A suspension prepared
by adding SiO2 (14.4 g, 240 mmol)18 and Na2CO3 (5.05 g, 47.6 mmol)
to 3-bromophenol (5.00 g, 28.9 mmol) in CCl4 (40 mL) was stirred at
25 °C and treated dropwise with 2-bromo-2-methylpropane (4.76 g,
34.7 mmol). The mixture was then heated at reflux for 18 h, cooled
to 25 °C, and filtered. The filtered solid was washed with CHCl3, and
the washings were added to the original filtrate. The combined organic
phases were washed with H2O and saturated aqueous NaCl, and then
they were dried with anhydrous MgSO4. Volatiles were removed by
evaporation under reduced pressure, and the residue was purified by
flash chromatography (silica, hexane (93%)/ethyl acetate (7%)) to give
5-bromo-2-(1,1-dimethylethyl)phenol (5.75 g, 25.1 mmol, 87%) as a
colorless oil.9
O(11)-Al(1)-O(31)-Al(3)
O(11)-Al(3)-O(31)-Al(1)
O(31)-Al(1)-O(11)-Al(3)
O(31)-Al(3)-O(11)-Al(1)
O(21)-Al(2)-O(41)-Al(4)
O(21)-Al(4)-O(41)-Al(2)
O(41)-Al(2)-O(21)-Al(4)
O(41)-Al(4)-O(21)-Al(2)
C(12)-C(11)-O(11)-Al(1)
C(12)-C(11)-O(11)-Al(3)
C(32)-C(31)-O(31)-Al(1)
C(32)-C(31)-O(31)-Al(3)
C(22)-C(21)-O(21)-Al(2)
C(22)-C(21)-O(21)-Al(4)
C(42)-C(41)-O(41)-Al(2)
C(42)-C(41)-O(41)-Al(4)
-12.4(1)
12.5(1)
12.5(1)
-12.3(1)
13.3(1)
-13.1(1)
-13.2(1)
13.4(1)
-104.8(2)
90.9(2)
87.6(2)
-110.8(2)
94.5(2)
-103.6(2)
-110.3(2)
80.2(3)
2,2′-Methylenebis[3-bromo-6-(1,1-dimethylethyl)phenol] (5).9
Diphenol 5 was prepared from 5-bromo-2-(1,1-dimethylethyl)phenol
by the published method.9
1,1′-Methylenebis[6-bromo-3-(1,1-dimethylethyl)-2-methoxyben-
zene] (6).9 A suspension prepared by adding K2CO3 (2.70 g, 19.5
mmol) to 2,2′-methylenebis[3-bromo-6-(1,1-dimethylethyl)phenol] (5;
0.917 g, 1.95 mmol) in acetone (20 mL) was stirred at 25 °C and treated
with iodomethane (5.70 g, 40.2 mmol). The mixture was then heated
at reflux for 36 h, cooled to 25 °C, and filtered. The filtered solid was
washed with CHCl3, and the washings were added to the original filtrate.
Volatiles were removed by evaporation under reduced pressure, the
residue was treated with saturated aqueous NH4Cl, and the resulting
mixture was extracted with CHCl3. The combined extracts were washed
with saturated aqueous NaCl and dried with anhydrous MgSO4.
Volatiles were removed by evaporation under reduced pressure, and
the residue was purified by flash chromatography (silica, hexane
(100%)) to give 1,1′-methylenebis[6-bromo-3-(1,1-dimethylethyl)-2-
methoxybenzene] (6; 0.923 g, 1.85 mmol, 95%) as a colorless solid.
An analytically pure sample was prepared by recrystallization from
hexane: mp 165-167 °C (lit.9 164-166 °C).
of the absolute values of the C(t-Bu)-C-O-Al dihedral angles
is 97.8(3)°. This conformational preference separates the ethyl
groups in dimer 9 into two diastereotopic sets, one oriented
toward the interior of the dimer and the other toward the exterior,
and each aluminum atom is bonded to one ethyl group of each
type. Exchange of the ethyl groups by simple rotations of intact
Al2O2 rings around axes defined by their O‚‚‚O diagonals is
presumably disfavored because this would create highly unfa-
vorable interactions with adjacent substituents on the aryl rings.
It is instructive to compare ∆G q for exchange in dimer 9
(18.2 ( 0.2 kcal mol-1 at 92 °C) with that observed for the
closely related exchange of i-butyl groups in structure 11 (17.0
( 0.2 kcal mol-1 at 55 °C).5 Even though the Al2O2 ring of
structure 11 is much more highly distorted than that of dimer
9, its opening is only slightly faster. This suggests that the
observed deformations are not enthalpically costly.
(16) Still, W. C.; Kahn, M.; Mitra, A. J. Org. Chem. 1978, 43, 2923.
(17) For a similar tert-butylation procedure, see: Kamitori, Y.; Hojo, M.;
Masuda, R.; Izumi, T.; Tsukamoto, S. J. Org. Chem. 1984, 49, 4161.
(18) Although the tert-butylation procedure of Hojo and co-workers
specifies the use of SiO2 that has been dried by heating under
vacuum,17 we found that SiO2 of the type normally employed for flash
chromatography16 could be used directly without special treatment.
Conclusions
Our results confirm that the conversion of hydroxyl groups
into metal alkoxides can serve as the basis of an effective
strategy for constructing strong multidentate Lewis acids from

Complexes of Strong Bidentate Lewis Acids
Inorganic Chemistry, Vol. 37, No. 11, 1998 2625
2,7-Bis(1,1-dimethylethyl)-1,8-dimethoxy-9H-fluorene (4).9 Di-
methyl ether 4 was prepared from 1,1′-methylenebis[6-bromo-3-(1,1-
dimethylethyl)-2-methoxybenzene] (6) by the published procedure.9
2,7-Bis(1,1-dimethylethyl)-9H-fluorene-1,8-diol (3). A solution of
2,7-bis(1,1-dimethylethyl)-1,8-dimethoxy-9H-fluorene (4; 225 mg,
0.665 mmol) in CH2Cl2 (10 mL) was stirred at -78 °C under dry Ar
and treated dropwise with a solution of BBr3 (666 mg, 2.66 mmol) in
CH2Cl2 (2.6 mL). The cooling bath was removed, and the temperature
was allowed to rise to 25 °C. After 18 h, the resulting mixture was
recooled to 0 °C, treated with H2O (15 mL), and extracted with CHCl3.
The combined extracts were washed with saturated aqueous NaCl and
dried with anhydrous MgSO4. Volatiles were removed by evaporation
under reduced pressure, and the residue was purified by flash chro-
matography (silica, hexane (93%)/ethyl acetate (7%)) to give 2,7-bis-
(1,1-dimethylethyl)-9H-fluorene-1,8-diol (3; 178 mg, 0.573 mmol, 86%)
as a colorless solid. An analytically pure sample was prepared by
(s, 2H), 7.31 (d, 8H, 3J ) 8.0 Hz), 7.39 (d, 2H, 3J ) 8.0 Hz), 7.48 (d,
3
3
2H, J ) 8.0 Hz), 7.90 (d, 8H, J ) 8.0 Hz); 13C NMR (100.6 MHz,
CDCl3) δ 21.8, 30.6, 35.0, 35.2, 53.3, 117.2, 125.6, 129.1, 132.5, 133.4,
137.4, 141.2, 145.7, 145.7, 168.2, 201.4.
Dimer 9 of Bis(diethylaluminum phenoxide) 8. A solution of 2,7-
bis(1,1-dimethylethyl)-9H-fluorene-1,8-diol (3; 14.8 mg, 0.0477 mmol)
in a mixture of pentane (0.6 mL) and CH2Cl2 (0.2 mL) was stirred at
25 °C under dry Ar and treated dropwise with neat Al(CH2CH3)3 (10.9
mg, 0.0955 mmol). After the addition was complete, the resulting
brown mixture was kept at 25 °C for 20 min. Partial evaporation of
solvent then yielded brown crystals of the dimer 9 of bis(diethylalu-
minum phenoxide) 8 (14.6 mg, 0.0153 mmol, 62%): 1H NMR (400
MHz, CDCl3) δ 0.28 (q, 8H, 3J ) 8.1 Hz), 0.35 (q, 8H, 3J ) 8.1 Hz),
3
3
0.65 (t, 12H, J ) 8.1 Hz), 0.87 (t, 12H, J ) 8.1 Hz), 1.55 (s, 36H),
4.75 (s, 4H), 7.52 (d, 4H, 3J ) 8.0 Hz), 7.55 (d, 4H, 3J ) 8.0 Hz); 13
NMR (100.6 MHz, CDCl3) δ 2.1, 5.2, 8.1, 8.7, 33.1, 36.4, 36.9, 115.7,
C
recrystallization from CHCl3: mp 176-178 °C; IR (melt) 3548 cm-1
;
128.9, 134.5, 139.6, 141.7, 148.8.
1H NMR (400 MHz, CDCl3) δ 1.48 (s, 18H), 3.64 (s, 2H), 4.80 (s,
X-ray Crystallographic Studies of Bis(trichlorotitanium phenox-
ide) 7, the 1:2 Complex of Bis(trichlorotitanium phenoxide) 7 with
4,4′-Dimethylbenzophenone, and the Dimer 9 of Bis(diethylalumi-
num phenoxide) 8. Data were collected on an Enraf-Nonius CAD-4
diffractometer. Cell constants were obtained using 25 well-centered
reflections. Five standard reflections were measured every 1 h of
exposure time and showed only small fluctuations. The data were
corrected for absorption by Gaussian integration. The structures were
solved by direct methods using either SHELXS-8519 or SHELXS-96.20
Full-matrix least-squares refinements on F2 were carried out with
SHELXL-93.21 All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were placed in ideal positions and refined using a
riding model with isotropic thermal parameters 1.2 times those of their
carrier atoms (1.5 times for methyl groups).
3
3
2H), 7.27 (d, 2H, J ) 8.0 Hz), 7.32 (d, 2H, J ) 8.0 Hz); 13C NMR
(100.6 MHz, CDCl3) δ 29.5, 30.0, 34.5, 112.4, 126.3, 128.1, 134.4,
141.4, 150.8; HRMS (EI) calcd for C21H26O2 m/e 310.1933, found m/e
310.1925.
Bis(trichlorotitanium phenoxide) 7. A solution of 2,7-bis(1,1-
dimethylethyl)-9H-fluorene-1,8-diol (3; 10.6 mg, 0.0341 mmol) in CH2-
Cl2 (0.5 mL) was stirred at 25 °C under dry Ar and treated dropwise
with neat TiCl4 (13.8 mg, 0.0727 mmol). After the addition was
complete, the resulting red mixture was kept at 25 °C for 20 min. Partial
evaporation of solvent then yielded red crystals of bis(trichlorotitanium
phenoxide) 7 (19.0 mg, 0.0308 mmol, 90%): mp 172-176 °C; 1H NMR
(400 MHz, CDCl3) δ 1.59 (s, 18H), 4.60 (s, 2H), 7.42 (d, 2H, 3J ) 8.0
Hz), 7.53 (d, 2H, 3J ) 8.0 Hz); 13C NMR (100.6 MHz, CDCl3) δ 30.5,
33.6, 35.2, 118.2, 126.4, 135.7, 137.0, 140.9, 168.9.
1:2 Complex of Bis(trichlorotitanium phenoxide) 7 with 4,4′-
Dimethylbenzophenone. A solution of 2,7-bis(1,1-dimethylethyl)-9H-
fluorene-1,8-diol (3; 27.0 mg, 0.0870 mmol) in CH2Cl2 (1.2 mL) was
stirred at 25 °C under dry Ar and treated dropwise with neat TiCl4
(32.9 mg, 0.173 mmol). After the addition was complete, the resulting
red mixture was kept at 25 °C for 15 min and was then treated with a
solution of 4,4′-dimethylbenzophenone (37.0 mg, 0.176 mmol) in CH2-
Cl2 (1.2 mL). Partial evaporation of solvent then yielded red crystals
of the 1:2 complex of bis(trichlorotitanium phenoxide) 7 with 4,4′-
dimethylbenzophenone as a solvate containing an equimolar amount
Acknowledgment. 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, acknowledgment is made to the donors of the
Petroleum Research Fund, administered by the American
Chemical Society, for support of this research.
Supporting Information Available: Tables of atomic coordinates
and isotropic thermal parameters, complete bond lengths and angles,
anisotropic thermal parameters, hydrogen atom coordinates and thermal
parameters, and distances to weighted least-squares planes for bis-
(trichlorotitanium phenoxide) 7, the CH2Cl2 solvate of its 1:2 complex
with 4,4′-dimethylbenzophenone, and the dimer 9 of bis(diethylalumi-
num phenoxide) 8 (70 pages). Ordering information is given on any
current masthead page.
1
of CH2Cl2 (79.0 mg, 0.0704 mmol, 81%): mp 102-110 °C; H NMR
(400 MHz, CDCl3) δ 1.60 (s, 18H), 2.45 (s, 12H), 4.77 (s, 2H), 5.30
(19) Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467.
(20) Sheldrick, G. M. SHELXS-96; University of Go¨ttingen: Go¨ttingen,
Germany, 1996.
(21) Sheldrick, G. M. SHELXL-93; University of Go¨ttingen: Go¨ttingen,
Germany, 1993.
IC9715411