Tetrakis(2-hydroxyphenyl)ethene and derivatives. A structurally preorganized tetradentate ligand system for polymetallic coordination chemistry and catalysis

Article abstract of DOI:10.1021/ja027436u

A topologically unique, conformationally constrained tetradentate ligand system for polymetallic coordination chemistry has been developed: tetrakis(2-hydroxyphenyl)ethene (1a) and substituted derivatives. The design exploits the planarity of the tetraphenylethylene core to impart rigidity to the roughly square oxygen binding array, while maintaining a degree of conformational mobility associated with rotation about the aryl-ethylene carbon-carbon bonds. Tetrakis(2-hydroxyphenyl)ethene derivatives are designed to promote multiple metal bridging over chelating coordination modes. The ligand is synthesized from anisole or 4-tert-butylanisole in four steps via the 2,2-dimethoxybenzophenone hydrazones 4a,b. The sterically hindered ortho-substituted tetraphenylethylene core is produced in high yield by acid-catalyzed decomposition of the corresponding diaryl diazomethane prepared in situ by oxidation of the hydrazone using nickel peroxide. Deprotection of the methyl ethers using boron tribromide gives tetrakis(2-hydroxyphenyl)ethene (1a), characterized by X-ray crystallography, and tetrakis(5-tert-butyl-2-hydroxyphenyl)ethene (1b). Sterically isolating substituents in the 3-position can be installed via Claisen rearrangement/hydrogenation, providing tetrakis(3-n-propyl-2-hydroxyphenyl)ethene (6) efficiently. To illustrate potential applications of this unprecedented ligand class, two coordination complexes are reported, including tetrakis(2-diethylaluminoxyphenyl)ethene (8), a structurally robust eight-membered-ring aluminum/oxygen crown complex characterized both in solution and in the solid state. Copyright

Full text of DOI:10.1021/ja027436u

Published on Web 08/06/2002
Tetrakis(2-hydroxyphenyl)ethene and Derivatives. A Structurally Preorganized
Tetradentate Ligand System for Polymetallic Coordination Chemistry and
Catalysis
Udo Verkerk, Megumi Fujita, Trevor L. Dzwiniel, Robert McDonald,† and Jeffrey M. Stryker*
Department of Chemistry, UniVersity of Alberta, Edmonton, Alberta, T6G 2G2 Canada
Received June 24, 2002
Scheme 1 a
The use of structurally preorganized polydentate ligand systems
to direct the assembly of polymetallic coordination complexes is
important to both supramolecular chemistry and catalysis.1 In this
context, calix[4]arenes2 (I) have been a widely used polyaryloxide
structural motif for organizing coordination complexes3 and model-
ing solid oxide supports and zeolite surfaces.4 The analogy to oxide
surfaces, however, is not particularly apt: calixarenes lack the
conformational rigidity inherent to surface binding sites, promoting
chelating over bridging coordination modes. The connectivity also
limits the introduction of sterically isolating substituents adjacent
to the binding sites,5 rendering metal complexes susceptible to
uncontrolled intermolecular aggregation.
a Conditions: (a) (i) BuLi, TMEDA, Et2O, -65 °C, (ii) Me2NCOCl,
(iii) H2O (3a, 77%; 3b, ca. 60%); (b) H2NNH2‚H2O, n-BuOH, ∆ (4a, 95%;
4b, quantitative); (c) (i) NiO2, CH3CN, 0 °C, (ii) catalytic TsOH (5a, 87%;
5b, 46% from 3b); (d) BBr3, CH2Cl2, -65 °C f room temperature (1a,
90%; 1b, 87%); (e) (i) C3H5Br, K2CO3, acetone, (ii) 200 °C, (iii) 600 psi
H2, Pd/C, MeOH (60% from 1a).
In this Communication, we report the development of an
alternative, topologically distinct tetradentate ligand system for
The structure of 1a was determined in the solid state by X-ray
crystallography,9 revealing a degree of both intramolecular and
intermolecular organization. Thus, the four aryl groups are nearly
perpendicular to the plane of the olefin and engage in pairwise
intramolecular hydrogen-bonding on opposite faces of the molecule
(Figure 1). The crystal is further organized into extended one-
dimensional ladders, linked by intermolecular hydrogen bonds (see
diagram in the Supporting Information). The increased structural
rigidity of the ligand thus disfavors the formation of a fully
intramolecular array of hydrogen bonds, as observed for calix[4]-
arenes in the cone conformation.2 In solution, the aryl rings rotate
rapidly, as established by variable-temperature 1H NMR spectros-
copy.
A convergent approach to 3-substituted tetrakis(2-hydroxyphe-
nyl)ethenes is complicated by steric effects: hydrazone formation
fails for benzophenones bearing large 3-substituents. The Claisen
rearrangement, however, provides an efficient ortho-alkylation
protocol (Scheme 1).14 Nucleophilic allylation and thermolysis
yields tetrakis(3-allyl-2-hydroxyphenyl)ethene9 cleanly; complete
hydrogenation of the side chains requires high pressure, providing
tetrakis(3-n-propyl-2-hydroxyphenyl)ethene 6 in high yield.9 Despite
the 3-substituents, rapid rotation of the aryl rings is maintained.
To confirm the potential of this preorganized ligand system for
applications to coordination chemistry, an investigation into both
main group and transition metal chemistry has been initiated.
Although designed for multiple metal coordination, ligand 1a also
accommodates tetradentate chelation in the case of larger polyvalent
metal ions. Thus, dark blue molybdenum(V) complex 79 was
prepared from the tetrasodium salt of 1a and (C5Me5)MoCl4.15 The
coordination chemistry, catalysis, and supramolecular chemistry:
tetrakis(2-hydroxyphenyl)ethene (1a) and substituted derivatives.
The design exploits the relative planarity of the ethylene core to
constrain the binding domain from inward collapse but retains a
degree of calixarene-like conformational freedom associated with
correlated rotation of the aryl groups.6 For the conformation that
places the aryl groups perpendicular to the ethylene plane and the
four hydroxy groups in the same hemisphere (e.g., II), the ligand
presents a roughly square array of oxygen atoms for metal binding
(vide infra). The design inherently accommodates the introduction
of alkyl substituents adjacent to the metal binding sites.
Although several tetrakis(2-substituted) tetraphenylethylene com-
pounds are known,7 oxygen-substituted derivatives are virtually
unprecedented.8 Nonetheless, an efficient four-step synthesis of
tetrakis(2-hydroxyphenyl)ethene (1a) has been developed (Scheme
1).9 The synthesis exploits the acid-catalyzed coupling of diaryl-
diazomethanes, a classical olefination procedure.10,11 Thus, 2,2′-
dimethoxybenzophenone (3a) was prepared by ortho-metalation of
anisole12 and converted to hydrazone 4a.9 Oxidation to the
diazomethane derivative was accomplished using nickel peroxide,13
and the crude product was coupled efficiently using catalytic
p-toluenesulfonic acid. Exhaustive demethylation of 5a under
standard conditions proceeded nearly quantitatively. The highly
soluble tetrakis(5-tert-butyl-2-hydroxyphenyl)ethene (1b) was pre-
pared analogously, although more elaborate product isolations were
required in this more lipophilic series.9 The syntheses of 1a and
1b are readily adaptable to larger scales.
* Address correspondence to this author. E-mail: jeff.stryker@ualberta.ca.
† Department of Chemistry Structure Determination Laboratory.
9
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J. AM. CHEM. SOC. 2002, 124, 9988-9989
10.1021/ja027436u CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
tetradentate aryloxy ligand environment. Differentiation of the
ligand binding sites, additional variation in peripheral substitution
pattern, and applications to polymetallic coordination chemistry and
catalysis are in progress.
Acknowledgment. Financial support from NOVA Chemicals
Corporation and the Natural Sciences and Engineering Research
Council of Canada is gratefully acknowledged.
Supporting Information Available: Experimental procedures and
complete characterization of all new compounds (PDF); details of the
crystallography for compounds 1a, 7, and 8 (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
Figure 1. ORTEP diagram of compound 1a.
References
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Figure 2. ORTEP diagram of molybdenum complex 7.
(4) See: Floriani C. Chem. Eur. J. 1999, 5, 19 and references therein. Giannini,
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(8) One tetrakis(2-alkoxyphenyl)ethene derivative has been reported, a double
crown ether isolated in 7% yield by oxidation of a 2,2′-crown ether-linked
benzophenone hydrazone, presumably via formation of an intermediate
carbene: von Itter, F. A.; Voegtle, F. Chem. Ber. 1985, 118, 2300.
Figure 3. ORTEP diagram of tetrakis(diethylaluminum) complex 8.
(9) Complete experimental details are provided as Supporting Information.
(10) Roberts, J. D.; Watanabe, W. J. Am. Chem. Soc. 1950, 72, 4869. Bethell,
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(11) McMurry reaction of benzophenone 3a or related 3,3′-disubstituted
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previously described;8 dehydrogenation of the tetraarylethanes failed.
(12) Lucas, P.; Mehdi, N. E.; Ho, H. A.; Belanger, D.; Breau, L. Synthesis
2000, 1253.
(13) Nakagawa, K.; Onoue, H.; Minami, K. Chem. Commun. 1966, 730.
structure, confirmed by X-ray crystallography (Figure 2), is closely
analogous to those of other Mo(V) tetraalkoxide complexes.16,17
The Mo-O bonds are nearly equidistant, and the ligand assumes
a slightly distorted propeller conformation, with the aryl rings canted
4-14° from perpendicular with respect to the ethene plane.
A stronger demonstration of the organizational potential of this
ligand template is provided by the polymetallic complex obtained
upon treatment of 1a with excess triethylaluminum in pentane.
Tetrakis(2-diethylaluminoxyphenyl)ethene (8) precipitates from
solution and was isolated in 65% yield; complex 8 is the only
product observed spectroscopically in solution.9 The 1H NMR
spectrum, invariant from -80 to 100 °C, shows four inequivalent
sets of ethyl groups, integrating to eight ethyl groups per ligand.
This suggests a robust C2-symmetric aluminum/oxygen structure
in solution; X-ray crystallography9 confirms this (AlO)4 structure
in the solid state (Figure 3).18
Neither the calix[4]arenes nor other conformationally less
constrained tetradentate aryloxy binding templates provide structur-
ally similar higher order polymetallic complexes.19,20 The confor-
mationally constrained tridentate ligand, tris(3,5-di-tert-butyl-2-
phenyloxy)methane, however, organizes a lower order but similarly
robust tris(aluminum/oxygen) crown complex analogous to complex
8.21
(14) Direct introduction of both o- and p-tert-butyl substituents by Friedel-
Craft alkylations led to inseparable product mixtures.
(15) Murray, R. C.; Blum, L.; Liu, A. H.; Schrock, R. R. Organometallics
1985, 4, 953.
(16) Walborsky, E. C.; Wigley, D. E.; Roland, E.; Dewan, J. C.; Schrock, R.
R. Inorg. Chem. 1987, 26, 1615.
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Chem. Eur. J. 2001, 7, 783 and references therein. Gibson, V. C.; Redshaw,
C.; Clegg, W.; Elsegood, M. R. J. Chem. Commun. 1998, 1969. (b) Cp*Ta-
(calix[4]arene): Acho, J. A.; Doerrer, L. H.; Lippard, S. J. Inorg. Chem.
1995, 34, 2542.
(18) Eight-membered Al/O and Al/S rings have been characterized crystallo-
graphically, although there is no evidence that such higher oligomers are
maintained in solution. (a) (Me2AlSAr)4: Taghiof, M.; Heeg, M. J.; Bailey,
M.; Dick, D. G.; Kumar, R.; Hendershot, D. G.; Rahbarnoohi, H.; Oliver,
J. P. Organometallics 1995, 14, 2903. (b) (Me2AlOLi)4: Storre, J.;
Schnitter, C.; Roesky, H. W.; Schmidt, H. G.; Noltemeyer, M.; Fleischer,
R.; Stalke, D. J. Am. Chem. Soc. 1997, 119, 7505. (c) (MesAlO)4:
Wehmschulte, R. J.; Power, P. P. J. Am. Chem. Soc. 1997, 119, 8387.
(19) Atwood, J. L.; Gardiner, M. G.; Jones, C.; Raston, C. L.; Skelton, B. W.;
White, A. H. Chem. Commun. 1996, 2487.
(20) Cottone, A., III; Scott, M. J. Organometallics 2000, 19, 5254.
(21) Cottone, A., III; Morales, D.; Lecuivre, J. L.; Scott, M. J. Organometallics
2002, 21, 418.
Tetrakis(2-hydroxyphenyl)ethene and derivatives thus provides
a versatile, topologically unique, and conformationally constrained
JA027436U
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