Calixarenes
We are interested in the application of multidentate
aryloxide ligands to prepare reactive metal complexes that may emulate aspects
of heterogeneous catalyst surfaces.
In this regard, calixarenes, a class of Òchalice‑likeÓ macrocyclic
phenols, offer considerable potential for providing a platform that mimics
oxygen rich surfaces.
The Calixarene Methylene Linker as a Binding
Functionality
[CalixBut(OH)4]
reacts with Mo(PMe3)6 and W(PMe3)4(h2–CH2PMe2)H
to yield products with analogous composition, namely {[CalixBut(OH)2(O)2]M(PMe3)3H2},
resulting from the oxidative addition of the O–H bonds of two adjacent
phenol groups to the metal center.
However, while the composition of the products are
similar, there is an important difference with respect to the manner by which
the methylene C–H groups interacts with the metal center. Specifically, the formally
16–electron fragment {[CalixBut(OH)2(O)2]W(PMe3)3H2}
undergoes oxidative addition of the C–H bond to give a metallated
trihydride [Calix-HBut(OH)2(O)2]W(PMe3)3H3,
whereas for molybdenum the corresponding 16–electron fragment merely
coordinates the C–H bond via an
agostic interaction to give [CalixBut(OH)2(O)2]Mo(PMe3)3H2.
The molecular structures of [CalixBut(OH)2(O)2]Mo(PMe3)3H2
and [Calix-HBut(OH)2(O)2]W(PMe3)3H3
are of particular interest because they provide the first evidence for the role
of the calixarene methylene linker as a binding functionality via
the formation of agostic and alkyl-hydride derivatives.
Solution 1H NMR spectroscopic studies
demonstrate that [Calix-HBut(OH)2(O)2]W(PMe3)3H3
is in equilibrium with its agostic isomer and that the [M(PMe3)3H2]
fragments of both the molybdenum and tungsten complexes [CalixBut(OH)2(O)2]M(PMe3)3H2
migrate rapidly around the phenolic rim of the calixarene.
In order to determine whether the calixarene framework is
responsible for the differences between the molybdenum and tungsten compounds, we
sought related compounds derived from non-macrocyclic phenols. Therefore, we examined the reactivity
of the simple diphenol CH2(ArMe2OH)2 [2,2Õ-methylenebis(4,6-dimethylphenol)]
towards Mo(PMe3)6 and W(PMe3)4(h2–CH2PMe2)H.
The methylene bridged diphenol CH2(ArMe2OH)2, which may be
viewed as a Òhalf–calixareneÓ, reacts with Mo(PMe3)6
at room temperature to give [k2,h2–CH2(ArMe2O)2]Mo(PMe3)3H2.
X-ray diffraction demonstrates that [k2,h2–CH2(ArMe2O)2]Mo(PMe3)3H2
possesses an agostic interaction akin to that of [CalixBut(OH)2(O)2]Mo(PMe3)3H2,
but the magnitude of the interaction is greater for the calixarene complex.
The reaction between W(PMe3)4(h2–CH2PMe2)H
and CH2(ArMe2OH)2
follows a very different course to the corresponding reaction of Mo(PMe3)6. Thus, instead of reacting with both
phenolic O–H groups, W(PMe3)4(h2–CH2PMe2)H
yields initially [k2–O,C–CH2(ArMe2OH){(C6H2Me)(CH2)O}]W(PMe3)4H2,
which is derived by reaction with an O–H bond and a C–H bond of an
adjacent methyl group. However,
the cleavage of the C–H bond represents a kinetic preference because heating
to 60ûC results in the formation of [k2,h2–CH2(ArMe2O)2]W(PMe3)3H2
derived from cleavage of the two O–H bonds.
X-ray diffraction studies demonstrate that [k2,h2–CH2(ArMe2O)2]W(PMe3)3H2
possesses an agostic interaction similar to that in the molybdenum analogue, in
marked contrast to its calixarene counterpart [Calix-HBut(OH)2(O)2]W(PMe3)3H3
which exists as a metallated isomer in the solid state. Thus, the calixarene ligand shows a
greater propensity to undergo oxidative addition than does the methylene-bridged
diphenoxide ligand. Nveretheless,
NMR spectroscopic studies, indicate that the metallated trihydride, [k3–CH(ArMe2O)2]W(PMe3)3H3,
is accessible in solution.
Exo and Endo Isomerism of Subvalent Tin and Germanium
Complexes Derived from 1,3–Diethers of p-tert-Butylcalix[4]arene
In their tetraphenolic form, calix[4]arenes have the
potential for serving as tetradentate tetraanionic X4 ligands. Modification of the calix[4]arene by
selective alkylation of the phenolic groups, however, yields molecules that may
serve in principle as tetradentate LX3, L2X2,
L3X and L4 ligands, thereby greatly expanding the
versatility of calix[4]arenes in coordination chemistry. For example, dialkylation of two
phenolic groups generates a molecule [CalixBut(OH)2(OR)2] that may serve as an L2X2
ligand in its deprotonated form and we have investigated the application of
such ligands to the chemistry of subvalent germanium and tin.
A series of mononuclear germanium and tin complexes
[CalixBut(O)2(OR)2]M
may be obtained via the reaction of
[CalixBut(OH)2(OR)2]
(R = Me, Et, Prn, Pri, Bun, C3H5,
CH2Ph, SiMe3) with M[N(SiMe3)2]2
(M = Ge, Sn).
Interestingly, [CalixBut(O)2(OR)2]M exist as exo and endo
isomers, which are differentiated according to whether the germanium is located
outside or inside the calixarene cavity.
The former is considered to be the kinetic product while the latter is
the thermodynamic product. The
ability to access the thermodynamically more stable endo–[CalixBut(O)2(OR)2]M
isomer, however, depends critically on the nature of both the metal and the
ether substituents. For example,
the ability to access the endo
isomer is easier for the germanium system.
Selected References
ÒFactors Influencing Coordination versus Oxidative Addition of C–H Bonds to Molybdenum and Tungsten: Structural and Spectroscopic Evidence that the Calixarene Framework Promotes C–H Bond Activation.Ó Daniela Buccella, Joseph M. Tanski, and Gerard Parkin Organometallics 2007, 26, 3275-3278.
ÒExo and Endo Isomerism of Subvalent Tin and Germanium Complexes
Derived from 1,3–Diethers of p-tert-Butylcalix[4]arene.Ó Tony
Hascall, Keliang Pang and Gerard Parkin Tetrahedron 2007, 63, 10826-10833 (Special issue on New Trends in Calixarene
Chemistry).
Òp-tert-Butylcalix[4]arene
Complexes of Molybdenum and Tungsten:
Reactivity of the Calixarene Methylene C–H Bond and the Facile
Migration of the Metal around the Phenolic Rim of the Calixarene.Ó Daniela Buccella and Gerard Parkin J.
Am. Chem. Soc. 2006, 128, 16358-16364.
ÒSubvalent Germanium and Tin Complexes Supported by a
Dianionic Calixarene Ligand:
Structural Characterization of Exo
and Endo Isomers of [Butcalix(TMS)2]Ge.Ó Tony Hascall, Arnold L. Rheingold, Ilia Guzei, and Gerard
Parkin J. Chem. Soc., Chem. Commun. 1998, 101-102.