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 “chalice‑like” 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 [2,2’-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 “half–calixarene”, 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 [k2O,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 60ŻC 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

“Factors 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.”  Daniela Buccella,  Joseph M. Tanski, and Gerard Parkin Organometallics 2007, 26, 3275-3278.

 

Exo and Endo Isomerism of Subvalent Tin and Germanium Complexes Derived from 1,3–Diethers of p-tert-Butylcalix[4]arene.”  Tony Hascall, Keliang Pang and Gerard Parkin Tetrahedron 2007, 63, 10826-10833 (Special issue on New Trends in Calixarene Chemistry).  

 

p-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.”  Daniela Buccella and Gerard Parkin J. Am. Chem. Soc. 2006, 128, 16358-16364.

 

“Subvalent Germanium and Tin Complexes Supported by a Dianionic Calixarene Ligand:  Structural Characterization of Exo  and Endo Isomers of [Butcalix(TMS)2]Ge.”  Tony Hascall, Arnold L. Rheingold, Ilia Guzei, and Gerard Parkin J. Chem. Soc., Chem. Commun.  1998, 101-102.