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Reports: UR454658-UR4: Interaction of Aromatic Hydrocarbons with Self-Assembled Cubic Supramolecular Metallocages

John D. Thoburn, PhD, Randolph-Macon College

The goal of this work is to synthesize new cubic, face-capped metallocomplexes that have a large void space capable of encapsulating polycyclic aromatic molecules and then study their dynamical host-guest properties with a variety of guests including petroporphyrin. We have made excellent progress on the synthesis of a cubic, self-assembled M8L6 porphyrin-based metallocomplex host (1) as well as some potential guests for use in a dynamics and molecular recognition studies.  One of the potential applications of this project is the separation of polycyclic aromatic petroleum products such as vanadyl porphyrins from asphaltene-like materials.  In this first year of the project we focused on synthetic targets. 

In order to probe the chiral recognition properties of the hosts, we will take advantage of the inherent handedness of the cage.  All eight metal centers have the same handedness (all Δ or all Λ).  Since it has already been established that large polycyclic aromatic molecules like coronene have a high affinity for these metallocomplexes, we will study the interactions between these chiral cubic host and a chiral hydrocarbon, pentahelicene 2. 

We have synthesized the necessary helicene albeit in very small quantities.  While the first four steps of the synthesis have all gone in good yield (grams), the last step, which involves treatment with potassium t-butoxide, has produced only very small amounts of pentahelicene.   

We should be able to remove the bromines in higher yields by using n-butyl lithium and quenching with water.  When the helicene 2 is synthesized we will look to see if 2-M, or its enantiomer 2-P, binds better to 1-Δ8  or  1-Λ8.

Concurrently, we are also working on an improved synthesis of host 1.  Although 1 is a known compound, its self-assembly is very sensitive to trace impurities formed during the reduction of its tetranitrophenylporphyrin precursor.  We are developing an new methodology for the synthesis that will directly introduce nitrogen into the porphyrin precursors via benzylcarbamate.  The carbamate can then be deprotected by catalytic hydrogenation and the by-products (CO2 and toluene) can be easily removed. 

The first step of this scheme has gone in quantitative yields and we are now working on the porphyrin synthesis.

The original cubic metallocomplex 1 suffers from low solubility in virtually all solvents.  In order to make these cubes more useful we are synthesizing metallocubes with pendant polyethylene (PEG) chains (3).  The best approach to incorporating the solubilizing PEG group into the metallocube involves the use of “click” chemistry, i.e. coupling a PEGylated azide with an acetylenated porphyrin:

We have successfully made the PEGylated azide, carried out the Sonogashira coupling the formylbromopyridine with TMS-acetylene, and are now working on the porphyrin synthesis.

      Two undergraduate researchers have worked on these projects thus far and their role in this endeavor has been essential.  The success of the projects rest in their hands, which in turn is dependent on the training they receive from the PI.  The challenges they have overcome in the laboratory are an important component of their scientific education, their preparation for graduate school and their careers.  One of the two students is working on her undergraduate degree while the other is now pursuing graduate studies in the area of supramolecular chemistry.       With support from the ACS-PRF the principle investigator has benefitted from the opportunity to present this work at a major in international symposium: the Gordon Research Conference on Self-Assembly and Supramolecular Chemistry, Ciocco, Italy, May, 2015.