Dr. Richard A. Bartsch
Principal Research Interests
- Design and Synthesis of Novel Organic Molecules for Ionic and Molecular Recognition
- Separation Science
- Metal Ion Complexation and Separation by Multidentate Ligands
- Preparation and Applications of Ionic Liquids
The central theme of Professor Bartsch’s research program is ionic and molecular recognition. Novel organic molecules for the complexation and separation of targeted ionic species (particularly metal ions) and molecules are designed and synthesized. Different types of metal ion complexants include acyclic (podand), cyclic (crown ether, calixarene), and bicyclic (calix-crown, cryptand) ligands. Using the concept of 'charge matching', a number of acidic functions equal to the charge of the metal ion is attached to the ligand. The presence of such ion-exchange sites markedly enhances the efficiency of metal ion complexation in separation systems and the ability to subsequently release the complexed metal ion from the ligand. In addition to customary proton-ionizable groups of carboxylic, phosphonic, and sulfonic acid functions, we use a new acidic group [-C(O)NHSO2X], in which the acidity can be 'tuned' by varying the electron-withdrawing ability of X.
Currently attention is focused upon the use of calixarene as a scaffold for attachment of pendant acidic groups through the phenolic oxygens. For calixarene, the limiting conformations of cone, partial cone, 1,3-alternate, and 1,2-alternate are shown below as structures (1-4), respectively.
With appropriate substitutions on the four phenolic oxygens, the conformation can be mobile or locked. For example, ligand (5) is conformationally mobile and ligand (6) in which the two methyl groups in (5) have been replaced in the butyl groups is locked in the cone conformation. Ligand (7) is conformationally locked in the 1,3-alternate conformation with the two acidic groups on the opposite side of calixarene framework.
Evaluation of these ligands by solvent extraction of metal ions from aqueous solutions into chloroform reveals that the both the extraction efficiency and selectivity are influenced by the conformations of similarly substituted ligands.
Connection of two phenolic oxygens of the calixarene scaffold with a polyether chain gives calixarene-crown ether ligands, also called calixcrowns. Attachment of two distal oxygens results in a calixarene-1,3-crown ether ligand (8); whereas linking two proximal oxygens provides a calixarene-1,2-crown ether analogue (9). (Both structures are shown with the calixarene unit in the cone conformation.)
Attachment of two ionizable groups to the phenolic oxygens gives di-ionizable calixarene-crown ethers in which the spatial relationship of the acidic side arms to the crown ether cavity is controlled by the conformation of the calixarene unit. The influence of such systematic structural variations upon the metal ion complexing ability of the ligands is being probed by solvent extraction.
In a collaborative research effort with Professor Edward Quitevis of this department, fundamental features of ionic liquids are being examined. Ionic liquids are organic salts that are liquids at room temperature. These novel media are highly polar with unique solvating abilities that can be 'tuned' by structural variations within the cationic and/or anionic components. Ionic liquids such as those shown below are prepared by a Bartsch coworker for study by the Quitevis Research Group.
- "Sorption of Heavy Metal Ions by Silica Gel-Immobilized, Proton-Ionizable Calixarenes.", J. Wang; D. Zhang; T. R. Lawson and R. A. Bartsch, Talanta 2009, 78, 477-483.
- "Effect of Cation Symmetry and Alkyl Chain Length on the Structure and Intermolecular Dynamics of 1,3-Dialkylimidazolium Bis(trifluoromethanesulfonyl-amide Ionic Liquids.", D. Xiao; L. B. Hines, Jr.; S. Li; R. A. Bartsch; E. L. Quitevis; O. Russina and A. Triolo, J. Phys. Chem. B. 2009, 113, 6426-6433.
- "Metal Ion Complexation in Acetonitrile by Di-ionized Calixarenes Bearing Two Dansyl Fluorophores.", Ű. Ocak, M. Ocak; K. Surowiec; R. A. Bartsch; M. G. Gorbunova; C. Tu and M. Surowiec, J. Incl. Phenom. Macrocycl. Chemistry. 2009, 63, 131-139.
- "Di-ionizable p-tert-Butylcalixarene-1,2-monothiacrown-3 Ligands in the Cone Conformation: Synthesis and Metal Ion Extraction.", D. Zhang; J. D. Crawford and R. A. Bartsch, Tetrahedron 2008, 64, 9843-9849.
- "Di-ionizable Calixarene-1,2-crown-5 and -crown-6 Ethers in Cone Conformations: Synthesis and Divalent Metal Ion Extraction.", C. Tu; K. Surowiec; J. Gega; D. W. Purkiss and R. A. Bartsch, Tetrahedron 2008, 64, 1187-1196.
- "Di-ionizable p-tert-Butylcalixarene-1,2-crown-3 Ligands in Cone and 1,2-Alternate Conformations: Synthesis and Metal Ion Extraction.", D. Zhang; X. Cao; D. W. Purkiss and R. A. Bartsch, Org. Biomol. Chem. 2007, 5, 1251-1259.
- "Effect of para-Substituents on Alkaline Earth Metal Ion Extraction by Proton Di-ionizable Calixarene-crown-6 Ligands in Cone, Partial-cone and 1,3-Alternate Conformations." H. Zhou; D. Liu; J. Gega; K. Surowiec; D. W. Purkiss and R. A. Bartsch, Organic & Biomolecular Chemistry. 2007, 5, 324-332.
- "Efficient Divalent Metal Cation Extractions with Di-ionizable Calixarene-1,2-crown-4 Compounds." C. Tu; K. Surowiec and R. A. Bartsch, Tetrahedron 2007, 63, 4184-4189.
- "Synthesis and Lead(II) Sorption of Silica Gel-Immobilized, Di-ionizable Calixarenes." D. Zhang; J. Wang; T. R. Lawson and R. A. Bartsch, Tetrahedron 2007, 63, 5076-5082.
- "Novel Calixarene-thiacrown Ether for Selective and Efficient Extraction of Ba(II), Pb(II) or Hg(II)." C. Tu; K. Surowiec and R. A. Bartsch, Journal of Inclusion Phenomena and Macrocyclic Chemistry 2007, 58, 361-366.