Organometallic catalytic sites on the periphery of the dendrimers consist of a metal center fixed by a metal-carbon σ-bond to the ligand system. This struc ture is known to be highly effective for the addition of polyhalogenalkanes to olefinic double bonds, both in monomolecular complexes and in dendrimeric complexes. The functionalized dendrimers devel oped by the Dutch group contain nickel(II) centers, which are particularly sensi tive to oxidation. The oxidized nickel(III) state, which is not usually encountered in organometallic complexes, is stabi lized in the dendrimers by the special characteristics of the N,C,N'-coordinating diaminoaryl ligand. This stabiliza tion leads to the catalytic activity of the dendrimers. The nickel-containing dendrimers catalyze the Kharasch addition at room temperature with dichloromethane as a solvent, methyl methacrylate as a sub strate, and carbon tetrachloride as a re agent. The catalytic activity is 20 to 30% less than that of monomeric organome tallic complexes but this, says van Koten, was expected. The recycle possibil ities may more than offset the decrease in activity. "The new dendrimers are nanoscopic particles in which the catalytic sites are precisely defined. This provides new op portunities for physical separations and catalyst recycle/' says van Koten. He also suggests that optimum catalytic performance may be improved with other dendrimer ligands. The principal virtue of the dendrimers is providing control of catalytic site dispersion. The Utrecht group is now looking ahead to application of the new materials with preselected catalytic properties to twophase reaction systems and, possibly, membrane reactors. A research group at the National Re newable Energy Laboratory (NREL) in Golden, Colo., is also interested in com bining the advantages of highly struc tured macromolecules with the catalytic chemistry of transition-metal phosphine complexes. The goal at NREL is to de velop suitable catalysts for electrochemi cal reduction of carbon dioxide. "This [process] is part of the NREL charter and ultimately we would like to utilize solar energy in a photovoltaic de vice to drive the electrochemical reduc tion of C0 2 ," says Daniel L. DuBois, a senior scientist at NREL. DuBois' group has focused on making
Polysilane dendrimer contains 12 diaminoaryl complexes
small dendritic molecules. The research ers take advantage of the recently devel oped approach for synthesizing poly phosphate ligands by the catalyzed ad dition of phosphorus-hydrogen bonds to activated olefins. More specifically, the NREL group has shown that metallation of small dendrimers that contain 12 to 15 phosphorus atoms with the complex [Pd(CH3CN)4](BF4)2 leads to dendrimers that show catalytic activity in the electro chemical reduction of C0 2 [Inorg. Chem., 33, 5482 (1994)]. The one-electron C0 2 reductions ob served in bulk electrolysis experiments carried out at NREL suggest to DuBois the formation of a Pd(I) species with Pd-Pd bonds. These reactions are anal ogous to the Pd(I) dimer formed through reduction of [Pd(triphenylphosphine)(CH3CN)](BF4)2 complexes. That would further suggest that higher turnover numbers are possible with greater sepa ration of the Pd sites in the dendrimers, a possibility DuBois says that future work in his lab will explore.
DuBois also says the catalytic activi ty and selectivity of the NREL dendrimer-supported catalysts are only slightly less than would be obtained from monomeric catalytic species. Fur thermore, DuBois thinks that refine ment of the catalytic system may yield higher activity and better selectivity from the dendrimer catalysts. Joseph Haggin
Monolayers enhance polymerfilmproperties Self-assembled monolayers of pyrrolecontaining alkanethiols enhance the ad hesion and conductivity of polypyrrole films on gold electrodes, according to chemists at Georgia Institute of Tech nology, Atlanta, and Louisiana State University, Baton Rouge. The work demonstrates that mono layers can play a significant role in im proving the growth and properties of FEBRUARY 6,1995 C&EN
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Just Published! Benign by Design: Alternative Synthetic Design for Pollution Prevention
SCIENCE/TECHNOLOGY
conducting polymers that one day might be used for fabricating nanoscale Monolayers are alkanethiols electronic devices. bearing substituted pyrroles At Georgia Tech, David M. Collard, an assistant professor of chemistry, and Curtis N. Sayre, a graduate student, re[^N(CH2)6SH port that films of poly(3-ethylpyrrole) on gold electrodes modified by pyrrole-thiAlkanethiol containing This new ol monolayers are extremely adhesive ^•substituted pyrrole ζ^βΜ volume and have high electrical conductivity describes [Uingmuir, 11, 302 (1995)]. Collard and the current Sayre prepared the films by electrooxistatus and dative polymerization of 34thylpyrrole potential H on gold substrates modified by spontaof synAlkanethiol containing neously adsorbed monolayers of alkanethetic 3-substituted pyrrole thiols bearing 3-substituted pyrroles. chemistry designed "The monolayer acts as a molecular to use and glue," says Collard. "The film sticks to genervery well to the gold surface and the kanethiols containing N-substituted ate fewer polymer is more highly conductive by pyrroles and alkane chain lengths of 11 yM 9m j.MJSSàétr^'- ' hazardous a factor of about three compared with three, five, and six carbon atoms. umW'^^^^^subpolymers deposited on an electrode In a paper published in the same isstances. without the monolayer." sue of hangmuir [11, 296 (1995)], Robin It presents research results on The two chemists deposited the L. McCarley, an assistant professor of the replacement of hazardous monolayers on gold by immersing the chemistry, and Robert J. Willicut, a feedstocks with biologically electrodes in dilute organic solutions of graduate student, describe the electroderived, innocuous feedstocks; of the pyrrole-containing alkanethiols. chemical and microscopic characterizahazardous reagents with visible They report that potentiostatic poly- tion of polypyrrole films deposited on light; and of phosgene, benzene, merization of 3-ethylpyrrole on un- gold surfaces modifed with a series of and halogens in a variety of modified gold "gives thick black films self-assembled alkanethiol monolayers industrially important reactions. In that are easily removed from the elec- containing pendant pyrrole groups. addition, the volume examines new techniques for carrying out trode surface by wiping the electrode They used these monolayers on gold transformations in environmentally with a paper towel." electrodes to enhance the nucleation and benign solvent systems and By contrast, deposition of poly(3- growth of polypyrrole films formed by provides examples of how alternaethylpyrrole) on electrodes modified electrochemical deposition from dilute tive synthetic design for pollution with monolayers produces films "so solutions containing the pyrrole and an prevention has been made comstrongly adherent to the electrode sur- electrolyte. Consequently, they observed mercially available. It also deface that the polymer must be removed "striking differences in the morphology scribes how to conduct a sourcewith coarse emery cloth, followed by ex- and adhesion of polypyrrole films reduction assessment and analyzes tensive polishing with alumina," accord- formed on modified versus unmodified computer-assisted synthetic gold surfaces." ing to the researchers. design. McCarley claims that the two hangCollard and Sayre used scanning Paul T. Anastas and Carol A. Farris, muir papers "describe the first use of electron microscopy (SEM) to compare Office of Pollution Prevention and Toxics, U.S. Environmental Protection the morphological differences between well-defined monolayers to control morAgency, Editors poly(3-ethylpyrrole) films deposited on phology, adhesion, nucleation, density, ACS Symposium Series No. 577 bare and modified electrodes. The SEM and conductivity of polypyrroles." He 208 pages (1994) Clothbound images showed that films deposited on notes that the monolayers serve as excelISBN 0-8412-3053-6 modified electrodes are smoother and lent "primers" supporting the growth of $59.95 ORDER FROM less porous than films deposited on very adherent and smooth polypyrrole American Chemical Society films. bare electrodes. Distribution Office Dept. 74 1155 Sixteenth Street. NW The Louisiana State chemists are inThe films are also compact. The Washington, DC 20036 Georgia Tech chemists suggest that terested in these types of monolayers Or CALL TOLL FREE 1-800-227-5558 the monolayers improve the packing because of "their potential in the con(in Washington, DC 872-4363) and of the films and conclude that "the struction of patterned surfaces using use your credit card! FAX: 202-872-6067. conductivity of these films is enhanced both optical lithography and scanned ACS Publications Catalog now as a consequence of this change in probe microscope fabrication methavailable on internet: gopher acsinfo.acs.org ods." The team is currently using inframorphology." Whereas Collard and Sayre investi- red spectroscopy and scanning tunnelgated monolayers of alkanethiols with ing microscopy to investigate pyrrolyl3-substituted pyrroles and alkane alkanethiol monolayers before and ACSIIIPUBLICATIONS chains of 10 or 11 carbon atoms, chem- after electrochemical oxidation. Essential Resources for the Chemical Sciences Michael Freemantle ists at Louisiana State examined al28
FEBRUARY 6,1995 C&EN
