The Power of Palladium(III) and the Power of Polymer Solar Cells

What's Hot in Chemistry; March/April 2011

Figure of Graphene, from the Wiki Commons (

by John Emsley

There are two new entrants to the current Hot Ten: paper #8, which is about a new polymer for solar cells and is discussed in more detail below, and paper #10, which reports graphene transistors that can operate at high frequencies; the latter report joins the six other graphene papers that continue to dominate the list.

For readers seeking some real chemistry, i.e., about chemical reactions, then we have to drop down the list to #12, but the search is worthwhile. This paper, published in Nature Chemistry (D.C. Powers, T. Ritter, 1[4]: 302-9, July 2009; 16 citations this period and 58 overall), is the work of Tobias Ritter and David Powers of Harvard University’s Department of Chemistry and Chemical Biology. It reports on the ability of palladium to act as a catalyst in the formation of carbon-chlorine, carbon-bromine, and carbon-oxygen bonds.

What makes the paper so noteworthy is the revelation that the palladium operates via a most unusual oxidation state: palladium(III). This discovery has important implications for the re-interpretation of previous research and for future investigation into palladium catalysis.

While palladium has been well-known as a catalyst for many years, the assumption has always been that it worked through Pd(II)-to-Pd(IV) redox reactions, and indeed compounds of the higher oxidation state have been synthesized, their crystal structures analyzed, and their activity demonstrated. Even so, the role of Pd(IV) has yet to be a proven intermediate state in catalysis.

Ritter and Powers’ research has centered on the complex [Pd(bhq)(OAc)]2, in which bhq is benzo[h]quinoline and OAc is acetate. In this molecule the two palladiums are each chelated by the bhq ligands and then linked to one another via two acetate bridges. As such, both metals are in the oxidation state Pd(II) as expected.

Harvard University’s Department of Chemistry and Chemical Biology.
The work of Tobias Ritter and David Powers hail from Harvard University’s Department of Chemistry and Chemical Biology. View the site.

However, when the complex reacts with a reagent such as one providing chlorine atoms, then these attach themselves temporarily to Pd whose oxidation state has perforce to be Pd(III). The chlorine atom then migrates to the adjacent carbon atom of the bhq molecule which is coordinated to the Pd. Ritter and Powers used the chlorinating agents dichloroiodobenzene and N-chlorosuccinimide, and they also showed that in similar reactions they could add bromo and acetate groups to bhq. In the latter case they also synthesized a deuterated acetate version.

Currently the Harvard group is exploring ways in which catalysis by palladium might be put to commercial use. "We are now seeking to develop new catalysts and materials which are based on electronic metal-to-metal communications, and similar to the ones disclosed in our paper," says Ritter, whose most recent publications concern the mechanisms whereby these remarkable palladium-catalyzed reactions take place (see J. Amer. Chem. Soc., 132: 14092 and 14530, 2010.)

As mentioned above, a new paper, #8, has muscled into the graphene gridlock of the Hot Ten. What it reports is a material for polymer solar cells which delivers a power conversion efficiency of 7.4%, the highest so far achieved for this type of cell. Up to now, solar sensitive material has either been high-cost inorganics, which work well, or low-cost organic polymers, which are much less efficient.

Now a group of chemists, headed by Gang Li of Solarmer Energy and Luping Yu of the Department of Chemistry of the University of Chicago, have shown that the latter type of cell can generate power comparable to the former, and their material can utilize a wider range of sunlight with a maximum response at 700 nm in the visible region.

They call their product PTB7/PC71BM, and it combines a benzothiophene polymer, which has fluorine and 2-ethylhexyl groups attached (i.e., PTB7), as the electron-releasing component, and phenyl-C71-butyric acid methyl ester (i.e., PC71BM), with its bulky fullerene units, as the electron-accepting layer. The blend of the two films was prepared from a mixed solvent consisting of dichlorobenzene and 1,8-diiodoctane.

Exotic as these polymers appear to be, they offer a much more sustainable type of solar cell because they are not limited by the availability of the rarer metals on which the inorganic types of solar cells currently rely.End

Dr. John Emsley is based at the Department of Chemistry, Cambridge University, U.K.

Click the tab above to view Hot Papers.
What's Hot in Chemistry
Rank Paper Cites This Period
Sep-Oct 10
Rank Last Period
Jul-Aug 10
X.S. Li, et al., "Large-area synthesis of high-quality and uniform graphene films on copper foils," Science, 324(5932): 1312-4, 5 June 2009. [U. Texas, Austin]     *453TF 40 1
A. Reina, et al., "Large area, few-layer graphene films on arbitrary substrates by chemical vapor deposition," Nano Letters, 9(1): 30-5, January 2009. [MIT, Cambridge]  *395IZ 38 2
3 D.C. Elias, et al., "Control of graphene’s properties by reversible hydrogenation: Evidence for graphane," Science, 323(5914): 610-3, 30 January 2009. [U. Manchester, U.K.; Inst. Microelectronics Tech., Chernogolovka, Russia; Cambridge U., U.K.; U. Nijmegen, Netherlands]   *400JB  34 3
M.D. Stoller, et al., "Graphene-based ultracapacitors," Nano Letters, 8(10): 3498-3502, October 2008. [U. Texas, Austin]  *358HD 34 4
L.Y. Jiao, et al., "Narrow graphene nanoribbons from carbon nanotubes," Nature, 458(7240): 877-80, 16 April 2009. [Stanford U., CA] *433CS 30 6
6 D.V. Kosynkin, et al., "Longitudinal unzipping of carbon nanotubes to form graphene nanoribbons," Nature, 458(7240): 872-6, 16 April 2009.  [Rice U., Houston, TX] *433CS 28 5
7 B. Kang, G. Ceder, "Battery materials for ultrafast charging and discharging," Nature, 458(7235): 190-3, 12 March 2009. [MIT, Cambridge, MA]  *417EQ 24 8
Y.Y. Liang, et al., "For the bright future—Bulk heterojunction polymer solar cells with power conversion efficiency of 7.4%," Adv. Materials, 22(20): E135-8, 25 May 2010.  [U. Chicago, IL; Solarmer Energy Inc., El Monte, CA]   *612IK 22
9 B. Lim, et al., "Pd-Pt bimetallic nanodendrites with high activity for oxygen reduction," Science, 324(5932): 1302-5, 5 June 2009.  [Washington U., St. Louis, MO; Brookhaven Natl. Lab., Upton, NY]  *453TF 20 9
10 Y.M. Lin, et al., "Operation of graphene transistors at gigahertz frequencies," Nano Letters, 9(1): 422-6, January 2009. [IBM T.J. Watson Res. Ctr., Yorktown Heights, NY]    *395IZ 19
SOURCE: Thomson Reuters Hot Papers Database. Read the Legend.


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