Organometallic chemistry of the actinide metals thorium and uranium has been undergoing a revival of sorts lately. This renewed interest is being sparked by a desire to better understand their electronic structure and to predict the behavior of actinide complexes with a broader range of ligands and under a wide array of reaction conditions.
Some of this chemistry is being carried out by Jaqueline L. Kiplinger and her colleagues at Los Alamos National Laboratory. "Our work is focused on trying to understand electronic structure, bonding, and reactivity in actinides, and tuning the interactions between these metals and ligands," she says.
Kiplinger's group is investigating actinide complexes containing multiply bonded functional groups to probe the fundamental involvement of 6d and 5f orbitals in bonding. "The extent of covalency in actinide-ligand bonding interactions is still a big question," she says. Ultimately, this basic research could lead to potential applications that may be of use to the Department of Energy, ranging from use in radioactive waste repositories to the design of selective chemical separation methods.
At the American Chemical Society national meeting in Philadelphia in August, several members of Kiplinger's group gave talks in the Division of Inorganic Chemistry describing their latest results. One of Kiplinger's goals during the past several years has been to make actinide alkylidene complexes (M=CR2). Although the alkylidenes have remained elusive, the Los Alamos chemists' explorations have led to the synthesis of a number of other novel thorium and uranium complexes.
ONE APPROACH the team has followed is to use pyridine N-oxide to induce hydrogen abstraction from a uranium metallocene dialkyl, (C5Me5)2MR2 (where Me = methyl, M = U or Th, and R = methyl or benzyl). Pyridine N-oxide is a typical oxygen-atom transfer reagent for transition metals, Kiplinger says. It's also been used to make actinide oxo complexes (M=O). Based on transition-metal chemistry, the team expected the reaction might lead to an alkylidene oxo complex.
However, postdoc Jaime A. Pool discovered something much different. One of the pyridine ring carbons undergoes rare actinide-mediated C–H bond activation, forming a cyclometallated complex and eliminating a hydrocarbon molecule. The four-membered ring that forms includes the metal atom bonded to a carbon atom of the pyridine ring, the pyridine nitrogen atom, and the oxygen atom of the pyridine N-oxide bound to the metal atom with a dative (lone pair) bond.
When excess pyridine N-oxide is reacted with the diphenyl thorium complex, (C5Me5)2ThPh2, Pool discovered that a unique thorium bis(oximate) complex is formed. The reaction is thought to proceed by the pyridine N-oxide oxygen atom first coordinating to thorium, followed by phenyl migration to the pyridine ring. The aromatic pyridine C–N bond is next cleaved and the ring opened. The resulting ligands are coordinated to the thorium via the nitrogen atoms, and the phenyl groups on the starting thorium complex end up at the terminal ends of ligands.
"The kind of reactivity we're seeing is quite different from the transition metals, and these examples are both new modes of reactivity for pyridine N-oxide," Kiplinger notes.
Additional studies suggested that the pyridine N-oxide oxygen atom actually isn't necessary for the reactions to proceed. They repeated some of the reactions with pyridine instead of pyridine N-oxide, and the general result was the same: C–N bond cleavage and ligand coordination to the metal via the nitrogen atoms, even at room temperature in some cases. "This chemistry rivals the best transition-metal systems known to cleave C–N bonds in pyridine," Kiplinger says.
Other experiments carried out by Pool suggest that catalytic C–N cleavage of aromatic rings by actinide complexes could offer insight into hydrodenitrogenation--the removal of nitrogen-containing compounds from petroleum feedstocks. Hydrodenitrogenation is necessary in petroleum refining to reduce nitrogen oxide emissions in fuels, as is hydrodesulfurization to remove sulfur compounds. One problem with current transition-metal catalysts is that C–N bond hydrogenolysis is slow, even at high temperature and pressure, Kiplinger explains.
In a related project, graduate student Kimberly C. Jantunen has been exploring the chemistry of thorium ketimido and hydrazonato complexes. In 2002, Kiplinger was part of a Los Alamos team that reported the synthesis of the uranium ketimido compound, (C5Me5)2U(N=CPh2)2. The ketimido, or azavinylidene, ligand is attractive because it can act as both a - and -electron donor, Kiplinger says. Jantunen has now synthesized thorium analogs by a different method.
Jantunen has worked in Kiplinger's lab during the past two summers in a collaborative project with Los Alamos spectroscopist David E. Morris. Jantunen is a graduate student at Simon Fraser University, Burnaby, British Columbia, where her research adviser is assistant chemistry professor Daniel B. Leznoff.
She prepared the thorium ketimido complexes by reacting excess benzonitrile with (C5Me5)2ThR2 (R = phenyl, benzyl, or methyl). The researchers believe the reaction proceeds by insertion of benzonitrile into the thorium-carbon bonds, with the alkyl or aryl groups on thorium migrating to the benzyl carbon of the benzonitrile molecules. Jantunen has shown that this method is a general high-yield route to prepare both the thorium and uranium ketimido complexes [Organometallics, 23, 4682 (2004)].
ELECTRONIC absorption and electrochemical studies of the actinide ketimido complexes suggest that they possess novel electronic structures [Organometallics, 23, 5142 (2004)]. For example, one feature of Jantunen's work is that the thorium complexes are intensely colored. "This is very unusual for thorium(IV) complexes, which tend to be colorless," Morris notes.
The iridescent colors prompted the researchers to carry out detailed molecular spectroscopy studies. Morris and postdoc Ryan E. Da Re observed optical emission from the thorium complexes and rare resonance Raman vibrational spectra for both the thorium and uranium complexes. "This molecular spectroscopic behavior is fairly common in transition-metal chemistry, but the only other example in tetravalent actinide chemistry is the uranocene systems, U(C8H8)2, that were studied 25 years ago," Morris says.
The thorium ketimido complexes also are reactive toward a variety of compounds in contrast to the uranium complexes, which display no reaction chemistry toward identical substrates, Kiplinger adds. This further suggests that there are differences in the ketimido nitrogen lone pair -bonding interactions with uranium and thorium, she says.
"We are seeing some unique reactivity that is inherent to the actinides, as well as remarkable molecular spectroscopic behavior," Kiplinger concludes. "It doesn't mean that actinide catalysts are going to be used anytime soon because of the precautions needed to handle the radioactive elements and a natural fear of radioactivity. But it's exciting chemistry that's refreshingly different."