Reactive Dications Tamed

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Topics Covered

carbene, Main-Group Chemistry

Courtesy of Paul Ragogna

A diiminopyridine ligand—a pyridine ring with bulky diisopropylphenyl wings—provides a stable home to a sulfur dication (S is yellow, N is blue, C is black, and H is not shown).

Opening the door to new chemical reactions, researchers at the University of Western Ontario are reporting the synthesis of the first air-stable group 16 dication complexes (J. Am. Chem. Soc., DOI: 10.1021/ja9073968). In these chemical species, the central element bears two reactive lone pairs of electrons, which could allow chemists to carry out a variety of new stoichiometric and catalytic reactions, say Caleb D. Martin, Christine M. Le, and Paul J. Ragogna, who made the compounds.

The new dicationic species stem from the Ragogna group’s attempts to prepare group 16 carbene analogs. In carbenes and carbene analogs, the central atom bears one lone pair of electrons. Ever since the first carbene stable enough to be isolated was reported in 1988, chemists have made a run on the rest of the main-group elements looking to make stable carbene analogs. Stable carbenes and carbene analogs of most elements in groups 13, 14, and 15 have been made by coordinating the element within a bidentate nitrogen heterocyclic framework. In these compounds, the formal charges on the main-group element range from anionic for group 13 to neutral for group 14 to cationic for group 15.

But stable carbene analogs of the group 16 elements sulfur, selenium, and tellurium have remained out of reach because imposing a positive charge on electronegative elements makes them extremely electrophilic and highly unstable. Ragogna’s group had previously come close to making stable dicationic group 16 carbenes with the same bidentate nitrogen ligands others have used. In the new work, the group used a diiminopyridine bearing bulky dialkylphenyl groups to trap the group 16 element with three nitrogen atoms. This arrangement sufficiently delocalizes the positive charge so that the compounds can be bottled and stored on a shelf for several weeks.

“This class of compounds and the route to it is significant, but not just for the stability,” comments Jack Passmore of the University of New Brunswick, who focuses on group 16 inorganic chemistry. “It opens a way up to prepare new classes of compounds.” However, it chemically remains to be seen whether they can act as a source of S²⁺, Se²⁺, or Te²⁺, Passmore adds.

The complexes as they exist now don’t do much in the way of exciting chemistry, Ragogna acknowledges. It will be tricky to release the naked dications in a controlled manner to do useful chemistry, he says. His group is currently attempting to do so with alkenes, alkynes, and main-group substrates.

Chemical & Engineering News

ISSN 0009-2347

Copyright © 2011 American Chemical Society