Creating Ligands with Multiple Personalities

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Science  22 Oct 2010:
Vol. 330, Issue 6003, pp. 455-456
DOI: 10.1126/science.1197652

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Ligands, once mere spectators in reactions catalyzed by transition metal complexes, are now being designed with additional functions that can augment the reactivity of the metal center. Classical ligands, such as the terpyridine in compound 1 (see the figure), often simply help to fill the coordination sphere of the metal and remain unchanged throughout a cycle of reactions. Even ligands that exert inductive effects—donating or withdrawing electrons to or from the metal's orbitals—can be tuned only by synthesizing a modified ligand. Multifunctional ligands that respond to effects such as changes in pH are now being developed, as illustrated by recent work by Hashiguchi et al. (1). They describe a ruthenium (Ru) complex 2 in which the ligand has a pair of labile protons (depicted in blue in the figure). The ligand is neutral but can lose one or two protons in strongly basic solution. This gain of one or two negative charges activates the Ru atom for carbon-hydrogen (C–H) bond reactions. In effect, three very different ligands can be generated from a single core structure simply by changing pH.