Synthesis and Characterization of Rhenium(V) Oxo Complexes Bearing PNP-Pincer Ligands

The
synthesis of a series of pyridine-based PNP-pincer rhenium-oxo
complexes, with phenyl (1, 3), tert-butyl (2), or cyclohexyl (4) groups on
the phosphorus atoms and either a ReO2X (1, 2) or a ReCl2O (3, 4) core is reported. The structures of these compounds were characterized
using 1H, 13C, and 31P NMR and using
X-ray crystallography for 1 and 4. Compounds 2 and 3 crystallize as the corresponding ReO­(OH)­X
compounds upon their reaction with moisture during the crystallization
process. The complexes were tested as catalysts in the oxidation reaction
of limonene with H2O2, the oxygen atom transfer
reaction from pyridine N-oxide to triphenylphosphine,
and the dehydration reaction of 1-phenylethanol, but all showed poor
catalytic performance. All complexes can be deprotonated at the benzylic
arm, and 2 can be deprotonated twice, accompanied by
dearomatization of the pyridine ring. 3 and 4 are highly stable in their oxidized forms. The noninnocent behavior
shown here could open up possibilities for cooperative catalysis.