Cyclometalation of Anthyridine-Based Ligands with Dirhodium Acetates: Structure and Catalytic Activity

Coordination
of 2,8-Ar2-5-phenylanthyridines (Ar = 2-thienyl, 1a; Ar = 2-ClC6H4–, 1b) with dirhodium tetraacetate yielded the cyclometalated complexes
[Rh2(OAc)3(metalated-1a)] (3a) and [Rh2(OAc)3(metalated-1b)] (3b), respectively. Under acidic conditions, cleavage
of the Rh–C bond in 3a,b took place
to give the corresponding coordination complexes 4a,b. Treatment of 3a,b with PPh3 led to the phosphine-cyclometalated species [Rh2(OAc)2{P,C-(C6H4)­PPh2}­(metalated-1a)] (5a) and
[Rh2(OAc)2{P,C-(C6H4)­PPh2}­(metalated-1b)] (5b), respectively. These new dirhodium complexes have been
structurally characterized by NMR spectroscopy, and some representative
compounds were also analyzed by X-ray methods. The use of these newly
prepared dirhodium complexes as catalysts for the allylic oxidation
of cyclohexenes was investigated.