Several years ago Walton and coworkers reported the crystal structures for two eight-coordinate rhenium(V) isomers with the formula [ReH2(mhp)(PPh3)2]+ (where mhp = the anion of 2-hydroxy-6-methylpyridine). Both isomers have dodecahedral coordination and are best distinguished from one another by the coordination of the chelating ligands. In one isomer, both of the chelating ligands constitute the four vertices of one of the dodecahedron's trapezoids while in the other isomer both chelating ligands occupy coordination sites on both of the dodecahedron's trapezoids. Subsequent to Walton's report, Crabtree and coworkers proposed a pseudorotational rearrangement for dodecahedral rhenium(V) centers supported entirely by monodentate ligands. Serendipitously, we identified isomers of certain rhenium(V) centers supported by monodentate ligands that included an unsymmetrical aromatic amine ligand. Those isomers related well to the report by Crabtree. Through a study of one such eight-cooridinate complex that included a chiral center, we found support for the notion that the rearrangement of ligands at such centers includes a change in the steric relationship of the B vertices of the dodecaheral coordination set consistent with Crabtree's proposed pseudorotational rearrangement mechanism. The work presented here bridges the report by Walton regarding dodecahedral centers supported by two chelating ligands and the work by Crabtree and ourselves which centered on dodecahedral centers supported entirely by monodentate ligands. This work examines eight-coordinate dodecahedral centers supported by a single cyclometallated imine-containing ligand. We extended our previous report of reactions between benzaldimines and ReH7(PPh3)2, to make the compounds for study, by expanding the aromatic group of the imine reactant to include a substituted pyrrole, a substituted thiophen, and substituted 2-hydroxy-aromatic amines. The deprotonated 2-hydroxy-aromatic amine-containing products were of particular interest due to a change in the metallocylce size from five members to six members and coordination of the ligand through oxygen and nitrogen rather than through carbon and nitrogen. Our variable temperature NMR studies of these chelating ligand-supported, eight-coordinate, centers demonstrate that such compounds exist, in solution, as isomers and allowed us to measure the activation entropy and activation enthalpy for the interconversion of the isomers.

Interconversion of eight-coordinate, dodecahedral, rhenium(V) isomers supported by a single chelating ligand

249th National Meeting (2015)

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