Heterobimetallic complexes stabilized by the P2N2 macrocyclic ligand system: synthesis and reactivity of a rhodium–copper system that activates molecular hydrogen

Abstract

The synthesis and reactivity of the bimetallic rhodium–copper complex, Rh(COE)[P₂N₂]Cu, which is stabilized by the P₂N₂ macrocycle, is reported. In the solid state, the rhodium and copper centers are on opposite sides of the macrocyclic ring with the Cu(I) in a linear environment and the Rh(I) in a square planar array. However, in solution a very symmetrical structure is suggested on the basis of the ¹H NMR data, which is consistent with at least two separate fluxional processes, rotation of the cyclooctene unit and movement of the Rh(I) unit between the two amido donors. Addition of H₂ to Rh(COE)[P₂N₂]Cu results in the formation of ([P₂N₂H]RhH(μ-H)₂Cu)₂via hydrogenation of the coordinated cyclooctene unit, oxidative addition of H₂ to the rhodium center and hydrogenolysis of the copper amido unit. Monitoring the reaction of H₂ by NMR spectroscopy indicated the formation of a number of intermediates which suggests hydrogenolysis of the copper amido linkage occurs to generate CuH in some form, along with Rh(COE)[P₂N₂H], which is converted to Rh(H)₂[P₂N₂H] by hydrogenation of the cyclooctene, which then recombines with the CuH present to generate the final product. Deuteration studies indicate that there is considerable H/D scrambling in the cyclooctane produced that we attribute to reversible beta-elimination, migratory insertion steps.