Effect of ligand modification on the reactivity of phosphinoamide-bridged heterobimetallic Zr/Co complexes

Abstract

The effect of modifying the N-aryl substituent (aryl = mesityl vs. m-xylyl) of the phosphinoamide ligands linking Zr and Co in tris(phosphinoamide)-linked heterobimetallic complexes has been investigated. Treatment of the metalloligand (ⁱPr₂PNXyl)₃ZrCl (2) (Xyl = m-xylyl) with CoI₂ affords the iodide-bridged product ICo(ⁱPr₂PNXyl)₂(μ-I)Zr(η^2-iPr₂PNXyl) (3) rather than the C₃-symmetric isomer observed using the N-mesityl derivative, ICo(ⁱPr₂PNMes)₃ZrCl. Upon two-electron reduction of complex 3, ligand rearrangement occurs to generate the three-fold symmetric reduced complex N₂Co(ⁱPr₂PNXyl)₃Zr(THF) (4). Comparison of 4 with the previously reported mesityl-substituted complex N₂Co(ⁱPr₂PNMes)₃Zr(THF) (1) reveals similar structural features but with a less sterically hindered Zr apical site in complex 4. An obvious electronic difference between these two complexes is also present based on the drastically different infrared N₂ stretching frequencies of 1 and 4. These notable differences lend themselves to different reactivity in both stoichiometric and catalytic reactions. Alkyl halide addition to complex 4 results in homo-coupling products resulting from alkyl radicals rather than the alkyl-bridged or intramolecular C–H activation products formed upon addition of RX to 1. This difference in reactivity with alkyl halides renders complex 3 a less effective catalyst for the Kumada cross-coupling of alkyl halides with n-octylMgBr than ICo(ⁱPr₂PNMes)₃ZrCl, as a greater proportion of homocoupling products are formed under catalytic conditions.