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

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 ((i)Pr2PNXyl)3ZrCl (2) (Xyl = m-xylyl) with CoI2 affords the iodide-bridged product ICo((i)Pr2PNXyl)2(μ-I)Zr(η(2-i)Pr2PNXyl) (3) rather than the C3-symmetric isomer observed using the N-mesityl derivative, ICo((i)Pr2PNMes)3ZrCl. Upon two-electron reduction of complex 3, ligand rearrangement occurs to generate the three-fold symmetric reduced complex N2Co((i)Pr2PNXyl)3Zr(THF) (4). Comparison of 4 with the previously reported mesityl-substituted complex N2Co((i)Pr2PNMes)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((i)Pr2PNMes)3ZrCl, as a greater proportion of homocoupling products are formed under catalytic conditions.