Subtle Differences Between Zr and Hf in Early/Late Heterobimetallic Complexes with Cobalt

The phosphinoamide-linked Co/Hf complexes ICo(Ph2PNiPr)3HfCl (4), ICo(iPr2PNMes)3HfCl (5), and ICo(iPr2PNiPr)3HfCl (6) have been synthesized from the corresponding tris(phosphinoamide)HfCl complexes (1–3) for comparison with the recently reported tris(phosphinoamide) Co/Zr complexes. Very minor structural and electronic differences between the Zr and Hf complexes were found when the N-iPr-substituted phosphinoamide ligands [Ph2PNiPr]− and [iPr2PNiPr]− were utilized. The reduction products [(THF)4Na-{N2-Co(Ph2PNiPr)3HfCl}2]Na(THF)6 (7) and N2-Co(iPr2PNiPr)3Hf (9) are also remarkably similar to the corresponding Zr/Co analogues. In the case of Hf/Co and Zr/Co complexes linked by the N-Mes ligand [iPr2PNMes]− (Mes = 2,4,6-trimethylphenyl), however, more pronounced differences in structure, bonding, and reactivity are observed. While differences associated with 5 are still modest, larger variations are observed when comparing the two-electron reduction product [N2-Co(iPr2PNMes)3Hf-X][Na(THF)5] (8) with its Zr congener. In addition to structural and spectroscopic differences, vastly different reactivity is observed, with 8 undergoing one-electron oxidation to form ClHf(MesNPiPr2)3CoN2 (11) in the presence of MeI, while a two-electron oxidative addition process occurs in a similar reaction with the Zr derivative. The activity of 5 toward Kumada coupling was investigated, finding significantly diminished activity in comparison to Co/Zr complexes.