Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes

Abstract

The benzylene-linked [PNP] scaffolds HN(CH₂-o-C₆H₄PPh₂)₂ ([A]H) and HN(C₆H₄-o-CH₂PPh₂)₂ ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe₂)₃ (^[A]1-M) and [B]M(NMe₂)₃ (^[B]1-M) were prepared and converted to the iodides [A]MI₃ (^[A]2-M) and [B]MI₃ (^[B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn₃ (^[A]3-M) and [B]MBn₃ (^[B]3-M) (M = Zr, Hf) were obtained via reaction with Bn₂Mg(OEt₂)₂. For zirconium, the benzylic ligand positions in ^[A]3-Zr and ^[B]3-Zr were found to cyclometalate readily, which led to the corresponding κ⁴-[PCNP]ZrBn₂ complexes ^[A]4-Zr and ^[B]4-Zr. As these complexes failed to hydrogenate cleanly, cyclometalated derivatives with only one alkyl substituent were targeted and the mixed benzyl chlorides κ⁴-[PCNP]MBnCl (^[B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of ^[B]5-Zr, the η⁶-tolyl complex [B]Zr(η⁶-C₇H₈)Cl (^[B]6-Zr) was generated cleanly, but the corresponding hafnium complex ^[B]5-Hf was found to decompose unselectively in the presence of H₂. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η⁶-arene complexes similar to ^[B]6-Zr may serve as zirconium(II) synthons, namely when reacted with 2,6-Dipp-NC (L) or pyridine (py). Both these substrates were shown to react cleanly with ^[B]6-Zr, which led to the formation of the bis-isocyanide complex [B]ZrCl(L)₂ (^[B]7-Zr) and the 2,2′-bipyridine derivative [B]ZrCl(bipy) (^[B]8-Zr), respectively. Upon reaction of [B]Zr(η⁶-C₇H₈)Cl (^[B]6-Zr) with NaBEt₃H, the cyclometalated derivative κ⁴-[PCNP]Zr(η⁶-C₇H₈) (^[B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI₃ (^[A]2-M) were treated with KBEt₃H, which led to the isolation of the cyclometalated hydrido complexes κ⁴-[PCNP]M(H)(κ³-Et₃BH) (^[A]10-M; M = Zr, Hf) featuring a κ³-bound triethyl borohydride moiety.