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

The benzylene-linked [PNP] scaffolds HN(CH-o-CHPPh) ([A]H) and HN(CH-o-CHPPh) ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe) ([A]1-M) and [B]M(NMe) ([B]1-M) were prepared and converted to the iodides [A]MI ([A]2-M) and [B]MI ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn ([A]3-M) and [B]MBn ([B]3-M) (M = Zr, Hf) were obtained via reaction with BnMg(OEt). For zirconium, the benzylic ligand positions in [A]3-Zr and [B]3-Zr were found to cyclometalate readily, which led to the corresponding κ-[PCNP]ZrBn 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 κ-[PCNP]MBnCl ([B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of [B]5-Zr, the η-tolyl complex [B]Zr(η-CH)Cl ([B]6-Zr) was generated cleanly, but the corresponding hafnium complex [B]5-Hf was found to decompose unselectively in the presence of H. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η-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) ([B]7-Zr) and the 2,2'-bipyridine derivative [B]ZrCl(bipy) ([B]8-Zr), respectively. Upon reaction of [B]Zr(η-CH)Cl ([B]6-Zr) with NaBEtH, the cyclometalated derivative κ-[PCNP]Zr(η-CH) ([B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI ([A]2-M) were treated with KBEtH, which led to the isolation of the cyclometalated hydrido complexes κ-[PCNP]M(H)(κ-EtBH) ([A]10-M; M = Zr, Hf) featuring a κ-bound triethyl borohydride moiety.