Mixed-sandwich complexes of zirconium(III) and hafnium(III): Potential analogues of [U(η-C8H6{SiiPr3-1,4}2)(η-Cp∗)] for cyclooligomerisation of CO

Reaction of ZrCl4 or HfCl4 with 2 equivalents of K2(COT{SiiPr3-1,4}2), followed by in situ treatment of the resultant sandwich complexes M(COT{SiiPr3-1,4}2) with a further equivalent of MCl4 affords the mono-COT chloro-bridged dimers [M(η-COT{SiiPr3-1,4}2)(μ-Cl)Cl]2, M = Zr (1); M = Hf (4). Further reaction of the latter with 2 equivalents of KCp∗ yields the crystallographically characterised M(IV) mixed-sandwich complexes [M(η-COT{SiiPr3-1,4}2)(η-Cp∗)Cl], M = Zr (2); M = Hf (5). Reduction of 2 with KC8 in toluene affords the monomeric Zr(III) mixed-sandwich complex [Zr(η-COT{SiiPr3-1,4}2)(η-Cp∗)], whereas the analogous reaction with 5 results in coupling of the COT rings to give dimeric [Hf(η-Cp∗)]2(μ-η7,η7-(C8H6{SiiPr3-3,6}2C8H6{SiiPr3-3,6}2)); both 2 and 5 have been structurally characterised.