Mixed-sandwich complexes of zirconium(III) and hafnium( III): Potential analogues of [U(eta-C8H6{(SiPr3)-Pr-i-1,4}(2))(eta-Cp*)] for cyclooligomerisation of CO

Reaction of ZrCl4 or HfCl4 with 2 equivalents of K-2(COT{(SiPr3)-Pr-i-1,4}(2)), followed by in situ treatment of the resultant sandwich complexes M(COT{(SiPr3)-Pr-i-1,4}(2)) with a further equivalent of MCl4 affords the mono-COT chloro-bridged dimers [M(eta-COT{(SiPr3)-Pr-i-1,4}(2))(mu-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(eta-COT{(SiPr3)-Pr-i-1,4}(2))(eta-Cp*)Cl], M = Zr (2); M = Hf (5). Reduction of 2 with KC8 in toluene affords the monomeric Zr(III) mixed-sandwich complex [Zr(eta-COT{(SiPr3)-Pr-i-1,4}(2))(eta-Cp*)], whereas the analogous reaction with 5 results in coupling of the COT rings to give dimeric [Hf(eta-Cp*)](2)(mu-eta(7), eta(7)-(C8H6{(SiPr3)-Pr-i-3,6}(2)C8H6{(SiPr3)-Pr-i-3,6}(2))); both 2 and 5 have been structurally characterised.