Carbon–hydrogen and metal–carbon bond formation and breakage in thermal reactions of bis(mesityl)(cyclo-octatetraene)zirconium(IV) derivatives. Crystal structures of [Zr(η8-C8H8)(mes)2], [{Zr(η8-C8H8)[η-OC(mes)2]}2], [{Zr(η8-C8H8)(µ-2,4-Me2-6-CH2C6H2)}2], and [{Zr(η8-C8H8)}2(µ-mes)(µ-2,4-Me2-6-CHC6H2)](mes = 2,4,6-Me3C6H2)

Abstract

The reaction of [Zr(η⁸-C₈H₈)Cl₂(thf)], (1), with Mg(mes)Br (thf = tetrahydrofuran, mes = 2,4,6-Me₃C₆H₂) in Et₂O afforded the very thermally labile complex [Zr(η⁸-C₈H₈)(mes)₂], (2). An X-ray analysis on complex (2) at 173 K gave insights on the possible stabilization of the hitherto unstable alkyl derivatives of (1), showing that the o-methyl groups protect the metal centre. The significant lability of the aryl ligand in complex (2) is shown by the facile double migration of the mes residue to CO forming [{Zr(η⁸-C₈H₈)[η-OC(mes)₂]}₂], (3). The carbonylation reaction has to be carried out at 0 °C, since complex (2) undergoes thermal decomposition at room temperature, the nature of which depends on the solvent used. In Et₂O complex (2) loses mesitylene at room temperature forming [{Zr(η⁸-C₈H₈)(µ-2,4-Me₂-6-CH₂C₆H₂)}₂], (7). Complex (7) has a triple-decker type structure in which two allyl units, coplanar with the η⁸-C₈H₈ rings, bridge the two zirconium atoms. Because of the allyl structure the aromaticity of the phenyl ring is lost. More drastic thermal conditions cause a hydrogen migration in (7) which rearranges to [{Zr(η⁸-C₈H₈)}₂(µ-mes)(µ-2,4-Me₂-6-CHC₆H₂)], (8), which contains an alkylidene ligand bridging two zirconium atoms. Complexes (2), (3), (7), and (8) were characterized by single-crystal X-ray analysis: (2), tetragonal, space group I4₁/a, a=b= 26.495(4), c= 13.711 (2)Å, Z= 16, R= 0.054 for 2 468 independent observed reflections; (3), monoclinic, space group C2/c, a= 25.856(9), b= 12.444(3), c= 15.571 (8)Å, β= 92.59(3)°, Z= 4, R= 0.112 for 1 659 independent observed reflections; (7), orthorhombic, space group Pbca, a= 7.831 (1), b= 19.368(2), c= 18.177(2)Å, Z= 4, R= 0.045 for 949 independent observed reflections; (8), triclinic, space group P, a= 16.469(3), b= 17.452(3), c= 10.373(2)Å, α= 92.35(1), β= 103.83(2), γ= 103.06(2)°, Z= 2, R= 0.041 for 3 792 independent observed reflections.