Chemistry of o-xylidene–metal complexes. Part 2. Preparation and properties of the o-xylidene–metal(IV) complexes [M(CH2C6H4CH2-o)(η-C5H4R)2](M = Ti, Zr, Hf, or Nb; R = H or SiMe3) and [{M′(CH2C6H4CH2-o)2(OC4H8)}n](M′= Ti, Zr, or Hf); X-ray crystal structures of the metallaindanes [M(CH2C6H4CH2-o)(η-C5H4R)2](R = H, M = Ti, Zr, or Hf; R = SiMe3, M = Nb)

Abstract

Reaction of the di-Grignard reagent o-C₆H₄(CH₂MgCl)₂(1) in tetrahydrofuran (thf) with the appropriate metallocene(IV) chloride, [M(η-C₅H₄R)₂Cl₂], yields the corresponding crystalline, thermally robust metallaindane complexes [[graphic omitted]H₂-o)(η-C₅H₄R)₂][R = H or SiMe₃, M = Ti, Zr, or Hf; R = SiMe₃, M = Nb]. The Zr complex with R = H affords o-xylene either upon photolysis or pyrolysis. N.m.r. spectra of the d⁰ complexes show magnetically equivalent pairs of C₅H₄R and CH₂ groups. The e.s.r. solution spectrum of the d¹ Nb complex was second order, with g^av= 1.9986 and a(⁹³Nb)= 77.2 G, calculated using a modified Breit–Rabi equation. Reduction of each of the d⁰ Ti, Zr, or Hf compounds with Na[C₁₀H₈] in thf results in scission of an M–CH₂ bond, as deduced by the appearance of a central triplet e.s.r. signal, with appropriate (except Hf) satellites, assigned to the d¹ complex [M(η-C₅H₄R)_n(o-CH₂C₆H₄CH₂Na)(thf)_x]^(2–n)+(n= 1 or 2); cyclic voltammetry on one of the Ti^IV complexes shows a pseudo-reversible one-electron reduction, and hence the metallate(III) ion [Ti(η-C₅H₅)₂{o-(CH₂)₂C₆H₄}]^– may be a transient intermediate. Reactions of the Group 4A metallaindanes with CO under ambient conditions are sensitive to the nature of M: the Ti and Zr complexes yield [M(η-C₅H₄R)₂(CO)₂] and indan-2-one; whereas the Hf complex affords two conformers of the seven-membered, bis-σ-acyl, the metallabenzocycloheptene-dione complex [[graphic omitted]O-o}(η-C₅H₄SiMe₃)₂], in which the two CO groups are believed to have an anti relationship. The two conformers coexist in equilibrium in solution in CH₂Cl₂ or CHCl₃ with one form (the less soluble) favoured at low temperature. Reaction of two equivalents of the di-Grignard reagent (1) with the appropriate Group 4A metal(IV) chloride in thf furnishes insoluble, pyrophoric powders of empirical formula M(CH₂C₆H₄CH₂-o)₂(thf), which may well be oligomers having bridging o-xylidenes. The X-ray structures of the four title metallaindanes [M(η-C₅H₅)₂{o-(CH₂)₂C₆H₄}](M = Ti, Zr, or Hf) and [Nb{η-C₅H₄(SiMe₃)}₂{o-(CH₂)₂C₆H₄}] confirm their formulae. Features of interest include (i) the bis-σ-co-ordinating mode of the chelating o-xylidene ligand; (ii) the bite angle (°) C^αMC^α′: 76.7(2) Ti, 77.3(2) Zr, 77.2(4) Hf, 72.4(7) Nb; (iii) the fold angle (°)[i.e. the angle subtended by the normals to (a) the MC^αC^α′ plane and (b) the C₄ plane of the MC₄ ring]: 41 Ti, 53 Zr, 53 Hf, 44 Nb; (iv) the 〈M–C^α〉 distances (Å): 2.203(6) Ti, 2.300(8) Zr, 2.28(1) Hf, 2.286(9) Nb; (v) the 〈M-C^β〉 distances (Å): 2.913(6) Ti, 2.857(8) Zr, 2.91 (1) Hf, 3.079(21) Nb; and (vi) the fact that the C₆ rings are planar while the α-CH₂'s lie 0.10 (Ti), 0.12 (Zr), 0.12 (Hf), or 0.07 Å(Nb) out of this plane on the same side of the C₆ plane as the metal atoms.