Titanium and zirconium complexes of the N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-butadiene ligand: syntheses, structures and uses in catalytic hydrosilylation reactions

Abstract

We report here a number of dianionic 1,4-diaza-1,3-butadiene complexes of titanium and zirconium synthesised by a salt metathesis reaction. The reaction of either CpTiCl₃ or Cp₂TiCl₂ with the dilithium salt of N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-butadiene [1; abbreviated (Dipp)₂DADLi₂] afforded the mono-cyclopentadienyl titanium complex [η⁵-CpTi((Dipp)₂DAD)Cl] (2) bearing a dianionic ene-diamide ligand, while the analogous reaction of zirconocene dichloride (Cp₂ZrCl₂) with the dilithium salt 1 gave the bis-cyclopentadienyl zirconium complex [Cp₂Zr{(Dipp)₂DAD}] (3). The metal dichloride complexes [Ti((Dipp)₂DAD)Cl₂] (4) and [{(Dipp)₂DADZrCl(μ-Cl)}₂(κ³-Cl)(Li)(OEt₂)₂] (5) were obtained by the reaction of 1 and anhydrous metal tetrachloride in a 1 : 1 molar ratio in diethyl ether at room temperature. Meanwhile, the homoleptic titanium complex [Ti{((Dipp)₂DAD)}₂] (6) was isolated in good yield by the treatment of 1 with TiCl₄ in a 1 : 2 molar ratio in diethyl ether. The complexes 2 and 5 were further reacted with neosilyl lithium to afford mono- and bis-alkyl complexes of titanium [η⁵-CpTi{(Dipp)₂DAD}(CH₂SiMe₃)] (7) and zirconium [Zr{(Dipp)₂DAD}(CH₂SiMe₃)₂] (8) respectively. Molecular structures of the complexes 2, 3, and 5–8 in the solid states were confirmed by single crystal X-ray diffraction analysis. The solid state structures of all the complexes reveal that the metal ions are chelated through the amido-nitrogen atoms and the olefinic carbons of the [(Dipp)₂DAD]²⁻ moiety, satisfying the σ²,π coordination mode. Compound 8 was used as a catalyst for the intermolecular hydrosilylation reaction of a number of olefins, and moderate activity of catalyst 8 was observed.