Electron-deficient pentamethylcyclopentadienyl-1,3-butadiene complexes of titanium, zirconium, and hafnium

Article abstract of DOI:10.1021/om00146a005

Electron-deficient conjugated diene complexes Cp*M(diene)Cl (M = Ti, Zr, Hf; diene = 2,3-dimethyl-1,3-butadiene, 2-methyl-1,3-butadiene, 1,3-butadiene; Cp* = η-C₅Me₅) were prepared either by reduction of Cp*MCl₃ in the presence of free diene (M = Zr, Hf), by reaction of Cp*TiCl₃ with the enediylmagnesium reagent [Mg(CH₂CMe=CMeCH₂)]·2THF, or through exchange of an η³-1-methallyl ligand between Cp*M(butadiene)(1-methallyl) and Cp*MCl₃. These 14-electron complexes form 16-electron adducts with a variety of Lewis bases. In all complexes the diene ligand assumes a nonfluxional s-cis conformation. NMR spectroscopy indicates that the bonding of the diene ligand has σ²,π-metallacyclopentene rather than η⁴-diene character. EHMO calculations on 14e and 16e model systems point out that on complexation of the Lewis base the metallacyclopentene character should become less pronounced. Both types of compounds were characterized by X-ray analysis. Cp*Hf(C₆H₁₀)Cl-C₅H₅N crystallizes in the orthorhombic space group Pna2₁ with a = 17.412 (3) A?, b = 8.164 (3) A?, c = 14.837 (3) A? (293 K), and Z = 4. The metallacyclopentene character of the diene ligand is apparent from the Hf-C(diene) distances [Hf-C_T = 2.274 (10)/2.278 (10) A?, Hf-C_C = 2.483 (10)/2.477 (9) A?] and the diene C-C distances [C_C-C_C = 1.384 (15) A?, C_C-C_T = 1.513 (14)/1.493 (14) A?]. Cp*Hf(C₆H₁₀)Cl crystallizes in the monoclinic space group C₂ with a = 8.820 (3) A?, b = 12.767 (3) A?, c = 14.486 (3) A?, β = 102.59 (1)° (100 K), and Z = 4. The diene C-C distances [C_C-C_C = 1-400 (12) A?, C_C-C_T = 1.397 (15)/1.525 (13) A?] indicate a large participation of the asymmetric π³,σ-resonance structure in the bonding of the diene fragment to the metal. The chlorine atom in Cp*M(diene)Cl can easily be substituted to form alkyl, aryl, allyl, and borohydride derivatives. The 14e alkyl complexes show agostic C-H-M interactions in NMR and IR spectra.