Cationic complexes of titanium(III); Phosphine substitution reactions

Abstract

The Ti(III) compound [Cp2Ti(CH3CN)2]2[ZnCl4], 1, has been prepared and crystallizes in the space group Pbca with a = 28.444(11) k,b = 15.370(4) Å, c = 15.206(5) Å, and Z = 8. The BPh4 salt (i.e., [Cp2Ti(CH3CN)2][BPh4], 2) is a convenient precursor for Ti(III) cationic phosphine complexes. Reaction of 2 with several monodentate phosphines has been monitored by epr spectroscopy. Compound 2 reacts with PMe3 in a stepwise fashion, to replace the coordinated CH3CN molecules, yielding [Cp2Ti(CH3CN)(PMe3)][BPh4], 3, and [Cp2Ti(PMe3)2][BPh4], 4. Complex 4 exhibits a quasi-reversible cyclic voltammogram at -0.40 V vs. SCE, in the presence of excess PMe3. This is indicative of a Ti(III)/Ti(II) redox couple thus generating the known Ti(II) species Cp2Ti(PMe3)2. Similar reactions of 2 with PEt3 show that only the monosubstitution product [Cp2Ti(CH3CN)(PEt3)]+, is formed even in the presence of excess phosphine. No substitution was seen with either PCy3 or PPh3. The steric and electronic factors affecting substitution are briefly considered.