Electrophilic reactivity of the zwitterionic titanocene monohalides Cp[η 5-C 5H 4B(C 6F 5) 3]TiX (X = Cl, Br): Reactions with water and methanol

Abstract

The paper reports new data evidencing for a high electrophilicity of the positively charged titanium atom in the previously described zwitterionic titanocene monochloride Cp[η 5-C 5H 4B(C 6F 5) 3]TiCl ( 1) and titanocene monobromide Cp[η 5-C 5H 4B(C 6F 5) 3]TiBr ( 2), containing a B(C 6F 5) 3 group in one of the C 5 rings. It has been established that on a contact of a toluene solution of these zwitterions with water vapour at 20 °C under Ar, a rapid protolytic cleavage of the otherwise inert B–C 6F 5 bond in the tris(pentafluorophenyl)borane moiety occurs to afford pentafluorobenzene and the corresponding halogenide hydroxide complex of titanocene Cp[η 5-C 5H 4B(C 6F 5) 2]TiX(μ-OH), where X = Cl ( 3), Br ( 4). An X-ray diffraction study of the complexes has shown that the hydroxide group in 3 and 4 is bonded via the oxygen atom both to the titanium and boron atoms. Under similar conditions, the interaction of zwitterion 1 with methanol gives rise to pentafluorobenzene and the chloride methoxide complex of titanocene Cp[η 5-C 5H 4B(C 6F 5) 2]TiCl(μ-OCH 3). It has been suggested that the driving force of the protolysis of the B–C 6F 5 bond in 1 and 2 is a sharp increase in the acidity of water or methanol molecule as a result of their complexation with the positively charged titanium centre in the starting zwitterion.