Preparation and Activation of Complexes of the Type [((mesityl)NCH2CH2)2NX]ZrMe2 (X = H, Me) with [Ph3C][B(C6F5)4] or [PhNMe2H][B(C6F5)4

Abstract

The zirconium dimethyl complexes [N2NX]ZrMe2 (N2NX = [(MesNCH2CH2)2NX]; Mes = mesityl; X = H (1a), Me (1b)), have “mer” structures in the solid state in which the amido nitrogens occupy “axial” positions in a trigonal bipyramid. The reaction of 1b with 1 equiv of [Ph3C][B(C6F5)4] followed by addition of diethyl ether yielded the ether adduct {[N2NMe]ZrMe(Et2O)}+ (with [B(C6F5)4]- as the anion), an X-ray study of which revealed it to be a fac trigonal-bipyramidal species in which the diethyl ether is coordinated in an apical position. The reaction of 1b with 1 equiv of [PhNMe2H][B(C6F5)4] led to {[N2NMe]ZrMe(NMe2Ph)}[B(C6F5)4], solution NMR studies of which suggest a structure analogous to that of {[N2NMe]ZrMe(Et2O)}+. Heating solutions of {[N2NMe]ZrMe(NMe2Ph)}[B(C6F5)4] led to C−H activation in one mesityl o-methyl group and formation of methane. The reaction of 1b with 0.5 equiv of [Ph3C][B(C6F5)4] yielded [{[N2NMe]ZrMe}2(μ-Me)][B(C6F5)4] (5b), an X-ray diffraction study of which revealed an almost linear (167.4°) methyl bridge linking two distorted TBP moieties through the apical positions (average Zr−C(bridge) = 2.48 Å, average Zr−C(terminal) = 2.24 Å). The equatorial methyl groups in 5b exchange readily between Zr centers, while the bridging methyl group and the equatorial methyl groups exchange relatively slowly on the NMR time scale, but still rapidly on the chemical time scale. Exchange of free [N2NMe]ZrMe2 with the [N2NMe]ZrMe2 fragment in 5b is also facile on the chemical time scale. The reaction of 1b with 1.0 equiv or more of [Ph3C][B(C6F5)4] led to formation of a cationic species (6b), two forms of which could be observed at low temperature. Activation of 1a with [Ph3C][B(C6F5)4] yielded only one cationic form of 6a at low temperatures. Exchange of methyl groups between 6a and 6b is slow on the chemical time scale. All evidence is consistent with the observation of different ion pairs of 6b at low temperatures.