Bis(amido)cyclodiphosph(III)azane Hafnium Complexes and Their Activation by Tris(perfluorophenyl)borane

Bis(amido) hafnium complexes [(RN)(t-BuNP)](2)HfCl2 (R = 2,6-i-Pr2C6H3 (5), 2,5-t-Bu2C6H3 (6)) bearing a N-tert-butyl cyclodiphosph(III)azane bridge were prepared by direct metalation of corresponding ligand precursors with Hf(NMe2)(4) followed by chlorination by Me3SiCl. Complexes 5 and 6 and the already known [(t-BuN)(t-BuNP)](2)HfCl2 (4) were subsequently converted to their methyl derivatives [(RN)(t-BuNP)](2)HfMe2 by the alkylation by MeMgBr in Et2O. The solid state structure of [(t-BuN)(t-BuNP)](2)HfMe2 (7) reveals a highly distorted trigonal-bypiramidal configuration at the metal center. Activation of the dimethyl complexes with B(C6F5)(3) led to the generation of corresponding cationic species, which were investigated by H-1, C-13, P-31, and F-19 NMR. In the solid state, the metal center of {[(t-BuN)(t-BuNP)](2)HfMe(mu-Me)B(C6F5)(3)} has a distorted trigonal-bypiramidal configuration and the [MeB(C6F5)(3)](-) anion is only weakly coordinated with cationic hafnium. In deuterobenzene solution {[(t-BuN)(t-BuNP)](2)HfMe(mu-Me)B(C6F5)(3)}, like in the solid state, exhibited C-s symmetry as a result of the additional coordination of the high electrophilic hafnium atom with one of the nitrogens of the P2N2 ring. This additional metal-ligand interaction in solution is absent in the case of the activated complexes 8 and 9, having bulky aryl substituents. The compounds 4 and 7 revealed moderate catalytic activity in the ethene polymerization, while hafnium complexes bearing aryl groups remained inactive in the same conditions.