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

Bis(amido) hafnium complexes [(RN)(t-BuNP)]2HfCl2 (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)]2HfCl2 (4) were subsequently
converted to their methyl derivatives [(RN)(t-BuNP)]2HfMe2 by the alkylation by MeMgBr
in Et2O. The solid state structure of [(t-BuN)(t-BuNP)]2HfMe2 (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 1H, 13C, 31P, and 19F NMR. In the solid state, the metal center of {[(t-BuN)(t-BuNP)]2HfMe(μ-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)]2HfMe(μ-Me)B(C6F5)3}, like in the solid state, exhibited Cs 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.