Terminal and Super-Basic Parent Imides of Hafnium.

ABSTRACT

A dinuclear hafnium complex containing the parent imido ligand [(PN)(PNC)Hf=NH{µ2 -K}]2 (2) (PN- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-Me3 C6 H2 ; PNC2- =(N-(2-Pi Pr2 -4-methylphenyl)-2,4,6-CH2 Me2 C6 H2 ), was prepared by reduction of the bisazide trans-[(PN)2 Hf(N3 )2 ] (1) with two equiv of KC8 . Encapsulation of K+ in 2 with crown-ether or cryptand affords the first discrete salt [K(encap)][(PN)(PNC)Hf≡NH] (encap=18-crown-6(THF)2 , 3; 2,2,2-Kryptofix, 4), featuring a terminal parent imide and possessing some of the shortest Hf-N bond lengths known to date. DFT calculations revealed formation of 2 to proceed via an extremely basic monomeric nitrido, [(PN)2 Hf≡N]- (A), having a computed pKBH+ of ∼57 followed by heterolytic splitting of an inert 1,2-CH bond of a benzylic methyl group across the Hf≡N triple bond in A. An electronic structure analysis reveals A to possess a covalent Hf≡N triple bond and of super-basic character. We also showcase reactivity of the Hf≡NH bond with various electrophiles.