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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-PiPr2-4-methylphenyl)-2,4,6-Me3C6H2; PNC2−=(N-(2-PiPr2-4-methylphenyl)-2,4,6-CH2Me2C6H2), was prepared by reduction of the bisazide trans-[(PN)2Hf(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)2Hf≡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.