Metallapyrimidines and Metallapyrimidiniums from Oxidative Addition of Pyrazolate N–N Bonds to Niobium(III), Niobium(IV), and Tantalum(IV) Metal Centers and Assessment of Their Aromatic Character

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Treatment of MCl 4 (4-tBupy) 2 (M = Nb, Ta) with 3 equiv of potassium 3,5-di- tert -butylpyrazolate afforded (2,2,6,6-tetramethyl-5-ketimidehept-3-en-3-imide)bis(3,5-di- tert -butylpyrazolate)chloroniobium(V) ( 1; 24%) and (2,2,6,6-tetramethyl-5-ketimidehept-3-en-3-imide)bis(3,5-di- tert -butylpyrazolate)chlorotantalum(V) ( 2; 27%) as deep red and yellow crystalline solids, respectively. Analogous treatment of NbCl 4 (THF) 2 with 3 equiv each of 4- tert -butylpyridine and potassium 3,5-di- tert -butylpyrazolate and excess Na/Hg in diethyl ether afforded (2,2,6,6-tetramethyl-5-ketimidohept-3-en-3-imide)bis(3,5-di- tert -butylpyrazolate)niobium(V) ( 3, 32%) as deep red crystals. X-ray crystallography established that 1 and 3 each contain two intact η 2 -3,5-di- tert -butylpyrazolate ligands as well as one 3,5-di- tert -butylpyrazolate ligand that has undergone N–N bond oxidative addition to the niobium center. In 1, one of the nitrogen atoms abstracted a hydrogen atom from tetrahydrofuran solvent, whereas no hydrogen atom abstraction occurred in 3 . These complexes represent rare examples of pyrazolate N–N bond cleavage. NICS calculations suggest that the niobacycles are weakly aromatic, in comparison to the highly aromatic 3,5-di- tert -butylpyrazolate ligands and pyrimidine.