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

Treatment of MCl4(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 NbCl4(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 η2-3,5-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.