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

Abstract

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...