Syntheses of Group 5 Amide Amidinates and Their Reactions with Water: Different Reactivities of Nb(V) and Ta(V)...

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe₂)₅ (M = Nb, Ta), for example, yields [M(μ₃-O)(NMe₂)₃]₄ (M = Nb, 1; Ta, 2) reported earlier. The similar reactivities of Nb(V) and Ta(V) compounds make it challenging, for example, to separate the two metals from their minerals. We have found that the reactions of H₂O with amide amidinates M(NMe₂)₄[MeC(NⁱPr)₂] (M = Nb, 3; Ta, 4) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex 3, the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(μ₃-O)(NMe₂)₃]₄ (1). For the Ta(V) complex 4, the amide ligands are released in the reaction with H₂O, leaving the amidinate ligand intact. [Ta(μ₃-O)(NMe₂)₃]₄ (2), the analogue of 1, was not observed as a product in the reaction of 4 with H₂O. To our knowledge, this is the first example of the formation of two different complexes that maintain the (V) oxidation state in both metals. The new complexes M(NMe₂)₄[MeC(NⁱPr)₂] (M = Nb, 3; Ta, 4) have been prepared by the aminolysis of M(NMe₂)₅ (M = Nb, Ta) with ⁱPrN(H)C(Me)=NⁱPr (5). The hydrolysis of 3 and 4 has been investigated by DFT electronic structure calculations. The first step in each hydrolysis reaction involves the formation of a hydrogen-bonded complex that facilitates a proton transfer to the amidinate ligand in 3 and protonation of an axial dimethylamide ligand in 4. Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in 3 and 4 have been computed by Natural Population Analysis (NPA), and these data are discussed relative to which of the ancillary ligands is protonated initially in the hydrolysis sequence. Ligand exchanges in 3 and 4 as well as the exchange in ⁱPrN(H)C(Me)=NⁱPr (5) were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s^-1 (3), 0.033(6) s^-1 (4), and 2.23(7) s^-1 (5), showing that the rate of the Nb complex Nb(NMe₂)₄[MeC(NⁱPr)₂] (3) is 13 times faster than that of its Ta analogue 4.