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

Chemistries of Nb(V) and Ta(V) compounds are essentially identical as a result of lanthanide contraction. Hydrolysis of M(NMe 2 ) 5 (M = Nb, Ta), for example, yields [M(μ 3 -O)(NMe 2 ) 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 H 2 O with amide amidinates M(NMe 2 ) 4 [MeC(N i Pr) 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(μ 3 -O)(NMe 2 ) 3 ] 4 ( 1 ). For the Ta(V) complex 4 , the amide ligands are released in the reaction with H 2 O, leaving the amidinate ligand intact. [Ta(μ 3 -O)(NMe 2 ) 3 ] 4 ( 2 ), the analogue of 1 , was not observed as a product in the reaction of 4 with H 2 O. 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(NMe 2 ) 4 [MeC(N i Pr) 2 ] (M = Nb, 3 ; Ta, 4 ) have been prepared by the aminolysis of M(NMe 2 ) 5 (M = Nb, Ta) with i PrN(H)C(Me)=N i Pr ( 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 i PrN(H)C(Me)=N i Pr ( 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(NMe 2 ) 4 [MeC(N i Pr) 2 ] ( 3 ) is 13 times faster than that of its Ta analogue 4 .