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) (M = Nb, Ta), for example, yields [M(μ-O)(NMe)] (M = Nb, ; Ta, ) 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 HO with amide amidinates M(NMe)[MeC(NPr)] (M = Nb, ; Ta, ) show that the niobium and tantalum analogues take different principal paths. For the Nb(V) complex , the amidinate and one amide ligand are liberated upon treatment with water, yielding [Nb(μ-O)(NMe)] (). For the Ta(V) complex , the amide ligands are released in the reaction with HO, leaving the amidinate ligand intact. [Ta(μ-O)(NMe)] (), the analogue of , was not observed as a product in the reaction of with HO. 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)[MeC(NPr)] (M = Nb, ; Ta, ) have been prepared by the aminolysis of M(NMe) (M = Nb, Ta) with PrN(H)C(Me)=NPr (). The hydrolysis of and 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 and protonation of an axial dimethylamide ligand in . Both proton transfers furnish an intermediate metal-hydroxide species. The atomic charges in and 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 and as well as the exchange in PrN(H)C(Me)=NPr () were probed by EXSY NMR spectroscopy, giving rate constants of the exchanges: 0.430(13) s (), 0.033(6) s (), and 2.23(7) s (), showing that the rate of the Nb complex Nb(NMe)[MeC(NPr)] () is 13 times faster than that of its Ta analogue .