Synthesis, Characterization, and Crystal Structures of Metal Amide Cage Complexes Containing a M 4 O 4 (M = Nb, Ta) Core Unit

Metal cage complexes [(Me 2 N) 3 MO] 4 (M = Nb, 3 ; Ta, 4 ) have been prepared from the reactions of M(NMe 2 ) 5 (M = Nb, 1 ; Ta, 2 ) with water. Single crystal X-ray diffraction studies of 3 and 4 reveal that they adopt cubane-like structures with M–O bridges. Variable-temperature NMR studies of –NMe A Me B rotations in 3 and 4 have been performed to give the following activation parameters for the exchanges: Δ H ≠ = −1.4(1.1) kJ/mol, Δ S ≠ = −209(8) J/mol K, G__ 30 8 K^^ = 6 4( 2) kJ/mol for 3 , and Δ H ≠ = −0.9(1.2) kJ/mol, Δ S ≠ = −2.1(0.2) × 10 2 J/mol K, G_ 30 8 K^^ = 6 3( 6) kJ/mol for 4 .