Synthesis and DFT, Multinuclear Magnetic Resonance, and X‑ray Structural Studies of Iminoacyl Imido Hydridotris(3,5-dimethylpyrazolyl)borate Niobium and Tantalum(V) Complexes

Reaction of alkyl imido [MTp*XR­(NtBu)] (M = Nb/Ta;
Tp* = HB­(3,5-Me2C3HN2)3; X = Cl, R = Me (1a/1b), CH2CH3 (2a/2b), CH2Ph
(3a/3b), CH2tBu (4a/4b), CH2SiMe3 (5a/5b), CH2CMe2Ph (6a/6b); X = R = Me (7a/7b)) complexes with 1 equiv of the isocyanide 2,6-Me2C6H3NC takes place with migration of
an alkyl group and leads to the formation of the series of chlorido
or methyl imido iminoacyl derivatives [MTp*X­(NtBu)­{C­(R)­NAr-κ2C,N}] (M = Nb/Ta; Ar = 2,6-Me2C6H3; X = Cl, R= Me (8a/8b), CH2CH3 (9a/9b), CH2Ph (10a/10b),
CH2tBu (11a/11b), CH2SiMe3 (12a/12b), CH2CMe2Ph (13a/13b); X = R = Me (14a/14b)). The molecular
structure of 10b was determined by X-ray diffraction
methods. An irreversible endo → exo isomerization was detected by 1H NMR in compounds 10a–13a. The insertion–isomerization
reaction coordinate was computed by DFT calculations.