Síntesis, reactividad, no rigidez estereoquímica y espectroscopia de rmn multinuclear de complejos tert-butil imido de niobio y tántalo (V)

Resumo

This section describes our latest results concerning mononuclear tert-butilimido niobium and tantalum (V) compounds. Complexes containing metal-ligand multiple bonds, particularly early transition metal-imido species, constitute an interesting subject because they are implicated as reagents or catalysts in oxidation and metathesis processes. The 1990´s have witnessed the spectacular development of a new generation of “non-metallocene” catalysts, a lot of examples show that this is a scientific area of constant activity. In recent years, many neutral groups imido complexes have been synthesized, although their functionalities were used as ancillary groups to support high-oxidation state metal centers in some cases. The imido ligand is a reactive group that participates in transformations such as imine and carbodiimide metathesis, imido-tooxo ligand exchange and the activation C-H bonds in aromatic hydrocarbons. More recently the synthesis of neutral and cationic imido tantalum compounds and their behavior in migratory insertion reactions with small unsaturated molecules have been reported. In these processes, the resulting [M{C(R)}E] (E = O, NR’; R, R’ =alkyl, aryl) moieties are a powerful key for C-C bond formation under mild reaction conditions and simultaneously, are versatile intermediates in transition metal promoted transformations. In this context, our research group completed a study about the insertion of isocyanide Ar-NC (Ar =2,6-Me2C6H3) into the M-Me bond of [MCp*Cl4-xMex] (M = Nb,18 Ta,19-20, 260 Cp*= 5 -C5Me5; x= 1- 4) complexes, in which a series of iminoacyl (x=1), zametalacyclopropane (x=2,3) and alkenylamido imido (x=3,4) derivatives were synthesized. In addition, new iminoacyl compounds [MLX(NAr){C(R)NAr} (M= Nb, L = 5 -C5H4SiMe3; M = Ta, L = 5 - C5Me5; X= Cl,R; Ar = 2,6-Me2C6H3) were obtained via insertion of isocyanide into M-Calkyl bond of alkyl chlorides or dialkyl imido complexes [MLXR(NAr)]; in contrast with previous results the insertion into alkyl methyl imido species implied the migration of the alkyliminoacyl and alkyl methyliminoacyl complexes, respectively.16 In view of these precedents and as an extension inside the research line of our group the main target of this Ph.D. work has been: The synthesis, reactivity and non-rigidity stereochemical studies of new imido niobium and tantalum complexes, using the high resolution Multinuclear Magnetic Resonance, particularly 15N.