Síntesis, reactividad, no rigidez estereoquímica y espectroscopia de RMN multinuclear de complejos "ter"-butil imido de niobio y tántalo (V)

Abstract

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 1990ºs 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.4-8 In recent years, many neutral groups imido complexes have been synthesized,9 although their functionalities were used as ancillary groups10-11 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-to-oxo ligand exchange and the activation C-H bonds in aromatic hydrocarbons.13, 15, 17-19, 223, 237 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.13, 15, 17, 223, 237 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), azametalacyclopropane (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. Conclusions: 1) New imido niobium and tantalum (M= Nb, Ta) imido complexes, trichlorido [MCl3(NtBu)Lx] (M=Nb, Ta; L=py, x =2; L=tmeda, x=1), dichlorido hydridotris(3,5-dimethylpyrazolyl)borate [MTp*Cl2(NtBu)] (M=Nb, Ta; L=BH(3,5-Me2C3HN2)3=Tp*), trialkyl [MR3(NtBu)] (M=Nb,Ta; R=Me, CH2Ph, CH2tBu, CH2CMe2Ph, CH2SiMe3; M=Ta, R=CH2CMeCH2) alkyl chlorido and dimethyl [MTp*XR(NtBu)] (M=Nb, Ta; X=Cl, R=Me, CH2CH3, CH2Ph, CH2tBu, CH2SiMe3, CH2CMe2Ph; X=R=Me) and amido [MTp*X(NMe2)(NtBu)] (M=Nb, Ta; X=Cl, Me, NMe2) have been prepared. 2) NMR spectroscopic data support a cis,mer-pseudooctaedral geometry of the trichlorido compounds. The bis-pyridine species show an exchange process between pyaxial and pyfree, whereas in the tmeda derivatives the bidentate ligand maintains a rigid axial/equatorial disposition. Trialkyl complexes exhibit a characteristic three-legged piano-stool geometry with three alkyl groups ? bonded to metal atom, except in the tris-2-methylallyl tantalum derivative in which the ligands are ?3-CH2CMeCH2 coordinated.Alkyl chlorido compounds [MTp*ClR(NtBu)] are fac-pseudooctahedral with two different conformational states of alkyl ligand (gauche-syn gauche-anti), observed in solution and solid state. 3) The insertion of 2,6-dimethylphenyl isocyanide into the M-C bond of alkyl imido complexes takes place with the formation of alkyl bis-iminoacyl and chlorido methyl iminoacyl compounds from trialkyl and alkyl chlorido species, respectively. In the last case, for M= Nb when the insertion is followed by 1H NMR spectroscopy we can observe an irreversible endo- --> exo- isomerization process whose rate is dependent of volume of the alkyl group, whereas for M=Ta only the direct formation of exo- isomers was observed.The amido complexes [MTp*X(NMe2)(NtBu)] reactivity was tested against trispentafluorophenylborane, resulting in: i- amido chloride derivatives are inert, ii-methyl species result in "cation like" amido compounds [MTp*(NMe2)(NtBu)]+ [BMe(C6F5)3]- (M = Nb, Ta), in which the exchange takes place between two enantiomers through a cationic transition state and iii- bis-amido complexes, Lewis acid H-abstracting a ligand dimethylamido methyl group, as the mechanism that we proposed, forming a cationic species containing an imine ligand-K2N [MTp*(NMe2)(NtBu){NMe(CH2)-K2N}]+ [BH(C6F5)3]- (M = Nb, Ta). 4) In amido imido complexes [MTp*X(NMe2)(NtBu)] the rotation about M-Namido bonded is restricted and its speed depends on the inductive effect (I) substituent NMe2 > Me > Cl, being slightly faster for niobium. However, cationic species [MTp*NMe2(NtBu)]+ [BMe (C6F5)3]- and [MTp*(NMe2){NMe(CH2)-K2N)(NtBu)]+ [BH(C6F5)3]- the speed of rotation is much greater than in neutral species. Furthermore, the process of ligand pseudorotation vector Tp* B-M around the carbocationic species [MTp*(NMe2)(NtBu){NMe(CH2)- K2N)(NtBu)]+ was observed and characterized. 5) In all the isoestructural compounds the chemical shifts 15Nimido-Nb are greater than 15Nimido-Ta due to the diamagnetic component of the shielding magnetic constant. However, the ligand resonances iminoacyl as much 15N and 13C are more shielded by niobium complexes and do not depend on the atomic number of the metal (M = Nb, Ta), but rather on its electronegativity.