Tri-chlorido, 2-methylallyl and 2-butenyl tert-butylimido niobium and tantalum complexes: synthesis, multinuclear NMR spectroscopy and reactivity

Pseudooctahedral complexes [MCl(3)(NtBu)L(2)] (M = Nb, L = py 1, ? tmeda 3; M = Ta, L = py 2, ? tmeda 4) have been studied by spectroscopic methods. By a VT (1)H NMR experiment a mutual exchange process between the py(ax) and py(free) in the complexes 1-2 was observed, whereas (13)C and (15)N NMR studies showed in the complexes 3-4 a tmeda ligand with an axial/equatorial coordination mode. The reaction of 2 with 3 equiv of Grignard reagent produces the methathesis products [TaR(3)(NtBu)] (R = CH(2)CMeCH(2)5, CH(2)CHCHCH(3)6) in which 2-methylallyl and 2-butenyl groups appear with a η(3)- and σ-coordination mode, respectively. When, toluene solutions of the compounds 5-6 were treated with 2 equiv of 2,6-dimethylphenylisocyanide the imido bisiminoacyl compounds [TaR(NtBu){C(R)NAr-κ(1)C}(2)] (Ar = 2,6-Me(2)C(6)H(3); R = CH(2)CMeCH(2)7, CH(2)CHCHCH(3)8) can be isolated, via an imido iminoacyl intermediate [TaR(2)(NtBu){C(R)NAr-κ(1)C}] (Ar = 2,6-Me(2)C(6)H(3); R = CH(2)CMeCH(2)9) as we have observed in the treatment of 5 with 1 equiv of isocyanide; however, the analogous reaction between 5 and COPh(2) leads to the formation of the trisalkoxo imido compound [Ta(OCPh(2)R)(3)(NtBu)] (R = CH(2)CMeCH(2)10). All new complexes were studied by IR and multinuclear NMR spectroscopy.