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

Abstract

Pseudooctahedral complexes [MCl₃(NtBu)L₂] (M = Nb, L = py 1, ½ tmeda 3; M = Ta, L = py 2, ½ tmeda 4) have been studied by spectroscopic methods. By a VT ¹H NMR experiment a mutual exchange process between the py_ax and py_free in the complexes 1–2 was observed, whereas ¹³C and ¹⁵N 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 [TaR₃(NtBu)] (R = CH₂CMeCH₂5, CH₂CHCHCH₃6) in which 2-methylallyl and 2-butenyl groups appear with a η³- 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-κ¹C}₂] (Ar = 2,6-Me₂C₆H₃; R = CH₂CMeCH₂7, CH₂CHCHCH₃8) can be isolated, via an imido iminoacyl intermediate [TaR₂(NtBu){C(R)NAr-κ¹C}] (Ar = 2,6-Me₂C₆H₃; R = CH₂CMeCH₂9) as we have observed in the treatment of 5 with 1 equiv of isocyanide; however, the analogous reaction between 5 and COPh₂ leads to the formation of the trisalkoxo imido compound [Ta(OCPh₂R)₃(NtBu)] (R = CH₂CMeCH₂10). All new complexes were studied by IR and multinuclear NMR spectroscopy.