A new type of monoanionic “scorpionate” ligand. Synthesis, spectroscopic characterisation and dynamic behaviour of some niobium(III) complexes

Abstract

The syntheses of novel scorpionate ligands in the form of the lithium derivative [{Li(H₂O)(bdmpza)}₄] 1 [bdmpza = bis(3,5-dimethylpyrazol-1-yl)acetate] or as the alcohol derivative 2,2-bis(3,5-dimethylpyrazol-1-yl)ethanol 2 (Hbdmpze) have been studied. These compounds are excellent precursors for the introduction of these scorpionate ligands into transition metal complexes. The complex [{NbCl₃(dme)}_n] (dme = 1,2-dimethoxyethane) reacted with 1 and 2, the latter with prior addition of BuⁿLi, to give the binuclear complexes [{NbCl₂(bdmpzx)}₂] (x = a, 3; x = e, 4) [bdmpze = 2,2-bis(3,5-dimethylpyrazol-1-yl)ethoxide]. Complex 3 reacted with 1 to give the complex [{NbCl(bdmpza)₂}₂] 5. In the same way the reaction of the mononuclear species [NbCl₃(dme)(RCCR′)] with 1 and 2, the latter with prior addition of BuⁿLi, gave the appropriate [NbCl₂(bdmpzx)(RCCR′)] complexes [x = a, R = R′ = Me, 6; Et, 7; Ph, 8; or SiMe₃, 9; R = Ph, R′ = Me, 10; R = Ph, R′ = Et, 11; R = Ph, R′ = SiMe₃, 12; x = e, R = R′ = Me, 13; Et, 14; or Ph, 15; R = Ph, R′ = Me, 16; R = Ph, R′ = Et, 17; R = Ph, R′ = SiMe₃, 18]. The structures of these complexes have been determined by spectroscopic methods. Variable-temperature NMR studies were carried out in order to study their dynamic behaviour in solution. The barriers to alkyne rotation have also been calculated. Reaction of 10 with LiCp′ (Cp′ = C₅H₄SiMe₃) gave the mixed Cp–bdmpza complex [NbCp′Cl(bdmpza)] 19.