19F NMR spectroscopy as useful tool for determining the structure in solution of coordination compounds of MF5 (M = Nb, Ta)

The salts [S(NMe2)3][MF6] (M = Nb, 2a; M = Ta, 2b) and [S(NMe2)3][M2F11] (M = Nb, 2c; M = Ta, 2d) have been prepared by reacting MF5 (M = Nb, 1a; M = Ta, 1b) with [S(NMe2)3][SiMe3F2] (TASF reagent) in the appropriate molar ratio. The solid state structure of 2b has been ascertained by X-ray diffraction. The 1:1 molar ratio reactions of 1a with a variety of organic compounds (L) give the neutral adducts NbF5L [L = Me2CO, 3a; L = MeCHO, 3b; L = Ph2CO, 3c; L = tetrahydrofuran (thf), 3d; L = MeOH, 3e; L = EtOH, 3f; L = HOCH2CH2OMe, 3g; L = Ph3PO, 3h; L = NCMe, 3i] in good yields. The complexes MF5L [M = Nb, L = HCONMe2, 3j; M = Nb, L = (NMe2)2CO, 3k; M = Ta, L = (NMe2)2CO, 3l; M = Nb, L = OC(Me)CHCMe2, 3m] have been detected in solution in admixture with other unidentified products, upon 2:1 molar reaction of 1 with the appropriate reagent L. The ionic complexes [NbF4(tht)2][NbF6], 4a, and [NbF4(tht)2][Nb2F11], 4b, have been obtained by combination of tetrahydrothiophene (tht) and 1a, in 1:1 and 2:3 molar ratios, respectively. The treatment of 1 with a two-fold excess of L leads to the species [MF4L4][MF6] [M = Nb, L = HCONMe2, 5a; M = Ta, L = HCONMe2, 5b; M = Nb, L = thf, 5c; M = Ta, L = thf, 5d; M = Nb, L = OEt2, 5e]. The new complexes have been fully characterised by NMR spectroscopy. Moreover, the revised 19F NMR features of the known compounds MF5L [M = Ta, L = Me2CO, 3n; M = Ta, L = Ph2CO, 3o; M = Ta, L = MePhCO, 3p; M = Ta, L = thf, 3q; M = Nb, L = CH3CO2H, 3r; M = Nb, L = CH2ClCO2H, 3s; M = Ta, L = CH2ClCO2H, 3t], TaF4(acac), TaF4(Me-acac) and [TaF(Me-acac)3][TaF6] (Me-acac = methylacetylacetonato anion) are reported.

The unambiguous 19F NMR characterization of the [MF6]− anions (M = Nb, Ta) in chlorinated solvents has allowed the discussion of the room temperature 19F NMR spectra of MF5 derivatives in CDCl3 or CD2Cl2 solutions, suggesting the usefulness of 19F NMR spectroscopy for previewing the structure of MF5 adducts in solution.Figure optionsDownload as PowerPoint slide