Structures and Unusual Rearrangements of Coordination Adducts of MX5 (M Nb, Ta; X = F, Cl) with Simple Diethers. A Crystallographic, Spectroscopic, and Computational Study.

High-yield reactions of the pentafluoride complexes MF5 (M=Nb, Ta) with a variety of diethers, ROCH2CH(R')OR", afford the ionic derivatives [MF4(K2-ROCH2CH(R')OR")2][MF6] (2a-2e) or [MF4(ROCH2CH(R')OR" )2][M2F11] (3a-3f) according to the metal/diether ratio employed. The structures of [MF4(K2-dme)2][MF6] [M=Nb (2a), Ta (2b); dme=MeOCH2CH2OMe] have been determined by X-ray diffraction. Moreover, the structure of the cationic part of 2a in the gas phase has been optimized by density functional theory calculations (the B3LYP/LANL2DZ method). An analogous computational study has allowed one to predict the existence of a heptacoordinated niobium cation within the species [NbCI4(dme)2][NbCl6]. This prediction has been confirmed by NMR identification at -60°C of the complexes [MCI4(K2-dme)(K1-dme)][MCI6] [M=Nb (4a), Ta (4b)], obtained by the addition of dme to MCI5 in CDCI3. Activation of the coordinated diether in 2 and 3 takes place in solution at high temperature and generally proceeds via cleavage of C-C bonds and coupling of the fragments produced. Thus, OR2 (R=Me, Et) and 1,4-dioxane have been obtained selectively upon thermal reaction of MF5 with rime, diglyme, or 1,2-diethoxyethane, followed by treatment with water. In analogous conditions, 1,2-dimethoxypropane converts mainly into acetone and dimethyl ether. The formation of dioxane from dme occurs in high yield also by using a catalytic amount of NbF5. The activation of dme by NbF5 follows a pathway different from that previously reported for NbCI5, and an explanation based on the thermodynamic parameters calculated at the B3LYP/LANL2DZ level is given: the formation of Me2O and dioxane is the most favored reaction between NbF5 and dme (ΔGr°= -15.16 kcal ∙ molNb-1), while the formation of MeCl and dioxane is the most favored reaction in the case of NbCl5/dme (ΔGr°=-53.13 kcal ∙ molNb-1). [ABSTRACT FROM AUTHOR]