Synthesis, characterization, and reactivity of niobium perfluoropinacolate complexes
Date
2024
DOI
Version
OA Version
Citation
Abstract
Six new niobium complexes supported by bidentate dianionic perfluoropinacolate, ({OC(CF3)2}2)2-, denoted as pinF, have been synthesized and fully characterized: [Nb(NMe2)(NHMe2)(pinF)2] (1), (Me2NH2)[Nb(pinF)3] (2), {K(THF)3}[Nb(pinF)3] (3), {K(THF)3}2[Nb2(μ2-O)2(pinF)4] (4), {K(18C6)(THF)2}2[Nb2(μ2-O)2(pinF)4] (5), and {K(MeCN)7/3}3[Nb2(μ2-O)2(pinF)4] (6). Complex 1 exhibits fluxionality of monodentate and bidentate ligands; various methods of recrystallization repeatedly provided solid-state evidence of a cis isomer (1-cis), while VT 1H and 19F and 2D 19F NMR provided solution-state evidence of a trans isomer (1-trans). The twist angles of 2 and 3 range from 28.87° to 32.17°, classifying the complexes as an intermediate between trigonal prismatic and octahedral geometry. The 18-crown-6 encapsulation of K+ countercations of 5 elongate the Nb–O bond distances compared to those of 4; the K+ countercations of 4 exhibit multiple K⋯F and K⋯O interactions with the four chelating pinF ligands. Each of the soluble, {Nb5+(pinF)} complexes exhibit solution-state 93Nb NMR resonance features that support previous 93Nb NMR measurements made with static/MAS methods.
The heteroleptic complex 1 was found to selectively dehydrate 2° and 3° alcohols to alkenes, historically observed by analogous insoluble niobium pentoxide (Nb2O5) and niobic acid (Nb2O5⋅nH2O). The dehydration of 1-phenylethanol to styrene under mild conditions exhibited no evidence for polymerization to styrene. Both 2° and 3° alcohol dehydration studies gave no evidence of condensation products. The transformation of 1 during dehydration was proposed from 1H, 19F, and 93Nb NMR spectroscopy.
Cyclic voltammetry was used to determine the electrochemical reduction and oxidation potential of Nb5+/4+ in the all-O-donor environments of 2, 3, 4, and 5. Despite observing E½ = -1.303 V of 2 and E½ = -1.263 V of 3 (both in MeCN), the Nb5+ center could not be reduced in the tris-pinF complex. One-electron reduction of the {Nb2(μ2-O)2(pinF)4} dimer formed the mixed-valent complex {K(MeCN)7/3}3[Nb2(μ2-O)2(pinF)4], which was the first crystal structure of Nb in all-O-donor environment: (1) in a formal oxidation state lower than 5+ and (2) in a coordination number lower than eight. The solid-state SCXRD and in silico DFT data point to 6 as a Robin-Day Class II dimer, and additional experiments with the dimer in solution for UV-Vis and Evans method suggest electronic communication between the two Nb centers.
Description
2024