Further insights into the chemistry of niobium and tantalum pentahalides with 1,2-dialkoxyalkanes: Synthesis of bromo- and iodoalkoxides, spectroscopic and computational studies

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• 作者：Riccardo Bini^a ; Fabio Marchetti^b ; ¹ ; ^{fabmar@dcci.unipi.it} ; Guido Pampaloni^b ; ^{pampa@dcci.unipi.it} ; Stefano Zacchini^c
• 关键词：Niobium ; Tantalum ; Diethers ; CO bond activation ; Oxygen abstraction
• 刊名：Polyhedron
• 出版年：2011
• 出版时间：17 May 2011
• 年：2011
• 卷：30
• 期：8
• 页码：1412-1419
• 全文大小：464 K

文摘

The room temperature reactions of a series of 1,2-dialkoxyalkanes ROCH₂CH(R′)OR′′ with MX₅ (M = Nb, Ta; X = Br, I) in 1:1 ratio result in single C–O bond cleavage and high-yield formation of the halo-alkoxides MBr₄[κ²-OCH₂CH(R′)OR′′] or [NbI₄{κ¹-OCH₂CH(R′)OR′′}]₂, and equimolar amounts of the corresponding alkyl halides RX. The reaction of NbBr₅ with 1,2-dimethoxyethane, dme, proceeds with preliminary formation of the ionic species [NbBr₄(κ²-dme)(κ¹-dme)][NbBr₆], 3b, which has been identified by solution NMR at low temperature and conductivity analyses. The gas-phase structure of 3b has been optimized by DFT calculations, confirming that the dme ligands adopt bidentate and monodentate coordination, respectively. Although the formation of NbOBr₃(dme), 4b, 1,4-dioxane and MeBr from NbBr₅/dme (ratio 1:2) is an exoergonic process (calculated  = −115.96 kcal mol⁻¹), it is inhibited at room temperature. High temperature conditions enhance the production of 1,4-dioxane at the expense of selectivity. The dinuclear species NbOBr₃(dme)NbBr₅ (Nb–O–Nb), 5b, (X-ray) has been isolated in modest yield as byproduct of the room temperature reaction of NbBr₅ with dme. In general, the 1:2 molar reactions of NbX₅ (X = Br, I) with ROCH₂CH(R′)OR′′ occur with the exclusion of nearly one equivalent of organic reactant.