Interaction of niobium and tantalum pentahalides with O-donors: coordination chemistry and activation reactions

Abstract

The chemistry of niobium and tantalum pentahalides, MX(5), with oxygen compounds is reviewed herein. The polynuclear structure of MX(5) is readily broken by addition of oxygen-containing organic molecules, L, to give either mononuclear or ionic dinuclear coordination adducts. Then activation of the organic ligand may take place favoured by several factors, i.e. low M-X bond energy, high temperature, presence of more than one oxygen function within L, L/M molar ratio ≥ 2. The activation reactions are often uncommon in the context of metal halides; they include the cleavage of C(sp3)-O, C(sp2)-O, C-H and C-C bonds, and eventual successive rearrangements proceeding with C-O or C-C couplings. The recently elucidated reactivity of MX(5) with limited amounts of oxygen compounds will be presented, and possible connections with the relevant MX(5)-directed syntheses reported in the literature will be outlined.

Structural Characterization of a Fluorido‐Amide of Niobium, and Facile CO2 Incorporation Affording a Fluorido‐Carbamate

“…The NMR spectra ( 1 H, 13 C, 19 F and 93 Nb) of 1a (CDCl 3 solution) all feature a single resonance, which suggests a rather symmetrical structure; the singlet at –158 ppm in the 19 F NMR spectrum indicates the presence of bridging fluorine atoms. [6b]…”

“…In spite of this, the interest in high‐valent fluorides of group 4 and 5 metals has progressively grown in the last decade due to their possible applications in organic synthesis, materials chemistry and as possible precursors of innocent, weakly coordinating anions . In particular, a variety of derivatives have been obtained and characterized from the reactions of easily available niobium and tantalum pentafluorides, MF 5 (M = Nb, Ta), with N, O and S donors as well as phosphines and arsines. [5a], …”

“…In this context, we decided to explore the possibility of preparing niobium(V) compounds with fluorido and dialkylamido ligands according to a strategy (use of an organic trimethylsilyl reactant) successfully employed for the synthesis of fluorido‐alkoxido[6b] and fluorido‐azido[5b], [5d] complexes. As a result, we present herein the structural characterization of the first ternary fluorido‐amide of niobium.…”

Cascade Reactions of α‐Phenylcinnamic Acid to Polycyclic Compounds Promoted by High Valent Transition Metal Halides

“…We were interested in the unexplored interaction of niobium pentafluoride with carboxylic acid chlorides. In general, the sluggishness of the organic reactant to cleave the Nb‐F‐Nb bridges in the tetranuclear structure of NbF 5 –, disfavours the formation of coordination compounds. Nevertheless, when an equimolar mixture of NbF 5 and α‐phenylcinnamoyl chloride, cleanly obtained from cis ‐PhCH=CPhC(=O)OH by means of PCl 5 (Scheme a, see SI for details), was heated in chloroform at reflux temperature, progressive conversion of the organic substrate occurred over 24 hours.…”

Stabilisation of [WF5]+ and WF5 by Pyridine: Facile Access to [WF5(NC5H5)3]+ and WF5(NC5H5)2

“…The WF 5 (NC 5 H 5 ) 2 adduct represents the first heptacoordinate adduct of at ransition-metal pentafluoride;N MR spectroscopic [25,26] and crystallographic [27] studies of MF 5 ·2C 5 H 5 N( M = Nb, Ta )r evealed that MF 5 undergo quantitative ligand-induced autoionisation to the ionic coordination isomers, [MF 4 (NC 5 H 5 ) 4 ] [MF 6 ], upon reactionw ith C 5 H 5 N. Furthermore,t he observation of ionic species upon reactions of MF 5 with excessm onodentate donorl igand,o rs toichiometric amounts of bidentate ligand,i su biquitous. [28,29] Uranium pentafluoride,h owever, forms am onocapped-trigonal-prismatic adduct with 2,2'bipy; [30] UF 5 ·2 HCN insteadm anifests as ao ne-dimensional coordination polymer with trigonal-dodecahedral U V centres. [31] [WF 5 (NC 5 H 5 ) n ] + + (n = 2, 3)…”