Bis(η⁵:η¹‐pentafulvene)niobium(V) Complexes: Efficient Synthons for Niobium Carbene and Imido Derivatives

Abstract

Transition-metal carbene complexes and their reactivities are a key topic of chemistry. They are an integral part of researches in catalysis, organic synthesis, coordination chemistry, and numerous other areas. In this context, we report the synthesis of a low-valent bis(η :η -(di-p-tolyl)-pentafulvene)niobium chloride. Owing to the π-η :σ-η coordination mode of the pentafulvenes and the resulting high nucleophilic character of the exocyclic carbon atom of the ligand, the bis(η :η -pentafulvene)niobium complex is able to achieve the umpolung of a coordinated vinyl unit and the resulting formation of the first η :η cyclic niobium Schrock carbene complex. This new synthetic route is, in comparison to classical α-hydrogen elimination reactions or thermolysis of diazo compounds, completely unprecedented. The reactivity of the cyclic carbene function and the remaining fulvene ligand is demonstrated by double N-H bond activation of primary amines to niobium imido complexes.

A niobium pentafulvene ethylene complex: synthesis, properties and reaction pathways

“…The Nb–C exo bonds of complex 2 (2.44–2.55 Å) are shorter than those in the reactant complex 1 (2.57–2.69 Å). 34 Consistently, the C ipso –C exo bonds of the fulvene ligands are longer in complex 2 than in complex 1 ( 1 : 1.42–1.43 Å; 2 : 1.45–1.46 Å) indicating a slightly stronger reduction of the ligand. The C ipso –C exo bond of the formed Cp′ ligand is elongated to a C sp 2 –C sp 3 single bond (1.51–1.54 Å) showing the protonation of the former pentafulvene ligand.…”

“…The Nb-C exo bonds of complex 2 (2.44-2.55 Å) are shorter than those in the reactant complex 1 (2.57-2.69 Å). 34 Consistently, the C ipso -C exo bonds of the fulvene ligands are longer in complex 2 than in complex 1 showing the protonation of the former pentafulvene ligand. 35 The Nb-C bond length of the formed η 2 -ethylene ligand is located in the range of 2.19-2.29 Å and the average C-C bond length of 1.43 Å is, as expected, considerably longer than an ethylene CvC double bond (1.33 Å) 36 but not as long as a typical single bond in cyclopropanes (1.51 Å).…”

“…Melting points were determined using a "Mel-Temp" from Laboratory Devices, Cambridge, or a Mettler Toledo MP30. The bis(η 5 :η 1 -(di-p-tolyl)pentafulvene) niobium chloride (1) 34 and the used deuterium labeled N-methylanilines 52,53 were synthesized according to known procedures. Visualizations of quantum chemical calculation results were created with VESTA 3 54 and the contour plot was created with Multiwfn.…”

Niobium Bis‐ and Mono(Pentafulvene) Complexes: E−H Bond Cleavage and Insertion Reactions (E=C, N, O)

“…This activates the metal carbon bond, making it more reactive than traditional metal alkyl complexes [1] . Before the first synthesis of bis( η 5 : η 1 ‐(di‐ para ‐tolyl)pentafulvene)niobium chloride ( 1 ), [9] stable group 5 pentafulvene complexes were limited to tantalum tuck‐in complexes [10–13] and non‐charge inverted vanadium compounds [14] . Having in mind diagonal relationships, we aimed to transfer the broad spectrum of titanium complex activated substrates to niobium chemistry [1,9,15] .…”

Synthesis and Reactivity of the Rhenium Fulvene Sandwich Complex [Re(η6-C5H4CH2)(η6-C6H6)]+

“…In contrast to iron or ruthenium, rhenium complexes bearing pentafulvene moieties as ligands are poorly, if at all, described in the literature. Most of the reported complexes comprise a fully methylated fulvene, synthesized either by photolysis of [Re­(η 5 -C 5 (CH 3 ) 5 )­(CO) 3 ] or via metal vapor synthesis (cocondensation). , In addition, the early transition metals Ti, Zr, Nb, Mo, and W are known to form complexes of fulvenes, typically disubstituted at the exocyclic carbon atom. − Thereby the bulky substituents prevent C–C coupling to form ansa -metallocenes . Apart from density functional theory (DFT) calculations published in 2009, complexes of the [Re­(η 6 -C 5 H 4 CH 2 )­(η 6 -C 6 H 6 )] + type have not been described so far .…”