Abstract
We report the synthesis of vanadium(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L1 and the general formula [VOCl(L1)]. A comparison of the structural (SC-XRD), electronic (UV–vis), and electrochemical (cyclic voltammetry) properties of 1 with the benzimidazolinylidene congener 2 (general formula [VOCl(L2)]) shows that the MIC is a stronger donor also for early transition metals with low d-electron population. Since electrochemical studies revealed both complexes to be reversibly reduced, the stronger donor character of MICs was not only demonstrated for the vanadium(V) but also for the vanadium(IV) oxidation state by isolating the reduced vanadium(IV) complexes [Co(Cp*)2][1] and [Co(Cp*)2][2] ([Co(Cp*)2] = decamethylcobaltocenium). The electronic structures of the compounds were investigated by computational methods. Complex 1 was found to be a moderate precursor for salt metathesis reactions, showing selective reactivity toward phenolates or secondary amides, but not toward primary amides and phosphides, thiophenols, or aryls/alkyls donors. Deoxygenation with electron-rich phosphines failed to give the desired vanadium(III) complex. However, treatment of the deprotonated ligand precursor with vanadium(III) trichloride resulted in the clean formation of the corresponding MIC vanadium(III) complex 6, which undergoes a clean two-electron oxidation with organic azides yielding the corresponding imido complexes. The reaction with TMS-N3 did not afford a nitrido complex, but instead the imido complex 10. This study reveals that, contrary to popular belief, MICs are capable of supporting early transition-metal complexes in a variety of oxidation states, thus making them promising candidates for the activation of small molecules and redox catalysis.
Highlights
This study reveals that, contrary to popular belief, mesoionic carbene (MIC) are capable of supporting early transition-metal complexes in a variety of oxidation states, making them promising candidates for the activation of small molecules and redox catalysis
We have extended the use of mesoionic carbenes with an early transition metal, vanadium
Electrochemical, and computational methods, we have shown that mesoionic carbenes are stronger donors than classical
Summary
Almost two decades after the first report of an abnormal 5imidazolinylidene carbene complex,[1] mesoionic carbenes have been developed into a distinguished ligand class.[2,3] Among them, 1,2,3-triazole derived mesoionic carbenes, namely 1,2,3triazolinylidenes,[4] stand out by their modular synthesis via the copper-catalyzed [3 + 2] cycloaddition between azides and alkynes.[5−7] After their initial reporting by Albrecht et al.,[8] they quickly became prominent synthetic targets for (electro-) catalysis,[9−23] supramolecular chemistry,[24−27] magnetism,[28] and photochemistry due to their versatile synthesis and comparatively straightforward handling.[29−36] Throughout these studies, a great effort has been made to decipher their electronic structure. Mesoionic carbenes are commonly believed to be strong σ-donor ligands paralleling heteroaryls; recent reports emphasize their π-accepting properties[37−39] as demonstrated by the isolation of a reduced triazolinylidene ligand.[40] most studies targeted hitherto late transition metals or main group elements, while early transition-metal complexes with mesoionic carbenes have been rarely explored.[41−44] Arguably, this can be attributed to the relatively weak bond between N-heterocyclic carbenes and early transition metals.[45] this weak bond may be enhanced by harnessing anionic linkers. This strategy has allowed the isolation of a number of interesting metal complexes[46−49] of the early transition metals,[50−71] the lanthanides,[72−80] and the actinides.[81−86]
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