Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

Abstract

We report the synthesis of vanadium(V) oxo complex 1 with a pincer-type dianionic mesoionic carbene (MIC) ligand L 1 and the general formula [VOCl(L 1 )]. 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(L 2 )]) 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-N 3 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.

Reactivity and Structure of a Bis-phenolate Niobium NHC Complex

“…The cyclic voltammograms of 1-Py in dichloromethane showed only two irreversible oxidations and no reduction processes (Figures S81 and S82), which is in stark contrast to the vanadium systems. 74 However, switching to THF as a solvent revealed the presence of two irreversible reductions close to the edge of the solvent window at −2.81 and −2.98 V vs Fc/[Fc] + couple as an internal standard (Figure S83 and Table S3). Removal of the pyridine co-ligand in 1 leads to a slight shift and a larger separation between the two reductive redox events now appearing at −2.70 and −3.03 V vs Fc/[Fc] + (Figure S84 and Table S3).…”

“…Albeit 1-Py and the pyridine-free complex 1 show irreversible reduction processes in their cyclic voltammogram, the isolation of a reduced niobium(IV) complex is more complicated compared to analogous vanadium complexes. 74 Among others, these new complexes cover a series of mesityl-substituted chalcogen and pnictogen donor complexes 2−5. Complexes 4 and 5 are potentially interesting precursors for the synthesis of extremely π-loaded 118 bis-imido and imido-phosphinidene complexes for catalysis and group transfer reactivity.…”

“…73 Furthermore, we recently investigated the coordination chemistry L 1 toward high-valent vanadium(V). 74 Here we extend our efforts in exploring the chemistry of early-transition-metal NHC complexes to the heavier congener of vanadium, namely, niobium.…”

Pincer‐Type Mesoionic Carbene Nickel(II) Complexes: Synthesis, Properties, Reactions, and Catalytic Application to the Suzuki–Miyaura Coupling Reaction of Aryl Bromides

“…Hence, comparisons between these complexes in terms of their structure and catalytic activity will provide important insights into the chemistry of MIC complexes. There are limited studies on the detailed comparison of imidazolylidene and triazolylidene complexes in the literature [59–62] . For instance, ruthenium complexes bearing cyclometalated aryl‐NHC ligands are catalyst precursors used in the transfer hydrogenation of aldehydes, ketones, and imines [59] .…”

“…There are limited studies on the detailed comparison of imidazolylidene and triazolylidene complexes in the literature. [59][60][61][62] For instance, ruthenium complexes bearing cyclometalated aryl-NHC ligands are catalyst precursors used in the transfer hydrogenation of aldehydes, ketones, and imines. [59] Different chemoselectivities, ketones and imines for the imidazolylidene complexes and aldehydes for the triazolylidene complexes, have been performed depending on the NHC ligand used.…”