A new chelating mesoionic carbene ligand, derived from 1,2,3-triazoles, with two redox-active tert-butyl-phenolate linkers has been synthesized and explored towards its reactivity and electrochemical properties in early transition metal chemistry.

We report the facile synthesis of five rare high-valent Mo­(VI)-NHC complexes with a dianionic OCO-pincer benzimidazolylidene ligand (L1) with the general formula L1MoO2 (1), L1Mo­(O)­(NtBu)­(THF) (2), L1Mo­(NtBu)­(NHtBu)Cl (3), L1Mo­(NtBu)2 (4), and L1 Mo­(NMes)2 (5). These complexes represent a complete series of high-valent molybdenum­(VI) complexes, obtained by consecutive exchange of the oxo for imido ligands from the dioxo complex (1) to the bis-imido complexes (4 and 5). The complexes can be synthesized in high yields following a simple deprotonation protocol, using either triethylamine or lithium diisopropylamide (LDA) as a base. All complexes have been characterized by various techniques, including 1H, 13C­{1H}, and 15N NMR spectroscopy, IR spectroscopy, and X-ray diffraction analysis. Except for the dioxo complex 1, all complexes adopt monomeric structures in the solid state. Compounds 1, 4, and 5 are rare examples of five-coordinated molybdenum­(VI) complexes in the solid state, while 2 is six-coordinated, having a weakly bound THF ligand in the axial position. The dioxo complex 1 was found to be stable under air and moisture in the solid state as well as in solution for several days. Furthermore, preliminary reactivity studies and an approximate determination of the pK a value of the imido ligands in 4 are presented.

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.

The high-valent molybdenum(VI) N-heterocyclic carbene complexes, (NHC)MoO 2 (1) and (NHC)MoO(N t Bu) (2) (NHC = 1,3bis(3,5-di-tert-butyl-2-phenolato)-benzimidazol-2-ylidene), are investigated toward their catalytic potential in the deoxygenation of nitroarenes. Using pinacol as the sacrificial and green reductant, both complexes are shown to be very active (pre)catalysts for this transformation allowing a reduction of the catalyst loading down to 0.25 mol %. Mechanistic investigations show μ-oxo bridged molybdenum(V) complexes [(NHC)MoO] 2 O (4) and [(NHC)Mo-(N t Bu)] 2 O (5) as well as zwitterionic pinacolate benzimidazolium complex 6, with a doubly protonated NHC ligand, to be potentially active species in the catalytic cycle. Both 4 and 5 can be prepared independently by the deoxygenation of 1 and 2 using triethyl phosphine (PEt 3 ) or triphenyl phosphine (PPh 3 ) and were shown to exhibit an unusual multireferenced ground state with a very small singlet−triplet gap at room temperature. Computational studies show that the spin state plays an unneglectable role in the catalytic process, efficiently lowering the reaction barrier of the deoxygenation step. Mechanistic details, putting special emphasis on the fate of the catalyst will be presented and potential routes how nitroarene reduction is facilitated are evaluated.

The photodynamics of two pseudoisomeric iron(ii) complexes reveal insights into reactive metal-centred states and hot branching dynamics. A new type of reactivity by triplet energy transfer from MC states enables oxygen sensitization activity.