Catalytic Deoxygenation of Nitroarenes Mediated by High-Valent Molybdenum(VI)–NHC Complexes

Abstract

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.

Mesoionic Carbenes in Low- to High-Valent Vanadium Chemistry

“…Thus, we initially aimed to generate a low-valent vanadium(III) complex by deoxygenating the parent vanadium(V) complex 1 with triethylphosphine. Although this strategy met with success for the deoxygenation of molybdenum(VI) benzimidazolinylidene complexes, 143,144 no useful reaction products could be isolated in the present case. Similarly, switching to other phosphines such as triphenylphosphine or trimethylphosphine turned out to be unproductive.…”

Thermal Decomposition Mechanisms of Volatile Molybdenum(VI) Bis-Imides

“…Molybdenum(VI) bis(imido) compounds have a wide range of applications, including their use as olefin metathesis catalysts, 1,2 oxygen atom transfer reagents, [3][4][5][6] and vapor phase deposition precursors. The reactivity of these industrially relevant compounds has been extensively studied, but the mechanism of their thermal decomposition remains poorly understood.…”

“…15 Chart 1. Known CVD and ALD precursors (A-H) that incorporate the bis(tertbutylimido)molybdenum(VI) framework and the general (RN)2MoCl2•L (L = neutral N,N'chelating ligand) compounds (1)(2)(3)(4)(5)(6)(7). R = Me, Et, i Pr; t Bu = tert-butyl; t Pe = tert-pentyl; Ad = 1adamantyl; t Oct = tert-octyl; xyl = 2,6-dimethylphenyl.…”

A mesoionic carbene complex of manganese in five oxidation states

“…Pincer-ligands have not only enabled the development of various robust catalysts, 1,2 and the stabilisation of highly reactive metal complexes, 3 but they also opened the way to isolating metal centres in high oxidation states. 4 However, pincers possessing an anionic 5 CNC coordination motif with a donating amido moiety are still scarce and only a handful have been reported to date.…”

“…The steric demand of this ligand allowed for the isolation of a series of highly reactive complexes such as a low-valent rhodium( i ) oxygen adduct, 8 or a T-shaped gold( i ) complex, 9 but it inhibited the formation of homoleptic metal complexes. However, given the propensity of NHC ligands to stabilize high-valent metal complexes, 2,10 such as in a hexa-NHC-Mn( iv ) complex by Smith and co-workers, 11 the formation of homoleptic MIC complexes is a desirable goal. In our ongoing interest in mesoionic carbenes, 12 we recently reported the synthesis and photophysical properties of a highly luminescent N -fused mesoionic carbene ligand and its s-block complexes 13 as well as its catalytic potential in nickel( ii ) mediated CO 2 -epoxide coupling.…”