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

The
high-valent molybdenum­(VI) N-heterocyclic
carbene complexes, (NHC)­MoO2 (1) and (NHC)­MoO­(NtBu) (2) (NHC = 1,3-bis­(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]2O (4) and [(NHC)­Mo­(NtBu)]2O (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 (PEt3) or triphenyl phosphine (PPh3) 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.