Despite
chromium being among the first transition metals ever reported to
bind to an NHC, chromium NHC complexes, especially in mid and high
oxidation states, have received scant attention. Herein, the synthesis,
characterization, and reactivity of a series of Cr(II) to Cr(V) complexes
bearing a 16-atom ringed dianionic tetra-NHC macrocycle are reported.
The Cr(II) dimer is diamagnetic and displays a very short Cr–Cr
quadruple bond, unprecedented for Cr-NHC complexes to date. Oxidative
cleavage of the Cr–Cr bond leads to the formation of a highly
stable diamagnetic Cr(IV) oxo complex. Similar reactions with organic
azides lead to paramagnetic Cr(IV) imide complexes. Notably, the Cr(IV)
oxo can be oxidized in a reversible reaction to yield a Cr(V) cationic
oxo complex, which is a very rare high oxidation state Cr-NHC-compound.
This Cr(V) oxo undergoes stoichiometric oxygen atom transfer. Similar
reactions were attempted with molybdenum and tungsten to form macrocyclic
NHC complexes, but only a molybdenum dimer could be isolated.
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