One Macrocyclic Ligand, Four Oxidation States: A 16-Atom Ringed Dianionic Tetra-NHC Macrocycle and Its Cr(II) through Cr(V) Complexes

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.