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

Abstract

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.

Actinide tetra-N-heterocyclic carbene ‘sandwiches’

“…In every case that we have previously reported with similar macrocycles (excluding a chiral macrocycle), 39 only one NHC environment is observed by 1 H NMR. [34][35][36]38 This result suggests that the NHCs are locked into position in solution giving 1 relative D 2 symmetry. Heating the solution to 50 C in CD 3 CN (Fig.…”

“…The synthesis of the octa-NHC actinide complexes is extremely challenging. For transition metal systems, we have typically deprotonated the 16-atom ring macrocyclic ligand [H 4 ( BMe2,-Me TC H )]Br 2 with n BuLi at low temperature prior to adding a metal salt, [34][35][36][37][38][39] but this methodology is unsuccessful with 5f salts, possibly due to lithium adduct formation. 44 Other strong bases, such as NaCH 2 Ph in diethyl ether, led to degradation of [H 4 ( BMe 2 ,Me TC H )]Br 2 .…”

“…For example, a chromium complex prepared with the 16-atom macrocycle, [H 4 ( BMe2,-Me TC H )]Br 2 , yields a relatively stable Cr(II) dimer, [( BMe 2 ,Me TC H ) Cr] 2 with a chromium-chromium quadruple bond; however, switching to the 18-atom macrocycle, [H 4 ( BMe 2 ,Et TC H )]Br 2 , yields the highly unstable paramagnetic monomeric complex ( BMe 2 ,-Et TC H )Cr. 35,38 Similarly, iron complexes with 16-atom rings are more stable, but not catalytically active for aziridination, while 18-atom rings yield less stable complexes that perform that reaction. 28,37,45,46 This trend of reduced stability for 18-atom rings translates to the actinide complexes here.…”

“…43,50 As we observed with chromium complexes of ( BMe2,Me TC H ) and ( BMe2,Et TC H ), adding a single additional carbon atom to each bridge dramatically changes the structure and properties at each metal ion, so the structure of 3 was particularly noteworthy. 30,35 In contrast to 1, complex 3 crystallizes in a higher symmetry space group (Fccc) and 3 has a square prismatic geometry about the uranium atom with precise D 2d symmetry (Fig. 2C).…”

“…1F). [34][35][36][37][38][39] We considered that the increased ionic radii of actinides versus transition metals could lead to actinide bis(NHC macrocycle) 'sandwiches', reminiscent of porphyrins, phthalocyanines, or even cyclooctatetraene (COT) sandwiches (e.g. (COT) 2 M), and, more importantly, that the strong s-donor capacity of the NHC ligand should impart unusual electronic structures on these complexes.…”