CHROMIUM(V) OXO COMPLEXES OF MACROCYCLIC TETRAAMIDO-N LIGANDS TAILORED FOR HIGHLY OXIDIZED MIDDLE TRANSITION-METAL COMPLEXES - A NEW O-18-LABELING REAGENT AND A STRUCTURE WITH 4 NONPLANAR AMIDES

Two chromium(V)-oxo complexes, [(CH3)4N][Cr(0)(η4-l)] and [(CH3)4N][Cr(O)(η4-2)], have been synthesized and characterized by X-ray crystallography and IR and EPR spectroscopies. Because exchange of the oxo ligand with water is slow, the easily synthesized, stable, crystalline 18O-labeled diperoxide (CH3)2(H18O18O)CCH2CH2C(18O18OH)(CH3)2 was prepared and used to conveniently synthesize 18O-labeled oxo complexes in high yields. The bonding of the two unique oxidation-resistant macrocyclic tetraamides to chromium is compared. The structural and EPR properties are consistent with a chromium-centered radical in each case and suggest that a chromium(V) oxidation state assignment is equally appropriate whether the ancillary ligand is the innocent [η4-1]4- or the potentially noninnocent [η4-2]4-. Both oxo complexes contain nonplanar amide groups. The distortions in [Cr(0)(η4-1)]- are more marked, and it is a unique species in containing four distinctly nonplanar amides. The discovery of these unusual structural parameters expands the class of nonplanar amides arising from ring constraint. © 1990, American Chemical Society. All rights reserved.