Reversible cleavage of the Cr-Cr multiple bond in (TAA Cr)2 (TAA = tetramethyldibenzotetraaza 14 annulene)

The dimeric (TAA Cr)2 was recrystallized from pyridine to form a low-spin monomeric complex (TAA Cr py2).py, indicating that molecular restructuring has occurred in the pyridine solution. The complex redissolved in hot toluene or THF to give the dimer, suggesting that chemical equilibrium exists between the two species. Cocrystallization also occurred when a pyridine solution of the dimer was mixed with THF/hexane. The crystal structure revealed one dimeric (TAA Cr)2 and two monomeric octahedral (TAA)Cr(py)2 molecules. The Cr-Cr multiple bond energy cannot be more than twice that of the Cr-py bond.