METAL(II) DERIVATIVES OF 3,5-DI-TERT-BUTYL-1,2-ORTHO-BENZOQUINONE - EPR STUDY OF CONFORMATION IN BIRADICALS

The metals M = Mg, Zn, Cd and Ba react with 3,5-di-tert-butyl-1,2-o-benzoquinone to form the biradical compounds M(TBSQ)2 (TBSQ = 3,4-di-tert-butyl-1,2-o-benzosemiquinone). In toluene M(TBSQ)2 reacts with neutral donor ligands (L = pyridine (py), 2,2'-bipyridine (bipy), and N,N,N',N'-tetramethylethanediamine (tmed) to produce 1:1 or 1:2 adducts. EPR spectra of M(TBSQ)2 in toluene solutions show that these exist as oligomers instead of mononuclear species. Analysis of the EPR of mixtures of M(TBSQ)2 (M = Mg, Zn, and Cd) and L (L = py, bipy, and tmed) in frozen solutions has demonstrated the existence of biradical species in which the interaction between two semiquinone ligands is purely dipolar in nature. For the monodentate ligand py both cis and trans isomers are identified, with the cis isomer by far the more abundant. Only the cis isomer is found for the bidentate ligands bipy and tmed. The tmed ligand produced additional species for M = Mg, Cd, and Ba, which are proposed as five-coordinate for M = Mg and eight coordinate for M = Cd and Ba.