ISOMERIZATION AND RACEMIZATION OF THE TRIS(4,4'-DIMETHYL-2,2'-BIPYRIDINE N,N'-DIOXIDE)CHROMIUM(III) COMPLEX

A new chromium(III) complex, [Cr(4,4'-Me2bpdo)3]3+ (4,4'-Me2bpdo = 4,4'-dimethyl-2,2'-bipyridine N,N'-dioxide) was prepared and resolved into a pair of enantiomers by an SP-Sephadex column chromatographic method using an achiral eluting agent Na2SO4. Both perchlorate and nitrate of the complex were found to crystallize in the lel2.ob(DELTA(lambda-lambda-delta), LAMBDA(delta-delta-lambda)) form. Upon dissolution in water, the complexes isomerized to the lel3(DELTA(lambda-lambda-lambda), LAMBDA(delta-delta-delta)) form with a rapid change in absorption spectra, and then racemized (DELTA reversible LAMBDA) slowly. The rate constants at 35.0-degrees-C were 4.39 x 10(-3) s-1 for the isomerization and 3.46 x 10(-5) s-1 for the racemization. These inversion reactions including conformational change (delta reversible lambda) and configurational change (DELTA reversible LAMBDA) are supposed to proceed by intramolecular mechanisms. The rates of isomerization and racemization of the 4,4'-Me2bpdo complex are ca. two and ten times slower than those of [Cr(bpdo)3]3+ (bpdo = 2,2'-bipyridine N,N'-dioxide), respectively.