Orientational isomerism in bridging 2-pyridyl complexes: orthometallation of (S)-nicotine and (R)-1-(4-pyridyl)ethanol in triosmium clusters

Diastereomeric orthometallated products, formed by treating [Os-3(CO)(10)(M eCN)(2)] with (S)-nicotine or (R)-1-(4-pyridyl)ethanol, have been partially separated by TLC but completely so by HPLC. The two diastereomers of [Os-3 (mu-H){mu-(R)-NC5H3CH(OH)Me-4}(CO)(10)] 1 and 2 and four isomers of [Os-3(m u-H){mu-(S)-NC5H3C4H7NMe}(CO)(10)], 3 to 6, two diastereomers each for the products of metallation at the 2 and 6 positions respectively, have been se parated and characterized by circular dichroism (CD) spectra to obtain thei r relative configurations. The CD spectra in the 230-500 nm wavelength rang e are totally characteristic of the configuration of the Os3CN group at ato ms. A crystal structure determination for isomer 2 has allowed absolute con figurations of all isomers to be established. There is little enantioselect ion in the orthometallation process and no detectable interconversion of is omers. The compound [Os-3(mu-H)(2){mu-(S)-NC5H3C4H7NMe}(2)(CO)(8)] was also obtained as a complex isomeric mixture which was not separated.