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

Abstract

Diastereomeric orthometallated products, formed by treating [Os₃(CO)₁₀(MeCN)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 [Os₃(µ-H){µ-(R)-NC₅H₃CH(OH)Me-4}(CO)₁₀] 1 and 2 and four isomers of [Os₃(µ-H){µ-(S)-NC₅H₃C₄H₇NMe}(CO)₁₀], 3 to 6, two diastereomers each for the products of metallation at the 2 and 6 positions respectively, have been separated and characterized by circular dichroism (CD) spectra to obtain their relative configurations. The CD spectra in the 230–500 nm wavelength range are totally characteristic of the configuration of the Os₃CN group at atoms. A crystal structure determination for isomer 2 has allowed absolute configurations of all isomers to be established. There is little enantioselection in the orthometallation process and no detectable interconversion of isomers. The compound [Os₃(µ-H)₂{µ-(S)-NC₅H₃C₄H₇NMe}₂(CO)₈] was also obtained as a complex isomeric mixture which was not separated.