Conformational analysis in diastereoisomer equilibria of square-pyramidal [C 5H 5(CO) 2Mo pyridine-2-imine]PF 6 complexes

Abstract

Synthesis and characterization of sonic new square-pyramidal pyridine-2-imine complexes [C 5,H 5,(CO) 2,MoNC 5,H 4,CX=NCH(R 1)(R 2)]PF 6 with X = CH 3, C 6H 5 and chiral amine, 1-phenyl-isobutylamine and amino acid methyl ester H 2NCH(COOCH 3)(R) with (R) = (CH 2C 6H 5) and (C 2H 5) have been reported. In combination with Mo chirality (R) and (S), mixtures of two diastereoisomeric pairs of enantiomers with racemic amine and amino acids were obtained which were separated by fractional cystallization. The diastereoisomers differ in the chemical shift of most of their 1H NMR signals and interconvert on heating in acetone- d 6 at 80°C for 80 hr and 40°C for 200 hr. On the basis of three conformational determining effects (i) C-H or C-alkyl of the asymmetric centre eclipses the ligand plane, (ii) MC 5H 5/C 6H 5 attraction and (iii) MC 5H 5/alkyl repulsion in order of decreasing significance, the chemical shifts of the C 5H 5 signals, their differences as well as the diastereoisomer ratio at equilibrium for all the complexes has been rationalised.