Inorganic asymmetric synthesis: Diastereoselective syntheses of mono-and dinuclear complexes containing octahedral, two-bladed propeller, bis(pyridine-2-aldehyde 2′-pyridylhydrazone)iron(II) stereocenters

A C₂ hexadentate diester derived from (5-hydroxymethyl)pyridine- 2-aldehyde 2′-pyridylhydrazone (5-HOCH₂PAPHY) and an enantiomerically pure (aS)-spirane dicarboxylic acid diastereoselectively reacts with iron(II) benzenesulfonate in methanol to furnish a 20% diastereomeric excess (de) of a two-bladed propeller, octahedral iron complex in which the P configuration of the newly created (±)-[Fe(PAPHY)₂] ²⁺ stereocenter predominates; when the reaction mixture is heated under reflux for 12 h, however, the excess of the diastereomer having the P configuration at iron increases to 80%, as determined by ¹H NMR spectroscopy. The configuration at iron in the major diastereomer of the complex was determined by comparison of the circular dichroism spectrum of the deprotonated complex with that of a related complex of known configuration. Repositioning of the hydroxymethyl group of the pyridine-2-aldehyde from the 5- to the 6-position produced a C₂-spirane ligand that generates a double-stranded diiron(II) helicate in >99% de. Single crystal X-ray structure determinations of the racemates of the protonated and deprotonated helicates revealed that the complexes crystallize diastereoselectively, that is, two ligand strands of aS configuration generate two octahedral iron(II) stereocenters of P configuration to give a dinuclear metal helicate of P configuration and vice versa for the ligand of aR configuration.