A Crystallographic Map of Chiral Recognition in π Complexes of Aromatic Aldehydes and a Chiral Transition Metal Lewis Acid: Enantioface Binding Selectivities in Solution Correlate to Distances between Metal and Carbon Stereocenters in the Solid State

Abstract

The π aromatic aldehyde complexes [(η5-C5H5)Re(NO)(PPh3)(η2-OCHAr)]+BF4- (1+BF4-; Ar = a, C6F5; b, 4-C6H4CF3; c, 4-C6H4Cl; d, C6H5; e, 4-C6H4CH3; f, 4-C6H4CH2CH3; g, 4-C6H4OCH3) exist as mixtures of configurational diastereomers (RS,SR/RR,SS or π/π') that differ in the OC enantioface bound to rhenium. Under standard conditions (0.000 71 M, CH2Cl2, 173 K), π/π' equilibrium ratios are 97:3, 89:11, 84:16, 80:20, 76:24, 79:21, and 74:26, respectively. Steric interactions between the aryl groups and cyclopentadienyl ligands destabilize the π' isomers. The crystal structures of (RS,SR)-1a−c,f+PF6- and (RS,SR)-1d+SbF6- show that the distances between the rhenium and carbon stereocenters (Å, a/b/c/d/f: 2.157(5)−2.161(9), 2.172(4), 2.176(4), 2.182(6)−2.188(9), 2.184(5)−2.199(6)) increase as π/π' ratios decrease. Stronger π accepting aldehydes give shorter bonds and higher chiral recognition. The aliphatic aldehyde complexes [(η5-C5H5)Re(NO)(PPh3)(η2-OCHR)]+BF4- exhibit higher π/π' ratios (R = CH3, 99.0:1.0; CH2CH3, 99.8:0.2; CH2CH2CH3, 99.5:0.5; CH(CH3)2 and C(CH3)3, >99.9:<0.1), and possible rationales are given. The π/π' ratios increase at higher concentration or lower temperature, and vary slightly with counteranion (BF4- > PF6- ≥ SbF6-).