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

The π aromatic aldehyde complexes [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(η 2 -O CHAr)] + BF 4 - ( 1 + BF 4 -; Ar = a, C 6 F 5; b, 4-C 6 H 4 CF 3; c, 4-C 6 H 4 Cl; d, C 6 H 5; e, 4-C 6 H 4 CH 3; f, 4-C 6 H 4 CH 2 CH 3; g, 4-C 6 H 4 OCH 3 ) exist as mixtures of configurational diastereomers ( RS, SR / RR, SS or π/π‘) that differ in the O C enantioface bound to rhenium. Under standard conditions (0.000 71 M, CH 2 Cl 2, 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 + PF 6 - and ( RS, SR )- 1d + SbF 6 - 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 [(η 5 -C 5 H 5 )Re(NO)(PPh 3 )(η 2 -O CHR)] + BF 4 - exhibit higher π/π‘ ratios (R = CH 3, 99.0:1.0; CH 2 CH 3, 99.8:0.2; CH 2 CH 2 CH 3, 99.5:0.5; CH(CH 3 ) 2 and C(CH 3 ) 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 (BF 4 - > PF 6 - ≥ SbF 6 - ).