A crystallographic map of chiral recognition in pi 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 pi aromatic aldehyde complexes [(eta(5)-C5H5)Re(NO)(PPh(3))(eta(2)-O=CHAr)](BF4-)-B-+(1(+)BF(4)(-); 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 pi/pi') that differ in the O=C enantioface bound to rhenium. Under standard conditions (0.000 71 M, CH2Cl2, 173 K), pi/pi' 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 pi' 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 (Angstrom, 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 pi/pi' ratios decrease. Stronger pi accepting aldehydes give shorter bonds and higher chiral recognition. The aliphatic aldehyde complexes [(eta(5)-C5H5)Re(NO)(PPh(3))(eta(2)-O=CHR)](BF4-)-B-+ exhibit higher pi/pi' 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 pi/pi' ratios increase at higher concentration or lower temperature, and vary slightly with counteranion (BF4- > PF6- greater than or equal to SbF6-).