Binuclear trans-Bis(β-iminoaryloxy)palladium(II) Complexes Doubly Linked with Pentamethylene Spacers: Structure-Dependent Flapping Motion and Heterochiral Association Behavior of the Clothespin-Shaped Molecules

摘要

The synthesis, structure, and solution-state behavior of clothespin-shaped binuclear trans-bis(β-iminoaryloxy)- palladium(II) complexes doubly linked with pentamethylene spacers are described. Achiral syn and racemic anti isomers of complexes 1–3 were prepared by treating Pd(OAc)_2 with the corresponding N,N'-bis(β-hydroxyarylmethylene)-1,5-pentanediamine and then subjecting the mixture to chromatographic separation. Optically pure (100% ee) complexes, (+)-anti-1, (+)-anti-2, and (+)-anti-3, were obtained from the racemic mixture by employing a preparative HPLC system with a chiral column. The trans coordination and clothespinshaped structures with syn and anti conformations of these complexes have been unequivocally established by X-ray diffraction studies. ~1H NMR analysis showed that (±)-anti-1, (±)-anti-2, syn-2, and (±)-anti-3 display a flapping motion by consecutive stacking association/dissociation between cofacial coordination planes in [D_8]toluene, whereas syn-1 and syn-3 are static under the same conditions. The activation parameters for the flapping motion (△H~# and DS~#) were determined from variable-temperature NMR analyses as 50.4 kJmol~(-1) and 60.1 Jmol~(-1)K~(-1) for (±)-anti-1, 31.0 kJmol~(-1) and -22.7 Jmol~(-1)K~(-1) for (±)-anti-2, 29.6 kJmol~(-1) and -57.7 Jmol~(-1)K~(-1) for syn-2, and 35.0 kJmol~(-1) and 0.5 Jmol~(-1)K~(-1) for (±)-anti-3, respectively. The molecular structure and kinetic parameters demonstrate that all of the anti complexes flap with a twisting motion in [D8]toluene, although (±)-anti-1 bearing dilated Z-shaped blades moves more dynamically than I-shaped (±)-anti-2 or the smaller (±)-anti-3. Highly symmetrical syn-2 displays a much more static flapping motion, that is, in a see-saw-like manner. In CDCl_3, (±)-anti-1 exhibits an extraordinary upfield shift of the 1H NMR signals with increasing concentration, whereas solutions of (+)-anti-1 and the other syn/anti analogues 2 and 3 exhibit negligible or slight changes in the chemical shifts under the same conditions, which indicates that anti- 1 undergoes a specific heterochiral association in the solution state. Equilibrium constants for the dimerizations of (±)- and (+)-anti-1 in CDCl_3 at 293 K were estimated by curvefitting analysis of the ~1H NMR chemical shift dependences on concentration as 26m~(-1) [K_(D(racemic))] and 3.2m~(-1) [K_(D(homo))], respectively. The heterochiral association constant [K_(D(hetero))] was estimated as 98m~(-1), based on the relationship K_(D(racemic))=1/2 K_(D(homo))+1/4 K_(D(hetero)). An inward stacking motif of interpenetrative dimer association is postulated as the mechanistic rationale for this rare case of heterochiral association.