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. Issue 23 (17th April 2014)

• Record Type: Journal Article
• Authors: Naito, Masaya; Souda, Hiroyuki; Koori, Hiroshi; Komiya, Naruyoshi; Naota, Takeshi
• Is Part Of: Chemistry. Volume 20:Issue 23(2014)
• Page Start: 6991
• Page End: 7000
• Publication Date: 2014-04-17
• Subjects: Chemistry -- Periodicals; 540
• DOI: 10.1002/chem.201305054
• Languages: English
• ISSNs: 0947-6539

Abstract

<abstract abstract-type="main" xml:lang="en"> <title>Abstract</title> <p>The synthesis, structure, and solution‐state behavior of clothespin‐shaped binuclear <italic>trans</italic>‐bis(β‐iminoaryloxy)palladium(II) complexes doubly linked with pentamethylene spacers are described. Achiral <italic>syn</italic> and racemic <italic>anti</italic> isomers of complexes <bold>1</bold>–<bold>3</bold> were prepared by treating Pd(OAc)<sub>2</sub> with the corresponding <italic>N</italic>, <italic>N′</italic>‐bis(β‐hydroxyarylmethylene)‐1, 5‐pentanediamine and then subjecting the mixture to chromatographic separation. Optically pure (100 % <italic>ee</italic>) complexes, (+)‐<italic>anti</italic>‐<bold>1</bold>, (+)‐<italic>anti</italic>‐<bold>2</bold>, and (+)‐<italic>anti</italic>‐<bold>3</bold>, were obtained from the racemic mixture by employing a preparative HPLC system with a chiral column. The <italic>trans</italic> coordination and clothespin‐shaped structures with <italic>syn</italic> and <italic>anti</italic> conformations of these complexes have been unequivocally established by X‐ray diffraction studies. <sup>1</sup>H NMR analysis showed that (±)‐<italic>anti</italic>‐<bold>1</bold>, (±)‐<italic>anti</italic>‐<bold>2</bold>, <italic>syn</italic>‐<bold>2</bold>, and (±)‐<italic>anti</italic>‐<bold>3</bold> display a flapping motion by consecutive stacking association/dissociation between cofacial coordination planes in [D<sub>8</sub>]toluene, whereas <italic>syn</italic>‐<bold>1</bold> and <italic>syn</italic>‐<bold>3</bold> are static under the same conditions. The activation parameters for the flapping motion (Δ<italic>H</italic><sup>≠</sup> and Δ<italic>S</italic><sup>≠</sup>) were determined from variable‐temperature NMR analyses as 50.4 kJ mol<sup>−1</sup> and 60.1 J mol<sup>−1</sup> K<sup>−1</sup> for (±)‐<italic>anti</italic>‐<bold>1</bold>, 31.0 kJ mol<sup>−1</sup> and −22.7 J mol<sup>−1</sup> K<sup>−1</sup> for (±)‐<italic>anti</italic>‐<bold>2</bold>, 29.6 kJ mol<sup>−1</sup> and −57.7 J mol<sup>−1</sup> K<sup>−1</sup> for <italic>syn</italic>‐<bold>2</bold>, and 35.0 kJ mol<sup>−1</sup> and 0.5 J mol<sup>−1</sup> K<sup>−1</sup> for (±)‐<italic>anti</italic>‐<bold>3</bold>, respectively. The molecular structure and kinetic parameters demonstrate that all of the <italic>anti</italic> complexes flap with a twisting motion in [D<sub>8</sub>]toluene, although (±)‐<italic>anti</italic>‐<bold>1</bold> bearing dilated Z‐shaped blades moves more dynamically than I‐shaped (±)‐<italic>anti</italic>‐<bold>2</bold> or the smaller (±)‐<italic>anti</italic>‐<bold>3</bold>. Highly symmetrical <italic>syn</italic>‐<bold>2</bold> displays a much more static flapping motion, that is, in a see‐saw‐like manner. In CDCl<sub>3</sub>, (±)‐<italic>anti</italic>‐<bold>1</bold> exhibits an extraordinary upfield shift of the <sup>1</sup>H NMR signals with increasing concentration, whereas solutions of (+)‐<italic>anti</italic>‐<bold>1</bold> and the other <italic>syn</italic>/<italic>anti</italic> analogues <bold>2</bold> and <bold>3</bold> exhibit negligible or slight changes in the chemical shifts under the same conditions, which indicates that <italic>anti‐</italic><bold>1</bold> undergoes a specific heterochiral association in the solution state. Equilibrium constants for the dimerizations of (±)‐ and (+)‐<italic>anti</italic>‐<bold>1</bold> in CDCl<sub>3</sub> at 293 K were estimated by curve‐fitting analysis of the <sup>1</sup>H NMR chemical shift dependences on concentration as 26 <sc>M</sc><sup>−1</sup> [<italic>K</italic><sub>D(racemic)</sub>] and 3.2 <sc>M</sc><sup>−1</sup> [<italic>K</italic><sub>D(homo)</sub>], respectively. The heterochiral association constant [<italic>K</italic><sub>D(hetero)</sub>] was estimated as 98 <sc>M</sc><sup>−1</sup>, based on the relationship <italic>K</italic><sub>D(racemic)</sub>=1/2 <italic>K</italic><sub>D(homo)</sub>+1/4 <italic>K</italic><sub>D(hetero)</sub>. An inward stacking motif of interpenetrative dimer association is postulated as the mechanistic rationale for this rare case of heterochiral association.</p> </abstract>