Syntheses and Characterization of Copper(II) Carboxylate Dimers Formed from Enantiopure Ligands Containing a Strong π···π Stacking Synthon: Enantioselective Single-Crystal to Single-Crystal Gas/Solid-Mediated Transformations

Tri- and tetrafunctional enantiopure ligands have been prepared from 1,8-naphthalic anhydride and the amino acids l-alanine, d-phenylglycine, and l-asparagine to produce (<i>S</i>)-2-(1,8-naphthalimido)propanoic acid (H<b>L</b><sub><b>ala</b></sub>), (<i>R</i>)-2-(1,8-naphthalimido)-2-phenylacetic acid (H<b>L</b><sub><b>phg</b></sub>), and (<i>S</i>)-4-amino-2-(1,8 naphthalimido)-4-oxobutanoic acid (H<b>L</b><sub><b>asn</b></sub>), respectively. Reactions of <b>L</b><sub><b>ala</b></sub><sup><b>–</b></sup> with copper(II) acetate under a variety of solvent conditions has led to the formation and characterization by X-ray crystallography of three similar copper(II) paddlewheel complexes with different axial ligands, [Cu<sub>2</sub>(<b>L</b><sub><b>ala</b></sub>)<sub>4</sub>(THF)<sub>2</sub>] (<b>1</b>), [Cu<sub>2</sub>(<b>L</b><sub><b>ala</b></sub>)<sub>4</sub>(H<b>L</b><sub><b>ala</b></sub>)] (<b>2</b>), and [Cu<sub>2</sub>(<b>L</b><sub><b>ala</b></sub>)<sub>4</sub>(py)(THF)] (<b>3</b>). A similar reaction using THF and <b>L</b><sub><b>phg</b></sub><sup><b>–</b></sup> leads to the formation of [Cu<sub>2</sub>(<b>L</b><sub><b>phg</b></sub>)<sub>4</sub>(THF)<sub>2</sub>] (<b>4</b>). With the exception of a disordered component in the structure of <b>4</b>, the naphthalimide groups in all of these compounds are arranged on the same side of the square, central paddlewheel unit, forming what is known as the chiral crown configuration. A variety of π···π stacking interactions of the 1,8-naphthalimide groups organize all of these complexes into supramolecular structures. The addition of the amide group functionality in the <b>L</b><sub><b>asn</b></sub><sup><b>–</b></sup> ligand leads to the formation of tetrameric [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)(MeOH)] (<b>5</b>), where reciprocal axial coordination of one of the amide carbonyl oxygen atoms between two dimers leads to the tetramer. Extensive supramolecular interactions in <b>5</b>, mainly the π···π stacking interactions of the 1,8-naphthalimide supramolecular synthon, support an open three-dimensional structure containing large pores filled with solvent. When crystals of [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)(MeOH)] are exposed to (<i>S</i>)-ethyl lactate vapor, the coordinated methanol molecule is replaced by (<i>S</i>)-ethyl lactate, bonding to the copper ion through the carbonyl oxygen, yielding [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)((<i>S</i>)-ethyl lactate)] (<b>6</b>) without a loss of crystallinity. With the exception of the replacement of the one axial ligand, the molecular structures of <b>5</b> and <b>6</b> are very similar. In a similar experiment of <b>5</b> with vapors of (<i>R</i>)-ethyl lactate, again a change occurs without a loss of crystallinity, but in this case the (<i>R</i>)-ethyl lactate displaces only slightly more than half of the axial methanol molecules forming [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py){((<i>R</i>)-ethyl lactate)<sub>0.58</sub>(MeOH)<sub>0.42</sub>}] (<b>7</b>). Importantly, in <b>7</b>, the (<i>R</i>)-ethyl lactate coordinates through the hydroxyl group. When crystals of [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)(MeOH)] are exposed to vapors of racemic ethyl lactate, the coordinated methanol molecule is displaced without a loss of crystallinity <i>exclusively</i> by (<i>S</i>)-ethyl lactate, yielding a new form of the tetramer [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)((<i>S</i>)-ethyl lactate)], in which the ethyl lactate in the pocket bonds to the copper(II) ion through the carbonyl oxygen as with <b>6</b>. Exposure of [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py){((<i>R</i>)-ethyl lactate)<sub>0.58</sub>(MeOH)<sub>0.42</sub>}] to <i>racemic</i> ethyl lactate yields a third form of [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)((<i>S</i>)-ethyl lactate)], where the three forms of [Cu<sub>4</sub>(<b>L</b><sub><b>asn</b></sub>)<sub>8</sub>(py)((<i>S</i>)-ethyl lactate)] have differences in the number of ordered (<i>S</i>)-ethyl lactate molecules located in the interstitial sites. These results demonstrate enantioselective bonding to a metal center in the chiral pocket of both <b>5</b> and <b>7</b> during single-crystal to single-crystal gas/solid-mediated exchange reactions.