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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

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journal contribution
posted on 17.10.2011, 00:00 by Daniel L. Reger, Jacob J. Horger, Agota Debreczeni, Mark D. Smith
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 (S)-2-(1,8-naphthalimido)propanoic acid (HLala), (R)-2-(1,8-naphthalimido)-2-phenylacetic acid (HLphg), and (S)-4-amino-2-(1,8 naphthalimido)-4-oxobutanoic acid (HLasn), respectively. Reactions of Lala 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, [Cu2(Lala)4(THF)2] (1), [Cu2(Lala)4(HLala)] (2), and [Cu2(Lala)4(py)(THF)] (3). A similar reaction using THF and Lphg leads to the formation of [Cu2(Lphg)4(THF)2] (4). With the exception of a disordered component in the structure of 4, 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 Lasn ligand leads to the formation of tetrameric [Cu4(Lasn)8(py)(MeOH)] (5), where reciprocal axial coordination of one of the amide carbonyl oxygen atoms between two dimers leads to the tetramer. Extensive supramolecular interactions in 5, 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 [Cu4(Lasn)8(py)(MeOH)] are exposed to (S)-ethyl lactate vapor, the coordinated methanol molecule is replaced by (S)-ethyl lactate, bonding to the copper ion through the carbonyl oxygen, yielding [Cu4(Lasn)8(py)((S)-ethyl lactate)] (6) without a loss of crystallinity. With the exception of the replacement of the one axial ligand, the molecular structures of 5 and 6 are very similar. In a similar experiment of 5 with vapors of (R)-ethyl lactate, again a change occurs without a loss of crystallinity, but in this case the (R)-ethyl lactate displaces only slightly more than half of the axial methanol molecules forming [Cu4(Lasn)8(py){((R)-ethyl lactate)0.58(MeOH)0.42}] (7). Importantly, in 7, the (R)-ethyl lactate coordinates through the hydroxyl group. When crystals of [Cu4(Lasn)8(py)(MeOH)] are exposed to vapors of racemic ethyl lactate, the coordinated methanol molecule is displaced without a loss of crystallinity exclusively by (S)-ethyl lactate, yielding a new form of the tetramer [Cu4(Lasn)8(py)((S)-ethyl lactate)], in which the ethyl lactate in the pocket bonds to the copper(II) ion through the carbonyl oxygen as with 6. Exposure of [Cu4(Lasn)8(py){((R)-ethyl lactate)0.58(MeOH)0.42}] to racemic ethyl lactate yields a third form of [Cu4(Lasn)8(py)((S)-ethyl lactate)], where the three forms of [Cu4(Lasn)8(py)((S)-ethyl lactate)] have differences in the number of ordered (S)-ethyl lactate molecules located in the interstitial sites. These results demonstrate enantioselective bonding to a metal center in the chiral pocket of both 5 and 7 during single-crystal to single-crystal gas/solid-mediated exchange reactions.