RESUMEN
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 (HL(ala)), (R)-2-(1,8-naphthalimido)-2-phenylacetic acid (HL(phg)), and (S)-4-amino-2-(1,8 naphthalimido)-4-oxobutanoic acid (HL(asn)), respectively. Reactions of L(ala)(-) 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(2)(L(ala))(4)(THF)(2)] (1), [Cu(2)(L(ala))(4)(HL(ala))] (2), and [Cu(2)(L(ala))(4)(py)(THF)] (3). A similar reaction using THF and L(phg)(-) leads to the formation of [Cu(2)(L(phg))(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 L(asn)(-) ligand leads to the formation of tetrameric [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(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 [Cu(4)(L(asn))(8)(py){((R)-ethyl lactate)(0.58)(MeOH)(0.42)}] to racemic ethyl lactate yields a third form of [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)], where the three forms of [Cu(4)(L(asn))(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.
RESUMEN
An enantiopure ligand built from connecting the π···π stacking 1,8-naphthalimide supramolecular synthon with L-asparagine, L(asn)(-), forms tetrameric [Cu(4)(L(asn))(8)(py)(MeOH)]. The methanol ligand, located in a chiral pocket, is replaced enantioselectively when exposed to racemic ethyl lactate vapor to yield [Cu(4)(L(asn))(8)(py)((S)-ethyl lactate)], in a single-crystal to single-crystal gas/solid transformation.
RESUMEN
Enantiopure, trifunctional carboxylate ligands synthesized by linking the strong π · · · π stacking 1,8-naphthalimide supramolecular synthon to three naturally occurring amino acids using the azide/alkyne click reaction have been prepared [amino acid = glycine (L(gly)(-)), alanine (L(ala)(-)), and serine (L(ser)(-))]. These ligands have been used to form complexes of the formula [M(L(amino acid))2(4,4'-bipy)(H2O)2] · xH2O (M = Fe, Co, Ni, Cu, Zn; x = 4.25-5.52) when mixed with an appropriate metal salt and 4,4'-bipyridine by layering methods. These complexes are isostructural, with the central metal atom coordinated to two κ(1)-carboxylate ligands, two water molecules, and one end each of two 4,4'-bipyridine ligands in a distorted octahedral environment. Each ligand is oriented in a trans arrangement. These complexes all have homochiral, helical, supramolecular, three-dimensional metal-organic framework structures, with the helical organization of the individual metallic units held together solely by strong, noncovalent π · · · π stacking interactions of the naphthalimide; the other two dimensions are organized mainly by the bipyridine ligands. The helices are extremely large; one turn of the helix travels â¼ 60 Å and has a diameter of ca. 40 Å. For the achiral ligand L(gly)(-), the nickel complex forms two types of homochiral crystals in the same tube, a clear example of spontaneous resolution. Despite the large size of the individual helices, they are tightly interconnected and nestled closely together. Part of the interconnection comes from the interstitial water molecules held inside the framework of the complexes in isolated pockets by hydrogen-bonding interactions. For both [Cu(L(ala))2(4,4'-bipy)(H2O)2] · 4.25H2O and [Co(L(ser))2(4,4'-bipy)(H2O)2] · 4.68H2O, the interstitial water molecules can be removed by placing the crystals under a vacuum for several hours, a process that can be reversed by exposure to atmospheric moisture. This removal/reintroduction of the interstitial water molecules takes place with no loss of crystallinity, representing dramatic examples of single-crystal to single-crystal transformations. The structures undergo little change other than the pockets holding the interstitial water molecules in the hydrated complexes become void spaces in the dehydrated complexes. The removal/reintroduction of the water molecules in these closely packed solids is facilitated by the "soft" π · · · π stacking interactions organizing one dimension of the structures. The observed magnetic and Mössbauer spectral properties are typical of isolated, magnetically dilute, paramagnetic pseudooctahedral divalent transition-metal complexes.