Controlling the self-assembly of homochiral coordination architectures of CuII by substitution in amino acid based ligands: synthesis, crystal structures and physicochemical properties

Abstract

Through the strategic design of ligands based on amino acids, structural diversity in chiral coordination architectures of Cu^II is demonstrated with six new examples: {[Cu(L-HTyrbenz)₂]·CH₃OH·H₂O}_n (1), {[Cu(L-HSerbenz)₂]·3H₂O}_n (2), {[Cu(L-HTyrthio)₂]·H₂O}_n (3), [Cu(L-HTyr4-pyr)₂(H₂O)]·2H₂O (4), [Cu(L-HSerthio)₂(H₂O)] (5), and [Cu(L-Phethio)₂(H₂O)]·3H₂O (6) [where L-H₂Tyrbenz = L-N-(benzyl)-tyrosine, L-H₂Serbenz = L-N-(benzyl)-serine, L-H₂Tyrthio = L-N-(methyl-2-thiophenyl)-tyrosine, L-H₂Tyr4-pyr = L-N-(methyl-4-pyridyl)-tyrosine, L-H₂Serthio = L-N-(methyl-2-thiophenyl)-serine and L-HPhethio = L-N-(methyl-2-thiophenyl)-phenylalanine]. For these 1 : 2 metal–ligand complexes, the availability of a donor atom (either from the phenolic OH group or the carboxylate group of one of the ligands) for bridging between the Cu^II centers results in the formation of coordination polymers (1–3), while no such availability allows a water molecule to occupy the fifth site around the Cu^II center to generate hydrogen bonded supramolecular assemblies (4–6). In 1, a coordination polymer is formed via a syn–anti bridging carboxylate, and the phenolic group has no role in its formation. To further emphasize this point, L-tyrosine in 1 is replaced with L-serine to form 2, in which an anti–anti bridging by the carboxylate group is observed. On the other hand, the formation of {[Cu(L-HTyrthio)₂]·H₂O}_n (3) results from the growth of a spiral polymer via the unique phenolic bridging with a distance of 10.806(9) Å between two Cu^II centers. On changing from the L-H₂Tyrbenz ligand to the L-H₂Tyr4-pyr ligand (1vs.4), the strong hydrogen bonding of the pyridyl nitrogen with the phenolic group does not allow the latter to bind to Cu^II. Similarly, on changing from L-H₂Tyrthio to L-H₂Serthio (3vs.5), the length of the –CH₂OH group in the latter is much less than the distance between the two Cu^II centers, therefore this group cannot occupy the fifth site and thus a water molecule is coordinated. This is further confirmed by reacting 5 with 2 eq. of L-H₂Tyrthio in methanol to form 3, while the reverse is not possible. All these compounds are characterized by a number of analytical methods, such as elemental analysis, FTIR, UV-Vis and circular dichroism spectroscopy, polarimetry, powder and single crystal X-ray diffraction and thermogravimetric analysis. Photoluminescence properties of all ligands containing the L-tyrosine group and their metal complexes (1, 3 and 4) are compared in solution at room temperature.