pH‐Responsive Switching Properties of a Water‐Soluble Metallamacrocyclic Phenylalaninehydroximate La(III)–Cu(II) Complex: Insight into Tuning Protonation Ligand States

Abstract

The water-soluble copper(II)-lanthanum(III) complex based on phenylalaninehydroximate ligands La(H 2 O) 4 -[15-MC CuPhalaha -5](Cl) 3 (1) has been synthesized and characterized structurally and spectroscopically. The X-ray structure of 1 demonstrated that the ring copper ions are placed in distorted square pyramidal geometries with various axial coordination ligands. UV/Vis absorption spectra reveal halochromic behavior of 1 in the aqueous solution which represents reversible color changes from blue to colorless in response to pH variations. This effect has been described as pH-induced "on-off-on" switch through protonation and deprotonation of the phenylalanine-[a] G. A. Razuvaev

Water-Soluble Bismuth(III) Polynuclear Tyrosinehydroximate Metallamacrocyclic Complex: Structural Parallels to Lanthanide Metallacrowns

“…Energies of copper(II) bonds with the oxime and carbonyl oxygen atoms are closer: 35.9–40.4 and 46.3–47.0 kcal/mol, respectively. In general, the bonding situation in the hydroximate chelates of the [M(15-MC Cu(II)Tyrha -5)] 3+ complexes is similar to that in the 15-metallacrowns-5 analyzed by us earlier with use of QTAIM in conjunction with the hybrid functionals τ-HCTHhyb [ 33 ] and M06 [ 29 , 42 , 43 ]. This makes the SR-PBE/rL2 calculations appropriate for description of the hydroximate 15-metallacrowns-5.…”

“…This leads to the more effective bonding with the HO − negative ion in 1 (as well with Cl − in the pyrazinohydroximate complex [ 29 ]). As a result, the Bi 3+ ion in the complex is characterized by the lower coordination number as compared with the La(III) 15-metallacrowns-5 where the La 3+ central ion bears four H 2 O molecules at the axial positions [ 33 ]. Accordingly, the higher DED accumulation in the axial positions of the Bi 3+ ion induces weakening of the Bi-O ox donor-acceptor interactions in comparison with the La-O ox contacts that has been described above.…”

“…The Bi 3+ ion is located at the center of the 15-MC-5 ring consisting of five [Cu(II)-N-O] repeating units. Following our interest in water-soluble Ln(III)-Cu(II) 15-MC-5 metallacrowns with aminohydroximate ligands [ 30 , 31 , 32 , 33 , 34 ], we describe here a general synthetic approach and comparable characterization of a new water-soluble metallamacrocyclic Bi(III)-Cu(II) 15-MC-5 complex derived from tyrosinehydroxamic acid.…”

Praseodymium Metallacrown-Based NMR Probe for Enantioselective Discrimination of Mandelate Anions in Water

“…The largest green-colored area reveals NCI between the MA phenyl ring and the metallacrown fragment. The red-orange coin-like domain arises from the stronger La–O­(oxime) ionic interaction typical for the bonding between a rare earth ion and the metallacrown scaffold. ,, VdW interactions of CH 3 (MC) and Ph (MA) in 5 are visualized as a small coin-like domain located between the interacting hydrogen atoms. Interestingly, the shorter distance between CH 3 (MC) and Ph (MA) in 5 (2.196 Å) by comparison with 4 (2.851 Å) does not lead to a larger RDG area.…”

“…Among the numerous metallacrown-based compounds, the family of 15-MC-5 includes intriguing examples of water-soluble polynuclear metallamacrocyclic lanthanide complexes. − In these water-soluble Ln­(III)–Cu­(II) aminohydroximate metallacrowns (MC), the nearly planar neutral metallamacrocycle bearing five [Cu­(II)–N–O] repeat units surrounds the central Ln 3+ ion, whereas the open axial site of the lanthanide coordination sphere is filled with weakly bound inner-shell water molecules. − Due to this unique structural feature the family of tetraaqua Gd­(III) 15-MC-5 metallacrowns has been suggested as a model for design of efficient high-field MRI contrast agents. , It should be noted that the paramagnetic lanthanide systems have traditionally been investigated by NMR spectroscopy, starting with the classic “lanthanide shift reagents.” − However, there are very few papers devoted to 15-MC-5 complexes, which can act as shift agents. ,, The use of the Ln­(III)–Cu­(II) 15-MC-5 complexes as lanthanide shift reagents is problematic because complicated exchange interactions between five paramagnetic Cu­(II) ions and Ln­(III) causes severe line broadening. Nevertheless, recently we have demonstrated that the Pr­(III) and Nd­(III) metallacrowns can exhibit clearly defined comparatively narrow NMR signals. , This can be applied to chiral recognition of enantiomers using chiral 15-MC-5 synthesized according to a simple protocol. , Since elucidation of the metallacrown structure more than 30 years ago by Pecoraro and co-workers, it has been established that chiral metallacrowns can be easily prepared by the reaction of resolved amino hydroxamic acids (e.g., l -alanine-, l -phenylalanine-, and l -tyrosine hydroxamic acid). , The beauty of this strategy lies in the judicious choice of a side chain functional group that can lead to the formation of a chiral metal crown.…”