Clarifying the Mechanism of Cation Exchange in Ca(II)[15-MC_Cu(II)Ligand-5] Complexes

Abstract

The calcium metallacrown Ca(II)[15-MC(Cu(II)N(Trpha))-5](2+) was obtained by self-assembly of Ca(II), Cu(II), and tryptophanhydroxamic acid. Its X-ray structure shows that the core calcium ion is well-encapsulated in the five oxygen cavity of the metallacrown scaffold. The kinetics of Ca-Ln core metal substitution was studied by visible spectrophotometry by addition of Ln(III) nitrate to solutions of Ca(II)[15-MC(Cu(II)N(Trpha))-5](2+) in methanol solution at pH 6.2 (Ln(III) = La(III), Nd(III), Gd(III), Dy(III), Er(III)) to obtain the corresponding Ln(III)[15-MC(Cu(II)N(Trpha))-5](3+) complexes on the hours time scale. The reaction is first order in the two reactants (second order overall) with different rate constants across the lanthanide series. In particular, the rate for the Ca-Ln substitution decreases from La(III) to Gd(III) and then increases slightly from Gd(III) to Er(III). This substitution reaction occurs with second order rate constants ranging from 0.1543(3) M(-1) min(-1) for La(III) to 0.0720(6) M(-1) min(-1) for Gd(III). By means of the thermodynamic log K constants for the same reaction previously reported, the rate constants for the inverse Ln-Ca substitution were also determined. In this study, we demonstrated that the substitution reaction proceeds through a direct metal substitution and does not involve the disassembly of the MC scaffold. These observations in concert allow the proposition of a hypothesis that the dimension of the core metals play the major role in determining the rate constants of the substitution reaction. In particular, the largest lanthanides, which do not require complete encapsulation in the MC cavity, displace the Ca(II) ion faster, whereas in the back reaction Ca(II) displaces the smaller lanthanides faster as they interact relatively weakly with the metallacrown oxygen cavity.

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

“…Nowadays, the popularity of water‐soluble polynuclear metallamacrocyclic Cu(II)‐Ln(III) complexes is largely due to their rich coordination chemistry, diverse properties and ease of synthesis . The remarkable feature of Cu(II)‐Ln(III) 15‐MC‐5 complexes is the deep color . It should be pointed out that the observed ligand‐field bands in visible region near 550 nm have provided the potential ability to obtain useful diagnostic information concerning the structures of transition metal ion‐peptide complexes .…”

Synthesis and Molecular Structures of CuII 15‐Metallacrown‐5 Complexes with Encapsulated CaII, PrIII and NdIII Ions

“…On the other hand, α‐aminohydroxamates, able to form only five‐ membered chelate rings, tend to form pentagonal 15‐MC‐5 structures, where the metallamacrocyclic circuit can be formed with metals as Cu II , Ni II , Zn II , Mn III ,6,9–10 and a central cavity can encapsulate a metal ion with suitable radius (Scheme , right). Up to now heteropolynuclear 15‐MC‐5 complexes containing Ln III ,8–16 Y III , Pb II , Hg II , Ag I , Na I , Ca II , and UO 2 2+ as central cavity ions have been obtained 17–21…”

“…Ni II and Zn II 15‐MC‐5 complexes with Ln III ions in the central cavity have been also synthesized; some of them structurally characterized 18,20. Cu II 15‐MC‐5 complexes with Na I and Ca II in the center of the cavity17,21 have been also reported, however, single‐crystal X‐ray diffraction analysis has not been carried out. Also, heterometallic Cu II 15‐MC‐5 complexes based on o ‐picHA with Pr III and Nd III in the central cavity have not been reported in the literature until now.…”

A Unique LnIII{[3.3.1]GaIII Metallacryptate} Series That Possesses Properties of Slow Magnetic Relaxation and Visible/Near‐Infrared Luminescence

“…Since their discovery in 1989, metallacrowns, inorganic structural analogues to crown ethers, have demonstrated a high potential in numerous applications, including host‐guest binding, gas adsorption, molecular magnetism, and lanthanide(III)‐based luminescence . Metallacrowns possess a high degree of tunability based on the choice of ligand and metal, which uniquely allows for the predictable design of complexes towards a specific application .…”

“…[26] Ligand fieldst hat are axially elongated promote an easya xis for oblate ions, while equatorially expanded ligand fields promote an easy-axis for prolate ions. [26] Since their discovery in 1989, [27,28] metallacrowns, inorganic structurala naloguest oc rown ethers, have demonstrated a high potentiali nn umerousa pplications, including host-guest binding, [29][30][31] gasa dsorption, [32] molecular magnetism, [33][34][35][36][37][38] and lanthanide(III)-basedl uminescence. [9,14,39] Metallacrowns possess ah igh degree of tunability based on the choice of ligand and metal,w hichu niquelya llows for the predictable design of complexes towards as pecific application.…”