Copper(II) 15-metallacrown-5 lanthanide(III) complexes derived from l-serine and l-threonine hydroxamic acids

Abstract

Formation of pentanuclear copper(II) complexes with serine hydroxamic acid (Serha) and threonine hydroxamic acid (Thrha) was studied in solution by electrospray ionization mass spectrometry (ESI-MS), absorption spectrophotometry and proton NMR spectroscopy. The presence of lanthanide(III) ions is essential for the self-assembly of the 15-metallacrown-5 compounds. The positive mode ESI-MS spectra of the solutions containing Serha or Thrha, copper(II), and lanthanide(III) ions (Ln = La, Nd, Gd,) in the ratio 5:5:1 showed only the peaks that could be unambiguously assigned to the following intact molecular ions: {La(NO 3 ) − } + and {La(NO 3 ) 2 [15-MC-5]} 2+ . The NMR spectra of the pentanuclear species revealed only one set of peaks indicating a five-fold symmetry of the complex. The UV-vis experiments revealed that upon addition of 0.2 equivalents of lanthanide(III) to the equimolar solutions containing Serha and copper(II), the self-assembly of the metallacrown structure was accomplished within 30 min.

Thermodynamics of Self-Assembly of Copper(II) 15-Metallacrown-5 of Eu(III) or Gd(III) with (S)-α-Alaninehydroxamic Acid in Aqueous Solution

“…The ternary systems Eu 3+ or Gd 3+ /Cu 2+ /α-Alaha were initially studied by in-cell potentiometric titration, but a marked drift in the emf values was observed; thus, batch titrations were performed (see the Experimental Section). This inertness in the Ln 3+ 15-MC-5 formation in water was also recently put into evidence by Pacco and co-workers for ( S )-serine- and ( S )-threoninehydroxamic acids: their spectrophotometric study showed that the formation equilibrium was reached only 30 min after reactant mixing, although we have measured a drift in the emf readings within 12 h . In the present investigation, the titrations were limited to pH 6.5 because at higher pH the formation of an insoluble purple gelatinous material was observed.…”

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 .…”

Elucidation of1H NMR Paramagnetic Features of Heterotrimetallic Lanthanide(III)/Manganese(III) 12-MC-4 Complexes

“…The structural information gleaned from the pseudo-contact shifts can confirm or refute if the solid-state structure is retained in solution or if a series of molecules made with the same ligand set but different lanthanide ions are isostructural in solution. − Almost all studies that have investigated a series of lanthanide complexes rely on complexes that contain one Ln III ion with a diamagnetic, organic ligand. Thus, little is known regarding the 1 H NMR behavior, and, in particular, the pseudo-contact shift properties of Ln III ions in the presence of paramagnetic 3 d transition metals. − Metallacrowns (MC) are a class of molecules that offer a window into such investigations, which may result in unpredicted properties.…”