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

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(NO3)[15-MC-5]−}+ and {La(NO3)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.