Nickel(II)-Dipeptidoamine-Based Tetrameric Complex: Structural Study in Solution and in Solid State

The coordination structure of M4L4H−8 macromolecules (M = Ni(II), Cu(II), Pd(II)) containing small peptidic ligands (L = Xaa-His or Xaa-His-Yaa) has been predicted primarily on the basis of spectroscopic and potentiometric data in the literature. In this work, the neutral tetranuclear nickel(II) complex 1 formed with four double-deprotonated ligands (L = α-methyl-alanyl-histamine) was prepared, and its crystal structure was determined (C36H56N16Ni4O4·4.5CH3OH·1.5H2O: a = 11.2645(4) Å, b = 23.5003(8) Å, c = 20.9007(7) Å, β = 102.321(1)°, monoclinic, P21/c, Z = 4). In complex 1, the metal ions have a square planar geometry with 4N donor set consisting of the N-terminal amino nitrogen, the deprotonated amide nitrogen, the imidazole N3 atom, and the deprotonated imidazole N1 atom of the adjacent ligand. The latter nitrogen atom provides the connection of the four NiLH−2 units forming a C1 symmetrical saddle-like shape. The complexation of L with Ni(II) ion has been studied by a potentiometric method combined with UV−visible spectrophotometric titration. At pH 8.0, the predominant species is M4L4H−8 with pK4oligomerization = 5.73. The tetranuclear structure of complex 1 was also studied in solution by 1H and 13C NMR spectroscopy suggesting a structure of symmetry S4. DFT calculations on optimized structure in symmetry C1 and S4 have been performed to explain the observed differences in solution and in solid state. The nuclearity was also confirmed in solution by ESI-HRMS analysis.