Mono-, di-, tetra- and heptanuclear copper(II) complexes with 4-(3,5-di-R-1H-pyrazol-1-yl)-6-methyl-2-(pyridin-2-yl)pyrimidines (R = H, Me): Syntheses, crystal structures and electrospray ionization mass spectrometry

A series of mono- and oligonuclear copper(II) complexes with new hybrid NNN-tridentate 4-(1H-pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands, 4-(1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L1) and 4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L2), have been synthesized. The reaction of CuCl2 with L1 in EtOH in the 1:1 metal-to-ligand molar ratio affords the mononuclear complex [CuL1Cl2]·0.5H2O (1). An increase of the metal-to-ligand molar ratio up to 4:1 as well as the use of less polar mixed solvent, EtOH/CHCl3, leads to mononuclear complex [CuL1Cl2] (2). Attempts to crystallize di- or oligonuclear copper(II) complex with L1 (the 8:1 metal-to-ligand molar ratio, EtOH/CHCl3) resulted in the formation of ionic halocuprate compound, [Cu7L14Cl12][Cu2Cl6] (3), containing unique heptanuclear cation, [Cu7L14Cl12]2+, and dinuclear anion [Cu2Cl6]2−. The reaction of CuCl2 with L2 in EtOH in the 1:1 metal-to-ligand molar ratio affords mononuclear complex [CuL2Cl2] (4). Di- and tetranuclear complexes, [Cu2L2Cl4] (5) and [Cu4L22Cl8] (6), can be crystallized from ethanolic solution when the 2:1 metal-to-ligand molar ratio is used. The former is the product of kinetic control, while the latter is that of thermodynamic one. Being coordinated, the molecules of L1 and L2 adopt tridentate chelating coordination mode binding copper atoms through pyrazolyl N2, pyrimidine N3 and pyridinyl N atoms and forming thus two five-membered chelate rings (CuN3C and CuN2C2). Depending on the metal-to-ligand molar ratio in the solution, electrospray ionization mass spectrometry (ESI-MS) evidenced the presence of mono- and oligonuclear copper species in the CuCl2–L1 and CuCl2–L2 solutions. The isolation of oligonuclear species in the solid state seems to be related with the absence of hydrogen-bond donor groups in the molecules of 4-(1H-pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands.