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-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands, 4-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L 1 ) and 4-(3,5-dimethyl-1 H -pyrazol-1-yl)-2-(pyridin-2-yl)-6-methylpyrimidine (L 2 ), have been synthesized. The reaction of CuCl 2 with L 1 in EtOH in the 1:1 metal-to-ligand molar ratio affords the mononuclear complex [CuL 1 Cl 2 ]·0.5H 2 O ( 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/CHCl 3 , leads to mononuclear complex [CuL 1 Cl 2 ] ( 2 ). Attempts to crystallize di- or oligonuclear copper(II) complex with L 1 (the 8:1 metal-to-ligand molar ratio, EtOH/CHCl 3 ) resulted in the formation of ionic halocuprate compound, [Cu 7 L 1 4 Cl 12 ][Cu 2 Cl 6 ] ( 3 ), containing unique heptanuclear cation, [Cu 7 L 1 4 Cl 12 ] 2+ , and dinuclear anion [Cu 2 Cl 6 ] 2− . The reaction of CuCl 2 with L 2 in EtOH in the 1:1 metal-to-ligand molar ratio affords mononuclear complex [CuL 2 Cl 2 ] ( 4 ). Di- and tetranuclear complexes, [Cu 2 L 2 Cl 4 ] ( 5 ) and [Cu 4 L 2 2 Cl 8 ] ( 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 L 1 and L 2 adopt tridentate chelating coordination mode binding copper atoms through pyrazolyl N 2 , pyrimidine N 3 and pyridinyl N atoms and forming thus two five-membered chelate rings (CuN 3 C and CuN 2 C 2 ). 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 CuCl 2 –L 1 and CuCl 2 –L 2 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-(1 H -pyrazol-1-yl)-2-(pyridin-2-yl)pyrimidine ligands.