Two novel homodinuclear Ni(II) and Cu(II) complexes with a 24-membered octadentate hexaazamacrocyclic ligand: stability and X-ray crystal structures

A dinucleating 24-membered hexaazadiphenol macrocyclic ligand, 3,6,9,17,20,23-hexaaza-29,30-dihydroxy-13,27-dimethyl-tricyclo[23,3,1,111,15]triaconta-1(28),11,13,15(30),25,26-hexaene (H2L or [24]BDBPH), is prepared by the NaBH4 reduction of the Schiff base obtained from [2+2] template condensation of 2,6-diformyl-p-cresol with diethylenetriamine. The ligand maintains dinuclear integrity for copper(II) and nickel(II) complexes, while facilitating the formation of non-bridged dicopper and bridged phenolic dinickel cores. Potentiometric equilibrium studies indicate that a variety of protonated, mononuclear and dinuclear complexes are formed with nickel(II) from p[H] 2 through 11 in aqueous solution. The stability constants and species distribution as a function of p[H] of the 1:1 and 1:2 [ligand:Ni(II)] complexes were determined in KCl supporting electrolyte (μ=0.100 M) at 25°C. The pH potentiometric studies show that the dinuclear complexes are formed via mononulear chelates in which two kinds of coordination patterns are proposed. Copper is complexed by half of the coordination sites of the dinucleating macrocycle (N3O−), and the nickel occupies salen-like sites of the macrocycle (N2O2 −), which form dibridged dinuclear complexes. The neutral homodinuclear complex [Cu2L](Cl)(Br) forms monoclinic crystals, of space group P21/c, with a=9.9125(17), b=13.559(2), c=22.713(4) Å, β=98.145(4)°, and Z=4. A dinulear Ni(II) complex forms with the formula [Ni2L(H2O)2](Br2)·2H2O. Dibridged phenolic oxygen atoms were observed. The complex crystallizes in monoclinic form, space group P21/c, with a=18.262(2), b=11.7520(15), c=15.755(2) Å, β=107.817(2)°, and Z=4.