Potentiometric titrations and nickel(II) complexes of four topologically constrained tetraazamacrocycles

The novel high spin Ni2+ complexes of the topologically constrained tetraaz amacrocycles (1-4) [4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (1); 4,10-dimethyl-1,4,7,10-tetraazabicyclo[6.5.2]pentadecane (2); 4,10-di methyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3); racemic-4,5,7,7,11,1 2,14, 14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4)] show str iking properties. Potentiometric titrations of the ligands 2 and 4 revealed them to be proton sponges, as reported earlier for 1 [1]. Ligand 3 is less basic, losing its last proton with a pK = 11.3(2). Despite high proton aff inities, complexation reactions in the absence of protons successfully yiel ded Ni2+ complexes in all cases. The X-ray crystal structures of Ni(1)(acac )(+), Ni(3)(acac)+ and Ni(1)(OH2)(2)(2+) demonstrate that the ligands 2 enf orce a distorted octahedral geometry on Ni2+ with two cis sites occupied by other ligands. Magnetic measurements and electronic spectroscopy on the co rresponding Ni(L)Cl-2. (L = 1 - 3) complexes reveal that all are high spin and six-coordinate with typical magnetic moments. In contrast, [Ni(4)Cl+] i s five-coordinate with a slightly higher magnetic moment and its own charac teristic electronic spectrum. The extra methyl groups on ligand 4 define a shallow cavity, sterically allowing only one chloride ligand to bind to the nickel(II) ion.