Potentiometric Titrations and Nickel(II) Complexes of Four Topologically Constrained Tetraazamacrocycles

Abstract

The novel high spin Ni2+ complexes of the topologically constrained tetraazamacrocycles (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-dimethyl-1,4,7,10-tetraazabicyclo[5.5.2]tetradecane (3); racemic-4,5,7,7,11,12,14,14-octamethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (4)] show striking 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 affinities, complexation reactions in the absence of protons successfully yielded 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 enforce a distorted octahedral geometry on Ni2+ with two cis sites occupied by other ligands. Magnetic measurements and electronic spectroscopy on the corresponding Ni(L)Cl2 (L = 1–3) complexes reveal that all are high spin and six-coordinate with typical magnetic moments. In contrast, [Ni(4)Cl+] is five-coordinate with a slightly higher magnetic moment and its own characteristic 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.