Solvent effects on Co-59 chemical shifts for tripositive Cobalt(III) complex ions

Interactions of tripositive cobalt(III) complex ions, such as Delta(lel(3))-[Co(chxn)(3)](3+) (chxn = (R,R)-1,2-cyclohexanediamine), [Co(oct-en)(2,3,2-tet)](3+) (oct-en = N-octylethylenediamine, 2,3,2-tet = 3,7-diazanonane-1,9-diamine), [Co(sep)(3)](3+) (sep = sepulchrate = 1,3,6,8,10,13,16, 19-octaazabicyclo[6.6.6]eicosane), and [Co(tn)(3)](3+) (tn = trimethylenediamine), in various solvents were studied using Co-59 chemical shifts together with the electronic absorption spectra. The results are discussed in comparison with those of [Co(en)(3)](3+), whose Co-59 chemical shift in various solvents had been previously reported to be fairly proportional to the donor numbers. There was a trend that the Co-59 chemical shift changes greatly if the acidic NH-hydrogen atoms of the ligands are favorable to form hydrogen bonding with the solvents. The magnitudes of the chemical-shift changes for [Co(tn)(3)](3+) were significantly smaller than those for the en and chxn complexes. This difference can be attributed to the more conformationally labile structure of the tn complex in solution.