Spin−Orbit-Induced Anomalous pH-Dependence in 1H NMR Spectra of CoIII Amine Complexes: A Diagnostic Tool for Structure Elucidation

The pH-dependent 1H NMR characteristics of a series of CoIII-(polyamin)-aqua and CoIII-(polyamin)-(polyalcohol) complexes, [Co(tach)(ino-κ3-O1,3,5)]3+ (13+), [Co(tach)(ino-κ3-Ο1,2,6)]3+ (23+), [Co(tach)(taci-κ-Ν1-κ2-O2,6)]3+ (33+), [Co(ditame)(H2O)]3+ (43+), and [Co(tren)(H2O)2]3+ (53+), were studied in D2O by means of titration experiments (tach = all-cis-cyclohexane-1,3,5-triamine, ino = cis-inositol, taci = 1,3,5-triamino-1,3,5-trideoxy-cis-inositol, tren = tris(2-aminoethyl)amine, ditame = 2,2,6,6-tetrakis-(aminomethyl)-4-aza-heptane). A characteristic shift was observed for H(−C) hydrogen atoms in the α-position of a coordinated amino group upon deprotonation of a coordinated oxygen donor. For a cis-H−C−N−Co−O−H arrangement, deprotonation of the oxygen donor resulted in an additional shielding (shift to lower frequency) of the H(−C) proton, whereas for a trans-H−C−N−Co−O−H arrangement, deprotonation resulted in a deshielding (shift to higher frequency). The effect appears to be of rather general natu...