Sucrose bis(1,10-phenanthroline) cobalt(III). Comparison of semi-empirical and ab initio geometrical optimizations
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The title compound (sucrose Co(phen)(2)) is a dication with the Delta-configuration at Co(III) and ligation through O-2(g) and OH-1(f). Earlier geometrical optimization with semi-empirical PM3(tm) parameters had fitted most of the structural evidence from absorption, CD and NMR spectroscopy, although the site of deprotonation was predicted to be OH-1(f) rather than OH-2(g) as shown by NMR spectroscopy. Both HF and DFT treatments have now been used and they correctly predict deprotonation of OH-2(g), in agreement with experimental data. The HF 3-21(G*) and 6-31(G*) treatments incorrectly show OH-1(f) as planar, although it is pyramidal from H-1 NMR coupling constants, but a DFT LACVP*/BP/6-31(G*) optimization gives the correct geometry. The earlier PM3(tm) optimization indicated extensive bowing of both phenanthrolines due to steric repulsions, but the ab initio treatments indicate limited bowing which fits the NMR chemical shifts of the phenanthroline hydrogens.