71155-87-8Relevant articles and documents
Carbon-13 and deuterium NMR spectroscopy of high-spin manganese(III) porphyrin halide and pyridine complexes
Goff, Harold M.,Hansen, Andrew P.
, p. 321 - 326 (2008/10/08)
Carbon-13 NMR spectroscopic measurements have been performed for high-spin manganese(III) porphyrins to evaluate effects of axial ligand binding and to correlate isotropic shift patterns with d-orbital occupation. A qualitative description of unpaired spin delocalization mechanisms is offered. No particular ordering of resonances is apparent for F-, Cl-, and I- adducts, but absolute shift values for the F- complex are larger and approach those for the stronger field 4-methylpyridine ligand. Resonances for α-pyrrole carbon atoms are downfield and cover a range from 383 to 492 ppm. Corresponding β-pyrrole carbon signals are upfield in the region from -72 to -166 ppm. The meso carbon signal exhibits a small upfield shift, which increases in magnitude with 4-methylpyridine displacement of the halide ligand. Previously elucidated carbon-13 shift correlations are consistent with predominant unpaired π-spin density at β-pyrrole and meso carbon sites of manganese(III) porphyrins. Negative π-spin density at the meso carbon atom is to be contrasted with earlier Hu?ckel calculations that predict large positive spin density at this position. Deuterium NMR spectroscopy revealed large downfield shifts for α-and β-deuterium atoms of coordinated pyridine-d5. Corresponding carbon-13 signals are also far downfield. These observations are readily explained by transmission of σ-spin density from the singly occupied dz2 orbital to the axial pyridine ligand.
