Effects of porphyrin deformation on the 13C and 1H NMR chemical shifts in high-spin five- and six-coordinate manganese(III) porphyrin complexes
Abstract
As an extension of our study to reveal the effect of porphyrin deformation on the 13C and 1H NMR chemical shifts, both five- and six-coordinate high-spin (S = 2) Mn(III) complexes such as Mn(Por)Cl and [Mn(Por)(CD3OD)2]Cl have been prepared, where Por is a porphyrin dianion such as TPP, OMTPP, and TiPrP. Molecular structures of five-coordinate Mn(OMTPP)Cl and Mn(TiPrP)Cl have been determined by the X-ray crystallographic analysis. As expected, Mn(OMTPP)Cl and Mn(TiPrP)Cl have exhibited a highly saddled and highly ruffled porphyrin core, respectively. The 13C NMR spectra have revealed that these complexes generally exhibit the α-pyrrole signals at the downfield positions and β-pyrrole an. meso signals at the upfield positions. The results suggest that the spin polarization of Mn(III)–NP σ bonds, which occurs in all the high-spin Mn(III) complexes, is the major factor to determine the chemical shifts of the porphyrin carbon signals (Cheng, R.-J.; Chang, S.-H.; Hung, K.-C. Inorg. Chem. 2007; 46: 1948–1950). Although th. meso and α-pyrrole signals are observed at the upfield and downfield positions, respectively, these signals are widely dispersed depending on the deformation mode of the porphyrin ring. The results have been explained in terms of the strong spin polarization of the Mn–NP bond together with the specific metal-porphyrin orbital interactions such as: (i) the a2u-dz2 interaction in five-coordinate complexes, (ii) the a2u-dxy interaction in ruffled complexes, and (iii) the a2u-dx2-y2 interaction in saddled complexes.
Dedicated to Professor Kevin M. Smith on the occasion of his 70th birthday and for his 50 years contribution to the porphyrin chemistry
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