大環變形對三價鐵-(Oi-PTPP)Cl及三價鐵-(OETPP)CN卟啉電子組態的影響

Abstract

Metalloporphyrin compounds are well known to be a considerable of models in the active site of hemopoteins with respect to the interpretations of the biological activity and electronic configuration of central metal. Recently, a series of nonplanar heme were identified by crystal structures or molecular dynamics calculations, which infer the universality of the porphyrin ring deformation extensively existed in nature. Octaethyltetraphenylporphyrin (OETPP) was the early noticed nonplanar porphyrin with saddle-shaped deformation. Interestingly, its corresponding FeIII(OETPP)Cl is quantum-mixed intermediate spin state, which is very common for cytochrome c', an electron transfer protein, but different to the planar iron(III) porphyrin such as FeIII (TPP)Cl and FeIII (OEP)Cl with pure high spin state ( S=5/2 ).

In this study, we report the synthesis and characterization of two five coordinate FeIII(OETPP)CN and FeIII(OiPTPP)Cl complexes. We employed spectroscopic techniques, including 1H NMR, 13C NMR, EPR, DFT theoretical calculations and the measurement of magnetic moment, to characterize and study the spin states with regard to porphyrin ring deformation. FeIII(OETPP)CN shows the reduced 1H NMR extent of FeIII(OETPP)Cl, ranging from 0 ppm to 25 ppm; the 13C NMR chemical shift of axial CN- ligand in a characteristic position of 13037 ppm, and the rest carbon peaks of the OETPP spread from -20 ppm to 400 ppm. The 77K EPR spectra are incompatible between pure solids, inclined to S = 3/2 and in the solvents, such as CH2Cl2 or CHCl3, with S = 1/2. This unusual conflict was proposed that the reassembly of axial CN- ligand and FeIII(OETPP) forms 6-coordinate FeIII(OETPP)(CN)2 in low temperature. The DFT calculations shows the diagnostic nuclear spin density which is proportional to the Fermi contact shift, the main contribution of paramagnetic NMR, with varied S = 3/2, 4A2 (dxy)2(dxz,yz)2(dz2)1, 4B1 (dxy)1(dxz2,yz1)(dz2)1,and 4B2 (dxy)1(dxz1,yz2)(dz2)1 and show the 4A2 most consistent to both of 1H NMR and 13C NMR data.

Octaisopropyltetraphenylporphyrin ( OiPTPP ) is designed for more saddle-shaped deformation. The 1H NMR spectrum of FeIII(OiPTPP)Cl shows the upfiledward shifts comparing to FeIII(OETPP)Cl or FeIII(OMTPP)Cl. 77K EPR shows an axial symmetry spectrum with g⊥= 4, g// =2. These signals show Fe(OiPTPP)Cl a pure intermediate-spin state (S=3/2). In the lack of single crystal structure of Fe(OiPTPP)Cl, we adopt structure optimization by ADF calculation. The energy of intermediate-spin state (S=3/2) is the lowest than high-spin (S=5/2) and low-spin (S=1/2) , showing S=3/2 is a ground state for Fe(OiPTPP)Cl. Additionally, the magnitude of saddle-shaped deformation of Fe(OiPTPP)Cl is larger than Fe(OETPP)Cl and Fe(OMTPP)Cl crystal structure. Molecular orbital analyses further present when porphyrin ring symmetry lower to C2v and consequently the dx2-y2 and dz2 have same symmetry (a1) to mix with porphyrin a2u orbital. This orbital interaction will not only raise the dx2-y2 orbital higher than planar porphyrin but also enlarge the gap between dx2-y2 and dxy, which stabilizes the S = 3/2 spin state accordingly.