A continuous wave and pulse electron paramagnetic resonance study of Co(II) (tetraphenylporphyrin) in different matrices

Abstract

The knowledge of the exact electronic structure of Co(II) porphyrins is important in view of their interest as models for the prosthetic groups in proteins and in view of their use as catalysts. Here, we present a continuous wave and pulse electron paramagnetic resonance (EPR) study of CoTPP (5,10,15,20-tetraphenyl-21H,23H-porphine cobalt(II)) in toluene and in ZnTPP at low temperatures. In both matrices two Co(II) complexes are observed. In toluene, the contributions can be ascribed to 1:1 and 2:1 toluene–CoTPP charge transfer complexes. In ZnTPP, one of the Co(II) complexes vanishes upon heating to 100°C and reappears upon storage in air. This behaviour and the derived EPR parameters suggest a ligation of water to CoTPP. The second complex in the magnetically diluted powder can be assigned to a ZnTPP–CoTPP charge transfer complex. The observed g and cobalt hyperfine values are discussed and compared with data from other Co(II) porphyrin systems. Using pulse EPR, the hyperfine and nuclear quadrupole couplings of the pyrrole nitrogens are determined for the 1:1 toluene–CoTPP and the ZnTPP–CoTPP complex. The observed nitrogen hyperfine interactions differ from the theoretically predicted values and are sensitive to a change in the π-acceptor. A qualitative analysis of the data is given using existing theoretical models.