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

The knowledge of the exact electronic structure of Co(II) porphyrins is imp ortant in view of their interest as models for the prosthetic groups in pro teins and in view of their use as catalysts. Here, we present a continuous wave and pulse electron paramagnetic resonance (EPR) study of CoTPP (5,10,1 5,20-tetraphenyl-21H,23H-porphine cobalt(II)) in toluene and in ZnTPP at lo w temperatures. In both matrices two Co(II) complexes are observed. In tolu ene, 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 hea ting to 100 degreesC 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-CoTP P charge transfer complex. The observed g and cobalt hyperfine values are d iscussed and compared with data from other Co(II) porphyrin systems. Using pulse EPR, the hyperfine and nuclear quadrupole couplings of the pyrrole ni trogens are determined for the 1:1 toluene-CoTPP and the ZnTPP-CoTPP comple x. The observed nitrogen hyperfine interactions differ from the theoretical ly predicted values and are sensitive to a change in the pi -acceptor. A qu alitative analysis of the data is given using existing theoretical models.