Evaluating π-π stacking effects in macrocyclic transition metal complexes using EPR techniques

The HYSCORE spectra for two different macrocyclic transition metal complexes, namely cobalt tetraphenylporphyrin (CoTPP) and a copper Salen derivative ([Cu(1)]), were examined in relation to their interactions with pyridine (Py) and methylbenzyl amine (MBA), respectively. In both cases weak hyperfine interactions were detected by HYSCORE, but the origin of these interactions was found to originate from completely different effects. In the CoTPPpy adduct, with axial coordination of the substrate (pyridine) to the metal centre, weak couplings from the pyrrole nitrogens of a neighbouring porphyrin complex were identified, and confirmed through a series of isotopic labelling and dilution experiments. This result represents the first ever identification by HYSCORE of π-π interactions between such porphyrin complexes in solution. In the [Cu(1)]MBA adduct, again with axial coordination of the substrate (MBA) to the copper centre, weak couplings were also identified in the HYSCORE spectra, which could easily be misinterpreted as arising from inter-molecular interactions between adjacent ligands. However, the origin of these couplings was clearly demonstrated to arise from intramolecular 13C-ligand interactions. These results demonstrate not only the sensitivity of the HYSCORE technique for detection of weak inter- and intra-molecular interactions in macrocyclic transition metal complexes, but additionally the need to consider dilution effects in the spectral assignments