The NMR spectra of porphyrins—14: Self-aggregation of zinc(II) meso-nitro and meso-dinitro octaethylporphyrins

Complex formation in zinc(II) meso-nitro-octa-ethylporphyrin (1) and the corresponding α,β-dinitro (2) and α, γ-dinitro (3) zinc(II) chelates has been studied using proton NMR at 220MHz. This allows complete resolution of all the distinct groups in the proton spectrum, and the large concentration dependence of the spectra of 1 and 2 can be analysed to afford the monomeric and monomer-dimer shifts for all protons in these molecules. In contrast 3 shows no concentration dependence, nor any change upon addition of pyrrolidine, which immediately dissociates the aggregates of 1 and 2. The monomeric ¹H and ¹³C shifts are reported, together with those of zinc(II) octaethylporphyrin (4), and the complete assignment given allows the substituent shifts of the meso nitro groups in the porphyrin to be obtained. Analysis of the monomer-dimer shifts in terms of the ring current model gives the detailed geometries of the dimers, which have an inter-ring separation of ca 4.5 Å and a lateral displacement from the vertical of ca 1.0 Å. The results also allow the distinction between two different molecular complexes considered previously, and fully confirm our earlier suggestions that binding is due to metal-to-porphyrin, rather than metal-to-substituent, interactions.