Nuclear magnetic resonance spectra of nitrogen-15 enriched octaethylporphyrin and its derivatives

Abstract

97%15N-Enriched octaethylporphyrin [15N4]OEPH2 and its mono- and di-N-methyl derivatives were synthesized by the use of Na 15NO2 as a strating material. 15N N.m.r. at 10 MHz and 1H n.m.r. spectra at 100 MHz were obtained for these compounds and the metal complexes of [15N4]OEP with MgII, ZnII, CdII, NiII, and FeII. 13C N.m.r. spectra at 25 MHz were also measured for the metal complexes. As shown previously 15N and 1H spectra of [15N4]OEPH2 in CDCl3 have indicated that the two central hydrogens move around the four nitrogen atoms rapidly enough to maintain four-fold symmetry on the n.m.r. time scale at higher temperatures, while they are fixed on two of the nitrogen atoms at lower temperatures. The temperature dependence of the spectra of [15N4]OEPMeH and [15N4]OEPMe2H+ has also revealed that the hydrogen of OEPMeH is fixed on the nitrogen located opposite to N-methyl group, while that of OEPMe2H+ exchanges rapidly between two nitrogens even below room temperatures. The 15N resonance of the NiII complex of [15N4]OEP is distinct from the other diamagnetic metal complexes and located at remarkably higher field. Similar distinct shifts were observed for the 13C resonance of the α-carbon of the Ni complex. The 1H spectrum of the methine proton of [15N4]OEPH2 was shown to be coupled to two 15N nuclei leading to a triplet. The metal complexes, however, showed more complicated spectral patterns. Their fine structures are due to ‘virtual coupling’ and arise from the coupling between 15N nuclei via the central metal ion. The extent of the coupling depends remarkably on the metal ions.