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 Mg^II, Zn^II, Cd^II, Ni^II, and Fe^II. 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 Ni^II 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.