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Elucidation of 1H NMR Paramagnetic Features of Heterotrimetallic Lanthanide(III)/Manganese(III) 12-MC‑4 Complexes

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journal contribution
posted on 05.07.2017, 13:15 by Corrado Atzeri, Vittoria Marzaroli, Martina Quaretti, Jordan R. Travis, Lorenzo Di Bari, Curtis M. Zaleski, Matteo Tegoni
The paramagnetic one-dimensional 1H NMR spectra of twelve LnIIINaI(OAc)4[12-MCMnIII(N)shi-4] complexes, where LnIII is PrIII–YbIII (except PmIII) and YIII, are reported. Their solid-state isostructural nature is confirmed in methanol-d4 solution, as a similar pattern in the 1H NMR spectra is observed along the series. Notably, a relatively well-resolved spectrum is reported for the GdIII complex. The chemical shift data are analyzed using the “all lanthanides” method, and the Fermi contact and pseudo-contact contributions are calculated for the lanthanide-induced shift (LIS). For the TbIII–YbIII complexes, the pseudo-contact contributions are typically 1 order of magnitude higher than the Fermi contact contributions; however, for the GdIII complex, the Fermi contact is the main contribution to the paramagnetic chemical shift. For the methyl protons of the axial acetate (OAc) ligands, the LIS is opposite in sign, with respect to that of the aromatic salicylhydroximate (shi3–) protons, because of structural rearrangements that occur upon dissociation of the NaI cation in solution. The calculated crystal field parameters (BLn) for the TbIII (360 cm–1), DyIII (250 cm–1), HoIII (380 cm–1), ErIII (410 cm–1), TmIII (620 cm–1), and YbIII (380 cm–1) complexes are not constant, likely as a consequence of the inaccuracy of the Bleaney’s constants and, to a smaller extent, the small structural changes that occur in solution. Overall, the metallacrown scaffold retains structural integrity and similarity in solution for the entire series; however, small structural features, which do not affect the overall similarity, do likely occur.