Low temperature NMR investigations of a series of polycrystalline metalloporphyrins

Abstract

This paper reports low temperature NMR investigations of a series of polycrystalline metalloporphyrins. The NMR signals from ligand nuclei are shifted by dipole-dipole and Fermi contact interactions with the unpaired electron spin(s) of the transition metal ion. In favorable instances one is able to completely analyze the spectra to obtain dipolar and Fermi coupling constants. The dipolar coupling constant can be further analyzed to yield effective electron-nuclei distances. The NMR shifts vary with the magnetic susceptibility of the sample and the NMR spectra can be used to monitor the temperature dependence of the magnetic susceptibility. This paper reports results from NMR studies of silver tetraphenylporphyrin (AgTPP), CuTPP, CoTPP, and VOTPP as well as copper protoporphyrin IX (Cu(PP-1x1) and Fe(PP-1X)Cl (hemin). IntroductionNMR spectra of solid transition metal complexes taken at very low temperatures can provide information about the interaction of the metals' unpaired electron spin(s) with nuclei in the surrounding ligands.1-6 The electron and nuclear spins may interact via dipole-dipole and/or Fermi contact interactions. The dipolar interaction depends on

Solid-State NMR Fermi Contact and Dipolar Shifts in Organometallic Complexes and Metalloporphyrins

“…These shifts are made up of both isotropic Fermi contact and anisotropic dipolar hyperfine interactions, which, especially at low temperatures, can be very large, on the order of 1 MHz. In early work, Kreilick et al investigated Cu II ( dl -alanine) 2 ·H 2 O ( 1 ) single crystals using 1 H NMR and reported both isotropic and anisotropic hyperfine interactions and used these to determine the relative positions of nuclei in the ligands with respect to the central metal. , This approach also enabled assignments for AgTPP ( 2 , TPP = meso -tetraphenylporphyrin), which were found to be in close agreement with single-crystal ENDOR results . The ENDOR hyperfine parameters for CuTPP ( 3 ) were also reported 4 and confirmed in later studies .…”

31P NMR as a Tool for Studying Incorporation of Ni, Co, Fe, and Mn into Aluminophosphate Zeotypes

“…As already mentioned, the 31 P NMR spectrum is affected by paramagnetic transition metal ions because of strong dipolar and hyperfine interactions of unpaired electrons with phosphorus nuclei. The influence of a single paramagnetic center can be expressed through an effective magnetic field of the form 19 where A iso is the isotropic hyperfine coupling constant, γ n is the nuclear gyromagnetic ratio, µ B is the Bohr magneton, r is the separation between the electronic and nuclear spins, and θ is the angle between the electron-nucleus vector and the external magnetic field. Note that the nucleus experiences the interaction with a single paramagnetic center only in samples with low fractions of homogeneously distributed transition metal ions.…”

“…However, even though a sum of dipolar contributions over all such sites was considered and even though anisotropic g tensors were considered instead of their isotropic values, the shape of the powder pattern induced by this interaction would still resemble the shape of the powder pattern induced by the chemical shift anisotropy. 19,20 It is worth noting that both terms in eq 1 depend on averaged electron spin polarization 〈S〉. This means that a transition metal ion with a larger spin quantum number S will induce larger NMR line broadening and, if A iso was fixed, a larger paramagnetic shift.…”