A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

<div> <p>The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. In contrast to the large number of standard NMR methods for diamagnetic compounds, the number of paramagnetic NMR methods is limited and spectral assignment often relies on theoretical models. We report a toolbox of 1D (¹H, proton-coupled ¹³C, selective ¹H‑decoupling ¹³C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods for the straightforward structural characterisation of paramagnetic complexes in solution and demonstrate its general applicability for fields from coordination chemistry to spin‑crossover complexes and supramolecular chemistry through the characterisation of Co^II and high-spin Fe^II mononuclear complexes as well as a Co₄L₆ cage. The toolbox takes advantage of the reduced signal overlap, decreased instrument time and greater sensitivity from the presence of the paramagnetic centre while overcoming the loss of structural information from the wide chemical shift dispersion and broad signals. In some circumstances, more structural information was revealed in the COSY spectra than would be observable for a diamagnetic analogue; as well as the expected through-bond cross-peaks, through-space and exchange cross-peaks were also observed for mononuclear complexes with multiple ligand environments and fast ligand exchange. With this toolbox, the standard characterisation of paramagnetic complexes and cages is now possible using NMR spectroscopic methods. </p> </div> <br>

A Paramagnetic NMR Spectroscopy Toolbox for the Characterisation of Paramagnetic/Spin-Crossover Coordination Complexes and Metal-Organic Cages

<div> <p>The large paramagnetic shifts and short relaxation times resulting from the presence of a paramagnetic centre complicate NMR data acquisition and interpretation in solution. In contrast to the large number of standard NMR methods for diamagnetic compounds, the number of paramagnetic NMR methods is limited and spectral assignment often relies on theoretical models. We report a toolbox of 1D (¹H, proton-coupled ¹³C, selective ¹H‑decoupling ¹³C, steady-state NOE) and 2D (COSY, NOESY, HMQC) paramagnetic NMR methods for the straightforward structural characterisation of paramagnetic complexes in solution and demonstrate its general applicability for fields from coordination chemistry to spin‑crossover complexes and supramolecular chemistry through the characterisation of Co^II and high-spin Fe^II mononuclear complexes as well as a Co₄L₆ cage. The toolbox takes advantage of the reduced signal overlap, decreased instrument time and greater sensitivity from the presence of the paramagnetic centre while overcoming the loss of structural information from the wide chemical shift dispersion and broad signals. In some circumstances, more structural information was revealed in the COSY spectra than would be observable for a diamagnetic analogue; as well as the expected through-bond cross-peaks, through-space and exchange cross-peaks were also observed for mononuclear complexes with multiple ligand environments and fast ligand exchange. With this toolbox, the standard characterisation of paramagnetic complexes and cages is now possible using NMR spectroscopic methods. </p> </div> <br>

EPR spectra of a new radiation-induced paramagnetic centre in kaolins

The EPR spectrum of a previously unreported paramagnetic centre formed in kaolin minerals by exposure to γ radiation is described. This centre, which is referred to here as the ‘C-centre’, was seen initially during an investigation of the radiation dose response of the EPR signal in the natural Lampang kaolin from northern Thailand, and its EPR properties are now presented for a purified sample from this material. They suggest a paramagnetic centre with rhombic symmetry based on O− associated with a single 27Al atom. Computer simulations suggest spin Hamiltonian parameters of g1 = 1.976, g3 = 2.0417 and A(27Al) = 2.10 mT, with g2 ≈ 2.01. This C-centre was also seen as a minor radiationinduced component in both crude and purified Ranong kaolin samples, along with a stronger signal from the B-centre radical. It seems to be associated with the kaolinite component, but was lost on annealing to 300°C after which only the signal from the A-centre was visible.