A Synergy and Struggle of EPR, Magnetometry and NMR: A Case Study of Magnetic Interaction Parameters in a Six-Coordinate Cobalt(II) Complex

Abstract

Herein, we combine for the first time SQUID magnetometry, cw-EPR, THz-EPR, and paramagnetic NMR spectroscopies to study the magnetic properties of a high-spin cobalt(II) heteroscorpionate complex. Complementary information provided by these methods allowed precise determination of the magnetic interaction parameters, thereby removing the ambiguity inherit to single-method studies. We systematically investigate the extent to which information about the magnetic interaction parameters can be deduced from reduced data sets. The detailed study revealed significant different magnetic properties in solid state and solution. To further exploit the information content of the solution NMR experimental results, we introduce the new concept of reduced paramagnetic shift. It allows for the determination of the magnetic axes and, subsequently, full NMR signal assignment. It is shown that even in complicated cases, in which common NMR analytics (integral intensities, relaxation factors, etc.) fail, it yields robust results.

Unravelling of a [High Spin—Low Spin] ↔ [Low Spin—High Spin] Equilibrium in Spin‐Crossover Iron(II) Dinuclear Helicates Using Paramagnetic NMR Spectroscopy

“…The signals of the two protons in the bridging phenyl group behave as expected for the process [HS‐HS] ↔ [HS‐LS] (Figure 4 c), so they may be used to quantify the observed SCO in the iron(II) helicates X@[Fe 2 L 3 ] . Of them, the “external” proton 9 (Figure 1) is most suitable for this purpose, as it is the farthest from the metal ions and thus experiences only little interference from different mechanisms of hyperfine interactions [35] . The simultaneous analysis of the temperature dependence of its chemical shift in the solutions of Cl@[Fe 2 L 3 ] and Br@[Fe 2 L 3 ] in [D 4 ]MeOH and [D 3 ]acetonitrile under an assumption that the geometry of the complexes in the pure states [HS‐HS] and [HS‐LS] is rather insensitive to the choice of the halogen anion and the solvent (for details see the Supporting Information) allowed obtaining the appropriate SCO curves from the variable‐temperature NMR spectra (Figure 5); the thermodynamic parameters (Supporting Information, Table S1) being typical of a thermally induced SCO in similar iron(II) complexes [36, 37] .…”

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

“…All sources of information are therefore valuable, and there is a growing interest in using paramagnetic NMR data to constrain electron SH parameters. [28][29][30][31] The use of electronic structure theory in the analysis of paramagnetic NMR data remains a growing field of research, and the clarification of its fundamental aspects remains a focus of current investigations. [32][33][34][35][36][37][38][39][40][41][42] Paramagnetic relaxation enhancement (PRE) is a useful source of information about electron-nucleus distances and also provides estimates of electron relaxation rates.…”

“…All sources of information are therefore valuable, and there is a growing interest in using paramagnetic NMR data to constrain electron SH parameters. [ 28 , 29 , 30 , 31 ] The use of electronic structure theory in the analysis of paramagnetic NMR data remains a growing field of research, and the clarification of its fundamental aspects remains a focus of current investigations. [ 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 , 42 ]…”

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts

“…However, direct measurement of zero‐field splitting (ZFS) and other electron spin Hamiltonian (SH) parameters in such systems is challenging, particularly in integer spin compounds. All sources of information are therefore valuable, and there is a growing interest in using paramagnetic NMR data to constrain electron SH parameters [28–31] . The use of electronic structure theory in the analysis of paramagnetic NMR data remains a growing field of research, and the clarification of its fundamental aspects remains a focus of current investigations [32–42] …”