Predicting paramagnetic ¹H NMR chemical shifts and state-energy separations in spin-crossover host-guest systems

Isley, W. C., III; Zarra, S.; Carlson, R. K.; Bilbeisi, R. A.; Ronson, T. K.; Nitschke, J. R.; Gagliardi, L.; Cramer, C. J.
Phys. Chem. Chem. Phys. 2014, 16, 10620 (doi:10.1039/c4cp01478b).

The behaviour of metal-organic cages upon guest encapsulation can be difficult to elucidate in solution. Paramagnetic metal centres introduce additional dispersion of signals that is useful for characterisation of host-guest complexes in solution using nuclear magnetic resonance (NMR). However, paramagnetic centres also complicate spectral assignment due to line broadening, signal integration error, and large changes in chemical shifts, which can be difficult to assign even for known compounds. Quantum chemical predictions can provide information that greatly facilitates the assignment of NMR signals and identification of species present. Here we explore how the prediction of paramagnetic NMR spectra may be used to gain insight into the spin crossover (SCO) properties of iron(II)-based metal organic coordination cages, specifically examining how the structure of the local metal coordination environment affects SCO. To represent the tetrahedral metal-organic cage, a model system is generated by considering an isolated metal-ion vertex: fac-ML₃²⁺ (M = Fe^II, Co^II; L = N-phenyl-2-pyridinaldimine). The sensitivity of the ¹H paramagnetic chemical shifts to local coordination environments is assessed and utilised to shed light on spin crossover behaviour in iron complexes. Our data indicate that expansion of the metal coordination sphere must precede any thermal SCO. An attempt to correlate experimental enthalpies of SCO with static properties of bound guests shows that no simple relationship exists, and that effects are likely due to nuanced dynamic response to encapsulation.