Solution and Solid-State Study of the Spin-Crossover [FeII(R-bik)3](BF4)2 Complexes (R = Me, Et, Vinyl)

The magnetic properties of three spin-crossover complexes, [Fe-II(R-bik)(3)](BF4)(2)center dot nH(2)O (1-3), based on bis(imidazolyl) ketone ligands, were investigated in solution and the solid state. Their properties were compared with those of the ketone-free analogue, [Fe-II(bim)(3)](OTf)(2) (4). The alkyl and vinyl R groups have weak influence on the transition temperature, T-1/2, in solution, while stronger differences are observed in the solid state, because different intermolecular interactions occur in 1-3. The spin-state equilibria in solution were followed by SQUID magnetometry and the Evans NMR spectroscopy method. Interestingly, the equilibria can also be simply and efficiently probed by following the temperature dependence of an adequately chosen H-1 chemical shift. Overall, these experiments give coherent results, with T-1/2 located between 320 and 335 K, a narrow range, in comparison with the solid state. DFT calculations have allowed the rationalization of the magnetic differences. The molecular-orbital and spin-density calculations reveal that the presence of the C=O group between the imidazolyl units in the ligands of 1-3 leads to an extended aromatic system, an effective pi-acceptor effect, stabilizing the LS state and reducing the LS-HS gap, in comparison with 4.