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The synthesis, structure, and magnetic properties of three clathrate derivatives of the spin-crossover porous coordination polymer {Fe(pyrazine)[Pt(CN)(4)]} (1) with five-membered aromatic molecules furan, pyrrole, and thiophene is reported. The three derivatives have a cooperative spin-crossover transition with hysteresis loops 14-29 K wide and average critical temperatures T-c=201 K (1.fur), 167 K (1.pyr), and 114.6 K (1.thio) well below that of the parent compound 1 (T-c=295 K), confirming stabilization of the HS state. The transition is complete and takes place in two steps for 1.fur, while 1.pyr and 1.thio show 50% spin transition. For 1.fur the transformation between the HS and IS (middle of the plateau) phases occurs concomitantly with a crystallographic phase transition between the tetragonal space groups P4/mmm and I4/mmm, respectively. The latter space group is retained in the subsequent transformation involving the IS and the LS phases. 1.pyr and 1.thio display the tetragonal P4/mmm and orthorhombic Fmmm space groups, respectively, in both HS and IM phases. Periodic calculations using density functional methods for 1.fur, 1.pyr, 1.thio, and previously reported derivatives 1.CS2, 1.I, 1.bz(benzene), and 1.pz(pyrazine) have been carried out to investigate the electronic structure and nature of the host-guest interactions as well as their relationship with the changes in the LS-HS transition temperatures of 1.Guest. Geometry-optimized lattice parameters and bond distances in the empty host 1 and 1.Guest clathrates are in general agreement with the X-ray diffraction data. The concordance between the theoretical results and the experimental data also comprises the guest molecule orientation inside the host and intermolecular distances. Furthermore, a general correlation between experimental T-c and calculated LS-HS electronic energy gap was observed. Finally, specific host-guest interactions were studied through interaction energy calculations and crystal orbital displacement (COD) curve analysis., The research reported herein was supported by the Spanish Ministerio de Ciencia e Innovacion (MICINN) and FEDER funds (CTQ2013-46275-P and CTQ2011-23862-C02-01), Generalitat de Catalunya (2009SGR-1459) and Generalitat Valenciana through PROMETEO/2012/049. D. A. thanks Conicyt-Chile for a predoctoral fellowship. Z.A.C. thanks the Spanish M.E.C.D. for a predoctoral FPU fellowship. M.O. thanks a Grant-In-Aid for Science Research for Innovative Areas "Coordination Programming (Area 2107, No. 22108512)" and for Scientific Research Program (A) (No. 23245014) from MEXT, Japan. The authors thankfully acknowledge the computer resources, technical expertise, and assistance provided by the Barcelona Supercomputer Center.

Hofmann type clatharates are host-guest compounds with the general formula M(NH3)2M'(CN)4.2G, in which M(NH3)2M'(CN)4 is the host lattice and G is benzene, the guest molecule. In previous studies, host-guest interactions have been investigated by analyzing the RT and LNT vibrational (infrared, far infrared and Raman) spectra of these clathrates. All the observed changes in the vibrational spectra of these clathrates are referred to a host-guest interaction originating from weak hydrogen bonding between the ammonia hydrogen atoms from the host lattice and the p electron cloud of the guest (benzene) molecules. In order to obtain an insight into the relative importance of the local crystalline field vs. the anharmonicity effects on the spectroscopic properties of the guest species upon enclathration, as well as to explain the observed band shifts and splittings, several quantum theoretical approaches are proposed.

An alternative mechanism is proposed for the observed splitting of the ν19 γ(CH) mode of the benzene guest molecules in Hofmann type clathrates to the one presented previously.1 According to the proposed model, the splitting is due to the vibrational Stark effect. A simple quantum theoretical model for this effect is derived, based on the degenerate-case stationary perturbation theory, both with and without explicit inclusion of the vibrational angular momentum. The model allows estimation of the relative field strengths at the enclathration sites of the guest molecules in a series of structurally similar clathrates. The derived equations may be used for calculation of the local field strength as well, if particular anharmonic coupling constants for enclathrated species are available. The predictions of the proposed method are fully in line with the observed room temperature-low temperature trends for a particular clathrate, as well as with the trends in the series of isostructural clathrate compounds for which experimental data are available.