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The adsorption mechanism of SO2in the Hofmann-like coordination polymer Fe(pz)[Pt(CN)4] is studied using inelastic neutron scattering and density functional theory calculations. We find that the most important spectral change upon gas adsorption is the blue shift of the low-energy peak found at 100 cm–1, a feature that is fully confirmed by the computed neutron-weighted phonon density of states. Our calculations suggest that the origin of this change is twofold: (i) an increase in the force constant of the cyanide out-of-plane movement due to the binding of the gas onto the Pt(CN)4plane and (ii) the hampered rotation of the pyrazine due to steric hindrance. The high-energy region of the neutron scattering data whose spectral weight is dominated by the internal vibrations of the pyrazine is negligibly affected by the presence of the gas as expected from a physisorption type of binding.

Understanding the origin of cooperativity and the equilibrium temperature of transition (T1/2) displayed by the spin-crossover (SCO) compounds as well as controlling these parameters are of paramount importance for future applications. For this task, the occurrence of polymorphism, presented by a number of SCO complexes, may provide deep insight into the influence of the supramolecular organization on the SCO behavior. In this context, herein we present a novel family of mononuclear octahedral FeIIcomplexes with formula cis-[Fe(bqen)(NCX)2], where bqen is the chelating tetradentate ligand N,N′-bis(8-quinolyl)ethane-1,2-diamine and X = S, Se. Depending on the preparation method, these compounds crystallize in either the orthorhombic or the trigonal symmetry systems. While the orthorhombic phase is composed of a racemic mixture of mononuclear complexes (polymorph I), the trigonal phase contains only one of the two possible enantiomers (Λ or Δ), thereby generating a chiral crystal (polymorph II). The four derivatives undergo SCO behavior with well-differentiated T1/2values occurring in the interval 90–233 K. On one hand, T1/2is about 110 K (polymorph I) and 87 K (polymorph II) higher for the selenocyanate derivatives in comparison to those for their thiocyanate counterparts. These differences in T1/2are ascribed not only to the higher ligand field induced by the selenocyanate anion but also to a remarkable difference in the structural reorganization of the [FeN6] coordination core upon SCO. Likewise, the higher cooperativity observed for the thiocyanate derivatives seems to be related to their stronger intermolecular interactions within the crystal. On the other hand, T1/2is about 53 K (thiocyanate) and 29 K (selenocyanate) higher for the trigonal polymorph II in comparison to those for the orthorhombic polymorph I. These differences, and the small changes observed in cooperativity, stem from the slightly different hetero- and homochiral crystal packing generated by the cis-[Fe(bqen)(NCX)2] molecules, which determines subtle adaptations in the intermolecular contacts and the FeIIcoordination core.

Deprotonation of the ionogenic tetradentate ligand 6,6′-bis(1H-tetrazol-5-yl)-2,2′-bipyridine [H2bipy(ttr)2] in the presence of FeIIin solution has afforded an anionic mononuclear complex and a neutral two-dimensional coordination polymer formulated as, respectively, NEt3H{Fe[bipy(ttr)2][Hbipy(ttr)2]}·3MeOH (1) and {Fe[bipy(ttr)2]}n(2). The anions [Hbipy(ttr)2]−and [bipy(ttr)2]2–embrace the FeIIcenters defining discrete molecular units 1with the FeIIion lying in a distorted bisdisphenoid dodecahedron, a rare example of octacoordination in the coordination environment of this cation. The magnetic behavior of 1shows that the FeIIis high-spin, and its Mössbauer spectrum is characterized by a relatively large average quadrupole splitting, ΔEQ= 3.42 mm s–1. Compound 2defines a strongly distorted octahedral environment for FeIIin which one [bipy(ttr)2]−anion coordinates the equatorial positions of the FeIIcenter, while the axial positions are occupied by peripheral N-tetrazole atoms of two adjacent {Fe[bipy(ttr)2]}0moieties thereby generating an infinite double-layer sheet. Compound 2undergoes an almost complete spin crossover transition between the high-spin and low-spin states centered at about 221 K characterized by an average variation of enthalpy and entropy ΔHav= 8.27 kJ mol–1, ΔSav= 37.5 J K–1mol–1, obtained from calorimetric DSC measurements. Photomagnetic measurements of 2at 10 K show an almost complete light-induced spin state trapping (LIESST) effect which denotes occurrence of antiferromagnetic coupling between the excited high-spin species and TLIESST= 52 K. The crystal structure of 2has been investigated in detail at various temperatures and discussed.