{[Hg(SCN)](μ-L)}: An Efficient Secondary Building Unit for the Synthesis of 2D Iron(II) Spin-Crossover Coordination Polymers.

We report an unprecedented series of two-dimensional (2D) spin-crossover (SCO) heterobimetallic coordination polymers generically formulated as {Fe(Hg(SCN))}·Solv, where x = 2 for L = tvp (trans-(4,4'-vinylenedipyridine)) (1tvp), bpmh ((1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine) (1bpmh·nCHOH; n = 0, 1), bpeh ((1E,2E)-1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine) (1bpeh·nHO; n = 0, 1) and x = 2.33 for L = bpbz (1,4-bis(pyridin-4-yl)benzene) (1bpbz·nHO; n = 0, 2/3). The results confirm that self-assembly of Fe, [Hg(SCN)], and ditopic rodlike bridging ligands L containing 4-pyridyl moieties favors the formation of linear [Fe(μ-L)] chains and in situ generated binuclear units {Hg(SCN)}. The latter act as bridges between adjacent chains generating robust 2D layers. The [FeN] centers are equatorially surrounded by four NCS groups and two axial N atoms of the organic ligand L. The compound 1tvp and the unsolvated form of 1bpmh undergo complete SCO centered at T = 177 and 226 K, characterized by the enthalpy and entropy variations ΔH = 12.3 and 10.5 kJ mol and ΔS = 69.4 and 48 J K mol, respectively. The almost complete SCO of the unsolvated form of 1bpeh occurs at ca. T = 119 K and exhibits a complete LIESST effect. Regardless of the degree of solvation, a half-spin conversion at T < 100 K occurs for 1bpbz·nHO, which becomes almost complete at p = 0.65 GPa. The labile solvent molecules present in 1bpmh·CHOH and 1bpeh·HO have a dramatic influence on the corresponding SCO behavior.