{[Hg(SCN) 3 ] 2 (μ-L)} 2- : 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 {FeII [(HgII (SCN)3 )2 ](L)x }·Solv, where x = 2 for L = tvp (trans-(4,4'-vinylenedipyridine)) (1tvp), bpmh ((1E,2E)-1,2-bis(pyridin-4-ylmethylene)hydrazine) (1bpmh·nCH3 OH; n = 0, 1), bpeh ((1E,2E)-1,2-bis(1-(pyridin-4-yl)ethylidene)hydrazine) (1bpeh·nH2 O; n = 0, 1) and x = 2.33 for L = bpbz (1,4-bis(pyridin-4-yl)benzene) (1bpbz·nH2 O; n = 0, 2/3). The results confirm that self-assembly of FeII , [HgII (SCN)4 ]2- , and ditopic rodlike bridging ligands L containing 4-pyridyl moieties favors the formation of linear [Fe(μ-L)]n 2n+ chains and in situ generated binuclear units {[HgII (SCN)3 ]2 (μ-L)}2- . The latter act as bridges between adjacent chains generating robust 2D layers. The [FeII N6 ] 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 T1/2 = 177 and 226 K, characterized by the enthalpy and entropy variations ΔH = 12.3 and 10.5 kJ mol-1 and ΔS = 69.4 and 48 J K-1 mol-1 , respectively. The almost complete SCO of the unsolvated form of 1bpeh occurs at ca. T1/2 = 119 K and exhibits a complete LIESST effect. Regardless of the degree of solvation, a half-spin conversion at T1/2 < 100 K occurs for 1bpbz·nH2 O, which becomes almost complete at p = 0.65 GPa. The labile solvent molecules present in 1bpmh·CH3 OH and 1bpeh·H2 O have a dramatic influence on the corresponding SCO behavior.