Polynuclear Spin Crossover Complexes: Synthesis, Structure, and Magnetic Behavior of [Fe₄(μ-CN)₄(phen)₄(L)₂)]⁴⁺ Squares

Three new tetranuclear compounds of formula [Fe4(μ-CN)4(phen)4(L)2)](PF6)4·G where L = tris(pyridin-2-ylmethyl)amine (TPMA) [G = 0] (1), (6-methylpyrid-2-ylmethyl)-bis(pyrid-2-ylmethyl)amine (MeTPMA) [G = 0] (2), or bis(6-methylpyrid-2-ylmethyl)-(pyrid-2-ylmethyl)amine (Me2TPMA) [G = NH4PF6] (3) and phen = 1,10-phenanthroline) have been synthesized and characterized. The three compounds crystallize in the C2/c space group and consist of [Fe4(μ-CN)4(phen)4(L)2)]4+ square shaped cations with two distinct iron(II) sites. The sites, associated with [Fe(phen)2(CN)2] and [Fe(L)(NC)2] moieties, are connected by cyanide bridging ligands and reside in different [FeN4C2] and [FeN6] ligand field strength environments. For 1, the structural features of both sites at 100 and 293 K are those of an iron(II) atom in the low-spin state, according to the magnetic properties. At 370 K the structure of the [FeN6] site is consistent with a quite complete change of spin state from the low-spin state to the high-spin state, a behavior confirmed by the magnetic study. Introduction of a methyl substituent in the sixth position of one or two pyridine groups to get MeTPMA or Me2TPMA derivatives, respectively, induce in 2 and 3 notable steric constraint in the [FeN6] site making longer the average Fe−N bond distances thereby weakening the ligand field strength and stabilizing the high-spin state. The [FeN4C2] site remains in the low-spin state in the three compounds.