Architectural Isomerism in the Three-Dimensional Polymeric Spin Crossover System {Fe(pmd)2[Ag(CN)2]2}: Synthesis, Structure, Magnetic Properties, and Calorimetric Studies

Two coordination polymers formulated [Fe(pmd)2[Ag(CN)2]2] (pmd = pyrimidine) have been synthesized and characterized. Both polymers, considered to be architectural isomers, display different crystal structures and magnetic properties. Isomer 1 crystallizes in the monoclinic C2/c space group with a = 6.9750(8) angstroms, b = 16.1700(9) angstroms, c = 14.2020(8) angstroms, beta = 97.954(2) degrees, V = 1586.37(14) angstroms3, and Z = 4. The crystal structure of isomer 2 has been studied at 250 and 150 K. At both temperatures, 2 displays the orthorhombic Pccn space group with a = 15.7700(2) [14.8950(2)] angstroms, b = 8.2980(4) [8.1580(4)] angstroms, c = 13.4180(6) [13.3480(5)] angstroms, V = 1755.87(14) [1621.96(10)] angstroms3, and Z = 4 for 250 [150] K. The iron(II) ions define distorted octahedral [FeN6] chromophores in both isomers. The equatorial positions are occupied by four [Ag(CN)2]- bridging ligands, which connect the defining layers of two iron(II) ions. Isomer 1 has two crystallographically distinct [Ag(CN)2]- groups; one is essentially linear, while the other is severely distorted [C(5)-Ag(2)-C(5i)] = 138.8(5) degrees. This fact facilitates the parallel interpenetration of two layers, which in addition show short Ag(1)....Ag(2) interactions (distance Ag(1)....Ag(2) = 2.9972(10) angstroms). Isomer 2 shows only one type of Ag atom, which is slightly bent [C-Ag-C = 161.54(12) degrees], and as a consequence, the layers defined are not interpenetrated. In both cases, the axial positions are occupied by the pmd ligands which interact with the Ag atoms of adjacent layers defining a 3D coordination polymer. Compound 1 is high spin in the whole range of temperatures, while 2 undergoes a cooperative high-spin <--> low-spin effect centered at ca. 184 K with a hysteresis loop ca. 5 K wide. The experimental enthalpy and entropy variations were 11.5 +/- 0.4 kJ mol(-1) and 64 +/- 3 J K(-1) mol(-1). Consistency between the experimental thermodynamic data and the magnetic data was checked in the frame of regular solution theory.