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Anion-Dependent Spin Crossover and Coordination Assembly Based on [Fe(tpa)]2+ [tpa = tris(2-pyridylmethyl)amine] and [N(CN)2]: Square, Zigzag, Dimeric, and [4+1]-Cocrystallized Complexes



A series of FeII complexes based on [Fe(tpa)]2+ building units and [N(CN)2] bridging ligands with different anions for charge balance has been prepared and characterized. They have a general formula [Fe(tpa){N(CN)2}]x·Y·Z, where x = 4 (14+), Y = 4ClO4, and Z = 2H2O for (ClO4)4(H2O)2; x = n (2n+), Z = 0, and Y = nClO4, nPF6, nAsF6, and nBPh4 for (ClO4)n, (PF6)n, (AsF6)n, and (BPh4)n, respectively; x = 2 (32+), Z = 0, and Y = 2BPh4 for (BPh4)2; x = 4 (44+), Y = 3SbF6 + [N(CN)2], and Z = [Fe(tpa){N(CN)2}2] (5) + 4H2O for (4+5). Single-crystal X-ray diffraction studies reveal that these complexes show various structures that are affected by different counteranions, i.e., square (ClO4)4(H2O)2, zigzag polymers [(ClO4)n, (PF6)n, (AsF6)n, and (BPh4)n], dimeric (BPh4)2, and [4+1]-cocrystallized (4+5). Magnetic studies show that the square complex (ClO4)4(H2O)2 undergoes a two-step complete spin crossover, whereas the dimer (BPh4)2 is high-spin in the whole temperature range, and all zigzag polymer anions show spin-crossover (SCO) behavior, the transition temperature (T1/2) of which increases with increasing anion size. Interestingly, complex (4+5) displays a gradual two-step spin crossover, which is different from that of (ClO4)4(H2O)2 and originates from the cocrystallized spin-crossover tetranuclear and monomeric species. Detailed studies on the crystal structures and magnetic properties have unveiled a remarkable anion-dependent formation of different structures and SCO behavior of these complexes.