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 Fe-II 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)center dot Y center dot Z, where x = 4 (1(4+)), Y = 4ClO(4)(-), and Z = 2H(2)O for 1 center dot(ClO4)(4)(H2O)(2); x = n (2(n+)), Z = 0, and Y = nClO(4)(-), nPF(6)(-), nAsF(6)(-), and nBPh(4)(-) for 2 center dot(ClO4)(n), 2 center dot(PF6)(n), 2 center dot(AsF6)(n), and 2 center dot(BPh4)(n), respectively; x = 2 (3(2+)), Z = 0, and Y = 2BPh(4)(-) for 3 center dot(BPh4)(2); x = 4 (4(4+)), Y = 3SbF(6)(-) + [N(CN)(2)](-), and Z = [Fe(tpa)(N(CN)(2)}(2)] (5) + 4H(2)O 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 1 center dot(ClO4)(4)(H2O)(2), zigzag polymers [2 center dot(ClO4)(n), 2 center dot(PF6)(n), 2 center dot(AsF6)(n), and 2 center dot(BPh4)(n)], dimeric 3 center dot(BPh4)(2), and [4+1]-cocrystallized (4+5). Magnetic studies show that the square complex 1 center dot(ClO4)(4)(H2O)(2) undergoes a two-step complete spin crossover, whereas the dimer 3 center dot(BPh4)(2) is high-spin in the whole temperature range, and all zigzag polymer 2.anions show spin-crossover (SCO) behavior, the transition temperature (T-1/2) of which increases with increasing anion size. Interestingly, complex (4+5) displays a gradual two-step spin crossover, which is different from that of 1 center dot(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.