Two ways of spin crossover in an iron(ii) coordination polymer associated with conformational changes of a bridging ligand.

1,4-Di(1-ethyl-1,2,3-triazol-5-yl)butane (bbtre) was prepared by lithiation of 1-ethyl-1,2,3-triazole, followed by alkylation with 1,4-dibromobutane. The ligand bbtre forms a three-dimensional network with Fe(ii), [Fe(bbtre)3](ClO4)2·2CH3CN, that exhibits thermally induced spin crossover (SCO). A change of temperature or change of spin state results in various types of structural transformation, leading to different structures that are stable in strictly defined temperature ranges. As a result, there are three spin crossover transitions arranged via two different paths. Thus, cooling below 280 K involves a HT(HS) → LT(HS) (HT, high temperature structure; LT, low temperature structure; HS, high spin) phase transition (PT), which is associated with conformational changes of the bbtre molecules and with deformation of the polymeric skeleton. In the LT phase incomplete and reversible LT(HS) ⇄ LT(HS/LS) spin crossover occurs (LS, low spin). In contrast, rapid cooling (of a sample not previously thermally treated) allows the HT(HS) → LT(HS) phase transition to be avoided, and so complete HT(HS) → HT1(LS) SCO occurs. This means that the PT plays the role of a switch, which allows a choice of one of two ways in which the SCO will proceed. After rapid cooling, further heating to 150 K and subsequent cooling results in a reversible HT1(HS) ⇄ HT1(LS) spin crossover (T↓1/2 = 130 K, T↑1/2 = 131 K). However, raising the temperature to 170-200 K leads to formation of a modulated structure HT2(HS) exhibiting the next reversible HT2(HS) ⇄ HT2(LS) SCO (T↓1/2 = 121 K, T↑1/2 = 123 K). Finally, heating above 200 K involves the HT2(HS) → LT(HS) PT and results in a LT(HS) structure exhibiting incomplete LT(HS) ⇄ LT(HS/LS) spin crossover.

"Normal" and "reverse" spin crossover induced by two different structural events in iron(ii) coordination polymer.

In [Fe(ebbtr)2(CH3CN)2](CF3SO3)2·4CH3CN spin crossover is associated with the occurrence of "normal" and "reverse" hysteresis loops separated by a region of stable HS form. This results from trans-trans → gauche-trans conformational changes of ebbtr molecules and anion reorientation, which occur in different ways during cooling and during heating.

HS⇄LS transition in iron(II) two-dimensional coordination networks containing tris(tetrazol-1-ylmethyl)methane as triconnected building block.

Novel tripodal ligand 1,1',1''-tris(tetrazol-1-ylmethyl)methane (111tz) and products of its reactions with perchlorate as well as with tetrafluoroborate salts of iron(II) are presented. The isostructural complexes, [Fe(111tz)2](ClO4)2 and [Fe(111tz)2](BF4)2, were isolated as two-dimensional (2D) coordination networks revealing a honeycomb-like pattern with cages occupied by disordered anions. 111tz molecules act as a tridentate ligand bridging three adjacent Fe(II) ions, and the nitrogen N4 atom of six tetrazole rings (tz) is placed in octahedron vertices of FeN6 chromophores. The complexes, crystallizing in the P3 space group, were characterized by variable-temperature single-crystal X-ray diffraction and variable-temperature magnetic susceptibility measurements. Variable-temperature magnetic susceptibility measurements show that both systems undergo abrupt and complete spin transition with T(1/2)(↑) = T(1/2)(↓) = 176 K for perchlorate and T(1/2)(↑) = 193.8 and T(1/2)(↓) = 192.8 K for the tetrafluoroborate analogue. Change of spin state in both complexes is accompanied by a thermochromic effect. The HS→LS transition in [Fe(111tz)2](ClO4)2 involves shortening of the Fe-N4 bond lengths from 2.171(2) Å (293 K) to 2.002(1) Å (100 K). In [Fe(111tz)2](BF4)2, lowering of temperature from 293 to 100 K is accompanied by shortening of the Fe-N4 distances from 2.179(2) to 1.987(2) Å, respectively. Perchlorate in [Fe(111tz)2](ClO4)2 or tetrafluoroborate anions in [Fe(111tz)2](BF4)2 are engaged in the formation of intermolecular contacts within as well as with the neighboring 2D layer.