How Nonequivalency of the Flexibility of the Ligand Bridges Leads to Anisotropy of Perturbation Transmission in a 3D Spin‐Crossover Coordination Network

Expanding the dimensionality of bis(tetrazolyl)alkane-based Fe(II) coordination polymers by the application of dinitrile coligands

Reactions between 1,2-di(tetrazol-2-yl)ethane (ebtz), 1,6-di(tetrazol-2-yl)hexane (hbtz) or 1,1'-di(tetrazol-1-yl)methane (1ditz) and Fe(BF4)2 in the presence of adiponitrile (ADN), glutaronitrile (GLN) or suberonitrile (SUN) resulted in the formation of coordination polymers [Fe(μ-ebtz)2(μ-ADN)](BF4)2 (1), [Fe(μ-hbtz)2(μ-ADN)](BF4)2 (2), [Fe(μ-1ditz)2(GLN)2](BF4)2·GLN (3) and [Fe(μ-1ditz)2(μ-SUN)](BF4)2·SUN (4). It was established that the application of dinitriles allows an increase in the dimensionality of the ebtz and hbtz based systems while maintaining the structure of the polymeric units characteristic of previously studied mononitrile based analogues. In 3 and 4, regardless of the type of dinitrile coligand, the motif of 2D polymeric layers constituted by 1ditz molecules remains preserved. However, the dimensionality of 1ditz based networks is governed by the coordination modes of dinitriles. 3, based on a shorter molecule of glutaronitrile, crystallizes as a two-dimensional (2D) coordination polymer. In this compound, dinitriles coordinate monodentately or play the role of guest molecules. The substitution of glutaronitrile with suberonitrile enables the bridging of neighboring polymeric layers, resulting in a 3D network. The intentional selection of bis(tetrazoles) and dinitriles as building blocks has led, as expected, to obtaining systems with the structure of the first coordination sphere consisting of four tetrazole rings and two axially coordinated nitrile molecules. It created the conditions required for the occurrence of thermally induced spin crossover. Magnetic measurements and single crystal X-ray diffraction studies were used for the characterization of the spin crossover properties of 1-4.

Extremely Slow Thermally-Induced Spin Crossover in the Two-Dimensional Network [Fe(bbtr)3 ](BF4 )2

Cooling [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T1/2 ↓ =76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T1/2 ↑ =89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr)3 ](ClO4 )2 in which spin crossover during cooling is preceded by phase transition at TPT =126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T1/2 (x) and TPT (x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr)3 ](BF4 )2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr)3 ](BF4 )2(1-x) (ClO4 )2x (0.1≤x≤0.9), revealed that at 295 K P1/2 value changes linearly with x indicating similar mechanism of spin crossover under elevated pressure in all systems under investigation. Variable pressure single crystal X-ray diffraction studies confirmed that in contrast to thermally induced spin crossover undergoing differently in tetrafluoroborate and perchlorate an application of high pressure removes this differentiation leading to a similar mechanism depending at first on start spin crossover and then P-3→P-1 phase transition occurs. In this report we have shown that 2D coordination polymer [Fe(bbtr)3 ](BF4 )2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) treated to date as spin crossover silent shows thermally induced spin crossover phenomenon. Spin crossover in tetrafluoroborate is extremely slow. Determination of the spin crossover curve required carrying measurement in the settle mode-cooling from 85 to 70 K took about 600 h (average velocity of change of temperature ca. 0.0004 K/min).

Single crystal-to-single crystal transformation - from two distinct to three distinct spin crossover centers in 2D coordination polymer [Fe(bbtr)3](CF3SO3)2

1,4-Di(1,2,3-triazol-1-yl)butane (bbtr) forms a two-dimensional (2D) coordination polymer (1) in a reaction with iron(II) triflate. In the crystal lattice there are two crystallographically unique iron(II) ions surrounded octahedrally by a 1,2,3-triazole ring coordinated through nitrogen atoms N3. Single crystal X-ray diffraction studies revealed that spin crossover for each crystallographically independent iron(II) ion proceeds at a different temperature (T_1/2(Fe1) = 201 K; T_1/2(Fe2) = 216 K), while the magnetic measurements showed that there is one step, complete thermally induced spin crossover (T_1/2 = 205 K). Complex 1 undergoes, with time, single crystal-to-single crystal transformation (SCSC) to the converted system (1c) from the R3̄ to the P6₃ space group, accompanied by significant changes in the lattice parameter c (a shortening of approximately one-third) and consequently unit cell volume. Structural transformation is associated with rebuilding of the polymeric layer as well as the anion network, which is reflected in the results of Mössbauer studies. In the polymorphic system (1c) there are three crystallographically independent iron(II) ions. The temperature dependence results for magnetic susceptibility indicated complete, one-step spin crossover very similar to that of 1; however, single-crystal X-ray diffraction studies of 1c revealed that spin crossover for each crystallographically independent iron(II) ion occurs in a different manner, revealing three elementary stages (T_1/2(Fe1) = 200 K; T_1/2(Fe2) = 212 K, T_1/2(Fe3) = 214 K).

Spatiotemporal Studies of the One-Dimensional Coordination Polymer [Fe(ebtz)2 (C2 H5 CN)2 ](BF4 )2 : Tug of War between the Nitrile Reorientation Versus Crystal Lattice as a Tool for Tuning the Spin Crossover Properties*

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF₄ )₂ ⋅6 H₂ O in different nitriles yields one-dimensional coordination polymers [Fe(ebtz)₂ (RCN)₂ ](BF₄ )₂ ⋅nRCN (n=2 for R=CH₃ (1) and n=0 for R=C₂ H₅ (2) C₃ H₇ (3), C₃ H₅ (4), CH₂ Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3-5 is complete and occurs above 160 K. In 2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T_1/2 ^↓ =78 K, T_1/2 ^↑ =123 K) and proceeds extremely slowly. Isothermal (80 K) time-resolved single-crystal X-ray diffraction studies revealed a complex nature for the HS→LS transition in 2. An initial, slow stage is associated with shrinkage of polymeric chains and with reduction of volume at 77 % (in relation to the difference between cell volumes V_HS -V_LS ) whereas only 16 % of iron(II) ions change spin state. In the second stage, an abrupt SCO occurs, associated with breathing of the crystal lattice along the direction of the Fe-nitrile bonds, while the nitriles reorient. HS→LS switching triggered by light (808 nm) reveals the coupling of spin state and nitrile orientation. The importance of this coupling was confirmed by studies of [Fe(ebtz)₂ (C₂ H₅ CN/C₃ H₇ CN)₂ ](BF₄ )₂ mixed crystals (2 a, 2 b), showing a shift of T_1/2 to higher values and narrowing of the hysteresis loop concomitant with an increase of the fraction of butyronitrile. This increase reduces the capability of nitrile molecules to reorient. Density functional theory (DFT) studies of models of 1-5 suggest a particular possibility of 2 to adopt a low (140-145°) value of its Fe-N-C(propionitrile) angle.

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

Structural phase transition in [Fe(bbtre)₃](ClO₄)₂·2CH₃CN (bbtre = 1,4-di(1-ethyl-1,2,3-triazol-5-yl)butane) plays the role of a switch, allowing spin crossover to be carried out in two ways.

Sliding Polymeric Layers and Anion Displacement Coupled with Spin Crossover in Two-Dimensional Networks of [Fe(hbtz)2 (CH3 CN)2 ](BF4 )2

The abrupt high spin (HS)→low spin (LS) transition (T^↓ _1/2 =136 K) in [Fe(hbtz)₂ (CH₃ CN)₂ ](BF₄ )₂ (hbtz=1,6-di(tetrazol-2-yl)hexane) is finished at 100 K and further thermal treatment influences the spin crossover. Subsequent heating involves a change of the spin state in the same way (T^↑ _1/2 =136 K) on cooling. In contrast, cooling below 100 K triggers different behavior and T^↑ _1/2 is shifted to 170 K. The extraordinary structural changes that occurred below 100 K are responsible for the observed diversity of properties. A unique feature of the low-temperature phase is the rebuilding of the anion network expressed by a shift of anions inside the polymeric layer at a distance of 1.2 Å as well as the relative shift of neighboring layers at over 4 Å. These structural alterations, connected with a phase transition, become the origin of the strain, which in most cases causes crystal cleaving. In a sample composed from crystals crushed as a result of the phase transition or as a result of mechanical crumbling, the hysteresis loop vanishes; however, annealing the sample allows to its partial restoration. A replacement of acetonitrile by other nitriles leads to preservation of the polymeric structure and spin crossover, but no phase transition follows.

Influence of conformational changes on spin crossover properties and superstructure formation in 2D coordination polymers [Fe(hbtz)2(RCN)2](ClO4)2

Spin-crossover, governed by conformational changes of nitrile molecule, triggers corrugation of polymeric skeleton and formation of superstructure in [Fe(hbtz)₂(allyl cyanide)₂](ClO₄)₂.

Role of Fe-N-C geometry flip-flop in bistability in Fe(tetrazol-2-yl)4(C2H5CN)2-type core based coordination network

[Fe(ebtz)(2)(C(2)H(5)CN)(2)](ClO(4))(2) was prepared in the reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(ClO(4))(2)·6H(2)O in propionitrile. The compound crystallizes as a one-dimensional (1D) network, where bridging of neighboring iron(II) ions by two ebtz ligand molecules results in formation of a [Fe(ebtz)(2)](∞) polymeric skeleton. The 1D chains are assembled into supramolecular layers with axially coordinated nitrile molecules directed outward. The complex in the high spin (HS) form reveals a very rare feature, namely, a bent geometry of the Fe-N-C(propionitrile) fragment (149.1(3)° at 250 K). The HS to low spin (LS) HS→LS transition triggers reorientation of the propionitrile molecule resulting in accommodation of a typical linear geometry of the Fe-N-C(nitrile) fragment. The switching of the propionitrile molecule orientation in relation to the coordination octahedron is associated with increase of the distance between the supramolecular layers. When the crystal is in the LS phase, raising the temperature does not cause reduction of the distance between supramolecular layers, which contributes to further stabilization of the more linear geometry of Fe-N-C(C(2)H(5)) and the LS form of the complex. Thus, a combination of Fe-N-C(C(2)H(5)) geometry lability and lattice effects contributes to the appearance of hysteretic behavior (T(1/2)(↓) ≈ 112 K, T(1/2)(↑) ≈ 141 K).