Robert Bronisz

Synthesis and structure of {[Zn(1,2-di(1,2,3,4-tetrazol-2-yl)ethane)3](ClO4)2}n. The first coordination polymer based on 2-substituted tetrazole

Abstract A 1,2-bis(tetrazol-2-yl)ethane (ebtz) was prepared by alkylation of 1H,2,3,4-tetrazole. The new ligand was characterised crystallographically and by IR and 1H NMR spectroscopy. First coordination polymer based on 2-substituted tetrazole of the formula {[Zn(ebtz)3](ClO4)2}n was synthesised. The molecular structure of macrocation consists of zinc(II) ions triple stranded by N4, N4′ coordinating ebtz molecules resulting in formation of one dimensional chain. The zinc(II) ion is surrounded octahedrally by six nitrogen atoms from tetrazole rings. The noncoordinated perchlorate anions are located in channels, which are placed between polycationic chains.

Thermal and Light-Induced Spin Switching Dynamics in the 2D Coordination Network of {[Zn1-xFex(bbtr)3](ClO4)2}∞ : The Role of Cooperative Effects

The thermal spin transition, the photoexcitation, and the subsequent spin relaxation in the mixed crystal series of the covalently linked two-dimensional network {[Zn(1-x)Fe(x)(bbtr)(3)](ClO(4))(2)}(∞) (x = 0.02-1, bbtr =1,4-di(1,2,3-triazol-1-yl)-butane) are discussed. In the neat compound, the thermal spin transition with a hysteresis of 13 K is accompanied by a crystallographic phase transition (Kusz, J.; Bronisz, R.; Zubko, M.; Bednarek, H. Chem. Eur. J.2011, 17, 6807). In contrast, the diluted crystals with x ≤ 0.1 stay essentially in the high-spin state down to low temperatures and show typical first order relaxation kinetics upon photoexcitation, and the structural phase transition is well separated from the spin transition. With increasing Fe(II) concentration, steeper thermal transitions and sigmoidal relaxation curves indicate increasingly important cooperative effects. Already at x = 0.38, the spin relaxation is governed by cooperative interactions between Fe(II) centers, and the crystallographic phase transition begins to influence the spin transition. The kinetic behavior of the thermal spin transition is reproduced within the framework of a dynamic mean-field model.

Persistent bidirectional optical switching in the 2D high-spin polymer {[Fe(bbtr)3](BF4)2}∞.

In the covalently linked 2D coordination network {[Fe(bbtr)(3)](BF(4))(2)}(∞), bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, the iron(II) centers stay in the high-spin (HS) state down to 10 K. They can, however, be quantitatively converted to the low-spin (LS) state by irradiating into the near-IR spin allowed (5)dd band and back again by irradiating into the visible (1)dd band. The compound shows true light-induced bistability below 100 K, thus, having the potential for persistent bidirectional optical switching at elevated temperatures.

A New Family of Spin-Crossover Complexes Based on a FeII(Tetrazolyl)4(MeCN)2-Type Core

A bitopic ligand 2-hydroxy-1-(tetrazol-1-yl)-3-(tetrazol-2-yl)propane (12pbtzOH) was synthesized and reacted with Fe(ClO4)(2).6H2O, giving a 1D coordination polymer {[Fe(12pbtzOH)2(CH3CN)2](ClO4)(2).2CH3CN} infinity that exhibits a high-spin to low-spin transition (T1/2(downward arrow)=T1/2(upward arrow) congruent with 104 K). This is an unprecedented example of an iron(II) complex containing Fe(tetrazolyl) 4(MeCN)2 cores.

On the Role of Intermolecular Interactions on Structural and Spin‐Crossover Properties of 2D Coordination Networks [Fe(bbtr)3]A2 (bbtr=1,4‐bis(1,2,3‐triazol‐1‐yl)butane; A=ClO4−, BF4−)

A series of complexes [M(bbtr)(3)]A(2) (M=Fe(II), Zn(II); bbtr=1,4-bis(1,2,3-triazol-1-yl)butane; A=ClO(4)(-), BF(4)(-)) and [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) (0<x<1) dilute systems was synthesized and characterized. Earlier studies on [Fe(bbtr)(3)](ClO(4))(2) (1⋅ClO(4)), which crystallizes in space group P3, revealed an abrupt spin transition with T(1/2)↓ ≈101 K and T(1/2)↑ ≈109 K. Variable-temperature measurements of the lattice parameters and change in Bragg peaks profiles of 1⋅ClO(4) showed a structural phase transition at 125 K leading to space group P1. Single-crystal X-ray diffraction studies allowed structures of the quenched high-spin phase and then a low-symmetry low-spin phase of 1⋅ClO(4) to be determined. We established that tetrafluoroborate analogue 1⋅BF(4) remains in the high-spin form in the temperature range 5-300 K. Contrary to 1⋅ClO(4), structural investigations did not reveal the presence of a structural phase transition in 1⋅BF(4). Analogous to 1⋅ClO(4), [Zn(bbtr)(3)](ClO(4))(2) (2⋅ClO(4)) undergoes a structural transition at 151 K, whereas at low-temperature [Zn(bbtr)(3)](BF(4))(2) (2⋅BF(4)) remains in space group P3. The structural phase transitions in both perchlorates are accompanied by similar reorganization of the intermolecular contacts, which leads to a shift of 2D layers. In effect, compression of the unit cell takes place, favouring the appearance of the spin transition in 1⋅ClO(4) in relation to 1⋅BF(4). The metal ion determines the temperature of the structural phase transition in 1⋅ClO(4) and 2⋅ClO(4). This property was exploited to separate the non-magnetic structural transformation and spin transition in 1⋅ClO(4). The spin and structural transitions in the [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2) systems take place at different temperatures, and the temperature difference between them increases with increasing zinc(II) content. This means that structural phase transition is necessary for appearance of the spin transition, but this transformation does not trigger directly the spin transition in 1⋅ClO(4).

Low-Spin→High-Spin Relaxation Dynamics in the Highly Diluted Spin-Crossover System [FexZn1−x(bbtr)3](ClO4)2

Whereas the neat polymeric iron(II) compound [Fe(bbtr)(3)](ClO(4))(2), bbtr = 1,4-di(1,2,3-triazol-1-yl)butane, shows a quantitative spin transition triggered by a crystallographic phase transition centered at 107 K with a 13 K wide hysteresis, the iron(II) complexes in the diluted mixed crystals [Fe(x)Zn(1-x)(bbtr)(3)](ClO(4))(2), x = 0.02 and 0.1, stay predominantly in the (5)T(2) high-spin state down to cryogenic temperatures. However, the (1)A(1) low-spin state can be populated as metastable state via irradiation into the spin-allowed (5)T(2)→(5)E ligand-field transition of the high-spin species in the near-infrared. The quantum efficiency of the light-induced conversion is approximately 10% at low temperatures and decreases rapidly above 160 K. The lifetime of the light-induced low-spin state decreases from 15 days at 40 K to 30 ns at 220 K, that is, by 14 orders of magnitude. In the high-temperature regime the activation energy for the low-spin→high-spin relaxation is 1840(20) cm(-1).

Light‐Induced Bistability in the 2 D Coordination Network {[Fe(bbtr)3][BF4]2}∞: Wavelength‐Selective Addressing of Molecular Spin States

Whereas the neat polymeric Fe(II) compound {[Fe(bbtr)3 ][ClO4 ]2 }∞ (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) shows an abrupt spin transition centered at 107 K facilitated by a crystallographic symmetry breaking, in the covalently linked 2D coordination network of {[Fe(bbtr)3 ][BF4 ]2 }∞ , Fe(II) stays in the high-spin state down to 10 K. However, strong cooperative effects of elastic origin result in reversible, persistent, and wavelength-selective photoswitching between the low-spin and high-spin manifolds. This compound thus shows true light-induced bistability below 100 K. The persistent bidirectional optical switching behavior is discussed as a function of temperature, irradiation time, and intensity. Crystallographic studies reveal a photoinduced symmetry breaking and serve to establish the correlation between structure and cooperative effects. The static and kinetic behavior is explicated within the framework of the mean-field approximation.

Significant Variation of the Singlet–Quintet Intersystem Crossing Rate Constant in an Iron(II) High-Spin Complex as a Function of Temperature†

In the dilute mixed-crystal system [Zn1−xFex(bbtr)3](ClO4)2, x=2 % (bbtr=1,4-di(1,2,3-triazol-1-yl)butane), the iron(II) centers are predominantly in the high-spin state. The low-spin state can be populated as a metastable state by irradiation with near-IR light; the rate constant of the low-spin→high-spin relaxation spans 14 orders of magnitude between 40 and 220 K

Synthesis and spin-crossover characteristics of polynuclear 4-(2´-hydroxy-ethyl)-1,2,4-triazole Fe(II) molecular materials

A new series of Fe(II) spin-crossover materials of formula [Fe(hyetrz)3](Anion)2•xH2O, where hyetrz = 4-(2′-hydroxy-ethyl)-1,2,4-triazole and Anion = Cl−, NO3−, Br−, I−, BF4−, ClO4−, PF6−, have been prepared and the spin transition characteristics studied. The structure of these compounds consists of linear chains in which the Fe(II) ions are linked by triple N1,N2-1,2,4-triazole bridges. Most of the hydrated compounds show non-classical spin-crossover behaviour associated with the removal of lattice water molecules, which initially stabilize the low-spin state. However for two of them, the perchlorate and the iodide compounds, the transition temperature is shifted to higher temperatures by dehydration. For the corresponding dehydrated compounds, the transition temperature T1/2 increases with decreasing anion radii. [Fe(hyetrz)3]I2 represents one of the few Fe(II) spin-crossover materials showing a spin transition in the close vicinity of room temperature (291 K) accompanied by a thermal hysteresis (12 K).

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).