Does the Structural Preorganization Contribute to the “Two-Way” Spin Crossover in 1,1′-Di(tetrazol-1-ylo)methane-Based Fe(II) Coordination Polymer Containing Conformationally Labile Adiponitrile Coligand?

Abstract

The reaction between 1,1′-di(tetrazol-1-ylo)methane (1ditz) and iron(II) tetrafluoroborate carried out in the presence of adiponitrile (ADN) afforded the coordination compound [Fe2(μ-1ditz)4(μ-ADN)(ADN)2](BF4)4·2ADN. The 1ditz molecules bridge the Fe(II) ions in two directions, resulting in a polymeric layer, whereas adiponitrile molecules join 1ditz-based units, extending the structure into the three-dimensional network. One of the two axial coordination sites of Fe(II) is occupied by monodentately coordinating adiponitrile. Dinitrile can also act as guest molecules occupying the area between the 1ditz-based layers. Cooling from room temperature triggers the structural phase transitions HS(P1; a, b, c) → HS(P2; 2a, b, c) → LS(P3; a, b, c) (P, phase; HS, high spin; and LS, low spin) associated with the conformational changes of the adiponitrile molecules. In the cooling mode, T 1/2 ↓ = 160 K, while in the heating mode T 1/2 ↑ is equal to 185 K. Depending on the further path of change of the temperature, it is possible (i) to perform the same cooling/heating cycle (T 1/2 ↓ = 160 K; T 1/2 ↑ = 185 K) after the sample is reheated above 240 K or (ii) to execute the cooling with T 1/2 ↓ = 167 K after the heating is stopped at 190 K, where the phase with superlattice (P2) does not appear. Both ways are reversible, and they are managed by conformational changes of the adiponitrile molecules.

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

“…Recently we have presented results of studies of three-dimensional coordination polymer [Fe 2 (μ-1ditz) 4 (μ-ADN)(ADN) 2 ](BF 4 ) 4 ·2ADN. 28 In this compound, adiponitrile molecules play a triple role: being non-coordinated guest molecules, coordinating monodentately and finally acting as bridging molecules. In this complex, polymeric layers (composed of Fe( ii ) ions bridged by 1ditz molecules in two directions) are crosslinked into a three-dimensional net by adiponitrile molecules.…”

“…The spin crossover in 3 was incomplete at 80 K, therefore we were not able to determine Fe–N distances characteristic of the low spin form. To estimate the contribution of the Fe( ii ) ions converted at 80 K to the LS form we utilized the d (Fe–N) HS − d (Fe–N) LS difference (0.193 Å) found for recently reported complex [Fe 2 (μ-1ditz) 4 (μ-ADN)(ADN) 2 ](BF 4 ) 4 ·2ADN 28 after cooling from 200 to 100 K. It allowed us to estimate that ca. 78%, 77% and 63% of the Fe( ii ) ions occupying Fe2, Fe1 and Fe3 sites adopted the LS form at 80 K, respectively.…”

“…Attempts to achieve partial replacement of tetrazole rings using small monodentate donors like thiocyanates afforded high spin products. 26 The exceptions are one-dimensional (1D) coordination polymers based on 2-hydroxy-1-(tetrazol-1-yl)-3-(tetrazol-2-yl)propane 27 and 1,1′-di(tetrazol-1-ylo)methane 28 in which [Fe(tetrazolyl) ] composition of the first coordination sphere, 29 heteroleptic systems can also be prepared. Among them, compounds were found in which structural transformations had a decisive influence on spin crossover transitions.…”

“…Moreover, recent studies revealed that di(tetrazol-1-yl)methane (1ditz) forms 3D coordination polymers in which layers, constituted by 1ditz bridging molecules, are crosslinked by adiponitrile molecules, resulting in the formation of a 3D net. 28 Hence, to assess the stability of the polymeric layer motif created by 1ditz, the studies were expanded to include other dinitrile coligands with different lengths of alkyl spacers.…”

“…Attempts to achieve partial replacement of tetrazole rings using small monodentate donors like thiocyanates afforded high spin products. 26 The exceptions are one-dimensional (1D) coordination polymers based on 2-hydroxy-1-(tetrazol-1-yl)-3-(tetrazol-2-yl)propane 27 and 1,1′-di(tetrazol-1-ylo)methane 28 in which [Fe(tetrazolyl) 4 (RCN) 2 ]-type cores (RCN – organic nitrile) are present. Nevertheless, there is a lack of other examples of Fe( ii ) complexes combining tetrazol-1-yl rings with organic nitrile donors.…”