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The role of the covalent connections of iron(II) spin-crossover centres in the transmission of perturbation produced by the HS ↔ LS transition has been investigated for the new 3D coordination polymer {[Fe(qbtr)3](ClO4)2}∞ [qbtr = 1,5-di(1,2,3-triazol-1-yl)pentane]. In this complex, a sequence of structural phase transitions occurs. A Ph1 (HS,HS) → Ph2 (HS,HS,HS) transition precedes spin crossover and is associated with an increased number of crystallographically unique iron(II) ions, whereas the change of spin state Ph2 (HS,HS,HS) → Ph3 (HS,LS) triggers another structural phase transition accompanied by a reduction in the number of the unique iron(II) sites. Iron(II) ions occupying two crystallographically unique Fe1 and Fe2 sites form the nodes of a coordination network in the Ph1 phase. The characteristic feature of the polymeric skeleton is the presence of three kinds of ligand bridges tethering the Fe1 ···Fe1, Fe1 ···Fe2 and Fe2 ···Fe2 sites. The ligand linkages differ from each other in rigidity, and the diversification of their properties arises from the susceptibility of the ligand molecules to undergo conformational changes. The high rigidity of the bridge established in the Ph2 phase between iron(II) ions occupying Fe2 sites allows for the propagation of the perturbation produced by the HS (Fe2) → LS (Fe2) transition through the crystal lattice. A strong compression along the Fe2 ···Fe2 bridging direction occurs. In contrast, the ligand molecules that form the two remaining types of bridge are disordered in the Ph2 phase and the change of spin state for the iron(II) ions occupying Fe2 sites induces their ordering, which leads to an increase in the separation between donor atoms. In effect, perturbation cannot be effectively transmitted along the Fe1 ···Fe1 and Fe1 ···Fe2 linkage directions because the 'elongation' of conformationally labile ligand molecules compensates for the shortening of the Fe2-N bond lengths. [ABSTRACT FROM AUTHOR]

(C3N2H5)2[CoCl4] (ICC) was characterized in a wide temperature range by the single-crystal X-ray diffraction method. Differential scanning calorimetry revealed two structural phase transitions: continuous at 245.5 K (from phase I to II) and a discontinuous one at 234/237 K (cooling/heating) (II → III). ICC adopts monoclinic space groups C2/c and P21/c in phase (I) and (III), respectively. The intermediate phase (II) appears to be incommensurately modulated. Dynamic properties of polycrystalline ICC were studied by means of dielectric spectroscopy and proton magnetic resonance (¹H NMR). The presence of a low frequency dielectric relaxation process in phase III reflects libration motion of the imidazolium cations. The temperature dependence of the ¹H spin-lattice relaxation time indicated two motional processes with similar activation energies that are by about an order of magnitude smaller than the activation energy obtained from dielectric studies. There are no abrupt changes in the ¹H relaxation time at the phase transitions indicating that the dynamics of the imidazolium rings gradually varies with temperature; that is, it does not change suddenly at the phase transition. Negative values of the Weiss constant and the intermolecular exchange parameter were obtained, confirming the presence of a weak antiferromagnetic interaction between the nearest cobalt centers. Moreover, the magnitude of zero field splitting was determined. The AC susceptibility measurements show that a slow magnetic relaxation is induced by small external magnetic field. [ABSTRACT FROM AUTHOR]

2D structurally related iron(ii) coordination networks {[Fe(hbtz)2(RCN)2](ClO4)2}∞ featuring, besides tetrazol-2-yl rings in the first coordination sphere, also axially coordinated propionitrile or allyl cyanide molecules (R = C3H5–, 1; R = C2H5–, 2) were synthesized. Thermally induced spin crossover (SCO) in 1 takes place in two poorly resolved stages (T11/2↓ = T11/2↑ = 198 K, T21/2↓ = 170 K, T21/2↑ = 171 K) whereas in 2 complete and relatively gradual one step SCO (T1/2↓ = T1/2↑ = 160 K) occurs. Diversification of the SCO properties of the complexes originates from the ability of coordinated allyl cyanide in 1 to undergo conformational alterations, which is not observed for propionitrile molecules in 2. SCO in 1 is accompanied by a non-monotonic change of the contribution of allyl cyanide conformers which is related to reconstruction of the network of intermolecular contacts established between polymeric layers. The coordination network 1 exhibits extraordinary elasticity and in the second stage SCO, accompanied by conformational changes of allyl cyanide, triggers a crystallographic phase transition which leads to the formation of a superstructure. What is important, the formation of the superstructure is not caused by long range ordering of HS and LS iron(ii) ions. The structural alteration is associated with corrugation of the polymeric skeleton and disappearance of nitrile disorder. Irradiation of a single crystal of 1 at 15 K with laser light (520 nm) allowed producing a novel low temperature HS phase of 1 in which, contrary to the high temperature HS phase, disordering of anion and allyl cyanide molecules is not observed and the corrugated nature of the polymeric layer, characteristic of the LS phase, is preserved. [ABSTRACT FROM AUTHOR]

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(BF 4 ) 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)](BF 4 ) 2 (1), [Fe(μ-hbtz) 2 (μ-ADN)](BF 4 ) 2 (2), [Fe(μ-1ditz) 2 (GLN) 2 ](BF 4 ) 2 ·GLN (3) and [Fe(μ-1ditz) 2 (μ-SUN)](BF 4 ) 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.

Cooling [Fe(bbtr) 3 ](BF 4 ) 2 (bbtr=1,4-di(1,2,3-triazol-1-yl)butane) triggers very slow spin crossover below 80 K (T 1/2 ↓ =76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T 1/2 ↑ =89 K). In contrast to isostructural perchlorate analogue [Fe(bbtr) 3 ](ClO 4 ) 2 in which spin crossover during cooling is preceded by phase transition at T PT =126 K in tetrafluoroborate phase transition does not occur to the beginning of spin crossover (80 K). Studies of mixed crystals [Fe(bbtr) 3 ](BF 4 ) 2(1-x) (ClO 4 ) 2x (0.5≤x≤0.9) showed that a phase transition precedes spin crossover, however, for x≅0.46 intersection of T 1/2 (x) and T PT (x) dependencies takes place. The application of pressure of 1 GPa shifts the spin crossover in [Fe(bbtr) 3 ](BF 4 ) 2 to a temperature above 270 K. High-pressure studies of neat tetrafluoroborate and perchlorate, as well as mixed crystals [Fe(bbtr) 3 ](BF 4 ) 2(1-x) (ClO 4 ) 2x (0.1≤x≤0.9), revealed that at 295 K P 1/2 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 ](BF 4 ) 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)., (© 2023 Wiley-VCH GmbH.)

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF 4 ) 2 ⋅6 H 2 O in different nitriles yields one-dimensional coordination polymers [Fe(ebtz) 2 (RCN) 2 ](BF 4 ) 2 ⋅nRCN (n=2 for R=CH 3 (1) and n=0 for R=C 2 H 5 (2) C 3 H 7 (3), C 3 H 5 (4), CH 2 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 (C 2 H 5 CN/C 3 H 7 CN) 2 ](BF 4 ) 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., (© 2020 Wiley-VCH GmbH.)

An amino acid containing octanuclear heterometallic {[MnCa] 2 } cluster has been synthesized, alongside a structurally-related trigonal prismatic [MnCa] 2+ cage.

The electrostatic self-assembly reaction of the [Cu(HL)] 2+ cation, where HL = 2-(2-aminoethylamino) ethanol, and the N 3 - or [Fe(CN) 6 ] 4- anion leads to the formation of two coordination polymers with the general formula of [Cu 6 (μ 1,1 -N 3 ) 6 (μ 1,3 -N 3 ) 2 (μ 1,1,3 -N 3 ) 2 (μ 1,1,1,3 -N 3 ) 2 (HL) 2 ] n (1) and {[Cu(HL)] 2 [Fe(CN) 6 ]·H 2 O} n (2), respectively. The resulting compounds have been structurally characterized by a single-crystal X-ray diffraction technique. Compound 1 possesses a rare 3D structure. It contains centrosymmetric hexanuclear repeating units, which act as six-connected nodes in the final network and copper(ii) ions are joined together by azide anions with four different types of bridging modes, μ 1,1 , μ 1,1,3 , μ 1,1,1,3 , and μ 1,3 . The structure of compound 2 is a 2D heterometallic Cu II /Fe II layer in which the [Cu(HL)] 2 nodes and the octahedral [Fe(CN) 6 ] 4- linkers are joined by μ 2 - and unusual μ 3 -CN bridging modes. Detailed static and dynamic magnetic analyses of 1 reveal a dominant ferromagnetic intracluster interaction and a ferromagnetic 3D ordering transition below T c = 5 K. The variable temperature magnetic susceptibility measurements of compound 2 show a very weak ferromagnetic coupling between the nearest Cu(ii) ions. Also, EPR spectroscopy of these compounds has been investigated in the solid state. Nanocrystals of compound 2 have also been synthesized by a sonochemical process under different reaction conditions. The results show that the crystallinity degree and uniform distribution of nanosheets are inversely dependent on the irradiation time.

The abrupt high spin (HS)→low spin (LS) transition (T ↓ 1/2 =136 K) in [Fe(hbtz) 2 (CH 3 CN) 2 ](BF 4 ) 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., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

A new family of [3 + 3] hexanuclear 3d-4f complexes [(μ 3 -CO 3 ){Co II Ln III L(μ 3 -OH)(OH 2 )} 3 ]-(ClO 4 )·mC 2 H 5 OH·nH 2 O (1-5) [Ln = La (1), Gd (2), Tb (3), Dy (4), and Ho (5)] have been prepared in moderate to high yields (62-78%) following a self-assembly reaction between the ligand 6,6',6''-(nitrilotris(methylene))tris-(2-methoxy-4-methylphenol) (H 3 L), Co(OAc) 2 ·4H 2 O and the lanthanide ion precursors in the mandatory presence of tetrabutylammonium hydroxide. During the reaction, atmospheric carbon dioxide is fixed in the product molecule as a bridging carbonato ligand which connects all the three lanthanide centers of this molecular assembly through a rare η 2 :η 2 :η 2 -μ 3 mode of bridging as revealed from X-ray crystallography. The metal centers in all these compounds, except the Gd III analogue (2), are coupled in antiferromagnetic manner while the nature of coupling in the CoGd complex is ferromagnetic. DFT calculations revealed that this ferromagnetic interaction occurs most likely by the Co II -Gd III superexchange, mediated via the bridging oxygen atoms. Only the Co II -Dy III compound (4) displayed a slow relaxation of the magnetization at a very low temperature as established by AC susceptibility measurements. The data provides an estimation of the activation energy U/k B = 9.2 K and the relaxation time constant τ 0 = 1.0 × 10 -7 s.

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.