Herein, 6-(1,2,3-triazol-1-ylmethyl)coumarin (L1), which can combine molecule photoreactivity and spin-crossover activity in Fe(ii) complexes, was prepared by reacting 6-(bromomethyl)coumarin with sodium salt 1,2,3-triazole. L1 forms mononuclear systems [Fe(L1)6](ClO4)2 & centerdot;nCH3CN (n = 0, 2). The solvated form (1) exhibits complete, one-step spin-crossover at T1/2 = 155 K. In 1, there is a competition between two coumarin fragments from two ligand molecules for access to a third fragment located between them. The distances between the double bonds of the lactone rings are equal to 3.70 and 3.92 & Aring;, and light irradiation at a wavelength of 365 nm results in [2 + 2] cycloaddition. Although photoconversion can proceed in two directions, this competition ultimately yields a two-dimensional polymer layer with a frustrated topology. The resulting complex (1c) also exhibits spin crossover, showing a slight shift to lower temperatures (T1/2 = 141 K). The non-solvated complex (2) exhibits a two-step spin-crossover (T11/2 = 83 K and T21/2 = 62 K). In contrast to 1, only one type of ligand pair exists for which topochemical requirements are fulfilled. In effect, a regular (2,2) polymeric layer is formed as a result of the [2 + 2] photocycloaddition (lambda = 365 nm). This results in an unprecedented shift in the spin crossover to higher temperatures up to 197 K. The photoconversion product (2c) is characterized by the presence of strong strains involving dimerized ligand molecules, particularly cyclobutane rings. DFT modelling based on the structure of the dimerized ligand reveals a significant difference in this strain, and the energetic effect of it (stress) is more than 240 kJ mol-1 higher for the ligands corresponding to the complex in the HS state. This is consistent with the observed 114 K shift of the transition temperature upon transformation from the initial mononuclear complex to the photoconverted 2D structure.

Reactions of the ditopic ligands 1-(5-(2-pyridyl)tetrazol-1-yl)-3-(tetrazol-2-yl)propane (5-p1tz2tz), 1-(5-(2-pyridyl)tetrazol-2-yl)-3-(tetrazol-2-yl)propane (5-p2tz2tz), and 1-(3-(2-pyridyl)-1,2,4-triazol-1-yl)-3-(tetrazol-1-yl)propane (3-p1tr1tz) with iron(II) tetrafluoroborate afforded a series of one-dimensional coordination polymers: [Fe(5-p1tz2tz)2](BF4)2 (1), [Fe(5-p2tz2tz)2](BF4)2 (2), [Fe(3-p1tr1tz)2](BF4)2·CH3CN (3) and [Fe(3-p1tr1tz)2](BF4)2·CH3OH (4). The first coordination spheres of all complexes are composed of two chelating pyridylazole units and two tetrazole rings. The two complexes involving pyridyltetrazole-form 1D chains with trans-coordinated monotetrazoles. They exhibit gradual spin transitions centred at approximately 350 K. For the pyridyl-1,2,4-triazole ligand, two polymorphs were isolated. One compound is a structural analogue of pyridyltetrazole complexes and exhibits high-spin down to 10 K. Modifying the synthetic procedure yields a 1D zig-zag chain with almost the same molecular volume, but with cis-coordinated monodentate tetrazole. This complex exhibits an abrupt spin-crossover accompanied by a hysteresis loop (T1/2 = 114 K, T1/2 = 131 K). Density functional theory (DFT) modelling of the above systems, combined with results for known mononuclear and polynuclear complexes of 1-propyltetrazole revealed how the spin-transition energies vary from the mononuclear species to the corresponding 1D chains.

The coupling between the spin state and the complex motion of the individual alkyl chains results in a unique thermal hysteresis exhibiting an "inverted" loop. Specifically, the high-spin (HS) → low-spin (LS) transition upon cooling occurs at a higher temperature than the LS → HS transition upon heating.

Cu(II) coordination compounds have ½ spin system; thus, they cannot exhibit single-molecule magnet behavior; however, under a static magnetic field, slow magnetic relaxation can occur. Such compounds are interesting because this type of magnetism is related to spin qubits. Nevertheless, examples of this type of Cu(II) compounds are still rare, and hence the search for novel synthetic approaches becomes a necessity. In this study we have presented the first example of photoreactive Cu(II) coordination compound exhibiting field induced slow magnetic relaxation. The introduction of an able to [2+2] photocyclization coumarin fragment resulted in the possibility to convert Cu(II) system, showing field induced slow magnetic relaxation at 16 K, to another one also exhibiting slow relaxation however shifted below 5 K. Light induced [2+2] cyclization occurs in the studied system practically quantitatively which opened the way to combine relaxation pathways in the one material via controllable partial photoconversion. This approach, to date unexplored, resulted in obtaining a half-photoconverted system exhibiting the characteristics close to the pre-conversion and post-conversion systems. Thus, controlled photoconversion has become a convenient way to modulate properties without having to synthesize compounds from their initially chemically modified building blocks.

The light induced [2 + 2] cyclization of the flexible coumarin-based ligand (L) converts the spin crossover active HS1 ⇆ LS1 mononuclear system [Fe(L)6](BF4)2·4CH3CN (1) into the high spin 1D coordination polymer (2). The contribution of the resulting high spin form HS2 is directly related to the degree of photoconversion and, at the same time, practically does not affect the properties of the remaining thermally active spin crossover centers (HS1). The origin of such a fundamental change in properties is an appearance of strain caused by ligand dimerization, which acts directly on the metal chromophores and is transmitted to the crystal lattice. The spin state of 2 can be changed by applying pressure as well as by light irradiation revealing a "hidden hysteresis" phenomenon (Appl. Phys. Lett., 2008, 93, 21906), referring to the appearance of the low spin state not accessible through thermal activation but through reversed-LIESST. A unique feature of 2 is the feasibility to attain any steady state within the hidden hysteresis region by combination of perturbations triggered by changes in temperature and light (808 nm HS2 → LS2 and 532 nm LS2 → HS2). Such states are stable within a time scale of several hours.

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 (T-1/2(down arrow)=76 K). The spin crossover (SCO) is accompanied by a hysteresis loop (T-1/2(up arrow)=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 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)](BF4)(2(1-x))(ClO4)(2x) (0.5 <= x <= 0.9) showed that a phase transition precedes spin crossover, however, for x congruent to 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)](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 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)](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).

Optically pure (RR)- and racemic (RR/SS)-trans-1,2-di(tetrazol-1-yl)cyclopentane were synthesized and used to prepare homo- and heterochiral Fe(II) coordination compounds. [Fe((RR/SS)-C7H10N8)(2)(CH3CN)(2)](BF4)(2) (1A), [Fe((RR/SS)-C7H10N8)(2)(C2H5CN)(2)](BF4)(2) (2A), [Fe((RR)-C7H10N8)(2)(CH3CN)(2)](BF4)(2)2CH(3)CN (1Bsolv), and [Fe((RR)-C7H10N8)(2)(C2H5CN)(2)](BF4)(2) (2B) form a family of one-dimensional coordination polymers. Fe(II) cations in these complexes are characterized by a heteroleptic coordination environment: the neighboring metal centers are bridged by two 1,2-di(tetrazol-1-yl)cyclopentane molecules, while the nitrile molecules (acetonitrile or propionitrile, respectively) occupy the axial positions. Independently of the kind of nitrile coligands, an ability to thermally induce spin crossover (SCO) is governed by chirality. 1Bsolv and 2B exhibit abrupt and complete SCO occurring at T-1/2 = 144 K and T-1/2 = 228 K, respectively. Desolvated form, 1B (of the same stoichiometry as 2B), also exhibits SCO (T-1/2 = 215 K). In contrast, an exchange within the polymeric chain of half of the RR molecules with the SS enantiomeric form results in formation of 1A and 2A, which remain in stable high-spin (HS) form down to 10 K. It has been shown that moving from a homochiral to a heterochiral system changes the structure of the polymeric unit (while maintaining the same polymer dimensionality and bridging fashion) that leads to the deep reorganization of the further coordination spheres, including the anion network.

Di(tetrazolyl)alkanes form with Fe(ii) chains or layers which can be extended into spin-crossover 2D and 3D networks by applying dinitriles.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

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(mu-1ditz)4(mu-ADN)(ADN)2](BF4)4 center dot 2ADN. The 1ditz molecules bridge the Fe(II) ions in two directions, resulting in a polymeric layer, whereas adiponitrile molecules join 1ditzbased 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 1ditzbased 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, T1/2 down arrow = 160 K, while in the heating mode T1/2 up arrow 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 (T1/2 down arrow = 160 K; T1/2 up arrow = 185 K) after the sample is reheated above 240 K or (ii) to execute the cooling with T1/2 down arrow= 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.

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 (T1/2(Fe1) = 201 K; T1/2(Fe2) = 216 K), while the magnetic measurements showed that there is one step, complete thermally induced spin crossover (T1/2 = 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 (T1/2(Fe1) = 200 K; T1/2(Fe2) = 212 K, T1/2(Fe3) = 214 K).

The resistance of Candida albicans and other pathogenic yeasts to azole antifungal drugs has increased rapidly in recent years and is a significant problem in clinical therapy. The current state of pharmacological knowledge precludes the withdrawal of azole drugs, as no other active substances have yet been developed that could effectively replace them. Therefore, one of the anti-yeast strategies may be therapies that can rely on the synergistic action of natural compounds and azoles, limiting the use of azole drugs against candidiasis. Synergy assays performed in vitro were used to assess drug interactions Fractional Inhibitory Concentration Index. The synergistic effect of fluconazole (1) and three synthetic lactones identical to those naturally occurring in celery plants—3-n-butylphthalide (2), 3-n-butylidenephthalide (3), 3-n-butyl-4,5,6,7-tetrahydrophthalide (4)—against Candida albicans ATCC 10231, C. albicans ATCC 2091, and C. guilliermondii KKP 3390 was compared with the performance of the individual compounds separately. MIC90 (the amount of fungistatic substance (in µg/mL) inhibiting yeast growth by 90%) was determined as 5.96–6.25 µg/mL for fluconazole (1) and 92–150 µg/mL for lactones 2–4. With the simultaneous administration of fluconazole (1) and one of the lactones 2–4, it was found that they act synergistically, and to achieve the same effect it is sufficient to use 0.58–6.73 µg/mL fluconazole (1) and 1.26–20.18 µg/mL of lactones 2–4. As fluconazole and phthalide lactones show synergy, 11 new fluconazole analogues with lower toxicity and lower inhibitory activity for CYP2C19, CYP1A2, and CYP2C9, were designed after in silico testing. The lipophilicity was also analyzed. A three-carbon alcohol with two rings was preserved. In all compounds 5–15, the 1,2,4-triazole rings were replaced with 1,2,3-triazole or tetrazole rings. The hydroxyl group was free or esterified with phenylacetic acid or thiophene-2-carboxylic acid chlorides or with adipic acid. In structures 11 and 12 the hydroxyl group was replaced with the fragment -CH2Cl or = CH2. Additionally, the difluorophenyl ring was replaced with unsubstituted phenyl. The structures of the obtained compounds were determined by 1H NMR, and 13C NMR spectroscopy. Molecular masses were established by GC-MS or elemental analysis. The MIC50 and MIC90 of all compounds 1–15 were determined against Candida albicans ATCC 10231, C. albicans ATCC 2091, AM 38/20, C. guilliermondii KKP 3390, and C. zeylanoides KKP 3528. The MIC50 values for the newly prepared compounds ranged from 38.45 to 260.81 µg/mL. The 90% inhibitory dose was at least twice as high. Large differences in the effect of fluconazole analogues 5–15 on individual strains were observed. A synergistic effect on three strains—Candida albicans ATCC 10231, C. albicans ATCC 2091, C. guilliermondii KKP 339—was observed. Fractional inhibitory concentrations FIC50 and FIC90 were tested for the most active lactone, 3-n-butylphthalide, and seven fluconazole analogues. The strongest synergistic effect was observed for the strain C. albicans ATCC 10231, FIC 0.04–0.48. The growth inhibitory amount of azole is from 25 to 55 µg/mL and from 3.13 to 25.3 µg/mL for 3-n-butylphthalide. Based on biological research, the influence of the structure on the fungistatic activity and the synergistic effect were determined.

[Zn(ebtz) 3 ](BF 4 ) 2 (1,2-di(tetrazol-2-yl)ethane) was a first example of coordination polymer based of 2-substituted tetrazole as donor group [1]. Expanding studies on Fe(II) complexes showed that species of [Fe(tetrazol-2-yl) 6 ]-type core exhibit thermally induced spin crossover (SCO) [2]. Further researches revealed an ability of 2-substituted tetrazole to the formation of coordination compounds in which metal ion (Cu(II), Fe(II)) is surrounded by four tetrazole rings and two alcohol or nitrile molecules. The complexes of the type [Fe(tetrazol-2-yl) 4 (RCN) 2 ] also exhibit SCO, which can be additionally affected by conformational changes of axially coordinated nitrile molecules [3]. It was established that the presence of a wide hysteresis loop in [Fe(ebtz) 2 (C 2 H 5 CN) 2 ](ClO 4 ) 2 is related to the reorientation of coordinated propionitrile molecules coupled with significant changes of separation between supramolecular layers [4]. In order to explain the role of coordinated nitrile molecules on spin crossover properties, we have carried out detailed studies depending on systematic exchange of the ones in series of [Fe(ebtz) 2 (RCN) 2 ](BF4 4 ) 2 [5]. We have focused on uncommon, very slow spin crossover observed in propionitrile derivatives. Measurements of the temperature dependence of magnetic susceptibility revealed thermal quenching of HS form after rapid cooling of the sample at 10 K. Measurements carried out at very slow scan rates showed an occurrence of hysteretic spin crossover (T 1/2  = 78 K, T 1/2  =123 K). It allowed to perform isothermal (80 K) time-resolved single-crystal X-ray diffraction studies for the HS → LS transition. Initially, it occurs very slow shrinkage of polymeric chains associated with reduced cell volume at 77% (concerning the difference between cell volumes V HS - V LS ) and only 16% of iron(II) ions adopt LS form. Fe this there is of propionitrile molecules connected with an increase of Fe-N-C(nitrile) angle from 143.6 to 161.6 . LIESST and r-LIESST studies performed at 14 K on single crystals confirmed that the contribution of switched Fe(II) ions strongly corresponds to the orientation of the nitrile molecule. These studies showed that stabilization of the spin form, produced by light irradiation, is dependent on the lattice-based effects. This property was utilized to manipulate spin crossover parameters by partial exchange of propionitrile by butyronitrile molecules. These studies showed that an increase in a fraction of butyronitrile molecules involves an increase of Fe-N-C(nitrile) angle resulting in a shift of SCO temperatures to higher values and in reduction of the width of the hysteresis loop.

Spin crossover occurs in octahedral coordination compounds of the 3d 4 -3d 7 electronic configuration of metal ions.The most spectacular changes are observed in Fe(II) complexes, where the HS→LS (HS -high spin, LS -low spin) transition is associated with shortening of Fe-N distance at about 0.2 Å.Although an ability to change of spin state is an intrinsic feature of the metal ion, the spin crossover properties of bulky, crystalline samples depend on the crystal structure of the coordination compound.Thus, different compositions of first coordination spheres of metal ions or presence in the crystal lattice crystallography unique molecules can result in the complex course of γHS(T) dependence (γHS(T) -the molecular ratio of molecules in HS form).Most often, a two-step spin crossover can be observed in such a situation.Our studies on iron(II) coordination polymers based on 1,4-di(1,2,3-triazol-1-yl)butane (bbtr) and its derivatives revealed a variety of spin crossover behaviours.[Fe(bbtr)3](ClO4)2 exhibits abrupt spin crossover accompanied by hysteresis loop (T1/2  = 112 K, T1/2  = 141 K) [1].Importantly spin crossover in this complex is accompanied by structural phase transition P-3→P-1 depending on the shift of neighbouring polymeric layers.The structural phase transition has not been found in the tetrafluoroborate analogue, and the complex [Fe(bbtr) 3 ](BF 4 ) 2 remains in the HS form in the range 10-300 K[2].The importance of structural changes on spin crossover properties showed our further studies using bbtr derivatives.An application of 1,4di(5-ethyl-1,2,3-triazol-1-yl)butane (ebbtr) leads to the formation of two-dimensional coordination polymers exhibiting unique spin crossover: "double"[3] and "normal and reverse"[4] transitions.The occurrence of uncommon spin transitions in these complexes is associated with significant structural changes.An application of regioisomeric ligand bbtre leads to forming a three-dimensional coordination network in which the multi-way spin crossover is strongly related to conformational changes of the bridging ligands [5].Studies of bbtr-based coordination polymers revealed the importance of counterion.Therefore, we have expanded our studies on the application of triflate derivatives.Synthesis performed between Fe(CF3SO3)2•6H2O and bbtr leads to forming a two-dimensional coordination polymer.The complex crystallizes in R-3 space group.The characteristic feature is the ordering of the half of bbtr bridging molecules and the presence of two crystallographically unique Fe(II) ions.Spin crossover is gradual and complete.Careful analysis of change of Fe-N distances revealed interesting phenomena.Namely, despite one-step spin crossover, both crystallographically unique Fe(II) ions change the spin state in different temperature ranges.Moreover, we have established the occurrence of very slow structural phase transition R-3→ P63.This structural transformation is associated with the vanishing of ligand disorder.Details concerning crystal structures of complexes before and after R-3→ P63 transformations on the poster.

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.

Reaction of 1,2-di(tetrazol-2-yl)ethane (ebtz) with Fe(BF4)(2).6 H2O 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=C2H5(2) C3H7(3), C3H5(4), CH2Cl (5)) exhibiting spin crossover (SCO). SCO in 1 and 3-5 is complete and occurs above 160 K. In2, it is shifted to lower temperatures and is accompanied by wide hysteresis (T-1/2(down arrow)=78 K,T-1/2(up arrow)=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 in2. 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)(C2H5CN/C3H7CN)(2)](BF4)(2)mixed crystals (2 a,2 b), showing a shift ofT(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 degrees) value of its Fe-N-C(propionitrile) angle.