Spin-dimer networks: engineering tools to adjust the magnetic interactions in biradicals †

Magneto-structural correlations in stable organic biradicals have been studied on the example of weakly exchange coupled models with nitronyl nitroxide and imino nitroxide spin-carrying entities. Here, heteroatom substituted 2,2 0 -diaza- and 3,3 0 -diaza-tolane bridged biradicals were compared with the hydrocarbon analogue, while a biphenyl model with its 2,2 0 -bipyridine counterpart. For a 3,3 0 -diazatolane bridge the torsional angle between the nitronyl nitroxides and the pyridyl rings increased heavily ( B 52–54 1 ) leading to a smaller theoretical intra-dimer exchange coupling value. However, a very large anti-ferromagnetic coupling was obtained experimentally. This could be appropriately explained by the presence of dominating inter-dimer exchange between the molecules. For the bis(imino nitroxide) with tolane bridge a field induced ordered state between 1.8 to 4.3 T in AC-susceptibility measurements was observed. In terms of a Bose Einstein condensate (BEC) of triplons this phenomenon could be described as a magnetic field induced ordered phase with 3D character.


Introduction
Magnetic dimers formed by weakly interacting antiferromagnetically coupled spin S = 1/2 centres are recognized as suitable candidates for exploring critical phenomena under well-controlled conditions. 1 When these systems are placed in a magnetic field strong enough to close the dimer gap, a gas of triplet excitations (triplons) is formed. 1 Depending on the topology of the dimerdimer couplings, various scenarios can be observed.Prominent examples include the Bose-Einstein condensation (BEC) of triplons in three-dimensionally-coupled dimer systems, as described by Tchernyshyov et al., 2 and the Luttinger-liquid behavior revealed in a one-dimensional spin-ladder system by Kra ¨mer and co-workers. 3ore recently, a Berezinskii-Kosterlitz-Thouless scenario was observed by Tutsch et al. in a two-dimensional coordinated copper polymer with large interlayer spacing. 46][7][8] Plasticity, flexibility and solubility in common organic solvents define the ease of various device fabrication.Advanced chemistry techniques permit small structural modifications in already achieved model systems in order to fine-tune their physical properties.In addition, organic-based magnetic materials offer a number of convenient tools extremely helpful in controlling the intra-and inter-dimer exchange coupling, such as p-p interactions, hydrogen bonding, etc. Nitronyl nitroxide (NN) radicals are well-known for their stability and bidentate character. 9,10NN radicals are among the most popular spin carriers used to construct molecule-based magnets. 10mportantly, in these radicals the spin density of the unpaired electron is delocalized over two semi-equivalent sites of coordination.This in turn allows the arrangement of the NN-molecules into a supramolecular network of interacting spins. 11Theoretical studies emphasized the importance of the mutual orientation and the relative distances in the crystal packing for promoting an efficient magnetic coupling. 11,12As was mentioned by Lahti, 13 minor changes in the crystal packing of biradicals could result in significant alterations of the magnetic behavior in the bulk material.In this regard the influence of the p-bridges on the intra-and intermolecular exchange interactions in conjugated biradical networks was studied on the example of the diazotolane dinitroxide models (Fig. 1).It was anticipated that heteroatom substitution could offer a particularly effective pathway for transmitting the magnetic interactions. 14ypically, imino nitroxides (INs) reveal a reduced J dimer value when compared to the corresponding NN biradicals.Taking this fact into consideration, already known bridges could be used, where otherwise a strong intramolecular exchange coupling was observed. 11Following this strategy, three bisimino nitroxides 3d, 4d, 5d were prepared (Fig. 1). 15n the current work we report the synthesis of a series of new nitroxide biradicals (Fig. 1), their characterization, including X-ray structural analysis and their magnetic properties, which are discussed with respect to the quantum chemical calculations based on the DFT approach.

Synthesis of nitroxide biradicals
Access to the family of diazatolane biradicals 1c and 2c required the synthesis of key precursors 1a and 2a, which were prepared following Sonogashira-Hagihara methodology. 16Interestingly, both structural isomers 1a and 2a could be achieved starting from the same precursor, i.e. commercially available 2,5-dibromopyridine 6 (Scheme 1).8][19][20][21] More precisely, reaction times and solvents played a crucial role in the course of electrophilic substitution, leading to predominant formation of the organolithium intermediate at the 2 or 5 position. 22,23In order to drive the reaction in the course of kinetically more favorable 2-bromo-5-carbaldehydepyridine 7, it was carried out in dry diethyl ether at À78 1C, and DMF was added to the mixture 20 min after the addition of n-BuLi was complete (Scheme 1).
Sonogashira-Hagihara coupling reactions can be successfully carried out under various conditions. 16For our systems it was found that by decreasing the reaction temperature from 80 1C (DMF/Et 3 N, 1 : 1) to B22 1C (CH 3 CN/Et 3 N, 1 : 1) the formation of several side-products was prevented.Therefore, the attachment of bromo derivative 8 to the pre-organized ethynyl-pyridine 10 was carried out at room temperature in the CH 3 CN/Et 3 N solvent mixture, which led to the corresponding product in a reasonable yield (72%).Notably, our attempts to remove dioxolane protective groups in the presence of dilute HCl (3%) resulted in precipitation of the acid by the triple bond.Thus, the final step towards 1a was performed under milder conditions.To a solution of the dioxolane precursor in an acetone-water mixture (7 : 1) a catalytic amount of p-TsOH acid (2 mol%) was added. 24Upon stirring the reaction mixture for 3 days at room temperature the target 2,2 0 -diazatolane 4,4 0 -dialdehyde 1a was obtained in nearly quantitative yield (81%).
Several synthetic procedures towards compound 3a were reported in the literature. 15Inspired by the efficiency and simplicity of the approach sketched in Scheme 2, commercially available 4,4 0 -dibromobiphenyl 14 was treated with n-BuLi in the presence of a catalytic amount of TMEDA.After addition of dry DMF, typical work-up and purification, biphenyl dialdehyde 3a was isolated in 58% yield.
In order to avoid further oxidation, and, therefore, to diminish the loss of the radical units, oxidation of N,N 0 -dihydroxyimidazolidine intermediate 1b with sodium periodate was carried out at B0-5 1C using an ice bath. 27The progress of the reaction could be conveniently monitored by TLC analysis of the reaction mixture aliquots.Synthesis of derivative 2c was achieved using an excess of MnO 2 in methanol.Imino biradicals 1d, 3d, and 4d were obtained following the procedure described by Tretyakov et al. 28,29 This method helped to avoid the harmful usage of acids and to synthesize the target molecules in better yields.As an illustration, transformation of imidazolidine 1b into the corresponding imino biradical 1d with an excess of MnO 2 in CH 3 NO 2 media is depicted in Scheme 3. To aid the interpretation of magnetic properties of the isomeric NN 1c and 2c related IN biradical 1d was also prepared.
The UV-Vis absorption spectra of 1c and 2c with maxima around 600 nm were typical for the nitronyl nitroxides, while the red bisiminonitroxides absorbed around 460 nm (Table S2, ESI †).The EPR spectra of bisnitronylnitroxides exhibited nine lines with A n /2 spacing for four equivalent nitrogens with two strongly coupled radicals.The bisiminonitroxides (1d, 3b, 4b, 5d) featured 13 lines, corresponding to the inequivalent nitrogens of the imidazoline ring (as presented in the Graphical abstract and Table S2, ESI †).

Crystal structure analysis
Crystals were obtained by slow diffusion of hexane in dichloromethane solutions of the nitroxide biradicals at room temperature.Deep-blue needle crystals of nitronyl nitroxides 1c and 2c and red blocks of imino nitroxides 1d, 3d, 4d, and 5d were then characterized using X-ray diffraction analysis.Selected torsion angles are reported in Table 1 (with some additional structural data given in the ESI †).
All nitroxide biradicals 1c, 2c, 1d, 3d, 4d, and 5d under study possess a planar backbone -the necessary prerequisite for the propagation of weak intramolecular magnetic interactions.Importantly, in such biradical models the torsion angles y (see Table 1) play a central role in the modulation of magnetic exchange interactions.
Not surprisingly, similar torsion angles y of about B241 result in a nearly identical degree of conjugation. 11,15Consequently, a comparable spin polarization and close values of the exchange integrals were expected for most of the obtained biradical models (see for comparison Table 1).In contrast to the described situation, the radical units in nitronyl nitroxide 2c were far more twisted (4501).From this crystal structure analysis it was assumed that nitronyl biradical 2c would exhibit a unique behavior among the compounds investigated here.
Biradical 1c adopts a triclinic space group with an inversion centre residing in the middle of the acetylene bridge (Fig. 2).Remarkably, in the asymmetric unit there are two crystallographically independent molecules with a nearly equal geometry.The dihedral angles of the mean plane of the imidazolidine ring and the two coplanar pyridine rings were found to be E251 for O1-N2-C7 (O2N3-C7) towards C5 and 221 for O3N5-C20 (O4-N6-C20) towards C18 (see Table 1).Here, the N(   The intermolecular contacts between NO groups of the first molecule with the pyridine rings (O4Á Á ÁC1 0 3.036, O4Á Á ÁC2 0 3.135 Å) of the neighboring biradical lead to V-shaped alignment of dimers (Fig. 2 and Fig. S1, ESI †).
Biradical 2c crystallizes in the P2 1 /c space group and its molecular structure is shown in Fig. 3. Compound 2c features a transoid arrangement with an inversion center of symmetry located in the C1-C1 0 bond, which is typical for the derivatives containing acetylene bridges (Fig. 3).In general, the structural data of 2c are rather similar to those described for the other nitronyl nitroxide biradicals in the literature (Table 1 and Table S1, ESI †). 11,14A remarkable difference is found in the largely increased torsion between the pyridyl ring and the imidazolidine fragment O1-N2-C7 (O2-N3-C7) with a dihedral angle of 53.72(11) (Fig. 3).
The crystal packing of 2c presents a beautiful example where the bidentate character of the nitronyl nitroxide fragment plays a major role in the spatial arrangement of the biradical chains.Here, the molecules of 2c form infinite zig-zag ribbons along the b axes (Fig. 3 and Fig. S3, ESI †).Notably, the neighboring molecules are organized in an alternate fashion, placing NN groups next to the pyridine plane and, thereby, stabilizing the crystal packing (Fig. S3, ESI †).
Imino biradical 1d crystallizes in the triclinic space group P% 1.There are two independent centrosymmetric half molecules in the asymmetric unit cell.(Fig. 4).The main structural characteristics are similar to those described for the corresponding nitronyl nitroxide derivative 1c.Thus, the pyridine rings form angles of E231 and E251 with the imidazolidine plane (Table 1).The N( 2 The packing arrangement of radical dimers 1d is shown in Fig. 4, and can be described as a ladder-like structure with biradicals of the first kind forming a stair, and nitroxides of the second type extending from both sides of each step (Fig. S4, ESI †).The structure is supported by numerous hydrogen bonds   arising from the pyridine p-bridges and the imino nitroxide chelating units.The short contacts of the NO groups and the hydrogen atoms of the pyridine core O211Á Á ÁH261 0 = 2.456, O212Á Á ÁH261 0 = 2.538 Å (Fig. 4) seems to be the most important.The shortest interchain distances of 3.352 Å (C6Á Á ÁC25 0 ) are found between the two neighboring diazatolane bridges (Fig. 4).The closest intermolecular contacts between NO groups and a pyridine ring are slightly larger, and are within the range of B2.4-2.7 (O1Á Á Á H41 0 , O1Á Á ÁH31 0 , respectively) and B3.1 Å (O1Á Á ÁC4 0 ).
Imino biradicals 3d and 4d feature isomorphic crystal structures.Thus, they possess the P2 1 /n space group with a center of symmetry located in the middle of the aromatic C-C 0 bond (Fig. 5 and Fig. S4, ESI †).The biphenyl bridge in 3d is surprisingly planar, and the dihedral angle between the mean plane of the benzene ring and the imidazolidine moiety is only 20.21.3d is slightly disordered, since the five membered ring can flip around 1801 such that the two NO groups could be oriented 'E' or 'Z' with 50% probability.The N(1)-O(1) 1.226(3) and N(2)-O(2) 1.213(3) Å bond lengths are in the standard range. 15aturally, for the isomorphic imino nitroxide derivatives 3d and 4d the main structural characteristics are very similar. 14,27hus, in 4d the O-N-C-N-O moiety is planar, with the N(3)-O(2) bond being slightly shorter than N(2)-O(1) 1.167(4) and 1.215(3) Å, respectively (Fig. S4, ESI †).Furthermore, in the asymmetric unit of 4d there are two crystallographically independent forms with the ratio of B2 : 3 belonging to different biradical chains (Fig. S4, ESI †).
An important feature already mentioned for the other biradicals of the current series is the abundance of close intermolecular contacts between the N-O entities and the aromatic moieties in the asymmetric unit.In particular, the hydrogen bonding O1Á Á ÁH31 0 2.580, O1Á Á ÁH123 0 2.696, and O1Á Á ÁC3 0 3.177 Å between the neighboring oxygen atoms and phenyl rings in 3d defines the formation of ribbon-like structures which are further organized in a zig-zag pattern.Likewise, multiple oxygen contacts (i.e.O1Á Á ÁH91 0 2.302, O2Á Á ÁH51 0 2.473, O2Á Á ÁH93 2.600, O1Á Á ÁC9 0 3.199 and O2Á Á ÁC5 0 3.085 Å) accompany the molecular ordering in the case of imino derivative 4d (Fig. S4, ESI †).Notably, the neighboring chains in 4d are connected with close O2Á Á ÁO2 0 2.801 Å contacts.
Imino nitroxide biradical 5d crystallizes in a monoclinic space group with P2 1 /n symmetry.The structure of compound 5d is shown in Fig. 6.In contrast to the previously described biradicals compound 5d has surprisingly small torsion angles between the radical unit and the adjacent phenyl ring of only about 61. 30 The overall tolane backbone is fairly planar.The N(2)-O(1) 1.271(1) Å bond distances are similar for this type of compound.

Magnetic characterization
To gain a more complete picture of the influence of a given p-spacer on the electronic properties, and especially on the intra-and inter-dimer exchange couplings, DFT calculations were performed.To get a first idea on the sizeable changes of the exchange interaction in the studied biradical models, geometry optimization with the B3LYP hybrid function and the 6-31G* basis set was carried out. 31Then the broken symmetry approach (BS) for the singlet and triplet states with the BLYP functional (to avoid Hartree-Fock contamination) and the same basis set were applied. 12Therefore, the direct exchange interaction for a weakly coupled spin dimer can be described as J/k B = E(BS) À E(T), since the spin expectation  values hS 2 (BS)i and hS 2 (T)i for the broken symmetry configuration are close to 1 and for the triplet configuration they are close to 2 such that their difference is close to unity. 32Notably, in the case of antiferromagnetic interactions J takes negative values.The calculations were then redone in accordance with the X-ray geometries, as listed in Table 2.
Applying a Quantum Design SQUID magnetometer the temperature dependence of the molar magnetic susceptibilities w mol (T) of microcrystalline samples 1c, 2c, 1d, 3d, 4d, and 5d was determined in the range of 2 K r T r 270 K in a magnetic field B = 1 T. The obtained data were corrected for the temperature-independent diamagnetic core contribution of the constituents. 33The magnetic contribution of the sample holder was determined in an independent experiment without a sample.The results are graphically displayed in the form of w mol T vs. T in the main panel, and w mol vs. T in the insets of Fig. 7 and 8 and in the ESI † (Fig. S8).AC-susceptibility measurements (w ac ) were performed only on 5d as a function of the magnetic field at T = 0.028 K using an ultra-high resolution AC-susceptometer adapted to a 3 He-4 He top-loading dilution refrigerator.The compensated-coil susceptometer was optimized for measuring small single crystals in the mg range.
Generally, all the biradicals under investigation (1c, 2c, 1d, 3d, 4d, 5d) featured a similar magnetic behavior in the studied temperature range.The observed w mol T values at 300 K of the nitroxides are around 0.7 cm 3 K mol À1 .Importantly, experimental data are rather close to the theoretical value of 0.75 cm 3 K mol À1 , which is expected for the two uncoupled spin S = 1/2 units (indicated by the broken line in Fig. 7).This indicates the high quality of the studied single crystals.
As a typical example, w mol T (T) of NN-biradical 1c is shown in Fig. 7. Upon decreasing the temperature w mol T features an approximately linear moderate decrease down to B120 K followed by a more pronounced drop below B50 K.This overall behavior reflects the dominant antiferromagnetic intra-dimer coupling J intra .The inset of Fig. 7 shows the molar magnetic susceptibility as a function of temperature.The Bleany-Bowers equation 34 for a model of an isolated dimer w iso with a mean-field correction was applied to fit the experimental data (solid line in the inset of Fig. 7).Using this fitting the sizes of the intra-dimer J intra and inter-dimer zJ 0 inter (where z is the number of the nearest neighbors) coupling constants were extracted: Thus, for 1c an experimental intra-dimer coupling constant J intra /k B = À5.4AE 0.2 K was obtained.This value is in tangible agreement with the coupling constant acquired from the broken-symmetry approach (Table 2).It should be mentioned that the experimental and theoretical J intra values of 1c are very similar to the ones reported for tolane NN (J intra /k B = À4.8K). 11 Table 2 Calculated J calc intra and experimentally obtained J exp intra intra-dimer exchange coupling constants.zJ 0 calc inter describes the inter-dimer coupling, where z is the number of the nearest neighbors.T max corresponds to the maximum in the w mol (T) plot a Optimized geometries were obtained from DFT calculations (B3LYP, 6-31G*), then single point calculations for the energy of the broken symmetry and the triplet state were applied (UBLYP, 6-31G* for the evaluation of J). 31 b The exchange interactions were evaluated for X-ray crystal structures applying the broken symmetry approach with the BLYP functional and the 6-31G* basis set.c Note that dominant magnetic exchange in 2c is between two adjacent NN units belonging to different biradicals (for details see the text and Fig. 3).In light of this it is reasonable to assume that a mere substitution of N for C in position X (as indicated in Fig. 1), if it does not severely affect the geometry of a given crystal structure, has no practical influence on the magnetic interactions.An unexpected result was acquired for NN derivative 2c where N was substituted with C in the Y position (Fig. 1).For this system the DFT calculations predict J intra values of a few Kelvin, i.e. similar to those found for the other materials under investigation.The magnetic properties of 2c determined experimentally are shown in Fig. 8.The w mol T data decreased gradually and leveled off around 20 K at a small value of approximately 0.02 cm 3 K mol À1 .In the inset of Fig. 8 a broad maximum in w mol at 53 K is clearly visible.From a theoretical fit 33 corrected with a Curie term for uncoupled S = 1/2 entities (solid red line in the inset of Fig. 8), an inter-dimer coupling constant of J inter /k B = À42.4K was obtained.This is significantly larger than J intra of 1c and tolane NN in ref. 11.This observation is conceivable in terms of the crystal structure peculiarities.As described above, the radical units in nitronyl nitroxide 2c are far more twisted (B531) in comparison to other biradical models described here.The further analysis of the X-ray data revealed exceptionally short contacts of B3.51 Å between two neighboring NO fragments in 3,3 0 -diazatolane nitronyl nitroxide 2c (Fig. S7, ESI †).Apparently, these short contacts are responsible for the unpredicted strong antiferromagnetic exchange interactions found in the solid.Taking only these intermolecular interactions into account and replacing the second radical unit by hydrogen, the value J inter /k B = À45.6K was calculated, which was very close to the experimentally obtained value of À42.4 AE 1.7 K (Table 2).Such short inter-dimer distances between spin centers could not be found in the other biradicals.
In accordance with the theoretical predictions and crystal structure analysis IN biradicals 1d, 3d, 4d and 5d featured weak antiferromagnetic intramolecular coupling.Their intra-dimer exchange constants are listed in Table 2 (see also Fig. S8, ESI †).Interestingly, in the case of IN-biradicals 3d and 4d the N-heteroatomic substitution has barely affected their magnetic properties in accordance to the previous statement.From the experimental data recorded for imino biradical 1d no clear maximum could be resolved, indicating an extremely weak intra-dimer coupling constant oÀ1.5 K.
Remarkably, a closer examination of the low-temperature (T o J intra /k B ) susceptibility data for IN-biradical 5d revealed a significant deviation from the isolated-dimer model.Typically, magnetization of the isolated-dimer systems changes in a step-wise manner at the saturation field B s and temperatures T o J intra /k B .Such field-temperature induced transitions correspond to the alteration in the spin-state population of the dimers, namely, the occupation of the low-lying triplet state.This phenomenon is reflected in the form of a single peak in susceptibility w, the field derivative of magnetization.Fig. 9 shows the AC-susceptibility of IN biradical 5d as a function of the magnetic field at 0.028 K (right scale) together with the magnetization of the material at the same temperature (left scale).
The AC susceptibility was measured only for IN biradical 5d since only for this sample a large single crystal was available.Polycrystalline samples have been shown to be critical for their intercrystalline contacts and depend on their powder to crystallinity content and several mg are still needed.The data highlight a well-pronounced double-peak feature in w ac , an evidence of a field-induced ordered state between B c1 = 1.8 T and B c2 = 4.3 T at 0.028 K.Such field-induced phases are expected for a coupled-dimer system where the lower edge of a band of magnetic excitations crosses the ground state at B c1 .Here, B c2 corresponds to the saturation field B s where the full magnetization of the system is obtained.The magnetization exhibits no hysteretic behavior.This feature is attributed to small magnetic intermolecular interactions between the neighboring biradical molecules mediated via hydrogen bonds.The strength of these interactions depends on the distance and the relative orientation of the radicals. 35,36The anomalies at the critical fields B c1 and B c2 are much sharper than the one found in a quasi-2D system. 11More likely, the inter-dimer coupling in 5d has a 3D character, and therefore, this field-induced ordered phase can be described in terms of a BEC of triplons.

Materials and methods
All chemicals and reagents were used as received from commercial sources (Acros Organics, Aldrich, Fluka, Lancaster, Merck and Strem) without additional purification.Solvents for synthesis were used as received, unless otherwise mentioned.ESR spectra were recorded in dilute, oxygen-free solutions in toluene, concentrations B10 À4 mol L À1 , using a Bruker ESP300 E X-band spectrometer equipped with an NMR gaussmeter (Bruker ER035), a frequency counter (Bruker ER041XK) and a variable temperature control continuous flow N 2 cryostat (Bruker B-VT 2000).The g-factor corrections were obtained using DPPH (g = 2.0037) as the standard.UV-Vis spectra were recorded in toluene solutions with a PerkinElmer spectrometer (UV/Vis/NIR Lambda 900) using a 1 cm optical path quartz cell at room temperature, This journal is © The Royal Society of Chemistry 2017 unless otherwise specified. 1H and 13 C NMR spectra were recorded on a Bruker DPX 250, Bruker DMX 300 spectrometer.Solid powders were pressed and IR spectra of the samples were recorded as they were (Nicolet 730 FT-IR spectrometer).Mass spectra (FDMS) were obtained on a VG Instruments ZAB-2 mass spectrometer.Elemental analyses were performed at the University of Mainz, Faculty of Chemistry and Pharmacy on a Foss Heraeus Varieo EL.The melting points were measured on a Bu ¨chi B-545 apparatus (uncorrected) by using open-ended capillaries.Crystallographic data for the reported structures of the biradicals have been deposited at the Cambridge Crystallographic Data Centre.‡ 2,3-Bis(hydroxyamino)-2,3-dimethylbutane (BHA) synthesis was described elsewhere. 11,151.General procedure for Sonogashira-Hagihara coupling reaction.An aryl bromide (21.7 mmol, 1 eq.) was transferred into a flame-dried flask in an argon stream.A mixture of dry DMF/NEt 3 (50 mL, 1 : 1) solvents was added through the rubber septum.The solution was carefully deaerated by purging with argon for 20-25 min, and a catalytic mixture of Pd(PPh 3 ) 2 Cl 2 (1.1 mmol, 0.05 eq.), PPh 3 (2.2 mmol, 0.1 eq.), and CuI (1.1 mmol, 0.05 eq.) was added at once.The resulting mixture was slightly heated (to 45 1C) and ethynyltrimethylsilane (32.6 mmol, 1.5 eq.) was added through the septum.After that the heating was increased to 80 1C.A white precipitate began to form after B15 min of heating.After the reported time (4 to 16 h), the mixture was cooled to ambient temperature, and the crystalline white solid of triethylamine hydrobromide was isolated by filtration.The orange-brown filtrate was concentrated, mixed with NH 4 Cl saturated aqueous solution (50 mL), and extracted with dichloromethane or diethyl ether (3 Â 40 mL).The organic fractions were combined, dried over magnesium sulfate, and concentrated with silica gel in vacuo.The residue was purified using column chromatography.
A2.Using the same set-up and the catalyst/reagent ratio a mixture of dry NEt 3 /CH 3 CN (1 : 1) as the solvent media was preferred.Here, after 5 minutes of stirring at room temperature formation of triethylamine hydrobromide salt was observed.The light yellow reaction mixture was stirred at room temperature for 17-19 h.The work-up was done in accordance with protocol A1.
B2.A solution containing BHA (1.3 eq. for each aldehyde group) and an aldehyde (1.0 eq.) in absolute deaerated methanol (10 mL/1 mmol) was stirred at room temperature for 12-72 h.The formed precipitate was filtered off, washed with cold (B5 1C) methanol and dried in air.The product was used for the next step without additional purification.

Conclusion
We have demonstrated that the synthesis and study of structurally similar compounds exhibiting different magnetic behaviors provide a reasonable approach for understanding the relationship between the substituents in the aromatic ring and the intramolecular exchange constant J intra .Unfortunately, the magnetic interactions based on structural peculiarities are difficult to predict as their strength strongly depends on the relative orientation This journal is © The Royal Society of Chemistry 2017 between the interacting magnetic orbitals.Here, the torsion angles y have crucial impacts on the overall p-conjugation in the nitroxide biradical systems.The latter in turn is responsible for the efficient communication between the nitroxide fragments.The crystal structure analysis shows that the synthesized biradicals are planar with relatively small torsions between the radical fragments and the aromatic bridges (with an exception in the case of derivative 2c).These are the essential prerequisites for obtaining a weak intramolecular coupling.A rapid increase of the torsion hinders the conjugation within the biradical system 2c, decreasing the intramolecular exchange interactions within the molecule.This structural feature causes an enhancement of the intermolecular coupling within 2c.As a result, an unprecedentedly high value of the coupling constant is observed in NN biradical 2c.This work illustrates the difficulties in the design and prediction of a target structure, and the need for an experimental proof.
Magnetic measurements, carried out on single-crystalline samples, confirmed that for biradicals 1c, 1d, 3d, 4d and 5d, weak antiferromagnetic intramolecular interactions are predominant.According to the magnetic characterization studied p-bridged nitroxides possess a moderate intra-dimer coupling in the range À2 to À6 K as derived from the fits based on an isolated dimer model.Furthermore, magnetic measurements and their interpretation revealed that NN biradical 2c exhibits surprisingly strong antiferromagnetic interactions, owing to the short (B3.5 Å) interdimer interaction.IN biradical 5d features a field-induced ordered phase assigned to 3D interdimer couplings, which accounts for a description in terms of a BEC of triplons.
In summary, we have found promising candidates in the quest for purely organic molecular magnets and crystalline networks with higher ordering.

Fig. 2
Fig. 2 Molecular structure (top) of biradical 1c with ORTEP drawn at the 50% probability level and crystal packing (bottom) with emphasized short contacts.

Fig. 4
Fig. 4 Molecular structure (top) with two biradical molecules per asymmetric unit of biradical 1d and short contacts and p-stacking specified in fragments of molecules from crystal packing.

Fig. 7 w
Fig.7w mol T as a function of temperature (solid orange circles) of 1c.The black broken line indicates the theoretical value of 0.75 cm 3 K mol À1 , expected for the two uncoupled spin S = 1/2 entities.Inset: Molar susceptibility w mol as a function of temperature (solid green circles) in a semilog representation, and a theoretical fit with a mean-field correction (red solid line).33

Fig. 8 w
Fig.8w mol T as a function of temperature (solid orange circles) of 2c.The black broken line indicates the theoretical value of 0.75 cm 3 K mol À1 , expected for the two uncoupled spin S = 1/2 entities.Inset: Molar susceptibility w mol as a function of temperature (solid green circles), and a theoretical fit with a mean-field correction (red solid line),33 including a Curie term corresponding to S = 1/2 entities.

Fig. 9
Fig. 9 Magnetization M(B) of 5d (solid red line, left scale) at 28 mK normalized to the saturation magnetization together with the magnetic AC-susceptibility w ac (B) (orange full circles, right scale).