Bis(triphenylphosphine)iminium Salts of Dioxothiadiazole Radical Anions: Preparation, Crystal Structures, and Magnetic Properties

Phenanthroline dioxothiadiazoles are redox active molecules that form stable radical anions suitable for the construction of supramolecular magnetic materials. Herein, the preparation, structures and magnetic properties of bis(triphenylphosphine)iminium (PPN) salts of [1,2,5]thiadiazole[3,4-f][1,10]phenanthroline 1,1-dioxide (L), [1,2,5]thiadiazole[3,4-f][4,7]phenanthroline 1,1-dioxide (4,7-L), 5-bromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (BrL), and 5,10-dibromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (diBrL) are reported. The preparation of new bromo derivatives of the L: 5-bromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (BrL) and 5,10-dibromo-[1,2,5]thiadiazolo[3,4-f][1,10]phenanthroline 2,2-dioxide (diBrL)—suitable starting materials for further derivatization—are described starting from a commercially available and cheap 1,10-phenanthroline. All PPN salts show antiferromagnetic interactions between the pairs of radical anions, which in the case of PPN(diBrL) are very strong (−116 cm−1; using Ĥ = −2JSS type of exchange coupling Hamiltonian) due to a different crystal packing of the anion radicals as compared to PPN(L), PPN(4,7-L), and PPN(BrL).


Materials and Methods
Chemicals and reagents were of analytical grade unless otherwise stated. Inert PureSolv-MD-5/7 solvent purification system with alumina filled columns and argon gas was used for the deoxygenation and dehydration of acetonitrile and tetrahydrofuran.

Materials and Methods
Chemicals and reagents were of analytical grade unless otherwise stated. Inert PureSolv-MD-5/7 solvent purification system with alumina filled columns and argon gas was used for the deoxygenation and dehydration of acetonitrile and tetrahydrofuran. 1,10-phenantroline-5,6-dione (phendione) was prepared according to literature procedure [39]. Some of the operations that required inert gas atmosphere were performed using Inert PureLab HE glove box filled with Ar gas.

Syntheses
2.1.1. 3-Bromo-1,10-phenantroline-5,6-dione (Br-phendione) and 3,8-Dibromo-1,10-phenantroline-5,6-dione (diBr-phendione) Bromo derivatives of phendione were prepared according to the modified literature procedure [40]. 1,10-phenantroline-5,6-dione (phendione) (20 g, 95.2 mmol) was dissolved in a chilled mixture of 120 mL 98% H 2 SO 4 and 60 mL 99% HNO 3 in a 500 mL round-bottom flask. KBr (20 g, 168 mmol) was added and the mixture was refluxed at 120 • C overnight. After 16 h, the mixture was allowed to cool down to room temperature and poured onto 500 mL of crushed ice, pH was carefully adjusted to around 5 using 0.25 M NaOH solution. The mixture was extracted with chloroform (6 × 150 mL) and the combined organic extracts were dried using MgSO 4 and evaporated to dryness. Column chromatography (SiO 2 , eluent: CHCl 3 :AcOEt:n-hexane, 10:2:1) afforded pure 3-bromo-1,10-phenantroline-5,6-dione (6.0 g, 22%), 3,8-dibromo-1,10-phenantroline-5,6-dione (3.0 g, 9%) and the unreacted phendione (0.9 g). 3-bromo-1,10-phenantroline-5,6-dione 1  3-bromo-1,10-phenantroline-5,6-dione (1.0 g, 3.5 mmol) was suspended in 35 mL of anhydrous ethanol, sulfamide (0.56 g, 5.8 mmol) was added and the mixture refluxed for seven days. After 24 h, another quantity (0.25 g) of sulfamide was added. Additions were repeated daily for one week. Next, the mixture was allowed to cool down to room temperature and filtered. The yellow solid was washed with two aliquots of ethanol (2 × 20 mL) and dried in vacuo to afford 1.1 g (91%) of the desired product. 1  3,8-dibromo-1,10-phenantroline-5,6-dione (150 mg, 0.4 mmol) was suspended in 20 mL of anhydrous ethanol, sulfamide (50 mg, 0.5 mmol) was added and the mixture refluxed for three days. After 24 h, another quantity (50 mg) of sulfamide was added. Additions were repeated daily for the duration of the synthesis. Next, the brownish-orange mixture was allowed to cool down to room temperature and filtered to afford a brownish-yellow which was washed with two aliquots of ethanol (2 × 10 mL) and dried under vacuum. The crude product was once more suspended in 20 mL of the ethanolic solution of sulfamide (50 mg) and refluxed for another 24 h. The mixture was allowed to cool down to room temperature and filtered. An orange solid was washed with 20 mL of ethanol, 10 mL of cold diethyl ether and dried under vacuum to afford 198 mg (17%) of the desired product. 1  This compound was obtained in a two-step procedure: the suspension of 310 mg of [1,2,5]thiadiazole [3,4- Product was obtained similarly to PPN(L). The suspension of 63 mg (0.23 mmol) of [1,2,5]thiadiazole [3,4-f] [4,7]phenanthroline 1,1-dioxide (4,7-L) in 35 mL of dry MeCN was stirred overnight under an inert atmosphere with a 1.0 g (6.6 mmol) of NaI. Next day the dark violet precipitate was separated from the mother solution by filtration and washed with dry MeCN (ca. 20 mL). The powder attains violet colour after washing. After vacuum drying for a few hours the 30 mg of crude Na(4,7-L) was used to obtain PPN(4,7-L). This was achieved by stirring MeCN (135 mL) suspension with 210 mg of PPNCl for two hours. The NaCl precipitate was filtered off and a clear violet solution was quickly condensed on a rotary evaporator to ca. 6 mL and transferred to the Ar-filled glovebox, where it was left for 2 h for crystallization. Large elongated block crystals were separated from the mother suspension by decantation, filtered and washed with a single drop of cold MeCN. Yield 49 mg (59% based on Na(4,7-L)). Anal. calcd. for PPN(4,7-L), C 48  350 mg of BrL (1.0 mmol) and 2.0 g of NaI (13.3 mmol) was sonicated for 20 min in anhydrous acetonitrile (50 mL). During sonication the yellow suspension turned dark violet due to the reduction of BrL to a radical anion BrL •− by iodide. The mixture was filtered and the dark violet precipitate was washed with three portions of anhydrous acetonitrile (3 mL each). The sodium salt Na(BrL) was dried under vacuum for 1 h (300 mg, 81%) and then suspended in dry tetrahydrofuran (50 mL). Solid PPNCl (1.15 g, 2.0 mmol) was added resulting in the color change of the liquid phase to deep violet and the precipitation of white sodium chloride. NaCl was removed by filtration and the violet THF solution was concentrated by rotary evaporation to ca. 15 mL. Large crystals of the THF solvate were obtained by slow vapor diffusion of dry diethyl ether onto the THF mother solution (two days). The violet crystals (ca. 1-3 mm) were separated by hand under the microscope from the smaller colorless crystals of PPNCl. Yield 90 mg (11% based on BrL). Anal 150 mg of diBrL (0.35 mmol) and 1.0 g of NaI (6.6 mmol) was sonicated for 10 min in anhydrous acetonitrile (20 mL). During the sonication, the yellow suspension turned dark violet due to the reduction of diBrL to a radical anion by iodide. The mixture was filtered and the dark violet precipitate was washed with three portions of anhydrous acetonitrile (2 mL each). The sodium salt Na(diBrL) was dried under vacuum for 1 h (95 mg, 60%) and then suspended in dry tetrahydrofuran (50 mL). Solid PPNCl (0.4 g, 0.7 mmol) was added in small portions resulting in the color change of the liquid phase to deep violet and the precipitation of sodium chloride. NaCl was removed by filtration and the violet THF solution was evaporated to dryness. The crude PPN(diBrL) was dissolved in ca. 15 mL of MeCN. Then the solution was evaporated to ca. 2 mL. The sample was left for an hour for crystallization and then the crystals were separated and purified similar to PPN(4,7-L). Yield

Other Physical Measurements
IR spectra were collected using Nicolet iN10 MX FT-IR microscope in the transmission mode. Cyclic voltammetry was performed using Mtm-anko M-161C electrochemical analyzer. Glassy carbon electrodes were used in both experiments. 1 H NMR spectra were measured using Bruker Avance II 300 MHz spectrometer. Elemental CHNS analysis were done with ELEMENTAR Vario Micro Cube CHNS analyzer.

Magnetic Measurements
The magnetic measurements were carried out using Quantum Design MPMS3 Evercool SQUID magnetometer. The samples were sealed in HDPE foil bags to protect them from the crystallization solvent loss. Corrections for the diamagnetism of the sample holder and the compounds themselves (Pascal constants) were applied [41].

X-ray Diffraction Data Collection/Refinement
Single crystal X-ray diffraction (XRD) data was collected on a Bruker D8 Quest Eco diffractometer equipped with Photon 50 TM CMOS detector and Mo-K α Triumph ® monochromator. The data were collected at low temperature using Kryoflex II low-temperature device. Data were integrated using SAINT [42], while multi-scan absorption corrections were applied using SADABS or TWINABS [43,44], all incorporated into APEX3 environment [45]. The structures were solved using SHELXT [46] and refined with SHELXL [47,48] software within the Olex2 package [49]. All hydrogen atoms were refined using riding model, and the non-hydrogen atoms were refined anisotropically using weighted full-matrix least-squares on F 2 . Disorder of THF crystalline solvent molecules in PPN(BrL) was refined using constraints. Ideal THF geometry was supported by IMGL library [50]. The occupancy factors of THF molecules are changed so that the refinement could be stable. The positions should not be taken as perfect because the model shows just one possibility. One of the phenyl rings of PPN cation is also strongly disordered, apparently over four positions, but it was refined using only two of them, where positive electron densities were the strongest. PPN(diBrL) was refined as two component twin with scales 0.6227 (6). In case of PPN(L) the occupation factor of the water oxygen atom was fixed at low value due to disorder of the solvent molecules.

Syntheses
The Br-phendione and diBr-phendione were synthesized by bromination of phendione using KBr in the mixture of concentrated HNO 3 and H 2 SO 4 acids ( Figure 1). The products were purified by column chromatography. Both were further used in the procedure of attaching the thiadiazole dioxide functional group to the phenanthroline backbone. This includes the reaction of alpha-diketone group with sulfamide in the anhydrous boiling ethanol (Figure 1). In contrast to the preparation of L or 4,7-L, reactions of sulfamide with Br-phendione and diBr-phendione require longer time and additional quantities of sulfamide added every 24 h of the reaction under reflux to achieve good yields. The synthesis of all PPN radical salts was carried out in two steps. In the first step, the reduction of the respective dioxothiadiazole derivative with sodium iodide led to the quantitative precipitation of poorly soluble sodium salts. In the second step, the metathesis of the obtained sodium salts using PPNCl resulted in the final PPN salts, which are very well soluble in THF, acetonitrile, dichloromethane, and chloroform (Figure 1).

BrL and diBrL-Crystal Structures and DFT Calculations
Crystal structures of BrL and diBrL were determined using single crystal X-ray diffraction (Table S1 in the Supplementary Materials). BrL crystallizes in the P-1 space group while diBrL in P2 1 /c. The asymmetric units of both compounds are presented in Figure 2a,b. BrL and diBrL molecules are equipped with bromine substituents which change the crystal packing of the molecules as compared to the non-substituted L. BrL forms π-π stacks along "a" crystallographic direction with the parallel off-centered arrangement of the molecules forced by steric hindrance of the SO 2 group (Figure S1a in the Supplementary Materials). The plane of each molecule in a stack is inclined relative to the direction of stack propagation. The structure of BrL seems to be similar to that of L with the layers of molecules forming a two-dimensional (2-D) network of double N···H-C hydrogen bonds (donor acceptor distance of 3.525 Å on one side of the molecule and 3.642 Å on the other). The presence of the Br substituent in BrL disrupts the H-bonding and the molecular packing as compared to L. One side of the molecule forms slightly different interactions, namely N···S close contacts (3.331 Å) which replace the N···H-C bonds ( Figure S1b in the Supplementary Materials). Also, the donor acceptor distances in N···H-C hydrogen bonds of BrL are a little shorter (3.467 Å) compared to those in L. The molecules of BrL do not lie completely flat, but are slightly tilted directing the bromine atoms slightly below the plane of the neighboring molecule.
The structure of the diBrL is completely different. The chains of parallel hydrogen bonded dimers interacting with each other through π-orbitals and bromine atoms are easily distinguished ( Figure S2 in the Supplementary Materials). The neighboring chains run in two perpendicular directions, and the source of closest contacts between them are N and O atoms of the dioxothiadiazole groups.
The DFT calculations (B3LYP exchange-correlation functional; 6-311++G(2d,2p) basis set) were performed for BrL and diBrL. Noteworthy, the inspection of molecular orbitals revealed that low-lying LUMO is not spread over the bromine atoms in contrast to the HOMO (Figure 3). The LUMOs for both BrL and diBrL molecules exhibit an antibonding character at the N-C bonds of the dioxothiadiazole group (nodal plane) and a bonding character at the dioxothiadiazole's S-N and C-C bonds, which is consistent with the observed shortening and elongation of the respective bonds discussed below.

BrL and diBrL-Cyclic Voltammetry
Electrochemical properties of brominated BrL and diBrL are similar to the previously reported L [35] and 4,7-L [34] featuring two distinct reduction processes (Figure 4). The first reduction leads to an anion radical and appears at ca. −441 mV vs Fc + /Fc. The second one results in a diamagnetic dianion and appears ca. 800 mV below the first reduction. The exact values of the reduction potentials are presented in Table 1. The first reduction potential of diBrL (−441 mV) and BrL (−471 mV) are shifted to less negative values as compared to L (−499 mV) and 4,7-L (−520 mV). This trend is also reflected in the HOMO-LUMO gap, which is much smaller for the brominated species (Table  1).

BrL and diBrL-Cyclic Voltammetry
Electrochemical properties of brominated BrL and diBrL are similar to the previously reported L [35] and 4,7-L [34] featuring two distinct reduction processes (Figure 4). The first reduction leads to an anion radical and appears at ca. −441 mV vs Fc + /Fc. The second one results in a diamagnetic dianion and appears ca. 800 mV below the first reduction. The exact values of the reduction potentials are presented in Table 1. The first reduction potential of diBrL (−441 mV) and BrL (−471 mV) are shifted to less negative values as compared to L (−499 mV) and 4,7-L (−520 mV). This trend is also reflected in the HOMO-LUMO gap, which is much smaller for the brominated species (Table 1).
Similarly to previously reported dioxothiadiazoles, a one electron reduction results in the contraction of the C-C and S-N bonds (by ca. 0.06 Å and 0.04 Å, respectively), as well as the elongation of C=N and S=O (by ca. 0.05 Å and 0.015 Å, respectively). These results line up with the shape of the LUMO, which is occupied in the radical form [34,35,37]. These particular bond lengths are indicative of the oxidation state of the molecule. Table 2 compares bond lengths of neutral and radical dioxothiadiazole-based compounds presented here.

PPN + Radical Salts-Magnetic Properties
The results of magnetic measurements are presented in both Figure 6 and Table 3. In all three salts that reveal layered structures (PPN(4,7-L), PPN(L), and PPN(BrL)) the χT(T) curves show very similar behavior. This dependence is constant above ca. 60 K and takes the values which are close to the theoretical 0.375 cm 3 K mol −1 spin-only value assuming S = 1 2 and g = 2.0 (Table 3 and Figure 6). Below ca. 60 K the χT(T) decreases and plummets below 15 K achieving values close to zero due to antiferromagnetic interactions between the radical anions. M(H) curves at 1.8 K differ slightly among these three compounds. The M(H) are slowly, almost linearly increasing with field (Figure 6b), until at some point the increase becomes steep, which again is typical for weak local antiferromagnetic interactions between neighboring spins. For PPN(4,7-L) the inflection point is located around 4 T, while for PPN(L)-around 6 T and for PPN(BrL) well above 7 T. The M(H) dependencies do not saturate at 7 T and the magnetization values at this field decreases along the series PPN(4,7-L), PPN(BrL), PPN(L), suggesting that the strongest magnetic interactions operate within the PPN(L) salt. Table 3. Values of χT at 300 K and 1.8 K, M(H) at 7T and the magnetic exchange constants J obtained from fitting of M(H) and χT(T) in the whole temperature range using PHI software [55]. χT(T) curve for PPN(diBrL) (Figure 6 black dots), on the other hand, decreases in the whole 340-80 K temperature range from 0.313 cm 3 K mol −1 at 340 K to 0.196 cm 3 K mol −1 at 80 K with a plateau-like feature around 0.186 cm 3 K mol −1 below this temperature. The signal starts to decrease again below 30 K and reaches a minimum of 0.158 cm 3 K mol −1 at 7 K. Near 2.0 K additional small increase of the χT(T) signal is observed which might be ascribed to very weak ferromagnetic interactions between the radical anions. The M(H) curve increases in a Brillouin like fashion reaching the value of 0.37 µ B (well below the expected 1.0 µ B for S = 1 2 and g = 2.0), but close to 0.5 µ B which suggests the presence of strong antiferromagnetic interactions between half of the radical anions in the compound. The magnetic data were fitted assuming local antiferromagnetic interactions between the pairs of anion radicals which in the case of PPN(4,7-L), PPN(L), and PPN(BrL) are transmitted through C-H···N hydrogen bonds ( Figure 5) and for PPN(diBrL) through the π-π contacts with different interplane distances (two types of radical pairs with two types of magnetic interactions (one very strong and the other very weak). Figure S8 presents the magnetic coupling scheme for all four compounds. The results of the simultaneous fitting of χT(T) and M(H) (PHI program [55]) using the following Hamiltonians (Equation (1) for PPN(4,7-L), PPN(L), and PPN(BrL) and Equation (2) for PPN(diBrL)) are collected in Table 3 and presented as solid lines in Figure 6 H where S 1 = S 2 = S 3 = S 4 = 1 2 are the spin numbers of the radical anions, g 1 = g 2 = g 3 = g 4 = 2.0 is the g-factor, µ B is the Bohr magneton, B is the magnetic field induction and J 12 and J 34 are the superexchange coupling constants-the fitting parameters with J 12 >> J 34 .
The magnetic interaction pathways between pairs of radical anions are justified by the presence of hydrogen bonded supramolecular pairs highlighted in Figure 5b Magnetic behavior of PPN(diBrL) is also strongly correlated with the structural packing. The stacks of diBr-L •− radical anions reveal four different π-contacts between them leading to two types of radical anion pairs within the infinite stack ( Figure S9 in the Supplementary Materials). The most efficient one controls the magnetic behavior of one half of the radical anions and results in very strong antiferromagnetic coupling (J 12 = −116(10) cm −1 ) that is comparable with the values reported for sodium salts of 4,7-L [34] and L [35] and other types of molecular magnets ( [56] and references therein). The antiferromagnetic interactions result in a χT value of 0.313 cm 3 K mol −1 at 340 K which is significantly lower than the expected 0.375 cm 3 K mol −1 for non-interaction S = 1/2 species. These interactions lead also to a plateau of 0.186 cm 3 K mol −1 below 70 K corresponding to half of the radical anions in the compound. The second exchange parameter J 34 is much weaker than J 12 but seems to be slightly underestimated as the fit does not reproduce the second step around 30 K where a further decrease of the χT(T) to 0.157 cm 3 K mol −1 occurs. This weaker exchange controls the magnetic behavior of the remaining half of radical anions and is responsible for the observation of the 0.186 cm 3 K mol −1 plateau below 70 K and the Brillouin-like M(H) curve reaching a saturation value approaching 0.5 µ B expected for half of the radicals in PPN(diBrL) (Figure 6b). Magnetic behavior of PPN(diBrL) is also strongly correlated with the structural packing. The stacks of diBr-L •− radical anions reveal four different π-contacts between them leading to two types of radical anion pairs within the infinite stack ( Figure S9 in the Supplementary Materials). The most

Conclusions
Two new derivatives of [1,2,5]thiadiazole [3,4- [34] and L [35]. The PPN salts show very good solubility in THF, acetonitrile, chloroform, and dichloromethane, which renders them suitable for the preparation of mixed-spin systems. All four radical anion-based compounds were characterized by means of single-crystal X-ray diffraction and magnetic measurements (SQUID magnetometry). PPN(L), PPN(4,7-L), and PPN(BrL) exhibit layered-type structures where the flat/weaved anionic layers are separated by layers of PPN + cations. PPN(diBrL), on the other hand, forms infinite chain-like π-π stacks of radical anions that are separated from each other by cations. The structures of the reported compounds directly influence the magnetic properties. The 'layered salts' show weak-to-moderate antiferromagnetic interactions despite slightly different substituents (bromine atoms) or the location of the nitrogen atoms, while the 'π-π-stacked-salt' exhibits very strong antiferromagnetic interactions transmitted through the direct overlap of the π orbitals of the radical anions. PPN(L), PPN(4,7-L), and PPN(BrL) constitute a rare example of a layered packing where the layers of radical anions are separated by the layers of cations (similar packing was observed for a few other PPN-based supramolecular systems [57]).