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Article

A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene)

Department of Chemistry, Virginia Tech, Blacksburg, VA 24061, USA
*
Author to whom correspondence should be addressed.
Magnetochemistry 2019, 5(3), 44; https://doi.org/10.3390/magnetochemistry5030044
Submission received: 25 June 2019 / Revised: 19 July 2019 / Accepted: 19 July 2019 / Published: 1 August 2019

Abstract

:
Using the structural and electronic tunability of molecules to control magnetism is a central challenge of inorganic chemistry. Herein, a ten-member family of the high-ordering temperature (Tc) molecule-based magnetic coordination networks of the form V[x-ClnPTCE]2·yCH2Cl2 (PTCE = phenyltricyanoethylene, y < 0.5) were synthesized and characterized, where x is (are) the position(s) and n is the number of chlorine substitutions on the phenyl ring. These chlorophenyltricyanoethelenes are tunable analogs of the more commonly investigated tetracyanoethylene (TCNE). Varying the number and position of chlorine substitution around the phenyl ring engendered a family of network solids with significantly different magnetic ordering temperatures ranging from 146 to 285 K. The Tcs of these ferrimagnets were rationalized with the aid of cyclic voltammetry and Density Functional Theory (DFT) calculations.

Graphical Abstract

1. Introduction

The ferrimagnet V[TCNE]2·yCH2Cl2 (where y < 0.5) was discovered in 1991, and improved syntheses have yielded samples with ordering temperatures (or critical temperatures, Tcs) up to 400 K [1,2,3]. As a room temperature magnetic semiconductor, films of V[TCNE]2 show potential for application in spintronics [4,5], a point emphasized by the extremely narrow ferromagnetic resonance spectra [6]. Efforts by us and others to tune the properties of V[TCNE]2-like magnets have involved the preparation of compounds with other transition metal ions [7], combinations of metal ions [8,9], and other conjugated polynitriles [10,11,12,13,14,15,16,17]. Recently, we reported a family of tunable fluorophenyltricyanoethylene-based magnets, V[x-FnPTCE]2, which ranged in Tc from 160 to 315 K [18,19,20]. The Tcs of the V[x-FnPTCE]2 networks displayed a significant dependence on the position of the fluorine substitution, and the trend was rationalized using a combination of steric and electronic effects based on electrochemical and computational data. Comprehensive and systematic magnetochemical studies of this kind offer insight into the factors that govern magnetic ordering within V[TCNE]2-like magnets, with the aim of eventually exploiting these factors to develop improved, higher-Tc materials. Herein, we extend these substituent-effect studies to analogous magnets of the form V[x-ClnPTCE]2. In particular, we were interested in the larger size of Cl relative to F, since we previously demonstrated that the larger size of F relative to H enhanced Tc by increasing the dihedral angle between the phenyl ring and olefin [18]. We were also interested in investigating the change in electronegativity between F and Cl to compare the impacts on Tc by the positional electronic effects of each halogen substitution. In order to facilitate these comparisons, ten new chlorophenyltricyanoethylene one-electron acceptors (Figure 1) were synthesized, the magnetic properties of the corresponding V[x-ClnPTCE]2 magnets were characterized, and the results were rationalized with the aid of measured and calculated acceptor properties.

2. Results

The chlorophenyltricyanoethylene acceptors were synthesized in moderate yield starting from the appropriately substituted chlorobenzaldehydes (Scheme 1). Most benzaldehydes were available commercially, while others were synthesized in one step from commercially available substituted benzenes (Supplementary Materials).
The reaction of two equivalents of a given acceptor with vanadium hexacarbonyl yields a black, insoluble solid, which is collected upon solvent removal as a free-flowing powder (Scheme 2) [1,2,3]. Elemental analysis indicates a 2:1 acceptor:vanadium ratio in the isolated product with less than 0.5 equivalents of residual CH2Cl2 in all cases, which is consistent with the stoichiometry and residual solvent observed for analogous magnets of this type. For the sake of simplicity, residual solvent will not be included in subsequent formulas.
The structures of these magnetic solids were inferred to be disordered, three-dimensional networks of VII cations bridged by radical anion forms of the organic acceptors. XANES/EXAFS studies of the related V[TCNE]2 ferrimagnet suggest that the VII ion is surrounded by six nitrile nitrogen atoms in a pseudo-octahedral ligand field [21,22]. Theoretical studies indicate that the lowest-energy structure of V[TCNE]2 includes two-dimensional layers of μ4-TCNE•− and VII spanned by μ2-TCNE•− struts. [23] In the case of the presented solids, this structure is likely to be inaccessible due to the steric bulk of the chlorinated phenyl rings and the presence of only three CN groups that can bridge the VII ions. The IR spectra of the neutral acceptors and the corresponding magnets exhibited characteristic C≡N stretching frequencies between 2100 and 2250 cm−1. The shift to lower energies for the C≡N bands of the magnets was consistent with the reduction of the nitrile bond order due to the presence of the radical anion located on the olefin. The IR data for the C≡N bands of the magnets and neutral acceptors are summarized in Table 1.
The measurement results of the magnetization versus applied field, M(H), are summarized in Table 2 and a representative plot of V[2,3,5,6-Cl4PTCE]2 is shown in Figure 2. The coercivities of ~5 G observed here were smaller than those observed for V[TCNE]2 (60 G) [1], but were consistent with the S = 3/2, (t2g)3(eg)0, 4A2 ground state of VII in a pseudo-octahedral or lower symmetry ligand field, which produces minimal single-ion magnetic anisotropy [24]. Magnetization reached saturation at 100 G, and the saturation magnetization values ranged from 4400 to 5000 emu·G/mol for these samples. The saturation values were far smaller than the ~28,000 emu·G/mol expected for ferromagnetic coupling and were much closer to the ideal value of 5585 emu·G/mol predicted (assuming g = 2) for S = 3/2 VII coupled antiferromagnetically to two S = 1/2 x-ClnPTCE radical anions. The presence of a diamagnetic dimer formation may contribute to lower rather than ideal saturation magnetization, analogous to [TCNE]22− dimers found in model compounds and related antiferromagnets [25,26,27]. Taken together, the above data allow these magnetic network solids to be conclusively categorized as soft ferrimagnets.
Field-cooled magnetization versus temperature data, M(T), were collected immediately after magnet synthesis (Figure 3), since previous studies have noted slight decreases in the Tc for aged samples [19]. The Tc for each sample was estimated by extrapolation to zero magnetization from the steepest portion of the M(T) curves. The reported Tc values were averaged from at least two preparations. Ranges for deviation in Tc (Table 2) were small (±2 K), demonstrating the reproducibility of the Tc data. During the M(T) experiments, sample magnetizations decreased slowly from 5 K until ~30 K below Tc (Figure 3). The relatively sharp transition near Tc suggests some degree of magnetic and structural uniformity in these materials. The Tc of each magnet ranged from 146 K to 285 K depending on the position of the chlorine substitution.

3. Discussion

In order to facilitate discussion of the observed substituent effects, it is useful to consider the change in the Tc of each magnet relative to the unsubstituted V[H5PTCE]2 magnet. The Tc values for the chlorine substituted magnets are tabulated below, and the Tc data for the analogous fluorine substituted magnets reported previously are provided for reference (Table 3) [18,20]. Among the three monosubstituted acceptors, chlorine substitution appears to have a similar, albeit larger effect on Tc as fluorine substitution. Namely, substitutions in the 2-position result in magnets with large increases in Tc (V[2-ClPTCE]2 Tc = 271 K, V[2-FPTCE]2 Tc = 257 K) relative to the unsubstituted acceptor (V[H5PTCE]2 Tc = 213 K). Substitution at the 3-position results in a smaller, yet significant increase in Tc (V[3-ClPTCE]2 Tc = 243 K, V[3-FPTCE]2 Tc = 233 K). Substitution in the 4-position results in significant depression of Tc (V[4-ClPTCE]2 Tc = 146 K, V[4-FPTCE]2 Tc = 160 K).
It was previously observed that the effect of multiple fluorine substitutions was to magnify the same changes observed for the monosubstituted samples. For instance, the 2-position enhancement and 4-position depression partially cancel out (V[2-FPTCE]2 Tc = 257 K vs. V[2,4-F2PTCE]2 Tc = 242 K vs. V[4-FPTCE]2 Tc = 160 K), and the same effect was found here for chlorine substitution (V[2-ClPTCE]2 Tc = 271 K vs. V[2,4-Cl2PTCE]2 Tc = 256 K vs. V[4-ClPTCE]2 Tc = 146 K). Similarly, multiple substitutions in the equivalent 2- and 6-positions magnified the Tc enhancement for both fluorine (V[2,6-F2PTCE]2 Tc = 300 K vs. V[2-FPTCE]2 Tc = 257 K) and chlorine (V[2,6-Cl2PTCE]2 Tc = 285 K vs. V[2-ClPTCE]2 Tc = 271 K) PTCE magnets, and multiple substitutions in the equivalent 3- and 5- positions magnified the Tc enhancement for both fluorine (V[3,5-F2PTCE]2 Tc = 263 K vs. V[3-FPTCE]2 Tc = 233 K) and chlorine (V[3,5-Cl2PTCE]2 Tc = 257 K vs. V[3-ClPTCE]2 Tc = 243 K) PTCE magnets. However, combining 2/6- and 3/5- substitutions revealed stark differences between chlorine and fluorine substitution. For fluorine substitution, the Tc was always enhanced by adding additional fluorine substituents to the four positions that enhance Tc (V[2,6-F2PTCE]2 Tc = 300 K vs. V[2,3,6-F3PTCE]2 Tc = 310 K vs. V[2,3,5,6-F4PTCE]2 Tc = 315 K), whereas the 2/6- position chlorine substituted magnet was adversely affected by adding 3/5- position substituents (V[2-ClPTCE]2 Tc = 271 K vs. V[2–3-Cl2PTCE]2 Tc = 265 K vs. V[2,5-Cl2PTCE]2 Tc = 265 K, and V[2,6-Cl2PTCE]2 Tc = 285 K vs. V[2,3,6-Cl3PTCE]2 Tc = 276 K vs. V[2,3,5,6-Cl4PTCE]2 Tc = 256 K). In the following discussion, we aim to disentangle the various electronic and structural factors that chlorine substitution has on the observed trends in Tc for these PTCE magnets.
In order to discuss the electronic effects of chlorine substitution, it is useful to first build a simple molecular orbital model of the VII-acceptor•− bonding in these systems. Carlegrim et al. examined analogous V[TCNE]2 systems by near-edge x-ray absorption fine structure and photoelectron spectroscopies [28]. Their results suggest that the π* singly occupied molecular orbital (SOMO) of the TCNE•− radical anion is very close in energy to the partially occupied t2g orbitals (POMO) of VII in a pseudo-octahedral ligand field, which facilitates strong covalent bonding and consequent strong ferrimagnetic communication (large magnetic exchange coupling). Ab initio DFT studies corroborate the placement of these energy levels for V[TCNE]2 [23,29]. The parent H5-PTCE ligand is a poorer electron acceptor (E1/2 = −0.40 vs. Ag/AgCl) than TCNE (E1/2 = +0.11 vs Ag/AgCl), thus the SOMO π* orbitals for H5-PTCE are assumed to lie higher in energy than the vanadium POMO t2g orbitals (Figure 4). The increased energy gap between the metal and ligand valance orbitals weakens the covalent bonding interaction, and the consequently smaller magnetic exchange coupling results in a lower Tc for V[H5-PTCE]2 magnets relative to V[TCNE]2 magnets. The addition of electron-withdrawing chlorine or fluorine substituents to the parent H5PTCE would be expected to lower the π* SOMO of the x-ClnPTCE acceptors, which should increase the VII-ClnPTCE /− covalent bonding and thus lead to larger magnetic exchange coupling and a higher Tc.
The predictions of our simple molecular orbital model were mostly borne out when we first limit the discussion to only those acceptors that have small calculated dihedral angles between the phenyl ring and the olefin, i.e., only those acceptors where electronics are expected to have the most important effect (H5-PTCE, 3-ClPTCE, 3,5-Cl2PTCE, and 4-ClPTCE). In particular, we noted the clear correlation (R = 0.991) between E1/2 and Tc for H5-PTCE, 3-ClPTCE, and 3,5-Cl2PTCE, where each additional chlorine substitution made it easier to reduce the acceptor (owing to a lower-energy SOMO) and raised the observed Tc of the corresponding magnet (Figure 5). Substituting the calculated electron affinities for the measured redox potentials provided a similarly good correlation to the Tc data (R = 0.97, see SI).
However, the 4-ClPTCE magnet is clearly an unexplained outlier. The 4-ClPTCE acceptor was slightly harder to reduce than 3-ClPTCE because chlorine (like fluorine) is partly electron donating by resonance in the ortho/para positions (Hammett parameters σp are less than σm for both chlorine and fluorine) [30]. However, neither the reduction potential nor the Hammett parameters can explain why the Tc of 4-ClPTCE magnet was lower than that of the unsubstituted H5-PTCE magnet. It is worth considering that aryl-chlorine (and aryl-fluorine) bonds can act as reasonable sites for oxidative addition, where a low valent metal attacks the aryl-halogen bond in a manner similar to nucleophilic aromatic substitution (NAS). Hence, we propose that the electron-rich V(0) may act as a nucleophile and bind to this site in the solid, generating magnetic coupling pathways that are deleterious to high Tc [31]. Several authors have shown that low valent metals can react with aryl-halogen bonds in the presence of electron withdrawing groups, and it has been shown that halogen substituents that are para to an electron withdrawing group are particularly vulnerable [32,33,34,35,36]. It is also worth noting that the strongly electron donating 4-MePTCE has a Tc well above that of either 4-ClPTCE or 4-FPTCE, which lends support to the proposal that NAS may play a role in the 4-X halogen substituted magnets [37].
Rationalization of the trend in Tc for the remaining chlorine-substituted magnets requires consideration of the role of steric factors. The large size of chlorine substitution (relative to hydrogen) in the 2/6-positions causes a substantial increase in the dihedral angle between the phenyl ring and the olefin. The increased dihedral angle would be expected to attenuate the electronic conjugation between the phenyl ring and the olefin, which should reduce the electronic effects of any chlorine substitutions on the VII-nitrile bonding interactions. The DFT-minimized gas-phase dihedral angles were found to be around 15° with no 2- or 6-position substitutions, while one 2-position substitution had a dihedral of ~38°, and substitution in both the 2- and 6- positions provided a dihedral of ~61° (Figure 6). The effect was similar, albeit reduced in magnitude for the fluorine substitutions, where substitution in both the 2- and 6-positions provided a dihedral of ~46°.
The high Tc values of the magnets with 2- and/or 6-position chlorine substitutions can be qualitatively rationalized by considering the effect of reduced phenyl-olefin conjugation on these systems. As the dihedral angle is increased, a greater portion of the unpaired spin density is localized onto the nitrile nitrogen atoms. Consequently, the amount of spin density on the phenyl ring is reduced, which is supported by the DFT calculations (Supplementary Materials). Since the nitrile nitrogen atoms are directly involved in bonding to the VII centers, it stands to reason that localization of increased spin density on those atoms would facilitate enhanced magnetic communication between the VII and acceptor and lead to a higher Tc. The proposed correlation between Tc and nitrile spin density (Figure 7) is at least partially supported by the data. At a low dihedral angle, the magnetic ordering temperatures spanned a wide range, which is consistent with the significant electronic effects observed in that regime. At higher dihedral angles and nitrile nitrogen spin densities, the ordering temperatures are more tightly clustered, consistent with the loss of electronic conjugation between the olefin and the ring. It is worth noting that the 2,6-Cl2PTCE magnet has both the highest Tc and highest nitrile spin density among this family of acceptors.
It remains an ongoing mystery as to why combining 2/6- and 3/5- substitutions results in a lower Tc for chlorine substituted magnets, but a higher Tc for fluorine substituted magnets. The larger dihedral angles of the 2/6- chlorine substituted acceptors would be expected to attenuate the beneficial effects of 3/5- substitution, but the reversal of the trend relative to fluorine substitutions is difficult to explain. Nonetheless, the Tc data for the 2,3-Cl2, 2,5-Cl2, 2,3,6-Cl3, and 2,3,5,6-Cl4 magnets all support that unexpected reversal of the trend. Other factors not considered by our current model must also play a significant role, although these effects are likely to be steric in nature given the electronic similarities of chlorine and fluorine substitution. For example, increased steric bulk for the 3/5-substitutions (or anywhere on the phenyl ring) may negatively impact the ability to form VII -PTCE•− contacts, and the consequently lower average molecular weight VII-PTCE•− networks would be expected to have a lower Tc.

4. Materials and Methods

General Considerations: Preparations of the air-sensitive magnetic samples were carried out in a nitrogen-filled Vacuum Atmospheres glovebox. Vanadium hexacarbonyl and its precursor salt, [Et4N][V(CO)6], were prepared according to procedure described in the literature [38]. Anhydrous dimethylformamide, n-butyllithium, N-chlorosuccinimide, 1,2,4,5-tetrachlorobenzene, 1-bromo-3,5-dichlorobenzene, and the chlorine substituted benzaldehydes were purchased from Alfa Aesar. Malononitrile, piperidine, and 1,2,4-trichlorobenzene were purchased from Aldrich. Magnesium turnings were purchased from Fischer and potassium cyanide was purchased from Acros. All reagents were used as received except as noted below. Dichloromethane was distilled from P2O5, followed by distillation from CaH2 under argon prior to use in the synthesis of the magnetic samples. Tetrahydrofuran was refluxed over sodium benzophenone ketyl and distilled under argon prior to use. All chlorinated acceptors were synthesized based on a modified literature procedure described below [39]. Flash chromatography was performed using Silicycle SiliaFlash P60 silica (230–400 mesh). Purity of the chlorophenyltricyanoethylenes was confirmed by sharp melting transitions and by 1H NMR spectroscopy. In all cases, acceptor purity was estimated to be greater than 99%. Melting points were uncorrected. NMR spectra were obtained using a JEOL EclipsePlus-500 spectrometer operating at 500 MHz for 1H and 125 MHz for 13C. Chemical shifts for 1H (or 13C) were reported in parts per million relative to the solvent resonance, taken as δ 7.26 (δ 77.16) for CDCl3. Elemental analyses were performed by Galbraith Laboratories, Knoxville, TN, USA.
Magnetic Measurements: Magnetic measurements were performed on a 7 T Quantum Design MPMS SQUID magnetometer. Powder samples were packed in glass tubes, and sealed under vacuum as previously described [40]. M vs. T experiments were performed by cooling to 5 K in a 100 G applied field and measured in a 5 G applied field upon warming from 5 to 300 K. Measurements of magnetization as a function of the applied magnetic field were performed at 5 K. Diamagnetic corrections were not applied to the magnetic data.
Electrochemistry Measurements: Cyclic voltammograms were recorded using a CH Instruments model 600A potentiostat. Measurements were performed on ~5 mM solutions in freshly distilled and degassed CH3CN with 0.1 M [n-Bu4N][PF6] as the supporting electrolyte. Data were collected between −1.2 V and 0.5 V at a scan rate of 0.1 V/s using a polished platinum working electrode, a platinum wire auxiliary electrode, and an aqueous Ag/AgCl reference electrode. Solution resistance was compensated 90% using positive-feedback IR compensation for all measurements. Redox potentials (E1/2) were reported as the average of the anodic and cathodic peaks relative to the Ag/AgCl reference. In order to facilitate comparison to previously published data where a glassy carbon working electrode was used instead of polished platinum, the measured redox potentials were adjusted so that the ferrocene internal standard was set to 0.45 V, which is the known potential of ferrocene vs. Ag/AgCl using a glassy carbon working electrode [41].
Density Functional Theory Calculations. Geometry optimization calculations were performed using the B3LYP density functional and 6-311++G(d,p) basis set within Gaussian09 [42]. Minimized structures for both the neutral and anionic forms of each chlorinated acceptor were calculated, and these coordinates are tabulated in the Supplementary Materials). Electron affinities (Eea) of the neutral acceptors were calculated by subtracting the zero-point energy self-consistent field energy values of the radical anion from the corresponding neutral acceptor. This method has been shown to provide estimations of Eea for a number of organic molecules with good comparison to the experimental data [43].
General procedure for the synthesis of the organic one-electron acceptors: The general procedure outlined in Scheme 1 has been described previously, and a representative example can be found in the Supplementary Materials [18,19,20].
Synthesis of 2-(2-chlorophenyl)-1,1,2-tricyanoethylene (2-ClPTCE). Yield: 43% over three steps. Mp: 127.5–127.9 °C. IR (KBr): νCN 2241 cm−1. 1H NMR δ 7.51 (m, 2H), 7.62 (m, 2H). 13C NMR δ 100.86, 109.63, 110.04, 112.39, 127.46, 128.26, 130.74, 131.53, 133.34, 134.97, 141.07. HRMS-FAB (m/z, [M + 2H]+) Calcd for C11H6ClN3: 215.02502, Found: 215.02474.
Synthesis of 2-(3-chlorophenyl)-1,1,2-tricyanoethylene (3-ClPTCE). Yield: 42% over three steps. Mp: 72.8–73.4 °C. IR (KBr): νCN 2240 cm−1. 1H NMR δ 7.59 (t, J = 8.0 Hz, 1H), 7.70 (m, 1H), 7.90 (m, 1H), 7.94 (t, J = 2.0 Hz, 1H). 13C NMR δ 94.61, 110.66, 110.90, 113.43, 127.54, 129.40, 129.91, 131.45, 135.62, 136.62, 140.61. HRMS-FAB (m/z, [M]+) Calcd for C11H4ClN3: 213.00937, Found: 213.01012.
Synthesis of 2-(4-chlorophenyl)-1,1,2-tricyanoethylene (4-ClPTCE). Yield: 21% over three steps. Mp: 172.4–172.9 °C. IR (KBr): νCN 2237 and 2230 cm−1. 1H NMR δ 7.61 (m, 2H), 7.97 (m, 2H). 13C NMR δ 93.08, 111.02, 111.15, 113.54, 126.91, 130.68, 130.93, 140.68, 142.93. HRMS-FAB (m/z, [M]+) Calcd for C11H4ClN3: 213.00937, Found: 213.00966.
Synthesis of 2-(2,3-dichlorophenyl)-1,1,2-tricyanoethylene (2,3-Cl2PTCE). Yield: 54% over three steps. Mp: 137.2–137.7 °C. IR (KBr): νCN 2243 cm−1. 1H NMR δ 7.41 (dd, 3J = 7.8 Hz, 4J = 1.6 Hz, 1H), 7.47 (t, J = 7.9 Hz, 1H), 7.77 (dd, 3J = 8.0 Hz, 4J = 1.6 Hz, 1H). 13C NMR δ 101.53, 109.33, 109.78, 112.07, 128.79, 128.89, 129.18, 131.66, 135.38, 135.90, 140.38. HRMS-FAB (m/z, [M]+) Calcd for C11H3Cl2N3: 246.97040, Found: 246.97069.
Synthesis of 2-(2,4-dichlorophenyl)-1,1,2-tricyanoethylene (2,4-Cl2PTCE). Yield: 42% over three steps. Mp: 102.8–103.5 °C. IR (KBr): νCN 2241 cm−1. 1H NMR δ 7.47 (d, J = 8.4 Hz, 1H), 7.50 (dd, 3J = 8.5 Hz, 4J = 1.9 Hz, 1H), 7.65 (d, J = 1.9 Hz, 1H). 13C NMR δ 101.12, 109.53, 109.89, 112.14, 125.78, 128.84, 131.53, 131.65, 134.37, 139.90, 141.31. HRMS-FAB (m/z, [M + 2H]+) Calcd for C11H5Cl2N3: 248.98605, Found: 248.98621.
Synthesis of 2-(2,5-dichlorophenyl)-1,1,2-tricyanoethylene (2,5-Cl2PTCE). Yield: 45% over three steps. Mp: 111.0–111.5 °C. IR (KBr): νCN 2242 and 2228 cm−1. 1H NMR δ 7.49 (dd, 4J = 2.2 Hz, 5J = 0.6 Hz, 1H), 7.56 (dd, 3J = 8.7 Hz, 5J = 0.5 Hz, 1H), 7.59 (dd, 3J = 8.7 Hz, 4J = 2.2 Hz, 1H). 13C NMR δ 101.94, 109.27, 109.71, 112.06, 128.46, 130.22, 131.55, 132.66, 134.55, 134.83, 139.41. HRMS-FAB (m/z, [M + 2H]+) Calcd for C11H5Cl2N3: 248.98605, Found: 248.98653.
Synthesis of 2-(2,6-dichlorophenyl)-1,1,2-tricyanoethylene (2,6-Cl2PTCE). Yield: 33% over three steps. Mp: 122.1–122.8 °C. IR (KBr): νCN 2244 and 2229 cm−1. 1H NMR δ 7.54 (s, 3H). 13C NMR δ 104.39, 108.89, 109.41, 111.63, 126.21, 129.41, 134.30, 134.41, 138.44. HRMS-FAB (m/z, [M + H]+) Calcd for C11H4Cl2N3: 247.97823, Found: 247.97889.
Synthesis of 2-(3,5-dichlorophenyl)-1,1,2-tricyanoethylene (3,5-Cl2PTCE). Yield: 35% over three steps. Mp: 142.3–143.3 °C. IR (KBr): νCN 2238 cm−1. 1H NMR δ 7.71 (t, 4J = 1.8 Hz, 1H), 7.83 (d, 4J = 1.8 Hz, 2H). 13C NMR δ 96.08, 110.22, 110.54, 113.11, 127.48, 130.68, 135.18, 137.39, 139.27. HRMS-FAB (m/z, [M + 2H]+) Calcd for C11H5Cl2N3: 248.98605, Found: 248.98652.
Synthesis of 2-(2,3,6-trichlorophenyl)-1,1,2-tricyanoethylene (2,3,6-Cl3PTCE). Yield: 38% over three steps. Mp: 135.1–135.8 °C. IR (KBr): νCN 2244 and 2226 cm−1. 1H NMR δ 7.50 (d, 3J = 8.8 Hz, 1H), 7.69 (d, 3J = 8.8 Hz, 1H). 13C NMR δ 104.63, 108.68, 109.22, 111.37, 127.57, 129.92, 132.28, 132.73, 133.94, 134.85, 137.81. HRMS-FAB (m/z, [M + H]+) Calcd for C11H3Cl3N3: 281.93926, Found: 281.94001.
Synthesis of 2-(2,3,5,6-tetrachlorophenyl)-1,1,2-tricyanoethylene (2,3,5,6-Cl4PTCE). Yield: 51% over three steps. Mp: 150.1–151.2 °C. IR (KBr): νCN 2247, 2238 and 2225 cm−1. 1H NMR δ 7.88 (s, 1H). 13C NMR δ 104.80, 108.50, 109.05, 111.13, 128.52, 130.97, 134.16, 134.85, 137.36. HRMS-FAB (m/z, [M]+) Calcd for C11H1Cl4N3: 314.89246, Found: 314.89288.
General procedure for the synthesis of V[ClxPTCE]2·yCH2Cl2 Magnets: Under a nitrogen atmosphere, vanadium hexacarbonyl (15.3 mg, 0.070 mmol) dissolved in dichloromethane (2 mL) was added quickly to a solution containing ClxPTCE (0.15 mmol) in dichloromethane (1 mL) with stirring. After 15 min, the black precipitate was collected on a medium porosity frit, rinsed with dichloromethane (2 × 2 mL), and dried in vacuo for 1 h to yield a black powder of the desired product.
Synthesis of V[2-ClPTCE]2. Yield: 32.5 mg (98%). IR (KBr): νCN 2210 and 2128 cm−1. Anal. Calcd (Found) C22H8Cl2N6V1: C 55.26 (55.29), H 1.69 (2.32), N 17.57 (17.35) %.
Synthesis of V[3-ClPTCE]2·0.05CH2Cl2. Yield: 33.5 mg (99%). IR (KBr): νCN 2208 and 2130 cm−1. Anal. Calcd (Found) C22H8Cl2N6V1·0.05CH2Cl2: C 54.90 (54.88), H 1.69 (1.85), N 17.42 (17.52) %.
Synthesis of V[4-ClPTCE]2·0.35CH2Cl2. Yield: 32.3 mg (91%). IR (KBr): νCN 2207 and 2128 cm−1. Anal. Calcd (Found) C22H8Cl2N6V1·0.35CH2Cl2: C 52.85 (52.82), H 1.73 (1.79), N 16.55 (16.76) %.
Synthesis of V[2,3-Cl2PTCE]2·0.05CH2Cl2. Yield: 32.3 mg (84%).IR (KBr): νCN 2204 and 2132 cm−1. Anal. Calcd (Found) C22H6Cl4N6V1·0.05CH2Cl2: C 48.04 (48.04), H 1.12 (1.14), N 15.24 (15.03) %.
Synthesis of V[2,4-Cl2PTCE]2·0.10CH2Cl2. Yield: 33.4 mg (86%). IR (KBr): νCN 2208 and 2135 cm−1. Anal. Calcd (Found) C22H6Cl4N6V1·0.10CH2Cl2: C 47.78 (48.15), H 1.12 (1.57), N 15.13 (15.28) %.
Synthesis of V[2,5-Cl2PTCE]2·0.30CH2Cl2. Yield: 38.7 mg (97%). IR (KBr): νCN 2198 and 2132 cm−1. Anal. Calcd (Found) C22H6Cl4N6V1·0.30CH2Cl2: C 46.78 (46.71), H 1.16 (1.29), N 14.68 (14.97) %.
Synthesis of V[2,6-Cl2PTCE]2·0.50CH2Cl2. Yield: 35.5 mg (86%). IR (KBr): νCN 2200 and 2128 cm−1. Anal. Calcd (Found) C22H6Cl4N6V1·0.50CH2Cl2: C 45.84 (45.82), H 1.20 (1.53), N 14.25 (14.50) %.
Synthesis of V[3,5-Cl2PTCE]2·0.30CH2Cl2. Yield: 37.3 mg (93%). IR (KBr): νCN 2199 and 2133 cm−1. Anal. Calcd (Found) C22H6Cl4N6V1·0.30CH2Cl2: C 46.78 (46.64), H 1.16 (1.29), N 14.68 (14.82) %.
Synthesis of V[2,3,6-Cl3PTCE]2·0.45CH2Cl2. Yield: 42.2 mg (92%). IR (KBr): νCN 2201 and 2136 cm−1. Anal. Calcd (Found) C22H4Cl6N6V1·0.45CH2Cl2: C 41.05 (41.05), H 0.77 (0.78), N 12.77 (13.17) %.
Synthesis of V[2,3,5,6-Cl4PTCE]2·0.20CH2Cl2. Yield: 48.6 mg (99%). IR (KBr): νCN 2198 and 2133 cm−1. Anal. Calcd (Found) C22H2Cl8N6V1·0.20CH2Cl2: C 37.99 (37.99), H 0.34 (<0.5), N 11.97 (12.03) %.

5. Conclusions

The foregoing results demonstrate that chlorine-substituted phenyltricyanoethylenes can be building blocks for the formation of magnetically ordered solids with Tcs from 146 to 285 K. Importantly, this work expands the list of organic one-electron acceptors that can react with V(CO)6 to form high Tc magnets. No single property of the acceptors yielded a satisfactory correlation with Tc, although smaller subsets of the data could be successfully rationalized according to electronic and steric effects. In general, chlorine substitutions were inductively electron withdrawing, making the corresponding acceptors easier to reduce, and the corresponding magnets had enhanced Tc relative to the unsubstituted H5PTCE parent. Substitution in the 2- and/or 6-position(s) caused a large increase in the dihedral angle between the phenyl ring and olefin, which localized spin onto the nitrile nitrogen atoms and produced a material with a higher Tc. The anomalously low Tc of 4-FPTCE previously reported was reproduced by the anomalously low Tc of 4-ClPTCE, for which we have proposed a few explanations including a potentially deleterious reaction.

Supplementary Materials

The following are available online at https://www.mdpi.com/2312-7481/5/3/44/s1: 1H and 13C NMR spectra of all organic acceptors, tests of Tc versus redox potential, radical, and radical anion dihedral angle, and additional computational details.

Author Contributions

The authors have contributed equally to the conceptualization and execution of this work and wrote the manuscript together.

Funding

This research was partially funded by the National Science Foundation (grant number 0518220) and the Virginia Tech ASPIRES program (partial funding for the purchase of the SQUID magnetometer).

Conflicts of Interest

The authors declare no conflicts of interest.

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Figure 1. Structures and names of the chlorine substituted (and unsubstituted parent) PTCE one-electron acceptors.
Figure 1. Structures and names of the chlorine substituted (and unsubstituted parent) PTCE one-electron acceptors.
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Scheme 1. Synthesis of organic one-electron acceptors (NCS = N-chlorosuccinimide).
Scheme 1. Synthesis of organic one-electron acceptors (NCS = N-chlorosuccinimide).
Magnetochemistry 05 00044 sch001
Scheme 2. Preparation of V[x-ClnPTCE]2 magnetic solids.
Scheme 2. Preparation of V[x-ClnPTCE]2 magnetic solids.
Magnetochemistry 05 00044 sch002
Figure 2. Magnetization hysteresis curve for V[2,3,5,6-Cl4PTCE]2 collected at 5 K.
Figure 2. Magnetization hysteresis curve for V[2,3,5,6-Cl4PTCE]2 collected at 5 K.
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Figure 3. Variable-temperature magnetization data for V[x-ClnPTCE]2 magnets. The legend indicates the substitution pattern of chlorine atoms in the organic unit.
Figure 3. Variable-temperature magnetization data for V[x-ClnPTCE]2 magnets. The legend indicates the substitution pattern of chlorine atoms in the organic unit.
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Figure 4. Molecular orbital schematic showing the relative energies of the valence orbitals on VII and the x-ClnPTCE one-electron acceptors.
Figure 4. Molecular orbital schematic showing the relative energies of the valence orbitals on VII and the x-ClnPTCE one-electron acceptors.
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Figure 5. Redox potential (taken as the average of the cathodic and anodic peaks measured by cyclic voltammetry and denoted as E1/2), vs. the ordering temperature (Tc) for planar V[x-ClnPTCE]2 magnets.
Figure 5. Redox potential (taken as the average of the cathodic and anodic peaks measured by cyclic voltammetry and denoted as E1/2), vs. the ordering temperature (Tc) for planar V[x-ClnPTCE]2 magnets.
Magnetochemistry 05 00044 g005
Figure 6. DFT-minimized structures of H5-PTCE (left), 2-ClPTCE (middle), and 2,6-Cl2PTCE (right) radical anions that show an increase in the dihedral angle as more chlorine substituents are added in the 2- and 6- positions.
Figure 6. DFT-minimized structures of H5-PTCE (left), 2-ClPTCE (middle), and 2,6-Cl2PTCE (right) radical anions that show an increase in the dihedral angle as more chlorine substituents are added in the 2- and 6- positions.
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Figure 7. Mulliken spin density vs. Tc for the chlorine-substituted PTCE magnets.
Figure 7. Mulliken spin density vs. Tc for the chlorine-substituted PTCE magnets.
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Table 1. Summary of CN stretching frequency data for neutral acceptors and resulting magnets.
Table 1. Summary of CN stretching frequency data for neutral acceptors and resulting magnets.
AcceptorNeutral AcceptorMagnet
νCN, cm−1νCN, cm−1
H5PTCE2235, 22332210, 2129
2-ClPTCE22412210, 2128
3-ClPTCE22402208, 2130
4-ClPTCE2237, 22302207, 2128
2,3-Cl2PTCE22432204, 2132
2,4-Cl2PTCE22412208, 2135
2,5-Cl2PTCE2242, 22282198, 2132
2,6-Cl2PTCE2244, 22292200, 2128
3,5-Cl2PTCE22382199, 2133
2,3,6-Cl3PTCE2244, 22262201, 2136
2,3,5,6-Cl4PTCE2247, 2238, 22252198, 2133
Table 2. Summary of magnetic data and associated acceptor properties.
Table 2. Summary of magnetic data and associated acceptor properties.
AcceptorTc (K)Var. in Tc (K)No. of Meas. M s   ( e m u · G m o l ) H c   ( e m u · G m o l ) E1/2 (V)EA (eV)Anion Dihedral (Degrees)
H5PTCE *213±2251002.0−0.402.6416.5
2-ClPTCE271±1347003.1−0.342.6738.2
3-ClPTCE243±1249004.8−0.282.8415.3
4-ClPTCE146±2244006.8−0.322.8114.8
2,3-Cl2PTCE265±2343004.2−0.292.8038.3
2,4-Cl2PTCE256±1250003.9−0.292.8337.0
2,5-Cl2PTCE265±1344004.1−0.262.8538.1
2,6-Cl2PTCE285±2349004.1−0.282.6662.1
3,5-Cl2PTCE257±2248004.2−0.203.0214.6
2,3,6-Cl3PTCE276±2246002.6−0.232.7960.9
2,3,5,6-Cl4PTCE256±1345003.6−0.202.8961.0
* Previous work [19].
Table 3. Comparison of Tc for the halogenated PTCE magnets (values for X = F taken from previous reports [18,20]).
Table 3. Comparison of Tc for the halogenated PTCE magnets (values for X = F taken from previous reports [18,20]).
AcceptorX = ClX = F
Tc (K)Tc (K)
2-XPTCE271257
3-XPTCE243233
4-XPTCE146160
2,3-X2PTCE265
2,4-X2PTCE256242
2,5-X2PTCE265
2,6-X2PTCE285300
3,5-X2PTCE257263
2,3,6-X3PTCE276310
2,3,5,6-X4PTCE256315

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Tatum, D.S.; Zadrozny, J.M.; Yee, G.T. A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene). Magnetochemistry 2019, 5, 44. https://doi.org/10.3390/magnetochemistry5030044

AMA Style

Tatum DS, Zadrozny JM, Yee GT. A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene). Magnetochemistry. 2019; 5(3):44. https://doi.org/10.3390/magnetochemistry5030044

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Tatum, David S., Joseph M. Zadrozny, and Gordon T. Yee. 2019. "A New Family of High Tc Molecule-Based Magnetic Networks: V[x-ClnPTCE]2·yCH2Cl2 (PTCE = Phenyltricyanoethylene)" Magnetochemistry 5, no. 3: 44. https://doi.org/10.3390/magnetochemistry5030044

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