A high-spin diradical dianion and its bridged chemically switchable single-molecule magnet
Triplet diradicals have attracted tremendous attention due to their promising application in organic spintronics, organic magnets and spin filters. However, very few examples of triplet diradicals with singlet-triplet energy gaps (DE ST ) over 0.59 kcal mol À1 (298 K) have been reported to date. In this work, we first proved that the dianion of 2,7-di-tert-butyl-pyrene-4,5,9,10-tetraone (2,7-tBu 2 -PTO) was a triplet ground state diradical in the magnesium complex 1 with a singlet-triplet energy gap DE ST ¼ 0.94 kcal mol À1 (473 K). This is a rare example of stable diradicals with singlet-triplet energy gaps exceeding the thermal energy at room temperature (298 K). Moreover, the iron analog 2 containing the 2,7-tBu 2 -PTO diradical dianion was isolated, which was the first single-molecule magnet bridged by a diradical dianion. When 2 was doubly reduced to the dianion salt 2K 2 , single-molecule magnetism was switched off, highlighting the importance of diradicals in single-molecule magnetism.
Introduction<!>Results and discussion<!>Crystal structures<!>Calculated electron spin density distribution and singlettriplet gap of 1<!>Magnetic characterization<!>Conclusions
<p>There is growing interest in radical chemistry because of the potential applications of radicals in various research elds such as organic chemistry, inorganic chemistry and materials science. 1 Diradicals are species containing two unpaired electrons that play pivotal roles in understanding the nature of chemical bonds and are of great interest in organic electronic device applications. 2 For example, Kubo and coworkers found that bisphenalenyl-based singlet diradicaloids exhibited high ambipolar mobilities in organic eld-effect transistors 3 and strong two-photon absorption properties. 4 Attributed to the small HOMO-LUMO gaps, several singlet diradicaloids show strong absorption in the near-infrared (NIR) region and can be applied to NIR-organic photodetectors. 5 Triplet diradicals are particularly attractive regarding their promising usage in organic magnets, organic spintronics, 6 spin lters, 7 memory devices, 8 etc. The well-known triplet diradical m-xylylene possesses a large singlet-triplet energy gap (DE ST ) of ca. 9.6 kcal mol À1 , 9 but it is only observable in solution at room temperature for hundreds of nanoseconds. 10 Rajca group reported a triplet aza-m-xylylene diradical with DE ST around 10 kcal mol À1 , which was persistent at ambient temperature in solution on the time scale of minutes. 11 To date, the number of stable triplet diradicals with DE ST over 0.59 kcal mol À1 (298 K), i.e., surpassing the thermal energy at room temperature, remains limited due to the intrinsic high reactivity. 2f,12 In contrast, the application of diradicals in synthesizing single-molecule magnets (SMMs) is much less explored, although monoradicals have been widely utilized in designing SMMs. 13 In the studied diradical-containing SMMs, the diradicals are mainly composed of neutral bis(imino nitroxide) moieties. 14 However, neutral diradicals oen suffer from weak coordination to metal ions. In recent years, a few stable triplet ground state diradicals with DE ST over 298 K have been realized by electron delocalization and steric protection strategies. 15 Thus, it is highly desirable to apply these high-spin diradicals to design SMMs. Shultz and coworkers obtained a series of bis(semiquinone) diradical dianion metal complexes with large DE ST over 298 K, but none of them are SMMs. 16 In this contribution, we report a readily accessible 2,7-di-tertbutyl-pyrene-4,5,9,10-tetraone (2,7-tBu 2 -PTO) diradical dianion in the magnesium and iron complexes LMg II (2,7-tBu 2 -PTO) Mg II L (L ¼ CH(MeCNAr) 2 , Ar ¼ 2,6-iPr 2 C 6 H 3 ) 1 and LFe II (2,7-tBu 2 -PTO)Fe II L 2, respectively. Complex 1 features a triplet ground state with DE ST ¼ 0.94 kcal mol À1 (473 K). Complex 2 represents the rst SMM connected by a strong ferromagnetically coupled diradical dianion. Moreover, 1 and 2 can be further doubly reduced to dianion salts 1K 2 and 2K 2 , where the 2,7-tBu 2 -PTO units are in the diamagnetic tetraanion state, and the single-molecule magnetism of 2K 2 is switched off.</p><!><p>Syntheses and two-electron reduction of the 2,7-tBu 2 -PTO diradical dianion ligated magnesium and iron complexes Pyrene-4,5,9,10-tetraones have been widely used as electronic materials due to their intrinsic strong p-electron acceptor ability. 17 They can accept as many as four electrons. However, the molecular and electronic structures of the reduced species, especially the doubly reduced species, have never been fully elucidated. 18 The cyclic voltammograms reveal that 2,7-tBu 2 -PTO can undergo four reversible one-electron reduction events with reduction potentials at À0.431 V, À0.710 V, À0.884 V and À1.556 V versus the Ag/Ag + electrode (Fig. S1 †), which indicates that the reduced species may be isolable.</p><p>The b-diketiminato-ligated magnesium dimer reported by Jones and coworkers, 19 which has been proven to be an elegant reducing agent in synthetic chemistry, 20 was utilized to reduce 2,7-tBu 2 -PTO. The reaction of 2,7-tBu 2 -PTO and one molar equivalent of LMg I -Mg I L in toluene at room temperature resulted in deep green crystals of 1 aer workup (Fig. 1). It has been spectroscopically and structurally characterized. Intrigued by the facile access of 1 and our previous phosphorus radical anion-coordinated Fe II complex exhibiting single-molecule magnetic properties, 21 we were interested in applying the 2,7-tBu 2 -PTO diradical dianion to construct SMMs. Then, the reaction of 2,7-tBu 2 -PTO and two molar equivalents of LFe I (toluene) 22 was performed in toluene at room temperature, from which reddish brown crystals of 2 were isolated (Fig. 1). Complexes 1 and 2 are highly sensitive to oxygen and moisture but can be stored for months in an N 2 -lled glovebox at ambient temperature.</p><p>The cyclic voltammetric investigations (Fig. S2 and S3 †) show that compounds 1 and 2 exhibit two reversible reduction waves (E red 1/2 ¼ À1.811 V and À1.548 V for 1; E red 1/2 ¼ À0.194 V and À0.974 V for 2 vs. Ag/Ag + ), indicating that 1 and 2 can be doubly reduced. In addition, a reversible oxidation process was observed at E 1/2 ¼ 0.077 V for 2, suggesting that it can also be oxidized, which might be attributed to the oxidation of the reduced form of 2,7-tBu 2 -PTO. Consequently, reactions of 1 and 2 with two molar equivalents of elemental potassium in THF at room temperature led to the formation of dianion salts 1K 2 and 2K 2 , respectively, which were isolated in moderate yields (Fig. 2). The solution NMR spectroscopic studies of 1K 2 at room temperature show well-resolved 1 H and 13 C signals in the diamagnetic window (Fig. S11 and S12, † respectively). The 1 H NMR spectrum exhibits a singlet resonance at d 8.42 ppm for the hydrogen atoms of the pyrene moiety. The proton signal for the g-H of the b-diketiminato ligand (d ¼ 5.10 ppm) in 1K 2 is slightly downeld shied compared to that of the b-diketiminato ligated magnesium dimer (d ¼ 4.81 ppm).</p><!><p>Crystals of 1 were obtained by cooling the toluene solution at À20 C (Fig. 3A). 23 It crystallizes in the monoclinic space group P2 1 /c and features a centrosymmetric geometry. The Mg atoms feature a tetrahedral geometry. The Mg-O distances (1.9803 (19)  and 1.9714( 19) Å) are comparable to those of the magnesium oxalate compound LMg(m-C 2 O 4 )MgL (1.9855(15) and 1.9858( 14) Å). 24 The C-O bonds are elongated, while the C7-C8 bond is shortened in comparison to those in 2,7-tBu 2 -PTO (Table 1) 25 in accordance with the reduction of carbonyl compounds. The C7-C8 bond length (1.443(3) Å) is between those of the C-C single bond (1.54 Å) and C]C double bond (1.34 Å), which shows the semiquinone character of the C 2 O 2 moieties. Moreover, due to the steric bulkiness of the L ligands, no intermolecular interaction is observed, which is normally present in pyrenes. 26 The molecular conguration of 2 is akin to that of 1 (Fig. 3B). The iron centers feature a tetrahedral geometry. The Fe-N bond lengths (1.963(2) and 1.968(2) Å) are comparable to those in the diazauorenylidene-substituted phosphaalkene radical anion coordinated iron complex (1.980(2) and 1.968(2) Å). 21 The bond lengths in the 2,7-tBu 2 -PTO moiety of 2 are comparable to those  in 1 (Table 1), which suggest that 2,7-tBu 2 -PTO is also most likely in the diradical dianion state. Therefore, two-electron transfer to 2,7-tBu 2 -PTO occurs in the reaction, and the iron atoms are in the oxidation state of two. Additionally, the zero-eld 57 Fe Mössbauer spectrum recorded at 80 K afforded an isomer shi value d ¼ 0.86 mm s À1 and a quadrupole splitting value DE Q ¼ 1.87 mm s À1 (Fig. S4 †), which conrms the presence of high-spin tetrahedral Fe II ions in 2. 21,27 Crystals of 1K 2 and 2K 2 suitable for X-ray diffraction analysis were grown from THF solutions at À20 C (Fig. 4). They crystallize in the monoclinic space group P 1. In comparison to precursors 1 and 2, the M-N bonds (2.044(2) and 2.045(2) Å for 1K 2 ; 1.997(2) and 1.9938( 19) Å for 2K 2 ) are slightly elongated, while the M-O bonds (1.9300 (18)  and 1.9506(18) Å for 1K 2 ;  1.9808(15) and 1.9811(16) Å for 2K 2 ) are shortened. The most pronounced change is the further increased C-O bond lengths (Table 1), which are close to those in the pyrocatecholate aluminum compound (1.369(5) and 1.380(4) Å). 28 Meanwhile, C7-C8 bonds (1.382(3) Å for 1K 2 ; 1.390(3) Å for 2K 2 ) are shortened, compared with those of 1 and 2. So 2,7-tBu 2 -PTO is in the tetraanion form. This result is consistent with the diamagnetic nature of 1K 2 demonstrated by NMR spectroscopy. Moreover, the isomer shi value d ¼ 0.86 mm s À1 and quadrupole splitting value DE Q ¼ 1.57 mm s À1 obtained from the zero-eld 57 Fe Mössbauer spectrum at 80 K of 2K 2 (Fig. S5 †) are close to those of 2, which prove the retention of the high-spin +2 oxidation state of the iron centers.  4), C1-C1A 1.449(5), C1-C6 1.400( 4), C2A-C7 1.445( 4), C7-C8 1.422( 4), C6-C8 1.443( 4), N1-Fe1-N2 95.08 (10), N1-Fe1-O2 128.34(9), N2-Fe1-O2 118.58(9), N1-Fe1-O1 117.52(9), N2-Fe1-O1 118.75( 9), and O2-Fe1-O1 80.97(8). Symmetry transformations were used to generate equivalent atoms labeled with 'A': Àx + 1, y + 1, and z + 1.</p><!><p>The theoretical calculations of 1 were performed. Geometry optimizations were performed at the (U)B3LYP/6-31G(d) level, and the stationary points were checked by frequency calculations. 29 The results from theoretical calculations are interpreted to suggest that 1 has a triplet ground state, and the calculated singlet-triplet gap is 0.91 kcal mol À1 . Moreover, the spin density distribution shows that it is mainly delocalized over the two C 2 O 2 moieties with contributions from the two central benzene rings (Fig. 5).</p><!><p>Electro-paramagnetic resonance (EPR) spectroscopy and superconducting quantum interference device (SQUID) measurements were performed to clarify the electronic structures of newly formed complexes. The EPR spectrum of 1 in the frozen toluene solution at 90 K (Fig. 6A) reveals a clear half-eld signal of the forbidden transition (Dm s ¼ AE2), which indicates the presence of a triplet-spin state species. The signals attributed to the Dm s ¼ AE1 transition are nely resolved, and parameters g x ¼ g y ¼ 2.0028, g z ¼ 2.0042, D ¼ 139 G and E ¼ 32 G are extracted from the spectral simulation with the SimFonia soware package. From the zero-eld splitting parameter D, the distance between the spin centers is estimated to be 5.85 Å, 2f which is close to the mean distance (6.05 Å) of C7 to C8A and O2 to O1A in the solid-state structure, which suggests that the spin density is delocalized over the C 2 O 2 moieties. In contrast, iron complexes 2 and 2K 2 are EPR-silent at room temperature and 90 K. SQUID measurements were performed on the powder sample of 1 (Fig. 6B). The c M T value at 300 K is 0.83 cm 3 mol À1 K, which is larger than the theoretical value of 0.75 cm 3 mol À1 K (S ¼ 1/2, g ¼ 2.0) for 2 free radicals. With decreasing temperature, the c M T values slowly increase to a maximum of 1.04 cm 3 mol À1 K at 8 K. With the further decrease in temperature, the c M T values slightly decline to 1.0 cm 3 mol À1 K at 1.8 K. This behavior obviously indicates a strong ferromagnetic coupling between two S ¼ 1/2 centers in 1, and the spins are completely parallel below 8 K. Moreover, the isothermal magnetization of 1.96 Nm B at 70 kOe and 1.8 K is close to the theoretical value of 2 Nm B (S ¼ 1, g ¼ 2) (Fig. S6 †), which provides evidence of ferromagnetic coupling. The temperature-dependent magnetizations were tted using the PHI program based on equation Ĥ ¼ À2J Ŝ1 Ŝ2 + gm B ŜH (S 1 ¼ S 2 ¼ 1/2). The best tting results are g ¼ 2.04(1), J ¼ 165.1(5) cm À1 , zj ¼ À0.0053(1) cm À1 and TIP ¼ 4.17 Â 10 À4 cm 3 mol À1 . Hence, complex 1 is a triplet ground state diradical with a singlet-triplet energy gap of 0.94 kcal mol À1 (473 K), and there is negligible intermolecular interaction. The experimental results are consistent with the theoretical calculations. Complex 1 is a rare example of stable diradicals with singlet-triplet energy gaps larger than 0.6 kcal mol À1 (300 K).</p><p>SQUID measurements on the powder samples of 2 and 2K 2 were also performed to gain more insights into their electronic structures and magnetic properties. The c M T value of 5.26 cm 3 mol À1 K at 300 K for 2 (Fig. 7) is far less than 6.75 cm 3 mol À1 K for two high-spin Fe II ions (S ¼ 2, g ¼ 2.0) and two organic radicals (S ¼ 1, g ¼ 2.0). This phenomenon may be due to   a strong antiferromagnetic coupling between the metal ions and the diradical ions, which results in dominant magnetic properties even above room temperature. With decreasing temperature, the c M T values rst gradually increase from 300 to 30 K, subsequently show a pronounced decrease, and nally reach the ultimate value of 5.48 cm 3 mol À1 K at 1.8 K. This phenomenon should be attributed to the competition between the intramolecular ferromagnetic and antiferromagnetic coupling, which forms a ferromagnetic experimental curve. The temperature and eld dependent magnetizations were tted to quantify the anisotropy parameters based on eqn (1) using the PHI program:</p><p>where J and J 1 -J 3 are the magnetic coupling constants between the spins of radicals and Fe II ions (Fig. 7, inset). A reasonable tting gives D ¼ À10.0(2) cm À1 , g ¼ 2.23(1), J 1 ¼ À634.7(4), J ¼ 116.1(1), J 2 ¼ À7.37 (10), J 3 ¼ À0.11(1) cm À1 and TIP ¼ 5.93 Â 10 À4 cm 3 mol À1 . The results indicate that the high-spin Fe II ions exhibit very strong antiferromagnetic coupling with the diradical unit. The coupling constant magnitude (À634.7(4) cm À1 ) is similar to that observed in the azophenine radical-bridged dinuclear iron complex (jJj $ 900 cm À1 ). 30 Moreover, the ferromagnetic coupling of the diradical dianion is retained in 2, and the coupling constant is slightly lower than that in 1 because the electronic structure has changed, such as the angle O1-Mg1-O2 from 83.87 for 1 to 80.97 for 2. The magnetization of 4.16 Nm B at 7 T and 1.8 K is far less than the expected value of 6.0 Nm B due to the strong anisotropy, which is also shown by the nonsuperposition of the M vs. H/T plots (Fig. S8 †).</p><p>The c M T value of 2K 2 at 300 K is 6.92 cm 3 mol À1 K, and the value rst slowly decreases from room temperature to 60 K and subsequently rapidly decreases to 3.82 cm 3 mol À1 K at 1.8 K (Fig. 7). These results suggest that the two high-spin Fe II ions are weakly antiferromagnetically coupled in 2K 2 . Fitting the c M T-T plots to eqn (2) using the PHI program 31 yields the parameters of D ¼ À6.45(10) cm À1 , g ¼ 2.12(2), J 3 ¼ À0.028(1) cm À1 and TIP ¼ 7.22 Â 10 À4 cm 3 mol À1 . The weak coupling between the Fe II ions is due to the closed-shell character of the 2,7-tBu 2 -PTO tetraanion, as observed in 1K 2 . The large difference in the D value in complexes 2 and 2K 2 can be attributed to the change in the local ligand eld aer the twoelectron reduction.</p><p>Alternating current (ac) susceptibility measurements were performed to evaluate the magnetization dynamics of 2 and 2K 2 . Obvious out-of-phase signals (c M 00 ) were observed for 2 under a zero eld, whereas no peak was observed below 1000 Hz (Fig. S9 †), which suggests the existence of quantum tunneling of magnetization (QTM). In contrast, the susceptibility measurements of 2K 2 showed that no c M 00 signals were observed with or without external elds, which suggests switching off the single-molecule magnetism aer the two-electron reduction of 2.</p><p>To suppress the QTM effect, a 1.0 kOe eld was applied to study the slow relaxation behavior of 2. U eff ¼ 71.1 K. The tting results indicate that the QTM effect has been suppressed well by the applied magnetic eld.</p><!><p>We have rst demonstrated that the dianion of 2,7-tBu 2 -PTO is a triplet ground state diradical in the coordination sphere of magnesium. This is a rare example of stable diradicals with singlet-triplet energy gaps larger than 0.59 kcal mol À1 (298 K), which surpasses the thermal energy at room temperature. The diradical dianion was applied to synthesize iron complex 2, which serves as the rst SMM bridged by a diradical dianion. Complexes 1 and 2 can be reduced to dianion salts 1K 2 and 2K 2 , respectively, where the 2,7-tBu 2 -PTO units are in diamagnetic tetraanion form. This work provides more insights into the molecular and electronic structures of the reduced species of PTO and demonstrates the potential of diradical dianions in constructing interesting magnetic materials. Syntheses and magnetic studies of other complexes bearing the reduced species of PTO are ongoing in our laboratory.</p>