A Stable Aluminum Tris(dithiolene) Triradical

A stable aluminum tris(dithiolene) triradical (3) was experimentally realized through a low-temperature reaction of the sterically demanding lithium dithiolene radical (2) with aluminum iodide. Compound 3 was characterized by single-crystal X-ray diffraction, UV–vis and EPR spectroscopy, SQUID magnetometry, and theoretical computations. The quartet ground state of triradical 3 has been unambiguously confirmed by variable-temperature continuous wave EPR experiments and SQUID magnetometry. Both SQUID magnetometry and broken-symmetry DFT computations reveal a small doublet–quartet energy gap [ΔEDQ = 0.18 kcal mol–1 (SQUID); ΔEDQ = 0.14 kcal mol–1 (DFT)]. The pulsed EPR experiment (electron spin echo envelop modulation) provides further evidence for the interaction of these dithiolene-based radicals with the central aluminum nucleus of 3.

■ RESULTS AND DISCUSSION Synthesis and UV−Vis Absorption Spectra.Triradical 3 was obtained as dark blue crystals (in 17% yield) via the 3:1 reaction of 2 with AlI 3 in hexane (at −78 °C) and subsequent recrystallization in a toluene/hexane mixed solvent (at −40 °C) (Scheme 1). 65Triradical 3 is thermally unstable above 115 °C.The reaction of 3 with O 2 (excess) in toluene gives the previously reported dithione dimer (4) 66 (aluminum oxide is deduced as a byproduct, Scheme 1).In addition, in polar solvents (such as THF, CH 3 CN, and CH 2 Cl 2 ), 3 immediately decomposes, giving an orange red slurry of 4.
Due to its extremely high sensitivity toward O 2 , 3 quickly changes color from dark blue to blue-green, even while being stored in a screw-cap cuvette (under an argon atmosphere) during the UV−vis spectral measurement.Consequently, only the UV−vis spectrum of the partially oxidized 3 was obtained, which contains four absorptions at 423, 435, 595, and 645 nm in the visible region (Figure 2a).Accompanying the complete decomposition of 3 (yellow solution), the two absorptions at 595 and 645 nm disappear, whereas the intensity of the absorption peaks at 423 and 435 nm increases (Figure 2b).
Electron Paramagnetic Resonance Spectra.The roomtemperature EPR spectrum (Figure 3a) of 3 (in toluene) only exhibits an unresolved broad singlet with a line width of ca.360 G, which corresponds to a relaxation time T 2 of ca.0.15 ns.The intense narrow line (in Figure 3a) is due to a monoradical admixture.Its integral intensity is less than 0.5% of the signal. 65The paramagnetic properties of 3 were further probed by variable-temperature (VT) EPR spectroscopy in toluene at 6−60 K, wherein the forbidden Δm = 2 and Δm = 3 transitions were observed at 1685 and 1080 G, respectively (see the full scan spectrum at 10 K in Figure 3c).The presence of these transitions, with the temperature dependence of the intensity following the Curie law (see Figure S1 in the Supporting Information), 65 supports the quartet ground state triradical character of 3. 28,32,67 The sharp Δm = 3 transition requires all three electron spins to be coupled to each other. 68Moreover, the integral ratio of the Δm = 1, 2, and 3 features is expected approximately to be 1:(D/B 0 ) 2 : (D/B 0 ) 4 , where D is the electron dipolar splitting, and B 0 is the magnetic field corresponding to the Δm = 1 resonance. 68Thus, for a weak dipolar coupling, the Δm = 3 feature cannot be observed due to the low intensity of its EPR line.An unusual feature of 3 is the relatively strong intensity of these forbidden transitions pointing to a very large value of D and very close distance between interacting electrons.Estimates based on the integral intensity ratio 68 between different allowed and forbidden transitions give D/B 0 in a 1/9 to 1/12 range (D = 280−370 G).This is consistent with the total field range of the Δm = 1 line (centered at ca. 3330 G), which is predicted to be about 4D including the smaller satellite lines spaced by 2D and 4D distances from the center.In our case, the satellite lines are not resolved but contribute to the shoulder feature in the field range of approximately 2600− 4100 G (Figure 3c), since they are broadened and low in intensity compared to the main line.This is likely due to a large D strain effect (Figure S2). 65Based on the formula 3 (where r is the interspin distance given in Å, and D is in Gauss) 69 and g = 2 for the Δm = 1 line, the distance between the three interacting electrons can be estimated to be in the range of 4.2−4.6Å.
The most intense signal (g = ca.2.01) of the lowtemperature EPR spectra of 3 corresponds to the Δm = 1 transition, which shows a partially resolved hyperfine pattern with a value of ca.5.0 G at temperatures below 20 K (Figure 3b).It should correspond to the hyperfine splitting on aluminum.For comparison, the hyperfine splitting on 25 Mg in our previously reported magnesium-based dithiolene radical was 2.3 G. 61 The hyperfine splitting on two equivalent 14 N nuclei of the dithiolene ligand (a N = ca. 1 G) 61 cannot be resolved within this intrinsic line width.The spectrum starts to broaden and lose resolution above 20 K (Figure 3b), which can be attributed to a decrease in the T 2 relaxation time with the increase of the temperature. 65Further evidence for the interaction of the unpaired electrons with the central aluminum nucleus in 3 was obtained from the pulsed 70 electron spin echo envelop modulation (ESEEM) spectroscopy. 65As seen in Figure 4, the strong ESEEM pattern corresponds to the nuclear Larmor frequency (3.82 MHz) of the aluminum nucleus at X-band (9.7 GHz) with which the free electrons interact.DFT computations of the simplified Λ-3-H model (quartet state, UB3LYP/6-311G** level) reveal that while the unpaired electrons are largely localized on the C 2 S 2 units of the three dithiolene ligands (the spin density of the C 2 S 2 unit = 0.77), the central aluminum bears a spin density of −0.04.
Solid-state EPR spectra of 3 (Figure 3d) show similar general features to those observed for 3 in toluene (Figure 3b,c).At room temperature, the spectrum is an unresolved singlet with a line width of ca.236 G (Figure S6). 65In the temperature range of 6−20 K, the spectrum remains an  exchange-narrowed singlet at g = ca.2.01 with a line width of 25 G (the line shape does not change, Figure S7). 65However, the spectrum starts to broaden above 20 K (Figure 3d).This broadening is almost perfectly Lorentzian with a width of ca. 15 G at 60 K (Figure S8), 65 giving T 2 < 4 ns at this temperature.The similar broadening observed at the same temperature interval for the powder and solution (in toluene) samples of 3 indicates that the short T 2 relaxation time is not due to intermolecular interactions or crystal field effects but related to the triradical nature of 3.
SQUID Magnetometry.Magnetic data on the powder of 3 were collected using SQUID magnetometry.The magnetic moment versus magnetic field measurements were performed at 2, 3, and 5 K with the field range of −50000 to 50000 Oe, being swept twice in both directions and showing no evidence of the sample hysteresis.As seen in Figure 5a, the M/M sat versus H/(T−θ) plot closely follows the Brillouin curve corresponding to S = 3/2, which unambiguously confirms the quartet ground state of triradical 3. 21 While the 1/χ versus T plot (Figure S9) approaches linearity (where χ is the magnetic susceptibility), the χT product versus T plot (Figure 5b) allows for more detailed analyses of the doublet−quartet equilibrium. 21The drop at low temperatures (T = 2−5 K) is related to paramagnetic saturation.We also observe a decrease in magnetization with the temperature increase from 25 to 200 K.When T = 200 K, the χT value is ca.85% of its maximum at lower temperature.This decrease reveals an admixture of a thermally populated doublet excited state. 21In the presence of the quartet/doublet equilibrium, the net magnetization should be described by eq S1 (see the Supporting Information). 65As seen in Figure 5b, the experimental χT versus T dependence can be reasonably well approximated by eq S1 65 with a doublet−quartet energy gap (ΔE DQ ) of 3J/k (0.18 kcal mol −1 , i.e., 90 K), which compares well to the theoretical value (ΔE DQ = 0.14 kcal mol −1 ) of the Λ-3-H model obtained by the broken-symmetry DFT computations. 23,65Based on the ΔE DQ value (0.18 kcal mol −1 ) of 3, a substantial (more than 5% of the spins) population of the doublet should be presented above 30 K. At 298 K, the populations of the quartet and doublet states should be similar, given the Boltzmann factor of ca.0.75.For the reported triradical species, 21,57 simulations of distinct EPR spectra corresponding to the quartet and doublet states were used to analyze the experimental data.However, for 3, the broadening of EPR lines due to the very short relaxation times (Figures 3b and S5) makes fine spectral features unresolved at temperatures with appreciable fraction of spins in the doublet state, thus preventing our study on the doublet−quartet equilibrium by EPR.
Molecular Structure and DFT Computations.X-ray structural analysis reveals that 3 exists as a pair of enantiomers with identical bonding parameters (Figure 6a).The central aluminum atom in 3 is coordinated by six sulfur atoms from three dithiolene ligands.The steric bulk of the dithiolene ligand (Figure S10) provides sufficient kinetic stability such that 3 can be isolated.The structural distortion from the regular octahedron for tris(bidentate ligand) complexes has been evaluated with both trigonal twist angle (ϕ) and s/h ratio (Figure 6b). 2,71,72The six-coordinate aluminum atom in 3 adopts an octahedral geometry with an elongated distortion [ϕ = 52.1°(av),s/h = 1.10 (av) vs ϕ = 60.0°,s/h = 1.22 for a regular octahedron 71 ], which compares to that [ϕ = 55.1°(av),s/h = 1.16 (av)] of the quartet state Λ-3-H model.While being marginally shorter than those (2.470Å, av) in Λ-3-H, the Al− S bonds in 3 [2.4092(7)−2.4307(7)Å] are somewhat longer than those [2.2785(10)−2.293(4)Å] for the four-coordinate aluminum-based ethene−tetrathiolate complex. 73 6) Å]. 74 The X-ray structural data support the monoanionic radical essence of the dithiolene ligands in 3. Thus, the central aluminum atom in 3 is in the formal oxidation state of +3.Natural bond orbital (NBO) analysis shows that while the aluminum atom in Λ-3-H has a positive natural charge of +0.57, each sulfur atom next to the central aluminum has a negative natural charge of −0.04.Our
Syntheses, spectral data, computations, and X-ray crystal determination (PDF) Cartesian coordinates for the Λ-3-H models (quartet state and broken-symmetry doublet state) (XYZ)

Figure 3 .
Figure 3. (a) Room-temperature X-band (9.4 GHz) continuous wave EPR (CW-EPR) spectrum of 3 in toluene.The intense narrow line (marked with *) is due to a monoradical admixture.(b) Main lines (Δm = 1, centered at ca. 3330 G) observed in the X-band (9.4 GHz) CW-EPR spectra of 3 in toluene at different temperatures normalized by intensity.(c) Less intense features observed in the zoom of the full-scan EPR spectrum of 3 at 10 K (marked by blue and red arrows) correspond to the forbidden Δm = 2 (centered at 1685 G) and Δm = 3 (centered at 1080 G) transitions, respectively.(d) Solid-state Xband EPR spectra of 3 at 20−60 K.The features corresponding to the forbidden transitions at Δm = 2 (centered at ca. 1669 G, blue arrow) and Δm = 3 (centered at 1070 G, red arrow) are shown in the inset plot (the spectrum was measured at 20 K).
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