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Half-Sandwich Metal Carbonyl Complexes as Precursors to Functional Materials: From a Near-Infrared-Absorbing Dye to a Single-Molecule Magnet Wenqing Wang,†,§ Jing Li,†,§ Lei Yin,‡ Yue Zhao,† Zhongwen Ouyang,‡ Xinping Wang,*,† Zhenxing Wang,*,‡ You Song,*,† and Philip P. Power*,∥ †

State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210093, China ‡ Wuhan National High Magnetic Field Center & School of Physics, Huazhong University of Science and Technology, Wuhan 430074, China ∥ Department of Chemistry, University of California, Davis, California 95616, United States S Supporting Information *

ABSTRACT: Chemical oxidations of piano-stool chromium/cobalt carbonyl complexes Cr(CO)3(η6,η5-C6H5C5H4)Co(CO)2 (1) and Cr(CO)3(η6,η6-C6H5C6H5) Cr(CO)3 (2) were investigated. Upon one-electron oxidation, 1 was transformed to a heterometalloradical species, 1•+. However, either one- or two-electron oxidation of 2 afforded a decomposition product, 3. Dipping 3 into pentane led to the formation of 4 via a crystal-to-crystyal transformation with the removal of solvent molecules. Complexes 1•+ and 4 were fully characterized by various spectroscopic techniques and single-crystal X-ray analysis. Cation 1•+ features a weak Cr−Co bond with a Wiberg bond order of 0.278. A near-infrared absorption band around 1031 nm was observed for 1•+, which is far red-shifted in comparison to previously reported dinuclear metalloradical species. Complex 4 contains a chromium(II) with a distorted pyramidal geometry and displays single-molecule magnetic properties.

1. INTRODUCTION Half-sandwich metal carbonyl complexes, such as (arene)Cr(CO)3 and (Cp)Co(CO)2, are a well-known class of organometallic species and are of fundamental importance in organic synthesis and organometallic chemistry.1 Their oxidation affords carbonyl-containing 17-electron organometallic radical cations, which are important in stoichiometric and catalytic transformations because of their high reactivity in comparison to 18-electron precursors.2 These highly reactive 17-electron radical cations can become stabilized when one CO is replaced by a stronger electron-donating ligand such as PR3 or when they are accompanied by a weakly coordinating anion.3 Further loss of an electron from (arene)Cr(CO)3 radical cations may © 2017 American Chemical Society

lead to decomposition by loss of ligands and formation of a “Cr(II)” species. This process has been studied by various electrochemical methods, but the identity of the Cr(II) species remains elusive.4 Their bimetallic complexes are of importance for the understanding of the nature of metal−metal bonds.5 Knowledge of the metal−metal interactions in the paramagnetic metal species is especially crucial for the fields of metalloproteins and metal-containing functional materials.6 A number of dinuclear metalloradicals with a metal−metal bond have been isolated.7 Received: June 30, 2017 Published: August 8, 2017 12069

DOI: 10.1021/jacs.7b06795 J. Am. Chem. Soc. 2017, 139, 12069−12075

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Journal of the American Chemical Society However, a hetero-dinuclear metalloradical containing a metal− metal hemibond has not been reported. By using weakly coordinating anions, we have demonstrated the stabilization of a 17-electron chromium(I) radical cation3j and cobalt carbonyl radical cations ([Co2Fv(CO)4]•+ and [Co2Cp2(CO)4]•+).7g The latter were the first stable species with metal−metal hemibonds. Herein, we report the oxidation of dinuclear half-sandwich carbonyl complexes (1 and 2, Scheme 1), which led to the first example of a hetero-dinuclear

Scheme 2. One-Electron Oxidation of 1

reversible two-electron oxidation peak in the cyclic voltammogram of 1 (Figure S1c, SI). The successful isolation of 1•+ encouraged us to study the oxidation of 2, which was prepared according to a literature method10a and may similarly produce a radical cation with an unusual Cr−Cr hemibond. However, the cyclic voltammogram of 2 shows a nonreversible oxidation peak with nBu4NPF6 as the supporting electrolyte at room temperature (Figure S2, SI)10b and no peaks with nBu4N[Al(ORF)4],10c which indicates that radical cation 2•+ is unstable under these conditions. Consequently, 2 was treated with 1 equiv of Ag[Al(ORF)4] in CH2Cl2, which afforded a yellow-green solution, but no crystals were obtained. However, concentrating and cooling of the filtrate of the reaction solution of 2 with 1 equiv of Ag[Al(ORH)4] [ORH = OC(CF3)2H]9 led to green crystals, identified as complex 3 (Scheme 3). A higher yield of 3 could

Scheme 1. Dinuclear Half-Sandwich Metal Carbonyl Complexes

Scheme 3. Formation of Crystals 3 and Crystal-to-Crystal Transformation from 3 to 4

metalloradical that features a Cr−Co half-bond and a rare example of a Cr(II) complex displaying single-molecule magnetic properties, respectively. The geometries and electronic structures of these species were investigated by singlecrystal X-ray diffraction, UV−vis, IR, EPR, and superconducting quantum interference device (SQUID) measurements, in conjunction with DFT calculations. Our work demonstrated an approach to functional materials from half-sandwich dinuclear metal carbonyl complexes by oxidation.

2. RESULTS AND DISCUSSION 2.1. Synthesis and Oxidation. Complex 1 was synthesized by modification of a literature method.8 The cyclic voltammetry of 1 in CH2Cl2 at room temperature with nBu4NPF6 as the supporting electrolyte revealed a nonreversible one-electron oxidation peak [Figure S1a, Supporting Information (SI)], indicating the instability of the radical cation 1•+ with PF6−. In contrast, a reversible one-electron oxidation peak in the cyclic voltammogram of 1 in CH2Cl2 at room temperature with n Bu4N[Al(ORF)4] [ORF = OC(CF3)3] was observed (Figure S1b, SI), indicating that 1•+ could be stable with the weakly coordinating anion of [Al(ORF)4]−. The treatment of 1 with 1 equiv of Ag[Al(ORF)4]9 in CH2Cl2 at room temperature afforded a yellow solution of the radical cation 1•+ (Scheme 2). Crystals of 1•+[Al(ORF)4]− were obtained by cooling the reaction solution at −25 °C. The isolated salt is thermally stable under a nitrogen atmosphere at room temperature. The reaction of 1•+ with one further equivalent of Ag[Al(ORF)4] or a direct treatment of 1 with 2 equiv of Ag[Al(ORF)4] in CH2Cl2 resulted in a fading of the solution color, but no crystals were obtained, which is consistent with the non-

be obtained by the reaction of 2 with 2 equiv of the silver salt. Crystals of 3 are stable only in solution but could be mounted with rapid handing on the single-crystal X-ray diffraction instrument under the protection of N2 flow at low temperature. Dipping crystals of 3 in pentane gave crystals of 4 in several 12070


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Journal of the American Chemical Society minutes with ejection of the CH2Cl2 of solvation from crystals of 3 (Scheme 3). 2.2. Crystal Structures. The structure of radical cation 1•+ is illustrated as a stereoview in Figure 1, together with a list of

Figure 1. Thermal ellipsoid (50%) drawing of 1•+: yellow, C; green, H; red, O; purple, Cr; blue, Co. Selected bond lengths (Å) and angles (deg): Cr1−Co1, 3.787(2); Cr1−C30, 1.896(9); Cr1−C31, 1.887(4); Cr1−C32, 1.887(4); Co1−C28, 1.770(6); Co1−C29, 1.769(4); C28− O5, 1.135(7); C29−O6, 1.136(9); C30−O7, 1.187(5); C31−O8, 1.135(9); C32−O9, 1.159(7); C30−Cr1−C32, 84.40(1); C30−Cr1− C31, 90.70(1); C31−Cr1−C32, 86.97(1); C28−Co1−C29, 96.61(2); Cr1−C30−O7, 178.1(7); Cr1−C31−O8, 178.8(3); Cr1−C32−O9, 178.4(1); Co1−C28−O5, 176.0(0); Co1−C29−O6, 178.2(3).

Table 1. Structural Parameters of 1•+, 1, and 1cis

1 DFT

DFT

X-ray

DFT

1.821 1.733 1.158 1.719 1.699 89.56 94.75

1.838 1.738 1.164 1.728 1.742 89.42 95.03

4.279 1.835 1.737 1.164 1.726 1.737 89.37 93.85

3.787 1.890 1.770 1.150 1.748 1.698 87.36 96.61

3.758 1.862 1.771 1.154 1.760 1.746 89.02 95.61

X-ray Cr−Co, Å avg Cr−C(O), Å avg Co−C(O), Å avg C−O, Å Cr−centroid(Ph) Co−centroid(Cp) av OC−Cr−CO, deg av OC−Co−CO, deg

1•+

1cis

8

Figure 2. Molecular structures of 3 (a) and 4 (b): yellow, C; green, H; red, O; and blue, Cr. Selected bond lengths (Å) and angles (deg) in 3: Cr2−O3, 2.048(3); Cr2−O4, 2.371(3); Cr2−O9, 2.046(3); Cr2− O10, 2.040(3); Cr2−O11, 2.103(3); Cr1−C13, 1.881(5); Cr1−C14, 1.858(6); Cr1−C15, 1.768(5); O1−C13, 1.138(6); O2−C14, 1.152(6); O3−C15, 1.194(5); Cr2−O3−C15, 147.2(3). In 4: Cr2− O3, 2.004(5); Cr2−O4, 2.448(4); Cr2−O9, 2.059(4); Cr2−O10, 2.024(4); Cr2−O11, 2.075(4); Cr1−C13, 1.856(9); Cr1−C14, 1.855(8); Cr1−C15, 1.771(7); O1−C13, 1.159(10); O2−C14, 1.138(10); O3−C15, 1.196(9); Cr2−O3−C15, 172.4(5).

selected bond distances and bond angles (Table 1). In comparison with the transoid configuration of the parent molecule 1,8 the radical cation shows a cisoid structure with an almost coplanar phen-Cp framework. The M−C(O) (M = Cr, Co) bonds lengthen slightly while the C−O bonds are shortened in comparison to the neutral parent molecules. The C−C bond length between phenyl and cyclopentadiene undergoes negligible change (1, 1.448 Å; 1•+, 1.445 Å). The Cr−Co distance in 1•+ [3.787(2) Å] is longer than a Cr−Co single bond length (2.4−2.6 Å). Since the van der Waals radius of Cr is unavaiable, the comparison of Cr−Co distance with the sum of the van der Waals radii is impossible. But the cisoid conformation indicates a weak interaction between chromium and cobalt in 1•+. Single-crystal X-ray diffraction shows that the crystals of 4 are formed by the loss of solvent molecule CH2Cl2 upon dipping of crystals of 3 in pentane (Figure 2). The crystals remain intact but undergo a change in space group from P1̅ (3) to Fdd2 (4). Channels containing the CH2Cl2 molecules were observed in

the crystal structure of 3 (Figure S3, SI), but these disappeared in 4. The molecular structure of 4 shows that the bisarene(Cr1) moiety is rotated away from the anionic center around the Cr2−C−O−Cr1 axis. In the structures of both 3 and 4, the average bond distance of Cr1−C(O) is 1.836 Å (3) and 1.827 Å (4). The Cr1−centroid distance is 1.730 Å in 3 and 1.740 Å in 4, respectively. The “arene−Cr1−(CO)” moiety together with the Cr1−centroid, Cr−C(O), and C−O distances are similar to those of neutral arene chromium tricarbonyl. This indicates that Cr1 atoms in 3 and 4 are zerovalent, while Cr2 atoms are divalent. The Cr2 atoms are penta-coordinated to four oxygen atoms, forming distorted square pyramidals. The O9, O10, and O11 atoms from anions and O3 atom from a carbonyl group define the equatorial square plane, with the Cr2 atoms deviating from the plane by 0.128 Å (in 3) and by 0.109 Å (in 4). The angles between the Cr2−O4 axis and the square 12071


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Journal of the American Chemical Society plane are 25.7° (in 3) and 26.9° (in 4). The Cr2−O4 bond distance in 3 [2.371(3) Å] is slightly shorter than that in 4 [2.448(3) Å]. 2.3. DFT Calculations and Comparison with X-ray Crystal Structures for 1•+. The X-ray crystal structure of 1•+ was well-reproduced by DFT calculations (Table 1) at the level of UB97D/SVP/SDDALL (Cr, Co).11 Of particular note, the calculated Cr−Co distance in 1•+ (3.758 Å) is close to that in the crystal structure [3.787(2) Å]. Consistent with the experimental data, one-electron oxidation causes the decrease of the C−O bond length but the increase of the M−C(O) bond lengths. The spin density distribution of 1•+ shows that the unpaired electron is localized on the Cr atom (0.61e) with a lower density at the Co atom (0.38e). The molecular orbitals of 1•+ display the Cr−Co σ* antibonding SOMO and the Cr−Co σ-bonding orbitals (Figure 3). The calculated Wiberg bond

Figure 4. (a) The solution EPR spectrum of 1•+ with simulation in CH2Cl2 (1.5 × 10−3 M) at 295 K. (b) The absorption spectrum of 1•+ in CH2Cl2 (1.0 × 10−4 M) at room temperature. Figure 3. (a) Cr−Co σ*-antibonding orbital (SOMO). (b) Cr−Co σbonding orbital (SOMO−3). (c) Electron spin density of 1•+.

The high-frequency (HF-EPR) experiments were carried out at 4.2 K on the polycrystalline powder samples of 4 to accurately determine the anisotropic parameters of the Cr(II) center. The change of the resonance fields with the frequency is shown in Figure 5a. One of the zero-field transitions is observed around 6.5 cm−1, which is attributed to the 3|D| zerofield resonance, leading to the |D| value of ∼2.17 cm−1. The least-squares fit to a complete two-dimensional array of the resonances gives the full set of spin Hamiltonian parameters, with S = 2, D = −2.15(3) cm−1, E = 0.01(1) cm−1, gx = gy = 1.90(5), and gz = 2.02(2). Simulations of the 170 GHz spectrum of 4 allowed us to determine the sign of D (Figure 5b). A positive D parameter does not fit, while a negative D value well matches the experiment, thus confirming the negative sign of D. 2.5. SQUID Measurements. The magnetic properties of 4 were measured using a MPMS-XL7 SQUID on a sample prepared from fresh crystals of 3 in a glovebox. The dc magnetic susceptibility measurements were collected between 1.8 and 300 K at an external field of 1.0 kOe for polycrystalline samples (Figure 6a). The χMT value is 3.58 cm3·mol−1·K at 300 K, which is significantly larger than the spin-only value of 3 cm3·mol−1·K expected for a high-spin Cr(II) with S = 2, g = 2. This is owing to the strong spin−orbital coupling for the spin center in a distorted square pyramid ligand field or some paramagnetic impurity. As the temperature is lowered, the χMT value decreases gradually until 6 K, followed by a rapid decrease to 1.71 cm3·mol−1·K at 1.8 K. The downturn could be the intrinsic magnetic anisotropy of the high-spin Cr(II) ion in 4. The magnetizations versus temperature isothermal plots were performed between 0 and 7 T in the temperature range from 1.8 to 20 K (inset in Figure 6a). The experimental χMT data and magnetizations were fitted concurrently using the PHI

order of the Cr−Co bond in 1•+ (0.278) is in accordance with the antibonding character of the SOMO, which is comparable to that of the Co−Co bond (0.290) in the dinuclear cobalt carbonyl radical cation [Co2Fv(CO)4]•+,7g indicating the formation of a Cr−Co hemibond. To check how the Cr−Co distance changes upon oxidation, a cis-isomer of 1 was also obtained as a minimum on the potential energy surface, which is more stable than the trans-isomer by ca. 0.7 kcal/mol. The Cr−Co distance (3.758 Å) in 1•+ is much shorter than that in 1cis (4.279 Å), strongly supporting the interaction between Cr and Co atoms in 1•+. 2.4. Spectroscopic Properties. The electronic nature of radical cation 1•+ was further investigated by EPR, IR, and UV−vis spectroscopy. The EPR spectrum (Figure 4a) of 1•+[Al(ORF)4]− solution at 295 K displays an eight-line signal coupling with the cobalt atom (59Co, I = 7/2, 100%; 53Cr, I = 3 /2, 9.55%) with g = 2.028, a = 10.5 G by simulation, which is due to spin density on the Co atom (Figure 3c). The EPR spectrum keeps isotropic but becomes broad in the frozen solution (Figure S4, SI). The CO stretching frequencies shown in their solid IR spectra (Figure S5, SI) increase from neutral parent molecules to radical cations, consistent with the shortening of C−O bond lengths upon oxidation. The UV−vis spectrum (Figure 4b) of 1•+[Al(ORF)4]− solution shows two broad absorptions around 574 (ε = 2.2 × 103 L·mol−1·cm−1) and 1031 nm (ε = 1.1 × 103 L·mol−1·cm−1). The longest absorption is significantly red-shifted in contrast to those of reported dinuclear metalloradical cations.7 From the timedependent DFT (TD-DFT) calculations (Figure S6, SI), these absorptions are assigned mainly to HOMO (β) → LUMO (β) and HOMO−3 (β) → LUMO (β) electronic transitions. 12072


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Figure 6. (a) χMT versus T curves for the powder sample of 4 in a 1.0 kOe dc field by MPMS-XL7 SQUID; the red line is the fitting result and the blue line is the simulated result without TIP using the PHI program with eq 1. Inset: Isothermal field-dependent magnetization at different temperatures. The solid lines are fitting results. (b) The temperature-dependence of out-of-phase (χ″) magnetization susceptibility of 4 at different frequencies by SQUID VSM. Inset: The reversed energy barrier fitting plot with ln(χM″/χM′) = ln(ω τ0) + U/ kBT.

Figure 5. (a) 2D field/frequency map of the EPR transition for 4. The solid squares represent experimental data. Red, blue, and black lines are simulations using best-fitted spin Hamiltonian parameters as in the text, with the magnetic field B parallel to the x, y, and z axis of the ZFS tensor, respectively. The green line represents an off-axis turning point, and the cyan line denotes the resonances from paramagnetic impurities. The vertical dashed lines represent the frequency (170 GHz) used in part b, at which the spectrum was recorded and simulated. (b) HF-EPR spectra of 4 with its simulations at 170 GHz and 4.2 K. Blue traces are spectra simulated using positive D values, while red traces are spectra simulated using negative D values. The asterisk (*) marks the signal from paramagnetic impurities.

no peaks were observed in the temperature dependence of ac susceptibilities. Fitting to ln(χM″/χM′) = ln(ωτ0) + U/kBT14 allows one to evaluate approximately U and τ0, giving the energy barrier ca. U = 14 (±0.2) K and τ0 = 8.8 (±0.5) × 10−8 s (inset in Figure 6b), which is in good agreement with the energy barrier obtained by U = |D|S2 (13.5 K).

program to avoid overparametrization and the influence of paramagnetic impurities with the following spin Hamiltonian12 2

3. CONCLUSION Chemical oxidations of piano-stool chromium/cobalt carbonyl complexes 1 and 2 were investigated. Upon one-electron oxidation, 1 was transformed to a heterometalloradical species 1•+. However, either one- or two-electron oxidation of 2 afforded a decomposition product, 3, which became crystals of 4 through an interesting crystal-to-crystal transformation upon dipping crystals of 3 into pentane.15 1•+ features a long and weak Cr−Co bond with a Wiberg bond order of 0.278, providing the first example of a heterodinuclear metalloradical with a metal−metal hemibond. A near-infrared absorption band around 1031 nm was observed for 1•+, which is far red-shifted compared to that previously reported for dimetalloradical species.7 The absorption around 1031 nm is typically interesting as near-infrared (NIR) spectral ranges with absorptions above 1000 nm are of importance owing to their promising applications in optical communications.16 The formation of 4 has proved the identity of the “Cr(II)” species upon two-electron oxidation of chromium tricarbonyl complex. The Cr(II) atom in 4 possesses a distorted pyramidal geometry and the complex displays single-molecule magnetic behavior. By far, most SIMs are centered at f-block metal elements.17

2

Ĥ = D[Sẑ − S(S + 1)/3] + E(Sx̂ − Sŷ ) + μB gSB̂

(1)

where D, E, S, and B represent the axial, rhombic zero-field splitting (ZFS) parameter, zero-field splitting parameter, the spin operator, and magnet field vector, and μB is the Bohr magneton. The best fit can be obtained with S = 2, D = −2.35 cm−1, g = 1.94, E = 0.011 cm−1, zj′ = −0.023 cm−1, and TIP = 0.00304 cm3·mol−1, indicating that 4 might be a singlemolecule magnet like the Mn(III) complex with a strong Jahn− Teller effect.13 The D and g values reasonably agree with those obtained from the HF-EPR data. The ac magnetic susceptibility was tested at 1.8 K under a different external dc field (0−3.0 kOe, Figure S7, SI). No χM″ signal was detected under zero dc field, due to fast quantum tunneling of magnetization (QTM). QTM can be hindered partly with a small external dc field, indicating that complex 4 might be a field-induced single-molecule magnet. To study the magnetic properties of 4 in detail, we selected 2.5 kOe as an external dc field to hinder QTM and as far as possible to avoid the Zeeman effect. Hence, ac measurement under 2.5 kOe was performed between 1.8 and 5.5 K (Figure 6b). Unfortunately, 12073


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diffractometer with Mo Kα (λ = 0.710 73 Å) radiation. Integrations were carried out with SAINT.19 Absorption corrections were done using the program SADABS.20 Crystal structures were solved by direct methods and refined by full-matrix least-squares based on F2 using the SHELXTL program.21 All non-hydrogen atoms were refined anisotropically. Details of the data collections and refinements are given in Table S1 (SI). Computational Details. All the geometry optimizations were performed with the B97D functional. SDDALL was applied for Cr and Co atoms and the basis set SVP for the rest of the atoms. Frequency calculations were carried out to confirm that all optimized geometries correspond to energy minima. All calculations were performed with the Gaussian 09 program suite.11

Compared to f-block complexes, the 3d transition-metal ions can be expected to suppress the orbital contributions to the magnetism that is required to develop magnetic anisotropy.17 Our work thus has demonstrated that oxidation of metal carbonyl complexes may lead to new species with promising physical properties. Investigation of oxidation of other dinuclear and polynuclear metal carbonyl complexes is under way.

4. EXPERIMENTAL SECTION All experiments were carried out under a nitrogen atmosphere by using standard Schlenk techniques and a glovebox. Solvents were dried prior to use. Biphenyl and chromium hexacarbonyl were purchased and used upon arrival. Cr(CO)3(η6,η6-C6H5C5H4)Co(CO)2 (1) was synthesized according to a modified method in the literature, with Na(DME)C5H5 used instead of KC5H5.8 Cr(CO)3(η6,η6-C6H5C5H4)Cr(CO)3 (2) was synthesized according to the reported method.10 Ag[Al(ORF)4] and Ag[Al(ORH)4] were prepared by following the literature procedure.9 Cyclic voltammetry was performed on a CHI660E electrochemical workstation, with platinum as the working and counter electrodes, Ag/Ag+ as the reference electrode, and 0.1 M n Bu4NPF6 or 0.1 M nBu4N[Al(ORF)4] as the supporting electrolyte. UV−vis spectra were recorded on Lambda 750 spectrometer. Infrared spectra were collected on a VECTOR22 FT-IR spectrometer. Elemental analyses were performed at Shanghai Institute of Organic Chemistry, the Chinese Academy of Sciences. The solution EPR spectra were obtained using a Bruker EMX plus-6/1 variabletemperature X-band apparatus and simulated with the software of WINEPR SimFonia. High-frequency electron paramagnetic resonance (HF-EPR) were performed on a locally developed instrument with the pulsed magnetic field,18 and simulations were performed using SPIN developed by Andrew Ozarowski of the National High Magnetic Field Laboratory, Florida State University. This program simulates powder or single-crystal EPR spectra for spin states with 1/2 < S < 5 using full diagonalization of the spin Hamiltonian matrix. Magnetic susceptibility measurements on polycrystalline samples of 4 were carried out on a SQUID magnetometer between 0 and 7 T in the temperature range from 1.8 to 300 K. The dc susceptibility measurements were collected in the temperature range 1.8−300 K under a dc field of 1000 Oe, and dc magnetization measurements were obtained in the temperature range 1.8−20 K under dc fields up to 7 T. The ac susceptibility measurements were performed at frequencies between 1 and 1000 Hz with an ac field of 2.5 kOe. Diamagnetic corrections were calculated from Pascal constants and applied to all the constituent atoms and sample holes. Preparation of 1·+[Al(ORF)4]−. Ag[Al(ORF)4] (0.32 g, 0.3 mmol) in CH2Cl2 was added to 1 (0.12 g, 0.3 mmol) dropwise at room temperature with stirring. Stirring was maintained at room temperature for 1 d. The resultant yellow solution was filtered to remove the gray precipitate (Ag metal). The filtrate was stored at −30 °C for 24 h to afford yellow crystals that were suitable for X-ray crystallography. These were determined to be 1·+[Al(ORF)4]−. Isolated yield: 74 mg, 18% (crystals). Mp: 119.5 °C (dec). Anal. Calcd (%) for C32H9O9AlCrCoF36: C, 28.28; H, 0.67. Found: C, 27.94; H, 0.87. Preparation of 3. A mixture of 2 (0.13 g, 0.3 mmol) and Ag[Al(ORH)4] (0.48 g, 0.6 mmol) in CH2Cl2 was stirred at room temperature for 1 day. The orange solution turned green, and gray solid (Ag metal) precipitated at last. After filtration, the filtrate was concentrated and stored at ca. −20 °C for 1 d to afford X-ray-quality crystals of 3. Yield: 170 mg, 29.8%. Crystal-to-Crystal Transformation from 3 to 4. Crystals of 3 were dipped into pentane (15 mL), which converted crystals of 3 into crystals of 4. Mp: 92.5 °C (dec). Anal. Calcd (%) for C39H18O11Al2Cr2F48: C, 27.04; H, 1.05. Found: C, 27.30; H, 1.36. X-ray Crystallographic Data Collection. Crystals of 1•+[Al(ORF)4]−, 3, and 4 were removed from a Schlenk flask under nitrogen and rapidly dipped into hydrocarbon oil. A selected crystal suitable for X-ray diffraction was attached to a glass fiber on a copper pin and immediately placed in the cold nitrogen stream of the diffractometer. X-ray data were collected at 123 K on a Bruker APEX DUO CCD

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b06795. Crystallographic data, IR spectra, SQUID measurements, and theoretical calculations (PDF) Crystallographic data of 1•+[Al(ORF)4]− in CIF format (CIF) Crystallographic data of 3 in CIF format (CIF) Crystallographic data of 4 in CIF format (CIF)

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] *[email protected] ORCID

Xinping Wang: 0000-0002-1555-890X Philip P. Power: 0000-0002-6262-3209 Author Contributions §

W.W. and J.L. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Natural Science Foundation of China (Grants 21525102, X.W.; 21690062, X.W.; and 91622115, Y.S.), the Major State Basic Research Development Program (Grants 2016YFA0300404, X.W.; 2013CB922102, Y.S.), the Natural Science Foundation of Jiangsu Province (Grant BK20140014, X.W.), and the US National Science Foundation (CHE-1565501, P.P.P) for financial support. We are thankful to Dr. Zaichao Zhang for assistance with the structure refinement and Lei Wang for assistance with the cyclic voltammetry performance. We are grateful to the High Performance Computing Center of Nanjing University for doing the numerical calculations in this paper on its IBM Blade cluster system.

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REFERENCES

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