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Isolable Borane-Based Diradical and Triradical Fused by a Diamagnetic Transition Metal Ion Lei Wang, Jing Li, Li Zhang, Yong Fang, Chao Chen, Yue Zhao, You Song, Liang Deng, Gengwen Tan, Xinping Wang, and Philip P. Power J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b10141 • Publication Date (Web): 29 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017
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Journal of the American Chemical Society
Isolable Borane-Based Diradical and Triradical Fused by a Diamagnetic Transition Metal Ion Lei Wang,† Jing Li,† Li Zhang,† Yong Fang,† Chao Chen,† Yue Zhao,† You Song,† Liang Deng,‡ Gengwen Tan,*,† Xinping Wang,*,† Philip P. Power*,§ †
State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, Collaborative Innovation Center of Advanced Microstructures, Nanjing University, Nanjing 210023, China ‡
State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345 Lingling Road, Shanghai 200032, China §
Department of Chemistry, University of California, Davis, CA 95616, USA
Supporting Information Placeholder ABSTRACT:
B
B
Complex Fe(bpy )3 (1, bpy = 5,5'bis(dimesitylboranyl)-2,2'-bipyridine) and its reduced species B [(18-c-6)K(THF)2]•[Fe(bpy )3] (2) were synthesized. Their solid state and electronic structures were investigated by single crystal X-ray crystallography, electron paramagnetic resonance (EPR) and UV-Vis spectroscopy, and SQUID B measurements. In 1 two bpy radical anions are fused by a II B diamagnetic Fe ion, while in 2 all three bpy ligands are in the radical state. Complex 1 possesses an open-shell singlet –1 ground state with a singlet-triplet gap of 0.18 kcal mol and 2 features an open-shell doublet ground state with a doublet–1 quartet gap of 1.4 kcal mol , as determined by SQUID measurements. The unpaired-electrons in 1 and 2 mainly delocalize over the boron atoms and the bipyrdine moieties with negligible spin density at the iron center. Complex 2 represents the first isolable example of boron-based triradicals.
Main-group element-based radicals are attracting increased attention in contemporary chemical research not only because of their importance in fundamental science, but also their potential applications arising from the interesting phys1 ical properties (optical, electronic, magnetic, etc.). Despite the challenges to obtain stable or persistent main-group element radicals due to the limited number of energetically accessible valence orbitals in main-group elements, a variety of stable main-group element-based radicals have been isolated with the aid of sterically congested and/or π1e-g, 1i-m conjugated ligands. The study of boron-based radicals is one of the most important areas in current radical chemis2 try. Since the first isolation of the trimesityl borane radical 3 4 anion, several stable boron-based monoradicals and diradi5, 4d cals with boranes as the spin carriers have been synthesized, by taking the advantage of the intrinsic electrondeficiency of three-coordinate boron atoms. In the diradicals, the two borane radical centers are coupled by π-conjugated bridges (A–E, Scheme 1) or through space (F, Scheme 1). In 2004, Bertrand and coworkers successfully isolated two bo-
ron-based singlet tetraradical G through catenation of two singlet 1,3-dibora-2,4-diphosphoniocyclobutane-1,3-diyl 6 diradicals in the para-position of the phenylene group. They are the only examples of isolable boron-based polyradicals.
Scheme 1. Structurally Characterized Boron-Based Diradicals and Tetraradicals (Mes = 2,4,6-Me3C6H2)
Considering the limited numbers of boron-based diradicals and polyradicals, we were interested in synthesizing triradicals based on boranes in an attempt to gain more insight into their structures and properties. Herein, we demonstrate a facile approach to access the stable borane-based diradical and triradical by fusing two and three diborane radical anions to a diamagnetic transition metal ion, respectively. First, we carried out the reaction of FeCl2 and three molar B B equiv of bpy (bpy = 5,5'-bis(dimesitylboranyl)-2,2'7 bipyridine), followed by the treatment with two molar equiv II of KC8 in THF at room temperature. The neutral Fe complex B Fe(bpy )3 (1) was isolated as dark-green crystals in 41% yield (Scheme 2). Compound 1 is highly air-sensitive, but is fairly stable under an inert atmosphere at room temperature. It
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was subsequently investigated by single crystal X-ray crystallography, EPR spectroscopy, and SQUID measurements.
Scheme 2. Syntheses of the FeII-Fused Diradical and Triradical Mes 2B
BMes2
3 Mes2 B N
N bpy
1) FeCl2 2) 2 KC8 THF
N Mes2 B Mes 2B
- 2 KCl - 16 C
BMes2
N
N Fe N
N N
B
BMes2
BMes2
Mes = 2,4,6-Me 3C6H2
1 Mes2B
Mes2B Mes 2B
N N N
BMes2
N
(18-c-6)K(THF) 2
Fe
KC8, 18-c-6 THF -8C
N N BMes2
BMes2
2
Figure 1. Thermal ellipsoid drawing of the molecular structure of 1 at 30% probability. The hydrogen atoms and the Mes groups (except the carbon atoms bonding to the boron atoms) are not shown for clarity. α, β , γ represent the three B o bpy ligands. Selected bond lengths (Å) and angles ( ): Fe1– N1 1.954(3), Fe1–N2 1.975(3), Fe1–N3 1.970(3), Fe1–N4 1.961(3), Fe1–N5 1.944(3), Fe1–N6 1.935(3), B1–C2 1.556(6), C5–C6 1.425(5), B2–C9 1.535(6), B3–C48 1.528(6), C51–C52 1.416(5), B4–C55 1.541(6), B5–C94 1.575(6), C97–C98 1.454(5), B6–C101 1.585(8); N1–Fe1–N2 82.06(13), N1–Fe1–N4 87.79(13), N1–Fe1– N6 95.37(13), N4–Fe1–N5 95.40(12), N5–Fe1–N6 81.50(13), N3– Fe1–N2 176.19(13). Crystals of 1 were isolated from a concentrated n-hexane solution at room temperature (Figure 1). It crystallizes in the triclinic space group P–1. The iron atom is coordinated by B three bpy ligands forming an octahedral configuration. The short Fe–N bond lengths (avg. 1.957(13) Å) indicate the presII 8 ence of a low-spin Fe center. All the boron centers feature
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a trigonal planar geometry. The mean B–C bond lengths to the bipyridine moieties in the diboranes α (1.546(6) Å) and β (1.535 (6) Å) are much shorter than that in C (1.580(6) Å), but are comparable to those in the diborane radical anion 4d bridged by a pyrene moiety (1.547 (6) Å). This suggests that the diboranes α and β have a radical anion character, whereas γ is still redox intact. The mean dihedral angles of the BC3 planes with the corresponding bipyridine planes in α and β o o are 19.5 and 20.8 , and the C5–C6 (1.425(5) Å) and C51–C52 (1.416(5) Å) bonds are largely shortened in comparison with B 7 those in (bpy )PtPh2 (1.482(6) Å), and [Ru(bpy)2(5,5’BP2bpy)][PF6]2 (1.479(6) Å; 5,5’-BP2bpy = 5,5’9 bis(BMes2phenyl)-2,2’-bpy), suggesting delocalization of the unpaired electron over the bipyridine moieties and the boron 10 centers in the diboranes α and β. The EPR spectrum of 1 in the solid state at 80 K revealed a broad signal at g = 2.0026. A signal at g ≈ 4 corresponding to the ∆ms = ±2 transition was observed, indicating that the triplet-spin state was accessible at the measurement conditions (Figure S1 in the supporting information (SI)). The SQUID measurements revealed that the χMT value steadily 3 -1 increased from 0 to 0.72 cm mol K when the temperature was elevated from 1.8 to 300 K (Figure 2a). Fitting the data 11 with the Bleaney–Bowers equation and Hamiltonian 2 afforded a small singlet–triplet energy gap of –1 ∆ES-T = –0.18 kcal mol . The spin density distribution shows that it is mainly delocalized over the boron atoms and the bipyridine moieties of diboranes α and β, whereas there is negligible spin density at the iron atom and the diborane γ 12 (Figure 2b), consistent with the solid state structure of 1.
Figure 2. (a) Temperature dependence of χMT under a 1.0 kOe field at 1.8–300 K of 1. The solid line is the fitting result 13 by PHI program. (b) Spin density distribution of 1 in the open-shell singlet state. The calculations were carried out at the UB3LYP/def2-TZVP level of theory. B
Since one of the bpy ligands in the diradical 1 was not reduced to the radical state, we were eager to further reduce it B to check whether the triradical 2 with three bpy ligands in the radical form could be obtained. Accordingly, we first carried out electrochemical studies of 1. The cyclic voltammogram showed six redox peaks in the range of –0.7 to –2.9 + V, versus the Ag/Ag electrode (Figure S2 in SI), suggesting complex 1 could undergo multiple one-electron redox reactions. Intrigued by the electrochemical behavior of 1, we performed one-electron reduction of 1 with one molar equiv of KC8 in the presence of 18-c-6 in THF at room temperature, B and upon work-up dark-brown crystals of [Fe(bpy )3][(18-c-
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Journal of the American Chemical Society 6)K(THF)2] (2) were obtained in 73% yield (Scheme 2). Moreover, we also carried out the reduction reactions with different ratios of KC8, however no crystalline products were obtained.
Figure 3. Thermal ellipsoid drawing of the molecular structure of 2 at 30% probability. The hydrogen atoms, the Mes groups (except the carbon atoms bonding to the boron atoms), the disordered atoms and the THF solvent molecules are not shown for clarity. Selected bond lengths (Å) and ano gles ( ): Fe1–N1 1.9358(19), Fe1–N2 1.9653(19), Fe1–N3 1.9627(19), B1–C2 1.530(4), C5–C6 1.414(3), B2–C9 1.525(4), B3– C48 1.539(4), C51–C51A 1.429(5), N3–Fe1–N2 98.27(8), N3–Fe1– N3A 81.72(11), N3A–Fe1–N2A 98.28(8), N2–Fe1–N2A 81.80(11), N1–Fe1–N1A 173.62(12). Crystals of 2 were obtained from a concentrated THF soluo tion at –20 C (Figure 3). It was isolated as a discrete salt with the potassium cation coordinated by 18-c-6 and two THF molecules. The anion possesses a C2v symmetry, and the iron center keeps an octahedral geometry. The Fe–N bond lengths (av. 1.955(2) Å) are comparable to those in 1, suggesting the II presence of a low-spin Fe in 2. The C–C bond lengths between the pyridines (1.414(3) and 1.429(5) Å) and the B–C bond lengths (1.525–1.539 Å) are close to those in the reduced diborane ligands α and β of 1, suggesting that all the diborane ligands feature a radical anion character and the unpaired-electrons delocalize over the three diborane ligands. Therefore, compound 2 is a triradical bearing three diborane II radical anions fused by a diamagnetic Fe ion. In addition, II the presence of a low-spin Fe ion in 2 was unambiguously 57 proven by Fe Mössbauer spectrum recorded at 80 K with –1 the isomer shift δ = 0.30 mm s and quadrupole splitting ΔEQ –1 = 0.64 mm s (Figure S3 in SI). The variable temperature magnetic susceptibility and isothermal field-dependence magnetization were performed on microcrystalline sample of 2 (Figure 4). The χMT values at 1.8 3 –1 K and 298 K are 0.38 and 0.52 cm mol K, respectively, the latter of which is much smaller than the expected value for 3 –1 three isolated organic radicals (1.125 cm mol K), indicating a fairly strong antiferromagnetic interaction between the radical centers and an open-shell doublet ground state of 2. The χMT vs T and M vs H results were well fitted with the 2 2 , spin isotropic Hamiltonian affording the best fitting result with the parameters J1 = – –1 –1 14 167.6 cm , J2 = –167.4 cm , g = 2.035. The doublet-quartet –1 –1 energy gap of 502.7 cm (1.4 kcal mol ) was obtained.
The EPR spectrum recorded in THF solution at 80 K revealed a broad resonance signal at g = 2.0012 and a weak half field signal at g ≈ 4 (Figure S4 in SI). The ∆ms = ±3 resonance signal was not observed, which is likely due to the large doublet-quartet energy gap as determined by SQUID measurements. The UV-Vis absorption spectra of 1 and 2 exhibit two long-wavelength absorptions at 849, 945 nm and 876, 979 nm, respectively (Figure S5).
Figure 4. Temperature dependence of χMT under a 1.0 kOe field at 1.8–300 K of 2. Insert: The diagram of the coupling situation between the three radical centers and isothermal field-dependence magnetization under low temperatures. 13 The solid line is the fitting result by PHI program. In summary, we have successfully isolated the unprecedented borane-based diradical 1 and triradical 2 through coordination of two and three diborane radical anions to a II diamagnetic Fe ion, respectively. The experimental and theoretical results reveal that the diradical 1 has an open-shell singlet ground state, and the triradical 2 features an openshell doublet ground state with a relatively large doubletquartet gap. These studies demonstrate that fusing the radical anions with a metal ion can serve as a practical methodology to construct main-group element-based polyradicals.
ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental details, Cyclic voltalmogramm of 1, Crystallographic data, EPR and UV-Vis absorption spectra of 1 and 2, and theoretical calculations (PDF) Crystallographic data of 1 in CIF formate (CIF) Crystallographic data of 2 in CIF formate (CIF)
AUTHOR INFORMATION Corresponding Author
[email protected] [email protected] [email protected] Notes The authors declare no competing financial interests.
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ACKNOWLEDGMENT We thank the National Key R&D Program of China (Grant 2016YFA0300404, X.W.), the National Natural Science Foundation of China (Grants 21525102, 21690062, X.W. and 21601082, G.T.), and the US National Science Foundation (Grant CHE-1565501, P.P.P) for financial support for financial support. We are grateful to the High Performance Computing Center of Nanjing University for doing the calculations on its IBM Blade cluster system. L.W. is in debt to the support of the program B for outstanding PhD candidate of Nanjing University.
The single-point energy calculations were carried out based on the coordinates of the crystal structure because of the high computational cost, see supporting information for details. (13) Chilton, N. F.; Anderson, R. P.; Turner, L. D.; Soncini, A.; Murray, K. S. J. Comput. Chem. 2013, 34, 1164. (14) The C2v symmetry of 2 dictates two exchange parameters as shown in the inset of Figure 4.
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