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Cyclic Alkyl(amino) Carbene-Stabilized Monoradicals of Organosilicon(IV) Compounds with Small Substituents Wenling Li,†,‡ Subrata Kundu,‡ Christian Köhler,‡ Jiancheng Li,‡ Sayan Dutta,∥ Zhi Yang,*,† Dietmar Stalke,*,‡ Regine Herbst-Irmer,‡ A. Claudia Stückl,‡ Brigitte Schwederski,§ Debasis Koley,*,∥ Wolfgang Kaim,*,§ and Herbert W. Roesky*,‡ †

School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, P. R. China Institut für Anorganische Chemie, Georg-August-Universität Göttingen, Tammannstrasse 4, D-37077 Göttingen, Germany § Institut für Anorganische Chemie, Universität Stuttgart, 70569 Stuttgart, Germany ∥ Department of Chemical Sciences, Indian Institute of Science Education and Research (IISER) Kolkata, Mohanpur 741 246, India

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‡

S Supporting Information *

ABSTRACT: Radicals and biradicals can easily be obtained when silicon(IV) compounds are reduced and stabilized by the Lewis base cyclic(alkyl)(amino)carbene (cAAC = C(CH2)(CMe2)2N-Dipp, Dipp = 2,6-i-Pr2C6H3) featuring strong σ-donor and π-acceptor properties. To obtain the cAACstabilized monoradicals with halogen and alkyl substituents at the silicon(IV) atoms, the Ar substituent (Ar = o-C6H4NMe2) was employed to act as a sidearm nitrogen donor ligand. The resulting monoradicals ArSiCl2(cAAC) (4) and ArSiRCl(cAAC) (R = Me 5; Et 6) were isolated by the reduction of ArSiCl2 (1) and ArSiRCl2 (R = Me 2; Et 3) with KC8 in the presence of cAAC with a strict ratio of 1:1:1. The three room-temperature stable radicals were kept for at least 3 months in the solid state under dry N2 atmosphere without decomposition. All the compounds were fully characterized by liquid injection field desorption/ionization−mass spectrometry, electron paramagnetic resonance spectroscopy, UV/vis spectroscopy, elemental analysis, and single-crystal X-ray crystallographic studies.

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INTRODUCTION Radicals were found to have enormous potential and can act as an initiator in some stereoselective reactions of chemical and biological processes.1−5 Most radicals are short-lived and unstable species,6 so the preparation of room-temperature stable radicals is one of the most fascinating topics over the last decades. Studies showed that a steric group around the central radical atom could protect the radical from further dimerization. Consequently, chemists paid significant attention to the design of bulky ligands and modification of substituents at the central element, for example, by Power,7 Niecke,8 Lappert, 9 Sekiguchi, 6 Bertrand, 10−12 Inoue, 13−16 and others.17−20 This research work resulted in the isolation of room-temperature stable radicals. It should be emphasized that several valuable articles about main group radicals were reviewed by many colleagues in the field.21−32 In general, stable radicals can be isolated by the reduction of the halogen derivatives of main group elements, especially for group 14 elements, via the dehalogenation route. In 2011, Sekiguchi and co-workers isolated a triplet ground-state bis(silyl radical) [m-{(t-Bu2MeSi)2Si}2C6H4] by the reductive elimination of KI with 2 equiv of KC8.33 In recent years, studies showed that carbenes, N-heterocyclic carbenes (NHCs) and cyclic(alkyl)(amino)carbenes (cAACs), were a good choice for the isolation of room-temperature unstable compounds and radicals.31,34−38 The comparison of NHC with © XXXX American Chemical Society

cAAC makes an efficient difference in stabilizing main group radicals owing to the stronger σ-donor and also better πacceptor properties of the latter.39−43 In 2010, Bertrand and co-workers reported on the preparation of a carbene-stabilized P2-radical cation and P2-dication42 and a radical cation (PN•+).43 Sekiguchi et al. obtained an NHC-stabilized silylene radical cation (I) via a one-electron oxidation of the NHCstabilized silylene.44 There are numerous reports related to carbene-stabilized silicon compounds, and their properties were studied by Robinson,45 Filippou,46,47 So,48 and our group.49−55 However, reports on the synthesis of cAACstabilized organosilicon monoradicals containing halide or alkyl substituents are limited in number. A breakthrough in the stable radical isolation is [(cAAC)2SiCl2] reported by our group, which was prepared via the ligand exchange using NHC-stabilized silylenes.50,56 Moreover, the corresponding biradicals [(cAAC)2SiX2] (X = F, Br) (II) were also synthesized.51,53 Treating (cAAC)2SiCl2 with 2 equiv of organolithium reagents R−Li (R = Me, Ph, and tBu) resulted in the reductive elimination of R−R (III) rather than formation of the excepted product (cAAC)2SiR2.53 However, the monoradical of the in situ replacement of Cl atoms was formed when we treated trichlorosilyl carbene monoradical Received: January 23, 2019

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DOI: 10.1021/acs.organomet.9b00041 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics (cAAC)SiCl3 with 3 equiv of PhLi.57 A similar cAACsupported monoradical (cAAC)SiCl2PPh2 (IV) was isolated by reduction, when the temperature was strictly controlled to prevent the formation of a byproduct (Figure 1).

Scheme 1. Synthesis of Monoradicals 4−6

desorption/ionization (LIFDI) mass spectra of compounds 4− 6 in toluene exhibited molecular ion peaks at 503.3, 483.3, and 497.3 m/z, respectively. The UV/vis spectra of monoradicals 4−6 were recorded in n-hexane, which showed absorption bands at 402, 434, and 436 nm, respectively (Supporting Information, Figures S1−S3). 4 and 5 crystallize in the triclinic space group P1̅ and are isomorphous (Figures 2 and 3, Table S1). 6 crystallizes in the

Figure 1. NHC- or cAAC-stabilized radicals.

In addition to the one-step synthesis, biradicals were also obtained by the reduction of chlorosilanes in the presence of cAAC. Reports on cAAC-supported organosilicon(IV) monoradicals containing small halogens and alkyl substituents at the silicon center are rare. Moreover, several intramolecularstabilized complexes were reported by Belzner, 58,59 Tamao,60,61 and Baceiredo et al.,62,63 in which a weak interaction between the donor and silicon is shown. Consequently, we selected a ligand with a nitrogen donor group to stabilize the unstable compounds. We report on a two-step synthetic route to isolate such monoradicals containing silicon halides. Herein, we describe the reduction of ArSiCl3 (1) and ArSiRCl2 (R = Me 2; Et 3) with KC8 in the presence of cAAC for the preparation of ArSiCl2(cAAC) (4) and ArSiRCl(cAAC) (R = Me 5; Et 6).

Figure 2. Structure of compound 4. Anisotropic displacement parameters are depicted at the 50% probability level. C-bound H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si(1)−C(9) 1.8264(12), Si(1)−C(1) 1.8743(11), Si(1)− Cl(1) 2.1037(6), Si(1)−Cl(2) 2.0629(6), Si(1)−N(1) 3.1627(12); C(1)−Si(1)−Cl(1) 104.96(4), C(1)−Si(1)−Cl(2) 111.77(4), Cl(2)−Si(1)−Cl(1) 101.37(3), C(9)−Si(1)−C(1) 116.98(5), C(9)−Si(1)−Cl(2) 113.36(4), C(9)−Si(1)−Cl(1) 106.61(4).

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RESULTS AND DISCUSSION Precursors 1−3 were prepared as colorless liquids, which remain stable for at least 6 months at room temperature under N2 atmosphere. Compounds 1−3 were characterized by nuclear magnetic resonance (NMR) spectroscopy. The 29Si NMR resonance of 3 (14.83 ppm) compares well with those of compounds 1 (δ = 11.39) and 2 (δ = 11.39 ppm) (for details, see the Supporting Information). Compounds 4−6 were isolated by reduction of arylchlorosilanes ArSiCl3, ArSiMeCl2, and ArSiEtCl2 with KC8 in the presence of 1 equiv of cAAC. The strict ratio of the precursors is 1:1:1, which seems to be very crucial for the formation of the products (Scheme 1). The color of the three radicals is yellow to orange. All compounds are stable in the solid state at room temperature for at least 3 months under dry N2 atmosphere. Broad 1H NMR resonances of the three compounds 4−6 indicate their radical character. The liquid injection field

Figure 3. Structure of compound 5. Anisotropic displacement parameters are depicted at the 50% probability level. C-bound H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si(1)−C(9) 1.8401(12), Si(1)−C(1) 1.8863(11), Si(1)− C(29) 1.8636(11), Cl(1)−Si(1) 2.1314(7), N(1)−Si(1) 3.1458(12); C(1)−Si(1)−Cl(1) 103.49(4), C(29)−Si(1)−C(1) 113.78(5), C(29)−Si(1)−Cl(1) 101.73(4), C(9)−Si(1)−C(29) 116.61(5), C(9)−Si(1)−C(1) 113.82(5), C(9)−Si(1)−Cl(1) 105.22(4). B


Article

Organometallics monoclinic space group P21/c (Figure 4) and has a similar structure to 4 and 5. The structures of these compounds

Figure 4. Structure of compound 6. Anisotropic displacement parameters are depicted at the 50% probability level. C-bound H atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Si(1)−C(9) 1.8441(12), Si(1)−C(1) 1.8916(12), Cl(1)− Si(1) 2.1393(9), Si(1)−C(29) 1.8726(12), Si(1)−N(1) 3.1598(13); C(1)−Si(1)−Cl(1) 103.35(4), C(29)−Si(1)−C(1) 115.14(5), C(29)−Si(1)−Cl(1) 101.90(4), C(9)−Si(1)−C(29) 116.47(5), C(9)−Si(1)−C(1) 113.28(5), C(9)−Si(1)−Cl(1) 104.30(4).

Figure 5. EPR spectrum of compound 4 in toluene at room temperature. Bottom: Experimental spectrum, top: spectrum simulated with g = 2.0051, 3.8 G line width (half Gaussian/half Lorentzian), and coupling constants a(35,37Cl) = 8.3 (6.9) G (1 Cl) and a(14N) = 5.6 G (1 N).

determined by the single-crystal X-ray diffraction showed that the Si atom is coordinated with carbon and chlorine atoms and exhibits a distorted tetrahedral geometry (Figures 2−4). Previously, we have reported the cAAC-stabilized radical (cAAC)SiCl 3 64 and diradicals (cAAC) 2 SiCl 2 5 0 and (cAACs)2Si2Cl4.65 The obtained bond distances are in good agreement with similar bond distances found in comparable compounds. A comparison of selected bond distances can be found in Table 1. The distance from NNMe2 to the Si atom is 3.1627(12) Å (4), which is between the normal coordinate distance [≤ 2.8 Å] and the sum of van der Waals radii of Si and N [3.65 Å]. Similar observations were made for 5 [3.1458(12) Å] and 6 [3.1598(13) Å]. This indicates a weak interaction between the Si and NNMe2 atoms.66 It seems that the lone pair density of the donor side-arm has an influence on the formation of the monoradicals. The electronic properties of the three radicals were analyzed by electron paramagnetic resonance (EPR) spectroscopy in solution at room temperature (Figures 5−7). Radicals 4, 5, and 6 display EPR spectra at g ≈ 2.005, which exhibit a typical 14N hyperfine splitting of about 5 Gauss for cAAC-stabilized radical compounds (4: 5.6 G; 5: 5.2 G; and 6: 5.1 G) and a coupling with one Cl atom in natural isotopic abundance (35Cl: 75.78%; 37 Cl: 24.22%; and gyromagnetic ratio 1.20). For a(35Cl), the coupling constants are 8.3 G (4), 9.1 G (5), and 9.3 G (6), and the a(37Cl) values (in parentheses in the captions) are 6.9 G (4), 7.6 G (5), and 7.7 G (6). The presence of detectable Cl


splitting for only one Cl atom in the case of 4 indicates a locked conformation (Figure 5) of the sterically crowded molecular structure with one strongly and one weakly coupling chlorine atomthe latter has not been observed in the spectral resolution (Figures 6 and 7) Quantum chemical calculations were performed at the UBP86/def2-TZVP//U-BP86/def2-SVP level to cast light on the bonding scenario in the compounds (4−6) at doublet ground electronic states. Mulliken spin density plots, depicted in Figure 8, reveal that in all the compounds, the unpaired electron is predominantly located on the carbene carbon atom with a significant contribution from the neighboring nitrogen

Table 1. Comparison of Selected Bond Distances for Structures 4−6 distance

Si−CcAAC

Si−CAr

4

1.8264(12)

1.8743(11)

5 6 (cAAC)SiCl364 (cAAC)2SiCl250 (cAAC)2Si2Cl465 (Cy-cAAC)SiPh357 (cAAC)SiMeCl−SiMeCl(cAAC)54 (cAAC)SiMe2−SiMe2(cAAC)54

1.8401(12) 1.8441(12) 1.8152(12) 1.8455(16), 1.8482(17) 1.846(5) 1.8704(17) 1.8466(12) 1.863(11)

1.8863(11) 1.8916(12)

Si−Cl 2.1037(6) 2.0629(6) 2.1314(7) 2.1393(9) 2.0396(4) to 2.0646(4) 2.0662(7), 2.0690(8) 2.061(2), 2.068(2)

Si−C29

NNMe2 to Si 3.1627(12)

1.8636(11) 1.8726(12)

3.1458(12) 3.1598(13)

1.8843(16) to 1.8957(17) 2.0860(8)

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1.9075(11) 1.882(13), 1.886(12) DOI: 10.1021/acs.organomet.9b00041 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Ar containing a side-arm electron donor could act as an intramolecular force to interact with the silicon atom, which seems to have a long-range influence on the formation of the monoradicals.

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EXPERIMENTAL SECTION

General Procedures. The experiments were carried out under N2 atmosphere using a standard Schlenk technique and a glovebox. The related solvents and C6D6 were thoroughly dried using a Na/K alloy before being distilled. NMR spectra were recorded using a Bruker AVANCE II 500 MHz NMR spectrometer. Elemental analysis was performed by the Analytical Laboratory, Institute of Inorganic Chemistry, University of Göttingen. The melting points were measured on a Bü chi B-540 melting point apparatus. LIFDI measurements were performed on a Joel AccuTOF spectrometer under inert atmosphere. UV/vis measurements were performed on a Varian Cary 5000 spectrometer. EPR spectra were recorded on a Bruker ELEXSYS E500 spectrometer, equipped with the digital temperature control system ER 4131VT using nitrogen as the coolant. All spectra were recorded at about 9.4 GHz microwave frequency, 0.5−1 G field modulation amplitude, 100 kHz field modulation frequency, and around 10 mW microwave power. The spectra were recorded at room temperature using toluene as the solvent. EPR simulations were performed using the program Bruker SimFonia, using half Gaussian/half Lorentzian line shapes. Hyperfine values are given in Gauss as the field unit, and conversion to frequency (MHz) requires multiplication with 2.8025. Reagents SiCl4, SiMeCl3, and SiEtCl3 (Aldrich) were used as received. The reaction precursors of (cAAC) and (o-C6H4NMe2)Li were synthesized according to previously reported procedures.67,68 Synthesis of Compound LSiCl2(cAAC) (4). Cold tetrahydrofuran (THF) (−78 °C) was added to a flask together with the ligand cAAC (0.285 g, 1.0 mmol) and the reducing agent KC8 (0.270 g, 2.0 mmol). Then, ArSiCl3 (0.254 g, 1.0 mmol) in THF (15 mL) was quickly added at −78 °C to the above suspension. The reaction was stirred at this temperature for 1 h and then allowed to warm up to room temperature. All solvents were removed under vacuum. The residue was then extracted with n-hexane (40 mL) and filtered. Yellow crystals suitable for X-ray diffraction analysis of 4 were grown from a concentrated n-hexane solution at 0 °C after 3 days (0.377 g, 75%). Decomp. at 127 °C. Elemental analysis found in % (calcd) for C28H41Cl2N2Si: C, 65.96 (66.64); H, 8.02 (8.19); N, 5.48 (5.55). MS (LIFDI, toluene) m/z: 503.2 (100%, [M+]). UV λab: 402 nm. Synthesis of compound LSiClMe(cAAC) (5). Cold THF (−78 °C) was added to a flask together with the ligand cAAC (0.285 g, 1.0 mmol) and KC8 (0.270 g, 2.0 mmol) at −78 °C. The addition of ArSiMeCl2 (0.234 g, 1.0 mmol) in THF (15 mL) to the suspension and the work up of the product were done as described for compound 4. Orange crystals suitable for X-ray diffraction analysis of 5 were grown from concentrated n-hexane solution at −32 °C after 1 week (0.396 g, 82%). Decomp. at 124 °C. Elemental analysis found in % (calcd) for C29H44ClN2Si: C, 71.25 (71.93); H, 9.06 (9.16); N, 5.72 (5.79). MS (LIFDI, toluene) m/z: = 483.3 (100%, [M+]). UV λab: 434 nm. Synthesis of Compound LSiClEt(cAAC) (6). Cold THF (−78 °C) was added to the flask together with the ligand cAAC (0.285 g, 1.0 mmol) and the reducing agent KC8 (0.270 g, 2.0 mmol) at −78 °C. ArSiEtCl2 (0.248 g, 1.0 mmol) in THF (15 mL) was added at −78 °C. For the work up, see compound 4. Yellow crystals suitable for X-ray diffraction analysis of 6 were grown from concentrated nhexane solution at −32 °C after 1 week (0.362 g, 73%). Decomp. at 125 °C. Elemental analysis found in % (calcd) for C30H46ClN2Si: C, 71.77 (72.32); H, 9.01 (9.31); and N, 5.45 (5.62). MS (LIFDI, toluene) m/z: 497.3 (100%, [M+]). UV λab: 436 nm. Crystal Structure Determination. Suitable crystals of 4−6 for X-ray structural analysis were mounted under cooling, argon atmosphere by using the X-Temp2 apparatus.69,70 The data were collected at 100(2) K and integrated with SAINT.71 SADABS72 was applied for multiscan absorption correction and 3λ correction.73 The


Figure 8. Mulliken spin density plots of the compounds (4−6) at the U-BP86/def2-TZVP//U-BP86/def2-SVP level (isosurface = 0.01 a.u.). Hydrogen atoms are omitted for clarity.

atom. Furthermore, the spin density at the silicon center is negligible. The calculated atomic spin densities are summarized in Table S8. The spin distributions are in accordance with the NPA charges on Si1 (4: +1.526 e, 5: +1.620 e, and 6: +1.647 e) and C9 (4: −0.326 e, 5: −0.298 e, and 6: −0.305 e) atoms. Additionally, we have performed time-dependent density functional theory (TDDFT) calculations to interpret the UV spectra of these compounds (Table S9). The theoretical UV spectrum of 4 exhibits two absorption bands at 360 and 464 nm. Similarly, 5 and 6 show two signals at (338, 432 nm) and (340, 432 nm), respectively. The higher-lying signal designates the β-highest occupied molecular orbital (HOMO) − 2 → βlowest unoccupied molecular orbital (LUMO) excitation, whereas the lower-lying absorption is characterized with βHOMO → β-LUMO excitation. It is worth mentioning that βHOMO represents the lone pair orbital located on the nitrogen (N1) atom, whereas β-HOMO − 2 is dominated by the π orbital distributed predominantly on the −Dipp group bonded to the N2 atom. On the other hand, β-LUMOs in all the compounds depict the π-symmetric unoccupied molecular orbital centered at the carbene carbon atoms. The molecular orbitals involved in the major electronic excitations are provided in Figure S5.

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CONCLUSIONS Herein, we report on cAAC-stabilized monoradicals containing halogen and alkyl substituents at the silicon atom. The employment of a two-step route as well as the donor capacity of the ligand is important for the isolation of the monoradicals. The room-temperature stable radicals 4−6 were obtained from the reduction of precursors 1−3 using KC8 in the presence of cAAC, respectively. The molar ratio of the reactants is strict for each reductive process. The employment of the suitable ligand D


Organometallics

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structures were solved by direct methods (SHELXT)74 and refined against F2 (SHELXL)75 within the SHelXle GUI.76 All hydrogen atoms were refined using the riding model. A summary of cell parameters, data collection, and structure solution and refinement is given in the Supporting Information. Computational Details. All computations reported in this article were performed employing a DFT method implemented in the Gaussian 09 suite of programs.77 Geometry optimizations were carried out using a gradient-corrected BP86 functional78,79 in conjunction with the Ahlrichs’ split valence plus polarization (def2SVP) basis set80,81 for all the atoms. BP86 is composed of Becke’s 1988 exchange and Perdew’s 1986 correlation functionals. No symmetry constraints were imposed during geometry optimizations. Frequency calculations were accomplished at the same level on the optimized geometries to characterize the nature of stationary points. All of the structures were verified as true minima on the potential energy surface in the absence of imaginary frequency. Single-point calculations were performed on optimized geometries using BP86 functional in combination with the def2-TZVP basis set for all the atoms. Tight wave function convergence criteria and an “ultrafine” (99 950) grid were used in numerical integration during single-point calculations. Natural bond orbital (NBO)82,83 analysis was performed at the U-BP86/def2-TZVP//U-BP86/def2-SVP level using the NBO Version 3.1 program. TDDFT calculations84 were accomplished in a n-hexane solvent at the same level and the solvation effect was simulated by a self-consistent reaction field approach using the SMD continuum solvation model,85 with the default parameters for nhexane (dielectric constant ε = 1.882). Orbital diagrams were rendered in Chemcraft86 and optimized geometries were prepared using the CYLview87 visualization software.

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ACKNOWLEDGMENTS The authors are grateful to the Deutsche Forschungsgemeinschaft (RO 224/68-1) and the Danish National Research Foundation (DNRF93) funded Centre for Materials Crystallography (CMC) for partial support. S.D. acknowledges the Council of Scientific and Industrial Research (CSIR), India, for the Senior Research Fellowship (SRF) and IISER Kolkata for the computational facility. D.K. acknowledges the funding from bilateral DST-DFG (INT/FRG/DFG/P-05/2017) scheme. This study is dedicated to Professor Hongping Zhu on the occasion of his 50th birthday.

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ASSOCIATED CONTENT

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00041. Experimental details, including NMR data and UV/vis spectra for the new compounds, the data of single-crystal X-ray structure determination part, and theoretical calculations (PDF) Accession Codes

CCDC 1891825−1891827 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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REFERENCES

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S Supporting Information *

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[email protected] (Z.Y.). [email protected] (D.S.). [email protected] (D.K.). [email protected] (W.K.). [email protected] (H.W.R.).

ORCID

Subrata Kundu: 0000-0002-2308-5452 Dietmar Stalke: 0000-0003-4392-5751 Regine Herbst-Irmer: 0000-0003-1700-4369 Debasis Koley: 0000-0002-7912-3972 Wolfgang Kaim: 0000-0002-8404-4929 Herbert W. Roesky: 0000-0003-4454-1434 Notes

The authors declare no competing financial interest. E


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