Isolation of Transient Acyclic Germanium (I) Radicals Stabilized by

Cyclic Alkylamino Carbenes. Mujahuddin M. Siddiqui,1 Samir Kumar Sarkar,1 Soumen Sinhababu,1 Paul Niklas Ruth,1 Regine. Herbst-Irmer,1 Dietmar Stalke ...
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Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkylamino Carbenes Mujahuddin M. Siddiqui, Samir Kumar Sarkar, Soumen Sinhababu, Paul Niklas Ruth, Regine Herbst-Irmer, Dietmar Stalke, Munmun Ghosh, Mingxing Fu, Lili Zhao, David Casanova, Gernot Frenking, Brigitte Schwederski, Wolfgang Kaim, and Herbert W. Roesky J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b13434 • Publication Date (Web): 11 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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Journal of the American Chemical Society

Isolation of Transient Acyclic Germanium(I) Radicals Stabilized by Cyclic Alkylamino Carbenes Mujahuddin M. Siddiqui,1 Samir Kumar Sarkar,1 Soumen Sinhababu,1 Paul Niklas Ruth,1 Regine Herbst-Irmer,1 Dietmar Stalke,*1 Munmun Ghosh,1 Mingxing Fu,2 Lili Zhao,2 David Casanova,3 Gernot Frenking*2,3,4 Brigitte Schwederski,5 Wolfgang Kaim,*5 Herbert W. Roesky*1 1Universität 2Institute

Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, 37077, Göttingen, Germany of Advanced Synthesis, Nanjing Tech University, Nanjing 211816, China

3Donostia

International Physics Center (DIPC), P.K. 1072, 20080 Donostia, Euskadi, Spain

4Universität 5Universität

Marburg, Fachbereich Chemie, Hans-Meerwein-Strasse, 35032 Marburg Stuttgart, Institute für Anorganische Chemie, Pfaffenwaldring 55, 70569 Stuttgart

Supporting Information Placeholder ABSTRACT: Despite the notable progress in the stabilization of main group radicals by NHCs and cAACs, no germanium radicals have been isolated so far due to synthetic challenges. Stabilization of neutral [:EIR]• (E = Si, Ge) radicals is an uphill task, as these reactive transient species are highly susceptible to dimerization. Herein, we report the synthesis of acyclic neutral germanium(I) radicals Cy-cAAC:GeN(SiMe3)Dip (1) and MecAAC:GeN(SiPh3)Mes (2) obtained by the reduction of [Ar(SiR3)NGeCl3] with KC8 in the presence of cAAC. Compounds 1 and 2 are well characterized by single crystal X-ray structural analysis, cyclic voltammetry and EPR spectroscopy. Furthermore, the structure and bonding of compounds 1 and 2 have been investigated by theoretical methods.

Radicals play an important role as key intermediates in numerous chemical and biological processes and have attracted interest of experimental as well as theoretical chemists from decades.1 The most common methods to access persistent and stable radical species are kinetic stabilization by the use of bulky ligands and electronic stabilization. The advent of singlet carbenes paved an alternative way to stabilize transient radical species by σdonation. This method had played an important part in the recent rapid development of the chemistry of main-group radical complexes.2 Over the years, several Gomberg type radicals [EIIIR3]• (E = Si, Ge, Sn; R = alkyl, aryl, silyl, amide, etc.) of heavier main group elements have been isolated and their redox properties have been studied.3 However, the isolation of neutral element(I) radicals [:EIR]• still remains a challenging task. The main reason behind the difficulty in isolation of these transient radicals is their high reactivity. In the past few years, several research groups attempted to synthesize neutral [:EIR]• radical derivatives by the alkali metal reduction of REX or REX3 (E= Si, Ge, Sn; X=Cl, Br, I), but all the attempts were futile. They are always susceptible to dimerization leading to [RË−ËR] (E = Si, Ge, and Sn) species.4 The radical chemistry of germanium is scarcely studied and apart from [GeIIIR3]• radicals (I) only a handful of examples are reported in literature (Chart 1). In 1997, Power and coworkers reported a cyclotrigermanyl radical [Ge(2,6-Mes2–C6H3)3]• (II) by

the reduction of Ge(2,6-Mes2–C6H3)Cl with 1 equiv. of KC8.5 In 2012, Jones and coworkers isolated the only example of [:EIR]• radical by incorporating it in an extremely bulky chelating βdiketiminate ligand.6 [(ButNacnac)Ge:]• (III) was obtained by the reduction of [(ButNacnac)GeCl] with 1 equiv of sodium naphthalenide in THF or by 0.5 equiv of the magnesium(I) dimer [{(MesNacnac)Mg}2 in toluene. However, a similar reduction of the less hindered precursors, [(MesNacnac)GeCl] and [(DipNacnac)GeCl] was unsuccessful. So and coworkers isolated a delocalized dianion radical {[LGe]•2-·Ca(THF)32+} (IV) by the reduction of [LGeCl] (L = 2,6-(CH=NAr)2C6H3, Ar = 2,6iPr2C6H3) with elemental calcium.7 Very recently, Gilroy and coworkers have reported hypervalent Ge(IV) radical anion (V) supported by formazanate ligand.8 To the best of our knowledge, no acyclic neutral Ge(I) radical has been reported so far.

Chart 1. Monomeric Germanium radical species reported. In recent years, N-heterocyclic carbenes (NHCs) and cyclic alkyl(amino) carbenes (cAACs) have been successfully utilized for stabilization of a variety of unstable species of main group and transition elements.2,9 As cAACs are stronger σ-donors and better π-acceptors than those of NHCs, thus, several hitherto transient radical species were successfully isolated by using cAACs. In comparison with the carbene stabilized silicon chemistry, the germanium chemistry is less explored and no carbene stabilized germanium radical species have been isolated so far.10 Our recent efforts to synthesize [:EIR]• radical species by the reduction of MeECl3 (E = Si, Ge) with KC8 in the presence of cAAC led to the isolation of interconnected silylenes and germylenes [(cAAC)MeE:]2 ( E= Si, Ge).11 Taking a cue from the importance of bulky β-diketiminate ligand in the isolation of III, we presumed the replacement of methyl substituents on germanium with bulky aromatic groups will prevent the dimerization of transient [:EIR]• radical species. Herein, we report a successful

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strategy for the isolation of transient acyclic Ge(I) radical compounds stabilized by cyclic alkylamino carbenes. Scheme 1. Synthesis of compounds 1 and 2.

Figure 2: EPR spectrum of 1 in toluene at room temperature. Bottom: experimental spectrum, top: spectrum simulated with g=2.007 and coupling constants 1N=5.3 G, 1H=2.1 G and 4H=1.05 G. A 1:1:3 molar ratio of [Ar(SiMe3)NGeCl3], cAAC: and KC8 was reacted in THF at − 78 °C, the resultant suspension was slowly warmed up to room temperature to get a dark purple solution. Solvent was evaporated in vacuum and the residue was extracted with n-hexane. Dark purple block shaped crystals of CycAAC:GeN(SiMe3)Dip (1) and Me-cAAC:GeN(SiPh3)Mes (2) were obtained in 55% and 63% yield at -30 °C after 12 hours (Scheme 1). Broad resonances were observed in the 1H NMR spectra of both the compounds indicating their radical nature. Compounds 1 and 2 were characterized by single crystal X-ray diffraction and elemental analysis.

The EPR spectra of radicals 1 (Figure 2) and 2 (SI, Figure S3) at room temperature in toluene and hexane solution respectively, are dominated by the typical 14N hyperfine splitting of about 5 Gauss for cAAC radical compounds12 (1: 5.3 G, 2: 5.0 G). Radical 1 exhibits additional 1H hyperfine coupling (Figure 2) for 4H (1.05 G) and 1H (2.1 G) attributed to hydrogen atoms in the cyclohexyl ring. Cyclohexyl and α-amino coupling confirm the spin concentration at the former carbene center. The isotropic g factors of the radicals (1: 2.0070; 2: 2.0075) are lower than the free electron value of 2.0023, suggesting some influence from the heavy element Ge with its high spin orbit coupling effect. The g Factor splitting (2.016, 2.004, 1.9975) at 171 K in frozen hexane solution (1) and (2.018, 2.004, 1.995) at 150 K in frozen toluene solution (2) confirms this notion (SI, Figures S4 and S5). However, a 73Ge hyperfine splitting was not observed due to the small nuclear magnetic moment, the low natural abundance of 7.73% and the distribution of intensity over ten lines (I = 9/2). The available information from hyperfine splitting’s and the g factor anisotropy g1-g3 of about 0.018 suggest that there is a rather balanced spin distribution between germanium and the ligand, as confirmed by calculations (see below). Therefore, the new germanium compounds presented here constitute borderline cases between cyclic diketiminato species (III) with mostly Ge-centered spin (g1-g3 = 0.033) and chelate compounds involving 1,4-diazadiene radical ligands.13

Figure 1. Molecular structure of 1; hydrogen atoms are omitted for clarity. Anisotropic displacement parameters are depicted at the 50% probability level. Selected bond lengths and angles: Ge1C1: 1.986(2) Å, Ge1-N2: 1.9060(18) Å, N1-C1: 1.366(3) Å, N2Ge1-C1: 114.91(9)°, N1-C1-Ge1: 111.38(15)°. Radicals 1 and 2 are stable in the solid state at room temperature as well as in hexane solutions at -30 °C under inert atmosphere for months, but the solution immediately lose their intense color upon exposure to air. Compound 1 melts in the range of 203 − 205 °C whereas compound 2 melts at 189 − 191 °C. The UV/vis spectra of 1 and 2 were recorded in n-hexane. Compound 1 shows absorption maxima at 249, 390 and 550 nm, compound 2 at 249, 325 and 555 nm respectively (SI, Figures S1− S2). The single crystal x-ray diffraction studies of 1 (Figure 1) and 2 (SI, Figure S10) revealed the Ge-C(cAAC) bond length to be slightly longer than the bond length reported for the corresponding germylones,9a where Ge is coordinated by two cAAC molecules (1: 1.986(2) Å, Ge(0) 1.954(2) Å / 1.9386(18) Å; 2: 1.979(3) Å, Ge(0): 1.9386(16) Å - 1.9440(15) Å), but significantly shorter than the bond length reported for Me2cAAC:GeCl2 (2.135(2) Å).9a

Figure 3: Cyclic voltammogram of THF solution of 1 containing of 0.1 M [n-Bu4N]PF6 as an electrolyte. a) at 400mV scan rate. b) The dotted area has been expanded in large (right). The redox properties of 1 and 2 was investigated by cyclic voltammetry measurements (Figure 3, SI, S6-S8). The cyclic voltammogram of 1 shows a one-electron electrochemically irreversible reduction with Ep,c = -1.22 V and Ep,a = -1.82 V (ΔE = -1.52 V, υ = 200mV/s) against Fc/Fc+ and a reversible reduction peak at E1/2 = -2.23 V (Figure 3). We believe the irreversible reduction corresponds to the formation of radical anion [1]-•, whereas the reversible reduction is due to the formation of radical

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Journal of the American Chemical Society dianion [1]2-• species. A very small hump at -1.83 V does not related to the first or second reduction peak, as confirmed from the scan rate dependency (SI S6). Similarly, the cyclic voltammogram of 2 shows the one-electron electrochemically irreversible peak at Ep,c = -1.35 V and Ep,a = -2.12 V (ΔE = -1.73 V, υ = 50mV/s) (Fc/Fc+) (SI, S7-S8). We carried out DFT (density functional theory) calculations at the UBP86-D3(BJ)/def2-SVP level of theory for 1 and 2 in order to understand the electronic structure of the molecules.14 Figure S11 shows the calculated equilibrium geometries of the two molecules and the most important bond lengths and angles, which are in good agreement with experiment. The calculations indicate that 1 and 2 have a doublet electronic ground state. The quartet state is calculated 47.7 kcal/mol (1) and 50.4 kcal/mol (2) higher in energy than the doublet state. If one calculates the energies of 1 and 2 using the experimentally determined geometries with other functional and larger basis sets, the quartet state is even 62 - 78 kcal/mol higher than the singlet state (Table 1). We calculated the electronic excitation energies and computed the simulated UV spectra of 1 and 2 in hexane at the UB3LYP/631G** level, which are shown in Figures S12 and S13. The theoretical spectra nicely reproduce the essential features of the experimentally observed bands (Figures S1 and S2). Inspection of the calculated transitions shows that the main orbital transitions responsible for the absorption bands are in both compounds the coupled HOMO-2→SOMO and SOMO→LUMO excitations (Figures S14 and S15). We analyzed the nature of the RGe-cAAC bonds in 1 and 2 with the EDA-NOCV method.15 Table 1 shows the numerical results. The calculations were carried out using the fragments RGe in the doublet state and cAAC in the singlet state. Test calculations using other spin state combinations gave significantly larger orbital values ∆Eorb, which is a probe for the choice of the best fragments.16 The data in Table 1 show that the covalent orbital interactions ∆Eorb have two major contributions ∆E(1) and ∆E(2), which provide > 80% of the orbital term. Table 1. EDA-NOCV results of 1 and 2 at the BP86D3(B)J/TZ2P level of theory. Energy values are given in kcal/mol. 1 fragments ∆Eint

RGe (doublet) cAAC (singlet)

2 RGe (doublet) cAAC (singlet)

-80.7

-78.5

∆Edisp[a]

-31.1 (9.1%)

-27.8 (8.3%)

∆EPauli

258.9

256.5

-170.9 (50.3%)

-170.3 (50.8%)

∆Eorb

-137.6 (40.5%)

-136.9 (40.9%)

∆E(1) [b]

-84.0 (61.1%)

-85.4 (62.4%)

∆E(2) [b]

∆Eelstat[a] [a]

-28.4 (20.6%)

-27.7 (20.2%)

∆E(3)

[b]

-6.5 (4.7%)

-6.4 (4.7%)

∆E(4)

[b]

-3.4 (2.5%)

-3.6 (2.6%)

-15.3 (11.1%)

-13.8 (10.1%)

∆Erest

The values in parentheses give the percentage contribution to the total attractive interactions ΔEdisp+ΔEelstat + ΔEorb. bThe values in parentheses give the percentage contribution to the total orbital interactions ΔEorb. a

deformation densities Δρ

ΔE(1) = -84.0 kcal/mol

ΔE(2) = -28.4 kcal/mol

interacting MOs

(LUMO)

(SOMO)

(HOMO)

(LUMO)

Figure 4. Shape of the most important interacting MOs of fragments of 1, plot of deformation densities Δρ of the pairwise orbital interactions and the associated interaction energies (ΔE). The direction of the charge flow is red→blue. Inspection of the deformation densities Δρ and the respective interacting orbitals, which are associated to ∆E(1) and ∆E(2), reveal that the strongest interaction comes from the donation of the cAAC lone-pair electrons to the vacant orbital of RGe. Figure 4 shows the shape of Δρ1 and the orbitals, which contribute to the cAAC→GeR donation in 1. The shape of the deformation densities and orbitals for 2 look very similar; they are shown in Figure S16. The second strongest orbital interaction is due to the π donation of the singly occupied π SOMO of RGe, which is mainly located at Ge, to the π LUMO of cAAC. The associated charge flow nicely agrees with the calculated total spin density of 1, which is shown in Figure 5. It corroborates the relevance of the spin delocalization, which may also be expressed in terms of the resonance structures shown in Scheme 1. The very similar spin density of 2 is shown in Figure S17.

Figure 5. Calculated spin density of 1 in the doublet state at the UM06-2X-D3/def2-TZVPP level. The computed numbers are taken from the Mulliken atomic spin densities.

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In conclusion, we have isolated transient [:GeIR]• species which are highly susceptible to dimerization. The acyclic neutral germanium(I) radicals (1 and 2) were stabilized using cyclic alkylamino carbene by the reduction of [Ar(SiR3)NGeCl3] with KC8. 1 and 2 were fully characterized using X-ray crystallography, CV and EPR spectroscopy. Theoretical calculations and simulation of EPR spectrum showed that the radical electron resides equally on the Ge atom and the carbene carbon atom. These Ge(I) radicals offer new examples of main group radical compounds stabilized by cAAC, which further establishes the significance of these ligands in the isolation of hitherto uncommon transient species. A challenge for the future is the preparation of corresponding species of silicon and carbon. ASSOCIATED CONTENT

Supporting Information The cif files of 1 and 2 [CCDC: 1880920 (1), 1880921 (2)], figures and the details of crystal structure refinements, UV−Vis spectra, EPR, CV measurements and theoretical investigations. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION

Corresponding Author * Herbert W. Roesky ([email protected]) * Dietmar Stalke ([email protected]) * Gernot Frenking([email protected]) * Wolfgang Kaim([email protected])

Notes The authors declare no competing financial interests. ACKNOWLEDGMENT H.W.R. is thankful to the DFG for financial support (RO 224/681). G.F. acknowledges financial support by the Deutsche Forschungsgemeinschaft. D.St. thanks the Danish National Research Foundation (DNRF93) funded Center for Materials Crystallography (CMC) for partial support. M.M.S. thanks SERB India for overseas postdoctoral fellowship. G.F. and L.Z. acknowledge financial support from Nanjing Tech University (grant number 39837123 and 39837132) and SICAM Fellowship from Jiangsu National Synergetic Innovation Center for Advanced Materials, Natural Science Foundation of Jiangsu Province for Youth (Grant No: BK20170964), National Natural Science Foundation of China (Grant No. 21703099), and the high performance center of Nanjing Tech University for computational support. M.M.S. thanks Professor Inke Siewert for fruitful suggestions. Dedicated to Professor Thomas Fässler on the occasion of his 60th birthday. REFERENCES (1) (a) Gomberg, M. An instance of trivalent carbon: Triphenylmethyl. J. Am. Chem. Soc., 1900, 22, 757–771. (b) H. Jkkk, in Stable Radicals: Fundamental and Applied Aspects of Odd-Electron Compounds, ed. Hicks, R. G. Wiley, Chichester, 2010. (c) Power, P. P. Persistent and stable radicals of the heavier main group elements and related species. Chem. Rev. 2003, 103, 789−809. (d) Lee, V. Y.; Sekiguchi, A. Stable silyl, germyl, and stannyl cations, radicals, and anions: Heavy versions of carbocations, carbon radicals, and carbanion. Acc. Chem. Res. 2007, 40, 410−419. (2) (a) Martin, C. D.; Soleilhavoup, M.; Bertrand, G. Carbenestabilized main group radicals and radical ions. Chem. Sci. 2013, 4, 3020−3030. (b) Soleilhavoup, M.; Bertrand, G. Cyclic (Alkyl)(Amino)Carbenes (CAACs): Stable Carbenes on the Rise. Acc.

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Journal of the American Chemical Society Silicon(I) Radical with Carbenes: A Cationic cAAC–Silicon(I) Radical and an NHC–Parent‐Silyliumylidene Cation. Angew. Chem., Int. Ed. 2017, 56, 7573−7578. (11) Kundu, S.; Samuel, P. P.; Luebben, A.; Andrada, D. M.; Frenking, G.; Dittrich, B.; Roesky, H. W. Carbene stabilized interconnected bisgermylene and its silicon analogue with small methyl substituents. Dalton Trans. 2017, 46, 7947-7952. (12) (a) Mondal, K. C.; Roesky, H. W.; Stückl, A. C.; Ehret, F.; Kaim, W.; Dittrich, B.; Maity, B.; Koley, D. Formation of Trichlorosilyl‐Substituted Carbon‐Centered Stable Radicals through the Use of π‐Accepting Carbenes. Angew. Chem. Int. Ed. 2013, 52, 1180411807. (b) Kundu, S.; Samuel, P. P.; Sinhababu, S.; Luebben, A. V.; Dittrich, B.; Andrada, D. M.; Frenking, G.; Stückl, A. C.; Schwederski, B.; Paretzki, A.; Kaim, W.; Roesky, H. W. Organosilicon Radicals with Si−H and Si−Me Bonds from Commodity Precursors. J. Am. Chem. Soc. 2017, 139, 11028−11031. (c) Sinhababu, S.; Kundu, S.; Paesch, A. N.; Herbst-Irmer, R.; Stalke, D.; Fernandez, I.; Frenking, G.; Stückl, A. C.; Schwederski, B.; Kaim, W.; Roesky, H. W. A Route to Base Coordinate Silicon Difluoride and the Silicon Trifluoride Radical. Chem. - Eur. J. 2017, 24, 1264−1268. (13) Tumanskii, B.; Pine, P.; Apeloig, Y.; Hill, N. J.; West, R. Radical Reactions of a Stable N-Heterocyclic Germylene: EPR Study and DFT Calculation J. Am. Chem. Soc. 2005, 127, 8248−8249.

(14) See Supporting Information for the description of the theoretical methods. (15) Mitoraj, M. P.; Michalak, A.; Ziegler, T. A Combined Charge and Energy Decomposition Scheme for Bond Analysis. J. Chem. Theory Comput. 2009, 5, 962−975. (16) Recent representative examples: (a) Mohapatra, C.; Kundu, S.; Paesch, A. N.; Herbst-Irmer, R.; Stalke, D.; Andrada, D. M.; Frenking, G.; Roesky, H. W. The Structure of the Carbene Stabilized Si2H2 May Be Equally Well Described with Coordinate Bonds as with Classical Double Bonds. J. Am. Chem. Soc. 2016, 138, 10429-10432. (b) Andrada, D. M.; Casalz-Sainz, J. L.; Pendas, A. M.; Frenking, G. Dative and ElectronSharing Bonding in C2F4. Chem. Eur. J. 2018, 24, 9083−9089. (c) Li, Z.; Chen, X.; Andrada, D. M.; Frenking, G.; Benkö, Z.; Li, Y.; Harmer, J. R.; Su, C.-Y.; Grützmacher, H. (L)2C2P2: Dicarbondiphosphide Stabilized by N-Heterocyclic Carbenes or Cyclic Diamido Carbenes. Angew. Chem., Int. Ed. 2017, 56, 5744−5749. (d) Zhao, L.; Hermann, M.; Holzmann, N.; Frenking, G. Dative Bonding in Main Group Compounds. Coord. Chem. Rev. 2017, 344, 163−204.

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