Equilibrium Coordination of NHCs to Si(IV) Species and Donor

(15) In the case of Me3SiCl, we do not see any adduct formation at room temperature. ... (17) This is due to the higher leaving group character of tri...
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Equilibrium Coordination of NHCs to Si(IV) Species and Donor Exchange in Donor−Acceptor Stabilized Si(II) and Ge(II) Compounds Avijit Maiti,† Debdeep Mandal,† Isabell Omlor,‡ Debabrata Dhara,† Lukas Klemmer,‡ Volker Huch,‡ Michael Zimmer,‡ David Scheschkewitz,*,‡ and Anukul Jana*,† †

Tata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad-500107, India Krupp-Chair of General and Inorganic Chemistry, Saarland University, 66123 Saarbrücken, Germany

‡

Inorg. Chem. Downloaded from pubs.acs.org by EAST CAROLINA UNIV on 03/13/19. For personal use only.

S Supporting Information *

dene)12 and NHCMe4 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene),12 based on their steric13 as well as electronic properties.14 In order to progressively decrease the Lewis acidity, we employed a series of methylchlorosilanes, SiCl4, MeSiCl3, Me2SiCl2, and Me3SiCl, in this study. We thus prepared NHCiPr 2 Me2-coordinated SiCl 4, 6 MeSiCl3, and Me2SiCl26 adducts using a modified procedure of Kuhn et al. by using benzene as a solvent instead of THF, which results in a significant improvement in yield. All compounds were isolated as white colorless solids (Scheme 1).15 In the case of

ABSTRACT: We report the reversible coordination of the N-heterocyclic carbene (NHC), NHC i P r 2 M e 2 (NHCiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-ylidene), to silicon(IV)-halides, SiCl4, MeSiCl3, Me2SiCl2, and Me3SiCl. Predicted as well as experimentally determined thermodynamic parameters of these equilibria confirm that the complexation constant increases with the Lewis acidity of the silicon halides. In contrast, the more σ-donating N-heterocyclic carbene, NHCMe4 (NHCMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene), does not show any signs of dissociation from the corresponding SiCl4 and Me2SiCl2 adducts even at higher temperatures. As a consequence, NHCiPr2Me2 in donor−acceptor stabilized Si(II)- and Ge(II)-dimethyl complexes, NHCiPr2Me2· GeMe2·Fe(CO)4 and NHCiPr2Me2·SiMe2·Fe(CO)4, is readily replaced by NHCMe4.

Scheme 1. Syntheses of NHCiPr2Me2·SiMenCl4−n

I

n recent years, a particular focus of main group chemistry has been the design of functional equivalents of transition metal complexes.1 In homogeneous catalysis, the reversible binding between transition metals and a variety of ligands and substrates is pivotal for closing catalytic cycles.2 N-Heterocyclic carbenes (NHCs) are well established as species active in organocatalysis.3 Since more recently, they have also been utilized for the stabilization of low-valent reactive intermediates.4 In organocatalysis, NHCs bind reversibly to the carbon center of the substrate and thus activate it for a variety of organic transformations.5 In the case of silicon, NHCs are known to allow for the isolation of hyper-coordinate,6 lowvalent,7 and low-coordinate species.8 In contrast, reports on the equilibrium coordination of NHCs to silicon centers are limited to the reactions with cyclotrisilenes.9 The observed exchange of NHCs in adducts of low-valent silicon10 and germanium species,11 however, suggests that coordination equilibria may be much more common than previously assumed. Moreover, there were no reports on the comparison of donor exchange rates in silicon and germanium systems. Here, we report the reversible coordination of NHCs with silicon(IV) compounds, the experimental determination of thermodynamic parameters, as well as the metathesis of NHCs in donor−acceptor stabilized silicon(II) and germanium(II) derivatives. We have considered two different NHCs, NHC iPr 2 Me 2 (NHCiPr2Me2 = 1,3-diisopropyl-4,5-dimethylimidazol-2-yli© XXXX American Chemical Society

Me3SiCl, we do not see any adduct formation at room temperature. Therefore, we prepared a sample containing NHCiPr2Me2 and Me3SiCl in a 1:1 ratio and measured 1H NMR at various temperatures (vide inf ra). The 1H NMR spectrum of NHCiPr2Me2·SiMeCl3 at room temperature shows a broad septet at δ = 5.69 ppm for the isopropyl C-H resonances. The 29Si NMR spectrum shows a singlet at δ = −82.8 ppm which is in between the corresponding signals of NHCiPr2Me2·SiCl4 (δ = −104.7 ppm) and NHCiPr2Me2·SiMe2Cl2 (δ = −72.6 ppm). NMR data for compounds NHCiPr2Me2·SiCl4 and NHCiPr2Me2·SiMe2Cl2 match those previously reported.6 To check whether there is an equilibrium between NHCiPr2Me2 and SiMenCl4−n, the 1H NMR spectra of 1:1 mixtures were recorded at variable temperatures in toluene-d8 in the presence of naphthalene as an internal standard. For the determination of the ratio between the NHC-Si(IV) adduct and the corresponding free NHCiPr2Me2, we employed the integrals of the isopropyl C-H signals in the 1H NMR spectra. In the case of NHCiPr2Me2·SiCl4, not even traces of the Received: January 26, 2019

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DOI: 10.1021/acs.inorgchem.9b00246 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry dissociation products were detected at 348 K. In the case of NHCiPr2Me2·SiMeCl3, there is no indication for the liberation of NHCiPr2Me2 at room temperature, although free NHCiPr2Me2 is increasingly observed at temperatures above 318 K. For NHCiPr2Me2·SiMe2Cl2, significant dissociation occurs even when a crystalline sample is dissolved at room temperature. In contrast, no traces of the adduct of NHCiPr2Me2 with Me3SiCl are observed even at 213 K in toluene, which is fully in line with the decreasing Lewis acidity. Müller et al. reported the quantitative assessment of the Lewis acidity of silylium ions with the Gutmann−Beckett method.16 It is known that NHCiPr2Me2 reacts with Me3SiOTf under the formation of the 2-trimethylsilyl substituted imidazolium salt.17 This is due to the higher leaving group character of triflate vs chloride. The dissociation of the triflate anion drastically increases the electrophilicity of the silicon center of Me3SiOTf compared to Me3SiCl.18 In order to determine the thermodynamic parameters for NHCiPr2Me2·SiMeCl3 and NHCiPr2Me2·SiMe2Cl2, naphthalene was added to the solutions as an internal standard. From the intensity of isopropyl C-H signals, we calculated the concentration of free NHCiPr2Me2 and the corresponding adduct at any given temperature. A van’t Hoff plot of the natural logarithm of the equilibrium constants against the inverse temperature provides information on the reaction enthalpies and entropies. The ΔG298 for NHCiPr2Me2·SiMeCl3 and NHCiPr2Me2·SiMe2Cl2 are −27.256 and −19.964 kJ·mol−1, respectively (Table 1). These thermodynamic parameters

Scheme 2. Synthesis of Lewis Acid and Lewis Base Stabilized SiMe2 and GeMe2

Compound 1 is prepared from the reaction of NHCiPr2Me2· SiMe2Cl2 and Na2Fe(CO)4 in 65% crystalline yield (Scheme 2).15 In the 1H NMR spectrum, iPr−CH of 1 is upfield shifted (δ = 5.09 ppm) compared to NHCiPr2Me2·SiMe2Cl2 (δ = 5.43 ppm). The 29Si NMR spectrum exhibits a resonance at δ = 26.3 ppm, downfield shifted in comparison to that of NHCiPr2Me2·SiMe2Cl2 (δ = −72.5 ppm). We obtained single crystals from hot saturated n-hexane solution, which were suitable for X-ray diffraction. The analysis of the molecular structure of 1 in the solid state reveals a fourcoordinate silicon center with two methyl groups, one NHC as a donor and one Fe(CO)4 moiety as an acceptor (Figure 1).

Table 1. Thermodynamic Parameters of the Equilibrium of NHCiPr2Me2·SiMenCl4−n Adduct NHCiPr2Me2· SiMenCl4−n

ΔH [kJ· mol−1]

ΔS [kJ·K−1· mol−1]

ΔG298 [kJ· mol−1]

NHCiPr2Me2·SiMeCl3 NHCiPr2Me2·SiMe2Cl2

−74.3 ± 2.9 −68.2 ± 2.7

−0.158 ± 0.008 −0.162 ± 0.008

−27.3 ± 2.4 −20.0 ± 2.5

Figure 1. Molecular structure of 1 in the solid state (thermal ellipsoids at 30%, all hydrogen atoms are omitted for clarity). Selected bond lengths [Å] and angles [deg]: C1−Si1 1.9707(9), Si1−C8 1.8852(9), Si1−Fe1 2.3323(4), Fe1−C9 1.7871(13), C9−O2 1.1485(17); C1−Si1−Fe1 115.259(29), C8−Si1−C7 99.654(39), Si1−Fe1−C9 176.819(42), Fe1−C9−O2 179.924(118).

indicate that NHCiPr2Me2 binds more strongly to more Lewis acidic silicon halides. Unsurprisingly, the entropic contribution is almost identical for both cases. Next, we considered the somewhat more strongly donating N-heterocyclic carbene, NHCMe4. We prepared NHCMe4coordinated SiCl46 and Me2SiCl219 adducts by modifying the procedure of Kuhn et al. and Inoue et al. respectively by using benzene as a solvent instead of THF.15 When we applied an analogous protocol to prepare NHCMe4-coordinated MeSiCl3 and Me3SiCl adducts, we obtained insoluble white solids instead. This could be due to the initial formation of a NHCMe4-stabilized silicon(IV) cation with a chloride as a counteranion with a limited stability, which subsequently decomposed to unidentified products. According to 1H NMR at variable temperature, there is no indication for reversibility of coordination in case of NHCMe4·SiCl4 and NHCMe4· SiMe2Cl2; no free NHCMe4 was detected even at 370 K. The stronger σ-donation of NHCMe4 compared to that of NHCiPr2Me2 results in stronger binding to the silicon centers of the silicon(IV)-halides. Subsequently, to address the donor metathesis in donor− acceptor stabilized silicon(II) and germanium(II) compounds, 20 we have prepared NHC iPr 2 Me2 and Fe(CO) 4 stabilized corresponding tetrel(II)-dimethyl derivatives, 1 and 2, respectively (Scheme 2).

In 1971, Marks et al. reported Lewis base (i.e., THF and pyridine) and Lewis acid (Fe(CO)4) stabilized dimethylgermylene.21 Here, to prepare the germanium analogue of 1, we considered the nucleophilic substitution reaction of corresponding germanium(II)-dichloride precursor, 3,22,23 instead of the reaction of the NHCiPr2Me2 with Me2GeCl2 followed by Na2Fe(CO)4 reaction that was used for the preparation of silicon-analogue 1. This is due to the unavailability of Me2GeCl2 in our hand. The reaction of two equivalents MeLi with 3 leads to 2 in a 78% crystalline yield (Scheme 2). The 1H NMR spectrum for 2 shows a resonance for the iPr− CH at δ = 4.96 ppm, which is significantly upfield shifted in comparison to 3 (δ = 5.88 ppm). Single crystals for X-ray diffraction were obtained from saturated hot n-hexane solution. The molecular structure analysis of 2 shows that it is almost perfectly isomorphous to 1 (Figure 2). The distance between carbenic carbon and the germanium center of 2 is 2.045 Å, which is larger than that of 1 (1.970 Å). With the donor−acceptor complexes of dimethyltetrylenes 1 and 2 in hand, we investigated whether the more nucleophilic B

DOI: 10.1021/acs.inorgchem.9b00246 Inorg. Chem. XXXX, XXX, XXX−XXX

Communication

Inorganic Chemistry

Figure 3. Molecular structure of 5 in the solid state (thermal ellipsoids at 30%, all hydrogen atoms are omitted for clarity). Selected bond lengths [Å] and angles [deg]: C1−Ge1 2.0196(22), Ge1−C8 1.9698(35), Ge1−Fe1 2.3847(6), Fe1−C12 1.7590(51), C12−O2 1.1402(71); C1−Ge1−Fe1 111.304(66), C8−Ge1−C9 99.770(175), Ge1−Fe1−C12 177.672(149), Fe1−C12−O2 179.677(435).

Figure 2. Molecular structure of 2 in the solid state (thermal ellipsoids at 30%, all hydrogen atoms are omitted for clarity). Selected bond lengths [Å] and angles [deg]: C1−Ge 2.0449(23), Ge−C8 1.9682(27), Ge−Fe 2.3904(2), Fe−C10 1.7780(35), C10−O3 1.1484(41); C1−Ge−Fe 115.518(64), C7−Ge−C8 99.305(113), Ge−Fe−C10 177.098(111), Fe−C10−O3 179.606(306).

N-heterocyclic carbene, NHCMe4 is able to replace the bulkier NHCiPr2Me2 in a metathesis reaction. According to previous reports by Filippou et al., NHCDip·SiI2 reacts with NHCiPr2Me2 and NHCMe4 under formation of silicon(II) monocation and dication, respectively.10 The absence of competent leaving groups in 1 and 2, allowed us to check the possibility of exchange reaction at the silicon(II) and germanium(II) center without the interference of cation formation. Reaction of one equivalent of NHCMe4 with compounds 1 or 2 at room temperature led to the appearance of signals of free NHCiPr2Me2 in the respective 1H NMR spectra (Scheme 3). In Scheme 3. Exchange of NHCiPr2Me2 by NHCMe4 in Donor− Acceptor Stabilized Si(II) and Ge(II)

Figure 4. Plot for the concentration of 1 with time in NHC-exchange reaction.

order to acquire an idea about the reaction mechanism, we monitored both reactions via 1H NMR. In the case of silicon, complete exchange of NHCiPr2Me2 with NHCMe4 took more than 120 h at room temperature to finally result in compound 4, the synthesis of which via a different approach had recently been reported by Inoue et al.19 In the case of germanium, the exchange of NHCiPr2Me2 with NHCMe4 under the formation of compound 5 is much faster and thus completed within 90 min at room temperature (Scheme 3). We obtained single crystals of compound 5 from saturated n-pentane/benzene at room temperature. X-ray structural analysis of 5 shows a four-coordinate germanium center just as in compound 2 (Figure 3). The bond distance between the carbenic carbon and germanium is 2.019 Å, which is slightly shorter than that of 2 (2.045 Å).20 The kinetics of the NHC exchange in 1 and 2 were analyzed by 1H NMR (Figure 4 and Figure 5). While the exchange of the NHC at the silicon center of 1 follows the second order rate law (rate constant of 0.0416 L·mol−1·min−1) and the half-

Figure 5. Plot for the concentration of 2 with time in NHC-exchange reaction.

life of the reaction is 466 min, the corresponding Ge analogue follows the first order rate law (rate constant of 0.0573 min−1) with the half-life of the reaction being 12.09 min. This indicates that the exchange of NHC for compound 1 is an associative and for compound 2 is a dissociative mechanism, which is in line with the relative stability of silylenes and C

DOI: 10.1021/acs.inorgchem.9b00246 Inorg. Chem. XXXX, XXX, XXX−XXX

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Braunschweig, H. Nitrogen fixation and reduction at boron. Science 2018, 359, 896−900. (2) (a) Osborn, J. A.; Jardine, F. H.; Young, J. F.; Wilkinson, G. The Preparation and Properties of Tris(triphenylphosphine)halogenorhodium(I) and Some Reactions thereof including Catalytic Homogeneous Hydrogenation of Olefins and Acetylenes and their Derivatives. J. Chem. Soc. A 1966, 1711−1732. (b) Meakin, P.; Jesson, J. P.; Tolman, C. A. The Nature of Chlorotris(triphenylphosphine)rhodium in Solution and Its Reaction with Hydrogen. J. Am. Chem. Soc. 1972, 94, 3240−3242. (c) Tanielyan, S. K.; Augustine, R. L.; Marin, N.; Alvez, G. Anchored Wilkinson Catalyst. ACS Catal. 2011, 1, 159−169. (d) Perea-Buceta, J. E.; Fernández, I.; Heikkinen, S.; Axenov, K.; King, A. W. T.; Niemi, T.; Nieger, M.; Leskela, M.; Repo, T. Diverting Hydrogenations with Wilkinson’s Catalyst towards Highly Reactive Rhodium(I) Species. Angew. Chem., Int. Ed. 2015, 54, 14321−14325. (3) (a) Marion, N.; Díez-González, S.; Nolan, S. P. N-Heterocyclic Carbenes as Organocatalysts. Angew. Chem., Int. Ed. 2007, 46, 2988− 3000. (b) Fèvre, M.; Pinaud, J.; Gnanou, Y.; Vignolle, J.; Taton, D. NHeterocyclic carbenes (NHCs) as organocatalysts and structural components in metal-free polymer synthesis. Chem. Soc. Rev. 2013, 42, 2142−2172. (c) Hopkinson, M. N.; Richter, C.; Schedler, M.; Glorius, F. An overview of N-heterocyclic carbenes. Nature 2014, 510, 485−496. (d) Naumann, S.; Dove, A. P. N-Heterocyclic Carbenes as Organocatalysts for Polymerizations: Trends and Frontiers. Polym. Chem. 2015, 6, 3185−3200. (4) (a) Wang, Y.; Robinson, G. H. Unique homonuclear multiple bonding in main group compounds. Chem. Commun. 2009, 5201− 5213. (b) Wang, Y.; Robinson, G. H. Carbene-stabilized main group diatomic allotropes. Dalton Trans 2012, 41, 337−345. (c) Martin, C. D.; Soleilhavoup, M.; Bertrand, G. Carbene-stabilized main group radicals and radical ions. Chem. Sci. 2013, 4, 3020−3030. (d) Sidiropoulos, A.; Jones, C.; Stasch, A.; Klein, S.; Frenking, G. N-Heterocyclic Carbene Stabilized Digermanium(0). Angew. Chem., Int. Ed. 2009, 48, 9701−9704. (e) Henne, F. D.; Dickschat, A. T.; Hennersdorf, F.; Feldmann, K.-O.; Weigand, J. J. Synthesis of Selected Cationic Pnictanes [LnPnX3−n]n+ (L = Imidazolium-2-yl; Pn = P, As; n = 1−3) and Replacement Reactions with Pseudohalogens. Inorg. Chem. 2015, 54, 6849−6861. (f) Cui, H.; Cui, C. Base-stabilized silaimine and its donor-free dimer derived from the reaction of NHCsupported silylene with SiCl4. Dalton Trans 2015, 44, 20326−20329. (g) Nesterov, V.; Reiter, D.; Bag, P.; Frisch, P.; Holzner, R.; Porzelt, A.; Inoue, S. NHCs in Main Group Chemistry. Chem. Rev. 2018, 118, 9678−9842. (5) Flanigan, D. M.; Romanov-Michailidis, F.; White, N. A.; Rovis, T. Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes. Chem. Rev. 2015, 115, 9307−9387. (6) Kuhn, N.; Kratz, T.; Bläser, D.; Boese, R. Carben-Komplexe des Siliciums und Zinns. Chem. Ber. 1995, 128, 245−250. (7) (a) Ghadwal, R. S.; Roesky, H. W.; Merkel, S.; Henn, J.; Stalke, D. Lewis Base Stabilized Dichlorosilylene. Angew. Chem., Int. Ed. 2009, 48, 5683−5686. (b) Filippou, A. C.; Chernov, O.; Schnakenburg, G. SiBr2(Idipp): A Stable N-Heterocyclic Carbene Adduct of Dibromosilylene. Angew. Chem., Int. Ed. 2009, 48, 5687− 5690. (c) Hickox, H. P.; Wang, Y.; Xie, Y.; Wei, P.; Schaefer, H. F., III; Robinson, G. H. Addition of Small Electrophiles to NHeterocyclic-Carbene-Stabilized Disilicon(0): A Revisit of the Isolobal Concept in Low-Valent Silicon Chemistry. J. Am. Chem. Soc. 2016, 138, 9799−9802. (d) Arz, M. I.; Straßmann, M.; Meyer, A.; Schnakenburg, G.; Schiemann, O.; Filippou, A. C. One-Electron Oxidation of a Disilicon(0) Compound: An Experimental and Theoretical Study of [Si2]+ Trapped by NHeterocyclic Carbenes. Chem. - Eur. J. 2015, 21, 12509−12516. (8) Wang, Y.; Xie, Y.; Wei, P.; King, R. B.; Schaefer, H. F., III; von R. Schleyer, P.; Robinson, G. H. A Stable Silicon(0) Compound with a Si = Si Double Bond. Science 2008, 321, 1069−1071. (9) (a) Leszczyńska, K.; Abersfelder, K.; Mix, A.; Neumann, B.; Stammler, H.-G.; Cowley, M. J.; Jutzi, P.; Scheschkewitz, D. Reversible Base Coordination to a Disilene. Angew. Chem., Int. Ed.

germylenes; for instance, arylchlorogermylene is isolable, whereas the silicon analogue needs an NHC-coordination to isolate corresponding arylchlorosilylene.24

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CONCLUSIONS In conclusion, after more than two decades of synthesis of the first NHC−silicon(IV) halides by Kuhn et al., we demonstrated for the first time that the binding between NHC and silicon(IV) halides is an equilibrium reaction and thus provided further experimental support for the description as a donor−acceptor interaction. In this context, with the donor− acceptor stabilized silicon(II)- and germanium(II)-dimethyl complexes, we have also investigated examples of reversible NHC coordination to low-valent group 14 centers. Strikingly, the kinetics of the NHC exchange reaction of NHCiPr2Me2 for NHCMe4 strongly suggest an associative mechanism for the silicon species (second order kinetics), while a dissociative mechanism appears to be active in the case of germanium (first order kinetics). This differing behavior is likely related to the relative electronegativity of silicon and germanium at almost identical atomic radii along with the increased stability of the lower oxidation state in the case of germanium, which is expected to be relevant for mechanistic considerations in group 14 in general.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b00246. Experimental Details (PDF) Accession Codes

CCDC 1875090−1875091 and 1875095 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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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

David Scheschkewitz: 0000-0001-5600-8034 Anukul Jana: 0000-0002-1657-1321 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the Tata Institute of Fundamental Research Hyderabad, Gopanpally, Hyderabad-500107, Telangana, India; SERB-DST (EMR/2014/001237), India; Research Group Linkage Programme of Alexander von Humboldt Foundation, Germany; and Saarland University, Germany.

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REFERENCES

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DOI: 10.1021/acs.inorgchem.9b00246 Inorg. Chem. XXXX, XXX, XXX−XXX