DOI: 10.1002/chem.201403854

Communication

& EPR Spectroscopy

Disupersilylperoxo Radical Anion [tBu3SiOOSitBu3]· : An Intermediate of Supersilanide Oxidation Alexandra Budanow,[a] Haleh Hashemi Haeri,*[b] Inge Snger,[a] Frauke Schçdel,[a] Michael Bolte,[a] Thomas Prisner,[b] Matthias Wagner,[a] and Hans-Wolfram Lerner*[a] Abstract: In the oxidative process of the supersilanide anion [SitBu3] , radical species are generated. The continuous wave (cw)-EPR spectrum of the reaction solution of Na[SitBu3] with O2 revealed a signal, which could be characterized as disupersilylperoxo radical anion [tBu3SiOOSitBu3]· affected by sodium ions though ion-pair formation. A mechanism is suggested for the oxidative process of supersilanide, which in a further step can be helpful in a better understanding of the oxidation process of isoelectronic phosphanes.

Scheme 1. Reaction of the supersilanide Na[SitBu3] with oxygen and N2O.

Over the past decades, silanides have been widely investigated in fundamental academic research.[1–4] To gain an insight into the stability of the silanides, one must look at their electronic nature first. The mechanism of silanide oxidation by O2 has not been fully elucidated to date, but is a topical subject. We assume that the oxidation process of isoelectronic phosphanes is analogous to the observed mechanism of the reaction between silanides and oxygen. Recently, we have reported that the synthesis of the supersiloxide Na[OSitBu3] can be achieved in two different ways: oxidation of Na[SitBu3][4, 5] with O2 at room temperature (Scheme 1 a) and the reaction of Na[SitBu3][4, 5] with N2O at 78 8C (Scheme 1 b).[6] These findings raise the question about the reaction mechanism underlying the oxidation process of Na[SitBu3].[7, 8] The reaction of Na[SitBu3] with oxygen suggested a mechanism, which is shown in Scheme 2: 1) At first, the silanide [SitBu3] releases one electron to O2 to give the supersilyl radical CSitBu3 (1) and NaO2. 2) In a second step, transient 1 reacts with O2 to form the supersilylperoxy radical COOSitBu3 (2). 3) Reaction of 2 with [SitBu3] gives the disupersilylperoxo radi[a] A. Budanow, I. Snger, F. Schçdel, Dr. M. Bolte, Prof. Dr. M. Wagner, Dr. H.-W. Lerner Institut fr Anorganische Chemie, Goethe-Universitt Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany) Fax: (+ 49) 69-798-29252 E-mail: [email protected] [b] Dr. H. H. Haeri, Prof. Dr. T. Prisner Institute of Physical and Theoretical Chemistry and Center of Biomolecular Magnetic Resonance (BMRZ), Goethe-Universitt Max-von-Laue-Strasse 7, 60438 Frankfurt am Main (Germany) Fax: (+ 49) 69-798-29404 E-mail: [email protected] Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201403854. Chem. Eur. J. 2014, 20, 10236 – 10239

Scheme 2. Oxidative process of the supersilanide anion [SitBu3] in the presence of oxygen.

cal anion [3] . 4) Finally, electron transfer from supersilanide anion [SitBu3] to [3] gives the siloxide [OSitBu3] and 1. Exposure of a 0.4 m solution of Na[SitBu3] in THF to dried air leads to the formation of a radical species. The recorded spectrum consists of a central part and two satellites, which are splitted by 1.15 mT from each other (Figure 1 a). The probability of having a 29Si nucleus (4.67 % natural abundance) can be calculated by using the coefficients of the binominal expansion to be 8.9 %, which is in consistency with the intensity ratio of the two satellites compared with the intensity of the central part of the spectrum. Therefore, the satellites can be assigned to the hyperfine coupling aiso(29Si) of the silicon nuclei. Higher resolved spectra are given in Figure 1 b and c. In these spectra, hyperfine splittings due to the protons can be also observed. The well-resolved spectrum in Figure 1 c shows rather narrow lines with line to line separation of 0.011 mT. To identify this radical, the spectrum was simulated by Easyspin software[9] by using the obtained (hyperfine coupling) values from DFT calculations as initial guesses (Figure 2). The simulated spectrum based on 1 differs unambiguously from the recorded spectrum of the reaction of Na[SitBu3] and O2 (Figure S1 in the Supporting Information).

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Communication DFT calculations on the optimized radical anion structure [3] in THF gave an average aiso(1H) of 0.012 mT and aiso(29Si) of 0.9 mT, which is in good agreement with the experimentally observed hyperfine couplings. The calculated g value (2.0079) and the experimental observed one (2.0081) are also in agreement within experimental error. The optimized structure and spin density of [3] are shown in Figure 2. Figure 3 represents simulations based on structure [3] . As can be seen, the experimental EPR spectrum is quantitatively simulated when, in addition to the 54 equivalent protons of [3] , two sodium ions are considered as well.

Figure 1. a) Cw-X-band (9.4 GHz) EPR spectrum of the reaction of Na[SitBu3] dissolved in THF and dried O2 at RT. High-resolution spectra are shown in b) and c) (recorded at modulation amplitude of 0.01 and 0.005 mT, respectively). Figure 3. Comparison of experimental and DFT-calculated EPR spectra based on structure [3] . Top: simulation with 54 equivalent protons, aiso(1H) = 0.011 mT ([3] ). Bottom: simulation based on DFT calculation of [3] with two equivalent sodium ions, aiso(23Na) = 0.023 mT (Na[3]).

Figure 2. DFT-calculated spin density of optimized structure [3] in THF, at 0.002 a.u. contour level. Hydrogen atoms are omitted for clarity. Selected bond lengths [] and angles [8]: O O 2.30, O Si 1.64, Ca Si 1.98, Ca Cb 1.54; O-O-Si 111, Ca-Si-Ca 110, and see the Supporting Information.

Concerning structure of compound 2, DFT calculations gave an average aiso(1H) value of 0.013 mT and aiso(29Si) of 0.38 mT. The calculated g value of 2.021 is in agreement with an experimentally reported value of 2.029 (for more information on 2, see the Supporting Information).[10] Chem. Eur. J. 2014, 20, 10236 – 10239

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The DFT calculations indicate that more than 90 % of the spin density is located on the oxygen nuclei, implying a huge hyperfine coupling for any atom covalently bond to the structure by the oxygen atoms. This problem can be addressed by using an ion-association concept. Examples of inorganic reactions that are seriously perturbed due to the ion association effect are reported,[11] which more likely occur in low dielectric constant solvents, such as THF. As shown in Figure 3, such a big coupling does not exist. Therefore, we had the idea of a noncovalent (electrostatic) interaction between the sodium metal cations and the anion radical by a solvent-shared ionpair formation (Na[3], Figure 3 bottom).[12] To prove this noncovalent interaction, we changed the metal ion from sodium to lithium, and let Li[SitBu3][4, 5] react with dried O2. As was predicted, we observed a changed continuous wave (cw)-EPR spectra (Figure 4). Simulation of this radical (Li[3]) revealed the presence of two nonequivalent lithium ions, one of them with a significantly higher coupling than the other (0.01 and 0.055 mT, respectively). The hyperfine splitting due to the 54 equivalent protons is 0.011 mT.

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Communication (THF)[OSitBu3]}2 suitable for X-ray diffraction analysis were obtained from a filtered heptane and {Na(THF)2[OSitBu3]}2 from a filtered hexane solution at room temperature (siloxides were prepared from tBu3SiOH with Li[nBu] and K, respectively; see the Supporting Information). The lithium silanolate {Li(THF)[OSitBu3]}2 features a planar four-membered Li2O2 ring (r.m.s. deviation 0.089 ). The O···O distance in {Li(THF)[OSitBu3]}2 is 2.811(4) , and the Li···Li dis-

Figure 4. Experimental RT EPR spectrum of radical Li[3] (exptl), and its simulation by using two nonequivalent lithium ions in addition to the 54 equivalent protons of [3] .

Lithium ions are smaller than sodium ions and form a larger solvation shell. Nevertheless, comparison of the simulated EPR spectra of the metal-containing samples and that of the bare radical anion [3] showed that lithium ions have a stronger impact on the anion radical EPR spectrum than sodium ions (Figure S8 in the Supporting Information). This is in agreement with Li cations having a higher affinity to anions than Na ions and generally tend to form stable contact ion pairs (e.g., Li[(Me3Si)3C Li C(SiMe3)3]), whereas sodium salts commonly reveal ion-separated pairs in donor solvents. Also, one has to keep in mind that [3] carries two bulky supersilyl groups, preventing close contact more for the sodium compared to the smaller lithium ions. These arguments support the experimental observation that Li[3] shows strong additional electrostatic interactions with the metal ions. Furthermore, we found that O2 reacts with potassium supersilanides K[SitBu3] in an analogous way to the reaction of O2 with M[SitBu3] (M = Li, Na), giving the related (end) products K[OSitBu3]. However, M[OSitBu3] (M = Li, Na, K) can be prepared more conveniently: Li[OSitBu3] from tBu3SiOH and Li or Li[nBu]; Na[OSitBu3] from Na[SitBu3] with N2O; K[OSitBu3] from tBu3SiOH and K (see the Supporting Information). These syntheses are more preferable than the oxidation of M[SitBu3] (M = Li, Na, K) with air, because the siloxides M[OSitBu3] (M = Li, Na, K) are thereby obtained in very good yields and high purity, without superdisilane tBu3SiSitBu3 as by-product. In this context, it is worth to mention that chalcogenolates, such as [OSitBu3] are a very interesting class of ligands, because they are used to stabilize transition-metal centers[13] and offer a variety of possible binding modes.[14] Therefore, convenient preparation routes for siloxides have raised interest in recent years. The crystal structure of {Li(THF)[OSitBu3]}2 (orthorhombic, P212121) is shown in Figure 5 and that of {Na(THF)2[OSitBu3]}2 (monoclinic, P21/c) in Figure 6 (the selected bond lengths and angles are listed in the related Figure captions). Crystals of {LiChem. Eur. J. 2014, 20, 10236 – 10239

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Figure 5. Solid-state structure of {Li(THF)[OSitBu3]}2. Displacement ellipsoids are drawn at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected distances [] and angles [8]: Li(1) O(1) 1.843(8), Li(1) O(2) 1.846(8), Li(1) O(11) 1.910(9), Li(1)···Li(2) 2.326(10), Li(2) O(2) 1.815(9), Li(2) O(1) 1.828(8), Li(2) O(21) 1.938(9), Si(1) O(1) 1.589(3), Si(1) C(3) 1.951(4), Si(2) O(2) 1.596(3), Si(2) C(6) 1.942(6), O(1)···O(2) 2.811(4); O(1)-Li(1)-O(2) 99.3(4), O(2)-Li(2)-O(1) 101.0(4), Li(2)-O(1)-Li(1) 78.7(4), Li(2)-O(2)-Li(1) 78.9(4).

Figure 6. Solid-state structure of one of two crystallographic independent molecules of {Na(THF)2[OSitBu3]}2. Displacement ellipsoids are drawn at the 50 % probability level. Hydrogen atoms are omitted for clarity. Selected distances []: Na(1) O(2) 2.241(3) Na(1) O(1) 2.255(3), Na(1) O(41) 2.378(3), Na(1) O(51) 2.384(4), Na(1)···Na(2) 2.902(2), Na(2) O(1) 2.234(3), Na(2) O(2) 2.253(3), O(1)···O(2) 3.363(4), O(1) Si(1) 1.580(3), O(2) Si(2) 1.581(3), Si(1) C(1) 1.925(5), Si(1) C(5) 1.945(6).

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Communication tance is 2.326(10) . The Li cations are coordinated by one THF molecule and the O atoms are bonded to a supersilyl residue. The sodium silanolate {Na(THF)2[OSitBu3]}2 crystallizes with two symmetry-independent molecules in the asymmetric unit. A four-membered Na2O2-ring is formed, which is not as planar as in the Li complex (r.m.s. deviation 0.167 and 0.123 ). The O···O distances in {Na(THF)2[OSitBu3]}2 are 3.363(4)  (3.333(4)  for the second molecule), and the Na···Na distances are 2.902(2)  (2.950(2)  for the second molecule). Although the oxygen atoms are bonded to an Si(tBu3)3 residue as in the Li complex, the Na cations are coordinated by two THF molecules each. As a result of that, two completely different molecules and crystal structures were formed. The THF-supported lithium silanolate crystallizes with one molecule in the asymmetric unit in the chiral space group P212121, whereas the THF-supported Na siloxide crystallizes with two molecules in the asymmetric unit in the centrosymmetric space group P21/c. Although the O···O distances in both silanolates are longer than the O O bond length of the disupersilylperoxo radical anion [3] (the O···O distance in {Li(THF)[OSitBu3]}2 is 0.51  and in {Na(THF)2[OSitBu3]}2 1.05  longer than that in [3] ), the O···O distance in the Li silanolate is significantly shorter than that in the Na siloxide.

Experimental Section All experimental and computational details, together with spectroscopic data, are given in the Supporting Information. CCDC-983899 ({Li(THF)[OSitBu3]}2), CCDC-983898 ({Na(THF)2[OSitBu3]}2), and CCDC938236 (K[OSitBu3]4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.

Acknowledgements We thank Olav Schiemann and Burkhard Endeward for fruitful discussions on experimental settings. All Gaussian 09 calculations were performed on resources provided by the Center for Scientific Computing (CSC) of the Goethe University Frankfurt

Chem. Eur. J. 2014, 20, 10236 – 10239

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by using CPU cluster “Fuchs” for High Performance Computing (FUCHS-HPC). Keywords: density functional calculations · EPR spectroscopy · oxygen · silyl anions · X-ray diffraction

[1] A. F. Holleman, E. Wiberg, Lehrbuch der Anorganischen Chemie, 102ednd edWalther de Gruyter, Berlin, 2007. [2] A. Sekiguchi, T. Fukawa, M. Nakamoto, V. Y. Lee, M. Ichinohe, J. Am. Chem. Soc. 2002, 124, 9865 – 9869. [3] N. Wiberg, Coord. Chem. Rev. 1997, 163, 217 – 252. [4] N. Wiberg, K. Amelunxen, H.-W. Lerner, H. Schuster, H. Nçth, I. Krossing, M. Schmidt-Amelunxen, T. Seifert, J. Organomet. Chem. 1997, 542, 1 – 18. [5] H.-W. Lerner, Coord. Chem. Rev. 2005, 249, 781 – 798. [6] H.-W. Lerner, S. Scholz, M. Bolte, Organometallics 2002, 21, 3827 – 3830. [7] In this context, it is worth to mention that superdisilane tBu3Si SitBu3, the by-product of the reaction between oxygen and Na[SitBu3], is more readily formed by oxidation of the supersilanide Na[SitBu3] with AgNO3 or TCNE. Previous investigations indicated that tBu3Si SitBu3 dissociates above 50 8C back into the supersilyl radicals CSitBu3 ; see Refs. [3–5, 8]. [8] N. Wiberg, H. Schuster, A. Simon, K. Peters, Angew. Chem. 1986, 98, 100 – 101; Angew. Chem. Int. Ed. Engl. 1986, 25, 79 – 80. [9] S. Stoll, A. Schweiger, J. Magn. Reson. 2006, 178, 42 – 55. [10] J. Howard, J. C. Tait, S. B. Tong, Can. J. Chem. 1979, 57, 2761 – 2766. [11] N. Hirota, J. Am. Chem. Soc. 1967, 89, 32 – 41; C. D. Stevenson, E. P. Wagner, R. C. Reiter, Inorg. Chem. 1993, 32, 2480 – 2482; M. J. S. Dewar, M. L. McKee, J. Am. Chem. Soc. 1978, 100, 7499 – 7505; M. J. S. Dewar, Y. Zheng, Inorg. Chem. 1991, 30, 3361 – 3362. [12] The small hyperfine splitting (0.023 mT), which was found in the spectrum of Na[3], showed that Na is not covalently bonded (therefore, no 385 line spectrum). Due to electrostatic interactions, mediated by the solvent, the Na ions, however, affect the anion radical system and cause a further splitting. Considering the overlapping of the narrow lines of the anion radical (line to line separation is only 0.011 mT), the result is a splitting pattern of 33 lines, which is different from the expected pattern of a solely radical anion [3] (theoretically, 55 lines). [13] P. T. Wolczanski, Chem. Commun. 2009, 740 – 757; P. T. Wolczanski, Polyhedron 1995, 14, 3335 – 3362. [14] T. I. Kckmann, M. Hermsen, M. Bolte, M. Wagner, H.-W. Lerner, Inorg. Chem. 2005, 44, 3449 – 3458; T. I. Kckmann, F. Schçdel, I. Snger, M. Bolte, M. Wagner, H.-W. Lerner, Organometallics 2008, 27, 3272 – 3278; T. Kckmann, F. Schçdel, I. Snger, M. Bolte, M. Wagner, H.-W. Lerner, Eur. J. Inorg. Chem. 2010, 468 – 475; F. Meyer-Wegner, M. Bolte, H.-W. Lerner, Inorg. Chem. Commun. 2013, 29, 134 – 137. Received: June 6, 2014 Published online on July 14, 2014

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