A Geometrically Compelled Disilene with λ4-Coordinate SiII Atoms

Compound 1 exhibits an ambivalent reactivity, reacting as disilene with ethylene, diphenylacetylene and benzophenone to give [2+2] cycloaddition produ...
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A Geometrically Compelled Disilene with #-Coordinate Si Atoms Arseni Kostenko, and Matthias Driess J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.8b11393 • Publication Date (Web): 29 Nov 2018 Downloaded from http://pubs.acs.org on November 30, 2018

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Journal of the American Chemical Society

A Geometrically Compelled Disilene with 4-Coordinate SiII Atoms Arseni Kostenko* and Matthias Driess* Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany.

Supporting Information Placeholder ABSTRACT: The first hypercoordinated disilene with the longest Si=Si distance (2.623(1) Å) reported to date, the 5,6-bis(amidinato silylenyl)acenaphtene (amidinato = PhC(NtBu)2) (1), is presented. It results from a spatially compelled double dative interaction between the two singlet silylene moieties, due to their close proximity. The dynamic behavior of the Si=Si bond contraction and elongation in 1 predicted by quantum chemical calculations could be confirmed experimentally by variable temperature NMR spectroscopy. Compound 1 exhibits an ambivalent reactivity, reacting as disilene with ethylene, diphenylacetylene and benzophenone to give [2+2] cycloaddition products, but as bis(silylene) towards Ni(cod)2 (cod = cycloocta-1,4-diene) to form the corresponding bis(silylene)Ni0(cod) complex.

twisted and bent Si=Si geometry.12 As expected, amidinatosubstituted, intramolecularly donor-supported N-heterocyclic silylenes (NHSi) with three-coordinate SiII atoms such as [PhC(NtBu)2]SiX (X = Cl, aryl, OR, NR2, PR2)13 and the bis-NHSis D14a, E14b (Chart 1) are reluctant to undergo Si=Si dimerization.15,16 To our surprise, we now learned that two NHSi groups can be forced to form a hypercoordinated Si=Si dimer if brought to a much closer distance than that in D and E.

Chart 1. Disilene vs Silylene Features

Si

Si

Si

 type

The ability of heavy Group 14 elements to form multiple bonds was not realized until 1973 when Lappert reported the first stable compound with a Sn-Sn double bond.1 This breakthrough contradicted the paradigm of the so-called “double-bond rule”2 and heralded the existence of many new classes of isolable lowcoordinate Group 14 element species, foremost in 1981 the synthesis of the first stable disilene (Si=Si)3 and silene (Si=C)4 by West and Brook, respectively. In contrast to ethylene, which adopts a planar D2h geometry, disilene has as a trans-bent equilibrium structure in which the Si=Si bond has partly π and charge transfer type character (Chart 1A).5 Therefore, disilene derivatives show an increased reactivity because of their low HOMO−LUMO gap and diradical character of the π bond.6a,b Moreover, disilene derivatives (R2Si=SiR2) can undergo thermal dissociation to two singlet silylene (R2Si:) fragments,6c a reaction, which is unparalleled for olefins; this is in accordance with the Carter-Goddard-MalrieuTrinquier (CGMT) model of heavy olefin analogues (E=E’),7 which predicts that E=E’ bond homolysis is drastically facilitated if the corresponding carbene-analogous fragments [E:] and [E’:] have a relatively large singlet-triplet energy gap (∆EST).7e In this way, incorporation of substituents R that stabilize the singlet state of a R2Si: fragment disfavor the formation of a Si=Si bond. This is especially evident for silylenes bearing amino substituents, since the strong donor-acceptor interaction between the lone-pair orbital of nitrogen with the vacant 3p acceptor orbital at silicon plays a significant role in the ∆EST tuning.7,8 Accordingly, tetraaminodisilenes (and the corresponding diaminosilylenes) prefer the formation of -amino-bridged dimers,9 with the exception of [(iPr2N)2Si:], which gives a weakly bonded Si···Si dimer (Chart 1B) at 77 K with an unusually long Si-Si distance of 2.472 Å.10 N-heterocyclic imino groups can also be used for the stabilization of a iminosilylene,11 which, in turn, affords the corresponding 1,2-diiminodisilene (Chart 1C) with an extremely

Si

2 (i-Pr)2N

It-BuN (TMS)3Si

N(i-Pr)2

Si

charge transfer type

(i-Pr)2N (i-Pr)2N

Si

Si

A

NIt-Bu

N N t-Bu

B t-Bu N N t-Bu

Si(TMS)3

t-Bu N

N(i-Pr)2

2.472 Å N(i-Pr)2

Ph

Si

2.324 Å NIt-Bu

Si

4.316 Å Si

Si O

t-Bu N N t-Bu

Ph

C

Ph

t-Bu N N t-Bu

Si

4.048 Å t-Bu N Si

N t-Bu

Ph

Fe

D

E

Herein, we report the synthesis, structure and reactivity of the first ‘spatially compelled’ hypercoordinated disilene 1 (Eq. 1) with fourcoordinate SiII atoms. Br

Br

1) 2eq. nBuLi/Et2O -40oC to rt

Ph

t-Bu N 2.623 Å Si Si N t-Bu

2) [PhC(NtBu)2]SiCl/Et2O -40oC to rt

t-Bu N N t-Bu

Ph (1)

1

Compound 1 results from salt-metathesis reaction of 5,6dilithioacenaphthene (prepared in situ from 5,6dibromoacenaphthene) with [PhC(NtBu)2]SiCl13a (Eq. 1), and could be isolated in the form of a black powder in 75% yields. An X-ray structure analysis of suitable single-crystals, obtained in diethyl ether solutions at ambient temperature, revealed the Si-Si distance of 2.623(1) Å (Figure 1), which is much longer than the Si-Si distances reported for disilenes (2.138-2.289 Å),5 the recently reported diiminodisilene C (2.324 Å)12 and even the transiently observed dimer of (iPr2N)2Si: (2.472 Å).10 In fact, the Si-Si distance in 1 is almost as long as the longest reported Si-Si single bond in (tBu3Si)2 (2.697 Å).17 Because of the Si-Si interaction in 1, the 29Si NMR shift of  -36.5 ppm is considerably different (high-field shifted) from the related amidinatosilylenes such as [PhC(NtBu)2]

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SiPh ( 30.2 ppm),13b D ( 17.4 ppm),14a and E ( 43.3 ppm),14b indicating an excess of electron density at the SiII atoms.

Figure 1. Molecular structure of 1 (left: top view; right: side view). Thermal ellipsoids are drawn at 50% probability level. H atoms are omitted for clarity. The nature of the Si-Si bonding interaction in 1 was elucidated by quantum chemical calculations. The HOMO exhibits Si lone-pair character, while the LUMO represents character of the Si 3p orbitals conjugated with the π system of the acenaphthene (Figure 2). The HOMO-2 and HOMO-11 show interactions between the Si centers that originate from the mixing of Si lone pairs and the 3p orbitals, thus constituting the σ and π bonds of a charge transfer type disilene (Chart 1 A, right).

Figure 2. Selected molecular orbitals of 1. To validate the Si-Si bonding interaction we employed the Quantum Theory of Atoms in Molecules (QTAIM)18. The QTAIM molecular graph of 1 is shown in Figure 3a displaying a (3,-1) bond critical point (BCP) between the Si atoms with electron density ρ = 0.051 e Bohr-3. Additionally, we studied the Si-Si attractor in 1 using the Localized Orbital Locator (LOL) function,19 which describes the features of bonding in terms of local kinetic energy. In 1 LOL exhibits a high localization region for the Si-Si attractor with ν = 0.674 as a local maximum (Fig. 3b) indicating a presence of a Si-Si bond, which is less localized than that of Mes2Si=SiMes2 (ν = 0.780). The Natural Bond Orbital (NBO) analysis of 1 shows significant n(Si1)→3p(Si2) and n(Si2)→3p(Si1) interactions (Fig. 4) with a donor acceptor stabilization energy of 69.4 kcal mol-1, that constitute a double Si-Si donor-acceptor bond with a low Wiberg Bond Index (WBI) of 0.72, compared with Mes2Si=SiMes2 (WBI = 1.73). Thus, the MO, AIM, and NBO analyses of 1 are consistent with the presence of a Si=Si bond that originates from a double dative interaction between the two NHSi groups, giving rise to the longest reported double donor-acceptor Si=Si distance.

Figure 3. (a) QTAIM molecular graph of 1 focusing on the Si-Si interaction. BCPs are shown in orange. (b) LOL surface map of 1 in the acenaphthene plane. Table 1. Comparison of 1, 1’, 1’’ and Mes2Si=SiMes2 r(Si-Si) (Å) ΔE (kcal mol-1) WBI(Si-Si) ρ (e Bohr-3) LOL ν(Si-Si)

1 2.623 0.0 0.72 0.051 0.674

1’ 2.317 9.9 1.14 0.086 0.678

1’’ 2.205 26.6 1.50 0.101 0.784

[Mes2Si]2 2.160 1.73 0.108 0.780

Figure 4. NBOs of 1. Lone pairs (top) and 3p orbitals (middle) of the SiII atoms interact to form two double donor-acceptor Si=Si bond (bottom). 1 is remarkably stable under exclusion of air and moisture both in the solid state (inert for months at room temperature) and in benzene solutions (no decomposition is observed after heating for 8h at 80°C). The UV-Vis spectrum of 1 in toluene shows absorption in the whole range of the visible spectrum, accounting for its striking black color (Figure S7). According to TD-DFT calculations the absorption originates from transitions at 391.9, 398.3, 404.5, 432.4, 498.6, 550.7, 574.8 and 607.5 nm (full description of the electronically excited states is presented in SI). 1 can be viewed as a cyclic diaryldisilene with additional 2coordinated amidinate ligands at each Si atom (Fig. 5, a). This leads to an extremely twisted structure with dihedral θ(NSiSiN) and CArSiSiCAr angles of 128.3 and 20.3˚, respectively, and a trans-bent geometry with the sum of angles around Si of 299.4˚. Strikingly, the singlet-triplet energy gap in 1 is only 18.2 kcal mol-1, which is much smaller than that of an independent aryl-amidinato-silylene unit such as the [PhC(NtBu)2]SiPh (47.1 kcal mol-1). The singlettriplet gap in 1 is comparable with that of 1,2-diiminodisilene (Chart 1C) 22.8 kcal mol-1.12 This value is higher compared with the twisted (t-Bu2MeSi)2Si=Si(Met-Bu2)2 (7.3 kcal mol-1),20 but the triplet state of 1 may still be accessible at room temperature. The

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Journal of the American Chemical Society effect of hypercoordination in 1 has been elucidated through theoretical calculations of the related isomers 1’ and 1’’, in which one or both of the amidinate ligands are merely 1-coordinated to silicon (Fig. 5 b, c). The change of coordination in 1’ (Fig. 5 b) is accompanied by a substantial contraction of the Si-Si distance to 2.317 Å, a value in the upper range of other previously reported disilenes (2.138-2.324 Å).5,12 This results in increased WBI of 1.14 and electron density ρ = 0.086 e Bohr-3 (Table 1). LOL shows no significant change in electrons localization with ν = 0.678. 1’ is 9.9 kcal mol-1 higher in energy than 1, which expresses the loss of intramolecular donor stabilization by one amidinate ligand. Further change of the second amidinate ligand from 2 to 1 to form 1’’ (Fig. 5 c) leads to an additional contraction of the Si-Si distance to 2.200 Å. 1’’ is 26.6 kcal mol-1 higher in energy than 1 and exhibits even higher values of WBI = 1.50, electron density ρ = 0.101 e Bohr-3 and LOL that shows higher electron localization of the SiSi attractor with ν = 0.784. These values closely resemble those of the classical disilene Mes2Si=SiMes2 (Table 1). (b)

(a) Ph

R 2.623 Å172.9o R N N Si Si N N R R

1

R N

Ph Ph

Si

R N

Si

NR

R N

Ph Ph

N R

1'

Figure 6. Relaxed potential energy surface scan of the elongation and contraction of the Si-Si bond in 1 (green), 1’ (blue), 1’’ (red) and Mes2Si=SiMes2 (yellow).

(c)

2.317 Å 145.1o

NR

2.205 Å

Si

Si

NR

N R

Ph

1''

Figure 5. Geometric parameters of 1 (a) vs the hypothetical mono 1- and 2-amidinato coordinated complex 1’ (b), and bis-1coordinated amidinato complex 1’’ (c). (R=t-Bu). Notably, the 2 coordination mode of the amidinate ligands in 1 leads to a very shallow potential energy surface of the Si-Si contraction and expansion. The fully optimized geometry of 1 with r(Si-Si) = 2.700 Å is negligibly lower in energy than the optimized structure with the experimental Si-Si distance of 2.623 Å. A relaxed surface scan of the Si-Si bond elongation and contraction of ±0.15 Å around the optimized value shows energy differences in range of 0.3 kcal mol-1 (Fig. 6), indicating a dynamic behavior of the Si-Si bond. In comparison, the 1- and 2-amidinato coordinated complex 1’, the bis-1-coordinated amidinato complex 1’’ and the Mes2Si=SiMes2 show much larger energy differences of 2.3, 3.4 and 4.6 kcal mol-1, respectively (Fig. 6). The 29Si NMR chemical shift of 1 at ambient temperature measured in C6D6 solution (δ = -36.5 ppm) and in the solid state (δ = -37.0 ppm) differ from the calculated value of the fully optimized structure (δ = -28.3 ppm) with r(Si-Si) = 2.700 Å. However, for the geometry with r(Si-Si) = 2.623 Å an excellent agreement with the experiment is obtained (δ = -36.0 ppm). It turned out that the 29Si chemical shift of 1 is very sensitive to the Si-Si distance: NMR calculations 21 of 1 with varying Si-Si bond lengths, show a linear correlation between the Si-Si distance and the calculated 29Si chemical shifts (Fig. 7). Additionally, NBO analysis shows that the amount of donor-acceptor n(Si1)→3p(Si2) and n(Si2)→3p(Si1) interactions is also closely related to the Si-Si distance. Thus, shortening of the Si-Si distance increases the double donoracceptor interaction between the Si atoms, resulting in excess of electron density on the 3p orbitals, which is accompanied by a stronger shielding.

Figure 7. Calculated correlation of Si-Si distance vs. 29Si chemical shift (blue) and vs. n→3p donation between the SiII atoms (red). Accordingly, a variable temperature (VT) NMR investigation of 1 in THF-d8 (Figure S8) shows that increasing the temperature results in 29Si chemical shifts at lower fields, going from  -43.4 ppm at 193 K to -38.0 ppm at 333 K, indicating an elongation of the Si-Si distance. In a control experiment, the bis(NHSi) E, in which the SiII atoms are too far apart for an attractive Si=Si interaction, was subjected to the similar VT NMR measurement. The difference of the 29Si chemical shifts between 193K and 333K amounts only 0.7 ppm, compared with  = 5.4 ppm for 1. Next we probed the reactivity of the compelled Si=Si bond in order to examine the ability of 1 to act as a disilene vs. bis-NHSi towards unsaturated organic substrates. In fact, 1 reacts with ethene, diphenylacetylene and benzophenone in [2+2] cycloaddition fashion across the Si=Si bond, similar to the features of other disilenes, to form 2, 3 and 4, respectively (Scheme 1, a-c). This is accompanied by the change of coordination mode from 2 to 1 for both (in 2, 3) or only one of the amidinate ligands (4) and shortening of the Si-Si distances from 2.623(1) Å in 1 to 2.377(8) Å in 2, 2.314(2) in 3 and 2.326(7) Å in 4. Oxidation of 1 with dioxygen or CO2 affords the 1,3-disila-2,4-dioxetane-like molecule 6 (Scheme 1, e). Unlike bis-NHSis that react with transition metals to form bis(NHSi)-metal complexes, disilenes form either side-on π-complexes or metallacycles,22 In the case of 1, the reaction with Ni(cod)2 (cod = cycloocta-1,4-diene) yields the corresponding bis(NHSi)Ni complex 5 (Scheme 1, d) with the Si-Si distance of 3.052(4) Å.

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Scheme 1. Reactivity of 1 H2C CH2 hexane rt 1h (a)

PhC CPh Et2O rt 5h (b)

Ph

R N N R

Si

Si

R N

N R 1 :R = t-Bu

Ph

Ph

Si

Ph

Ph

Ni(cod)2 Et2O rt 12h -cod (d)

Ph

(e)

Si

Ph

Ph NR

R N NR

Ph2CO Et2O rt 1h (c)

O2 or CO2 toluene rt 1h

Si

R N

NR

Si Si Si

R N

2 X-ray Ph Ph Si

NR

Si

Ph N R 3 X-ray Ph Ph R NR N O Si Si N Ph N R R 4 X-ray cod R Ni R N N Si Si Ph N N R R 5 X-ray R N N R

Si

O O

Si

R N N R

Ph

6 X-ray

In conclusion, the close proximity of the two NHSi moieties inflicted by acenaphthene scaffold in 1 enforce the constitution of a charge-transfer type disilene with 4-coordinate SiII atoms. The unique dynamic behavior of this Si=Si bond and the extraordinarily long Si-Si distance of 2.623(1) Å in 1 provide disilene- as well as bis-silylene-like reactivity.

ASSOCIATED CONTENT Supporting Information Experimental procedures, characterizations, crystallographic analyses, and computational data. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Authors *[email protected] *[email protected]

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT We thank the Deutsche Forschungsgemeinschaft for financial support. A.K. is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship. We thank Paula Nixdorf for her assistance in collecting X-ray crystallographic data, Dr. Shenglai Yao for his assistance in structure solution and MarcelPhilip Lücke for his help with the experimental work.

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Journal of the American Chemical Society Leszczynska, K. An Acyclic Imino-Substituted Silylene: Synthesis, Isolation, and its Facile Conversion into a Zwitterionic Silaimine. Angew. Chem., Int. Ed. 2012, 51, 8589−8593. [12] Wendel, D.; Szilvasi, T.; Jandl, C.; Inoue, S.; Rieger. B. Twist of a Silicon−Silicon Double Bond: Selective Anti-Addition of Hydrogen to an Iminodisilene. J. Am. Chem. Soc. 2017, 139, 9156−9159. [13] (a) So, C.-W.; Roesky, H. W.; Magull, J.; Oswald, R. B. Synthesis and Characterization of [PhC(NtBu)2]SiCl: A Stable Monomeric Chlorosilylene. Angew. Chem., Int. Ed. 2006, 45(24), 3948-3950. (b) Mo, Z.; Dr. Szilvási, T.; Zhou, Y.-P.; Yao, S.; Driess, M. An Intramolecular Silylene Borane Capable of Facile Activation of Small Molecules, Including Metal-Free Dehydrogenation of Water. Angew. Chem. Int. Ed. 2017, 56,3 699 –3702. (c) So C.-W.; Roesky, H. W.; , Gurubasavaraj, P. M.; Oswald, R. B.; Gamer, M. T.; Jones, P. G.; Blaurock, Steffen. Synthesis and Structures of Heteroleptic Silylenes. J. Am. Chem. Soc. 2007, 129(39), 12049-12054. [14] (a) Wang, Y; Kostenko, A.; Yao, S.; Driess, M. Divalent SiliconAssisted Activation of Dihydrogen in a Bis(N-heterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins. J. Am. Chem. Soc. 2017, 139(38), 13499-13506. (b) Luecke, M.P; Porwal, D.; Kostenko, A.; Zhou, Y.-P.; Yao, S.; Keck, M; Limberg, C.; Oestreich, M.; Driess, M. Bis(silylenyl)-substituted ferrocene-stabilized η6arene iron(0) complexes: synthesis, structure and catalytic application. Dalt. Trans. 2017, 46(47), 16412-16418. [15] The NHSi described here cannot dimerize to a disilene but dimerizes to the corresponding disilyldiaminodisilene instead. Schmedake, T. A.; Haaf, M.; Apeloig, Y.; Müller, T.; Bukalov, S.; West, R. Reversible Transformation between a Diaminosilylene and a Novel Disilene. J. Am. Chem. Soc. 1999, 121, 9479. [16] Selected recent reviews on silylene chemistry: (a) Kira, M. An isolable dialkylsilylene and its derivatives. A step toward comprehension of heavy unsaturated bonds. ChemComm 2010, 46, 2893. (b) Yao, S; Xiong, Y.; Driess, M. Zwitterionic and Donor-Stabilized N-Heterocyclic Silylenes (NHSis) for Metal-Free Activation of Small Molecules. Organometallics 2011, 30, 1748. (c) Asay, M.; Jones, C.; Driess, M. N-Heterocyclic Carbene Analogues with Low-Valent Group 13 and Group 14 Elements: Syntheses, Structures, and Reactivities of a New Generation of Multitalented Ligands. Chem. Rev. 2011, 111, 354. (d) Sen, S. S.; Khan, S.; Samuel, P. P.; Roesky H. W. Chemistry of functionalized silylenes. Chem. Sci. 2012, 3, 659.

[17] Wiberg, N.; Schuster, H.; Simon, A.; Peters, K. Hexa-tertbutyldisilane–the Molecule with the Longest Si-Si Bond. Angew. Chem. Int. Ed. Engl. 1986, 25, 79-80. [18] Bader, R.F.W. Atoms in Molecules: A Quantum Theory. Oxford University Press, Oxford, 1990. [19] LOL presents a covalent bond as a local maximum of the value ν between bound centers. ν = [0, 1] and attains large values (ν > 1/2) in regions where the electron density is dominated by a localized orbital. (a) Schmider, H.L.; Becke, A.D. Chemical content of the kinetic energy density. J. Mol. Struct: THEOCHEM 2000, 527, 51. (b) Schmider, H.L.; Becke, A.D. Two functions of the density matrix and their relation to the chemical bond. J. Chem. Phys. 2002, 116, 3184. (c) Jacobsen, H. Localized-orbital locator (LOL) profiles of chemical bonding. Can. J. Chem. 2008, 86: 695–702. [20] Kostenko, A.; Tumanskii, B.; Karni, M.; Inoue, S.; Prof. Ichinohe, M.; Sekiguchi A.; Apeloig Y. Observation of a Thermally Accessible Triplet State Resulting from Rotation around a Main‐Group π Bond. Angew. Chem. Int. Ed. 2015, 54, 12144 –12148. [21] Karni, M.; Apeloig, Y.; Takagi, N.; Nagase, S. Ab Initio and DFT Study of the 29Si NMR Chemical Shifts in RSi⋮SiR. Organometallics 2005, 24(26), 6319-6330. [22] (a) Pham, E. K.; West, R. Synthesis and characterization of the first transition-metal .eta.2-disilene complexes J. Am. Chem. Soc. 1989, 111, 7667. (b) Pham, E. K.; West, R. Platinum .eta.2-disilene complexes: syntheses, reactivity, and structures Organometallics 1990, 9, 1517. (c) Hashimoto, H.; Sekiguchi, Y.; Iwamoto, T.; Kabuto, C.; Kira, M. Synthesis and X-ray Structure of a Platinum η2-Disilene Complex. Organometallics 2002, 21, 454. (d) Hashimoto, H.; Sekiguchi, Yu.; Sekiguchi, Y.; Iwamoto, T.; Kabuto, C.; Kira, M. Comparison of structures between platinum and palladium complexes of a tetrasilyldisilene Can. J. Chem. 2003, 81, 1241. (e) Kira, M.; Iwamoto, T.; Sekiguchi, Yu.; Kabuto, C. 14-Electron Disilene Palladium Complex Having Strong π-Complex Character. J. Am. Chem. Soc. 2004, 126, 12778. (f) D. H. Berry, J. H. Chey, H. S. Zipin and P. J. Carroll, J. Am. Chem. Soc., 1990, 112, 452. (g) Hong, P.; Damrauer, N. H.; Carroll, P. J.; Berry D. H. Disilene complexes of molybdenum and tungsten. Organometallics 1993, 12, 3698. (h) Hashimoto, H.; Suzuki, K.; Setaka, W.; Kabuto C.; Kira, M. Iron Complexes of (E)- and (Z)-1,2Dichlorodisilenes J. Am. Chem. Soc. 2004, 126, 13628. (i) Fischer, R.; Zirngast, M.; Flock, M.; Baumgartner, J.; Marschner, C. Synthesis of a Hafnocene Disilene Complex. J. Am. Chem. Soc. 2005, 127, 70.

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