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DOI:10.1002/ejic.201500108
Acyclic Amido-Containing Silanechalcogenones Yuk-Chi Chan,[a] Yongxin Li,[a] Rakesh Ganguly,[a] and Cheuk-Wai So*[a]
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Keywords: N ligands / Silicon / Chalcogens / Carbene Homologs Reactions of the amidinate-stabilized silicon(II) bis(trimethylsilyl)amide [LSiN(SiMe3)2], {L = PhC(NtBu)2, 1} with a stiochiometric amount of elemental sulfur, selenium and tellurium afforded the first stable silanechalcogenones comprising an acyclic amido substituent [L{N(SiMe3)2}Si=E] {E = S
(2), Se (3) or Te (4)}. All compounds were characterized by X-ray crystallography and multinuclear NMR spectroscopy. The results illustrate that these compounds possess some silicon–chalcogen double bond character.
Introduction
In contrast, stable monomeric silanechalcogenones bearing an acyclic amido ligand are rare and underdeveloped. For example, the silanoic amide [HC(CMeNAr)2Si(NH2)=O] (Ar = 2,6-iPr2C6H3) moiety comprising an acyclic NH2 substituent, was synthesized by reaction of the DMAP-stabilized heterocyclic silanone complex [HC(CMeNAr){C(=CH2)NAr}Si(DMAP)=O] (DMAP = N,N-dimethylaminopyridine) with dry ammonia.[6d] However, the silanoic amide is unstable in solution and undergoes tautomerization as evidenced by the solid-state structure of K in which the silanoic amide coordinates with its tautomer. Moreover, reaction of the amidinato silylene bearing an acyclic amido substituent [LSiNR2] {EIII, L = PhC(NtBu)2, R = Ph, Me} with N2O cannot afford the monomeric silanones “L(NR2)Si=O”, instead the dimeric siloxane products [LSi(NR2)(μ-O)]2 were formed.[8] So far, only the silanethione [LSi(S)NPh2] comprising the acyclic NPh2 substituent was synthesized by reaction of [LSiNPh2] with PhNCS with elimination of PhNC.[5d] The selenium and tellurium analogues are still unknown. As such, we are interested in investigating whether a bulkier acyclic amido ligand N(SiMe3)2 is able to stabilize four-coordinate silanechalcogenones. Herein, we report the oxidation of the amidinate-stabilized silicon(II) bis(trimethylsilyl)amide [LSiN(SiMe3)2] (1)[9] with chalcogens.
Stable silanechalcogenones with the composition [R2Si=E] (E = O, S, Se, Te) have attracted much attention as they exhibit distinct electronic properties compared with the lighter analogues [R2C=E].[1] Because of poor overlapping of the π orbitals between the silicon and chalcogen atoms, the Si=E bonds are highly polar and reactive. They readily undergo head-to-tail oligomerization reactions at low temperatures. In the past two decades, several research groups utilized the advantages of kinetic and thermodynamic effects of sterically hindered substituents, which are incorporated at the Si atom, to isolate stable compounds comprising a Si=X bond. The Si=E double bond nature in the following examples were elucidated by X-ray crystallography, DFT calculations and by their reactivities. The most spectacular example is the intramolecular N-donor-stabilized diarylsilanethione AI, which was easily synthesized by reaction of the corresponding diarylsilane with CS2 (Figure 1).[2] Following this pioneer work, research groups of Okazaki and Kira synthesized a series of stable donor-free diaryl- (B)[3] and cyclic dialkylsilanechalcogenones (C).[4] In addition, a variety of silanechalcogenones D–L derived by amidinate, β-diketiminate and other N-donor ligands were isolated.[5] More recently, the research groups of Driess, Roesky, Filippou and Ueno illustrate that compounds H–N containing a Si=O skeleton can be stabilized by a Lewis base at the Si atom and/or a Lewis acid at the O atom.[6] Moreover, the base-stabilized silanoic acid, sila-β-lactone and silacyclopropan-1-one were also reported.[7] [a] Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371 E-mail:
[email protected] http://www.ntu.edu.sg/home/cwso/ Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/ejic.201500108. Eur. J. Inorg. Chem. 2015, 3821–3824
Results and Discussion Reactions of 1 with one equivalent of elemental sulfur, selenium and tellurium afforded the amidinate-stabilized amidosilanethione, -selone and -tellone [L{N(SiMe3)2}Si=E] {E = S (2), Se (3) and Te (4), Scheme 1}, respectively, which comprise an acyclic amido ligand. Compound 1 quickly reacted (12 h) with elemental sulfur to give a yellow reaction mixture. In contrast, the reactions of 1 with elemental selenium and tellurium required 2–3 d to consume
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Figure 1. Stable silanechalcogenones.
most of the selenium and tellurium powder, which gave a dark orange and greenish yellow mixture with some black powder, respectively, probably the unreacted selenium and tellurium. The reaction mixtures were then filtered and concentrated to afford compounds 2–4 as thermally stable airand moisture-sensitive colorless and yellow crystalline solids, respectively. The reaction rate of 1 towards elemental S, Se and Te is consistent with their relative oxidizing powers. All compounds are soluble in THF and toluene and were characterized by X-ray crystallography and multinuclear NMR spectroscopy.
Scheme 1. Synthesis of compounds 2–4.
Compound 2 crystallizes in the monoclinic space group P21/c, while 3 and 4 crystallize in the triclinic space group P1¯. They are isostructural, and their molecular structures are shown in Figures S1 and S2 and Figure 2, respectively. The amidinate ligands are bidentately bonded to the Si1 Eur. J. Inorg. Chem. 2015, 3821–3824
atoms, which adopt a tetrahedral geometry. The Si–N bonds [2: 1.7191(13), 3: 1.7168(15), 4: 1.706(5) Å] are comparable with that of [L(NPh2)Si=S] [1.7358(16) Å]. The Si1–S1 bond [1.9935(6) Å] in 2 shows a good agreement with the four-coordinate amidinato silanethiones with an acyclic sulfide [L(StBu)Si=S] [1.984(8) Å][5c] and amido substituent [L(NPh2)Si=S] [1.9814(8) Å].[5d] It is slightly longer than that in the three-coordinate cyclic dialkylsilanethione CI [1.9575(7) Å][4] and the diarylsilanethione BI [1.948(4) Å].[3a] Moreover, the Si1–Se1 [2.1384(5) Å] and Si1–Te1 bonds [2.3720(15) Å] in 3 and 4 are comparable with those in the four-coordinate NHC-stabilized N-heterocyclic silaneselone and -tellone [FIII: 2.1457(9); FIV: 2.383(2) Å], respectively.[5e] They are intermediate between those in the three-coordinate cyclic dialkylsilanechalcogenones [CII: 2.0963(5); CIII: 2.3210(6) Å][4] and the fivecoordinate bis(amidinato)chalcogenones [DII: 2.1632(7) Å, DIII: 2.4017(6) Å].[5a] These indicate a considerable double bond character in the Si=E bonds in 2–4. The Si1–N3 bonds in 2–4 [1.7191(13) Å (2), 1.7168(15) Å (3), 1.706(5) Å (4)] are slightly shorter than that in 1 [1.769(7) Å], as the oxidation state of silicon atoms increases from +2 to +4. Compounds 2–4 display a set of resonances assignable to the amidinate ligand and SiMe3 substituents. The 29Si NMR spectra of 2–4 display resonances for the Si atom in the Si=X bonds (2: δ = –16.9, 3: –20.0, 4: –47.6 ppm) and in the SiMe3 substituents (2: δ = 1.1, 7.2; 3: 2.1, 8.2; 4: 3.6,
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Figure 2. Molecular structure of [L{N(SiMe3)2}Si=Te] (4) (30 % ellipsoids probability). Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [°]: Si1–Te1 2.3720(15), Si1–N1 1.830(5), Si1–N2 1.829(5), Si1–N3 1.706(5), N1–Si1–N2 71.9(2), N1–Si1–N3 113.0(2), N2–Si1–N3 113.7(2), N1–Si1–Te1 112.92(17), N2–Si1–Te1 112.2(16), N3–Si1–Te1 122.40(17), N1–C5–N2 107.0(4).
9.9 ppm). The 29Si NMR resonances for the Si=X bonds show an upfield shift compared with those of three-coordinate silanechalcogenones B (I: δ = 166.56; II: 174 ppm) and C (I: δ = 216.8; II: 227.7; III: 229.5 ppm).[3,4] Their negative chemical shifts are similar to those of four-coordinate silanechalcogenones (EI: δ = –17.5 ppm; FIII: –38.4, 39.1 ppm; G: –30.3 ppm; FIV: –51.8, –52.2 ppm).[5b,5e,5g] In addition, the 29Si NMR spectra of 3 and 4 show satellites due to the coupling of silicon atom with selenium (1J29Si-77Se = 284 Hz) and tellurium (1J29Si-125Te = 492 Hz) atoms, respectively. The coupling constants of 3 and 4 are larger than that in the silaneselenide and -telluride containing bridging Si–E single bonds [(Me3Si)3Si]2E (E = Se, 151.2 Hz; Te: 347.2 Hz).[10] Moreover, the 77Se and 125Te NMR spectra of 3 and 4 display one signal at δ = –451 and –1093 ppm, which are comparable to D (I: δ = –496; II: –1199 ppm)[5a] and F (III: δ = –470 ppm; IV: = –1010 ppm),[5e] respectively. They are upfield shifted compared with that of the three-coordinate silaneselone (BII: δ = 635 ppm) and -tellone (BIII: δ = 731 ppm).[3] Furthermore, the considerable Si=E double-bond character is supported by the π씮π* and n씮π* transition absorptions in the UV/Vis spectra of 2, 3 (333, 348 nm) and 4 (352, 368 nm). Both π씮π* and n씮π* transition bands of 2 and 3 are red-shifted as compared with 4. Similar absorption bands and red shift can also be observed in L, which have multiple bond character in the Si=E bonds.[5j] As a result, these results, along with the Si=E bond lengths and NMR spectroscopic data, demonstrate that the character of the Si–E bonds in 2–4 are intermediate between a Si=E double bond and a strongly delocalized Si+–E– single bond.
Conclusions The first amidinate-stabilized silanechalcogenones containing an acyclic amido substituent were prepared by the straight-forward oxidation of the corresponding amidinato silicon(II) bis(trimethylsilyl)amide with chalcogens. Their Eur. J. Inorg. Chem. 2015, 3821–3824
X-ray structures and NMR spectroscopy conclusively show that these compounds possess some silicon–chalcogen double bond character.
Experimental Section General Procedures: All manipulations were carried out under an inert atmosphere of argon gas by standard Schlenk techniques. THF and toluene were dried and distilled from Na/K alloy prior to use. [D6]benzene was dried and distilled from K metal prior to use. LSiN(SiMe3)2 was prepared according to the literature procedure.[9] S8, Se and Te powder were purchased from Alfa Aesar and used without further purification. The 1H, 13C, 29Si, 77Se and 125 Te NMR spectra were recorded on a JEOL ECA 400 spectrometer. The NMR spectra were recorded in C6D6, and the chemical shifts are relative to SiMe4 for 1H, 13C, 29Si, Ph2Se for 77Se and Ph2Te for 125Te, respectively. Elemental analyses were performed by the Division of Chemistry and Biological Chemistry, Nanyang Technological University. Melting points were measured in sealed glass tubes and were not corrected. Preparation of [L{N(SiMe3)2}Si=S] (2): A solution of LSiN(SiMe3)2 (0.220 g, 0.52 mmol) in THF (10 mL) was added to a suspension of S8 powder (16.8 mg, 0.0655 mmol) in THF (10 mL) at room temperature. The yellow solution was stirred at room temperature for 12 h. Volatiles in the mixture were removed under reduced pressure and the residue was extracted with toluene. After filtration and concentration of the filtrate, compound 2 was obtained as colorless crystals. Yield: 101 mg (42.3 %). M.p. 268.9 °C. 1 H NMR (395.9 MHz, 25 °C, [D6]benzene): δ = 0.42 (s, 9 H, SiMe3), 0.86 (s, 9 H, SiMe3), 1.25 (s, 18 H, tBu), 6.81–6.94 (m, 3 H, Ph), 7.07–7.09 (d, 1 H, Ph), 7.18 ppm (s, 1 H, Ph). 13C{1H} NMR (99.5 MHz, 25 °C, [D6]benzene): δ = 5.0, 6.8 (SiMe3), 30.7 (CMe3), 55.0 (CMe3), 127.0, 130.0, 130.1 (Ph), 173.3 ppm (NCN); 29 Si{1H} NMR (78.65 MHz, 25 °C, [D6]benzene): δ = 1.1, 7.2 (SiMe3), –16.9 ppm (NSiN). C21H41N3SSi3 (451.9): calcd. C 55.82, H 9.15, N 9.30; found C 55.64, H 8.91, N 9.20. UV/Vis (THF): λmax = 348, 333 nm. Preparation of [L{N(SiMe3)2}Si=Se] (3): A solution of LSiN(SiMe3)2 (0.220 g, 0.52 mmol) in THF (10 mL) was added to a suspension of Se powder (53 mg, 0.67 mmol) in THF (10 mL) at room
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www.eurjic.org temperature. The yellow suspension was stirred at room temperature for 1 d to form an orange mixture with some black precipitate. After addition of toluene (10 mL), the mixture was filtered. After concentration of the filtrate, compound 3 was obtained as colorless crystals. Yield: 144 mg (27.5 %). M.p. 268.0 °C. 1H NMR (395.9 MHz, 25 °C, [D6]benzene): δ = 0.43 (s, 9 H, SiMe3), 0.87 (s, 9 H, SiMe3), 1.29 (s, 18 H, tBu), 6.82–6.95 (m, 3 H, Ph), 7.08–7.10 (d, 1 H, Ph), 7.18 ppm (s, 1 H, Ph). 13C{1H} NMR (99.5 MHz, 25 °C, [D6]benzene): δ = 5.2, 7.1 (SiMe3), 30.8 (CMe3), 55.3 (CMe3), 127.2, 130.1, 130.5 (Ph), 172.7 ppm (NCN). 29Si{1H} NMR (78.65 MHz, 25 °C, [D6]benzene): δ = 2.1, 8.2 (SiMe3), –20.0 ppm (1J29Si-77Se = 284 Hz) (NSiN). 77Se{1H} NMR (76.3 MHz, 25 °C, [D6]benzene): δ = –451 ppm. UV/Vis (THF): λmax = 348, 333 nm. Preparation of [LSi{N(SiMe3)2}=Te] (4): A solution of LSiN(SiMe3)2 (0.210 g, 0.50 mmol) in THF (10 mL) was added to a suspension of Te powder (68 mg, 0.53 mmol) in THF (10 mL) at room temperature. The yellow suspension was stirred at room temperature for 3 d to form a greenish yellow mixture with some black precipitate. Volatiles in the mixture were removed under reduced pressure, and the residue was extracted with toluene. After filtration and concentration of the filtrate, compound 4 was obtained as yellow crystals. Yield: 98 mg (36.0 %). M.p. 223.8 °C (decomposed). 1H NMR (395.9 MHz, 25 °C, [D6]benzene): δ = 0.41 (s, 9 H, SiMe3), 0.92 (s, 9 H, SiMe3), 1.34 (s, 18 H, tBu), 6.83–6.94 (m, 3 H, Ph), 7.15–7.18 ppm (d, 2 H, Ph). 13C{1H} NMR (99.5 MHz, 25 °C, [D6]benzene): δ = 5.5, 7.8 (SiMe3), 31.1 (CMe3), 55.7 (CMe3), 127.0, 130.2, 130.6 (Ph), 171.4 ppm (NCN). 29Si{1H} NMR (78.65 MHz, 25 °C, [D6]benzene): δ = 3.6, 9.9 (SiMe3), –47.6 ppm (1J29Si-77Se = 492 Hz) (NSiN). 125Te{1H} NMR (126.24 MHz, 25 °C, [D6]benzene): δ = –1093 ppm. UV/Vis (THF): λmax = 368, 352 nm. X-ray Data Collection and Structural Refinement: Intensity data for compounds 2–4 were collected by using a Bruker APEX II diffractometer. The crystals of 2–4 were measured at 103(2) K. The structures were solved by a direct phase determination (SHELXS2013) and refined for all data by full-matrix least-squares methods on F2. All non-hydrogen atoms were subjected to anisotropic refinement. The hydrogen atoms were generated geometrically and allowed to ride on their respective parent atoms; they were assigned appropriate isotopic thermal parameters and included in the structure-factor calculations. CCDC-1044014, CCCD-1044015 and CCDC-1044016 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via http:// www.ccdc.cam.ac.uk/data_request/cif. Supporting Information (see footnote on the first page of this article): X-ray structures of 2 and 3 and the crystallographic data of 2–4 are presented.
Acknowledgments This work was supported by the AcRF Tier 1 grant (RG 22/12).
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