Synthesis of a 1-Aryl-2,2-chlorosilyl(phospha)silene Coordinated by an N-Heterocyclic Carbene.

Phosphasilenes, P=Si doubly bonded compounds, have received considerable attention due to their unique physical and chemical properties. We report on the synthesis and structure of a chlorophosphasilene coordinated by an N-heterocyclic carbene (NHC), which has the potential of functionalization at the Si-Cl moiety. Treatment of a silylphosphine, ArPH-SiCl₂RSi (Ar = bulky aryl group, RSi = Si(SiMe₃)₃) with two equivalents of Im-Me₄ (1,3,4,5-tetramethylimidazol-2-ylidene) afforded the corresponding NHC-coordinated phosphasilene, ArP=SiClRSi(Im-Me₄) as a stable compound. Bonding properties of the P=Si bond coordinated to an NHC will be discussed on the basis of theoretical calculations.

1. Introduction

There has been much interest in the chemistry of multiple bond compounds between heavier main group elements due to their unique π-electron systems different from those of second row elements such as pan class="Chemical">olefin, class="Chemical">pan class="Chemical">imine, and azo compounds. Several homonuclear multiple bond compounds between heavier main group elements have been synthesized as stable compounds by utilizing sterically demanding substituents attached to the reactive π-bond moiety [1]. Since the first isolation of a stable phosphasilene, a Si=P compound, reported by Bickelhaupt in 1984 [2], heteronuclear multiple bond compounds have also attracted many chemists because of their expected unique characters different from those of homonuclear systems [1,3,4]. Especially, phosphasilenes are of great interest because they can act as a strong π-accepting [5,6,7,8,9,10,11] ligands for transition metals. Up to now, several phosphasilenes have been synthesized as stable compounds including a “half-parent” phosphasilene, HP=Si(Tip)(SitBu3) (1) by Driess et al. [12]. Thus, phosphasilenes have generated much interest as heteronuclear π bond compounds between heavier group 14 and 15 elements in the expectation of their variety of electronic, optical, and magnetic properties when they are “functionalized” by further functional groups [13,14,15,16,17,18,19,20]. In this context, Tamao et al. have reported the synthesis of π-functionalized phosphasilenes with organic π-conjugated systems (2, 3) and their Au complexes (Scheme 1) [21,22]. From these point of view, a proper phosphasilene synthon as a “building block” has been desired for synthetic studies of “functionalized” P=Si compounds.

Scheme 1

Examples of stable functionalized phosphasilenes and N-heterocyclic carbene (NHC)-coordinated phosphasilenes.

As a pospan class="Chemical">sible “building block” for a class="Chemical">pan class="Chemical">phosphasilene, some “functionalized” phosphasilenes have been reported. In general, such multiple bond compounds between heavier main group elements are highly reactive, accordingly they are difficult to isolate as stable compounds if they have a functional group instead of a sterically demanding group. In most cases, such functional phosphasilenes have been isolated as a donor–coordinated phosphasilene synthon bearing a strong donor such as an N-heterocyclic carbene (NHC) [23]. Functional phosphasilenes are interesting targets because, for example, the above-mentioned “half parent” phosphasilene 1 [5,6,7,8,9,10,11] has a PH moiety, which should be a nucleophilic building block of the P=Si moiety because of the lone pair of the P atom and its H+–P− polarity. H,N-Functionalized phosphasilene 4 was isolated by Cui et al. [24], where the H,N-functionality afforded nucleophilicity at the P moiety. It should be noted that 4 was generated by the addition of BPh3 to the NHC-coordinated analogue 5. Thus, it can be concluded that an NHC-coordinated phosphasilene should be an appropriate precursor under mild conditions. In addition, Roesky et al. reported cyclic(amino)(alkyl)carbene (CAAC)-coordinated 2,2-dichlorophosphasilene 6 [25], which was described as a carbene-dichlorosilylene stabilized phosphinidene. Moreover, it would be expected to behave as an electrophilic phosphasilene building block synthon because of the Si–Cl moiety, though it would be difficult to remove the CAAC moiety because of its high σ-donating ability [26]. One of the conceivable promising “building blocks” for a phosphasilene should be a phosphasila-vinylidene analogue, RP=Si: which would exhibit an amphiphilic, nucleophilic and electrophilic, character because of its P=Si: moiety. Recently, Filippou et al. reported the synthesis of the NHC-coordinated phosphasila-vinylidene derivative 7 [27], which was synthesized by the reaction of IDipp-SiCl2 (IDipp = 1,3-bis(2,6-diisopropylphenyl)-imidazol-2-ylidene) [28] with Mes*P(SiMe3)Li (Mes* = 2,4,6-tri-t-butylphenyl) [29] via 1,2-elimination of Me3SiCl [30,31,32]. Because of the strong σ-coordination of IDipp, 7 exhibits not a silavinylidene character but the corresponding silyl anion character 7′ which has an isoelectronic structure to a diphosphene. Based on these considerations, we have designed, and report hereafter, a silyl-chloro substituted phosphasilene representing a phosphasilenylidene synthon because of its potential α-silylchloride elimination.

2. Results and Discussions

Treatment of pan class="Chemical">silyl-substituted trichlorosilane 8, (Me3class="Chemical">pan class="Chemical">Si)3Si–SiCl3 [33], with ArPHLi (9, Ar = Tbt or Tbb, Tbt = 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl, Tbb = 2,6-bis[bis(trimethylsilyl)methyl]-4-t-butylphenyl) [34] afforded the corresponding dichloro(phosphino)silanes 10a,b (Scheme 2). The reaction of 10a with MN(SiMe3)2 (M = Li, Na, K) as base that was expected to generate the corresponding phosphasilene ArP=Si(Cl)[Si(SiMe3)3], resulted in no observable reaction. In contrast, treatment of 10a with n- or t-butyllithium gave a complicated and unidentifiable product mixture. When 10a was treated with Im-Me4 (1,3,4,5-tetramethylimidazol-2-ylidene) [35], the NHC-coordinated 1-aryl-2,2-chlorosilylphosphasilene 11a was obtained as a stable orange-red compound. The 31P-NMR spectra (C6D6) of 11a showed singlet signals in the upper-field region (δP = −117), indicating a considerable contribution of phosphanide character represented by the canonical structure 11a′.

Scheme 2

Synthesis of NHC-coordinated phosphasilene 11a.

The structural parameters in the pan class="Chemical">crystalline states of class="Chemical">pan class="Chemical">dichloro(phosphino)silanes 10a,b and NHC-coordinated phosphasilene 11a were determined by the X-ray crystallographic analysis (Figure 1) [36]. The Si–P bond length in 11a [2.1319(11) Å] is apparently shorter than those in the precursors 10a,b [2.2463(8), 2.2465(8) for 10a, 2.2281(8) for 10b] and typical Si–P single bonds. This structural feature suggested some degree of P–Si multiple bond character in 10a despite its Si atom is already saturated due to the Si–P, Si–Si, Si–Cl, and Si–C single bonds. The detailed bond character of P–Si bond in 11a has been revealed on the basis of theoretical calculations.

Figure 1

Molecular structures of (a) 10a (two independent molecules in the unit cell); (b) 10b; and (c) 11a with atomic displacement parameters set at 50% probability. All hydrogen atoms other than those at the P atoms are omitted for clarity and only selected atoms are labeled. Selected bond lengths (Å) (a) 10a: P1–Si1, 2.2463(8); Si1–Si2, 2.3318(8); Si1–Cl1, 2.0636(8); Si1–Cl2, 2.0853(8) (molecule A); P2–Si3, 2.2465(8); Si3–Si4, 2.3370(8); Si3–Cl3, 2.0764(9); Si3–Cl4, 2.0699(9) (molecule B). (b) 10b: P–Si1, 2.2281(8); Si1–Si2, 2.3203(7); Si1–Cl1, 2.0691(7); Si1–Cl2, 2.0672(7). (c) 11a: Si1–P, 2.1319(11); Si1–Cl, 2.1907(11); Si1–Si2, 2.3707(11); Si1–C, 1.1949(3). Numbers in parenthesis correspond to the standard deviation of the values.

Optimized structural parameters of pan class="Chemical">11c at B3PW91/6-31+G(d,class="Chemical">p) level, which has a class="Chemical">pan class="Gene">Bbp group (2,6-[CH(SiMe3)2]2-C6H3) instead of the Tbt group of 11a were found to be in good agreement with those experimentally observed by X-ray crystallographic analysis. NBO (Natural Bond Orbital) calculations [37] of 11c showed two chemical bonds between the P and Si atoms, indicating the existence of a P=Si π bond. The first P–Si bond is σ-type bonding, 54% P (sp5.54) + 46% S (sp1.67), suggesting its typical Si–P σ-bond character. The second one is π-type bonding, 85% P (sp51.0) + 15% Si (sp22.74d13.43). The predominant contribution to this chemical bond seems to be caused by the coordination of the lone pair of P (3p orbital) to the vacant 4d orbital of the Si atom. As it can be deduced from the NBO values (Figure 2), this π-bond is rather weak, i.e., it is mostly localized on the P atom. The latter also reveals the shielding of the nucleus and, hence, provides some explanation for the highfield shifted 31P resonance relative to 11a; this also lends support to the predominant contribution of the canonical structure of 11a′. However, the Wiberg bond index (WBI) of the P–Si bond was computed as 1.42, showing its multiple bond character to some extent. This bonding character should not arise from the steric hinderance exerted by the bulky aryl group and silyl group, because the less hindered model compound 11d was found to exhibit bonding characters similar to those of 11c. That is, the P–Si bond in 11d was composed of σ- and π-type NBOs, 50% P (sp4.92) + 50% Si (sp1.52) and 89% P (sp46.43) + 11% Si (sp38.31d27.66), where the WBI of Si–P bond is 1.37. It can be expected that such π-bonding character between the Si and P atoms would promote the elimination of the Im-Me4 moiety from 11a to give the corresponding donor free phosphasilene with the Si=P double bond. Indeed, the dissociation of the Si–Im-Me4 coordination in 11c to give the corresponding phosphasilene 12c and Im-Me4 were computed as ΔG = +28 kcal/mol [38,39] while the Si–NHC dissociation of the less hindered model 11d to give 12d was highly endothermic (ΔG = +34 kcal/mol), suggesting that the steric demand would reduce the dissociation energy of the Si–NHC coordination (Scheme 3) [40]. Regarding the ideally generated phosphasilene 12c, it was confirmed that it exhibits considerable π-bond character of the P=Si bond on the basis of NBO calculations. That is, the P=Si bond in 12c was composed of σ- and π-bonds, 54% P (sp4.74) + 46% Si (sp1.34) (σ-bond) and 63% P (p1.00) + 37% Si (p1.00d0.17) (π-bond), with WBI = 1.78. At the same time 12d, the less hindered model, exhibits similar P=Si bonding properties with WBI = 1.87, 52% P (sp4.02) + 48% Si (sp1.28) (σ-bond) and 61% P (p1.00) + 39% Si (p1.00d0.02) (π-bond), indicating that the sterically demanding substituents would not perturb the P=Si bonding characters in the phosphasilenes.

Figure 2

NBO (Natural Bond Orbital) calculations for NHC-coordinated and unligated phosphasilenes. (a,b) B3PW91/6-31+G(d,p)//B3PW91/3-21G(d), (c,d) MP2/6-311G(2d). WBI, Wiberg bond index.

Scheme 3

Theoretical calculations of NHC-coordinated and non-coordinated phosphasilenes. (a) B3PW91-D3BJ/6-31+G(d,p)//B3PW91/6-31+G(d,p); (b) MP2/6-311G(2d).

3. Materials and Methods

3.1. General Information

All manipulations were carried out under an pan class="Chemical">argon atmosclass="Chemical">phere uclass="Chemical">pan class="Chemical">sing either Schlenk line techniques or glove boxes. Solvents were purified by the Ultimate Solvent System, Glass Contour Company (Nikko Hansen & Co., Osaka, Japan) [41]. 1H-, 13C-, 28Si-, and 31P-NMR spectra were measured on a JEOL AL-300 spectrometer (JEOL Ltd., Tokyo, Japan) or a Bruker Avance-600 spectrometer (Bruker, Billerica, MA, USA). High-resolution mass spectra (HRMS) were measured on a Bruker micrOTOF focus-Kci mass spectrometer (on ESI-positive mode) (Bruker). Elemental analysis was carried out by using Micro Corder JM10 (J-Science Lab Co., Ltd., Kyoto, Japan) at the Microanalytical Laboratory, Institute for Chemical Research, Kyoto University. TMS3SiSiCl3 was prepared according to literature procedure [33].

3.2. Synthesis

3.2.1. General Procedure for the Synthesis of Phosphinodichlorosilanes 10a and 10b

To a solution of class="Chemical">TbtPH2 or class="Chemical">pan class="Chemical">TbbPH2 (0.6 mmol each) in 5 mL diethyl ether was added 0.412 mL of n-BuLi (0.66 mmol, 1.6 M in n-hexane) at ambient temperature. The reaction mixture was stirred for 1 h followed by the addition of 229.2 mg (0.6 mmol) of (Me3Si)3SiSiCl3 in 5 mL of n-hexane. The solution was further stirred for 6 h. After the reaction was completed (31P-NMR check), all volatiles were removed in vacuo (7 × 10−3 mbar). The crude mixture was dissolved in n-hexane and filtered over celite to remove the inorganic salt. The crude product was recrystallized (in the case of 10a) by slow evaporation of an n-hexane solution at ambient temperature.

Data for 10a: Yield = 78.8 mg (0.083 mmol, 14%), white solid. class="Chemical">1H-NMR (600 MHz, C6D6, 28 °C): δ = 0.18 (s, 18H, class="Chemical">pan class="Chemical">SiMe3), 0.31 (s, 18H, SiMe3), 0.34 (s, 18H, SiMe3), 0.42 (s, 27H, Si(SiMe3)3), 1.48 (s, 1H, para-CH(SiMe3)2), 2.69 (d, 1H, 4JP,H = 2.4 Hz, CH(SiMe3)2), 2.79 (d, 1H, 4JP,H = 5.4 Hz, CH(SiMe3)2), 4.46 (d, 1H, 1JP,H = 224.3 Hz, P-H), 6.66 (s, 1H, C6H5-H), 6.66 (s, 1H, C6H5-H). 13C{1H} NMR (151 MHz, C6D6, 28 °C): δ = 1.0 (s, SiMe3), 1.7 (s, SiMe3), 1.8 (s, SiMe3), 2.6 (s, Si(SiMe3)3), 29.8 (d, 3JP,C = 6.1 Hz, CH(SiMe3)2), 30.1 (d, 3JP,C = 11.3 Hz, CH(SiMe3)2), 30.9 (s, para-CH(SiMe3)2), 121.3 (d, 1JP,C = 17.7 Hz, P-C), 122.9 (s, Ar), 128.3 (s, Ar), 143.8 (s, Ar), 150.1 (d, 2JP,C = 12.1 Hz, Ar), 150.7 (d, 2JP,C = 17.3 Hz, Ar). 29Si{1H} NMR (119.2 MHz, C6D6, 28 °C): δ = −8.9 (s, Si(SiMe3)2), 2.2 (s, CH(SiMe3)2), 3.2 (s, CH(SiMe3)2), 39.5 (d, 1JP,Si = 110.7 Hz, P-SiCl2); central Si-atom of Si(SiMe3)3 was not observed. 31P{1H} NMR (253 MHz, C6D6, 28 °C): δ = −113.5 (ssat, 1JP,Si = 110.7 Hz). HRMS: m/z calcd. for C36H87Cl2PSi11+H+, 931.3444; found, 931.3488. Elemental analysis: calcd. for C36H87 Cl2PSi11: C 46.45, H 9.42 found: C 45.62, H 9.29.

Data for 10b: The class="Chemical">31P-NMR of the reaction mixture revealed the formation of two class="Chemical">products [δ = –115.9 (1JP,H = 223.4 Hz) (10b) and -147.8 (1JP,H = 210.4 Hz)] in 85:15 ratio. The latter class="Chemical">pan class="Chemical">signal was identified to be TbbP(H)SiMe3. Compound 10b precipitated in small amounts only after keeping the crude mixture in n-hexane solution over three weeks at −20 °C; the crystals thus obtained were used for the X-ray diffraction study.

3.2.2. Synthesis of NHC-coordinated Phosphasilene 11a

A quantity of 93 mg (0.1 mmol) of 10a was dissolved in 3 mL of pan class="Chemical">THF and a solution of 25 mg (0.2 mmol) of class="Chemical">pan class="Chemical">1,3,4,5-tetramethyl-imidazol-2-ylidene in 3 mL of THF was added while stirring at ambient temperature. During the addition a solid precipitate was observed and the color turned to orange-red. The solution was stirred for additional 15 min before all volatiles were removed in vacuo (7 × 10−3 mbar). The crude mixture was dissolved in n-hexane and filtered over celite to remove imidazolium salt. After keeping the n-hexane solution at r.t. for one day of the crude mixture some crystals precipitated of which X-ray diffraction and NMR analysis was performed.

Data for 11a: Orange-red solid. class="Chemical">1H-NMR (600 MHz, class="Chemical">pan class="Chemical">toluene-d8, 28 °C): δ = 0.14 (s, 18H, SiMe3), 0.21 (d, 18H, 6JP,H = 1.7 Hz, SiMe3), 0.40 (s, 18H, SiMe3), 0.47 (s, 27H, Si(SiMe3)3), 1.35 (s, 6H, N-Me), 1.43 (s, 1H, para-CH(SiMe3)2), 3.64 (s, 6H, C-Me), 4.09 (s, 1H, CH(SiMe3)2), 4.16 (s, 1H, CH(SiMe3)2), 6.58 (s, 1H, C6H5-H), 6.70 (s, 1H, C6H5-H). 13C{1H} NMR (151 MHz, toluene-d8, 28 °C): δ = 1.2 (d, 5JP,C = 3.0 Hz, SiMe3), 1.5 (s, SiMe3), 1.7 (s, SiMe3), 2.0 (s, SiMe3), 2.2 (s, SiMe3), 3.8 (d, 4JP,C = 3.5 Hz, Si(SiMe3)3), 8.1 (s, C-Me), 29.6 (s, CH(SiMe3)2), 29.9 (s, CH(SiMe3)2), 30.1 (s, para-CH(SiMe3)2), 36.0 (s, N-Me), 121.8 (s, C=C), 126.4 (s, Ar), 137.7 (d, 1JP,C = 69.3 Hz, P-C), 137.9 (d, JP,C = 1.1 Hz, Ar),150.7 (s, Ar), 153.9 (s, C2). 29Si{1H} NMR (119.2 MHz, toluene-d8, 28 °C): δ = −111.9 (d, 2JP,Si = 47.6 Hz, Si(SiMe3)2), −22.4 (d, 1JP,Si = 225.5 Hz, P-SiCl(Im-Me4)), −9.1 (d, 3JP,Si = 8.4 Hz, Si(SiMe3)3), 1.5 (s, CH(SiMe3)2), 1.8 (s, CH(SiMe3)2), 2.1 (s, CH(SiMe3)2). 31P{1H} NMR (253 MHz, toluene-d8, 28 °C): δ = −117.1 (ssat, 1JP,Si = 225.5 Hz). HRMS: m/z calcd. for C43H98ClN2PSi11 + H+, 1019.4685; found, 1019.4689.

3.3. Computational Methods

The level of theory and the bapan class="Chemical">sis sets used for the structural oclass="Chemical">ptimization are contained within the main text. Frequency calculations confirmed minimum energies for all oclass="Chemical">ptimized structures. All calculations were carried out uclass="Chemical">pan class="Chemical">sing the Gaussian 09 Rev. D.01 (Gaussian, Inc., Wallingford, CT, USA) program package. Computational time was generously provided by the Supercomputer Laboratory in the Institute for Chemical Research of Kyoto University.

3.4. X-ray Crystallographic Analysis

pan class="Chemical">Single class="Chemical">pan class="Chemical">crystals of 10a, [10b·C6H14], and 11a were obtained from recrystallization from n-hexane. Intensity data were collected on a RIGAKU Saturn70 CCD system with VariMax Mo Optics (Rigaku, Tokyo, Japan) using Mo Kα radiation (λ = 0.71075 Å). Crystal data are shown in the references. The structures were solved by a direct method (SIR2004 [42]) and refined by a full-matrix least square method on F2 for all reflections (SHELXL-97 [43]). All hydrogen atoms were placed using AFIX instructions, while all other atoms were refined anisotropically. Supplementary crystallographic data were deposited at the Cambridge Crystallographic Data Centre (CCDC; under reference numbers: CCDC–1501067, 1501065, and 1501066 for 10a, [10b·C6H14], and 11a, respectively). Copies of the data can be obtained, free of charge, via www.ccdc.cam.ac.uk/data_request.cif (accessed on 26 August 2016).

4. Conclusions

The pan class="Gene">NHC-adduct of class="Chemical">pan class="Chemical">chlorosilylphosphasilene has been synthesized and structurally characterized. It is evidenced that even the NHC-adduct 11a exhibited P=Si multiple bond character in which, on the basis of theoretical calculations, a d-orbital of the Si atom is involved. Furthermore, the NHC-adduct 11a should be a possible precursor for the corresponding “functionalized” phosphasilene 12a because the dissociation of NHC is computed to be exothermic. Further investigations on the generation of the phosphasilenes 12a and application of 10a and 10b towards utilization as a building block for P=Si moiety are currently in progress.