Low-Energy Electronic Transition in SiB Rings - Organometallics (ACS

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Low-Energy Electronic Transition in SiB Rings Tapas K. Purkait, Eric M. Press, Eric A. Marro, Maxime A. Siegler, and Rebekka S. Klausen* Department of Chemistry, Johns Hopkins University, 3400 N. Charles Street, Baltimore, Maryland 21218, United States

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S Supporting Information *

ABSTRACT: Five- and six-membered rings containing Si−B bonds were synthesized by salt metathesis of magnesium disilanides with tetramethylpiperidinyldichloroborane (TMPBCl2). Materials were characterized by 1H, 11B, 13C, and 29Si NMR spectroscopy, as well as X-ray crystallography. Insights from crystallography facilitate conformational analysis of the cyclosilanes and elucidation of conformation-dependent optical properties. Crystallography also supports assignment of π character to the BN bond of the tetramethylpiperidinylborane fragment. The SiB rings have unique optical properties compared to all silicon rings. A 350 nm electronic transition only observed in SiB rings is assigned to a σ−π* transition on the basis of density functional theory calculations.

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INTRODUCTION The incorporation of main group elements such as B, P, and Se into conjugated materials promises the ability to modulate electronic transitions while retaining atomic-level structural precision.1,2 A fundamental understanding of the influence of the main group substituent on molecular properties is essential for the design and synthesis of tailored structures. In the case of boron-containing π-conjugated materials, p−π conjugation is invoked in which conjugation is extended through the vacant p-orbital of boron.3−5 Boron-functionalized π-conjugated materials have found wide application,6 based on the unique properties conferred by boron that can be exploited for sensing,7−10 nonlinear optics,11−13 and n-type charge transport.14−16 Far less is understood about the electronic properties of boron-functionalized σ-conjugated silicon materials.17−19 Sodium-mediated Wurtz coupling of boron and silicon halides yielded a SiB copolymer and ceramic precursor.20 Matsumi et al. reported the dehydrocoupling copolymerization of phenylsilane (PhSiH3) with mesitylborane (MesBH2), yielding colorless low-molecular-weight polymers.21 DFT calculations suggested p-σ type conjugation, in which orbital density delocalized uniformly across a linear Si−B−Si chain. Theoretical studies of B-substituted polysilanes suggested a lowering of the band gap upon heteroatom substitution.22 The synthesis of molecular monosilylboranes 23,24 is better precedented than polysilylboranes, and these compounds have found wide utility as reagents in organic synthesis.25 With respect to optoelectronic properties, Rayez et al. reported the synthesis of compounds with the general formula Ph2RSi− BMes2 that exhibited solvatochromic emission attributed to charge transfer character.26 Hybrid σ−π organosilanes were also shown to exhibit photoinduced charge transfer.27−29 Doping in silicon, in which the intentional introduction of impurity atoms modulates the electronic properties of the © XXXX American Chemical Society

intrinsic semiconductor, inspires the pursuit of boronfunctionalized molecular cyclosilanes. We report the synthesis and optical characterization of two new SiB rings (Si4B and Si5B), as well as all silicon analogs. Si5B is the first example of a six-membered SiB ring. The possibility of two mechanisms for electron delocalization, σ−p conjugation and charge transfer, motivates the detailed characterization of optoelectronic properties. A low-energy electronic transition is uniquely observed in the SiB rings. Computational studies support assignment of this feature to a σ → π* transition from the Si framework to the BN π*. The LUMO’s of the SiB rings are lowered due to the presence of boron, but the HOMO’s are also raised in energy, suggestive of p−σ conjugation through the BN system. Conformational effects on σ-conjugation are discussed.

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RESULTS AND DISCUSSION Synthesis. The targeted SiB rings were synthesized by coupling nucleophilic disilanides 1a,b with tetramethylpiperidinyldichloroborane 2. We previously reported 1a,b as intermediates in the synthesis of site-specifically functionalized cyclohexasilanes via coupling to chlorosilanes.30−32 The cyclohexasilanes are precursors to poly(cyclosilane)s, a class of conjugated polymers inspired by crystalline silicon.30,32,33 The addition of magnesium salts to the coupling reaction suppresses undesired fragmentation by modulating silanide nucleophilicity.32,34 As observed in the cyclohexasilane syntheses, the addition of magnesium bromide to the dipotassium disilanides results in cleaner reactions (Scheme 1). Five-membered ring Si4B was isolated in 74% yield, and six-membered ring Si5B was isolated in 34% yield. In the case of Si5B, a change in solvent from Received: November 2, 2018

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DOI: 10.1021/acs.organomet.8b00804 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

observed between substituents on boron and nitrogen. The twisting is attributed to relieving unfavorable steric interactions between the TMP methyl substituents and the phenyl substituents on silicon. The five-membered ring of Si4B adopts an envelope-like conformation (Figure 2c). Overall, the structure of Si4B is similar to the related five-membered SiB cycle reported by Marschner et al., which also features an envelope-like conformation.36 We find that Si5B adopts a boatlike conformation (Figure 2d). The structural parameters of the BN bond are again consistent with a double bond. Bond angles around the B atom range between 108.5 and 126.0°, with the endocyclic Si−B−Si angle falling at the lower end of the range. The sum of all bond angles is 360°, indicating a trigonal planar boron center. The BN bond is short (1.409(3) Å), and a ca. 30 torsion angle indicates a twisted double bond. Cyclosilanes Si5 and Si6 exhibit typical SiSi bond lengths and endocyclic bond angles (Table 2). No evidence of ring strain is apparent. Si5 crystallizes in an envelope conformation, while Si6 adopts a chair conformation in the solid state (Figure 1c,d). Density Functional Theory (DFT) Calculations: Geometry and Electronic Structure. DFT calculations elucidate the influence of boron incorporation on the properties of the SiB rings. Geometry optimizations were performed (CAM B3LYP/6-31G(d)). The SiB compounds converged to structures similar to the crystal structures. The BN bond possesses double-bond character as assessed by bond lengths and the sum of the bond angles (Figure S1). Predicted ring conformations are also similar to the X-ray crystal structures with Si4B adopting an envelope-like conformation and Si5B adopting a boat-like conformation. Isodensity plots of the HOMO−1, HOMO, and LUMO of all four compounds and associated orbital energies are shown in Figure 2. In all cases, the HOMO and HOMO−1 are close in energy and concentrated on the cyclic framework. However, significant differences are observed in the LUMO’s of SiB and Si rings. The LUMO’s of the SiB rings are on the BN bond and have π* symmetry. In both cases, the LUMO energy is 0.5 eV lower in energy than that of the cyclosilane of comparable ring size. An isodensity plot shows that the LUMO of Si5 is significantly delocalized across both the σ framework and the exocyclic phenyl rings. The LUMO of Si5 has π symmetry: Above and below the ring, fused, delocalized orbitals reminiscent of benzene are observed. This π symmetry may aid in mixing with the exocyclic phenyl rings. The LUMO of cyclohexasilane Si6 is also found on both the Si and Ph groups. Figure S2 depicts the frontier molecular orbitals of TMPBH2. The LUMO+1 of TMPBH2 is also localized on the BN bond and has π* symmetry. The energy of this orbital (+0.13 eV) is much higher than the LUMO energies of Si4B and Si5B. Isodensity plots of the highest occupied molecular orbitals in the SiB rings shows that in the HOMO−1 and HOMO orbital density is localized on the ring system. In Si4B, orbital densities in the HOMO−1 are delocalized along the Ph2Si− (SiMe2)2−SiPh2 backbone. Minimal orbital density is seen on exocyclic methyl or phenyl substituents. In the HOMO, orbital density is on the Ph2Si−B−SiPh2 fragment, suggesting p−σ delocalization. Si5B shows the same alternation between the Si and Si−B−Si fragments in the HOMO−1 and HOMO. The HOMO and HOMO−1 of the Si5 cyclosilane also alternate between halves of the ring system, an effect that is

Scheme 1. Synthesis of Si−B Rings and Chemical Structure of Cyclosilane Analogs Si5 and Si6a

Synthesis of Si−B rings: (a) five-membered and (b) six-membered. (c) Chemical structure of cyclosilane analogs Si5 and Si6. 18-cr-6 = 18-crown-six. a

THF to toluene suppressed the formation of 1,1,2,2tetraphenylhexamethylcyclopentasilane, an undesired intramolecular coupling product. Si4B and Si5B are indefinitely stable to storage in a nitrogen atmosphere glovebox. Neat samples partially decomposed in air within a 5 h period to an unidentified mixture of products. Hengge reported the first synthesis of a five-membered SiB ring via coupling of a dianion to a chloroboranes, a strategy later employed by Marschner as well.35,36 Neither Hengge nor Marschner reported the synthesis of six-membered SiB cycles, and a CSD search suggests such a structure has not been crystallographically characterized. All silicon analogs Si5 and Si6 were synthesized by treating 1a,b with magnesium bromide, followed by dichlorodimethylsilane (Scheme 1c). Si5 is a novel compound, while we previously reported the synthesis and X-ray crystal structure of Si6 as an intermediate in the synthesis of macrocyclic polysilanes.32 Single Crystal X-ray Crystallography. Single crystals of Si4B and Si5B suitable for X-ray structure determination were grown from solution at −30 °C (see the Experimental Section for details). Crystal structures are shown in Figure 1. Table 1 summarizes selected structural parameters. In Si4B, the BN bond has double-bond character. Bond angles between 120−128° are observed around the BN bond, with the exception of the smaller, endocyclic Si−B−Si angles. A trigonal planar geometry is observed at boron (sum of all bond angles around B is 360°). The BN bond length of ca. 1.408(2) Å is significantly shorter than the 1.58(2) Å BN bond distance of ammonia borane (H3N−BH3).37 Our observed short BN bond distance is consistent with other aminoboranes with π-bond character.38 The tetrasubstituted BN bond is modestly twisted from planarity, with a ca. 30° torsion angle B


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Organometallics

Figure 1. (a, b) X-ray crystal structures of Si4B and Si5B. Hydrogens omitted for clarity. Side-on views of ring conformations in Si4B and Si5B are shown. For clarity, in side-on views, methyl and phenyl groups and hydrogens are omitted. (c, d) X-ray crystal structures of Si5 and Si6. Hydrogens omitted for clarity. Side-on views of ring conformations in Si5 and Si6 are shown. Blue = silicon; black = carbon; purple = boron; light blue = nitrogen. Displacement ellipsoids shown at 50% probability level. Carbon atoms of tetramethylpiperdine ring are shown as hollow ellipsoids. Selected elements are labeled for reference to Tables 1 and 2 C


Article

Organometallics Table 1. Selected Geometric Parameters for the Experimental Crystal Structures of Si4B and Si5B Si4B bond lengths (Å)

bond angles (deg)

torsion angles (deg)

B−N Si(1)−B Si(4)−B Si(1)−B−Si(4) Si(1)−B−N Si(4)−B−N C(33)−N−B C(29)−N−B Si(1)−B−N−C(33) Si(4)−B−N−C(29)

Si5B 1.408(2) 2.0717(17) 2.1174(17) 105.44(7) 126.15(11) 128.41(11) 123.78(12) 120.97(12) −32.90(19) −30.42(19)

B−N Si(1)−B Si(5)−B Si(1)−B−Si(5) Si(1)−B−N Si(5)−B-N C(31)−N−B C(35)−N−B Si(1)−B−N−C(35) Si(5)−B−N−C(31)

1.409(3) 2.065(2) 2.131(3) 108.50(11) 126.04(18) 125.43(16) 121.38(18) 123.83(18) 34.4(3) 37.7(3)

the photophysics of cyclic oligosilanes are less well-studied than linear oligosilanes, although hybrid C−Si and mediumsized rings have been used to probe conformational effects.48,49 The photophysical properties of short chain linear oligosilanes with the general formula SinMe2n+2 (n = 4, 6) are well-studied as model systems for higher polysilanes. At room temperature in solution, the anti43 conformation is typically dominant. In the anti conformation, the highest occupied molecular orbital (HOMO) has σ symmetry and is delocalized across the silicon framework, and the lowest unoccupied molecular orbital (LUMO) has similar symmetry and delocalization.49−52 Changes in conformation disrupt σorbital conjugation. In addition, the symmetry of the LUMO changes from σ to π, so the lowest energy electronic transition is σπ* instead of σσ*. The combination of these effects results in significantly hypsochromically shifted absorption in cisoid linear oligosilanes. The enforced cisoid conformation of the 5-membered cyclosilanes (Figure 5) suggests that their photophysical properties should be more comparable to shorter chain systems than linear silanes with the same number of silicon atoms. Indeed, the 195 nm transition in Si4B and Si5 is comparable to hexamethyldisilane (Si2Me6) and blue-shifted relative to decamethyltetrasilane (n-Si4Me10). DFT calculations on disilanes indicate that short chain systems are unique relative to longer oligosilanes.53 While the HOMO in all cases is σ(SiSi), in disilanes the LUMO includes contributions from SiC antibonding orbitals. Isodensity plots of the LUMO’s in Si5 and Si6 show contributions from both Si and Ph rings. The ca. 195 nm transition in six-membered rings Si5B and Si6 is assigned to the oligosilane backbone by analogy. Phenyl-Terminated Oligosilanes. The broad ca. 250 nm transition observed in all four rings is attributed to the exocyclic phenyl rings. As first reported by Gilman in the 1960s, phenyl-substituted oligosilanes have bathochromically shifted onset of absorbance compared to methyl-functionalized systems.54,55 The contribution of σ,π-conjugation to these phenomena has been previously described.56,57 Mixing of Ph and Si orbitals is apparent in our frontier molecular orbital calculations (Figure 2). Solvatochromism. No changes in UV−vis absorbance spectra were observed in several solvents investigated (Figure S3). As excited states with charge transfer character typically show solvatochromic emission but not solvatochromic absorbance, the emissive properties of all cyclosilanes were investigated. Fluorescence spectroscopy was attempted in a variety of solvents, however weak to no emission was observed for all ring systems in this study (Figure S4). Permethylated diand trisilanes are not emissive materials, unlike longer

likely due to the enforced bent conformation (vide infra). The Si6 HOMO and HOMO−1 are more evenly distributed around the Si framework. In both materials, the HOMO and HOMO−1 are σ(SiSi) in nature. Our calculations predict that SiB rings should have significantly smaller HOMO−LUMO gaps and red-shifted electronic transitions compared to all silicon rings due to a combination of the HOMO-raising effect of p−σ delocalization and the LUMO-lowering effect of the low-lying BN π*.39−41 In the following sections, experimental and theoretical evidence for the unique Si−B low-energy electronic transition is presented. UV−Vis Spectroscopy and Time-Dependent DFT (TDDFT). UV−vis absorption spectra of SiB compounds were recorded in n-pentane at room temperature to obtain the widest possible spectral window (Figure 3a,b). UV−vis spectra of cyclosilane control compounds were recorded under similar conditions (Figure 3c,d). All four compounds have absorption bands at ca. 195 and 250 nm, but only the SiB rings have an additional weak feature at ca. 350 nm (Figure 3e,f and Table 3). The broad low-energy transition unique to Si4B and Si5B must involve the low-lying orbitals associated with the BN π*. The ca. 195 nm transition is assigned to energy states associated with the oligosilane backbone, while the ca. 250 nm transition is associated with the phenyl rings. These assignments are consistent with our DFT calculations, which show that SiB rings have significantly lower energy LUMO’s than all Si rings (Figure 2) in which orbital density is centered on the BN π* orbital. The weakness of the ca. 350 nm transition (ε ∼ 1000 L mol−1 cm−1) is consistent with a formally disallowed electronic transition, such as σ → π*. Assignment of the higher energy features in the UV−vis absorbance spectrum is consistent with prior work on oligosilanes. Cisoid Oligosilane. Oligosilane conformation strongly influences σ-electron delocalization, with corresponding effects on molecular orbital structure and energies, electronic transitions, and photophysical properties.42 Oligosilanes populate more conformations than alkanes and Michl and West have proposed an expanded nomenclature for oligosilane conformations based on the torsion angle ω (Figure 4).43 The preferred conformations are substituent-dependent. In permethyloligosilanes, anti and gauche/ortho conformations are both energy minima.43,44 σ-Conjugation, or the stabilization afforded by the interaction of σ-orbitals, is strongest in the fully extended anti conformation, while cisoid kinks disrupt σconjugation.45 The Ladder C model provides an explanation for the observed conformation dependence.46,47 We note that D


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Organometallics

Figure 2. Frontier molecular orbitals of Si4B, Si5, Si5B, and Si6 calculated at the PBE0/6-311+G(2d,p)//CAM-B3LYP/6-31G(d) level. Hydrogens omitted for clarity. Molecular orbitals visualized with an isodensity value of 0.04. The HOMO−1’s and HOMO’s of all compounds have the largest density on the ring system. The largest LUMO densities of Si4B and Si5B are a BN π* bond, while the LUMO densities of Si5 and Si6 are on the silicon and aryl π system.

Table 2. Selected Geometric Parameters for the Experimental Crystal Structures of Si5 and Si6 Si5 bond lengths (Å) bond angles (deg)

Si(1)−Si(5) Si(4)−Si(5) Si(1)−Si(5)−Si(4) mean Si−Si−Si

Si6 2.3582(4) 2.3656(5) 101.704(16) 103.9

E

Si(1)−Si(2) Si(2)−Si(3) Si(1)−Si(2)−Si(3) mean Si−Si−Si

2.3677(5) 2.3605(5) 111.186(19) 111.38


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Organometallics

Figure 3. UV−visible absorbance spectra of (a) Si4B, (b) Si5B, (c) Si5, and (d) Si6. Spectra recorded in n-pentane at room temperature ([compound] = 1.0 × 10−5 M). Offset and cropped UV−visible spectra of (e) five-membered and (f) six-membered rings highlighting a broad transition at ca. 350 nm in Si−B interelement cycles. Spectra recorded in n-pentane at room temperature ([compound] = 5.0 × 10−5 M).

oligosilanes.49,51,59 Full simulated spectra are reported in the Supporting Information. Theory reproduces the experimental observation of a low energy excitation unique to the SiB rings (Figure 6). As observed experimentally, the intensity of this feature is somewhat more pronounced in the Si4B than the Si5B ring. TD-DFT assigns the lowest energy feature in Si4B to a combination of HOMO to LUMO (47%) and HOMO−1 to

oligosilane chains. DFT and ab initio calculations indicate that the Si3Me8 excited singlet state S1 potential energy surface is shallow and in the vicinity of a funnel.58 Conformational flexibility contributes to rapid internal conversion. Calculated Spectra. The nature of the excitations observed for all compounds were further probed with TD-DFT calculations. The PBE0/6-311+G(2d,p) level of theory was selected for its prior success in calculating the excited states of F


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Organometallics Table 3. UV−Visible Spectroscopic Data for Si4B, Si5B, Si5, and Si6a compound Si4B

Si5B Si5 Si6

λ (nm)

ε (L mol−1 cm−1)

195 258 340 194 269 363 194 247 194 251

131000 24300 903 206000 22800 1100 150500 28800 158000 34300

a

Conditions: n-pentane, room temperature.

Figure 4. Dihedral angles in oligosilanes and Michl−West nomenclature.43

Figure 6. Cropped simulated UV−vis spectra of (a) five-membered rings Si4B and Si5 and (b) six-membered rings Si5B and Si6. TDDFT PBE0/6-311+G(2d,p)//CAM-B3LYP/6-31G(d). Only SiB compounds show a feature in the 350−400 nm region.

Figure 5. Side-on views of crystal structures of five-membered rings Si4B and Si5 indicating torsion angle ω (∠Si(1)−Si(2)−Si(3)-Si(4)) and cisoid conformation of oligosilane backbone. One phenyl group and hydrogens omitted for clarity. Si(2) is not visible (behind Si(3)).

Table 4. Comparison of Predicted and Experimental σ(SiSi)−π*(BN) transitions Si4B

LUMO (48%) excitations, both of which are σ → π*. The HOMO and HOMO−1 of Si4B differ in whether the σ framework is localized on exclusively SiSi bonds or a mixture of SiSi and SiB bonds. The lowest energy transition in the Si5B ring is also HOMO to LUMO (σ → π*). Theory slightly underestimates the energy of these transitions (Table 4). The experimental onset of absorbance in Si4B is 416 nm, while theory predicts an onset of absorbance at 460 nm. Similarly, Si5B is predicted to have an onset of absorbance at 438 nm compared to the experimental onset at 406 nm. Figures S5 and S6 summarize results with other theoretical methods. Functionals which include at least 60% long-range Hartree−Fock exchange energies60 (LC-ωPBE61 and ωB97X-

Si5B

entry

experiment

theory

experiment

theory

λmax λonset Eg

340 416 2.98

362 460 2.70

363 406 3.05

371 438 2.83

D)62 were employed to simulate UV−vis spectra of the SiB and Si rings due to evidence of improved accuracy in charge transfer systems.63,64 While both LC-ωPBE and ωB97X-D accurately predicted a low-energy 300−450 nm feature in Si4B and Si5B, high-energy features associated with the ring backbone were blue-shifted relative to experiment. Furthermore, these functionals dramatically overpredicted HOMO− LUMO gaps in Si4B and Si5B. For these reasons, the results G


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Organometallics

tetramethylpiperidide36,65 (1.0 equiv, 20 mmol, 2.94 g). The sealed Schlenk flask was removed from the glovebox and connected to a Schlenk line. Then, lithium 2,2,6,6-tetramethylpiperidide was suspended in pentane and cooled to −40 °C in an acetonitrile/dry ice bath. A solution of BCl3 (1.0 equiv, 20 mmol, 20 mL, 1.0 M solution in hexane) was added to the suspension dropwise via syringe over 10 min. The reaction mixture was stirred for 30 min at −40 °C, and then allowed to warm to room temperature overnight (∼15 h). After 15 h, a precipitate was filtered away with a fritted Schlenk tube into a 250 mL Schlenk flask. The filtrate was concentrated under reduced pressure to obtain a yellow oil. The product was distilled at 85 °C under reduced pressure (0.9 Torr) by short path distillation to yield 2 as a colorless oil which solidified to a white waxy solid over time (Yield: 3.45 g, 78%). NMR spectra were consistent with the reported spectra. 1H NMR (400 MHz, benzene-d6) δ 1.41 (m, 6H), 1.38(s, 12H). 11B {1H} (128 MHz, benzene-d6) δ 33.0. 13C {1H} (101 MHz, benzene-d6) δ 56.6, 35.3, 31.4, 14.4. Synthesis of Si4B. In a glovebox, a solution of 1a (1.0 equiv, 0.5 mmol, 0.546 g) in THF (10 mL) was prepared in a 50 mL roundbottomed flask equipped with stir bar. MgBr2·OEt2 (1.1 equiv, 0.55 mmol, 143 mg) was added to the solution of 1a, and the reaction mixture was stirred at room temperature for 1 h. The red reaction mixture turned yellow within 5 min and eventually became cloudy with a white precipitate. After 1 h, the white precipitate was removed by gravity filtration through a plug of Celite into a 20 mL vial with a stir bar. 2 (1.0 equiv, 0.5 mmol, 111 mg) was added by spatula transfer to the vial containing the filtrate and the reaction was stirred for 3 h at room temperature, during which a white precipitate was observed. The reaction mixture was concentrated under reduced pressure. n-Pentane (15 mL) was added, and the insoluble material removed by gravity filtration through a plug of Celite. The light yellow filtrate was concentrated to a yellow solid under reduced pressure. The solid was dissolved in heptane, and Si4B crystallized from heptane at −30 °C as light yellow crystals (Yield: 234 mg, 74%). 1H NMR (400 MHz, CD2Cl2) δ 7.37 (dd, J = 7.6, 1.9 Hz, 8H), 7.25− 7.19 (m, 12H), 1.67 (m, 6H), 1.18 (s, 12H), 0.06 (s, 12H). 11B {1H} NMR (128 MHz, CD2Cl2) δ 63.77. 13C {1H} NMR (101 MHz, CD2Cl2) δ 140.52, 137.95, 127.97, 127.65, 60.82, 36.75, 34.04, 14.63, −4.21. 29Si {1H} NMR (79 MHz, CD2Cl2) δ −32.52, −43.46. HRMS: [M + H]+ Calcd for C37H51BNSi4: 632.3192. Found: 632.3199. FTIR: 3131 (w), 3064 (m), 3041 (m), 3009 (s), 2959 (s), 2888 (s), 2784 (w), 2094 (w), 1945 (w), 1884 (w),1810 (w), 1587 (m), 1484 (s), 1423 (s), 1362 (s), 1244 (s), 1173 (s),1085 (s), 1092 (s), 990 (m), 950 (s), 841 (s), 788 (s), and 700 (s) cm−1. Anal. Calcd for C37H50BNSi4: C, 70.32, H, 7.98, N, 2.22. Found: C, 68.25, H, 8.09, N, 1.98. Low carbon content in elemental analysis for silicon compounds may be due to incomplete combustion related to SiC formation. Synthesis of Si5B. In a glovebox, a solution of 1b (1.0 equiv, 0.220 mmol, 252.0 mg) in THF (7.0 mL) was prepared in a 20 mL vial equipped with a stir bar. MgBr2·OEt2 (1.1 equiv, 0.242 mmol, 62.5 mg) was added as a solid to the solution of 1b, and the reaction mixture was stirred at room temperature for 1 h. The red reaction mixture turned yellow within 15 min and eventually became cloudy with a white precipitate. After 1 h, the reaction mixture was concentrated to a yellow solid under vacuum. Toluene (6.0 mL) was added, providing a yellow solution with a suspended white solid. The white solid was removed by gravity filtration of through a cotton plug into another 20 mL vial equipped with a stir bar. Residual silanide solution was transferred with 2 × 0.5 mL of toluene and passed through the cotton plug into the yellow solution. A solution of 2 (1.0 equiv, 0.5 mmol, 111 mg) in 1.0 mL toluene was prepared in a 1 dram vial. 2 was added dropwise via pipet to the magnesium silanide solution and the reaction mixture was stirred for 1.5 h at room temperature. The reaction mixture became light yellow with a white precipitate. The reaction mixture was concentrated under reduced pressure to yield a yellow solid. The solids were suspended in benzene and separated from the insoluble material by gravity filtration through a cotton plug. The light yellow filtrate was concentrated to a yellow solid under reduced pressure. The yellow solid was dissolved in 15 mL of near boiling n-heptane, then slowly cooled to −30 °C to

calculated with the PBE0 functional were deemed the most physically relevant.

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CONCLUSIONS We show the synthesis of different size SiB rings from nucleophilic disilanides and electrophilic chloroboranes, including the first six-membered SiB ring. A change in solvent suppressed an undesired intramolecular coupling reaction, enabling clean isolation of the target compounds. Materials are characterized both spectroscopically (1H, 11B, 13C, 29Si NMR) and crystallographically. Our study provides new understanding to the diversity of ways in which boron substitution influences conjugation. The pale-yellow SiB rings are characterized by steady-state UV−vis absorbance spectroscopy, revealing a low-energy feature unique to the mixed heterocycles. Calculations support assignment of the low-energy feature to σ → π* transitions from the silicon backbone of the ring to the BN π*. We also provide computational evidence for a HOMO-raising effect upon boron substitution.

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EXPERIMENTAL SECTION

General Procedures. All experiments were performed under an atmosphere of dry nitrogen or argon with the rigid exclusion of air and moisture using standard Schlenk techniques or in a nitrogen glovebox. All glassware was oven-dried overnight in a 175 °C oven. Reaction solvents were dried on a J. C. Meyer Solvent Dispensing System (SDS) using stainless-steel columns packed with neutral alumina (except for toluene which is dried with neutral alumina and Q5 reactant, a copper(II) oxide oxygen scavenger), following the manufacturer’s recommendations for solvent preparation and dispensation unless otherwise noted. Compounds 1a, 1b, and Si6 were synthesized according to literature procedures.31,32 All remaining reagents were purchased from commercial sources. NMR spectra were recorded on either a Bruker Avance 300, 400 or III HD 400 MHz Spectrometer and chemical shifts are reported in parts per million (ppm). Spectra were recorded in benzene-d6 or dichloromethane-d2, with tetramethylsilane or the residual solvent peak as the internal standard (1H NMR: C6H6 δ = 7.16; CH2Cl2, δ = 5.32). 11B NMR spectra are externally referenced to boron trifluoride diethyl etherate (BF3·Et2O, δ = 0 ppm). All reported 29Si {1H} spectra use a DEPT pulse sequence. In 29Si NMR spectra, signals from Si bound to boron are weak and broad due to the quadrupolar relaxation of boron. Mass spectrometry (MS) and high-resolution mass spectrometry (HRMS) were either performed in the Department of Chemistry at Johns Hopkins University using a VG Instruments VG70S/E magnetic sector mass spectrometer with EI (70 eV) or in the Columbia University Department of Chemistry mass spectrometry facility using a Waters XEVO G2XS QToF mass spectrometer equipped with a UPC2 SFC inlet, electrospray ionization (ESI) probe, atmospheric pressure chemical ionization (APCI) probe, and atmospheric solids analysis probe (ASAP). All column chromatography was performed on a Teledyne ISCO Combiflash Rf+ using Redisep Rf silica columns. IR spectra were collected on either a Thermo Scientific Nicolet iS5 spectrometer equipped with iD5 ATR laminated diamond crystal attachment or a Thermo Scientific Nicolet Nexus 670 FT-IR. Elemental analysis was performed by Robertson Microlit Laboratories. UV−vis spectroscopy was performed on a Shimadzu UV-1800 Spectrophotometer. All UV−vis measurements were collected in anhydrous pentane at room temperature with quartz cuvettes. All solutions were prepared in the glovebox. Photoluminescence spectra were obtained using a PTi Photon Technology International Fluorometer (Quanta Master 40) with a 75-W Ushio Xenon short arc lamp and operated with Felix32 Version 1.2 software. Synthesis of 2. The synthesis of 2 was adapted from reported procedures. In a glovebox, an oven-dried 100 mL Schlenk flask equipped with a stir bar was charged with lithium 2,2,6,6H


Organometallics

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provide Si5B as yellow crystals (Yield: 52.3 mg, 34%). 1H NMR (400 MHz, CD2Cl2) δ 7.48−7.40 (m, 8H), 7.28−7.20 (m, 12H), 1.70− 1.58 (m, 6H), 1.19 (s, 12H), 0.07 (s, 6H), −0.18 (s, 12H). 11B {1H} NMR (128 MHz, CD2Cl2) δ 66.30. 13C {1H} NMR (101 MHz, CD2Cl2) δ 141.64, 139.12, 128.09, 127.74, 60.68, 37.08, 34.02, 14.67, −2.73, −4.96. 29Si {1H} NMR (79 MHz, CD2Cl2) δ −34.74, −40.01, −45.01. HRMS: [M + H]+ Calcd for C39H57BNSi5: 690.3430. Found: 690.3434. FTIR: 3127 (w), 3065 (m), 3043 (m), 3011 (s), 2965 (s), 2937 (m), 2888 (s), 2785 (w), 2094 (w),1945−1810 (,w), 1580 (m), 1478 (s), 1423 (s), 1362 (s), 1241 (s), 1085 (s), 1091 (s), 950 (s), 800 (s), 733 (s), and 700 (s) cm−1. Anal. Calcd for C39H56BNSi5: C, 67.88, H, 8.18., N, 2.03 Found: C, 65.30, H, 8.48, N, 1.95. Low carbon content in elemental analysis for silicon compounds may be due to incomplete combustion related to SiC formation. Synthesis of Si5. In a glovebox, a solution of 1a (1.0 equiv, 0.5 mmol, 0.546 g) in THF (10 mL) was prepared in a 50 mL roundbottomed flask equipped with stir bar. MgBr2·OEt2 (1.1 equiv, 0.55 mmol, 143 mg) was added to the solution of 1a and the reaction mixture was stirred at room temperature for 1 h. The red reaction mixture turned yellow within 5 min of addition and eventually became cloudy with a white precipitate. Dimethyldichlorosilane (1.1 equiv, 0.55 mmol, 66 μL) was added dropwise by syringe. The reaction mixture became colorless with a white precipitate. The solution was stirred for 3 h at room temperature. The reaction mixture was concentrated to a white solid under reduced pressure. The solids were extracted with pentane (15 mL), and insoluble material was removed by vacuum filtration. The colorless filtrate was concentrated to a white solid under reduced pressure. The crude product was purified by automated column chromatography with 100% hexanes to obtain Si5 as a white solid (Yield: 0.22 g, 81%). Si5 was dissolved in hot hexanes, then slowly cooled to room temperature to obtain X-ray quality crystals. 1H NMR (400 MHz, CD2Cl2) δ 7.46−7.39 (m, 8H), 7.38− 7.24 (m, 12H), 0.38 (s, 6H), 0.34 (s, 12H). 13C {1H} NMR (101 MHz, CD2Cl2) δ 136.93, 136.32, 128.68, 128.20, −3.43, −4.45. 29Si {1H} NMR (79 MHz, CD2Cl2) δ −33.30, −40.12, −42.00. HRMS: Calcd for C30H39Si5: 539.1898. Found: 539.1892. FTIR: 3130 (w), 3065 (m) 3047 (m), 3008 (w), 2990 (w), 2944 (m), 2888 (m), 2787 (w), 1948−1823 (w), 584 (w),1483 (m), 1426 (s), 1243 (m), 1091 (s), 1027(w), 995 (w), 837 (m), 805(s), 732 (s), and 696 (s) cm−1. Anal. Calcd for C30H38Si5: C, 66.84, H, 7.11. Found: C, 66.87, H, 7.39. Single Crystal X-ray Crystallography. All reflection intensities were measured at 110(2) K using a SuperNova diffractometer (equipped with Atlas detector) with Mo Kα radiation (λ = 0.71073 Å) using the program CrysAlisPro (version 1.171.36.32 for Si4B and Si5; version 1.171.39.29c for Si5B). The same program was used to refine the cell dimensions and for data reduction. The structure was solved with the program SHELXS-2014/766 and was refined on F2 with SHELXL-2014/7.66 Numerical absorption corrections based on Gaussian integration over a multifaceted crystal model were applied using CrysAlisPro. The temperature of the data collection was controlled using the system Cryojet (manufactured by Oxford Instruments). The H atoms were placed at calculated positions using the instructions AFIX 23, AFIX 43 or AFIX 137 with isotropic displacement parameters having values 1.2 or 1.5 Ueq of the attached C atoms. The structures of Si4B, Si5B, and Si5 are ordered. For Si5B, the crystal that was mounted on the diffractometer was a composite of two crystals (one larger and one smaller components). The two components are related by a 1.4720° rotation along the reciprocal axis 0.9583a* + 0.2348b* + 0.1630c*. The structure refinement was performed the HKLF 5 format. The BASF scale factor refines to 0.203(4). Computational Methods. All DFT calculations were performed using the Gaussian 09 package.67 Geometries were optimized using the CAM-B3LYP functional with the 6-31G(d) basis set. No symmetry restrictions were applied to geometry optimization. All optimized structures possess zero imaginary frequencies. TD-DFT calculations were performed using the PBE0 functional with the 6311+G(2d,p) basis set on CAM-B3LYP optimized geometries. The 20 lowest excitations were calculated.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00804. NMR spectra, crystal measurement and refinement data for compounds studied by XRD, supplemental figures (PDF) Cartesian Coordinates contains the computed Cartesian coordinates of all of the molecules reported in this study (XYZ) Accession Codes

CCDC 1872455−1872457 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 Author

*E-mail: [email protected]. ORCID

Rebekka S. Klausen: 0000-0003-4724-4195 Present Address

T.K.P.: Eurofins Lancaster Laboratories, Lancaster, PA 17601. Author Contributions

E.M.P. and E.A.M. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy (DOE), Office of Science, Basic Energy Sciences under Award # DE-SC0013906 (molecular synthesis). DFT calculations were conducted using scientific computing services at the Maryland Research Computing Center (MARCC).

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REFERENCES

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