Homolytic Cleavage Reactions of a Neutral Doubly Base Stabilized

The neutral doubly base stabilized diborane(4) [C2H2(NCH2C6H4)2CB]2 (3a), featuring an ... Construction of Linear and Branched Tetraboranes by 1,1- an...
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Homolytic Cleavage Reactions of a Neutral Doubly Base Stabilized Diborane(4) Levy L. Cao and Douglas W. Stephan* Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 S Supporting Information *

ABSTRACT: The neutral doubly base stabilized diborane(4) [C2H2(NCH2C6H4)2CB]2 (3a), featuring an electron-precise B(sp3)− B(sp3) σ bond, was generated by one-electron reduction of the corresponding borenium precursor with Cp*2Co followed by dimerization. Compound 3a reacts with TEMPO, (PhC(O)O)2, PhNO, (tht)AuCl (tht = tetrahydrothiophene), and S8. The resulting species, (C2H2(NCH2C6H4)2CB)(L) (L = ONC5H6Me4 (4), OC(O)Ph (5), PhNO (6), Cl (7)) and [(C2H2(NCH2C6H4)2CB)2(S4)] (8), have been fully characterized and are consistent with homolytic cleavage of the B−B bond in 3a.

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INTRODUCTION In contrast to C−C bonds, B−B bonds are far less common. Nonetheless, the advent and reactivity of diborane(4) species has had a significant impact on organic1−3 and organometallic chemistry,4,5 and indeed such species have been widely applied. A variety of strategies have been developed to access such B−B bonded compounds, including haloborane reductions,6−8 dehydrocoupling,9−20 borylene coupling,21−24 hydroborations,25 and nucleophilic substitutions.26−31 This can be attributable to the relatively low bond dissociation energy of homonuclear sp3−sp3 boron−boron bonds (ca. 293 kJ/mol vs 345 kJ/mol for an analogous C−C bond).32 Despite this issue, several reports have described the synthesis of base stabilized diboranes and diboranes(6) (Figure 1). The species [(IPr)-

BH2]2 (A) was reported by Robinson et al.33 and was derived from the reductive coupling of the carbene adduct (IPr)BBr3 in reaction with KC8. Finze and co-workers34 reported the structurally related dianion [B(CN)3]2− (B), prepared by reduction of [FB(CN)3]− with tBuLi or KC8. Dehydrocoupling of a diboron precursor was used to generate the species [HB(μhpp)]2 (C; hpp = 1,3,4,6,7,8-hexahydro-2H-pyrimido[1,2α]pyrimidinate), and a subsequent derivatization afforded the dicationic derivative [(Me2HN)HB(μ-hpp)]22+.35−40 More recently, Fontaine et al.41 have demonstrated spontaneous dehydrocoupling from hydroborane to afford species D. Grigsby and Power42 isolated the dimeric 9H-9-borafluorene dianion E, while Wagner et al.43−46 described a radical coupling affording the dianionic species F and species G. Tamao et al.47 prepared the B−B bonded species H via diborane reduction. Brauschweig et al.25 afforded species I from hydroborations of diborane precursors, while most recently Kinjo and coworkers48 have reported the one-electron oxidation of organoboron L2PhB: (L = oxazol-2-ylidene) to give the dicationic diborane species [L2PhB−BPhL2][X]2 J. While the above reports have focused on the synthesis of diboranes, fewer have probed the subsequent reactivity of these species. Himmel and co-workers12 described the reaction of [HB(μ-hpp)]2 with disulfides and elemental sulfur (S8), affording [(RS)B(μ-hpp)]2 and [(μ-S)HB(μ-hpp)]2, respectively. The Braunschweig group has demonstrated the conversion of carbene-stabilized boron−boron single bonded species to those containing double and triple boron−boron bonded compounds.2,49,50 On the other hand, a 2015 report by Finze and co-workers34 showed the hexacyanodiborane(6) dianion [B2(CN)6]2− to be remarkably water and air stable. Earlier last year, Kinjo et al. described the reactions of the

Figure 1. Examples of related diborane species.

Received: July 11, 2017

© XXXX American Chemical Society

A

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computed to be significantly lower in energy than 2 equiv of the corresponding monomeric B-based radical, and the bond dissociation enthalpy was calculated to be 42.9 kcal/mol (M06-2X/Def2-TZVP//M06-2X/Def2-SVP). The computed structure predicts a B−B bond length of 1.901 Å, which is significantly longer than most of the known B(sp3)−B(sp3) bond distances, which range from 1.753 to 1.849 Å (see the Supporting Information).38,44−47 The 11B NMR chemical shift of 3a was computed at the GIAO-B97-2/Def2-TZVP//M062X/Def2-SVP level of theory,54,55 and the value correlates well with the experimentally observed value (−14.5 ppm; see the Supporting Information). The highest occupied molecular orbital (HOMO) of 3a was primarily derived from the B−B bond, while the lowest occupied molecular orbital (LUMO) was diffused across the whole molecule with E(HOMO− LUMO) = 4.23 eV (Figure 2a). The Wiberg bond index (WBI)

dicationic diborane J with isonitriles and AuCl at elevated temperatures to give the isocyano- and chloride-bound boronium cations, respectively.48 In this paper, we access a unique doubly base stabilized diborane(4) from the reduction of a planar borenium cation. This species reacts with a variety of reagents, including TEMPO, (PhC(O)O)2, PhNO, (tht)AuCl, and S8, via facile homolytic cleavage of the B−B bond at room temperature.

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RESULTS AND DISCUSSION In recent work, we have reported the reaction of (IBn)BH3 (1a) with trityl cation, prompting the subsequent borylations of the pendant benzyl groups to afford the borenium cationic salt [C2H2(NCH2C6H4)2CB][B(C6F5)4] (2a) (Scheme 1).51 The Scheme 1. Generation and Reactivity of 3a

Figure 2. (a) Plot of frontier molecular orbitals for 3. (b) NBO for B− B bonding. (c) Plot of the gradient vector field of ρ(r) and the contour map of the Laplacian of the electron density, ∇ρ(r), of 3.

of the central B−B bonding orbital from the natural bond orbital (NBO) calculations56,57 (M06-2X/Def2-TZVP//M062X/Def2-SVP level) was found to be 0.67, and each boron center beats a natural atomic charge of 0.006. A bonding NBO between the boron centers was found with an occupancy of 1.65 e−. Similarly, atoms in molecules58 studies reveal a bond critical point along the B−B bond path with ρ = 0.098 and ∇2ρ = −0.126, indicating a covalent bonding interaction between the B centers (Figure 2). All of these results suggested that we have successively generated a doubly base stabilized diborane(4) species that is analogous to the species G and J (Figure 1) prepared by Wagner and co-workers45,46 and Kinjo et al.,48 respectively. The benzamidazolium analogue of 2b was prepared following an analogous synthetic protocol. The precursor carbene adduct 1b was converted to the borenium cation [C6H4(NCH2C6H4)2CB][B(C6F5)4] (2b). The formulations of these species were confirmed by spectroscopy and with X-ray data (Figure 3). Treatment with Cp*2Co proceeded to give rise to a sharp singlet resonance at −14.9 ppm in the 11B NMR spectrum with the complete consumption of 2b, consistent with the formation of 3b, containing a four-coordinate boron center (Scheme 2). While compound 3b exhibited similar sensitivity to 3a and was similarly resistant to crystallization, its synthesis permitted a crossover experiment to be performed. To this end, a combination of 3a and 3b was monitored by 11B NMR spectroscopy. The mixture revealed resonances attributable to both 3a and 3b as well as resonances consistent with the

orientation of the empty p orbital of 2a relative to the plane of this borenium cation suggests extended conjugation and led us to investigate one-electron reduction of 2a. Compound 2a does not react with THF, as evidenced by NMR spectroscopy (see the Supporting Information). A cyclic voltammogram (CV) of 2a performed in THF, using 0.1 M [nBu4N][B(C6F5)4] as electrolyte, showed a quasi-reversible one-electron-reduction wave at −1.55 V vs Cp2Fe, suggesting the formation of a new species upon reduction. On the basis of this potential, the reaction of 2a with Cp*2Co was subsequently performed at room temperature in C6D5Br. After 5 min, the reaction gives rise to a sharp singlet resonance at −16.8 ppm in the 11B NMR spectrum with the complete consumption of 2a, suggesting the formation of the new species 3a containing a four-coordinate boron center (Scheme 1). The 1H NMR spectrum of 3a shows two doublets in the benzylic region, in contrast to 2a, where only a single resonance was attributable to the benzylic protons. These data are consistent with the formation of a tetrahedral geometry at boron.51 Similar results were reproduced in THF, C6H5Cl, or 1,2-C6H4Cl2; however, efforts to separate 3a from the reduction byproduct [Cp*2Co][B(C6F5)4] were complicated by the degradation of 3a after a few hours, even at low temperatures (see the Supporting Information). The structure of 3a was probed via computational studies using density functional theory (DFT) methods at the M062X/Def2-SVP level of theory.52,53 The singlet state of 3a was B

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Figure 3. ORTEP depiction of 1b (top) and the cation of 2b (bottom). Color scheme: C, black; N, blue; B, yellow-green. Hydrogen atoms are omitted for clarity except for the B−H bonds in 1b. Selected metric parameters (bond lengths in Å and angles in deg): 1b, B−C 1.585(5); 2b, B−C(10) 1.536(8), B−C(17) 1.547(7), B−C(6) 1.562(6), C(10)−B−C(17) 112.8(5), C(10)−B−C(6) 111.9(5), C(17)−B−C(6) 135.3(5).

formation of the mixed species 3c. A similar spectrum was obtained from the reaction of a 1:1 mixture of 2a and 2b with Cp*2Co (see the Supporting Information). These observations with the support of the computational results (see the Supporting Information for 3b) suggested the generation of 3a with a weak B(sp3)−B(sp3) bond. Due to the sensitive nature of 3a, reactivity studies were performed in situ by adding a substrate 15 min after addition of Cp*2Co to 2a. In this fashion, TEMPO was used initially (Scheme 2). The reaction mixture features a single new resonance at −6.3 ppm in the 11B NMR spectrum. Following workup, the product 4 was isolated as a white solid in 95% yield (Scheme 1). Recrystallization afforded X-ray quality crystals, allowing the connectivity of 4 to be confirmed as [C2H2(NCH2C6H4)2CB(ONC5H6Me4)] (Figure 4), in which the B center is quaternized via formation of a new B−O bond (1.500(4) Å). The geometry about B is distorted in a pseudotetrahedron, with B−CNHC and B−CPh

Figure 4. ORTEP depiction of the molecular structures of 4 (top) and 5 (bottom). Color scheme: C, black; N, blue; B: yellow-green; O, red. Hydrogen atoms are omitted for clarity. Selected metric parameters (bond lengths in Å and angles in deg): 4, B−O 1.500(4), B−C(10) 1.610(5), B−C(17) 1.630(4), B−C(6) 1.652(5), O−N(3) 1.455(3), O−B−C(10) 114.8(3), O−B−C(17) 101.4(2), C(10)−B−C(17) 104.4(2), O−B−C(6) 116.6(2), C(10)−B−C(6) 100.8(2), C(17)− B−C(6) 118.8(3), N(3)−O−B 120.5(2); 5, O(1)−B 1.547(5), C(17)−B 1.613(6), C(10)−B 1.597(6), C(6)−B 1.632(6), B− O(1)−C(18) 120.5(3), C(17)−B−O(1) 109.4(3), C(10)−B−O(1) 112.2(3), C(10)−B−C(17) 106.8(4), C(6)−B−O(1) 103.6(3), C(6)−B−C(17) 121.1(4), C(6)−B−C(10) 103.7(3).

distances of 1.610(4), 1.652(5), and 1.630(4) Å and C−B−C angles of 118.9(3), 100.9(2), and 104.4(3)°. In a similar fashion, 3a generated in situ was allowed to react with 1 equiv of benzoyl peroxide, (PhC(O)O)2 (Scheme 1).

Scheme 2. Synthesis of 3a and Its Reactivity with 3b

C

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Organometallics The resulting species 5 gives rise to a single resonance at −9.2 ppm in the 11B NMR spectrum. Following workup, this species was isolated as a white powder in 93% yield. X-ray crystallographic analysis confirmed that 5 is [C2H2(NCH2C6H4)2CB(O(O)CPh)] (Figure 4). The structure was analogous to that described for 4 with a B−O bond length in 5 of 1.547(5) Å. Compound 3a was also shown to react with 2 equiv of PhNO to give the clean paramagnetic species 6 (Scheme 1). The product gave rise to an EPR spectrum and was completely NMR silent. Simulation of the EPR spectrum afforded a g value of 2.0012 and hyperfine couplings of 3.6 G to B, 10.0 and 1.0 G to N, and 1.2 and 2.9 G to H atoms (Figure 5; see the

Figure 6. ORTEP depiction of the molecular structure of 7. Color scheme: C, black; N, blue; B, yellow-green; Cl, green. Hydrogen atoms are omitted for clarity. Selected metric parameters (bond lengths in Å and angles in deg): Cl−B 1.959(2), C(17)−B 1.600(3), C(10)−B 1.577(3), C(6)−B 1.617(3), C(10)−B−C(17) 107.6(2), C(10)−B− C(6) 104.4(2), C(17)−B−C(6) 121.7(2), C(10)−B−Cl 106.9(1), C(17)−B−Cl 108.4(1), C(6)−B−Cl 107.1(1).

Figure 5. (a) Simulated and (b) experimental EPR spectra for 6.

Supporting Information). Spin density and Fermi contact coupling constants computed at the uM06-2X/Def2-TZVP// uM06-2X/Def2-SVP level of theory were consistent with experimental data and affirmed that the unpaired electron density is primarily located on the PhNO fragment. These data are thus consistent with the formation of the nitrogen-centered radical [C2H2(NCH2C6H4)2CB(PhNO)]• (6) and suggest B− N coordination, although this could not be unambiguously confirmed, despite repeated attempts to crystallize 6 (Scheme 1). Compound 3a also reacts with 2 equiv of (tht)AuCl to generate a black solid (elemental gold) in suspension in C6H5Cl (Scheme 1). After filtration and subsequent workup the product 7 was ultimately isolated as a white solid in 81% yield. The resulting product gives a sharp singlet at −10.9 ppm in the 11B NMR spectrum. Following slow evaporation of the solvent, colorless crystals of 7 were obtained. An X-ray diffraction study confirmed the formulation to be [C2H2(NCH2C6H4)2CBCl] (Figure 6). The geometry about B is again pseudotetrahedral with a B−Cl bond distance of 1.959(2) Å. These data imply that chlorination of the diborane proceeds with the concurrent reduction of Au(I), a strategy also exploited by Kinjo and co-workers.48 In that previous work, chlorination of the dication affords elemental Au and the boronium cation [L2PhBCl]+. The corresponding reaction of 3a with excess S8 prompted the formation of the species 8 (Scheme 1), which gave rise to a new and sharp signal in the 11B NMR spectrum at −14.0 ppm. Two multiplet resonances arise in the benzylic region, a spectral pattern which differs from those seen for 4, 5, and 7. The formulation of 8 was confirmed by X-ray diffraction. In the solid-state structure, the molecule 8 sits on a crystallographic 2fold axis with the asymmetric unit being comprised of a chelated-boron center linked to an S2 unit. The overall molecular structure is comprised of four sulfur atoms in a chain linking the two boron atoms, and thus the molecular formulation for 8 is [(C2H2(NCH2C6H4)2CB)2(S4)] (Figure 7). The B−S distance was determined to be 2.016(5) Å,

Figure 7. ORTEP depiction of the molecular structure of 8. Color scheme: C, black; N, blue; B, yellow-green; S, yellow. Hydrogen atoms are omitted for clarity. Selected metric parameters (bond lengths in Å and angles in deg): C(6)−B 1.638(8), C(10)−B 1.577(7), C(17)−B 1.595(8), S(1)−B 2.016(5), S(1)−S(2) 2.077(2), S(2)−S(2)′ 2.078(3), C(17)−B−C(10) 108.7(5), C(6)−B−C(10) 104.7(4), C(17)−B−C(6) 124.4(4), C(10)−B−S(1) 106.7(3), C(17)−B− S(1) 109.9(3), C(10)−B−S(1) 101.2(3), B−S(1)−S(2) 105.4(2), S(1)−S(2)−S(2) 108.35(7).

significantly longer than the B−S bonds (1.886(2), 1.894(2) Å) reported in [(μ-S)HB(μ-hpp)]2,12 presumably a reflection of the more electron rich nature of the boron center in 8. The S− S distances were found to be 2.077(2) and 2.078(3) Å with a B−S−S angle of 105.4(2)°. The low computed bond dissociation energy of 3a and the reactivity described herein are consistent with a series of reactions that proceed via homolytic cleavage of the B−B bond in 3a. Indeed, the reactivity of 3a with (PhC(O)O)2 is similar to that described for boron radicals in reports by the groups of D

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Organometallics Braunschweig59 and Yamashita.60 In some sense, this B−B reactivity is reminiscent of the dissociation of hexaphenylethane generating trityl radical, as first described in the seminal work of Gomberg.61 While the generation and characterization of Bbased radicals have prompted numerous studies,59,60,62−76 the present report is, to the best of our knowledge, the first to describe reactivity consistent with the generation of a transient radical by homolytic cleavage of a B−B bond.

during combustion and has been previously reported for other boron compounds. In the case of compounds 3−8, where an acceptable elemental analysis was not obtained, the purity was confirmed via HRMS. The fragility of 5−8 results in the observation of the B cation in the HRMS data. Synthesis of BIBnBH3 (1b). Following the same procedure as for 1a, with 1,3-dibenzyl-1H-benzo[d]imidazole-3-ium bromide (1.503 g 3.96 mmol), 1b was obtained as a white solid in 35% yield (430 mg) after recrystallization from cold toluene/pentane (1/1 v/v) at −35 °C, which gave crystals suitable for a single-crystal X-ray diffraction study. 1 H NMR (400 MHz, C6D6, 298 K): δ 7.22 (m, 4H, Ph-H), 6.98 (m, 6H, Ph-H), 6.75 (m, 4H, NCPh-H), 5.44 (s, 4H, Ph-CH2), 2.33 (q, 1 JBH = 85 Hz, 3H, BH3). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ 135.87, 132.60, 128.92, 128.57, 128.38, 128.15, 125.28, 123.22, 111.23, 49.21 (Ph-CH2). 11B NMR (128 MHz, C6D6, 298 K): δ −35.1 (q, 1JBH = 85 Hz). Anal. Calcd for C21H21BN2: C, 80.79; H, 6.78; N, 8.97. Found: C, 80.78; H 6.86; N, 8.64. Synthesis of [C6H4(NCH2C6H4)2CB][B(C6F5)4] (2b). In an inertatmosphere glovebox, 1b (350.3 mg, 1.12 mmol) was weighed into a thick-walled bomb charged with a magnetic stirrer. The compound was dissolved in 10 mL of chlorobenzene, and a solution of trityl tetrakis(pentafluorophenyl)borate (1.035 g, 1.12 mmol) in 10 mL of chlorobenzene was added dropwise with stirring. The mixture was stirred for 30 min, over which time gas evolution subsides. The bomb was sealed, and the contents were stirred at 130 °C for 26 h. After the reaction mixture was cooled, volatiles were removed in vacuo. The residue was dissolved in dichloromethane and filtered through Celite. After the volatiles were removed in vacuo, toluene (3 × 5 mL) and pentane (3 × 5 mL) were used to wash the residue. The residue was redissolved in dichloromethane and recrystallized by layering with pentane to give crystals that were washed with toluene (3 × 5 mL) and pentane (3 × 5 mL) and dried in vacuo to give a colorless crystal (836.7 mg, 76%). 1H NMR (500 MHz, C6D5Br, 298 K, toluene omitted): δ 8.51, (d, 2H, JHH = 8 Hz), 7.57 (t, 2H, JHH = 8 Hz), 7.47− 7.41 (m, 4H), 7.37, (dt, 2H, JHH = 4, 6 Hz), 7.15 (t, JHH = 8 Hz, 2H), 5.17 (s, 4H). 11B NMR (128 MHz, C6D5Br, 298 K): δ 45.2 (s, br), −16.1 (s). 13C{1H} NMR (126 MHz, CDCl3, 298 K, partial): δ 142.56, 137.98, 137.85, 136.27, 132.38, 129.51, 129.08, 129.00, 128.19, 127.70, 125.26, 113.15, 48.70. 19F NMR (C6D5Br, 376 MHz, 298 K): δ −131.6 (s, br, 8F, o-C6F5), −162.0 (t, 4F, p-C6F5, JFF = 20 Hz), −165.9 (t, br, 8F, m-C6F5, JFF = 19 Hz). Anal. Calcd for C52H24B2F20N2: C, 57.92; H, 2.24; N, 2.60. Found: C, 58.11; H, 2.44; N, 2.36. Generation of [C2H2(NCH2C6H4)2CB]2 (3a) and [C8H4(NCH2C6H4)2CB]2 (3b). These compounds were prepared in a similar fashion, and thus only one preparation is detailed. Under an inert atmosphere, a solution of 2a or 2b (10 mg, 0.010 mmol) in 0.5 mL of C6D5Br was placed in a vial with Cp*2Co (3 mg, 0.010 mmol) at room temperature. The reaction mixture turned from green to brown-yellow at room temperature, and a solid crashed out from the solution after 10−30 min. NMR spectra of 3a and 3b were taken 10 min after the reaction started. Data for 3a are as follows. 1H NMR (400 MHz, C6D5Br, 298 K, toluene and [Cp*2Co][B(C6F5)4] omitted): δ 7.62 (s, br, 4H, Ph), 7.16−7.00 (m, 8H, Ph), 6.69 (s, br, 4H, Ph), 7.26 (s, 4H, NCH), 4.24 (d, 2JHH = 16 Hz, 4H, CHaHb−), 3.11 (d, 2JHH = 16 Hz, 4H, CHaHb−). 11B NMR (128 MHz, C6D5Br, 298 K): −16.8. HRMS (DART): m/z [M − H]+ calcd for [C34H27B2N4]+ 513.2422, measured 513.2433. Data for 3b are as follows. 1H NMR (400 MHz, C6D5Br, 298 K, toluene and [Cp*2Co][B(C6F5)4] omitted): δ 7.62 (s, br, 4H, Ph), 7.16−7.00 (m, 20H, Ph), 4.50 (d, 2JHH = 15 Hz, 4H, CHaHb−), 3.34 (d, 2JHH = 15 Hz, 4H, CHaHb−). 11B NMR (128 MHz, C6D5Br, 298 K): −14.9. HRMS(ESI): m/z [M − H]+ calcd for [C42H31N4B2]+ 613.2735, measured 613.2741. Synthesis of [C2H2(NCH2C6H4)2CB(ONC5H6Me4)] (4). Following the procedure for generation of 3a (102 mg of 2a was used, reaction was performed in 4 mL of THF), 10 min after the reaction started, TEMPO (16 mg, 0.10 mmol) was added to the reaction mixture. The reaction mixture was stirred for 1 h, and violate was removed in vacuo. The desired product was extracted from toluene solution (3 × 2 mL)

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CONCLUSIONS In conclusion, the doubly base stabilized diborane(4) 3a contains a weak B−B bond that reacts with TEMPO and peroxide to give the diamagnetic species 4 and 5, respectively. Reaction with PhNO yields the radical 6, with the unpaired spin largely centered at the PhNO fragment. Reaction with (tht)AuCl affords the haloborane 7 and Au metal, while the reaction with S8 gives 8, a species in which two boron centers are linked by an S4 unit. These reactions provide rare illustrations of homolytic B−B bond cleavage. We are continuing to study the reactivity of reduced boron species, and further reports will follow in due course.

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EXPERIMENTAL SECTION

General Considerations. All manipulations were performed under an atmosphere of dry, oxygen-free N2 by means of standard Schlenk or glovebox techniques (Innovative Technology glovebox equipped with a −35 °C freezer). Toluene (C7H8), dichloromethane (DCM), and pentane were collected from a Grubbs-type column system manufactured by Innovative Technology. DCM, pentane, bromobenzene (C6H5Br), o-dichlorobenzene (o-C6H4Cl2), and toluene were stored over 4 Å molecular sieves. Molecular sieves, type 4 Å (pellets, 3.2 mm diameter), purchased from Sigma-Aldrich were activated prior to use by iteratively heating with a 1050 W Haier microwave for 5 min and cooling under vacuum. The process was repeated until no further moisture was released upon heating. Benzene-d6 and bromobenzene-d5, purchased from Cambridge Isotope Laboratories, were degassed and stored over 4 Å molecular sieves in the glovebox for at least 8 h prior to use. Tetrahydrofuran (THF) was dried by sodium-benzophenone overnight followed by distillation. Chloroform-d, purchased from Cambridge Isotope Laboratories, and chlorobenzene (C6H5Cl) were dried by stirring over CaH2 for several days followed by distillation. Unless otherwise mentioned, chemicals were purchased from Sigma-Aldrich, Strem Chemical, or TCI. 1,3Dibenzylimidazolium bromide and 1,3-dibenzyl-1H-benzo[d]imidazole-3-ium bromide were prepared using literature methods.77 NMR spectra were recorded on a Bruker Avance III 400 MHz, Agilent DD2 500 MHz, or Agilent DD2 600 MHz spectrometer, and spectra were referenced to residual solvents of C6D5Br (1H 7.28 ppm for meta proton), C6D6 (1H 7.16 ppm; 13C 128.06 ppm), or CDCl3 (1H 7.26 ppm; 13C 77.16 ppm) or externally (11B, (Et2O)BF3; 19F, CFCl3). Chemical shifts (δ) are reported in ppm, and the absolute values of the coupling constants (J) are given in Hz. In some instances, signal and/ or coupling assignment was derived from 2D NMR experiments. A Fisher Scientific Centrific Model 228 instrument (fixed speed 3000 rpm) was used for a centrifuge. Electron paramagnetic resonance (EPR) measurements were performed at 298 K using a Bruker ECSEMX X band EPR spectrometer equipped with an EP4119HS cavity. Simulations were performed using PEST WinSIM software. Mass spectrometry was obtained using a Bruker Autoflex MALDI-TOF instrument, while high-resolution mass spectrometry was obtained using a JEOL Accu TOF Model JMS-T1000LC apparatus (with DART ion source). Compounds 1a and 2a were prepared as previously reported.51 For each new compound attempts were made to confirm purity by elemental analysis. In some cases, elemental analysis was reproducibly low on carbon content, while providing satisfactory H and N values. This phenomenon is attributed to the formation of boron carbides E

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Organometallics and filtered through Celite. After it was dried in vacuo, the residue was washed with pentane (3 × 2 mL), giving an off-white solid (40 mg, yield 97%). Single crystals for X-ray studies were obtained from slow evaporation of a saturated benzene solution to give colorless crystals. 1 H NMR (400 MHz, C6D6, 298 K, toluene omitted): δ 8.54 (d, 3JHH = 8 Hz, 2H, Ph), 7.37 (t, 3JHH = 8 Hz, 2H, Ph), 7.19 (t, 3JHH = 8 Hz, 2H, Ph), 6.94 (d, 3JHH = 8 Hz, 2H, Ph), 5.79 (s, 2H, NCH), 5.34 (d, 2JHH = 15 Hz, 2H, NCH2Ph), 4.09 (d, 2JHH = 15 Hz, 2H, NCH2Ph), 1.47− 1.35 (m, 4H, CH2), 1.19 (s, 6H, CH3), 0.87 (d, 3JHH = 7 Hz, 2H, CH2), 0.72 (s, CH3, 6H). 13C{1H} NMR (126 MHz, C6D6, 298 K): δ 138.0, 135.7, 129.3, 128.6, 128.4, 128.3, 128.2, 128.1, 128.0, 126.9, 125.7, 125.2, 125.1, 59.6 (NCH), 52.3 (NC(CH3)2), 40.8, 34.7, 20.6, 18.2. 11B NMR (128 MHz, C6D6, 298 K): δ −6.3. HRMS (DART): m/z [M + H]+ calcd for [C26H33BN3O]+ 414.2717, measured 414.2713. Synthesis of [C2H2(NCH2C6H4)2CB(O(O)CPh)] (5). Following the procedure for generation of 3a (102 mg of 2a was used, reaction was erformed in 4 mL of C6H5Cl), 10 min after the reaction started, (PhC(O)O)2 (12 mg, 0.05 mmol) was added to the reaction mixture. The reaction mixture was stirred for 1 h, and violate was removed in vacuo. The desired product was extracted from benzene solution (3 × 2 mL), and filtered through glass fiber. After it was dried in vacuo, the residue was washed with pentane (3 × 2 mL), giving an off-white solid (35 mg, yield 93%). Single crystals for X-ray studies were obtained by layering a saturated toluene solution with pentane to give colorless crystals. 1H NMR (400 MHz, C6D5Br, 298 K): δ 8.39 (d, 3JHH = 11 Hz, 2H, Ph), 7.95 (d, 3JHH = 11 Hz, 2H, Ph), 7.31 (m, 3H, Ph), 7.15 (m, 3H, Ph), 7.02 (m, 3H, Ph), 6.36 (s, 2H, NCH), 5.23 (d, 2JHH = 18 Hz, 2H, NCH2Ph), 4.48 (d, 2JHH = 18 Hz, 2H, NCH2Ph). 13C{1H} NMR (126 MHz, C6D5Br, 298 K, partial): δ 138.1, 133.8, 127.6, 127.1, 118.3, 51.2 (NC(CH3)2), 40.85, 34.55, 20.65, 18.18. 11B NMR (128 MHz, C6D5Br, 298 K): δ −9.2. LRMS (MALDI): m/z [M]+ calcd for [C24H19BN2O2]+ 378.15, measured 378.38; HRMS (DART): m/z [2a]+ calcd for [C17H14BN2]+ 257.1250, measured 257.1248. Generation of [C2H2(NCH2C6H4)2CB(ONPh)] (6). Following the procedure for generation of 3a (10 mg of 2a was used), after 10 min, the solution was filtered through Celite, and nitrosobenzene (2 mg, 0.01 mmol) was added. The reaction mixture turned yellow immediately, and the EPR spectrum for 5 was recorded: g = 2.0012, AB = 3.6 G, AN = 10.0, 1.0 G, AH = 2.8, 1.2. 3.0, 1.2, 2.9 G. HRMS (DART): m/z [2a]+ calcd for [C17H14BN2]+ 257.1250, measured 257.1248. Synthesis of [C2H2(NCH2C6H4) 2CBCl] (7). Following the procedure for generation of 3a (102 mg of 2a was used, reaction was performed in 4 mL of THF), 10 min after the reaction started, (tht)AuCl (32 mg, 0.10 mmol) was added to the reaction mixture. The reaction mixture was stirred for 1 h, and violate was removed in vacuo. The desired product was extracted from toluene solution (3 × 2 mL) and filtered through a glass filter. After it was dried in vacuo, the residue was washed with pentane (3 × 2 mL), giving an colorless solid (24 mg, yield 81%). Single crystals for X-ray studies were obtained by layering a saturated toluene solution with pentane to give colorless crystals. 1H NMR (400 MHz, CDCl3, 298 K): δ 8.25 (d, 3JHH = 8 Hz, 2H, Ph), 7.39 (dt, 3JHH = 7 Hz, 4JHH = 1.7 Hz, 2H, Ph), 7.24 (d, 3JHH = 1 Hz, 1H, Ph), 7.22 (d, 3JHH = 1 Hz, 1H, Ph), 7.21 (d, 3JHH = 2 Hz, 1H, Ph), 7.19 (br, 1H, Ph), 7.17 (s, 2H, NCH), 5.57 (d, 2JHH = 16 Hz, 2H, NCH2Ph), 5.11 (d, 2JHH = 16 Hz, 2H, NCH2Ph). 13C{1H} NMR (126 MHz, CDCl3, 298 K): δ 133.0, 127.6, 126.2, 125.8, 119.5 51.4 (NC(CH3)2). 11B NMR (128 MHz, CDCl3, 298 K): δ −10.9. HRMS (DART): m/z [2a]+ calcd for [C17H14BN2]+ 257.1250, measured 257.1249. Synthesis of [(C2H2(NCH2C6H4)2CB)2(S4)] (8). Following the procedure for generation of 3a (204 mg of 2a was used, reaction was performed in 8 mL of C6H5Cl), 10 min after the reaction started, S8 (26 mg, 0.10 mmol) was added to the reaction mixture. The reaction mixture was stirred for 1 h, and violate was removed in vacuo. The desired product was extracted from benzene solution (3 × 2 mL) and filtered through a glass filter. After it was dried in vacuo, the residue was washed with pentane (3 × 2 mL), giving a light yellow solid (40 mg, yield 63%). Single crystals for X-ray studies were obtained from

slow evaporation of a pentane into a saturated dichloromethane solution at −35 °C to give light yellow crystals. 1H NMR (400 MHz, Tol-d8, 298 K): δ 8.46 (m, 4H, Ph-H), 7.34 (m, 4H, Ph-H), 7.20 (m, 4H, Ph-H), 6.88 (m, 4H, Ph-H), 5.96 (m, 4H, −NCH−), 5.20 (m, 4H, −N−CH2−Ph), 4.20 (m, 4H, −N−CH2−Ph). 13C{1H} NMR (126 MHz, Tol-d8, 298 K, partial): δ 133.8, 133.7, 118.7, 118.1, 50.9 (−NC(CH3)2−). 11B NMR (128 MHz, Tol-d8, 298 K): δ −14.0 (s). LRMS (MALDI): m/z [M]+ calcd for [C34H28BN4S4]+ 642.14, measured 642.33. HRMS (DART): m/z [2a] + calcd for [C17H14BN2]+ 257.1250, measured 257.1247. X-ray Data Collection, Reduction, Solution, and Refinement. Single crystals were coated with Paratone-N oil, mounted using a glass fiber pin, and frozen in the cold nitrogen stream of the goniometer. Data sets were collected on a Bruker Apex II diffractometer. The data were collected at 150(±2) K for all crystals. Data reduction was performed using the SAINT software package, and absorption corrections were applied using SADABS.78 The structures were solved using XS and refined by full-matrix least squares on F2 using XL as implemented in the SHELXTL suite of programs. Carbon-bound hydrogen atoms were placed in calculated positions using an appropriate riding model and coupled isotropic temperature factors. Electrochemistry. Cyclic voltammetry experiments were performed using a BASi-Epsilon Model RDE-2 instrument. A standard three-electrode cell configuration was employed using a glassy-graphite working electrode, a platinum-wire counter electrode, and a silver wire as a reference electrode. Formal redox potentials were referenced to the ferrocene/ferrocenium redox couple. [nBu4N][B(C6F5)4] was used as supporting electrolyte. The scanning direction was from positive to negative potential. Computations. All calculations were computed using the Gaussian 09 program.79 Geometry optimizations were performed at the (u)M06-2X functional.80 The Def2-SVP basis set was used for all atoms. The stationary nature of the converged-upon geometry was confirmed by carrying out a frequency calculation and ensuring the absence of imaginary frequencies. A stability calculation was also performed on the geometries of 3a and 3b to confirm their singlet lowest energy states. Attempts to locate the transition state of the dimerization process of 3a were performed by optimizing the structure of 3a with various frozen B−B bond lengths (0.2 Å per step from 2.1 to 3.1 Å) at the M06-2X/Def2-SVP level, and the triplet state of 3a was performed at the uM06-2X/Def2-SVP level. Geometry optimizations of the literature-known diboranes were performed at the M06-2X/Def2-SVP level for all atoms with the solid-state structures as the starting point81−85 and checked with the absence of imaginary frequencies. Frontier orbital energies and isotropic shifts for 3a and 3b were computed at the GIAO-B97-2/Def2-TZVP//M062X/Def2-SVP level of theory using the parent compound 2a (δ(11B) 40.2 ppm) as the standard.86 Natural bond orbital (NBO) analysis was carried out at the M06-2X/Def2-TZVP//M06-2X/Def2-SVP level with the NBO 6.0 program.87 Atoms in molecules (AIM) analysis of 3a was performed with the MultiWin 3.3.8 program.88 For compound 6, initial estimates of the hyperfine coupling constants were obtained by evaluating the Fermi contact coupling constants for each nucleus using the expectation value of the spin operator and the spin density at that nucleus. To obtain the requisite flexibility in the core region of the nuclei, the electronic structures of 6 and 6′ were obtained by deconstructing the Def2-TZVP basis set into its constituent primitive functions for all atoms. The reported Fermi contact couplings were converted into gauss using the experimentally obtained g factor of 6.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00522. Experimental procedures, spectroscopic data and crystallographic data (PDF) Cartesian coordinates for the calculated structures (XYZ) F

DOI: 10.1021/acs.organomet.7b00522 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Accession Codes

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CCDC 1501224−1501225, 1541763−1541765, and 1558211− 1558212 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 data_ [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 for D.W.S.: [email protected]. ORCID

Douglas W. Stephan: 0000-0001-8140-8355 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to acknowledge the Canadian Foundation for Innovation, project number 19119, and the Ontario Research Fund for funding of the Centre for Spectroscopic Investigation of Complex Organic Molecules and Polymers. D.W.S. gratefully acknowledges the financial support of the NSERC of Canada and is grateful for the award of a Canada Research Chair and an Einstein Fellowship at TU Berlin. L.L.C. is grateful for the award of an Ontario Graduate Scholarship and insightful discussions with Dr. Timothy Burrow, Dr. Timothy Johnstone, Dr. Liu (Leo) Liu, and Mr. Alexander E. Waked and the NMR staff at the University of Toronto. The computational work was made possible by the facilities of the Shared Hierarchical Academic Research Computing Network (SHARCNET: http://www.Sharcnet.ca) and Compute Canada.

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