Diverse Bonding Activations in the Reactivity of a Pentaphenylborole

Mar 21, 2017 - Department of Chemical Sciences, Indian Institute of Science Education ... (D.K.), *E-mail: [email protected]. ... For a more comprehensi...
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Diverse Bonding Activations in the Reactivity of a Pentaphenylborole toward Sodium Phosphaethynolate: Heterocycle Synthesis and Mechanistic Studies Yan Li,† Rahul Kumar Siwatch,† Totan Mondal,‡ Yongxin Li,† Rakesh Ganguly,† Debasis Koley,*,‡ and Cheuk-Wai So*,† †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371 Singapore ‡ Department of Chemical Sciences, Indian Institute of Science Education and Research Kolkata, Mohanpur 741 246, India S Supporting Information *

ABSTRACT: The reaction of the pentaphenylborole [(PhC)4BPh] (1) with sodium phosphaethynolate·1,4-dioxane (NaOCP(1,4-dioxane)1.7) afforded the novel sodium salt of phosphaboraheterocycle 2. It comprises anionic fused tetracyclic P/B-heterocycles that arise from multiple bond activation between the borole backbone and [OCP]−anion. Density functional theory calculations indicate that the [OCP]− anion prefers the form of phosphaethynolate −O−CP over phosphaketenide O CP− to interact with two molecules of 1, along with various B−C, C−P, and C−C bond activations to form 2. The calculations were verified by experimental studies: (i) the reaction of 1 with NaOCP(1,4-dioxane)1.7 and a Lewis base such as the Nheterocyclic carbene IAr [:C{N(Ar)CH}2] (Ar = 2,6-iPr2C6H3) and amidinato amidosilylene [{PhC(NtBu)2}(Me2N)Si:] afforded the Lewis base-pentaphenylborole adducts [(PhC)4B(Ph)(LB)] (LB = IAr (3), :Si(NMe2){(NtBu)2CPh} (4)), respectively; (ii) the reaction of 1 with the carbodiimide ArNCNAr afforded the seven-membered B/N heterocycle [B(Ph) (CPh)4C(NAr)N(Ar)] (5). Compounds 2−5 were fully characterized by NMR spectroscopy and X-ray crystallography.

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INTRODUCTION Boroles are singlet, highly Lewis acidic, and antiaromatic fivemembered 4π electron systems that possess rich chemistry for building boron-containing heterocycles.1Although the preparation of a borole can date to 1969,2 the reactivity of boroles with unsaturated molecules via Diels−Alder and coordination/ringexpansion pathways was not intensively explored until recent investigation by research groups of Piers, Braunschweig, and Martin.3 In cases of coordination/ring-expansion pathways, the reactions are usually driven by unfavorably antiaromatic borole rings, which proceed through the coordination of Lewis acidic boron centers with unsaturated molecules, such as alkynes,4 ketones, isocyanates,3h carbon monoxide,3b organic azides,3k,5 and diazo compounds,6 and subsequent cleavage of B−C bonds, to give boron-containing six- and seven-membered heterocycles. Among these reactions, it is noteworthy that boroles reacted with triply bonded substrates resulting in a diversity of reaction mechanisms. First, carbon monoxide and © 2017 American Chemical Society

acetonitrile reacted with the pentaphenylborole [(PhC)4BPh] to afford donor−acceptor adducts, which were rearranged to ring-expanded products [C(O)B(Ph)(CPh)4] (A; Scheme 1) and [C(CH3)NB(Ph)(CPh)4]2 (B) through 1,1- and 1,2insertions in solution, respectively.3b In addition, the reaction of diphenylacetylene with [(PhC)4BPh] formed the sevenmembered borepin [(PhC)6BPh] (C) via the rearrangement of a bicyclic Diels−Alder intermediate.4a,c When the phenyl substituents of [(PhC)4BPh] are replaced by C6F5, the resulting pentaarylborole [(PhFC)4BPhF] (PhF = C6F5) reacted with diphenylacetylene to go through a phenyl migration and ringexpansion mechanism to yield the six-membered boraheterocycle [C(CPh2)B(PhF)(CPhF)4] (D) as the major product.4 The reaction can also undergo a Diels−Alder pathway to yield the borepin [B(PhF)(CPhF)6] as the minor product. Moreover, Received: January 15, 2017 Published: March 21, 2017 4112

DOI: 10.1021/acs.inorgchem.7b00128 Inorg. Chem. 2017, 56, 4112−4120

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Inorganic Chemistry Scheme 1. Representative Ring-Expansion Products by the Reaction of Boroles with Triply Bonded Moleculesa

a

Carbon monoxide: A; acetonitrile: B; alkyne: C and D; azides: E, F and G (TMS = SiMe3; Mes = 2,4,6-Me3C6H2); 1-adamantylphosphaalkyne: H.

Scheme 2. Synthesis of Compound 2

to orange. After 1 h of the reaction, the 31P NMR spectrum showed the appearance of a sharp singlet at δ −70 ppm, indicating the consumption of NaOCP (δ −392 ppm)8b and the formation of a sole product. In the second NMR-scale reaction, a 2:1 molar ratio of 1 and [NaOCP(1,4-dioxane)1.7] was employed. The 31P NMR spectrum showed a completed consumption of the latter. In this context, 1 mmol of compound 1 reacted with 0.5 mmol of NaOCP in toluene from −78 °C to room temperature, and this conversion smoothly afforded compound 2 (Scheme 2), which was isolated as pale yellow crystals in 62% yield. The 11B NMR spectrum of 2 recorded in deuterated tetrahydrofuran (THF-d8) shows a broad signal at δ 24.1 ppm and a sharp singlet at δ −2.94 ppm, indicating the presence of two boron centers with different coordinative environments. In line with the observation of above-mentioned NMR-scale reactions, the 31P NMR signal of 2 (δ −70.5 ppm) is significantly low-field shifted relative to that of the 1-phospha-6-boratricyclohept-3-ene (H, δ −130.7 ppm).7 Compound 2 is stable in an argon atmosphere for at least three months but slowly decomposes to white powder when exposed in air and moisture. Compound 2 was studied by X-ray crystallography (Figure 1). In the first intuition, the C−P bond in the [OCP]− (O2C9P1) is cleaved. Compound 2 appears to be formed by an anionic phosphorus atom (P1) undergoing a [1 + 2] cycloaddition with the CC bond (C1C2) of a fused boraheterocycle, which is afforded by the insertion of a carbon monoxide moiety (O2C9) into two borole rings (B1C5C6C7C8 and B2C1C2C3C4), along with the phenyl migration (from B1 to C9). The phosphorus atom (P1) is also coordinated with one of the boron centers (B1) in the fused bora-heterocycle. Accordingly, the C1−C2 bond (1.552(7) Å) is a single bond, which is significantly longer than the C3−C4

the interactions of boroles with organoazides led to various Ninsertion reactions due to different reaction conditions and substituents of boroles and organoazides, which afforded a variety of ring-expanded B−N heterocycles, such as the eightmembered BN3C4 heterocycle [N(SiMe3)B(Ph){C(Ph)}4N2] (E), 1,2-azaborine [N(SiMe3)B(Ph){C(Ph)}4] (F), and 1,2azaborinine-substituted azo dye [N(N2Mes)B(Mes){C(Ph)}4] (Mes = 2,4,6-Me3C6H2; G).3k,5 Furthermore, Martin et al. reported the synthesis of the fused P/N-heterocycle, 1phospha-6-boratricyclohept-3-ene, by the reaction of the pentaphenylborole [(PhC)4BPh] with 1-adamantylphosphaalkyne (Ad-CP) (Ad = 1-adamantyl; H).7 Recently, sodium phosphaethynolate (NaOCP), which has an [O−CP]− anionic structure, gained limelight due to its importance as an anionic phosphorus transfer reagent for building a variety of phosphorus-containing four-, five-, and sixmembered heterocycles.8 Intrigued by the chemistry of borole and sodium phosphaethynolate, we are interested in investigating whether the latter can be utilized to expand a borole ring to form an unprecedented P/B heterocycle. Herein, we report the reactivity of the pentaphenylborole [(PhC)4BPh] toward NaOCP. In addition, DFT calculations were performed to understand the reaction mechanism. Further experimental studies: (1) the reactions of [(PhC)4BPh], NaOCP, and Lewis bases (the N-heterocyclic carbene and amidinato amidosilylene) and (2) the reaction of [(PhC)4BPh] with a carbodiimide were performed to verify DFT calculations.

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RESULTS AND DISCUSSION

In the first NMR-scale experiment, the reaction of the pentaphenylborole [(PhC)4BPh] (1) and sodium phosphaethynolate·1,4-dioxane [NaOCP(1,4-dioxane)1.7] in a 1:1 molar ratio in C6D6 resulted in gradual color change from deep blue 4113

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Inorganic Chemistry

DFT calculations at BP86-D3BJ/def2-TZVP(SMD)//BP86D3/def2-SVP level of theory were performed to gain insight into the mechanism and driving force for the formation of 2.14 The formation of 2 from respective reactants (1 and NaOCP) is highly exothermic as well as exergonic (ΔHLSol/ΔGLSol = −236.0/−138.2 kcal/mol), indicating a facile product formation. To effectively reduce the computation cost, two meta-Ph substituents of 1 were replaced by H atoms, and this model reactant is designated as 1M. To check the reliability of this model reactant, the formation of anionic B/P heterocycle products (1/1M→2/2M) in both cases were calculated and compared. The geometrical feature reveals similar heterocycle framework as evident from the overlay figure (Figure S5). The comparable energy values [ΔHLSol (1→2/1M→2M) = −236.0/ −240.6 kcal/mol] suggest that 1M can be used as a suitable reactant model for investigating the mechanistic pathways for the heterocycle formation. In the initial reaction step, there are two possible modes of attack to the electrophilic B1 atom in 1M, namely, O- and Pattack from [OCP]−. Although both the intermediates formed after nucleophilic coordination show comparable energetics (ΔH = −39.1/−39.9 kcal/mol for O/P attack at BP86-D3/ def2-SVP level), it is quite evident that the B−P coordinated species will not afford 2M. It is also noteworthy to mention that the negative natural population analysis (NPA) charge on the O1 atom is greater (−0.518 e) than that on the P1 (−0.413 e) atom of [OCP]− indicating a clear electronic preference for the O-attack to occur. The nucleophilic O1 attack of NaOCP to the Lewis acidic B1 center of 1M furnishes the intermediate 1MA following a barrierless transformation (Figure 2). The formation of 1MA from the reactants 1M and NaOCP is highly exothermic by −27.1 kcal/mol (Figure 2). Molecular orbital analysis reveals that the lowest unoccupied molecular orbital (LUMO) of 1M is mainly contributed by the empty π orbital of the C2−B1−C3 skeleton, while the O1 lone pair significantly contributes to the highest occupied molecular orbital (HOMO) of NaOCP (Figure S3, see the Supporting Information). Moreover, the boron-centered electrophilicity of 1M is supported by the NPA charge of 0.816 e at the B1 atom. During this nucleophilic addition (1M→1MA), a direct coordination of the O1 to B1 center allows the electron density on the B1 atom to increase as reflected from a significant decrease of positive NPA charge (qB1 = 0.490/0.816 e in 1MA/ 1M). As the electron density of OCP fragment (|q| = 0.304 e) is transferred to the B1 center, the C1−O1 bond gets elongated by 0.049 Å. In the subsequent step, the relatively more electronrich C2 (qC2 = −0.297 e) atom of intermediate 1MA readily reacts with another molecule of 1M to furnish intermediate 1MB. To our surprise, a genuine transition state could not be located even after multiple attempts.14 However, it is important to note that, in comparison to 1MA, the formation energy of 1MB is exothermic both in gas phase (ΔH{1MA→1MB}= −17.5 kcal/ mol) and under solvent environment (ΔHLsol{1MA→1MB} = −13.1 kcal/mol). Eventually, the inclusion of the entropy contribution in this combination step is still exergonic (ΔG{1MA→1MB} = −2.2 kcal/mol) in gas phase but slightly endergonic (ΔGLsol{1MA→1MB} = 2.2 kcal/mol) in toluene (Figure S3). Subsequently, gradual progress of the electron-rich C6 atom toward the C1 center in 1MB results in the formation of the intermediate 1MC. This reaction is best viewed as an intramolecular nucleophilic attack of the C6 center to the electrophilic C1 center of OCP fragment in 1MB. The frontier

Figure 1. X-ray crystal structure of 2: perspective view of the molecule.9 Hydrogen atoms, toluene molecules, and disorder in phenyl substituents are omitted for clarity. Thermal ellipsoids are drawn at 15% probability. Selected bond lengths (Å) and angles (deg): C1−B2 1.600(9), C1−C2 1.552(7), C2−C31.537(7), C3−C4 1.347(7), C4− B1 1.615(8), C9−B1 1.643(9), O2−C9 1.456(7), B2−O2 1.372(8), C1−P1 1.915(5), C2−P1 1.839(5), B1−P1 1.977(6), B1−C5 1.612(8), C5−C6 1.348(7), C6−C7 1.501(8), C7−C8 1.367(10), C8−C9 1.541(8), B1−C9 1.643(9), Na1−P1 2.902(3), Na1−O1 2.272(5); C1−C2−C3 112.5(4), C2−C3−C4 116.7(4), C3−C4−B1 118.0(5), C4−B1−P1 101.0(3), C4−B1−C5 113.7(5), C4−B1−C9 110.0(5), C5−B1−P1 114.2(4), C5−B1−C9, 112.2(4), C9−B1−P1 104.9(4), C1−B2−O2 122.8(6), O2−B2−C64 113.7(6), C64−B2− C1 123.5(6), C1−P1−B1 95.8(2), C2−P1−B1 94.2(2), C1−P1−C2 48.8(2).

bond (1.347(7) Å), as well as the C5−C6 (1.348(7) Å) and C7−C8 bonds (1.367(10) Å) of the C5C6C7C8 butadiene skeleton. The phosphorus atom is covalently bonded to the C1 and C2 atoms to form a three-membered ring. The C−P1 bonds are unequal (1.915(5), 1.839(5) Å) and comparable with those of the phosphacyclopropane moiety in [Cp(CO)2FeP{C(SiMe3)2-C(H)(SiMe3)}] (1.872(7), 1.861(6) Å)10 and the 1-phospha-6-boratricyclohept-3-ene (1.8701(10), 1.9672(18) Å).7 The B1−P1 bond length (1.977(6) Å) falls in the range of typical B−P single bonds (1.93−2.00 Å).11 The Na−P bond length (2.902(3)Å) is comparable with that of the disilylated sodium monophosphanides [Na{P(SiPhtBu2)2}]2 (2.887(3), 2.897(3) Å).12 In addition, the Na1−C contacts between the phenyl rings and sodium center are ranged from 2.802(6) to 2.943(8) Å. The two boron atoms exhibit different geometries. The B1 atom adopts a tetrahedral geometry, while the B2 atom exhibits a trigonal planar geometry (the sum of the bond angles at the B2 center is 360°). In this context, the B1 and B2 atoms correspond to the relatively high-field (s, δ −2.94 ppm) and low-field shifted (br, δ 24.1 ppm) signals in the 11B NMR spectrum, respectively. The B1−C bond lengths (1.612(8)− 1.634(9) Å) are comparable with typical B−C single bonds (cf. l.55−1.64 Å).13 4114

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Figure 2. Energy profile for the reaction of 1M with NaOCP at BP86-D3BJ/def2-TZVP(SMD)//BP86-D3/def2-SVP level. The energy convention can be found in the Computational Details.

molecular orbital (FMO) analysis of 1MB rationalizes that the HOMO is mainly on the C5−C6 π-orbital, whereas the LUMO +1 locates on the vacant pz orbital at the C1 atom (Figure S3). The activation barrier for the transformation from 1MB→1MC is 27.5 kcal/mol, which is the rate-limiting step for the transformation from 1M→1ME. The single imaginary mode in 1MB-TS depicts the C1−C6 bond formation with concomitant weakening of the C1−P1 bond. The weakening of the C1−P1 bond is reflected in the Wiberg bond order of 2.08 in 1MB-TS relative to 2.61 in 1MB. The C1−C6 distance in 1MB-TS is reduced by 1.862 Å, and the C1−P1 bond is increased by 0.063 Å. In the resulting intermediate 1MC, the C4−P1 and C1−P1 bond lengths are 1.877 and 1.772 Å, respectively. The overall reaction energy for this intramolecular nucleophilic attack is highly exothermic (ΔHLsol{1MB→1MC} = −19.9 kcal/mol) as well as exergonic (ΔGLsol{1MB → 1MC} = −17.9 kcal/mol), suggesting an adequate thermodynamical driving force for the formation of 1MC (Figure 2 and Scheme S1). In the following step, the electron-rich C3 center attacks the electron-deficient P1 atom (qC3/qP1 = −0.512/0.666 e) with simultaneous formation of the B2−C1 bond by utilizing the electron density from the C1P1 double bond (Figure 2). This step surmounts an energy barrier of 21.7 kcal/mol while passing through the transition state 1MC-TS, leading to the intermediate 1MD. The transition vector animates formation of the B2−C1 bond with simultaneous weakening of the C1−P1 bond in 1Mc-TS. The C1−P1 bond length (1Mc/1Mc-TS = 1.772/1.964 Å) in 1MC-TS

is significantly longer than that in the intermediate 1MC, while the B2···C1 distance (1Mc/1Mc-TS = 2.855/2.204 Å) in 1Mc-TS is comparatively shorter than that in the intermediate 1MC (Figure S4). These suggest a late transition state along the reaction coordinate. Gratifyingly, our calculated results reveal that the formation of 1MD from 1MC is slightly endothermic or rather isoenergetic (ΔHLsol{1MC→1MD} = 2.5 kcal/mol). This endothermicity may attribute to the formation of fused strained structure of 1MD. The structural feature of 1MD is unique in that the P1 atom forms a three-membered ring with the C3 and C4 atoms (C3−P1−C4 = 48.4°). The calculated C3−P1 and C4−P1 bond lengths are 1.935 and 1.875 Å, respectively, while the B2− C1 (1.757 Å) bond is significantly shorter than the B2···C1 distance in the intermediate 1MC. In the final reaction step, −Ph group migrates from the B2 to C1 center to furnish the monomeric product 1ME. It is found that the migration of the −Ph group occurs via the transition state 1MD-TS, and the barrier height for the migration is 23.6 kcal/mol relative to the intermediate 1MD. The corresponding single eigenmode (425i cm−1) animates the synchronous breaking of the B2−CPh bond and formation of the C1−CPh bond. In 1MD-TS, the B2−CPh bond lengthens to 2.167 Å, while the C1···C7 distance gets shortened to 2.056 Å. In 1ME, the C1−P1 bond (C···P distance: 2.950 Å) is completely broken with concurrent formation of the B2−C1 bond (1.695 Å), while the P1 atom forms three single bonds with the B2 (2.010 Å), C3 (1.965 Å), and C4 (1.875 Å) atoms. Eventually, monomeric complex 1ME, readily dimerizes 4115

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Inorganic Chemistry Scheme 3. Synthesis of Compounds 3 and 4

to give the dimeric product 2M. The dimerization process (1ME→2M) is accompanying with the energy liberation of −43.2 kcal/mol. Since 1,4-dioxane in [NaOCP(1,4-dioxane)1.7] is a donor molecule, it can coordinate with the Lewis acidic boron atom of 1 to hinder the nucleophilic attack of NaOCP, which may result in another reaction mechanism such as Diels−Alder reaction mechanism in the reaction of 1 with Ad-CP.7 To verify the above-mentioned reaction mechanism, which does not include the presence of 1,4-dioxane, the reactions of 1 and [NaOCP(1,4-dioxane)1.7] in the presence of Lewis bases, IAr (:C{N(Ar)CH}2),15 and [LSiNMe2] (L = PhC(NtBu)2)16a were performed. In these reactions, compound 2 was not observed; instead, the IAr−pentaphenylborole adduct [IArBC4Ph5] (3) and the amidinato amidosilylene−pentaphenylborole adduct [L(Me2N)SiBC4Ph5] (4) were formed, respectively (Scheme 3). In addition, further reactions of 3 and 4 with [NaOCP(1,4dioxane)1.7] show no reactivity. It is postulated that the Lewis bases (IAr, [LSiNMe2]) compete with OCP− anion toward the boron center of 1. The coordination of the Lewis bases and 1 remarkably decreases the antiaromaticity of the borole backbone, which results in preventing further interaction of OCP− with 1. The formations of 3 and 4 illustrate three facts: (1) the nucleophilicity of OCP− is weaker than that of IAr (:C{N(Ar)CH}2) and [LSiNMe2] but stronger than that of 1,4dioxane; (2) the reaction of 1 with NaOCP proceeds through the nucleophilic attack of −O−CP to the boron center in the first step, which is in line with the DFT calculations; (3) the possible coordination of 1,4-dioxane toward 1 does not affect the reaction mechanism. Compound 3 was isolated as a yellow crystalline solid in good yield (91%). Similar NHC−pentaphenylborole [IMesB(Cl)(CPh)4] (IMes = :C{N(Mes)CH}2, Mes = 2,4,6Me3C6H2) was reported by Braunschweig et al.17 In addition, the NMR spectroscopic and crystallographic data18 of 3 are comparable with those of [IMes-B(Cl)(CPh)4]. Compound 4 was isolated as a colorless crystalline solid in 82% yield. Its 29Si NMR signal (δ 22.9 ppm) is broad due to the quadrupolar moment of the Bborole nucleus. It also shows a significant low-field shift compared with that of [LSiNMe2] (δ −2.62 ppm),16a as the lone-pair electrons on the Si(II) center are donated to the vacant orbital on the boron center. The Xray crystal structure of 4 (Figure 3) shows that the Si1−B1 bond (2.040(7) Å) is slightly longer than that of the amidinato silylene−borane adduct [LSi(H)(BH3)] (1.9624(5)Å).16b Besides compound 4, other silylene−borane adducts were synthesized.16c,d [OCP]− is a superposition of the resonance structures of phosphaethynolate −O−CP and phosphaketenide OC P−.8c Although DFT calculations for the reaction of 1 with [NaOCP(1,4-dioxane)1.7] show that the nucleophilic O-attack of phosphaethynolate −O−CP is preferred over the nucleophilic P-attack of phosphaketenide OCP− toward

Figure 3. X-ray crystal structure of 4. Hydrogen atoms, toluene molecules, and disorder in tBu substituents are omitted for clarity. Thermal ellipsoids are drawn at 30% probability. Selected bond lengths (Å) and angles (deg): B1−C18 1.636(8), B1−C24 1.626(8), B1−C27 1.638(7), B1−Si1 2.040(7), N3−Si1 1.691(5), N1−Si1 1.866(4), N2−Si1 1.834(5); C24−B1−C27 99.1(4), N3−Si1−B1 119.3(2), N3−Si1−N1 107.2(2), N3−Si1−N2 107.8(2), N1−Si1−N2 70.7(2).

the B center, it is still interesting to evaluate how the phosphaketenide OCP− interacts with 1. In other words, we intended to assess the reaction pathway of an allene-like skeleton toward 1. First, the reaction of 1M with neutral silyl phosphaketene OCP−SiR3 was calculated. Unfortunately a similar intermediate of the type 1MA was not found, since repeated optimizations resulted in decoordination of the combining reactants.19 As such, the carbodiimide ArNC NAr (Ar = 2,6-iPr2C6H3), which is an allene analogue similar to OCP−, was tested to react with 1. The reaction of compound 1 with ArNCNAr in a 1:1 molar ratio in toluene at room temperature afforded the B/N heterocycle [N(Ar)BPh(CPh)4C(NAr)] (5) as a yellow crystalline solid in excellent yield (90%; Scheme 4). In its 11B NMR spectrum, compound 5 shows a broad signal at δ 40 ppm, which is close Scheme 4. Synthesis of Compound 5

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formation of 2 as illustrated by the above-mentioned DFT calculations.

to that of the 1,2-azaborinine [N(Mes)B(Ph)(CPh)4] (δ 35.9 ppm).5b Compound 5 was analyzed by X-ray crystallography (Figure 4). The BNC5 ring of 5 is puckered. The endocyclic

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CONCLUSION An unprecedented sodium salt of the phosphaboraheterocycle anion 2 was synthesized via the reaction of the pentaphenylborole [(PhC)4BPh] (1) with [NaOCP(1,4-dioxane)1.7]. Compound 2 is anionic fused P/B-heterocycles. It was formed by various B−C, C−P, and C−C bond activation between the borole backbone and [OCP]− anion, which was illustrated by both DFT calculations and experimental studies. Further reactivity of 1 with other allene analogues is currently investigated.

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EXPERIMENTAL SECTION

All reactions and handling of reagents were performed under an atmosphere of argon using standard Schlenk techniques or a glovebox. Compound 1,2a NaOCP(1,4-dioxane)1.7,8b LSiNMe2 (L = PhC(NtBu)2),16 and IAr (:C{N(Ar)CH}2, Ar = 2,6-iPr2C6H3)15 were synthesized according to the literatures. Further, the 1,4-dioxane content in NaOCP was estimated according to the known 31P NMR spectroscopic method.8b ArNCNAr was purchased from TCI and used directly. Solvents were purified by an M-Braun solvent drying system. Solution NMR spectra were recorded on a JEOL ECA or JEOL ECASL 400 NMR spectrometer. Deuterated solvents C6D6 and THF-d8 were dried over Na/K alloy with stirring for 2 d, followed by distillation and degassing. 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. Synthesis of 2. Toluene (30 mL) was added to a mixture of 1 (0.44 g, 1.00 mmol) and NaOCP(1,4-dioxane)1.7 (0.12 g, 0.50 mmol) at −78 °C under stirring. The reaction mixture was warmed to room temperature and kept stirring for 2 h at this temperature. The blue suspension slowly turned to dark orange solution, which was then filtered. The filtrate was concentrated under vacuum to one-third of its original volume. Compound 2 was isolated as slight yellow crystals, which were dried under vacuum. Single crystals of compound 2 suitable for X-ray crystallography were grown from its concentrated toluene solution at 0 °C. Yield: 0.33 g (62%, based on NaOCP). mp 182 °C (dec). Anal. calcd (%) for C142H108B4Na2O4P2·C7H8 (2· toluene): C, 84.31; H, 5.51. Anal. found: C, 84.62; H, 5.52. 1H NMR (THF-d8, 298 K, 400 MHz): δ 8.27 (d, 2H, C6H5), 7.77 (d, 2H, C6H5), 7.42 (d, 2H, C6H5), 7.32 (d, 2H, C6H5), 7.25−5.75 (m, 42H, C6H5).13C NMR (THF-d8, 298 K, 100 MHz): δ 153.3, 152.5, 150.4, 150.1, 147.8, 144.8, 143.8, 143.3, 142.9, 141.7, 139.8, 138.2, 137.5, 137.0, 134.2, 132.3, 132.2, 130.5, 130.4, 129.6, 129.5, 128.8, 128.7, 128.0, 126.3, 126.2, 126.1, 125.8, 125.3, 125.1, 125.0, 124.9, 124.5, 123.8, 123.1, 123.0, 122.9, 122.6, 122.1, 120.8, 65.5, 40.1, 20.6, 14.8 ppm. 11B NMR (THF-d8, 298 K, 128 MHz): δ 24.1 (br, s, BC(Ph)(O)), −2.94 (s, C3BP) ppm. 31P NMR (THF-d8, 298 K, 162 MHz): δ −70.5 ppm. Reaction of 1, NaOCP(1,4-dioxane)1.7, and IAr to Form 3. Toluene (30 mL) was added to a mixture of 1 (0.44 g, 1.00 mmol), NaOCP(1,4-dioxane)1.7 (0.24 g, 1.00 mmol), and IAr (0.39 g, 1.00 mmol) at room temperature. The reaction mixture initially was yellow and was stirred for 1 h. The resulting brown suspension was then filtered, and the filtrate was concentrated under vacuum to one-third of its original volume. Compound 3 was isolated as yellow crystals and dried under vacuum. Single crystals of compound 3 suitable for X-ray crystallography were grown from its concentrated toluene solution at 0 °C. Yield: 0.73 g (91%, based on 1). mp: 146 °C (dec). Anal. calcd (%) for C61H61BN2: C, 87.96; H, 7.38; N, 3.36. Anal. found: C, 88.29; H, 7.52; N, 3.03. 1H NMR (C6D6, 298 K, 400 MHz): δ 7.50 (d, 2H, C6H5), 7.15−6.72 (m, 29H, C6H5 + Ar-H), 6.50 (s, 2H, C-H (IAr)), 3.06 (br, 4H, iPr-H), 0.93 (d, 24H, iPr-Me, 3JH−H = 6.8 Hz). 13C NMR (C6D6, 298 K, 100 MHz): δ 159.9 (Ccarbene), 149.4, 145.6, 140.9, 137.8, 137.5, 136.4, 131.1, 131.0, 130.0, 129.0, 127.5, 126.8, 126.6, 125.4,

Figure 4. X-ray crystal structure of 5. Hydrogen atoms and a toluene molecule are omitted for clarity. Thermal ellipsoids are drawn at 30% probability. Selected bond lengths (Å) and angles (deg): B1−N1 1.417(2), B1−C17 1.571(2), C16−C17 1.349(2), C15−C16 1.483(2), C14−C15 1.353(2), C13−C14 1.504(2), C13−N1 1.428(2), C13− N2 1.268(2); C17−B1−N1 120.5(1), C13−N1−B1 122.1(1), N1− C13−C14 117.4(1), N1−C13−N2 116.8(1), N2−C13−C14 125.3(1).

B1−N1 bond (1.417(2) Å) is comparable with that of the 1,2azaborinine [N(4-iPrC6H4)B(Mes)(CPh)4] (1.434(3) Å),5a which indicates the delocalization of lone-pair electrons on the N1 atom to the vacant p orbital on the B1 atom. It is also consistent with the angular sum of 359.91° and 358.96° around the B1 and N1 atoms, respectively, which confirm their trigonal planar geometries. The bond length analysis of B1 to C13 bond (via C17, C16, C15, C14) suggests alternating single and double bonds and therefore excludes the possibility of a genuine electronic delocalization in the seven-membered ring of compound 5. Moreover, the exocyclic C13−N2 bond length (1.268(2) Å) is comparable with typical CN double bonds, indicating that the π-electrons of the C13−N2 bond do not delocalize into the BNC5 ring. The reaction of 1 with ArN CNAr appears to proceed by the coordination of one nitrogen atom from ArNCNAr with the boron atom, followed by the insertion of the CN double bond into the C−B bond. Similar reaction mechanism was found in the reaction of 1 with adamantyl isocyanate and isothiocyanate to form the B/N seven-membered heterocycles [N(Ad)B(Ph)(CPh)4C(E)] (E = O, S).3h,20 When the adamantyl group was replaced with less steric-hindered aryl ring, the B/N sevenmembered heterocycles further underwent a Diels−Alder reaction to form fused heterocycles.20 These experimental studies suggest that the phosphaketenide OCP− may undergo similar reaction mechanism via coordination of the P center with the B atom of 1, followed by insertion of the CP bond to form P/B heterocycle intermediate “PB(Ph)(CPh)4C(O)”. However, this reaction pathway cannot lead to the 4117

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Inorganic Chemistry

(NBO)27 analysis was performed at BP86-D3BJ/def2-TZVP(SMD)// BP86-D3/def2-SVP level using NBO Version 3.1 program implemented in Gaussian package. Accurate calculation of Gibbs free energy in the solution phase is still a challenging task for computational chemists because of significant consumption of translational degrees of freedom upon moving from the gas phase to solvent, and no standard procedure is readily available to account for this deficiency. It has been found that if the number of molecules alters during the reaction, the Gibbs energy change is enormous due to overestimation of entropy contribution.28 Therefore, it is more reliable to represent the enthalpy changes over Gibbs free energies in ligand association and dissociation steps.29 In view with this argument, we reported only the solventcorrected enthalpy (ΔHLSol) values at BP86-D3BJ/def2-TZVP(SMD)//BP86-D3/def2-SVP level, if otherwise not mentioned. Wiberg bond indices (WBI)30 were calculated at the same level of theory. Optimized geometries and orbital diagrams are illustrated using Chemcraft and CYLview drawing.31 We tried to optimize a transition state connecting the intermediates 1MA and 1MB by considering the following combinations of methods and basis sets: BP86-D3/def2-SVP, BP86-D3/6-31G*, M06-L/def2-SVP, M06-L/631G*, B3LYP-D3/def2-SVP, and B3LYP-D3/6-31G*. Despite such extensive and careful search, we are unable to locate a transition state connecting the intermediates 1MA and 1MB.

124.4, 124.1, 124.0, 123.7, 28.7, 26.6, 22.2, 21.1 ppm. 11B NMR (C6D6, 298 K, 128 MHz): δ −6.62 (s) ppm. Reaction of 1, NaOCP(1,4-dioxane)1.7, and LSiNMe2 to Form 4. Toluene (30 mL) was added to a mixture of 1 (0.44 g, 1.00 mmol), NaOCP(1,4-dioxane)1.7 (0.24 g, 1.00 mmol), and LSiNMe2 (0.30 g, 1.00 mmol) at room temperature. The reaction mixture was stirred for 30 min at this temperature, and the blue solution turned to slight yellow. The resulting solution was filtered, and the filtrate was concentrated under vacuum to one-third of its original volume. Compound 4 was isolated as colorless crystals and dried under vacuum. Single crystals of compound 4 suitable for X-ray crystallography were grown from its concentrated toluene solution at 0 °C. Yield: 0.73 g (82%, based on 1). mp 136 °C (dec). Anal. calcd (%) for C51H54BN3Si: C, 81.90; H, 7.28; N, 5.62. Anal. found: C, 82.20; H, 7.40; N, 5.85. 1H NMR (C6D6, 298 K, 400 MHz): δ 8.10 (d, 2H, C6H5), 7.56 (t, 2H, C6H5), 7.39−7.33 (m, 6H, C6H5), 7.02−6.68 (m, 20H, C6H5), 2.42 (s, 6H, NMe2), 1.30−0.7 (br, 18H, tBu-H). 13C NMR (C6D6, 298 K, 100 MHz): δ 175.5 (PhC), 151.6, 146.2, 141.7, 136.1, 131.0, 130.2, 130.0, 129.5, 128.9, 127.1, 125.0, 124.9, 124.0, 53.3, 39.1, 31.7, 31.2, 30.5, 22.7, 14.0, 1.11 ppm. 11B NMR (C6D6, 298 K, 128 MHz): δ −10.1 (s) ppm. 29Si NMR (C6D6, 298 K, 99 MHz): δ 22.9 (s) ppm. Synthesis of 5. Toluene (30 mL) was added to a mixture of 1 (0.44 g, 1.00 mmol) and Ar−NCN−Ar (0.36 g, 1.00 mmol, Ar = 2,6-iPr2C6H3) at room temperature. The reaction mixture was stirred for 1 h at this temperature, during which the blue solution slowly turned to slight yellow. The resulting suspension was then filtered, and the filtrate was concentrated under vacuum to one-third of its original volume. Compound 5 was isolated as slight-yellow crystals and dried under vacuum. Single crystals of compound 5 suitable for X-ray crystallography were grown from its concentrated toluene solution at 0 °C. Yield: 0.73 g (90%, based on 1). mp 167 °C (dec). Anal. calcd (%) for C59H59BN2: C, 87.82; H, 7.37; N, 3.47. Anal. found: C, 87.36; H, 7.60; N, 3.10. 1H NMR (C6D6, 298 K, 400 MHz): δ 7.41−6.58 (m, 31H, C6H5 + Ar-H), 4.03 (sept, 1H, iPr-H, 3JH−H = 6.8 Hz), 3.79 (sept, 1H, iPr-H, 3JH−H = 6.8 Hz), 3.43 (sept, 1H, iPr-H, 3JH−H = 6.8 Hz), 1.80 (sept, 1H, iPr-H, 3JH−H = 6.8 Hz), 1.69 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz), 1.48 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz), 1.41 (d, 3H, iPr-Me, 3 JH−H = 6.8 Hz), 0.94 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz), 0.77 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz), 0.62 (d, 3 H, iPr-Me, 3JH−H = 6.8 Hz), 0.47 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz), 0.35 (d, 3H, iPr-Me, 3JH−H = 6.8 Hz). 13 C NMR (C6D6, 298 K, 100 MHz): δ 153.8, 150.3, 147.6, 147.3, 143.8, 143.7, 142.3, 140.6, 139.8, 139.4, 139.1, 138.5, 137.0, 134.1, 133.4, 132.3, 132.0, 130.7, 130.6, 129.0, 128.2, 127.1, 126.9, 126.8, 126.5, 126.4, 126.2, 125.8, 125.4, 124.4, 123.9, 123.8, 123.1, 122.7, 31.6, 30.3, 28.5, 28.3, 28.0, 25.6, 24.7, 24.6, 24.4, 23.5, 23.3, 22.7, 22.1, 20.3, 14.0 ppm. 11B NMR (C6D6, 298 K, 128 MHz): δ 40.0 (br, s) ppm. Computational Details. All calculations were performed with the Gaussian09 suite of program.21 The geometries of the saddle points were optimized using Becke and Perdew’s BP8622 in conjugation with def2-SVP23 basis set for all the atoms. Spherical basis functions (5D, 7F) were used in all cases. Additionally, the effect of dispersion was incorporated using Grimme-D3 approximation.24 Geometry optimizations were performed without any symmetry constraints. Frequency calculations were accomplished on the optimized geometries at the same level of theory (BP86/def2-SVP) to determine the nature of stationary points (minima or saddle point on the potential energy surface) and also to obtain the respective thermochemical energy values. The intermediates were verified as true minima by the absence of negative eigenvalue, whereas the transition states were characterized by a single imaginary frequency corresponding to the reaction coordinate. Intrinsic reaction coordinate (IRC) calculations were performed to designate the true transition states. For further validation, single-point BP86 calculations including D3 version of Grimme’s dispersion with Becke−Johnson damping (D3BJ) were performed on optimized geometries in conjugation with def2-TZVP basis set for all the atoms.25 Solvation energies in toluene (ε = 2.374) were evaluated by a self-consistent reaction field (SCRF) approach using the SMD continuum solvation model.26 Natural bond orbital

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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.inorgchem.7b00128. X-ray data for compounds 2−5 (CIF) Selected NMR spectra of 2−5; Table S1 giving selected X-ray crystallographic data of compounds 2−5; Figures S1 and S2 giving X-ray crystal structures of 2 and 3; Figures S3 and S4, Scheme S1 and Table S2 giving selected theoretical data of 2 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (D.K.) *E-mail: [email protected]. (C.-W.S.) ORCID

Cheuk-Wai So: 0000-0003-4816-9801 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Y.L. synthesized compounds 2−5. R.K.S. prepared NaOCP. T.M. performed DFT calculations. Yx.L. and R.G. analyzed X-ray crystal data. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by ASTAR SERC PSF and AcRF Tier 1 (RG 7/13) grants (C.-W.S.). T.M. is thankful to the CSIR for the SRF fellowship. D.K. acknowledges IISER-Kolkata for startup grant and SERB for DST fast track fellowship (SR/FT/SC72/2011).

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REFERENCES

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Article

NOTE ADDED AFTER ASAP PUBLICATION Due to a production error, this paper was published on the Web on March 21, 2017, with the incorrect artwork for Scheme 4. The corrected version was reposted on March 22, 2017.

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DOI: 10.1021/acs.inorgchem.7b00128 Inorg. Chem. 2017, 56, 4112−4120