Diverse Bonding Activations in the Reactivity of a Pentaphenylborole Toward Sodium Phosphaethynolate : Heterocycle Synthesis and Mechanistic Studies

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Diverse bonding activations in the reactivity of a pentaphenylborole toward sodium phosphaethynolate : heterocycle synthesis and mechanistic studies

Li, Yan; Siwatch, Rahul Kumar; Mondal, Totan; Li, Yongxin; Ganguly, Rakesh; Koley, Debasis; So, Cheuk‑Wai

2017

Li, Y., Siwatch, R. K., Mondal, T., Li, Y., Ganguly, R., Koley, D., & So, C.‑W. (2017). Diverse bonding activations in the reactivity of a pentaphenylborole toward sodium phosphaethynolate : heterocycle synthesis and mechanistic studies. Inorganic chemistry, 56(7), 4112–4120. doi:10.1021/acs.inorgchem.7b00128 https://hdl.handle.net/10356/139442 https://doi.org/10.1021/acs.inorgchem.7b00128

This document is the Accepted Manuscript version of a Published Work that appeared in final form in Inorganic chemistry, copyright © American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see https://doi.org/10.1021/acs.inorgchem.7b00128

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Diverse Bonding Activations in the Reactivity of a Pentaphenylborole toward Sodium Phosphaethynolate: Heterocycle Synthesis and Mechanistic Studies

Yan Li,a Rahul Kumar Siwatch,a Totan Mondal,b Yongxin Li,a Rakesh Ganguly,a Debasis

Koley*band Cheuk-Wai So*a aDivision of Chemistry and Biological Chemistry, School of Physical and Mathematical

Sciences, Nanyang Technological University, 637371 Singapore bDepartment of Chemical Sciences, Indian Institute of Science Education and Research Kolkata,

Mohanpur 741 246, India

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 which arise from multiple bond activation between the borole backbone and [OCP]⁻anion. DFT 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 N-heterocyclic 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.

Introduction

Boroles are singlet, highly Lewis acidic and antiaromatic five-membered 4π electron systems which possess rich chemistry for building boron-containing heterocycles.1Although the preparation of a borole can date back to 1969,2 the reactivity of boroles with unsaturated molecules via Diels−Alder and coordination/ring-expansion 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 azides3k,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 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 1,2- insertions in solution, respectively.3b In addition, the reaction of diphenylacetylene with

[(PhC)4BPh] formed the seven-membered borepin [(PhC)6BPh] (C) via the rearrangement of a

4a,c bicyclic Diels-Alder intermediate. When the phenyl substituents of [(PhC)4BPh] are replaced

F F F by C6F5, the resulting pentaarylborole [(Ph C)4BPh ] (Ph = C6F5) reacted with diphenylacetylene to go through a phenyl migration and ring expansion mechanism to yield the

F F 4 six-membered boraheterocycle [C(=CPh2)B(Ph )(CPh )4] (D) as the major product. The

F F reaction can also undergo a Diels−Alder pathway to yield the borepin [B(Ph )(CPh )6] as the minor product. Moreover, the interactions of boroles with organoazides led to various N- insertion 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 eight- membered BN3C4 heterocycle [N(SiMe3)B(Ph){C(Ph)}4N2](E), 1,2-azaborine

[N(SiMe3)B(Ph){C(Ph)}4] (F) and 1,2-azaborinine-substituted azo dye

3k,5 [N(N2Mes)B(Mes){C(Ph)}4] (Mes = 2,4,6-Me3C6H2; G). Furthermore, Martin et al. reported the synthesis of the fused P/N-heterocycle, 1-phospha-6-boratricyclohept-3-ene, by the reaction of the pentaphenylborole [(PhC)4BPh] with 1-adamantylphosphaalkyne (Ad-C≡P) (Ad = 1- adamantyl; H).7

Scheme 1. Representative ring expansion products by the reaction of boroles with triply bonded molecules (carbon monoxide: A; acetonitrile: B; alkyne: C and D; azides: E, F and G (TMS =

SiMe3; Mes = 2,4,6-Me3C6H2); 1-adamantylphosphaalkyne: H)

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 six-membered 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), (2) the reaction of [(PhC)4BPh] with a carbodiimide, were performed to verify DFT calculations.

Results and Discussion

Scheme 2. Synthesis of compound 2.

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 to orange. After 1 hour of the reaction, the 31P

NMR spectrum showed the appearance of a sharp singlet at  -70 ppm, indicating the consumption of NaOCP ( −392 ppm)8b and the formation of a sole product. In the second

31 NMR-scale reaction, a 2:1 molar ratio of 1 and [NaOCP(1,4-dioxane)1.7] was employed. The P

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 oC to room temperature and this conversion smoothly afforded compound 2, which was isolated as pale yellow crystals in 62

11 % yield (Scheme 1). The B NMR spectrum of 2 recorded in THF-d8 shows a broad signal at 

24.1 ppm and a sharp singlet at  - 2.94 ppm, indicating the presence of two boron centers with different coordinative environments. In line with the observation of abovementioned NMR-scale reactions, the 31P NMR signal of 2 ( -70.5 ppm) is significantly low field shifted relative to that of the 1-phospha-6-boratricyclohept-3-ene (H,  -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.

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 (o): C1-B21.600(9), C1-C2

1.552(7), C2-C31.537(7), C3-C41.347(7), C4-B11.615(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).

Compound 2 was studied by X-ray crystallography. 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 boraheterocycle. Accordingly, the C1-C2 bond (1.552(7) Å) is a single bond, which is significantly longer than the C3-C4 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

10 [Cp(CO)2FeP{C(SiMe3)2-C(H)(SiMe3)}] (1.872(7), 1.861(6) Å) 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)-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 360o). In this context, the B1 and B2 atoms correspond to the relatively high-field (s,  -2.94 ppm) and low-field shifted (br,  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

DFT calculations at BP86-D3BJ/def2-TZVP(SMD)//BP86-D3/def2-SVP level of theory were carried out 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

Sol Sol exergonic (∆HL /∆GL = -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

Sol M M the overlay figure (Figure S5). The comparable energy values [∆HL (1→2/1 →2 ) = -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.

Sol ∆HL (kcal/mol) Sol (∆ GL ) (kcal/mol)

1M 0.0 (0.0) M NaOCP 1 B-TS -12.7 (19.8) M 1 A -27.1 M (-14.7) 1 1M -TS M D M 1 C-TS 1 B -34.0 -38.4 -40.2 (-3.1) (-5.9) (-12.5) M Energy (kcal/mol) Energy M 1 D 1 C -57.6 -60.1 (-25.9) M (-30.4) 1 E -98.8 (-67.2)

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.

In the initial reaction step, there are two possible modes of attack to the electrophilic B1 atom in

1M, viz. O- and P-attack from [OCP]⁻. Although both the intermediates formed after nucleophilic coordination show comparable energetics (∆H = -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 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

M furnishes the intermediate 1 A following a barrierless transformation (Figure 2). The formation

M M of 1 A from the reactants 1 and NaOCP is highly exothermic by -27.1 kcal/mol (Figure 2).

Molecular orbital analysis reveals that the 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 HOMO of NaOCP (Figure S3, see the SI). 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

M M 1 1 1 (1 →1 A), a direct coordination of the O to B center allows the electron density on the B

1 atom to increase as reflected from a significant decrease of positive NPA charge (qB =

M M 0.490/0.816 e in 1 A/1 ). 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

2 2 M more electron rich C (qC = -0.297 e) atom of intermediate 1 A readily reacts with another

M M molecule of 1 to furnish intermediate 1 B. 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

M M M M 1 A, the formation energy of 1 B is exothermic both in gas phase (∆H{1 A→1 B}= -17.5

sol M M kcal/mol) and under solvent environment (∆HL {1 A→1 B} = -13.1 kcal/mol). Eventually, the

M M inclusion of the entropy contribution in this combination step is still exergonic (∆G{1 A→1 B}

sol M M = -2.2 kcal/mol) in gas phase but slightly endergonic (∆GL {1 A→1 B} = 2.2 kcal/mol) in toluene (Figure S3). Subsequently, gradual progress of the electron rich C6 atom towards the C1

M M center in 1 B results in the formation of the intermediate 1 C. This reaction is best viewed as an intramolecular nucleophilic attack of the C6 center to the electrophilic C1 center of OCP fragment

M M in 1 B. The FMO (Frontier Molecular Orbital) analysis of 1 B rationalizes that the HOMO is

5 6 1 mainly on the C –C π-orbital whereas the LUMO+1 locates on the vacant pz orbital at the C

M M atom (Figure S3). The activation barrier for the transformation from 1 B→1 C is 27.5 kcal/mol,

M M which is the rate limiting step for the transformation from 1 →1 E. The single imaginary mode

M 1 6 1 1 in 1 B-TS depicts the C –C bond formation with concomitant weakening of the C –P bond.

1 1 M The weakening of the C –P bond is reflected in the Wiberg bond order of 2.08 in 1 B-TS

M 1 6 M 1 1 relative to 2.61 in 1 B. The C –C distance in 1 B-TS is reduced by 1.862 Å and the C –P bond

M 4 1 1 1 is increased by 0.063 Å. In the resulting intermediate 1 C, the C –P and C –P bond lengths are

1.877 and 1.772 Å, respectively. The overall reaction energy for this intramolecular nucleophilic

sol M M attack is highly exothermic (∆HL {1 B→1 C} = -19.9 kcal/mol) as well as exergonic

sol M M (∆GL {1 B→1 C} = -17.9 kcal/mol), suggesting an adequate thermodynamical driving force

M 3 for the formation of 1 C (Figure 2 and Scheme S1). In the following step, the electron rich C

1 3 1 center attacks the electron deficient P atom (qC /qP = -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

M M transition state 1 C-TS, leading to the intermediate 1 D. The transition vector animates

2 1 1 1 M formation of the B −C bond with simultaneous weakening of the C −P bond in 1 c-TS. The

1 1 M M M C −P bond length (1 c/1 c-TS = 1.772/1.964 Å) in 1 C-TS is significantly longer than that in

M 2 1 M M M the intermediate 1 C, while the B ···C distance (1 c/1 c-TS = 2.855/2.204 Å) in 1 c-TS is

M comparatively shorter than that in the intermediate 1 C (Figure S4). These suggest a late transition state along the reaction coordinate. Gratifyingly, our calculated results reveal that the

M M sol M M formation of 1 D form 1 C is slightly endothermic or rather isoenergetic (∆H {1 C→1 D} =

2.5 kcal/mol). This endothermicity may attribute to the formation of fused strained structure of

M M 1 1 D. The structural feature of 1 D is unique in that the P 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

2 1 M the B ···C distance in the intermediate 1 C. In the final reaction step, –Ph group migrates from

2 1 M the B to C center to furnish the monomeric product 1 E. It is found that the migration of the –

M Ph group occurs via the transition state 1 D-TS and the barrier height for the migration is 23.6

M -1 kcal/mol relative to the intermediate 1 D. The corresponding single eigenmode (425i cm ) animates the synchronous breaking of the B2−CPh bond and formation of the C1−CPh bond. In

M 2 Ph 1 7 1 D-TS, the B −C bond lengthens to 2.167 Å, while the C ···C distance gets shortened to

M 1 1 2.056 Å. In 1 E, the C −P 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

3 4 M (2.010 Å), C (1.965 Å) and C (1.875 Å) atoms. Eventually, monomeric complex 1 E, readily

M M M dimerizes to give the dimeric product 2 . The dimerization process (1 E→2 ) is accompanying with the energy liberation of -43.2 kcal/mol.

Scheme 2. Synthesis of compounds 3 and 4.

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 other reaction mechanism such as Diels-Alder reaction mechanism in the reaction of 1 with Ad-

C≡P.7 In order to verify the abovementioned 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

15 16a bases, IAr (:C{N(Ar)CH}2) and [LSiNMe2] (L = PhC(NtBu)2) 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. In addition, further reactions of 3 and 4 with [NaOCP(1,4-dioxane)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 formation of 3 and 4 illustrate three facts: (1) the nucleophilicity

⁻ of OCP is weaker than IAr (:C{N(Ar)CH}2) and [LSiNMe2], but stronger than 1,4-dioxane; (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.

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 (o): 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-

B1119.3(2), N3-Si1-N1 107.2(2), N3-Si1-N2 107.8(2), N1-Si1-N2 70.7(2).

Compound 3 was isolated as a yellow crystalline solid in good yield (91 %). Similar NHC- pentaphenylborole [IMes-B(Cl)(CPh)4] (IMes =:C{N(Mes)CH}2, Mes = 2,4,6-Me3C6H2) 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 (

22.9 ppm) is broad due to the quadrupolar moment of the Bborole nucleus. It also shows a

16a significant low-field shift compared with that of [LSiNMe2] ( -2.62 ppm) as the lone pair electrons on the Si(II) center is donated to the vacant orbital on the boron center. The X-ray crystal structure of 4 (Figure 3) shows that the Si1-B1 bond (2.040(7) Å) is slightly longer than

16b that of the amidinato silylene-borane adduct [LSi(H)(BH3)] (1.9624(5)Å). Besides compound

4, other silylene-borane adducts were synthesized.16c,d

Scheme 3. Synthesis of compound 5.

[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 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

M the type 1 A was not found since repeated optimizations resulted in decoordination of the

19 combining reactants. 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

11 [N(Ar)BPh(CPh)4C(=NAr)] (5) as a yellow crystalline solid in excellent yield (90 %). In its B

NMR spectrum, compound 5 shows a broad signal at  40 ppm, which is close to that of the 1,2-

5b azaborinine [N(Mes)B(Ph)(CPh)4] ( 35.9 ppm). Compound 5 was analysed by X-ray crystallography (Figure 4). The BNC5 ring of 5 is puckered. The endocyclic B1−N1 bond

(1.417(2) Å) is comparable with that of the 1,2-azaborinine [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 359.91o and 358.96o around the B1 and N1 atoms, respectively, which confirm their trigonal planar geometry. 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

3h,20 the B/N seven-membered heterocycles [N(Ad)B(Ph)(CPh)4C(=E)] (E = O, S). When the adamantyl group was replaced with less steric hindered aryl ring, the B/N seven-membered heterocycle 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 a P/B-heterocycle intermediate “PB(Ph)(CPh)4C(=O)”. However, this reaction pathway cannot lead to the formation of 2 as illustrated by the abovementioned DFT calculations.

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

(o): 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).

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.

Experimental Section

All reactions and handling of reagents were performed under an atmosphere of argon using

2a 8b standard Schlenk techniques or a glove box. Compound 1, NaOCP(1,4-dioxane)1.7, LSiNMe2

16 15 (L = PhC(NtBu)2) and IAr (:C{N(Ar)CH}2, Ar = 2,6-iPr2C6H3) were synthesized according to the literature. 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 a 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 days, 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

o 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 2h at this temperature. The blue suspension slowly turned to dark orange solution, which was then filtered. The filtrate was concentrated under vacuum to 1/3 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 %,

o based on NaOCP). Mp: 182 C (dec.). Elemental analysis calcd (%) for C142H108B4Na2O4P2.C7H8

1 (2.toluene): C, 84.31; H, 5.51. Anal. found: C, 84.62; H, 5.52. H NMR (THF-d8, 298 K, 400

MHz):  8.27 (d, 2H, C6H5), 7.77 (d, 2H, C6H5), 7.42 (d, 2H, C6H5), 7.32 (d, 2H, C6H5), 7.25-

13 5.75 (m, 42H, C6H5). C NMR (THF-d8, 298 K, 100 MHz):  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,

11 123.8, 123.1, 123.0, 122.9, 122.6, 122.1, 120.8, 65.5, 40.1, 20.6, 14.8 ppm. B NMR (THF-d8,

31 298 K, 128 MHz):  24.1 (br, s, BC(Ph)(O)), -2.94 (s, C3BP) ppm. P NMR (THF-d8, 298 K,

162 MHz):  -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 1/3 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 oC

(dec.). Elemental analysis calcd (%) for C61H61BN2: C, 87.96; H, 7.38; N, 3.36. Anal. found: C,

1 88.29; H, 7.52; N, 3.03. H NMR (C6D6, 298 K, 400 MHz):  7.50 (d, 2H, C6H5), 7.15-6.72 (m,

3 29H, C6H5 + Ar-H), 6.50 (s, 2H, C-H (IAr)), 3.06 (br, 4H, iPr-H), 0.93 (d, 24H, iPr-Me, JH-H =

13 6.8 Hz). C NMR (C6D6, 298 K, 100 MHz):  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, 124.4, 124.1, 124.0, 123.7, 28.7,

11 26.6, 22.2, 21.1ppm. B NMR (C6D6, 298 K, 128 MHz):  -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 1/3 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

o (82 %, based on 1). Mp: 136 C (dec.). Elemental analysis calcd (%) for C51H54BN3Si: C, 81.90;

1 H, 7.28; N, 5.62. Anal. found: C, 82.20; H, 7.40; N, 5.85. H NMR (C6D6, 298 K, 400 MHz): 

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

13 (s, 6H, NMe2), 1.30-0.7 (br, 18H, tBu-H). C NMR (C6D6, 298 K, 100 MHz):  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,

11 39.1, 31.7, 31.2, 30.5, 22.7, 14.0, 1.11 ppm. B NMR (C6D6, 298 K, 128 MHz):  -10.1 (s) ppm.

29 Si NMR (C6D6, 298 K, 99 MHz):  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 1/3 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 o C (dec.). Elemental analysis calcd (%) for C59H59BN2: C, 87.82; H, 7.37; N, 3.47. Anal. found:

1 C, 87.36; H, 7.60; N, 3.10. H NMR (C6D6, 298 K, 400 MHz):  7.41-6.58 (m, 31H, C6H5 + Ar-

3 3 H), 4.03 (sept, 1H, iPr-H, JH-H = 6.8 Hz), 3.79 (sept, 1H, iPr-H, JH-H = 6.8 Hz), 3.43 (sept, 1H,

3 3 3 iPr-H, JH-H= 6.8 Hz), 1.80 (sept, 1H, iPr-H, JH-H = 6.8 Hz), 1.69 (d, 3H, iPr-Me, JH-H = 6.8

3 3 Hz), 1.48 (d, 3H, iPr-Me, JH-H = 6.8 Hz), 1.41 (d, 3H, iPr-Me, JH-H = 6.8 Hz), 0.94 (d, 3H, iPr-

3 3 3 Me, JH-H = 6.8 Hz), 0.77 (d, 3H, iPr-Me, JH-H = 6.8 Hz), 0.62 (d, 3 H, iPr-Me, JH-H = 6.8 Hz),

3 3 13 0.47 (d, 3H, iPr-Me, JH-H = 6.8 Hz), 0.35 (d, 3H, iPr-Me, JH-H = 6.8 Hz). C NMR (C6D6, 298

K, 100 MHz):  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,

11 24.7, 24.6, 24.4, 23.5, 23.3, 22.7, 22.1, 20.3, 14.0 ppm. B NMR (C6D6, 298 K, 128 MHz): 

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 carried out 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 (ɛ = 2.374) were evaluated by a self-consistent reaction field (SCRF) approach using the SMD continuum solvation model.26 Natural bond orbital (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

Sol argument, we have reported only the solvent corrected enthalpy (∆HL ) 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 have tried to optimize a

M M transition state connecting the intermediates 1 A and 1 B by considering the following combinations of methods and basis sets: BP86-D3/def2-SVP, BP86-D3/6-31G*, M06-L/def2-

SVP, M06-L/6-31G*, B3LYP-D3/def2-SVP, and B3LYP-D3/6-31G*. Despite such extensive

M and careful search, we are unable to locate a transition state connecting the intermediates 1 A

M and 1 B.

ASSOCIATED CONTENT

Supporting Information.

The Supporting Information is available free of charge on the ACS Publication website.

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 – S2 giving X-ray crystal structures of 2 and 3; Figures S3 - S4,

Scheme S1 and Table S2 giving selected theoretical data of 2. (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail (C.-W. So): [email protected]

*E-mail (D. Koley): [email protected]

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. analyze X-ray crystal data.

ACKNOWLEDGMENT

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 start- up grant and SERB for DST fast track fellowship (SR/FT/SC-72/2011).

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SYNOPSIS

The sodium salt of the phosphaboraheterocycle anion 2 was synthesized via the reaction of the pentaphenylborole 1 with [NaOCP(1,4-dioxane)1.7]. Compound 2 is anionic fused P/B- heterocycles, which were formed by various B-C, C-P and C-C bond activation between the borole backbone and [OCP]⁻anion.