A Boron Analogue of Vinylidene Dication Supported by Phosphines

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A Boron Analogue of Vinylidene Dication Supported by Phosphines Wei Lu, Yongxin Li, Rakesh Ganguly, and Rei Kinjo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b13068 • Publication Date (Web): 16 Jan 2018 Downloaded from http://pubs.acs.org on January 16, 2018

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Journal of the American Chemical Society

A Boron Analogue of Vinylidene Dication Supported by Phosphines Wei Lu,† Yongxin Li,‡ Rakesh Ganguly‡ and Rei Kinjo†* †

Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, and ‡NTU-CBC Crystallography Facility, Nanyang Technological University, 637371, Singapore ABSTRACT: In the presence of a catalytic amount of heavier tetrylene dichlorides an allenic diborene 1 undergoes a 1,3hydrogen shift to afford a terminal diborene 2, which can be deemed a boron analogue of vinylidene dication stabilized by Lewis bases. X-ray diffraction analysis and computational studies revealed that 2 involves a conjugative interaction between the C=C and B=B -orbitals. The reaction of 2 with ZnBr2 afforded the corresponding isolable complex 3, in which two boon centers coordinate to the Zn atom asymmetrically. Over the past 35 years, the chemistry of homoatomic multiply bonded species based on p-block elements other than carbon has attracted considerable attention owing to their peculiar structural and electronic features.1 Among them, the construction of multiple bonds between two boron atoms has been considered as one of the most challenging tasks because of the intrinsic electron deficient nature of boron. Indeed, the simplest boron-boron multiply bonded species, namely diborenes I, in which each boron center is surrounded by only six electrons, has never been isolated to date (Figure 1a).2 In order to achieve eight-electron system at the B atoms in diborenes, early studies employed a reduction protocol which led to the formation of dianionic species.3 In 2007, Robinson and coworkers have elegantly demonstrated that by employing N-heterocyclic carbenes (NHCs) as supporting ligands, a metal-free and neutral diborene IDip·HB=BH·IDip (IDip= 1,3bis(2,6-diisopropylphenyl)imidazol-2-ylidene) can be synthesized.4 Since this milestone, a variety of diborenes supported by NHCs or phosphines have been developed and structurally characterized.5,6 While most of the isolated diborenes exhibit trans geometry II7 with respect to the two Lewis base ligands, a few derivatives with cis-configuration III8 have also been documented (Figure 1b), which has allowed to comprehend the bonding nature of the B=B unit in detail.

Figure 1. a) Lewis base-free diborene I, its isomer I’ and vinylidene dication; b) Lewis base-stabilized diborenes II, III; c) Terminal diborene IV. L = Lewis base. Contrary to the recent dramatic advance in the chemistry of internal diborenes, never described has been its constitutional isomer, terminal diborene I’, in which one of the two boron atoms bears two substituents and forms a double bond with the other substituent-free boron atom (Figure 1a). Compound I’ is a neutral

analogue of vinylidene dication, which has never been detected thus far.9 We reasoned that similar to II and III, the coordination of Lewis bases should stabilize I’ to lead the corresponding adduct IV (Figure 1c). Very recently, we have reported the synthesis of a neutral allenic diborene 1 involving the B(sp)=B(sp2) double bond.10 Interestingly, a computational calculation indicated that the isomer of 1 corresponding to IV is thermodynamically more stable (~18 kcal∙mol−1) than 1. The thermolysis reaction of 1 at 80 oC, however, afforded a complex mixture, which prompted us to examine the additive effect under the ambient condition. Herein, we report the synthesis, X-ray diffraction analysis and computational studies of IV. We also present the coordination reaction between IV and ZnBr2. Scheme 1: Synthesis of 2 (Ar = 2,6-diisopropylphenyl).

We initially postulated that a base would deprotonate one of the H atoms at the CH2 moiety in the C3BN ring of 1. Subsequent transfer of the proton to the central boron atom via a formal 1,3-H shift should yield the isomer corresponding to IV. To bear out this hypothesis, we have treated 1 with various bases. However, when employing 4-dimethylaminopyridine (DMAP), 1,4diazabicyclo[2.2.2]octane (DABCO), IDip and potassium bis(trimethylsilyl)amide (KHMDS), no reactions were observed even with a stoichiometric amount. Probably, the H atom is not so acidic as to be deprotonated. Indeed, neither NaH nor CaH2 reacted with 1. Serendipitously, we found that a catalytic amount of the heavier tetrylenes dichlorides ECl2 (E = Ge, Sn) promoted the desired formal 1,3-H shift (Scheme 1). Thus, in the presence of GeCl2·dioxane (9.6 mol%), a full conversion of compound 1 was confirmed within 2 hours at room temperature to give product 2 in 87% NMR yield. Likewise, when 1 was treated with 7.2 mol% SnCl2 in C6D6, 2 was gained in 85% NMR yield after 12 hours. Meanwhile, the reactions of 1 with a catalytic amount of other Lewis acids such as ZnBr2, AlCl3 and Ph3B did afford compound 2 and a few unidentified products were detected (see the Supporting Information). In the 1H NMR spectrum of 2, a broad signal for the BH appears at δ = 5.5 ppm, which is overlapped with that of the proton in the BN five-membered ring. The chemical shift is comparable to those of the reported B(sp2)H (δ = 4.71-6.14 ppm),4a,4d,11 and in good agreement with the computational result (δ = 5.13 ppm) with the optimized molecule opt-2 (see the Supporting Information,

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Table S6). The 11B NMR spectrum shows a broad singlet for the BH at δ = 52.6 ppm which is massively shifted upfield compared with that (δ = 86.4 ppm) of 1, whereas a broad peak for the B·(PMe3)2 appears at δ = −18 ppm which is slightly downfieldshifted with respect to that (δ = −25.5 ppm) of 1. The calculated 11B NMR chemical shifts with opt-2 (BH: δ = 52.3 ppm, B·(PMe3)2: δ = −22.5 ppm) are nearly identical to those of the experimental results. The 31P NMR resonances of 2 appear at δ = −13.0 ppm and δ = −18.3 ppm, respectively. The solid-state IR spectrum of 2 exhibits a weak B-H stretching vibration at 2384 cm−1 (Figure S5), which is in accordance with the computational estimation (2501 cm−1) (Figure S25).12

veals that the charges at the B-B moiety are +0.118 for the B2 atom and −1.017 for the B3 atom, respectively, which agrees well with the 11B NMR chemical-shifts and suggests the pronounced polar nature of the B-B double bond.

Figure 3: Plots of the HOMO (left), LUMO (middle) and HOMO−1 (right) of 2.

Figure 2: Solid-state structure of 2 (hydrogen atoms except for those on the B2 and C14 are omitted for clarity). Single crystals of 2 were obtained by slow evaporation of a pentane solution, and the solid-state structure was unambiguously determined by X-ray diffractometry (Figure 2). The C13, H1, B2, B3, P1 and P2 atoms are coplanar, and both phosphine ligands are bound to the B3 atom. The BN five-membered ring is significantly twisted with respect to the H1B2B3 plane with a torsion angle (C14-C13-B2-B3) of 58.9(5)°. The B2-B3 bond distance (1.579(4) Å) is comparable to those of 1 (1.560 (4) Å) and IDip·HB=BH·IDip (1.561(18) Å), indicative of its double bond character. The elongated B2-C13 (1.583(4) Å) and the contracted C13-C14 (1.350(4) Å) bond distances compared with those in 1 (1.424(4) Å and 1.538(4) Å, respectively) are consistent well with the bond rearrangement via the 1,3-hydrogen shift. The distances of the B3-P1 (1.863(3) Å) and the B3-P2 (1.849(3) Å) bonds are similar to those of 1 (1.854(3) and 1.855(3) Å, respectively) and close to the reported B-P double bond (1.763-1.859 Å),13 indicating the strong bonding interaction between the B and P atoms. These metric data confirms that 2 exhibits to the structural property of IV bearing the two supporting phosphine ligands only at one side of the B-B unit. DFT calculations were carried out to interrogate the origins of the structure of 2 (Figure 3). The HOMO is composed of the B-B π-bonding orbital with some extension to the two phosphine atoms which exhibits anti-phase conjugation with the C13-C14 π orbital, while the LUMO is mainly dominated by the C13-B2 bonding orbital with contributions from the p-orbitals at the C14 and B3 moieties. The HOMO−1 is largely represented by the inphase π-bonding orbital over the C14-C13-B2-B3 unit, as well as, the π orbital of the B-N-C moiety in the C3BN five-membered ring. These MOs indicate a conjugative interaction between the C=C and B=B -orbitals. Natural Bond Orbital (NBO) analysis including the second order perturbation analysis confirms the delocalization of electrons in the B-B π-orbital to the mainly unoccupied d-orbital of the phosphine atoms, which is in line with the observed short B-P bonds distances (Table S8 and S9). Wiberg bond index value of 1.404 for the B-B bond documents its double bond character. Natural Population Analysis (NPA) re-

Scheme 2 illustrates one of the plausible catalytic cycles for the ECl2-promoted 1,3-hydrogen migration of 1. The reaction should be initiated by the coordination of 1 to ECl2 to form 1·ECl2. The zwitterionic nature of 1·ECl2 may allow the migration of one of the H atoms at the CH2 moiety in the C3BN ring to the dicoordinate boron center,14 concomitant with the shift of electrons of the C=B moiety to form a C=C double bond in the C3BN ring. The resulting 2·ECl2 would undergo the dissociation of the ECl2 fragment to furnish 2. Our preliminary computational study with GeCl2 indicates that the proposed cycle is thermodynamically favorable (for the details, see the Supporting Information). Thus, it has been revealed that the initial complexation between 1 and GeCl2 is exergonic (ΔG = −10.4 kcal·mol−1), and 2·GeCl2 is more stable than 1·GeCl2 by 17.4 kcal·mol−1. While the simple dissociation of 2·GeCl2 to 2 and GeCl2 requires 9.1 kcal·mol−1, the total energy involving the subsequent complexation of GeCl2 with 1 is estimated to be −1.2 kcal·mol−1. The presence of 2∙ECl2 in the catalytic cycle has been confirmed by the reaction of 1 with a stoichiometric amount of GeCl2, which afforded 2∙GeCl2 as orange crystals. Complex 2∙GeCl2 has been fully characterized by the standard spectroscopic means and an Xray diffraction analysis (Scheme2, bottom).

Scheme 2: A proposed catalytic cycle (top). The value in parentheses indicates the calculated free energy difference (kcal∙mol−1) by using E = Ge. Solid-state structure of

2∙GeCl2 (bottom).

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Journal of the American Chemical Society We are grateful to Nanyang Technological University (NTU) and the Singapore Ministry of Education (MOE2015-T2-2-032) for financial support.

2∙GeCl2 Formation of the proposed intermediate 2∙ECl2 implies the coordination ability of 2 toward Lewis acid, which prompted us to investigate the reaction of 2 with metal Lewis acid. When a C6D6 solution of 2 and ZnBr2 (1 equiv.) was stirred at room temperature overnight, a dark red solution was formed. After work-up, complex 3 was obtained as a white powder in 38% yield (Scheme 3).7h,15 In the 11B NMR spectrum of 3, two broad singlets appear at δ = 65.7 ppm and δ = −26.2 ppm, which correspond to the boron atoms of HB and B∙(PMe3)2, respectively. An X-ray diffraction analysis of 3 revealed the dimeric structure linked by the Zn2Br2 four-membered ring unit (Figure 4). While the B2 center is nearly planar, a pyramidal geometry is observed around the B3 atom with respect to the Zn center (sum of the bond angles except for the B-Zn bond: ∑B2 = 359.3°, ∑B3 = 343.9°), The B2-B3 bond distance (1.652(5) Å) is slightly longer than that (1.579(4) Å) of 2, and the Zn(II) center interacts asymmetrically with the central B2 moiety (B2-Zn1: 2.754(8) Å, B3-Zn1: 2.147(3) Å). Scheme3: Reaction of 2 with ZnBr2.

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3.

Figure 4: Solid-state structure of 3 (hydrogen atoms except for those on B2 and C14 are omitted for clarity).

4.

Collectively, we have presented the synthesis of a neutral terminal diborene 2 from an allenic diborene 1 via a formal 1,3hydrogen shift catalyzed by heavier tetrylene dichlorides. The complexation reaction with ZnBr2 afforded 3, demonstrating the coordination ability of 2 toward Lewis acid. 5.

ASSOCIATED CONTENT Supporting Information Synthesis, NMR spectra, crystallographic data (CIF) and computational details including Cartesian coordinates for stationary points. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Notes

The authors declare no competing financial interest

ACKNOWLEDGMENT

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