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Crystalline Tetraatomic Boron (0) Species Wei Lu, Yongxin Li, and Rei Kinjo J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b02173 • Publication Date (Web): 21 Mar 2019 Downloaded from http://pubs.acs.org on March 21, 2019

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Journal of the American Chemical Society

Crystalline Tetraatomic Boron (0) Species Wei Lu, Yongxin Li, Rei Kinjo* Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, 637371, Singapore

Supporting Information Placeholder ABSTRACT:

Reaction of a 1,2-diboraallene 1 with tetradibromodiborane B2Br4 coordinated by a cyclic (alkyl)(amino)carbene (cAAC) in a 3:1 ratio affords a neutral tetraboron species 2 which possesses a planar tatraatomic boron(0) unit with the average oxidation state of zero. X-ray diffraction analysis and computational studies indicate that 2 features the delocalized electrons in the part of the framework of the B4 unit, as well as, the conjugated π-system over the C-B3-C moiety.

Atomic boron clusters featuring elemental boron in the formal zero oxidation state have attracted considerable attention not only because of their intriguing electronic structures involving both delocalized multicenter-twoelectron bonds in the  framework and the conjugated system, but also their potential application in materials science.1 Meanwhile, small elemental boron (0) species (Bn, n ≤ 6), despite its significance for elucidation of the essential BB bonding and electronic nature of elemental boron clusters, have scarcely been reported.2 The groups of Zhou, Schleyer, Xu and Frenking have pioneered experimental and theoretical investigation of a series of homoleptic boron carbonyl complexes [Bn(CO)m, n ≤ 4] (Figure 1a).3 The one boron-centered tricarbonyl complex I possesses a tilted oneelectron donor carbonyl ligand.3a The dicarbonyl complex of B2 II features a liner structure with a B-B triple bond character.3b The four-centered cyclic boron-dicarbonyl complex III exhibits a diradical property.3c These species are generated at low temperature in solid noble-gas matrices or the gas phase and can only be detected spectroscopically. Since the landmark study by Robinson and coworkers demonstrating that the neutral parent diborene [NHC(H)B=B(H)NHC] can be prepared as an isolable species by employing N-heterocyclic carbenes (NHCs) as the supporting ligands (Figure 1b),4 various low-valent main group species have been synthesized as isolable species by making use of Lewis-bases.5 Regarding the boron system (Figure 1c), a NHCs-supported diatomic boron IV featuring a genuine B-B triple bond was isolated in 2012 by Braunschweig and coworkers.6 Shortly afterward, the same group reported the relevant B2 species supported by cyclic (alkyl)(amino)carbenes (cAACs) V exhibiting a cumulenelike bonding property,7 and its reaction with a small NHC (1,3-dimethylimidazol-2-ylidene) affording a three carbenes-

containing species VI.8 Despite the recent development and potential utility of elemental boron species,10 the structurally identified small elemental boron (0) complexes are still limited to B2 species,9,10 and as far as we are aware, tetraatomic boron (0) species has never been isolated thus far. Recently, our group has reported the isolation of a cAACand phosphine-coordinated diatomic boron species 1,11 namely 1,2-diboraallene. In the course of the investigation of the reactivity of 1, we pondered that 1 could be a potential candidate as the precursor for the synthesis of larger Bn architectures.3b,3c,12 In this contribution, we show that the synthesis of a neutral B4 species starting from 1 is indeed feasible (Figure 1d). We also present the outcome of single crystal X-ray diffraction analysis and computational studies. a) Transient monoelemental boron (Bn: n = 1, 2, 4) carbonyl complexes (only observed spectroscopically) B

B(CO)3

B

OC

I

B

CO

B

OC

B

CO

B

II

III

b) First Isolable diborene by Robinson H

NHC B

B

NHC

H

c) Isolable B2 species (identified crystallographycally) NHC

B

B

NHC

B

cAAC

B V

IV

PMe3

cAAC cAAC

B

cAAC

B

cAAC

B

B

NHC 1

VI

PMe3

d) This work: Isolable B4 species Me3P 1

PMe3 B

cAAC

B

B

B

cAAC

Figure 1. a) Elemental boron (Bn: n = 1, 2, 4) species observed at low temperature in matrices or gas phase; b) Robinson’s diborene; c) Structurally identified neutral B2 species. NHC = N-heterocyclic carebene; cAAC = cyclic (alkyl)(amino)carbene; d) Present work.

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Reaction of three equivalents of compound 1 with cAACB2Br4 adduct in toluene afforded a red solution, and after purification 2 was gained as red crystals in 30% isolated yield, accompanied by a zwitterionic boronium species 3 as the byproduct (Scheme 1). Compound 2 was characterized by standard spectroscopic, analytic and crystallographic techniques. Particular diagnostic are the 11B NMR signals at −45.3 ppm and 26.4 ppm, that are assigned to the PMe3ligated boron B(PMe3)2 and the cAAC-bounded boron atoms B(cAAC), respectively. These are shifted upfield from those of the starting material 1 (B(PMe3)2 = −25.5 ppm; B(cAAC) = 86.4 ppm). The 11B NMR resonance for the central dicoordinated boron (B-B-B) could not be detected, probably due to overlap with the signal for the B atoms in the cAAC rings (MeOB = 37.0 ppm), as well as, the broadening of the signal associated with the coupling with quadrupolar boron (11B, I = 3/2) in non-spherically symmetric environment. This inference is borne out with the calculated 11B NMR resonances for the computationally optimized molecule opt2 (BBB = 46.0 ppm, MeOB = 36.3 ppm B(PMe3)2 = −42.5 ppm, B(cAAC) = 24.5 ppm), in line with experimentally observed values (Table S16). In the 31P NMR spectrum of 2, a broad singlet for PMe3 appears at −13.4 ppm, which is slightly shifted downfield with respect to that (−20.9 ppm) of 1.

longer than those (1.68-1.80 Å) reported for B(sp2)-B(sp3) single bonds.15 The B2-C1 (1.456(3) Å) and B4-C2 (1.454(3) Å) distances are only slightly longer than the B(sp)-C(sp2) distances observed in 1 (1.424(4) Å) and VI (1.423(2) Å), comparable to those in V (1.458(2) Å and 1.459(2) Å), and marginally shorter than cAAC-borylene interaction (1.46-1.52 Å).8b,16 These metric data indicate notable interaction between the B4 unit and cAAC fragments. The B3–P1 (1.887(2) Å) and B3–P2 (1.891(2) Å) bond lengths are slightly elongated in comparison with those in 1 (1.854(3) Å and 1.855(3) Å). To the best of our knowledge, 2 represents the first structurally authenticated B4 species supported by cAAC and trimethylphosphine ligands, in which the average oxidation state for the boron atoms is zero. a)

Dip cAAC

B

PMe3

B

MeO B

cAACB2Br4 (1 equiv.)

+

PMe3

Me3Si Me3Si

(3 equiv.)

1

N

b)

cAAC

Toluene, r.t.

Me3P

Me3Si

SiMe3

B

Me3Si B MeO

PMe3

B N Dip

B

Br

SiMe3 B N

+ cAAC

B

Dip 2 (30%) (73%: NMR yield)

B B

OMe Br

PMe3 PMe3

3

Scheme 1. Synthesis of 2 (Dip = 1,3-diisopropylphenyl).

Reddish single crystals of 2 were obtained by slow evaporation of a pentane solution at room temperature. An X-ray diffraction study revealed that the solid-state structure of 2 involves a four-membered B4 fragment, two cAAC and two PMe3 units (Figure 2). The B1 atom is formally dicoordinate whereas the B3 atom is tetracoordinate with ligation by two PMe3. The tricoordinate B2 and B4 atoms are bound to cAAC unit. All six atoms (B1, B2, B3, B4, C1, C2) are in a perfectly planar arrangement (sum of the internal angles of the B4 unit = 360.0°), and two cAAC rings are nearly coplanar in a cis configuration (N1-C1-B2-B3 torsion angle = 178.2(4)°; N2-C2-B4-B3 torsion angle = 177.1(1)°). The B2-B1B4 bond angle of 128.38(16)° is significantly wider than the B2-B3-B4 moiety (99.35(12)°), and it is comparable to that (134.8(2)°) reported for the acyclic C3 bent allene.13 The B1-B2 and B1-B4 distances (1.564(3) Å and 1.554(3) Å, respectively) are essentially identical, which falls within the typical range of B=B double bonds observed in base-stabilized diborenes.4,6a,14 In contrast, conspicuously long are the B2-B3 (1.833(3) Å) and B3-B4 (1.849(3) Å) contacts, that are slightly

Figure 2. Solid-state structure of 2. a) top view and b) side view. (hydrogen atoms except for those on C14, C30 and solvent molecules are omitted for clarity). Thermal ellipsoids are set at the 30% probability level. To elucidate the origins of the bonding and electronic properties of 2, we performed molecular orbital (MO) analysis and natural bond orbital analysis (NBO) using a density functional theory (DFT) method (Figure 3). The optimized geometry opt-2 at the B3LYP/6-311G(d,p) level of theory provides excellent agreement with the metrics of 2 determined crystallographically, which allows assessment of the key molecular orbitals. The LUMO mainly corresponds to the p-orbitals on the B1, C1 and C2 atoms. While the HOMO is dominated by the antiphase B2-C1 and B4-C2 πtype orbitals with some contribution from the N lone pairs, the HOMO−1 comprises the conjugated π-type orbital over the C1-B2-B1-B4-C2 moiety. This -system resembles to that of pentadienyl cation with four -electrons.17 The HOMO−2 is responsible for the σ-framework of the B4 moiety involving the B1-B2, B2-B3, B3-B4 and B4-B1 bonding interaction. Particular diagnostic is the HOMO−3 featuring the B2-B3 and B4-B3 -bonding interaction with involvement of the in-


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Journal of the American Chemical Society plane p-orbital of B1, which thus can also be deemed an inplane B2-B1-B4 -type orbital with the lone pair-like character at the central B1 atom. While the HOMO−1 and HOMO−3 of opt-2 are reminiscent of the frontier molecular orbitals of divalent carbon(0) species L2C(0),13,18 those electronic structures are essentially disparate from each other due to the lack of the resonance form involving two lone pairs at the B1 in opt-2 (Scheme S1). NBO analysis gave Wiberg bond index (WBI) values of 1.31 (B1-B2), 1.31 (B1-B4), 0.69 (B2-B3) and 0.68 (B3-B4), indicative of multiple bond character of B1-B2 and B1-B4 bonds and single bond character of B2-B3 and B3-B4 bonds, respectively. The WBI value of 1.42 for both B2-C1 and B4-C2 contacts propose the multiple bond nature, consistent with the presence of -type bonding MOs (HOMO and HOMO−1) over those moieties. Delocalization of π-electrons from the B2-C1 and B4-C2 orbitals to the p-orbital of B1 is manifested by the second order perturbation analysis with the stabilization energy of 24.97 kcal∙mol−1 and 24.63 kcal∙mol−1, respectively (Tables S17-S18). Significantly, donation of electron density from the B2-B3 and B3-B4 σ-bonds to the in-plane p-orbital of B1 is found with the second order perturbation energies of 151.21 kcalmol−1 and 153.02 kcalmol−1, respectively (Tables S17-S18). These data indicate that the B4 unit involves delocalized electrons in both  and -systems. Note that diamondshaped tetraboron unit featuring the delocalized bonding system is ubiquitous in a myriad of two-dimensional boron materials.1 Natural Population Analysis (NPA) reveals that the B1 atom (qB1: +0.057) is more electron deficient with respect to the cAAC bounded boron (qB2: −0.163; qB4: −0.154) and the B3 (qB3: −0.711) atoms (Table S19). The UV-vis spectrum of 2 in toluene shows the absorption maxima at 541 nm, which is assigned to the HOMO-LUMO transition. The simulated spectrum by a time-dependent density functional theory (TD-DFT) concurs well with experimental data (Figure S28).

Figure 3. Plots of the frontier molecular orbitals of opt-2. One of the plausible reaction pathways for the formation of 2 is shown in Scheme 2. The reaction can be initiated by a redox reaction between the cAACB2Br4 adduct and 1 to furnish a dibromodiboron intermediate Int-1 and a zwitterionic boronium species 3. The former may undergo dimerization to yield a tetrabromocyclotetraborane Int-2. Subsequent PMe3 transfer from 3 to Int-2 affords Int-3 and an unsymmetrical diborene intermediate Int-4, which may partially dimerize to form Int-3. A stoichiometric redox reaction between Int-3 and Int-4 gives rise to a tetrabromodiborane 4 and dibromocyclotetraborane Int-5. The latter can be reduced by 1 to generate the tetraatomic boron species supported by two PMe3 and two cAAC ligands, accompanying the formation of 3. The last step involves isomerization of Int-6 via a phosphine transfer to afford 2. Concomitantly, compound 4 can also be reduced by 1 to give 3 and a half equivalent of Int-5, which may be followed by the above-mentioned steps to afford 2. Overall, a 3:1 mixture of 1 and cAACB2Br4 may produce one equivalent of 2 and two equivalents of 3. Br cAACB2Br2

cAACB2Br4 +

Redox

1

x2

Int-1

0.5

cAAC Br

B B

B B

+ cAACB2Br2PMe3)2 3 PMe3 Transfer

Br Int-2

PMe3

Br Redox

cAACB2Br2PMe3 Int-4

+ 0.5

Br cAAC

cAAC Br

B B

B B

Br cAAC

Br

Me3P

Int-3 PMe3

0.5 Int-4 cAAC + B 0.5 cAACB2Br4PMe3 + 0.5 Br 4

Br

B B

cAAC

B PMe3

1.5 1

Redox

Int-5 0.5 1

1.5 3 + 0.5 2

Redox PMe3

0.5 2

Isomerization

B 0.5 cAAC

B

B

cAAC + 0.5 3

B PMe3 Int-6

Scheme 2. A proposed reaction pathway for the formation of 2. To support the proposed reaction pathway, we have carried out a variable-temperature NMR study on the reaction of cAACB2Br4 with an equimolar amount of 1. The instant generation of 3 was detected at low temperature, and warming up of the reaction mixture led to the formation of 4 concomitant with a complex mixture, presumably corresponding to Int-5 (Figure S6-S11). To this mixture, addition of two equivalents of 1 resulted in a full



consumption of 4 as well as the mixture, and the formation of 2 associated with 3 were confirmed. A preliminary computational study reveals that 2 is thermodynamically more stable by 15.16-16.79 kcal∙mol−1 than Int-6 (Table S20), supporting the last step of the reaction pathway. We have also confirmed that the reaction of 4 with four equivalents of KC8 gives a mixture involving 2 (36% NMR yield), indicating that the formation of 2 by reduction of 4 is also feasible, and thus it could be involved as the alternative path (Figures S12S13). In conclusion, we have demonstrated that 1,2-diboraallene 1 can be used as a precursor for the preparation of a B4 species 2 supported by Me3P and cAAC ligands. The latter can be isolated as a bottleable species and characterized by various means. X-ray diffractometry and computational studies indicate that 2 features not only the partial delocalization of electrons in the -framework of the B4 unit, but also the conjugated π-system over the C-B3-C moiety analogous to pentadienyl cation. Investigation on the reactivity of 2 is currently underway.

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 We are grateful to Nanyang Technological University (NTU) and the Singapore Ministry of Education (MOE2015-T2-2032) for ﬁnancial support.

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SYNOPSIS TOC

Dip MeO

B

TMS TMS

N B B +

PMe3 PMe3

cAACB2Br4

Me3P

Me3Si Me3Si MeO

B

PMe3

SiMe3 SiMe3

B B N Dip

B

B N

B

OMe

Dip


6

Crystalline Tetraatomic Boron (0) Species - American Chemical Society

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