Synthesis of 9-Borafluorene Analogues Featuring a Three-Dimensional 1,1’-Bis(o-carborane) Backbone

Abstract

The synthesis of [1,1’-bis(o-carboranyl)]boranes was achieved through the deprotonation of 1,1’-bis(o-carborane) reagents followed by salt metathesis with (ⁱPr)₂NBCl₂. X-ray crystallography confirms planar central BC₄ rings and Gutmann-Beckett studies reveal an increase in Lewis acidity at the boron center in comparison to their biphenyl congener, 9-borafluorene.

Polyhedral carborane clusters are viewed as three-dimensional aromatic analogues to the ubiquitous two-dimensional aromatic arenes (e.g. benzene).¹ These species share high delocalization within the cage and ring resulting in high kinetic stability.² The significant difference is that carboranes exhibit a aromaticity while benzene is a classical n aromatic molecule. Due to their unique steric profile and electronic structure, o-carboranes have been explored as a substitute for phenyl groups in molecules. The lability of the C-H vertices (pKa = 22 c.f. benzene = 43) of o-carborane facilitates selective derivatization to incorporate carboranes into molecular architectures.³ 1,1’-Bis(o-carborane, B) can be viewed as a three-dimensional analogue to a biphenyl unit, a common ligand scaffold in organometallic chemistry (A, Figure 1).⁴ The facile manipulation and high stability has resulted in complexes featuring B being investigated in medicine and electronic materials.^{2a, 5}

Fig 1.

Relationship of biphenyl (A) to 1,1’-bis(o-carborane) (B) and the corresponding chelated boranes investigated in this work.

9-Borafluorenes (1A) contain a biphenyl backbone linked by a tricoordinate boron center and have been recognized as attractive targets for molecular sensors⁶, reagents for the synthesis of polycyclic aromatic hydrocarbons⁷ as well as components in organic light emitting diodes (OLEDs)⁸ and organic photovoltaics (OPVs).⁹ The vacant p_z orbital on the boron center extends conjugation throughout the three fused rings. We envisioned that 1,1’-bis(o-carborane) could replace the biphenyl framework in 9-borafluorenes to generate a species with a three-dimensional backbone.

The initial strategies to access the target [1,1’-bis(o-carboranyl)]boranes were inspired by effective methods for the synthesis of 9-borafluorenes, specifically transmetallation of a stannole or dilithiated species with RBX₂.¹⁰ The corresponding [1,1’-bis(o-carboranyl)]stannole^4j was recently reported and the [1,1’-bis(o-carboranyl)]dilithium species¹¹ has been generated and utilized in situ. Unfortunately, all attempts to access the [1,1’-bis(o-carboranyl)]borane via these reagents were unsuccessful (Tables S-1 and S-2). In addition, the transmetallation reaction with the [1,1’-bis(o-carboranyl)]magnesium species did not generate the desired boracycle (Table S-3). Potassium bis(trimethylsilyl)amide [K(HMDS)] is also an effective base for the deprotonation of the C-H vertices and the resultant salt, K_z[B], is easier to generate and offers enhanced solubility in comparison to the dilithiated reagent.^4k,12 After several attempts using a variety of conditions (Table S-4), the room temperature generation of K₂[B] in THF followed by addition of (ⁱPr)₂NBCl₂ proved to be an effective method to furnish the desired [1,1’-bis(o-carboranyl)]borane 1B. Acquiring a ¹¹B{¹H} NMR spectrum of the crude reaction mixture showed a three-coordinate peak at 32.9 ppm, slightly shifted from (ⁱPr)₂NBCl₂ (31.3 ppm), coupled with the disappearance of one of the diagnostic signals corresponding to B (−2.2 ppm) and emergence of a doublet at 1.8 ppm, suggesting restricted rotation about the C-C bond in B.¹³ After isolation, the product was dissolved in CDCl₃ and the subsequent ¹H NMR spectrum contained no C-H carborane signal at 3.51 ppm, indicating successful deprotonation of the carboranyl moieties and the product was isolated in 89% yield (Scheme 1). The identity of 1B was further confirmed based on single crystal X-ray diffraction studies (Figure 2). The synthetic route was compatible with the octa-methylated variant 1BMe₈¹⁴ featuring a ¹¹B{¹H} NMR resonance at 33.7 ppm corresponding to the (ⁱPr)₂NB-center, and a singlet at 6.3 ppm resulting from k²-C,C’-chelation of the bis(o-carborane). X-ray diffraction studies confirmed the structural identity of 1BMe₈, which was isolated in 67% yield (Figure 2).

Scheme 1.

Synthesis of 1B and 1BMe₈.

Fig 2.

Solid-state structures of 1B and 1BMe₈. Thermal ellipsoids are depicted at 50% probability and hydrogen atoms are removed for clarity. The diisopropyl group in 1BMe₈ is positionally disordered and only the major component is shown.

A notable structural feature of 1B and 1BMe₈ are highly planar central BC₄ rings (maximum deviation from planarity = 029 Å and 0.011 Å, respectively), which is comparable to their borafluorene counterpart 1A (0.020 Å). The boron atom of the central ring and adjacent nitrogen atom of 1B are trigonal planar [Σ_angles: B(1) = 360.0(18)° and N(1) = 360.0(17)°, Table 1]. Positional disorder of the isopropyl groups on the nitrogen atom of 1BMe₈ prevents an in-depth analysis of the metrical parameters of the substituents. The endocyclic carbon-carbon bonds of 1B and 1BMe₈ are longer than 1A^10b [1B: C(1)-C(2) 1.649(3) Å, C(2)-C(3) 1.528(3) Å, and C(3)-C(4) 1.649(3) Å, 1BMe₈:, C(1)-C(2) 1.652(3) Å, C(2)-C(3) 1.524(3) Å, and C(3)-C(4) 1.646(3) Å, 1A: C(1)-C(2) 1.418(3) Å, C(2)-C(3) 1.474(3) Å, and C(3)-C(4) 1.413(3) Å] but contracted from the parent B^3j [C(1)-C(2) 1.630(3) Å, C(2)-C(3) 1.528(3) Å, and C(3)-C(4) 1.649(3) Å]. The B-N bond lengths of 1B and 1BMe₈ are slightly shorter compared to previously reported B-N length of 1A [1.371(3) Å and 1.384(4) Å c.f. 1.396(3) Å]^10b, ¹⁵, indicating strong π-donation from the nitrogen lone pair to boron.¹⁶

Table 1.

Salient bond lengths (Å) and angles [°] in compounds 1B, 1BMe₈, and 1A.

	1B	1BMe8	1A
B(1)-C(1)	1.631(3)	1.622(4)	1.593(3)
C(1)-C(2)	1.649(3)	1.652(3)	1.418(3)
C(2)-C(3)	1.528(3)	1.524(3)	1.474(3)
C(3)-C(4)	1.649(3)	1.646(3)	1.413(3)
C(4)-B(1)	1.630(3)	1.626(4)	1.601(3)
B(1)-N(1)	1.371(3)	1.384(4)	1.396(3)
N(1)-B(1)-C(4)	126.06(19)	125.50(2)	128.97(13)
C(1)-B(1)-N(1)	125.61(18)	125.40(2)	127.51(19)
C(1)-B(1)-C(4)	108.33(16)	109.00(2)	103.44(17)
B(1)-N(1)-C(5)	119.94(17)		120.90(2)
B(1)-N(1)-C(8)	120.09(18)		119.76(18)
C(5)-N(1)-C(8)	119.96(16)		119.35(19)

The UV-Vis spectra of 1B and 1BMe₈ in CH₂Cl₂ (Figure 3A) exhibit absorption maxima at 232 and 233 nm, respectively, blueshifted from 1A (248 nm).^10b Cyclic voltammetry (CV) measurements conducted on 1B show a quasi-reversible one-electron reduction wave at −0.93 V and a second irreversible one-electron reduction at −1.86 V versus the ferrocenium/ferrocene couple (Fc⁺/Fc). In comparison, 1BMe₈ exhibits only an irreversible reduction at −2.09 V whereas 1A showed only a reversible reduction at −2.95 V, indicating that the bis(o-carboranyl) backbone imparts an electron-withdrawing effect facilitating reduction (Figure 3B).^10b

Fig 3.

A) UV-Vis absorption emission spectra for 1B and 1BMe₈ obtained from solutions of CH₂CI₂ (λ= 232 and 233 nm respectively). B) Cyclic voltammograms of 1B and 1BMe₈ recorded in anhydrous tetrahydrofuran with 0.1 M [NⁿBu₄l [PF₆] and referenced to the ferrocenium/ferrocene redox couple (Fc⁺/Fc; scan rate = 0.1 V/s).

In order to understand the electronic effects of the bis(o-carboranyl) ligand scaffold, density functional theory (DFT) calculations were carried out. The geometries of 1A, 1B, and 1BMe₈ were optimized based on the X-ray structure of 1B at the PBE-D3(BJ):TZP level, and single-point calculations were carried out at the B3LYP-D3(BJ):TZ2P level of theory (Figure S-13). The frontier orbital diagrams for 1B and 1BMe₈ are similar, where the highest occupied molecular orbital (HOMO) is predominantly of π-character with respect to the B-N fragment, and the lowest occupied molecular orbital (LUMO) primarily resides on the bis(o-carboranyl) borane fragment. In contrast to the HOMO for 1A is entirely on the biphenyl fragment with no contribution from the amine, and the LUMO for 1A is localized on the biphenyl borane fragment. The HOMO-LUMO gaps for 1B and 1BMe₈ are comparable (5.99 eV and 6.03 eV, respectively), and significantly larger than 1A (4.17 eV). These data corroborate similar absorption maxima for 1B and 1BMe₈ as well as a bathochromic shift relative to the absorption maximum of 1A (Figure 3A). The calculated higher-lying LUMO for 1BMe₈ (−1.74 eV) relative to that of 1B (−2.05 eV) is consistent with the observed more negative reduction potential for 1BMe₈ (−2.09 V and −1.86 V, respectively; Figure 3B).

To experimentally gauge Lewis acidity, the Gutmann-Beckett method was utilized.¹⁷ This method involves the addition of an excess of Et₃PO to a solution of the borane and monitoring the change in chemical shift of the ³¹P{¹H} NMR signal (δ_31Psample - 41.0). Multiplying this value by 2.21 gives the acceptor number (AN), where a greater AN signifies stronger Lewis acidity. The AN of 1A is 13.5 in C₆D₆^10b and performing the analogous study with 1B gave an AN value of 15.3. Methyl substitution at the peripheral boron vertices have an inductive effect, in this case acting as electron-withdrawing groups.^{3k, 18} Subsequent Gutmann-Beckett studies of 1BMe₈ corroborated this hypothesis with an AN of 20.3, aligning with an increase of Lewis acidity at the boron center.

In summary, we have taken advantage of the lability of the C-H bonds of 1,1’-bis(o-carborane) to access 9-borafluorene analogues with a three-dimensional backbone. These species represent the first examples of 1,1’-bis(carboranyl)boranes and feature a highly planar central ring with enhanced Lewis acidity in comparison to 9-borafluorenes. Methyl substitution at the 8,9,10,12-B-vertices results in an increase of the overall Lewis acidity of the molecule. The results demonstrate the potential of utilizing bis(o-carboranes) as biphenyl analogues to create unique boracyclic architectures.