Diazaborolyl-boryl push-pull systems with ethynylene-arylene bridges as 'turn-on' fluoride sensors

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Diazaborolyl-boryl push–pull systems with ethynylene–arylene bridges as

‘turn-on’ ﬂuoride sensors†

Lothar Weber,*^aDaniel Eickhoff,^aJan Kahlert,^aLena Böhling,^aAndreas Brockhinke,^a

Hans-Georg Stammler,^aBeate Neumann^aand Mark A. Fox^b

Received 23rd February 2012, Accepted 21st June 2012

DOI: 10.1039/c2dt30438d

Two linear π-conjugated systems with 1,3-diethyl-1,3,2-benzodiazaborolyl [C₆H₄(NEt)₂B–] as a donor

group and dimesitylboryl (–BMes₂) as acceptor were synthesised with –ethynylene–phenylene-

(–CuC-1,4-C₆H₄–, 3) and –ethynylene–thiophene– (–CuC-2,5-C₄H₂S– 12) bridges between the boron

atoms. An assembly (20) consisting of two diazaborolyl-ethynylene–phenylene-boryl units, [C₆H₄(NCy)-

(N′)B–CuC-1,4-C₆H₄-BMes₂] joined via a 1,4-phenylene unit at the nitrogen atoms (N′) of the

diazaborolyl units was also synthesised. The three push–pull systems, 3, 12 and 20, form salts on ﬂuoride

addition with the BMes₂groups converted into (BMes₂F)⁻anions. The molecular structures of 3, 12 and

(NBu₄)(12·F) were elucidated by X-ray diffraction analyses. The borylated systems 3, 12 and 20 show

intense blue luminescence in cyclohexane with quantum yields (Φ_ﬂ) of 0.99, 0.44 and 0.94, respectively,

but weak blue-green luminescence in tetrahydrofuran (Φ_ﬂ= 0.02–0.05). The charge transfer nature of

these transitions is supported by TD-DFT computations with the CAM-B3LYP functional. Addition of

tetrabutylammonium ﬂuoride to tetrahydrofuran solutions of 3 and 20 resulted in strong violet-blue

luminescence with emission intensities up to 46 times more than the emission intensities observed prior

to ﬂuoride addition. Compounds 3 and 20 are demonstrated here as remarkable ‘turn-on’ ﬂuoride sensors

in tetrahydrofuran solutions.

BMes₂groups are often strongly coloured and/or luminescent,⁸

Introduction

which renders them useful as colorimetric or luminescent

sensors for ﬂuoride ions.^9–13

Conjugated organic molecules and polymers with three-coordi-

nate boron units as building blocks have attracted considerable

interest because of their linear and non-linear optical and elec-

tronic properties, which make them potentially useful in func-

tional materials.¹Three-coordinate boron generally behaves as a

π-acceptor due to its vacant p-orbital, which stabilises the

LUMO of an adjacent conjugated π-electron system and thus

lowers the HOMO–LUMO gap of these molecules. This ﬁeld of

research has been dominated by the use of the dimesitylboryl

group (BMes₂, Mes = 2,4,6-Me₃C₆H₂), in which the unsaturated

boron centre is stabilised towards oxidation and hydrolysis by

the steric shielding of the four ortho-methyl groups.^2–5The

BMes₂group is considered to have an acceptor strength between

that of NO₂- and CN-groups.^6,7Such electron-deﬁcient com-

pounds are efﬁcient electron-transporting and/or emitting layers

in organic light-emitting diodes (OLEDs).⁵Compounds with

In the past decades, the chemistry of a different class of three-

coordinate boron compounds, namely 1,3,2-diazaboroles, has

rapidly developed.^14–21Some of these compounds show strong

luminescence when irradiated by UV light. For synthetic reasons

the 1,3-diethyl-1,3,2-benzodiazaborole unit is the most fre-

quently employed representative, and compounds containing this

group as a substituent are moderately air-stable.^16,22–27As the

BMes₂group is known as an effective acceptor (A) and the ben-

zodiazaborolyl unit has been suggested to be a π-donor (D),²⁷

the novel “push–pull” systems [D-bridge-A] with 1,4-phenylene,

4,4′-biphenylene, 2,5-thiophene and 5,5′-dithiophene scaffolds

(I–IV, Chart 1) have been investigated recently.²⁸Photophysical

studies on these compounds reveal blue-green ﬂuorescence

and Stokes shifts for the ﬁrst three representatives of

7820–9760 cm⁻¹in THF, whereas the Stokes shift for the last

compound is signiﬁcantly smaller (5510 cm⁻¹in THF). Thereby

the π-donating strength of the 1,3-diethyl-1,3,2-benzodiazaboro-

lyl substituent was found to lie between that of the MeO and the

Me₂N groups. It is well documented that the absorption as well

as the emission bands of a boron-functionalised conjugated

π-system can be shifted to lower energies by elongation of the

latter.²⁹

^aFakultät für Chemie der Universität Bielefeld, 33615 Bielefeld,

Germany. E-mail: lothar.weber@uni-bielefeld.de

^bDepartment of Chemistry, Durham University, South Road, Durham,

DH1 3LE, UK

†Electronic supplementary information (ESI) available: Additional

photophysical data, ﬂuoride titration data, TD-DFT data, additional MO

ﬁgures and molecular orbital contributions. CCDC 858762–858765. For

ESI and crystallographic data in CIF or other electronic format see DOI:

10.1039/c2dt30438d

The 2-arylethynyl-1,3,2-benzodiazaboroles, V–X (Chart 1),

were shown to be highly luminescent with quantum yields (Φ_ﬂ)

10328 | Dalton Trans., 2012, 41, 10328–10346

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Scheme 2 Synthesis of 5.

treatment of the organolithium species with an equimolar

amount of 2-bromo-1,3,2-benzodiazaborole 1. The lithiating

reagent, LiN(SiMe₃)₂was used here as nBuLi reacts with the

CN group of 4. Diazaborole 5 was isolated as colourless needles

in 69% yield (Scheme 2).

Compound 12, where the benzodiazaborolyl and dimesityl-

boryl units are separated by a 2,5-thiophene-diyl bridge was

synthesised from 2-(trimethylsilylethynyl) thiophene 8³²by

treatment with an equimolar amount of n-butyllithium in diethyl

ether at room temperature and the subsequent addition of

an ether solution of dimesityl ﬂuoroborane (9).³³After the

addition of brine to the reaction mixture, crude 2-(silylethynyl)-

5-dimesitylboryl-thiophene was isolated in ca. 95% yield. Com-

plete characterisation of intermediate 10 was disclaimed in

favour of its subsequent desilylation by means of potassium

carbonate in a mixture of dichloromethane and methanol to

afford 2-(ethynyl)-5-dimesitylboryl-thiophene (11) in 62% yield.

The synthesis of target compound 12 was accomplished by

lithiation of 11 and coupling with 2-bromo-1,3,2-benzodiazabor-

ole in n-hexane, and 12 was obtained as large colourless crystals

in 54% yield. Compound 12 was converted into its crystalline

ﬂuoride adduct [nBu₄N]⁺[12·F]⁻by addition of 1 equiv. of

TBAF·3H₂O in C₆D₆(Scheme 3).

The compound 2-isopropylamino-1-bromobenzene 13, was

used for the synthesis of a bisborole with two diazaborole mole-

cules (like V and 3) linked by a spacer. The synthesis of the

precursor triphenylene-tetraamine 14 was accomplished by a

Hartwig–Buchwald coupling between p-phenylenediamine and

two equiv. of 2-isopropylamino-1-bromobenzene 13 in boiling

toluene utilising an in situ prepared catalyst (prepared from

1,3-bis-2′,6′-diisopropylphenyl)imidazolium chloride (IPr·HCl),

sodium tert-butanolate and Pd(OAc)₂in toluene) (Scheme 4),

(71% yield).³⁴Treatment of 14 with 2 equiv. of boron tribromide

in the presence of an excess of calcium hydride in CH₂Cl₂

afforded the 1,4-bis(2′-bromo-1′,3′,2′-benzodiazaborol-1-yl)-1′-

benzene derivative 15 as colourless crystals in 62% yield.

Combination of the bis(bromoborole) 15 with 2 equiv. of in situ

generated lithium phenylacetylide in n-hexane furnished the

bis-benzodiazaborole 16 as a microcrystalline solid in 53% yield

after crystallisation from an n-hexane–CH₂Cl₂mixture.

Chart 1

Scheme 1 Syntheses of 3 and [nBu₄N]⁺[3·F]⁻.

of 1.00 and 0.96 in cyclohexane and THF, respectively for V.²⁷

Thus, it was logical to insert an acetylenic unit between the ben-

zodiazaborolyl group and the adjacent arene or heteroarene ring

in the push–pull systems of I and III, respectively. Here the

syntheses, characterisation and photophysics of these compounds

and of two assemblies, where two linear 2-aryl-1,3,2-benzo-

diazaborole units are linked via a phenylene unit, are described.

The intriguing ﬂuoride-sensing properties of the push–pull

systems are also explored as such molecules have potential use

as luminescent sensors for anions such as ﬂuoride.

Results and discussion

Whereas the product from 15 with lithiated 4-dimesitylboryl-

phenylacetylene³⁰could not be isolated as a pure compound, the

cyclohexyl bromoaniline 17 was successfully used to obtain a

soluble bisborole 20 with two BMes₂groups. Triphenylene tetra-

amine 18 was synthesised analogously from 2 equiv. of bromo-

aniline 17 and 1,4-phenylenediamine as colourless crystals in

51% yield. Two-fold cyclocondensation with 2 equiv. of BBr₃

led to the bis(bromodiazaborolyl)benzene derivative 19 (43%

yield). The synthesis of target molecule 20 was completed by

reaction of 2 equiv. of lithiated 4-dimesitylboryl-phenylacety-

lene³⁰and 19 in toluene. Product 20 was isolated from the

The reaction of in situ lithiated 4-dimesitylboryl-phenylacetylene

2³⁰with an equimolar amount of 2-bromo-1,3-diethyl-1,3,2-ben-

zodiazaborole 1¹⁶in n-hexane at room temperature led to the for-

mation of diazaborolylated 4-dimesitylboryl-phenylacetylene

3 as colourless platelets in 90% yield (Scheme 1). The combi-

nation of equimolar amounts of 3 with tetrabutylammonium

ﬂuoride (TBAF) resulted in the formation of the salt [nBu₄N]-

[3·F].

For comparison with closely related derivatives 3 and V–X,

the cyano compound 5 was synthesised here by the lithiation of

4-cyanophenylacetylene 4³¹in THF and the subsequent

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Scheme 3 Syntheses of 12 and [nBu₄N]⁺[12·F]⁻.

12, 15, 16, 19 and 20 display singlets in the narrow range of

20.2–23.5 ppm for the boron nuclei of the benzodiazaborole

units. Singlet resonances at 74.4–74.9 ppm were observed for

the dimesitylboryl groups at phenylene units, whereas the corres-

ponding singlets seen in the thiophene derivatives, 10–12, were

signiﬁcantly shielded (65.2–66.8 ppm). Singlet resonances at δ =

21.4 and 5.0 ppm were registered for the benzodiazaborole part

and the BMes₂F unit, respectively, for the anions [3·F]⁻and

[12·F]⁻.

X-ray crystallography

Molecular structures were determined for the two benzodiaza-

borolyl-functionalised dimesitylboryl(hetero)arylacetylenes 3,

12, [nBu₄N]⁺[12·F]⁻and the 1,4-bis(benzodiazaborol-5-yl)-

benzene 15 (Fig. 1–3). Bond lengths and angles of interest are

listed in Table 1.

Molecule 3 is constructed from a benzodiazaborole ring that is

linked to the arylalkynyl unit by a B(1)–C(11) single bond of

1.528(3) Å, which compares well with the corresponding bond

length in V (1.524(2) Å). The length of the triple bond C(11)–

C(12) is 1.205(3) Å. Valence angles at the nearly linear bridge

between the two rings B(1)–C(11)–C(12) and C(11)–C(12)–

C(13) are 177.8(2)° and 178.8(2)°. The planes between the

central benzene ring and the heterocycle are twisted by 88.2°.

The benzene ring is attached in the 4-position to the dimesityl-

boryl group by a B(2)–C(16) single bond [1.565(3) Å]. B–C

bonds at the mesityl groups in 3 are in the expected range [B(2)–

C(19) 1.581(3), B(2)–C(28) 1.578(3) Å]. The plane deﬁned by

the C(16), C(19) and C(28) atoms, including the boron atom, is

twisted out of the plane of the benzene ring by 42.2°. The inter-

planar angles between this plane and the mesityl groups are

Scheme 4 Syntheses of 16 and 20.

reaction residue as a microcrystalline colourless solid by continu-

ous extraction with n-hexane over a period of two weeks.

Compounds 3, 5, 12, 16 and 20 are stable to oxygen and

moisture, whereas the bromo derivatives 15 and 19 decompose

in air. The new compounds are well soluble in benzene, toluene,

dichloromethane and chloroform and only poorly soluble in

alkanes. The ¹¹B{¹H} NMR spectra of all new compounds 3, 5,

10330 | Dalton Trans., 2012, 41, 10328–10346

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Fig. 3 Molecular structure of the bisborole 15. Hydrogen atoms are

omitted for clarity.

planes of the heterocycles enclose a dihedral angle of 44.9°,

which is about half the size of the corresponding angle in 3. The

5-position of the thiophene ring is substituted by a dimesityl-

boryl unit via a B(2)–C(16) single bond of 1.541(3) Å. As given

in 3 the relevant mesityl B–C bonds B(2)–C(17) [1.579(3) Å]

and B(2)–C(26) [1.578(3) Å] are as expected. The plane deﬁned

by the atoms C(16), C(17) and C(26), including atom B(2), is

twisted out of the thiophene plane by 20.4°. The interplanar

angles between this plane and the mesityl groups are 58.0° and

60.2°.

Fig. 1 Molecular structures of 3 and 12. Hydrogen atoms are omitted

Crystals of the adduct [nBu₄N]⁺[12·F]⁻contain 4 pairs of

independent molecules (A–D) in the asymmetric unit (Fig. 2;

Table 1). Bond lengths and bond angles were essentially identi-

cal within 3 esds. Signiﬁcant deviations were found for the angle

C–C–C at the acetylene bridge which for molecule B is more

bent (176.8(3)°) than for the three other molecules (178.1(3)–

178.7(3)°). The angle B–C–C varies from 169.2(2)° in D via

173.5(2)° in A and 175.0(2)° in B to 176.8(3)° in C. The inter-

planar angles between the heterocycles vary in the series 40.4

(B) < 43.8 (D) < 46.8 (C) < 52.4° (A), which are not signiﬁ-

cantly different to the respective angle in 12 (44.9°). The geo-

metric parameters within the Mes₂BF unit (Table 1) are

comparable to those in other triaryl ﬂuoroborates.^9a,10eNo other

signiﬁcant changes in the structural parameters of the BDB–

CuC–C₄H₂S– part of 12 upon ﬂuoride addition were observed.

Compound 15 may be described as a benzene ring that is sub-

stituted at the para-positions by two 2-bromo-3-isopropyl-1,3,2-

benzodiazaborol-1-yl units via N(1)–C(10) single bonds of

1.427(4) Å. (Fig. 3) The molecule possesses a centre of inver-

sion in the middle of the benzene spacer. The planes of benzo-

diazaborole rings and the benzene unit enclose dihedral angles

of 51.7°. Bond lengths and angles within the benzodiazaborole

for clarity.

Fig. 2 Structure of one independent molecule (A) (out of four) in the

unit cell of the anion [12·F]⁻in [Bu₄N][12·F]. Hydrogen atoms are

omitted for clarity.

part of 15 are comparable to

3

and other benzo-

diazaboroles.^{19,21–23,25–28}In summary, molecule 15 is closer to

planarity than the BMes₂analogue 3.

52.9° and 57.7°. Bond lengths and bond angles within the

benzodiazaborole part of 3 are similar to those of numerous

diazaboroles studied before.^{17,19,22,25–27}

The molecule 12 may be described as a benzodiazaborole

which is connected with a 2-thienylethynyl unit via the B(1)–

C(11) single bond of 1.520(3) Å. Both heterocycles are linked

by an essentially linear ethynyl bridge with a triple bond C(11)–

C(12) of 1.207(3) Å and angles B(1)–C(11)–C(12) and C(11)–

C(12)–C(13) of 176.4(2)° and 178.2(3)°, respectively. The

Photophysics

Neutral species

Photophysical measurements for the new compounds 3, 5, 12,

16 and 20 in cyclohexane and THF solutions are summarised in

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Table 1 Selected bond lengths and angles for 3, 12, [12·F]⁻and 15

3

12

[12·F]^−a

15

Bond lengths (Å)

B–N

B(1)–N(1) 1.425(3)

B(1)–N(2) 1.427(3)

B(1)–N(1) 1.425(3)

B(1)–N(2) 1.422(3)

B(1)–N(1) 1.422(3)

B(1)–N(2) 1.427(3)

B(1)–N(1) 1.433(5)

B(1)–N(2) 1.411(5)

B–C

B(1)–C(11) 1.528(3)

B(2)–C(16) 1.565(3)

B(2)–C(19) 1.581(3)

B(2)–C(28) 1.578(3)

B(1)–C(11) 1.520(3)

B(2)–C(16) 1.541(3)

B(2)–C(17) 1.579(3)

B(2)–C(26) 1.578(3)

B(1)–C(11) 1.533(3)

B(2)–C(16) 1.646(3)

B(2)–C(17) 1.667(3)

B(2)–C(26) 1.648(3)

B–Hal

C–C

B(2)–F(1) 1.477(2)

B(1)–Br(1) 1.925(5)

C(11)–C(12) 1.205(3)

C(12)–C(13) 1.437(2)

C(13)–C(14) 1.398(3)

C(14)–C(15) 1.390(3)

C(15)–C(16) 1.403(3)

C(16)–C(17) 1.401(3)

C(17)–C(18) 1.381(3)

C(13)–C(18) 1.402(3)

C(11)–C(12) 1.207(3)

C(12)–C(13) 1.425(3)

C(13)–C(14) 1.371(3)

C(14)–C(15) 1.404(3)

C(15)–C(16) 1.375(3)

C(11)–C(12) 1.211(3)

C(12)–C(13) 1.422(3)

C(13)–C(14) 1.378(3)

C(14)–C(15) 1.411(3)

C(15)–C(16) 1.383(3)

C(10)–C(11) 1.389(5)

C(11)–C(12A) 1.389(5)

C(10)–C(12) 1.389(5)

C–N

C–S

N(1)–C(1) 1.395(2)

N(2)–C(2) 1.401(2)

N(1)–C(7) 1.455(2)

N(2)–C(9) 1.463(2)

N(1)–C(1) 1.392(2)

N(2)–C(2) 1.391(2)

N(1)–C(7) 1.460(3)

N(2)–C(9) 1.462(3)

N(1)–C(1) 1.396(3)

N(2)–C(2) 1.396(3)

N(1)–C(7) 1.463(3)

N(2)–C(9) 1.459(3)

N(1)–C(1) 1.401(5)

N(2)–C(2) 1.401(5)

N(2)–C(7) 1.477(5)

N(1)–C(10) 1.427(4)

S(1)–C(13) 1.711(2)

S(1)–C(16) 1.722(2)

S(1)–C(13) 1.734(2)

S(1)–C(16) 1.732(2)

Bond Angles (°)

C–C–C

C(11)–C(12)–C(13)

178.8(2)

C(11)–C(12)–C(13)

178.2(3)

B(1)–C(11)–C(12)

176.4(2)

C(11)–C(12)–C(13) 178.7(2)

[176.3(2);178.1(2); 178.7(2)]

B(1)–C(11)–C(12) 173.5(2)

[175.0(2);176.8(2); 169.2(2)]

B–C–C

B(1)–C(11)–C(12)

177.8(2)

Torsion Angles (°)

N(1)–B(1)⋯C(13)–C(14)

89.6

N(1)–B(1)⋯C(13)–C(14)

44.9

N(2)–B(1)⋯C(13)–C(14) 48.0

[42.1; 48.7;38.8]

C(1)–N(1)–C(10)–C(12)

49.5

C(19)–B(2)–C(16)–C(15)

42.5

C(15)–C(16)–B(2)–C(26)

21.2

C(17)–B(2)–C(16)–C(15) 83.8

[81.4; 85.2;85.3]

C(28)–B(2)–C(16)–C(17)

37.0

C(15)–C(16)–B(2)–C(17)

160.4

C(26)–B(2)–C(16)–C(15) 48.7

[49.1; 42.9;47.7]

F(1)–B(2)–C(16)–C(15) 165.2

[166.6; 161.6; 163.6]

F(1)–B(2)–C(16)–S(1) 9.2 [14.6; 18.4; 11.4]

^aBond lengths and bond angles shown here are from molecule A.

Table 2. For comparison, the reported absorption and emission

data for related systems I–V (Chart 1) in cyclohexane and THF

solutions are included.^27,28Photophysical data for compounds 3,

12 and 20 in other solvents (toluene, chloroform, dichloro-

methane and acetonitrile) are listed in Table S1† and shown in

Fig. S1 and S2.†

The absorption maxima of 3 and 5 are shifted to lower ener-

gies with respect to the closely related analogues, V–X

(Chart 1), showing that such π-acceptors, CN and BMes₂, lower

the HOMO–LUMO energy gaps. The absorption spectra of the

bisdiazaborole 16 in cyclohexane and THF show absorption

maxima that are identical to the parent monodiazaborole, V, but

the extinction coefﬁcients are at least twice as large for 16 com-

pared to V. The absorption bands of compound 20 are compar-

able to that of the closely related monodiazaborole 3. The

extinction coefﬁcients in cyclohexane are similar for both com-

pounds but the extinction coefﬁcient in THF is doubled for 20

compared to 3. The larger extinction coefﬁcients observed for

the bisdiazaboroles compared to the closely related monodiaza-

boroles are simply due to twice as many chromophores present

in a solution of similar molarity when comparing the linked

species with the ‘monomeric’ ones.

All the neutral diazaboroles listed in Table 2 exhibit intense

blue/green luminescence under UV irradiation in cyclohexane

solutions with high quantum yields of 0.94–0.99 for three

compounds, 3, 16 and 20, containing the phenylene-acetylene

moieties. Compound 16 also has a high quantum yield Φ_ﬂof

0.82 in THF but the BMes₂systems 3 and 20 have very low Φ_ﬂ

values of 0.02–0.03 in THF. These quantum yields mirror those

found for the related systems, the unsubstituted derivative V and

the BMes₂derivatives I and II respectively. The Stokes shifts

observed in the regions of 4000–7000 cm⁻¹in cyclohexane and

7000–10 000 cm⁻¹in THF are typically found in related benzo-

diazaboroles reported elsewhere.^19p,27,28

10332 | Dalton Trans., 2012, 41, 10328–10346

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Table 2 Photophysical data for 3, 5, 12, 16, 20 and related compounds, I–V

In cyclohexane solution In THF solution

Solvatochromic shifts

Stokes

λ_{max, abs}

[nm]

ε

λ_{max, em}shift

λ_{max, abs}

[nm]

ε

λ_{max, em}shift

Absorption Emission

[L mol⁻¹cm⁻¹

]

[nm]

[cm⁻¹

]

Φ_ﬂ

[L mol⁻¹cm⁻¹

]

[nm]

[cm⁻¹

]

Φ_ﬂ

[cm⁻¹

]

[cm⁻¹

]

3

5

12

16

20

I

342

343

372

307

344

327

328

21 300

7400

421

411

441

379

429

408

399

439

430

345

5400

4900

4200

6600

5800

6000

5400

6200

2500

3600

0.99 342

0.22 339

0.44 374

0.96 307

0.94 343

0.99 329

0.99 331

0.81 350

0.85 392

1.00 307

18 100

12 000

24 900

32 700

38 200

31 600

25 900

20 100

32 800

18 300

503

477

516

399

495

477

489

482

500

394

9300

8500

7300

7700

9000

9400

9800

7900

5500

7200

0.02

0.05 −300

0.05 200

0

3900

3300

1100

3100

3400

4300

1600

3100

3600

22 500

44 300

22 200

29 600

23 900

19 400

11 400

10 400

0.82

0

0.03 −100

0.08 200

0.02 300

0.46 400

0.45 200

II

III 345

IV 389

V

307

0.96

0

bands were suggested to arise from acetonitrile binding to the

boron atom.

The shorter wavelengths reﬂect negative solvatochromic shifts

of 1500 cm⁻¹for 3, 2900 cm⁻¹for 12 and 1100 cm⁻¹for 20 and

mirror such shifts observed in their absorption spectra. The high-

energy emission bands observed for 3, 12 and 20 are likely to

arise from formation of acetonitrile adducts where each adduct

has the boron atom of the BMes₂group bound to the nitrogen of

the acetonitrile molecule. This hypothesis is supported by com-

putations (vide infra).

Comparison with the photophysical data for the reported

push–pull systems I–IV reveals no trends or similarities between

these compounds. Somewhat similar data between thiophene 12

and the dithiophene derivative IV are, however, noted.

Fig. 4 Emission spectra for 3, 12 and 20 in acetonitrile.

Fluoride species

Small positive solvatochromic shifts were observed in the

Selected data from absorption and emission spectra recorded for

the anions generated from addition of ﬂuoride ions are listed in

Table 3. The absorption maxima for these ﬂuoride salts are

higher in energies compared to their neutral species indicating

that the HOMO–LUMO energy gap is increased in these salts

with respect to their neutral species.

All the ﬂuoride salts give violet-blue ﬂuorescence emissions

with maxima between 350 and 410 nm in both cyclohexane and

THF solutions. The Stokes shifts in these salts are generally

lower than in their neutral analogues. The solvatochromic shifts

measured on emission are varied with only 100 cm⁻¹for [12·F]⁻

up to 3100 cm⁻¹for [3·F]⁻(Fig. 5). Different transitions are

thus likely for the two monodiazaborole anions where the

excited state for [12·F]⁻is much less polar than for [3·F]⁻. The

observed solvatochromism of [3·F]⁻suggests a transition from

the π-phenylene–ethynylene scaffold to the benzodiazaborole

unit, whereas the absence of solvatochromism in [12·F]⁻points

to a local π–π* transition on the thiophene scaffold.

absorption spectra recorded in cyclohexane, THF, dichloro-

methane, chloroform and toluene solutions (Tables 2 and S1†).

However, a negative solvatochromic effect of 300 cm⁻¹was

found for nitrile 5 in THF and implies that the ground-state geo-

metry for 5 is relatively polar (and thus more stable in polar sol-

vents) compared to the other diazaboroles observed here.

Negative solvatochromic shifts between 600 and 1500 cm⁻¹

were observed in the absorption spectra for 3, 12 and 20

recorded in acetonitrile, a solvent with a high dielectric constant,

with the largest negative shift found for compound 12

(Table S1†).

Large solvatochromic effects between 3000 and 4000 cm⁻¹

were observed from the emission maxima on going from cyclo-

hexane to THF in most diazaboroles. One exception is com-

pound 16 which has a shift value of only 1100 cm⁻¹and may

adopt a less polar excited state than other compounds listed in

Table 2. The solvatochromic shifts also correspond to solvent

polarities in other solvents except for acetonitrile (Table S1†).

Two emission bands were observed in acetonitrile for 3 (394 and

550 nm), 12 (391 and 567 nm) and 20 (410 and 546 nm) with

the longer wavelength expected from the highly polar solvent

(Fig. 4). Similar bands have been reported for other push–pull

systems with BMes₂groups in acetonitrile.^13aThe reported

The photophysical data for [3·F]⁻in Table 3 are very similar

to the photophysical data shown in Table 2 for the neutral deriva-

tive V (Chart 1). The BMes₂F⁻group behaves like a spectator in

the transitions involved in the low energy absorption and emis-

sion bands or like a donor such as Me and OMe in the neutral

diazaboroles VI and VII, respectively, where the photophysical

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Table 3 Photophysical data for the salts, [nBu₄N][3·F], [nBu₄N][12·F] and the anions [20·F]⁻and [20·2F]²⁻

In cyclohexane solution In THF solution

Solvatochromic shifts

Absorption Emission

Stokes

λ_{max, abs}

ε

λ_{max, em}shift

λ_{max, abs}

ε

λ_{max, em}shift

[nm]

[L mol⁻¹cm⁻¹

]

[nm]

[cm⁻¹

]

Φ_ﬂ

[nm]

[L mol⁻¹cm⁻¹

]

[nm]

[cm⁻¹

]

Φ_ﬂ

[cm⁻¹

]

[cm⁻¹

]

[3·F]⁻

308

331

316

27 600

23 830

33 760

29 040

353

382

380

4100

4000

5100

5700

0.51 310

0.07 334

0.24 318

0.12 313

36 000

29 590

41 300

36 760

393

384

408

401

6800

3900

6900

7000

0.49 200

0.06 100

0.26 200

0.25 100

3100

100

2200

1400

[12·F]⁻

[20·F]⁻

[20·2F]²⁻312

Fig. 5 Emission spectra for 3 and 12 before and after ﬂuoride addition.

data are also similar. The [3·F]⁻anion may thus be viewed as an

analogue of the monodiazaboroles V–VI.

As there are two diazaborole units with BMes₂groups in 20,

it was of interest to see whether one equivalent of ﬂuoride ion

would clearly generate [20·F]⁻exclusively and whether there are

unusual transitions observed between the fragment containing

the anionic BMes₂F⁻group and that with the neutral BMes₂

group (Scheme 5).

Titration experiments of compound 20 with nBu₄NF in THF

were monitored by UV-vis and ﬂuorescence spectroscopy to

examine the effect of sequential addition of ﬂuoride ions on the

bisdiazaborole. Titrations of 20 in cyclohexane with nBu₄NF

addition were complicated by the low solubilities of the three

species 20, [20·F]⁻and [20·2F]²⁻. Therefore, only titrations of

20 in THF solutions are discussed here.

One remarkable observation is the ‘turn-on’ ﬂuorescence for 3

in THF on addition of ﬂuoride anion. Compound 3 has a weak

blue-green emission in THF but, on ﬂuoride addition, an intense

blue-violet emission is observed (Fig. 5). The emission band

intensity of 3 at 3000 units increases to 145 000 units corres-

ponding to [3·F]⁻on ﬂuoride addition. Thus, the emission

maximum intensity is increased 46 times. This ‘turn-on’

ﬂuorescence is evident for 3 in THF but not in other solvents

like cyclohexane (Fig. 5). No such ‘turn-on’ ﬂuorescence is

shown for the thiophene diazaborole 12 in solvents such as

THF and cyclohexane. In fact, ‘turn-off’ ﬂuorescence takes place

for 12 in cyclohexane with the emission intensity reduced by

one-tenth and the quantum yield decreased by a factor of 6

(Fig. 5).

Fig. 6 depicts the absorption and emission spectra for the

three species, 20, [20·F]⁻and [20·2F]²⁻in THF where the

anions were formed by addition of 1 and 2 equiv. of ﬂuoride

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Scheme 5 Sequential ﬂuoride addition of 20.

Fig. 6 Absorption and emission spectra for 20, [20·F]⁻and [20·2F]²⁻. The anions were generated by 1 and 2 equivalents of F⁻respectively.

during the titration. Fig. S3† shows the changes and isosbestic

points in the absorption spectrum of 20 in THF as nBu₄NF is

added in intervals. Addition of 1 equiv. of nBu₄NF leads to a

blue-shift of the absorption band maximum at 343 nm for 20 to

a maximum at 318 nm assigned to the adduct [20·F]⁻. Addition

of a second equiv. of nBu₄NF gave an absorption spectrum with

a maximum at 313 nm which may be assigned to the dianion

[20·2F]²⁻. In addition, a weak low-energy band appeared.

neutral bisdiazaborole 16 has no low energy band at a longer

wavelength than 320 nm, so considering [20·2F]²⁻as a deriva-

tive of 16 is not straightforward here.

The monoanion [20·F]⁻has no observable absorption band at

a longer wavelength than 320 nm. This is surprising as the

ﬂuoride-free part of the molecule in conformer A should still

exhibit an absorption similar to the non-perturbed precursor

(Scheme 5). Such transitions would still be expected for confor-

mer B where the anionic BMes₂F group and neutral BMes₂

group are in close proximity. A statistical mixture of 20, [20·F]⁻

and [20·2F]²⁻is expected to show a low energy band at

340–370 nm with 25% of the band intensity from 20 and 25%

from [20·2F]²⁻. Values of only 10% from 20 and 10% from

[20·2F]²⁻were estimated assuming that there is no low energy

The fact that the dianion [20·2F]²⁻is formed from 2 equiv. of

nBu₄NF shows that the barrier for the addition of a second

ﬂuoride ion to the monoanion [20·F]⁻is low. If [3·F]⁻is viewed

as a derivative of V based on their photophysical data then

[20·2F]²⁻might be compared with 16. Indeed, Tables 2 and 3

conﬁrm very similar values. However, on closer inspection, the

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Fig. 7 Frontier orbitals of compound 3 (left) and 12 (right). HLG = HOMO–LUMO gap.

band contribution from [20·F]⁻itself. The spectra for [20·F]⁻in

Fig. 6 are likely to correspond to a mixture of 20, [20·F]⁻and

[20·2F]²⁻where at least 90% of [20·F]⁻is present.

For the arylethynyl systems 3, 12, [3·F]⁻and [12·F]⁻nearly

coplanar orientations of the diazaborole units and the aryl rings

with torsion angles of less than 9° were found as global minima.

Rotation barriers between the rings of only 0.6–1.6 kcal mol⁻¹

indicate that all possible rotamers may be present at ambient

temperature.²⁷Accordingly, the pronounced interplanar angles

observed for 3, 12 and [12·F]⁻by X-ray crystallography are

likely to arise from crystal packing forces.

In the emission spectrum of 20 in THF, addition of 1 equiv. of

nBu₄NF led to a blue-shift of the band maximum from 495 nm

for 20 to 408 nm, which we assign to the adduct [20·F]⁻. Two

equivalents of nBu₄NF for one equivalent of 20 gave an

emission at 401 nm corresponding to dianion [20·2F]²⁻

.

The observed emission intensity for the monoanion [20·F]⁻is

at least ten-fold the emission intensity for the neutral bisdiaza-

borole 20, whereas the emission band intensity for the dianion

[20·2F]²⁻is ﬁve times more intense than that of 20. The large

emission intensities of the anions compared to that of the neutral

bisdiazaborole show that addition of ﬂuoride ions also promotes

ﬂuorescence emission, i.e. ‘turn-on’ ﬂuorescence, for 20 in THF.

However, the relative emission intensity decreases on going from

[20·F]⁻to [20·2F]²⁻. The ﬂuorescence intensity is clearly

reduced (rather than increased as might have been expected by

the two-fold ﬂuoride addition) when the second diazaborolyl

unit in [20·F]⁻is converted to the ﬂuoride anion.

The geometry of 20 is considered as two molecules of 3

linked by a ‘spacer’ and, likewise, the geometry of [20′·2F]²⁻

regarded as two [3·F]⁻molecules linked by a spacer. Thus,

[20′·F]⁻could be viewed as containing independent molecules

of 3 and [3·F]⁻. The different moieties, however, suggest that

there may be interactions between the two units. The most stable

optimised geometry for [20′·F]⁻is the cisoid form where the

two diazaborole-π-BMes₂scaffolds are close together (con-

former B in Scheme 5 as opposed to conformer A by 5.1 kcal

mol⁻¹). This contrasts with the most stable conformers found for

20′ and [20′·2F]²⁻where transoid forms are adopted as given in

Scheme 5. It may be argued that the different charges within the

units result in favourable attraction between the two units in

[20′·F]⁻. Known compounds with B–F–B chelation have B–F

distances of 1.49–1.64 Å.^10a,35The two computed B–F distances

in 20′F are 1.48 and 4.66 Å which rule out the ﬂuoride ion

being chelated between the two boron centres here.

Computations

Geometry computations

Geometries of the molecules 3, 12, the model compound 20′ as

well as the ﬂuoride adducts [3·F]⁻, [12·F]⁻, [20′·F]⁻and

[20′·2F]²⁻were optimised by DFT calculations at the B3LYP/

6-31G* level of theory without any symmetry constraints. In

20′, the cyclohexyl substituents at the nitrogen atoms of 20 were

replaced by methyl groups and the para-methyl groups of the

mesityl rings by hydrogen atoms to reduce computational

efforts. The computed geometric parameters of 3, 12 and

[12·F]⁻are in reasonable agreement with the experimental geo-

metry data (see Table S3†).

Molecular orbital computations

The frontier molecular orbitals for the neutral molecules 3 and

12 are depicted in Fig. 7 and selected molecular orbital con-

tributions are listed in Tables S4 and S5.† As expected from

earlier studies, the HOMOs of 3 and 12 are mainly located on

the benzodiazaborolyl moieties whereas the LUMOs are domi-

nated by the boron p-orbitals of the dimesitylboryl groups with

contributions of the (hetero)arylene–ethynylene bridges.

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Fig. 8 Frontier orbitals of ﬂuoride adducts [3·F]⁻(left) and [12·F]⁻(right).

The addition of ﬂuoride converts the electron-poor dimesityl-

boryl group into an electron-rich fragment. Hence, the LUMOs

of the anions shown in Fig. 8 are located at the phenylene–

ethynylene and thiophene–ethynylene scaffolds for [3·F]⁻and

[12·F]⁻respectively. The HOMOs in [3·F]⁻and [12·F]⁻are,

however, very different with the HOMO located at the mesityl

groups in [3·F]⁻and at the thiophene–ethynylene scaffold in

[12·F]⁻. Tables S6 and S7† list selected molecular orbital con-

tributions for [3·F]⁻and [12·F]⁻.

The molecular orbital make-ups in the N-phenylene-bridged

bisboroles 20′ and its dianion [20′·2F]²⁻are like those in the

monoborole systems 3 and [3·F]⁻respectively (Fig. S4 and S5;

Tables S8 and S9†). However, the phenylene moiety contributes

to 15% and 1% of the HOMO and LUMO in 20′ respectively

and to 4% and 5% of the HOMO and LUMO in [20′·2F]²⁻

respectively. These small contributions of the phenylene ‘spacer’

suggest that photophysical processes in 20 and 3 as well as in

[20′·2F]²⁻and [3·F]⁻are broadly similar.

weak HOMO–LUMO transitions for [3·F]⁻(Table 4). The calcu-

lated energies of these transitions are underestimated by some

0.6 eV compared to the energies for the observed absorption

maxima in cyclohexane solutions. A mixture of rotamers is

expected in solution due to the very low rotation barrier

between the diazaborolyl unit and the phenylene or thiophene

ring so perpendicular geometries were also investigated. Calcu-

lated transition energies for the perpendicular geometries are

overestimated compared to observed values (Table S11†). If a

mixture of rotamers is assumed to exist in solutions then agree-

ment between experimental and computed absorption values are

acceptable at the B3LYP functional. This may be fortuitous

as B3LYP does not generally model long-range charge transfer

transitions adequately – especially in molecules where the

HOMO–LUMO overlap is negligible.^36,37

The CAM-B3LYP functional is considered as an appropriate

method for long-range charge transfer transitions and thus

TD-DFT data obtained with this functional are also listed in

Table 4. The calculated excitation energies are overestimated by

only 0.11–0.18 eV compared to observed absorption maxima. If

we take into account that a mixture of rotamers exist in solution

then the calculated excitation energies from the mixture would

be considerably overestimated (Table S11, also see Tables S12–

S14† for MO compositions of perpendicular geometries). Never-

theless, and more importantly, the trend between observed and

calculated values is excellent with the CAM-B3LYP functional.

The ﬁrst vertical excitation transitions predicted from the

CAM-B3LYP functional are varied. Fig. 9 and Table 4 show that

only in the case of [12·F]⁻, the HOMO → LUMO transition is

dominant and corresponds to a local π → π* transition on the

thiophene–ethynylene bridge. The dominant contribution to the

The HOMO and other high-lying occupied orbitals and the

LUMO and other low-lying unoccupied orbitals in [20′·F]⁻

show even less phenylene unit contributions at ca. 1% (Fig. S6;

Table S10†). This suggests that the phenylene link is acting as a

true spacer and that [20′·F]⁻is behaving like two separated

chromophores, 3 and [3·F]⁻.

TD-DFT calculations

Time-dependent DFT (TD-DFT) calculations at B3LYP/6-31G*

on the co-planar geometries of 3, 12, [3·F]⁻and [12·F]⁻predict

strong HOMO–LUMO transitions for 3, 12 and [12·F]⁻but

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Table 4 TD-DFT data for the ﬁrst vertical excitation using B3LYP and CAM-B3LYP functionals

Calc. B3LYP

λ_max(eV)

2.95

Calc. CAM-B3LYP

Obs.

λ_max(eV)

Osc. str.

0.63

Major transition (κ_ia)

λ_max(eV)

3.81

Osc. str.

1.16

Major transitions

3

3.63

3.34

4.03

3.75

HOMO → LUMO (0.93)

HOMO → LUMO (0.90)

HOMO → LUMO (0.69)

HOMO → LUMO (0.81)

HOMO → LUMO (0.56)

HOMO-3 → LUMO (0.30)

HOMO → LUMO (0.59)

HOMO-2 → LUMO (0.29)

HOMO → LUMO (0.35)

HOMO-2 → LUMO (0.54)

HOMO → LUMO (0.65)

12

2.81

0.70

3.52

1.38

[3·F]⁻

[12·F]⁻

3.49

0.12

4.20

1.08

3.46

0.72

3.86

1.12

Fig. 9 Orbital transitions for 3, 12, [3·F]⁻and [12·F]⁻with corresponding κ_iavalue for the CAM-B3LYP functional.

S₀→ S₁transition for [3·F]⁻at CAM-B3LYP is the HOMO-2 →

LUMO transition which is also a local π → π* transition on the

bridge along with substantial HOMO → LUMO charge transfer

character. For 3 and 12, the nature of the ﬁrst vertical excitations

are similar with dominant charge transfer HOMO → LUMO

contributions (π borolyl → π*-bridge/boryl) and substantial local

π-aryl/boryl → π*-bridge/boryl contributions.

The small solvatochromic behaviour observed for [12·F]⁻com-

pared to [3·F]⁻is supported by the absence of charge transfer

character predicted for [12·F]⁻. The substantial solvatochromic

shifts observed in the emission data for 3 and 12 are in agree-

ment with the dominant charge-transfer transitions predicted for

3 and 12 by CAM-B3LYP.

Although the CAM-B3LYP TD-DFT data are appropriate for

comparison with absorption data, the nature of these transitions

computed may apply to observed emission data provided that the

S₀and S₁geometries and their MOs are assumed to be similar.

Fluoride ion afﬁnities

The afﬁnities of compounds, 3, 12 and 20′ towards ﬂuoride

anions were calculated and compared with selected molecules in

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For those representatives with only BMes₂groups, the ﬁrst

ﬂuoride ion afﬁnities ranged from 117.3 kcal mol⁻¹in XVIII to

119.2 kcal mol⁻¹in 1,4-bis(dimesitylboryl)benzene (XVI).

These values clearly exceed the ﬂuoride ion afﬁnities in 1,3,2-

benzodiazaborole

derivatives

V,

VI

and

XI–XIV

(96.1–105.3 kcal mol⁻¹). The ﬁrst ﬂuoride ion afﬁnities in com-

pounds 3, 12 and both conformers 20′(A) and 20′(B), where the

ﬂuoride is added to the BMes₂group, are 113.3–119.1 kcal mol⁻¹

in accordance with our experimental ﬁndings.

For the addition of a second F⁻ion to the second BMes₂sub-

stituent in compounds XV–XVIII, lower ﬂuoride ion afﬁnities

of 61.2–84.0 kcal mol⁻¹were calculated. For 20′, the second

ﬂuoride ion afﬁnities of 97.3–106.0 kcal mol⁻¹were found and

support the facile ﬂuoride addition of [20·2F]²⁻from [20·F]⁻

observed here.

The second ﬂuoride addition to compounds 3, 12 and I–IV

goes to the boron atom in the benzodiazaborole ring with low

ﬂuoride ion afﬁnities of 47.9–65.3 kcal mol⁻¹. The values of

47.9 kcal mol⁻¹for III and 65.3 for IV are in accord with

reported²⁴experimental data for III and IV where the second

ﬂuoride could not be added to the borolyl group of III and 41

equivalents of ﬂuoride were required to ensure the second

ﬂuoride addition to IV.

Chart 2

The inﬂuence of acetonitrile as solvent in 3, 12 and 20

Table 5 Calculated ﬂuoride ion afﬁnities in kcal mol⁻¹for 3, 12, 20′

and related systems. Et₂bdb = C₆H₄(NEt)₂B–, Mebdb = μ-N(C₆H₄NMe)B–,

Th = 2-C₄H₃S

The photophysical data for 3, 12 and 20 in acetonitrile solutions

revealed absorption and emission bands with negative solvato-

chromic shifts. Such behaviour has been observed for other

push–pull systems involving BMes₂groups as acceptors else-

where.^13aOne possibility is that 1 : 1 acetonitrile adducts exist in

solution where the acetonitrile is weakly bound via nitrogen to

the boron atom in the BMes₂group which had been suggested

previously.^13a

1st

F⁻

2nd

F⁻

Compound

Et₂bdb-CC-1,4-C₆H₄-BMes₂

Et₂bdb-CC-2,5-C₄H₂S-BMes₂

1,4-[Mebdb-CC-1′,4′-C₆H₄-BMes₂]₂C₆H₄

via conformer A

3

12

20′

118.0

118.1

113.3 106.0

56.2

54.4

The acetonitrile afﬁnities of 3 and 12 based on the optimised

geometries of 3·NCMe and 12·NCMe adducts are only 10.4 and

12.0 kcal mol⁻¹, respectively. TD-DFT data on these adducts at

the CAM-B3LYP functional (3·NCMe 4.25 eV; 12·NCMe

3.98 eV) resemble that of the anions [3·F]⁻and [12·F]⁻

(Table 4). However, the observed absorption bands for 3 and 12

in acetonitrile (3.71 and 3.51 eV respectively) are not close to

the CAM-B3LYP energies. The ¹¹B NMR data of 3 and 12 in

CD₃CN solutions where the two resonances are observed as

expected for the neutral species. At the ground states, com-

pounds 3 and 12 are relatively polar (more stabilised in more

polar solvents) and thus negative solvatochromic shifts are

observed in the absorption spectra.

One argument for the unusual emission data of 3 and 12

observed here in acetonitrile is that mixtures of both the neutral

species and the weakly-bound acetonitrile adduct exist as the

acetonitrile afﬁnities of 3 and 12 in the excited states may be

substantial unlike in the ground states. The high-energy emission

bands of 391 and 394 nm in 3 and 12 respectively (Fig. 4) corres-

pond to the weakly-bound 3·NCMe and 12·NCMe adducts

partly because these adducts have much higher ﬂuorescence

intensities than 3 and 12 in acetonitrile. Compounds 3 and 12

have low quantum yields in acetonitrile with Φ_ﬂvalues of 0.03

at 550 nm for 3 and 0.08 at 567 nm for 12. When excited at the

1,4-[Mebdb-CC-1′,4′-C₆H₄-BMes₂]₂C₆H₄

via conformer B

Et₂bdb-1,4-C₆H₄-BMes₂

Et₂bdb-1,4-C₆H₄-1,4-C₆H₄-BMes₂

Et₂bdb-2,5-C₄H₂S-BMes₂

Et₂bdb-2,5-C₄H₂S-2,5-C₄H₂S-BMes₂

Et₂bdb-CC-Ph

Et₂bdb-CC-C₆H₄Me

Et₂bdb-Th

Et₂bdb-2,5-C₄H₂S-Th

Et₂bdb-2,5-C₄H₂S-2,5-C₄H₂S-Et₂bdb

1,3,5-(Et₂bdb-2′,5′-C₄H₂S)₃C₆H₃

Mes₂B-1,6-C₁₄H₈-BMes₂

Mes₂B-1,4-C₆H₄-BMes₂

Mes₂B-1,4-C₆H₄–1,8-C₁₀H₆-1,4-C₆H₄-

BMes₂

20′

119.1

97.3

I

II

III

IV

V

VI

XI

XII

XIII

XIV

XV

XVI

116.7

116.8

115.8

115.3

96.7

96.1

99.7

103.2

105.0

105.3

118.6

119.2

48.4

64.7

47.9

65.3

—

64.0

75.1

72.4

61.2

73.3

XVII 117.3

Mes₂B-1,4-C₆H₄-SiPh₂–1,4-C₆H₄-BMes₂

XVIII 118.0

84.0

Charts 1 and 2 (Table 5). Fluoride anion additions of diazabor-

oles III, IV, VI and XI–XIV have been explored by spectro-

scopic means but no salts were isolated.^24,25Detailed ﬂuoride

additions have been carried out on diboron compounds (Chart 2,

XV–XVIII) with various bridges between the two BMes₂

groups elsewhere.^4jThese are looked at here for comparison

with the bisdiazaborole 20 to estimate the afﬁnity strengths of

the second BMes₂group towards the ﬂuoride anion.

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high-energy emission wavelengths, the observed excitation

spectra are different from the absorption spectra of 3 and 12 with

the band maxima at 310 and 325 nm from acetonitrile solutions

of 3 and 12 respectively. These excitation spectra are likely to

correspond to 3·NCMe and 12·NCMe as the respective values

of 4.00 eV and 3.80 eV are in good agreement with the

calculated CAM-B3LYP values of 4.25 eV and 3.98 eV for these

adducts, respectively.

The much weaker high-energy emission band observed for

20·NCMe relative to 20 suggests that (i) the acetonitrile afﬁnity

for the excited state of 20 is lower than the afﬁnities for the

excited states of 3 and 12 or (ii) the ﬂuorescence intensity for

20·NCMe is low or (iii) both. Nevertheless, when excited at the

high-energy emission wavelength, the observed excitation spec-

trum has a band at 317 nm which is likely to arise from the

acetonitrile adduct, 20·NCMe.

(2-trimethylsilylethynyl)thiophene (8),³²dimesityl ﬂuoroborane

(9)³³and 1,3-bis(2′,6′-diisopropylphenyl)imidazolium chloride³⁴

were prepared according to literature methods. 4-Bromobenzo-

nitrile (6), 2-bromoaniline, p-phenylenediamine, boron tribromide,

sodium tetrahydridoborate, palladium acetate and tetrabutyl-

ammonium ﬂuoride trihydrate (TBAF·3H₂O) were purchased

commercially. Titration experiments were performed with a

commercially available tetrabutylammonium ﬂuoride solution

(1 M in THF).

NMR spectra were recorded at room temperature with a

Bruker AM Avance DRX-500 spectrometer {¹H, ¹¹B, ¹³C} by

using TMS and BF₃·OEt₂as external standards. Mass spectra

were taken with a VG autospec sector ﬁeld mass spectrometer

(Micromass).

Absorption spectra were measured with a UV/Vis double-

beam spectrometer (Shimadzu UV-2550). The setup used to

acquire excitation–emission spectra (EES) was similar to that

employed in commercial static ﬂuorimeters: the output of a con-

tinuous Xe-lamp (75W, LOT Oriel) was wavelength-separated by

a ﬁrst monochromator (Spectra Pro ARC-175, 1800 L mm⁻¹

grating, Blaze 250 nm) and then used to irradiate a sample. The

ﬂuorescence was collected by mirror optics at right angles and

imaged on the entrance slit of a second spectrometer while com-

pensating astigmatism at the same time. The signal was detected

by a back-thinned CCD camera (RoperScientiﬁc, 1024 × 256

pixels) in the exit plane of the spectrometer. The resulting

images were spatially and spectrally resolved. As the next step,

one averaged ﬂuorescence spectrum was calculated from the raw

images and stored in the computer. This process was repeated for

different excitation wavelengths. The result is a two-dimensional

ﬂuorescence pattern with the y-axis corresponding to the exci-

tation, and the x-axis to the emission wavelength. A wavelength

range of λ_ex= 230–450 nm (in 1 nm increments) for the exci-

tation and λ_em= 200–800 nm for the detection was employed.

The time to acquire a complete EES is typically less than

15 min. Post-processing of the EES includes subtraction of the

dark current background, conversion of pixel to wavelength

scales, and multiplication with a reference ﬁle to take the

varying lamp intensity as well as grating and detection efﬁciency

into account. For all measurements, samples were contained in

quartz cuvettes of 10 × 10 mm²(Hellma type 111-QS, suprasil,

optical precision). They were prepared with distilled and dried

THF or cyclohexane, with concentrations varying from 1 to

8 μM according to their optical density. The quantum yields

were determined using POPOP (p-bis-5-phenyl-oxazolyl-

(2)-benzene) (Φ_ﬂ= 0.93) as the standard.

Conclusions

In conclusion, two linear π-conjugated systems with 1,3-diethyl-

1,3,2-benzodiazaborolyl [C₆H₄(NEt)₂B] as a donor group and

the dimesitylboryl unit (–BMes₂) as acceptor with ethynylene–

phenylene (–CuC-1,4-C₆H₄, 3) and ethynylene–thiophene

(–CuC-2,5-C₄H₄S, 12) bridge between the boron atoms have

been prepared. An assembly (20) constructed by two benzodi-

azaborolyl ethynyl-phenylene-boryl units [C₆H₄(NCy)N′B–

CuC-1,4-C₆H₄-BMes₂] with a 1,4-phenylene bridge at nitrogen

atoms (N′) was also achieved. The borylated compounds 3, 12

and 20 show intense blue luminescence in cyclohexane with

quantum yields of 0.99, 0.44 and 0.94, respectively, but weak

blue-green luminescence in tetrahydrofuran (Φ_ﬂ= 0.02–0.05). In

acetonitrile, however, two emission bands are observed for 3, 12

and 20, and the higher energy one is dominating for 3 and 12,

whereas the low energy band is favoured for 20. This obser-

vation has been rationalised by a partial adduct formation of the

acetonitrile with the boron atom of the Mes₂B unit. The charge

transfer character of these transitions is supported by TD-DFT

computations with the CAM-B3LYP functional. Addition of

tetrabutylammonium ﬂuoride to tetrahydrofuran solutions of 3,

12 and 20 give violet-blue ﬂuorescence emissions at higher

energy and with lower Stokes shifts than their neutral analogues.

The observed solvatochromism of [3·F]⁻suggests a transition

from the π-phenylene–ethylene scaffold to the benzodiazaborole

unit whereas the absence of solvatochromism in [12·F]⁻points

to a local π–π* transition on the thiophene part. The emission

intensities of adducts [3·F]⁻and [20·F]⁻increase by a factor of

46 as compared to those of the neutral 3 and 20, which makes

derivatives 3 and 20 to remarkable “turn on” ﬂuoride sensors in

tetrahydrofuran solutions.

2(4′-Dimesitylboryl-phenylethynyl)-1,3-diethyl-1,3,2-

benzodiazaborole (3)

Experimental section

A solution of 4-dimesitylboryl-phenylacetylene (2) (0.62 g,

1.77 mmol) in n-hexane (20 mL) was added dropwise with

1.11 mL (1.77 mmol) of 1.6 M solution of n-butyllithium in

n-hexane at 20 °C. After 30 min of stirring neat 2-bromo-1,3-

diethyl-1,3,2-benzodiazaborole (1) (0.44 g, 1.77 mmol) was

added. The mixture was stirred for 16 h at room temperature and

then ﬁltered. The ﬁltrate was concentrated until it became cloudy

and cooled overnight at −20 °C. Compound 3 separated as

General

All manipulations were performed under dry, oxygen-free argon

by using Schlenk techniques. Solvents were dried by standard

methods and freshly distilled prior to use. The compounds

2-bromo-1,3-diethyl-1,3,2-benzodiazaborole (1),¹⁶4-dimesityl-

boryl-phenylacetylene (2),³⁰4-cyanophenylacetylene (4),³¹

10340 | Dalton Trans., 2012, 41, 10328–10346

This journal is © The Royal Society of Chemistry 2012

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1

colourless platelets (0.83 g, 90% yield). H NMR (C₆D₆), δ =

borolyl B–CuC peak was not observed. ¹¹B{¹H} NMR (C₆D₆):

δ = 20.7 (s) ppm. MS/EI: m/z (%) = 299.3(98), [M⁺], 284.2

(100), [M]⁺− CH₃. C₁₉H₁₈BN₃(299.18): calcd C. 76.28,

H 6.06, N 14.05; found C 75.66, H 6.38, N 13.73.

3

1.20 (t, J_HH= 7.2 Hz, 6H, CH₂CH₃), 2.10 (s, 12H, o-CH₃-

mesityl), 2.20 (s,6H, p-CH₃-mesityl), 3.75 (q, J_HH= 7.2 Hz,

3

4H, CH₂CH₃), 6.79 (s, 4H, CH-mesityl), 6.95 (m, 2H, diazabor-

olyl CHvCH–CHvCH), 7.12 (m, 2H, diazaborolyl CHvCH–

CHvCH), 7.57 (m, 4H, H-phenylene) ppm. ¹³C{¹H} NMR

(C₆D₆): δ = 15.9 (s, CH₂CH₃), 21.0 (s, p-CH₃-mesityl), 23.4

(s, o-CH₃-mesityl), 38.1 (s, CH₂CH₃), 107.0 (s, B–CuC), 109.1

(s, diazaborolyl CHvCH–CHvCH), 119.3 (s, diazaborolyl

CHvCH–CHvCH), 126.6 (s, CuC–C), 128.6 (s, CH-mesityl),

131.6 (s, CH-phenylene), 136.1 (s, CH-phenylene), 136.9

(s, diazaborolyl C₂N₂), 138.6 (s, p-C-mesityl), 140.7 (s, o-C-

mesityl), 141.6 (s, BC-mesityl), 146.7 (s, BC-phenylene) ppm.

The borolyl B–CuC peak was not observed. ¹¹B{¹H} NMR

(C₆D₆): δ = 21.1 (s, BN₂), 74.4 (s, BMes₂) ppm. MS/EI:

m/z (%) = 522.5(100), [M]⁺, 402.4(35), M⁺− MesH, 387.4(33)

M⁺− MesH − CH₃. C₃₆H₄₀B₂N₂(522.34): calcd C 82.78,

H 7.72, N 5.36; found C 82.74, H 7.84, N 5.34.

2-(Dimesitylboryl)-5-(trimethylsilylethynyl)thiophene (10)

A solution of 2- (trimethylsilylethynyl)thiophene (0.80 g,

4.44 mmol) in diethyl ether (20 mL) was deprotonated at room

temperature by treatment with 2.77 mL (4.44 mmol) of a 1.6 M

solution of n-butyllithium in n-hexane. After 1.5 h of stirring, a

solution of dimesityl ﬂuoroborane (1.2 g, 4.44 mmol) in diethyl

ether (25 mL) was added and stirring was continued for 16 h.

The resulting mixture was combined with 50 mL of brine. The

organic phase was separated and the aqueous phase was

extracted with diethyl ether (2 × 50 mL). The combined ether

phases were dried with anhydrous Na₂SO₄. After ﬁltration the

solvent was removed in vacuo. The residue (1.81 g, 95%) was

identiﬁed by NMR spectroscopy and subsequently desilylated to

1

give compound 11. H NMR (C₆D₆): δ = 0.16 (s, 9H, SiMe₃),

Formation of [nBu₄N]3·F]⁻

2.15 (s, 12H, o-CH₃-mesityl), 2.18 (s, 6H, p-CH₃-mesityl), 6.76

3

(s, 4H, m-H-mesityl), 7.12 (d, J_HH= 1.9 Hz, 1H, H-thiophene),

Equimolar amounts of 3 (27.5 mg, 0.053 mmol) and tetrabutyl-

ammonium ﬂuoride trihydrate (TBAF·3H₂O) (18.3 mg,

0.058 mmol) were dissolved in 0.8 mL of C₆D₆. Layering the

solution with 4 mL n-pentane gave the 1 : 1 adduct as a colour-

less solid. H NMR (C₆D₆): δ = 0.78 (t, J_HH= 7.5 Hz, 12H,

(CH₂)₃CH₃), 1.03 (m, 16H, CH₂CH₂CH₂CH₃), 1.24 (t, J_HH

7.2 Hz, 6H, CH₂CH₃), 2.30 (s, 6H, p-CH₃-mesityl), 2.34

(m, 8H, CH₂(CH₂)₂CH₃), 2.45 (s, 12H, o-CH₃-mesityl), 3.82

(q, J_HH= 7.2 Hz, 4H, CH₂CH₃), 6.84 (s, 4H, H-mesityl), 6.96

3

7.13 (d, J_HH= 1.9 Hz, H-thiophene) ppm. ¹³C{¹H} NMR

(C₆D₆): δ = –0.5 [s, Si(CH₃)₃], 21.1 (s, p-CH₃-mesityl), 23.4

(s. o-CH₃-mesityl), 98.4 (s, CuC-SiMe₃), 102.8 (s, CuC-SiMe₃),

128.7 (s, CH-mesityl), 134.4 (s, CH-thiophene), 136.4 (s,

C-thiophene), 138.8 (s, p-C-mesityl), 139.9 (s, CH-thiophene),

140.8 (s, o-C-mesityl), 141.2 (s, BC-mesityl), 152.7 (s, BC-thio-

phene) ppm. ¹¹B{¹H} NMR (C₆D₆): δ = 65.2 ppm.

1

3

=

3

(m, 2H, diazaborolyl CHvCH–CHvCH), 7.10 (m, 2H, diaza-

borolyl CHvCH–CHvCH), 7.73 (m, 4H, H-phenylene) ppm.

¹¹B{¹H} NMR (C₆D₆): δ = 5.1 (s, 1B, BMes₂F), 21.3 (s, 1B,

BN₂) ppm. ¹⁹F{¹H} NMR (C₆D₆): −172.2 (s) ppm.

2-(Dimesitylboryl)-5-(ethynyl)thiophene (11)

A sample of K₂CO₃(2 g) was added to the solution of com-

pound 10 (1.81 g, 4.22 mmol) in a mixture of CH₂Cl₂(50 mL)

and methanol (50 mL). After stirring the slurry for 3 h at 20 °C

water (100 mL) was added. The aqueous layer was extracted

with CH₂Cl₂(2 × 50 mL). The combined organic layers were

dried (Na₂SO₄) and then ﬁltered. Solvent was evaporated

in vacuo and the residue was chromatographed on Silica 60

(Merck) in a short column (l = 30 cm, ∅ = 5 cm) with cyclo-

hexane as an eluent. Product 11 was isolated as a yellow solid

2(4′-Cyanophenylethynyl)-1,3-diethyl-1,3,2-benzodiazaborole (5)

The solution of 4-cyanophenylacetylene (0.26 g, 2.05 mmol) in

15 mL of THF was combined at 20 °C with the solution of an

equimolar amount of LiN(SiMe₃)₂, freshly made from 0.33 g

HN(SiMe₃)₂(2.05 mmol) and 1.28 mL (2.05 mmol) of a 1.6 M

n-butyllithium solution in n-hexane. Stirring of the resulting

black solution was continued for 30 min, before a sample of 1

(0.51 g, 2.05 mmol) was added. After 16 h of stirring the sol-

vents and all volatile components were removed in vacuo. The

solid residue was triturated with 30 mL of warm n-hexane (ca.

60 °C) and ﬁltered. Storing the ﬁltrate overnight at −35 °C

afforded 0.42 g (69% yield) of product 5 as colourless needles.

1

(0.93 g, 62% yield). H NMR (C₆D₆): δ = 2.15 (s, 12H, o-CH₃-

mesityl), 2.18 (s, 6H, p-CH₃-mesityl), 2.98 (s, 1H, CuCH),

3

6.76 (s, 4H, m-H-mesityl), 7.08 (d, J_HH= 3.8 Hz, 1H, H-thio-

phene), 7.12 (d, ³J_HH= 3.8 Hz, 1H, H-thiophene) ppm. ¹³C{¹H}

NMR (C₆D₆): δ = 21.1 (s, p-CH₃-mesityl), 23.4 (s, o-CH₃-

mesityl), 77.1 (s, CuCH), 85.0 (s, CuCH), 128.6 (s, CH-

mesityl), 134.2 (s, CuC–C), 134.9 (s, CH-thiophene), 138.9

(s, p-C-mesityl), 139.8 (s, CH-thiophene), 140.8 (s, o-C-

mesityl), 141.2 (s, BC-mesityl), 152.8 (s, BC-thiophene) ppm.

¹¹B{¹H} NMR (C₆D₆): δ = 65.9 ppm.

3

¹H NMR (C₆D₆): δ = 1.20 (t, J_HH= 7.2 Hz, 6H, CH₂CH₃),

3

3.73 (q,³J_HH= 7.2 Hz, 4H, CH₂CH₃), 6.84 (d, J_HH= 8.2 Hz,

2H, H-phenylene), 6.95 (m, 2H, diazaborolyl, CHvCH–

3

CHvCH), 7.04 (d, J_HH= 8.2 Hz, 2H, H-phenylene), 7.12

(m, 2H, diazaborolyl CHvCH–CHvCH) ppm. ¹³C{¹H} NMR

(C₆D₆): δ = 16.1 (s, CH₂CH₃), 38.3 (s,CH₂CH₃), 104.9

(s, B–CuC), 109.4 (s, diazaborolyl CHvCH–CHvCH), 112.3

(s, C–CN), 118.1 (s, C–CN), 119.7 (s, diazaborolyl CHvCH–

CHvCH), 127.1 (s, CuC–C), 131.8 (s, CH-phenylene), 132.1

(s, CH-phenylene), 136.9 (s, diazaborolyl C₂N₂) ppm. The

2-(5′-Dimesitylboryl-2′-thienylethynyl)-1,3-diethyl-1,3,2-

benzodiazaborole (12)

The solution of 11 (0.73 g, 2.05 mmol) in n-hexane (25 mL)

was treated at room temperature with 1.28 mL (2.05 mmol) of a

1.6 M solution of n-butyllithium in n-hexane. After 30 min

This journal is © The Royal Society of Chemistry 2012

Dalton Trans., 2012, 41, 10328–10346 | 10341

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3

4

0.80 g (3.17 mmol) of neat 1 was added and stirring was contin-

ued for 16 h. The slurry was ﬁltered and the ﬁlter-cake was

washed with hot n-hexane (ca. 60 °C). The ﬁltrate was freed

from all volatile components. Crystallisation of the crude

material from n-hexane yielded 0.58 g (54%) of product 12 as

(d, J_HH= 5.7 Hz, 1H, NH), 6.37 (dd, J_HH= 7.7 Hz, J_HH

=

1.4 Hz, 1H, H-phenylene), 6.40 (m, 1H, H-phenylene), 6.99

(m, 1H, H-phenylene), 7.37 (dd, J_HH= 7.9 Hz, J_HH= 1.3 Hz,

1H, H-phenylene) ppm.

3

4

1

3

large colourless crystals. H NMR (C₆D₆): δ = 1.14 (t, J_HH

=

N,N′-(Bis-2′-isopropylamino-phenyl)-p-phenylenediamine (14)

7.2 Hz, 6H, CH₂CH₃), 2.19 (s, 6H, p-CH₃-mesityl), 2.20

3

(s, 12H, o-CH₃-mesityl), 3.69 (q, J_HH= 7.2 Hz, 4H, CH₂CH₃),

Sodium tert-butanolate (0.039 g, 0.40 mmol), 1,3-bis(2,6-diiso-

propylphenyl)imidazolium chloride (0.175 g, 0.37 mmol) and

0.041 g (0.18 mmol) of palladium acetate were dissolved with

stirring in hot toluene (10 mL, 80 °C). This solution was added

to a mixture of 4.00 g (36.99 mmol) of p-phenylenediamine,

15.84 g (73.98 mmol) of 13 and 7.11 g (73.98 mmol) of sodium

tert-butanolate in 500 mL of toluene. The mixture was heated

for 48 h at 110 °C, whereby solid NaBr separated. The dark solu-

tion was washed with a degassed saturated NH₄Cl solution

(3 × 150 mL) and brine (1 × 150 mL) before the organic phase

was dried with Na₂SO₄. Removal of solvent in vacuo gave a

dark blue oil. This oil was crystallised twice from toluene to

yield product 14 (9.80 g, 71.0%) as colourless crystals. ¹H NMR

6.78 (s, 4H, H-mesityl), 6.92 (m, 2H, diazaborolyl CHvCH–

CHvCH), 7.09 (m, 2H, diazaborolyl CHvCH–CHvCH), 7.21

3

(d, J_HH= 3.9 Hz, 1H, H-thiophene), 7.23 (d, J_HH= 3.9 Hz,

1H, H-thiophene) ppm. ¹³C{¹H} NMR (C₆D₆): δ = 16.0

(s, CH₂CH₃), 21.1 (s, p-CH₃-mesityl), 23.5 (s, o-CH₃-mesityl),

38.3 (s, CH₂CH₃), 99.6 (s, B–CuC), 109.3 (s, diazaborolyl

CHvCH–CHvCH), 119.5 (s, diazaborolyl CHvCH–

CHvCH), 128.7 (s, CH-mesityl), 134.7 (s, CH-thiophene),

136.6 (s, CuC–C), 137.0 (s, C₂N₂), 138.9 (s, p-C-mesityl),

140.1 (s, CH-thiophene), 140.9 (s, o-C-mesityl), 141.2 (s, BC-

mesityl), 153.2 (s, BC-thiophene) ppm. The borolyl B–CuC

peak was not observed. ¹¹B{¹H} NMR (C₆D₆): δ = 20.9

(s, BN₂), 66.8 (s, BMes₂) ppm. MS/EI: m/z (%) = 528.3(100)

[M]⁺, 408.2(23), [M]⁺− MesH, 393.1(18) [M]⁺− MesH −

CH₃. C₃₄H₃₈B₂N₂S (528.37): calcd C 77.29, H 7.25, N 5.30;

found C 75.84, H 7.41, N 5.26.

3

(C₆D₆): δ = 0.92 (d, J_HH= 6.3 Hz, 12H, CH(CH₃)₂), 3.37

(sept., ³J_HH= 6.3 Hz, 2H, CH(CH₃)₂), 3.85 (s, 2H, NH), 4.43 (s,

2H, NH), 6.53 (s, 4H, 1,4-phenylenediamine CvCH–CHvC),

6.67, 6.72, 7.04, 7.09 (4m, 8H, CH–CHvCH–CH) ppm. ¹³C

{¹H} NMR (C₆D₆): δ = 22.7 (s, CH(CH₃)₂), 43.8 (s, CH-

(CH₃)₂), 111.9 (s, CH-1,2-phenylenediamine), 117.3 (s, CH-1,2-

phenylenediamine), 118.0 (s, 1,4-phenylenediamine CvCH–

CHvC), 123.3 (s, CH-1,2-phenylenediamine), 125.1 (s,

CH-1,2-phenylenediamine), 130.8 (s, 1,4-phenylenediamine

CvCH–CHvC), 139.0 (s, C-1,2-phenylenediamine), 142.5 (s,

C-1,2-phenylenediamine) ppm. MS/EI: m/z (%) = 374.3(100)

[M⁺], 288.1(29) [M⁺− 2CH(CH₃)₂]. C₂₄H₃₀N₄(374.52): calcd

C 76.97, H 8.07, N 14.96; found C 77.17, H 8.02, N 14.93.

Formation of [nBu₄N][12·F]

Equimolar amounts of 12 (21.5 mg, 0.04 mmol) and

TBAF·3H₂O (15.4 mg, 0.049 mmol) were combined in C₆D₆

(0.8 mL). Layering the clear solution with n-pentane (3 mL)

gave the 1 : 1 adduct as colourless crystals. ¹H NMR (C₆D₆): δ =

3

0.83 (t, J_HH= 7.5 Hz, 12H, (CH₂)₃CH₃), 1.06 (m, 16H, CH₂

3

CH₂CH₂CH₃), 1.24 (t, J_HH= 7.2 Hz, 6H, CH₂CH₃), 2.29

(s, 6H, p-CH₃-mesityl), 2.35 (m, 8H, CH₂(CH₂)₂CH₃), 2.53

3

(s, 12H, o-CH₃-mesityl), 3.81 (q, J_HH= 7.2 Hz, 4H, CH₂CH₃),

1,4-Bis[(2′-bromo-3′-isopropyl-1′,3′,2′-benzodiazaborol-1′-yl)]-

benzene (15)

6.83 (s, 4H, H-mesityl), 6.96 (m, 2H, CHvCH–CHvCH), 7.03

(m, 1H, H-thiophene), 7.10 (m, 2H, CHvCH–CHvCH), 7.43

(m, 1H, H-thiophene) ppm. ¹¹B{¹H} NMR (C₆D₆): δ = 5.0

(s, 1B, BMes₂F), 21.4 (s, 1B, BN₂) ppm. ¹⁹F{¹H} NMR (C₆D₆):

−163.1 (s) ppm.

A slurry of calcium hydride (2.80 g, 66.0 mmol) in CH₂Cl₂

(50 mL) was simultaneously combined with two separate solu-

tions of 14 (5.00 g, 13.35 mmol) and boron tribromide (7.34 g,

29.37 mmol) each in 50 mL of CH₂Cl₂. The slurry was stirred

16 h at room temperature, then ﬁltered and the ﬁltrate was liber-

ated from volatile components in vacuo. The residue was crystal-

lised from toluene to give product 15 (4.50 g, 61%) as

colourless crystals. Single crystals of the material were grown by

2-Isopropylamino-1-bromobenzene (13)

Molecular sieves (20 g, 3 Å) were added to a solution of 2-bro-

moaniline (15.0 g, 87.2 mol) in acetone (150 mL) and the result-

ing mixture was vigorously stirred for 24 h. It was ﬁltered and

the ﬁltrate was evaporated to dryness to afford a yellow oil.

NaBH₄(9.9 g, 261.6 mmol) was added at 0 °C to a solution of

this oil in methanol (150 mL). The reaction mixture was stirred

for 16 h, during which the ice bath warmed up to ambient temp-

erature. Then 150 mL of a 1 M aqueous NaOH solution was

added. The resulting mixture was extracted with CH₂Cl₂(3 ×

100 mL) and the combined organic layers were dried with an-

hydrous Na₂SO₄. After removal of solvents the residue was

puriﬁed by short path distillation (2 × 10⁻⁵bar, 60 °C bath temp-

erature) to give 15.6 g (84%) of product 13 as colourless liquid.

1

layering a saturated CHCl₃solution with n-pentane. H NMR

3

(C₆D₆): δ = 1.55 (d, J_HH= 6.3 Hz, 12H, CH(CH₃)₂), 4.47

3

(sept., J_HH= 6.3 Hz, 2H, CH(CH₃)₂), 6.93 (m, 2H, CH-diaza-

borolyl), 7.01 (m, 2H, CH-diazaborolyl), 7.06 (m, 2H, CH-

diazaborolyl), 7.22 (m, 2H, CH-diazaborolyl), 7.38 (s, 4H, 1,4-

phenylendiamine CvCH–CHvC) ppm. ¹³C{¹H} NMR (C₆D₆):

δ = 21.8 (s, CH(CH₃)₂), 46.5 (s, CH(CH₃)₂), 110.8, 110.9,

119.9, 120.4 (4s, CH-diazaborolyl), 128.4 (s, 1,4-phenylendi-

amine CvCH–CHvC), 135.7 (s, 1,4-phenylendiamine CvCH–

CHvC), 137.5, 137.6 (2s, C-diazaborolyl) ppm. The borolyl

B–CuC peak was not observed. ¹¹B{¹H} NMR (C₆D₆): δ =

22.9 (s) ppm. C₂₄H₂₆B₂Br₂N₄(551.92): calcd C 52.23, H 4.75,

N 10.15, found C 51.41, H 4.31, N 10.01.

3

¹H NMR (C₆D₆): δ = 0.88 (d, J_HH= 6.3 Hz, 12H, CH(CH₃)₂),

3

3.69 (d of sept., J_HH= 5.7, 6.3 Hz, 1H, CH(CH₃)₂), 4.13

10342 | Dalton Trans., 2012, 41, 10328–10346

View Article Online

1,4-Bis[2′-phenylethynyl-3′-isopropyl-1′,3′,2′-benzodiazaborol-

1′-yl)]benzene (16)

palladium acetate (0.024 g, 0.11 mmol) were dissolved with stir-

ring in hot toluene (5 mL, 80 °C). This solution was added to a

mixture of p-phenylenediamine (1.17 g, 10.8 mmol), 5.50 g

(21.6 mmol) of 17 and sodium tert-butanolate (2.29 g,

23.8 mmol) in 200 mL of toluene. The solution was heated for

72 h at 110 °C, whereby solid NaBr precipitated. Then the dark

solution was extracted with degassed saturated solutions of

NH₄Cl (3 × 50 mL) and NaCl (1 × 50 mL). The organic layer

was dried with Na₂SO₄, ﬁltered and freed from solvent in vacuo

to afford a dark blue oil. Two-fold recrystallisation from

n-hexane furnished 2.5 g (51% yield) of product 18 as colourless

crystals. ¹H NMR (C₆D₆): δ = 0.94 (m, 6H, C₆H₁₁), 1.07

(m, 4H, C₆H₁₁), 1.39 (m, 2H, C₆H₁₁), 1.52 (m, 4H, C₆H₁₁), 1.87

(m, 4H, C₆H₁₁), 3.11 (m, 2H, N–CH), 4.01 (s, 1H, NH), 4.55

(s, 1H, NH), 6.31 (s, 4H, 1,4-phenylendiamine CvCH–

CHvC), 6.56 (m, 2H, H-1,2-phenylenediamine), 6.72 (m, 2H,

H-1,2-phenylenediamine), 7.01 (m, 2H, H-1,2-phenylene-

diamine), 7.08 (m, 1H, H-1,2-phenylenediamine) ppm. ¹³C{¹H}

NMR (C₆D₆): δ = 24.6 (s, CH₂), 25.5 (s, CH₂), 31.5 (s, CH₂),

51.4 (s, CH–C₆H₁₁), 111.5, 117.7 (2s, CH-1,2-phenylene-

diamine), 118.1 (s, 1,4-phenylendiamine CvCH–CHvC), 123.8,

124.7 (2s, CH-1,2-phenylenediamine), 130.6 (s, 1,4-phenylen-

diamine CvCH–CHvC), 138.5, 142.9 (2s, C-1,2-phenylene-

diamine) ppm.

Lithium phenylacetylide was prepared from phenylacetylene

(0.43 g, 4.19 mmol) and 2.61 mL (4.19 mmol) of a 1.6 M

hexane solution of n-butyllithium at 20 °C. After 20 min a slurry

of 15 (2.61 g, 4.19 mmol) in n-hexane (20 mL) was added.

After a period of 4 d of stirring at room temperature it was

ﬁltered. The ﬁltrate was discarded and the ﬁlter-cake was con-

tinuously extracted with n-hexane for 7 d. During this time a col-

ourless precipitate separated, which was crystallised from

CH₂Cl₂–n-hexane to give product 16 (0.63 g, 53% yield) as a

1

3

microcrystalline solid. H NMR (CDCl₃): δ = 1.77 (d, J_HH

=

3

6.3 Hz, 12H, CH(CH₃)₂), 4.56 (sept., J_HH= 6.3 Hz, 2H, CH-

(CH₃)₂), 7.11 (m, 2H, CH-diazaborolyl), 7.19 (m, 2H, CH-diaza-

borolyl), 7.30–7.38 (m, 2H, CH-diazaborolyl and 6H, m -and

p-CH-phenyl), 7.41 (m, 2H, CH-diazaborolyl), 7.53 (m, 4H,

o-CH-phenyl), 7.72 (s, 4H, 1,4-phenylendiamine CvCH–

CHvC) ppm. ¹³C{¹H} NMR (CDCl₃): δ = 23.4 (s, CH(CH₃)₂),

46.2 (s, CH(CH₃)₂), 107.3 (s, B–CuC), 109.8 (s, CH-diazaboro-

lyl), 110.3 (s, CH-diazaborolyl), 119.3 (s, CH-diazaborolyl),

120.0 (s, CH-diazaborolyl), 123.1 (s, CuC–C), 126.6 (s,

1,4-phenylendiamine CvCH–CHvC), 128.4 (s, m-CH-phenyl),

128.9 (s, p-CH-phenyl), 131.9 (s, o-CH-phenyl), 133.2 (s,

1,4-phenylendiamine CvCH–CHvC), 136.7, 137.9 (2s,

C-diazaborolyl) ppm. The borolyl B–CuC peak was not

observed. ¹¹B{¹H} NMR (CDCl₃):

δ = 20.2 (s) ppm.

1,4-Bis[(2′-bromo-3′-cyclohexyl-1′,3′,2′-benzodiazaborol-1′yl)]-

C₄₀H₃₆B₂N₂(594.36): calcd C 80.83, H 6.11, N 9.43; found C

80.19, H 6.10, N 9.23.

benzene (19)

A slurry of CaH₂(1.1 g, 16.4 mmol) in CH₂Cl₂(30 mL) was

simultaneously combined with two separate solutions of 18

(2.3 g, 5.1 mmol) and boron tribromide each in 30 mL of

CH₂Cl₂. The slurry was stirred for 16 h at ambient temperature,

ﬁltered and the ﬁltrate was evaporated to dryness. The residue

was crystallised from toluene to afford 1.38 g (43%) of 19 as a

colourless solid. ¹H NMR (C₆D₆): δ = 1.07 (m, 2H, C₆H₁₁),

1.18 (m, 4H, C₆H₁₁), 1.51 (m, 2H, C₆H₁₁), 1.66 (m, 4H, C₆H₁₁),

1.83 (m, 4H, C₆H₁₁), 2.19 (m, 4H, C₆H₁₁), 3.97 (m, 2H, NCH),

6.95 (m, 2H, CH-diazaborolyl), 7.05 (m, 2H, CH-diazaborolyl),

7.07 (m, 2H, CH-diazaborolyl), 7.22 (m, 2H, CH-diazaborolyl),

7.14 (s, 4H, 1,4-phenylendiamine CvCH–CHvC) ppm. ¹³C

{¹H} NMR (C₆D₆): δ = 25.4, 26.3, 32.2 (3s, CH₂), 54.9 (s, CH–

C₆H₁₁), 110.6, 110.7, 119.8, 120.3 (4s, CH-diazaborolyl), 128.1

(s, 1,4-phenylendiamine CvCH–CHvC), 135.7 (s, 1,4-pheny-

lendiamine CvCH–CHvC), 137.4, 137.5 (2s, C-1,2-pheny-

lenediamine) ppm. The borolyl B–CuC peak was not observed.

¹¹B{¹H} NMR (C₆D₆): δ = 23.5 (s) ppm. C₃₀H₃₄BBr₂N₄

(632.05): calcd C 57.01, H 5.42, N 8.86; found: C 56.15,

H 5.17, N 8.24.

2-Cyclohexylamino-1-bromobenzene (17)

A mixture of 2-bromoaniline (5.00 g, 29.07 mmol), cyclohexa-

none (8.56 g, 87.2 mmol) and p-toluene sulfonic acid in 80 mL

of benzene was reﬂuxed with a Dean-Stark trap attached for

24 h. Solvent and excess of cyclohexanone were removed at

60 °C in vacuo. The resulting oil was dissolved in methanol

(100 mL) and treated with NaBH₄(3.4 g, 90.0 mmol) in an ice

bath. The reaction mixture was stirred for 16 h during which the

bath reached ambient temperature. An aqueous solution of

sodium hydroxide (150 mL, 1 M) was added and the organic

layer was removed. The aqueous layer was extracted with

CH₂Cl₂(3 × 100 mL) before the combined organic phases were

dried with Na₂SO₄. It was ﬁltered and the ﬁltrate was evaporated

to dryness. Remaining starting material was removed by ﬁltration

of a toluene solution over a pad (l = 15 cm, ∅ = 5 cm) of silica

60. After washing with toluene the elute was freed from solvent.

The subsequent short path distillation of the residue (10⁻⁶bar,

90 °C bath temperature) furnished product 17 as colourless

liquid (yield: 4.50 g, 61%). ¹H NMR (CDCl₃): δ = 1.26 (m, 3H,

C₆H₁₁), 1.39 (m, 2H, C₆H₁₁), 1.65 (m, 1H, C₆H₁₁), 1.77 (m, 2H,

C₆H₁₁), 2.04 (m, 2H, C₆H₁₁), 3.30 (m, 1H, NCH), 4.23 (s, 1H,

NH), 6.51 (m, 1H, H-phenylene), 6.64 (m, 1H, H-phenylene),

7.14 (m, 1H, H-phenylene), 7.40 (m, 1H, H-phenylene) ppm.

1,4-Bis[2′-p-dimesitylboryl-phenylethynyl-3′-cyclohexyl-1′,3′,2′-

benzodiazaborol-1-yl)]benzene (20)

A toluene solution (40 mL) of 4-dimesitylboryl-phenylacetylene

(1.05 g, 3.0 mmol) was treated at 20 °C with 1.88 mL of a

1.6 M n-hexane solution (20 mL) of n-butyllithium (3.0 mmol).

After 20 min a toluene solution (20 mL) of compound 19

(0.95 g, 1.5 mmol) was added at 20 °C and stirring was contin-

ued for 16 h. Solvent and volatile components were removed

N,N′-(Bis-2′-cyclohexylamino-phenyl)-p-phenylenediamine (18)

Sodium tert-butanolate (0.023 g, 0.29 mmol), 1,3-bis(2,6-diiso-

propylphenyl)imidazolium chloride (0.092 g, 0.22 mmol) and

Dalton Trans., 2012, 41, 10328–10346 | 10343

View Article Online

Table 6 Crystallographic data of 3, 12, [Bu₄N][12·F] and 15

3

12

[Bu₄N][12·F]

15

Empirical formula

M_r[g mol⁻¹

Crystal dimension (mm)

Crystal system

Space group

a [Å]

b [Å]

c [Å]

α [°]

β [°]

C

₃₆H₄₀B₂N₂

C₃₄H₃₈B₂N₂S

528.34

0.21 × 0.06 × 0.04

Triclinic

ˉ

P1

C₅₀H₇₄B₂FN₃S

789.80

0.31 × 0.16 × 0.08

Triclinic

ˉ

P1

C₂₄H₂₆B₂Br₂N₄

551.93

0.33 × 0.21 × 0.16

Monoclinic

P2₁/c

9.1033(5)

7.6616(4)

17.2575(9)

90

90.917(2)

90

1203.48(11)

2

1.523

4.412

556

]

522.32

0.30 × 0.30 × 0.04

Monoclinic

P2₁

8.4437(2)

9.4089(2)

19.3916(4)

90

97.7092(11)

90

1526.66(6)

2

1.136

0.064

560

3.0–27.5

32 337

3700

8.1825(4)

13.9608(5)

14.0110(7)

82.544(3)

83.797(2)

74.747(2)

1526.52(12)

2

1.149

0.131

564

2.9–25.0

16 943

18.230(3)

22.799(4)

24.997(4)

106.930(11)

99.728(14)

94.965(11)

9694(3)

8

1.082

0.876

3440

1.9–66.4

53 217

γ [°]

V [Å]

Z

ρ_calc[g cm⁻³

]

μ [mm⁻¹

F (000)

Θ [°]

]

4.9–72.0

20 670

2343

No. reﬂ. collected

No. reﬂ. unique

R (int)

5359

0.046

30 945

0.0250

0.052

0.0439

No. reﬂ. [I > 2σ(I)]

Reﬁned parameters

GOF

3303

369

1.024

0.0364

0.0879

0.188/−0.165

3741

360

1.017

0.0488

0.1340

0.204/−0.227

25 380

2101

1.017

0.0494

0.1406

0.904/−0.291

2342

148

1.083

0.0500

R_f[I > 2σ(I)]

wR_F₂(all data)

Largest diff. peak and hole [e Å⁻³

Remarks

0.1184

]

2.729/−1.804

Largest diff. peak near Br(1) (0.77 Å)

in vacuo and the residue was stirred in 60 mL of n-hexane. After

ﬁltration the ﬁltrate was discarded and the ﬁlter-cake was con-

tinuously extracted with n-hexane over a period of 14 d. Thereby

a colourless precipitate separated, which was collected by ﬁl-

tration and subsequently recrystallised from a toluene–n-hexane

mixture. Product 20 was obtained as a colourless microcrystal-

line solid (0.21 g, 12% yield). ¹H NMR (C₆D₆): δ = 1.23

(m, 6H, C₆H₁₁), 1.61 (m, 2H, C₆H₁₁), 1.73 (m, 4H, C₆H₁₁), 2.05

(s, 12H, o-CH₃-mesityl), 2.08 (m, 4H, C₆H₁₁), 2.18 (s, 6H,

p-CH₃-mesityl), 2.29 (m, 4H, C₆H₁₁), 3.91 (m, 2H, NCH), 6.76

(s, 4H, CH-mesityl), 6.99 (m, 2H, CH-diazaborolyl), 7.05 (m,

2H, CH-diazaborolyl), 7.19 (m, 2H, CH-diazaborolyl), 7.31 (m,

(graphite monochromator, λ = 0.71073 Å) at 100 K (12 at

200 K), while [Bu₄N][12·F] and 15 were measured on a Bruker

AXS X8 with Cu_Kαradiation at 100 K. Crystallographic pro-

grams used for structure solution and reﬁnement were

SHELXS-97 and SHELXL-97.³⁸The structures were solved by

direct methods and were reﬁned by using full-matrix least-

squares of F²of all unique reﬂections with anisotropic thermal

parameters for all non-hydrogen atoms. Hydrogen atoms were

included at calculated positions with U(H) = 1.2 U_eqfor CH₂

groups and U(H) = 1.5 U_eqfor CH₃groups. Crystallographic

data for the compounds are listed in Table 6.

3

2H, CH-diazaborolyl), 7.43 (d, J_HH= 8.4 Hz, 1,4-phenylene),

3

Computational methods

7.48 (d, J_HH= 8.4 Hz, 1,4-phenylene), 7.56 (s, 4H, 1,4-pheny-

lendiamine CvCH–CHvC) ppm. ¹³C{¹H} NMR (C₆D₆): δ =

21.0 (s, p-CH₃-mesityl), 23.5 (s, o-CH₃-mesityl), 25.6, 26.9,

32.5 (3s, CH₂), 54.2 (s, CH–C₆H₁₁), 107.3 (s, B–CuC), 109.7,

110.2, 119.1, 119.8 (4s, CH-diazaborolyl), 126.2 (s, CuC–C),

126.4 (s, 1,4-phenylendiamine CvCH–CHvC), 128.7 (s, CH-

mesityl), 131.2 (s, CH-1,4-phenylene), 133.1 (s, 1,4-phenylen-

diamine CvCH–CHvC), 136.0 (s, CH-1,4-phenylene), 136.6,

137.8 (2s, C-diazaborolyl), 138.9 (s, p-C-mesityl), 140.6 (s,

o-C-mesityl), 141.5 (s, BC-mesityl), 146.5 (s, BC-1,4-pheny-

lene) ppm. The borolyl B–CuC peak was not observed.

¹¹B{¹H}NMR (C₆D₆): δ = 21.0 (s, BN₂), 74.9 (s, BMes₂) ppm.

C₈₂H₈₆B₄N₄(1170.8) calcd C 84.12, H 7.40, N 4.79; found

C 83.69, H 7.25, N 4.31.

All computations were carried out with the Gaussian 03 and

Gaussian 09 packages.³⁹Geometries were optimized by DFT

calculations at the B3LYP/6-31G* level of theory.^40,41TD-DFT

calculations with the CAM-B3LYP⁴²functional were performed

on the B3LYP optimized geometries. Frequency calculations on

the fully optimized geometries showed no imaginary frequen-

cies. MO ﬁgures were generated with Molekel⁴³and the Gauss-

Sum 2.2 package was used to calculate the orbital

contributions.⁴⁴

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10346 | Dalton Trans., 2012, 41, 10328–10346

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Diazaborolyl-boryl push-pull systems with ethynylene-arylene bridges as 'turn-on' fluoride sensors

DOI: 10.1039/c2dt30438d

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