Backbone Boron-Functionalized Imidazoles/Imidazolium Salts: Synthesis, Structure, Metalation Studies, and Fluoride Sensing Properties

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Backbone Boron-Functionalized Imidazoles/Imidazolium Salts:

Synthesis, Structure, Metalation Studies, and Fluoride Sensing

Properties

Iruthayaraj Avinash,^†Sabeeha Parveen,^†and Ganapathi Anantharaman*

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ABSTRACT: Incorporation of a Lewis acidic BMes₂(Mes = mesityl) moiety at

the backbone of the imidazole ring was achieved by metal−halogen exchange

procedure. Among them, two isomeric boron-phosphine functionalized imidazoles

(3 and 6), monoboron-functionalized imidazoles (4 and 5), and its corresponding

imidazolium salts were synthesized and thoroughly characterized. The solid-state

structure of 3 reveals a dimeric B−N adduct that possesses six-membered [C−B−

N]₂ring, and 5 crystallizes as tetrameric B−N adduct that forms an interesting 16-

membered macrocycle, whereas 4 and 6 were obtained as monomeric BMes₂-

substituted imidazoles. 6 behaves as a P^N-type ligand upon the coordination with

CuI to aﬀord luminescent L₂Cu₄I₄-type metal complexes (10 and 11) whose

photophysical properties were also studied. The presence (in 10) and the absence

(in 11) of BMes₂made a remarkable impact on ﬂuorescence emission causing shift

from the green (10) to orange (11) region. The ﬂuoride sensing properties of

BMes₂-containing imidazoles (4 to 9) were studied using UV−vis and ﬂuorescence spectroscopy.

between the boryl and the donor group are drawing more

interests due to their added advantages in (a) increasing the

electrophilicity of boron, (b) tuning of luminescent properties,

and (c) oﬀering the coordination site for metal ions (Chart

1).^5g,h,11g,12Besides the anion sensing, metal complexes with

BAr₂-substituted ligands were extensively used in electro-

luminescent materials and the related optoelectronic applica-

tions.¹³Among them, pyridine-containing boranes were more

explored, whereas imidazole-containing ambiphilic/donor−

acceptor molecules are elusive.

Functionalization of imidazoles/imidazolium salts with

Lewis base/donor group has been well-studied due to the

interesting coordination chemistry and ubiquitous application

in catalysis, medicine, and materials.¹⁴However, functionaliza-

tion of imidazoles/imidazolium salts with Lewis acidic/

acceptor groups are less explored.¹⁵

INTRODUCTION

■

Triarylboranes are one of the prominent classes of organo-

boron compounds whose applications are widespread in

nonlinear optics (NLO),¹organic light-emitting diodes

(OLEDs),²catalysis,³small molecule activation,⁴and in

anion sensing.⁵The empty pπ-orbital on boron induces the

charge transfer transition from the π-conjugated aryl rings or

the donor groups attached to it, which is responsible for the

luminescence property of most of the triarylboranes and their

related applications. The strong Lewis acidic nature of

triarylboranes enables them as excellent candidates for sensing

the nucleophilic anions like ﬂuoride and cyanide.⁶Moreover,

the ﬂuorescent nature of triarylboranes promotes them as

optical sensors. Though the ﬂuoride ions are essential for

dental and skeletal health,⁷excess of ﬂuoride contamination in

drinking water and their consequence in human health and

environment has engrossed the chemist’s attention in the

pursuit of eﬃcient and stable ﬂuoride sensors.⁸Among the few

eﬀective analytical methods⁹and molecular sensors¹⁰in the

literature, compounds and metal complexes containing

organoboron functionalities^5h,6b,11have received recent

attention due to (i) strong aﬃnity and selectivity of boron

toward ﬂuoride, (ii) switchable photophysical properties of

donor−acceptor-containing triaryl boranes that makes them a

ﬂuorescent/colorimetric ﬂuoride sensors, and (iii) facile

synthesis of stable and soluble species. Besides the generally

used aryl linkers, organoboron compounds containing

heterocyclic linkers like pyridine, picoline, and thiophene

Our research group has been working on the chemistry of

various backbone and N-functionalized imidazoles/imidazo-

lium salts as precursors for metal-NHC (NHC = N-

heterocyclic carbene) complexes and coordination polymers

and their applications in catalysis.¹⁶

Received: February 5, 2020

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inorganic/organic functional groups at the backbone of

imidazole.¹⁷The metal−halogen exchange procedure, which

was successfully applied to introduce heteroatom functional

groups (e.g., SPh, PPh₂, SiMe₃) at the backbone of the

imidazoles by our group,^16d,f,gwas employed in this work to

achieve boron substitution. Metal−Halogen exchange reaction

of 4-iodo-5-diphenyl-phosphino-1-methylimidazole (2) with

isopropyl magnesium chloride (ⁱPrMgCl) followed by the

addition of ﬂuorodimesitylborane yielded 3 as the major

product (Scheme 1). The ¹H NMR spectrum of 3 (Figure S7,

Chart 1. BAr₂-Functionalized Heterocyclic Compounds

Scheme 1. Synthesis of 3

In our pursuit toward novel and ubiquitous heteroatom-

functionalized imidazoles/imidazolium salts,^16b,d,gwe envis-

aged to synthesize imidazole-based ambiphilic (donor−accept-

or) molecules by introducing Lewis acid borane functionality

at the backbone of imidazoles along with a Lewis basic donor

functional group (Chart 2). In this paper, we describe the

Chart 2. Backbone Heteroatom-Functionalized Imidazoles/

Imidazolium Salts from Our Laboratory

Supporting Information) shows a peculiar pattern of six

distinct peaks for CH₃protons of mesityl (Mes) group (four

ortho and two para CH₃protons) ranging from 1.14 to 2.19

ppm (Table 1). Furthermore, four diﬀerent signals were

observed for meta aryl protons of the mesityl group ranging

from 5.79 to 6.58 ppm. The aryl protons of the PPh₃groups

resonate from 6.86 to 7.32 ppm, and a singlet peak was

observed for imidazole C-H at 7.89 ppm. This unique splitting

pattern of mesityl protons was also observed in imidazol-2-yl-

borane-nitrogen (B−N) dimer adduct reported by Okada et al.

in 1995, which suggests the formation of imidazole-4-yl-borane

dimer adduct in the present case.^15cIn the ¹³C NMR spectrum

of 3, the number of carbon peaks for the mesityl group

validates the splitting pattern observed in the ¹H NMR

spectrum. The ³¹P NMR spectrum of 3 displays a solitary

singlet peak at −32.1 ppm. The formation of dimer adduct was

corroborated by the electrospray ionization−mass spectrome-

try (ESI-MS) spectrum, which shows m/z values 1029.5531

[M + H]⁺and 909.4569 [M-(Mes)]⁺in addition to 515.2796

that corresponds to the monomeric fragment of 3. Further, the

structure of 3 was unambiguously established by the X-ray

diﬀraction (XRD) analysis.

To achieve monomeric tricoordinate borane for applications

such as anion sensing and small molecule activation, attempts

were made to break the dimer by quaternization of the

nitrogen or phosphorus atom using various reagents (Scheme

1). But 3 was very sluggish toward the quaternization reaction,

which always led to the mixture of P-methylated and N-

methylated products with traces of unreacted starting material

that were inseparable.

The stability of 3 is probably due to the formation of a stable

six-membered (B−N−C)₂ring. It was hypothesized that the

change of the position of boron and phosphorus substitution at

the backbone imidazole should avoid the formation of the

stable six-membered ring. Therefore, the substitution of BMes₂

group at the C-5 position of imidazole was achieved for the

ﬁrst time by employing a metal−halogen exchange reaction of

synthesis of an assortment of boron-functionalized imidazoles,

their photophysical properties, and applications in ﬂuoride

sensing. Moreover, the synthesis of luminescent copper

complexes of boron-functionalized phosphinoimidazole and

their photophysical properties are also described.

RESULTS AND DISCUSSION

■

Synthesis of BMes₂/PPh₂-Functionalized Imidazoles/

Imidazolium Salts 3−9. As introduced by Knochel and co-

workers, the metal−halogen exchange procedure starting from

4,5-dihaloimidazole is an eﬃcient method to introduce

B

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1

Table 1. Pertinent H NMR Chemical Shift Values of Compounds 3−9

1

chemical shift values (δ, ppm) of H NMR

ortho-CH₃of mesityl

compounds

group

para-CH₃of mesityl group

meta-H of mesityl group

Im-H

3

1.14 (s, 6H,), 1.69 (s, 6H), 1.83 (s,6H), 2.02 (s, 6H), 5.79 (s, 2H), 5.87 (s, 2H), 6.52 (s, 2H),

7.89 (s, 2H, C2-H_Im

)

2.11 (s, 6H), 2.19 (s, 6H)

6.58 (s, 2H)

4

5

1.96 (s, 6H), 2.07 (s, 2.26 (s, 3H) 2.29 (s, 3H)

6H)

6.73−6.81 (two singlets merged, 4H, m-

C₆H₂)

7.51 (s, 1H, C2-H_Im

2.05 (s, 48H)

2.27(s, 24H)

6.78 (s, 16H)

7.24 (C4-H_im, merged with residual NMR solvent), 7.66

(s, 4H, C2-H_Im

7.34−7.45 (1H, C4−H_immerged with PPh2 protons)

9.32 (s, 1H, C2-H_Im

)

6

7

2.02 (s, 12H)

2.04 (s, 12H)

2.25 (s, 6H)

6.74 (s, 4H)

6.83 (s, 4H)

2.24−2.33 (two singlets

merged, 6H)

)

8

9

2.04 (s,12H)

2.03 (s,12H)

2.29 (s,6H)

6.84 (s,4H)

7.01 (s, 1H, C4-H_im), 10.70 (s, 1H, C2-H_Im

)

7.05 (s, 1H, C4-H_im), 9.52 (s, 1H, C2-H_Im

)

Scheme 2. Synthesis of BMes₂-Substituted Imidazoles and Imidazolium Salts

i

4,5-diiodo-1-methylimidazole with PrMgCl followed by the

addition of dimesitylﬂuoroborane to give 4 (Scheme 2). 4 was

characterized by NMR (¹H and ¹³C) spectroscopy and ESI-MS

techniques.

phosphino-1-methylimidazole (6) as the major product, where

the phosphine group was substituted at the C2 position rather

than the expected C4 position. A trace amount of compound A

is identiﬁed only through the ESI-MS spectrum, but the

attempts to separate the pure compound from the impurities

were unsuccessful. Both 5 and 6 were characterized by NMR

spectroscopy and ESI-MS techniques. The ¹H NMR spectrum

of 5 and 6 shows two distinct singlet peaks for ortho-CH₃and

para-CH₃protons of BMes₂group in 2:1 ratio in contrast to

the appearance of four distinct peaks for 4 (Table 1). The

above observation infers the absence of substitution at the C-4

position in 5 and 6, which allows the free rotation of the B−

1

The H NMR spectrum of 4 displayed an unusual pattern,

where four distinct peaks (two for o-CH₃and two for p-CH₃

protons) were observed for CH₃protons of the mesityl group

in a 2:2:1:1 ratio ranging from 1.96 to 2.29 ppm rather than

the usual two distinct peaks for ortho and para CH₃protons in

a 2:1 ratio (Table 1). The appearance of four nonequivalent

methyl groups hints at the restriction of B−C_Mesrotation,

which is probably due to the presence of bulky iodine atom at

the adjacent carbon. Moreover, N−CH₃protons resonate at

3.19 ppm, and the m-C₆H₂protons of the mesityl group and

the C2-H of imidazole are observed around 6.73−6.81 and

7.51 ppm in 4:1 ratio, respectively. The presence of [M + H]⁺

peak at 457.1305 in the ESI-MS spectrum further corroborated

the formation of 4.

C

_Mesbond that leads to the usual splitting pattern of CH_3(Mes)

protons in contrast to 4, where the bulky iodine atom at the C-

4 position leads to four unique splittings for the CH_3(Mes)

1

protons. In the H NMR spectrum of 5, C2−H and C4−H

resonate at 7.66 and 7.24 ppm, respectively, whereas in the

case of 6, the ¹H NMR signal for C4−H was merged with the

aryl protons of the C2-substituted PPh₂moiety around 7.34−

7.45 ppm (Table 1). The ¹³C NMR spectrum of 6 further

supports the interpretation derived from the ¹H NMR

spectrum. It displays a doublet peak for C2 of imidazole at

154.8 ppm, and a clear downﬁeld shift was observed for the C2

signal of imidazole in 6 (154.8 ppm) compared to that of 4

(145 ppm). The presence of a doublet splitting pattern and

downﬁeld shift suggests the presence of phosphine substitution

i

The reaction of 4 with PrMgCl and subsequent quenching

with either water or electrophile (PPh₂Cl) led to the formation

of monoboron-substituted imidazole (5) or unsymmetrically

substituted boron-phosphine (6 and A)-based imidazoles,

respectively (Scheme 2). In the former case, 5 was isolated as

colorless solid, which was stable to air and moisture, whereas,

in the latter case, the puriﬁcation of the crude product by

column chromatography aﬀorded 5-dimesitylboryl-2-diphenyl-

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Figure 1. (a) Crystal structure of 3 (50% ellipsoid probability); hydrogens are omitted for clarity. (b) Close view of six-membered imidazole-

borane ring in 3. (c) Crystal structure of 4 (50% ellipsoid probability).

at the C2 position. The ³¹P NMR spectrum of 6 displayed a

single singlet peak at −25.9 ppm. Further, the formation of the

respective products was corroborated by the presence of [M +

H]⁺peaks 331.2343 (for 5) and 515.2787 (for 6) in the ESI-

MS spectrum.

The single crystals of 4 were obtained from the slow

evaporation of ethyl acetate/hexane mixture. Compound 4

crystallizes in the orthorhombic P2₁2₁2₁space group. The

selected bond parameters of 4 are given in Table 2. The solid-

state structure of 4 features a central tricoordinate boron atom

that is attached to two mesityl groups and the C5 carbon of the

imidazole in a trigonal planar geometry with bond angles

119.6(7)° [C5−B1−C7], 117.7(7)° [C5−B1−C16], and

122.7(6)° [C7−B1−C16] (Figure 1c and Table 2). The

B1−C5 bond distance (1.536 Å) in 4 is signiﬁcantly shorter

than the B1−C4 bond distances (1.581(11) and 1.578(11) Å)

in 3 and almost closer to the similar C2-BMes₂substituted

imidazole.^15dThe distance between the boron atom and the

iodine is found to be 3.667 Å, which is slightly shorter than the

van der Waals radii, yet there is restricted rotation of C−B_Mes

group, which was also manifested by the interesting splitting

pattern in H NMR spectrum of 4. More interestingly the

presence of asymmetric substitutions such as C4−I and

C5−BMes₂, were manifested with the bond distances in the

imidazole ring like (a) longer C4−C5 and C5−N1 bonds than

the ideal CC and CN distances and (b) marginally

shorter C4−N3 than CN distance.

Contrary to the case of dimeric imidazole-borane adduct 3,

BMes₂-substituted imidazoles 4 and 5 can be easily

quaternized by methylating reagents such as (CH₃)₃OBF₄/

CH₃I/CH₃OTf to aﬀord the corresponding imidazolium salts

(7−9) in good yields (Scheme 2). The formation of 7−9 was

authenticated by NMR spectroscopy (Table 1) and ESI-MS

analysis. However, the attempts to synthesize N-methylated or

P-methylated product of 6 via quaternization using the above

reagents were unsuccessful, as it always led to the formation of

multiple products, which were inseparable.

The ¹¹B NMR signals for 4−6 and 8 in the upﬁeld region

[4.35 ppm (4), 1.87 ppm (5), 4.11 ppm (6), and 1.46 ppm

(8)] suggest the possible Lewis acid−base interactions

(intermolecular/intramolecular) in solution. Thus, attempts

were made to obtain the single crystals of the compounds to

study the nature of interactions in the solid state in which 4

and 5 could be successfully crystallized.

18

1

Solid-State Structures of BMes₂-Functionalized Imi-

dazoles 3, 4, and 5. Slow evaporation of dichloromethane

(DCM) solution of 3 aﬀorded single crystals suitable for XRD

analysis. Compound 3 crystallizes in the monoclinic C2/c

space group; the crystal structure of 3 is given in Figure 1a and

1,b and the selected bond parameters are given in Table 2. The

solid-state structure of 3 conﬁrms the substitution of the

BMes₂group at the C4 carbon and the presence of PPh₂group

at the C5 carbon of the imidazole as established by

spectroscopic and spectrometric analysis. Further, the crystal

structure reveals a dimeric intermolecular imidazole-borane

(B_Mes-N_Im) adduct that forms a boat-like six-membered [−C−

B−N−]₂ring (Figure 1b) that has two tetracoordinate boron

atoms that are connected by two mesityl groups, the C4

carbon of the imidazole and nitrogen atom of the second

imidazole unit in a distorted tetrahedral geometry. The C−B−

N angle measures 100.26(13)° (C4−B1−N3A) and

100.27(13)° (C4A−B1A−N3), whereas the C−B−C and

N−B−C angles range from 105° to 116°. The bond distances

B−N [1.629(2) and 1.642(2) Å] and B−C_Im[1.641(2) and

1.634(3) Å] in 3 are found to be slightly longer than the

similar C2-borane substituted B−N dimer products.^15c,d

Single crystals of 5 were obtained by the slow evaporation of

a DCM/hexane (5:1) mixture of 5; the structure of 5 is given

in Figure 2, and the pertinent bond parameters are given in

Table 2. Compound 5 crystallizes in the tetragonal P4₂/n space

group, and the asymmetric unit contains a central tricoordinate

boron atom connected to two mesityl groups and the C5

carbon of imidazole ring. The structure of 5 reveals an

unprecedented tetrameric 16-membered macrocycle with

strong intermolecular B−N_Iminteraction (Figure 2b). Com-

pound 5 features four symmetric tetracoordinate boron atoms

where each boron atom is connected to two mesityl groups, C5

carbon of imidazole, and to the nitrogen atom of the

neighboring imidazole unit in a distorted tetrahedral geometry.

The bond angle between C5−B1−C7, C5−B1−N3, and C7−

B1−N3 ranges from 105.33° to 109.39°, which lies closer to

ideal angle for tetrahedral geometry, whereas, surprisingly, the

bond angle of C7−B1−C16 (120.17°) remains closer to the

ideal angle of trigonal planar geometry. The B−N bond

distance in 5 (1.616 Å) is shorter than that of the dimeric B−N

adduct 3 [(1.629(2) and 1.642(2) Å)] and comparable to the

B−N distance in C2-bound dimeric B−N adducts.^15c,dThe

C5−B distance (1.636(4) Å) in 5 is almost comparable to

D

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C4−B1 distances [1.634(2) Å and 1.641(2) Å] in dimer

adduct 3 and longer than the C5−B1 distance [1.536(10) Å]

in monomeric compound 4. Similarly, the lengthening of the

N3−C4 bond distance is observed in 5 (1.387(3) Å) as

compared to that of 4 (1.330(10) Å), which explains the

electron donation of the N3 atom to the boron atom of the

neighboring imidazole unit in 5.

Metalation Studies. P^N-type bidentate ligands are

known to provide interesting copper complexes or clusters

that exhibit intense luminescence, which are applicable in

optoelectronic devices.¹⁹Mostly, phosphine-containing pyr-

idine, triazole, and oxazole ligands²⁰are used in preparing

(P^N)₂Cu₄I₄-type clusters, whereas phosphinoimidazole-con-

taining Cu₄I₄clusters are rare.²¹The luminescence of such

copper clusters can be tuned across the visible light region by

factors such as solvent, temperature, mechanical stress, and

substituents. Variation of N-substitution in the imidazole of

(P^N)₂Cu₄I₄-type clusters has shown subtle variation in the

emission maxima wavelength.^21bOn the basis of the above

information, it was envisaged that the substitution of the

BMes₂group at the backbone of the imidazole should create a

considerable impact on the electronic communication in the

molecule, which in turn should alter the emission wavelength

of the copper complexes.

Hence, metalation of BMes₂-containing P^N-type bidentate

ligand 6 was performed by a layering method using hexane and

acetonitrile as solvents to obtain yellowish-green crystals (10),

which displayed a bright green luminescence under UV light

(365 nm) (Scheme 3). Compound 10 is insoluble in most of

the common organic solvents and is very sparingly soluble in

DMSO-d₆. However, the formation of a tetranuclear copper

complex of Cu₄I₄L₂type was conﬁrmed through single-crystal

X-ray diﬀraction analysis (Figure 3a). The solution of 10 that

remained after the crystals were ﬁltered out, was kept for

crystallization at room temperature. Surprisingly, after a week,

the slow evaporation of the above solution has aﬀorded

colorless crystals that exhibit orange ﬂuorescence under UV

light (365 nm) (Scheme 3). The X-ray diﬀraction study

revealed a similar Cu₄I₄L₂-type tetranuclear copper complex

(11) but with the absence of the BMes₂group at the backbone

of the imidazole (Figure 3b). Further, the crystals of 11 were

also obtained from the slow evaporation of CHCl₃/dimethyl

sulfoxide (DMSO) mixture of 11 (which was dissolved in the

hot condition and then cooled to rt).

The selected bond parameters of 10 and 11 are given in

Figure 3 (vide infra) and Table S2 (Supporting Information).

Compound 10 crystallizes in the triclinic P1 space group with

one-half of the molecule as an asymmetric unit, while the other

part of the molecule is generated by inversion. The tetranuclear

copper core forms a rectangular plane with the intermetallic

angles of 89.74 (4)° (Cu2−Cu1−Cu2ⁱ) and 90.26(4)° (Cu1−

Cu2−Cu1ⁱ). Each copper is connected by P or N of the 6, and

the Cu₄I₄core is formed by two bridging μ₂-iodide at the

equatorial position between opposite Cu atoms with bond

distances of 2.617(11) Å (Cu1−I2ⁱ= Cu1ⁱ-I2) and 2.588(10)

Å (Cu2−I2 = Cu2ⁱ-I2ⁱ) and two μ₂-iodide in the apical

position (above and below the plane) between the adjacent Cu

atoms with bond distances of 2.612(12) Å (Cu1−I1 = Cu1ⁱ-

I1ⁱ) and 2.578(11) Å (Cu2−I1 = Cu2ⁱ-I1ⁱ) (Figure 3a). Cu1−

N3 and Cu2−P1 possess the bond distances of 1.972(6) and

2.194(2) Å, respectively. The C5−B1 and C4−C5 bond

distances are found to be 1.541(11) and 1.375(10) Å. The

Cu₄I₄unit exhibits two diﬀerent Cu−Cu distances, with a

E

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Figure 2. Crystal structure of 5 (50% ellipsoid probability); hydrogens are omitted for clarity. (b) Close view of 16-membered macrocyclic ring.

Scheme 3. Metalation Studies of 6

shorter distance between the copper atoms of opposite ligands

(2.504(14) Å) and the longer distance between the copper

atoms of the same ligands (2.829(13) Å). These Cu−Cu bond

parameters are almost similar to those of the complex

Cu₄I₄(Ph₂Ppy)₂.^20a,e

the intermetallic angle, which in turn inﬂuences the other bond

parameters such as Cu−Cu bond distances in 11. However,

other factors like solvent and the electronic property of the

ligands cannot be ignored. Because of the poor solubility of 10

and 11 in common organic solvents, NMR studies could not

be performed.

Compound 11 crystallizes in the C2/c space group, and as in

the case of 10, one-half the molecule appears as the

asymmetric unit, whereas the other half is generated by

inversion. The equatorial bridging μ-iodide ions are bound to

the copper atoms of the opposite ligands with the distances of

2.579(13) and 2.641(12) Å as in the case of 10. In contrast to

the structure of 10, the apical iodides in 11 are triply bridged

(μ₃) above and below the plane in a symmetrical pattern

(Figure 3b). The μ₃-bridged iodides are bound to one of the

N-coordinated copper atoms and two P-coordinated copper

atoms. The Cu₄I₄unit of 11 also displays two diﬀerent Cu−Cu

bond distances, 2.464(14) and 2.784(15) Å, which are shorter

than the parameters of compound 10. The Cu−Cu bond

distances in 10 and 11 fall in the range that was observed for

similar octahedral-shaped L₂Cu₄X₄(X = I, Br, Cl) type clusters

(for shorter side of Cu₄X₄: 2.46 to 2.72 Å; for longer side: 2.67

to 3.03 Å).^19b,d,20,22The C4−C5 bond distance of the

imidazole in 11 (1.352(11) Å) is shorter than that of 10.

The Cu₄core of 11 forms a planar parallelogram with an acute

intermetallic angle of 79.23(5)° (Cu2−Cu1−Cu2ⁱ= Cu2ⁱ−

Cu1ⁱ−Cu2) and obtuse angle of 100.78(5)° (Cu1−Cu2−Cu1ⁱ

= Cu1ⁱ−Cu2ⁱ−Cu1), which contrasts with the nearly

rectangular Cu₄plane in the case of 10. The formation of

parallelogram Cu₄I₄core in 11 is probably due to the presence

of μ₃-bridging of the apical iodide ions, which changes from

The shortest Cu−Cu distance in 10 (2.504(13) Å) and 11

(2.464(14) Å) clearly indicates the strong cuprophilic

interactions, which are also reﬂected in their bright

luminescent behavior in solid state under UV light (Scheme

3). The direct synthesis of 11 from 1-methyl-2-

(diphenylphosphine)imidazole and CuI was previously de-

scribed by Baumann and co-workers in a patent ﬁled by them,

where they had synthesized a series of luminescent Cu₄I₄

clusters supported by various heterocyclic ligands for their

application in optoelectronics.^21b

Photophysical Studies. BMes₂-substituted imidazoles 5

and 6 exhibit bluish-green luminescence in solution state under

UV light (365 nm), whereas imidazolium salts 8 and 9 display

pale violet luminescence in solution under UV light (365 nm).

However, no luminescence behavior was observed in the case

of dimeric imidazole-borane adduct (3) and C4-iodo

substituted compounds (4 and 7). These interesting properties

of BMes₂-functionalized imidazoles prompted us to study their

photophysical properties. The absorption spectra of BMes₂-

substituted imidazoles recorded in tetrahydrofuran (THF) (4

and 5) and dimethylformamide (DMF) (6) at room

temperature shows an intense broad absorption band centered

at 332 nm (4), 316 nm (5), and 333 nm (6) (Figure 4a). The

absorption spectra of imidazolium salts 7−9 in CH₃CN at

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Figure 3. (a) Crystal structure of 10 with 50% ellipsoid probability (left) and close view of Cu₄I₄unit of 10 with its bond distances (Å) (right). (b)

Crystal structure of 11 with 50% ellipsoid probability (left) and close view of Cu₄I₄unit of 11 with its bond distances (Å) (right).

room temperature show a broad band centered at 346 nm (7)

and 326 nm (8 and 9) (Figure 4b). The molar absorptivity

values (ε) of 4−9 are given below Figure 4. The emission

spectra of luminescent compounds 5, 6, 8, and 9 were

recorded in the respective solvents at room temperature.

Compounds 5 and 6 display broad emission bands in the blue

region centered at 460 nm (5) and 480 nm (6) when excited at

316 and 333 nm, respectively, whereas BMes₂-substituted

imidazolium salts in CH₃CN display an emission band

centered at 427 and 428 nm, respectively, in the violet region

(Figure 4c). The ﬂuorescence quantum yield (φ_f) for the

compounds 5, 6, 8, and 9 was measured in the respective

solvents at room temperature with sulfuric acid solution of

quinine sulfate (φ_f= 0.54) as standard, and the φ_fvalues are

found to be 0.152 (5), 0.095 (6), 0.123 (8), 0.136 (9).²³

Because of the insoluble nature of luminescent Cu₄I₄

complexes 10 and 11 in common organic solvents, photo-

physical studies were performed using solid-state ﬂuorescence

spectroscopy. The solid-state ﬂuorescence spectrum of 10

shows emission maximum at 518 nm (λ_ex= 440 nm) in the

green region of the spectrum, and the emission maximum of

11 was red-shifted to 580 nm (λ_ex= 380 nm) in the orange

region, whereas the ligand 6 shows emission maxima in the

blue region at 425 nm (λ_ex= 330 nm) (Figure 5). Both the

compounds did not display dual emission (both high-energy

and low-energy emission) as observed in Cu₄I₄(R₂PCH₂py)₂

reported by Thompson’s group.^20eThe signiﬁcant shift in the

emission of BMes₂containing 10 from the unsubstituted

counterpart 11 clearly illustrates that the nature of the

functional group at the backbone of imidazole can tune the

electronic properties of the copper clusters and thereby their

luminescence properties.

Fluoride Sensing Studies. The ﬂuoride sensing proper-

ties of BMes₂-functionalized imidazole/imidazolium com-

pounds 4−9 were evaluated using tetrabutylammonium

ﬂuoride (TBAF) as ﬂuoride source. The titration of 4−9 in

organic solvents against TBAF was monitored using a UV−vis

spectrophotometer. The solvents for titration were chosen

according to the stability/consistency of the absorption

spectrum of the respective compounds. The abstraction of

ﬂuoride ion was indicated by the gradual decrease in the

absorbance without much change in the wavelength (λ_max

)

against the incremental addition (0.1 equiv) of TBAF (Figure

6). The absorbance completely quenches after the addition of

1 equiv of TBAF indicating the complete disruption of the

conjugation due to the binding of ﬂuoride to the borane

moiety. In the case of 6, appearance of a new band around 275

nm was observed as the TBAF solution was gradually added,

whereas, in the case of other compounds such secondary

signals were not observed.

Titrations of luminescent compounds 5, 8, and 9 against

TBAF were also monitored by ﬂuorescence spectroscopy

(Figure 7). The abstraction of ﬂuoride ion was clearly

indicated by a steady decrease in the emission band against

the incremental addition (0.1 equiv) of TBAF. The emission

band was almost completely quenched after the addition of 1

equiv of TBAF, thus enabling the compounds 5, 8, and 9 as

eﬃcient turn-oﬀ ﬂuorescent sensors.

The formation of ﬂuoroborate species upon the addition of

TBAF to the above imidazole-based boranes was also indicated

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Figure 5. Solid-state emission spectra of 6, 10, and 11. The spectrum

was recorded at the excitation wavelength (λ_ex) of 330 (6), 440 (10),

and 380 (11) nm.

indicating the formation of ﬂuoroborate species. Further the

structure of 8-F was unambiguously established by single-

crystal XRD analysis, and the structure of 8-F along with

selected bond parameters are given in Figure 8. The solid-state

structure of 8-F features a tetracoordinate boron atom that is

connected to two mesityl groups, C4 carbon of the imidazole

ring, and the ﬂuorine atom in a distorted tetrahedral geometry

(Figure 8). The B−F bond distance in 8-F (1.462 Å) is typical

of that obtained in the case of other ﬂuoroborates known in

literature.^24,25Also the C−C and C−N distances in the

imidazole ring are similar to those of 5. To investigate the

selectivity of the above boranes toward ﬂuoride ion, the

titration of 4−9 with other anions such as Cl⁻, Br⁻, I⁻and

−

HSO₄was performed (Figures S34 −S36 in the Supporting

Information). The absence of quenching even after the

addition of 50 equiv of the above anions indicates that the

compounds 4−9 can bind to the ﬂuoride ion more selectively

and eﬀectively than the all other anions.

The ﬂuoride binding constants calculated from the UV

absorbance values of the titration by (a) the Benessi-

Henderson equation (linear regression method) and (b)

analysis using the bindﬁt online package (nonlinear regression

method) are given in Table 3. The values calculated by both

methods are almost closer, and no signiﬁcant deviation is

observed. The binding constant value for all the compounds

lies in the range of 10⁴M⁻¹. The iodo-substituted imidazole-

based borane 4 is found to have a lower binding constant value

(7.8 × 10³and 1.4 × 10⁴M⁻¹) than the rest of the compounds,

and this might be partly attributed to the presence of bulkier

iodine atom at the adjacent carbon. In addition, the variations

in bond distances between the ring atoms in 4 in comparison

with those of 5 suggest the absence of eﬀective delocalization

of electrons in the imidazole ring thus reducing the

electrophilicity of boron. However, no correlation could be

arrived at for the values obtained for other compounds. The

binding constant value of imidazolium salt 7 is slightly (5

times) greater than its parent imidazole 4, which explains the

enhancement of electrophilicity of boron by the electron-

deﬁcient imidazolium ring. Surprisingly, no considerable

change is observed between the ﬂuoride binding constant of

Figure 4. (a) Absorption spectra of 4−6, (b) absorption spectra of

7−9, and (c) emission spectra of 5, 6, 8, and 9. The molar

absorptivity (M⁻¹cm⁻¹) of 4−9: ε₃₃₂= 83 823 (4), ε₃₁₆= 94 838 (5),

ε₃₃₃= 24 450 (6), ε₃₄₆= 9521 (7), ε₃₂₆= 7908 (8), ε3₂₆= 12 953 (9).

by ¹⁹F NMR signal around −180 ppm, which is typical for

similar ﬂuoroborate species.^15b,24

The ﬂuoroborate species 8-F could be isolated from the

direct reaction of 8 with TBAF·3H₂O in CH₃CN. The ¹⁹F

NMR spectrum of 8-F displays a distinct peak at −180.8 ppm

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Figure 6. Absorption spectra of 4−9 upon the titration with incremental addition of TBAF (0.1 to 1.0 equiv).

Figure 7. Emission spectra of 5, 8, and 9 upon the titration with incremental addition of TBAF (0.1 to 1.0 equiv). The ﬂuorescence spectrum was

recorded at excitation wavelengths of 327 (5), 337 (8), and 335 (9) nm.

the BMes₂-functionalized imidazole 6 and the corresponding

imidazolium salts (7−9).

presence of the ﬂuoroborate product as well as tetrabutylam-

monium iodide (TBAI) (Figure S31, Supporting Information).

The ﬂuoride binding constant values of related BMes₂-

containing aryl boranes and heteroaromatic boranes are given

in Table 4. Except for the compounds in entries 1, 3, and 4

(Table 4),^{5b,6b,11b,26,27}the binding constant values for all other

compounds are in the range of 10⁴M⁻¹as also in the case of

compounds 4−9. The above data suggest that imidazole-based

trisubstituted boranes 4−9 are good and competitive sensors

for ﬂuoride ions among the similar triarylboranes.

CONCLUSION

■

Two isomeric borane and phosphine-functionalized imidazoles

(3 and 6) and borane-functionalized imidazoles (4 and 5) and

the corresponding imidazolium salts (7−9) were synthesized

conveniently using the metal−halogen exchange procedure.

Both 3 and 5 form an interesting dimeric 6-membered ring and

a 16-membered macrocycle, respectively. The metalation study

of P^N-type ligand 6 with CuI yielded a luminescent

tetranuclear L₂Cu₄I₄-type cluster 10. The chloroform/DMSO

mixture of 10 in the presence of air aﬀorded a BMes₂cleaved

L₂Cu₄I₄cluster 11. The signiﬁcant shift in the emission of

BMes₂containing 10 from the unsubstituted counterpart 11

patently demonstrates that the presence of BMes₂at the

backbone of imidazole can alter the luminescent properties of

metal clusters eﬀectively. Although various heterocyclic rings

were used to tune the luminescent property of the copper

clusters to give a wide range of emissions, the usage of borane

To assess the ﬂuoride binding ability of BMes₂-function-

alized imidazolium salt 8 in water, a direct reaction between

TBAF·3H₂O in D₂O (0.05M) and 8 in CDCl₃(0.05M) was

performed in an NMR tube and was monitored by ¹⁹F NMR

spectroscopy. The ﬂuorine NMR displayed the appearance of a

new signal at −180 ppm indicating the formation of

ﬂuoroborate species (Figure S32, Supporting Information).

The complete disappearance of the TBAF signal in the ¹⁹F

NMR spectrum illustrates the facile abstraction of ﬂuoride ion

1

by 8 in water medium. Further, the H NMR shows the

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Table 4. Fluoride Binding Constant Values of Relevant

BMes₂-Containing Organoborane Compounds in the

Literature

Figure 8. Crystal structure of 8-F (hydrogen atoms are omitted for

clarity). Selected bond length (Å) and angle (deg): B1−F1 1.462(2);

C4−C5 1.367(3); C5−B1 1.636(3); C4−N3 1.383(3); C2 N3

1.324(3); C2 N1 1.327(3); C5−N1 1.403(3); C8−B1−C17

112.99(16); C5−B1−C17 103.76(16); C8−B1−C5 119.44(17);

F1−B1−C5 104.27(15); F1−B1−C8 104.31(15); F1−B1−C17

111.87(16).

Table 3. Fluoride Binding Constant Values of 4−9

a

Method of calculation: (A) using UV titration, calculated by

nonlinear regression analysis; (B) using UV titration data, calculated

by the LabFit program; (C) using UV titration data, calculated from

the 1:1 binding isotherm curve; (D) using UV and ﬂuorescence

titration data, calculated from the 1:1 binding isotherm curve.

substituent on such ligands in copper clusters is not known. A

combination of such systems can be immensely important to

synthesize various luminescent copper clusters for optoelec-

tronic applications. The synthesis of novel luminescent metal

complexes containing the BMes₂group using 6 with various

other metals for their optoelectronic applications is currently

under progress in our laboratory. Further, the sensing

properties of the BMes₂-containing compounds 4−9 toward

ﬂuoride ion were studied using UV−vis spectroscopy and

found to display good activity in common organic solvents.

Compound 8 was found to abstract ﬂuorine eﬀectively in

D₂O/CDCl₃mixture. This study can be a prelude for the

synthesis of more imidazole-based borane molecules for anion

sensing. Further, the possibility of appending (a) water-soluble

substituents to the nitrogen atom of imidazole and (b)

electron-withdrawing substituents in phenyl/imidazole rings in

4, 5, and 6 can provide the scope for designing water-soluble

and more eﬃcient anion sensors, which is a part of the ongoing

research in our laboratory.

EXPERIMENTAL SECTION

General Procedure and Materials. All the reactions and

manipulations were performed under a dry nitrogen atmosphere

■

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using standard Schlenk line techniques unless otherwise mentioned.

Solvents were dried according to the standard literature procedures,

and they were freshly distilled prior to use. All glassware was dried in

an oven at 140 °C overnight. TBAF·3H₂O, chlorodiphenylphosphine,

bromomesitylene, and copper iodide were commercially procured and

used as received. Isopropylmagnesium chloride (ⁱPrMgCl) was freshly

prepared before use. Compounds 1 and 2 were prepared using

previously reported procedures.^28,16d

6.58 (s, 2H, m-C₆H₂), 6.86−6.94 (m, 8H, P(C₆H₅), 7.10−7.17 (m,

6H, P(C₆H₅), 7.21 (t, 4H, P(C₆H₅), 7.32 (t, 2H, P(C₆H₅), 7.89 (s,

2H, C2_Im-H)). ¹³C NMR (CDCl₃, 125 MHz, δ, ppm) 20.7, 20.9, 26.1,

26.9, 27.0, 28.5 (C₆H₂-CH₃), 36.0 (N-CH₃), 127.2, 127.6, 128.4,

128.6, 129.1, 129.8, 130.0, 130.3, 130.7, 133.1, 133.3, 134.3, 139.5,

140.9, 142.5, 144.0, 147.4. ³¹P NMR (160 MHz, δ, ppm): −32.1. ¹¹

B

NMR: signal for BMes₂in 3 could not be observed. IR (KBr, v,

̅

cm⁻¹): 3285(m), 2950(m), 2918(s), 2855(m), 2724(w), 1716(w),

1631(w), 1543(m), 1479(m), 1433(s), 1374 (m), 1296(s), 1283(s),

1233(m), 1187(m), 1160(m), 1089(m), 1071(m), 1026(m), 958(w),

927(w), 843(s), 835(s), 799(m), 740(m), 696(s), 666(m), 616(w),

575(w), 560(w), 536(w), 507(m), 448(w). ESI-MS m/z calcd. for

monomer unit C₃₄H₃₇BN₂P⁺, 515.2782; found, 515.2780.

1

Instrumentation. H, ¹³C, ³¹P, ¹⁹F, and ¹¹B NMR spectra were

obtained on a JEOL-DELTA 400 or 500 MHz or Bruker NanoBay

300 MHz spectrometer. The spectra were recorded in CDCl₃as the

solvent. ¹H and ¹³C NMR chemical shifts were referenced with

respect to tetramethylsilane (TMS). ³¹P, ¹⁹F, and ¹¹B NMR chemical

shifts were referenced with respect to 85 % H₃PO₄, CFCl₃, and BF₃·

OEt₂, respectively. Infrared spectra (IR) were recorded as KBr pellets

on a Fourier transform infrared (FT-IR) Bruker Vector model or

PerkinElmer-Spectrum Two. ESI-MS spectra were recorded on a

Waters-Q-TOF Premier-HAB213 spectrometer. Elemental analyses

were performed with an EAI Exeter analytical, INC CE-440 elemental

analyzer or PerkinElmer-Series-II CHNS/O analyzer 2400. Melting

points reported are uncorrected. UV−Vis absorption measurements

were recorded on a JASCO V-670 spectrophotometer, and

ﬂuorescence (emission) measurements were obtained using a

commercial ﬂuorimeter, Fluorolog, HORIBA JobinYvon.

Synthesis of 4. To a THF (35 mL) mixture of 4,5-diiodo-1-

methylimidazole (7.0 g, 21.0 mmol) and lithium chloride (0.98 g, 23.1

mmol) in a Schlenk ﬂask was added freshly prepared 1.0 M THF

i

solution of PrMgCl (23.1 mL, 23.1 mmol) at −20 °C. The mixture

was warmed to room temperature, and the stirring was continued for

45 min, where a formation of white slurry was observed. Then, the

mixture was again cooled to −20 °C, and Mes₂BF (5.63 g, 21 mmol)

in THF (20 mL) was cannulated to the above mixture. The mixture

was allowed to reach room temperature, and then the stirring was

continued for 5 h. Then, the solvent was evaporated under vacuum,

and the pale yellow colored residue was extracted with DCM (20

mL), which was followed by the addition of half saturated aqueous

NH₄Cl solution (20 mL). The organic layer was separated, dried over

MgSO₄, and evacuated to dryness to give pale yellow colored solid as

a crude product. The product was puriﬁed by column chromatog-

raphy (silica gel, 100−200 mesh, 15:85 mixture of ethyl acetate and

hexane) to aﬀord the title compound as a colorless solid. Colorless

crystals were obtained by slow evaporation of hexane/ethyl acetate

(1:1) mixture. Yield: 5.90 g (62%). mp 230 °C. Anal. Calcd for

C₂₂H₂₆BIN₂; C, 57.92; H, 5.74; N, 6.14. Found: C, 57.76; H, 5.76; N,

Syntheses. Preparation of Fluoro(dimesityl)borane (BMes₂F).

BMes₂F was prepared using a simpliﬁed and modiﬁed procedure than

the already reported procedure.²⁹To the magnesium turnings (1.95 g,

80 mmol) and a granule of iodine in a double-necked ﬂask ﬁtted with

a condenser was added THF (10 mL) to give a purple solution. A

small fraction of bromomesitylene (0.25 mL) was added to the above

mixture and was stirred until the decolorization of the solution. Then

the THF solution (70 mL) of bromomesitylene (12.25 mL, 80.0

mmol) was slowly and carefully cannulated to the above mixture at ice

bath temperature, and the ice bath was removed after the completion

of exothermic heat generation. Then the reaction mixture was stirred

at room temperature, until the generation of the MesMgBr, which was

indicated by the complete dissolution of magnesium in 2 h. Then the

MesMgBr solution was cannulated to the Schlenk ﬂask containing

BF₃·Et₂O (4.93 mL, 40 mmol) at ice bath temperature. After addition,

the solution was allowed to warm to room temperature, and the

stirring was continued for 12 h, during which the formation of white

precipitate was observed. Then the solvent was removed under

vacuum, and the resulting solid was extracted with dry hexane (2 × 30

mL). The hexane mixture was ﬁltered through a pad of diatomaceous

earth under strict inert condition, and the ﬁltrate was evacuated to

give the product (Mes₂BF) as a white solid, which was dried in vacuo

and was stored in the glovebox for further use. Yield: 10.0 g (93.3%).

Note: In the case of the appearance of pink coloration in the solid

(due to moisture), the compound can be recrystallized from pentane

1

6.28%. H NMR (CDCl₃, 500 MHz; δ, ppm): 1.96 (s, 6H, o-CH₃),

2.07 (s, 6H, o-CH₃), 2.26 (s, 3H, p-CH₃), 2.29 (s, 3H, p-CH₃), 3.19

(s, 3H, N−CH₃), 6.73−6.81 (two singlets merged, 4H, m-C₆H₂), 7.51

(s, 1H, C2_Im-H). ¹³C NMR (CDCl₃, 125 MHz, δ, ppm) 21.3, 21.4,

22.2, 23.2 (Mes-CH₃), 34.7 (N-CH₃), 100.9 (Im-CI), 128.5, 128.7,

139.6, 139.8, 140.3, 141.2, 145.0. ¹¹B NMR (CDCl₃, 95 MHz, δ,

ppm) 4.35. IR (KBr, v, cm⁻¹): 3095(w), 2919(w), 1631(w), 1605(s),

̅

1546(w), 1490(m), 1413(s), 1412(m), 1374(m), 1345(s), 1309(m),

1286(w), 1227(s), 1209(s), 1180(m), 1153(w), 1101(w), 1027(w),

951(w), 923(w), 863(m), 844(s),835(s), 738(w),720(m), 699(m),

654(m), 628(m), 586(w), 555(w), 514(w). ESI-MS m/z calcd. for

+

C₂₂H₂₇BIN₂, 457.1312 [M + H]⁺; found, 457.1305.

Synthesis of 5. To a stirred solution of 4 (1.2 g, 2.63 mmol) in

i

THF (20 mL), freshly prepared PrMgCl (2.7 mmol) was added at

−78 °C and allowed slowly to come to room temperature. The

solution was stirred for 4 h, and water (1 mL) was added. The

reaction mixture was stirred for an additional 4 h. Then the volatiles

were removed under vacuum, and a pale yellow colored residue was

obtained. The residue was dissolved in DCM (20 mL) and mixed

with half saturated ammonium chloride (20 mL). The layers were

separated, and the organic layer was dried over anhydrous MgSO₄. A

colorless semi solid was obtained upon removal of DCM, which was

puriﬁed by column chromatography (silica gel 100−200 mesh using a

25:75 mixture of ethyl acetate and hexane mixture). Colorless crystals

of 5 were obtained by the slow evaporation of DCM/hexane (5:1)

mixture. Yield: 0.50 g (58.2%). mp 170 °C. Anal.Calcd for

C₈₈H₁₀₈B₄N₈: C, 80.00; H, 8.24; B, 3.27; N, 8.48, found C, 79.19;

H, 8.01; N, 7.59; ¹H NMR (CDCl₃, 400 MHz; δ, ppm): 2.05 (s, 48H,

o-CH₃), 2.27 (s, 24H, p-CH₃), 3.29 (s, 12H, N−CH₃), 6.78 (s, 16H,

m-C₆H₂), 7.66 (s, 4H, C2_Im-H). The value for C4_Im-H is merged with

residual NMR solvent. ¹³C NMR (CDCl₃, 75 MHz; δ, ppm): 21.2,

22.4, 22.9 (Mes-CH₃), 34.3 (N-CH₃), 126.9, 128.5, 137.7, 139.3,

140.6. ¹¹B NMR (CDCl₃, 95 MHz; δ, ppm): 1.87. IR (KBr, cm⁻¹:

3160(w), 2942(m), 2915(m), 1606(m), 1552(m), 15 223(m),

1447(s), 1375(m), 1233(m), 1184(w), 1132(m), 1081(m),

1021(m), 881(w), 831(s), 800(m), 781(m), 733(m), 708(m),

1

to aﬀord the product as colorless crystals. H NMR (CDCl₃, 400

MHz, δ, ppm): 2.30 (s, 12H, o-CH₃), 2.33 (s, 6H, p-CH₃), 6.86 (s,

4H, m-C₆H₂). ¹⁹F NMR (CDCl₃, 375 MHz, δ, ppm): −14.1.

Synthesis of 3. To a THF mixture (50 mL) of 4-iodo-5-

diphenylphosphino-1-methylimidazole (2) (2.0 g, 5.09 mmol) and

i

LiCl (0.23 g, 5.6 mmol) in a Schlenk ﬂask was added PrMgCl (2.80

mL, 5.6 mmol, 2.0 M in THF) at −20 °C. The mixture was warmed

to room temperature, and the stirring was continued. After 1.5 h, the

mixture was again cooled to −20 °C, and Mes₂BF (1.5 g, 5.09 mmol)

in THF (10 mL) was cannulated to the above mixture. The reaction

mixture was allowed to reach the room temperature, and the stirring

was continued. After 8 h, the solvent was evaporated under vacuum.

The residue was puriﬁed by column chromatography (silica gel, 100−

200 mesh, 5:95 mixture of ethyl acetate and hexane) to give the title

compound as a colorless solid. Yield: 1.06 g (41.2% with respect to

the reactant 2). mp 175 °C. Anal. Calcd for C₆₈H₇₂B₂N₄P₂; C, 79.38;

1

H, 7.05; N, 5.45. Found: C, 79.30; H, 7.31; N, 5.34%. H NMR

(CDCl₃, 500 MHz; δ, ppm): 1.14 (s, 6H, Mes−CH₃), 1.69 (s, 6H,

Mes−CH₃), 1.83 (s, 6H, Mes−CH₃), 2.02 (s, 6H, Mes−CH₃), 2.11

(s, 6H, Mes−CH₃), 2.19 (s, 6H, Mes−CH₃), 2.98 (s, 6H, N−CH₃),

5.79 (s, 2H, m-C₆H₂), 5.87 (s, 2H, m-C₆H₂),, 6.52 (s, 2H, m-C₆H₂),

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+

675(m), 662(m), 555.62(m). ESI-MS m/z calcd. for C₂₂H₂₈BN₂

Synthesis of 8-F. To a stirred solution of imidazolium salt 8 (0.096

g, 0.2 mmol) in acetonitrile was added TBAF·3H₂O (0.063 g, 0.2

mmol), and allowed to stir at room temperature for 4 h. The clear

solution thus formed was evaporated to dryness to obtain a colorless

product. Crystals were obtained by slow evaporation of CHCl₃/

(monomeric unit), 331.2340 [M + H]⁺; found, 331.2343.

Synthesis of 6. To a THF (15 mL) mixture of 4 (1.0 g, 2.19 mmol)

and LiCl (0.12 g, 2.84 mmol) in a Schlenk ﬂask was transferred a

freshly prepared 1.0 M THF solution of PrMgCl (2.9 mL, 2.84

i

1

mmol) at −20 °C. The mixture was warmed to room temperature,

and the stirring was continued. After 2 h, the mixture was cooled to

−20 °C, and chlorodiphenylphosphine (0.483 g, 2.19 mmol) was

added to the above mixture. Then the mixture was allowed to warm to

room temperature, and the stirring was continued. After 8 h, the

solvent was evaporated under vacuum, and the residue was dissolved

in DCM followed by the addition of half saturated aqueous NH₄Cl

solution (10 mL). The organic layer was separated, ﬁltered over a pad

of diatomaceous earth, dried over MgSO₄, and then evacuated to

dryness to give the crude product. The crude product was puriﬁed by

column chromatography (neutralized silica gel, 100−200 mesh, 5:95

mixture of ethyl acetate and hexane) to aﬀord 6 as colorless solid.

Yield: 0.68 g (45%). mp 125 °C. Anal. Calcd for C₃₄H₃₆BN₂P: C,

79.38; H, 7.05; N, 5.45. Found: C, 79.21; H, 7.00; N, 5.35. ¹H NMR

(CDCl₃, 500 MHz; δ, ppm): 2.02 (s, 12H, o-CH₃), 2.25 (s, 6H, p-

CH₃), 3.30 (s, 3H, N−CH₃), 6.74 (s, 4H, m-C₆H₂), 7.34−7.45 (m,

11H, P(C₆H₅)₂+ C4_Im-H). ¹³C NMR (CDCl₃, 125 MHz, δ, ppm)

21.2, 22.8 (Mes-CH₃), 33.9 (N-CH₃), 128.3, 128.7, 129.5, 134.0,

134.2, 138.7, 140.5, 147.0, 154.8. ³¹P NMR (160 MHz, δ, ppm): 25.9

toluene. Yield: 0.41 g (48.3%). mp 185 °C. H NMR (CDCl₃, 400

MHz; δ, ppm): 2.00 (s,12H, o-CH₃), 2.20 (s, 6H, p-CH₃), 3.69 (s,

3H, N−CH₃), 3.84 (s, 3H, N−CH₃), 6.27 (s,1, C4_Im-H), 6.65 (s, 4H,

m-C₆H₂), 8.13 (s,1H,C2_Im−H). ¹³C NMR (CDCl₃, 100 MHz; δ,

ppm): 20.8, 23.5 (Mes-CH₃), 35.0, 39.0 (N-CH₃), 124.7, 128.6,

128.7, 132.3, 133.4, 137.1, 141.5. ¹⁹F NMR (CDCl₃, 375 MHz; δ,

ppm): −180.68. ¹¹B NMR spectra could not be obtained. IR (KBr, v,

̅

cm⁻¹): 2956(s), 2924(s), 2854(s), 1604(w), 1578(w), 1463(m),

1378(m), 1231(w), 1150(w), 1128(w), 1028(m), 921(w), 895 (w),

838(w), 820(w), 802(w), 731(w), 714(w), 618(w). ESI-MS m/z

+

calcd. for C₂₃H₃₀BN₂, 345.2502; found, 345.2508.

Synthesis of 9. To a stirred solution of compound 5 (0.066 g,

0.050 mmol) in DCM was added methyl triﬂuoromethanesulfonate

(0.022 mL, 0.2 mmol), and stirring continued for 18 h at room

temperature. The solvent was concentrated to 1 mL under vacuum,

and diethyl ether (15 mL) was added to it. The colorless precipitate

thus obtained was ﬁltered and again washed with diethyl ether. The

precipitate thus obtained was dried well to give the title compound.

Yield: 0.052 g (52.74%) mp 140−145 °C. Anal. Calcd for

C₂₄H₃₀BF₃N₂O₃S: C, 58.31; H, 6.12; N, 5.67; Found: C 57.36; H

(s, PPh₂). ¹¹B NMR (CDCl₃, 95 MHz, δ, ppm) 4.11. IR (KBr, v,

̅

1

cm⁻¹): 3086(w), 3057(w), 2917(w), 1631(w), 1604(s), 1547(w),

1480(s), 1434(s), 1376(m), 1346(w), 1324(m), 1269(m), 1237(m),

1223(m), 1206(m), 1155(s), 1091(w), 1027(w), 957(w), 842(s),

836(s), 748(m), 741(m), 720(m), 696(s), 588(w), 522(w), 507(s),

444(w). ESI-MS m/z calcd. for C₃₄H₃₇BN₂P⁺, 515.2782 [M + H]⁺;

found, 515.2787 [M + H]⁺.

5.303; N 5.79%. H NMR (CDCl₃, 400 MHz; δ, ppm): 2.03 (s,12H,

o-CH₃), 2.29 (s, 6H, p-CH₃), 3.53 (s, 3H, N−CH₃), 3.96 (s, 3H, N−

CH₃), 6.84 (s, 4H, m-C₆H₂), 7.07 (s, 1H, C4_Im-H), 9.52 (s,1H, C2_Im-

H). ¹³C NMR (CDCl₃, 100 MHz; δ, ppm): 21.3, 23.0 (Mes-CH₃),

36.3, 36.4 (N-CH₃), 129.1, 133.0, 133.5, 140.8, 141.4, 141.9, 142.9.

¹¹B NMR spectra could not be obtained. IR (KBr, v, cm⁻¹): 3148(w),

̅

Synthesis of 7. A Schlenk ﬂask containing 4 (0.40 g, 0.87 mmol)

and trimethyloxonium tetraﬂuoroborate (0.14 g, 0.96 mmol) was

charged with DCM and was stirred for 16 h at room temperature. The

solvent was evaporated under vacuum, and the resulting crude

product was washed with a 1:4 mixture of THF and diethyl ether (2 ×

15 mL) and dried to aﬀord 7 as a colorless solid. Yield; 0.39 g (80%).

mp 200 °C. Anal. Calcd for C₂₃H₂₉B₂F₄IN₂: C, 49.51; H, 5.24; N,

2921(m), 2963(m), 1606(m), 1585(m), 1547(w), 1441(m),

1421(m), 1377(w), 1259(s), 1225(s), 1159(s), 1094(m), 1031(s),

912(w), 841(m), 801(m), 757(w), 732(w), 702(w), 689(w), 640(s),

+

574(w), 517(m). ESI-MS m/z calcd. for C₂₃H₃₀BN₂, 345.2497;

found, 345.2500.

Synthesis of 10. Acetonitrile solution of CuI (0.042 g, 0.22 mmol,

2.2 mL of solvent) was layered above the hexane solution of 4 (0.0514

g, 0.1 mmol, 2.2 mL of hexane) in a glass tube and kept for

crystallization at room temperature. After 12 h, formation of pale

green crystals was observed. The solution was decanted, and the

crystals were washed with acetonitrile (3 × 3 mL) and dried in vacuo

to give the title compound. Yield: 0.079 g (88%). mp 260 °C

(decomp.). Anal. Calcd for C₆₈H₇₂B₂Cu₄I₄N₄P₂: C, 45.61; H, 4.05; N,

3.13. Found: C, 45.67; H, 4.09; N, 3.11%. NMR spectra of 10 could

not be recorded due to poor solubility in common organic solvents.

1

5.02. Found: C, 49.44; H, 5.18; N, 5.12%. H NMR (CDCl₃, 500

MHz; δ, ppm): 2.04 (s, 12H, o-CH₃), 2.24−2.33 (two singlets

merged, 6H, p-CH₃), 3.45 (s, 3H, N−CH₃), 3.89 (s, 3H, N−CH₃),

6.83 (s, 4H, m-C₆H₂), 9.32 (s, 1H, Im-H). ¹³C NMR (CDCl₃, 125

MHz, δ, ppm) 21.4, 21.5, 22.5 (Mes-CH₃), 37.2, 38.6 (N−CH₃), 88.7

(Im-CI), 129.0, 129.4, 138.8, 139.5, 141.9, 143.1, 144.2. ¹¹B NMR

−

(CDCl₃, 95 MHz, δ, ppm) −2.27 (BF₄) and peak for BMes₂could

not be observed. IR (KBr, v, cm⁻¹): 3174(w), 2957(m), 2919(m),

̅

IR (KBr, v, cm⁻¹): 2918(m), 2853(m), 1735(w), 1605(s), 1481(m),

1605(s), 1568(m), 1545(w), 1510(m), 1449(m), 1418(m), 1377(w),

1338(s), 1242(m), 1220(m), 1193(w), 1156(m), 1083(s), 1028(s),

957(w), 926(w), 878(m), 844(m), 759(w), 713(w), 695(m),

619(m), 556(w), 533(w), 520(m). ESI-MS m/z calcd. for

̅

1438(s), 1432(s), 1374(m), 1342(w), 1317(m), 1277(m), 1233(m),

1207(m), 1178(m), 1117(w), 1093(w), 1027(w), 996(w), 953(w),

839(s), 743(m), 722(m), 689(s), 590(m), 571(m), 552(m), 529(m),

508(s), 471(m), 450(m).

+

C₂₃H₂₉BIN₂471.1463; found, 471.1469.

Synthesis of 8. To a stirred solution of 5 (1.0 g, 0.76 mmol) in dry

acetonitrile was added methyl iodide (0.2 mL, 3.04 mmol), and the

solution was reﬂuxed for 18 h. The solvent was concentrated to 1 mL

under vacuum, and diethyl ether (15 mL) was added to it. White

colored precipitate thus obtained was ﬁltered and again washed with

diethyl ether. The precipitate was dried under vacuum to obtain the

title compound. Yield: 1.0 g (70.4%) mp 230−232 °C (with

decomp.). Anal. Calcd for C₂₃H₃₀BIN₂: C, 58.50; H, 6.40; N, 5.93.

Found: C, 57.69; H, 6.37; N, 6.16%. ¹H NMR (CDCl₃, 400 MHz; δ,

ppm): 2.04 (s, 12H, o-CH₃), 2.29 (s,6H, p-CH₃), 3.59 (s, 3H, N−

CH₃), 4.07 (s, 3H, N−CH₃), 6.84 (s, 4H, C₆H₂(meta)), 7.01 (s, 1H,

C4_Im-H), 10.70 (s, 1H, C2_Im-H). ¹³C NMR (CDCl₃, 100 MHz; δ,

ppm): 21.3, 23.2(Mes-CH₃), 36.5, 37.0 (N-CH₃), 129.1, 133.0, 140.8,

141.4, 141.6 142.6. ¹¹B NMR (CDCl₃, 95 MHz; δ, ppm): 1.46. IR

Synthesis of 11. Compound 10 (0.020 g, 0.011 mmol) was

dissolved in a DMSO/acetonitrile (1:4) mixture in hot condition and

then kept for crystallization by a slow evaporation method. After a

week, colorless crystals were obtained, which show bright orange

ﬂuorescence under UV light. Yield: 0.010 g (0.0077 mmol, 70.2%).

mp 250 °C (decomp). Anal. Calcd for C₃₂H₃₀Cu₄I₄N₄P₂: C, 29.69; H,

2.34; N, 4.33. Found: C, 30.01; H, 2.42; N, 4.48%. NMR spectra of

(BMes₂PPh₂Im)₂Cu₄I₄could not be recorded due to poor solubility

in common organic solvents. IR (KBr, v, cm⁻¹): 3148(w), 3050(w),

̅

1618(w), 1525(w), 1479(m), 1454(m), 1432(s), 1407(m), 1354(w),

1329(w), 1278(m), 1181(w), 1144(w), 1095(m), 1025(w), 997(w),

859(w), 843(w), 762(s), 748(m), 739(s), 692(s), 539(s), 513(m),

465(m), 438(w).

Fluorescent Quantum Yield (φ_f) Measurements. Emission

quantum yields were determined relative to quinine sulfate in 0.1

M H₂SO₄at room temperature (φ_f= 0.546). The absorbances of all

the samples and the standard at the speciﬁc wavelength were chosen

below 0.2. The quantum yields were calculated by a previously

reported procedure.²³

(KBr, v, cm⁻¹): 3151(w), 2916(m), 1727(w), 1606(s), 1583(m),

̅

1543(w), 1427(s), 1340(m) 1286(w), 1240(s), 1215(m), 1160(s),

1082(w), 1032(w), 955(w), 878(w), 865(w), 841(s), 817(m),

731(w), 707(m), 681(m), 624(w), 603(s), 517(w). ESI-MS m/z

+

calcd. for C₂₃H₃₀BN₂, 345.2497; found, 345.2500.

L

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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry

pubs.acs.org/IC

Article

X-ray Crystallography. The crystal data were collected on a Bruker

SMART APEX CCD diﬀractometer. Data were collected using

graphite-monochromated Mo Kα radiation (λ = 710 73 Å) at 153 or

273 or 293 K. All the structures were solved by direct methods using

Olex2 and reﬁned with the SHELXL (2018/3) reﬁnement package

using least-squares minimization.³⁰All the hydrogen atoms were

included in idealized positions, and a riding model was used. Non-

hydrogen atoms were reﬁned with anisotropic displacement

parameters.

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ASSOCIATED CONTENT

* Supporting Information

The Supporting Information is available free of charge at

https://pubs.acs.org/doi/10.1021/acs.inorgchem.0c00348.

■

sı

Crystallographic data and NMR spectra of the reported

compounds, ﬂuoride binding constant calculations, and

the graphs of selectivity experiments (PDF)

Accession Codes

̌

Klikar, M.; Mikysek, T.; Giannetas, V.; Fakis, M.; Bures, F.

CCDC 1980250−1980253, 1980330, and 1980331, contain

the supplementary crystallographic data for this paper. These

data can be obtained free of charge via www.ccdc.cam.ac.uk/

data_request/cif or by emailing data_request@ccdc.cam.ac.uk,

or by contacting The Cambridge Crystallographic Data

Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +

44 1223 336033.

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Organoboron Compounds BAr₂R (Ar = Mesityl, R = 7-Azaindolyl- or

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AUTHOR INFORMATION

Corresponding Author

■

Ganapathi Anantharaman − Department of Chemistry, Indian

Institute of Technology, Kanpur, Uttar Pradesh 208016, India;

orcid.org/0000-0001-7740-5581; Phone: (+91)-0512-

679-7517; Email: garaman@iitk.ac.in

Authors

Iruthayaraj Avinash − Department of Chemistry, Indian

Institute of Technology, Kanpur, Uttar Pradesh 208016, India

Sabeeha Parveen − Department of Chemistry, Indian Institute

of Technology, Kanpur, Uttar Pradesh 208016, India

Complete contact information is available at:

https://pubs.acs.org/10.1021/acs.inorgchem.0c00348

Author Contributions

^†I.A. and S.P. have contributed equally to this work.

̈

C.; Alahmadi, A. F.; Meng, B.; Liu, J.; Jakle, F.; Wang, L. A p-π *

conjugated triarylborane as an alcohol-processable n-type semi-

conductor for organic optoelectronic devices. J. Mater. Chem. C

2019, 7, 7427−7432.

Notes

The authors declare no competing ﬁnancial interest.

(3) (a) Blackwell, J. M.; Sonmor, E. R.; Scoccitti, T.; Piers, W. E.

B(C₆F₅)₃-Catalyzed Hydrosilation of Imines via Silyliminium

Intermediates. Org. Lett. 2000, 2, 3921−3923. (b) Gevorgyan, V.;

Rubin, M.; Benson, S.; Liu, J.-X.; Yamamoto, Y. A Novel B(C6F5)3-

Catalyzed Reduction of Alcohols and Cleavage of Aryl and Alkyl

Ethers with Hydrosilanes. J. Org. Chem. 2000, 65, 6179−6186.

(c) Eisenberger, P.; Bailey, A. M.; Crudden, C. M. Taking the F out of

FLP: Simple Lewis Acid-Base Pairs for Mild Reductions with Neutral

Boranes via Borenium Ion Catalysis. J. Am. Chem. Soc. 2012, 134,

17384−17387. (d) Hounjet, L. J.; Bannwarth, C.; Garon, C. N.;

Caputo, C. B.; Grimme, S.; Stephan, D. W. Combinations of Ethers

and B(C₆F₅)₃Function as Hydrogenation Catalysts. Angew. Chem.,

Int. Ed. 2013, 52, 7492−7495. (e) Yin, Q.; Kemper, S.; Klare, H. F.

T.; Oestreich, M. Boron Lewis Acid-Catalyzed Hydroboration of

Alkenes with Pinacolborane: BArF₃Does What B(C₆F₅)₃Cannot Do!

Chem. - Eur. J. 2016, 22, 13840−13844. (f) Andrea, K. A.; Kerton, F.

M. Triarylborane-Catalyzed Formation of Cyclic Organic Carbonates

and Polycarbonates. ACS Catal. 2019, 9, 1799−1809. (g) Carden, J.

L.; Gierlichs, L. J.; Wass, D. F.; Browne, D. L.; Melen, R. L. Unlocking

ACKNOWLEDGMENTS

■

I.A. and S.P. thank IIT Kanpur for their doctoral fellowships.

We thank Mr. S. K. Sachan for his help in solving the crystal

structures. The authors thank Science and Engineering Research

Board, India, for the ﬁnancial support (Project No. SR/s1IC-

14-2006) and IIT Kanpur for infrastructure.

DEDICATION

■

This article is dedicated to Prof. Goutam Kumar Lahiri on the

occasion of his 60th birthday.

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https://dx.doi.org/10.1021/acs.inorgchem.0c00348

Inorg. Chem. XXXX, XXX, XXX−XXX

Article Doi

Backbone Boron-Functionalized Imidazoles/Imidazolium Salts: Synthesis, Structure, Metalation Studies, and Fluoride Sensing Properties

DOI: 10.1021/acs.inorgchem.0c00348

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Full text of DOI:10.1021/acs.inorgchem.0c00348

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