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Volume 39 | Number 3 | 2010

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Volume 39 | Number 3 | 21 January 2010 | Pages 657–964

Dalton Transactions

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Triarylborane Conjucated acacH Ligands and their BF2 Complexes: Facile Synthesis and Intriguing Optical Properties

5

Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX DOI: 10.1039/b000000x

ABSTRACT: A facile synthetic route for a new class of organoborane compounds (Mes)2B-arene-acacH

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and (Mes)2B-arene-acacBF2 (Mes = mesityl and arene = C6H4 or C6Me4) have been reported. The new dyads exhibit intriguing photophysical properties. A small structural change in spacer connecting the two chromophores leads to fine tuning of photophysical properties. The dyad containing 2,3,5,6-tetramethyl phenyl spacer acts as selective “turn-on” chemodosimetric sensor for cyanide ion. Steric crowding around the boron centre significantly alters anion binding events. From NMR titration studies it is established that fluoride and cyanide follow different binding mechanisms which lead to intriguing optical properties in the reported probes.

Introduction 15

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Recently, luminescent triarylborane (TAB) containing extended π-systems have received much attention because of their ability to function as potential electron transport and emissive layer in Organic Light Emitting Diodes (OLED)1. The intrinsic electron deficiency of TAB has also been exploited in Non Linear Optics (NLO)2 and energy harvesting materials3. Further, in the last two decades the coordinative unsaturation of TAB has been well exploited for the selective detection of small anions such as fluoride, cyanide and neutral molecules4. In this regard seminal reports from Yamaguchi4a and Gabbai et al4c should be noted. In recent times, considerable research effort has been devoted to incorporate versatile functional groups such as -COOH, -CHO, -NH2, -COCH3 to TAB systems5-9, as they can be utilized for constructing novel functional materials. For example, carboxylic acid containing TAB has been used as a potential ligand in e assembling metalorganic frameworks (MOF)5a-c, , homobimetallic paddlewheel complexes6 and luminescent lanthanide carboxylates7. Wang et al elegantly demonstrated the utility of amine containing TAB in constructing novel blue phosphorescent molecules, and also the potential of these molecules in OLEDs8. Very recently, we have demonstrated the utility of aldehyde containing TAB in constructing dual fluorescence emissive TAB-BODIPY dyads9 (BODIPY: boron dipyromethane). A small structural and the resulting conformational change in these dyads leads to a significant change in their photophysical properties and ability to bind to fluoride ion. The intriguing photophysical properties of TAB-BODIPY dyads encouraged us to introduce new functional groups to TAB unit, which can be utilized for constructing novel dyads. In this connection, β-diketones caught our attention because they are well-known chelating ligands for several p-block10, d-block11 and f-block12 metals in organometallic chemistry. We envision that This journal is © The Royal Society of Chemistry [year]

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incorporation of boryl moiety into the β-diketonate ligand would open up new opportunities for the development of novel functional materials. Accordingly, we designed and synthesized boryl appended acacH ligands and TAB-acacBF2 dyads and investigated their photophysical properties and the results are reported in this paper.

Result and discussion 55

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Synthesis & characterization The synthetic protocol employed to construct –B(Mes)2 appended acacH ligands (4a and 4b) and respective TABacacBF2 dyads (5a and 5b) is shown in Scheme 1. The precursor compounds 2a, 2b, 3a and 3b were synthesized according to known literature procedures13a, e, f. Modified Maverick’s procedure11b was used for preparing compounds 4a and 4b. Compound 4a was obtained by treating 3a with 2,2,2-trimethoxy4,5-dimethyl-1,3,2-dioxaphosphole, followed by methanolysis. Under similar conditions, 4b could not be obtained from 3b. Nonetheless, 4b was prepared under slightly modified reaction conditions (see experimental procedures). 5a and 5b were obtained by treating the respective precursors (4a for 5a and 4b for 5b) with BF3⋅OEt2 in the presence of triethylamine (TEA). Mes

R

R

B Mes

R Br

a

R

B Mes

R

R

2a, R = H 2b, R = CH3

R O

Mes

b

R

B Mes

R

R OH

Mes

R

O R

3a, R = H (76.9%) 3b, R = CH3 (58.5%)

c

R

4a, R = H, (50.1%) 4b, R = CH3, (59.8%)

R

Mes

O F B O F

B Mes R

R

5a, R = H, (36%) 5b, R = CH3, (90.1%)

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Scheme 1: a) (i) n-BuLi/THF, -78 ºC, 1 hr (ii) DMF, 6 h, 25°C, 2N HCl. b) (i) 2,2,2-Trimethoxy-4,5-dimethyl-1,3,2-dioxaphosphole, 12 hr, 25 °C (for 4b, 60 ºC). (ii) Methanol, reflux 4 hr. c) BF3.OEt2, Et3N, Toluene, 65 ºC, 12 hr.

[journal], [year], [vol], 00–00 | 1

Dalton Transactions Accepted Manuscript

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George Rajendra Kumar and Pakkirisamy Thilagar*

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GooF

1.032

1.047

1.031

[a]R1 = Σ││Fo│ − │Fc││/ Σ│Fo│. [b]wR2 = [Σ{w(Fo2 – Fc2)2}/Σ{w(Fo2)2}]1/2

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Molecular Structures 35

Single crystals of 4a, 5a and 5b suitable for X-ray diffraction14 studies were obtained from slow evaporation of respective solutions in “hexane diethylether mixture”. Molecular structures of compounds 4a, 5a and 5b are shown in Figure 1. The tricoordinate boron atom in all the compounds has a trigonal planar configuration with the sum of angles around boron being 360o. The BF2-diketonate unit in both 5a and 5b are puckered and -BF2 unit significantly deviates from the mean plane of BF2diketonate ring15. The deviation is considerably higher for 5b (32.20°) than for 5a (20.04°). The dihedral angle between BC2 (mesityl carbon) and spacer which connects –B(Mes)2 and BF2diketonate unit in 5b (60.7°) is twofold higher than the value observed for 5a (26.5°). The presence of additional methyl groups at the ortho-positions of dimesitylboryl group and BF2diketonate unit twisted the two chromophores (boryl and borondiketonate) significantly in 5b. On the basis of these results one can tentatively conclude that electronic communication between B(Mes)2 and BF2-diketonate in 5b should be less than that in 5a. Photophysical studies

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Figure 1: Molecular structures of 4a (left), 5a (middle) and 5b (right). All aryl and methyl hydrogen atoms are removed for clarity. 20

Table 1: Crystallographic data for 4a, 5a and 5b 4a

5a

5b

Empirical formula

C29H33B O2

C29H32B2F2O2 ⋅C6 H12

C33H40B2 F2 O2

Fwt

424.36

556.32

528.27

T (K)

296(2)

273(2)

296(2)

λ(Å)

0.71073

0.71073

0.71073

Crystal lattice

monoclinic

orthorhombic

monoclinic

Space group

P 2 1/c

P2(1) 2(1) 2(1)

C 2/c

a=13.3757(8)Å b=11.6597(6)Å

a=8.714(2)Å

a=42.366(1)Å

b=15.194(4)Å

b=8.600(3)Å

c=16.4980(9)Å

c=23.608(6)Å

c=15.977(5)Å

β=109.092(3)°

α, β, γ = 90.00°

β = 91.14(3)°

2431.4(2)

3125.9(1)

5820(3)

Unit cell dimensions (a, b, c) (Å), V (Å3) Z, ρ

4, 1.159

4, 1.182

8, 1.206

µ (cm-1)

0.070

0.078

0.081

F(000)

912.0

1192

2256

size (mm)

0.6×0.4×0.2

0.1×0.1×0.1

0.15×0.12×0.12

θ range (°)

2.44-29.95

2.49-27.92

2.42-30.29

TMax & Tmin

0.986, 0.967

0.992, 0.992

0.9904, 0.9880

Unique refln

4846

5325

5140

R1[a], wR2[b] (I > 2sI)

R1 = 0.0515, wR2 = 0.1317

R1= 0.0666, wR2 = 0.1972

R1=0.0681, wR2 = 0.1693

Largest diff. peak and hole ( e.A-3)

0.318, -0.255

0.675, -0.490

0.352, -0.280

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Preliminary photophysical studies were carried out on 4a, 4b, 5a and 5b. All compounds show similar type of absorption profile, having a broad band (280-350 nm) and a weak shoulder at ∼270 nm. These bands can be ascribed to π-pπ (B) transition of the boryl unit, and π−π* transitions of phenyl-acac unit (Figure 2) respectively. Upon excitation in the region 260-350 nm, hexane solutions of 4a, 4b, 5a, and 5b show a single broad emission at ∼375 nm attributable to boryl unit.4f Fluorescence quantum yields of 4a, 4b, 5a and 5b are summarized in Table 2. Recently Mayoral et al and Poon et al reported that acac-BF2 complexes exhibit more quantum yield compared to free acacH ligands18. In present case both ligands and the respective BF2 complexes show nearly similar quantum yields.

Figure 2: UV-Vis absorption (left in CHCl3, 3×10-5 M) and fluorescence (right, in Hexane, 3×10-5 M) spectra of 4a (λex = 308 nm), 4b (λex = 316 nm), 5a (λex = 306 nm) and 5b (λex = 306 nm).

Interestingly, compounds 5a and 5b and show dual emissions (~390 and ~500 nm) in DMSO. It is important to mention here that on standing the yellow colour DMSO solutions of 5a and 5b slowly become colorless. These results indicate that both 5a and 5b are not stable in DMSO. Hence the appearance of two different emission peaks in DMSO can be attributable to 5a and 5b and their respective decomposed products (can be free ligand 4a and 4b). This journal is © The Royal Society of Chemistry [year]

Dalton Transactions Accepted Manuscript

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Compounds 4a, 4b, 5a, and 5b were characterized by NMR (1H, 13 C, 11B, 19F) spectroscopy and HRMS spectrometric techniques. The 1H NMR spectra of 3a and 3b show the aldehyde proton signals at 10.07 & 10.67 ppm respectively. The appearance of enol proton signal at 16.65 ppm (4a) and 16.51 ppm (4b) confirms the conversion of formyl-group into acacH in 4a and 4b. The relative integration value of enol proton suggests that enol form is predominant in CDCl3. The aryl C-H protons of mesitylene moieties of 4b and 5b give rise to two distinct resonances at 6.73 and 6.72 ppm in CDCl3, indicating the nonequivalence of the two mesityl units in 4b and 5b. From the single crystal structure of 5b, it is found that the dihedral angle between duryl spacer and –B(Mes)2 unit is 60.7 °; possibly this twisted arrangement is retained in solution rendering the two C-H protons of the mesitylene groups nonequivalent.

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high dipole moment, thus 5b shows more pronounced red shift in polar environments.

Table 2: Fluorescence Quantum yield of 4a, 4b, 5a and 5b (Ф) in solvents of different polarity (3×10-5 M)

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Toluene

CH2Cl2

CH3CN

DMSO

4a

0.02

0.04

0.06

0.08

0.10

4b

0.04

0.06

0.10

0.09

0.08

5a

0.01

0.07

0.03

0.02

0.03

5b

0.04

0.02

0.02

0.03

0.04

The excitation spectra (Figure S21 & S22) of 4a, 4b 5a and 5b reproduce the respective absorption spectrum. These results suggest that the emission bands are genuine and have only intramolecular origin. To get further insight into the electronic nature of the excited state of 4a, 4b, 5a and 5b photoluminescence spectra of all compounds were carried out in solvents with different polarity (Figure 3). In all the cases, the emission bands are red shifted with an increase in the solvent polarity. All the compounds shows significant Stokes shift and are summarised in Table 3. Compound 5b displays larger Stokes shift compared to 4a, 4b and 5a. Large Stokes shifts usually result from charge transfer and significant geometric rearrangements in the excited state. To rationalize this, the Lipperted –Mataga plot4i was derived for all the four compounds using the relationship between solvent polarity parameter ∆f and stoke shift ∆ν (Figure 4). ∆f = [(D-1)/(2D+1)] – [(n2-1)/(2n2+1)]

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Table 3: Observed stoke shifts (Δν) for 4a, 4b, 5a and 5b -1

Δν(cm )

25

30

35

40

45

(2)

∆µ is change in dipolemoment [∆µ = µg - µe, where µg and µe are dipolemoment in ground and excited states respectively]. hPlank’s Constant, c- velocity of light and a- Onsagar radius. The Lipperted –Mataga plot for 4a, 4b, 5a and 5b are shown in Figure 4. All the compounds exhibits linear relationship between ∆ν and ∆f. Using Onsagar values (a, since we don’t have crystal structure for 4b, it’s a value is assumed as ~8.4Å based on crystal structure of 4a), and slope from Lippert-Mataga plots, ∆µ is calculated to be ~21D, ~17D, ~22D, ~48D for 4a, 4b, 5a and 5b respectively. Since the changes in absorption wavelength (Figure S16 & S17) is significantly small for all the compounds compared to changes in fluorescence emission wavelength with solvent polarity, the calculated dipolemoment change (∆µ) can be considered to be contribution mainly from exited state (∆µ = µe). The excited state dipole moment values of 4a, 4b, 5a and 5b clearly indicates that polar ICT (intramolecular charge transfer) state is the emitting state in all these compounds. The DFT computational results are also in-line with the above inference (Figure 5 and 6). The larger dipolemoment (~48D) observed for 5b is directly related to the bulky duryl substituents which would favour an ICT in the emitting state due to the orthogonally oriented TAB and BF2 moiety. Generally, the intramolecular charge transfer states are stabilized in polar solvents due to their This journal is © The Royal Society of Chemistry [year]

Hexane

Toluene

CH2Cl2

CH3CN

DMSO

4a

4746

6082

6787

7566

8169

4b

3636

3987

4396

5298

5549

5a

6013

6388

7554

7957

8348

5b

5303

7732

11210

12945

13986

(1)

D and n represents dielectric constant and refractive index of a solvent, respectively. ∆ν = (νA-νF) cm-1 = (2∆µ2/hca3) ∆f + Constant

Figure 3. Fluorescence emission spectra of 4a (top left, λex = 308 nm), 4b (top right, λex = 316 nm), 5a (bottom left, λex = 306 nm) and 5b (bottom right, λex = 306 nm) (1×10-5 M) in solvents of different polarity.

Figure 4. Lippert-Mataga plots for 4a, 4b, 5a and 5b.

DFT Computational Studies 60

To get further insight into the electronic nature of compounds 4a, 4b, 5a and 5b, DFT computational studies were performed. The hybrid B3LYP19 functional has been used for all calculations as incorporated in Gaussian 09 package20, mixing the exact Hartree-Fock-type exchange with Becke’s exchange functional21 Journal Name, [year], [vol], 00–00 | 3

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ΦF

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Anion binding studies

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Figure 6: Frontier-Molecular-Orbitals for 5a and 5b. (isovalue = 0.04). Hydrogen atoms are omitted for clarity.

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Triarylborane is a well-known receptor for small anions such as fluoride and cyanide. The presence of –BMes2 unit in 4a, 4b, 5a and 5b gave an opportunity to evaluate their potential for the detection of anions such as F− and CN−. Compounds 5a and 5b are found to be unstable in presence of F/CN and give rise to complex spectral responses. The possible interpretations were made based on the available experimental data.

Figure 7: UV-Vis absorption titration spectra of 5a (left, 1×10-5 M) and 5b (right 1×10-5 M) in CHCl3 against TBAF (0.1 equiv = 2μL).

Figure 5: Frontier-Molecular-Orbitals for 4a and 4b. (isovalue = 0.04). Hydrogen atoms are omitted for clarity.

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twisted molecular arrangement, excited state charge separation may be more favourable in 5b compared to 5a. Hence compound 5b shows large Stokes shifts in polar solvents.

The FMOs of compound 5a and 5b are shown in Figure 6. As shown, the localisations of HOMOs and LUMOs are similar in case of both 5a and 5b. The HOMOs are dominated by the mesityl centred π-orbitals, the LUMOs are concentrated on the acacBF2 moieties. The close electronic structures (and HOMOLUMO band gap) of compound 5a and 5b corroborates well to their similar UV-Vis absorption profiles. The distributions of the FMOs suggest possibility of intramolecular charge separation upon electronic excitation. Due to the structural rigidity and

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Figure 8: Fluorescence titration spectra of 5a (left, 1×10-5 M, λex = 306 nm) and 5b (right, 1×10-5 M, λex = 306 nm) in CHCl3 against TBAF (0.1 equiv = 2μL).

Binding constants of 4a, 5a and 5b towards fluoride and cyanide are summarized in Table 4. The results obtained from UV-Vis and fluorescence titrations for 4a against F−/CN− (Figure S18, & S19) suggest that the anions bind to boryl center thereby quenching the absorption and emission originating from boryl unit. Under similar conditions, 4b does not show any changes in the absorption and emission profiles (Figure S20 & S21, slight increament in emission intensity may be due to the interaction of anion with enol proton). This suggests that the boron center in 4b is highly shielded by the duryl spacer. For 5a, addition of 1 equivalent (eq) of F− in the form of tetrabutylammonium fluoride (TBAF) quenches the intensity of absorption band at ~300 nm. In fluorescence titrations, addition of fluoride to 5a quenches the emission band at 393 nm. An excess F− causes appearance of a new broad band at ∼447 nm. In contrast, the addition of fluoride to 5b induces very little changes in the absorption profile. In fluorescence emission studies the intensity of band at 468 nm is decreased and concomitantly a higher energy band at 401 nm gained intensity (Figure 7 & 8). This can be due to different binding modes of fluoride with 5a and 5b. This journal is © The Royal Society of Chemistry [year]

Dalton Transactions Accepted Manuscript

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and that developed by Lee-Yang-Parr for the correlation contribution.22 We used 6-31G(d) basis set for all the atoms which provides reasonably good quality results in reasonable timescales. All ground state geometry optimizations were followed by subsequent frequency test to establish stationary points The optimized structures nearly reproduced the crystal structures. The FMOs (Frontier molecular orbitals) of the compounds are shown in Figure 5-6. As evident from Figure 5, the HOMO in compound 4a is mesityl centred π-orbital. Whereas for 4b, the HOMO is duryl-spacer dominated π-orbital. However, the LUMO of both 4a and 4b can be described as boron centred pπ* orbital. The loss of conjugation in compound 4b results in minor destabilisations of the FMOs but the effective HOMO-LUMO band gap in 4a and 4b nearly the same. This result supports the experimentally observed similar UV-Vis profiles for these two compounds.

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Excess fluoride causes the acac-BF2 unit to dissociate and hence the emission intensity at 401 nm slowly gains its intensity. In both 5a and 5b, the emission band becomes broad in presence of excess equivalents of fluoride. This may be due to possible existence of multiple species. Both 5a and 5b exhibit a “turn-on” fluorescence response (Figure 9 & 10) upon excitation at 306 nm, in the presence of 1 eq of tetrabutylammonium cyanide (TBACN). A new fluorescence emission band at ∼440 nm gradually gains in intensity. In the presence of additional amount of CN- both 5a and 5b show completely different fluorescence responses. In case of 5a, the band at ∼440 nm fully disappears, whereas for 5b the intensity of band at ∼440 nm is increased up to the addition of 5 eq of CN−. This indicates that the binding mechanism of CN- beyond 1 eq is different for 5a and 5b.

Figure 9: UV-Vis absorption titration spectra of 5a (left, 1×10-5 M) and 5b (right 1×10-5 M) in CHCl3 against TBACN (0.1 equiv = 2μL).

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Figure 10: Fluorescence titration spectra of 5a (left, 1×10-5 M, λex = 306 nm) and 5b (right, 1×10-5M, λex = 306 nm) in CHCl3 against TBACN (0.1 equiv = 2μL).

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Similar observation has been reported by Gabbai et al for sterically encumbered B(Mes)3 compound4d. In 11B NMR titrations, in the presence of an excess of TBAF 5a/5b shows signals corresponding to F3B-acac (∼0.75 ppm, q) and BF4−(∼1.01 ppm, s) species (supporting information). From 19F titrations of 5a/5b against TBAF, we propose that in the case of 5a, fluoride concomitantly binds to both the tri-coordinate boron center and the acac-BF2 unit thereby causing the dissociation of acac-BF2 complex and also quenching the absorption and emission bands corresponding to the boryl unit. For 5b, the changes in the optical properties in presence of fluoride ion are attributable to the changes associated only with -acacBF2 unit16. This journal is © The Royal Society of Chemistry [year]

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Figure 11: Fluorescence response of 4a (top left, 1×10-5 M, λex = 308 nm), 5a (top right, 1×10-5 M, λex = 306 nm) and 5b (bottom, 1×10-5 M, λex = 306 nm) in CHCl3 in the presence of F-/CN- and various other anions Table 4: Binding constants* -

5

K(F )×10 M

-1

-

5

K(CN ) ×10 M

4a

1.202

0.247

5a

0.9879

0.4861

5b

0.0763

0.1807

-1

*

65

(Binding constants are calculated from UV-Vis absorption titrations by plotting (1 – I/I0)/[Anion] against I/I0 (see supporting information)17. For 4a, 5b K value calculated for changes in absorption corresponds to TAB and acac-BF2 units respectively. For

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To get further insight into the anion binding event, NMR titrations were carried out in the presence of fluoride in CDCl3. In 19 F NMR titrations, addition of 0.2 molar equivalents of TBAF to 5a gives rise to a broad signal at -172.6 ppm corresponding to F-BAr3 apart from –BF2 (137.95 and 138.01 ppm) signals. Upon addition of further TBAF to 5a, the, fluoride ion concomitantly binds to both BAr3 and -acacBF2 units followed by the dissociation of the acacBF2 complex to give rise to a new multiplet at -145 and a singlet at -154 ppm (less intensity peak) corresponding to F3B-acac-TAB and BF4−species respectively (in addition to F-BAr3 and –BF2) (Supporting Information). The NMR spectrum becomes more complex upon addition of more than one equivalent of TBAF. This may be due to the concurrent presence of more than two species in the titration mixture (Supporting Information Figure S24). In contrast, under similar conditions 5b does not show any new peak in the region corresponding to F-BAr3 species. Apparently, the tricordinate boron center in 5b is well protected by the presence of additional methyl groups, and it is not accessible to fluoride ion and this result is in line with literature4d.

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5a, K value calculated for changes in absorption corresponds to TAB unit)

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Conclusions

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In summary, synthesis of TAB containing versatile acacH ligands and their -BF2 complexes has been accomplished and their photophysical properties have been investigated. The dyad containing duryl spacer exhibits dual emission in polar solvents and also shows “turn-on” chemodosimetric response for cyanide ion. The boryl appended acacH may be useful as a new ligand suitable for binding p-block, d-block and f-block metals. Further study in this direction is in progress.

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Experimental Section 95

Materials and Methods

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n-Butyllithium (1.6 M in hexanes) and 2,3-butanedione were purchased from Aldrich chemicals. Trimethylphosphite obtained from Spectrochem (India). All reactions were carried under an atmosphere of purified Nitrogen using standard schlenck techniques. THF, triethylamine, and toluene were distilled over sodium prior to use. HPLC grade solvents (hexane, toluene, dichloromethane, acetonitrile, and DMSO) were used for absorption and emission spectroscopic measurements. All the UV-Vis absorption and fluorescence titrations are carried out at room temperature in freshly distilled solvents including NMR titrations. Quantum yields are calculated using anthracene as standard. 1 19 13 The (400 MHz) H NMR, (376 MHz) F NMR, (100 MHz) C 11 NMR and (160 MHz) B NMR were recorded on a Bruker Advance 400 MHz NMR spectrometer. High resolution mass spectra were obtained from Q-TOF instrument by electrospray ionization (ESI). Electronic absorption spectra were recorded on a Perkin Elmer LAMBDA 750 UV/visible spectrophotometer. Fluorescence emission studies were done using Horiba JOBIN YVON Fluoromax-4 spectrometer. 6 | Journal Name, [year], [vol], 00–00

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The precursors 2a and 2b were prepared according to the 13b, c literature procedures . Synthesis of 3a: To a solution of 2a (3 g, 7.4 mmol) in THF was added dropwise n-BuLi (1.6 M, 4.63 mL, 7.4 mmol) under N2 atmosphere at -78 °C and stirred at this temperature for 60 min. To the reaction mixture was added DMF (1.14 mL, 14.8 mmol) and the reaction mixture was allowed to warm to room temperature and stirring continued. After 6 hr, 10 mL of 2N HCl was added and continued the stirring for another 4 h. After addition of water, the reaction mixture was extracted with ethylacetate. The combined organic extract was washed with water, dried over Na2SO4 and concentrated under reduced pressure afforded crude product, which was further purified by silica gel column chromatography (petroleum ether/ EtOAc 1 (99:1)). Yield: 2.0 g in 76.9%. H NMR (400 MHz, CDCl3, δ ppm) 10.07 (s, 1H), 7.85 (d, J = 8 Hz, 2H), 7.66 (d, J = 8 Hz, 13 2H), 6.83 (s, 4H), 2.32 (s, 6H), 1.98 (s, 12H). C NMR (100 MHz, CDCl3, δ ppm) 193.3, 152.8, 141.3, 140.8, 139.9, 138.4, 136.4, 129.5, 128.9, 23.9, 21.8. Synthesis of 3b: Compound 3b was prepared following a procedure similar to that used for compound 3a. The quantities involved and characterization data are as follows. 2b (4.8 g, 10.4 mmol) n-BuLi (1.6 M, 6.5 mL, 10.4 mmol) DMF 1 (1.6 mL, 20.8 mmol) Yield: 2.5 g (58.5 %). H NMR (400 MHz, CDCl3, ppm): δ = 10.67 (s, 1H), 6.75 (s, 4H), 2.32 (s, 6H), 2.27 13 (s, 6H), 2.03 (s, 6H), 1.96 (d, J = 5.2 Hz, 12H). C NMR (100 MHz, CDCl3, ppm): δ = 197.6, 141.6, 141.2, 140.2, 136.5, 135.7, 134.5, 129.4, 23.8, 23.2, 21.7, 20.5, 15.7. Synthesis of 4a: Mixture of 3a (1 g, 2.8 mmol) and 2,2,2Trimethoxy-4,5-dimethyl-1,3,2-dioxaphosphole (2.96 g, 14.1 mmol) in a 50 mL schlenck flask was stirred under purified N2 atmosphere for 12 hr. Methanol (5 mL) was added to the reaction mixture and the stirring was continued for another 4 hr at 65 ºC. The resultant colorless precipitate was filtered and washed with methanol gave pure 4a (0.6 g) in 50.1% 1 yield. m.p = 168ºC. H NMR (400 MHz, CDCl3, ppm): δ = 16.66 (s, 1H, OH), 7.51 (d, J =8 Hz, 2H), 7.15 (d, J = 8 Hz, 2H), 13 6.83 (s, 4H), 2.31 (s, 6H), 2.01 (s, 12H), 1.90 (s, 6H). C NMR (100 MHz, CDCl3, ppm): δ = 190.5, 145.0, 141.4, 140.6, 140.3, 11 138.6, 136.4, 130.5, 128.1, 115.0, 23.9, 23.2, 21.0. B NMR (160 MHz, CDCl3, ppm): δ = 75.6. HRMS (TOF) calcd: C29H33BO2 + (M +Na ) 447.2471, found 447.2501 Synthesis of 4b: Mixture of 3b (1 g, 2.4 mmol), and 2, 2, 2Trimethoxy-4,5-dimethyl-1,3,2-dioxaphosphole (2.55 g, 12.2 mmol) in a 50 mL schlenck flask was stirred under purified N2 atmosphere at 60 ºC for 12 hr. Methanol (5 mL) was added to the reaction mixture and the stirring was continued for another 4 hr at reflux condition. The resultant colorless precipitate was filtered and washed with methanol gave pure 1 4b (0.7 g) in 59.8% yield. H NMR (400 MHz, CDCl3, ppm): δ = 16.52 (s, 1H), 6.75 (d, 4H, J = 8 Hz), 2.27 (s, 6H), 2.02 - 1.96 13 (m, 24H) 1.75 (s, 6H). C NMR (100 MHz, CDCl3, ppm): δ = 190.9, 141.4, 141.1, 139.8, 136.4, 136.3, 133.5, 129.4, 129.3, 114.13, 11 23.8, 23.7, 23.2, 21.7, 20.9, 17.3. B NMR (160 MHz, CDCl3, + ppm): δ = 71.7. HRMS (TOF) calcd: C33H41BO2 (M + Na )

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To investigate the binding mechanism, 19F NMR titrations of 5a/5b against TBACN were carried out. In the presence of one equivalent of CN− both 5a and 5b show a new signal at ∼ -150 ppm with concomitant disappearance of the 19F resonances corresponding to acacBF2 unit. In the presence of an excess of CN−, for 5a the signal at -150 ppm is downfield shifted to -149 ppm, at the same time a multiplet appeared at -145 ppm. Under similar conditions for 5b, no change was observed for the signal at -150 ppm. Control experiment was carried out with the model compound (ph-acac-BF2) which is devoid of –B(Mes)2 unit. In the presence of an excess of cyanide, the model compound replicated the spectrum of 5b⋅CN−. This result supports our inference that the initial amount of added CN− opens up the acacBF2 unit and give rise to a new fluorescent species [5a(CN−) for 5a and 5b(CN−) for 5b], which is responsible for the new emission peak at 429 nm. As observed in fluoride titrations, the sterically crowded tricoordinate boron in 5b is not accessible for cyanide ion. Hence, the fluorescence of the intermediate species is unquenched in the presence of even large excess of CN−. In case of 5a the second equivalent of CN− binds to tricoordinate boron center leading to the formation of nonfluorescent species 5a(CN−)2. Fluorescence titrations of 4a, 5a and 5b with other anions do not show any affinity towards them (Figure 11).

Single-crystal X-ray diffraction studies were carried out with a Bruker SMART APEX diffractometer equipped with 3-axis goniometer.

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503.3097 found 503.3093 peak at 535.3325 is due to 4b-MeOH adduct.

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Synthesis of 5a: Under purified N2 atm, a toluene solution of 4a (0.1 g, 0.24 mmol) with Et3N (0.16 mL, 1.18 mmol) was heated at 60 ºC for 30 min. BF3.OEt2 (0.15 mL, 1.18 mmol) was added and the reaction mixture was refluxed for 12 hr. The resulting mixture was concentrated under reduced pressure. After addition of water, organic layer was extracted with DCM. The combined dichloromethane solution washed with water and dried over Na2SO4. Removal of all the volatiles under reduced pressure gave crude product which was further purified by recrystallization (Hexane/diethylether). 1 Yield 0.04 g (36%). H NMR (400 MHz, CDCl3, ppm): δ = 7.59 (d, J = 8 Hz, 2H), 7.15 (d, J = 8 Hz, 2H), 6.84 (s, 4H), 2.31 (s, 13 6H), 2.12 (s, 6H), 2.00 (s, 12H). C NMR (100 MHz, CDCl3, ppm): δ = 191.3, 141.2, 139.6, 137.4, 136.5, 130.6, 128.8, 116.7, 24.1, 11 23.8, 21.7. B NMR (160 MHz, CDCl3, ppm): δ = 0.39(s, broad, 19 BF2), 77.5(broad, TAB moiety) F NMR (376 MHz, CDCl3) δ = -137.95, -138.01(two singlets, 2F). HRMS (TOF) calcd: + C29H32B2O2F2 (M + Na ) 495.2454, found 495.2454. Synthesis of 5b: Compound 5b was prepared following a procedure similar to that used for compound 5a. The quantities involved and characterization data are as follows. 5b (0.15 g, 0.31 mmol), Et3N (0.22 mL, 1.6 mmol) and 1 BF3.OEt2 (0.19 mL, 1.6 mmol). Yield 0.1 g (90.9%). H NMR (400 MHz, CDCl3, ppm): δ = 6.75(d, J = 10 Hz, 4H), 2.27 (s, 13 6H), 2.03-1.95(m, 30 H). C NMR (100 MHz, CDCl3, ppm): δ = 191.4, 144.5, 141.5, 140.9, 140.1, 136.9, 133.1, 132.3, 129.5, 129.4, 11 115.7, 23.7, 23.2, 21.7, 20.9, 17.2. B NMR (160 MHz, CDCl3, 19 ppm): δ = 0.58(s, broad, BF2), 69.0 (broad, TAB moiety) F NMR (376 MHz, CDCl3) δ = -139.06 to -139.00 (two singlets, + 2F) HRMS (TOF) calcd: C33H40B2F2O2 (M +Na ) 551.3080, found 551.3022.

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PT thanks; the Department of Science and Technology (DST) New Delhi, and IISc-STC Bangalore for the financial support. GRK thanks UGC New Delhi for JRF and thank Mr. Sanjoy Mukherjee for DFT calculations.

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Inorganic and Physical Chemistry Department, Indian Institute of Science, Bangalore-560012, India. Fax: (+91) 8023601552; Tel: +91-802293-3353; E-mail: [email protected] † Electronic Supplementary Information (ESI) available: 1H and 13C NMR spectra, NMR titrations. CCDC 963972 (4a), 963973 (5a) and 963974 (5b). See DOI: 10.1039/b000000x/ 1 (a) H. Doi, M. Kinoshita, K. Okumoto, Y. Shirota, Chem. Mater., 2003, 15, 1080; (b) C. D. Entwistle, T. B. Marder, Chem. Mater., 2004, 6, 4574; (c) W. L. Jia, J. M. Moran, Y. Y. Yuan, Z. H. Lu, S. Wang, J. Mater. Chem., 2005, 15, 3326; (d) D. Reitzenstein, C. Lambert, Macromolecules., 2009, 3, 773; (e) Y. L. Rao, D. Schoenmakers, L. Y. Chang, J. S. Lu, Z. H. Lu, Y. Kang, S. Wang, Chem. Eur. J., 2012, 18, 11306. 2 (a) Z. Q. Liu, Q. Fang, D. X. Cao, D. Wang, G. B. Xu, Org. Lett., 2004, 17, 2933; (b) Z. Yuan, C. D. Entwistle, J. C. Collings, D. A. Jov, A. S. Batsanov, J. A. K. Howard, N. J. Taylor, H. M. Kaiser, D. E. Kaufmann, S. Y. Poon, W. Y. Wong, C. Jardin, S. Fathallah, A. Boucekkine, J. F. Halet, T. B. Marder, Chem. Eur. J., 2006, 12, 2758; (c) L. Ji, Q. Fang, M. S. Yuan, Z. Liu, Y. X. Shen, H. F. Chen, Org. Lett., 2010, 22, 5192; (d) S. Nikolay, Makarov, S. Mukhopadhyay, K.

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DOI: 10.1039/C3DT52768A

Triarylborane Conjucated acacH Ligands and their BF2 Complexes: Facile Synthesis and Intriguing Optical Properties George Rajendra Kumar and Pakkirisamy Thilagar*

A facile synthetic route for a new class of organoborane compounds (Mes)2B-arene-acacH and (Mes)2B-arene-acacBF2 (Mes = mesityl and arene = C6H4 or C6Me4) have been reported. The new dyads exhibit intriguing photophysical properties. Steric crowding around the boron centre significantly alters anion binding events, which lead to intriguing optical properties in the reported probes.

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Department of Inorganic and Physical Chemistry, Indian Institute of Science, Bangalore 560012, India. E-mail: [email protected];Fax: 0091-80-23601552; Tel: 0091-80-22933353