Synthesis and studies of covalently linked meso-furyl boron-dipyrromethene-ferrocene conjugates

Journal of Organometallic Chemistry 697 (2012) 65e73

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Synthesis and studies of covalently linked meso-furyl boron-dipyrrometheneferrocene conjugates Tamanna K. Khan a, Raghuvir R.S. Pissurlenkar b, Mushtaque S. Shaikh b, M. Ravikanth a, * a b

Department of Chemistry, Indian Institute of Technology, Powai, Mumbai 400 076, India Department of Pharmaceutical Chemistry, Bombay College of Pharmacy, Santacruz (E), Mumbai 400 098, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 18 August 2011 Received in revised form 6 October 2011 Accepted 11 October 2011

Four meso-furyl BODIPY-ferrocene conjugates 1e4 in which one or more ferrocene groups were connected directly to BODIPY core or meso-furyl group were synthesized by coupling of appropriate bromo meso-furyl BODIPYs with a-ethynylferrocene under mild Pd(0) coupling conditions. The compounds were characterized by HR-MS mass, NMR, absorption, electrochemistry and fluorescence techniques. The absorption studies of compounds 1e4 showed charge transfer band in addition to BODIPY absorption bands indicating that the BODIPY and ferrocene moieties interact within the conjugates. On the other hand, the charge transfer band is absent in meso-phenyl BODIPY-ferrocene conjugate due to the orthogonal arrangement of ferrocene appended meso-phenyl group with BODIPY core which prevents the interaction between the two moieties. The electrochemical studies showed strong oxidation due to ferrocene moiety and reduction due to meso-furyl BODIPY unit. The compounds 3 and 4 which contain two and three ferrocenyl groups respectively were oxidized at the same potential with two and three electrons involved in the redox process. The compounds 1e4 are weakly fluorescent due to electron transfer from ferrocene unit to BODIPY unit. However, the fluorescence can be restored by oxidizing the ferrocene to ferrocenium ion which prevents the electron transfer between the two moieties. The computational studies support the experimental results. Ó 2011 Elsevier B.V. All rights reserved.

Keywords: Meso-furyl boron-dipyrromethene Ferrocene conjugates Charge transfer Weakly fluorescent

1. Introduction Ferrocene appended fluorophores such as porphyrins, phthalocyanines, perylene tetracarboxylic diimide etc have received tremendous attention owing to their potential applications in molecular devices and redox switches [1e6]. Ferrocenes have excellent reversible redox properties, high stability and more importantly, the electron donating abilities of the ferrocene unit can be reversibly modulated by the chemical or electrochemical redox reaction [7]. On the other hand, among fluorophores presently available, boron-dipyrromethene dyes (BODIPYs) received special attention because of their excellent thermal and photochemical stability, high fluorescence yields, negligible triplet state formation, intense absorption profile, good solubility and chemical robustness [8e10]. The electronic properties of BODIPYs can be fine tuned by using several different approaches such as introduction of functional substituents on the carbon framework, enlargement of the chromophore, substitution of the fluorine atoms by O- or C-

* Corresponding author. Tel.: þ91 022 2576 7176; fax: þ91 022 2576 7152. E-mail address: [email protected] (M. Ravikanth). 0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.jorganchem.2011.10.016

donors or replacing the meso-CH position with a nitrogen bridge to name few such modifications [8e19]. Hence these dyes are widely used as biomolecular labels, chromogenic probes and cation sensors, drug delivery agents, fluorescent switches, electroluminescent films, laser dyes, light-harvesters and sensitizers for solar cell applications [8e10]. Interestingly, the reports on BODIPYferrocene conjugates are few inspite of their synergistic properties [20e24]. Recently, two independent groups simultaneously reported BODIPY dyes I & II possessing one and two redox-active vinyl ferrocene groups at 3- and 3,5-positions, respectively (Chart 1) [20,21]. They demonstrated reversible electrochromism in these conjugates. We recently showed that the ferrocene connected directly to BODIPY framework III can be used as potential redox fluorescent switch whereas the ferrocene connected to mesophenyl group of BODIPY IV cannot be used (Chart 1) [22]. This is rationalized on the basis of magnitude of interaction between BODIPY and ferrocene units. In BODIPY-ferrocene conjugate IV, the meso-phenyl group is orthogonal to BODIPY core resulting in weak communication between BODIPY core and ferrocene group whereas in compound III, the ferrocene unit is directly attached to BODIPY framework leading to stronger interaction between the units. Thus, our earlier study indicated that if there is sufficient

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communication between the redox active group and fluorophore, the conjugates containing fluorophore and redox active units can act as potential redox fluorescent switches [22]. However, the compound IV is failed to act as redox fluorescent switch because of phenyl spacer which prevented better communication between ferrocene and BODIPY units. Recently, it is showed that the electronic properties of meso-furyl BODIPYs are significantly altered compared to meso-phenyl BODIPYs which is attributed to more inplane orientation of meso-furyl group with BODIPY core [25]. Thus, we thought that if we use furyl group as spacer instead of phenyl group, the communication between BODIPY and ferrocene would be better and such conjugates can act as redox fluorescent switches. In this paper, we report the synthesis of four covalently linked meso-furyl BODIPY-ferrocene conjugates and explored their photophysical and redox properties. Furthermore, we demonstrated that by using the furyl group as spacer between BODIPY and ferrocene units, the conjugate can be used as potential redox fluorescent switch. 2. Results and discussion To synthesize the mono-ferrocenyl connected BODIPYs 1 and 2, the required precursors 5 and 6 respectively were prepared by following standard methods (Scheme 1). The meso-(5-bromo-2furyl)-dipyrromethane 7 was prepared first by treating one equivalent of 5-bromo furan-2-aldehyde with 25 equivalents of pyrrole in the presence of a catalytic amount of BF3$OEt2 at room temperature for 15 min. The crude compound was purified by silica gel column chromatography and isolated pure compound 7 in 60% yield. In the next step, the compound 7 was first oxidized with DDQ in CH2Cl2 at room temperature for 10 min and then treated with triethylamine followed by BF3$OEt2 for 30 min. After standard work-up, the crude compound was purified by silica gel column chromatography and afforded 5 as dark red fluorescent solid in 26%

yield. The 3-bromo BODIPY 6 was prepared in 25% yield by oxidizing the 3-bromo meso-furyl dipyrromethane 8 with DDQ followed by treatment with BF3$OEt2 at room temperature (Scheme 1). The compound 8 was prepared in turn by treating meso-(2furyl)-dipyrromethane [26] with one equivalent of N-bromosuccinimide (NBS) at 78  C under nitrogen atmosphere for 1 h followed by flash silica column chromatographic purification. The BODIPYferrocene conjugates 1 and 2 were prepared by coupling of 5 and 6 with a-ethynyl ferrocene [27] respectively in toluene/Et3N in the presence of catalytic amounts of AsPh3/Pd2(dba)3 at 50  C for 2 h (Scheme 1). The crude compounds were purified by silica gel column chromatography and afforded conjugates 1 and 2 in 50e60% yield. The BODIPY-ferrocene conjugates 3 and 4 were prepared in one pot starting with meso-(5-bromo-2-furyl)-dipyrromethane 7 (Scheme 2). The compound 7 was treated with NBS in THF at room temperature for 30 min and the resulted mixture of bromo compounds without isolation was reacted with BF3$OEt2 in CH2Cl2 followed by oxidation with DDQ. The mixture of 3-bromo- and 3,5dibromo meso-(5-bromofuryl)BODIPYs 9 and 10 respectively was subjected to flash silica gel column chromatography to remove impurities and no attempt was made to separate the bromo BODIPYs 9 and 10. The mixture containing bromo substituted BODIPYs 9 and 10 was coupled with a-ethynyl ferrocene in THF/Et3N in the presence of catalytic amounts of AsPh3/Pd2(dba)3 at 50  C. The crude mixture containing ferrocenyl appended BODIPYs were subjected to silica gel column chromatography and isolated compound 3 in 40% yield and compound 4 in 35% yield (Scheme 2). The BODIPY-ferrocene conjugates 1-4 are freely soluble in common organic solvents such as CHCl3, CH2Cl2, toluene etc and were confirmed by HR-MS mass spectra. The compounds 1e4 were characterized by various spectroscopic and electrochemical techniques. 1H, 13C, 19F and 11B NMR were used to characterize the compounds 1e4 and the comparison of 1H NMR spectra of

Fe

Br Br O

NH

HN

O 1) CH2Cl2, DDQ 2) Et3N, BF3.OEt2

7

O

NH

HN 8

Fe N

N

B

Toluene/ Et3N Pd2(dba)3/AsPh3

F F 5 (26%)

N

N B

F

1

H

O

1) THF, N-Bromosuccinimide 2) CH2Cl2, DDQ 3) Et3N, BF3.OEt2

O

H

O

Fe

N

Toluene/ Et3N Pd2(dba)3/AsPh3

N B F F 6 (25%)

Br

F

N

N B F

F

Scheme 1. Synthetic scheme for the preparation of BODIPY-ferrocene conjugates 1e2.

2

Fe

T.K. Khan et al. / Journal of Organometallic Chemistry 697 (2012) 65e73

67

Fe

Br O O N Br O

NH HN

N 1) THF, N-Bromosuccinimide 2) CH2Cl2 , DDQ

H

N B

F 9 +

3) Et3N, BF3 .OEt2

F

Br

N B

F

F 3 (40%)

Fe

Fe

Br Toluene/ Et3 N Pd2(dba)3/ AsPh3

O

Fe

8 N Br

N B

F

10

F

O

Br N

N B

F Fe

F

4 (35%)

Fe

Scheme 2. Synthetic scheme for the preparation of BODIPY-ferrocene conjugates 3e4.

compounds 1 and 2 with meso-phenyl BODIPY-ferrocene conjugate IV is presented in Fig. 1. As clear from the Fig. 1 that in compound 1, the six pyrrole protons appeared as three sets of signals and the proton which is adjacent to meso-furyl group (type c) experienced 0.5 ppm downfield shift compared to type c proton of compound IV. This is attributed to the in-plane orientation of meso-furyl group with BODIPY core resulting in better conjugation in compound 1 compared to compound IV where meso-phenyl group is out-ofplane with BODIPY core. The ferrocenyl group appeared as sets of three signals in both compounds in 4.2e4.5 ppm region with slight changes in their chemical shifts. The three sets of signals for ferrocenyl protons were also observed earlier in the same region in BODIPY derivatives possessing one or two redox active vinyl ferrocene units [20e22]. Similarly, in compound 2, because of unsymmetrical substitution, the five pyrroles of BODIPY core appeared as five signals and ferrocene appeared as set of three signals. In compounds 3 and 4, due to increase in the number of ferrocenyl groups, the number of pyrrole signals decreased and the signals corresponding to ferrocenyl group increased. In all these conjugates, except change in the number of signals corresponding to BODIPY core and ferrocenyl group, there is no significant changes in their chemical shifts. Furthermore, the compounds 1e4 showed a quartet at approximately 147 ppm in 19F NMR and a triplet at w0.9 ppm in 11B NMR as observed for any other BODIPY systems. The compounds 1e4 were studied by absorption, electrochemical and fluorescence techniques. The comparison of absorption spectra of compounds 1 and 2 along with compound IV is shown in Fig. 2 and the relevant data for all compounds presented in Table 1. Generally, BODIPYs exhibit one strong absorption band at w500 nm along with a broad transition at higher energy region [28]. As clear from the absorption spectra, all compounds 1e4

including IV exhibited a sharp strong band at w530e570 nm. However, the meso-furyl BODIPY-ferrocene conjugates showed a bathochromic shift compared to compound IV due to presence of furyl group at meso-position which enhances p-electron delocalization and decreases the HOMO-LUMO energy gap. The bathochromic shifts observed among compounds 1e4 is due to presence of one or two ethynyl groups present at the BODIPY core. However, the most interesting feature of the absorption spectra of compounds 1e4 is the presence of broad ill-defined band at higher wavelength region which is assigned to intramolecular charge transfer (ICT) band due to charge transfer from ferrocene to BODIPY unit as observed earlier for vinyl and ethynyl substituted BODIPY ferrocence units [20e22]. The intensity of charge transfer band increases in intensity with the increase of the number of ferrocenyl groups on BODIPY core. The charge-transfer band is completely absent in compound IV indicating the absence of interaction between BODIPY core and ferrocenyl group. However, the compound 1 shows ill-defined charge transfer band supporting the interaction between ferrocenyl group and BODIPY core. The intense charge transfer band observed for compounds 2e4 supports strong interaction between the ferrocenyl and BODIPY units. Thus, the absorption study indicates that the ferrocenyl group(s) present directly on the BODIPY core involves in strong interaction; the ferrocenyl group present on the meso-furyl group show some interaction with BODIPY unit but the ferrocenyl group present at the meso-phenyl group does not interact with the BODIPY unit. The electrochemical properties of BODIPY-ferrocene conjugates 1e4 along with compound IV were followed by cyclic voltammetry and differential pulse voltammetry in CH2Cl2 using tetrabutylammonium perchlorate as supporting electrolyte. The comparison of cyclic voltammograms along with differential pulse

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Fig. 1. Comparison of 1H NMR spectra of compounds 1 and 2 along with IV in selected region recorded in CDCl3.

voltammograms of compounds 1e4 recorded using same concentration is presented in Fig. 3 and the data are presented in Table 1. In general, the BODIPYs exhibit one reversible reduction and one irreversible oxidation or sometimes no oxidation was noted. The earlier study revealed that meso-furyl BODIPYs are electron deficient and easier to reduce compared to meso-phenyl BODIPYs [26]. This trend is also observed for compounds 1e4 which are easier to reduce than meso-phenyl BODIPY IV. Furthermore, the conjugates

1.0

Absorbance

0.8

1e4 exhibit one reversible oxidation corresponding to ferrocene to ferrocenium ion in 0.65e0.69 V region which is shifted slightly to more positive compared to compound IV indicating that ferrocenyl group in compounds 1e4 are difficult to oxidize due to distribution of orbital coefficient of ferrocene(s) onto boron-dipyrromethene moiety. However, the earlier reported BODIPY-vinylferrocene conjugates I and II, the ferrocene to ferrocenium oxidation is relatively easier which is due to better electronic communication between BODIPY and vinylferrocene units [20,21]. These observations are in line with the absorption studies. Furthermore, it is clear that the number of electrons involved in the redox process for compounds 3 and 4 are different from compounds 1 and 2 (Fig. 3). This is due to the difference in the number of ferrocenyl groups present in compounds 1e4. To know the number of electrons involved in the oxidation process of compounds 1e4, we used the following Bard’s equation:

0.6 Table 1 Absorption and Electrochemical redox data of BODIPY-ferrocene conjugates 1e4 along with IV.

0.4

Absorption data

Electrochemical data BODIPY (Fc/Fþ c)

0.2 0.0 500

600

700

800

Wavelength (nm) Fig. 2. Comparison of normalized absorption spectra of compounds 1 () and 2 (.) along with IV (---) recorded in CHCl3.

C. No.

l (nm) (log ε)

C. T. band l (nm)

Eoxi 1/2 (V)

Ered 1/2 (V)

IV 1 2 3 4

360(sh) 505(4.6) 465(sh) 530(3.8) 436(4.2) 554(4.4) 478(4.1) 561(4.3) 478(4.0) 570(4.3)

e 610 646 668 690

0.635 0.696 0.650 0.664 0.653

0.750 0.616 0.625 0.568 0.541

T.K. Khan et al. / Journal of Organometallic Chemistry 697 (2012) 65e73

Fig. 3. Comparison of cyclic voltammogram of compounds (a) 1, (b) 2, (c) 3 and (d) 4 along with differential pulse voltammogram recorded in CH2Cl2 containing 0.1 M TBAP as the supporting electrolyte recorded at a scan speed of 50 mV/s.

nd ¼ ðIc =Ir ÞðCc =Cr ÞðMc =Mr Þ0:275 Ic, Cc, Mc are the intensities, concentrations and molecular masses of compounds 1e4 and Ir, Cr, Mr for free ferrocene reference respectively. Thus, according to the above equation, the number of electrons involved in the oxidation of ferrocene to ferrocenium ion are found to be one for compounds 1 and 2, two for compound 3 and three for compound 4. This indicates that compounds 3 and 4 which contain two and three ferrocene groups respectively undergo oxidation at the same potential. The steady-state fluorescence properties of BODIPY-ferrocene conjugates 1e4 were investigated. The studies indicated that BODIPY-ferrocene conjugates are very weakly fluorescent because of electron transfer from ferrocene moiety to BODIPY unit [20e22,24]. Our earlier studies on BODIPY-ferrocene conjugate III showed that the fluorescence can be restored if electron transfer from ferrocene to BODIPY is eliminated by oxidizing ferrocene to ferrocenium ion using any oxidizing agent whereas it cannot be

69

restored in compound IV [22]. We attributed this to lack of communication between BODIPY and ferrocene moieties in compound IV because of orthogonal arrangement of ferrocene substituted meso-phenyl group with BODIPY. However, we anticipated that the fluorescence of compound 1 in which ferrocene is attached to meso-furyl group can be restored since meso-furyl group is more in-plane orientation with BODIPY and the absorption and electrochemical studies supports the interaction between BODIPY and ferrocene moieties. Thus, we carried out fluorescence titration of compound 1 as well as reference compound IV in CHCl3 with increasing amounts of Fe(ClO4)3 and presented in Fig. 4. Upon addition of increasing amounts of Fe(ClO4)3 to compound IV, there is no significant increase in the fluorescence supporting our earlier observations. However, compound 1 on titration with Fe(ClO4)3 showed a significant increase in the intensity of fluorescence band at w600 nm and the quantum yield reaches saturation for the completely oxidized compound. This is clearly evident in the colours of the solution for compounds IV and 1 before and after oxidation. The compound IV did not show any change in its colour before and after oxidation of ferrocene moiety whereas the colour of the compound 1 changes from light pink to red. Similar observations were made with compounds 2e4 in which one or two ferrocene moieties are connected directly to BODIPY core. Thus, our studies clearly indicate that the compound IV which cannot be used as redox fluorescent switch because of lack of interaction between fluorophore and redox active group can be transformed to redox switch by using appropriate spacer such as furyl which helps in bringing necessary interaction between two moieties as shown with BODIPY-ferrocene conjugate 1. To understand the differences in the electronic properties of two ferrocene BODIPYs 1 and IV, the quantum chemical calculations were performed using DFT method with a ccdz-pv** þ basis set. For the optimized structures, full population analyses studies were done and the Table 2 briefs the HOMO-LUMO energy data of compounds 1, IV and their oxidized products 1þ and IVþ. The contours of electronic distribution in HOMO and LUMO on these molecules suggested that there were not very significant differences between compounds 1 and IV. For both the compounds 1 and IV, it was observed that orbital coefficient in the HOMO is located majorly in the ferrocene part of the molecules while it gets delocalized over other half portion of molecule in their LUMO (Fig. 5a). In particular, for the compound 1, the meso-2-furyl ring retains much of orbital coefficient as compared to phenyl ring of compound IV. This is in total contrast to the orbital coefficient pattern of simple meso-2-furyl BODIPY [25] and meso-4-phenyl BODIPY [28] derivatives where the orbital coefficients are majorly located in BODIPY core. The comparison of the HOMO-LUMO gap energies suggests that the compound 1 would absorb at slightly higher wavelength (Table 2). Similarly, the orbital coefficient in HOMO-LUMO of their corresponding oxidized products 1þ and IVþ were also calculated. It was observed that oxidation of ferrocene to ferrocenium ion in both compounds 1þ and IVþ, the orbital coefficient in HOMO-LUMO are remarkably perturbed. As clear from the Fig. 5b, that the orbital coefficient in the HOMO shift towards the BODIPY portion in compound 1þ which is like meso-2-furyl BODIPY. This could be the most probable reason why compound 1 fluoresces on oxidation. However, the magnitude of electronic delocalization is not to the same extent in compound IVD which explains the distinct behaviour of compound IV on oxidation. 3. Conclusions In conclusion, we synthesized four meso-furyl BODIPY-ferrocene conjugates 1e4 by coupling the functionalized meso-furyl BODIPYs

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a

4.0 3.5

Intensity

3.0 2.5 2.0 1.5 1.0 0.5 0.0 550

600

650

700

Wavelength (nm)

b

0.0010

Fe(CIO4)3

6

Intensity

5 4

Quantumyield

7

0.0008 0.0006 0.0004 0.0002 0.0000

3

0

100

200

300

400

Fe(ClO4 )3

2 1 0 550

600 650 Wavelength (nm)

700

750

Fig. 4. Fluorescence spectral changes of (a) IV and (b) 1 (10 mM) on the addition of Fe(ClO4)3 in CHCl3 (lex ¼ 488 nm). The concentration of Fe(ClO4)3 was varied from 0 to 300 mM. The inset show the plot of 4F as a function of concentration of Fe(ClO4)3.

with ethynylferrocene under mild Pd(0) coupling conditions. The number of ferrocenyl group(s) connected to BODIPY was varied from one to three by connecting the ferrocene group(s) to the mesofuryl group as well as directly to the BODIPY core. In all four compounds, the absorption studies indicated the presence of charge transfer band because of electron transfer from ferrocene to BODIPY core. This further supports the interaction between the ferrocene and BODIPY moieties in compounds 1e4. However, in meso-phenyl BODIPY compound in which the ferrocene is attached to meso-phenyl group, the charge transfer band is absent indicating that the two moieties in meso-phenyl BODIPY do not interact strongly. The fluorescence properties of compounds 1e4 indicated

that the BODIPY fluorescence is completely quenched due to electron transfer from ferrocene to BODIPY core. However, the fluorescence can be restored for compounds 1e4 by oxidizing ferrocene to ferrocenium ion which stops the electron flow between ferrocene and BODIPY moieties. Under similar experimental conditions, the quenched fluorescence of meso-phenyl BODIPY cannot be restored because of orthogonal arrangement of meso-phenyl group with BODIPY core which prevents interaction between fluorophore and redox active group. The computational data supported the experimental results. More studies are required to quantify these observations. 4. Experimental section

Table 2 HOMO-LUMO energy gaps of compounds 1, IV and their oxidized products 1þ and IVþ. C. No.

HOMO

LUMO

HOMO-LUMO gap energy

1 1D IV IVD

137.068 185.177 135.509 182.305

75.158 157.272 72.997 154.277

61.910 27.904 62.512 28.029

Note: all values in Kcal/mol.

4.1. Chemicals THF and toluene were dried over sodium benzophenone ketyl and chloroform was dried over calcium hydride prior to use. BF3$OEt2 and 2,3-dichloro-5,6-dicyano-1,4- benzoquinone (DDQ) obtained from Spectrochem (India) were used as obtained. All other chemicals used for the synthesis were reagent grade unless otherwise specified. Column chromatography was performed on silica (60e120 mesh) or alumina.

T.K. Khan et al. / Journal of Organometallic Chemistry 697 (2012) 65e73

71

spectra were recorded on Varian spectrometer operating at 96.3 MHz. TMS was used as an internal reference for recording 1H (of residual proton; d 7.26) and 13C (d 77.0 signal) in CDCl3. Absorption and steady-state fluorescence spectra were obtained with PerkineElmer Lambda-35 and PC1 Photon Counting Spectrofluorometer manufactured by ISS, USA instruments respectively. Fluorescence spectra were recorded at 25  C in a 1 cm quartz fluorescence cuvette. The fluorescence quantum yields (Ff) were estimated from the emission and absorption spectra by comparative method at the excitation wavelength of 488 nm using Rhodamine 6G (Ff ¼ 0.88) [29] as standard. Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) studies were carried out with electrochemical system utilizing the three electrode configuration consisting of a glassy carbon (working electrode), platinum wire (auxillary electrode) and saturated calomel (reference electrode) electrodes. The experiments were done in dry dichloromethane using 0.1 M tetrabutylammonium perchlorate as supporting electrolyte. Half wave potentials were measured using DPV and also calculated manually by taking the average of the cathodic and anodic peak potentials. High-resolution mass spectrum was obtained from Q-TOF instrument by electron spray ionization (ESI) technique. 4.3. Computational studies

Fig. 5. The HOMO-LUMO and energy gaps for compounds (a) 1 and IV; (b) 1þ and IVþ

4.2. Instrumentation 1

H NMR spectra (d in ppm) were recorded using Varian VXR 300, 400 MHz and Bruker 400 MHz spectrometer. 13C NMR spectra were recorded on Bruker operating at 100.6 MHz 19F NMR spectra were recorded on Varian spectrometer operating at 282.2 MHz 11B NMR

I

I

N

B

F

N

N

F

F

B

N F Fe

Fe

Fe

II

I Fe CH3

N

N B

F

F III

Fe

N F

N B F IV

Chart 1. Structures of compounds IeIV.

The QM calculations were carried out using the program Jaguar [30] v7.7 in the Schrödinger Suite 2010 (Schrödinger LLC, USA) running on Intel Xeon processor based workstation with Cent OS 5.5 Enterprise Linux. The structures of the Compounds 1 and IV (Chart 1) were generated in silico by attaching ferrocene moiety to meso-phenyl [28] and meso-2-furyl [25] BODIPY moieties. The structures were energy optimized using quantum mechanics with density functional theory (DFT) [31] and B3LYP [32] gradient corrected correlation functional method in conjugation with ccdzpv**þ basis set [33] and parameters. 4.3.1. Meso-(5-bromo-2-furyl)-dipyrromethane (7) In a 250 mL round-bottomed flask, freshly distilled 5-bromo furfural (1 g, 5.7 mmol) and pyrrole (15.8 mL, 0.22 mmol) were dissolved in 50 mL dichloromethane under an argon atmosphere. BF3$OEt2 (72 mL, 0.57 mmol) was added to initiate the reaction and the reaction mixture was stirred for 15 min at room temperature. The reaction mixture was diluted with CH2Cl2 and washed thoroughly with 0.1 M NaOH solution and water. The organic layers were combined, dried over Na2SO4 and concentrated on a rotary evaporator. The crude compound was subjected to silica gel column using petroleum ether/ethylacetate (95/5, v/v) and afforded pure meso-(5-bromo-2-furyl)-dipyrromethane 7 as a white solid in 60% yield (1 g). M. p. 120  C; 1H NMR (400 MHz, CDCl3, d in ppm): 5.45 (s, 1 H, eCH), 6.09 (s, 2 H, b-Py), 6.13 (d, J ¼ 3.2 Hz, 1 H, furan), 6.16 (d, J ¼ 2.9 Hz, 2 H, b-Py), 6.21 (d, J ¼ 3.2 Hz, 1 H, furan), 6.69 (s, 2 H, b-Py). 13C NMR (100 MHz, CDCl3, d in ppm): 38.16, 107.19, 107.39, 108.59, 108.63, 109.82, 112.15, 117.85, 117.96, 121.02, 129.08, 129.36, 156.48. LCMS calcd for (C13H11BrN2O): m/z 290.00 found: [M]þ 290.03. 4.3.2. 4,4-Difluoro-8-(5-bromo-2-furyl)-4-bora-3a,4a-diazas-indacene (5) Compound 7 (500 mg, 1.71 mmol) was taken in dichloromethane (30 mL) and oxidized with DDQ (390 mg, 1.71 mmol) at room temperature. The reaction was allowed to stir for 1 h at room temperature. Triethylamine (10 mL, 68.7 mmol) followed by BF3$OEt2 (10.8 mL, 85.9 mmol) was added, and stirred at room temperatutre for 30 min. The solvent was removed on rotary evaporator and the resultant crude compound was purified by

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silica gel column chromatography using petroleum ether/ethyl acetate (95/5, v/v) to afford pure compound 5 as dark red solid in 26% yield. (150 mg) 1H NMR (400 MHz, CDCl3, d in ppm): 6.59 (s, 2 H, b-Py), 6.66 (d, J ¼ 2.9 Hz, 1 H, furan), 7.14 (d, J ¼ 2.9 Hz, 1 H, furan), 7.44 (s, 2 H, b-Py), 7.90 (s, 2 H, b-Py). 13C NMR (100 MHz, CDCl3, d in ppm): 115.58, 118.71, 122.69, 129.57, 130.51, 131.34, 131.94, 143.55, 150.39. HR-MS mass calcd. for (C13H8BBrF2N2O): 336.9945 found 336.9959. 4.3.3. 3-Bromo-8-(2-furyl)-4-bora-3a,4a-diaza-s-indacene (6) This compound was prepared in sequence of steps in one pot reaction. Meso-furyl- dipyrromethane [26] 8 (500 mg, 2.32 mmol) was treated with one equivalent of N-bromosuccinimide (413 mg, 2.32 mmol) in dry THF (50 mL) at 78  C under nitrogen for 1 h. The reaction mixture was brought to room temperature and subjected to flash column chromatography using CH2Cl2. The solvent was removed on rotary evaporator under vacuum to obtain concentrated solution of brominated meso-furyl- dipyrromethane which was treated with DDQ (527 mg, 2.32 mmol) at room temperatute for 1 h. Triethylamine (12.9 mL, 93 mmol) and BF3$Et2O (14.6 mL, 116 mmol) was then added and stirred the reaction mixture for additional 1 h at room temperature. The reaction mixture was washed successively with 0.1 M NaOH solution and water. The organic layers were combined, dried over Na2SO4, filtered, and evaporated. The crude compound was subjected to silica gel column chromatography and the required a-bromo derivative of BODIPY 6 was collected as second band using petroleum ether/ dichloromethane (95:5). The solvent was removed on rotary evaporator under vacuo and afforded pure 6 as red powder in 25% yield. (200 mg) 1H NMR (400 MHz, CDCl3, d in ppm): 6.58 (m, 1 H, bPy), 6.69 (d, J ¼ 4.3 Hz, 2 H, b-Py þ furan), 7.12 (d, J ¼ 3.6 Hz, 1 H, furan), 7.38 (d, J ¼ 4.0 Hz, 1 H, b-Py), 7.46 (d, J ¼ 4.3 Hz, 1 H, b-Py), 7.81 (s, 1 H, furan), 7.91 (s, 1 H, b-Py). HR-MS mass calcd. for (C13H8BBrF2N2O): 336.9945 found 336.9959. 4.4. General procedure for the synthesis of BODIPY-ferrocene conjugates 1e4 Ferrocenyl acetylene and appropriate BODIPY containing bromo functional group 5e8 were dissolved in dry toluene/Et3N (6 mL, 5:1) in a 25 mL, two-necked, round-bottomed flask fitted with a reflux condenser, gas inlet and gas outlet tubes for nitrogen purging. The reaction vessel was placed in an oil bath preheated to 35  C. After purging the flask with nitrogen for 15 min, AsPh3 (3.5 equiv.) and Pd2(dba)3 (0.44 equiv.) were added, and the reaction mixture was stirred at 50  C for 2 h. TLC analysis of the reaction mixture indicated the appearance of a dark new spot apart from the two minor spots corresponding to starting precursors. The solvent was removed under reduced pressure, and the crude compound was purified by silica gel column chromatography. The excess AsPh3 and the small amounts of unreacted starting precursors were removed with petroleum ether and the required pure BODIPYferrocene conjugates were then collected with petroleum ether/ ethyl acetate. 4.4.1. BODIPY-ferrocene conjugate 1 Column chromatographic purification on silica using petroleum ether/dichloromethane (80/20) afforded compound 1 as green solid in 60% yield. 1H NMR (400 MHz, CDCl3, d in ppm): 4.29 (s, 5 H), 4.36 (s, 2 H), 4.60 (s, 2 H), 6.60 (s, 2 H, b-Py), 6.82 (d, J ¼ 3.0 Hz, 1 H, furan), 7.24 (d, J ¼ 3.3 Hz, 1 H, furan), 7.53 (s, 2 H, b-Py), 7.88 (s, 2 H, b-Py). 13C NMR (100 MHz, CDCl3, d in ppm): 149.02, 148.62, 132.02, 130.44, 128.06, 130.44, 122.58, 118.47, 117.55, 98.85, 75.57, 72.07, 70.50, 70.09. 19F NMR (282.2 MHz, CDCl3, d in ppm): 145.5 (q, JB-F ¼ 56.4 Hz). 11B NMR (96.3 MHz, CDCl3, d in ppm): 0.47 (t,

JB-F ¼ 28.2 Hz). HR-MS mass calcd. for C25H17BF2FeN2O 466.0761 found 466.0751. 4.4.2. BODIPY-ferrocene conjugate 2 Column chromatographic purification on silica using petroleum ether/dichloromethane (85/15) afforded compound 2 as blue solid in 50% yield. 1H NMR (400 MHz, CDCl3, d in ppm): 4.34 (s, 5 H), 4.38 (t, J ¼ 1.8, 2 H), 4.67 (t, J ¼ 1.8 Hz, 2 H), 6.58 (m, 1 H, b-Py), 6.69 (d, J ¼ 4.3 Hz, 2 H, b-Py D furan), 7.12 (d, J ¼ 3.6 Hz, 1 H, furan), 7.38 (d, J ¼ 4.0 Hz, 1 H, b-Py), 7.46 (d, J ¼ 4.3 Hz, 1 H, b-Py), 7.81, (s, 1 H, furan), 7.91 (s, 1 H, b-Py). 13C NMR (100 MHz, CDCl3, d in ppm): 149.03, 147.23, 143.52, 142.25, 134.92, 130.81, 129.52, 128.73, 125.59, 123.73, 119.61, 118.16, 113.38, 104.87, 80.31, 72.50, 70.71, 70.31. 19F NMR (282.2 MHz, CDCl3, d in ppm): 145.4 (q, JB-F ¼ 56.4 Hz). 11B NMR (96.3 MHz, CDCl3, d in ppm): 0.71 (t, JB-F ¼ 27.3 Hz). HR-MS mass calcd. for C25H17BF2FeN2O 467.0830 found m/z 467.0829. 4.4.3. BODIPY-ferrocene conjugate 3 Column chromatographic purification on silica using petroleum ether/dichloromethane (85/15) afforded compound 3 as a purple solid in 40% yield. 1H NMR (400 MHz, CDCl3, d in ppm): 4.30 (s, 5 H), 4.34e4.37 (m, 7 H), 4.38 (s, 2 H), 4.60 (t, J ¼ 1.8, 2 H), 4.68 (t, J ¼ 1.8 Hz, 2 H), 6.58 (d, J ¼ 4.0 Hz, b-Py), 6.71 (d, J ¼ 4.0 Hz, 1 H, bPy), 6.81 (d, J ¼ 3.6 Hz, 1 H, furan), 7.16 (d, J ¼ 3.6 Hz, 1 H, furan), 7.42 (d, J ¼ 4.0 Hz, 1 H, b-Py), 7.52 (d, J ¼ 4.0 Hz, 1 H, b-Py), 7.90 (s, 1 H, bPy). 13C NMR (100 MHz, CDCl3, d in ppm): 62.54, 63.54, 70.02, 70.38, 70.49, 70.73, 72.03, 72.53, 117.47, 118.12, 121.38, 123.84, 128.35, 128.67, 130.55, 132.09, 132.35, 133.92, 138.17, 141.84, 142.45, 149.01. 19 F NMR (282.2 MHz, CDCl3, d in ppm): 145.1 (q, JB-F ¼ 56.8). 11B NMR (96.3 MHz, CDCl3, d in ppm): 0.72 (t, JB-F ¼ 28.2 Hz). HR-MS mass calcd. for C37H25BF2Fe2N2O 674.0727 found m/z 674.0744. 4.4.4. BODIPY-ferrocene conjugate 4 Column chromatographic purification on silica using petroleum ether/dichloromethane (90/10) afforded compound 4 as blue solid in 35% yield. 1H NMR (400 MHz, CDCl3, d in ppm): 4.30 (s, 5 H), 4.35e4.36 (m, 6 H), 4.38 (s, 10 H), 4.60 (t, J ¼ 1.8 Hz, 2 H), 4.68 (t, J ¼ 1.8 Hz, 4 H), 6.70 (d, J ¼ 4.2 Hz, 2 H, b-Py), 6.79 (d, J ¼ 3.6 Hz, 1 H, furan), 7.07 (d, J ¼ 3.6 Hz, 1 H, furan), 7.41 (d, J ¼ 3.9 Hz, 2 H, b-Py). 19 F NMR (282.2 MHz, CDCl3, d in ppm): 145.8 (q, JB-F ¼ 56.5 Hz). 11B NMR (96.3 MHz, CDCl3, d in ppm): 0.97 (t, JB-F ¼ 26.9 Hz). HR-MS mass calcd. for (C49H33BF2Fe3N2O): 882.0702 found m/z 882.0694. Acknowledgments MR thanks Department of Atomic Energy (DAE-BRNS) for financial support. TKK thanks Indian Institute of Technology, Bombay for fellowship. Authors also thank, Dr. Evans C. Coutinho at Bombay College of Pharmacy for providing computational facilities. Appendix. Supplementary material Complete characterization data of compounds, 1H, 1H-1H COSY NMR and 13C NMR spectra. Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j. jorganchem.2011.10.016. References [1] P.D. Beer, C. Blackburn, J.F. McAleer, H. Sikanyika, Inorg. Chem. 29 (1990) 378e381. [2] P.D. Beer, E.L. Tite, A. Ibbotson, J. Chem. Soc. Dalton Trans. (1991) 1691e1698. [3] H. Plenio, R. Diodone, J. Organomet. Chem. 492 (1995) 73e80. [4] P.D. Beer, Chem. Soc. Rev. 18 (1989) 409e450. [5] S. Rai, G. Gayatri, G.N. Sastry, M. Ravikanth, Chem. Phys. Lett. 467 (2008) 179e185.

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