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Synthesis, structure and spectral and electrochemical properties of 3-pyrrolyl BODIPY-metal dipyrrin complexes† V. Lakshmi,a Way-Zen Leeb and M. Ravikanth*a The stable dipyrrin substituted 3-pyrrolyl BODIPY (α-dipyrrin 3-pyrrolyl BODIPY) was synthesized by oxidation of dipyrromethane substituted 3-pyrrolyl BODIPY with 2,3-dichloro-5,6-dicyano-benzoquinone in CH2Cl2 at room temperature. The α-dipyrrin 3-pyrrolyl BODIPY is characterized by using HR-MS, 1D, 2D NMR and absorption spectroscopic techniques. The absorption spectrum of α-dipyrrin 3-pyrrolyl BODIPY showed a characteristic absorption band at 630 nm and a charge transfer band at 717 nm due to intramolecular charge transfer from the dipyrrin unit to the 3-pyrrolyl BODIPY unit. The 3-pyrrolyl BODIPYmetal dipyrrin complexes (Pd(II), Re(I) and Ru(II)) were prepared by treating α-dipyrrin 3-pyrrolyl BODIPY with appropriate metal salts in toluene–triethylamine at 100 °C and purified by silica gel column chromatography. The crystal structure obtained for the 3-pyrrolyl BODIPY-Pd(II) dipyrrin complex showed that the 3-pyrrolyl BODIPY and metal dipyrrin moieties are aligned to each other with an angle of 41.9°. The

Received 30th June 2014, Accepted 28th August 2014

absorption studies showed a strong band at ∼620 nm corresponding to 3-pyrrolyl BODIPY moiety and a

DOI: 10.1039/c4dt01970a

weak band at ∼530 nm corresponding to metal dipyrrin unit with complete disappearance of the charge transfer band at 717 nm. The complexes are electron deficient and exhibited only reversible/quasi-revers-

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ible reductions in cyclic voltammetry.

Introduction Pyrrolyl dipyrrin or prodigiosin possess potent immunosuppressive, antimicrobial, antimalarial and cytotoxic properties and are isolated from microorganisms.1 Because of their biological activity, many pyrrolyl dipyrrins have been isolated, synthesized and used for various studies.2 BF2 complexes of pyrrolyl dipyrrins have been used as biological labels because of their excellent photophysical properties, and the Invitrogen company markets several types of BF2-complexes of pyrrolyl dipyrrins3 in small quantities at rather expensive prices. In recent years, we and other researchers developed very simple facile routes for the synthesis of BF2 complexes of meso-aryl substituted pyrrolyl dipyrrins 1 (Chart 1) and showed that the presence of pyrrolyl group at 3-position of BODIPY resulted in better photophysical properties.4 Subsequently, we also showed that the α-position of uncoordinated pyrrole group of 3-pyrrolyl BODIPY can be functionalized with functional

a Department of Chemistry, Indian Institute of Technology, Bombay, Powai, Mumbai 400 076, India. E-mail: [email protected] b Instrumentation Center, Department of Chemistry, National Taiwan Normal University, Ting-Chow Road, Taipei, 11677, Taiwan † Electronic supplementary information (ESI) available: Spectral data. CCDC 1005193. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4dt01970a

16006 | Dalton Trans., 2014, 43, 16006–16014

groups, such as bromo, formyl, cyano, nitro and trimethylsilylacetylene groups, to obtain very useful functionalized5 3-pyrrolyl BODIPYs 2 (Chart 1). Furthermore, we also showed that the 3-pyrrolyl BODIPY containing a formyl functional group at

Chart 1

Molecular structures of 3-pyrrolyl BODIPY based systems.

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Scheme 1

Paper

Synthesis of α-dipyrrin 3-pyrrolyl BODIPY 4.

α-position of an appended pyrrole ring can be used to prepare 3-dipyrromethanyl BODIPY 3 (Scheme 1) by treating it with excess pyrrole under acid catalyzed conditions.5 During these studies, we realized that it is possible to isolate stable α-dipyrrin 3-pyrrolyl BODIPY 4 (Scheme 1) by oxidizing 3-dipyrromethanyl BODIPY 3 with DDQ under standard conditions. The dipyrrins are known to form metal complexes because of the presence of two nitrogen donor atoms, which helps in the complexation to the metals, and thus dipyrrins have emerged as versatile ligands in coordination chemistry.6 Thus, it is possible to use α-dipyrrin 3-pyrrolyl BODIPY 4 as a ligand to synthesize 3-pyrrolyl BODIPY-metal dipyrrin complexes. Such multifunctional systems containing BODIPY and metal dipyrrin units are quite interesting from several view points. For example, the multicomponent systems made of subunits belonging to such classes of compounds can exhibit photoinduced intercomponent electron and/or energy transfer processes, possibly leading to valuable functions such as charge separation and/or energy migration.7 Furthermore, linking of metal dipyrrin system to the light absorbing unit, such as BODIPY, would enhance the excited state energy transfer to the triplet states and such complexes would have applications in electroluminescence,6c,8 luminescent molecular probes,9 photocatalysis,10 triplet–triplet annihilation upconversion.11 A perusal of literature revealed that there are very few examples available on covalently linked BODIPY-metal dipyrrin complexes.12 To the best of our knowledge, there is no report on 3-pyrrolyl BODIPY-metal dipyrrin complexes. Because of our interest in 3-pyrrolyl BODIPY chemistry, we explored the coordination chemistry of α-dipyrrin 3-pyrrolyl BODIPY 4 to synthesize 3-pyrrolyl BODIPY-metal dipyrrin complexes. Herein, we report the synthesis and characterization of first examples of 3-pyrrolyl BODIPY-M(II/I) dipyrrin M4 (M = Pd(II), Re(I) and Ru(II)) complexes (Chart 1). The spectral and electrochemical properties of these 3-pyrrolyl BODIPY-metal dipyrrin complexes are also described.

Results and discussion Synthesis and characterization of ligands 3 and 4 The α-formyl 3-pyrrolyl BODIPY 2a was prepared by treating 3-pyrrolyl BODIPY 1a under Vilsmeier–Haack reaction con-

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ditions as reported earlier.5 The dipyrromethane substituted 3-pyrrolyl BODIPY 3 was prepared by treating 2a with excess pyrrole in CH2Cl2 under mild acid catalyzed conditions followed by column chromatographic purification on silica gel and afforded 3 as dark solid in 85% yield (Scheme 1). The compound 3 was characterized by HR-MS, 1H, 19F, 11B NMR and absorption techniques. In 1H NMR of compound 3, the meso-phenyl protons appeared as a multiplet in the 7.48–7.53 region and the pyrrole protons appeared as nine sets of signals. The meso CH proton appeared as a singlet at 5.67 ppm and the two NH protons of dipyrromethane moiety appeared as broad singlet at 8.19 ppm, whereas the bridging pyrrole NH proton appeared as a singlet at 10.4 ppm. The pyrrole protons of compound 3 were identified and assigned based on detailed COSY and NOESY studies (Fig. 1). The α-pyrrole proton of BODIPY moiety (a-type) of compound 3 appeared as a broad singlet in the downfield region at 7.62 ppm as shown in Fig. 1b. The a-type proton signal at 7.62 ppm showed crosspeak correlation with a resonance at 6.42 ppm, which we identified as b-type pyrrole proton of the BODIPY moiety. The resonance at 6.62 ppm was due to c-type pyrrole proton of BODIPY moiety as this resonance showed cross-peak correlation with b-type resonance at 6.42 ppm and also with NOE correlation with multiplet resonance of meso-phenyl protons (Fig. 1b). Furthermore, the multiplet resonance (7.48–7.53 ppm) of meso-phenyl protons showed NOE correlation with a signal at 6.92 ppm, which we identified as the d-type pyrrole proton of BODIPY moiety of compound 3. The resonance at 6.86 ppm was identified as the e-type pyrrole proton as this resonance showed cross peak correlation with the d-type pyrrole proton at 6.92 ppm. The e-type resonance at 6.86 ppm showed NOE correlation with a resonance at 6.96 ppm, which we identified as f-type proton of the bridging pyrrole. The multiplet resonance in 6.19–6.22 ppm region was due to the g-type bridging pyrrole proton as this resonance showed cross-peak correlation with the f-type bridging pyrrole proton at 6.96 ppm (Fig. 1b). The dipyrromethanyl CH protons at 5.67 ppm showed cross-peak correlation with a multiplet resonance in 6.13–6.16 ppm region, which we recognized as the h-type proton of dipyrromethane. The multiplet resonance in 6.19–6.22 ppm region showed cross-peak correlation with multiplet resonance in 6.13–6.16 region, which we identified as the i-type pyrrole protons of dipyrromethanyl moiety along

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Fig. 1 (a) The 1H NMR comparison of compounds (i) 3 and (ii) 4; (b) NOESY spectrum of compound 3 in the selected region recorded in CDCl3 (δ in ppm).

with the g-type bridging pyrrole proton. The resonance at 6.75 ppm was identified as the j-type pyrrole protons of dipyrromethanyl moiety as this resonance showed cross-peak correlations with i-type, as well as NH protons (Fig. 1b). Thus, we identified all the resonances of compound 3 by detailed 1D and 2D NMR spectral analysis. The compound 3 showed strong absorption band at 591 nm along with a shoulder band at 554 nm (Fig. 2) and also a band at 408 nm due to S0→S2 transition, which is commonly noted for BODIPYs.13 However, compared to 3-pyrrolyl BODIPY4a 1a, the compound 3 experienced ∼15 nm bathochromic shift in its absorption band maxima. The compound 3 showed a strong emission band at 622 nm with a quantum yield of 0.16. In the next step, the compound 3 was oxidized by DDQ followed by column chromatographic purification to afford the desired α-dipyrrin 3-pyrrolyl BODIPY ligand 4 in 96% yield. The HR-MS peak at 476.1856 confirmed the composition of compound 4. In 1H NMR of compound 4, the CH proton at 5.67 ppm disappeared, and the other protons of the 3-pyrrolyl BODIPY and dipyrrin moieties, especially the α-protons of dipyrrin moiety, experienced downfield shifts compared to those of compound 3, which is attributed to the enhanced π-delocalization. The NH protons of the dipyrrin moiety, as well as the bridging pyrrole NH proton were not observed in the 1H NMR spectrum of compound 4 because of their involvement in rapid tautomerism. In the absorption spectrum, the

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Fig. 2 Normalized absorption spectra of compounds 3 (dashed line) and 4 (dotted line) along with protonated dipyrrin 4H+ (solid line) recorded in CH2Cl2.

compound 4 showed a bathochromically shifted S0→S1 transition at 630 nm along with a shoulder band at 589 nm and S0→S2 transitions at higher energy side (Table 2). In addition, we also noted a charge transfer band at 717 nm due to intramolecular charge transfer from the dipyrrin moiety to the 3-pyrrolyl BODIPY moiety (Fig. 2). We subjected the compound 4 for systematic protonation studies by titrating compound 4

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Scheme 2

Paper

Synthesis of 3-pyrrolyl BODIPY-metal dipyrrin complexes, Pd4, Re4 and Ru4.

with dilute TFA (ESI†). Upon protonation, the absorption bands of 4·H+ were bathochromically shifted with disappearance of the charge transfer band at 717 nm compared to that of compound 4. Synthesis and characterization of metal complexes Pd4, Re4 and Ru4 The dipyrrins are known to form complexes with metal ions and the coordination chemistry of dipyrrins is well developed. We explored the complexation ability of compound 4 with various metal salts under standard reaction conditions.14 Though we carried out various metal complexation reactions of compound 4, we were successful in obtaining only Pd4, Re4 and Ru4 complexes (Scheme 2). The compound 4 was treated with Pd(acac)2 in toluene in the presence of triethylamine at 100 °C for 1 h followed by column chromatographic purification on silica and afforded Pd4 in 52% yield. The Re4 was prepared by treating compound 4 with Re(CO)5Cl in toluene– triethylamine for 1 h at 100 °C followed by addition of one equivalent of PPh3 for the exchange of chloride ions and stirred at 100 °C for additional 1 h. The crude reaction mixture was purified by silica gel column chromatography and afforded pure Re4 in 54% yield. Similarly, compound 4 was reacted with [Ru( p-cymene)Cl2]2 under similar reaction conditions and the crude compound was purified to afford Ru4 in 52% yield. The BODIPY-metal dipyrrin complexes Pd4, Re4 and Ru4 are freely soluble in common organic solvents and their identities were confirmed by corresponding molecular ion peaks in HR-MS. X-ray crystallography of Pd4 We attempted to grow single crystals for the BODIPY-metal dipyrrin complexes and fortunately obtained the single crystal

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suitable for X-ray diffraction for Pd4. The crystal was obtained by slow diffusion of n-hexane into CH2Cl2 solution for Pd4 at room temperature over a period of 10 days. The compound Pd4 was crystallized in monoclinic with a P21/c space group and the crystal structure is shown in Fig. 3. The crystallographic parameters are presented in Table 1. The boron complexed pyrrolyl dipyrrin moiety and palladium complexed dipyrrin moiety in Pd4 were arranged in two different planes with an angle of 41.9°. In Pd4, all the three pyrrole rings of pyrrolyl BODIPY moiety are in one plane unlike 3-pyrrolyl BODIPY4b 1b where the pyrrole group at 3-position is slightly deviated from the BF2-dipyrrin plane. The torsional angle between the meso-phenyl group and boron complexed 3-pyrrolyl dipyrrin moiety is 53.2° in Pd4, whereas it was 51.6° in 3-pyrrolyl BODIPY 1b. Similar to 3-pyrrolyl BODIPY4b 1b, in Pd4 complex, the 3-pyrrolyl NH is involved in hydrogen bonding with fluorides of BF2. However, F1-HN3 and F2-HN3 hydrogen bond distances are almost equal (2.334 Å and 2.243 Å respectively) unlike 3-pyrrolyl BODIPY 1b in which the F1-HN3 and F2-HN3 hydrogen bond distances are quite different from each other (2.093 Å, 2.453 Å respectively). The bond distance between boron dipyrrin unit and pyrrole (C19–C18) is slightly shortened in Pd4 by 0.01 Å compared to 3-pyrrolyl BODIPY 1b. The B-F1 and B-F2 bond distances are 1.385 Å and 1.362 Å, respectively, in Pd4. Whereas these bond distances are equal in 3-pyrrolyl BODIPY 1b (1.388 Å). Furthermore, the palladium complexed dipyrrin moiety of Pd4 showed a 42.6° torsional angle with respect to the meso-pyrrole group. The Pd(II) ion is almost in square planar geometry by coordinating with one acetylacetonate (acac) unit and two nitrogens in same plane. The bond angles N1–Pd–N2 (89.47°), O1–Pd–O2 (92.47°), N1– Pd–O2 (89.60°), N1–Pd–O1 (178.38°), N2–Pd–O2 (177.77°) are also supporting the square planar geometry at Pd(II) center in

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Pd4. The Pd–N1 and Pd–N2 bond distances are 1.966 Å and 1.967 Å, while Pd–O1 and Pd–O2 bond distances are 1.985 Å and 2.013 Å, respectively. These Pd–O and Pd–N bond distances are slightly shorter than those of other reported complexes such as [FcPd(acac)].15 The compounds Pd4, Re4 and Ru4 were characterized in detail by using 1D and 2D NMR, absorption and cyclic voltammetry techniques. The comparison of 1H NMR spectra of Pd4, Re4 and Ru4 are shown in Fig. 4a and NOESY spectrum along with assignments for Pd4 is shown in Fig. 4b. As shown in Fig. 4a, all BODIPY-metal dipyrrin complexes show similar 1H NMR features like BODIPY-dipyrrin 4. All resonances were identified and assigned using 1D and 2D NMR techniques. In the 1H NMR of BODIPY-metal dipyrrin complexes (Pd4, Re4, Ru4), the resonances, especially the protons of metal dipyrrin moiety, experienced slight shifts in their resonances compared to those in compound 4. For example, in Pd4, the j-type protons, which are nearer to the metal ion, experienced a 0.2 ppm downfield shift compared to compound 4. Thus, the metal complexation slightly altered the electronic properties of BODIPY and dipyrrin moieties in complexes Pd4, Re4 and Ru4 compared to those in BODIPY-dipyrrin 4.

Photo-optical and electrochemical properties Fig. 3 The crystal structure of compound Pd4 with 50% probability (a) perspective view and (b) side view. The hydrogen atoms were omitted for clarity.

Table 1 Crystal data and structure refinement parameters for compound Pd4

Parameters

Pd4 0.5 (CH2Cl2)

Mol. formula moiety Mol. formula sum For. weight Temp (K) Cryst sym Space group λ (Å) a (Å) b (Å) c (Å) α (°) β (°) γ (°) Volume (Å3) Z μ (mm−1) Dcalcd (Mg m−3) F(000) θ range (°) e data/unique Rint Data/restraints/parameters GOF on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data) Largest diff. peak/hole (e Å−3) Max. and min. transmission

C33H26BF2N5O2Pd(CH2Cl2)0.5 C33.5H27BClF2N5O2Pd 722.26 200 Monoclinic P21/c 0.71073 13.2813(18) 27.766(4) 8.8018(14) 90 100.405(5) 90 3192.5(8) 4 0.716 1.503 1460 2.14 to 25.13 20 256/5698 0.0877 5698/0/397 0.865 R1 = 0.0597, wR2 = 0.1169 R1 = 0.1437, wR2 = 0.1393 0.643 and −0.790 e Å−3 0.9937 and 0.7958

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The absorption spectra of compounds Pd4, Re4 and Ru4 were recorded in CH2Cl2 and presented in Fig. 5. All BODIPY-metal dipyrrin complexes showed similar types of absorption spectra, and the morphologies of the absorption spectra matched closely with the absorption spectrum of 4H+ (Table 2). All BODIPY-metal dipyrrin complexes showed strong band at ∼620 nm along with a shoulder band that corresponds to 3-pyrrolyl BODIPY moiety and an ill-defined broad band at ∼530 nm that corresponds to metal dipyrrin moiety. However, the metal complexes Pd4, Re4 and Ru4 absorb at a lower wavelength region compared to that for 4H+, which absorbs strongly at 700 nm. The fluorescence properties of 4 and its metal complexes Pd4, Re4 and Ru4 were recorded in CH2Cl2 and the relevant data are included in Table 2. The compound 4 is very weakly fluorescent because of the charge transfer between the two moieties and exhibited one band at 677 nm corresponding to 3-pyrrolyl BODIPY moiety. The complexes Pd4, Re4 and Ru4 are also very weakly fluorescent and showed one weak emission band at ∼660 nm, which is due to 3-pyrrolyl BODIPY moiety. Thus, the covalent linking of metal dipyrrin moieties quenches the fluorescence of 3-pyrrolyl BODIPY moiety in Pd4, Re4 and Ru4 by enhancing various non-radiative decay channels.16 The ultra-fast photophysical studies are required to quantify these observations. The electrochemical properties of Pd4, Re4 and Ru4 were investigated by cyclic voltammetry and differential pulse voltammetry using 0.1 M TBAP as a supporting electrolyte in CH2Cl2 and presented in Fig. 6. The complexes Pd4, Re4 and Ru4 showed two well-defined reversible/quasi-reversible one electron reductions, but did not show any well defined oxidations. The one electron reductions were evidenced from the

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Fig. 4 (a) The 1H NMR comparison of compounds (i) Pd4, (ii) Re4 and (iii) Ru4; (b) NOESY spectrum of compound Pd4 in the selected region recorded in CDCl3 (δ in ppm).

Fig. 5 Comparison of normalized absorption spectra of Pd4 (dotted line), Re4 (solid line) and Ru4 (dashed line).

ipc/ipa ratio of unity (ipc and ipa are cathodic and anodic peak currents respectively). On the basis of the electrochemical data exhibited by 3-pyrrolyl BODIPY5 and the metal dipyrrin refer-

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ence compounds,6b the peaks were tentatively assigned to BODIPY or metal dipyrrin units in Pd4, Re4 and Ru4 complexes. The BODIPY-dipyrrin compound 4 showed one reversible reduction at −0.51 V, which was assigned to 3-pyrrolyl BODIPY unit. The complexes Pd4, Re4 and Ru4 showed one reversible reduction in −0.54 to −0.57 V region corresponding to 3-pyrrolyl BODIPY unit and a second reversible reduction in −0.81 to −0.92 V region (Table 3), which corresponds to the reduction of metal-dipyrrin moiety. Since both 3-pyrrolyl BODIPY and metal dipyrrin units in Pd4, Re4 and Ru4 are electron deficient, the complexes showed only reversible/quasireversible reduction suggesting that Pd4, Re4 and Ru4 complexes are stable only under electrochemical reduction conditions. In conclusion, we synthesized stable α-dipyrrin 3-pyrrolyl BODIPY and explored various metal complexation reactions. We successfully synthesized stable three 3-pyrrolyl BODIPYmetal dipyrrin complexes (Pd(II), Re(I), Ru(II)) and characterized by various spectroscopic techniques. The crystal structure obtained for the 3-pyrrolyl BODIPY-Pd(II) dipyrrin complex showed that the 3-pyrrolyl BODIPY and Pd(II) dipyrrin moieties are inclined to each other with an angle of 41.9°. The absorp-

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Table 2

Dalton Transactions The photophysical data of compounds 3, 4, Pd4, Re4 and Ru4

Comp λabs (log ε) [nm] [mol−1 dm3 cm−1]

λem (nm)

3 4 Pd4 Re4 Ru4

622 677 660 658 659

408 (4.65), 554 (4.97) 591 (5.21) 411 (4.55), 452 (4.43), 589 (sh), 630 (4.66), 717 (4.22) 415 (4.63), 472 (4.29), 525 (4.46), 623 (4.82) 413 (4.43), 466 (4.25), 523 (4.25), 619 (4.69) 417 (4.56), 462 (4.26), 532 (4.32), 620 (4.70)

tion spectral studies showed the absorption features of both the moieties with a strong absorption band at ∼620 nm corresponding to 3-pyrrolyl BODIPY moiety and a weak absorption band at ∼530 nm corresponding to a metal dipyrrin moiety. The electrochemical studies indicated that the complexes are electron deficient and stable only under reduction conditions.

Experimental section Chemicals All the general chemicals and solvents were procured from S.D. Fine Chemicals, India. Column chromatography was performed using silica gel and neutral alumina obtained from Sisco Research Laboratories, India. Tetrabutylammonium perchlorate was purchased from Fluka and used without further purifications. All NMR solvents were used as received. Solvents like dichloromethane, toluene, triethylamine, tetrahydrofuran (THF) and hexane were purified and distilled by standard procedures. General

Fig. 6 Cyclic voltammograms (solid line) and differential pulse voltammograms (dotted line) of compounds (i) 4, (ii) Pd4, (iii) Re4 and (iv) Ru4 recorded in CH2Cl2 by using 0.1 M TBAP at 50 mV s−1 scan rate.

Table 3 Half-wave potential data (Ep recorded in CH2Cl2

(red))

of 3, 4, Pd4, Re4 and Ru4

X-ray crystallography

Ep (red) (V vs. SCE) Comp

I

II

3 4 Pd4 Re4 Ru4

−0.69 −0.51 −0.54 −0.58 −0.57

— — −0.81 −0.92 −0.92

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All the NMR spectra (δ values, ppm) were recorded with 400 or 500 MHz spectrometers. Tetramethylsilane (TMS) was used as an internal reference for recording 1H (of residual proton; δ = 7.26 ppm) and 13C (δ = 77.2 ppm) spectra in CDCl3. Chemical shift multiplicities are reported as s = singlet, d = doublet, t = triplet, q = quartet and m = multiplet. Boric acid and trifluorotoluene were used as reference standards for recording 11B and 19 F NMRs. The HRMS spectra were recorded with a Bruker maxis impact using an electron spray ionization method, TOF analyser. Cyclic voltammetric (CV) and differential pulse voltammetric (DPV) studies were carried out with an electrochemical system utilizing a three-electrode configuration consisting of a glassy carbon (working) electrode, platinum wire (auxiliary) electrode, and a saturated calomel (reference) electrode. The experiments were performed in dry CH2Cl2 with 0.1 M tetrabutylammonium perchlorate (TBAP, n-Bu4NClO4) as the supporting electrolyte. Half-wave potentials (Ep) were measured from DPV. All the potentials were calibrated versus a saturated calomel electrode by the addition of ferrocene as an internal standard, taking E1/2 (Fc/Fc+) = 0.48 V, vs. SCE. The quantum yield for compound 3 was calculated using Sulforhodamine B reference (ϕ = 0.69 in ethanol, λex = 530 nm) and corrected for changes in the refractive index of solvent.17 The fluorescence spectra of metal complexes Pd4, Re4 and Ru4 were recorded at λex = 600 nm in CH2Cl2.

X-ray intensity data measurements of compound Pd4 were carried out on a Bruker SMART APEX II CCD diffractometer with graphite-monochromatized Mo Kα (λ = 0.71073 Å) radiation at 200 (2) K. The data were collected with ω scan width of 0.5° at different settings of φ and 2θ with a frame time of 5 s keeping the sample-to-detector distance fixed at 50 mm. The X-ray data collection was monitored by APEX2 program

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(Bruker, 2006).18 SHELX-97 was used for the structure solution and full matrix least-squares refinement on F2.19 The packing solvent molecule of CH2Cl2 between two adjacent asymmetric units of Pd4 was severely disordered. Therefore, the disordered CH2Cl2 molecule was squeezed by PLATON program. Squeeze void volume and the electron count are consistent with the disordered CH2Cl2 molecule in the structure of Pd4. CCDC 1005193 contains the ESI crystallographic data for this paper. Dipyrromethanyl substituted 3-pyrrolyl-BODIPY (3) To a stirred solution of an aldehyde 2a (100 mg, 0.28 mmol) and pyrrole (2.8 mmol) in 50 mL of dichloromethane (CH2Cl2), TFA (0.028 mmol) was carefully added under nitrogen atmosphere and stirred at room temperature for 30 min. The reaction mixture was diluted with CH2Cl2 and thoroughly washed with aqueous sodium hydroxide solution (5%). The combined organic layers were collected, dried over Na2SO4 and evaporated under reduced pressure. The crude oily product was subjected to silica gel column chromatography using petroleum ether–ethylacetate (80/20) as eluent and afforded pure compound 3 as a dark solid in 85% (0.112 g). 1H NMR (500 MHz, CDCl3, δ in ppm) 5.67 (s, 1 H) 6.13–6.16 (m, 2 H) 6.19–6.22 (m, 3 H) 6.42 (dd, J = 3.91, 2.19 Hz, 1 H) 6.62 (d, J = 3.62 Hz, 1 H) 6.74–6.78 (m, 2 H) 6.86 (d, J = 4.77 Hz, 1 H) 6.92 (d, J = 4.77 Hz, 1 H) 6.96 (dd, J = 3.72, 2.58 Hz, 1 H) 7.48–7.53 (m, 5 H) 7.62 (s, 1 H) 8.19 (s, 2 H) 10.45 (br. s, 1 H); 13C NMR (126 MHz, CDCl3, δ in ppm) 38.1, 107.5, 108.9, 110.9, 116.0. 118.2, 119.4, 121.1, 123.3, 125.0, 128.5, 128.6, 129.9, 130.0, 130.6, 133.2, 133.5, 134.7, 136.7, 137.8, 139.3, 141.3, 151.4; 11B NMR (160 MHz, CDCl3, δ in ppm) 1.39 (t, 1 B); 19F NMR (376 MHz, CDCl3, δ in ppm) −141.1 (q, 2 F); UV-Vis (CH2Cl2, λmax/nm, εmax/mol−1 dm3 cm−1): 408 (4.65), 554 (4.97) 591 (5.21); HRMS calcd for (C28H22BF2KN5): 516.1573 (M + K)+, found 516.1569 (M + K)+; Anal. Calcd (%) for C28H22N5: C 70.46, H 4.65, N 14.67; found C 70.53, H 4.59, N 14.64. α-Dipyrrin 3-pyrrolyl BODIPY 4 A benzene solution of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ, 0.42 mmol) was added slowly to an ice-cold solution of substituted dipyrromethanyl-3-pyrrolyl BODIPY 3 (100 mg, 0.21 mmol) in 50 mL CH2Cl2 and stirred for 3 h. The solvent was removed under reduced pressure and purified by neutral alumina column chromatography with dichloromethane–triethylamine (98 : 2) to afford the α-dipyrrin 3-pyrrolyl BODIPY 4 as an ash coloured solid in 96% (96 mg) yield. 1H NMR (500 MHz, CDCl3, δ in ppm) 6.54 (dd, J = 3.81, 1.90 Hz, 1 H) 6.59 (d, J = 3.05 Hz, 2 H) 6.81 (d, J = 3.64 Hz, 1 H) 7.00 (t, J = 4.77 Hz, 3 H) 7.15 (d, J = 3.45 Hz, 1 H) 7.21–7.23 (m, 2 H) 7.51–7.60 (m, 5 H) 7.71–7.76 (m, 2 H) 7.82 (s, 1 H); 11B NMR (160 MHz, CDCl3, δ in ppm) 1.42 (t, 1 B); 19F NMR (470 MHz, CDCl3, δ in ppm) −140.1 (br. s, 2 F); UV-Vis (CH2Cl2, λmax/nm, εmax/mol−1 dm3 cm−1): 411 (4.55), 452 (sh, 4.43), 630 (4.66), 717 (4.22); HRMS calcd for (C28H21BF2N5): 476.1858 (M + H)+, found 476.1856 (M + H)+; Anal. Calcd (%) for C28H20N5: C 70.76, H 4.24, N 14.73; found C 70.68, H 4.28, N 14.69.

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3-Pyrrolyl BODIPY-Pd(II) dipyrrin complex (Pd4) Samples of the ligand 4 (50 mg, 0.11 mmol) and Pd(acac)2 (32 mg, 0.11 mmol) were dissolved in dry toluene under nitrogen atmosphere. Dry triethylamine (0.029 mL, 0.22 mmol) was added and heated to 100 °C for 1 h. The progress of the reaction was monitored by thin layer chromatography, which showed the disappearance of starting material and appearance of a major new, less polar spot. The solvent was removed under reduced pressure and the crude compound was purified by silica gel chromatography with petroleum ether–ethyl acetate (95/5) to afford the desired pure product Pd4 as a grey coloured crystalline solid in 52% (37 mg) yield. 1H NMR (500 MHz, CDCl3, δ in ppm) 2.11 (s, 6 H) 5.49 (s, 1 H) 6.46–6.48 (m, 1 H) 6.52 (dd, J = 4.40, 1.56 Hz, 2 H) 6.71–6.73 (m, 1 H) 6.74–6.76 (m, 1 H) 6.98 (s, 2 H) 7.09–7.11 (m, 1 H) 7.25 (dd, J = 4.40, 1.16 Hz, 2 H) 7.43–7.63 (m, 5 H) 7.70 (br. s, 1 H) 7.96 (s, 2 H) 10.9 (br. s, 1 H); 13C NMR (126 MHz, CDCl3, δ in ppm) 26.6, 101.4, 116.8, 117.6, 118.1, 118.3, 121.2, 125.8, 126.7, 128.5, 130.2, 130.6, 131.1, 132.8, 133.0, 133.9, 134.6, 135.5, 135.7, 138.7, 141.0, 146.6, 186.9; 11B NMR (160 MHz, CDCl3, δ in ppm) 1.41 (t, 1 B); 19F NMR (470 MHz, CDCl3, δ in ppm) −140.1 (q, 2 F); UV-Vis (CH2Cl2, λmax/nm, εmax/mol−1 dm3 cm−1): 415 (4.63), 472 (4.29), 525 (4.46), 623 (4.82); HRMS calcd for (C33H27BF2N5O2Pd): 680.1272 (M + H)+, found 680.1264 (M + H)+; Anal. Calcd (%) for C33H26N5: C 58.30, H 3.85, N 10.30; found C 58.02, H 3.79, N 10.24. 3-Pyrrolyl BODIPY-Re(I) dipyrrin complex (Re4) Samples of the ligand 4 (50 mg, 0.11 mmol) and Re(CO)5Cl (38 mg, 0.11 mmol) were dissolved in dry toluene under nitrogen atmosphere and dry triethylamine (0.029 mL, 0.22 mmol) was added and heated to 100 °C for 1 h. The triphenylphosphine (28 mg, 0.11 mmol) was then added and the heating was continued for further 1 h. The reaction progress was monitored by thin layer chromatography, which showed the disappearance of the starting material and appearance of major new, less polar spot. The solvent was removed under reduced pressure, and the crude compound was purified by silica gel chromatography with petroleum ether–ethyl acetate (95/5) and afforded the desired pure product Re4 as a grey coloured solid in 54% (56 mg) yield. 1H NMR (500 MHz, CDCl3, δ in ppm) 6.37 (dd, J = 4.20, 1.14 Hz, 2 H) 6.47 (dd, J = 3.72, 2.19 Hz, 1 H) 6.71 (d, J = 3.81 Hz, 1 H) 6.96–7.10 (m, 10 H) 7.12–7.30 (m, 8 H) 7.31–7.36 (m, 3 H) 7.50–7.61 (m, 5 H) 7.66 (s, 1 H) 7.72 (s, 2 H) 10.70 (br. s, 1 H). 13C NMR (126 MHz, CDCl3, δ in ppm) 117.4, 118.1, 119.3, 121.2, 124.8, 126.3, 128.5, 128.6, 130.0, 130.1, 130.2, 130.5, 130.6, 130.9, 131.4, 133.1, 133.6, 133.7, 133.8, 133.9, 134.6, 135.9, 136.5, 136.9, 137.9, 138.0, 140.7, 150.4, 154.7, 196.7. 11B NMR (160 MHz, CDCl3, δ in ppm) 1.45 (t, 1 B). 19F NMR (376 MHz, CDCl3, δ in ppm) −140.0 (br. S, 2 F); UV-Vis (CH2Cl2, λmax/nm, εmax/mol−1 dm3 cm−1): 413 (4.43), 466 (4.25), 523 (4.25), 619 (4.69); HRMS calcd for (C49H35BF2N5O3PRe): 1008.2101 (M + H)+, found 1008.2106 (M + H)+; Anal. Calcd (%) for C49H37N5: C 58.28, H 3.69, N 6.94; found C 58.37, H 3.72, N 6.98.

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3-Pyrrolyl BODIPY-Ru(II) dipyrrin complex (Ru4) Samples of ligand 4 (50 mg, 0.11 mmol) and [Ru( p-cymene)Cl2]2 (67 mg, 0.11 mmol) were dissolved in dry toluene under nitrogen atmosphere, and dry triethylamine (0.029 mL, 0.22 mmol) was added and heated to 100 °C for 1 h. The TLC analysis showed the disappearance of starting material and appearance of major, highly polar new spot. The solvent was removed under reduced pressure, and the resulted crude compound was purified by silica gel chromatography with ethyl acetate and afforded the desired pure product Ru4 as a grey coloured solid in 52% (37 mg) yield. 1H NMR (500 MHz, CDCl3, δ in ppm) 1.10 (d, J = 6.87 Hz, 6 H) 2.23 (s, 3 H) 2.46 (septet, 1 H) 5.30 (q, J = 5.98 Hz, 4 H) 6.46 (dd, J = 3.81, 2.10 Hz, 1 H) 6.59–6.61 (m, 2 H) 6.70 (d, J = 2.48 Hz, 2 H) 6.98 (s, 2 H) 7.08–7.10 (m, 1 H) 7.20 (d, J = 4.20 Hz, 2 H) 7.49–7.58 (m, 5 H) 7.68 (s, 1 H) 8.04 (s, 2 H) 10.85 (br. S, 1 H); 13C NMR (126 MHz, CDCl3, δ in ppm) 18.8, 22.3, 30.8, 84.9, 85.0, 100.5, 102.6, 116.7, 117.7, 118.4, 119.1, 121.2, 125.5, 126.5, 128.5, 130.2, 130.6, 130.9, 133.0, 134.5, 134.6, 138.4, 141.9, 155.4; 11B NMR (160 MHz, CDCl3, δ in ppm) 1.40 (t, 1 B); 19F NMR (470 MHz, CDCl3, δ in ppm) −140.2 (q, 2 F); UV-Vis (CH2Cl2, λmax/nm, εmax/mol−1 dm3 cm−1): 417 (4.56), 462 (4.26), 532 (4.32), 620 (4.70); HRMS calcd for (C38H33BF2N5Ru): 710.1851 (M + H)+, found 710.1850 (M + H)+; Anal. Calcd (%) for C38H33N5: C 61.26, H 4.46, N 9.40; found C 61.40, H 4.49, N 9.36.

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Acknowledgements MR and VL acknowledge the financial support from Council of Scientific and Industrial Research, Govt. of India.

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Synthesis, structure and spectral and electrochemical properties of 3-pyrrolyl BODIPY-metal dipyrrin complexes.

The stable dipyrrin substituted 3-pyrrolyl BODIPY (α-dipyrrin 3-pyrrolyl BODIPY) was synthesized by oxidation of dipyrromethane substituted 3-pyrrolyl...
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