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Photoinduced intramolecular energy transfer and anion sensing studies of isomeric RuIIOsII complexes derived from an asymmetric phenanthroline–terpyridine bridge† Dinesh Maity, Chanchal Bhaumik, Debiprasad Mondal and Sujoy Baitalik* Two heterobimetallic Ru(II)–Os(II) complexes of compositions [(bpy)2MII( phen-Hbzim-tpy)M’II(tpyPhCH3)]4+, where MII = Ru and M’II = Os (4) and MII = Os and M’II = Ru (5), phen-Hbzim-tpy = 2-(4-(2,6di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10]phenanthroline, bpy = 2,2’-bipyridine, and tpy-PhCH3 = 4’-(4-methylphenyl)-2,2’:6’,2’’-terpyridine have been synthesized and characterized by elemental analyses, ESI mass spectrometry, and 1H NMR and UV-vis absorption spectroscopy. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated and compared with that of monometallic model compounds [(bpy)2MII( phen-Hbzim-tpy)]2+ [MII = Ru (1) and MII = Os (2)] and [( phen-Hbzim-tpy)RuII(tpy-PhCH3)]2+ (3). The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations, whereas the monometallic complexes display one-electron oxidation in the positive potential window. Steady state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer takes place from the Ru-center to the Os-based component in both the heterometallic dyads in all the solvents. The complexes under investigation contain an imidazole NH proton which became appreciably acidic due to metal coordination and can be utilized for recognition of selective anions in solution either via hydrogen bonding interaction or by proton transfer. Accordingly, the anion binding properties of the two heterobimetallic complexes as well as parent brid-
Received 10th August 2013, Accepted 11th October 2013
ging ligand, phen-Hbzim-tpy, have been studied in solutions using absorption, steady state and time-
DOI: 10.1039/c3dt52186a
resolved luminescence spectral measurements. The metalloreceptors act as sensors for F−, CN− and AcO− ions. It is evident from sensing studies that in the presence of excess of selective anions, deprotona-
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tion of the imidazole N–H proton occurs in all cases.
Introduction Molecular photonics is an emerging area of science with regards to controlling the passage of information quanta along nanoscale molecular wires.1–3 Intramolecular electron and energy transfer processes in di- and multinuclear complexes whose mononuclear entities are linked by conjugated bridging ligands have received considerable attention in this area because cooperative interactions between metal centers in these complexes give rise to properties that are useful for constructing photomolecular devices.4,5 The use of polypyridine
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India. E-mail: [email protected]; Fax: +91 33 24146584; Tel: +91 33 24146666 † Electronic supplementary information (ESI) available: Fig. S1–S30. See DOI: 10.1039/c3dt52186a
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complexes of Ru(II) and Os(II) as building blocks is very appealing in this area because of their unique combination of photophysical, photochemical and electrochemical properties and thermal and photochemical stabilities.6–8 In particular, the Os(II) polypyridines expand the Ru(II) analogue’s light absorbing properties to longer wavelengths because of the 1GS → 3 MLCT absorption.4a Covalently linked two-component systems, dyads, are the simplest class of supramolecular architecture for the study of photo-induced energy and electron transfer processes.9–18 The directional flow of electrons or energy is facilitated when there is asymmetry in the multicomponent system.9–15 Such asymmetry can be introduced either by using ligands which differ in donor–acceptor properties or using heterometallic sites. In principle, such intramolecular charge transfer in binuclear complexes can be exploited for the realization of miniaturized sub-nanoscale devices where electrons or photons dispatch information between specific
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sites within distances of the order of a few angstroms.4,5,9–15 The features associated with bridging ligands such as length, rigidity, topology, conjugation, charge and the numbers of dissociable hydrogen atoms play important roles in this context.4a,9–15 Two types of polypyridine bridging ligands are generally used for this study. The first type containing bidentate chelating sites like 2,2′-bipyridine (bpy) or 1,10-phenanthroline ( phen), either directly coupled or linked by a spacer, are found to be in high usage.1 In terms of structure, however, such a choice is not ideal because substitution of a single position in these ligands will lead to the formation of diastereomeric products. The second type of ligand, containing tridentate chelating sites such as 2,2′:6′,2′′-terpyridine is more appealing from the viewpoint of constructing linear, rodlike polynuclear complexes.4 However, usually such complexes are practically non-luminescent at room temperature and their excited state lifetime (τ = 0.25 ns for [Ru(tpy)2]2+)4 is also very short, representing major barriers for their use as photosensitizers. Consequently, much effort has been devoted to design and synthesize tridentate polypyridine ligands that can produce Ru(II) complexes with enhanced emission quantum yields and excited-state lifetimes.4,18 As compared to the trisbidentate (N–N)3 and bis-terdentate (N–N–N)2 complexes, rigidly-linked homo- and heterometallic assemblies comprising the combination of a bis-terdentate and a tris-bidentate complex are extremely rare, despite the very interesting properties that could be expected from the unique attributes of each type of complex.19–22
Recently, we reported the synthesis, redox activities, and photo-induced electron- and energy transfer processes of three homodimetallic Ru(II)–Ru(II) and two heterodimetallic Ru(II)– Rh(III) complexes based on the use of heteroditopic phenHbzim-tpy as a bridging ligand.23 A key challenge is not only to control the strength but also the direction of the interaction between the two metal centers in both the ground and the excited state. This can be achieved either through the structural modification of the bridging ligand and/or the variation of the nature of the coordinating metal centers. In this context, we report herein the synthesis, characterization, and physicochemical properties of two heterobimetallic complexes of composition [(bpy)2RuII( phen-Hbzim-tpy)OsII(tpy-PhCH3)]4+ and [(bpy)2OsII( phen-Hbzim-tpy)RuII(tpy-PhCH3)]4+, where phen-Hbzim-tpy = 2-(4-(2,6-di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10] phenanthroline, bpy = 2,2′bipyridine, and tpy-PhCH3 = 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (Chart 1). In the first compound, the Ru(II) is bound to phen-Hbzim-tpy via the bidentate N–N site and the Os(II) via the tridentate N–N–N site, while in the second isomer Ru(II) is bound via the tridentate N–N–N site and the Os(II) via the bidentate N–N site of phen-Hbzim-tpy. In contrast to related symmetrical bridging ligands, each metal center in these complexes is geometrically and photophysically distinct that is, (N–N)3 versus (N–N–N)2. As will be seen, the extent and directionality of the interaction between the metal centers is found to be highly dependent on the position of the metal center. Additionally, the present complexes owing to the presence of
Chart 1
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an imidazole NH proton in the second coordination sphere which became appreciably acidic due to metal coordination can be utilized for multi-channel recognition of selective anions either via hydrogen bonding interaction or by proton transfer reaction in solution.24,25 As will be seen, consequent to the interaction with selective anions (such as F−, CN− and AcO−), remarkable changes in the photophysical properties of the complexes occur. Another objective of this study is also to enquire if the rate of the energy transfer in the heterometallic dyads might be modulated by changing the solvents, interacting with selective anions as well as by changing the pH of the solution.
Experimental Materials Reagent grade chemicals obtained from commercial sources were used as received. Solvents were purified and dried according to standard methods. 1,10-Phenanthroline and tetrabutylammonium (TBA) salt of the anions were purchased from Sigma-Aldrich. 1,10-Phenanthroline-5,6-dione,26 4′-( p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH3),24e,27 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCHO),28 and bridging 2-(4(2,6-di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5-f ][1,10]phenanthroline ( phen-Hbzim-tpy) ligand23,29 were synthesized according to the literature procedures. cis-[Ru(bpy)2Cl2]·2H2O,30 cis-[Os(bpy)2Cl2],31 [(tpy-PhCH3)RuCl3],10 and [(tpy-PhCH3) OsCl3]10 were also prepared by employing the literature procedures. AgClO4 was prepared from silver carbonate and perchloric acid and recrystallized from benzene. The monometallic Ru(II) complex of composition [(bpy)2Ru( phenHbzim-tpy)](ClO4)2 (1) was prepared by the previously published literature procedure.23,29 Synthesis of the metal complexes The complexes were prepared under oxygen- and moisture-free dinitrogen using standard Schlenk techniques. [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2). A mixture of cis-Os(bpy)2Cl2 (285 mg, 0.5 mmol) and phen-Hbzim-tpy (1.10 g, 2.0 mmol) in 100 ml of 1 : 1 (v/v) ethanol–water mixture was heated under reflux for 48 h with constant stirring. After cooling to room temperature, the undissolved suspended material was removed by filtration. The filtrate was concentrated to approximately 30 mL on a rotary evaporator and the solution was filtered again. To the filtrate an aqueous solution (5 mL) of NaClO4 (1.0 g) was added where a dark brown product deposited. The crude product was purified by silica gel column chromatography eluting with acetonitrile. On recrystallization from acetonitrile–methanol (1 : 2 v/v) mixture in the presence of a few drops of aqueous 10−4 M perchloric acid afforded a black microcrystalline compound (380 mg, Yield: 60%). Anal. Calcd for C54H41N11Cl2O10Os: C, 51.26; H, 3.27; N, 12.18. Found: C, 51.24; H, 3.30; N, 12.19. 1H NMR (500 MHz, DMSO-d6, δ/ppm, see Scheme S1, ESI† for proton numbering): 8.90 (d, 2H, J = 8.0 Hz, H3″), 8.85 (d, 4H, J =
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8.5 Hz, 2H3′ + 2H3″), 8.81 (d, 4H, J = 8.0 Hz, 2H6 + 2H9), 8.70 (d, 2H, J = 8.0 Hz, H3), 8.53 (d, 2H, J = 8.5 Hz, H8), 8.28 (d, 2H, J = 8.5 Hz, H7), 8.08 (t, 2H, J = 7.5 Hz, H4), 8.02–7.98 (m, 4H, 2H4′ + 2H11), 7.90 (t, 2H, J = 8.0 Hz, H10), 7.84 (t, 2H, J = 6.7 Hz, H4′), 7.76 (d, 2H, J = 5.5 Hz, H6′), 7.58 (t, 2H, J = 6.0 Hz, H5), 7.52–7.47 (m, 4H, 2H5′ + 2H6′), 7.25 (t, 2H, J = 6.7 Hz, H5′). ESI-MS ( positive, CH3CN) m/z = 515.59 (100%) [(bpy)2Os( phenHbzim-tpy)]2+, m/z = 1130.03 (10%) [(bpy)2Os( phen-bzimtpy)]+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 650(br)(2750), 486(12 830), 435(sh)(11 750), 318(sh) (36 500), 292(81 080). [(tpy-PhCH3)Ru(tpy-Hbzim-phen)](ClO4)2·3H2O (3). A mixture of Ru(tpy-PhCH3)Cl3 (75 mg, 0.14 mmol), AgBF4 (92 mg, 0.47 mmol) and 30 mL acetone were refluxed with continuous stirring for 4 h. After the solution cooled down to room temperature, the precipitated AgCl was removed by filtration. The filtrate containing [(tpy-PhCH3)Ru(acetone)3]3+ was added slowly to a methanol–chloroform solution of phen-Hbzim-tpy (160 mg, 0.30 mmol) and the solution was refluxed for 5 h, during which time the color of the solution changed from violet to orange-red. After cooling to room temperature, the undissolved suspended material was removed by filtration and kept overnight at a refrigerator. A deep-red colored compound that deposited was filtered, washed with chloroform and ether and then dried under vacuum. The compound was redissolved in the minimum volume of acetonitrile and then subjected to silica-gel column chromatography (eluent: acetonitrile). The eluent was rotary evaporated to a small volume and then underwent an anion exchange reaction with NaClO4 give rise to the desired compound. The compound was finally recrystallized from acetonitrile–methanol (1 : 1 v/v) mixture in presence of a few drops of aqueous 10−4 M perchloric acid (110 mg, Yield: 64%). Anal. Calcd for C56H44N10Cl2O11Ru: C, 55.82; H, 3.68; N, 11.62. Found: C, 55.84; H, 3.72; N, 11.60. 1H NMR (300 MHz, DMSO-d6, δ/ppm): 9.56 (s, 2H, H3′), 9.44 (s, 2H, H3′), 9.10 (d, 2H, J = 7.9 Hz, H6), 9.01 (d, 2H, J = 8.1 Hz, H6), 8.85 (d, 2H, J = 7.9 Hz, H9), 8.62 (nr, 4H, 2H7 + 2H8), 8.30 (d, 2H, J = 8.0 Hz, H8), 7.98 (t, 4H, J = 7.7 Hz, H4), 7.71 (d, 2H, J = 5.5 Hz, H11), 7.67 (t, 2H, J = 7.4 Hz, H10), 7.51–7.48 (m, 6H, 4H3 + 2H7), 7.31 (t, 4H, J = 6.0 Hz, H5), 2.46 (s, 3H, CH3). ESI-MS (positive, CH3CN) m/z = 476.37 (100%) [(tpy-PhCH3)Ru(tpyHbzim-phen)]2+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 500 (46 420), 378(38 580), 326(sh)(54 420), 312(63 420), 278(70 750). [(bpy)2Ru( phen-Hbzim-tpy)Os(tpy-PhCH3)](ClO4)4·H2O (4). A mixture of [(bpy)2Ru( phen-Hbzim-tpy)](ClO4)2·4H2O (1) (120 mg, 0.1 mmol) and [Os(tpy-PhCH3)Cl3] (67 mg, 0.11 mmol) in 20 mL degassed ethylene glycol was heated overnight at 200 °C with continuous stirring. The resulting black solution was cooled to room temperature and then poured into 10 mL aqueous solution of NaClO4·H2O (1.0 g) and stirred for few minutes where a black precipitate appeared. The precipitate was collected by filtration and washed several times with water and then dried under vacuum. The compound was purified in the same way as 3 except using a mixture of CH3CN and 10% aqueous KNO3 (10 : 1 v/v) as the eluent in the column chromatography. Yield: 120 mg, 64%. Anal. Calcd for C76H56N14Cl4O17RuOs: C, 48.80;
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H, 3.02; N, 10.48. Found: C, 48.78; H, 3.04; N, 10.47. 1H NMR (500 MHz, DMSO-d6, δ/ppm): 9.56 (s, 2H, H3′), 9.48 (s, 2H, H3′), 9.16–9.08 (m, 6H, 4H6 + 2H9), 8.87 (d, 2H, J = 7.9 Hz, H3″), 8.84 (d, 2H, J = 7.9 Hz, H3″), 8.68 (d, 2H, J = 8.2 Hz, H8), 8.64 (d, 2H, J = 8.2 Hz, H7), 8.31 (d, 2H, J = 7.9 Hz, H8), 8.20 (t, 2H, J = 7.2 Hz, H4), 8.10 (t, 4H, J = 8.7 Hz, 2H4 + 2H4′), 7.95–7.89 (m, 4H, 2H4′ + 2H6′), 7.83 (d, 2H, J = 5.5 Hz, H6′), 7.63–7.55 (m, 6H, 2H5′ + 2H10 + 2H11), 7.45 (d, 2H, J = 5.8 Hz, H7), 7.40 (d, 4H, J = 5.8 Hz, H3), 7.34 (t, 2H, J = 5.8 Hz, H5′), 7.23–7.17 (m, 4H, H5), 2.52 (s, 3H, CH3). ESI-MS ( positive, CH3CN) m/z = 484.92 (100%) [(bpy)2Ru( phen-bzim-tpy)Os (tpyPhCH3)]3+; UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 672(12 330), 494(64 500), 460(br)(50 170), 424(sh)(37 000), 352(sh)(72 000), 314(123 500), 287(188 750). [(bpy)2Os( phen-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)4·2H2O (5). A mixture of [Ru(tpy-PhCH3)Cl3] (75 mg, 0.14 mmol), AgBF4 (92 mg, 0.47 mmol) and 30 mL acetone were refluxed with continuous stirring for 4 h. After the solution cooled down to room temperature, the precipitated AgCl was removed by filtration. 40 mL of EtOH was then added to the filtrate. Acetone was removed by rotary evaporation. To the resulting solution was added an ethanol solution of [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2) and refluxed for 12 h. On cooling, a reddish black colored compound that deposited was filtered and then dried under vacuum. Purification of the compound by column chromatography and subsequent recrystallization was done in the same way as 3. Yield: 155 mg, 58%. Anal. Calcd for C76H58N14Cl4O18OsRu: C, 48.34; H, 3.09; N, 10.38. Found: C, 48.31; H, 3.12; N, 10.40. 1H NMR (500 MHz, DMSO-d6, δ/ppm): 9.55 (s, 2H, H3′), 9.46 (s, 2H, H3′), 9.10 (d, 4H, J = 8.2 Hz, H6), 8.91 (d, 2H, J = 8.2 Hz, H9), 8.85 (d, 2H, J = 8.2 Hz, H3″), 8.82 (d, 2H, J = 7.9 Hz, H3″), 8.71 (d, 2H, J = 8.8 Hz, H8), 8.66 (d, 2H, J = 8.8 Hz, H7), 8.36 (d, 2H, J = 7.6 Hz, H8), 8.07–8.00 (m, 6H, 4H4 + 2H4′), 7.89 (t, 2H, J = 7.5 Hz, 2H4′), 7.75 (d, 2H, J = 5.8 Hz, H6′), 7.56–7.47 (m, 12H, 4H3 + 2H6′ + 2H7 + 2H10 + 2H11), 7.27–7.23 (m, 8H, 4H5 + 4H5′), 2.48 (s, 3H, CH3). ESI-MS ( positive, CH3CN) m/z = 363.65 (44%) [(bpy)2Os( phenHbzim-tpy)Ru(tpy-PhCH3)]4+; m/z = 484.52 (100%) [(bpy)2Os ( phen-bzim-tpy)Ru(tpy-PhCH3)]3+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 658(br)(4580), 494(69 500), 439(br)(33 830), 352(sh)(60 580), 372(sh)(85 750), 308(sh)(109 170), 289(162 000). Caution! AgClO4 and perchlorate salt of the metal complexes used in this study are potentially explosive and therefore should be handled in small quantities with care.
Physical measurements Elemental (C, H, and N) analyses were performed on a PerkinElmer 2400II analyzer. 1H NMR and {1H–1H} COSY spectra were obtained on a Bruker Avance DPX 500 MHz spectrometer using DMSO-d6 solution. Electrospray ionization mass spectra (ESI-MS) were obtained on a Micromass Qtof YA 263 mass spectrometer. Electronic absorption spectra were obtained with a Shimadzu UV 1800 spectrophotometer at room temperature. The binding studies of the receptor with different anions were carried out in acetonitrile solution. The binding/
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equilibrium constants were evaluated from the absorbance data using eqn (1).32 Aobs ¼ ðA0 þ A1 K½GT Þ=ð1 þ K½GT Þ
ð1Þ
where Aobs is the observed absorbance, A0 is the absorbance of the free receptor, A∞ is the maximum absorbance induced by the presence of a given anionic guest, [G]T is the total concentration of the guest, and K is the binding constant of the host– guest entity. To determine the ground state pKa values of the complexes, spectrophotometric titrations were carried out with a series of acetonitrile–water (3 : 2 v/v) solutions containing the same amount of complex (10−5 M) and pH adjusted in the range 2.5–12. Robinson–Britton buffer was used in the study.33 Due to the solubility limitation, pKa determination of phen-Hbzimtpy was carried out in dimethylsulfoxide–water (3 : 2 v/v). The pH measurements were made with a Beckman Research Model pH meter. The pKa values were evaluated from the titration data and using eqn (2). pH ¼ pK a log
A A0 Af A0
ð2Þ
Steady state luminescence spectra were recorded on a Perkin-Elmer LS55 fluorescence spectrophotometer. Photoluminescence titrations were carried out with the same sets of solutions as were made with spectrophotometry. Luminescence quantum yields were determined by a relative method using [Ru(bpy)3]2+ as the standard. Time-correlated singlephoton-counting (TCSPC) measurements were carried out for the luminescence decay of complexes. The samples were excited at 450 nm for the metal complexes and at 370 nm for phen-Hbzim-tpy using a picosecond diode laser (IBH Nanoled07) in an IBH Fluorocube apparatus. The luminescence decay data were collected on a Hamamatsu MCP photomultiplier (R3809) and were analyzed by using IBH DAS6 software. The electrochemical measurements (CV and SWV) were carried out with a BAS epsilon electrochemistry system. A three-electrode assembly comprising a Pt (for oxidation) or glassy carbon (for reduction) working electrode, Pt auxiliary electrode, and Ag/AgCl reference electrode and tetraethylammonium perchlorate (TEAP) was used as supporting electrolyte. All the potentials reported in this study were referenced against the Ag/AgCl electrode, which under the given experimental conditions gave a value of 0.36 V for the ferrocene/ferrocenium couple. Experimental uncertainties were as follows: absorption maxima, ±2 nm; molar absorption coefficients, 10%; emission maxima, ±5 nm; excited-state lifetimes, 10%; luminescence quantum yields, 20%; redox potentials, ±10 mV; pKa ±0.2.
Results and discussion Synthesis and characterization of the complexes The outline of the syntheses of the Ru–Os and Os–Ru isomers derived from the heteroditopic phen-Hbzim-tpy bridge is
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Scheme 1
presented in Scheme 1. The monometallic Os(II) complex [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2) was isolated after refluxing the reactants for 48 h in ethanol–water (1 : 1 v/v) mixture under weakly acidic conditions to keep the imidazole proton intact. For the synthesis of the Ru–Os isomer (4), 1 was boiled with [(tpy-PhCH3)OsCl3] in ethylene glycol. For the formation of the Os–Ru isomer (5), an ethanolic solution of 2 was refluxed with [(tpy-PhCH3)Ru(EtOH)3]3+. All the compounds were then purified by column chromatography by using an appropriate eluent. Finally, the compounds were recrystallized from acetonitrile–methanol (1 : 1 v/v) mixture in the presence of a few drops of aqueous 10−4 M perchloric acid to keep the imidazole NH proton intact. The compounds were characterized by elemental (C, H and N) analyses, ESI mass spectrometric, UV-vis, and 1H NMR spectroscopic measurements and the results are given in the Experimental section. The ESI-mass spectra of the complexes and their simulated isotopic patterns are shown in Fig. S1–S3 (ESI†). As the molecular formulae as well as the masses of 4 and 5 are the same, it is expected that the ESI mass spectra of the two heterobimetallic compounds will also be same. The ESI mass spectrum of 4 shown in Fig. S3 (ESI†) presented a cluster of peaks with
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m/z = 484.92, which corresponds to those of [(bpy)2Ru( phenbzim-tpy)Os(tpy-PhCH3)]3+, while 5 shows a cluster of peak at m/z = 484.52 corresponding to the species [(bpy)2Os( phenbzim-tpy)-Ru(tpy-PhCH3)]3+. Thus, the peak positions (m/z values) of the tri-positive species as mentioned above are almost the same for both 4 and 5 as expected, although small differences are observed in their experimental isotopic distribution patterns. NMR spectra The 1H NMR spectra of the complexes have been recorded in DMSO-d6 at room temperature and the assignments made for the observed chemical shifts, according to the numbering (Fig. S4, ESI†), are listed in the Experimental section. The spectral assignments of the complexes have been made with the help of their {1H–1H} COSY spectra, relative areas of the peaks, and taking into consideration the usual ranges of J values for bpy and tpy-PhCH3.23,24 The 1H NMR spectra of the complexes show the occurrence of a fairly large number of resonances, some of which are overlapped with each other. The most up-field singlet at 2.52 ppm in 4 and 2.48 ppm in 5 (not shown in Fig. S4, ESI†) accounting
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for three protons, is clearly due to –CH3 protons of the coordinated tpy-PhCH3 moiety. It is of interest to note that the H3′ protons which appear as a singlet at 8.89 ppm in 1 and 8.85 ppm in 2 are considerably shifted to the down-field region in both the heterobimetallic complexes when the free terpyridine site in 1 and or 2 is coordinated with a second metal center (RuII or OsII). The 1H NMR spectra of the two heterometallic dyads look extremely complicated, because of the presence of many signals and extensive overlapping. Fig. S4 (ESI†) also shows that there are significant differences in the spectral patters displayed by the two heterometallic isomers. Thus, the presence of two closely situated singlets at 9.56 and 9.48 ppm in 4 and 9.55 and 9.46 ppm in 5, clearly indicate the two different chemical environment of H3′ protons. It is also to be noted that the H3 proton of the tpy moiety of phen-Hbzimtpy in the bimetallic complexes shifts to a significantly up-field region compared to the monometallic complexes (1 and 2) because this proton lies above the shielding region of a pyridine ring of another tpy capping ligand. It is to be noted that a singlet appearing at 14.64 for 4 and 14.66 ppm for 5 is due the imidazole NH proton of the coordinated phen-Hbzim-tpy moiety. Absorption spectra The absorption spectral characteristics of both the Ru–Os complexes (4 and 5) along with monometallic precursor compounds (1–3) were studied in different solvents and the absorption maxima and molar extinction coefficients (ε) are given in Table 1 and Tables S1 and S2 (ESI†). In Fig. 1, the UVvis spectra of equimolar solutions of the five complexes in acetonitrile are displayed. The absorption spectra of the complexes are in general of a similar type in all the solvents. For example, in acetonitrile solution, complexes 1–5 exhibit several very high-intensity absorption bands in the 280–380 nm (ε = 38 580–191 000 M−1 cm−1) spectral region and moderately
Table 1
Fig. 1 UV-vis absorption spectra of equimolar solutions of complexes 1–5 in acetonitrile at room temperature.
strong absorption band and shoulder in the 460–500 nm (ε = 17 240–66 560 M−1 cm−1) region (Fig. S5–S8, ESI†). Three Os(II) containing complexes (2, 4, and 5) show additional broad and weaker spectral features (ε = 4180–8700 M−1 cm−1) in the longer wavelength region (600–720 nm). Based on the extensive investigations performed on [M(bpy)3]2+ and [M(tpy-PhCH3)2]2+ (M = RuII and OsII) and related complexes4,6,18,24 it can be assigned that the high-intensity absorption bands in the 280–380 nm spectral region are due to π–π* transition of bipyridine and spin-allowed ligand-centered (LC) transitions of the bridging phen-Hbzim-tpy ligand, while the absorption bands in the 460–500 nm region are due to spin-allowed overlapping M(dπ) → bpy and M(dπ) → phen-Hbzim-tpy (M = RuII and OsII) charge transfer transitions (MLCT). In this broad-band system, the overlapping contributions from two types of ligands cannot be distinguished although the expectation is that the MLCT transition involving the phen-Hbzim-tpy ligand should be at a slightly lower energy than those involving bpy ligands. In the heterobimetallic complexes (4 and 5), there is another
Spectroscopic and photophysical data of 1–5 in acetonitrile solution
Luminescence At 298 Ka
At 77 Kb
Compounds
Absorption λmax/nm (ε/M−1 cm−1)
λmax/nm τ/ns
Φ/10−3 kr/105 s−1
knr/107 s−1
λmax/nm
Φ
1
460(19 740), 426(sh)(15 670), 326(br)(49 280), 288(109 721) 650(br)(5570), 486(25 990), 435(sh)(23 800), 318(sh)(73 930), 292(164 240) 500(46 420), 378(38 580), 326(sh)(54 420), 312(63 420), 278(70 750) 672(12 330), 494(64 500), 460(br)(50 170), 424(sh)(37 000), 352(sh)(72 000), 314(123 500), 287(188 750) 658(br)(4580), 494(69 500), 439(sh)(33 830), 352(sh)(60 580), 327(sh)(85 750), 308(sh)(109 170), 289(162 000) 474(10 400) 490(28 000) 657(3650), 477(13 750) 667(6600), 490(26 000)
607
151
297
19.59
0.46
589
0.31
720
44.31
83.15
18.76
2.07
711
0.18
658
1.79, 3.34 6.90
38.65, 20.65 55.48, 29.73 645
0.17
750
101
164
16.24
0.82
733
0.20
722
42.52
82.26
19.35
2.16
704
0.15
629 640 718 734
0.25 1010 s−1).11 Similar very fast energy transfer processes have reported for the complexes in which the chelating units are separated by one or two phenyl rings.10,12 In contrast, compounds in which the spacers contain a saturated moiety such as bicycle[2,2,2]octane, adamantine etc. interposed between two phenyl rings have been found to slow down the Ru → Os energy transfer rate remarkably (ken ∼ 106 s−1).9,10 Previously, we have reported moderately fast (ken = 6.1 × 107 s−1) Ru → Os energy transfer where the bridging unit contains an imidazolylbis(benzimidazole) [H2Imbzim]− anion.34 Similar order of ken has been reported for a triazolate anion-bridged RuIIOsII complex.13 In the present case with heteroditopic bridge, the Ru → Os energy transfer rate is very fast (ken ∼ 109 s−1). Energy transfer processes occur in two ways, viz. the Dexter-type35 through bond electron exchange or the Förster-type36 through space Coulombic interaction. However, in most cases it is not easy to ascertain which particular mechanism plays the dominant role. Electron exchange becomes important where the spacer is either highly conjugated or relatively short, while the Coulombic mechanism becomes important when the spacer is either saturated or the transition dipoles are well-separated.9
Dalton Transactions
Anion sensing studies of the complexes through different channels Sensing of the anions by the metalloreceptors 4 and 5 has been monitored by observing the spectral changes that occur in acetonitrile solutions. As shown in Fig. 6, the MLCT peaks at 494 and shoulder at 462 nm for 4 and at 495 and 436 nm, respectively for 5, remain practically unchanged upon addition of 5 equiv. of Cl−, Br−, I−, NO3− and ClO4− ions to their acetonitrile solutions (2.5 × 10−5 M). On the other hand, following the addition of 5 equiv. of F−, CN−, and AcO−, the MLCT band and shoulder becomes red-shifted to 502 and 467 nm for 4 and to 500 and 438 nm for 5, respectively indicating that interactions occur between the metalloreceptors and anions. The small red-shift of the MLCT bands can be attributed to the second-sphere donor–acceptor interactions between metalcoordinated phen-Hbzim-tpy and the anions. Such interactions (hydrogen bonding or proton transfer, vide infra) increase the electron density at the metal center leading to lowering of the MLCT band energies.24,25f–h,37 In order to gain quantitative insight into the sensor–anion interaction, spectrophotometric titrations have been carried out with F−, CN− and AcO− ions. The spectral changes that occur for 4 as a function of CN− and F− are shown in Fig. 7. As may be noted, with the increase of CN− and F− up to 1 equiv., the MLCT bands in the successive absorption curves undergo red shifts during which they pass through three isosbestic points at 471, 454, and 380 nm for CN− (Fig. 7a) and at 472,
Fig. 6 Changes in UV-vis absorption and luminescence spectra of 4 (a and c, respectively) and 5 (b and d, respectively) in acetonitrile solution upon the addition of different anions as TBA salts.
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Fig. 7 Changes in UV-vis absorption and luminescence spectra of 4 in acetonitrile solution (2.0 × 10−5 M) upon incremental addition of CN− (a and c, respectively) and F− (b and d, respectively) ions (5.0 × 10−3 M). The insets show the fit of the experimental absorbance (a and b) and luminescence (c and d) data to a 1 : 1 binding profile.
452, and 378 nm for F− (Fig. 7b). On the other hand, as the CN− and F− ions are added to the solution of 5, the MLCT absorption maxima at 495 and 436 nm get shifted to longer wavelengths viz. 500 and 438 nm, respectively with concurrent development of three isosbestic points at 380, 334, and 296 nm for CN− (Fig. S16a, ESI†) and at 378, 332, and 296 nm for F− (Fig. S16b, ESI†) upon addition of 1 equiv. of the respective anions. Continual addition beyond 1 equiv. does not produce any observable spectral change in the UV-vis spectra of either 4 or 5. The spectral behavior observed with AcO− as the guest anions is almost identical to that of F− and CN− for both 4 and 5 (Fig. S17 and S18, ESI†). By using eqn (1), the equilibrium constant K for the receptor–anion interaction has been evaluated and the values are given in Table 3. It may be noted that the values of K for the receptors with F−, CN− and AcO− are grossly of six orders of magnitude. Comparable values of the equilibrium constants of related diruthenium(II) complex with anionic guests such as F−, CN− and AcO− have been reported previously.24,25 It was reported previously that a suitably substituted H-bond donor receptor functionality undergoes deprotonation in the presence of excess anions, leading to classical Brønsted acid–base chemistry.38 To examine such a possibility, spectrophotometric titrations of the metalloreceptors (4 and 5) have also been carried out with a solution of TBAOH (Fig. S19 and
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Table 3 Equilibrium/binding constantsa,b (K/M−1) for phen-Hbzim-tpy in DMSO and for 4 and 5 in MeCN towards various anions at 298 K
From absorption spectra Anions
phen-Hbzim-tpy K
4 K
5 K
F− AcO− CN− H2PO4−
7.49 × 104 7.38 × 103 N.A.c 3.25 × 103
3.24 × 106 3.17 × 106 3.66 × 106 N.A.c
2.81 × 106 2.71 × 106 2.96 × 106 N.A.c
From emission spectra Anions
phen-Hbzim-tpy K
4 K
5 K
F− AcO− CN− H2PO4−
5.04 × 104 6.93 × 103 N.A.c 1.95 × 103
3.11 × 106 3.08 × 106 3.48 × 106 N.A.c
2.84 × 106 2.70 × 106 3.12 × 106 N.A.c
a b
t-Butyl salts of the respective anions were used for the studies. Estimated errors were
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PAPER
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Photoinduced intramolecular energy transfer and anion sensing studies of isomeric RuIIOsII complexes derived from an asymmetric phenanthroline–terpyridine bridge† Dinesh Maity, Chanchal Bhaumik, Debiprasad Mondal and Sujoy Baitalik* Two heterobimetallic Ru(II)–Os(II) complexes of compositions [(bpy)2MII( phen-Hbzim-tpy)M’II(tpyPhCH3)]4+, where MII = Ru and M’II = Os (4) and MII = Os and M’II = Ru (5), phen-Hbzim-tpy = 2-(4-(2,6di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10]phenanthroline, bpy = 2,2’-bipyridine, and tpy-PhCH3 = 4’-(4-methylphenyl)-2,2’:6’,2’’-terpyridine have been synthesized and characterized by elemental analyses, ESI mass spectrometry, and 1H NMR and UV-vis absorption spectroscopy. The absorption spectra, redox behavior, and luminescence properties of the complexes have been thoroughly investigated and compared with that of monometallic model compounds [(bpy)2MII( phen-Hbzim-tpy)]2+ [MII = Ru (1) and MII = Os (2)] and [( phen-Hbzim-tpy)RuII(tpy-PhCH3)]2+ (3). The complexes display very intense, ligand-centered absorption bands in the UV and moderately intense MLCT bands in the visible regions. The bimetallic complexes show two successive one-electron reversible metal-centered oxidations, whereas the monometallic complexes display one-electron oxidation in the positive potential window. Steady state and time-resolved luminescence data at room temperature show that an efficient intramolecular electronic energy transfer takes place from the Ru-center to the Os-based component in both the heterometallic dyads in all the solvents. The complexes under investigation contain an imidazole NH proton which became appreciably acidic due to metal coordination and can be utilized for recognition of selective anions in solution either via hydrogen bonding interaction or by proton transfer. Accordingly, the anion binding properties of the two heterobimetallic complexes as well as parent brid-
Received 10th August 2013, Accepted 11th October 2013
ging ligand, phen-Hbzim-tpy, have been studied in solutions using absorption, steady state and time-
DOI: 10.1039/c3dt52186a
resolved luminescence spectral measurements. The metalloreceptors act as sensors for F−, CN− and AcO− ions. It is evident from sensing studies that in the presence of excess of selective anions, deprotona-
www.rsc.org/dalton
tion of the imidazole N–H proton occurs in all cases.
Introduction Molecular photonics is an emerging area of science with regards to controlling the passage of information quanta along nanoscale molecular wires.1–3 Intramolecular electron and energy transfer processes in di- and multinuclear complexes whose mononuclear entities are linked by conjugated bridging ligands have received considerable attention in this area because cooperative interactions between metal centers in these complexes give rise to properties that are useful for constructing photomolecular devices.4,5 The use of polypyridine
Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700032, India. E-mail: [email protected]; Fax: +91 33 24146584; Tel: +91 33 24146666 † Electronic supplementary information (ESI) available: Fig. S1–S30. See DOI: 10.1039/c3dt52186a
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complexes of Ru(II) and Os(II) as building blocks is very appealing in this area because of their unique combination of photophysical, photochemical and electrochemical properties and thermal and photochemical stabilities.6–8 In particular, the Os(II) polypyridines expand the Ru(II) analogue’s light absorbing properties to longer wavelengths because of the 1GS → 3 MLCT absorption.4a Covalently linked two-component systems, dyads, are the simplest class of supramolecular architecture for the study of photo-induced energy and electron transfer processes.9–18 The directional flow of electrons or energy is facilitated when there is asymmetry in the multicomponent system.9–15 Such asymmetry can be introduced either by using ligands which differ in donor–acceptor properties or using heterometallic sites. In principle, such intramolecular charge transfer in binuclear complexes can be exploited for the realization of miniaturized sub-nanoscale devices where electrons or photons dispatch information between specific
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Dalton Transactions
sites within distances of the order of a few angstroms.4,5,9–15 The features associated with bridging ligands such as length, rigidity, topology, conjugation, charge and the numbers of dissociable hydrogen atoms play important roles in this context.4a,9–15 Two types of polypyridine bridging ligands are generally used for this study. The first type containing bidentate chelating sites like 2,2′-bipyridine (bpy) or 1,10-phenanthroline ( phen), either directly coupled or linked by a spacer, are found to be in high usage.1 In terms of structure, however, such a choice is not ideal because substitution of a single position in these ligands will lead to the formation of diastereomeric products. The second type of ligand, containing tridentate chelating sites such as 2,2′:6′,2′′-terpyridine is more appealing from the viewpoint of constructing linear, rodlike polynuclear complexes.4 However, usually such complexes are practically non-luminescent at room temperature and their excited state lifetime (τ = 0.25 ns for [Ru(tpy)2]2+)4 is also very short, representing major barriers for their use as photosensitizers. Consequently, much effort has been devoted to design and synthesize tridentate polypyridine ligands that can produce Ru(II) complexes with enhanced emission quantum yields and excited-state lifetimes.4,18 As compared to the trisbidentate (N–N)3 and bis-terdentate (N–N–N)2 complexes, rigidly-linked homo- and heterometallic assemblies comprising the combination of a bis-terdentate and a tris-bidentate complex are extremely rare, despite the very interesting properties that could be expected from the unique attributes of each type of complex.19–22
Recently, we reported the synthesis, redox activities, and photo-induced electron- and energy transfer processes of three homodimetallic Ru(II)–Ru(II) and two heterodimetallic Ru(II)– Rh(III) complexes based on the use of heteroditopic phenHbzim-tpy as a bridging ligand.23 A key challenge is not only to control the strength but also the direction of the interaction between the two metal centers in both the ground and the excited state. This can be achieved either through the structural modification of the bridging ligand and/or the variation of the nature of the coordinating metal centers. In this context, we report herein the synthesis, characterization, and physicochemical properties of two heterobimetallic complexes of composition [(bpy)2RuII( phen-Hbzim-tpy)OsII(tpy-PhCH3)]4+ and [(bpy)2OsII( phen-Hbzim-tpy)RuII(tpy-PhCH3)]4+, where phen-Hbzim-tpy = 2-(4-(2,6-di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5][1,10] phenanthroline, bpy = 2,2′bipyridine, and tpy-PhCH3 = 4′-(4-methylphenyl)-2,2′:6′,2″-terpyridine (Chart 1). In the first compound, the Ru(II) is bound to phen-Hbzim-tpy via the bidentate N–N site and the Os(II) via the tridentate N–N–N site, while in the second isomer Ru(II) is bound via the tridentate N–N–N site and the Os(II) via the bidentate N–N site of phen-Hbzim-tpy. In contrast to related symmetrical bridging ligands, each metal center in these complexes is geometrically and photophysically distinct that is, (N–N)3 versus (N–N–N)2. As will be seen, the extent and directionality of the interaction between the metal centers is found to be highly dependent on the position of the metal center. Additionally, the present complexes owing to the presence of
Chart 1
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an imidazole NH proton in the second coordination sphere which became appreciably acidic due to metal coordination can be utilized for multi-channel recognition of selective anions either via hydrogen bonding interaction or by proton transfer reaction in solution.24,25 As will be seen, consequent to the interaction with selective anions (such as F−, CN− and AcO−), remarkable changes in the photophysical properties of the complexes occur. Another objective of this study is also to enquire if the rate of the energy transfer in the heterometallic dyads might be modulated by changing the solvents, interacting with selective anions as well as by changing the pH of the solution.
Experimental Materials Reagent grade chemicals obtained from commercial sources were used as received. Solvents were purified and dried according to standard methods. 1,10-Phenanthroline and tetrabutylammonium (TBA) salt of the anions were purchased from Sigma-Aldrich. 1,10-Phenanthroline-5,6-dione,26 4′-( p-methylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCH3),24e,27 4′-(p-formylphenyl)-2,2′:6′,2″-terpyridine (tpy-PhCHO),28 and bridging 2-(4(2,6-di( pyridin-2-yl)pyridine-4-yl)phenyl)-1H-imidazole[4,5-f ][1,10]phenanthroline ( phen-Hbzim-tpy) ligand23,29 were synthesized according to the literature procedures. cis-[Ru(bpy)2Cl2]·2H2O,30 cis-[Os(bpy)2Cl2],31 [(tpy-PhCH3)RuCl3],10 and [(tpy-PhCH3) OsCl3]10 were also prepared by employing the literature procedures. AgClO4 was prepared from silver carbonate and perchloric acid and recrystallized from benzene. The monometallic Ru(II) complex of composition [(bpy)2Ru( phenHbzim-tpy)](ClO4)2 (1) was prepared by the previously published literature procedure.23,29 Synthesis of the metal complexes The complexes were prepared under oxygen- and moisture-free dinitrogen using standard Schlenk techniques. [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2). A mixture of cis-Os(bpy)2Cl2 (285 mg, 0.5 mmol) and phen-Hbzim-tpy (1.10 g, 2.0 mmol) in 100 ml of 1 : 1 (v/v) ethanol–water mixture was heated under reflux for 48 h with constant stirring. After cooling to room temperature, the undissolved suspended material was removed by filtration. The filtrate was concentrated to approximately 30 mL on a rotary evaporator and the solution was filtered again. To the filtrate an aqueous solution (5 mL) of NaClO4 (1.0 g) was added where a dark brown product deposited. The crude product was purified by silica gel column chromatography eluting with acetonitrile. On recrystallization from acetonitrile–methanol (1 : 2 v/v) mixture in the presence of a few drops of aqueous 10−4 M perchloric acid afforded a black microcrystalline compound (380 mg, Yield: 60%). Anal. Calcd for C54H41N11Cl2O10Os: C, 51.26; H, 3.27; N, 12.18. Found: C, 51.24; H, 3.30; N, 12.19. 1H NMR (500 MHz, DMSO-d6, δ/ppm, see Scheme S1, ESI† for proton numbering): 8.90 (d, 2H, J = 8.0 Hz, H3″), 8.85 (d, 4H, J =
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Paper
8.5 Hz, 2H3′ + 2H3″), 8.81 (d, 4H, J = 8.0 Hz, 2H6 + 2H9), 8.70 (d, 2H, J = 8.0 Hz, H3), 8.53 (d, 2H, J = 8.5 Hz, H8), 8.28 (d, 2H, J = 8.5 Hz, H7), 8.08 (t, 2H, J = 7.5 Hz, H4), 8.02–7.98 (m, 4H, 2H4′ + 2H11), 7.90 (t, 2H, J = 8.0 Hz, H10), 7.84 (t, 2H, J = 6.7 Hz, H4′), 7.76 (d, 2H, J = 5.5 Hz, H6′), 7.58 (t, 2H, J = 6.0 Hz, H5), 7.52–7.47 (m, 4H, 2H5′ + 2H6′), 7.25 (t, 2H, J = 6.7 Hz, H5′). ESI-MS ( positive, CH3CN) m/z = 515.59 (100%) [(bpy)2Os( phenHbzim-tpy)]2+, m/z = 1130.03 (10%) [(bpy)2Os( phen-bzimtpy)]+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 650(br)(2750), 486(12 830), 435(sh)(11 750), 318(sh) (36 500), 292(81 080). [(tpy-PhCH3)Ru(tpy-Hbzim-phen)](ClO4)2·3H2O (3). A mixture of Ru(tpy-PhCH3)Cl3 (75 mg, 0.14 mmol), AgBF4 (92 mg, 0.47 mmol) and 30 mL acetone were refluxed with continuous stirring for 4 h. After the solution cooled down to room temperature, the precipitated AgCl was removed by filtration. The filtrate containing [(tpy-PhCH3)Ru(acetone)3]3+ was added slowly to a methanol–chloroform solution of phen-Hbzim-tpy (160 mg, 0.30 mmol) and the solution was refluxed for 5 h, during which time the color of the solution changed from violet to orange-red. After cooling to room temperature, the undissolved suspended material was removed by filtration and kept overnight at a refrigerator. A deep-red colored compound that deposited was filtered, washed with chloroform and ether and then dried under vacuum. The compound was redissolved in the minimum volume of acetonitrile and then subjected to silica-gel column chromatography (eluent: acetonitrile). The eluent was rotary evaporated to a small volume and then underwent an anion exchange reaction with NaClO4 give rise to the desired compound. The compound was finally recrystallized from acetonitrile–methanol (1 : 1 v/v) mixture in presence of a few drops of aqueous 10−4 M perchloric acid (110 mg, Yield: 64%). Anal. Calcd for C56H44N10Cl2O11Ru: C, 55.82; H, 3.68; N, 11.62. Found: C, 55.84; H, 3.72; N, 11.60. 1H NMR (300 MHz, DMSO-d6, δ/ppm): 9.56 (s, 2H, H3′), 9.44 (s, 2H, H3′), 9.10 (d, 2H, J = 7.9 Hz, H6), 9.01 (d, 2H, J = 8.1 Hz, H6), 8.85 (d, 2H, J = 7.9 Hz, H9), 8.62 (nr, 4H, 2H7 + 2H8), 8.30 (d, 2H, J = 8.0 Hz, H8), 7.98 (t, 4H, J = 7.7 Hz, H4), 7.71 (d, 2H, J = 5.5 Hz, H11), 7.67 (t, 2H, J = 7.4 Hz, H10), 7.51–7.48 (m, 6H, 4H3 + 2H7), 7.31 (t, 4H, J = 6.0 Hz, H5), 2.46 (s, 3H, CH3). ESI-MS (positive, CH3CN) m/z = 476.37 (100%) [(tpy-PhCH3)Ru(tpyHbzim-phen)]2+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 500 (46 420), 378(38 580), 326(sh)(54 420), 312(63 420), 278(70 750). [(bpy)2Ru( phen-Hbzim-tpy)Os(tpy-PhCH3)](ClO4)4·H2O (4). A mixture of [(bpy)2Ru( phen-Hbzim-tpy)](ClO4)2·4H2O (1) (120 mg, 0.1 mmol) and [Os(tpy-PhCH3)Cl3] (67 mg, 0.11 mmol) in 20 mL degassed ethylene glycol was heated overnight at 200 °C with continuous stirring. The resulting black solution was cooled to room temperature and then poured into 10 mL aqueous solution of NaClO4·H2O (1.0 g) and stirred for few minutes where a black precipitate appeared. The precipitate was collected by filtration and washed several times with water and then dried under vacuum. The compound was purified in the same way as 3 except using a mixture of CH3CN and 10% aqueous KNO3 (10 : 1 v/v) as the eluent in the column chromatography. Yield: 120 mg, 64%. Anal. Calcd for C76H56N14Cl4O17RuOs: C, 48.80;
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H, 3.02; N, 10.48. Found: C, 48.78; H, 3.04; N, 10.47. 1H NMR (500 MHz, DMSO-d6, δ/ppm): 9.56 (s, 2H, H3′), 9.48 (s, 2H, H3′), 9.16–9.08 (m, 6H, 4H6 + 2H9), 8.87 (d, 2H, J = 7.9 Hz, H3″), 8.84 (d, 2H, J = 7.9 Hz, H3″), 8.68 (d, 2H, J = 8.2 Hz, H8), 8.64 (d, 2H, J = 8.2 Hz, H7), 8.31 (d, 2H, J = 7.9 Hz, H8), 8.20 (t, 2H, J = 7.2 Hz, H4), 8.10 (t, 4H, J = 8.7 Hz, 2H4 + 2H4′), 7.95–7.89 (m, 4H, 2H4′ + 2H6′), 7.83 (d, 2H, J = 5.5 Hz, H6′), 7.63–7.55 (m, 6H, 2H5′ + 2H10 + 2H11), 7.45 (d, 2H, J = 5.8 Hz, H7), 7.40 (d, 4H, J = 5.8 Hz, H3), 7.34 (t, 2H, J = 5.8 Hz, H5′), 7.23–7.17 (m, 4H, H5), 2.52 (s, 3H, CH3). ESI-MS ( positive, CH3CN) m/z = 484.92 (100%) [(bpy)2Ru( phen-bzim-tpy)Os (tpyPhCH3)]3+; UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 672(12 330), 494(64 500), 460(br)(50 170), 424(sh)(37 000), 352(sh)(72 000), 314(123 500), 287(188 750). [(bpy)2Os( phen-Hbzim-tpy)Ru(tpy-PhCH3)](ClO4)4·2H2O (5). A mixture of [Ru(tpy-PhCH3)Cl3] (75 mg, 0.14 mmol), AgBF4 (92 mg, 0.47 mmol) and 30 mL acetone were refluxed with continuous stirring for 4 h. After the solution cooled down to room temperature, the precipitated AgCl was removed by filtration. 40 mL of EtOH was then added to the filtrate. Acetone was removed by rotary evaporation. To the resulting solution was added an ethanol solution of [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2) and refluxed for 12 h. On cooling, a reddish black colored compound that deposited was filtered and then dried under vacuum. Purification of the compound by column chromatography and subsequent recrystallization was done in the same way as 3. Yield: 155 mg, 58%. Anal. Calcd for C76H58N14Cl4O18OsRu: C, 48.34; H, 3.09; N, 10.38. Found: C, 48.31; H, 3.12; N, 10.40. 1H NMR (500 MHz, DMSO-d6, δ/ppm): 9.55 (s, 2H, H3′), 9.46 (s, 2H, H3′), 9.10 (d, 4H, J = 8.2 Hz, H6), 8.91 (d, 2H, J = 8.2 Hz, H9), 8.85 (d, 2H, J = 8.2 Hz, H3″), 8.82 (d, 2H, J = 7.9 Hz, H3″), 8.71 (d, 2H, J = 8.8 Hz, H8), 8.66 (d, 2H, J = 8.8 Hz, H7), 8.36 (d, 2H, J = 7.6 Hz, H8), 8.07–8.00 (m, 6H, 4H4 + 2H4′), 7.89 (t, 2H, J = 7.5 Hz, 2H4′), 7.75 (d, 2H, J = 5.8 Hz, H6′), 7.56–7.47 (m, 12H, 4H3 + 2H6′ + 2H7 + 2H10 + 2H11), 7.27–7.23 (m, 8H, 4H5 + 4H5′), 2.48 (s, 3H, CH3). ESI-MS ( positive, CH3CN) m/z = 363.65 (44%) [(bpy)2Os( phenHbzim-tpy)Ru(tpy-PhCH3)]4+; m/z = 484.52 (100%) [(bpy)2Os ( phen-bzim-tpy)Ru(tpy-PhCH3)]3+. UV-vis [CH3CN; λmax/nm (ε/M−1 cm−1)]: 658(br)(4580), 494(69 500), 439(br)(33 830), 352(sh)(60 580), 372(sh)(85 750), 308(sh)(109 170), 289(162 000). Caution! AgClO4 and perchlorate salt of the metal complexes used in this study are potentially explosive and therefore should be handled in small quantities with care.
Physical measurements Elemental (C, H, and N) analyses were performed on a PerkinElmer 2400II analyzer. 1H NMR and {1H–1H} COSY spectra were obtained on a Bruker Avance DPX 500 MHz spectrometer using DMSO-d6 solution. Electrospray ionization mass spectra (ESI-MS) were obtained on a Micromass Qtof YA 263 mass spectrometer. Electronic absorption spectra were obtained with a Shimadzu UV 1800 spectrophotometer at room temperature. The binding studies of the receptor with different anions were carried out in acetonitrile solution. The binding/
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Dalton Transactions
equilibrium constants were evaluated from the absorbance data using eqn (1).32 Aobs ¼ ðA0 þ A1 K½GT Þ=ð1 þ K½GT Þ
ð1Þ
where Aobs is the observed absorbance, A0 is the absorbance of the free receptor, A∞ is the maximum absorbance induced by the presence of a given anionic guest, [G]T is the total concentration of the guest, and K is the binding constant of the host– guest entity. To determine the ground state pKa values of the complexes, spectrophotometric titrations were carried out with a series of acetonitrile–water (3 : 2 v/v) solutions containing the same amount of complex (10−5 M) and pH adjusted in the range 2.5–12. Robinson–Britton buffer was used in the study.33 Due to the solubility limitation, pKa determination of phen-Hbzimtpy was carried out in dimethylsulfoxide–water (3 : 2 v/v). The pH measurements were made with a Beckman Research Model pH meter. The pKa values were evaluated from the titration data and using eqn (2). pH ¼ pK a log
A A0 Af A0
ð2Þ
Steady state luminescence spectra were recorded on a Perkin-Elmer LS55 fluorescence spectrophotometer. Photoluminescence titrations were carried out with the same sets of solutions as were made with spectrophotometry. Luminescence quantum yields were determined by a relative method using [Ru(bpy)3]2+ as the standard. Time-correlated singlephoton-counting (TCSPC) measurements were carried out for the luminescence decay of complexes. The samples were excited at 450 nm for the metal complexes and at 370 nm for phen-Hbzim-tpy using a picosecond diode laser (IBH Nanoled07) in an IBH Fluorocube apparatus. The luminescence decay data were collected on a Hamamatsu MCP photomultiplier (R3809) and were analyzed by using IBH DAS6 software. The electrochemical measurements (CV and SWV) were carried out with a BAS epsilon electrochemistry system. A three-electrode assembly comprising a Pt (for oxidation) or glassy carbon (for reduction) working electrode, Pt auxiliary electrode, and Ag/AgCl reference electrode and tetraethylammonium perchlorate (TEAP) was used as supporting electrolyte. All the potentials reported in this study were referenced against the Ag/AgCl electrode, which under the given experimental conditions gave a value of 0.36 V for the ferrocene/ferrocenium couple. Experimental uncertainties were as follows: absorption maxima, ±2 nm; molar absorption coefficients, 10%; emission maxima, ±5 nm; excited-state lifetimes, 10%; luminescence quantum yields, 20%; redox potentials, ±10 mV; pKa ±0.2.
Results and discussion Synthesis and characterization of the complexes The outline of the syntheses of the Ru–Os and Os–Ru isomers derived from the heteroditopic phen-Hbzim-tpy bridge is
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Scheme 1
presented in Scheme 1. The monometallic Os(II) complex [(bpy)2Os( phen-Hbzim-tpy)](ClO4)2·2H2O (2) was isolated after refluxing the reactants for 48 h in ethanol–water (1 : 1 v/v) mixture under weakly acidic conditions to keep the imidazole proton intact. For the synthesis of the Ru–Os isomer (4), 1 was boiled with [(tpy-PhCH3)OsCl3] in ethylene glycol. For the formation of the Os–Ru isomer (5), an ethanolic solution of 2 was refluxed with [(tpy-PhCH3)Ru(EtOH)3]3+. All the compounds were then purified by column chromatography by using an appropriate eluent. Finally, the compounds were recrystallized from acetonitrile–methanol (1 : 1 v/v) mixture in the presence of a few drops of aqueous 10−4 M perchloric acid to keep the imidazole NH proton intact. The compounds were characterized by elemental (C, H and N) analyses, ESI mass spectrometric, UV-vis, and 1H NMR spectroscopic measurements and the results are given in the Experimental section. The ESI-mass spectra of the complexes and their simulated isotopic patterns are shown in Fig. S1–S3 (ESI†). As the molecular formulae as well as the masses of 4 and 5 are the same, it is expected that the ESI mass spectra of the two heterobimetallic compounds will also be same. The ESI mass spectrum of 4 shown in Fig. S3 (ESI†) presented a cluster of peaks with
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m/z = 484.92, which corresponds to those of [(bpy)2Ru( phenbzim-tpy)Os(tpy-PhCH3)]3+, while 5 shows a cluster of peak at m/z = 484.52 corresponding to the species [(bpy)2Os( phenbzim-tpy)-Ru(tpy-PhCH3)]3+. Thus, the peak positions (m/z values) of the tri-positive species as mentioned above are almost the same for both 4 and 5 as expected, although small differences are observed in their experimental isotopic distribution patterns. NMR spectra The 1H NMR spectra of the complexes have been recorded in DMSO-d6 at room temperature and the assignments made for the observed chemical shifts, according to the numbering (Fig. S4, ESI†), are listed in the Experimental section. The spectral assignments of the complexes have been made with the help of their {1H–1H} COSY spectra, relative areas of the peaks, and taking into consideration the usual ranges of J values for bpy and tpy-PhCH3.23,24 The 1H NMR spectra of the complexes show the occurrence of a fairly large number of resonances, some of which are overlapped with each other. The most up-field singlet at 2.52 ppm in 4 and 2.48 ppm in 5 (not shown in Fig. S4, ESI†) accounting
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Dalton Transactions
for three protons, is clearly due to –CH3 protons of the coordinated tpy-PhCH3 moiety. It is of interest to note that the H3′ protons which appear as a singlet at 8.89 ppm in 1 and 8.85 ppm in 2 are considerably shifted to the down-field region in both the heterobimetallic complexes when the free terpyridine site in 1 and or 2 is coordinated with a second metal center (RuII or OsII). The 1H NMR spectra of the two heterometallic dyads look extremely complicated, because of the presence of many signals and extensive overlapping. Fig. S4 (ESI†) also shows that there are significant differences in the spectral patters displayed by the two heterometallic isomers. Thus, the presence of two closely situated singlets at 9.56 and 9.48 ppm in 4 and 9.55 and 9.46 ppm in 5, clearly indicate the two different chemical environment of H3′ protons. It is also to be noted that the H3 proton of the tpy moiety of phen-Hbzimtpy in the bimetallic complexes shifts to a significantly up-field region compared to the monometallic complexes (1 and 2) because this proton lies above the shielding region of a pyridine ring of another tpy capping ligand. It is to be noted that a singlet appearing at 14.64 for 4 and 14.66 ppm for 5 is due the imidazole NH proton of the coordinated phen-Hbzim-tpy moiety. Absorption spectra The absorption spectral characteristics of both the Ru–Os complexes (4 and 5) along with monometallic precursor compounds (1–3) were studied in different solvents and the absorption maxima and molar extinction coefficients (ε) are given in Table 1 and Tables S1 and S2 (ESI†). In Fig. 1, the UVvis spectra of equimolar solutions of the five complexes in acetonitrile are displayed. The absorption spectra of the complexes are in general of a similar type in all the solvents. For example, in acetonitrile solution, complexes 1–5 exhibit several very high-intensity absorption bands in the 280–380 nm (ε = 38 580–191 000 M−1 cm−1) spectral region and moderately
Table 1
Fig. 1 UV-vis absorption spectra of equimolar solutions of complexes 1–5 in acetonitrile at room temperature.
strong absorption band and shoulder in the 460–500 nm (ε = 17 240–66 560 M−1 cm−1) region (Fig. S5–S8, ESI†). Three Os(II) containing complexes (2, 4, and 5) show additional broad and weaker spectral features (ε = 4180–8700 M−1 cm−1) in the longer wavelength region (600–720 nm). Based on the extensive investigations performed on [M(bpy)3]2+ and [M(tpy-PhCH3)2]2+ (M = RuII and OsII) and related complexes4,6,18,24 it can be assigned that the high-intensity absorption bands in the 280–380 nm spectral region are due to π–π* transition of bipyridine and spin-allowed ligand-centered (LC) transitions of the bridging phen-Hbzim-tpy ligand, while the absorption bands in the 460–500 nm region are due to spin-allowed overlapping M(dπ) → bpy and M(dπ) → phen-Hbzim-tpy (M = RuII and OsII) charge transfer transitions (MLCT). In this broad-band system, the overlapping contributions from two types of ligands cannot be distinguished although the expectation is that the MLCT transition involving the phen-Hbzim-tpy ligand should be at a slightly lower energy than those involving bpy ligands. In the heterobimetallic complexes (4 and 5), there is another
Spectroscopic and photophysical data of 1–5 in acetonitrile solution
Luminescence At 298 Ka
At 77 Kb
Compounds
Absorption λmax/nm (ε/M−1 cm−1)
λmax/nm τ/ns
Φ/10−3 kr/105 s−1
knr/107 s−1
λmax/nm
Φ
1
460(19 740), 426(sh)(15 670), 326(br)(49 280), 288(109 721) 650(br)(5570), 486(25 990), 435(sh)(23 800), 318(sh)(73 930), 292(164 240) 500(46 420), 378(38 580), 326(sh)(54 420), 312(63 420), 278(70 750) 672(12 330), 494(64 500), 460(br)(50 170), 424(sh)(37 000), 352(sh)(72 000), 314(123 500), 287(188 750) 658(br)(4580), 494(69 500), 439(sh)(33 830), 352(sh)(60 580), 327(sh)(85 750), 308(sh)(109 170), 289(162 000) 474(10 400) 490(28 000) 657(3650), 477(13 750) 667(6600), 490(26 000)
607
151
297
19.59
0.46
589
0.31
720
44.31
83.15
18.76
2.07
711
0.18
658
1.79, 3.34 6.90
38.65, 20.65 55.48, 29.73 645
0.17
750
101
164
16.24
0.82
733
0.20
722
42.52
82.26
19.35
2.16
704
0.15
629 640 718 734
0.25 1010 s−1).11 Similar very fast energy transfer processes have reported for the complexes in which the chelating units are separated by one or two phenyl rings.10,12 In contrast, compounds in which the spacers contain a saturated moiety such as bicycle[2,2,2]octane, adamantine etc. interposed between two phenyl rings have been found to slow down the Ru → Os energy transfer rate remarkably (ken ∼ 106 s−1).9,10 Previously, we have reported moderately fast (ken = 6.1 × 107 s−1) Ru → Os energy transfer where the bridging unit contains an imidazolylbis(benzimidazole) [H2Imbzim]− anion.34 Similar order of ken has been reported for a triazolate anion-bridged RuIIOsII complex.13 In the present case with heteroditopic bridge, the Ru → Os energy transfer rate is very fast (ken ∼ 109 s−1). Energy transfer processes occur in two ways, viz. the Dexter-type35 through bond electron exchange or the Förster-type36 through space Coulombic interaction. However, in most cases it is not easy to ascertain which particular mechanism plays the dominant role. Electron exchange becomes important where the spacer is either highly conjugated or relatively short, while the Coulombic mechanism becomes important when the spacer is either saturated or the transition dipoles are well-separated.9
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Anion sensing studies of the complexes through different channels Sensing of the anions by the metalloreceptors 4 and 5 has been monitored by observing the spectral changes that occur in acetonitrile solutions. As shown in Fig. 6, the MLCT peaks at 494 and shoulder at 462 nm for 4 and at 495 and 436 nm, respectively for 5, remain practically unchanged upon addition of 5 equiv. of Cl−, Br−, I−, NO3− and ClO4− ions to their acetonitrile solutions (2.5 × 10−5 M). On the other hand, following the addition of 5 equiv. of F−, CN−, and AcO−, the MLCT band and shoulder becomes red-shifted to 502 and 467 nm for 4 and to 500 and 438 nm for 5, respectively indicating that interactions occur between the metalloreceptors and anions. The small red-shift of the MLCT bands can be attributed to the second-sphere donor–acceptor interactions between metalcoordinated phen-Hbzim-tpy and the anions. Such interactions (hydrogen bonding or proton transfer, vide infra) increase the electron density at the metal center leading to lowering of the MLCT band energies.24,25f–h,37 In order to gain quantitative insight into the sensor–anion interaction, spectrophotometric titrations have been carried out with F−, CN− and AcO− ions. The spectral changes that occur for 4 as a function of CN− and F− are shown in Fig. 7. As may be noted, with the increase of CN− and F− up to 1 equiv., the MLCT bands in the successive absorption curves undergo red shifts during which they pass through three isosbestic points at 471, 454, and 380 nm for CN− (Fig. 7a) and at 472,
Fig. 6 Changes in UV-vis absorption and luminescence spectra of 4 (a and c, respectively) and 5 (b and d, respectively) in acetonitrile solution upon the addition of different anions as TBA salts.
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Fig. 7 Changes in UV-vis absorption and luminescence spectra of 4 in acetonitrile solution (2.0 × 10−5 M) upon incremental addition of CN− (a and c, respectively) and F− (b and d, respectively) ions (5.0 × 10−3 M). The insets show the fit of the experimental absorbance (a and b) and luminescence (c and d) data to a 1 : 1 binding profile.
452, and 378 nm for F− (Fig. 7b). On the other hand, as the CN− and F− ions are added to the solution of 5, the MLCT absorption maxima at 495 and 436 nm get shifted to longer wavelengths viz. 500 and 438 nm, respectively with concurrent development of three isosbestic points at 380, 334, and 296 nm for CN− (Fig. S16a, ESI†) and at 378, 332, and 296 nm for F− (Fig. S16b, ESI†) upon addition of 1 equiv. of the respective anions. Continual addition beyond 1 equiv. does not produce any observable spectral change in the UV-vis spectra of either 4 or 5. The spectral behavior observed with AcO− as the guest anions is almost identical to that of F− and CN− for both 4 and 5 (Fig. S17 and S18, ESI†). By using eqn (1), the equilibrium constant K for the receptor–anion interaction has been evaluated and the values are given in Table 3. It may be noted that the values of K for the receptors with F−, CN− and AcO− are grossly of six orders of magnitude. Comparable values of the equilibrium constants of related diruthenium(II) complex with anionic guests such as F−, CN− and AcO− have been reported previously.24,25 It was reported previously that a suitably substituted H-bond donor receptor functionality undergoes deprotonation in the presence of excess anions, leading to classical Brønsted acid–base chemistry.38 To examine such a possibility, spectrophotometric titrations of the metalloreceptors (4 and 5) have also been carried out with a solution of TBAOH (Fig. S19 and
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Table 3 Equilibrium/binding constantsa,b (K/M−1) for phen-Hbzim-tpy in DMSO and for 4 and 5 in MeCN towards various anions at 298 K
From absorption spectra Anions
phen-Hbzim-tpy K
4 K
5 K
F− AcO− CN− H2PO4−
7.49 × 104 7.38 × 103 N.A.c 3.25 × 103
3.24 × 106 3.17 × 106 3.66 × 106 N.A.c
2.81 × 106 2.71 × 106 2.96 × 106 N.A.c
From emission spectra Anions
phen-Hbzim-tpy K
4 K
5 K
F− AcO− CN− H2PO4−
5.04 × 104 6.93 × 103 N.A.c 1.95 × 103
3.11 × 106 3.08 × 106 3.48 × 106 N.A.c
2.84 × 106 2.70 × 106 3.12 × 106 N.A.c
a b
t-Butyl salts of the respective anions were used for the studies. Estimated errors were
