New cyclometallated Ru(II) complex for potential application in photochemotherapy?

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Cite this: DOI: 10.1039/c3pp50327e

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New cyclometallated Ru(II) complex for potential application in photochemotherapy?† Bryan A. Albani,a Bruno Peña,b Kim R. Dunbar*b and Claudia Turro*a In an effort to create a molecule that absorbs further into the optimum window for photochemotherapy (PCT), the new cyclometallated complex [Ru(biq)2( phpy)](PF6) (1, biq = 2,2’-biquinoline, phpy− = deprotonated 2-phenylpyridine) was synthesized, characterized and compared to the known photoactive complexes [Ru(biq)2(bpy)](PF6)2 (2, bpy = 2,2’-bipyridine) and [Ru(biq)2( phen)](PF6)2 (3, phen = 1,10-phenanthroline), both of which undergo exchange of one biq ligand when irradiated with red light in coordinating solvents. Excited state ligand dissociation in 2 and 3 is believed to be related to the steric

Received 15th September 2013, Accepted 5th November 2013

hindrance afforded by the presence of two coordinated biq ligands. The ligand exchange quantum yield of 2 is ∼2-fold greater than that of 3, which was shown to be cytotoxic when irradiated with visible light. Cyclometallation results in a red shift of the MLCT absorption maximum of 1 by ∼100 nm relative to

DOI: 10.1039/c3pp50327e

those of 2 and 3, but, although 1 exhibits a distorted octahedral geometry, photoinduced ligand exchange does not occur. DFT calculations were used to aid in our understanding of the lack of photo-

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chemistry of 1 which is explained by the destabilization of the eg(σ*) orbitals upon cyclometallation.

Introduction Ruthenium(II) polypyridyl complexes with extended aromatic ligands have been shown to interact with DNA as chemotherapeutic agents and molecular light switches through intercalation and electrostatic interactions.1–4 Recent studies indicate that certain Ru(II) complexes have the ability to undergo photoinduced ligand exchange forming covalent bonds with DNA in a manner akin to cisplatin, such that these lesions may result in cell death. Unlike traditional photodynamic therapy (PDT) agents that rely on the generation of singlet oxygen for action, these photo-cisplatin analogs achieve cell death via mechanisms that are independent of oxygen; in order to differentiate the two methods, the latter is referred to as photochemotherapy (PCT).5–7 PCT involving transition metal complexes has generally focused on the exchange of monodentate ligands upon irradiation.5,8,9 The photoinduced ligand exchange of bidentate ligands bound to ruthenium(II), however, is well documented for sterically strained complexes including those with ligands such as 2,2′-biquinoline (biq).10 For example, the

a Department of Chemistry and Biochemistry, The Ohio State University, Columbus, OH, USA. E-mail: [email protected] b Department of Chemistry, Texas A&M University, College Station, TX, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional crystal structure and crystallographic data, calculations, 1H NMR data, emission spectra. See DOI: 10.1039/c3pp50327e

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photoinduced exchange of a biq ligand in [Ru(biq)2(bpy)]2+ (bpy = 2,2′-bipyridine) in CH3CN results in the formation of the intermediate cis-[Ru(biq)(bpy)(CH3CN)2]2+, which can be used in the synthesis of tris-heteroleptic Ru(II) complexes of the type [Ru(biq)( phen)(L)]2+ in the presence of a variety of bidentate ligands, L.10 It was not until decades later that [Ru(biq)(phen)2]2+ and [Ru(biq)2(phen)]2+ (phen = 1,10-phenanthroline) were shown to exhibit cytotoxicity upon irradiation with visible light, while being relatively non-toxic under similar conditions in the dark.11 Both Ru(II) complexes undergo ligand dissociation in water following the absorption of visible light to generate the corresponding bis-aqua complexes; the latter species covalently bind to DNA in vitro and these adducts are believed to result in cell death.6,11 Photoinduced ligand exchange occurs in complexes with 3 LF (ligand field) dd states that are thermally accessible from the lower-lying energy 3MLCT (metal-to-ligand charge transfer) state(s).12–15 The thermal population of the metal centered 3LF state results in electron density on the eg-type orbitals with Ru–L(σ*) character, thus resulting in ligand dissociation.12–15 The exchange of bidentate ligands, however, is unusual because both bonds must be cleaved upon MLCT excitation. Bulky biq ligands that sterically strain the conventional octahedral geometry in Ru(II) complexes are believed to lower the energy of the 3LF state relative to the 3MLCT state, thus resulting in enhanced photochemistry.6,11 The maximum of the MLCT absorption of [Ru(biq)2( phen)]2+ located at 550 nm in H2O is outside the optimal excitation range for PCT which is 600–850 nm.11 Therefore,

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Fig. 1

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Schematic representations of the molecular structures of 1–3.

biquinoline complexes that are capable of undergoing photochemical ligand exchange, but with lower energy absorption, are desirable. Cyclometallated Ru(II) complexes have been utilized as light harvesters in solar energy conversion schemes because their MLCT absorption bands are broader and at lower energies relative to the diimine analogs.16–18 The present work focuses on the synthesis and characterization of [Ru(biq)2( phpy)](PF6) (1) ( phpy− = deprotonated 2-phenylpyridine). The photophysical properties and photochemistry of 1 were investigated and compared to those of [Ru(biq)2(bpy)](PF6)2 (2) and [Ru(biq)2( phen)](PF6)2 (3). A schematic representation of the molecular structures of 1–3 is displayed in Fig. 1.

Experimental section Materials The starting materials RuCl3·3H2O (Sigma-Aldrich), 2,2′-biquinoline (Acros, TCI), 2,2′-bipyridine (Sigma-Aldrich), 1,10-phenanthroline (Sigma-Aldrich), and ascorbic acid (Sigma-Aldrich) were used as received without further purification. Ethanol (Decon Laboratories Inc. 200 proof ) was used as received for the synthesis of 2 and 3. For the preparation of 1, ethanol (KORTEP, 200 proof ) was dried over Mg/I2 and was distilled prior to use. Standard Schlenk-line techniques (N2 atmosphere) were used to maintain anaerobic conditions during preparation of the compounds. All solvents used for chromatography (EMD Chemicals) were used as received without further purification. Analytical Thin Layer Chromatography (TLC) was performed on aluminum-backed sheets coated with aluminum oxide neutral 60 F254 adsorbent (0.20 mm thickness, EMD Chemicals). Flash chromatography was carried out with alumina (activated, basic, Brockmann I) from Sigma-Aldrich. The compounds [Ru( phpy)(CH3CN)4](PF6),19 Ru(biq)2Cl2,20 2,21 and 321 were prepared following reported procedures. Instrumentation The 1H NMR spectrum of 1 was measured on a Varian Inova 500 MHz spectrometer whereas those of 2 and 3 were recorded on a Bruker 400 MHz DPX ultrashield system. Electrospray ionization (ESI) mass spectrometry data were collected on a


Bruker micrOTOF instrument with methanol as the eluent. Electronic absorption spectral measurements were carried out using a Hewlett Packard 8453 diode array spectrometer and electrochemical studies were performed on a BAS CV-50W voltammetric analyzer. Emission spectra were obtained on a Horiba Fluormax-4 spectrometer for 2 and 3, and near IR emission in 1 was obtained on a home built instrument utilizing a germanium detector. Photolysis and quantum yield experiments were carried out using a 150 W Xe short arc lamp (USHIO) in an upright Milliarc lamp housing unit (PTI) powered by a LPS-220 power supply (PTI) equipped with a LPS-221 igniter (PTI). The desired wavelength range was attained using band-pass filters (Thorlabs, fwhm ∼10 nm) or 3 mm thick (2 mm for 610 nm) long-pass filters (CVI MellesGriot). Gel imaging was performed with a Bio Rad Gel Doc 2000 transilluminator. [Ru(biq)2( phpy)](PF6) (1) The ligand 2,2′-biquinoline (100 mg, 0.39 mmol) was added to a yellow suspension of [Ru( phpy)(CH3CN)4][PF6] (0.10 g, 0.18 mmol) in ethanol (15 mL), and the mixture was refluxed for 5 h. The resulting dark green solution was reduced to dryness, and the residue was purified by column chromatography (basic Al2O3, CH3CN–CH2Cl2, gradient from 0% to 25% CH3CN). The first green band was collected and reduced to dryness. The solid residue was dissolved with CH2Cl2 (10 mL) and hexanes (8 mL) was added slowly. The green precipitate was collected by filtration and washed with CH2Cl2–hexanes 1 : 1 (3 × 20 mL). Yield: 0.089 g (53%). 1H NMR (500 MHz, CD3CN): δ 8.93 (d, 1H, 3J = 8.5 Hz), 8.84 (d, 1H, 3J = 8.5 Hz), 8.78 (d, 1H, 3J = 9.0 Hz), 8.70 (d, 1H, 3J = 8.5 Hz), 8.64 (d, 1H, 3 J = 9.0 Hz), 8.60 (d, 1H, 3J = 9.0 Hz), 8.28 (d, 1H, 3J = 8.5 Hz), 8.24 (d, 1H, 3J = 9.0 Hz), 8.04 (dd, 1H, 3J = 8.0 Hz, 4J = 1.5 Hz), 7.96 (dd, 1H, 3J = 8.0 Hz, 4J = 1.5 Hz), 7.73 (dd, 1H, 3J = 8.0 Hz, 4 J = 1.5 Hz), 7.62 (d, 2H, 3J = 8.5 Hz), 7.43–7.33 (m, 4H), 7.31 (ddd, 1H, 3J = 8.0 Hz, 3J = 6.5 Hz, 4J = 1.0 Hz), 7.27 (m, 2H), 7.22 (ddd, 1H, 3J = 8.0 Hz, 3J = 6.5 Hz, 4J = 1.0 Hz), 7.13 (d, 1H, 3 J = 8.0 Hz), 7.08 (d, 1H, 3J = 9.0 Hz), 6.94 (m, 2H), 6.88 (ddd, 1H, 3J = 8.5 Hz, 3J = 7.0 Hz, 4J = 1.5 Hz), 6.85–6.79 (m, 3H), 6.66 (ddd, 1H, 3J = 8.5 Hz, 3J = 7.0 Hz, 4J = 1.5 Hz), 6.31 (ddd, 1H,


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J = 7.5 Hz, 3J = 7.5 Hz, 4J = 1.5 Hz), 6.25 (dd, 1H, 3J = 8.0 Hz, J = 1.0 Hz). Elem. anal. calcd for [Ru(biq)2(phpy)](PF6): C, 61.8%; N, 7.67%; H, 3.54%. Found: C, 61.4%; N, 7.61%; H, 3.66%.

4

Methods


1

H NMR spectroscopy was performed in (CD3)2CO (acetone-d6) or CD3CN and all resonances were referenced to the residual protonated solvent peak. Cyclic voltammetry experiments were performed in a three-electrode cell with a Pt working electrode, a Pt wire auxiliary electrode, and a saturated Ag/AgCl reference electrode. The samples were dissolved in distilled CH3CN containing 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte, and bubbled with N2 for 10 minutes prior to each measurement. The cyclic voltammetry data were recorded at a scan rate of 100 mV s−1 with ferrocene being added to the sample after each measurement to serve as an internal standard (+0.40 V vs. SCE in CH3CN).22 The chloride salt of each complex was used for experiments performed in H2O, which were obtained using an Amberlite IRA-410 ion exchange resin, prepared by soaking the powder in a 1 M HCl solution at 50 °C for 3 days, with methanol as the eluent. Emission spectra were measured at both 298 K and 77 K in CH3CN in a 1 × 1 cm quartz cuvette using an excitation wavelength corresponding to the maximum of the MLCT absorption for 2 and 3, and 405 nm for 1. Elemental Analysis was performed by Atlantic Microlab Inc. The quantum yields (Ф) for photoinduced ligand exchange of the biq ligand in H2O were measured for complexes 2 and 3 with 550 nm and 600 nm irradiation wavelengths using the appropriate band-pass filters.23 The moles of complex reacted were quantitated using electronic absorption spectroscopy by monitoring the decrease in MLCT absorption maximum of each complex as a function of irradiation time (moles reacted per s) at early irradiation times, and Reinecke’s salt was used as an actinometer to determine the intensity (Einstein s−1) of the Xe arc lamp at the desired wavelength.23,24 Dark green single crystals of 1 were grown by slow diffusion of hexanes into a dichloromethane solution of the compound in a fine tube. X-ray data were collected at 291 K on a Bruker APEX II CCD X-ray diffractometer equipped with a graphite monochromated MoKα radiation source (λ = 0.71073 Å). The data sets were integrated with the Bruker SAINT software package.25 The absorption correction (SADABS)26 was based on fitting a function to the empirical transmission surface as sampled by multiple equivalent measurements. Solution and refinement of the crystal structures was carried out using the SHELX27 suite of programs as implemented in X-SEED.28 The structure was solved by direct methods with all non-hydrogen atoms being refined with anisotropic displacement parameters using a full-matrix least-squares technique on F 2. Hydrogen atoms were fixed to parent atoms and refined using the riding model. The supercoiled pUC18 (Bayou Biolabs) used in the gel electrophoresis mobility shift assays was purified using a standard QIAprep® Spin Miniprep Kit (QIAGEN). The DNA was bound to a miniprep spin column, washed with 500 μL PB buffer,


Paper with 750 μL PE buffer, and extracted from the column with warm water. The purified DNA was then linearized using a QIAquick® GelExtraction Kit (QIAGEN). For the linearization, 55 μL of water, 25 μL of pUC18, 10 μL of React 4 Buffer (Invitrogen), and 10 μL Sma1 enzyme (Invitrogen) were added to a small Eppendorf tube and heated at 30 °C for 1 h and the enzyme was then deactivated by incubating the sample at 65 °C for 10 minutes. After this procedure, QG buffer (300 μL) and isopropanol (100 μL) were added to the mixture, was added to a spin column and washed with 750 μL of PE buffer, and was then extracted from the column with 40 μL of water. The agarose gel electrophoresis was carried out using 1× TBE buffer ( pH = 8.28) at room temperature for one hour at 95 V powered by an EC 105 voltmeter produced by E-C Apparatus Corporation. Gels were then stained in ethidium bromide solutions (0.5 μg mL−1) for 30 minutes then soaked in water for 30 minutes. Calculations were performed with density functional theory (DFT) using the Gaussian 09 program.29 The B3LYP30–32 functional along with the 6-31G* basis set for H, C, and N33 and the SDD energy consistent pseudopotentials were used for Ru.34 Optimization of full geometries was carried out with the respective programs and orbital analysis was performed in Gaussview version 3.09.35 Following optimization of the molecular structures, frequency analysis was performed to ensure the existence of local minima on the potential energy surfaces. Electronic absorption singlet-to-singlet transitions were calculated using time-dependent DFT (TD-DFT) methods with the polarizable continuum model (PCM) that mimicked the solvation effect of CH3CN in Gaussian 09.36

Results and discussion X-ray crystal structure of 1 The molecular structure of 1 is depicted in Fig. 2 with crystallographic data being compiled in Tables 1 and S1†

Fig. 2 Thermal ellipsoid plot of the [PF6]− salt of complex 1 (ellipsoids drawn at 50% probability).


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Table 1 Selected bond distance, angles, and dihedral angles for the cation [Ru(biq)2( phpy)]+ (1)

Bond lengths (Å)


Ru1–C1 Ru1–N1 Ru1–N2 Ru1–N3 Ru1–N4 Ru1–N5

Bond angles (°) 2.095(4) 2.087(4) 2.092(3) 2.148(3) 2.091(4) 2.096(3)

C1–Ru1–N1 N2–Ru1–N3 N4–Ru1–N5 N1–Ru1–N3 N3–Ru1–N4 C1–Ru1–N4

78.8(2) 76.8(1) 76.7(1) 90.8(1) 98.3(1) 92.4(2)

Dihedral angles (°) N2–C20–C21–N3 C19–C20–C21–C22 N4–C38–C39–N5 C37–C38–C39–C40 N2–Ru1–N3–C29 N4–Ru1–N5–C47

−9.9(6) −11.9(8) −0.9(6) 0.9(8) −165.1(4) 166.2(4)

(CCDC 961134). Compound 1 crystallizes in the monoclinic space group P21/n and there are two interstitial dichloromethane molecules in the asymmetric unit. The coordination sphere of the metal center consists of five nitrogen atoms and one carbon atom in a distorted octahedral environment. The Ru1–C1 bond length of 2.095(4) Å in 1 is longer than the corresponding bond distances in [Ru(bpy)2(phpy)]+, 2.044(1) Å,37 and in [Ru(phen)2(phpy)]+, 2.036(7) Å,38 which may be attributed to the steric repulsion between the biq ligands. Three of the Ru–N bond lengths with the biq ligands, Ru1–N2, Ru1–N4 and Ru1–N5, are similar (∼2.09 Å) and within the range of those found in the closely related compound 3 and [Ru(phen)2(biq)]2+, 2.079(2) to 2.0112(3) Å.11 In contrast, the Ru1–N3biq bond located trans to the C donor atom of phpy− is the longest, 2.148(3) Å, reflecting the strong trans influence of the phenyl ring of phpy−. The angle between adjacent biq ligands, N3–Ru1–N4 is 98.3(1)° which is larger than the angles formed between each biq and phpy−, viz., N1–Ru1–N3 and C1–Ru1–N4, which are 90.8(1) and 92.4(2)°, respectively (Table 1). The more obtuse N3–Ru1–N4 angle is likely a consequence of steric repulsion between the two bulky biq ligands. The biq ligand trans to the C1 atom of phpy− is twisted about the C–C bond, N2–C20–C21–N3, −9.9(6)°, whereas such a distortion is not observed in the other biq ligand, N4–C38–C39–N5, 0.9(6)° (Table 1). In addition, the quinoline moieties of both biq ligands are bent by ∼15° out of the plane formed with the metal center, a distortion that was also observed in 3.11 Electronic absorption, emission, and electrochemistry The absorption profiles of 2 and 3 are in good agreement with previously published data (Fig. 3).21 Complex 2 exhibits maxima at 549 nm (ε = 6600 M−1 cm−1), 482 nm (ε = 4800 M−1 cm−1), and 407 nm (ε = 2800 M−1 cm−1) in CH3CN, and those for 3 are observed at 552 nm (ε = 9600 M−1 cm−1), 480 nm (ε = 7100 M−1 cm−1), and 409 nm (ε = 3900 M−1 cm−1) in the same solvent. The transitions at ∼410 nm are assigned as Ru(t2g)→L(π*) (L = bpy, phen) 1MLCT in 2 and 3, respectively, whereas those at ∼480 nm and ∼550 nm are attributed to Ru(t2g)→biq(π*) transitions.21 Three 1MLCT absorption peaks


Fig. 3 (a) Electronic absorption spectra for 1–3 in CH3CN and (b) normalized emission spectra of 1 and 2 at 77 K in CH3CN.

are also observed in 1, but are red-shifted as compared to the corresponding bands in 2 and 3 (Fig. 3a), with Ru(t2g)→phpy−(π*) absorption maximum at 455 nm (ε = 1700 M−1 cm−1), and bands associated with Ru(t2g)→biq(π*) transitions at 545 (ε = 2200 M−1 cm−1) and 640 nm (ε = 5200 M−1 cm−1). The red shift in 1 is attributed to an increase in energy of the highest occupied molecular orbital (HOMO) resulting from cyclometallation, since the energy of the biq(π*) orbitals is expected to remain relatively unchanged.16,39 The carbon to metal bond provides significant ligand character to the HOMO, which is typically nearly solely metal in character in Ru(II) polypyridyl complexes.16,39 Emission in the near-IR spectral region is observed from complex 1 in CH3CN with maxima at 1030 nm and 980 nm at 298 K and 77 K, respectively (λexc = 405 nm) shown in Fig. 3b, which is significantly red shifted from that of 2 and 3 with maxima at 748 nm and 747 nm at room temperature, respectively, and at 740 nm and 735 at 77 K, respectively, in agreement with published results.21 Cyclic voltammetry reveals quasi-reversible metal-centered oxidation events, E1/2(Ru3+/2+), at +1.44 V vs. SCE for 2 and 3 in CH3CN, which is typical of Ru(II) polypyridyl complexes.40,41


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In the case of the cyclometallated complex, 1, the observed Ru(III/II) couple is observed at a less positive potential, with E1/2(Ru3+/2+) = +0.65 V vs. SCE. This cathodic shift in oxidation potential is typical of cyclometallated complexes relative to bpy or phen and results from the increased electron density on the metal provided by the covalent bonding of the phpy− ligand.39 The increased electron density on the metal also increases the π-backbonding to the pyridyl rings of the biq ligands which leads to a cathodic shift in the sequential ligand centered reductions for 1 relative to those of 2 and 3.39 Quasi-reversible reduction waves, assigned as reduction of the biq ligands, are observed with E1/2(Ru2+/+) = −1.06 and −1.31 vs. SCE for 1 in CH3CN, whereas the analogous reduction events of the biq ligands occur at −0.80 V and −1.03 V vs. SCE for 2 and at −0.80 V and −1.05 V vs. SCE for 3 in the same solvent. A third reduction process is observed for 2 and 3 in CH3CN with the values E1/2(Ru2+/+) = −1.59 V and E1/2(Ru2+/+) = −1.56 V vs. SCE, respectively, assigned to the reduction of the bpy and phen ligands. The reduction of the phpy− ligand in 1 is not observed and must occur outside the spectral analysis window for CH3CN. Photochemistry The photoreactivity of 1–3 was evaluated by monitoring the changes to the electronic absorption spectrum of each complex as a function of irradiation time. As expected based on prior work, the irradiation of 2 and 3 in H2O and CH3CN with visible light (λirr ≥ 530 nm or λirr ≥ 630 nm) results in exchange of one biq ligand with two molecules of coordinating solvent, generating [Ru(biq)(L)(H2O)2]2+ and [Ru(biq)(L)(CH3CN)2]2+ (L = bpy, phen), respectively (Fig. 4 and S6†), but the complexes are not reactive when kept in the dark under similar experimental conditions (Fig. S4 and S5†).10,11 In contrast, complex 1 is not photochemically active in CH3CN (λirr ≥ 530 nm, Fig. S7†) or H2O based on absorption changes or when monitored by mass spectrometry as a function of irradiation time (λirr ≥ 530 nm). The quantum yield for the exchange of the biq ligand in 2 in H2O with λirr = 550 nm and λirr = 600 nm irradiation were measured to be 0.068(2) and 0.053(3), respectively. These values are factors of 3.4 and 2.2 greater than those measured for 3, Φ550 = 0.020(3) and Φ600 = 0.024(2) at each wavelength, respectively. A similar trend was observed between the sterically hindered complexes [Ru(bpy)2(dmbpy)]2+ and [Ru(bpy)2(dmdpq)]2+ (dmbpy = 6,6′-dimethyl-2,2′-bipyridine, dmdpq = 7,10-dimethyl-pyrazino[2,3-f ]-1,10-phenanthroline) in which the exchange of the dmdpq ligand possessing fused aromatic rings occurred less efficiently than the analogous dmbpy complex,6 suggesting that less rigid bidentate ligands enhance the ligand exchange. Calculations DFT and TD-DFT calculations were performed to gain a better understanding of the electrochemistry, photophysical properties, and photochemistry observed for the three complexes. The DFT calculations reveal metal-based HOMO, HOMO−1,


Fig. 4 Electronic absorption spectral changes of (a) 2 (40 μM) with increasing irradiation times, tirr, 0, 1, 2, 3, 5, 10, 15, 20, 30, 45, 60, 90, and 180 min and (b) 3 (15 μM) at tirr = 0, 2, 5, 10, 20, 30, and 60 min in H2O (λirr ≥ 630 nm).

and HOMO−2 levels representing the dxy, dxz, and dyz orbitals for complexes 1–3. These HOMOs are calculated at nearly identical energies for 2 and 3, but that of 1 is destabilized by ∼0.6 eV and exhibits significant phpy− ligand character and electron density on the metal–carbon bond (Fig. 5). The HOMO of 2 was arbitrarily set at 0.0 eV. The differences in the calculated energies of the HOMOs agree with the cathodic shift in the experimental oxidation potential of 0.75 V between 1 and 2 or 3. The additional ease in oxidation by ∼0.15 V may be related to the overall +1 charge of 1 as compared to +2 of 2 and 3 which renders removal of an electron from the former more favorable. The LUMO and LUMO+1 orbitals of 1–3 are calculated to be localized on the biq ligands and to lie at similar energies in the three complexes. Although 1 is more difficult to reduce than 2 and 3 by ∼0.2 V, this difference may also be related to the overall charge of the complexes. TD-DFT calculations reveal that the lowest vertical singlet excited states of 1, 2, and 3 possess significant contributions, ∼88% for 1 and ∼97% for 2 and 3, from HOMO→LUMO transitions, but low oscillator strengths, with maxima at 840 nm ( f = 0.0013), 586 nm ( f = 0.0003), and 587 nm ( f = 0.0004), respectively (Tables S2–S4†). More intense absorption bands with f > 0.01 are predicted at 686 nm ( f = 0.027) for 1, at 523 nm ( f = 0.076) for 2, and at 527 nm ( f = 0.086) for 3 calculated to possess 83%, 93%, and 96% contribution from HOMO−1→LUMO transitions, respectively. These calculated absorption maxima are similar to the experimental values, 640 nm, 549 nm, and 552 nm, for 1, 2, and 3, respectively.


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Photochemical & Photobiological Sciences the related cyclometallated complex [Ru(phpy)(bpy)(CH3CN)2]+.42 Upon irradiation, one bond is weakened due to population of the lower lying energy 3LF state resulting in ligand dissociation and solvent coordination, but the remaining CH3CN does not exchange with extended photolysis because the higher-lying 3 LF state is not populated.


Gel mobility assays

Fig. 5 Calculated (a) MO diagrams showing the frontier orbitals and (b) electron densities of the HOMOs (isovalue = 0.04) of 1 and 2.

In order to understand the lack of photodissociation of the biq ligand in 1 as compared to 2 and 3, the orbitals and transitions involved in the process must be considered. As stated previously, ligand dissociation is believed to occur via thermal population of the 3LF states from the low-lying 3MLCT state(s).12–15 Experimental data, calculations, and previous work all indicate that the HOMO in 1 lies at a higher energy than those of 2 and 3, but that the energies of the LUMOs remain relatively unchanged among the three complexes.16 It is expected that cyclometallation results in a larger gap between the 3MLCT state and 3LF states because the eg(σ*) orbitals are destabilized due to the increased covalent interaction provided by the Ru–C bond, significantly raising the energy of the eg orbitals with Ru–L(σ*) character.16 The eg(σ*) orbitals in 2 are calculated as the LUMO+9 (dz2) and LUMO+10 (dx2−y2), are relatively close in energy (ΔE = 0.26 eV). In contrast, the eg(σ*) orbitals in 1 are calculated to be LUMO+10 (dz2) and LUMO+17 (dx2−y2) with an energy difference of 3.22 eV. The relative energies of the two eg(σ*) orbitals is related to the energies of the 3LF states involving the population of each d-orbital, dz2 and dx2−y2. In order for photodissociation of the bidentate ligand to take place, both Ru–N(biq) bonds must be broken. Because the 3LF state associated with the dx2−y2 orbital lies at a very high energy in 1, it is not likely to be accessible upon irradiation with visible light. It is proposed that, although one Ru–N(biq) bond may break upon irradiation, it quickly re-coordinates to the metal. This explanation is also consistent with the observed exchange of only one CH3CN in


Gel electrophoresis mobility shift assays were carried out in order to compare the photoinduced DNA binding of 2 and 3. It is well documented that cisplatin thermally binds to linearized DNA and reduces its migration through an agarose gel in a concentration-dependent manner.43 The same pattern is observed for 2 and 3 upon irradiation with low energy light, but not in the dark (Fig. 6). In Fig. 6, lanes 1 and 8 contain 1 kb DNA ladder, lanes 2–7 were loaded with 50 μM pUC18 DNA, and lanes 3–6 contain increasing concentrations of 2 or 3. In Fig. 6a and 6c, the samples in lanes 3–6 were irradiated for 15 minutes with λirr ≥ 630 nm light prior to loading. It is evident in Fig. 6a that, as the concentration of 2 is increased, the DNA mobility decreases, whereas no shift in mobility is observed when the samples are incubated in the dark for 20 min under similar experimental conditions (Fig. 6b). These results are indicative of covalent binding of 2 to DNA only upon irradiation. Fig. 6c and 6d display the results for complex 3 irradiated under the same conditions as 2 (Fig. 6a) and in the dark, respectively. The reduced effect of 3 as compared to 2 on the DNA mobility can be attributed to the ∼2-fold lower Ф600 value of the former, such that a smaller amount of the photoproduct that binds to DNA is formed. Increased DNA binding was observed for 3 when the irradiation times were doubled, but it nevertheless remained lower than the effect observed for 2 (Fig. S8†).

Conclusions The new cyclometallated complex [Ru(biq)2( phpy)](PF6) (1) was synthesized and characterized by various methods including X-ray crystallography. The photophysical electrochemical and photochemical properties of 1 were compared to those of known photoactive complexes [Ru(biq)2(bpy)](PF6)2 (2) and [Ru(biq)2( phen)](PF6)2 (3). Complexes 2 and 3 undergo exchange of one of the biq ligands when irradiated with λirr ≥ 630 nm light in water to generate the corresponding complexes [Ru(biq)2(bpy)(H2O)2](PF6)2 and [Ru(biq)2( phen)(H2O)2](PF6)2. The resulting bis-aqua photoproducts covalently bind to linearized ds-DNA. The quantum yield for ligand exchange for 2, Φ600 = 0.053(3), was measured to be 2.2-fold greater than that determined for 3, Φ600 = 0.024(2), at the same irradiation wavelength, 600 nm, thereby lending support to the hypothesis that less rigid ancillary ligands lead to more efficient biq ligand exchange. The differences in ligand exchange quantum yields of 2 and 3 were correlated to the DNA binding ability of the complexes.


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Fig. 6 Imaged ethidium bromide-stained agarose gels of 50 μM linearized pUC18 plasmid (10 mM phosphate buffer, pH = 7.8) in the presence of various concentrations of complex: lanes 1 and 8, 1 kb DNA molecular weight standard; lanes 2 and 7, linearized plasmid alone; lanes 3–6, 25, 50, 75, 100 μM of 2 (a) irradiated (λirr ≥ 630 nm, 15 min), (b) incubated in the dark (20 min, 298 K) and 3 (c) irradiated (λirr ≥ 630 nm, 15 min), and (d) incubated in the dark (20 min, 298 K).

Since the compounds [Ru(bpy)3](PF6)2 and [Ru( phen)3](PF6)2 are stable upon irradiation relative to the biq complexes 2 and 3, steric bulk was thought to be a key criteria for photoinduced bidentate ligand exchange. Although the crystal structure of 1 reveals an elongated Ru–N(biq) bond, the complex does not display photoinduced ligand substitution in coordinating solvents under similar conditions as those used for 2 and 3. The difference in reactivity of the cyclometallated complex 1 is ascribed to increased energy of the metal-centered 3LF states resulting from the bonding of the strong σ-donor phpy− ligand, a finding supported by DFT calculations. Complexes 2 and 3 are good candidates as PCT agents owing to their covalent DNA binding upon irradiation with light in the photodynamic window. Although the absorption maximum of 1 is red-shifted relative to those of 2 and 3, the lack of photochemistry of the former compound indicates that the use of cyclometallated Ru(II) complexes may not be a good strategy for the design of new PCT agents.

Acknowledgements K.R.D. and C.T. thank the National Science Foundation for partial support of this work (CHE-1213646) and the Ohio Supercomputer Center.

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