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Synthesis, structure, redox, NLO and DNA interaction aspects of [{(L–)2RuII}3(3-L)]3+ and [(L)2RuII(NC5H4S−)]+

[L3− = 1,3,5-triazine-2,4,6-trithiolato, L– = arylazopyridine]†

Sanjib Kar,a Biswajit Pradhan,b Rajeev Kumar Sinha,b Tapanendu Kundu,b Prashant Kodgire,c K. Krishnamurthy Rao,c Vedavati G. Puranikd and Goutam Kumar Lahiri*aa Department of Chemistry, Indian Institute of Technology–Bombay, Powai, Mumbai 400076,

India. E-mail: lahiri@chem.iitb.ac.inb Department of Physics, Indian Institute of Technology–Bombay, Powai, Mumbai 400076, Indiac School of Biosciences and Engineering, Indian Institute of Technology–Bombay, Powai,

Mumbai 400076, Indiad Centre for Materials Characterisation, National Chemical Laboratory, Pune 411008,

Maharashtra, India

Received 3rd March 2004, Accepted 21st April 2004First published as an Advance Article on the web 6th May 2004

The trinuclear complexes [{(L–)2RuII}3(3-L)](ClO4)3, [1](ClO4)3–[3](ClO4)3 {L = trianionic form of 1,3,5-triazine-2,4,6-trithiol; NpC5H4NNa–C6H4(R), R = H (L), m-Me (L″), p-Me (L)} and the analogous mononuclear complex [(L)2RuII(NC5H4S−)]ClO4 [4]ClO4 were synthesized. Crystal structures of [1](ClO4)3 and [4]ClO4 were determined. [1]3+–[3]3+ exhibit three successive oxidative couples corresponding to RuIIRuIIRuIII RuIIRuIIRuII; RuIIRuIIIRuIII RuIIRuIIRuIII; RuIIIRuIIIRuIII RuIIRuIIIRuIII where the mixed valent states are moderately coupled. The complexes display multiple reductions associated with the azo functions of the ancillary ligands (L–). The energy of the RuII-based lowest energy MLCT transitions (533–558 nm) involving the * level of azoimine chromophore of L– varies depending on the nuclearity as well as substituents in the ligand framework and follows the order: [1]3+ > [2]3+ > [3]3+ > [4]+. The complexes exhibit reasonably high third-order non-linear optical properties with = (0.90–2.45) × 10−29 esu. The interactions of the trinuclear complexes [{(L)2RuII}3(3-L)]3+ [1]3+, [{(bpy)2RuII}3(3-L)]3+ [5]3+ and [{(phen)2RuII}3(3-L)]3+ [6]3+ (bpy = 2,2-bipyridine and phen = 1,10-phenanthroline) with the circular and linear forms of p-Bluescript DNA show reduced ethidium bromide fluorescence on gel electrophoresis.

Introduction1,3,5-Triazine-2,4,6-trithiol (H3L) (also known as 2,4,6-trimercap-totriazine or trithiocyanuric acid) which exists in either thiol (IA) or thione (IB) (Scheme 1) form, is an effective analytical reagent to remove univalent and divalent heavy metals (Hg2+, Ag+, Cd2+, Pb2+, Cu2+) from waste water.1 The recent systematic investigations of the coordination chemistry of H3L reveal that it can function as a versatile ambidentate ligand with a variety of coordination modes including monodentate N- or S- donor (II),2 bidentate chelating [N,S]− donor (III),3 or bridging two metal ions through the two bidentate [N,S]− donor sets (IV) (Scheme 1).4

The trinucleating mode of [L]3− using all three available [N,S]− donor sets (V) is limited to only two sets of metal complexes [{(5-CH3C5H4)2TiIII}3(3-L)]5 and [{(bpy)2/(phen)2RuII}3(3-L)]3+ (bpy = 2,2-bipyridine, phen = 1,10-phenanthroline).6 However, structur-ally authenticated trinucleating motif of the type (V) (Scheme 1) has not been reported up to now. Moreover, H3L is known to form supramolecular layered and channel structures on co-crystallisation with different organic molecules via extensive hydrogen bonding interactions.7

The present work originates from our interest in developing newer classes of polynuclear ruthenium complexes incorporating selective combinations of bridging and ancillary functionalities which can exhibit (i) bridging function mediated intermetallic electronic coupling in the mixed valent states(s);6,8 (ii) non-linear optical properties suitable for photonic devices;6 and (iii) DNA

cleavage properties suitable for therapeutic applications.9 We have thus designed triruthenium complexes incorporating the relatively less explored trianionic thiolato based bridging unit ([L]3−) in combination with the strongly -acidic 2-arylazopyridine ancil-lary ligands [{(L–)2RuII}3(3-L)](ClO4)3, [1](ClO4)3–[3](ClO4)3, {NpC5H4NNa–C6H4(R), R = H (L), m-Me (L″), p-Me (L)}. The analogous mononuclear complex [(L)2RuII(NC5H4S)](ClO4), [4](ClO4) has also been synthesized. The trinucleating motif of L3− has been authenticated for the first time via the crystal structure determination of the representative complex [1](ClO4)3. The crystal structure of the analogous mononuclear complex [4]ClO4 has also been determined. The spectro-electrochemical and nonlinear optical properties of all the complexes have been studied. The interactions of [1]3+ and the recently reported6 analogous trinuclear complexes

† Electronic supplementary information (ESI) available: Table S1: Short contacts in [1](ClO4)3·H2O. Table S2: C–HO interactions in [4]ClO4·C6H6. Fig. S1: Packing diagram of [4](ClO4)·C6H6 (down the b-axis). Figs. S2 and S3: Mass spectral data for [3](ClO4)3 and [4]ClO4. Fig. S4: Experimental layout for nonlinear absorption and z-scan measurements. Description of the z-scan technique used. See http://www.rsc.org/suppdata/dt/b4/b403332a/

Scheme 1

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[{(bpy)2RuII}3(3-L)](ClO4)3, [5](ClO4)3 and [{(phen)2RuII}3(3-L)](ClO4)3, [6](ClO4)3 with the linear and circular forms of p-Blue-script DNA have also been explored.

Results and discussionSynthesis

The reaction of the sodium salt of 1,3,5-triazine-2,4,6-trithiol (Na3L) with the ruthenium starting complexes ctc-[RuII(L –)2(H2O)2]2+ [L– = NpC5H4–NNa–C6H4(R); R = H (L); m-Me (L″); p-Me (L)] (ctc, cis-trans-cis with respect to the aqua molecules, pyridine (Np) and azo (Na)-nitrogens, respectively10) in a 1 : 3 molar ratio in ethanol under a dinitrogen atmosphere followed by chromatographic purification using a silica gel column resulted in trinuclear com-plexes [{(L–)2RuII}3(3-L)]3+, [1–3]3+ (Scheme 2). All attempts to synthesize dinuclear [{(L–)2RuII}2(L)]2+ (motif-IV, Scheme 1) and mononuclear [{(L–)2RuII}(L)]+ (motif-III, Scheme 1) derivatives using appropriate 1 : 2 and 1 : 1 molar ratios of ctc-{Ru(L–)2

2+} and L, respectively, have failed altogether. On every occasion the trinuclear species [1–3]3+ were obtained exclusively, irrespective of the metal–ligand ratio used.

[2.434(2) and 2.437(2) Å]11 and [{(bpy)2RuII}2((CH3)C4N2H-OS)](ClO4)2 [2.432(11) Å].8f The Ru–N (L3−) distances [2.105(7) Å (average)] in [1](ClO4)3 and Ru–N(7) (pyridine-2-thiolato) distance [2.085(4) Å] in [4]ClO4 are reasonably longer than the Ru–N (pyri-dine) distances involving L, 2.021(8) and 2.021(4) Å (average) in [1](ClO4)3 and [4]ClO4, respectively. The presence of strongly -ac-ceptor azo (NN) function of L trans to the Ru–N bond (involv-ing L3− or pyridine-2-thiolato) possibly weakens the latter. The azo function of one of the terminal ligands (L) around a metal centre is trans to the nitrogen centre of the bridging ligand (L3−) in [1](ClO4)3 or pyridine-2-thiolato in [4]ClO4 whereas the azo function of the other terminal ligand around the same metal ion is trans to the thio-lato group. The RuII–N(azo) distance is expected to be shorter than the RuII–N (pyridine) distance of L due to the d(RuII) → p*(azo) back-bonding as has been observed in other structurally character-ised ruthenium–azopyridine complexes.10,13–16 However, RuII → azo back-bonding in [1](ClO4)3 or [4]ClO4 is apparent only when the azo function of L is trans to the nitrogen atom of the bridging ligand (L3−) or pyridine-2-thiolato. Therefore, out of the two RuII–N (azo) distances per metal unit only one is appreciably shorter than the corresponding Ru–N (pyridine) distance (Table 1). In conse-quence to the d(RuII) → p*(azo) back-bonding the average azo (NN) distance (1.283 Å) of the coordinated L is longer than that observed in the free azo ligands (1.25 Å).17

Scheme 2

Therefore, the analogous mononuclear derivative [(L)2RuII(NC5H4S−)]+ [4]+ (Scheme 2) was prepared via the reac-tion of ctc-[RuII(L)2(H2O)2]2+ and pyridine-2-thiol and in presence of CH3CO2Na base under a dinitrogen atmosphere followed by chromatographic purification using a silica gel column.

The complexes were isolated as their perchlorate salts. The dia-magnetic trinuclear [1]3+–[3]3+ and mononuclear [4]+ complexes exhibited 1 : 3 and 1 : 1 conductivities, respectively, in acetonitrile and gave satisfactory microanalytical and mass spectral data (see Experimental section).

Crystal structures [1](ClO4)3 and [4]ClO4

The crystal structures of the representative trinuclear complex [1](ClO4)3 and the mononuclear one [4]ClO4 are shown in Figs. 1 and 2 and selective bond distances and angles are given in Table 1. The bridging ligand L3− is bonded to the three ruthenium centres using all three available [N,S]− donor sets in [1](ClO4)3 (motif-V, Scheme 1). The two pyridine (Np) and two azo (Na)-nitrogen donors associated with the terminal ligands (L) around each ruthenium ion constitute the trans and cis pairs, respectively, in both trinuclear and mononuclear complexes. Thus the trans and cis geometry of the precursor {Ru(L)2} fragment10 has been retained in both [1]3+ and [4]+. The RuN5S coordination sphere is distorted octahedral as can be seen from the angles subtended at each metal ion (Table 1). Distortion is primarily due to the customary N–Ru–N bite angles involving the azopyridine ligand (average 76.27 and 76.28° for [1]3+ and [4]+, respectively) and N–Ru–S bite angles involving the bridging ligand L3− (average 67.43°) in [1]3+ and the pyridine-2-thiolato ligand (68.23°) in [4]+. Thus the four-membered chelate rings originating from the bridging ligand (L3−) in [1]3+ and from the pyridine-2-thiolato ligand in [4]+ suffer from a considerable strain.11,12 The RuII–S distances, 2.430(3), 2.437(3) and 2.434(3) Å in [1](ClO4)3 and 2.421(2) Å in [4]ClO4 are bit longer than those found in RuII(SC6H4NNC5H4N)2 [2.376(2) and 2.359(2) Å],13 but comparable with the other complexes incorporating sterically hindered four-membered pyridine-2-thiolato based chelate rings,[RuII(bpy)2(NC5H4S−)]ClO4 [2.434(3) Å],12 RuII((NC5H4S−)2(PPh3)2

Fig. 1 ORTEP diagram of [1](ClO4)3·H2O. Perchlorate anions and solvent molecule are removed for clarity. Ellipsoids are drawn at 50% probability.

The crystal structure of the trinuclear complex [1](ClO4)3 shows the presence of three disordered perchlorate moieties along with two water molecules having half occupancy. The three larger {Ru(L)2} fragments pull the central six-membered bridging ring and constraints on this ring were applied during the refinement. Several –, C–H and C–HO interactions along with intra-molecular C–HN and C–HS short contacts are observed in the crystal structure (Table S1, ESI†). When the molecules are viewed down the b-axis one can see ClO4

− and H2O molecules lie in a row (Fig. S1, ESI†)].

The crystal structure of [4]ClO4 shows the presence of one per-chlorate ion along with one benzene molecule as solvent of crystal-lisation. Several C–H interactions are observed in the crystal structure and oxygen atoms of perchlorate ion make C–HO short contacts (Table S2, ESI†). Solvent benzene molecules nicely fit in the cavity formed by the complex molecule (Fig. 3).

1H NMR spectra

The 1H NMR spectrum of the mononuclear complex [4]+ in CDCl3 exhibits the calculated number of twenty two partially overlapping

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aromatic protons in the region 9.5–6.5 ppm (Fig. 4(a)), consistent with the non-equivalent nature of all the five aromatic rings. The 1H NMR spectra of the trinuclear complexes ([1]3+–[3]3+) are com-plicated due to the presence of a large number of aromatic protons with similar chemical shifts (Fig. 4(b)). However, the direct com-parison of the integrations of the clearly resolved methyl signals with those of the overlapping aromatic signals reveals that the cal-culated number of 54 signals for [1]3+ and 51 signals for [2]3+ and

[3]3+ are present. As a consequence of the low-symmetric structure six closely spaced but distinct methyl signals are observed for the complexes, [2]3+ [: 2.04, 2.09, 2.11, 2.13, 2.16, 2.21 ppm] and [3]3+ [: 2.22, 2.23, 2.25, 2.26, 2.30, 2.32 ppm] (Fig. 4(b)).

Electronic spectra

The complexes systematically exhibit three intense bands in the UV region (Table 2, Fig. 5) which are believed to be intraligand transitions.18 In addition, one intense band in the range 533–558 nm associated with a shoulder at the higher energy part (510 nm) has also been observed systematically for all the complexes. These are tentatively assigned as MLCT transitions involving * accep-

Table 1 Selected bond distances (Å) and angles (°) for [1](ClO4)3·H2O and [4](ClO4)·C6H6

[1](ClO4)3·H2O [4](ClO4)·C6H6

Ru(1)–N(4) 1.989(9) Ru(3)–N(15) 2.012(8) Ru–N(6) 1.978(4) Ru(1)–N(3) 1.996(8) Ru(3)–N(13) 2.020(8) Ru–N(3) 2.029(4) Ru(1)–N(1) 2.018(9) Ru(3)–N(18) 2.033(8) Ru–N(1) 2.037(4) Ru(1)–N(6) 2.033(8) Ru(3)–N(16) 2.038(9) Ru–N(4) 2.042(4) Ru(1)–N(21) 2.122(7) Ru(3)–N(20) 2.088(7) Ru–N(7) 2.085(4) Ru(1)–S(1) 2.430(3) Ru(3)–S(3) 2.434(3) Ru–S 2.421(2) Ru(2)–N(9) 1.986(7) N(2)–N(3) 1.289(10) N(2)–N(3) 1.284(6) Ru(2)–N(7) 2.033(9) N(5)–N(6) 1.292(10) N(5)–N(6) 1.283(6) Ru(2)–N(10) 2.042(9) N(8)–N(9) 1.256(10) Ru(2)–N(12) 2.045(8) N(11)–N(12) 1.290(10) Ru(2)–N(19) 2.105(7) N(14)–N(15) 1.301(10) Ru(2)–S(2) 2.437(3) N(17)–N(18) 1.274(9) N(4)–Ru(1)–N(3) 96.8(3) N(10)–Ru(2)–N(19) 88.6(2) N(6)–Ru–N(3) 99.16(17) N(4)–Ru(1)–N(1) 173.2(3) N(12)–Ru(2)–N(19) 102.2(2) N(6)–Ru–N(1) 96.90(17) N(3)–Ru(1)–N(1) 76.5(4) N(9)–Ru(2)–S(2) 102.4(2) N(3)–Ru–N(1) 76.19(18) N(4)–Ru(1)–N(6) 77.0(4) N(7)–Ru(2)–S(2) 86.8(2) N(6)–Ru–N(4) 76.38(17) N(3)–Ru(1)–N(6) 98.9(3) N(10)–Ru(2)–S(2) 94.4(3) N(3)–Ru–N(4) 105.28(17) N(1)–Ru(1)–N(6) 104.5(3) N(12)–Ru(2)–S(2) 166.6(3) N(1)–Ru–N(4) 173.25(17) N(4)–Ru(1)–N(21) 91.5(2) N(19)–Ru(2)–S(2) 67.6(2) N(6)–Ru–N(7) 164.58(17) N(3)–Ru(1)–N(21) 164.6(2) N(15)–Ru(3)–N(13) 77.1(4) N(3)–Ru–N(7) 95.37(17) N(1)–Ru(1)–N(21) 94.9(3) N(15)–Ru(3)–N(18) 85.5(3) N(1)–Ru–N(7) 91.53(17) N(6)–Ru(1)–N(21) 95.60(19) N(13)–Ru(3)–N(18) 104.3(3) N(4)–Ru–N(7) 94.86(17) N(4)–Ru(1)–S(1) 95.6(3) N(15)–Ru(3)–N(16) 100.4(4) N(6)–Ru–S 98.12(12) N(3)–Ru(1)–S(1) 98.7(2) N(13)–Ru(3)–N(16) 177.5(3) N(3)–Ru–S 161.30(13) N(1)–Ru(1)–S(1) 84.8(2) N(18)–Ru(3)–N(16) 75.4(4) N(1)–Ru–S 94.70(13) N(6)–Ru(1)–S(1) 161.6(2) N(15)–Ru(3)–N(20) 171.5(3) N(4)–Ru–S 85.70(12) N(21)–Ru(1)–S(1) 67.51(19) N(13)–Ru(3)–N(20) 96.0(2) N(7)–Ru–S 68.23(14) N(9)–Ru(2)–N(7) 75.4(4) N(18)–Ru(3)–N(20) 101.2(2) N(9)–Ru(2)–N(10) 99.0(4) N(16)–Ru(3)–N(20) 86.5(2) N(7)–Ru(2)–N(10) 174.4(3) N(15)–Ru(3)–S(3) 107.2(2) N(9)–Ru(2)–N(12) 88.7(3) N(13)–Ru(3)–S(3) 87.8(2) N(7)–Ru(2)–N(12) 103.5(4) N(18)–Ru(3)–S(3) 164.3(2) N(10)–Ru(2)–N(12) 76.2(4) N(16)–Ru(3)–S(3) 92.9(3) N(9)–Ru(2)–N(19) 168.0(3) N(20)–Ru(3)–S(3) 67.19(19) N(7)–Ru(2)–N(19) 96.9(2)

Fig. 2 ORTEP diagram of [4](ClO4)·C6H6. Perchlorate anion and solvent molecule are removed for clarity. Ellipsoids are drawn at 40% probability.

Fig. 3 Packing diagram of [4](ClO4)·C6H6 (down the c-axis).

1 7 5 4 D a l t o n T r a n s . , 2 0 0 4 , 1 7 5 2 – 1 7 6 0

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tor levels of azopyridine and triazine ligands.6,8i The energy of the lowest energy MLCT transition probably involving the * level of azoimine chromophore of the azopyridine ligand varies depending on the nuclearity as well as the substituents in the ligand frame-work and follows the order: [1]3+ > [2]3+ > [3]3+ > [4]+. On the other hand, higher energy shoulders remain unaffected on moving from [1]3+ → [3]3+. For the starting complexes ctc-[RuII(L–L)2Cl2], the (d)RuII → *(L–L) MLCT transitions appear near 580 nm.19 Thus the t2g level of RuII in the present set of complexes ([1]3+–[3]3+ and [4]+) is further stabilised and the effect is more pronounced in the case of trinuclear species. This has also been reflected in their redox potentials (see later). Though the spectral profile of the tri-nuclear species ([1]3+–[3]3+) is similar to that of the mononuclear analogue [4]+, the intensity of the MLCT transition is higher in [1]3+–[3]3+ as compared to the mononuclear one [4]+ (Table 2).8d

Electrochemical properties

Metal redox. The monomeric complex [4]+ exhibits irreversible Ru(II) → Ru(III) oxidation process at 1.05 V vs. SCE (Fig. 6(a)), although the analogous bipyridine derivative [Ru(bpy)2(NC5H4S−)]+ oxidises reversibly at 0.54 V vs. SCE.12 Thus a substantial stabilisa-tion of the ruthenium(II) state has been taken place on switching from bpy to L which further emphasises the relatively stronger -acidic character of L. The ruthenium(III)–ruthenium(II) couple of [Ru(L)3]2+ appears at 2.10 V.20 Thus substitution of one strongly -acidic L by a -donating thiolato based ligand (pyridine-2-thio-lato) decreases the Ru(III)/Ru(II) potential by ~1.0 V. The reduction of overall charge of the complex molecule, +2 in [Ru(L)3]2+ to +1 in [4]+ provides further electrostatic stabilisation of the oxidised trivalent Ru(III) state. Though the lowering of the Ru(III)/Ru(II) potential in moving from [Ru(L)3]2+ to [4]+ is understandable, the

irreversible characteristic of the voltammogram of [4]+ is not clear. However, the built in steric constrains in [4]+ due to the four-mem-bered chelate ring arising from the pyridine-2-thiolato ligand, might be responsible for this.

Fig. 4 1H NMR spectra of (a) [4]+ (400 MHz) and (b) [1]3+ (300 MHz) in CDCl3. Inset shows the methyl signals of [2]3+.

Fig. 5 Electronic spectra of [1]3+ (− − −); [2]3+ (− · −); [3]3+() and [4]+ (—) in CH3CN. Inset shows electronic spectra in the range 625–460 nm.

Fig. 6 Cyclic (—) and differential pulse (− − −) voltammograms of(a) [4]+ and (b) [3]3+ in CH3CN.

The trinuclear complexes ([1]3+–[3]3+) exhibit three successive quasi-reversible one-electron oxidation processes in the range 1.6–2.0 V vs. SCE (Table 1, Fig. 6(b)). Coulometric oxidation of the first couple (couple 1) results in continuous current, implying the unstable nature of the oxidised species, therefore the one-electron nature of each oxidation step is tentatively assigned by comparing its differential pulse voltammetric current height with that of the ligand reductions (see later). The observed responses are consid-ered to be RuIIRuIIRuIII → RuIIRuIIRuII (couple I); RuIIRuIIIRuIII → RuIIRuIIRuIII (couple II); RuIIIRuIIIRuIII → RuIIRuIIIRuIII (couple III).6,8b,e,21 The first oxidation potential (couple I) decreases reason-ably on introduction of one electron-donating methyl group in the pendant phenyl ring of the azopyridine ligand (Table 2). The separa-tions in potentials between the successive couples in [1]3+–[3]3+ are 120, 130 and 150 mV, respectively, for couple I/couple II and 180, 260 and 220 mV, respectively, for couple II/couple III. The observed separations of ca. 120–260 mV between the successive redox pro-

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1 7 5 6 D a l t o n T r a n s . , 2 0 0 4 , 1 7 5 2 – 1 7 6 0 D a l t o n T r a n s . , 2 0 0 4 , 1 7 5 2 – 1 7 6 0 1 7 5 7

cesses correspond to the comproportionation constant values of 1.1 × 102 (Kc1)/1.1 × 103 (Kc2) for [1]3+; 1.6 × 102 (Kc1)/2.6 × 104 (Kc2) for [2]3+ and 3.5 × 102 (Kc1)/5.4 × 103 (Kc2) for [3]3+ [using the equation RTln Kc = nF(E)22] which indicate moderate intermetal-lic electronic coupling across the bridging unit in the mixed valent states.23 The intermetallic electronic coupling increases on introduc-tion of an electron-donating methyl group in the azopyridine frame-work of [2]3+and [3]3+. The Kc values of the present set of complexes particularly with respect to Kc1/Kc2 for [1]3+ and Kc1 for [2]3+ and [3]3+ are slightly lower than those of the corresponding bipyridine [{(bpy)2RuII}3(3-L)]3+ [5]3+ [4.4 × 103 (Kc1)/1.7 × 104 (Kc2)] and phenanthroline [{(phen)2RuII}3(3-L)]3+ [6]3+ complexes (1.6 × 103 (Kc1)/1.2 × 104 (Kc2)).6 The lowering of Kc value with the increase in -acidity of the ancillary ligands (bipyridine → azopyridine) implies that the efficiency of the bridging function mediated inter-metallic electronic coupling process decreases with the decrease in electron density on the metal centres.

The electrogenerated first-step oxidised RuIIRuIIRuIII congeners (14+–34+) were found to be unstable even at 273 K which had es-sentially precluded to perform any further spectroscopic studies on the mixed valent species so far.

Ligand reduction. The complexes display multiple reversible re-ductions (Table 2, Fig. 6). Since the azo function of the coordinated azopyridine ligand is known to undergo two successive one-elec-tron reductions18 and the other associated ligands L3− and pyridine-2-thiolate do not exhibit any reductions within the experimental potential limit of −2.0 V vs. SCE,6,8f,12 therefore in corroboration with the earlier observations, we believe that the observed responses are essentially azopyridine based reductions. Theoretically four and twelve one-electron reductions are expected from the mononuclear and trinuclear complexes, respectively, and a few of them are indeed detected within the experimental potential limit (Table 2). Electro-chemically generated mono-reduced species (12+–32+ and 4) in ace-tonitrile were found to be reasonably stable at room-temperature (the one-electron nature of the first reduction couple was confirmed via constant potential coulometry). The reduced species could also be generated via chemical reduction by using hydrazine hydrate. However, we were unable to isolate the reduced species in the pure solid state as during the work up process it got re-oxidised. We have managed to record the EPR spectra of 12+–32+ and 4 by quick freez-

ing the reduced solution at 77 K (liquid nitrogen). The complexes exhibit one sharp EPR signal near g = 2.0 characteristic of a ligand based radical, however, no hyperfine splittings due to the nitrogen centre was resolved.8

Spectroelectrochemical correlation

Trinuclear complexes [1]3+–[3]3+ exhibit lowest energy MLCT transitions of the type t2g(RuII) → ligand LUMO (where the LUMO is essentially dominated by the azo-imine chromophore of the azopyridine ligands, L–) in the range 533–542 nm (Table 2). The quasi-reversible first ruthenium(III)–ruthenium(II) couple (couple I) and the first ligand based reduction appear in the ranges 1.68 → 1.57 V and −0.36 → −0.41 V, respectively. The MLCT transition involves the excitation of the electron from the filled t2g

6 orbital of Ru(II) to the lowest * orbital of the azoimine function. The energy of the MLCT transition can be predicted from the experimentally observed electrochemical data with the help of eqns. (1) and (2).12 Here E0(RuIII–RuII) is the

(MLCT) = 8065(E0) + 3000 (1)

E0 = E0(RuIII–RuII) − E0(L–) (2)

formal potential (in V) of the first Ru(III)/Ru(II) couple (couple 1), E0(L–) that of the first ligand reduction and MLCT is the energy of the charge-transfer band in cm−1. The factor 8065 is used to convert the potential difference E from V into cm−1 unit and the term 3000 cm−1 is of empirical origin. The calculated and experimentally observed MLCT values are listed in Table 2. Here the calculated val-ues for all the three complexes lie within 700 cm−1 of the experimen-tally determined MLCT energies, which are in good agreement.12

Non-linear optical properties

In recent years there has been a growing research interest in devel-oping materials which can exhibit nonlinear optical properties for potential applications in optoelectronics and phototechnologies.24 In addition to organic materials, transition metal complex based materials have also been found to be effective in this regard.25 Thus, the third-order nonlinear optical properties of all the four complexes ([1]3+, [2]3+, [3]3+ and [4]+) were investigated in acetonitrile solvent using the open and closed aperture z-scan technique26,27 (ESI†).

Table 2 UV-visa and electrochemical dataa

Electrochemical data

UV-vis data, max/nm Metal oxidation, Ligand reduction, (MLCT)/cm−1

Compound (/dm3 mol−1 cm−1) E2980/V (Ep/mV) E298

0/V (Ep/mV) Ed/V Obs.a Calc.e

[1](ClO4)3 533 (18750), 1.68 (80) (couple I), −0.36 (65), 2.04 18761 19452 510 (14662),b 1.80 (90) (couple II), −0.88 (70), 442 (11063), 2.00 (120) (couple III) −0.99 (70), 358 (44947), −1.83 (80), 318 (52246), −2.0 (130) 204 (116326) [2](ClO4)3 540 (22370), 1.61 (90) (couple I), −0.41 (80), 2.02 18518 19291 510 (17624),b 1.74 (90) (couple II), −0.92 (70), 360 (55095), 2.0 (110) (couple III) −1.03 (85), 322 (62462), −1.94 (130), 206 (152266) −2.06 (140) [3](ClO4)3 542 (19720), 1.57 (80) (couple I), −0.41 (80), 1.98 18450 18968 510 (14620),b 1.72 (80) (couple II), −0.92 (62), 368 (55470), 1.94 (110) (couple III) −1.07 (80), 328 (53590), −1.94 (130), 204 (146406) −2.08 (120) [4](ClO4) 558 (12987), 1.05c −0.40 (64), — — — 532 (10328),b −0.95 (80), 314 (26650), −1.76 (130) 288 (24680), 214 (48158)a In CH3CN. b Shoulder. c Irreversible, Epa value is considered. d Calculated using eqn. (2). e Calculated using eqn. (1).

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The open aperture transmittance vs. scan distance for [1]3+ is shown in Fig. 7. The signature clearly indicates the saturation of absorption. This is because the charge transfer bands are close to the excitation wavelength (532 nm) as is also evident from the absorp-tion spectrum of the complexes. This saturable absorber behaviour of complexes may arise due to the presence of an excited state hav-ing a lifetime greater than the pulse width.

nise the effect of the trinuclear complex [1]3+, and recently reported two other similar trinuclear complexes [{(bpy)2RuII}3(3-L)](ClO4)3 [5](ClO4)3 and [{(phen)2RuII}3(3-L)](ClO4)3 [6](ClO4)3 on both the circular and linear forms of DNA. In this regard each of the three complexes at concentrations varying from 0.005 to 0.5 mM, were incubated with the circular form of p-Bluescript SK as well as the linearised form under conditions described in the Experimental sec-tion and analysed on a 0.7% agarose gel electrophoresis. All three complexes show a reduced ethidium bromide fluorescence of circu-lar DNA, starting at a concentration of 0.2 mM, that was completely abolished at 0.5 mM (Fig. 9(a)–(c)). The complexes also reduces the fluorescence intensity of linear DNA (Fig. 9(d)–(f)). [1]3+ is found to be most effective even at 0.3 mM (Fig. 9(f)), whereas [5]3+ and [6]3+ appear to be more effective at concentrations greater than 0.4 mM. Thus our results strongly suggest that all the three complexes interact with DNA, both circular as well as linear. At present we are unable to assess the exact nature of the interactions between these three compounds and DNA that leads to the reduction in fluorescence intensity. However, there are several possibilities. Firstly, the interaction of these compounds with DNA may prevent the intercalation of ethidium bromide into DNA. Alternatively, the binding to DNA could result in the quenching of ethidium bromide fluorescence. Experiments are in progress to elucidate the mecha-nism of interaction.

ConclusionsThe present work illustrates the following important features: (i) to the best of our knowledge the complex [1](ClO4)3 represents the first example of a structurally characterised trinuclear complex incorporating 1,3,5-triazine-2,4,6-trithiolate (L3−) as a bridging ligand. (ii) [1]3+–[3]3+ exhibit successive electron-transfer pro-cesses to RuIIRuIIRuIII RuIIRuIIRuII (couple 1); RuIIRuIIIRuIII RuIIRuIIRuIII (couple II); RuIIIRuIIIRuIII RuIIRuIIIRuIII (couple III) where the mixed-valent states are electronically moderately coupled and the extent of coupling decreases with the decrease in electron density on the metal ions via the involvement of the ancillary functionalities. (iii) The complexes show reasonably good second molecular hyperpolarisabilities, suitable for photonic device appli-cations. The trinuclear complexes {[1]3+–[3]3+} are found to be more

Fig. 7 Open aperture z-scan for [1]3+ in CH3CN. Dots represent experi-mental points and the solid curve is the theoretical fit.

The closed aperture transmittance as a function of the scan distance is shown in Fig. 8. The pre-focal maxima and post-focal minima indicate the negative third-order nonlinearity of the com-plex. The respective values of all the nonlinear parameters are given in Table 3. The second-order molecular hyperpolarisability () for the mononuclear and trinuclear complexes follows the order [2]3+ > [1]3+ > [3]3+ > [4]+. Thus among the three trinuclear complexes, [2]3+ has the largest , whereas [1]3+ and [3]3+ are comparable with the value of [1]3+ being slightly larger. The observed enhancement in nonlinearity for the trinuclear derivatives ([1]3+–[3]3+) with respect to [4]+ is probably due to the change in dipole moments as well as the resonances. The increment in the dipole moment enhances the value whereas shift in resonance decreases it.28 The value of [1]3+ is ~2 times larger than [4]+. This is essentially a reflection of the nucleation effect while moving from [4]+ to [1]3+. The dipole moment of the complex [1]3+ being larger than that of [3]3+([1]3+ = 18750, [3]3+ = 19720) should enhance the value but this enhance-ment is essentially compensated and slightly dominated by the 9 nm shift in absorption band (max[1]3+ = 533 nm, max[3]3+ = 542 nm) relative to the excitation wavelength (532 nm). In the case of [2]3+, the dipole moment contribution is dominating over the wavelength shift ([2]3+ = 22370, max[2]3+ = 540 nm) which results in higher value compared to [1]3+ and [3]3+.

The magnitudes of (10−29 esu) obtained for these complexes are comparable to those observed for many other ruthenium com-plexes.29 It may be noted that the observed values for the pres-ent set of trinuclear complexes ([1]3+–[3]3+) ((1.64–2.45) × 10−29 esu) are slightly lower than those of the recently reported similar low-symmetric trinuclear complexes, [{(byp)2RuII}3(3-L)]3+ [5]3+ (4.5 × 10−29) and [{(phen)2RuII}3(3-L)]3+ [6]3+ (5.09 × 10−29).6 Thus, the effect of electronic aspects of the ancillary ligands (-acidic bipyridine/phenanthroline in [5]3+/[6]3+ to strongly -acidic azopyridine in [1]3+–[3]3+) has been reflected in their values. Both the two photon absorption (singlet to singlet) and excited state ab-sorption (triplet to triplet) could be the reason of the observed non-linearity. However, the relative contribution of these mechanisms is not defined. Therefore, the derived parameters can be regarded as effective parameters.

Interactions of [1]3+, [5]3+ and [6]3+ with DNA

As ruthenium complexes involving polypyridine ligands are well established to interact with DNA,30 we therefore intended to scruti-

Table 3 Non-linear optical parameters

1020n2/cm2 1015/cm 1012|(3)|/ 1029||/Compound 0/cm s erg−1 s erg−1 esu esu

[1](ClO4)3 4.64 2.01 −2.1 7.56 1.93[2](ClO4)3 5.46 2.56 −2.6 9.55 2.45[3](ClO4)3 4.74 1.73 −1.65 6.41 1.64[4](ClO4) 5.164 1.87 −2.05 7.08 0.90

Fig. 8 Closed aperture z-scan for [1]3+. Inset represents the refractive part of the z-scan.

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effective in this regard as compared to the mononuclear analogue [4]+. The effect of substituents in the ancillary ligand framework of {[1]3+–[3]3+} on the third-order nonlinearity has been addressed. (iv) The trinuclear complexes [1]3+, [5]3+ and [6]3+ interact with the cir-cular as well as linear forms of p-Bluescript DNA and the effectivity varies depending on the nature of the ancillary functionalities.

ExperimentalMaterials

The starting complexes ctc-[Ru(L–)2(H2O)2](ClO4)2·H2O were prepared according to the reported procedure.31 The trinucleating bridging ligand, trisodium salt of 1,3,5-triazine-2,4,6-trithiol (Na3L) was purchased from Aldrich, USA. Other chemicals and solvents were of reagent grade and used as received. For electrochemical studies HPLC grade acetonitrile was used. Commercial tetraeth-ylammonium bromide was converted to pure tetraethylammonium perchlorate (TEAP) by following an available procedure.32 Plasmid p-Bluescript was isolated from E. coli DH5. The p-Bluescript was linearised with EcoRI restriction endonuclease.Physical measurements

The solution electrical conductivity was checked using a Systronic conductivity bridge, 305. Infrared spectra were taken on a Nicolet spectrophotometer with samples prepared as KBr pellets. 1H NMR spectra were obtained on a 300/400-MHz Varian FT-NMR spec-trometer. UV-Visible spectra were recorded on a Jasco-570 spec-trophotometer. Cyclic voltammetric and coulometric measurements were carried out using a PAR model 273A electrochemistry system. A glassy-carbon working electrode, a platinum wire auxiliary elec-trode and a saturated calomel reference electrode (SCE) were used in a three-electrode configuration. Tetraethylammonium perchlo-rate (TEAP) was the supporting electrolyte and the concentration of the solution was 10−3 M. The half wave potential E0

298 was set equal to 0.5(Epa + Epc), where Epa and Epc are anodic and cathodic cyclic voltammetric peak potentials, respectively. The scan rate used was 50 mV s−1. A platinum gauze working electrode was used in coulometric experiments. All electrochemical experiments were carried out under a dinitrogen atmosphere. The elemental analyses were carried out using a Perkin-Elmer 240C elemental analyser. The electrospray mass spectra were recorded on a Micromass Q-ToF mass spectrometer.Preparation of complexes [1](ClO4)3–[3](ClO4)3

The complexes were prepared by following the same general proce-dure. The details are given for [1](ClO4)3.

[{(L)2RuII}3(3-L)](ClO4)3, [1](ClO4)3. The ligand Na3L (19 mg, 0.047 mmol) was added to the starting complex ctc-[Ru-(L)2(OH2)2](ClO4)2·H2O (100 mg, 0.140 mmol) in absolute ethanol (20 cm3). The resulting mixture was heated to reflux under a dini-trogen atmosphere for 24 h. The concentrated solution was kept in the refrigerator overnight. The precipitate which formed on cooling was filtered and washed thoroughly with ice-cold water followed by cold ethanol and diethyl ether. The product was recrystallised from acetonitrile–benzene (1 : 4). Yield: 66% (57 mg). Anal. Calc. for C69H54N21Cl3O12S3Ru3 [1](ClO4)3: C, 44.20; H, 2.90; N, 15.69. Found: C, 44.09; H, 2.59; N, 15.96%. M/−1 cm2 mol−1 in aceto-nitrile at 298 K: 346. Electrospray mass spectral data: m/z = 1776 corresponding to {[1](ClO4)2}+ (calc. molecular weight: 1775.63). IR data/cm−1: (ClO4

−): 1098, 634; (NN): 1350. /ppm (J/Hz): 9.42 (4.76), 9.29 (5.85), 9.17 (5.49), 8.99 (7.3), 8.84 (6.6, 7.7), mul-tiplet centred at 8.62 ppm, multiplet centred at 8.36 ppm, multiplet centred at 8.05 ppm, 7.6 (5.86), multiplet centred at 7.23 ppm, 7.01 (7.7, 8.0), 6.78 (7.3), multiplet centred at 6.73 ppm.

For [2](ClO4)3: yield: 69% (60 mg). Anal. Calc. for C75H66N21Cl3O12S3Ru3 [2](ClO4)3: C, 45.98; H, 3.40; N, 15.01. Found: C,45.69; H, 3.20; N, 14.82%. M/−1 cm2 mol−1 in acetoni-trile at 298 K: 336. Electrospray mass spectral data m/z = 1860.12 corresponding to {[2](ClO4)2}+ (calc. molecular weight: 1859.78). IR data/cm−1: (ClO4

−): 1104, 624; (NN): 1341. /ppm (J/Hz): 9.51 (5.5), 9.39 (5.86), 9.07 (5.12), 9.04 (7.7), multiplet centred at 8.95 ppm, multiplet centred at 8.87 ppm, 8.75 (5.5), multiplet cen-tred at 8.45 ppm, 8.28 (7.7, 7.7), multiplet centred at 8.14 ppm, 7.94 (6.9, 7.6), 7.75 (5.9, 4.5), 7.6 (5.86), 7.42 (5.5), multiplet centred at 7.1 ppm, 6.9 (8.0), 6.77, 6.68 (8.0), 6.56 (8.2), 6.49 (7.7), 6.40 (8.0). Methyl signals are observed at 2.04, 2.09, 2.11, 2.13, 2.16, 2.21.

For [3](ClO4)3: yield: 63% (55 mg). Anal. Calc. for C75H66N21Cl3-O12S3Ru3[3](ClO4)3: C, 45.98; H, 3.40; N, 15.01. Found: C, 45.74; H, 3.25; N, 14.72%. M/−1 cm2 mol−1 in acetonitrile at 298 K: 332. Mass spectral data: m/z = 1859.52 corresponding to {[3](ClO4)2}+ (calc. molecular weight: 1859.78) (Fig. S2, ESI†). IR data/cm−1: (ClO4

−): 1117, 623; (NN): 1349. /ppm (J/Hz): 9.41 (5.5), 9.31 (6.2), 9.2 (4.0, 3.8), 9.11 (5.5), 8.86 (3.7), 8.61 (5.2), multiplet cen-tred at 8.38 ppm, multiplet centred at 8.12 ppm, multiplet centred at 7.94 ppm, multiplet centred at 7.69 ppm, multiplet centred at 7.52 ppm, multiplet centred at 6.94 ppm, 6.8 (8.2), 6.67 (8.3), 6.58 (6.6, 6.9). Methyl signals are observed at 2.22, 2.23, 2.25, 2.26, 2.30, 2.32.

Preparation of [(L)2RuII(NC5H4S−)]ClO4, [4]ClO4. The pyri-dine-2-thiol ligand (16 mg, 0.140 mmol) followed by anhydrous CH3CO2Na (12 mg, 0.140 mmol) were added to the precursor com-plex ctc-[Ru(L)2(OH2)2](ClO4)2·H2O (100 mg, 0.140 mmol) in ab-solute ethanol (20 cm3). The resulting mixture was heated to reflux under a dinitrogen atmosphere for 12 h. The precipitate thus formed on cooling the concentrated solution was filtered off and washed thoroughly with ice-cold water followed by cold ethanol and di-ethyl ether. The product was recrystallised from acetonitrile–ben-zene (1 : 6). Yield: 80% (76 mg). Anal. Calc. for C27H22N7ClO4SRu [4]ClO4: C, 47.86; H, 3.27; N, 14.48. Found: C, 47.49; H, 3.56; N, 14.35%. M/−1 cm2 mol−1 in acetonitrile at 298 K: 140. Electro-spray mass spectral data: m/z = 577.98, corresponding to {[4]}+ (calc. molecular weight, 577.65) (Fig. S3, ESI†). IR data/cm−1: (ClO4

−): 1091, 630; (NN): 1328. /ppm (J/Hz): 9.12 (6.0), 8.78 (8.0), 8.72 (7.6), multiplet centred at 8.2 ppm, 7.96 (5.6), 7.63 (5.6, 6.4), multiplet centred at 7.4 ppm, multiplet centred at 7.24 ppm, 7.01 (7.24, 7.25), 6.92 (8.4), 6.81 (8.4), 6.75 (8.4).

Crystal structure determination

Single crystals of [1](ClO4)3 and [4]ClO4 were grown by slow dif-fusion of their acetonitrile solution in benzene followed by slow evaporation. Crystal data and data collection parameters are listed in Table 4. X-ray data of the complexes were collected on a Bruker SMART APEX CCD diffractometer using Mo-K radiation. The structures were solved and refined by full-matrix least-squares on F2 using SHELX-97 (SHELXTL).33 SADABS correction was applied.

Fig. 9 DNA binding study for [1]3+, [5]3+ and [6]3+ by agarose gel elec-trophoresis using both circular and linear p-Bluescript DNA. Lane C is the DNA control with no metal complex added. The other lanes are labelled with metal complex of different concentrations: 0.5, 0.4, 0.3, 0.2, 0.1, 0.05 and 0.005 mM, respectively. Each of these lanes contains 100 ng plasmid DNA.

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All the data were corrected for Lorentzian polarization, and absorp-tion effects. Hydrogen atoms except those attached to the water molecule in [1](ClO4)3 were included in the refinement process as per the riding model.

CCDC reference numbers 232966 and 232967.See http://www.rsc.org/suppdata/dt/b4/b403332a/ for crystal-

lographic data in CIF or other electronic format.

Non-linear optical parameter measurement

The experiments were performed with the second harmonic ( = 532 nm) of an indigenously developed Q-switched nanosecond Nd:YAG laser. The laser pulse width (FWHM) was 6 ns and the pulse repetition rate was 6 Hz. The schematic diagram of the experimental set-up is shown in Fig. S4 (ESI†). The laser beam was focused on the sample by using a lens of focal length 25 cm. The spot radius of the beam at the focal plane of the lens was measured as 27 m. The corresponding Rayleigh range of the beam was 4.3 mm. The transmitted laser beams for the open and closed aperture experi-ments were collected using photodiodes PD2 and PD3, respectively, whereas the reference beam was collected with the aid of photodi-ode PD1. In the closed aperture experiment, an aperture A of radius 0.5 mm was placed before the photodiode, collecting the transmit-ted beam. The linear transmittance of the aperture was calculated as ~0.1. The corresponding open aperture and closed aperture data were processed by a data acquisition system. The acetonitrile solu-tion of the samples (concentration for [1]3+–[3]3+ = 2.5 × 10−4 mol dm−3 and for [4]+ = 5 × 10−4 mol dm−3) were placed in a 3 mm path length quartz cuvette and scanned through 15 mm on either side of the focal plane of lens with the aid of a stepper motor controlled translational stage.

DNA measurement

The interactions of circular and linear p-Bluescript DNA with the complexes [1]3+, [5]3+ and [6]3+ were analysed by agarose gel elec-trophoresis as previously described.30 The concentration of the DNA was determined by staining with ethidium bromide and observing on a UV illuminator. In a typical experiment 2 L of p-Bluescript DNA (100 ng) was incubated in an eppendorf tube with 18 L of metal complex of different concentrations (0.5, 0.4, 0.3, 0.2, 0.1, 0.05 and 0.005 mM) in 1 : 8 DMSO–H2O. We observed that the DMSO solvent does not induce any changes in DNA. The samples were incubated for 1 h at 25 °C and 15 L was analysed in a 0.7% agarose electrophoresis. Gels were then stained with 0.5 g mL−1 ethidium bromide for 1 h and documented with UV illumination using a KDs120 gel documentation system from Kodak Digital Science.

AcknowledgementsFinancial support received from the Council of Scientific and In-dustrial research, New Delhi (India) is gratefully acknowledged.

Special acknowledgement is made to the Sophisticated Analytical Instrument Facility, Indian Institute of Technology, Bombay, for providing the NMR and EPR facilities.

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Table 4 Crystallographic data for complexes [1](ClO4)3·H2O and [4](ClO4)·C6H6

[1](ClO4)3·H2O [4](ClO4)·C6H6

Empirical formula C69H56Cl3N21O13S3Ru3 C33H28N7ClO4SRuMr 1893.09 755.20Crystal symmetry Monoclinic MonoclinicSpace group P21/n P21/ca/Å 15.716(8) 10.104(2)b/Å 23.618(12) 30.680(5)c/Å 20.997(11) 10.729(2)/° 90.586(8) 94.295(3)V/Å3 7793(7) 3316.5(10)Z 4 4T/K 293(2) 293(2)Dc/g cm−3 1.614 1.512/mm−1 0.832 0.665Data, restraints, parameters 13698, 73, 1018 5835, 0, 424Final R1, wR2 0.0723, 0.1388 0.0554, 0.1158

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1 7 6 0 D a l t o n T r a n s . , 2 0 0 4 , 1 7 5 2 – 1 7 6 0

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