Polyhedron 72 (2014) 115–121

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Polyhedron

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Heteroleptic Ru(II) complexes containing aroyl hydrazoneand 2,20-bipyridyl: Synthesis, X-ray crystal structures, electrochemicaland DFT studies

http://dx.doi.org/10.1016/j.poly.2014.01.0310277-5387/� 2014 Elsevier Ltd. All rights reserved.

⇑ Corresponding authors. Tel.: +34 868 88 7489; fax: +34 868 88 4149 (A. Espinosa).Tel.: +91 33 2668 0521; fax: +91 33 2668 2916 (S.K. Chattopadhyay).

E-mail addresses: artuesp@um.es (A. Espinosa), shch20@hotmail.com(S.K. Chattopadhyay).

1 Current address: Research Institute for Science and Technology, Tokyo Universityof Science, 2641 Yamazaki, Noda-shi, Chiba 278-8510, Japan.

Bipinbihari Ghosh a, Sumita Naskar a, Subhendu Naskar b, Arturo Espinosa c,⇑, Sam C.K. Hau d,Thomas C.W. Mak d, Ryo Sekiya e,1, Reiko Kuroda e,1, Shyamal Kumar Chattopadhyay a,⇑a Department of Chemistry, Bengal Engineering and Science University, Shibpur, Howrah 711103, Indiab Department of Applied Chemistry, Birla Institute of Technology, Mesra, Ranchi 835215, Jharkhand, Indiac Department of Organic Chemistry, University of Murcia, Campus de Espinardo, 30100 Murcia, Spaind Department of Chemistry, The Chinese University of Hong Kong, Shatin, New Territories, Hong Kong, Chinae Department of Life Sciences, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo 153-8902, Japan

a r t i c l e i n f o

Article history:Received 20 September 2013Accepted 27 January 2014Available online 5 February 2014

Keywords:Ru(II)Aroyl hydrazone2,20-BipyridylCyclic voltammetryDFT calculationsX-ray structure

a b s t r a c t

Four heteroleptic Ru(II) complexes [Ru(bpy)2(L)]X (L = bidentate N, O� donor hydrazone ligands, X = ClO4

or PF6) have been synthesized and characterized. Single crystal X-ray structures of two of the complexesare also reported. The complexes show strong absorptions in the 400–600 nm region of the visible radi-ation, with very similar room temperature emission spectra to that of reference [Ru(bpy)3]X2 but with thelowest energy MLCT (metal to ligand charge transfer) band bathochromically shifted by about 50 nm.Results obtained from both electrochemical experiments and DFT calculations show remarkable differ-ences in the HOMO–LUMO properties of the hydrazone complexes compared to [Ru(bpy)3]X2. TDDFT cal-culations have been used to understand the nature of the electronic transitions.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Metal complexes of aroyl hydrazones continue to be an attrac-tive area of research in view of the simplicity of their synthesiswith main-group elements, transition metals and lanthanides,which exhibit a variety of molecular architectures as well as veryinteresting chemical, electrochemical, spectroscopic, magneticand biological properties [1–25]. Thus, several Fe(II) and Fe(III)complexes of these ligands are known to show spin crossover phe-nomenon with potential technological applications [25–26]. Re-cently a benzoyl hydrazone derivatized coumarin ligand has beenshown to be a Cu(II) selective fluorescent chemosensor whichcan be used for bioimaging [27].

The electron transfer reactivity of the MLCT (metal to ligandcharge transfer) excited states in polypyridyl-based complexeshave long been used in many processes including electron transfer

reagents and as dyes for solar photocells [28]. In particular, despitepossessing excellent photophysical properties which make themuseful for a variety of applications [29], ruthenium polypyridylcomplexes are not suitable for live cell imaging due to their poorcellular uptake [30]. On the contrary, aroyl hydrazones have dem-onstrated adequate lipophilicity and ability to permeate cell mem-branes [31]. Therefore, mixed chelate complexes of Ru(II)containing both polypyridine and aroyl hydrazone ligands maypossess adequate balance between lipophilicity and suitablephotophysical properties for use in live cell imaging.

Herein, we report the synthesis, X-ray crystal structure andspectroscopic and redox properties of ruthenium(II) complexes offour aroyl hydrazones (Scheme 1). The electronic structure andelectronic spectra of two representative complexes are explainedon the basis of DFT and TD-DFT calculations.

2. Experimental

2.1. Materials

Thiophene-2-carbaldehyde, furan-2-carbaldehyde, ethyl benzo-ate and ethyl salicylate were purchased from Aldrich. All other

Scheme 1. Deprotonated aroyl hydrazone ligands used.

116 B. Ghosh et al. / Polyhedron 72 (2014) 115–121

chemicals and solvents were of reagent grade and used as received.Cis-Ru(bpy)2Cl2�2H2O was prepared following a published proce-dure [32]. Solvents for spectroscopic and cyclic voltammetry mea-surements were of HPLC grade and obtained from Merck orAldrich. Elemental analyses were performed on a Perkin–Elmer2400 C, H, N analyzer. Infrared spectra were recorded as KBr pelletson a JASCO FT-IR-460 spectrophotometer. UV–Vis spectra were re-corded using a JASCO V-530 spectrophotometer. Cyclic voltammo-grams were recorded in acetonitrile solutions, containing 0.1 MTEAP as supporting electrolyte, using a CH1106A potentiostat, withPt disc/glassy carbon working electrode, Pt wire as a counter elec-trode and Ag, AgCl/saturated KCl as the reference electrode. Theferrocene/ferrocenium couple was observed at E0 (DEp) = 0.48 V(100 mV) under these experimental conditions.

2.2. Synthesis of ligands

The ligands were prepared by a procedure previously described[15].

L1H: Yield: 70%. Anal. Calc. for C12H10N2O2: C, 67.28; H, 4.67; N,13.08. Found: C, 67.02; H, 4.63; N, 12.88%. ESI-MS, m/z: 215[M+H]+, 237 [M+Na]+; 1H NMR (DMSO-d6): d (ppm): 11.80 (s,1H), 8.34 (s, 1H); 7.55 (m, 3H), 5.34 (d, 1H), 6.64 (s, 1H); selectedIR bands (cm�1): 3245 mNAH, 1644 mamideI, 1570 mC@N, 1539 mamideII.

L2H: Yield: 73%. Anal. Calc. for C12H10N2OS: C, 62.61; H, 4.34; N,12.17. Found: C, 62.12; H, 4.33; N, 12.02%. ESI-MS, m/z: 231[M+H]+, 253 [M+Na]+; NMR (DMSO-d6): d (ppm): 11.82 (s, 1H),8.67 (s, 1H), 7.89 (s, 2H), 7.67 (d, 1H), 7.54 (m, 4H), 7.14 (t, 1H); se-lected IR bands (cm�1): 3255 mNAH, 1643 mamideI, 1600 mC@N 1552mamideII.

L3H: Yield: 75%. Anal. Calc. for C12H10N2O3: C, 62.60; H, 4.34; N,12.17. Found: C, 62.23; H, 4.27; N, 11.88%. ESI-MS, m/z: 231[M+H]+, 253 [M+Na]+; 1H NMR (DMSO-d6): d (ppm): 11.79 (s,2H), 8.35 (s, 1H), 7.85 (d, 2H), 7.44 (t, 1H), 6.95 (m, 3H), 6.65 (m,1H); selected IR bands (cm�1): 3247 mNAH, 1635 mamideI, 1635 mC@N,1537 mamideII.

L4H: Yield: 72%. Anal. Calc. for C12H10N2O3: C, 62.60; H, 4.34; N,12.17. Found: C, 62.23; H, 4.27; N, 11.88%. ESI-MS, m/z: 247[M+H]+, 269 [M+Na]+; 1H NMR (DMSO-d6): d (ppm): 11.81 (s,2H), 8.66 (s, 1H), 7.85 (d, 1H), 7.70 (d, 1H), 7.50 (d, 1H), 7.43 (t,1H), 7.15 (t, 1H), 6.95 (t, 2H); selected IR bands (cm�1): 3248 mNAH,1623 mamideI, 1623 mC@N, 1556 mamideII.

2.3. Synthesis of complexes

The Ru(II) complexes 1–4 were synthesized following a generalprocedure outlined below for complex 1.

[Ru(bpy)2L1]ClO4 (1): To a methanolic solution (40 mL) of ligandL1H (0.214 g, 1.0 mmol), Et3N (0.101 g, 1.0 mmol) and cis-Ru(bpy)2

Cl2�2H2O (0.52 g, 1.0 mmol) were added and the reaction mixturewas refluxed for 6–8 h. The volume of the solution was reducedto 20 mL. After addition of NaClO4 to the solution, it was left forslow evaporation. Crystalline precipitates of the desired compound

were obtained and isolated by filtration. The crystalline residuewas washed with cold methanol and diethyl ether and dried inair. Yield: 57%. Anal. Calc. for C32H25N6O6ClRu: C, 52.97; H, 3.45;N, 11.59. Found: C, 52.65; H, 3.19; N, 11.95%. ESI-MS, m/z:626[(M+H)–ClO4]+; 1H NMR (DMSO-d6): d (ppm): 8.75 (m, 4H),8.51 (m, 2H), 8.42 (d, 1H), 8.15 (m, 2H), 8.01 (m, 2H), 7.91 (d,1H), 7.73 (m, 4H), 7.49 (d, 1H), 7.38 (m, 4H), 7.22 (t, 1H), 6.88 (s,1H) , 6.67 (d, 1H), 6.31 (m, 1H); selected IR bands (cm�1): 1633mamideI, 1592 mC@N.

[Ru(bpy)2L2]ClO4 (2): Yield: 51%. Anal. Calc. for C32H25N6O5

ClSRu: C, 51.82; H, 3.37; N, 11.34. Found: C, 52.18; H, 3.17; N,11.55%. ESI-MS, m/z: 643[(M+H)–ClO4]+; 1H NMR (DMSO-d6): d(ppm): 9.05 (s, 1H), 8.95 (d, 1H), 8.79 (d, 1H), 8.66 (m, 2H), 8.41(d, 1H), 8.31 (d, 1H), 8.18 (t, 2H), 7.99 (m, 3H), 7.76 (t, 2H), 7.64(t, 1H), 7.37 (m, 7H), 7.01 (t, 1H), 6.54 (t, 1H) , 6.47 (t, 1H); selectedIR bands (cm�1): 1640 mamideI, 1595 mC@N.

[Ru(bpy)2L3]ClO4 (3): Yield: 54%. Anal. Calc. for C32H25N6O7

ClRu: C, 51.82; H, 3.37; N, 11.34. Found: C, 51.65; H, 3.17; N,10.95%. ESI-MS, m/z: 643[(M+H)–ClO4]+; 1H NMR (DMSO-d6): d(ppm): 12.49 (s, 1H), 8.83 (d, 3H), 8.73 (m, 2H), 8.51 (d, 1H), 8.41(d, 1H), 8.17 (m, 2H), 8.03 (t, 1H), 7.73 (m, 5H), 7.53 (d, 1H), 7.37(m, 1H), 7.27 (m, 2H), 6.89 (m, 2H), 6.76 (m, 2H), 6.36 (m, 1H),4.75 (d, 1H); selected IR bands (cm�1): 1622 mamideI, 1595 mC@N. Sin-gle crystals suitable for X-ray crystallography for this compoundwas obtained in the form of hexafluorophosphate salt [Ru(bpy)2-

L3]PF6 (3a): Anal. Calc. for C32H25N6O3F6PRu: C, 48.75; H, 3.17; N,10.66. Found: C, 48.82; H, 3.30; N, 10.95%.

[Ru(bpy)2L4]ClO4 (4): Yield: 56%. Anal. Calc. for C32H25N6O6

ClSRu: C, 50.73; H, 3.30; N, 11.09. Found: C, 50.22; H, 3.17; N,11.29%. ESI-MS, m/z: 659[(M+H)–ClO4]+; 1H NMR (DMSO-d6): d(ppm): 12.51 (s, 1H), 9.16 (s, 1H), 8.98 (d, 1H), 8.73 (m, 3H), 8.35(m, 2H), 8.20 (t, 2H), 8.01 (t, 1H), 7.71 (m, 4H), 7.47 (m, 2H), 7.30(b s, 3H), 7.04 (t, 1H), 6.87 (d, 1H), 6.76 (t, 1H), 6.61 (s, 1H), 6.50(d, 1H); selected IR bands (cm�1): 1623 mamideI, 1594 mC@N.

2.4. X-ray crystallography

Data were collected for 3a on a Bruker AXS Kappa Apex II Duodiffractometer at 173 K using frames of oscillation range 0.3�, with2� < h < 28�, and for 4 on a Bruker SMART APEXCCD diffractometerat 123(2) K, using Mo Ka radiation (0.71073 Å). Data were cor-rected for absorption effects with the SADABS program [33], solvedby direct methods and refined by full-matrix least squares on F2

using SHELX-97 [34]. The non-hydrogen atoms were refined withanisotropic displacement parameters. All hydrogen atoms wereplaced at calculated positions and refined as riding atoms usingisotropic thermal parameters. For 4 two Cl atoms for the perchlo-rate anion was located, which are only 0.330 Å away. By doing thisrather than locating one Cl atom of unusually long thermal elip-soid, we could locate four oxygen atoms of reasonable structurefor each of the two Cl atoms, which we named A and B. A summaryof the crystallographic data is presented in Table 1.

2.5. Computational details

Quantum chemical calculations were performed with the ORCAelectronic structure program package [35]. All geometry optimiza-tions were run with tight convergence criteria using the B3LYP [36]functional together with the new efficient RIJCOSX algorithm [37]and the def2-TZVP basis set [38]. The [SD(28,MWB)] effective corepotential (ECP) was used for Ru atoms [39]. In all optimizationsand energy evaluations, the latest Grimme’s semiempirical atom-pair-wise correction, accounting for the major part of the contribu-tion of dispersion forces to the energy, was included [40]. Solventeffects (acetonitrile) were taken into account via the COSMOsolvation model [41]. Reported energies are uncorrected for the

Table 1Crystal data and structure refinement for 3a, 4.

3a 4

Empirical formula C32H25F6N6O3RuP C32H25ClN6O6RuSFormula weight 787.62 758.16T (K) 173(2) 123(2)k (Å) 0.71073 0.71073Crystal system triclinic monoclinicSpace group P�1 C2/cUnit cell dimensionsa (Å) 11.4745(5) 30.2910(18)b (Å) 12.2373(6) 9.4890(5)c (Å) 12.6987(6) 24.8290(14)a (�) 95.228(1) 90b (�) 105.478(1) 124.108(1)c (�) 111.459(1) 90V (Å3) 1563.5(1) 5909.0(6)F(000) 792 3072Dcalc (g cm�3) 1.673 1.704Z 2 8Absorption coefficient (mm�1) 0.634 0.751Reflections collected: total,

unique27157, 5652 17800, 6766

Observed reflections [I > 2r(I)] 5285 5820Maximum and minimum

transmission0.8325 and 0.7422 0.9022 and 0.7433

Data/restraints/parameters 5652/0/442 6766/0/471Goodness-of-fit (GOF) on F2 1.005 1.123Final R indices [I > 2r(I)] R1 = 0.0240,

wR2 = 0.0604R1 = 0.0570,wR2 = 0.1395

R indices (all data) R1 = 0.0264,wR2 = 0.0620

R1 = 0.0668,wR2 = 0.1454

Largest differences in peak andhole (e �3)

�0.401 and 1.17 �0.522 and 1.568

B. Ghosh et al. / Polyhedron 72 (2014) 115–121 117

zero-point vibrational term. Figs. 3 and 5–7 were generated withVMD [42].

3. Results and discussions

3.1. Synthesis

The protonated forms of the aroyl hydrazone ligands L1H–L4Hwere prepared by a procedure described earlier [15]. The synthesisof Ru(II) complexes 1–4 was achieved by refluxing equimolecularamounts of the corresponding aroyl hydrazone ligand with

Fig. 1. Molecular structure of 3a along with atom numbering sch

cis-Ru(bpy)2Cl2�2H2O (bpy: 2,20-bipyridine) and triethylamine inmethanolic solution for 6–8 h, followed by anion exchange withsodium perchlorate. The complexes were satisfactorily character-ized by spectroscopic and analytical techniques. Single crystalssuitable for X-ray crystallography for complex 3 were obtained inthe form of hexafluorophosphate salt 3a.

3.2. Description of X-ray crystal structures of 3a and 4

Single crystal X-ray analysis revealed that in the solid state both3a and 4 consist of a monomeric [Ru(bpy)2(N^O)]+ (N^O = L3 or L4)cation and one PF6

� or ClO4� anion, as shown in Figs. 1 and 2.In both complexes the central Ru(II) cation adopts a distorted

octahedral geometry, being coordinated by five N and one O atoms.Four of these N atoms belong to two chelating bpy ligands,whereas the mono-anionic hydrazone ligand binds in a bidentatemanner through the imine nitrogen and enolate oxygen atoms.The most relevant metrical parameters are collected in Table 2.The best least-squares plane in both complexes is defined by atomsRu1, N2, N1, N3 and O2 for 3a and Ru1, N2, N3, N4 and O1 for 4, theroot-mean-square deviation (rmsd) of the constituent atoms fromthe least-squares plane being 0.041 and 0.026 Å for [Ru(bpy)2(L3)]+

and [Ru(bpy)2(L4)]+, respectively. The axial N(4)–Ru(1)–N(6) bondangle for [Ru(bpy)2(L3)]+ is 168.0(1)� and the axial N(1)–Ru(1)–N(5) bond angle for [Ru(bpy)2(L4)]+ is 171.87(14)�, respectively.The average Ru-Nbpy distances [2.048(4) and 2.048(5) Å for L3

and L4 respectively] in both molecules are very similar to that ob-served for [Ru(bpy)3]2+ (2.056(6) Å) [43]. The Ru-Nimine and Ru-Odistances are also very similar to those observed earlier for anotherRu(II) complex of the same ligand [15]. It may be noted that thehydrazone ligands in the present complexes exhibit a more openarrangement (E configuration with respect to C(3)@N(4) bond inScheme 1) with a C(2)–C(3)–N(4)–N(5) torsion angle of 170.9(2)�for L3 and 174.6(4)� for L4 (neglecting the atoms having site occu-pancy factor SOF = 0.393). For previously reported complexes ofthe type trans,trans,trans-[Ru(PPh3)2(N^O)2] the configuration ofthe N^O ligand was found to be Z with respect to the azomethineC(3)@N(4) bond, with C(2)–C(3)–N(4)–N(5) torsion angle of0.7(6)� for L1 and 3.77(11)� for L4 [15]. Similar Z configurationwas also observed in another report on Ru(II) hydrazone com-plexes of L1 and L2 [44].

eme; thermal ellipsoids are drawn at 30% probability level.

Fig. 2. Molecular structure of 4 along with atom numbering scheme; thermalellipsoids are drawn at 30% probability level. The thiophene ring and perchlorateion both exhibit twofold disorder, and in each case the atoms with ’B’ labelsrepresent an alternative orientation.

Table 2Selected bond distances (Å) and angles (�) for 3a and 4.

Complex 3a Complex 4

Bond distancesRu1–N1 2.063(2) Ru1-N1 2.044(4)Ru1–N2 2.036(2) Ru1-N2 2.057(3)Ru1–N3 2.045(2) Ru1-N3 2.053(3)Ru1–N4 2.047(1) Ru1-N4 2.038(3)Ru1–N6 2.089(1) Ru1-N5 2.061(4)Ru1–O2 2.072(2) Ru1-O1 2.072(2)

Bond anglesN1–Ru1–N6 84.6(1) N1–Ru1–N4 86.3(1)N1–Ru1–N2 78.9(1) N1–Ru1–N2 78.5(1)N1–Ru1–O2 95.5(1) N1–Ru1–O1 94.0(1)N1–Ru1–N4 101.1(1) N1–Ru1–N3 97.8(2)N1–Ru1–N3 177.6(1) N1–Ru1–N5 171.9(1)N2–Ru1–N3 98.6(1) N2–Ru1–N5 97.94(13)N2–Ru1–N4 86.5(1) N2–Ru1–N4 99.38(14)N2–Ru1–N6 105.2(1) N2–Ru1–N3 176.04(13)N2–Ru1–O2 172.8(1) N2–Ru1–O1 88.42(11)N3–Ru1–O2 87.0(1) N3–Ru1–O1 93.30(11)N3–Ru1–N4 79.0(1) N3–Ru1–N4 78.84(13)N3–Ru1–N6 95.8(1) N3–Ru1–N5 85.91(12)N4–Ru1–O2 90.2(1) N4–Ru1–O1 172.10(12)N4–Ru1–N6 168.0(1) N4–Ru1–N5 101.51(15)N6–Ru1–O2 78.6(1) N5–Ru1–O1 78.49(11)

Table 3Cyclic voltammetry dataa of complexes 1–4 in MeCN solutions.

Complex E0/V (DEp/mV) valuesb

1 0.84(76), 1.44, �1.50(74), �1.76(104)2 0.83(64), 1.56, �1.49(62), �1.76(104)3 0.91(68), 1.51, �1.49(49), �1.73(81)4 0.90(76), 1.67, �1.50(75), �1.77(127)

a Scan rate is 100 mV/s.b Potentials are referred against Ag/AgCl electrode.

Table 4Computeda electronic parameters (eV) for ligands and derived L(bpy)2Ru(II)complexes.

HOMO Ip[b] LUMO EA

[c] H-L gap

bpy �6.670 8.379 �1.530 0.123 5.140L1-H �5.837 7.526 �1.813 0.265 4.025L3-H �5.672 7.316 �1.701 0.245 3.971[(bpy)3Ru]2+ �11.191 12.149 �7.665 6.433 3.526[L1(bpy)2Ru]+ �7.742 8.794 �5.041 3.877 2.700[L3(bpy)2Ru]+ �7.738 8.952 �5.142 3.980 2.595

a At the optimization B3LYP-D/def2-TZVP level.b Vertical ionization potential.c Vertical electron affinity.

118 B. Ghosh et al. / Polyhedron 72 (2014) 115–121

3.3. Electrochemical studies

On the positive side of the Ag/AgCl electrode, complexes 1–4show a quasi-reversible response at 0.8–0.9 V, which is assignedto Ru(III)/Ru(II) couple (Table 3, ESI Figs. S1–S8). At more positivepotential, around 1.4–1.6 V, another irreversible oxidation withmuch larger anodic peak current is observed, which in a firstapproximation might be attributed to ligand centered oxidation(vide infra). The Ru(III)/Ru(II) potential in the complexes reportedhere are at a more positive values than those of neutralmono(hydrazone) complexes [RuCl2(dmso)2LH] (LH = L1H or L2H)or bis(hydrazone) complexes [Ru(PPh3)2(L)2] (L = L1, L2, L3, L4) re-ported earlier[15,44]. Consistent with previous observations [15]the Ru(III)/Ru(II) potential for the salicyloyl hydrazone complexes

3 and 4 are more positive compared to their corresponding benzoylhydrazone analogs 1 and 2. Within the furyl-appended complexes1 and 3, this fact can be explained by the higher positive (Mulliken)charge at the metal center in the case of 3 (qMull = 0.22 au) in com-parison to 1 (qMull = 0.21 au), as well as by the slightly higherHOMO in 3 (Table 4). However, unlike the reported observation[15], the Ru(III)/Ru(II) potentials are nearly the same for the furyl(X = O) and thienyl (X = S) series (Scheme 1).

On the negative side of the reference electrode there are twoquasi-reversible reductions at �1.50 and �1.75 V, which are as-signed to addition of electrons to the bpy(p⁄) orbitals, as boththe LUMO and LUMO+1 possess bpy(p⁄) character (vide infra).The bpy(p⁄) based LUMO is situated sufficiently high up in energycompared to the conduction band edge potential of anatase TiO2

(�0.5 V versus SCE) used in dye sensitized solar cells (DSSC), so thatelectron transfer from the excited state of the Ru(II) complexes tothe conduction band of TiO2 should be thermodynamically highlyfavourable [45].

All four Ru(II) complexes 1–4 display lower (less positive) E0-

{Ru(III)/Ru(II)} potentials than reference [Ru(bpy)3]2+ (1.3 V). Thisis in agreement with the DFT calculations, showing that both Ru(II)species 1 and 3 are more easily oxidized than [Ru(bpy)3]2+ aspointed out by their lower vertical ionization potential (IP) andhigher energy of HOMO (see Table 4). In the same vein, the formalpotentials for the two successive bpy centered reductions of 1–4are more negative than in [Ru(bpy)3]2+ (�1.3 and �1.5 V, respec-tively) [46]. It is also worth noting that both the HOMO andHOMO-2 of complex 3 contain an appreciable amount of hydra-zone ligand character (Fig. 3), so that the two oxidations describedearlier, may not be correctly described as sequential pure metaland ligand oxidations, since the first oxidation has significant li-gand centered character as well. Furthermore, the hole in the ex-cited state should be expected to be delocalized, similar to theeffect produced by thiocyanate ligands in Grätzel’s N3 (red) andBlack dyes [47].

3.4. Electronic spectra

The electronic spectral data of the complexes are listed in Ta-ble 5. In the visible region all the complexes 1–4 show a strongband at 480–490 nm, with a shoulder at 450 nm, and another

Fig. 3. Calculated (B3LYP-D3/def2-TZVP) Kohn–Sham isosurfaces (0. 04 isovalue) ofrepresentative frontier molecular orbitals (FMO) for complex [L3(bpy)2Ru]+.

Table 5UV–Vis spectra of the complexes in MeCN solutions.

Complex kmax/nm(emax M�1 cm�1)

1 242(55379), 292(109448), 322(44303), 401(23200),453(sh,20557), 492(21867)

2 245(82184), 294(159257), 324(48072), 407(24948),457(sh,26171), 490(28312)

3 244(84912), 292(201652), 335(73443), 400(31458),451(sh,29732), 484(32484)

4 244(57973), 292(118301), 331(37711), 406(15860),447(sh,18001), 482(19836)

Fig. 4. Electronic spectra of complexes 1–4 recorded in acetonitrile solution.

Fig. 5. Calculated Kohn–Sham isosurfaces (0.04 isovalue) for HOMO-1 (left) andLUMO+1 (right) in [(bpy)3Ru]2+.

B. Ghosh et al. / Polyhedron 72 (2014) 115–121 119

band/shoulder at around 400 nm (Fig. 4; ESI Fig. S9, Table 5). The450–490 nm bands have similar profile to that of [Ru(bpy)3]2+

[48], but they are bathochromically shifted by 40–50 nm in com-plexes 1–4 and have been assigned as MLCT transitions. Theband/shoulder at 400 nm is due to charge transfer from the hydra-zone to the bpy ligand (vide infra).

DFT calculations show that in [(bpy)3Ru]2+ HOMO as well as al-most degenerate HOMO-1 and HOMO-2 are ruthenium-centeredd-type AOs, whereas MOs below are p-type bpy orbitals (Fig. 5)[49]. All LUMOs are p⁄-type bpy orbitals, with some mixing withRu d AOs in the case of degenerate LUMO+1 and LUMO+2. TD-DFT calculations (B3LYP-D3/def2-TZVP) predict a low energy (LE)band for [(bpy)3Ru]2+ at ca. 430 nm. This band consists of two tran-sitions at 429.6 nm, f = 0.122, and 428.9 nm, f = 0.125, both havingMLCT character from essentially degenerate HOMO-1 and HOMO-2to degenerate LUMO+1 and LUMO+2 (Fig. 5).

The electronic transitions are easily visualized by means of thetransition electron differences (Fig. 6, only one transition shown)corresponding to both predicted bands, which show electron den-

sity flowing from ruthenium-centered filled d-type AOs (negativeelectron difference values, in red) to mostly bpy-centered p-typeorbitals (positive values, in yellow), therefore confirming theirMLCT character. Indeed, in the ground state both the Mullikenand the Löwdin population analyses reveal high occupancy of4dxy (1.770/1.734 e), 4dxz (1.788/1.744 e) and 4dyz (1.838/1.788 e)orbitals and half-filled character of 4dz2 ð0:630=0:819 eÞ and4dx2�y2 ð0:691=0:870 eÞ.

In [L3(bpy)2Ru]+ the predicted LE transition is bathochromicallyshifted to ca. 486 (f = 0.089), with two other close bands of moder-ate intensity at 398 (f = 0.056) and 385 nm (f = 0.042), in very goodagreement with experiment, and corresponding to transitions fromRu d-type AOs to bpy-centered p⁄ orbitals (Fig. 7). It is worth men-tioning that HOMO-2 to HOMO correspond to ruthenium-centeredMOs whereas LUMO and LUMO+1 are p-type MOs centered at thebpy ligands (see Fig. 3). Another important absorption is predictedat 345 nm (f = 0.034) and it corresponds to a L3 intraligand phenolto imine transition.

In the case of complex [L1(bpy)2Ru]+, the lowest energy absorp-tion is expected to occur at 496 nm (f = 0.076; HOMO-2 ? LUMO)(Fig. 8), together with other weaker absorptions at 404 nm(f = 0.038), 391 nm (f = 0.041), 390 nm (f = 0.033) and 366 nm(f = 0.034), all of them of Ru ? bpy in nature. Lacking the possibil-ity for intraligand transition in the aroylhydrazone moiety, theRu ? imine transition is observed to be hypsochromically shiftedat 326 nm (f = 0.231), in agreement with the experiment.

Fig. 6. Calculated (B3LYP-D3/def2-TZVP) electron difference isosurfaces (isovalues0.005, yellow and �0.005, red) for predicted optical transition at 430 nm in[(bpy)3Ru]2+. (Color online.)

Fig. 7. Calculated (B3LYP-D3/def2-TZV P) electron difference isosurfaces (isovalues0.005, yellow and �0.005, red) for predicted optical transition at 486 (a), 398 (b)and 345 nm (c) in [L3(bpy)2Ru]+. (Color online.)

Fig. 8. Calculated (B3LYP-D3/def2-TZVP) electron difference isosurfaces (isovalues0.005, yellow and �0.005, red) for predicted optical transition at 496 (a) and326 nm (b) in [L1(bpy)2Ru]+. (Color online.)

Fig. 9. Emission spectra of the isoabsorptive solution of the complexes inacetonitrile solution at room temperature.

120 B. Ghosh et al. / Polyhedron 72 (2014) 115–121

3.5. Emission spectra

When excited at 454 nm all four complexes 1–4 exhibit broademission band with kmax(emission) = 612 nm at room temperature.The emission intensities of isoabsorptive solutions of the four

complexes are very similar, with compounds 1 and 4 showingslightly higher emission intensity than the other two (Fig. 9) andalso very similar in position and band profile to that of [Ru(bpy)3]2+

(Fig. S10), therefore being probably originated from a 3MLCT statearising from a dp

5p⁄bpy1 configuration [46–48].

3.6. Liphophilicity of the complexes

As stated in the introduction, it was expected that mixed che-late complexes of Ru(II) containing both polypyridine and aroylhydrazone ligands may possess better lipophilicity than the homo-leptic Ru(II) polypyridyl complexes. The partition coefficient be-tween 1-octanol and water, logP, was inspected as a convenientquantification descriptor for the lipophilicity of the complexes[50], as this feature determines, to a large degree, many pharmaco-kinetic properties of drugs, such as the rate of passive diffusionacross biological membranes for their potential use in live cellimaging. The values computed according to available standard pro-cedures [51,52] suggest moderate to high lipophilicity of the com-plexes which increases in the order 3 (logP = 2.62) < 4(logP = 3.09) < 1 (logP = 3.40) < 2 (logP = 3.88).

4. Conclusions

We have synthesized and characterized four ruthenium(II)complexes which absorb strongly over the 400–600 nm regionwith lowest energy MLCT band bathochromically shifted with re-gard to [Ru(bpy)3]X2 species. The LUMO of the molecules are ofbpy(p⁄)-type, whereas the HOMO are Ru-centered but containingappreciable contribution from the hydrazone ligands, so the holein the excited state might be expected to be delocalized, similarto the effect of thiocyanate ligands in N3 and Black dyes. Moreover,this will also lead to higher electronic coupling between the oxi-dized dye and an external electron donor like I�, and the electrontransfer reaction is also thermodynamically quite favourable inview of the sufficiently positive value of the Ru(III)/Ru(II) redoxpotentials. The LUMO of the molecules are at sufficiently higherenergy compared to that of the conduction band edge potentialof anatase TiO2 (�0.5 V versus SCE), used in dye sensitized solarcells (DSSC). Thus the ruthenium complexes reported in this paperare suitable candidates for developing them further for applicationas dyes in DSSC. The adequate lipophilicity of the complexes alsomake them potential candidate for use in cell imaging.

B. Ghosh et al. / Polyhedron 72 (2014) 115–121 121

Acknowledgments

S.K.C. acknowledges University Grants Commission (UGC) andAll India Council of Technical Education (AICTE) for funding. B.G.thanks Bengal Engineering and Science University (BESU) for aInstitute fellowship.

Appendix A. Supplementary data

CCDC 983055 and 954005 contain the supplementary crystallo-graphic data for compounds 3a and 4, respectively. These data canbe obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Cen-tre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk. Supplementary data asso-ciated with this article can be found, in the online version, athttp://dx.doi.org/10.1016/j.poly.2014.01.031.

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