Polyhedron 39 (2012) 59–65

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Polyhedron

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Synthesis, characterization and theoretical studies of ruthenium(II) complexeswith the quinone functionalized polypyridine ligand, Nqphen

Ramiro Díaz a,⇑, Angélica Francois a, Mauricio Barrera b, Bárbara Loeb b

a Faculty of Natural Resources, Universidad Católica de Temuco, Manuel Montt 56, Temuco, Chileb Faculty of Chemistry, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile

a r t i c l e i n f o a b s t r a c t

Article history:Received 6 December 2011Accepted 15 March 2012Available online 28 March 2012

Keywords:Acceptor ligandQuinoneRuthenium complexesTDDFT study

0277-5387/$ - see front matter � 2012 Elsevier Ltd. Ahttp://dx.doi.org/10.1016/j.poly.2012.03.020

⇑ Corresponding author. Tel.: +56 45 205413; fax: +E-mail address: ramdiazh@uct.cl (R. Díaz).

The synthesis, spectral characterization, electrochemical properties and TDDFT theoretical study of thecomplexes [Ru(dmbpy)2(Nqphen)](PF6)2 (2a) and [Ru(tmbpy)2(Nqphen)](PF6)2 (2b) is reported. Nqphenis the quinone substituted acceptor ligand [3,2-a:20 ,30-c]-benzo[3,4]-phenazine-11,16-quinone, dmbpy is4,40-dimethyl-2,20-bipyridine and tmbpy is 4,40 ,5,50-tetramethyl-2,20-bipyridine. No major difference isobserved in the spectroscopic and electrochemical properties of the complexes, reflecting that the pres-ence of the electron withdrawing Nqphen ligand governs their behavior. Molecular orbital calculationsshow that the LUMO is centered on the quinonic fragment of Nqphen, while the HOMO orbital has mainlya metal character. The calculations results at TDFT level were consistent with the experimental data, andpermitted their detailed interpretation.

� 2012 Elsevier Ltd. All rights reserved.

1. Introduction (Scheme 1A) [9], and a series of rhenium(I) complexes with this li-

An interesting property of the polypyridine complexes of Ru(II)is its capacity of to reach charge-separate excited states (CSES)through metal to ligand charge transfers (MLCT) [1]. The incorpo-ration into the molecule of a ligand with electron acceptor charac-teristics favors the formation of these CSES and gives directionalityto the photo-induced electron transfer [2]. Ligands into whichquinone residues have been incorporated have aroused consider-able interest, due to the versatility of this functional group, whichcan be converted into hydroquinone by the transfer of two protonsand two electrons [3]. Indeed, this redox couple is involved invarious biological electron transport systems [4]. Ru(II) complexeswith ligands which have been functionalized with quinones havebeen studied as molecular switches [5], since the quinones deacti-vate the luminescent excited states of these complexes, due to anintramolecular photoinduced electron transfer (PET) quenchingits MLCT state by the quinone fragment [6]. On the other hand,the hydroquinone formed is a good electron donor and does notdeactivate the excited state, allowing the luminescence to berecovered. For this reason, ligands which are structurally flat havebeen studied as luminescent sensors and DNA excision agents [7].If additionally a ligand with electron donor properties is incorpo-rated into the complex, D–C–A (D = electron donor, C = chromo-phore, A = electron acceptor) systems are formed [2,8].

The structurally flat quinone containing acceptor liganddipyrido[3,2-a:20,30-c]benzo[3,4]phenazine-11,16-quinone (Nqphen)

ll rights reserved.

56 45 205285.

gand, fac-[Re(CO)3(Nqphen)L]+ [10] with L a para substituted pyri-dine, have been reported previously by our group. The electronacceptor effect of the quinonic group in the Re(I) complexes is evi-denced by comparing the electrochemical properties of these Re(I)complexes with complexes with similar aromatic and flat electronaccepting ligands but without a quinonic group [11], where the lat-ter have greater redox potential.

On the other hand, a ESR and theoretical study on Nqphen andother quinone related ligands, e.g. 12,17-dihydronaphtho-[2,3-h]dipyrido[3,2-a:20,30-c]phenazine-12,17-dione, Aqphen [12](Scheme 1B), shows different behaviors associated mainly withtheir structural differences.

In this work the synthesis, characterization and TDDFT theoreticalstudy of the Ru(II) complexes with Nqphen ligand [Ru(dmbpy)2(Nq-phen)](PF6)2 and [Ru(tmbpy)2(Nqphen)](PF6)2, is presented, wheredmbpy is 4,40-dimethyl-2,20-bipyridine and tmbpy is 4,40,5,50-tetra-methyl-2,20-bipyridine. The electronic and electrochemical proper-ties of the synthesized complexes were analyzed and compared withthose reported for similar complexes. Molecular calculations wereused to explain their electronic and electrochemical behavior. Theexcitation energies and oscillator strengths for the two cationiccomplexes at the optimized geometry in the ground state were alsoobtained.

2. Experimental

All complexes were characterized by conventional spectro-scopic techniques (1H NMR, IR, UV–Vis), elemental analysis andelectrochemistry.

Scheme 1. Quinone and methyl substituted ligands.

60 R. Díaz et al. / Polyhedron 39 (2012) 59–65

2.1. Measurements

UV–Vis spectra were recorded on a Shimadzu UV 3101PC.1H NMR spectra were recorded on a Bruker AC/200 MHz spectrom-eter with TMS as reference. Infrared spectra were recorded in KBrpellets in a Bruker Vector-22 FTIR spectrometer. Cyclic voltamme-try was performed using Bas CV-50 W 2.3 MF-9093 equipment.The measurements were recorded in acetonitrile. Tetrabutylam-monium hexafluorophosphate (HBTA) (Aldrich) was used aselectrolyte.

2.2. Materials

RuCl3�H2O, 4,40-dimethyl-2,20-bipyridine (dmbpy), tetrabutyl-ammonium hexafluorophosphate (TBAPF6) and deuterate solventswere obtained from Aldrich. 4,40,5,50-tetramethyl-2,20-bipyridine(tmbpy) was kindly provided by Dr. T.J. Meyer of North CarolinaUniversity at Chapel Hill, USA. Nqphen free ligand was preparedaccording to published procedures [9].

The solvents used for synthesis were p.a. grade, while forelectrochemical measurements spectroscopic grade solvents wereused.

2.3. Theoretical studies

All the calculations were performed with the ADF package [14].Geometrical optimizations were carried out under C2 symmetryusing the PW91 [15] exchange correlation functional, which wasselected among other functionals (LDA, BP86, BLYP), as best repro-ducing [Ru(bpy)3]2+ crystalline structure. Standard DZP basis set

was employed for elements of the first series (C, N, O and H) whilefor ruthenium a TZP basis set was chosen. TDDFT calculations werealso performed for the ligand and metal complexes, employing theSAOP [16] exchange potential, ZORA TZP basis set for rutheniumand DZ basis set for C, H, O, and N. This exchange functional waschosen as being that which best reproduces the experimentalUV–Vis spectra. Other exchanges functional tested in this studywere PBE, BLYP, OLYP, LB94.

A solvent effect study (acetonitrile) was included only for com-plexes through the COSMO model where the shape of the cavity isdefined with the solvent excluded surface and allinger radius. Onlythe first 30 excitations were considered.

2.4. Synthesis

2.4.1. The precursors complexes cis-Ru(dmbpy)2Cl2�2H2O (1a) and cis-Ru(tmbpy)2Cl2�2H2O (1b)

These compounds were synthesized by a modification of a pub-lished procedure for cis-Ru(bpy)2Cl2�2H2O complex [17], where1,4-hydroquinone was added as a reductant, to give better yieldsthan reported. Yields of 87% for 1a and 84% for 1b were obtained.

2.4.2. [Ru(dmbpy)2(Nqphen)](PF6)2 (2a)0.12 g (0.25 mol) of cis-Ru(dmbpy)2Cl2�2H2O and 0.10 g

(0.28 mmol) of Nqphen were suspended in 30 ml of ethanol/watermixture at 2:1. The mixture was heated at reflux for 18 h under aninert atmosphere. During this time, the color of the mixture chan-ged from violet, typical of the precursory complex, to red. Theresulting solution was cooled to r.t. and 0.1 g of NH4PF6 dissolvedin 10 ml of distilled water was added. The mixture was stirred for1 h, during which time a red precipitate formed. The solid was fil-tered and washed with abundant water and diethyl ether and puri-fied by column chromatography (Al2O3; toluol/acetonitrile 1:1).The solution was concentrated by rotary evaporation, and theresulting product precipitated with diethyl ether, filtered and thendried under high vacuum. 0.154 g of product was obtained with 60%yield.

Anal. Calc. for RuC46H34N8O2P2F12 (M = 1121.83 g/mol): C,49.251; H, 3.0548; N, 9.9885. Found: C, 48.91117; H, 3.16211; N,9.78025%.

2.4.3. [Ru(tmbpy)2(Nqphen)](PF6)2 (2b)The synthesis of this complex was performed using a similar

method to 2a. 0.167 g of product was obtained with 60.3% yield.Anal. Calc. for RuC50H42N8O2P2F12 (M = 1177.93 g/mol): C,

50.983; H, 3.5939; N, 9.5127. Found: C, 50.21526; H, 3.764416;N, 9.394043%.

3. Results and discussion

3.1. Synthesis of the complexes

Synthesis of the homoleptic complexes 1a and 1b first requiredthe replacement of the chloride ions from RuCl3�3H2O by dmbpyand tmbpy respectively. The modification outlined in this work isthe use of hydroquinone for reduction ‘‘in situ’’ of Ru(III) to Ru(II)under the reaction conditions. The excess LiCl ensures the forma-tion of the bis-complex.

The two heteroleptic complexes were readily prepared with rel-atively similar yield (�60%), by allowing a slight excess of Nqphenligand to react with 1a or 1b complexes, in a boiling mixture ofethanol/water (2:1) for 18 h. The reaction was monitored by chro-matography on aluminum foil; the disappearance of the violetband of the precursor complexes was followed by the appearanceof the red band of the product. The cationic complexes were solu-ble in polar solvents such as MeOH and MeCN.

R. Díaz et al. / Polyhedron 39 (2012) 59–65 61

Fig. 1 shows the 1H NMR spectrum of complex 2a. Identificationof the molecule protons and assignment of the signs are also in-cluded. Table 1 summarizes the 1H NMR data for all complexessynthesized.

The proton chemical shifts were assigned on the basis of peakintegration, coupling constants and by comparison with Nqphenfree ligand [9] and a Re(I) complex [10].

Two sets of protons in the bpy rings were observed due to therelative degree of the diamagnetic anisotropy effect from adjacentchloride and aromatic rings in neutral complexes and aromaticrings of bpy or Nqphen in cationic complexes.

In cationic complexes, the H3, H5 and H6 protons are close to theNqphen ligand, while H30 , H50 and H60 are close to the other bpyrings (see insert in Fig. 1). As Nqphen has more electron acceptorproperties than bpy, a higher de-shielding effect on protons H3,H5 and H6 is expected; these protons were therefore assigned tolower fields than H30 , H50 and H60 , respectively.

The presence of the Nqphen ligand in the cationic complexesproduces a strong de-shielding effect on all aromatic protons ofbpy ligands as compared to the neutral complexes. Even themethyl groups are de-shielding, although to a lesser extent thanthe aromatic protons.

The Nqphen pattern in the complexes is similar to that of freeligand. All protons on Nqphen ligand are displaced to low fieldsdue to coordination, except for Ha protons. This behavior of Ha

would be an effect of the backbonding of the metal, which shieldsthese protons.

The two extra methyl groups in tmbpy as compared to dmbpydo not affect Nqphen protons significantly; however a shift to highfield is observed in bpy protons of tmbpy as compared to dmbpydue to the donor shielding effect of extra methyl groups.

The integrals for the bpy ligand protons reflect a 1:1 relation ofthis ligand with respect to Nqphen, indicating that the cationiccomplexes were obtained in a pure form.

3.2. FTIR spectra

The FTIR spectra show that the stretching vibration frequencyfor the carbonyl groups of the Nqphen free ligand appears at1686 cm�1. This value is lower than in the parent compound, naph-toquinone, where it occurs at 1778 cm�1. This shift can be attrib-uted to the presence of the pyrazine ring, which is acting as an

N

N

O

Ode

c a d b e

6 3, 3’

8.28.69.09.49.8

Fig. 1. 1H NMR spectra of [Ru(dmbpy)2(N

electron donor, and delivering electronic density to the quinoniccarbon skeleton which results in an enlargement of the C–O dis-tance and a lowering of its stretching vibration frequency. Whenthe free ligand is coordinated to the [Ru(dmbpy)2]+2 fragment, anelectronic charge density flow occurs from the ligand to the posi-tively charged ruthenium atom. As a consequence of this process,the bipyridine component fragment of the Nqphen ligand becomeselectron deficient, giving rise to an increase of its electron-attract-ing strength and its ability to remove electron density from thepyrazine ring. The net result is an increase in the vibration fre-quency of carbonyl to 1690 cm�1 in 2a and 1692 cm�1 in 2b, as aconsequence of the lesser electronic density delivered to thequinonic ring by the pyrazine fragment.

3.3. Molecular orbital structure

For the purposes of the present study, the Nqphen ligand shouldbe decomposed into three fragments containing bipyridine (B),pyrazine (P) and naphtoquinone (Q) molecular groups, as shownin Fig. 2.

The use of this decomposition will facilitate the study of itsmolecular structure and its influence on the optical and redoxproperties of the complexes [18]. Table 2 shows the electronicstructure of the Nqphen free ligand in terms of the first four occu-pied and unoccupied molecular orbitals.

The lowest unoccupied molecular orbital (LUMO) is composedmainly of the Q fragment, thus the highest probability of findingan electron in this orbital would be found on the quinonic frag-ment which is located on the rear side of the molecule. Lookingat lower energy, the highest occupied molecular orbital (HOMO)appears to be composed mostly of the two quinonic (Q) and pyraz-inic (P) fragments, and consequently the electronic density will belocated mostly on the center and rear side of the molecule, givingthis orbital a PQ character. Molecular orbitals containing the bipyr-idine fragment are found at higher energies, near 4.0 eV.

The molecular orbital diagram for complexes 2a and 2b arerather similar and a representative picture showing the commonfeatures of both complexes is displayed in Fig.3. Details of the com-position of the first 10 molecular orbitals in terms of their fragmentcomponents are displayed in Table 3.

From Fig. 3 it can be seen that the HOMO is located at �7.36 eVand composed of ruthenium d-orbitals. The HOMO-1 and HOMO-2

7.0

CH3 CH3’NRu

N

N

N CH3'

CH3

N

N

CH3

CH3'

ab

c

6

35

3'

5' 6'

(PF6)2

5 5’

6’

7.47.8 2.42.8

qphen)](PF6)2, at 200 MHz in CD3CN.

Table 11H NMR chemical shifts (ppm) for the cationic (in CD3CN) and neutral (CD3Cl) complexes (200 or 400 MHz).

Position 2a 2b 1a 1b

Ha 8.76 (dd); Jab = 5.41, Jac = 1.23 8.72 (dd); Jac = 1.23 – –Hb 8.25 (dd); Jba = 5.41, Jbc = 8.21 8.22 (dd); Jba = 5.41, Jbc = 8.12 – –Hc 9.84 (dd); Jcb = 8.12, Jca = 1.23 9.78 (dd); Jcb = 8.12, Jca = 1.23 – –Hd 8.57 (dd); Jde = 5.91, Jde0 = 3.20 8.56 (dd); Jde = 5.66, Jde0 = 3.2 – –He 8.19 (dd); Jed = 5.91, Jde0 = 3.30 8.19 (dd); Jed = 5.66, Jde0 = 3.2 – –H3 8.83 (s) 8.68 (s) 7.96 (s) 7.80 (s)H30 8.78 (s) 8.63 (s) 7.91 (s) 7.74 (s)H5 7.57 (dd); J56 = 5.9, J53 = 1.2 – – –H50 7.29 (dd); J5060 = 5.7, J5030 = 1.2 – 6.85 –H6 8.07 (dd); J65 = 5.91 7.89 (dd) 9.73 9.60H60 7.95 (dd); J6050 = 5.7 7.86 (dd) 7.33 7.22CH3 2.70 (s) 2.61 (s), 2.57 (s) 2.54 (s) 2.45, 2.39CH30 2.58 (s) 2.50 (s), 2.39 (s) 2.37 (s) 2.27, 2.05

N

N

N

N

O

O

B P Q

Fig. 2. Fragment decomposition of the Nqphen ligand.

Table 2Molecular orbital composition of Nqphen free ligand.

MO Energy Q P B Character

L3 �4.13 3 3 94 BL2 �4.23 23 24 54 BL1 �5.05 18 58 23 PL �5.55 69 25 6 QH �7.44 47 52 1 PQH1 �7.63 87 11 2 QH2 �7.98 2 1 96 BH3 �8.21 0 0 100 B

H = HOMO, H1 = HOMO-1, L = LUMO, L1 = LUMO+1.

62 R. Díaz et al. / Polyhedron 39 (2012) 59–65

orbitals are degenerated and also contain metallic orbitals,whereas HOMO-3 and HOMO-4 contains fragments of the Nqphenligand. At higher energies, the LUMO is found at -5.72 eV and ismostly composed of Q fragment (see Fig. 2). The LUMO+1(�5.37 eV) is centered on the middle and the front side of the li-gand (P and B fragments), while a mixture of Nqphen and dmbpyorbital is found on the LUMO+2 (�5.02 eV).

It is interesting to mention that while the composition of theLUMO in the free ligand and in the complex is almost the same;a lowering in the energy of this orbital of 0.17 eV is observed whenthe ligand forms part of the complex. This difference indicates thatupon coordination, the LUMO of Nqphen becomes more stabilizedand thus, is a better electron acceptor. These results are consistentwith electrochemical data, as discussed below.

3.4. Experimental and calculated UV–Vis spectra

Table 4 summarizes the UV–Vis data for all the synthesizedcomplexes. Data for Nqphen free ligand are also included for

comparison. Fig. 4 shows the experimental and calculated UV–Vis spectra for Nqphen free ligand and 2a complex.

The experimental UV–Vis spectrum of the neutral precursorcomplex 1a and 1b shows three well defined bands. In 1b, the pres-ence of the additional methyl substitutes in the bpy ligand pro-vokes an important band shift towards higher energy. This is dueto the fact that the electron donor effect of methyl groups destabi-lizes the orbit p⁄ of the bpy ligand [19]. The band which appears atthe highest energy was assigned to intraligand transitions (IL,p ? p⁄) while the two following bands were assigned to metal toligand charge transfer (MLCT, d ? p⁄).

The experimental UV–Vis spectra of the cationic complexes arevery similar, only with slight differences in the absorption maxima.This would indicate that the spectrum of these complexes is dom-inated by transitions involving principally the Nqphen ligand, com-mon to both complexes. An intense band appears in the ultravioletzone, assigned to intraligand transitions (IL, p ? p⁄). A shoulder isobserved at approximately 310 nm, which was assigned to transi-tions on the Nqphen ligand, by comparison with the lower energyband (317 nm) which appears in the free ligand spectrum.

The band in the visible region, assigned to metal to ligandcharge transfer (TCML, d ? p⁄), appears at higher energy than thechlorate precursors. Moreover, the bands that appear in the limitof UV zone in the precursors complexes are not observed in cat-ionic complexes, possibly because they collapse with TCML bandsin the visible zone or are masked by the IL transitions on theNqphen ligand.

Results obtained from TDDFT calculation for the free Nqphen li-gand and [Ru(dmbpy)2(Nqphen)]+2 complex are displayed in Table5. For the free ligand, three electronic transitions (2A, 2B, 2C) arefound at 387, 311 and 300 nm. The main electronic transition withoscillator strength 0.16 occurs at 311 nm and can be comparedwith the experimental value of 317 nm. According to TDDFT, thiselectronic transition is governed mainly through excitation 2B,which involves electronic density moving from the QP fragmentsto the B fragment, and can be classified as IL transition. The shoul-der appearing at 387 nm, corresponding to excitation 2A, can alsobe identified with an IL process since it involves an electronic den-sity displacement from the B to the Q fragment, which occurs inthe opposite direction to that in excitation 2B.

Three main electronic transitions are found for the complex (1A,1B, and 1C), located at 435, 430, and 425 nm with oscillatorstrength 0.06, 0.11 and 0.05, respectively. An accurate analysis ofthese electronic transitions can be achieved by means of the tran-sition density analysis method [18] which shows that, in general,the bands are composed of 92% metal to ligand charge transfer(MLCT), 7% ligand to ligand (LL) and 1% intraligand (IL). If MLCTexcitations is considered, 63% goes to the Nqphen ligand and 29%to the dmbpy ligand.

Fig. 3. Molecular orbital diagram for the [Ru(dmbpy)2(Nqphen)](PF6)2.

Table 3Molecular orbital composition of complex [Ru(dmbpy)2(Nqphen)](PF6)2.

MO Energy Q P B Ru dmbpy

L6 �3.94 0 0 0 0 100L5 �4.69 23 15 59 1 1L4 �4.85 0 0 1 7 98L3 �4.87 1 5 37 8 48L2 �5.02 2 10 37 1 49L1 �5.37 12 45 42 0 0L �5.72 64 27 9 0 0H �7.36 0 0 3 88 9H1 �7.49 0 0 2 81 18H2 �7.50 0 0 11 79 10H3 �7.60 53 45 1 0 0H4 �7.75 89 11 0 0 0H5 �8.57 42 54 4 0 0H6 �8.98 0 0 0 0 100

Table 4UV–Vis spectroscopic data for neutral and cationic complexes.

Complexa,b kabs (nm)

IL MLCT

Nqphenc 242, 275, 317 –1a 296 377, 5601b 259 355, 5482a 284, �310 h 4452b 288, �310 h 441

a Cationic complexes such as PF6� salt.

b All complexes in MeCN.c Free ligand in CHCl3.

R. Díaz et al. / Polyhedron 39 (2012) 59–65 63

No contributions to the MLCT of HOMO to LUMO transitions areobserved, due to the spatial distance existing in the molecule

between these molecular fragments, since the HOMO is locatedon the metal and LUMO is located on the Q fragment of the Nqphenligand.

3.5. Electrochemical experiment

The results for the cyclic voltammetry measurements for cat-ionic complexes are summarized in Table 6. As with the resultsUV–Vis spectroscopy, the results for the two cationic complexespresent only slight differences. The reversible reduction processeswere assigned compared with the results for the Nqphen free ligandand its B, P and Q fragments, and the energies of molecular orbitaldetermined by theoretical calculations. (see Fig. 2 and Table 3).

The first reduction process was assigned to the Q fragment,which according to the results of the theoretical study is the zonewhich shows the greatest sensitivity for reception of electronicdensity. The second reduction process was assigned to the P andB fragments and the third reduction potential would be on B frag-ment of Nqphen ligand and on bpy ligand, which are the most dif-ficult to reduce and where is located mostly the LUMO+2 orbital.

In the cationic complexes, the reduction process of the coordi-nated Nqphen ligand occurs at lower potential than for the free li-gand. This effect, which is caused by the coordination to the metal,is also detected when analyzing FTIR and UV–Vis data, as well aswhen comparing the value of the LUMO calculated energies forthe free and coordinated ligand.

The oxidation observed is assigned to the RuII/III processes andno difference is observed between the two cationic complexes.The similarity in the results for the two complexes implies thatthe difference in the number of methyl groups on the bpy doesnot affect the redox properties of the complexes. This result has al-ready been discussed with the FTIR and UV–Vis data.

Fig. 4. Experimental (black) and calculated (red) UV–Vis spectra of (A) Nqphen free ligand (in CCl3) and (B) [Ru(dmbpy)2(Nqphen)](PF6)2 (in MeCN). (Color online.)

Table 5Summary of TDDFT calculation for the Nqphen free ligand and [Ru(dmbpy)2(Nqphen)](PF6)2 complex.

Compound Electronic transition k (nm) Oscillator strength Main excitations Assignment

Nqphen 2A 387 0.04 94% H5 ? L IL (B ? Q)a

2B 311 0.16 61% H6 ? L1 IL (BP ? P)25% H10 ? L IL (B ? Q)5% H5 ? L2 IL (Q ? P)

2C 300 0.09 67% H7 ? L2 IL (Q ? P)15% H10 ? L IL (B ? Q)10% H8 ? L IL (Q ? Q)

[Ru(dmbpy)2(Nqphen)](PF6)2 1A 435 0.06 38% H2 ? L4 TCML (M ? B0)b

29% H1 ? L3 TCML (M ? B0 + M ? N)21% H2 ? L5 TCML (M ? N)6% H1 ? L2 TCML (M ? B0 + M ? N)

1B 430 0.11 56% H1 ? L4 TCML (M ? B0)12% H2 ? L2 TCML (M ? B0 + M ? N)10% H4 ? L3 IL + LL7% H2 ? L3 TCML (M ? B0 + M ? N)

1C 425 0.05 74% H2 ? L5 TCML (M ? N)14% H1 ? L2 TCML (M ? B0 + M ? N)7% H3 ? L4 LL

a See Fig. 2 for meaning of B, P and Q.b M = Ru; B0 = dmbpy ligand; N = Nqphen ligand.

Table 6Electrochemical data for cationic complexes in MeCN. Potentials given in V vs. SCE;supporting electrolyte 0.1 M Bu4NPF6.

Complex E1/2(ox) (RuIII/II) E1/2(red)

Q(0/�1,�1/�2) P + B(0/�1) B + bpy(0/�1)

Nqphena – �0.65 �1.23 �1.532a +1.30 �0.44 �0.92 �1.402b +1.30 �0.45 �0.94 �1.42

a Free ligand.

64 R. Díaz et al. / Polyhedron 39 (2012) 59–65

When the results of complexes 2a and 2b are compared withthose reported for the similar complex [Ru(bpy)2(Aqphen)](PF6)2

[13], two reduction processes appear in the latter at low potential(�0.22 and�0.84 V), which were assigned to the formation of sem-iquinone and to the dianion in the quinone fragment. In complexes2a and 2b on the other hand, a broad signal is observed at low po-tential; this would contain both processes on the quinone fragment(Q(0/�1,�1/�2)), which collapse into a single signal because of theproximity of their potentials. This type of behavior has also beenreported for the [Ru(phen)2(Aqphen)](PF6)2 (phen = Phenenathro-line) complex [20], where one signal is observed for the quinonereduction. In both complexes with Aqphen ligand, the quinonereduction potential is lower than for the analogous complexes withNqphen.

This difference would be given by the structural difference ofthe Aqphen and Nqphen ligands. Due to the structural break in Aq-phen, the two quinonic carbonyls are not equivalent, and for thisreason two different signals appear in the complex [Ru(bpy)2(Aq-phen)](PF6)2. The above is given as possible explanation for thelower value of the first reduction process in the Aqphen free ligandas compared to Nqphen; however the linearity of the latter pro-duces a greater electronic delocalization [12].

4. Conclusion

In this article we have reported the synthesis of new Ru (II)complexes with the quinone substituted ligand, Nqphen. The re-sults obtained in the spectroscopic studies, electrochemical mea-surements and elementary analyses, were consistent with theproposed molecular structures.

The difference in the number of methyl substituents on the bpyligands produces no important modifications in the spectroscopicand electrochemical properties of the cationic complexes. From thisit may be concluded that the properties of the complexes are mainlygoverned by the properties and influence of the Nqphen ligand.

The theoretical calculations allowed assigning and explainingsome electronic transitions and the redox processes in the Nqphenfree ligand and its complexes; the results correlate well with thosefound experimentally. Because the LUMO in the complexes is lo-cated in the Q fragment of the Nqphen ligand, and the HOMO in

R. Díaz et al. / Polyhedron 39 (2012) 59–65 65

the metal, no electronic transitions are found involving these OM,since they are also spatially distant.

The electron acceptor capacity of the Q fragment of thecoordinated Nqphen ligand is high compared with other molecularfragments, and it is to be expected that the photoexcited electronfinally resides there, after intramolecular migration from thehigher energy OM, as has been proposed in Re complexes withthe Aqphen ligand [13].

Acknowledgement

Authors acknowledge at Fondecyt for support through Grants1070799 and 1110991.

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