Synthesis, characterization, spectroscopic and electrochemical studies of donor-acceptor ruthenium(II) polypyridine ligand derivatives with potential NLO applications

Article abstract of DOI:10.1016/j.poly.2014.09.004

In this work we report the preparation of six new donor-metal-acceptor (D-M-A) type complexes of ruthenium(II) with the highly absorbing chromophoric ligand 4,4′-bis(2-(4-methoxyphenyl)styryl)-2,2′-bipyridine, (L-OCH₃, donor moiety) and substituted polypyridinic ligands with electron acceptor character (NN-A). The NN-A studied ligands were pyrazino[2,3-f][1,10]phenanthroline (ppl), 11-R-dipyrido[2,3-a:2′,3′-c]phenazine (dppz-R; R is H, NO₂, or CN) and 10,11-[1,4-naphtalenedione]dipyrido[3,2-a:2′,3′-c]phenazine (Aqphen). The complexes were characterized by IR, NMR, UV-Vis spectroscopy and cyclic voltammetry. The potential NLO response of the complexes was evaluated by solvatochromic studies. Although the communication between D and A exists, the effect of the change of the acceptor moiety on the properties of the complexes is small and the behavior of the complexes is governed mainly by the donor ligand. The Metal to Ligand Charge Transfer bands (MLCT) exhibited by all complexes in the visible region have dominant electronic density transfer character from the metal to the chromophoric L-OCH₃ ligand. The hypsochromic shift of this low energy absorption band on going from a less polar (benzene) to a more polar solvent (acetonitrile) indicated that a redistribution of the electronic density among the metal and the donor ligand is observed. This behavior permits to predict a NLO response for these types of complexes. The combination of high molar absorptivity with intraligand charge transfer (ILCT) mixing into the MLCT bands are encouraging for the generation of new materials with interesting NLO properties.

Full text of DOI:10.1016/j.poly.2014.09.004

Polyhedron 85 (2015) 511–518
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Synthesis, characterization, spectroscopic and electrochemical studies
of donor–acceptor ruthenium(II) polypyridine ligand derivatives
with potential NLO applications
César Zúñiga a, Irma Crivelli b, Bárbara Loeb a,
⇑
a Facultad de Química, Pontificia Universidad Católica de Chile, Casilla 306, Santiago, Chile
b Facultad de Ciencias, Universidad de Chile, Las Palmeras 3425, Santiago, Chile
a r t i c l e i n f o
a b s t r a c t
Article history:
In this work we report the preparation of six new donor–metal–acceptor (D–M–A) type complexes of
ruthenium(II) with the highly absorbing ‘‘chromophoric’’ ligand 4,40-bis(2-(4-methoxyphenyl)styryl)-
2,20-bipyridine, (L-OCH3, donor moiety) and substituted polypyridinic ligands with electron acceptor
character (NN-A). The NN-A studied ligands were pyrazino[2,3-f][1,10]phenanthroline (ppl), 11-R-dipyr-
ido[2,3-a:20,30-c]phenazine (dppz-R; R is H, NO2, or CN) and 10,11-[1,4-naphtalenedione]dipyrido[3,2-
a:20,30-c]phenazine (Aqphen). The complexes were characterized by IR, NMR, UV–Vis spectroscopy and
cyclic voltammetry. The potential NLO response of the complexes was evaluated by solvatochromic stud-
ies. Although the communication between D and A exists, the effect of the change of the acceptor moiety
on the properties of the complexes is small and the behavior of the complexes is governed mainly by the
donor ligand. The Metal to Ligand Charge Transfer bands (MLCT) exhibited by all complexes in the visible
region have dominant electronic density transfer character from the metal to the chromophoric L-OCH3
ligand. The hypsochromic shift of this low energy absorption band on going from a less polar (benzene)
to a more polar solvent (acetonitrile) indicated that a redistribution of the electronic density among the
metal and the donor ligand is observed. This behavior permits to predict a NLO response for these types
of complexes. The combination of high molar absorptivity with intraligand charge transfer (ILCT) mixing
into the MLCT bands are encouraging for the generation of new materials with interesting NLO properties.
Ó 2014 Elsevier Ltd. All rights reserved.
Received 18 March 2014
Accepted 8 September 2014
Available online 28 September 2014
Keywords:
Ruthenium complexes
Solvatochromic effect
MLCT
NLO
Donor-Acceptor effect
1. Introduction
D–A pair but also on the nature of the p-conjugated spacer
[12–15]. Materials expected to generate high non-linear optical
The nonlinear optical (NLO) properties of various classes of
metal complexes have been systematically explored in the search
for new and optimized NLO materials. Both early [1–4] and more
recent [5–10] review articles of metal complexes indicate the
breadth of the active research in this field, with the majority of
studies focusing on quadratic nonlinearities in complexes with a
donor(D)–bridge–acceptor(A) composition. The introduction of an
organometallic fragment offers further possibilities for modifica-
tion and tuning of the electronic and/or optical properties of these
materials. Connection of the metal in an electron donor or acceptor
group to another acceptor or donor group, either directly or via a
responses should fulfil the following conditions: (i) a low energy
CT transition related to a small
D(HOMO–LUMO) which can imme-
diately be derived from UV–Vis spectra; (ii) a large transition
dipole moment related to the oscillator strength, which can be cal-
culated from the integrated intensity of the CT band; (iii) a large
change in the dipole moment Dleg [16]. In this context, a number
of NLO dipolar and octupolar metal complexes, mainly based on
bipyridine ligands, have been reported in the literature and
recently reviewed [17,18]. Complexes of ruthenium(II) with poly-
pyridine ligands displayed a low energy Metal to Ligand Charge
Transfer (MLCT) bands with good molar extinction coefficients
conjugated
p
-network, leads to intense and low-energy intramo-
(e) due to electron delocalization [19–21], which changed the elec-
lecular charge transfer (ICT) transitions [11]. The NLO activity of
this class of compounds depends not only on the strength of the
tric dipole moment associated with the charge-transfer (CT) state
as indicated by the solvatochromic effect on this band [16].
In this work we report the preparation, characterization and the
evaluation of potential NLO response of new metallic complexes of
ruthenium(II) with the highly absorbing ‘‘chromophoric’’ ligand
⇑
Corresponding author.
E-mail address: bloeb@puc.cl (B. Loeb).
http://dx.doi.org/10.1016/j.poly.2014.09.004
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

512
C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
4,40-bis(2-(4-methoxyphenyl)styryl)-2,20-bipyridine,
(L-OCH3,
0.66 mL (5.42 mmol) in 10 mL of dry THF was added over a
donor moiety) and substituted polypyridinic ligands with electron
acceptor character (NN-A, acceptor group) such as pyrazino[2,
3-f][1,10]phenanthroline (ppl), 11-R-dipyrido[2,3-a:20,30-c]phena-
zine (dppz-R; R: H, NO2, CN) and 10,11-[1,4-naphtalenedi-
one]dipyrido[3,2-a:20,30-c]phenazine (Aqphen):
5 min period. The mixture was stirred at 0 °C for 5 h and then at
room temperature for 16 h. Then it was quenched with 2 mL of
methanol and 18 mL of water, whereupon the color changed from
orange to a light yellow. The product was extracted with CH2Cl2
(3 ꢁ 30 mL). The organic phase was dried over anhydrous Na2SO4
O
R
N
N
N
N
N
N
N
N
N
N
N
O
NN-A
N
R = H, CN, NO2
Based on the solvatochromic effect observed, a potential NLO
response is anticipated.
and the solvent was evaporated to give the product. The solid
was washed three times with diethyl ether and was filtered and
dried under vacuum.
Yield: 56%. IR (cmꢀ1): 3385
(CH aliphatic); 1598 (C@N); 1249 m
m(OAH); 3061
m(CH aromatic); 2835
2. Experimental
m
m
m
(CAOAC asymmetric); 1033
(CAOAC symmetric). 1H NMR (CDCl3, 400 MHz, d ppm): 3.07
2.1. General methods and materials
(m, 4H, Hx); 3.79 (s, 6H, OCH3); 4.27 (m 1H0); 6.86 (d, J = 8.8 Hz,
4H, H9); 7.08 (d, J = 5.0 Hz, 2H, H5); 7.28 (d, J = 8.8 Hz, 4H, H8);
8.26 (s, 2H, H3); 8.50 (d, J = 5.1 Hz, 2H, H6).
Organic solvents were purified by standard methods. All chem-
icals were reagent-grade, purchased from Aldrich and used as
received unless otherwise specified. 4,40-Bis(2-(4-methoxy-
phenyl)ethyl)-2,20-bipyridine [L-OCH3] was prepared by a modifi-
cation of the synthesis reported in the literature [22] that will be
described in Section 2.3, and the precursor complex, cis-[Ru(L-
OCH3)2Cl2], was prepared according to a literature procedure [23].
IR spectra were obtained using a Bruker VECTOR 22 FT-IR spec-
trophotometer in solid mode (KBr cell, 0.2 mm in length). 1H NMR
and 1H–1H COSY spectra were obtained on a Bruker AVANCE 400
spectrometer, all spectra being referred to TMS as an internal stan-
dard. UV–Vis absorption spectra were recorded on a Shimadzu
UV–Vis–NIR 3101 PC160 instrument using 1-cm quartz cells. Elec-
trochemistry measurements (cyclic voltammetry) were carried out
with a BAS CV-50 W unit. A conventional three-electrode configu-
ration was used, consisting of a platinum working electrode, a plat-
inum wire as the auxiliary electrode, and Ag/AgNO3 as the
reference electrode. All solutions were prepared in acetonitrile,
freshly distilled over P4O10 with 0.1 M TBAH (tetrabutylammonium
hexafluorophosphate) as the supporting electrolyte, and thor-
oughly degassed with N2 prior to each experiment. Cyclic voltam-
2.3. Synthesis of 4,40-bis(2-(4-methoxyphenyl)styryl)-2,20-bipyridine
[L-OCH3]
1.0 g
(2.10 mmol)
of
4,40-bis(2-hydroxy-2-(4-methoxy-
phenyl)styryl)-2,20-bipyridine was dissolved in 50 mL of 10% aque-
ous H2SO4 solution; the mixture was heated at 90 °C for 2 h and
then cooled in an ice bath and neutralized with 1 M aqueous NaOH
to yield a light yellow precipitate. The product was filtered, washed
with water, then acetone and dried under vacuum, giving L-OCH3
as a yellow solid. The numbering of hydrogen atoms is shown in
Fig. 1 in order to follow the assignment of the 1H NMR signals of
the L-OCH3 ligand.
Yield: 73%. Anal. Calc. for C28H24N2O2: C, 79.98; H, 5.75; N, 6.66.
Found: C, 80.59; H, 5.80; N, 6.79%. IR (cmꢀ1): 3027
m
(CH aromatic);
(CAOAC asymmetric);
(CAOAC symmetric). 1H NMR (CDCl3, 400 MHz, d ppm):
2833
1028
m(CH aliphatic); 1580 m(C@ N); 1249 m
m
3.85 (s, 6H, OCH3); 6.94 (d, J = 8.7 Hz, 4H, H9); 7.00 (d, J = 16.3 Hz,
mograms were run at a sweep rate of 100 mV sꢀ1. The reported E
½
OCH 3
values were calculated as the semidifference between the Ep corre-
H5
Hx
H3
sponding to the cathodic and anodic waves: E = (Ec + Ea)/2. Ele-
½
mental analyses were performed using a CE Instruments model
EA 1108 elemental analyser.
H6
H9
N
N
Hy
H8
2.2. Synthesis of 4,40-bis(2-hydroxy-2-(4-methoxyphenyl)styryl)-2,20-
bipyridine
The synthetic method was adapted from the synthesis of an
analogous compound [23]. 0.50 g (2.71 mmol) of 4,40-Dimethyl-
2,20-dipyridyl was dissolved in 20 mL of dry tetrahydrofuran
(THF) at 0 °C, under nitrogen, and added dropwise to a solution
of diisopropylamine (3.5 mL, 24.9 mmol). The mixture was stirred
at 0 °C for 75 min; then a solution of 4-Methoxybenzaldehyde
OCH 3
Fig. 1. Proton numbering for the 1H NMR signals of the L-OCH3 ligand.

C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
513
Hc
130.4 (C8); 133.1 (C3); 133.3 (C14); 134.9 (C10); 152.6 (C1); 152.8
(C16).
Hd
C15
Hb
C16
N
C14
C13
Ha
2.5. Synthesis of Tris(4,40-bis(2-(4-methoxyphenyl)styryl)-2,20-bipyr-
idine)ruthenium bis(hexafluorophosphate) [Ru(L-OCH3)3](PF6)2 (1)
N
N
C10
CN
He
C17
C18
C12
C5
C11
C6
C9
C8
54 mg (0.26 mmol) RuCl3ꢂ3H2O and 330 mg (0.78 mmol) of
L-OCH3 were stirred at reflux in 40 mL EtOH/H2O (1:1) under nitro-
gen atmosphere for 24 h. The ethanol was removed by rotary evap-
oration and a solution of 254 mg (1.56 mmol) NH4PF6 in water
(10 mL) was added, after which a deep-red precipitate appeared,
as depicted in Scheme 1. The solid was filtered off, washed with
water and diethyl ether. The crude product was purified by chro-
matography in aluminum oxide prepared with CH3Cl, and the
desired compounds were eluted with acetone. The deep red-wine
band was collected and the solvent removed by rotary evaporation,
the solid was then dissolved in a minimum volume of acetone, and
the solid precipitated with ethyl ether.
N
C4
C3
C7
C1
Ha´
Hd´
C2
Hb´
Hc´
Fig. 2. Proton and carbon numbering for the NMR signals of the dppz-CN ligand.
2H, Hx); 7.37 (dd, J = 5.1 Hz/1.2 Hz, 2H, H5); 7.42 (d, J = 16.3 Hz, 2H,
Hy); 7.52 (d, J = 8.7 Hz, 4H, H8); 8.52 (s, 2H, H3); 8.65 (d, J = 5.1 Hz,
2H, H6).
Yield: 24%. Anal. Calc. for RuC84H72N6O6P2F12: C, 61.05; H, 4.39;
N, 5.09. Found: C, 61.68; H, 4.45; N, 5.12%. IR (cmꢀ1): 3028
m
(CH
(CAOAC
(PAF6).
2.4. Synthesis of 11-cyanodipyrido[2,3-a:20,30-c]phenazine (dppz-CN)
aromatic); 2836
m(CH aliphatic); 1596
m(C@N); 1252 m
asymmetric); 1028
m(CAOAC symmetric); 842 and 557
m
1.00 g (4.80 mmol) of 1,10-phenanthroline-5,6-dione and 0.64 g
(4.80 mmol) of 3,4-diaminobenzonitrile were dissolved in 80 mL of
methanol and the mixture was refluxed for 5 h. The mixture was
allowed to cool and the resultant precipitate filtered and washed
with MeOH and diethyl ether to give a light red solid that was
dried in vacuum. The numbering of hydrogen atoms is shown in
Fig. 2 in order to follow the assignment of the 1H NMR signals of
the dppz-CN ligand.
1H NMR (acetone-d6, 400 MHz, d ppm): 3.84 (s, 18H, OCH3); 7.00
(d, J = 8.4 Hz, 12H, H9); 7.30 (d, J = 16.5 Hz, 6H, Hy); 7.66
(d, J = 9.1 Hz, 18H, H8, and H5,); 7.76 (d, J = 16.1 Hz, 6H, Hx); 8.02
(t, 6H, H6); 9.00 (s, 6H, H3).
2.6. General procedure for the synthesis of heteroleptic ruthenium
complexes
Yield: 86%. Anal. Calc. for C19H9N5: C, 74.27; H, 2.94; N, 22.79.
Found: C, 74.26; H, 2.94; N, 22.84%. IR (cmꢀ1): 3060
m
(CH aro-
150 mg (0.148 mmol) cis-Ru(L-OCH3)2Cl2, silver hexafluoro-
phosphate 75 mg (0.296 mmol) and 0.148 mmol of the appropriate
polypyridinic ligand were dissolved in 50 mL of dimethylformam-
ide (DMF). The resulting solution was then refluxed and stirred for
8 h under nitrogen atmosphere, during which the solution changed
to a deep red-wine color. After cooling, the reaction mixture was
matic); 2227 m(CBN); 1584 m
(C@N). 1H NMR (CDCl3, 400 MHz, d
ppm): 7.68–7.73 (m, 2H, Hc y Hc0); 7.95 (dd, J = 1.7 Hz/8.8 Hz,
1H, Ha0); 8.29 (d, J = 8.8 Hz, 1H, He); 8.57 (d, J = 1.3 Hz, 1H, Ha);
9.22–9.25 (m, 2H, Hd and Hd0); 9.35–9.42 (m, 2H, Hb and Hb0).
13C {1H} NMR (CDCl3, d ppm): 123.5 (C2); 123.6 (C15); 129.9 (C7);
H3CO
+2
H3CO
OCH
3
N
N
N
N
EtOH/H O
2
RuCl 3 * 3H2O
3 L-OCH3
+
Ru
Ref lux
NH PF
4
6
N
N
OCH
3
H3CO
H3CO
Scheme 1. Synthesis of Tris(4,40-bis(2-(4-methoxyphenyl)styryl)-2,20-bipyridine)ruthenium bis(hexafluorophosphate). The numbering of hydrogen atoms is shown in Fig. 1
in order to follow the assignment of the 1H NMR signals of the L-OCH3 ligand.

514
C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
filtered through a pad of Celite. The solvent of the deep red-wine
solution was removed by rotary evaporation and the crude product
was then dissolved in a minimal amount of acetone. A saturated
aqueous ammonium hexafluorophosphate solution was added
and the precipitate collected and washed with diethyl ether. The
crude product was filtered off and purified by chromatography in
aluminum oxide in the same way as described in Section 2.5.
2H, Hx0); 7.76 (d, J = 5.1 Hz, 2H, H5); 7.78 (d, J = 16.5 Hz, 2H, Hx);
0
7.97 (d, J = 5.8 Hz, 2H, H6 ); 8.15(d, J = 5.8 Hz, 2H, H6); 8.13 (dd,
J = 5.1 Hz/8.4 Hz, 2H, Hc, Hc0); 8.72 (d, J = 8.8 Hz, 2H, Hd, Hd0);
8.74 (d, J = 5.1 Hz, 1H, Ha); 8.82 (d, J = 9.5 Hz, 1H, Ha0); 9.03
0
(s, 2H, H3 ); 9.06 (s, 2H, H3); 9.28 (s, 1H, He); 9.78 (dd, J = 3.7 Hz/
7.7 Hz, 2H, Hb, Hb0).
2.6.5. [Ru(L-OCH3)2Aqphen](PF6)2 (6)
2.6.1. [Ru(L-OCH3)2ppl](PF6)2 (2)
Yield: 33%. Anal. Calc. for RuC82H60N8O6P2F12: C, 59.89; H, 3.68;
N, 6.81. Found: C, 58.80; H, 3.63; N, 6.79%. IR (cmꢀ1): 3071
m
(CH
(C@ N);
(CAOAC symmetric); 842
m
(PAF6). 1H NMR (acetone-d6, 400 MHz, d ppm): 3.84 (d,
Yield: 44%. Anal. Calc. for RuC70H56N8O4P2F12 (+1.0 acetone): C,
57.60; H, 4.10; N, 7.36. Found: C, 56.40; H, 4.05; N, 7.35%. IR
aromatic); 2837
1254 (CAOAC asymmetric); 1027
and 557
m(CH aliphatic); 1671 m(C@ O); 1596 m
(cmꢀ1): 3072
N); 1253
840 and 557
m(CH aromatic); 2837 m(CH aliphatic); 1596 m(C@
m
m
m
(CAOAC asymmetric); 1027
m
(CAOAC symmetric);
(PAF6). 1H NMR (acetone-d6, 400 MHz, d ppm):
0
m
J = 12.4 Hz, 12H, OCH3); 6.97 (d, J = 8.6 Hz, 4H, H9 ); 7.02 (d,
J = 8.5 Hz, 4H, H9); 7.24 (d, J = 16.7 Hz, 2H, Hy0); 7.32 (d,
0
3.85 (s, 12H, OCH3); 6.99 (d, J = 8.7 Hz, 4H, H9 ); 7.02 (d,
J = 8.7 Hz, 4H, H9); 7.24 (d, J = 16.1 Hz, 2H, Hy0); 7.33 (d,
J = 16.8 Hz, 2H, Hy); 7.53 (t, 2H, H5 ); 7.60 (d, J = 8.5 Hz, 4H, H8 );
0
0
7.66 (d, J = 8.2 Hz, 4H, H8); 7.74 (d, J = 15.7 Hz, 2H, Hx0); 7.76 (d,
0
J = 16.5 Hz, 2H, Hy); 7.47 (d, J = 5.5 Hz, 2H, H5 ); 7.62 (d,
0
J = 8.7 Hz, 4H, H8 ); 7.67 (d, J = 8.7 Hz, 4H, H8); 7.70 (d, J = 16.8 Hz,
J = 5.1 Hz, 2H, H5);7.79 (d, J = 15.4 Hz, 2H, Hx); 7.96 (d, J = 8.7 Hz,
2H, Hx0); 7.76 (d, J = 5.8 Hz, 2H, H5); 7.79 (d, J = 16.5 Hz, 2H, Hx);
7.89 (d, J = 6.2 Hz, 2H, H6 ); 8.13 (dd, J = 5.1 Hz/8.4 Hz, 2H, Hc);
8.15 (d, J = 6.2 Hz, 2H, H6); 8.71 (d, J = 5.1 Hz, 2H, Hd); 9.02 (s,
2H, H3 ); 9.06 (s, 2H, H3); 9.36 (s, 2H, Ha); 9.66 (d, J = 8.1 Hz, 2H,
1H, Hg); 7.99 (d, J = 9.6 Hz, 1H, Hg ); 8.02 (d, J = 5.9 Hz, 2H, H6 );
0
0
8.13 (t, 1H, Hc); 8.16 (d, J = 6.3 Hz, 2H, H6); 8.19 (t, 1H, Hc0); 8.27
(d, J = 7.5 Hz, 1H, Hf); 8.34 (d, J = 7.7 Hz, 1H, Hf0); 8.75 (d,
J = 5.4 Hz, 2H, Hd, Hd0); 8.74 (d, J = 5.1 Hz, 2H, Ha, Ha0); 9.03 (s,
0
0
0
Hb).
2H, H3 ); 9.06 (s, 2H, H3); 9.28 (s, 1H, He);9.72 (d, J = 8.0 Hz, 1H,
Hb); 9.79 (d, J = 7.7 Hz, 1H, Hb0).
2.6.2. [Ru(L-OCH3)2dppz](PF6)2 (3)
Yield: 23%. Anal. Calc. for RuC74H58N8O4P2F12 (+0.2 acetone): C,
58.72; H, 3.91; N, 7.34. Found: C, 58.86; H, 3.92; N, 7.35%. IR
3. Results and discussion
(cmꢀ1): 3031
(C@N); 1253
842 and 557
m(CH aromatic); 2837 m(CH aliphatic); 1596
3.1. Synthesis and characterization
m
m(CAOAC asymmetric); 1027 m(CAOAC symmetric);
m
(PAF6). 1H NMR (acetone-d6, 400 MHz, d ppm): 3.84
Each acceptor ligand (NN-A) was prepared according to the fol-
lowing literature procedures: Pyrazino[2,3-f][1,10]phenanthroline
(ppl) [24], Dipyrido[2,3-a:20,30-c]phenazine (dppz) [25], 11-nitrodi-
pyrido[2,3-a:20,30-c]phenazine (dppz-NO2) [26] and 10,11-[1,4-
naphtalenedione]dipyrido[3,2-a:2’,3’-c]phenazine (Aqphen) [27].
Transition metal complexes cis-[Ru(L-OCH3)2(NN-A)](PF6)2: In
general, all the complexes described in this work were prepared
by reaction of the appropriate polypyridine ligand with the corre-
sponding metal precursor cis-[Ru(L-OCH3)2Cl2] as depicted in
Scheme 2.
Characterization of the new products was achieved by means of
IR, 1H and COSY 1H–1H NMR spectroscopy and elemental analysis.
The solids of the new complexes were recrystallized from
Me2CO/Et2O (1:1) and obtained as spectroscopically pure, air and
thermally stable, red to dark-red microcrystalline solids in moder-
ate yields (23%–51%).
0
(d, 12H, OCH3); 6.97 (d, J = 8.8 Hz, 4H, H9 ); 7.00 (d, J = 8.6 Hz, 4H,
H9); 7.23 (d, J = 16.3 Hz, 2H, Hy0); 7.32 (d, J = 16.0 Hz, 2H, Hy);
0
0
7.51 (d, J = 5.6 Hz, 2H, H5 ); 7.61 (d, J = 8.8 Hz, 4H, H8 ); 7.65 (d,
J = 7.7 Hz, 4H, H8); 7.69 (d, J = 5.6 Hz, 2H, H5); 7.73 (d, J = 16.0 Hz,
0
0
2H, Hx ); 7.81 (d, J = 16.0 Hz, 2H, Hx); 7.97 (d, J = 5.9 Hz, 2H, H6 );
8.02 (d, J = 6.0 Hz, 2H, H6); 8.13 (t, 2H, Hc); 8.18 (d, J = 9.5 Hz, 2H,
He); 8.46 (d, J = 9.6 Hz, 2H, Ha); 8.67 (d, J = 5.0 Hz, 2H, Hd); 9.06
0
(s, 2H, H3 ); 9.09 (s, 2H, H3); 9.74 (d, J = 7.9 Hz, 2H, Hb).
2.6.3. [Ru(L-OCH3)2dppz-CN](PF6)2 (4)
Yield: 51%. Anal. Calc. for RuC75H57N9O4P2F12 (+0.6 acetone): C,
58.60; H, 3.88; N, 8.01. Found: C, 57.74; H, 3.83; N, 7.98%. IR
(cmꢀ1): 3072
m
m
(CH aromatic); 2837
(C@ N); 1253 (CAOAC asymmetric); 1027
(CAOAC symmetric); 842 and 557
(PAF6). 1H NMR (acetone-
m(CH aliphatic); 2229
m(CBN); 1596
m
m
m
d6, 400 MHz, d ppm): 3.84 (d, 12H, OCH3); 6.67 (d, J = 9.0 Hz, 4H,
The most remarkable common features observed in the IR spec-
tra of these new compounds (1–6) were: (i) the existence of a
sharp intense band at ca. 1249–1253 cmꢀ1, due to the asymmetric
stretching vibration of the CAOAC group of the 4,40-bis(2-(4-
methoxyphenyl)styryl)-2,20-bipyridine ligand, and a sharp med-
ium intensity band at 1027 cmꢀ1 due to the symmetric vibration
0
0
H9 ); 7.01 (d, J = 8.9 Hz, 4H, H9); 7.24 (d, J = 16.5 Hz, 2H, Hy );
0
7.33 (d, J = 16.2 Hz, 2H, Hy); 7.51 (d, J = 6.0 Hz, 2H, H5 ); 7.61 (d,
0
J = 8.7 Hz, 4H, H8 ); 7.66 (d, J = 8.7 Hz, 4H, H8); 7.70 (d, J = 16.5 Hz,
2H, Hx0); 7.75 (d, J = 10.2 Hz, 2H, H5); 7.78 (d, J = 16.4 Hz, 2H,
0
Hx); 7.97 (d, J = 6.0 Hz, 2H, H6 ); 8.03 (d, J = 6.0 Hz, 1H, Ha); 8.15
(d, J = 5.9 Hz, 2H, H6); 8.17 (dd, J = 2.5 Hz/6.0 Hz, 2H, Hc, Hc0);
of the CAOAC group; (ii) a very strong
m(PF6) band at ca. 840–
8.37 (dd, J = 1.7 Hz/8.8 Hz, 1H, Ha0); 8.66 (d, J = 8.9 Hz, 1H, He);
842 cmꢀ1 and a sharp and strong d(PAF) band at 557 cmꢀ1. Also,
0
0
8.73 (d, J = 5.5 Hz, 2H, Hd, Hd ); 9.02 (s, 2H, H3 ); 9.04 (s, 2H, H3);
the IR spectra of compound
5 exhibited a strong band at
9.76 (t, 2H, Hb, Hb0).
1511 cmꢀ1 and a sharp medium intensity band at 1347 cmꢀ1
attributed to the NO2 group, whereas for complex 4 the CBN
stretching vibration appeared at 2227 cmꢀ1 in the IR spectrum.
Finally, compound 6 showed a sharp medium intensity band at
2.6.4. [Ru(L-OCH3)2dppz-NO2](PF6)2 (5)
Yield: 40%. Anal. Calc. for RuC74H57N9O6P2F12 (+0.5 acetone): C,
57.09; H, 3.81; N, 7.94. Found: C, 56.47; H, 3.77; N, 7.93%. IR
1671 cmꢀ1 attributed to the
m(C@O) group, which is compatible
(cmꢀ1): 3072
N); 1511 and 1347
(CAOAC symmetric); 842 and 557
m
(CH aromatic); 2837
m
(CH aliphatic); 1596
(CAOAC asymmetric); 1027
(PAF6). 1H NMR (acetone-
m(C@
with the presence of the naphthalenedione fragment.
m(NO2); 1253
m
m
m
d6, 400 MHz, d ppm): 3.84 (s, 12H, OCH3); 6.97 (d, J = 8.4 Hz, 4H,
0
0
H9 ); 7.01 (d, J = 8.4 Hz, 4H, H9); 7.24 (d, J = 16.5 Hz, 2H, Hy );
0
7.32 (d, J = 16.1 Hz, 2H, Hy); 7.51 (d, J = 5.5 Hz, 2H, H5 ); 7.60 (d,
Scheme 2. Synthesis of heteroleptic ruthenium complexes. NN-A = ppl, dppz-R; R:
0
J = 8.4 Hz, 4H, H8 ); 7.66 (d, J = 8.7 Hz, 4H, H8); 7.74 (d, J = 15.7 Hz,
H, NO2, CN, and Aqphen.

C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
515
Table 1a
1H NMR results for the free L-OCH3 ligand, and for complexes (1–6).
0
0
0
0
0
0
0
Complex
OCH3
H3
H3
H5
H5
H6
H6
H8
H8
H9
H9
Hx
Hx
Hy
Hy
L-OCH3
3.85
3.84
3.85
3.84
3.84
3.84
3.84
8.52
9.00
9.06
9.09
9.04
9.06
9.06
7.37
7.66
7.76
7.69
7.75
7.76
7.76
8.65
8.02
8.15
8.02
8.15
8.15
8.16
7.52
7.66
7.67
7.65
7.66
7.66
7.66
9.94
7.00
7.02
7.00
7.01
7.01
7.02
7.00
7.76
7.79
7.81
7.78
7.78
7.79
7.42
7.30
7.33
7.32
7.33
7.32
7.32
1
2
3
4
5
6
9.02
9.06
9.02
9.03
9.03
7.47
7.51
7.51
7.51
7.53
7.89
7.97
7.97
7.97
8.02
7.62
7.61
7.61
7.60
7.60
6.99
6.97
6.67
6.97
6.97
7.70
7.73
7.70
7.74
7.74
7.24
7.23
7.24
7.24
7.24
Table 1b
1H NMR results for complexes (2–6) NN-A ligand.
0
0
0
0
0
0
Complex
Ha
Ha
Hb
Hb
Hc
Hc
Hd
Hd
He
Hf
Hf
Hg
Hg
2
3
4
5
6
9.36
8.46
8.03
8.74
8.74
9.66
9.74
9.76
9.78
9.72
8.13
8.13
8.17
8.13
8.13
8.71
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
8.67
8.73
8.72
8.75
8.18
8.66
9.28
9.28
8.37
8.82
9.79
8.19
8.27
8.34
7.96
7.99
H3CO
+2
H9
H9
Hc
Hd
Hb
H8
Hx
H8
H9´
N
N
N
N
Ha
H8´
H3CO
H9´
Hy
H3
H5
H6
Hy´
H3
ppl (2)
'
H8
´
N
Hc
Hx´
Hd
Hb
N
N
Ha
N
N
H5'
N
N
N
He
Ru
A
H6'
N
R
Ha´
Hd´
Hb´
dppz (3-5)
R=H, CN, NO2
N
Hc´
Hg´
Hf ´
Hg
Hf
Hc
H3CO
Hd
Hb
O
N
N
N
N
O
He
Ha
Hd´
Hb´
H3CO
Aqphen (6)
Hc´
Fig. 3. Proton numbering for the 1H NMR spectra of the complexes.
The 1H NMR spectra are in agreement with the proposed struc-
tures of the complexes. These spectra are included as S1–S8 in
Supplementary material. Tables 1a and 1b and Fig. 3 show the
results and the assignments of the 1H NMR signals. In comparison
appears at lower field (9.66–9.79 ppm). The shift in this signal,
when compared with the corresponding free ligand, is fundamen-
tally due to the anisotropic effect of the nitrogen atom in the pyr-
azine fragment. It can also be seen that the change in the signal
position, although smooth, tends to vary with the acceptor
strength of the ligand. As its strength increases, the electronic den-
sity around the proton of the phenanthroline fragment decreases
and the signal shifts to lower fields.
0
with the free ligands, the H6, H6 protons as well as the Hd proton in
ortho position with respect to the phenanthrolinic nitrogen atom
in the NN-A ligand, appear at low field (around 8.72 ppm) as a
consequence of their coordination to the metal.
Otherwise, the signal of the Hb proton of the NN-A ligand, the
proton in para position to the pyrazine N of the acceptor ligand,
With regard to the signals of the L-OCH3 ligands, the expected
and characteristic pattern can be observed: the doublet of the Hx

516
C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
H6
H6’
Hb
Hd
H5’
H5
Hc
Fig. 4. COSY 1H–1H NMR spectrum of the [Ru(L-OCH3)2(ppl)](PF6)2 complex.
Table 2
1,2
[Ru(L-OCH3)2ppl]2+
Oxidation potentials for complexes 1–6.
[Ru(L-OCH3)2dppz-NO2]2+
[Ru(L-OCH3)2Aqphen]2+
[Ru(L-OCH3)2dppz]2+
[Ru(L-OCH3)2dppz-CN]2+
[Ru(L-OCH3)3]2+
Complex
Oxidation
1,0
0,8
0,6
0,4
0,2
0,0
E1/2 Ru(II)/Ru(III) (V)
DE (mV)
[Ru(L-OCH3)3](PF6)2(1)
1.06
1.10
1.13
1.16
1.12
1.14
116
162
98
153
94
[Ru(L-OCH3)2ppl](PF6)2(2)
[Ru(L-OCH3)2dppz](PF6)2(3)
[Ru(L-OCH3)2dppz-CN](PF6)2(4)
[Ru(L-OCH3)2dppz-NO2](PF6)2(5)
[Ru(L-OCH3)2Aqphen](PF6)2(6)
118
and Hy protons (7.23–7.81 ppm) is shifted by the presence of the
electron density acceptor groups, and the signal of the donor group
OCH3 at around 3.84 ppm.
Finally, 2D-NMR spectra were registered for the new complexes
in order to corroborate the assignments given above. Fig. 4 shows
the spectrum for the [Ru(L-OCH3)2(ppl)](PF6)2 complex. The corre-
lations between some protons of the L-OCH3 ligand, and of the NN-
A ligand, are shown.
200
300
400
500
600
wavelength [nm]
Fig. 5. Absorption spectra of complexes 1–6 in acetonitrile.
withdrawing groups in dppz has little effect on the RuIII/II couple.
This is because of the long distance from these groups to the metal
center and the ‘‘blocking’’ effect of the pyrazine ring [26]. Never-
theless, an experimental tendency was observed for the acceptor
character of the ligands: ppl < NO2dppz < dppz < Aqphen <
CNdppz.
3.2. Cyclic voltammetry
Electrochemical data for complexes 1–6 were measured in ace-
tonitrile solution; the data are compiled in Table 2. In the anodic
region, the complexes exhibited quasi-reversible one-electron
redox waves due to oxidation of the ruthenium metal. For all com-
plexes the oxidation potential assigned to the RuIII/II couple was
found at higher values than for the homoleptic complex as a con-
sequence of increased back-bonding to the lower-lying p⁄ orbitals
of the electron-acceptor ligands. The introduction of electron-
3.3. UV–Vis absorption spectra
The electronic absorption spectra of the Ru(II) complexes were
measured in acetonitrile at room temperature, Table 3. In the UV
Table 3
Electronic absorption data for complexes 1–6.
Complex
MLCT#
ILCT
p–
p⁄
IL p–
p⁄/n–p⁄
[Ru(L-OCH3)3](PF6)2 (1)
488 (5.24)
479 (4.51)
475 (4.34)
467 (3.89)
470 (4.50)
472 (5.19)
359 (12.62)
368 (9.19)
369 (10.72)
367 (9.06)
365 (11.00)
372 (12.04)
296 (8.72)
299 (6.94)
281 (7.92)
277 (10.59) sh 296
298 (9.90)
[Ru(L-OCH3)2ppl](PF6)2 (2)
[Ru(L-OCH3)2dppz](PF6)2 (3)
[Ru(L-OCH3)2dppz-CN](PF6)2 (4)
[Ru(L-OCH3)2dppz-NO2](PF6)2 (5)
[Ru(L-OCH3)2Aqphen](PF6)2 (6)
294 (10.89)
#
In CH3CN at room temperature (e
104 Mꢀ1 cmꢀ1).

C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
517
region, three intense and sharp bands were observed correspond-
ing to intra-ligand transitions (IL). The high-intensity absorption
observed with a relatively low energy cost; in consequence a
NLO response would be predicted. The magnitude of the electronic
density redistribution, on passing from ground to excited state,
seems to be influenced by the acceptor substituent in the R-dppz
fragment in complexes. It is known that both, nitro and cyano
groups have high electron acceptor character. Looking at the elec-
trochemical results, the tendency indicates that in the ground state
the cyano moiety is only slightly more acceptor than nitro, Never-
theless the solvent effect is clearly different when comparing the
more polar solvent, ACN, and the non-polar solvent, benzene,
pointing to nitro as a better acceptor.
In 2010, Le Bozec and co-workers [8] published theoretical
results for beta values for M(II) complexes with 4,40-bis(X-styryl)-
2,20bipyridine as donor ligand. When X is a nitro or cyano group,
the visible band is assigned to ILCT and LLCT transitions, the
kmaximun of the cyano substituted complex appearing at a slightly
higher energy than the equivalent nitro complex. Theoretical calcu-
lations of the ground state dipolar moment give similar values for
both, but when calculating the bSHG parameter, a high value for
X = nitro is observed, bSHG (nitro) > bSHG (cyano), independent of
the elected functional used for the calculations. The results reported
in the present work seem to be in agreement with Le Bozec results
since complexes with different acceptor substituent groups have a
different solvent effect. In our complexes, 4,40-bis(X-styryl)-
2,20bipyridine is the donor moiety, with X = AOCH3, while dppz-Y
(with Y = cyano, nitro) is the acceptor one. As previously mentioned,
Table 4, the MLCT band (visible region) was assigned as a MLCT to
4,40-bis(X-styryl)-2,20bipyridine. Taken into account that the visible
band should also have a contribution from the MLCT to dppz-Y
transition, and since both associated dipolar moments are vectori-
ally in opposite directions, the NLO response should be related to
a btotal = |b (MLCT donor)| ꢀ |b (MLCT dppz-Y)|. Considering a series
of complexes, with the same donor ligand i (as is the case here
reported), the btotal value should decrease as the acceptor character
of the Y substituent increases, giving therefore a lesser solvent
effect. According to this, for the complexes studied in this work,
the results indicate that nitro (with a lesser solvent effect) is a bet-
ter acceptor group in comparison with the cyano moiety.
band around 350 nm can be ascribed mainly to a p–
p⁄ IL transition,
although some MLCT contribution should be considered [28–30].
In the visible range, complexes 1–6 exhibited very intense,
broad Metal to Ligand Charge Transfer (MLCT) absorption bands,
as is usually observed for related complexes [29] (see Fig. 5). In this
case the same ILCT contribution may be present [26]. The lowest
energy absorption maximum (488 nm,
e
= 5.24 ⁄ 104 Mꢀ1 cmꢀ1
)
was obtained for the homoleptic complex 1, which contains only
L-OCH3 ligands, and that therefore can be taken as a ‘‘blank’’ or
‘‘reference’’. Considering (i) the position of the visible absorption
band in the homoleptic complex 1; (ii) that the presence of an
acceptor group in the NN-A ligand shifts those bands to higher
energy; and (iii) the intraligand bands are at higher energies, the
MLCT is assigned as metal to the chromophoric ligand charge
transfer, although overlapping MCLT transitions to L-OCH3 and
NN-A are most likely the reason for the broadening. The blue-shift
in the MLCT bands is due to decreased electron density at Ru due to
increased back-bonding to the acceptor ligands and to weak
electronic coupling between the ruthenium and the low-lying p⁄
orbitals in the acceptor ligands.
3.4. Solvatochromic studies
Electronic absorption spectra for the new complexes were mea-
sured in different solvents, progressing from less polar to more
polar: benzene, acetone, and acetonitrile; the values of the corre-
sponding absorption maxima are recorded in Table 4. The UV–Vis
spectra of solutions of 2–6 exhibit MLCT absorption in the
20000–25000 cmꢀ1 range. For acetone, and acetonitrile, the effect
of solvent polarity in the absorption maxima is small, although
these maxima are ill-defined due to the broad nature of these
bands. A similar behavior was observed in dichloromethane. The
slight solvent dependence when comparing polar solvents can also
be ascribed to overlapping MCLT transitions to L-OCH3 and NN-A.
In this context, the solvent discussion will be centered in compar-
ing the more polar solvent, ACN, and the non-polar solvent, ben-
zene. The 400–500 nm band shows negative solvatochromism
[16], indicated by an hypsochromic shift of the absorption band
when going from a less polar, benzene, to a more polar solvent,
acetonitrile. This phenomenon is less marked for complex (5), that
undergoes a small hypsochromic shift of 785 cmꢀ1, while for com-
plexes (2, 3, 4 and 6) a noticeable blue shift in the MLCT band
A briefly consideration to the possibility of the presence of an
ILCT in the chromophoric donor ligand must be done. In Le Bozec
and co-workers paper, some complexes have Zn as metal center.
In these complexes, no MLCT is possible. Associated to the ILCT
band a rather high value of b0 was measured [8]. Therefore it is
not possible to discard completely a contribution of this transition
to the visible absorption band in the complexes here reported.
If this would be the case, ILCT should reinforce the effect of the
MLCT to dppz ligand. The solvent effect for the complexes with
cyano and nitro substituents is different but, according to the dis-
cussion above, to define which is the variable responsible of such
difference is not simple, when there are contribution of transitions
with different vectorial directions.
(D
k = 2138–2894 cmꢀ1) was noted.
The hypsochromic character of the solvent effect shows that the
ground state is more polar and more stabilized in polar solvents
than the MLCT excited state. This is in agreement with the assign-
ment of the visible absorption band and electrochemical results. As
seen from the first oxidation process, the HOMO orbital is centred
on the metal, with a small change in its energy when the polypy-
ridinic LL-A ligand changes.
Finally, considering the two state model expressions for beta, it
can be seen that the variables that generate good values for the
NLO parameter are, among others, high molar absorptivity and a
large change in dipole moment between the ground and excited
Looking in particular to the solvent effect, a redistribution of the
electronic density among the metal and the donor ligand is
states (Dl). For the complexes reported here, the molar absorptiv-
ity is high, while
Dl is moderate. Therefore, a NLO response should
Table 4
Electronic absorption data for complexes 1–6 in different solvents.
be expected.
Complexes
MLCT ACN MLCT Acetone MLCT Benzene
cmꢀ1
cmꢀ1
cmꢀ1
[Ru(L-OCH3)3](PF6)2 (1)
[Ru(L-OCH3)2ppl](PF6)2 (2)
[Ru(L-OCH3)2dppz](PF6)2 (3)
[Ru(L-OCH3)2dppz-CN](PF6)2 (4) 21413
[Ru(L-OCH3)2dppz-NO2](PF6)2 (5) 21277
[Ru(L-OCH3)2Aqphen](PF6)2 (6)
20492
20877
21053
20492
21053
20833
21053
21186
21186
–
Acknowledgment
18762
18519
18519
20492
19048
The authors thank Fondecyt Chile (Project Nos. 1070799 and
1110991), and Dr. Hubert Le Bozec and Javier Concepción for
helpful discussions.
21186

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C. Zúñiga et al. / Polyhedron 85 (2015) 511–518
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Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.poly.2014.09.004.
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