Synthesis, crystal structure, and second-order nonlinear optical properties of ruthenium(II) complexes with substituted bipyridine and phenylpyridine ligands

Article abstract of DOI:10.1002/ejic.200600258

Two new ruthenium(II) complexes of formula [Ru(bpy)₂-(L ¹)][PF₆] and [Ru(bpy)₂(L²)][PF ₆]₂ are reported. HL¹ is a (nitrophenyl) ethenyl-substituted phenylpyridine liga

Full text of DOI:10.1002/ejic.200600258

FULL PAPER
DOI: 10.1002/ejic.200600258
Synthesis, Crystal Structure, and Second-Order Nonlinear Optical Properties
of Ruthenium(II) Complexes with Substituted Bipyridine and Phenylpyridine
Ligands
Laurence Labat,[a] Jean-François Lamère,[a] Isabelle Sasaki,[a] Pascal G. Lacroix,*[a]
Laure Vendier,[a] Inge Asselberghs,[b] Javier Pérez-Moreno,[b] and Koen Clays[b]
Keywords: Ligand design / Donor–acceptor systems / Ruthenium / Nonlinear optics / Density functional calculations
Two new ruthenium(II) complexes of formula [Ru(bpy)2-
(L1)][PF6] and [Ru(bpy)2(L2)][PF6]2 are reported. HL1 is a (ni-
trophenyl)ethenyl-substituted phenylpyridine ligand, and L2
is the bipyridine analogue of HL1. The X-ray crystal structure
of [Ru(bpy)2(L1)][PF6] has been solved, and the compound is
found to crystallize in the monoclinic C2/c space group. The
electronic spectrum of the cyclometalated derivative
[Ru(bpy)2(L1)][PF6] exhibits a low-lying transition that is red-
shifted from 454 to 546 nm relative to that of the parent bi-
pyridine-based complex, which reveals an important charge-
transfer character. To support this assumption, the nonlinear
optical properties were investigated by the hyper-Rayleigh
scattering technique and indicate a molecular static hyperpo-
larizability (β0) equal to 230×10–30 cm5 esu–1
.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2006)
ing octupolar topologies,[9,10] and the observation of switch-
able NLO responses upon metal oxidation in some cases,[11]
a possibility which could attract great interest in relation
with the concept of molecular switches.[12,13]
Introduction
Molecules with π-conjugated electronic structures have
been attracting considerable interest due to their potential
nonlinear optical (NLO) properties and applications in op-
toelectronic and photonic devices.[1,2] While the first investi-
gations were initially focused on purely organic systems,[3]
the search for alternative organometallic and inorganic de-
rivatives has gradually intensified in the last fifteen
years.[4–6] Incorporation of metal atoms into π-conjugated
systems introduces many new variables, such as diversity of
oxidation states, intensely colored charge-transfers, ad-
ditional magnetic capabilities, and a diversity of new top-
ologies, which leads to NLO chromophores of much greater
complexity than that of the first generation of “push-pull”
organic molecules.[6]
For instance, ruthenium complexes have provided intri-
guing molecular materials with unusual electronic capabili-
ties since the 1960s, in particular as mixed-valence RuIIRuIII
complexes.[7] In (pyridine)ruthenium() complexes, for ex-
ample, the metal center is engaged in π-bonding with the
organic ligating groups and can be involved in strongly al-
lowed metal-to-ligand charge-transfer (MLCT), and hence
provides various candidates for NLO applications. Indeed,
ruthenium has led to the most investigated family of (pyri-
dine)metal complexes in nonlinear optics.[8] The reasons for
this interest are the possibility of designing various promis-
In the last few years several cyclometalated counterparts
of (polypyridine)ruthenium complexes have been prepared
and their physical properties reported.[14–17] In fact, cyclo-
metalated complexes have found widespread interest as spe-
cies with promising properties in various research fields[15c]
owing to the strong σ-donor ability of the cyclometalating
ligand. Basically, replacement of a nitrogen donor by a for-
mal carbanion donor drastically increases the electron
density around the metal atom and the crystal-field
strength.[14,15a] As a consequence, the dǞπ* back-donation
is reinforced, which leads to a potential enhancement of the
overall intramolecular charge transfer, a property which is
highly desirable for the design of NLO chromophores. The
enhancement of the donating capabilities of the rutheni-
um() center has been evidenced experimentally by the ob-
servation of intense charge-transfer properties in RuII/RuIII
complexes with bis(cyclometalating) ligands,[15b,18] and by a
shift of the value of the RuII/RuIII oxidation potential of
about 900 mV upon replacement of one nitrogen atom by
one carbon atom in tris(bipyridine)ruthenium()-like com-
plexes.[14]
Recently, we have developed a new synthetic strategy
aimed at providing an access to numerous phenylpyridine
ligands substituted in para (R1) and meta (R2) positions
with respect to the nitrogen atom (Scheme 1).[19] In the per-
spective of nonlinear optics, the presence of an acceptor (A)
in the meta position is the most desirable to favor an intense
[a] Laboratoire de Chimie de Coordination du CNRS,
205 route de Narbonne, 31077 Toulouse, France
[b] Department of Chemistry, University of Leuven,
Celestijnenlaan 200D, 3001 Leuven, Belgium
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P. G. Lacroix, et al.
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charge-transfer process from electron-rich metal–carbon
moieties, as indicated in this scheme, and finally to provide
a sizeable NLO response.
Results and Discussion
Synthesis and Characterization
Two methods for the synthesis of HL1 and L2, which
contain the electron-withdrawing substituent NO2, were en-
visioned: by condensing formylbipyridine or formyl-
(phenyl)pyridine with the appropriate phosphonate
(method A) or by condensing the phosphonate 3a,b derived
from the bipyridine or the phenylpyridine with p-nitrobenz-
aldehyde (method B). Method B was chosen as the synthe-
sis of 5-(bromomethyl)-2,2Ј-bipyridine (2b) from 1b[20] has
already been described[21] and was extended here to 5-
methyl-2-phenylpyridine (1a). The first step involves the
conversion of the methyl group to the corresponding bro-
momethyl group by treatment with N-bromosuccinimide
(NBS) and azoisobutyronitrile (AIBN). The bromomethyl
Scheme 1.
Following this idea, we report here our attempt to design derivatives 2a,b were then converted into the phosphonate
a set of two (phenylpyridine and bipyridine)ruthenium derivatives 3a,b.[22] The final step was the Wadsworth–Em-
complexes with potential MLCT capabilities. In the first mons reaction of compounds 3a,b with the commercially
section, their synthesis and characterizations will be pre- available p-nitrobenzaldehyde, which leads to the styryl de-
sented. Then, the crystal structure of the cyclometalated rivatives HL1 and L2 in 29% and 37% yields, respec-
complex will be discussed. Finally, the optical and nonlin- tively.[22] The double-bond linkages were confirmed to be
ear optical properties of both derivatives will be compared (E) by 1H NMR analysis based on the value of the JCH=CH
in relation to the enhanced “push-pull” character induced coupling constant (ca. 16 Hz).
by the Ru–C linkage. The investigated molecules of formula
The syntheses of the two complexes are not similar. The
[Ru(bpy)2(L1)][PF6] and [Ru(bpy)2(L2)][PF6]2 are shown in tris(bipyridine) complex was obtained by the classical
Scheme 2.
method, i.e., heating of 1 equiv. of [Ru(bpy)2]Cl2 with
Scheme 2.
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Ruthenium() Complexes with Substituted Bipyridine and Phenylpyridine Ligands
FULL PAPER
1 equiv. of L2 in ethanol, whereas for the cyclometalated
complex 1 equiv. of [Ru(bpy)2]Cl2 was treated with 5 equiv.
of the phenylpyridine HL1 in the presence of silver trifl-
ate.[19] The complexes were characterized by the usual meth-
ods. The labelling of the atoms is the one used for the X-
ray structure. Characteristic peaks of cyclometalated com-
pounds were observed in the NMR spectra of [Ru(bpy)2-
(L1)][PF6] and the peaks of the spectra were fully
assigned.[14,23,24] Nevertheless, for complex [Ru(bpy)2-
(L2)][PF6]2, the unambiguous assignment of all the signals
was not possible: since the electronic environments of nu-
merous pyridine hydrogen atoms are nearly similar, their
signals occur in a very narrow chemical-shift range.
Structural Data
The crystallographic data are summarized in the Experi-
mental Section, and the X-ray molecular structure of
[Ru(bpy)2(L1)][PF6] is presented in Figures 1 and 2. The
compound crystallizes in the monoclinic C2/c space group
(Z = 8). The asymmetric unit cell (Figure 1) is built up from
Figure 2. View of the crystal cell of [Ru(bpy)2(L1)][PF6] in the ac
plane.
one ruthenium complex, one PF6– anion, and one molecule [Ru(bpy)2(nitrophenpy)]+ structure reported in the litera-
of diethyl ether. In the crystal, [Ru(bpy)2(L1)][PF6] is orga- ture.[16] Another, more surprising, structural feature is the
nized in extended layers of complexes in the bc plane (Fig- angle of 37.0(3)° observed between the phenyl group carry-
ure 2). The π-conjugated skeleton of the substituted phenyl- ing the nitro substituent and the pyridine ring of the phen-
pyridine ligands lies parallel to the layers, the direction of ylpyridine ligand. This rotation suggests that the ni-
highest polarizability (phenyl Ǟ NO2) being roughly [001]. trophenyl group is not significantly involved in the charge-
The ruthenium atom lies in a distorted octahedral envi- transfer process in the solid state. In order to verify whether
ronment. Metal–ligand bond lengths are presented in the torsion arises from crystal packing or from intrinsic
Table 1. Interestingly, and in contrast with previously re- electronic effects, the gas-phase geometries of [Ru(bpy)2-
ported X-ray data,[16] the Ru–C(1) distance of 2.033(4) Å is (L1)][PF6] and [Ru(bpy)2(L2)][PF6]2 were investigated within
not significantly shortened with respect to the average Ru– the framework of density functional theory (DFT).
N distance 2.036(4) Å observed for the bipyridine N(3),
The calculated coordination spheres are compared in
N(4) atoms. Nevertheless, a tendency for increased ruthe- Table 1. The tendency for slightly increases bond lengths
nium–nitrogen bond lengths in the trans position with re- obtained by DFT is observed here, as is the case in model
spect to the Ru–C bond is unambiguously seen [Ru–N(6) = Ru(phenpy) complexes (see Experimental Section). In the
2.133(4) Å]. Along this line, the increase of this latter bond case of the bipyridine-based derivative, the six bond lengths
length is indeed the most noticeable geometrical change in- are very similar, with an average value of 2.1175 Å. By con-
troduced in the coordination sphere of the metal atom by trast, the coordination sphere of the cyclometalated deriva-
the presence of a carbon atom, in agreement with the tive exhibits the usual structural features of [Ru(bpy)2-
Figure 1. Asymmetric unit cell of [Ru(bpy)2(L1)][PF6]. Hydrogen atoms are omitted for clarity.
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P. G. Lacroix, et al.
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Table 1. Ruthenium–ligand bond lengths [Å] in [Ru(bpy)2(L1)][PF6] recorded in solvents of different polarities are gathered in
and [Ru(bpy)2(L2)][PF6]2.
Table 2 and presented in Figure 4, where the energy transi-
N [25]
[Ru(bpy)2(L1)][PF6]
[Ru(bpy)2(L2)][PF6]2
tions are drawn vs. the Reichardt solvent parameter ET
.
X-ray
DFT
X-ray
DFT
A first comparison conducted on the two transitions of the
cyclometalated complex, located at 370 and 546 nm, con-
firms that the low-lying transition exhibits the largest solva-
tochromic shift (slope equal to 499 in Figure 4b, vs. 414 in
Figure 4a). This is consistent with the anticipation of an
intense charge-transfer effect associated with the low-lying
transition of these ruthenium complexes. In a second com-
parison, the low-energy transitions are compared for
[Ru(bpy)2(L1)][PF6] (Figure 4b) and the bipyridine-based
[Ru(bpy)2(L2)][PF6]2 (Figure 4c). A reduction of the slope
to 369 in Figure 4c clearly shows that the solvatochromic
shift (and hence the charge transfer properties) are limited
in [Ru(bpy)2(L2)][PF6]2. All together, these experimental
data confirm the initial intuition that cyclometalated deriv-
atives are promising candidates for the design of molecules
with enhanced charge-transfer capabilities.
Ru–C(1)
Ru–N(1)
Ru–N(3)
Ru–N(4)
Ru–N(5)
Ru–N(6)
2.033(4)
2.072(3)
2.033(3)
2.039(4)
2.068(4)
2.133(4)
2.049
2.131
2.088
2.096
2.124
2.233
n.a.
n.a.
n.a.
n.a.
n.a.
n.a.
2.117
2.120
2.118
2.116
2.116
2.118
(phenpy)]+ complexes, namely a shortening of the Ru–C
bond (2.049 Å) and elongation of the Ru–N bond (2.233 Å)
trans to the Ru–C bond. Importantly, the angles between
the nitrophenyl and pyridine rings are 4.4° and 3.1° for
[Ru(bpy)2(L1)]+ and [Ru(bpy)2(L2)]2+, respectively. This re-
sult suggests that, to a large extent, the torsion angle of 37°
observed in the crystal structure arises from a solid-state
effect and is not an intrinsic electronic behavior of the chro-
mophores.
Table 2.Absorption maxima (λmax [nm]) of the lowest-energy op-
tical transitions for [Ru(bpy)2(L1)][PF6] and [Ru(bpy)2(L2)][PF6]2 in
solvents of different polarities.
Optical Properties
N[a]
Solvent
ET
[Ru(bpy)2(L1)][PF6] [Ru(bpy)2(L2)][PF6]2
The UV/Vis spectra of [Ru(bpy)2(L1)][PF6] and [Ru- MeOH
0.765
0.654
0.617
0.552
0.472
0.321
0.259
369
370.5
370.5
370
372
371.5
372.5
547.0
549.0
550.0
548.5
550.0
553.5
555.5
361.5
362.0
362.0
362.5
365.0
362.0
365.5
453.0
453.5
453.5
453.0
454.0
456.0
456.5
(bpy)2(L2)][PF6]2 are shown in Figure 3. At first glance,
EtOH
1-PrOH
both spectra are grossly similar and reveal three intense
2-PrOH
transitions located at 296 (65150), 370 (51850), and 546
MeCN
(14500 mol–1 Lcm–1) nm for the cyclometalated complex,
CH2Cl
CHCl3
and at 288 (64850), 365 (44900), and 454
(12250 mol–1 Lcm–1) nm for the parent bipyridine-based de-
rivative. It is interesting to note that the only significant
difference in both spectra is a redshift of 92 nm (3711 cm–1)
observed for the low-lying transition depending on the pres-
ence of an Ru–N or Ru–C linkage. This considerable shift
suggests that the low-energy transition is the main signature
of the charge transfer arising between the ruthenium atom
and the substituted bipyridine or phenylpyridine ligand.
[a] Reichardt parameter.
A theoretical support is provided from the calculated
electronic spectra. Functionals specifically designed for TD-
DFT calculations of electronic spectra (LB94, SOAP,
GRAC) are not implemented in our version of Gaussian,
therefore the spectra were calculated with ZINDO on the
basis of the gas-phase DFT structures. The data are gath-
ered in Table 3, where they are compared to the experimen-
tal values. There is an important blueshift in the computed
wavelengths for both complexes. However, this seems to be
a general tendency frequently observed in the calculated
spectra of inorganic chromophores.[26] The fact that the
magnitude of this shift is especially pronounced in the case
of the low-energy transition of [Ru(bpy)2(L1)][PF6], which
is assumed to arise from an intense metal–phenyl charge
transfer, may be additionally related to an enhancement of
the ruthenium–ligand distance within the DFT computa-
tion. Apart from these energy differences, a qualitative
agreement between experiment and calculation is observed
on two important points: (i) both complexes exhibit a very
intense high-energy transition and a less intense one located
Figure 3. UV/Vis spectra of [Ru(bpy)2(L1)][PF6] (1) and [Ru(bpy)2-
(L2)][PF6]2 (2) recorded in acetonitrile.
In order to further analyze the nature of the electronic at lower energy; (ii) while the high-energy transitions are
transitions of ruthenium complexes, the solvatochromic grossly located at the same energy range in both derivatives,
properties have been investigated. Solvatochromism is usu- the low-lying transition is considerably red-shifted in the
ally associated with a large dipole moment change upon case of the cyclometalated compound.
electronic excitation, and is therefore an interesting probe of
To further understand the nature of the charge-transfer
large charge-transfer capabilities. The absorption maxima behavior, the low-lying transitions can be analyzed at the
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Ruthenium() Complexes with Substituted Bipyridine and Phenylpyridine Ligands
FULL PAPER
citations involved in the 1 Ǟ 7 transition lead to the same
47% of ground-state electron density in orbital 117. How-
ever, and in striking contrast with [Ru(bpy)2(L2)][PF6]2, the
electron density in orbital 117 is shared between L1 (61.5%)
and the ruthenium atom (31.8%). An additional donating
contribution of the ruthenium atom is provided by orbital
115 (70.0% ruthenium-based). These differences lead to a
significant enhancement of the donating character of the
ruthenium atom in [Ru(bpy)2(L1)][PF6]. The overall charge-
transfer behavior associated with the 1 Ǟ 7 transition is
shown in Figure 5, which clearly indicates that the ruthe-
nium–phenyl fragment is deeply involved as the donating
moiety of the chromophore.
Figure 5. Charge transfer associated with the low-lying electronic
transition of [Ru(bpy)2(L1)][PF6]. White (black) contributions are
indicative of increase (decrease) of electron density occurring upon
electron transition.
Additional experimental evidence revealing the enhance-
ment of the electron density around the metal center is pro-
vided by the electrochemical studies of the RuII/RuIII oxi-
dation process. Single-electron waves are observed in both
complexes, leading to E1/2 values equal to 0.49 and 1.30 V
for [Ru(bpy)2(L1)][PF6] and [Ru(bpy)2(L2)][PF6]2, respec-
tively. The effect of cyclometalation on the stability of the
RuIII state is illustrated by a cathodic shift of about 810 mV.
This behavior demonstrates the strong σ-donating character
of the anionic ligand, and the enhancement of the donating
ability upon replacement of a nitrogen atom by a carbon
atom, in the coordination sphere of the ruthenium atom.[19]
Figure 4. Absorption maxima [cm–1] in solvents of different po-
larity (Reichardt constant ETN) for [Ru(bpy)2(L1)][PF6] [band at
370 (A) and 546 nm (B)] and for [Ru(bpy)2(L2)][PF6]2 [band at
454 nm (C)].
orbital level. In the case of [Ru(bpy)2(L2)][PF6]2, three
dominant excitations contribute to the 1 Ǟ 2 transition
(Table 3), which leads to 47% of the ground-state electron
density in orbital 117. However, this orbital is located on
the L2 fragment only (98.1% of the electron density), which
Nonlinear Optical Properties
Before describing the NLO response of [Ru(bpy)2-
results in a ligand-to-ligand charge-transfer process. The (L1)][PF6] and [Ru(bpy)2(L2)][PF6]2, it is important to em-
contribution of the metal atom to 1 Ǟ 2 is weak and is phasize that the values are not overestimated by multipho-
provided by orbital 114 (70.0% centered on the ruthenium ton fluorescence. This is experimentally verified by per-
atom). In the case of [Ru(bpy)2(L1)][PF6], the dominant ex- forming femtosecond hyper-Rayleigh scattering (HRS) ex-
Table 3. Experimental and ZINDO calculated spectra for [Ru(bpy)2(L1)][PF6] and [Ru(bpy)2(L2)][PF6]2.
Compound
UV/Vis spectra
λ [nm] ε [mol–1Lcm–1
ZINDO data Assignment Composition of CI expansion
Orbitals (% electron density on RuII)
]
λ [nm]
f
of the low-lying transitions[a]
[Ru(bpy)2(L1)][PF6]
546
14500
412
0.43
1 Ǟ 7
0.529 χ117Ǟ123 – 0.436 χ117Ǟ120
0.371 χ115Ǟ119
+
119 (6.1%), 120 (0.3%), 123 (0.9%),
115 (70.0%), 117 (31.8%)
370
454
51900
12300
344
397
0.72
0.42
1 Ǟ 11
1 Ǟ 2
[Ru(bpy)2(L2)][PF6]2
0.529 χ117Ǟ118 – 0.454 χ114Ǟ118
0.434 χ117Ǟ119
–
118 (0.2%), 119 (4.2%), 114 (70.0%),
117 (1.6%)
364
44900
358
0.87
1 Ǟ 10
[a] Orbital 117 is the HOMO and orbital 118 the LUMO in both compounds.
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P. G. Lacroix, et al.
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periments as a function of modulation frequency.[27] The derivative with respect to the parent metal–bipyridine mole-
finite lifetime associated with any fluorescence results in a cule. The last few years have witnessed a growing interest
demodulation (reduction of amplitude) for higher fre- in (pyridine)ruthenium() complexes in relation with the
quencies.[28] As exemplified in Figure 6, no such demodula- concept of NLO switching obtained by a redox effect.[11]
tion is observed in the case of [Ru(bpy)2(L1)][PF6], thus im- It therefore seems promising to design such cyclometalated
plying that the HRS signal is not comprised of any ruthenium chromophores that exhibit large NLO responses
multiphoton fluorescence contribution.[29]
in addition to reversible and accessible redox properties.
Along these lines, these complexes could undoubtedly at-
tract potential interest from the perspective of materials sci-
ence.
Experimental Section
Starting Materials and Equipment: Reagents and solvents are com-
mercially available and were used as received. 1a,[19] 1b,[20] and
[Ru(bpy)2Cl2][30] were prepared according to literature procedures.
1H and 13C NMR spectra were recorded at 298 K with a Bruker
1
AM 250 spectrometer (250 MHz for H NMR and 62.9 MHz for
13C) or a Bruker Avance 500 spectrometer for 2D NMR. The atom
labelling used for the assignment is given in Scheme 2. DCI and EI
mass spectra were obtained with a Thermo Finnigan TSQ 7000
and FAB mass spectra were recorded with a quadrupolar Nermag
R 10-10 instrument using an NBA matrix. UV/Vis spectra were
recorded with a Hewlett Packard 8452A spectrophotometer. Ele-
mental analyses were performed by the “Service de Microanalyses
du Laboratoire de Chimie de Coordination” (Toulouse) with a Per-
kin–Elmer 2400 Serie II instrument. Cyclic voltammetry data were
recorded with an Autolab PGSTAT 100 potentiostat, in acetonitrile
Figure 6. Experimental β values as a function of modulation fre-
quency for [Ru(bpy)2(L1)][PF6], showing the frequency-indepen-
dent response of the multiphoton-fluorescence-free HRS signal.
The HRS data are gathered in Table 4 for both deriva-
tives. In the case of [Ru(bpy)2(L2)][PF6]2, the detected signal
falls within the limit of the uncertainty of the present HRS using a Pt disk (1 mm diameter) as the working electrode, a Pt
gauze as the auxiliary electrode, and SCE as the reference electrode.
nBu4NPF6 (0.1 ) was used as the supporting electrolyte (purum
electrochemical grade Fluka, purified by sublimation). All voltam-
metric experiments were performed at ambient temperature in a
home-made, airtight three-electrode cell connected to a vacuum/
argon line. The solutions used for the electrochemical studies were
typically 10–3  in ruthenium complex. Prior to measurements, the
solutions were deoxygenated by bubbling with argon gas for 15 min
and the working electrode was polished with a polishing machine
(Presi P230); during experiments, a stream of argon was passed
setup; therefore the hyperpolarizability was not determined
with full accuracy and the reported data are the upper β
limit only. It must be pointed out that, as the second har-
monic (400 nm) is close to the charge-transfer band located
at 546 and 454 nm for [Ru(bpy)2(L1)][PF6] and [Ru(bpy)2-
(L2)][PF6]2, respectively, the experimental β values are nec-
essarily overestimated by resonance. Therefore, the param-
eter to take into account as the intrinsic hyperpolarizability
(independent of the laser frequency) is β0 (vide infra). An
examination of Table 4 reveals a significant enhancement of over the solution.
the NLO response in the cyclometalated derivative, with an
5-(Bromomethyl)-2-phenylpyridine (2a): 5-Methyl-2-phenylpyridine
intrinsic hyperpolarizability equal to 230×10–30 cm5 esu–1.
(1a; 2.11 g, 12.5 mmol), NBS (2.22 g, 12.9 mmol), and AIBN
(36 mg) were refluxed in dry CCl4 (58 mL) for 2 h. After cooling,
the suspension was filtered. The solvent was removed in vacuo and
n-hexane (30 mL) was added to the remaining paste. The mixture
was stirred in an ice bath for 1 h, whereupon a white fleecy precipi-
tate appeared. This was filtered off to yield the pure product
(837 mg, 27%). 1H NMR (CDCl3): δ = 8.68 (s, 1 H), 7.97 (m, 2
H), 7.75 (m, 2 H), 7.45 (m, 3 H), 4.52 (s, 2 H) ppm. C12H10BrN
(248.1) + 0.25 H2O: calcd. C 57.05, H 4.19, N 5.54; found C 56.81,
H 3.95, N 5.57. DCI MS (NH3): m/z (%) = 248.1 (33), 250.1 (34)
[MH]+.
Table 4. Experimental hyperpolarizabilities [10–30 cm5 esu–1] for the
ruthenium complexes, recorded using the hyper-Rayleigh scattering
method at 800 nm (β) and at zero frequency (β0).
β
β0
[Ru(bpy)2(L1)][PF6]
[Ru(bpy)2(L2)][PF6]2
500
Ͻ 200
230 ( 50)
Ͻ 40
5-(Bromomethyl)-2,2Ј-bipyridine (2b): Same procedure as for the
synthesis of 2a, starting from 5-methyl-2,2Ј-bipyridine (1b). Yield:
Conclusions
1
Two related ruthenium() complexes have been investi-
gated to compare their MLCT capabilities and hence the
hyperpolarizabilities of metal–phenylpyridine- vs. metal–bi-
pyridine-based chromophores. We have reported on a set of
computational, spectroscopic, electrochemical, and nonlin-
ear optical measurements which all prove that a strongly
68%. The H NMR spectrum is identical to those of previous re-
ports.[31,32]
Diethyl (2-Phenylpyridin-5-yl)methylphosphonate (3a): Under nitro-
gen, 2a (1.01 g, 4 mmol) was dissolved in triethyl phosphite (7 mL)
and the mixture was refluxed for 1 d. After cooling, the brown oil
was purified by two successive chromatographic separations on alu-
enhanced MLCT behavior takes place in a cyclometalated mina (99% CH2Cl2/1% CH3OH). Yellow oil (1.31 g, 87%). The
3110 Eur. J. Inorg. Chem. 2006, 3105–3113
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Ruthenium() Complexes with Substituted Bipyridine and Phenylpyridine Ligands
FULL PAPER
compound was used in the next step without further purification
(triethyl phosphite could not be totally eliminated). 1H NMR
(CDCl3): δ = 8.55 (s, 1 H), 7.95 (m, 2 H), 7.70 (m, 2 H), 7.42 (m,
was carried out on alumina (3% MeOH in CH2Cl2) to yield the
pure complex (15 mg, 24%). 1H NMR (CD3COCD3): δ = 8.78 (td,
J = 8.2 and 1.1 Hz, 1 H, H36), 8.68 (ddd, J = 8.1, 1.1 and 0.6 Hz,
3 H), 4.05 (m, 4 H), 3.14 (d, JH,P = 21.6 Hz, 2 H), 1.26 (t, J = 1 H, H33), 8.62 (ddd, J = 8.2, 1.3 and 0.8 Hz, 1 H, H23), 8.60
7.2 Hz, 6 H) ppm.
(ddd, J = 8.3, 1.2 and 0.8 Hz, 1 H, H26), 8.21 (d, J = 8.8 Hz, 2 H,
H16, H18), 8.24–8.17 (m, 3 H, H11, H37, H9), 8.16 (ddd, J = 5.7,
1.5 and 0.7 Hz, 1 H, H30), 8.10 (ddd, J = 5.4, 1.5 and 0.8 Hz, 1
H, H39), 8.06 (ddd, J = 5.8, 1.4 and 0.8 Hz, 1 H, H29), 8.00–7.90
(m, 6 H, H22, H32, H8, H5, H27, H20), 7.71 (d, J = 8.9 Hz, 2 H,
H15, H19), 7.64 (ddd, J = 7.6, 5.4 and 1.2 Hz, 1 H, H38), 7.42–
7.38 (m, 3 H, H21, H31, H28), 7.29 (d, J = 16.4 Hz, 1 H, H13),
7.21 (d, J = 16.4 Hz, 1 H, H12), 6.92 (dt, J = 7.2 and 1.2 Hz, 1 H,
H4), 6.85 (dt, J = 7.2 and 1.3 Hz, 1 H, H3), 6.52 (dd, J = 7.4 and
1 Hz, 1 H, H2) ppm. 13C NMR (CD3COCD3): δ = 194.27 (C1),
167.39 (C7), 157.90 (C34), 157.09 (C24), 156.80 (C25), 155.34
(C35), 154.27 (C30), 150.56 (C29), 150.04 (C20) 149.95 (C8), 149.23
(C39), 147.10 (C17), 145.26 (C6), 143.34 (C14), 136.59 (C37),
135.37 (C2), 135.19 (C32), 134.04 (C22), 133.77 (C27), 132.22 (C9),
131.17 (C10), 128.62 (C3), 128.46 (C12), 127.98 (C13), 127.37
(C38), 127.27 (C15, C19), 126.57 (C21), 126.40 (C31), 126.27 (C28),
124.85 (C5), 123.97 (C16, C18), 123.63 (C33), 123.41 (C36), 123.09
(C26), 123.08 (C23), 120.95 (C4), 118.39 (C11) ppm. The metathe-
sis was carried out as follows: the complex with the triflate counter-
ion was dissolved in the minimum amount of methanol then a satu-
rated aqueous solution of NH4PF6 was added until no more pre-
cipitation occurred. The precipitate was filtered off, rinsed with dis-
tilled water and dried under vacuum. C39H29F6N6O2PRu (859.7) +
H2O: calcd. C 52.41, H 3.50, N 9.40; found C 52.38, H 3.30, N
9.12. FAB-MS: m/z = 715 [M–PF6]+.
[Ru(bpy)2(L2)][PF6]2: L2 (48 mg, 0.158 mmol) was dissolved in
EtOH (25 mL) under N2 and the mixture heated to reflux.
[Ru(bpy)2Cl2] (85 mg, 0.158 mmol) was then added and the mixture
refluxed for 3 h. After cooling in an ice bath, addition of NH4PF6
(saturated solution in water) led to the precipitation of the complex.
The orange precipitate was filtered off, rinsed with water and di-
ethyl ether, and dried in vacuo (115 mg). The compound was ob-
tained by two successive chromatographic purifications on alumina
(CH3CN) and finally crystallization from acetone, which yielded
the pure, brick-red complex (80 mg, 50%).1H NMR (CD3COCD3):
δ = 8.88–8.82 (m, 6 H, H8, H5, H26, H23, H33, H36), 8.60 (dd, J
= 8.5, 1.9 Hz, 1 H, H9), 8.30–8.06 (m, 7 H, H11, H4, H27, H25,
H32, H37, H39), 8.24 (d, J = 9 Hz, 2 H, H16, H18), 8.11 (dd, J =
5.6, 7 Hz, 1 H, H2), 8.09–8.06 (m, 3 H, H29, H20, H30), 7.76 (d,
J = 8.8 Hz, 2 H, H15, H19), 7.64–7.58 (m, 5 H, H3, H31, H38,
H21, H28), 7.57 (d, J = 16.4 Hz, 1 H, H12), 7.21 (d, J = 16.5 Hz, 1
H, H13) ppm. 13C NMR (CD3COCD3): δ = 157.33, 157.24, 157.21,
157.04, 156.30, 152.07 (C11), 151.92 and 151.83 and 151.75 (C2,
C29, C20, C30), 151.05, 147.62, 142.51, 138.09, and 138.00 (C37,
C32, C25, C27), 136.53, 133.84 (C9), 132.01 (C12), 127.93, 127.88,
127.86, 127.84 (C15, C19), 127.75, 127.04 (C13), 124.60, 124.50,
124.48, 124.44, 124.35, 124.04, 114.38 ppm. C38H29F12N7O2P2Ru
(1006.7): calcd. C 45.34, H 2.90, N 9.74; found C 45.19, H 2.77, N
9.49. FAB-MS: m/z = 862 [M–PF6]+, 717 [M–2PF6]+.
Diethyl (2,2Ј-Bipyridin-5-yl)methylphosphonate (3b): Same pro-
cedure as for the synthesis of 3a, starting from 2b and using 98%
CH2Cl2/2% CH3OH as eluent for the two chromatographic separa-
tions. Some traces of triethyl phosphite remained and the com-
1
pound was used without any further purification (Yield: 76%). H
NMR (CDCl3): δ = 8.66 (d, J = 4.2 Hz, 1 H), 8.56 (s, 1 H), 8.36
(d, J = 3.6 Hz, 1 H), 8.33 (d, J = 3.8 Hz, 1 H), 7.80 (m, 2 H), 7.28
(m, 1 H), 4.08 (q, J = 7 Hz, 4 H), 3.18 (d, JH,P = 22 Hz, 2 H), 1.35
(t, J = 6.8 Hz, 6 H) ppm.
5-(p-Nitrostyryl)-2-phenylpyridine (HL1): In a Schlenk flask, 3a
(348 mg, 1.14 mmol) was dissolved in THF (21 mL) and cooled in
an ice bath. n-Butyllithium (1.6  in hexane; 0.72 mL, 1.15 mmol)
was added dropwise and the solution was stirred at room tempera-
ture for 1 h. A THF solution (6 mL) of p-nitrobenzaldehyde
(174 mg, 1.15 mmol) was then added slowly and the color changed
from yellow to brown. The mixture was refluxed for 3 h. After cool-
ing to room temperature, the mixture was hydrolyzed with water
(10 mL). The precipitate was filtered off, washed with diethyl ether,
and purified by column chromatography on alumina (80% CH2Cl2/
20% n-pentane). The product was recovered as a lemon-yellow
1
powder (200 mg, 37% yield). H NMR (CDCl3): δ = 8.86 (d, J =
2.1 Hz, 1 H, H6), 8.27 (d, J = 8.8 Hz, 2 H, H11, H13), 8.07 (dd, J
= 8.7 and 1.4 Hz, 2 H, H2Ј, H6Ј), 7.98 (dd, J = 8.3 and 2.3 Hz, 1
H, H4), 7.81 (d, J = 8.3 Hz, 1 H, H3), 7.70 (d, J = 8.8 Hz, 2 H,
H10, H14), 7.53 (dd, J = 7.6 and 7.1 Hz, 2 H, H3Ј, H5Ј), 7.47 (t,
J = 7.3 Hz, 1 H, H4Ј), 7.31 (d, J = 16.3 Hz, 1 H, H7), 7.26 (d, J =
16.3 Hz, 1 H, H8) ppm. 13C NMR (CDCl3): δ = 155.22 (C2),
148.96 (C6), 147.08 (C12), 143.18 (C9), 138.59 (C1Ј), 133.87 (C4),
130.36 (C5), 129.41 (C4Ј), 129.35 (C7), 128.91 (C5Ј), 127.89 (C8),
127.10 (C10, C14), 126.89 (C6Ј), 124.27 (C11, C13), 120.48 (C3)
ppm. C19H14N2O2 (302.3): calcd. C 75.48, H 4.67, N 9.27; found
C 75.14, H 4.59, N 9.12. DCI MS (NH3): m/z = 303 [MH]+.
5-(p-Nitrostyryl)-2,2Ј-bipyridine (L2): According to the procedure
described above for the synthesis of HL1, 3b yielded compound L2
(29% yield) after purification by chromatography on alumina with
0.5% MeOH in CH2Cl2. 1H NMR (CDCl3): δ = 8.84 (d, J =
1.9 Hz, 1 H, H6), 8.73 (ddd, J = 4.7, 1.6 and 0.8 Hz, 1 H, H6Ј),
8.51 (d, J = 8.3 Hz, 1 H, H3), 8.47 (d, J = 8 Hz, 1 H, H3Ј), 8.29
(d, J = 8.9 Hz, 2 H, H11, H13), 8.06 (dd, J = 8.3 and 2.3 Hz, 1 H,
H4), 7.88 (td, J = 7.8 and 1.6 Hz, 1 H, H4Ј), 7.71 (d, J = 8.8 Hz,
2 H, H10, H14), 7.37 (ddd, J = 5.9; 4.9 and 1 Hz, 1 H, H5Ј), 7.34
(d, J = 16.5 Hz, 1 H, H7), 7.30 (d, J = 15.5 Hz, 1 H, H8) ppm. 13
C
NMR (CDCl3): δ = 155.70 (C2), 155.32 (C2Ј), 149.15 (C6Ј), 148.56
(C6), 147.16 (C12), 143.07 (C9), 137.25 (C4Ј), 133.99 (C4),
131.99(C5) 129.27 (C7), 128.50 (C8), 127.18 (C10, C14), 124.28
(C11, C13), 124.05 (C5Ј), 121.36 (C3Ј), 121.22 (C3) ppm.
C18H13N3O2 (303.3): calcd. C 71.28, H 4.32, N 13.85; found C
70.78, H 3.94, N 13.53. EI-MS: m/z = 303 [M]+.
Crystallographic Studies: Single crystals of [Ru(bpy)2(L1)][PF6]
suitable for X-ray structure determination were grown by slow dif-
fusion of diethyl ether into a concentrated solution of the complex
in acetonitrile. Data were collected at 180 K with a Stoe Imaging
Diffraction System (IPDS) equipped with a graphite-monochro-
mated Mo-Kα radiation source (λ = 0.71073 Å) and an Oxford
Cryosystems Cryostream Cooler Device. Empirical absorption cor-
rections were applied (Tmin = 0.811, Tmax = 0.963).[33] The structure
was solved by direct methods using SIR92,[34] and refined by means
of least-squares procedures on F2 with the aid of the program
[Ru(bpy)2(L1)][PF6]: [Ru(bpy)2Cl2] (40 mg, 0.073 mmol) and AgOTf
(41 mg, 0.16 mmol) were added to a hot solution of HL1 (110 mg,
0.36 mmol) in MeOH/CH2Cl2 (1:1, 10 mL) and the mixture was
heated at 60 °C for 1 h. After cooling, the AgCl precipitate was
filtered through glass fiber. The filtrate was concentrated and the
crude product was purified by chromatography on alumina using
two eluents: 0.5% MeOH in CH2Cl2 to recover the unreacted ex-
cess of ligand, then 3% MeOH in CH2Cl2 to isolate a dark-violet
complex. A second chromatographic purification of this complex
Eur. J. Inorg. Chem. 2006, 3105–3113
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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3111

P. G. Lacroix, et al.
FULL PAPER
SHELXL97[35] included in the software package WinGX version
1.63.[36] Final unit-cell parameters were obtained by means of a
least-squares refinement of a set of well-measured reflections. The
atomic scattering factors were taken from the International Tables
for X-ray Crystallography.[37] All non-hydrogen atoms were refined
anisotropically, and in the last cycles of refinement a weighting
scheme was used, where weights are calculated with the following
sitions between the 10 highest occupied molecular orbitals and the
10 lowest unoccupied ones were chosen to undergo CI mixing.
NLO Measurements: The experimental β measurements were per-
formed with the frequency-resolved femtosecond hyper-Rayleigh
scattering (HRS)[46] technique at 800 nm. This setup was used as
described before.[27] The samples were dissolved in acetonitrile and
crystal violet (CV) dissolved in methanol was used as the reference
(βxxx = 338×10–30 cm5 esu–1).[27] A concentration series ws mea-
sured and the slopes of the linear fitting compared. The internal
field-correction factor [(n2 +2)/3]3, where n is the refractive index
at the NaD line, was applied to correct for the difference in solvent
(nMeOH = 1.328 and nMeCN = 1.343). The compounds were ana-
lyzed as dipolar structures with Cϱv symmetry. To correct for the
difference in symmetry between the dipolar compound and octupo-
lar reference, the following appropriate correction factors were
used:[46b,47]
formula: w = 1/[σ2(Fo2) + (aP)2 + bP] where P = (Fo + 2Fc )/3.
Further details are listed in Table 5. The drawing of the molecule
was produced with the program ORTEP32[38] with 50% probability
displacement ellipsoids for non-hydrogen atoms. CCDC-602439
contains the supplementary crystallographic data for this paper.
These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
2
2
Table 5. Crystal data for [Ru(bpy)2(L1)][PF6] (1).
Empirical formula
Formula mass
Crystal size [mm]
Crystal system
Space group
a [Å]
C29H29N6O2Ru·C4H10O·PF6
933.84
(1)
0.425×0.125×0.0625
monoclinic
C2/c
32.661(7)
9.5543(19)
26.044(5)
90.92(2)
8126(3)
1.527
180
0.501
φ
2.22 Ͻ θ Ͻ 26.10
37036
7614
7614
543
b [Å]
c [Å]
The determined apparent hyperpolarizability was found to be inde-
pendent of the modulation frequency, and the fluorescence-free hy-
perpolarizability was calculated by averaging the obtained hyperpo-
larizabilities at different modulation frequencies. Due to the weak-
ness of the 2ω light intensity in the HRS phenomenon, measure-
ments have to be performed close to the resonance, which leads to
an artificial enhancement of the NLO signal. In this case, the pa-
rameter of interest is the intrinsic hyperpolarizability β(0), which is
independent of the laser frequency and can be estimated within a
two-level model according to the following expressions:[48]
β [°]
V [Å3]
ρcalcd. [gcm–3]
T [K]
µ (Mo-Kα) [mm–1]
Scan mode
θ range [°]
Measured reflections
Independent reflections
Observed reflections
No of parameters
H atoms
(2)
calculated
0.0403
R1
wR2
0.0491
0.518
–1.143
∆ρmax [eÅ–3]
∆ρmin [eÅ–3]
(3)
Theoretical Methods: Gas-phase geometries for cations [Ru(bpy)2-
(L1)]+ and [Ru(bpy)2(L2)]2+ were fully optimized using the
Gaussian-03 program package[39] within the framework of DFT at
the B3LYP/6-31G*/LANL2DZ(Ru) level.[40,41] The starting metri-
cal parameters for the calculations were taken from the present
crystal structure of [Ru(bpy)2(L1)]+. In the absence of a crystal
structure for [Ru(bpy)2(L2)][PF6]2, the starting model was built up
from the previously reported structures of [Ru(bpy)3][PF6]2,[42] and
4-hydroxy-4Ј-nitrobiphenyl.[43] Due to the large size of the mole-
cule, time-consuming vibrational analyses were not performed on
the ruthenium complexes. Nevertheless, we checked that the DFT
approach provides an accurate description of the molecular geome-
try in the case of the few previously reported [Ru(bpy)(phenpy)]+
complexes,[16,19] although with a tendency for slightly inflated coor-
dination spheres in the DFT-computed structures. The all-valence
INDO (intermediate neglect of differential overlap) method[44] was
employed to calculate the electronic transitions in order to analyze
the origin of the charge transfer at the atomic level. Calculations
were performed using the INDO/1 Hamiltonian incorporated in
the commercially available MSI software package ZINDO.[45] The
monoexcited configuration interaction (MECI) approximation was
employed to describe the excited states. The 100 lowest-energy tran-
Acknowledgments
The authors thank Dr. A. Sournia-Saquet for electrochemical mea-
surements, CALMIP (Calcul en Midi Pyrénées, Toulouse) for par-
allel computation facilities, and a Programme d’Action Intégrée
(PAI-Tournesol) for financial support.
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Eur. J. Inorg. Chem. 2006, 3105–3113

Ruthenium() Complexes with Substituted Bipyridine and Phenylpyridine Ligands
FULL PAPER
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Received: March 24, 2006
Published Online: July 5, 2006
Eur. J. Inorg. Chem. 2006, 3105–3113
© 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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