Syntheses, spectroscopic and molecular quadratic nonlinear optical properties of dipolar ruthenium(II) complexes of the ligand 1,2-phenylenebis(dimethylarsine)

Article abstract of DOI:10.1039/b409432h

Six new complex salts trans-[Ru^IICl(pdma)₂L][PF ₆]_n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-pyridyl)hexa-1,3,5-triene (bph), n = 1,5; L = N-methyl-4-[(e)-2-(4-pyridyl)ethenyl]pyridinmm (Mebpe^-), n = 2, 7; L = N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb⁺), n = 2, 8; L = N-methyl-4-{(E,E,E)-6-(4-pyridyl)hexa1,3,5-trienyl}pyridinium (Mebph⁺), n = 2,9; L = bis(4-pyridyl)acetylene (bpa), n = 1, 10; L = N-methyl-4-[2-(4-pyridyl)-ethynyl]pyridinium (Mebph⁺), n = 2, 11] have been prepared. The electronic absorption spectra of 5 and 7-11 display intense, visible metal-to-ligand charge-transfer (MLCT) bands, with λ_max values in the range 434-492 nm in acetonitrile. Cyclic voltammetric studies reveal reversible Ru^III/II waves with _E1/2 values in the range 1.06-1.15 V vs. Ag-AgCl, Natogether with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B. J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7-11 have been investigated by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole moment changes Δμ₁₂ for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities β₀ according to the two-state equation β₀ = 3Δμ₁₂(μ₁₂) ^2/(E_max)²(μ₁₂= transition dipole moment, E_max = MLCT energy). MLCT absorption and electrochemical data show that a trans-{Ru^IICl(pdma)₂}⁺ centre is considerably less electron-rich than a {Ru^II(NH₃) ₅}^2- unit. Although the β₀ responses of the pdma complexes are only a little smaller than those of their {Ru ^II(NH₃)₅}^2- analogues, this result is partly attributable to unexpected changes in the relative π₁₂ values on freezing. Thus, substantial increases in μ₁₂ for the arsine compounds act to partially offset the β₀-decreasing influence of their higher E_max values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained for the salts 1-2.5MeCN, 4·2MeCN, 7 and 11, but only 1-2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such as may show bulk NLO effects.

Full text of DOI:10.1039/b409432h

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F U L L P A P E R
Syntheses, spectroscopic and molecular quadratic nonlinear
optical properties of dipolar ruthenium(II) complexes of the ligand
1,2-phenylenebis(dimethylarsine)
Benjamin J. Coe,*a Josephine L. Harries,a James A. Harris,a,b Bruce S. Brunschwig,b
Simon J. Coles,c Mark E. Lightc and Michael B. Hursthousec
a
Department of Chemistry, University of Manchester, Oxford Road, Manchester, UK M13 9PL.
E-mail: b.coe@man.ac.uk
Molecular Materials Research Center, Beckman Institute, California Institute of Technology,
b
MC 139-74, 1200 East California Blvd., Pasadena, CA 91125, USA
EPSRC National Crystallography Service, School of Chemistry, University of Southampton,
c
Highfield, Southampton, UK SO17 1BJ
Received 21st June 2004, Accepted 29th July 2004
First published as an Advance Article on the web 24th August 2004
Six new complex salts trans-[RuIICl(pdma)2L][PF6]n [pdma = 1,2-phenylenebis(dimethylarsine); L = (E,E,E)-1,6-bis(4-
pyridyl)hexa-1,3,5-triene (bph), n = 1, 5; L = N-methyl-4-[(E)-2-(4-pyridyl)ethenyl]pyridinium (Mebpe+), n = 2, 7; L = N-
methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-dienyl]pyridinium (Mebpb+), n = 2, 8; L = N-methyl-4-{(E,E,E)-6-(4-pyridyl)hexa-
1,3,5-trienyl}pyridinium (Mebph+), n = 2, 9; L = bis(4-pyridyl)acetylene (bpa), n = 1, 10; L = N-methyl-4-[2-(4-pyridyl)-
ethynyl]pyridinium (Mebpa+), n = 2, 11] have been prepared. The electronic absorption spectra of 5 and 7–11 display
intense, visible metal-to-ligand charge-transfer (MLCT) bands, with max values in the range 434–492 nm in acetonitrile.
Cyclic voltammetric studies reveal reversible RuIII/II waves with E1/2 values in the range 1.06–1.15 V vs. Ag–AgCl, to-
gether with irreversible L-based reduction processes. Along with a number of previously reported related compounds (B.
J. Coe et al., J. Chem. Soc., Dalton Trans., 1996, 3917; 1997, 591; 2000, 797), salts 5 and 7–11 have been investigated
by using Stark (electroabsorption) spectroscopy in butyronitrile glasses at 77 K. These studies have afforded dipole mo-
ment changes 12 for the MLCT transitions which have been used to calculate molecular static first hyperpolarizabilities
0 according to the two-state equation 0 = 312(12)2/(Emax)2 (12 = transition dipole moment, Emax = MLCT energy).
MLCT absorption and electrochemical data show that a trans-{RuIICl(pdma)2}+ centre is considerably less electron-rich
than a {RuII(NH3)5}2+ unit. Although the 0 responses of the pdma complexes are only a little smaller than those of their
{RuII(NH3)5}2+ analogues, this result is partly attributable to unexpected changes in the relative 12 values on freezing.
Thus, substantial increases in 12 for the arsine compounds act to partially offset the 0-decreasing influence of their higher
Emax values when compared with the analogous pentaammine species. Single crystal X-ray structures have been obtained
for the salts 1·2.5MeCN, 4·2MeCN, 7 and 11, but only 1·2.5MeCN adopts a non-centrosymmetric space group (Fdd2) such
as may show bulk NLO effects.
dinated to pyridinium ligands, with the original primary objective
Introduction
of gaining detailed crystallographic data. The syntheses, proper-
Future optoelectronic and photonic data processing devices are
ties and crystal structures of some of these complexes have been
likely to be based on molecular materials which exhibit nonlin-
previously reported,9 but without any accompanying NLO data.
ear optical (NLO) properties.1 Fundamental research into such
Such data was originally not obtained for the following reasons:
materials has recently increasingly involved organotransition metal
(i) MLCT absorption and electrochemical studies clearly show
complexes which can display very marked NLO effects, combined
that a trans-{RuIICl(pdma)2}+ centre is a rather less effective elec-
with various other potentially useful properties such as redox and/
tron donor when compared with an ammine-bearing unit such as
or magnetic behaviour.2 The establishment of detailed structure–
{RuII(NH3)5}2+, suggesting that the  values of the pdma complexes
activity correlations for first hyperpolarizabilities , which govern
will be smaller than those of their ammine analogues; (ii) the MLCT
quadratic molecular NLO effects, is the primary objective of most
bands of the pdma complexes are found very close to 532 nm, pre-
current work with such metal-based chromophores. Our contribu-
cluding the aquisition of meaningful HRS data when using the stan-
tion to this field has focused on hyper-Rayleigh scattering (HRS)3
dard 1064 nm Nd3+-YAG laser fundamental with which the ammine
and electronic Stark effect (electroabsorption)4 spectroscopic stud-
complexes have been investigated. However, since this previous
ies with various RuII pyridyl ammine complexes of pyridinium-
study was reported,9 we have carried out Stark spectroscopic experi-
substituted ligands.5 We have found that such dipolar chromophores
ments with our ammine complexes and found that this technique is
can exhibit very large static first hyperpolarizabilities 0 that are
a useful approach to deriving  values for such chromophores.5e–i
associated with intense, low energy metal-to-ligand charge-transfer
Although the Stark approach is indirect, the resonance effects
(MLCT) transitions.5 The MLCT absorption and NLO properties
which often limit the utility of direct HRS measurements can be
of these complexes can be tuned via changes in ligand structure,
discounted. We have therefore now synthesised some further new
in general accordance with the widely used two-state model,6 and
dipolar RuII pdma complexes to expand the existing series, and
can also be readily and reversibly switched by exploiting the RuIII/II
investigated these compounds by using Stark spectroscopy.
redox couple.7
Because of continual difficulties with growing single crystals
suitable for X-ray diffraction studies, we have obtained only scant
structural information on our RuII ammine complexes.5a,c,d Inspired
by earlier studies involving complexes of the chelating ligand 1,2-
phenylenebis(dimethylarsine) (pdma),8 we have prepared a series
of complexes in which trans-{RuIICl(pdma)2}+ centres are coor-
Experimental
Materials and procedures
The compound pdma was obtained from Dr G. Reid, University
of Southampton. (E,E,E)-1,6-Bis(4-pyridyl)hexa-1,3,5-triene
T h i s j o u r n a l i s
©
T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2
2 9 3 5

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(bph)10 and the salts trans-[RuIICl(pdma)2(NO)][PF6]2,11 N-methyl-
4-[(E)-2-(4-pyridyl)ethenyl]pyridinium hexafluorophosphate
added and the acetone removed in vacuo. The solution was heated
under reflux for 2 h, cooled to room temperature, and diethyl ether
was added to afford a dark red–orange precipitate. This solid was
filtered off and the excess bph was removed by two reprecipitations
from acetone–diethyl ether. The product was further purified by
precipitation from acetone–aqueous NH4PF6, then acetone–diethyl
ether to afford a brick-red solid. Recrystallization from acetoni-
trile–diethyl ether afforded a dark red solid: yield 47 mg (35%).
H (CD3COCD3) 8.51 (2 H, d, J 7.2, C5H4N), 8.32 (4 H, m, 2C6H2),
7.84 (4 H, m, 2C6H2), 7.53 (2 H, d, J 6.7, C5H4N), 7.42 (2 H, d, J
7.3, C5H4N), 7.31–7.13 (2 H, m, 2CH), 7.09 (2 H, d, J 6.7, C5H4N),
6.75–6.66 (3 H, m, 3CH), 6.52 (1 H, d, J 15.5, CH), 1.90 (12 H, s,
4AsMe), 1.78 (12 H, s, 4AsMe) (Found: C, 39.14; H, 4.06; N, 2.57.
Calc for C36H46N2As4ClF6PRu: C, 39.74; H, 4.26; N, 2.57%). m/z:
([Mebpe+]PF6)5d and N-methyl-4-[(E,E)-4-(4-pyridyl)buta-1,3-
dienyl]pyridinium hexafluorophosphate ([Mebpb+]PF6)5i were
prepared according to published procedures. The compound
bis(4-pyridyl)acetylene (bpa) was prepared via modification of a
literature procedure,12 with chromatographic purification resulting
in improved yields. All other reagents were obtained commercially
and used as supplied. All reactions were conducted under an argon
atmosphere. Products were dried at room temperature in a vacuum
desiccator (CaSO4) for ca. 24 h prior to characterization.
General physical measurements
Proton NMR spectra were recorded on a Varian Gemini 200
spectrometer and all shifts are referenced to SiMe4. The fine split-
ting of pyridyl or phenyl ring AA′BB′ patterns is ignored and the
signals are reported as simple doublets, with J values referring to
the two most intense peaks. Elemental analyses were performed
by the Microanalytical Laboratory, University of Manchester
and UV/VIS spectra were obtained by using a Hewlett Packard
8452A diode array spectrophotometer. Mass spectra were recorded
using +electrospray on a Micromass Platform spectrometer (cone
voltage 80 V), with the exception of compound 8 for which the
MALDI technique was used.
Cyclic voltammetric measurements were carried out with an
EG&G PAR model 283 potentiostat/galvanostat. An EG&G PAR
K0264 single compartment microcell was used with a Ag–AgCl
reference electrode (3 mol dm−3 NaCl, saturated AgCl), a plati-
num-disc working electrode and platinum-wire auxiliary electrode.
Acetonitrile(HPLCgrade)wasdistilledbeforeuseandtetra-n-butyl-
ammonium hexafluorophosphate, twice recrystallized from ethanol
and dried in vacuo, was used as supporting electrolyte. Solutions
containing ca. 10−3 mol dm−3 analyte (0.1 mol dm−3 electrolyte)
were deaerated by purging with N2. All E1/2 values were calculated
from (Epa + Epc)/2 at a scan rate of 200 mV s−1.
−
943 ([M − PF6 ]+).
trans-[RuIICl(pdma)2(Mebpe+)][PF6]2 7. This was prepared and
purified in an identical fashion to 5 by using trans-[RuIICl(pdma)2-
(NO)][PF6]2 (75 mg, 0.073 mmol), NaN3 (4.9 mg, 0.075 mmol) in
acetone (5 cm3) and [Mebpe+]PF6 (125 mg, 0.365 mmol) in place
of bph. The product was obtained as dark red needles: yield 43 mg
(49%). H (CD3COCD3) 8.91 (2 H, d, J 6.9, C5H4N), 8.31 (4 H, m,
2C6H2), 8.22 (2 H, d, J 6.9, C5H4N), 7.84 (4 H, m, 2C6H2), 7.76–7.58
(4 H, m, 2CH, C5H4N), 7.32 (2 H, d, J 6.7, C5H4N), 4.48 (3 H, s,
C5H4N–Me) 1.91 (12 H, s, 4AsMe), 1.79 (12 H, s, 4AsMe) (Found:
C, 33.71; H, 3.63; N, 2.30. Calc. for C33H45As4ClF12N2P2Ru: C,
−
33.14; H, 3.79; N, 2.34%). m/z: 1050 ([M − PF6 ]+).
trans-[RuIICl(pdma)2(Mebpb+)][PF6]2 8. This was prepared
and purified in an identical fashion to 7, but using [Mebpb+]PF6
(134 mg, 0.364 mmol) in place of [Mebpe+]PF6. Dark red needles
were obtained: yield 53 mg (59%). H (CD3COCD3) 8.52 (2 H, d,
J 6.7, C5H4N), 8.29 (4 H, m, 2C6H2), 7.98 (2 H, d, J 6.9, C5H4N),
7.91 (4 H, m, 2C6H2), 7.61–7.52 (1 H, m, CH), 7.43 (2 H, d, J 6.7,
C5H4N), 7.28–7.19 (1 H, m, CH), 6.99 (2 H, d, J 6.6, C5H4N), 6.86
(2 H, m, 2CH), 4.28 (3 H, s, C5H4N–Me), 1.91 (12 H, s, 4AsMe),
1.73 (12 H, s, 4AsMe) (Found: C, 34.33; H, 3.56; N, 2.32. Calc.
for C35H47As4ClF12N2P2Ru: C, 34.40; H, 3.88; N, 2.29%). m/z: 932
Syntheses
−
([M − 2PF6 ]+).
Bis(4-pyridyl)acetylene. CAUTION: This compound has been
reported to cause painful blistering of the skin several days after
contact; use gloves when handling. Br2 (3.5 cm3, 10.8 g, 68 mmol)
was added dropwise to a stirred solution of trans-1,2-bis(4-
pyridyl)ethylene (3.52 g, 19.3 mmol) in HBr (48%, 46.5 cm3) at
0 °C. The mixture was stirred at 120 °C for 2 h, then cooled to
room temperature. Chilling in ice caused the precipitation of an
orange solid which was filtered off and washed with water then
stirred in aqueous NaOH (2 mol dm−3, 120 cm3) for 30 min. The
resulting white solid, 1,2-dibromo-1,2-bis(4-pyridyl)ethane, was
filtered off, washed with a large amount of water and dried: yield
4.0 g (61%). H (CDCl3) 8.69 (4 H, d, J 5.9, C5H4N), 7.39 (4 H, d, J
5.9, C5H4N), 5.27 (2 H, s, CH). Finely cut Na (1.2 g, 52 mmol) was
stirred in t-BuOH (120 cm3) at 80 °C under argon until dissolution
was complete (overnight). 1,2-Dibromo-1,2-bis(4-pyridyl)ethane
(4.0 g, 11.7 mmol) was added in portions and the mixture was
stirred under argon at 80 °C for 4 h. The mixture was cooled to
40 °C and EtOH was added (20 cm3), followed by water (20 cm3,
CAUTION!). The brown solution was extracted with CHCl3 until
the extracts became colourless (ca. 6 × 50 cm3), the extracts dried
(CaCl2), and evaporated to dryness. The solid was dissolved in
a minimum volume of CHCl3, loaded onto a silica gel column
(200 g), and eluted with diethyl ether–THF (85:15). The major
band was collected and evaporated to afford a white solid: yield
2.0 g (95%). H (CDCl3) 8.60 (4 H, d, J 6.0, C5H4N), 7.35 (4 H,
d, J 6.0, C5H4N) (Found: C, 79.92; H, 4.42; N, 15.31. Calc. for
C12H8N2: C, 79.98; H, 4.47; N, 15.54%).
trans-[RuIICl(pdma)2(Mebph+)][PF6]2 9. A solution of 5
(50 mg, 0.046 mmol) in DMF (1.5 cm3) and methyl iodide (0.5 cm3)
was stirred for 24 h at room temperature. The excess methyl iodide
was removed in vacuo and addition of aqueous NH4PF6 to the
dark red solution gave a brick-red precipitate which was filtered
off, washed with water and dried. Purification was effected by
precipitation from acetone–diethyl ether: yield 48 mg (84%). H
(CD3COCD3) 8.84 (2 H, d, J 6.9, C5H4N), 8.32 (4 H, m, 2C6H2),
8.14 (2 H, d, J 6.7, C5H4N), 7.88 (4 H, m, 2C6H2), 7.74–7.66 (1 H,
m, CH), 7.53 (2 H, d, J 6.9, C5H4N), 7.27–7.18 (1 H, m, CH), 7.12
(2 H, d, J 6.9, C5H4N), 6.96 (1 H, d, J 15.5, CH), 6.89–6.74 (2 H,
m, 2CH), 6.66 (1 H, d, J 15.7, CH), 4.48 (3 H, s, C5H4N–Me), 1.90
(12 H, s, 4AsMe), 1.78 (12 H, s, 4AsMe) (Found: C, 35.59; H, 3.50;
N, 2.25. Calc. for C37H49As4ClF12N2P2Ru: C, 35.61; H, 3.96; N,
−
2.24%). m/z: 1102 ([M − PF6 ]+).
trans-[RuCl(pdma)2(bpa)]PF6 10. This was prepared and puri-
fied in an identical fashion to 5 by using bpa (221 mg, 1.23 mmol) in
place of bph. The product was obtained as a red–brown solid: yield
79 mg (63%). H (CD3COCD3) 8.70 (2 H, br s, C5H4N), 8.30 (4 H,
m, 2C6H2), 7.83 (4 H, m, 2C6H2), 7.77 (2 H, d, J 6.7, C5H4N), 7.50
(2 H, d, J 6.6, C5H4N), 7.19 (2 H, d, J 6.7, C5H4N), 1.90 (12 H, s,
4AsMe), 1.80 (12 H, s, 4AsMe) (Found: C, 36.81; H, 3.72; N, 2.67.
Calc. for C32H40As4ClF6N2PRu: C, 37.18; H, 3.90; N 2.71%). m/z:
−
888 ([M − PF6 ]+).
trans-[RuIICl(pdma)2(bph)]PF6 5. A solution of trans-[RuIICl-
(pdma)2(NO)][PF6]2 (125 mg, 0.122 mmol) and NaN3 (8.1 mg,
0.125 mmol) in acetone (10 cm3) was stirred at room temperature
for 2 h. Butan-2-one (10 cm3) and bph (286 mg, 1.22 mmol) were
trans-[RuIICl(pdma)2(Mebpa+)][PF6]2 11. This was prepared
and purified in an identical fashion to 9 by using 10 (50 mg,
0.048 mmol) in place of 5. The product was obtained as a dark
red solid: yield 53 mg (92%). H (CD3COCD3) 9.06 (2 H, d, J 6.7,
2 9 3 6
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2

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Table 1 Crystallographic data and refinement details for complex salts 1·2.5MeCN, 4·2MeCN, 7 and 11
1·2.5MeCN
4·2MeCN
7
11
Formula
M
C29H43.50As4ClF6N4.50PRu
1036.35
C36H48As4ClF6N4PRu
1117.95
C33H45As4ClF12N2P2Ru
1195.85
C33H43As4ClF9N2P1.50Ru
1121.35
Crystal system
Space group
Orthorhombic
Fdd2
Monoclinic
P21/n
Monoclinic
P21/c
Monoclinic
P21/c
a/Å
b/Å
c/Å
21.9898(5)
43.4293(11)
16.3636(4)
12.2140(3)
13.1090(4)
26.6750(6)
90.7850(13)
4270.62(19)
4
20.4769(2)
9.44240(10)
24.6388(3)
112.9200(10)
4387.82(8)
4
20.2299(11)
9.5405(4)
24.5138(14)
112.535(2)
4370.0(4)
4
/°
U/Å3
15627.3(7)
16
Z
T/K
120(2)
3.928
120(2)
3.600
120(2)
3.563
120(2)
3.544
/mm−1
Reflections collected
Independent reflections (Rint)
Final R1, wR2 [I > 2(I)]a
(all data)
35833
37632
14172
28451
6876 (0.0936)
0.0443, 0.1086
0.0509, 0.1131
7489 (0.0656)
0.0392, 0.0895
0.0699, 0.1007
7731 (0.0221)
0.0558, 0.1564
0.0617, 0.1628
8355 (0.0774)
0.0527, 0.1243
0.0923, 0.1372
a The structures were refined on Fo2 using all data; the values of R1 are given for comparison with older refinements based on Fo with a typical threshold of
Fo > 4(Fo).
C5H4N), 8.30 (4 H, m, 2C6H2), 8.21 (2 H, d, J 6.9, C5H4N), 7.83
(6 H, m, 2C6H2 and C5H4N), 7.26 (2 H, d, J 6.9, C5H4N), 4.57 (3 H, s,
C5H4N–Me), 1.91 (12 H, s, 4AsMe), 1.85 (12 H, s, 4AsMe) (Found:
C, 33.21; H, 3.26; N, 2.41. Calc. for C33H43As4ClF12N2P2Ru: C,
where Emax is the energy of the MLCT maximum (in wavenumbers).
The degree of delocalization cb and electronic coupling matrix
element Hab for the diabatic states are given by
2
−
33.20; H, 3.63; N, 2.35%). m/z: 1048 ([M − PF6 ]+).
(3)
X-Ray structural determinations
Crystals of complex salts 1·2.5MeCN, 4·2MeCN, 7 and 11
were obtained by slow diffusion of diethyl ether vapour into
acetonitrile solutions. The crystals chosen for diffraction
studies had the following approximate dimensions and appear-
ances: 0.20 × 0.10 × 0.08 mm, yellow block (1·2.5MeCN);
0.15 × 0.10 × 0.10 mm, orange rod (4·2MeCN); 0.60 × 0.40 ×
0.20 mm, red block (7); 0.40 × 0.20 × 0.02 mm, dark red plate (11).
The crystal of 11 contained unrefinable solvent (probably aceto-
nitrile) which was removed by using the SQUEEZE procedure.
Although 11 is a bis-hexafluorophosphate salt, the asymmetric unit
contains 1.5 PF6− anions, with one being completed due to crystallo-
graphic symmetry, giving rise to F9P1.5 in the crystallographic
formula. Crystallographic data and refinement details are presented
in Table 1.
(4)
If the hyperpolarizability tensor 0 has only nonzero elements along
the MLCT direction, then this quantity is given by
(5)
Results and discussion
CCDC reference numbers 240011 (1·2.5MeCN), 240012
(4·2MeCN), 240013 (7) and 240014 (11).
See http://www.rsc.org/suppdata/dt/b4/b409432h/ for crystallo-
graphic data in CIF or other electronic format.
Stark spectroscopy
The Stark apparatus, experimental methods and data analysis
procedure were exactly as previously reported,5e,13 with the only
modification being that a Xe arc lamp was used as the light source
in the place of a W filament bulb. Butyronitrile was used as the
glassing medium, for which the local field correction fint is estimated
as 1.33.5e,13 The Stark spectrum for each compound was measured
a minimum of three times using different field strengths, and the
signal was always found to be quadratic in the applied field. A two-
state analysis of the MLCT transitions gives
2
ab2 = 122 + 412
(1)
where ab is the dipole moment difference between the diabatic
states, 12 is the observed (adiabatic) dipole moment difference,
and 12 is the transition dipole moment. Analysis of the Stark spec-
tra in terms of the Liptay treatment4 affords 12, and the transition
dipole moment 12 can be determined from the oscillator strength
fos of the transition by
|12| = [fos/(1.08 × 10−5Emax)]1/2
(2)
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2
2 9 3 7

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Table 2 UV/VIS and electrochemical data for complex salts 5 and 7–11 in acetonitrile
E1/2/V vs. Ag–AgCl
(Ep/mV)b

max/nm
c
Complex salt
(max/dm3 mol−1 cm−1)
Emaxa/eV
Assignment
E1/2[RuIII/II
]
Epc
5 trans-[RuIICl(pdma)2(bph)]PF6
446 (18800)
362 (37300)
492 (13000)
316 (28500)
486 (15700)
356 (37900)
472 (15000)
394 (26800)
434 (6000)
2.78
3.43
2.52
3.92
2.53
3.48
2.58
3.15
2.86
4.33
2.54
4.13
d→*
→*
d→*
→*
d→*
→*
d→*
→*
d→*
→*
d→*
→*
1.06 (90)
1.10 (95)
−1.07
−0.79
−0.80
−0.80
−1.10
−0.98
7 trans-[RuIICl(pdma)2(Mebpe+)][PF6]2
8 trans-[RuIICl(pdma)2(Mebpb+)][PF6]2
9 trans-[RuIICl(pdma)2(Mebph+)][PF6]2
10 trans-[RuCl(pdma)2(bpa)]PF6
1.09 (105)
1.07 (110)
1.12 (95)
286 (22100)
488 (11500)
300 (21900)
11 trans-[RuIICl(pdma)2(Mebpa+)][PF6]2
1.15 (105)
a Solutions ca. 3–8 × 10−5 mol dm−3. b Solutions ca. 10−3 mol dm−3 in analyte and 0.1 mol dm−3 in NBun4PF6 at a platinum disc working electrode with a scan
rate of 200 mV s−1. Ferrocene internal reference E1/2 = 0.43 V, Ep = 90–110 mV. c For an irreversible reduction process.
Table 3 MLCT absorption and selected cyclic voltammetric data for complex salts 1–15 in acetonitrile
E/V vs. Ag–AgClb
c
Complex salt

maxa/nm
Emaxa/eV
a/mol−1 dm3 cm−1
E1/2[RuIII/II
]
E1/2[L+/0] or Epc
1d
2d
3e
422
558
418
434
446
486
492
486
472
434
488
510
520
536
544
2.94
2.22
2.97
2.86
2.78
2.55
2.52
2.53
2.58
2.86
2.54
2.43
2.38
2.31
2.28
6100
12000
8400
14300
18800
8300
13000
15700
15000
6000
11500
12500
10000
10400
10400
1.22
1.48
1.10
1.08
1.06
1.14
1.10
1.09
1.07
1.12
1.15
1.15
1.15
1.16
1.16
−1.48c
−0.35
−1.49c
−1.33
−1.07c
−0.74
−0.79c
−0.80c
−0.80c
−1.10c
−0.98c
−0.58
−0.51
−0.38c
−0.34
4e
5
6f
7
8
9
10
11
12f
13f
14f
15f
a Solutions ca. 3–8 × 10−5 M. b Solutions ca. 10−3 M in analyte and 0.1 M in NBun PF6 at a platinum bead/disc working electrode with a scan rate of 200 mV
4
s−1. Ferrocene internal reference E1/2 = 0.43 V. c For an irreversible reduction process. d Ref. 8a. e Ref. 8b. f Ref. 9.
band is observed on moving from n = 0 to 1, but the MLCT energy
Emax increases slightly as the chain is further extended up to three
E-ethylene units (Table 3). This is very unusual optical behav-
iour because the intramolecular charge-transfer (ICT) bands of
donor–acceptor polyene chromophores normally red-shift on elon-
gation of conjugated chains, irrespective of whether terminal metal
centres are present.14 To our knowledge, the only previous reports
of donor–acceptor polyenes in which the ICT/MLCT bands blue-
shift with increasing conjugation length involve compounds hav-
ing tetrathiafulvalenyl donor groups with various strong electron
acceptors including 2,2-dicyanovinyl,15 and also complexes related
to 6–9, but containing RuII ammine centres, which have recently
been studied in our laboratory.5f,i
Within the 4,4′-bipyridinium-based series 6 and 12–15, Emax
decreases steadily as the N-pyridinium substituent changes in the
order R = Me > Ph > 4-AcPh > 2,4-DNPh > 2-Pym (Table 3). Simi-
lar behaviour has also been observed in RuII ammine complexes
of the same ligands,5c,d and this trend is attributable primarily to a
continual stabilisation of the ligand-based LUMO as the electron-
withdrawing ability of R increases, as reflected in the ligand first
reduction potentials (see below). Replacing a trans-CHCH with
a CC bridge does not significantly affect the MLCT energies,
according to the data for the pairs 4/10 and 7/11.
Synthetic studies
The new complex salts 5 and 7–11 were prepared in order to pro-
vide an extensive series of systematically modified compounds
together with the existing 1–4,8 69 and 12–15.9 The complexes in
5 and 10 were prepared via reactions of the neutral pro-ligands
bph or bpa with the sodium azide-treated complex precursor trans-
[RuIICl(pdma)2(NO)]2+,11 and 7 and 8 were prepared similarly but
using the pro-ligand salts of the appropriate N-methylated cations,
i.e. [Mebpe+]PF6 and [Mebpb+]PF6, respectively. The complexes in
9 and 11 were synthesized via treatment of 5 and 10, respectively,
with methyl iodide.All of the new compounds show diagnostic pro-
ton NMR spectra, and mass spectra and elemental analyses provide
further confirmation of identity and purity.
Electronic absorption spectroscopic studies
The UV/Visible absorption spectra of salts 5 and 7–11 have been
measured in acetonitrile and the results are presented in Table 2.
These spectra feature intense absorptions in the UV region due to
→* intraligand charge-transfer (ILCT) transitions, together with
intense, broad d(RuII)→*(L) (L = pyridyl ligand) visible MLCT
bands. Data for the MLCT bands are also collected in Table 3
together with data for the other compounds studied in this work.8,9
As expected, methylation of the uncoordinated pyridyl nitrogen
in 1, 3–5 or 10 (to give 2, 6, 7, 9 or 11, respectively) leads to large
red-shifts in the MLCT bands (Tables 2 and 3). These red-shifts
are accompanied by substantial increases in intensity for 1 and 10.
Within the polyene series 6–9, an initial red-shift of the MLCT
Electrochemical studies
The new complex salts 5 and 7–11 were studied by cyclic voltam-
metry in acetonitrile and the results are presented in Table 2. All
of the complexes show reversible RuIII/II oxidation waves, together
2 9 3 8
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2

View Article Online
Table 4 MLCT absorption and Stark spectroscopic data for complex salts 1–4, 6–9 and 11–15a
c
d
2e
g
Salt

max/nm
Emax/eV
fos
12b/D
12 /D
ab /D
cb
Habf/cm−1
0 /10−30 esu
1
2
3
4
6
7
8
9
11
12
13
14
15
424
545
422
506
491
515
506
501
509
516
525
536
544
2.92
2.28
2.94
2.45
2.53
2.41
2.45
2.48
2.44
2.40
2.36
2.31
2.28
0.10
0.31
0.21
0.26
0.41
0.33
0.94
1.04
0.38
0.37
0.41
0.46
0.53
3.0
6.0
4.3
5.3
6.6
6.0
10.0
10.5
6.4
6.4
6.7
7.3
7.8
8.2
6.1
10.2
13.5
14.0
21.5
19.4
20.7
28.7
30.6
22.3
21.6
22.4
23.1
25.1
0.10
0.27
0.11
0.07
0.13
0.09
0.14
0.14
0.09
0.10
0.10
0.11
0.11
7000
8200
7300
4900
6900
5600
6900
6900
5600
5700
5700
5900
5700
10
50
28
11.1h
18.6
14.3
16.9
20.6
22.2
18.2
17.4
17.9
17.9
19.7
100
113
123
401
468
146
144
170
206
268
a Measured in butyronitrile glasses at 77 K. b Calculated from eqn (2). c Calculated from fint12 using fint = 1.33. d Calculated from eqn (1). e Calculated from
eqn (3). f Calculated from eqn (4). g Calculated from eqn (5). h Inferior data fit.
with irreversible pyridyl ligand-based reduction processes. Data for
the RuIII/II waves are also collected in Table 3 together with data for
the other compounds studied in this work.8,9
–* transition (this complex salt exhibits a true MLCT band at

2
max = 880 nm at 298 K in 1:1 glycerol–water).13 Although the cb
for 2 is smaller than that of its pentaammine analogue, it is still
considerably larger than those for the other compounds studied,
which at ca. 0.1, are consistent with MLCT processes. Within the
4,4′-bipyridinium-based series 6 and 12–15, 0 increases steadily,
approximately doubling on replacement of a N-Me with a N-(2-
Pym) substituent. As expected according to the two-state model,
this trend parallels the red-shifting of the MLCT bands, and similar
behaviour of 0 has been observed in the related RuII ammine com-
plexes of the same ligands.5c,d
The large red-shifts in the MLCT bands caused by methylation of
the uncoordinated pyridyl nitrogens in 1, 3–5 or 10 (to give 2, 6, 7,
9 or 11, respectively) are shown by the electrochemical data to arise
primarily from stabilisation of the ligand-based LUMOs. However,
the Ru-based HOMOs also show some stabilisation, especially
on moving from 1 to 2 in which the extent of electronic coupling
between the pyridinium nitrogen and the Ru centre is expected to
be the largest amongst the complexes studied. The corresponding
changes in the RuIII/II E1/2 values in the n = 1 or 3 compounds are only
small. Within the polyene series 6–9, the RuIII/II E1/2 value decreases
slightly on moving from n = 0 to 1, but subsequently shows smaller
decreases. This observation can be ascribed to the lessening influ-
ence of the electron-withdrawing N-methylpyridinium group as the
polyene chain extends. Replacing a trans-CHCH with a CC
bridge causes slight increases in the RuIII/II E1/2 values (see data for
the pairs 4/10 and 7/11), indicating that the RuII centres become less
electron-rich. This effect can be attributed to decreased -donating
and/or increased -accepting ability of the pyridyl ligand.
Structural studies
Having investigated the molecular quadratic NLO properties of
complex salts 1–4, 6–9 and 11–15, the next steps towards poten-
tially useful materials are the preparation and study of crystalline
samples in which bulk NLO effects may be observed. We have
obtained single crystal X-ray structures for 1·2.5MeCN, 4·2MeCN,
7 and 11. Representations of the molecular structures of the com-
plex cations are shown in Figs 1–4, respectively, and selected inter-
atomic distances and angles are presented in Table 5.
The dihedral angles between the pyridyl rings are as follows:
5.21(23)° (4·2MeCN), 12.31(62)° (7) and 12.66(3)° (11), in each
case consistent with significant electronic coupling throughout
the conjugated ligand systems. The ca. 0.03 Å shortening of the
Ru–N distance in 11 when compared with 7 can be attributed to a
greater extent of -back-bonding in the former due to the electron-
withdrawing nature of the ethynyl unit, consistent with the electro-
chemical data. Unfortunately, although the cations in 4·2MeCN,
Stark spectroscopic studies
We have studied complex salts 1–4, 6–9 and 11–15 by using Stark
spectroscopy in butyronitrile glasses at 77 K, and the results are
presented in Table 4. Satisfactory data fits could only be obtained
for the MLCT bands, and not for the high-energy ILCT bands of
these compounds. As observed previously with RuII ammine com-
plex salts,5e–i,13 the Emax values generally decrease on moving from
acetonitrile solution to butyronitrile glass (Tables 3 and 4), but an
opposite effect is observed for 2. Within the polyene series 6–9, the
12 values increase steadily with n, as is normal for donor–acceptor
polyenes, whilst a large increase in 12 is observed on moving from
n = 1 to 2. It is reasonable to assume that the lowest energy (i.e.
MLCT) electronic transitions will dominate the NLO responses
of these chromophores, so 0 values have been derived by using
eqn. (5). The 0 values for 6–9 appear to increase on moving from
n = 0 to 3, but the large estimated relative errors (±20%) on these
numbers mean that the values for the pairs 6/7 and 8/9 may not be
significantly different.
As expected, increasing the ligand electron acceptor strength by
N-methylation of 1, 3 and 4 causes increases in 0, this effect being
largest in the pyrazine complexes. The complex in 2 also has the
2
largest values of cb and Hab, consistent with this species having the
greatest degree of delocalization and electronic coupling of those
2
studied. The maximum possible value of cb (0.5) corresponds to
essentially complete delocalization of the orbitals involved in the
electronic transition. Such a situation has been observed for the
visible absorption band (max = 538 nm at 298 K in 1:1 glycerol-
water) of [RuII(NH3)5L][BF4]3 (L = N-methylpyrazinium), which
is therefore best described as arising from a non-directional
Fig. 1 Structural representation of the complex salt 1·2.5MeCN (50%
probability ellipsoids).
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2
2 9 3 9

View Article Online
Table 5 Selected interatomic distances (Å) and angles (°) for complex salts
1·2.5MeCN, 4·2MeCN, 7 and 11
1·2.5MeCN
4·2MeCN
7
11
Ru1–N1
Ru1–As1
Ru1–As2
Ru1–As3
Ru1–As4
Ru1–Cl1
2.118(10)
2.4270(15)
2.4290(15)
2.4320(15)
2.4259(15)
2.436(3)
2.114(4)
2.110(4)
2.081(5)
2.4133(7)
2.4228(7)
2.4122(7)
2.4179(7)
2.4127(11) 2.4094(7)
2.4210(12) 2.4163(7)
2.4226(12) 2.4210(7)
2.4179(12) 2.4159(7)
2.4292(14) 2.441(3)
2.4360(16)
N1–Ru1–As1
N1–Ru1–As2
N1–Ru1–As3
N1–Ru1–As4
94.0(3)
93.0(3)
93.9(3)
94.4(3)
92.52(13)
93.82(12)
90.96(13)
92.70(12)
84.90(2)
176.46(3)
95.26(2)
94.17(2)
92.27(16)
92.19(12)
91.63(13)
94.99(12)
94.43(13)
85.61(2)
172.72(3)
93.25(2)
95.39(3)
91.84(16)
94.67(16)
94.39(16)
85.51(4)
172.98(5)
93.16(4)
95.33(4)
173.67(5)
85.25(4)
As1–Ru1–As2 172.98(6)
As1–Ru1–As3
As1–Ru1–As4
As2–Ru1–As3
As2–Ru1–As4
84.71(5)
94.65(5)
94.60(5)
85.03(5)
173.46(3)
85.27(2)
173.87(3)
84.99(2)
As4–Ru1–As3 171.77(6)
N1–Ru1–Cl1
As1–Ru1–Cl1
As2–Ru1–Cl1
As3–Ru1–Cl1
As4–Ru1–Cl1
178.6(3)
87.30(8)
85.69(8)
86.95(8)
84.83(8)
178.47(13) 179.11(18) 178.59(13)
88.01(4)
87.66(4)
88.54(4)
85.81(4)
87.85(7)
87.29(8)
85.23(7)
86.48(8)
87.62(4)
86.96(5)
85.23(4)
86.97(5)
Fig. 2 Structural representation of the complex salt 4·2MeCN (50% prob-
ability ellipsoids).
Fig. 3 Structural representation of the complex salt 7 (50% probability
ellipsoids).
7 and 11 display relatively large quadratic NLO responses at the
molecular level, these salts all adopt centrosymmetric crystal pack-
ing structures, so are not expected to show any bulk NLO effects.
On the other hand, 1·2.5MeCN does crystallize non-centrosymmet-
rically, but the chromophore in this compound has a comparatively
low 0 value because the unmethylated pyrazine ligand is not an
especially strong -acceptor. However, we have previously reported
the crystal structure of the acetone solvate of salt 13 which adopts
the non-centrosymmetric orthorhombic space group Pna21.9 This
material may therefore be worthy of further NLO studies, such as
SHG screening or electro-optic measurements.
Fig. 4 Structural representation of the complex salt 11 (50% probability
ellipsoids).
and for their {RuII(NH3)5}2+ analogues5e,f,i are presented in Table 6.
Representative UV-VIS absorption spectra of the n = 2 complex salt
8 and of its {RuII(NH3)5}2+ analogue 8A are shown in Fig. 5.
In every case, the MLCT bands of the arsine complexes are
blue-shifted by ca. 0.6 eV when compared with those of their penta-
ammine analogues at 77 K. Previous comparative electrochemical
studies have shown that this effect is largely attributable to a marked
relative stabilisation of the Ru-based HOMOs in the arsine species.9
Although the molar extinction coefficients of the arsine complexes
are ca. 40–50% lower than those of their pentaammine counterparts
at room temperature,9 the fos and 12 values for the arsine chromo-
phores are actually higher in each case at 77 K, because their band
intensities increase substantially on freezing. For an example
comparison, the room-temperature 12 values for salts 8 and 8A are
5.5 and 6.8 D, respectively. It is noteworthy that the temperature
Comparisons with ruthenium(II) ammine complexes
We have found previously that the MLCT absorptions of complexes
with electron-donating trans-{RuIICl(pdma)2}+ centres are found to
high energy and have smaller molar extinction coefficients when
compared with those of their RuII ammine analogues.9 We had
therefore assumed that such arsine complexes will have consider-
ably smaller molecular quadratic NLO responses than their ammine
counterparts. The additional Stark spectroscopic data now available
allow us to make direct quantitative comparisons to test this prem-
ise. Selected data for the two series of complex salts 6–9 and 12–14
2 9 4 0
D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2

View Article Online
Table 6 MLCT absorption and Stark spectroscopic data for salts 6–9, 12–14 and their {RuII(NH3)5}2+ analogues.a
c
d
2e
g
Salt

max/nm
Emax/eV
fos
12b/D
12 /D
ab /D
cb
Hab/cm−1f
0 /10−30 esu
6
491
645
515
681
506
675
501
669
516
696
525
718
536
731
2.53
1.92
2.41
1.82
2.45
1.84
2.48
1.85
2.40
1.78
2.36
1.73
2.31
1.70
0.41
0.20
0.33
0.23
0.94
0.43
1.04
0.36
0.37
0.22
0.41
0.20
0.46
0.22
6.6
5.2
6.0
5.5
10.0
7.9
10.5
7.2
6.4
5.7
6.7
5.8
7.3
5.8
14.3
13.8
16.9
16.2
20.6
22.4
22.2
27.1
17.4
15.3
17.9
17.0
17.9
16.3
19.4
17.3
20.7
19.6
28.7
27.4
30.6
30.6
21.6
19.1
22.4
20.6
23.1
20.0
0.13
0.10
0.09
0.09
0.14
0.09
0.14
0.06
0.10
0.10
0.10
0.09
0.11
0.09
6900
4700
5600
4100
6900
4300
6900
3500
5700
4300
5700
3900
5900
4000
113
120
123
175
401
482
468
475
144
186
170
229
206
225
6Ah
7
7Ah
8
8Ai
9
9Ai
12
12Ah
13
13Ah
14
14Ah
a Measured in butyronitrile glasses at 77 K. Analogous {RuII(NH3)5}2+ complex salts denoted by XA (data for 15A not available). b Calculated from eqn (2).
c Calculated from fint12 using fint = 1.33. d Calculated from eqn (1). e Calculated from eqn (3). f Calculated from eqn (4). g Calculated from eqn (5). h Ref 5e.
i Refs 5f,i.
pentaammine salts are expected to be considerably larger than those
of their arsine counterparts. Nevertheless, the arsine complexes do
show reasonably large molecular quadratic NLO responses, which
are accompanied by considerable gains in visible transparency,
thermal stability and also crystallising ability when compared
with their pentaammine analogues. The electronic and optical
properties of the arsine complexes show trends which mirror those
observed previously in their ammine counterparts. In particular, the
unusual linear optical behaviour of RuII ammine pyridyl polyene
complexes is also found in the corresponding arsine species, but
clear evidence for an accompanying decrease in  on chain exten-
sion is not observed. Further studies (such as long wavelength HRS
and/or theoretical calculations) will be required to establish whether
the behaviour of the arsine pyridyl polyene complexes is actually
fundamentally different from that of the ammine species, and also to
derive a greater understanding of the effects of temperature changes
on the optical properties.
Fig. 5 UV/VIS absorption spectra of the salts 8 (dashed line) and 8A (full
line) at 298 K in acetonitrile.
Acknowledgements
We thank the EPSRC for support (a PhD studentship and grant
GR/M93864), and are also grateful to Dr Lathe A. Jones for the
improved synthesis of bis(4-pyridyl)acetylene (bpa).
dependence of 12 is much more pronounced for the arsine salts than
for the related ammine compounds.
The 12 values show no consistent variation between the two
types of complex, but are generally of similar magnitude (with
the exception of those for 9 and 9A). However, ab and cb2 are
1 Nonlinear Optical Properties of Organic Molecules and Crystals, ed.
generally a little larger and Hab is somewhat larger for the arsine
complexes. The values of , calculated from eqn. (5), are slightly
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Conclusion
MLCT absorption and electrochemical data clearly show that
a trans-{RuIICl(pdma)2}+ centre is less electron-rich than a
{RuII(NH3)5}2+ unit. Analyses of Stark spectroscopic data accord-
ing to the two-state model show that the 0 responses of the arsine
complexes are only a little smaller than those of the analogous
{RuII(NH3)5}2+ species in butyronitrile glasses at 77 K. However,
this somewhat unexpected result can be traced to changes in the
relative band intensities on freezing and is therefore not applicable
to normal room temperature conditions where the 0 values of the
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D a l t o n T r a n s . , 2 0 0 4 , 2 9 3 5 – 2 9 4 2
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