Synthesis, physical and photophysical properties of Ru(II) pyridyl-pyrimidine and pyridyl-pyrimidinone complexes: Effect of substituents on the pyrimidine ring

Article abstract of DOI:10.1016/S0020-1693(99)00321-7

[Ru(bpy)₂((EtO)₂pypmR)]²⁺ (R = H, Me and pypm = 2-(2-pyridyl)pyrimidine) forms by ethoxide substitution of 4,6-halo groups when Ru(bpy)₂CO₃·2H₂O reacts with 4,6-dihalo-2-(2-pyridyl)pyrimidi

Full text of DOI:10.1016/S0020-1693(99)00321-7

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Inorganica Chimica Acta 296 (1999) 114–126
Synthesis, physical and photophysical properties of Ru(II)
pyridyl-pyrimidine and pyridyl-pyrimidinone complexes: effect of
substituents on the pyrimidine ring
Wen-Mei Xue, W.J. Perez, D. Paul Rillema *
The Department of Chemistry, Wichita State Uni6ersity, Wichita, KS 67260-0051, USA
Received 13 May 1999
Abstract
[Ru(bpy)2((EtO)2pypmR)]2+ (R=H, Me and pypm=2-(2-pyridyl)pyrimidine) forms by ethoxide substitution of 4,6-halo
groups when Ru(bpy)2CO3·2H2O reacts with 4,6-dihalo-2-(2-pyridyl)pyrimidine in ethanol. When these ligands react with
Ru(bpy)2Cl2·2H2O in acetone, [Ru(bpy)2(ClpypnR)]+ (R=H, Me, Ph and pypn=2-(2-pyridyl)pyrimidinone) forms by a
hydrolytic reaction. The complexes give rise to metal-to-ligand charge transfer (MLCT) absorptions in the 320–480 nm region and
p“p* transitions below 300 nm. The lowest energy dp“p* absorptions of [Ru(bpy)2(ClpypnR)]+ shift to the red region of the
spectrum when compared with those of [Ru(bpy)2(pypmR)]2+ and [Ru(bpy)2((EtO)2pypmR)]2+. Emission spectra maxima are
observed from 606 to 653 nm with the high energy absorptions associated with the pypm derivatives and the low energy
absorptions associated with the pypn derivatives. The oxidation potentials for Ru3+/2+ couples vary from +1.33 to +0.95 V
versus SSCE; ClpypnR ligands make Ru2+ more easily oxidizable. Reductions are ligand-centered and sequential for each ligand
p* system; the order of decreasing ease of reduction in the complexes is pypmRꢀ(EtO)2pypmR\ClpypnR (R=H, Me)\
ClpypnPh. Excited-state Ru2+*/+ couples range from +0.77 to +0.46 V; those of Ru3+/2+* range from −0.56 to −0.96 V.
A variable-temperature emission lifetime study reveals the presence of a low-lying 3dd state with an energy gap between the
3MLCT and the 3dd (DE) ranging from 1800–3920 cm−1, the thermal deactivation rate constants from 3dd (k1) vary from
1.4×1010 to 5×1014 −1. The ground state pKa values of the complexes determined by spectrometric titration range from −4.1
s
to −6.7. The excited state pKa*(app) values measured by fluorescence titration range from 0.5 to 3.03. © 1999 Elsevier Science
S.A. All rights reserved.
Keywords: Pyrimidine ring; Pyridyl-pyrimidine complexes; Pyridyl-pyrimidinone complexes; Ru(II) complexes; Oxidation potentials; MLCT
absorptions
molecules can be obtained by examining the ground-
state and excited-state behavior of molecules.
1. Introduction
In the past, most studies of this type were devoted to
organic molecules. Only recently have reports appeared
focused on inorganic compounds. Some initial studies
of transition metal complexes have been reviewed by
Vos [1]. We [2] and others [3] have been concerned
about ruthenium(II) complexes containing ligands with
attached carboxylic acid groups or heterocyclic ligands
with remote nitrogen donor atoms. These complexes
fall into the MLCT classification where the basicity of
the ligand is increased in the excited state compared to
the ground state.
The excited-state acid–base properties of compounds
can differ significantly from those in the ground state.
These changes can be explained by the difference in
electron distribution in the ground state and the excited
state. In the case of metal-to-ligand charge transfer
(MLCT), the metal center becomes more acidic and the
ligand becomes more basic. However, the opposite is
true for ligand-to-metal charge transfer (LMCT). Thus,
important information about the electronic structure of
We have previously reported the acid–base proper-
ties of a series of ruthenium(II) complexes where the
* Corresponding author.
0020-1693/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved.
PII: S0020-1693(99)00321-7

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
115
ligands have been systematically varied from 2,2%-
bipyridine (bpy) to 2,2%-bipyrimidine (bpm) to 2,2%-
bipyrazine (bpz) to 2-(2-pyridyl)pyrimidine. In this
paper we examine derivatives of 2-(2-pyridyl)pyrimidine
complexes where substituents on the pyrimidine ring
have been varied to demonstrate the ability of further
adjusting excited state acid–base properties. In addition
to the insight provided concerning that issue, we also
uncovered new chemistry related to the ease of sub-
stituent substitution on the pyrimidine ring of deriva-
tized 2-(2-pyridyl)pyrimidine ligands coordinated to
ruthenium(II), which under normal conditions for the
free ligand are difficult to carry out.
solvent was removed by rotary evaporation and the
solid was recrystallized from toluene.
1. Anal. Calc. for C30H25N7RuP2F12: C, 41.20; H,
2.88; N, 11.21. Found: C, 42.00; H, 3.80; N, 10.36%.
MS (positive FAB): m/z 730 (M++PF6), 585 (M+).
1H NMR (CD3COCD3): 2.24 (s, 3H, CH3), 7.56 (m,
4H), 7.65(m, 1H), 8.02(m, 4H), 8.21(m, 8H), 8.80(m,
4H), 8.97(d, 1H).
2. Anal. Calc. for C33H31N7O2RuP2F12: C, 41.78; H,
3.29; N, 10.34. Found: C, 41.46; H, 2.91; N, 10.36%.
MS (positive FAB): m/z 804 (M++PF6), 659 (M+).
1H NMR (CD3COCD3): 0.71 (t, 3H, CH3), 1.44 (t, 3H,
CH3), 3.84–3.99 (m, 2H, CH2), 4.67 (q, 2H, CH2), 6.49
(s, 1H), 7.51 (m, 2H), 7.60 (m, 3H), 7.90 (m, 2H), 8.00
(m, 2H), 8.09–8.23 (m, 5H), 8.48 (d, 1H), 8.79 (m, 4H),
8.90 (d, 1H).
2. Experimental
3. Anal. Calc. for C34H33N7O2RuP2F12: C, 42.42; H,
3.46; N, 10.18. Found: C, 43.58; H, 2.54; N, 10.54%.
MS (positive FAB): m/z 963 (M++2PF6), 818 (M++
2.1. Materials
1
PF6), 673 (M+). H NMR (CD3COCD3): 0.72 (t, 3H,
The cpmpounds 5-methyl-2-(2-pyridyl)pyrimidine
(Mepypm) [4], 4,6 - dichloro - 2 - (2 - pyridyl)pyrimidine
(Cl2pypm) [4], 4,6 - dibromo - 2 - (2 - pyridyl)pyrimidine
(Br2pypm) [4], 4,6-dichloro-5-methyl-2-(2-pyridyl)pyri-
midine (Cl2pypmMe) [4], 4,6 - dibromo - 5 - methyl - 2-
(2-pyridyl)pyrimidine (Br2pypmMe) [4], 4,6-dichloro-5-
phenyl - 2 - (2 - pyridyl)pyrimidine (Cl2pypmPh) [4],
Ru(bpy)2Cl2·2H2O [5] and Ru(bpy)2CO3·2H2O [6] were
prepared according to literature methods. Commer-
cially purchased tetrabutylammonium hexafluorophos-
phate (TBAH) was of electrometric grade (Sachem) and
was used without further purification. All solvents were
purchased commercially as HPLC grade and were used
without further purification. Sulfuric acid (96.0%) was
reagent A.C.S. grade from Fischer Scientific. Distilled
water was further purified by passage through a Mil-
lipore purification train.
CH3), 1.48 (t, 3H, CH3), 2.07 (s, 3H, CH3-pypm), 3.62
(m, 1H, CH2), 3.88 (m, 1H, CH2), 4.72 (m, 2H, CH2),
7.51 (m, 2H), 7.58 (m, 3H), 7.90 (m, 2H), 8.02 (m, 1H),
8.07 (m, 1H), 8.12–8.24 (m, 5H), 8.54 (m, 1H), 8.77–
8.84 (m, 4H), 8.88 (m, 1H).
2.3. [Ru(bpy)2(ClpypnR)](PF6) (R=H, 4; R=Me, 5;
R=Ph, 6)
A 0.20 g (0.38 mmol) sample of Ru(bpy)2Cl2·2H2O
was dissolved in 100 ml acetone and the solution was
stirred at room temperature for 15 min, after which
0.24 g (0.94 mmol) of AgPF6 was added. The solution
was allowed to reflux for 2 h, after which the precipi-
tated AgCl was removed by filtration. To the filtrate
was added 0.76 mmol of ligand in 10 ml acetone (0.17
g of 4,6-dichloro-2-(2-pyridyl)pyrimidine (Cl2pypm), or
0.18 g of 4,6-dichloro-5-methyl-2-(2-pyridyl)pyrimidine
(Cl2pypmMe), or 0.23 g of 4,6-dichloro-5-phenyl-2-(2-
pyridyl)pyrimidine (Cl2pypmPh)), and the solution was
refluxed for 3 h. The reaction mixture was cooled to
room temperature and filtered; the solvent was reduced
via a rotary evaporator to a volume of approximately 5
ml, which was then chromatographed using a neutral
alumina column with a 3:1 chloroform–acetone mix-
ture as the eluant. The first orange-colored fraction was
collected, and the solvent was reduced to a volume of
approximately 5 ml. The pure product was precipitated
by addition of diethyl ether to the solution, collected by
suction filtration and dried in a vacuum oven at room
temperature.
2.2. Preparation of [Ru(bpy)2(pypmMe)](PF6)2 (1) and
[Ru(bpy)2((EtO)2-pypmR)](PF6)2 (R=H, 2; R=Me, 3)
A 0.051 g (1.0×10−4 mol) sample of Ru(bpy)2-
(CO3)·2H2O was dissolved in 5 ml of absolute ethanol.
A slight stoichiometric excess of the ligand pypmMe,
Cl2pypm or Cl2pypmMe was added and the reaction
mixture was heated at reflux for 2 h. The color of the
solution changed from purple to reddish-orange. The
reaction mixture was cooled to room temperature and 2
ml of an aqueous solution containing saturated
NH4PF6 was added. The solution volume was slowly
reduced using rotary evaporation until precipitation
was complete. The solid was collected by suction filtra-
tion and was purified by passing it over a neutral
alumina column previously developed with methylene
chloride. The red–orange fraction eluted from the
column using a 10:0.5 ratio of CH2Cl2/CH3OH. The
4. Yield: 0.21 g (71%). Anal. Calc. for C29H21-
N7ClORuPF6: C, 45.53; H, 2.77; N, 12.82. Found: C,
45.66; H, 2.96; N, 12.70%. MS (positive FAB): m/z
620 (M+). 1H NMR (CDCl3): 6.02 (s, 1H), 7.17
(m, 1H), 7.43 (m, 4H), 7.56 (d, 1H), 7.63 (m, 2H),

116
W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
7.70 (dd, 1H), 7.83 (m, 2H), 7.97 (m, 3H), 8.15 (d, 1H),
8.24 (d, 1H), 8.35 (dd, 1H), 8.45 (d, 2H), 8.60 (dd, 1H).
Excited state lifetimes were determined by exciting
the samples at 450 nm using an Opotek optical para-
5. Yield: 0.20
g
(67%). Anal. Calc. for
metric oscillator pumped by
a
frequency-tripled
C30H23N7ClORuPF6: C, 46.25; H, 2.98; N, 12.59; Cl,
4.55. Found: C, 46.68; H, 3.18; N, 12.16; Cl, 5.13%. MS
Nd:YAG laser (continuum surlite, run at 1.5 mJ per 10
ns pulse). Spectral regions were isolated using a Hama-
matsu R955 PMT in a cooled housing (−15°C,
Amherst) coupled to an Acton SpectraPro 275
monochromator. Variable emission lifetimes from 160
to 330 K were determined by adding a Cryo Industries
EVT cryostat controlled by a Lakeshore 805 tempera-
ture controller to the system above. The cryostat was
modified in-house by adding a large copper thermal
mass and then calibrated with an auxiliary thermocou-
ple using ice-cold water as the reference junction.
The ground state pKa was determined by spectro-
metric titration. Sample solutions of approximately 3×
10−5 M were prepared by mixing appropriate volumes
of concentrated H2SO4 with the same volume of the
complex stock solution and diluting with deionized
water in order to obtain acidities around the expected
pKa value. Absorption spectra were measured on air-
equilibrated solutions in the same cuvette; deionized
water served as the solvent blank.
The excited state pKa*(app) was measured by
fluorescence titration. Air-equilibrated aqueous solu-
tions of different pH values, but a constant sample
concentration (approximately 3×10-5 M) and constant
ionic strength of 0.1 N, were prepared by adding appro-
priate volumes of diluted H2SO4 with the same volume
of the complex stock solution and diluting with 0.1 N
Na2SO4 aqueous solution. The pH values were deter-
mined using a Mettler–Toledo MP 220 pH meter at
294 K with a glass electrode calibrated with standard
buffers.
1
(positive FAB): m/z 634 (M+). H NMR (CDCl3): 2.17
(s, 3H, CH3), 7.17 (t, 2H), 7.43 (m, 4H), 7.63 (m, 3H),
7.81 (q, 2H), 7.96 (m, 3H), 8.15 (d, 1H), 8.25 (d, 1H),
8.40 (m, 3H), 8.57 (d, 1H).
6. Yield: 0.24 g (74%). Anal. Calc. for C35H25-
N7ClORuPF6: C, 49.98; H, 3.00; N, 11.66; Cl, 4.21.
Found: C, 49.65; H, 3.37; N, 11.33; Cl, 4.04%. MS
1
(positive FAB): m/z 696 (M+). H NMR (CDCl3): 7.05
(t, 1H), 7.19–7.29 (m, 7H), 7.40 (m, 3H), 7.47 (q, 2H),
7.59 (m, 3H), 7.69 (t, 1H), 7.85 (t, 1H), 7.95 (m, 3H),
8.07 (d, 1H), 8.22 (d, 1H), 8.38 (t, 2H), 8.50 (d, 1H),
8.60 (d, 2H).
2.4. Physical measurements
1H NMR spectra were obtained with a Varian Inova
400 FT-NMR spectrometer. Elemental analyses were
carried out by M-H-W Laboratories, Phoenix, AZ.
Mass spectra were measured by the Mass Spectroscopy
Laboratory, University of Kansas. UV–Vis spectra
were obtained with a Hewlett–Packard 8452A diode
array spectrophotometer. Cyclic voltammograms were
obtained with an EG&G PAR 263A potentiostat/gal-
vanostat. The measurements were made using a plat-
inum disk working electrode, a platinum gauze counter
electrode, and an Ag/AgNO3 (0.1 M in acetonitrile)
reference electrode and recorded with an IBM 325T
computer. The supporting electrolyte was 0.1 M
TBAH. Ferrocene was added as an internal standard.
All samples were purged with argon prior to measure-
ment. Emission spectra were obtained for each complex
in a 4:1 ethanol–methanol solution at room tempera-
ture and in the glassy state at 77 K with a Spex
Fluorolog 212 spectrofluorometer. All emission spectra
were corrected for instrument response. Absolute emis-
sion quantum yields were measured by the method of
Demas and Crosby [7] using [Ru(bpy)3]2+ as the stan-
dard where the emission quantum yield is 0.089 in 4:1
ethanol–methanol with 436 nm excitation [8]. Samples
and standard solutions were purged with argon before
measurement. The quantum yield of the sample was
determined according to Eq. (1),
3. Results
3.1. Preparation of compounds
The structure and abbreviations of the ligands are
shown in Fig. 1. The preparation of [Ru(bpy)2-
(pypmMe)]2+ was carried out by the method reported
for other [Ru(bpy)2(L–L)]2+ complexes using Ru-
(bpy)2CO3·2H2O as the starting material [9]. However,
when 4,6-dichloro- and 4,6-dibromo-2,2-(pyridyl)-
pyrimidine were reacted with Ru(bpy)2CO3·2H2O by
the same procedure, a nucleophilic displacement of
4,6-halo substituents by ethoxy groups which came
from the solvent occurred (Scheme 1). The products
were well characterized by elemental analysis, positive
FAB mass spectroscopy, and proton NMR spec-
troscopy. The 1H NMR spectrum of 1 had a single
peak at 2.24 ppm assigned as originating from CH3 in
the 5-position of the pyrimidine ring; it moved to 2.07
ppm in 3. A multiplet near 4.72 ppm from one CH2
ƒs=ƒr(Br/Bs)(Ds/Dr)
(1)
where the subscripts s and r refer to the sample and
standard reference solution, respectively; D is the inte-
grated emission intensity and ƒ is the luminescence
quantum yield. The quantity B is calculated by B=1−
10−AL, where A is the absorbance at the excitation
wavelength and L is the optical pathlength.

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
117
Fig. 1. Structure and abbreviations of the ligands.
group and a triplet around 1.48 ppm from the CH3
group of one ethoxide were also observed in the H
CH2 groups, and two groups of triplet signals around
1.47 and 0.71 ppm were observed for the two CH3
groups.
When 4,6-dichloro-2-(2-pyridyl)pyrimidine reacted
with Ru(bpy)2Cl2·2H2O in acetone following the proce-
dure for preparation of [Ru(bpy)2(pypm)]2+ in an
attempt to synthesize [Ru(bpy)2(Cl2pypmR)]2+, an un-
expected, but not unreasonable hydrolytic reaction in
1
NMR spectrum of 3. Similar absorptions of the other
ethoxy group appeared as two sets of multiplets located
at 3.62 and 3.88 ppm and as a triplet located at 0.72
ppm. The ethoxide group signals of 2 were similar to
that of 3; a quartet around 4.68 and a multiplet located
in the 3.84–3.99 ppm range were observed for the two
Scheme 1.
Scheme 2.

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W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
Fig. 2. UV–Vis absorption spectra of [Ru(bpy)2(pypmMe)]2+ (1, —), [Ru(bpy)2((EtO)2pypmMe)]2+ (3, - - - -) and [Ru(bpy)2(ClpypnMe)]+
(5, . . . . .) in 4:1 ethanol–methanol at room temperature.
the 4- or 6-position of the pyrimidine ring was ob-
served. The proposed reaction mechanism is shown in
Scheme 2 where pyrimidine was converted into pyrim-
idinone while coordinated to Ru(II). The products were
confirmed by elemental analysis, positive FAB mass
reported that the dp“p1*(bpy) and dp“p1*(pypm)
transitions could not be separated from one another
because the absorption maxima of the first MLCT
(dp“p1*) bands are similar in energy for the various
1
spectroscopy and H NMR spectroscopy.
Table 1
UV–Vis spectral data for ruthenium complexes at room temperature a
3.2. Electronic spectra
Complex
umax (nm) (m×104 M−1 cm−1
)
The absorption spectra of [Ru(bpy)2(pypmMe)]2+
(1), [Ru(bpy)2((EtO)2pypmMe)]2+ (3) and [Ru(bpy)2-
(ClpypnMe)]+ (5) are shown in Fig. 2 for comparison.
The lowered symmetry of the complexes removes the
degeneracy of the p* levels, resulting in the appearance
of broad MLCT bands. The UV–Vis absorption data
and assignments of Ru(II) complexes reported here
along with those of [Ru(bpy)3]2+ and [Ru(bpy)2
-(pypm)]2+ are listed in Table 1. The assignments of
the electronic absorption spectra were made on the
basis of the well-documented optical transitions in
[Ru(bpy)3]2+, where a set of low-energy transitions
were assigned as a MLCT transition (dp“p1*) and
another set of transitions located approximately 7000
cm−1 higher in energy were assigned as the second
MLCT band (dp“p2*) [10]. For mixed-ligand com-
plexes, the interpretation of absorption bands is com-
plex since there are multiple dp“p1* and dp“p2*
transitions. From Table 1 and Fig. 2 it is found that the
dp“p1* bands of [Ru(bpy)2(pypm)]2+, [Ru(bpy)2-
(pypmMe)]2+ (1), [Ru(bpy)2((EtO)2pypm)]2+ (2) and
[Ru(bpy)2((EtO)2pypmMe)]2+ (3) are similar in energy
(around 450 nm), and they are also similar to that of
[Ru(bpy)3]2+; it is difficult to separate the dp“
p1*(bpy) and dp“p1*((EtO)2pypmR) transitions into
individual assignments. This was expected since it was
d“p1*
d“p2*
p“p*
[Ru(bpy)2(pypm)]2+ b 449 (1.1) 317 (sh)
285 (5.2)
257 (2.0)
246 (2.3)
425 (sh)
1
2
448 (1.3) 326 (sh, 0.99) 286 (7.0)
420 (1.1) 246 (2.7)
454 (1.4) 348 (sh, 0.75) 288 (7.3)
436 (1.3) 256 (sh, 2.3)
244 (2.6)
454 (1.3) 350 (sh, 0.63) 288 (7.2)
434 (1.1) 254 (sh, 2.1)
238 (2.7)
472 (1.4) 354 (sh, 0.89) 290 (6.6)
3
4
5
446 (1.2) 320 (sh, 1.5)
256 (3.0)
248 (2.9)
472 (1.1) 360 (sh, 0.90) 292 (4.8)
448 (1.0) 340 (sh, 1.2)
322 (sh, 1.4)
256 (2.4)
246 (2.4)
234 (2.3)
292 (2.9)
256 (1.3)
244 (1.4)
285 (8.7)
250 (2.5)
238 (3.0)
6
476 (0.65) 326 (0.83)
446 (0.55)
[Ru(bpy)3]2+ c
451 (1.4) 345 (0.65)
323 (0.65)
a In 4:1 ethanol–methanol.
b From data reported in Ref. [11].
c From Ref. [24].

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
119
Table 2
Redox potentials of ruthenium complexes at room temperature a
Complex
E1/2 (V) (DEp, mV)
3+/2+*
1/2
2+*/+
Oxidation (RuIII/II
)
Reduction
E
E
1/2
E1/2
E1/2(1)
E1/2(2)
E1/2(3)
[Ru(bpy)2(pypm)]2+ b
1
2
3
+1.35
−1.20
−1.49
−1.73
−0.56
−0.72
−0.74
−0.76
−0.94
−0.96
−0.94
−0.72
0.71
0.77
0.73
0.72
0.48
0.46
0.47
0.68
+1.33(78)
+1.25(82)
+1.26(86)
+0.98(73)
+0.95(50)
+0.97(82)
+1.27
−1.28(76)
−1.26(80)
−1.30(72)
−1.44(82)
−1.46(78)
−1.44(73)
−1.31
−1.49(74)
−1.51(86)
−1.51(76)
−1.72(96)
−1.76(92)
−1.73(119)
−1.50
−1.73(82)
−1.81(92)
−1.79(90)
4
5
6
[Ru(bpy)3]2+ c
−1.77
a All samples measures in 0.1 M TBAH/CH3CN; potentials are vs. SSCE; error in potentials was 0.02 V; scan rate=200 mV (s).
b From data reported in Ref. [11].
c From Ref. [24].
and [Ru(bpy)3]2+; the oxidation potentials ranged from
+1.25 to +1.35 V; the three reductions ranged from
−1.20 to −1.31, −1.49 to −1.51 and −1.73 to
−1.81 V, respectively. However, complexes 4–6 con-
taining minus one charged pypn ligands differed
markedly in their redox potentials from the previous
three: E1/2(RuIII/II) varied between +0.95 and +0.98
V; only two reductions were observed which were
nearly constant in each series, E1/2(1) ca. −1.44 V,
E1/2(2) ca. −1.76 V; E1/2(3) was not observed in the
solvent window.
Ru(II) complexes containing bpy and pypm ligands
[11]. However, the dp“p1* band of [Ru(bpy)2-
(ClpypnR)]+ (4–6) is red-shifted about 1000 cm−1
from the others. The assignment for this transition will
be made in Section 4.
Introducing a methyl group in the 5-position of
pyrimidine, 4,6-diethoxy-pyrimidine and 4-chloro-6-
pyrimidinone did not have a great effect on the UV–
Vis absorption spectra when comparing the data of
[Ru(bpy)2(pypm)]2+ with 1, 2 with 3, and 4 with 5.
Introducing a 5-phenyl group lowers the lowest unoccu-
pied molecular orbital energy but only shifts the lowest
energy absorption from 472 (4 and 5) to 476 nm (6).
The shift is small compared to reported Ru(II) tris-
bipyridyl complexes where 4,4%-methyl substituents red-
shifted the dp“p1* band from 450 to 455 nm and
4,4%-phenyl substituents red-shifted it to 473 nm [11].
So, the lowest energy MLCT absorption of the com-
plexes studied here is not very sensitive to the electronic
properties of 5-substituents. Hence, it is not a good
position to vary substituents for tuning ground and
excited state electronic properties.
3+/2+*
2+*/+
E1/2
and E1/2
were calculated by Eqs. (2)
and (3), respectively, where
E13/2+/2+*=E1/2(RuIII/II)−Eem
E12/2+*/+ =E1/2(1)+Eem
(2)
(3)
E1/2(RuIII/II) is the reduction potential for the Ru(III/II)
couple, E1/2(1) is the first reduction potential and Eem is
the excited-state luminescent energy in electron volts
(determined from the luminescence maximum at room
temperature). The excited-state redox potentials indi-
cate that the complexes can behave as both photooxi-
dants and photoreductants. As photoreductants, the
driving force varied from −0.56 to −0.96 V and is in
the order: pypmRꢀ(EtO)2pypmR\ClpypnR, whereas
as photooxidants, the potentials (E12/2+*/+) varied from
0.77 to 0.46 V in the order: pypmRꢀ(EtO)2pypmR\
ClpypnR (R=Me, H)ꢀClpypnPh.
In summary, the profiles of the lowest energy dp“p*
transition did not differ significantly between the com-
plexes and those of [Ru(bpy)3]2+ but the transition
energies varied in the order: [Ru(bpy)3]2+
ꢀ
\
[Ru(bpy)2(pypmR)]2+ꢀ[Ru(bpy)2((EtO)2pypmR)]2+
[Ru(bpy)2(ClpypnR)]+. Substituents located in the 5-
position on the pypm and pypn ligands did not influ-
ence the lowest energy transition significantly.
3.4. Luminescence properties
3.3. Electrochemical properties
The emission spectra of the complexes at 77 K
displayed vibrational components whereas the com-
plexes showed broad emission at room temperature
similar to those reported for [Ru(bpy)3]2+ [8,12]. The
emission maxima and lifetimes at 77 and 298 K, as well
as quantum yields at 298 K, are tabulated in Table 3.
Redox properties were determined by cyclic voltam-
metry and are summarized in Table 2. The positions
and shapes of the cyclic voltammograms did not differ
greatly between complexes 1–3, or [Ru(bpy)2(pypm)]2+

120
W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
Fig. 3 shows a comparison of the glassy-state emission
spectra of 1, 3 and 5 at 77 K in 4:1 ethanol–methanol.
The emission maxima at 298 K fell in the range 606
nm for [Ru(bpy)2(pypmMe)]2+ (1) to 653 nm for
[Ru(bpy)2(ClpypnMe)]+ (5); emission quantum yields
varied from 0.056 for the former to 0.0015 for the
latter. The emission lifetimes at room temperature in
4:1 ethanol–methanol were around 0.40 ms except for
[Ru(bpy)2((EtO)2pypm)]2+ (2) which was relatively
short-lived with a ~ of 0.13 ms. The order of emission
energy both at room temperature and at 77 K was:
plotted versus the absolute temperatures, and fit to Eq.
(4) by using the program ENZFITTER
,
k
obs=~−1=ko+k1 e−DE/RT
(4)
where ko is the sum of radiative and nonradiative rate
constants (ko=kr+knr), DE is the energy gap between
two excited states, and k1 is the thermal deactivation
rate constant between two energy levels. Fig. 4 shows a
plot of kobs versus T for each complex. A fit of the data
to Eq. (4) permits the evaluation of ko, k1 and DE. The
parameters obtained are listed in Table 4 together with
kr and knr which were calculated from kr=ƒem/~ and
knr=1/~−kr. The energy gap (DE) for all compounds
fell in the range 1800–3920 cm−1 and ko in the range
1.11×106 to 3.8×106 s−1. The thermal deactivation
[Ru(bpy)2(pypmR)]2+\[Ru(bpy)2((EtO)2pypmR)]2+
\
[Ru(bpy)2(ClpypnR)]+. A linear correlation was found
between the emission and the lowest MLCT absorption
energies at 298 K with a correlation coefficient of 0.97,
indicating that the emission was derived from the low-
est energy MLCT excited state.
rate constants k1 varied from 1.4×1010 to 5×1014 s−1
.
3.6. Protonation of the ground states
3.5. Temperature-dependent emission lifetimes
Inasmuch as the pH scale does not accurately reflect
the activity of H+ in highly acidic solution, the Ham-
mett acidity function (Ho) was used instead of pH for
The temperature-dependent emission lifetimes ob-
tained in 4:1 ethanol–methanol for each complex were
Table 3
Emission maxima, quantum yields and excited-state lifetimes in 4:1 ethanol–methanol at 298 and 77 K
Complex
298 K
77 K
lmax (nm)
t (ms) a
ƒ
em
umax (nm)
~ (ms) a
1
2
3
4
5
6
606
624
612
645
653
650
0.45
0.13
0.51
0.37
0.40
0.36
0.056
0.0085
0.0015
0.016
0.015
0.015
577, 620
589, 634
585, 630
603, 647
608, 653
606, 653
5.04
4.02
3.30
4.79
4.50
5.04
a Error= 9 0.01
Fig. 3. Emission spectra of [Ru(bpy)2(pypmMe)]2+ (1, —), [Ru(bpy)2((EtO)2pypmMe)]2+ (3, - - - -) and [Ru(bpy)2(ClpypnMe)]+ (5, . . . . .) at 77
K in 4:1 ethanol–methanol.

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
121
Fig. 4. Temperature-dependent lifetime data of the complexes with a computer-generated fit using Eq. (4) and the parameters in Table 4, (a)–(f)
represents complexes 1–6, respectively.
Table 4
Kinetic decay parameters
b
c
Complex
T (K) a
ko (s−1
)
k1 (s−1
)
DE (cm−1
)
kr (s−1
)
knr (s−1
)
1
2
3
4
5
6
210–330
200–330
160–290
200–330
200–330
200–330
(1.1190.05)×106
(3.890.1)×106
(1.490.5)×106
(1.7790.08)×106
(2.0090.08)×106
(1.9190.09)×106
(391)×1013
(592)×1014
3570980
3920990
3920990
25009200
18009100
2960950
1.3×105
6.6×104
3.0×103
4.3×104
3.8×104
4.2×104
2.1×106
7.7×106
2.0×106
1.4×106
2.5×106
2.7×106
(1.190.4)×1013
(291)×1011
(1.490.8)×1010
(2.690.6)×1012
a Temperature range studied.
b kr=ƒem/~.
c knr=1/~−kr.
[Ru(bpy)2((EtO)2pypmMe)]2+ (3) in the visible region.
There are two isobestic points at 417 and 490 nm. The
other complexes gave comparable results. In general for
the complexes studied here, the color of the solutions
changed from yellow to lilac with increasing acidity; the
lowest energy MLCT absorption band decreased in
protonation of the ground states. Values of Ho as a
function of the weight percent of H2SO4 are given in
the literature [13].
The absorption spectra of the complexes changed as
their solutions were made increasingly acidic. Fig. 5
shows a typical spectral change for protonation of

122
W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
intensity while a new band, red-shifted by 100 nm,
appeared. The spectral shifts can be attributed to the
protonation of the uncoordinated nitrogen bases on the
pyrimidine and pyrimidinone ligands with the concomi-
tant stabilization of the p* orbitals. Based on the fact
that the MLCT absorption bands of [Ru(bpy)3]2+ re-
mained unchanged in 0–96% H2SO4 [14], it is unlikely
that solvatochromic effects play any part in the spectral
changes seen here.
pypmMe)]2+ (3) and [Ru(bpy)2(Clpypn)]+ (4) are
shown in Fig. 6.
The pKa values for the protonated complexes ranged
from −4.1 to −6.7; the ground state complexes are
very weak bases, requiring high concentrations of acid
to achieve protonation. The values are of the same
order of magnitude as previously observed for other
similar complexes [15].
Plots of absorbance versus Ho were constructed from
the data, and the titration curves were fit to Eq. (5)
using the ENZFITTER program,
3.7. Protonation of the excited-states
The lifetimes and the emission intensities of the lu-
minescent states decreased sharply in acidic solutions.
No emission was observed from the protonated species
as shown in Fig. 7. Fig. 8 shows pH dependencies of
the emission intensity and the lifetime for [Ru(bpy)2-
(pypmMe)]2+ (1), [Ru(bpy)2((EtO)2-pypm)]2+ (2) and
[Ru(bpy)2(ClpypnPh)]+ (6); ~o and Io values in neutral
solution were taken as the standards of comparison.
The variations of I and ~ as a function of pH were
A=(a+b×10x−c)/(1+10x−c
)
(5)
where A is the absorbance of the solution, a and b are
the plateau absorbance values of the protonated and
unprotonated species, respectively, c is the mid-point of
the titration curve (pKa), and x is the value of Ho. The
pKa values are listed in Table 5; examples of curve
fittings for [Ru(bpy)2pypmMe)]2+ (1), [Ru(bpy)2((EtO)2-
Fig. 5. Typical visible absorption spectral change with increasing acidity (direction of arrows) in the order of Ho of −1.96, −5.18, −5.92,
−6.23, −6.71, −7.05 and −7.84 for [Ru(bpy)2((EtO)2pypmMe)]2+ (3).
Table 5
Values of monitoring wavelength, pKa, pKa*(app) and kq for the complexes at room temperature
Complexes
umon (nm) a
pKa
~
(ms) b
pKa*(app, I) c
pKa*(app, ~) d
kq (s−1
)
o
[Ru(bpy)2(pypm)]2+ e
540
550
550
550
550
550
578
−4.490.3
−4.190.1
−4.190.1
−6.790.1
−6.090.1
−5.690.1
−6.490.1
2.4
1
2
3
4
5
6
0.43
0.12
0.062
0.13
0.22
0.19
3.0490.01
1.5290.05
0.490.1
2.7190.04
1.9390.03
2.4190.02
3.0390.01
1.6490.02
0.590.1
2.7690.03
2.0090.04
2.4590.01
(2.5590.06)×109
(2.890.9)×108
(591)×107
(4.090.5)×109
(3.990.7)×108
(1.490.1)×109
a Monitoring wavelength for pKa determination.
b In 0.1 N Na2SO4 aqueous solution at 298 K.
c Determined by emission intensity.
d Determined by lifetime data.
e From the data reported in Ref. [15].

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
123
Fig. 6. Plots of absorbance vs. Ho with a computer-generated fit using Eq. (6) for [Ru(bpy)2pypmMe)]2+ (1, ), [Ru(bpy)2((EtO)2pypmMe)]2+
(3, ꢀ) and [Ru(bpy)2(Clpypn)]+ (4, ꢁ).
Fig. 7. Emission spectrum of [Ru(bpy)2pypmMe)]2+ (1) as a function of pH at room temperature. The emission intensity decreases as pH
decreases in the order of 5.78, 4.18, 3.56, 2.95, 2.67, 1.81 and 0.91.
exactly parallel. By fitting the data using Eq. (5), where
A is I/Io or ~/~o, a and b are the plateau I/Io or ~/~o
values of the protonated and unprotonated species,
respectively, x is the pH and c is the pKa*(app),
pKa*(app) values were obtained and are listed in Table
5. They ranged from 0.4 to 3.04. Purging with Ar had
no effect on the shape of the titration curve or the
values of pKa*(app).
A possible mechanism for the pH-dependent lumines-
cence is proton-induced quenching [16]. Because Stern–
Volmer plots are inaccurate for estimating rate
constants for the quenching by H+ (kq) due to the
spread of [H+] values across the pH range, Sun and
Hoffman [17] suggested calculating kq using Eq. (6).
These values are listed in Table 5.
4. Discussion
4.1. Preparation of compounds
Our experiments demonstrate that [Ru(bpy)2-
(X2pypmR)]2+ (X=Cl, Br) cannot be obtained by the
reaction of Ru(bpy)2CO3·2H2O with X2pypmR in
ethanol, a nucleophilic substitution reaction occurred
and the complexes [Ru(bpy)2((EtO)2pypmR)]2+ (1–3)
formed instead (Scheme 1). This nucleophilic substitu-
tion was not unexpected owing to the fact that alkoxy-
lation of 4,6-halo substituents in aromatic nitrogen-
containing heterocycles occurs readily in alcohol, except
in the presence of strongly electron-releasing sub-
stituents in the ring [18].
kq=1/~oKa*(app)
(6)

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W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
Fig. 8. Dependence of emission intensity (I) or lifetime (~) as a function of pH for the complexes [Ru(bpy)2pypmMe)]2+ (1,
and ꢁ),
[Ru(bpy)2((EtO)2pypm)]2+ (2, ꢀ, and ꢂ) and [Ru(bpy)2(ClpypnPh)]+ (6, " and ꢃ), solid symbols represent I/Io and open symbols represent
~/~o, — (I/Io) and . . . . . (~/~o) are fitted by Eq. (6).
Nucleophilic attack by OH− in either the 4- or
6-position of the coordinated X2pypmR ligand leads to
pyrimidinones. Unfortunately we do not know for sure
which possibility occurred because we were unable to
grow a single crystal for an X-ray crystallographic
determination, but physical properties suggest the exo-
cyclic ketone neighbors the remote nitrogen (vide infra).
Hydrolysis of pyrimidines generally requires vigorous
acid or alkaline conditions [16]. The fact that the
observed hydrolysis occurred here under such moderate
conditions suggests an increase in the electrophilic abil-
ity at the 4,6-positions of pyrimidine due to the pres-
ence of Ru(II), or excess AgPF6 in the reaction solution
has a tendency to draw chloride away so as to increase
the attacking ability of OH−. Our attempt to hydrolyze
the other chloride by dissolving the products in 1:1
acetone–water, adding excess AgPF6 and refluxing the
mixture for 4 h failed. After purification of the prod-
ucts, the same results of elemental analysis, mass spec-
tra, 1H NMR, cyclic voltammograms, UV–Vis and
emission spectra were obtained, which leads to the
conclusion that the other chloride is difficult to hy-
drolyze under our reaction conditions. This suggests
that a selective hydrolysis has been achieved for coordi-
nated 4,6-dichloro-2-(2-pyridyl)pyrimidine, which is
difficult to control for uncoordinated pyrimidine
derivatives [18].
state in competition with thermally activated up-con-
version to a higher lying metal-centered state (3dd), or a
3
so-called ‘fourth’ MLCT state. The presence of the 3dd
state or the fourth MLCT state introduces additional
decay pathways including the possibility of photosub-
3
stitution by populating the dd state [19,20]. The partic-
ipation of two thermally accessible upper states can be
documented by analysis of lifetime data acquired over a
range of temperatures. The overall relaxation rate is
given in general by the sum of the individual terms as
shown in Eq. (7),
kobs=1/t=kr+knr+kdd exp(−DEdd/RT)
+k4th exp(−DE4th/RT)
+krx exp(−DEdd/RT)
(7)
where k4th as well as kdd may each represent a sum of
radiative and nonradiative decay rate constants for the
depopulation of the corresponding state. It can be
−
pointed out that in the case of PF6 salts in noncoordi-
4.2. Photophysical properties
As presented in Scheme 3, the general model of the
photophysics of Ru(II) complexes studied here involved
excitation into the MLCT manifold, relaxation to the
luminescent MLCT state, and deactivation of the latter
state via radiative and nonradiative decay to the ground
Scheme 3.

W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
125
nating solvents, as in the case here, the krx term be-
comes unimportant and the last term in Eq. (7) can be
neglected.
differed considerably from the other complexes studied
here. The oxidation potentials (Ru3+/2+) decreased to
approximately 0.97 V. According to Crutchley and
Lever [23], the stronger electron donor capacity of a
ligand gives rise to a lesser effective nuclear charge on
the metal center which destabilizes the metal dp or-
bitals. This interpretation would suggest that ClpypmR
are strong electron donor ligands which have the effect
of increasing the energy of the metal orbitals resulting
in complexes that are more easily oxidizable.
For the two series of complexes studied here, the
ligand associated with the lowest lying excited state is
unclear from electrochemistry results. The reductions
for the [Ru(bpy)2(pypm)]2+ series fall in the same range
as those of [Ru(bpy)3]2+; for the [Ru(bpy)2(pypn)]+
series, only two reductions were observed. But one can
deduce the lowest lying excited state by examining
excited state properties as well as redox properties.
Excited state properties responding to substituent ef-
fects indicate that the excited state resides on that
ligand; no response indicates the excited state lies on a
different ligand, which in the pypm or pypn series is the
bipyridine ligand.
For the majority of known cases [8,19,20], the tem-
perature dependence of the lifetime data can be ade-
quately fit using Eq. (4) which incorporates only one
thermally dependent term. Our previous work [20b]
3
showed that for most complexes the dd state domi-
nated the temperature dependence of the emission life-
times, where typical values of the parameters in Eq. (4)
were koꢀ106 s−1, k1ꢀ1012–1014
s
−1, and DEꢀ2500–
3
4500 cm−1. If the 4th MLCT state were responsible
for the temperature dependence, the typical values
would be koꢀ106
s
−1, k1ꢀ106–108 s−1 and DEꢀ
300–800 cm−1 [20f]. The parameters of the complexes
studied here were generally among the region of typical
values for a 3dd state and indicated that the luminescent
3
MLCT state underwent deactivation via the dd state.
4.3. The ground state and excited state
Redox potentials have often been used to assign the
ground state and the first excited state in ruthenium(II)
polypyridyl complexes. The first oxidation is associated
with removal of an electron from a metal-centered dp
orbital; the first reduction is associated with addition of
an electron to the p* orbital of the polypyridyl ligand.
Each of these reactions is illustrated by Eqs. (8) and (9).
For example,
4.4. The [Ru(bpy)2(pypm)]2+ series
The first reductions and emission properties vary
with the substituents on the pypm ligand. The redox
potentials for R=CH3 shifts from −1.20 (R=H) to
−1.28 V. The addition of two alkoxy groups causes a
shift to −1.26 V. Addition of a CH3 group to the
alkoxylated ligand causes an additional shift to −1.30
V. Comparison of the emission properties of com-
pounds 1 (R=CH3), 2 (X=OEt, R=H) and 3 (X=
OEt, R=CH3) gave sequential shifts as follows:
emission lifetimes at 77 K in 4:1 ethanol–methanol
decreased from 5.0 to 4.0 to 3.3 ms and emission
quantum yields changed from 0.056 to 0.0085 to
0.0015, respectively. Radiative rate constants also de-
[(bpy)2Ru(L–L)]2+ −e “ [(bpy)2Ru(L–L)]3+
[(bpy)2Ru(L–L)]2+ +e “ [(bpy)2Ru(L–L−)]+
(8)
(9)
[Ru(bpy)3]2+ is oxidized at 1.27 V and is reduced at
−1.31 V versus SCE in acetonitrile at room tempera-
ture. But in mixed-ligand compounds, the energies of
the p* orbitals of the ligands often differ leading to
localization of the added electron on one specific ligand
which is distinguishable from the others by its redox
potential. For example, the first reduction of the com-
plex [(bpy)2Ru(bpz)]2+, where bpz is 2,2%-bipyrazine,
occurs at −0.91 V versus SCE, while the two
bipyridine ligands are reduced at −1.45 and −1.68 V
versus SCE, respectively [21]. Since a direct correlation
between the MLCT energies and the energy gap repre-
sented by the difference between the first oxidation
potential of the metal center and the first reduction
potential of the ligand has been reported [22], the
highest occupied molecular orbital is assigned to the dp
orbital on ruthenium(II) and the lowest lying excited
state is assigned to the ligand having the most positive
reduction potential, which for [(bpy)2Ru(bpz)]2+ is the
bpz ligand.
creased from 1.3×105 to 6.6×104 to 3.0×103 s−1
,
respectively. Therefore, L–L in Eq. (9) is the pypm
ligand.
4.5. The [Ru(bpy)2(pypn)]+ series
For the pypn series, substituents varied in the 5-posi-
tion (R=H, CH3, phenyl). The values for the first
reduction and emission properties remained fairly con-
stant for this series of complexes. E1/2(1)ꢀ1.44 V ver-
sus SCE, ~ (77 K)ꢀ4.5 ms, ƒemꢀ0.015, krꢀ4.2×104
s
−1. The reason for this may be related to the negative
charge on the pypn ligand making it less responsive to
the substituent. Because excited-state acid–base proper-
ties of the pypn complexes were sensitive to the pH of
the solution (vide infra), the ligand L–L in Eq. (9) is
the pypn ligand similar to the situation found in the
The oxidation potential (Ru3+/2+) varied from 1.35
V for [Ru(bpy)2(pypm)]2+ to 1.26 V for 3. The poten-
tials of the complexes containing pyrimidinone ligands

126
W.-M. Xue et al. / Inorganica Chimica Acta 296 (1999) 114–126
Cherry, L.J. Henderson Jr., Inorg. Chem. 23 (1984) 983. (c) P.A.
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pypm series. Thus, the first observed reduction is asso-
ciated with the pypn ligand and the low lying dp“ p*
transition is associated with transfer of an electron
from the ruthenium center to the pypn ligand. The
second reduction is shifted approximately 20 mV more
negative than found for the second reduction in the
pypm series. It has been argued that a lesser effective
nuclear charge on the metal center would have the
synergistic effect of raising the energy of the ligand p*
level through charge interaction [23].
4.6. Ground-state and excited-state pKa
In their ground states, [Ru(bpy)2(pypm)]2+ com-
plexes are very weak bases (pKaꢀ −4). But [Ru(bpy)2-
(pypn)]+ complexes are even weaker bases by
approximately two orders of magnitude. One might
expect the reverse since the pypn ligand would have
more electron density due to the presence of the nega-
tive charge and render the remote nitrogen more basic.
The reason for this anomaly may be related to the fact
that the remote nitrogen is adjacent to an electron
withdrawing group. The fact that the effect is large
favors location of the exocyclic ketone at this site.
While the ground state pKa values remain relatively
constant for each series, the excited state pKa values
show differences in behavior. First, pKa* (app) for the
pypm series increased by approximately six orders of
magnitude compared to the ground state pKa. But,
pKa* (app) of the pypn series increased even more
(approximately eight orders of magnitude). These varia-
tions are consistent with assigning the p* level of pypm
as the lowest excited state in the Ru(bpy)2(pypm)]2+
series and of pypn as the lowest excited state in the
Ru(bpy)2(pypn)]+ series.
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We thank the Office of Basic Energy Sciences of the
United States Department of Energy for support of the
investigation and the National Science Foundation for
the 400 MHz NMR and the laser lifetime equipment.
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