Photoinduced ligand transformations in a ruthenium complex of dimethoxytetrapyridotetraazapentacene

Article abstract of DOI:10.1039/c0dt00982b

The dinuclear ruthenium(ii) complex [(phen)₂Ru(tatpOMe)Ru(phen) ₂]⁴⁺ (2⁴⁺; phen is 1,10-phenanthroline and tatpOMe is 10,21-dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a: 2′3′-c:3′′,2′′-l:2′′′, 3′′′-n]pentacene

Full text of DOI:10.1039/c0dt00982b

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www.rsc.org/dalton | Dalton Transactions
Photoinduced ligand transformations in a ruthenium complex of
dimethoxytetrapyridotetraazapentacene
Shreeyukta Singh, Norma R. de Tacconi, David Boston and Frederick M. MacDonnell*
Received 7th August 2010, Accepted 16th September 2010
DOI: 10.1039/c0dt00982b
The dinuclear ruthenium(II) complex [(phen)2Ru(tatpOMe)Ru(phen)2]4+ (24+; phen is
1,10-phenanthroline and tatpOMe is 10,21-dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a:2¢3¢-
c:3¢¢,2¢¢-l:2¢¢¢,3¢¢¢-n]pentacene) has been synthesized and characterized by 1H NMR, ESI mass
spectroscopy and elemental analysis. Loss of methoxy group from bridging ligand of complex 24+ due
to irradiation is observed by 1H NMR and photochemistry. The interrelated electronic properties
UV-Vis, electrochemistry, photochemistry and molecular orbital calculation are analyzed and discussed
on the bridging ligand of the complex 24+.
nar aromatic acceptor ligands related to the well-known dppz
Introduction
ligand. Mononuclear and dinuclear ruthenium(II) complexes of
the tetraazatetrapyridophenazine (tatpp) ligand display unusual
photochemical activity in which the tatpp ligand is reduced
by up to 2 electrons upon visible light irradiation in the pres-
ence of sacrificial donors.16–19 The dinuclear ruthenium complex,
[(phen)2RuII(tatpp)RuII(phen)2]4+, 14+, has also been shown to
bind DNA via intercalation and cleave DNA in a process
that is enhanced under low oxygen conditions.20 The closely
related complex, [(phen)2RuII(tatpq)RuII(phen)2]4+, 34+, is also
photochemically active and can undergo up to four tatpq ligand-
based reductions via photon driven processes.19,21
Ruthenium(II) polypyridyl complexes continue to enjoy consider-
able attention as chromophores for molecular light-to-chemical
energy conversion schemes.1–4 The development of molecular
photocatalysts capable of driving conversion of common sub-
strates into fuels, i.e. H+ into H2 and CO2 into CH3OH, needs
to address the multi-electron requirements of the substrates and
still couple this with the single-photon, single-electron excitation
common for most chromophores. One strategy has been to build
photocatalysts capable of photodriven multi-electron storage, such
that they absorb multiple photons over time and build-up reducing
(or oxidizing equivalents) within their structure. Brewer and co-
workers were first to demonstrate this in 1994 in a trimetallic
Ru–Ir–Ru complex which was later extended to Ru–Rh–Ru
systems.5–7 In the latter Rh-based system, photon absorption by
the Ru components drives a Rh(III/I) reduction and this center
is subsequently able to reduce protons to H2. A number of other
molecular8–11 and supramolecular12–15 Ru–Pt or Ru–Pd bimetallic
systems have also been shown to produce H2 photochemically in
the presence of sacrificial donors.
While both 14+ and 34+ can photochemically store multiple
electrons reversibly on the bridging ligands, neither complex has
shown much promise as for solar H2 generation owing to the
modest reduction potentials of these stored electrons. In this
work, we explore the substitution of the central hydrogens in
tatpp with methoxy electron-donating groups (tatpOMe) in an
effort to shift the reduction potentials of the resulting ruthenium
complex, [(phen)2RuII(tatpOMe)RuII(phen)2]4+ (24+), to more neg-
ative values. If this complex retains the photochemical activity
of the parent tatpp complex, then we could aim to photoreduce
this new complex by 2-electrons and potentially store them at
more negative reduction potentials. In this report, we describe
the synthesis and characterization of the tatpOMe ligand and
its dinuclear ruthenium(II) complex, as well as an evaluation
We have been exploring the photochemistry of ruthenium
complexes, shown in Fig. 1, containing a unique class of pla-
Department of Chemistry and Biochemistry, University of Texas at Arling-
ton, Arlington, TX, 76019-0065, USA. E-mail: macdonn@uta.edu
Fig. 1 Structures of dinuclear ruthenium(II) complexes having central bridging ligand tatpp, tatpOMe and tatpq.
11180 | Dalton Trans., 2010, 39, 11180–11187
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of its electrochemical and photochemical properties for use as
a photocatalyst for photon-driven multi-electron storage and
transfer.
by single-point calculations based on the gas phase optimized
structures using the polarizable continuum model (PCM). The
orbital analysis was completed with Gaussview.
Synthesis
Experimental
3,6-dimethoxybenzene-4,5-bis(p-toluenesulfonamido)benzene.
Chemicals
To
a solution of 5.0 g (0.03 mmol) 1,2-diamino-3,6-
dimethoxybenzene24 dissolved in 10 mL pyridine was added
3 equivalents (9.6 g, 0.09 mmol) of p-toluensulfonyl chloride in
three portions. The resulting solution was heated to 35 ◦C for
1 h. and then poured into a large beaker containing 1000 mL
H2O. The resulting precipitate was filtered, and recrystallised
from ethanol. Yield: 13 g (92%). M.p. 125–128 ◦C 1H NMR
(CDCl3) d = 2.39 (s, 6H), 3.39 (s, 6H). 6.59 (s, 2H, NH), 6.96 (s,
2H), 7.20 (d, 2JHH = 8.1 Hz, 4H), 7.58 (d, 2JHH = 8.1 Hz, 4H).
The compounds 1,10-phenathroline (phen), hydrated ruthe-
nium(III)chloride, p-benzoquinone, toulensulfony-chloride were
purchased from Aldrich or Alfa and used without fur-
ther purification. Ru(phen)2Cl2,22 1,10-Phenanthroline-5,6-dione
(phendione),23 1,4-dimethoxy-2,3-dinitrobenzene,24 1,2-diamino-
3,6-dimethoxybenzene24 were prepared according to literature
procedures. All organic solvents were of analytical grade and
used as received unless stated otherwise. For electrochemical
measurements, the acetonitrile (MeCN, Aldrich) was dried on
alumina and distilled under nitrogen prior to use. The supporting
electrolyte Bu4NPF6 (Aldrich) was dried overnight under vacuum
at 60 ◦C and stored under nitrogen.
4,5-Dinitro-3,6-dimethoxy-4,5-bis(p-toluenesulfonamido)ben-
zene. 3,6-Dimethoxybenzene-4,5-bis(p-toluenesulfonamido)-
benzene (10 g, 20.9 mmol) was dissolved in 15 mL of acetic acid
and heated to 60 ◦C. A mixture of fuming nitric acid (4 mL) acetic
acid (2 mL) was slowly added to the first solution and then refluxed
for 1 h at 100◦–110 ◦C. The resulting solution was poured into a
large beaker containing 1000 mL water. The resulting precipitate
was isolated by filtration and recrystallized from ethanol. Yield:
10.7 g (90%). M.p. 122–125 ◦C. 1H NMR (CDCl3) d = 2.45 (s, 6H),
3.44(s, 6H). 7.25 (s, 2H, NH), 7.34 (d, 2JHH = 8.1 Hz, 4H), 7.74 (d,
2JHH = 8.1 Hz, 4H).
Physical Measurements
1H and 13C NMR spectra were obtained on a JEOL Eclipse Plus
500 or 300 MHz spectrometer using CD3CN as the solvent unless
otherwise noted. Chemical shifts are given in ppm and referenced
to TMS. UV-Visible spectra were obtained on a Hewlett–Packard
HP8453A spectrophotometer in MeCN.
1,2-Diamino-3,6-dimethoxy-4,5-bis(p-toluenesulfonamido)ben-
zene. 4, 5-Dinitro-3, 6-dimethoxy-4, 5-bis(p-toluenesulfona-
mido)benzene (1.0 g, 1.8 mmol) and 10% Pd on C (30 m) were
suspended in 20 mL ethanol in a steel pressure reactor. The
resulting mixture was hydrogenated for 24 h. at room temperature
at 5 bar H2. After cooling and venting, the reaction mixture
was filtered using Celite and the resulting ethanol solution
concentrated to dryness using a rotatory evaporator to yield a
pink colored crystalline product. Yield 0.70 g (80%), M.p. 86–
90 ◦C. 1H NMR (CDCl3) d = 2.38 (s, 6H), 3.48(s, 6H). 7.25 (s, 2H,
NH), 7.19 (d, 2JHH = 8.1 Hz, 4H), 7.56 (d, 2JHH = 8.1 Hz, 4H).
Photochemical Reaction
The UV-Visible spectra were obtained using a Hewlett–Packard
HP8453A spectrophotometer. All solutions were sealed in a
quartz cuvette and degassed for 10 min with nitrogen prior to
irradiation. The cuvettes were irradiated using 150 W halogen
lamp (StockerYale Imagelite Model 21AC) clear glass, light bulb.
The progress of the photochemical reaction was monitored by
recording the absorption spectra. All absorption measurements
were done at room temperature using a sample concentration of
16 mM dissolved in MeCN.
10,13-dimethoxy-11,12-bis(p-toluenesulfamido)-dipyrido[3,2-
a:2¢,3¢-c]phenazine. 1,2-Diamino-3,6-dimethoxy-4,5-bis(p-tolu-
enesulfonamido)benzene (100 mg, 0.19 mmol) and 1,10-
phenanthroline-5,6-dione (21.7 mg, 0.18 mmol) were suspended in
50 mL of ethanol and refluxed for 6 h. The solution was filtered hot
and the precipitate washed with ethanol and dried under vacuo.
Yield 130 mg (98%), M.p. 130–132 ◦C. 1H NMR (CDCl3) d = 2.40
(s, 6H), 3.99(s, 6H). 7.27 (d, 2JHH = 8.1 Hz, 4H), 7.54 (s, 2H, NH),
Electrochemistry
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) experiments were conducted in dry acetonitrile (Aldrich,
99.93+%, HPLC grade) with 0.1 M NBu4 PF6 (Sigma) as sup-
porting electrolyte using a CH600C electrochemical analyzer (CH
Instruments). A glassy carbon disc (1.5 mm diameter disk, Cypress
Systems) was used as working electrode in conjunction with a Pt
wire and a premium “no leak” Ag/AgCl/3M KCl (Cypress, model
EE009) as counter and reference electrode respectively. Potentials
are quoted with respect to NHE. Oxygen was exhaustedly removed
from the electrochemical cell by bubbling purified nitrogen.
n
2
7.73(d, JHH = 8.1 Hz, 4H), 7.78 (dd, JHH = 9.0 Hz, 2H), 9.28 (d,
JHH = 6.0 Hz, 2H), 9.45 (d, JHH = 9.0 Hz, 2H).
11,12-Diamino-10,13-dimethoxy-dipyrido[3,2-a:2¢,3¢-c]phena-
zine. Detosylation of 10,13-dimethoxy-11,12-bis(p-toluene-
sulfamido)-dipyrido[3,2-a:2¢,3¢-c]phenazine (1 g, 1.4 mmol) was
accomplished by dissolving the solid in 15 mL concentrated
sulfuric acid and then heating this solution to 80 ◦C for 4 h in
a water bath. The resulting dark violet solution was then added
dropwise to ice water and filtered. The resulting filtrate was
treated with a saturated solution of NaHCO3 until the pH was ~6
to 7, during which the free diamine precipitated from solution.
Molecular Orbital Calculations
The molecular and electronic structure calculations for 14+ and
24+ were performed with density functional theory (DFT) using
the Gaussian03 program package using the B3LYP functional.25
A
cc-pVDZ basis set was used for hydrogen, carbon, nitrogen, and
a LanL2DZ basis set for ruthenium. Solvent effects were modeled
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The product was isolated by filtration and dried. Yield 0.37 g
evaporation. Addition of an aqueous solution of NH4PF6 to the
ethanol solution results in the formation of a precipitate which
is isolated by filtration and washed with 10 mL of chloroform.
This product is further purified by metatheses between the Cl-
(68%), M.p. 128–130 ◦C. 1H NMR (CDCl3) d = 4.14(s, 6H), 7.95
(dd, JHH = 9.0 Hz, 2H), 9.28 (d, JHH = 6.0 Hz, 2H), 9.65(d, JHH
=
9.0 Hz, 2H).
-
and PF6 salts in dark. Yield: 240 mg (64%). Anal. Calc. For
10,21-Dimethoxy-9,10,20,33-tetraazatetrapyrido[3,2-a:2¢3¢-
c:3¢¢,2¢¢-l:2¢¢¢,3¢¢¢-n]pentacene (tatppOMe). 11,12-Diamino-
10,13dimethoxy-dipyrido[3,2-a:2¢,3¢-c]phenazine (100 mg,
C80H50F24N16O2P4: C, 46.89; H, 2.46; N, 10.49. Found: C, 46.42;
H, 2.38; N, 10.52. 1H NMR (CDCN3) d = 4.81(s, 6H), 7.64–7.84
(dd, JHH = 9.0 Hz), 8.1 (d, 1H), 8.22 (d, 1H) 8.26 (s), 8.61 (apparent
triplet) 9.64 (d, JHH = 9.0 Hz, 2H). ESI-MS (m/z): 1906 [34+-PF6]+1,
879.67 [34+-2PF6]+2, 538.67 [34+-3PF6]+3, 367.87 [34+]4+.
0.26 mmol) and 1,10-phenanthroline-5,6-dione (5.6 mg,
0.26 mmol) were dissolved in 15 mL acetic acid and refluxed
overnight, during which time a precipitate formed. After cooling
the reaction mixture filtered and the solid washed with 100 mL
water. Yield 121 mg, (92%), M.P. 190–192 ◦C. Anal. Calc. For
C32H18N8O2: C, 70.32; H, 3.32; N, 20.50. Found: C, 70.02; H,
Results and discussion
3.15; N, 20.39. 1H NMR (CDCl3) d = 4.88(s, 6H), 7.84 (dd, JHH
=
Synthesis
9.0 Hz, 2H), 9.32 (d, JHH = 6.0 Hz, 2H), 9.70(d, JHH = 9.0 Hz,
2H). 13C NMR (CDCl3) d: 65.3, 124.5, 127.5, 134.0, 134.5, 141.9,
147.7, 149.1, 153.3.
The complete synthetic route followed for preparation of the
ligand, tatpOMe, and the dinuclear ruthenium(II) complex,
[(phen)2Ru(tatpOMe)Ru(phen)2][PF6]4, is shown in Fig. 2. The
amine functions on 1,2-diamino-3,6-dimethoxybenzene are pro-
tected by tosylation and the dinitro compound formed by treat-
ment with fuming nitric acid and sulfuric acid to obtain 4,5-
dinitro-3,6-dimethoxy-4,5-bis(p-toluenesulfonamido)benzene fol-
lowing the methods developed by Starns et.al. for the
non-methoxylated compound.26,27 This compound is con-
verted to the diamine by reduction with H2 over a Pd/C
catalyst. The resulting 1,2-diamino-3,6-dimethoxy-4,5-bis(p-
toluenesulfonamido)benzene is coupled with one equivalent of
1,10-phenanthroline-5,6-dione to give the bis N-tosylated dppz
[(phen)2Ru(tatppOMe)Ru(phen)2][PF6]4, 3[PF6]4. All proce-
dures for this synthesis were conducted under anaerobic and low-
light conditions. The reaction vessel was wrapped in aluminium
foil during the reflux period to avoid light exposure. TatppOMe
(100 mg, 18 mmol) and Ru(phen)2Cl2 (90 mg, 0.18 mmol) were
suspended in a chloroform–ethanol (50 : 50) mixture under a N2
atmosphere. The resulting slurry was refluxed for 1 day after which
a second 90 mg (0.18 mmol) portion of Ru(phen)2Cl2 was added.
The resulting solution was refluxed for a total of 8 days and
then filtered hot and most of the chloroform removed by rotary
Fig. 2 Synthesis of tatpOMe and [(phen)2Ru(tatpOMe)Ru(phen)2] [PF6]4.
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derivative.27 After detosylation, the resulting 11,12-diamino-
10,13-dimethoxy-dipyrido[3,2-a:2¢,3¢-c]phenazine is coupled with
one equivalent of 1,10-phenanthroline-5,6-dione to yield the dark
brown colored ligand tatpOMe in 31% overall yield. Unlike the
insoluble tatpp ligand, tatpOMe is slightly soluble in chloroform
and dichloromethane, thereby allowing a straightforward NMR
analysis of the highly symmetric structure (D2h point group). The
1H NMR of tatppOMe in CDCl3 shows 4 peaks with a singlet for
-OMe groups at 4.88 ppm and the characteristic AMX splitting for
the aromatic Ha, Hb, and Hc in the 7–10 ppm region. The proximity
-0.24 V by DPV. It is an irreversible process and is indicative of
significant structural changes in the ligand upon reduction, which
we now know to be associated with loss of the methyl or methoxy
functions (vide infra).
In the dinuclear ruthenium(II) complex 24+, a total of four redox
processes are observed in the DPV data shown in Fig. 3. The
reversible redox process centered at +1.58 V is assigned to the
two Ru2+/3+ couples. As seen with complex 14+ at +1.60 V, the two
one-electron couples occur as a single peak indicating the two
Ru(II) centers are sufficiently far apart that electronic coupling
is negligible. The remaining three electrochemical processes of
complex 24+ are associated with reduction or oxidation of the
coordinated tatpOMe ligand (Fig. 3). An irreversible oxidative
process at +1.31 V is associated with a tatpOMe0/+ couple which
is shifted positive by 100 mV relative to the same process in the
free ligand at +1.21 V (see Table 1). The positive shift is attributed
to the increase in overall charge upon complex formation.
of Hc to the pyrazine nitrogen lone pairs causes a distinctive
28–31
downfield shift in related compounds
and is seen here at
9.70 ppm. Elemental analysis data also support the proposed
structure.
The
dinuclear
ruthenium(II)
complex,
[(phen)2Ru-
(tatpOMe)Ru(phen)2][PF6]4, is prepared by a prolonged refluxing
of the slightly soluble tatpOMe ligand with 2 equivalents of
1
Ru(phen)2Cl2 in 1 : 1 chloroform–ethanol. As expected, the H
NMR spectrum of [(phen)2Ru(tatppOMe)Ru(phen)2][PF6]4 in
deuteroacetonitrile is very similar to that of 1[PF6]4. The notable
differences being the large singlet at 4.81 ppm for the two methoxy
groups in 24+ and the absence of the peak corresponding to central
hydrogens in tatpp. The Hc protons of the tatpOMe ligand are
observed the most downfield in 24+ at 9.64 ppm, which is in a
similar range as the similarly situated protons in 14+ at 9.70 ppm
and 34+ at 9.68 ppm.29 A complete spectrum and peak assignment
is given in the supporting information. ESI-MS data shows 4
peaks with m/z of 1906, 879, 538 and 367 which correspond
well with the calculated m/z ratios for the 34+cation with 0 to 3
associated PF6- counterions. Elemental analysis data also support
the proposed structure.
Electrochemistry
The redox properties of the tatpOMe in CH2Cl2 and complex
2[PF6]4 in MeCN were analyzed by cyclic voltammetry (CV) and
differential pulse voltammetry (DPV) and the data are collected
in Table 1 along with that for complex 1[PF6]4. The redox data
provides information regarding the frontier molecular orbital
energies of the free tatpOMe ligand and the complex 24+. All of the
electron transfer processes for the tatpOMe ligand are irreversible
and therefore peak potentials (instead of half-wave potentials)
obtained from DPV data are listed for the free ligand in Table 1.
The first oxidative process at +1.21 V reveals that the energy of the
tatpOMe HOMO is below that typically found for metal-based
HOMO typical of [Ru(phen)3]2+ and closely related derivatives
whose potential is approximately +1.5 V vs. NHE.3 On the other
side, the ligand LUMO is shown to be accessible at a potential of
Fig. 3 DPV runs on a glassy carbon electrode (1.5 mm diameter) for
80 mM [2](PF6)4 in acetonitrile containing 0.1 M TBAPF6 as supporting
electrolyte. Four individual runs are shown encompassing the potential
windows for reduction (0.4 V/-0.9 V) and oxidation (0.4 V/-1.9 V)
processes respectively. Each potential window was scanned in positive-
and negative-going potential direction. Pulse amplitude = 0.01 V, step
size = 0.001 V, pulse duration = 0.05 s, and pulse period = 0.2 s.
Complex 14+ does not show a tatpp0/+ couple prior to the Ru2+/3+
(see Table 1) indicating that the HOMO of coordinated tatpp is
lower in energy than that for coordinated tatpOMe. It seems that
the electron-donating methoxy groups destabilize the tatpOMe
Table 1 Electrochemical data for [(phen)2Ru(tatpp)Ru(phen)2]4+ and [(phen)2Ru(tatpOMe)Ru(phen)2]4+ in acetonitrile and tatpOMe in CH2Cl2
Oxidation/Va
Reduction/Va
1st
—
1.21 (ir)
1.31 (ir)
2nd
1st
2nd
[(phen)2Ru(tatpp)Ru(phen)2]4+ (in MeCN)
tatpOMe (in CH2Cl2)
1.60 (2)
1.43 (ir)
1.58 (2)
-0.02 (1)
-0.24 (ir)
-0.05 (1)
-0.51 (1)
-0.56 (ir)
-0.52
[(phen)2Ru(tatpOMe)Ru(phen)2]4+ (in MeCN)
a Potentials were measured vs. Ag/AgCl/3M KCl reference electrode and converted to NHE by adding 0.197 V. Number of electrons is shown in
parenthesis and irreversible processes are indicated with “ir”.
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Table 2 Absorption data of the dinuclear ruthenium complexes and
Zn(II) adducts of tatpp, tatpOMe and in MeCN
HOMO relative to tatpp such that it becomes the HOMO for the
overall complex.
The two redox processes of complex 24+ at -0.05 V and -0.52 V
are certainly associated with the reduction of the coordinated
tatpOMe ligand because the reductions associated with the
terminal phen groups are known to require reduction potentials
more negative than -1.2 V. These reductions occur at potentials
nearly identical with those for 14+ but unlike the 14+/3+ and 13+/2+
couples which are of similar shape and intensity,18,19 these two
reductions show very different peak areas and intensities as shown
in Fig. 3. The magnitude of the process at -0.52 V is unusually
large, even larger than the 2-electron Ru2+/3+ couple, suggesting a
multi-electron process. On the other hand, the magnitude of the
first reductive couple at -0.05 V is less than half the intensity of the
Ru2+/3+ couple. As these processes appear to be mostly reversible,
it seems reasonable to assign the -0.05 V process to a tatpOMe0/-
couple whereas the larger peak at -0.52 V process is indicative of
a multi-electron transfer process with at least 2 electrons. It is also
noteworthy that the irreversible reductive process observed for the
free tatpOMe ligand at -0.24 V is now shifted to -0.05 V for the
respective tatpOMe0/- couple in 24+ because of the effect of Ru(II)
coordination.
abs, 298 K: lmax, nm (e ¥ 10-4,
Compound
M-1 cm-1)
[(phen)2Ru(tatpp)Ru(phen)2][PF6]4
[(phen)2Ru(tatpq)Ru(phen)2][PF6]4
444 (6.30), 422 (4.91), 396
(3.70), 323 (8.48)
441 (3.60), 370 (4.57), 318 (5.16)
[(phen)2Ru(tatpOMe)Ru(phen)2][PF6]4 448 (4.20), 430 (4.04), 350 (9.03)
tatppOMe (in CH2Cl2)
Zn-tatppOMe adduct
Zn-tatpp adduct
462 (1.26), 434 (0.861), 410
(0.513)
456 (1.01), 430 (0.934), 405
(0.809)
448 (1.75), 423 (1.30), 400
(0.460), 380 (0.460), 323 (4.57)
an intense transition at 323 nm and a structured transition with
peaks at 400, 423, and 447 nm. Given the close structural similarity
of the tatpp and tatpOMe ligands and the similarity of their
absorption spectra, it seems reasonable to assume isoelectronic
transitions are occurring in both ligands. From previous studies
of the tatpp ligand, we have assigned the lowest energy transition
in both ligands as a p(HOMO)-p*(LUMO) transition and the
higher energy transition at 323 nm as a p(HOMO)–p*(LUMO+1)
transition.17 All of the ligand-centered transitions in tatpp are
observed at higher energy than the isoelectronic transitions in
tatpOMe. Given the electrochemical data (Fig. 3) and this red
shift in peaks for tatpOMe (Fig. 4), it is reasonable to assume
that the methoxy substituents destabilize the ligand HOMO with
respect to the original tatpp moiety.
The electronic absorption spectra of the ruthenium complexes
14+, 24+, and 34+ in MeCN are shown in Fig. 5 and their absorption
bands are reported in Table 2. The spectra for 14+ and 24+ are
similar excepting for the intensity of the transitions in the 410–
450 nm region. The spectrum of 14+ was shown to be comprised
of two overlapping bands (the MLCT and structured LC band) in
the 410–450 region17 which is also apparent in 24+ with the lesser
intensity being due to the lower extinction coefficient for the LC
transition in tatpOMe in this region. The MLCT band for 14+
and 24+peaks at ~ 450 nm and is associated with a Ru(II) dp →
ligand (p*) transition that occurs at virtually the same energy as
seen for [Ru(phen)3]2+. This transition can be assigned to Ru(II)
dp → phen (p*)in 14+ and 24+ both of which also show broad
lower energy shoulders between 500 and 600 nm. These broad and
Electronic Absorption Spectroscopy
The electronic absorption spectra for the free tatpOMe ligand in
CH2Cl2, the Zn(II) adduct of tatpOMe in MeCN, and the Zn(II)
adduct of tatpp in MeCN are shown in Fig. 4 and the respective
absorption bands are compiled in Table 2. The free tatpp ligand is
generally insoluble but can be dissolved in MeCN upon addition of
a 5–10 fold molar excess of Zn(BF4)2, presumably due to formation
of the bis Zn(II) adduct. As tatpOMe is soluble in CH2Cl2, both
the free ligand spectrum and that of its Zn(II) adduct in MeCN are
compared in Fig. 4. The free tatpOMe in CH2Cl2 shows an intense
LC transition at 350 nm and weaker structured LC transitions with
distinct peaks at 410 nm, 434 nm, and 461 nm. The spectrum of the
Zn(II)-tatpOMe adduct is very similar to that of tatpOMe except
that the structured peaks are shifted to higher energy at 405, 430,
and 457 nm upon coordination to the Zn(II) ions. This blue shift is
likely due to stabilization the tatpOMe HOMO upon coordination
to the electron withdrawing and positively charged Zn(II) ions. By
comparison, the Zn(II)-tatpp adduct also shows two strong peaks:
Fig. 4 Absorption spectra of the free tatpOMe ligand in CH2Cl2, the
tatpOMe ligand in MeCN with 10 eq Zn(BF4)2, and the tatpp ligand in
MeCN with 10 eq Zn(BF4)2.
Fig. 5 Electronic absorption spectra of the 14+, 24+, and 34+ (16 mM) in
acetonitrile.
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moderately intense, low-energy transitions are likely to be MLCT
bands associated with the tatpp and tatpOMe ligands, which have
low energy p* orbitals as indicated by their redox properties. The
degree of electronic coupling of these low energy ‘redox orbitals’
with the Ru dp HOMO has been the subject of considerable study
in these and related dppz and tpphz complexes.28,32–39
For dppz28,32–34 and tpphz,35–39 it has been shown that two accep-
tor orbitals play significant roles in the overall complexes behavior.
The ‘redox orbital’ is the lowest lying ligand MO and is the first
MO populated upon chemical or electrochemical reduction. The
‘optical orbital’ is a MO with acceptor characteristics energetically
and symmetrically similar to the LUMO on a bipyridine ligand
and is usually the LUMO+1 for dppz or tpphz when coordinated to
Ru(II). This ‘optical orbital’ is the MO populated upon excitation
into the complex MLCT band at ca. 480 nm. For tpphz and
dppz, the electronic absorption spectra do not show any significant
absorptions below this 480 nm band and thus it appears that the
oscillator strength for the Ru(II) dp → dppz (or tpphz) p*(LUMO)
is very small or zero. This has been explained by MO calculations
which show little or no p orbital density on the coordinated
nitrogens for the LUMO.28,35,39 Similar arguments have been made
for the tatpp and tatpq ligands.29
Molecular Orbital Calculations
In order to interpret and compare the electrochemical and optical
data for 14+ and 24+, MO calculations were performed on the
structures using DFT theory as described in the experimental
section. Fig. 6 shows the energy, orbital density, and symmetry for
the relevant frontier orbitals on 14+ and 24+. For both 14+ and 24+,
the LUMO+1 MOs (p1*) were identified as having ‘bipyridine-like’
orbital symmetry and density on the tatpp or tatpOMe ligands.
The energy of these two p1* MO’s were arbitrarily set to zero for
subsequent comparisons. The LUMOs (p0*) for 14+ and 24+ were
both approximately 1 eV lower than the p1* MOs and showed
large orbital density on the central portions of the ligands and
little or no orbital density on the nitrogens coordinated to the
Ru(II) ions, which is similar to the situation observed in dppz
and tpphz complexes. The highest occupied MOs with Ru dp
character in both complexes have nearly equal energies of ca. -3.6
eV relative to the p1* however the relative order of MO’s has now
changed. This Ru dp orbital corresponds to the HOMO for 14+
and is the HOMO-1 for 24+. The HOMO for 24+ is largely localized
on the central portion tatpOMe ligand and thus the first ‘redox
orbital’ that should be accessed during the oxidation of the 24+ is
a ligand-centered MO. This description corresponds nicely with
the electrochemical data in which an irreversible oxidation peak
was seen at +1.32 V prior to the Ru2+/3+ couple at +1.54 V in
the DPV of 24+. We had assigned this oxidation to the coordinated
tatpOMe0/+ couple and can show it is irreversible because tatpOMe
oxidation or reduction has been found to lead to demethylation
or demethoxylation (vide infra).
Fig. 6 MO energy diagram of the relevant frontier molecular orbitals for
complexes 14+ and 24+
.
functions in tatpOMe is to raise the energy of the ligand HOMO,
in this case to something above the Ru dp orbitals. A similar effect
is seen in the LUMO’s for 14+ and 24+ with the energy of the
LUMO for 24+ being found at higher energy than the LUMO for
14+, however the magnitude of the effect is much less significant
on the unoccupied orbitals.
The broad shoulder observed in the optical spectra of 14+ and
24+ between 500 and 600 nm is not observed in either spectrum
of the Zn(II) adducts of respective ligands in MeCN ruling out
a LC type transition. Similarly, this shoulder is not observed in
the spectrum of [Ru(phen)3]2+ in MeCN. These two observations,
the broadness of the shoulders, plus the appreciable extinction
coefficients (ca. 10,000 M-1 cm-1 for 14+ and 6,000 M-1 cm-1 for
24+) are suggestive of MLCT type transitions involving the tatpp
or tatpOMe ligands. From the MO picture, two types of MLCT’s
are possible; a MLCT involving a Ru dp →p0* (MLCT0) transition
or a Ru dp →p1* (MLCT1). There are two reasons we favor the
latter transition as the origin of the shoulder in 14+ and 24+. First,
the MO calculations show no p orbital density on the coordinated
nitrogens in the p0* orbital of 14+ and 24+. The analogous MLCT0
On the other hand, the tatpp complex 14+ does not show a ligand-
centered occupied MO until the HOMO-6, indication that the
first oxidative process seen electrochemically should be the Ru2+/3+
couple which is what is observed at +1.54 V. The intervening
orbitals, HOMO-1 to HOMO-5, are all related to the Ru dp
orbitals. From these data and calculations, we can now see that the
largest impact of replacing the hydrogens in tatpp with methoxy
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transition in Ru(II)dppz and Ru(II)tpphz complexes, which have
very similar electronic structures, is not observed, therefore it is
not expected in 14+ and 24+. Second, if the MLCT1 transition in
14+ and 24+ is observed at 450 nm, the nearly 1 eV drop in energy
for the MLCT0 transitions would be expected to give peaks at
wavelengths near 700 nm, which is much lower in energy than
what is observed. If, however, the electronic origin of the shoulder
is the MLCT1 transition, the spectra can be explained as follows:
strong Ru dp →p* (phen) transitions occur around 450 nm and
are overlapping with a broad the MLCT1 transition for tatpp and
tatpOMe having a maxima closer to 510 nm. The energy of the p1*
orbital in tatpp and tatpOMe is a little lower in energy than the
phen p* orbital of similar symmetry due the electron withdrawing
central portions of these ligands.
observed upon further irradiation, however, exposure of the final
solution to air rapidly bleaches all peaks above 500 nm. The final
spectrum mirrors that as seen for complex 34+ with dominant
peaks at 444 nm and 370 nm and a shoulder at 315 nm. ESI-mass
spectrometric analysis of this final solution reveals a dominant
parent ion peak at 1872 m/z which corresponds to the m/z for
+
{[(phen)2Ru(tatpq)Ru(phen)2][PF6]3} .
From these data, it is clear that complex 24+ was converted
to the quinone complex 34+ during the photochemical process
and subsequent air oxidation. The evolution of the absorption
data support demethylation of the methoxy substituents upon 1-
electron reduction of the tatpOMe ligand in 24+. Melloni et al. have
shown that the reductive cleavage of o-, m-, p-dimethoxybenzene
ethers can be induced via potassium reduction under anaerobic
conditions.40,41 Both demethylation or demethoxylation products
are observed and are favored by one or two-electron reductions
mechanisms, respectively. In this case, reductive quenching of the
3MLCT state in 24+ traps a single electron on the tatpOMe ligand
favoring demethylation.
Photochemistry
The photochemical activity of complex 24+ was examined under
similar conditions which resulted in multi-electron reduction of
complexes 14+ and 34+.19 Irradiation of the complex in MeCN
containing 0.25 M TEA under anaerobic conditions leads to
the spectral evolution shown in Fig. 7. After 5 s irradiation,
peaks appear at 878 and 1000 nm along with an increase in
the intensity of the band at 450 nm. This is analogous the
changes initially observed during the irradiation of 14+ under
similar conditions and corresponds to the formation of the
radical complex [(phen)2RuII(tatpOMe∑-)RuII(phen)2]3+ by MLCT
excitation and reductive quenching with TEA. By 20 s, these
two peaks at long wavelength are replaced by an intense peak
at 635 nm plus a very broad but less intense band peaking
at 1050 nm. This change is not seen during the irradiation
of 14+ and is instead consistent with the formation of the
one-electron reduced complex [(phen)2RuII(tatpq∑-)RuII(phen)2]3+,
which has been characterized as having a very broad peak at
1050 nm, and the formation of the two-electron reduced species
We also noted that complex 24+ appeared to be sensitive to
photochemical degradation in the absence of sacrificial donors.
Irradiation of 24+ with visible light in anaerobic MeCN solution,
leads to a nearly complete bleaching of the tatpOMe LC peaks
leaving only the broad Ru dp→p* (phen) type MLCT at 450 nm
and no new absorptions at longer wavelengths. Exposure of this
unknown intermediate to air does not return the spectrum to its
original 24+ but instead results in a spectrum which has features
of both 14+ and 34+ complex with peaks at 315 nm, 370 nm, and a
structured band at 420 and 445 nm. A fitting of the composite
spectrums of 14+ and 34+perfectly recreates the observed final
spectrum in Fig. 8 with a mixture of 21% 14+ and 79% 34+,
supporting a photochemical process involving both demethylation
and demethoxylation. An NMR scale experiment reveals complete
loss of the methoxy peak at 4.70 ppm and the appearance of a
methanol peak at 3.25 ppm. If the experiment is conducted in air,
irradiation leads to 100% conversion of 24+ to 34+, as determined
by both NMR and ESI-mass spectrometry.
[(phen)2RuII(tatpq2-)RuII(phen)2]2+ which has a strong absorption
19,21
at 635 nm
Continued irradiation for a total of 80 s bleaches
the 1050 nm peak and increases the intensity of the 635 nm
peak, supporting conversion of [(phen)2RuII(tatpq∑-)RuII(phen)2]3+
to [(phen)2RuII(tatpq2-)RuII(phen)2]2+. No further changes are
Fig. 7 Changes in the absorption spectra of complex [2][PF6]4 upon
visible light irradiation for various times under anaerobic conditions in
acetonitrile with 0.25 M TEA and subsequently, after air oxidation.
Fig. 8 Changes in the absorption spectra of complex [2][PF6]4 (25 mM)
upon visible light irradiation under anaerobic conditions in acetonitrile
without added TEA and subsequently, after air oxidation.
11186 | Dalton Trans., 2010, 39, 11180–11187
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This photoreactivity was surprising as the 14+, and 34+,
complexes are stable to irradiation in air or under N2.
However, we had noted that structurally similar ligands can
have surprisingly different sensitivities in a previous study of
the ruthenium complexes tatpp and benzodipyrido[3,2-a:2¢,3¢-
c]phenazine. Irradiation of the latter complex in the presence of
O2 yields the quinone, dipyrido[3,2-a:2¢,3¢-c]benzo[3,4]phenazine-
11,16-quinone, whereas the tatpp complex is not changed.31
15 P. Lei, M. Hedlund, R. Lomoth, H. Rensmo, O. Johansson and L.
Hammarstrom, J. Am. Chem. Soc., 2008, 130, 26–27.
16 N. de Tacconi, R. Chitakunye, F. M. Macdonnell and R. O. Lezna,
J. Phys. Chem. A, 2008, 112, 497–507.
17 N. R. de Tacconi, R. O. Lezna, R. Konduri, F. Ongeri, K. Rajeshwar
and F. M. MacDonnell, Chem.–Eur. J., 2005, 11, 4327–4339.
18 R. Konduri, N. R. de Tacconi, K. Rajeshwar and F. M. MacDonnell,
J. Am. Chem. Soc., 2004, 126, 11621–11629.
19 R. Konduri, H. Ye, F. M. MacDonnell, S. Serroni, S. Campagna and
K. Rajeshwar, Angew. Chem., Int. Ed., 2002, 41, 3185–3187.
20 T. K. Janaratne, A. Yadav, F. Ongeri and F. M. MacDonnell, Inorg.
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Conclusion
21 K. L. Wouters, N. R. Tacconi, R. Konduri, R. O. Lezna and F. M.
MacDonnell, Photosynth. Res., 2006, 87, 41–55.
Substitution of the central hydrogens in tatpp for methoxy groups
in tatpOMe gives a new electron-rich bridging ligand which
was expected to have multi-electron storage capacity like that
of tatpp. The electrochemical and optical properties of the free
ligand, its Zn(II)-adducts, or its dinuclear ruthenium complex 24+
reveal strong similarities to tatpp, with some notable exceptions.
Electrochemistry reveals the redox processes centered on the
tatpOMe ligand to be far less well-behaved and irreversible which
was ultimately shown to be a consequence of ligand decomposition
upon reduction or oxidation. The methoxy groups are shown to
raise the energy of the HOMO and LUMO relative to tatpp but
the greater effect is on the HOMO. Due to the high reactivity
of tatpOMe towards demethylation and demethoxylation upon
reduction or oxidation, the photochemistry of 24+ is not suitable
for multi-electron-storage or transfer.
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Acknowledgements
The authors wish to thank Professor Peter Kroll and Kenneth
Abayan for assistance with the electronic structure calculations.
We also want to thank the US National Science Foundation
(Grants CHE-0911720 (FMM and NT) and CHE-0840509
(500 MHz NMR)) and the Robert A. Welch Foundation (FMM,
Y-1301) for financial support.
27 Y. Shirai, A. J. Osgood, Y. Zhao, Y. Yao, L. Saudan, H. Yang, C. Yu-
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