Variable noninnocence of substituted azobis(phenylcyanamido)diruthenium complexes

Article abstract of DOI:10.1021/ic502487x

The synthetic chemistry of substituted 4,4′-azobis(phenylcyanamide) ligands was investigated, and the complexes [{Ru(tpy)(bpy)}₂( μ-L)][PF₆]₂, where L = 2,2′:5,5′-tetramethyl-4,4′-azobis(phenylcyanamido) (Me₄adpc^2-), 2,2′-dimethyl-4,4′-azobis(phenylcyanamido) (Me₂adpc^2-), unsubstituted (adpc^2-), 3,3′-dichloro-4,4′-azobis(phenylcyanamido) (Cl₂adpc^2-), and 2,2′:5,5′-tetrachloro-4,4′-azobis(phenylcyanamido) (Cl₄adpc^2-), were prepared and characterized by cyclic voltammetry and vis-near-IR (NIR) and IR spectroelectrochemistry. The room temperature electron paramagnetic resonance spectrum of [{Ru(tpy)(bpy)}₂( μ-Me₄adpc)]³⁺ showed an organic radical signal and is consistent with an oxidation-state description [Ru^II, Me₄adpc^?-, Ru^II]³⁺, while that of [{Ru(tpy)(bpy)}₂( μ-Cl₂adpc)]³⁺ at 10 K showed a low-symmetry Ru^III signal, which is consistent with the description [Ru^III, Cl₂adpc^2-, Ru^II]³⁺. IR spectroelectrochemistry data suggest that [{Ru(tpy)(bpy)}₂( μ-adpc)]³⁺ is delocalized and [{Ru(tpy)(bpy)}₂( μ-Cl₂adpc)]³⁺ and [{Ru(tpy)(bpy)}₂( μ-Cl₄adpc)]³⁺ are valence-trapped mixed-valence systems. A NIR absorption band that is unique to all [{Ru(tpy)(bpy)}₂( μ-L)]³⁺ complexes is observed; however, its energy and intensity vary depending on the nature of the bridging ligand and, hence, the complexes oxidation-state description.

Full text of DOI:10.1021/ic502487x

Article
pubs.acs.org/IC
Variable Noninnocence of Substituted
Azobis(phenylcyanamido)diruthenium Complexes
Mohommad M. R. Choudhuri,† Mahdi Behzad,‡ Mousa Al-Noaimi,§ Glenn P. A. Yap,⊥ Wolfgang Kaim,∥
Biprajit Sarkar,∥ and Robert J. Crutchley*,†
†Chemistry Department, Carleton University, Ottawa, Ontario K1S 5B6, Canada
‡Department of Chemistry, Semnan University, Semnan 35131-19111, Iran
§Chemistry Department, Faculty of Science, The Hashemite University, Zarka 13115 Jordan
⊥Department of Chemistry & Biochemistry, University of Delaware, Newark, Delaware 19716, United States
∥Institut fur Anorganische Chemie, Universitat Stuttgart, Pfaffenwaldring 55, Stuttgart D-70550, Germany
̈
̈
S
* Supporting Information
ABSTRACT: The synthetic chemistry of substituted 4,4′-
azobis(phenylcyanamide) ligands was investigated, and the
complexes [{Ru(tpy)(bpy)}2(μ-L)][PF6]2, where L =
2,2′:5,5′-tetramethyl-4,4′-azobis(phenylcyanamido)
(Me4adpc2−), 2,2′-dimethyl-4,4′-azobis(phenylcyanamido)
(Me2adpc2−), unsubstituted (adpc2−), 3,3′-dichloro-4,4′-
azobis(phenylcyanamido) (Cl2adpc2−), and 2,2′:5,5′-tetra-
chloro-4,4′-azobis(phenylcyanamido) (Cl4adpc2−), were prepared and characterized by cyclic voltammetry and vis−near-IR
(NIR) and IR spectroelectrochemistry. The room temperature electron paramagnetic resonance spectrum of [{Ru(tpy)-
(bpy)}2(μ-Me4adpc)]3+ showed an organic radical signal and is consistent with an oxidation-state description [RuII, Me4adpc•−,
RuII]3+, while that of [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ at 10 K showed a low-symmetry RuIII signal, which is consistent with the
description [RuIII, Cl2adpc2−, RuII]3+. IR spectroelectrochemistry data suggest that [{Ru(tpy)(bpy)}2(μ-adpc)]3+ is delocalized
and [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ and [{Ru(tpy)(bpy)}2(μ-Cl4adpc)]3+ are valence-trapped mixed-valence systems. A NIR
absorption band that is unique to all [{Ru(tpy)(bpy)}2(μ-L)]3+ complexes is observed; however, its energy and intensity vary
depending on the nature of the bridging ligand and, hence, the complexes’ oxidation-state description.
superexchange, the mixing of the acceptor wave function with
the bridging ligand highest occupied molecular orbital
(HOMO) effectively extends the acceptor wave function onto
the bridging ligand. It is then possible to conceive of a situation
in which the acceptor wave function is entirely on the bridging
ligand (i.e., the ligand is oxidized) and the noninnocence of the
bridging ligand is realized. The spectroscopic consequence of
this transition from valence-trapped, delocalized to a non-
innocent state has not been previously examined to our
knowledge.
In past research, it was shown that π interactions between
donor−acceptor−donor groups, NCN−diphenylazo−NCN, of
the bridging ligand 4,4′-azobis(phenylcyanamide) dianion
(adpc2−) created a continuous π-superexchange pathway that
spanned the entire ligand.5,6
INTRODUCTION
■
Studies of mixed-valence coordination complexes have played
an important role in understanding electron transfer and
confirming the Marcus−Hush theory.1 These studies had
immediate application to biological electron transfer,2 but
present day application is focused on the creation of novel
electooptic materials and molecular devices.3 To achieve
success in these applications, it is necessary to understand
how mixed-valence properties can be expressed and controlled.
The vast majority of mixed-valence complexes are symmetric
and are composed of electron-donor and -acceptor metal ions
bridged by a polyatomic ligand. Because the metal ions are well
separated, direct overlap of donor and acceptor wave functions
is not possible and metal−metal coupling occurs through the
superexchange mechanism using the bridging ligand orbitals.4
For many types of bridging ligands, metal−metal coupling is
often weak, and this gives rise to a valence-trapped mixed-
valence state. However, if there is a good symmetry and energy
match between donor and acceptor wave functions and the
bridging ligand orbitals, strong metal−metal coupling can result
in the formation of a delocalized state in which the odd electron
is shared equally between metal ions. Electron-transfer or hole-
transfer superexchange can occur using the bridging ligand
empty or filled orbitals, respectively. In the case of hole-transfer
The planarity of this bridging ligand was confirmed in a
crystal structure of [{Ru(tpy)(bpy)}2(μ-adpc)][PF6]2, where
tpy is 2,2′:6′,2″-terpyridine and bpy is 2,2′-bipyridine.5
Electrochemical and spectroscopic studies of the mixed-valence
complex [{Ru(tpy)(bpy)}2(μ-adpc)]3+ were consistent with a
delocalized case or a class III system in the Robin and Day
Received: October 10, 2014
© XXXX American Chemical Society
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classification,7 having a large comproportionation constant Kc =
1013 and a low-energy near-IR (NIR) band centered at 1920
nm (ε = 6000 M−1 cm−1) in N,N-dimethylformamide (DMF).
Hartree−Fock calculations of free adpc2− showed that both
HOMO and lowest unoccupied molecular orbital (LUMO) of
adpc2− provided a pathway for hole- and electron-transfer
superexchange, respectively.5
fore, reveals a transition between oxidation-state descriptions
for these noninnocent systems.
RESULTS AND DISCUSSION
■
Synthesis. Only 4,4′-azodianiline is available commercially
albeit at significant expense, and because of this, the syntheses
of substituted azodianiline derivatives were investigated. These
are described in the Supporting Information (SI).
In a later study,8 gas-phase density functional theory (DFT)
calculations of [{Ru(tpy)(bpy)}2(μ-adpc)]3+ were consistent
with a ligand-centered radical species [RuII, adpc•−, RuII]3+ and,
if correct, this would require reinterpretation of the complex’s
electrochemical and spectroscopic data. However, DFT
calculations of noninnocent systems, while excellent in most
cases,9 are not always reliable because of the limits of the
method and, in particular, inadequate modeling of solvent
perturbations. For example, in a recent study,10 electron
paramagnetic resonance (EPR) spectroscopy unambiguously
assigned the oxidation-state description [RuII, dicyd2−, RuIII]3+
to the mixed-valence complex, [{Ru(NH3)5}2(μ-dicyd)]3+,
where dicyd2− is a 1,4-dicyanamidobenzene dianion, even
though gas-phase DFT calculations supported the dicyd radical
anion description, [RuII, dicyd•−, RuII]3+. DFT calculations on
this molecule could only be reconciled with experimental
results when specific solvent−solute donor−acceptor inter-
actions were incorporated in the model.
Preparation of the substituted 4,4′-azobis(phenylcyanamide)
ligands required three different procedures from the corre-
sponding azodianilines (Schemes 1−3). Method 1 (Scheme 1)
proved to be the most dependable route to azobis-
(phenylcyanamide)s, giving relatively modest yields of 50−
55%, while method 2 (Scheme 2) gave nearly quantitative
yields (95%) for the preparation of Me2adpcH2 and
unsubstituted adpcH2. Methods 2 and 3 (Scheme 3) gave a
trace or no yield at all for chlorinated azodianilines, and this was
probably due to deactivation of the amines by the chlorinated
arene groups. The synthesis of Cl4adpcH2 could only be
achieved following Scheme 1 but with poor purity.
Thallium salts of Cl2adpc2− and Me2adpc2− were prepared
following the published procedure5 for Tl2[adpc] and gave
good elemental analyses. However, Tl2[Cl4adpc] could not be
prepared in high purity, and all attempts to make the Tl+ or
[AsPh4]+ salt of Me4adpc2− were unsuccessful.
An EPR study of [{Ru(tpy)(bpy)}2(μ-adpc)]3+ would have
been expected to provide substantive proof of this complex’s
oxidation-state description. Unfortunately, we were unable to
observe an EPR signal of this complex in DMF down to liquid-
helium temperatures. Although this apparently rapid (en-
hanced) EPR relaxation can be traced to significant
participation of a heavy metal with a high spin−orbit coupling
constant to the singly occupied molecular orbital (SOMO),
thus indirectly suggesting a metal-based spin, we, nevertheless,
decided to synthesize a series of substituted adpc2− ligands,
which when incorporated into a mixed-valence complex, should
provide a range of properties that would be sufficient to
establish an oxidation-state description.
The [{Ru(tpy)(bpy)}2(μ-L)][PF6]2 complexes, where L is
Me2adpc2−, adpc2−, Cl2adpc2−, and Cl4adpc2−, were prepared
by the salt metathesis reaction of [Ru(tpy)(bpy)Cl][PF6] with
Tl2[L] in refluxing DMF, under argon. The ligand substitution
reaction of Me4adpc2−, prepared by deprotonation of the
neutral ligand with butyllithium, with 2 equiv of the solvato
complex [Ru(tpy)(bpy)(DMF)]2+, prepared in situ by adding
TlPF6 to [Ru(tpy)(bpy)Cl][PF6], yielded [{Ru(tpy)(bpy)}2(μ-
Me4adpc)][PF6]2. Recrystallization of [{Ru(tpy)(bpy)}2(μ-
Me2adpc)][PF6]2 resulted in good purity and high yields of
∼90%. The other complexes required column chromatography,
and the final yields of pure complexes were 30−40%. The
complexes were air-stable, and acetonitrile and DMF solutions
of the complexes remained unchanged for several days after
preparation.
This study reports the syntheses of substituted azobis-
(phenylcyanamide) ligands R2R′2-adpc2− (Figure 1). In
NMR Spectroscopy. The 1H NMR spectra of the
complexes R2R′2adpcH2 and Tl2[R2R′2-adpc] salts in dimethyl
sulfoxide (DMSO)-d6 were obtained, and the data are placed in
the SI (Tables S1−S3). For all dinuclear complexes, a total of
14 peaks were observed in the region between 9.70 and 7.15
ppm for tpy and bpy protons, while the signals from the
protons on the bridging adpc2− ligand appear in the region
between 7.20 and 5.65 ppm. Integration confirms that the
complexes are symmetrical dinuclear systems. An unambiguous
assignment of tpy and bpy protons as well as those on adpc2−
ligands was possible using a 2D COSY technique in
combination with the observed splitting pattern and corre-
sponding J values (Figure S13 in the SI).
X-ray Crystallography. The X-ray crystal structure of
[{Ru(tpy)(bpy)}2(μ-adpc)][PF6] has been previously pub-
lished5 and shows that the Ru2+ ion occupies the
pseudooctahedral coordination sphere of nitrogen donor
atoms and that the adpc2− ligand is approximately planar
with two cyanamide groups in an anti conformation and the azo
group in a trans conformation. We were interested to see if
substituents ortho to the azo group might induce the bridging
ligand to adopt a nonplanar geometry. Good-quality crystals of
Figure 1. Substituted 4,4′-azobis(phenylcyanamide) dianion (R2R′2-
adpc2−) ligands.
addition, the complexes [{Ru(tpy)(bpy)}2(μ-R2,R′2adpc)]-
[PF6]2 were prepared and characterized by cyclic voltammetry
(CV), vis-NIR and IR spectroelectrochemistry, and EPR
spectroscopy. EPR spectroscopy showed an organic radical
signal for [{Ru(tpy)(bpy)}2(μ-Me4adpc)]3+ and an axial RuIII
signal for [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+. This study, there-
[{Ru(tpy)(bpy)}2(μ-L)][PF6], where L = Cl4adpc2−
,
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Scheme 1. Method 1
Scheme 2. Method 2
Scheme 3. Method 3
Cl2adpc2−, Me2adpc2−, and Me4adpc2−, could not be grown.
Nevertheless, suitable yellow-orange crystals of the tetraphe-
nylarsonium salt of 2,2′-dimethyl-4,4′-azobis-
(phenylcyanamide) were grown from a boiling mixture of
acetone and water (2:1). Crystallography data and bond lengths
and angles have been placed in the SI (Tables S4−S9). An
ORTEP drawing of the Me2adpc2− ion is shown in Figure S14
in the SI and shows that the planarity of Me2adpc2− is retained
even in the presence of two methyl groups ortho to the azo
group.
here for comparison.11 The voltammograms of the thallium
salts of anionic ligands R2R2′adpc2− in DMF showed two
closely spaced oxidation waves in the region between 0.3 and
0.7 V vs NHE, and both couples were shifted to positive
potentials when electron-donating methyl substituents were
replaced by chloro substituents.
The voltammograms of dinuclear [Ru(tpy)(bpy)}2(μ-
R2R2′adpc)][PF6]2 complexes present two quasi-reversible
oxidation waves in the region between 0.6 and 1.7 V vs
NHE. The first redox couple E1 is further stabilized by about
400 mV from electron-releasing Me4 to electron-withdrawing
Cl4 substituents, while the second redox couple E2 does not
vary much in its position. The assignment of these oxidation
waves will be discussed on the basis of the results obtained from
EPR and IR and UV−vis−NIR spectroelectrochemical studies.
EPR Spectroscopy. The EPR spectroscopic studies were
performed on the singly oxidized dinuclear [{Ru(tpy)-
(bpy)}2(μ-L)]3+ complexes, where L = Me4adpc2− and
Cl2adpc2− in an acetonitrile solution, and their spectra are
shown in Figure 2. Unfortunately, acetonitrile solutions of
[{Ru(tpy)(bpy)}2(μ-L)]3+, where L = Me2adpc2−, adpc2−, and
Cl4adpc2−, remained EPR-silent even at liquid-helium temper-
atures. [{Ru(tpy)(bpy)}2(μ-Me4adpc)]3+ gave a room temper-
ature isotropic spectrum in acetonitrile (Figure 2A) that is
relatively narrow with g = 1.96. This is very similar to the EPR
spectrum of the potassium salt of the 1,4-dicyanamide radical
anion12 for which g = 2.0023 and strongly suggests a SOMO
ELECTRONIC ABSORPTION SPECTROSCOPY
■
The electronic absorption data of the thallium salts of anionic
ligands, dinuclear complexes, and their oxidized products in
DMF are compiled in Table 1. Assignments are based on
previous studies.5,11 All free R2R2′adpc2− ions show an intense
absorption at 500−550 nm, which can be assigned to
cyanamide-to-azo intramolecular charge-transfer (ICT) tran-
s i t i o n s . T h e d i n u c l e a r [ { R u ( t p y ) ( b p y ) } 2 ( μ -
R2R2′adpc)]2+complexes show the same ICT transition as
well as intense metal-to-ligand charge-transfer (MLCT)
transitions for RuII-to-tpy and -bpy chromophores in the
same spectral region.
Electrochemistry. The CV data for the dinuclear
complexes in acetonitrile and the thallium salts of ligands in
DMF are shown in Table 2. The CV data for the mononuclear
complex [Ru(tpy)(bpy)(2,4-Cl2pcyd)][PF6] are also included
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a
a
Table 1. Quantitative Electronic Absorption Data for
Tl2[R2R2′adpc] and [{Ru(tpy)(bpy)}2(μ-R2R2′adpc)][PF6]2
Complexes in DMF and Their Spectroelectrochemical
Oxidation Products
Table 2. CV Data for Tl2[R2R2′adpc] and Ru(tpy)(bpy)
Complexes
ΔE = E2 −
ligand/complex
Tl2[Me2adpc]
E1
E2
E1
b
b
b
b
b
b
0.29 (60)
0.40 (60)
0.47 (67)
0.43 (70)
0.54 (67)
0.62 (67)
0.14
0.14
0.15
compound
absorption
Me2adpc2−
Me2adpc•−
338 (4345), 515 (43600), 545 (37400)
Tl2[adpc]
Tl2[Cl2adpc]
430 (12000), 514 (33900), 667 (2440), 726 (9100),
1118 (760), 1256 (1780)
c
[Ru(tpy)(bpy)(2,4-Cl2pcyd)]
[PF6]
1.04
adpc2−
adpc•−
340 (6700), 500 (54400), 523 (51500)
d
d
d
d
[{Ru(tpy)(bpy)}2(μ-Me4adpc)]
0.67 (59)
0.70 (64)
1.59 (74)
1.56 (82)
0.92
0.86
421 (8400), 498 (31100), 658 (5030), 714 (17400),
1310 (3900)
[PF6]2
Cl2adpc2−
Cl2adpc•−
346 (6600), 506 (51000), 533 (50600)
[{Ru(tpy)(bpy)}2(μ-Me2adpc)]
[PF6]2
426 (15000), 505 (37600), 663 (2910), 719 (9130),
1175 (928), 1341 (2060)
d
d
d
d
[{Ru(tpy)(bpy)}2(μ-adpc)][PF6]2 0.80 (59)
1.56 (71)
1.60 (80)
0.76
0.66
[{Ru(tpy)(bpy)}2(μ-Cl2adpc)]
0.94 (66)
[{Ru(tpy)(bpy)}2(μ- 319 (58500), 501(62300)
Me4adpc)]2+
[PF6]2
d
d
[{Ru(tpy)(bpy)}2(μ-Cl4adpc)]
1.06 (114) 1.61 (74)
0.55
[{Ru(tpy)(bpy)}2(μ- 486 (42500), 652 (11300), 820 (10500), 1678 (5300)
Me4adpc)]3+
[PF6]2
a
In V vs NHE (anodic and cathodic peak separation in mV), scan rate
[{Ru(tpy)(bpy)}2(μ- 453 (42000), 696 (14500)
Me4adpc)]4+
0.1 V/s, in 0.1 M tetrabutylammonium hexafluorophosphate, Fc+/Fc
b
c
d
[{Ru(tpy)(bpy)}2(μ- 321 (62000), 499 (69100)
used as the internal reference. In DMF. Taken from ref 11. In
acetonitrile.
Me2adpc)]2+
[{Ru(tpy)(bpy)}2(μ- 479 (44500), 652 (12800), 823 (12200), 1758 (7000)
Me2adpc)]3+
[{Ru(tpy)(bpy)}2(μ- 452 (44700), 652 (8400), 823 (7700)
Me2adpc)]4+
[{Ru(tpy)(bpy)}2(μ- 293 (81900), 317 (72100), 490 (69100)
adpc)]2+
[{Ru(tpy)(bpy)}2(μ- 292 (86900), 314 (73700), 471 (41300), 652
adpc)]3+
(13900), 812 (12700), 1920 (10000)
[{Ru(tpy)(bpy)}2(μ- 312 (73400), 456 (41900), 856 (12200)
adpc)]4+
[{Ru(tpy)(bpy)}2(μ- 321 (62000), 499 (69100)
Cl2adpc)]2+
[{Ru(tpy)(bpy)}2(μ- 482 (42300), 667 (13100), 833 (8700), 1923 (10400)
Cl2adpc)]3+
[{Ru(tpy)(bpy)}2(μ- 487 (25000), 668 (5300), 834 (6400), 1124 (11500)
Cl2adpc)]4+
[{Ru(tpy)(bpy)}2(μ- 321 (63700), 534 (60000)
Cl4adpc)]2+
[{Ru(tpy)(bpy)}2(μ- 517 (35900), 677 (7900), 876 (7700), 1923 (6600)
Cl4adpc)]3+
[{Ru(tpy)(bpy)}2(μ- 462 (31700), 677 (5660), 876 (7000), 1120 (7020)
Cl4adpc)]4+
a
In DMF, 0.1 M TBAH, λ in nm, ε in L mol−1 cm−1 in parentheses.
that is mostly localized on the bridging ligand and corresponds
to the oxidation-state description [RuII, Me4adpc−, RuII]. On
the other hand, [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ remained
EPR-silent at room temperature and gave a broad anisotropic
EPR signal at 110 K (Figure 2B) with g1, g2, and g3 = 2.16, 2.09,
and 1.96, respectively, gav = 2.075, and g anisotropy Δg = g1 −
g3 = 0.20. The spectral pattern is very similar to that of
mononuclear [Ru(tpy)(bpy)(2,4-Cl2pcyd)]2+, for which g1, g2,
and g3 = 2.34, 2.10, and 1.92, respectively,13 and is typical of a
low-spin RuIII d5 ion,13,14 and this suggests the oxidation-state
description of [RuII, Cl2adpc2−, RuIII]3+ for [{Ru(tpy)-
(bpy)}2(μ-Cl2adpc)]3+. However, this formal description
should be qualified based on DFT calculations of [Ru-
(acac)2(L)]n, where n = 1−, 0, and 1+, acac is acetylacetonate,
and L is a redox-active ligand, which suggested that, for a
similar range of g values, a pure RuIII oxidation-state description
is inaccurate and that significant spin density is located on the
redox-active ligand.15
Figure 2. EPR spectra of [{Ru(tpy)(bpy)}2(μ-L)]3+ in acetonitrile:
(A) L = Me4adpc2− at 295 K; (B) L = Cl2adpc2− at 10 K.
density with a change in the R and R′ substituents from
electron-donating to electron-withdrawing properties. The
greater stability of Cl2adpc2− to oxidation (Table 2) compared
to Me4adpc2− is the probable reason for this transition. The
consistency of this interpretation will be examined by the
complexes’ IR and vis−NIR spectroelectrochemistry.
IR Spectroelectrochemistry. For ruthenium cyanamide
complexes, the ν(NCN) band frequency is dependent on the
ruthenium oxidation state. The reason for this is that the
cyanamide anion possesses two resonance forms
The EPR spectroscopy of the [{Ru(tpy)(bpy)}2(μ-
R2R′2adpc)]3+ complexes suggests that a transition has
occurred from mostly ligand-centered to metal-centered spin
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which contribute to bonding in the cyanamide group and the
frequency of ν(NCN). The population of resonance forms can
be perturbed by coordination to a metal ion and the nature of
the coordination sphere. When the cyanamide group is bonded
to the π-acid RuIII ion, the contribution of resonance form A
increases and ν(NCN) approaches that of an organic
carbodiimide (range of 2100−2150 cm−1).16 If the cyanamide
group is bonded to the π-donor RuII ion, resonance form B
increases its contribution and ν(NCN) approaches that of a
neutral cyanamide (ca. 2250 cm−1).17 Thus, for a given RuII
coordination sphere, oxidation to RuIII results in a shift of
ν(NCN) to lower frequencies. This has been shown in IR
spectroelectrochemical studies of [Ru(tpy)(bpy)(2,4-
Cl2pcyd)]+, [Ru(NH3)5(2,3,5,6-Cl4pcyd)]+, and [Ru-
(NH3)3(bpy)(2,3-Cl2pcyd)]+ complexes.13,18 Because the IR
time scale is on the order of 1013 s−1, the number of ν(NCN)
bands observed is a good measure of whether a mixed-valence
complex is valence-trapped or delocalized.19 For symmetric
mixed-valence complexes incorporating a bridging adpc2−
ligand, two cyanamide ν(NCN) bands are predicted if
valence-trapped, whereas only one is predicted if delocalized.
If instead the bridging adpc2− ligand is oxidized to a radical
anion, delocalization within the radical is expected to result in
the observation of only one ν(NCN) band. The one-electron
oxidation of [Ru(tpy)(bpy)}2(μ-adpc)]2+ to [Ru(tpy)-
(bpy)}2(μ-adpc)]3+ and two-electron oxidation to [Ru(tpy)-
(bpy)}2(μ-adpc)]4+ are summarized in terms of possible
oxidation-state descriptions in Scheme 4.
Figure 3. IR spectroelectrochemical oxidation of (A) [{Ru(tpy)-
(bpy)}2(μ-Me4adpc)]2+ forming [{Ru(tpy)(bpy)}2(μ-Me4adpc)]3+
and (B) the trication complex forming [{Ru(tpy)(bpy)}2(μ-
Me4adpc)]4+ in DMF, 0.1 M TBAH: (A) 0−0.58 V and (B) 0.58−
0.70 V vs Ag/AgCl.
Scheme 4. Oxidation-State Descriptions for One- and Two-
Electron Oxidation of [{Ru(tpy)(bpy)}2(μ-adpc)]2+
IR spectroelectrochemistry in the region of ν(NCN) for the
[{Ru(tpy)(bpy)}2(μ-L)][PF6]2 complexes, where L =
Me4adpc2−, adpc2−, and Cl2adpc2−, in DMF is shown in
Figures 3−5. Those for L = Me2adpc2− and Cl4adpc2− are very
similar to those for L = Me4adpc2− and Cl2adpc2−, respectively,
and have been placed in the SI (Figures S15 and S16). All
complexes show good reversibility (>95%) upon single-electron
oxidation to [{Ru(tpy)(bpy)}2(μ-L)]3+, while the second
oxidation is not very reversible (less than 90% recovery of
the initial spectrum).
Figure 3 shows the IR spectra resulting from the one- and
two-electron oxidations of [{Ru(tpy)(bpy)}2(μ-Me4adpc)]2+.
In Figure 3A, oxidation to the trication is accompanied by a
large decrease in the intensity of the ν(NCN) band at 2160
cm−1 together with a slight shift to higher frequencies. On the
basis of EPR studies (Figure 2A), this change is associated with
the formation of radical anion Me4adpc and the oxidation-state
description [RuII, Me4adpc•−, RuII]3+ in Scheme 4. Further
oxidation (Figure 3B) shows the growth of a weak band at 2030
cm−1, which is consistent with anionic cyanamide bound to
RuIII and suggests an oxidation-state description [RuII,
Figure 4. IR spectroelectrochemical oxidation of (A) [{Ru(tpy)-
(bpy)}2(μ-adpc)]2+ forming [{Ru(tpy)(bpy)}2(μ-adpc)]3+ and (B)
the trication complex forming [{Ru(tpy)(bpy)}2(μ-adpc)]4+ in DMF,
0.1 M TBAH: (A) 0−0.66 V and (B) 0.66−0.80 Vvs Ag/AgCl.
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consistent with a valence-trapped oxidation-state description
[RuII, Cl2adpc2−, RuIII]3+. Further oxidation forming [{Ru-
(tpy)(bpy)}2(μ-Cl2adpc)]4+ shows a loss of the low-frequency
band and a further decrease in the ν(NCN) band at 2160 cm−1.
A single band is consistent with the oxidation-state description
[RuIII, Cl2adpc2−, RuIII]4+ or [RuII, Cl2adpc0, RuII]4+ in Scheme
4, but the latter state description seems unlikely because
ν(NCN) for this description should have a much greater
frequency.
As shown by Figures 3−5, significant differences in the
observed ν(NCN) bands occur depending on the appropriate
oxidation-state description in Scheme 4. It remains to explore
how these oxidation-state descriptions influence the electronic
absorption spectroscopy of these systems.
Vis−NIR Spectroelectrochemistry. The vis−NIR spec-
troelectrochemical oxidation of Cl2adpc2− in DMF is
representative of the R2R′2adpc2− ions. As shown in Figure 6,
Figure 5. IR spectroelectrochemical oxidation of (A) [{Ru(tpy)-
(bpy)}2(μ-Cl2adpc)]2+ forming [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ and
(B) the trication complex forming [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]4+ in
DMF, 0.1 M TBAH: (A) 0−0.66 V and (B) 0.66−0.82 V vs Ag/AgCl.
Figure 6. Vis−NIR spectroelectrochemical oxidation of Cl2adpc2−
forming radical anion Cl2adpc•− in DMF, 0.1 M TBAH: 0−0.80 V vs
Ag/AgCl.
Me4adpc•−, RuIII]4+. For [{Ru(tpy)(bpy)}2(μ-adpc)]2+, oxida-
tion gives significantly different spectral changes in the region
of ν(NCN) (see Figure 4).
The main feature of both parts A and B of Figure 4 is a single
ν(NCN) band, and this is consistent with a symmetric state in
which the oxidation-state descriptions are delocalized [RuII,
adpc2−, RuIII]3+ and [RuIII, adpc2−, RuIII]4+, respectively.
However, there are additional weak bands that are difficult to
rationalize. In Figure 4A, oxidation to the mixed-valence
oxidation to the radical anion Cl2adpc•− results in a moderate
decrease in the ICT band1 at 500 nm and the growth of two
bands at 715 and 1315 nm of weaker intensity. Further
oxidation of the radical to Cl2adpc0 proved to be irreversible.
The vis−NIR spectroelectrochemical oxidation of [{Ru-
(tpy)(bpy)}2(μ-L)]2+ complexes, where L = Me4adpc2−,
adpc2−, and Cl2adpc2−, in DMF is shown in Figures 7−9, and
spectral data are found in Table 1. Those for L = Me2adpc2−
and Cl4adpc2− have been placed in the SI (Figures S17 and
S18). For one-electron oxidation, the spectroscopic changes
seen in Figures 7A−9A are similar but show significant
differences that must be related to the noninnocence of the
R2R′2adpc2− ligand and the change in the oxidation-state
description of the complexes. In Figure 7A, oxidation of
[{Ru(tpy)(bpy)}2(μ-Me4adpc)]2+ to the trication results in a
decrease in the ICT band at 501 nm and the growth of bands at
652, 820, and 1678 nm. On the basis of the trication’s EPR
spectrum (Figure 2A), the assigned oxidation-state description
[RuII, (Me4adpc•−, RuII]3+, and the spectrum of Cl2adpc•−
(Figure 6), it seems reasonable to assign the bands at 652, 820,
and 1678 nm to ICT transitions of Me4adpc•− with possibly
some MLCT character mixed into the low-energy bands. For
[{Ru(tpy)(bpy)}2(μ-adpc)]3+ (Figure 8A), a broad absorption
appears at ca. 1000 nm, which for [{Ru(tpy)(bpy)}2(μ-
Cl2adpc)]3+ (Figure 9A) resolves itself to a band at 1124 nm.
Upon two-electron oxidation to [{Ru(tpy)(bpy)}2(μ-
Cl2adpc)]4+ (Figure 9B), this band becomes a dominant
feature with λmax = 1124 nm, and on the basis of the oxidation-
state description [RuIII, Cl2adpc2−, RuIII]4+, we assign this band
to a cyanamide-to-RuIII LMCT transition.11
complex [{Ru(tpy)(bpy)}2(μ-adpc)]3+ results in a moderate
−1
decrease in the intensity of the band at 2160 cm
that is
accompanied by the growth of a weak band at 2265 cm−1. The
band at 2265 cm−1 is similar to that seen for protonated
cyanamides (adpcH2 has ν(NCN) = 2225 cm−1) and may be
due to a neutral radical cyanamide group ν(NCN). However,
this would require valence trapping of adpc•−, with a neutral
radical cyanamide at one end and an anionic cyanamide at the
other. This is unlikely because there is no barrier for
delocalization and indeed only one ν(NCN) band is observed
for a complex with Me4adpc•− (Figure 3A). Further oxidation
(Figure 4B) only slightly decreases the band at 2160 cm−1 and
slightly increases the weak band at 2250 cm−1 but sees the
growth of a weak intensity band at 2030 cm−1 that may be a
ν(NCN) due to an anionic cyanamide bound to RuIII. Thus,
there appear to be ν(NCN) bands that are consistent with all
three oxidation-state descriptions of the tetracation in Scheme
4. The oxidation of [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]2+ shows
significantly less complexity (Figure 5).
In Figure 5A, oxidation forming the mixed-valence complex
[{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ results in a decrease of the
ν(NCN) band at 2160 cm−1 and the growth of a band at 2040
cm−1. The observation of two ν(NCN) bands and an EPR
spectrum of a low-symmetry RuIII ion (Figure 2b) is entirely
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Figure 7. Vis−NIR spectroelectrochemical oxidation of [{Ru(tpy)-
(bpy)}2(μ-Me4adpc)}]2+ forming (A) [{Ru(tpy)(bpy)}2(μ-
Me4adpc)]3+ and (B) [{Ru(tpy)(bpy)}2(μ-Me4adpc)]4+ in DMF, 0.1
M TBAH: (A) 0−0.48 V and (B) 0.50−0.58 V vs Ag/AgCl.
Figure 9. Vis−NIR spectroelectrochemical oxidation of (A) [{Ru-
(tpy)(bpy)}2(μ-Cl2adpc)]2+ forming [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+
and (B) forming [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]4+ in DMF, 0.1 M
TBAH: (A) 0−0.78 V and (B) 0.81−0.98 Vvs Ag/AgCl.
R2R′2adpc substituent. These changes to the NIR band for
the trication complexes are summarized in Table 3.
Table 3. NIR Band Properties of [{Ru(tpy)(bpy)}2(μ-L)]3+
Complexes
a
L
Eop, cm−1 intensity ε, L mol−1 cm−1 oscillator strength f
Me4adpc2−
Me2adpc2−
adpc2−
Cl2adpc2−
Cl4adpc2−
5960
5668
5208
5200
5200
5300
7000
0.074
0.098
0.141
0.167
0.118
11000
10400
6600
a
The oscillator strength was calculated by summing the oscillator
strengths of Gaussian peaks used to model the NIR band envelope.
As supported by EPR and IR spectroelectrochemcal studies,
the [{Ru(tpy)(bpy)}2(μ-L)]3+ complexes, where L = adpc2−,
Cl2adpc2−, and Cl4adpc2−, possess an oxidation-state descrip-
tion [RuII, R2R′2adpc2−, RuIII]3+, and it is then appropriate to
describe their NIR band as an intervalence transition. Although
the data are limited in Table 3, it is suggested that the shift in
Eop to higher energies with substituent donor properties is an
indicator of increasing radical anion R2R′2adpc character. We
attempted to model this change using theory, but DFT
calculations including solvent perturbations would not
converge.
Figure 8. Vis−NIR spectroelectrochemical oxidation of [{Ru(tpy)-
(bpy)}2(μ-adpc)}]2+ forming (A) [{Ru(tpy)(bpy)}2(μ-adpc)]3+ and
(B) [{Ru(tpy)(bpy)}2(μ-adpc)]4+ in DMF, 0.1 M TBAH: (A) 0−0.75
V and (B) 0.80−1.00 Vvs Ag/AgCl.
CONCLUSION
■
The synthetic chemistry of substituted azobis-
(phenylcyanamide) ligands is presented. Electron-withdrawing
substituents deactivate azodianilines and limit the synthetic
pathways to cyanamides. The complexes [{Ru(tpy)(bpy)}2(μ-
Another important change that occurs in Figures 7−9 is that
the low-energy NIR band red shifts and becomes noticeably
broader with the donor/withdrawing properties of the
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L)][PF6]2, where L = Me4adpc2−, Me2adpc2−, adpc2−,
Cl2adpc2−, and Cl4adpc2−, were prepared and characterized by
CV and vis−NIR and IR spectroelectrochemistry. The EPR
spectrum of [{Ru(tpy)(bpy)}2(μ-Me4adpc)]3+ showed an
organic radical signal and is consistent with an oxidation-state
description [RuII, Me4adpc•−, RuII]3+, while that of [{Ru(tpy)-
(bpy)}2(μ-Cl2adpc)]3+ showed a low-symmetry RuIII signal,
which is consistent with the description [RuIII, Cl2adpc2−,
RuII]3+. IR spectroelectrochemistry of the [{Ru(tpy)(bpy)}2(μ-
L)]3+ complexes suggested that, for L = Me4adpc2− and
Me2adpc2− complexes, the bridging ligand is a radical anion,
while for L = adpc2−, Cl2adpc2−, and Cl4adpc2−, the oxidation-
state description [RuIII, L2−, RuII]3+ is appropriate. Further-
more, IR data suggest that [{Ru(tpy)(bpy)}2(μ-adpc)]3+ is
delocalized and [{Ru(tpy)(bpy)}2(μ-Cl2adpc)]3+ and [{Ru-
(tpy)(bpy)}2(μ-Cl4adpc)]3+ are valence-trapped mixed-valence
systems. The transition between oxidation-state descriptions of
the complexes [{Ru(tpy)(bpy)}2(μ-L)]3+ is also examined by
its effect on the complexes’ electronic spectra. For [{Ru(tpy)-
(bpy)}2(μ-L)]3+, the NIR band is unique to the trication, but its
energy and intensity vary depending on its oxidation-state
description.
as working and counter electrodes and Ag/AgCl as the quasi-reference
electrode. EPR spectra of the complexes were recorded in acetonitrile
at room temperature to 10 K by using a Bruker system EMX, and a
ESR 900 continuous-flow cryostat from Oxford Instruments was used
for this purpose. Jandel Peakfit was used to model the NIR charge-
transfer bands.
Syntheses of Azobis(phenylcyanamide) Ligands. 3,3′-Di-
chloro-4,4′-azobis(phenylcyanamide)·0.25H2O, Cl2adpcH2. A boil-
ing solution of benzoyl chloride (2 g, 14.2 mmol) in 20 mL of acetone
was added dropwise to a refluxing solution of NH4SCN (1.08 g, 14.2
mmol) in 20 mL of acetone for 30 min. To the mixture was added a
boiling solution of 3,3′-dichloro-4,4-azodianiline (1.25 g, 4.7 mmol) in
acetone (100 mL) and the reflux continued for 12 h. The resulting
suspension was added to 1 L of distilled water to afford precipitation of
the orange benzoylthiourea derivative, which was then filtered, washed
with water, and then added to a boiling solution of 2 M NaOH (400
mL) and stirred vigorously for about 20 min until a deep-red solution
formed. The solution was cooled to 60−65 °C and treated with a
solution of lead acetate (5.36 g, 14.2 mmol) in distilled water (20 mL)
while stirring to effect precipitation of black PbS. After 5 min, the
reaction mixture was filtered through a Buchner funnel into an ice-
cooled suction flask. The dark-red filtrate was then acidified with
glacial acetic acid (100 g, 1.67 mol), precipitating a dark-green
product. Recrystallization from 1.6 L of boiling acetone afforded 1.05 g
of a yellowish-green product. Yield: 71.3%. Anal. Calcd for
C14H8.5N6O0.25Cl2: C, 50.10; H, 2.55; N, 25.04. Found: C, 50.25; H,
In summary, the tricationic dinuclear ruthenium complexes
are shown by EPR and IR and vis−NIR spectroelectrochem-
istry to have properties that switch between oxidation-state
descriptions [RuII, adpc•−, RuII]3+ and [RuIII, adpc2−,
RuII]3+depending on the donor−acceptor properties of the
substituents on adpc2−. This study reveals in a single system a
transformation from bridging ligand noninnocence to mixed-
valent class III and II properties.
1
2.64; N, 25.00. H NMR (300 MHz, DMSO-d6): 10.51 (2H, s), 7.96
(2H, d), 7.93 (2H, dd), 7.39 (2H, d). IR (KBr): ν(NCN) 2248 cm−1.
Mass spectrum: m/z 331 (M+).
2,2′:5,5′-Tetrachloro-4,4′-azobis(phenylcyanamide), Cl4adpcH2.
The synthesis was similar to that above with some modification: 4
equiv of NH4SCN and benzoyl chloride were used for the preparation
of benzoyl thiocyanate, which was then refluxed with 1 equiv of
2,2′:5,5′-tetrachloro-4,4′-azodianiline in almost double the volume of
1
acetone for 36 h to generate the crude product. H NMR showed the
EXPERIMENTAL SECTION
crude to be made of mostly Cl2adpcH2 (∼80%) and an impurity
(likely monocyanamide). The poor solubility frustrated attempts at
purification, and so the compound was used as isolated to make its
thallium salt (see below). 1H NMR (300 MHz, DMSO-d6): 7.71 (2H,
s), 7.27 (2H, s). IR (KBr): ν(NCN) 2251 cm−1.
■
Materials. All of the reagents and solvents were reagent-grade or
better and were purchased from Aldrich, Acros, and Strem Chemicals.
The syntheses of reagent azodianilines and complexes are described in
the SI. Indium−tin oxide (ITO) glass plates were purchased from
Delta Technologies, and gold foil was purchased from Buckbee Mears.
Tl2[adpc] and [{Ru(tpy)(bpy)}2(μ-adpc)][PF6]2 have been previously
reported.1 The reagent complexes Ru(tpy)Cl3·H2O and [Ru(tpy)-
(bpy)Cl][PF6]2 were prepared following modified literature proce-
dures (see the SI).20,21
2,2′:5,5′-Tetramethyl-4,4′-azobis(phenylcyanamide)·0.4acetone·
0.2CH3COOH, Me4adpcH2. The synthesis was similar to that of
Cl2adpcH2 except that crude Me4adpcH2 was recrystallized from
acetone/water/glacial acetic acid (1:1:1) to give yellow crystalline
flakes in 50% final yield. Anal. Calcd for C19.6H21.2N6O0.8: C, 66.57; H,
1
6.04; N, 23.77. Found: C, 66.35; H, 5.69; N, 23.70. H NMR (300
Instrumentation. BOMEM Michelson MB100 Fourier transform
MHz, DMSO-d6): 9.67 (2H, s), 7.45 (2H, s), 7.04 (2H, s), 2.65 (6H,
s), 2.21 (6H, s). IR (KBr): ν(NCN) 2233 cm−1. Mass spectrum: m/z
318 (M+). The presence of acetone and acetic acid was confirmed by
1H NMR. Integration was consistent with the above formula.
4,4′-Azobis(phenylcyanamide), adpcH2. adpcH2 was prepared
previously5 using method 1 (Scheme 1) and gave a reported yield
of 50%. Two alternative procedures are presented below.
Method 2 (Scheme 2). To 4,4′-azodianiline (1.0 g, 4.7 mmol) in 60
mL of 40% DMF in glacial acetic acid was added half of a solution of
CNBr (1.20 g, 11.3 mmol) in 20 mL of 40% DMF in glacial acetic
acid, and the resulting solution was stirred at −10 to −15 °C for 30
min. The rest of the CNBr solution was then added to the reaction
mixture, followed by the dropwise addition of 20 mL of a 1 M NaOH
solution over 1 h. The reaction mixture was permitted to rise to room
temperature and stirred for 24 h, precipitating golden-yellow
microcrystalline adpcH2, which was filtered, washed with a copious
amount of water, and dried overnight. Yield: 1.21 g, 95%. Anal. Calcd
for adpcH2·H2O, C14H12N6O: C, 59.99; H, 4.32; N, 29.98. Found: C,
59.70; H, 4.39; N, 29.76. 1H NMR (300 MHz, DMSO-d6): 10.61 (2H,
s), 7.88 (4H, d), 7.12 (4H, d). IR (KBr): ν(NCN) 2225 cm−1, in
agreement with the literature.1
infrared and Cary 5 UV−vis−NIR spectrophotometers were used to
1
record IR and electronic spectra, respectively. H NMR spectra were
obtained at ambient temperature using a Bruker AMX-400 NMR or a
Bruker 300 Ultra Shield spectrometer and referenced to tetrame-
thylsilane (0.00 ppm). CV studies were performed with a Metrohm
Autolab PGSTAT30 potentiostat/galvanostat on argon-saturated
solutions of the complexes in DMF with 0.1 M TBAH as the
supporting electrolyte. A three-electrode arrangement consisting of a
platinum disk working electrode (BAS; 1.6 mm diameter), a platinum
wire auxiliary electrode, and Ag/AgCl quasi-reference electrode was
used. The electrochemical cell consisted of a double-jacketed glass
container with an internal volume of 15 mL. Ferrocene (E° = 0.665 V
vs NHE)22 was used for calibration. UV−vis−NIR spectroelec-
trochemistry of all dinuclear complexes and anionic ligands was
performed by using an optically transparent thin-layer electrochemistry
(OTTLE) cell.18,23 The cell had interior dimensions of roughly 1 × 2
cm with a path length of 0.2 cm and was fitted with a AgCl/Ag quasi-
reference electrode. ITO-coated glass served as working and counter
electrodes. UV−vis−NIR spectra were obtained on a Cary 5
spectrophotometer, and the potential was controlled by using a BAS
CV-27 potentiostat. At any given potential, the system was allowed to
come to equilibrium (i ≈ 0 μA) before the spectrum was taken. IR
spectroelectrochemistry was performed using the OTTLE cell
described above using a gold foil (500 line/in. and 60% transmittance)
Method 3 (Scheme 3). To a solution of 4,4′-azodianiline (1.0 g, 4.7
mmol) in tetrahydrofuran (THF; 100 mL) was added dropwise about
4.5 mL (10.62 mmol) of a 2.5 M BuLi solution in hexane at −30 to
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−40 °C, and the mixture was stirred for 30 min, during which time the
dark-brown solution turned red because of formation of the
corresponding amide. The solution was then brought to room
temperature, and CS2 (0.7 mL, 11.2 mmol) was added. The resulting
mixture was then stirred at room temperature for 24 h, during which
time an orange precipitate was formed. The precipitate was filtered,
washed with THF, and dried overnight to obtain 0.85 g of pure lithium
dithiocarbamate (yield: 48%). To a solution of dithiocarbamate (0.3 g,
0.80 mmol) in acetonitrile (50 mL) was added concentrated NH3 (30
mL), and the temperature of the solution was brought down to 0 to
−5 °C. Diacetoxyiodobenzene (DIB; 0.26 g, 0.80 mmol) was added
portionwise to the solution over a period of 30 min, during which time
a light-yellow precipitate of sulfur started to separate out, and the
mixture was stirred for 2 h at room temperature. The rest of DIB (0.26
g, 0.80 mmol) was added again over a period of 30 min and stirring
continued for 12 h to effect the complete precipitation of yellow sulfur.
The orange-red filtrate, collected after removal of sulfur, was added to
500 mL of water and acidified with glacial acetic acid to afford a very
fine yellow-orange precipitate of adpcH2. The yellow precipitate was
filtered, washed with water, and dried overnight to obtain 0.2 g of the
pure product. Yield: 95% from dithiocarbamate. The 1H NMR
spectrum was identical with that of adpcH2 prepared using method 2.
2,2′-Dimethyl-4,4′-azobis(phenylcyanamido)·0.1H2O,
Me2adpcH2. Method 1. Yield: 55%. Method 2. Yield: 97%. Method 3.
Yield: 95%. Anal. Calcd for C16H14.2N6O0.1: C, 65.78; H, 4.90; N,
28.77. Found: C, 65.74; H, 4.86; N, 28.48. 1H NMR (300 MHz,
DMSO-d6): 10.54 (2H, s), 7.65 (2H, d), 6.96 (2H, s), 6.90 (2H, dd),
2.67 (6H, s). IR (KBr): ν(NCN) 2243 cm−1.
Syntheses of Tl2[R2R2′adpc] Salts. Tl2[Me2adpc]. Me2adpcH2
(0.25 g, 0.93 mmol) was dissolved in a boiling acetone/water mixture
(2:1, 300 mL), and about 1 mL of triethylamine was added. An orange
precipitate quickly formed upon the addition of a solution of thallium
acetate (0.45 g, 1.86 mmol) in an acetone/water mixture (2:1, 50 mL).
The resulting mixture was then boiled for an additional 5 min and
slowly cooled at −20 °C. Filtration afforded a bright-red-orange
microcrystalline solid, which was washed with a cold water/acetone
mixture and dried overnight to obtain 0.46 g of pure Tl2[Me2adpc].
Yield: 71%. Anal. Calcd for C16H12N6Tl2: C, 27.55; H, 1.74; N, 12.06.
Found: C, 27.48; H, 1.52; N, 11.91. 1H NMR (300 MHz, DMSO-d6):
7.36 (2H, d). 6.49 (2H, s). 6.40 (2H, d). 2.45 (6H, s). IR (KBr):
ν(NCN) 2083 and 2061 cm−1.
was chilled overnight at −20 °C and filtered to remove a fine white
precipitate of TlCl. The dark-brown filtrate was rotaevaporated to
dryness, redissolved in acetone (30 mL), and added to anhydrous
ether (1 L) to precipitate 0.40 g of the crude product. The dinuclear
complex was purified using an alumina column (grade III, Brockman
acidic, 330 g) and toluene/acetonitrile and DMF as eluents. The first
three bands containing starting materials and mononuclear impurities
were eluted by 1:1, 1:2, and 1:3 toluene/acetonitrile. The fourth
reddish-brown band was identified as the dinuclear complex and eluted
with 1:1 DMF/acetonitrile. The resulting mass after removal of the
solvent was redissolved in acetone (20 mL), filtered, and treated with
anhydrous diethyl ether (300 mL) to precipitate the dark-brown
product. Recrystallization of the complex from its saturated solution in
DMF by slow diffusion of diethyl ether, followed by vacuum drying,
afforded 0.20 g of the pure complex. Yield: 38%. Anal. Calcd for
C68H56N16P2F12ORu2: C, 50.88; H, 3.52; N, 13.96. Found: C, 51.07;
H, 3.87; N, 14.19. 1H NMR (300 MHz, DMSO-d6): 9.68 (2H, d), 8.97
(2H, d), 8.92 (4H, d), 8.80 (4H, d), 8.70 (2H, d), 8.41 (2H, t), 8.33
(2H, t), 8.11 (6H, dd), 7.86 (2H, t), 7.75 (4H, d), 7.55−7.39 (6H, m),
7.20−7.13 (2H, m), 7.12 (2H, s), 5.61 (2H, s), 2.21 (6H, s), 1.80 (6H,
s). IR (KBr): ν(NCN) 2152 cm−1.
[{Ru(tpy)(bpy)}2(μ-Me2adpc)][PF6]2·H2O. A mixture of [Ru(tpy)-
(bpy)Cl][PF6] (0.38 g, 0.56 mmol) and Tl2[Me2adpc] (0.20 g, 0.28
mmol) was refluxed in DMF (50 mL) for 48 h. The reaction mixture
was then chilled at −20 °C and filtered through Celite to remove a fine
white precipitate of TlCl. The dark-brown filtrate was then
rotaevaporated to dryness to yield 0.51 g of the crude product,
which after recrystallization from its saturated solution in DMF by
slow diffusion of diethyl ether, followed by filtration and vacuum
drying, gave 0.41 g of the pure dark-brown complex. Yield: 93%. Anal.
Calcd for C66H52N16P2F12ORu2: C, 50.26; H, 3.32; N, 14.21. Found:
1
C, 50.27; H, 3.13; N, 13.86. H NMR (300 MHz, DMSO-d6): 9.69
(2H, d), 9.00 (2H, d), 8.94 (4H, d, 8.82 (4H, d), 8.73 (2H, d), 8.44
(2H, t), 8.35 (2H, t), 8.13 (6H, dd), 7.89 (2H, t), 7.77 (4H, d), 7.49
(6H, dd), 7.18 (4H, dd), 5.95 (2H, d), 5.87 (2H, s), 2.31 (6H, s). IR
(KBr): ν(NCN) 2164 cm−1.
[{Ru(tpy)(bpy)}2(μ-adpc)][PF6]2. This complex was prepared by
following literature procedures.5
[{Ru(tpy)(bpy)}2(μ-Cl2adpc)][PF6]2·DMF. A mixture of [Ru(tpy)-
(bpy)Cl][PF6] (0.61 g, 0.91 mmol) and Tl2[Cl2adpc] (0.33 g, 0.45
mmol) in 100 mL of DMF was refluxed for 48 h. After removal of a
fine white precipitate of TlCl, the dark-brown filtrate was concentrated
to 20 mL and added to diethyl ether (600 mL) to precipitate the crude
product (0.71 g). About 0.41 g of the crude product was then
dissolved in 80 mL of 1:1 acetonitrile/toluene and purified by column
chromatography using an alumina column (50 cm × 3 cm, grade III,
Brockman acidic, 250 g). The first three bands containing starting
materials and mononuclear impurities were eluted with 1:1 and 2:1
acetonitrile/toluene. The fourth dark-brown band containing the
target compound was eluted with 3:1 acetonitrile/toluene, filtered, and
evaporated to dryness to obtain the dark-brown solid. Recrystallization
was achieved by slow diffusion of diethyl ether into a saturated
solution of the complex in DMF (30 mL), which after filtration and
vacuum drying gave 0.36 g of the pure dark-brown complex. Yield:
36%. Anal. Calcd for C67H51N17Cl2P2F12ORu2: C, 48.09; H, 3.07; N,
14.23. Found: C, 47.97; H, 3.45; N, 14.26. 1H NMR (300 MHz,
DMSO-d6): 9.67 (2H, d), 8.98 (2H, d), 8.93 (4H, d), 8.81 (4H, d),
8.72 (2H, d), 8.50−8.30 (4H, m), 8.12 (6H, m), 7.89 (2H, t), 7.76
(4H, d), 7.49 (6H, m), 7.24 (2H, dd), 7.18 (2H, t), 5.88 (2H, d). IR
(KBr): ν(NCN) 2164 cm−1.
Tl2[adpc]. This compound was prepared by following literature
procedures.5
Tl2[Cl2adpc]. This compound was prepared following the method
used to prepare Tl2[Me2adpc]. Yield: 55%. Anal. Calcd for
C14H6N6Tl2: C, 22.77; H, 0.82; N, 11.39. Found: C, 22.92; H, 1.19;
1
N, 11.37. H NMR (300 MHz, DMSO-d6): 7.55 (2H, d), 7.44 (2H,
dd), 6.96 (2H, d). IR (KBr): ν(NCN) 2099 cm−1.
Tl2[Cl4adpc]. Crude Tl2[Cl4adpc] was prepared following the
method used for Tl2[Me2adpc]. 1H NMR (300 MHz, DMSO-d6):
7.59 (2H, s), 6.95 (2H, s). IR (KBr): ν(NCN) 2115 cm−1. This salt
did not give acceptable elemental analysis and was used as isolated.
Preparation of [AsPh4]2[Me2adpc]. Me2adpcH2 (0.10 g, 0.93
mmol) was dissolved in 50 mL of 2 M NaOH(aq). To this was
added a solution of [AsPh4]Cl·H2O (0.35 g, 0.80 mmol) in 20 mL of
water, and the mixture was stirred for 15 min. The red precipitate was
collected and washed with 30 mL of water. Recrystallization from
boiling acetonitrile gave red needle-shaped crystals of [As-
Ph4]2[Me2adpc] that were suitable for crystallographic analysis.
1
Yield: 0.25 g, 60%. H NMR (300 MHz, DMSO-d6): 7.36 (2H, d),
[{Ru(tpy)(bpy)}2(μ-Cl4adpc)][PF6]2·5H2O. The complex was pre-
pared by following the above procedure with some modification: A
mixture of [Ru(tpy)(bpy)Cl] (0.58 g, 0.43 mmol) and crude
Tl2[Cl4adpc] (0.35 g) was refluxed in DMF (130 mL) for 4 days to
yield 0.69 g of the crude product. Column chromatography of 0.30 g
of the crude product was performed using an alumina column (grade
IV, Brockman acidic, 300 g). The fourth deep-purple band was eluted
with pure acetonitrile and 10% methanol (in acetonitrile) to yield a
dark-brown complex that lacks the [PF6] anion [recognized by IR for
ν(PF6): 842 cm−1]. This complex was then stirred in the boiling
6.49 (2H, s), 6.40 (2H, d), 2.45 (6H, s). IR (KBr): ν(NCN) 2083 and
2061 cm−1.
Syntheses of Complexes. [{Ru(tpy)(bpy)}2(μ-Me4adpc)][PF6]2·
H2O. To a solution of Me4adpcH2 (0.10 g, 0.314 mmol) in anhydrous
DMF (90 mL) was added 2.5 M BuLi in hexane (0.3 mL, 0.698 mmol)
under an inert atmosphere at −40 °C, and the mixture was stirred for
30 min at the same temperature. The resulting dark-red solution was
then brought to room temperature, treated with [Ru(tpy)(bpy)Cl]-
[PF6] (0.42 g, 0.628 mmol) and TlPF6 (0.22 g, 0.628 mmol), and
refluxed under an inert atmosphere for 2 days. The resulting mixture
I
DOI: 10.1021/ic502487x
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
mixture of DMF and water (1:1, 40 mL) in the presence of excess
NH4PF6 (5 equiv) for 10 min, and the resulting mixture was cooled to
room temperature to obtain [PF6] salt of the complex. The pure dark-
brown complex (0.1 g) was obtained by slow diffusion of diethyl ether
into a saturated solution of the complex in DMF. Yield: 13%. Anal.
Calcd for C64H52N16Cl4P2F12O5Ru2: C, 43.70; H, 2.98; N, 12.74.
Found: C, 43.85; H, 3.10; N, 13.00. 1H NMR (300 MHz, DMSO-d6):
9.64 (2H, d), 8.97 (2H, d), 8.93 (4H, d), 8.81 (4H, d), 8.71 (2H, d),
8.50−8.28 (4H, m), 8.12 (6H, m), 7.88 (2H, t), 7.76 (4H, d), 7.48
(6H, m), 7.43 (2H, s), 7.18 (2H, t), 5.88 (2H, s). IR (KBr): ν(NCN)
2166 cm−1.
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Crystallography. Diffraction data for a suitable crystal of
[AsPh4]2[Me2adpc] were collected on a 1K Siemens Smart CCD
using Mo Kα radiation (λ = 0.71073 Å) at 120(2) K using an ω-scan
technique and corrected for absorptions using equivalent reflections.
Unit cell parameters were obtained from 60 data frames, 0.3° ω,
from three different sections of the Ewald sphere. The systematic
absences in the diffraction data are uniquely consistent with the
reported space group. Direct methods were used to solve the
structures, and refinement was done with full-matrix least-squares
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ASSOCIATED CONTENT
* Supporting Information
Synthetic procedures of organic precursors and their H NMR
spectra, figure showing the COSY NMR of [{Ru(tpy)-
(bpy)}2(μ-Me2adpc)](PF6)2, three tables of NMR data of the
complexes R2R′2adpcH2 and Tl2[R2R′2adpc] salts, an ORTEP
diagram of the Me2adpc2− and tetraphenylarsonium ions
showing atomic labeling, six tables of crystallographic data
and bond lengths, angles, and dihedral angles, and four spectra
of IR and vis−NIR spectroelectrochemistry. This material is
available free of charge via the Internet at http://pubs.acs.org.
■
S
1
AUTHOR INFORMATION
Corresponding Author
*E-mail: robert_crutchley@carleton.ca.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by Carleton University and the Natural
Sciences and Engineering Research Council of Canada. We
thank Serge Gorelsky for attempting the DFT calculations of
the [{Ru(tpy)(bpy)}2(μ-adpc)]3+ ion.
REFERENCES
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DOI: 10.1021/ic502487x
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