Long-lived, red-emitting excited state of a Ru(II) complex of a diaminotriazine ligand

Article abstract of DOI:10.1016/j.poly.2015.11.009

Heteroleptic complexes of Ru(II) with tridentate ligand Py-DAT2 (2,6-bis(4,6-diamino-1,3,5-triazin-2-yl)pyridine) and with derivatives of tridentate ligands tpy (2,2′:6′,2″-terpyridine) or dpt (2,6-dipyridyltriazine) have been synthesized and characterized. Changing the second ligand from tpy to dpt was found to profoundly alter the electronic and photophysical properties of the complexes. Structural analysis of the Py-DAT2/tpy complex by single-crystal XRD revealed the presence of a 2D terpyridyl embrace and an extended array of hydrogen bonds involving water of crystallization and the amino groups of Py-DAT2.

Full text of DOI:10.1016/j.poly.2015.11.009

Polyhedron xxx (2015) xxx–xxx
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Long-lived, red-emitting excited state of a Ru(II) complex of a
diaminotriazine ligand
a,
Amlan K. Pal a, Adam Duong a,b, James D. Wuest a, , Garry S. Hanan
⇑
⇑
a Département de Chimie, Université de Montréal, 2900 Édouard-Montpetit, Montréal, Québec H3T 1J4, Canada
b Département de Chimie, Biochimie et Physique, Université de Québec à Trois-Rivières, 3351 Boulevard des Forges, Trois-Rivières, Québec G9A 5H7, Canada
a r t i c l e i n f o
a b s t r a c t
Article history:
Heteroleptic complexes of Ru(II) with tridentate ligand Py-DAT2 (2,6-bis(4,6-diamino-1,3,5-triazin-2-yl)
pyridine) and with derivatives of tridentate ligands tpy (2,20:60,200-terpyridine) or dpt (2,6-dipyridyltri-
azine) have been synthesized and characterized. Changing the second ligand from tpy to dpt was found
to profoundly alter the electronic and photophysical properties of the complexes. Structural analysis of
the Py-DAT2/tpy complex by single-crystal XRD revealed the presence of a 2D terpyridyl embrace and
an extended array of hydrogen bonds involving water of crystallization and the amino groups of Py-DAT2.
Ó 2015 Elsevier Ltd. All rights reserved.
Received 13 August 2015
Accepted 5 November 2015
Available online xxxx
Keywords:
Ruthenium complexes
Hydrogen bonding
Electrochemistry
Photophysical behavior
DFT calculations
Ru(II) complexes of polypyridyl ligands continue to be studied
intensively because they have valuable photophysical [1–3] and
redox [4,5] properties. The complexes have many applications, such
as in light-harvesting devices [6,7], in dye-sensitized solar cells [8],
and as catalysts for the oxidation of water [9]. Although Ru(II) com-
plexes of bidentate ligands such as bpy (2,20-bipyridine) and phen
(1,10-phenanthroline) have attractive photophysical properties,
they are chiral, and their incorporation in large supramolecular
assemblies is problematic unless they are specially designed
[10,11]. Increasing attention has therefore been given to complexes
of tpy (2,20:60,200-terpyridine), which are achiral and more easily
incorporated in assemblies of higher nuclearity for use in electron
transfer and energy transfer [12]. In addition, studies in crystal
engineering have shown that embraces of tpy units arising from
face-to-face and edge-to-face aromatic interactions can help to con-
trol how complexes are organized in the solid state [13]. Although
[Ru(tpy)2]2+ complexes are easy to synthesize, their photophysical
properties are unsuitable for many applications; for example, the
lifetime of the excited state is only about 0.25 ns at 25 °C, whereas
introducing strong r-donating carbene or cyclometalating ligands
[15,16], (c) expanding the chelate ring [17], and (d) incorporating
an auxiliary chromophore [18]. These approaches act by destabiliz-
ing the 3MC state. However, a relatively unexplored approach is to
stabilize the 3MLCT state by incorporating additional atoms of N in
the central or peripheral pyridyl rings of tpy [19,20].
We have now examined complexes of the tridentate ligand
Py-DAT2 (2,6-bis(4,6-diamino-1,3,5-triazin-2-yl)pyridine), which
resembles tpy but incorporates extra endocyclic atoms of N in
the peripheral pyridyl units and also adds exocyclic NH2 groups
that can take part in intercomplex hydrogen bonds. Two new
heteroleptic Ru(II) complexes with Py-DAT2 and either BrPh-tpy
(4-bromophenyl-2,20:60,200-terpyridine) or BrPh-dpt (4-bro-
mophenyl-2,6-dipyridyltriazine) have been prepared as the
salts [Ru(Py-DAT2)(BrPh-tpy)][(NO3)2] (1) and [Ru(Py-DAT2)
(BrPh-dpt)][(PF6)2] (2), and they have been characterized by vari-
ous techniques, including single-crystal XRD. As planned, hydrogen
bonding helps to determine the solid-state packing of complex 1.
In addition, tpy-complex, 1 has electrochemical and photophysical
properties that are notably different from those of dpt-complex, 2,
and the origin of these differences is probed by calculations using
density functional theory (DFT).
that of [Ru(bpy)3]2+ is about 1
ls, due to fast thermal conversion
of emissive triplet metal-to-ligand charge transfer (3MLCT) states
into non-emissive triplet metal-centered (3MC) states. Recent
studies have explored various strategies for increasing the
excited-state lifetimes of [Ru(tpy)2]2+ complexes, such as (a) adding
electron-donating or withdrawing groups to tpy [4,5,14], (b)
1. Results and discussion
The ligand Py-DAT2 was synthesized by the reported method
[21]. Precursor complexes of RuCl3 with substituted tpy or dpt
ligands were made according to a recently developed protocol
⇑
Corresponding authors.
E-mail addresses: adam.duong@uqtr.ca (A. Duong), james.d.wuest@umontreal.ca
(J.D. Wuest), garry.hanan@umontreal.ca (G.S. Hanan).
http://dx.doi.org/10.1016/j.poly.2015.11.009
0277-5387/Ó 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: A.K. Pal et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.11.009

2
A.K. Pal et al. / Polyhedron xxx (2015) xxx–xxx
[22]. Heteroleptic complexes [Ru(Py-DAT2)(BrPh-tpy)][(NO3)2] (1)
and [Ru(Py-DAT2)(BrPh-dpt)][(PF6)2] (2) were prepared by micro-
wave-assisted heating of solutions of precursor RuCl3 complexes
and Py-DAT2 in ethylene glycol (Scheme 1). Subsequent purifica-
tion of the products by column chromatography, followed by anion
metathesis and drying under vacuum, afforded complexes 1 and 2
in analytically pure form in good yields. The microwave-assisted
method was faster and more effective than conventional methods
of heating in solution.
Ru(II) complexes 1 and 2 were characterized by 1H and 13C NMR
spectroscopy in solution, high-resolution ESI mass spectrometry
(HR ESI-MS), elemental analysis, UV–vis spectroscopy, and cyclic
voltammetry. In addition, the structure of complex 1 was solved
by single-crystal X-ray diffraction (XRD). The NMR spectra of com-
plexes 1 and 2 displayed a high degree of symmetry [23], as
expected for a structure with an effective C2 axis (Figs. S1–S4 in
ESI). In both complexes, the NH2 groups closest to the metal center
are shifted downfield (ꢀ8 ppm), whereas the more distant NH2
groups appear at ꢀ5 ppm, presumably because the former engage
in intramolecular hydrogen bonds with the N atom of the central
pyridine or triazine ring of the tpy or dpt ligands (Fig. 1). In the
HR ESI-MS spectra, the most abundant peaks were found to corre-
spond to species [M-2(Y)]2+ formed by loss of the ions Y = NO3 or
PF6. Peaks corresponding to [M-(Y)]+ were also evident.
Crystals suitable for analysis by XRD were grown by allowing
vapors of diethyl ether to diffuse into a solution of complex 1 in
a 1:1 (v/v) mixture of MeCN and MeOH. The structure reveals coor-
dinatively saturated Ru(II) in a distorted octahedral geometry, with
both ligands bound in meridional fashion (Fig. 1; see also Table S1
in ESI for specific crystallographic parameters; CCDC 1025738).
The N–Ru–N trans angles subtended by ligands Py-DAT2 and
BrPh-tpy are similar to those observed in other Ru(II) complexes
containing tpy ligands (N1–Ru1–N3 = 158.4(2)°, N4–Ru1–
N8 = 156.4(2)°). The observed bond distances and angles are in
good agreement with those estimated by computation (Table S2
and Fig. S5 in ESI).
Fig. 1. ORTEP view of the structure of complex 1, as found in crystals grown from
diethyl ether/MeCN/MeOH. Anions, solvated water molecule and atoms of hydro-
gen have been omitted for clarity. Ellipsoids correspond to a 30% probability level.
Coordination by Py-DAT2 promises to offer a general strategy in
which well-known tridentate heteroaromatic ligands are replaced
to create complexes with similar topologies but now endowed
with the additional ability to engage in intercomplex hydrogen
bonds that help dictate organization in the solid state. Such com-
plexes have been called metallotectons [24]. In complexes of sim-
ple tpy ligands, assembly is typically controlled by tpy embraces
involving onset face-to-face aromatic interactions [13,25–27]. In
contrast, packing in crystals of complex 1 is guided primarily by
intermolecular N–Hꢁ ꢁ ꢁN hydrogen bonds involving paired
diaminotriazinyl groups of the Py-DAT2 ligands, which leads to
the formation of zigzag chains (Fig. 2) [24,28–29]. In addition, N–
Hꢁ ꢁ ꢁO hydrogen bonds involving NH2 groups of Py-DAT2, included
molecules of water, and NOꢂ3 (Fig. 2) also help determine the
observed packing, as well as weak intermolecular Brꢁ ꢁ ꢁH–C interac-
tions and C–Hꢁ ꢁ ꢁ
p interactions. The chains are further joined to
form sheets by aromatic interactions and multiple hydrogen bonds
involving molecules of water and NOꢂ3 (Fig. 3). The cationic sheets
are separated by intervening anionic layers containing hydrogen-
bonded molecules of water and NOꢂ3 (Fig. 4).
The UV–vis absorption spectra of complexes 1 and 2 were
recorded in degassed MeCN solutions at 25 °C. At higher energy
(<300 nm), ligand-centered (LC)
p ?
p⁄ transitions are observed
for both complexes. In the case of complex 1, a high-energy transi-
tion (245 nm) is predominantly a tpy-based LC transition, whereas
another (275 nm) has mixed character involving an LC-transition
Fig. 2. Representation of the structure of complex 1, showing chains formed
primarily by intermolecular N–Hꢁ ꢁ ꢁN hydrogen bonds between paired diaminotri-
azinyl groups of the Py-DAT2 ligands, reinforced by N–Hꢁ ꢁ ꢁO hydrogen bonds
involving NH2 groups of Py-DAT2, included molecules of water, and NOꢂ3 . Hydrogen
bonds are shown as dotted lines, and atoms of carbon appear in gray, nitrogen in
blue, oxygen in red, bromine in orange, and hydrogen in white. (Color online.)
Scheme 1. Syntheses of heteroleptic complexes [Ru(Py-DAT2)(BrPh-tpy)][(NO3)2]
(1) and [Ru(Py-DAT2)(BrPh-dpt)][(PF6)2] (2).
Please cite this article in press as: A.K. Pal et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.11.009

A.K. Pal et al. / Polyhedron xxx (2015) xxx–xxx
3
24 nm compared to that of [Ru(tpy)2]2+ (kmax = 451 nm), but the
corresponding transition of complex 2 is red-shifted by 17 nm
compared to that of [Ru(tpy)2]2+. This presumably arises because
the p⁄ level of BrPh-tpy in complex 1 lies at higher energy than that
of tpy in [Ru(tpy)2]2+, whereas the p⁄ level of BrPh-dpt in the com-
plex is expected to lie at lower energy than that of tpy in [Ru
(tpy)2]2+, due to the electron-withdrawing effects of the extra
atoms of N in the triazinyl core [32].
The redox behavior of complexes 1 and 2 in dry degassed MeCN
was assessed by cyclic voltammetry at 25 °C (Table 1 and Fig. S7 in
ESI). At negative potentials, the free ligands exhibited an irreversible
reduction, followed by a quasi-reversible reduction at more nega-
tive potentials. As expected, terpyridine BrPh-tpy is less easily
reduced than dipyridyltriazine BrPh-dpt. At positive potentials,
complex 1 exhibited a quasi-reversible oxidation and an irreversible
oxidation at 1.25 V and 1.99 V, respectively. The first is assigned to
metal-based oxidation because the HOMO is predominantly (66%)
metal-based (Fig. 5). At negative potentials, several quasi-reversible
reductions and irreversible reductions are observed, which may be
assigned to ligand-based reductions in a coarse approximation, as
suggested by DFT calculations, which reveal that the LUMO to
LUMO+4 of complex 1 have significant (91–99%) contributions from
the ligand (Table S6 in ESI). The first reduction at ꢂ1.04 V, which is
associated with BrPh-tpy, has a peak-to-peak separation of 36 mV
and is thus linked to a two-electron process. Complex 2 displays
metal-based monoelectronic quasi-reversible oxidation at 1.37 V
in the anodic region and three quasi-reversible and irreversible
ligand-based reduction peaks, as suggested by DFT calculations
(Table S7 in ESI). For both complexes, irreversible peaks at ꢀ
ꢂ1.8 V are due to adsorption on the primary glassy carbon elec-
trode, and the transfer of electrons associated with these peaks
could not be determined unambiguously. The higher energy calcu-
lated for the HOMO of complex 1 (EHOMO = ꢂ6.13 eV) compared
with those of [Ru(tpy)2]2+ (EHOMO = ꢂ6.18 eV) and complex 2
(EHOMO = ꢂ6.33 eV) is in good agreement with the lower oxidation
potential of complex 1 compared to that of [Ru(tpy)2]2+ and com-
plex 2, thus confirming the electron-richness of complex 1, relative
to the behavior of [Ru(tpy)2]2+ and complex 2 (Fig. 5).
Fig. 3. View of the structure of crystals of complex 1, showing how aromatic
interactions define cationic sheets in the ac plane.
The luminescence of the complexes was examined by phospho-
rescence spectroscopy in dry degassed MeCN. Upon excitation at
the lowest-energy absorption maxima, complex 1 was found to
be emissive at 735 nm, whereas complex 2 proved to be non-emis-
sive at 25 °C. The associated excited-state lifetime was determined
to be 12.9 ns for complex 1. Surprisingly, the emission maximum
of complex 1 was found to be red-shifted by more than 100 nm rel-
ative to that of [Ru(tpy)2]2+ (kem = 629 nm), whereas the 1MLCT
Fig. 4. View along the c axis of the structure of crystals of complex 1, showing how
cationic sheets of the metallotecton (highlighted in alternating blue and yellow) are
separated by intervening anionic layers containing hydrogen-bonded NOꢂ3 and H2O.
(Color online.)
Table 1
Redox behavior of BrPh-tpy, BrPh-dpt, complex 1, complex 2, and reference complex
[Ru(tpy)2]2+
.
Compound
BrPh-tpy
E1/2(ox)a
E1/2(red)a
–
–
–
ꢂ1.90 ꢂ2.09
–
–
–
–
–
–
(major) and singlet metal-to-ligand charge transfer (1MLCT) transi-
tion (minor), as predicted by TD-DFT calculations (Fig. S6 and
Table S4 in ESI). A lower-energy transition (451 nm) has a mixed
character of two major transitions, H-1 ? L (53%) and H ? L + 2
(36%) (where H = HOMO and L = LUMO). It is predominantly a
1MLCT transition, which also borrows energy from higher-energy
LC transitions (Table S4 in ESI) [30,31]. In the case of complex 2,
the transition at 285 nm is ascribed to an LC transition involving
the BrPh-dpt moiety, whereas the lower-energy transition at
492 nm is predominantly a 1MLCT transition, which also borrows
energy from higher-energy LC transitions, as suggested by TD-
DFT calculations (Table S5 in ESI). It is noteworthy that the low-
est-energy 1MLCT transition for complex 1 is blue-shifted by
(irr)b
(70)
BrPh-dpt
–
ꢂ1.42 ꢂ2.01
(irr)b
(180)
1
2
1.99 (irr)b 1.25 ꢂ1.04 ꢂ1.21 ꢂ1.75 ꢂ1.93 ꢂ2.06
(86) (36)
(106)
(irr)b
(126)
–
(59)
–
–
–
1.37 ꢂ0.61 ꢂ1.12 ꢂ1.80
(67) (65)
1.31 ꢂ1.23 ꢂ1.47
(60) (70) (69)
(241)
(irr)b
–
2+c
[Ru(tpy)2]
–
–
a
Potentials are in volts vs. SCE for MeCN solutions (0.1 M in Bu4NPF6) recorded at
25 °C. The difference between cathodic and anodic peak potentials (millivolts) is
given in parentheses.
Irreversible; potential is given for the anodic and cathodic waves, for oxidation
and reductions, respectively.
b
c
From Ref. [33].
Please cite this article in press as: A.K. Pal et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.11.009

4
A.K. Pal et al. / Polyhedron xxx (2015) xxx–xxx
Québec, the Canada Foundation for Innovation, the Canada
Research Chairs Program, NanoQuébec, the Centre for Self-Assem-
bled Chemical Structures, and the Université de Montréal for finan-
cial support.
Appendix A. Supplementary data
CCDC 1025738 contains the supplementary crystallographic
data for complex 1. These data can be obtained free of charge via
http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the
Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail:
deposit@ccdc.cam.ac.uk. Supplementary data associated with this
article can be found, in the online version, at http://dx.doi.org/10.
1016/j.poly.2015.11.009.
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ing Research Council of Canada, the Ministère de l’Éducation du
Please cite this article in press as: A.K. Pal et al., Polyhedron (2015), http://dx.doi.org/10.1016/j.poly.2015.11.009