Observation of an inversion in photophysical tuning in a systematic study of luminescent triazole-based osmium(II) complexes

Article abstract of DOI:10.1021/acs.inorgchem.9b00915

In a systematic survey of luminescent bis(terdentate) osmium(II) complexes, a tipping point involving a reversal in photophysical tuning is observed whereby increasing stabilization of the ligand-based lowest unoccupied molecular orbital (LUMO) results in a blue shift in the optical absorption and emission bands. The complexes [Os(N^N′^N″)₂]²⁺ [N^N′^N″ = 2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine (Os1), 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os2), 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2′-bipyridyl (Os3), 2-(pyrid-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os4), 2-(pyrazin-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyridine (Os5), and 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2′-bipyrazinyl (Os6)] have been prepared and characterized, and all complexes display phosphorescence ranging from the orange to near-IR regions of the spectrum. Replacement of the central pyridine in the ligands of Os1 by the more π-accepting pyrazine in Os2 results in a 55 nm red shift in the triplet metal-to-ligand charge-transfer-based emission band, while a larger red shift of 107 nm is observed for the replacement of one of the triazole donors in the ligands of Os1 by a second pyridine ring in Os3 (λ^em_max = 702 nm). Interestingly, replacement of the central pyridine ring in the ligands of Os3 by pyrazine (Os4, λ^em_max = 702 nm) fails to result in a further red shift in the emission band. Reversal of the relative positions of the pyridine and pyrazine donors in Os5 (λ^em_max = 733 nm) compared to Os4 does indeed result in the expected red shift in the emission with respect to that for Os3 based on the increased π-acceptor character of the ligands present. However, an inversion in emission tuning is observed for Os6, in which the incorporation of a second pyrazine donor in the ligand architecture results in a blue shift in the optical absorption and emission maxima (λ^em_max = 710 nm). Electrochemical studies reveal that while incorporating pyrazine in the ligands indeed results in an expected anodic shift in the first reduction potential through stabilization of the ligand-based LUMO, there is also a concomitant anodic shift in the Os^II/Os^III-based oxidation potential. This stabilization of the metal-based highest occupied molecular orbital (HOMO) thus nullifies the effect of stabilization of the LUMO in Os4 compared to Os3, resulting in these complexes having coincident emission maxima. For Os6, stabilization of the HOMO through the incorporation of two pyrazine donors in the ligand structure now exceeds stabilization of the LUMO, resulting in a larger HOMO?LUMO gap and a counterintuitive blue shift in the optical properties in comparison with those of Os5. While it is known that the replacement of ligands (e.g., replacing bipyridyl with bipyrazinyl) can result in a larger HOMO?LUMO energy gap through greater stabilization of the HOMO, these results importantly allow us to capture the tipping point at which this inversion in photophysical tuning occurs. This therefore enables us to explore the limits available in emission tuning with a relatively simple and minimalist ligand structure.

Full text of DOI:10.1021/acs.inorgchem.9b00915

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Observation of an Inversion in Photophysical Tuning in a Systematic
Study of Luminescent Triazole-Based Osmium(II) Complexes
Paul A. Scattergood,*,†,‡ James Roberts,† Salem A. E. Omar,† and Paul I. P. Elliott*,†,‡
†Department of Chemistry and ‡Centre for Functional Materials, University of Huddersfield, Queensgate, Huddersfield HD1 3DH,
U.K.
S
* Supporting Information
ABSTRACT: In a systematic survey of luminescent bis(terdentate) osmium(II) complexes, a tipping point involving a reversal
in photophysical tuning is observed whereby increasing stabilization of the ligand-based lowest unoccupied molecular orbital
(LUMO) results in a blue shift in the optical absorption and emission bands. The complexes [Os(N^N′^N″)2]2+ [N^N′^N″ =
2,6-bis(1-phenyl-1,2,3-triazol-4-yl)pyridine (Os1), 2,6-bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os2), 6-(1-benzyl-1,2,3-triazol-
4-yl)-2,2′-bipyridyl (Os3), 2-(pyrid-2-yl)-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (Os4), 2-(pyrazin-2-yl)-6-(1-benzyl-1,2,3-
triazol-4-yl)pyridine (Os5), and 6-(1-benzyl-1,2,3-triazol-4-yl)-2,2′-bipyrazinyl (Os6)] have been prepared and characterized,
and all complexes display phosphorescence ranging from the orange to near-IR regions of the spectrum. Replacement of the
central pyridine in the ligands of Os1 by the more π-accepting pyrazine in Os2 results in a 55 nm red shift in the triplet metal-
to-ligand charge-transfer-based emission band, while a larger red shift of 107 nm is observed for the replacement of one of the
triazole donors in the ligands of Os1 by a second pyridine ring in Os3 (λem = 702 nm). Interestingly, replacement of the
max
central pyridine ring in the ligands of Os3 by pyrazine (Os4, λemmax = 702 nm) fails to result in a further red shift in the emission
band. Reversal of the relative positions of the pyridine and pyrazine donors in Os5 (λemmax = 733 nm) compared to Os4 does
indeed result in the expected red shift in the emission with respect to that for Os3 based on the increased π-acceptor character
of the ligands present. However, an inversion in emission tuning is observed for Os6, in which the incorporation of a second
pyrazine donor in the ligand architecture results in a blue shift in the optical absorption and emission maxima (λem = 710
max
nm). Electrochemical studies reveal that while incorporating pyrazine in the ligands indeed results in an expected anodic shift in
the first reduction potential through stabilization of the ligand-based LUMO, there is also a concomitant anodic shift in the
OsII/OsIII-based oxidation potential. This stabilization of the metal-based highest occupied molecular orbital (HOMO) thus
nullifies the effect of stabilization of the LUMO in Os4 compared to Os3, resulting in these complexes having coincident
emission maxima. For Os6, stabilization of the HOMO through the incorporation of two pyrazine donors in the ligand structure
now exceeds stabilization of the LUMO, resulting in a larger HOMO−LUMO gap and a counterintuitive blue shift in the
optical properties in comparison with those of Os5. While it is known that the replacement of ligands (e.g., replacing bipyridyl
with bipyrazinyl) can result in a larger HOMO−LUMO energy gap through greater stabilization of the HOMO, these results
importantly allow us to capture the tipping point at which this inversion in photophysical tuning occurs. This therefore enables
us to explore the limits available in emission tuning with a relatively simple and minimalist ligand structure.
component white-light systems,14−18 in addition to finding use
INTRODUCTION
Transition-metal complexes exhibiting phosphorescence in the
■
as luminescent chemosensors.19−21 There has also been a
notable drive toward the development of complexes that not
red/near-IR (NIR) region have been the subject of extensive
research.1 For example, red and NIR emitters have been widely
only display red/NIR emission but also absorb light at longer
investigated as the phosphor within light-emitting electro-
chemical cells2−7 and organic-light-emitting devices,8−13
including functioning as the low-energy aspect within multi-
Received: March 29, 2019
© XXXX American Chemical Society
A
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wavelengths. These photophysical characteristics are ideal for
achieving effective luminescent cellular imaging agents, where
the occurrence of both the absorption and emission of light
within the biologically transparent region is highly desir-
able.22−25 Further, coordination complexes with electronic
absorption profiles extending into the NIR have additionally
been identified as necessary in order to improve the efficiency
of dye-sensitized solar cells, harvesting photons from an often
neglected region of the solar emission spectrum.26−28
emission at 595 nm and preferentially localizes within the
mitochondria of HeLa and U2OS cell lines, allowing for
luminescence imaging by confocal microscopy.36 These initial
studies also revealed that the homoleptic triazole-containing
complexes exhibited significant luminescence quenching in the
presence of oxygen, with the sensitization of singlet oxygen
thus providing the basis for the development of potential dual-
mode photodynamic theranostic agents.
While offering promise, the absorption and emission bands
exhibited by the complexes in our initial investigations were
not ideally situated in the optical spectrum so as to optimally
align with the biologically transparent window. To expand
upon our previous studies, we were motivated to design and
develop new terdentate ligand architectures in order to shift
the absorption and emission characteristics of the resultant
osmium(II) complexes firmly into the deep-red/NIR region.
Because of the synthetic versatility of the 1,2,3-triazole motif
for ligand design, the aforementioned singlet-oxygen-sensitiz-
ing activity, and also the reported facile conjugation of
complexes to biologically relevant targeting moieties37 through
1,2,3-triazole-based linkers, we were minded to retain this
heterocycle in our ligands appearing in the systematic survey
reported here. In order to maintain a relatively simple ligand
architecture for reasons of facile synthetic accessibility and
concerns over resultant complex solubility, these triazole
donors were therefore combined in both symmetric and
asymmetric terdentate ligands with more electron-withdrawing
pyridine and pyrazine donor rings. Through this approach we
were confident in achieving a lowering of the energy of the
ligand-based lowest unoccupied molecular orbital (LUMO)
and thus a red shift of the optical absorption and emission
bands of the complexes.
While the parent terdentate ligand 2,2′:6′,2″-terpyridine
(tpy) is ubiquitous in the coordination chemistry of photo-
active metal complexes, we note that derivatives and analogues
often carry peripheral substituents, primarily upon the pyridyl
rings, with the core of the ligand framework remaining intact.38
Less attention has been paid to the synthesis and development
of unsubstituted tpy analogues containing higher azines or
alternative N-donor heterocycles. Pyrazine-based ligands in
complexes of osmium(II) are relatively rare in the literature
but have been reported, for example, in investigations of
electron transfer and delocalization,39 in addition to being
featured within higher-chelating-ligand structures, facilitating
coordination to two metal centers and thus the formation of bi-
and multimetallic systems.40 The group of Brewer has
extensively explored use of the 2,3-bis(2-pyridylpyrazine)
ligand,41,42 including in osmium(II)-containing multimetallic
systems displaying NIR absorption,43 while Campagna and co-
workers have utilized the same framework and derivatives
thereof in the synthesis of multimetallic dendrimers, which
function as light-harvesting antenna.44 While the use of
pyrazine as a bridging ligand is more widespread, its
employment within polyazine ligands of monometallic
complexes is relatively sparse. For example, Ruminski and
co-workers have investigated a homoleptic osmium(II)
complex of dipyrido-2,3-a;3′,2′-j-phenazine ([Os(dpop′)2]2+),
which has an UV−visible absorption profile extending to ∼800
nm and displays weak phosphorescence with λem = 795 nm.45
In this contribution, we explore the design and synthesis of
new symmetrical and asymmetrical terdentate ligand archi-
tectures featuring pyridine, pyrazine, and 1,2,3-triazole donor
moieties and investigate their coordination chemistry with
Over the last few decades, considerable attention has been
paid to coordination complexes of kinetically inert d6 metals
such as rhenium(I), ruthenium(II), iridium(III), and osmium-
(II).29 The photophysical properties of these complexes are
well understood and documented, with the excited state
frequently dominated by long-lived triplet metal-to-ligand
charge-transfer (3MLCT) states from which phosphorescence
occurs and from where further electron-transfer events are
possible. With a view toward achieving low-energy photo-
luminescence and potential applications in luminescence
cellular imaging, complexes of osmium(II) offer several
advantageous photophysical properties. First, the high spin−
orbit coupling constant associated with the heavy-metal center
gives rise to formally spin-forbidden ground state-to-3MLCT
state electronic absorption bands of appreciable extinction
coefficient, which occur at significantly lower energy than the
corresponding spin-allowed transitions which populate the
singlet metal-to-ligand charge-transfer (1MLCT) states.30
Further, these excitation bands are typically red-shifted
compared with those observed for comparable complexes of
the group 8 congener ruthenium(II), with photoluminescence
from osmium(II) complexes occurring in the deep-red-to-NIR
spectral region. For example, bis(terdentate) complexes of
osmium(II) featuring 6-[5-(trifluoromethyl)pyrazol-3-yl]-2,2′-
bipyridine ligands have previously been reported to display
appreciable panchromatic electronic absorption profiles and
low-energy luminescence with λem = 655−935 nm.31 These
properties are ideal for potential cellular imaging agents,
enabling a greater depth of tissue penetration for excitation,
reducing biological damage through the use of lower-energy
excitation sources, and avoiding autofluorescence from
chromophores within the biological material.
While complexes of d6 metals, particular those of ruthenium-
(II) and iridium(III), have been extensively developed for
luminescence biological imaging applications, the use of
osmium(II) complexes for this purpose is rather rare. Keyes
and co-workers32 have reported an osmium(II) polypyridyl−
polyargenine conjugate for live cell imaging, while Chao and
co-workers33 have investigated a benzimidazolylpyridine-
containing osmium(II) complex as a lysosomal tracker that
displays deep-red emission with λem = 736 nm. Very recently,
Zhang and co-workers34 have reported emissive osmium(II)
polypyridyl complexes featuring iminopyridine ligands that
permit NIR luminescence imaging of RNA and nucleoli of live
cells. Our own group has previously investigated 1,2,3-triazole-
based complexes of osmium(II), with complexes in the series
[Os(bpy)3−n(pytz)n]2+ [bpy = 2,2′-bipyridyl; pytz = 1-benzyl-
4-(pyrid-2-yl)-1,2,3-triazole; n = 0−3] displaying phosphor-
escence within the deep-red spectral region.35 The homoleptic
species [Os(pytz)3]2+ was found to result in the luminescent
staining of lysosomes and endosomes within two cancer cell
lines. In a related study, we have also prepared the osmium(II)
complex [Os(btzpy)2]2+ of the terdentate ligand 2,6-bis(1-
phenyl-1,2,3-triazol-4-yl)pyridine (btzpy), which displays
B
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Scheme 1. Synthetic Route to Terdentate Ligands 1 and 3
Scheme 2. Synthetic Route to Terdentate Ligands 7 and 10
Scheme 3. Synthesis of the Terdentate Ligands 14, 17, and 18
osmium(II). These bis(terdenate) complexes are emissive in
the red/NIR region, with not only the identity but the specific
positioning of the azines within the ligand framework having a
significant effect upon the photophysical and electrochemical
properties of the complexes as a whole. Further, we show that
while the expected increase in the electron-withdrawing
character does indeed stabilize the LUMO of the complexes,
the incorporation of pyrazine donors also has a significant
stabilizing effect on the predominantly Os d-orbital-based
highest occupied molecular orbital (HOMO). Thus, we
observe a tipping point in our series where stabilization of
the HOMO outweighs stabilization of the LUMO and the
trend in photophysical tuning becomes inverted.
singlet at δ 5.59, while the benzylic aromatic protons fall within
the multiplets at δ 7.26−7.43.
The 1,2,3-triazole-appended 2,2′-bipyridyl ligand 7 was
prepared via a four-step procedure starting from 2-bromopyr-
idine (Scheme 2). Briefly, palladium-catalyzed Stille cross-
coupling of the stannane 4 with a stoichiometric quantity of
2,6-dibromopyridine afforded 6-bromo-2,2′-bipyridyl (5),
which subsequently underwent palladium-catalyzed Sonoga-
shira cross-coupling with ethynyltrimethylsilane to give the
corresponding ethynyl-substituted bipyridine 6. A further
CuAAC reaction with benzyl azide furnished the desired
ligand 7 with a modest yield of 44%. We were additionally able
to introduce a pyrazine heterocycle into the terdentate ligand
structure (10) by following an analogous synthetic route
utilizing 2,6-dibromopyrazine (Scheme 2). 1H NMR spectra of
the ligands 7 and 10 feature the characteristic singlet triazole
ring resonances at δ 8.17 and 8.18, respectively. The placement
of the pyrazine ring in the central position of the tris-
heterocycle ligand 10 leads to a loss of symmetry for the
pyrazine moiety, resulting in the observation of two downfield
singlet resonances in the 1H NMR spectrum at δ 9.43 and 9.54
attributed to the 3 and 5 positions, respectively, assigned
through nuclear Overhauser effect (NOE) correlation data.
In order to determine the effect of the relative positions of
the pyridine and pyrazine donors on the photophysical
RESULTS AND DISCUSSION
■
In a fashion similar to that of the previously reported synthesis
of 1,46,47 the pyrazine-containing ligand 2,6-bis(1-benzyl-1,2,3-
triazol-4-yl)pyrazine (3) was conveniently prepared through
the copper-catalyzed alkyne−azide cycloaddition (CuAAC) of
2,6-bis(ethynyltrimethylsilyl)pyrazine (2) and benzyl azide
1
(Scheme 1). The H NMR spectrum of 3 is simple, with
singlets at δ 8.05 and 9.30 corresponding to the triazole ring
and equivalent pyrazinyl protons, respectively. The methylene
protons of the benzyl substituents are observed as a further
C
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Figure 1. Structures of the osmium(II) complexes investigated in this work.
properties of subsequent complexes, we targeted ligand 14
featuring a central pyridine and peripheral pyrazine rings
(Scheme 3). An obvious synthetic strategy would be one
directly analogous to that described above, employing a
stannylpyrazine reagent. However, despite reports concerning
the preparation of 2-(tributylstannyl)pyrazine,48,49 we were
unable to successfully isolate this species in any appreciable
yield. These difficulties, in addition to the inherent toxicity of
tin reagents and problems frequently encountered during the
purification of Stille cross-coupling products, led us to seek an
alternative synthetic solution. Burke and co-workers have
recently reported the robust preparation of a range of 2-
heterocyclic N-methyliminodiacetic acid (MIDA) boronates,
suitable as coupling partners in palladium-catalyzed cross-
coupling reactions.50 While 2-heterocyclic boronic acids are
generally unstable and difficult to handle, MIDA boronates are
found to be both air- and moisture-stable and are readily
prepared. These reagents have additionally been trialled within
Suzuki-type reactions, where they have proven to be effective
in providing the in situ “slow release” of unstable but reactive
boronic acids, thereby functioning as effective building blocks
in the synthesis of a range of heterocyclic organic frame-
works.51
The 2-pyrazinyl MIDA boronate (11) was prepared via the
reported procedure50 and obtained with a yield of 53%. Initial
attempts to cross-couple 11 with a stoichiometric quantity of
2,6-dibromopyrazine gave predominantly the bis-substituted
product. Consequently, the 1,2,3-triazole moiety was appended
first via 2-bromo-6-ethynylpyridine and a subsequent CuAAC
reaction with benzyl azide to produce 13 (Scheme 3). Further
reaction with pyrazine MIDA boronate 11 was carried out
following a two-pot procedure, successfully furnishing the
target ligand 14 with a modest yield of 45%.
as a singlet at δ 8.91, assigned through a strong NOE
correlation with the methylene protons of the benzyl group,
themselves giving rise to a singlet at δ 5.79. COSY spectra
allow the protons on the 5 and 6 positions of the peripheral
pyrazine ring to be identified as a pair of strongly coupled
resonances at δ 9.62 and 8.72, with the proton on the 3
position, together with those of the central pyrazine ring
appearing as three singlet resonances at δ 9.44, 9.37, and 8.74.
The lack of coupling interactions and absence of obvious
correlation signals in NOESY NMR spectra preclude the
specific assignment of these resonances.
Finally, 11 was utilized further to access the tris-pyrazinyl
ligand 18 (Scheme 3). Surprisingly, only two reports have
previously been made concerning the synthesis of this polyaza
species,38,49 both of which rely on the Stille coupling of a
stannylpyrazine with chloropyrazines. Here, employment of
the Suzuki coupling of 11 with 2,6-dibromopyrazine gives 18
with a moderate yield of 28%.
The osmium(II) complexes of the reported terdentate
ligands (Os1−Os6; Figure 1) were all conveniently prepared
as their hexafluorophosphate salts by the reaction of 1 equiv of
the appropriate ligand with [OsCl6][NH4]2 in refluxing
ethylene glycol followed by treatment with NH4PF6.
Purification by either column chromatography or recrystalliza-
tion gave the bis(terdentate) complexes as brown-to-dark-
green powders. 1H NMR analysis of the complexes gave
spectra similar to those of the free ligands, although with
protons on the coordinating fragments being marginally
deshielded. For example, the triazole ring and pyrazinyl
protons of ligand 3 are observed at δ 8.37 and 9.18,
respectively, in deuterated acetonitrile (MeCN-d3), while the
corresponding resonances for Os2 appear at δ 8.66 and 9.28.
Attempts were also made to prepare the bis(terdentate)
osmium(II) complex of 2,2′:6′,2″-terpyrazine (Os7) both in
an analogous manner to Os1−Os6 and via an alternative route
involving the reaction between [{Os(C6H6)Cl2}2] and 4 equiv
of ligand 18 in refluxing ethanol (EtOH)/water (H2O). All
We were additionally able to apply this synthetic method-
ology and the use of a pyrazinyl MIDA boronate to produce
the triazole−bipyrazine ligand 17 (Scheme 3). The triazole
ring proton of 17 is readily observed in the 1H NMR spectrum
D
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Figure 2. UV−visible electronic absorption spectra recorded for MeCN solutions of Os1−Os6 (inset: magnification of the region containing bands
3
for direct singlet ground state to MLCT state transitions).
Table 1. Summarised Photophysical Data for Os1−Os6
a
a
,
b
b
,
c
a
c
d
λem /nm Φem
/
Φem /%
τem /ns
τem /ns
λem /nm (77
a
λabs /nm (ε/dm3 mol−1 cm−1
)
(RT)
% (air)
(degassed)
(air)
(degassed)
K)
e
Os1 530 (2960), 436 (5570), 385 (19400), 338 (13550), 297 (68475), 288
595
0.8
9.3
3.5
2.9
2.5
1.8
63
937
564, 606 (sh)
(48600), 242 (49000)
f
Os2 570 (2400), 452 (3900), 409 (14550), 339 (7900), 302 (37300), 248
650
1.1
1.0
1.2
1.1
1.7
269
88
924
253
240
216
625, 673 (sh)
676, 741 (sh)
687, 746 (sh)
703, 769 (sh)
(25800), 229 (33260)
g
Os3 631 (2830), 581 (3320), 462 (10450), 400 (9900), 302 (67550), 268
702
(27775), 259 (34930)
g
Os4 633 (2480), 587 (2620), 493 (4330), 442 (13280), 312 (52125), 265
702
155
135
186
(30650), 243 (33970)
g
Os5 656 (2910), 596 (3320), 523 (4735), 471 (10500), 441 (9430), 405
733
(8850), 358 (10675), 315 (54075), 263 (40435)
g
Os6 641 (2230), 590 (2620), 452 (9780), 430 (9750), 396 (8830), 328
710
2.9
288
690, 752 (sh)
(39370), 319 (40025), 269 (33380), 243 (33000)
a
b
c
d
e
f
Aerated MeCN. Relative to [Ru(bpy)3][PF6]2, Φem = 0.018 in aerated MeCN.53 Degassed MeCN. 4:1 EtOH/MeOH. λex = 500 nm. λex
=
g
550 nm. λex = 580 nm.
synthetic attempts resulted in the production of a very dark-
green intractable powder, which remained highly insoluble
LUMO as the pyridine moiety is replaced with the more π-
accepting pyrazine.
after metathesis with NH4PF6 and NH4BArF salts. Indeed,
Electronic absorption spectra of Os3−Os6 are panchro-
matic, displaying intense absorbance bands in the UV region in
addition to strong bands within the visible region, which tail-
off at ∼700 nm. For Os3, an intense band centered at 302 nm
is assigned to π → π* intraligand transitions, while the 1MLCT
4
very poor solubility has been encountered previously in
complexes of this terpyrazine ligand.38 We have thus been
unable to confirm the successful synthesis of Os7 and so
discount it from further experimental discussions.
3
and MLCT absorption envelopes are observed within the
UV−visible electronic absorption spectra were recorded for
MeCN solutions of Os1−Os6 and are shown in Figure 2, with
summarized spectroscopic data presented in Table 1. The
spectrum of Os2 exhibits an intense absorbance at 302 nm,
which is assigned to a π → π* transition localized on the ligand
(3), shifted to lower energy compared to a similar ligand-based
transition observed for the previously reported complex Os1
(297 nm).36 Os2 displays further electronic absorbance
features within the visible region, with those between 370
and 450 nm assigned to 1MLCT transitions and weaker bands
at 520−630 nm attributed to the direct population of 3MLCT
states, an electronic absorbance feature characteristic of
osmium(II) polypyridyl type complexes as a consequence of
the high spin−orbit coupling constant of the metal center.30,52
The 1,3MLCT bands observed for Os2 are shifted to lower
energy compared to those of Os1, indicative of a lower-energy
regions 400−500 and 550−680 nm, respectively. These
charge-transfer bands are stabilized in energy with respect to
Os1−Os2, primarily as a consequence of the partial
replacement of 1,2,3-triazole moieties with pyridyl units and
subsequent stabilization of the ligand-based LUMO. The
incorporation of more efficient π-accepting units in the form of
pyrazine into the ligand set of complex Os4 may be reasonably
expected to further stabilize the 1,3MLCT states. However,
when the absorbance profile of Os4 is compared to that of
Os3, the positions of the charge-transfer bands appear to be
unchanged. Moving from Os4 to Os5, where the positions of
the pyridyl and pyrazinyl units within the ligands are
exchanged, it is likely that the LUMO remains mostly
pyrazine-based and as such is now positioned much further
away from the 1,2,3-triazole unit, which has an appreciable
destabilizing influence as a consequence of its poor π-acceptor
E
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ability. This therefore might be expected to lead to a reduction
in the energy of the LUMO for Os5 over that of Os4. Indeed,
in agreement with this reasoning, the 1,3MLCT bands of Os5
appear at lower energy with respect to those of Os4, with the
1MLCT maximum recorded at 471 nm and the 3MLCT
absorbance tailing off beyond 720 nm. While Os6 with its
bis(pyrazinyl)-containing ligands may be reasonably expected
to give further stability to charge-transfer transitions, it is
interesting to note that the 1,3MLCT bands are of a similar
spectral position to those recorded for Os3 and Os4 and are, in
fact, blue-shifted relative to those of Os5.
intensity is affected by the presence of oxygen in all cases;
however, the level of quenching in aerated solutions compared
to degassed solutions generally diminishes as the emission
bands become progressively more red-shifted and the lifetime
of the excited state becomes shorter. Os2 exhibits an emission
maximum at 650 nm, shifted by some 1420 cm−1 (55 nm) to
lower energy than Os1 as a result of replacement of the central
pyridyl moiety with pyrazine and subsequent stabilization of
the 3MLCT state. This observation is in agreement with UV−
visible electronic absorption data (vide supra) and the
expectation of a significantly stabilized ligand-based LUMO.
Emission bands for Os3−Os6 are red-shifted still further, with
emission maxima beyond 700 nm placing the observed
phosphorescence within the NIR region. In accordance with
their electronic absorption spectra, Os3 and Os4 have
identically positioned emission maxima (λem = 702 nm),
whereas the lower-lying 3MLCT state in Os5 results in a lower-
energy emission with a maximum at 733 nm. It is noteworthy
that the specific placement of the three heterocycles within the
isomeric terdentate ligands of Os4 and Os5 has an appreciable
influence on the photophysical properties, with a flanking
pyrazine moiety evidently resulting in a more stabilized
LUMO. Mirroring the unexpectedly blue-shifted charge-
transfer absorption bands recorded for Os6 relative to those
of Os5, emission from Os6 is noted at 710 nm. These
Complexes Os1−Os6 were found to be emissive in aerated
MeCN solutions from the orange to deep-red/NIR spectral
regions (Figure 3 and Table 1), with broad, featureless bands
3
observations clearly indicate that the MLCT state of Os5 is
stabilized over that of Os6, despite the ligand-localized LUMO
of the latter likely to be lower-lying by virtue of the inclusion of
four π-accepting pyrazinyl units. We also note that while the
emission quantum yield for the lowest-energy emitter (Os5) is
small (∼1%), it remains comparable to both the other
complexes within this series and previously reported osmium-
(II) polypyridyl complexes, particularly those that emit in the
deep-red/NIR region,31 which are known to be weak emitters
at room temperature (RT).7,30,35
Photoluminescence lifetimes were recorded for all com-
plexes Os1−Os6 in both aerated and degassed MeCN
solutions (Table 1). The emission lifetime for each complex
was found to be elongated in the absence of oxygen, indicating
the occurrence of luminescence from an excited state of triplet
Figure 3. Normalized corrected emission spectra recorded for aerated
MeCN solutions of Os1−Os6 at RT.
suggesting that the luminescence originates from states having
3
predominantly MLCT character. It is pertinent to note that
the emissive 3MLCT state in Os3−Os6 can be accessed
through direct excitation into the spin-forbidden 3MLCT
absorption band at wavelengths ≥600 nm, ideal for biological
imaging applications, for example, where excitation within the
biological transparent region is highly desirable. The emission
Figure 4. Cyclic voltammograms for 1.5 mmol dm−3 MeCN solutions of complexes Os1−Os6 recorded at RT at 100 mV s−1. Solutions contained
0.2 mol dm−3 NBu4PF6 as the supporting electrolyte. All potentials are shown against the Fc+/Fc couple.
F
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Table 2. Summarized Electrochemical Data for 1.5 mmol dm−3 MeCN Solutions of Complexes Os1−Os6 Measured at RT at a
a
Scan Rate of 100 mV s−1
complex
Eox/V
E
red/V
(Eox − Ered)/V
b
Os1
Os2
Os3
Os4
Os5
Os6
+0.64 (67)
+0.88 (77)
+0.56 (72)
+0.87 (77)
+0.80 (76)
+1.09 (68)
−2.00
2.64
2.46
2.23
2.23
2.11
2.18
b
−1.58 (61), −1.99
−1.67 (67), −1.93 (72)
−1.36 (76), −1.72
−1.31 (66), −1.61 (66)
b,c
−1.09 (77), −1.39 (69), −2.07 (83)
a
Potentials are shown in V vs Fc+/Fc. Anodic−cathodic peak separations (ΔEa,c) for reversible couples are shown in mV within parentheses (ΔEa,c
b
c
for Fc+/Fc was typically 70 mV). Cathodic peak potential. Process not shown in Figure 4; see Figure S32.
character and confirming our assignment to phosphorescence
from a MLCT state. Indeed, we have previously found that
in line with the assumption that this ligand results in the most
stabilized LUMO of all complexes within the series.
3
Os1 is an efficient sensitizer of singlet oxygen [Φ(1O2) = 57%]
with the intensity of phosphorescence undergoing a 43-fold
reduction between degassed and oxygenated MeCN solu-
tions.36 Further inspection of the degassed photoluminescence
lifetimes for Os1−Os6 reveals a close agreement with the
energy gap law.54,55 Complex Os1 displays the highest
emission energy and correspondingly the longest lifetime of
937 ns, which is seen to shorten across the series as the
emission energy decreases. The lowest-energy emitter, Os5,
displays the shortest lifetime of 135 ns, marginally shorter than
that of Os6, where the emission maximum is shifted to slightly
shorter wavelength (Figure 3). Photoluminescence quantum
yields for degassed solutions are also found to mirror this
trend, again in good agreement with the energy gap law, with
Os1 being the most efficient emitter within the series (Φ =
9.3%), decreasing systematically with the steady reduction in
energy of photoluminescence to Os5 (Φ = 1.8%).
Low-temperature emission spectra were recorded for Os1−
Os6 at 77 K in EtOH/methanol (MeOH) glass mixtures
(Figure S31). The emission profiles reveal additional vibronic
structure, with maxima shifted to higher energy relative to the
solution-state spectra as a result of rigidochromic effects. While
the emission profiles of Os3 and Os4 are now separated, with
maxima at 676 and 687 nm, respectively, the general trend in
the emission energy across the series remains unchanged in
frozen solvent glass, with Os5 still exhibiting the lowest-energy
emission with a maximum at 703 nm.
Cyclic voltammograms recorded for complexes Os1−Os6
are shown in Figure 4 with summarized electrochemical data
presented in Table 2. At least one reduction process is
observed for each complex Os1−Os6 within the available
electrochemical solvent window, all of which are assigned to
ligand-based processes. The trend in the potential of the
reductive electrochemistry is generally in agreement with our
initial expectations. Replacement of the central pyridine in the
ligands of Os1 by pyrazine in the ligands of Os2 results in an
anodic shift of 0.42 V. The first reductions for Os4 and Os5
appear at more positive potential than that of Os3 because of
stabilization of the ligand-based LUMO, again owing to the
incorporation of pyrazine donors within the ligand structure. In
agreement with earlier interpretations based on spectroscopic
data, it is noted that the positioning of the pyrazine moiety in a
flanking rather than central position within the ligand structure
results in enhanced stabilization of the LUMO, with the first
reduction of Os5 appearing at a slightly more positive potential
than that of Os4. The presence of two π-accepting pyrazine
moieties within each ligand of Os6 results in the appearance of
the most anodically shifted first reduction potential at −1.09 V,
All complexes exhibit a reversible oxidation process, which is
assigned to the OsII/OsIII couple. On the basis of our previous
work, together with that of others, we initially expected the
potential of this oxidation process, although perturbed, to be
relatively insensitive to the changing nature of the ligands
across the series owing to the HOMO being predominantly Os
d orbital in character.7,12,35,56,57 However, the electrochemical
data reveal this couple to also be significantly affected by the
incorporation of pyrazine units within the ligand set, with the
first oxidation potentials for Os2, Os4, and Os5 appearing
within the region +0.80−0.88 V versus ferrocene/ferrocenium
(Fc+/Fc), shifted anodically by ca. 0.25 V compared to those of
Os1 and Os3. The use of bis(pyrazinyl)-containing ligands
within complex Os6 results in an even greater positive shift in
the first oxidation potential, appearing at +1.09 V.
It is therefore evident that, unlike in our previous studies
where the relative energy of the ligand-based LUMO broadly
dictates the overall observed changes and trends in the
photophysical properties of the complexes, for this series, the
significant variance in the energy of the HOMO makes a key
contribution to the spectroscopic properties. The introduction
of one pyrazine ring into the terdentate ligand architectures
generally leads to stabilization and a red shift in the absorption
and emission bands, but the extent of this tuning is
undermined by concomitant stabilization of the HOMO with
that of the LUMO. For Os4, the spectroscopic changes by
virtue of stabilization of the LUMO with respect to that of Os3
through the replacement of a pyridine by pyrazine are canceled
out by stabilization of an almost equal magnitude of the
HOMO. When two pyrazine rings are incorporated in each
ligand in Os6, additional stabilization observed for the HOMO
outweighs stabilization of the LUMO, resulting in an increased
HOMO−LUMO gap. Thus, the trend in the HOMO−LUMO
energy gap revealed through electrochemistry perfectly
matches those trends observed in the electronic absorption
and luminescence spectra (vide supra) and explains the
reversal in MLCT energy tuning observed upon going from
Os5 to Os6.
Upon examination of the literature, we note that these
results on pyrazine ligand-based stabilization of the HOMO
are in agreement with previously reported data on ruthenium-
(II) and osmium(II) complexes. While the first reduction
potential for [Ru(bpz)3]2+ (bpz = 2,2′-bipyrazine) appears
0.63 V to more positive potential than that for [Ru(bpy)3]2+,
the RuII/RuIII oxidation of the former is anodically shifted to a
greater degree (0.71 V),58 resulting in a blue shift in both
3
1MLCT absorptions and the MLCT-based emission band
(from 609 nm for [Ru(bpy)3]2+ to 600 nm for [Ru(bpz)3]2+ in
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Figure 5. Molecular orbital energy-level diagram for complexes Os1−Os6 and plots of the HOMOs and LUMOs in each case.
H2O).59 An analogous blue shift in emission is observed for
[Os(bpz)3]2+ (700 nm in MeCN) compared to [Os(bpy)3]2+
(724 nm), where the oxidation potential of the former is
anodically shifted by 0.70 V with respect to that of the latter,
while the first reduction potential of [Os(bpz)3]2+ is positively
shifted by 0.59 V.60
donor in the terdentate ligands, are each stabilized to the same
extent in comparison to the frontier orbitals of Os3, leading to
an almost identical HOMO−LUMO energy gap. This is in
agreement with the electrochemical data, the resultant and
unexpected lack of a red shift in the optical absorption
spectrum, and a coincident emission maximum for this
complex compared to Os3. Placement of the pyrazine as
the outer ring of the terdentate ligands in Os5 leads to a
comparable energy of the HOMO with respect to that of Os4,
but removal of the destabilizing influence of a neighboring
triazole moiety results in stabilization of the LUMO by a
further 0.1 eV. This is again in agreement with the
electrochemical data and experimentally observed red shift in
the absorption and emission profiles of Os5 compared to those
of Os4.
While the replacement of both pyridine donors in the
ligands for Os3 with pyrazine in the ligands for Os6 leads to
significant stabilization of the LUMO, this also leads to greater
stabilization of the HOMO, resulting in an enlargement of the
HOMO−LUMO gap compared to Os5. The calculated data
are therefore in agreement with the experimental electro-
chemical data, confirming the observed inverted tuning of the
1MLCT and 3MLCT energies through an increase in the
number of π-accepting pyrazine moieties in the ligand
architecture.
To complement and corroborate our experimental spectro-
scopic and electrochemical studies, we carried out density
functional theory (DFT) calculations to determine the nature
of the frontier orbitals and the influence of the ligands on their
relative energies. The calculated relative energies of the
HOMO and LUMO for the series of complexes (Figure 5)
are in excellent agreement with the experimental electro-
chemical data (vide supra). Replacement of the central
pyridine in the ligands in Os1 by pyrazine as in Os2 results
in stabilization of the LUMO by 0.44 eV, with a concomitant
lesser stabilization of the HOMO leading to a reduction in the
HOMO−LUMO gap by 0.2 eV. The larger π system
associated with the triazolylbipyridine ligands in Os3 results
in a stabilization of the LUMO comparable to that for Os2
with respect to Os1. The results confirm the electrochemical
data, which show that the red shift in the optical absorption
and emission profiles for Os3 derives from the HOMO
undergoing little or no modulation in energy by virtue of the
number of triazole donors in the ligand set in comparison with
Os1.
While we were not able to synthetically isolate the
terpyrazine (tpz) complex [Os(tpz)2]2+ (Os7), we might
predict that it would possess absorption and emission spectra
further blue-shifted compared to those of Os6 based on the
experimental data for bpz complexes compared to those of
The replacement of either the central or outer pyridine ring
by pyrazine in the ligands in Os4 and Os5 again leads to
stabilization of the HOMO as well as the LUMO. The HOMO
and LUMO in Os4, incorporating the pyrazine as the central
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their bpy analogues.58−60 We therefore also optimized the
ground state of complex Os7 in our DFT calculations in order
to determine whether the further inclusion of pyrazine donors
into the ligand set would lead to further inverted
optoelectronic tuning. Because of problems in converging the
ground-state geometry, D2d symmetry was imposed during
optimization. The calculated molecular orbital energies
confirm the prediction of further stabilization in the energy
of the LUMO (−3.30 eV) by 0.15 eV relative to that of Os6.
However, a more significant stabilization by 0.26 eV of the
HOMO is observed (−6.68 eV), leading to enlargement of the
HOMO−LUMO gap of Os7 by 0.12 eV compared to that of
Os6. On the basis of the trends and correlations of calculated
data and their agreement with the experimental electro-
chemical and spectroscopic data for these complexes, one
could confidently predict that the UV−visible absorption and
emission profiles of Os7 would indeed appear further blue-
shifted compared to those of Os6.
The data presented here, complemented with those
previously reported for related bidentate systems, show that
while the incorporation of pyrazine donors does indeed lead to
stabilization of the ligand-based LUMO and a resultant red
shift in the optical absorption and emission bands, this is
accompanied by stabilization of the HOMO. Increasing the
electron-withdrawing character of the ligands may continue to
result in red-shifted spectral features until a tipping point is
reached, whereby further stabilization of the HOMO exceeds
that of the LUMO and is manifested by an inversion in the
photophysical tuning behavior. Through sequential modifica-
tion of the ligand architecture whereby the π-acceptor
character is progressively tuned through variation of the
number and positions of the pyridine and pyrazine rings, rather
than the wholesale replacement of oligopyridyl with oligopyr-
azinyl-based ligands, we are able to capture this tipping point
and the associated inversion in the photophysical properties.
EXPERIMENTAL SECTION
■
Os136 and benzyl azide61 were prepared as previously described.
Caution! Care should be taken in the preparation of triazole-containing
compounds utilizing organic azide starting materials because these
precursors are potentially explosive. Minimal C-to-N atom ratios of
2.5:1 to 3:1 are recommended to mitigate this risk if the organic azide
is to be isolated prior to use rather than prepared and used in situ. All
reagents were purchased from Alfa Aesar, Acros Organics, Sigma-
Aldrich, and Fluorochem and used as received. All synthetic
manipulations were carried out under an atmosphere of dry N2
employing standard Schlenk line techniques. Deaeration of solvents
(Fisher Scientific) was performed through vigorous bubbling with N2
for a period of at least 15 min. Dry tetrahydrofuran (THF) was
obtained by distillation over CaH2 and stored under an atmosphere of
N2. Dry N,N-dimethylformamide (DMF) was purchased from Acros
and stored under an atmosphere of dry N2. NMR spectra were
recorded on a Bruker Ascend 400 MHz spectrometer, with all
chemical shifts reported in ppm, calibrated relative to the residual
solvent signal (CHCl3, 1H δ 7.26, 13C δ 77.16; MeCN, 1H δ 1.94, 13
C
δ 1.32, 118.26; acetone, 1H δ 2.17, 13C δ 29.84, 206.26). High-
resolution mass spectrometry (HRMS) was performed on an Agilent
6210 time-of-flight instrument with a dual electrospray ionization
source. Cyclic voltammograms were measured using a PalmSens
EmStat3 potentiostat with PSTrace electrochemical software (version
4.8). Analyte solutions (typical concentration 1.5 mmol dm−3) were
prepared using N2-saturated dry MeCN, freshly distilled from CaH2.
All measurements were conducted at RT under a stream of dry N2 at
potential scan rates ranging from 50 to 500 mV s−1. NBu4PF6 was
used as the supporting electrolyte, being recrystallized from EtOH
and oven-dried prior to use, with a typical solution concentration of
0.2 mol dm−3. The working electrode was glassy carbon, with
platinum wire utilized as the counter electrode. The reference
electrode was Ag/AgCl, being chemically isolated from the analyte
solution by an electrolyte-containing bridge tube tipped with a porous
frit. Ferrocene was employed as an internal reference, with all
potentials quoted relative to the Fc+/Fc couple. UV−visible electronic
absorption spectra were recorded on an Agilent Cary-60 spectropho-
tometer, utilizing quartz cuvettes of 1 cm path length. Emission
spectra were recorded on a Fluoromax-4 spectrophotometer utilizing
quartz cuvettes of 1 cm path length and corrected for both the
detector response and solvent Raman signals. “Degassed” solutions
were prepared via three repeat “freeze−pump−thaw” cycles. Quantum
yields (Φem) are quoted relative to [Ru(bpy)3][PF6]2 in aerated
MeCN, with all complexes being excited at a single wavelength with
common optical density. Quantum yields are thus determined from
the ratio of the integrated area under the peaks. Because emission
bands for the osmium complexes tail into the NIR region, outside the
effective range of the spectrophotometer, an experimental uncertainty
of 20% is assumed. Luminescence lifetimes were measured with an
Edinburgh Instruments Mini-τ, equipped with a picosecond diode
laser (404 nm, 56 ps) excitation source.
CONCLUSIONS
■
A series of new phosphorescent osmium(II) complexes have
been reported that display emission from the red to NIR with
the emission intensity sensitive to the presence of oxygen. In
pushing the absorption and emission maxima toward the
biologically transparent window, the complexes are attractive
potential candidate prototypes for the further development of
targeted dual-mode theranostic agents for confocal imaging
microscopy and photodynamic therapy applications. We will
be pursuing this work shortly and will report the results in due
course.
Synthesis of 2,6-Bis(ethynyltrimethylsilyl)pyrazine (2). 2,6-
Dibromopyrazine (1.50 g, 6.30 mmol), Pd(PPh3)2Cl2 (233 mg, 0.33
mmol, 5 mol %), and CuI (127 mg, 0.66 mmol, 10 mol %) were
added to a deaearated mixture of dry THF/Et3N (1:1, v/v; 50 mL).
Ethynyltrimethylsilane (3.6 mL, ρ = 0.709 g mL−1, 25.9 mmol) was
added and the reaction solution stirred at 50 °C for 16 h. The reaction
solution was cooled to RT and filtered through a short silica pad (2
cm) and the filtrate reduced in volume. Purification was achieved via
column chromatography (SiO2, CH2Cl2). Yield: 1.11 g, 65%. 1H
NMR (CDCl3, 400 MHz): δ 0.27 (s, 18H), 8.54 (s, 2H). 13C NMR
(CDCl3, 101 MHz): δ −0.35, 100.26, 100.45, 139.32, 146.00. HRMS
(ESI). Calcd for C14H21N2Si2 (MH+): m/z 273.1243. Found: m/z
273.1244.
Importantly, this systematic survey of the photophysical
properties across the series of osmium(II) complexes also
reveals an initially unexpected and counterintuitive inversion of
spectroscopic tuning with increasing ligand π-acceptor
character. While the LUMOs in these complexes are
progressively stabilized by this approach, stabilization in the
energy of the HOMO is also observed, which reaches a tipping
point whereupon stabilization of the latter outcompetes that of
the former, leading to an inversion and blue shifting in the
tuning of the optical absorption and emission properties. This
work therefore provides important results with regard to the
limitations of photophysical tuning in complexes in which a
relatively austere and minimalist electron-withdrawing ligand
architecture is incorporated.
Synthesis of 2,6-Bis(1-benzyl-1,2,3-triazol-4-yl)pyrazine (3).
2 (1.10 g, 4.03 mmol), CuSO4·5H2O (0.77 g, 3.07 mmol), sodium
ascorbate (1.22, 6.16 mmol), K2CO3 (3.56 g, 25.7 mmol), and benzyl
azide (1.37 g, 10.3 mmol) were combined in 1:1 (v/v) THF/H2O
(100 mL). tert-Butyl alcohol (tBuOH; 20 mL) and pyridine (3.5 mL)
I
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were added and the resultant mixture stirred at RT for 16 h. The
organic solvents were removed by rotary evaporation to leave an
aqueous suspension, to which was added CHCl3 (150 mL), additional
H2O (60 mL), and concentrated aqueous NH3 (15 mL). The biphasic
mixture was stirred rapidly at RT for 1 h. The organic layer was
separated, washed successively with dilute aqueous NH3 (200 mL),
saturated brine (200 mL), and H2O (200 mL), then dried over
MgSO4, and evaporated to dryness. Purification was performed via
column chromatography (SiO2, 1% MeOH/CH2Cl2), affording the
title compound as a white solid. Yield: 1.25 g, 79%. 1H NMR (CDCl3,
400 MHz): δ 5.59 (s, 4H), 7.26−7.34 (m, 4H), 7.34−7.43 (m, 6H),
8.05 (s, 2H), 9.30 (s, 2H). 13C NMR (CDCl3, 101 MHz): δ 54.56,
122.97, 128.25, 129.08, 129.36, 134.37, 140.87, 144.91, 146.36.
HRMS (ESI). Calcd for C22H19N8 (MH+): m/z 395.1727. Found: m/
z 395.1729. Calcd for C22H18N8Na (M + Na+): m/z 417.1547.
Found: m/z 417.1547. Anal. Calcd for C22H18N8: C, 66.99; H, 4.60;
N, 28.41. Found: C, 66.98; H, 4.42;N, 28.29.
155.54, 156.58. HRMS (ESI). Calcd for C15H17N2Si (MH+): m/z
253.1161. Found: m/z 253.1156.
Synthesis of 6-(1-Benzyl-1,2,3-triazol-4-yl)-2,2′-bipyridine
(7). 6 (1.10 g, 4.36 mmol), benzyl azide (0.57 g, 4.29 mmol),
K2CO3 (1.19 g, 8.62 mmol), CuSO4·5H2O (0.42 g, 1.69 mmol), and
sodium ascorbate (0.68 g, 3.43 mmol) were added to 1:1 (v/v) THF/
H2O (100 mL). tBuOH (20 mL) and pyridine (3.5 mL) were added,
and the reaction mixture was then stirred for 16 h at RT. The organic
solvents were removed by rotary evaporation to afford an aqueous
suspension, to which was added CHCl3 (100 mL), additional H2O
(50 mL), and concentrated aqueous NH3 (15 mL). The biphasic
mixture was stirred rapidly at RT for 40 min and the organic layer
then separated. The organic phase was washed successively with dilute
aqueous NH3 (200 mL) and brine (200 mL), followed by H2O (200
mL), and then dried over MgSO4. Purification was carried out via
column chromatography (Al2O3, gradient elution, CH2Cl2 to 2%
MeOH/CH2Cl2), giving the product as a white solid. Yield: 0.59 g,
1
44%. H NMR (CDCl3, 400 MHz): δ 5.62 (s, 2H), 7.27−7.43 (m,
Synthesis of 2-(Tri-n-butylstannyl)pyridine (4). The synthesis
was carried out following a previously published procedure:62 To a
solution of 2-bromopyridine (3 mL, ρ = 1.657 g mL−1, 31.5 mmol) in
dry THF (120 mL) at −78 °C was added, dropwise, nBuLi (13.3 mL,
2.5 M in hexanes, 33.3 mmol). The mixture was stirred for a further 1
h at −78 °C before the quick addition of tri-n-butyltin chloride (8.6
mL, ρ = 1.2 g mL−1, 31.7 mmol). Stirring was maintained at −78 °C
for 3 h before the solution was allowed to warm to RT and then
quenched through the addition of a saturated aqueous solution of
NH4Cl (30 mL). The reaction mixture was extracted into ethyl
acetate (3 × 50 mL) with the combined organic layers, then washed
with saturated brine (100 mL) and H2O (100 mL), and dried over
MgSO4. Evaporation of the solvent in vacuo yielded a light-brown oil,
which was stored in the refrigerator and used without further
purification. Yield: 11.30 g, 97%. Characterization data matched those
previously reported.62 1H NMR (CDCl3, 400 MHz): δ 0.84−0.90 (m,
9H), 1.08−1.14 (m, 6H), 1.28−1.37 (m, 6H), 1.50−1.59 (m, 6H),
7.10 (ddd, J = 1.3, 4.9, and 7.7 Hz, 1H), 7.39 (dt, J = 1.2 and 7.4 Hz,
1H), 7.48 (td, J = 1.7 and 7.5 Hz, 1H), 8.73 (d, J = 4.8 Hz, 1H).
Synthesis of 6-Bromo-2,2′-bipyridine (5). 2,6-Dibromopyr-
idine (5.69 g, 24.0 mmol), 4 (8.00 g, 21.7 mmol), and Pd(PPh3)4
(1.50 g, 1.30 mmol, 6 mol %) were combined in thoroughly deaerated
toluene (30 mL) and heated to reflux for 12 h. After cooling to RT,
the solvent was removed by rotary evaporation and the resulting
residue redissolved in CH2Cl2 (30 mL). Extraction of the organic
phase with 3 × 50 mL portions of 6 M aqueous HCl provided an
aqueous solution that was subsequently neutralized with 10% aqueous
NH3 solution. The aqueous phase was then extracted with CH2Cl2 (3
× 30 mL), with the combined organic layers being washed with H2O
(100 mL), dried over MgSO4, and evaporated to dryness. Purification
was achieved via column chromatography (SiO2, gradient elution,
0.5% MeOH/CH2Cl2 to 1% MeOH/CH2Cl2), affording the product
6H), 7.79 (td, J = 1.2 and 7.8 Hz, 1H), 7.90 (t, J = 7.9 Hz, 1H), 8.17
(s, 1H), 8.20 (dd, J = 0.8 and 7.9 Hz, 1H), 8.32 (dd, J = 0.8 and 7.7
Hz, 1H), 8.40 (d, J = 7.9 Hz, 1H), 8.67 (d, J = 4.3 Hz, 1H). 13C NMR
(CDCl3, 101 MHz): 54.49, 120.29, 120.32, 121.19, 122.19, 123.90,
128.23, 128.94, 129.31, 134.74, 136.90, 137.96, 149.23, 149.33,
149.76, 155.82, 156.07. HRMS (ESI). Calcd for C19H16N5 (MH+):
m/z 314.1400. Found: m/z 314.1402. Calcd for C19H15N5Na (M +
Na+): m/z 336.1220. Found: m/z 336.1220. Anal. Calcd for
C19H15N5: C, 72.83; H, 4.83; N, 22.35. Found: C, 72.94; H, 4.86;
N, 22.43.
Synthesis of 2-Bromo-6-(pyridin-2-yl)pyrazine (8). 2,6-
Dibromopyrazine (3.55 g, 14.9 mmol), 2-(tributylstannyl)pyridine
(5.51 g, 14.9 mmol), and Pd(PPh3)4 (1.07 g, 0.926 mmol, 6 mol %)
were added to deaerated toluene (100 mL) and heated at 110 °C for
21 h. The dark-red-brown mixture was cooled to RT and the solvent
removed under reduced pressure. The resulting oily residue was
redissolved in CH2Cl2 (100 mL) and extracted with 2 × 100 mL
portions of aqueous 6 M HCl. The combined aqueous layers were
then neutralized with a 30% aqueous NH3 solution, resulting in the
formation of a light-brown precipitate. The aqueous suspension was
extracted with 2 × 100 mL portions of CH2Cl2, with the combined
organic phases then being dried over MgSO4 and the solvent
removed. Purification was achieved via column chromatography
(SiO2, gradient elution 0.5% MeOH/CH2Cl2 to 0.75% MeOH/
CH2Cl2), with the product eluting immediately before a yellow band.
The title compound was obtained as a white solid. Yield: 1.50 g, 43%.
1H NMR (CDCl3, 400 MHz): δ 7.38 (ddd, J = 1.2, 4.7, and 7.5 Hz,
1H), 7.85 (td, J = 1.8 and 7.7 Hz, 1H), 8.36 (d, J = 8.0 Hz, 1H),
8.68−8.72 (m, 2H), 9.56 (s, 1H). 13C NMR (CDCl3, 101 MHz): δ
122.08, 125.09, 137.32, 140.08, 141.03, 147.03, 149.72, 151.90,
152.87. HRMS (ESI). Calcd for C9H7N3Br (MH+): m/z 235.9818.
Found: m/z 235.9823.
1
as a white solid. Yield: 1.38 g, 27%. H NMR (CDCl3, 400 MHz): δ
7.30−7.35 (m, 1H), 7.49 (d, J = 7.8 Hz, 1H), 7.66 (t, J = 8.00 Hz,
1H), 7.82 (td, J = 1.6 and 7.9 Hz, 1H), 8.35−8.43 (m, 2H), 8.66 (d, J
= 4.5 Hz, 1H). 13C NMR (CDCl3, 101 MHz): δ 119.86, 121.64,
124.41, 128.13, 137.17, 139.37, 141.74, 149.37, 154.64, 157.50.
HRMS (ESI). Calcd for C10H8N2Br (MH+): m/z 234.9865. Found:
m/z 234.9867.
2-(Ethynyltrimethylsilyl)-6-(pyridin-2-yl)pyrazine (9). 8 (1.36
g, 5.76 mmol), Pd(PPh3)2Cl2 (215 mg, 0.307 mmol, 5 mol %), and
CuI (102 mg, 0.536 mmol, 9 mol %) were added to deaerated 1:1 (v/
v) dry THF/Et3N (80 mL). Ethynyltrimethylsilane (1.6 mL, ρ =
0.709 g mL−1, 11.5 mmol) was added and the reaction mixture then
stirred at 60 °C for 16 h. The resultant dark-brown solution was
filtered through a short silica pad and the filtrate evaporated to
dryness. Purification was carried out via column chromatography
(SiO2, 1% MeOH/CH2Cl2), affording the product as a pale-yellow oil.
Yield: 1.14 g, 78%. 1H NMR (CDCl3, 400 MHz): δ 0.31 (s, 9H), 7.37
(ddd, J = 0.8, 4.8, and 7.5 Hz, 1H), 7.84 (td, J = 1.7 and 7.7 Hz, 1H),
8.42 (d, J = 7.9 Hz, 1H), 8.68−8.73 (m, 2H), 9.53 (s, 1H). 13C NMR
(CDCl3, 101 MHz): δ −0.21, 99.50, 101.14, 122.05, 124.81, 137.20,
138.51, 141.71, 147.54, 149.56, 150.64, 153.72. HRMS (ESI). Calcd
for C14H16N3Si (MH+): m/z 254.1108. Found: m/z 254.1117.
2-(1-Benzyl-1,2,3-triazol-4-yl)-6-(pyridin-2-yl)pyrazine (10).
9 (1.14 g, 4.50 mmol) and benzyl azide (610 mg, 4.58 mmol) were
Synthesis of 6-(Ethynyltrimethylsilyl)-2,2′-bipyridine (6). 5
(1.22 g, 5.19 mmol), Pd(PPh3)2Cl2 (182 mg, 0.26 mmol, 5 mol %),
and CuI (99 mg, 0.52 mmol, 10 mol %) were added to a deaerated
1:1 (v/v) mixture of dry THF/Et3N (60 mL). Ethynyltrimethylsilane
(1.8 mL, ρ = 0.709 g mL−1, 13.0 mmol) was added and the reaction
solution then heated to 60 °C for 16 h. The reaction mixture was
allowed to cool to RT and passed through a short (2 cm) silica pad
and the filtrate evaporated. The residue was purified via column
chromatography (SiO2, 1% MeOH/CH2Cl2), affording the title
1
compound. Yield: 1.09 g, 83%. H NMR (CDCl3, 400 MHz): δ 0.29
(s, 9H), 7.27−7.34 (m, 1H), 7.48 (d, J = 7.6 Hz, 1H), 7.73−7.83 (m,
2H), 8.35 (d, J = 8.0 Hz, 1H), 8.47 (d, J = 8.0 Hz, 1H), 8.65 (d, J =
4.4 Hz, 1H). 13C NMR (CDCl3, 101 MHz): δ −0.08, 94.61, 104.12,
120.67, 121.74, 124.10, 127.68, 137.01, 137.08, 142.58, 149.18,
t
added to 1:1 (v/v) THF/H2O (120 mL). BuOH (20 mL) was
added, followed by K2CO3 (1.08 g, 7.82 mmol), CuSO4·5H2O (463
mg, 1.85 mmol), sodium ascorbate (768 mg, 3.87 mmol), and
J
DOI: 10.1021/acs.inorgchem.9b00915
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
pyridine (3.5 mL). The reaction mixture was stirred rapidly at RT for
27 h, and the organic solvents were then removed via rotary
evaporation. To the resulting aqueous suspension was then added
CHCl3 (150 mL), additional H2O (50 mL), and concentrated
aqueous NH3 (12 mL). The biphasic mixture was stirred rapidly at
RT for 1 h and the organic layer separated. The aqueous phase was
extracted with further 2 × 50 mL portions of CHCl3, with the
combined organic layers then being washed successively with dilute
aqueous NH3 (2 × 100 mL), brine (1 × 100 mL), and H2O (100
mL). The organic phase was dried over MgSO4 and the solvent
removed to leave a light-brown solid, which was purified by column
chromatography (SiO2, gradient elution 1% MeOH/CH2Cl2 to 2.5%
MeOH/CH2Cl2), affording the product as an off-white solid after
mixture was stirred rapidly for 1 h at RT. The organic layer was
removed and the aqueous phase extracted with a 50 mL portion of
CHCl3. The combined organic layers were washed with dilute
aqueous NH3 (100 mL), followed by brine (100 mL), dried over
MgSO4, and then evaporated to dryness. The crude solids were
purified by column chromatography (SiO2, 1% MeOH/CH2Cl2) to
1
give the title compound as a white powder. Yield: 0.73 g, 46%. H
NMR (CDCl3, 400 MHz): δ 5.56 (s, 2H), 7.28−7.35 (m, 2H), 7.35−
7.42 (m, 4H), 7.60 (t, J = 7.7 Hz, 1H), 8.08 (s, 1H), 8.12 (dd, J = 0.6
and 7.7 Hz, 1H). 13C NMR (CDCl3, 101 MHz): δ 54.57, 118.94,
122.79, 127.09, 128.40, 129.07, 129.34, 134.32, 139.30, 141.75,
147.55, 151.44. HRMS (ESI). Calcd for C14H12N4Br (MH+): m/z
315.0240. Found: m/z 315.0233. Calcd for C14H11N4BrNa (M +
Na+): m/z 337.0059. Found: m/z 337.0053.
1
thorough drying in vacuo. Yield: 1.09 g, 77%. H NMR (CDCl3, 400
Synthesis of 2-Pyrazinyl-6-(1-benzyl-1,2,3-triazol-4-yl)-
pyridine (14). Anhydrous Cu(OAc)2 (146 mg, 0.80 mmol), tribasic
K3PO4 (780 mg, 3.68 mmol), and 10 4 Å molecular sieves were added
to thoroughly deaerated dry DMF (20 mL). Diethanolamine (160 μL,
ρ = 1.097 g mL−1, 1.67 mmol) was added and the mixture heated to
85 °C for 15 min. The resulting deep-blue mixture was then
transferred via cannula to a reaction flask containing 13 (435 mg, 1.38
mmol), 11 (575 mg, 2.44 mmol), tribasic K3PO4 (790 mg, 3.72
mmol), anhydrous KOAc (146 mg, 1.49 mmol), Pd XPhos G1 (77
mg, 0.10 mmol), and 10 4 Å molecular sieves. The reaction mixture
was heated to 100 °C for 20 h, cooled to RT, and then diluted
through the addition of CHCl3 (100 mL) and H2O (150 mL). The
organic layer was separated and the aqueous phase extracted with a
further 100 mL portion of CHCl3. The combined organic layers were
then washed with H2O (2 × 200 mL), dilute aqueous NH3 (2 × 100
mL), followed by brine (100 mL), dried over MgSO4, and then
evaporated to dryness. Purification was carried out by column
chromatography (Al2O3, gradient elution, 0.1% MeOH/CH2Cl2 to
0.2% MeOH/CH2Cl2), giving the product as a white solid. Yield: 194
MHz): δ 5.64 (s, 2H), 7.33−7.45 (m, 6H), 7.82 (td, J = 1.7 and 7.7
Hz, 1H), 8.18 (s, 1H), 8.35 (d, J = 7.9 Hz, 1H), 8.71 (d, J = 4.6 Hz,
1H), 9.43 (s, 1H), 9.54 (s, 1H). 13C NMR (CDCl3, 101 MHz): δ
54.46, 121.43, 123.02, 124.49, 128.15, 128.96, 129.25, 134.34, 136.96,
141.44, 141.89, 144.43, 146.56, 149.48, 149.96, 153.97. HRMS (ESI).
Calcd for C18H15N6 (MH+): m/z 315.1353. Found: m/z 315.1381.
Calcd for C18H14N6Na (M + Na+): m/z 337.1172. Found: m/z
337.1174.
Synthesis of 2-Pyrazinyl N-Methyliminodiacetic Acid Boro-
nate (11). Following the procedure previously reported by Burke and
co-workers,50 2-iodopyrazine (2.0 mL, ρ = 2.086 g mL−1, 20.2 mmol)
and triisopropyl borate (4.7 mL, ρ = 0.815 g mL−1, 20.3 mmol) were
added to dry THF (70 mL) and cooled to −78 °C. nBuLi (8.1 mL, 2.5
M in hexanes, 20.2 mmol) was added dropwise and the solution
stirred for 1 h at −78 °C and then allowed to warm to RT with further
stirring for 3 h. Separately, a three-necked flask equipped with a
dropping funnel, a thermometer, and a distillation apparatus was
charged, under N2, with a previously prepared solution of N-
methyliminodiacetic acid (5.35 g, 36.3 mmol) in dimethyl sulfoxide
(DMSO; 30 mL), which was subsequently heated to 120 °C. The
boronate solution was then transferred via cannula to the dropping
funnel and added to the hot reaction mixture slowly, dropwise, at such
a rate so as to maintain the internal temperature between 110 and 120
°C. THF was rapidly distilled during the course of the addition, after
which the DMSO solvent was also removed by distillation under
reduced pressure at 50 °C. The resulting brown residue was dried
under high vacuum overnight at 50 °C. Purification was carried out by
column chromatography (SiO2, gradient elution, 5% MeCN/Et2O to
MeCN), affording the product as a light-brown crystalline solid,
which was stored in the refrigerator. Yield: 2.50 g, 53%. Character-
ization data matched those previously reported.50 1H NMR (MeCN-
d3, 400 MHz): δ 2.61 (s, 3H), 4.00 (d, J = 16.6 Hz, 2H), 4.18 (d, J =
16.9 Hz, 2H), 8.53 (d, J = 2.6 Hz, 1H), 8.68 (dd, J = 1.6 and 2.6 Hz,
1H), 8.77 (d, J = 1.6 Hz, 1H).
Synthesis of 2-Bromo-6-(1-benzyl-1,2,3-triazol-4-yl)-
pyridine (13). 2,6-Dibromopyridine (5.00 g, 21.1 mmol), Pd-
(PPh3)2Cl2 (730 mg, 1.04 mmol, 5 mol %), and CuI (403 mg, 2.11
mmol, 10 mol %) were added to a deaerated 7:1 (v/v) mixture of dry
THF/Et3N (80 mL). Ethynyltrimethylsilane (2.65 mL, ρ = 0.709 g
mL−1, 19.1 mmol) was added and the reaction solution stirred at 40
°C for 6 h. The dark-brown solution was cooled to RT and passed
through a short (2 cm) silica plug and the filtrate evaporated to
dryness. Column chromatography (SiO2, 3:7 CH2Cl2/hexane)
afforded a white solid (1.95 g), which was found by 1H NMR
(CDCl3) analysis to contain a mixture of the desired 2-bromo-6-
(ethynyltrimethylsilyl)pyridine (12) and a small quantity of unreacted
2,6-dibromopyridine, which was used in the subsequent step without
further purification, as has been previously reported.63 12 (1.27 g,
mixture as detailed above), benzyl azide (0.69 g, 5.18 mmol), and
K2CO3 (1.55 g, 11.2 mmol) were added to 1:1 (v/v) THF/H2O (120
mL). tBuOH (20 mL) was added, followed by CuSO4·5H2O (0.65 g,
2.60 mmol), sodium ascorbate (1.00 g, 5.04 mmol), and pyridine (3
mL). The reaction mixture was stirred at RT for 23 h, after which the
organic solvents were removed by rotary evaporation. To the resulting
aqueous suspension was then added CHCl3 (150 mL), additional
H2O (50 mL), and concentrated aqueous NH3 (12 mL). The biphasic
1
mg, 45%. H NMR (CDCl3, 400 MHz): δ 5.62 (s, 2H), 7.32−7.44
(m, 5H), 7.91 (t, J = 7.9 Hz, 1H), 8.16 (s, 1H), 8.24 (dd, J = 0.8 and
8.0 Hz, 1H), 8.26 (dd, J = 0.8 and 8.0 Hz, 1H), 8.57 (d, J = 2.5 Hz,
1H), 8.58−8.60 (m, 1H), 9.62 (d, J = 1.3 Hz, 1H). 13C NMR
(CDCl3, 101 MHz): δ 54.53, 120.55, 120.90, 122.35, 128.29, 129.01,
129.34, 134.57, 138.13, 143.42, 143.70, 144.58, 148.81, 150.08,
150.99, 153.82. HRMS (ESI). Calcd for C18H15N6 (MH+): m/z
315.1353. Found: m/z 315.1350. Calcd for C18H14N6Na (M + Na+):
m/z 337.1172. Found: m/z 337.1164. Anal. Calcd for C18H14N6: C,
68.78; H, 4.49; N, 26.73. Found: C, 68.89; H, 4.57; N, 26.61.
2-Bromo-6-(1-benzyl-1,2,3-triazol-4-yl)pyrazine (16). 2,6-Di-
bromopyrazine (7.00 g, 29.4 mmol), Pd(PPh3)2Cl2 (0.94 g, 1.34
mmol, 4.5 mol %), and CuI (0.51 g, 2.68 mmol, 9.1 mol %) were
added to a mixture of dry THF (75 mL) and Et3N (15 mL).
Ethynyltrimethylsilane (4.1 mL, ρ = 0.709 g mL−1, 29.6 mmol) was
added and the reaction solution heated to 30 °C for 6 h. The dark-
red-brown solution was cooled to RT and filtered through a short
silica pad. The filtrate was evaporated to dryness and the resultant
residue subjected to column chromatography (SiO2, 7:3 hexane/
CH2Cl2), affording an orange oil. 1H NMR analysis revealed the
product to be comprised of a mixture of 2,6-dibromopyrazine, 2, and
2-bromo-6-(ethynyltrimethylsilyl)pyrazine (15) in a 0.5:0.5:1 respec-
tive molar ratio. This mixture was used in the following step without
1
further purification. Yield (based on 15): 2.90 g, 39%. Relevant H
NMR (CDCl3, 400 MHz) analysis for 15: δ 0.28 (s, 9H), 8.58 (s,
1H), 8.59 (s, 1H).
The above mixture of substituted pyrazines (5.50 g, calcd to
contain 2.75 g, 10.8 mmol, of 15) was combined with excess benzyl
azide (3.31 g, 24.9 mmol), CuSO4·5H2O (2.75 g, 11.0 mmol), sodium
ascorbate (4.12 g, 20.7 mmol), and K2CO3 (5.00 g, 36.23 mmol) in a
t
1:1 (v/v) solution of THF/H2O (300 mL). BuOH (30 mL) was
added, followed by pyridine (6 mL), and the resultant suspension
stirred rapidly at RT for 18 h. The organic solvent was removed by
rotary evaporation to give an aqueous suspension, to which was added
CHCl3 (200 mL), concentrated aqueous NH3 (20 mL), and
additional H2O (50 mL). The biphasic mixture was stirred rapidly
at RT for 1 h and the organic layer then removed. The aqueous phase
K
DOI: 10.1021/acs.inorgchem.9b00915
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was extracted with a further 100 mL portion of CHCl3, and the
combined organic layers were then washed successively with dilute
aqueous NH3 (10%; 2 × 100 mL) and brine (1 × 100 mL). The
organic phase was dried over MgSO4 and the solvent removed to
leave an oily residue, which was purified by column chromatography
(SiO2, 1.5% MeOH/CH2Cl2), yielding the title compound as a white
The solids were recrystallized from CH2Cl2/hexanes, giving the title
complex as a dark-brown powder. Yield: 347 mg, 80%. 1H NMR
(MeCN-d3, 400 MHz): δ 5.37 (s, 8H), 7.15 (d, J = 7.3 Hz, 8H),
7.28−7.41 (m, 12H), 8.66 (s, 4H), 9.28 (s, 4H). 13C NMR (MeCN-
d3, 101 MHz): δ 56.73, 127.70, 129.42, 130.10, 130.18, 133.84,
140.57, 145.78, 149.48. HRMS (ESI). Calcd for [C44H36N16Os]2+
(M2+): m/z 490.1456. Found: m/z 490.1457. Anal. Calcd for
C44H36N16P2F12Os: C, 41.64; H, 2.86; N, 17.66. Found: C, 41.77;
H, 2.67; N, 17.76.
1
solid. Yield: 2.39 g, 70%. H NMR (CDCl3, 400 MHz): δ 5.59 (s,
2H), 7.29−7.35 (m, 2H), 7.36−7.43 (m, 3H), 8.10 (s, 1H), 8.57 (s,
1H), 9.33 (s, 1H). 13C NMR (CDCl3, 101 MHz): δ 54.66, 123.71,
128.40, 129.20, 129.40, 134.02, 139.75, 140.17, 145.12, 145.98,
146.41. HRMS (ESI). Calcd for C13H11N5Br (MH+): m/z 316.0192.
Found: m/z 316.0190. Calcd for C13H10N5BrNa (M + Na+): m/z
338.0012. Found: m/z 338.0011. Calcd for C26H20N10Br2Na (2M +
Na+): m/z 653.0132. Found: m/z 653.0087.
Synthesis of Os3. [(NH4)2OsCl6] (150 mg, 0.34 mmol) and 7
(225 mg, 0.72 mmol) were combined in ethylene glycol (25 mL) and
heated to reflux for 16 h. The reaction mixture was cooled to RT and
treated with an aqueous solution (25 mL) of NH4PF6 (275 mg, 1.69
mmol). The resulting dark-green precipitate was collected by
filtration, washed with H2O, and dried in vacuo. The solids were
purified by column chromatography (Al2O3, 4:1 CH2Cl2/MeCN),
followed by recrystallization from MeCN/Et2O, giving the desired
complex as a dark-green powder. Yield: 100 mg, 27%. 1H NMR
(MeCN-d3, 400 MHz): δ 5.33 (s, 4H), 7.06−7.13 (m, 6H), 7.23 (d, J
= 5.5 Hz, 2H), 7.26−7.38 (m, 6 H), 7.76 (td, J = 1.5 and 7.8 Hz, 2H),
7.87 (t, J = 8.0 Hz, 2H), 8.34 (d, J = 8.0 Hz, 2H), 8.39 (d, J = 8.0 Hz,
2H), 8.55 (s, 2H), 8.56 (d, J = 8.0 Hz, 2H). 13C NMR (MeCN-d3,
101 MHz): δ 56.44, 121.44, 121.73, 125.40, 127.20, 128.51, 129.21,
130.00, 130.02, 134.15, 136.94, 138.53, 150.99, 151.97, 153.30,
156.92, 161.11. HRMS (ESI). Calcd for [C38H30N10Os]2+ (M2+): m/z
409.1129. Found: m/z 409.1148. Anal. Calcd for C38H30N10P2F12Os:
C, 41.23; H, 2.73; N, 12.65. Found: C, 41.20; H, 2.69; N, 12.57.
Synthesis of Os4. [(NH4)2OsCl6] (153 mg, 0.35 mmol) and 10
(236 mg, 0.75 mmol) were combined in ethylene glycol (20 mL) and
heated to reflux for 17 h. The reaction mixture was cooled to RT and
treated with an aqueous solution (10 mL) of NH4PF6 (337 mg, 2.06
mmol). The resulting dark-green precipitate was collected by
filtration, washed with H2O, followed by Et2O, and dried in vacuo.
The solids were purified by column chromatography [SiO2, 1:1:10
(v/v/v) H2O/saturated aqueous KNO3/MeCN], which after
subsequent counterion metathesis gave the title complex as a dark-
green powder. Yield: 172 mg, 45%. 1H NMR (MeCN-d3, 400 MHz):
δ 5.35 (s, 4H), 7.08−7.21 (m, 6H), 7.23−7.40 (m, 8H), 7.86 (t, J =
7.6 Hz, 2H), 8.57 (d, J = 7.9 Hz, 2H), 8.66 (s, 2H), 9.38 (s, 2H), 9.64
(s, 2H). 13C NMR (MeCN-d3, 101 MHz): δ 56.68, 125.86, 127.65,
128.75, 129.39, 130.02, 130.09, 133.80, 139.64, 141.94, 142.62,
144.63, 149.05, 150.41, 154.40, 158.59. HRMS (ESI). Calcd for
[C36H28N12Os]2+ (M2+): m/z 410.1082. Found: m/z 410.1090. Anal.
Calcd for C36H28N12OsP2F12: C, 38.99; H, 2.55; N, 15.16. Found: C,
38.88; H, 2.60; N, 15.03.
Synthesis of 6-(1-Benzyl-1,2,3-triazol-4-yl)-2,2′-bipyrazine
(17). Anhydrous Cu(OAc)2 (200 mg, 1.10 mmol) and tribasic K3PO4
(970 mg, 4.57 mmol) were added to dry, thoroughly deaerated DMF
(20 mL) along with 10 4 Å molecular sieves. Diethanolamine (210
μL, ρ = 1.097 g mL−1, 2.19 mmol) was added and the solution heated
to 85 °C with stirring for 10 min. The resulting bright-blue solution
was then transferred via cannula to a reaction vessel containing 16
(608 mg, 1.92 mmol), 11 (661 mg, 2.81 mmol), tribasic K3PO4 (800
mg, 3.77 mmol), anhydrous KOAc (184 mg, 1.87 mmol), Pd XPhos
G1 (97 mg, 0.13 mmol), and 10 4 Å molecular sieves. The reaction
mixture was heated to 100 °C for 22 h, cooled to RT, and then
diluted through the addition of CHCl3 (100 mL) and H2O (150 mL).
The organic layer was removed and the aqueous phase extracted with
a further portion (100 mL) of CHCl3. The combined organic layers
were washed successively with H2O (200 mL), dilute aqueous NH3 (2
× 100 mL), followed by brine (100 mL), dried over MgSO4 and the
solvent was removed in vacuo. The residue was purified by column
chromatography (Al2O3, 0.1% MeOH/CH2Cl2), giving an off-white
powder. The solids were redissolved in CH2Cl2 (15 mL) and slowly
triturated with excess hexanes to afford the pure title compound as a
white solid. Yield: 128 mg, 21%. 1H NMR (acetone-d6, 400 MHz): δ
5.79 (s, 2H), 7.33−7.45 (m, 3H), 7.45−7.50 (m, 2H), 8.72−8.75 (m,
2H), 8.91 (s, 1H), 9.37 (s, 1H), 9.44 (s, 1H), 9.62 (d, J = 0.9 Hz,
1H). 13C NMR (acetone-d6, 101 MHz): δ 54.71, 125.36, 129.09,
129.37, 129.88, 136.82, 142.27, 142.65, 144.20, 145.03, 146.11,
146.59, 146.71, 149.28, 149.99. HRMS (ESI). Calcd for C17H14N7
(MH+): m/z 316.1305. Found: m/z 316.1296. Calcd for C17H13N7Na
(M + Na+): m/z 338.1125. Found: m/z 338.1116. Anal. Calcd for
C17H13N7: C, 64.75; H, 4.16; N, 31.09. Found: C, 64.81; H, 3.96; N,
31.03.
Synthesis of 2,2′:6′,2″-Terpyrazine (18). 2,6-Dibromopyrazine
(325 mg, 1.36 mmol), 11 (1.11 g, 4.72 mmol), tribasic K3PO4 (2.14 g,
10.1 mmol), Pd XPhos G1 (80 mg, 0.11 mmol), anhydrous
Cu(OAc)2 (274 mg, 1.51 mmol), diethanolamine (0.3 mL, ρ =
1.097 g mL−1, 3.13 mmol), and 20 4 Å molecular sieves were added to
an oven-dried flask. Dry, deaerated DMF (15 mL) was added and the
mixture heated to 100 °C for 17 h. After cooling to RT, the mixture
was diluted through the addition of CHCl3 (70 mL) and H2O (100
mL). The organic phase was removed and the aqueous layer extracted
with a further 50 mL portion of CHCl3. The combined organic layers
were subsequently washed with dilute aqueous NH3 (100 mL),
followed by brine (2 × 200 mL), and dried over MgSO4. The solvent
was removed under reduced pressure, and the remaining brown solids
were dried thoroughly under high vacuum. The crude solids were
then suspended in stirring MeOH (30 mL), collected by filtration,
and washed with hexane to give the pure title compound as beige
solids. Yield: 89 mg, 28%. NMR characterization was found to be in
agreement with that reported in the literature.38 1H NMR (CDCl3,
400 MHz): δ 8.71 (s, 4H), 9.68 (s, 2H), 9.75 (s, 2H). HRMS (ESI).
Calcd for C12H9N6 (MH+): m/z 237.0888. Found: m/z 237.0887.
Synthesis of Os2. [(NH4)2OsCl6] (150 mg, 0.34 mmol) and 3
(282 mg, 0.72 mmol) were combined in ethylene glycol (25 mL) and
heated to reflux for 16 h. The reaction mixture was cooled to RT and
treated with an aqueous solution (25 mL) of NH4PF6 (165 mg, 1.01
mmol). The resulting dark-colored precipitate was collected by
filtration, washed with H2O, followed by Et2O, and dried in vacuo.
Synthesis of Os5. [(NH4)2OsCl6] (62 mg, 0.14 mmol) and 14
(90 mg, 0.29 mmol) were combined in ethylene glycol (8 mL) and
heated to reflux for 7 h. The reaction mixture was cooled to RT and
treated with an aqueous solution (8 mL) of NH4PF6 (112 mg, 0.69
mmol). The resulting precipitate was collected by filtration, washed
with H2O, followed by Et2O, and dried in vacuo. The solids were
subsequently redissolved in MeCN (12 mL), refrigerated for 5 h, and
then passed quickly through a short (2 cm) Celite pad. The addition
of excess Et2O to the filtrate reprecipitated a dark-green powder,
which was purified further by column chromatography [Al2O3, 1:1:10
(v/v/v) H2O/saturated aqueous KNO3/MeCN]. Subsequent
counterion metathesis furnished the desired complex as dark-green
solids. Yield: 93 mg, 60%. 1H NMR (MeCN-d3, 400 MHz): δ 5.35 (s,
4H), 7.14 (d, J = 7.1 Hz, 4 H), 7.28−7.40 (m, 8H), 7.98 (t, J = 8.2
Hz, 2H), 8.11 (d, J = 3.3 Hz, 2H), 8.42 (d, J = 8.0 Hz, 2H), 8.58 (s,
2H), 8.72 (d, J = 8.1 Hz, 2H), 9.50 (d, J = 0.5 Hz, 2H). 13C NMR
(MeCN-d3, 101 MHz): δ 56.60, 121.83, 121.99, 127.54, 129.42,
130.02, 130.09, 133.83, 138.28, 145.88, 147.53, 149.78, 151.32,
151.75, 155.66, 156.93. HRMS (ESI). Calcd for [C36H28N12Os]2+
(M2+): m/z 410.1082. Found: m/z 410.1104. Calcd for
[C36H28N12PF6Os]+ (M+): m/z 965.1817. Found: m/z 965.1825.
Anal. Calcd for C36H28N12OsP2F12: C, 38.99; H, 2.55; N, 15.16.
Found: C, 38.80; H, 2.46; N, 15.04.
Synthesis of Os6. [(NH4)2OsCl6] (77 mg, 0.17 mmol) and 17
(114 mg, 0.36 mmol) were combined in ethylene glycol (10 mL) and
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heated to reflux for 7 h. The reaction mixture was cooled to RT and
treated with an aqueous solution (10 mL) of NH4PF6 (146 mg, 0.89
mmol). The resulting precipitate was collected by filtration, washed
with H2O, followed by Et2O, and dried in vacuo. Purification was
carried out by column chromatography [SiO2, 0.06:1:1:10 (v/v/v/v)
Et3N/H2O/saturated aqueous KNO3/MeCN], which, after counter-
ion metathesis, afforded the product as a dark-green solid. Yield: 58
mg, 30%. 1H NMR (MeCN-d3, 400 MHz): δ 5.36 (s, 4H), 7.16 (d, J
= 6.9 Hz, 4H), 7.29−7.40 (m, 6H), 7.45 (d, J = 3.2 Hz, 2H), 8.21 (d,
J = 3.3 Hz, 2H), 8.73 (s, 2H), 9.46 (s, 2H), 9.69 (s, 2H), 9.81 (s, 2H).
13C NMR (MeCN-d3, 101 MHz): δ 56.85, 128.08, 129.58, 130.05,
130.19, 133.54, 142.36, 142.79, 145.12, 146.30, 148.67, 148.94,
149.50, 150.02, 154.38. HRMS (ESI). Calcd for [C34H26N14Os]2+
(M2+): m/z 411.1034. Found: m/z 411.1044. Anal. Calcd for
C34H26N14Os P2F12: C, 36.76; H, 2.36; N, 17.65. Found: C, 36.89;
H, 2.44; N, 17.83.
Computational Details. The ground-state geometries of the
complexes were optimized in the gas phase at the B3LYP level of
theory64 using the Stuttgart−Dresden relativistic small-core effective
core potential65 and basis set for the osmium atom and 6-311G* basis
sets66 for all other atoms using the NWChem software package.67
Molecular orbital energies and isosurface plots were then calculated in
single-point calculations at the same level of theory using the SMD
solvation model (MeCN).68 HOMO and LUMO plots were
produced using the Gabedit69 viewer software. Time-dependent
DFT calculations were also carried out including the solvation model,
with the first 50 singlet and 10 triplet roots determined.
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́
́
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00915.
NMR spectroscopy and mass spectrometry character-
ization data, additional electrochemical data, low-
temperature (77 K) photoluminescence spectra, opti-
mized geometry coordinates for Os1−Os7, and
calculated UV−visible absorption spectra for Os1−Os6
(PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: p.scattergood@hud.ac.uk.
*E-mail: p.i.elliott@hud.ac.uk.
ORCID
Paul A. Scattergood: 0000-0001-9070-5933
Paul I. P. Elliott: 0000-0003-1570-3289
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The authors thank the University of Huddersfield (PIPE and
PAS) and the State of Libya (SAEO) for supporting this work.
Via our membership of the U.K.’s HEC Materials Chemistry
Consortium, which is funded by EPSRC (Grants EP/L000202
and EP/R029431), this work used the ARCHER U.K.
National Supercomputing Service (http://www.archer.ac.uk).
We acknowledge Orlando De Azevedo for useful synthetic
discussions and also thank Stephen Boyer at London
Metropolitan University for carrying out elemental micro-
analysis.
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