Synthesis and photophysical characterization of a bis-pincer osmium complex

Article abstract of DOI:10.1016/j.poly.2014.06.051

The neutral bis-pincer transition metal complex Os(PCP)₂ (PCP = 2,6-(CH₂PPh₂)₂C₆H₃, Ph = C₆H₅) was prepared via two synthetic routes with up to 42% yield. The lemon-ye

Full text of DOI:10.1016/j.poly.2014.06.051

Polyhedron 84 (2014) 136–143
Contents lists available at ScienceDirect
Polyhedron
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Synthesis and photophysical characterization of a bis-pincer osmium
complex
⇑
Yifei Liu, Peter I. Djurovich, Ralf Haiges, Mark E. Thompson
Department of Chemistry, University of Southern California, Los Angeles, CA 90089, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
The neutral bis-pincer transition metal complex Os(PCP)2 (PCP = 2,6-(CH2PPh2)2C6H3, Ph = C6H5) was pre-
pared via two synthetic routes with up to 42% yield. The lemon-yellow complex is air stable as a neat
solid and in solution, and is thermally stable as can be purified by high vacuum zone sublimation at
280-240-200 °C under 1 ꢀ 10ꢁ6 torr. The pincer ligands coordinate in a pseudo-octahedral arrangement
around the metal center. X-ray crystallography reveals that the complex is in pseudo-D2 geometry sym-
metry due to the twist caused by the methylenes. NMR studies at varied temperatures between 223 and
343 K along with low temperature inversion recovery studies show the fluxional behavior and exchang-
ing process between the two enantiomers (Ea = 8.6 kcal/mol). The Os(PCP)2 shows no emission in solution
Received 14 May 2014
Accepted 26 June 2014
Available online 11 July 2014
Keywords:
Pincer ligand
Osmium complex
Dynamic NMR
Phosphorescence
Metal centered excited state
at room temperature, but emits at 77 K in 2-MeTHF glass (U = 60%). At room temperature, the neat solid
of Os(PCP)2 shows yellow emission of 3% quantum yield. TDDFT predicted S0 ? S1 transition shows that
the lowest singlet state (S1) is metal-ligand (the Os-Xylyl fragments) to ligand’ (the PPh2 fragments)
charge transfer (ML-L’CT) in character with a small contribution from the higher lying metal centered
(1MC) states. The large spin–orbit coupling constant of osmium leads to effective intersystem crossing
to a triplet state with increased MC character. The DFT predicted triplet spin surface of Os(PCP)2 indicates
that the emission is dominated by 3MC state. Significant Os–P bonds elongation at the lowest triplet
excited state also strongly supports the conclusion.
Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction
N^C^N or C^N^C types of chelates have also attracted interest for
their photophysical properties [20].
Bis-terdentate chelated transition metal complexes such as
[Ru(terpy)2]2+, [Os(terpy)2]2+, and [Ir(terpy)2]3+ with bis(2,20:60,200-
terpyridine) (terpy) and related complexes have been intensively
studied for their interesting photophysical, photochemical, and
electrochemical properties, and potential applications in molecular
electronics such as photoinduced electron and energy transfer
organometallic rods, molecular wires and switches, and self-
assembled light-harvesting systems [1–10]. The terdentate chelat-
ing ligands are structurally more appealing than bidentate chelat-
ing analogues for octahedral metal complexes as they lack optical
isomers and enable a controllable linear arrangement for the
design of functional molecular materials [11]. Terdentate carbene
ligands have been employed in homo- and heteroleptic ruthenium
complexes for interesting photophysical, especially photolumines-
cence properties [12–19]. Neutral or ionic heteroleptic iridium
complexes containing two terdentate ligands of terpyridine,
Among the array of choices for chelates of bis-terdentate tran-
sition metal complexes, a particular type of terdentate ligand, the
pincer (pincer = L-X-L0-X-L, where X = CH2, NH, O, etc.) named after
the chelating mode with the metal center is much less studied.
Transition metal complexes chelated with pincer ligands were first
reported in the 1970s and used diphosphine ligands of the general
structure [2,6-(CH2PR2)2C6H4, PCP-H] (R = alkyl or aryl). These
ligands can be readily cyclometalated onto metals such as Ni, Ir,
Rh, Pt and Pd [21] and thus organometallic complexes containing
mono-pincer chelates have been extensively studied thereafter,
especially in catalysis chemistry [22,23]. To the best of our
knowledge, the only example of a metal complex with bis-pincer
coordination in literature is the main group metal complex
Mg(P0CP0)2, where C is a metalated 2,6-xylyl group and P0 is a
PMe2 substituent, reported by Muller and co-workers in 1994 [24].
Unlike in terpy-based ligands, where the pyridyl ligands (L) are
conjugated through sp2–sp2 linkages, conjugation between L-moi-
eties in pincer ligands is interrupted by the sp3 hybridized linkages
of the X-groups. Pincer ligands should therefore be poor acceptors
since their unoccupied orbitals are at energies higher than that of
⇑
Corresponding author. Tel.: +1 2137404389.
E-mail address: met@usc.edu (M.E. Thompson).
http://dx.doi.org/10.1016/j.poly.2014.06.051
0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

Y. Liu et al. / Polyhedron 84 (2014) 136–143
137
terpy. Similarly, the triplet energies of the pincer ligands should be
higher than terpy based ligands due to the lack of conjugation in
the molecular structure. Inspired by the literature studies on
homoleptic and heteroleptic phosphorescent iridium complexes,
the weak accepting properties and high triplet energy of the pincer
ligands suggest that the phosphorescence of bis-pincer osmium
complexes will show high energy emissions, significantly blue-
shifted from [Os(terpy)2]2+ [25–27]. Therefore, we were interested
in preparing the neutral bis-pincer based transition metal complex
of Os(PCP)2, with Os(II) metal center and typical PCP pincer ligand
of [2,6-(CH2PR2)2C6H3] (R = C6H5). Herein the synthetic routes,
molecule structure, and photophysical characterization of the
complex are fully examined.
d = 3.62, 3.52 (AB, JAB = 14.4 Hz, 8H, CH2). 31P NMR (243 MHz,
C6D6, 343 K): d = 6.1 (s). 31P NMR (243 MHz, C7D8, 223 K):
d = 11.2 (s). LC–MS ESI (m/z): 1138.50 ([M], calc. 1138.28).
2.3. X-ray crystallography
The X-ray intensity data were measured on a Bruker APEXDUO
CCD system using radiation from
a Mo Ka fine-focus tube
(k = 0.71073 Å) with a TRIUMPH monochromator. A clear yellow
pear shape specimen of C64H54OsP4, approximate dimensions
0.16 mm ꢀ 0.09 mm ꢀ 0.04 mm, was used for the X-ray crystallo-
graphic analysis. A total of 2520 frames were collected. The frames
were integrated using the Bruker SAINT V8.18C software. The inte-
gration of the data using a triclinic unit cell yielded a total of 75643
reflections to a maximum h angle of 27.51 (0.68 Å resolution), of
which 11025 were independent (average redundancy 7.761,
completeness = 95.5%, Rint = 7.75%). The final cell constants of
2. Experimental
2.1. Materials and methods
a = 12.6896(11) Å,
volume = 2511.0(4) Å3, are based upon the refinement of the
XYZ-centroids of 9013 reflections above 2h (I) with
b = 14.1378(12) Å,
c = 14.7794(13) Å,
The syntheses were carried out under a nitrogen atmosphere
using standard Schlenk techniques. Solvents were distilled from
sodium benzophenone (hexanes, THF), or calcium hydride
(2-propanol). The starting materials OsCl2(PPh3)3 and 1,3-(PPh2
CH2)2C6H4 (PCP-H) were prepared according to literature methods
[28–30]. Other chemicals and solvents were ordered from Sigma-
AldrichÒ, and used as received. 1H NMR, 31P NMR, gCOSY NMR at
223 K, inversion recovery NMRs at 233 K, and VT-1H NMR spectra
were measured by Varian 600 NMR Spectrometer. The 13C NMR
cannot be resolved due to the poor solubility of Os(PCP)2. The
chemical shifts were referenced to a deuterated solvent. Mass
spectra were obtained on a Shimadzu LCMS-2020 quadrupole mass
spectrometer equipped with a column oven (T = 40 °C), a PDA
photodetector (200–800 nm) and a MS spectrometer (LC–MS
2020; m/z range: 0–2000; ionization modes: ESI/APCI).
r
1.40 < 2h < 27.51°. Data were corrected for absorption effects using
the multi-scan method (SADABS). The ratio of minimum to maximum
apparent transmission was 0.766.
The structure was solved and refined using the Bruker SHELXTL
ꢀ
Software Package, using the space group triclinic P1, with Z = 2 for
the formula unit, C64H54OsP4. The final anisotropic full-matrix
least-squares refinement on F2 with 415 variables converged at
R1 = 6.78%, for the observed data and wR2 = 11.25% greater than
2r
(F2). The goodness-of-fit was 1.072. The largest peak in the final
difference electron density synthesis was 1.352 eꢁ/Å3 and the
largest hole was ꢁ1.041 eꢁ/Å3. On the basis of the final model,
the calculated density was 1.504 g/cm3 and F(000), 1148 eꢁ.
2.4. Photophysical characterization
2.2. Synthesis of Os(PCP)2 (PCP = 2,6-(PPh2CH2)2C6H3)
The UV–Vis spectra were recorded on a Hewlett-Packard 4853
diode array spectrometer. Steady-state emission measurements
were performed using a QuantaMaster model C-60SE spectrofluo-
rimeter (Photon Technology International) with an excitation
wavelength of 350 nm in 2-MeTHF glass (77 K), in the solid state,
and in the PMMA film under N2 protection. Phosphorescent life-
times were measured by time-correlated single-photon counting
with IBH Fluorocube instrument equipped with a 405 nm LED exci-
tation source. Quantum yield (298 K) was measured using a
Hamamatsu C9920 system equipped with a xenon lamp, calibrated
integrating sphere, and Model C10027 photonic multichannel ana-
lyzer. The low temperature (77 K) quantum yield was determined
relative to the quantum yield of Ir(ppy)3 at 77 K (Ø = 1) [27].
Route 1: The Schlenk tube with OsCl2(PPh3)3 (240 mg,
0.23 mmol) and PCP-H (327 mg, 0.69 mmol) was evacuated for
30 min. Then 80 ml of dry and deaerated 2-propanol was added.
The solution mixture was refluxed for 24 h, cooled to room temper-
ature, and filtered over Al2O3 plug. The filtrate solution was then
evaporated over rotavap. The solid residue was washed with
hexanes and diethyl ether. Then the dried solid was dissolved in
15 ml THF, and precipitated with dropwise addition of 30 ml hex-
anes. The precipitate was dried under vacuum and obtained as
110 mg lemon yellow solid (42%). Route 2: OsCl(PPh3)(PCP) was
synthesized following literature process and isolated in 50% yield.
The AgOTf (27 mg, 0.10 mmol) and OsCl(PPh3)(PCP) (100 mg,
0.10 mmol) were reacted in THF at room temperature for 1 h. The
solution was filtered, and the solvent was evaporated. The in situ
generated Os(OTf)(PPh3)(PCP) (90% yield) was directly used for
the next step without further purification. The Schlenk tube with
Os(OTf)(PPh3)(PCP) (100 mg, 0.09 mmol) and PCP-H (43 mg,
0.09 mmol) was evacuated for 30 min. Then 20 ml of dry and deaer-
ated 2-propanol was added. The solution mixture was refluxed for
24 h, cooled to room temperature, and then followed by the same
workup as in Route 1 (yield based on OsCl2(PPh3)3 = 31%).
2.5. Theoretical calculations
The theoretical calculations were carried out by Schrödinger
2013 Materials Science Suite, using the density function theory
(DFT) with Lee-Yang-Parr correlation functional B3LYP. The basis
set used was LACVP⁄⁄. Time-dependent DFT (TD-DFT) calculations
were performed with GAUSSIAN 03 program to understand more
about the ground state to excited states transitions of the complex.
The same hybrid functional of B3LYP was employed, and the basis
set was LANL2DZ/6-31G⁄.
1H NMR (600 MHz, C6D6, 343 K): d = 7.39 (d, J = 7.2 Hz, 4H, xylyl
CH), d = 7.31 (t, J = 6.6 Hz, 2H, xylyl CH), d = 6.92–6.73 (m,
40H, PPh2), d = 3.60 (s, 8H, CH2). 1H NMR (600 MHz, C7D8, 223 K):
d = 7.83 (m, 4H, o-Ph), d = 7.63 (d, J = 3.6 Hz, 4H, o-Ph), d = 7.54
(d, J = 7.2 Hz, 4H, xylyl CH), d = 7.48 (t, J = 7.2 Hz, 2H, xylyl CH),
d = 7.34 (t, 4H, J = 6.6 Hz, m-Ph), d = 6.82 (t, 4H, J = 7.2 Hz, m-Ph),
d = 6.68 (d, J = 6.0 Hz, 4H, o-Ph), d = 6.41 (t, J = 6.6 Hz, 4H, p-Ph),
d = 5.76 (t, J = 7.2 Hz, 4H, m-Ph), d = 5.49 (d, J = 6.0 Hz, 4H, o-Ph),
2.6. Electrochemistry
Cyclic voltammetry (CV) and differential pulse voltammetry
(DPV) measurements were performed using an EG&G
Potentiostat/Galvanostat model 283. Dry DMF was used as solvent
under
a N2 atmosphere with 0.1 M tetra(n-butyl)ammonium

138
Y. Liu et al. / Polyhedron 84 (2014) 136–143
hexafluorophosphate as the supporting electrolyte. A glassy carbon
working electrode and a platinum counter electrode were used,
with a silver wire as the pseudoreference electrodes. The oxidation
feature absent in OsCl(PPh3)(PCP). The bond lengths of the four
OsAP bonds are nearly identical, ranging between 2.3348(19) Å
and 2.3419(19) Å, and only slightly longer than the OsAP bond
lengths of the OsCl(PPh3)(PCP) (2.313(4) Å and 2.312(4) Å). Like-
wise, comparison with [OsCl(=C=CHPh)(PPh3)(PCP)], where the
vinylidene substituent (trans to Cl) is strongly electron-withdraw-
ing, shows similar distances for the Os–CPCP (2.133(5) Å) and
Os–PPCP (2.3889(14) Å and 2.3547(15) Å) bonds [31]. The phenyl
rings in Os(PCP)2 are arranged around the complex into two dis-
tinct sets of pairs, designated A and B (Fig. 1, middle). The phenyl
rings in the A-type pairs are slip-stacked and in close enough prox-
potential was measured relative to
a ferrocenium/ferrocene
(Fc+/Fc) redox couple as an internal standard. Electrochemical
reversibility is determined by cyclic voltammetry.
3. Results and discussion
3.1. Synthetic routes of Os(PCP)2
imity to undergo
whereas rings in the B-type pairs are further apart (shortest CAC
distance 3.7 Å) and have no -overlap.
The deviation from an ideal octahedral geometry in Os(PCP)2 is
due to small chelate bite angles in the PCP ligand (average C–Os–
P = 76.5°). Thus, while the axially ligated carbon atoms are near lin-
ear [C(31)–Os(1)–C(47) = 178.60(3)°], bond angles for the trans
disposed P(1)-Os(1)–P(3) and P(2)–Os(1)–P(4) are 152.21(7)° and
153.75(7)°, respectively. The pseudo tetrahedral configuration of
the bridging methylenes (average P–CH2–C angle = 105°) causes
p–p interactions (shortest CAC distance = 3.2 Å)
The bis-pincer complex Os(PCP)2 was prepared in a reaction of
excess PCP-H ligand precursor (3 eq) with OsCl2(PPh3)3 (1 eq) in
refluxing anhydrous 2-propanol for 24 h (Scheme 1, Route 1). Dur-
ing the course of the reaction the solution changed color from
green to purple. After filtration and workup, the complex was iso-
lated as a lemon-yellow powder (42% yield) that is air-stable as a
neat solid and in solution. However, the solubility of Os(PCP)2 in
general is poor, being most soluble in aromatic solvents. The com-
plex is thermally stable as crystals can be obtained by gradient
sublimation at 280-240-200 °C under vacuum (1 ꢀ 10ꢁ6 torr).
The stepwise preparation of Os(PCP)2 was also investigated. A
reaction between PCP-H (1 eq) and OsCl(PPh3)(PCP) (1 eq) [31] in
2-propanol, surprisingly, only results in the formation of an intrac-
table black solution. However, on the basis of previous work with
related Ru complexes [32], substitution of chloride with triflate
to form Os(OTf)(PPh3)(PCP), followed by reaction with PCP-H
(Route 2) led to Os(PCP)2 in overall 31% yield (based on OsCl2
(PPh3)3). Interestingly, in the case of the Ru cogener of Os(PCP)2
one ligand forgoes metalation and instead forms an agostic
C–Hꢂ ꢂ ꢂRu interaction, even despite several attempts at deprotona-
tion with strong bases [32,33]. Apparently, for osmium the metal–
C bond is strong enough that deprotonation of the aryl can occur
without added base to form a stable dimetalated product.
p
the phosphines in the PCP ligand to twist in
conformation away from the plane of the xylyl group (average
Os–C–CH2–P dihedral angle = 24.6°). A projection view down the
a transoid
C
C(31)-Os(1)–C(47) axis (Fig. 1, right) illustrates the relative orien-
tation of the xylyl moieties with respect to the phosphines. The
xylyl planes are tilted 46° with respect to each other, while the
projected angle of the two P–Os–P planes of the PCP ligands is per-
pendicular. The transoid twist in the PCP ligands leads to a pseudo-
D2 symmetry for the complex, and hence the presence of
enantiomers, as opposed to the D2d symmetry found in M(terpy)2
complexes [1].
D and K
n+
3.3. NMR characterization
3.2. X-ray crystallography
The Os(PCP)2 complex undergoes a dynamic exchange process
in fluid solution at ambient temperatures. The 1H NMR spectrum
measured in C6D6 at 298 K shows broad and featureless resonances
for the phenyl rings (d = 6.10–7.10 ppm) and only one singlet
(d = 3.64 ppm) for the eight diastereotopic methylene protons
(Fig. SI-1). No signal is observed in 31P NMR at 298 K. NMR spectra
recorded in C6D6 at higher temperatures are more structured. For
X-ray diffraction analysis was carried out on a crystal of
Os(PCP)2 obtained from sublimation. The unit cell of Os(PCP)2
crystal contains a pair of enantiomers in a triclinic P1 space group.
ꢀ
Perspective drawings of the complex are depicted in Fig. 1 (left,
middle). Selected bond distances and angles are given in Table 1.
The molecular geometry of Os(PCP)2 shows that the pincer ligands
are arranged in pseudo-octahedral environment around the metal
center (Fig. 1, left). The Os(1)-C(31) and Os(1)-C(47) bond distances
(2.151(7) Å and 2.149(7) Å respectively) are longer than the OsAC
bond distances observed for monopincer osmium complex
OsCl(PPh3)(PCP) (2.04(1) Å) [31]. The OsAC bonds are elongated
due to the trans-effect between the two opposing aryl rings, a
example, the 31P NMR at 343 K shows
a single resonance
(d = 6.1 ppm) indicating that the phosphorus atoms on the pincer
ligands are in equivalent environments (Fig. SI-2). Sharp reso-
nances are also observed for the xylyl protons (d = 7.39, d,
J = 7.2 Hz, 4H; d = 7.31, t, J = 6.6 Hz, 2H) while three broad signals
further upfield (d = 6.73-6.92 ppm) are assigned to protons on
freely rotating phenyl rings of the PCP ligand (Fig. SI-3).
Route 2:
OsCl(PPh3)(PCP)
yield: 50%
1 eq. PCP-H
1 eq. PCP-H
1 eq. AgOTf
OsCl2(PPh3)3
Os(OTf)(PPh3)(PCP)
yield: 90%
2-propanol
reflux 24h
THF, 1h
Route 1:
2-propanol
X
3 eq. PCP-H
2-propanol
reflux 24h
1 eq. PCP-H
2-propanol
reflux 24h
reflux 24h
P'
P'
Os
P'
Total yield:
Route 1: 42%
Route 2: 31%
P' = PPh2
P'
Scheme 1. The synthetic route for Os(PCP)2.

Y. Liu et al. / Polyhedron 84 (2014) 136–143
139
Fig. 1. Two perspective views of Os(PCP)2 shown in 50% probability thermal ellipsoids (left, middle). Hydrogen atoms and the phenyl rings on phosphorus (left) are omitted
for clarity. Projection view down the C(31)–Os(1)–C(C47) axis of Os(PCP)2 (right). Solid and dashed lines represent the xylyl planes.
Table 1
Selected bond lengths (Å) and angles (degree) of Os(PCP)2.
Bond lengths (Å)
Os(1)–C(31)
Os(1)–P(1)
Os(1)–P(3)
2.151(7)
2.338(2)
2.341(2)
Os(1)–C(47)
Os(1)–P(2)
Os(1)–P(4)
2.149(7)
2.3348(19)
2.3419(19)
Bond angles (°)
P(1)–Os(1)–P(3)
P(1)–Os(1)–C(47)
P(3)–Os(1)–C(47)
C(31)–Os(1)–C(47)
152.21(7)
76.1(2)
76.1(2)
178.60(3)
P(2)–Os(1)–P(4)
P(2)–Os(1)–C(31)
P(4)–Os(1)–C(31)
153.75(7)
77.1(2)
76.7(2)
Highly resolved NMR spectra are observed at low temperature
(223 K) in toluene-d8 (C7D8) (Figs. SI-4–6). The 31P NMR shows a
single resonance (d = 11.2 ppm) while splitting of the diastereotop-
ic methylenes is clearly evident in the 1H NMR (d = 3.62, 3.52 ppm,
JAB = 14.4 Hz). Eight well-resolved resonances, along with two
more signals obscured by residual protons on the solvent, are
observed for the protons on the PPh2 moieties indicating that the
phenyl rings are situated in two distinct environments. Correla-
tions between the various protons of the phenyl rings are clearly
revealed in the gCOSY spectrum. Resonances appear upfield for
ortho (o)-, meta (m)- and para (p)-protons on one set of phenyls
Fig. 2. Variable temperature 1H NMR spectra (600 MHz, C7D8) for the methylene
protons of Os(PCP)2.
expected for a diastereotopic methylene protons converts to single
coalesced resonance on warming from 223 to 273 K (Fig. 2). This
signal sharpens as temperature increases from 273 to 343 K.
A model for the fluxional behavior leading to racemization of
Os(PCP)2 is illustrated in Fig. 3. At 223 K, Os(PCP)2 can be repre-
sented as
D- or K-enantiomers with D2 symmetry undergoing
slow exchange of the A- and B-type phenyls on the NMR timescale
(see SI). As temperature increases, the rate of exchange increases
until coalescence is achieved at 273 K for the methylenes and
283 K for the phenyls (Fig. SI-9). Above this temperature, the dia-
stereotopic methylene protons undergo a fast exchange process
as the phenyl rings rotate more rapidly. At 343 K, the enantiomers
are in rapid exchange and display an NMR spectrum consistent
with a complex in a pseudo-D2d symmetric environment. Rate con-
stants (k) were calculated by analyzing the NMR spectra for the
methylenes at temperatures between 248 and 268 K. An Arrhenius
plot of ln(k) versus 1/T gives a good linear fit, with r2 = 0.966
(Fig. SI-11) [35]. The line-shape analysis indicates that the activa-
tion energy for racemization of Os(PCP)2 is 8.6 kcal/mol.
0
at do = 5.49 ppm, dm = 5.76 ppm, dp = 6.41 ppm and dm = 6.82 ppm
0
with the remaining signal at do = 7.63 ppm. Signals from the other
set of phenyls appear downfield (do = 6.68 ppm, dm = 6.95 ppm,
0
0
dp = 7.05 ppm, dm = 7.34 ppm and do = 7.83 ppm). The NMR data
is consistent with the complex being in a D2 configuration similar
to what is found in the crystal structure. The upfield resonances are
tentatively assigned to protons shielded by the
p-stacked phenyl
rings labelled A in Fig. 1 (middle) whereas the other signals belong
to the B-type phenyls. The exchange process at 233 K was probed
further by performing an inverse recovery experiment (Figs. SI-7
and 8).[34] A 180° pulse applied to the ortho-proton resonance
at d = 5.49 ppm was found to transfer magnetization to all the
other ortho-protons in the phenyl rings during relaxation. This
behavior indicates that the exchange involves simultaneous
rotation of the phenyl rings and interchange between the A- and
B-sites (racemization).
3.4. Electrochemistry
Variable temperature 1H NMR measurements in C7D8 were car-
ried out between 223 K and 343 K (Figs. SI-9 and 10). The xylyl pro-
tons display sharp signals throughout the entire temperature
range, undergoing only slight changes in bandwidth and a gradual
shift upfield with increasing temperature. In contrast, resonances
from the phenyl protons collapse into the baseline as temperature
is increased from 223 to 283 K. Upon further warming to 343 K
these aromatic signals reappear as a broad set of multiplets
between d = 6.4 and 6.8 ppm. In addition, the doublet pattern
The electrochemical properties of Os(PCP)2 were examined by
cyclic voltammetry and differential pulse voltammetry in CH2Cl2
solution (Fig. SI-12). A reversible oxidation process is observed at
E(10/2/+) = –0.345 V (versus Fc+/Fc) followed by second quasi-revers-
ible process at E(1+//22+) = 0.441 V (versus Fc+/Fc). Os(PCP)2 can also
be oxidized using ferrocenium hexafluorophosphate in solution
to form a stable blue cation (see below). The Os(PCP)2 complex is
more readily oxidized compared to [Os(terpy)2]2+ derivatives [1].
However, the stability of Os(PCP)2 in the formal +2 oxidation state

140
Y. Liu et al. / Polyhedron 84 (2014) 136–143
microsecond lifetime (s = 8.0 ls) indicative of phosphorescence.
The radiative rate constant is of 7 ꢀ 104 sꢁ1, and the non-radiative
rate constant is 5 ꢀ 104 sꢁ1. The phosphorescence is at lower
energy than that observed from free PCP-H under the same condi-
tions (kmax = 426 nm,
luminescent as a neat solid at room temperature (kmax = 556 nm).
The quantum yield ( = 0.03) and lifetime ( = 0.3 s, 63%; 2.0 s,
s = 1.0 s) [38]. The Os(PCP)2 complex is also
U
s
l
l
37%) in the solid state lead to an estimated radiative rate constant
of ca. 3 ꢀ 104 sꢁ1, while the non-radiative rate is two orders of
magnitudes larger. Cooling the solid to 77 K decreases non-radia-
tive decay. However, the measured lifetimes (
6.3 s, 62%) indicate that the emission efficiency is less than unity.
The biexponential decay is likely due to combination of
s = 2.2 ls, 38%;
l
a
Fig. 3. Process proposed for the racemization of Os(PCP)2 through a D2d transition
molecular and aggregate emission in the solid state. Discussion
of the nature of the emissive state will follow from an analysis of
the computational studies for Os(PCP)2.
state.
is in contrast to bis-cyclometalated, Os(ppy)2(bpy) (ppy = 2-
phenylpyridyl; bpy = 2,20-bipyridyl) [36]. The oxidation potential
of the latter complex, which has cis-configured phenyl rings, is
ca. 400 mV more cathodic than Os(PCP)2 and is isolated as an OsIII
species. Apparently, the four PPh2 moieties impart greater stabil-
ization to the metal center than four pyridyl groups, even in spite
the unfavorable trans configuration of the metalated aryl rings in
Os(PCP)2.
3.6. DFT calculations
The electronic structure of Os(PCP)2 was investigated using
density functional theory (DFT) and time-dependent (TD-DFT)
methods starting from coordinates of the single crystal structure.
Bond lengths within the coordination sphere calculated for
Os(PCP)2 in the gas-phase are slightly longer than values in the
crystal structure. The frontier orbitals are shown in Fig. 5. The
HOMO and HOMO-1 of Os(PCP)2 are near degenerate and predom-
inantly localized on the Os center (46%, 38%) and xylyl (L, 31%,
29%), with the remaining contribution from the PPh2 substituents
(L0). The LUMO, LUMO + 1 and LUMO + 2 are also near degenerate
and are localized on L0 with varying participation from Os. For
example, the LUMO has 3% contribution from Os, whereas the
amount is 14% for the LUMO + 1 and 2% for LUMO + 2. TD-DFT cal-
culations were used to determine the S0 ? Sn and S0 ? Tn transi-
tion energies and oscillator strengths (f) of Os(PCP)2. The S0 ? S1
transition calculated for Os(PCP)2 (k = 441 nm, f = 0.0193) is close
to the onset in the experimental absorption spectrum (Table SI-3).
Poor overlap between the frontier orbitals of Os(PCP)2 leads to
the small oscillator strength. The S0 ? S1 transition primarily
involves HOMO ? LUMO + 1 (94%) mixed with higher HOMO ?
LUMO + 10 (2%) and HOMO ? LUMO + 20 (4%) configurations. The
LUMO + 10 and LUMO + 20 are primarily metal-centered (MC)
and have substantial contributions (3% and 20%, respectively) from
antibonding orbitals on Os. On the basis of orbital parentage, the
S0 ? S1 transition can be described as principally a metal-xylyl
(ML) to PPh2 (L0) charge transfer transition (ML-L0CT) configura-
tionally mixed with metal centered (1MC) transitions. The other
lowest S0 ? Sn (n = 2–10) transitions are all similarly dominated
by the ML-L0CT transitions mixed with varying amounts of 1MC
transitions. Likewise, the S0 ? T1 transition (k = 491 nm) involves
HOMO ? LUMO + 1 (78%) with notable participation from
HOMO ? LUMO + 20 (16%). DFT calculation of the geometry opti-
3.5. Electronic spectroscopy
Absorption and emission spectra for Os(PCP)2 are shown in
Fig. 4, and photophysical data are listed in Table SI-2. The complex
displays strong absorption at 250 nm that gradually diminishes
into weaker transitions down to 450 nm (kmax = 335 nm,
e
= 2700 L molꢁ1 cmꢁ1, and kmax = 384 nm, = 1800 L molꢁ1 cmꢁ1).
e
Absorption from 250 to 300 nm is assigned to ligand based
(p ?
p⁄) transitions on the basis of a similar intense absorbance
found in the PCP-H ligand precursor. The bands at lower energy
are assigned to charge transfer transitions (CT) since they are
absent in the free ligand (Fig. SI-13). The extinction coefficients
for the CT transitions are comparable to values found in other
osmium (II) complexes, but are relatively weak when compared
to related iridium (III) complexes [26,37]. Upon oxidation with fer-
rocenium ion the CT transitions undergo a small blue shift that is
accompanied by a concomitant appearance of broad transitions
for the cation (kmax = 519 nm,
kmax = 591 nm,
The Os(PCP)2 complex is non-emissive in fluid solution or when
doped (10%) in a PMMA matrix at room temperature. However, a
broad, featureless emission (kmax = 546 nm) is observed (quantum
e
= 450 L molꢁ1 cmꢁ1 and
e
= 480 L molꢁ1 cmꢁ1; Fig. SI-14).
yield
U
= 0.6) in a dilute solution of 2-MeTHF at 77 K (Fig. 4) with a
0.30
abs.
1.0
tddft
mized triplet state gives a self-consistent field (
aration between S0 and T1 that is in good agreement with the
emission energy ( SCF = 2.42 eV, k = 512 nm). The spin density
DSCF) energy sep-
0.25
77K 2MeTHF
298K solid
0.8
D
0.20
0.15
0.6
0.4
surface for this state is predominantly localized on the osmium
center (65%) with an obvious contribution from the d(x2-y2) orbital
(Fig. 6). The Os–P bonds are elongated (2.53 Å) compared to values
in the optimized singlet ground state (2.41 Å), whereas the Os–C
bond lengths are little changed. The contribution from the PCP
ligand is relatively small, which suggests that the emission from
Os(PCP)2 is dominated by the 3MC state.
The results from the DFT calculations can be used to construct a
qualitative potential energy diagram illustrating surfaces for
lowest singlet and triplet states of Os(PCP)2 (Fig. 7). The lowest sin-
glet state (S1) is ML-L0CT in character with a small contribution
from the higher lying 1MC states. The large spin–orbit coupling
0.10
0.05
0.2
0.0
300
400
500
600
700
Wavelength (nm)
Fig. 4. Absorption spectrum of Os(PCP)2 in CH2Cl2 solution (black line) and the
relative oscillator strengths calculated using TD-DFT (green lines) along with
emission spectra recorded in 2-MeTHF glass at 77 K (blue line) and as a neat solid at
room temperature (red dashed line). (Color online.)

Y. Liu et al. / Polyhedron 84 (2014) 136–143
141
LUMO+1
(-0.64 eV)
LUMO+2
(-0.63 eV)
LUMO+20
(1.09 eV)
LUMO
(-0.66 eV)
HOMO
(-4.25 eV)
HOMO-1
(-4.30 eV)
Fig. 5. Frontier orbitals involved in the S0 ? Sn (n = 1–6) and S0 ? T1 transitions of Os(PCP)2.
rather low quantum yield for the solid state Os(PCP)2 at room tem-
perature. The quantum yield of the complex is less than unity even
at 77 K due to the domination from the 3MC state, indicating that
the barrier to thermal crossing into the 3MC state is shallow.
It has been widely accepted that the weak emission of
[Ru(terpy)2]2+ at room temperature was due to the thermally acti-
vated crossing from the lowest triplet MLCT state to a short-lived
3MC state, which then undergoes a non-radiative relaxation to
the ground state [1]. Recent computational studies on [Ru(ter-
py)2]2+ show that dissociation of Ru–N bonds is the primary path-
way for non-radiative decay [39]. The low temperature (77 K)
emission of [Ru(terpy)2]2+ (k0-0 = 598 nm,
U
= 0.48) is from 3MLCT
of low energy level compared with the Os(PCP)2 complex. The
3MC state of [Ru(terpy)2]2+ is inaccessible at 77 K. The related
[Os(terpy)2]2+ complexes showed relatively stronger and long-lived
Fig. 6. Spin density surface calculated for the triplet state of Os(PCP)2.
luminescence (k0-0 = 714 nm,
U = 0.01) even in solution and at room
temperature because the complexes has even lower 3MLCT, and
larger 3MLCT/3MC energy gaps due to the stronger ligand field of
osmium. The low temperature (77 K) emission of [Os(terpy)2]2+
(k0-0 = 689 nm,
U
= 0.12) is of 3MLCT character. On contrary, emis-
sions at room temperature and low temperature (77 K) observed of
complex Os(PCP)2 are dominated by 3MC state. The PCP-H ligand is
of much higher triplet energy than terpy, and pushes up the 3MLCT
state of Os(PCP)2 significantly. However, the PCP-H as a poor donor
ligand only weakly destabilizes the ligand field of Os(II), and the
3MC state is accessible at both room temperature and low temper-
ature (77 K). It is noteworthy that emission at 77 K from Os(PCP)2
is much brighter than that from [Ru(terpy)2]2+ and [Os(terpy)2]2+
even though it originates from a 3MC state. Apparently non-
radiative decay is greatly suppressed at 77 K.
4. Conclusion
Fig. 7. Energy diagram illustrating the radiative and non-radiative decay channels
involving the singlet and triplet states of Os(PCP)2.
In summary, we have demonstrated the synthetic routes and
characterization of the bis-pincer organometallic osmium complex
Os(PCP)2.
D and K enantiomers of Os(PCP)2 undergo relatively
constant of osmium leads to effective intersystem crossing to a
triplet state with an increased MC character. The high triplet
energy of the PCP ligand favors such mixing between the 3ML-L0CT
and 3MC states. Any further elongation of the Os–P bond will
increase participation from the 3MC state. The T1 state can then
readily undergo non-radiative deactivation to the ground state,
either vibronically or through a surface crossing, leading to the
slow exchange processes that can be observed on the NMR time
scale. As expected, the poorly conjugated pincer chelate greatly
increases the energy of metal and ligand based unoccupied orbitals
of the complex Os(PCP)2. The large spin–orbit coupling constant of
the Os eventually leads to a lowest triplet state with significant
amount of metal character, rather than dominant ligand contribu-
tions of our initial prediction. The consequence of this is that the

142
Y. Liu et al. / Polyhedron 84 (2014) 136–143
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This work is sponsored by Universal Display Corporation and
NSF CRIF grant 1048807 (X-ray Crystallography). We are grateful
to Dr. Travis J. Williams, Dr. Allan Kershaw, graduate students Xin-
ping Wu and Jeff A. Celaje for important discussions and technical
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Appendix A. Supplementary data
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CCDC 1000470; contains the supplementary crystallographic
data for Os(PCP)2. These data can be obtained free of charge via
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Cambridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: (+44) 1223 336 033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with this
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