Chelating and Bridging Roles of 2-(2-Pyridyl)benzimidazole and Bis(diphenylphosphino)acetylene in Stabilizing a Cyclic Tetranuclear Platinum(II) Complex

Article abstract of DOI:10.1021/acs.inorgchem.9b02274

The reaction of complex [Pt(Me)(DMSO)(pbz)], 1, (pbz = 2-(2-pyridyl)benzimidazolate) with [PtMe(Cl)(DMSO)₂], B, followed by addition of bis(diphenylphosphino)acetylene (dppac), gave the novel tetranuclear platinum complex [Pt₄Me₄(μ-dppac)₂(pbz)₂Cl₂], 2, bearing both the pbz and dppac ligands. In this structure, the pbz ligands are both chelating and bridging to stabilize the tetraplatinum framework. The tetranuclear Pt(II) complex was fully characterized by NMR spectroscopy, X-ray crystallography, and mass spectrometry, and its electronic structure was investigated and supported by DFT calculations.

Full text of DOI:10.1021/acs.inorgchem.9b02274

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Chelating and Bridging Roles of 2‑(2-Pyridyl)benzimidazole and
Bis(diphenylphosphino)acetylene in Stabilizing a Cyclic Tetranuclear
Platinum(II) Complex
S. Masoud Nabavizadeh,*,†,‡ Fatemeh Niroomand Hosseini,§ Fatemeh Niknam,†
Peyman Hamidizadeh,† S. Jafar Hoseini,† Peter C. Ford,‡ and Mahdi M. Abu-Omar*,‡
†Professor Rashidi Laboratory of Organometallic Chemistry, Department of Chemistry, College of Sciences, Shiraz University,
Shiraz 71467-13565, Iran
‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, California 93106, United States
§Department of Chemistry, Shiraz Branch, Islamic Azad University, Shiraz 71993−37635, Iran
S
* Supporting Information
ABSTRACT: The reaction of complex [Pt(Me)(DMSO)(pbz)], 1,
(pbz = 2-(2-pyridyl)benzimidazolate) with [PtMe(Cl)(DMSO)2], B,
followed by addition of bis(diphenylphosphino)acetylene (dppac),
gave the novel tetranuclear platinum complex [Pt4Me4(μ-
dppac)2(pbz)2Cl2], 2, bearing both the pbz and dppac ligands. In
this structure, the pbz ligands are both chelating and bridging to
stabilize the tetraplatinum framework. The tetranuclear Pt(II)
complex was fully characterized by NMR spectroscopy, X-ray
crystallography, and mass spectrometry, and its electronic structure
was investigated and supported by DFT calculations.
compound using 1,2,3-tetrazolate as a building block (VI,
Chart 1) was reported15 in which all triazolate nitrogen atoms
are coordinated to 3 different Pt(II) ions. Casas and co-
workers synthesized16 a tetranuclear platinum(II)−platinum-
(IV) mixed-valence complexes by addition of a stoichiometric
amount of a Pt(II) reagent [cis-[Pt(C6F5)2(thf)2] to some
binuclear platinum(IV) complexes (VII, Chart 1).
Cyclometalated complexes have attracted attention because
of their several applications in science.17−21 Of particular
interest are cycloplatinated complexes bearing strong field
ligands such as benzo[h]quinolinate (bhq) and 2-phenyl-
pyridinate (ppy) (VIII, Chart 1).22−27 The cycloplatinated(II)
compounds with the [Pt(C∧N)R] moiety (C∧N= cyclo-
metalated ligand and R= alkyl, aryl or halide) are very
attractive due to their ease of preparation through C−H
activation of ligands like 2-phenylpyridine (Hppy) and
benzo[h]quinoline (Hbhq) (VIII, Chart 1) with a suitable
Pt(II) precursor complex.24−33 Earlier, we have studied the
reaction of the double rollover cycloplatinated(II) complex
[Pt2(μ-bpy-2H)(Me)2(κ1-dppm)2], bpy= 2,2′-bipyridine (in
which each metallic center bears a monodentate dppm ligand)
with the known monomeric cycloplatinated (II) complex
[Pt(Me)(bhq)(SMe2)] in a 1:2 molar ratio. This reaction
INTRODUCTION
■
The construction of multinuclear metal assemblies has been a
growing area of research during the past decade.1,2 Develop-
ments based on the use of different ligands as the building unit
for the synthesis of compounds have been reported for
binuclear Pt complexes,3−6 although to the best of our
knowledge, tetranuclear complexes are rare.7−9
Wilson and Lippard reported10 a tetranuclear platinum
complex in which two acetate-bridged diplatinum(III) units
are connected by two bridging amido ligands (I, Chart 1). The
syntheses of tetranuclear platinum complexes in which the
metal centers are supported by phosphido ligands have been
reported by Fornies and co-workers (II, Chart 1).11 Stang and
́
Cao reported12 the preparation and characterization, via self-
assembly, of platinum- and palladium-based cationic, tetranu-
clear, macrocyclic complexes (III, Chart 1). In these
complexes, 4,4′-bipyridine acts as a spacer ligand between
the Pt centers. Youngs and co-workers also reported13 the
synthesis of tetraplatinum square heterocycle which had large
cavities and in which the rings were composed of only
platinum and butadiyne fragments (IV, Chart 1). They used
chelating phosphines in order to enforce the required cis
geometry at the platinum atom. Yam et al.14 synthesized a V-
shape luminescent tetranuclear platinum(II) alkynyl complex
(V, Chart 1), and its electrochemical behavior was studied by
cyclic voltammetry. More recently, a tetranuclear Pt(II)
Received: July 29, 2019
© XXXX American Chemical Society
A
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Chart 1. Examples of Some Tetraplatinum Complexes
resulted in the unsymmetrical tetranuclear complex [{(bhq)-
(Me)Pt(μ-dppm)Pt(Me)}2(μ-bpy-2H)] (IX, Chart 1).29
The 2-(2-pyridyl)benzimidazole (Hpbz) ligand normally
acts as an anionic N∧N bidentate ligand through N−H bond
activation of the imidazole unit (instead of C−H activation in
Hppy and Hbhq ligands).34−38 Hpbz derivatives have been
used as “bipyridine analogues” in some complexes with metal
centers such as Cu(I, II),39 Zn(II),40 Ru(II),41 Os(II),42 and
Re(I).43,44 The ease of synthesis and the commercial
availability of Hpbz make it an attractive ligand. Although
there are many reports on preparation of metal complexes in
which pbz acts as a N∧N bidentate ligand, complexes bearing
this ligand as a bridging ligand between two metal centers have
not been studied until now.
Reactions of biphosphine ligands to form polynuclear
complexes with metal centers supported by these bridging
ligands have produced rich and interesting chemistry.30,45−49
However, to the best of our knowledge, there are no reported
complexes where 2-(2-pyridyl)benzimidazole acts concurrently
as a chelating and bridging ligand. This, together with our
interest in the design and synthesis of organoplatinum(II)
complexes,50−58 prompted us to exploit the 2-(2-pyridyl)-
benzimidazole and bis(diphenylphosphino)acetylene (dppac)
units as the backbone for the construction of a cyclic
tetraplatinum(II) complex. Here we report an interesting
tetranuclear organoplatinum(II) complex in which, for the first
time, the pbz anion acts as both a chelating and a bridging
ligand to stabilize the tetraplatinum framework. In this
B
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solution, refinement, and creation of publication data. Structure
determination was carried out using intrinsic methods. Crystallo-
graphic information is reported in Table 1. Crystallographic data is
complex, dppac also serves as a bridging ligand. The electronic
structure of this complex, supported by theoretical calculations,
is also investigated.
Table 1. Crystal and Structure Refinement for Complex 2·
EXPERIMENTAL SECTION
■
1
4(CH3)2CO
General Remarks. The H and 31P NMR spectra were recorded
on a Bruker Avance DPX 400 MHz spectrometer using TMS (1H)
and H3PO4 (31P) as references. CDCl3 was used as solvent for NMR
spectroscopy. All chemical shifts and coupling constants are reported
in ppm and Hz, respectively. Microanalyses were done using a
Thermo Finnigan Flash EA-1112 CHNSO rapid elemental analyzer,
and melting points were measured on a Buchi 530 instrument. UV−
vis absorption spectra were recorded on a PerkinElmer Lambda 25
spectrophotometer using a cuvette with 1.00 cm path length. The
complexes [Pt(Me)2(DMSO)2], A,59 and [Pt(Me)(DMSO)(pbz)],
1,60 were synthesized by known methods. Bis(diphenylphosphino)-
acetylene (dppac) and 2-(2-pyridyl)benzimidazole (Hpbz) were
purchased from commercial sources.
empirical formula
formula weight
temperature (K)
wavelength (Å)
crystal system
C92H92Cl2N6O4P4Pt4
2320.85
100(2)
0.71073
triclinic
space group
P1
̅
a/Å
b/Å
c/Å
β/°
volume/Å3
Z
11.207(3)
11.658(3)
17.085(5)
94.774(7)
2151.6(10)
1
Synthesis of Pt Complexes. [Pt(Me)Cl(DMSO)2], B. This
compound had been previously synthesized by the reaction of
[PtCl2(DMSO)2] with SnMe4 for 24 h with a yield of 55%.61 In an
alternative method, 381 mg (1.00 mmol) of A and 421 mg (1.00
mmol) of [PtCl2(DMSO)2] were added to 50 mL of chloroform or
acetone and stirred for 6 h. The solvent was then removed by
evaporation to give 640 mg of compound B (1.59 mmol). Yield: 80%.
1H NMR in CDCl3: trans isomer: δ 0.99 [s, 2JPtH = 78.9 Hz, 3H, Me];
density (calculated)/(mg/m3)
absorption coefficient/mm−1
F(000)
1.791
6.672
1124
theta range for data collection
reflections collected
independent reflections, [R(int)]
max. and min transmission
data/restraints/parameters
goodness-of-fit on F2
R1/wR2 [I > 2σ(I)]
1.229−27.266°
18876
9565, [0.0356]
0.7455 and 0.5607
9565/0/511
0.976
3
3.42, [s, JPtH = 24.2 Hz, 12H, Me of DMSO]; cis isomer: δ 1.17 [s,
2JPtH = 79.0 Hz, 3H, Me]; 3.21, [s, 3JPtH = 8.8 Hz, 6H, DMSO trans to
3
Me]; 3.49 [s, JPtH = 38.2 Hz, 6H, DMSO trans to Cl].
[Pt4(Me)4(μ-dppac)2(pbz)2Cl2], 2. [Pt(Me)Cl(DMSO)2], B, (401
mg, 1.00 mmol) was added to a solution of 1 (482 mg, 1.00 mmol) in
acetone (100 mL). This was stirred at room temperature for 24 h, and
then dppac (394 mg, 1.00 mmol) was added to the solution. After 20
h, the solvent was removed, and the yellow residue was washed with
cold diethyl ether to give 800 mg of 2 (0.381 mmol). Yield: 76%. Mp
> 250 °C. Anal. Calcd for C80H68Cl2N6Pt4P4: C, 46.0; H, 3.2; N,
4.0%. Found, C, 45.8; H, 3.4; N, 4.1%. NMR in CDCl3: δ(1H) 0.56
0.0294/0.0596
deposited with the Cambridge Crystallographic Data Centre, No.
CCDC 1874226. Copies of this information may be obtained free of
charge from The Director, CCDC, 12 Union Road, Cambridge, CB2
1EZ, UK. Fax: + 44(1223)336−033, e-mail: deposit@ccdc.cam.ac.uk,
or www.ccdc.cam.ac.uk.
Computational Details. Gaussian 09 was used64 to fully optimize
the structure at the B3LYP level of density functional theory (DFT).
The effective core potential of Hay and Wadt with LANL2DZ was
selected for Pt65 and the 6-31G(d) basis set was chosen for all other
atoms. To estimate and confirm the optimized structures of the
molecules, frequency calculations were carried out to obtain only real
frequencies for the optimized structures. The calculations for the
electronic absorption spectra using time-dependent DFT (TD-DFT)
were done at the same level of theory (B3LYP). The “Chemissian”
software66 is used to show the compositions of molecular orbitals and
also to plot the theoretical absorption spectra.
2
3
[d, JPtH = 83.5 Hz, JPH = 7.3 Hz, 6H, Me ligands trans to N of 2-
pyridyl ring], 1.09 [d, 2JPtH = 72.6 Hz, 3JPH = 4.7 Hz, 6H, Me ligands
3
3
trans to N of benzimidazole ring]; 8.15 [dd, JPtH = 43.6 Hz, JHH
=
6.5 Hz, 4JPH = 5.3 Hz, 2H, H6]; 7.84 and 7.33 [m, Ph of dppac]; 7.45
3
3
[m, H6′ and H3′]; 6.90 [d, JHH = 5.4 Hz, 2H, H3]; 6.79 [t, JHH
=
=
3
3
15.2 Hz, 2H, H5′]; 6.58 [t, JHH = 12.7 Hz, 2H, H5]; 6.31 [t, JHH
3
15.1 Hz, 2H, H4′]; 6.07 [d, J(HH) = 8.6 Hz, 2H, H4]. δ(31P) 27.9 [s,
1JPtP = 2545 Hz, 2P, trans to benzimidazole ring], 20.6 [s, 1JPtP = 4392
Hz, 2P, trans to Cl]. ESI-TOF-MS: m/z 2107.4 [2 + H3O]+, 1717.4
[2 − dppac + Na]+ (Chart 2).
Chart 2. NMR Labeling for Complex 2
RESULTS AND DISCUSSION
■
Complex Synthesis and Characterization. The detailed
synthetic route for the tetranuclear complex [Pt4(Me)4(μ-
dppac)2(pbz)2Cl2], 2, is depicted in Scheme 1. The precursor
complex [Pt(Me)(DMSO)(pbz)], 1, was prepared through the
reaction of [Pt(Me)2(DMSO)2], A, with Hpbz in 1:1 ratio. In
the key step, complex B was added to a solution of 1 in acetone
to produce an intermediate which is suggested to be the
dimeric Pt complex [Cl(Me)(DMSO)Pt(μ-pbz)Pt(Me)-
(DMSO)], C. Then, dppac was added in situ to the solution
to result in the assembly of 2 (in 76% yield) as a yellow solid.
The structure of new complex 2 was fully characterized using
multinuclear NMR spectroscopy (1H and 31P{1H} NMR),
electrospray ionization mass spectroscopy (ESI-MS), and
single-crystal X-ray crystallography. The 31P{1H} NMR
spectrum is illustrated in Figure 1A (details of the NMR
spectra are provided in the Experimental Section). The
31P{1H} NMR spectrum of 2 displays two distinct singlets
Crystallographic Data. The structure of complex 2 was
determined from single-crystal X-ray diffraction data at 100(2) on a
Bruker KAPPA APEX II diffractometer equipped with an APEX II
CCD detector using a TRIUMPH monochromator with a Mo Kα X-
ray source (k = 0.71073 Å). The crystal was mounted on a cryoloop
under Paratone-N oil and kept under nitrogen. The SADABS62
method is used for absorption correction of the data, and the
SHELXTL63 software is used for subsequent calculations, structure
C
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Scheme 1. Route to Preparation of Complex 2
1
Figure 1. (A) 31P and (B) H (aliphatic region) NMR spectra of complex 2.
with equal intensities, having platinum satellites. They appear
at δ = 20.6 (1JPtP = 4392 Hz) and 27.9 (1JPtP = 2545 Hz),
which are respectively assigned to the P trans to Cl and the P
trans to ligating N atom of benzimidazole ring, in agreement
with the higher trans influence of N compared to Cl. It should
technique is presented in Figure 2, and the numerical data
summarized in Table 1 (see the caption of Figure 2 for selected
geometrical parameters). To our knowledge, complex 2 is the
first example of a tetranuclear Pt complex having both bridging
2-(2′-pyridyl)benzimidazolate and bis(diphenylphosphino)-
acetylene ligands. In this structure, pbz ligands act as both
chelating and bridging ligand to stabilize the tetraplatinum
framework. The molecular structure of complex 2 confirms the
Pt4(dppac)2(pbz)2 framework. As shown in Figure 2, the solid
state structure exhibits distorted four-coordinated square
planar geometry around the platinum centers. The methyl
groups are trans to the pyridyl and benzimidazole rings of pbz
ligand. The largest deviation from ideal square planar geometry
belongs to the N3−Pt2−N2 angle at 78.01(13)° which is
associated with the Pt(pbz) chelate ring. One of the P atoms of
dppac is located trans to the ligating N of benzimidazole ring
(Pt2−P2 = 2.1910(13) Å) and another one is trans to the Cl
atom (Pt1−P1 = 2.1754(12) Å). The Me groups are located
trans to the ligating N of the pyridyl (Pt2−C26 = 2.054(4) Å)
and benzimidazole (Pt1−N1 = 2.122(3) Å) rings (See Figure
2). Therefore, the structure of complex 2 confirms that “Pt(μ-
pbz)Pt” moieties are connected through dppac bridging ligand
1
be noted that the JPtP values of 4264 and 3081 Hz were
previously reported for the P atom trans to Cl in complexes
[Pt(Me)(dppe)Cl] (dppe = Ph2PCH2CH2PPh2) and
[PtCl2(dppm)] (dppm = Ph2PCH2PPh2),67 respectively. On
1
the basis of the H NMR spectrum of 2 (see Figure 1B), two
different Me groups are observed, being trans to pyridyl and
benzimidazole rings. The Me group trans to ligating N of
pyridyl was observed at δ = 0.56 as a doublet with 2JPtH = 83.5
Hz and 3JPH = 7.3 Hz, while the Me group trans to the ligating
N of benzimidazole ring appeared at δ = 1.09 ppm with a
2
3
considerably lower JPtH = 72.6 Hz and JPH = 4.7 Hz values.
This observation indicates a stronger trans influence of the
ligating N of benzimidazole ring in comparison to the 2-pyridyl
ring.
Crystal Structure. Yellow crystals of complex 2 were
obtained upon diffusion of n-hexane vapor into the acetone
solution. The structure obtained by X-ray crystallography
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Figure 3. Expanded (left) and simulated (right) isotopic pattern ESI-
MS spectrum of complex [2 + H3O]+, showing the expected intensity
due to isotopic distribution. The small differences between simulation
and experimental patterns may be related to small impurities of
carrier-solvent used for the experiment.
Figure 2. (top) Structure of complex 2. All hydrogen atoms and
solvent molecules are omitted for clarity. Selected geometrical
parameters (Å, deg): Pt1−Cl1 2.3597(12); Pt1−C1 2.064(4); Pt1−
P1 2.1754(12); Pt1−N1 2.122(3); Pt2−N2 2.078(4); Pt2−N3
2.134(3); Pt2−P2 2.1910(13); Pt2−C26 2.054(4); Cl1−Pt1−N1
88.28(10); Cl1−Pt1−P1 175.11(4); Cl1−Pt1−C1 86.89(12); N1−
Pt1−P1 96.52(10); P2−Pt2−N3 104.73(10); P2−Pt2−C26
83.69(13); N3−Pt2−N2 78.01(13); N2−Pt2−C26 93.63(16).
(bottom) 18-membered Pt4P4N4C6 central core of tetranuclear
complex 2.
in which the P−C−C−P fragment is almost linear. The
terminal Me ligands are trans in relation to each other. It
should be noted that this kind of bridged dppac complex where
a dppac molecule acts as a bridging ligand between two metal
fragments is not common.3,67,68
ESI-MS result further confirms the molecular formulas
formation by demonstrated isotopically well-resolved peak at
m/z = 2107.4 which is related to [2 + H3O]+. The peak
matched well with its calculated theoretical isotope distribu-
tions (see Figure 3).
Absorption Spectrum. Complex 2 is stable in most
common organic solvents including acetone, CH2Cl2, CHCl3,
and CH3CN, which allows the investigation of its absorption
spectrum in a wide range of solvents. Figure 4A shows the
absorption spectrum of 2 in several common organic solvents,
and it is notable that the maxima of the two intense bands in
the 300−400 nm region are essentially independent the
solvent, occurring, for example, at 312 and 358 nm in CH2Cl2
(ε = 3.00 × 104 and 2.60 × 104 M−1 cm−1, respectively). In
order to gain further insight into the nature of electronic
transitions, TD-DFT calculations were performed on the
optimized ground state geometries. As shown in Figure 4B, the
computed absorption spectrum (bars) is in good agreement
with its experimental spectrum. The numerical data for the
selected calculated wavelengths together with their assign-
ments are listed in Table 2, while plots of key molecular
orbitals are shown in Figure 5. The lower energy absorption
band is concluded to be an admixture of ligand (L′ = Cl) to
ligand (L = pbz) charge transfer (L′LCT) and metal (Pt) to L
charge transfer (MLCT) transitions, while the higher energy
Figure 4. (A) UV−visible absorption spectrum of complex 2 at
different solvents (1.0 × 10−5 M); dashed line shows the UV−visible
spectrum of acetone, UV cutoff of acetone is 330 nm. (B) Overlaid
experimental absorbance (spectrum) and calculated TD-DFT (bar)
for complex 2 in CH2Cl2.
band is attributed to an admixture of L′LCT, MLCT and
intraligand (IL) transitions.
In addition to the contours and energies of several highest
occupied molecular orbitals (HOMOs) and lowest unoccupied
molecular orbitals (LUMOs) for the ground-state structure of
the tetranuclear platinum complex 2, Figure 5 also lists the
corresponding partial MO compositions (percentages). The
calculated HOMO−LUMO energy gap is 3.588 eV. Analysis of
the frontier molecular orbitals reveals that the maximum
contributions to HOMO and HOMO−1 belong to the choride
ligand and Pt metal center (in PtMeClNP moiety) with a small
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Table 2. TD-DFT Computed Spectrum at the Ground State Geometry of 2 Compared to the Experimental Spectrum in
a
CH2Cl2 Solution 1 × 10−5
M
b
b
c
λcal(f)
λmax(exp)
transitions (probability)
assignments
345 (0.480)
312
HOMO−3 − LUMO+1 (46)
HOMO−2 − LUMO (35)
HOMO − LUMO (63)
L′LCT, MLCT, IL
IL, L′LCT, MLCT
L′LCT, MLCT
365 (0.150)
358
HOMO−1 − LUMO+1 (29)
L′LCT, MLCT
a
b
c
Only transitions with a probability higher than 20% are reported. In nm. L = pbz, L′= Cl, L″ = dppac.
Figure 5. Representative frontier orbitals and the relative composition energy levels for complex 2.
contribution of dppac. These calculations further show that
LUMO and LUMO+1 are significantly localized on the pbz
bridging ligand, while LUMO+2 is distributed over the dppac
ligand.
via www.ccdc.cam.ac.uk/data_request/cif, or by emailing
data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
CONCLUSIONS
■
Reaction of [Pt(Me)(DMSO)(pbz)] with [Pt(Me)Cl-
(DMSO)2] in the presence of dppac leads to a rare
tetranuclear Pt(II) complex in which two 2-(2-pyridyl)-
benzimidazolate ligands act as both chelating and bridging
ligands to stabilize dinuclear Pt(II) units. Dppac ligands
bridging these units via P-donor atoms create a cyclic
tetranuclear platinum complex. Investigation of electronic
structure and related DFT and TD-DFT calculations of this
tetranuclear structure indicate that the very strong near UV
absorption bands have very mixed MLCT, LL′CT, and IL
character.
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: nabavizadeh@shirazu.ac.ir (S.M.N.).
*E-mail: abuomar@chem.ucsb.edu (M.M.A.).
ORCID
S. Masoud Nabavizadeh: 0000-0003-3976-7869
Fatemeh Niroomand Hosseini: 0000-0002-5856-8104
S. Jafar Hoseini: 0000-0003-4514-1096
Peter C. Ford: 0000-0002-5509-9912
Mahdi M. Abu-Omar: 0000-0002-4412-1985
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b02274.
Notes
■
S
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the Iran National Science Foundation (Grant No.
97012633), Shiraz University, the Islamic Azad University
(Shiraz Branch), and the Department of Chemistry and
Biochemistry at UCSB for financial support.
Computational details for complex 2 (PDF)
Accession Codes
CCDC 1874226 contains the supplementary crystallographic
data for this paper. These data can be obtained free of charge
F
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W.-H.; Kwong, W.-L.; Che, C.-M. Highly phosphorescent platinum
(II) emitters: photophysics, materials and biological applications.
Chem. Sci. 2016, 7, 1653−1673.
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