Air-Stable Blue Phosphorescent Tetradentate Platinum(II) Complexes as Strong Photo-Reductant

Article abstract of DOI:10.1002/anie.201808642

Strong photo-reductants have applications in photo-redox organic synthesis involving reductive activation of C?X(halide) and C=O bonds. We report herein air-stable Pt^II complexes supported by tetradentate bis(phenolate-NHC) ligands having peripheral electron-donating N-carbazolyl groups. Photo-physical, electrochemical, and computational studies reveal that the presence of N-carbazolyl groups enhances the light absorption and redox reversibility because of its involvement into the frontier MOs in both ground and excited states, making the complexes robust strong photo-reductant with E([Pt]^+/*) over ?2.6 V vs. Cp₂Fe^+/0. The one-electron reduced [Pt]^? species are stronger reductants with E_PC([Pt]^0/?) up to ?3.1 V vs. Cp₂Fe^+/0. By virtue of the strong reducing nature of these species generated upon light excitation, they can be used in light-driven reductive coupling of carbonyl compounds and reductive debromination of a wide range of unactivated aryl bromides.

Full text of DOI:10.1002/anie.201808642

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Accepted Article
Title: Air-Stable Blue Phosphorescent Tetradentate Platinum(II)
Complexes as Strong Photo-reductant
Authors: Kai Li, Qingyun Wan, Chen Yang, Xiao-Yong Chang, Kam-
Hung Low, and Chi-Ming Che
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808642
Angew. Chem. 10.1002/ange.201808642
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10.1002/anie.201808642
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Air-Stable Blue Phosphorescent Tetradentate Platinum(II)
Complexes as Strong Photo-reductant
Kai Li, Qingyun Wan, Chen Yang, Xiao-Yong Chang, Kam-Hung Low, and Chi-Ming Che*
Abstract: Strong photo-reductants have demonstrated important
applications in photo-redox organic synthesis involving reductive activation
of C-X(halide) and C=O bonds. We report herein air-stable Pt(II)
complexes supported by tetradentate bis(phenolate-NHC) ligands having
peripheral electron-donating N-carbazolyl groups. Photo-physical,
electrochemical, and computational studies reveal that the presence of N-
carbazolyl groups enhances the light absorption and redox reversibility
because of its involvement into the frontier MOs in both ground and excited
states, rendering the complexes robust strong photo-reductant with
E([Pt]+/*) over -2.6 V vs Cp2Fe+/0. The one-electron reduced [Pt]- species
are stronger reductants with EPC([Pt]0/-) up to -3.1 V vs Cp2Fe+/0. By virtue of
the strong reducing nature of these species generated upon light excitation,
these Pt(II) complexes can be used in light-driven reductive coupling of
carbonyl compounds and reductive debromination of a wide range of
unactivated aryl bromides.
Scheme 1. (a) Two pathways for photo-reduction reactions. (b) Correlation
among ground state and excited state redox potentials and emission energy.
Strong photo-reductants have appealing applications in reductive C-C
bond formation reactions that involve radical/radical ion intermediates,
and which are usually accomplished under harsh conditions in ground
PC: photo-catalyst; S: organic substrate. (c) Structures of literature-reported
complexes having strongly reducing excited states (potentials: V vs Cp2Fe+/0).
state.[1] As depicted in Scheme 1, the substrate scope for light-induced
single-electron transfer (SET) process is confined by ground state and
excited state redox potentials of the photo-catalyst (PC), i.e. E([PC]+/*)
and E([PC]0/-).[2,3] For the widely used photo-reductant [Ir(ppy)3], its
E([PC]+/*) is -1.73 vs SCE. Despite the E([PC]0/-) of -2.19 V, access to
[Ir(ppy)3]- is disfavored by the low excited state reduction potential for
E([PC]*/-). In this context, a few organic dyes have proven to be strong
photo-reductants.[4] By elegant design of consecutive photon-induced
electron transfer, triplet-triplet annihilation (TTA) based up-conversion,
or triplet sensitization/electron transfer processes, visible-light can be
used to generate the strongly reducing species.[4a,c-e] It is noted that
the multi-component system is challenging to design because of the
presence of multiple, plausible energy/electron transfer processes.[5]
Therefore, the search for new powerful photo-reductant remains a
crucial step to the development of new photo-redox catalysis for
synthetic chemistry in terms of expanding substrates and for devising
new reactions of synthetic interest.
We propose to develop strong photo-reductants with blue
phosphorescent tetradentate platinum(II) complexes[10] for the following
reasons. First, the high energy (Eem) and tens of microseconds lifetime of
the triplet excited states of blue phosphorescent metal complexes are
instrumental to achieve strongly reducing excited state species ([PC]*) for
efficient bimolecular photochemical reactions. Second, the strong chelate
effect of tetradentate ligand scaffold renders the platinum photo-catalyst
robust against demetalation both in the ground and excited states. Third,
Pt(II) complexes with open coordination sites may activate chemical
bonds via inner-sphere atom transfer process. In this work, we chose
Pt(II) complexes supported by the bis(phenolate-NHC) ligands bearing
electron-donating N-carbazolyl groups (1 and 2, Figure 1). The analogue
bearing strong electron-withdrawing CF3 (3) and the other complexes,
[Pt(ppy)(acac)], [Pt(pim)(acac)] and [Ir(ppy)3] were studied for comparison.
As up to now, a few metal complexes of Cu(I), W(0), Mo(0),[8] and
Cr(0)[9] have been shown to display strongly reducing excited states. But
these complexes also have low redox potentials of ca. -(0.38‒0.72) V vs
Cp2Fe+/0 for the M+/0 couple (Scheme 1), which disfavors regeneration of
the metal complexes through reduction of their oxidized forms with
common sacrificial electron donors, e.g. tertiary amines. Moreover, the air-
sensitivityofthelow-valentcomplexes mayhampertheir practical use.
[6]
[7]
[*] Dr. K. Li, Q. Wan, Dr. C. Yang, Dr. X.-Y. Chang, Dr. K.-H. Low, Prof.
Dr. C.-M. Che
Figure 1. Chemical structures of Pt(II) complexes 1‒3 and reference complexes.
Synthetic route (Scheme S1) and characterization data (multinuclear
NMR and high-resolution mass spectra and elemental analyses) of 1–3
are given in the Supporting Information. Complexes 1–3 were prepared in
5889% yields by a convenient reaction of [Pt(DMSO)2Cl2] with
bis(imidazolium) salts (synthesized from commercially available
inexpensive materials). X-ray single crystal structures of 1 and 2 have
been determined.[11] In both crystal structures, the peripheral carbazolyl
groups are significantly twisted with respect to the phenolate plane. The
State Key Laboratory of Synthetic Chemistry, Institute of Molecular
Functional Materials, HKU-CAS Joint Laboratory on New Materials,
and Department of Chemistry, The University of Hong Kong, Pokfulam
Road, Hong Kong (China)
E-mail: cmche@hku.hk
Prof. Dr. C.-M. Che
HKU Shenzhen Institute of Research and Innovation, Shenzhen
518053 (China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201808642
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COMMUNICATION
crystal structure of 2 shows a roughly planar molecular backbone with a
PtPt and π-π contact of ~3.5 Å in each pair of molecules (Figure 2). In
contrast, abentconformationandamuchshorter PtPtdistanceof ~3.1Å
are observed in the crystal structure of 1 (Figure S1). The complexes are
well soluble in DMF, but poorly soluble in MeCN and alcohol. DMF was
selected as the solvent for the electrochemical, photo-physical and
photochemical studies. The photo-stability of 2 in degassed solution was
examined (Figure 3). Continuous irradiation of a [D7]DMF solution of 2
usingaUVlamp(365nm, 12W)for96hintheabsenceofoxygendidnot
inducenotablechange.
π(phenolate/carbazole)-π*(NHC) (3ILCT) excited states perturbed by the
Pt(II) center. The PLQYs () of 2 in toluene and PMMA (5 wt%) reach
40%. In sharp contrast, 3 shows the highest emission energy but with a
very low  value (1% in DMF). A thermally accessible non-radiative state
is likely the reason. The emission properties of 1−3 in other states are
providedintheSupportingInformation(TableS2andFigureS3).
Table 1: Photo-physical data of 1‒3 in DMF at room temperature.
Absorption[a]
Emission[a,b]
λmax / nm (τ / µs); 
λmax / nm (ε / 104 M-1 cm-1)
1
2
3
284 (4,79), 308 (sh, 1.88), 345 (2.14), 359 (1.92)
288 (4.87), 298 (5.08), 352 (2.17), 364 (1.78)
274 (sh, 2.50), 291 (1.60), 332 (1.35), 345 (1.50)
446 (6.1); 20%
447 (6.7); 24%
434 (0.16); 1%
(a)
[a] in DMF (concentration ~2 × 10-5 M). [b] λex = 350 nm (1 and 2) or 345 nm (3).
(a)
(b)
(b)
5x104
1.0
0.8
0.6
0.4
0.2
0.0
4x104
3x104
2x104

max = 447 nm
 = 24%
 = 6.7 s
LUMO
1x104
0
300
400
500
600
HOMO
 / nm
Figure 4. (a) UV/Vis absorption and emission spectra of 2 in DMF at room
temperature; concentration ~ 2 × 10–5 M. (b) DFT calculated HOMO and LUMO of 2 in
S0 state.
Figure 2. Crystal structure of 2. (a) Top view. (b) Side view.
Table 2: Rate constants for the quenching of 2* (λex = 350 nm) by various
organic substrates (S) in DMF at room temperature.
E(S0/-) / V vs SCE
kq / M-1 s-1
Acetophenone
Benzaldehyde
-2.14[a]
-1.81[a]
1.33 × 109
2.53 × 109
4-Chlorobenzonitrile
Benzonitrile
-2.03[a]
-2.31[a]
-2.35[b]
-2.21[b]
-2.78[b]
-2.66[b]
1.88 × 108
2.71 × 106
2.18 × 107
1.47 × 107
4.30 × 106
4.96 × 105
Chloroform
Benzyl chloride
2-Chlorobromobenzene
4-Fluorobromobenzene
[a] Half reduction potential. [b] Cathodic peak potential.
Figure 3. 1H NMR spectral traces of 2 in degassed [D7]DMF upon UV lamp
irradiation (365 nm, 12 W).
DFT and TDDFT calculations were performed on 1 and 2 as well as
Pt-H to decipher the effect of N-carbazolyl groups on their electronic
structures (Figure 4b and Figures S4‒S6). In the S0 state, the electron
density of HOMO for 1 and 2 distributes over the N atoms of carbazolyl
groups and the phenolate moieties, in comparison to that for analogue Pt-
H. The increased oscillator strength for 1 and 2 is also reproduced from
calculations (Figure S7). Unexpectedly, the calculations show that 1 and 2
undergodifferent S0→T1 excitedstategeometrydistortions andthefrontier
MOs of 1 and 2 in T1 state have different contour plots. As for Pt-H and
previous reported complexes, the NHC ring that dominantly contributes to
the SOMO is twisted in T1 state for 1. Meanwhile, the N-carbazolyl group
rotates by ~14o to have a smaller torsion angle with the phenolate ring,
due to the decrease of electron density on N atom upon photo-excitation.
Incontrast, thesubstitutionoftBugroups resultsintheT1 stateof2tohave
mainly π(tBu-carbazolyl1 + phenolate)→π*(tBu-carbazolyl2) charge-
transfer character. As a result, NHC twisting has not been observed in the
calculation. Inviewof their quitesimilar absorption and emission properties
insolutions, it might not be appropriate toassignthe T1 states of 1and 2to
Complexes 1−3 exhibit intense absorptions in DMF in UVA region (λ
= 315‒400 nm) (Table 1, Figure 4a, and Figure S2) with ε on the order of
[10a,b]
104 M-1 cm-1. With respect to previous studies,
the lowest-energy
absorptions are assigned to mixed π(phenolate)→π*(NHC) (1ILCT) and
dπ(Pt)→π*(NHC) (1MLCT) transitions. Notably, the presence of N-
carbazolyl results in increased molecular absorptivity for 1 and 2 in the
lowest-energy 1ILCT/1MLCT transitions compared to that of 3, revealing
the involvement of carbazolyl groups in the transition. A blue shift of ca. 20
nm was observed for 3 relative to 1 and 2, likely due to the stabilization of
HOMO level arising from strong electron-withdrawing effect of CF3.
Solvatochromic study for 2 reveals a slight blue shift of the lowest energy
absorption maximum by 4 nm upon changing the solvent from toluene
(λmax = 368 nm) to DMF (λmax = 364 nm) (Figure S2). Absorption from
S0→T1 transition has not been located. Complexes 1 and 2 display strong
blue emission insolutions and in PMMA films at room temperature (Figure
4a and Figure S2), which are assigned to be predominantlyfrom thetriplet
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be different. Instead, the T1 states of 1 and 2 are assigned tentatively to
have π(tBu-carbazolyl1 + phenolate)→π*(tBu-carbazolyl2 + NHC) charge-
transfercharacter withvaryingdegreeof eachcomponents.
constants depicted in Table 2. When 3 or Pt-Me was used, lower yields
(< 40%) were obtained for acetophenone, probably due to the weaker
light absorption and/or the very short excited state lifetime; Pt-F gave a
Electrochemical studies by cyclic voltammetry revealed that 2 and
3 show oxidation waves at 0.35 V and 0.50 V (Epa) vs Cp2Fe+/0,
respectively (Figure 5). Notably, this oxidation process is more
reversible for 2 than for 3, presumably due to the contribution from
carbazoyl groups to HOMO in 2. From the Latimer diagram, the
excited-state redox potential E([Pt]+/*) of 2 is estimated to be -2.63 V
vs Cp2Fe+/0, revealing that the excited state reducing power is
comparable to those of the most powerful photo-reductants of W(0),[7]
Mo(0)[8] and organic dye.[4] It is noted that the one-electron reduced
species [Pt]- (for 2) with E[Pt]0/- of -3.05 V vs Cp2Fe+/0 is a stronger
reductant. For 3, the CF3 substitution has no appreciable effect on the
potential of the reduction peak. Previously reported Pt-F and Pt-Me[10b]
were also studied for comparison. Complex Pt-F shows similar E([Pt]+/*)
and E[Pt]0/- values as 2 while Pt-Me shows a more cathodic E[Pt]0/-
wave (Figure S8). Therefore, the reduction potentials of both [Pt]* and
[Pt]- for 3 and Pt-F are virtually the same as those of 2 (Figure S8).
The strong absorption, long emission lifetime, and increased redox
reversibility of 2 suggest that it is a promising, strong photo-reductant.
higher yield than 3 and Pt-Me; the use of [Pt(ppy)(acac)],
[Pt(pim)(acac)], or [Ir(ppy)3] resulted in low yields of 9−18% (Table S5).
A plausible reaction pathway is proposed in Scheme S2.
Table 3: Light-driven reductive coupling of aromatic carbonyls.[a,b]
[a] Reaction conditions: Aromatic carbonyl (0.2 mmol), catalyst (1 mol%), DIPEA
(0.4 mmol), DMF (2 mL), xenon lamp (λ > 370 nm). [b] Product yield determined by
1H NMR spectroscopy using an internal standard. [c] Reaction time of 24 h.
Reductive radical dehalogenation that avoids the use of toxic
organotin reagent is a useful tool in organic synthesis.[12] Photo-redox
catalysis via electron transfer provides an appealing approach to the
generation of radical anions because of the mild reaction conditions and
good functional group tolerance. However, examples of photo-catalysts
that are able to reduce unactivated alkyl and aryl bromides upon light
irradiation are sparse.[2c,4] We examined the applicability of 2 for
reductive debromination of unactivated aryl bromides. With reference to
the protocol reported by Stephenson and co-workers,[2b] in the presence
of 2 (1.5 mol%), DIPEA (5 equiv), and formic acid (10 equiv) in DMF, 4a
was completely reduced to give 5a in 96% yield after 4 h of irradiation
with UVA LED (365 nm, 4.5 W). Control experiments showed that Pt(II)
catalyst, DIPEA, and light are all essential for this transformation (Table
S6). The presence of formic acid can improve the reaction. A range of
Ar-Br bearing steric bulky (4d) and strongly electron-donating (4h,i)
substituents were converted to Ar-H in good yields (74−81%, Table 4).
Carboxyl, hydroxyl, and amide groups can be tolerated in this
transformation as well in reasonable yields (4j−l). For reduction of 4a,
the use of 3, Pt-F, or Pt-Me gave 5a in 46−61% yields under the same
reaction conditions; [Pt(ppy)(acac)] or [Pt(pim)(acac)] resulted in low
yields of 9−16% (Table S6). The reduction potential of [Pt(ppy)(acac)]
(E([Pt]+/*) = -2.07 V vs SCE) is insufficient to reduce unactivated Ar-Br.[2g]
In spite of a high emission energy, the short triplet excited state lifetime
of [Pt(pim)(acac)] is postulated to render a low catalytic efficiency.[13]
[Ir(ppy)3] was found to be ineffective for the photochemical reduction of
4a under thesameconditions.
Previous works on photo-redox catalyzed debromination pointed to
radical mechanism. Afew alkyl and arylbromides 4m−p weresubjected to
the reaction conditions and found to undergo cyclization to afford products
in moderate to excellent yields (Scheme S3), substantiating the radical
mechanism.[14] Intermolecular radical-radical coupling was also
demonstrated on benzyl chloride (4q) to furnish the desired homocoupled
product (Scheme S4). The Pt(II) complexes may activate halocarbons via
outer-sphere electron transfer and/or inner-sphere atom abstraction
processes. Photoreactions of 2 with a series of organic halides were
investigated by Stern-Volmer quenching studies. The rate constants for
the quenching of emission of 2 are found to depend on the reduction
1.6x10-4
2
3
1.2x10-4
0.5
8.0x10-5
4.0x10-5
-3.03
0.35
0.0
-4.0x10-5
-3.05
-8.0x10-5
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
0.5
Potential / V vs CP2Fe+/0
Figure 5. Cyclic voltammograms of 2 and 3 in DMF.
The redox properties of 2* were first examined through Stern-
Volmer experiments with a panel of pyridinium salts having reduction
potentials (E1/2) from -0.67 to -1.68 V vs SCE. The diffusion-corrected
quenching rate constants are close to diffusion controlled limit, being in
the range of 1.81‒5.17 × 109 s-1 (Figure S9 and Table S3). The strong
reducing power of 2* is also revealed by its emission being quenched
by aryl carbonyls with rate constants on the order of 109 M-1 s-1 (Table
2 and Figures S10−S11). The emission of 2 is quenched by
benzonitrile (Table 2 and Figure S10) with a relative low quenching
rate constant; presumably, the excited state reduction potential of 2 is
close to the reduction potential of benzonitrile at -2.76 V vs Cp2Fe+/0.
This is in agreement with the estimated value of -2.63 V for 2 from
electrochemical and spectroscopic measurements. Importantly, the
emission of 2 can be quenched by diisopropylethylamine (DIPEA)
(Figure S12) with a rate constant of 3.71 × 105 M-1 s-1, revealing the
possibility to generate [2]- that can act as a stronger reductant.
The light-induced reductive coupling of carbonyl compounds with 2
as catalyst was examined. With DIPEA as sacrificial electron donor, a
panel of aromatic carbonyls underwent pinacol coupling reaction to
afford diols in yields of 65−91% (Table 3 and Table S4). Control
experiments (Table S5) confirmed the essential roles of Pt(II) catalyst,
DIPEA, and light for this process. Unlike the cases where amine
cations or external additives are required to activate carbonyl
groups,[3e,f] the substrates in this work are proposed to be reduced
directly by 2*; this is also revealed by the large oxidative quenching rate
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potential rather than the bond dissociation energy of the R-X substrates
(Table 2 and Figure S13). For example, kq of 4-chlorobenzonitrile (1.88 ×
108 M-1 s-1) is much faster than that for a variety of alky and aryl bromides.
Thus, the quenching is mainly attributed to outer-sphere electron-transfer
from 2* to R-X. Based upon the emission quenching experiments with R-
Br and DIPEA, a plausible reaction mechanism involving both oxidative
andreductiveprocessesis proposed(SchemeS5).
[2]
Table 4: Light-driven reductive hydrodebromination of aryl bromides.[a,b]
[3]
[4]
[5]
[a] Reaction conditions: substrate (0.1 mmol), 2 (1.5‒5 mol%), DIPEA (0.5 mmol),
formic acid (40 µL), and UVA LED (365 nm, 4.5 W). [b] Product yield determined by
1H NMR spectroscopy using an internal standard. [c] 5 mol% 2 used.
In summary, strongly electron-donating carbazolyl groups have been
incorporated into blue phosphorescent Pt(II) complexes supported by
tetradentate bis(phenolate-NHC) ligands todevelop air stablestrong photo-
reductants. The Pt(II) complex displays strong excited state reducing ability
with E([Pt]+/*) over -2.6 V vs Cp2Fe+/0. The one-electron reduced species is
stronger reductant with Epc([Pt]-/0) at -3.1 V vs Cp2Fe+/0. Together with long
excited state lifetime, the complex is able to drive light-induced reductive
coupling of aromatic carbonyl compounds and reductive debromination of
unactivated aryl bromides in good to excellent yields under mild conditions.
This work highlights that Pt(II) complexes supported bytetradentateligands
arepromisingrobustphoto-catalystsfororganicsynthesis.
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CCDC 1417213 and 1845252 contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre.
This work was supported by the National Key Basic Research Program
of China(No. 2013CB834802), the Hong Kong Research Grants Council
(HKU17330416), the University Grants Committee Areas of Excellence
Scheme (AoE/P-03/08), CAS-Croucher Foundation Funding Scheme for
Joint Laboratories, and the Science and Technology Innovation
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Keywords: photo-redox catalysis • photo-reductant • Pt(II)
complex• radical dehalogenation • C-C bond formation
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10.1002/anie.201808642
Angewandte Chemie International Edition
COMMUNICATION
Entry for the Table of Contents
COMMUNICATION
Highly reducing triplet excited states
of phosphorescent Pt(II) complexes
bearing tetradentate bis(phenolate-NHC)
ligands having peripheral N-carbazolyl
groups were observed, with E([Pt]+/*)
being -2.6 V vs Cp2Fe+/0. These air-stable
Pt(II) complexes show intense blue
emission and catalyze light-driven
Dr. Kai Li, Qingyun Wan, Dr. Chen
Yang, Dr. Xiao-Yong Chang, Dr. Kam-
Hung Low, and Prof. Chi-Ming Che*
Page No. – Page No.
Air-Stable
Blue
Phosphorescent
Tetradentate Platinum(II) Complexes
as Strong Photo-reductant
reductive
coupling
of
carbonyl
compounds and reductive debromination
of a variety of unactivated aryl bromides.
This article is protected by copyright. All rights reserved.