Photophysics and Photoredox Catalysis of a Homoleptic Rhenium(I) Tris(diisocyanide) Complex

Article abstract of DOI:10.1021/acs.inorgchem.7b03258

Herein a homoleptic rhenium(I) complex bearing three chelating diisocyanide ligands and its photophysical properties are communicated. The complex emits weakly from a high-energy triplet metal-to-ligand charge-transfer excited state with an 8 ns lifetime in deaerated CH₃CN at 22 °C and is shown to act as an efficient photoredox catalyst comparable to [Ir(ppy)₃] (ppy = 2-phenylpyridine) in representative test reactions.

Full text of DOI:10.1021/acs.inorgchem.7b03258

Communication
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Photophysics and Photoredox Catalysis of a Homoleptic Rhenium(I)
Tris(diisocyanide) Complex
Christopher B. Larsen and Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, CH-4056 Basel, Switzerland
S
* Supporting Information
Using [Ru(bpy)3]2+ and [Ir(ppy)3] as examples, the metal-
centered (RuII and IrIII, respectively) ground-state oxidation
ABSTRACT: Herein a homoleptic rhenium(I) complex
bearing three chelating diisocyanide ligands and its
photophysical properties are communicated. The complex
emits weakly from a high-energy triplet metal-to-ligand
charge-transfer excited state with an 8 ns lifetime in
deaerated CH3CN at 22 °C and is shown to act as an
efficient photoredox catalyst comparable to [Ir(ppy)3]
(ppy = 2-phenylpyridine) in representative test reactions.
potentials of these complexes are 1.29 and 0.77 V vs SCE,
respectively, and the ligand-based (bpy vs ppy, respectively)
reductions are −1.33 and −2.19 V versus saturated calomel
electrode (SCE), yielding E00 values of 2.10 and 2.50 eV,
respectively.6 When these values are inserted into the Rehm−
Weller equation, it is unsurprising that [Ir(ppy)3] is the more
powerful photoreductant with E1/2(C+/C*) equal to −1.73 V
versus SCE, compared to −0.81 V versus SCE for [Ru(bpy)3]2+.
Our recent investigations into photoluminescent earth-
abundant zerovalent d6 metal complexes have identified
chelating diisocyanides as promising ligands for the development
of strongly photoreducing complexes.26−28 These ligands are
more difficult to reduce than bpy or ppy, and their strong σ-
donor character makes metal-based oxidations more facile. In
combination with their strong π-accepting character, these
attributes also increase the energy of deactivating ligand-field
states relative to desirable MLCT states. Furthermore, the
strong-field character of isocyanides is akin to that of carbon-
yls,29,30 stabilizing the low-valent rhenium(I).31,32 Prior studies
of rhenium(I) complexes with monodentate isocyanide ligands
demonstrated that this compound class has very promising
photophysical properties,33−35 but analogues with chelating
ligands seem to be unknown so far.
hotoredox catalysis is rapidly emerging as a useful method
P
for C−C bond formations.1,2 d6 precious-metal complexes
are the prototypical photoredox catalysts and have played a
pivotal role in the development of photoredox catalysis as a
field.3,4 Despite growing interest in organic and earth-abundant-
metal complex photoredox catalysts,5−8 d6 precious-metal
complexes remain the first choice for many photoredox
transformations, with [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine)
and [Ir(ppy)3] (ppy = 2-phenylpyridine) among the most
popular.1−3,9 This largely stems from their ease of synthesis,
favorable photophysical properties, tunability, and robustness.
Interestingly, despite significant attention having been paid to
the photoluminescent properties of [Re(CO)3(α-diimine)(X)]
complexes (X = Cl−, Br−, CN−, SCN−, py)10−16 and their
performance in the catalytic photoreduction of CO2,15,17−24
rhenium(I) complexes have been essentially unused as catalysts
in organic photoredox transformations.
We have therefore prepared a homoleptic rhenium(I)
tris(diisocyanide) complex, [Re(L1)3]PF6 (Scheme 1), with the
goal of developing a strong photoreductant, anticipating that the
chelating nature of the ligand would increase the robustness
compared to rhenium(I) complexes with monodentate
d6 metal complexes typically exhibit photoluminescent metal-
to-ligand charge-transfer (MLCT) excited states.3 The energies
of these excited states are easily tunable through the choice of
metal and ligand design. A key feature of such complexes is that
they are both better oxidants and reductants in their emissive
3MLCT excited states than in their electronic ground states. The
magnitude by which the oxidation potential of a complex is
altered between ground and excited states may be estimated
using the Rehm−Weller equation:25
a
Scheme 1. Synthesis of [Re(L1)3]PF6
E
1/2(C+/C*) ≈ E1/2(C+/C0) − E00
where E1/2(C+/C0) and E1/2(C+/C*) are the ground- and
excited-state state oxidation potentials, respectively, and E00 is
the energy of the electronic origin for the excited state (the
difference in energy between the zeroth vibrational levels of the
ground and excited states). With the goal of increasing the
reducing power of a catalyst and therefore increasing its substrate
scope, the pertinent design principles are to combine an easily
oxidized metal [to decrease E1/2(C+/C0)] with difficult-to-
reduce ligands (to increase E00 for the MLCT excited state).
a
(i) L1, Na(Hg), THF, rt, 15 h; (ii) AgPF6, NH4PF6, THF, 76%.
Received: December 27, 2017
© XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.7b03258
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
arylisocyanide ligands.27 The relevant photophysical properties
of the complex are presented, in combination with the
photoredox catalytic performance for two representative test
reactions.
Electrochemical studies performed on [ReCl(CO)3(N−N)]
complexes typically exhibit an irreversible ReII/ReI oxidation
near the solvent limit,41 and for the [Re(2,6-dimethylphenyliso-
cyanide)6]+ complex, a potential of 1.08 V versus SCE was
reported.40 [Re(L1)3]PF6 exhibits such an oxidation at 1.36 V
versus SCE in CH3CN (Figure S3). In combination with the
excited-state energy (see above), an estimated E1/2(C+/C*) of
−1.60 V versus SCE can be obtained. [Re(L1)3]PF6 is
photostable in deaerated CH3CN, with its electronic absorption
spectrum in deaerated CH3CN remaining unchanged after
irradiation at 405 nm (1 W) for 24 h (Figure S2). This is an
important finding in view of the known photolability of the
[Re(2,6-dimethylphenylisocyanide)6]+ complex, which under-
goes efficient ligand exchange in the presence of halide anions.42
Many organic photoredox catalysts have excited-state lifetimes
that are similarly as short as that of [Re(L1)3]PF6,5,43 and it
therefore seemed reasonable to perform photoredox catalysis
experiments with this complex, particularly in view of its
comparatively high reducing power and photostability.
Rhenium(I) complexation of L1 was achieved through
adaptation of the method of Wang et al.36 for the synthesis of
[Re(Mebpy)3]PF6 from [ReCl4(THF)2] (THF = tetrahydrofur-
an) to afford [Re(L1)3]PF6 in 76% yield (Scheme 1). L1 and
[ReCl4(THF)2] were prepared according to literature proce-
dures.26,37
Electronic absorption and emission spectra of [Re(L1)3]PF6 in
CH3CN are presented in Figure 1 and in THF, CH2Cl2, and
CH3CN in Figure S1. Both absorption and emission profiles
exhibit only minor differences across this solvent range.
Our initial approach was to try and match the excited-state
oxidation potential of the complex with the reduction potential of
a known substrate of interest. To this end, we chose the
dimerization of benzyl bromide (E0red = −1.3 V vs SCE)1 as a test
reaction (Table 1).44 Reactions were performed in flame-sealed
a
Table 1. Benzyl Bromide Dimerization
irradiation
wavelength/ benzyl bromide
dibenzyl
toluene
Figure 1. Electronic absorption (blue trace) and emission (red trace; λex
= 350 nm) spectra of [Re(L1)3]PF6, recorded in deaerated CH3CN at 22
°C. Inset: Photoluminescence decay of [Re(L1)3]PF6 recorded in
deaerated CH3CN (λex = 405 nm). (*) Stray excitation light. The black
dotted trace represents the ligand UV−vis spectrum in CH3CN.
catalyst
[Re(L1)3]
PF6
nm
consumption/% formation/% formation/%
405
100
49
81
45
19
4
[Ru(bpy)3]
(PF6)2
455
a
Conditions: 20 h of continuous irradiation in vacuum flame-sealed
1
NMR tubes. Conversions obtained from H NMR integrals. See the
The electronic absorption spectrum of [Re(L1)3]PF6 in
CH3CN (blue trace) exhibits a maximum at 318 nm (ε =
34000 M−1 cm−1) not present in the ligand spectrum (dotted
black trace), which tails past 400 nm and is assigned as MLCT in
nature. Upon photoexcitation into this MLCT absorption feature
(λex = 350 nm) in deaerated CH3CN at 22 °C, photo-
luminescence is observed at ∼480 nm (red trace in Figure 1)
with a quantum yield of 0.6% and lifetime of 8 ns (inset in Figure
1). Photoluminescence is quenched in the presence of oxygen
Supporting Information for details.
NMR tubes containing the substrate (0.1 M), catalyst (1 mol %),
and N,N-diisopropylethylamine (DIPEA; 4 mol equiv) as a
sacrificial electron donor in deaerated CD2Cl2. Over the course
of 20 h under 405 nm (1 W) irradiation at room temperature,
[Re(L1)3]PF6 completely converted the substrate, with 81%
dimerizing to afford dibenzyl and 19% presumably abstracting a
hydrogen atom from trace water present in the solvent to afford
toluene. Catalyst-based photoluminescence was still observable
after consumption of the substrate with no obvious loss of
intensity, indicating that the catalyst is stable over long reaction
times. As a control experiment, [Ru(bpy)3](PF6)2 [E1/2(C+/C*)
= −0.72 V vs SCE] converted only 49% of the substrate under
455 nm irradiation for the same duration at room temperature,
yielding dibenzyl and toluene in 45% and 4% yield, respectively.
The different excitation source was used to better coincide with
the absorption maximum of the [Ru(bpy)3](PF6)2 MLCT
absorption band. No reaction occurred in the absence of either
catalyst or light.
3
and is therefore characterized as MLCT in nature. From the
intersection of the absorption and photoluminescence spectra, a
3MLCT excited-state energy of 2.96 eV is estimated.
The low quantum yield and short excited-state lifetime may be
explained by the thermal population of ligand-field excited states.
These states are typically not readily accessible in [Re(CO)3(α-
diimine)(X)] complexes;10−16,38 however, our complex has a
higher-energy 3MLCT state than [Re(CO)3(α-diimine)(X)]
complexes,16,39 and the isocyanide ligands destabilize ligand-field
states to a lesser degree than carbonyls, resulting in thermally
accessible ligand-field states. A previously studied homoleptic
rhenium(I) hexakis(arylisocyanide) complex with monodentate
ligands was nonemissive at room temperature because of the
thermal occupation of ligand-field states.40 Thus, the chelating
ligand leads to markedly improved photophysical properties.
Encouraged by the catalytic performance of [Re(L1)3]PF6 in
this test reaction, we chose to explore the scope of the complex
for more challenging catalytic photoreductive transformations.
B
DOI: 10.1021/acs.inorgchem.7b03258
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Communication
Given an E1/2(C+/C*) of −1.60 V versus SCE for [Re(L1)3]PF6,
the reduction of iodobenzene [E0red ≈ −1.6 V vs SCE in dimethyl
sulfoxide (DMSO)]45 to benzene was chosen as a demanding
transformation (Table 2). Reactions were once again performed
in flame-sealed NMR tubes with deaerated solvent, using
conditions modified from Nguyen et al. for [Ir(ppy)3] (Table
2).46
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: oliver.wenger@unibas.ch.
ORCID
Oliver S. Wenger: 0000-0002-0739-0553
Notes
The authors declare no competing financial interest.
a
Table 2. Reduction of Halobenzenes
ACKNOWLEDGMENTS
■
C.B.L. acknowledges a Swiss Government Excellence Postdoc-
toral Scholarship for Foreign Scholars. The authors thank Drs. L.
A. Buldt and X. Guo for insightful discussion. Financial support
̈
by the Swiss National Science Foundation through Grant
200021_156063/1 is gratefully acknowledged.
catalyst
[Re(L1)3]PF6
X
conversion/%
100
I
b
[Ir(ppy)3]
I
100
[Re(L1)3]PF6
[Ir(ppy)3]
Br
Br
22
18
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.7b03258.
■
S
Experimental procedures, variable-solvent UV−vis and
emission spectra, UV−vis photostability experiment,
cyclic voltammetry, and photoredox catalysis NMR
experiments (PDF)
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DOI: 10.1021/acs.inorgchem.7b03258
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