Long-Lived, Strongly Emissive, and Highly Reducing Excited States in Mo(0) Complexes with Chelating Isocyanides

Article abstract of DOI:10.1021/jacs.9b07373

Newly discovered tris(diisocyanide)molybdenum(0) complexes are Earth-abundant isoelectronic analogues of the well-known class of [Ru(α-diimine)₃]²⁺ compounds with long-lived ³MLCT (metal-to-ligand charge transfer) excited

Full text of DOI:10.1021/jacs.9b07373

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Cite This: J. Am. Chem. Soc. XXXX, XXX, XXX−XXX
Long-Lived, Strongly Emissive, and Highly Reducing Excited States
in Mo(0) Complexes with Chelating Isocyanides
†
†
†
̈
Patrick Herr, Felix Glaser, Laura A. Buldt, Christopher B. Larsen, and Oliver S. Wenger*
Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland
S
* Supporting Information
ABSTRACT: Newly discovered tris(diisocyanide)molybdenum(0) com-
plexes are Earth-abundant isoelectronic analogues of the well-known class
3
of [Ru(α-diimine)3]2+ compounds with long-lived MLCT (metal-to-ligand
charge transfer) excited states that lead to rich photophysics and
photochemistry. Depending on ligand design, luminescence quantum yields
up to 0.20 and microsecond excited state lifetimes are achieved in solution at
room temperature, both significantly better than those for [Ru(2,2′-
bipyridine)3]2+. The excited Mo(0) complexes can induce chemical reactions
that are thermodynamically too demanding for common precious metal-
based photosensitizers, including the widely employed fac-[Ir(2-phenyl-
pyridine)3] complex, as demonstrated on a series of light-driven aryl−aryl
coupling reactions. The most robust Mo(0) complex exhibits stable
photoluminescence and remains photoactive after continuous irradiation
exceeding 2 months. Our comprehensive optical spectroscopic and
photochemical study shows that Mo(0) complexes with diisocyanide chelate ligands constitute a new family of luminophores
and photosensitizers, which is complementary to precious metal-based 4d6 and 5d6 complexes and represents an alternative to
nonemissive Fe(II) compounds. This is relevant in the greater context of sustainable photophysics and photochemistry, as well
as for possible applications in lighting, sensing, and catalysis.
discovered that chelating diisocyanide ligands give access to
INTRODUCTION
■
3
Cr(0) and Mo(0) complexes with emissive MLCT excited
The long-lived luminescent metal-to-ligand charge transfer
(MLCT) excited states of complexes with precious d6 metals
such as Ru(II), Ir(III), Os(II), or Re(I) form the basis for
many photophysical and photochemical applications, for
example in luminescent devices,1 sensing,2,3 solar energy
conversion,4 and photoredox catalysis.5 There is a long-
standing interest in obtaining complexes with similarly
favorable electronic structures made from cheaper first-row
transition metal elements.6−8 Fe(II) is an obvious and much
explored target,9−15 because iron is the most abundant and
cheapest transition metal in Earth’s crust. However, such 3d6
complexes have energetically low-lying metal-centered (MC)
excited states that deactivate the MLCT states very
efficiently.16−18 Different concepts have been explored to
states.36 The Cr(0) complex exhibited an excited-state lifetime
of 2.2 ns and a luminescence quantum yield of ca. 10−5 in
tetrahydrofuran (THF) solution at room temperature,37 while
3
the Mo(0) complex had a MLCT lifetime of 166 ns and a
quantum yield of 0.023 in toluene.38 Herein, we report how
improved ligand design can enhance both of these properties
in Mo(0) complexes by an order of magnitude, and it becomes
evident that the initially communicated single example is not
merely an academic curiosity, but instead tris(diisocyanide)-
molybdenum(0) complexes represent a new family of robust
4d6 complexes with very favorable photophysical properties.
Moreover, we demonstrate that excitation of these complexes
can induce chemical reactions that are thermodynamically too
challenging for typical precious metal-based photosensitizers,
such as the well-known fac-[Ir(ppy)3] complex. For this
purpose, we investigated base-promoted homolytic aromatic
substitution (BHAS) reactions,39 which were performed in an
intramolecular fashion on a series of substrates specifically
designed to elucidate the catalytic properties of the Mo(0)
complexes.
3
lengthen MLCT lifetimes in Fe(II) complexes, including the
optimization of metal coordination geometries,19−21 the use of
push−pull ligand environments,22,23 and the enhancement of
ligand fields with N-heterocyclic24−27 or mesoionic carbenes.28
Recently, these efforts culminated in the discovery of two
Fe(III) complexes that luminesce from a ligand-to-metal
charge transfer (LMCT) excited state29,30 and an Fe(II)
complex with a 3MLCT lifetime of 528 ps in solution at room
temperature.31
Our study complements ongoing research on photoactive
complexes with other Earth-abundant metal elements such as
Building on early studies of group 6 d6 metal complexes with
isocyanide ligands32,33 and recent reports of luminescent W(0)
complexes with monodentate isocyanides,34,35 we recently
Received: July 11, 2019
© XXXX American Chemical Society
A
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Fe,9−15 Cu,40−50 Cr,51−54 Co,55 Ni,56−58 Zr,59,60 W,34,35,61−63
and Ce,64 as well as current research on new metal-free organic
photosensitizers.65−69
RESULTS AND DISCUSSION
■
Synthesis and Infrared Spectroscopy. Monodentate
isocyanide ligands play an important role in organometallic
chemistry as alternatives to CO that can be further
functionalized,70,71 and one attractive option is to create
polydentate chelating isocyanides.72−80 In our initial commu-
nication, we reported that a m-terphenyl backbone is useful to
obtain a diisocyanide ligand that binds Mo(0) in chelating
fashion, giving access to a homoleptic complex (Scheme 1a,
Scheme 1. Molecular Structures of (a) [Mo(LMe)3] and (b)
[Mo(LtBu)3]
Figure 1. Cyclic voltammograms of (a) [Mo(LMe)3] and (b)
[Mo(LtBu)3] recorded in deaerated THF with 0.1 M TBAPF6 at 20
°C. The potential scan rates were 0.1 and 0.2 V/s, respectively. The
differences in anodic and cathodic peak currents are 86 mV (a) and
116 mV (b). (c, d) Latimer diagrams for [Mo(LMe)3] and
3
[Mo(LtBu)3], based on an energy of 2.2 eV for the emissive MLCT
excited state (Figure S7b).
Table 1. Electrochemical Potentials (in V vs SCE) in the
3
Electronic Ground State and in the Long-Lived MLCT
Excited State
E0 (M+/M0)
E0 (M+/*M0)
a
[Mo(LtBu)3]
−0.08
−0.02
−0.24
−0.32
0.77
−2.3
−2.2
a b
,
[Mo(LMe)3]
c
[Mo(CN-C6H5)6]
[Mo(CN-2,6‑iPrC6H5)6]
fac-[Ir(ppy)3]
[Mo(LMe)3]) that is structurally and electronically similar to
[Ru(bpy)3]2+ (bpy = 2,2′-bipyridine).38 Sterically demanding
groups in α-position to the ligating isocyanides usually stabilize
low-valent transition metal species, rendering them less
susceptible to electrophilic attack.71,81 We therefore reasoned
that bulkier tert-butyl (rather than methyl) substituents could
minimize interactions between the Mo(0) center and the
solvent, as well as rigidifying the complex, potentially leading
to decreased nonradiative 3MLCT deactivation. In the
resulting new [Mo(LtBu)3] complex (Scheme 1b), this is
indeed the case, as our detailed comparative photophysical
study with [Mo(LMe)3] will show. We had previously used LtBu
to obtain a luminescent Cr(0) complex,37 and the synthesis of
LMe was reported in our initial Mo(0) communication.38 The
new [Mo(LtBu)3] complex was prepared by stirring a solution
of [MoCl2(THF)4] and 3 equiv of LtBu in THF over Na/Hg
(see Supporting Information page S2 for details). The complex
was isolated as a red powder that can be handled under air.
The CN stretch frequencies in the free LMe and LtBu
ligands are 2113 and 2112 cm−1 (Figure S4), respectively. In
the [Mo(LMe)3] and [Mo(LtBu)3] complexes, the CN
vibrations are broad and intense, and their maxima are shifted
to 1939 and 1951 cm−1, respectively, due to π-backbonding.82
Electrochemistry. In cyclic voltammetry, oxidation of
Mo(0) to Mo(I) in deaerated THF with 0.1 M TBAPF6 results
in quasi-reversible waves from which potentials of −0.02 and
−0.08 V vs saturated calomel electrode (SCE) can be
determined for [Mo(LMe)3] and [Mo(LtBu)3], respectively
(Figure 1). These potentials are shifted anodically by 0.2−0.3
V relative to those for previously reported Mo(0) complexes
with monodentate arylisocyanides (Table 1).83
c
d
−1.7
a
b
This work, measured in THF with 0.1 M TBAPF6 (Figure 1). From
c
ref 38, measured in THF with 0.1 M TBAPF6. From ref 83, measured
in THF with 0.1 M TBAPF6 versus Ag/AgCl. From ref 84.
d
chelate complexes.85 It seems likely that the latter effect is
dominant, because the bite angle of the chelating LMe and LtBu
ligands is expected to become less favorable with increasing
oxidation state due to decreasing Mo−C bond distances.
Sweeps over greater potential ranges than those in Figure 1
produce lower-quality voltammograms, and ligand reduction
was undetectable. However, the observable Mo(0/I) redox
wave is the most important one, because it is directly relevant
for the photochemistry of [Mo(LMe)3] and [Mo(LtBu)3]
presented below.
UV−Vis Absorption, Luminescence, and Transient
Absorption Spectroscopy. The UV−vis spectra of [Mo-
(LMe)3] and [Mo(LtBu)3] exhibit MLCT absorptions between
350 and 550 nm (Figure 2a and b) that cause the orange−red
color of these complexes. Ligand-based π−π* transitions
appear at shorter wavelengths, and thus the overall spectra are
reminiscent of that of isoelectronic [Ru(bpy)3]2+.36 However,
the MLCT absorptions in [Mo(LMe)3] and [Mo(LtBu)3] are
broader and have higher extinction coefficients by factors of
approximately 1.5 and 2.0, respectively.
Upon excitation at 500 nm, both Mo(0) complexes
luminesce in deaerated solution at room temperature (Figure
2c and d). The emission is broad and unstructured, and there
is a sizable red shift of the emission band maximum when
increasing the solvent polarity from n-hexane to toluene and
THF (540 cm−1 in total for [Mo(LMe)3] and 410 cm−1 for
[Mo(LtBu)3]). These observations are compatible with MLCT
In principle, this shift could be due to either stabilization of
the Mo(0) state or destabilization of the Mo(I) form in the
B
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Figure 2. UV−vis absorption spectra of (a) [Mo(LMe)3] and (b)
[Mo(LtBu)3] in THF at 20 °C. Normalized photoluminescence
spectra of (c) [Mo(LMe)3] and (d) [Mo(LtBu)3] in deaerated solvents
at 20 °C. Excitation occurred at 500 nm.
Figure 3. Transient absorption spectra measured after excitation of
10−5 M solutions of (a) [Mo(LMe)3] and (b) [Mo(LtBu)3] in
deaerated toluene at 532 nm with laser pulses of ∼10 ns duration. The
signals were time-integrated over 200 ns immediately after excitation.
Luminescence decays (c, e) recorded from these solutions at 595/615
nm and recoveries of the MLCT bleaches (d, f) at 485 nm. See
Figures S8 and S10 for analogous data recorded in other solvents.
emission, in analogy to [Ru(bpy)3]2+. A key finding is that the
luminescence quantum yield (ϕ) of the [Mo(LtBu)3] complex
is much higher than that of [Mo(LMe)3] in all investigated
solvents. For example, in deaerated toluene at 20 °C, the
complex with the more sterically demanding tert-butyl-
substituted diisocyanide ligand exhibits ϕ = 0.203, whereas
the complex with the methyl substituents only has ϕ = 0.023
(Table 2). This shows that the aforementioned design
principle, according to which better shielding of the metal
center from the chemical environment and enhanced complex
rigidity should lead to improved photophysical properties, is
indeed successful. [Ru(bpy)3]2+ has a luminescence quantum
yield of 0.095 under optimized conditions in deaerated
CH3CN, more than a factor of 2 lower than [Mo(LtBu)3].86
Such strong luminescence is remarkable for a d6 MLCT
emitter made from an Earth-abundant transition metal and
compares very favorably to most Cu(I) diimine com-
plexes.43,87−89 The latter represent an extremely well
investigated class of compounds for which similar lumines-
cence quantum yields required many optimization attempts.
Time-resolved luminescence and transient UV/vis absorp-
tion spectroscopy were used to explore the decay character-
istics of the emissive excited states and to confirm their MLCT
nature. The transient absorption spectra in Figure 3a and b
were averaged over a period of 200 ns immediately after
excitation at 532 nm with laser pulses of ca. 10 ns duration. In
both cases a negative signal (ground-state bleach) is detected
near 500 nm, coincident with the lowest-energy 1MLCT
absorption bands in Figure 2a and b. Additionally, excited-state
absorption bands near 345 nm are observed for both
complexes, reminiscent of the absorption bands near 370 nm
transitions on the transiently reduced bpy ligand.90 Such short-
wavelength excited-state absorption bands combined with
ground-state 1MLCT bleaches are in fact diagnostic features of
MLCT excited states in photoactive Ru(II) and Fe(II)
complexes.22,31
As expected, the MLCT luminescence at 595/615 nm and
the MLCT bleach at 485 nm exhibit the same decay for a given
complex (Figure 3c/d and e/f), confirming that the same
excited state is monitored by emission and transient absorption
spectroscopies. However, the MLCT lifetimes (τ) extractable
from these data sets are markedly different for [Mo(LtBu)3] and
[Mo(LMe)3] in all investigated solvents (Figure S10). For
instance, in deaerated toluene at 20 °C, τ = 166 ns for
[Mo(LMe)3], while for the [Mo(LtBu)3] complex, a biexponen-
tial decay with τ1 = 1110 ns (85%) and τ2 = 2330 ns (15%) is
observed (Table 2). Thus, the factor-of-10 increase in
luminescence quantum yield when going from [Mo(LMe)3]
to [Mo(LtBu)3] (see earlier) is accompanied by an equivalent
increase in excited-state lifetime, compatible with the view that
the rate for radiative decay from the emissive MLCT states is
similar in both complexes, whereas the rate for nonradiative
decay is decreased by an order of magnitude in [Mo(LtBu)3].
This is additional confirmation for the success of the design
concept outlined in the Introduction, and it is evident that the
bulkier tert-butyl substituents lead to much improved photo-
physical properties relative to the methyl substituents.
3
for MLCT-excited [Ru(bpy)3]2+, which are caused by π−π*
3
Table 2. MLCT Excited-State Lifetimes (τ) and Luminescence Quantum Yields (ϕ) in Deaerated Solvents at 20 °C
n-hexane
τ2 (ns)
2370 (24%)
toluene
τ2 (ns)
2330 (15%)
THF
τ2 (ns)
τ1 (ns)
ϕ
τ1 (ns)
ϕ
τ1 (ns)
ϕ
a
[Mo(LtBu)3]
1040 (76%)
225 (100%)
0.190
0.045
1110 (85%)
166 (100%)
0.203
0.023
610 (73%)
74 (100%)
1610 (27%)
0.058
0.006
b
[Mo(LMe)3]
a
b
This work, extracted from the data in Figure S10; determined using [Ru(bpy)3]2+ in aerated CH3CN as standard (ϕ = 0.018).86 From ref 38.
C
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In all investigated solvents, the luminescence decays and
MLCT bleach recoveries are consistently monoexponential for
[Mo(LMe)3], while for [Mo(LtBu)3] biexponential kinetics were
observed in all experiments. The biexponential decays are
tentatively attributed to the presence of different conformers in
the latter. In our previously published crystal structure of
[Cr(LtBu)3], the three LtBu ligands are not equivalent,37 and
interconversion between different conformations seems to be
sterically hindered by the tert-butyl substituents. In [Mo-
(LMe)3], the methyl substituents impose less hindrance, and
interconversion between different conformers can therefore
occur on a faster time scale, leading to single-exponential
MLCT decays. Related dynamic interconversions have been
observed for some Ru(II) complexes.91
nucleophiles over extended irradiation times, rather than to
develop new photoredox reactions. Toward this end, we
concentrated on net redox-neutral cross-coupling between aryl
halides and arenes, which can be considered electron-catalyzed
reactions.39 A specific example is the intramolecular reaction
between an aryl iodide and pyrrole (Scheme 2). Base-
Scheme 2. Intramolecular Base-Promoted Homolytic
Aromatic Substitution (BHAS) via Photoredox Catalysis
Going from toluene to THF, the MLCT lifetimes and
luminescence quantum yields decrease in parallel in both
complexes (Table 2). Qualitatively, this can be understood in
the framework of the energy gap law,92 because the MLCT
state is energetically stabilized in more-polar THF (red shifts
observable in Figure 2c and d), leading to a smaller energy gap
to the ground state and consequent faster nonradiative
relaxation. Different propensities for energy dissipation via
molecular vibrations of the various solvents could further
contribute to the observable lifetime and quantum yield
changes between n-hexane, toluene, and THF. It is likely that
the solvent nucleophilicity also plays a role with respect to the
excited-state lifetime, especially for [Mo(LMe)3], with more
nucleophilic solvents more efficiently deactivating the excited
state. However, nucleophilicity and energy-gap law effects are
difficult to disentangle in low-polarity solvents. The MLCT
lifetimes of both complexes are strongly oxygen-sensitive,
confirming the triplet nature of these excited states.
promoted homolytic aromatic substitution (BHAS) reactions
of this type have been identified as a viable alternative to Pd-
and Rh-based C−C bond formation, but they often require
harsh conditions involving high temperatures and strong
bases.95−100 Photochemical approaches to BHAS not relying
on UV excitation have been relatively scarce so far.101−106
First, we tested the performance of the [Mo(LMe)3] complex
as a photocatalyst for the reaction in Scheme 2, which can be
1
conveniently followed by monitoring the benzylic H NMR
resonances of substrate and product (Figure S13). Using 50
mM substrate 1a in deaerated C6D6, 2 equiv of 2,2,6,6-
tetramethylpiperidine (TMP) as base, and a photocatalyst
loading of 5 mol % in a flame-sealed NMR tube at room
temperature, 19% of the substrate was converted to product 1b
after irradiation at 470 nm with a light-emitting diode (LED)
(ca. 14 W; see Supporting Information page S16 for details)
for 1 h (Table 3, entry 1). Maximum conversion of 31% was
For the photoredox investigations below, the excited-state
oxidation potential (Eox*) is relevant, and this in turn requires
Table 3. Performance of [Mo(LMe)3], [Mo(LtBu)3], and fac-
3
[Ir(ppy)3] in the Light-Driven BHAS Reaction of Scheme
knowledge of the energy of the photoactive MLCT state.
a
2
Therefore, in addition to the emission experiments reported
earlier, luminescence spectra were measured at 77 K (Figure
S7) to estimate the energy of the electronic origin (E00) of the
3MLCT emission more accurately. This procedure yields an
entry substrate
complex
irradiation time/h conversion/%
1
2
3
4
5
1a
1a
1a
1a
2a
[Mo(LMe)3]
[Mo(LMe)3]
[Mo(LtBu)3]
fac-[Ir(ppy)3]
[Mo(LtBu)3]
1
18
1
1
18
19
31
100
4
E
00 value of 2.2 eV for both complexes, leading to excited-state
oxidation potentials of −2.2 and −2.3 V vs SCE for
[Mo(LMe)3] and [Mo(LtBu)3], respectively, as indicated in
Figure 1c and d (gray shaded areas). Thus, both complexes
should be far stronger photoreductants than isoelectronic
[Ru(bpy)3]2+ (Eox* = −0.9 V vs SCE) and fac-[Ir(ppy)3] (Eox*
= −1.73 V vs SCE).84
7
a
Conditions: 50 mM substrate, 5 mol % metal complex, and 2 equiv
of 2,2,6,6-tetramethylpiperidine (TMP) in deaerated C6D6 at ca. 40
°C. Irradiation at 470 nm with 14 W LED. See Supporting
Information pages S16−S19.
Photochemical Studies. In our preliminary communica-
tion we reported that the [Mo(LMe)3] complex is able to
photocatalyze the rearrangement of an acylcyclopropane
substrate to a 2,3-dihydrofuran product,38 but we noticed
that the presence of nucleophiles in the reaction mixture
caused degradation of the Mo complex over time. We
speculated that this degradation is caused by ligation of
nucleophiles to the metal center in Mo(I) intermediates
formed transiently in the course of the photocatalysis reaction
cycle; seven-coordinate molybdenum isocyanide complexes are
well-known.93,94 We will now show that in the new
[Mo(LtBu)3] complex this problem no longer persists,
presumably because the bulkier tert-butyl substituents more
efficiently shield the metal from the chemical environment.
The main purpose of the studies below was to explore the
photochemical properties of this new family of d6 MLCT
emitters and to test their robustness in the presence of
observed after 18 h (Table 3, entry 2; Figure S13). Catalyst-
based luminescence could not be detected after this time,
implying decomposition of [Mo(LMe)3]. The new [Mo(LtBu)3]
complex, however, catalyzed the reaction to completion within
1 h (Table 3, entry 3; Figure S14). In comparison, use of fac-
[Ir(ppy)3] (Eox* = −1.73 V vs SCE) under identical conditions
resulted in nearly negligible conversion over the same time
period (Table 3, entry 4; Figure S15), and reactions did not
reach conclusion even after >100 h of continuous irradiation.
As fac-[Ir(ppy)3] absorbs weakly at wavelengths longer than
450 nm and may therefore create a biased comparison, the
experiments with fac-[Ir(ppy)3] were repeated under 405 nm
irradiation, which showed little difference to the results
obtained with 470 nm irradiation (Figure S16).
D
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t
The photochemical BHAS reaction is initiated by a reductive
dehalogenation step (see below), and the promising results
observed above for [Mo(LtBu)3] encouraged us to explore the
reaction of Scheme 2 with a bromo-substituted substrate (2a),
which is more difficult to reduce than the iodoarene of 1a.
However, only very modest conversion was observable, and it
required extremely long irradiation times (Table 3, entry 5;
Figure S17). Over the course of these experiments, we found
that the [Mo(LtBu)3] complex still exhibited stable photo-
luminescence and was catalytically active after continuous
irradiation exceeding 2 months.
presence of a weak base (TMP) rather than BuOK, and it
seems plausible that this renders deprotonation of intermediate
B inefficient, making oxidation of B to C necessary before the
t
proton can be abstracted by TMP. With BuOK the reverse
order of steps is commonly observed (deprotonation followed
by oxidation), and this can give rise to a radical chain
mechanism because the radical anion intermediate formed
after the deprotonation step is strongly reducing (see
Supporting Information page S15 for further details).100 In
the mechanism of Scheme 3, the neutral radical intermediate B
is not sufficiently reducing to propagate a radical chain via
electron donation to another equivalent of substrate.
A proposed mechanism for the photochemical BHAS
reaction with [Mo(LtBu)3] is presented in Scheme 3. Due to
To gain further insight into the photochemical performance
of [Mo(LtBu)3], we explored the carefully selected range of
substrates in Table 4, allowing us to probe the influence of
Scheme 3. Proposed Mechanism for the Light-Driven Base-
Promoted Homolytic Aromatic Substitution (BHAS)
a
Reaction of Scheme 2
Table 4. BHAS Reactions To Explore the Photochemical
a
Reactivity of [Mo(LtBu)3]
a
Mo(0), *Mo(0), and Mo(I) denote the [Mo(LtBu)3] complex in its
3
initial ground state, its long-lived MLCT-excited state, and its one-
electron oxidized form, respectively. TMP is 2,2,6,6-tetramethylpiper-
idine.
its high reducing power (Eox* = −2.3 V vs SCE, Figure 1d), the
3MLCT-excited Mo(0) complex is able to induce the initial
reductive dehalogenation of iodobenzene substrate 1a (Ered
≈
a
Conditions were similar to those given in Table 3, but using a
−2.2 eV)107 in an efficient manner. In contrast, the
photoexcited fac-[Ir(ppy)3] complex is substantially less
reducing (Eox* = −1.73 V vs SCE),84 and this is likely the
main reason for its comparatively poor performance as a
catalyst of the reaction in Scheme 2 (Table 3, entry 4).
Bromobenzene has a reduction potential of ca. −2.4 V vs SCE
(in dimethylformamide (DMF));107 hence, reductive dehalo-
genation of substrate 2a becomes inefficient even for
photoexcited [Mo(LtBu)3] (Table 3, entry 5), especially in
nonpolar C6D6. Photodriven reductive dehalogenation reac-
tions are increasingly well-investigated,108−111 and the general
observation is that aryl bromides are substantially more
difficult to reduce than aryl iodides,107,112 and aryl chlorides
are even more challenging.106,113−115 The liberation of I− in
this initial step (and ligation of I− to Mo(I), see earlier)38 is
likely responsible for the rather rapid degradation of [Mo-
(LMe)3] and the modest conversion achievable with this first-
generation complex (Table 3, entries 1 and 2). Aryl radicals
usually have very short lifetimes, but in the specific case of aryl
radical A (Scheme 3), intramolecular reaction with the radical
interceptor pyrrole to form the cyclic intermediate B is rather
efficient.116 Oxidation of intermediate B by Mo(I) then
reinstates the photosensitizer to its initial (electronic ground)
state and leads to radical cation C, which can be subsequently
deprotonated to afford the final cyclization product. An
important difference to thermal BHAS reactions is the
photoreactor with active cooling to stabilize the temperature of the
reaction mixture at 25 °C throughout the complete irradiation
process. See Supporting Information pages S16−S21 for details.
structural and electronic variations on the light-driven BHAS
reaction in a systematic manner. Substrate 3a differs from 1a
by the length of the alkyl linker, and the resulting increased
conformational flexibility causes a need for longer reaction
times (Table 4, entries 1 and 2; Figure S19). Substrate 4a has
the same ethylene linker as 3a, but the reaction time had to be
lengthened further from 4 to 145 h to reach a similar level of
conversion (Table 4, entry 3; Figure S20). This can be
attributed to the fact that tert-butylphenyl is a far less efficient
interceptor for the aryl radical than pyrrole,116 rendering the
step from A to B in Scheme 3 inefficient. Substrate 5a (Table
4, entry 4) has essentially the same linker length as 4a, but one
CH2 group has been replaced by an O atom. This makes the
reaction even slower, now requiring 450 h instead of 145 h
(Figure S21). The reason for this is likely the electron-
donating nature of the alkoxy linker, rendering the reduction
potential of the iodobenzene unit more negative and making
reductive deiodination in the initial reaction step more difficult.
In substrate 6a (Table 4, entry 5) the alkoxy linker is reversed
compared to 5a, with the O atom connected to the tert-
butylphenyl rather than the iodobenzene unit. This reduces the
E
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J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Article
necessary reaction time to 7 h (for 81% conversion; Figure
S22), likely because the electron-donating nature of the alkoxy
linker now mostly acts on the tert-butylphenyl unit, making the
deiodination of 6a less difficult and the oxidation step
converting intermediate B to C (Scheme 3) more facile. The
reactivities observable for the range of substrates 1a−6a thus
seem compatible with the mechanism in Scheme 3. Evidently
the [Mo(LtBu)3] complex is much more robust in these
photochemical experiments than [Mo(LMe)3], making the new
tert-butylated complex far superior not just in terms of
photophysical properties (see earlier) but also for photo-
chemical applications involving challenging reduction steps not
feasible with [Ru(bpy)3]2+ or fac-[Ir(ppy)3].
AUTHOR INFORMATION
Corresponding Author
*oliver.wenger@unibas.ch
ORCID
■
Oliver S. Wenger: 0000-0002-0739-0553
Author Contributions
†P.H., F.G., and L.A.B. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
Funding from the Swiss National Science Foundation through
Grant no. 200021_178760 is gratefully acknowledged.
■
SUMMARY AND CONCLUSIONS
■
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ASSOCIATED CONTENT
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