Design of Luminescent Isocyano Rhenium(I) Complexes: Photophysics and Effects of the Ancillary Ligands

Article abstract of DOI:10.1021/acs.inorgchem.8b02536

Despite the well-reported MLCT [dπ(M) → π?(CNR)] transitions in the isocyano transition metal complexes, emissive complexes with phosphorescence derived from MLCT [dπ(M) → π?(CNR)] were not extensively studied. To provide insights into the design strategy

Full text of DOI:10.1021/acs.inorgchem.8b02536

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Design of Luminescent Isocyano Rhenium(I) Complexes:
Photophysics and Effects of the Ancillary Ligands
Kin-Cheung Chan,† Ka-Ming Tong,† Shun-Cheung Cheng, Chi-On Ng, Shek-Man Yiu,
and Chi-Chiu Ko*
Department of Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong, China
S
* Supporting Information
ABSTRACT: Despite the well-reported MLCT [dπ(M) →
π*(CNR)] transitions in the isocyano transition metal
complexes, emissive complexes with phosphorescence derived
from MLCT [dπ(M) → π*(CNR)] were not extensively
studied. To provide insights into the design strategy of
phosphorescent rhenium(I) complexes with an emissive
3MLCT [dπ(Re) → π*(CNR)] excited state, a series of
pentaisocyano rhenium(I) complexes have been synthesized.
In contrast to most of the reported penta- or hexaisocyano
rhenium(I) complexes with unsubstituted or alkyl- or
monohalo-substituted phenylisocyanide ligands, which only
exhibit photoluminescence in 77 K glassy medium, the
solutions of all of these complexes were found to show phosphorescence at room temperature. Detailed study on their emission
properties revealed that they are derived from the 3MLCT [dπ(Re) → π*(CNR)] excited state mixed with LL′CT character. It
has been shown that the strong electron-withdrawing substituents on the isocyanide ligands can lower the energy of the MLCT
[dπ(Re) → π*(CNR)] state and raise the deactivating ligand-field state. These effects are the crucial criteria to render the
pentaisocyano rhenium(I) complexes emissive. Moreover, the emission properties in terms of energy, lifetime, and quantum
yields can also be enhanced by the ancillary ligand.
only display phosphorescence in 77 K glassy medium, but they
are generally nonemissive at room temperature.3a,5
To understand the deactivation of the MLCT [dπ(Re) →
INTRODUCTION
■
Transition metal complexes with metal-to-ligand charge transfer
(MLCT) excited state have been extensively reported because
they found applications in many different areas.1 Most of the
MLCT excited states showing rich photophysical and photo-
chemical behavior are designed based on the π-conjugated
multidentate ligands; study of the emissive MLCT excited state
of monodentate ligands is much less reported.2 Previously, we
have designed readily tunable luminescent rhenium(I) diimine
luminophores with isocyanide ligands.3 Our spectroscopic study
has revealed that these isocyano rhenium(I) diimine complexes
exhibit both MLCT [dπ(Re) → π*(CNR)] and [dπ(Re) →
π*(diimine)] transitions.3 As the MLCT [dπ(Re) → π*-
(diimine)] exited state is lower-lying, the phosphorescence of
these complexes is derived from the MLCT [dπ(Re) →
π*(diimine)] exited state. Although the emissive MLCT
[dπ(M) → π*(CNR)] excited state was reported in the
1970s,2a−c luminescent complexes and their outstanding
photoredox properties derived from this type of excited state
only gained popularity in recent years owing to the development
of metal complexes with bidentate isocyanide ligands by Wenger
and co-workers.4 Surprisingly, rhenium(I) complexes with these
bidentate isocyanide ligands are not strongly luminescence.4g
Similarly, most of the reported penta- and hexaisocyano
rhenium(I) complexes with monodentate isocyanide ligands
3
π*(CNR)] excited state and design phosphorescent rhenium(I)
3
complexes derived from the MLCT [dπ(Re) → π*(CNR)]
excited state, a series of rhenium(I) complexes with mono-
dentate isocyanide ligands of diverse electronic natures
{[Re(CNR)5X]; CNR = 2,4,6-Cl3C6H2NC, 4-(SF5)C6H4NC,
3,5-(CF3)2C6H3NC, and 4-(EtOOC)C6H4NC; X = I, CN,
CNB(C6F5)3} have been synthesized. The detailed photo-
physical and electrochemical properties of these complexes have
also been reported.
RESULTS AND DISCUSSION
■
Synthesis and Characterization. The substituted phenyl-
isocyanide ligands with electron-withdrawing substituents were
synthesized from their corresponding formamide by adopting
the method developed by Luo et al.,13 in which triphenylphos-
phine and iodine were used for dehydration. Compared to the
most commonly used dehydration reactions, as presented by
Ugi et al.,12 this method is more effective in terms of the reaction
yields as well as the purity of the crude products. These
advantages are particularly noticeable in the preparation of
Received: September 6, 2018
© XXXX American Chemical Society
A
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Scheme 1. Synthetic Route for Pentaisocyano Rhenium(I) Complexes
phenylisocyanides with electron-withdrawing substituents, such
as trifluoromethyl and pentafluorosulfanyl groups.
from linearity. This is likely attributed to the π-back-bonding
interaction between the rhenium metal center and isocyanide
ligands, as well as the steric repulsion between the adjacent
isocyanide ligands. A careful analysis of the bonding parameters
of the axial isocyanide ligand and the equatorial isocyanide
ligands revealed that the average Re−C distance of equatorial
isocyanide ligands (2.03 Å) is slightly longer than that of axial
isocyanide ligands (1.98 Å). Moreover, the average CN−C
bond angle of equatorial isocyanides (169.4°) is slightly larger
than that of axial isocyanide ligands (167.6°). This is suggestive
of a stronger π-back-bonding interaction from the metal center
toward the axial ligand because the corresponding trans ligand
(anionic ligand) is competing to a relatively lesser extent for π-
back-bonding interaction, due to the weaker π-accepting ability
of the anionic ligands, compared to the neutral isocyanide
ligands.3a−c
UV−vis Spectroscopy. The electronic absorption proper-
ties of the pentaisocyano rhenium(I) complexes (1−4) in
dichloromethane have been investigated. Their UV−vis
absorption spectra are shown in Figure 2, and the absorption
data are summarized in Table 1. All of the complexes showed
intense absorptions in the high-energy UV region (λ ≤ 350 nm),
with an extinction coefficient in the order of 103 dm3 mol−1
cm−1, which are assigned to be the LC (π → π*) transitions of
substituted phenylisocyanide ligands.5b,8 Apart from the very
intense LC transitions, the complexes also display poorly
resolved absorption shoulders in the near UV−visible region
(379−440 nm), which are assigned to MLCT [dπ(Re) →
π*(CNR)] transitions as similar to those of penta- and
hexaisocyano rhenium(I) complexes,5b isocyano rhenium(I)
diimine complexes,3 and homoleptic isocyano complexes.2a−c
The sensitivity of this absorption shoulder to the polarity of the
solvent medium is in agreement with the charge transfer nature
of the MLCT transition (Figure S1). Although the absorptivity
of MLCT transitions is usually in the order of 104 dm3 mol−1
cm−1, the much larger extinction coefficient of these MLCT
[dπ(Re) → π*(CNR)] transitions can be rationalized by the
additive effect of the MLCT transitions of the five isocyanide
ligands as well as the overlapping with the slightly higher energy
LC transitions. This assignment is in agreement with the
computation and electrochemical results.
All isocyano rhenium(I) complexes were prepared from the
ligand substitution reactions of the hexaiodorhenate(IV)
precursor [ReI6]2−, with corresponding isocyanide ligands and
the subsequent reduction reaction by hydrazine (Scheme 1).
With the iodo pentaisocyano rhenium(I) complex precursors
(1a−4a), the iodide ligand can be readily replaced through the
halide abstraction with AgOTf and the coordination of another
ancillary ligand, such as cyanide, to afford the cyano complexes
(1b−4b). The isocyanotris(pentafluorophenyl)borato com-
plexes (1c−4c) can be prepared by the reactions between the
cyano complexes with tris(pentafluorophenyl)borane in di-
chloromethane under an inert atmosphere using a similar
procedure to that for other isocyanoborato complexes.3g,6
Further purification of these complexes was achieved by the
slow diffusion of n-hexane or n-pentane into concentrated
dichloromethane or diethyl ether solutions of the complexes,
which give analytically pure complexes as yellow to colorless
crystals. All of the complexes have been characterized by 1H and
19F NMR spectroscopy, IR spectroscopy, mass spectrometry,
and elemental analysis. Complexes 1c, 2a, 2c, 3c, and 4a were
also structurally characterized by X-ray crystallography.
IR and NMR Spectroscopy. The cyanide, isocyanotris-
(pentafluorophenyl)borate, and isocyanide ligands were char-
acterized by the IR active CN stretches in the range of 2034−
2202 cm−1, which are in the typical range of these ligands
reported in other related complexes.6,7 These complexes were
also characterized by 1H and 19F NMR spectroscopy. With a C4
symmetry of the complexes, the four equatorial isocyanide
ligands are chemically equivalent but in a different chemical
environment compared to the axial isocyanide ligand. Therefore,
1
two sets of H and 19F NMR signals of the isocyanide ligands
with the integral ratio of 4:1 were observed in the spectra of
these complexes. The octahedral coordination geometry and the
arrangement of the isocyanide ligands in 1c, 2a, 2c, 3c, and 4a
were unambiguously confirmed by the X-ray crystal structures.
X-ray Crystal Structure Determination. The perspective
drawings of 1c, 2a, 2c, 3c, and 4a are shown in Figure 1. The
crystal structure determination data, and selected bond
distances and bond angles are summarized in Tables S1, S2
and Tables S3−S7 (Supporting Information), respectively. In
these crystal structures, the complexes adopted an octahedral
geometry with the bond angles at the rhenium metal center (L−
Re−L′) close to 90° or 180°. As for the bond angles CN−C of
isocyanide and isocyanoborate ligands, they are in the range of
157.3−177.8° and 164.9−174.8°, respectively, and deviated
The assignment of the lowest energy absorption shoulders to
the MLCT [dπ(Re) → π*(CNR)] transitions is also supported
by the sensitivity of the absorption energy of these shoulders
toward changing the ancillary ligand. In general, these MLCT
absorptions are gradually blue-shifted upon increasing the π-
accepting ability of the ancillary anionic ligand from iodide to
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Figure 1. Perspective drawings of (a) 1c, (b) 2a, (c) 2c, (d) 3c, and (e) 4a with atomic labeling. Hydrogen atoms have been omitted for clarity.
Thermal ellipsoids are shown at the 30% probability level.
cyanide and to isocyanoborate. This is reflected in the trends of
the energy of these absorption shoulders in the order of 1a (440
nm) < 1b (400 nm) < 1c (383 nm); 2a (400 nm) < 2b (396 nm)
< 2c (381 nm); 3a (400 nm) < 3b (391 nm) < 3c (379 nm); 4a
(417 nm) < 4b (408 nm) < 4c (397 nm) (Figure 2). These
trends can be rationalized by the better stabilization of dπ(Re)
by the ancillary ligand with better π-accepting ability (iodide <
cyanide < isocyanoborate). On the other hand, these MLCT
absorption shoulders for complexes with the same ancillary
ligand also show the expected energy dependence on the π-
accepting ability of isocyanide ligands (Figure 3).
have been investigated. In contrast to the reported pentaiso-
cyano rhenium(I) complexes,3a,5b such as complexes 5a−7a,
which are nonemissive in the solution state at room temperature,
the solutions of complexes 1−4 with different anionic ancillary
ligands [I (1a−4a), CN (1b−4b), and CNB(C6F5)3 (1c−4c)]
display blue to yellow phosphorescence with a structureless
emission band (Figure 4) upon excitation with λ < 400 nm. The
emission data are summarized in Table 2. These emission
properties are assigned to phosphorescence mainly derived from
3
the MLCT [dπ(Re) → π*(CNR)] state mixed with LL′CT
character. The assignment is supported by the energy depend-
ency on the π-accepting ability on the anionic ancillary ligand as
well as the solvent dependency (Figure S2), similar to the
situation regarding MLCT absorptions.
Emission Spectroscopy. The emission properties of the
pentaisocyano rhenium(I) complexes in dichloromethane
solution at 298 and 77 K EtOH/MeOH (4:1 v/v) glass medium
C
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Figure 2. Overlaid UV−vis absorption spectra of (a) 1a−1c, (b) 2a−
2c, (c) 3a−3c, and (d) 4a−4c in dichloromethane at 298 K.
Figure 4. Normalized emission spectra of (a) 1a−1c, (b) 2a−2c, (c)
3a−3c, and (d) 4a−4c at 298 K in dichloromethane.
Table 1. UV−vis Absorption Data for 1a−1c, 2a−2c, 3a−3c,
and 4a−4c in CH2Cl2 Solution
Table 2. Emission Data for 1a−1c, 2a−2c, 3a−3c, 4a−4c, and
5a−7a
absorption λabs/nm (ε/M−1 cm−1
)
medium (T/K) emission λem/nm (τo) quantum yield Φem × 103
1a 253 (69 780), 302 (52 495), 343 (65 670), 368 sh (49 005), 399 sh (28
315), 440 sh (13 035)
a
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
CH2Cl2 (298)
510 (<10 ns)
492 (3.9 μs)
480 (13 ns)
458 (7.3 μs)
454 (111 ns)
440 (16.9 μs)
518 (<10 ns)
502 (6 μs)
487 (<10 ns)
463 (6.7 μs)
448 (<10 ns)
437 (7.3 μs)
502 (<10 ns)
489 (4.2 μs)
471 (<10 ns)
463 (4.4 μs)
440 (<10 ns)
440 (4.9 μs)
560 (29 ns)
514 (6.4 μs)
529 (210 ns)
475 (7.6 μs)
473 (400 ns)
451 (8.7 μs)
478 (3.5 μs)
471 (2.5 μs)
469 (2.5 μs)
1.2
2.0
9.2
0.1
0.2
0.6
0.1
0.1
0.2
5.5
1b 248 sh (68 345), 255 (74 795), 264 (63 135), 338 (79 155), 360 sh (65
b
glass (77)
565), 400 sh (36 555)
a
CH2Cl2 (298)
1c 253 (81 065), 328 (81 250), 350 sh (63 560), 373 sh (46 430),
b
383 sh (37 060)
glass (77)
a
2a 269 sh (41 035), 339 (47 885), 359 sh (40 900), 400 sh (15 095)
2b 261 sh (43 475), 335 (58 850), 353 sh (54 555), 396 sh (26 760)
CH2Cl2 (298)
b
glass (77)
a
2c 254 sh (54 020), 324 (66 070), 349 (58 635), 365 sh (51 205),
CH2Cl2 (298)
381 sh (28 340)
b
glass (77)
3a 275 (48 590), 336 (56 470), 362 sh (43 090), 400 sh (15 745)
3b 263 (42 950), 330 (59 935), 349 sh (53 810), 391 sh (26 315)
3c 258 (55 190), 321 (67 575), 346 (58 220), 379 sh (27 090)
4a 273 sh (59 700), 360 (70 575), 417 sh (26 985)
a
CH2Cl2 (298)
b
glass (77)
a
CH2Cl2 (298)
b
glass (77)
4b 246 (87 895), 267 sh (57 140), 369 (74 065), 408 sh (44 395)
a
CH2Cl2 (298)
4c 244 (92 320), 262 sh (70 025), 345 (74 040), 363 (75 040),
b
glass (77)
397 sh (37 425)
a
CH2Cl2 (298)
5a 244 (80 510), 273 (68 400), 329 (84 505), 361 sh (48 200),
b
glass (77)
398 sh (18 510)
a
CH2Cl2 (298)
6a 235 (56 880), 275 (61 740), 319 (66 720), 356 sh (27 240),
b
377 sh (16 420)
glass (77)
a
7a 241 (66 860), 269 (53 145), 325 (74 340), 361 sh (32 375),
CH2Cl2 (298)
381 sh (22 195)
b
glass (77)
a
CH2Cl2 (298)
14
72
b
glass (77)
a
CH2Cl2 (298)
b
glass (77)
glass (77)
glass (77)
glass (77)
c
b
5a
6a
7a
c
c
b
b
a
b
c
Excitation at λ < 400 nm. EtOH/MeOH (4:1 v/v). Nonemissive in
CH2Cl2 solution at 298 K.
corresponding lower-lying nonradiative LF excited state, as
compared to that of 1a−4a. The difference in their emission
properties can be explained by the much stronger π-accepting
properties of isocyanide ligands in 1−4 [CNC6H2Cl3-2,4,6;
CNC6H4(SF5)-4; CNC6H3(CF3)2-3,5; and CNC6H4(COOEt)-
4] than those in 5a−7a [CNC6H4Cl-4, CNC6H4F-4, and
CNC6H3(CH3)2-2,6]. The increase in the π-accepting ability of
isocyanide ligands in these complexes can lower the MLCT
[dπ(Re) → π*(CNR)] energy. The effect can be visualized by
comparing the MLCT [dπ(Re) → π*(CNR)] absorption
Figure 3. Overlaid UV−vis absorption spectra of 1a−4a, [Re-
(CNC6H4Cl-4)5I] (5a),3a [Re(CNC6H4F-4)5I] (6a),3a and [Re-
(CNC6H4(CH3)2-2,6)5I] (7a)3a in dichloromethane solution at 298
K. Inset shows the MLCT absorption shoulders and tailings of these
complexes in the region of 450−510 nm.
The nonemissive nature of 5a−7a can be attributed to the
higher MLCT energy, which can be efficiently quenched by the
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(Figure 3), the low-temperature MLCT phosphorescence of
these complexes (Table 2, Figure S3), and computational study
(see below). On the other hand, the LF splitting should also be
increased by five stronger π-accepting isocyanides in these
complexes.9 As a result of these effects, the pentaisocyano
rhenium(I) complexes with strong π-accepting isocyanides
become emissive in the solution state at room temperature.
Similarly, the increase in the π-accepting properties of the
anionic ancillary ligand raises not only the energy of MLCT
[dπ(Re) → π*(CNR)] but also the nonemissive deactivating LF
excited state,6b leading to the enhancement of their emission
properties. In general, the emission of the complexes becomes
blue-shifted with an increase in quantum yield when the anionic
ancillary ligand changes from iodide to stronger π-accepting
cyanide and to the strongest π-accepting isocyanoborate. The
increase in the emission lifetime is also observed in the series of
1a−1c and 4a−4c; however, the change in the emission lifetime
along the series of 2a−2c and 3a−3c cannot be experimentally
confirmed as their lifetimes are too short to be measured with
our instrument. It is also worth noting that the isocyanoborato
complexes 1c−4c show a deep blue and structureless emission,
which is not easily achievable with other phosphorescent
transition metal complex systems. The emission of these
complexes is higher in energy than those of isocyanoborato
rhenium(I) diimine complexes,6 which is consistent with the
higher-lying MLCT [dπ(Re) → π*(CNR)] excited state than
the MLCT [dπ(Re) → π*(diimine)].3,5b,6
mixed MLCT and ligand-centered emissive excited state, which
generally exhibit a structured emission band, the emission bands
of these complexes remain structureless at low temperature. This
is due to the much higher energy of the ligand-centered excited
state in these isocyano rhenium(I) complexes, as the substituted
phenylisocyanides are much less conjugated than the bidentate
diimine or cyclometalated ligands.
Electrochemistry. The electrochemical properties of 1−4
have been investigated by cyclic voltammetry. The data are
summarized in Table 3, and the representative cyclic voltammo-
Table 3. Electrochemical Data of 1a−1c, 2a−2c, 3a−3c, and
a
4a−4c in Acetonitrile Solution (0.1 M nBu4NPF6) at 298 K
b
b
oxidation E1/2/V vs SCE
reduction E1/2/V vs SCE
c
c
complex
[ΔEp/mV] (Epa/V vs SCE)
[ΔEp/mV] (Epc/V vs SCE)
d
e
1a
1b
1c
2a
2b
2c
3a
3b
3c
4a
4b
4c
(+0.95)
(+1.15)
(−1.48)
(−1.61)
(−1.65)
(−1.69)
(−1.74)
(−1.76)
(−1.62)
(−1.83)
(−1.78)
(−1.60)
(−1.67)
(−1.76)
d
e
e
+1.33 [70]
d
e
(+0.76)
e
+0.89 [64]
+1.15 [68]
e
d
e
(+0.86)
e
+0.95 [73]
+1.20 [71]
e
d
e
(+0.67)
e
+0.77 [75]
+1.06 [73]
e
In 77 K EtOH/MeOH (4:1 v/v) glass medium, complexes 1−
4 with different ancillary ligands also display very intense
photoluminescence upon excitation with λ < 400 nm. The
emission of these complexes is blue-shifted relative to their room
temperature emission, which is probably due to the rigid-
ochromism commonly observed in other MLCT phosphor-
escent rhenium(I) emitters.1g,f,3,6,10 The emission of these
complexes also follows the same energy dependence on the
change of ancillary ligand (Figure 5), similar to that of the room
a
b
Working electrode: glassy carbon; scan rate, 100 mV s−1. E1/2 is
(Epa + Epc)/2, where Epa and Epc are peak anodic and peak cathodic
potentials, respectively. ΔEp is |Epa − Epc|. Irreversible oxidation;
anodic peak potential (Epa/V vs SCE). Irreversible reduction;
c
d
e
cathodic peak potential (Epc/V vs SCE).
Figure 6. Cyclic voltammograms of the (a) oxidative scan and (b)
reductive scan of 3c in acetonitrile solution (0.1 M nBu4NPF6) with
scan rate of 100 mV s−1.
Figure 5. Normalized emission spectra of (a) 1a−1c, (b) 2a−2c, (c)
3a−3c, and (d) 4a−4c in 77 K EtOH/MeOH (4:1 v/v) glass medium.
grams of 3c are shown in Figure 6. According to the oxidative
scan, all of the complexes display a quasi-reversible or
irreversible oxidation in the range of +0.67 to +1.33 V vs SCE.
This is attributed to the metal-based Re(I/II) oxidation.3−6 The
oxidation potentials are in the order of isocyanoborato
complexes (1c−4c) > cyano complexes (1b−4b) > iodo
complexes (1a−4a). With the same ancillary ligand, the
oxidation potentials are in line with the π-accepting ability of
temperature emission. Therefore, they are also attributed to a
3
predominantly MLCT [dπ(Re) → π*(CNR)] excited state
origin.5b Complexes 5a−7a also show MLCT phosphorescence
(Figure S3, Table 2), which is higher in energy compared to
those of complexes 1a−4a, in the low temperature glass
medium. In contrast to the most blue-emitting luminescent
polypyridyl or cyclometalated complexes that derived from a
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isocyanide ligands {(2,4,6-Cl3C6H2NC) > [3,5-(CF3)2C6H3-
NC] > (4-(SF5)C6H4NC) > [4-(EtOOC)C6H4NC]}. These
trends are due to better stabilization of the dπ(Re) with better π-
accepting ligands. In the reductive scan, these complexes display
irreversible reduction in the range of −1.48 to −1.83 V vs SCE.
These reductions are tentatively assigned to the ligand-centered
reduction in isocyanide.
The potential difference between the metal-centered
oxidation and the isocyanide ligand-centered reduction can be
used to reflect the energy of the MLCT [dπ(Re) → π*(CNR)]
state.11 A linear correlation between the emission energy and the
potential difference in the plot of the emission energy against
potential difference (Figure 7) further supports the emission
origins of these complexes derived from the 3MLCT [dπ(Re) →
π*(CNR)] excited state.
Figure 8. Computed alpha MO energies and the contour surfaces of
SOMO electron densities for the T1 state of 1a (left) and 5a (right).
lower-energy SOMO, i.e., the predominant dπ(Re) orbital. As a
consequence, the triplet MLCT [dπ(Re) → π*(CNR)] energy
is lowered for complexes with better π-accepting isocyanide
ligands.
On the other hand, computation results also revealed that
there is a higher contribution of the d-orbital character of the
rhenium in the higher-lying SOMO of 5a compared to those in
1a−4a, 4b, and 4c (Table S9, Supporting Information). This
suggests the involvement or mixing of an LF excited state
character in the lowest-lying triplet state of 5a, which accounts
for the effective quenching of the emissive MLCT state. As a
result, 5a is nonemissive at room temperature. It is also worth
noting that the contribution of the rhenium d-orbital in the
higher-lying SOMO of 4a−4c is in the order of 4a > 4b > 4c.
This corroborates that the replacement of the iodo ancillary
ligand with the stronger π-accepting cyano or isocyanoborato
ancillary ligand could destabilize the deactivating LF excited
state and explains the enhanced emission properties for
complexes with better π-accepting anionic ancillary ligand.
Figure 7. A plot of Eem (298 K, CH2Cl2) versus potential difference of
the metal-centered oxidation and the isocyanide ligand-based reduction
with linear least-squares fit.
Computational Studies. To have a better understanding of
the nature of the emissive excited state of these pentaisocyano
rhenium(I) complexes and the influence of the electronic effects
of isocyanide ligands, DFT calculations on selected iodo
complexes (1a−5a) have been carried out. To gain insights
into the effects of the anionic ancillary ligand on various
electronic states, DFT calculations on 4b and 4c have also been
performed. Although the computed energy gaps between the
optimized T1 and S0 states of the complexes are underestimated
compared to the experimental values, the trend along these
complexes is consistent with that of the experimental results
(Table S8, Supporting Information). The contour maps of the
electron density of singly occupied molecular orbitals (SOMOs)
of complexes 1a−5a, 4b, and 4c are shown in Figure S4
(Supporting Information). According to the contour maps, the
lower-energy SOMOs of all of these complexes mainly consist of
the dπ orbital of rhenium mixed with some contribution from
the π orbital of the ancillary ligand, whereas the higher-energy
SOMOs are mostly contributed by the π* orbitals of the
isocyanide ligands. These results are consistent with the
CONCLUSION
■
To conclude, a series of charge-neutral luminescent pentaiso-
cyano rhenium(I) complexes {[Re(CNR)5X]; CNR = 2,4,6-
Cl3C6H2NC, 4-(SF5)C6H4NC, 3,5-(CF3)2C6H3NC, and 4-
(EtOOC)C6H4NC; X = I, CN, CNB(C6F5)3} have been
synthesized and characterized by IR spectroscopy, 1H NMR, 19F
NMR spectroscopy, ESI-mass spectrometry, and elemental
analysis.
In contrast to most of the reported penta- or hexaisocyano
rhenium(I) complexes with unsubstituted or alkyl- or
monohalo-substituted phenylisocyanide ligands, which only
exhibit emission in 77 K glassy medium, the solutions of all of
these complexes were found to show photoluminescence at
room temperature. Detailed photophysical and electrochemical
study together with DFT calculation revealed that the emission
3
of these complexes is derived from the MLCT [dπ(Re) →
π*(CNR)] excited state mixed with some LL′CT character. The
presence of strong electron-withdrawing substituents on the
isocyanide ligands, which can lower the MLCT [dπ(Re) →
π*(CNR)] and raise the LF deactivating state, is found to be
crucial in the design of emissive isocyano rhenium(I) complexes.
On the other hand, the emission properties in terms of energy,
lifetime, and quantum yields can also be enhanced by the anionic
ancillary ligand. The emission colors of these pentaisocyano
rhenium(I) complexes ranged from deep blue to yellow, and
importantly, their emission bands remain structureless in the
blue region.
3
assignment of the predominantly MLCT [dπ(Re) → π*-
(CNR)] emission origin with LL′CT character.
The influence of the electronic properties of the isocyanide
ligands on the lowest triplet state can be elucidated from the
calculated MO energy-level diagrams of 5a and 1a (Figure 8).
The increase in the π-accepting ability of isocyanide ligands from
(4-ClC6H4NC) in 5a to (2,4,6-Cl3C6H2NC) in 1a should lead
to the stabilization of both SOMOs. Due to the direct
attachment of electron-withdrawing substituents to the
isocyanide ligands, the higher-energy SOMO (mainly the
π*(CNR) orbital) is significantly more stabilized than the
This work highlights the importance of the ligand design on
the energy of various electronic states of the transition metal
F
DOI: 10.1021/acs.inorgchem.8b02536
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.47 (d, J = 7.3 Hz,
8H, m-phenyl H’s of the equatorial isocyanide ligands), 8.01 (d, J = 8.1
Hz, 2H, o-phenyl H’s of the axial isocyanide ligand), 8.07 (d, J = 7.3 Hz,
8H, o-phenyl H’s of the equatorial isocyanide ligands). IR (KBr disc), v/
cm−1: 1717 ν(CO); 2060 (br), 2159 ν(CN). ESI−MS: m/z 1227
[M + K]+. Elemental analyses, Calcd for 4a (found) %: C 50.51 (50.81)
H 3.81 (3.91) N 5.89 (5.85).
General Synthetic Procedure for [Re(CNR)5CN]. A mixture of
[Re(CNR)5I] (500 mg, 1.0 mol equiv), KCN (1.2 mol equiv), and
AgOTf (1.2 mol equiv) in degassed ethanol (20 mL) were heated at 40
°C overnight under an inert atmosphere of argon. The resulting mixture
was filtered, and the solvent was removed under reduced pressure. The
residue was purified by column chromatography on silica gel using a
mixture of dichloromethane and acetone as eluent.
[Re(CNC6H2Cl3-2,4,6)5(CN)] (1b). Yield: 307 mg, 0.25 mmol, 66%.
1H NMR (400 MHz, CDCl3, 298 K): δ 7.33 (s, 2H, m-phenyl H’s of the
axial isocyanide ligand), 7.38 (s, 8H, m-phenyl H’s of the equatorial
isocyanide ligands). IR (KBr disc), v/cm−1: 2034 (br), 2163 ν(CN).
ESI−MS: m/z 1283 [M + K]+. Elemental analyses, Calcd for 1b·
0.5Et2O (found) %: C 35.61 (35.82) H 1.18 (1.14) N 6.56 (6.94).
{Re[CNC6H4(SF5)-4]5(CN)} (2b). Yield: 252 mg, 0.19 mmol, 54%. 1H
NMR (300 MHz, CDCl3, 298 K): δ 7.33 (d, J = 8.9 Hz, 2H, m-phenyl
H’s of the axial isocyanide ligand), 7.47 (d, J = 8.5 Hz, 8H, m-phenyl H’s
of the equatorial isocyanide ligands), 7.79 (m, 10H, o-phenyl H’s of the
axial and equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ
82.89 (m, 5F, axial F’s of SF5), 63.05 (m, 20F, equatorial F’s of SF5).
ESI−MS: m/z 1397 [M + K]+. IR (KBr disc), v/cm−1: 2051 (br), 2165
ν(CN). Elemental analyses, Calcd for 2b·0.5C6H14 (found) %: C
33.43 (33.33) H 1.94 (1.95) N 6.00 (6.12).
{Re[CNC6H3(CF3)-3,5]5(CN)} (3b). Yield: 275 mg, 0.20 mmol, 59%.
1H NMR (300 MHz, CDCl3, 298 K): δ 7.74 (s, 2H, o-phenyl H’s of the
axial isocyanide ligand), 7.80 (s, 1H, p-phenyl H’s of the axial isocyanide
ligand), 7.88 (s, 12H, o- and p-phenyl H’s of the equatorial isocyanide
ligands). 19F NMR (282 MHz, CDCl3) δ −63.14 (s, 24F, CF3 F’s of the
equatorial isocyanide ligands), −63.18 (s, 6F, CF3 F’s of the axial
isocyanide ligand). IR (KBr disc), v/cm−1: 2060 (br), 2130 ν(CN).
ESI−MS: m/z 1446 [M + K]+. Elemental analyses, Calcd for 3b
(found) %: C 39.24 (39.18) H 1.07 (1.38) N 5.97 (5.89).
{Re[CNC6H4(COOEt)-4)]5(CN)} (4b). Yield: 210 mg, 0.19 mmol, 46%.
1H NMR (300 MHz, CDCl3, 298 K): δ 1.40 (m, 15H, CH3 H’s of the
ethyl groups), 4.38 (m, 10H, CH2 H’s of the ethyl groups), 7.31 (d, J =
8.3 Hz, 2H, m-phenyl H’s of the axial isocyanide ligand), 7.44 (d, J = 8.4
Hz, 8H, m-phenyl H’s of the equatorial isocyanide ligands), 8.06 (m,
10H, o-phenyl H’s of the axial and equatorial isocyanide ligands). ESI−
MS: m/z 1127 [M + K]+. IR (KBr disc), v/cm−1: 1717 ν(CO); 2056
(br), 2164 ν(CN). Elemental analyses, Calcd for 4b (found) %: C
56.29 (56.29) H 4.17 (4.39) N 7.72 (7.46).
General Procedure for [Re(CNR)5CNB(C6F5)3]. A mixture of
[Re(CNR)5(CN)] (50 mg, 1.0 mol equiv) and B(C6F5)3 (1.2 mol
equiv) was dissolved in anhydrous dichloromethane (20 mL) under an
inert atmosphere of argon. The resulting solution was stirred overnight
at room temperature. After removing the solvent under reduced
pressure, the residue was purified by column chromatography on silica
gel using a solvent mixture of hexane and dichloromethane as eluent.
Further purification was achieved by the slow diffusion of pentane or
hexane into the concentrated dichloromethane or diethyl ether
solutions of the complexes to yield yellow to white crystalline solids
depending on the nature of the isocyanide ligands.
complexes. Judicious functionalization of the isocyanide ligands
could lead to the design of luminescent isocyano rhenium(I)
complexes from the MLCT [dπ(Re) → π*(CNR)] excited
state. Moreover, it also provides strategies to tune the energy of
the emissive excited state and improve the phosphorescent
properties in terms of quantum yield and lifetime. This should
lead to a new class of luminescent materials developed from the
transition metal isocyanides.
3
EXPERIMENTAL SECTION
■
Materials and Reagents. Rhenium metal powder, silver
trifluoromethanesulfonate (AgOTf), and potassium cyanide (KCN)
were purchased from International Laboratory Company Ltd. Tris-
(pentafluorophenyl)borane [B(C6F5)3] and hydrazine hydrate were
purchased from Acros Organics. These reagents were used without
further purification. All solvents were of analytical reagent grade and
were used without further purification. Potassium hexaiodorhenate(IV)
and substituted phenylisocyanides 2,4,6-Cl3C6H2NC, 4-SF5C6H4NC,
3,5-(CF3)2C6H3NC, and 4-(EtOOC)C6H4NC were prepared accord-
ing to our previously reported procedures3a,d using the methods
developed by Ugi12 and Luo.13 The previously reported pentaisocyano
rhenium(I) complexes, Re(CNC6H4Cl-4)5I] (5a), [Re(CNC6H4F-
4)5I] (6a), and [Re(CNC6H4(CH3)2-2,6)5I] (7a) were prepared
according to our reported procedure.3a
General Synthetic Procedure for [Re(CNR)5I] (1a−4a). These
complexes were synthesized based on our previously reported
procedures.3a,d To an acetone (10 mL) solution of K2[ReI6] (500
mg, 1.0 mol equiv) at 0 °C, isocyanide ligand (CNR) (5.5 mol equiv)
was added. The resulting solution was stirred overnight at room
temperature. After removing the solvent under reduced pressure, the
residue was redissolved in methanol (10 mL). Hydrazine hydrate (2
mL) was slowly added to the resulting mixture and stirred for 30 min.
Thereafter, dichloromethane (30 mL) was added and washed with
water (10 mL × 3). After removal of solvent, the residue was purified by
column chromatography on silica gel using a mixture of petroleum ether
and dichloromethane as eluent. Further purification was achieved by
the slow diffusion of pentane or hexane into concentrated dichloro-
methane or diethyl ether solutions of the complexes to give yellow
crystalline solids.
1
[Re(CNC6H2Cl3-2,4,6)5I] (1a). Yield: 440 mg, 0.33 mmol, 67%. H
NMR (300 MHz, CDCl3, 298 K): δ 7.28 (s, 2H, m-phenyl H’s of the
axial isocyanide ligand), 7.38 (s, 8H, m-phenyl H’s of the equatorial
isocyanide ligands). IR (KBr disc), v/cm−1: 2041 (br), 2160 ν(CN).
ESI−MS: m/z 1384 [M + K]+. Elemental analyses, Calcd for 1a
(found) %: C 31.25 (31.51) H 0.75 (1.10) N 5.21 (5.29).
1
{Re[CNC6H4(SF5)-4]5I} (2a). Yield: 270 mg, 0.19 mmol, 38%. H
NMR (300 MHz, CDCl3, 298 K): δ 7.23 (d, J = 9.0 Hz, 2H, m-phenyl
H’s of the axial isocyanide ligand), 7.50 (d, J = 9.0 Hz, 8H, m-phenyl H’s
of the equatorial isocyanide ligands), 7.73 (d, 2H, J = 9.0 Hz, o-phenyl
H’s of the axial isocyanide ligand), 7.80 (d, J = 9.0 Hz, 8H, o-phenyl H’s
of the equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ
83.67 (m, 5F, axial F’s of SF5), 63.30 (m, 20F, equatorial F’s of SF5). IR
(KBr disc), v/cm−1: 2054 (br), 2157 ν(CN). ESI−MS: m/z 1458 [M
+ H]+. Elemental analyses, Calcd for 2a·0.5Et2O (found) %: C 29.71
(29.89) H 1.68 (1.38) N 4.68 (4.92).
1
{Re[CNC6H3(CF3)-3,5]5I} (3a). Yield: 448 mg, 0.30 mmol, 61%. H
NMR (300 MHz, CDCl3, 298 K): δ 7.63 (s, 2H, o-phenyl H’s of the
axial isocyanide ligand), 7.70 (s, 1H, p-phenyl H’s of the axial isocyanide
ligand), δ 7.86 (s, 4H, p-phenyl H’s of the equatorial isocyanide
ligands), 7.88 (s, 8H, o-phenyl H’s of the equatorial isocyanide ligands).
19F NMR (282 MHz, CDCl3) δ −63.14 (s, 24F, CF3 F’s of the
equatorial isocyanide ligands), −63.18 (s, 6F, CF3 F’s of the axial
isocyanide ligand). IR (KBr disc), v/cm−1: 2065 (br), 2129 ν(CN).
ESI−MS: m/z 1508 [M + H]+. Elemental analyses, Calcd for 3a·
0.5Et2O (found) %: C 36.52 (36.56) H 1.30 (1.21) N 4.53 (4.56).
{Re[CNC6H4(COOEt)-4)]5I} (4a). Yield: 210 mg, 0.18 mmol, 36%. 1H
NMR (300 MHz, CDCl3, 298 K): δ 1.39 (m, 15H, CH3 H’s of the ethyl
groups), δ 4.37 (m, 10H, CH2 H’s of the ethyl groups), δ 7.23 (d, J = 8.1
{Re(CNC6H2Cl3-2,4,6)5[CNB(C6F5)3]} (1c). Yield: 55 mg, 0.03 mmol,
78%. 1H NMR (400 MHz, CDCl3, 298 K): δ 7.38 (s, 2H, m-phenyl H’s
of the axial isocyanide ligand), 7.40 (s, 8H, m-phenyl H’s of the
equatorial isocyanide ligands). 19F NMR (376 MHz, CDCl3) δ
−133.25 (dd, J = 23.3, 7.9 Hz, 6F, o-phenyl F’s of the isocyanoborate),
−160.02 (t, J = 20.4 Hz, 3F, p-phenyl F’s of the isocyanoborate),
−165.55 (td, J = 22.6, 7.8 Hz, 6F, m-phenyl F’s of the isocyanoborate).
IR (KBr disc), v/cm−1: 2052 (br), 2153, 2202 ν(CN). ESI−MS: m/z
1796 [M + K]+. Elemental analyses, Calcd for 1c·0.5Et2O (found) %: C
37.50 (37.71) H 0.84 (0.75) N 4.69 (4.72).
{Re[CNC6H4(SF5)-4]5[CNB(C6F5)3]} (2c). Yield: 36 mg, 0.02 mmol,
52%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.39 (d, J = 8.8 Hz, 8H, m-
G
DOI: 10.1021/acs.inorgchem.8b02536
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
phenyl H’s of the equatorial isocyanide ligands), 7.44 (d, J = 8.9 Hz, 2H,
m-phenyl H’s of the axial isocyanide ligand), 7.82 (m, 10H, o-phenyl
H’s of the axial and equatorial isocyanide ligands). 19F NMR (282 MHz,
CDCl3) δ 84.09 (m, 5F, axial F’s of SF5), 62.86 (m, 20F, equatorial F’s
of SF5), −133.86 (dd, J = 23.8, 8.6 Hz, 6F, o-phenyl F’s of the
isocyanoborate), −158.83 (t, J = 20.4 Hz, 3F, p-phenyl F’s of the
isocyanoborate), −164.84 (td, J = 23.1, 7.8 Hz, 6F, m-phenyl F’s of the
isocyanoborate). IR (KBr disc), v/cm−1: 2057 (br), 2148, 2198 ν(C
N). ESI−MS: m/z 1909 [M + K]+. Elemental analyses, Calcd for 2c·
0.5Et2O (found) %: C 35.27 (34.94) H 1.32 (1.54) N 4.41 (4.13).
{Re[CNC6H3(CF3)-3,5]5[CNB(C6F5)3]} (3c). Yield: 30 mg, 0.02 mmol,
44%. 1H NMR (300 MHz, CDCl3, 298 K): δ 7.80 (s, 8H, o-phenyl H’s
of the equatorial isocyanide ligands), 7.82 (s, 2H, o-phenyl H’s of the
axial isocyanide ligand), 7.88 (s, 1H, p-phenyl H’s of the axial isocyanide
ligand), 7.91 (s, 4H, p-phenyl H’s of the equatorial isocyanide ligands).
19F NMR (282 MHz, CDCl3) δ −63.11 (s, 6F, CF3 F’s of the axial
isocyanide ligand), −63.34 (s, 24F, CF3 F’s of the equatorial isocyanide
ligands), −134.54 (dd, J = 23.1, 8.0 Hz, 6F, o-phenyl F’s of the
isocyanoborate), −158.66 (t, J = 20.3 Hz, 3F, p-phenyl F’s of the
isocyanoborate), −164.96 (td, J = 22.1, 7.6 Hz, 6F, m-phenyl F’s of the
isocyanoborate). IR (KBr disc), v/cm−1: 2064 (br), 2127, 2206 ν(C
N). ESI−MS: m/z 1959 [M + K]+. Elemental analyses, Calcd for 3c·
0.5Et2O (found) %: C 40.51 (40.53) H 1.03 (0.88) N 4.29 (4.39).
{Re[CNC6H4(COOEt)-4)]5[CNB(C6F5)3]} (4c). Yield: 30 mg, 0.02
reference. All solutions for electrochemical studies were deaerated with
prepurified nitrogen gas prior to measurements.
X-ray Crystal Structure Determination. The crystal structures
were determined on an Xcalibur, Sapphire3, Gemini Ultra diffrac-
tometer using graphite monochromatized Cu Kα (λ = 1.54178 Å)
radiation. The structures were solved by direct methods employing the
SHELXL-97 program15 on a PC. Re and many non-H atoms were
located according to direct methods. The positions of other non-H
atoms were found after successful refinement by full-matrix least-
squares using the SHELXL-97 program on a PC.15 All non-H atoms
were refined anisotropically in the final stage of least-squares
refinement. The positions of hydrogen atoms were calculated based
on riding mode with thermal parameters equal to 1.2 times that of the
associated carbon atoms, and participated in the calculation of final R
indices.
Computational Details. All calculations were done using the
Gaussian 09 package, version B.01.16 The structures of the ground state
(S0) and the lowest triplet state (T1) of the complexes 1a−4a, 4b, 4c,
and [Re(CNC6H4Cl-4)5I] (5a)3a were optimized using Density
Functional Theory (DFT) with the B3LYP hybrid functional.17
A
combination of 6-31+G(d) (for elements H, B, C, N, O, F, S, and Cl)
and LANL2DZ18 (for Re and I) basis sets was employed. Vibrational
analysis was performed to ensure that all optimized structures are
located at potential energy minima. The effect of the solvent was taken
account by the polarized continuum model with integral equation
formalism (IEF-PCM).19
1
mmol, 41%. H NMR (300 MHz, CDCl3, 298 K): δ 1.41 (t, J = 7.1
Hz, 15H, CH3 H’s of the ethyl groups), 4.41 (d, J = 7.1 Hz, 10H, CH2
H’s of the ethyl groups), 7.37 (d, J = 8.1 Hz, 2H, m-phenyl H’s of the
axial isocyanide ligand), 8.09 (m, 10H, o-phenyl H’s of the axial and
equatorial isocyanide ligands). 19F NMR (282 MHz, CDCl3) δ
−133.76 (dd, J = 23.9, 8.3 Hz, 6F, o-phenyl F’s of the isocyanoborate),
−159.30 (t, J = 20.5 Hz, 3F, p-phenyl F’s of the isocyanoborate),
−165.03 (td, J = 23.1, 8.0 Hz, 6F, m-phenyl F’s of the isocyanoborate).
IR (KBr disc), v/cm−1: 1722 ν(CO), 2066 (br); 2144, 2193 ν(C
N). ESI−MS: m/z 1639 [M + K]+. Elemental analyses, Calcd for 4b·
0.5CH2Cl2 (found) %: C 50.82 (50.73) H 2.82 (3.09) N 5.12 (4.86).
Physical Measurements and Instrumentation. 1H and 19F
NMR spectra were recorded on a Bruker AV300 (300 MHz) or AV400
(400 MHz) FT-NMR spectrometer. Chemical shifts (δ, ppm) were
reported relative to tetramethylsilane (Me4Si). IR spectra were
obtained from KBr discs by using a PerkinElmer Spectrum 100 FTIR
spectrophotometer. All ESI mass spectra were recorded on a PE-SCIEX
API-3200 single quadrupole mass spectrometer. Elemental analyses of
all compounds were performed on an Elementar Vario MICRO Cube
elemental analyzer. Electronic absorption spectra were recorded on a
Hewlett-Packard 8452A diode array spectrophotometer. Steady-state
emission at room temperature and at 77 K were recorded on an
Edinburgh Instruments FLS980 fluorescence spectrometer and LP920
laser flash photolysis system with a flashlamp pumped Q-switched
Nd:YAG laser using 355 nm excitation; lifetimes were measured with
the photomultiplier tube of the instruments. Measurements of the
EtOH/MeOH (4:1 v/v) glass samples at 77 K were carried out with
dilute EtOH/MeOH sample solutions contained in a quartz tube inside
a liquid nitrogen-filled quartz optical Dewar flask. Solutions were
rigorously degassed on a high-vacuum line in a two-compartment cell
with no less than four successive freeze pump-thaw cycles. The
luminescence quantum yields were determined by using the optical-
dilution method as described by Demas and Crosby using quinine
sulfate in aqueous sulfuric acid (0.05 M) as reference standard.14 Cyclic
voltammetry measurements were performed by using a CH instrument,
Inc., Model CHI 620 Electrochemical Analyzer. Electrochemical
measurements were performed in acetonitrile solutions with 0.1 M
nBu4NPF6 as the supporting electrolyte at room temperature with a
glassy carbon electrode (CH Instruments, Inc.) with a diameter of 0.3
mm as a working electrode, a platinum wire coil as the counter
electrode, and a Ag/AgNO3 (0.1 M in acetonitrile) electrode (CH
Instruments, Inc.) as the reference electrode. The working electrode
surface was polished with an 1 μm α-alumina slurry and then a 0.3 μm
α-alumina slurry (Linde) on a microcloth (Buehler Co.). The
ferrocenium/ferrocene couple (FeCp2+/0) was used as the internal
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02536.
■
S
Crystal and structure determination data, selected bond
lengths and bond angles, computed energy gaps between
the optimized T1 and S0 states, and contour maps of the
electron density of SOMOs of complexes 1a−5a, 4b, and
4c (PDF)
Accession Codes
CCDC 1822767−1822771 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
AUTHOR INFORMATION
Corresponding Author
*E-mail: vinccko@cityu.edu.hk. Phone: (+852)-3442-6958.
Fax: (+852)-3442-0522.
■
ORCID
Chi-Chiu Ko: 0000-0002-2426-3206
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work has been supported by the Hong Kong University
Grants Committee Area of Excellence Scheme (AoE/P-03-08)
and General Research Fund (Project Nos. CityU 11303515 and
CityU 11306217) from the Research Grants Council of the
Hong Kong SAR, China. K.-C.C. acknowledges receipt of a
University Postgraduate Studentship and a Research Tuition
Scholarship administrated by City University of Hong Kong.
H
DOI: 10.1021/acs.inorgchem.8b02536
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Inorganic Chemistry
Article
strong reductant for photoredox catalysis. Angew. Chem., Int. Ed. 2016,
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55, 11247−11250. (b) Buldt, L. A.; Wenger, O. S. Chromium(0),
molybdenum(0), and tungsten(0) isocyanide complexes as lumino-
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