Structure, Spectroscopy, and Electrochemistry of Manganese(I) and Rhenium(I) Quinoline Oximes

Article abstract of DOI:10.1021/acs.inorgchem.8b02862

Reactions of α- and β-diimine quinoline aldoximes with Mn(I) and Re(I) tricarbonyl halides afford quinoline oxime complexes. Both Mn(I) and Re(I) complexes experience severe geometric strain due to ligand steric interactions: 6-membered metallocycles exhibit more pronounced distortions than 5-membered ones, consistent with density functional theory structural analyses. Such distortions likely also affect reactivity patterns, as evidenced by Re(I)-induced deoximation of a quinoline variant containing a CF₃-ketoxime.

Full text of DOI:10.1021/acs.inorgchem.8b02862

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Structure, Spectroscopy, and Electrochemistry of Manganese(I) and
Rhenium(I) Quinoline Oximes
†
†
†
,†
Danh X. Ngo, Wesley W. Kramer, Brendon J. McNicholas, Harry B. Gray,*
,
†,‡
and Bradley J. Brennan*
†
Division of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, California 91125, United States
School of Molecular Sciences, Arizona State University, Tempe, Arizona 85287, United States
‡
*
S Supporting Information
ABSTRACT: Reactions of α- and β-diimine quinoline aldoximes with Mn(I) and Re(I)
tricarbonyl halides afford quinoline oxime complexes. Both Mn(I) and Re(I) complexes
experience severe geometric strain due to ligand steric interactions: 6-membered
metallocycles exhibit more pronounced distortions than 5-membered ones, consistent
with density functional theory structural analyses. Such distortions likely also affect
reactivity patterns, as evidenced by Re(I)-induced deoximation of a quinoline variant
containing a CF -ketoxime.
3
INTRODUCTION
RESULTS AND DISCUSSION
■
■
Quinoline is a redox-active ligand that can function as an
electron acceptor, similar to bipyridine (bpy). Quinoline
analogues are synthetically attainable, providing routes to
tunable ligands, and amine condensation reactions offer a
second imine for enhanced metal binding. Starting from a 2- or
Synthesis. Quinoline oxime ligands were prepared from
quinoline aldehydes (Schemes 1−3). Scheme 1 depicts the
synthesis of ligand 4 and its corresponding Re(I) complex
1
11
(
Re4im). Addition of TMS-CF to quinoline-8-carbaldehyde,
3
catalyzed by K CO , affords the TMS-protected alcohol 1, and
2
3
cleavage of the TMS protecting group with TBAF yields
alcohol 2. Oxidation of 2 with 2-iodoxybenzoic acid (IBX)
affords the hydrated ketone 3, likely owing to the electron-
8
-quinoline aldehyde analogue, the resulting quinoline diimine
has an α- or β-diimine structure. With hydroxylamine as the
amine, quinoline oximes with a diimine and a pendent proton
donor can be obtained. Metal complexes with diimine/oxime
ligands have been employed as electrocatalysts for hydrogen
evolution reaction (HER) and CO reduction.
work highlights how pendent proton donors on bpy and
related diimine ligands enhance the catalytic efficiencies of
Mn(I) and Re(I) complexes.
We investigated redox-active, substituted quinoline diimines
12
withdrawing nature of the CF group. Reaction of 3 with
3
hydroxylamine yields oxime ligand 4. Quinoline-2-carbalde-
hyde oxime (5) and quinoline-8-carbaldehyde oxime (6) were
synthesized from quinoline-2-carbaldehyde and quinoline-8-
carbaldehyde, respectively, by reaction of hydroxylamine in
ethanol in the presence of sodium acetate (Schemes 2 and 3).
Upon coordination to Re(I) using Re(CO) Cl, 4 undergoes
spontaneous deoximation to form Re4im, where the hydroxyl
group has been replaced by a hydrogen atom. H NMR of
Re4im exhibited a 1:1:1 triplet at δ = 5.96 ppm with J = 53.2
2
3−8
Recent
2
3
−5
5
containing a pendent proton donor for comparisons to bpy-
1
6
−8
based frameworks.
Employing a quinoline oxime diimine
scaffold, we obtained Mn(I) and Re(I) complexes and
investigated their structural, electronic, and electrochemical
properties. Notably, our density functional theory (DFT)/
time-dependent DFT (TDDFT) calculations have shed light
on structural distortions in the ground states as well as shifts in
MLCT absorption bands in the visible spectra of the
complexes. Interestingly, an efficient deoximation reaction
14
Hz, characteristic of hydrogen coupled to the N S = 1
13
nucleus. Mass spectrometry confirmed loss of the hydroxyl
group. Attempts to prepare complexes of 4 with Mn(I) using
Mn(CO) Br were unsuccessful, as an inseparable mixture of
5
products was obtained. Crude NMR and IR data suggest that
the desired Mn(I) diimine oxime product could be present
along with a deoximated decomposition product.
occurred upon binding of a CF -ketoxime to Re(I), yielding a
Reactions of ligand 5 with Mn(CO) Br and Re(CO) Cl
3
5
5
primary imine. This decomposition reaction, first discovered
afforded complexes Mn-5 and Re-5, respectively. Mn-6 and
Re-6 were prepared in a similar manner using ligand 6.
by Fischer and Czarkie, did not occur in the aldoxime
9
,10
analogue. Based on DFT analysis of ligand-induced strain in
the Re(I) complexes, we suggest how this relatively rare
transformation is promoted.
Received: October 13, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02862
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
a
Scheme 1. Synthesis of Re-4im
a
(
a) Me SiCF (1.5 equiv), 1 mol % K CO , DMF, rt; (b) TBAF (1.2 equiv), THF, rt; (c) IBX (2 equiv), EtOAc, reflux; (d) NH OH·HCl (2
3
3
2
3
2
equiv), pyridine (4 equiv), EtOH, reflux; (e) Re(CO) Cl, toluene, 95 °C.
5
a
Scheme 2. Syntheses of Mn-5 and Re-5
a
(
a) NH OH·HCl (2 equiv), NaOAc (4 equiv), EtOH, reflux; (b) Mn(CO) Br, Et O, reflux; (c) Re(CO) Cl, toluene, 90 °C.
2
5
2
5
a
Scheme 3. Syntheses of Mn-6 and Re-6
a
(
a) NH OH·HCl (2 equiv), NaOAc (4 equiv), EtOH, rt; (b) Mn(CO) Br, Et O, reflux; (c) Re(CO) Cl, toluene, 90 °C.
2
5
2
5
Notably, 5 and 6, which lack the CF moiety of 4, retain the
oxime functional group after coordination. Oxime proton H
lengths are given in Table 1. No intramolecular hydrogen
bonding was observed in the crystal structures. The most
notable discrepancy in bond length is for the M−N bond,
where the oxime complexes have longer bonds than their bpy
analogues due to weaker σ-donating capabilities.
3
1
NMR shifts for the free ligands 5 and 6 are 9.19 and 8.31 ppm,
respectively. Downfield shifts of OH proton resonances from
those for the parent ligands were observed for the Re(I)
complexes, while upfield shifts were seen for the Mn(I)
analogues.
The structure of Re4im is that of the deoximation product
1
indicated by H NMR and mass spectrometry, while the oxime
Metal-assisted deoximation of oxime ligands to a variety of
products, including primary imines, has been reported
functional group remains intact in the structures of Mn-5, Re-
5, Mn-6, and Re-6. An interesting feature of the structures is
the substantial out-of-plane distortion of the chelated quinoline
from the equatorial plane of the metal complexes (Δ, Table 2,
Figure 2).
9
,10,14−17
previously.
While definitive mechanisms have not
been established, it has been suggested that the electrophilicity
16,18,19
of the oxime moiety is a key factor in deoximation.
Our
results favor this proposal, as the electron withdrawing CF3
moiety (as well as nitrogen coordination to the metal center)
in the quinoline oxime Re4im would increase the electro-
philicity of the oxime carbon atom. The lack of an electron-
withdrawing substituent in the aldoxime analogue Re-6
appears to disfavor deoximation.
Previously observed to a lesser degree in Re(I) biquinoline
complexes (16.8°), this distortion is due to the steric
interaction of the aromatic protons of the quinoline ring
with the CO ligands as noted by Kurz et al. The steric clash,
which is apparent in space-filling models (Figure 2), causes
deformation of the ligand upon chelation. The quinoline
ligands in Mn-6 (36.6°) and Re-6 (34.1°) are distorted out of
the equatorial plane more so than those of Mn-5 (26.2°) and
20
Molecular Structures. Structures of Re4im, Mn-5, Re-5,
Mn-6, and Re-6 are shown in Figure 1, and relevant bond
B
DOI: 10.1021/acs.inorgchem.8b02862
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Inorganic Chemistry
Article
Figure 1. Structures of Mn(I) and Re(I) complexes with ellipsoids set at 50% probability. Hydrogen atoms and cocrystallized THF solvent
molecules omitted for clarity.
Table 1. Average M−C and M−N Bond Lengths for Mn(I)
Re-5 (26.0°). This distortion is due to the clash of quinoline
protons with the CO ligands in the 6-member metallocycle;
the bite angles in Mn-6 (85.8°) and Re-6 (83.1°) are larger
than those in the 5-membered metallocycle [Mn-5 (77.4°) and
and Re(I) Quinoline Oxime Complexes
average M−C bond length average M−N bond length
complex
Re4im
Mn-5
Mn-6
Re-5
(Å)
(Å)
Re-5 (73.5°)]. For reference, Mn(bpy)(CO) Br and Re(bpy)-
1.917(2)
1.811(1)
1.806(2)
1.919(3)
1.917(2)
1.823(2)
2.170(2)
2.061(4)
2.057(2)
2.186(2)
2.181(2)
2.048(2)
3
(
CO) Cl have Δ angles of 4.1° and 4.3°, respectively, between
3
21,22
the equatorial plane and the bpy ligand.
The distortion in Re4im (23.4°), in which the ligand lacks
the oxime hydroxyl group, exhibits a smaller Δ than aldoxime
Re-6, suggesting that the oxime contributes to molecular
strain, likely due to an oxime hydrogen interaction with the
carbonyl ligand, similar to the steric interaction of the
quinoline hydrogen.
Re-6
Mn(bpy)
a
b
(
CO) Br
3
Re(bpy)
CO) Cl
1.92(1)
2.171(8)
(
3
a
b
Reference 19. Reference 20.
Vibrational and Electronic Spectra. Data from IR
spectra of the Mn(I) and Re(I) quinoline oxime complexes
C
DOI: 10.1021/acs.inorgchem.8b02862
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Table 2. Spectroscopic Properties of Mn(I) and Re(I)
Complexes
CO
stretches,
OH stretch,
MLCT bands,−3
ν(CO)
ν(OH)
λ(nm) [ε × 10
−
1
−1
−1
−1
complex
Re4im
(cm
)
(cm
)
Δ
M
cm ]
2025, 1922,
904
(NH) 3213
23.4°
421 [4.9]
458 [2.9]
412 [4.9]
388 [3.4]
382 [4.3]
416
1
Mn-5
Re-5
2031, 1933,
899
3376
26.2°
26.0°
36.6°
34.1°
4.1°
1
2029, 1930,
910
3280
1
Mn-6
Re-6
2032, 1943,
927
3107
1
Figure 3. UV−vis spectra of Re(I) quinoline oxime complexes in
MeCN. Inset: MLCT bands in the visible region with peak energies
Re-6 > Re-5 > Re4im.
2028, 1923,
904
3095
1
Mn(bpy)
2025, 1939,
1925
2022, 1919,
1899
a
b
(
CO) Br
3
Re(bpy)
CO) Cl
4.3°
371
(
3
a
b
Reference 19. Reference 20.
in MeCN are summarized in Table 1 (full spectra are shown in
Figures S18−S33). As expected, each complex displays three
distinct CO stretches with frequencies similar to those seen in
3
−8,21−27
analogous Mn(I) and Re(I) complexes.
Broad OH
stretches are observed for metal complexes of 5 and 6. Re4im
−
1
displays an NH stretch at 3213 cm .
UV−vis spectra of the Mn(I) and Re(I) complexes are
shown in Figures 3 and 4, with MLCT energies and extinction
coefficients set out in Table 1. The MLCT peaks for Mn-6
Figure 4. UV−vis spectra of Mn(I) quinoline oxime complexes in
MeCN. Inset: MLCT bands in the visible region with peak energies
Mn-6 > Mn-5.
(
388 nm) and Re-6 (382 nm) are at higher energies than those
of Mn-5 (458 nm) and Re-5 (412 nm), indicating that the
ligand 6 LUMO is energetically higher than that of ligand 5.
The MLCT absorption of Re4im redshifts from that of Re-6.
This shift is attributable to LUMO stabilization owing to
Kohn−Sham (KS) orbitals of Mn-5 and Mn-6 are shown in
Figure 5; those of other complexes are shown in Figures S40
and S41. In all cases, the calculated HOMOs are metal-halide
π* orbitals between metal d and halide p . The LUMOs in
electron-withdrawal by the CF group (DFT calculated LUMO
3
energies: Re4im (−3.29 eV) and Re-6 (−3.00 eV).
DFT Calculations. Density functional calculations were
performed to gain further insight into the electronic structures
of the Mn(I) and Re(I) quinoline oxime and quinoline
ketimine complexes. Geometry optimization yielded bond
angles and lengths in good agreement with crystal structures.
Calculated and experimental values are given in Tables S4−S9.
In support of the proposed role of steric hindrance in
determining the geometry, calculated structures reveal
substantial distortion of the quinoline ring out of the equatorial
plane. Calculated angles Δ were smaller than experimental
values by 5−10°; however, the trends still hold, with Δ for Mn-
yz
y
these complexes are primarily localized on the diimine ligand
with a small contribution from d . The calculated HOMO−
xy
LUMO gaps (ΔE
, Table S3) agree with observed
HOMO−LUMO
MLCT energies (Mn-6 > Mn-5 and Re-6 > Re-5 > Re4im).
TD-DFT results (Table 3) are in line with experimentally
observed trends in MLCT energies (Figures 3 and 4). Each
complex has at least one low-energy transition (fosc > 0.01)
from a metal-based orbital (HOMO−2, HOMO−1, HOMO)
to a ligand-based orbital (LUMO, LUMO+1). Interestingly,
the calculations suggest that several transitions contribute to
the MLCT absorptions of Mn-6 and Re-6.
6
and Re-6 greater than those of Mn-5 and Re-5.
Figure 2. Steric clash in Re-5. Left: Space-filling model illustrating the overlap between a quinoline ring proton and CO van der Waals’ spheres.
Right: Structural model of superimposed octahedral equatorial (red) and quinoline (blue) planes used to calculate Δ.
D
DOI: 10.1021/acs.inorgchem.8b02862
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Inorganic Chemistry
Article
The difference in MLCT energies between complexes of
ligands 5 and 6 can be rationalized from their molecular
structures and orbital compositions (Figures 5, S40, and S41).
While the ligands are structurally similar, their electronic
properties are not. Ligand 5 is an α-diimine similar to bpy with
a conjugated diimine bridge: α-diimines have a low-lying π*
orbital that can participate in π-backbonding interactions with
8
,28,29
the metal center.
The diimine bridge in ligand 6 is not
directly conjugated, with the imines cross-conjugated via the
quinoline ring. Thus, the metal character in the LUMOs of
Mn-6 and Re-6 is lower than that of Mn-5 and Re-5 (Table
S3). While the diimine bridge is π-conjugated in Mn-5, there is
a node at the phenyl-pyridine (quinoline) bridge in Mn-6
(
Figure 5). This cross-conjugation dramatically affects the
spectroscopic properties of metal complexes incorporating
ligand 6; notably, the MLCT feature is shifted to higher
energies in Mn-6 and Re-6 spectra (relative to Mn-5 and Re-
5
).
Electrochemistry. Cyclic voltammetry was conducted on
all complexes under an inert atmosphere in MeCN with 0.1 M
TBAPF as supporting electrolyte. Reduction potentials are
6
summarized in Table 4 (representative cyclic voltammograms
Table 4. Electrochemistry of Mn(I) and Re(I) Quinoline
Oxime Complexes
cathodic peak potentials (V vs Fc/Fc+)
complex
first reduction
second reduction
a
d
Re4im
−1.34
−2.03
−2.50
a
Re-5
Re-6
−2.01
−1.63
−1.79
−1.54
a
a
a
Mn-5
Mn-6
−2.01
−1.85
−2.10
−1.88
Figure 5. DFT-calculated orbitals for Mn-5 (left) and Mn-6 (right).
The HOMO−2, HOMO−1, and HOMO orbitals have primarily
Mn(I) character. The LUMO and LUMO+1 orbitals have primarily
ligand character. Cross-conjugation is apparent in the LUMO of Mn-
b
d
Re(bpy)(CO) Cl
Mn(bpy)(CO) Br
−1.72
3
c
−1.64
3
a
6
with a node between the two imines; there is no equivalent node in
Electrochemistry performed under inert atmosphere at 100 mV/s
(500 mV/s in the case of Re-5) with ∼1 mM compound in MeCN
the LUMO of Mn-5.
b
c
containing 0.1 M TBAPF as supporting electrolyte. Ref 1. Ref 2.
6
d
E1/2
.
Table 3. Calculated (TD-DFT) Transition Energies for
Mn(I) and Re(I) Quinoline Oxime Complexes (Transition
Compositions with Contributions Greater than 5%)
(
CVs) are shown in Figure 6; all other CVs are in Figures
exptl MLCT
energy (eV)
calcd MLCT
energy (eV)
electronic transition
composition
complex
calcd fosc
S34−38). The first electrochemical reduction of complex
Re4im is electrochemically reversible (red trace), as evidenced
by a peak current ratio near unity. Plots of current density
versus square root of the scan rate are linear, indicating a
diffusion-controlled process (Figure S34). This reversibility
could result from the noninnocence of the imine functional
group. Previous investigations of a bis-2-pyridyl ketone Re(I)
tricarbonyl complex showed that inclusion of an oxime imparts
Re4im
2.94
2.71
3.01
3.20
2.91
2.44
2.42
2.47
0.0111 HOMO→LUMO
1 (94.4%)
+
Mn-5
Re-5
0.0327 HOMO−1→
LUMO (96.6%)
0.0785 HOMO−1→
LUMO (95.0%)
0.0289 HOMO−1→
LUMO (92.7%)
0.0372 HOMO−3→
LUMO (34.4%)
Mn-6
30
electrochemical irreversibility. The authors attributed this
finding to the oxime acting as an electron sink.
3
.11
HOMO−2→
All cathodic features for Mn-5, Re-5, Mn-6, and Re-6 are
electrochemically irreversible. One possible mechanism for this
decomposition is dissociation of halide, while ligand
dissociation for Re4im occurs during the second reduction.
Halide dissociation after the first reduction is common for
Mn(I) analogues and is also observed for certain Re(I)
LUMO (53.4%)
Re-6
3.25
2.45
0.0655 HOMO→LUMO
(
94.6%)
2
3
.99
.33
0.0299 HOMO−2→
LUMO (90.7%)
0.0519 HOMO−1→
21,23,31
LUMO+1
complexes.
(
85.9%)
The first electrochemical reduction of Re4im is anodically
shifted 290 mV from that of Re-6, primarily due to the effect of
the electron-withdrawing CF group. DFT calculations suggest
HOMO→LUMO
1 (5.7%)
+
3
the ligand greatly affects the LUMO energies (by 290 meV)
E
DOI: 10.1021/acs.inorgchem.8b02862
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Figure 6. Cyclic voltammograms (100 mV/s) of 1 mM Re4im, 1 mM Re-5 (500 mV/s), 1 mM Mn-5, and 3 mM Mn-6 under an inert atmosphere
in MeCN with 0.1 M TBAPF as supporting electrolyte.
6
(
Table S3). As the LUMOs are ligand-based, ligand-centered
proton source (Figure 7). With added TFE, a modest i /i
cat p
+
reduction occurs first in both cases.
ratio of ∼4 at −1.95 V vs Fc/Fc was determined. The catalytic
Re(bpy)(CO) Cl electrocatalysis of CO to CO reduction
activity of Mn-6 was further analyzed by bulk electrolysis in the
3
2
+
occurs by both proton-dependent and proton-independent
pathways. Because Mn(bpy)(CO) Br electrocatalysis of CO
reduction only occurs in the presence of an added proton
source, we investigated Re4im, Mn-5, Re-5, Mn-6, and Re-6
electrocatalysis in CO -saturated MeCN solutions containing
0
(
presence of CO and TFE at −2.0 V vs Fc/Fc . Although care
2
2
3
was taken to prevent photodecomposition by excluding
ambient light, the head space analysis following bulk
electrolysis experiments indicated 776% Faradaic efficiency
for CO production. Our calculations show that 7.2 μmol CO is
produced while complete decomposition of the Mn-6 would
result in 60 μmol. Therefore, we attribute of the formation of
CO to decomposition. A loss of color from the catholyte also
suggested decomposition.
3
2
3
1
2
.1 M TBAPF6 with and without added trifluoroethanol
TFE). Cyclic voltammograms of Mn-6 in the presence of
CO with and without added 0.5 M TFE are shown in Figure 7
2
Electrochemical analysis without CO2 shows that the
complexes appear to have features similar to those of
Re(bpy)(CO) Cl and Mn(bpy)(CO) Br. Re4im is readily
3
3
+
/0
reduced (−1.34 V vs Fc ), but is not an active electrocatalyst
for reduction of CO . And, although the other Re(I)
1
2
2
complexes have potentials similar to those of known catalysts,
they also are not active. Similar to previous reports, the Mn(I)
analogues decompose under catalytic conditions.
Our results are in line with those of other researchers who
have found that complexes containing quinoline-based ligands
6
,20
do not catalyze electrochemical CO reduction. No reasons
2
for their inactivity were given. One possible decomposition
pathway is bound CO bridging the metal and imine carbon, as
Figure 7. Cyclic voltammograms (100 mV/s) of ∼3 mM Mn-6 in
2
MeCN containing 0.1 M TBAPF under N (black), CO (red), and
was shown in a Re(I) iminopyridine complex, thereby blocking
6
2
2
32
with added TFE (blue).
catalytic pathways. Another is that the geometrically strained
quinoline ligand dissociates during electrochemical reduction,
a hypothesis supported by the decomposition of Mn-6.
(
all other CVs are shown in Figures S35−38). Re4im and Re-6
did not show any new electrochemical features or increased
CONCLUDING REMARKS
current in the presence of CO , nor with added TFE. Re-5 and
■
2
Mn-5 showed increased current but no new redox events that
would suggest electrocatalytic behavior.
Only Mn-6 displayed new electrochemical features in the
Our work shed light on the role of conjugation in diimine
ligands, highlighting differences between α-diimines, which
feature a low-lying π* orbital, and β-diimines, where imines are
cross-conjugated in the quinoline ring. Severe structural strain
presence of CO , with increased current in the presence of a
2
F
DOI: 10.1021/acs.inorgchem.8b02862
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Inorganic Chemistry
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was apparent from inspection of the structures of both Mn(I)
and Re(I) quinoline oxime complexes. Quinoline hydrogens,
and to a lesser degree quinoline oximes, lead to distortion of
the quinoline from the octahedral plane. Deviations from
planarity are as high as 36.6° (Mn-6). We conclude that strain-
induced metal-complex distortion is the primary cause of
electrochemical irreversibility, disfavoring sustained, electro-
solvent was removed under reduced pressure to obtain an orange
1
solid. Yield 94%. H NMR (500 MHz, CDCl ) δ 8.96 (dd, J = 4.1, 1.8
3
Hz, 1H), 8.14 (d, J = 8.3 Hz, 1H), 8.13 (t, J = 8.1 Hz, 1H), 7.83 (dd, J
=
4
8.2, 1.4 Hz, 1H), 7.61 (dd, J = 8.1, 7.3 Hz, 1H), 7.40 (dd, J = 8.3,
.1 Hz, 1H), 6.81 (q, J = 6.6 Hz, 1H), 0.12 (s, 9H). 1 C NMR (126
3
MHz, CDCl ) δ 149.96, 145.67, 136.20, 134.21, 129.32, 128.83,
3
1
27.87, 126.22, 125.04 (q, J = 280 Hz), 121.20, 66.39 (q, J = 32.4
19
Hz), −0.25. F NMR (282 MHz, CDCl ) δ −77.57 (d, J = 6.5 Hz).
MS(FAB+): m/z = 300.1030 (calcd for M + H 300.1032).
3
+
catalytic CO reduction.
2
2
,2,2-Trifluoro-1-(quinolin-8-yl)ethanol (2). A solution of 1 (2.020
EXPERIMENTAL SECTION
Materials. Anhydrous dimethylformamide (DMF) and anhydrous
tetrahydrofuran (THF) were obtained from commercial sources and
stored under an inert atmosphere. Ethyl acetate (EtAc) was dried
using activated 4 Å molecular sieves. Diethyl ether (Et O) was
stabilized with BHT. Metal coordination precursors Re(CO) Cl and
g, 6.7 mmol) in 25 mL of THF was placed under N , cooled to 0 °C,
2
■
and 8.5 mL of TBAF (1.0 M in THF) was added (8.5 mmol). The
reaction solution was stirred at room temperature for 16 h; 120 mL of
dichloromethane were added, and the solution was washed with 3 ×
5
0 mL water. The organic layer was dried with Na SO and filtered,
2 4
2
and the solvent removed under reduced pressure to yield an orange
5
1
solid. Yield: 87%. H NMR (500 MHz, CDCl ) δ 9.01 (s, 1H), 8.87
3
Mn(CO) Br were obtained from Strem, quinoline-8-carbaldehyde
5
(
dd, J = 4.3, 1.8 Hz, 1H), 8.29 (dd, J = 8.4, 1.8 Hz, 1H), 7.91 (dd, J =
.2, 1.4 Hz, 1H), 7.70 (ddt, J = 7.1, 1.6, 0.8 Hz, 1H), 7.62 (dd, J = 8.2,
.1 Hz, 1H), 7.52 (dd, J = 8.3, 4.3 Hz, 1H), 5.49 (q, J = 7.8 Hz, 1H).
and trimethylsilyl trifluoromethane (TMSCF ) from Matrix Scientific,
3
8
7
quinoline-2-carbaldehyde from Acros, tetra-n-butylammonium fluo-
ride (TBAF) as a 1.0 M solution in THF from Aldrich, and 2-
iodoxybenzoic acid (SIBX, 39 wt % stabilized by isophthalic acid and
benzoic acid) from TCI Chemicals. Tetra-n-butylammonium
1
3
C NMR (126 MHz, CDCl ) δ 148.35, 147.06, 137.40, 131.03,
3
1
29.20, 129.02, 128.75, 126.25, 125.15 (q, J = 286 Hz), 121.31, 75.74
1
9
(
q, J = 31.8 Hz). F NMR (282 MHz, CDCl ) δ −77.86 (d, J = 7.8
hexafluorophosphate (TBAPF ) was recrystallized from ethanol and
3
6
+
Hz). MS(FAB+): m/z = 228.0634 (calcd for M + H 228.0636).
,2,2-Trifluoro-1-(8-quinolinyl)-ethanone hydrate (3). Following
dried prior to use in electrochemical studies. The remaining solvents
and chemicals were obtained from commercial sources and used
without further purification.
2
40
a generic procedure, 5.07 g (7.1 mmol) SIBX was added to a
solution of 791 mg (3.5 mmol) 2 in 30 mL of dry EtOAc. The
Methods. NMR spectra were taken on Varian 500 or 300 MHz
instruments. UV−visible spectroscopy was performed in MeCN or
MeOH using a Varian Cary 50 Bio spectrophotometer. Infrared
spectra were taken on a Thermo Scientific Nicolet iS5 FTIR
Spectrometer with an iD5 ATR accessory. Electrochemistry was
performed using a glassy carbon working electrode, Ag/AgCl
reference electrode, and platinum wire counter electrode using a
Gamry Reference 600, Bio-Logic VSP-300, or Bio-Logic SP-200
potentiostat. Cyclic voltammetry was performed in MeCN with 0.1 M
TBAPF supporting electrolyte under an N or CO atmosphere.
reaction mixture was placed under N and stirred while refluxing for
2
2
4 h, cooled to room temperature, and filtered. The filtrate was
transferred to a separatory funnel and washed with 50 mL of
NaHCO and 3 × 50 mL water. The resultant organic layer was dried
3
with Na SO , filtered, and the solvent was removed under reduced
2
4
pressure. The solid was dissolved in 75 mL of dichloromethane and
flushed through a 15 cm plug of silica using dichloromethane as
eluent. The solvent was removed under reduced pressure to obtain a
1
light green solid. Yield: 66%. H NMR (500 MHz, CD CN). In
3
6
2
2
acetonitrile, a 7:1 equilibrium mixture of hydrated ketone and ketone
was observed. The peaks for the two species were resolvable. δ
Ferrocene was used as an internal standard. Bulk electrolysis
measurements were carried out in a gastight cell with a glassy carbon
(
(
1
ketone) 8.99 (dd, J = 4.2, 1.7 Hz, 1H), 8.45 (d, J = 1.8 Hz, 1H), 8.44
plate, a 10 mM Ag/AgNO quasi-reference electrode, and a platinum
3
d, J = 1.7 Hz, 1H), 8.26 (dd, J = 8.2, 1.4 Hz, 1H), 8.03 (dd, J = 7.1,
mesh counter electrode separated by a glass frit. The cell was purged
with CO2 for 1 h and sealed. After 1-h electrolysis, 10 mL of
headspace was sampled with a gastight syringe and injected into an
Agilent 7890a gas chromatograph. A calibration curve was obtained
1
.4 Hz, 1H), 7.78 (d, J = 7.1 Hz, 1H). H NMR (500 MHz, CD CN)
3
δ (hydrated ketone) 8.92 (dd, J = 4.3, 1.8 Hz, 1H), 8.49 (dd, J = 8.4,
1
1
1
.8 Hz, 1H), 8.47 (s, 2H), 8.20 (d, J = 7.4 Hz, 1H), 8.10 (dd, J = 8.3,
.4 Hz, 1H), 7.75 (dd, J = 8.2, 7.4 Hz, 1H), 7.65 (dd, J = 8.4, 4.3 Hz,
using mixtures of different percent volumes of CO in CO . High
2
1
H). H NMR (300 MHz, CD OD). In methanol, only the dimethyl
resolution mass spectrometry was performed using a Waters LCT
Premiere XE instrument (ESI-TOF, positive ion mode) or a Jeol
3
acetal was observed. δ 8.90 (dd, J = 4.4, 1.7 Hz, 1H), 8.51 (dd, J = 8.4,
1.8 Hz, 1H), 8.11 (dd, J = 8.3, 1.4 Hz, 1H), 8.04 (d, J = 7.2 Hz, 1H),
+
MSRoute (FAB ) instrument using a 3-nitrobenzyl alcohol matrix.
7
.74 (t, J = 7.8 Hz, 1H), 7.64 (dd, J = 8.4, 4.3 Hz, 1H), 5.49 (s, 1H).
Crystals suitable for X-ray diffraction (XRD) were grown by slow
diffusion of pentane into THF solutions of the desired metal complex.
See Supporting Information for all XRD data.
Computational Details. DFT and TD-DFT calculations were
performed using an Orca 2.9.1 software package. All calculations were
performed using the PBE0 functional with a triple-ζ def2-TZVP basis
1
3
C NMR (126 MHz, CD CN) δ 152.89, 149.86, 146.68, 139.68,
3
137.82, 132.69, 132.53, 131.75, 131.53, 129.75, 129.37, 128.76,
127.87, 127.50, 126.47, 124.17, 124.07, 122.91, 96.78 (q, J = 32.4
1
9
Hz). F NMR (282 MHz, CD OD) δ −86.25. MS(FAB+): m/z =
3
+
244.0582 (calcd for M + H 244.0585).
3
3
set applied to all atoms. For Mn complexes, the RI-J approximation
and corresponding auxiliary def-2-TZVP/J basis set were used to
2,2,2-Trifluoro-1-(8-quinolinyl)-ethanone oxime (4). Following a
41
similar procedure with modification, 870 μL (3.29 mmol) of
34,35
accelerate the calculations.
Calculations implemented the zeroth-
pyridine were added to a 15 mL of ethanolic solution of 201 mg (0.83
36
order regular approximation (ZORA) method. All complexes were
optimized starting from XRD geometries subjected to tight SCF as
well as default geometry convergence criteria. Converged geometries
were confirmed by the absence of imaginary frequencies in numerical
frequency calculations. Population analyses was carried out using a
mmol) of 3 and 116 mg of NH
2
OH·HCl (1.67 mmol). The reaction
solution was placed under N , stirred for 1 h at room temperature,
2
and then stirred while refluxing for 24 h. The reaction solution was
concentrated to 5 mL by removal of solvent under reduced pressure,
and 5 mL of water was added. The solution was concentrated under
reduced pressure, and the product precipitated. The solid was filtered,
washed with 40 mL of distilled water, and dried to obtain a white
3
7,38
Lo
̈
wdin population analysis.
The 20 lowest singlets were
calculated for all molecules using TD-DFT as implemented in ORCA.
1
Synthesis. 8-(2,2,2-Trifluoro-1-(trimethylsilyloxy)ethyl)quinoline
solid. Yield: 70% with ∼60:40 ratio of oxime isomers. H NMR (500
39
(
1). Following a generic procedure, 26 mg K CO (0.19 mmol) was
MHz, CD CN) δ 10.16 (br s, 0.6H), 8.95 (m, 1H), 8.39 (ddt, J = 8.4,
2
3
3
added to a 40 mL DMF solution of 2.51 g (16 mmol) of quinoline-8-
1.8, 0.5 Hz, 1H), 8.14−8.10 (m, 1H), 7.85−7.73 (m, 0.4H), 7.73−
1
9
carbaldehyde and 3.5 mL (24 mmol) of TMSCF . The reaction
7.66 (m, 1.6H), 7.60 (m, 1H). F NMR (282 MHz, CD CN) δ
3
3
1
3
mixture was placed under N and stirred at room temperature for 28
−65.23 (33%), −67.57 (67%). C NMR (126 MHz, CD CN) δ
2
3
h. The reaction was washed with 25 mL brine and 2 × 25 mL water.
151.09, 150.84, 147.16, 136.39, 136.24, 131.12, 130.62, 130.50,
130.22, 129.82, 128.18, 128.07, 127.52, 126.18, 126.08, 122.16,
The resultant organic layer was dried with Na SO , filtered, and the
2
4
G
DOI: 10.1021/acs.inorgchem.8b02862
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
1
22.13. MS(FAB+): m/z = 241.0586 (calcd for M + H 241.0589).
acetate was added to a 15 mL ethanolic solution of 101 mg (0.64
mmol) of quinoline-8-carbaldehyde and 90 mg (1.3 mmol) of
NH OH·HCl. The reaction solution was stirred for 21 h under N . A
drop of HCl was added, and the reaction mixture was stirred for 3 h.
Saturated sodium bicarbonate solution (2.5 mL) was added, and 1 mL
of ethanol with 10 mL of water was added. The EtOH was removed
under reduced pressure to precipitate the product. The resultant
−1
Selected IR data (ATR, range) ν/cm : 3172 (νO−H).
Re(CO) (2,2,2-trifluoro-1-(quinolin-8-yl)ethan-1-imine)Cl
Re4im). Following a similar procedure with modification, a 5 mL
toluene solution of 54 mg (0.15 mmol) of Re(CO) Cl and 34 mg of 4
0.14 mmol) was placed under N , heated at 95 °C, and stirred for 24
h. The reaction mixture was cooled to room temperature, and the
precipitate was filtered, washed with 5 mL toluene, and dried.
Additional product was obtained by concentration of the filtrate under
reduced pressure, addition of an equivalent amount of hexanes, and
cooling to −20 °C. The precipitate in toluene/hexanes was filtered
3
2
2
42
(
5
(
2
mixture in water was filtered and dried to afford an off-white solid.
1
Yield: 68%. H NMR (500 MHz, CD CN) δ 9.34 (s, 1H), 9.19 (s,
3
1
8
=
H), 8.98 (dd, J = 4.2, 1.8 Hz, 1H), 8.36 (dd, J = 8.3, 1.8 Hz, 1H),
.25 (dd, J = 7.3, 1.4 Hz, 1H), 8.03 (dd, J = 8.2, 1.4 Hz, 1H), 7.67 (t, J
and washed with 5 mL toluene. The red-brown solids were combined.
Yield: 64%. H NMR (500 MHz, CD CN) δ 9.69 (dd, J = 5.2, 1.7 Hz,
H), 8.72 (dd, J = 8.3, 1.7 Hz, 1H), 8.49−8.41 (m, 2H), 7.91 (t, J =
.9 Hz, 1H), 7.70 (dd, J = 8.2, 5.2 Hz, 1H), 5.96 (t, J = 53.2 Hz, 1H).
13
7.7 Hz, 1H), 7.58 (dd, J = 8.3, 4.1 Hz, 1H). C NMR (126 MHz,
1
3
CD CN) δ 150.28, 146.41, 145.55, 136.37, 130.17, 129.58, 128.31,
3
1
7
1
26.39, 125.52, 121.72. MS(FAB+): m/z = 173.0722 (calcd for M +
+
−1
−1
H 173.0715). Selected IR data (ATR, 4000−500 cm ) ν/cm :
3
19
F NMR (282 MHz, CD CN) δ −65.56. TOF MS−ES+: m/z =
3
170 (ν ).
O−H
4
M
94.9972, (calcd for M − Cl 495.00). UV−vis (CH CN) λ nm (ε
3
max
Mn(CO) (quinoline-8-carbaldehyde oxime)Br (Mn-6). Following
−
1
−1
3
cm ): 217 (40000), 246 (15000), 302 (9400), 420 (4900).
43
a similar procedure with modification, a 40 mL Et O solution of 63
−1
−1
2
Selected IR data (ATR, 3500−500 cm ) ν/cm : 3213 br, 3140 sh
). Selected IR data (MeCN, 2200−1800 cm ) ν/cm : [2025,
N−H
mg (0.23 mmol) of Mn(CO) Br and 40 mg (0.23 mmol) of 6 was
−
1
−1
5
(
1
ν
placed under N and stirred while refluxing for 24 h. The reaction
2
922, 1904 (ν
)].
CO
mixture was cooled to 0 °C, filtered, and the precipitate was washed
Quinoline-2-carbaldehyde Oxime (5). Following a published
1
42
with 20 mL of Et
2
O to obtain a yellow solid. Yield: 70%. H NMR
procedure with modification, 347 mg (2.55 mmol) of sodium
acetate was added to a 15 mL ethanolic solution of 105 mg (0.67
mmol) of quinoline-2-carboxaldehyde and 92 mg (1.3 mmol) of
NH OH·HCl. The reaction solution was placed under N and stirred
(
500 MHz, CDCl ) δ 9.78 (s, 1H), 8.83 (s, 1H), 8.59 (s, 1H), 8.38 (s,
3
1
1
3
2
H), 8.05 (d, J = 7.4 Hz, 1H), 7.88 (s, 1H), 7.77 (s, 1H), 7.64 (s,
H). TOF MS−ES+: m/z = 305.9175 (calcd for M − 3(CO)
2
2
−1
−1
05.9200). UV−vis (CH CN) λ nm (ε M cm ): 217 (43000),
3
max
while refluxing for 24 h. Fifteen milliliters of cold distilled water was
then added. The resulting precipitate was filtered, washed with 15 mL
of distilled water and 6 mL of 1:1 EtOH/H O, and dried to obtain a
−
1
95 (7600), 388 (3400). Selected IR data (ATR, 4000−500 cm ) ν/
−
1
−1
cm : 3107 (ν ). Selected IR data (MeCN, 2200−1800 cm ) ν/
O−H
2
−1
1
cm : [2032, 1943, 1927 (νCO)]. Anal. Found: C, 31.50; H, 1.86; N,
yellow solid. Yield: 53%. H NMR (500 MHz, CD CN) δ 10.66 (br s,
3
6.65. Calcd: C, 32.67; H, 1.69; N, 5.86.
1
8
8
8
1
1
H), 9.60 (d, J = 3.2 Hz, 1H), 8.31 (d, J = 8.6 Hz, 1H), 8.31 (s, 1H),
.07 (dq, J = 8.5, 0.9 Hz, 1H), 7.99 (d, J = 8.6 Hz, 1H), 7.98 (dd, J =
.4, 1.4 Hz, 1H), 7.81 (ddd, J = 8.4, 6.9, 1.5 Hz, 1H), 7.66 (ddd, J =
Re(CO) (quinoline-8-carbaldehyde oxime)Cl (Re-6). Following a
3
42
similar procedure with modification, a 5 mL toluene solution of 36
1
3
mg (0.10 mmol) of Re(CO) Cl and 18 mg (0.11 mmol) of quinoline-
.1, 6.9, 1.2 Hz, 1H). C NMR (126 MHz, CD CN) δ 152.67,
5
3
8
-carbaldehyde oxime 6 was placed under N , heated at 90 °C, and
50.58, 147.76, 136.52, 129.92, 129.08, 128.12, 127.89, 127.17,
2
+
stirred for 4 h. The reaction mixture was cooled to 0°, filtered, and the
17.33. MS(FAB+): m/z = 173.0720 (calcd for M + H 173.0715).
−
1
−1
precipitate was washed with 2 mL of toluene, affording a yellow solid.
Selected IR data (ATR, 3500−500 cm ) ν/cm : 3168 (νO−H).
Mn(CO) (quinoline-2-carbaldehyde oxime)Br (Mn-5). Following
a similar procedure with modification, a 20 mL Et O solution of 40
mg (0.15 mmol) of Mn(CO) Br and 25 mg (0.14 mmol) of 5 was
placed under N and stirred while refluxing for 6 h. The mixture was
cooled to room temperature, filtered, and the precipitate was washed
with 15 mL of Et O, affording a bright orange solid. Yield: 60%. H
NMR (500 MHz, CD CN) δ 8.86 (d, J = 8.9 Hz, 1H), 8.74 (s, 1H),
1
Yield: 54%. H NMR (500 MHz, CD CN) δ 9.67 (dd, J = 5.2, 1.7 Hz,
3
3
43
1
1
1
H), 9.43 (s, 1H), 8.71 (dd, J = 8.4, 1.4 Hz, 1H), 8.70 (d, J = 0.6 Hz,
H), 8.31 (dd, J = 8.4, 1.4 Hz, 1H), 8.10 (ddd, J = 7.3, 1.6, 0.6 Hz,
H), 7.86 (dd, J = 8.2, 7.2 Hz, 1H), 7.69 (dd, J = 8.3, 5.3 Hz, 1H).
2
5
2
−
1
−1
UV−vis (CH CN) λmax nm (ε M cm ): 242 (16000), 296 (7500),
3
1
382 (4200). MS(FAB+): m/z = 442.9750 (calcd for M − Cl
−1 −1
2
443.0041). Selected IR data (ATR, 4000−500 cm ) ν/cm : 3095
−1 −1
3
8
1
.60 (d, J = 8.2 Hz, 1H), 8.17−8.11 (m, 1H), 8.08 (ddd, J = 8.6, 6.9,
(νO−H). Selected IR data (MeCN, 2200−1800 cm ) ν/cm : [2028,
1923, 1904 (νCO)]. Anal. Found: C, 31.09; H, 1.59; N, 5.63. Calcd:
C, 32.67; H, 1.69; N, 5.86.
.5 Hz, 1H), 7.89 (d, J = 8.2 Hz, 1H), 7.85 (t, J = 7.5 Hz, 1H). UV−
vis (CH CN) λ nm (ε M− cm ): 207 (40000), 256 (23000), 300
1
−1
3
max
(
7000), 322 (6400), 458 (2900). TOF MS−ES+: m/z = 310.9869,
(
calcd for M − Br 310.9865). Selected IR data (ATR, 3500−500
ASSOCIATED CONTENT
−
1
−1
cm ) ν/cm : 3376 (ν ). Selected IR data (MeCN, 2200−1800
cm ) ν/cm : [2031, 1933, 1899 (ν
■
O−H
−
1
−1
)]. Anal. Found: C, 39.57;
*
S
Supporting Information
CO
H, 2.22; N, 6.81. Calcd: C, 39.93; H, 2.06; N, 7.16.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02862.
Re(CO) (quinoline-2-carbaldehyde oxime)Cl (Re-5). Following a
similar procedure with modification, a 5 mL toluene solution of 50
3
42
mg (1.4 mmol) of Re(CO) Cl and 24 mg (1.4 mmol) of 6 was placed
5
under N , heated at 90 °C, and stirred for 4 h. The reaction mixture
2
Experimental information on XRD analysis, UV−visible
absorption spectra of the ligands, a summary of UV−
visible absorbance data, additional electrochemical
analyses, H and F NMR spectra, IR spectra, and
DFT/TD-DFT calculation summaries (PDF)
was cooled to room temperature, filtered, washed with 4 mL toluene,
1
and dried to obtain an orange-red solid. Yield: 77%. H NMR (500
MHz, CD CN) δ 10.66 (br s, 1H), 9.07 (s, 1H), 8.72 (d, J = 5.8 Hz,
3
1
19
1
H), 8.71 (d, J = 6.7 Hz, 1H), 8.17 (dd, J = 8.2, 1.5 Hz, 1H), 8.10
(ddd, J = 8.7, 6.9, 1.5 Hz, 1H), 8.00 (d, J = 8.5 Hz, 1H), 7.89 (ddd, J
=
8.1, 6.9, 1.0 Hz, 1H). TOF MS−ES+: m/z = 443.0035, (calcd for M
1 −1
−
Cl 443.0041). UV−vis (CH CN) λ
nm (ε M− cm ): 205
max
Accession Codes
3
(
(
(
37000), 256 (21000), 332 (7200), 412 (4900). Selected IR data
CCDC 1567369−1567373 contain the supplementary crys-
tallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
−
1
−1
ATR, 4000−550 cm ) ν/cm : 3280 (νO−H). Selected IR data
−
1
−1
MeCN, 2200−1800 cm ) ν/cm : [2029, 1930, 1910 (ν
)].
CO
Anal. Found: C, 32.52; H, 1.73; N, 5.78. Calcd: C, 32.67; H, 1.69; N,
.86.
Quinoline-8-carbaldehyde Oxime (6). Following a published
5
42
procedure with modification, 347 mg (2.55 mmol) of sodium
H
DOI: 10.1021/acs.inorgchem.8b02862
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
(
13) Wasylishen, R. E.; Friedrich, J. O. Deuterium Isotope Effects on
AUTHOR INFORMATION
Corresponding Authors
E-mail: hbgray@caltech.edu.
E-mail: bradley.brennan@asu.edu.
■
1
the Nitrogen Chemical Shift and J (N,H) in the Ammonium Ion. J.
Chem. Phys. 1984, 80 (1), 585−587.
(
*
14) Garnovskii, D. A.; Bokach, N. A.; Pombeiro, A. J. L.; Haukka,
*
M.; Fraus
PdII-Mediated Nitrile-Oxime Coupling. Eur. J. Inorg. Chem. 2005,
005 (17), 3467−3471.
15) Andrusenko, E. V.; Kabin, E. V.; Novikov, A. S.; Bokach, N. A.;
́
to da Silva, J. J. R.; Kukushkin, V. Y. Microwave-Assisted and
ORCID
2
(
Harry B. Gray: 0000-0002-7937-7876
Funding
This work was funded by NSF CCI Solar Fuels (CHE-
30124). Additional support was provided by the Resnick
Sustainability Institute at Caltech and a Dr. and Mrs. Daniel C.
Harris SURF Fellowship.
Notes
Starova, G. L.; Zolotarev, A. A.; Kukushkin, V. Y. Highly Reactive
Ni -Bound Nitrile-Oxime Coupling Intermediates Stabilized by
Substituting Conventional Nitriles with a Dialkylcyanamide: Ni -
Bound Nitrile-Oxime Coupling Intermediates. Eur. J. Inorg. Chem.
2015, 2015 (29), 4894−4904.
(
II
1
II
16) Pramanik, S.; Roy, S.; Ghorui, T.; Ganguly, S.; Pramanik, K.
Iridium(III) Mediated Reductive Transformation of Closed-Shell
Azo-Oxime to Open-Shell Azo-Imine Radical Anion: Molecular and
Electronic Structure, Electron Transfer, and Optoelectronic Proper-
ties. Inorg. Chem. 2016, 55 (4), 1461−1468.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank Larry Henling, Michael Takase, David VanderVelde,
Mona Shahgholi, and Naseem Torian for experimental
assistance and discussions.
(
17) Mu, G.; Cong, L.; Wen, Z.; Wu, J. I.-C.; Kadish, K. M.; Teets,
T. S. Homoleptic Platinum Azo-Iminate Complexes via Hydro-
genative Cleavage of Formazans. Inorg. Chem. 2018, 57 (15), 9468−
9477.
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