A new mechanism of metal-ligand cooperative catalysis in transfer hydrogenation of ketones

Article abstract of DOI:10.1016/j.poly.2020.114508

We report iridium catalysts IrCl(η⁵-Cp^?)(κ²-(2-pyridyl)CH₂NSO₂C₆H₄X) (1-Me, X = CH₃ and 1-F, X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η⁵-Cp^?)(κ²-(C₅H₄N)CHNSO₂Ar) (2), IrH(η⁵-Cp^?)(κ²-(2-pyridyl)CH₂NSO₂Ar) (3), and Ir(η⁵-Cp^?)(κ³-(2-pyridyl)CH₂NSO₂Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R₂CHOH ? 3 + R₂CO) is suggested by kinetic isotope effects, Eyring parameters (ΔH^≠ = 29.1(8) kcal mol^?1 and ΔS^≠ = ?17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle.

Full text of DOI:10.1016/j.poly.2020.114508

Polyhedron 182 (2020) 114508
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
A new mechanism of metal-ligand cooperative catalysis in transfer
hydrogenation of ketones
Ivan Demianets a, Valeriy Cherepakhin a, Alexander Maertens a, Paul J. Lauridsen a,
Shaama Mallikarjun Sharada b, Travis J. Williams a,
⇑
a Donald P. and Katherine B. Loker Hydrocarbon Institute and Department of Chemistry, University of Southern California, Los Angeles, CA 90089-1661, United States
b Mork Family Department of Chemical Engineering and Materials Science and Department of Chemistry, University of Southern California, Los Angeles CA 90089
a r t i c l e i n f o
a b s t r a c t
*
We report iridium catalysts IrCl(g j
5-Cp )( 2-(2-pyridyl)CH2NSO2C6H4X) (1-Me, X = CH3 and 1-F, X = F) for
Article history:
Received 3 December 2019
Accepted 10 March 2020
Available online 12 March 2020
transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand
cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series
of catalytic intermediates: Ir(g j g j
5-Cp )( 2-(C5H4N)CHNSO2Ar) (2), IrH( 5-Cp )( 2-(2-pyridyl)CH2NSO2Ar)
*
*
*
(3), and Ir(
g
5-Cp )(
j
3-(2-pyridyl)CH2NSO2Ar) (4). The structures of 1-Me and 4-Me were established
rate-determining, concerted hydrogen transfer step
To John Bercaw on the occasion of his 75th
birthday, for showing us again and again to
see the whole board while not skipping the
details.
by single-crystal X-ray diffraction.
A
(2 + R2CHOH ¢ 3 + R2CO) is suggested by kinetic isotope effects, Eyring parameters (D
H– = 29.1
(8) kcal molꢀ1 and
D
S– = ꢀ17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT,
a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton
Keywords:
Ketone
Alcohol
shuttle.
Ó 2020 Elsevier Ltd. All rights reserved.
Iridium
Transfer hydrogenation
Metal hydride
1. Introduction
previously reported a family of iridium- and ruthenium-based
hydrogen transfer catalysts B–D (Fig. 1), that employ PN and NC
Substrate activation via dual-site metal-ligand cooperation is a
thought-provoking area in catalysis [1–5], particularly because
such mechanisms enable reactivity, selectivity, and tunability
unavailable to metals alone: for example, asymmetric hydrogena-
tion of unsaturated polar bonds, where cooperation between a
ruthenium center and an amido ligand results in dihydrogen split-
ting and delivery of the proton and hydride to a substrate [6–8]. A
different mechanism of cooperative hydrogen transfer was discov-
ered by Milstein et al. [9], who showed reversible dearomatization
of pyridine-based pincer complexes A (Fig. 1).
The community generally accepts that the Milstein PNP and
PNN pincer complexes frequently operate via a metal-ligand coop-
erative mechanism, and the role of the ligand backbone in catalysis
is being studied actively in these and related hydrogenation sys-
tems [10–13]. However, controversy surrounds many bifunctional
mechanism proposals [13–16], including documented cases of
non-cooperative, metal-centered catalysis by complexes in which
cooperation was designed or expected [17–23]. Our group
CH-acidic bidentate ligands [24–27]. Contrary to their original
design and unlike their pincer congeners, B–D do not operate by
the expected cooperative catalysis mechanism, and their deproto-
nated/dearomatized forms do not participate in any catalytic reac-
tion that we have found.
*
A series of complexes based on isolobal Cp IrIII and (p-cymene)
RuII fragments and bidentate pyridyl-sulfonamide ligands have
demonstrated a high catalytic activity in transfer hydrogenation
of quinolinium salts [28], imines [29], aldehydes, and ketones
[30]. Here we describe two complexes of this class, 1-Me and 1-F
(Fig. 1), that enable transfer hydrogenation of ketones with 2-pro-
panol, via metal-ligand cooperation, through reversible dearomati-
zation of the pyridine moiety. The iridium atom and dearomatized
N-picolylbenzenesulfonamide ligand cooperate in a concerted
abstraction of C–H and O–H hydrogen atoms: this contrasts the
PNN Milstein complexes in which chemistry happens more readily
on the phosphorus side of the pincer. Since the concerted nature of
proton-hydride transfer in Milstein-type catalytic systems is still
an open question [13], and since B–D do not utilize the cooperative
mechanism, we wanted to characterize our catalytic system with
experimental and computational data.
⇑
Corresponding author.
E-mail address: travisw@usc.edu (T.J. Williams).
https://doi.org/10.1016/j.poly.2020.114508
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

2
I. Demianets et al. / Polyhedron 182 (2020) 114508
Fig. 1. Hydrogen transfer catalysts.
2. Results and discussion
2.1. Synthesis and reactivity of complexes 1
In this section we describe the synthesis of complexes 1 (1-Me
and 1-F) and a reactivity study that reveals a series of catalytically
relevant species 2, 3, and 4 (Schemes 1 and 2). We began this pro-
ject by exploring the reactivity of 1-Me, but later we made a fluo-
rinated analog 1-F for the purpose of 19F NMR studies (Scheme 1).
These complexes were synthesized with high yields (>90%) in
respective reactions among pentamethylcyclopentadienyliridium
*
(III) chloride [Cp IrCl2]2, the corresponding N-(picolyl)-benzenesul-
fonamide ligand H[NNX] (X = Me, F), and triethylamine. Complexes
1 are air and water tolerant, therefore an aqueous workup is effi-
cient and convenient. The structure of 1-Me was established by
single-crystal X-ray diffraction (Scheme 1). Both compounds are
analogous in reactivity and spectral properties.
Scheme 2. Formation of catalytically competent complexes 2, 3, and 4. Molecular
structure of 4-Me shown with 50% probability ellipsoids.
We demonstrated temperature- and solvent-dependant
dynamic behaviour of the CH2 protons in complex 1-Me, which
is a consequence of stereoinversion of the iridium atom enabled
by reversible chloride dissociation. The rate of the stereoinversion
correlates with the solvent dielectric constant; for example,
according to 1H NMR, in CD2Cl2 (
e
= 9.1) the diastereotopic protons
appear as two broad peaks at 4.76 and 4.52 ppm, whereas in CD3-
OD ( = 33) they give a sharp singlet at 4.56 ppm. We conducted a
e
variable temperature NMR experiment in CD2Cl2 and (CD3)2CO/
CD3OD (5:1 by volume) solutions and found that while the coales-
cence temperature of the methylene peaks in CD2Cl2 is way above
its boiling point, in (CD3)2CO/CD3OD the peaks coalesce at ꢀ12 °C,
which corresponds to the rate constant of iridium inversion
kc = 512 sꢀ1 and the free energy of activation
D
G– = 12 kcal molꢀ1
(Fig. S28).
Complexes 1 undergo deprotonation at the methylene site by t-
BuOK to give dark-blue solutions of the corresponding complexes 2
(Scheme 2). These compounds are moisture-, air-, and light-sensi-
tive, and our isolation attempts led to their decomposition. Never-
theless, complexes 2 were characterized by 1H NMR in CD2Cl2
solution, and their identity was established based on the character-
istic singlets of the backbone CH fragment at 8.11 (1-Me) and 8.12
(1-F) ppm. This assignment is consistent with the COSY data that
suggest spin isolation of the CH fragment (Figs. S5 and S6). The
deprotonation also effects the pyridine protons H3 and H4, whose
peaks are shifted downfield to the olefin region and appear at
5.80 and 6.41 ppm, respectively, thus indicating the loss of pyri-
Scheme 1. Synthesis of 1-Me and 1-F. Molecular structure of 1-Me shown with 50%
probability ellipsoids.

I. Demianets et al. / Polyhedron 182 (2020) 114508
3
dine aromaticity. Analogous dearomatization of the pyridine sys-
tem was observed previously for the pincer complexes [9] and
our dehydrogenation catalysts C and D [24,25].
Complexes 2 are convenient precursors for the synthesis of cat-
alytically relevant compounds 3 and 4 (Scheme 2). The hydride
complexes 3 can be prepared by reacting the corresponding com-
plex 2 with formic acid in CH2Cl2. According to 1H NMR, the chem-
ical shifts of the hydride ligands in 3-Me and 3-F (ꢀ10.39 and
ꢀ10.36 ppm, respectively) illustrate a negligible effect of the
methyl or fluorine group on the metallic catalytic center. IR spectra
of 3-Me and 3-F are consistent with the Ir–H bond, as they demon-
strate metal-hydride vibration bands at 2100 and 2118 cmꢀ1
,
respectively. We find that species 2 undergoes a slow transforma-
tion to 4 in a CH2Cl2 solution, and the process is accelerated by
direct sunlight (Scheme 2). Single-crystal X-ray diffraction study
suggests that 4-Me features an aryl–iridium bond that originates
from phenylene metalation. Although the mechanistic details of
this transformation are unknown, the intramolecular C–H activa-
tion is selective over intermolecular processes even in benzene or
toluene solutions. Alternatively, 4-F can be accessed from the
hydride complex 3-F by heating it with acetophenone and 2-pro-
panol without a base (Scheme 2). Interestingly, 3-F does not trans-
fer its hydride to acetophenone in the absence of added 2-
propanol, and without the ketone that serves as a hydrogen accep-
tor, the conversion of 3-F reaches only 53%.
We performed a series of catalytic tests to compare the activity
of complexes 1, 3, and 4, and to evaluate the effect of added base
(Table 1). Entries 2, 6, and 7 show that complexes 1, 3, and 4 are
equally effective in transfer hydrogenation of acetophenone with
2-propanol, since they all give the same yield of 1-phenylethanol
within the error of experiment. Entry 3 shows that air does not
deactivate the catalyst but slightly reduces the reaction yield to
85%. Entries 1, 2, 4, and 5 demonstrate that transfer hydrogenation
by 1-Me is enabled by general base catalysis, since the base
strength correlates with the acetophenone conversion. Impor-
tantly, bases that cannot deprotonate complexes 1 inhibit the reac-
tion. In contrast to our catalytic system, a homolog of 1-Me with an
ethylene group in the ligand backbone, reported by O’Connor et al.
[30d], shows a different reactivity in transfer hydrogenation of
ketones with 2-propanol: the complex facilitates the base-free
reaction and is deactivated by a base.
Fig. 2. 19F NMR spectra of iridium species derived from 1-F in the reaction between
2-propanol and acetophenone. Species of unknown structure are marked with
asterisks.
fied by spiking the reaction mixture with the pure compounds.
When 1-F is treated with 2-propanol, acetophenone, and t-BuOK
at room temperature the reaction mixture temporarily turns dark
blue due to formation of 2-F, and then slowly turns yellow. At this
point, the corresponding 19F NMR spectrum contains four peaks at
ꢀ109.90, ꢀ110.40, ꢀ110.60, and ꢀ111.26 (3-F) ppm (Fig. 2A). The
hydride 3-F is the only form that persists during the reaction.
When the mixture is brought to 85 °C, the catalytic transfer hydro-
genation begins, the mixture turns dark blue again, and the peak of
2-F appears at ꢀ107.40 ppm (Fig. 2B). Generation of 2-F in the cat-
alytic mixture is temperature dependent, since it exists as a minor
intermediate during the catalysis at 85 °C and disappears at room
temperature (Fig. 2C and D). Complex 4-F was detected as a minor
catalytic species at ꢀ112.44 ppm. While the spectra contain signals
that we were unable to isolate, we see that they equilibrate with
the known species, and MALDI data show that these are ligated
*
forms of the [Cp Ir(
j
2-(2-pyridyl)CH2NSO2C6H4F)]+ cation.
2.2. Kinetic studies
We tracked speciation of precatalyst 1-F in 2-propanol–ace-
tophenone transfer hydrogenation by variable temperature in situ
19F NMR study (Fig. 2). The peaks of 2-F, 3-F, and 4-F were identi-
Complexes 2 are the most reactive among the observed cat-
alytic intermediates, and therefore we believe that the active cata-
lyst systems involve these species. Our further kinetic and
theoretical studies support this hypothesis.
Standard homogeneity tests [31,32], i.e., mercury drop test,
quantitative poisoning, and well-behaved kinetics suggest that
transfer hydrogenation of acetophenone with 1 and 2-propanol
proceeds via homogeneous catalysis, and therefore, the system is
suitable for the NMR kinetic studies (Fig. S21). We endeavored to
learn the kinetic isotope effect (KIE) values for CH and OH groups
Table 1
Catalyst and base screening.a
of 2-propanol and the activation parameters (D D
H– and S–) to
get an insight to the rate-determining step of the catalytic
mechanism.
Entry
[Ir]
Base, conditions
Yield, %b
We measured the rates of the reaction under identical condi-
tions (acetophenone, 1-Me, and t-BuOK) to enable KIE determina-
tions among five 2-propanol isotopologues ((CH3)2CHOH,
(CD3)2CHOH, (CD3)2CDOH, (CD3)2CHOD, and (CD3)2CDOD). Kinetic
isotope effects were derived from the obtained rate constant values
(Table 2). The combined isotope effect (kCHOH/kCDOD = 2.15(3))
matches the product of the average separate OH and CH isotope
effects 2.16(6). Secondary KIE was determined by comparing the
rate constants of (CH3)2CHOH and (CD3)2CHOH, kCH3/kCD3 = 1.69
(18)/1.38(2) = 1.23(13). These combined data are consistent with
a mechanism where the hydride and proton transfer occur in a sin-
gle kinetically relevant step.
1
2
3
4
5
6
7
1-Me
1-Me
1-Me
1-Me
1-Me
3-Me
4-F
NaH
95
92
85
38
10
88
88
t-BuOK
t-BuOK, air
K2CO3
CH3COOCs
t-BuOK
t-BuOK
a
Reaction conditions: acetophenone (60 mg, 0.500 mmol), 2-propanol (0.5 mL), a
catalyst (5.00 mmol, 1 mol%), and a base (25.00 mmol, 5 mol%) were stirred in a
closed reactor under nitrogen at 85 °C for 14 h.
The yields of 1-phenylethanol were calculated from 1H NMR spectra of the
b
reaction samples.

4
I. Demianets et al. / Polyhedron 182 (2020) 114508
Table 2
Kinetic isotope effect data.
3. Conclusions
In conclusion, we present the first example of bidentate transi-
tion metal complexes, 1-Me and 1-F, that operate a via metal-
ligand cooperative mechanism previously known only in pincer
scaffolds only. The structurally-novel iridium complexes supported
by bidentate N-picolylbenzenesulfonamide ligands undergo rever-
sible deprotonation and formation of 2, which appears to be the
active catalyst. Kinetic isotope effect data and proton-hydride fide-
lity are consistent with the concerted nature of the hydrogen atom
transfer from 2-propanol to 2. Eyring study and DFT calculations
provided consistent activation parameter values, and the opti-
mized structure of the nine-membered cyclic transition state, TS,
which is stabilized by a second alcohol molecule that shuttles
the proton from the alcohol to the catalyst.
Alcohol
k, sꢀ1
KIE
(CD3)2CHOH
(CD3)2CDOH
(CD3)2CHOD
(CD3)2CDOD
(CH3)2CHOH
1.38(2) ꢁ 10ꢀ4
7.63(1) ꢁ 10ꢀ5
1.22(1) ꢁ 10ꢀ4
6.40(9) ꢁ 10ꢀ5
1.69(18) ꢁ 10ꢀ4
kCHOH/kCHOD
kCDOH/kCDOD
kCHOH/kCDOH
kCHOD/kCDOD
kCHOH/kCDOD
1.13(2)
1.19(2)
1.80(4)
1.91(4)
2.15(3)
Proton-hydride fidelity in our catalytic system was established
based on the 1H NMR data obtained during the KIE study
(Fig. S18). Analysis of the deuteration sites in 1-phenylethanol pro-
duct showed full deuteration of the CH site only when a C-deuter-
4. Experimental
4.1. General procedures
ated isotopologue, (CD3)2CDOD or (CD3)2CDOH, is used. We also
note that C-deuteration is independent of O-deuteration in the
alcohol. Thus the absence of proton-hydride scrambling suggests
a concerted proton-hydride abstraction.
All synthetic procedures were performed under nitrogen either
in a Vacuum Atmospheres glovebox or in a closed reactor. Ace-
tophenone, 4-toluenesulfonyl chloride, 4-fluorobenzenesulfonyl
*
chloride, 2-picolylamine, [Cp IrCl2]2, potassium tert-butoxide, for-
An Eyring study was conducted to determine the activation
parameters of 2-propanol–acetophenone transfer hydrogenation.
Variable temperature 1H NMR kinetics were recorded to determine
four rate constants within 50–90 °C (Figs. S19 and S20). The
enthalpy and entropy of activation were derived using Eyring
mic acid (97%), and NaHCO3 were purchased from common ven-
dors and used without further purification. NMR solvents
(CD2Cl2, CDCl3, and C6D6), 2-propanol, and triethylamine were
dried and distilled over CaH2. Hexane and CH2Cl2 were dried using
a solvent purification system.
equation and were found to be
D
associative event in the rate-determining step.
D
H– = 29.1(8) kcal molꢀ1 and
1H, 13C, and 19F NMR spectra were acquired on Varian Mercury
400, VNMRS-500, and VNMRS-600 spectrometers and processed
using MestReNova 12.0.1. All chemical shifts are reported in ppm
and referenced to the residual 1H or 13C solvent peaks. Following
abbreviations are used: (s) singlet, (bs s) broad singlet, (d) doublet,
(t) triplet, (dd) doublet of doublets, etc. NMR spectra of all metal
complexes were taken in 800 J. Young tubes (Wilmad or Norell) with
Teflon valve plugs. MALDI-MS spectra were acquired on Bruker
Autoflex Speed MALDI Mass Spectrometer. X-ray crystallography
data were obtained on a Bruker APEX DUO single-crystal diffrac-
tometer equipped with an APEX2 CCD detector, Mo fine-focus
and Cu micro-focus X-ray sources. IR spectra were obtained using
Jasco FT/IR-4600 FT-IR Spectrometer.
S– = ꢀ17(19) eu. The strongly negative
D
S– value indicates an
2.3. Computational study
We employed hybrid, dispersion-corrected DFT, and reaction
path search methods to determine the structure of the transition
state in the rate-determining step of 2-propanol–acetophenone
transfer hydrogenation. Our DFT model replaces 2-isopropanol,
*
Cp , and Ts groups with H2O, Cp, and Tf, respectively, to simplify
the calculations. We considered two potential transition states TS
and TS0: the former contains a nine-membered ring with an extra
alcohol molecule acting as a proton shuttle (Scheme 3), whereas
in the latter the hydrogen transfer happens directly between 2-
propanol and the metal complex (Fig. S23). The computed enthalpy
4.2. Catalytic procedure
of activation for TS is
with the enthalpy of activation obtained in the Eyring study,
H– = 29.1(8) kcal molꢀ1. The other transition state TS0 has a much
higher enthalpy of activation,
H– = 42.6 kcal molꢀ1, which is
D
H– = 26.2 kcal molꢀ1, which is consistent
A solution of acetophenone (60 mg, 0.500 mmol), complex 1
(5.00 mmol, 1 mol%) and a base (25.00 mmol, 5 mol%, t-BuOK,
KOH, NaH, K2CO3, or K3PO4) in isopropanol (0.5 mL) was stirred
in a closed reactor under nitrogen at 85 °C.
D
D
energetically unrealistic and in discord with the experiment.
Therefore, we conclude that TS could be relevant to the rate-deter-
mining step, and that the catalyst most likely operates via a proton
shuttle mechanism. Similar proton shuttles have been invoked in
hydrogenation reactions of other bifunctional complexes [33,34].
4.3. Synthesis of ligands
N-Picolylbenzenesulfonamide ligands H[NNMe
were synthesized in good yields 85–90% according to a reported
procedure by Prim et al. [35].
]
and H[NNF]
H[NNF]: Colorless crystals. 1H NMR (400 MHz, CDCl3): d 8.41 (d,
J = 4.7 Hz, 1H, Py), 7.82 (dd, J = 9.0, 5.1 Hz, 2H, C6H4), 7.59 (td,
J = 7.7, 1.7 Hz, 1H, Py), 7.20–7.11 (m, 2H, Py), 7.07 (t, J = 8.6 Hz,
2H, C6H4), 6.52 (br s, 1H, NH), 4.25 (d, J = 5.0 Hz, 2H, CH2). 13C
{1H} NMR (101 MHz, CDCl3): d 164.99 (d, J = 254.5 Hz), 154.87,
149.02, 137.07, 136.02 (d, J = 3.4 Hz), 129.97, 129.88, 122.82,
122.20, 116.31, 116.09, 47.50. 19F NMR (564 MHz, CDCl3): d
ꢀ105.60. IR (KBr, cmꢀ1): 3075 (br), 2867 (br), 1591, 1492, 1335,
1165, 839, 552.
Scheme 3. Rate-determining step and its transition state.

I. Demianets et al. / Polyhedron 182 (2020) 114508
5
4.4. Synthesis 1-Me and 1-F
6.99 (d, J = 7.9 Hz, 2H, C6H4), 6.91 (t, J = 6.7 Hz, 1H, Py), 4.76 (d,
J = 17.2 Hz, 1H, CH2), 4.43 (d, J = 17.2 Hz, 1H, CH2), 2.26 (s, 3H,
Me), 1.81 (s, 15H, 5Me), ꢀ10.39 (s, 1H, IrH). 13C{1H} NMR
(151 MHz, CD2Cl2): d 166.07, 152.71, 141.33, 140.20, 136.51,
128.48, 127.86, 123.46, 119.79, 87.73, 61.60, 21.34, 10.19. IR
*
[Cp IrCl2]2 (100 mg, 0.125 mmol), ligand (H[NNMe] or H[NNF],
0.250 mmol), and triethylamine (38 mg, 0.375 mmol) were stirred
with CH2Cl2 (5 mL) for 3 h at room temperature. Then, all volatiles
were removed in vacuum, the residue was suspended in hexanes
(5 mL) and filtered. The solid was washed with hexane (5 mL)
and then with water (3 ꢁ 5 mL). The yellow solid was dried on
the filter and then crystallized from CH2Cl2/hexanes to give the
product.
1-Me: Orange crystals (156 mg, 93%). Crystals suitable for X-ray
analysis were obtained by slow evaporation of CH2Cl2/hexane solu-
tion. 1H NMR (600 MHz, CD2Cl2): d 8.51 (d, J = 5.2 Hz, 1H, Py), 7.79
(d, J = 8.2 Hz, 2H, C6H4), 7.70 (td, J = 7.7, 1.5 Hz, 1H, Py), 7.26 (t,
J = 6.9 Hz, 1H, Py), 7.21 (d, J = 7.9 Hz, 1H, Py), 7.10 (d, J = 8.0 Hz,
2H, C6H4), 4.76 (br s, 1H, CH2), 4.52 (br s, 1H, CH2), 2.30 (s, 3H,
Me), 1.69 (s, 15H, 5Me). 13C{1H} NMR (101 MHz, CD2Cl2): d
164.66, 151.57, 141.08, 140.91, 138.34, 128.98, 128.40, 124.97,
120.68, 86.82, 58.10, 21.40, 9.65. IR (KBr, cmꢀ1): 1569, 1474,
1447, 1402, 1380, 1355, 1275, 1225, 1134, 1104, 1084, 1056,
1031, 1001, 961, 932, 816, 768, 723, 709, 661, 620, 591, 556,
(KBr, cmꢀ1): 2907 (br), 2100 (
mIrH), 1449, 1289, 1137, 1092, 667,
558. MALDI-MS: m/z calcd for [C23H29IrN2O2S]+ 590.16, found
589.3.
3-F: Pale-yellow crystals (71 mg, 75%). 1H NMR (600 MHz, CD2-
Cl2): d 8.34 (d, J = 5.7 Hz, 1H, Py), 7.73–7.65 (m, 4H, C6H4), 7.53 (td,
J = 7.7, 1.5 Hz, 1H, Py), 7.20 (d, J = 7.8 Hz, 1H, Py), 6.91 (t, J = 6.6 Hz,
1H, Py), 6.84 (t, J = 8.9 Hz, 2H, C6H4), 4.79 (d, J = 17.2 Hz, 1H, CH2),
4.42 (d, J = 17.2 Hz, 1H, CH2), 1.81 (s, 15H, 5Me), ꢀ10.36 (s, 1H, IrH).
13C{1H} NMR (151 MHz, CD2Cl2): d 165.69, 163.67 (d, J = 248.5 Hz),
152.87, 140.43, 136.65, 130.30 (d, J = 8.6 Hz), 123.53, 119.79,
114.61 (d, J = 22.0 Hz), 87.78, 61.54, 10.19. 19F NMR (564 MHz,
CD2Cl2): d ꢀ112.03. IR (KBr, cmꢀ1): 2118 (
mIrH), 1592, 1495, 1284,
1137, 670, 543.
4.7. Synthesis of 4-Me and 4-F
536, 506, 485, 482, 477, 468, 452. MALDI-MS: m/z calcd for [C23
-
Complex 1 (0.160 mmol) and potassium tert-butoxide (90 mg,
0.801 mmol) were stirred with CH2Cl2 (6 mL) for 30 min to give
a dark blue solution of 2. Then, the reaction was continued outside
glovebox under direct sunlight for one hour. After the mixture
turned red it was filtered through a PTFE syringe filter. The result-
ing solution was dried under vacuum and crystallized twice from
CH2Cl2/hexanes to give an off-white crystalline material.
H28IrN2O2S]+ 589.15, found 589.32.
1-F: Yellow crystals (141 mg, 90%). 1H NMR (600 MHz, CD2Cl2):
d 8.51 (d, J = 6.4 Hz, 1H, Py), 7.97 (dd, J = 9.0, 5.4 Hz, 2H, C6H4), 7.71
(td, J = 7.7, 1.5 Hz, 1H, Py), 7.27 (t, J = 7.3 Hz, 1H, Py), 7.22 (d,
J = 8.5 Hz, 1H, Py), 6.96 (t, J = 8.9 Hz, 2H, C6H4), 4.78 (d,
J = 16.9 Hz, 1H, CH2), 4.47 (d, J = 18.0 Hz, 1H, CH2), 1.69 (s, 15H,
5Me). 13C{1H} NMR (151 MHz, CD2Cl2):
d
164.17 (d,
4-Me: Pale-yellow powder (75 mg, 80%). Crystals suitable for X-
ray analysis were obtained by slow evaporation of CH2Cl2/pentane
solution. 1H NMR (600 MHz, CD2Cl2): d 8.27–8.23 (m, 1H), 8.11 (s,
1H), 7.47–7.40 (m, 2H), 7.38 (dt, J = 8.9, 1.4 Hz, 1H), 7.20 (d,
J = 8.0 Hz, 2H), 6.41 (dd, J = 9.0, 6.2 Hz, 1H, CH2), 5.80 (dd,
J = 7.4, 6.2 Hz, 1H, CH2), 2.38 (s, 3H, Me), 1.78 (s, 15H, 5Me). 13C
{1H} NMR (101 MHz, CD2Cl2): d 166.92, 149.87, 146.58, 145.59,
139.72, 136.87, 136.00, 124.07, 124.02, 122.02, 119.64, 88.43,
60.57, 21.03, 8.63. IR (KBr, cmꢀ1): 2914, 1341, 1259, 1215, 1148,
1124, 1086, 849, 833, 802, 764, 728, 683, 664, 617, 601, 583,
559, 541, 510, 483, 464, 456. MALDI-MS: m/z calcd for [C23H27IrN2-
O2S]+ 588.14, found 587.3.
J = 249.0 Hz), 164.39, 151.56, 140.08 (d, J = 2.9 Hz), 138.45,
131.02 (d, J = 8.7 Hz), 125.08, 120.74, 115.15 (d, J = 22.0 Hz),
86.93, 57.98, 9.65. 19F NMR (564 MHz, CD2Cl2): d ꢀ111.48. IR
(KBr, cmꢀ1): 1591, 1497, 1277, 1136, 940, 667, 558. MALDI-MS:
m/z calcd for [C22H25FIrN2O2S]+ 593.13, found 593.35.
4.5. Synthesis of 2-Me and 2-F
Complex 1 (0.080 mmol) and potassium tert-butoxide (45 mg,
0.400 mmol) were stirred with CD2Cl2 (1.0 mL) for 20 min to give
a dark blue solution of 2. It was filtered through a PTFE syringe fil-
ter and then analyzed by 1H NMR. Complex 2 is moisture-, air-, and
light-sensitive, therefore any isolation attempt led to
decomposition.
4-F: Pale-yellow crystals (68 mg, 72%). 1H NMR (600 MHz, CD2-
Cl2): d 8.29 (d, J = 5.2 Hz, 1H, ArH), 7.58 (td, J = 7.7, 1.4 Hz, 1H, ArH),
7.26–7.12 (m, 3H, ArH), 7.03 (t, J = 6.6 Hz, 1H, ArH), 6.57 (td, J = 8.8,
2.5 Hz, 1H, ArH), 5.11 (d, J = 18.3 Hz, 1H, CH2), 4.41 (d, J = 18.3 Hz,
1H, CH2), 1.73 (s, 15H, 5Me). 13C{1H} NMR (151 MHz, CD2Cl2): d
167.25, 163.67 (d, J = 250.3 Hz), 150.38, 149.37, 145.85, 137.67,
124.75, 124.53 (d, J = 8.7 Hz), 121.50 (d, J = 17.1 Hz), 120.27,
110.40 (d, J = 23.1 Hz), 89.19, 61.01, 9.06. 19F NMR (564 MHz, CD2-
Cl2): d ꢀ113.54. IR (KBr, cmꢀ1): 2921, 1557, 1267, 1144, 862, 549.
2-Me: 1H NMR (600 MHz, CD2Cl2): d 8.25 (d, J = 6.8 Hz, 1H, Py),
8.11 (s, 1H, CH), 7.43 (d, J = 7.7 Hz, 2H, C6H4), 7.37 (d, J = 8.9 Hz, 1H,
Py), 7.20 (d, J = 7.8 Hz, 2H, C6H4), 6.40 (d, J = 6.7 Hz, 1H, Py), 5.80 (t,
J = 6.6 Hz, 1H, Py), 2.38 (s, 3H, Me), 1.78 (s, 15H, 5Me).
2-F: 1H NMR (600 MHz, CD2Cl2): d 8.25 (d, J = 7.1 Hz, 1H, Py),
8.12 (s, 1H, CH), 7.59–7.52 (m, 2H, C6H4), 7.39 (d, J = 9.0 Hz, 1H,
Py), 7.12–7.05 (m, 2H, C6H4), 6.42 (ddd, J = 9.0, 6.2, 1.1 Hz, 1H,
Py), 5.81 (t, J = 7.4 Hz, 1H, Py), 1.78 (s, 15H, 5Me). 19F NMR
(564 MHz, CD2Cl2): d ꢀ107.79.
CRediT authorship contribution statement
4.6. Synthesis of 3-Me and 3-F
Ivan Demianets: Conceptualization, Methodology, Formal anal-
ysis, Investigation. Valeriy Cherepakhin: Formal analysis, Investi-
gation. Alexander Maertens: Investigation. Paul J. Lauridsen:
Investigation. Shaama Mallikarjun Sharada: Formal analysis,
Supervision. Travis J. Williams: Conceptualization, Formal analy-
sis, Supervision, Funding acquisition.
Complex 1 (0.160 mmol) and potassium tert-butoxide (90 mg,
0.801 mmol) were stirred with CH2Cl2 (5 mL) for 20 min to give
a dark blue solution of 2. Then, 2 drops of formic acid were added
to cause a sharp color change to orange. The mixture was stirred
for another 10 min with NaHCO3 (50 mg) and then filtered through
a PTFE syringe filter. The resulting solution was dried under vac-
uum and crystallized twice from CH2Cl2/hexanes to afford the
product.
Declaration of Competing Interest
3-Me: Pale-yellow crystals (89 mg, 95%). 1H NMR (600 MHz,
CD2Cl2): d 8.35 (d, J = 5.7 Hz, 1H, Py), 7.55 (d, J = 8.2 Hz, 2H,
C6H4), 7.53 (td, J = 7.7, 1.5 Hz, 1H, Py), 7.20 (d, J = 7.8 Hz, 1H, Py),
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.

6
I. Demianets et al. / Polyhedron 182 (2020) 114508
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This work was sponsored by the National Science Foundation
(CHE-1856395) and the Hydrocarbon Research Foundation. We
thank the NSF (DBI-0821671, CHE-0840366, CHE-1048807) and
the NIH (S10 RR25432) for analytical instrumentation. ID and VC
acknowledge Sonosky fellowship support from the USC Wrigley
Institute for Environmental Studies. We thank Dr. Ralf Haiges for
help with X-ray crystallography.
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