Light-promoted C-Cl bond-forming reductive elimination of a metal-metal bonded PdIII-PdIIIcomplex

Article abstract of DOI:10.1039/d1cc03344a

The excited-state reductive elimination (RE) activities of a metal-metal bonded PdIII2complex, [(2-phenylpyridyl)Pd(μ-acetate)Cl]₂, are described. The C-Cl bond-forming RE reaction is accelerated by up to five orders of magnitude upon visible p

Full text of DOI:10.1039/d1cc03344a

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Light-promoted C–Cl bond-forming reductive
elimination of a metal–metal bonded PdIII–PdIII
complex†
Cite this: Chem. Commun., 2021,
57, 7673
Received 23rd June 2021,
Accepted 7th July 2021
Jeongcheol Shin,
Kuldeep Gogoi and Kiyoung Park
*
DOI: 10.1039/d1cc03344a
rsc.li/chemcomm
The excited-state reductive elimination (RE) activities of a metal– been assessed yet. Thus, inspired by the photochemical proper-
metal bonded Pd2III complex, [(2-phenylpyridyl)Pd(l-acetate)Cl]2, are ties of analogous metal–metal bonded complexes,2a,2b we
described. The C–Cl bond-forming RE reaction is accelerated by up to have characterized the electronic structure of a binuclear
five orders of magnitude upon visible photoexcitation, which induces PdI2II complex, [(2-phenylpyridyl; phpy)PdIII(m-acetate)Cl]2 (1)
the Pd–Pd bond dissociation. This ligand field variation reduces the by employing electronic absorption (Abs), magnetic circular
energy cost for intramolecular charge transfer (ICT) from coupling dichroism (MCD), and resonance Raman (rR) spectroscopies in
substrate to the metal center involved in the RE reaction. The combination with density functional theory (DFT) computa-
correlation found between the ligand field and the RE activity tions, and examined its photochemical properties.
indicates the ICT energy as a fundamental descriptor for RE reactions.
Our previous study on high-valent Ni complexes has shown
that the C–C bond-forming RE activity is determined by the
Binuclear metal–metal bonded complexes have been widely energy separation between the electron-donating C-based MO
utilized to perform diverse chemistries such as C–H functiona- and the electron-accepting Ni-based MO.6 This energy separa-
lization, halogen atom abstraction, and DNA cleavage.1 As their tion is a function of the Ni–C covalency and the ligand field of
frontier molecular orbitals (FMOs) involve metal–metal bond- the Ni center. Thus, considering that photoexcitation can alter
ing and antibonding interactions, these systems often display the metal–metal bond order and thus ligand field,7 we antici-
unique photophysical and photochemical properties.2 For pated that the photoexcitation of the PdI2II complex would affect
4À
instance, Pt2(m-pyrophosphito)4
becomes more active for its RE activity, providing a basis for investigating correlations
hydrogen and halogen atom abstraction upon visible to near- between the ligand field and the RE activity.
UV photoexcitation, which is associated with the electronic
At À35 1C under dark conditions, 1 was stable (Fig. S1, ESI†),
transition from the Pt2 5dz2–5dz2 s*-antibonding MO (ds*) to but 442 nm irradiation induced the decay of 1 with a decrease
the Pt2 6p–6p s-bonding MO (ps).2,3 This transition results in a in its characteristic Abs bands at 17 500 cmÀ1 (570 nm) and
long-lived triplet state (t B 9 ms) and a significant variation in 24 000 cmÀ1 (415 nm) (Scheme 1 and Fig. 1a). The final
the Pt–Pt bond order.
spectrum obtained after 2 hour irradiation displayed the Abs
Binuclear Pd2 species appear as active intermediates in high- bands at 24 500 cmÀ1 (410 nm) and 29 000 cmÀ1 (305 nm),
valent Pd-mediated cross-coupling reactions.4 In the presence indicative of the PdII product (Fig. S2, ESI†). The yield of the
of hypervalent iodine or N-halosuccinimide reagents, PdII C–Cl coupled product 2 was assessed with 1H NMR spectro-
(acetate)2 catalyst can convert to (m-1,3-carboxylato)-bridged scopy, by following a protocol previously reported.5b This
PdI2II dimers, which then can undergo C–Cl, C–Br, C–O, and
C–C cross-coupling reactions with an improved reductive elim-
ination (RE) activity than corresponding PdIV monomers.5
While the PdI2II dimers involve Pd–Pd bonds that can be variable
with electronic transitions, their excited-state chemistry has not
Department of Chemistry, Korea Advanced Institute of Science and Technology (KAIST),
Daejeon 34141, Republic of Korea. E-mail: kiyoung.park@kaist.ac.kr
† Electronic supplementary information (ESI) available: Methods and materials,
details of photoreactions, NMR, Abs, resonance Raman, and MCD spectra of Pd
Scheme 1 Excited-state C–Cl bond-forming RE reaction of 1.
Chem. Commun., 2021, 57, 7673–7676
complexes, and DFT-computed results. See DOI: 10.1039/d1cc03344a
This journal is © The Royal Society of Chemistry 2021
| 7673

View Article Online
Communication
ChemComm
Fig. 1 (a) Abs-monitored decay of 1 upon 442 nm excitation at À35 1C.
The time interval between spectra is 5 min. Plots in (b) and (c) show
changes in the concentration of 1, [1], and the natural logarithm of [1]
versus irradiation time, respectively, obtained with 785 (black), 532 (green),
442 (red), and 266 (blue) nm laser excitations with a normalized power of
50 mW. (d) 1st-order rate constants of the photo-induced RE reactions of
1, overlaid with the Abs spectrum.
Fig. 2 (a) Raman spectra of 1 obtained with 785 nm (top, black) and
532 nm (top, blue) laser excitation, and DFT-calculated Raman spectrum
of 1 (bottom). (b) rR enhancement profile of the Pd-ligand vibrations at
analysis confirmed that the photoexcitation induced the C–Cl
bond-forming RE reaction from 1 (Fig. S3, ESI†).
162 (J), 242 (&), 279 (}
), 306 (n), and 372 cmÀ1 (,). (c) Raman spectra of
[(phpy)PdII(OAc)]2 (top) and 1 (bottom) obtained with 532 nm laser excita-
tion. (d) rR enhancement profile of the phpy vibrations at 1035 (J),
1316 (&), 1485 (}
), 1579 (n), and 1604 cmÀ1 (,). Abs spectrum of 1 is
overlaid with the rR profiles. Further details are in Fig. S11, ESI.† (e) DFT-
calculated normal modes obtained with the B3LYP functional and the
combined basis sets of LANL2TZ(f) for Pd and 6-311g(d,p) for others.
Various excited states were accessed with 10 different excita-
tion energies, 830, 785, 633, 532, 488, 442, 405, 355, 325, and
266 nm. The C–Cl coupling rates were measured by monitoring
the Abs decay at 21 000 cmÀ1 (475 nm), where the PdII product
does not contribute (Fig. 1a). Except for the 785 and 830 nm
laser lines, where 1 does not absorb and thus no reaction
occurred, other energy lines effectively induced the C–Cl cou-
pling from 1. This photoreaction followed 1st-order kinetics,
indicating that the C–Cl bond formation is an intramolecular
reaction (Fig. 1b and c). The possibility of photo-induced Pd–Cl
bond dissociation preceding the C–Cl bond formation could be
excluded, as exogenous chloride ions or radical trapping agents
(a-phenyl-N-tert-butylnitrone) did not affect the reaction rate
(Fig. S8 and S9, ESI†).
excitations (Fig. 2). The low-energy features can be associated
with Pd-ligand stretching vibrations. Considering that features at
279 and 372 cmÀ1 are also displayed in the rR spectrum of
[(phpy)PdII(OAc)]2, these are assigned to the Pd–acetate (OAc)
and Pd–phpy stretching vibrations. Features at 162, 242, and
306 cmÀ1 that appear upon addition of PhICl2 to [(phpy)
PdII(OAc)]2 can be related to the Pd–Pd and Pd–Cl bonds. The
previous rR study of an analogous d7–d7 complex, [Pt(m-pyro-
phosphito)2Cl]24À, reported the Pt–Pt and Pt–Cl stretching vibra-
tions at 158 and 304 cmÀ1,8 which match well the rR features at
162 and 306 cmÀ1 of 1. Thus, the features at 162 and 306 cmÀ1
can be attributed to the Pd–Pd and symmetric Pd–Cl stretching
vibrations, respectively. This assignment is supported by DFT-
calculated normal modes, which further revealed that the phpy-
Pd–Cl bending vibration can mix with the Pd–Cl stretching
vibration and thus match the rR feature at 242 cmÀ1 with mild
resonance enhancement (Fig. 2e). The rR profiles of these vibra-
tions indicate that the electronic transition at 17 500 cmÀ1
(570 nm) involves a significant distortion along the Pd–Cl and
Pd–Pd bonds.
The trend in k’s as a function of excitation energy is parallel
to the Abs spectrum of 1, indicating comparable quantum
efficiencies throughout the visible range (Fig. 1d and Table
S1, ESI†). The maximum rate constant k of 0.16 minÀ1 was
obtained with the 442 nm laser excitation, being 5000 times
greater than that of dark conditions (2.9 Â 10À5 minÀ1
)
obtained from the extrapolation of higher-temperature k’s
using the Eyring equation (Fig. S1, ESI†). A higher-power
427 nm LED light (45 W) promoted the RE reaction even further,
displaying 105 times improved activity with k B 4.0 minÀ1. The
final yields of the photoreactions were comparable to those of
thermal reactions at elevated temperatures (Fig. S3 and S4, ESI†).
To gain insights into the electronic structure of 1, rR spectro-
scopy has been utilized. Compared to the off-resonance spectrum
obtained with 785 nm excitation, features at energies below
400 cmÀ1 and above 1000 cmÀ1 displayed enhanced intensities
in the rR spectra obtained with 633, 532, 515, and 488 nm
The high-energy rR features at 1000–1700 cmÀ1 are almost
identical for 1 and [(phpy)PdII(OAc)]2 (Fig. 2c). These vibrations
also appear in the spectra of IrIII(phpy)3 and H-phpy,9 and thus
can be attributed to the in-plane vibrations of the phpy ligand
(Fig. S11, ESI†). Although rR spectra with an excitation energy
7674 | Chem. Commun., 2021, 57, 7673–7676
This journal is © The Royal Society of Chemistry 2021

View Article Online
ChemComm
Communication
despite the different nature of the transitions (Table S1, ESI†)
indicates that the photoreaction of 1 would follow Kasha’s rule,
i.e., the RE reaction likely occurs from a common state, such as the
T1 state. The feasibility of ISC to the T1 state was also assessed by
DFT computations. The DFT-optimized structure of 1 at the T1 state
has the electronic configuration of (Pd2 ds)1(Pd2 ds*)1 and a
significantly elongated distance between the Pd centers (3.14 Å for
T1 vs. 2.71 Å for S0) with a majorly reduced Mayer bond order (0.065
for T1 vs. 0.419 for S0). The DFT-calculated spin–orbit coupling
between the singlet and triplet states of the (ds)1(ds*)1 configu-
ration of 1 (22.2) is comparable to that of [PtIII(m-pyrophosphito)2Cl]2
(23.1), supporting that the visible photoexcitation of 1 can lead to the
metal–metal bond dissociated T1 state (Table S6, ESI†).
Fig. 3 (a) 5 T 273 K MCD and (b) 273 K Abs spectra of 1, with dashed sum
spectra of colored Gaussian bands. (c) TDDFT-simulated Abs spectrum of
1, and (d) frontier MOs responsible for colored transitions in (c).
To understand the enhanced RE activity of the excited states,
the reaction coordinates of the T1 and S0 states have been
higher than 488 nm could not be collected due to the fast compared (Fig. 4a). The T1-state 5-coordinated (5C) Pd2III
photo-decay of 1 even under liquid nitrogen, the intensity complex is calculated to involve a smaller kinetic barrier of
profiles of the phpy vibrations show a growing trend as the DG‡ = 16.8 kcal molÀ1 than the S0-state 6-coordinated (6C) PdI2II
excitation energy approaches the Abs band at 419 nm. This complex with DG‡ = 22.5 kcal molÀ1. As the products of the two
enhancement profile implies that electronic transitions at pathways differ in the Pd oxidation state, the calculated thermo-
20 000–28 000 cmÀ1 (500–355 nm) involve charge transfer from dynamics is less driven for the T1 state (DG0 = +0.2 kcal molÀ1
)
the phpy ligand.
than the S0 state (DG0 = À1.7 kcal molÀ1). Thus, a difference in
To resolve electronic transitions further, the MCD and Abs the intrinsic barriers of the two pathways is more significant
spectra of 1 were iteratively deconvoluted with Gaussian func- (DG‡int = 16.7 kcal molÀ1 for the T1 state versus 23.3 kcal molÀ1 for
tions. At least five different transitions, bands 1–5 in Fig. 3, the S0 state). The experimental barrier (DG‡532 nm = 16.9 kcal molÀ1
)
compose the Abs features at 15 000–32 000 cmÀ1 (670–310 nm). obtained with the excitation to band 1 matches well the DFT-
Band 1, with the highest efficiency for the C–Cl coupling calculated barrier of the T1 state, supporting that the photoactivity
reaction, displays the highest MCD-to-Abs intensity ratio, which of 1 would derive from the T1 state. Under dark conditions,
is indicative of Pd-based transitions rather than electric-dipole though, the T1 state may also be thermally accessible, given a
allowed transition such as charge transfer (CT) transitions small energy difference of DGT1-S0 B 3.1 kcal molÀ1. The experi-
(Tables S1 and S4, ESI†). Consistently, time-dependent DFT mental barrier of the dark condition, DG‡dark = 20.7 kcal molÀ1
,
(TDDFT) computation shows that band 1 is associated with the is better reproduced by the pathway that involves a spin
Pd2 ds-to-ds* transition, which should alter the Pd–Pd bond
order significantly, agreeing with the rR enhancement profiles
of the Pd–Pd stretching vibration. TDDFT-calculated transi-
tions that correspond to bands 2 and 3 are phpy-to-Pd2 ds*
CT transitions, which also match the rR enhancement of the
phpy-based vibrations. Bands 4 and 5 can be associated with
the acetate-to-Pd2 ds* CT and Pd2 ds-to-dx2Ày2 transitions,
respectively (Fig. 3). Additionally, the low-efficiency band at
39 000 cmÀ1 (255 nm) in Fig. 1d can be attributed to the p-to-p*
transition of the phpy ligand, as shown in relevant complexes such
as IrIII(phpy)3, PdII(phpy)2, and PdII(ethylenediamine)(phpy).10
The electronic spectral assignment and rR profile of 1
indicate that all the electronic transitions in the visible range
involve the Pd2 ds* acceptor MO and thus induce a significant
variation in the Pd–Pd bond order. Related d7–d7 complexes, such as
[Re0(bis(diphenylphosphino)methane)(CO)3]2, [RhII(m-acetate)2(py)]2,
[PtIII(m-C6H3-5-Me-2-AsPh2)2Cl]2, and [PtIII(m-pyrophosphito)2Cl]2, are
known to undergo intersystem crossing (ISC) to a long-lived triplet
state (t = 3–30 ms) with the reduced metal–metal bond order upon
UV-vis photoexcitation (Table S5, ESI†).7,11 The excited-state lifetime
of 1, though, could not be measured, as the fast photo-decaying
nature of 1 prevented good signal-to-noise data acquisition for
(red) and T1 (blue) of 1 drawn with Gibbs free energy. Energies in
Fig. 4 (a) Reaction coordinate calculations for C–Cl RE reactions of S0
luminescence or transient spectroscopy. However, the fact that all
parentheses are values compared to R(T1). (b) FMO changes upon the
visible photoexcitation displays comparable quantum efficiencies geometric change from R to TS.
This journal is © The Royal Society of Chemistry 2021
Chem. Commun., 2021, 57, 7673–7676 | 7675

View Article Online
Communication
ChemComm
five orders of magnitude. The spectroscopic and computational
analyses indicate that visible light photoexcitation can effec-
tively dissociate the Pd–Pd bond of 1. The resulting 5C Pd2III
species is more RE-active than the ground-state 6C PdI2II species,
because of a weaker ligand field and thus a reduced ICT energy
cost. This finding on the correlation between the ligand field
and the RE activity establishes the ICT energy cost as a funda-
mental descriptor for RE reactions, which can be useful for
understanding perturbations in RE activity and designing new
catalysts with optimized RE activities.
This work was financially supported by Samsung Science
and Technology Foundation (SSTF-BA1801-05). We thank
Center for Catalytic Hydrocarbon Functionalizations, IBS for
computational and rR spectroscopic resources.
Fig. 5 Variations in the ICT energy depending on ligand field.
crossover from the S0 reactant to the T1 transition state with
DG‡ = 19.9 kcal molÀ1 (Fig. S22, ESI†).
Conflicts of interest
The increased RE activity of the excited-state 5C PdI2II species
relative to the ground-state 6C PdI2II species demonstrates the
effect of ligand field on the RE activity. The reaction coordi-
nates show that upon conversion from reactant (R) to transition
state (TS), electrons are transferred from the Cl-based
s-bonding MO to the Pd-based lowest-unoccupied s*-
antibonding MO (#134 - #148 for S0 and #135 - #147 for
T1 in Fig. 4b). The energy separations between these frontier
MOs are 4.11 and 3.52 eV for the 6C and 5C species, respec-
tively, indicating that the internal charge transfer (ICT) process
would require B13.6 kcal molÀ1 more energy for the 6C
species than the 5C species (Fig. 5). This ICT energy difference
parallels the intrinsic barrier differences of the two species
(DDG‡int B 6.6 kcal molÀ1), considering that geometric relaxa-
tion would occur. The lower ICT energy cost of the 5C species
originates from the lower-energy Pd dz2-based unoccupied MO,
which is significantly stabilized upon Pd–Pd bond dissociation
(Fig. 5).
This correlation found between the RE activity and the
ligand field suggests the ICT energy as a fundamental descrip-
tor of RE reactions. A previous study on the C–C coupling
activity of PtIV complexes showed that the 5C PtIV species are
more kinetically competent for the RE reaction than the 6C PtIV
species (Fig. S23, ESI†).12 This ligand-field effect on the C–C
coupling activity can also be understood by a variation in the
ICT energy, being parallel to the ground- vs. excited-state Pd2III
RE activities (Fig. S16, ESI†). The same principle can also be
applied to the lower RE activity of the mononuclear (phpy)2
PdIV(Cl)(OAc) species, 40% formation of 2 in 24 hours at
80 1C.13 Compared to 1, the PdIV monomer contains a stronger
ligand field with an additional C ligand and more covalent
Pd–C bonds. Thus, the energy separation between the ICT-
There are no conflicts to declare.
Notes and references
1 (a) I. G. Powers and C. Uyeda, ACS Catal., 2017, 7, 936–958;
(b) H. M. L. Davies and K. B. Liao, Nat. Rev. Chem., 2019, 3,
347–360; (c) J. F. Berry and C. C. Lu, Inorg. Chem., 2017, 56,
7577–7581; (d) H. T. Chifotides and K. R. Dunbar, Acc. Chem. Res.,
2005, 38, 146–156; (e) K. Matsumoto and M. Ochiai, Coord. Chem.
Rev., 2002, 231, 229–238.
2 (a) D. M. Roundhill, H. B. Gray and C. M. Che, Acc. Chem. Res., 1989,
22, 55–61; (b) J. E. Bercaw, A. C. Durrell, H. B. Gray, J. C. Green,
N. Hazari, J. A. Labinger and J. R. Winkler, Inorg. Chem., 2010, 49,
1801–1810; (c) R. M. van der Veen, C. J. Milne, A. El Nahhas,
F. A. Lima, V. T. Pham, J. Best, J. A. Weinstein, C. N. Borca,
R. Abela, C. Bressler and M. Chergui, Angew. Chem., Int. Ed., 2009,
48, 2711–2714.
3 (a) C. M. Che, L. G. Butler, H. B. Gray, R. M. Crooks and
W. H. Woodruff, J. Am. Chem. Soc., 1983, 105, 5492–5494;
(b) D. J. Thiel, P. Livins, E. A. Stern and A. Lewis, Nature, 1993,
362, 40–43.
4 D. C. Powers and T. Ritter, Acc. Chem. Res., 2012, 45, 840–850.
5 (a) D. C. Powers and T. Ritter, Organometallics, 2013, 32, 2042–2045;
(b) D. C. Powers and T. Ritter, Nat. Chem., 2009, 1, 302–309.
6 J. Shin, S. Gwon, S. Kim, J. Lee and K. Park, J. Am. Chem. Soc., 2020,
142, 4173–4183.
7 (a) A. E. Stiegman, V. M. Miskowski and H. B. Gray, J. Am. Chem.
Soc., 1986, 108, 2781–2782; (b) M. A. Bennett, S. K. Bhargava,
E. C.-C. Cheng, W. H. Lam, T. K.-M. Lee, S. H. Priver, J. Wagler,
A. C. Willis and V. W.-W. Yam, J. Am. Chem. Soc., 2010, 132,
7094–7103; (c) A. E. Stiegman and V. M. Miskowski, J. Am. Chem.
Soc., 1988, 110, 4053–4054.
8 C. M. Che, W. P. Schaefer, H. B. Gray, M. K. Dickson, P. B. Stein and
D. M. Roundhill, J. Am. Chem. Soc., 1982, 104, 4253–4255.
9 S. H. Lai, J. W. Ling, Y. M. Huang, M. J. Huang, C. H. Cheng and
I. C. Chen, J. Raman Spectrosc., 2011, 42, 332–338.
10 (a) M. G. Colombo, T. C. Brunold, T. Riedener, H. U. Gudel,
M. Fortsch and H. B. Burgi, Inorg. Chem., 1994, 33, 545–550;
(b) M. Maestri, D. Sandrini, V. Balzani, A. Vonzelewsky,
C. Deuschelcornioley and P. Jolliet, Helv. Chim. Acta, 1988, 71,
1053–1059; (c) M. A. Ivanov and M. V. Puzyk, Opt. Spectrosc., 2001,
91, 869–872.
relevant FMOs of the PdIV complex is calculated to be 1.21 eV 11 P. M. Bradley, B. E. Bursten and C. Turro, Inorg. Chem., 2001, 40,
greater than that of the Pd2 6C species (Fig. 5 and Fig. S16,
1376–1379.
12 (a) J. Procelewska, A. Zahl, G. Liehr, R. van Eldik, N. A. Smythe,
ESI†). Accordingly, the RE reaction barrier is calculated to be
B. S. Williams and K. I. Goldberg, Inorg. Chem., 2005, 44, 7732–7742;
(b) A. T. Luedtke and K. I. Goldberg, Inorg. Chem., 2007, 46,
8496–8498.
higher for the mononuclear system (DG‡ = 28.1 kcal molÀ1 for
the 6C PdIV monomer vs. 22.5 for the 6C PdI2II dimer).
13 S. R. Whitfield, New Oxidation Reactions of Palladium and Platinum:
Synthetic and Mechanistic Investigations., University of Michigan,
2008.
In conclusion, we have observed that electronic excitation
can accelerate the C–Cl bond-forming RE reaction of 1 by up to
7676 | Chem. Commun., 2021, 57, 7673–7676
This journal is © The Royal Society of Chemistry 2021