Three-way cooperativity in d8 metal complexes with ligands displaying chemical and redox non-innocence

Article abstract of DOI:10.1002/chem.201403276

Reversible proton- and electron-transfer steps are crucial for various chemical transformations. The electron-reservoir behavior of redox non-innocent ligands and the proton-reservoir behavior of chemically non-innocent ligands can be cooperatively utilized for substrate bond activation. Although site-decoupled proton- and electron-transfer steps are often found in enzymatic systems, generating model metal complexes with these properties remains challenging. To tackle this issue, we present herein complexes [(cod_-H)M(μ-L^2-)M(cod_-H)] (M=Pt^II, [1] or Pd^II, [2], cod=1,5-cyclooctadiene, H₂L=2,5-di-[2,6-(diisopropyl)anilino]-1,4-benzoquinone), in which cod acts as a proton reservoir, and L^2- as an electron reservoir. Protonation of [2] leads to an unusual tetranuclear complex. However, [1] can be stepwise reversibly protonated with up to two protons on the cod^-H ligands, and the protonated forms can be stepwise reversibly reduced with up to two electrons on the L^2- ligand. The doubly protonated form of [1] is also shown to react with OMe^- leading to an activation of the cod ligands. The site-decoupled proton and electron reservoir sources work in tandem in a three-way cooperative process that results in the transfer of two electrons and two protons to a substrate leading to its double reduction and protonation. These results will possibly provide new insights into developing catalysts for multiple proton- and electron-transfer reactions by using metal complexes of non-innocent ligands.

Full text of DOI:10.1002/chem.201403276

DOI: 10.1002/chem.201403276
Full Paper
&
Electron Transfer
8
Three-Way Cooperativity in d Metal Complexes with Ligands
Displaying Chemical and Redox Non-Innocence
[a]
[b]
[b]
[b]
[a]
Naina Deibel, Stephan Hohloch, David Schweinfurth, Fritz Weisser, Anita Grupp, and
[
b]
Biprajit Sarkar*
Dedicated to Prof. Konrad Seppelt on the occasion of his 70th birthday
Abstract: Reversible proton- and electron-transfer steps are
crucial for various chemical transformations. The electron-re-
servoir behavior of redox non-innocent ligands and the
proton-reservoir behavior of chemically non-innocent li-
gands can be cooperatively utilized for substrate bond acti-
vation. Although site-decoupled proton- and electron-trans-
fer steps are often found in enzymatic systems, generating
model metal complexes with these properties remains chal-
lenging. To tackle this issue, we present herein complexes
usual tetranuclear complex. However, [1] can be stepwise re-
versibly protonated with up to two protons on the cod li-
ÀH
gands, and the protonated forms can be stepwise reversibly
2
À
reduced with up to two electrons on the L ligand. The
doubly protonated form of [1] is also shown to react with
À
OMe leading to an activation of the cod ligands. The site-
decoupled proton and electron reservoir sources work in
tandem in a three-way cooperative process that results in
the transfer of two electrons and two protons to a substrate
leading to its double reduction and protonation. These re-
sults will possibly provide new insights into developing cata-
lysts for multiple proton- and electron-transfer reactions by
using metal complexes of non-innocent ligands.
2
À
II
II
[
(cod )M(m-L )M(cod )] (M=Pt , [1] or Pd , [2], cod=1,5-
ÀH ÀH
cyclooctadiene, H L=2,5-di-[2,6-(diisopropyl)anilino]-1,4-ben-
2
2À
zoquinone), in which cod acts as a proton reservoir, and L
as an electron reservoir. Protonation of [2] leads to an un-
Introduction
ically non-innocent ligands are capable of acting as proton-
storage sites, and this concept has been used for catalysis with
[3]
Proton- and electron-transfer processes are elementary steps
that are crucial for various bond-activation reactions used by
metal complexes of pincer-type ligands.
1,5-Cyclooctadiene (cod) and its related counterparts can
[
1]
2
2
enzymatic systems for substrate conversion. For example, in
cytochrome P450, protonated arginine and lysine act as
proton sources, and nicotinamide adenine dinucleotide phos-
phate (NADPH) as an electron source to make the activation of
bind in a chelating h ,h fashion to transition-metal centers
and are known to be susceptible to deprotonation reactions in
the metal-bound state. In doing so, the hapticity of such li-
2
2
2
1
gands changes from h ,h to h ,h , whereby deprotonation of
O possible, which then eventually leads to CÀH oxygenation
cod leads to a change of one of the olefin donors to a carba-
2
[
1b]
[4a–g]
2
3
reactions.
Combining proton-transfer steps with electron-
nionic donor (Scheme 1).
Coordination of the h ,h (p-allyl)
[4i–j]
transfer steps has the obvious advantage of maintaining low
reduction potentials for the electron-transfer processes. In en-
zymatic systems, the source of electrons and protons are usu-
ally different, and these centers cooperate at the active site to
type is also known for the deprotonated form of cod.
Li-
gands, such as 2,5-di-[2,6-(diisopropyl)anilino]-1,4-benzoqui-
none (H L), on the other hand, are typical examples of redox
2
[2]
non-innocent ligands. Such ligands in their deprotonated and
metal-coordinated form are capable of undergoing various re-
versible electron-transfer steps, and can thus exist in different
[
1]
make substrate activation possible. Redox non-innocent li-
gands are excellent tools for electron storage, and this proper-
ty of such ligands has been recently utilized for performing
[
2]
catalytic bond-activation reactions. On the other hand, chem-
[a] Dr. N. Deibel, Dr. A. Grupp
Institut fꢀr Anorganische Chemie
Universitꢁt Stuttgart, Pfaffenwaldring 55, 70569, Stuttgart (Germany)
[
b] S. Hohloch, Dr. D. Schweinfurth, F. Weisser, Prof. Dr. B. Sarkar
Institut fꢀr Chemie und Biochemie, Anorganische Chemie
Freie Universitꢁt Berlin, Fabeckstrasse 34-36, 14195, Berlin (Germany)
E-mail: biprajit.sarkar@fu-berlin.de
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201403276. It contains experimental details.
Scheme 1. Chemical non-innocence of cod (top) and redox non-innocence
of diimino-quinone ligands (bottom).
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[
5]
2
2
2
1
redox forms (Scheme 1). Even though there have been some
reports on transition-metal complexes that contain cod and
h ,h to h ,h , with the formation of a 1,2-h-6-s-cycloocta-1,4-
[4c–f]
dienyl (alkene/allyl) ligand.
The eventual formation of a neu-
[
6]
a redox-active ligand, to the best of our knowledge, reports
on stepwise reversible proton- and electron-transfer studies in
tral species, which is less reactive, is a likely reason for this
base-induced transformation. Remarkably, the position of de-
protonation at the cod ligand is dependent on the metal
center, as can be seen from a comparison of the X-ray crystal
structures of [1] and [2] (Scheme 2).
[6d–j]
metal complexes combining these ligands remain rare.
We have been interested in various facets of metal com-
plexes of redox-active ligands, including performing bond-acti-
[
7]
vation reactions with them. For generating such metal com-
plexes, we have used redox non-innocent ligands that contain
one or more [NR] groups because of the possibilities to tune
the steric and electronic properties that the R groups offer in
We were next interested in exploring if the cod ligands in
ÀH
[1] and [2] can be protonated back to cod leading to the con-
version of the allyl ligand to an olefin and a concomitant
change in the hapticity of the ligands. Gratifyingly, the proto-
nation of [1] with one equivalent of HDMF[OTf] (DMF=N,N-di-
[
8]
such ligands. Herein, we present the metal complexes
2À
II
II
À
[
(cod )M(m-L )M(cod )] (M=Pt , [1] or Pd , [2]), in which the
methylformamide, OTf =triflate) led to a protonation of one
ÀH
ÀH
deprotonated form of cod is bound to the metal centers. The
protonation products of these complexes are reported. Fur-
thermore, we show that in [1], reversible stepwise protona-
tion/deprotonation of the cod ligands, and reversible reduc-
of the cod ligands leading to the formation of [(cod )Pt(m-
ÀH ÀH
2À
L) Pt(cod)](OTf), [3]. The formation of [3] and the change of
one of the ligands from carbanionic to olefin were proven by
NMR spectroscopy. Surprisingly, protonation of the palladium
complex [2] with one equivalent of HDMF[OTf] led to the for-
mation of an unusual tetranuclear complex, the structure of
which was confirmed by single-crystal X-ray diffraction (see
2À
tion/oxidation of the L ligands are possible. We finally show
that these remote proton- and electron-transfer sites can work
in tandem for performing substrate bond activation. Com-
pounds [1] and [2], as well as all their various reduced and
protonated forms, have been characterized by an arsenal of
[9]
below). Based on literature reports, we propose two different
reaction pathways for the reaction of [1] and [2] with HDMF-
[OTf] (Scheme 3).
structural,
methods.
electrochemical,
and
spectroelectrochemical
Protonation of [1] proceeds through a metal-assisted con-
certed protonation of cod . This process leads to the release
ÀH
of DMF, and the change of allyl to alkene. However, for [2], the
II
initial step is likely an oxidative addition to the Pd center,
Results and Discussion
À
+
whereby OTf and HDMF are added to the palladium center.
[9]
Reductive elimination leads to the release of the ligand H L.
Synthesis and characterization
2
The thus formed reactive complex containing a labile DMF
ligand can react with unreacted [2] present is solution to gen-
erate the tetranuclear complex [4]. In compound [4], the O
The complexes [1] and [2] were synthesized by the reactions
of [Pt(cod)Cl ] or [Pd(cod)Cl ], respectively with H L in the pres-
2
2
2
1
ence of NEt . H NMR spectra of [1] and [2] indicated the loss
2À
II
3
donors of L act as a bridge between two Pd centers
Scheme 3). The detection of free H L during the protonation
of one proton each from the cod ligands, which was later con-
firmed by X-ray crystal structures of these complexes (see
below). The deprotonation step likely proceeds through an ini-
(
2
of [2], as well as the known propensity of palladium to readily
undergo oxidative-addition and reductive-elimination reactions
lend support to the proposed mechanism. The diplatinum
2
2
tial coordination of cod ligand to the metal centers in the h ,h
mode through the olefinic double bonds, as was found in the
complex [3] can be further protonated with another equivalent
precursor complex [Pt(cod)Cl ]. The coordinated cod is then
2À
2
of HDMF[OTf] to generate [(cod)Pt(m-L) Pt(cod)](OTf) , [5]. The
2
deprotonated by NEt3 present in the reaction mixture
cod ligands in [5] are all bound to the metal centers through
olefinic double bonds (Scheme 3).
The doubly protonated com-
(Scheme 2). This process leads to the generation of metal-coor-
dinated cod , and a change in the coordination mode from
ÀH
plex [5] was reacted with
NaOMe. This reaction leads to
complex [6], in which the cod li-
gands have been activated by
the formation of new CÀOMe
bonds through nucleophilic
attack (Scheme 4). The addition
À
of OMe to the cod ligands
leads to a p/s rearrangement
and to the change in their hap-
2
2
2
1
ticity from h ,h to h ,h . Such re-
arrangements are text-book ex-
amples of converting olefin com-
[4a,b]
plexes to alkyl complexes.
[
4d]
Scheme 2. Mechanism of formation of [1] and [2].
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the cod rings are longer than
ÀH
the two bond lengths men-
tioned above (Table S2 in the
Supporting Information). The
II
bond lengths within the Pd ana-
logue [2] are similar to [1],
except that the position of the
double bond neighboring the
carbanionic donor is now at
C5CÀC6C. This is a consequence
of the different positions of de-
protonation of the cod ligands
in [1] and [2], as has been dis-
cussed above. The metal–metal
distances in both [1] and [2] are
around 8 ꢁ (Table S3 in the Sup-
porting Information).
In the centrosymmetric tetra-
II
nuclear Pd complex [4], two of
II
the Pd centers are each bound
Scheme 3. Addition of protons to [1], and the corresponding coordination changes at the metal centers (top).
Proposed reaction mechanism for the protonation reactions of [1] and [2] (bottom); only half of the molecule is
shown for clarity with [2] on top and [1] at the bottom. Adapted from Reference [9].
to the O,N donors of the bridg-
2
À
2
1
ing L ligand, and to a h ,h -co-
ordinated cod ligand
Figure 2). The other two Pd
centers are each bound to a tri-
ÀH
II
(
flato ligand and a cod ligand
ÀH
with the fourth coordination site
being occupied by an O donor
2
À
II
of L , which bridges two Pd
centers. The bonding inside the
Scheme 4. Reaction of [5] with NaOMe leading to the formation of [6].
cod ligand is similar to the di-
ÀH
nuclear complexes discussed
II
All the complexes were characterized by NMR spectroscopy,
elemental analysis, and mass spectrometry (see the Experimen-
tal Section in the Supporting Information). NMR spectroscopy
already pointed to the different hapticity of the cod ligands in
the various complexes. This fact was further confirmed by the
single-crystal X-ray diffraction studies on the metal complexes.
above. The distance between the O,N-coordinated Pd centers
II
is 8.1 ꢁ. Additionally, the distances between the Pd centers co-
ordinated through the same O donor is 3.75 ꢁ, and that be-
II
tween the two anti Pd centers is 8.42 ꢁ. Thus, as has been dis-
II
cussed above, protonation of the Pd complex [2] results in
this unusual tetranuclear complex [4].
Complex [5], which is the doubly protonated form of [1], is
centrosymmetric. Additionally, [5] has a S2 axis running
through the cod ligands and the central quinone ring
Crystal structures
2
2
Single crystals of the complexes [1], [2], [4], [5], and [6] suita-
ble for diffraction studies were grown and used for X-ray dif-
fraction studies (see the Experimental Section in the Support-
ing Information). Crystallographic details are listed in Table S1,
and selected bond lengths and bond angles are given in
Tables S2–S4 in the Supporting Information. The metal centers
in all the complexes have a distorted square-planar coordina-
tion geometry (Figure 1). In compounds [1] and [2], each
(Figure 3). Both cod ligands in [5] are h ,h -coordinated to the
II
Pt centers. Accordingly, the C1CÀC1C bond lengths of 1.37 ꢁ
and the C4CÀC4C bond lengths of 1.35 ꢁ are the shortest dis-
tances within the cod rings (Table S2 in the Supporting Infor-
mation). The PtÀPt distance in [5] is 7.8 ꢁ.
In complex [6], the activated cod ligands are bound to the
II
2
1
Pt centers in a h ,h fashion (Figure 3). The C1CÀC8C distance
of 1.37 ꢁ is the shortest CÀC distance within the activated cod
ligands (Table S2 in the Supporting Information). This data thus
clearly points to alkene/alkyl coordination in complex [6] and
2
1
cod ligand displays a h ,h coordination. The distance be-
ÀH
tween the C1C and C8C atoms (the additional “C” refers to
À
atoms from the cod ligands) in [1] is 1.39(2) ꢁ, and these two
the p/s rearrangement induced by the addition of OMe to
II
2
atoms of cod are bound to the Pt center in a h coordina-
the cod ligands. The C9CÀO2C distance of 1.39 ꢁ establishes
the formation of CÀOMe bonds at the cod ligands.
ÀH
tion mode. Additionally, the C3CÀC4C bond length is 1.35(2) ꢁ,
and this bond forms the “allyl part” of the coordination togeth-
In complexes [1], [2], [4], and [6] that contain an activated
cod ligand, the bond lengths between the metal centers and
the carbanionic C donor of the cod rings are slightly shorter
1
II
er with the carbanionic C5C atom that is bound h to the Pt
center. As was expected, all other CÀC bond lengths within
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Figure 1. ORTEP plots of [1] and [2]. Hydrogen atoms have been omitted for
clarity.
than the metal–olefinic C-donor bond lengths (Table S2 in the
Supporting Information). Additionally, in all four complexes,
À
the C donor of the activated cod ligands is always trans to an
O donor of the central bridging quinone ring (Figures 1–3).
Figure 2. Ball-and-stick plot of [4] (top). A perspective view of [4] showing
the different PdÀPd distances (bottom). Hydrogen atoms have been omitted
for clarity.
The stronger MÀC(anionic) bond leads to a longer MÀO-
(quinone) bond, possible due to its stronger trans influence
when compared to a C=C bond.
A look at the bond lengths within the bridging quinone
2À
Cyclic voltammetry
ligand (L ) revealed CÀO bond lengths that are closer to
a single bond, and C=N bond lengths that are closer to
a double bond (Table S2 in the Supporting Information). Ac-
cordingly, the neighboring C3ÀC1 (1.397 ꢁ for [2]) bonds are
shorter and the C2ÀC3 (1.430 ꢁ) bonds are longer. These data
The complexes [1]–[6] display a reversible one-electron reduc-
tion step. In addition, irreversible oxidation processes were ob-
served for [1] and [2]. Furthermore, the complexes [1], [3], [5],
and [6] showed a second one-electron reduction wave (Fig-
ures 4, S1 and S2 in the Supporting Information, and Table 1).
By using THF as a solvent, which has a larger potential window
on the negative side, the second reduction wave of [1] is more
clearly visible (Figure S1 in the Supporting Information, top).
2À
point to the best description of L as containing negatively
À
charged O and neutral imine type donors, with the negative
charge being preferentially located on the more electronega-
[
8]
tive oxygen atoms.
In all the complexes, the metal atoms and the central qui-
none ring are almost co-planar. The diisopropyl-phenyl sub-
stituents on the N atom of the quinone ring are perpendicular
to the central quinone ring Figures 1–3.
II
The redox potentials of the Pd complex [2] are cathodically
shifted compared to those of its Pt analogue [1]. This proba-
bly leads to the expected second reduction for [2] falling out-
II
side the dichloromethane window (Table 1).
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Figure 4. Cyclic voltammograms of [1], [3], and [5] in CH
at 298 K. Scan rate: 100 mVs . Ferrocene/ferrocenium was used as an inter-
nal standard.
2
Cl
2
/0.1m Bu
4
NPF
6
À1
[
a]
Table 1. Redox potentials from cyclic voltammetry.
ox1
red1
red2
Compound
E
[V]
E
[V]
E
[V]
[
[
b]
b]
[b]
[1]
[2]
[3]
[4]
[5]
[6]
0.70
0.61
–
–
–
À1.36
À1.69
À0.83
À1.95
À0.21
À1.29
À2.09
–
À1.57
–
À0.91
À1.98
[
c]
–
[
a] Half-wave potentials from cyclic voltammetric measurements in
CH
2
Cl
2
/0.1m Bu
4
NPF
6
for reversible processes at 298 K, scan rate
À1
100 mVs ; ferrocene/ferrocenium was used as an internal standard.
[
b] Epa for irreversible process. [c] At À408C.
to the formation of [5], makes the reduction potentials of [5]
more than 1 V more facile compared to [1]. The increase in
positive charge of the complexes on protonation is responsible
for this huge shift in the redox potentials of the complexes.
Complex [6], which contains cod rings with the OMe groups
has redox potentials that are comparable to [1] (Figure S2 in
the Supporting Information, Table 1). Thus, the addition of
À
a OMe group to cod seems to have the same influence on
the redox potentials of the complex as the deprotonation of
cod. This is probably so, because an anionic ligand is generat-
ed in each case.
Figure 3. ORTEP plots of [5] (top). A perspective view of [5] to point out the
center of inversion and the S axis (middle, see text for discussion). Ball-and-
stick plot of [6] (bottom). Hydrogen atoms and counteranions are omitted
2
II
The tetranuclear Pd complex [4] displays a single redox
for clarity.
event at À1.95 V, which is chemically reversible only at low
temperatures. The lower reversibility of this redox wave is
2
À
Remarkably, complex [3], which is the mono-protonated
form of [1], displayed two one-electron reduction steps, the
potentials of which are anodically shifted by about 500 mV
compared to [1]. Along the same lines, [5], which is the mono-
protonated form of [3], displayed reduction potentials that are
about 600 mV anodically shifted compared to [3] (Figure 4 and
Table 1). Thus, the transfer of two protons to [1], which leads
likely associated with the bridging mode of the O atoms of L
in [4] (Figure 2). Such a fragile coordination is expected to
break-up on changing the redox state of the complex, possibly
leading to a lower reversibility of the redox process. The reduc-
tion potential for [4] is cathodically shifted by about 600 mV
II
compared to the dinuclear Pd complex [2].
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To shed more light on the redox processes discussed above,
spectroelectrochemical measurements and DFT calculations
were carried out on the complexes.
[
a]
Table 2. EPR data of the one-electron-reduced complexes.
1
4
[b]
[b]
[c]
Compounds
g
iso
a
iso ( N)
aiso (M)
CÀ
[
[
[
1]
2]
3]
1.999
2.002
1.999
2.002
1.999
1.999
2ꢂ3.15
2ꢂ3.65
4.5; 2.2
2ꢂ3.9
2ꢂ3.3
2ꢂ3.3
2ꢂ13.2
2ꢂ1.2
CÀ
[d]
EPR, UV/Vis/NIR spectroelectrochemistry, and spin-density
calculations
CÀ
[c]
20.0; 9.7
CÀ
CÀ
CÀ
[d]
[4]
5]
[6]
2ꢂ1.1
[c]
[
2ꢂ14.3
2ꢂ14.4
[c]
The one-electron reduced forms of complexes [1]–[6] were in-
vestigated by X-band EPR spectroscopy. The reduced com-
plexes display similar spectra in fluid solutions with signals
centered around g=2.0 (Figure 5, Table 2). The spectra of the
reduced forms of complexes [1], [2], and [4]–[6] were simulat-
ed by considering a hyperfine coupling of about 3–4 G to two
[
0
a] X-band EPR data obtained from in situ generated species in CH
2
Cl
2
/
À
.1m Bu NPF at 298 K, for 4
4 6
C
at 275 K. The anion-radical sign signifies
the one-electron-reduced state of the complexes and not the total
charge. [b] Isotropic hyperfine coupling constants in Gauss obtained from
1
95
1
simulation. [c] Hyperfine coupling to
Pt, I= =
2
, natural abundance=
1
05
33.3%. [d] Hyperfine coupling to
2.2%.
Pd, I=5/2, natural abundance=
14
equivalent N nuclei. Additionally, satellites to two equivalent
2
on the spin-density distribution of the one-electron
reduced form of that complex.
To shed further light on the EPR results, spin-densi-
ty calculations were carried out for the one-electron
reduced forms of selected complexes. Structural opti-
mizations were carried out by using DFT method
with the BP86 functional. A look at the Lçwdin spin-
density plot shows the spin density to be predomi-
nantly localized on the bridging ligand (Figure 6). For
example, for the one-electron reduced form of [1],
the two platinum centers combined contain a total
spin density of only 3.2%, whereas 83% of the spin
density is localized on the central bridging ligand.
The two central carbon atoms of the bridge (and ac-
cordingly the protons attached to them) contain neg-
ligible spin density. The nitrogen atoms of the bridg-
ing ligand contain a total spin density of 29.2%. De-
spite this high spin density on the nitrogen atoms
(
compared to the platinum centers), the experimental
195
hyperfine coupling observed for Pt is larger than
14
that of N owing to the larger value of the intrinsic
1
95
isotropic hyperfine coupling constant for
Pt as
14
compared to N. Similar results from spin-density cal-
culations were obtained for the one-electron reduced
forms of [2] and [5] (Figure S3 in the Supporting In-
formation). For the one-electron reduced form of [3],
which contains one cod and one cod ligands, the
ÀH
spin-density distribution was found to be non-sym-
metrical (Figure S4 in the Supporting Information).
For that species, one of the nitrogen atoms contain
about 9.5% spin density, and the other contains
about 20% spin density. A similar non-symmetrical
spin-density distribution is also seen for the platinum
centers for that compound. Thus, the spin-density calculations
nicely corroborate the experimental EPR results.
Figure 5. X-band EPR spectra of the mono-reduced forms of the complexes at 298 K. The
anion-radical sign signifies the reduced state of the complexes and not the real charge.
1
05
195
Pd or Pt nuclei were also considered in the simulations
Table 2). Remarkably, for the reduced state of the mono-pro-
(
II
tonated Pt complex [3], an asymmetric spin distribution was
observed with hyperfine coupling to two inequivalent
The observation of the EPR signals for the one-electron re-
duced complexes in fluid solutions at ambient temperature
(except for the reduced form of [4], in which the lower degree
of chemical reversibility forced us to record a spectrum at
275 K), the g value close to 2.0, the hyperfine couplings ob-
14
N
195
nuclei of 4.5 and 2.2 G, respectively. Accordingly, the Pt cou-
pling for this complex was also found to be different for the
two Pt nuclei with values of 20 and 9.7 G. Thus, the inequiva-
lence of the two cod rings in [3] seems to have an influence
14
2À
served to the two N nuclei of the L ligand, and the results
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Figure 6. Lçwdin spin-density plot for the one-electron reduced form of [1].
See text for explanation.
from spin-density calculations are clear indications of the re-
duction predominantly taking place on the bridging quinone
[
8c,f]
ligand in all the investigated complexes.
X-ray crystallo-
graphic and NMR studies (see below) have clearly established
the cod ligand as the proton-transfer site. The EPR studies de-
2À
liver compelling evidence for the quinone bridge L being the
electron-transfer site. Hence, the protonation of a remote cod
ligand drastically influences the redox potentials (see electro-
chemistry section above) of the complexes for electron-transfer
steps that occur on the bridging quinone ligand.
Figure 7. Changes in the UV/Vis/NIR spectrum of [5] during spectroelectro-
[
10]
chemistry in an OTTLE cell in CH Cl /0.1m Bu NPF .
2
2
4
6
All investigated complexes display a main broad absorption
band in the visible region (Figures 7, S5, and Table S5 in the
Supporting Information), which is assigned to a mixture be-
One-electron reduction of the complexes leads to a loss of
intensity of the initial band in the visible region. New struc-
tured bands were observed at lower energies for the one-elec-
tron reduced states of all the complexes (Figures 7, S5, and
Table S5 in the Supporting Information). These bands are as-
signed to a mixture of ligand-to-metal charge transfer (LMCT)
2
À
tween an intra-ligand transition within the L bridge, and
II
a metal-to-ligand charge transfer (MLCT) transition from Pt to
2À
L . It should be noted that ligands, such as H L, indeed dis-
2
play an absorption band in the visible region, which is respon-
[
8]
3·À
sible for their characteristic red color.
transitions and transitions within the radical ligand L formed
II
The Pd complex [2] exhibits an absorption band at l=
20 nm, which appears at a higher energy compared to its Pt
on one-electron reduction of the complexes. Such structured
bands in the visible region are typical for radicals of bridging li-
gands, such as the one described herein (free, as well as metal-
II
4
counterpart [1], for which this band appears at about 470 nm.
As can be seen from Table 1, the difference between the oxida-
tion and reduction potentials for [2] is larger than that for [1],
and this fact seems to get reflected in the long wavelength
bands of the complexes. Stepwise protonation of [1] to form
[8]
bound, ones). It was possible to obtain the spectroscopic sig-
natures of the one- and two-electron reduced forms of the
protonated diplatinum complexes [3] and [5]. In both cases,
reduction by a further electron leads to a loss of the structured
bands, and the appearance of a broad band (Figures 7, S5, and
Table S3 in the Supporting Information). This band is assigned
[
3] and [5] leads to a shift of the band in the visible region to
higher energies; additional weak band at longer wavelengths
were observed for [3] and [5] (Table S5 in the Supporting Infor-
mation). The absorption band for complex [6] in the visible
region appeared at 475 nm, which is comparable to the band
4À
to an LMCT transition from a L aromatic bridging ligand to
II
the Pt centers. The bands in the UV region do not show much
variation for the different complexes in their various oxidation
states.
II
observed for [1]. The tetranuclear Pd complex [4] displayed
an absorption spectrum in the visible region, which is almost
identical to its dinuclear counterpart [2] (Figure S5 and
Table S5 in the Supporting Information).
Thus, the spectroelectrochemistry measurements deliver the
spectroscopic signatures for all the complexes in the various
protonated and redox forms. This information will be used in
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the following to elucidate reactivity studies with these metal
complexes.
it is seen that the cod ligand can be reversibly activated by
forming a CÀOMe bond, which can then be reversible broken
with a proton source. This conversion is associated with the
coordination change of the cod ligands from an alkene/alkene
form to an alkyl/alkene form. Recently, such a conversion has
Reactivity studies on and with the complexes
II
II
Pd complex [2] leads to the formation of a tetranuclear com-
been reported for a mononuclear Pt complex by making use
[6a]
plex on protonation (see above). We first attempted to check
the reversibility of the protonation steps that lead to the con-
version of [1] through [3] to [5]. UV/Vis spectroscopy was used
to investigate this phenomenon, because the spectroscopic
signatures of all the forms are known. Gratifyingly, the reaction
of [5], which has an intense absorption band at l=433 nm,
of electron transfer from a redox-active ligand.
We have
shown herein an example, in which such a transformation can
be performed through protonation. In both cases, it is the
change in the total charge of the complex that probably
makes such transformations possible. The reactions shown in
Scheme 5 thus clearly prove the chemical non-innocence of
[4h–j]
with NEt , leads to stepwise changes in its UV/Vis spectrum
the cod ligands in the complexes presented herein.
3
leading first to the formation of a species, UV/Vis spectrum of
Compound [8], which is a highly air-sensitive compound,
was characterized by NMR spectroscopy (Figure S8 in the Sup-
porting Information) and UV/Vis spectroscopy. The UV/Vis
spectrum of this chemically isolated species is identical to that
observed during the in situ two-electron reduction of [5] ob-
served during spectroelectrochemical measurements. To the
best of our knowledge, [8] is the first example of a chemically
isolated complex that has a tetra-reduced form of a quinone-
derived ligand containing a [O,N,O,N] donor set. Even though
such aromatic bridges have been stabilized in metal complexes
for quinone ligands containing
which is identical to that of [3]. Further addition of NEt gener-
3
ates a spectrum with an intense band at l=470 nm, which
matches exactly the spectrum of [1] (Figure S6 in the Support-
ing Information). Deprotonation/protonation is also associated
with color changes that can be followed with the naked eye.
Thus, compound [5], which is green, first changes its color to
brown green when one proton is removed to form [3], further
deprotonation of [3] generates complex [1], which is brown
(Scheme 5). Complex [1] can, of course, be protonated back
[5g]
a [O,O,O,O] donor set,
it is
usually difficult to stabilize corre-
sponding aromatic systems that
contain one or more NR donors
because of the higher negative
reduction potentials of such
compounds. Remarkably, in the
present case, it is the protona-
tion of the remote cod ligand
that helps in shifting the reduc-
tion potentials in the positive di-
rection, and makes the isolation
of compound [8] possible. To-
gether with the spectroelectro-
+
À
Scheme 5. Reactions with H and OMe for the diplatinum complexes. Double bonds are shown in the top
Scheme to point to the generality of this process (not necessarily restricted to cod).
via [3] to [5], as has been discussed above. These reversible
protonation/deprotonation reactions are associated with the
change in the coordination mode of cod, which binds as an
alkene/alkene ligand in the neutral state, and as a carbanion/
chemical results the conversion of [5] to [8] establishes the
redox non-innocent nature of L in these complexes.
2À
Having unequivocally established the stepwise, reversible
protonation steps for the cod ligands (Scheme 5) and the re-
2À
alkene ligand in the negative cod form.
versible electron-transfer steps for the L ligand (Scheme 6) in
the investigated complexes, we were next interested in prob-
ing if these site-decoupled protons and electrons can be trans-
ferred in a three-way cooperative process to a substrate. After
taking redox potentials into consideration, 3,5-di-tert-butyl-
benzoquinone was chosen as a substrate and its conversion to
ÀH
The complex [6] that contains methoxy-activated cod rings
can also be protonated back to [5] via [7] by using [HDMF]-
(OTf) (Scheme 5). The protonation reactions in this case have
also been followed by UV/Vis spectroscopy. Thus, the band at
l=474 nm for [6] first changes to an intermediate position
before eventually shifting to 433 nm, which is the main ab-
sorption band in the visible region for [5] (Figure S7 in the
Supporting Information). Even though we have not character-
ized the intermediate species [7] for this transformation by any
other method, the stepwise changes in the UV/Vis spectrum
coupled with the clean formation of [5] on double protonation
make us believe that [7] was indeed formed during this proto-
nation reaction. Compound [5] can be converted back to [6]
by reacting it with NaOMe, as has been discussed above. Thus,
Scheme 6. Chemical reduction of complex [5].
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um complex resulted in the generation of an unusual tetranu-
clear palladium complex, in which the cod ligand retains its
ÀH
charge and coordination mode. On the other hand, the diplati-
num complex can be reversibly protonated at the cod li-
ÀH
gands, whereby those ligands undergo a s/p rearrangement
and change their coordination mode from an allyl/alkene to an
alkene/alkene mode. Additionally, we have shown that the cod
ligands can also be activated by reaction of the protonated di-
platinum complex with NaOMe through nucleophilic attack,
which leads to the formation of a new CÀOMe bond at the
cod ligands and a change in their coordination mode from
alkene/alkene to alkyl/alkene. For the diplatinum complexes,
À
all protonation reactions, as well as the reaction with OMe ,
have been shown to be completely reversible. Reversible elec-
tron transfers have been observed at the bridging quinone li-
gands for all the complexes. The locus of the electron-transfer
steps at the bridging quinone ligands has been unequivocally
established by using UV/Vis and EPR spectroelectrochemical
methods. We have also shown that the protonation of the
Scheme 7. Reaction cycle displaying transfer of two protons and two elec-
trons to a substrate.
3,5-di-tert-butyl-catechol was probed by using [8] as an elec-
remote cod ligands leads to a shift of the bridging quino-
ÀH
tron and proton source (Scheme 7).
noid ligand reduction by ꢀ500 mV/proton. Thus, the protonat-
ed complex displays reduction potentials that are more than
one Volt positively shifted compared to its non-protonated
counterpart. Taking advantage of this low reduction potential,
we have managed to chemically isolate the first example of
a dinuclear complex containing an aromatic bridge of a qui-
none-derived ligand with a [O,N,O,N] donor set. The doubly
protonated and doubly reduced diplatinum complex has been
utilized in a three-way cooperative process to transfer two pro-
tons from two different cod ligands and two electrons from
the bridging quinone ligand to reduce 3,5-di-tert-butyl-benzo-
quinone to 3,5-di-tert-butyl-catechol (Scheme 7). The protona-
tion at the remote cod ligands helps to control reduction po-
tentials of electron-transfer steps occurring at the site-decou-
pled quinone bridge. It is the achievement of low reduction
potentials that finally makes electron and proton transfer to
the substrate possible.
To perform this reaction, [5] was reduced in situ with cobal-
tocene to [8], and the conversion was followed by UV/Vis
spectroscopy. This conversion led to a color change from
green to violet. Once [8] was formed, the substrate 3,5-di-tert-
butyl-benzoquinone was added to the reaction mixture, which
led to an immediate color change to orange. The UV/Vis spec-
trum of the mixture changed accordingly (Figure S9 in the
Supporting Information). After stirring for 24 h, the reaction
mixture was worked up (see the Experimental Section in the
Supporting Information), and the isolated solid was subjected
to NMR spectroscopy. The NMR spectrum of the isolated solid
in CD Cl showed signals that correspond to [1], 3,5-di-tert-
2
2
butyl-catechol and cobaltocenium (Figure S10 in the Support-
ing Information). Thus, compound [8] transfers two electrons
and two protons to 3,5-di-tert-butyl-benzoquinone leading to
its conversion to the catechol form. In the process, [8] gets
converted to [1]. The cobaltocenium signal observed in the
NMR spectrum arises from the oxidized form of cobaltocene,
which is used for the in situ reduction of [5] to [8]. The isolat-
ed compound also displays a UV/Vis spectrum that matches
with the main transition of [1] (Figure S9 in the Supporting In-
formation). Compound [1] can now be protonated to convert
it to [5], which closes the cycle (Scheme 7).
Thus, it is seen that two protons, one each from a different
cod ligand, and two electrons from the bridging quinone
ligand in [8] can be transferred to a substrate leading to its
double protonation and double reduction. It is intriguing that
such site-decoupled proton and electron sources can work in
tandem to transfer electrons and protons to a particular sub-
strate in a three-way cooperative process.
Control of redox potentials through remote protonation
steps is common in biological systems, and biocatalysis usually
works through transfer of electrons and protons. Most man-
made homogeneous catalysts on the other hand take advant-
age of oxidative addition and reductive elimination steps for
substrate activation and transformation. However, in recent
years, examples of substrate activation by consecutive proton-
and electron-transfer steps have appeared with man-made cat-
[2c]
alysts. We believe that the proof of concept that we have
delivered in this work of site decoupled proton and electron
transfer sites that can work in tandem for performing substrate
bond activation will further contribute to develop concepts for
new kinds of catalysis. Additionally, we have also shown that
spectroelectrochemistry is a powerful technique to determine
spectroscopic signatures of reactive intermediates that appear
in bond-activation processes. These spectroscopic signatures
can then be utilized to elucidate the pathway of substrate acti-
vation in such complexes containing non-innocent ligands. We
note that all proton- and electron-transfer steps observed in
these complexes are ligand based.
Conclusion
We have presented herein diplatinum and dipalladium com-
plexes that contain anionic cod stopper ligands and a redox-
ÀH
active quinonoid bridging ligand. Protonation of the dipalladi-
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Received: April 27, 2014
Published online on September 22, 2014
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