Synthesis and redox chemistry of Pd(ii) complexes of a pincer verdazyl ligand

Article abstract of DOI:10.1039/c9dt02549a

A series of palladium(ii) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL (L = Cl, CH₃CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(ii) complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH₃CN, 12: L = Cl) or its closed-shell monoanionic charge state (10: L = CH₃CN, 9: L = Cl). The charge state of the ligand was determined using X-ray crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically interconverted between radical ligand (6, 12) and reduced, closed-shell anion (9, 10) using decamethylferrocene as the reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.

Full text of DOI:10.1039/c9dt02549a

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DOI: 10.1039/C9DT02549A
Journal Name
ARTICLE
Synthesis and Redox Chemistry of Pd(II) Complexes of a Pincer
Verdazyl Ligand
a
b
a
Corey A. Sanz, Brian O. Patrick and Robin G. Hicks*
Received 00th January 20xx,
Accepted 00th January 20xx
A series of palladium(II) complexes containing a redox-active, tridentate verdazyl ligand of general formula (verdazyl)PdL
DOI: 10.1039/x0xx00000x
(L = Cl, CH CN) are synthesized. The tetrazine core of tridentate verdazyl ligand 5 is flanked by two pyridyl groups, creating
3
a geometry in which the ancillary ligand L is bound trans to the verdazyl ring in the square planar metal complexes. Pd(II)
www.rsc.org/
complexes were isolated with the verdazyl ligand in either its neutral radical charge state (6: L = CH
3
CN, 12: L = Cl) or its
closed-shell monoanionic charge state (10: L = CH CN, 9: L = Cl). The charge state of the ligand was determined using X-ray
3
crystallography and NMR, EPR, and IR spectroscopy. The cyclic voltammograms of radical complexes 6 and 12 each contain
a reversible one-electron reduction wave and an irreversible one-electron oxidation wave. The complexes can be chemically
intercovnerted between radical ligand (6, 12) and reduced, closed-shell anion (9,10) using decamethylferrocene as the
reductant and a mixture of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone and fluoroboric acid as the oxidant.
hydrogen atom transfer reactions when bound to palladium.6
Another example describes a C-H amination reaction that
generates a phenyl-substituted pyrollidine molecule from an
Introduction
7
aliphatic azide using a palladium-RAL catalyst. The redox-active
The incorporation of redox-active ligands (RALs) into
coordination complexes has become increasingly popular in the
last couple of decades. One motivation for studying RAL
complexes lies in the design of cheaper catalysts using first-row
transition metals. The accessibility of ligand redox chemistry
amidophenolate ligand on the palladium catalyst plays a key
role by triggering the radical reaction via single-electron
transfer to the coordinated azide. However, the limited catalytic
1
7
b
turnover of this system (turnover number = 2.8) demonstrates
that there is still much more to learn about the fundamentals of
palladium-RAL chemistry.
2
has allowed researchers to develop first-row transition metal
catalysts that are capable of carrying out multi-electron
Our group is interested in the coordination chemistry of a
3
chemistry. Comparatively, RAL complexes of precious metals
8
class of tetrazine-based stable radicals known as verdazyls. We
have received far less attention. This is especially true for
palladium, a transition metal that is capable of performing a
wide variety of catalytic transformations. Wieghardt and co-
workers have synthesized and electrochemically characterized
a number of palladium complexes with bidentate RALs of similar
structure to the classical dioxolene ligand. More recent studies
include chemical oxidation and reduction of these complexes to
generate an electron-transfer series for each set of ligands. In
all cases, the redox events were assigned as ligand-centred and
the palladium atom remained in the Pd(II) oxidation state.
The availability of ligand redox chemistry provides an
opportunity for these complexes to engage in single-electron
transfer processes, contrasting the traditional two-electron
chemistry of palladium. Brown and co-workers have shown that
a pair of redox-active iminosemiquinone ligands can engage in
recently began investigating the redox chemistry of verdazyl
radicals and discovered that they could be reversibly reduced or
9
oxidized by a single electron. The dearth of studies on the
reactivity of RAL-Pd complexes prompted us to explore the
redox chemistry of dichloro-palladium complex 1, which
4
1
0
contains a neutral bidentate verdazyl radical ligand. One-
electron reduction of this complex generated 0.5 equivalents of
binuclear complex 2 (Scheme 1). Both of the verdazyl ligands in
5
2
are in their singly-reduced monoanionic charge state and both
of the palladium atoms have retained their Pd(II) oxidation
states. Two-electron oxidation of the binuclear complex
restored the original radical complex 1.
O
O
2
FeCp*2, CH2Cl2
N
N
N
N
N
N
a. Department of Chemistry, University of Victoria, PO Box 3065 STN CSC, Victoria,
B.C. V8W3V6, Canada. E-mail: rhicks@uvic.ca
Crystallography Laboratory, Department of Chemistry, University of British
-2 [FeCp*2][Cl]
N
N
Pd
Ph2
P
PdCl2
PPh2
Cl
Cl
b.
2
Pd
PhICl2, CH2Cl2
-PhI
P
N
Columbia, Vancouver, BC V6T1Z1, Canada
Ph2
N
N
N
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any supplementary
information available should be included here]. See DOI: 10.1039/x0xx00000x
O
1
2
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Scheme 1: Reversible redox chemistry of a phosphine-substituted verdazyl ligand.10 Blue
tetrazine rings represent neutral radical verdazyls and red rings represent anionic
verdazyls.
Scheme 3: Synthesis of complex 6
View Article Online
DOI: 10.1039/C9DT02549A
The X-ray structure of radical complex 6 is shown in Fig 1.
The bond metrics within the tetrazine ring are consistent with
We have also explored the redox chemistry of bis(verdazyl)
1
0-11,15
other verdazyl radical palladium complexes
and the
1
1
palladium complexes. Oxidation of 3, a homoleptic Pd(II)
complex that contains two monoanionic verdazyl ligands,
resulted in the formation of bis(radical) complex 4 (Scheme 2).
In this reaction, ligand-centred oxidation was accompanied by
partial dissociation of one of the verdazyl radical ligands.
Reduction of 4 resulted in chelation of the formerly
monodentate radical ligand to regenerate 3.
distorted square planar geometry around the metal (sum of
angles around Pd = 360.0°) is consistent with palladium(II). An
interesting feature of 6 is the Pd-N3 bond length (1.883(3) Å),
which is the shortest bond known between palladium and a
1
0-11,15
verdazyl nitrogen atom (this bond is typically over 2 Å).
This Pd-N bond is also short in comparison to the central Pd-N
bond of other complexes with similar non-verdazyl NNN-
chelating ligands. For example, palladium complexes with
terpyridine or substituted-terpyridine ligands all have central
O
iPr
[BF4]2
O
iPr
iPr
iPr
N
N
N
N
N
N
N
N
O
16
Pd-N bonds that are over 1.9 Å in length. Brook and co-
1
2
)
)
2 AgBF4, CH2Cl2
DMSO
N
iPr
iPr
N
N
N
N
workers report an unusually short Ni-N3 bond (1.979(5) Å) in an
Pd
iPr
N
N
N
N
N
N
14
octahedral nickel(II) complex based on 5 as a ligand.
2
FeCp*2, CH2Cl2
Pd
iPr
-2 [FeCp* ][BF ]
2
4
N
O
S
O
3
4
Scheme 2: Redox chemistry of (bis)verdazyl palladium complexes containing bidentate
verdazyl ligands.11
The partial ligand dissociation that was observed during the
oxidation of 3 highlights a potential problem with the verdazyl
ligand system, namely verdazyl ligand dissociation. Chirik and
co-workers have also reported issues with partial ligand
dissociation in iron complexes of a redox-active bis(imino)-
pyridine ligand.12 The resulting open coordination site can
change the way that substrates bind and react with the metal
centre. However, partial ligand dissociation can also be a
desirable property. Redox-switchable hemilabile ligands add an
extra element of control to the coordination sphere of a
4
Fig. 1: X-ray structure of 6. Hydrogen atoms and BF ions removed for clarity; thermal
1
3
metal. In this work, we try to eliminate the possibility of ligand ellipsoids drawn at 50% probability. Selected bond lengths (Å): Pd1‒N3 1.883(3), Pd1‒N5
1
4
dissociation by utilizing a tridentate verdazyl ligand (5). The 2.050(3), Pd1‒N6 2.027(3), Pd1‒N7 2.006(3), N1‒N2 1.352(4), N3‒N4 1.380(4), N2‒C2
1
.305(5), N3‒C2 1.313(5), O1‒C1 1.199(4).
redox chemistry of a series of palladium complexes of 5 is
explored and compared to that of other verdazyl-Pd complexes.
The effect of the ancillary ligands on the redox potentials is also
discussed.
-1
The IR spectrum of 6 contains a CN stretch at 2343 cm ,
1
7
which is characteristic of acetonitrile bound to palladium.
1
8
Consistent with many other verdazyl metal complexes, the
C=O stretching frequency of 6 (1732 cm ) is higher than that of
free ligand 5 (1689 cm ) . The EPR spectrum of 6 (Fig. 2) shows
coupling to four inequivalent nitrogen atoms. The general shape
of the spectrum, which is typical for verdazyl radical ligands,
arises from the four different hyperfine coupling constants
-1
-1 14
Results and discussion
Synthesis of verdazyl-radical acetonitrile complex 6
Verdazyl radical 5 was prepared according to the original
synthesis reported by Brook and co-workers, which did not
14
associated with each N nucleus (spin = 1, see Table S1 in ESI).
1
4
include a purification step. A crude sample of 5 was mixed
with one equivalent of (CH CN) Pd(BF to generate palladium
complex 6 as a green solid with a yield of 28% (Scheme 3).
The hyperfine coupling constants in the spectrum of 6 are in the
3
4
)
4 2
8
usual range for verdazyl ligands. The g-value (2.0033) is also
consistent with other verdazyl-palladium complexes and is in
1
0-11,15a
the appropriate range for a ligand-centred radical.
In
[
BF4]2
O
O
addition, no hyperfine coupling to palladium is observed in the
EPR spectrum of 6. These results suggest that the unpaired
electron is localized primarily on the verdazyl ligand and not the
metal, a feature that is consistent with previously characterized
N
N
N
N
N
N
N
N
N
N
(CH3CN)4Pd(BF4)2
CH3CN
Pd
NCCH3
N
N
1
0-11,15a
verdazyl-palladium complexes.
Complex 6 is completely
6
5
NMR silent due to the radical nature of the verdazyl ligand.
28%
2
| J. Name., 2012, 00, 1-3
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View Article Online
DOI: 10.1039/C9DT02549A
O
O
N
N
N
N
N
N
N
N
HN
NH
NaIO4, H2O
NaIO3, H2O
H
-
N
N
8
7
74%
(CH3CN)2PdCl2
CH3CN
-HCl
[
BF4]
O
O
N
N
N
N
N
Pd
AgBF4
N
N
N
N
N
Pd
CH Cl /CH CN
2
2
3
-AgCl
NCCH3
Cl
3
460
3510
Field (G)
3560
N
N
1
0
9
88%
72%
Fig. 2: EPR spectrum of radical/acetonitrile complex 6 in acetonitrile (black trace) and the
corresponding simulated spectrum (red trace)
Scheme 4: Synthesis of palladium complexes with reduced verdazyl ligands
Synthesis of verdazyl-anion acetonitrile complex 9
The coordination chemistry of leuco verdazyl 7 was explored
with palladium(II) chloride. Leuco verdazyl 7 was reacted with
The low yield of 6 in the reaction shown in Scheme 3 may
result from the lack of purification of verdazyl 5 prior to the
reaction. We turned to IR spectroscopy as a means of assessing
the purity of crude 5 and found that the spectrum contains
multiple C=O stretches, which contrasts the characterization of
other verdazyl derivatives which only have one diagnostic C=O
(
3 2 2 2
CH CN) PdCl , which was generated in-situ by refluxing PdCl in
acetonitrile, to generate monochloro palladium complex 9 as a
crystalline red solid (Scheme 4).
The X-ray structure of 9 is shown in Fig. 3. The complex
consists of both a verdazyl ligand and a chloride ion bound to
palladium. The metal centre adopts a distorted square planar
geometry (sum of angles around Pd = 360.5°), which suggests
that the palladium atom has an oxidation state of (II). Previous
work from our group has established that the N-N bond lengths
-1 19
stretch near 1680 cm . The complex C=O region suggests that
the sample consists of more than one compound. In addition,
the cyclic voltammogram (CV) of crude 5 is made up of many
indistinct peaks that vary in size and reversibility, unlike the CVs
of other verdazyls which normally consist of discrete reversible
in the tetrazine ring are sensitive to the charge state of the
verdazyl ligand.
10-11
Reduced (monoanionic) verdazyl ligands
9
oxidation and reduction processes. We interpret these results
have longer N-N bonds than their neutral radical counterparts
as a consequence of the increased amount of electron density
in the N-N antibonding verdazyl SOMO. The N-N bonds in 9 are
longer than those of radical complex 6, consistent with our
assignment of the verdazyl ligand of 9 being in its reduced
monoanionic charge state (Table 1). The N-N bonds in 9 are also
as an indication that the samples of crude 5 are not very pure.
We made several attempts to purify 5 but our efforts were
unsuccessful;
column
chromatography
resulted
in
decomposition and all recrystallization attempts failed.
The complications associated with the purification of 5
prompted us to explore other ways of coordinating the
tridentate verdazyl ligand to palladium. We discovered that the
ligand could be isolated as a pure crystalline solid in its “leuco”
form (7). Unlike radical 5, the verdazyl ring in leuco verdazyl 7 is
in its one-electron reduced (but protonated) state. The
synthesis of 7 was adapted from the original synthesis of radical
of similar length to those of other palladium complexes that
contain monoanionic verdazyl ligands.
10-11
Another notable
feature of 9 is the distortion of the tetrazine ring from planarity
see Figure S1 in ESI). This is commonly observed for reduced
(
verdazyl ligands and contrasts the relatively planar
conformation adopted by neutral radical verdazyl ligands
has been shown computationally that the 8π electrons
belonging to an anionic verdazyl ring makes these compounds
10-11
. It
1
4
5
.
Oxidation of tetrazane 8 with one equivalent of sodium
periodate (compared the 1.5 equivalents used in the synthesis
of 5) generated 7 as a yellow solid (Scheme 4). A similar
synthetic procedure for 7 was recently reported by Brook and
coworkers.20
21
antiaromatic. This antiaromatic character accounts for the
distortion of the reduced verdazyl ring from planarity. The N2-
C2 bond of 9 (1.299(3) Å) is much shorter than the adjacent N3-
C2 bond (1.376(3) Å). This feature is consistent among other
1
0-11
reduced verdazyl ligands
and can be rationalized with the
resonance structure depicted in Scheme 4 in which the verdazyl
N donor atom is formally “amido-like” in nature. Formally, a
C=N double bond between N2 and C2 creates a negative charge
on the N3 atom that is stabilized by the nearby electropositive
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metal centre. All of the structural metrics in complex 9 indicate 4). 10 is the one-electron reduced form of radicaVliewcoAmrticpleleOxnlin6e,
a physical oxidation state of Vd(-)Pd(II)Cl and also confirm that which also contains an acetonitrile ligandD.OI: 10.1039/C9DT02549A
no redox chemistry has occurred in the reaction of leuco
verdazyl 7 with PdCl to generate 9.
The X-ray structure of 10 is shown in Fig. 4. The N-N bonds
of 10 are lengthened with respect to radical complex 6 (see
Table 1), which indicates that the verdazyl ligand is in its
reduced state. The tetrazine core also shows the typical
distortion from planarity that is common among reduced
verdazyl ligands (see Figure S1 in ESI). In addition, the N2-C2
bond (1.293(8) Å) is significantly shorter than the N3-C2 bond
2
(1.369(8) Å). These features of the ligand combined with the
nearly square planar geometry around the palladium atom (sum
of angles around Pd = 360.4°) allow us to assign a physical
+
oxidation state of [Vd(-)Pd(II)] to 10. This designation also
confirms that no redox chemistry has occurred during the
transformation of 9 to 10. This is particularly important given
the reaction employed to prepare 10: silver(I) salts are often
used as single-electron oxidants, but in this case the role of the
silver ion is to abstract the chloride atom from 9. Silver(I) is a
+
22
weak oxidant in acetonitrile (E° = +0.04 V vs Fc/Fc ), which
explains why chloride abstraction is favored over electron
Fig. 3: X-ray structure of 9. Hydrogen atoms removed for clarity; thermal ellipsoids drawn
at 50% probability. Selected bond lengths (Å): Pd1‒N3 1.934(2), Pd1‒N5 2.036(2),
Pd1‒N6 2.020(2), Pd1‒Cl1 2.3284(6), N1‒N2 1.400(3), N3‒N4 1.417(3), N2‒C2 1.299(3),
N3‒C2 1.376(3), O1‒C1 1.233(3).
transfer (the first oxidation wave in the cyclic voltammogram of
+
9
1
occurs at +0.19 V vs Fc/Fc , see Table 2). The bond metrics of
0 can also be compared to chloro complex 9 (Table 1). Only
subtle differences exist between the monoanionic verdazyl
ligands in each of the two complexes, which suggests that the
ancillary ligand (chloro or acetonitrile) has little effect on the
structural aspects of the reduced verdazyl ligand.
Table 1: Selected bond lengths (Å) for chloro complexes 12 and 9 and acetonitrile
complexes 6 and 10
Bond
N1-N2
1.365(4)
1.400(3)
1.352(4)
N3-N4
1.352(3)
1.417(3)
1.380(4)
1.438(6);
1.423(7)
N2-C2
1.321(3)
1.299(3)
1.305(5)
1.293(8);
1.298(8)
N3-C2
1.330(3)
1.376(3)
1.313(5)
1.369(8);
1.372(7)
1
2
9
6
1
1
.415(8);
.385(7)
1
0a
a
Two molecules in the asymmetric unit
1
The H and 13C NMR spectra of 9 (see ESI) contain signals in
the normal chemical shift range for diamagnetic compounds,
which is consistent with the fact that the verdazyl is in a closed-
shell (monoanionic) charge state. Furthermore, the C=O
stretching frequency of 9 is also indicative of a reduced verdazyl
ligand. We have previously established that reduced verdazyl
1
0-
ligands exhibit a lower ν(C=O) than neutral verdazyl radicals.
1
1
4
Fig. 4: X-ray structure of 10. Hydrogen atoms and BF ion removed for clarity; thermal
Reduction of the radical ligand puts more electron density
ellipsoids drawn at 50% probability. Two molecules of 10 were found in the asymmetric
unit. Selected bond lengths (Å): Pd1‒N3 1.915(5); 1.912(4), Pd1‒N5 2.033(5); 2.050(4),
Pd1‒N6 2.016(4); 2.009(5), Pd1‒N7 2.035(6); 2.014(6), N1‒N2 1.415(8); 1.385(7), N3‒N4
into the verdazyl π system and consequently increases the
extent of electron donation into the C=O π* orbital. The lower
-1
ν(C=O) of 9 (1683 cm ) in comparison to that of radical complex 1.438(6); 1.423(7), N2‒C2 1.293(8); 1.298(8), N3‒C2 1.369(8); 1.372(7), O1‒C1 1.23(1);
-1
1.226(8).
6
(1732 cm ) is consistent with this argument.
Synthesis of verdazyl-anion chloro complex 10
Acetonitrile complex 10 is very similar spectroscopically to
1
13
chloro complex 9. The H and C NMR spectra of 10 (see ESI),
which look much like the spectra of 9, are characteristic of a
diamagnetic compound. Furthermore, the ν(C=O) of 10 appears
In order to draw direct comparisons between the neutral
and monoanionic charge states of the tridentate verdazyl
ligand, we targeted an acetonitrile-palladium complex with a
monoanionic verdazyl ligand. An acetonitrile solution of
reduced complex 9 was treated with one equivalent of AgBF
afford acetonitrile complex 10 via chloride abstraction (Scheme
-1
at the same frequency as the ν(C=O) of 9 (1683 cm ). This
demonstrates that the verdazyl ligands in both 9 and 10 are in
their monoanionic redox state (vide supra). The difference in
overall charge between the two complexes (neutral for 9 and 1+
4
to
4
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for 10) appears to have no effect on the ν(C=O) of the verdazyl
ligand in the solid state.
The CVs of reduced palladium complexes 9 and 10 each
contain two one-electron oxidations (Fig. 5, black and red trace,
View Article Online
DOI: 10.1039/C9DT02549A
respectively) and a one-electron reduction. Both oxidations
appear at more positive potentials in comparison to the CV of
free verdazyl 5 (Table 2). A shift in redox potentials to more
positive values upon coordination is common for verdazyl
ligands and is likely due to the proximity of the verdazyl ring to
an electropositive metal center. The first oxidation wave in the
CVs of 9 and 10 occurs at +0.19 V and +0.43 V, respectively. The
shape of these oxidation waves demonstrates that one-electron
oxidation is chemically reversible for both compounds.
However, the second oxidation process in each CV (9: +1.23 V,
Electrochemical Properties of Verdazyl-Palladium Complexes
The redox properties of the palladium complexes and the
free ligand were investigated electrochemically. The cyclic
voltammograms (CVs) of reduced complexes 9 and 10 and free
verdazyl 5 are shown in Fig. 5. In-situ deprotonation of leuco
verdazyl 7 using sodium tert-pentoxide generates the anion of
5
, which reveals two reversible oxidations; the one-electron
relationship between deprotonated 7 and 5 permits the CV of
1
0: +1.41 V), which would presumably generate a verdazyl
the former to serve as proxy for the latter. Like most other
cation, is not reversible based on the lack of a return wave in
the cathodic sweep in each trace. This type of behaviour has
been observed in other metal complexes of bidentate verdazyl
9
verdazyl radicals, the CV of 5 contains an oxidation and a
reduction wave, both of which are chemically reversible. The
oxidation and reduction potentials of 5 are similar to those
1
0-11,15b
ligands.
In previous papers we have attributed the
reported for other verdazyl radicals that also contain N-aryl
irreversibility to dissociation of the resulting cationic verdazyl
ligand. Partial dissociation of an electrochemically-generated
verdazyl cation has also been observed for zinc complex 11,
where the verdazyl group is located on the terminal end of the
substituents.9
,15b
When compared to the redox potentials of
verdazyls with N,N'-dialkyl substituents, however, the redox
9
events of 5 occur at more positive potentials.
1
8
tridentate ligand. In the case of compounds 9 and 10,
however, the verdazyl moiety is flanked by two pyridine groups
in a rigid scaffold. These features were expected to help keep
the cationic verdazyl ring bound to the palladium atom. Even if
the verdazyl N-Pd bond is broken, a return wave would still be
1
8
expected on the cathodic sweep following oxidation. The
irreversibility of the second oxidation in the CVs of 9 and 10 is
more likely to be a result of the fully oxidized complex reacting
with something else in solution, potentially the acetonitrile
solvent or trace amounts of water. The reduction waves in the
CVs of 9 and 10 occur at -1.74 V and -1.37 V, respectively. The
peak heights of these reductions suggests one-electron
processes; the irreversible nature of these reductions precludes
definitive locus assignment (Pd and the pyridyl units are both
possibilities).
N
Fig. 5: Cyclic voltammogram of 10 (red trace), 9 (black trace), and 5 (blue trace). The
vertical axis is current and the arrows show both the direction and starting point for each
scan. All CVs were recorded as 1 mM solutions in acetonitrile at a scan rate of 100 mV/s
N
Zn
N
N
N
N
O
Cl Cl
4 4
(all solutions contain 0.1 M Bu NBF as electrolyte).
1
1
Table 2: Electrochemical parameters for free radical 5 and its palladium complexes.
+
Potentials are in V vs Fc/Fc and the CVs were recorded in acetonitrile.
The redox events in the CV of acetonitrile complex 10 are
shifted to more positive potentials with respect to chloro
complex 9 (Table 2). This difference may be a consequence of
the overall charge on each complex (1+ for 10 versus neutral for
9). The more positive potentials found in the CV of 10 may also
be a result of the increased π-donor capabilities of the chloride
ligand in comparison to acetonitrile. On a related note, the CV
of radical/acetonitrile complex 6 (see ESI) is essentially identical
to that of anionic-verdazyl/acetonitrile complex 10, which is not
surprising given that the two compounds are both members of
Ancillary
Ligand
-
Verdazyl Charge
State
E°2nd red
E°1st red
E°ox
5
6
Free radical ligand
Neutral radical
Monoanionic
Neutral radical
Monoanionic
-
-0.84
+0.43
-1.36b
+1.39b
CH
CH
3
CN
CN
+0.42
b
+0.43a
b
1
1
0
2
3
-1.37
+1.41
-1.75b
-1.74b
+1.23b
+1.23b
Cl
Cl
+0.19
+0.19a
9
a
b
1
st oxidation potential (9 and 10 contain reduced verdazyl ligands)
Irreversible process, peak potential given (anodic peak for oxidations and the same electron-transfer series.
cathodic peak for reductions)
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J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
Chemical Redox Reactions
charged nitrogen atom in the reduced verdazyl ligand
View Article Online
DOI: 10.1039/C9DT02549A
(compared to the neutral verdazyl ligand of 12).
The reversibility of the first oxidation process in the CVs of
complexes 9 and 10 suggests that these compounds may be
amenable to chemical oxidation to their corresponding radical
complexes. We tried reacting several different oxidants with
both 9 and 10, many of which failed (silver(I) salts, nitrosonium
+
(
NO ) salts, iodobenzene dichloride (PhICl
2
)); although chemical
reactions are evident based on color changes, tractable
products could not be isolated from these reactions. We then
turned to 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) as
an oxidant, but this reagent alone was not strong enough to
oxidize either 9 or 10. However, the addition of acid to a
solution containing both the metal complex and DDQ resulted
in successful one-electron oxidation of both 9 and 10. The
reduction potentials of quinones have been shown to shift to
more positive values at lower pH, rendering DDQ a stronger
4
Fig. 6: X-ray structure of 12. Hydrogen atoms and BF ion removed for clarity; thermal
2
3
oxidant in an acidic solution. Thus, treatment of either 9 or 10 ellipsoids drawn at 50% probability. Selected bond lengths (Å): Pd1‒N3 1.921(2), Pd1‒N5
with DDQ and aqueous fluoroboric acid (HBF ) resulted in the
4
2
1
.052(2), Pd1‒N6 2.035(2), Pd1‒Cl1 2.2997(8), N1‒N2 1.365(4), N3‒N4 1.352(3), N2‒C2
.321(3), N3‒C2 1.330(3), O1‒C1 1.217(4).
formation of radical complexes 12 or 6, respectively (Scheme 5,
forward reactions). Complex 6 was mentioned previously,
where it was synthesized from a crude sample of radical 5 with
a much lower yield (28%, Scheme 3). Both 6 and 12 can each be
reduced by a single electron using decamethylferrocene
-1
The ν(C=O) of radical/chloro complex 12 (1732 cm )
indicates that the verdazyl ligand is in its neutral radical charge
state. This is the exact same frequency as the ν(C=O) for radical/
acetonitrile complex 6, despite the difference in overall charge
between the two molecules. A similar situation was observed
for reduced complexes 9 and 10 where both molecules
2
(FeCp* ) to regenerate complexes 10 and 9, respectively
(Scheme 5, reverse reactions).
-1
exhibited the same ν(C=O) (1683 cm ). These results show that
the ν(C=O) of the tridentate verdazyl ligand can be used as a
measure of the ligand’s redox state but not of the overall charge
[
BF4]
[BF4]2
O
O
0.5 DDQ, HBF4 (aq)
CH3CN
N
N
N
N
N
N
N
N
N
N
Pd
Pd
on the complex. The IR spectrum of 12 also contains a BF stretch
NCCH3
NCCH3
N
FeCp*2
N
-1
at 1067 cm , which confirms the presence of a BF
4
counterion
CH3CN/CH2Cl2
following oxidation of 9 with DDQ and fluoroboric acid (Scheme
5). The EPR spectrum of chloro complex 12 (see ESI) is very
similar to that of acetonitrile complex 6. The g-value and
hyperfine coupling constants are in good agreement with those
of 6 and suggest that the spin density is mainly localized on the
verdazyl ring (see Table S1 in ESI). As was the case with 6, chloro
complex 12 has no discernible NMR spectrum.
1
0
6
76%
75%
[
BF4]
O
O
N
N
N
N
N
0.5 DDQ, HBF4 (aq)
CH3CN
N
N
N
N
N
Pd
Pd
Cl
Cl
N
FeCp*2
CH3CN/CH2Cl2
N
9
12
83%
96%
Conclusions
Scheme 5: Redox reactions of verdazyl-palladium complexes
Herein, we have investigated the redox chemistry of a series
The X-ray structure of radical/chloro complex 12 is shown in of tridentate-verdazyl palladium complexes. The results of this
Fig. 6. Like other verdazyl palladium complexes, the metal study are broadly consistent with our previous work in that the
centre in 12 is best described as having a distorted square verdazyl-palladium complexes exhibit ligand-centred redox
planar geometry, consistent with palladium(II) (sum of angles chemistry and the palladium(II) ions seem to remain redox
around Pd = 360.0°). The N-N bond lengths, which are inert. The tridentate verdazyl ligand was also able to withstand
shortened with respect to reduced complex 9 (Table 1), are ligand dissociation when switching between its neutral and
consistent with the verdazyl ligand existing in its neutral radical anionic charge states, an issue that complicated the reactivity
charge state. In addition, the planar nature of the verdazyl ring of the bidentate-verdazyl palladium complexes shown in
(
see Figure S1 in ESI) and the similarity between the N2C2 and Scheme 2. However, electrochemical oxidation of the neutral
N3C2 bond lengths are other indications of a verdazyl radical radical ligand to a verdazyl cation was irreversible, suggesting
ligand. The Pd-Cl bond of reduced complex 9 is longer than that that the tridentate verdazyl ligand may not be stable in its
of radical complex 12. We believe that the longer Pd-Cl bond in cationic charge state. This observation may be specific to
9
is a result of the stronger trans influence from the negatively palladium complexes; we have previously observed reversible
6
| J. Name., 2012, 00, 1-3
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
ARTICLE
ligand oxidation in Zn and Ru-verdazyl radical complexes. The structures were obtained at a temperature of 9Vi0ewKArtiuclseiOnnglinae
ancillary ligand (chloro or acetonitrile) appears to have little Bruker X8 APEX II diffractometer with grapDhOiIt:e10m.10o3n9o/Cch9DroTm02a54te9dA
effect on the spectroscopic details of the palladium complexes. Mo-Kα radiation (λ = 0.71073 Å).
The redox processes of acetonitrile complexes 6 and 10,
however, occur at more positive potentials than those of chloro 1-isopropyl-3,5-di(pyridin-2-yl)-6-oxo-leucoverdazyl (7)
complexes 9 and 12. The combination of RALs with palladium
has opened up a new avenue of reactivity to explore. In contrast
to the traditional two-electron chemistry commonly associated
with this metal, RALs provide an opportunity to carry out radical
reactions via single-electron processes.
It should be noted that the synthesis of this compound has
recently been reported by another group using a similar
20
procedure. Tetrazane 8 (0.0852 g, 0.287 mmol) was dissolved
in 4 mL of water and heated at 60 °C until all of the solid was
dissolved. The solution was cooled for 3 minutes and then
added to a solution of sodium metaperiodate (0.0632 g, 0.295
mmol) in 2 mL of water. The reaction mixture immediately
turned a dark orange color and it was allowed to stir one hour.
The resulting mixture was extracted with 10 mL of
dichloromethane and after evaporation of the solvent the crude
product was isolated as an orange oil. The oil was purified on a
neutral alumina column, eluting with 10:1 dichloromethane:
ethyl acetate. The neutral alumina (Brockmann grade I, 60
mesh) was deactivated prior to use by adding 15% w/w water.
The first fraction was collected off of the column and the solvent
was evaporated to afford 7 as a yellow solid. Yield: 0.0630g,
Experimental section
General Methods
All reactions were carried out under a nitrogen atmosphere
using standard Schlenk techniques unless otherwise noted.
However, strict air-free conditions are likely not necessary for
these reactions considering the stability of the reactants and
products in open air. Acetonitrile was dried over calcium
hydride and distilled under an argon atmosphere unless
otherwise noted. All other solvents were used directly from the
bottle. All reagents were purchased from Sigma-Aldrich and
used as received. Verdazyl radical 5 and tetrazane 8 were
7
4%. The solid can be handled/stored in air without any signs of
decomposition. This compound is also stable in solutions
exposed to air for at least 24 hours. FT-IR (KBr) – C=O 1689
-1
-1 1
1
4
1
13
1
3
cm , NH 3254 cm . H NMR (CDCl , 300 MHz): δ N-H: 10.73 (s),
prepared using literature methods.
H and C{ H} NMR
Aromatic protons: 8.64 (d, 1H, J = 5.0 Hz), 8.35 (d, 1H, J = 5.1
Hz), 8.25 (d, 1H, J = 8.7 Hz), 8.07 (d, 1H, J = 8.0 Hz), 7.74 (t of d,
spectra were recorded on a Bruker AV300 instrument (300
MHz). FT-IR spectra were recorded on a Perkin-Elmer Spectrum
One FT-IR spectrometer as KBr discs. The EPR spectrum of 6 was
recorded on a Bruker EMX spectrometer at room temperature
1
7
H, J = 7.8 Hz, J = 1.7 Hz), 7.63 (t of d, 1H, J = 8.0 Hz, J = 1.8 Hz),
.32 (ddd, 1H, J = 7.6 Hz, J = 5.0 Hz, J = 1.1 Hz), 7.32 (ddd, 1H, J
=
7.2 Hz, J = 5.1 Hz, J = 0.9 Hz), CH(CH
3
)
2
: 4.71 (sep, 1H, J = 6.5
(295 K) and referenced externally using DPPH (g = 2.0036). The
1
3
1
Hz), CH(CH
3 2
) : 1.36 (d, 6H, J = 6.7 Hz). C{ H} NMR (CDCl
3
, 75.5
EPR spectrum of 12 was recorded on a Bruker Elexsys E500
MHz): δ 150.4 (quat. C), 149.2 (quat. C), 148.7 (Aryl C-H), 147.9
quat. C), 147.4 (quat. C), 146.4 (Aryl C-H), 137.9 (Aryl C-H),
36.6 (Aryl C-H), 124.8 (Aryl C-H), 120.7 (Aryl C-H), 118.0 (Aryl
series continuous wave EPR spectrometer at room temperature
(
1
(295 K) and referenced externally using DPPH (g = 2.0036). EPR
solutions were sparged with nitrogen for 15 minutes, with
-3
C-H), 113.2 (Aryl C-H), 47.9 (CH(CH
3
)
2
), 20.1 CH(CH
3
)
2
. HRMS
approximate sample concentrations of 10 M. The simulated
+
(
ESI/Orbitrap): m/z 297.14588 ([M+H] ; Anal. Calcd for
spectrum of 6 was generated using the WinSim 2002 EPR
+
2
4
15 16 6 16 6
C H N OH : m/z 297.14584). Anal. Calcd for C15H N O:
simulation program. The simulated spectrum of 12 was
2
5
C, 60.80; H, 5.44; N, 28.36. Found: C, 60.51; H, 5.50; N, 28.20.
Mp 68-70 °C
generated using the EasySpin computational package for
Matlab. Elemental analyses were performed by Canadian
Microanalytical Service Ltd., Delta, BC, Canada. UV-Vis spectra
were recorded on an Agilent 8453 spectrometer. Cyclic
voltammetry experiments were carried out using a Bioanalytical
(
1-isopropyl-3,5-di(pyridin-2-yl)-6-oxoverdazyl-
anion)chloropalladium(II) (9)
Systems Epsilon voltammetric analyzer with a scan rate of 100 Method 1: Synthesis using 7 and PdCl2:
mV/s. Typical electrochemical cells consisted of a three- PdCl (0.0383 g, 0.216 mmol) was added 20 mL of freshly
2
electrode setup including a glassy carbon working electrode, a distilled acetonitrile and the solution was refluxed under a
platinum wire counter electrode, and a silver wire reference nitrogen atmosphere until all of the solid had dissolved. The
+
-
electrode. Solutions of analyte (~1 mM) that contain Bu
4
N BF
4
3 2 2
orange solution of (CH CN) PdCl was allowed to cool for 5
as an electrolyte (~0.1 M) were referenced against an internal minutes and then a solution of leuco verdazyl 7 (0.0630 g, 0.213
ferrocene standard (~1 mM). The CV of 5 was obtained by mmol) in 3 mL of acetonitrile was added. The solution
adding one molar equivalent of sodium tert-pentoxide to a immediately turned dark red and it was allowed to stir under
solution of leuco verdazyl 7. High resolution mass spectra were nitrogen for one hour. The reaction mixture was filtered and the
obtained using a Thermo Ultimate 3000 Orbitrap instrument solvent was removed on the rotary evaporator. The crude red
using an electrospray ionization source. In all cases, the positive solid was purified using column chromatography (10:1
ion mode was selected for mass spectrometry data acquisition dichloromethane : ethyl acetate, silica). Yield: 0.0645 g, 72%.
and the experimental isotope pattern of each palladium
complex was matched with the calculated pattern. X-ray crystal Method 2: Synthesis using 12 and FeCp*2:
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ARTICLE
Journal Name
0
.0138 g (0.0263 mmol) of radical/chloro complex 12 was (open to air). The solvent was removed onVietwhAerticlreoOtnalirnye
dissolved in 4 mL of acetonitrile (solvent used directly out of the evaporator and the crude brown/red solDidOIw: 1a0s.10e3x9t/rCa9cDteTd02w54i9thA
bottle) to afford a green solution. Separately, 0.0086 g (0.0264 3x60 mL boiling toluene. Extraction details: the crude solid was
mmol) of FeCp*
yellow solution). The two solutions were mixed to form a red reflux. The red product slowly dissolved while the green
solution, which was stirred for 1 hour (open to air). The solvent [FeCp* ][BF ] remained undissolved. The red solution was
2
was dissolved in 0.5 mL of dichloromethane suspended in 60 mL of toluene and the solvent was brought to
(
2
4
was removed on the rotary evaporator and the brown/red solid decanted from the green solid and the process was repeated
was purified using column chromatography (see Method 1). another two times. The toluene fractions were combined and
Yield: 0.0096 g, 83%.
put in the freezer overnight. A red solid precipitated and was
collected by filtration. The filtrate was reduced to 8 mL and a
second crop of crystals was collected. Yield: 0.0143 g, 75%.
Characterization data for 9:
The solid can be handled/stored in air without any signs of
decomposition. This compound is also stable in solutions Characterization data for 10:
exposed to air for at least 24 hours. FT-IR (KBr) – C=O 1683 The solid can be handled/stored in air without any signs of
-
1 1
cm . H NMR (CDCl
3
, 300 MHz): δ Aromatic protons: 8.76 (d, 1H, decomposition. This compound is also stable in solutions
-1
J = 5.5 Hz), 8.48 (d of d, 1H, J = 6.0 Hz, J = 1.6 Hz), 8.28 (d, 1H, J exposed to air for at least 24 hours. FT-IR (KBr) – BF 1068 cm ,
-1
-1
-1 1
=
7
1
=
8.9 Hz), 7.96 (t of d, 1H, J = 7.8 Hz, J = 1.5 Hz), 7.86 (d, 1H, J = C=O 1683 cm , CN (acetonitrile) 2347 cm , 2361 cm . H NMR
.7 Hz), 7.69 (ddd, 1H, J = 9.4 Hz, J = 7.1 Hz, J = 1.8 Hz), 7.42 (ddd, (CD CN, 300 MHz): δ Aromatic protons: 8.26 (d, 1H, J = 5.4 Hz),
H, J = 7.6 Hz, J = 5.5 Hz, J = 1.7 Hz), 6.88 (ddd, 1H, J = 7.3 Hz, J 8.16 (d, 1H, J = 9.0 Hz), 8.11 (t of d, 1H, J = 7.8 Hz, J = 1.4 Hz),
3
5.9 Hz, J = 1.3 Hz), CH(CH
3 2
)
1
: 4.73 (sep, 1H, J = 6.6 Hz), 7.94 (d, 1H, J = 5.9 Hz), 7.78-7.86 (m, 2H), 7.55 (t of d, 1H, J = 6.6
3
1
CH(CH
3 2
) : 1.31 (d, 6H, J = 6.7 Hz). C{ H} NMR (CDCl
3
, 75.5 MHz): Hz, J = 1.6 Hz), 6.97 (t of d, 1H, J = 6.6 Hz, J = 1.3 Hz), CH(CH
: 1.26 (d, 6H, J = 6.8 Hz).
CN, 75.5 MHz): δ 155.0 (quat. C), 154.1 (quat.
C-H), 125.8 (Aryl C-H), 123.6 (Aryl C-H), 117.2 (Aryl C-H), 115.3 C), 153.0 (quat. C), 150.7 (Aryl C-H), 148.4 (Aryl C-H), 145.5
3 2
) :
3 2
δ 156.0 (quat. C), 154.2 (quat. C), 153.9 (quat. C), 150.3 (Aryl C- 4.63 (sep, 1H, J = 6.7 Hz), CH(CH )
H), 148.5 (Aryl C-H), 146.3 (quat. C), 138.9 (Aryl C-H), 138.9 (Aryl
1
3
1
C{ H} NMR (CD
3
(
λ
Aryl C-H), 48.3 (CH(CH
3
)
2
3
), 20.0 (CH(CH
3
)
2
). UV-vis (CH
3
CN) – (quat. C), 141.8 (Aryl C-H), 141.2 (Aryl C-H), 128.1 (Aryl C-H),
-1
-1
3
max 301 nm (ε = 10.7x10 M cm ), λmax 382 nm (ε = 4.9x10
124.9 (Aryl C-H), 119.2 (Aryl C-H), 116.2 (Aryl C-H), 49.4
), 20.1 (CH(CH ). UV-vis (CH CN) – λmax 296 nm (ε =
Cl+; Anal. Calcd for 11.0x10 M cm ), λmax 376 nm (ε = 5.0x10 M cm ), λmax 488
-1
-1
3
-1
-1
M cm ), λmax 498 nm (ε = 2.1x10 M cm ). HRMS (ESI/ (CH(CH
Orbitrap): m/z 839.03687 ((VdPd)
3
3
)
2
)
3 2
3
-1
-1
3
-1
-1
2
+
+
3
-1
-1
(C
H N
15 15 6
OPd)
Calcd for C15
1.15; H, 3.55; N, 18.99. Mp 230-236 °C
2
Cl / C30
OPdCl: C, 41.21; H, 3.46; N, 19.22. Found: C, (M ; Anal. Calcd for C17
for C17 OPdBF : C, 38.55; H, 3.43; N, 18.51. Found: C, 36.43;
30 12 2 2
H N O Pd Cl : m/z 839.03715). Anal. nm (ε = 2.0x10 M cm ). HRMS (ESI/Orbitrap): m/z 442.06068
+
+
H
15
N
6
18 7
H N OPd : m/z 442.06022). Anal. Calcd
4
H N
18 7
4
H, 3.43; N, 17.25. Mp 234-240 °C
(
1-isopropyl-3,5-di(pyridin-2-yl)-6-oxoverdazyl-
anion)palladium(II)(acetonitrile) (tetrafluoroborate) (10)
Method 1: Synthesis using 9 and AgBF
(1-isopropyl-3,5-di(pyridin-2-yl)-6-
oxoverdazyl)palladium(II)(acetonitrile) bis(tetrafluoroborate) (6)
4
:
Verdazyl-anion/chloro complex 9 (0.2664 g, 0.609 mmol) was Method 1: Synthesis using 5 and (CH
3 4 4 2
CN) Pd(BF ) :
dissolved in 7 mL of dichloromethane (dried over 4 Å molecular Verdazyl radical 5 was prepared according to the procedure
1
4
sieves) to produce a dark red solution. 20 mL of distilled reported by Brook et. al. using 0.1465 g (0.493 mmol) of
acetonitrile was then added to the flask and it was stirred to tetrazane 5. As described by Brook, all of the crude red oil was
become homogeneous. 0.1193 g of AgBF
added to the reaction and it was stirred for 15 hours. The white was dissolved in 50 mL of freshly distilled acetonitrile to afford
AgCl precipitate was removed by filtration and the solvent was clear orange solution. 0.2172 (0.489 mmol) of
removed on the rotary evaporator. The resulting red solid was (CH CN) Pd(BF was added to the reaction and it was allowed
4
(0.613 mmol) was used in the next reaction without further purification. The oil
a
g
3
4
4 2
)
redissolved in 30 mL of acetonitrile. Insoluble AgCl was still to stir for 2.5 hours under a nitrogen atmosphere. The solution
present in the solution and so it was filtered through a short was now a brownish-green color and a dark solid precipitate
Celite plug. Evaporation of the solvent yielded a red solid, which had formed. The reaction was stored in the freezer overnight
was recrystallized from refluxing anhydrous ethanol (20 mL). and the green solid was isolated by filtration the following day.
Yield: 0.2854 g, 88%.
Yield: 0.0863 g, 28%.
Method 2: Synthesis using 6 and FeCp*
2
:
4
Method 2: Synthesis using 10 and DDQ/HBF :
Radical/acetonitrile complex 6 (0.0367 g, 0.0595 mmol) was 0.0845 g (0.160 mmol) of verdazyl-anion/acetonitrile complex
dissolved in 20 mL acetonitrile (solvent used directly out of the 10 was dissolved in 30 mL of acetonitrile to generate a deep red
bottle) to generate a green solution. Separately, 0.0194 g solution. 0.0189 g (0.0833 mmol) of 2,3-dichloro-5,6-dicyano-
(
0.0595 mmol) of FeCp*
2
was dissolved in 4 mL of 1,4-benzoquinone (DDQ) was added to the reaction and it was
dichloromethane (yellow solution). The two solutions were stirred for 5 minutes. The solution was still red at this point but
mixed to form a red solution, which was stirred for 40 minutes appeared slightly darker than it was prior to the addition of
8
| J. Name., 2012, 00, 1-3
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
Journal Name
DDQ. 2 drops of aqueous fluoroboric acid (HBF
added to the solution and it immediately changed to a vibrant
green color. The solution was stirred for 20 minutes and was
then dried over anhydrous sodium sulfate (~0.2 g). The solution
was concentrated to a volume of approximately 5 mL and was
then layered onto 20 mL of dichloromethane. Green crystals
deposited after several days of slow diffusion. Yield: 0.0753 g,
ARTICLE
4
, 48%) was
Conflicts of interest
View Article Online
DOI: 10.1039/C9DT02549A
In accordance with our policy on Conflicts of interest please
ensure that a conflicts of interest statement is included in your
manuscript here. Please note that this statement is required
for all submitted manuscripts. If no conflicts exist, please state
that “There are no conflicts to declare”.
7
6%.
Acknowledgements
Characterization data for 6:
The solid can be handled/stored in air without any signs of The acknowledgements come at the end of an article after the
decomposition. This compound is also fairly stable in solutions conclusions and before the notes and references.
exposed to air for at least 24 hours. However, slow evaporation
of acetonitrile solutions will result in reduction of the radical
verdazyl ligand. FT-IR (KBr) – BF 1054 cm , C=O 1732 cm , CN
-1
-1
Notes and references
-
1
-1
(
3
acetonitrile) 2343 cm , 2360cm . UV-vis (CH CN) – λmax 294
3
-1
-1
3
-1
-1
nm (ε = 16.0x10 M cm ), λmax 413 nm (ε = 3.1x10 M cm ), 1.
max 454 nm (ε = 2.6x10 M cm ), λmax 479 nm (ε = 2.7x10
M cm ), λmax 583 nm (ε = 2.3x10 M cm ), λmax 630 nm (ε =
.3x10 M cm ). HRMS (ESI/Orbitrap): m/z 442.06058 (M ;
(a) O. R. Luca and R. H. Crabtree, Chemical Society Reviews
2013, 42, 1440-1459; (b) V. Lyaskovskyy and B. de Bruin,
ACS Catalysis, 2012, 2, 270-279; (c) P. J. Chirik, Inorganic
Chemistry, 2011, 50, 9737-9740; (d) F. N. Castellano,
Dalton Transactions, 2012, 41, 8493-8501.
P. J. Chirik and K. Wieghardt, Science, 2010, 327, 794-795.
(a) M. W. Bouwkamp, A. C. Bowman, E. Lobkovsky and P. J.
Chirik, Journal of the American Chemical Society 2006, 128,
3
-1
-1
3
λ
-1
-1
3
-1
-1
3
-1
-1
+
3
+
Anal. Calcd for C17
2
H
18
N
7
OPd : m/z 442.06022). EPR (CH
3
CN): g =
1
5
2.
.0033, a( N): 4.4 G (1N), 5.2 G (1N), 5.7 G (1N), 8.0 G (1N).
Anal. Calcd for C17 OPdB : C, 33.13; H, 2.94; N, 15.91.
Found: C, 32.78; H, 2.98; N, 15.46. Mp 250-258 °C
3
.
H
18
N
7
2 8
F
1
3340-13341; (b) A. L. Smith, K. I. Hardcastle and J. D.
Soper, Journal of the American Chemical Society 2010, 132,
4358-14360; (c) P. J. Chirik, Accounts of Chemical
(
(
1-isopropyl-3,5-di(pyridin-2-yl)-6-oxoverdazyl)chloropalladium(II)
tetrafluoroborate) (12)
1
Research, 2015, 48, 1687-1695; (d) P. Chaudhuri, M. Hess,
J. Müller, K. Hildenbrand, E. Bill, T. Weyhermüller and K.
Wieghardt, Journal of the American Chemical Society,
0
.0343 g (0.0785 mmol) of verdazyl-anion/chloro complex 9 was
added to 50 mL of acetonitrile. The solution was heated gently
until all of the solid had dissolved. 0.0089 g (0.0379 mmol) of
1
999, 121, 9599-9610; (e) B. Bagh, D. L. J. Broere, V. Sinha,
P. F. Kuijpers, N. P. van Leest, B. de Bruin, S. Demeshko, M.
A. Siegler and J. I. van der Vlugt, Journal of the American
Chemical Society, 2017, 139, 5117-5124.
2
,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) was added
to the red solution and it was stirred for 5 minutes. The solution
was still red/bergundy at this point but appeared darker than it
was prior to the addition of DDQ. 1 drop of aqueous fluoroboric
acid (HBF , 48%) was added to the solution and the reaction
4
immediately turned green. The solution was stirred for 25
minutes and was then dried over anhydrous sodium sulfate
4
5
.
.
(a) D. Herebian, E. Bothe, E. Bill, T. Weyhermüller and K.
Wieghardt, Journal of the American Chemical Society,
2001, 123, 10012-10023; (b) P. Chaudhuri, C. N. Verani, E.
Bill, E. Bothe, T. Weyhermüller and K. Wieghardt, Journal
of the American Chemical Society, 2001, 123, 2213-2223;
(
c) D. Herebian, E. Bothe, F. Neese, T. Weyhermüller and K.
Wieghardt, Journal of the American Chemical Society,
003, 125, 9116-9128.
(~0.2 g). The solution was concentrated to a volume of
approximately 3 mL and was then layered onto 10 mL of
toluene. Green crystals deposited after several days of slow
diffusion. Yield: 0.0396 g, 96%. The solid can be handled/stored
in air without any signs of decomposition. This compound is also
fairly stable in solutions exposed to air for at least 24 hours.
However, slow evaporation of acetonitrile solutions will result
in reduction of the radical verdazyl ligand. FT-IR (KBr) – BF 1067
2
(a) S. Kokatam, T. Weyhermüller, E. Bothe, P. Chaudhuri
and K. Wieghardt, Inorganic Chemistry, 2005, 44, 3709-
3
717; (b) K. Ray, T. Weyhermüller, F. Neese and K.
Wieghardt, Inorganic Chemistry, 2005, 44, 5345-5360; (c)
S.-L. Kokatam, P. Chaudhuri, T. Weyhermuller and K.
Wieghardt,
Dalton
Transactions,
2007,
DOI:
-
1
-1
3
3
cm , C=O 1732 cm . UV-vis (CH CN) – λmax 297 nm (ε = 10.8x10
10.1039/B614745C, 373-378.
-
1
-1
3
-1
-1
M cm ), λmax 319 nm (ε = 10.6x10 M cm ), λmax 435 nm (ε = 6.
K. M. Conner, A. C. Arostegui, D. D. Swanson and S. N.
Brown, Inorganic Chemistry, 2018, 57, 9696-9707.
(a) D. L. J. Broere, B. de Bruin, J. N. H. Reek, M. Lutz, S.
Dechert and J. I. van der Vlugt, Journal of the American
Chemical Society, 2014, 136, 11574-11577; (b) D. L. J.
Broere, N. P. van Leest, B. de Bruin, M. A. Siegler and J. I.
van der Vlugt, Inorganic Chemistry, 2016, 55, 8603-8611.
R. G. Hicks, Ed., Stable Radicals: Fundamental and Applied
Aspects of Odd-Electron Compounds, John Wiley and Sons,
Ltd., Chichester, 2010.
3
-1
-1
3
-1
-1
3
.5x10 M cm ), λmax 470 nm (ε = 3.6x10 M cm ), λmax 573
nm (ε = 1.7x10 M cm ), λmax 627 nm (ε = 1.8x10 M cm ).
ESI/Orbitrap): m/z 839.03674 ((VdPd)
3
-1
-1
3
-1
-1
7.
+
(
(
(
2
Cl ; Anal. Calcd for
+
+
C
15
H
N
15 6
OPd)
CN): g = 2.0039, a( N): 4.7 G (1N), 5.8 G (1N), 6.1 G (1N),
.4 G (1N). Anal. Calcd for C15 OPdClBF : C, 34.38; H, 2.89;
2 30 12 2 2
Cl / C30H N O Pd Cl : m/z 839.03715). EPR
1
5
CH
3
8
H N
15 6
4
8
.
N, 16.04; Cl, 6.77. Found: C, 33.53; H, 3.14; N, 15.52; Cl, 7.07.
Mp 248-256 °C
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 9
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Pleas De ad l to o nn oT tr aa nd sj au cs t ti omn as rgins
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Page 11 of 11
Dalton Transactions
View Article Online
DOI: 10.1039/C9DT02549A
Synthesis and Redox Chemistry of Pd(II) Complexes of a Pincer Verdazyl
Ligand
a
b
a
Corey A. Sanz, Brian O. Patrick and Robin G. Hicks*
0/+/2+
O
N
N
N
N Pd L/X
N
N
Palladium (II) complexes of a “pincer” verdazyl ligand undergo a variety of ligand-centered electron
transfer reactions.