Structural and reactivity comparison of analogous organometallic Pd(iii) and Pd(iv) complexes

Article abstract of DOI:10.1039/c2dt32127k

The tetradentate ligands ^RN4 (^RN4 = N,N′-di-alkyl-2,11-diaza[3,3](2,6)pyridinophane, R = Me or iPr) were found to stabilize cationic (^RN4)PdMe₂ and ( ^RN4)PdMeCl complexes in both Pd^III and Pd^IV oxidation states. This allows for the first time a direct structural and reactivity comparison of the two Pd oxidation states in an identical ligand environment. The Pd^III complexes exhibit a distorted octahedral geometry, as expected for a d⁷ metal center, and display unselective C-C and C-Cl bond formation reactivity. By contrast, the Pd^IV complexes have a pseudo-octahedral geometry and undergo selective non-radical C-C or C-Cl bond formation that is controlled by the ability of the complex to access a five-coordinate intermediate. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2dt32127k

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COMMUNICATION
Structural and reactivity comparison of analogous organometallic Pd(III) and
Pd(IV) complexes†
Fengzhi Tang,a Fengrui Qu,a Julia R. Khusnutdinova,a Nigam P. Rathb and Liviu M. Mirica*a
Received 14th September 2012, Accepted 3rd October 2012
DOI: 10.1039/c2dt32127k
The tetradentate ligands RN4 (RN4 = N,N′-di-alkyl-2,11-
isolation and characterization of analogous mononuclear
PdIII and PdIV complexes. Isolation of these complexes enabled
diaza[3,3](2,6)pyridinophane, R = Me or iPr) were found to
stabilize cationic (RN4)PdMe2 and (RN4)PdMeCl complexes
a direct structural and reactivity comparison of the two Pd oxi-
dation states in an identical ligand environment.
in both PdIII and PdIV oxidation states. This allows for the
first time a direct structural and reactivity comparison of the
two Pd oxidation states in an identical ligand environment.
Results and discussion
The PdIII complexes exhibit a distorted octahedral geometry,
as expected for a d7 metal center, and display unselective
C–C and C–Cl bond formation reactivity. By contrast, the
PdIV complexes have a pseudo-octahedral geometry and
undergo selective non-radical C–C or C–Cl bond formation
that is controlled by the ability of the complex to access a
five-coordinate intermediate.
Synthesis and properties of PdIII and PdIV complexes
The MeN4 and iPrN4 ligands were synthesized using reported and
slightly modified synthetic procedures8 and were used to prepare
the PdII precursors (MeN4)PdIIMeCl (1), (iPrN4)PdIIMeCl (2),
(
MeN4)PdIIMe2 (3), and (iPrN4)PdIIMe2 (4) (Scheme 1).9 Cyclic
voltammetry (CV) studies of complexes 1–4 in MeCN reveal
two oxidation waves assigned to PdII/PdIII and PdIII/PdIV redox
couples, respectively (Fig. 1 and Table 1).5,9,10 Interestingly,
these oxidation potentials increase with the increasing steric bulk
of the ligand from MeN4 to iPrN4 to tBuN4. For example, the
Introduction
The role of Pd0/PdII intermediates in palladium-catalyzed reac-
tions has been studied extensively for the past four decades.1 In
addition, PdIV and PdIII complexes have recently been proposed
as catalytically active intermediates in various oxidative organic
transformations that complement the conventional Pd0/II cataly-
sis.2 In this context, development of ligand systems that can
stabilize both PdIII and PdIV complexes in a similar coordination
environment can provide key insight into the mechanism of
these important catalytic transformations,3 especially for systems
in which either one or both oxidation states have been proposed
to be responsible for the observed reactivity.4
We have recently reported the first mononuclear organometal-
lic PdIII complexes – stabilized by a tetradentate ligand N,N′-
di-tert-butyl-2,11-diaza[3,3](2,6)pyridinophane
(
tBuN4), and
Scheme 1 Synthesis of (RN4)PdIII and (RN4)PdIV complexes.
studied their C–C bond formation reactivity.5 We have also
investigated the involvement of PdIII species in aerobically-
induced C–C bond formation from PdII precursors6 and in Khar-
asch radical additions.7 Continuing our study of high valent
Pd chemistry, herein we report the use of N-methyl (MeN4) and
N-isopropyl (iPrN4) pyridinophane ligand analogs that lead to the
aDepartment of Chemistry, Washington University, One Brookings
Drive, St. Louis, Missouri 63130-489, USA. E-mail: mirica@wustl.edu
bDepartment of Chemistry and Biochemistry, University of Missouri-
St. Louis, One University Boulevard, St. Louis, Missouri 63121-4400,
USA
†Electronic supplementary information (ESI) available. CCDC 884994.
For ESI and crystallographic data in CIF or other electronic format see
DOI: 10.1039/c2dt32127k
Fig. 1 CVs of (a) RN4PdIIMeCl and (b) RN4PdIIMe2 complexes. Con-
ditions: 0.1 M Bu4NClO4–MeCN or CH2Cl2, 100 mV s−1. The CV data
for [(tBuN4)PdIIIMeCl]+ were taken from ref. 5.
14046 | Dalton Trans., 2012, 41, 14046–14050
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Table 1 Spectroscopic properties of PdIII complexes
EpcIII/II, EpaII/III, E1/2III/IV (ΔEp)a (mV)
UV-vis (MeCN), λ (nm) (ε, M−1 cm−1
)
EPR, gx, gy, gz (AN, G)b
1+
−620/−135, −480/−20,c +205 (130)
−535, +10, +428 (65)
606 (615), 475 (480), 316 (3550)
658 (810), 517 (580), 312 (4020)
723 (1100), 545 (490), 368 (3300)
620 (405), 490 (430), 365 (1500)
666 (623), 500 (500), 339 (4545)
741 (360), 560 (160), 350 (2300)
2.212, 2.101, 2.014 (23.0)
2.228, 2.115, 2.011 (21.3)
2.239, 2.134, 2.005 (19.5)
2.168 (16.0), 2.168 (16.0), 1.989 (22.5)
2.184 (15.0), 2.184 (15.0), 1.988 (20.4)
2.222 (12.0), 2.191 (13.8), 1.986 (18.0)
2+
A+d
3+
−464, +134, +587 (63)
−1030, −520, −395 (75)
−965, −420, −135 (100)
−774, −341, +53 (67)
4+
B+e
a Measured for PdII precursors by CV vs. Fc+/Fc in 0.1 M Bu4NClO4–MeCN or CH2Cl2, scan rate 100 mV s−1; ΔEp is the peak potential separation
for the PdIII/IV couple. b In 3 : 1 PrCN : MeCN, 77 K; AN’s are superhyperfine coupling constants to the axial N atoms. c The duplicate Epa
and
II/III
values are likely due to two conformations present in solution (ref. 5). d CV and UV-vis data for [(tBuN4)PdIIIMeCl]+ (A+) are from ref. 5.
II/III
Epc
e UV-vis and EPR data for [(tBuN4)PdIIIMe2]+ (B+) are from refs. 5 and 6, respectively.
III/IV
E1/2
values of MeN4 complexes 1 and 3 are ∼400 mV lower
than those for the analogous tBuN4PdII complexes (Table 1).5
The presence of such low PdIII/IV oxidation potentials and an
appreciable separation between the PdII/III and PdIII/IV potentials
suggest that both PdIII and PdIV species can be stabilized by
MeN4 and iPrN4 (vide infra).
One-electron oxidation of RN4PdIIMeCl complexes 1 and 2
by controlled potential electrolysis (CPE) in 0.1 M Bu4NClO4–
MeCN or chemical oxidation with 1 equiv. ferrocenium hexa-
fluorophosphate (Fc+PF6−) generate the PdIII species [(MeN4)
PdIIIMeCl]+ (1+) and [(iPrN4)PdIIIMeCl]+ (2+), while oxidation
Fig. 2 ORTEP representation (50% probability ellipsoids) of the
cations of [1+]ClO4 (left) and [12+](ClO4)2 (right). Selected bond dis-
tances (Å) and angles (°): [1+]ClO4, Pd1–C1 2.021(1), Pd1–Cl1i 2.344(3),
Pd1–N1 2.085(2), Pd1–N1i 2.085(2), Pd1–N3 2.302(3), Pd1–N4
2.338(3), N3–Pd1–N4 149.12(1); [12+](ClO4)2, Pd1–C1 2.072(2), Pd1–
Cl1 2.2917(6), Pd1–N1 1.966(2), Pd1–N2 2.063(2), Pd1–N3 2.098(2),
Pd1–N4 2.105(2), N3–Pd1–N4 158.94(7).
of 3 and 4 with 1 equiv. Fc+PF6 yields [(MeN4)PdIIIMe2]+ (3+)
−
and [(iPrN4)PdIIIMe2]+ (4+), respectively (Scheme 1).9 The X-ray
structures of [1+]ClO4, [2+]ClO4, [3+]ClO4, and [4+]ClO4 all
reveal the presence of the expected distorted octahedral geometry
at the d7 PdIII center (Fig. 2–4 and S33†).10,11 Moreover, the Pd–
Naxial distances increase in ∼0.05 Å steps when going from
MeN4 to iPrN4 to tBuN4 due to the increasing steric clash
between the N-substituents and the equatorial ligands, as can be
observed in the space filling models of the PdIII complexes
(Fig. S33–S34). The presence of the less bulky N-substituent
also leads to a more symmetric structure for the PdIII complexes.
For example, while 3+ exhibits a more symmetric C2v geometry,
4+ has a C2 symmetry due to the bulkier iPrN4 ligand.9
The EPR spectra of complexes 1+–4+ reveal anisotropic
signals corresponding to PdIII centers with a dz2 ground state, as
expected for a d7 ion (Table 1, Fig. S28–S32†).5,6,9,10 This is
further supported by the presence of superhyperfine coupling to
the two axial N atoms (AN = 12–23 G) that are observed in glas-
sing solvent mixtures at 77 K.9 Interestingly, the shorter Pd–
Naxial distances due to the size of the N-substituents are also
responsible for the increasing trend in the AN superhyperfine
coupling constants from tBuN4 to iPrN4 to MeN4 (Table 1),
suggesting that these EPR parameters can be used to approxi-
mate the strength of Pd–Naxial interactions in PdIII complexes.9
In addition, the same ligand steric effects likely lead to a red
shift of the visible absorption bands with the increasing size of
the N-substituent in [(RN4)PdIIIMeCl]+ and [(RN4)PdIIIMe2]+
complexes (Table 1 and Fig. S23–S24†).12
Fig. 3 ORTEP representation (50% probability ellipsoids) of the
cations of [3+]ClO4 (left) and [32+](PF6)2 (right). Selected bond dis-
tances (Å) and angles (°): [3+]ClO4, Pd1–C1 2.0432(10), Pd1–C1i
2.0433(10), Pd1–N1 2.1393(7), Pd1–N1i 2.1393(7), Pd1–N2 2.4013
(11), Pd1–N3 2.3506(12), N2–Pd1–N3 145.37(4); [32+](PF6)2, Pd1–C1
2.042(3), Pd1–C2 2.039(3), Pd1–N1 2.054(2), Pd1–N2 2.058(3), Pd1–
N3 2.129(3), Pd1–N4 2.116(2), N3–Pd1–N4 156.70(10).
(Scheme 1).9 The X-ray structures of [12+](ClO4)2, [32+](PF6)2,
and [42+](PF6)2 confirm the presence of PdIV centers with a
pseudo-octahedral geometry and reveal Pd–Naxial distances that
are 0.24–0.27 Å shorter than those in the PdIII analogs 1+, 3+,
and 4+, respectively (Fig. 2–4), in line with the preferred octa-
hedral geometry for a d6 PdIV center vs. the distorted geometry
of a d7 PdIII ion.10,11 Moreover, the MeN4 ligand can easily
accommodate the shorter Pd–Naxial distances and the more
compact structure of the octahedral PdIV centers, as seen in the
Further chemical oxidation of 1+ and 2+ with 1 equiv. nitroso-
nium tetrafluoroborate (NO+BF4−) in MeCN generates [(MeN4)
PdIVMeCl]2+ (12+) and [(iPrN4)PdIVMeCl]2+ (22+), while treat-
−
ment of 3 and 4 with 2 equiv. Fc+PF6 yields the PdIV com-
plexes [(MeN4)PdIVMe2]2+ (32+) and [(iPrN4)PdIVMe2]2+ (42+)
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Scheme 2 Reactivity of [(RN4)PdIIIMeCl]+ complexes.19
Fig. 4 ORTEP representation (50% probability ellipsoids) of the
cations of [4+]ClO4 (left) and [42+](PF6)2 (right). Selected bond dis-
tances (Å) and angles (°): [4+]ClO4, Pd1–C1 2.033(4), Pd1–C2 2.042
(4), Pd1–N1 2.165(3), Pd1–N2 2.132(3), Pd1–N3 2.429(3), Pd1–N4
2.452(3), N3–Pd1–N4 145.61(11); [42+](PF6)2, Pd1–C1 2.057(11), Pd1–
C2 2.028(11), Pd1–N1 2.034(9), Pd1–N2 2.048(9), Pd1–N3 2.177(9),
Pd1–N4 2.182(9), N3–Pd1–N4 155.5(3).
Scheme 3 Reactivity of [(RN4)PdIVMeCl]2+ complexes.19
presence of visible light to generate both ethane and methyl
chloride in up to 17% and 21% yields, respectively (Scheme 2).
Formation of the two products is suppressed completely in the
presence of an effective alkyl radical trap, TEMPO,9 suggesting
a radical mechanism.5 By contrast, the thermolysis of (RN4)PdIV-
MeCl complexes leads to selective formation of MeCl: 12+ gen-
erates 22% MeCl in 4 h at 70 °C, while 22+ undergoes reductive
elimination even at RT to yield 52% MeCl (Scheme 3).16 The
observed reactivity is not affected by TEMPO, indicative of a
non-radical mechanism. Moreover, addition of 2 equiv. Cl− to
12+ leads to quantitative formation of MeCl and (MeN4)PdIICl2
in 1 h at RT, suggesting an SN2 reductive elimination mechan-
ism.17 The observed higher yield of MeCl for 22+ vs. 12+ is
likely due to the greater steric bulk of the N-iPr vs. N-Me groups
that facilitates formation of a five-coordinate intermediate via
either chloride or axial amine dissociation.18 Overall, these reac-
tivity studies suggest that preferential formation of PdIV vs. PdIII
species during oxidatively-induced reactions leads to a selective
C–Cl bond formation, likely due to the increased electrophilicity
of PdIV vs. PdIII centers in SN2 reductive elimination.
Fig. 5 Space filling models of the cations of [32+](PF6)2 and [42+
(PF6)2.
]
space filling models of 32+ (Fig. 5). By contrast, the N-iPr
groups of iPrN4 in 42+ cause a distortion of the macrocyclic
ligand and push the Me ligands out of the equatorial plane to
accommodate the shorter Pd–Naxial distances of the PdIV center
(Fig. 5 and S36–S37†).9 These structural properties also provide
strong support for the observed trend of decreasing PdIII/IV oxi-
dation potentials from tBuN4 to iPrN4 to MeN4 (Fig. 1).13 Interest-
ingly, 12+–42+ are uncommon dicationic organometallic
complexes and are expected to exhibit increased electrophilic
reactivity.14,15
Similar comparative reactivity studies were performed for the
[(RN4)PdIIIMe2]+ and [(RN4)PdIVMe2]2+ species. Photolysis of
the PdIII complexes 3+ and 4+ produces ethane in ∼30% yield
(Scheme 4).16 Interestingly, the formation of ethane is only par-
tially suppressed in the presence of TEMPO, suggesting the
potential involvement of both radical and non-radical mechan-
isms (vide infra). By comparison, while almost no ethane is
formed upon thermolysis of 32+ at 70 °C, 42+ generates up to
54% ethane via a non-radical mechanism (Scheme 5).9 This dra-
matic reactivity difference may be due to the symmetric geome-
try of 32+ that does not favor formation of a 5-coordinate
intermediate, whereas the bulkier N-iPr groups lead to a geo-
metric distortion in 42+ that promotes the formation of a 5-coor-
dinate intermediate necessary for ethane elimination.18,20 In
addition, photolysis of both 32+ and 42+ generates ethane in
∼50% yield, both in the absence and presence of TEMPO,
suggesting a non-radical mechanism of ethane formation. There-
fore, we propose that a photo-induced dissociation of an axial N
Reactivity comparison of PdIII and PdIV complexes
The involvement of PdIV and/or PdIII intermediates in various
catalytic reactions has recently been demonstrated or proposed.2
In this regard, our ability to isolate PdIII and PdIV complexes in
an identical ligand environment provides a unique opportunity to
systematically probe their reactivity in organometallic reactions.
The (RN4)PdIIIMeCl complexes 1+ and 2+ are stable in the solid
state or in solution in the dark, yet they undergo unselective and
low-yielding C–C and C–Cl bond formation reactions in the
donor generates
a
5-coordinate intermediate [(κ3-RN4)
14048 | Dalton Trans., 2012, 41, 14046–14050
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Scheme 4 Reactivity of [(RN4)PdIIIMe2]+ complexes.19
Scheme 7 Proposed mechanisms for the photo-induced ethane elimin-
ation from 3+ (and 4+).
Scheme 8 Proposed mechanism for the photo-induced ethane elimin-
ation from 32+ (and 42+).
Scheme 5 Reactivity of [(RN4)PdIVMe2]2+ complexes.19
mechanism involving a homolytic cleavage of a PdIII–Me bond
to form a Me radical that can subsequently bind to the PdIII
center of another molecule of 3+ (Scheme 7, path A).5 Alterna-
tively, a non-radical dissociation of an axial PdIII–Naxial bond
upon irradiation can form a transient intermediate [(κ3-MeN4)
PdIIIMe2]+, which can undergo Me group transfer via dispropor-
tionation with another molecule of 3+ to yield 5+ (Scheme 7,
path B).6,22 The partial suppression of ethane formation by
TEMPO suggests that both mechanisms may be operative during
photolysis and can generate species 5+, which is responsible for
ethane formation.22
Scheme 6 Crossover experiments for the photolysis of 3+ and 32+
.
PdIVMe2]2+, which is likely to undergo facile reductive elimin-
ation (vide infra).18,20
Interestingly, crossover studies for the PdIV complexes yield
different results. Photolysis of a 1 : 1 mixture of [(MeN4)-
PdIV(CH3)2](PF6)2, [32+](PF6)2, and [(MeN4)PdIV(CD3)2](PF6)2,
[32+-d6](PF6)2, yields 23% of CH3CH3, and no crossover
product CH3CD3 is formed (Scheme 6).9 Since the typical yield
of ethane upon photolysis of 32+ is ∼46%, the result suggests a
1 : 0 : 1 ratio of CH3CH3, CH3CD3, and CD3CD3. The observed
ratio is consistent with a non-radical mechanism involving a
photo-induced dissociation of an axial N donor to give the
5-coordinate intermediate, followed by rapid concerted reductive
elimination of ethane (Scheme 8).18,20 Overall, these reactivity
studies suggest that, by contrast to PdIII analogs, the PdIV com-
plexes undergo selective C–C and C–heteroatom bond formation
that most likely involve non-radical mechanisms.
Crossover experiments were performed to gain insight into the
possible mechanisms of ethane formation in the photolysis of
[(RN4)PdIII(Me)2]+ and [(RN4)PdIV(Me)2]2+ complexes. Photo-
lysis of a 1 : 1 mixture of [(MeN4)PdIII(CH3)2]ClO4, [3+]ClO4, and
[(MeN4)PdIII(CD3)2]+, [3+-d6]ClO4, leads to formation of both
CH3CH3 and CH3CD3 in 10% yield. Given the typical yield of
∼30% ethane upon photolysis of [3+]ClO4 under the same con-
ditions, the result suggests a CH3CH3 : CH3CD3 : CD3CD3 ratio
of 1 : 1 : 1 (Scheme 6). The observed ratio of ethane isotopologs
is consistent with the formation of a [(κ3-MeN4)PdIVMe3]+ inter-
mediate (5+) that undergoes a fast rearrangement of the Me
groups and thus can reductively eliminate any two of three Me
groups.6 In addition, the 1 : 1 : 1 ratio does not support a radical
mechanism involving the coupling of two Me radicals or an
SH2-like mechanism involving a Me radical attacking the Me
group of another PdIII–Me molecule, as these mechanisms
would lead to a 1 : 2 : 1 statistical ratio of ethane isotopologs.21
The proposed intermediate 5+ can be generated through a radical
Conclusion
In summary, we have successfully isolated and characterized
analogous organometallic PdIII and PdIV complexes stabilized by
This journal is © The Royal Society of Chemistry 2012
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the tetradentate ligands MeN4 and iPrN4. The unprecedented iso-
lation of both PdIII and PdIV complexes with an identical ligand
environment allowed a direct structural and organometallic reac-
tivity comparison. The d7 PdIII centers prefer a distorted octa-
hedral geometry, while the d6 PdIV centers adopt a more
compact octahedral geometry. In addition, reactivity studies
show that PdIII centers lead to photo-induced radical formation
and unselective C–C and C–Cl bond formation, although transi-
ent PdIV intermediates can also be involved. By comparison,
PdIV centers undergo selective C–C or C–Cl bond formation
through non-radical reductive elimination mechanisms. Overall,
these studies suggest that both PdIII and PdIV species could act
as intermediates in various oxidatively-induced C–C and C–hetero-
atom bond formation reactions, while PdIV centers tend to
exhibit a more selective and higher yielding reductive elimin-
ation reactivity. Our current research efforts aim to provide
insight into the involvement of PdIII and/or PdIV species in stoi-
chiometric and catalytic organic transformations employing
high-valent Pd intermediates.
6 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, J. Am. Chem. Soc.,
2012, 134, 2414.
7 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, Angew. Chem., Int.
Ed., 2011, 50, 5532.
8 (a) F. Bottino, M. Di Grazia, P. Finocchiaro, F. R. Fronczek, A. Mamo
and S. Pappalardo, J. Org. Chem., 1988, 53, 3521; (b) C. M. Che,
Z. Y. Li, K. Y. Wong, C. K. Poon, T. C. W. Mak and S. M. Peng, Polyhe-
dron, 1994, 13, 771.
9 See ESI.†
10 L. M. Mirica and J. R. Khustnutdinova, Coord. Chem. Rev., 2012, 256,
DOI: 10.1016/j.ccr.2012.04.030.
11 (a) A. J. Blake, A. J. Holder, T. I. Hyde and M. Schröder, J. Chem. Soc.,
Chem. Commun., 1987, 987; (b) A. J. Blake, L. M. Gordon, A. J. Holder,
T. I. Hyde, G. Reid and M. Schröder, J. Chem. Soc., Chem. Commun.,
1988, 1452; (c) A. R. Dick, J. W. Kampf and M. S. Sanford, J. Am.
Chem. Soc., 2005, 127, 12790.
12 The detailed characterization of the observed spectroscopic trends for
complexes 1+–4+ is ongoing and will be reported elsewhere.
13 The PdIV species [(tBuN4)PdIVMeCl]2+ can be generated electrochemi-
cally at low temperatures, yet it is unstable at RT (ref. 6).
14 Only one other dicationic organometallic complex has been reported to
date: W. Oloo, P. Y. Zavalij, J. Zhang, E. Khaskin and A. N. Vedernikov,
J. Am. Chem. Soc., 2010, 132, 14400.
15 While a few monoaryl PdIV complexes have been isolated to date
(T. Furuya and T. Ritter, J. Am. Chem. Soc., 2008, 130, 10060; N. D. Ball
and M. S. Sanford, J. Am. Chem. Soc., 2009, 131, 3796; P. L. Arnold,
M. S. Sanford and S. M. Pearson, J. Am. Chem. Soc., 2009, 131, 13912;
Ref. 14), only one other monoalkyl PdIV complex has been reported:
D. Shabashov and O. Daugulis, J. Am. Chem. Soc., 2010, 132, 3965.
16 Thermolysis in the dark of PdIII complexes leads to unspecific decompo-
sition and formation of methane. In addition, photolysis or thermolysis of
PdII precursors leads to formation of methane and Pd black, along with
small amounts of ethane or MeCl.
Notes and references
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Metal Chemistry: From Bonding to Catalysis, University Science Books,
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17 (a) S. S. Stahl, J. A. Labinger and J. E. Bercaw, Angew. Chem., Int. Ed.,
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3 D. C. Powers, E. Lee, A. Ariafard, M. S. Sanford, B. F. Yates,
A. J. Canty and T. Ritter, J. Am. Chem. Soc., 2012, 134, 12002.
4 (a) W.-Y. Yu, W. N. Sit, K.-M. Lai, Z. Zhou and A. S. C. Chan, J. Am.
Chem. Soc., 2008, 130, 3304; (b) T. S. Mei, X. S. Wang and J. Q. Yu,
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5 J. R. Khusnutdinova, N. P. Rath and L. M. Mirica, J. Am. Chem. Soc.,
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18 (a) J. M. Racowski, A. R. Dick and M. S. Sanford, J. Am. Chem. Soc.,
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met. Chem., 2011, 35, 61.
19 During the reactivity studies for RN4PdIII/PdIV complexes, the
RN4PdIIMe(solv) or RN4PdII(solv)2 products are unstable and lead to for-
mation of methane and other PdII products (see ESI†).
20 (a) J. Procelewska, A. Zahl, G. Liehr, R. van Eldik, N. A. Smythe,
B. S. Williams and K. I. Goldberg, Inorg. Chem., 2005, 44, 7732;
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