Tetracoordinate diamidato-bis(phosphanyl) metal systems: Synthesis, characterization, and electrochemical analysis of palladium(II) complexes

Article abstract of DOI:10.1016/j.ica.2006.12.011

Square planar diamidato-bis(phosphanyl) palladium(II) complexes have been prepared and characterized. Most products precipitate out of THF solution on reaction of the parent ligand with Pd(OAc)₂ in the presence of 2 equiv. of base at 50 °C over

Full text of DOI:10.1016/j.ica.2006.12.011

Inorganica Chimica Acta 360 (2007) 2455–2463
www.elsevier.com/locate/ica
Tetracoordinate diamidato-bis(phosphanyl) metal systems:
Synthesis, characterization, and electrochemical analysis
of palladium(II) complexes
a
b
c
a,
*
Rebecca A. Swanson , Brian O. Patrick , Michael J. Ferguson , Christopher J.A. Daley
a
Department of Chemistry, Western Washington University, Bellingham, WA 98225-9150, United States
b
Department of Chemistry, University of British Columbia, Vancouver, BC, Canada V6T 1Z3
c
Department of Chemistry, University of Alberta, Edmonton, Alta., Canada T6G 2G2
Received 1 December 2006; accepted 6 December 2006
Available online 15 December 2006
Abstract
Square planar diamidato-bis(phosphanyl) palladium(II) complexes have been prepared and characterized. Most products precipitate
out of THF solution on reaction of the parent ligand with Pd(OAc)2 in the presence of 2 equiv. of base at 50 ꢀC overnight. The solution
and solid state structure of each complex is reported based on multinuclear NMR and X-ray analyses, respectively. The effects of car-
boxamido nitrogen coordination on the stabilization of the metal center from reduction were studied using cyclic voltammetry. The irre-
versible peak reduction potential of each complex was greater by approximately ꢀ340 mV to ꢀ590 mV as compared to Pd(PPh3)2Cl2
indicating that carboxamido nitrogen coordination protects the Pd(II) center from reduction.
ꢁ 2006 Elsevier B.V. All rights reserved.
Keywords: Trost ligand; Palladium complexes; Amidato-metal bonding; Protection from reduction; X-ray crystal structures
1. Introduction
the A-cluster is the active site responsible for interconver-
sion of C1 carbon species such as CO2 and CO via redox
reactions [6,9]. The exact role of the distal nickel center
in the A-cluster remains to be elucidated, however, it is
noted that its coordination sphere is similar to that found
in NHase being bonded through a peptide backbone
Metal-amidato complexes have attracted much atten-
tion over the last couple of decades with studies in areas
such as structure and reactivity [1], catalyst development
[2], medicinal chemistry [3], and bioinorganic chemistry
[4]. Of note, in nature there are a few fascinating systems
(Fig. 1) where such coordination occurs including: the P-
cluster of nitrogenase [5], the A-cluster of CODH/acetyl
Co-A synthase (CODH) [6], and the active site of nitrile
hydratase (NHase) [7]. The P-cluster is indispensable for
nitrogenase activity as it is believed to participate in the
transfer of electrons required to reduce dinitrogen [5,8].
In its oxidized form, the P-cluster is bonded to the enzyme
through multiple Cys–S–Fe bonds as well as with an ami-
dato bond from one of the Cys. In the CODH enzyme,
Cys–Gly–Cys unit as
a diamidato-dithiolate center.
NHases are mononuclear non-heme iron(III) or non-corrin
cobalt(III) containing enzymes that hydrolyze nitriles to
their corresponding amides. In NHase, the mononuclear
metal center is bonded in a tetradentate plane via two car-
boxamido nitrogens and the Cys thiolates from a Cys–Ser–
Cys unit where the thiolates have been post-translationally
modified to sulfinic and sulfenic units. A third unmodified
Cys thiolate binds the metal center below the plane. The
stable high oxidation state in NHase is a rare case as tran-
sition metal-thiolate species (notably Fe(III)–S systems) are
prone to undergo autoreduction [10]; the metal(III) center
in NHase shows no redox activity but behaves solely as a
*
Corresponding author. Tel.: +1 360 650 3167; fax: +1 360 650 2826.
E-mail address: daley@chem.wwu.edu (C.J.A. Daley).
0020-1693/$ - see front matter ꢁ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2006.12.011

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R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
Fig. 1. Structures of complexes containing metal-amidato bonding found in nature. The P-cluster of nitrogenase in the oxidized form (left), the A-cluster
of CODH (M = metal; center), and the active site of NHase (right).
Lewis acid [7]. It is the carboxamido nitrogen bonding that
has been proposed to protect the metal center from reduc-
tion, and thus redox activity, as compared to similar non-
coordinated carboxamido nitrogen species [11]. Based on
the intriguing examples above, along with the numerous
other complexes reported, continued investigation into
the properties of metal systems that contain amidato bond-
ing is of great interest.
The Trost ligand (R,R)-1 (Fig. 2) contains carboxamido
nitrogens that could coordinate in a manner similar to
that of the distal nickel center in CODH and the metal
active site in NHase where the thiolate units are substi-
tuted for phosphane units. As such, we are interested in
examining the properties of metal complexes bound by
(R,R)-1 in a tetracoordinate diamidato-bis(phosphanyl)
form. We began with the palladium complexes, which
are also of interest to the catalytic community as Trost-
based palladium systems are well known as highly efficient
catalysts for reactions such as enantioselective allylic alky-
lations. The active catalytic species are believed to be
1-Pd(0) and 1-Pd(II)(R)(X) type complexes with the tetra-
coordinate (R,R)-1-Pd deemed catalytically inactive [12].
their electrochemistry, and compare them to other similar
palladium complexes.
2. Experimental
2.1. General
All reactions were performed using standard Schlenk
techniques or in a N2-filled MBraun glovebox. Solvents were
purified using an Innovative Technologies Solvent Purifica-
tion System and were further deoxygenated with N2-bub-
bling prior to use. 2-(Diphenylphosphino)benzoic acid
(97%; Alfa Aesar), (1R,2R)-(+)-1,2-diaminocyclohexane-
N,N0-bis(20-diphenylphosphinobenzoyl) ((R,R)-1; 98%,
STREM), and (1R,2R)-(+)-1,2-diaminocyclohexane-N,
N0-bis(20-diphenylphosphino-1-naphthoyl) ((R,R)-2; 94%,
STREM) were purchased from suppliers and used without
further purification unless stated otherwise. NMR data were
obtained on a Varian INOVA 500 MHz spectrometer. The
chemical shifts for proton and carbon were referenced to
tetramethylsilane (TMS) at 0.00 ppm and the chemical shifts
for phosphorus were referenced to 85% H3PO4 at 0.00 ppm.
X-ray analyses were performed by Dr. Patrick at the Univer-
sity of British Columbia or by Dr. Ferguson at the University
of Alberta.
In contrast, the Suss-Fink group has recently reported
¨
that the achiral complex 4-Pd is likely the catalytically
active species in Suzuki cross-coupling reactions [13].
Therefore, the complete characterization of diamidato-
bis(phosphanyl) palladium complexes is of broad scientific
interest. Herein we report on the direct synthesis of (R,R)-
1-Pd and its analogues (R,R)-2-Pd and ( )-3-Pd, their
structural characterization in solution and the solid state,
2.2. Synthesis of ( )-3
The synthesis was modified slightly from that reported
by Trost [14]. To a 100 mL side-arm flask were added 2-
(diphenylphosphino)benzoic acid (2.0004 g, 6.5310 mmol),
dicyclohexylcarbodiimide (1.4820 g, 7.1841 mmol), N,
N-dimethyl-4-aminopyridine (DMAP; 0.0208 g, 0.1703
mmol), and CH2Cl2 (40 mL) to dissolve the solids. The
solution was stirred for 35 min then ( )-1,2-diphenylethy-
lenediamine (0.6942 g, 3.266 mmol) was added as a solid
under a strong flow of N2. The reaction flask was rinsed
with CH2Cl2 (4 mL) via cannula to ensure all the diamine
was in solution. The chalky yellow mixture was stirred
for a further 20 h under N2. The solution was filtered
through a Celite plug (1.5 cm · 6 cm) into a 100 mL round
bottom flask. The Celite was washed with CH2Cl2 (2 · 7
mL) until the eluent was colorless. The solvent was
removed under vacuum yielding the crude product as a
yellow solid. The solid was purified by column chromatog-
raphy (silica column 4 cm · 11 cm) using 1:3 diethyl
ether:hexanes (200 mL) followed by 30% ethyl acetate:hex-
Fig. 2. Ligands with the potential to form tetradentate diamidato-
bis(phosphanyl) metal complexes.

R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
2457
anes (800 mL), and 50% ethyl acetate:hexanes (400 mL).
After removal of solvent on a rotovap, 3 was collected as
a yellow solid that was further recrystallized from
CH2Cl2:hexanes in a freezer. The off-white solid was col-
lected by filtration in 54.43% yield (1.402 g, 1.777 mmol).
1H NMR analysis was identical to that reported [7] and
( )-3 was used without further purification.
(0.0163 g, 0.408 mmol; 54% w/w in oil), and Pd(OAc)2
(0.0393 g, 0.175 mmol). Compound ( )-3-Pd was obtained
1
in 92.7% yield (0.145 g, 0.162 mmol). H NMR (CDCl3,
500 MHz) d 6.42 (dd, 2H, Hi), 6.57 (d, 2H, Ha), 6.65 (dd,
4H, ortho-H), 6.81 (t, 4H, Hm), 7.12 (m, 8H, para-
H + meta-Hb + para-Hb), 7.18 (t, 2H, Hh), 7.32 (t, 4H,
meta-H0), 7.37 (m, 4H, ortho-Hb), 7.50 (m, 4H,
Hg + para-H0), 7.58 (dd, 4H, ortho-H0), 8.69 (dd, 2H, Hf).
13C{1H} NMR (CDCl3, 126 MHz) d 125.46 (m), 125.55,
127.30, 127.79, 128.50, 128.83, 130.18, 131.60, 131.75,
132.02, 132.28, 133.03, 135.91, 142.93, 143.27, 165.43.
31P{1H} NMR (CDCl3, 202 MHz) d 26.49 (s).
2.3. Synthesis of (R,R)-1-Pd
To a 50 mL side-arm flask was added NaH (0.0195 g,
0.478 mmol; 54% w/w in oil) with a stirbar and THF
(10 mL). To this was added a THF (10 mL) solution of 1
(0.150 g, 0.217 mmol) and Pd(OAc)2 (0.049 g, 0.218 mmol)
via cannula. The yellow solution was heated to 50 ꢀC for
17 h. After this time a yellow solid precipitated out of solu-
tion. The solution was filtered to yield (R,R)-1-Pd (0.146 g,
0.184 mmol) in 84.6% yield. 1H NMR (CDCl3, 500 MHz) d
1.25 (m, 2H, He), 1.45 (m, 2H, Hb), 1.60 (m, 2H, Hd), 2.49
(br ‘d’, 2H, Hc), 3.70 (m, 2H, Ha), 6.72 (td, 2H, Hi), 7.11
(m, 4H, ortho-H), 7.13–7.23 (m, 14H, Hh + ortho-
H0 + meta-H,H0), 7.38 (t, 2H, para-H0), 7.42 (t, 2H, para-
H), 7.51 (t, 2H, Hg), 8.51 (dd, 2H, Hf). 13C{1H} NMR
(CDCl3, 126 MHz) d 26.27, 31.89, 67.91, 123.86, 124.27,
128.7 (m), 129.16, 129.59, 130.91, 130.95, 131.45, 131.74,
131.98 (m), 133.07 (m), 133.51 (m), 145.37 (m), 167.47.
31P{1H} NMR (CDCl3, 202 MHz) d 25.66 (s).
2.6. X-ray crystallography
The crystal data for (R,R)-1-Pd, (R,R)-2-Pd, and ( )-3-
Pd are collected in Table 1. Crystals of (R,R)-1-Pd, (R,R)-
2-Pd, and ( )-3-Pd suitable for X-ray structure determina-
tion were obtained by vapor diffusion of diethyl ether into
a concentrated solution of the compound of interest in
CH2Cl2.
Data for (R,R)-1-Pd and (R,R)-2-Pd were collected at
the University of British Columbia on a Bruker X8 APEX
CCD diffractometer utilizing graphite monochromated
MoKa radiation at ꢀ100 ꢀC. Unit cell parameters were
obtained from a least-squares refinement of the setting
angles of 6793 reflections ((R,R)-1-Pd) or 7498 reflections
((R,R)-2-Pd) from the data collection. The space group
was determined to be P212121 ([No. 19]) for both (R,R)-
1-Pd and (R,R)-2-Pd. The data were corrected for absorp-
tion effects using the multi-scan technique (SADABS) [15]. See
Table 1 for a summary of the crystal data and X-ray data
collection information. The structures were solved by direct
methods [16] and refined using full-matrix least squares [17]
of F2. All refinements were performed using the SHELXTL
crystallographic software package [18]. The non-H atoms
were treated anisotropically. Hydrogen atoms were
assigned positions on the basis of the geometries of their
attached carbon atoms and were given thermal parameters
20% greater than those of the attached carbons. For (R,R)-
2.4. Synthesis of (R,R)-2-Pd
In a 50 mL side-arm flask containing a stirbar, NaH
(0.0130 g, 0.325 mmol; 54% w/w in oil) and (R,R)-2
(0.112 g, 0.142 mmol) were dissolved in THF (10 mL). To
this was added a THF (5 mL) solution of Pd(OAc)2
(0.0318 g, 0.142 mmol) via cannula from a 25 mL side-arm
flask. The stirred yellow solution was heated to 50 ꢀC for
24 h. Note: no solid precipitates out of solution. The solvent
was removed under vacuum to yield (R,R)-2-Pd (0.113 g,
0.126 mmol) in 89.1% yield. 1H NMR analysis of the prod-
uct showed no visible impurities but the product was crystal-
lized from CH2Cl2:Et2O prior to use in cyclic voltammetry
1-Pd, the final model refined to values of R1(F) = 0.027 (for
P
1
studies. H NMR (CDCl3, 500 MHz) d 1.14 (m, 2H, Hb),
7936 data with F o2 P 2 ðF o2ÞÞ and wR2(F2) = 0.0660 (for
1.52 (m, 2H, He), 1.75 (m, 2H, Hd), 3.25 (br ‘d’, 2H, Hc),
3.94 (m, 2H, Ha), 6.49 (t, 2H, Hi), 7.04–7.14 (m, 8H,
ortho-H0 + meta-H0), 7.22–7.28 (m, 4H, meta-H), 7.35 (t,
2H, para-H), 7.40 (t, 2H, para-H0), 7.46–7.58 (m, 10H,
H2 + H3 + Hh + ortho-H), 7.71 (d, 2H, H4), 8.59 (d, 2H,
H1). 13C{1H} NMR (CDCl3, 126 MHz) d 25.86, 31.43,
68.82, 123.42 (m), 125.60, 127.28, 127.33, 127.60, 127.98,
128.18, 128.42 (m), 128.60–128.88 (m), 130.75, 131.51 (m),
131.69, 133.26 (m), 134.91 (m), 135.19, 147.97 (m), 176.74
(m). 31P{1H} NMR (CDCl3, 202 MHz) d 23.66 (s).
all 8535 independent data). For (R,R)-2-Pd, the final model
refined to values of R1(F) = 0.043 (for 6982 data with
P
F 2o P 2 ðF o2ÞÞ and wR2(F2) = 0.097 (for all 7745 indepen-
dent data).
Data for ( )-3-Pd was collected at the University of
Alberta on a Bruker Platform/SMART 1000 CCD diffrac-
tometer using MoKa radiation at ꢀ80 ꢀC. Unit cell param-
eters were obtained from a least-squares refinement of the
setting angles of 8136 reflections from the data collection.
The space group was determined to be P21/n (an alternate
setting of P21/c [No. 14]). The data were corrected for
absorption through use of the program SADABS [19]. See
Table 1 for a summary of crystal data and X-ray data col-
lection information. The structures were solved using direct
methods (SHELXS-86) [20], and refinement was completed
2.5. Synthesis of ( )-3-Pd
Compound ( )-3-Pd was synthesized in a similar man-
ner to 1-Pd using ( )-3 (0.1382 g, 0.1752 mmol), NaH

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R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
Table 1
Selected crystallographic data for (R,R)-1-Pd, (R,R)-2-Pd, and ( )-3-Pda
(R,R)-1-Pd
(R,R)-2-Pd
( )-3-Pd
Formula
C
795.10
9.7744 (3)
17.5108 (6)
21.0956 (7)
90.0
90.0
90.0
3610.7 (2)
4
44H38N2O2P2Pd
C52H42N2O2P2Pd Æ CH2Cl2
980.14
11.9129 (4)
13.2944 (4)
28.6030 (8)
90.00
90.00
90.00
4530.0(2)
4
C52H40N2O2P2Pd Æ CH2Cl2
978.13
Formula weight (g/mol)
˚
a (A)
11.0579 (7)
19.6816 (12)
20.7039 (12)
90.0
95.747 (1)
90.0
˚
b (A)
˚
c (A)
a (ꢀ)
b (ꢀ)
c (ꢀ)
3
˚
V (A )
4483.3 (5)
4
Z
Space group
Crystal system
qcalc (g/cm3)
T (K)
P212121 (#19)
orthorhombic
1.463
173
0.644
37726
3.02–55.60
8535
P212121 (#19)
orthorhombic
1.437
173
0.643
24207
3.38–50.00
7745
P21/n (#14)
monoclinic
1.449
193
0.649
34152
2.86–52.76
9171
l (MoKa) (mmꢀ1
Total reflections
2h Range (ꢀ)
)
Unique reflections
Parameters
460
559
559
Flack v
ꢀ0.04 (1)
0.027, 0.066
1.07
ꢀ0.02 (3)
0.043, 0.097
1.05
c
R1b, wR2
0.0299, 0.0855
1.051
Sb
a
See CCDC 629352, 629353, and 629354 deposition files for more information.
P
P
P
P
2
2
R1 = iFoj ꢀ jFci/ jFoj, I P 2r(I); wR2 ¼ f ½wðF 2o ꢀ F c2Þ ꢁ= ½wðF o2Þ ꢁg1=2, all data.
b
c
P
2
1=2
S ¼ f ½wðF 2o ꢀ F 2c Þ ꢁ=ðn ꢀ pÞg
.
using the program SHELXL-93 [21]. Hydrogen atoms were
assigned positions on the basis of the geometries of their
attached carbon atoms and were given thermal parameters
20% greater than those of the attached carbons. For ( )-3-
dium are of interest as the similar ligand 4 (Fig. 2) devel-
oped by Burger et al. was found to only form the
bis(phosphanyl) palladium(II) complex (4-PdCl2) [22]
where the phosphanes coordinate trans to each other and
the carboxamido nitrogens do not participate in the bond-
ing (Scheme 1) unless high heat and a stronger base was
used, in which case the 4-Pd complex can be formed [13].
Even on reaction of 4-PdCl2 with excess NEt3 as base, for-
mation of the corresponding diamidato-bis(phosphanyl)
palladium(II) complex (4-Pd) was not observed at room
temperature [22]. In contrast, 1 has been shown to form
a cis coordinate bis(phosphanyl) palladium(0) complex
[12], believed to be the active coordination mode in numer-
ous enantioselective catalyzed reactions, as well as the
diamidato-bis(phosphanyl) palladium(II) complex 1-Pd
[12,23] that is reported to be catalytically inactive for the
aforementioned reactions. The first report of the formation
of 1-Pd was on opening a solution of the bis(phosphanyl)
Pd, the final model refined to values of R1(F) = 0.0299 (for
P
8211 data with F o2 P 2 ðF o2ÞÞ and wR2(F2) = 0.0855 (for
all 9171 independent data).
2.7. Electrochemistry
Measurements were carried out at 20 2 ꢀC using a
standard three-electrode arrangement with a platinum wire
as the counter electrode and a Ag/AgCl double-junction
reference electrode in CH2Cl2 containing 0.1 M
(Bu4N)(PF6) supporting electrolyte. All potentials are
quoted with respect to this electrode. Cyclic voltammo-
grams (500 mV/s) were recorded with a BAS CV-50 W
(BioAnalytical System Instruments) system using a glassy
carbon working electrode and 0.15–0.25 mM solutions of
the sample in CH2Cl2 electrolyte solution. Oxygen was
removed from each sample by purging the solutions with
high-purity nitrogen.
3. Results and discussion
3.1. Synthesis and characterization
Palladium complexes of the Trost ligand 1 are of interest
as they may be, or may model, active catalytic species in
enantioselective palladium-catalyzed reactions such as
allylic alkylations. The coordination modes of 1 with palla-
Scheme 1.

R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
2459
palladium(0) complex to air [12]. More recently, Campos
et al. obtained 1-Pd as a byproduct on workup of a cata-
lytic asymmetric allylic alkylation reaction as confirmed
by its X-ray structure [23]. However, the isolation, yield,
and further characterization of 1-Pd were not reported in
either case. In order to investigate this complex further,
we set forth to develop a straightforward synthesis of
(R,R)-1-Pd along with its analogues (R,R)-2-Pd and ( )-
3-Pd.
We successfully synthesized (R,R)-1-Pd on reaction of
(R,R)-1 with Pd(OAc)2 in THF at 50 ꢀC for 17 h. The prod-
uct (R,R)-1-Pd precipitates out of solution and can be iso-
lated by filtration in high yield (Scheme 1: 84.6%). It is
noted that unlike the reactivity of 4, reaction of (R,R)-1
with Pd(OAc)2 in CH2Cl2 at room temperature did form
some of the desired (R,R)-1-Pd when no base was added,
however the reaction was not clean as several other uniden-
tified complexes were also present. Furthermore, reaction
of (R,R)-1 with Pd(OAc)2 in THF at room temperature
for 24 h in the presence of base did precipitate (R,R)-1-
Pd but in only a modest quantity (28.7% yield). The com-
plex (R,R)-1-Pd was formed cleanly and in the best yield
when base was added and the mixture was heated.
A possibility for the lack of the amidato bonding of 4 to
the Pd(II) center at room temperature may be the nature of
the carboxamido nitrogens themselves. The nitrogens of 1–
3 are bonded to aliphatic carbons whereas they are bonded
to an aromatic ring in 4. While the lack of carboxamido
nitrogen coordination may be a result of the less nucleo-
philic nature of the carboxamido nitrogens in 4, the fact
that the analogous diamidato bis(phosphanyl) platinum(II)
complex (4-Pt) [23] is formed in the absence of base sug-
gests that the nucleophilic nature of the nitrogens is not
the primary factor to coordination. As noted, while
(R,R)-1-Pd was formed without heat in the presence of
base and even in the absence of base, the most efficient syn-
thesis of (R,R)-1-Pd required heating the solution in base.
The byproduct (R,R)-1-Pd reported by Campos [23] was
formed after prolonged heating of a basic ethanol solution
of THF and further purified by crystallization from
CH2Cl2:Et2O (89.1% yield).
3.2. Solution structure
Complex (R,R)-1-Pd was characterized in solution by
multinuclear NMR analyses. Two immediate features
1
noted in the H NMR spectrum (Fig. 3) were the disap-
pearance of the amide protons, strongly indicating coordi-
nation of the carboxamido nitrogens, and a signature peak
that appeared at 8.51 ppm. The latter peak is the only sig-
nal observed down-field of 8 ppm and was an indication of
successful coordination with all the complexes. In solution,
1
(R,R)-1-Pd has C2-symmetry based on the number of H
NMR signals and integration and is further confirmed by
the single peak observed in the 31P{1H} NMR spectrum
1
at 25.66 ppm (singlet). All H NMR signals were assigned
based on multidimensional NMR analyses including
HMQC, HMBC, COSY, NOESY, and ROESY. Protons
Ha were readily identified (Fig. 3) at 3.70 ppm as they are
the only protons expected in this region. The 1H NMR sig-
nal at 6.72 ppm was assigned as Hi based on integration of
the signal and the loss of coupling observed in the 1H{31P}
NMR spectrum. The remaining cyclohexyl backbone pro-
tons Hb–Hd and the remaining aromatic backbone protons
Hf–Hj were then assigned via analysis of the COSY,
HMBC, and NOESY spectra with respect to the Ha and
Hi signals, respectively. The signature peak at 8.51 ppm
corresponds to Hf, which was confirmed from its cross-cou-
pling to the carbonyl carbon in the HMBC spectrum.
Assignment of the auxiliary phenyl protons on the phos-
phorus arms was then completed [24].
Similarly to (R,R)-1-Pd, complexes (R,R)-2-Pd and ( )-
3-Pd were characterized in solution by multinuclear NMR
analysis; both display C2-symmetry in solution based on
the number and integration of the 1H NMR signals
(Fig. 3) as well as the single 31P{1H} NMR signals
observed for each complex ((R,R)-2-Pd, 23.66 ppm; ( )-
1
3-Pd, 26.49 ppm). The H NMR spectra lacked the N–H
and the achiral analogue 4-Pd reported by the Suss-Fink
proton signals indicating coordination of the carboxamido
nitrogens, and each complex had the characteristic H sig-
¨
1
[13] group was only isolated on heating a toluene solution
in the presence of K2CO3. These reports further support
our theory that while the nucleophilic nature of the carbox-
amido nitrogens may be a factor, heat is the key compo-
nent to carboxamido nitrogen coordination to the Pd(II)
center. While Trost reported the clean formation of 1-Pd
at room temperature, the reaction occurred on exposure
of the bis(phosphanyl) coordinated Pd(0) complex of 1 to
air. The resultant reaction pathway involves oxidation of
Pd(0) to Pd(II) that may provide a lower barrier to the for-
mation of 1-Pd. The palladium complexes of (R,R)-2 and
( )-3 were also synthesized to further examine the proper-
ties of these structurally related ligands. Complex ( )-3-Pd
precipitated out of the THF solution overnight (92.7%
yield) like that of (R,R)-1-Pd however (R,R)-2-Pd remained
in solution as the naphthyl backbone increased its solubil-
ity in THF. Complex (R,R)-2-Pd was isolated on removal
nal at ꢂ 8.5 ppm indicating tetradentate coordination sim-
ilar to that found with (R,R)-1-Pd. The latter signals were
identified as Hf for ( )-3-Pd from COSY, HMBC, and
NOESY analyses based on the signal Hi that was assigned
from integration and 1H{31P} NMR analyses. H1 for
(R,R)-2-Pd was assigned in a similar manner and had the
added identifier of a strong NOE to the cyclohexyl back-
bone protons Hc. The backbone phenyl group protons
(on the ethylenediamine fragment) of ( )-3-Pd were distin-
guished from those of the phosphane via a strong NOE
between Ha and ortho-Hb. Based on these data, the com-
plexes were identified as C2-symmetric diamidato-bis(phos-
phanyl) square planar complexes. Analysis of the solid
state structures of each complex was performed in order
to compare them to each other and with their solution
structures.

2460
R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
Fig. 3. 1H NMR spectra of (R,R)-1-Pd, (R,R)-2-Pd, and ( )-3-Pd in CDCl3. Only the (S,S)-3-Pd enantiomer is depicted for assignment purposes, but the
product is racemic. Proton assignments are as denoted except for the residual CHCl3 and water signals which are denoted with an ‘x’. The region between
4.2 and 6.2 ppm contains no peaks and has been omitted for clarity.
3.3. Solid state structure
and 2; Fig. 4). While the structure of (R,R)-1-Pd has been
reported, we report our structure here for completeness to
the series of complexes in this manuscript and because of
the differences noted from the original report. Of experi-
mental note, our structure was collected at ꢀ100 ꢀC and
The products were characterized in the solid state by X-
ray crystallography using crystals obtained via diethyl
ether vapor diffusion into a concentrated solution of
(R,R)-1-Pd, (R,R)-2-Pd, or ( )-3-Pd in CH2Cl2 (Tables 1
3
˚
the unit cell volume was determined to be 3610.7 A while
Table 2
Selected bond distances (A) and angles (ꢀ) for complexes (R,R)-1-Pd, (R,R)-2-Pd, ( )-3-Pd
˚
(R,R)-1-Pd
(R,R)-2-Pd
( )-3-Pd
Pd–N(1)
Pd–N(2)
Pd–P(1)
2.0674(18)
2.0687(16)
2.2525(5)
2.2549(6)
1.493(3)
Pd–N(1)
Pd–N(2)
Pd–P(1)
2.089(4)
2.073(4)
2.2793(14)
2.2532(12)
1.483(6)
1.474(6)
1.522(7)
Pd–N(1)
Pd–N(2)
Pd–P(1)
2.0523(17)
2.0560(17)
2.2480(5)
2.2389(5)
1.480(3)
Pd–P(2)
Pd–P(2)
Pd–P(2)
N(1)–C(1)
N(2)–C(6)
C(1)–C(6)
N(1)–C(x)
N(2)–C(y)
C(1)–C(6)
N(1)–C(1)
N(2)–C(2)
C(1)–C(2)
1.485(2)
1.527(3)
1.480(3)
1.524(3)
N(1)–Pd–N(2)
N(1)–Pd–P(1)
N(1)–Pd–P(2)
N(2)–Pd–P(1)
N(2)–Pd–P(2)
P(1)–Pd–P(2)
N(1)–C(1)–C(6)
N(2)–C(6)–C(1)
83.94(7)
90.86(5)
N(1)–Pd–N(2)
N(1)–Pd–P(1)
N(1)–Pd–P(2)
N(2)–Pd–P(1)
N(2)–Pd–P(2)
P(1)–Pd–P(2)
N(1)–C(1)–C(6)
N(2)–C(6)–C(1)
83.55(15)
88.21(11)
171.47(12)
169.59(12)
89.27(11)
98.39(5)
N(1)–Pd–N(2)
N(1)–Pd–P(1)
N(1)–Pd–P(2)
N(2)–Pd–P(1)
N(2)–Pd–P(2)
P(1)–Pd–P(2)
N(1)–C(1)–C(2)
N(2)–C(2)–C(1)
84.09(7)
87.47(5)
166.99(5)
174.70(5)
83.05(5)
102.14(2)
106.95(17)
108.92(17)
171.07(5)
171.35(5)
88.40(5)
100.17(2)
106.08(16)
107.06(17)
106.1(4)
110.6(4)

R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
2461
Fig. 4. ORTEP plot at 50% probability level of (R,R)-1-Pd, (R,R)-2-Pd, and (S,S)-3-Pd. Note that 3-Pd is racemic and the unit cell contains two molecules
of (S,S)-3-Pd and two of (R,R)-3-Pd but only one enantiomer is shown for clarity. The ancillary phenyl groups and hydrogen atoms (except those on the
carbon atoms adjacent to the amide nitrogens) are omitted for clarity. In the case of (R,R)-2-Pd and ( )-3-Pd a molecule of CH2Cl2 co-crystallized but it is
omitted for clarity.
the Campos structure [23] was collected at room tempera-
ture and the unit cell volume was determined to be
3600.13 A . This is an interesting structural observation
chemistry of the complexes was examined via cyclic vol-
tammetry to determine their peak reduction potentials
(Table 3). As a baseline complex Pd(PPh3)2Cl2 was chosen
as it contains two triarylphosphane ligands similar to
(R,R)-1-Pd, (R,R)-2-Pd, and ( )-3-Pd but has no metal-
amidato bonding [26]. Cyclic voltammograms of (R,R)-1,
(R,R)-2, and ( )-3 in the absence and in the presence of
base were run in order to determine if the ligands them-
selves (free or deprotonated) have any electrochemical fea-
tures in the range analyzed with the palladium complexes.
There were no peaks observed for any of them on sweeping
the range from 0 V to ꢀ2.2 V to 1.0 V and back to 0 V (ver-
sus Ag/AgCl) in CH2Cl2 solution containing 0.1 M
(Bu4N)(PF6) as electrolyte. Under the same conditions,
Pd(PPh3)2Cl2 has an irreversible peak reduction potential
at ꢀ1.25 V [27]. By comparison, each of the amidato com-
plexes has a higher peak reduction potential in the order of
ꢀ1.59 V ((R,R)-2-Pd), ꢀ1.76 V ((R,R)-1-Pd), and ꢀ1.84 V
(( )-3-Pd). Based on the silent voltammograms of the free
and deprotonated ligands, the reduction peaks observed
are due to the reduction of the metal center. It is noted that
while Pd(PPh3)2Cl2 showed a clear irreversible reduction
peak, complexes (R,R)-1-Pd, (R,R)-2-Pd, and ( )-3-Pd
had quasi-reversible reduction peaks. The latter quasi-
reversible assignment is based on the ꢀ50 mV to ꢀ66 mV
shift in the peak reduction potential of the complexes on
changing the scan rates from 100 mV/s to 600 mV/s. On
comparison of Pd(PPh3)2Cl2 to (R,R)-1-Pd, (R,R)-2-Pd,
and ( )-3-Pd, the results indicate that the amidato bonding
does protect the Pd(II) center from reduction. This stabil-
ization of the Pd(II) state may be contributing to the inac-
tivity of (R,R)-1-Pd (and (R,R)-2-Pd and ( )-3-Pd) as
3
˚
as normally the cell volume decreases with decreasing tem-
perature yet the opposite is observed. Another difference is
that while they are close, all the Pd–X (X = N or P) bond
lengths of our structure are longer (>3r) than those of the
Campos structure. Finally, our structure refined to a better
R-factor of 2.65% as compared to 4.69% reported by Cam-
pos [25]. The complexes are all square planar about the pal-
ladium as confirmed by the summation of the bond angles
between the coordinating atoms (P(1)–Pd–N(1) + N(1)–
Pd–N(2) + N(2)–Pd–P(2) + P(2)–Pd–P(1) = 360ꢀ ideally
for square planar; (R,R)-1-Pd = 360.0(2)ꢀ; (R,R)-2-
Pd = 359.4(4)ꢀ; ( )-3-Pd = 360.1(2)ꢀ). It is noted that
( )-3-Pd maintains pseudo-C2-symmetry in the solid state
while the C2-symmetry is broken in (R,R)-1-Pd and
(R,R)-2-Pd as the cyclohexyl backbone is out of the square
plane set by the phosphorus, nitrogen, and palladium
atoms. Within each complex, the Pd–N(1) and Pd–N(2)
bond lengths are equivalent (within 3r) as are the Pd–
P(1) and Pd–P(2) bond lengths for complex (R,R)-1-Pd
(Table 2). The Pd–P(1) versus Pd–P(2) bond lengths of
˚
˚
(R,R)-2-Pd (D ꢂ 0.02 A) and ( )-3-Pd (D ꢂ 0.01 A) are
slightly asymmetric. Overall, the bond lengths in each com-
plex are in the range of those reported for similar com-
plexes including 4-Pd [13].
3.4. Electrochemical analysis
Studies of active site synthetic analogues and the native
enzyme of nitrile hydratase have shown that amidato coor-
dination protects the Fe(III) (and Co(III)) mononuclear
metal center from reduction even in the presence of coordi-
nated thiolate groups [7]. In contrast, higher oxidation
state transition metal complexes with coordinated thiolate
ligands generally undergo autoreduction [10]. Further-
more, amidato coordination has been shown to stabilize
the higher oxidation states of various metals in the absence
of coordinated thiolates [11]. The reason for the increased
stability has been associated with the strong r-donor nat-
ure of the carboxamido nitrogen anion. To determine if
the amidato coordination in (R,R)-1-Pd, (R,R)-2-Pd, and
( )-3-Pd further stabilized the Pd(II) center the electro-
Table 3
Reduction peak potentials (Ep) of Pd(II) complexes from cyclic voltam-
metry studiesa
Entry
Compound
Ep reductionb (V)
1
2
3
4
a
1-Pd
2-Pd
3-Pd
Pd(PPh3)2Cl2
ꢀ1.76c
ꢀ1.59c
ꢀ1.84c
ꢀ1.25
Voltammograms were measured at 500 mV/s in CH2Cl2 solutions
containing 0.1 M (Bu4N)(PF6) as electrolyte.
b
Measured vs. Ag/AgCl reference electrode.
Quasi reversible reduction peak observed.
c

2462
R.A. Swanson et al. / Inorganica Chimica Acta 360 (2007) 2455–2463
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required to complete the catalytic cycle. These results con-
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4. Conclusions
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bis(phosphanyl) palladium(II) complexes that readily coor-
dinate in a square planar tetradentate fashion. Each com-
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the solid state by X-ray crystallographic analyses and their
electrochemistry has been studied using cyclic voltamme-
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amido nitrogen bonding to Pd(II) has on stabilizing the
complex from reduction. As observed with other high oxi-
dation state metal complexes, the carboxamido nitrogen
bonding to the Pd(II) centers in (R,R)-1-Pd, (R,R)-2-Pd,
and ( )-3-Pd protected the complexes from reduction as
noted by the increased peak reduction potentials that were
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Acknowledgements
We are pleased to acknowledge the financial support of
the Research Corporation (Cottrell College CC6100) and
the Bureau for Faculty Research at Western Washington
University. All NMR data were recorded on a Varian IN-
OVA 500 MHz spectrometer that was funded by the Na-
tional Science Foundation (NSF-MRI #0216604). We
thank Dr. James Vyvyan of Western Washington Univer-
sity for supplying ( )-1,2-diphenylethylenediamine.
(d) S.M. Redmore, C.E.F. Rickard, S.J. Webb, L.J. Wright, Inorg.
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Appendix A. Supplementary material
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CCDC 629352, 629353, and 629354 contain the supple-
mentary crystallographic data for this paper. The data can
be obtained free of charge via http://www.ccdc.cam.ac.uk/
conts/retrieving.html, or from the Cambridge Crystallo-
graphic Data Centre, 12 Union Road, Cambridge CB2
1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.ica.2006.12.011.
mun. 9 (2006) 1151.
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Giacovazzo, A. Guagliardi, A.G.G. Moliterni, G. Polidori, R.
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2
[17] Least squares function minimized: wðF 2o ꢀ F c2Þ .
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2463
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refinement, which could improve its R-factor. However, we are
inclined to have more confidence in the final model we obtained,
although the differences between the two models are subtle.
[26] Complex Pd(PPh3)2(OAc)2 was not used as it has been shown to
spontaneously autoreduce to form Pd(0) complexes and O@PPh3. How-
ever, it is noted that in the presence of excess PPh3, the reduction potential
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[27] Pd(PPh3)2Cl2 also shows an irreversible oxidation peak on the reverse
scan (ꢀ2.2 V to +1.0 V) at +0.124 V which characterized the complex
generated by the reduction at ꢀ1.25 V. Similar results have been
observed with carbene–palladium(II) and mixed carbene/phosphane–
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S
are based on F 2o;
conventional R-factors R1 are based on Fo, with Fo set to zero
for negative F 2o. The observed criterion of F o2 > 2rðF 2oÞ is used only
for calculating R1 and is not relevant to the choice of reflections for
refinement. R-factors based on F 2o are statistically about twice as
large as those based on Fo, and R-factors based on all data will be
even larger.
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¨
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[24] The individual phenyl group proton ABC-spin systems were identified
but their assignment to a specific auxiliary phenyl group was