Palladium complexes of bis(acyclic diaminocarbene) ligands with chiral N-substituents and 8-membered chelate rings

Article abstract of DOI:10.1016/j.jorganchem.2009.06.007

Reaction of cis-dichloridobis(p-trifluoromethylphenylisocyanide)palladium(II) with N,N′-bis[(R)-1-phenylethyl]-1,3-diaminopropane afforded an enantiomerically pure, C₁-symmetric bis(acyclic diaminocarbene)PdCl₂ complex in 41% yield.

Full text of DOI:10.1016/j.jorganchem.2009.06.007

Journal of Organometallic Chemistry 694 (2009) 3297–3305
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Journal of Organometallic Chemistry
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Palladium complexes of bis(acyclic diaminocarbene) ligands with chiral
N-substituents and 8-membered chelate rings
*
Yoshitha A. Wanniarachchi, Sri S. Subramanium, LeGrande M. Slaughter
Department of Chemistry, Oklahoma State University, Stillwater, OK 74078, USA
a r t i c l e i n f o
a b s t r a c t
Reaction of cis-dichloridobis(p-trifluoromethylphenylisocyanide)palladium(II) with N,N0-bis[(R)-1-phen-
ylethyl]-1,3-diaminopropane afforded an enantiomerically pure, C1-symmetric bis(acyclic diaminocar-
bene)PdCl2 complex in 41% yield. The X-ray crystal structure of the complex revealed that three of the
four carbene nitrogens are twisted out of conjugation with the carbene units, apparently as a result of
steric interactions between one phenyl group and the propylene backbone of the chelate. A similar reac-
tion with N,N0-bis[(R)-1-(1-naphthyl)ethyl]-1,3-diaminopropane did not lead to an isolable bis(carbene)
complex, instead forming significant amounts of bis(ammonium) salt as a decomposition product. How-
ever, reaction of the same palladium isocyanide precursor with a mixture of all diastereomers of N,N0-
bis[1-(1-naphthyl)ethyl]-1,3-diaminopropane provided an achiral, Cs-symmetric palladium bis(acyclic
diaminocarbene) complex derived exclusively from the (R,S) diamine in 20% yield. An X-ray structure
showed that the (R,S) stereochemistry allows the bulky naphthyl groups to adopt an orientation that
avoids steric interactions with the backbone that likely lead to the instability of the homochiral analogue.
The two palladium carbene complexes catalyzed the aza-Claisen rearrangement of an allylic imidate to
an allylic amide in 24–34% yield, with an enantiomeric excess of 8% ee for the [(R)-1-phenylethyl]-substi-
tuted complex.
Article history:
Received 16 March 2009
Received in revised form 3 June 2009
Accepted 5 June 2009
Available online 11 June 2009
Keywords:
Carbene ligands
Chelate ligands
Acyclic diaminocarbenes
Palladium
Chirality
Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction
ligand properties of ADCs that could potentially provide certain
advantages over NHCs: ADCs can act as stronger
r-donors than
Cyclic diaminocarbene ligands, more commonly known as N-
heterocyclic carbenes (NHCs), are now firmly established as one
of the most important classes of ancillary ligands in transition me-
tal coordination chemistry [1] and catalysis [2]. The ability of NHCs
to boost the activities of organometallic catalysts relative to the
corresponding phosphine-ligated systems [3] has been frequently
NHCs [11,16], their larger N–C–N angles can place steric bulk or
chiral substituents nearer to the metal center [11,12,15], and hin-
dered rotation about the carbene C–N bonds [10,17] provides con-
formational flexibility [15] that could lead to ‘‘flexible steric bulk
[18].” Investigations of ADCs as ancillary ligands in catalysis have
been very few [14,19,20], but they have been shown to provide
comparable or better activity relative to analogous NHC systems
in certain types of Pd-catalyzed coupling reactions [14,20]. Related
acyclic alkylaminocarbenes have also shown promise in catalysis
[21].
Reports of ADC coordination chemistry since Alder’s work have
largely utilized synthetic routes involving the free carbenes [11–
15], which creates limitations on the types of ADCs that can be
used. In contrast to imidazolium-derived NHCs [22], free ADCs
are thermodynamically inclined to dimerize to enetetramines
[23], although sufficient steric bulk as in A can prevent this [10].
In order to expand the range of available ADC structures, we sought
routes to these ligands that avoid free carbene generation. We
turned to the well-established, but recently little-studied, reaction
of protic amines with coordinated isocyanides [24] as a one-step,
metal-templated route to coordinated ADCs [25]. This chemistry
was inadvertently discovered by Chugaev as early as 1915 in the
synthesis of the first known carbene complex, a chelated platinum
attributed to the higher
r-donating abilities of the NHCs [4],
although there is growing recognition that NHCs may not act as
stronger donors than phosphines under all circumstances [5,6].
Whereas a large and growing literature documents the organome-
tallic chemistry of the imidazolium-derived imidazolin-2-ylidenes
and their backbone-saturated analogues [7], which represent the
most popular classes of NHCs, the related acyclic diaminocarbenes
(ADCs) [8] have been largely neglected as ancillary ligands. The
presence of two nitrogen substituents on the carbene carbon atoms
of ADCs should provide them with electronic stabilization similar
to that of NHCs [9], in accordance with Alder’s seminal work dem-
onstrating the stability of free bis(diisopropylamino)carbene (A,
Fig. 1) [10]. The limited amount of ADC coordination chemistry
that has appeared following Alder’s work [11–15] points out some
* Corresponding author. Tel.: +1 405 744 5941; fax: +1 405 744 6007.
E-mail address: lms@chem.okstate.edu (LeGrande M. Slaughter).
0022-328X/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2009.06.007

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Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
precursor influence the preferred geometry of the bis(ADC) che-
lates formed in these one-step ligand assembly processes.
2. Results and discussion
2.1. Synthesis of chiral diamines
Our motivation for this study was to examine how the use of
diamines with bulky, chiral N-substituents and a flexible hydrocar-
bon tether would influence the chelate structures of bis(ADC) li-
gands formed upon reaction with cis-coordinated isocyanide
ligands. A propylene linker was chosen for the initial studies, be-
cause the resulting 8-membered bis(ADC) chelate ring should have
greater conformational flexibility compared to the 7-membered
chelate structures previously reported by us (C and D, Fig. 1)
[6,29], thus potentially allowing the chiral groups to approach
the metal in some ligand conformations. The chiral N-substituents
chosen were 1-phenylethyl and 1-(1-naphthyl)ethyl groups, pro-
viding two bis(ADC) precursors with different amounts of steric
bulk. Palladium complexes of chelating imidazole-based bis(NHC)
ligands containing these same chiral N-substituents have recently
been reported by Herrmann and co-workers [31], although no
examples containing three-carbon linkers were prepared. The need
to prepare the chiral imidazole precursors in a low-yielding reac-
tion (ꢀ33%) is one disadvantage of this previous work that is
avoided in the approach to bis(carbene) ligands reported here.
The chiral diamine precursors were prepared by a modification
of a procedure reported by Horner and Dickerhof [32] and Feringa
and co-workers [33], with 1,3-dibromopropane used in place of the
dichloroalkane. Heating 1,3-dibromopropane with 2 equiv of either
1-phenylethylamine or 1-(1-naphthyl)ethylamine at 150 °C in tol-
uene for 12 h, followed by cooling to room temperature, led to iso-
lation of the bis(dialkylammonium) dibromide salts 1 and 2 as
intermediates that could be washed with solvents to provide clea-
ner products in the subsequent step (Scheme 1). The bis(ammo-
nium) salts were characterized by two inequivalent 1H NMR
Fig. 1. Representative free ADC (A) and bis(ADC) complexes (B–D).
bis(ADC) complex resulting from nucleophilic addition of hydra-
zine to cis methylisocyanide ligands (B, Fig. 1) [26]. Complex B
was not recognized as a carbene complex until 1970, when its
structure was elucidated by Rouschias and Shaw [27] and Burke
et al. [28].
Our earlier work has shown that a small ‘‘library” of palladium
complexes containing variants of the Chugaev-type bis(ADC) can
be prepared by a common synthetic procedure and screened to
identify an air-stable, moderately active Suzuki–Miyaura cross
coupling catalyst [19]. Subsequently, we extended this synthetic
approach to chiral bis(ADC) palladium complexes C and D (Fig. 1)
by reaction of arylisocyanide complexes with chiral secondary dia-
mines [6,29]. In addition to providing the first chiral ADC ligands,
this one-step synthetic route provides a strategy for systematically
varying the chiral structure and chelate ring size. The 7-membered
chelate rings of C and D were found to engender non-ideal C–Pd–C
bite angles of 82–87°, which we viewed as a likely cause of the
lower effective donor abilities of these ligands relative to common
bis(NHC) or bis(phosphine) ligands [6]. This allowed the use of C
and D as electrophilic catalysts for aza-Claisen rearrangement
reactions [6], an unusual departure from the prevailing use of
NHCs and ADCs to stabilize electron-rich metal centers. In addition,
the 7-membered chelate structure constrains the bis(ADCs) of C
and D to C1-symmetric conformations with the chiral elements rel-
atively remote from the metal center. While the influence of the
chiral backbone on the metal coordination sphere was sufficient
to provide modest enantiomeric excesses of 30–59% ee in aza-Cla-
isen rearrangement reactions [6], the need for highly enantioselec-
tive catalysts led us to pursue other ligand architectures that might
provide enhanced chiral environments about the metal. The devel-
opment of structurally diverse chiral bis(ADC) ligands is particu-
larly important given that reported examples of chiral bis(NHC)
ligands are still relatively scarce [30]. Herein we report extension
of the isocyanide-based synthetic route to a novel chiral bis(ADC)
architecture based on diamines with chiral N-substituents and
flexible, achiral backbones that form 8-membered chelate rings.
While this strategy does not place the chiral centers in closer prox-
imity to the metal compared with chiral backbone-containing
complexes C and D, we hypothesized that the achiral chelate back-
bone might engender enough flexibility to create a dynamic chiral
environment in which chiral groups approach the metal center
during conformational changes. Although bis(ADC) complexes
with enhanced chiral coordination environments and utility as
enantioselective catalysts did not result from this work, it did pro-
vide important insights into how structural features of the diamine
+
signals for the diastereotopic RR0NH2 groups. Treatment of 1 and
2 with aqueous NaOH provided the corresponding diamines 3
and 4 in 51–77% overall yields. The same procedure was used for
the enantiomerically pure diamines (3, 4a) and a diastereomeric
mixture (4b). The (S,S) isomer of N,N0-bis[1-phenylethyl]-1,3-
diaminopropane (3) was previously synthesized and characterized
by Feringa and co-workers [33]. Reactions of the (S,S) isomer of
N,N0-bis[1-(1-naphthyl)ethyl]-1,3-diaminopropane (4) have been
reported by Equey and Alexakis, but no details of synthesis and
characterization were provided [34].
Scheme 1. Synthesis of chiral diamines used to construct bis(ADC) ligands.

Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
3299
Fig. 2. Molecular structure of complex 7, with thermal ellipsoids shown at the 50%
probability level. Hydrogen atoms have been removed for clarity, except for those
on N1, N2, C3, and C5. Selected bond lengths (Å), angles (°), and torsion angles (°):
Pd1–C1 1.981(4), Pd1–C2 2.012(3), C1–N1 1.347(4), C1–N3 1.335(4), C2–N2
1.338(4), C2–N4 1.330(4), N1–C1–N3 116.6(3), N2–C2–N4 118.9(3), C1–N3–C3
125.3(3), C1–N3–C7 119.6(3), C3–N3–C7 113.5(3), C2–N4–C5 123.1(3), C2–N4–C8
120.0(3), C5–N4–C8 115.4(3), C1–Pd–C2 84.06(13), N1–C1–N3–C3 14.1(5), N1–C1–
N3–C7 178.7(3), N2–C2–N4–C5 ꢁ9.0(5), N2–C2–N4–C8 ꢁ174.0(3).
Scheme 2. Synthesis of enantiomerically pure palladium bis(ADC) complexes with
homochiral N-substituents.
2.2. Synthesis and structural analysis of palladium bis(ADC) complexes
Bis(p-trifluoromethylphenylisocyanide) palladium complex 5
(Scheme 2) was chosen as a bis(ADC) synthon because it has pre-
viously been shown to react with structurally diverse diamines
to give stable bis(ADC) complexes [6,29], including one exhibiting
substantial steric strain [35]. Stirring a dichloromethane solution
of 5 with N,N0-bis[(R)-1-phenylethyl]-1,3-diaminopropane (3) led
to very slow formation of a white, microcrystalline precipitate,
requiring 2 d of reaction to obtain reasonable yields. This is in con-
trast to the much faster formation of the 7-membered chelate
bis(ADC) complexes C and D (Fig. 1), which occurred within 2 h
[6,29]. The 1H NMR spectrum of the complex in DMSO-d6 exhibits
two inequivalent NH resonances at 9.63 and 9.56 ppm, two
inequivalent Ph(CH3)CH quartets at 5.88 and 5.71 ppm, and an
overlapped pair of Ph(CH3)CH doublets at 1.43 ppm. These data
are consistent with a chelating bis(ADC)PdCl2 complex 6 possess-
ing C1 symmetry (Scheme 2), with the two chiral N-substituents
occupying stereochemically distinct environments. In addition,
each of the six protons on the propylene linker gives rise to a un-
ique multiplet, with chemical shifts ranging from 5.62 to
0.72 ppm. This suggests that the propylene backbone of the chelate
toward the back of the chelate ring and the one attached to C3 ob-
liquely facing the coordination plane. The 8-membered chelate
ring is chair-like, with C9 of the propylene linker folded away from
the Pd atom. The need to avoid steric interactions of the CH2 units
of C7 and C8 with the bulky N-substituents explains why the che-
late is locked into this conformation in solution rather than inter-
converting with the boat conformer. The ligand geometry does not
appear to provide a substantially asymmetric coordination envi-
ronment at the palladium center: the phenyl group on C3 is too
distant from the coordination sphere to exert significant steric
influence, and the propylene backbone presents no appreciable
asymmetry to the coordination sphere. However, there are two
indications of potential strain in the bis(ADC) ligand caused by
the steric influence of the chiral N-substituents. First, the hydrogen
atoms on C3 and C5 are forced into close proximity with the neigh-
boring N–H hydrogens (1.86 and 1.85 Å, respectively), presumably
to avoid worse steric interactions between the chiral groups and
the propylene tether that would otherwise arise. Second, the sub-
stituents on the carbene nitrogens are twisted substantially out of
ring is conformationally rigid in solution. Because bis(ADC)PdCl2
6
the NCN planes, disrupting the favorable
moieties. This is most evident in the torsion angles between the
NCN planes and the -carbons of the chiral N-substituents: the
p-conjugation of the ADC
was not sufficiently soluble in DMSO-d6 to obtain a 13C NMR spec-
trum, it was converted to the more soluble bis(ADC)PdBr2 analogue
7 by treatment with excess NaBr in aqueous CH3CN, following a
procedure reported previously by us (Scheme 2) [6]. The 13C
NMR spectrum of 7 revealed two inequivalent carbene resonances
at 193.6 and 191.6 ppm, slightly downfield of the range of 184–
190 ppm seen for the analogous 7-membered chelate bis(ADC)
complexes C and D [6,29].
a
N1–C1–N3–C3 torsion angle is 14.1(5)°, and the N2–C2–N4–C5
torsion angle is ꢁ9.0(5)°. The N2–C2–N4–C8 torsion angle of
ꢁ174.0(3) also indicates a substantial deviation from co-planarity
at the end of the propylene linker that is closest to a phenyl ring
of the N-substituents, suggesting that steric repulsions are occur-
ring. The Pd–Ccarbene distances of 1.981(4) and 2.012(3) Å are sim-
ilar to values observed in the related 7-membered chelate bis(ADC)
complexes C and D (Fig. 1) [6,29].
An X-ray crystallographic analysis of a crystal of 7 obtained by
slow evaporation of an acetonitrile solution of the complex con-
firmed a C1-symmetric conformation of the chiral bis(ADC) ligand
(Fig. 2). The complex crystallized in the chiral orthorhombic space
group P212121, and X-ray anomalous dispersion effects substanti-
ated the (R,R) stereochemistry of the ligand. The two (R)-1-phenyl-
ethyl N-substituents both adopt the same orientation relative to
the nitrogen to which they are attached, apparently to avoid steric
interactions of the phenyl groups with each other or with the pro-
pylene linker. This results in the two phenyl groups pointing to dif-
ferent parts of the molecule, with the one attached to C5 oriented
We anticipated that using N,N0-bis[(R)-1-(1-naphthyl)ethyl]-
1,3-diaminopropane (4a) to construct a bis(ADC) ligand could lead
to a more asymmetric environment at the metal center by virtue of
the significantly bulkier naphthyl group. However, reaction of 4a
with palladium arylisocyanide synthon 5 under identical condi-
tions to those used with diamine 3 (Scheme 3) resulted in forma-
tion of a yellow precipitate whose 1H NMR spectrum matched
that of the bis(ammonium) salt 2a. Further stirring did not afford
any isolable metal complex, and the use of rigorously dried

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Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
Fig. 3. Molecular structure of complex 8, with thermal ellipsoids shown at the 50%
probability level. Hydrogen atoms have been removed for clarity, except for those
on N1, N2, C3, and C5. Selected bond lengths (Å), angles (°), and torsion angles (°):
Pd1–C1 1.993(3), Pd1–C2 2.000(3), C1–N1 1.350(4), C1–N3 1.331(4), C2–N2
1.348(4), C2–N4 1.330(4), N1–C1–N3 117.6(3), N2–C2–N4 118.3(3), C1–N3–C3
123.5(3), C1–N3–C7 119.8(2), C3–N3–C7 116.7(2), C2–N4–C5 123.8(2), C2–N4–C8
119.4(2), C5–N4–C8 116.8(2), C1–Pd–C2 84.4(1), N1–C1–N3–C3 4.1(4), N1–C1–N3–
C7 ꢁ177.6(3), N2–C2–N4–C5 ꢁ1.8(4), N2–C2–N4–C8 ꢁ179.3(2).
Scheme 3. Formation of an achiral, Cs-symmetric palladium bis(ADC) complex by
reaction of palladium arylisocyanide precursor 5 with a diastereomeric mixture of
diamines (1-Np = 1-naphthyl).
acetonitrile under a dry nitrogen atmosphere gave the same result.
We postulated that a bis(ADC) complex may have formed and
rapidly decomposed (vide infra).
compared with chiral complex 7. The substituents on N3 and N4
of the carbene moieties are nearly co-planar with the NCN units,
with the NCN–Clinker and NCN–CN-subst torsion angles showing
deviations from planarity of 0.7(2)–4.1(4)°. This is in contrast to
the deviations of 1.3(3)–14.1(5)° observed in complex 7. One nota-
ble similarity to 7 is that the methine C–H groups of 8 are in close
proximity to the N–H hydrogens, with respective HꢂꢂꢂH distances of
1.82 and 1.87 Å.
A comparison of the structures of 7 and 8 with those of related
palladium bis(ADC) complexes is instructive. The respective C–Pd–
C bite angles of 84.06(13)° and 84.41(11)° for 7 and 8 are little
changed compared with the 7-membered chelate bis(ADC) com-
plexes C and D (Fig. 1), which exhibited respective C–Pd–C angles
of 82.3(2)° and 86.6(1)° [6,29]. Differences are apparent, however,
in the dihedral angles between the carbene NCN planes and the
PdC2X2 planes. The average dihedral angles of 87.7° and 85.6° ob-
served for 7 and 8, respectively, are significantly larger than the
average dihedral angles of 77.8° in C and 69.0° in D. Thus, the
use of a longer linker results in no significant change in the
bis(ADC) bite angle, but instead permits the carbene units to rotate
to a near-perpendicular arrangement. The carbene N–C–N angles
of 7 and 8 (average angles 117.8° and 118.0°, respectively) are
not significantly different from those of C (average 116.8°) and D
(average 115.6°), indicating little difference in the amount of strain
within the carbene units.
A structural comparison of 7 and 8 with reported examples of
bis(NHC) palladium complexes having 8-membered chelate rings
is also informative [36]. The three structurally characterized exam-
ples of such complexes are shown in Fig. 4 [37–39]. Similarly to 7
and 8, these all contain saturated three-carbon tethers. The C–Pd–C
bite angles are quite similar to those seen in bis(ADC) complexes 7
and 8: 84.8(2)° in E [37], 85.0(2)° in F [38], and 87.6(1)° in G [39].
However, the dihedral angles between the carbene NCN planes and
the PdC2X2 planes are significantly smaller, averaging 83.2° for E,
81.0° for F, and 73.8° for G, in comparison with respective values
of 87.7° and 85.6° for 7 and 8. The smaller dihedral angles for the
bis(NHC) ligands relative to the bis(ADC) ligands having the same
chelate ring size can be explained as a result of the smaller carbene
NCN angles of ꢀ107° inherent to imidazole-derived carbene li-
gands, in contrast to the larger NCN angles of ꢀ118° found in the
The apparent inability to form a stable bis(ADC) complex from
the (R,R) naphthyl diamine 4a raised the question of whether such
a complex could be prepared from the corresponding (R,S) diamine.
Lacking a readily available means of resolving the (R,S)-meso dia-
mine from the (R,R)/(S,S) racemate, we decided to examine the
reaction of palladium arylisocyanide synthon 5 with the unre-
solved diastereomeric mixture 4b. As in the reaction of enantiome-
rically pure 4a, the solid initially formed upon treatment of
palladium isocyanide precursor 5 with the diastereomeric mixture
of diamines 4b in acetonitrile was identified as a bis(ammonium)
salt by 1H NMR. However, removal of this solid by filtration after
3 h followed by continued stirring of the clear filtrate for 2 d at
25 °C afforded a white precipitate in poor yield (20% relative to
Pd) whose 1H and 13C NMR spectra were consistent with
bis(ADC)PdCl2 complex 8 (Scheme 3). In contrast to complexes 6
and 7, the NMR spectral data indicate twofold symmetry in the
bis(ADC) ligand of 8. Only one NH resonance is visible at
9.73 ppm in the 1H NMR spectrum. A single set of resonances is
present for the 1-(1-naphthyl)ethyl N-substituents, with
a
Np(CH3)CH quartet at 6.28 ppm and a Np(CH3)CH doublet at
1.92 ppm. Four distinct multiplets are present for the propylene
linker, indicating a symmetric but conformationally rigid tether
in the bis(ADC) backbone. The 13C NMR spectrum of 8 contains a
single carbene resonance at 191.3 ppm as well as one set of peaks
for the 1-(1-naphthyl)ethyl N-substituents and the CF3–Ph group.
A crystal of 8 suitable for X-ray crystallographic analysis was
obtained by slow evaporation of a DMSO solution of the complex.
The X-ray structure revealed that the complex has Cs symmetry,
with the two halves of the bis(ADC) ligand related by a (non-crys-
tallographic) mirror plane (Fig. 3). The 8-membered chelate ring of
8 adopts a chair-like conformation similar to that seen in 7. The 1-
(1-naphthyl)ethyl N-substituents have (R) and (S) stereochemistry
at C3 and C5, respectively, confirming that the complex itself is
achiral and derived from the meso diamine. This stereochemical
arrangement allows both bulky naphthyl groups to orient them-
selves toward the outside of the complex, avoiding any steric inter-
actions with the propylene linker. Instead, the much less sterically
demanding methyl groups are directed toward the back of linker.
This appears to result in significantly less strained carbene units

Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
3301
Fig. 4. Structurally characterized palladium complexes of bis(NHC) ligands with 8-
membered chelate rings.
Scheme 4. Proposed mechanism for the decomposition of a strained ADC ligand to
generate an ammonium salt.
acyclic carbene moieties of 7 and 8. Assuming identical bite angles
and perpendicular dihedral angles, the smaller NCN angles in a
bis(NHC) complex would result in a greater distance between the
two nitrogen atoms connected to the three-carbon tether in com-
parison to an analogous bis(ADC) complex (Fig. 5), creating addi-
tional strain in the chelate ring of the bis(NHC) ligand. This
would be relieved by rotation of the carbene units away from a
perpendicular arrangement, which would bring the tethered nitro-
gen atoms closer together. It is notable that the dimethyl substitu-
tion on the tethers of E and F may result in more rigid linkers that
could affect the ability of the NHC rings to attain optimal dihedral
angles. Thus, complex G, which exhibits the smallest NCN– PdC2X2
dihedral angles, is probably the best comparison to use for analyz-
ing bis(NHC) versus bis(ADC) ligand geometries.
ative to a phenyl group, thus rendering the carbene units unstable.
It is also interesting to note that another, stereochemically inequiv-
alent isomer of complex 8 could form from the meso diamine, with
(S) stereochemistry at C3 and (R) stereochemistry at C5. However,
this isomer is not observed. The arguments given above predict
that this isomer would have both bulky naphthyl groups oriented
toward the back of the chelate ring, resulting in highly strained
and unstable diaminocarbene moieties.
A further question is whether the bis(ADC) ligands predicted to
be unstable by the above arguments actually form and then
decompose during the reactions of the diamines with palladium
isocyanide precursor 5. We have previously reported one example
of a strained, tetrasubstituted Chugaev-type bis(ADC) complex
formed from 5 in which the chelate-forming 1,2 addition of an
N–H group across the isocyanide CN bond was observed to be
reversible in solution [35]. However, the isolation of bis(ammo-
nium) salts from reactions of 4a and 4b with 5 suggests that a dif-
2.3. Discussion of the effect of ligand structure on complex stability
A comparison of the structures of 7 and 8 provides a rationale
for the inability to form a stable bis(ADC) complex from enantio-
merically pure (R,R) naphthyl diamine 4a (Scheme 3). The 1-aryl-
ethyl N-substituents in both structures adopt an orientation with
the methine C–H pointing toward the neighboring N–H group,
and the aryl and methyl groups pointing either behind the chelate
ring or toward the outside of the complex. Steric strain leading to
deviations from substituent co-planarity at the ‘‘upper” carbene
nitrogens N3 and N4 is minimized if the larger aryl groups face
the outside of the complex, as observed in the (R,S) naphthyl com-
plex 8. When both 1-arylethyl groups have the same stereochem-
ical configuration as in complex 7, one aryl group must point
toward the back of the chelate ring, where steric interactions with
the propylene tether and possibly the methyl group of the other N-
substituent are significant. A homochiral naphthyl analogue of
complex 7 would likely display even greater deviations from pla-
narity at N3 and N4 due to the greater bulk of a naphthyl group rel-
ferent process is occurring.
A proposed pathway to the
bis(ammonium) salt decomposition product is shown in Scheme
4. A carbene nitrogen is twisted out of conjugation with the NCN
unit due to steric strain, causing it to pyramidalize. This renders
the nitrogen susceptible to attack by small traces of H2O or acid
in the acetonitrile solvent, affording a free secondary amine and
an N-protonated isocyanide ligand. The latter species is rapidly
deprotonated by the liberated amine to generate the ammonium
salt. In principle, this process regenerates the isocyanide complex,
which may react with an amine in a more favorable conformation
to give a stable bis(ADC) palladium complex. However, the low
yield obtained in the formation of 8 (20% per Pd) suggests that
other decomposition processes may be operating as well.
2.4. Catalytic studies
We have previously reported that palladium complexes C and D
(Fig. 1), which have chiral bis(ADC) ligands with 7-membered che-
late rings, act as precatalysts in the enantioselective aza-Claisen
rearrangement of an allylic imidate to an allylic amide, providing
up to 70% yield and enantiomeric excesses of 30–59% ee [6]. We
wished to examine how the activity and enantioselectivity of this
reaction would compare when the 8-membered chelate bis(ADC)
complexes 6 and 8 were utilized. An identical protocol to that
reported previously by us was followed: the precatalyst was acti-
vated with one equiv of AgBAr4F ([BArF4]ꢁ = tetrakis[3,5-bis(trifluoro-
methyl)phenyl]borate) in dichloromethane solution in the presence
of the benchmark benzimidate substrate 9 [40,41], followed by heat-
ing at 40 °C for 2 d (Scheme 5). The enantiomerically pure precata-
lyst (R,R)-6 provided the desired [3,3]-rearrangement product 10 in
34% yield, with the undesired [1,3]-rearrangement product 11 and
the amide elimination product 12 also obtained in 9% and 14% yields,
Fig. 5. Rationale for the smaller NCN–PdC2X2 dihedral angles observed in bis(NHC)
versus bis(ADC) chelates with three-carbon linkers.

3302
Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
Scheme 5. Aza-Claisen rearrangements of benzimidate 9 catalyzed by 8-membered chelated palladium bis(ADC) complexes.
respectively. This product distribution is comparable to those ob-
tained with enantiomerically pure samples of precatalysts C and D
[6]. However, the measured enantiomeric excess was only 8% ee of
the (S)-(+) isomer of 10, making 6 a significantly less enantioselec-
tive precatalyst than either of the 7-membered chelate palladium
bis(ADC) complexes. Achiral naphthyl-substituted bis(ADC) complex
8 provided an even poorer yield of 24% of the desired product 10, as
well as side products 11 and 12 in respective yields of 7% and 14%.
Thus, neither of the 8-membered chelate complexes provides an
advantage over the previously studied bis(ADC) complexes in the
yield of the [3,3]-rearrangement product.
boiled over and distilled from CaH2 before use. Dichloromethane
(Pharmco) was washed with concentrated H2SO4, water, aqueous
NaHCO3, and again water, and then distilled from P2O5 prior to
use. Hexanes (Pharmco), toluene (Pharmco) and diethyl ether
(Acros) were dried over and distilled from Na/benzophenone ketyl
prior to use. DMSO for crystal growth (Acros, reagent grade) was
used as received. Water was purified by an E-pure system (Barn-
stead) and had a resistivity of P17.6 MX cm. NMR solvents were
purchased from Cambridge Isotopes Laboratories. C6D6 and
DMSO-d6 were dried by stirring over activated 4 Å molecular sieves
followed by vacuum distillation at room temperature and were
stored in a nitrogen glove box before use. CD2Cl2 was dried by a
similar procedure and then stored over and distilled from P2O5
prior to use. (R)-(+)-1-Phenylethylamine (99+%), (R)-(+)-1-(1-
naphthyl)ethylamine (99+%), and 1,3-dibromopropane (98%) were
purchased from Acros. ( )-1-(1-Naphthyl)ethylamine (98%)
was purchased from Aldrich. Bis(p-trifluoromethylphenylisocya-
nide)PdCl2 (5) was prepared as previously reported [29]. Silver
tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (AgBArF4) [42] and
(E)-2-hexenyl-N-[4-(trifluoromethyl)phenyl]benzimidate (9) [40]
were prepared by literature procedures. All other materials were
purchased from Acros and used as received.
3. Conclusion
The one-step assembly of chelating bis(ADC) ligands by reaction
of diamines with palladium bis(isocyanide) synthons has been
extended to examples containing achiral propylene linkers and
chiral 1-arylethyl N-substituents. Use of enantiomerically pure
N,N0-bis[(R)-1-phenylethyl]-1,3-diaminopropane (3) provided
a
C1-symmetric, chiral bis(ADC) complex with slightly strained
diaminocarbene moieties as judged by the structure of 7. No stable
bis(ADC) complex could be obtained from homochiral N,N0-bis[(R)-
1-(1-naphthyl)ethyl]-1,3-diaminopropane, but reaction of palladium
arylisocyanide precursor 5 with a mixture of diastereomers of the
same naphthyl-substituted diamine afforded an achiral, Cs-sym-
metric bis(ADC) complex 8. The favored bis(ADC) conformations
in both complexes appear to minimize steric interactions between
aryl groups and the propylene linker which might disrupt the
NMR spectra were recorded on Varian Unity INOVA 400 MHz
and 600 MHz spectrometers. Reported chemical shifts are refer-
enced to residual solvent peaks (13C, 1H). Elemental analyses were
performed by Desert Analytics (Tucson, Arizona) or Midwest
Microlab (Indianapolis, Indiana).
p-conjugation within the carbene moieties. The propylene linkers
4.2. Syntheses
in these complexes are surprisingly rigid in solution, precluding
the conformational flexibility that would be required to bring the
chiral N-substituents in close proximity to the palladium center.
The formation of a single stable bis(ADC) complex 8 from a diaste-
reomeric mixture of the diamine is particularly interesting, because
it suggests that reactions of metal isocyanide precursors with dia-
stereomeric mixtures might be utilized in future work to select the
most stable chelate structure obtainable from a given diamine
without the need to resolve the diamine into its meso and homo-
chiral components. Finally, the new bis(ADC) complexes gave
unimpressive yields in the catalytic aza-Claisen rearrangement of
an allylic imidate, as well as poor enantioselectivity (8% ee) in
the case of (R,R)-6. These results indicate that this particular
bis(ADC) ligand design, incorporating chiral N-substituents and
rigid 8-membered chelate rings, does not provide the electronic or
asymmetric properties needed for effective enantioselective electro-
philic palladium catalysis. Similar bis(ADC) designs with longer, more
flexible achiral backbones are a worthy target for future studies.
4.2.1. N,N0-Bis[(R)-1-phenylethyl]-1,3-diaminopropane (3)
(R)-(+)-1-Phenylethylamine (0.10 mL, 0.78 mmol) and 1,3-
dibromopropane (0.040 mL, 0.39 mmol) were dissolved in tolu-
ene (5 mL) in a sealable glass vessel. The sealed vessel was
placed in an oil bath at 150 °C, and the reaction mixture was
stirred for 12 h. Upon cooling to room temperature, white nee-
dle-shaped crystals precipitated from the reaction mixture. The
precipitate was filtered, washed with toluene and diethyl ether,
and dried in vacuo. The 1H NMR spectrum of this solid character-
ized it as the bis(dialkylammonium) dibromide salt 1. Yield:
102 mg, 59%. 1H NMR (400 MHz, DMSO-d6): d 9.18 (br s, 2 H,
NH2), 9.02 (br s, 2 H, NH2), 7.53–7.38 (m, 10 H, Ar), 4.36 (s, 2
H, PhCH), 2.90 (br s, 2 H, NCH2), 2.63 (br s, 2 H, NCH2), 1.99
3
(unresolved m, 2 H, CH2), 1.55 (d, JH,H = 6.4 Hz, 6 H, CH3). The
bis(ammonium) salt was dissolved in dichloromethane (20 mL),
and 20 mL of a 20% (w/v) aqueous solution of NaOH was added.
The mixture was placed in a separatory funnel, and the organic
layer was separated. The aqueous layer was extracted twice
more with 20 mL of distilled dichloromethane, and the combined
organic layers were dried over anhydrous Na2SO4. Removal of
solvent on a rotary evaporator followed by drying in vacuo
yielded 2 as yellow oil. Yield: 56 mg, 51% overall. NMR spectral
data matched those previously reported by Feringa et al. for
the (S,S) isomer [33].
4. Experimental
4.1. General procedures
All manipulations were performed under air unless otherwise
noted. Acetonitrile (Pharmco) was pre-dried over CaCl2, then

Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
3303
4.2.2. N,N0-Bis[1-(1-naphthyl)ethyl]-1,3-diaminopropane,
diastereomeric mixture (4b)
diethyl ether (5 mL), and dried in vacuo overnight. Yield: 85 mg
(0.095 mmol), 80%. 1H NMR (400 MHz, DMSO-d6): d 9.64 (s, 1 H,
3
The same procedure employed for diamine 3 was followed,
starting with 0.10 mL (0.62 mmol) of ( )-1-(1-naphthylethyl-
amine) and 0.032 mL (0.31 mmol) of 1,3-dibromopropane to yield
the bis(ammonium) dibromide salt 2b as an intermediate in 71%
yield (0.12 g). 1H NMR (400 MHz, DMSO-d6): d 9.41 (br s, 2 H,
NH), 9.58 (s, 1 H, NH), 8.23 (d, JH,H = 8.0 Hz, 2 H, CF3–Ph), 8.05
3
(d, JH,H = 8.0 Hz, 2 H, CF3–Ph), 7.65–7.43 (m, 6 H, Ar), 7.35–7.13
3
(m,
8
H, Ar), 5.92 (q, JH,H = 6.4 Hz,
1
H, PhCH), 5.74 (q,
3JH,H = 6.4 Hz, 1 H, PhCH), 5.64 (m, 1 H, CH2), 5.49 (m, 1 H, CH2),
3.82 (m, 1 H, CH2), 3.28 (m, 1 H, CH2), 1.54 (m, 1 H, CH2), 1.43
(2d unresolved, 6 H, CH3), 0.76 (m, 1 H, CH2). 13C NMR (101 MHz,
DMSO-d6): d 193.6 (carbene), 191.6 (carbene), 143.8 (CF3–Ph ipso),
143.7 (CF3–Ph ipso), 138.4 (Ph ipso), 137.8 (Ph ipso), 128.9 (Ph),
128.7 (Ph), 128.6 (Ph), 128.4 (Ph), 127.4 (Ph), 126.5 (Ph), 124.9
(CF3–Ph meta), 124.8 (CF3–Ph meta), 124.1 (2 q overlapped,
3
NH2), 9.13 (br s, 2 H, NH2), 8.22 (d, JH,H = 8.4 Hz, 2 H, naphthyl),
8.05–7.93 (m, 4 H, naphthyl), 7.86 (br s, 2 H, naphthyl), 7.68–
7.53 (m, 6 H, naphthyl), 5.27 (s, 2 H, NpCH), 3.06 (br s, 2 H,
NCH2), 2.82 (br s, 2 H, NCH2), 2.07 (br s, 2 H, CH2), 1.63 (br s, 6
H, CH3). After deprotonation with NaOH, the diastereomeric mix-
ture of diamines 4b was obtained in 66% overall yield (0.078 g).
1H NMR (400 MHz, DMSO-d6): d 8.24–8.10 (m, 2 H, naphthyl),
7.88–7.80 (m, 2 H, naphthyl), 7.74 (d, 3JH,H = 8.0 Hz, 2 H, naphthyl),
7.64 (d, 3JH,H = 7.2 Hz, 2 H, naphthyl), 7.49–7.34 (m, 6 H, naphthyl),
1
2JC,F = 30 Hz, CF3–Ph para), 124.1 (2 q overlapped, JC,F = 272 Hz,
CF3), 122.3 (CF3–Ph ortho), 121.7 (CF3–Ph ortho), 56.6 (NCMe),
55.8 (NCMe), 54.3 (NCH2), 53.5 (NCH2), 29.2 (NCH2CH2), 18.0
(CH3), 15.4 (CH3). Anal. Calc. for C35H34Br2F6N4Pd: C, 47.19; H,
3.85; N, 6.29. Found: C, 47.20; H, 3.90; N, 6.40%.
3
4.48 (q, JH,H = 6.3 Hz, 2H, NpCH), 2.51–2.30 (m, 4 H, NCH2), 2.06
3
(br s, 2 H, NH), 1.60–1.48 (m, 2 H, CH2), 1.33 (d, JH,H = 6.3 Hz, 6
H, CH3). 13C NMR (101 MHz, C6D6): d 142.2 (2 s overlapped, naph-
thyl ipso), 134.6 (naphthyl), 132.0 (naphthyl), 129.3 (naphthyl),
128.2 (2 s overlapped, naphthyl), 128.0 (2 s overlapped, naphthyl),
127.8 (2 s overlapped, naphthyl), 127.4 (naphthyl), 126.0 (2 s over-
lapped, naphthyl), 125.8 (naphthyl), 125.5 (naphthyl), 123.6 (2 s
overlapped, naphthyl), 123.3 (2 s overlapped, naphthyl), 54.7
(NCMe), 54.6 (NCMe), 46.9 (NCH2), 46.6 (NCH2), 31.3 (NCH2CH2),
31.2 (NCH2CH2), 24.1 (CH3), 24.0 (CH3). HRMS (ESI), m/z: 383.25
[M+H]+, 405.25 [M+Na]+. Homochiral N,N0-bis[(R)-1-(1-naph-
thyl)ethyl]-1,3-diaminopropane 4a was prepared similarly from
1,3-dibromopropane (0.16 mL, 1.6 mmol) and (R)-(+)-1-(1-naph-
thyl)ethylamine (0.50 mL, 3.1 mmol), affording the bis(ammo-
nium) dibromide salt 2a in 92% yield (0.75 g) and then the
diamine in 77% overall yield (0.45 g). 1H NMR spectral data of 4a
were identical to those of the diastereomeric mixture 4b, but with
sharper peaks.
4.2.5. N,N0-Bis[1-(1-naphthyl)ethyl]-bis(ADC) palladium dichloride
complex (8)
Under a nitrogen atmosphere, a solution of N,N0-Bis[1-(1-naph-
thyl)ethyl]-1,3-diaminopropane 4b (67 mg, 0.18 mmol; mixture of
all diastereomers) in 4 mL of acetonitrile was added dropwise into
a
solution of bis(p-trifluoromethylphenylisocyanide)PdCl2
5
(91 mg, 0.18 mmol) in 30 mL of acetonitrile while the latter was
stirred. Over the next 3 h a yellow precipitate, identified by 1H
NMR as a bis(ammonium) salt of the diamine, formed in the reac-
tion mixture. The precipitate was removed by filtration through a
fine frit, and the clear filtrate was stirred for an additional 2 d at
25 °C. During this time, bis(ADC) complex 8 precipitated as a white
solid. The solid was collected by filtration in air, washed with ace-
tonitrile and dichloromethane, and dried in vacuo overnight. Yield:
33 mg, 0.037 mmol, 20%. 1H NMR (400 MHz, DMSO-d6): d 9.73 (s, 2
H, NH), 8.17 (d, 3JH,H = 8.2 Hz, 4 H, CF3–Ph), 7.92 (d, 3JH,H = 8.2 Hz, 4
H, CF3–Ph), 7.79 (d, 2 H, 3JH,H = 7.2 Hz, naphthyl), 7.60–7.54 (m, 4 H,
naphthyl), 7.52–7.45 (m, 2 H, naphthyl), 7.47–7.32 (m, 2 H, naph-
4.2.3. N,N0-Bis[(R)-1-phenylethyl]-bis(ADC) palladium dichloride
complex (6)
3
thyl), 7.28 (d, 4H, 3JH,H = 8.8 Hz, naphthyl), 6.28 (q, JH,H = 6.2 Hz, 2
A solution of N,N0-bis[(R)-1-phenylethyl]-1,3-diaminopropane 3
(30 mg, 0.11 mmol) in 4 mL of acetonitrile was added dropwise
H, NpCH), 5.48 (m, 2 H, CH2), 3.44 (m, 2 H, CH2), 2.19 (m, 1 H, CH2),
3
1.92 (d, JH,H = 6.2 Hz, 6 H, CH3), 1.90 (m, 1 H, CH2). 13C NMR
into a solution of bis(p-trifluoromethylphenylisocyanide)PdCl2
5
(151 MHz, DMSO-d6): d 191.3 (carbene), 144.0 (CF3–Ph ipso),
133.5 (naphthyl), 133.4 (naphthyl), 130.7 (naphthyl), 129.2 (naph-
thyl), 128.7 (naphthyl), 127.0 (naphthyl), 126.2 (naphthyl), 125.4
(naphthyl), 125.2 (naphthyl), 125.0 (CF3–Ph meta), 124.5 (q,
(60 mg, 0.11 mmol) in 22 mL of acetonitrile while the latter was
stirred. The reaction mixture was stirred for 2 d at 25 °C, during
which time bis(ADC) complex 6 precipitated as a white, microcrys-
talline solid. The solid was collected by filtration, washed with ace-
tonitrile and dichloromethane, and dried in vacuo overnight. Yield:
1
2JC,F = 33 Hz, CF3–Ph para), 124.2 (q, JC,F = 272 Hz, CF3), 123.2
(naphthyl), 122.5 (CF3–Ph ortho), 54.3 (NCMe), 53.8 (NCH2), 30.3
(NCH2CH2), 19.2 (CH3). Anal. Calc. for C35H34Cl2F6N4Pd: C, 57.25;
H, 4.25; N, 6.21. Found: C, 57.49; H, 4.36; N, 6.49%.
34 mg, 41%. 1H NMR (400 MHz, DMSO-d6): d 9.63 (s, 1 H, NH), 9.56
3
(s,
1
H, NH), 8.15 (d, JH,H = 8.4 Hz,
2
H, CF3–Ph), 8.02 (d,
3JH,H = 8.4 Hz, 2 H, CF3–Ph), 7.57–7.42 (m, 6 H, Ar), 7.30–7.15 (m,
3
3
8 H, Ar), 5.88 (q, JH,H = 6.4 Hz, 1 H, PhCH), 5.71 (q, JH,H = 6.4 Hz,
1 H, PhCH), 5.62 (m, 1 H, CH2), 5.54 (m, 1 H, CH2), 3.85 (m, 1 H,
CH2), 3.29 (m, 1 H, CH2), 1.56 (m, 1 H, CH2), 1.43 (2d unresolved,
6 H, CH3), 0.72 (m, 1 H, CH2). Anal. Calc. for C35H34Cl2F6N4Pd: C,
52.42; H, 4.27; N, 6.99. Found: C, 52.13; H, 4.12; N, 6.91%.
4.3. X-ray crystallography
4.3.1. General crystallographic procedures
X-ray diffraction data were collected on a Bruker SMART APEX II
diffractometer with a CCD detector using a combination of / and
x
scans. The crystal-to-detector distance was 6.0 cm. A Bruker Kryo-
flex liquid nitrogen cooling device was used for low-temperature
data collections. Unit cell determination and data collection uti-
lized the Bruker APEX2 software package [43]. Data integration
employed SAINT [44]. Multiscan absorption corrections were imple-
mented using SADABS [45]. X-ray diffraction experiments employed
4.2.4. N,N0-Bis[(R)-1-phenylethyl]-bis(ADC) palladium dibromide
complex (7)
Bis(ADC)PdCl2 complex 6 (95 mg, 0.12 mmol) was suspended in
acetonitrile (60 mL) in a round bottomed flask. NaBr (512 mg,
5 mmol) and 2.0 mL of deionized water were added. The flask
was sealed with a polyethylene stopper, and the mixture was stir-
red for 12 h at 25 °C. The resultant turbid solution was filtered
through a sintered glass frit, and the filtrate was concentrated to
2 mL under reduced pressure without applying heat. Deionized
water (10 mL) was added, resulting in formation of a grey precip-
itate that floated on the liquid surface. The solid was collected by
suction filtration, washed with deionized water (20 mL) and
graphite-monochromated Mo Ka radiation (k = 0.71073 Å). Struc-
tures were solved by direct methods and refined by full-matrix
least-squares on F2 using the SHELXTL software suite [46]. Non-
hydrogen atoms were assigned anisotropic temperature factors,
with hydrogen atoms included in calculated positions (riding mod-
el) except as indicated. Further details of the structural determina-
tions are presented in Table 1 and in the text below.

3304
Y.A. Wanniarachchi et al. / Journal of Organometallic Chemistry 694 (2009) 3297–3305
Table 1
of each product purified by flash chromatography using a 10:90 mix-
ture of ethyl acetate and hexanes.
Details of X-ray crystallographic structural determinations of palladium bis(ADC)
complexes 7 and 8.
For determination of enantiomeric excess, a 2 mg sample of
purified allylic amide 10 was dissolved in 2 mL of 95:5 n-hexane/
EtOH to make a solution of roughly 1000 ppm concentration. An
aliquot of this solution was diluted to 30–50 ppm to be used for
HPLC analysis. Enantiomeric excess was determined using a Beck-
man System Gold HPLC equipped with a 25 cm Chiralpak AD–H
column (Chiral Technologies Inc.) and a UV detector (254 nm).
7
8ꢂDMSO
Formula
Mr
T (K)
Crystal size (mm)
Color, habit
Crystal system
Space group
a (Å)
C35H34Br2F6N4Pd
890.88
115(2)
C43H38Cl2F6N4PdꢂC2H6OS
980.20
125(2)
0.33 ꢃ 0.07 ꢃ 0.04
Colorless rod
Orthorhombic
P212121
7.3123(1)
17.5737(2)
26.6940(3)
90
3430.29(7)
4
1.725
2.935
1768
0.19 ꢃ 0.17 ꢃ 0.02
Colorless plate
Monoclinic
P21/c
13.6755(6)
24.5368(11)
14.0915(6)
114.997(1)
4285.5(3)
4
Sample injection volume was 20 lL, and a Shimadzu CR501 Chro-
matopac integrator was used to determine the enantiomeric ratio.
Test runs of a racemic mixture of 10 were performed before the %
ee measurement to ensure baseline separation and reliable inte-
grated peak ratios. The absolute configuration was determined as
previously described [6].
b (Å)
c (Å)
b (°)
V (Å3)
Z
dcalcd. (g cmꢁ3
)
1.519
0.672
2000
l
(mmꢁ1
F(0 0 0)
)
Acknowledgments
h Range (°)
Index ranges
1.53ꢁ25.63
ꢁ8 6 h 6 8
ꢁ21 6 k 6 21
ꢁ32 6 l 6 32
26 414
6452 [0.048]
5906
0.0265, 0.0538
0.0323, 0.0557
1.016
1.64ꢁ28.28
ꢁ18 6 h 6 18
ꢁ32 6 k 6 32
ꢁ18 6 l 6 18
55 796
10 627 [0.067]
7726
0.0432, 0.0925
0.0720, 0.1048
1.010
We thank the National Science Foundation (CAREER Award
CHE-0645438) for funding this research and Prof. Joseph M. Tanski
(Vassar College) for assistance in determining the crystal structure
of complex 8. The Oklahoma State Regents for Higher Education
provided funds for purchase of the APEX II diffractometer.
Reflections measured
Reflections unique [Rint
]
Reflections observed [I > 2
R1, wR2 [I>2 (I)]
R1, wR2 (all data)
Goodness of fit (GoF) on F2
Peak/hole (e Åꢁ3
Absolute structure parameter
r(I)]
r
Appendix A. Supplementary material
)
0.516/ꢁ0.354
1.248/ꢁ1.094
0.005(6)
–
CCDC 711505 and 711254 contain the supplementary crystallo-
graphic data for complexes 7 and 8. These data can be obtained
free of charge from The Cambridge Crystallographic Data Centre
via www.ccdc.cam.ac.uk/data_request/cif.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.jorganchem.2009.06.007.
4.3.2. X-ray crystallographic analysis of complex 7
Data were collected in 50 s scans, with a target data redundancy
of 4.0 including 75% of Friedel pairs. Friedel opposites were not
merged in the final reflection data used for structure solution
and refinement, in order to allow absolute structure determination
by anomalous dispersion effects. The N–H hydrogen atoms at-
tached to N1 and N2 were restrained to a distance of 0.88 Å, with
other positional parameters of these atoms allowed to refine freely.
For refinement as the (R,R) enantiomer, Flack x = 0.005(6), R1
References
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