Substituent effects on the stereoelectronic and chemical properties of a novel phosphapalladacycle

Article abstract of DOI:10.1002/ejic.200700171

A novel phosphapalladacycle was prepared by direct cyclopalladation of the ligand (diphenylmethyl)diphenylphosphane (2). The corresponding μ-chloro five-membered phosphapalladacycle 3 was obtained by heating a toluene solution of 2 and Pd(OAc)₂ followed by chloride ion metathesis. An optical resolution procedure was then employed by separation of the (S)-prolinato derivatives 4 by fractional crystallization followed by treatment with dilute hydrochloric acid to yield both enantiopodes of the dimers 3 in the optically active forms. The current phosphapalladacycle was noted to display different properties with respect to the analogous phosphapalladacycle prepared from (diphenylmethyl)di-tert-butylphosphane. For instance, the phosphapalladacycle based on the (diphenylmethyl)di-tert-butylphosphane was shown to be conformationally rigid. In contrast, the analogous five-membered phosphapalladacycle of phosphane 2 was noted to exist in both the δ and λ conformations in the solid state, while a flattened conformation was observed in solution. In addition, the (diphenylmethyl)di-tert-butylphosphane- based phosphapalladacycle was noted to exhibit a much more improved regioselectivity towards ancillary ligands such as (S)-prolinate and triphenylphosphane than 2. Furthermore, the μ-chloro dimer 3 of the current palladacycle was shown to be lacking in thermodynamic stability of the Pd-C bond that was noted for the tBu analogue. In contrast, the Pd-C bonds of dimer 3 were immediately ruptured in the presence of concentrated HCl to give rise to the binuclear complex 5, in which the phosphane was coordinated in the η¹-P coordination mode. Wiley-VCH Verlag GmbH & Co. KGaA, 2007.

Full text of DOI:10.1002/ejic.200700171

FULL PAPER
DOI: 10.1002/ejic.200700171
Substituent Effects on the Stereoelectronic and Chemical Properties of a Novel
Phosphapalladacycle
Joseph Kok-Peng Ng,[a] Shuli Chen,[b] Geok-Kheng Tan,[a] and Pak-Hing Leung*[b]
Keywords: Palladium / Phosphane ligands / Chirality / NMR spectroscopy / Substituent effects
A novel phosphapalladacycle was prepared by direct cyclo-
palladation of the ligand (diphenylmethyl)diphenylphos-
phane (2). The corresponding µ-chloro five-membered phos-
phapalladacycle 3 was obtained by heating a toluene solu-
tion of 2 and Pd(OAc)2 followed by chloride ion metathesis.
An optical resolution procedure was then employed by sepa-
ration of the (S)-prolinato derivatives 4 by fractional crystalli-
zation followed by treatment with dilute hydrochloric acid to
yield both enantiopodes of the dimers 3 in the optically active
forms. The current phosphapalladacycle was noted to display
different properties with respect to the analogous phos-
phapalladacycle prepared from (diphenylmethyl)di-tert-bu-
tylphosphane. For instance, the phosphapalladacycle based
on the (diphenylmethyl)di-tert-butylphosphane was shown
to be conformationally rigid. In contrast, the analogous five-
membered phosphapalladacycle of phosphane 2 was noted
to exist in both the δ and λ conformations in the solid state,
while a flattened conformation was observed in solution. In
addition, the (diphenylmethyl)di-tert-butylphosphane-based
phosphapalladacycle was noted to exhibit a much more im-
proved regioselectivity towards ancillary ligands such as (S)-
prolinate and triphenylphosphane than 2. Furthermore, the
µ-chloro dimer 3 of the current palladacycle was shown to be
lacking in thermodynamic stability of the Pd–C bond that
was noted for the tBu analogue. In contrast, the Pd–C bonds
of dimer 3 were immediately ruptured in the presence of con-
centrated HCl to give rise to the binuclear complex 5, in
which the phosphane was coordinated in the η1-P coordina-
tion mode.
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2007)
We have previously contributed to the field of palladacy-
cle chemistry by the exploitation of chiral, nitrogen-based
palladacycles for the syntheses of optically active phospha-
nes of various functionalities.[9] To further improve the syn-
thetic efficiency of these invaluable phosphanes in terms of
enhanced chemo-, regio-, diastereo- and enantioselectivit-
ies, it is essential to develop new variants of this palladacy-
cle motif. Apart from modifying the organic framework of
the cyclopalladated ligand,[8m,9d,9e] we have explored the
possibility of replacing the original nitrogen atom with its
phosphorus and arsenic congeners.[4] The work reported
herein serves to describe the development and characteris-
tics of a new phosphorus-based palladacycle that was pre-
pared in the optically active form. The palladacycle was
constructed from the ligand (diphenylmethyl)diphenylphos-
phane and reveals features that are characteristically dif-
ferent from that of the structurally analogous (di-
phenylmethyl)di-tert-butylphosphane.[10]
Introduction
Metal complexes of phosphorus-based ligands have
played central roles in organometallic and synthetic chemis-
try.[1] In particular, the chiral forms of these metal com-
plexes have also continued to be invaluable as agents in
catalytic asymmetric syntheses.[2] Suitable forms of these li-
gands may be transformed to give rise to palladacycles.[3–5]
The latter (or cyclopalladated complexes) are a well-known
class of compounds in organometallic chemistry, and they
can be derived from ligands containing other heteroatom
donors such as nitrogen and sulfur, apart from those of
phosphorus. The syntheses and applications of many of
these cyclopalladated complexes are currently being re-
ported.[6] The properties of a ligand could be enhanced as
a result of cyclopalladation. For instance, Lewis demon-
strated that while a cyclopalladated triarylphosphite was ef-
fective as a catalyst for halogenating C=C bonds, the same
task could not be accomplished by its non-cyclopalladated
analogue.[7] Since then, the synthetic utilities of these
systems as homogeneous catalysts were further
illustrated.[3a–3j,5a–5e,8]
Results and Discussion
Synthesis of the Phosphapalladacycle in the Optically
Active Form
[a] Department of Chemistry, National University of Singapore
Kent Ridge, Singapore 119260
[b] Division of Chemistry and Biological Chemistry, Nanyang
Technological University
The preparation of the prochiral phosphane 2 was based
on the reported procedure that was applied to the synthesis
Singapore 637616
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Stereoelectronic and Chemical Properties of a Novel Phosphapalladacycle
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Scheme 1. Synthesis of compounds 2 and 3.
of its tBu analogue,[11] i.e. treatment of (diphenylmethyl)-
lithium (1) with the corresponding chlorodiphenylphos-
phane (Scheme 1), which afforded the product as a white
solid. Earlier attempts had failed to synthesize 2 by the
more common methods of halide displacement from either
(a) chlorodiphenylphosphane with a Grignard reagent or
(b) an alkyl halide by using sodium diphenylphosphanide,
which was found to be efficient for structurally analogous
phosphanes.[4] The structure of compound 2 was confirmed
by the doublet resonance of the α-CH proton in the 1H
NMR spectrum (CDCl3). This proton spin couples with the
2
31P nucleus with JP,H = 6.8 Hz. Importantly, this doublet
resonance is crucial as an indicator of the successful merg-
ing of the diphenylmethyl and diphenylphosphanyl skel-
etons.
As illustrated in Scheme 1, treatment of the phosphane 2
with palladium(II) acetate followed by chloride ion metath-
esis led to the chloro-bridged dimeric complex 3 of the pho-
sphapalladacycle. Complex 3 was thus isolated as air-stable
pale yellow blocks in 61% yield. The dimer was detected by
31P NMR spectroscopy (CDCl3) as a series of overlapping
peaks at δ = 70.4–70.8 ppm at room temperature (300 K).
However, upon cooling to 223 K, the resonance was repro-
duced as four resolved singlets at δ = 70.7, 70.8, 70.9 and
Figure 1. X-ray molecular structure of meso-3. Selected bond
lengths [Å] and bond angles [°]: Pd1–C1, 2.017(3); Pd1–P1,
2.183(1); Pd1–Cl1, 2.441(1); Pd1–Cl1A, 2.407(1); C1–Pd1–P1,
79.43(8); C1–Pd1–Cl1, 174.43(8); C1–Pd1–Cl1A, 97.87(8); P1–
Pd1–Cl1, 96.95(3); Cl1–Pd1–Cl1A, 97.87(8); Pd1–Cl1–Pd1A,
94.39(3).
71.4 ppm. This supports the chiral and racemic nature of atives 4 (Scheme 2). The presence of the two extra isomers
the phosphapalladacycle in solution, which is exemplified was a consequence of geometric (E,Z) isomerism resulting
by the presence of the six equilibrating isomers, namely, the from the nonsymmetrical nature of the (S)-prolinate. Four
chiral syn- and anti-(R,R)/(S,S) and the achiral, meso syn- singlets were therefore present in the 31P NMR spectrum
and anti-(R,S) isomers.
(CDCl3). The racemic (and therefore chiral) nature of the
In the solid state, an X-ray diffraction study of 3 revealed phosphapalladacycle was emphasized by categorizing the
that the complex was crystallized as the achiral, meso-anti four singlets into two pairs of resonances with overall equal
isomer. The molecular structure is presented in Figure 1. intensities. These were observed at δ = 62.7, 71.0 and 64.4,
The structure consists of a symmetrical plane that bisects 72.1 ppm with relative intensities of 4:9 and 3:10 corre-
the ClCl axis, each half of the dimer being the mirror image spondingly.
of the other. Moreover, the central four-membered {Pd2(µ-
The separation of the two enantiomeric forms of the
Cl)2}cycle is flat, and its four atoms form a perfect plane. phosphapalladacycle was then accomplished by the slow
The stereogenic states of the two α-C atoms are manifested fractional crystallization of the mixture of four dia-
by the attachments of four different substituents. ortho-Pal- stereomers from a solution of CH2Cl2/diethyl ether. The
ladation generates diastereotopicity on the two PPh rings. (R,S)- and (S,S)-4 complexes were obtained sequentially in
One of them is considered “axial”, since its P1–Cipso bond 59 and 77% yields. The absolute configurations of these
subtends an angle of 18.5° with the normal to the m.c.pl. derivatives were determined from their X-ray molecular
(mean coordination plane), while the value of 62.4° applies structures. These are presented in Figure 2 and Figure 3 for
to the equivalent bond of the “equatorial” PPh ring.
(R,S)-4 and (S,S)-4 [as the (Z)-isomers], respectively, while
Addition of the chiral ligand, (S)-prolinate, to the dimer the selected bond lengths and bond angles are presented in
3 gave rise to a mixture of two pairs of diastereomeric deriv- Table 1. Their absolute configurations were confirmed from
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FULL PAPER
Scheme 2. Formation of (S)-prolinate derivatives 4.
their respective Flack parameters of 0.02(3) and 0.04(4).
Like the previously reported (S)-prolinate complexes, the
secondary stereogenic nitrogen atom from the (N,O) chelate
was noted to adopt the (S) configuration in both dia-
stereomers.[9e,12] Both diastereomers are presented as the
(Z) geometrical isomer and they are noted to have similar
bond lengths and bond angles.
Figure 3.. -ray molecular structure of Z-(SC,SCSN)-4.
Table 1. Selected bond lengths [Å] and angles [°] of (Z)-(RC,SCSN)-
and (Z)-(SC,SCSN)-4.
Z-(RC,SCSN)-4
Z-(SC,SCSN)-4
Pd1–C1
Pd1–O1
Pd1–N1
Pd1–P1
P1–C7
C6–C7
C1–C6
P1–C20
2.006(4)
2.083(3)
2.118(3)
2.219(1)
1.845(4)
1.513(6)
1.408(5)
1.823(4)
1.826(5)
1.514(6)
177.1(2)
97.5(1)
Pd1–C1
Pd1–O1
Pd1–N1
Pd1–P1
P1–C7
C6–C7
C1–C6
P1–C20
2.015(4)
2.084(3)
2.095(4)
2.203(1)
1.849(5)
1.518(7)
1.406(6)
1.815(5)
1.814(5)
1.495(7)
178.9(2)
97.9(2)
81.1(1)
81.0(1)
99.9(1)
178.7(1)
103.5(2)
102.5(3)
118.6(4)
120.8(3)
114.7(4)
105.0(4)
P1–C14
C7–C8
P1–C14
C7–C8
C1–Pd1–O1
C1–Pd1–N1
O1–Pd1–N1
C1–Pd1–P1
O1–Pd1–P1
N1–Pd1–P1
C7–P1–Pd1
C6–C7–P1
C1–C6–C7
C6–C1–Pd1
C26–C27–C28
N1–C30–C29
C1–Pd1–O1
C1–Pd1–N1
O1–Pd1–N1
C1–Pd1–P1
O1–Pd1–P1
N1–Pd1–P1
C7–P1–Pd1
C6–C7–P1
C1–C6–C7
C6–C1–Pd1
C26–C27–C28
N1–C30–C29
80.5(1)
80.8(1)
101.20(8)
178.3(1)
101.1(1)
103.8(3)
117.2(4)
120.7(3)
113.2(4)
104.7(4)
solved dimers 3 had an apparent broad singlet at δ = 70.8–
70.9 ppm. The variable temperature (VT) 31P{1H} NMR
spectra of (R,R)-3, summarized in Figure 4, indicate that
Figure 2. X-ray molecular structure of Z-(RC,SCSN)-4.
The two optical antipodes of the chloro dimer 3 were the appearance of the NMR resonance signals is tempera-
then readily obtained by mixing the respective (S)-prolinate ture-dependent, as an additional singlet emerges upon cool-
derivatives 4 with 1  HCl. Thus, (R,R)-3 was isolated as ing. This resonance is shifted progressively downfield rela-
a yellowish-green amorphous powder in quantitative yield tive to the parent signal. The effect is completely reversible,
(Scheme 3), with [α]D = –382° (CH2Cl2). Notably, such acid so that warming the NMR sample causes the signal to co-
treatment did not lead to any loss of optical activity, as alesce with the latter, albeit rather weakly shifted upfield.
verified from the 31P NMR spectroscopic data of the (S)- The appearance of two sets of singlets indicates the exis-
prolinate complexes 4, obtained by a re-derivatization of tence of the syn and anti regioisomers that are possible
the optically resolved forms of 3. At room temperature within the dimeric nature of 3. The fact that other signals
(300 K), the 31P{1H} spectra (CDCl3) of the optically re- originating from the achiral, meso syn- and anti-(R,S) iso-
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Stereoelectronic and Chemical Properties of a Novel Phosphapalladacycle
FULL PAPER
mers were not observed (but were present for the unre- On the other hand, this angle was found to be 96.8° (or
solved parent dimer) points to a good degree of optical pu- 83.2° as the adjacent angle) for (R,S)-4 and as such, the
rity for the obtained (R,R)-3.
phenyl substituent at the α-C atom is considered equatori-
ally disposed. The possibility of the α-C phenyl substituent
to adopt two different orientations indicates a lack of cru-
cial intrachelate steric interactions to enforce it to perma-
nently adopt an axial orientation. The absence of such an
interaction is due to the rotational ability of this substituent
along the C7–C8 bond. Evidently, this could be supported
by the diversification of the angle between the mean planes
of this phenyl substituent and the palladated {C(1–6)}
phenylene ring by 24.6° from 99.2° in meso-3 to 74.6° in
(SC,SC)-4, despite the common axial disposition of their α-
C phenyl substituents. It should be noted from the structure
of (RC,SC)-4 that, when the substituent is equatorially dis-
posed, the mean plane of this phenyl ring is almost perpen-
dicular to that of the palladated phenylene ring, at 83.1°.
When the α-C phenyl substituent is in this rotameric state,
any unfavourable steric repulsion with the palladated ring
may thus be effectively minimized. In support of this state-
ment, the shortest distance between the two aromatic rings,
i.e. that between the H5 and H13 protons, at 3.350 Å, may
be mentioned. A separation of this magnitude for these two
atoms far exceeds the sum of van der Waals radii for two
hydrogen atoms (2.4 Å) and is considered too large to gen-
erate any kind of instability to the conformational state of
the phosphapalladacycle ring when the α-C phenyl substitu-
ent is equatorially disposed. For comparison, a close inter-
action of this kind at 2.148–2.234 Å between the α-C pro-
ton and the nearest proton of the palladated naphthylene
ring was found for the conformationally rigid ortho-pallad-
ated [1-(1Ј-naphthyl)ethyl]diphenylphosphane system.[4a] In
other words, in the current phosphapalladacycle, the pre-
vention of any unfavourable steric contacts between the two
aromatic rings may be accomplished simply by the adoption
of a suitable rotameric state for the equatorially disposed
α-C phenyl substituent or by the alternate axial orientation
of this substituent. In contrast, the solid state studies of
the tBu-based phosphapalladacycle (S)-7 revealed an axial
orientation of the α-C phenyl substituent.[10] The alternate
equatorially disposed orientation of this substituent was
prevented by the close interaction of this phenyl ring with
the tBu groups or the H5 atom of the palladated phenyl
ring. Any form of steric relief that could be brought about
Scheme 3. Regeneration of the dimer 3 in the optically active form.
Figure 4. VT 31P NMR spectra of (R,R)-3.
Substituent Effects on Phosphapalladacycle Characteristics
While the palladacycle obtained from (diphenylmethyl)- by the adoption of a certain rotameric state of this phenyl
di-tert-butylphosphane structurally resembles the current ring seems remote, since it is surrounded by three groups in
one (the two differ only in terms of the substituents on the this conformational state.
phosphorus atom), the properties of the two systems were
noticeably dissimilar. First, the replacement of the tBu2P
with the Ph2P group seems to evoke a conformational flexi-
bility on the phosphapalladacycle ring. Comparison of the
meso-3, (R,S)- and (S,S)-4 molecular structures (Figure 1,
Figure 2 and Figure 3) reveals the possibility of the ring to
adopt either one of the two conformational states. This is
because in meso-3 and (S,S)-4, the phenyl substituent at the
The lack of conformational rigidity of the current –PPh2
stereogenic α-C atom is noted to adopt the axial orienta- based phosphapalladacycle is not restricted to the solid
tion, as indicated by the respective angles of 7.0 and 9.5° state but extends to solution. This was observed in the 2D
subtended by this substituent to the normal of the m.c.pl. 1H-1H NOESY NMR spectrum (Figure 5) of the β-dike-
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J. K.-P. Ng, S. Chen, G.-K. Tan, P.-H. Leung
FULL PAPER
tonate derivative, (R)-6, which was obtained as an amorph-
ous powder, with [α]436 = –1123° (CH2Cl2), from the direct
treatment of the dimer (R,R)-3 with sodium acetylacetonate
1
in acetone. From its H NMR spectrum (CDCl3), two visi-
bly distinguished sets of ortho-, meta- and para-PPh protons
appear, as the two PPh rings are diastereotopic. Assignment
1
of the signals was made by a combination of H{31P}, 2D
1H-1H COSY and NOESY NMR spectroscopic experi-
ments. The four aromatic protons of the ortho-palladated
ring form a spin system, as observed from the COSY data.
The most upfield aromatic signal was assigned to the H5
Figure 6. Newman projections for two possible flattened palladacy-
cle conformations for complex (R)-6. Curved arrows depict NOE
proton, in recognition of its NOE contact (interaction C) contacts.
with the α-CH proton. The latter spin couples with the ad-
jacent 31P nucleus and was presented as a simple doublet
resonance (2JP,H = 13.3 Hz). A rather unexpected long-
range coupling with the 31P nucleus was detected for the
H2 proton, the corresponding coupling constant being a
rather significant 4.0 Hz. The five aromatic protons of the
α-C phenyl substituent are collectively represented as an ex-
tensively overlapping multiplet at δ = 7.00–7.05 ppm. The
1
2D H-1H NOESY NMR spectrum points to a rather flat-
tened palladacycle ring in which each PPh ring was ob- comprises the four protons (H2–H5) of the ortho-palladated
served to distinctly interact through NOE with only either phenylene ring, while the other set of signals is representa-
the methine proton or the phenyl ring at the stereogenic α- tive of the remaining five aromatic protons belonging to the
C centre (interactions A and D). Such a conformational α-C phenyl substituent. The most upfield aromatic signal
picture may be satisfied by either one of the two Newman was assigned the H5 proton on account of its NOE contact
1
representations in Figure 6.
with the α-C methine proton from the 2D H-1H NOESY
NMR spectrum (Figure 7 and Figure 8). Among the (H2–
H5) set of aromatic protons, such an NOE interaction is
only possible for this proton, given its closest proximity to
the α-CH proton. The remaining three aromatic protons of
the same set may next be unambiguously assigned by using
the H5 proton as a starting point by considering either (i)
the coupling information from the COSY data or (ii) the
negative, off-diagonal, i.e. NOE signals among these pro-
tons from the NOESY NMR spectrum.
As an added peculiarity, the spectral pattern of the five
aromatic protons of the α-C phenyl substituent corresponds
to a rather complicated ABCDE spin system, in which both
ortho protons are chemically nonequivalent on the NMR
time scale, a characteristic that is also shared by the meta
protons. The detection of an off-diagonal, opposite-phased
EXSY signal (signal I, in Figure 8) between the two ortho
1
protons in the 2-D H-1H NOESY NMR spectrum points
to the exchangeability between these two protons, which
may be provided by the rotational capability of the Cα–Ci
bond of the phosphapalladacycle, but such capability must
be rather restricted even in solution to allow a differentia-
tion and hence signal-doubling of both ortho protons (as
well as the meta protons).
Figure 5. Expanded 2D 1H-1H NOESY NMR spectrum of (R)-6
(CDCl3). Selected NOE interactions: A: α-CH–o-PPh; B: α-CH–o-
Ph of α-CPh; C: α-CH–H5; D: o-PPh–o-Ph of α-CPh; E: o-PPh–
m-PPh; F: H2–H3. Solvent signal: ᭿, CDCl3.
In contrast, the phosphapalladacycle ring of the tBu2P
In the expanded 2-D 1H-1H NOESY NMR spectrum
equivalent is conformationally rigid, on the basis of a sim- (Figure 7), NOE contacts were observed between the α-CH
ilar 2D 1H-1H NOESY NMR study performed on the anal- protons and both sets of the tBu substituents (interactions
ogous β-diketonate complex 7 (whose preparation was re- C and D). Moreover, similar interactions between the two
ported earlier[10]) containing the (S) configuration at the α- ortho protons of the α-C phenyl substituent and only one
1
C centre. The aromatic signals in its H NMR spectrum of the two available tBu substituents were noted (interac-
may be divided into two separate sets of spin patterns ac- tions A and B). Consequently, these NOE patterns, together
counting for a total of nine aromatic protons. One of these with the absence of similar NOE contacts between the α-C
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Figure 9. Sawhorse and Newman projections for the λ and δ con-
formations for complex (S)-7 (CDCl3). Curved arrows depict NOE
contacts.
Figure 7. Expanded 2D 1H-1H NOESY NMR spectrum of (S)-7
(CDCl3). Aliphatic–aromatic NOE interactions: A: equatorial tBu–
o-α-Ph; B: equatorial tBu–o-α–Ph; C: equatorial tBu–α–CH; D: ax-
ial tBu–α–CH; E: acac CH–acac Me; F: acac CH–acac Me. Solvent
signals: ᭿, H2O, o, CH2Cl2, ᮀ, CDCl3.
A mentionable observation among these aromatic signals
is the presence of the long-range 1H–31P spin-spin couplings
for the H2 and H4 protons. A rather significant coupling
constant of 3.9 Hz over four bonds was found for the for-
mer, while a coupling constant of 2.1 Hz over possibly five
bonds was detected for the latter. Such a phenomenon is
unusual, when similar couplings for the other two aromatic
protons belonging to the same phenylene ring do not exist.
A similar feature was also noted for the corresponding H2
proton in (R)-6.
The large steric bulk of the tBu groups has resulted in a
better regioselectivity with respect to the coordination of
arriving ancillary ligands to the palladacycle. This was
noted in the reaction of the chloro-bridged dimer 8 with
(S)-prolinate, in which the strong degree of regioselectivity
is evident from the product distribution of 99:1 and 98:2
(E/Z) for its (S,S) and (R,S) derivatives, correspondingly.[10]
Furthermore, compound 8 reacted with triphenylphos-
phane to give the trans isomer (Ϯ)-9 exclusively (Scheme 4).
This is far superior with respect to the current phosphapal-
ladacycle 3, which generated a mixture of cis and trans iso-
mers (Ϯ)-10 in 1.1:1 ratio. Both geometrical isomers were
readily distinguished from each other by a consideration of
2
Figure 8. Expanded 2D 1H-1H NOESY NMR spectrum of (S)-7
(CDCl3). Selected aromatic–aromatic NOE interactions: G: H2–
H3; H: o-α-Ph–m-α-Ph; J: H4–H5. EXSY signal: I: o-α-Ph–o-α-Ph.
Solvent signal: ᮀ, CDCl3.
the JP,P coupling constants. These were determined to be
29.0 Hz for cis-10 and were more pronounced at 418.1 and
390.6 Hz for trans-(Ϯ)-10 and trans-(Ϯ)-9, respectively. For
trans-(Ϯ)-9, only one set of signals was observed in its H
1
NMR spectrum expectedly and the trans-(P, P) geometry
phenyl ortho protons and the remaining tBu group (i.e. the was further inferred from the relatively high-field position
axially disposed tBu group; the other tBu group must be of the phenylene aromatic proton at δ = 6.24 ppm. The best
the equatorially disposed one) must be known in support evidence for this geometry was provided by its molecular
of the λ conformation and the rigid state of the phos- structure. The structure (with the accompanying numbering
phapalladacycle with (S) absolute configuration in solution. scheme) is presented in Figure 10. The chiral nature of the
As a comparison, this set of NOE interactions is impossible phosphapalladacycle is obvious from the stereogenic α-C
for the case of the δ conformation for the same absolute (S) atom, by the ortho-palladation of only one of the two avail-
configuration at the α-C centre, as illustrated by the by the able phenyl rings (i.e. the C1–C6 phenyl ring) on the phos-
Sawhorse and Newman projections along the P–α-C bond phane moiety. Analysis of the extent of the tetrahedral dis-
in Figure 9.
tortion on the central Pd atom reveals that it is located in
Eur. J. Inorg. Chem. 2007, 3124–3134
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3129

J. K.-P. Ng, S. Chen, G.-K. Tan, P.-H. Leung
FULL PAPER
a highly congested environment: the dihedral angle between
the {P1–Pd1–C1} and {Cl1–Pd1–P2} planes has a large
value of 23.0°. Such a large value is probably an indication
of the need to relieve steric repulsion due to the presence
of the phosphapalladacycle bulky tBu substituents and the
PPh3 phenyl rings. The overcrowding around the central Pd
atom was further demonstrated by the mean vertical dis-
placement of 0.2483 Å from planarity of the five atoms of
the m.c.pl.
Scheme 4. Formation of triphenylphosphane adducts (Ϯ)-9 and
(Ϯ)-10.
Figure 10. X-ray molecular structure of trans-(Ϯ)-9. Selected bond
lengths [Å] and bond angles [°]: Pd1–P1, 2.295(1); Pd1–C1;
2.014(2); Pd1–O1, 2.375(1); Pd1–Cl1, 2.3878(5); C1–Pd1–P1,
80.41(5); C1–Pd1–P2, 96.99(5); P1–Pd1–P2, 161.01(2); C1–Pd1–
Cl1, 167.63(5); P1–Pd1–Cl1, 97.77(2); P2–Pd1–Cl1, 88.63(2).
Notably, the current Ph2P-based phosphapalladacycle
also discloses a profound difference in the Pd–C bond reac-
tivity. To recall, the Pd–C bond of the tBu2P analogue was
noted to be only kinetically unstable and the phosphapalla-
dacycle ring opens and closes reversibly by Pd–C bond
cleavage in refluxing acetone containing concentrated uct 5 from this procedure and perform an X-ray diffraction
HCl.[10] This effect was attributed to the thermodynamic study. From Figure 11, the complex has an overall anti
stability imparted to the palladacycle by the steric influence structure that reveals the occupation of both diagonally op-
of the bulky tBu substituents. In stark contrast to its tBu2P posite terminal ends of the dimer by two additional chloride
analogue, the presence of a few drops of concentrated HCl ligands. Moreover, the η1-P type of coordination mode
in a CDCl3 solution of the chloro dimer 3 led to a perma- adopted by the phosphane ligand is apparent here. Unlike
nent ring opening of the phosphapalladacycle by cleavage the ortho-palladated structure, however, the central four-
of the Pd–C bond (Scheme 5). The transformation was im- membered {Pd2(µ-Cl)2}cycle is not planar, since each Pd
mediate, as observed from the colour change from pale yel- atom deviates from planarity by an angle of 6.45°. It is
low to orange-red. The fact that the Pd–C bond cleavage of worth mentioning that the P–Pd bond has weakened as a
the current phosphapalladacycle was permanent was af- result of palladacycle ring opening. This bond length has
firmed by the possibility to even isolate the binuclear prod- increased from 2.1832(7) Å in the ortho-palladated structure
Figure 11. X-ray molecular structure of compound 5. Selected bond lengths [Å] and bond angles [°]: Pd1–P1, 2.237(3); Pd1–Cl1, 2.260(3);
Pd1–Cl2, 2.323(3); Pd1–Cl2A, 2.414(3); P1–Pd1–Cl2, 96.5(1); P1–Pd1–Cl1, 86.9(1); P1–Pd1–Cl2A, 177.4(1); Cl1–Pd1–Cl2, 171.9(1), Cl2–
Pd1–Cl2A, 85.8(1); Cl1–Pd1–Cl2A, 90.7(1).
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Stereoelectronic and Chemical Properties of a Novel Phosphapalladacycle
FULL PAPER
solution of (diphenylmethyl)lithium was then added dropwise to an
ice-chilled thf solution (5 mL) of chlorodiphenylphosphane (0.98 g,
4.46 mmol) with rapid stirring. The addition led to an immediate
decolourization of the (diphenylmethyl)lithium reagent. When de-
colourization was no longer observed upon further addition of the
(diphenylmethyl)lithium reagent, the addition was discontinued,
the ice-bath was removed, and the mixture was allowed to stir at
room temperature for a further 30 min. It was then concentrated
to dryness, washed with freshly deoxygenated water (20 mL) and
extracted with CH2Cl2 (3ϫ50 mL). The organic extracts were com-
bined, dried with MgSO4 and filtered. The product was finally ob-
tained as a white solid upon removal of solvents by distillation and
further drying under reduced pressure. Yield: 1.54 g (98.0%). 1H
3 to 2.237(3) Å in the monodentate state. Thus, the possibil-
ity of stabilizing the P–Pd bond as a result of ortho-pallad-
ation is noteworthy. The P–C bond strengths have not var-
ied significantly, however.
Scheme 5. Permanent Pd–C bond cleavage to generate the achiral
binuclear dimer 5.
2
NMR (300 MHz, CDCl3): δ = 4.63 (d, JP,H = 6.8 Hz, 1 H,
Ph2CH), 7.03–7.26 (m, 20 H, aromatic protons) ppm. 31P{1H}
NMR (121 MHz, CDCl3): δ = –3.2 (s) ppm.
Di-µ-chlorobis{[(diphenylphosphanyl)(phenyl)methyl]phenyl-C2,P}-
dipalladium(II) (3): The phosphane 2 (1.57 g, 4.46 mmol) and palla-
dium(II) acetate (1.00 g, 4.46 mmol) were suspended in toluene
Conclusions
The novel palladacycle based on the ligand (di-
phenylmethyl)diphenylphosphane was prepared in the op- (100 mL), and the reacting mixture was stirred at 50 °C for 2 h to
give a deep reddish brown mixture with deposition of palladium
black. The mixture was concentrated to dryness by using a rotary
evaporator to yield a crude reddish-black residue. 31P{1H} NMR
(121 MHz, CDCl3): δ = 68.4 (s) ppm. The residue was then sus-
pended in acetone (15 mL), to which a solution of lithium chloride
(0.50 g, 11.8 mmol) in acetone/methanol (1:3, v/v) was added. The
mixture was then stirred for 90 min, concentrated to dryness and
re-suspended in CH2Cl2. This suspension was filtered through a
plug of Celite to yield an orange solution, which was further
washed with water (40 mL). The organic extract was concentrated
tically active forms by optical resolution of its (S)-prolinate
derivatives. This palladacycle was shown to display proper-
ties which were different from those of its bulkier analogue,
the (diphenylmethyl)di-tert-butylphosphane-based pallada-
cycle. First, the five-membered ring of the current pallada-
cycle assumes both available conformations in solution and
in the solid state. From solid-state X-ray structure studies,
the conformational flexibility of this system was directly at-
tributed to the absence of intrachelate interactions in either
the λ or the δ conformations. Second, the bulkiness of the and chromatographed on a silica gel column from which the chloro
dimer was eluted as a pale yellowish-green fraction by using
CH2Cl2–hexanes (1:2, v/v). Crystallization from CH2Cl2/diethyl
ether yielded pale yellow blocks; m.p. (decomp.) 232–236 °C. Yield:
1.36 g (61.3 %). C50H40Cl2P2Pd2 (986.52): calcd. C 60.9, H 4.1;
found C 60.6, H 4.1. 1H NMR (300 MHz, CDCl3): δ = 5.27 (d,
2JP,H = 13.6 Hz, 1 H, α-CH), 6.98–8.11 (br. overlapping m, 19 H,
aromatic protons) ppm. 31P{1H} NMR (121 MHz, CDCl3): δ =
70.4, 70.6, 70.8 (br. overlapping s) ppm; (223 K) four sets of signals
at δ = 70.7 (s), 70.8 (s), 70.9 (s) and 71.4 (s) ppm.
tBu groups also brings about a more improved regioselec-
tivity with respect to the coordination of ancillary tri-
phenylphosphane and (S)-prolinate ligands for the latter
palladacycle. Third, in the absence of such steric effects
originating from the tBu groups, the Pd–C bond of the for-
mer does not possess the type of thermodynamic stability
that was observed for the latter. Currently, both pallada-
cycles are being tested in various asymmetric applications.
sym-Dichlorodi-µ-chlorobis[(diphenylmethyl)diphenylphosphane]di-
palladium(II) (5): To a CH2Cl2 solution (3 mL) of the racemic com-
plex 3 (0.11 g, 0.12 mmol) was added an excess of conc. HCl
(30 µL, 37% wt/wt, spec. gravity 1.19) with a micropipette. The
mixture was allowed to stir vigorously for 1 h at room temperature,
after which it was diluted with water (5 mL) and CH2Cl2 (20 mL).
The orange-red organic layer was separated and was washed with
water (2ϫ5 mL) and dried with MgSO4. The excess CH2Cl2 was
removed, from which the product was obtained as fine orange-red
crystals from CH2Cl2/hexanes exhibiting low solubility; m.p. (de-
comp.) 231–233 °C. Yield: 0.088 g (72.7 %). C50H42Cl4P2Pd2
(1059.44): calcd. C 56.7, H 4.0; found C 56.3, H 4.3. 1H NMR
(300 MHz, CDCl3): δ = 6.35 (d, 2JP,H = 15.6 Hz, 1 H, α-CH), 7.19–
7.48 (m, 20 H, aromatic protons) ppm. 31P{1H} NMR (121 MHz,
CDCl3): δ = 36.8 (s) ppm.
Experimental Section
General: Reactions involving air-sensitive compounds were per-
formed under a positive pressure of argon. Routine 1H NMR spec-
tra were recorded at 300 MHz or 500 MHz with a Bruker ACF 300
or Bruker AMX 500 NMR spectrometer. All the 31P{1H} NMR
spectra were recorded at 120 MHz or 202 MHz with a Bruker ACF
300 or Bruker AMX 500 NMR spectrometer. Unless stated other-
wise, all NMR spectroscopic experiments were performed at room
temperature (300 K). Melting points were determined with a Büchi
melting point B-545 apparatus and are uncorrected. Optical rota-
tions were measured on the specified solution in 1-dm or 0.1-dm
cells at 25 °C with a Perkin–Elmer Model 341 polarimeter. Elemen-
tal analyses were performed by the Elemental Analysis Laboratory
of the Department of Chemistry at the National University of Sin-
gapore.
Chloro{[(؎)-(diphenylphosphanyl)(phenyl)methyl]phenyl-C2,P}-
(triphenylphosphane)palladium(II) [(؎)-10]: Triphenylphosphane
(0.012 g, 0.045 mmol) was added to a CH2Cl2 solution (3 mL) of
the dimer 3 (0.022 g, 0.022 mmol), and the mixture was stirred for
30 min, after which the pale yellow solution was concentrated in
vacuo, and the product was obtained as pale yellow crystals in
CH2Cl2/hexanes; m.p. 200–202 °C. Yield: 0.020 g (58.8 %).
(Diphenylmethyl)diphenylphosphane (2):
A
solution of (di-
phenylmethyl)lithium was prepared by syringing n-BuLi in hexanes
(3.7 mL, 5.9 mmol, 1.6 ) into a thf solution (5 mL) of di-
phenylmethane (1.00 g, 5.94 mmol) and allowing the mixture to stir
at room temperature for 3 h. The freshly prepared, intensely red
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J. K.-P. Ng, S. Chen, G.-K. Tan, P.-H. Leung
FULL PAPER
C43H35ClP2Pd (755.55): calcd. C 68.4, H 4.7; found C 68.5, H 4.4.
blocks with 97.1% de (from 31P{1H} NMR spectroscopic data),
which gradually desolvated upon standing at room temperature;
m.p. 191–193 °C. [α]D = –109°, [α]578 = –114°, [α]546 = –131°, [α]436
= –242°, [α]365 = –397° (c 1.1, CH2Cl2). Yield: 0.34 g (59.0%, based
on the theoretical yield of one diastereomer). C30H28NO2PPd
1H NMR (300 MHz, CDCl3) two sets of signals in 1:0.87 ratio for
both geometric isomers in favour of the cis isomer, δ = 5.16 (br. d,
2
2JP,H = 13.7 Hz, 1 H, α- CH of trans isomer), 5.31 (d, JP, H
=
13.3 Hz, 1 H, α-CH of cis isomer), 6.32–8.61 (aromatic protons of
both isomers) ppm. 31P{1H} NMR (202 MHz, CDCl3) two sets of (571.93): calcd. C 63.0, H 4.9, N 2.5; found C 63.0, H 4.8, N 2.6.
signals in 1:0.87 ratio for both geometric isomers in favour of the
1H NMR (CDCl3, 500 MHz) two sets of signals in 1:0.28 ratio in
favour of the E-isomer, δ = 1.41 (m, 1 H, γ-H of E-isomer), 1.77–
1.87 (overlapping m, γ-H of both isomers), 2.04–2.14 (overlapping
m, γ-H of Z-isomer, β-H of E-isomer), 2.19–2.39 (overlapping m,
β-H of Z-isomer, β-H and N-H of E-isomer), 2.45–2.56 (overlap-
ping m, β-H of Z-isomer, δ-H of E-isomer), 2.80 (m, 1 H, δ-H of
E-isomer), 3.36 (m, 1 H, δ-H of Z-isomer), 3.56 (br. overlapping
m, 2 H, δ-H and N-H of Z-isomer), 3.93 (m, 1 H, prolinate α-H
of E-isomer), 4.21 (m, 1 H, prolinate α-H of Z-isomer), 5.20 (d,
2
cis isomer, δ = 18.1 (d, JP,P = 29.0 Hz, PPh3 of cis diastereomer),
26.6 (d, 2JP,P = 418.1 Hz, PPh3 of trans diastereomer), 58.6 (d, 2JP,P
2
= 418.1 Hz, palladacycle P of trans diastereomer), 71.0 (d, JP,P
29.0 Hz, palladacycle P of cis diastereomer) ppm.
=
[(S)-Prolinato-N,O]{(؎)-1-[(diphenylphosphanyl)(phenyl)methyl]-
phenyl-C2,P}palladium(II) (4): A methanol solution (9 mL) of po-
tassium (S)-prolinate (0.312 g, 0.680 mmol) was added to the dimer
3 (1.00 g, 1.02 mmol) suspended in CH2Cl2 (10 mL), and the mix-
ture was stirred for 1 h at room temperature. The resulting white
suspension was concentrated to dryness, re-suspended in CH2Cl2
(8 mL), washed with water (2ϫ10 mL) and dried with MgSO4. The
colourless solution was then concentrated to dryness by using a
rotary evaporator and was further dried at 70 °C in vacuo for 3 h
2
2JP,H = 13.0 Hz, 1 H, palladacycle α-H of Z-isomer), 5.46 (d, JP,H
3
= 12.5 Hz, 1 H, palladacycle α-H of E-isomer), 6.70 (d, JH,H
=
7.7 Hz, 2 H, o-α-Ph of E-isomer), 6.73 (d, 3JH,H = 7.4 Hz, 1 H, 5-H
of E-geometrical isomer), 6.92–7.46 (m, aromatic protons of both
geometric isomers), 7.52–7.58 (overlapping m, aromatic protons of
both geometric isomers), 7.73 (ddd, 3JH,H = 8.5Hz, 4JH,H = 1.6 Hz,
to yield a flaky white solid; m.p. (decomp.) 225–227 °C. [α]D
=
3
3JP,H = 11.1 Hz, 2 H, o-PPh of Z-isomer), 7.81 (ddd, JH,H
=
+107°, [α]578 = +114°, [α]546 = +129°, [α]436 = +263°, [α]365 = +636°
(c 1.1, CH2Cl2). Yield: 1.15 g (99.1%). C30H28NO2PPd (571.93):
4
3
8.0 Hz, JH,H = 1.6 Hz, JP,H = 11.0 Hz, 2 H, o-PPh of E-isomer),
3
4
8.13 (ddd, JH,H = 7.7 Hz, JH,H = 1.1 Hz, JP,H = 4.0 Hz, 1 H, 2-
H of E-isomer) ppm. 31P{1H} NMR (CDCl3, 202 MHz) two sig-
nals with relative intensities of 2.8:10, δ = 64.5 (s, Z-isomer), 72.2
(s, E-isomer) ppm.
1
calcd. C 63.0, H 4.9, N 2.5; found C 62.5, H 5.1, N 2.7. H NMR
(CDCl3, 500 MHz) four sets of signals in the ratio of 0.4:0.3:0.9:1.0,
δ = 1.32–1.44 [overlapping m, γ-H of E-(R,S)- and E-(S,S)-dia-
stereomers], 1.75 [m, γ-H of E-(S,S)-diastereomer], 1.81–1.86 [over-
lapping m, γ-H of Z-(S,S)-diastereomer, γ-H of E- and Z-(R,S)-
diastereomers], 2.02–2.41 [series of m, γ-H of Z-(S,S)-and Z-(R,S)-
diastereomers, both β-H of E-(R,S)-diastereomers, both β-Hs of E-
(S,S)-diastereomers, β-H of Z-(R,S)-diastereomer, both β-H of Z-
(S,S)-diastereomers, δ-H of E-(S,S)-diastereomer, N-H of E-(R,S)-
diastereomer], 2.44–2.61 [series of m, β-H of Z-(R,S)-diastereomer,
δ-H of E-(R,S)-diastereomer, δ-H of E-(S,S)-diastereomer, N-H of
E-(S,S)-diastereomer], 2.80 [m, δ-H of E-(R,S)-diastereomer], 3.35
[m, δ-H of Z-(R,S)-diastereomer], 3.41–3.51 [overlapping m, both
δ-Hs of Z-(S,S)-diastereomer], 3.55 [δ-H of Z-(R,S)-diastereomer],
3.61–3.69 [overlapping m, N-H of both Z-(R,S)- and Z-(S,S)-dia-
stereomers], 3.92 [m, α-H of E-(R,S)-diastereomer], 3.99 [m, α-H
of E-(S,S)-diastereomer], 4.14 [m, α-H of Z-(S,S)-diastereomer],
Isolation of (SC,SCSN)-(Prolinato-N,O){[(diphenylphosphanyl)-
(phenyl)methyl]phenyl-C2,P}palladium(II) [(SC,SCSN)-4]: Subse-
quent slow evaporation of the remaining mother liquor above af-
forded the more soluble diastereomer (SC,SCSN)-4 as large colour-
less blocks with 91.0% de (from 31P{1H} NMR spectroscopic data);
m.p. (decomp.) 272–274 °C. [α]D = +303°, [α]578 = +318°, [α]546
=
+369°, [α]436 = +728°, [α]365 = +1580° (c 1.1, CH2Cl2). Yield: 0.45 g
(76.8 %, based on the theoretical yield of one diastereomer).
C30H28NO2PPd (571.93): calcd. C 63.0, H 4.9, N 2.5; found C 63.2,
1
H 4.7, N 2.6. H NMR (CDCl3, 500 MHz) two sets of signals in
1: 0.43 ratio in favour of the E-isomer, δ = 1.34 (m, 1 H, γ-H of E-
isomer), 1.75 (m, 1 H, γ-H of E-isomer), 1.85 (m, 1 H, γ-H of Z-
isomer), 2.08–2.13 (overlapping m, γ-H of Z-isomer, β-H and δ-H
of E-isomer), 2.23 (m, 1 H, β-H of E-isomer), 2.38 (overlapping m,
2 H, both β-H of Z-isomer), 2.50–2.61 (overlapping m, 2 H, δ-H
and N-H of E-isomer), 3.43–3.52 (overlapping m, 2 H, both δ-H
protons of Z- isomer), 3.65 (br. m, 1 H, N-H of Z-isomer), 4.00
(m, 1 H, prolinate α-H of E-isomer), 4.14 (m, 1 H, prolinate α-H
2
4.22 [m, α-H of Z-(R,S)-diastereomer], 5.08 [d, JP,H = 13.9 Hz,
palladacycle α-H of Z-(S,S)-diastereomer], 5.20 [d, 2JP,H = 13.0 Hz,
palladacycle α-H of Z-(R,S)-diastereomer], 5.40 [d, 2JP,H = 12.5 Hz,
palladacycle α-H of E-(S,S)-diastereomer], 5.49 [d, 2JP,H = 12.5 Hz,
2
palladacycle α-H of E-(R,S)-diastereomer], 6.71 [d, JH,H
=
2
7.49 Hz, o-α-Ph of E-(R,S)-diastereomer], 6.74 [d, 2JH,H = 7.77 Hz,
2-H of E-(R,S)-diastereomer], 6.78 [m, o-α-Ph of E-(S,S)-dia-
stereomer], 6.85 [d, 2JH,H = 6.94 Hz, 5-H of E-(S,S)-diastereomer],
6.93–7.58 [series of m, aromatic protons of all four diastereomers],
7.72–7.85 [partially overlapping m, o-PPh of Z-(R,S)-, o-PPh of
E-(R,S)-and o-PPh of E-(S,S)-diastereomers], 8.10–8.15 [partially
overlapping m, 2-H proton of E-(R,S)- and E-(S,S)-diastereomers]
ppm. 31P{1H} NMR (CDCl3, 202 MHz) four signals in relative in-
tensities of 4:3:9:10, δ = 62.7 (s), 64.4 (s), 71.0 (s) and 72.1 (s) ppm
respectively.
of Z-isomer), 5.08 (d, JP,H = 13.8 Hz, 1 H, palladacycle α-H of Z-
2
isomer), 5.40 (d, JP,H = 12.7 Hz, 1 H, palladacycle α-H of E-iso-
3
mer), 6.78 (m, 2 H, o-α-Ph), 6.85 (d, JH,H = 7.3 Hz, 1 H, 5-H
of E-isomer), 7.00–7.58 (overlapping m, aromatic protons of both
geometric isomers), 7.78 (ddd, 3JH,H = 8.1 Hz, 4JH,H = 1.4 Hz, 3JP,H
3
= 10.9 Hz, 2 H, o-PPh of E-geometrical isomer), 8.10 (ddd, JH,H
= 7.8 Hz, 4JH,H = 1.4 Hz, JP,H = 4.1 Hz, 1 H, 2-H of E-geometrical
isomer) ppm. 31P{1H} NMR (CDCl3, 202 MHz) two sets of signals
with relative intensities of 4.3:10 in favour of the E-isomer, δ = 62.7
(s, Z-geometrical isomer), 71.1(s, E-geometrical isomer) ppm.
(R,R)-di-µ-Chlorobis{[(diphenylphosphanyl)(phenyl)methyl]phenyl-
C2,P}dipalladium(II) [(R,R)-3]: Dilute HCl (10 mL, 1 ) was added
to a CH2Cl2 solution (10 mL) of the diastereomer (RC,SCSN)-4
(0.2347 g, 0.410 mmol), and the resulting solution was vigorously
stirred for 5 min at room temperature. The aqueous layer was sepa-
rated, and the procedure was repeated once with the same amount
of HCl. The organic layer was then removed, washed with water
(2ϫ10 mL), dried with MgSO4 and the solvents evaporated to dry-
Separation of (RC,SCSN) and (SC,SCSN) Diastereomers
Isolation of (RC,SCSN)-(Prolinato-N,O){[(diphenylphosphanyl)-
(phenyl)methyl]phenyl-C2,P}palladium(II) [(RC,SCSN)-4]: Diethyl
ether was slowly added at regular intervals to a moderately concen-
trated CH2Cl2 solution of the above 1:1 mixture of both dia-
stereomers (1.168 g, 2.04 mmol), from which the less soluble dia-
stereomer (RC,SCSN)-4 was isolated after two days as colourless
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Stereoelectronic and Chemical Properties of a Novel Phosphapalladacycle
FULL PAPER
4
4
3
ness in vacuo to afford the (R,R)-3 as a greenish-yellow powder
(attempts to crystallize the product by using a variety of solvents
have been unsuccessful thus far); m.p. (decomp.) 196–200 °C. [α]D
= –382°, [α]578 = –400°, [α]546 = –468°, [α]436 = –946° (c 0.5,
CH2Cl2). Yield: 0.20 g (99.0%). C50H40Cl2P2Pd2 (986.52): calcd. C
8.0 Hz, JH,H = 1.8 Hz, JH,H = 1.3 Hz, JP,H = 11.2 Hz, 2 H, o-
3
4
PPh), 8.02 (ddd, JH,H = 7.8 Hz, JH,H = 1.3 Hz, JP,H = 4.0 Hz, 1
H, 2-H) ppm. 31P{1H} NMR (CDCl3, 202 MHz): δ = 66.4 (s) ppm.
Chloro{(؎)-1-[(di-tert-butylphosphanyl)(phenyl)methyl]phenyl-
C2,P}(triphenylphosphane)palladium(II) [(؎)-9]: Triphenylphos-
phane (0.069 g, 0.262 mmol) was added to a CH2Cl2 solution of
the corresponding chloro dimer 3 (0.108 g, 0.119 mmol), and the
resulting mixture was allowed to stir for 16 h under a positive pres-
sure of argon. The product was isolated as pale yellow blocks from
CH2Cl2/hexanes upon slow crystallization; m.p. 216–217 °C. Yield:
0.145 g (85.3 %). C39H43ClP2Pd (715.57): calcd. C 65.5, H 6.1;
found C 65.4, H 6.1. 1H NMR (CDCl3, 500 MHz): δ = 1.21 (d,
1
60.9, H 4.1; found C 61.2, H 4.0. H NMR (CDCl3, 300 MHz): δ
2
= 5.22 (d, JP,H = 13.7 Hz, 1 H, α-CH), 6.89–8.03 (br. overlapping
m, 19 H, aromatic protons) ppm. 31P{1H} NMR (121 MHz,
CDCl3): δ = 70.8 (s), (253 K) 2 sets of signals with relative inten-
sities of 1: 0.65, δ = 70.8 (s), 71.1(s) ppm.
(S,S)-di-µ-Chlorobis{[(diphenylphosphanyl)(phenyl)methyl]phenyl-
C2,P}dipalladium(II) [(S,S)-3]: Dilute HCl (7 mL, 1 ) was added
to a CH2Cl2 solution (10 mL) of the (SC, SCSN)-4 (0.1642 g,
0.287 mmol), and the resulting solution was vigorously stirred for
5 min at room temperature. The aqueous layer was separated, and
the procedure was repeated once with the same amount of HCl.
The organic layer was next removed, washed with water
(2ϫ10 mL), dried with MgSO4 and the solvents evaporated to dry-
ness in vacuo to afford the (S,S)-3 as a greenish-yellow powder;
3JP,H = 12.8 Hz, 9 H, axial tBu), 1.40 (d, JP,H = 13.1 Hz, 9 H,
3
2
4
equatorial tBu), 4.54 (dd, JP,H = 11.7 Hz, JP,H = 5.1 Hz, 1 H, α-
CH), 6.24 (m, 1 H, 2-H), 6.64–6.71 (m, 2 H, 3-H, 4-H), 6.80 (dd,
3JH,H = 7.6 Hz, JH,H = 1.4 Hz, 1 H, 5-H), 6.94 (br. dd, JH,H
=
4
3
3JH,H = 7.5 Hz, 1 H, m-α-Ph), 7.17 (m, 1 H, p-α-Ph), 7.24–7.40 (m,
m-PPh3, p-PPh3, o-α-Ph, m-α-Ph, signals of free PPh3), 7.70 (m, 6
H, o-PPh3), 8.46 (br. d, 3JH,H = 7.8 Hz, 1 H, o-α-Ph) ppm. 31P{1H}
m.p. (decomp.) 198–202 °C. [α]D = +368°, [α]578 = +384°, [α]546
=
2
NMR (202 MHz, CDCl3): δ = –5.0 (s, free PPh3), 24.8 (d, JP,P
=
+446°, [α]436 = +908° (c 0.5, CH2Cl2). Yield: 0.141 g (99.5 %).
390.6 Hz, coordinated PPh3), 100.5 (d, 2JP,P = 390.6 Hz, palladacy-
cle P), four overlapping sets of signals at 111.8, 111.9, 112.1, 112.2
(s, signals of racemic dimer) ppm.
C50H40Cl2P2Pd2 (986.52): calcd. C 60.9, H 4.1; found C 60.9, H
1
2
3.9. H NMR (CDCl3, 300 MHz): δ = 5.22 (d, JP,H = 13.7 Hz, 1
H, α-CH), 6.89–8.03 (br. overlapping m, 19 H, aromatic protons)
ppm. 31P{1H} NMR (121 MHz, CDCl3): δ = 70.9 (s) ppm.
X-ray Diffraction Studies of the Complexes meso-3, (Z)-(RC,SCSN)-
4, (Z)-(SC, SCSN)-4, 5 and trans-(؎)-9: Crystal data for these five
complexes and a summary of the crystallographic analyses are
given in Table 2. Diffraction data were collected on a Siemens
SMART CCD diffractometer with Mo-Kα radiation (graphite
monochromator) by using ω-scans. SADABS absorption correc-
tions were applied, and refinements by full-matrix least-squares
were based on SHELXL 93.[13] All non-hydrogen atoms were re-
fined anisotropically. Hydrogen atoms were introduced at fixed dis-
tance from carbon atoms and were assigned fixed thermal param-
eters.
(R)-(Acetylacetonato-O,OЈ){1-[(diphenylphosphanyl)(phenyl)methyl]-
phenyl-C2,P}dipalladium(II) [(R)-6]: To an acetone solution (3 mL)
of the dimer (R,R)-3 (0.139 g, 0.l41 mmol) was added Na acac·H2O
(0.040 g, 0.282 mmol), and the mixture was vigorously stirred at
room temperature for 2 h until a white suspension was obtained.
The suspension was filtered through a short plug of Celite, and the
colourless solution was concentrated to dryness in vacuo to yield
a white amorphous powder; m.p. 86–89 °C. [α]436 = –1123°, [α]365
= –2397° (c 0.7, CH2Cl2). Yield: 0.089 g (56.7%). C30H27O2PPd
(556.92): calcd. C 64.7, H 4.9; found C 64.9, H 4.8. 1H NMR
(CDCl3, 500 MHz): δ = 1.84 (s, 3 H, acac-Me), 2.15 (s, 3 H, acac-
2
Me), 5.22 (d, JP,H = 13.3 Hz, 1 H, α-CH), 5.39 (s, 1 H, acac-CH),
CCDC-634583 (for meso-3), -634584 [for (Z)-(RC,SCSN)-4],
6.96 (dd, JH,H = 7.6 Hz, JH,H = 1.42 Hz, 1 H, 5-H), 7.00–7.05 -634585 [for (Z)-(SC,SCSN)-4], -634586 (for 5), and -634587 [for
3
4
(m, 6 H, 4-H, o-Ph, m-Ph, p-Ph protons of α-C-Ph substituent),
trans-(Ϯ)-9] contain the supplementary crystallographic data for
7.09 (m, 1 H, 3-H), 7.13 (m, 2 H, m-PPh), 7.29 (dtt, 3JH,H = 7.4 Hz, this paper. These data can be obtained free of charge from The
5
4JH,H = 1.29Hz, JP,H = 2.1 Hz, 1 H, p-PPh), 7.45 (ddd, 3J(H,H) =
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
3
3
8.5, 4JH,H = 1.3, JP,H = 11.5 Hz, 2 H, o-PPh), 7.74 (dddd, JH,H
=
Table 2. Crystallographic data for complexes meso-3, Z-(RC,SCSN)-4, Z-(SC,SCSN)-4, 5 and trans-(Ϯ)-9.
meso-3
Z-(RC,SCSN)-4
Z-(SC,SCSN)-4
5
trans-(Ϯ)-9
Formula
M
Space group
Crystal system
a [Å]
b [Å]
c [Å]
C50H40Cl2P2Pd2
986.46
I41/a
Tetragonal
29.912(2)
29.912(2)
10.5243(14)
90
C30H28NO2PPd
571.90
P21
Monoclinic
9.866(2)
9.1675(18)
14.831(3)
90
C30H28NO2PPd
571.90
C50H42Cl4P2Pd2
1059.38
C2/c
Monoclinic
10.4912(14)
16.715(2)
25.710(3)
90
C39H43ClP2Pd
715.52
P21/n
Monoclinic
12.4345(5)
15.4637(6)
19.4561(8)
90
P212121
Orthorhombic
8.7813(7)
9.6673(7)
31.100(3)
90
α [°]
β [°]
90
90
107.856(4)
90
1276.9(5)
2
90
90
2640.1(4)
4
93.834(4)
90
4498.5(11)
4
106.7020(10)
90
3583.3(2)
4
γ [°]
V [Å3]
9416.6(16)
8
Z
T [K]
λ [Å]
223(2)
0.71073
0.977
0.0370
0.0983
–
293(2)
0.71073
0.817
0.0382
0.0876
223(2)
0.71073
0.790
0.0519
0.1029
295(2)
0.71073
1.143
0.0810
0.1541
295(2)
0.71073
0.707
0.0360
0.0853
µ [mm–1]
R1 (obs. data)
wR2 (obs. data)
Flack parameter
0.02(3)
0.04(4)
–
–
Eur. J. Inorg. Chem. 2007, 3124–3134
© 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
3133

J. K.-P. Ng, S. Chen, G.-K. Tan, P.-H. Leung
FULL PAPER
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Acknowledgments
We thank the National University of Singapore for the financial
support of this research and the award of a research scholarship to
J. K.-P. N. We also thank the Nanyang Technological University
for the research scholarship to S. C.
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Received: February 5, 2007
Published Online: May 10, 2007
3134
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Eur. J. Inorg. Chem. 2007, 3124–3134