A new type of chiral porphyrin: Organopalladium porphyrins with chiral chelating diphosphine ligands

Article abstract of DOI:10.1016/j.jorganchem.2005.10.014

Peripherally palladated Ni(II) porphyrins have been prepared using enantiopure chiral chelating diphosphines as supporting ligands on the attached Pd(II) fragment. Both enantiomers of the following complexes have been obtained in good yields, using oxidative addition of the bromoporphyrin starting material 5-bromo-10,20-diphenylporphyrinatonickel(II) (NiDPPBr (1)) to the [Pd⁰L] complex generated in situ from Pd₂dba₃ and the chiral ligand L: [PdBr(NiDPP)(CHIRAPHOS)] (2a,b) [CHIRAPHOS = 2,3-bis(diphenylphosphino)butane], [PdBr(NiDPP)(Tol-BINAP)] (3a,b) [Tol-BINAP) = 2,2′-bis(di-p-tolylphosphino)-1,1′-binaphthyl] and [PdBr(NiDPP)(diphos)] [diphos = 1,2-bis(methylphenylphosphino)benzene] (4a,b). The induced asymmetry in the porphyrin was readily detected by ¹H NMR and CD spectroscopy. The porphyrin chiroptical properties are strongly dependent upon the structure of the chiral ligand, such that a monosignate CD signal, and symmetric and asymmetric exciton couplets were observed for 4a, 2b, and 3a,b, respectively.

Full text of DOI:10.1016/j.jorganchem.2005.10.014

Journal of Organometallic Chemistry 691 (2006) 2162–2170
www.elsevier.com/locate/jorganchem
A new type of chiral porphyrin: Organopalladium porphyrins
with chiral chelating diphosphine ligands
a
b
b
a,
*
Margaret J. Hodgson , Victor V. Borovkov , Yoshihisa Inoue , Dennis P. Arnold
a
Synthesis and Molecular Recognition Program, School of Physical and Chemical Sciences,
Queensland University of Technology, G.P.O. Box 2434, Brisbane 4001, Australia
Entropy Control Project, ICORP, JST, 4-6-3 Kamishinden, Toyonaka-shi, Osaka 560-0085, Japan
b
Received 15 August 2005; received in revised form 4 October 2005; accepted 5 October 2005
Available online 11 November 2005
Abstract
Peripherally palladated Ni(II) porphyrins have been prepared using enantiopure chiral chelating diphosphines as supporting ligands
on the attached Pd(II) fragment. Both enantiomers of the following complexes have been obtained in good yields, using oxidative
addition of the bromoporphyrin starting material 5-bromo-10,20-diphenylporphyrinatonickel(II) (NiDPPBr (1)) to the [Pd0L] complex
generated in situ from Pd2dba3 and the chiral ligand L: [PdBr(NiDPP)(CHIRAPHOS)] (2a,b) [CHIRAPHOS = 2,3-bis(diphenylphosph-
ino)butane], [PdBr(NiDPP)(Tol-BINAP)] (3a,b) [Tol-BINAP) = 2,20-bis(di-p-tolylphosphino)-1,10-binaphthyl] and [PdBr(NiDPP)-
(diphos)] [diphos = 1,2-bis(methylphenylphosphino)benzene] (4a,b). The induced asymmetry in the porphyrin was readily detected by
1H NMR and CD spectroscopy. The porphyrin chiroptical properties are strongly dependent upon the structure of the chiral ligand,
such that a monosignate CD signal, and symmetric and asymmetric exciton couplets were observed for 4a, 2b, and 3a,b, respectively.
Ó 2005 Elsevier B.V. All rights reserved.
Keywords: Organopalladium porphyrin; Chiral porphyrin; Chiral diphosphine; Circular dichroism
1. Introduction
porphyrins. In some cases, these groups are designed to
offer additional supramolecular coordination properties
to increase the number of contact points between the por-
phyrin-based receptor and the substrate [2]. In addition,
intrinsically achiral bis(porphyrins) have been used as chi-
ral reporter groups due to the induction of chirality in the
porphyrin chromophore by chiral hosts. These supramolec-
ular systems have been effectively applied for the determi-
nation of the absolute configuration of various chiral
compounds [12,13], selective sensing of saccharides, cys-
teine polyion, and mandelic acid [14–16], and chiral mem-
ory elements [17,18].
Chiral chelating diphosphine ligands have assumed
importance in enantioselective reactions catalyzed by tran-
sition metal complexes, such as those involving palladium
[19]. We have tried to combine porphyrin-based chiral
receptors and potential transition metal catalytic sites,
taking advantage of the fact that enantiopure chiral
diphosphine ligands are now commercially available.
Porphyrins have fascinated scientists for many years due
to their unique roles in biology. In recent years, a vast
array of novel porphyrinoid structures has been prepared
and investigated for both fundamental scientific and
applied technological reasons [1]. The introduction of chi-
rality into porphyrin structures may lead to major applica-
tions in chiral recognition [2–8] and enantioselective
catalysis [9–11], using the coordinating ability and catalytic
propensity of the metal ion coordinated centrally within
the chiral environment. Most of these chiral porphyrins
have been prepared using chiral substituent groups
attached to peripheral meso and/or b carbons (Scheme 1)
of the ring, particularly using, for ease of synthesis, chiral
attachments to meso-aryl groups in 5,10,15,20-tetraaryl-
*
Corresponding author.
E-mail address: d.arnold@qut.edu.au (D.P. Arnold).
0022-328X/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2005.10.014

M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
2163
active transition metal site near the periphery may lead to
new unique enantioselective processes and functions. We
now report our initial studies of the incorporation of chiral
diphosphine ligands into organopalladium(II) porphyrins.
This work has been carried out, as proof of concept, with
the Ni(II) complex of 5,15-diphenylporphyrin (NiDPP) as
the porphyrin core. Measurement of the circular dichroism
(CD) spectra has revealed chiral induction in the porphyrin
unit by the bulky chiral ligands. This paper reports the syn-
thesis and characterization, particularly by NMR and CD
spectroscopy, of three pairs of enantiomeric complexes,
using three different classes of chiral diphosphine
(Scheme 2).
Ar
M
β
N
N
N
N
meso
M'X(L )
2
Ar
Scheme 1. The general structure of peripherally metallated porphyrins
(M0 = Pd, Pt).
‘‘Organometallic porphyrins’’ of various structural types
have been known for many years [20], but since 1998, we
have been studying a previously neglected class, namely
peripherally palladated and platinated porphyrins, of the
type represented by the general structure shown in Scheme
1, in which the metallo substituent is attached at one (or
two) meso positions. We have prepared numerous exam-
ples in which we varied the lateral aryl substituents
(Ar = phenyl, p-tolyl, 3,5-di-tert-butylphenyl) on the
10,20 porphyrin carbons, the centrally coordinated metal
ion (M = 2H, MnCl, Co, Ni, Zn, InCl), the neutral ligands
(L2 = monodentate aryl or alkyl phosphines, chelating
diphosphine, chelating diamine), and the external ligand
(X = Br, Cl, NO3, CF3SO3, pyridines, pyridyl porphyrins).
Syntheses, spectra, crystal structures, electrochemistry and
uses as synthetic intermediates have been described [21–28].
For chelating diphosphines, only the Pd(II) complexes have
been prepared [21,22,27], while for monodentate phos-
phines and chelating diamines, both Pd(II) and Pt(II) sys-
tems are known [23–25]. Palladium catalysis has recently
been used to prepare chiral 5,15-disubstituted derivatives
of diarylporphyrins, but the ligands on the palladium were
not themselves chiral [29]. It is expected that the combina-
tion of a chiral porphyrin receptor with a catalytically
2. Results and discussion
2.1. Synthesis
The suite of new chiral derivatives 2–4 was prepared
according to the methods we have used previously for achi-
ral diphosphine ligands, namely oxidative addition of
meso-bromoporphyrin (1) to Pd(0) diphosphine precursors
[21,22,27]. The latter were prepared in situ by reaction of
the diphosphine ligand with Pd2dba3 (dba = dibenzylide-
neacetone) in hot toluene under argon atmosphere (Scheme
2). The enantiomeric pairs of three types of chiral diphos-
phine (Scheme 2) were employed, in order to explore the
influences of the different types on the NMR and CD spec-
tra of the porphyrins. The first, represented by CHIRA-
PHOS [2,3-bis(diphenylphosphino)butane], is chiral by
virtue of the two chiral carbons in the backbone. The sec-
ond is Tol-BINAP [2,20-bis(di-p-tolylphosphino)-1,10-
binaphthyl], which is chiral due to atropisomerism about
the binaphthyl bond. The final one [abbreviated here
as ‘‘diphos’’, 1,2-bis(methylphenylphosphino)benzene] is
Ph
Ph
8
12
2a: L = (R,R)-CHIRAPHOS
13
17
7
3
2b: L = (S,S)-CHIRAPHOS
Pd dba /L
N
N
N
N
N
N
N
N
2
3
3a: L = (R)-Tol-BINAP
3b: L = (S)-Tol-BINAP
PdBr(L)
Br
Ni
Ni
toluene, 105 ˚C
4a: L = (R,R)-diphos
4b: L = (S,S)-diphos
18
2
1
Ph
Ph
Me
Ph
Ph
Ni
Ph
Ph
Ni
P
P
P
N
N
N
N
N
N
N
N
N
N
N
Pd
Br
Pd
Br
Ni
N
Pd
Br
P
P
P
Ph
Me
Ph
Ph
Ph
2a
4a
3a
Scheme 2. Synthetic method, list of compounds, structures of the chiral complexes, [represented by the respective (R)- or (R,R)-isomers] and the
numbering of the porphyrin ring carbons.

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M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
chiral at both phosphorus atoms [30]. The reactivity of the
PdL fragments clearly decreases in the order
diphos > CHIRAPHOS > Tol-BINAP, as judged by the
rate of completion of the reaction. This is in the same order
as the expected decrease in electron density on the phos-
phorus atoms attached to the Pd(0), although it is also
the order of increasing bulkiness of the ligand. On the other
hand, the ligand that appeared to generate the most reac-
tive Pd(0) nucleophile in our previous studies, 1,10-
bis(diphenylphosphino)ferrocene (dppf), is also rather
bulky [22], so a decision on the relative influences of these
factors is not yet clear.
PHOS (58.9, 41.8), Tol-BINAP (44.2, 36.3), diphos (25.0,
11.9 ppm). The H NMR spectra are, as expected, rather
complex, and deserve more discussion, as these are the first
chiral organopalladium complexes to be isolated for
porphyrins.
1
The downfield region of the spectrum of the (R,R)-
CHIRAPHOS complex 2a (Fig. 1) gives a clear indication
of the asymmetry of the complex, by the presence of eight
separate one-proton doublets for the eight porphyrin b
protons, in addition to a singlet for the unique 15-meso
proton (Schemes 1 and 2 show the numbering and nomen-
clature of the porphyrin carbons). One of the phenyl
groups of the diphosphine ligand appears at rather high
field, in the 5–5.5 ppm range, because of its average posi-
tion in the porphyrin shielding zone. Two-dimensional
COSY and ROESY spectra were collected to try to assign
the protons completely, but while all through-bond corre-
lations were obtained, some dipolar couplings could not
be identified, including those for the porphyrin 10,20-phe-
nyl groups with their adjacent b protons. Therefore, the lat-
ter could not be assigned individually, but only pair-wise.
The methyl groups on the five-membered chelate ring
appear as doublets of doublets due to the coupling with
their partner CH and the adjacent 31P. These couplings
were defined by J-resolved spectra. The CH protons
appeared as complex multiplets. Dipolar couplings from
these backbone protons to ortho-protons of the P-phenyl
groups were observed in some, but not all, cases, leaving
some ambiguities in the assignments of the phenyl groups.
A minor impurity with similar chemical shifts was also
present; this is the corresponding chloro complex, formed
by exchange of the bromide for chloride in the chlorinated
solvent. We have observed this behaviour for other organ-
opalladium and -platinum porphyrins [21–23], and con-
firmed the origin of the impurities. Upon standing in the
solvent during the acquisitions of the 2D spectra, the char-
acteristic peaks of unsubstituted NiDPP slowly increased in
intensity, but the proportion decomposed reached only ca.
5% after 24 h.
The reactions were followed by thin layer chromatogra-
phy (TLC) until the starting bromoporphyrin 1 was con-
sumed. The Pd porphyrins decompose on the TLC plates,
forming NiDPP, so the formation of the Pd species cannot
be followed directly by TLC [27]. The diphos complexes
4a,b precipitated from hot toluene within 15 min, while
the other two ligands yielded complexes that were soluble
in toluene. The diphos complexes are rather unstable in
chlorinated solvents, decomposing to NiDPP and (presum-
ably) Pd(diphos) dihalide complexes. This decomposition
prevented a thorough study of the complexes of this ligand,
as their low solubility prevented examination in other sol-
vents. The Tol-BINAP complexes 3a,b were the most stable
in solution, and could be recrystallised from CH2Cl2/hex-
ane. The CHIRAPHOS complexes 2a,b were isolated by
the precipitation with hexane from the concentrated reac-
tion mixture in toluene. The products were characterised
1
by the H and 31P{1H} NMR, FAB-mass, and UV–Vis
and CD spectra, and in the case of {PdBr(NiDPP)[(S,S)-
(CHIRAPHOS)]} (2b), by CHN analysis.
2.2. NMR spectra
As is typical for chelating bisphosphine palladium(II)
complexes, the 31P{1H} NMR spectra exhibit a pair of
doublets due to P–P coupling between the non-equivalent
31P nuclei. The 2J(P–P) values decrease in the order
CHIRAPHOS (38.5) > Tol-BINAP (37.4) > diphos (23.3
Hz), while the chemical shifts lie in the order CHIRA-
The complexes 3a,b containing the Tol-BINAP ligand
1
gave very complex H NMR spectra, as expected, due to
9.60
9.40
9.20
9.00
8.80
8.60
8.40
Fig. 1. Porphyrin proton region of the 400 MHz 1H NMR spectrum of {PdBr(NiDPP)[(R,R)-CHIRAPHOS]} (2a) in CDCl3. The minor doublets near
9.20 and 8.95 ppm are due to NiDPP, and the other small signals are due to the chloro analogue of 2a.

M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
2165
the asymmetry and the large number of non-equivalent
aromatic protons. We will discuss this spectrum in detail,
because some interesting consequences of the ligand struc-
ture are apparent. One of the tolyl CH3 singlets appears at
quite high field, 0.31 ppm. The methyl groups for the other
three tolyl groups resonate at 1.70, 2.06 and 2.53 ppm. This
large spread is due to the magnetic anisotropy of the por-
phyrin ring, and perhaps also the binaphthyl rings. For
comparison, note that the methyl groups of the free ligand
resonate as two singlets near 2.2 ppm. The remaining sig-
nals for 3 span the range 4.6–10 ppm, as shown in Fig. 2
(upper). Individual assignments of the porphyrin ring pro-
tons are again not possible, although the pairs on each pyr-
role ring were readily identified by COSY and ROESY
spectra. The 2,3/7,8 pairs (see Scheme 2 for ring number-
ing) resonate as doublets at 10.02, 8.76/9.81, 8.59 ppm.
The 13- and 17-Hs were identified from the dipolar cou-
pling to the meso-H as doublets at 8.98 and 8.96 ppm,
and their scalar coupled partners appear at 8.80 and
8.71 ppm, respectively. The fact that two of these protons
resonate so far downfield (i.e. 10.02, 9.81 ppm; compare
the CHIRAPHOS analogues at 9.41, 9.27), is rather strik-
ing for a Ni(II) porphyrin. It appears, therefore, that these
protons come under the deshielding influence of one or
more of the aryl rings of the ligand. Severe out-of-plane
distortion of the porphyrin ring due to the presence of
the bulky substituent would normally induce upfield shifts
due to decreased aromaticity.
be achieved from the room temperature spectrum. It may
be helpful to examine Fig. 2 in conjunction with the struc-
tures in Fig. 3, which are different views of the minimized
structure of the (R)-Tol-BINAP complex calculated at
PM3(tm) semi-empirical level using the SPARTAN 02 pro-
gram. The signals at 4.67 and 4.84, representing 4 Hs, are
due to the m- and o-Hs of one tolyl ring. In an expanded
view, these show the expected P–H couplings as well as
the o-coupling between neighbouring protons on an aryl
ring. The ROESY correlation of the 4.67 ppm signal with
the highest field methyl signal (0.31 ppm) suggests this is
the ring that lies over the plane of the porphyrin (Fig. 3),
as this methyl group would surely experience the greatest
upfield shift due to the magnetic anisotropy of the porphy-
rin macrocycle. These signals will be mentioned again
below, when the effects of lowering the temperature are
described. From the o-protons, there is a dipolar coupling
to a signal (another pseudo-triplet, actually a doublet of
doublets due to P–H and H–H coupling) at 8.32 ppm. This
signal integrates for only one H, so it would appear to be
the o-H on one side of a neighbouring tolyl ring. This pro-
ton also exhibits the only dipolar coupling that was
observed from a non-porphyrin proton to one of the b pro-
tons, the one at 8.91 ppm. However, from this point, the
correlation path was lost, and a single assignment eluded
our efforts. The BINAP protons appear as several triplets
and doublets across the aromatic region, overlapping with
some of the porphyrin 10,20-phenyl signals.
By examining the integrals, it became apparent that
some protons seemed to be ‘‘missing’’. This matter was
resolved by examination of spectra at reduced tempera-
tures. But we will first describe the assignments that can
Returning to the problem of the ‘‘missing’’ tolyl group,
it can be seen in the room temperature spectrum in Fig. 2,
that there is a very broad feature in the region 5–6.5 ppm
(shown by the horizontal bracket in Fig. 2, upper). Upon
10.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
5.5
5.0
4.5
Fig. 2. Porphyrin and phosphine aryl region of the 400 MHz 1H NMR spectrum of {PdBr(NiDPP)[(R)-Tol-BINAP]} (3a) in CDCl3; upper: 288 K; lower:
218 K. In the text, particular mention is made of the region shown by the horizontal bracket (upper) and the signals marked by the arrows (lower).

2166
M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
Fig. 3. The PM3 calculated equilibrium geometry for {PdBr(NiDPP)[(R)-Tol-BINAP]} (3a); left: top view; right: side view.
cooling to 258 K, this region reveals several signals, and
indeed at 218 K (Fig. 2, lower), features at 5.30 and
5.64 ppm become notably sharper, with identical shapes
to the 4.67/4.84 ppm pair discussed above. Simultaneously,
the latter signals have broadened and shifted slightly
upfield. In a mixture of CD2Cl2/CDCl3 at 193 K, this
region appears as one broad signal, suggesting that, at even
lower temperatures, the two sides of this ring would be
observed to be non-equivalent. The 5.30/5.64 signals inte-
grate for only one H each. These results can be explained
by rotation of the tolyl groups being slowed such that the
two sides of the rings are detectably non-equivalent. The
higher field set has a much lower coalescence temperature,
due in part to a much smaller chemical shift difference
between the pairs on each side of the ring. By examination
of the model in Fig. 3, it seems that the highest field set rep-
resents the ring that lies across and nearly parallel to the
porphyrin, while the set that is very broad at room temper-
ature may be from the protons of the ring that is close to
the porphyrin but nearly orthogonal to it. Other effects
of the lowering of the temperature are seen in the downfield
region, in the emergence of two sharp doublets at 8.35 and
8.58 ppm, whose presence was unsuspected on viewing the
room temperature spectrum. From their typical chemical
shifts, these are o-Hs of one of the 10,20-Ph rings. Overall,
the combination of these experiments has given a picture of
the complex that is fairly well matched to the calculated
structure, although we would have liked to obtain a crystal
structure to define some of these aspects somewhat more
clearly. Unfortunately, efforts to grow single crystals were
unproductive so far, as only powders were obtained, and
long storage in solution led to decomposition.
reactions may still be possible if the active species survive
long enough for reactions such as transmetallations to
occur.
2.3. UV–Vis and CD spectra
A major point of interest with these complexes, and
indeed any porphyrins made asymmetric by appending chi-
ral fragments, is how does the porphyrin chromophore
react to the asymmetric environment. In Figs. 4–6, the
UV–Vis and CD spectra of 2b, 3a,b, and 4a are shown
(optical spectra of 4a were recorded immediately after
preparation of the solutions to reduce the effect of unavoid-
able decomposition).
5
0
-5
-10
-15
2.0
1.5
1.0
0.5
0.0
The diphos complexes 4 exhibited similar asymmetry in
the downfield region, and because of the relative simplicity
and rigidity of the ligand, the 1D spectrum is much less
complex. In common with the other complexes, the phenyl
group on the P cis to DPP appears well upfield, the ortho
protons resonating at 5.48 ppm. As noted above, it is unli-
kely that the complexes of this ligand could be used for any
applications that depend on long-term storage, due to their
instability in solution. However, catalytic asymmetric
300
350
400
450
500
550
600
Wavelength/nm
Fig. 4. CD spectrum (upper) and visible absorption spectrum (lower) of
{PdBr(NiDPP)[(R,R)-diphos]} 4a in CH2Cl2.

M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
2167
an intense Soret band (corresponding to the allowed por-
phyrin B transition) and visible bands of low intensities
(corresponding to the quasi-allowed porphyrin Q transi-
tions) (Figs. 4–6, lower). It is of note that while the absorp-
tion maxima of 2 and 3 are exactly the same, the maxima of
4 are hypsochromically shifted by 5–7 nm. These high-
energy shifts are apparently a result of smaller degree of
the porphyrin ring distortion due to the less bulky diphos
substituent in 4.
10
0
-10
-20
On the basis of CD analysis (Figs. 4–6, upper), these
complexes exhibit different efficiency of asymmetry trans-
fer from a chiral substituent to the porphyrin core. Thus,
in contrast to similar UV–Vis absorption properties, their
chiroptical properties are rather dissimilar, reflecting their
structural and conformational peculiarities. In general,
CD in monomeric porphyrins can be induced by the chi-
ral deformations of the porphyrin ring and/or by the
through-space excitonic interactions between the porphy-
rin electronic transition and a different type of electronic
transition (such as carbonyl or aromatic electric dipole
moments), which are spatially fixed in a close proximity
to the porphyrin ring in a chiral fashion. In the former
case, the optical response is typically a monosignate
CD signal reflecting inherent optical activity of the por-
phyrin electronic transitions [31,32], whilst the latter
results in a bisignate couplet, often of unsymmetrical
shape, if several excitonic couplings are involved [33,34].
For example, an appreciable coupled-oscillator interac-
tion between the porphyrin p–p* transitions and p–p*
transitions of aromatic groups was demonstrated in the
case of heme containing proteins [35]. However, the CD
outcome may be even more complicated, if both asymme-
try transfer mechanisms operate simultaneously. In the
chiral complexes studied, we may also expect existence
of these two channels of chirality transfer because of con-
siderable ruffling of the porphyrin core as clearly seen in
Fig. 3 and the possibility of multiple excitonic interac-
tions between the porphyrin and aromatic electronic
transitions.
1.5
1.0
0.5
0.0
300
350
400
450
500
550
600
Wavelength/nm
Fig. 5. CD (upper) and visible absorption (lower) spectra of
{PdBr(NiDPP)[(S,S)-CHIRAPHOS]} 2b in CH2Cl2.
80
60
40
20
0
-20
-40
-60
-80
Hence, in the case of less crowded 4a, the CD spectrum
consists of two moderate negative Cotton effects (CE) at
425 and 413 nm (De = ꢀ14 and ꢀ5 cmꢀ1 Mꢀ1, respectively)
corresponding to the porphyrin B transitions (Fig. 4). This
monosignate spectral pattern is apparently due to the
inherent chiral deformations caused by the diphos substitu-
ent, whilst excitonic interactions of the porphyrin B transi-
tions with phenyl electronic transitions apparently are
rather weak or just cancelled each other due to a particular
spatial arrangement. The situation is drastically changed
upon increasing the substituentÕs bulkiness in 2b. Surpris-
ingly, a well defined and nearly symmetrical bisignate sig-
nal is clearly observed in the CD spectrum (Fig. 5). This
couplet consists of two CEs of opposite signs with positive
and negative CEs at 430 (De = +16 cmꢀ1 Mꢀ1) and 414
(De = ꢀ21 cmꢀ1 Mꢀ1) nm, respectively. The CE positions
are bathochromically shifted in comparison to those of
4a, following the corresponding UV–Vis patterns, and their
2.0
1.5
1.0
0.5
0.0
300
350
400
450
500
550
600
Wavelength/nm
Fig. 6. CD spectrum of {PdBr(NiDPP)[(R)-Tol-BINAP]} (3a) (upper,
solid line), {PdBr(NiDPP)[(S)-Tol-BINAP]} (3b) (upper, dashed line), and
visible absorption spectrum (lower) in CH2Cl2.
The UV–Vis spectra of 2–4 show similar spectral
patterns in the region of 300–600 nm, which are entirely
typical of Ni(II) porphyrin complexes, characterized by

2168
M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
intensities are markedly enhanced; the zero crossing point
is at 422 nm that matches exactly with the UV–Vis absorp-
tion maximum of the Soret band. These CD features are
typical for exciton coupling and apparently arise from the
through-space interactions between the porphyrin and phe-
nyl electronic transitions. Judging from the positive sign of
the induced CD, the (S,S)-CHIRAPHOS substituent and
porphyrin chromophore couple with each other in an over-
all clockwise orientation [36].
4. Experimental
4.1. General
Syntheses involving zerovalent metal precursors were
carried out in an atmosphere of high-purity argon using
conventional Schlenk techniques. The organopalladium
porphyrins were handled in air once isolated. Reagents
and ligands were used as received from Sigma–Aldrich,
unless stated below. 5-Bromo-10,20-diphenylporphyrin
and its nickel complex were prepared according to known
procedures [37]. The enantiomerically pure chiral diphos
ligands were supplied by Dr. S.B. Wild, Research School
of Chemistry, The Australian National University, and
the Tol-BINAP ligands (Strem) were supplied by Dr.
M.L. Williams, School of Science, Griffith University. All
solvents were of analytical reagent grade. Toluene and
diethyl ether were stored over sodium wire. Dichlorometh-
ane and chloroform were stored over anhydrous sodium
carbonate. THF was distilled immediately before use from
sodium/benzophenone under an atmosphere of high-purity
argon. Analytical TLC was performed using Merck silica
gel 60 F254 plates and column chromatography was per-
Further increasing of the substituent bulkiness by intro-
ducing a binaphthyl group in 3a,b results in even more pro-
found effects in the CD spectra. In particular, the induced
CD signal is of complex bisignate shape (Fig. 6) and con-
sists of two non equivalent CEs in the region of Soret tran-
sitions at 436 and 418 nm, of different intensities (De = 15
and 80 cmꢀ1 Mꢀ1, respectively). The overall optical activ-
ities of 3a and 3b are further enhanced in comparison to
those of 2b and 4a, while the zero crossing point is batho-
chromically shifted from the maximum of Soret band by
7 nm due to asymmetry of the induced couplet. These
CD features are a result of the increased rotational strength
of the coupling electronic transitions and multicomponent
interchromophoric through-space interactions caused by
the additional binaphthyl group. The sign of the CD cou-
plet is directly dependent upon the substituentÕs absolute
configuration with the (R)-enantiomer 3a giving a negative
first CE and (S)-enantiomer 3b inducing CE of opposite
sign, thus yielding the corresponding mirror images.
According to the exciton chirality method [36], the sign
of the induced chirality reflects the corresponding clock-
wise and anticlockwise orientations of the substituent–por-
phyrin interactions for 3b and 3a, although their
complexity makes it difficult to assign the individual
couplings.
1
formed using Merck silica gel (230–400 mesh). H NMR
spectra were recorded on a Bruker Avance 400 MHz spec-
trometer and 31P spectra were recorded on a Varian Unity
300 MHz instrument in CDCl3 solutions, using CHCl3 as
the internal reference at 7.26 ppm for 1H spectra, and exter-
nal 85% H3PO4 as the zero reference for proton-decoupled
31P spectra. The low temperature and 2D NMR studies
were performed at the Centre for Molecular Architecture,
Central Queensland University, on a Bruker Avance
400 MHz instrument. In Australia, UV–Vis spectra were
recorded on a Varian Cary 3 spectrometer using dichloro-
methane as solvent. FAB mass spectra were recorded by
the Mass Spectrometry Service, Research School of Chem-
istry, The Australian National University, using m-nitrob-
enzyl alcohol as matrix. Microanalyses were performed by
the Microanalytical Service, Department of Chemistry,
The University of Queensland. UV–Vis and CD spectra
were measured in Japan at room temperature in CH2Cl2
solutions on a Shimadzu UV-3101PC spectrometer and
JASCO J-720 spectropolarimeter, respectively. CD
scanning conditions were as follows: scanning rate =
50 nm/min, bandwidth = 2 nm, response time = 1 s,
accumulations = 4.
3. Conclusion
We have demonstrated the original aim, i.e. the genera-
tion of a chiral porphyrin environment using single enanti-
omers of chelating diphosphine ligands on peripherally
metallated organopalladium porphyrins. The Tol-BINAP
ligand offers the most promise for further study, as it
induces the strongest effects on the CD spectra, as well as
forming fairly stable products. Clearly, for any application
where coordination at the central metal ion of the metallo-
porphyrin is required, Ni(II) is not the ideal metal ion to
choose. However, we have found in the past that Ni por-
phyrins give the cleanest NMR behaviour, hence their
use in these preliminary studies. In future, we would like
to extend this work into: (i) other centrally coordinated
metals such as Zn(II), Mg(II) (for chiral recognition); (ii)
five- or six-coordinate metals such as Co(III), Mn(III),
Ru(II) (for enantioselective catalysis using the central
metal); and (iii) less labile transition metals on the periph-
ery, especially Pt, Rh, or Ir (to afford more stable
compounds).
4.2. Synthesis of {PdBr(NiDPP)[(R,R)-CHIRAPHOS]}
(2a)
Toluene (5 ml) was added to a Schlenk flask and heated
to 105 °C under a stream of Ar. (R,R)-CHIRAPHOS
(17.0 mg, 0.0399 mmol) was added, followed by Tris-
(dibenzylideneacetone)-dipalladium(0) (Pd2dba3, 18.3 mg,
0.0199 mmol). The dark purple colour of the palladium
starting material faded, leaving a clear dark yellow
solution. After stirring for a further 5 min, Br(NiDPP)

M.J. Hodgson et al. / Journal of Organometallic Chemistry 691 (2006) 2162–2170
2169
(11.9 mg, 0.0199 mmol) was added, and the mixture was
stirred at 105 °C for 2 h, and the reaction progress was
monitored by TLC (CH2Cl2:hexane, 1:1). After cooling
to room temperature, the volume was reduced to about
one-third under vacuum, and hexane was added to precip-
itate the product, which was filtered, washed with hexane
and vacuum dried to yield a fine dark purple powder
25.0 (d, JPP 37.4 Hz). UV–Vis: kmax (e/103 Mꢀ1 cmꢀ1) 422
(196), 530 (15.8), 560 sh (4.8) nm. FAB-MS: weak cluster
peaking at 1382.3 (M+ calcd. 1382.2).
4.5. Synthesis of {PdBr(NiDPP)[(S)-Tol-BINAP]} (3b)
This complex was prepared by the method above using
(S)-Tol-BINAP. The enantiomers exhibited identical
NMR and UV–Vis spectra, as expected.
1
(17.1 mg, 76%). H NMR: d 0.80 (dd, J(P–H) 12.8, J(H–
H) 6.8 Hz, 3H, CH3 nearer to P cis to DPP), 1.03 (dd,
J(P–H) 10.2, J(H–H) 6.8 Hz, 3H, CH3 nearer to P trans
to DPP), 2.13 (m, J(P–H) 11.3 Hz, 1H, CH nearer to P
trans to DPP), 2.65 (m, J(P–H) 11.1 Hz, 1H, CH nearer
to P cis to DPP), 5.24 (m, 4H, o- and m-H of one PPh),
5.56 (qt, 1H, p-H of one PPh), 7.07 (dt, 2H, m-H of one
PPh), 7.23 (dt, 1H, p-H of one PPh), 7.40 (br dd, 2H, o-
H of one PPh), 7.5–7.72 (m, 16H, PPh and 10,20–Ph),
8.08–8.2 (overlapping m, 4H, o-H of two PPh), 8.29, 8.45
(each d, 1H, J = 4.7 Hz, 2- and 8-b H), 8.73, 8.75 (each
d, 1H, J = 4.7 Hz, 12- and 18-b H), 8.89, 8.99 (each d,
1H, J = 4.7 Hz, 13- and 17-b H), 9.27, 9.41 (each d, 1H,
J = 4.7 Hz, 3- and 7-b H), 9.57 (s, 1H, meso-H) ppm. 31P
NMR: d 41.8 (d, JPP 38.5 Hz), 58.9 (d, JPP 38.5 Hz).
4.6. Synthesis of {PdBr(NiDPP)[(R,R)-diphos]} (4a)
Toluene (6 ml) was added to a Schlenk flask and heated
to 105 °C under a stream of Ar. (R,R)-diphos (19.4 mg,
0.0602 mmol) was added, followed by Pd2dba3 (27.5 mg,
0.0300 mmol). The dark purple colour of the palladium
starting material lightened to a dark orange solution. After
stirring for
a further 10 min, Br(NiDPP) (11.9 mg,
0.0199 mmol) was added. The reaction mixture was main-
tained at 105 °C under Ar and monitored by TLC
(CH2Cl2-hexane, 3:7). After 15 min, a dark precipitate
was noted. After 30 min, the reaction mixture was cooled
and the precipitate was collected by vacuum filtration
and dried under high vacuum to yield 4a (57%) as a dark
4.3. Synthesis of {PdBr(NiDPP)[(S,S)-CHIRAPHOS]}
(2b)
1
purple powder. H NMR: d 1.02 (d, J(P–H) 10.6 Hz, 3H,
CH3), 2.59 (d, J(P–H) 9.1 Hz, 3H, CH3), 5.51 (dd, 2H, o-
H on PPh cis to DPP), 6.01 (dt, 2H, m-H on PPh cis to
DPP), 6.69 (br t, 1H, p-H on PPh cis to DPP), 7.4–7.7 (ser-
ies of m, 15H, PPh, ligand Ph and m-, p-H of 10,20-Ph),
7.8–7.9 (m, 4H, o-H of 10,20-Ph), 8.14, 8.73, 8.76, 8.77,
8.85, 9.06, 9.07, and 9.41 (each d, 1H, b-H), 9.72 (s, 1H,
meso-H) ppm. 31P NMR: d 44.2 (d, JPP 23.3 Hz), 36.3 (d,
JPP 23.3 Hz). UV–Vis: kmax (e/103 Mꢀ1 cmꢀ1) 417 (215),
525 (17), 553 sh (5.1) nm. FAB-MS: 1026.2 (M+ calcd.
1024.02).
This complex was prepared by the method above for 2a
using 10 ml of toluene, (S,S)-CHIRAPHOS (12.8 mg,
0.0300 mmol), Pd2dba3 (13.7 mg, 0.0150 mmol) and
Br(NiDPP) (11.9 mg, 0.0199 mmol) to yield a fine purple
powder (22.2 mg, 99%). NMR spectra were identical
to those of the above enantiomer. UV–Vis: kmax (e/
103 Mꢀ1 cmꢀ1) 422 (184), 530 (14.9), 560 sh (4.4) nm.
FAB-MS (most abundant mass): 1131.18 (MH+ calcd.
1131.09). Found: C, 63.76; H, 4.23; N, 4.49. C60H47-
BrN4NiP2Pd requires C, 63.72; H, 4.19; N, 4.95%.
4.7. Synthesis of {PdBr(NiDPP)[(S,S)-diphos]} (4b)
4.4. Synthesis of {PdBr(NiDPP)[(R)-Tol-BINAP]} (3a)
This complex was prepared by the method above using
(S,S)-diphos. The enantiomers exhibited identical NMR
spectra. These complexes were both unstable in CDCl3
solution, with significant decomposition to form NiDPP
evident within 1 h.
Toluene (10 ml) was added to a Schlenk flask and heated
to 105 °C under a stream of Ar. (R)-Tol-BINAP (54.3 mg,
0.0800 mmol) was added, followed by Pd2dba3 (18.0 mg,
0.0197 mmol). The dark purple colour of the palladium
starting material lightened to a dark orange solution. After
stirring for a further 10 min, Br(NiDPP) (11.9 mg,
0.0199 mmol) was added. The reaction mixture was main-
tained at 105 °C under Ar for 5 h and monitored by TLC
(ethyl acetate:hexane, 3:7). The mixture was cooled to room
temperature and the solvent removed under high vacuum.
The product 3a was recrystallised from CH2Cl2–hexane in
80% yield as a fine dark purple powder. 1H NMR (293 K):
d 0.31, 1.70, 2.06 and 2.53 (each s, 3H, CH3), 4.67 (dd,
2H, Ptol), 4.84 (dd, 2H, Ptol), 5–6.5 (vbr, Ptol), 6.60 (m,
4H, Ptol), 6.9–7.8 (m, Naph, Ptol and 10,20-Ph), 7.9–8.1
(m, Ptol), 8.31 (t, 1H, Ptol), 8.3–8.6 (vbr, Ptol), 8.59, 8.71,
8.76, 8.80, 8.96, 8.98, 9.81, 10.02 (each 1H, d, b-H), 9.58
(1H, s, meso-H) ppm. 31P NMR: d 11.9 (d, JPP 37.4 Hz),
Acknowledgements
We thank Dr. S.B. Wild and Dr. M.L. Williams for gifts
of chiral ligands. M.J.H. thanks the Faculty of Science,
Queensland University of Technology, for a Post-graduate
Scholarship. We thank Dr. M.R. Johnston and the Centre
for Molecular Architecture, Central Queensland University
for the use of their NMR facilities for parts of the work.
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