Synthesis and physical properties of biquinoxalinyl bridged bis-porphyrins: Models for aspects of photosynthetic reaction centres

Article abstract of DOI:10.1039/b304161a

The synthesis of biquinoxalinyl-bridged bis-porphyrin 4 and metallated derivatives 5-11 was achieved in high yields. UV-visible spectroscopy and electrochemical experiments indicated weak orbital coupling of the two quinoxalinyl units but minimal orbital

Full text of DOI:10.1039/b304161a

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Synthesis and physical properties of biquinoxalinyl bridged
bis-porphyrins: models for aspects of Photosynthetic Reaction
Centres
Maxwell J. Crossley,* Paul J. Sintic, Robin Walton and Jeffrey R. Reimers
School of Chemistry, The University of Sydney, NSW 2006, Australia.
E-mail: m.crossley@chem.usyd.edu.au
Received 17th April 2003, Accepted 17th June 2003
First published as an Advance Article on the web 7th July 2003
The synthesis of biquinoxalinyl-bridged bis-porphyrin 4 and metallated derivatives 5–11 was achieved in high yields.
UV-visible spectroscopy and electrochemical experiments indicated weak orbital coupling of the two quinoxalinyl
units but minimal orbital coupling between the two porphyrins. The weak electronic communication is attributed
to non-planarity, on average, of the molecule because of rotation about the inter-quinoxalinyl connection. This,
combined with poor coupling across the fused junctions between the porphyrin and quinoxalinyl units, results in
minimal inter-porphyrin communication. As a result, the biquinoxalinyl linkage is appropriate for inclusion in more
elaborate synthetic compounds, such as the tris-porphyrin 1, that are designed to model the charge-separation
apparatus of Photosynthetic Reaction Centres.
from the zinc() porphyrin to the gold() porphyrin. Addi-
Introduction
tionally, as both linkages facilitate poor inter-porphyrin
Porphyrin arrays are often used to study electron- and energy-
conjugation, photoinduced charge separation is expected to be
transfer processes due to their great similarity with natural
systems. In the Photosynthetic Reaction Centres (PRCs)
long-lived. On the route to tris-porphyrin 1, we have developed
syntheses of bis-porphyrin systems with each of the required
porphyrinic macrocycles are assembled into arrays where light
linkers. In a preliminary communication we reported the syn-
energy is converted into electrical energy and then chemical
thesis of bis-porphyrin 4 and its dizinc() derivative 5.17 We
energy with high efficiency.1,2 The structure of the PRCs of
have also examined the photophysics of the monogold()
purple bacteria3 and Photosystems I and II of the cyano-
adduct 10.18
bacterium Synechococcus elongatus4,5 show a very similar
The route to these compounds uses the porphyrin-2,3-dione
2 and porphyrin-2,3,12,13-tetraone systems that we developed
arrangement of the porphinoid pigments involved in charge
separation. In order to mimic this natural process, a number of
in other work for the synthesis of laterally-bridged oligo-
synthetic systems have been designed in which two or more
porphyrins such as the bis-porphyrins and analogous tris-,
porphyrins are linked by conjugated bridges through either the
meso- or β-pyrrolic positions on the porphyrin macrocycle,6–9
through β-positions by supramolecular assemblies10–12 or by
direct linkage.13,14 These systems were designed to create an
energy gradient to enable the unidirectional transfer of energy
or electrons with possible photovoltaic and photonic appli-
cations.15 In addition, most of these systems have been designed
with the aim of maximising the electronic communication
between the porphyrins.
A target of our work is the covalently linked tris-porphyrin 1
which we propose as a model for the typically weakly coupled
(bacterio)chlorophyll molecules found in the special pair and
primary electron acceptor of the bacterial PRC. This model
contains a Tröger’s base linkage that establishes C2 symmetry
in the charge separation special pair thereby mimicking an
important aspect of the PRC. Also, it contains a biquinoxalinyl
linkage. Tris-porphyrin 1 is also an accurate distance model for
the PRC with inter-chromophoric distances that closely match
the natural system (Table 1).16 The insertion of metal ions into
the terminal chromophores of the tris-porphyrin 1 creates a
redox gradient that should facilitate multistep electron-transfer
Table 1 Distance16 (Å) between macrocyclic rings of tris-porphyrin 1
(anti-planar χ = 180Њ) compared to the donor–acceptor pair (D–A pair)
found in the PRC
Porphyrin units
Edge-to-edge
Centre-to-centre
ZnP–FbP
FbP–AuP
ZnP–AuP
D–A pair
9.2
10.7
27.8
9.5
16.5
18.7
34.9
16.5
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 3
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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Scheme 1 Reagents and conditions: i, CH2Cl2, 2 h; ii, metal salt (see text); iii, KAuCl4, NaOAc, AcOH–CHCl3, ∆, then KPF6 anion exchange;
iv, Zn(OAc)2ؒ2H2O, MeOH–CHCl3, ∆, 4 h (Ar = 3,5-But2C6H3).
tetrakis-, pentakis- and hexakis-porphyrins.19–22 The inter-
porphyrin linkages in these compounds were generated by
reactions with 1,2,4,5-benzenetetraamine, which establishes a
tetraazaanthracene bridge and results in a porphyrin-centre to
porphyrin-centre distance of 16.9 Å. Use of 3,3Ј-diamino-
benzidine 3 analogously results in the formation of biquin-
oxalinyl linked bis-porphyrins (Scheme 1).
were acquired at 220 and 230 K and showed no broadening of
proton signals providing evidence that the molecule must be
rapidly twisting about the inter-quinoxalinyl connection.
Metallated derivatives 5–11 were also prepared and physical
properties of these molecules were determined to gain a pre-
liminary understanding of the electronic processes occurring in
the biquinoxalinyl bis-porphyrin system.
In this paper we give full details of the synthesis of the bis-
porphyrins 4–11, together with model quinoxalino[2,3-b]-
porphyrins 13–18 and we describe electrochemical, absorption
spectroscopy, and electronic-structure modelling studies that
assess the suitability of this linker to be used in systems desig-
ned to exhibit photo-induced energy- and electron-transfer
properties.
Dizinc() biquinoxalinyl bis-porphyrin 5 was obtained in
86% yield by treatment of the free-base bis-porphyrin 4 with
zinc() acetate dihydrate (Scheme 1). The absence of NH
stretches in the infrared spectrum of the dizinc() bis-porphyrin
5 indicated that metallation was complete. The zinc() bis-
porphyrins prepared in this work were all found to demetallate
rather easily. Accordingly, chromatography was avoided where
possible and when it was used only short plugs of silica were
employed. The demetallation of products was particularly
facile during chromatography in light.
Results and discussion
Dicopper() biquinoxalinyl bis-porphyrin 6 was prepared in
77% yield by adding a slurry of copper() acetate monohydrate
in methanol to a solution of the free-base bis-porphyrin 4 in
chloroform and heating the mixture at reflux for 3 hours
(Scheme 1). Dinickel() bis-porphyrin 7 was prepared by the
addition of nickel() acetate tetrahydrate in glacial acetic acid
to a chloroform solution of bis-porphyrin 4 in 81% yield
(Scheme 1). Metallation using N,N-dimethylformamide and the
metal chloride was not possible as the free-base bis-porphyrin 4
is insoluble in this solvent.
Synthesis
Free-base biquinoxalinyl bis-porphyrin 4 was synthesised by
addition of 3,3Ј-diaminobenzidine 3 to excess 2,3-dioxo-
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin2indichloro-
methane at room temperature (Scheme 1). A rapid reaction
ensued and the product precipitated from solution over 2 hours.
Chromatography and crystallisation afforded the bis-porphyrin
4 in 89% yield. Reaction in chloroform gave a similar yield of 4
but without precipitation of the product. When stoichiometric
amounts of dione 2 and tetraamine 3 (2 : 1) were employed, the
yield of 4 was only 64%. Incompletely cyclised products were
never observed. It would seem that the two orthogonal aryl
groups flanking the dione unit provide a steric pathway that
directs the incoming diamine group so that it always reacts in a
double condensation mode.
Dipalladium() biquinoxalinyl bis-porphyrin 8 was prepared
in 98% yield by adding palladium() chloride to a solution of
free-base bis-porphyrin 4 in toluene and glacial acetic acid and
heating the mixture at reflux for 72 hours (Scheme 1).
Monozinc() biquinoxalinyl bis-porphyrin 9 was obtained by
treatment of 4 with one equivalent of zinc() acetate dihydrate
(Scheme 1). This reaction resulted in a mixture of the free-base
4, monozinc() 9 and dizinc() 5 bis-porphyrins. This mixture
was easily separated using flash chromatography and the
monozinc() product 9 was obtained in 35% yield. Chromato-
graphy was performed immediately with protection from light
as the unstable metallation product 9 rapidly demetallated.
Zinc()-gold() bis-porphyrin 11 was prepared by first heat-
ing free-base biquinoxalinyl bis-porphyrin 4 in refluxing acetic
Spectroscopy confirmed the structure of 4 and 1H NMR
revealed the symmetry of the compound. The resonances of the
protons on the rings joined by the inter-quinoxalinyl connec-
tion displayed the expected AMX splitting pattern for reson-
ances of protons on 1,2,4-trisubstituted benzene rings. The
spectrum showed a higher degree of symmetry than that
expected for a biquinoxalinyl bridged bis-porphyrin with a non-
1
planar conformation. Low temperature H NMR spectra of 4
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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acid and chloroform for 72 hours in the presence of potassium
tetrachloroaurate() and sodium acetate (Scheme 1). The
formation of the monogold() adduct 10 in a yield above that
expected statistically was the result of its partial precipitation
during the reaction, coupled with a slow rate of metallation.
Anion exchange was then performed on the crude product by
stirring in the presence of potassium hexafluorophosphate. The
product was purified by flash chromatography to give [mono-
gold() bis-porphyrin]PF6 10 in a yield of 60%. [Monogold()
bis-porphyrin]PF6 10 was then heated with zinc() acetate di-
hydrate (2 eq.) in chloroform and methanol, followed by basic
work up, anion exchange and flash chromatography to give
[zinc()-gold() bis-porphyrin]PF6 11 in 87% yield (Scheme 1).
The overall yield for the two step reaction was 52%. Micro-
analysis failed to give meaningful results probably because
of the weakly bound nature of the anion, which led to diffi-
culties in recrystallisation of 10 and 11 from chloroform–light
petroleum and possible partial counterion exchange. These
gold() chelated derivatives have applications in photo-induced
electron-transfer. The photophysical analysis of 10 has been
reported elsewhere,18 while the zinc()-gold() bis-porphyrin
11 is still under investigation.23 The alternative sequence of
metallation by way of the monozinc() adduct 9 is not
appropriate as zinc() porphyrins readily demetallate under the
conditions required to insert gold().
Monomer quinoxalino[2,3-b]porphyrins were also syn-
thesised in order to provide comparisons with bis-porphyrins
4–11 when determining physical properties. 5,10,15,20-Tetra-
kis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin 13 ful-
filled this role and was synthesised in 92% yield by stirring
a
solution of porphyrin-2,3-dione 2 in dichloromethane
with o-phenylenediamine 12 (5 eq.) at room temperature for
1
6 hours (Scheme 2). The H NMR (400 MHz) spectrum of
quinoxalino[2,3-b]porphyrin 13 shows the C2v symmetry of the
compound. Two multiplets at δ 7.72–7.75 and 7.82–7.85 indi-
cated the four quinoxaline protons. The absence of carbonyl
stretches in the infrared spectrum, mass spectroscopy and
elemental analysis confirmed the structure of 13.
The zinc(), copper(), nickel(), palladium() and gold()
chelates 14–18 of quinoxalino[2,3-b]porphyrin 13 were also
prepared in 82, 89, 64, 88, and 51% yields, respectively, using
the standard metallation methods to complete the series of
comparisons (Scheme 2).
Modelling
The structures of the derivatives of 4 and 13 in which the 3,5-di-
tert-butylphenyl groups are replaced with hydrogens were
determined using the B3LYP density functional calculations24
with the 3-21G basis set25 by Gaussian-98.26 For biquinoxalinyl
bis-porphyrin 4, the calculated dihedral angle about the inter-
quinoxalinyl connection is 40Њ. Electronic couplings were
obtained from electron affinities estimated as the unoccupied-
orbital eigenvalues and are thus somewhat crude approx-
imations. The vertical excitation energies of 13 were evaluated
by our program27 using INDO/S.28
Scheme 2 Reagents and conditions: i, CH2Cl2, 6 h; ii, see text.
bis-porphyrin 19 which has four rather than two distinct
reversible one-electron reductions occurring at low voltage.
Changes in the voltammograms on dimer formation originate
through incompletely screened electrostatic interactions operat-
ing between the two halves, and from couplings of the fragment
molecular orbitals of each half. The relatively minor changes
observed for the biquinoxalinyl bis-porphyrins indicate that the
Coulombic screening is nearly complete and the orbital inter-
actions are minor whilst the profound effects observed for 19
indicate significant orbital interactions and inter-porphyrin
coupling within this rigid fused molecule.
The differences between the cyclic voltammograms of the
quinoxalino-porphyrins and the biquinoxalinyl bis-porphyrins
largely comprise small changes in the potentials and significant
peak broadening. These provide information concerning the
specific chemistry of the species and some indication of residual
inter-orbital interactions. In order to understand these effects,
pictures of the orbitals of a free-base mono-porphyrin are
Electrochemical properties
The electronic properties of the biquinoxalinyl bis-porphyrin
system were investigated using cyclic voltammetry. Potentials
for the quinoxalino-porphyrins (MPQ) 13–17 and their corre-
sponding biquinoxalinyl bis-porphyrins [(MPQ-)2] 4–8 are pro-
vided (Table 2), as are the observed voltammograms for the
free-base molecules (Fig. 1). These results are characteristic of
those for the other molecules, with the greatest variations found
in fact for the nickel() species; some specific issues affecting the
zinc() derivatives are also discussed later. Basically, the vol-
tammograms show only minor variations associated with inter-
connection of the quinoxalinyl units to form the bis-por-
phyrins. This is in stark contrast to the behaviour of conjugated
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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Table 2 Summary of redox potentials (in V) obtained for free-base and dimetallated biquinoxalinyl bis-porphyrins 4–8 and free-base and metal-
lated quinoxalino-porphyrins 13–17. All measurements were obtained in THF, scan rate 100 mV sϪ1, referenced to Ag/AgCl and containing 0.1 M
TBAP
Macrocycle
reduction (E1/2
)
Bridge reduction (Ep)
Oxidation (E1/2)
Compound
R1
R2
R3
R4
R5
Ox1
Ox2
∆ (1st Ox Ϫ 1st Red)
4
13
5
14
6
15
7
16
8
17
(2HPQ-)2
2HPQ
(ZnPQ-)2
ZnPQ
(CuPQ-)2
CuPQ
(NiPQ-)2
NiPQ
(PdPQ-)2
PdPQ
Ϫ1.02
Ϫ1.03
Ϫ1.18
Ϫ1.18
Ϫ1.03
Ϫ1.07
Ϫ1.07a
Ϫ1.04
Ϫ1.04
Ϫ1.04
Ϫ1.21
Ϫ1.25
Ϫ1.81a
Ϫ1.55
Ϫ1.44
Ϫ1.46
Ϫ1.44
Ϫ1.46
Ϫ1.46
Ϫ1.45
Ϫ2.24
Ϫ2.24
Ϫ2.40b
Ϫ2.27
Ϫ2.26
Ϫ2.15
Ϫ2.36
Ϫ2.25
Ϫ2.32
Ϫ2.14
Ϫ2.64
Ϫ2.64
Ϫ2.80
Ϫ2.95
Ϫ2.76
Ϫ2.77
Ϫ2.72
Ϫ2.66
Ϫ2.78
Ϫ2.76
Ϫ2.79
Ϫ2.80
1.28a
1.26a
1.04
1.02
1.21
1.20
1.12
1.10
1.37
1.38
2.30
2.29
2.22
2.20
2.24
2.27
2.19
2.14
2.41
2.42
1.22
1.13
a Ep value. b Peak distorted due to absorption.
Fig. 1 Cyclic voltammograms obtained upon the reduction of free-
base quinoxalino[2,3-b]porphyrin 13 (top) and free-base biquinoxalinyl
bis-porphyrin 4 (bottom) in THF containing 0.1 M TBAP.
provided (Fig. 2) as obtained from density-functional calcu-
lations. This shows the lowest-unoccupied molecular orbital
and the following four orbitals, named L1 to L5, respectively.
Most importantly, the orbitals can be partitioned into those
that are largely localised on the porphyrin (L1 and L2), those
localised on the appended quinoxaline (L3 and L5), and one
largely porphyrin orbital with some quinoxaline delocalisation
(L4). The orbital L4 is known not to contribute to the electro-
chemistry of porphyrins and to make only a minor contribution
to their spectroscopy and is discussed no further. In biquin-
oxalinyl bis-porphyrins, coupling between the monomeric units
Fig. 2 The nature of the five lowest-energy unoccupied orbitals from
B3LYP calculations of 13 and the coupling J between matching orbitals
(Ln–Ln) in the biquinoxalinyl bis-porphyrin 4.
is related to the orbital coefficient on the atoms that form
the inter-quinoxalinyl connection; quantitatively, the values
of the orbital couplings J obtained from density-functional
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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Table 3 Summary of redox potentials (in V) obtained for bis-porphyrin 9 and quinoxalino-porphyrins 13 and 14. Measurements were obtained in
THF, scan rate 100 mV sϪ1, referenced to Ag/AgCl and containing 0.1 M TBAP
Compound
Macrocycle reduction (E1/2
)
Bridge reduction (Ep)
Oxidation (E1/2)
9
13
14
Ϫ1.00
Ϫ1.03
Ϫ1.17a
Ϫ1.25
Ϫ1.18
Ϫ1.75
Ϫ2.33a
Ϫ2.24
Ϫ2.27
Ϫ2.73
Ϫ2.64
Ϫ2.95
1.06
1.02
1.32a, b
1.26b
1.13
Ϫ1.81b
a Broad peaks, indicating a superposition of redox process on both the ZnP and FbP in 9. b Ep value.
calculations on the corresponding free-base bis-porphyrin are
also provided (Fig. 2).
Reductions R1 and R2 (Table 2) are reversible one-electron
processes that have been assigned to the porphyrin sub units.29
Isolated porphyrins display similar reductions associated with
just one orbital, identified as L1 by the density-functional calcu-
lations (Fig. 2). Similar reductions involving orbital L2 are not
expected within the accessible voltage range, and the reductions
R3 and R4 have been attributed to the quinoxaline subunits.29
From the calculations, these processes are assigned to orbital
L3. The degree of broadening of R3 observed for the bis-
porphyrins is greater than that observed for R1 and R2, as
anticipated by the calculated increase in coupling J from 0.030
to 0.114 eV (Fig. 2); intuitively, R3 is expected to separate into
two resolvable components if the coupling is as high as the
value of 0.114 eV predicted for R4, however, supporting the
orbital assignment. The free-base species 4 and 13 (Fig. 1) also
display a resolved fifth reduction named R5 at Ϫ2.80 V, just
before the onset of solvent reduction.
Strong adsorption of dizinc() biquinoxalinyl bis-porphyrin
5 to the electrode, impaired analysis of this system by electro-
chemical studies. It cannot be determined whether the first
quinoxaline bridge reduction is split or if the cyclic voltam-
mogram is distorted by deposition (Table 2). In contrast, the
Fig.
3
Cyclic voltammograms obtained upon the reduction of
nickel() quinoxalino[2,3-b]porphyrin 16 (top) and dinickel() biquin-
cyclic voltammogram of monozinc() biquinoxalinyl bis-
porphyrin 9 revealed that each chromophore has equivalent
electrochemical behaviour to the corresponding free-base and
zinc() quinoxalino[2,3-b]porphyrins 13 and 14 (Table 3). Thus,
the asymmetric system 9 is weakly coupled displaying similar
trends to those observed for the free-base, dicopper() and di-
palladium() biquinoxalinyl bis-porphyrin chelates 4, 6, and 8.
Substantial differences were observed when dinickel()
biquinoxalinyl bis-porphyrin 7 was compared to nickel()
quinoxalino[2,3-b]porphyrin 16 (Fig. 3 and Table 2). Cycles of
the first reduction, of the first two reductions and of the first
three reductions revealed that the first macrocyclic reduction in
the bis-porphyrin 7 is irreversible but the second macrocyclic
reduction (E1/2 = 1.44 V) is reversible and approximately equal
to that of the model quinoxalino[2,3-b]porphyrin 16. The quin-
oxaline reductions in the bis-porphyrin 7 occurred at more
negative potentials than the reductions of 16 and the current
increased. The origin of the enhancement is yet to be deter-
mined but it might be due to distortions of the porphyrin
macrocycle affecting the overall structure. Such distortions are
a common feature of nickel() porphyrins and a phenomenon
used to explain anomalous results in other electrochemical
studies.30,31
oxalinyl bis-porphyrin 7 (bottom) in THF containing 0.1 M TBAP.
all the above biquinoxalinyl bis-porphyrins, oxidation poten-
tials were almost identical to respective monomeric com-
ponents, again indicative of weak coupling. The separation
between the first oxidation and the first reduction, the HOMO-
LUMO gap, for all the derivatives 4–8 fell within the expected
range of 2.25 0.20 V.
We have previously reported the photophysical properties
of monogold() biquinoxalinyl bis-porphyrin 10.18 Electro-
chemical investigation of this metal() system when com-
pared to its constituent monomeric components, free-base
and gold() quinoxalino[2,3-b]porphyrins 13 and 18 (Fig. 4),
revealed that the system is weakly coupled. The bis-porphyrin
10 undergoes eight one-electron reductions that can be assigned
to the free-base macrocycle (FbP), the gold() macrocycle
(AuP) and biquinoxalinyl bridge components. The first redox
process is assigned to the AuP and is a metal-centred Au/Au
reduction.32 The following macrocycle-centred reductions in 10
were similar to the model compounds (indicating weak com-
munication) with the exception of the sixth and seventh reduc-
tions at Ep = Ϫ2.14 and Ϫ2.38 V, which are tentatively assigned
to the first quinoxaline reductions of the FbP and AuP macro-
cycles in 10. These peak potentials appear to have been split by
240 mV when compared to the molecular components, com-
ponents that are reduced at virtually the same potential as each
other. This could be an indication of significantly enhanced
interaction across the inter-quinoxalinyl connection. Alter-
natively, we note that the corresponding digold() molecule
could not be readily isolated as it underwent decomposition of
the linkage, and the triply reduced gold–free base compound
may likewise be chemically reactive. Oxidation redox processes
in 10 were poorly resolved and no useful information could be
obtained.
Oxidation processes were also examined for the free-base and
metal() biquinoxalinyl bis-porphyrins (Tables 2 and 3). The
dimetallated adducts gave
a single well defined ‘quasi-
reversible’ oxidation, with the exception of the dizinc()
analogue 5 which gave two reversible one-electron oxidative
processes. Likewise, the metallated quinoxalino[2,3-b]-
porphyrins showed an identical trend to their corresponding
bis-porphyrin analogues. Multistep electron-transfer processes
occur in copper(), nickel() and palladium() bis-porphyrins
6–8 as the peak current intensity is approximately twice the
size of the corresponding first reduction. The presence of
the free-base macrocycle in bis-porphyrin 4 and monozinc()
bis-porphyrin 9 resulted in irreversible oxidative processes. In
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polarized Soret components. For the zinc() mono-porphyrin
14, the two components of the Soret band are individually iden-
tifiable, with one of these moving to significantly lower energy
compared to the free-base analogue. This feature is retained in
the spectrum of the dizinc() species 5. However, a shoulder in
the spectrum of both molecules is apparent at ca. 470 nm, and it
is clear that the lower-energy Soret component loses signifi-
cantly more intensity to this shoulder in the bis-porphyrin than
it does in the monomer. Hence it is clear that this 470 nm shoul-
der is affected by some inter-chromophore coupling process.
INDO/S calculations predict that the 470 nm shoulder arises
from a porphyrin to quinoxaline transition involving the bridge
orbital L5 (Fig. 2). A similar transition is also observed in the
spectrum of the significantly coupled bis-porphyrin 19, and
both INDO/S as well as ab initio CIS, RPA, and CASPT2 calcu-
lations35 interpret it analogously. The observed spectral shape
changes on dimerisation are thus attributed to intermediate-
strength coupling acting between the two quinoxalines in the
biquinoxalinyl species. No indication of inter-porphyrin
coupling is evident, however. For the copper() and nickel()
porphyrins (Fig. 5), the metal induces larger red shifts of the
lower-energy Soret component than those found for zinc()
porphyrins. This enhances its interaction with the 470 nm por-
phyrin to quinoxaline transition, making the latter more pro-
nounced. For the dinickel() bis-porphyrin 7, the lower Soret
and porphyrin to quinoxaline transitions are of equal intensity
and individually resolved at 464 and 472 nm, respectively.
Conclusions
An efficient synthesis of biquinoxalinyl bridged bis-porphyrins
has been developed. Through spectroscopic, electrochemical,
and modelling studies we have shown that the porphyrin
macrocycles contained in biquinoxalinyl bis-porphyrin 4 and all
described metallated derivatives are all weakly coupled to each
other.
Fig. 4 Cyclic voltammograms obtained upon the reduction of free-
base quinoxalino[2,3-b]porphyrin 13 (top), gold() quinoxalino[2,3-b]-
porphyrin 18 (middle) and monogold() biquinoxalinyl bis-porphyrin
10 (bottom) in THF containing 0.1 M TBAP.
The minimal coupling between porphyrin rings in this system
can be attributed to the nature of the inter-quinoxalinyl linkage
and to the degree of mixing between the porphyrin and quin-
oxaline orbitals. The electronic absorption spectra indicate that
transitions of a porphyrin to quinoxaline nature in monomeric
species are significantly perturbed in the spectra of the corre-
sponding biquinoxalinyl bis-porphyrins. This is interpreted as
arising from coupling of the quinoxaline based orbitals L5
(Fig. 2) across the inter-biquinoxalinyl connection. The corre-
sponding cyclic voltammograms show even less perturbations
in going from quinoxalino-porphyrins to biquinoxalinyl bis-
porphyrins, even for reduction processes involving quinoxaline
orbitals. This is interpreted as arising from the involvement of a
different, much less coupled bridge orbital, L3. The weak inter-
porphyrin couplings observed in the systems studied indicate
that these compounds provide essential key features required in
model systems for the operation of PRCs.
Future work will involve functionalising the biquinoxalinyl
bis-porphyrin into larger multiporphyrin arrays. Recently, we
have successfully synthesised zinc()-free-base-gold() tris-
porphyrin array 1, in which the biphenyl linker has been func-
tionalised to extended Tröger’s base bis-porphyrin, to act
as an accurate distance model of the PRC. It is encouraging
that a molecular component of tris-porphyrin 1, monogold()
biquinoxalinyl bis-porphyrin 10, exhibits weak ground-state
interaction and rapid electron-transfer.18 The synthesis and
photophysical properties of 1 will be reported elsewhere.
Comprehensive electrochemical analysis of the quin-
oxalino[2,3-b]porphyrins 13–18, and the biquinoxalinyl bis-
porphyrin systems 5–11, in different solvents and other varied
conditions is currently in progress.33
Absorption spectroscopy
Comparison of UV-visible absorption spectra of model bis-
porphyrin systems with those of their monomeric species pro-
vides evidence of intramolecular interaction. If there were no
intramolecular interactions, the absorption spectrum would be
the sum of the monomer spectra. Changes in spectral features,
such as splitting, broadening and shifting are indicative of
intramolecular interactions.7 Whole magnitudes of any
observed splittings are proportional to the degree of coupling
within the molecule34 but such splittings are often difficult to
observe. The observed spectra for the free-base, copper(),
nickel(), and zinc() monomeric and dimeric quinoxalinyl
porphyrins (Fig. 5) in deacidified chloroform show, in some
cases, signs of significant interactions but in no case can
individual band splittings be resolved and hence quantitative
analysis provided.
The spectrum of the free-base biquinoxalinyl bis-porphyrin 4
has the same shape as its monomeric component, free-base
quinoxalino[2,3-b]porphyrin 13, with slight broadening and a
red-shift for the Soret band maxima of 10 nm (Fig. 5a). Little
evidence is seen for inter-chromophoric coupling.
Analysis of the Soret-band spectra is somewhat complicated
as in all porphyrins this band has two distinct components that
may or may not be individually resolvable. In the quinoxalinyl
porphyrin monomers, these two components are polarised in
the π plane, one in the direction of the bridge, with the other
perpendicular. In bis-porphyrins, inter-porphyrin coupling is
manifest through splittings of the energies of the analogously
Experimental
General procedures
Melting points were recorded on a Riechert melting point stage
and are uncorrected. Microanalyses were performed by the
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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Fig. 5 UV-visible absorption spectra (CHCl3) of: (a) free-base biquinoxalinyl bis-porphyrin 4 and free-base quinoxalino[2,3-b]porphyrin 13,
(b) dicopper() biquinoxalinyl bis-porphyrin 6 and copper() quinoxalino[2,3-b]porphyrin 15, (c) dinickel() biquinoxalinyl bis-porphyrin 7 and
nickel() quinoxalino[2,3-b]porphyrin 16, (d) dizinc() biquinoxalinyl bis-porphyrin 5 and zinc() quinoxalino[2,3-b]porphyrin 14.
Australian Microanalytical Service, National Analytical
Laboratories, Ferntree Gully, Victoria, or the Microanalytical
Unit, The University of New South Wales, Australia.
mm). All solvents used for chromatography were redistilled
before use, unless otherwise stated. Light petroleum refers to
the fraction of bp 60–80 ЊC. Where solvent mixtures were used,
the proportions are given by volume. All solvents used for re-
crystallisations were redistilled before use, unless otherwise
stated.
Electrochemistry was performed using a BAS 100 Electro-
chemical Analyzer. All measurements were made at room
temperature, unless otherwise stated, on a sample dissolved in
Infrared spectra were determined on a Digilab FTS 20/80 or
on a Perkin-Elmer Model 1600 FT-IR spectrophotometer as
solutions in the stated solvents. Unless otherwise stated,
electronic spectra were determined as chloroform solutions
with an Hitachi 150–20 or Cary 5E UV-Vis-NIR spectro-
photometer. 1H NMR spectra were recorded on a Bruker
WM-400 (400 MHz) or AMX-400 (400 MHz) spectrometer,
with tetramethylsilane (Me4Si) as the internal standard. Signals
are recorded in terms of chemical shift (δ) in ppm from Me4Si,
multiplicity, coupling constants (in Hz) and assignments, in that
order. 31P NMR spectra were acquired on a Bruker DPX-400
(162 MHz) spectrometer. 31P NMR chemical shifts are refer-
enced to external, neat trimethyl phosphite taken to be 140.85
ppm at room temperature. The following abbreviations for
multiplicity are used: s, singlet; d, doublet; t, triplet; m, multi-
plet; h, heptet; dd, doublet of doublets; ABq, AB quartet.
Matrix assisted laser desorption ionisation time of flight
(MALDI-TOF) mass spectra without a matrix were recorded
on a VG TofSpec spectrometer. Electron impact (EI) ionisation
mass spectra were recorded on a Kratos MS902 mass spectro-
meter which was connected to a Kratos DS 90 data handling
system. Fast atom bombardment high resolution mass spectra
(FAB-HRMS) were recorded on a VG ZAB-2SEQ instrument
at the Research School of Chemistry, Australian National
University.
freshly distilled tetrahydrofuran, with 0.1
M tetrabutyl-
ammonium perchlorate (TBAP) as the supporting electrolyte.
A glassy carbon working electrode, platinum wire auxiliary
electrode and an Ag–AgCl–KCl (sat) reference electrode were
used. The solutions were de-oxygenated by saturation with oxy-
gen-free argon. The tetrahydrofuran was purified in three steps;
first it was dried over sodium wire; and then over sodium wire
and benzophenone under nitrogen, and finally freshly distilled
over lithium aluminium hydride under nitrogen. The electrolyte
was purified by recrystallisation from an ethyl acetate–ether
mixture, air dried, and then stored under vacuum over P2O5.
Synthesis of biquinoxalinyl bridged bis-porphyrins
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin} 4. 2,3-Dioxo-
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin 2 (174 mg,
0.159 mmol, 3 eq.) and 3,3Ј-diaminobenzidine 3 (11.5 mg, 0.054
mmol, 1 eq.) were dissolved in dichloromethane (10 cm3) and
stirred for 2 h. The reaction was followed by TLC (dichloro-
methane–light petroleum, 1 : 1) until no further reaction was
observed to have taken place, and the solvent was removed. The
residue was purified by flash chromatography (SiO2, chloro-
Column chromatography was routinely carried out using
flash chromatography on Merck silica gel Type 9385 (230–
400 mesh). Analytical thin layer chromatography (TLC) was
performed on Merck silica gel 60 F254 precoated sheets (0.2
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
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form–light petroleum, 1 : 1). The front-running, tan major frac-
tion was collected and evaporated to dryness. This fraction was
recrystallised to yield brown microcrystals of 6Љ,6ٞ-{5,5Ј,10,
10Ј,15,15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b:2Ј,3Ј-bЈ]bisporphyrin} 4 (112 mg, 89%), mp > 300 ЊC
(from dichloromethane and light petroleum). The reaction was
repeated using a 2 : 1 ratio of diketone–tetraamine but the yield
was only 80.6 mg (64%) (Found: C, 84.6; H, 8.5; N, 7.4.
C164H190N12 requires C, 84.6; H, 8.2; N, 7.2%); νmax(CHCl3)/
cmϪ1 3351, 2966, 2906, 2670, 1593, 1478, 1364, 1249 and 922;
λmax(CHCl3)/nm 303 (log ε 4.66), 373sh (4.79), 407sh (5.25),
423sh (5.46), 444 (5.56), 511sh (4.28), 532 (4.64), 570 (4.03), 601
(4.38), 653 (3.29) and 738 (2.09); δH(400 MHz; CDCl3; Me4Si)
Ϫ2.47 (4 H, s, inner NH), 1.51–1.60 (144 H, m, t-butyl H), 7.85
(2 H, t, J 1.8, aryl H), 7.86 (2 H, t, J 1.8, aryl H), 8.03 (2 H, d,
J8Љ,7Љ 9.0, biquinoxalinyl H-8), 8.04–8.07 (8 H, m, aryl H), 8.08
(4 H, d, J 1.8, aryl H), 8.12 (4 H, d, J 1.8, aryl H), 8.13 (4 H, d,
J 1.8, aryl H), 8.17 (2 H, dd, J7Љ,8Љ 9.0, J7Љ,5Љ 2.0, biquinoxalinyl
H-7), 8.21 (2 H, d, J5Љ,7Љ 2.0, biquinoxalinyl H-5), 8.79 (4 H, s,
H-12 and H-13), 9.01 and 9.10 (4 H, ABq, JAB 5.0, β-pyrrolic
H), 9.02 and 9.11 (4 H, ABq, JAB 5.0, β-pyrrolic H); m/z
(MALDI-TOF) 2329 (Mϩ requires 2329). Evaporation of the
second brown fraction afforded the starting material 2,3-dioxo-
5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)chlorin 2 (45 mg).
flash chromatography (SiO2, carbon tetrachloride–light petrol-
eum, 1 : 1). The major dark brown band was collected, evapor-
ated to dryness and the residue was recrystallised to give 6Љ,6ٞ-
{5,5Ј,10,10Ј,15,15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quin-
oxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dicopper() 6 (80 mg,
77%) as brown microcrystals, mp > 300 ЊC (from chloroform
and light petroleum) (Found: C, 79.5; H, 7.8; N, 6.7. C164
-
H186N12Cu2 ϩ 0.25 CHCl3 requires C, 79.5; H, 7.6; N,
6.8%); νmax(CHCl3)/cmϪ1 2966, 2934, 2906, 2871, 1594, 1478,
1467, 1394, 1364, 1351, 1298, 1248, 1159, 1010, 944 and 938;
λmax(CHCl3)/nm 412 (log ε 5.44), 453 (5.29), 478sh (5.16), 531
(4.37), 569 (4.61) and 611 (4.53); m/z (MALDI-TOF) 2453 (Mϩ
requires 2453).
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dinickel(II) 7.
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butylphenyl)-
quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin}
4 (155 mg, 0.066
mmol) was dissolved in chloroform (50 cm3) and a solution of
nickel() acetate tetrahydrate (80 mg, 0.68 mmol) in glacial
acetic acid (30 cm3) was added. The solution was heated at
reflux for 4 h. The organic layer was washed with water (2 × 100
cm3), and the water extracted with chloroform (30 cm3). The
combined organic extracts were dried over anhydrous sodium
sulfate and filtered. The solvent was removed and the residue
was purified by flash chromatography (SiO2, carbon tetrachlor-
ide–light petroleum, 1 : 1). The major dark brown fraction was
collected, evaporated to dryness and the residue was recrystal-
lised. 6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dinickel()
7 was obtained as dark brown microcrystals (131 mg, 81%), mp
> 300 ЊC (from chloroform and light petroleum) (Found: C,
78.7; H, 7.9; N, 6.7. C164H186N12Ni2 ϩ 0.5 CHCl3 requires C,
79.0; H, 7.5; N, 6.7%); νmax(CHCl3)/cmϪ1 2967, 2906, 2869,
1594, 1470, 1465, 1395, 1365, 1355, 1299, 1248, 1159, 1013 and
940; λmax(CHCl3)/nm 363sh (log ε 4.53), 415 (5.32), 456 (5.18),
471 (5.18), 524 (4.32), 566 (4.48) and 609 (4.38); δH(400 MHz;
CDCl3; Me4Si) 1.44–1.55 (144 H, m, t-butyl H), 7.70 (2 H, t, J 2,
aryl H), 7.71 (2 H, t, J 2, aryl H), 7.74 (4 H, d, J 2, aryl H), 7.75
(4 H, d, J 2, aryl H), 7.82 (2 H, t, J 2, aryl H), 7.84 (4 H, d, J 2,
aryl H), 7.85 (4 H, d, J 2, aryl H), 7.87 (2 H, t, J 2, aryl H),
7.95 (2 H, d, J8Љ,7Љ 9.0, biquinoxalinyl H-8), 8.09 (2 H, dd,
J7Љ,8Љ 9.0, J7Љ,5Љ 2.0, biquinoxalinyl H-7), 8.18 (2 H, d, J5Љ,7Љ 2.0,
biquinoxalinyl H-5), 8.69 (4 H, s, H-17, H-18, H-17Ј and
H-18Ј), 8.76 and 8.86 (4 H, ABq, JAB 5, β-pyrrolic H), 8.77 and
8.88 (4 H, ABq, JAB 5, β-pyrrolic H); m/z (MALDI-TOF) 2444
(Mϩ requires 2443).
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dizinc(II) 5.
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butylphenyl)-
quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin} 4 (88 mg, 0.038 mmol)
was dissolved in chloroform (7 cm3). A solution of zinc() acet-
ate dihydrate (191 mg, 0.87 mmol) in methanol (4 cm3) was
added and the solution heated at reflux for 4 h. The solvent was
removed and the residue was dissolved in chloroform (20 cm3),
washed with water (2 × 20 cm3), dried over anhydrous sodium
sulfate, filtered and the solvent removed. The residue was
recrystallised as quickly as possible, minimising exposure to
light. The dark brown microcrystals of 6Љ,6ٞ-{5,5Ј,10,10Ј,15,
15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,
3Ј-bЈ]bisporphyrinato}dizinc() 5 were collected (80 mg, 86%),
mp > 300 ЊC (from dichloromethane and methanol). An analyt-
ically pure sample of dizinc() bis-porphyrin 5 was obtained by
passing the compound through a short plug of silica, minimis-
ing exposure to light (Found: C, 77.1; H, 7.9; N, 6.6. C164
-
H186N12Zn2 ϩ 1.5 CH2Cl2 requires C, 77.1; H, 7.4; N, 6.5%);
νmax(CHCl3)/cmϪ1 2966, 2906, 2869, 1593, 1478, 1463, 1394,
1364, 1346, 1297, 1248, 1155, 1002 and 938; λmax(CHCl3)/nm
360 (log ε 4.69), 419sh (5.41), 432 (5.44), 457sh (5.34), 540
(4.32), 579 (4.61) and 624 (4.46); δH(400 MHz; CDCl3; Me4Si)
1.48–1.58 (144 H, m, t-butyl H), 7.81 (2 H, t, J 1.9, aryl H), 7.82
(2 H, t, J 1.9, aryl H), 7.99 (2 H, t, J 1.9, aryl H), 8.02 (4 H, d,
J 1.9, aryl H), 8.02–8.05 (2 H, m, aryl H), 8.05 (4 H, d, J 1.9,
aryl H), 8.09 (2 H, d, J 9.0, biquinoxalinyl H-8), 8.11 (4 H, d,
J 1.9, aryl H), 8.12 (4 H, d, J 1.9, aryl H), 8.22 (2 H, dd, J7Љ,8Љ 9.0,
J7Љ,5Љ 2.0, biquinoxalinyl H-7), 8.25 (2 H, d, J 2.0, biquinoxalinyl
H-5), 8.92 (4 H, s, H-12 and H-13), 9.02 and 9.08 (4 H, ABq,
JAB 5.0, β-pyrrolic H), 9.03 and 9.09 (4 H, ABq, JAB 5.0,
β-pyrrolic H); m/z (MALDI-TOF) 2455 (Mϩ requires 2456).
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dipalladium(II)
8.
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin}
4
(100 mg,
0.0429 mmol) and palladium() chloride (70 mg, 0.395 mmol)
were dissolved in toluene (15 cm3) and glacial acetic acid
(15 cm3). The mixture was then heated at reflux for 72 h. The
mixture was then diluted with chloroform (100 cm3) and
washed with water (2 × 100 cm3), sodium carbonate solution
(10%, 2 × 100 cm3) then water (2 × 100 cm3), dried over
anhydrous sodium sulfate and filtered. The filtrate was evapor-
ated to dryness and the residue purified by chromatography
over silica (chloroform–light petroleum; 1 : 1). The major band
was collected and the solvent removed to give 6Љ,6ٞ-{5,5Ј,10,10Ј,
15,15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:
2Ј,3Ј-bЈ]bisporphyrinato}dipalladium() 8 (107 mg, 98%) as a
red solid, mp > 300 ЊC (Found: C, 78.4; H, 8.05; N, 6.4.
C164H186N12Pd2 ϩ C7H8 (toluene) requires C, 78.1; H, 7.4;
N, 6.4%). νmax(CHCl3)/cmϪ1 2964s, 2904m, 2868m, 1594s,
1477m, 1464w, 1427w, 1394w, 1364s, 1320w, 1300m and 1248m;
λmax(CHCl3)/nm 356 (log ε 4.69), 407 (5.35), 446 (5.29), 466sh
(5.22), 514 (4.39), 553 (4.73) and 590 (4.63); δH(400 MHz;
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}dicopper(II) 6.
A slurry of cupric() acetate monohydrate (20 mg, 0.098 mmol)
in methanol (4 cm3) was added to 6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,
20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]-
bisporphyrin} 4 (99 mg, 0.043 mmol) in chloroform (30 cm3)
and heated at reflux for 3 h. The reaction mixture was checked
by TLC (chloroform–light petroleum, 1 : 1) and found to be
complete, thus the solvent was removed. The residue was dis-
solved in chloroform (40 cm3) and washed with water (2 ×
50 cm3), dried over anhydrous sodium sulfate, filtered and the
solvent removed completely. The compound was purified by
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CDCl3; Me4Si) 1.49–1.57 (144 H, m, t-butyl H), 7.80–7.81 (4 H,
m, aryl H), 7.96–7.99 (10 H, m, aryl H and biquinoxalinyl H-8),
8.01–8.03 (4 H, m, aryl H), 8.06 (8 H, t, J 1.8, aryl H), 8.16 (2 H,
dd, J 1.9 and 8.8, biquinoxalinyl H-7), 8.19 (2 H, d, J 1.8,
biquinoxalinyl H-5), 8.85 (4 H, br s, β-pyrrolic H), 8.86–8.89
(4 H, m, β-pyrrolic H), 8.960 (2 H, d, J 4.9, β-pyrrolic H) and
8.962 (2 H, d, J 4.9, β-pyrrolic H); m/z (MALDI-TOF) 2536
(Mϩ requires 2535).
filtered. The filtrate was evaporated to dryness and the residue
dissolved in chloroform (15 cm3). The organic phase was then
stirred with a saturated solution of potassium hexafluoro-
phosphate (1.86 g, 10.0 mmol) in water (10 cm3) for 18 h.
Chloroform (100 cm3) was then added and the mixture washed
with water (4 × 100 cm3), dried over anhydrous sodium sulfate
and filtered. The filtrate was evaporated to dryness and the
residue purified by chromatography over silica (chloroform–
methanol; 100 : 5). The first major band was collected and the
solvent removed to give unreacted free-base biquinoxalinyl bis-
porphyrin 4 (50 mg). The second major band was collected and
the solvent removed to give hexafluorophosphate[6Љ,6ٞ-{5,5Ј,
10,10Ј,15,15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}]aurate() 10 (157 mg, 60%)
as a reddish-brown solid, mp > 300 ЊC; νmax(CHCl3)/cmϪ1 3016s,
2962s, 2856m, 1726m, 1593m, 1466m, 1364m and 1248m;
λmax(CH2Cl2)/nm 424sh (log ε 5.22), 447 (5.42), 535 (4.49), 595
(4.32) and 654sh (3.50); δH(400 MHz; CDCl3; Me4Si) Ϫ2.49
(2 H, br s, inner NH), 1.40–1.50 (144 H, m, t-butyl H), 7.81–
7.83 (2 H, m, H-7Љ and H-5Љ or H-8Љ), 7.91–7.93 (2 H, m, aryl H
and biquinoxalinyl H), 8.01 (1 H, t, J 1.0, aryl H), 8.03 (2 H, d,
J 1.6, aryl H), 8.04 (2 H, d, J 1.7, aryl H), 8.05–8.06 (3 H, m,
aryl H), 8.07–8.08 (3 H, m, aryl H), 8.11–8.12 (6 H, m, aryl H),
8.13–8.14 (6 H, m, aryl H), 8.23 (1 H, d, J 2.0, H-5ٞ, H-8ٞ or
H-5Љ), 8.28 (1 H, d, J 2.0, H-5ٞ, H-8ٞ or H-5Љ), 8.34 (1 H, dd,
J 8.9 and 2.0, H-7ٞ), 8.80 (2 H, s, H-12 and H-13 on FbP), 9.04
and 9.13 (2 H, ABq, JAB 4.6, β-pyrrolic H on FbP), 9.05 and
9.12 (2 H, ABq, JAB 4.6, β-pyrrolic H on FbP), 9.24 (2 H, s,
H-12Ј and H-13Ј on AuP), 9.25–9.28 (2 H, m, β-pyrrolic H on
AuP), 9.35 (1 H, d, J 5.1, β-pyrrolic H on AuP) and 9.36 (1 H, d,
J 5.1, β-pyrrolic H on AuP); δp(162 MHz; CDCl3) Ϫ145.34 (1 P,
h, J 713, PF6); m/z (MALDI-TOF) 2524.5 ([M-PF6]ϩ requires
2524); m/z (FAB-MS) Found: [M–PF6]ϩ 2523.0. ([M–PF6]ϩ
requires 2524).
6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}zinc(II) 9. 6Љ,
6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis(3,5-di-tert-butylphenyl)-
quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin}
4 (200 mg, 0.086
mmol) was dissolved in chloroform (25 cm3) and heated at
reflux with zinc() acetate dihydrate (19 mg, 0.088 mmol, 1 eq.)
in methanol (1.5 cm3) for 2 h. The reaction was followed by
TLC (chloroform–light petroleum, 1 : 1). The solvent was
removed and the residue was dissolved in chloroform (30 cm3),
washed with water (2 × 50 cm3), dried over anhydrous sodium
sulfate, filtered and the solvent removed. The product mixture
was separated using flash chromatography (chloroform–light
petroleum, 2 : 3). The first tan band eluted was evaporated to
dryness and recrystallised to give free-base biquinoxalinyl bis-
porphyrin 4 (54 mg, 27%). The second brown band was treated
in the same way and yielded 6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-
octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bis-
porphyrinato}zinc() 9 (72 mg, 35%), mp > 300 ЊC (from
chloroform and light petroleum). In a similar fashion, the last
brown band was found to yield dizinc() biquinoxalinyl bis-
porphyrin 5 (42 mg, 20%). Analytically pure samples of the
zinc() bis-porphyrin 9 were obtained by passing the compound
through a short plug of silica, minimising exposure to light
(Found: C, 82.1; H, 8.0; N, 7.1. C164H188N12Zn requires C, 82.3;
H, 7.9; N, 7.0%); νmax(CHCl3)/cmϪ1 3220, 2965, 2867, 1594,
1478, 1364, 1295, 1236 and 1151; λmax(CHCl3)/nm 415sh (log
ε 5.40), 437 (5.50), 532 (4.52), 574 (4.42), 600 (4.30) and 618
(4.26); δH(400 MHz; CDCl3; Me4Si) Ϫ2.44 (2 H, s, inner NH),
1.51 (18 H, s, t-butyl H), 1.52 (18 H, s, t-butyl H), 1.54 (18 H, s,
t-butyl H), 1.545 (18 H, s, t-butyl H), 1.547 (18 H, s, t-butyl H),
1.55 (18 H, s, t-butyl H), 1.57 (18 H, s, t-butyl H), 1.58 (18 H, s,
t-butyl H), 7.80–7.82 (4 H, m, aryl H), 7.98–8.01 (2 H, m,
aryl H and possible biquinoxalinyl H), 8.02–8.08 (12 H, m,
aryl H and possible biquinoxalinyl H), 8.11–8.12 (8 H, m, aryl
H), 8.13–8.19 (3 H, m, biquinoxalinyl H), 8.24 (1 H, d,
J5ٞ,7ٞ 2.0 Hz, biquinoxalinyl H-5ٞ), 8.80 (2 H, s, H-12 and
H-13 on FbP), 8.83 (2 H, s, H-12Ј and H-13Ј on ZnP), 9.01–
9.04 (4 H, m, β-pyrrolic H on FbP) and 9.07–9.10 (4 H, m,
β-pyrrolic H on ZnP); m/z (MALDI-TOF) 2392.5 (Mϩ requires
2393).
Base line material from the column chromatography was
collected and the solvent removed to give trace quantities of
impure bis-chelated gold() 6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-
octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bis-
porphyrin} as a red-brown solid. m/z (MALDI-TOF) 2932,
2787, 2772, 2723, 1658 and 1590 ([M-2PF6]ϩ requires 2719).
Hexafluorophosphate[6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-
octakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]-
bisporphyrinato}]zinc(II)aurate(III)
11.
Hexafluorophos-
phate[6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-octakis(3,5-di-tert-butyl-
phenyl)-quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}]aurate()
10 (90 mg, 0.034 mmol) and zinc() acetate dihydrate (25 mg,
0.111 mmol) were dissolved in a solution of chloroform (14 cm3)
and methanol (1 cm3). The mixture was then heated at reflux
for 4 h. The mixture was then diluted with chloroform
(100 cm3) and washed with water (2 × 100 cm3), dried over
anhydrous sodium sulfate and filtered. The filtrate was evapor-
ated to dryness and the residue dissolved in chloroform
(15 cm3). The organic phase was then stirred with a saturated
solution of potassium hexafluorophosphate (2.79 g, 15.0 mmol)
in water (15 cm3) for 18 h. Chloroform (100 cm3) was then
added and the mixture was washed with water (4 × 100 cm3),
dried over anhydrous sodium sulfate and filtered. The solvent
was removed under vacuum and the residue purified by
chromatography over silica (chloroform–methanol; 100 : 6).
The major polar band was collected, the solvent removed and
the residue recrystallised from a chloroform–light petroleum
solution (1 : 5) to afford hexafluorophosphate[6Љ,6ٞ-{5,5Ј,10,
10Ј, 15,15Ј,20,20Ј-octakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b:2Ј,3Ј-bЈ]bisporphyrinato}]zinc()aurate() 11 (80 mg,
87%) as green-blue crystals, mp > 300 ЊC; νmax(CHCl3)/cmϪ1
3688w, 3606w, 2965s, 2868m, 1718w, 1593m, 1477m, 1427w,
Hexafluorophosphate[6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-octakis-
(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphyrin-
ato}]aurate(III) 10. 6Љ,6ٞ-{5,5Ј,10,10Ј,15,15Ј,20,20Ј-Octakis-
(3,5-di-tert-butylphenyl)quinoxalino[2,3-b:2Ј,3Ј-bЈ]bisporphy-
rin} 4 (240 mg, 0.103 mmol), potassium tetrachloroaurate()
(100 mg, 0.265 mmol) and sodium acetate (80 mg, 0.975 mmol)
were dissolved in a solution of chloroform (15 cm3) and glacial
acetic acid (18 M, 15 cm3). The reaction mixture was then
heated at reflux for 24 h. The solution was then allowed to cool
and fresh potassium tetrachloroaurate() (100 mg, 0.264
mmol) and sodium acetate (90 mg, 1.097 mmol) were added
with additional glacial acetic acid (18 M, 1 cm3). The mixture
was then heated at reflux for another 24 h, allowed to cool and
further potassium tetrachloroaurate() (61 mg, 0.161 mmol),
sodium acetate (59 mg, 0,72 mmol) and glacial acetic acid
(18 M, 1 cm3) were added. The mixture was heated at reflux for
a further 24 h, allowed to cool and then diluted with chloro-
form (150 cm3). The mixture was then washed with water
(2 × 150 cm3), sodium carbonate solution (10%, 2 × 150 cm3),
water (2 × 150 cm3), dried over anhydrous sodium sulfate and
1394w, 1364m, 1297w, 1248w, 1222s, 1214s and 1157w; λmax
-
(CHCl3)/nm 354 (log ε 4.71), 392brsh (4.93), 421sh (5.27), 445
(5.42), 547 (4.46), 576 (4.47), 587brsh (4.42) and 615 (4.27);
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
2785

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δH(400 MHz; CDCl3; Me4Si) 1.53–1.59 (144 H, m, t-butyl H),
7.81–7.83 (2 H, m, aryl H), 7.92–7.94 (3 H, m, aryl H), 8.00–
8.03 (4 H, m, aryl H and biquinoxalinyl H), 8.04–8.06 (6 H, m,
aryl H and biquinoxalinyl H), 8.07–8.14 (11 H, m, aryl H and
biquinoxalinyl H), 8.18 (1 H, dd, J7Љ,8Љ 8.8, J7Љ,5Љ 1.9, H-7Љ),
8.28–8.30 (2 H, m, biquinoxalinyl H), 8.36 (1 H, dd, J7ٞ,8ٞ 8.9,
J7ٞ,5ٞ 1.9, H-7ٞ), 8.94 (2 H, s, H-12 and H-13 on ZnP), 9.04
and 9.08 (2 H, ABq, JAB 4.6, β-pyrrolic H on ZnP), 9.05
and 9.09 (2 H, ABq, JAB 4.6, β-pyrrolic H on ZnP), 9.25 (2 H,
s, H-12Ј and H-13Ј on AuP), 9.26–9.27 (2 H, m, β-pyrrolic H
on AuP), 9.35 (1 H, d, J 5.2, β-pyrrolic H on AuP) and 9.36
(1 H, d, J 5.1, β-pyrrolic H on AuP); δp(162 MHz; CDCl3)
Ϫ145.34 (1 P, h, J 713, PF6); m/z (MALDI-TOF) 2588
([M–PF6]ϩ requires 2588).
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b]porphyrinato}copper(II) 15. 5,10,15,20-Tetrakis(3,5-di-
tert-butylphenyl)quinoxalino[2,3-b]porphyrin 13 (143 mg, 0.123
mmol) was dissolved in dichloromethane (30 cm3) and a slurry
of copper() acetate monohydrate (245 mg, 1.22 mmol) in
methanol (25 cm3) was added. The mixture was heated at reflux
for 4 h and the solvent removed. The residue was dissolved in
dichloromethane (50 cm3), washed with water (2 × 100 cm3),
dried over anhydrous sodium sulfate, filtered and evaporated to
dryness. The residue was recrystallised to afford brown micro-
crystals of {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quin-
oxalino[2,3-b]porphyrinato}copper() 15 (134 mg, 89%), mp >
300 ЊC (from dichloromethane) (Found: C, 80.3; H, 7.7; N, 7.1.
C82H94N6Cu requires C, 80.3; H, 7.7; N, 6.9%); νmax(CHCl3)/
cmϪ1 2967, 2906, 2869, 1594, 1478, 1466, 1426, 1394, 1364,
1347, 1300, 1248, 1181, 1120, 1010, 944, 899, 862 and 827;
λmax(CHCl3)/nm 341 (log ε 4.41), 406 (5.11), 441 (5.10), 528
(3.89), 562 (4.19) and 606 (4.22); m/z (EI) 1226 (Mϩ, 100%).
Synthesis of quinoxalino[2,3-b]porphyrins
5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]-
porphyrin 13. 2,3-Dioxo-5,10,15,20-tetrakis(3,5-di-tert-butyl-
phenyl)chlorin 2 (246 mg, 0.225 mmol) and o-phenylene-
diamine 12 (120 mg, 1.10 mmol) were dissolved in dichloro-
methane (11 cm3) and stirred for 6 h. The solvent was removed.
The residue was dissolved in chloroform (50 cm3), washed with
water (2 × 100 cm3), dried over anhydrous sodium sulfate and
filtered. The solvent was removed and the residue was purified
using flash chromatography (SiO2, dichloromethane–light
petroleum, 2 : 3). The front running major red band was
collected and evaporated to dryness. This residue was recrystal-
lised to afford red microcrystals of 5,10,15,20-tetrakis(3,5-di-
tert-butylphenyl)quinoxalino[2,3-b]porphyrin 13 (241 mg, 92%),
mp > 300 ЊC (from dichloromethane and methanol) (Found: C,
84.7; H, 8.2; N, 7.3. C82H96N6 requires C, 84.5; H, 8.3; N,
7.2%); νmax(CHCl3)/cmϪ1 3350, 2967, 2905, 2869, 1593, 1478,
1364 and 1248; λmax(CHCl3)/nm 356 (log ε 4.36), 413sh (5.08),
434 (5.33), 493sh (3.70), 530 (4.27), 566 (3.84), 599 (4.03) and
651 (2.95); δH(400 MHz; CDCl3; Me4Si) Ϫ2.47 (2 H, s, inner
NH), 1.49 (36 H, s, t-butyl H), 1.53 (36 H, s, t-butyl H),
7.72–7.75 (2 H, m, quinoxalino H), 7.80 (2 H, t, J 1.8, aryl H),
7.82–7.85 (2 H, m, quinoxalino H), 7.93 (2 H, t, J 1.8, aryl H),
7.98 (4 H, d, J 1.8, aryl H), 8.11 (4 H, d, J 1.8, aryl H), 8.80
(2 H, s, H-12 and H-13), 8.99 and 9.07 (4 H, ABq, JAB 5.0, H-7,
H-8, H-17 and H-18); m/z (MALDI-TOF) 1166 (Mϩ requires
1166).
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b]porphyrinato}nickel(II) 16. 5,10,15,20-Tetrakis(3,5-di-
tert-butylphenyl)quinoxalino[2,3-b]porphyrin 13 (157 mg, 0.135
mmol) was dissolved in dichloromethane (30 cm3) and a solu-
tion of nickel() acetate tetrahydrate (160 mg, 1.36 mmol) in
glacial acetic acid (15 cm3) was added. The mixture was heated
at reflux for 4 h, washed with water (2 × 100 cm3), sodium
carbonate solution (5%, 2 × 100 cm3), water (2 × 100 cm3),
dried over anhydrous sodium sulfate, filtered and evaporated to
dryness. The residue was recrystallised to afford brown micro-
crystals of {5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quin-
oxalino[2,3-b]porphyrinato}nickel() 16 (105 mg, 64%), mp >
300 ЊC (from dichloromethane and methanol) (Found: C, 80.5;
H, 7.7; N, 7.1. C82H94N6Ni requires C, 80.6; H, 7.8; N, 6.9%);
νmax(CHCl3)/cmϪ1 2967, 2906, 2869, 1594, 1478, 1466, 1394,
1364, 1353, 1299 and 1248; λmax(CHCl3)/nm 339sh (log ε 4.39),
355 (4.45), 410 (5.01), 436 (5.09), 530 (4.03), 536sh (4.02), 560
(4.03) and 602 (4.09); δH(400 MHz; CDCl3; Me4Si) 1.42, 1.46
and 1.55 (72 H, s, t-butyl H), 7.70 (2 H, m, aryl Hp), 7.70 (4 H,
d, J 1.8, aryl H), 7.72–7.76 (2 H, m, quinoxalino H), 7.75 (2 H,
m, aryl H), 7.78–7.82 (2 H, m, quinoxalino H), 7.84 (4 H, d,
J 1.8, aryl H), 8.68 (2 H, s, H-12 and H-13), 8.76 and 8.90 (4 H,
ABq, JAB 5, H-7, H-8, H-17 and H-18); m/z (EI) 1222 (Mϩ,
100%).
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b]porphyrinato}zinc(II) 14. A solution of zinc() acetate
dihydrate (220 mg, 1.0 mmol) in methanol (1 cm3) was heated at
reflux with 5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)quin-
oxalino[2,3-b]porphyrin 13 (116 mg, 0.10 mmol) in dichloro-
methane (20 cm3) for 30 min. The solution was evaporated to
dryness, dissolved in dichloromethane (20 cm3), washed with
water (2 × 50 cm3), dried over anhydrous sodium sulfate and
filtered. The solvent was removed and the residue was purified
using flash chromatography (SiO2, dichloromethane–light
petroleum, 2 : 3). The main red band was collected, evaporated
to dryness and recrystallised to give {5,10,15,20-tetrakis(3,5-
di-tert-butylphenyl)quinoxalino[2,3-b]porphyrinato}zinc() 14
(100 mg, 82%), mp > 300 ЊC (from dichloromethane and
methanol) (Found: C, 78.95; H, 7.8; N, 6.95. C82H94N6Zn ϩ
H2O requires C, 79.0; H, 7.8; N, 6.7%); νmax(CHCl3)/cmϪ1 2959,
2932, 2906, 2869, 1593, 1478, 1462, 1394, 1364, 1342, 1298,
1174, 1135, 1118, 1015, 1007 and 938; λmax(CHCl3)/nm 337 (log
ε 4.39), 412sh (5.10), 424 (5.11), 447 (5.09), 538 (3.80), 574
(4.20) and 618 (4.15); δH(400 MHz; CDCl3; Me4Si) 1.48 (36 H,
s, t-butyl H), 1.53 (36 H, s, t-butyl H), 7.77–7.82 (2 H, m,
quinoxalino H), 7.80 (2 H, t, J 1.8, aryl H), 7.88–7.92 (2 H, m,
quinoxalino H), 7.93 (2 H, t, J 1.8, aryl H), 7.97 (4 H, d, J 1.8,
aryl H), 8.10 (4 H, d, J 1.8, aryl H), 8.91 (2 H, s, H-12 and
H-13), 9.00 and 9.05 (4 H, ABq, JAB 5.0, H-7, H-8, H-17 and H-
18); m/z (MALDI-TOF) 1229 (Mϩ requires 1229).
{5,10,15,20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino-
[2,3-b]porphyrinato}palladium(II) 17. 5,10,15,20-Tetrakis(3,5-
di-tert-butylphenyl)quinoxalino[2,3-b]porphyrin 13 (103 mg,
0.09 mmol) and palladium() chloride (91 mg, 0.513 mmol)
were dissolved in toluene (20 cm3) and glacial acetic acid
(20 cm3). The mixture was then heated at reflux for 48 h. The
mixture was then diluted with dichloromethane (40 cm3) and
washed with water (100 cm3), sodium carbonate solution (10%,
2 × 100 cm3), water (2 × 100 cm3), dried over anhydrous sodium
sulfate and filtered. The filtrate was evaporated to dryness and
the residue purified by chromatography over silica (dichloro-
methane–light petroleum; 1 : 3). The major band was collected
and the solvent removed to give {5,10,15,20-tetrakis(3,5-di-
tert-butylphenyl)quinoxalino[2,3-b]porphyrinato}palladium()
17 (98 mg, 88%) as a red solid, mp > 300 ЊC (from dichloro-
methane and methanol) (Found: C, 77.7; H, 7.5; N, 6.4.
C82H94N6Pd requires C, 77.5; H, 7.5; N, 6.6%); νmax(CHCl3)/
cmϪ1 2964s, 2904m, 2869m, 1594s, 1477m, 1464w, 1427w,
1393w, 1364s, 1301w and 1248m; λmax(CHCl3)/nm 334sh (log
ε 4.41), 345 (4.47), 401 (4.99), 436 (5.09), 510 (3.88), 545 (4.29)
and 586 (4.37); δH(400 MHz; CDCl3; Me4Si) 1.48 (36 H, s,
t-butyl H), 1.52 (36 H, s, t-butyl H), 7.75–7.80 (4 H, m, aryl H
and quinoxaline H), 7.84–7.87 (2 H, m, quinoxaline H), 7.91–
7.93 (6 H, m, aryl H), 8.05 (4 H, d, J 1.8, aryl H), 8.83 (2 H, s,
H-12 and H-13), 8.84 and 8.92 (4 H, ABq, J 4.9, β-pyrrolic H);
m/z (MALDI-TOF) 1271 (Mϩ requires 1270).
O r g . B i o m o l . C h e m . , 2 0 0 3 , 1, 2 7 7 7 – 2 7 8 7
2786

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8 K. A. Jolliffe, T. D. M. Bell, K. P. Ghiggino, S. J. Langford and
Hexafluorophosphate{5,10,15,20-tetrakis(3,5-di-tert-butyl-
phenyl)quinoxalino[2,3-b]porphyrinato}aurate(III) 18. 5,10,15,
20-Tetrakis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphy-
rin 13 (100 mg, 0.086 mmol), potassium tetrachloroaurate()
(80 mg, 0.211 mmol) and sodium acetate (70 mg, 1.30 mmol)
were dissolved in a solution of chloroform (15 cm3) and glacial
acetic acid (18 M, 15 cm3). The reaction mixture was then
heated at reflux for 24 h. The mixture was then diluted with
chloroform (100 cm3) and washed with water (3 × 100 cm3),
sodium carbonate solution (10%, 2 × 100 cm3), water (2 ×
100 cm3), dried over anhydrous sodium sulfate and filtered. The
filtrate was removed under vacuum and the residue dissolved in
chloroform (20 cm3). The organic phase was then stirred with a
saturated solution of potassium hexafluorophosphate (1.86 g,
10.1 mmol) in water (20 cm3) for 24 h. The mixture was then
diluted with chloroform (100 cm3) and washed with water
(4 × 100 cm3), dried over anhydrous sodium sulfate and filtered.
The filtrate was evaporated to dryness and the residue purified
by chromatography over silica (chloroform–methanol; 100 : 5).
The front running band was collected and the solvent removed
to yield unreacted free-base quinoxalino[2,3-b]porphyrin 13
(25 mg).
The major polar band was collected and the solvent evapor-
ated to dryness to afford hexafluorophosphate{5,10,15,20-tetra-
kis(3,5-di-tert-butylphenyl)quinoxalino[2,3-b]porphyrinato}-
aurate() 18 (60 mg, 51%) as a red-brown solid, mp > 300 ЊC;
(FAB-HRMS found: [M–PF6]ϩ 1359.7206. C82H94N6Au
requires 1359.7212); νmax(CHCl3)/cmϪ1 3688w, 3018w, 2965s,
2870m, 1734s, 1594m, 1477w, 1395m, 1365w, 1300m, 1247m,
1214m, 1124w, 1064w and 1032w; λmax(CHCl3)/nm 344 (log
ε 4.37), 392 (4.58), 418brsh (4.83), 436 (5.28), 510sh (3.88),
545 (4.05) and 587 (4.10); δH(400 MHz; CDCl3) 1.48 (36 H, s,
t-butyl H), 1.53 (36 H, s, t-butyl H), 7.91 (2 H, t, J 1.6, aryl
H), 7.93 (4 H, d, J 1.6, aryl H), 7.94–7.98 (4 H, m, quinoxalino
H), 8.01 (2 H, t, J 1.6, aryl H), 8.07 (4 H, d, J 1.6, aryl H), 9.23
(2 H, s, H-12 and H-13), 9.24 and 9.33 (4 H, ABq, JAB 5.1, H-7,
H-8, H-17 and H-18); δP(162 MHz; CDCl3) Ϫ145.34 (1 P, h,
J 713, PF6); m/z (MALDI-TOF) 1360.5 ([M–PF6]ϩ requires
1361).
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Acknowledgements
We thank the Australian Research Council for research grants
to M. J. C. and the Australian Post-graduate Research Award to
R. W. and P. J. S.
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