New meso-substituted trans-A2B2 di(4-pyridyl) porphyrins as building blocks for metal-mediated self-assembling of 4 + 4 Re(i)-porphyrin metallacycles

Article abstract of DOI:10.1039/c3ob40452h

The reaction between 5-(4-pyridyl)dipyrrylmethane and aromatic aldehydes affords meso-arylsubstituted trans-A₂B₂ di(4-pyridyl)porphyrins which are key building blocks in the metal-mediated self-assembling of supramolecular structures

Full text of DOI:10.1039/c3ob40452h

Organic &
Biomolecular Chemistry
View Article Online
View Journal
PAPER
New meso-substituted trans-A2B2 di(4-pyridyl)-
porphyrins as building blocks for metal-mediated
Cite this: DOI: 10.1039/c3ob40452h
self-assembling of 4 + 4 Re(I)–porphyrin metallacycles†
Mariangela Boccalon, Elisabetta Iengo* and Paolo Tecilla*
The reaction between 5-(4-pyridyl)dipyrrylmethane and aromatic aldehydes affords meso-arylsubstituted
trans-A2B2 di(4-pyridyl)porphyrins which are key building blocks in the metal-mediated self-assembling
of supramolecular structures. A careful optimization of the reaction conditions allowed us to obtain 5,15-
diphenyl-10,20-di(4-pyridyl)porphyrin (P1), and two analogues bearing on the meso-phenyl substituents
two dipropyl- (P4) or dihexyl-alkyl chains (P5), with yields ranging from 53 to 63%. Porphyrin P1 reacts
with Re(CO5)Br to give the expected 4 + 4 Re(I)–porphyrin metallacycle which has been fully characterized
by means of infrared, NMR and UV-Vis (absorption and emission) spectroscopies and by guest inclusion
studies. Unexpectedly the addition of alkyl chains to the porphyrin fragment, which increase the solubi-
lity of the porphyrin in organic solvents, has the opposite effect on the adduct with Re(I). Indeed, the
reaction between Re(CO5)Br and porphyrins P4,5 gives very insoluble materials, hampering their com-
plete characterization.
Received 5th March 2013,
Accepted 22nd April 2013
DOI: 10.1039/c3ob40452h
www.rsc.org/obc
appealing features: rigid and planar geometries, high chemical
stability, inherent symmetry, intense electronic absorption
Introduction
Self-assembling of tailored building blocks is a widely used bands in the visible region, a relatively long fluorescence decay
strategy for obtaining complex molecular architectures in solu- time, and facile tunability of their optical and redox properties
tion and in the solid state. Among the different approaches by metallation/functionalization.2 Conjugation of these chromo-
that can be exploited to guide the self-assembling process, the phoric ligands with several metal ions (Pd(II), Pt(II), Ru(II), Re(I))3
metal-mediated directional-bonding has emerged over the has led to a variety of coordination adducts with interesting
years as a general, high yielding synthetic strategy that gives potential applications in the fields of optoelectronic, catalysis,
access to a variety of 2D (rhomboids, squares, rectangles, tri- molecular recognition, etc.
angles, etc.) and 3D (trigonal pyramids and prisms, cubes,
A particularly interesting class of metal-bridged arrays is
cuboctahedra, double squares, adamantanoids, dodecahedra, that formed by porphyrin ligands with Re(I) complexes.
and a variety of other cages) supramolecular ensembles.1 One Indeed, this metal ion, usually introduced using the Re(CO)5X
of the most attractive characteristics of the metal-mediated self- (X = Cl or Br) precursor, ensures high thermodynamic stability
assembling approach is the relatively high degree of control that of the supramolecular adducts at room temperature. At high
can be achieved on the geometry and on the thermodynamic temperatures, however, rhenium–N bonds are labile enough to
and kinetic stability of the supramolecular adducts. This is allow for conversion of kinetic structures (such as open oligo-
obtained by the appropriate selection of organic ligands with a mers) to thermodynamic structures during the assembly
topologically well-defined set of donor atoms which are com- process. The group of J. T. Hupp has shown that in weakly co-
bined with metal fragments with the desired properties such as ordinating solvents such as a mixture of toluene and tetrahydro-
the bite angle, number and geometry of the binding sites and furan, the 1 : 1 combination of Re(CO)5X and a rigid or semi-
stability of the metal–ligand interaction. In this context, por- rigid dipyridyl ligand (L) generally produces molecular squares
phyrin-based ligands and in particular meso substituted pyridyl- (Fig. 1) in high yield.4 The strong trans labilizing effect of CO
porphyrins have attracted increasing interest because of several allows a pair of cis carbonyl ligands, and only those, to be
replaced first with solvent molecules and then with the pyridyl
donors, while maintaining a fac geometry of the residual ones.
Department of Chemical and Pharmaceutical Sciences, University of Trieste, via
With ditopic ligands, the number of Re–N bonds is maximized
Giorgieri 1, I-34127 Trieste, Italy. E-mail: eiengo@units.it, ptecilla@units.it
by forming cyclic as opposed to open structures and strain is
†Electronic supplementary information (ESI) available: Additional experimental
minimized by forming structures having square geometries
information, including NMR, IR and UV-Vis spectra. See DOI:
10.1039/c3ob40452h
(although triangular assemblies are known).5
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
purification. In particular, with meso substituted pyridyl-
porphyrins the standard condensation procedures are often
scarcely effective and a careful optimization of the experimental
parameters is required to ensure a satisfactory yield of the
desired product. We are interested in meso-substituted trans-
A2B2 dipyridylporphyrins with two trans meso positions occu-
pied by 4-pyridyl residues and the other two functionalized with
conformationally rigid moieties, which may be further trans-
formed into useful functional groups. We thus chose 5,15-
diphenyl-10,20-di(4-pyridyl)porphyrin (trans-DPyDPhP, P1) as a
lead compound for the optimization of the synthetic
procedures.
trans-A2B2 dipyridylporphyrins may be prepared with the
conventional Adler’s synthesis by mixing the two aldehydes
and pyrrole in acidic conditions.14 This approach is still fol-
lowed, particularly when also the isomers other than the trans-
A2B2 are desired, but the yields are usually in the range 2–4%
and the purification of the six regioisomers formed is quite
time consuming.15 A more convenient method for the direct
synthesis of meso-substituted trans-A2B2 porphyrins is the reac-
tion of a dipyrromethane with an aldehyde in acidic con-
ditions proposed by Lindsey (Scheme 1).16 With this approach
the reported yields of trans-DPyDPhP,17 and of other related
porphyrins with differently substituted phenyl rings, are in the
6–16% range.18 Moving from this standpoint, we investigated a
set of reaction conditions in order to increase the synthetic
efficiency. To ensure flexibility in the substitution pattern of
the phenyl rings we started with 5-(4-pyridyl)dipyrrylmethane
which is easily prepared in 65% yield from 4-pyridinecarboxy-
aldehyde and pyrrole.19 The dipyrromethane was reacted with
benzaldehyde in the presence of trifluoroacetic acid (TFA)
under different reaction conditions, as summarized in Table 1.
All the reactions were carried out using equimolar concen-
trations of dipyrromethane and aldehyde (6.4 × 10−3 M) and
varying the number of equivalents of TFA, the solvent, the
temperature and the reaction time. The reaction is very sensi-
tive to the different parameters and in several conditions we
Fig. 1 Formation of a 4 + 4 Re(I)–L metallacycle.
The 4 + 4 Re(I)–L metallacycles thus obtained have a
roughly cubical box structure with edges, in the case of L =
trans-dipyridylporphyrin, about 2 nm long. The porphyrin
ligands may rotate around the Re–pyridyl bond and compu-
tational studies suggest that in the lowest energy conformers
the four tetrapyrrolic rings assume a tilted disposition thus
reducing the inner volume of the molecular box.6 However,
Hupp and co-workers have demonstrated that Re(I)-metalla-
cycles with zincated porphyrin are able to incorporate inside
the cavity different N-donor ligands that bind axially to the
Zn(II) ions.7 The ability of the Re(I)-metallacycle to bind sub-
strates, combined with its photo-luminescence properties, is at
the basis of the development of epoxidation catalysts,8 chemo-
sensors9 and of polymeric materials with interesting properties
for chemical sensing and molecular sieving.10
We have recently reported that a 4 + 4 Re(I)–porphyrin metalla-
cycle, in which the porphyrins are functionalized with peripheral
carboxylic acid residues, is able to form a dimeric nanopore in a
phospholipid bilayer showing tuneable ionophoric activity.11 In
this case the robustness of the metallacycle was essential to pre-
serve its structural integrity in the complex and competitive
membrane environment. Indeed, the attempt to use Pd(II), which
gives kinetically labile complexes, and di(meso-3-pyridyl)porphy-
rins to assemble a cyclic structure directly into liposomes led to a
complex mixture of linear and cyclic adducts.12
Stimulated by this finding and following our interest in syn-
thetic ionophores13 we decided to widen the library of avail-
able 4 + 4 Re(I)–porphyrin metallacycles. Indeed, despite their
interesting properties, the number of such adducts described
in the literature is extremely limited and this is probably a con-
sequence of the non-trivial synthesis of the porphyrin precur-
sors, of the scarce solubility of the Re(I) adducts, and of their
difficult characterization. In this manuscript we report on the
synthesis and characterization of new trans-dipyridylporphyr-
ins designed to improve their solubility in organic solvents,
and our subsequent efforts to prepare and characterize new
4 + 4 Re(I)–porphyrin metallacycles.
Results and discussion
Synthesis of meso-substituted trans-A2B2 di(4-pyridyl)-
porphyrins
Despite the many literature reports, the synthesis of porphyr-
Scheme 1 (a) Pyrrole as a solvent, 85 °C, 15 h, 65%; (b) TFA, DCM, 20 min.,
ins is still facing problems of low yields and difficult 0 °C; (c) r.t., 1 h, 53% from 1.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
Table 1 Optimization of the trans-DPyDPhP synthesisa
Entry
TFA
Temp.
Time
Solvent
Yield
1
2
3
4
5
6
7
8
9
0.22 eq.
0.22 eq.
1 eq.
1 eq.
1 eq.
rt
rt
rt
rt
Reflux
rt
0 °C
0 °C
0 °C
Overnight
Overnight
Overnight
Overnight
Overnight
Overnight
3 hours
DCM
MeOH
DCM
MeOH
MeOH
MeOH
DCM
Traces
Traces
Traces
Traces
Traces
Traces
12%
5 eq.
10 eq.
10 eq.
43 eq.
20 minutes
20 minutes
DCM
DCM
23%
53%
a All reactions were carried out using equimolar amounts of
dipyrromethane and aldehyde (2.24 mmol) in 350 mL of solvent. The
reaction was quenched by the addition of DDQ (4.48 mmol).
obtained only traces of the desired product. The best results
were obtained for a high number of equivalents of the acid, a
short reaction time and a low temperature (entry 9). In these
conditions the yield in isolated product is 53%, which is pretty
high for porphyrin synthesis. Using this optimized procedure
we also obtained 39% yield in the synthesis of 5,15-di(4-car-
boxymethylphenyl)-10,20-di(4-pyridyl)porphyrin (P2).11
Porphyrins and porphyrins adducts, in particular their
neutral Re(I) derivatives, are generally poorly soluble in organic
solvents and an increased solubility is usually obtained by the
insertion of alkyl chains in positions 3,4 of the β-pyrrolic ring.
We therefore prepared the dipyrromethanes 4a,b and 520 by con-
densation of the substituted pyrrole, obtained via the Barton–
Zard synthesis,21 and the appropriate aldehyde (Scheme 2).
Each dipyrromethane was then reacted with the complemen-
tary aldehyde in order to obtain porphyrins P3a,b. However,
either under the optimized reaction conditions reported
above, or by increasing the temperature, the reaction times
and also testing different procedures,22 the desired porphyrins
were never obtained. Although there are several examples of
meso tetra-aryl porphyrins per-alkyated on the β-pyrrolic rings,
a search of the literature shows that the only known compound
containing meso-pyridyl groups is the 2,8,12,18-tetrabutyl-
3,7,13,17-tetramethyl-5,10,15,20-tetra-(4-pyridyl)-porphyrin
which was prepared by Adler’s method.23 Clearly, the high
steric hindrance of the alkyl substituents together with the
peculiar reactivity of the pyridyl moiety makes the preparation
of this type of porphyrins quite difficult.
Scheme 2 (a) p-TSA, toluene, reflux, 48 h, 3a = 81.6%, 3b = 39.8%; (b) NaOH,
ethylene glycol, reflux, 24 h, 4a = 88.4%, 4b = 53.4%; (c) TFA, DCM, 20 min.,
0 °C; (d) r.t., 1 h, no product was isolated.
Scheme 3 (a) H2SO4, MeOH, reflux, 18 h, 96%; (b) 1-bromoalkane, K2CO3,
TBABr, acetone, reflux, 18 h, 8a = 95%, 8b = 90%; (c) CuCN, DMF, reflux, 48 h;
(d) LiOH, THF–MeOH, TBABr, r.t., 18 h, 10a = 70%, 10b = 81% from 8; (e) hydro-
xylamine hydrochloride, 4-methylmorpholine, isobutyl chloroformate, DCM, r.t.,
18 h, 11a = 57%, 11b = 67%; (f) LiAlH4, THF, r.t., 4 h, 12a = 79%, 12b = 75%.
Due to the synthetic problems illustrated above, we decided
to insert the alkyl chains on the meso aryl rings. We therefore
prepared aldehydes 12a,b bearing two propyloxy or hexyloxy step (reflux at 150–160 °C in a strongly acidic medium) pro-
chains in meta position and a cyano group in para (Scheme 3). moted the partial hydrolysis of the methyl ester. Therefore, the
This last one could be eventually easily transformed into some following hydrolysis step was conducted directly on the crude
useful functional groups.
of the reaction affording the desired carboxylic acids 10a and
The synthesis started from commercially available 4-bromo- 10b in 70% and 81% yield, respectively, calculated from 8. The
3,5-dihydroxybenzoic acid which was transformed into its acids were transformed into the corresponding Weinreb
methyl ester in 96% yield. The ester was then reacted with amides 11a and 11b with 57% and 67% yield, respectively, by
propyl- or hexylbromide by Williamson ether synthesis to give treatment with hydroxylamine hydrochloride in the presence
8a24 and 8b in 95% and 90% yield, respectively. Nucleophilic of 4-methylmorpholine as a base and isobutyl chloroformate
substitution of the bromoderivate 8 with CuCN gave the cyano as a condensing agent. Final reduction with LiAlH4 gave alde-
compound 9. The drastic reaction conditions required for this hydes 12a and 12b with 79% and 75% yield, respectively.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
Scheme 4 (a) TFA, DCM, 20 min., 0 °C; (b) r.t., 1 h, P4 = 61%, P5 = 63%; from 12.
Reaction of aldehydes 12a,b with dipyrromethane 1 under
optimized conditions (TFA 43 eq., DCM, 0 °C, 20 min, followed
by oxidation with DDQ) gave porphyrins P4 and P5 in 61% and
63% yield, respectively (Scheme 4). The two porphyrins were
fully characterized by 1H-NMR, 13C-NMR, IR and UV-Vis spectro-
scopies and ESI-MS spectrometry. As expected, porphyrins P4
and P5 are very well soluble in the common organic solvents.
Scheme 5 (a) THF–toluene 4 : 1, 48 h, reflux, 87%; (b) CHCl3, overnight, r.t., 72%.
Synthesis and characterization of 4 + 4 Re(I)–porphyrin
metallacycles
aggregates in solution and the insufficient volatility, which
strongly hampers the use of mass spectrometry experiments.
In addition, rotational freedom of the porphyrin edges around
the metal–pyridyl bond may lead to a population of confor-
mers, which is further complicated by the presumably stati-
stical distribution of the syn/anti isomers derived by the
orientation of the halide ligand on each rhenium corner that
can be up or down with respect to the square framework.4 As a
consequence, the structural data present in the literature are
scarce and the determination of the size and nuclearity of
such metallacycles (4 + 4 vs. 3 + 3 or else) is based on one gel
permeation chromatography study,25 on the determination of
the diffusion coefficients using pulsed-field-gradient NMR,‡26
and on one reported FAB mass spectrum.7a We have recently
reported the characterization of one such metallacycle by
means of infrared, NMR and UV-Vis (absorption and emission)
spectroscopies which, combined with guest inclusion studies,
gave support to the cyclic 4 + 4 structure.11 We, therefore,
applied the same methodological approach for the characteriz-
ation of [Re(CO)3(P1)Br]4.
The assembling of the 4 + 4 Re(I)–porphyrin metallacycles fol-
lowed the procedure reported by Hupp and co-workers.7a Equi-
molar amounts of porphyrin P1 and Re(CO)5Br (0.162 mmol)
dissolved in 4 : 1 THF–toluene (80 mL) were heated at reflux
for 48 hours (Scheme 5). Precipitation of the crude product
and repeated washing cycles with diethyl ether afforded metal-
lacycle [Re(CO)3(P1)Br]4 as a maroon microcrystalline powder
in 87% yield. The zincated derivative [Re(CO)3(Zn·P1)Br]4 was
obtained by treatment of a chloroform solution of [Re-
(CO)3(P1)Br]4 with a saturated solution of Zn(OAc)2 in metha-
nol. After stirring overnight the solvents were evaporated and
the residue was washed thoroughly with methanol to eliminate
the excess of Zn(OAc)2. [Re(CO)3(Zn·P1)Br]4 was obtained as a
purple solid in 72% yield. While the [Re(CO)3(P1)Br]4 is
soluble in common solvents such as CHCl3, DMSO, and THF,
its zincated derivative is less soluble.
Following the original 1997 report by Hupp and co-workers,7a
only about seven tetraporphyrin metallacycles with Re-(CO)3X
(X = Cl− or Br−) corners differing in the nature of the
trans-di(pyridyl)porphyrin edges have been described.4,7b,11
Moreover, full characterization of these metallacycles remains
problematic and partially unachieved. Difficulties in gaining
an unambiguous characterization derive from several inherent
properties of these neutral tetraporphyrin assemblies, in par-
‡The reported NMR characterization of these species is very poor. Apart our pre-
vious communication (ref. 11), in which we reported some 1H NMR data, this is
the first paper, to our knowledge, in which a 4 + 4 Re(I)–dipyridylporphyrin
metallacycle has been fully characterized by 1H NMR. The only other two
1H NMR spectra we found in the literature, refer to 4 + 4 Pt(II)–porphyrin metal-
ticular their low solubility and high tendency to form lacycles (ref. 3d and 3f).
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
Fig. 2 Selected regions of the 1H-NMR spectra (500 MHz, CDCl3) of (a) por-
phyrin P1, (b) [Re(CO)3(P1)Br]4, and (c) [Re(CO)3(Zn·P1)Br]4.
Fig. 3 IR spectra in KI of P1 (dashed line) and [Re(CO)3(P1)Br]4 (solid line).
Fig. 2 shows the downfield region of the 1H-NMR spectra
(500 MHz, CDCl3) of porphyrin P1, metallacycle [Re(CO)3-
(P1)Br]4 and its zincated analogue [Re(CO)3(Zn·P1)Br]4.
The number of resonances in the 1H-NMR spectrum
of [Re(CO)3(P1)Br]4 is consistent with the formation of a highly
symmetric cyclic structure, with proton chemical shifts in
good agreement with those reported for other related metalla-
cycles (assignments were done with a 2D HH-COSY exper-
iment, see ESI†). The spectrum indicates that all the available
pyridyl fragments are bound to Re(I). The doublet of the H2,6
-
Py protons is downfield shifted as compared to the parent por-
phyrin as a consequence of coordination of the pyridyl group
to the Re(I) center (Δδ = 0.48 ppm), and the doublet of the
Fig. 4 UV-Vis spectra of P1 (black), [Re(CO)3(P1)Br]4 (red), and [Re(CO)3-
(Zn·P1)Br]4 (blue) in chloroform at ca. 2 × 10−5 M concentration (calculated with
respect to the porphyrin). The inset shows an enlargement of the Q bands
region.
H
3,5-Py protons is also downfield shifted but to a minor extent
(Δδ = 0.15 ppm) due to the bigger distance from the coordi-
nation site. The spectrum shows relatively sharp peaks
together with broader and less intense bands, which are prob-
ably partly due to the presence of conformers resulting from a
slow rotation of the porphyrins edges around the metal-pyridyl
bond and/or to a distribution of geometrical isomers, and
partly to small amounts of impurities. As a matter of fact, the
spectrum seems to average and clean upon metallation of the
430.2 nm) and the expected collapse of the four Q-bands
into two.
An indirect proof of the zinc(II) tetraporphyrin metallacycle
structure can be obtained by guest inclusion studies.
[Re(CO)3(Zn·P1)Br]4 can accommodate a cruciform ligand of
appropriate dimension within its cavity, and it has a perfect
complementarity with 5,10,15,20-tetra(4-pyridyl)porphyrin
(H2-TPyP, Fig. 5 top). Thus the metallacycle is expected to form
a strong host–guest complex, via four contemporary zinc–
pyridyl bonds with H2-TPyP. The results of a fluorescence titra-
tion of [Re(CO)3(Zn·P1)Br]4 with H2-TPyP are shown in Fig. 5
(bottom).
Addition of H2-TPyP induced a progressive quenching of
the emission intensity of the metallacycle. For concentrations
of H2-TPyP > [Re(CO)3(Zn·P1)Br]4 the spectra are influenced by
the presence of free H2-TPyP (maximum emission at 647 and
715 nm, dashed black line), which produces an increase in
fluorescence. Fitting the fluorescence data at different wave-
lengths, with a 1 : 1 binding model that takes into account also
the emission of the free porphyrin, gives a log Kass = 7.06 (s.d.
0.12) in agreement with the reported data7a,10 and this con-
firms the formation of a strong 1 : 1 complex between metalla-
cycle [Re(CO)3(Zn·P1)Br]4 and H2-TPyP (Fig. 6).
1
porphyrin cores (Fig. 2c). The H NMR of [Re(CO)3(Zn·P1)Br]4
shows also a downfield shift of the resonances of the βH
protons of the pyrrolic rings, which is consistent with the elec-
tron withdrawing effect of the central zinc(II) ion.
Infrared spectroscopy in the carbonyl stretching region is
distinctive for the presence of fac-tricarbonyl rhenium(I)
fragments: as shown in Fig. 3, three strong CO stretching
bands appear at ν = 2027, 1922 and 1892 cm−1 in the IR
spectrum of [Re(CO)3(P1)Br]4, which are absent in that of
the parent porphyrin P1. Coordination of the pyridyl nitrogen
to the Re(I) complex is also supported by the bathochromic
shift of 4.8 nm (λmax P1 = 418 nm, λmax [Re(CO)3(P1)Br]4
=
422.8 nm) in the Soret region of the UV-Vis spectrum (Fig. 4),
in agreement with net removal of electron density from
the porphyrin ring upon rhenium–pyridyl bond formation.
Metallation of the porphyrins with Zn(II) induces a further
bathochromic shift of the Soret band (λmax [Re(CO)3(Zn·P1)Br]4 =
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
Fig. 6 Fluorescence emission of [Re(CO)3(Zn·P1)Br]4 at 616 nm (blue), 661 nm
(black), and 718 (red) with increasing concentration of H2-TPyP (data from
Fig. 5). The lines are the calculated titration curves on the basis of a 1 : 1 binding
model and log Kass = 7.06. The arrows indicate the ordinate axis of reference.
Fig. 7 Selected regions of the 1H-NMR spectra (500 MHz, DMSO-d6) of (a) por-
phyrin P4, (b) the product obtained from the reaction of P4 and Re(CO)5Br at
65 °C. The empty and full circles indicate the resonances pairwise connected in
the HH-COSY spectrum.
to that corresponding to a metallacycle with four equivalent
porphyrins in free (and fast) rotation around the pyridyl–
rhenium bond (Fig. 7). The spectrum recorded in DMSO-d6 at
65 °C in fact shows the presence of two sets of resonances rela-
tive to the pyridyl protons: one at δ = 9.35 and 8.46 ppm and
the other at δ = 9.06 and 8.22 ppm, which are pairwise con-
nected in the HH-COSY spectrum (see ESI†), as indicated in
Fig. 7 by empty and full circles, respectively. The individual
signals of these two sets integrate for 2H. In between, and par-
tially overlapping with these sets (δ = 9.2–8.8 ppm), there is a
group of four multiplets, integrating for 2H each, for a total of
8H, assigned to the βH. These observations might be consist-
ent with the formation of the species [Re(CO)3(P4)2Br] bearing
two porphyrins with only one metal-coordinated pyridyl frag-
ment each and four non-equivalent βH protons, thus repre-
senting a “corner” of the desired metallacycle. Indeed, a
comparison of the pyridyl protons chemical shifts of this
species with those of the parent porphyrin P4 (Fig. 7) shows
that the upfield set of pyridyl resonances (δ = 9.06 and
8.22 ppm, full circles) may well correspond to those of
unbound pyridyl fragments, present in a 1 : 1 ratio as com-
pared with the resonances of rhenium-bound ones. However, a
closer inspection of the 1H NMR signals multiplicities is in
disagreement with such a conclusion. In fact, each pyridyl
proton signal consists of closely overlapping doublets (typically
arising from the presence of a syn/anti isomer distribution
Fig. 5 Top: structure of the 1 : 1 complex of [Re(CO)3(Zn·P1)Br]4 with H2-TPyP.
Bottom: fluorimetric titration of [Re(CO)3(Zn·P1)Br]4 (CHCl3, 1 × 10−6 M) with
H2-TPyP (CHCl3, 0–3 × 10−6 M), λexc = 425 nm. The black dashed curve is the
emission spectra of H2-TPyP (CHCl3, 1 × 10−6 M, λexc = 425 nm).
We then studied the assembling reaction between Re(CO)5Br
and porphyrins P4 or P5 with the scope of obtaining a new
class of 4 + 4 metallacycles with a broader range of solu-
bility, thanks to the presence of the alkyl chains on the por-
phyrin fragments. However, the results were somewhat
disappointing. Reaction of P4 with the Re(I) complex in the
conditions illustrated above, and also increasing the reaction
time and/or changing the ratio of THF–toluene, gives a purple
solid very poorly soluble in THF, CHCl3 and DMSO, and in-
soluble in DCM, MeOH, acetone, CH3CN and hexane. The
UV-Vis and IR spectra are very similar to those reported above
for [Re(CO)3(P1)Br]4, thus indicating that all the pyridines are
coordinated to the Re(I) centre (see ESI†). However, the
1H-NMR spectrum (DMSO-d6) is more complex than expected,
with relatively broad resonances (see ESI†). By increasing the
temperature, the proton resonances become sharper but the
high number of peaks in the aromatic region clearly indicates
the presence of a structure with a lower symmetry with respect
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
derived by the different possible orientations of the halide organic solvents, resulted counterproductive in the final tar-
ligand in Re(I)–dipyridyl metallacycles),4 each βH protons geted 4 + 4 Re(I)–porphyrin metallacycles. Indeed, using por-
signal consists of two partially overlapping doublets, and the phyrins bearing alkyl chains on the two meso phenyl
resonances of the ortho phenyl protons appear as three par- substituents, we were unable to provide an unambiguous
tially overlapping singlets.§ Taken together, these observations characterization of the isolated species, although several data
are indicative of the presence of the expected 4 + 4 porphyrin suggest the formation of the metallacycle. In any case, the
metallacycle, with the porphyrin edges in a rigid somewhat solubility of the product obtained decreases on increasing the
tilted conformation (present as a statistical distribution of geo- length of the alkyl chains. We are now investigating this effect
metrical syn/anti isomers) with very distinct proton magnetic and screening alternative substituents of the porphyrin frag-
environments deriving from the orientation of the protons ments and the chemical modification of the cyano substi-
which point inside (toward the shielding cone of the other por- tuents on the phenyl rings of the metallacycles to obtain more
phyrin rings) or outside the metallacycle. This possibility has soluble systems and therefore widening their application in
been already suggested to explain the crowded aromatic region the realization of supramolecular functional architectures.
of the 1H NMR spectra of two 4 + 4 Pt(II) porphyrin metalla-
cycles.3d,f‡ Careful TLC analysis of the isolated product indicates
the presence of a single spot with high mobility (Rf = 0.86,
SiO2, CHCl3–EtOH, 98/2), typical of a discrete porphyrin metal-
lacycle, and very similar to those found for [Re(CO)3(P1)Br]4
Experimental
Materials and general methods
described above, and [Re(CO)3(P2)Br]4.11 Additional NMR All commercially available reagents were purchased from
experiments (2D ROESY, and DOSY) did not give any further Aldrich, Fluka and Strem Chemicals and used without purifi-
element to unambiguously assign the nature and 3D structure cation unless otherwise mentioned. Solvents were purchased
of this species.
from Aldrich, VWR, Fluka and Riedel, and deuterated solvents
The results with P5 were more disappointing. The purple from Cambridge Isotope Laboratories and Aldrich. Reactions
solid obtained from the reaction with Re(CO)5Br is practically were monitored by TLC on Merck silica gel plates (0.25 mm) or
insoluble in all organic solvents. Therefore, the characteriz- on Fluka aluminium oxide plates and visualized by UV light,
ation was limited to the acquisition of the UV-Vis and IR I2, or by KMnO4–H2SO4. Chromatography was performed on
spectra which show coordination of the pyridyl nitrogen to Merck silica gel 60F-254 (230–400 mesh) or Merck aluminium
Re(I) (see ESI†) and to TLC analysis, which indicates the absence oxide 90 standardized and the solvents employed were of
of unreacted porphyrin and the presence of a single spot.
analytical grade. Size exclusion chromatography was carried
out using SephadexTM LH-20 (Amersham Biosciences). NMR
spectra were recorded on a Varian 500 spectrometer (operating
at 500 MHz for proton and at 125 MHz for carbon) or on a Jeol
400 or 270 spectrometer (operating at 400 or 270 MHz for
proton and at 100 MHz or 67.8 for carbon, respectively).
Chemical shifts (δ) are reported in ppm using the solvent
residual signal as an internal reference and the multiplicity of
each signal is designated by the conventional abbreviations: s,
singlet; d, doublet; t, triplet; q, quartet; m, multiplet; br, broad;
dd, doublet of doublets. Coupling constants ( J) are quoted in
Hz. Electrospray ionization (ESI) measurements were performed
on a Perkin Elmer APII at 5600 eV by Dr Fabio Hollan, Depart-
ment of Chemical and Pharmaceutical Sciences, University of
Trieste, Italy. Melting points (m.p.) were measured with a Büchi
SHP-20 apparatus and are not corrected. Infrared spectra (IR)
were recorded on a Perkin-Elmer FT-IR/Raman 2000 instrument
in the transmission mode; samples were prepared as KI pellets.
UV-Visible Spectra were recorded on a Shimadzu UV-1800
spectrophotometer. Fluorescence spectra were recorded on a
Varian Cary Eclipse Fluorescence spectrophotometer.
Conclusions
Despite the body of work on porphyrin synthesis present in
the literature there is still a large margin for improvement, in
particular given the strong sensitivity of the acid catalyzed con-
densation reaction to the substitution pattern of the target por-
phyrin. We have shown that a careful control of the reaction
conditions led to high yield in the preparation of meso substi-
tuted trans-A2B2 dipyridylporphyrins bearing two variably sub-
stituted aromatic groups in the meso positions. These types of
porphyrins are of particular importance being the key building
blocks in the design and self-assembling of metal-mediated
discrete structures, and may as well extend the pool of func-
tional building units for infinite 2D and/or 3D hollow struc-
tures. Presently, we have exploited this feature in the
preparation of a new 4 + 4 Re(I)–porphyrin metallacycle which
has been fully characterized by means of infrared, NMR and
UV-Vis (absorption and emission) spectroscopies and by guest
inclusion studies. Quite unexpectedly the addition of alkyl
chains to the porphyrin fragment, which is a common
approach employed to increase the solubility of porphyrins in
5-(4-Pyridyl)dipyrromethane (1),19 3,4-dimethyl-1H-pyrrole-
2-carboxylic acid ethyl ester (2a),21b 4-ethyl-3-methyl-1H-
pyrrole-2-carboxylic acid ethyl ester (2b),21a 5-(4-carbomethoxy-
phenyl)-2,8-dimethyl-3,7-diethyl-1,9-dicarboxydipyrromethane
(5),20 methyl-4-bromo-3,5-dihydroxy benzoate (7)24 and methyl-
4-bromo-3,5-dipropoxy benzoate (8a)24 have been prepared
with improved and/or modified literature procedures.
§Moreover, ML2 “corners” gives usually well resolved 1H-NMR spectra with a
characteristic upfield shifts of the proton resonances of the meso phenyl substi-
tuents, which is not observed in this case (see ref. 3i).
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
5,10,15,20-Tetra(4-pyridyl)porphyrin (H2-TPyP) was purchased 2,8-CH3), 2.31 (q, J = 7.5 Hz, 4H, 3,7-CH2CH3), 4.24 (q, J =
from Aldrich. [ReBr(CO)5] was prepared as reported in the 7.1 Hz, 4H, COOCH2CH3), 5.54 (s, 1H, 5-H), 7.02 (d, J = 5.2 Hz,
literature.27
2H, H3,5-Py), 8.46 (br, 2H, NH), 8.55 (d, J = 5.2 Hz, 2H,
5,15-Diphenyl-10,20-di-(4-pyridyl)porphyrin (P1 – optimized H2,6-Py). 13C-NMR (125 MHz, CDCl3): 10.47 (2,8-CH3), 14.47
conditions). 5-(4-Pyridyl)dipyrromethane (0.500 g, MW (COOCH2CH3), 14.95 (3,7-CH2CH3), 17.29 (3,7-CH2CH3), 39.59
=
223.27, 2.24 mmol) and freshly distilled benzaldehyde (5-C), 60.01 (COOCH2CH3), 118.86 (1,9-C), 123.28 (C2,6-Py),
(238 mg, MW = 106.12, 2.24 mmol) were dissolved in 350 mL 124.94 (3,7-C), 127.11 (2,8-C), 129.44 (4,6-C), 149.02 (C1-Py),
of anhydrous DCM under Ar. The mixture was cooled at 0 °C 150.40 (C3,5-Py), 161.70 (COOCH2CH3). ESI-MS (m/z): 452.4
with an ice bath and TFA (7.15 mL, MW = 114.02, d = 1.535, [M + H+], 474.4 [M + Na+], 490.3 [M + K+].
96.3 mmol) was added drop-wise. The reaction mixture was
A mixture of the above product 3 (0.774 mmol) and NaOH 4
stirred at 0 °C for 20 minutes and DDQ (1.015 g, MW = 227.00, N (20 mL) was heated at reflux in ethylene glycol (40 mL) for
4.48 mmol) was then added. The mixture was stirred at room 24 hours under Ar. After cooling, the reaction mixture was
temperature for 1 hour. The organic phase was washed with diluted with water (150 mL) and extracted with toluene (4 ×
saturated NaHCO3 and water, dried over anhydrous Na2SO4 70 mL). The organic solvent was washed with water (2 ×
and the solvent removed. Purification by column chromato- 70 mL), dried over anhydrous Na2SO4, filtrated and concen-
graphy (silica, CHCl3–EtOH from 100/0 to 98/2) afforded a trated affording the desired product 4.
purple solid. Recrystallization (CHCl3–hexane) afforded
370 mg of product. Yield 53%.
5-(4-Pyridyl)-2,3,7,8-tetramethyl dipyrromethane, 4a. Brown
solid, yield 88.4%. 1H-NMR (400 MHz, CDCl3): 1.82 (s, 6H,
1H-NMR (500 MHz, δ-CDCl3): −2.84 (s, 2H, NH), 7.76–783 3,7-CH3), 2.03 (s, 6H, 2,8-CH3), 5.47 (s, 1H, 5-H), 6.42 (s, 2H,
(m, 6H, H3,4,5-Ph), 8.17 (d, J = 5.6 Hz, 4H, H3,5-Py), 8.21 (d, 1,9-H), 7.02 (d, J = 5.6 Hz, 2H, H3,5-Py), 7.81 (br, 2H, NH), 8.35
J = 6.7 Hz, 4H, H2,6-Ph), 8.81 (d, J = 4.5 Hz, 4H, βH), 8.91 (d, (d, J = 5.6 Hz, 2H, H2,6-Py). 13C-NMR (100 MHz, CDCl3): 9.11
J = 4.5 Hz, 4H, βH), 9.04 (d, J = 5.5 Hz, 4H, H2,6-Py). UV-Vis (3,7-CH3), 10.50 (2,8-CH3), 40.50 (5-C), 114.27 (1,9-C), 115.07
spectrum (λmax (nm), relative intensity (%)) in CH2Cl2: 418 (C2,6-Py), 119.02 (3,7-C), 123.70 (2,8-C), 125.95 (4,6-C), 149.87
(100), 513.2 (6.6), 548.2 (2.9), 587.8 (2.3), 647.2 (1.8). ESI-MS (C1-Py), 151.55 (C3,5-Py). ESI-MS (m/z): 280.3 [M + H+], 302.3
(m/z): 617.4 [M + H+], 639.4 [M + Na+]. IR (KI pellets): ˜ν = 1597, [M + Na+], 318.0 [M + K+].
1475, 1407 (m, ˜νC–C, Ar), 728, 735 (m, ˜νC–H, Ar). M.p. >300 °C.
5-(4-Pyridyl)-2,8-dimethyl-3,7-diethyl dipyrromethane, 4b.
Brown solid, yield 53.4%. 1H-NMR (400 MHz, CDCl3): 0.92
(t, J = 7.5 Hz, 6H, 3,7-CH2CH3), 2.03 (s, 6H, 2,8-CH3), 2.30 (q,
J = 7.5 Hz, 4H, 3,7-CH2CH3), 5.49 (s, 1H, 5-H), 6.39 (s, 2H,
Synthesis of 5-(4-pyridyl)-2,3,7,8-tetraalkyl-dipyrromethane
(4a,b)
A solution of 3,4-dialkyl-1H-pyrrole-2-carboxylic acid ethyl ester 1,9-H), 7.04 (d, J = 5.4 Hz, 2H, H3,5-Py), 7.54 (br, 2H, NH), 8.40
(2, 5.98 mmol) in 20 mL of toluene anhydrous was heated at (d, J = 5.4 Hz, 2H, H2,6-Py). 13C-NMR (100 MHz, CDCl3): 10.42
reflux under Ar. A catalytic amount of p-TSA (about 50 mg) was (2,8-CH3), 15.25 (3,7-CH2CH3), 17.60 (3,7-CH2CH3), 40.01 (5-C),
added followed by 4-pyridinecarboxaldehyde (0.28 mL, d = 114.56 (1,9-C), 118.48 (C2,6-Py), 121.68 (3,7-C), 123.70 (2,8-C),
1.137, MW = 107.11, 2.99 mmol). The reaction mixture was 125.71 (4,6-C), 149.88 (C1-Py), 152.18 (C3,5-Py). ESI-MS (m/z):
refluxed for 48 hours and then cooled to room temperature, 308.3 [M + H+], 320.3 [M + Na+], 346.3 [M + K+].
washed with
a saturated aqueous solution of NaHCO3
Attempts to prepare porphyrin P3a,b
(3 × 10 mL), with water (1 × 10 mL), dried over anhydrous
Na2SO4, filtrated and concentrated. Purification by column In the attempts to prepare porphyrin P3a,b we reacted dypyrro-
chromatography (silica gel, DCM–AcOEt from 100/0 to 20/80) methane 4a,b with methyl 4-formylbenzoate or compound
afforded the pure product 3.
5 with 4-pyridinecarboxaldehyde in a 1 : 1 ratio testing
1,9-Ethoxycarbonyl-5-(4-pyridyl)-2,3,7,8-tetramethyl dipyrro- varying combinations of: solvent (DCM or MeOH), acid
methane, 3a. Yield = 81.6%, Rf: 0.15 (SiO2, DCM–AcOEt, 6/4). catalyst and its concentration (TFA from 1 to 43 eq.; BF3·Et2O
1H-NMR (500 MHz, CDCl3): 1.24 (t,
J = 7.1 Hz, 6H, catalytic), temperature (from 0 °C to reflux), reaction
COOCH2CH3), 1.82 (s, 6H, 3,7-CH3), 2.26 (s, 6H, 2,8-CH3), 4.10 time (from 20 min to overnight). For the other general con-
(q, J = 7.1 Hz, 4H, COOCH2CH3), 5.53 (s, 1H, 5-H), 7.02 (d, J = ditions and for the procedure of oxidation with DDQ see the
5.7 Hz, 2H, H3,5-Py), 8.46 (d, J = 5.7 Hz, 2H, H2,6-Py), 9.28 (br, synthesis of P1. In any case we did not obtain the desired
2H, NH). 13C-NMR (125 MHz, CDCl3): 8.81 (3,7-CH3), 10.49 porphyrin.
(2,8-CH3), 14.19 (COOCH2CH3), 39.82 (5-C), 59.86
(COOCH2CH3), 118.37 (1,9-C), 118.64 (3,7-C), 121.83 (C2,6-Py),
Synthesis of 4-cyano-3,5-dialkyloxy benzaldehyde (12a,b)
127.53 (2,8-C), 130.36 (4,6-C), 149.11 (C1-Py), 150.23 (C3,5-Py), 4-Bromo-3,5-dihydroxy benzoic acid (6, 530 mg, MW = 233.02,
162.11 (COOCH2CH3). ESI-MS (m/z): 424.3 [M + H+], 446.2 2.27 mmol) was dissolved in 30 mL of methanol. H2SO4
[M + Na+], 462.1 [M + K+].
(0.50 mL, MW = 98.078, d = 1.835, 9.35 mmol) was added and
1,9-Diethoxycarbonyl-5-(4-pyridyl)-2,8-dimethyl-3,7-diethyl the mixture was heated at reflux for 18 hours. The solvent was
dipyrromethane, 3b. Yield = 39.8%, Rf: 0.32 (SiO2, DCM– evaporated and the residue was re-dissolved in 50 mL of water
AcOEt, 6/4). 1H-NMR (500 MHz, CDCl3): 0.91 (t, J = 7.5 Hz, 6H, and 30 mL of AcOEt. The aqueous layer was extracted with
3,7-CH2CH3), 1.31 (t, J = 7.1 Hz, 6H, COOCH2CH3), 2.28 (s, 6H, AcOEt (3 × 30 mL) and the combined organic layers were
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
washed with water (2 × 50 mL), dried on anhydrous Na2SO4, fil-
trated and the solvent was removed under reduced pressure 70% from 8a. H-NMR (400 MHz, CDCl3): 1.05 (t, J = 7.4 Hz,
affording 541 mg of methyl-4-bromo-3,5-dipropoxy benzoate 6H, OCH2CH2CH3), 1.91 (m, 4H, OCH2CH2CH3), 4.10 (t, J =
4-Cyano-3,5-dipropoxy benzoic acid (10a). White solid, yield
1
(7) as a colourless oil. Yield 96%.
6.5 Hz, 4H, OCH2CH2CH3) 7.24 (s, 2H, Ar), 11.18 (br, 1H,
1H-NMR (500 MHz, CD3OD): 3.85 (s, 3H, COOCH3), 7.04 (s, COOH). 13C-NMR (100 MHz, CDCl3): 10.40 (OCH2CH2CH3),
2H, Ar), 9.8 (br, 1H, COOH). 13C-NMR (100 MHz, CD3OD): 22.27 (OCH2CH2CH3), 71.60 (OCH2CH2CH3), 96.65 (C4-Ar),
51.33 (COOCH3), 103.75 (C4-Ar), 107.27 (C2-Ar), 129.69 (C1-Ar), 105.48 (C2-Ar), 112.96 (CN), 134.43 (C1-Ar), 162.07 (C3-Ar),
155.32 (C3-Ar), 166.74 (COOCH3). IR (KI disks): ˜ν = 1704 (s, 170.70 (COOH). IR (KI pellets): ˜ν = 3200 (s, ˜νCOOH), 2963–2890
˜νCvO), 860 (m, ˜νAr).
(m, ˜νCH), 2228.9 (s, ˜νCN), 1724 (s, ˜νCvO), 763 (s, ˜νAr).
The ester 7 (206 mg, MW = 247.04, 0.834 mmol), the appro-
4-Cyano-3,5-dihexyloxy benzoic acid (10b). White solid, yield
1
priate 1-bromoalkane (2.5 mmol) and tetrabutylammonium 81% from 8b. H-NMR (400 MHz, CDCl3): 0.94 (t, J = 6.7 Hz,
bromide (54 mg, MW = 322.38, 0.17 mmol) were dissolved in 6H, O(CH2)5CH3), 1.33 (m, 8H, O(CH2)3(CH2)2CH3), 1.46
5 mL of acetone. After addition of anhydrous K2CO3 (300 mg, (m, 4H, O(CH2)2CH2(CH2)2CH3), 1.83 (m, 4H, OCH2CH2-
MW = 138.21, 2.17 mmol) the reaction mixture was refluxed (CH2)3CH3), 4.01 (t, J = 6.5 Hz, 4H, OCH2(CH2)4CH3) 7.23
for 18 hours. The solvent was then removed and the residue (s, 2H, Ar), 10.5 (br, 1H, COOH) 13C-NMR (67.8 MHz,
taken up in as much diethyl ether and water as needed to CDCl3): 13.84 (O(CH2)5CH3), 22.40 (O(CH2)4CH2CH3), 25.35
obtain two homogeneous phases. The aqueous phase was (O(CH2)3CH2CH2CH3), 28.6 (O(CH2)2CH2(CH2)2CH3), 31.29
extracted with diethyl ether. The combined organic phases (OCH2CH2(CH2)3CH3), 69.47 (OCH2(CH2)4CH3), 96.10 (C4-Ar),
were dried over anhydrous Na2SO4, filtered, and the ether 105.22 (C2-Ar), 112.86 (CN), 134.72 (C1-Ar), 162.13 (C3-Ar),
evaporated to obtain the alkylated product.
169.81 (COOH).
Methyl-4-bromo-3,5-dipropoxy benzoate (8a). Brown oil,
To a solution of acid 10 (8.67 mmol) in 60 mL of DCM
1
yield 95%. H-NMR (500 MHz, CDCl3): 1.04 (t, J = 7.4 Hz, 6H, cooled to 0 °C, 4-methylmorpholine (1.90 mL, MW = 101.15,
OCH2CH2CH3), 1.82 (m, 4H, OCH2CH2CH3), 3.87 (s, 3H, d = 0.92, 17.3 mmol) and isobutyl chloroformate (1.12 mL,
COOCH3), 4.05 (t, J = 6.4 Hz, 4H, OCH2CH2CH3) 7.12 (s, 2H, MW = 136.58, d = 1.053, 8.67 mmol) were added. The resulting
Ar). 13C-NMR (100 MHz, CDCl3): 10.61 (OCH2CH2CH3), 22.55 mixture was stirred for 20 minutes at 0 °C, and then hydroxyl-
(OCH2CH2CH3), 52.39 (COOCH3), 70.99 (OCH2CH2CH3), amine hydrochloride (930 mg, MW = 97.55, 9.54 mmol) was
106.34 (C2-Ar), 107.73 (C4-Ar), 129.89 (C1-Ar), 156.58 (C3-Ar), added. The reaction mixture was stirred for 1 hour at 0 °C, and
166.60 (COOCH3).
for 18 hours at room temperature. The mixture was diluted
Methyl-4-bromo-3,5-dihexyloxy benzoate (8b). Yellow oil, with water and the organic phase was washed with water, citric
1
yield 90%. H-NMR (500 MHz, CDCl3): 0.94 (t, J = 6.7 Hz, 6H, acid 5% and brine, dried over anhydrous Na2SO4, filtered and
O(CH2)5CH3), 1.32 (m, 8H, O(CH2)3(CH2)2CH3), 1.48 (m, 4H, the solvent was removed under reduced pressure. Purification
O(CH2)2CH2(CH2)2CH3), 1.81 (m, 4H, OCH2CH2(CH2)3CH3), by column chromatography (silica gel, CHCl3–MeOH from
3.88 (s, 3H, COOCH3), 4.02 (t, J = 6.9 Hz, 4H, OCH2(CH2)4CH3) 100/0 to 90/10) gave the Weinreb amide 11.
7.17 (s, 2H, Ar). 13C-NMR (100 MHz, CDCl3): 13.95 (O(CH2)5CH3),
4-Cyano-N-methoxy-N-methyl-3,5-dipropoxybenzamide (11a).
1
22.45 (O(CH2)4CH2CH3), 22.62 (O(CH2)3CH2CH2CH3), 28.99 Yellow oil, yield 57%. H-NMR (500 MHz, CDCl3): 1.05 (t, J =
(O(CH2)2CH2(CH2)2CH3), 31.46 (OCH2CH2(CH2)3CH3) 52.19 6.4 Hz, 6H, OCH2CH2CH3), 1.91 (m, 7.0 Hz, 4H,
(COOCH3), 69.42 (OCH2(CH2)4CH3), 106.22 (C2-Ar), 107.73 OCH2CH2CH3), 3.32 (s, 3H, NCH3), 3.53 (s, 3H, OCH3), 4.01
(C4-Ar), 129.79 (C1-Ar), 156.53 (C2-Ar), 166.40 (COOCH3).
(t, J = 7.4 Hz, 4H, OCH2CH2CH3), 6.75 (s, 2H, Ar). 13C-NMR
J
=
The alkyl derivative 8 (12.4 mmol) was added to a solution (67.8 MHz, CDCl3): 10.14 (OCH2CH2CH3), 22.04 (OCH2CH2CH3),
of CuCN (8.00 g, MW = 89.56, 89.0 mmol) in 200 mL of DMF 33.23 (NCH3), 61.32 (OCH3), 70.73 (OCH2CH2CH3), 93.34
and the reaction mixture was heated to reflux (150–160 °C) for (C4-Ar), 103.76 (C2-Ar), 113.31 (CN), 140.15 (C1-Ar), 161.77
48 hours. After cooling to room temperature a solution of (C3-Ar), 168.38 (CO). IR (KI pellets): ˜ν = 2227.94 (s, ˜νCN),
FeCl3 (29 g in 80 mL of water and 20 mL of HCl conc.) was 1644.62 (s, ˜νCvO), 914 (m, ˜νAr).
added. The mixture was heated at 70–80 °C for 2 hours and
4-Cyano-N-methoxy-N-methyl-3,5-dihexyloxybenzamide (11b).
1
then extracted with CHCl3. The combined extracts were Yellow oil, yield 67%. H-NMR (400 MHz, CDCl3): 0.86 (t, J =
washed with a saturated NaCl solution, dried over anhydrous 7.0 Hz, 6H, O(CH2)5CH3), 1.30 (m, 8H, O(CH2)3 (CH2)2CH3),
Na2SO4 and the solvent was removed in vacuo to afford a 1.46 (m, 4H, O(CH2)2CH2(CH2)2CH3), 1.80 (m, 4H, OCH2-
brown oil which is a mixture of ester 9 and acid 10. The CH2(CH2)3CH3), 3.32 (s, 3H, NCH3), 3.66 (s, 3H, OCH3), 4.03
product was taken up in a mixture of THF–MeOH 1/1 (t, J = 6.5 Hz, 4H, OCH2(CH2)4CH3), 6.74 (s, 2H, Ar). 13C-NMR
(120 mL), 1.2 g of LiOH·H2O were added and the reaction (67.8 MHz, CDCl3): 14.07 (O(CH2)5CH3), 22.59 (O(CH2)4CH2CH3),
mixture was stirred for 18 hours. The mixture was concentrated 22.55 (O(CH2)3CH2CH2CH3), 28.01 (O(CH2)2CH2(CH2)2CH3), 28.86
and taken up with water and acidified with HCl 6 N to pH = (OCH2CH2(CH2)3CH3), 31.49 (NCH3), 61.32 (OCH3), 69.51
2. The product was extracted with AcOEt (3 × 30 mL), washed (OCH2(CH2)4CH3), 93.49 (C4-Ar), 103.84 (C2-Ar), 113.36 (CN),
with water (2 × 20 mL) and brine (1 × 20 mL), dried over an- 140.12 (C1-Ar), 161.80 (C3-Ar), 168.40 (CO).
hydrous sodium sulphate, filtered, and the solvent evaporated
A mixture of Weinreb amide 11 (1.67 mmol) in 40 mL of
to afford the desired acid 10.
THF anhydrous was stirred and cooled to 0 °C under Ar. Then
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
3.3 mL of
a
2
M
LiAlH4 solution in THF anhydrous C56H51N8O4 899.4033). IR (KI pellets): ˜ν = 2990–2855 (m, ˜νC–H),
(6.67 mmol) was added drop-wise. The mixture was stirred at 2249 (s, ˜νCN), 1615 (s, ˜νN–H), 1565 (s, ˜νC–C Ar), 1475 (s, ˜νCN),
room temperature for 4 hours; then, after cooling to 0 °C, a 1237 (s, ˜νC–O–C), 805, 783 (s, ˜νC–H Ar).
saturated solution of NH4Cl and water was slowly added. The
5,15-Di[4-cyano-3,5-dihexyloxy
phenyl]-10,20-di[pyridyl]-
1
product was extracted with AcOEt (3 × 100 mL), washed with porphyrin (P5). M.p. >300 °C, yield 63%. H-NMR (500 MHz,
water (2 × 100 mL) and brine (1 × 100 mL), dried over an- δ-CDCl3): −2.93 (s, 2H, NH), 0.87 (m, 12H, O(CH2)5CH3),
hydrous Na2SO4, filtered and the solvent was removed under 1.32 (m, 16H, O(CH2)3(CH2)2CH3), 1.50 (m, 8H, O(CH2)2-
reduced pressure to give the desired aldehyde 12.
CH2(CH2)2CH3), 1.90 (m, 8H, OCH2CH2(CH2)3CH3), 4.15
4-Cyano-3,5-dipropoxybenzaldehyde (12a). Yellow solid, (t, J = 6.4 Hz, 8H, OCH2(CH2)4CH3), 7.51 (s, 4H, o-Ph) 8.14
1
yield 79%. H-NMR (400 MHz, CDCl3): 1.06 (t, J = 7.4 Hz, 6H, (d, J = 5.5 Hz, 4H, H3,5-Py), 8.86 (d, J = 4.4 Hz, 4H, βH),
OCH2CH2CH3), 1.85 (m, 4H, OCH2CH2CH3), 4.08 (t, J = 6.5 Hz, 8.96 (d, J = 4.4 Hz, 4H, βH), 9.07 (d, J = 5.5 Hz, 4H, H2,6-Py).
4H, OCH2CH2CH3), 6.99 (s, 2H, Ar), 9.93 (bs, 1H, CHO). 13C-NMR (125 MHz, δ-CDCl3): 13.82 (O(CH2)5CH3), 22.37
13C-NMR (100 MHz, CDCl3): 10.18 (OCH2CH2CH3), 22.06 (O(CH2)4CH2CH3), 25.41 (O(CH2)3CH2CH2CH3), 28.80 (O(CH2)2-
(OCH2CH2CH3), 71.03 (OCH2CH2CH3), 96.87 (C4-Ar), 104.64 CH2(CH2)2CH3), 31.32 (OCH2CH2(CH2)3CH3), 69.59 (OCH2-
(C2-Ar), 112.98 (CN), 140.63 (C1-Ar), 162.70 (C3-Ar), 191.08 (CH2)4CH3), 91.89 (C4-Ar), 111.70 (C2-Ar), 113.70 (CN), 117.75
(CHO).
and 119.47 (Cmeso), 129.34 (C3,5-Py), 148.23 (C1-Ar); 148.59
4-Cyano-3,5-dihexyloxybenzaldehyde (12b). Yellow solid, (C3,5-Py); 149.88 (C1-Py); 160.39 (C3-Ar). UV-Vis spectrum
1
yield 75%. H-NMR (400 MHz, CDCl3): 0.87 (t, J = 6.5 Hz, 6H, (λmax (nm), relative intensity (%)) in CH2Cl2: 419.0 (100), 513
O(CH2)5CH3), 1.30 (m, 8H, O(CH2)3(CH2)2CH3), 1.44 (m, 4H, (5.3), 545.0 (1.9), 582.5 (2.2), 642.5 (1.9). ESI-MS (m/z): 1068.4
O(CH2)2CH2(CH2)2CH3), 1.83 (m, 4H, OCH2CH2(CH2)3CH3), [M + H+]. HRESI-MS m/z 1067, 6180 [M + H+] (calcd for
4.09 (t, J = 6.5 Hz, 4H, OCH2(CH2)4CH3), 6.97 (s, 2H, Ar), C68H75N8O4 1067.5911). IR (KI pellets): ˜ν = 2959–2955–2872
9.92 (bs, 1H, COOH). 13C-NMR (67.8 MHz, CDCl3): 14.07 (m, ˜νC–H), 2229 (s, ˜νCN), 1623 (s, ˜νN–H), 1602 (s, ˜νC–C Ar), 1429
(O(CH2)5CH3), 22.60 (O(CH2)4CH2CH3), 25.54 (O(CH2)3- (s, ˜νCN), 1130 (s, ˜νC–O–C), 806, 783 (s, ˜νC–H Ar).
CH2CH2CH3), 28.93 (O(CH2)2CH2(CH2)2CH3), 31.48 (OCH2-
CH2(CH2)3CH3), 69.77 (OCH2(CH2)4CH3), 93.09 (C4-Ar), 104.68
(C2-Ar), 113.15 (CN), 140.64 (C1-Ar), 162.66 (C3-Ar), 191.02
Synthesis of 4 + 4 Re(I)–porphyrin metallacycles
(CHO).
[Re(CO)3(P1)Br]4. 5,15-Diphenyl-10,20-di(4-pyridyl)porphyrin
(P1, 100 mg, MW = 616.71, 0.162 mmol) and Re(CO)5Br
(66 mg, MW = 406.163, 0.162 mmol) were dissolved in 80 mL
Synthesis of porphyrins P4,5
5-(4-Pyridyl)dipyrrylmethane (0.213 g, MW = 223.27, 0.95 mmol) of a mixture of freshly distilled 4 : 1 THF–toluene and heated
and 4-cyano-3,5-dialkyloxybenzaldehyde (0.95 mmol) were dis- at reflux for 48 h under Ar. After cooling, 50 mL of hexane was
solved in 200 mL of anhydrous DCM under Ar. The mixture added to promote product precipitation. The purple product
was cooled at 0 °C with an ice bath and TFA (3.0 mL, MW = was centrifuged and then re-precipitated using chloroform and
114.02, d = 1.535, 40.8 mmol) was added drop-wise. The re- ethyl ether to give 136 mg of solid which was washed several
action mixture was stirred at 0 °C for 20 minutes and DDQ times with diethyl ether. Rf: 0.83 (SiO2, CHCl3–EtOH, 98/2).
(433 mg, MW = 227.00, 1.9 mmol) was then added. The Yield 87%. 1H-NMR (500 MHz, δ-CDCl3): −2.89 (s, NH) 7.61
mixture was stirred at room temperature for 1 hour. The (m, m,p-Ph), 8.05 (m, o-Ph), 8.32 (d, J = 5.6 Hz, H3,5-Py);
organic phase was washed with saturated NaHCO3 and water, 8.80–8.85 (d + d, J = 4.6 Hz, βH), 9.52 (d, J = 6.1 Hz, 16H, H2,6-
dried over anhydrous Na2SO4 and the solvent was removed. Py). UV-Vis spectrum (λmax (nm), relative intensity (%)) in
Purification by column chromatography (silica, CHCl3–EtOH CH2Cl2: 422.8 (100), 515.4 (6.1), 553.2 (3.7), 590.6 (2.1), 650
from 100/0 to 98/2) afforded a purple solid. Recrystallization (2.1). IR (KI pellets): ˜ν
=
2027–1922–1892 (m, ˜νCO
)
fac
(CHCl3–hexane) afforded the pure porphyrin.
1597–1475–1407 (m, ˜νC–C, Ar), 728–735 (m, ˜νC–H Ar).
[Re(CO)3(Zn·P1)Br]4. The above product (72 mg, MW =
5,15-Di[4-cyano-3,5-dipropoxyphenyl]-10,20-di[pyridyl]-
1
porphyrin (P4). M.p. >300 °C, yield 61%. H-NMR (500 MHz, 3867.41, 1.9 × 10−5 mol) was dissolved in chloroform (70 mL)
δ-CDCl3): −2.87 (s, 2H, NH); 1.10 (t, J = 7.4 Hz, 12H, O and a solution of zinc acetate dihydrate (19 mg, MW = 219.5,
CH2CH2CH3), 1.98 (m, 8H, OCH2CH2CH3), 4.15 (t, J = 6.4 Hz, 8.7 × 10−5 mol) in methanol (less than 2 mL) was added. The
8H, OCH2CH2CH3), 7.44 (s, 4H, o-Ph), 8.20 (d, J = 5.5 Hz, 4H, mixture was stirred in the dark overnight. The solvents were
H3,5-Py); 8.90 (d, J = 4.4 Hz, 4H, βH), 9.00 (d, J = 4.4 Hz, 4H, removed under reduced pressure and the purple solid was
βH), 9.10 (d, J = 5.4 Hz, 4H, H2,6-Py). 13C-NMR (125 MHz, washed with methanol and filtrated to give 56 mg of purple
δ-CDCl3): 10.48 (OCH2CH2CH3), 22.42 (OCH2CH2CH3), 71.05 product. Rf: 0.72 (SiO2, CHCl3–EtOH, 98/2). Yield 72%.
(OCH2CH2CH3), 91.95 (C4-Ar), 111.71 (C2-Ar), 113.98 (CN), 1H-NMR (500 MHz, δ-CDCl3): 7.61 (m, 24H, m,p-Ph), 8.08 (m,
117.74 and 119.41 (Cmeso), 129.27 (C3,5-Py), 148.17 (C1-Ar), 16H, o-Ph), 8.34 (m, 16H, H5,6-Py), 8.92 (m, 16H, βH), 8.96 (m,
148.48 (C3,5-Py), 149.76 (C1-Py), 160.25 (C3-Ar). UV-Vis spec- 16H, βH), 9.49 (m, 16H, H2,6-Py). UV-Vis spectrum (λmax (nm),
trum (λmax (nm), relative intensity (%)) in CH2Cl2: 419.5 (100), relative intensity (%)) in CH2Cl2: 430.2 (100), 557.6 (3.1), 607.8
507 (6.8), 544.5 (3.1), 587.5 (3.1), 649.0 (2.1). ESI-MS (m/z): (1.0). IR (KI pellets): ˜ν = 2027.5–1922–1890 (s, ˜νCvO fac), 1611
900.1 [M + H+]. HRMALDI-MS m/z 899.4003 [M + H+] (calcd for (s, ˜νC–C, C–N), 796.5 (s, ˜νC–H Ar).
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013

View Article Online
Organic & Biomolecular Chemistry
Paper
[Re(CO)3(P4)Br]4. 5,15-Di[4-cyano-3,5-dipropoxyphenyl]-10,20-
di(4-pyridyl)porphyrin (P4, 260 mg, MW = 899.05, 0.289 mmol)
and Re(CO)5Br (117 mg, MW = 406.163, 0.289 mmol) were dis-
solved in 220 mL of freshly distilled 4 : 1 THF–toluene and
heated at reflux for 48 h under Ar. After cooling, 200 mL of
hexane was added to promote product precipitation. The
product was centrifuged and then re-precipitated using chloro-
form and ethyl ether. Purification by size exclusion chromato-
graphy (SEC) (stationary phase: SephadexTM LH-20, mobile
phase: freshly distilled THF) afforded 345 mg of purple solid.
Rf: 0.86 (SiO2, CH3Cl–EtOH, 98/2). Quantitative yield.
E. Iengo and E. Alessio, Coord. Chem. Rev., 2006, 250,
1471; (c) S. J. Lee and J. T. Hupp, Coord. Chem. Rev.,
2006, 250, 1710; (d) I. Beletskaya, V. S. Tyurin,
A. Y. Tsivadze, R. Guilard and C. Stern, Chem. Rev., 2009,
109, 1659.
3 (a) E. Alessio, M. Casanova, E. Zangrando and E. Iengo,
Chem. Commun., 2012, 48, 5112; (b) N. P. E. Barry, O. Zava,
J. Furrer, P. J. Dyson and B. Therrien, Dalton Trans., 2010,
39, 5272; (c) E. Iengo, F. Scandola and E. Alessio, Struct.
Bonding, 2006, 121, 105; (d) T. Milic, J. C. Garno,
J. D. Batteas, G. Smeureanu and C. M. Drain, Langmuir,
2004, 20, 3974; (e) N. Fujita, K. Biradha, M. Fujita,
S. Sakamoto and K. Yamaguchi, Angew. Chem., Int. Ed.,
2001, 40, 1718; (f) J. Fan, J. A. Whiteford, B. Olenyuk,
M. D. Levin, P. J. Stang and E. B. Fleisher, J. Am. Chem.
Soc., 1999, 121, 2741; (g) P. J. Stang, J. Fan and B. Olenyuk,
Chem. Commun., 1997, 1453; (h) C. M. Drain and
J.-M. Lehn, J. Chem. Soc., Chem. Commun., 1994, 2313;
(i) E. Iengo, B. Milani, E. Zangrando, S. Geremia and
E. Alessio, Angew. Chem., Int. Ed., 2000, 39, 1096.
4 J. T. Hupp, Struct. Bonding, 2006, 121, 145.
1H-NMR (500 MHz, δ-DMSO-d6): −2.94 (s, 8H, NH), 1.02
(m, OCH2CH2CH3), 1.83 (m, OCH2CH2CH3), 4.22 (m, OCH2-
CH2CH3), 7.67 (m, 16H, o-Ph), 8.22 (m, 8H, H3,5-Py), 8.46
(m, 8H, H3,5-Py), 8.88 (m, 8H, βH), 8.93 (m, 8H, βH), 9.06–9.10
(m, 24H, βH + H2,6-Py), 9.35 (m, 8H, H2,6-Py). UV-Vis spec-
trum (λmax (nm), relative intensity (%)) in CH2Cl2: 422.5 (100),
514.5 (7.4), 550.0 (3.7), 586.5 (3.7), 643.5 (1.8). IR (KI pellets):
˜ν = 2990–2855 (m, ˜νC–H), 2029–1930–1916 (s, ˜νCvO fac), 2249
(s, ˜νCN), 1615 (s, ˜νN–H), 1565 (s, ˜νC–C Ar), 1475 (s, ˜νCN), 1237
(s, ˜νC–O–C), 805, 783 (s, ˜νC–H Ar).
[Re(CO)3(P5)Br]4. 5,15-Di[4-cyano-3,5-dihexyloxyphenyl]-10,20-
di(4-pyridyl)porphyrin (P5, 200 mg, MW = 1067.37, 0.187 mmol)
and Re(CO)5Br (76 mg, MW = 406.163, 0.187 mmol) were dis-
solved in 200 mL of freshly distilled 4 : 1 THF–toluene and
heated at reflux for 48 h under Ar. After cooling, 200 mL of
hexane was added to promote product precipitation. The
product was centrifuged and then re-precipitated using chloro-
form and ethyl ether. Purification by size exclusion chromato-
graphy (SEC) (stationary phase: SephadexTM LH-20, mobile
phase: freshly distilled THF) afforded 670 mg of a purple solid.
Rf: 0.88 (SiO2, CHCl3–EtOH, 98/2). Yield: 75%.
UV-Vis spectrum (λmax (nm), relative intensity (%)) in
CH2Cl2: 424 (100), 511 (5.3), 552.5 (4.7), 591.0 (2.0), 646.0 (1.0).
IR (KI pellets): ˜ν = 2959–2955–2872 (m, ˜νC–H), 2029–1929–1908
(s, ˜νCvO fac), 2229 (s, ˜νCN), 1623 (s, ˜νN–H), 1602 (s, ˜νC–C Ar), 1429
(s, ˜νCN), 1130 (s, ˜νC–O–C), 806, 783 (s, ˜νC–H Ar).
5 (a) S.-S. Sun and A. J. Lees, J. Am. Chem. Soc., 2000, 122,
8956; (b) S.-S. Sun and A. J. Lees, Inorg. Chem., 1999, 122,
8956; (c) E. Zangrando, M. Casanova and E. Alessio, Chem.
Rev., 2008, 108, 4979.
6 L. Miljacic, L. Sarkisov, D. E. Ellis and R. Q. Snurr, J. Chem.
Phys., 2004, 121, 7228.
7 (a) R. V. Slone and J. T. Hupp, Inorg. Chem., 1997, 36, 5422;
(b) S. Bélanger, M. H. Keefe, J. l. Welch and J. T. Hupp,
Coord. Chem. Rev., 1999, 190–192, 29.
8 M. L. Merlau, M. del Pilar Mejia, S. T. Nguyen and
J. T. Hupp, Angew. Chem., Int. Ed., 2001, 40, 4239.
9 (a) G. A. Mines, B.-C. Tzeng, K. J. Stevenson, J. Li and
J. T. Hupp, Angew. Chem., Int. Ed., 2002, 41, 154;
(b) R. C. Bailey and J. T. Hupp, J. Am. Chem. Soc., 2002, 124,
6767.
10 (a) S. Bélanger and J. T. Hupp, Angew. Chem., Int. Ed., 1999,
38, 2222; (b) M. E. Williams and J. T. Hupp, J. Phys. Chem.
B, 2001, 105, 8944; (c) K. F. Czapleski, J. T. Hupp and
R. Q. Snurr, Adv. Mater., 2001, 13, 1895.
11 M. Boccalon, E. Iengo and P. Tecilla, J. Am. Chem. Soc.,
2012, 134, 20310.
Acknowledgements
European Reintegration Grant PERG02-GA-2007-224823, Fonda-
zione Beneficentia Stiftung, and MIUR-PRIN 2010JMAZML_007
are gratefully acknowledged for financial support.
12 U. Devi, J. R. D. Brown, A. Almond and S. J. Webb, Lang-
muir, 2011, 27, 1448.
13 (a) S. Licen, F. De Riccardis, I. Izzo and P. Tecilla, Curr. Drug
Discovery Technol., 2008, 5, 86; (b) D. Montesarchio,
C. Coppola, M. Boccalon and P. Tecilla, Carbohydr. Res.,
2012, 356, 62; (c) F. De Riccardis, I. Izzo, D. Montesarchio
and P. Tecilla, Acc. Chem. Res., DOI: 10.1021/ar4000136.
Notes and references
1 (a) R. Chakrabarty, P. S. Mukherjee and P. J. Stang,
Chem. Rev., 2011, 111, 6810; (b) N. J. Young and B. P. Hay, 14 E. Iengo, E. Zangrando, R. Minatel and E. Alessio, J. Am.
Chem. Commun., 2013, 49, 1354; (c) T. R. Cook, Chem. Soc., 2002, 124, 1003.
Y.-R. Zheng and P. J. Stang, Chem. Rev., 2013, 113, 734; 15 X.-J. Zhu, P. Wang, H. W. C. Leung, W.-K. Wong,
(d) M. M. J. Smulders, I. A. Riddell, C. Browne and
J. R. Nitschke, Chem. Soc. Rev., 2013, 42, 1728.
W.-Y. Wong and D. W. J. Kwong, Chem.–Eur. J., 2011, 17,
7041.
2 (a) E. Iengo, E. Zangrando and E. Alessio, Acc. Chem. Res., 16 B. J. Littler, Y. Ciringh and J. S. Lindsey, J. Org. Chem.,
2006, 39, 841; (b) F. Scandola, C. Chiorboli, A. Prodi,
1999, 64, 2864.
This journal is © The Royal Society of Chemistry 2013
Org. Biomol. Chem.

View Article Online
Paper
Organic & Biomolecular Chemistry
17 D. A. Jose, A. D. Shukla, G. Ramakrishna, D. K. Palit,
N. Patel, V. Tamez, J. A. Grosso, M. Peddicord and
H. N. Ghosh and A. Das, J. Phys. Chem. B, 2007, 111, 9078.
V. A. Palaniswamy, Tetrahedron Lett., 2006, 47, 5481.
18 (a) E. Kühn, V. Bulach and M. W. Hosseini, Chem. 22 S. Banfi, A. Manfredi, G. Pozzi, A. Quici and A. Trebicka,
Commun., 2008, 5104; (b) A. M. Shultz, O. K. Farha, Gazz. Chim. Ital., 1996, 126, 179.
J. T. Hupp and S. T. Nguyen, J. Am. Chem. Soc., 2009, 131, 23 G. M. Mamardashvili, O. E. Storonkina and
4204; (c) D. T. Gryko and M. Tasior, Tetrahedron Lett., 2003,
44, 3317.
19 C. Ruzié, L. Michaudet and B. Boitrel, Tetrahedron Lett.,
2002, 43, 7423.
N. Zh. Mamardashvili, Russ. J. Coord. Chem., 2004, 30, 388.
24 F. Durola, D. Hanss, P. Roesel, J.-P. Sauvage and
O. S. Wenger, Eur. J. Org. Chem., 2007, 125.
25 C. R. Graves, M. L. Merlau, G. A. Morris, S.-S. Sun,
S. T. Nguyen and J. T. Hupp, Inorg. Chem., 2004, 43, 2013.
20 S. L. Gould, G. Kodis, P. A. Liddell, R. E. Palacios, A. Brune,
D. Gust, T. A. Moore and A. L. Moore, Tetrahedron, 2006, 26 W. H. Otto, M. H. Keefe, K. E. Splan, J. T. Hupp and
62, 2074. C. K. Larive, Inorg. Chem., 2002, 41, 6172.
21 (a) P. Bobál and D. A. Lightner, J. Heterocycl. Chem., 2001, 27 S. P. Schmidt, W. C. Trogler and F. Basolo, Inorg. Synth.,
38, 527; (b) A. Bhattacharya, S. Cherukuri, R. E. Plata,
1990, 28, 160.
Org. Biomol. Chem.
This journal is © The Royal Society of Chemistry 2013