Conformational flexibility of 1,4-naphthiporphyrin promotes a palladium-mediated contraction of naphthalene to isoindene

Article abstract of DOI:10.1021/om2004139

The carbaporphyrinoids 5,10,15,20-tetraphenyl-1,4-naphthiporphyrin and 5,20-diphenyl-10,15-ditolyl-24-thia-1,4-naphthiporphyrin, which contain a naphthalene ring embedded in the macrocyclic framework, were obtained in a modification of the "3 + 1" approach using the naphthalene analogue of tripyrrane. The 1,4-naphthiporphyrin and 24-thia-1,4-naphthiporphyrin are planar except for the tilt of the naphthalene moiety toward the center of the macrocycle, as determined by X-ray crystallography. Extensive NMR studies demonstrated that the diprotonation of 1,4-naphthiporphyrin forced a remarkable conformational change due to flipping of the naphthalene moiety, which afforded the extended macrocyclic structure. The reaction of palladium(II) chloride with 1,4-naphthiporphyrin in acetonitrile yielded palladium(II) 1,4-naphthiporphyrin, which preserved the folded conformation of the free base. The remarkable, facile palladium(II)-stimulated contraction of naphthalene to isoindene produced the first complex of metal(II) benzocarbaporphyrin which acquires a unique tetrahedral carbon coordination mode.

Full text of DOI:10.1021/om2004139

ARTICLE
pubs.acs.org/Organometallics
Conformational Flexibility of 1,4-Naphthiporphyrin Promotes a
Palladium-Mediated Contraction of Naphthalene to Isoindene
Bartosz Szyszko and Lechoszaw Latos-Graz_yꢀnski*
Department of Chemistry, University of Wroczaw, 14 F. Joliot-Curie Street, Wroczaw 50 383, Poland
S Supporting Information
b
ABSTRACT: The carbaporphyrinoids 5,10,15,20-tetraphenyl-
1,4-naphthiporphyrin and 5,20-diphenyl-10,15-ditolyl-24-thia-
1,4-naphthiporphyrin, which contain a naphthalene ring em-
bedded in the macrocyclic framework, were obtained in a
modification of the “3 + 1” approach using the naphthalene
analogue of tripyrrane. The 1,4-naphthiporphyrin and 24-thia-
1,4-naphthiporphyrin are planar except for the tilt of the
naphthalene moiety toward the center of the macrocycle, as determined by X-ray crystallography. Extensive NMR studies
demonstrated that the diprotonation of 1,4-naphthiporphyrin forced a remarkable conformational change due to flipping of the
naphthalene moiety, which afforded the extended macrocyclic structure. The reaction of palladium(II) chloride with 1,4-
naphthiporphyrin in acetonitrile yielded palladium(II) 1,4-naphthiporphyrin, which preserved the folded conformation of the free
base. The remarkable, facile palladium(II)-stimulated contraction of naphthalene to isoindene produced the first complex of
metal(II) benzocarbaporphyrin which acquires a unique tetrahedral carbon coordination mode.
’ INTRODUCTION
neighboring ring atoms in benzenoid hydrocarbons trade
locations.27 To the best of our knowledge, the cleavage of
naphthalene via metal ion insertion has not been reported yet. In
the context of the present work, it is essential to recall that the
remarkable breaking of an aromatic CꢀC bond by insertion of
tungsten into an unstrained fused aromatic ring—quinoxaline (1,4-
diazanaphthalene)—afforded an o-diisocyanobenzene derivative.28
Herein we have found that due to geometrical constraints of
the porphyrinoid macrocycle an intramolecular rearrangement of
palladium(II) 1,4-naphthiporphyrin has been efficiently pro-
moted, affording the facile palladium(II)-stimulated contraction
of naphthalene to isoindene, both built in the carbaporphyrinoid
structure.
Carbaporphyrinoids provide a unique macrocyclic platform
which is suitable for exploring organometallic chemistry confined
to a peculiar macrocyclic environment.1ꢀ4 The close proximity of
CH or CC bonds to the metal ion enforces an unusual coordination
geometry and eventually unique reactivity. The internal carbon,
susceptible to substitution or oxidation reactions, can be modified
by alkylations,5 halogenations,6 nitration,7 acetoxylation,8 C-cya-
nide addition,9 oxygen insertion or hydroxylation,10 sulfur inser-
tion,11 formation of ketal,12 pyridination,13 amination,14 or phospha-
nylation.15 Recently we have demonstrated that the palladium(II)
p-benziporphyrin altered the fundamental reactivity of the
benzene unit, allowing facile palladium(II)-stimulated contrac-
tion of p-phenylene to cyclopentadiene.16 Eventually the very
first complexes of 21-carbaporphyrin were formed.17,18 As a matter
of fact, the cleavage or contraction of benzene and its derivatives
have been rarely detected.19ꢀ22
Similarly to the case for benzene, the extremely rare examples
of naphthalene to indene ring contraction have been reported
over the years in the literature, including 2-naphthylnitrene
contraction to 3-cyanoindene under mild conditions,23 the
synthesis of perfluoro-2-cyanoindene via N-chlorination of per-
fluoro-2-aminonaphthalene,24 and formation of perchloro-3-vi-
nylindene via extensive chlorination of 2-methylnaphthalene.25
Interestingly, the iodine(III)- or thallium(III)-promoted ring
contraction of 1,2-dihydronaphthalenes to indanes seems to be
a useful synthetic procedure.26 Automerization of naphthalene
(e.g., of [1-13C] to [2-13C]naphthalene) and related processes
detected for benzenoid hydrocarbons involve reversible contrac-
tion of a benzene nucleus to a five-membered-ring intermediate
benzofulvene. This is the crucial step for the mechanism by which
’ RESULTS AND DISCUSSION
Synthesis and Characterization of Carbaporphyrinoids.
5,10,15,20-Tetraphenyl-1,4-naphthiporphyrin (1) is a carbapor-
phyrinoid with a 1,4-naphthalene ring embedded in the tripyrrolic
framework. Formally 1,4-naphthiporphyrin may be formed by
perimetral fusion of the benzene ring to p-benziporphyrin. In fact
1,4-naphthiporphyrin 1 and 24-thia-1,4-naphthiporphyrin 2 are
obtained in a simple modification of the “3 + 1” approach1,29
described for p-benziporphyrin,30,31 expanded p-benziporphyrins,32
and heterocarbaporphyrins33,34 using the naphthalene analogue of
tripyrrane 3 (Scheme 1).35
The X-ray structures of 1 and 2 were obtained (Figure 1). Both
macrocycles are practically planar, except for the tilt of the
naphthalene moiety toward the center of the corresponding
Received: May 18, 2011
Published: July 29, 2011
r
2011 American Chemical Society
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macrocycles, as defined for the 1-i or 2-i conformation (i, in; o,
out). The dihedral angles between the naphthalene ring (B) and
the four meso carbon atom (Cmeso plane) planes for 1-i and 2-i
are respectively 71 and 68°.
single-bond limit for C(sp2)ꢀC(sp2). Marked alternations of
CRꢀCmeso distances (1-i, C(4)ꢀC(5) = 1.462(3) Å, C(5)ꢀC-
(6) = 1.383(3) Å, C(9)ꢀC(10) = 1.441(3) Å, C(10)ꢀC(11) =
1.375(3) Å; 2-i, C(4)ꢀC(5) = 1.485(2) Å, C(5)ꢀC(6) =
1.371(2) Å, C(9)ꢀC(10) = 1.446(2) Å, C(10)ꢀC(11) =
1.384(2) Å) resembling those observed for nonaromatic 3-aza-
m-benziporphyrin and 24-thia-3-aza-m-benziporphyrin have
been also detected.36 These structural features and the high
values of the dihedral angles between the naphthalene ring and
four meso carbon atoms plane confirm that the naphthalene
incorporated in the framework of 1 or 2 limits essentially the
macrocyclic aromatic delocalization while retaining the strong
conjugation of the tripyrrolic brace. For instance there is an
appreciable effect of the conjugation on the thiophene fragment.
The bond lengths within the thiophene ring are altered. Thus, the
CRꢀCβ bonds in 2-i (1.442(2) and 1.440(2) Å) are longer than
the CβꢀCβ distances (1.353(2) Å), whereas the reverse is true
for tetrathiaporphyrinogen.37 The C(11)ꢀC(12) (1.378(3) Å)
and C(15)ꢀC(16) (1.382(3) Å) distances reflect some double-
bond character in the bonds that link the thiophene ring to the
macrocyclic frame.
The C(20)ꢀC(1) and C(4)ꢀC(5) distances (1-i, 1.454(3)
and 1.462(3) Å; 2-i, 1.483(2) and 1.485(2) Å) approach the
Scheme 1. Synthesis of 1,4-Naphthiporphyrin 1 and 24-Thia-
1,4-naphthiporphyrin 2a
As determined by 1H NMR, the folded conformations 1-i and
2-i detected in the solid state are preserved in solution in the
solvents probed (toluene-d8, chloroform-d, dichloromethane-d2)
over the whole temperature range accessible for a given solvent.
Feasible exchange between two degenerate nonplanar conforma-
tions in a seesaw fashion was not detected (see Scheme 2), which
is in contrast with the p-benziporphyrin30,38 and 24-thia-p-
benziporphyrin33 conformational flexibility.
Scheme 2. Putative Seesaw Conformational Equilibrium of 1
a Both ultimate conformations are shown. Legend: (a) 1 equiv of
pyrrole, 2 equiv of benzaldehyde, Et2O BF3 in dichloromethane, room
3
temperature, 2 h followed by oxidation with DDQ, 3%; (b) 1 equiv of
2,5-bis(tolylhydroxymethyl)thiophene, Et2O BF3 in dichloromethane,
3
room temperature, 2 h followed by oxidation with DDQ, 22%.
Figure 1. Crystal structures of 1-i and 2-i: (top) perspective view; (bottom) side view with aryl groups omitted for clarity. The thermal ellipsoids
represent 50% probability.
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Figure 2. 1H NMR spectra of (a) 1-i, (b) 1-i(H2O), (c) [1-i-H]+, and (d) [1-o-H2]2+. Spectra a, b, and d were obtained in chloroform-d at 220 K, and
spectrum c was obtained in dichloromethane-d2 at 175 K. Resonances are labeled as in Scheme 1.
Figure 3. 1H NMR spectra of (a) 2-i and (b) [2-i-H2]2+ (chloroform-d, 220 K). The residual resonances of the monocationic form of 2 are marked with
asterisks.
The resonance assignments which are given above selected
groups of peaks in Figures 2, 3, and 5 were made on the basis of
relative intensities and detailed two-dimensional NMR studies
(COSY, NOESY, 1H, 13C, HMQC, HMBC). In the cases of 1-i
and 2-i the NOE effects, including the structurally unique
H(31) H(7), H(31) ortho-Ph(5), and H(22) ortho-Ph(5)
correlations allowed the determination of the relay of connectiv-
ities imposed by the structural constraints of the folded con-
formation of 1 or 2.
In principle, 1,4-naphthiporphyrin 1 and 24-thia-1,4-naphthi-
porphyrin 2 may acquire a macrocyclic aromatic structure
involving several accessible delocalization routes differentiated
3 3 3
3 3 3
3 3 3
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Scheme 3. Selected Resonance Structures of 1,4-Naphthi-
porphyrin 1-o Which Demonstrate 18e and 22e Macrocyclic
π-Delocalization Pathwaysa
π-delocalization pathway for the entire macrocycle (β-H pyrrole,
7.19, 6.52, 6.74 ppm;NH, 10.29 ppm).8 Notably, the resonances of
naphthalene (H(21), H(31) and H(22), H(32)) are shifted visibly
upfield (1.3 and 1.8 ppm, respectively) with respect to 3, applied
here as a suitable reference. They are a manifestation of the weak
diatropic ring current. One can note that 1-i acquires a conforma-
tion with both naphthalene rings in the macrocyclic shielding zone.
Water Binding. The neutral form of 1-i produces the clearly
observable N(24)H resonance, providing that the sample and the
deuterated solvent are carefully dried prior to 1H NMR spectro-
scopic exploration. In fact, the 1H NMR spectrum of 1-i at 220 K
demonstrated the noticeable upfield relocation of NH (ca. 0.6
ppm) and naphthalene (H(21) 0.2 ppm) resonances as refer-
enced to the spectrum collected at 300 K, consistent with the
increased aromaticity contribution of conformers due to the
larger folding of the molecule.
The titration studies, wherein aliquots of chloroform-d satu-
rated with water were added to an initial sample of dry 1-i in
chloroform-d, resulted in considerable broadening of the NH
resonance (40 μL of chloroform-d saturated with water added at
300 K) from 16 to 360 Hz accompanied by its upfield relocation
(from 3.59 to 3.46 ppm). Distinct differences in the spectro-
scopic patterns of samples after careful drying and after con-
trolled addition of water at 300 K have been detected at spectra
collected at 220 K (Figure 2b). Namely, two broadened reso-
nances at the 3 to ꢀ3 ppm region have been readily assigned to
the NH and one water molecule bound by 1-i. These findings led
us to postulate that 1,4-naphthiporphyrin could be acting as a
water-binding receptor.41,42 The following equilibrium has been
considered, wherein the water molecule would be bound to the
center of the molecule via hydrogen bonds:
a The unfolded structure is chosen for clarity; two different delocaliza-
tion routes are shown for identical 1-o-2 and 1-o-4.
1-i þ H2O h 1-iðH2OÞ
ð1Þ
The fast exchange of 1-i and 1-i(H2O) produces a single
resonance (220 K) for each hydrogen atom of 1-i, reflected by
smooth changes of all chemical shifts following the gradual
increase of water concentration. Significantly, the exchange of
the NH and coordinated H2O protons is relatively slow, afford-
ing the separation of respective resonances, but is clearly
reflected by the EXSY correlation at NOESY map at 220 K
(Figure S2 in the Supporting Information). The coordinated
water signal is shifted upfield compared to that for H2O in
chloroform-d. This is a reflection of the aromatic ring current
effect of the naphthiporphyrin moiety and the influence it has on
the bound H2O resonance. In accord with this equilibrium, the
position and intensity of the combined H2O resonance was
found to depend on the presence (and total concentration) of
water in the system (Figure S1 in the Supporting Information).
In particular, this binding of water has been accompanied by the
downfield relocation of naphthalene H(21) and H(22) reso-
nances and marked downfield position of all others, as expected
due to the gain in aromatic nature.
by the contribution of p-phenylene-related motives (1-o-2, 1-o-3;
18e macrocyclic π-delocalization pathways) or the participa-
tion of the macrocyclic rim (1-o-4, 1-o-5, 22e macrocyclic
π-delocalization pathways) accessible for both folded and un-
folded conformations of 1 and 2 (Scheme 3).
Actually, the extent of macrocyclic π-conjugation is expected
to be controlled by the relative position of the naphthalene ring
and the tripyrrolic brace defined by the dihedral angle between
naphthaleneꢀCmeso planes (Scheme 2). Conceivably these
π-subunits of the macrocyclic systems can accept a variety of
nonorthogonal orientations which allow eventually the substan-
tial overlap of π-orbitals for folded 1-o or unfolded 1-i con-
formations, which are clearly distinguished by the aromaticity
1
related to H NMR spectroscopic features (see discussions of
variable-temperature studies, palladium(II) coordination, or
water binding below).
The pyrrolic part of the 1-i spectrum is placed in the relatively
upfield region (H(7), 7.18 ppm; H(8), 7.94 ppm; H(12), 7.10
ppm; chloroform-d, 300 K), and these resonances are accom-
panied by the N(24)H signal at the peculiar position of 3.56 ppm.
The aromatic ring current effect of 1-i is relatively small because
of the severe macrocyclic distortion, as reflected here by the
difference in β-H and NH chemical shifts in the 3.54ꢀ4.36
ppm range.1,39,40 Thus, β-H and NH chemical shifts resemble
these of carbaporphyrinoids at the borderline cases of porphyrinoid
aromaticity.1,39,40 In the limiting case of m-benziporphyrin the
strongly aromatic structure of benzene completely blocks a
Protonation of 1 and 2. Two discrete stages of protonation, i.e.
the monocationic [1-i-H]+ and dicationic [1-o-H2]2+ species, have
1
been detected during the H NMR titration of 1-i with trifluor-
oacetic acid (chloroform-d, 220 K; dichloromethane-d2, 175 K)
(Figure 2c,d). The separate sets of resonances of neutral 1-i,
monocationic [1-i-H]+, and dicationic [1-o-H2]2+ species have
been readily identified. These observations have been attributed to
a slow exchange among ionic forms in the system under the
conditions of our experiment. Importantly, the second protonation
step is accompanied by a remarkable conformational change, i.e. a
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Scheme 4. Protonation of 1,4-Naphthiporphyrin 1
Scheme 6. Insertion of Palladium(II) into 1,4-Naphthipor-
phyrin 1a
a Reaction conditions: palladium(II) chloride (3.3 equiv), acetonitrile,
room temperature, 48 h, 87%.
Scheme 5. Protonation of 24-Thia-1,4-naphthiporphyrin 2
flip of the naphthalene moiety, which affords the extended (open)
macrocyclic structure of [1-o-H2]2+ (Scheme 4). The spectro-
scopic evidence includes a spectacular upfield/downfield swap of
the H(21), H(31), H(22), H(32) and H(21), H(22) resonances,
which reflects the relocation of these protons between the well-
defined shielding/deshielding zones of the aromatic macrocycle.
The NOE effects, including specifically the N(24)H H(21,22)
3 3 3
Figure 4. Crystal structure of 4: (top) perspective view; (middle) side view
with phenyl groups omitted for clarity; (bottom) geometry of the interac-
tion between palladium(II) and the naphthalene moiety. The thermal
ellipsoids represent 50% probability. Selected bond distances (Å): PdꢀN-
(23) = 2.046(2), PdꢀN(24) = 2.017(3), PdꢀN(25) = 2.044(2).
and N(23)H H(21,22) correlations, allowed the determination
3 3 3
of the relay of connectivities imposed uniquely by the structural
constraints of the unfolded structure [1-o-H2]2+. The monopro-
tonated species reveal the overall pattern consistent with the folded
conformation, although the broadening of resonances (dichloro-
methane-d2, 175 K), accounted for tentatively by the [1-i-H0]+ h
[1-i-H00]+ tautomeric equilibrium due to the NH exchange
(Scheme 4), renders a detailed analysis unfeasible.
The geometry of 4-i as determined by X-ray crystallography
reflects the balance among constraints of the macrocyclic ligand,
the size of the palladium(II) ion, and the predisposition of
palladium(II) for a square-planar geometry, resembling even-
tually the structure of palladium(II) vacataporphyrin and
palladium(II) p-benziporphyrin (Figure 4).16,43
The naphthalene-in conformational motif of 1 (see Figure 1)
is conserved. The dihedral angle between the naphthalene ring
(B) and the Cmeso plane equals 46.4° and is markedly smaller in
comparison to 1-i. The macrocyclic six-membered ring (A)
shows a slight boatlike deformation (C(1) and C(4) are dis-
placed from the C(2)C(3)C(21)C(22) plane by ca. 0.3 Å),
and the dihedral angle between C(1)C(2)C(3)C(4) and
24-Thia-1,4-naphthiporphyrin 2 differs in the macrocyclic
flexibility as compared to 1,4-naphthiporphyrin 1 (Scheme 5).
Thus the folded structure seems to be conserved for the neutral
form in solution. In contrast to the case for 1, the diprotonation
favors the well-defined folded structure, as revealed by the
appropriate 1H NMR data (Figure 3).
Coordination of Palladium(II). Reaction of palladium(II)
chloride with 1,4-naphthiporphyrin 1-i in acetonitrile results in
the formation of the four-coordinate palladium(II) 1,4-naphthi-
porphyrin 4-i (Scheme 6).
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Figure 5. 1H NMR spectra of (a) 4 (chloroform-d, 220 K), (b) 6 (chloroform-d, 300 K), and (c) 7 (chloroform-d, 300 K). Resonance assignments
follow the numbering of 1,4-naphthiporphyrin (a) or benzocarbaporphyrin (b, c).
C(1)C(21)C(22)C(4) planes equals 30°, which is noticeably
emphasized by the canted position of the fused (B) benzene ring.
Actually, the peculiar orientation of the naphthalene and Cmeso
planes seen in Figure 4 (the side-on projection) nicely reveals the
macrocyclic cleft formation which is defined by 1,4-naphthalene
and macrocyclic arms and is clearly reflected by the displacement
of the naphthalene bridgehead C(2) and C(3) atoms from the
Cmeso plane (C(2), 1.63 Å; C(3), 1.64 Å). The bond distances
within the tripyrrolic and naphthalenic framework of 4 indicate
that the macrocycle reveals the bond length pattern expected
for aromatic carbaporphyrinoids,1,39 including 6 (see below).
In particular, the C(20)ꢀC(1) and C(4)ꢀC(5) distances
(1.392(3) and 1.395(3) Å) approach the aromatic bond limit.
The aromaticity of 4 is clearly demonstrated by the equalization
of CRꢀCmeso distances (C(4)ꢀC(5) = 1.395(3) Å, C(5)ꢀC(6) =
1.428(2) Å, C(9)ꢀC(10) = 1.398(2) Å, C(10)ꢀC(11) =
1.394(3) Å). These geometrical features, consistent with spectro-
scopically detected aromatic behavior, remain in contrast with
those determined for 1-i and 2-i. The distinct feature of the
structure is the very pronounced bending of the chloride ligand
toward the naphthalenic unit. Significantly, the N(23)ꢀPdꢀN-
(25) fragment (170.1(1)°) is slightly distorted from linearity,
while N(24)ꢀPdꢀCl is bent (155.6(1)°) as a consequence of
strain induced by incorporation of a metalꢀchloride bond into a
macrocyclic ring. In particular, the naphthalene unit approaches
(Pd C = 3.3 Å)44 but clearly longer than normally observed
3 3 3
PdꢀC bond lengths, as the typical values for palladium(II)ꢀC-
(η2) alkene complexes are on the order of 2.18 Å.45,46 Thus, the
palladium(II) ion interacts weakly with the bridging carbon
center of naphthalene in a η2 fashion, engaging the C(2) and
C(3) carbon atoms. The projection of the palladium(II) ion onto
the C(2)C(3)C(21)C(22) plane (C4 plane) lies close to the
center of the C(2)ꢀC(3) bond (Figure 4). A similar weak
interaction has been observed for metal(II) p-benziporphyrin
complexes both in solution and in the solid state, although in this
case the hydrogen-substituted carbon atoms C(21) and C(22)
are involved.16,30,31 Importantly, the peculiar interactions of
metal(II) ions with naphthalene, which essentially include the
bridgehead carbon atoms, were reported for the product of
π-complexation of naphthalene to trimeric mercury(II) perfluoro-
o-phenylene.47
The stereoisomer 4-i has been unambiguously identified as a
1
single species in solution. Evidently 4-i shares a common H
NMR spectral pattern with the free base 1-i, although its ring
current effect is more pronounced (Figure 5a). In particular, the
upfield relocations of H(21), H(31), H(22), and H(32) and the
downfield positions of H(21) and H(22) in comparison to those for
3 and 1-i have been noted. Significantly, the observed H(7)
3 3 3
H(31) NOE effect is consistent with the folded structure 4-i.
Contraction. Palladium(II) 1,4-naphthiporphyrin 4-i dissol-
ved in acetonitrile undergoes reactions after the heterogeneous
addition of potassium carbonate at room temperature over a
palladium(II) at distances shorter (Pd C(2) = 2.94 Å,
3 3 3
Pd C(3) = 2.93 Å) than the expected van der Waals contact
3 3 3
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Scheme 7. Contraction of Palladium(II) 1,4-Naphthipor-
phyrin 4-i
Figure 6. Crystal structure of 6: (top) perspective view; (bottom) side
view with phenyl groups omitted for clarity. The thermal ellipsoids
represent 50% probability. Selected bond distances (Å): PdꢀN(22) =
2.021(2), PdꢀN(23) = 2.048 (2), PdꢀN(24) = 2.015(2), PdꢀC(21) =
2.107(2). The side view presents the geometry of the interaction
between palladium(II) and the formyl substituent.
(Figure 6). The macrocycle reveals the bond length pattern
expected for aromatic carbaporphyrinoids.1,39 The peculiar loca-
tion of the 21-formyl substituent forces the relatively short
Pd C(formyl) distance (2.512(2) Å).1,29,49
3 3 3
The square-planar geometry with a direct bond between the
transition metal and the tetrahedrally hybridized isoindene
carbon was demonstrated by X-ray analysis. In spite of intense
exploration the metallocarbaporphyrinoids 6 and 7 are the very
first representatives of benzocarbaporphyrins which contain an
isoindene moiety and preserve the tetrahedral geometry around
the inner carbon.1,17,18,29,50 The created coordination center
provides an environment that allows exceptional stabilization
of the metal(II) ion by this carbaporphyrinoid. Typically the β-
alkylated benzocarbaporphyrins and meso-tetraphenylbenzipor-
phyrins bind metal(III) cations, as their coordination center is
suitably prearranged to stabilize higher oxidation states
(silver(III), gold(III)).1,29,49,51 Thus, the C(21) adaptability of
benzocarbaporphyrin has been recognized for the very first time.
In fact, palladium(II) coordination traps the unprecedented tautomer
of benzocarbaporphyrin 8 (Scheme 8), although its dicationic
form was reported to be formed under strongly acidic conditions.52
A feasible mechanism of contraction as inferred from the
reaction products and in straightforward analogy to contraction
of palladium(II) p-benziporphyrin16 is expected to comprise the
following major steps (Scheme 7):
period of 12 h under anaerobic conditions. Palladium(II) 21-
formyl-21-benzocarbaporphyrin 6 (major product) and
palladium(II) 21-benzocarbaporphyrin 7 (traces) have been
spectroscopically identified as the final products (Scheme 7).
On the synthetic scale after chromatographic workup 6 was
obtained in 21% yield. The 1H NMR spectra of 6 and 7
(Figure 5b,c) resemble the basic features of aromatic carba-
porphyrinoids1,31,48 and reveal striking similarities to the recently
reported palladium(II) 21-carbaporphyrin complexes.16 The
1
analytically specific H (3.48 ppm) coupled with 13C (170.8
• 4-i to 4-o rearrangement of palladium(II) 1,4-naphthi-
porphyrin
ppm) resonances of 21-formyl in 6 and H(21) (ꢀ2.93 ppm) in 7
readily confirmed their identity. The 13C chemical shifts deter-
mined for the C(21) carbon atoms of 6 (69.8 ppm) and 7 (47.3
ppm) definitely reflect the tetrahedral geometry around the
coordinating carbon atom.
The coordinating environment of palladium(II) in 6 corre-
sponds to a conventional square-planar structure with the N(22),
N(23), N(24), and C(21) atoms occupying equatorial positions
• addition of palladium(II) and hydroxide to the C(21)ꢀ
C(22) bond
• β-elimination53
• competing contractions via 1,2-hydride shift or cheletropic
extrusion of carbon oxide
Activation of the naphthalene moiety of 4-o toward a com-
bined addition of palladium(II) and hydroxide is of particular
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ARTICLE
importance. This step is feasible due to the conformational
flexibility of 1,4-naphthiporphyrin, demonstrated previously dur-
ing its protonation. Following the 4-i to 4-o rearrangement the
palladium(II) ion interacts with the naphthalene in a η2 fashion,
involving C(21) and C(22) carbon atoms. The palladium(II)
and hydroxide addition to the activated C(21)ꢀC(22) bond is
expected to form palladium(II) 22-hydroxy-1,4-dihydronaphthi-
porphyrin 5. The subsequent contraction of 5, accompanied by a
relief of strain energy of the embedded conjugated dihydro-
naphthalene unit, results in the formation of the new aromatic
palladium(II) porphyrinoids 6 and 7.
purged with nitrogen for 20 min. Subsequently, boron trifluorideꢀ
diethyl etherate (200 μL) was added, the setup was fitted with a reflux
condenser, and the solution was refluxed for 48 h under a protective
atmosphere. After this time the solution was neutralized by addition of
triethylamine (2 mL). The solvents were removed using a rotary
evaporator, and the crude residue was chromatographed on silica gel
70-230 using 1% triethylamine in dichloromethane as eluent. The
desired product eluted as the first fraction. The solvent was removed
1
under reduced pressure, yielding 3. Yield: 1095 mg (50%). H NMR
(chloroform-d, 298 K, mixture of isomers): δ 8.03 (AA0BB0, 2H), 7.78
(b, 2H), 7.36 (AA0BB0, 2H), 7.27 (t, 4H, 3J = 7.0 Hz), 7.21 (t, 2H, 3J = 7.3
Hz), 7.18ꢀ7.16 (m, 4H), 6.93 and 6.92 (2s, 2H), 6.68ꢀ6.66 (m, 2H),
6.20 (b, 2H), 6.14ꢀ6.12 (m, 2H), 5.81ꢀ5.79 (m, 2H). 13C NMR
(chloroform-d, 298 K): δ 142.82, 138.33, 133.31, 132.04, 128.98,
128.57, 126.73, 126.40, 125.89, 124.61, 117.09, 108.45 and 108.43,
108.25 and 108.17, 46.71. HR-MS (ESI+, TOF): m/z [M + H]+
439.2173, calcd for C32H27N2+ 439.2169.
’ CONCLUSION
The palladium(II) 1,4-naphthiporphyrin provides a unique
environment to alter the fundamental reactivity of the naphtha-
lene unit. Accordingly, the palladium(II)-stimulated contraction
of naphthalene to isoindene has been discovered, yielding
eventually the first complex of metal(II) benzocarbaporphyrin
with a unique tetrahedral carbon coordination mode. In more
general terms, it is essential to recall here that benzocarbapor-
phyrins were originally generated by a peculiar contraction of the
azulene seven-membered ring built into the azuliporphyrin
structure 9. As a result, a family of aromatic benzocarbaporphyr-
ins was identified and characterized.54,55
5,10,15,20-Tetraphenyl-1,4-naphthiporphyrin (1). 1,4-Bis(phenyl(2-
pyrolyl)methyl)naphthalene (3; 1850 mg, 4.2 mmol), pyrrole (290 μL,
4.2 mmol), and benzaldehyde (1280 μL, 8.4 mmol) were added to dry
dichloromethane (900 mL) under nitrogen. Boron trifluorideꢀdiethyl
etherate (100 μL) was then added, and the reaction mixture was protected
from light and stirred for 2 h. 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ; 2860 mg, 12.6 mmol) was subsequently added, and the reaction
1
mixture was stirred for another /2 h. After this time the solvent was
evaporated under reduced pressure and the dark residue was subjected to
chromatography (grade II alumina, dichloromethane). The desired
product was eluted as a green band. Subsequent chromatography on
alumina with ethyl acetate/hexane as eluent (v/v 3/7) gave 1. Yield: 95
mg (3%). 1H NMR (600 MHz, chloroform-d, 298 K): δ 8.07 (d, 4H, 3J =
7.7 Hz, 5,20-o-Ph), 7.95 (d, 2H, 3J = 4.7 Hz, 7,18-H), 7.74 (t, 4H, 3J = 7.7
Hz, 5,20-m-Ph), 7.62ꢀ7.52 (d, t, d, 6H, 10,15-o-Ph, 5,20-p-Ph, 10,15-o-
Ph), 7.47 (m, 6H, 10,15-m,p-Ph), 7.19 (d, 2H, 3J = 4.7 Hz, 8,17-H), 7.11
(s, 2H, 12,13-H), 6.89 (s, 2H, 21,22-H), 6.30 (AA0BB0, 2H, 22,32-H), 6.07
(AA0BB0, 2H, 21,31-H), 3.59 (b, 1H, NH). 13C NMR (126 MHz,
chloroform-d, 298 K): δ 164.50, 158.83, 148.25, 146.26, 144.55, 141.90,
141.10, 136.37, 134.85, 133.78, 133.23, 133.09, 132.55, 130.13, 128.71,
128.40, 127.28, 127.23, 124.31, 124.19, 123.98, 115.72. HR-MS (ESI+,
Scheme 8. Contractions of 1 and 9 Isomers to the Common
Molecular Target 8
+
FT-MS): m/z [M + H]+ 676.2747, calcd for C50H34N3 676.2747.
UVꢀvis (CH2Cl2, 298 K): λmax (log ε) 353 (4.36), 463 (4.46),
Thus, the two carbaporphyrinic isomers 1,4-naphthipor-
phyrin 1 and azuliporphyrin 9 undergo conceptually related
transformations (Scheme 8) which involve ring contractions
appropriately of isomeric azulene (seven-membered ring) or
naphthalene (six-membered ring) units, affording eventually an
isoindene moiety built into the common transformation target—
benzocarbaporphyrin 8.
627 nm (4.18).
1
5,10,15,20-Tetraphenyl-1,4-naphthiporphyrin 2H+ (1 2H+). H
3
3
NMR (600 MHz, chloroform-d, 220 K): δ 8.48 (dd, 3J = 4.9 Hz, 4J =
1.3 Hz, 8,17-H), 8.44 (d, 3J = 7.3 Hz, 5,20-o-Ph), 7.93 (d, 3J = 7.0 Hz,
10,15-o-Ph), 7.87ꢀ7.84 (d covered with dd, 5,20-o-Ph, 7,18-H), 7.79
(d, 3J = 7.3 Hz, 10,15-o-Ph), 7.71ꢀ7.64 (m, remaing meso-Ph signals),
7.48 (AA0BB0, 21,31-H), 7.43 (d, 4J = 1.0 Hz, 12,13-H), 7.04 (AA0BB0,
22,32-H), 6.10 (b, 24-NH), 5.38 (b, 23,25-NH), 4.04 (s, 21,22-H).
UVꢀvis (CH2Cl2, 298 K): λmax (log ε) 379 (4.34), 476 (4.70), 720
(4.30).
’ EXPERIMENTAL SECTION
5,20-Diphenyl-10,15-ditolyl-1,4-naphthi-24-thiaporphyrin (2). 1,4-
Bis(phenyl(2-pyrolyl)methyl)naphthalene (3; 1090 mg, 2.9 mmol), and
2,5-bis(tolylhydroxymethyl)thiophene (845 mg, 2.9 mmol) were added
to dry dichloromethane (900 mL) under nitrogen. Boron trifluorideꢀ
diethyl etherate (200 μL) was then added, and the reaction mixture was
protected from light and stirred for 2 h. 2,3-Dichloro-5,6-dicyano-
1,4-benzoquinone (DDQ; 1941 mg, 8.6 mmol) was subsequently added,
Solvents and Reagents. Methanol, hexane, toluene, acetonitrile,
and 1,2-dichloroethane were used without purification. Dichloro-
methane and pyrrole were distilled over calcium hydride. Chloroform-d
was prepared directly before use by running through a basic alumina
column. Reagents not listed here were used as received.
2,5-Bis(tolylhydroxymethyl)thiophene has been synthesized as de-
scribed in the literature.56
1
and the reaction mixture was stirred for another /2 h. After this time
Synthetic Procedures and Analytical Data. 1,4-Bis(phenyl-
(2-pyrolyl)methyl)naphthalene (3). In a 250 mL round-bottomed flask
equipped with a reflux condenser and magnetic stirrer, 1,4-bis(phenyl-
hydroxymethyl)naphthalene (11; 1.7 g, 5 mmol) obtained analogously
to the previously reported 1,4-bis(phenylhydroxymethyl)benzene (see
the Supporting Information)32 was dissolved in a mixture of dry pyrrole
(10 mL, 0.144 mol) and chloroform (15 mL) and the solution was
solvent was evaporated under reduced pressure and the dark residue was
subjected to chromatography (grade II alumina, dichloromethane). The
desired product was eluted as a green band. Subsequent chromatography
on alumina (grade II) with hexane as eluent gave 2. Yield: 446 mg (22%).
1H NMR (600 MHz, chloroform-d, 220 K): δ 8.03 (d, 4H, 3J = 7.6 Hz,
5,20-o-Ph), 7.88 (d, 2H, 3J = 4.6 Hz, 7,18-H), 7.63 (t, 4H, 3J = 7.1 Hz,
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ARTICLE
5,20-m-Ph), 7.59 (t, 2H, 3J = 7.2 Hz, 5,20-p-Ph),7.50 (s, 2H, 12,13-H),
calcd for C50H32N3OPd+ 796.1575. UVꢀvis (CH2Cl2, 298 K): λmax
(log ε) 318 (3.54), 370 (3.75), 405 (3.72), 478 (3.92), 667 nm (3.50).
Palladium(II) 5,10,15,20-Tetraphenyl-21-benzocarbaporphyrin
3
7.43ꢀ7.40 (2d, 4H, 10,15-Tol), 7.30 (d, 4H, J = 8.0 Hz, 10,15-Tol),
3
7.16 (d, 2H, J = 4.6 Hz, 8,17-H), 6.88 (AA0BB0, 2H, 21,31-H), 6.76
1
(AA0BB0, 2H, 22,32-H), 6.35 (s, 2H, 21,22-H), 2.47 (s, 6H, 10,15-Me-
Tol). 13C NMR (151 MHz, chloroform-d, 220 K): δ 166.11, 157.95,
150.77, 148.04, 142.19, 140.22, 137.12, 136.77, 136.02, 134.59, 133.64,
131.67, 131.57, 131.52, 131.38, 129.20, 129.17, 128.60, 128.40, 128.33,
125.95, 124.38, 21.40. HR-MS (ESI+, TOF): m/z [M + H]+ 721.2665,
calcd for C52H37N3S+ 721.2672. UVꢀvis (CH2Cl2, 298K): λmax (log ε)
356 (4.35), 438 (4.30), 623 nm (4.10).
(7). Yield: traces. H NMR (600 MHz, chloroform-d, 300 K): δ 8.45
(d, 2H, 3J = 4.7 Hz, 7,18-H), 8.37 (d, 2H, 3J = 4.7 Hz, 8,17-H), 8.34 (s,
2H, 12,13-H), 8.22 (d, 2H, 3J = 7.2 Hz, 5,20-o-Ph), 8.16 (d, 2H, 3J = 6.9
Hz, 10,15-o-Ph), 8.14 (d, 2H, 3J = 7.2 Hz, 5,20-o-Ph), 8.11 (d, 2H, 3J =
6.9 Hz), 7.87 (t, 2H, 3J = 7.3 Hz, 5,20-m-Ph), 7.80 (t, 2H, 3J = 7.3 Hz,
5,20-m-Ph), 7.74ꢀ7.69 (m, 8H, 5,20-p-Ph, 10,15-m,p-Ph), 7.42ꢀ7.40
(AA0BB0, 2H, 21,31-H), 7.36ꢀ7.35 (AA0BB0, 2H, 22,32-H), ꢀ2.93 (s,
1H, 21-H). 13C NMR (151 MHz, chloroform-d, 300 K): δ 152.22,
148.71, 143.17, 142.90, 141.69, 141.36, 136.40, 134.17, 133.92, 133.64,
132.41, 129.54, 129.33, 128.49, 128.33, 128.21, 127.96, 127.91, 127.53,
126.88, 126.86, 126.82, 124.62, 47.27. HR-MS (ESI+, TOF) m/z [M-H]ꢀ
766.1498, calcd for C50H30N3OPdꢀ 766.1480. UVꢀvis (CH2Cl2,
298 K): λmax (log ε) 397 (3.84), 464 (3.71), 639 nm (4.07).
5,20-Diphenyl-10,15-Ditolyl-1,4-naphthi-24-thiaporphyrin 2H+
3
(2 2H+). 1H NMR (600 MHz, chloroform-d, 220 K): δ 9.25 (d, 3J = 4.6
3
Hz, 7,18-H), 9.00 (s, 12,13-H), 8.90 (d, 3J = 6.9 Hz, 5,20-o-Ph), 8.77 (d,
3J = 4.6 Hz, 8,17-H), 8.33 (s, 21,22-H), 8.28 (d, J = 7.5 Hz, 10,15-o-
3
Tol), 8.19 (d, 3J = 6.5 Hz, 5,20-o-Ph), 7.91 (d, 3J = 7.3 Hz, 10,15-m-Tol),
7.73 (d, 3J = 7.1 Hz, 10,15-o-Tol), 7.69 (d, 3J = 7.3 Hz, 10,15-m-Tol),
5.19 (broad AA0BB0, 22,32-H), 3.21 (broad AA0BB0, 21,31-H), 1.18 (b,
23,25-NH). UVꢀvis (CH2Cl2, 298 K): λmax (log ε) 274 (4.84), 402
(4.56), 441 (4.61), 507 (4.87), 776 nm (4.69).
1
NMR Spectroscopy. H NMR spectra were recorded on a high-
field spectrometer (1H 600.15 and 500.13 MHz), equipped with a
broad-band inverse gradient probehead. Spectra were referenced to the
residual solvent signal (chloroform-d, 7.24 ppm). Two-dimensional
NMR spectra were recorded with 2048 data points in the t2 domain
and up to 1024 points in the t1 domain, with a 1 s recovery delay.
Mass Spectrometry. High resolution and accurate mass spectra
were recorded using the electrospray technique.
Chloropalladium(II) 5,10,15,20-tetraphenyl-1,4-naphthiporphyrin
(4). 5,10,15,20-Tetraphenyl-1,4-naphthiporphyrin (1; 29 mg, 43 μmol)
and palladium(II) chloride (23 mg, 0.13 mmol) were added to dry
acetonitrile (50 mL) under nitrogen and stirred for 48 h. After this time
solvent was evaporated under reduced pressure and the dark brown
residue was subjected to chromatography (silica gel 70-230, 3% metha-
nol/97% dichloromethane solution). 4 was eluted as the main orange-
green fraction. The product was recrystallized from dichloromethane/
hexane solution. Yield: 30.6 mg (87%). 1H NMR (600 MHz, chloroform-
d, 220 K): δ 9.22 (s, 2H, 21,22-H), 9.16 (d, 2H, 3J = 7.5 Hz, 5,20-o-Ph),
8.58 (d, 2H, 3J = 4.7 Hz, 7,18-H), 8.39 (s, 2H, 12,13-H), 8.38 (d, 2H, 3J =
4.7 Hz, 8,17-H), 8.02 (d, 2H, 3J = 7.0 Hz, 10,15-o-Ph), 7.89 (t, 2H, 3J = 7.5
Hz, 5,20-p-Ph), 7.87 (d, 2H, 3J = 7.5 Hz, 10,15-o-Ph), 7.75ꢀ7.63 (m, 6H,
10,15-m,p-Ph), 7.60 (t, 2H, 3J = 7.5 Hz, 5,20-m-Ph), 7.43 (t, 2H, 3J = 7.5
Hz, 5,20-m-Ph), 6.82 (d, 2H, 3J = 7.5 Hz, 5,20-o-Ph), 4.58 (AA0BB0, 2H,
21,31-H), 3.32 (AA0BB0, 2H, 22,32-H). 13C NMR (151 MHz, chloroform-
d, 220 K): δ 163.29, 158.64, 142.85, 142.42, 142.15, 141.99, 140.73,
140.44, 140.18, 136.18, 135.61, 134.92, 133.33, 132.58, 132.16, 130.42,
128.74, 128.43, 128.16, 128.03, 127.09, 126.95, 125.44, 124.07, 120.53.
HR-MS (ESI+, TOF) m/z [M ꢀ Cl]+ 780.1633, calcd for C50H32N3Pd+
780.1631. UVꢀvis (CH2Cl2, 298 K): λmax (log ε) 452 (4.83), 506 (4.87),
592 (3.99), 649 (4.05), 682 (4.05), 746 nm (4.18).
Synthesis of 6 and 7. Chloropalladium(II) 5,10,15,20-tetraphenyl-
1,4-naphthiporphyrin (4; 13.0 mg, 16 μmol) and potassium carbonate
(90 mg) were added to acetonitrile (50 mL) and stirred under nitrogen
at room temperature overnight. The dark green mixture was evapo-
rated, and the residue was subjected to chromatography (silica gel 70-
230, dichloromethane). Traces of 7 were eluted as the first green
fraction, followed by the more polar main product 6. 7 was purified by
another round of chromatography (silica gel 70-230, hexane/dichlor-
omethane 3/7 v/v).
UVꢀVis Spectroscopy. Electronic spectra were recorded on a
diode-array spectrophotometer.
X-ray Crystallography. X-ray-quality crystals were prepared by
slow evaporation of solutions of 1, 2, and 4 dissolved in dichloro-
methane/hexane and 6 dissolved in toluene. Data were collected at 100
K on a Xcalibur PX-k geometry diffractometer, with Mo KR radiation
(λ = 0.710 73 Å). Data were corrected for Lorentz and polarization
effects. Crystal data are compiled in Table S1 (Supporting Information).
The structures were solved by the heavy-atom method with SHELXS-97
and refined by full-matrix least-squares methods by using SHELXL-97
with anisotropic thermal parameters for the non-H atoms. Scattering
factors were those incorporated in SHELXS-97.
’ ASSOCIATED CONTENT
S
Supporting Information. Text, figures, tables, and CIF
b
files giving crystal data, bond lengths and angles, anisotropic
thermal parameters, and a full set of UVꢀvis electronic spectra.
This material is available free of charge via the Internet at http://
pubs.acs.org.
’ AUTHOR INFORMATION
Corresponding Author
*E-mail: llg@wchuwr.pl. Home page: http://llg.chem.uni.wroc.pl.
’ ACKNOWLEDGMENT
Palladium(II) 5,10,15,20-Tetraphenyl-21-formyl-21-benzocarba-
1
porphyrin (6). Yield: 2.7 mg (21%). H NMR (600 MHz, chloro-
Financial support from the Ministry of Science and Higher
Education (Grant No. N204 013536) is kindly acknowledged.
We thank Professor Tadeusz Lis for valuable discussions.
form-d, 300 K): δ 8.39 (d, 2H, 3J = 4.9 Hz, 7,18-H), 8.31 (d, 2H, 3J = 4.9
Hz, 8,17-H), 8.30 (d partially covered by 8,17-H signal, 2H, 5,20-o-Ph),
8.26 (s, 2H, 12,13-H), 8.10 (d, 2H, 3J = 6.7 Hz, 10,15-o-Ph), 8.07ꢀ8.04
(2d, 4H, 10,15-o-Ph, 5,20-o-Ph), 7.87 (t, 2H, 3J = 7.6 Hz, 5,20-m-Ph),
7.82 (t, 2H, 3J = 7.5 Hz, 5,20-m-Ph), 7.76 (t, 2H, 3J = 7.4 Hz, 5,20-p-Ph),
7.73ꢀ7.67 (m, 6H, 10,15-m,p-Ph), 7.48ꢀ7.46 (AA0BB0, 2H, 21,31-H),
7.44ꢀ7.42 (AA0BB0, 2H, 22,32-H), 3.48 (s, 1H, 21-CHO). 13C NMR
(151 MHz, chloroform-d, 300 K): δ 170.81, 155.96, 147.17, 143.83,
142.82, 141.05, 139.18, 134.14, 133.87, 133.85, 133.81, 133.63, 130.17,
129.88, 129.85, 129.38, 128.81, 128.67, 128.56, 128.23, 127.85, 126.96,
126.89, 125.07, 69.81. HR-MS (ESI+, TOF) m/z [M + H]+ 796.1554,
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