Tetraphenyl-p-benziporphyrin: A carbaporphyrinoid with two linked carbon atoms in the coordination core

Article abstract of DOI:10.1021/ja017852z

Replacement of one of the pyrrole rings with a p-phenylene unit transforms porphyrin into p-benziporphyrin (1), an aromatic carbaporphyrinoid that locates two connected carbon atoms in the coordination core. p-Benziporphyrin forms a complex with cadmium(II) (2) with an unprecedented η² Cd(II)-arene interaction. Copyright

Full text of DOI:10.1021/ja017852z

Published on Web 03/21/2002
Tetraphenyl-p-benziporphyrin: A Carbaporphyrinoid with Two Linked Carbon
Atoms in the Coordination Core
Marcin Ste¸pien´ and Lechosław Latos-Graz˘ yn´ski*
Department of Chemistry, UniVersity of Wrocław, 14 F. Joliot-Curie St., Wrocław 50 383, Poland
Received December 21, 2001
Carbaporphyrinoids are porphyrin analogues that possess at least
one CH unit replacing a pyrrolic nitrogen in the coordination core.
This “internal carbon” atom normally belongs to a carbo- or
heterocyclic ring substituting one of the pyrroles.1-6 It may
participate in coordination of metal ions leading to stable organo-
metallic complexes with often unusual properties.4
5,10,15,20-Tetraphenyl-p-benziporphyrin (1a) is a novel car-
baporphyrinoid with a p-phenylene ring embedded in the tripyrrolic
framework. 1 may be considered to have two adjacent CH units in
the macrocyclic core. The coordinating properties of 1 are exempli-
fied by a cadmium(II) complex.
1 is an aromatic isomer of the nonaromatic 6,11,16,21-tetra-
phenyl-m-benziporphyrin,5 related to hexaalkyl-m-benziporphyrin
and its modifications including aromatic oxybenziporphyrin.6
1 is obtained in a simple modification of the synthesis described
for tetraphenyl-m-benziporphyrin (Scheme 1).5 After chromato-
graphic workup compound 1 was obtained in 1% yield.7
Figure 1. Crystal structures of 1b and (1a-H2)Cl2‚2CH2Cl2‚H2O (50%
thermal ellipsoids, one of two symmetry-independent molecules shown for
1b). Solvent molecules and hydrogen atoms (except for NH) are omitted
for clarity.
Scheme 1. Synthesis
1b and the dication (1a-H2)Cl2 have been found to adopt similar
conformations in the solid state (Figure 1). The macrocycle is planar
(1b) or only slightly ruffled (1a-H22+) except for the tilt of the
p-phenylene moiety. The dihedral angle between the six-membered
ring and the N3 plane is almost invariant and equals 48° and 44°
for the two independent molecules of 1b, and 43° for 1a-H22+. The
dication coordinates two chloride ions through hydrogen bonds. In
2+
the structures of 1b and 1a-H2 the phenylene moiety displays a
slight but statistically significant distortion from the idealized
benzene geometry. The C(2)-C(3) and C(21)-C(22) bond lengths
vary in the range 1.365(2)-1.390(2) Å and are systematically
shorter than the remaining distances within the six-membered ring
(1.394(2)-1.415(3) Å). Along with the lengths of C(1)-C(20) and
C(4)-C(5) bonds (1.454(3)-1.464(2) Å), which are shorter than
the respective distances in 22-acetoxy-m-benziporphyrin,5 they
indicate that the phenylene is partly conjugated with the tripyrrolic
brace.
Scheme 2
Scheme 3
At 168 K the phenylene protons 2,3-H and 21,22-H have strongly
differentiated chemical shifts (7.68 and 2.32 ppm, respectively).
They are a clear manifestation of the diatropic ring current, which
deshields the external and shields the internal protons. At higher
temperatures the two signals coalesce into a singlet due to a dynamic
process. The molecule switches rapidly between two equivalent
conformations, which differ by a flip of the p-phenylene (Scheme
2).
Activation parameters of this process determined for 1c are ∆Hq
) 34.0(3) kJ/mol and ∆Sq ) 9.6(1.3) J/(mol‚K). Such conforma-
tional flexibility is not typical of regular porphyrins but was
observed in certain porphyrin analogues.8
To account for the structural and spectroscopic characteristics
of 1, it is necessary to include the quinoid canonical form III in
the description of p-benziporphyrin (Scheme 3). In conjunction with
the Kekule´ structures I and II it defines two 18e macrocyclic
π-delocalization pathways B and C, which may coexist with the
[6]annulene aromaticity of the benzene ring (A). While 1 is the
first porphyrinoid to exhibit the delocalization pattern shown in
Scheme 3 it was shown earlier that p-phenylene moieties, when
* Corresponding author. E-mail: llg@wchuwr.chem.uni.wroc.pl.
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10.1021/ja017852z CCC: $22.00 © 2002 American Chemical Society

C O M M U N I C A T I O N S
as in the free base (the tilt angle is 45°). However the six-membered
ring shows a slight boatlike deformation (C(1) and C(4) are
displaced from the C4 plane by ca. 0.15 Å).
In conclusion, p-benziporphyrin, an isomer of m-benziporphy-
rin,5,6 is a new aromatic porphyrinoid, which preserves the essential
features of the [18]porphyrin(1.1.1.1) frame and can coordinate
metal ions using the (Cn,N,N,N) coordination core. The p-phenylene
ring may be able to participate in various metal-arene bonding
modes ranging from η2 to η.6
Acknowledgment. Financial support from the State Committee
for Scientific Research KBN of Poland (Grant 4 T09A 147 22)
and the Foundation for Polish Science is kindly acknowledged.
Figure 2. 1H NMR spectrum of 2a (CDCl3, 298 K). Resonance assignments
(obtained from COSY and NOESY maps) follow the numbering given in
Scheme 1.
Supporting Information Available: Synthetic procedures, UV-
vis spectra, NMR data (including a 1H-113Cd 1D HMQC spectrum of
2a) (PDF) and crystallographic data for 1b, (1a-H2)Cl2, and 2 (CIF).
This material is available free of charge via the Internet at http://
pubs.acs.org.
References
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Figure 3. Structure of 2‚2CDCl3 (50% thermal ellipsoids; solvent molecules
and hydrogen atoms omitted for clarity). Inset presents the geometry of
interaction between cadmium(II) and p-phenylene.
incorporated into an annulenoid structure, can likewise participate
in the overall delocalization.9 The simultaneous accessibility of local
and macrocyclic aromaticity distinguishes 1 from the m-benzipor-
phyrins with an isolated [6]annulene subsystem,5,6a and from
oxybenziporphyrin,6b where macrocyclic aromaticity is achieved
by transforming the benzene moiety into semiquinone.
1a reacts smoothly with CdCl2 yielding chlorocadmium(II)
tetraphenyl-p-benziporphyrin (2a), wherein the macrocycle acts as
a monoanionic ligand. Complexation constrains the conformational
dynamics of the macrocycle, and the 1H NMR spectrum of 2a taken
at 298 K contains sharp p-phenylene signals (δ2,3 - δ21,22 ) 6.57
ppm) with no signs of exchange (Figure 2). Coordination through
the three nitrogen donors is confirmed by the presence of 111/113Cd
satellites seen for all the â-pyrrolic signals. In addition a weak scalar
coupling (4.4 Hz) is observed between 111/113Cd and protons
21,22-H, which results from spatial proximity between the cadmium
ion and p-phenylene (the coupling is absent for 2,3-H).
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occupying the equatorial positions. Cd(II) is displaced by 0.524(1)
Å from the N3 plane (Figure 3).
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close to the center of the C(21)-C(22) bond, so the metal ion
interacts with the benzene ring in a η2 fashion. To the best of our
knowledge, this arrangement has no precedent in cadmium chem-
istry although is well-documented for Hg(II) and Ag(I) arene
complexes.14,15 The orientation of the phenylene ring in 2 is similar
(12) The shortest nonbonding distance between cadmium and arene reported
so far was a 2.7 Å η1 contact found in Cd(O-2.6-Ph2C6H3)2. Darensbourg,
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