Macrocyclic diiminodipyrromethane complexes: Structural analogues of Pac-Man porphyrins

Article abstract of DOI:10.1039/b308443d

The complexation of palladium(II) by a unique family of [2+2] diiminodipyrromethane macrocycles yields compounds that adopt structures reminiscent of Pac-Man porphyrins.

Full text of DOI:10.1039/b308443d

Macrocyclic diiminodipyrromethane complexes: structural analogues of
Pac-Man porphyrins†
Gonzalo Givaja, Alexander J. Blake, Claire Wilson, Martin Schröder* and Jason B. Love*
School of Chemistry, University of Nottingham, University Park, Nottingham, UK NG7 2RD.
E-mail: jason.love@nottingham.ac.uk; Fax: +44 115 9513563; Tel: +44 115 8467167
Received (in Cambridge, UK) 23rd July 2003, Accepted 22nd August 2003
First published as an Advance Article on the web 9th September 2003
The complexation of palladium(II) by a unique family of
[2+2] diiminodipyrromethane macrocycles yields com-
pounds that adopt structures reminiscent of Pac-Man
porphyrins.
products are present. The X-ray crystal structure of the salt 2a
was determined and confirms that [2+2] cyclisation has
occurred (Fig. 1). The tetraprotonated macrocycle adopts a
bowl-like conformation with one TsO2 group (S1) hydrogen
bonded through O2 and O3 to opposing pairs of iminopyrrole
units. Two further TsO2 groups (S2 and S4) interact with the
remaining iminopyrrole units through O5 and O11, re-
spectively. This conformation suggests a possible mode of
complexation for transition metals as two N4 compartments are
defined by the resultant wedge at the dipyrromethane meso-
carbons.
The macrocyclic salts can be neutralised by the addition of
NEt3, which quantitatively forms the free ligands H4[L] as
poorly soluble, amorphous, yellow powders, although addition
of a protic solvent such as d4-methanol to a slurry of H4[L] in
CDCl3 results in immediate dissolution. The 1H NMR spectrum
of H4[L1] reveals the retention of the characteristic resonance of
the imine proton at 8.10 ppm. The relative simplicity of the
synthesis and the high yields afforded mean that multigram
quantities can be readily prepared.
Polypyrrolic macrocycles exhibit a rich and diverse chemistry.
For example, the flexible frameworks of calixpyrroles can
accommodate a variety of transition metals and f-block
elements in a range of oxidation states, from which elegant
transformations of the macrocycle itself or the activation of
small molecules such as N2 can ensue.1 Calixpyrroles are also
extremely efficient and sometimes selective anion binding
agents.2 Furthermore, Schiff-base polypyrrolic lanthanide com-
plexes such as motexafin lutetium are receiving considerable
attention as photodynamic therapy agents,3 and similar imino-
polypyrroles can complex actinide cations such as uranyl and
late first-row transition metals.4 Also, the larger “accordion”
porphyrins, iminopolypyrrolic macrocycles that contain two
distinct donor compartments, have been shown to functionally
model binuclear metalloenzymes.5
Following our successful syntheses of acyclic, N-donor
elaborated dipyrromethanes,6 we were interested in using the
diformyldipyrromethane synthon 1 in the preparation of
polypyrrolic macrocycles. We report herein the acid-template
synthesis of the new, compartmentalised diiminodipyrro-
methane macrocycles H4[L] and their complexation of Pd(II).
The addition of p-toluenesulfonic acid to a stirred mixture of
dialdehydes 1a or 1b and 1,2-diaminobenzene in methanol
cleanly generates the [2+2] Schiff-base condensation products
2a or 2b, respectively, in high yield as orange, microcrystalline
solids (Scheme 1).† Electrospray mass spectrometry of the
reaction mixture showed that no higher order cyclisation
Addition of triethylamine to a stirred mixture of Pd(OAc)2
and either macrocycle in CH2Cl2 results in the clean generation
of the orange dipalladium compounds [Pd2(L1)] 3a and
[Pd2(L2)] 3b, respectively. The electrospray mass spectrum of
the reaction medium of 3a showed only one signal at 872 amu
with the correct isotopic pattern for [Pd2(L1)]+. The X-ray
crystal structures of both 3a and 3b were determined and were
found to be similar (Fig. 2).‡ In 3a, Pd1 and Pd2 are both
coordinated by diiminodipyrrolide donor compartments and
adopt similar square planar geometries (S angles Pd1 = 360.03,
Pd2 = 359.85°) with standard Pd–N bond lengths. The
presence of the rigid o-aryl spacer between the donor compart-
ments has a profound effect on the overall molecular geometry
with face-to-face p-stacking of the o-aryl groups enforcing a
unique wedge-like arrangement of the two PdN4 square planes.
This is unlike bimetallic accordion porphyrins, in which flexible
Scheme 1 The synthetic route to the dipalladium complexes, 3.
Fig. 1 Structure of the macrocyclic salt 2a. Omitted for clarity are one TsO2
molecule (S3, half occupancy with MeO2) not hydrogen bonding to the
macrocycle, MeOH solvate molecules and all H atoms.
† Electronic supplementary information (ESI) available: experimental and
crystallographic details. See http://www.rsc.org/suppdata/cc/b3/b308443d/
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This journal is © The Royal Society of Chemistry 2003

stacking in the o-aryl backbone. The solid state structure of 3a
(and by inference, 3b) appears to be retained in solution. The 1H
NMR spectrum of 3a reveals the presence of two distinct methyl
groups at 1.61 and 1.50 ppm, assigned to the meso-substituents
that lie syn and anti to the cleft, respectively. A dynamic process
that would render these environments equivalent in solution is
precluded as Pd–N(imino) bond cleavage is a prerequisite in
order to progress through a symmetrical, flattened macrocyclic
conformation.
We are at present investigating the luminescent properties of
3a and 3b, and the syntheses of transition metal complexes
incorporating L. Preliminary results suggest that, as with
bimetallic cofacial diporphyrins,8 dicobalt(II) complexes of L
react spontaneously with O2; thus, reaction between Co(OAc)2
and H4[L1]/NEt3 in air forms the oxo-complex [Co2O(L1)] as
the sole product (by ESMS, 735 amu).
We thank the Royal Society (J.B.L, University Research
Fellowship), the University of Nottingham, and the EPSRC for
support.
Notes and references
† Electronic supplementary information (ESI) available: experimental and
crystallographic details. See http://www.rsc.org/suppdata/cc/b3/b308443d/
‡ Satisfactory combustion analyses were obtained for all reported com-
pounds. Crystal data: 2a: C64.5H72N8O12.5S3.5, triclinic, a = 12.9007(10),
b = 16.2779(13), c = 18.0091(14) Å, a = 75.638(1), b = 75.199(1), g =
85.938(1) °, U = 3542.0(5) Å3, T = 150(2) K, space group P1, Z = 2,
Fig. 2 Solid state structures of the dipalladium compounds 3a and 3b (50 %
ellipsoids). H-atoms are omitted for clarity.
¯
m(Mo-Ka) = 0.181 mm21, 32396 reflections measured, 16861 unique (Rint
= 0.005) which were used in all calculations. The final wR(F2) was 0.2704
(all data). 3a:
C38.25H33N8O0.25Pd2, triclinic, a = 9.4559(8), b =
alkyl chain linkers between the two donor compartments lead to
greater conformational flexibility but is more reminiscent of
single-pillared, or ‘Pac-Man’ porphyrins (Fig. 3).7,8 It is
significant that while ligands H4[L] appear highly flexible, the
dipalladium complexes 3a and 3b are highly rigid and
synthetically more available than their Pac-Man analogues.
Although the presence of the sp3-CMe2 group in the dipyrrolide
backbone precludes full conjugation of the N4-donor set, the
planarity observed and structural similarity to single-pillared
porphyrins warrants comparison. In 3a, the Pd…Pd distance of
3.76 Å is similar to that seen in the cofacial, single-pillared
diporphyrin complex Pd2(DPX) (dibenzoxanthene pillar)
(Pd…Pd = 3.97 Å).9 However, the ‘bite angle,’ q, between the
PdN4 planes of 56.5° in 3a is much greater than that observed in
Pd2(DPX), where the porphyrin rings are effectively co-planar
(q = 3.9°);§ the use of single dibenzofuran pillars results in
larger bite angles (q = 20–25°) and can accommodate metal–
metal distances from 3.5 to 7.8 Å.10 A torsional twist is evident
in 3a (F = 19°) and 3b (F = 22°) (Fig. 2). These values lie
between those of the torsionally rigid Pac-Man porphyrins (F =
2–7°) and the more flexible cofacial, amide-linked diporphyrins
(e.g. dicopper hexyldiporphyrin-7, F = 43.2°, Fig. 3).11 This
relatively large twist in 3a and 3b is presumably a consequence
of destabilising, steric interactions between the meso-groups of
the dipyrrolide unit, and the need to maximise face-to-face p-
13.2076(11), c = 14.3084(12) Å, a = 74.331(2), b = 86.576(2), g =
81.503(2) °, U = 1701.3(4) Å3, T = 150(2) K, space group P1, Z = 2,
¯
m(Mo-Ka) = 1.099 mm21, 14533 reflections measured, 7481 unique (Rint
= 0.032) which were used in all calculations. The final wR(F2) was 0.0704
(all data). 3b: C60H45N8O0.5Pd2, triclinic, a = 10.3192(14), b = 13.238(2),
c = 14.403(2) Å, a = 88.446(2), b = 77.151(2), g = 75.866(2) °, U =
2374.4(9) Å3, T = 150(2) K, space group P1, Z = 2, m(Mo-Ka) = 0.810
mm21, 20521 reflections measured, 10455 unique (Rint = 0.030) which
were used in all calculations. The final wR(F2) was 0.0964 (all data). CCDC
216097–216099. See http://www.rsc.org/suppdata/cc/b3/b308443d/ for
crystallographic data in .cif or other electronic format.
§ q = Pd1–X–Pd2 angle, where X = bisector of normal between aryl ring
centroids;F = normal 2 dihedral angle of PdN4 and aryl C6 plane.
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Fig. 3 Examples of Pac-Man, accordion and cofacial porphyrins.
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