Induced fit process in the selective distal binding of imidazoles in zinc(II) porphyrin receptors

Article abstract of DOI:10.1021/ic0341643

The respective affinities of various imidazole derivatives, imidazole (ImH), 2-methylimidazole (2-MeImH), 2-phenylimidazole (2-PhImH), N-methylimidazole (N-Melm), 2-methylbenzimidazole (2-MeBzImH), and 4,5- dimethylbenzimidazole (4,5-Me₂BzImH), for two phenanthroline (Phen) strapped zinc(II) porphyrin receptors porphen-Zn 1-Zn and 2-Zn have been studied. The formation of a supplementary H-bond considerably enhances the affinity of the zinc(II)-porphen receptor for imidazoles unsubstituted on the pyrrolic nitrogen (ImH) versus N-substituted imidazoles such as N-Melm. The ImHs?porphen-Zn complexes are formed with association constants up to 4 orders of magnitude superior to those measured either for N-Melm as substrate or TPP-Zn as receptor. Distal or proximal binding of the substrates was determined by ¹H NMR measurements and titration. In two cases, the very high stability of the inclusion complex enabled the use of 2D NMR techniques. Excellent correlation between solution and solid-state structures has been obtained. A total of six X-ray structures are detailed in this article showing that the evolution of the shape of the zinc(II) receptor is mostly dependent on the steric constraints induced by the substitution on the imidazole. Hindered guests also progressively induce considerable mobility restrictions and severe distortions on the receptor, especially in the case of 2-MeBzImH and 2-PhImH.

Full text of DOI:10.1021/ic0341643

Inorg. Chem. 2003, 42, 3779−3787
Induced Fit Process in the Selective Distal Binding of Imidazoles in
Zinc(II) Porphyrin Receptors†
Dharam Paul,‡ Fre´de´ric Melin,‡ Caroline Hirtz,‡ Jennifer Wytko,‡ Philippe Ochsenbein,§ Michel Bonin,|
,‡
Kurt Schenk,| Patrick Maltese, and Jean Weiss*
Laboratoire d’Electrochimie, UMR 7512 au CNRS, UniVersite´ Louis Pasteur, 4 rue Blaise Pascal,
F-67070 Strasbourg, France, Sanofi-Synthe´labo, 371 rue du Prof. Blayac, F-34000 Montpellier,
France, De´partement Sciences Physiques, UniVersite´ de Lausanne, CH-1015 Lausanne-Dorigny,
Switzerland, and Laboratoire de Chimie Supramole´culaire ISIS, UniVersite´ Louis Pasteur,
4 rue Blaise Pascal, F-67070 Strasbourg, France
Received February 14, 2003
The respective affinities of various imidazole derivatives, imidazole (ImH), 2-methylimidazole (2-MeImH),
2-phenylimidazole (2-PhImH), N-methylimidazole (N-MeIm), 2-methylbenzimidazole (2-MeBzImH), and 4,5-
dimethylbenzimidazole (4,5-Me2BzImH), for two phenanthroline (Phen) strapped zinc(II) porphyrin receptors porphen−
Zn 1-Zn and 2-Zn have been studied. The formation of a supplementary H-bond considerably enhances the affinity
of the zinc(II)−porphen receptor for imidazoles unsubstituted on the pyrrolic nitrogen (ImH) versus N-substituted
imidazoles such as N-MeIm. The ImHs⊂porphen−Zn complexes are formed with association constants up to 4
orders of magnitude superior to those measured either for N-MeIm as substrate or TPP−Zn as receptor. Distal or
proximal binding of the substrates was determined by 1H NMR measurements and titration. In two cases, the very
high stability of the inclusion complex enabled the use of 2D NMR techniques. Excellent correlation between
solution and solid-state structures has been obtained. A total of six X-ray structures are detailed in this article
showing that the evolution of the shape of the zinc(II) receptor is mostly dependent on the steric constraints
induced by the substitution on the imidazole. Hindered guests also progressively induce considerable mobility
restrictions and severe distortions on the receptor, especially in the case of 2-MeBzImH and 2-PhImH.
Introduction
site for copper(II/I) to an iron porphyrin.8 The latter approach
has been widely used by the group of Collman to prepare
functional analogues of heme protein active sites.9 Due to
their design, ditopic ligands built on porphyrins exhibit a
structure/reactivity relationship similar to strapped, picket-
Ditopic ligands based on porphyrins are of great interest
for modeling naturally occurring bimetallic active sites such
as cytochrome c oxidase in which copper and hemic iron
are involved in the fixation and the four electron reduction
of molecular dioxygen.1-5 Two main approaches have been
developed that consist of either in situ forming of bridged
copper(II) and iron(III) complexes,6,7 for example starting
from copper(I) and iron(II), or covalently linking a binding
(5) For characterization of different states of the enzyme, see: (a) Rich,
P. R.; Breton, J. Biochemistry 2001, 40, 6441. Morgan, J. E.;
Verkhovsky, M. I.; Palmer, G.; Wilkstro¨m, M. Biochemistry 2001,
40, 6882. Hagen, W. R. Biochim. Biophys. Acta 1982, 708, 82. (b)
Dunham, W. R.; Sands, R. H.; Shaw, R. W.; Beinert, H. Biochim.
Biophys. Acta 1983, 748, 73. (c) Hagen, W. R.; Dunham, W. R.; Sands,
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* To whom correspondence should be addressed. E-mail: jweiss@
chimie.u-strasbg.fr.
† This article is dedicated to Dr. Christiane Dietrich-Buchecker on the
occasion of her 61st birthday.
‡ Laboratoire d’Electrochimie, Universite´ Louis Pasteur.
§ Sanofi-Synthe´labo Recherche.
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| De´partement Sciences Physiques, Universite´ de Lausanne.
Laboratoire de Chimie Supramole´culaire ISIS, Universite´ Louis Pasteur.
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Kaderli, S.; Zuberbu¨hler, A. D. J. Am. Chem. Soc. 2001, 123, 6183
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10.1021/ic0341643 CCC: $25.00 © 2003 American Chemical Society
Published on Web 05/21/2003
Inorganic Chemistry, Vol. 42, No. 12, 2003 3779

Paul et al.
fence, or “basket handle” porphyrins, namely the topographi-
cal control of axial base coordination, the presence of straps
mimicking the distal site of natural hemoproteins while the
open face of the porphyrin is considered as the proximal
site.10
In Nature, the complementarity of numerous additive weak
interactions is usually responsible for highly organized
assemblies. The role of weak distal interactions (e.g. steric,
electrostatic, H-bonding) is emphasized by the CO/O2
discrimination in hemoproteins11 and by the interest in
hindered heme for the study of magnetic properties. Indeed,
the combination of axial auxiliary binding on metallopor-
phyrins and peripheral interactions such as H-bonding or
hydrophobic π-stacking has resulted in an elegant approach
of modeling heme-dependent proteins.12 The properties of
iron porphyrins and 2-methylimidazole (2-MeImH) appear
to be highly dependent on the position of the axial ligand
which has been shown to be a determining factor for the
catalytic properties of hemes.13 A structure-property rela-
tionship study has established correlation between heme
properties and 2-MeImH alignment in the case of cytochrome
c, showing that axial base binding in heme proteins and
related topographic changes are of major importance in the
fine-tuning of heme properties.13 Recent reports in the field
of superstructured porphyrins have shown that axial ligand
selection can be achieved by proper positioning of H-binding
sites within the straps or the basket handles.14 The recognition
Figure 1. Two receptors used for the recognition of N-unsubstituted
imidazoles.
of chiral amino acids demonstrates that the information
learned from modeling heme proteins can be utilized as input
into the design of selective receptors based on zinc(II)
porphyrins.7 On the basis of these considerations, the com-
bination of strong direct interactions with the metallic core
of a porphyrin and weak interactions located around the axial
ligand(s) is more frequently utilized to control the selectivity
of the axial ligation on zinc or iron porphyrins. Several recent
papers have even outlined the importance of geometric
changes that occur in hemoproteins upon ligand binding and
its particular involvement regarding signal transduction.15
Thus, topographic control of the axial base positioning and
the geometrical changes resulting from this binding, are
relevant to many metalloporphyrins properties.
We have previously described the synthesis of a highly
rigid phenanthroline (Phen) strapped porphyrin 1 (porphen)16
and its zinc complex 1-Zn (Figure 1),17 the use of this ligand
to access copper and silver mixed-valence homobinuclear
complexes,18 and preliminary studies of axial base binding
that have led to the unexpected formation of inclusion
complexes with N-unsubstituted imidazoles.19 On the basis
of the same synthetic approach, a meso-substituted derivative
(8) For examples see: (a) Gunter, M. J.; Mander, L. N.; Murray, K. S.;
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Schwenninger, R.; Boitrel, B. Angew. Chem., Int. Ed. 1998, 37, 3397.
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Science 1997, 275, 949.
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J. P.; Herrmann, P. C.; Fu, L.; Eberspacher, T. A.; Eubanks, M.;
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Chem. Soc. 1997, 119, 3481. (c) Momenteau, M. Models of Hemo-
protein Active Sites. In Supramolecular Control of Structure and
ReactiVity; Hamilton, A., Ed.; Wiley: New York, 1996; p 155. (d)
Momenteau, M.; Reed, C. A. Chem. ReV. 1994, 94, 659.
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Te´treau, C.; Momenteau, M.; Lavalette, D. Inorg. Chem. 1990, 29,
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Trans. 2001, 1841 and reference cited. (b) Casella, L.; Monzani, E.;
Fantucci, P.; Gullotti, M.; De Gioia, L.; Strini, A.; Chillemi, F. Inorg.
Chem. 1996, 35, 439 and references therein. (c) Munro, O. Q.;
Marques, H. M. Inorg. Chem. 1996, 35, 3752. (d) Nakamura, M.;
Ikeue, T.; Neya, S.; Funasaki, N.; Nakamura, N. Inorg. Chem. 1996,
35, 3731. (e) Uno, T.; Hatano, K.; Nishimura, Y. J. Am. Chem. Soc.
1994, 116, 4107. (f) Nakamura, M.; Tajima, K.; Tada, K.; Ishizu, K.;
Nakamura, N. Inorg. Chim. Acta 1994, 224, 113. (g) Walker, F. A.;
Simonis, U. J. Am. Chem. Soc. 1991, 113, 8652. (h) Hatano, K.; Safo,
M. K.; Walker, F. A.; Scheidt, W. R. Inorg. Chem. 1991, 30, 1643.
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Rohmer, M. M.; Strich, A.; Veillard, A.; Theor. Chim. Acta 1984, 65,
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2845.
(14) For recent reviews on molecular recognition involving zinc(II)-
porphyrins, see: (a) Ogoshi, H.; Mizutani, T.; Hayashi, T.; Kuroda,
Y. In The Porphyrin Handbook; Kadish, K. M., Smith, K. M., Guilard,
R., Eds.; Academic Press: New York, 2000; Vol. 6, p 1. (b) Weiss,
J. J. Inclusion Phenom. Macrocyclic Chem. 2001, 40, 1. (c) Middel,
O.; Verboom, W.; Reinhoudt, D. N. J. Org. Chem. 2001, 66, 3998.
(15) (a) Haruta, N.; Aki, M.; Ozaki, S.; Watanabe, Y.; Kitagawa, T. I.
Biochemistry 2001, 40, 6956 and references cited. (b) Trent, J. T.,
III; Hvitved, A. N.; Hargrove, M. S. Biochemistry 2001, 40, 6155. (c)
Perry, C. B.; Chick, T.; Ntlokwana, A.; Davies, G.; Marques, H. M.
J. Chem. Soc., Dalton Trans. 2002, 449. (d) Ellison, M. K.; Schulz,
C. E. Scheidt, W. R. Inorg. Chem. 2002, 41, 2173.
(16) Wytko, J. A.; Graf, E.; Weiss, J. J. Org. Chem. 1992, 57, 1015.
(17) Ochsenbein, P.; Bonin, M.; Schenk, K.; Froidevaux, J.; Wytko, J.;
Graf, E.; Weiss, J. Eur. J. Inorg. Chem. 1999, 2, 1175.
(18) Giraudeau, A.; Gisselbrecht, J. P.; Gross, M.; Weiss, J. J. Chem. Soc.,
Chem. Commun. 1993, 1103.
(12) Kuroda, Y.; Kato, Y.; Higashioji, T.; Hasegawa, J.; Kawanami, S.;
Takahashi, M.; Shiraishi, N.; Tanabe, K.; Ogoshi, H. J. Am. Chem.
Soc. 1995, 117, 10950.
3780 Inorganic Chemistry, Vol. 42, No. 12, 2003

Imidazoles in Zinc(II) Porphyrin Receptors
Scheme 1a
a Key: (i) NBS (2 equiv), pyridine, 0 °C, CHCl3, 30 min, 69%; (ii) Zn(AcO)2, THF, 30 min, 99%; (iii) 9% mol Pd(PPh3)4 (vs R-B(OH)2), 2 M Na2CO3(aq)
/MeOH/toluene (1:1:10), 80%.
Table 1. log Ka of Axial Ligands with TPP-Zn, 1-Zn, and
2-Zn (Figure 1) has been prepared, bearing 3,5-dimethylphen-
yl or 5-m-xylyl substituents in two meso positions. This paper
2-Zn in CH2Cl2
pKaa
(substrate) log Kassn Soretb Q bandsb Phenb
describes how, for these metalloporphyrins, the presence of
the phenanthroline strap allows for both the selective distal
or proximal coordination of axial imidazole derivatives and
for the enhanced binding of N-unsubstituted imidazole
(ImH). The topographic adjustments occurring in these
receptors to accommodate various substrates will be detailed.
Binding studies have been performed on receptors 1-Zn and
2-Zn with imidazole (ImH), 2-methylimidazole (2-MeImH),
2-phenylimidazole (2-PhImH), 2-methylbenzimidazole (2-
MeBzImH), 4,5-dimethylbenzimidazole (4,5-Me2BzImH),
and N-methylimidazole (N-MeIm) using 1H NMR and UV-
visible titrations. Several zinc-porphyrin imidazole inclusion
complexes have been characterized by single-crystal X-ray
diffraction, and the changes of shape observed in the
receptors due to substrate binding will be discussed. A highly
distorted porphyrin ring is obtained upon coordination of
2-MeBzImH in 2-Zn, providing a remarkable example of a
distorted â-unsubstituted porphyrin.
receptor
substrate
TPP-Zn ImH
TPP-Zn N-MeIm
TPP-Zn 2-MeImH
6.65
7.33
7.56
6.65
7.33
7.56
7.50
6.65
7.33
7.56
7.50
6.4
4.8 ( 0.2 428 562, 604
5.3 ( 0.2 428 564, 604
5.4 ( 0.2 428 564, 604
1-Zn
1-Zn
1-Zn
1-Zn
2-Zn
2-Zn
2-Zn
2-Zn
2-Zn
ImH
6.1 ( 0.2 428 560, 598 284, 310
4.7 ( 0.1 424 556, 592 286, 308
7.3 ( 0.3 428 560, 596 284, 308
N-MeIm
2-MeImH
2-PhImH
ImH
N-MeIm
2-MeImH
2-PhImH
2-MeBzImH
6.4 ( 0.3 422
5.9 ( 0.3 440 575, 616 283, 311
3.3 ( 0.2 437 571, 611 285
552 284, 310
6.6 ( 0.3 440 575, 615 283, 309
4.4 ( 0.3 441 576, 617 283, 309
5.7 ( 0.2 441 573, 614 283, 309
a pKa value obtained from the literature.24 λmax in nm. Standard
concentrations used: [substrate] ) 2 × 10-2 M and [1-Zn] ) [2-Zn] ) 3
× 10-6 and 4 × 10-5 M for determination of log Ka from Soret bands and
Q-bands, respectively.
b
As preliminary studies for the paramagnetic iron(II)
complexes, the affinities of the two diamagnetic zinc(II)
receptors 1-Zn and 2-Zn for a variety of imidazoles have
been studied to estimate their ability to form pentacoordinated
complexes of a divalent metal. The results obtained have
focused our attention on the unique binding properties of
this series of phenanthroline-strapped porphyrins.
Results and Discussion
Derivatization of Porphen 1. An advantage of the
synthesis leading to porphyrin 1 is the large scale (up to gram
quantities) on which this strapped porphyrin can be pre-
pared.16 Thus, this phenanthroline-strapped porphyrin may
be considered as a starting material for more elaborate
architectures. In this study, substitution of the remaining free
meso positions has been achieved in two steps, beginning
with NBS bromination of the unsubstituted meso positions
in chloroform. Treatment of 1 with excess NBS afforded
the dibromide 3 in 69% yield. As indicated in Scheme 1,
metalation with zinc acetate in THF afforded a protection
of the porphyrin core and enabled a clean coupling reaction
involving the dibromide 3-Zn and the m-xylylboronic acid20
in classical Suzuki conditions to give the receptor 2-Zn in
80% yield (see Supporting Information for details). Attempts
to prepare the free base derivative 2-H2 from the dialdehyde
precursor previously involved in the synthesis of 1 and a
m-xylyldipyrrylmethane have resulted in poor isolated yields
of porphyrin.
UV-Visible Titrations and Stability Constants. In this
study, Job plots obtained from UV-visible titrations of
receptors 1-Zn or 2-Zn with various imidazoles have
confirmed a 1:1 stoichiometry in each case, and thus, for all
titrations performed,21 data have been analyzed considering
a pentacoordinated zinc(II) atom. Except for a few examples,
in which in the solid state the zinc(II) has been shown to be
hexacoordinated,22 axial ligation of N-donor ligands usually
affords pentacoordinated zinc(II) ions.23 The results of UV-
visible titrations in CH2Cl2 are collected in Table 1.
For comparison, association constants of ImH, N-MeIm,
and 2-MeImH with (meso-tetraphenylporphyrinato)zinc(II),
TPP-Zn, used as a reference, have been determined or
collected from literature (see Table 1).
Spectral changes observed during the coordination of axial
auxiliary ligands on zinc(II) porphyrins have been exhaus-
(21) For the titration method and treatment of the data, see: (a) Kirskey,
C. H.; Hambright, P. Storm, C. B. Inorg. Chem. 1969, 10, 2141. (b)
Anzai, K.; Hosokawa, T.; Hatano, K. Chem. Pharm. Bull. 1986, 34,
1865.
(19) Froidevaux, J.; Ochsenbein, P.; Bonin, M.; Schenk, K.; Maltese, P.;
Gisselbrecht, J.-P.; Weiss, J. J. Am. Chem. Soc. 1997, 119, 12362.
(20) Schmid, M.; Eberhardt, R.; Klinga, M.; Leskela, M.; Rieger, B.
Organometallics 2001, 20, 2321.
(22) For examples of hexacoordination, see: Scheidt, W. R.; Eigenbrodt,
C. W.; Ogiso, M.; Hatano, K. Bull. Chem. Soc. Jpn. 1987, 60, 3529.
(23) For a compilation of Zn-N distances, see: Cheng, B.; Scheidt, W.
R. Inorg. Chim. Acta 1995, 237, 5.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3781

Paul et al.
1
Figure 2. 2D H NMR NOESY of the inclusion complex 2-MeImH⊂1-Zn, 300 MHz. CHDCl2 (5.30 ppm) and trace of hexane (triplet at 0.90 ppm and
broad signal at 1.3 ppm) from crystallization are present, together with residual water (singlet at 1.5 ppm).
tively analyzed in the literature, and the bathochromic shifts
of the Soret and Q-bands observed in the pentacoordinated
complexes of 1-Zn and 2-Zn are consistent with previously
described results.23
series, N-MeIm , ImH , 2-MeImH, as well as a drastic
enhancement of the stability constants exhibited with protic
imidazoles. The reason for this enhancement obviously
involves the phenanthroline moiety, as intensity changes in
the phenanthroline absorption bands (284 and 310 nm) were
observed during all titrations with ImHs. Evidence for
H-bonding of the ImHs to the Phen nitrogens has been
Titration experiments have shown that from 0 to 1 equiv
of substrate, in the case of N-unsubstituted imidazoles, not
only the Soret and the Q-bands are affected by the coordina-
tion of the imidazole but also the Phen bands in the UV
region of the spectrum. Similar experiments performed either
with TPP-Zn as receptor or with N-MeIm and pyridine as
substrates show no changes in the visible region of the
spectra, neither from 0 to 1 equiv nor in the presence of
slight excesses of the substrates. When huge excesses
(averaging 80 equiv) of N-MeIm or pyridine are added, the
typical red shifts of both the Soret band and the Q-band are
observed.
Prior to further investigation of this phenomenon, the
stability constants for each substrate with 1-Zn or 2-Zn and
for a reference receptor TPP-Zn were calculated on the basis
of titration experiments (Table 1). As a consequence of
increasing pKa values, the stability constants usually observed
with zinc(II) porphyrins follow the order ImH < N-MeIm
< 2-MeImH. For our receptors 1-Zn and 2-Zn, the higher
association constants for ImHs in comparison with that
obtained for N-MeIm clearly indicate that additional interac-
tions must occur during their binding. As shown in Table 1,
the presence of a hindered strap typically reduces the
statistical approach of the axial base to only one side of the
porphyrin. In the case of 1-Zn and 2-Zn, the presence of the
Phen strap introduces a discrepancy in the binding strength
1
obtained from H NMR data (see Supporting Information)
and confirmed by both 2D 1H NMR and single-crystal X-ray
diffraction.
1
2D H NMR Studies. 2D NMR experiments could be
performed due to the high stability of 2-MeImH complexes
with the two receptors 1-Zn (ROESY) and 2-Zn (NOESY).
These experiments allowed unambiguous peak assignments
and correlate exceptionally well with the corresponding solid-
state structures (vide infra). Fully assigned ROESY plots for
2-MeImH⊂1-Zn are provided as Supporting Information, as
they confirm previous assignments obtained by selective
irradiations in NOEMULT.19 For 2-MeImH⊂2-Zn, Figure
2 depicts highly significant correlations observed in the
region comprised between the HImMe resonance at δ ) -2.15
ppm and the HImN-H signal at δ ) 13.05 ppm. The large
chemical shift range covered by this plot has primarily
allowed the assignment of correlations among imidazole
HIm5′, HIm4′, and HImMe protons and neighboring protons on
2-Zn (expanded NOESY from 2 to 9 ppm is available as
Supporting Information). In particular, correlation of HIm4′
with Hm, Ho, Hh, and Hb confirms, first of all, the location
of the imidazole plane between the spacers and the proximity
of its CdC bond with one of the xylyl groups and, second,
3782 Inorganic Chemistry, Vol. 42, No. 12, 2003

Imidazoles in Zinc(II) Porphyrin Receptors
Table 2. Crystallographic Data for Inclusion Complexes 2-PhImH⊂1-Zn, 2-MeImH⊂2-Zn, 2-MeBzImH⊂2-Zn, and 2-PhImH⊂2-Zn
param
2-PhImH⊂1-Zn
2-MeImH⊂2-Zn
2-MeBzImH⊂2-Zn
2-PhImH⊂2-Zn
empirical formula
fw
C65H40N8Zn
998.42
C100H85N8Zn
1464.13
C81.4H59.1Cl2N8 Zn
1286.16
C84H62Cl6N8Zn
1461.49
crystal system
orthorhombic
monoclinic
monoclinic
monoclinic
lattice params
a (Å)
b (Å)
c (Å)
21.453(4)
11.001(2)
20.066(4)
25.060(5)
13.692(5)
23.533(5)
101.551(5)
7911(4)
P21/c
14.859(3)
18.671(4)
24.067(5)
93.10(3)
6667(2)
P21/n
13.441(3)
22.398(5)
24.133(5)
100.80(3)
7137(2)
P21/c
â (deg)
V (Å3)
4735.3(16)
Pnam
4
1.400
5.73
-30
56.22
5879
space group
Z (no. of molecules/unit cell)
4
4
4
D(calcd) (g cm-3
)
)
1.229
3.65
-113
48.06
8656
1054
1.281
5.01
-113
48
6935
885
1.360
6.22
-93
51.94
9228
995
µ(Mo KR) (cm-1
data collcn temp (°C)
2θmax (deg)
no. of observns used (I > 2σ(I))
no. of variables
373
max shift/error on last LS cycle
goodness of fit (GoF)a
resids:a R1; wR2
0.051
0.924
0.0492; 0.0943
none
0.002
1.136
0.0554; 0.1553
none
0.664
0.084
0.982
0.0451; 0.1232
none
0.348
-0.622
0.920
0.0549; 0.1376
none
0.792
abs corr
largest peak in last diff map (e Å-3
)
0.259
2
2
2
2
2
a GoF ) {∑[w(Fo - Fc )2]/(Nref - Nvar)}1/2; R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) {∑[w(Fo - Fc )2]/∑[w(Fo )]2}1/2
.
Table 3. Distances between Imidazole N-H (H82) and the Phen
allows an unambiguous assignment of the Hb resonance.
Located further above the porphyrin ring, HIm5′ is in spatial
contact only with Ho and not with Hm. The correlation of
HImMe with Ho and Hm confirms that the phenyl spacers are
freely rotating but that the imidazole is fixed in the cavity,
Nitrogen Atoms (N42, N45)a
complex
H82-N42
H82-N45
2-PhImH⊂2-Zn
2-MeImH⊂2-Zn
2-MeBzImH⊂2-Zn
2-PhImH⊂1-Zn
2.034
2.075
1.868
2.185
2.260
2.426
2.606
2.185
HImMe correlating with Hk and Hb′/a, as well as with HMe3
.
All correlation assignments have been made using the HMeIm
/
a Distances in Å.
Hk, H4′Im/Hb, and H4′Im/Hh correlations as a starting point.
Despite the fact that 2-MeBzImH is tightly bound in the
cavity of 2-Zn, it was not possible to perform 2D NMR
experiments due to the extreme broadening of the spectrum.
In the case of 2-PhImH, complexation within the cavity is
clearly indicated by the chemical shifts displacements of Ho
and Hm, but a weaker complexation is observed in a slowly
exchanged inclusion complex.
density. The distances measured between H82 and the Phen
nitrogen atoms N42 and N45 vary from 1.87 to 2.61 Å (Table
3), confirming the presence of a strong bifurcated hydrogen
bond.
A ruffling of the tetrapyrrolic macrocycle is generally
observed upon coordination of the guest to the central zinc
atom, and all the structures present a more or less pronounced
tilt of the Phen strap over the porphyrin ring. This tilt (65°)
was previously observed in the free base crystallographic
structure and was assigned to packing interactions. For ImHs
inclusion complexes, several factors contribute to reduce or
enhance the degree of tilting.
When compared with structures of ImH⊂1-Zn and 2-Me-
ImH⊂1-Zn, the structure of 2-PhImH⊂1-Zn (Figure 3)
shows additional distortion of the porphyrin macrocycle to
accommodate the aromatic substituent of the imidazole guest.
The Phen plane is brought closer to the normal to the
porphyrin plane, reducing the tilt of the strap above the
porphyrin plane (dihedral angle 78.1°) and forcing the Zn-
NIm bond to deviate 8.9° from the normal to the porphyrin
plane.
These small changes induced by steric constraints also
allow additional stabilizing interactions to be developed
between the phenyl ring of the imidazole and the porphyrin
core. The mean distance of 3.55(3) Å between the nearly
parallel planes (2.2° dihedral angle) of the porphyrin nucleus
and the phenyl substituent of the substrate is perfectly
adequate for efficient π-π interactions. In addition, an offset,
positioning the meso carbon of the porphyrin on the normal
X-ray Diffraction Studies. Six crystallographic structures
of inclusion complexes with receptors 1-Zn and 2-Zn have
been obtained. Although structures of ImH⊂1-Zn, and
2-MeImH⊂1-Zn have already been briefly described,17
structural features concerning these complexes will be used
for comparison throughout this discussion. Single crystals
were obtained by slow diffusion of hexane in 1:1 mixtures
of receptors and substrates in CH2Cl2, except for 2-PhImH
and 2-Zn for which a 10-fold excess of the substrate was
used (Table 2). All crystals were air stable except for
complexes of 2-MeBzImH⊂2-Zn and 2-PhImH⊂2-Zn, which
required handling in mineral oil in an open glass capillary.
It should be noted that in the case of 2-MeBzImH⊂2-Zn,
2-PhImH⊂1-Zn, and 2-PhImH⊂2-Zn, solution studies did
not show full binding of the substrate for equimolar mixtures
of substrate and receptor. Despite many attempts, crystal-
lization of the complexes resulting from proximal coordina-
tion with N-alkylimidazoles has never led to single crystals.17
As a general feature, all the coordination complexes
display distal inclusion of the imidazole guests. In each
structure, the pyrrolic proton of the imidazole guest (H82)
has been located by the presence of residual electronic
Inorganic Chemistry, Vol. 42, No. 12, 2003 3783

Paul et al.
Figure 3. Front (left) and top view (right) of 2-PhImH⊂1-Zn displaying relevant distances discussed in the text. Ct is an artificially generated centroid on
the phenyl spacer.
Figure 4. Front view (left) and side view (right) of the 2-MeImH⊂2-Zn inclusion complex. Significant distances and angles discussed in text are represented.
Table 4. Projection of Zn-NIm and NPhen-NHIm Distances, along the NPhen-Zn Axis, in the Solid-State Structures in 1-Zn and 2-Zn Inclusion
Complexes with Various Imidazole Substrates and Evaluation of the Space Left for Imidazole Bindinga
no.
Param
ImH⊂1-Znb 2-MeImH⊂1-Znb 2-PhImH⊂1-Znc 2-MeImH⊂2-Znc 2-MeBzImH⊂2-Znc 2-PhImH⊂2-Znc
1
2
3
4
5
Zn-NIm
Im-NPhen(av)
tot. 1 + 2
Zn-NPhen(av)
1.9
2.8
4.7
6.4
1.7
1.9
2.6
4.5
6.7
2.2
1.9
2.6
4.5
6.8
2.3
1.9
2.7
4.6
6.6
2.0
1.9
2.7
4.6
6.8
2.2
1.9
2.7
4.6
6.7
2.1
N
approximated space left (4 - 3)
a N---N distance in free imidazole ca. 2.2 Å. b Reference 19. c This work.
to the phenyl plane of the 2-PhImH guest, is observed as
shown in Figure 3 (right). Two aromatic H atoms of this
phenyl substituent also develop stabilizing interactions with
the phenyl spacers that separate the phenanthroline and the
porphyrin; the two phenyl protons H2 and H6 on the substrate
point toward the center of the π cloud of each phenyl spacer
of 1-Zn. The distance between the centroid of the phenyl
spacer and the phenyl C2 atom of the 2-PhImH substrate
bearing the hydrogen is quite short (3.62(2) Å) suggesting
that strong CH---π interactions are established, at least in
the solid state.26
distance between the pyridinic nitrogen of the imidazole and
the zinc cation is comparable with literature data,21 as is the
distance of 2.89(2) Å between a Schiff base nitrogen atom
of the phenanthroline and the imidazole pyrrolic N.25
The crystal structure of 2-MeImH⊂2-Zn (Figure 4) when
1
compared to 2D H NMR data is extremely representative
of its solution structure. Additional information is provided
by the distances of 3.07(2) and 2.89(2) Å between the
pyrrolic nitrogen of the imidazole and the phenanthroline
nitrogens. These distances are consistent with a strong,
bifurcated hydrogen bond. Significant NPhen-NHimidazole
distances relevant to the binding strength are collected in
Table 4. The 2.07(2) Å distance between the zinc(II) atom
and the pyridinic nitrogen of the imidazole shows that the
strength of the coordination bond is unaffected by substitution
either of the substrate or of the porphyrin periphery. Contrary
to rough predictions made from CPK models, our results
clearly establish that the xylyl substituents are not bulky
enough to inhibit the “distal” binding of the imidazole.
Despite the 8.9° angle of the zinc-nitrogen bond with the
normal to the mean plane of the porphyrin, the 2.07(3) Å
(24) Kadish, K. M.; Shive, L. R.; Bottomley, L. A. Inorg. Chem. 1981,
20, 1274.
(25) Steiner, T.; Desiraju, G. R. J. Chem. Soc., Chem. Commun. 1998, 891.
(26) For discussion, see: Desiraju, G. R. Acc. Chem. Res. 2002, 35, 565
and references cited. For a database survey, see: Umezawa, Y.;
Tsuboyama, S.; Takahashi, H.; Uzawa, J.; Nishio, M. Tetrahedron
1999, 55, 10047. For recent examples, see: Yamakawa, M.; Yamada,
I.; Noyori, R. Angew. Chem., Int. Ed. Engl. 2001, 40, 2818.
3784 Inorganic Chemistry, Vol. 42, No. 12, 2003

Imidazoles in Zinc(II) Porphyrin Receptors
Figure 5. Front view (left) and side view (right) of the distorted 2-MeBzImH⊂2-Zn inclusion complex. A side view shows the puckered porphyrin ring
and the deviated zinc-N bond.
Even more surprising has been the success in crystallizing
the 2-MeBzImH complex (vide infra), which was unexpected
considering the lower association constant (Table 1). In the
case of 2-methyl-substituted imidazoles, CH-π interactions
may further contribute to the stabilization of the inclusion
complexes. In the structure of 2-MeImH⊂2-Zn, the hydrogen
atoms (H84a-c) of the methyl substituent (2-MeIm) have
been located by X-ray diffraction. Two of these hydrogen
atoms (H84a,b) are pointing toward the porphyrin ring while
the third (H84c) is pointing upward in the direction of the
Phen nucleus. H84a,b are respectively located at 3.00 and
2.83 Å from the aromatic porphyrinic nitrogen atoms N24
Figure 6. Side view of the distorted 2-PhImH⊂2-Zn inclusion complex.
and N21 and 3.26 and 2.92 Å from the center of the pyrrolic
rings containing N24 and N21. H84c interacts with the
the changes is greater in the case of 2-MeBzImH because
aromatic phen nitrogen atoms N42 and N45 at distances of
of the extended aromatic character of the guest. In fact, all
2.86 and 2.93 Å. Both distances are consistent with rather
distortions tend to maximize stabilizing π-π and CH-π
strong CH-π interactions according to literature data.26
interactions without weakening the driving forces of the distal
binding i.e. the Zn-N coordination bond and the bifurcated
H-bond. These two interactions remain almost unchanged
in their bond lengths throughout the series of 2-Zn imidazole
The molecular structure of 2-MeBzImH⊂2-Zn shown in
Figure 5 provides evidence for the tremendous flexibility of
the receptor, as considerable geometric changes are observed
in the framework.
Two types of distortions arise from inclusion of the
complexes (Table 4). Assuming that solid-state and solution
structures correlate quite well in this series, it should be
benzimidazole substrate, one affecting the strap through
pointed out that the coordination of ImH or 2-MeBzImH
rotations around the four single bonds connecting the
induces a very similar red shift of 2-Zn Soret and Q-bands.
tetrapyrrolic core with the Phen nucleus via the meso phenyl
This suggests that distortion does not have a strong effect
substituents and the other concerning the porphyrin ring itself.
on the absorption maxima in this series of inclusion
The phenyl spacer on the left-hand side of the strap (Figure
complexes, despite the various types and various amplitudes
5) is rotated to allow CH-N interactions in the solid state,
between the Ho proton and the imidazole pyrrolic nitrogen
(d ) 3.19(2) Å), and CH-π interactions with the imidazole
N3dC2 double bond (3.29(2) Å). The other phenyl spacer,
of distortions observed.27
Finally, in the case of 2-PhImH⊂2-Zn, the distortions
observed in both the substrate and the receptor are introduced
in the structure to preserve interactions already observed in
on the right-hand side, is rotated so that it weakly stacks
the absence of the xylyl substituents, for 2-PhImH⊂1-Zn
with the substrate, which is also slightly tilted in the cavity
defined by the strap. The bifurcated H-bond is again
unsymmetrical (3.16(2) and 2.82(2) Å) but still strong, and
the plane of the porphyrin is puckered along the anchoring
(Figure 6). The dihedral angle of 19.2° between the mean
plane of the porphyrin and the phenyl substituent on the
imidazole is larger than in the case of receptor 1-Zn, but
stacking of the these two aromatic moieties is still favored.
point of the strap to accommodate the steric hindrance of
the benzimidazole. The Zn-N bond deviation from the
(27) There is currently a controversy concerning the origins of red-shifted
normal to the plane defined by the four porphyrin nitrogen
atoms is small and the axial coordination still strong with a
bond length of 2.08(2) Å. Although the distortions affecting
the strap are observed for other substrates, the amplitude of
optical spectra in nonplanar porphyrins; see for example: (a) Parusel,
A. B. J.; Wondimagegn, T.; Ghosh, A. J. Am. Chem. Soc. 2000, 122,
6371. (b) Wertsching, A. K.; Koch, A. S.; DiMagno, S. G. J. Am.
Chem. Soc. 2001, 123, 3932. (c) Ryeng, H.; Ghosh, A. J. Am. Chem.
Soc. 2002, 124, 8099.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3785

Paul et al.
Figure 8. Longitudinal and lateral distortions of the Phen pocket.
bulkier substrate 2-MeBzImH, in the case of ImH no
explanation other than a poor fit was valid, considering the
very small geometrical differences observed in comparison
to the structure of the free receptor 1-Zn.17
Additional information is provided by the lateral distortions
observed in the receptor for which significant distances
identified in Figure 8 are listed in Table 5. When compared
to the free base 1 that displays distances A and B of 7.01(3)
and 9.70(2) Å, respectively,17 imidazole complexation pro-
vokes a widening of the cavity defined by the spacers, as A
is nearly constant (6.94 Å) and B is clearly increased to 10.47
Å in the structure of ImH⊂1-Zn.
Figure 7. Rough estimation of the adequacy of the substrate/receptor.
An additional CH-π interaction is observed with a distance
of 3.92(2) Å displayed in Figure 6, between the centroid
generated on the 2-phenyl and the xylyl aromatic proton in
the ortho position on the distal side. This additional CH-π
interaction can explain the twist, and it emphasizes how the
system generates a stabilizing interaction to compensate for
the energy cost due to the larger dihedral angle between the
stacked porphyrinic and phenyl rings. Stronger CH-π
interactions also contribute to the global stability of the
inclusion complex with distances of 3.73(2) and 3.38(2) Å
between the ortho CH of the phenyl substituent and a
centroid generated on each spacer of the Phen strap.
It should be noted that the shortest B distance (9.26 Å) is
observed for the inclusion of 2-MeBzImH in 2-Zn, thus
allowing maximum overlap between the aromatic walls of
the cavity and the substrate. The variations in the width of
the cavity seem to be directly correlated to the electron-rich
character of the substrate, most probably to maximize the
lipophilic interactions between the substrate and the receptor.
The more electron rich the imidazole is, the smaller the
width of the Phen pocket. For bulkier substrates, this lateral
shrinking permits a deeper penetration of the guest in the
host and prevents a too drastic elongation of the H-bonds,
thus compensating for the energy lost in the distortion of
the porphyrin plane, as is the case for 2-MeBzImH⊂2-Zn.
The receptor adjusts to allow guest insertion within the strap
with changes that cost only a minimum of energy to the
system. Ruffling of the porphyrin macrocycle allows the
imidazole nucleus to enter deeper within the phenanthroline
pocket, to preserve the overlap between the porphyrin
electronic density and the phenyl substituent of the substrate,
thus maintaining stabilizing π-π interactions.
A global estimation of the adjustments made by the
receptor to accommodate the various substrates may be
performed using the distances measured along the longitu-
dinal axis of the cavity. Distances collected in Table 4 are
identified in Figure 7 and show that a simple estimation of
the space remaining for the substrate in each receptor can
be made by subtracting projections of both the Zn-NIm
coordination bond and the averaged NHIm-NPhen distance
(corresponding to the H-bond) along the axis connecting the
zinc atom and the Phen nitrogen (Zn-NPhen). From this
estimation, it is reasonable to compare the space left for the
substrate and the theoretical space needed for a perfect fit
of the substrate in the receptor which is roughly 2.2 ( 0.1
Å, depending on the substitution of the imidazole ring.
From Table 4 data, the adequacy of the space available
for ImHs insertion and the N---N distance in free imidazoles
favors the formation of inclusion complexes in most cases.
As shown in all side views of complexes, the tilt of the
phenanthroline strap over the porphyrin plane, already
observed in the structure of the free base,17 decreases for a
bulkier substitution in the 2 position of the imidazole. When
bulk is introduced on the opposite side of the imidazole
(positions 4 and 5), the tilt of the phenanthroline strap is not
required for the insertion of ImH in the pocket. Instead, the
distortion affects the porphyrin ring as demonstrated by the
structure of 2-MeBzImH⊂2-Zn in Figure 5.
Conclusion and Perspectives
The phenanthroline-strapped receptors described in this
paper display “induced fit” related distortions to accom-
modate bulky substrates on the hindered face of a zinc
porphyrin. The binding site is selected on the basis of a strong
metal to ligand coordination interaction that is finely tuned
by establishing a bifurcated hydrogen bond, with a final
adjustment achieved via weak interactions. Four basic
distortions are observed at different levels: the porphyrin
ring distortion; the longitudinal cavity distortion; the strap
tilt above the plane of the porphyrin; the deviation of the
Zn-Nimidazole bond versus the normal to the porphyrin mean
plane. The porphyrin ring distortion is related to the steric
hindrance in the 4 and 5 positions of the imidazole. The tilt
It should be noted that the poorest fit is observed for the
unsubstituted imidazole ImH and that, for the corresponding
inclusion complex ImH⊂1-Zn, the largest tilt of the Phen
strap over the porphyrin ring is observed.19 Of all the
substrates, ImH and 2-MeBzImH are the only substrates for
1
which fast exchange is observed in solution by H NMR.
Although this exchange is easily understandable for the
3786 Inorganic Chemistry, Vol. 42, No. 12, 2003

Imidazoles in Zinc(II) Porphyrin Receptors
Table 5. Lateral and Longitudinal Distortions in the Inclusion Complexes in the Solid State
param
ImH⊂1-Zna
2-MeImH⊂1-Zna
2-PhImH⊂1-Znb
2-MeImH⊂2-Znb
2-MeBzImH⊂2-Znb
2-PhImH⊂2-Znb
dist A (Å)
dist B (Å)
av C1, C2 (Å)
6.94
10.47
6.37
6.75
9.36
6.74
6.86
9.43
6.76
6.88
9.52
6.57
6.81
9.26
6.75
6.83
9.36
6.74
a Reference 19. b This work. A: distance between opposite meso carbons (C5 and C15) bearing the strap. B: distance between the extremities of the
phenyl spacer. C1 and C2: distances between Zn and the Phen nitrogen atoms.
of the strap is allowed when no substituent is present in the
2 position of the imidazole. The deviation of the Zn-Nimidazole
from the normal to the porphyrin plane and the longitudinal
cavity distortion are observed, if one or both of the previous
adjustments are precluded or insufficient. These studies
demonstrate how small geometric adjustments may be
performed by initially rigid structures in order to produce
energetically optimized supramolecular complexes. This very
strong imidazole binding implies that if the coordination site
of these Phen-strapped porphyrins is to be reserved exclu-
sively for exogenic ligands, the use of N-alkylimidazoles as
axial bases is a sin equa non condition. Detailed studies
concerning the thermodynamics and kinetics of axial base
ligation in these receptors need to be performed to assess,
in particular, entropic and enthalpic contributions to this
highly selective binding. Zinc(II) and iron(III/II) complexes
are currently being studied by electrochemistry to determine
how controlled small geometric variations influence the redox
properties of heme and analogues. These results demonstrate
that the synthetic availability of phenanthroline-strapped
porphyrin 1 may be considered as a versatile starting material
to easily access more elaborate superstructured porphyrins.
Further functionalization of the porphyrinic core with “built-
in” proximal ligands for iron(III/II) is currently in progress,
as well as the construction of self-assembled photodyads
through imidazole binding.28
Acknowledgment. The CNRS is gratefully acknowledged
for financial support (Special Grant PCV 98-081), and D.P.
thanks the French Government for an EGIDE postdoctoral
fellowship.
Supporting Information Available: Experimental details for
the synthesis of 2-Zn, UV-vis titration of 1-Zn with ImH, 1H NMR
titrations for 1-Zn with 2-PhImH and 2-MeImH and for 2-Zn with
2-PhImH and 2-MeBzImH, ROESY of 2-MeImH⊂1-Zn, NOESY
of 2-MeImH⊂2-Zn, and ORTEP plots and atom numbering for
2-MeImH⊂2-Zn, 2-PhImH⊂1-Zn, 2-PhImH⊂2-Zn, and 2-MeB-
zImH⊂2-Zn. This material is available free of charge via the
Internet at http://pubs.acs.org.
IC0341643
(28) Wytko, J. A.; Paul, D.; Koepf, M.; Weiss, J. Inorg. Chem. 2002, 41,
3699.
Inorganic Chemistry, Vol. 42, No. 12, 2003 3787