Translocation-coupled transmetalation at the origin of a dinuclear lead porphyrin complex: Implication of a hanging-atop coordination mode

Article abstract of DOI:10.1039/c2cc00067a

Translocation of a lead cation from the N-core of a porphyrin to a hanging carboxylate group is coupled to a transmetalation process with a second lead cation, leading to a dinuclear species. A novel hanging-atop coordination mode is responsible for the dynamic and stereocontrolled binding of lead to the porphyrin core. The Royal Society of Chemistry 2012.

Full text of DOI:10.1039/c2cc00067a

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COMMUNICATION
Translocation-coupled transmetalation at the origin of a dinuclear lead
porphyrin complex: implication of a hanging-atop coordination modew
a
a
b
a
a
´
Stephane Le Gac, Btissam Najjari, Luca Fusaro, Thierry Roisnel, Vincent Dorcet,
¸
Michel Luhmer,b Eric Furet,a Jean-Francois Haleta and Bernard Boitrel*a
Received 4th January 2012, Accepted 16th February 2012
DOI: 10.1039/c2cc00067a
Translocation of a lead cation from the N-core of a porphyrin to a
hanging carboxylate group is coupled to a transmetalation process
with a second lead cation, leading to a dinuclear species. A novel
hanging-atop coordination mode is responsible for the dynamic and
stereocontrolled binding of lead to the porphyrin core.
Multimetal porphyrin entities have demonstrated indisputable
utility in the building of functional supramolecular assemblies
with cooperative or responsive properties, in the design of
materials for light-energy conversion, or in the study of
biomimetic systems.1 In this context, a fine control of metalation
processes in porphyrins appended with another metal binding
site is of primary importance for the design of novel metallo-
architectures. To our knowledge, translocation of a metal ion
from the porphyrin N-core to an appended binding site
remains unknown.2
Coordination chemistry of porphyrins towards Pb(II) cation
is largely unexplored, and structural data available so far are
limited to a few examples.3 1 : 1 complexes have been described
in the crystalline state in which the 4-coordinate Pb(II) ion is
significantly out-of-plane, 1.30–1.44 A away from the mean
plane of the macrocycle.4 Such a displacement is due to the
large ionic radius of Pb(II) (0.98 A with a coordination number
of 4) and to the stereochemically active 6s2 lone pair that
points away from the porphyrin core and acts as a phantom
ligand. We have recently described a unique coordination
mode occurring in a dinuclear centro-symmetrical complex
provided by the adjunction of overhanging carboxylate
groups.5 Each Pb(II) ion is s-linked to a nitrogen atom of
the N-core and to an oxygen atom of the COOÀ function, in a
cis geometry, and is located 1.795 A out of the porphyrin mean
plane. We were interested in structural variations of the ligand
and particularly in an intermediate situation in which a
single carboxylate group dangles over the porphyrin core.
Scheme 1 Complexation of Pb(II) by ligands 1 and 2 (DMSO-d6
solution, 15 equiv. DIPEA).
As developed hereafter, such an environment allowed a case
study of the dynamic and stereocontrolled binding of lead with
a coupled translocation–transmetalation process.
The singly strapped, mono-carboxylic acid, porphyrin 1
(Scheme 1) was obtained from the corresponding diethyl
malonate derivative 2 upon treatment with KOH in refluxing
ethanol (ESIw). Complexation properties of 1 towards Pb(II)
were studied at 25 1C by 1H NMR titration experiments in
DMSO-d6 with an excess of diisopropylethylamine (DIPEA).
The addition of 1 equiv. of PbCl2 to 1 led to the quantitative
formation of a Cs symmetrical species (Fig. 1a). No change in
the spectrum was observed with an excess of this salt, indicating
the formation of a mononuclear complex. The Soret band
in the corresponding UV-vis spectra is red-shifted by 52 nm
which is consistent with a classic out-of-plane coordination.4d
The fact that equilibria are reached instantaneously at room
temperature clearly contrasts with harsh metalation conditions
required for ‘naked’ porphyrins (e.g. refluxing pyridine).4 This
strongly suggests that the binding is assisted by the deconvolution6
of a chloride anion by the carboxylate of the strap, leading
to the side-selective formation of a sitting-atop (SAT) inter-
mediate prior to metal insertion (Scheme 1). Thus, in the
a
´
Universite de Rennes 1—ENSCR, Sciences Chimiques de Rennes,
´
´
UMR CNRS 6226, 263 avenue du General Leclerc, CS 74205, 35042
Rennes Cedex, France. E-mail: Bernard.Boitrel@univ-rennes1.fr;
Fax: (+33)2 2323 5637; Tel: (+33)2 2323 5856
b
´
Laboratoire de RMN Haute Resolution, Universite Libre de Bruxelles,
CP 160/08, 50 av. F.-D. Roosevelt, B-1050, Bruxelles, Belgium
´
w Electronic supplementary information (ESI) available: Full experi-
mental details. CCDC 845672 and 855999. For ESI and crystallo-
graphic data in CIF or other electronic format see DOI: 10.1039/
c2cc00067a
c
3724 Chem. Commun., 2012, 48, 3724–3726
This journal is The Royal Society of Chemistry 2012

Fig. 1 1H NMR spectra for the complexation of Pb(II) by ligand 1
(DMSO-d6, 500 MHz, 15 equiv. DIPEA, region of the Ha signal).
(a) Titration with PbCl2, and subsequent addition of NaOAc;
(b) titration with Pb(OAc)2.
Fig. 2 X-Ray crystal structure of 1PbÁPbOAc: ORTEP view at the
30% probability level (hydrogen atoms omitted, dashed lines indicate
H-bonds). Distances [A]: Pb1–Pb 2 3.727; Pb2(/Pb1) to 24-atom mean
plane 1.383(/2.323); Pb1–O1 2.449, Pb1–O2 2.380, Pb1–O3 2.587,
Pb1–O4 2.432, Pb1–O5 2.596, Pb2–N1 2.399, Pb2–N2 2.413,
Pb2–N3 2.405, Pb2–N4 2.381.
mononuclear complex, the lead cation is likely located in an
inside position relative to the strap, with the hanging COOÀ
group being no longer coordinated (1Pb, Scheme 1).7 This
conclusion is strengthened by the X-ray structure of the
mononuclear lead complex 2Pb obtained with the diethyl
malonate precursor of 1 (ESIw). The Pb(II) cation is out-of-
plane 4-coordinate to the N-core; however, at the opposite of
another nonrelated strapped porphyrin,4c the metal is in an
inside position relative to the strap (Scheme 1). In DMSO-d6
solution, Pb(II) was inserted into 2 upon standing overnight at
room temperature (ESIw). This, as in 1Pb, is consistent with the
participation of the hanging group in the metalation process.
Similarly, Pb(NO3)2 led to 1Pb as the major species even
with an excess of the metal salt (ESIw). Conversely, a second
complexation process was observed when Pb(OAc)2 was used
for the titration of 1. Besides the formation of 1Pb, addition of
up to 3 equiv. of this metal salt led to the quasi-quantitative
formation of a new species (Fig. 1b). The same species was
formed upon addition of a slight excess of NaOAc to a 2 : 1
mixture of PbCl2 or Pb(NO3)2 and 1 (Fig. 1a, ESIw). A ROESY
2D NMR experiment revealed that one acetate counteranion
belongs to this new complex (ESIw). The high field shift of
B1.60 ppm for the CH3 protons indicates a bound acetate
located above the porphyrin plane. The overall process with
Pb(OAc)2 is compatible with the successive formation of mono-
and dinuclear complexes and is characterized by the apparent
association constants Ka = 1.3 Â 104 MÀ1 and Ka = 9.3 Â
ligands is ascribed to the lone pair of lead which, as shown below,
points towards the N-core of the macrocycle. Both the exogenous
acetate and DMSO ligands are oriented at almost 901 relative to
the carboxylate of the strap and are H-bonded to one amide NH
group (dashed lines in Fig. 2). The Pb–Pb distance of 3.727 A
amounts to 92% of the sum of lead van der Waals radii, which
indicates no metal–metal interactions. It is noteworthy that very
similar inter-molecular Pb–Pb distances are found in mono-
nuclear lead porphyrin complexes stacked in a closely related
arrangement.4b From these structural data, the Pb1 ion appears
as ‘‘floating’’ above the porphyrin, in what we define as a
hanging-atop (HAT) coordination mode.
The formation of 1PbÁPbOAc from 1Pb is instantaneous at
room temperature. This, again, strongly contrasts with the
prolonged heating required for lead insertion into unfunctiona-
lized porphyrins. Low activation energy could be due to the
deformation of the porphyrin macrocycle induced by the out-of-
plane coordination of lead in 1Pb, similarly to what is described
in transmetalation processes with unfunctionalized porphyrins.9
A distortion-mediated mechanism related to that involved in
chelatase-catalyzed insertions of metal ions into porphyrins10
was presumably observed with a closely related ligand in the
case of Pb(II) - Bi(III) transmetalation.11 Therefore, the second
lead ion (Pb2) is expected to approach from the naked side,
opposite to the first lead atom Pb1 (transmetalation process),
the latter being picked back up by the carboxylate group of the
strap to yield 1PbÁPbOAc (translocation process, Scheme 1). In
the overall mechanism leading to 1PbÁPbOAc, transmetalation
and translocation are however coupled and do operate in a
strong cooperative way (no translocation, no transmetalation).
In other words, insertion of Pb2 and departure of Pb1 are
assisted by the carboxylate of the strap, only if a suitable
counteranion as acetate is present in the media. This unique
HAT coordination mode thus allows a lead cation to be stereo-
selectively coordinated to the N-core of the porphyrin, i.e. either
inside or outside the strap, in an apparent switching process.
The counteranion dependence of the dinuclear complex, and
the high field shift of the Pb1-bound acetate observed in the
1H NMR spectrum of 1PbÁPbOAc, are consistent with the
1
2
104 MÀ2, respectively (ESIw).8
Single crystals were successfully grown from the NMR sample
resulting from the titration of 1 by Pb(OAc)2. X-Ray diffraction
analysis confirmed the bimetallic nature of the complex, namely
1PbÁPbOAc (Scheme 1). The crystal structure is depicted in Fig. 2.
One lead cation (Pb2) is 4-coordinate to the N-core of the
porphyrin, opposite to the strapped side and 1.383 A out of the
porphyrin mean plane. The other lead cation (Pb1) is not bound
to the porphyrin N-core (average Pb1–N = 3.243 A), but is
hanged at 2.323 A above the porphyrin mean plane. Its coordi-
nation sphere involves two dihapto carboxylate ligands, an
intramolecular one from the strap and an exogenous one, and
is completed by one DMSO ligand, with lead at the apex of a
distorted pyramid. This strongly hemidirected distribution of the
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This journal is The Royal Society of Chemistry 2012
Chem. Commun., 2012, 48, 3724–3726 3725

with an acetate counterion, and is established owing to the
6s2 stereochemically active lone pair that favorably orients the
bound anion. The out-of-plane, outside-the-strap geometry,
binding of Pb2 should favour the inward orientation of the
HAT Pb(II), of which the 6s2 lone pair can ‘accommodate’ in
the dome-shaped porphyrin core. This coordination mode is
responsible for a unique translocation-coupled transmetalation
process, in which a lead cation bound to the N-core can be
formally switched from an inside to an outside position, or in
other words, can be stereoselectively incorporated in the
strapped macrocycle, in a dynamic way. The hanging carboxylic
acid approach provides a new opportunity to further extend the
frontiers of porphyrin-like coordination chemistry.
Fig. 3 (a) DFT-optimized structure of 1PbÁPbOAc (hydrogen atoms
removed) and deformation electron-density isosurface for the coordi-
nation sphere of Pb1 (contour value: +0.008 e bohrÀ3). (b) Structural
type observed for 1PbÁPbOAc (second coordination sphere omitted).
Notes and references
inward orientation of Pb1 in solution as in the crystal structure.
Density functional calculations (DFT) were performed to
probe the role of the lone pair of the lead cations in this new
HAT binding mode. Calculations were carried out at the
B88P86/LANL2DZ12,13 level of theory using Gaussian09.14
A full in vacuo geometry optimization of 1PbÁPbOAc was first
performed starting from the X-ray structure. The computed
geometry does not show significant modifications of the
complex (Fig. 3a). The lone pair of each lead cation was
indirectly localized using the deformation electron-density
map.15 In a hemidirected coordination geometry, the observed
residual electron density in the vicinity of a lead atom is
attributed to a trans deformation of the lone pair orbital
relative to the proximal ligands.16 Logically, the lone pair of
Pb1 and Pb2 points towards and away from the N-core of the
macrocycle, respectively (Fig. 3a, ESIw). Because of a quasi-
symmetrical N4 environment, the lone pair of Pb2 keeps an
axial symmetry whereas that of Pb1 appears distorted and
slightly directed towards the N2 atom, but without effect on
Pb1–N2 length. Therefore, there is evidence neither for a
specific interaction between the lone pair of Pb1 and the
N-core of the macrocycle, nor for any repulsion, and a faithful
structural type for 1PbÁPbOAc can be drawn as in Fig. 3b. The
role of the counteranion was also investigated by DFT. First,
the H-bonding interaction between the AcOÀ and the NHCO
was evaluated to be B10 kcal molÀ1. Second, the bound AcOÀ
was replaced by a nitrate and DFT calculations of the complex
1PbÁPbNO3 did not evidence significant differences with
1PbÁPbOAc, the bound nitrate being at a distance of the strap
compatible with H-bonding interaction with the NHCO
(ESIw).17 In contrast, in the optimized structure of a hypo-
thetical complex 1PbÁPbCl, the chloride anion is standing B6 A
away from the NHCO of the strap and does not exhibit any
intramolecular stabilizing interactions. Therefore, this second
sphere of coordination defined by the intramolecular H-bond
with the strap could be stronger with the AcOÀ, if not specific,
and explains why the formation of a dinuclear complex relies
on the presence of this counterion.
1 (a) I. Beletskaya, V. S. Tyurin, A. Y. Tsivadze, R. Guilard and
C. Stern, Chem. Rev., 2009, 109, 1659; (b) J. K. Sanders, Coordination
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2 References for metal ion translocation with synthetic receptors:
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Acc. Chem. Res., 2001, 34, 488; (b) L. Fabbrizzi, F. Foti,
S. Patroni, P. Pallavicini and A. Taglietti, Angew. Chem., Int.
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3 C. M. Lemon, P. J. Brothers and B. Boitrel, Dalton Trans., 2011,
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Chem., 1980, 19, 2057; (b) M. J. Plater, S. Aiken, T. Gelbrich,
M. B. Hursthouse and G. Bourhill, Polyhedron, 2001, 20, 3219;
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B. Boitrel, Dalton Trans., 2007, 3684; (d) Z. Halime, M. Lachkar,
T. Roisnel, P. Richard and B. Boitrel, Inorg. Chem., 2007, 46, 6338.
5 Z. Halime, M. Lachkar, T. Roisnel, E. Furet, J.-F. Halet and
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Metalloporphyrins, ed. K. M. Smith, Elsevier, Amsterdam, 1975, p. 157.
7 1M (1M) refers to a metal ion coordinated in an inside (outside)
position relative to the strap.
8 HRMS (ESI-TOF) measurements: at a 1 : 1 1/Pb(OAc)2 ratio (negative
ion mode), m/z calcd for C66H49N6O8Pb: 1261.3384 [1-3H+Pb]À;
found: 1261.3389. At a 1 : 3 1/Pb(OAc)2 ratio (positive ion mode),
m/z calcd for C66H49N6O8Pb2: 1469.3143 [1-3H+2Pb]+; found:
1469.3149 (AcOÀ is loss in the ionisation process).
9 (a) C. Jr. Grant and P. Hambright, J. Am. Chem. Soc., 1969,
91, 4195; (b) M. Tabata, W. Miyata and N. Nahar, Inorg. Chem.,
1995, 34, 6492.
10 P. Hambright and P. B. Chock, J. Am. Chem. Soc., 1974, 96, 3123.
11 S. Le Gac, B. Najjari, N. Motreff, P. Remaud-Le Saec, A. Faivre-
Chauvet, M.-T. Dimanche-Boitrel, A. Morgenstern, F. Bruchertseifer,
M. Lachkar and B. Boitrel, Chem. Commun., 2011, 47, 8554.
12 (a) A. D. Becke, Phys. Rev. A, 1988, 38, 3098; (b) J. P. Perdew,
Phys. Rev. B: Condens. Matter Mater. Phys., 1986, 33, 8822.
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14 M. J. Frisch, et al., Gaussian 09, revision A.02, Gaussian Inc.,
Wallingford, CT, 2009.
15 Deformation electron-density isosurfaces were obtained by subtracting
from the molecular density of 1PbÁPbOAc, the self-consistent com-
puted densities of the [Pb]2+ and [1Pb+DMSO+AcO]2À (for Pb2) or
[1+DMSO+PbOAc]2À (for Pb1) units, kept with their respective
geometries in the dinuclear complex optimized with Gaussian 09 (ESIw).
16 L. Shimoni-Livny, J. P. Glusker and C. W. Bock, Inorg. Chem.,
1998, 37, 1853.
In summary, we have described a new hanging-atop (HAT)
coordination mode in a dinuclear lead complex with an over-
hanging carboxylic acid porphyrin. The HAT lead cation does
not interact with the N-core but adopts an inward geometry
stabilized through a second sphere of coordination with the strap.
This intramolecular secondary interaction is best achieved
17 The minor species observed with 1Pb in the NMR titration with
Pb(NO3)2 could therefore correspond to a dinuclear species.
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3726 Chem. Commun., 2012, 48, 3724–3726
This journal is The Royal Society of Chemistry 2012