Regioselective reactivity of an asymmetric tetravalent Di[dihydroxotin(IY)] bis-porphyrin host driven by hydrogen-bond templation

Article abstract of DOI:10.1002/chem.200801775

Local molecular environment effects on the rates of ligand exchange at an asymmetric di[dihydroxotin(IV)] bis-porphyrin 5 are examined. The host 5 possesses four non-equivalent tin(IV)-ligand binding sites that are distinguished by their position relative to a shallow cavity, by the steric environment at each binding site and by electronic-structure differences between the constituent porphyrin and quinoxalinoporphyrin macrocycles. These design features of the asymmetric host are confirmed by X-ray crystal structure analysis. Binding experiments with monodentate carboxylic acids and bidentate dicarboxylic acids show significant differences in the rate of ligand exchange at each of the four tin(IV) binding sites. For monodentate carboxylic acids, binding preferentially occurs at the exterior porphyrin site. Further addition of carboxylic acid results in sequential binding at the quinoxalinoporphyrin sites and lastly at the interior site on the porphyrin, with high regioselectivity. These selective binding outcomes are immediately apparent by NMR spectroscopy. A series of 2D NMR spectroscopy experiments allowed identification of the preferred binding sites at the host. This positively identifies that steric hindrance and electron-withdrawing functionality on the porphyrin macrocycle impede ligand exchange. However, these effects are overcome by dicarboxylic acid guests, which form ditopic hydrogen-bond interactions between the intracavity hydroxo ligands in the initial stage of ligand exchange, leading to regioselective binding between the tin(IV) sites within the cavity. It is envisaged that the factors identified herein that define regioselective ligand exchange at host 5 will find wider application in supramolecular systems incorporating tin(IV) porphyrins.

Full text of DOI:10.1002/chem.200801775

FULL PAPER
DOI: 10.1002/chem.200801775
Regioselective Reactivity of an Asymmetric Tetravalent
Di[dihydroxotin(IV)] Bis-Porphyrin Host Driven by Hydrogen-Bond
Templation
Peter R. Brotherhood, Ian J. Luck, Iain M. Blake, Paul Jensen, Peter Turner, and
Maxwell J. Crossley*[a]
Abstract: Local molecular environment
effects on the rates of ligand exchange
at an asymmetric di[dihydroxotin(IV)]
bis-porphyrin 5 are examined. The host
ligand exchange at each of the four
tin(IV) binding sites. For monodentate
carboxylic acids, binding preferentially
occurs at the exterior porphyrin site.
Further addition of carboxylic acid re-
sults in sequential binding at the qui-
noxalinoporphyrin sites and lastly at
the interior site on the porphyrin, with
high regioselectivity. These selective
binding outcomes are immediately ap-
parent by NMR spectroscopy. A series
of 2D NMR spectroscopy experiments
allowed identification of the preferred
binding sites at the host. This positively
identifies that steric hindrance and
electron-withdrawing functionality on
the porphyrin macrocycle impede
ligand exchange. However, these ef-
fects are overcome by dicarboxylic acid
guests, which form ditopic hydrogen-
bond interactions between the intra-
cavity hydroxo ligands in the initial
stage of ligand exchange, leading to re-
gioselective binding between the
tin(IV) sites within the cavity. It is en-
visaged that the factors identified
herein that define regioselective ligand
exchange at host 5 will find wider ap-
plication in supramolecular systems in-
corporating tin(IV) porphyrins.
5
possesses four non-equivalent
tin(IV)–ligand binding sites that are
distinguished by their position relative
to a shallow cavity, by the steric envi-
ronment at each binding site and by
electronic-structure differences be-
tween the constituent porphyrin and
quinoxalinoporphyrin
macrocycles.
These design features of the asymmet-
ric host are confirmed by X-ray crystal
structure analysis. Binding experiments
with monodentate carboxylic acids and
bidentate dicarboxylic acids show sig-
nificant differences in the rate of
Keywords: host–guest systems
·
NMR spectroscopy · porphyrinoids ·
supramolecular chemistry · Trçgerꢀs
base
Introduction
assembly phenomena in artificial supramolecular systems
has afforded many structures that have strong potential for
application in technology. As in nature, artificial systems are
assembled with selectivity that is dictated by thermodynamic
control. Multiple labile forces between components may act
in concert to define the assembly outcome,[3] or conditions
are employed under which covalent bond formation be-
tween components is reversible.[4] The topic of this paper,
ligand exchange chemistry of dihydroxotin(IV) porphyrins,
is a system that offers the potential to use both labile inter-
action to define selective assembly of components and, ulti-
mately, covalent strength association of components.[5]
Tin(IV) porphyrins have been used in supramolecular
chemistry due to their spontaneous, chemoselective, cova-
lent strength binding of oxyanions at the six-coordinate
tin(IV) metal centre.[6–9] Many examples also utilise the in-
teresting optical and electronic properties of the tin(IV) por-
phyrin.[10] These studies have largely generated complexes
Biological systems are adept in the use of labile interactions
to define selectivity in molecular interactions through equili-
bration to a thermodynamically or kinetically stable confor-
mation.[1] These conformations are often then stabilised by
the formation of covalent bonds between assembled compo-
nents. Stabilisation of protein quaternary structure by disul-
fide bonds is an obvious example.[2] Mimicry of biological
[a] P. R. Brotherhood, Dr. I. J. Luck, Dr. I. M. Blake, Dr. P. Jensen,
Dr. P. Turner, Prof. M. J. Crossley
School of Chemistry, F11
The University of Sydney, NSW 2006 (Australia)
Fax : (+61)2-9351-3329
E-mail: m.crossley@chem.usyd.edu.au
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.200801775.
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with symmetric axial ligation about the six-coordinate
tin(IV) metal centre.
Ligand exchange at dihydroxotin(IV) porphyrins proceeds
by initial outer-sphere hydrogen-bond equilibrium between
an incoming protic oxygen molecule (typically a carboxylic
acid) and the oxygen of the bound hydroxo ligand.[5–8,11]
Subsequently, the protonated hydroxo ligand (water) disso-
ciates and the carboxylate anion may then associate by an
ester-like bond at the tin(IV) centre. Once bound at the
tin(IV) centre, carboxylate ligands tend only to exchange in
the presence of free acid of comparable or greater strength
than that bound.[11,12] It is the initial hydrogen-bond equilib-
rium that offers the potential for kinetic or thermodynamic
control over the site of interaction of the carboxylic acid
with the dihydroxotin(IV) porphyrin host.
Herein we describe the design, synthesis and crystallo-
graphic characterisation of an asymmetric tetravalent di[di-
hydroxotin(IV)] bis-porphyrin host molecule 5 that possess-
es four non-equivalent binding sites, and binding studies
with this host and carboxylic acids. These allow us to identi-
fy factors that are symptomatic of the host asymmetry, and
that result in the highly regioselective binding of carboxylate
anions at specific sites on host 5. This work identifies factors
that may be utilised to control facial selectivity in ligand ex-
change at tin(IV) porphyrins, expanding their utility in
supramolecular chemistry.
Results and Discussion
Synthesis and crystal structure analysis of asymmetric host
5: The precursor to host 5, free-base, asymmetric bis-por-
phyrin 4, was synthesised by the acid-catalysed condensation
of the 2-aminoporphyrin 1 and the aminoquinoxalinopor-
phyrin 2 with formaldehyde (Scheme 1).[13–15] A two molar
equivalent excess of the less-reactive 1 was employed. The
three major products from the crude reaction mixture could
be isolated by chromatography over silica gel and separation
was best achieved as their dizinc(II) chelates. The homo-
coupled dizinc(II) Trçgerꢀs base bis-porphyrin[16] was isolat-
ed in 52% yield and extended dizinc(II) Trçgerꢀs base bis-
quinoxalinoporphyrin[14,17] was obtained in 48% yield. Once
isolated, dizinc(II) asymmetric bis-porphyrin 3 was demetal-
lated by treatment with hydrochloric acid to yield the free-
base asymmetric bis-porphyrin 4 in 14% overall yield from
2.
Scheme 1. Synthesis of di[dihydroxotin(IV)] asymmetric host 5.
1
ate,[21] see Scheme 1. H NMR and high-resolution electro-
spray ionisation Fourier transform ion cyclotron resonance
(HR-ESI-FT/ICR) spectra for 3, 4 and 5 are provided in the
Supporting Information. Recrystallisation of 5 from di-
chloromethane/acetonitrile yielded crystals suitable for X-
ray crystal structure analysis.
Formation of the asymmetric bis-porphyrin product dem-
onstrates the applicability of the Trçgerꢀs base reaction to
the cross-condensation of arylamines of quite different elec-
tronic structures, in this case a macrocyclic heteroaromatic
amine 1 and an aromatic fused aniline 2.[18] This reaction, in
a single step, has also imparted asymmetry, chirality, concav-
ity and rigidity on the host framework.
Free-base asymmetric bis-porphyrin 4 was then treated
with tin(II) chloride in pyridine at reflux,[19,20] and hydro-
lysed to di[dihydroxotin(IV)] host 5 in 80% yield by heating
at reflux in 4:1 tetrahydrofuran/aqueous potassium carbon-
Crystal structure analysis allows identification of many of
the design features of host 5, depicted in Figure 1. The
asymmetric surface is immediately evident. The concavity
imparted by the methanodiazocine Trçgerꢀs base bridge re-
sults in the formation of a shallow cavity at one face of the
host, into which two of the hydroxo-ligand–carboxylate in-
teraction sites project. The remaining two binding sites of
the host, one on the porphyrin and one on the quinoxalino-
porphyrin, project outwards from the exterior of the cavity.
The distance between the tin(IV) metal centres across the
interior of the cavity is 12.8 ꢂ, which is sufficient to allow
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Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis-porphyrin
FULL PAPER
proton on this meso-aryl substituent to the quinoxalinopor-
phyrin, hereafter referred to as the cavity aryl proton (indi-
cated by the orange arrows in Figure 1), resonates character-
istically upfield at d=5.99 ppm, confirming its orientation in
close proximity to the shielding effect of the porphyrin mac-
rocycle. Only protons in close vertical proximity above or
below the plane of the porphyrin macrocycle fall within this
shielding zone.[23] The other aryl protons on 5 resonate be-
tween d=7.59 and 8.32 ppm. The cavity aryl proton reso-
nates at even higher field in the free-base 4 and dizinc(II)
analogues of host 5, at d=5.15 and 4.96 ppm, respectively
(see Figures S1 and S3 in the Supporting Information).
As well as the steric and cavity influences on the tin(IV)
ligation sites identified by crystallography, there are known
electronic-structure differences between the porphyrin mac-
rocycle and the quinoxalinoporphyrin macrocycle that may
affect the rate of ligand exchange at each tin(IV) metal
centre. It is known from the electrochemistry of monomeric
quinoxalinoporphyrins that the addition of the quinoxalino
group activates the reduction of the porphyrin macrocycle
and the reduction of electroactive metal centres bound at
the inner periphery.[18,24] These electronic effects may also
influence the strength of tin(IV)–hydroxo bonds, the basicity
of the hydroxo oxygen, and its capacity to accept hydrogen
bonds from incoming carboxylic acid guests, thereby slowing
the rate of ligand exchange at the quinoxalinoporphyrin
macrocycle compared with the rate of exchange at the por-
phyrin. We describe experiments designed to test this hy-
pothesis later in the discussion.
These factors, the presence of a shallow cavity, the steric
environment at each of the binding sites and the electronic
differences between the two macrocycles, operate in concert
such that there are significant differences in the propensity
of ligand exchange at each of these four sites. The experi-
ments subsequently described in this paper identify the op-
eration of these factors in leading to regioselective ligand
exchange at 5. We have identified, by NMR spectroscopy,
the preferred site for ligand exchange as being on the exteri-
or of the porphyrin macrocycle (indicated by the green
arrows in Figure 1a and b) and positively identified the fac-
tors that lead to binding regioselectivity.
Figure 1. X-ray crystal structure of 5. Distances are given in ꢂ. a) View
into the cavity. b) Side view, (3,5-tBu2C6H3 groups removed for clarity).
c) View into the cavity showing the proximity of the cavity aryl group to
the hydroxo ligand at the interior face of the porphyrin. d) Full structure
(solvent and disorder removed) view into cavity. The cavity aryl proton is
indicated with the orange arrows. The “preferred” site of ligand exchange
at ꢀ1 molar equivalents monodentate guest (identified by NMR spec-
troscopy) is indicated by the green arrows.
Binding studies with monodentate carboxylic acids: Formic
acid (6), acetic acid (7) and pivalic acid (8) were selected for
their variation in steric demand immediately about the car-
boxylate functionality. When these acids are introduced in-
dependently to host 5 in ꢀ1 molar equivalent aliquots,
ligand exchange occurs primarily at only one of the four
sites on the host, to form new dihydroxotin(IV)–carboxyla-
tohydroxotin(IV) complexes 5·6, 5·7 and 5·8 in 60–80%
yield, depending on the carboxylic acid used. The remaining
carboxylic acid binds to the host in a random fashion, either
to form other 1:1 complexes or complexes with higher car-
boxylate/5 ratios. At the completion of binding no free car-
boxylic acid is detected.
the ditopic binding of aliphatic dicarboxylic acids in the
order of six to ten carbons in length.[22] Note the steric envi-
ronment about these intracavity binding sites. In the solid
state, the binding sites at the interior and exterior face of
the quinoxalinoporphyrin and at the exterior face of the por-
phyrin are sterically unencumbered. The binding site at the
interior face of the porphyrin, however, is obscured from
one side by a 3,5-di-tert-butylphenyl group appended at a
meso position of the quinoxalinoporphyrin. This arrange-
ment is indicated in Figure 1c and d. Indeed, the para-aryl
1H–119Sn HMQC provides major evidence that regioselec-
tive ligand exchange occurs at only one of the four binding
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M. J. Crossley et al.
sites on host 5 with the initial ꢀ1 molar equivalent aliquot
of carboxylic acid guest. The 119Sn chemical shift is sensitive
to the functionality of the porphyrin macrocycle and to the
nature of axial ligation and there are characteristic chemical
shift ranges for dihydroxotin(IV) nuclei, hydroxocarboxyla-
totin(IV) nuclei and dicarboxylatotin(IV) nuclei.[25] The
1H–119Sn HMQC for complex 5·7, depicted in Figure 2a,
identifies the b-pyrrolic resonances and the acetate reso-
nance corresponding to the hydroxoacetato 119Sn nucleus
and confirms the assignment of the initial site of ligand ex-
change discussed below.
Regioselective binding is also evident by 1D 1H NMR
1
spectroscopy. There are three regions of the H NMR spec-
trum in which this is most easily identified: the methanodia-
zocine bridge resonances (d=4 to 5 ppm), the resonance for
the cavity aryl proton (d=6 to 7 ppm), identified in
Figure 1, and the resonance for the bound ligand (d=0 to
À2 ppm). The protons in each of these environments are
particularly sensitive to changes in the pattern of ligation
about the host and dispersion of these signals is good, allow-
1
ing identification of signals due to new complexes. H NMR
spectra for complexes 5·6, 5·7 and 5·8 show six new doublets
in the methanodiazocine bridge region and a single new res-
onance for the cavity aryl proton. For 5·7 and 5·8, single car-
boxylate ligand environments are apparent at d=À1.15 and
À1.30 ppm, respectively. The bound formate resonance for
5·6 coincides with the tert-butyl proton resonances. These
1H NMR spectra are shown in Figure S6 in the Supporting
Information.
An important control experiment is one in which the out-
come of random ligand exchange is examined. An addition
of 1 to ꢀ3 molar equivalents of 7 results in formation of
many complexes resulting from non-selective ligand ex-
change at the three remaining sites on the host once binding
Figure 2. NMR spectroscopy characterisation of regioselective 1:1 acetate
binding at host 5. a) Complete 1H–119Sn HMQC (400 MHz, CDCl3,
300 K) of host 5+0.95 molar equivalents acetic acid 7 (5·7). Note the
presence of only one new hydroxoacetatotin(IV) environment (119Snd=
À601 ppm). C Ar indicates the signal for the cavity aryl proton defined
in Figure 1. b) Expansion of boxed regions in a.
1
is complete at the first, “preferred” site. The H NMR spec-
tra for mixtures of complexes thus generated are too com-
1
plicated to interpret and the H–119Sn HMQC at two molar
equivalents of 7 shows many 119Sn environments, see Figur-
These titration experiments demonstrate regioselective
es S7 and S8 in the Supporting Information.
ligand exchange upon addition of ꢀ1 molar equivalent of 6,
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Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis-porphyrin
FULL PAPER
7 or 8 to host 5. These results show that the morphology of
host 5 and the electronic effects at each macrocycle act to
direct ligand exchange preferentially to only one of the four
binding sites. The next sections are devoted to identification
of the preferred binding site and the factors that are impor-
tant in its selection.
Identification of the preferred binding site by NMR: Detec-
tion of NOE and through-bond interactions[9a,b] between the
nuclei of bound 7 and 8 in complexes 5·7 and 5·8, and the
nuclei of the host framework, allow paths of connectivity to
be established such that the initial site of ligand exchange
can be identified. During the course of these investigations,
1
we also made a full H and 119Sn NMR assignment for host 5
and its di[diacetatotin(IV)] complex.[26] Identical pathways
exist in these complexes to those that allow the assignment
of 5·7 and 5·8.
The experiments used to identify these paths of connectiv-
ity in host 5 and the carboxylate complexes were NOESY,
ROESY employing varied mixing times, double quantum fil-
tered
correlation
spectroscopy
(DQF-COSY),
1H–119Sn HMQC, 1H–13C HMBC and 1H–13C HSQC. For
these experiments, 5·7 and 5·8 were generated by adding
0.9 molar equivalents of 7 and 0.7 molar equivalents 8, re-
spectively. It was found that the ROESY experiments gave
better results at lower temperature. The spectra discussed in
this section were obtained at 250 K, after sufficient time at
room temperature was allowed for binding to go to comple-
tion.
For 5·7 and 5·8, two paths of NOE and through-bond con-
nectivity were identified, which show that the preferred site
of exchange at ꢀ1 molar equivalent of guest is at the exteri-
or cavity position of the porphyrin macrocycle (this position
is indicated in Figure 1 by the green arrows). The first of
these paths is shown in Figure 3 by the red arrows. In Fig-
ure 3a the bound ligand is 7, however, this connectivity was
also established for 5·8. The quinoxalino proton adjacent to
the methanodiazocine bridge is the “anchor” in the frame-
work of host 5 to which the position of the bound ligand can
be tied. This quinoxalino proton experiences NOE interac-
tion (interaction 1 (red) in Figure 3) with one of the bridge
methylene protons. This bridge proton couples with its part-
ner on the same carbon (interaction 2 (red) in Figure 3),
which in turn connects through space to an ortho proton of
a meso-aryl group on the porphyrin macrocycle (interaction
3 (red) in Figure 3). This pathway establishes the position of
this ortho aryl proton as being exterior to the cavity of host
5. This aryl proton then connects (interaction 4 in Figure 3)
through NOE to the bound ligand protons. Further evidence
for the facial assignment comes from the connectivity (inter-
actions A and B in Figure 3) between the endocyclic bridge
methylene proton and the ortho-aryl proton projecting into
the cavity (interaction A in Figure 3). Coupling of the interi-
or cavity and exterior cavity ortho protons (interaction B in
Figure 3. a) ROESY (400 MHz, CDCl3, 250 K) spectrum showing one
pathway of connectivity (red arrows) that allows assignment of the posi-
tion at which the first molar equivalent of 7 binds to host 5, forming 5·7
(see red arrows in part b). Dashed arrows indicate spin-coupling. b) Dia-
grammatic representation of the NOE (solid arrows) and spin-coupled
(dashed arrows) connectivity for 5·7 and 5·8 that identify the initial site
of complexation. Red indicates pathway one and blue indicates pathway
two (see text). Arrows and symbols in part b correspond to those in part
a. The I ring is the cavity aryl group defined in Figure 1.
in Figure 3b. NOE connectivity (interaction 1 (blue) in
Figure 3) to one of the tert-butyl groups appended to the
cavity aryl group (I ring, identified by its NOE connectivity
to the cavity aryl proton) allows identification of the signal
arising from a II ring ortho proton. DQF-COSY (interaction
2 (blue) in Figure 3) distinguishes the interior and exterior
1
Figure 3) was detected by DQF-COSY and H–13C HMBC.
The second path of connectivity defining the initial site of
complexation for 5·7 and 5·8 is indicated by the blue arrows
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M. J. Crossley et al.
cavity ortho protons on the B ring and subsequent NOE
connectivity (interaction 3 in Figure 3) defines the position
of the guest ligands on the asymmetric host 5 surface. The
bound ligand also connects through NOE to the other two
ortho protons on the same face of the porphyrin macrocycle
indicated in Figure 3.
Together these two paths of connectivity can only be ex-
plained by the binding of ligands at the exterior face of the
porphyrin. Measurements, in the crystal structure, of the dis-
tances between the host nuclei identified in these experi-
ments were also in good agreement with the connectivities
determined by ROESY. The next section describes control
experiments that allow rationalisation of the factors leading
to this binding selectivity.
Factors that determine binding selectivity: We discussed
above, in light of the crystal structure data for host 5, the
steric environment at each of the binding sites and the pres-
ence of a shallow cavity at one face of the host. We have
also drawn attention to the electronic differences between
porphyrins and quinoxalinoporphyrins.[18,24] The following
series of experiments specifically identifies the importance
of each of these factors.
Figure 4. 1H–119Sn HMQC (400 MHz, CDCl3, 300 K) showing expansions
of the b-pyrrolic proton region. a) Host 5. b) Host 5+6.0 molar equiva-
lents 6, 15 min, di[diformatotin(IV)] complex. c) Host 5+6.0 molar
equivalents 7, 30 min, di[diacetatotin(IV)] complex. d) Host 5+8.0 molar
equivalents 8, 36 h, di[dipivalatotin(IV)] complex and one isomer of the
3:1 pivalato–host complex.
Steric influences: Addition of ꢁ4 molar equivalents of
guests 6, 7 and 8 to host 5 results in ligand exchange at all
four binding sites on the host to form the di[dicarboxylato-
tin(IV)] complexes of host 5. The di[diformatotin(IV)] and
di[diacetatotin(IV)] complexes form within 15 and 30 min,
respectively, upon addition of six molar equivalents 6 or 7.
It is not possible, however, to form the di[dipivalatotin(IV)]
complex in quantitative yield under these conditions. Addi-
tion of eight molar equivalents of 8 to host 5 and a reaction
time of 36 h results in the formation of a mixture comprised
of the di[dipivalatotin(IV)] complex and only one of the
four possible dipivalatotin(IV)–hydroxopivalatotin(IV) com-
plexes (hereafter referred to as the 3:1 complex).
1H–119Sn HMQC of the mixture, depicted in Figure 4d,
allows detection of a single hydroxopivalatotin(IV) nucleus
along with two dipivalatotin(IV) nuclei. The dipivalato-
tin(IV) nucleus of the 3:1 complex is coincident with those
signals for the di[dipivalatotin(IV)] complex.
acetato to pivalato, there is increasing downfield shift in the
resonance for this proton that correlates with the steric
demand of the ligand, see Figure 5. This is consistent with
steric interaction between the bound ligand and the cavity
aryl group and displacement of the cavity aryl proton away
from the major shielding zone of the porphyrin macrocycle.
The chemical shift for the cavity aryl proton in the 3:1 com-
plex is very similar to that in host 5, which indicates that in
this complex this binding site maintains a hydroxo ligand.
The proximity of the cavity aryl proton to this binding site
was also detected by ROESY experiments on host 5.[26] To-
gether these experiments clearly show the steric impedance
to ligand exchange at the interior cavity site of the porphy-
rin macrocycle.
This result shows that the steric environment at one of
the binding sites offers a significant deterrent to ligand ex-
change for 8, the bulkiest of the carboxylate guests. From
the crystal structure analysis for host 5 (Figure 1) it is clear
that this hindered binding site is that on the interior cavity
position of the porphyrin, which is partially obscured by the
cavity aryl group identified in Figure 1b. Complete di[dipi-
valatotin(IV)] complex formation does not occur because
steric interaction at this binding site results in mechanical
strain in the tin(IV)–pivalato bond, activating it to hydroly-
sis by water released during binding.
Macrocyclic electronic structure influences: The effect of the
electronic differences between the porphyrin and quinoxali-
noporphyrin macrocycles of host 5 on ligand exchange rates
was tested in such a way as to control the morphology of
the host. The binding of 7 to a 1:1 molar ratio of model
compounds dihydroxotin(IV) porphyrin 9a[21] and dihydrox-
otin(IV) quinoxalinoporphyrin 10a[21] was examined by em-
ploying concentrations such that the NMR spectroscopy ti-
tration experiment was precisely analogous to those per-
formed on host 5.
1
This H NMR spectroscopy titration, depicted in Figure 6,
Examination of the chemical shifts for the cavity aryl
proton (identified in Figure 1 by the orange arrows) in the
di[dicarboxylatotin(IV)] complexes shows that as ligands in-
crease in steric demand from hydroxo, through formato and
shows that the presence of the quinoxalino group at the por-
phyrin periphery significantly reduces the rate of ligand ex-
change at the tin(IV) centre. The electron-withdrawing
effect of the quinoxalino group increases the Lewis acidity
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Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis-porphyrin
FULL PAPER
Figure 5. 1H NMR spectrum (400 MHz, CDCl3, 300 K) for the cavity aryl
proton (Ar) defined in Figure 1. a) Host 5. b) Host 5+6.0 molar equiva-
lents 6, 15 min, di[diformatotin(IV)] complex. c) Host 5 +6.0 molar equi-
valents 7, 30 min, di[diacetatotin(IV)] complex. d) Host 5+8.0 molar
equivalents 8, 36 h di[dipivalatotin(IV)] complex and one isomer of the
3:1 pivalato–host complex (3:1). Note the downfield shift with the in-
creasing steric demand of the ligands. The 3:1 complex maintains a hy-
droxo ligand adjacent to the cavity aryl group.
Figure 6. a) 1H NMR (400 MHz, CDCl3, 300 K) titration of a 1:1 molar
ratio mixture of 9 and 10 with 7. i) 0.6, ii) 1.2, iii) 2.0 and iv) 3.0 equiva-
lents 7. Each acetate environment was identified by 1H–119Sn HMQC.
^
b) Plot showing the proportion of binding site consumed on 9 ( ) and 10
&
( ) after each addition of aliquot and 30 min binding time.
Hydrogen-bonding mode: The experiments described above
show a clear influence on the rate of ligand exchange at
binding sites on host 5 by both steric factors and the elec-
tron-withdrawing effect of the fused quinoxaline group.
These factors work in concert to direct the first molar equiv-
alent aliquot of monodentate carboxylic acid guest to bind
at their preferred site, the exterior, sterically non-congested
site on the electronically favoured porphyrin macrocycle.
However, these factors are overridden in the binding of di-
carboxylic acid guests of appropriate length to bridge the
two tin(IV) centres across the interior of the cavity.
of the tin(IV) metal centre, thereby reducing the basicity of
the hydroxo ligand oxygen (the hydrogen-bond acceptor in
the initial stage of acidolysis) and strengthening the hydroxo
ligand bond to the tin(IV) metal centre. As a result, ligand
exchange kinetics are more rapid at the porphyrin tin(IV)
centre and these sites bind the majority of the initial aliquot
of carboxylic acid guest.
Adipic acid 11 and subaric acid 12 form ditopic hydrogen-
bond interactions with host 5 between the intracavity hy-
droxo ligands in the initial stage of the ligand exchange pro-
cess. This organises the guest into a regioselective interac-
1
tion with the tetravalent host, which is detected by H NMR
spectroscopy (see Figure 5b and Figure S9b in the Support-
Chem. Eur. J. 2008, 14, 10967 – 10977
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10973

M. J. Crossley et al.
ing Information). The guest is then trapped to the host in
this orientation by tin(IV)–carboxylate bond formation. This
exclusive intracavity binding may be predicted by the “mo-
lecular ruler” concept.[22] At completion of ditopic binding
The binding of 11 and 12 subjects them to the asymmetric
environment of the host 5 cavity. In this environment each
of the methylene protons of the aliphatic backbone are non-
equivalent and give rise to complex splitting patterns in the
1H NMR spectrum. For 11 these eight resonances are clearly
resolved in the upfield region of the spectrum typical for
binding modes between porphyrin centres,[5] see Figure 7d.
As subsequent aliquots of 11 and 12 are added, binding
occurs at the exterior of the host. Guests bound at the exte-
rior are also subject to a non-symmetric environment, but
rapid rotation about methylene–methylene bonds reduces
dispersion of the signals for the diastereotopic protons ap-
pended to each carbon. The signals for the interior and exte-
rior bound ligands are readily identified by chemical shift.
The electronic effect differences between the porphyrin
and the quinoxalinoporphyrin identified in the previous sec-
tion are also evident in the titration of 11 and 12 with host
5. When the second molar equivalent aliquot of diacid is
added to host 5, one major set of external bound-ligand sig-
nals appears. Addition of the third molar equivalent aliquot
results in appearance of a second set of externally bound
ligand signals 0.1 to 0.2 ppm upfield of the original signals,
which indicates that ligand exchange is favoured at one of
the exterior cavity sites, most likely that at the exterior face
of the porphyrin macrocycle. See signals marked 1 and 2 in
Figure 7e and f and Figure S9 in the Supporting Informa-
tion.
1
the methanodiazocine bridge region of the H NMR spec-
trum simplifies to six geminally coupled doublets, indicative
that there is one host–guest complex in solution (See Fig-
ure 7c inset and Figure S9c, inset in the Supporting Informa-
tion).
These experiments with dicarboxylic acids 11 and 12 show
that while the steric and electronic influences on monoden-
tate ligand binding are significant, these are overcome by
guests that possess the facility to form ditopic hydrogen
bonds between the interior cavity hydroxo ligands in the ini-
tial stage of the ligand exchange process. This ditopic hydro-
gen-bonding mode represents the thermodynamic product
of the pre-equilibrium and essentially renders the first
tin(IV)–carboxylate bond formation to be an intramolecular
process, resulting in the rapid kinetic trapping of this mode
of interaction.
Conclusion
Figure 7. 1H NMR (400 MHz, CDCl3, 300 K) spectrum showing the bind-
ing of 11 to host 5. a) Host 5. Inset shows the methanodiazocine bridge
region. b) Host 5+0.4 molar equivalents 11, 30 min. c) Host 5+0.4 equiv-
alents 11, 17 h. Inset shows methanodiazocine bridge region, six doublets
of a single complex. d) Host 5+0.9 equivalents 11, 17 h. e) Host 5+
2.0 equivalents 11, 1 h. f) Host 5+3.0 equivalents 11, 10 h. The C5-meth-
ylene for external adipate is obscured beneath the tBu signals. 1 and 2,
see text for explanation.
Steric and electronic influences and the operation of cavity
effects are shown to result in exquisite control over hydro-
gen-bonding propensity, ligand exchange, and hence, carbox-
ylate binding to a tetravalent di[dihydroxotin(IV)] bis-por-
phyrin host (5) designed to identify the significance of these
factors. These steric, electronic and cavity-effect properties
operate by influencing the kinetics and/or thermodynamics
of hydrogen-bond interactions in the pre-equilibrium to
ligand exchange. Ligand exchange may then proceed from
this dynamic hydrogen-bonded mode of interaction with dis-
sociation of a molecule of water and tight binding of the
oxygen anion at the tin(IV) centre, siphoning material out
of the hydrogen-bond pre-equilibrium to yield a tin(IV)-
bound complex that reflects the incidence of favoured inter-
actions established by hydrogen bonding. This results in the
10974
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Chem. Eur. J. 2008, 14, 10967 – 10977

Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis-porphyrin
FULL PAPER
methyleneH), 4.85 (d, J=17.5 Hz, 1H; methyleneH), 4.96 (dd, J=
1.8 Hz, 1H; ArH), 7.48 (d, J=9.2 Hz, 1H; quinoxalinoH), 7.53 (dd, J=
1.8 Hz, 1H; ArH), 7.60 (d, J=9.2 Hz, 1H; quinoxalinoH), 7.72–7.75 (m,
4H; ArH), 7.77 (dd, J=1.8 Hz, 1H; ArH), 7.79 (dd, J=1.8 Hz, 1H;
ArH), 7.85 (dd, J=1.8 Hz, 1H; ArH), 7.88 (dd, J=1.8 Hz, 1H; ArH),
7.89 (dd, J=1.8 Hz, 1H; ArH), 7.91 (dd, J=1.8 Hz, 1H, ArH), 7.96
(brdd, J=1.8 Hz, 1H; ArH), 8.08 (dd, J=1.8 Hz, 1H; ArH), 8.13 (dd,
J=1.8 Hz, 1H; ArH), 8.14 (dd, J=1.8 Hz, 1H; ArH), 8.17 (dd, J=
1.8 Hz, 1H; ArH), 8.25 (dd, J=1.8 Hz, 1H; ArH), 8.28 (dd, J=1.8 Hz,
1H; ArH), 8.41 (d, J=4.9 Hz, 1H; b-pyrrolicH), 8.70 (d, J=4.7 Hz, 1H;
b-pyrrolicH), 8.77–8.80 (m, 3H; b-pyrrolicH), 8.87 and 8.88 (ABq, J=
4.9 Hz, 2H; b-pyrrolicH), 8.97 (d, J=4.7 Hz, 1H; b-pyrrolicH), 9.00–9.03
(m, 3H; b-pyrrolicH), 9.07 ppm (d, J=4.7 Hz, 1H; b-pyrrolicH).
facility for simple carboxylic acid guests to bind with signifi-
cant site specificity on the asymmetric surface of host 5.
It is worth noting that all of the information required to
direct this regioselective binding at host 5 was installed with
a single facile condensation of 1 and 2 with formaldehyde to
generate the methanodiazocine Trçgerꢀs base linker between
the macrocycles. The Trçgerꢀs base condensation of these
two achiral and electronically distinct arylamines defines the
steric environment at each binding site, the presence of the
cavity, the rigidity and the asymmetry of the host.
This work shows there is potential for expanding the ap-
plication of tin(IV) porphyrins in the generation of molecu-
lar devices, beyond the formation of complexes that are
symmetrical about the six-coordinate tin(IV) metal centre.
Tuning the outcome of the hydrogen-bond pre-equilibrium
to ligand exchange and kinetic trapping of this mode of in-
teraction offers chemoselective and regioselective binding of
oxygen anions at the tin(IV) centre as well as covalent
strength association.
The third major fraction was collected to yield dizinc(II) extended Trç-
gerꢀs base bis-porphyrin[14,17] (53 mg, 48%) with identical spectral proper-
ties to those previously reported. Compound 3 (37 mg) was dissolved in
dichloromethane (20 mL) and shaken with hydrochloric acid (32% w/v,
10 mL) for 5 min. The organic layer was washed with water (4ꢃ50 mL)
and evaporated. The residue was purified by chromatography over silica
(dichloromethane/light petroleum, 1:2). The major band was collected,
evaporated to dryness and the residue was recrystallised from dichloro-
methane/methanol (1:1) to yield 4 as a purple microcrystalline powder
(27 mg, 14%). M.p.>3008C; 1H NMR (400 MHz, CDCl3, 300 K): d=
À2.49 (brs, 2H; inner NH), À2.21 (brs, 1H; inner NH), À2.17 (brs, 1H;
inner NH), À0.14 (brs, 18H; tBuH), 1.35 (s, 18H; tBuH), 1.39 (s, 9H;
tBuH), 1.43 (s, 18H; tBuH), 1.46 (s, 18H; tBuH), 1.51 (brs, 9H; tBuH),
1.53–1.54 (m, 36H; tBuH), 1.58 (s, 9H; tBuH), 1.65 (s, 9H; tBuH), 3.89
(d, J=18.1 Hz, 1H; methyleneH), 3.99 (d, J=12.2 Hz, 1H; methyle-
neH), 4.07 (d, J=17.4 Hz, 1H; methyleneH), 4.20 (brd, J=18.1 Hz, 1H;
methyleneH), 4.52 (d, J=12.2 Hz, 1H; methyleneH), 4.74 (d, J=
17.4 Hz, 1H; methyleneH), 5.15 (dd, J=1.5 Hz, 1H; ArH), 7.37 (d, J=
9.3 Hz, 1H; quinoxalinoH), 7.49 (brdd, 2H; ArH), 7.50 (d, J=9.3 Hz,
1H; quinoxalinoH), 7.55 (dd, J=1.7 Hz, 1H; ArH), 7.72 (dd, J=1.7 Hz,
1H; ArH), 7.73–7.76 (m, 3H; ArH), 7.79 (dd, J=1.7 Hz, 1H; ArH),
7.84 (dd, J=1.7 Hz, 1H; ArH), 7.86–7.87 (m, 2H; ArH), 7.89–7.90 (m,
3H; ArH), 7.97 (dd, J=1.7 Hz, 1H; ArH), 8.00 (brs, 2H; ArH), 8.07
(dd, J=1.7 Hz, 1H; ArH), 8.14–8.16 (m, 2H; ArH), 8.30 (dd, J=1.7 Hz,
1H; ArH), 8.32 (dd, J=1.7 Hz, 1H; ArH), 8.42 (d, J=5.1 Hz, 1H; b-pyr-
rolicH), 8.62–8.66 (m, 3H; b-pyrrolicH), 8.72 (d, J=4.9 Hz, 1H; b-pyrro-
licH), 8.81–8.88 (m, 6H; b-pyrrolicH), 8.92 ppm (d, J=5.1 Hz, 1H; b-
pyrrolicH); IR (CHCl3): n˜ =3337 (w), 2964 (s), 2905 (m), 2868 (m), 1593
(s), 1477 (m), 1466 (m), 1458 (m), 1425 (w), 1394 (w), 1364 (m), 1292
(w), 1263 (w), 1248 (m), 1126 (w), 999 (w), 922 (m), 883 cmÀ1 (m); UV/
Vis (CHCl3): lmax (loge): 423 (5.50), 440 sh (5.41), 523 (4.46), 601 (4.12),
650 nm (3.72); MS (ESI): m/z: 2295.38 [M]+; MS (HR-ESI-FT/ICR): m/
Experimental Section
General procedures: 1H NMR spectra were recorded on a Bruker DPX-
400 (400 MHz) spectrometer and signals are quoted in ppm relative to
the residual protiated solvent peak.[27] The 2D NMR spectra discussed
were recorded on the same instrument by using standard Bruker pulse
programs. Temperature was controlled by using a Bruker B-VT 2000 var-
iable temperature unit. 119Sn NMR (149 MHz) chemical shifts are quoted
relative to the external standard dihydroxo[5,10,15,20-tetrakis-(3,5-di-tert-
butylphenyl)porphyrinato]tin(IV)
(
119Sn=À569.2 ppm)[21] CDCl3 was
dried and deacidified by filtration through a plug of anhydrous potassium
carbonate and activated neutral alumina prior to use. Other deuteriated
solvents were used as received. See the Supporting Information for addi-
tional details and routine synthetic procedures.
Synthesis of 4: Freshly prepared 1[16] (185 mg, 0.170 mmol) and 2[14]
(101 mg, 86.0 mmol) were dissolved in tetrahydrofuran (16 mL) and nitro-
gen was bubbled through the solution for 10 min. A solution of hydro-
chloric acid (32% w/v, 2 mL) in ethanol (4 mL) was added and nitrogen
was passed over the solution for 10 min. Formaldehyde (37% w/v,
0.45 mL) was added and the reaction was heated at 658C under nitrogen
for 36 h. The reaction was diluted with diethyl ether (100 mL), washed
with water (4ꢃ100 mL) and the organic solvent was removed. The resi-
due was precipitated from dichloromethane/methanol (1:1) to yield an
amorphous purple residue (205 mg) composed of three major products,
as determined by TLC (1:1 dichloromethane/light petroleum). The resi-
due was dissolved in chloroform (20 mL) and treated with zinc(II) ace-
tate dihydrate (300 mg, 1.37 mmol) dissolved in methanol (10 mL) at
reflux for 5 min and the organic solvents were evaporated to dryness.
TLC analysis (1:1 dichloromethane/light petroleum) indicated complete
consumption of the starting material. The residue was purified by chro-
matography over silica (dichloromethane/light petroleum, 1:2). The first
major fraction was evaporated to yield dizinc(II) Trçgerꢀs base bis-por-
phyrin[16] (103 mg, 52%) with identical spectral properties as those re-
ported previously. The second major band was collected to yield 3
(37 mg) as a purple residue. This material was not analytically pure, but
identification by 1H NMR spectroscopy was possible. 1H NMR
(400 MHz, CDCl3, 300 K): d=À0.23 (brs, 18H; tBuH), 1.33 (s, 18H;
tBuH), 1.38 (s, 9H; tBuH), 1.42 (brs, 9H; tBuH), 1.44 (s, 9H; tBuH),
1.47 (s, 9H; tBuH), 1.48 (brs, 9H; tBuH), 1.49 (9H; s, tBuH), 1.51 (s,
9H; tBuH), 1.53 (s, 9H; tBuH), 1.56 (s, 18H; tBuH), 1.57 (s, 9H;
tBuH), 1.65 (s, 9H; tBuH), 3.99 (d, 1H; J=18.4 Hz, methyleneH), 4.02
(d, 1H; J=12.4 Hz, methyleneH), 4.15 (d, 1H; J=17.5 Hz, methyle-
neH), 4.46 (d, J=18.4 Hz, 1H; methyleneH), 4.53 (d, J=12.4 Hz, 1H;
z: calcd for C161H192
N
12 +2H+: 1148.2785; found: 1148.2740 [M+2H]2+
.
Synthesis of 5: Bis-porphyrin
4
(20.0 mg, 8.71 mmol) and anhydrous
tin(II) chloride (50.0 mg, 0.264 mmol) were combined and pyridine
(4 mL) was added. The mixture was heated to reflux with efficient stir-
ring for 12 h. The cooled reaction was diluted with diethyl ether
(100 mL), washed with water (4ꢃ100 mL) and the solvent was evaporat-
ed under reduced pressure until there was no trace of pyridine. The resi-
due was purified by chromatography over neutral alumina (class IV, di-
chloromethane/light petroleum, 1:1) and the major band was collected
and evaporated. The residue was dissolved in tetrahydrofuran (100 mL)
and a solution of anhydrous potassium carbonate (560 mg, 4.05 mmol) in
distilled water was added. The mixture was heated at reflux and stirred
efficiently for 12 h, diluted with freshly distilled diethyl ether (100 mL),
washed with deionised water (3ꢃ100 mL), dried over anhydrous sodium
sulfate, filtered and evaporated. The residue was recrystallised from
freshly distilled dichloromethane/acetonitrile (1:1) to yield 5 as lustrous
purple crystals suitable for X-ray crystallography (18 mg, 80%). M.p.>
3008C; 1H NMR (400 MHz, CDCl3, 300 K): d=À7.44 (brs, 1H; OH
ligand), À7.05 (brs, satellites 2J
U
2
(brs, satellites J
N
1.35 (brs, 18H; tBuH), 1.46 (s, 9H; tBuH), 1.49 (s, 9H; tBuH), 1.499 (s,
9H; tBuH), 1.503 (s, 9H; tBuH), 1.51 (s, 9H; tBuH), 1.54 (s, 9H;
tBuH), 1.55 (s, 9H; tBuH), 1.58 (s, 9H; tBuH), 1.59 (s, 9H; tBuH), 1.60
(s, 9H; tBuH), 1.620 (s, 9H; tBuH), 1.626 (s, 9H; tBuH), 4.11 (d, J=
Chem. Eur. J. 2008, 14, 10967 – 10977
ꢁ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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10975

M. J. Crossley et al.
12.5 Hz, 1H; methyleneH), 4.17 (d, J=18.5 Hz, 1H; methyleneH), 4.38
(d, J=17.6 Hz, 1H; methyleneH), 4.46 (d, J=12.5 Hz, 1H; methyle-
neH), 4.59 (d, J=18.5 Hz, 1H; methyleneH), 4.90 (d, J=17.6 Hz, 1H;
methyleneH), 5.99 (brdd, J=1.7 Hz, 1H; ArH), 7.59 (dd, 1.8 Hz, 1H;
ArH), 7.61 (d, J=9.1 Hz, 1H; quinoxalinoH), 7.70 (d, J=9.1 Hz, 1H;
quinoxalinoH), 7.83–7.84 (m, 3H; ArH), 7.88 (dd, 1.7 Hz, 1H; ArH),
7.93 (dd, 1.5 Hz, 1H; ArH), 7.95 (dd, 1.8 Hz, 1H; ArH), 7.80–7.81 (m,
2H; ArH), 8.04 (dd, 1.7 Hz, 1H; ArH), 8.08–8.10 (m, 2H; ArH), 8.16
(dd, 1.7 Hz, 1H; ArH), 8.17–8.18 (m, 2H; ArH), 8.26 (dd, 1.7 Hz, 1H;
ArH), 8.30 (dd, 1.7 Hz, 1H; ArH), 8.31–8.32 (m, 2H; ArH), 8.92 (d, J=
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4.9 Hz, satellites 4J
satellites 4J
(H,Sn)=10 Hz, 1H; b-pyrrolicH), 8.99–9.07 (m, 5H; b-pyrro-
licH), 9.10 (d, J=4.9 Hz, satellites 4J
(H,Sn)=10 Hz, 1H; b-pyrrolicH),
9.13–9.18 (m, 3H; b-pyrrolicH), 9.22 ppm (d, J=4.6 Hz, satellites 4J-
(H,Sn)=10 Hz, 1H; b-pyrrolicH); 119Sn NMR (149 MHz, CDCl3,
ACHTUNGTRENNUNG(H,Sn)=10 Hz, 1H; b-pyrrolicH), 8.93 (d, J=4.9 Hz,
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
ACHTUNGTRENNUNG
SnMe4): d=À571.3, À570.9 ppm; IR (CHCl3): n˜ =3693 (w), 3618 (w),
2964 (s), 2905 (m), 2870 (m), 1593 (s), 1477 (m), 1468 (m), 1427 (m),
1394 (m), 1364 (m), 1344 (w), 1294 (w), 1265 (s), 1248 (m), 1229 (m),
1194 (w), 1169 (w), 1150 (w), 1128 (w), 1055 (w), 1032 (w), 1020 cmÀ1
(w); UV/Vis (CHCl3): lmax (loge)=429 (5.63), 466 (5.38), 565 (4.53), 584
[5] P. R. Brotherhood, R. A.-S. Wu, P. Turner, M. J. Crossley, Chem.
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(4.38), 598 (4.36), 640 nm (4.29); MS (ESI): m/z: 1288.8 [MÀOH]2+
;
1281.5 [MÀ2OH]2+
;
MS (HR-ESI-FT/ICR): m/z: calcd for
C161H190N12O2Sn2: 1281.1614; found: 1281.1619 [MÀ2OH]2+
.
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1H NMR binding experiments: Host 5 (4 mg, 1.5 mmol) was dissolved in
CDCl3 (600 mL, ꢂ2.5 mm) in 5 mm ID NMR tubes. Standard solutions of
carboxylic acid guests 6, 7 and 8 were prepared in CDCl3 (2.00 mL,
ꢂ0.100m). Standard solutions of dicarboxylic acid guests 11 and 12 were
prepared in [D6]DMSO/CDCl3 (10:90, 2.00 mL, ꢂ0.100m). Precise molar
equivalent aliquots of guests 6–8, 11 and 12 were added to host 5 by mi-
crolitre syringe and the mixture was immediately stirred. Monitoring by
1H NMR spectroscopy then commenced (ꢀ2 min post aliquot addition)
and continued until a cessation in spectral changes indicated binding had
reached equilibrium. Further characterisation by the NMR spectroscopy
techniques indicated was then performed on these equilibrium systems.
CCDC-699388 contains the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
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Regioselective Reactivity of an Asymmetric Tetravalent Di[dihydroxotin(IV)] Bis-porphyrin
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Received: August 28, 2008
Published online: November 4, 2008
Chem. Eur. J. 2008, 14, 10967 – 10977
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www.chemeurj.org
10977