Investigations of rotamers in diaxial Sn(IV)porphyrin phenolates-towards a molecular timepiece

Article abstract of DOI:10.1016/j.tet.2008.05.127

An approach to the formation of molecular timepieces is outlined based on differentiating between rotamers in diaxial Sn(IV) porphyrin phenolates. Two models are explored in detail. The first explores how the rates of rotation of the diaxial ligands is di

Full text of DOI:10.1016/j.tet.2008.05.127

Tetrahedron 64 (2008) 8394–8401
Contents lists available at ScienceDirect
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journal homepage: www.elsevier.com/locate/tet
Investigations of rotamers in diaxial Sn(IV)porphyrin phenolatesdtowards
a molecular timepiece
*
Sheshanath V. Bhosale, Connie Chong, Craig Forsyth, Steven J. Langford , Clint P. Woodward
School of Chemistry, Monash University, Clayton Victoria 3800, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
An approach to the formation of molecular timepieces is outlined based on differentiating between
rotamers in diaxial Sn(IV) porphyrin phenolates. Two models are explored in detail. The first explores
how the rates of rotation of the diaxial ligands is discriminated based on steric hindrance of the two
porphyrin macrocycle faces at low temperature. The second model explores a ‘stopwatch’ function based
on the ligation of Ag(I) ions to a 5,15-dipyridylporphyrinato tin(IV) complex bearing 3-hydroxypyridine
ligands. The complexation inhibits rotation of the axial ligand, a result, which can be reversed by pre-
cipitation of Ag(I) using tetraethylammonium bromide. X-ray crystallography has also been used to
characterize two Ag(I) 5,15-dipyridylporphyrinato tin(IV)complexes. The two isoforms differ in their
supramolecular organization. One structure is formed through a cofacial stack linking each porphyrin by
Ag(I) coordination. The other displays a sheet-like coordination polymer structure.
Received 10 March 2008
Received in revised form 21 May 2008
Accepted 30 May 2008
Available online 5 June 2008
Keywords:
Porphyrin
Coordination
Tin(IV)
Molecular dynamics
Rotamers
Ó 2008 Elsevier Ltd. All rights reserved.
Supramolecular chemistry
Clock
1. Introduction
used in a unique memory element for molecular based computing.9
In each case, stimuli (e.g., electronic or photonic) produce a change
Mechanical devices that function as a result of the rotary motion
of some component are common within our macroscopic world.
Some familiar examples are helicopters, propellers, internal com-
bustion engines, ATP synthase and flagellae to name but a few. In
these cases, the rotation leads to mechanical work. However, rota-
tional mechanical devices can also be used for measurement, typi-
callyas aresultof(random)changes in rotary motionasderived from
environmental stimuli including thermal energy. Examples here
includegyroscopes, windvanesandwindturbines, andindeedmany
macroscopic rotors have been demonstrated at a molecular level.1,2
One class of driven rotor is the analogue clock, stopwatch or
timepiecedinstruments that measure and record time with a dial.3
In chemical terms, a timepiece is an entity that kinetically iso-
merizes through 86,400 dependent rotational isomers in a 24 h
period. The dependency lies in the defined transition from one
isomer to the next. Each of these rotational isomers, or rotamers, is
measurable and hence instructive. Molecular rotamers, particularly
displaying two states have been demonstrated elegantly in the
fields of supramolecular chemistry.4 Three example classes are the
in the system leading to the predominance of a new and different
rotamer, which is addressable by spectroscopic means.
It occurred to us that the metalloporphyrin macrocycle, func-
tionalized at all 12 outer peripheral positions could lead to a series
of rotamers that mimic the basis of an analogue timepiece (Fig. 1).
In this simple model, the four meso positions can be likened to the
3, 6, 9 and 12 positions of the timepiece. The eight remaining
12
11
1
10
2
3
9
8
4
6
fluorescent molecular rotors,5 the rotary motors of Feringa and
the catenanes of Sauvage7 and Stoddart.8 The latter system being
5
7
6
Figure 1. Metalloporphyrins bearing two axial ligands could conceivably act as a mo-
lecular timepiece in which the 12 positions around the porphyrin’s outer periphery
represent the 12 hourly positions on an analogue clock face.
* Corresponding author. Tel.: þ61 3 9905 4569; fax: þ61 3 9905 4597.
E-mail address: steven.langford@sci.monash.edu.au (S.J. Langford).
0040-4020/$ – see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2008.05.127

S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
8395
b
-pyrrolic positions, which are evenly spaced between the meso
one face over the other as a thermodynamic barrier, but without
totally impeding the desired rotational motion. One way of
approaching this is to use monofunctional meso aryl groups (e.g., 2-
methoxyphenyl units) antipodally displaced at any or all of the
5,10,15,20 positions of the porphyrin. Such functionalization,
positions, lead to the complete face numbering of the timepiece.
The hands of the timepiece could be conceived from one or two
axial ligands whose M–X–L bond angle (M¼metalloporphyrin
metal, X¼heteroatom, L¼ligand) is 90<ꢀ<180 to allow for cir-
cumnavigation of the porphyrin macrocycle and interaction with
judicial functional groups around the outer periphery.
In terms of mobility, three aspects of the metalloporphyrin are
important. The first is axial ligation, which leads to an axis upon
which each ligand can rotate about the plane of the porphyrin.
Secondly, the ability of meso groups, in particular, to rotate per-
pendicular to the porphyrin plane leading to atropisomerism. The
third level of mobility is in the porphyrin plane itself in which
ruffling or saddle formation is likely, leading to discrimination of
regions around the macrocycle on geometric grounds. Of these, the
former are more important from a design perspective.
though, leads to a mixture of a and b atropisomers, which are either
typically hard to separate or interconvert readily at ambient con-
ditions. One way around this complication is to prepare porphyrins
such as 1 bearing bulky methoxy groups at the 2,6-positions of one
meso aryl group and a single and different substituents at the an-
tipodal meso aryl group (Scheme 1). In this case, even if there is
rotation about the meso aryl bond, the same structure is generated
and atropisomerism is negated.
PhCHO
2. The concept of a molecular timepiece
HCl(aq)
N
NH
HN
H
54% yield
2
There are a number of design challenges to be addressed based
on this concept:
Br
OMe
1.
CHO
CHO
OMe
+
/ TFA / DCM / 24 h
(1) The choice of metalloporphyrin. In this case, diamagnetic met-
alloporphyrins that contain a metal ion centre with pseudo
octahedral geometry would serve as the best candidates,
though square pyramidal metal ions would also satisfy the
definition of a clock. The strength of ligand binding should be
such that ligand exchange is unlikely, especially under variable
temperature conditions (ꢁ60 ꢀC<T< 60 ꢀC) for pragmatic use.
Hence association constants (Ka values) of >105 Mꢁ1 per ligand
should be achieved.
(2) Controlling the motion of the axial ligands equates to the two
arms of the timepiece. So discrimination of the dynamics of the
two hands should be possible based on steric or electrostatic
interactions.
3
4
2. DDQ / DCM / 2 h
X
N
Y
14% yield
10% yield
9% yield
1 X = Y = OMe, Z = Br, R = H
5 X = Y = R = Z = OMe
6 X = Z = Br, Y = R = H
H
N
N
H
N
Z
R
Scheme 1.
(3) The ability to distinguish between the 12 positions of the face is
required to allow for optimum addressability. In the easiest
Porphyrin 1 was prepared as outlined in Scheme 1. Dipyrryl-
situation, distinguishing between the meso and
sitions represents the first challenge.
b
-pyrrolic po-
methane 2 was prepared in 54% yield following modification of an
existing method.12 Dipyrrylmethane 2 was chosen because of its
ease of synthesis, crystallinity and relative stability. Reaction of 2
with equimolar amounts of aldehydes 3 and 4 under acidic con-
ditions leads to a mixture of porphyrinogens, which were not iso-
lated but oxidized (DDQ) to a mixture of the corresponding
porphyrins 1, 5 and 6 in almost equivalent yields after chroma-
tography. The fact that a statistical yield of porphyrins was not
achieved indicates the interplay between the added reactivity of
ortho electron donating aldehydes and the steric bulk of 2,6-di-
substituted aldehydes in porphyrinogen formation. The judicial
choice of methoxy substituents is also pragmatic, allowing easy
separation of the complicated porphyrin mixture based by column
chromatography and by giving a set of probe protons with which to
monitor by NMR spectroscopy.
(4) Most analogue timepieces have the hands rotating in the same
direction based on the internal mechanism. Similarly, for mo-
lecular motion to be useful it must be directional, leading from
an initial state ‘A’ to a second state ‘B’, then a third ‘C’, etc. or at
the very least, readable at each state.4 Of greater challenge will
be designing systems whereby the movement of one hand is
dependent on the movement (and position) of the other.
(5) Finally, can stopwatch effects be employed, i.e., ways to control
the STOP–START motion of the hands by restricting the rotation
of one or both hands based on an external input?
In this paper, we describe two approaches that we hope advance
the concept of a molecular timepiece. Our focus is on the use of
Sn(IV) porphyrin phenolates to demonstrate this principle because
of the inherent simplicity of this form of axial ligation and the
flexibility in choice of phenolic ligand. More importantly, we know
from previous work that both the fundamental requirements and
that of concept (1) are satisfied in this class of metalloporphyrin.10
Sn(IV) porphyrins are also advantageous in that their oxophilic
character can be exploited as ways of imparting control over the
dynamics of the ligands.11
Oxidative insertion of the Sn(IV) metal centre in 1 (Scheme 2), 5
and6 usingstannouschlorideinpyridineyields the dihydroxy Sn(IV)
porphyrins 7–9, respectively, in 88–93% yield.14 Tin(IV)porphyrin
phenolates13 are the stable products of the equilibrium-based con-
densation reaction of substituted phenols with tin(IV)porphyrin
dihydroxide in an organic medium. Condensation of 7 with 10
(2 equiv) in chloroform solution at reflux for 1 h (Scheme 2) yields
the porphyrin phenolate complex in quantitative yield.
1H NMR spectra of 7 and 7$102 are shown for comparison in
Figure 2. Integration readily confirms the stoichiometry to be 2:1,
consistent with the formation of 7$102 (Scheme 1).
3. An approach to control the dynamics of axial rotamers
based on steric effects
The very large chemical shift changes (Dd) observed for the
aromatic signals associated with the axial ligands in the complex
7$102 (>4 ppm in the case of ortho protons) are a result of the
In order to differentiate the two faces of a porphyrin by
restricting rotation, an amount of steric bulk could be introduced to

8396
S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
MeO
Table 1
MeO
MeO
H
O
Ph
a
Ph
Associated changes in chemical shift (Dd
11$122 and [11$122$Ag2]$2OTf in CDCl3 solution at 300 K
) for selected probe protons for 7$102,
N
SnCl2
N
N
N
N
N
N
H
Sn
H
N
Pyridine
90 °C / 1 h
OMe
MeO
Ph
Hc
O
O
N
Ph
Hf
Br
Br
H
Hb
1
7
Br
He
OMe
Ha
Hd
Sn
Sn
/ CHCl3
Ha'
O
Br
10
R
OH
Hb'
O
OH
Ph
N
N
N
N
Sn
OH
8
MeO
k1rot
OMe
Ph
Br
7.82
11.122
MeO
O
Ph
Attributed
resonance
Dd 7$102
Dd 14$122
Dd [14$122 $Ag2]$2OTf
MeO
N
N
N
N
Sn
OMe
OH
Ph
N
N
N
MeO
Sn
Ha
Hb
ꢁ4.46
ꢁ1.51
ꢁ4.64
ꢁ1.54
d
d
d
Ph
O
N
Br
d
d
MeO
Ph
OH
0
OMe
Ha
d
d
0
Hb
d
d
MeO
9
k2rot
Hc
Hd
He
Hf
ꢁ6.25
ꢁ5.95
ꢁ2.14
ꢁ0.61
ꢁ0.27
ꢁ0.48
0.45
0.21
7.102
d
d
d
k1rot > k2rot
a
Dd
¼d
(complex)ꢁ
d(free phenol).
Scheme 2.
incrementally to 240 K did not result in the expected broadening of
ligand aromatic proton signals associated with the more hindered
set (Fig. 2c). What is seen is the separation of OMe resonances of the
ligand upon cooling, suggesting that at lower temperature, differ-
entiation between these signals is achieved. The difference in
broadness and height between the two signals may be indicative of
the processes envisaged, though this is speculative. These changes
are also accompanied by complication in the meso aryl proton
resonances. Further cooling leads to significant broadening of the
whole spectrum as a result of sample crystallization.
time-averaged orientation and proximity of these nuclei to the
porphyrin ring current (Table 1). The changes are similar to that
experienced for Sn(IV)TPP$102 in CDCl3.13 The aromatic region is
also complicated, resolving each AA0 system of the
b
-pyrrolic pro-
tons due to the symmetry invoked by the antipodal meso sub-
stitution patterns. Evident are the two groups of signals at 1.9–2.2
and 5.2–5.4 attributable to the two different axial ligands as a
d
d
result of their different environments based on 5,15-substitution
patterns. The sharpness of each 1H NMR signal at 400 MHz and
300 K indicates free rotation of the p-methoxybenzene ligands
around the outer periphery. Warming the sample to 333 K (not
shown) leads to identical spectra. Interestingly, cooling down
4. Controlling the dynamics of rotamers by metal ion
coordination
The inconclusive nature of the results obtained using a steric
approach and a means of better addressability for the different
isomers formed prompted a change in design direction. Metal ion
coordination can be used to dictate both the geometry of a ligand
complex and also as a point of dynamic control.11,15 Hence it
seemed plausible to us that the rotation of axial ligands could be
controlled in this manner to give a primitive stopwatch function or
a means of controlling the rates of rotation. This second approach
was based on the coordination of Ag(I) to a bipyridine ligand con-
stituted from the 5,15-dipyridylporphyrin 11 and 3-hydroxy-
pyridine 12 (Scheme 3). Porphyrin 13 was prepared from the
dipyrrylmethane 2 and aldehyde 14 in 28% yield using the method
already outlined in Scheme 1. Metallation of 13 (SnCl2/Py/air) leads
to the dihydroxy Sn(IV)porphyrin 11 in 91% yield. Condensation of
11 with 12 (2 equiv) in chloroform solution at reflux for 1 h yields
the porphyrin phenolate complex 11$122 in quantitative yield. In-
tegration of the 300 MHz 1H NMR spectrum readily confirms the
stoichiometry of the complex formed between 11 and 12 to be 2:1.
The sum of the porphyrin ring currents experienced by the protons
on the pyridine linker again leads to large chemical shift changes in
the 1H NMR spectrum (Table 1). The changes are similar to that
experienced for Sn(IV)TPP$122 in CDCl3, therefore, we can conclude
that no apparent interaction between the ligated and meso pyri-
dines occurs.13 Again, the sharpness of each 1H NMR signal at
400 MHz and 300 K indicates free rotation of the axial pyridine
ligands around the outer periphery (Fig. 3).
Figure 2. The 400 MHz 1H NMR spectra of (a) 7 at 300 K and 7$102 in CDCl3 at (b)
300 K and (c) 240 K;
*
represents the p-methoxyphenol internal standard.

S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
8397
N
CHO
β-pyrrolic H
1.
N
Ph
N
N
N
N
H
H
O
Ph
N
14
N
N
N
N
N
N
Sn
NH
HN
TFA / DCM / 24 h
(a)
Ph
N
2. DDQ, 2 h
Ph
O
2
13
28% yield
meso ArH
N
SnCl2 / pyridine
90 °C / 1 h
“Go”
N
Hf
N
H
O
Ph
O
Ph
N
12
OH
N
N
N
N
N
N
Sn
O
N
N
N
Sn
N
He
CHCl3
Ph
N
Ph
O
H
f
e
11.12
N
2
11
g
N
a
b
c
O
"GO"
Ph
h
N
d
N
N
N
N
Sn
O
(b)
/ MeCN
Et4NBr
N
Ph
= Ag+
N
N
“Stop”
O
Ph
N
N
N
N
N
Sn
O
N
Ph
N
2+
[11.12 ]Ag 2 OTf
(c)
2
2
"STOP"
Scheme 3.
“Go”
4.1. NMR studies on metal binding
Addition of 2.2 equiv of AgOTf to a solution of 11$122 in MeCN-d3
leads to a broadening of all signals and a concomitant change in
chemical shifts for probe protons (Table 1) consistent with a change
in the normal dynamics associated with 11$122 at 300 K and
300 MHz (Fig. 3 a,b). The nature of this dynamics can be in-
vestigated further by variable temperature NMR studies at
400 MHz (Fig. 4). Cooling the solution from 300 to 293 K (Fig. 4f)
leads to a sharpening of signals He and Hf, and further resolution
into three unique forms in the ratio 1:2:1,16 which persist to 243 K.
Warming the solution to 323 K leads to a broadening of these probe
protons and partial coalescence inferring restricted rotation on the
NMR timescale being relaxed as a result of the elevation in tem-
9
8
7
6
ppm
Figure 3. Partial 300 MHz 1H NMR spectra of (a) 11$122 at 300 K in CDCl3, (b) after the
addition of 2.2 equiv of AgOTf in MeCN and (c) after the addition of 2.2 equiv of Et4NBr.
The mode of binding shown in (b) is tentatively assigned based on NMR effects.
case, the phenolate groups lie in an anti orientation with respect to
each other with an Sn–O–C angle of 122.2ꢀ.
In the case of Figure 5, a pseudo one-dimensional array is
formed along the crystallographic a-axis in which the Ag(I) ion
adopts
a
distorted tetrahedral geometry (N–Ag–N angles¼
perature. Addition of 2.2 equiv of Et4NBr to
a solution of
113.1<ꢀ<120.6) linking porphyrin units in a cofacial manner
[11$122$Ag2]$2OTf leads to decomplexation by the precipitation of
AgBr and generation of the original spectrum of 11$122 (Fig. 3c).
Similar addition of AgOTf to a MeCN-d3 solution of 7$102, which is
not capable of formal coordination, leads to no appreciable effect. In
primitive terms, then, the results obtained for the addition of Agþ to
11$122 lead to a situation of ‘STOP–GO’ control.
0
through a bipyridine arrangement defined as Nmeso–Ag(I)–Nligand
(Fig. 5b). The tetrahedral geometry is further defined by a MeCN
solvent molecule (Ag(1)–N(6) distance¼2.515(10) Å) and a liber-
ated 3-hydroxypyridine ligand (Ag(1)–N(5) distance¼2.290(8) Å).
The Ag–Nligand (2.266(8) Å) and Ag–Nmeso bonding distances
(2.313(8) Å) are indifferent, which is surprising based on what one
would reasonable assume is a negative inductive effect of the Sn(IV)
centre, weakening the Ag–Nligand bond. Solvent molecules (PhMe)
sit within the voids between stacks in a unidirectional manner
along the b-axis direction while the TfOꢁ counterions lie in a cor-
rugated fashion in the direction of the c-axis.
Figure 6 shows the different isoforms, whose structure is
dominated by a two-dimensional sheet array in which a zig-zag
arrangement of porphyrin units is seen. The connectivity between
porphyrins lies in two directions and is linked by Ag(I) ions in
a distorted tetrahedral geometry (N–Ag–N angles¼99.6<ꢀ<122.6)
bearing three pyridine ligands (1ꢃmeso and 2ꢃligand) and a MeCN
solvent molecule. The Ag–N bond distances are commensurate
with the first structure. Disordered solvent (PhMe) and counterions
4.2. X-ray crystallographic studies on Ag(I) complexes
Dark red single crystals of 11$122 suitable for X-ray crystallo-
graphy were grown by treatment of 11$122 with silver(I) trifluoro-
acetic acid in a 5:1 mixture of toluene and acetonitrile. Two crystal
forms were isolated and subjected to X-ray crystallographic anal-
ysis. The two molecular structural isoforms of [11$122$Ag2]$2OTf
and the different macromolecular structures of the arrays formed
are shown in Figures 5 and 6. In both cases, the metal centres of 11
are coordinated octahedrally via the four inner peripheral nitrogens
of the porphyrin ligand and axially via the two phenolic oxygens
(av. bond lengths: Sn–N¼2.09ꢂ0.2 Å and Sn–O¼2.05 Å). In each

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S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
He
7
Hf
9
8
6
ppm
Figure 4. Variable temperature 400 MHz 1H NMR spectra of [11$122$Ag2]$2OTf in
CDCl3 (a) 243, (b) 253, (c) 263, (d) 273, (e) 283, (f) 293, (g) 313 and (f) 323 K.
sit in free spaces between the sheets with some measure of
distortion.
The fact that in both cases an intermolecular interaction is ob-
served over an intramolecular Ag(I) complex may be the result of
both crystal packing forces and the method of preparation. As such,
we do not believe that the coordination behaviour in the solid state
is an accurate reflection of the Ag(I) binding mode in the NMR
experiments conducted.
Figure 5. (a) Stick representation of part of the linear chain showing the Sn(IV) en-
vironment in [11$122$Ag2]. Only one disordered component of the phenyl ring is
shown. (b) View of an extended portion of the chain showing the Ag(I) coordination
environment. Hydrogen atoms, lattice CF3SOꢁ3 anions and PhMe have been omitted for
clarity.
5. Outlook
An approach that mimics some aspects of the motion and con-
trol of a clock is outlined based on differentiating between rotamers
in diaxial Sn(IV) porphyrin phenolates. Of the two models explored,
exploitation of coordination or electrostatic interactions between
the axial ligands and the meso positions of the porphyrin macro-
cycle (in the first instance) seems like a more plausible approach.
This effect can be exploited in both a primitive stopwatch and to
differentiate rotation of the axial ligands. Diffusion control is
a complication for a pragmatic system though it has been used to
demonstrate the principle in this case. The concern underlying
a steric approach is the balance between restriction and impedance
of motion. X-ray crystallography has also been used to characterize
two Ag(I) 5,15-dipyridylporphyrinato tin(IV)complexes, though the
coordination polymer topology obtained does not support the
premise of the solution based results ideally. We are now in the
process of developing new systems based on these results to ad-
vance this challenging area further.
6. Experimental section
6.1. General methods
All starting materials were purchased from Aldrich and used as-
received. 1H NMR spectra were recorded using the Bruker DPX 300
spectrometer operating at 300 and 75 MHz, and reference against

S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
8399
Figure 6. (a) Stick representation of part of the two-dimensional sheet of [11$122$Ag2] showing the two unique Sn environments. Hydrogen atoms and the lattice CF3SO3 anion and
PhMe have been omitted for clarity. (b) View of an extended portion of the two-dimensional sheet.
residual solvent (CDCl3). Variable temperature studies were carried
out on a Bruker DRX 400 spectrometer. High resolution mass
spectra (HRMS) were recorded on an Agilent 6220 Accurate-Mass
TOF LC/MS in positive-ion mode. X-ray crystallographic measure-
ments were carried out using a Bruker Nonius X8 Apex II CCD
structural analyses have been deposited with the Cambridge
Crystallographic Data Centre, CCDC 680785 and 680786 for struc-
tures of [11$122$Ag2]$2OTf. Copies of this information may be
obtained free of charge from The Director, CCDC, 12 Union Road,
Cambridge, CB2 1EZ, UK (fax: þ44 1223 336033; e-mail: depos-
it@ccdc.cam.ac.uk or http://www.ccdc.cam.ac.uk).
diffractometer using graphite monochromatized Mo K
a
radiation
[
l
(Mo K
a
)¼0.71069 Å] at 123.0ꢂ1 K. The cell constants and the
orientation matrices for data collection were obtained from a least
squares refinements. Structural solutions were obtained by direct
methods (SHELXS-97)17a followed by successive Fourier-difference
methods, and refined by full matrix least squares on Fo2bsd (SHELXL-
97).17b The data were corrected for Lorentz and polarization effects
and processed using Nonius software. Crystallographic data for the
6.2. Synthetic procedures
6.2.1. 5-Phenyl-2,20-dipyrrylmethane 212
A solution of benzaldehyde (2.0 mL, 20 mmol) and 20-fold excess
pyrrole (28 mL, 400 mmol) were combined in a three neck round-
bottom flask. The mixture was stirred in the dark at room

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S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
temperature under argon for 10 min followed by dropwise addition
of trifluoroacetic acid (TFA, 0.5 mL). Completion of reaction was
monitored by TLC; after 30 min stirring at room temperature the
excess pyrrole was distilled and recovered. The crude product was
purified by column chromatography on flash silica (40–60 mesh)
elutingwith CH2Cl2/hexane/Et3Nin the ratioof 100:50:1. Presenceof
dipyrrole was checked by TLC, under UV light, with dipyrrylmethane
spots dark brown in appearance. The crude product was recrystal-
lized from ethanol, giving 2 (54%, 2.78 g) as cream coloured crystals.
resulting solution was stirred for 1 h. Water (30 mL) was added, and
the solid collected by vacuum filtration, washed with water, and
dried by suction filtration. The filter cake was digested in situ with
chloroform (3ꢃ20 mL), which dissolved the purple product, leaving
a brown residue of tin salts. The filtrate was dried over anhydrous
Na2SO4 and concentrated to w5 mL on a rotary evaporator. A col-
umn of alumina (20 g, neutral, activity V) was prepared in CHCl3
and the concentrate was applied to the column and washed onto
the alumina with a further 20 mL of solvent. The product was
eluted with CHCl3 and the purple band was concentrated (w3–
5 mL). Hexane (20 mL) was carefully layered on top of the CHCl3
solution, the flask stoppered and the mixture left to crystallize. The
crystals were filtered, washed with hexane and dried in vacuum
desiccators to give purple crystalline solid.
Mp101–102 ꢀC (lit.:12 100–101 ꢀC);1HNMR(CDCl3, 300 MHz):
(br s, 2H), 7.22–7.32 (m, 5H), 6.71 (m, 2H), 6.14–6.18 (m, 2H), 5.92 (s,
d 7.92
2H), 5.48 (s, 1H); 13C NMR (75 MHz, CDCl3):
d 142.6, 133.2, 129.3,
128.9, 127.5, 117.7, 109.3, 107.8, 44.6. ESI-MS m/z 222 (MþH)þ.
6.2.2. Synthesis of porphyrins 1, 5 and 6
5-Phenyl-2,20-dipyrrylmethane 2 (0.70 g, 3.2 mmol), 2-bromo-
benzaldehyde (0.299 g,1.6 mmol) and 2,6-dimethoxybenzaldehyde
(0.265 g, 1.6 mmol) were dissolved in dichloromethane (400 mL) in
a 1 L two neck round-bottom flask. Reaction mixture was stirred in
the dark, under a slow stream of nitrogen for 30 min, and then
6.2.3.1. Preparation of porphyrin 7. Yield: 93%, purple crystals, mp
>350 ꢀC; 1H NMR (CDCl3, 300 MHz):
d
9.11 (m, 6H), 8.95 (m, 2H),
8.38–8.33 (m, 6H), 7.82 (br s, 1H), 7.81–7.76 (m, 8H), 7.08 (m, 2H),
3.58 (s, 3H), 3.54 (s, 3H); 13C NMR (75 MHz, CDCl3):
160.7, 160.5,
d
152.9, 149.8, 147.6, 146.0, 140.8, 134.8, 134.6, 128.0, 126.6, 126.5,
118.0, 115.4, 114.4, 111.6, 111.5, 104.3, 104.2, 55.9, 55.8, 55.7, 55.2;
trifluoroacetic acid (150 mL) was added via syringe in one portion.
The reaction mixture was stirred for 24 h at room temperature,
resulting in a dark red solution. A solution of 2,3-dichloro-5,6-
dicyanobenzoquinone (800 mg) in 50 mL of dichloromethane was
then added in one portion and the mixture stirred for an additional
2 h. The solvent was evaporated to dryness and the crude mixture
was purified by column chromatography (SiO2, DCM/acetone
(98:2)). The fast moving fraction was identified as porphyrin 6, the
second band was identified as the target porphyrin 1 and the final
band was identified as porphyrin 5.
UV–vis (CHCl3) lmax (nm, log
3
)¼429 (5.54), 561 (4.26), 600 nm
(3.98); IR (nmax, CHCl3): 3627, 3043, 2946, 2837, 1485, 1422, 1260,
1213 cmꢁ1; HRMS-ESI (MeOH) m/z obsd 901.0826, calcd 901.0831
(C47H34BrN4O3Sn (Mꢁ2OHþOMe)þ).
6.2.3.2. Preparation of porphyrin 8. Yield: 89%, purple crystals, mp
>350 ꢀC; 1H NMR (CDCl3, 300 MHz):
d
9.16–9.09 (m, 4H), 9.00–8.94
(m, 4H), 8.37 (m, 6H), 7.83 (m, 2H), 7.82–7.79 (m, 10H); 13C NMR
(75 MHz, CDCl3): 155.9, 155.7, 154.6, 154.3, 141.8, 141.3, 135.5,
135.2, 133.4, 132.7, 132.0, 130.9, 128.8, 128.7, 127.3, 127.3, 126.4; UV–
d
6.2.2.1. Porphyrin 6. Yield: 9%, purple solid, mp >300 ꢀC; 1H NMR
vis (CHCl3) lmax (nm, log
3
)¼429 (5.48), 561 (4.15), 600 nm (3.88);
(CDCl3, 300 MHz):
8.31–8.12 (m, 6H), 8.01 (m, 4H), 7.65–7.79 (m, 8H), ꢁ2.93 (s, 2H);
13C NMR (75 MHz, CDCl3):
160.8, 160.3, 144.5, 142.9, 142.6, 134.8,
132.2, 130.5, 130.2, 128.0, 127.8, 126.9, 126.8, 126.0, 120.8, 119.4,
118.6; UV–vis (CHCl3) lmax (nm, log
)¼419 (5.43), 514 (4.15), 549
(3.85), 589 (3.85), 646 nm (3.76); IR (nmax, CHCl3): 3695, 3035, 1471,
d
8.89 (d, J¼4.5 Hz, 4H), 8.74 (d, J¼5.1 Hz, 4H),
IR (nmax, CHCl3): 3631, 3053, 2986, 1422 cmꢁ1; HRMS-ESI m/z obsd
903.9424, calcd 903.9495 (C44H26Br2N4OSn (MꢁOH)þ).
d
6.2.3.3. Preparation of porphyrin 9. Yield: 88%, purple crystals, mp
3
>350 ꢀC. 1H NMR (CDCl3, 300 MHz):
d
8.75 (br s, 8H), 8.37–8.32 (m,
4H), 7.80–7.76 (m, 8H), 7.08 (s, 2H), 7.02 (s, 2H), 3.53 (s, 12H); 13C
NMR (75 MHz, CDCl3): 161.3, 160.9,153.2,150.8,150.1, 148.0, 147.5,
147.4, 141.4, 135.0, 132.4, 131.7, 128.0, 126.7, 119.3, 118.5, 115.7, 114.7,
111.7, 104.6, 56.3, 56.1; UV–vis (CHCl3) lmax (nm, log
1005 cmꢁ1
;
HRMS-ESI m/z obsd 771.0751, calcd 771.0759
d
(C44H29Br2N4 (MþH)þ).
3)¼428 (5.46),
6.2.2.2. Porphyrin 1. Yield: 14%, purple solid, mp >300 ꢀC; 1H NMR
560 (4.16), 600 nm (3.94); IR (nmax, CHCl3): 3585, 3026, 2932, 2840,
1472,1422,1252,1205 cmꢁ1; HRMS-ESI (MeOH) m/z obsd 883.1925,
calcd 883.1942 (C49H39N4O5Sn (Mꢁ2OHþOMe)þ).
(CDCl3, 300 MHz):
d
8.47 (m, 6H), 8.29 (d, J¼5.1 Hz, 2H), 7.90–7.81
(m, 6H), 7.64 (br s, 1H), 7.45–7.32 (m, 8H), 6.76 (m, 2H), 3.21 (s, 3H),
3.14 (s, 3H), ꢁ3.05 (br s, 2H); 13C NMR (75 MHz, CDCl3):
d 160.7,
160.6, 143.1, 142.3, 135.2, 134.6, 132.0, 130.3, 129.8, 127.7, 127.6,
6.2.4. Preparation of Sn(IV)(bis-p-methoxyphenolate) 7$102
The Sn(IV)porphyrin(OH)2 complex 7 was added to 2 equiv of p-
methoxyphenol 10 in base washed CDCl3 and the solution was left
to stir at 60 ꢀC overnight. Yield: quantitative; 1H NMR (CDCl3,
127.5, 125.8, 120.0, 119.6, 117.9, 112.9, 104.4; UV–vis (CHCl3) lmax
(nm, log
3
)¼419 (5.53), 515 (4.26), 550 (3.88), 588 (3.90), 642 nm
(3.73); IR (nmax, CHCl3): 3694, 3024, 3009, 2838, 1471, 1432, 1349,
1251, 1009 cmꢁ1; HRMS-ESI m/z obsd 753.1856, calcd 753.1865
(C46H33BrN4O2 (MþH)þ).
300 MHz):
d
9.13 (d, J¼7.9 Hz,
b
-pyrrolic-HSn, 2H), 9.06 (d, J¼8.2 Hz,
-pyrrolic-HSn, 2H), 8.79 (d,
b-pyrrolic-HSn, 2H), 8.64–8.60 (m, meso-ArHSn, 3H), 7.91–
b
-pyrrolic-HSn, 2H), 8.91 (d, J¼7.9 Hz,
b
J¼8.2 Hz,
6.2.2.3. Porphyrin 5. Yield: 10%, purple solid, mp >300 ꢀC; 1H NMR
7.62 (m, meso-ArHSn, 10H), 7.37 (m, meso-ArHSn, 4H), 5.25 (d,
J¼8.8 Hz, ArH, 2H), 5.18 (d, J¼8.3 Hz, ArH, 2H), 3.61 (s, 6H), 3.54 (s,
6H), 2.17 (d, J¼8.8 Hz, 2H), 1.92 (d, J¼8.9 Hz, ArH, 2H); 13C NMR
(CDCl3, 300 MHz):
d 8.75 (s, 8H), 8.18 (m, 4H), 7.71 (m, 8H), 6.98 (s,
4H), 3.5 (s, 12H), ꢁ3.13 (s, 2H); 13C NMR (75 MHz, CDCl3):
d 160.9,
142.8, 134.8, 139.4, 127.7, 126.8, 121.8, 104.7, 104.6, 56.4; UV–vis
(75 MHz, CDCl3): d 160.1, 147.0, 146.4, 141.0, 135.5, 135.4, 135.3,
(CHCl3) lmax (nm, log
3
)¼419 (5.47), 515 (4.21), 548 (3.87), 589
135.2, 127.4, 127.4, 127.3, 126.5, 121.7, 121.4, 120.8, 119.8, 118.2, 115.3,
114.3, 111.4, 111.1, 105.6, 104.7, 56.2, 55.9, 55.8, 55.8, 55.7, 55.2; UV–
(3.89), 643 nm (3.77); IR (nmax, CHCl3): 3695, 3053, 3018, 2986,
2838, 1472, 1422, 1349, 1252, 1224, 1005 cmꢁ1; HRMS-ESI m/z obsd
735.2965, calcd 735.2971 (C48H38N4O4 (MþH)þ).
vis (CHCl3) lmax (nm, log
IR (nmax
3
)¼429 (5.56), 561 (4.28), 600 nm (3.89);
,
CHCl3): 3024, 2931, 2856, 1472, 1422, 1344, 1260,
1219 cmꢁ1; HRMS-ESI (MeOH) m/z obsd 901.0830, calcd 901.0831
(C47H34BrN4O3Sn (Mꢁ102þOMe)þ).
6.2.3. Typical experimental procedure for the preparation of tin
complexes
The porphyrin (50 mg) and powdered SnCl2$2H2O (40 mg) were
stirred and refluxed in pyridine (20 mL) for 1 h. The solution was
cooled to 50 ꢀC, then concentrated NH3 (5 mL) was added, and the
6.2.5. 5,15-Bisphenyl-10,20-bis(2-pyridyl)porphyrin 13
A round-bottom flask fitted with a rubber septum was charged
with freshly distilled dichloromethane (320 mL) and purged with

S.V. Bhosale et al. / Tetrahedron 64 (2008) 8394–8401
8401
bubbling Ar for 20 min. 5-Phenyl-2,20-dipyrrylmethane 2 (0.65 g,
2.93 mmol) was added along with pyridine-2-carbaldehyde 14
(278 L, 2.93 mmol) and the resulting solution was stirred for
30 min under Ar. Trifluoroacetic acid (150 L) was added via sy-
(CHCl3) lmax (nm, log
(
3
)¼426 (5.43), 560 (4.16), 599 nm (3.91); IR
nmax, CHCl3): 3024, 3005, 1462, 1428, 1005 cmꢁ1; HRMS-ESI
m
(MeOH) m/z obsd 765.1424, calcd 765.1425 (C43H29N6OSn
m
(Mꢁ122þOMe)þ).
ringe in one portion. The reaction mixture was stirred for 24 h at
room temperature, resulting in a dark red solution. A solution of
2,3-dichloro-5,6-dicyanobenzoquinone (800 mg) in 50 mL of
dichloromethane was then added in one portion and the mixture
stirred for an additional 2 h. The solvent was evaporated to dryness
and the crude mixture was purified by flash chromatography
(DCM/methanol, 98.5:1.5), with the desired porphyrin 13 obtained
in 28% yield (249 mg) as a purple solid. Mp >350 ꢀC. 1H NMR
6.2.8. Ag(I) NMR experiments with 11$122
To a solution of Sn(IV)(bispyridinephenolate) 11$122 in CDCl3
was added 2.2 equiv of silver triflate (AgOTf) in CH3CN and the
mixture stirred at room temperature for 2 h. The solvent was
evaporated, the residual purple solid dissolved in CDCl3 and their
1H NMR spectra recorded at various temperatures. The addition of
2 equiv of Et4NBr resulting in precipitation of AgBr leads to the
spectrum corresponding to that of the free ligand Sn(IV)(bis-
pyridinephenolate) 11$122.
(CDCl3, 300 MHz):
d
ꢁ2.80 (s, 2H), 7.74 (m, 8H), 8.10 (t, J¼6.9 Hz,
2H), 8.23 (m, 6H), 8.88 (d, J¼9.9 Hz, 8H), 9.14 (d, J¼3.9 Hz, 2H); 13
C
NMR (75 MHz, CDCl3): d 161.1, 148.9, 142.4, 135.1, 135.0, 130.6, 128.0,
127.0, 122.7, 120.7, 118.9; UV–vis (CHCl3): lmax (nm, log
3
)¼407
Acknowledgements
(5.28), 502 (4.01), 536 (3.70), 576 (3.63), 628 (3.35); IR (nmax
,
CHCl3): 3696, 3023, 1464, 1427, 1005 cmꢁ1
; HRMS-ESI obsd
This work was funded under the Australian Research Council’s
Discovery project (DP0878220). One of the authors (S.J.L.) wishes to
acknowledge Professor J. Fraser Stoddart for providing inspiration
over a decade ago to always think ‘outside the square’.
617.2454, calcd 617.2448 (C42H28N6 (MþH)þ).
6.2.6. Preparation of [5,15-bisphenyl-10,20-bis(2-pyridyl)-
porphyrinato]tin(IV)dihydroxide (11)
5,15-Bisphenyl-10,20-bis(2-pyridyl)porphyrin 11 (50 mg) and
powdered SnCl2$2H2O (40 mg) were stirred and refluxed in pyri-
dine (5 mL) for 1 h. The solution was cooled to 50 ꢀC, then con-
centrated NH3 (2.5 mL) was added and heated with stirring for 1 h.
Water (30 mL) was added, and the solid collected by vacuum fil-
tration, washed with water and dried by suction filtration. The filter
cake was digested in situ with chloroform (3ꢃ10 mL), which dis-
solved the purple product, leaving a brown residue of tin salts. The
filtrate was dried over anhydrous Na2SO4 and concentrated to
w10 mL on a rotary evaporator. A column of alumina (100 g, neu-
tral, activity V) was prepared in CHCl3 and the concentrate was
applied to the column and eluted with CHCl3, with the purple band
concentrated (w3–5 mL) by rotary evaporation. Hexane (20 mL)
was carefully layered on top of the CHCl3 solution, the flask stop-
pered and the mixture left to crystallize. The purple crystals of 14
were filtered, washed with hexane and dried in vacuum desiccator.
References and notes
1. (a) Kottas, G. S.; Clarke, L. I.; Horinek, D.; Michl, J. Chem. Rev. 2005, 105, 1281; (b)
Khuong, T.-A. V.; Nunez, J. E.; Godinez, C. E.; Garcia-Garibay, M. A. Acc. Chem.
Res. 2006, 39, 413; (c) Wintjes, N.; Bonifazi, D.; Cheng, F.; Kiebele, A.; Sto¨hr, M.;
Jung, T.; Spillmann, H.; Diederich, F. Angew. Chem., Int. Ed. 2007, 46, 4089.
2. Molecular Devices and Machines; Balzani, V., Venturi, M., Credi, A., Eds.; Wiley
VCH: Weinheim, 2003.
3. The term clock is a more general one for the sake of discussion, however,
a quick search of chemical literature confuses the term with kinetic events such
as the iodine clock. Hence the term timepiece is employed. Definitions come
from EncartaÒ World English Dictionary Ó 1999 Microsoft Corporation.
4. (a) Magnera, T. F.; Michl, J. Top. Curr. Chem. 2005, 262, 63; (b) Kelly, T. R.; De
Silva, H.; Silva, R. A. Nature 1999, 401, 150; (c) Kelly, T. R. Acc. Chem. Res. 2001, 34,
514; (d) Kay, E. R.; Leigh, D. A.; Zerbetto, F. Angew. Chem., Int. Ed. 2007, 46, 72.
5. (a) Haidekker, M. A.; Akers, W. J.; Fischer, D.; Theodorakis, E. A. Opt. Lett. 2006,
31, 2529; (b) Ghiggino, K. P.; Hutchison, J. A.; Langford, S. J.; Latter, M. J.; Lee, M.
A. P.; Lowenstern, P. R. Adv. Funct. Mater. 2007, 17, 805; (c) Haidekker, M. A.;
Brady, T. P.; Lichlyter, D.; Theodorakis, E. A. J. Am. Chem. Soc. 2006, 128, 398.
6. Feringa, B. Acc. Chem. Res. 2001, 34, 504.
7. Collin, J.-P.; Dietrich-Buchecker, C.; Gavin
J.-P. Acc. Chem. Res. 2001, 34, 477.
8. Saha, S.; Stoddart, J. F. Chem. Soc. Rev. 2007, 36, 77.
9. Flood, A. H.; Peters, A. J.; Vignon, S. A.; Steuerman, D. W.; Tseng, H.-R.; Kang, S.;
Heath, J. R.; Stoddart, J. F. Chem.dEur. J. 2004, 10, 6558.
10. (a) Hawley, J. C.; Bampos, N.; Sanders, J. K. M.; Abraham, R. J. Chem. Commun.
1998, 661; (b) Langford, S. J.; Lau, V.-L.; Lee, M. A. P.; Lygris, E. J. Porphyrins
Phthalocyanines 2002, 6, 748.
11. A molecular gate based on a silver lock was recently reported. This system has
the potential to function as a clock using the definitions invoked in this paper,
see: Guenet, A.; Graf, E.; Kyritsakas, N.; Allouche, L.; Hosseini, M. W. Chem.
Commun. 2007, 2935.
Yield (57 mg, 91%); mp >350 ꢀC. 1H NMR (300 MHz, CDCl3):
(s, 8H,
-pyrrolic-HSn), 7.75–7.86 (m, 8H), 8.14–8.63 (m, 8H); 13C
NMR (75 MHz, CDCl3): 159.5, 159.2, 149.3, 149.1, 146.6, 140.7,
135.5, 135.4, 135.2, 133.4, 133.3, 132.4, 130.9, 128.6, 127.3, 121.7,
121.6, 119.8; UV–vis (CHCl3) lmax (nm, log
d
9.21
˜a, P.; Jiminez-Molero, M. C.; Sauvage,
b
d
3)¼426 (5.48), 560 (4.22),
600 nm (3.86); IR (nmax, CHCl3): 3631, 3052, 3023, 2986, 1461, 1422,
1226, 1213 cmꢁ1; HRMS-ESI (MeOH) m/z obsd 765.1419, calcd
765.1425 (C43H29N6OSn (Mꢁ2OHþOMe)þ).
12. Littler, B. J.; Miller, M. A.; Hung, C.-H.; Wagner, R. W.; O’Shea, D. F.; Boyle, P. D.;
Lindsey, J. S. J. Org. Chem. 1999, 64, 1391.
13. Langford, S. J.; Lee, M. A.-P.; Macfarlane, K. J.; Weigold, J. A. J. Inclusion Phenom.
2001, 41, 135.
14. Arnold, D. P. J. Chem. Educ. 1988, 65, 1111.
15. Sato, D.; Akutagawa, T.; Takeda, S.; Noro, S.-I.; Nakamura, T. Inorg. Chem. 2007,
46, 363.
16. We are assuming that intramolecular complexation is faster than in-
termolecular complexation. The nature of the different magnetic environments
is not known at this point though it could be as a result of different stereo-
isomeric forms induced by complexation.
17. (a) Sheldrick, G. M. SHELXS-97; University of Go¨ttingen: 1997; (b) Sheldrick, G. M.
SHELXL-97; University of Go¨ttingen: Go¨ttingen, 1997.
6.2.7. Preparation of Sn(IV)(bispyridinephenolate) 11$122
Sn(IV) porphyrin 11 (28.4 mg, 3.45
of 3-hydroxypyridine (6.5 mg, 6.83 mol) in base washed CDCl3
and the solution was left to stir at 60 ꢀC overnight. Yield: quanti-
tative, purple solid, mp >350 ꢀC. 1H NMR (300 MHz, CDCl3):
9.16
(s, 8H, -pyrrolic-HSn), 7.86 (m, meso-ArHSn, 8H), 8.13–8.48 (m, 8H),
7.00 (d, J¼1.3 Hz, 2H, ArH), 5.57 (m, 2H, ArH), 3.25 (d, J¼2.7 Hz, 2H,
ArH), 2.17 (m, 2H, ArH); 13C NMR (75 MHz, CDCl3):
213.0, 159.8,
mmol) was added to 2 equiv
m
d
b
d
159.5, 150.3, 149.6, 147.4, 146.8, 140.9, 139.1, 138.7, 135.2, 133.4,
133.2, 131.4, 130.9, 128.6, 124.2, 122.4, 121.7, 119.8, 113.4; UV–vis