A supramolecular array assembled via the complementary binding properties of ruthenium(II) and tin(IV) porphyrins

Article abstract of DOI:10.1039/b010148f

A new porphyrin trimer has been self-assembled by employing the mutually non-interfering coordination properties of the ruthenium(II) and tin(IV) centers to form a multi-metal array. The photo- and electro-chemical properties of this array are also reported.

Full text of DOI:10.1039/b010148f

View Article Online / Journal Homepage / Table of Contents for this issue
A supramolecular array assembled via the complementary binding
properties of ruthenium(II) and tin(IV) porphyrins
Bhaskar G. Maiya,*a Nick Bampos,*b A. Asok Kumar,a Neil Feederb and
Jeremy K. M. Sandersb
a School of Chemistry, University of Hyderabad, Hyderabad, 500046, India
b University Chemical L aboratory, University of Cambridge, L ensÐeld Road, Cambridge, UK
CB2 1EW
Received (in Cambridge, UK) 19th December 2000, Accepted 20th April 2001
First published as an Advance Article on the web 21st May 2001
A new porphyrin trimer has been self-assembled by employing the mutually non-interfering coordination properties
of the ruthenium(II) and tin(IV) centers to form a multi-metal array. The photo- and electro-chemical properties of
this array are also reported.
both step-wise and one-pot methods. In the former method,
the pyridine-bound ruthenium(II) porphyrin 2 was assembled
Introduction
Ðrst with the “pendantÏ carboxylic acid functionality being
readily available for further coordination. The acid itself is not
soluble in the organic solvents in which the porphyrin is
soluble but the ruthenium porphyrin coordination to the
pyridyl group solubilises the acid and allows us to obtain
solutions of known concentration; the reaction reaches com-
pletion over a couple of hours. In the second preparation, two
equivalents of 2 were reacted with the dihydroxy tin(IV)
porphyrin 3 to obtain 4 in essentially quantitative yield. The
array was also prepared in one pot by simply mixing 1, 3 and
pyridine-4-carboxylic acid in the appropriate stoichiometry.
Attempts were made to grow single crystals of 2 and 4. As
yet, crystals of 4 suitable for X-ray di†raction have not been
isolated, but complex 2 crystallized out in the P1 space group.
The structure of 2 thus obtained clearly shows the H-bonded
dimeric nature of the complex with an OÈHÉ É ÉO distance of
1.843 A (Fig. 1). Both the coordinated pyridyl ligand (RuÈN
2.201 A) and CO [RuÈCO 1.839(4) A] are near perpendicular
to the porphyrin plane. Therefore, self-assembly of array 4 can
be supposed to proceed with the breaking of these weak
H-bonds prior to the competing SnÈO interaction.
The structure of 4 was probed in solution by 1H NMR
spectroscopy. The near perpendicular juxtaposition of the
axial pyridine ligand with respect to the plane of the ruthe-
nium porphyrin, as evident in the crystal structure of 2
(dihedral angle approx 90¡), is expected to be retained in array
4. As such, the a and b protons of the bound pyridine in 2 are
shielded by the porphyrin ring-current5 and resonate at 1.76
and 5.79 ppm, respectively (identiÐed by COSY and NOESY
experiments). The corresponding resonances in array 4 are
seen to be further shifted upÐeld to 2.67 and 0.50 ppm respec-
tively due to the additional shielding e†ect exerted by the
tin(IV) porphyrin, as expected. Self-assembly of 4 is also
accompanied by disappearance of the axial hydroxy proton
resonances ([7.49 ppm) of 3.
The recent resurgence in the coordination chemistry of
metallo-porphyrins is largely due to the realization that
metalÈligand interactions provide a convenient means of con-
structing functional supramolecular arrays.1 The range of
metals and ligands and also the multitude of interactions
between them permit what is otherwise a synthetically cum-
bersome modulation of the photophysical and electrochemical
properties of the resulting supramolecular species.2 To this
end, the well known binding of zinc(II) and ruthenium(II) to
nitrogen and that of tin(IV) to oxygen has recently been
employed to construct heterometallic porphyrin oligomers.3
These oligomers were designed to exploit the complementary
geometries and cooperative binding properties of the tailor-
made metalloporphyrin building blocks. Building on these
results we set out to determine if this theme can be “ampliÐedÏ
to generate self-assembled, axially directed porphyrin arrays
without relying on extended and inefficient syntheses of
designed host and guest precursors. Our e†orts in this direc-
tion resulted in the design of a prototype trimeric array 4, the
construction of which relies on the spontaneous self-assembly
driven by the independent coordination properties of the
ruthenium(II) and tin(IV) centers (Scheme 1). Unlike the case
with the previously reported heterometallic porphyrins men-
tioned above which are characterized by the incorporation of
the nitrogen and oxygen donor functionalities into their
superstructures, the complementary binding achieved here is
by an external, bifunctional ligand leading to the self-assembly
of 4. In addition to describing aspects related to the design,
construction and spectral characterization of 4, we also report
the photophysical and electrochemical properties of this novel
trimeric array.
Results and discussion
The UV/vis spectrum of the trimer is essentially a super-
position of the spectra of its constituent components with the
molar absorption coefficients at the peak maxima being close
to the sum of those of 2 and 3. This observation, coupled with
The monomeric precursors employed in this study; the
Ðve-coordinated
(5,10,15,20-tetrakis(3,5-di-tert-butylphenyl)
porphyrinato)ruthenium(II) carbonyl,
1
and the six-co-
ordinated (5,10,15,20-tetrakis(4-methylphenyl)porphyrinato)-
tin(IV) dihydroxide, 3 porphyrins, were prepared by modiÐ-
cation of known procedures.4 Trimer 4 was constructed by
the fact the j
values of 4 are also similar to those of 2 and
max
3, clearly provides evidence for the absence of any electronic
interaction between the porphyrin planes in this array. This is
DOI: 10.1039/b010148f
New J. Chem., 2001, 25, 797È800
797
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2001

View Article Online
Scheme 1
not surprising if one considers that the array has “axial-
bondingÏ type architecture6 and that the coordinated
pyridine-acid spacer dictates that the separation distances
between the porphyrin planes should be [4 A (CPK molecu-
lar model). Further support for the presence of non-
interacting porphyrin monomers in
4
comes from
electrochemical experiments. Oxidation and reduction poten-
tials of the trimer, as measured by di†erential pulse voltam-
metry (see Experimental), were found to be nearly identical to
those of 2 and 3. In contrast, the excited state properties of the
tin(IV) porphyrin component of the array were found to be
quite di†erent from that of 3, while the ruthenium(II) porphy-
rin component was found to be non-Ñuorescent, as is the case
with the monomeric porphyrins 1 and 2. The Ñuorescence
quantum yield (/ ) of the tin(IV) porphyrin part of the trimer is
f
only 0.007 (j \ 600 nm where the absorption is almost
exc
exclusively due to the tin(IV) porphyrin of the trimer) as
against the corresponding reference compound 3 (/ \ 0.024).
f
Analogously, the Ñuorescence life-time (q) of the trimer was
found to be 0.63 ^ 0.05 ns while that of 3 is 1.48 ^ 0.1 ns. This
quenching of Ñuorescence observed for the trimer 4 can be
interpreted in terms of a photo-induced electron transfer
(PET) from the axial ruthenium(II) porphyrin to the excited
state of the basal tin(IV) porphyrin.¤ Indeed, the free energy
change (*G¡) for such
a PET process is exoergic by
0.32 ^ 0.04 eV, as estimated from the redox potential data of
the trimer and the singlet state energy of the tin(IV) porphyrin
(2.04 ^ 0.03 eV).
¤ Excitation energy transfer (between the two chromophores) and
heavy atom e†ect (due to the ruthenium centre) are the other possible
quenching mechanisms but, no direct/indirect evidence has been
obtained to show that these processes are operative in this system. In
addition, Ñuorescence of 3 was found to be unquenched in the pres-
ence of externally added 1 (2 or [2 mol equivalents).
Fig. 1 X-Ray structure of 2 recorded at 180 K. The Ðgure shows the
dimer of molecules linked through the hydrogen bonded carboxylate
groups. The t-butyl groups are disordered over two orientations, only
one of which is shown for clarity. Hydrogen atoms have also been
omitted from these groups.
798
New J. Chem., 2001, 25, 797È800

View Article Online
Trimer 4. 0.032 g (0.024 mmol) of complex 2 was dissolved
in 5 ml of CH Cl . To this, 0.010 g (0.012 mmol) of 3 was
Conclusion
2
2
In summary, a photochemically-functional, axial-bonding type
hybrid porphyrin trimer has been assembled by an advanta-
geous utilization of the well-known hard and soft acidÈbase
(HSAB) principle and metal ion recognition by a ditopic
ligand. Clearly, this new strategy provides scope for building
more elaborate arrays having interesting photochemical func-
tions. Such studies are currently in progress.
added and the resulting solution was sonicated for a few
minutes. Evaporation of the solvent Ðrst under a steady
stream of N and then under vacuum gave the desired
2
product in quantitative yield. 1H NMR (400 MHz, CDCl ): d
3
0.50 (d, 4H, J \ 6.9 Hz, pyridyl-H ), 2.67 (d, 4H, pyridyl-H ),
1.39 (s, 72H, t-butyl), 1.43 (s, 72H, t-butyl), 2.62 (s, 12H, tolyl-
a
b
CH ), 7.32 (d, 8H, J \ 7.9 Hz, Sn aryl-H), 7.45 (d, 8H, Sn
3
aryl-H), 7.49 (br s, 8H, Ru aryl-H), 7.64 (br s, 8H, Ru aryl-H),
7.83 (br s, 8H, Ru aryl-H), 8.31 (s, 16H, Ru pyrrole-H), 8.64 (s,
18H, Sn pyrrole-H); 13C NMR (100 MHz, CDCl ): d 21.5
Experimental
3
(tolyl CH ), 31.6 and 31.7 [2 ] t-butyl CH ], 34.8 and 34.9
3
3
1H NMR (1D and 2D) spectra were recorded on Bruker
DRX-400 or DRX-500 spectrometers respectively in CDCl .
Resonances were referenced relative to residual solvent
[2 ] t-butyl C(CH ) ], 118.6 (pyridyl C ), 120.2 (Ru aromatic
3 3
b
CH), 122.1 (Ru aromatic C), 127.5 (Sn aromatic CH), 127.8 (Sn
aromatic C), 128.6 and 129.6 (2 ] Ru aromatic CH), 131.1 (Sn
b-pyrrole C), 132.9 (Ru b-pyrrole C), 134.0 (Sn aromatic CH),
3
signals. UV/vis spectra were recorded with either a Uvikon
model 810 or a Jasco Model 7800 UV-Visible spectrophotom-
eter using 10 mm oven-dried cuvettes. Concentration of the
samples used for these measurements ranged from about
2 ] 10~6 M (Soret bands) to 5 ] 10~5 M (Q bands). Steady-
state Ñuorescence spectra were recorded using a Jasco Model
FP-777 spectroÑuorimeter. The emitted quanta were detected
at right angles to the incident beam. The utilized concentra-
tions of the Ñuorophores were such that the optical densities
(O.D.) at the excitation wavelengths were always less than 0.2
in CH Cl . The Ñuorescence quantum yields (/ ) were esti-
137.6 (pyridyl C), 137.9 (Sn aromatic C), 142.7 (pyridyl C ),
b
142.2 (Sn porphyrinic C), 146.4 (Ru porphyrinic C), 147.9 and
148.3 (Ru aromatic C), three unidentiÐed resonances; UV/vis
j
(CH Cl )/nm (log[e/M~1 cm~1]): 417 (5.38), 425 (5.37),
max
2 2
532 (4.26), 562 (4.10), 602 (3.92), E
]1.60, [0.93, [1.36, [1.76.
(V vs. SCE): ]0.79,
1@2
Crystallography
2
2
f
Crystals of 2 were grown by the layered addition of dry
mated by integrating the areas under the Ñuorescence curves
and by using (5,10,15,20-(tetraphenyl)porphyrinato)zinc(II)
(/ \ 0.036 in CH Cl ) as the standard.7 Fluorescence life
hexane onto a dry CDCl solution of 2 and data collected on
3
a Nonius Kappa CCD di†ractometer:
C
H
N O Ru,
89 111
5 3
2
2
0.20 ] 0.15 ] 0.10 mm, T \ 180(2) K, M \ 1399.90, triclinic,
space group P1, a \ 13.342(2), b \ 16.113(2), c \ 25.563(2) A,
a \ 80.020(10), b \ 77.700(10), c \ 69.650(10)¡, U \ 5004.6(10)
times were measured by the time-correlated single photon
counting method at the National Centre for Ultrafast Pro-
cesses (Chennai, India) using their pico-second laser excitation
and detection facility. The samples (solvent CH Cl ) were
AŽ
3, Z \ 2, D \ 0.929 Mg m~3, j \ 0.710 69 A,
c
2
2
F(000) \ 1496, k \ 0.197 mm~1, R \ 0.0779 [15 643 reÑec-
excited at 440 nm and the emission was monitored at
610 ^ 10 nm in each case. The count rates employed ranged
typically from 103 to 104 s~1. The dark count was \40 s~1.
Deconvolution of the data was carried out by the method of
iterative reconvolution of the instrument response function
and the assumed decay function, as described earlier.8 Di†er-
ential pulse voltammetric experiments (CH Cl and 0.1 M
tetrabutylammonium perchlorate, TBAP) were performed on
a Princeton Applied Research (PAR) 174A polarographic
analyzer coupled with a PAR 175 universal programmer and
a PAR RE 0074 xÈy recorder, as detailed in our previous
studies.6 Ferrocene was used to calibrate the potential axis.
Distilled solvents were used throughout and when used dry,
were freshly obtained from solvent stills. CH Cl was distilled
1
tions with I [ 2p(I)], wR \ 0.2352 for 22 773 independent
2
reÑections and 922 parameters.
CCDC reference number 152817. See http://www.rsc.org/
suppdata/nj/b0/b010148f/ for crystallographic data in CIF or
other electronic format.
2
2
Acknowledgements
We thank the EPSRC (J. K. M. S.) and Council of ScientiÐc
and Industrial Research, New Delhi, India (B. G. M.) for
Ðnancial support of this work. B. G. M. gratefully acknow-
ledges the Royal Society of Chemistry (London) for the
award of a Journals Grant for International Authors and the
National Centre for Ultrafast Processes (Chennai, India) for
extending their time-resolved Ñuorescence measurement
facility. The authors are also grateful to Dr Andrew Bond for
help with the crystallographic data.
2
2
from CaH under argon.
2
Synthesis
Compound 2. 0.030 g (0.0126 mmol) of 1 was dissolved in 5
ml of CH Cl . To this solution, 0.0155 g (0.126 mmol) of
2
2
pyridine-4-carboxylic acid was added and the resulting
mixture was sonicated and then heated at 40 ¡C for a few
minutes. The contents were passed through a pad of Celite
and the Ðltrate was evaporated to give the desired product in
References
1
J. K. M. Sanders, N. Bampos, Z. Clyde-Watson, S. L. Darling,
J. C. Hawley, H.-J. Kim, C. C. Mak and S. J. Webb, in T he
Porphyrin Handbook, ed. K. M. Kadish, K. M. Smith and R.
Guilard, Academic Press, London, 2000, vol. 3, pp. 1È48.
quantitative yield. 1H NMR (400 MHz, CDCl ): d 1.47 (s,
3
36H, t-butyl), 1.50 (s, 36H, t-butyl), 1.76 (d, 2H, J \ 6.8 Hz,
2
For recent examples, see: (a) S. L. Darling, E. Stulz, N. Feeder,
N. Bampos and J. K. M. Sanders, New J. Chem., 2000, 24, 261;
(b) F. Felluga, P. Tecilla, L. Hillier, C. A. Hunter, G. Licini and P.
Scrimin, Chem. Commun., 2000, 1087; (c) K. Chichak, M. C.
Walsh and N. R. Branda, Chem. Commun., 2000, 847; (d) U.
Michelsen and C. J. Hunter, Angew. Chem., Int. Ed., 2000, 39,
764; (e) S. L. Darling, C. C. Mak, N. Bampos, N. Feeder, S. J.
Teat and J. K. M. Sanders, New J. Chem., 1999, 23, 359; ( f ) K.
Chichak and N. R. Branda, Chem. Commun., 1999, 523; (g) W.-H.
Leung, J. L. C. Chim, W. Lai, L. Lam, W.-T. Wong, W. H. Chan
and C.-H. Yeung, Inorg. Chim. Acta., 1999, 290, 28.
pyridyl-H ), 5.79 (d, 2H, pyridyl-H ), 7.72 (br s, 4H, aryl-H),
a
b
7.87 (br s, 4H, aryl-H), 8.02 (br s, 4H, aryl-H), 8.65 (s, 8H,
pyrrole-H); 13C NMR (100 MHz, CDCl ): d 31.7 (t butyl
3
3 3
CH ), 34.9 and 35.0 [2 ] t-butyl C(CH ) ], 120.5 (aromatic
3
CH), 121.3 (pyridyl C ), 122.7 (aromatic C), 128.8 (aromatic
b
CH), 129.7 (aromatic CH), 131.9 (b-pyrrole C), 141.6 and 143.7
(porphyrin C), 145.3 (pyridyl C ), 148.2 and 148.5 (aromatic
C); UV/vis j
532 (4.25), E (V vs. SCE): ]0.79, [1.76.
a
(CH Cl )/nm (log[e/M~1 cm~1]) 415 (5.25),
max
1@2
2 2
New J. Chem., 2001, 25, 797È800
799

View Article Online
3
4
5
H.-J. Kim, N. Bampos and J. K. M. Sanders, J. Am. Chem. Soc.,
1999, 121, 8120.
J. H. Fuhrop and K. M. Smith, in Porphyrins and Metalloporphy-
rins, ed. K. M. Smith, Elsevier, Amsterdam, 1975, p. 769.
R. J. Abraham, G. R. Bedford, D. McNeille and B. Wright, Org.
Magn. Reson., 1980, 14, 418.
6
(a) T. A. Rao and B. G. Maiya, Chem. Commun., 1995, 939; (b) L.
Giribabu, T. A. Tao and B. G. Maiya, Inorg. Chem., 1999, 38,
4971.
D. J. Quimby and F. R. Longo, J. Am. Chem. Soc., 1975, 19, 5111.
M. Sirish and B. G. Maiya, J. Porphyrins Phthalocyanines, 1998,
2, 327.
7
8
800
New J. Chem., 2001, 25, 797È800