A simple alternative method for preparing Sn(IV) porphyrins

Article abstract of DOI:10.1142/S1088424610002124

Sn(IV) porphyrins are highly desirable for various applications because of their stability, their preference for oxygen-donor ligands and because they possess properties which can be easily characterized using various spectroscopic techniques. The most established method for the preparation of the Sn(IV) porphyrins is refluxing the porphyrin with SnCl ₂·2H ₂O in pyridine as solvent. Although this method works efficiently, we found that the work-up results in a lot of unwanted materials and using large quantities of pyridine as solvent is not good under laboratory conditions. In this paper, we show that the Sn(IV) porphyrins can be prepared easily by treating porphyrin with SnCl ₂·2H ₂O in chloroform, di chloromethane and toluene as solvent containing 25-50% ethanol as co-solvent. The reaction works smoothly and involves simple work-up and straightforward chromatographic purification. The method works efficiently for meso and β-substituted porphyrins. The spectral and electrochemical properties of various Sn(IV) were studied and our studies showed that the properties are sensitive to the nature of substituent present at the meso-position. Copyright

Full text of DOI:10.1142/S1088424610002124

Journal of Porphyrins and Phthalocyanines
J. Porphyrins Phthalocyanines 2010; 14: 361–370
DOI: 10.1142/S1088424610002124
Published at http://www.worldscinet.com/jpp/
A simple alternative method for preparing Sn(IV) porphyrins
Vijayendra S. Shetti and Mangalampalli Ravikanth*◊
Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400 076, India
Received 24 September 2009
Accepted 14 October 2009
ABSTRACT: Sn(IV) porphyrins are highly desirable for various applications because of their stability,
their preference for oxygen-donor ligands and because they possess properties which can be easily
characterized using various spectroscopic techniques. The most established method for the preparation
of the Sn(IV) porphyrins is refluxing the porphyrin with SnCl2·2H2O in pyridine as solvent. Although
this method works efficiently, we found that the work-up results in a lot of unwanted materials and using
large quantities of pyridine as solvent is not good under laboratory conditions. In this paper, we show
that the Sn(IV) porphyrins can be prepared easily by treating porphyrin with SnCl2·2H2O in chloroform,
dichloromethane and toluene as solvent containing 25–50% ethanol as co-solvent. The reaction works
smoothly and involves simple work-up and straightforward chromatographic purification. The method
works efficiently for meso and β-substituted porphyrins. The spectral and electrochemical properties
of various Sn(IV) were studied and our studies showed that the properties are sensitive to the nature of
substituent present at the meso-position.
KEYWORDS: tin porphyrin, alternative method, chloroform, ethanol, meso-substituents, spectral
properties.
NMR, absorption, fluorescence, electrochemical and other
INTRODUCTION
techniques. We are interested in the synthesis of covalent
Tin(IV) porphyrins are widely used for various applica-
and non-covalent unsymmetrical porphyrin arrays con-
tions [1]. The preferential coordination of the Sn(IV) por-
taining two or more different porphyrin subunits to study
phyrins to oxy anionic ligands has been used by Sanders
singlet-singlet energy transfer from one porphyrin subunit
et al. [2] and others [3] for the construction of novel elabo-
rate multi-porphyrin arrays. Langford et al. [4] and others
to another [9]. In continuation of our ongoing search for
building new porphyrin arrays, we are interested in using
[5] used the six-coordinate tin(IV) porphyrins as building
dihydroxotin(IV) porphyrin building blocks to construct
blocks in the design of functional materials with uniform
multiporphyrin arrays containing heteroatom-substituted
channels. These compounds have also been investigated
porphyrin subunits as axial ligands. Unlike Zn(II) porphy-
for applications such as catalysis [6], biomedicine [7] and
rin building blocks, the Sn(IV) porphyrin building blocks
synthesis of nanomaterials [8]. Tin(IV) porphyrins offer
are more stable and possess photophysical properties sim-
the chemist many advantages due to the particular prop-
ilar to those of Zn(II) porphyrins, hence dihydroxotin(IV)
erties conferred by the highly charged main group metal
porphyrins are better building blocks for the construction
center. The Sn(IV) complexes are diamagnetic and usually
of multiporphyrin arrays. Towards achieving the goal of
six-coordinate with trans-diaxial anionic or occasionally
the synthesis of Sn(IV) porphyrin-based multiporphyrin
neutral ligands which can be manipulated in most cases
arrays containing heteroporphyrins as axial ligands, we
[1]. Sn(IV) porphyrins can be characterized easily by
first attempted the synthesis of dihydroxo(5,10,15,20-
tetratolylporphyrinato)tin(IV) [SnTTP(OH)2] by follow-
ing standard literature methods [10].
š
SPP full member in good standing
The Sn(IV) porphyrins were first synthesized in 1948
by Rothemund and co-workers by refluxing the porphyrin
with SnCl2·2H2O in pyridine, followed by work-up and
*Correspondence to: Mangalampalli Ravikanth, email:
ravikanth@chem.iitb.ac.in, tel: +91 22-2576-7176, fax: +91
22-2572-3480
Copyright © 2010 World Scientific Publishing Company

362
V.S. SHETTI AND M. RAVIKANTH
Porphyrin
Our studies indicate that the
dihydroxotin(IV)tetratolylpor-
phyrin 1 can be prepared by
treating 5,10,15,20-tetratolyl-
SnCl2.2H2O
High boiling solvents (Pyridine, DMF, Acetic acid)
porphyrin
(H2TTP)
with
SnCl2·2H2O in chloroform-
ethanol (1:1) under mild reflux-
ing conditions followed by
one column chromatographic
purification (Scheme 2). Our
modified method is beneficial
over pyridine method because
of the following reasons:
(1) the work-up is simple,
(2) low-boiling solvents such
as CHCl3 can be used which
can be removed easily, and
(3) it requires just one straight-
forward column chromato-
graphicpurification.Themethod
works very efficiently and was
demonstrated by synthesizing
Crude Sn(IV) porphyrin (Cl)2
Water work-up followed
by alumina column
chromatography
K2CO3 / THF/ H2O reflux
Water work-up
Sn(IV) porphyrin (OH)2
Scheme 1. Literature methods for the preparation of dihydroxoSn(IV) porphyrin
dihydroxotin(IV) derivatives of various porphyrin mac-
rocycles 2–6. The effects of various meso substituents on
spectral, electrochemical and photophysical properties of
Sn(IV) porphyrins are also described.
column chromatographic purification (Scheme 1) [10a].
Later, Adler et al. (1970) and Gouterman et al. (1973)
synthesized Sn(IV) porphyrins by refluxing porphyrin
with SnCl2·2H2O in high-boiling solvents like dimethyl-
formamide [10b] and glacial acetic acid [10c] (Scheme 1).
Among these methods, the pyridine method is most widely
used to synthesize Sn(IV) porphyrins. The Sn(IV) porphy-
rins prepared by pyridine method have two chloride groups
as axial ligands before passing through the column and
changed to two hydroxo groups after the crude compound
was subjected to basic alumina column chromatographic
purification. However, in a recent report by Crossley and
co-workers, it was noted that the pyridine method gave
Sn(IV) porphyrins containing mixture of axial ligands
such as chloro and hydroxo groups after column chro-
matographic purification instead of Sn(IV) porphyrins
containing two axial hydroxyl groups. Hence, Crossley
and co-workers [10d] developed an alternative method
to synthesize Sn(IV)porphyrins containing two axial
hydroxo groups. In Crossley’s method, the dichlorotin(IV)
porphyrin was obtained first by refluxing porphyrin with
SnCl2·2H2O in pyridine for 3 h followed by precipitation
with excess water. The dichlorotin(IV)porphyrin complex
was then treated with K2CO3 in THF-H2O (4:1) at reflux-
ing temperature for 3 h and the compound was purified
on neutral alumina column to afford dihydroxotin(IV)por-
phyrin 1 in 90% yield (Scheme 1). We initially attempted
to prepare Sn(IV) derivative of 5,10,15,20-tetratolylpor-
phyrin 1 by pyridine method. Although the reaction
worked smoothly, we found that the work-up was tedious
and laborious. Furthermore, the pyridine is not an environ-
mentally friendly solvent to handle in large quantities [11].
Thus, we explored various alternative simple reaction con-
ditions to prepare dihydroxotin(IV)tetratolylporphyrin 1.
EXPERIMENTAL
Chemicals
The free-base porphyrins were synthesized by follow-
ing literature procedures [18]. All general chemicals and
solvents were procured from S.D. Fine Chemicals, India.
Column chromatography was performed using basic alu-
mina obtained from Sisco Research Laboratories, India.
Tetrabutylammonium perchlorate was purchased from
Fluka and used without further purifications.
Instrumentation
1H NMR spectra were recorded with a Varian
400 MHz instrument using tetramethylsilane as an
internal standard. 13C NMR spectra were recorded on a
Varian spectrometer operating at 100.6 MHz. 119Sn NMR
was recorded with a Varian spectrometer operating at
111.8 MHz using tetraphenyltin as an internal standard
(Ph4Sn = 0 ppm). All NMR measurements were carried
out at room temperature in deuterochloroform. Absorp-
tion and steady-state fluorescence spectra were obtained
with Perkin-Elmer Lambda 35 and Lambda 55 instru-
ments, respectively. The fluorescence quantum yields (Φf)
of Sn(IV) porphyrins were estimated from the emission
and absorption spectra by comparative method [15b].
The time-resolved fluorescence decay measurements
[17] were carried out at magic angle using a picosecond
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 362–370

A SIMPLE ALTERNATIVE METHOD FOR PREPARING SN(IV) PORPHYRINS
363
Cl
Sn
Cl
N
N
N
N
NH
N
N
SnCl2.2H2O
CHCl3\EtOH
HN
Dichloro[5,10,15,20-tetrakis(p-tolyl)porphyrinato]tin(IV)
basic alumina
column chromatography
OH
N
N
N
N
Sn
OH
Dihydroxo[5,10,15,20-tetrakis(p-tolyl)porphyrinato]tin(IV)
Scheme 2. Synthesis of dihydroxoSn(IV) porphyrin 1 by alternative method
diode laser-based time correlated single photon counting
General synthesis of dihydroxoporphyrinatotin(IV)
1–6
(TCSPC) fluorescence spectrometer from IBH, UK. All
the decay curves were fitted to single exponential func-
tions using IBH software. The radiative and non-radiative
rate constants, kr and knr, were calculated by the follow-
ing equations:
Asampleofporphyrin(59µmol)wasdissolvedin10mL
of chloroform in a one-neck round-bottom flask. The solu-
tion of SnCl2·2H2O (0.59 mmol) in 10 mL of ethanol was
added to it and the reaction mixture was stirred at reflux-
ing temperature. The porphyrin solution turned green on
addition of SnCl2·2H2O. The progress of the reaction was
monitored by TLC analysis and absorption spectroscopy.
The metalation was completed in four hours as judged by
absorptionspectroscopy.Triethylamine(0.5mL)wasadded
to the reaction mixture and the reaction mixture was stirred
for an additional five minutes. The solvent was removed
on rotary evaporator and crude compound was subjected
to alumina column chromatographic purification. The
residual unreacted free-base porphyrin was collected first
with CH2Cl2 and the desired dihydroxoporphyrinatotin(IV)
was eluted with CH2Cl2/1–2% CH3OH. The solvent was
removed on rotary evaporator and afforded the pure Sn(IV)
porphyrin 1–6 as purple solid.
Σ K = 1/τf
kr = Φfk
(1)
(2)
(3)
knr = k- kr
MALDI-TOF spectra were obtained from Axima-CFR
manufactured by KratosAnalyticals. CyclicVoltammetric
(CV) and Differential Pulse Voltammetric (DPV) studies
were carried out with BAS electrochemical system uti-
lizing the three-electrode configuration consisting of a
glassy carbon (working electrode), platinum wire (auxil-
iary electrode) and saturated calomel (reference electrode)
electrodes in dry dichloromethane using 0.1 M tetrabu-
tylammonium perchlorate as supporting electrolyte.
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 363–370

364
V.S. SHETTI AND M. RAVIKANTH
Dihydroxo[5,10,15,20-tetrakis(p-tolyl)porphyri-
nato]tin(IV) [SnTTP(OH)2] 1. Yield 90%, mp > 300 °C.
Anal. calcd. for C48H38N4O2Sn: C, 70.17; H, 4.66; N,
8.17%. Found: C, 70.13; H, 4.70; N, 8.20. 1H NMR (400
MHz; CDCl3; Me4Si): δH, ppm -7.43 (2H, br s, axial OH),
2.7 (s, 12H, tolyl), 7.61 (d, J = 7.8 Hz, 8H, Ar), 8.20
(d, J = 7.8 Hz, 8H, Ar) 9.13 (s, satellite, J = 10.7 Hz, 8H,
β-pyrrole). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm
21.4, 121.2, 127.6, 132.4, 135.0, 137.8, 138.4, 147.0.
MS (MALDI-TOF): m/z 807 (calcd. for [M - OH]+ 807);
1609 (calcd. for [M2 - 2OH]+ 1609).
Dihydroxo[octaethylporphyrinato]tin(IV)
[SnOEP(OH)2] 2. Yield 80%, mp > 300 °C. Anal. calcd.
for C36H46N4O2Sn: C, 63.08; H, 6.76; N, 8.17%. Found:
C, 63.12; H, 6.71; N, 8.19. 1H NMR (400 MHz; CDCl3;
Me4Si): δH, ppm -8.06 (2H, br s, axial OH), 2.03 (t, J =
7.6 Hz, 24H, CH3), 4.20 (q, J = 7.6 Hz, 16H, CH2), 10.47
(s, satellite, J = 2.8 Hz, 4H, meso-H). 13C NMR (100
MHz; CDCl3; Me4Si): δC, ppm 18.7, 20.1, 97.5, 143.7,
144.5. MS (MALDI-TOF): m/z 671 (calcd. for [M - OH]+
671); 1337 (calcd. for [M2 - 2OH]+ 1337).
β-pyrrole). 13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm
119.0, 129.8, 133.1, 146.2, 148.9. MS (MALDI-TOF):
m/z 769 (calcd. for [M]+ 769); 755 (calcd. for [M - OH]+
755); 1505 (calcd. for [M2 - 2OH]+ 1505).
RESULTS AND DISCUSSION
In search of an alternative method to the existing,
widely used pyridine method [10a] for preparing Sn(IV)
porphyrins, we have chosen three solvents – CHCl3,
CH2Cl2 and toluene – in which porphyrins are readily sol-
uble. The Sn(IV) insertion studies were carried out by tak-
ing H2TTP as a model compound in CHCl3 as solvent and
ethanol as co-solvent. Although we discussed our results
from the use of CHCl3 as solvent, the results were simi-
lar when we used toluene and CH2Cl2 as solvents along
with ethanol as co-solvent. The attempts to prepare Sn(IV)
TTP(OH)2 1 were initiated by treating 3 mM of H2TTP in
CHCl3 with 10 equivalents of SnCl2·2H2O in ethanol (1:1
v/v) at refluxing temperature. The progress of the reaction
was monitored by TLC analysis and absorption spectros-
copy by taking aliquots of the reaction mixture at regular
intervals and checking the absorption spectra after addi-
tion of triethylamine. It was noted that the color of the
porphyrin solution turned to green soon after the addition
of alcoholic SnCl2·2H2O. The absorption spectroscopy of
the green solution showed the formation of dication of
H2TTP. The absorption spectral analysis at frequent inter-
vals indicated that the absorption bands corresponding to
Sn(IV) derivative started to appear only after two hours;
the reaction was completed in 4 h. After the completion
of the reaction, triethylamine was added to the compound
and the greenish color of the reaction mixture was changed
to dark pink with green tinge. The solvent was removed
on rotary evaporator and the crude compound was sub-
jected to basic alumina column. The unreacted small
amount of H2TTP was removed initially and the required
compound 1 was then collected in 90% yield as purple
solid. For comparison purpose, compound 1 was also
synthesized by pyridine method [10a]. A comparison of
various spectroscopic data of 1 prepared by both methods
showed the data to be exactly identical. To confirm that the
two hydroxyl groups are the axial ligands in 1 prepared
by CHCl3/ethanol method, we used 1H & 119Sn NMR and
Dihydroxo[5,10,15,20-tetrakis(2-thienyl)porphy-
rinato]tin(IV) [SnTThP(OH)2] 3. Yield 65%, mp >
300 °C. Anal. calcd. for C36H22N4O2S4Sn: C, 54.76; H,
1
2.81; N, 7.10%. Found: C, 54.78; H, 2.78; N, 7.08. H
NMR (400 MHz; CDCl3; Me4Si): δH, ppm -7.3 (2H, br
s, axial OH), 7.54 (m, 4H, thienyl), 7.93 (d, J = 5.0 Hz,
4H, thienyl), 8.02 (m, 4H, thienyl), 9.32 (s, satellite, J =
10.1 Hz, 8H, β-pyrrole). 13C NMR (100 MHz; CDCl3;
Me4Si): δC, ppm 114.0, 119.3, 126.4, 128.8, 131.0, 133.0,
135.0, 141.6, 147.9, 148.3. MS (MALDI-TOF): m/z 791
(calcd. for [M]+ 791); 775 (calcd. for [M - OH]+ 775);
1544 (calcd. for [M2 - 2OH]+ 1544).
Dihydroxo[5,10,15,20-tetrakis(2-furyl)porphyri-
nato]tin(IV) [SnTFP(OH)2] 4. Yield 65%, mp > 300 °C.
Anal. calcd. for C36H22N4O6Sn: C, 59.62; H, 3.06; N,
7.72%. Found: C, 59.64; H, 3.03; N, 7.73. 1H NMR (400
MHz; CDCl3; Me4Si): δH, ppm -7.33 (2H, br s, axial OH),
7.09 (m, 4H, furyl), 7.45 (m, 4H, furyl), 8.19 (s, 4H,
furyl), 9.52 (s, satellite, J = 13.6 Hz, 8H, β-pyrrole). MS
(MALDI-TOF): m/z 729 (calcd. for [M]+ 729).
Dihydroxo[5,10,15,20-tetrakis(pentafluorophenyl)
porphyrinato]tin(IV) [SnTPFPP(OH)2] 5. Yield 86%,
mp > 300 °C. Anal. calcd. for C44H10F20N4O2Sn: C, 46.96;
H, 0.90; N, 4.98% Found: C, 46.92; H, 0.95; N 4.96. 1H
NMR (400 MHz; CDCl3; Me4Si): δH, ppm -7.31 (2H, br
s, axial OH), 9.23 (s, satellite, J = 9.1 Hz, 8H, β-pyrrole).
13C NMR (100 MHz; CDCl3; Me4Si): δC, ppm 105.2,
132.9, 136.7, 139.2, 145.5, 147.1, 148.0. MS (MALDI-
TOF): m/z 1109 (calcd. for [M - OH]+ 1109).
1
MALDI-TOF mass techniques (Fig. 1). In H NMR, we
observed a broad signal in the upfield region at -7.4 ppm
(Fig. 1ai), corresponding to the axial OH protons, as noted
previously for Sn(IV)porphyrin prepared by pyridine
method. Similarly, in 1H NMR, we also noted one singlet
for eight β-pyrrole protons along with satellite peaks
(Fig. 1aii), as noted earlier. In 119Sn NMR, only one signal
at -570 ppm (Fig. 1b) was observed which is the character-
istic feature of Sn(IV)porphyrin having two axial hydroxy
groups [1]. If the axial ligands are two chloro groups, the
signal was anticipated at -580 ppm in 119Sn NMR. If the
mixture of axial ligands are present, it is expected to show
more than one signal in 119Sn NMR. Furthermore, recently
Dihydroxo[5,10,15,20-tetrakis(4-pyridyl)porphy-
rinato]tin(IV) [SnTPyP(OH)2] 6. Yield 60%, mp >
300 °C. Anal. calcd. for C40H26N8O2Sn: C, 62.44; H,
1
3.41; N, 14.56% Found: C, 62.46; H, 3.38; N 14.57. H
NMR (400 MHz; CDCl3; Me4Si): δH, ppm -7.36 (2H, br
s, axial OH), 8.29 (d, J = 5.9 Hz, 8H, m-Py), 9.14 (d, J =
5.9 Hz, 8H, o-Py), 9.18 (s, satellite, J = 12.8 Hz, 8H,
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 364–370

A SIMPLE ALTERNATIVE METHOD FOR PREPARING SN(IV) PORPHYRINS
365
indicated that the two chloride groups were
present as axial ligands, as noted earlier in
pyridine method.
The insertion of Sn(IV) ion into H2TTP
was then tested under various other experi-
mental conditions. When the Sn(IV) inser-
tion was carried out by treating the H2TTP
in CHCl3 with ten equivalents of alcoholic
SnCl2·2H2O solution at room temperature
instead of refluxing conditions, no meta-
lation was observed until 36 h, suggest-
ing that the mild refluxing conditions are
necessary for Sn(IV) insertion into the
porphyrin macrocycle. The reaction was
also carried out by changing the num-
ber of equivalents of SnCl2·2H2O. When
we used five equivalents instead of ten
equivalents of SnCl2·2H2O, no metalation
was observed for 24 h. However, when
we carried out the reaction with 20 and
40 equivalents of SnCl2·2H2O, the reac-
tion was completed in 2 h with no further
improvement in the yields. Thus, the opti-
mized reaction conditions for the prepara-
tion of 1 are: treating the porphyrin with
ten equivalents of SnCl2·2H2O in CHCl3/
ethanol (1:1) at refluxing temperature for
4 h, followed by simple work-up and col-
umn chromatographic purification.
To understand the necesssity of alcohol
in the synthesis of 1, the following experi-
mentswerecarriedout:(1)Thereactionwas
performed by addition of ten equivalents
of solid SnCl2·2H2O to H2TTP dissolved
in reagent grade CHCl3 which contains
0.6–1% ethanol as stabilizer at refluxing
temperature conditions. The Sn(IV) inser-
tion was noticed with reagent grade CHCl3.
1
Fig. 1. Spectroscopic evidences for the formation of 1: (a) partial H NMR
However, the rate of Sn(IV) insertion was
very slow and yielded 1 only in 30% after
30 h; (2) The Sn(IV) insertion into porphy-
rin was also carried out with ethanol free
spectrum of 1 recorded in CDCl3 showing (i) axial OH proton (ii) β-pyrrole
protons with satellites, (b) 119Sn NMR spectrum of 1 recorded in CDCl3,
(c) MALDI-TOF mass spectrum of 1
CHCl3 [12]. We observed that no Sn(IV) insertion into
porphyrin macrocycle even after 48 h under refluxing con-
ditions; (3) The reaction was carried out by changing the
ratio of CHCl3 to ethanol; we used 1:1 v/v for optimized
reaction conditions. When metalation was carried out
in 1:0.5 v/v of CHCl3/ethanol, the reaction worked effi-
ciently in the same duration of time. However, when we
used 1:0.25 v/v of CHCl3/ethanol, the required time for
complete metalation was almost double and the yield
also dropped to 50%. Thus, 1:0.5 v/v of CHCl3/ethanol
is compulsory for efficient metalation under these condi-
tions; (4) To understand the effect of different alcohols,
we carried out the reaction with various other alcohols
under the same reaction conditions, i.e. ten equivalents of
SnCl2·2H2O, CHCl3/alcohol in 1:1 v/v and 4 h of reflux
Crossley and co-workers used MALDI-TOF mass spec-
tral analysis to differentiate dichlorotin(IV)porphyrins
and dihydroxotin(IV)porphyrins [10d]. They showed
that under the conditions of MALDI-TOF mass spectral
analysis, the dichlorotin(IV)porphyrins show parent ion
resulting from the loss of one chloro ligand whereas the
dihydroxotin(IV)porphyrins show (M - OH)+ peak and
associated dimer peak with the loss of two hydroxo groups
corresponding to (M2 - 2 × OH)+. The Sn(IV)porphyrin
1 prepared by CHCl3/ethanol method showed similar
MALDI-TOF mass spectral features (Fig. 1c), supporting
the argument that the two hydroxyl groups were present
as axial ligands. However, the spectral analysis of crude
compound before subjecting to column chromatography
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 365–370

366
V.S. SHETTI AND M. RAVIKANTH
conditions. The reaction worked equally efficiently when
we used methanol (90% yield) instead of ethanol. How-
ever, alcohols such as 2-methoxyethanol (12 h, 85%),
butan-1-ol (8 h, 80%), dodecyl alcohol (12 h, 80%) and
cyclohexanol (15 h, 80%) assisted in Sn(IV) metalation,
but these require longer reaction periods and yields were
relatively low compared to ethanol and methanol (see
Supporting Information). No metalation was observed
when isopropyl and tert-butyl alcohols were used which
may be due to the poor solubility of SnCl2·2H2O in these
two solvents. Thus, it is important to choose the alcohol
which solubilizes the SnCl2·2H2O for rapid metalation.
We carried out the experiments to know the role of
triethylamine. Since some amount of H2TTP was con-
verting to dicationic form on addition of SnCl2·2H2O and
the reaction mixture was always green in color, we per-
formed the reaction by adding triethylamine soon after
the addition of SnCl2·2H2O. The reaction mixture color
was changed from green to purple but the metalation did
not occur even after 24 hours. Similarly, when we treated
the porphyrin in CHCl3 containing 0.1% v/v triethylam-
ine with alcoholic solution of SnCl2·2H2O, the Sn(IV)
insertion was not noticed. The addition of triethylamine
to alcoholic solution of SnCl2·2H2O gave unknown white
precipitate which was not soluble even after heating.
Thus, the presence of triethylamine during the reaction
prevents metalation. Furthermore, when the triethylam-
ine was not added to the reaction mixture at the end of the
reaction, the metalation yields were not affected. These
studies indicate that the addition of triethylamine was not
needed in the reaction although we added small amount
of triethylamine to the reaction mixture at the end to neu-
tralize the acid generated in the reaction.
We also tested different Sn(II) salts for metal inser-
tion reaction. Addition of Sn(CH3)2Cl2, Sn(C6H5)2Cl2 and
Sn(n-Bu)2Cl2 in ethanol to H2TTP in CHCl3 gave green
coloration due to formation of dication but no metalation
occurred under refluxing conditions even after one week.
Also, no metalation was noticed with SnCl4 under any
reaction conditions which we explored.Thus, SnCl2·2H2O
is the only salt which is cheap and readily available and
can be used for preparation of Sn(IV) porphyrins.
Applications
To show the applicability of the method, we pre-
pared Sn(IV) derivatives of five other porphyrin mono-
mers 2–6 shown in Chart 1. The Sn(IV) insertion into
S
O
OH
OH
Sn
OH
N
N
N
N
N
N
N
N
N
N
N
N
S
O
Sn
Sn
S
O
OH
OH
OH
S
O
SnOEP(OH)2
2
SnTThP(OH)2
3
SnTFP(OH)2
4
F
F
F
F
N
F
F
F
F
F
F
F
F
OH
Sn
N
N
N
N
OH
Sn
N
N
N
F
F
N
N
OH
N
F
OH
F
F
F
F
N
F
SnTPFPP(OH)2
5
SnTPyP(OH)2
6
Chart 1. Various dihydroxotin(IV) porphyrin derivatives
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 366–370

A SIMPLE ALTERNATIVE METHOD FOR PREPARING SN(IV) PORPHYRINS
367
the octaethylporphyrin (OEP) worked smoothly and
as singlet in metal-free porphyrins appeared as one
strong central signal surrounded by small satellite peaks
in Sn(IV) derivatives [14]. The satellite peaks appeared
due to coupling of 119Sn and 117Sn with β-pyrrole protons
and the coupling constant 4JSn-H is normally in the range
of 10–13 Hz [14]. In the case of Sn(IV)OEP(OH)2, the
couplingwasobservedbetween119Snandmeso-Hprotons
[1]. Furthermore, the β-pyrrole protons were down-
field-shifted in Sn(IV)TTP(OH)2 compared to H2TTP.
The axial OH protons in Sn(IV) derivatives experienced
porphyrin ring current effect and appeared as broad sig-
nal at -7.3 ppm in tetraarylporphyrins and at -8.0 ppm
in octaethylporphyrin (Table 1). The presence of meso-
substituents such as pyridyl, pentafluorophenyl, thie-
nyl and furyl groups in place of tolyl groups alters the
electronic properties and effects were clearly seen in
SnOEP(OH)2 2 was isolated in decent yield (12 h, 80%)
with one straightforward column chromatographic puri-
fication, However, the rate of Sn(IV) insertion into OEP
was three times slower compared to Sn(IV) insertion into
the TTP. Similarly, the Sn(IV) derivatives of 5,10,15,20-
meso-tetrathienylporphyrin (SnTThP(OH)2) 3 (12 h, 65%),
5,10,15,20-meso-tetrafurylporphyrin (SnTFP(OH)2)
4
(20 h, 65%), 5,10,15,20-meso-tetrapentafluorophenylpor-
phyrin (SnTPFPP(OH)2) 5 (15 h, 86%) were prepared in
decent yields under similar reaction conditions. The Sn(IV)
derivative of 5,10,15,20-meso-tetra(4-pyridyl)porphyrin
(SnTPyP(OH)2) 6 required 40 h duration and yielded the
pure compound in 60% yield after one column chromato-
graphic purification. Thus, the method works efficiently
with various types of porphyrins and requires simple col-
umn chromatographic purification to obtain the Sn(IV)
derivatives in pure form in decent yields.
1
β-pyrrole protons in H NMR spectra. On replacement
of meso-tolyl groups with six-membered meso-aryl
groups such as pyridyl and pentafluorophenyl groups
or five-membered meso-thienyl and meso-furyl groups,
the β-pyrrole protons experienced downfield shifts and
maximum shifts were noted for SnTFP(OH)2 4 (Table
1). However, the changes in δSn in 119Sn NMR are very
minimal, indicating that the metal ion is not very sensi-
tive to meso-substituents (Table 1).
Spectral and electrochemical properties of Sn(IV)
porphyrins 1–6
Interestingly, although the Sn(IV) derivatives of por-
phyrins with axial hydroxo ligands are very attractive
components for various applications because of their
favorable optical and coordination properties, the system-
atic investigation of their spectral and electrochemical
[13] properties are scarce in literature. Hence, we inves-
tigated systematically the NMR, optical, electrochemical
and photophysical studies of six Sn(IV) derivatives of
different macrocycles.
Absorption properties
The Sn(IV) porphyrins exhibit normal spectra with
one strong Soret band and two Q bands [1]. The com-
parison of normalized Soret band spectra of 1, 2, and 4
are shown in Fig. 2 and the data are presented in Table 2.
The position and the intensity of the absorption bands
depend on the kind of substituent present at the meso-
position. In 1, the three absorption bands appeared at 429,
563 and 603 nm which were blue-shifted and appeared at
407, 537 and 574 nm in 2 with no meso-substituents. The
absorption bands of 5 and 6 were blue-shifted compared
to SnTTP(OH)2 1 but the absorption bands of 3 and 4
were red-shifted. Thus, the replacement of six-membered
NMR studies
NMR is very informative about the structure of
Sn(IV) porphyrins. The six Sn(IV) porphyrin complexes
were characterized by 1H and 119Sn NMR spectroscopy;
the selected data are presented in Table 1. In general,
the signals corresponding to β-pyrrole and axial OH pro-
1
tons of Sn(IV) porphyrins in H NMR spectra are quite
informative. The eight β-pyrrole protons which appear
1
Table 1. H NMR and 119Sn NMR chemical shifts of various
Sn(IV) porphyrins recorded in CDCl3
Porphyrin
1H NMR (δ in ppm)
119Sn NMR
(δ in ppm)
OH
β-pyrrole
resonance
resonance
1
2a
-7.43
-8.06
-7.24
-7.33
-7.31
-7.36
9.13
10.47
9.31
9.52
9.23
9.18
b
-570.20
-569.24
-568.97
—
3
4b
5
-567.15
-569.39
6
The chemical shift of meso-H. 119Sn resonance was not
Fig. 2. Comparison of normalized Soret band absorption spec-
tra of (a) 2 (b) 1 (c) 4 recorded in dichloromethane
a
obtained due to poor solubility.
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 367–370

368
V.S. SHETTI AND M. RAVIKANTH
Table 2. Absorption and electrochemical data for various Sn(IV) porphyrins 1–6 recorded in dichloromethane
Compound
Soret band, nm (log ε)
Q bands, nm (log ε)
Reduction potentials, V vs. SCE
I
II
I
II
1
2
3
4
5
6
429 (5.93)
407 (5.44)
437 (5.51)
448 (4.43)
422 (5.61)
424 (5.55)
563 (4.30)
537 (4.04)
569 (4.12)
578 (3.29)
553 (4.33)
558 (4.20)
603 (4.24)
574 (3.99)
614 (3.94)
635 (3.46)
588 (sh)
-0.96
-1.18
-0.56
-0.45
-0.46
-0.52
-1.36
-1.59
-0.96
-0.86
-0.96
-0.92
596 (3.70)
Fig. 3. Comparison of the normalized emission spectra of (a) 2
(b) 1 (c) 4 recorded at λex = 430 nm in dichloromethane
Fig. 4. Fluorescence decay profiles and weighted residual fits
of (a) 4 (b) 1
aryl groups with five-membered thienyl and furyl groups
resulted in red shifts in absorption bands and maximum
shifts were noted for meso-furyl substituted Sn(IV) por-
phyrin 4 [15].
(Fig. 3) which were blue-shifted in the case of 2, 5 and
6. The quantum yields calculated by comparative method
[16] and singlet excited state lifetime (Fig. 4) measured
by single photon counting method [17] were decreased in
the case of 2 and 6 compared to 1.
Fluorescence properties
The Sn(IV) porphyrins were shown as good energy
donors and the photophysical properties are comparable
to those of Zn(II) porphyrins. The comparision of fluores-
cence spectra of 1, 2 and 4 is shown in Fig. 3 and the pho-
tophysical data of Sn(IV) porphyrins 1–6 are presented
in Table 3. The photophysical parameters of Sn(IV) por-
phyrins are sensitive to the nature of meso-substituent.
The compound 1 showed two bands at 609 and 662 nm
krad and knr, the radiative and non-radiative rate constants,
respectively, were also decreased and increased, respec-
tively, for 5 and 6 compared to 1. However, compounds 3
and 4 showed only one fluorescence band which was batho-
chromically shifted compared to 1.. These observations sug-
gest that the five membered meso-furyl and meso-thienyl
groups alter the electronic properties of the porphyrin more
effectively compared to six-membered aryl groups.
Table 3. Photophysical data for various Sn(IV) porphyrins 1–6 recorded
in dichloromethane
Electrochemical properties
The redox properties of Sn(IV) porphy-
rins were probed through cyclic voltammetric
studies with tetrabutylammonium perchlorate
as supporting electrolyte (0.1 M) in dichlo-
romethane as solvent. A comparison of reduc-
tion waves of Sn(IV) porphyrins 1, 3–6 is
shown in Fig. 5 and the data are presented in
Table 2. The compound 1 showed one irre-
versible oxidation and two reversible or quasi-
reversible reductions (Fig. 5). These redox
processes are due to oxidation and reduction
Compound
λem, nm
Фfa
τf, 10-9 s
kr, 106 s-1
knr, 108 s-1
1
2
3
4
5
6
609 662 0.025
576 630 0.024
1.21
0.55
0.73
1.14
0.89
1.06
20.66
43.63
42.46
12.28
4.60
8.05
17.74
13.27
8.64
637
656
—
—
0.031
0.014
593 647 0.0041
598 652 0.017
11.18
9.27
16.03
a Quantum yields were calculated by taking ZnTPP (Фf = 0.036) as refer-
ence at λex = 560 nm.
Copyright © 2010 World Scientific Publishing Company
J. Porphyrins Phthalocyanines 2010; 14: 368–370

A SIMPLE ALTERNATIVE METHOD FOR PREPARING SN(IV) PORPHYRINS
369
be tuned upon introduction of suitable substituents at
meso-positions.
Acknowledgements
MR thanks Council of Scientific and Industrial
Research (CSIR) and Department of Science and Tech-
nology (DST) for financial support and VS thanks CSIR
for fellowship.
Supporting information
A full list of 1H NMR, 119Sn NMR and MALDI-TOF
spectra of selected compounds are given in the supple-
mentary material. This material is available free-of-
charge via the Internet at http://www.worldscinet.com/
jpp/jpp.shtml.
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J. Porphyrins Phthalocyanines 2010; 14: 369–370

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