Synthesis and reactivity of porphyrin complexes of group IV p-elements

Article abstract of DOI:10.1023/A:1020407029048

The composition of products and the general regularities of complex formation of meso-tetraphenylporphine with Si(IV), Ge(IV), Sn(II), and Pb(II), as well as the kinetics and mechanism of dissociation of the resulting complexes under the action of proton-

Full text of DOI:10.1023/A:1020407029048

Russian Journal of General Chemistry, Vol. 72, No. 6, 2002, pp. 968 973. Translated from Zhurnal Obshchei Khimii, Vol. 72, No. 6, 2002,
pp. 1035 1040.
Original Russian Text Copyright
2002 by Molodkina, Lomova, Mozhzhukhina.
Synthesis and Reactivity of Porphyrin Complexes
of Group IV p-Elements
O. V. Molodkina, T. N. Lomova, and E. G. Mozhzhukhina
Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo, Russia
Received July 17, 2000
Abstract The composition of products and the general regularities of complex formation of meso-
tetraphenylporphine with Si(IV), Ge(IV), Sn(II), and Pb(II), as well as the kinetics and mechanism of dissocia-
tion of the resulting complexes under the action of proton-donor reagents were studied. The nature of chemical
and steric factors that operate to stabilize mixed-ligand acido porphyrin p-metal complexes was revealed.
Pophyrin complexes of Si, Ge, and Pb are almost
unexplored. The synthesis, electrochemical reactions,
and modification of the complexes of Si with octa-
ethylporphine (H2OEP) and etioporphyrin (H2EP)
with OH, OMe, OPh, Me, Ph, or OSi(Me)3 acido
ligands have been reported [1 5]. The synthesis, struc-
ture, and spectral and electrochemical properties of
complexes (X)2GeP [P is the dianion of porphine (H2P),
H2OEP, or tetraphenylporphine (H2TPP); X is Cl, OH,
Et, OMe, or OEt] [2,3, 6 8] have been studied. The
complexes of Pb with H2OEP, H2TPP, and meso-
porphyrin (H2MP) of the composition PbP have been
synthesized in [9 11], and tetravalent Pb complexes
results in [12]. The structure and practical application
of Sn porphyrin complexes are presently widely
studied [13, 14], but no quantitative information
concerning their stability is yet available.
In this work we made an attempt to reveal general
regularities of the synthesis of porphyrin complexes
of Group IV p-elements, and, on an example of
H2TPP, to detemine factors that contribute into the
stability of metal porphyrins in solutions.
The experimental synthetic results for the com-
plexes (OH)2SiTPP, (Cl)2GeTPP, (Cl)2SnTPP,
(AcO)2PbTPP, and PbTPC (Table 1) show that these
have been described in [9, 12]. Data of different compound are easiest to prepare by complex forma-
authors on the stability and spectra of such complexes
are contradictory: According to [9], [PbOEP]2+ is
unstable in organic solvents in the presence of small
amounts of water, which contradicts the calculation
tion of porphyrins with p-element salts in high-boiling
solvents. Therewith, tetravalent silicon and germa-
nium salts give complexes in which the formal charge
of the complexing atom is +4.
Table 1. Synthesis, isolation, and purification of Si(IV), Ge(IV), Pb(IV), and Sn(IV) complexes with H2TPP and Pb(II)
complexes with H2TPC (meso-tetraphenylchlorine)
Complex-
ing agent
Yield,
%
Complex
Solvent T, K Isolation method
Purification method
(OH)2SiTPP
SiCl4
Pyridine
458 Extraction with
CHCl3, washing
with H2O
Double chromatography on Al2O3, solvent 38
and eluent CHCl3
Cl2GeTPP
Cl2SnTPP
GeCl4
SnCl2
Quinoline 510 Extraction with
Chromatography on Al2O3, solvent and 93
eluent CHCl3
hexane
DMF
423 Precipitation with Double chromatography on Al2O3, solvent 65
H2O
and eluent CHCl3
PbTPC
Pb(OAc)2 Pyridine
388 Extraction with
CHCl3, washing
with H2O
Chromatography on Al2O3 in CHCl3,
7a
(AcO)2PbTPP Py
eluent CHCl3 C6H6; chromatography on 93a
Al2O3 in CHCl3, eluent DMF
a
Relative yields of the complexes.
1070-3632/02/7206-0968$27.00 2002 MAIK Nauka/Interperiodica

SYNTHESIS AND REACTIVITY OF PORPHYRIN COMPLEXES
The reactions of H2TPP with salts of double-
969
charged tin(II) and lead(II) cations (Table 1) are ac-
companied by redox processes. The synthesis of the
tin(IV) complex gives SnTPP which is readily oxidi-
zed with atmospheric oxygen to (Cl)2SnTPP [15].
Redox reactions involving metal cation, solvent, and
atmospheric oxygen in the complex formation of
Pb(OAc)2 with H2TPP yield two metal porphyrins,
PbTPC and (AcO)2PbTPP [16]. Atomic absorp-
tion and IR spectroscopy show that the complex
(AcO)2PbTPP contains Py in the coordination sphere.
In solutions such species exists only in the presence
of excess Py [17], and, therefore, in what follows,
dealing with solutions of (AcO)2PbTPP Py in acidic
media, we consider this complex as acido porphyrin
[(AcO)2PbTPP]. The experimental evidence for the
chlorin and porphyrin structure of the complexes,
respectively, is given in [16]. The spectral characteris-
A
1
2
500
600
700 , nm
Fig. 1. Electronic absorption spectrum of (OH) SiTPP in
17.8 M H SO (373 K). : (1) 0 and (2)
2
.
2
4
log kapp = log kv + nlog [H2SO4].
(2)
tics of the lead(II) and lead(IV) complexes coincide Here kapp and kv are the apparent and true rate con-
with those reported in [16].
stants, [H2SO4] is the concentration of unionized
H2SO4 molecules in concentrated aqueous sulfuric
acid. The log kv values and the true activation para-
meters are given in Table 2. From the resulting data
it follows that dissociation (1) obeys kinetic equa-
tion (3).
As follows from the composition of the synthesized
complexes, Group IV p-elements prefer the six-
coordinate form with a mixed coordination sphere
[(X)n 2MTPP].
The complexes (OH)2SiTPP and (AcO)2PbTPP are
very stable. They undergo no dissociation in usual
conditions (298 K, 1 atm) on treatment with H2O
AcOH, H2SO4 AcOH, and H2SO4 H2O.
+
dc
(OH)2SiTPP HSolv/d
= kv[(OH)2SiTPP H+Solv][H2SO4]2.
(3)
The electronic absorption spectrum of the complex
(OH)2SiTPP in sulfuric acid (Fig. 1) differs sharply
from the spectrum in CHCl3 (see Experimental). The
observation of a characteristic two-band visible absorp-
tion spectrum (Fig. 1) suggests [18] formation in conc.
H2SO4 of the ion-molecular associate (OH)2SiTPP
The kinetic regularities of (OH)2SiTPP dissociation
in H2SO4 are similar to those found earlier for
(Cl)2GeTPP [19].
(AcO)2PbTPP is very stable. It does not dissociate
in conc. H2SO4 and monohydrate even at high tem-
peratures (373 K). The electronic absorption spectrum
of the complex shows that it exists in conc. H2SO4 as
the associate (AcO)2PbTPP H+Solv (Fig. 3).
H+Solv
.
Thermostating solutions of (OH)2SiTPP in 17
18 M H2SO4 at 340 360 K results in that the spec-
trum of the associate with a proton transforms, accord-
ing to Eq. (1), into the spectrum of sulfonated H4TPP2+
(Fig. 1) with preserved isosbestic points. This follows
from Fig. 1 and the results of additional spectrophoto-
metric study of the reaction products after their re-
pricipitation and extraction with an organic solvent.
Table 2. Kinetic parameters of dissociation of (OH)2SiTPP
in concentrated sulfuric acida
b
1
298 K
kapp
c 0H SO , M
107, s
log k Tv
kapp
T, K
log kv
(OH)2SiTPP H+Solv + 3HS+olv
2
4
17.28
17.85
18.00
18.14
18.33
2.92
3.74
4.43
6.69
298
348
353
358
7.7
5.8
5.7
5.5
H2S2O7
2+
H4TPP(SO3H)2x+ + (OH)2Si
.
(1)
Solv
The formal first-order rate constants of reaction (1)
increase with increasing initial and equilibrium con-
centration of H2SO4 (Table 2, Fig. 2). Dependence
(2) is fairly (R 0.996 0.998) fitted by a straight line
with a slope (tan = n) close to 2 (Fig. 2).
16.89
a
1
1
Activation parameters: E 78 kJ/mol,
Obtained by extrapolation of the log k 1/T dependence.
S
140 J mol
K .
b
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002

970
MOLODKINA et al.
The complex PbTPC, unlike the Pb(IV) complex,
4
dissociates in usual conditions in aqueous sulfuric
log kapp
acid and its mixtures with AcOH at the instant of
1
3
dissolution. In the system ethanol (3 8) 10
M
2
3
3
AcOH (Fig. 4), it dissociates at a measurable rate.
Therewith, the electronic absorption spectrum of
lead(II) tetraphenylchlorin transforms into the spec-
trum of the final product H2TPP according to Eq. (4).
2
tan = 2
1.1
1.2 log [H2SO4]
[O]
PbTPC + 2H+Solv
H2TPC
H2TPP. (4)
2+
Pb
Solv
Fig. 2. Dependence of log k
on log [H SO ] for
app
2
4
(OH) SiTPP in conc. H SO . Temperature, K: (1) 348,
(2) 353, and (3) 358 K.
2
2
4
As seen from Fig. 4, H2TPC does not accumulate,
and the electronic absorption spectrum of the reaction
mixture at intermediate points definitely reveals two
colored compounds, PbTPC and H2TPP. This fact
implies that the second stage of reaction (4) is not
limiting.
A
The first-order dissociation rate constants kaTpp
linearly increases with increasing initial concentration
of acetic acid c0AcOH (Table 3, Fig. 5). The prevalence
in the mixed solvent EtOH AcOH at the specified
HOAc concentrations of equilibrium (5) over the
equilibria of autoprotolysis of EtOH and AcOH and
the proportionality of c0AcOH and [EtOH2+]2 at
moderate AcOH concentrations [Eq. (6)] allow us to
write the third-order reaction kinetic equation (7).
420
500
580
660
740 , nm
Fig. 3. Electronic absorption spectrum of (OAc) PbTPP
in 18.66 M H SO .
2
2
4
The lability, low E values, and the negative
S
values for reaction (4) (Table 3) point to an SEN3
mechanism of PbTPC dissociation in EtOH AcOH.
This mechasim has been rigorously substantiated for
metal porphyrins in mixed proton-donor solvents
[17, 20].
A
K
EtOH + AcOH
EtOH+2 + AcO ,
(5)
(6)
(7)
K = [EtOH+2]2/[AcOH] = [EtOH+2]2/c 0AcOH
dcPbTPC/d = (1/K)kv[PbTPC][EtOH+2]2.
,
1
The above results, as well as published quantitative
data [15, 19, 21] on the stability of the complexes
(Cl)2SnTPP and (Cl)2GeTPP allow general regularities
of the reactivity of acido porphyrin complexes of
Group IVb p-metals in sulfuric acid solutions to be
revealed. In concentrated sulfuric acid, these com-
plexes exist as two spectrally distinguishable forms,
molecular [(Cl)2SnTPP and (Cl)2GeTPP] and an ion-
2
molecular
associate
[(OH)2SiTPP HS+olv
and
420 440 460 480 500 520 540 , nm
(AcO)2PbTPP H+Solv].
Fig. 4. Electronic absorption spectrum of PbTPC in
A common feature of dissociation of hexacoordi-
nate complexes of H2TPP with Si, Sn, and Ge is that
they involve H2SO4 molecules as proton donors
3
EtOH AcOH (c
7.88 10 M, 290 K). : (1) 0 and
AcOH
(2)
.
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002

SYNTHESIS AND REACTIVITY OF PORPHYRIN COMPLEXES
971
298 K
Table 3. Kinetic parameters of dissociation of PbTPC in
Table 4. Kinetic stabilities (k
), covalent radii (rc) of
app
ethanol acetic acid
complexing atoms, and positions of the first absorption
bands ( max) in the electronic absorption spectra
I
c 0AcOH 103,
E,
S ,
k
104,
298 K
app
1
1
298 K
1
I
M
kJ/mol
J mol 1 K
s
k
104, s
rc,
[23]
,
app
max
Complex
(c 0H SO , M)
ml
2
4
3.50
4.40
5.80
6.55
7.88
63 8
44 5
29 3
23 2
23 3
127 21
178 15
221 7
241 5
237 7
0.7
1.3
2.7
3.8
5.6
(OH)2SiTPP
(Cl)2GeTPP
(Cl)2SnTPP
0.017 (18.33)
0.200 (18.66) [19] 1.93
0.030 (18.66) [15] 2.11
1.89
612
592 [19]
601 [15]
584
(AcO)2PbTPP Stable (18.66)
2.14
[Eqs. (3), (8), (9)] rather that H3O+ cations as with
similar phthalocyanine complexes [22].
As known [17], chemical stabilization of metal
porphyrins with -type donor acceptor bonds definite-
ly shows up in a hypsochromic shift of the first ab-
I
dc(Cl) GeTPP/d = kv[(Cl)2GeTPP][H2SO4]2,
(8)
sorption band ( max) in the electronic absorption spec-
2
trum. The positions of the complexes in series (10)
disagree with this tendency but are nicely consistent
with the increasing sum of the covalent radii of the
metal atom and the coordinating nitrogen atom along
the silicon subgroup. Analysis of X-ray diffraction
data for (X)2MTPP [24, 25] shows that all complexes
of tetravalent element of the silicon subgroup are
planar. Therewith, the rM+N values for the Sn(IV) and
Pb(IV) complexes (Table 4) are the most close to
2.01 , which corresponds to D4h symmetry ideal for
metal porphyrins in terms of internal strain. Coordina-
tion of the smaller Si(IV) and Ge(IV) atoms (Table 4)
produces radial compression of the macroring in
(Cl)2GeTPP and (OH)2SiTPP and, as a result, de-
creases the covalent contribution into the M N bonds.
The complex (OH)2SiTPP exhibits the lowest kinetic
dc(Cl) SnTPP/d
2
= [(Cl)2SnTPP](k1[H2SO4]2 + k2[H2SO4]).
(9)
The existence of different forms of the complexes
and the observation of different reaction orders in the
concentration of unionized H2SO4 suggests that these
complexes, while being similar in composition and
the structure of the coordination sphere, dissociate by
different mechanisms.
Complexes Xn 2MTPP may dissociate either
after trans cis isomerization of acido ligands [15] or
after dissociation of the acido complex by the bonds
metal acido ligand donor atom. Mixed-ligand com-
plexes of tetravalent tin, germanium, and silicon with
phthalocyanine [22] dissociate by the metal nitrogen
bond, when acido ligands have been removed from
the first coordination sphere. Unlike metal phthalo-
cyanines, the complexes (Cl)2SnTPP and (Cl)2GeTPP
do not dissociate under the same conditions (aqueous
H2SO4, Brand’s region). It is only 100% H2SO4, i.e.
an H2SO4 H2O mixture with the highest dielectric
constant, in which M Cl and then M N bonds tend
to dissociate.
1
kapp 104,
10
1
s
2
3
5
Since the k2v98 K rate constants relate to different-
order reactions, the kinetic stabilities of the metal
porphyrins in the series Si Ge Sn Pb should be
298 K
app
compared by the k
values at the same cH0 SO
2 4
(Table 4). At H2SO4 concentrations close to 100%
(18.66 M), the kinetic stability of the complexes
2
3
4
5
6
7 cA0cOH , M
Fig. 5. Dependence of the rate constants k of dissocia-
decreases in series (10), in parallel with the increasing
298 K
app
k
values.
app
(OAc)2PbTPP < (Cl)2SnTPP < (Cl)2GeTPP
< (OH)2SiTPP.
tion of PbTPC on the initial concentration of AcOH in
EtOH. Temperature, K: (1) 308, (2) 313, and (3) 315.
(10)
RUSSIAN JOURNAL OF GENERAL CHEMISTRY Vol. 72 No. 6 2002

972
MOLODKINA et al.
stability in series (10), probably, because of the
highest ionic contribution into the Si O and Si N
bonds. The strongest Pb OAc and Pb N bonding in
Tetraphenylporphinesilicon dihydroxide
(OH)2SiTPP. IR spectrum, cm : pyrrole rings: 800
1
(
(
C H), 992 (
,
,
C N), 1336 ( C N), 1440
C C
C H
(AcO)2PbTPP is confirmed by the largest hypso-
C=N), 1520 (in-plane vibrations); benzene rings: 696
I
chromic shift of
in the electronic absorption
and 740 ( C H), 1068 and 1176 ( C H), 1496, 1595 sh
and 1576 ( C=C), 2920 and 3045 ( C H); 460 (Si Nporf);
560 and 608 (Si O). Electronic absorption spectrum
(CHCl3), max, nm (log ): 652 (4.05), 612 (4.227),
571 (4.501), 524 (4.22), 410 (5.14).
max
spectrum (Table 4).
Thus, the stability of tetraphenylporphyrin com-
plexes of silicon subgroup metals is determined by
the strength of their metal nitrogen bonds, effects
of steric shielding of the MN4 center by acido ligands,
and the tendency for ion-molecular association with
solvent. The effect of radial compression on complex
formation of Ge(IV) with H2TPP destabilizes the
resulting metal porphyrin. The high charge and the
small radius of the Si(IV) cation ensure ionic contribu-
tion into the stability of the (OH)2SiN4 coordination
entity.
(Pyridine)tetraphenylporphinelead diacetate
1
(AcO)2PbTPP Py. IR spectrum, cm : pyrrole rings:
798 ( C H), 976 (
,
,
C N), 1340 ( C N), 1440
C C C H
(
C=N), 1523 (in-plane vibrations); benzene rings: 704,
778 ( C H), 1072, 1159 ( C H), 2860, 2920 ( C H);
pyridine: 700, 1460, 1560; 405 (Pb Npyridine); 450
(Pb Nporf); 1655 and 1734 (O C O). Electronic
absorption spectrum (CHCl3), max, nm: 584, 444,
345 sh, 310 s; intensity decreases in the series IV >
III > II > I.
In going from (AcO)2PbTPP to PbTPC, a sharp
decrease in kinetic stability is observed. The desta-
bilization of the latter complex is explained by the
decreased Zeff on the metal atom and electron density
on the macroring (hydrogenation of one bond), as
well as the larger size of the Pb2+ ion [12]. These
factors all prevent planar arrangement of the cation in
the metal chlorine.
Tetraphenylchlorinelead PbTPC. IR spectrum,
1
cm : pyrrole rings: 796 ( C H), 1000 (
,
,
C C
C N), 1328 ( C N), 1440 ( C=N), 1525 (in-plane viCbrHa-
tions); benzene rings: 704 i 752 ( C H), 1072 and 1176
(
C H), 1470, 1576 and 1592 ( C=C), 2860, 2920 and
1
3020, 3080 ( C H); 416 (Pb N). H NMR spectrum
(CDCl3, HMDS), , ppm: 7.73 m (12H, Hm, Hp),
EXPERIMENTAL
8.16 m (8H, Ho), 8.77 d (4H, C4H2N), 8.89 t (6H,
C4H4N). Electronic absorption spectrum (CHCl3),
max, nm (log ): 652 (3.37), 609 (3.52), 551 (3.25),
516 (3.4), 466 (4.33), 419 (4.5), 355 (3.76).
As complexing agents for synthesis of (OH)2SiTPP
and (AcO)2PbTPP were used chemical grade SiCl4
and analytical grade Pb(OAc)2 3H2O. H2TPP
(Armenian Branch, Inst. Khim. Reaktivov Osobo Chist.
Khim. Veshchestv) was purified by chromatography
on Al2O3 (Brockmann activity grade II) in chloroform.
The synthesized complexes were purified on neutral
Al2O3. Organic solvents were dried by known pro-
cedures [20].
The kinetics of dissociation of PbTPC and
(OH)2SiTPP in EtOH AcOH and H2SO4 H2O mix-
tures, respectively, were studied spectrophotometrical-
ly. The apparent rate constants kTapp, activation
energies E, and reaction orders in the concentrations
of complex and proton-donor species were determined
by least-squares treatment of the log (A0 A )/(A
A ) , log k (1/T), and log kapp [H+Solv] dependences
using Microsoft Excel. The activation entropies were
Monohydrate of H2SO4 was prepared from che-
mical grade oleum with potentiometric control [26].
The concentration of H2SO4 in H2SO4 H2O mixtures
was determined by acid base titration with an
accuracy of no less than 0.01%.
calculated by the equation
(E/T) 19.1log T 205.
S
= 19.1log kTapp
+
The electronic absorption spectra were measured on
Specord M-400 and SF-26 instruments. The IR spec-
tra were obtained on a Specord M-80 instrument in
KBr and assigned analytically using data in [27 29].
ACKNOWLEDGMENTS
The work was financially supported by the Russian
Foundation for Basic Research (project no. 97-03-
33489).
1
The H NMR spectra were measured on a Tesla BS-
497 spectrometer (100 MHz, internal reference TMS).
The synthesis and physicochemical characteristics
of Cl2SnTPP and Cl2GeTPP were described earlier
[15,19]. The complexes of silicon(IV) and lead(IV)
with H2TPP and of lead(II) with H2TPC (meso-tetra-
phenylchlorine) were prepared by complex formation
of porphyrins with metal salts in pyridine (Table 1).
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