Electrochemistry and Spectroscopy of Sulfate Complexes of (Tetraphenylporphyrinato)manganese

Article abstract of DOI:10.1002/hlca.200490002

The electrochemical and spectroscopic properties of [Mn ₂(tpp)₂(SO₄)] (H₂tpp = tetraphenylporphyrin = 5,10,15,20-tetraphenyl-21H,23H-porphine) were studied to characterize the stability of this compound as a function of solvent, redox state, and sulfate concentration. In non-coordinating solvents such as 1,2-dichloroethane, the dimer was stable, and two cyclic voltammetric waves were observed in the region for Mn^III reduction. These waves correspond to reduction of the dimer to [Mn^II(tpp)] and [Mn ^III(tpp)(OSO₃)]^-, and reduction of [Mn ^III(tpp)(OSO₃)]^- to [Mn ^II(tpp)(OSO₃)]^2-, respectively. In the coordinating solvent DMSO, [Mn₂(tpp)₂(SO₄)] was unstable and dissociated to form [Mn^III(tpp)(DMSO) ₂]⁺. A single voltammetric wave was observed for Mn ^III reduction in this solvent, corresponding to formation of [Mn ^II(tpp)(DMSO)]. In non-coordinating solvent systems, addition of sulfate (as the bis(triphenylphosphoranylidene)ammonium (PPN⁺) salt) resulted in dimer dissociation, yielding [Mn^III(tpp)(OSO ₃)]^-. Reduction of this monomer produced [Mn ^II(tpp)(OSO₃)]^2-. In DMSO, addition of SO ₄^2- led to displacement of solvent molecules forming [Mn^III(tpp)(OSO₃)]^-. Reduction of this species in DMSO led to [Mn^II(tpp)(DMSO)].

Full text of DOI:10.1002/hlca.200490002

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Helvetica Chimica Acta Vol. 87 (2004)
Electrochemistry and Spectroscopy of Sulfate Complexes of
(Tetraphenylporphyrinato)manganese
by Kevin Lawson, Jeff Gross, and Philip W. Crawford*
Department of Chemistry, Southeast Missouri State University, Cape Girardeau, Missouri 63701, USA
The electrochemical and spectroscopic properties of [Mn2(tpp)2(SO4)] (H2tpp ˆ tetraphenylporphyrin ˆ
5,10,15,20-tetraphenyl-21H,23H-porphine) were studied to characterize the stability of this compound as a
function of solvent, redox state, and sulfate concentration. In non-coordinating solvents such as 1,2-
dichloroethane, the dimer was stable, and two cyclic voltammetric waves were observed in the region for
MnIII reduction. These waves correspond to reduction of the dimer to [MnII(tpp)] and [MnIII(tpp)(OSO3)]À, and
reduction of [MnIII(tpp)(OSO3)]À to [MnII(tpp)(OSO3)]2À, respectively. In the coordinating solvent DMSO,
‡
[Mn2(tpp)2(SO4)] was unstable and dissociated to form [MnIII(tpp)(DMSO)2] . A single voltammetric wave
was observed for MnIII reduction in this solvent, corresponding to formation of [MnII(tpp)(DMSO)]. In non-
‡
coordinating solvent systems, addition of sulfate (as the bis(triphenylphosphoranylidene)ammonium (PPN )
salt) resulted in dimer dissociation, yielding [MnIII(tpp)(OSO3)]À. Reduction of this monomer produced
[MnII(tpp)(OSO3)]2À. In DMSO, addition of SO24À led to displacement of solvent molecules forming
[MnIII(tpp)(OSO3)]À. Reduction of this species in DMSO led to [MnII(tpp)(DMSO)].
Introduction. The chemistry of porphyrinatometals has been of considerable
interest during the past 25 years due to the importance of these compounds in a number
of biological processes [1]. Synthetic porphyrinatometals have often been used as
models for the chemistry occurring within natural systems that contain metalloproteins.
As a result, porphyrinatometals containing a wide variety of anionic axial ligands have
been synthesized and characterized to date. One such ligand, the sulfate ion (SO24À ),
has been shown to react with (tetraphenylporphyrinato)manganese(III) to form a
porphyrinatomanganese(III) dimer, [Mn2(tpp)2(SO4)] (H2tpp ˆ tetraphenylpor-
phyrin ˆ 5,10,15,20-tetraphenyl-21H,23H-porphine), which is bridged by the sulfato
ligand [2]. This complex has been characterized by spectroscopic methods (UV/VIS
and IR spectroscopy) and X-ray crystallography, and its photochemical reactions have
been examined [2]. Structurally, this dimeric species is similar to that of the analogous
iron derivative, [Fe2(tpp)2(SO4)] [3][4]. For both [Mn2(tpp)2(SO4)] and [Fe2-
(tpp)2(SO4)], the sulfato ligand bridges the two metal atoms in a monodentate fashion
[2 4]; this coordination was confirmed by both the X-ray structures and IR spectra. In
addition, the VIS spectrum of [Mn2(tpp)2(SO4)] dimer showed no unusualfeatures
compared to other porphyrinatomanganese complexes, indicating only a weak
interaction between the two porphyrin moieties [2].
Although the structure and spectroscopic properties of [Mn2(tpp)2(SO4)] have
been characterized, the electrochemical properties of this substance have not
previously been examined. In addition, the association/dissociation equilibria of this
complex in coordinating solvents and in the presence of excess ligand have not been
investigated. In this report, we present a study of the electrochemical and spectroscopic

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121
properties of the sulfato-bridged dimer [Mn2(tpp)2(SO4)]. The major aim was to
characterize the dimer×s equilibria, and to understand its stability as a function of
solvent strength, redox state, and sulfate concentration. The iron analog, [Fe2-
(tpp)2(SO4)], has been shown to be stable only in non-coordinating solvents [5]. In
coordinating solvents and in the presence of excess sulfate, the iron derivative
dissociates [5][6]. The behavior of [Mn2(tpp)2SO4] will, therefore, be compared to that
of the corresponding iron derivative under similar conditions.
Experimental Part
Materials. Chloro[5,10,15,20-tetraphenyl-21H,23H-porphinato(2À)-kN21,kN22,kN23,kN24]manganese
([Mn(tpp)(Cl]), bis(triphenylphosphoranylidene)ammonium chloride ((PPN)(Cl), CHCl3, CH2Cl2, benzene,
acetone, and tetrabutylammonium borohydride ((Bu4N)BH4) were purchased from Aldrich Chemical Co.
ClCH2CH2Cl, dimethylsulfoxide (DMSO), dimethylformamide (DMF), Na2SO4, and (Bu4N)ClO4 were
purchased from Fisher Scientific. Acetone was obtained from Curtin Matheson Scientific. All solvents were of
the highest purity available and were used without further purification. Small amounts of (Bu4N)ClO4 were
dried in vacuo at 908 for 4 6 h prior to use and stored in a dessicator. Care must be exercised during the heating
and handling of TBAP (Bu4N)ClO4 since all perchlorates have the potential to decompose explosively.
The {m-[sulfato(2À)-kO:kO']bis[5,10,15,20-tetraphenyl-21H,23H-porphinato(2À)-kN21,kN22,kN23,kN24]di-
manganese ([Mn2(tpp)2SO4)], was synthesized from [Mn(tpp)Cl] and Ag2SO4 in a biphasic CH2Cl2/H2O system
[2]. The product was characterized by comparison of the UV/VIS and IR spectra with the reported spectra.
Bis(triphenylphosphoranylidene)ammonium sulfate ((PPN)2SO4) was prepared by the metathesis reaction
of (PPN)Cland excess Na 2SO4 in H2O [7]. The product, recrystallized from acetone/Et2O, exhibited distinctive
IR bands corresponding closely to those of the free sulfate ion [8].
Cyclic Voltammetry. CHI-Instruments 6000 voltammetric analyzer; scan rates from 100 mV/s to 300 mV/s.
Half-wave potentials (E1/2) were measured as the average of the cathodic and anodic peak potentials [9]. A
three-electrode Bioanalytical Systems (BAS) cell was used for all experiments, consisting of a BAS Pt-disk
(1.6 mm) electrode, a BAS Pt-wire auxiliary electrode, and a BAS Ag/AgNO3 (0.1m in MeCN) reference
electrode (SRE). Deoxygenation of all solns. was accomplished by passing a gentle, constant stream of
prepurified N2 through the soln. for 15 min and maintaining a blanket of the inert gas over the soln. during the
experiment. In the case of CH2Cl2 and ClCH2CH2Cl, the N2 was saturated with the solvent prior to entering the
electrochemical cell to minimize evaporation. Sulfate-ion concentrations were varied by adding appropriate
amounts of (PPN)2SO4 directly to the test solns., followed by several minutes of additional deoxygenation.
Spectroelectrochemical Experiments. They were performed with an optically transparent thin-layer
electrode (OTTLE). The cell design has been described previously [10]. This cell had a Pt-mesh working
electrode and an optical path length of 0.2 mm. The reference was a BAS Ag/AgNO3 (0.1m in MeCN) reference
electrode (SRE). Test solns. were deoxygenated for 30 min. The VIS-spectral data were obtained following
controlled potential electrolysis at the desired potentials. Potential control was maintained using a BAS CV-27
voltammograph.
VIS Spectra. Perkin-Elmer Lambda-3 UV/VIS spectrophotometer connected to an IBM computer (also for
spectroelectrochemical experiments). Besides the OTTLE cell, a 1-mm quartz cell was used to obtain the Soret
region, and a 1-cm quartz cell was used to measure the spectral data in the long wavelength (500 800 nm)
region.
IR Spectra. Analect RFX-30-FT IR spectrometer; solid samples as KBr pellets, soln. samples in a 0.1-mm
NaCl liquid cell (Wilmad Glass Co.).
Chemical Reduction of [Mn(tpp)Cl]. Addition of an excess of the reducing agent (Bu4N)BH4 to either a
CH2Cl2 or a DMSO soln. of [Mn(tpp)Cl] while under a blanket of N2 maintained in a glove bag gave the soln. for
VIS analysis. The UV/VIS cells were sealed tightly with rubber septa to exclude O2 prior to removing the solns.
from the N2 atmosphere.
Results. Spectroscopy. The VIS spectrum of [Mn2(tpp)2(SO4)] in CH2Cl2 has been
reported previously [2]. The spectrum for the sulfato-bridged dimer is similar to those
reported for high-spin porphyrinatomanganese(III) complexes [11][12]. VIS-Spectral

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data for [Mn2(tpp)2(SO4)] in several different solvents were recorded and are listed in
Table 1. The VIS spectra of [Mn(tpp)Cl] were also determined for comparison. The
spectrum of the sulfato-bridged dimer in the non-coordinating solvent CHCl3 between
400 and 750 nm exhibited a Soret band with an absorbance maximum at 466 nm, Qa and
Qb band maxima at 577 and 612 nm, respectively, and an additional broad band at
530 nm (Fig. 1), in close agreement with the reported spectrum [2]. By comparison, the
spectrum of [Mn(tpp)Cl] in CHCl3 exhibited absorbances similar to the literature
values [13], with a Soret band at 479 nm and other VIS bands at 528, 583, and 618 nm.
The spectralfeatures observed for both [Mn (tpp)2(SO4)] and [Mn(tpp)Cl] in other
2
non-coordinating (or weakly coordinating) solvents were similar to those in CHCl3
(Table 1). The solution IR spectrum of [Mn2(tpp)2(SO4)] in ClCH2CH2Cl, after
background subtraction, was essentially the same as that observed for the spectrum of
the solid.
Table 1. VIS Spectra (Tetraphenylporphyrinato)manganese(III) Complexesa)
Solvent
Soret bandb)
Other VIS bandsb)
[Mn2(tpp)2(SO4)]
CHCl3
CH2Cl2
466 (215)
466 (172)
468 (75.8)
468 (119)
468
440(sh), 466 (185)
439 (sh), 465 (258)
479 (115)
530 (10.9), 577 (22.5), 612 (18.9)
528 (7.1), 575 (15.4), 612 (13.5)
531 (4.3), 577 (8.9), 613 (7.3)
532 (6.2), 577 (12.8), 613 (10.5)
527, 572, 609
c
)
benzene
toluene
MeCOMed)
DMF
524 (14.2), 568 (23.0), 604 (17.9)
523 (12.6), 567 (23.1), 604 (17.9)
528 (6.3), 583 (10.8), 618 (11.7)
529 (6.6), 583 (11.2), 618 (13.0)
530 (5.6), 583 (8.7), 619 (10.5)
523 (5.8), 585 (9.3), 621 (11.2)
532 (5.7), 586 (9.2), 622 (11.2)
518 (6.2), 567 (11.2), 603 (8.7)
515 (13.1), 567 (17.4), 602 (15.0)
521 (3.5), 574 (7.2), 608 (5.9)
531 (7.0), 578 (13.1), 613 (12.1)
531 (5.6), 579 (10.5), 615 (10.3)
531 (3.4), 578 (6.3), 615 (6.3)
530 (6.1), 575 (11.0), 614 (10.9)
528 (5.7), 576 (10.0), 614 (8.9)
DMSO
CHCl3
CH2Cl2
[Mn(tpp)Cl]
478 (119)
ClCH2CH2Cl478 (101)
benzene
toluene
DMSO
DMSO
CH2Cl2
CHCl3
ClCH2CH2Cl470 (119)
MeCOMe
DMF
477 (124)
478 (12.5)
465 (132)
465 (124)
470 (51.8)
469 (159)
‡e
[Mn(tpp)]
)
[Mn(tpp)(OSO3H)]c)
[Mn(tpp)(OSO3)]À
469 (66.9)
441 (80.7), 465 (82.3)
440 (sh), 466 (109)
DMSO
a) VIS Spectraldata between 400 and 750 nm. b) lmax in nm, e (in parentheses) in cmÀ1 ¥ mmÀ1
.
c) [2]. d) Sample
not completely soluble in this solvent system. e) [14].
In the coordinating solvent DMSO, the spectra for both [Mn2(tpp)2(SO4)] and
[Mn(tpp)Cl] were essentially the same and similar to that reported for
‡
[Mn(tpp)(DMSO)2] [14], with a Soret band maximum at 465 nm, Q bands at 567
and 603 nm, and a broad band at ca. 520 nm (Table 1, Fig. 2). A small absorbance was
observed at 439 nm in the spectrum of the sulfato-bridged dimer that was not observed
in the spectrum of [Mn(tpp)Cl].
‡
Addition of sulfate ion (as the PPN salt) to solutions of [Mn(tpp)2(SO)4] in
various solvents produced significant changes in the VIS spectra (Figs. 1 and 2). In

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123
Fig. 1. VIS Spectrum of [Mn(tpp)2(SO4)] (0.054 mm) in CHCl3 in the presence and absence of sulfate:0 m m
(PPN)2SO4 (solid line) and 10.4 mm (PPN)2SO4 (dashed line) (360 490 nm in 1-mm cell; 490 700 nm in 1-cm
cell)
CHCl3, addition of excess sulfate shifted the Soret band from 466 to 469 nm (Fig. 1).
Only minor shifts in the wavelengths of the longer wavelength bands were observed.
However, the excess sulfate ion resulted in a definite increase in the absorbances of
each of these bands. The changes observed were nearly stoichiometric with addition of
sulfate, and no isosbestic points were observed. In addition, the spectrum remained
unchanged at higher concentrations of the ligand, i.e., at greater than 1 equiv. of added
sulfate. Similar spectral results were observed in other non-coordinating/weakly
coordinating solvents. The IR spectrum of the dimer [Mn2(tpp)2(SO4)] in ClCH2CH2Cl
following the addition of 15 equiv. of sulfate exhibited non-porphyrin bands at 1286,
‡
1109, 1032, 943, and 880 cmÀ1 following subtraction of solvent, PPN , and free sulfate
bands.
In the coordinating solvent DMSO, addition of excess sulfate ion to the
[Mn2(tpp)2(SO4)] solution resulted in a spectrum with absorbances at 441, 466, 528,
576, and 615 nm (Fig. 2,a). Isosbestic points were detected at 456, 470, 502, 576, 593,
and 609 nm during the spectral changes in this solvent for the dimer as the sulfate
concentration was increased from 0 to 23 equiv. (Fig. 2,b).
Addition of excess sulfate ion to solutions of [Mn(tpp)Cl] under analogous
conditions produced identical spectra to those of the dimer following sulfate addition.
In addition, isosbestic points were also observed in the spectrum of [Mn(tpp)Cl] in
DMSO at the same wavelengths (456, 470, 502, 576, 593, and 609 nm) as those noted for
the dimer in DMSO as the amount of sulfate was increased.
‡
Chemicalreduction of [Mn(tpp)Cl] with borohydride (BH À4 ) (as the Bu4N salt) in
ClCH2CH2Clproduced a spectrum consistent with a five-coordinate, anion-complexed
manganese(II) species [15][16], yielding absorbance maxima at 613, 573, and 441 nm.
The same spectrum was observed for the reduced product in the presence of added

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Fig. 2. a) VIS Spectrum of [Mn2(tpp)2(SO4)] (0.052 mm) in DMSO in the presence and absence of sulfate:0 m m
(PPN)2SO4 (solid line) and 11.6 mm (PPN)2SO4 (dotted line) (360 490 nm in 1-mm cell; 490 700 nm in 1-cm
cell). b) VIS-Spectral changes for [Mn2(tpp)2(SO4)] (0.052 mm) in DMSO with varying concentrations of
(PPN)2SO4 (0 11.6 mm) (360 480 nm in 1-mm cell; 480 700 nm in 1-cm cell)
sulfate ion. Borohydride reduction of [Mn(tpp)Cl] in DMSO lead to a spectrum
consistent with that of [MnII(tpp)(DMSO)] [15][16], with absorbances observed at
606, 567, and 435 nm. This spectrum was unaltered after addition of sulfate ion to the
solution. Addition of only 1 equiv. of borohydride to a solution of [Mn2(tpp)2(SO4)] in

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125
CH2Cl2 resulted in a spectrum possessing Soret bands at both 435 and 467 nm,
indicative of the presence of both MnII and MnIII species.
To better understand and characterize the products of the reactions between the
porphyrinatomanganese complexes and the sulfate ion, the molar absorptivity ratio of
bands Qb and Qa was calculated for the various spectra (Table 2) [16]. By using the
terminology of Boucher [11], these bands are numbered III and IV, respectively. In the
non-coordinating solvent ClCH2CH2Cl, the ratio eIII/eIV is 1.21 for [Mn(tpp)Cl], in
close agreement with the reported value of 1.17 [16]. Similar values (1.08 1.21) were
also observed in other non-coordinating solvents. In DMSO, this ratio for [Mn(tpp)Cl]
was 0.78. This latter value is somewhat lower than the reported value of 0.99 [16], but
could be repeated multiple times under the conditions used in this study. Addition of a
large excess of sulfate to [Mn(tpp)Cl] solutions in various solvent systems produced
spectra with similar ratios, i.e., eIII/eIV ˆ 0.89 1.00, thus implying that the same product
is formed in each solvent. The same ratios (0.93 1.00) were also obtained after
addition of sulfate to the [Mn2(tpp)2(SO4)] in all solvents utilized. Following the
chemicalreduction of [Mn(tpp)Cl] in ClCH 2CH2Cl, the ratio eIII/eIV was determined to
be 1.08, in keeping with the reported values for [MnII(tpp)Cl]À in non-coordinating or
weakly coordinating solvent systems [16]. A ratio of 1.03 was calculated for the reduced
product following sulfate addition to the solution. In DMSO, the reduced species gave
eIII/eIV ˆ 0.90 both in the absence and presence of added sulfate, implying that sulfate
does not coordinate to MnII in this solvent system.
Table 2. Spectroscopic Properties as a Function of Solvent for (Tetraphenylporphyrinato)manganese(III)
Complexes
a
b
Solvent
eIII/eIV
)
eIV/eV )
[Mn2(tpp)2(SO4)]
[Mn(tpp)Cl]
CHCl3
CH2Cl2
benzene
toluene
DMF
DMSO
CHCl3
CH2Cl2
0.84
0.88
0.82
0.82
0.78
0.77
1.08
1.16
0.10
0.090
0.12
0.11
0.12
0.090
0.094
0.094
ClCH2CH2Cl1.21
benzene
toluene
DMSO
CH2Cl2
0.086
0.088
1.20
1.22
0.78
0.82
0.92
0.075
0.074
0.085
0.14
[Mn(tpp)(OSO3H)]
[Mn(tpp)(OSO3)]À
CHCl3
0.082
ClCH2CH2Cl0.98
MeCOMe
DMF
1.00
0.99
0.89
0.094
0.13
0.092
DMSO
a) Molar absorptivity ratios of bands III (l 599 618 nm) and IV (l 564 582 nm). b) Molar absorptivity ratios
of bands IV (l 564 582 nm) and V (Soret band, l 462 474 nm).
Electrochemistry. The redox properties of [Mn2(tpp)2(SO4)] were investigated by
cyclic voltammetry. A representative voltammogram for this porphyrinatomanganese

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complex in ClCH2CH2Clis provided in Fig. 3. All reductions were found to be
diffusion-controlled as evidenced by constant current functions at all scan rates
employed [9][17]. Three waves were exhibited during cyclic voltammetry of the
sulfato-bridged [Mn2(tpp)2(SO4)] (0.5 mm) in Cl CHCH2Clbetween À0.2 and À2.2 V
2
(vs. SRE) (Fig. 3). The first two waves, waves Ia and Ib, appeared in the potential
region for the MnIII/MnII redox couple [18]. Wave Ia, observed at a potential of ca.
À0.55 V, appeared as a shoulder to wave Ib at Epc ˆ À0.71 V. Qualitatively, the
voltammogram was similar to that expected for a reversible two-electron charge
transfer in which DE8 ˆ À90 mV [17], with the exception that only one oxidation wave
was observed during the reverse sweep. This wave occurred at Epa ˆ À0.47 V. The third
wave, wave II, was observed at a potentialof À 1.91 V and corresponds to reduction of
the porphyrin ring to form the anion radical[18]. This process was irreversibel in this
solvent system, in agreement with the reported behavior of [Mn(tpp)(ClO4)] under
similar conditions [18]. The voltammetric response of [Mn(tpp)Cl] was analogous to
that reported previously [18][19].
Fig. 3. Cyclic voltammetry of [Mn(tpp)2(SO4)] (0.52 mm) in ClCH2CH2Cl:0 m m (PPN)2SO4 (solid line) and
10.0 mm (PPN)2SO4 (dotted line). Conditions: Pt-disk working electrode, SRE, (Bu4N)ClO4 (0.1m) supporting
electrolyte, 100 mV/s

Helvetica Chimica Acta Vol. 87 (2004)
127
The spectroelectrochemistry of the sulfato-bridged [Mn2(tpp)2(SO4)] in CH2Cl2
was examined by using an OTTLE cell. The results for the Soret region are shown in
Fig. 4. As the potentialwas stepped through and the negative of the first two
voltammetric waves, the Soret band for the MnIII species was replaced by a broad band
at 440 nm. This feature is associated with the spectra of five-coordinate porphyrina-
tomanganese(II) complexes that are axially coordinated by anionic ligands
[15][16][20]. It seems noteworthy that the Soret band associated with the MnIII
species was not diminished completely at potentials at which the MnII species begins
to develop, i.e., potentials more positive than À0.6 V. In the long-wavelength region,
the Qa and Qb bands at 577 and 612 nm shifted to 572 and 612 nm, respectively. No
isosbestic points were observed during the spectroelectrochemical reduction of
[Mn2(tpp)2(SO4)] in this solvent system.
Fig. 4. Spectroelectrochemistry of [Mn2(tpp)2(SO4)] (0.089 mm) in CH2Cl2 with (Bu4N)ClO4 (0.1m) in OTTLE
cell: À 0.2 V vs. SRE (solid line) and À1.0 V vs. SRE (dashed line)
The cyclic voltammetric behavior of [Mn2(tpp)2(SO)4)] (0.05 mm) in DMSO
between À0.2 and À2.0 V differed significantly from that in ClCH2CH2Cl( Fig. 5).
Only two voltammetric waves were observed during the reduction, as is typically
observed for porphyrinatomanganese complexes [18]. The first wave for the dimer,
representing the MnIII/MnII redox couple, occurred at E1/2 ˆ À0.68 V. The second
reduction, representing the first ring reduction, was observed at E1/2 ˆ À1.71 V. By
comparison, [Mn(tpp)Cl], which completely dissociates in DMSO to form
‡
[Mn(tpp)(DMSO)2] [18], yielded reduction waves at À0.66 V and À1.69 V.
The spectroelectrochemistry of the sulfato-bridged dimer was also carried out in
DMSO. Stepping the potentialnegative of the Mn III/MnII redox couple produced a
II
spectrum with a Soret band at 434 nm, typicalof five-coordinate [Mn (tpp)(S)] (S ˆ
axial solvent molecule) [15][16]. The reduced product also exhibited absorbances at
522, 563, and 605 nm.

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Fig. 5. Cyclic voltammetry of [Mn(tpp)2(SO4)] (0.52mm) in DMSO:0 m m (PPN)2SO4 (solid line) and 30.0 mm
(PPN)2SO4 (dotted line). Conditions: Pt-disk working electrode, SRE, (Bu4N)ClO4 (0.1m) supporting
electrolyte, 100 mV/s
‡
Addition of sulfate ion (as the PPN salt) to solutions of [Mn2(tpp)2(SO4)] in
ClCH2CH2Clresutled in the disappearance of wave Ia ( Fig. 3). At sulfate/dimer
concentrations greater than 1:2, this reduction process could no longer be observed in
the voltammogram. In addition, wave Ib became more quasi-reversible, and its peak
current increased by 1/3 with increasing ligand concentration. At 10 mm sulfate, DEp ˆ
0.45 V (vs. 0.25 V for the dimer) and Epc ˆ À0.86 V for wave Ib. At sulfate
concentrations greater than 10 :1, the cathodic peak potential was essentially
independent of sulfate concentration. Wave II was unaffected by addition of sulfate
below concentrations of 10 :1. Addition of sulfate concentrations above this amount
shifted Epc to slightly more-positive values, and enhanced the reversibility of wave II
due to the growth of a new anodic process at À1.77 V. At 10 mm sulfate, Epc ˆ À1.87 V
and E1/2 ˆ À1.82 V. The E1/2 for wave II was independent of sulfate concentration.
Cyclic voltammetric reduction of [Mn(tpp)Cl] in the presence of a large excess of
sulfate ion resulted in a voltammogram similar to that of [Mn2(tpp)2(SO4)] under
analogous conditions.
Spectroelectrochemical reduction of [Mn2(tpp)2(SO4)] in CH2Cl2 in the presence of
added sulfate was also investigated. Stepping the potential through the region of the
MnIII/MnII redox couple produced a VIS spectrum with a Soret band at 441 nm.
Significantly, isosbestic points were observed at ca. 405 and 455 nm as the potentialwas
stepped through the region of MnIII reduction.
Addition of excess sulfate ion to [Mn2(tpp)2(SO4)] in DMSO also resulted in some
changes in the voltammetric behavior. However, these changes were not as dramatic as

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those observed in the non-coordinating solvent system. The potential of the first wave
shifted negatively with increasing ligand concentration. However, between 0 and
30 mm sulfate, E1/2 differed by only 25 mV. The wave for the first ring reduction was
unaffected by added sulfate in DMSO. [Mn(tpp)Cl] yielded similar voltammetric
results in DMSO in the presence of added sulfate. The one-electron reduction of the
dimer in DMSO with added sulfate by using the OTTLE cell produced a spectrum with
a Soret band at 434 nm, consistent with [MnII(tpp)(DMSO)] [15][16].
Discussion. Effect of Solvent on [Mn2(tpp)2(SO4)] Structure and Electrochemistry.
[Mn2(tpp)2(SO4)] was apparently stable in non-coordinating solvents during the time
period over which the experiments were performed. No evidence of decomposition was
observed in the VIS spectrum of the dimer in non-coordinating solvents such as CHCl3
within 30 min. In addition, the solution IR spectrum of the dimer in ClCH2CH2Cl, after
appropriate background subtraction, was the same as that of the solid. The
voltammetric and VIS-spectral data indicate, however, that the sulfato-bridged dimer
was not stable in DMSO. Dissociation of the dimer occurred in this solvent, forming
‡
primarily the solvent-coordinated complex, [Mn(tpp)(DMSO)2] , although a small
amount of a sulfato-coordinated species may be present in this solution also. Analogous
to the behavior of the corresponding iron derivative [5][6], the equilibria of Eqns. 1
and 2 are proposed to occur for the dimer in DMSO based on the data. In this solvent
system, the reaction of Eqn. 1 apparently goes to completion, whereas the reaction of
Eqn. 2 favors the products at equilibrium. Although the chemistry of the sulfato-
bridged dimer in DMF was not investigated in detail( Table 1), it seems reasonable to
assume that dissociation of the dimer also occurs in this solvent given the similarities
between the spectra for the dimer in both DMF and DMSO. [Fe2(tpp)2(SO4)] has been
shown to dissociate according to Eqns. 1 and 2 in DMSO [5][6].
‡
[Mn2(tpp)2(SO4)] ‡ 2DMSO>[Mn(tpp)(DMSO)2] ‡ [Mn(tpp)(SO4)]À
(1)
(2)
[Mn(tpp)(SO4)]À ‡ 2DMSO>[Mn(tpp)(DMSO)2] ‡ SO24À
‡
In non-coordinating solvents, reduction of the sulfato-bridged complex apparently
involves dimer cleavage concerted with electron transfer. The mechanism according to
Eqns. 3 5 is proposed based on the electrochemical and spectrochemical data.
[Mn2(tpp)2(SO4)] ‡ eÀ>[MnII(tpp)] ‡ [MnIII(tpp)(SO4)]À
[Mn(tpp)(SO4)]À ‡ eÀ>[Mn(tpp)(SO4)]2À
[Mn(tpp)(SO4)]2À>[Mn(tpp)] ‡ SO24À
(3)
(4)
(5)
The reaction of Eqn. 3 was observed as wave Ia during the cyclic voltammetric
reduction of the sulfato-bridged adduct, and the reaction of Eqn. 4 as wave Ib. The
following results support this mechanism: a) The dimer [Mn2(tpp)2(SO4)] was stable in
the non-coordinating solvent systems, and two waves were observed in the potential
region for the MnIII/MnII redox couple. b) No isosbestic points were observed during

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the spectroelectrochemical reduction of the dimer in CH2Cl2, supporting the presence
of more than two species, such as [Mn2(tpp)2(SO4)], [MnIII(tpp)(SO4)]À, [MnII(tpp)],
and [MnII(tpp)(OSO3)]2À, in the solution during the spectral changes. c) Both MnII and
MnIII species were observed in the spectrum following one-electron borohydride
reduction of the dimer. d) The spectrum of the totally reduced product clearly showed
the presence of a porphyrinatomanganese(II) adduct. e) Spectraland electrochemical
data with added sulfate indicated that wave Ib is due to a sulfato-complexed MnIII
species different from the dimer. f) Wave Ia was apparently irreversible, i.e., the
oxidation observed at À0.47 V was coupled to wave Ib. Such behavior is expected if
dimer cleavage occurs following the initial reduction. Reoxidation would involve
bringing two monomeric porphyrinato complexes together at the electrode surface, a
process expected to be kinetically less facile than reoxidation of an intact dimeric
species. The electrochemical behavior exhibited for [Mn2(tpp)2(SO4)] in non-
coordinating solvents was consistent with that reported for the analogous iron
derivative under similar conditions [5].
The apparent lack of spectral features attributable to [Mn(tpp)] [21] in the
spectroelectrochemical results in CH2Cl2 must be addressed, since both [Fe(tpp)] and
[Fe(tpp)(SO4]2À have been observed during spectroelectrochemical reduction of
[Fe2(tpp)2(SO4)] under similar conditions [5]. Comparison of the relative e values for
the anion-complexed MnII adduct generated during spectroelectrochemistry of the
dimer with added sulfate vs. that in the absence of added ligand shows that the former is
1.7 times the latter. This result would be expected if spectroelectrochemical reduction
in the absence of added ligand does not convert all MnIII species to [MnII(tpp)(SO4]2À.
Although the exact reason is not known, spectral overlap may mask the spectrum of
[Mn(tpp)], a conclusion supported by the broadness of the Soret band observed in the
absence of added sulfate.
In DMSO, reduction of [Mn2(tpp)2(SO4)] involved the bis-solvent-coordinated
MnIII complex, forming [MnII(tpp)(DMSO)]. This is supported by both the spectroe-
lectrochemistry of the dimer in this solvent and the nearly identical E1/2 values for the
dimer and [Mn(tpp)Cl].
Further reduction of the MnII state in both solvent systems involves porphyrinato-
ring reduction.
Effect of Added Sulfate on [Mn2(tpp)2(SO4)] Structure and Electrochemistry. The
sulfato-bridged dimer reacted with additional ligand in all solvents studied to form a
sulfato-coordinated porphyrinatomanganese(III) monomer. In non-coordinating sol-
vents, the monomer was produced as the result of dimer dissociation according to
Eqn. 6. In DMSO, this occurred as the result of solvent displacement according to
Eqn. 7.
[Mn2(tpp)2(SO4)] ‡ SO24À >2 [Mn(tpp)(SO4)]À
(6)
(7)
‡
[Mn(tpp)(DMSO)2] ‡ SO24À >[Mn(tpp)(SO4)]À ‡ 2DMSO
Severalpieces of evidence support this conuclsion.
a) The IR spectrum of
[Mn2(tpp)2(SO4)] in ClCH2CH2Cl with a large excess of added sulfate, following
appropriate background subtraction, was consistent with [Mn(tpp)(SO4)]À. Qualita-

Helvetica Chimica Acta Vol. 87 (2004)
131
tively, the IR spectrum of the product species was similar to that reported for the
corresponding hydrogen sulfate adduct [2]. The splitting of the free sulfate band at
1104 cmÀ1 into bands at 1109 and 1032 cmÀ1 was consistent with monodentate
coordination of sulfate [8]. b) The VIS spectrum observed for the reaction product
was different from that observed for [Mn2(tpp)2(SO4)] and [Mn(tpp)Cl], and the
reported spectrum of [Mn(tpp)(HSO4)] [2]. c) The VIS spectrum and the eIII/eIV ratio
of the product were similar to those reported for other monomeric [Mn(tpp)X]
complexes containing −hard× axial ligands [13]. Additionally, the eIII/eIV and eIV/eV ratios
calculated for the product were different from those observed for the parent dimer and
the reported spectrum of [Mn(tpp)(HSO4)]. d) The spectralproperties of this suflate
complex were similar in the various solvent systems, indicating that the same or similar
species were generated in each case. In addition, reaction of [Mn(tpp)Cl] with excess
sulfate led to a product with the same VIS-spectral characteristics. Reaction of both
[Fe(tpp)2(SO4)] and [Fe(tpp)Cl] has been shown to lead to formation of a monomeric,
sulfato-coordinated FeIII adduct [5][6].
‡
Sulfate apparently does not bond as strongly to [MnIII(tpp)] as it does to the
analogous iron derivative. Addition of a high excess concentration of sulfate resulted
only in a 70 mV shift in E1/2 for [Mn(tpp)Cl] in ClCH2CH2Cl, whereas a 320-mV
difference in E1/2 between [Fe(tpp)Cl] and the sulfate complex has been observed [5].
This may indicate a significantly greater degree of ionic bonding between the axial
sulfato ligand and porphyrinatomanganese complexes compared to their iron counter-
parts [11][13]. As with the porphyrinatoiron complexes [5], the sulfate ion forms a
stronger complex with the porphyrinatomanganese adduct in non-coordinating
solvents than in coordinating solvents, where the dimer exists primarily as a bis-
solvent-coordinated complex in the absence of excess sulfate. This phenomenon may be
due in part to stronger solvation of the dianionic sulfate ion in the more strongly
coordinating solvents. As a result, higher concentrations of sulfate are needed before
this ligand can coordinate to the metal.
Reduction in the presence of excess sulfate ion involves the sulfato-coordinated
monomer in both non-coordinating solvents and DMSO. The negative shift in potential
with increasing sulfate concentration is consistent with stabilization of MnIII over MnII.
In CH2Cl2, the spectroelectrochemical results indicate that sulfate remained coordi-
nated to the metal following the metal-centered reduction (Eqn. 4). The quasi-
reversibility of this process may be the result of a large out-of-plane displacement that
the Mn-atom of the sulfato complex undergoes upon reduction. This would have the
effect of increasing the inner-shell activation barrier and decreasing the rate of electron
transfer [15]. Alternatively, the quasi-reversibility could be due to the slow rate of
ligand coordination [5]. In DMSO, the solvent binds preferentially to the reduced state,
even in the presence of a large excess of the sulfate ion. Thus, a solvent molecule
displaces the sulfate ion upon reduction of [MnIII(tpp)(OSO3)]À in this solvent.
The slight shift in potential and the enhanced reversibility for the reduction of
[MnII(tpp)] in ClCH2CH2Clat higher suflate concentrations ( Fig. 3) indicates slight
stabilization of the reduced ring species over [Mn(tpp)] under these conditions. Such
stabilization has been attributed previously to interactions between the porphyrinato p
ring and coordinating solvent molecules [18][22]. The exact cause of these observations
in the present study is not known and was not investigated further. However, it is

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conceivable that a weak p interaction could occur between the porphyrinato radical
‡
anion and the phenylgroups of the PPN ion under these conditions.
The financialsupport of the Grants and Research Funding Committee at Southeast Missouri State
University is gratefully acknowledged. We would also like to acknowledge Dr. Frank Schultz of the Department
of Chemistry at Indiana University-Purdue University at Indianapolis for the use of the OTTLE cell during the
spectroelectrochemical experiments and for helpful suggestions, and Dr. Michael Ryan of the Department of
Chemistry at Marquette University for his generous gift of (PPN)Cl.
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Received July 2, 2003