Platinum(IV) tetraphenylporphyrin dibromide complexes: Synthesis, characterization, and electrochemistry

Article abstract of DOI:10.1016/S0277-5387(00)00349-1

Platinum(IV) porphyrins of the type [Pt(IV) (p-X)₄TPP]Br₂ where (p-X)₄TPP is a para-substituted tetraphenylporphyrin have been synthesized by direct oxidation of their Pt(II) precursors with Br₂. Both the Pt(II) and Pt(IV) complexes have been characterized by visible and ¹H NMR spectroscopy and their electrochemical properties. In both the oxidation and reduction processes, a substituent effect is evident, and the reduction of Pt(IV) → Pt(II) is identified. The oxidation and reduction potentials of the [Pt(IV) (p-X)₄TPP] Br₂ complexes are compared to those of the [Pt(II)(p-X)₄TPP] precursors and [Pt(IV) (p-X)₄TPP]Cl₂ analogs. Addition of I₂ or I^- to Pt(II) porphyrins does not result in oxidative-addition to form Pt(IV) porphyrins. Instead, the addition of I^- to [Pt(IV) (p-X)₄TPP] Br₂ results in the reduction of the platinum to form the corresponding [Pt(II) (p-X)₄TPP)]. (C) 2000 Elsevier Science Ltd.

Full text of DOI:10.1016/S0277-5387(00)00349-1

www.elsevier.nl/locate/poly
Polyhedron 19 (2000) 1057–1062
Platinum(IV) tetraphenylporphyrin dibromide complexes:
synthesis, characterization, and electrochemistry
U
U
L.M. Mink a, 1, J.W. Voce a, J.E. Ingersoll a, V.T. Nguyen a, R.K. Boggess b, 2, H. Washburn b,
D.I. Grove b
a Chemistry Department, California State University, San Bernardino, CA 92407-2397, USA
b Department of Chemistry and Physics, Radford University, Radford, VA 24142-6949, USA
Received 6 December 1999; accepted 15 February 2000
Abstract
Platinum(IV) porphyrins of the type [PtIV(p-X)4TPP]Br2 where (p-X)4TPP is a para-substituted tetraphenylporphyrin have been syn-
thesized by direct oxidation of their Pt(II) precursors with Br2. Both the Pt(II) and Pt(IV) complexes have been characterized by visible and
1H NMR spectroscopy and their electrochemical properties. In both the oxidation and reduction processes, a substituent effect is evident, and
the reduction of PtIV™PtII is identified. The oxidation and reduction potentials of the [PtIV(p-X)4TPP]Br2 complexes are compared to those
of the [PtII(p-X)4TPP] precursors and [PtIV(p-X)4TPP]Cl2 analogs. Addition of I2 or Iy to Pt(II) porphyrins does not result in oxidative-
addition to form Pt(IV) porphyrins. Instead, the addition of Iy to [PtIV(p-X)4TPP]Br2 results in the reduction of the platinum to form the
corresponding [PtII(p-X)4TPP)].
q2000 Elsevier Science Ltd All rights reserved.
Keywords: Platinum(II); Platinum(IV); Bromide; Porphyrins; Electrochemistry; Voltammetry
1. Introduction
study of platinum(IV) dibromide compounds [19–21]. The
compounds in this paper are prepared in a fashion similar to
[PtIV(p-X)4TPP]Cl2 complexes previously reported [22].
The chemical and electrochemical reactivity of Pt(II) and
Pt(IV) tetraphenylporphyrins with the halogens chlorine,
bromine, and iodine is discussed in this research.
An extensive amount of research has focused on metallo-
porphyrins owing to the critical roles they play in electron
transfer, photosynthesis, and oxygen transfer processes [1].
Their reactivity is found to be dependent upon the central
metal, the substituents on the porphyrin ring, and the axially
coordinated ligands. Platinum porphyrins have been investi-
gated as oxygen sensing probes [2–6], photosensitizers
[7–9], potential antitumor agents [10], and molecular con-
ductors [11,12]. Most of the recent studies have focused on
Pt(II) porphyrins. Pt(IV) porphyrins of the type [PtIV(p-
X)4TPP]Br2, where XsCN, H, CH3, and OCH3, have been
synthesized from the corresponding [PtII(p-X)4TPP] pre-
cursors through direct oxidative-addition of Br2(l). Stable
[PtIV(p-X)4TPP]Br2 complexes are formed with axial Bry
ligands that generate large D splitting values in the Pt(IV)
(5d, t62ge0g) through s donation [13]. Considerable attention
has recently been devoted to the development of plati-
num(IV) dichloride complexes as pro-drug antitumor agents
[14–18], whereas relatively few reports have involved the
2. Experimental
2.1. General
1H NMR spectra were recorded at 270.167 MHz with a
JEOL ECLIPSE spectrometer (reference d 7.24, CHCl3).
UV–Vis spectra were obtained with a Beckman DU-7500
diode array instrument using quartz cells with a path length
of 1.0 cm. Cyclic voltammetry was carried out by the con-
ventional three-electrode method in CH2Cl2 (distilled from
CaH2) with Bu4NPF6 (0.10 M) as a supporting electrolyte.
Electrochemical measurements were made with either a PAR
174A electrochemical analyzer or an Amel model 551 poten-
tiostat and a model 567 function generator. The concentration
of the complex was usually between (0.5–1.0)=10y3 M.
Ferrocene was used as an internal standard. All potentials are
UCorresponding author. Tel. q1-909-880-5478; fax: q1-909-880-7066;
1
e-mail: lmink@csusb.edu
UCorresponding author.
2
0277-5387/00/$ - see front matter q2000 Elsevier Science Ltd All rights reserved.
PII S0277-5387(00)00349-1
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L.M. Mink et al. / Polyhedron 19 (2000) 1057–1062
referenced to the E$ (E$s$(EpaqEpc)) for the (C5H5)2Fe/
(C5H5)2Feq couple; E$ for ferrocene was typically
q0.48"0.02 V versus the AgNAgClNNaCl(saturated) BAS
reference electrode, near the 0.45 V reported for the ferrocene
couple versus an SCE reference electrode [23].
dominated by the Soret, a, and b bands near 420, 573, and
536 nm, respectively (Table 1). These bands exhibit 20–25
nm bathochromic shifts (to longer wavelengths)withrespect
to the precursor [PtII(p-X)4TPP] complexes and are similar
in absorption values to their [PtIV(p-X)4TPP]Cl2 analogs
[22]. A slight bathochromic shift of ;4 nm is detected for
the Soret band for the [PtIV(p-X)4TPP]Br2 complexes com-
pared to their chloride counterparts. Thus, there is a substan-
tial effect on the electronic spectra of the complexes as a
result of the PtII™PtIV oxidation but little effect due to var-
iance in the halogens at the axial positions. The bathochromic
shifts resulting from PtII™PtIV havebeeninterpretedasbeing
indicative of a decrease in metal d to porphyrin p*-bonding
[24]. As was observed for [PtII(p-X)4TPP] and [PtIV(p-
X)4TPP]Cl2, the Soret, a, and b bands of the [PtIV(p-
X)4TPP]Br2 complexes do not display any pronounced
substituent effect.
Reagent grade chemicals and solvents were purchased
from Aldrich Chemical Company, Fisher Scientific, and
Spectrum. Platinum(II) chloride was obtained from Alfa
Aesar, chlorine, from Matheson Gas Products, and bromine
from Janssen Chimica. Chromatographic grade aluminum
˚
oxide, activated neutral, standard grade 150 mesh, 58 A and
˚
silica gel (Merck grade 9385, 230-400, mesh 60 A) were
purchased from Aldrich Chemical Company. Elementalanal-
yses were performed by Desert Analytics, Tucson, AZ.
2.2. Synthesis of [PtIV(p-X)4TPP]Br2 (XsCN, H, OCH3,
CH3)
In the 1H NMR spectra, a distinct decrease in shielding of
all proton resonances is evident upon oxidation of [PtII(p-
X)4TPP] to [PtIV(p-X)4TPP]Br2 (Fig. 1), and a similar
trend was observed for the analogous [PtIV(p-X)4TPP]Cl2
complexes [22]. In comparing the 1H NMR of the [PtIV(p-
X)4TPP]Br2 to the [PtIV(p-X)4TPP]Cl2 complexes, thebro-
mide complexes display a slight upfield chemical shift of all
1H NMR resonances (;0.02 ppm). The upfield chemical
shift may be indicative of increased electron density about
the platinum center for the dibromide complexes. Increased
electron density about the platinum center would allow for
increased platinum–porphyrin p*-backbonding resulting in
increased shielding of the porphyrin proton resonances. This
would be anticipated as Pt(IV) forms a weaker bond with
the bromide than with the chloride ion [25], and this feature
is evident in the electrochemical results. As was the case for
the [PtIV(p-X)4TPP]Cl2 complexes, the JPt–H pyrrole cou-
pling decreases from ca. 11 Hz for Pt(II) to ca. 8 Hz for
Pt(IV) owing to the relativedecreaseinplatinum6scharacter
(Table 2).
The synthesis of the [PtII(p-X)4TPP] complexes has been
previously reported [22]. The [PtIV(p-X)4TPP]Br2 com-
plexes were synthesized through direct addition of Br2 to the
corresponding [PtII(p-X)4TPP] complexes. A Br2(l)/CHCl3
solution was prepared by the addition of 12 drops of Br2(l) to
10 cm3 of CHCl3, and 50 drops of the mixture were added
slowly to [PtII(p-X)4TPP] (0.022 mmol) dissolved in 20
cm3 of CHCl3 at 08C. The reaction was monitored by UV–
Vis spectroscopy and judged complete in 2 h by the gradual
decrease of the Soret band near 403 nm associated with
[PtII(p-X)4TPP] and the appearance of the Soret band near
421 nm associated with [PtIV(p-X)4TPP]Br2. The solution
was concentrated to 5 cm3 by rotary evaporation, and an
equal volume of cold methanol precipitated the [PtIV(p-
X)4TPP]Br2 crystalline solid. The complex was dissolved in
a minimum amount of toluene and loaded onto a 3 cm=18
cm silica gel column. Non-reacted [PtII(p-X)4TPP] was
eluted as a light orange band with toluene. [PtIV(p-
X)4TPP]Br2 was collected upon elution with 2% trifluoro-
acetic acetic acid (TFA) in chloroform and appeared as a dark
red–purple band. ConcentrationoftheTFA/chloroformsolu-
tion to 25 cm3 and addition of an equal volume of cold
methanol precipitated the red–purple [PtIV(p-X)4TPP]Br2
crystalline solid. [PtIV(p-X)4TPP]Br2 was collected by fil-
tration and then washed with cold methanol and water.
[PtIV(p-CN)4TPP]Br2 was not purified by column chroma-
tography owing to limited solubility. Final yields were in the
range of 75%. Elemental analyses gave the following results.
Calc. for [Pt(p-H)4TPP]Br2 (C44H28PtN4Br2): C, 54.62; H,
2.92; N, 5.79; Br, 16.52. Found: C, 53.95; H, 2.81; N, 5.57;
Br, 16.09%. Calc. for [Pt(p-OCH3)4TPP]Br2 (C48H36Pt-
N4O4Br2): C, 53.00; H, 3.34; N, 5.15; Br, 14.69. Found: C,
52.64; H, 3.01; N, 4.89; Br, 14.60%.
A porphyrin ring generally shows three or four well
defined, quasireversible sites of redox activity. The LUMO
of the porphyrin ring is considered to be the eg (p*) orbital
and the HOMO either an a2u (p) or a1u (p) orbital. In earlier
Table 1
Spectral data for [PtII(p-X)4TPP] and [PtIV(p-X)4TPP]Br2 complexes a
Soret band
b band
a band
PtII(p-CN)4TPP
403
427
402
426
407
430
405
427
511
538
510
535
512
537
511
535
541
573
[PtIV(p-CN)4TPP]Br2
[PtII(TPP)]
540
[PtIV(TPP)]Br2
b
[PtII(p-CH3O)4TPP]
[PtIV(p-CH3O)4TPP]Br2
[PtII(p-CH3)4TPP]
[PtIV(p-CH3)4TPP]Br2
539
573
539
3. Results and discussion
b
As reported for the [PtIV(p-X)4TPP]Cl2 complexes, the
UV–Vis spectra of the [PtIV(p-X)4TPP]Br2 complexes are
a Wavelengths in nanometers.
b Not observed.
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Fig. 1. NMR spectra of the aromatic region (270 MHz, reference d 7.24, CHCl3): (a) [PtII(p-CH3O)4TPP], (b) [PtIV(p-CH3O)4TPP]Br2.
Table 2
1H NMR chemical shifts and coupling constants a
H-pyr
H-ortho
H-meta
H-para
PtII(p-CN)4TPP
[PtIV(p-CN)4TPP]Br2
[PtII(TPP)]
8.68 (JPt–Hs11)
8.26
8.38
8.15
8.25
8.05
8.16
8.03
8.12
8.07
8.21
7.74
7.78
7.26
7.30
7.53
7.57
b
9.10 (JPt–Hs8)
8.76 (JPt–Hs12)
9.03 (JPt–Hs8)
8.78 (JPt–Hs10)
9.05 (JPt–Hs8)
8.76 (JPt–Hs11)
9.030 (JPt–Hs8)
7.75
7.79
[PtIV(TPP)]Br2
[PtII(p-CH3O)4TPP]
(–OCH3s4.13)
(–OCH3s4.10)
(–CH3s2.69)
(–CH3s2.70)
[PtIV(p-CH3O)4TPP]Br2
[PtII(p-CH3)4TPP]
[PtIV(p-CH3)4TPP]Br2
a Chemical shifts in ppm, coupling constants in Hz.
b NMR in acetonitrile solvent.
work, we were able to observe the four redox waves for
H2(TPP), but the oxidation at the most positive potential was
distorted and did not display good redox characteristics[22].
Substitution of the para protons by –CN to give H2(p-
CN)4TPP provides a ligand that does exhibit all four well
defined sites of redox activity (Table 3). The substituent
effect observed in other H2(p-X)4TPP ligands is evident for
H2(p-CN)4TPP as the first oxidation is shifted by q0.23 V
and the first reduction by q0.16 V relative to H2(TPP).
These are substantially larger changes than observed in other
H2(p-X)4TPP ligands as, for example, the first oxidation of
H2(p-Cl)4TPP relative to H2(TPP) is only q0.09 V.
In several instances, the electrochemistry of the [PtII(p-
X)4TPP] complexes also shows four sites of redox activity
attributable to the porphyrin ring, and the substituent effect
is observed in the complexes as well as the free ligands.
Reaction of the [PtII(p-X)4TPP] complexes with Cl2 pro-
duced the [PtIV(p-X)4TPP]Cl2 complexes. The four sites of
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L.M. Mink et al. / Polyhedron 19 (2000) 1057–1062
Table 3
Electrochemical data for the [PtIV(p-X)4TPP]Br2 complexes
H2(TPP) a
q1.01
q1.02(93)
q0.55(85)
q0.78(88)
q0.91(80)
q1.14(80)
q0.89(100)
q0.85(90)
q0.76(95)
y1.67(70)
y1.51(105)
y1.64(70)
y2.04(90)
y1.84(100)
y1.87(80)
y1.86(95)
y2.00(80)
y1.83(110)
y2.04(90)
H2(p-CN)4TPP a
[PtII(p-CN)4TPP] a
[PtIV(p-CN)4TPP]Br2
b
y0.50
y0.75
y0.80
y0.81
a
[PtIV(TPP)]Br2
a
[PtIV(p-CH3)4TPP]Br2
a
[PtIV(p-CH3O)4TPP]Br2
a Electrochemistry performed in CH2Cl2. Scan rates were 200 mV sy1. Potentials (mV) are referenced to the ferrocene/ferrocinium redox couple. Numbers
in parentheses are DEp (mV) and the numbers in italics are the PtIV™PtII(V) reductions.
b Electrochemistry performed in CH3CN.
redox activity remain, and a fifth irreversible reductionattrib-
utable to PtIV™PtII that showed a substantial substituent
effect was observed. With the preparation of the correspond-
ing [PtIV(p-X)4TPP]Br2 and [PtIV(p-X)4TPP]Cl2 com-
plexes, the effect of the axial ligand as well as the substituents
on the porphyrin ring can be studied. As was found for the
[PtIV(p-X)4TPP]Cl2 complexes, only the first oxidation and
first reduction waves of the ligand were always well defined.
Replacement of the two axial Cly ligands on the [PtIV(p-
X)4TPP]Cl2 complexes with Bry caused only a slight neg-
ative shift (-0.05 V) in both of these waves. However, a
substantial positive shift (between 0.19 and 0.31 V) in the
[PtIV(p-X)4TPP]Br2 complexes relative to the [PtIV(p-
X)4TPP]Cl2 complexes is observed for the irreversible
PtIV™PtII reduction (Table3). Thus, theaxialligands,which
are directly coordinated to the Pt(IV), have a pronounced
effect on that reduction, and the effect is in the direction
expected as Cly should form the stronger bond to Pt(IV),
making [PtIV(p-X)4TPP]Cl2 more difficult to reduce [25].
The reduction process for both the Cly and Bry complexes
can be described by:
scan D), the oxidation at q0.25 V, and impurities on the
porphyrin peak are present. The cyclic voltammograms
clearly indicate that the oxidation at q0.25 V and the small
impurities on the q0.89 V oxidation and reduction are asso-
ciated with the reduction of the PtIV and can be described by
the overall process:
[PtIV(TPP)]Br2q2ey™[PtII (TPP)]q2Bry
(2)
Either simultaneously or in a subsequent step, once the
Pt(IV) is reduced, the Bry becomes non-coordinating and is
irreversibly oxidized at q0.25 V. The small peaks associated
with the porphyrin ring at q0.89 V are due to the small
quantity of [PtII(TPP)] formed near the electrode surface.
Cyclic voltammetry of the [PtIV(p-CH3O)4TPP]Br2 com-
plex gave similar results. Proof that the q0.25 V peak was
due to Bry was obtained when a cyclic voltammogram of
tetra(n-butyl)ammonium bromide gave the same peak.
An attempt was made to determine whether a six-coordi-
nate Pt(II) species might at least temporarily result from the
reduction above. If reaction (2) is a two-step process:
[PtIV(TPP)]Br2q2ey™[PtII (TPP)]Br2
[PtII (TPP)]Br2™[PtII (TPP)]q2Bry
(3)
(4)
[PtIV(p-X)4TPP]X2q2ey™[PtII(p-X)4TPP]q2Xy
(1)
the addition of a large excess of Bry may retard reaction (4)
and reaction (3) may become partially reversible. The sup-
To illustrate, for the [PtIVTPP]Br2 complex, if the poten-
tial scan is initiated at 0.00 V in the anodic direction (Fig. 2,
scan A), the first porphyrin oxidation and, upon reversal of
the potential scan the subsequent reduction, is observed at
q0.89 V. The PtIV™PtII reduction isthen observedaty0.75
V, followed by the reduction, and upon reversal of the poten-
tial scan, the subsequent oxidation of the porphyrin ring at
y1.84 V. When the scan is initiated at 0.00 V in the cathodic
direction (Fig. 2, scan B), the PtIV™PtII reduction and first
reduction and subsequent oxidation of the porphyrin ring are
again observed. However, a new irreversible oxidation at
q0.25 V appears as do some small impuritiesassociatedwith
the porphyrin oxidation–reduction at q0.89 V. If scan A is
repeated but allowed to go through two cycles (Fig. 2, scan
C), the first cycle is identical to scan A but the second cycle
has the oxidation at q0.25 V and the small impurities asso-
ciated with the q0.89 V peak. When scan A is repeated and
allowed to go through the PtIV™PtII reduction, but not the
ring reduction and subsequent oxidation at y1.84 V (Fig. 2,
Fig. 2. Cyclic voltammograms for [PtIV(TPP)]Br2 in CH2Cl2 at a scan rate
of 100 mV sy1; scan A initiated at 0.0 V in an anodic direction; scan B
initiated at 0.0 V in a cathodic direction; scan C initiated at 0.0 V in an
anodic direction, multiple cycles; scan D initiated at 0.0 V in an anodic
direction, reversed after the Pt(IV) to Pt(II) reduction.
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1061
porting electrolyte tetra(n-butyl)ammonium hexafluoro-
Acknowledgements
phosphate was replaced with tetra(n-butyl)ammonium
bromide so that the [Bry] was 0.10 M. The electrochemical
results at scan rates to 500 mV sy1 were identical to the
previous results and failed to reveal any evidence of a revers-
ible nature to the PtIV™PtII reduction.
Partial support for this work was provided by the National
Science Foundation’s Division of Undergraduate Education
through grant DUE 9452076 for the purchase of the nuclear
magnetic resonance spectrometer and by a Cottrell College
Science Award of Research Corporation (LM). The authors
thank Alfa Aesar, a Johnson Matthey Company, for their
metal loan program, and CSUSB Associated Students, Inc.
RKB wishes to acknowledge support by a Cottrell College
Science Award by Research Corporation and the National
Science Foundation (TFI-802595) for funds to purchase the
electrochemical equipment.
For the [PtIV(p-X)4TPP]Br2 complexes, the data of Table
3 illustrate that the first oxidation and the first reduction of
the porphyrins do exhibit a substituent effect similar to that
found for the [PtIV(p-X)4TPP]Cl2 complexes. The electron-
donating CH3O– and CH3– groups shift the first oxidation
and the first reduction to a more negative potential, and the
electron-withdrawing –CN group shifts them toward a more
positive potential. As has been observed with other Pt(II)
systems, the addition of I2 or Iy in the presence of the oxi-
dizing agent hydrogen peroxide, H2O2, to Pt(II) porphyrins
does not result in oxidative-addition generating Pt(IV) por-
phyrins [26–29]. The addition of Iy to [PtIV(p-X)4TPP]Br2
results in reduction of the platinum to form the corresponding
Pt(II) porphyrins, [PtII(p-X)4(TPP)]. However, addition
of iodide to [PtIV(p-X)4TPP]Cl2 does not result in platinum
reduction. This reduction process of Pt(IV) complexes by
iodide has previously been observed and is believed to pro-
ceed through iodide–halide bond formationpriortothereduc-
tion in which the halogen acts as a bridging ligand [30–33].
The fact that iodide effectively reduces[PtIV(p-X)4TPP]Br2
and not [PtIV(p-X)4TPP]Cl2 is attributed to the more posi-
tive reduction potentials(byca. 0.2V)of thebromidespecies
compared to their chloride counterparts. The enhanced ease
of reduction of Pt(IV) bromide complexes compared to anal-
ogous Pt(IV) chloride complexes has previously been attrib-
uted to a weaker Pt(IV)–Br bond, and bromide being a good
bridging ligand during the reduction process [25,28,29,34–
38].
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4. Conclusions
[PtIV(p-X)4TPP]Br2 complexes are readily accessible
through direct oxidative-addition of Br2 to [PtII(p-X)4TPP].
The [PtII(p-X)4TPP] porphyrins are non-reactive toward I2
or iodide in the presence of an oxidizing agent. Iodide effec-
tively reduces the [PtIV(p-X)4TPP]Br2 complexes whereas
the [PtIV(p-X)4TPP]Cl2 complexes are unaffected.Replace-
ment of axial Cly ligands on the Pt(IV) by Bry has little
effect on the electrochemistry of the complex other than to
move the reduction of the Pt(IV) to considerably more pos-
itive potentials. The reduction results in the loss of the axial
ligands and is described by:
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´
´
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