Spectral, magnetic and electrochemical properties of metal oxa- and oxathia-porphyrins

Article abstract of DOI:10.1039/a801934g

Metal derivatives (Cu^II, Ni^II) of monooxa-, dioxa- and oxathia-tetraphenylporphyrins and their one-electron oxidised and reduced species have been studied. Electronic spectra of the monooxa and oxathia derivatives exhibit split Soret bands and a complicated pattern of Q-bands revealing their lower symmetry. The spectra of one-electron reduced species show only marginal shifts while the one-electron oxidised product of copper monooxaporphyrins show broad, red-shifted bands. Cyclic voltammetric studies indicated one-electron metal-centered reduction at fairly low potentials forming copper(I) and nickel(I) porphyrins. The ring-oxidised product exhibits weak antiferromagnetic interaction between the unpaired electrons of copper and the porphyrin ring. The ESR spectra of the copper dioxa- and oxathia-porphyrins exhibit rhombic symmetry with unusually small metal hyperfine couplings. A comparison of A^Cu values and the E_1/2 values for metal reduction suggests that distortion towards tetrahedral symmetry and the presence of a soft donor atom like sulfur in porphyrins are required to generate spectral and electrochemical properties like those observed for the type I copper center in proteins.

Full text of DOI:10.1039/a801934g

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Spectral, magnetic and electrochemical properties of metal oxa- and
oxathia-porphyrins†
Bashyam Sridevi,a Seenichamy Jeyaprakash Narayanan,a Alagar Srinivasan,a
Tavarekere K. Chandrashekar*,a and Japyesan Subramanianb
a Department of Chemistry, Indian Institute of Technology, Kanpur-208016, U.P., India
b Department of Chemistry, Pondicherry University, Pondicherry-605014, India
Metal derivatives (CuII, NiII) of monooxa-, dioxa- and oxathia-tetraphenylporphyrins and their one-electron
oxidised and reduced species have been studied. Electronic spectra of the monooxa and oxathia derivatives
exhibit split Soret bands and a complicated pattern of Q-bands revealing their lower symmetry. The spectra
of one-electron reduced species show only marginal shifts while the one-electron oxidised product of copper
monooxaporphyrins show broad, red-shifted bands. Cyclic voltammetric studies indicated one-electron metal-
centered reduction at fairly low potentials forming copper() and nickel() porphyrins. The ring-oxidised product
exhibits weak antiferromagnetic interaction between the unpaired electrons of copper and the porphyrin ring. The
ESR spectra of the copper dioxa- and oxathia-porphyrins exhibit rhombic symmetry with unusually small metal
hyperfine couplings. A comparison of ACu values and the E values for metal reduction suggests that distortion
₁
₂
towards tetrahedral symmetry and the presence of a soft donor atom like sulfur in porphyrins are required to
generate spectral and electrochemical properties like those observed for the type I copper center in proteins.
Replacement of pyrrole N atoms in the porphyrin core by other
heteroatoms such as S, Se, Te and O alters the core sizes
and electronic structure.1,2 Recent studies on these systems
have resulted in the synthesis of diverse porphyrins such as
tetrathiaporphyrin dication,3 tetraoxaporphyrin dication,4
tetraoxa[22]porphyrin5 and tetraoxa[26]porphyrin dications
([22] and [26] refer to the number of π electrons),6 dithia-,
diselena-, monothia-, monoselena- and monotellura-
porphyrins.1,2 The tetrathia and tetraoxa derivatives cannot
bind metal ions because of their dicationic nature while metal
complexes of dithia and diselena derivatives have not been
reported. However, monothiaporphyrin forms complexes with
metal ions such as FeII, CuII, NiII and RhII, etc.;7,8 only the
nickel() complex of monoselenaporphyrin has been reported.9
The crystal structure of nickel() thiaporphyrin revealed a
distorted square pyramid around the metal with an axial
chloride ligand, leading to five-co-ordination.7,8 The unique
feature of the structure is the co-ordinated thiophene ring which
is bent out of the plane of the remainder of the ligand core and
bound to the metal in a η1 fashion. Further, it has been shown
that the derivatives of CuII and NiII undergo one-electron metal-
centered reduction10 to form the corresponding complexes of
CuI and NiI and the nickel() ion has a nearly planar geometry
and surprisingly short Ni᎐N and Ni᎐S bonds.11
and oxathia-porphyrins revealed that the oxaporphyrins behave
more like the parent tetraphenylporphyrin in their spectro-
scopic properties while their electrochemical properties
resemble those of thiaporphyrins.
In this paper we report the metal binding (CuII and NiII)
properties of monooxa-, dioxa- and oxathia-porphyrins and
their one-electron reduced and oxidised species. Specifically,
electronic absorption spectra have been used to study ground-
state properties while the redox chemistry has been monitored
by cyclic voltammetry. The changes occurring in the electron
delocalisation pathway and the metal–ligand bond strength
upon binding of CuII have been studied by ESR spectroscopy. It
has been shown that the first reduction is metal centered,
corresponding to the formation of complexes of CuI and NiI,
while the first oxidation of copper monoxaporphyrins is ring
based with a weak antiferromagnetic coupling between the
metal and the ring unpaired electrons.
Results
The metallation of oxaporphyrins can readily be done using
literature methods.12 The copper() and nickel() derivatives of
monooxa and monothia derivatives are stable both in the solid
and solution states, the dioxa derivative on a chromatographic
column undergoing gradual demetallation while the oxathia
derivative on standing for few hours in the solid decomposes to
give the corresponding free base porphyrin. The electronic
absorption spectra of various neutral, one-electron-reduced and
-oxidised copper oxaporphyrins are shown in Fig. 1 and the
data are in Table 1. In general, the optical bands are red shifted
both in the Soret and Q-band region relative to the parent
[M(tpp)] (M = Cu or Ni) and the magnitude of the shifts are
smaller for oxaporphyrins compared to their thiaporphyrin
counterpart.14 The ε values are significantly reduced for the
core-modified porphyrins relative to the parent [M(tpp)].16
The EPR spectra of the copper porphyrins described here
were recorded in toluene–CH2Cl2 (1:1) at room and liquid
nitrogen temperature. Fig. 2 compares room temperature
spectra of [CuLS(Cl)] and [CuLO(Cl)]. Only the former exhib-
ited superhyperfine couplings from the co-ordinated nitrogens
at room temperature. However, at low temperature, these
couplings are observed for monooxaporphyrins while the
However, relatively few reports2 exist on the synthesis of
oxaporphyrins and mixed oxathiaporphyrins in which one or
more pyrrole rings have been replaced by furan rings and/or
thiophene rings. To the best of our knowledge there are only
two recent reports on the metal (NiII) binding properties of
oxaporphyrins by Latos-Grazynski and co-workers12 and
Gross et al.13 Both monooxa- and dioxa-porphyrins form
nickel() complexes, which can easily be reduced to the corres-
ponding nickel() derivative and the crystal structures reveal a
nearly in-plane co-ordination of the furan moiety as opposed to
the side-on co-ordination observed for the thiophene and
selenophene porphyrins.7 We have been interested in the core
modification of tetraarylporphyrins and our earlier studies on
thiaporphyrins have revealed many interesting similarities
and differences in optical, electrochemical and photochemical
properties.14 Further, our recent study15 on free base oxa-
† Non-SI units employed: G = 10Ϫ4 T, µB ≈ 9.27 × 10Ϫ24 J TϪ1
.
J. Chem. Soc., Dalton Trans., 1998, Pages 1979–1984
1979

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Table 1 Electronic spectral data of various copper and nickel porphyrins in CH2Cl2
λmax/nm (10Ϫ4 ε/Ϫ1 cmϪ1
)
Porphyrin
Soret band
Q-bands
[CuLS(Cl)]
[CuLO(Cl)]
[CuLO,Me(Cl)]
[CuLO,OMe(Cl)]
[CuLO,Br(Cl)]
[CuL2OCl2]
[CuLOSCl2]
[NiLS(Cl)]
463 (8.5)
430 (7.0)
434 (11.5)
436 (5.5)
434 (10.6)
417 (7.6)
454 (12.4)
466 (8.0)
428 (8.6)
417 (13.5)
429 (9.2)
416 (7.9)
421 (10.1)
—
418 (7.0)
—
437 (12.5)
428 (17.6)
416 (8.4)
—
705 (5.4)
625 (1.4)
634 (2.1)
639 (1.9)
636 (3.0)
670 (3.0)
557 (12.2)
572 (6.3)
575 (7.1)
580 (4.5)
579 (8.2)
600 (2.6)
—
534 (8.0)
538 (9.3)
542 (5.9)
538 (10.9)
507 (8.1)
688 (12.6), 644 (13.2), 578 (11.9), 538 (14.4)
710 (6.9), 675 (7.0), 612 (8.8), 547 (11.6), 511 (22.7)
738 (1.6), 707 (1.7), 633 (2.8), 579 (8.6), 551 (7.6)
688 (4.6), 665 (4.5), 604 (4.1), 504 (14.0)
[NiLO(Cl)]
[NiL2OCl2]
R1
Table 2 Spin Hamiltonian parameters evaluated from ESR spectra for
various copper porphyrins
Cu
||
Cu
N
Porphyrin
A
A⊥
A⊥
g||
g⊥
α2
R3
R2
[Cu(tpp)]
209
159
118
115
32.8
28.4
25.7
25.5
27.2
n.r.
Ayy
16.4
14.2
12.9
12.7
13.6
n.r.
Azz
102
99
2.1870
2.2120
2.2493
2.2506
2.2561
2.2569
gxx
2.032
2.056
2.036
2.042
2.040
2.036
gyy
0.8277
0.7218
0.6540
0.6528
0.6535
0.6390
gzz
[CuLS(Cl)]
[CuLO(Cl)]
[CuLO,Me(Cl)]
R3
R2
N
N
N
O
Cl
M
[CuLO,OMe(Cl)] 114
[LO,Br(Cl)]
110
Axx
75
R1
[CuL2OCl2]
[CuLOSCl2]
30
29
2.01
2.01
2.08
2.085
2.29
2.31
78
The A values are in 10Ϫ4 cmϪ1. n.r. = Not resolved.
Monooxaporphyrin
R1
R2
R3
M = Cu2+
M = Ni2+
[NiLO(Cl)]
H
H
H
H
H
H
H
H
H
[CuLO(Cl)]
Me
OMe
Br
[CuLO,Me(Cl)]
[CuLO,OMe(Cl)]
[CuLO,Br(Cl)]
Ph
Ph
O
N
N
N
O
S
N
Cl
M
Cl
M
Ph
Ph
Ph
Ph
Cl
Cl
O
Ph
Ph
Dioxaporphyrin
Oxathiaporphyrin
M = Cu2+ [CuL2OCl2]
M = Ni2+ [NiL2OCl2]
M = Cu2+ [CuLOSCl2]
dioxa- and oxathia-porphyrins did not show any superhyperfine
couplings in frozen solution or in the solid diluted in a
diamagnetic matrix (free base dioxaporphyrin) even at low tem-
perature (Fig. 3). The spin Hamiltonian parameters17 evaluated
from the observed spectra are listed in Table 2. In general, the
EPR spectra reveal the presence of a distorted tetragonal
copper site17,18 in the monooxa derivative while the dioxa and
oxathia derivatives show rhombic spectra. Replacement of a
N-donor atom of a pyrrole ring by O or S results in a decrease
in the ACu values with a concomitant increase in g values.19 The
nitrogen superhyperfine splittings observed for some derivatives
show only a minor variation on going from [Cu(tpp)] to its
oxa and thia derivatives, suggesting a predominantly isotropic
interaction.17,18
Fig. 1 Electronic absorption spectra of (A) copper monooxa deriv-
atives and their one-electron-oxidised and -reduced species in CH2Cl2.
The concentrations used were: Soret band, 2.6 × 10Ϫ6; and Q-bands,
2.6 × 10Ϫ5 . (B) Copper dioxa and oxathia derivatives and one-
electron-reduced copper dioxaporphyrin in CH2Cl2; concentrations as
in (A)
as the supporting electrolyte show one-electron quasi-reversible
(∆Ep 90–140 mV) reduction at fairly low potentials except
for [NiLO(Cl)] and [CuLOSCl2] which show irreversible reduc-
tion. Fig. 4 compares cyclic voltammograms of [NiLS(Cl)],
[CuLS(Cl)] and [CuLO(Cl)]. In all the cases the separation
between the anodic and cathodic peaks was dependent on the
Cyclic voltammetric studies in CH2Cl2 containing NBu4ClO4
1980
J. Chem. Soc., Dalton Trans., 1998, Pages 1979–1984

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Table 3 Reduction potential for the MII–MI coupling for various
copper and nickel porphyrins
II
I
₁
Porphyrin
E (M –M )/V
∆Ep/mV
₂
[CuLS(Cl)]
[CuLO(Cl)]
[CuLO,Me(Cl)]
[CuLO,OMe(Cl)]
[CuLO,Br(Cl)]
[CuL2OCl2]
[CuLOSCl2]
[NiLS(Cl)]
Ϫ0.05
130
131
145
118
114
105
294
106
350
97
Ϫ0.22
Ϫ0.24
Ϫ0.25
Ϫ0.24
Ϫ0.37
Ϫ0.11
Ϫ0.06
Ϫ0.42
Ϫ0.36
[NiLO(Cl)]
[NiL2OCl2]
Fig. 2 Room temperature EPR spectra of copper monooxa (LO) and
monothia (LS) complexes in toluene–CH2Cl2 (1:1). The concentrations
used were ≈10Ϫ3 ; dpph = diphenylpicrylhydrazyl
Fig. 4 Cyclic voltammograms of copper monooxa-, monothia- and
nickel monothia-porphryins in CH2Cl2 recorded at 100 mV sϪ1 scan
speed; 0.1  NBu4ClO4 was used as the supporting electrolyte in CH2Cl2
nature. Repeated magnetic susceptibility measurements at room
temperature show a small magnetic moment in the range 0.8–
1.1 µB suggesting a weak antiferromagnetic interaction. This
value may be contrasted with 2.40 µB measured for [Cu(tpp)]
radical cation in CD2Cl2.21 A recent study from this laboratory
on a distorted copper() porphyrin cation radical also showed
an antiferromagnetic interaction between the two unpaired
electrons of the metal and ring both in the solid and solution
state.22 Chemical oxidation of [CuL2OCl2] and [CuLOSCl2]
resulted in demetallation leading to free base dications as
revealed by the absorption spectra.
Fig. 3 (a) The EPR spectrum of copper dioxaporphyrin frozen
in toluene–CH2Cl2 (1:1). The concentration used was ≈10Ϫ3 . (b)
Simulated spectrum; spin Hamiltonian parameters in Table 2
Discussion
Electronic absorption spectra
scan rate and an illustration of this effect for [CuLOSCl2] along
with cyclic voltammograms of the dioxa derivatives of CuII and
NiII are shown in Fig. 5. The half wave potentials (Table 3)
evaluated for this reduction in all the cases are outside the range
accessible to porphyrin ring reduction in the same solvent. For
example, the first ring reductions of the free oxa, dioxa and
oxathia base derivatives fall in the range Ϫ0.90 to Ϫ1.20 V.15
Based on earlier work on thiaporphyrins from this laboratory14
and others7 and the electronic spectra of the ring-reduced
products,20 this reduction is assigned to the metal-centered
reduction corresponding to the formation of porphyrins of CuI
and NiI. It is pertinent that Latos-Grazynski and co-workers12
have recently isolated the nickel() complexes of oxa- and dioxa-
porphyrins.
The chemical oxidation of copper() monooxa derivatives
with a one-electron oxidant leads to the formation of ring-
oxidised products and the absorption spectra exhibit the
changes which would be expected upon ring oxidation20 [Fig.
1(A)]. The oxidised species are EPR silent both in the solid and
solution state at room temperature suggesting their diamagnetic
The absorption spectra of the metal oxa and thia derivatives
show characteristic Soret and Q-bands in the region 400–800
nm.16 The monooxa, monothia and oxathia derivatives exhibit
split Soret bands and a complicated pattern of Q-bands, unlike
[M(tpp)] (M = Cu or Ni), revealing their lower symmetry. On
the other hand, the spectra of metal dioxaporphyrins are not
too much different from those of the corresponding free base
derivatives.15 The one-electron-reduced species show small blue
shifts of absorption bands while the one-electron-oxidised
[CuLO(Cl)] exhibits significant changes. This is not surprising
considering the fact that the first ring reduction involves the
metal center and on going from MII to MI the porphyrin ring
does not undergo any major change in its π-electron conju-
gation. On the other hand, the oxidation of the porphyrin ring
affects the porphyrin π-electron conjugation because of the
removal of an electron from the π system10,20 and is expected to
show broad red-shifted Q-bands and the Soret band as
observed. The ε values of the metal derivatives are about 50%
smaller than those of the corresponding free base derivatives,
suggesting a decreased π-electron conjugation upon metal
J. Chem. Soc., Dalton Trans., 1998, Pages 1979–1984
1981

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Fig. 6 Calculated optimised structures of [CuLO(Cl)], [CuL2OCl2] and
[CuLOSCl2]. The meso phenyl rings are omitted for clarity
Fig. 5 Cyclic voltammograms of (A) the copper oxathiaporphyrin in
CH2Cl2 at various scan speeds. (B) Comparison of cyclic voltam-
mograms of copper and nickel dioxaporphyrins in CH2Cl2 at scan
speed 200 mV sϪ1
energy of the b1 molecular orbital because of its antibonding
character. The gradual decrease in the value of α2 observed on
distortion (Table 2) suggests a lowering of the energy of the b1
molecular orbital relative to that of [Cu(tpp)] and a weaker
Cu᎐N σ bond in the distorted porphyrins. Strong support for
such a conclusion comes from (a) the observed small decrease
interaction. This probably is due to the distorted geometry of
the porphyrin ring around the metal (see below).
N
ESR spectra
in A⊥ values for copper() monooxaporphyrins relative to
[Cu(tpp)] and (b) comparison of measured bond distances for
Cu᎐N in [Cu(tpp)]26 [1.981(7) Å] and [CuLS(HCO3)]7 (1.993,
2.042 and 2.067 Å).
The spread of g values, g|| in particular, for CuX4 systems with
X = S, O and N is well documented.19,23 Generally, the g|| values
decrease and increase respectively for X = S and O donors
relative to X = N. This has been interpreted in terms of the
relative hard and soft nature of the donor atoms.24 Thus, on
going from CuN4 to CuN3S for a monothiaporphyrin the value
is expected to decrease slightly if the core structure around the
metal remains square planar in CuN3S. However, the g|| value
increases slightly (∆g|| = 0.025) for CuN3S suggesting a distor-
tion from the regular planar geometry. The dioxa and the
oxathia derivatives result in further distortion around the metal
centre and the lower symmetry is reflected in the rhombic spec-
tra observed for these complexes.
The decrease in the A values has been attributed to the direct
mixing of 4s metal orbitals with the ground state of CuII caused
by the symmetry lowering.7 This is substantiated by recent work
of Solomon and co-workers25 on the reduced and oxidised site
of blue copper proteins. They have ascribed the small EPR
hyperfine splittings observed for the oxidised form of blue
copper proteins to the extremely high covalency of the copper
The energy-optimised structures calculated for the monooxa,
dioxa and oxathia copper() derivatives shown in Fig. 6 further
confirm the existence of distortion. However, it should be
pointed out that the geometry-optimised structures give only a
rough idea about the geometry around the metal centre and this
should be taken as a qualitative explanation. A comparison of
structures revealed maximum distortion for the oxathia deriv-
ative which is not surprising because of the different co-
ordination modes of thiophene sulfur and furan oxygen. It is
known from the crystal structures reported for nickel() com-
plexes of thia- and oxa-porphyrins that the furan oxygen co-
ordinates in a η1 fashion without serious distortion while the
thiophene sulfur is bent out of the plane of the porphyrin ring
upon co-ordination.11 However, the observed distortion for
[CuL2OCl2] is surprising in view of the reported structure of
[NiL2OCl2] which shows a pseudo-octahedral geometry in which
the pyrrole and furan rings are coplanar. It is possible that Cu2ϩ
being a Jahn–Teller ion can still be distorted and this is not
expected for the corresponding nickel() derivative. The obser-
vation that the Soret band of [CuL2OCl2] is broad compared to
that of [NiL2OCl2] probably suggests a lower symmetry in the
copper() complexes. Only crystal structures can confirm this
point.
2 Ϫ y2
3dx
HOMO which is highly anisotropic with 42% Cu and
36% S (cysteine) and <2% on each histidine nitrogen. It has
been shown that only the mixing of the unoccupied 4s/4p
2 Ϫ y2
orbitals and the half occupied copper 3dx
orbital with the
filled ligand levels contributes to the strength of the individual
Cu᎐L bonds. In the present study the changes occurring upon
distortion can at least be evaluated for the Cu᎐N in-plane
σ bond through the covalency factor α2 which represents the
bonding coefficient of the b1 molecular orbital.18 The stronger
the in-plane σ bond between the CuII and N, the higher is the
Electrochemical studies
The electrochemical data of the complexes establish the pres-
ence of one-electron metal-centered reduction in each case and
1982
J. Chem. Soc., Dalton Trans., 1998, Pages 1979–1984

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the reduction potentials are raised to more positive values by
partial replacement of N-donors by S and/or O. However, the
Conclusion
It has been demonstrated that the core-modified oxa- and
oxathia-porphyrins bind metal ions and their spectral and
electrochemical properties parallel those observed for the
thiaporphyrins. The metallation of oxathiaporphyrin estab-
lishes the co-ordination of pyrrole nitrogen, thiophene sulfur
and furan oxygen in a single porphyrin unit. The unusually
small ACu values observed for [CuL2OCl2] and [CuLOSCl2] repre-
sent the lowest values observed to date for any copper()
porphyrin. Furthermore, this study has demonstrated that the
maximum distortion towards tetrahedral symmetry does not
necessarily shift the reduction potential for the MII–MI couple
to more positive values at least for porphyrin systems, and
the presence of a soft donor atom is necessary to generate the
low ACu values and the more positive reduction potentials
observed for blue copper proteins. Probably the copper() com-
plex of a dithiaporphyrin could highlight this observation.
Unfortunately, the dithiaporphyrin reported in which the two
thiophene rings are opposite to each other does not form a
copper() complex. Currently, we are exploring the synthesis
of core-modified expanded porphyrins in which a CuN2S2
co-ordination geometry can be achieved.
₁
individual E values vary depending on the metal and the
₂
number and nature of the donor atoms. For the copper por-
phyrins, E varies as [CuLS(Cl)] > [CuLO(Cl)] > [CuL2OCl2]
₁
₂
while for the nickel porphyrins [NiLS(Cl)] > [NiL2O(Cl)] >
[NiLO(Cl)]. In both cases the thiaporphyrins show more posi-
tive values compared to those of the oxaporphyrins. This estab-
lishes that the presence of a soft donor atom like S may be one
of the factors responsible for the more positive values by stabil-
isation through its π-acceptor capability.23,27 Comparison of
Ni᎐S bond distances measured from the crystal structures of
thiaporphyrins of NiII and NiI establishes this possibility
(NiII᎐S 2.296 and NiI᎐S 2.143). If, on the other hand, the mag-
nitude of distortion towards tetrahedral geometry is the only
deciding factor for the more positive E value, then [CuL2OCl2]
₁
₂
and [CuLOSCl2] would be expected to show more positive E
₁
₂
₁
values relative to the others. Since the measured E values do
₂
not show this trend, clearly both the geometrical changes upon
reduction and the nature of the donor atom in the porphyrin
II
₁
core are important in deciding the magnitude of E for the M –
₂
MI couple. This is further substantiated by the conclusions
of Solomon and co-workers25 in their electronic structure studies
on the reduced and oxidised site of blue copper proteins. It
has been pointed out that the two factors which are import-
ant for high reduction potentials are: (a) the weakening of
the copper–thioether axial bond in the oxidised state and (b)
the formation of a strong copper–thiolate equatorial π bond
Experimental
Syntheses of complexes
Free base oxa- and dioxa-porphyrins were prepared and charac-
terised following the reported procedure2c,12 and the oxathia-
porphyrin was prepared and characterised as described in our
earlier work.15 All the chemicals used for the syntheses were
reagent grade unless otherwise specified. Solvents for spectro-
scopic measurements were purified and dried according to the
standard methods.
2 Ϫ y2
through the overlap of π orbitals on S with the 3dx
orbital
on Cu. However, it should be mentioned that recent work on
bis(pyrazole) complexes of CuII with only N and O donors also
revealed high reduction potentials for the CuII–CuI couple and
this has been explained on the basis of the availability of an
appropriate ligand geometry for stabilisation of CuI.28
In general, the reversibility of the MII–MI couple depends on
the structural change accompanying the reduction process.29
If no hindrance is expected to the adoption of a tetrahedral
stereochemistry upon reduction, then the redox couple exhibits
Nernstian behaviour, while any structural change such as ligand
loss or ligand substitution under the reaction conditions results
in a quasi-reversible or irreversible couple. For example, the
irreversible nature of [Cu(Me4cyclam)]2ϩ (Me4cyclam = 1,4,8,11-
tetramethyl-1,4,8,11-tetraazacyclotetradecane) reduction is
associated with ligand substitution in the reduced form,
while the reversible nature of the [Cu(dth)2]2ϩ–[Cu(dth)2]ϩ
(dth = 2,5-dithiahexane) couple is assigned to the adoption of
tetrahedral symmetry without hindrance.19,23 The deviation
from the electrochemical reversibility of porphyrin complexes
of Cu and Ni observed in the present study suggests that the
MII–MI reduction is accompanied by a relatively slow structural
rearrangement because of the macrocyclic constraints. That
the peak separation depends on the scan rate further justifies
such a conclusion.
[CuLO(Cl)]. A solution of CuCl2ؒ2H2O (0.040 g, 1.173 mmol)
in ethanol (5 cm3) was added to a solution of 5,10,15,20-
tetraphenyl-21-oxaporphyrin (0.030 g, 0.048 mmol) in dichloro-
methane (50 cm3) and the mixture refluxed for 3 h with stirring.
It was cooled and then the solvent was removed under reduced
pressure. The solid was dissolved in dichloromethane and
washed several times with water. The organic fraction was dried
over anhydrous sodium sulfate and chromatographed on a
basic alumina column using dichloromethane–methanol
(95:5). The first fraction was identified as free base and the
metallated porphyrin was eluted next with dichloromethane–
methanol (90:1). The solid obtained after evaporation of sol-
vent under reduced pressure was dried in vacuum which gave
pure [CuLO(Cl)] (0.027 g, 79%). FAB mass spectrum: m/z 678
(M Ϫ Cl) (calc. 678.27) (Found: C, 74.4; H, 3.8; N, 5.5.
C44H28ClCuN3O requires C, 74.04; H, 3.95; N, 5.88%). A
similar procedure was followed for other copper() mono-
oxaporphyrins.
[CuL2OCl2]. A solution of CuCl2ؒ2H2O (0.030 g, 0.879 mmol)
in ethanol (5 cm3) was added to a solution of 5,10,15,20-
tetraphenyl-21,23-dioxaporphyrin (0.040 g, 0.065 mmol) in
dichloromethane (50 cm3) with stirring. The mixture was heated
under reflux for 8 h. It was cooled and the solvent removed
under reduced pressure. The solid was washed with water till
the filtrate was colourless, then extracted with dichloromethane
and the solution concentrated. An equal volume of hexane was
added and kept for crystallisation. Attempted purification by
column chromatography using alumina resulted in demetal-
lation. The solid residue was separated by filtration which gave
the pure product [CuL2OCl2] (0.022 g, 46.1%). FAB mass spec-
trum: m/z 680 (M Ϫ 2Cl) (calc. 680.26) (Found: C, 70.1; H, 3.5;
N, 3.6. C44H28Cl2CuN2O2 requires C, 70.35; H, 3.72; N, 3.75%).
Magnetic studies
Earlier magnetic studies21,30 on the [Cu(tpp)] ϩ radical indicate
that the radical cation is completely diamagnetic in the solid
state and paramagnetic (S = 1) in solution. The diamagnetism
in the solid was explained on the basis of the dimeric nature of
the radical cation and the crystal21 structure reveals the pres-
ence of a ruffled core which removes the orthogonality between
᝽
2 Ϫ y2
the metal dx
orbital and the porphyrin a1 (π) orbital,
facilitating d–π coupling. However, in solution, it was assumed
that [Cu(tpp)] ϩ is monomeric with a planar core where a strict
᝽
orthogonality of magnetic orbitals prevents d–π coupling. The
observed weak antiferromagnetic coupling for the copper
monooxa radicals here again supports the presence of distorted
cores at the metal both in solution and in the solid as revealed
by EPR spectra.
[CuLOSCl2]. This complex was prepared using the same pro-
J. Chem. Soc., Dalton Trans., 1998, Pages 1979–1984
1983

View Article Online
E. Pacholska, P. J. Chmielewski, M. M. Olmstead and A. L. Balch,
Angew. Chem., Int. Ed. Engl., 1995, 34, 2252.
3 E. Vogel, P. Rohrig, M. Sicken, B. Knipp, A. Herrmann, M. Pohl,
H. Schmickler and J. Lex, Angew. Chem., Int. Ed. Engl., 1989, 28,
1651.
4 M. Pohl, M. Schmickler, J. Lex and E. Vogel, Angew. Chem., Int. Ed.
Engl., 1991, 30, 1693.
cedure as above. When kept for a long time even in the solid
state slow demetallation of copper to form the free base
occurred. Attempts to record the FAB mass spectrum resulted
in demetallation giving the free base. Combustion analysis also
indicated demetallation. Only UV/VIS and ESR spectra were
used to infer the formation of [CuLOSCl2].
5 G. Markl, H. Sauer, P. Kreitmeier, T. Burgermeister, F. Kastner,
G. Adolin, H. Noth and K. Polborn, Angew Chem., Int. Ed. Engl.,
1994, 33, 1151.
[CuLO(Cl)] ϩ[SbCl6]Ϫ. The complex [CuLO(Cl)] (0.010 g,
᝽
0.014 mmol) in dichloromethane (10 cm3) was stirred for 10 min
under argon and 1.1 equivalent of tris(p-bromophenyl)-
ammonium hexachloroantimonate (0.012 g, 0.015 mmol)
solution in dichloromethane (5 cm3) was added and stirred
for about 3 h. The progress of the reaction was monitored
by checking the absorption spectra at different time intervals.
After completion of the reaction the solution was filtered,
solvent evaporated under reduced pressure and the product
obtained recrystallised from dichloromethane–hexane (yield
0.010 g, 80%) (Found: C, 50.34; H, 2.51; N, 4.35. C44H28Cl7-
CuN3OSb requires C, 50.42; H, 2.69; N, 4.01%).
6 E. Vogel, Pure Appl. Chem., 1993, 63, 143.
7 L. Latos-Grazynski, J. Lisowski, M. M. Olmstead and A. L. Balch,
J. Am. Chem. Soc., 1987, 109, 4428; Inorg. Chem., 1989, 28, 1183;
J. Lisowski, M. Grzeszczuk and L. Latos-Grazynski, Inorg. Chim.
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Olmstead and A. L. Balch, Inorg. Chem., 1989, 28, 3328.
8 R. P. Pandian and T. K. Chandrashekar, J. Chem. Soc., Dalton
Trans., 1993, 119.
9 L. Latos-Grazynski, E. Pacholska, P. J. Chmielewski, M. M.
Olmstead and A. L. Balch, Inorg. Chem., 1996, 35, 566.
10 P. Chmielewski, M. Grzeszczuk, L. Latos-Grazynski and
J. Lisowski, Inorg. Chem., 1989, 28, 3546.
11 L. Latos-Grazynski, M. M. Olmstead and A. L. Balch, Inorg.
Chem., 1989, 28, 4065.
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Tetrahedron Lett., 1997, 38, 2383.
The nickel() derivatives of the monooxa- and dioxa-
porphyrins were prepared following the reported procedure.12
Measurements
The electronic spectra were recorded on a Shimadzu UV-160
spectrophotometer, proton NMR spectra on a Bruker 200 MHz
spectrometer. Analyses (C, H, N) were done on a Heraeus Carlo
Erba 1108 elemental analyser. The FAB mass spectra were
recorded using a JEOL SX-120/DA6000 spectrometer with Ar
as the FAB gas, ESR spectra on a Varian E-109 X-band spec-
trometer at room and liquid nitrogen temperature. Cyclic vol-
tammetric and controlled potential coulometric studies were
done on a EG/G Par model 273A polarographic analyser inter-
faced to a computer. A three-electrode system consisting of a
platinum working electrode, a platinum-mesh counter electrode
and a commercially available saturated calomel electrode
(SCE) as the reference electrode were used. This reference elec-
trode was separated from the bulk of solution by a fritted
glass bridge filled with the solvent–supporting electrolyte mix-
ture. Half-wave potentials were measured as the average of the
cathodic and anodic peak potentials. Solid-state magnetic sus-
ceptibility measurements were done by the Faraday technique
using a locally built magnetometer. The set-up consisted of an
electromagnet with constant-gradient pole gaps (Polytronic
Corporation, Bombay, India) and a Sartorius M25-D/S balance
(Germany). The system was calibrated using Hg[Co(SCN)4].
Susceptibilities were corrected for diamagnetic contribution.
The energy-optimised structures were calculated using
HYPERCHEM version 5.0 software31 on a Pentium 120 MHZ
personal computer. A MMϩ force field was employed with the
use of the Polare-Ribere conjugated gradient with convergence
limit at 0.001.
14 R. P. Pandian, D. Reddy, N. Chidambaram and T. K.
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15 B. Sridevi, S. J. Narayanan, A. Srinivasan, M. V. Reddy and T. K.
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17 J. M. Assour, J. Chem. Phys., 1965, 43, 2477.
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19 J. Bednarek and S. Schlick, J. Am. Chem. Soc., 1990, 112, 5019;
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20 J. H. Fuhrhop, The Porphyrins, ed. D. Dolphin, Academic Press,
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21 B. S. Erler, W. F. Scholz, Y. J. Lee, W. R. Scheidt and C. A. Reed,
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22 M. Ravikanth, A. Misra, T. K. Chandrashekar, S. Sathiah and
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23 H. Yokoi and A. W. Addison, Inorg. Chem., 1977, 16, 1341;
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Acknowledgements
T. K. C. thanks the Department of Science and Technology
and Council of Scientific and Industrial Research for the finan-
cial support. We thank Professor R. N. Mukherjee for the help
in measurement of magnetic susceptibility and Professor Dogra
for help in calculations.
27 E. R. Dockal, T. E. Jones, W. F. Sokol, R. J. Engerer, D. B.
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