Resonance Raman characterization of five-coordinate dioxygen adducts of porphyrinatocobalt(II) complexes formed in low temperature O2 matrices

Article abstract of DOI:10.1039/a805124k

Resonance Raman (RR) spectra of five-coordinate dioxygen adducts of cobalt(tetramesitylporphine) and cobalt(tetramesopropylporphine) formed in low temperature O₂ matrices are reported. For the first time, all three vibrations expected for the C

Full text of DOI:10.1039/a805124k

View Article Online / Journal Homepage / Table of Contents for this issue
Resonance Raman characterization of Ðve-coordinate dioxygen
adducts of porphyrinatocobalt(II) complexes formed in low
temperature O matrices¤
2
Leonard M. Proniewicz,*a,b Antoni Kulczycki,c Aleksandra Wese¡ucha-Birczyn
 ska,b
Halina Majcherczyka and Kazuo Nakamoto*d
a Chemical Physics Division, Department of Chemistry, Jagiellonian University, 3 Ingardena Str.,
0-060 Cracow, Poland
3
b Regional L aboratory of Physicochemical Analysis and Structural Research, Jagiellonian
University, 3 Ingardena Str., 30-060 Cracow, Poland
c Institute of Material Science and Ceramics, University of Mining and Metallurgy (AGH),
30 Mickiewicz Ave., 30-059 Cracow, Poland
d Chemistry Department, Marquette University, Milwaukee, W I 53201-1881, USA
Recei¿ed (in Montpellier, France) 2nd July 1998, Accepted 29 September 1998
Resonance Raman (RR) spectra of Ðve-coordinate dioxygen adducts of cobalt(tetramesitylporphine) and
cobalt(tetramesopropylporphine) formed in low temperature O matrices are reported. For the Ðrst time, all three
2
vibrations expected for the CowO fragment, m(OwO), m(CowO ) and d(CowOwO), were observed for 16O and
2
2
2
1
8O isotopomers. These bands have been assigned based on oxygen isotope shift data as well as normal
2
coordinate analysis of the CowO moiety. This work presents also the Ðrst observation of two distinct m(OwO)
2
bands due to the Cow16Ow18O and Cow18Ow16O isomers in “end-onÏ type dioxygen adducts. The mechanism
of resonance enhancement, the e†ect of the axial ligand on the m(OwO) and m(CowO ) frequencies and the
2
di†erences in metalwO bonding between cobalt and iron porphyrins are discussed.
2
Resonance Raman (RR) spectroscopy is a potentially e†ective
probe for studying the structure and bonding of dioxygen
adducts of heme proteins and their model compounds.1 In
favourable cases, spectral features associated with m(OwO),
cycle mode. At that time we could not locate the m(Cow16O )
2
vibration because it is very weak and overlapped by a strong
TPP-d band at 383 cm~1. However, the broad shoulder at
8
369 cm~1, observed for the 18O adduct only, was assigned to
2
m(MwO ) and d(MwOwO) (M being a metal), where m and d
the m(Cow18O ) mode of Co(TPP-d )18O .12
In this work we report the simultaneous observation of all
2
2
8
2
are stretching and bending vibrations, respectively, can be
identiÐed in the RR spectra by using isotopically labeled com-
pounds. Unfortunately, resonance enhancement of the
m(OwO) mode of native heme proteins and iron porphyrins
has not been successful for those containing nitrogeneous
axial ligands. This is probably due to the absence of the
three modes expected for the CowO moiety, m(OwO),
2
m(CowO ) and d(CowOwO), of Ðve-coordinate Co(TMP)O
2
2
and Co(MPR)O , where TMP and MPR denote the tetra-
2
mesitylporphinato and tetramesopropylporphinato dianions,
respectively. Our assignments are based on 16O /18O isotope
2
2
FewO charge transfer (CT) transition in the visible range.2
shifts. We also performed normal coordinate analysis (NCA)
based on the generalized valence force Ðeld (GVFF) model13
and discuss the e†ect of the CowOwO bond angle variation
2
However, the m(FewO ) and d(FewOwO) vibrations have
2
been observed for a number of heme proteins.3,4 On the other
hand, the m(OwO) and m(CowO ) vibrations of dioxygen
on the CowO stretching frequency. Additionally, we calcu-
2
2
adducts of Co-substituted hemoglobin, myoglobin and their
lated potential energy distributions (PED) by using a standard
model compounds can be resonance enhanced.5h7 However,
these spectra are rather difficult to interpret because of the
appearance of several oxygen isotope sensitive bands that
arise from vibrational coupling between m(OwO) and internal
modes of the nitrogeneous axial ligand.8h10
procedure14 to estimate the degree of vibrational mixing
between m(CowO ) and d(CowOwO).
2
Experimental
Although a large amount of RR data is available on six-
coordinate dioxygen adducts of cobalt porphyrins, only two
studies have been reported, thus far, on Ðve-coordinate
Compounds
H TMP was synthesized by the method of Hill and Wil-
2
Co(porphyrin)O 11,12 since these compounds are extremely
liamson.15 To remove tetramesitylchlorine contamination,
2
unstable and can be formed only in low temperature matrices.
H TMP was reÑuxed with 2,3-dichloro-5,6-dicyano-1,4-ben-
2
The m(CowO ) stretch of Co(TPP)O , where TPP denotes
zoquinone (DDQ) in toluene, extracted with sodium dithionite
2
2
the tetraphenylporphinato dianion, was Ðrst assigned to the
band at 345 cm~1 in the RR spectrum of Co(TPP) cocon-
densed with O in a low temperature O matrix.11 Recently,
and then chromotographed over basic alumina with freshly
distilled dichloromethane.16 H MPR was purchased from
2
Midcentury (Posen, IL) and used without further puriÐcation.
2
2
by using a b-pyrrole deuteriated analogue of TPP, TPP-d ,
The cobalt complexes of these porphyrins were prepared by
8
which has no vibrations in the 350 cm~1 region, we12 were
able to show that the 345 cm~1 band is due to a TPP macro-
reaction of cobalt(II) chloride, CoCl , with the respective
2
porphyrin in the presence of 2,6-dimethylpyridine in reÑuxing
dry tetrahydrofuran, THF, under a nitrogen atmosphere. The
complexes obtained were then puriÐed by chromatography on
¤
Non-SI unit employed: 1 Torr B 133 Pa.
New J. Chem., 1999, 71È76
71

View Article Online
an alumina column according to the procedure of Collman et
al.17
Samples of 16O (prepuriÐed, 99.9%) and 18O (97% pure)
2
2
were purchased from Matheson Gas and ICON Services,
respectively. “ScrambledÏ dioxygen (16O : 16O 18O : 18O \
2
2
1
: 2 : 1) was prepared via electrical discharge of an equimolar
mixture of 16O and 18O . Ozone formed during this process
2
2
was decomposed by activated 4A molecular sieves inserted
into a reaction Ñask. The Ðnal mixing ratio of all oxygen iso-
topic molecules was determined by Raman measurements of
the reaction mixture.
Spectral measurements
RR spectra of Ðve-coordinate base-free dioxygen adducts of
cobalt porphyrins were obtained by using the miniature oven
technique.18 Thus, a sample of porphyrinatocobalt(II) was
placed in a miniature graphite oven and heated to about 150È
1
80 ¡C in a vacuum (B10~5 Torr) to remove any volatile con-
taminants. Then, the sample was vaporized by heating the
oven to 300È350 ¡C and co-condensed with O on a copper
2
cold tip, which was cooled to about 25 K by a CTI Model 21
closed-cycle helium refrigerator. A back-scattering geometry
was set up with a cylindrical lens to produce a line focus on
the sample surface to avoid possible thermal decomposition of
the samples.19
RR spectra were recorded on a Spex Model 1403 double
monochromator Ðtted with a Hamamatsu R-928 photomulti-
plier and a Spex DM1B computer. Four scans were accumu-
lated to obtain the RR spectra presented in this work. A
Coherent Innova 100-K3 krypton ion laser was used for 406.7
nm excitation. The power at the sample was kept at ca. 5 mW.
A spectral band pass of 4 cm~1 was routinely used. The accu-
racy of a frequency reading was ^1 cm~1.
Fig. 1 Resonance Raman spectra of Co(TMP) co-condensed with
dioxygen at 25 K (406.7 nm excitation). (A) 16O ; (B) 18O ; (C)
2
2
scrambled O .
2
non-equivalent m(16Ow18O), which are characteristic of the
end-on dioxygen geometry.
Results and discussion
Co(TMP)O2
In the low frequency region the RR spectrum of the 16O
2
adduct of Co(TMP) exhibits two oxygen isotope sensitive
The RR spectrum (406.7 nm excitation) of the 16O adduct
of Co(TMP) is shown in Fig. 1(A). In the high frequency
region this adduct is characterized by the band at 1270 cm~1,
which is shifted to 1200 cm~1 upon 16O /18O substitution
2
bands at 404 (as a shoulder on a very strong porphyrin band
at around 390 cm~1) and 220 cm~1 [Fig. 1(A)]. Upon
1
6O /18O substitution these bands are downshifted; the 404
2
2
2
2
cm~1 band disappears since it is apparently shifted under the
strong 390 cm~1 macrocycle band, while the 220 cm~1 band
is shifted to 210 cm~1. Based on our previous work12 and also
on NCA calculations (vide infra), we assign the 404 and 220
[
Fig. 1(B)]. These bands are easily identiÐed as m(16Ow16O)
and m(18Ow18O), respectively, of Co(TMP)O . The observed
2
isotopic shift of 70 cm~1 is in good agreement with the theo-
retically predicted value for the OwO diatomic harmonic
oscillator (72 cm~1). These frequencies are also in good agree-
ment with those reported previously for Co(TPP-d )O (RR
cm~1 bands to the m(Cow16O ) and d(Cow16Ow16O)
2
modes, respectively. We also tried to determine the
8
2
m(Cow18O ) frequency from the di†erence spectrum (A) [ (B)
studies12), Co(TPP)O and Co(OEP)O (IR studies11,20).
2
2
2
and by using a Ðtting procedure. However, due to inadequate
As was demonstrated previously,11 the dioxygen in “base-
experimental conditions we were not able to get a clear-cut
result.
freeÏ Co(TPP)O takes an end-on geometry. This is based on
2
the evidence that when a metal porphyrin reacts with 16O 18O
When scrambled oxygen is used [Fig. 1(C)] the band at 404
cm~1 practically disappears since its intensity is 1/4 of that
presented in Fig. 1(A). Obviously, the expected
m(Cow16O18O) and m(Cow18O16O) bands are hidden under
the porphyrin band. Thus, the only oxygen isotope sensitive
band seen is at 215 cm~1. Although four CowOwO bending
vibrations are expected when scrambled dioxygen is reacted
with the cobalt(II) porphyrin, we observe only a broad and
symmetric band at 215 cm~1 since their frequencies are
expected to be close (within 10 cm~1).
in scrambled dioxygen, the end-on adduct having non-
equivalent oxygen atoms exhibits two m(16Ow18O) bands
Mnamely m[(metal)w16Ow18O] and m[(metal)w18Ow16O]N,
while the side-on adduct, having equivalent oxygen atoms,
shows only one m(16Ow18O). In this study we also performed
experiments with scrambled dioxygen [Fig. 1(C)]. In the high
frequency region, two new bands appear in addition to those
already discussed. One is clearly observed at 1241 cm~1, while
the second occurs at around 1230 cm~1 as a shoulder on
the 1234 cm~1 porphyrin band. Thus, these bands are
readily assignable to the m[(Co)w16Ow18O] and
m[(Co)w18Ow16O] vibrations, respectively.11,21 The present
Co(MPR)O2
results clearly demonstrate that the O molecule binds to
Co(TMP) in the asymmetric end-on fashion. Such end-on
structures are also known for six-coordinate dioxygen adducts
Next we measured the RR spectra of Co(MPR)O to
2
2
observe the m(Cow16O ) and m(Cow18O ) vibrations, which
2
2
are expected to have less interference from the porphyrin core
of
cobalt
porhyrins,1,8h10
cobalt-substituted
heme
deformation mode near 390 cm~1.1,26 Fig. 2(A) shows the RR
proteins1,5h7 and cobalt Schi† bases of the g1 type.22h25
However, this is the Ðrst report on the RR observation of two
spectrum (406.7 nm excitation) of the 16O adduct of
2
Co(MPR). It exhibits three bands at 1262, 402 (shoulder) and
72
New J. Chem., 1999, 71È76

View Article Online
tation (vide infra) seem to suggest that the direct mechanism is
not responsible for the observed phenomenon. It is interesting
to note that the m(CowO ) and m(OwO) stretches of (six-
2
coordinate) oxycobaltmyoglobin, CoMbO , and oxycobalthe-
2
moglobin, CoHbO , are enhanced through the L ] Co CT
transition [p*(p*O /d ) ] r*(d Co/p*)] underlying the Soret
2
2
2
z
g
xz
g
band.5 These observations seem to suggest that the nature of
the CowO CT transition is a†ected strongly upon coordi-
2
nation of a base ligand in the position trans to the bound
dioxygen.
In metal(porphyrin)L (D symmetry) or metal(porphyrin)L
2
4h
(C
symmetry) complexes, the out-of-plane vibration (A and
A , respectively) can couple with in-plane porphyrin core
4
v
1g
1
vibrations of the same symmetry and borrow intensity from
the latter.29 Spiro30 suggests, however, that such vibrational
coupling would be rather weak since the internal coordinates
are orthogonal. He prefers direct coupling of the axial ligand
vibration to the in-plane electronic transition. Thus, in
Co(porphyrin)O , the m(CowO ) mode of A symmetry may
2
2
1
be enhanced by the p ] p* transition of the porphyrin core
since it alters the CowO and OwO bond lengths. If so, their
excitation proÐles should follow that of the electronic absorp-
tion spectrum. Then the observed enhancement should be the
strongest in the Soret region and become much weaker in the
a and b regions.1,29 Even in the Soret region, however, excita-
tion of the m(CowO ) and m(OwO) modes is expected to be
rather weak since the p ] p* transitions are highly localized
2
in the porphyrin core. Thus, the CowO and OwO bond dis-
tances may not change appreciably27 and the RR enhance-
Fig. 2 Resonance Raman spectra of Co(MPR) co-condensed with
dioxygen at 25 K (406.7 nm excitation). (A) 16O ; (B) 18O . (A) [ (B)
ment of the m(CowO ) and m(OwO) modes may be weak. In
2
2
2
is the di†erence spctrum.
fact, we observed only a very weak m(18Ow18O) band at 1200
cm~1 for Co(TMP)18O with excitation at 568.2 nm from a
2
krypton ion laser. This band disappears upon 18O /16O iso-
2
20 cm~1. These bands are identiÐed as the m(16Ow16O),
2
2
topic substitution. Obviously, the m(16Ow16O) band is too
weak to be observed, since it is hidden under a medium to
strong porphyrin band (probably B mode, which corre-
m(Cow16O ) and d(Cow16Ow16O) modes, respectively, con-
Ðrming our previous assignments.12 Upon 16O /18O substi-
2
2
2
tution these bands are shifted to 1190, 386 and 204 cm~1,
respectively [Fig. 2(B)]. As can be seen in the middle trace, the
di†erence spectrum (A) [ (B) clearly shows the presence of
two bands at 402 and 386 cm~1.
2
g
sponds to l of the TPP macrocycle).31 Thus, we conclude
2
7
that the m(CowO ) and m(OwO) modes of Ðve-coordinate
2
dioxygen adducts of Co(porphyrins) are probably resonance
enhanced through the indirect mechanism proposed by
Spiro.30
Mechanism of RR enhancement
Fig. 1 and 2 show that only weak RR enhancement can be
InÑuence of ligand basicity on the m(OwO) frequency
achieved for all three modes expected for the CowO moiety
2
When the O molecule binds to Co(II) (d7), the CowO bond
of Co(TMP)O and Co(MPR)O by using 406.7 nm excita-
2 2
is formed mainly by r donation from d (Co) to the anti-
2
2
2
z
tion. No marked di†erences were noted for RR enhancements
by excitation at 413.1 and 415.4 nm.
bonding p*(O ) orbital, which is only slightly counteracted by
g
2
p donation in the oposite direction. There is abundant evi-
dence to indicate that nearly one electron is transferred to the
dioxygen via r-bonding.22,23 Thus, the canonical form of the
Thus far, at least three mechanisms have been proposed to
account for resonance enhancement of axial vibrations such as
m(CowO ) in metalloporphyrin complexes, namely one direct
2
dioxygen adduct can be expressed as Co(III)wO ~. In general,
and two indirect mechanisms. The direct mechanism assumes
2
the m(OwO) frequency decreases as the negative charge on the
the presence of a metalwO charge transfer (CT) transition.
2
2pp* orbital of dioxygen increases via electron donation from
the porphyrin core through the Co atom. Thus, the more
acidic the porphyrinato ligand, the less electron density on the
metal ion and less r donation to the dioxygen, resulting in a
In cobalt porphyrins, the axial ligand (L) ] Co CT transition
may occur from Ðlled r or p ligand orbitals to vacant metal d
orbitals, whereas Co ] L CT transition may occur from the
partially Ðlled d orbitals to vacant p ligand orbitals.1 Since
such CT transitions are localized in the CowL linkage, the
CowL distance is expected to change appreciably when the
laser wavelength is chosen in the Co ] L CT region.27
Although we have not been able to measure the electronic
spectrum of Co(porphyrin)O in low temperature O matrices,
weaker CowO bond and a stronger OwO bond. Accord-
2
ingly, the observed order of the m(OwO) frequencies:
Co(TPP-d )O (1281 cm~1) [ Co(OEP)O (1275 cm~1) [
8
2
2
Co(TMP)O (1270 cm~1) [ Co(MPR)O (1262 cm~1) reÑects
2
2
the order of ligand basicity of these porphyrins. Although the
2
2
opposite trend is expected for m(CowO ),32 it was not possible
we anticipate the presence of a strong Soret band at around
10 nm for arylporphyrins and around 390 nm for alkyl-
2
to conÐrm it for Ðve-coordinate Co(por)O since their fre-
2
4
quencies were too close (404 ^ 2È3 cm~1) (Table 1).
porphyrins.28 The fact that the m(CowO ) and m(OwO) vibra-
2
tions are only weakly enhanced by violet lines from a krypton
Normal coordinate analysis
ion laser does not necessarily exclude the direct mechanism of
RR enhancement of these modes. However, a very weak
Previously we carried out NCA on a triatomic CowOwO
enhancement of these vibrations in Co(MPR)O by violet
moiety of Co(MPR)O by using a set of four force con-
2
2
lines (practically no enhancement by using argon ion blue and
stants.12 In this work we performed NCA on a bent
green lines) and the observation of m(OwO) upon yellow exci-
CowOwO fragment of Co(TMP)O . Since no X-ray data are
2
New J. Chem., 1999, 71È76
73

View Article Online
Table 1 The m(OwO), m(MwO ), and d(MwOwO) frequencies (in
2
cm~1) of Ðve-coordinate dioxygen adducts of cobalt and iron porphy-
rins
Compound
m(OwO)
m(MwO )
d(MwOwO)
References
2
Co(TPP-d )O
1281
1275
1270
1262
1195
1192
1188
?
216
217
220
220
345
?
12
8
Co(OEP)O
2
2
407
404
402
508
509
516
21, 12b
this work
this work
19
19
19
Co(TMP)O
Co(MPR)O2
2
Fe(TPP-d )O
Fe(OEP)O
8
2
2
Fe(TMP)O2
343
available on Ðve-coordinate Co(porphyrin)O2
complexes, we
used the molecular parameters, which were varied in the fol-
lowing ranges: the OwO distance (R ) from 1.15 to 1.30 A
Ž
,
1
the CowO distance (R ) from 1.90 to 2.10 A
Ž
, and the
2
CowOwO angle (h) from 90¡ to 180¡. Since we were not able
to observe the m(Cow18O ), these parameters were adjusted so
2
that the calculated frequency of m(Cow18O ) becomes 385
2
cm~1, that is, almost the same frequency as that observed for
Co(MPR)18O . The best Ðt of data including both dioxygen
2
isotopomers was obtained for: R \ 1.20 A
Ž
, R \ 2.00 A
Ž
and
1
2
h \ 137.5¡. These values are very close to those used in the
Fe(Pc)O calculations,33 where Pc is the phthalocyanato
2
dianion. A set of six force constants obtained was: K(R ) \
1
7
.43, K(R ) \ 1.63, H(h) \ 0.26, K(R , R ) \ 0.10, K(R ,
2
1
2
1
h) \ 1.8 ] 10~5, and K(R , h) \ 0.13 mdyn AŽ ~1. Then we cal-
2
culated frequency dependences of the m(16Ow16O),
m(Cow16O ), d(Cow16Ow16O) modes (and the same for the
2
1
8O derivative) as a function of h. The results shown in Fig. 3
2
clearly indicate that the m(OwO) frequencies do not change
much with variation of h, whereas the m(CowO ) and
2
d(CowOwO) frequencies change dramatically. For example,
in the range from 130¡ to 150¡, a slight change of h (i.e., of 5¡)
induces
a
downshift of m(CowO ) and an upshift of
2
d(CowOwO) by as much as 10 cm~1. Thus, observation of
these low frequency modes is highly important in estimating
the CowOwO angle.
We also calculated the potential energy distributions (PED)
for m(16Ow16O), m(Cow16O ) and d(Cow16Ow16O) as a
2
function of h as presented in Fig. 4. These results show very
small degrees of mixing among these three modes.
Fig. 4 Potential energy distributions (PED) for: (A) m(16Ow16O);
(
B) m(Cow16O ); (C) d(Cow16Ow16O) as
a
function of the
2
Cow16Ow16O angle (h).
Metal ion e†ect: Co(III) versus Fe(III) in Ðve- and
six-coordinate porphyrin dioxygen adducts as heme protein
model compounds
Five-coordinate M(porphyrin)O cannot be directly used as
2
model compounds for heme proteins. However, they can serve
as a potential probe for understanding the inÑuence of the
steric, electronic and other environmental factors on dioxygen
binding in naturally occurring heme proteins. The main diffi-
culty in studying these base-free compounds is that they are
very unstable and can be obtained exclusively in low tem-
perature matrices.
In contrast to Co(porphyrin), reaction of
O
with
2
Fe(porphyrin) in the gas phase leads to formation of two types
of Ðve-coordinate dioxygen adducts.19,21 The major product
exhibits the m(16Ow16O) mode in the 1188È1223 cm~1 range,
while the minor one shows it at around 1105 cm~1.19,21 These
Fig. 3 Plots of theoretical m(OwO), m(CowO ) and d(CowOwO) as
2
a function of the CowOwO angle (h). The curves are calculated by
using K(OwO), K(CowO) and H(CowOwO) values of 7.43, 1.63,
and 0.26 mdyn AŽ ~1, respectively (see text for details).
74
New J. Chem., 1999, 71È76

View Article Online
bands have been assigned to the end-on and the side-on
isomers, respectively.21 The latter is thermally unstable and
converts reversibly to the end-on isomer upon raising the tem-
perature. These two Fe(porphyrin)O2 isomers are formed
because there are two possible electronic ground state con-
Ðgurations that have been proposed for the square planar
Fe(TPP): (d )2(d d )3(d )1 34 and (d )2(d )2(d d )2.35
in M(por)(base)(O ). Consequently, coordination of a base
2
ligand causes a downshift of the m(OwO) band by “onlyÏ 50
cm~1 in iron complexes but by 100È150 cm~1 in six-
coordinate cobalt dioxygen compounds.
The metalwO stretching frequencies of six-coordinate
2
dioxygen adducts of cobaltous and ferrous porphyrin model
compounds are in fairly good agreement with those of hemo-
globin (HbO ), myoglobin (MbO ) and their cobalt analogues
2
z
2
xy
xz yz
xy
z
xy yz
The former tends to favour the formation of the end-on,
whereas the latter tends to prefer the symmetric side-on struc-
ture. Since the energy gap between these two electronic states
is small, both isomers are produced in the reaction of
Fe(II)(porphyrin) with O . The nature of the FewO bond in
2
2
(Table 2). On the other hand, previous infrared45 and RR
studies5,7,19,46 of these compounds show several oxygen
isotope sensitive bands in the m(OwO) region that have
caused controversy in their assignments. Yu and coworkers5,6
and Potter et al.45 attributed the observed multiple band
pattern to two discrete conformers of the dioxygen adducts.
Later, Kincaid and coworkers8h10 have shown that the
observed spectra can be interpreted in terms of a single con-
former and that the appearance of the multiple oxygen isotope
sensitive bands arises from “vibrational resonance couplingÏ of
the m(OwO) mode with internal modes of the trans-axial his-
tidylimidazole fragment. Using this approach the exact
m(OwO) frequency can be calculated from the observed RR
spectra. The existence of a single dioxygen conformer has also
been conÐrmed by Miller and Chance47 in their recent FT-IR
2
2
these end-on dioxygen adducts is not fully understood.
However, it has been suggested22,23 that the FewO bond is
2
formed by r donation from the antibonding p* of O to the
g
2
3d
orbital of the iron atom, which is counteracted by p don-
ation from the iron 3d orbitals to the dioxygen antibonding
2
z
p
p* orbitals.
g
Table 1 lists the m(OwO), m(MwO ) and d(MwOwO) fre-
2
quencies of the known Co and Fe(por)O end-on adducts. As
2
is clearly seen, all cobalt complexes exhibit m(OwO) at much
higher and m(MwO ) at much lower frequencies than the cor-
2
responding iron complexes. In general, the m(OwO) frequency
reÑects the charge density on the dioxygen: the larger the
studies on MbO .
2
negative charge on the 2pp* orbital of dioxygen the lower the
g
m(OwO) and the higher the m(MwO ) frequencies. Thus, the
2
low m(MwO ) and high m(OwO) frequencies reÑect weaker
2
Acknowledgements
MwO and stronger m(OwO) bonds in the Co than in the Fe
2
complexes. In other words, the results presented in Table 1
This work was supported by grants 2P 303 060 05 from the
Polish State Committee for ScientiÐc Research (KBN) (to
LMP) and DMB-8613743 from the National Science Founda-
tion (to KN).
suggest that the net negative charge on the dioxygen is less in
the Ðve-coordinate cobalt than in the analogous iron com-
plexes.
When a base ligand (in most cases imidazole, pyridine or
their derivatives) coordinates to the axial position (trans to
dioxygen), the m(OwO) band shifts markedly to lower fre-
quency while the m(MwO ) band shifts upward as shown in
2
References
Table 2. These trends indicate clearly that the trans axial
ligand increases electron donation to the metal d orbitals, thus
1
See for example: (a) Biological Applications of Raman Spectros-
copy, ed. T. G. Spiro, Wiley-Interscience, New York, 1987, vol. 3;
(b) T. Kitagawa and T. Ogura, Prog. Inorg. Chem., 1997, 45, 431.
D. A. Case, B. H. Huynh and M. Karplus, J. Am. Chem. Soc., 1979,
enhancing the MwO bond strength and consequently
2
weakening the OwO bond by increasing the net negative
2
3
4
charge on the antibonding p* (O ) orbitals. This e†ect was
g
2
a
101, 4433.
quantitatively demonstrated for
series of Co(TPP-
W. A. Oertling, R. T. Kean, R. Wever and G. T. Babcock, Inorg.
Chem., 1990, 29, 2633.
d )(base)O
complexes, where the m(OwO) frequency
8
2
decreased linearly as the pK of the base increased.44
(a) S. Hirota, T. Ogura, E. H. Appelman, K. Shinzawa-Itoh, S.
Yoshikawa and T. Kitagawa, J. Am. Chem. Soc., 1994, 116, 10564;
(b) S. Jeyarajah, L. M. Proniewicz, H. Bronder and J. R. Kincaid,
J. Biol. Chem., 1994, 269, 31047.
a
Observed changes in the m(OwO) and m(MwO ) fre-
2
quencies are more dramatic for cobalt than iron complexes, as
expected from the di†erences in the nature of the CowO and
2
5
M. Tsubaki and N.-T. Yu, Proc. Natl. Acad. Sci. USA, 1981, 78,
3581.
FewO bonds.22,23 This metal ion e†ect must be attributed to
2
the multiple bond character of the FewO bond relative to
2
6 H. C. Mackin, M. Tsubaki and N.-T. Yu, Biophys. J., 1983, 41,
the single CowO bond. In fact, the m(FewO ) mode is at
3
49.
2
2
around 570 cm~1 while the m(CowO ) mode is near 520 cm~1
7 A. Bruha and J. R. Kincaid, J. Am. Chem. Soc., 1988, 110, 6006.
8
2
L. M. Proniewicz, K. Nakamoto and J. R. Kincaid, J. Am. Chem.
Soc., 1988, 110, 4541.
Table 2 The m(OwO) and m(MwO ) frequencies (in cm~1) of six-
2
9 L. M. Proniewicz, A. Bruha, K. Nakamoto, E. Kyuno and J. R.
Kincaid, J. Am. Chem. Soc., 1989, 111, 7050.
0 L. M. Proniewicz and J. R. Kincaid, J. Am. Chem. Soc., 1990, 112,
coordinate dioxygen adducts of various cobalt and iron porphyrins
and heme proteins
1
6
75.
Compound
m(OwO)
m(MwO )
References
2
11 M. Kozuka and K. Nakamoto, J. Am. Chem. Soc., 1981, 103, 2162.
2 (a) A. Wese¡uchaÈBirczynska, K. Nakamoto and L. M. Pronie-
wicz, J. Mol. Struct., 1992, 275, 95; (b) L. M. Proniewicz, A.
1

Co(OEP-d )(py)O
1145
1142
1151
?
1157
1159
1138
1138
1107
1103
521
519
527
574
575
568
537
539
567
572
36
37
36, 6
38
37
39, 26
7, 19, 5
7, 5
40, 41
42, 43
4
2
Co(TPP)(pip)O
Co(PF)(DMI)O
2
Wese¡uchaÈBirczy n ska, K. Nakamoto, in XIIIth International
2
Conference on Raman Spectroscopy., eds N.-T. Yu and X.-Y. Li, J.
Fe(OEP)(pip)O
2
Wiley and Sons, New York, 1992, p. 540.
Fe(TPP)(pip)O
2
Fe(PF)(NMI)O
2
13 (a) A. Kulczycki, Spectrochim. Acta, Part A, 1985, 41, 1427; (b) A.
Kulczycki, J. Mol. Struct., 1991, 263, 151.
CoHbO2
14 Y. Morino and K. Kuchitsu, J. Chem. Phys., 1952, 20, 1809.
15 C. L. Hill and M. M. Williamson, J. Chem. Soc., Chem., Commun.,
1985, 1228.
16 K. Rousseau and D. Dolphin, T etrahedron L ett., 1974, 48, 4251.
17 J. P. Collman, J. I. Brauman, K. M. Doxsee, T. R. Halbert, S. E.
Hayes and K. S. Suslick, J. Am. Chem. Soc., 1978, 100, 2761.
18 W. Scheuermann and K. Nakamoto, Appl. Spectrosc., 1978, 32,
CoMbO
2
HbO
2
MbO
2
Abbreviations: py \ pyridine; pip \ piperidine; DMI \ 1,2-dimethyl-
imidazole; NMI \ N-methylimidazole; PF \ “picket-fenceÏ porphy-
rin.
2
51.
New J. Chem., 1999, 71È76
75

View Article Online
1
2
2
2
9 L. M. Proniewicz, I. R. Paeng and K. Nakamoto, J. Am. Chem.
34 M. Zerner, M. Gouterman and H. Kobayashi, T heor. Chim. Acta,
1966, 6, 363.
Soc., 1991, 113, 3294.
0 M. W. Urban, K. Nakamoto and J. R. Kincaid, Inorg. Chim. Acta,
35 J. P. Collman, J. L. Hoard, N. Kim, G. Lang and C. A. Reed, J.
Am. Chem. Soc., 1975, 97, 2676.
1
984, 61, 77.
1 T. Watanabe, T. Ama and K. Nakamoto, J. Phys. Chem., 1984, 88,
40.
2 R. D. Jones, D. A. Summerville and F. Basolo, Chem. Rev., 1979,
9, 139.
36 K. Bajdor, J. R. Kincaid and K. Nakamoto, J. Am. Chem. Soc.,
1984, 106, 7741.
4
37 K. Nakamoto, I. R. Paeng, T. Kuroi, T. Isobe and H. Oshio, J.
Mol. Struct., 1988, 189, 293.
7
2
2
3 M. H. Gubelmann and A. F. Williams, Struct. Bonding, 1983, 55, 1.
4 (a) K. Bajdor, K. Nakamoto, H. Kanatomi and I. Murase, Inorg.
Chim. Acta, 1984, 82, 207; (b) W. Kanda, H. Okawa, S. Kida, J.
38 L. M. Proniewicz and H. Majcherczyk, Univ. Iagell. Acta Chim.,
1993, 36, 115.
39 J. P. Collman, J. I. Brauman, T. R. Halbert and K. S. Suslick,
Proc. Natl. Acad. Sci. USA, 1976, 73, 3333.
G o ral and K. Nakamoto, ibid., 1988, 146, 193.
2
5 K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coor-
dination Compounds, Wiley-Interscience, New York, 5th edn., 1997,
part B.
40 C. H. Barlow, J. C. Maxwell, W. J. Wallace and W. S. Caughey,
Biochem. Biophys. Res. Commun., 1973, 55, 91.
41 H. Brunner, Naturwissenschaften, 1974, 61, 129.
42 J. C. Maxwell, J. A. Volpe, C. H. Barlow and W. S. Caughey,
Biochem. Biophys. Res. Commun., 1974, 58, 166.
43 M. Tsubaki, K. Nagai and T. Kitagawa, Biochemistry, 1980, 19,
379.
2
2
2
2
3
6 J. M. Burke, J. R. Kincaid, S. Peters, R. R. Gagne, J. P. Collman
and T. G. Spiro, J. Am. Chem. Soc., 1978, 100, 6083.
7 F. Inagoki, M. Tasumi and T. Miyazawa, J. Mol. Spectrosc., 1974,
50, 286.
8 M. Gouterman, in T he Porphyrins, ed. D. Dolphin, Academic
Press, New York, 1978, vol. III, part A, ch. 1.
44 J. R. Kincaid, L. M. Proniewicz, K. Bajdor, A. Bruha and K.
Nakamoto, J. Am. Chem. Soc., 1985, 107, 6775.
45 W. T. Potter, M. P. Tucker, R. A. Houtchens and W. S. Caughey,
Biochemistry, 1987, 26, 4699.
9 T. Kitagawa, M. Abe, Y. Kyogoku, H. Ogoshi, E. Watanabe and
Z. Yoshida, J. Phys. Chem., 1976, 80, 1181.
0 T. G. Spiro, in Iron Porphyrins, ed. A. B. P. Lever and H. B. Gray,
Addison-Weseley, Reading (MA), 1983, vol. 2, p. 91.
46 L. M. Proniewicz and J. R. Kincaid, Coord. Chem. Rev., 1997, 161,
81.
3
3
3
1 X.-Y. Li, Ph.D. Dissertation, Princeton University, 1988.
2 N.-T. Yu and E. A. Kerr, in ref. 1, p. 39.
47 L. M. Miller and M. R. Chance, J. Am. Chem. Soc., 1994, 116,
9662.
3 K. Bajdor, H. Oshio and K. Nakamoto, J. Am. Chem. Soc., 1984,
106, 7273.
Paper 8/05124K
76
New J. Chem., 1999, 71È76