Comparative FTIR study of the cobalt and iron porphyrin reactions with CO. Does cobalt porphyrin form a bis-carbonyl complex in the Ar matrix?

Article abstract of DOI:10.1016/j.ica.2019.119011

The adducts of Co(II)(TPP) and Fe(II)TPP (TPP is meso-tetraphenyl-porphyrinato dianion) with carbon monoxide in Ar matrix at 10 K have been studied by FTIR spectroscopy using CO, C¹⁸O and their equimolar mixture. It is shown that both metals ma

Full text of DOI:10.1016/j.ica.2019.119011

Accepted Manuscript
Comparative FTIR study of the cobalt and iron porphyrin reactions with CO.
Does cobalt porphyrin form a bis-carbonyl complex in the Ar matrix?
G.G. Martirosyan, H.V. Adonts, G.S. Hovhannisyan, T.S. Kurtikyan
PII:
S0020-1693(19)30580-8
https://doi.org/10.1016/j.ica.2019.119011
119011
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Reference:
ICA 119011
Inorganica Chimica Acta
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25 April 2019
25 June 2019
10 July 2019
Please cite this article as: G.G. Martirosyan, H.V. Adonts, G.S. Hovhannisyan, T.S. Kurtikyan, Comparative FTIR
study of the cobalt and iron porphyrin reactions with CO. Does cobalt porphyrin form a bis-carbonyl complex in
the Ar matrix?, Inorganica Chimica Acta (2019), doi: https://doi.org/10.1016/j.ica.2019.119011
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Comparative FTIR study of the cobalt and iron porphyrin reactions with CO. Does cobalt
porphyrin form a bis-carbonyl complex in the Ar matrix?
G. G. Martirosyan1*, H. V. Adonts1, G. S. Hovhannisyan1 and T. S. Kurtikyan1*.
The Scientific Technological Centre of Organic and Pharmaceutical Chemistry NAS RA,
Molecule Structure Research Centre, 26 Azatutyan Av., Yerevan, 0014, Armenia
Corresponding author.
E-mail address: ggmartirosyan@gmail.com
Abstract.
The adducts of Co(II)(TPP) and Fe(II)TPP (TPP is meso-tetraphenyl-porphyrinato dianion) with
carbon monoxide in Ar matrix at 10 K have been studied by FTIR spectroscopy using CO, C18O
and their equimolar mixture. It is shown that both metals may form mono-carbonyl adducts with
CO, however formation of bis-carbonyl adduct was confirmed only for iron porphyrin. These
results refute an earlier assumption regarding existence of Co(TPP)(CO)2 in Ar matrix.
Keywords.
FTIR spectroscopy; Iron(II) and Cobalt(II) porpyrin; carbonyl complexes; Ar matrix;
Introduction.
The coordination chemistry of carbon monoxide binding with metalloporphyrins and
related model complexes has a long history and has been of particular interest in elucidation of
the nature of the heme binding site. [1-3]. Numerous studies has focused on probing the
1

interaction of CO with Fe(II) and Ru(II) metalloporphyrin model complexes, to better understand
the factors that govern CO binding in heme centers of the proteins. The investigations have also
been extended to the six-coordinate CO- adduct of Fe(II) and Ru(II) porphyrins with donor or
acceptor ligands trans to carbon monoxide, revealing high sensitivity of CO to the bonding
changes [4-9]. While spectroscopic data on the reactions of iron(II) and ruthenium(II) porphyrins
with CO are well documented, significantly less information is available on Co(II) porphyrin
complexes containing bond CO. Both, mono- and bis-coordinated carbonyls have been reported
for iron and ruthenium porphyrin complexes at room temperature and low temperature noble gas
matrix [10-14]. In contrast, only low temperature binding of CO to the Co porphyrins have been
achieved. The reaction of cobalt(II) porphyrins with CO has been initially studied by electron
paramagnetic resonance (EPR) in frozen solution. It has been shown that Co(TPP) forms a 1:1
adduct with CO at 90 K [15, 16]. We have also reported that only mono-carbonyl complex was
detected by infrared spectroscopy upon reaction of CO gas with low temperature sublimed layers
of Co(II)(TPP) at 77K [17]. The only bis-carbonyl Co(TPP)(CO)2 adduct was reported to form at
15 K in Ar matrix and studied by IR and Raman spectroscopy [18]. However, comparison and
analysis of the currently available vibrational data for iron, ruthenium and cobalt porphyrins
revealed some discrepancies in interpretation of the IR spectra and assignment of the bis-
carbonyl adduct Co(TPP)(CO)2 in Ar matrix.
It is well established, that due to competition for metal dp electrons the frequency of CO
vibration in bis-carbonyl of iron and ruthenium porphyrins is noticeably higher ( ~ 60 cm-1) than
in the monocarbonyl complex. For example, CO frequencies of mono and bis-carbonyl
complexes Fe(TPP) in Ar matrix are reported at 1974 and 2036 cm-1 respectively [13]. Similarly,
the stretching frequency modes for Ru(TPP)(CO) and Ru(TPP)(CO)2 are observed at 1951 and
2

2015 cm-1 in the same matrix [14]. At the same time the ν(CO) of mono and bis-carbonyl
complexes of Co(TPP) were reported at 2073 and 2077 cm-1 in Kr matrix, while the same adduct
in the Ar matrix revealed only a single band at 2078 cm-1 [18 ]. The authors suggested that ν(CO)
of the 1:l and 1:2 adducts are probably overlapped in Ar matrix.
An infrared spectroscopy is a powerful tool for studying the structure and bonding of
carbonyl adducts of heme proteins and model compounds. The spectral features associated with
CO binding to the metallo-complexes can be easily identified in the IR spectra and assigned by
using isotopically labeled CO. At the same time FTIR spectroscopy is a unique tool to
differentiate between coordination numbers for simple diatomic molecules applying the mixtures
of isotopes [19]. Indeed, mixed isotopic spectra are extremely important for determination of the
number of CO subunits in the metal carbonyls. With the equal mixture of CxO + CyO (if oxygen
atom is isotope labeled) the mono-carbonyl metal porphyrin complex should display a doublet
containing only the bands of the pure isotopic molecules Me(Por)(CxO) and Me(Por)(CyO) with
1:1 intensities ratio. In contrast, the bis-carbonyl is spectroscopically manifested by a formation
of an additional band between the bands of doublet, due to the mixed isotopic molecule
Me(Por)(CxO)(CyO). For the strong antisymmetric C-O vibrations of the Me(TPP)(CxO)2,
Me(TPP)(CxO)(CyO) and Me(TPP)(CyO)2 the intensities of the IR bands should follow the 1:2:1
(statistical weights) distribution [19]. In this paper we report comparative FTIR spectral study of
the Co(TPP) and Fe(II)(TPP) reactions with CO, C18O and their equimolar mixture in Ar matrix
at 10 K.
2. Experimental section
Material and Measurements: H2TPP was purchased from PorphyChem. Co(TPP) and
Fe(TPP)(Py)2 were synthesized and purified according to the published methods [20,21]. The
3

metalloporphyrin was placed in Knudsen cell and heated at 420-450 K for 3-4 h to remove
solvates or/and protective pyridine ligands (in case of iron porphyrin). Then metalloporphyrin
was vaporized under continuous vacuum conditions at 500-550 K and co-deposited with
previously prepared CO/Ar mixture on a CsI window cooled to 10 K by a closed cycle helium
cryostat DE-202. The temperature of the substrate was controlled with LakeShore 331
temperature controller. The gases, Ar (99.9995%, Linde), CO (99.9 %) and C18O (99.5%, with
81.4 % isotopic enrichment, Institute of Isotopes, Republic of Georgia) were used without further
purification. COx/Ar mixtures were prepared in a glassy bulb using a vacuum system equipped
with mercury manometer. Equimolar mixtures of CO + C18O were prepared taking into the
account C18O isotope enrichment. The infrared spectra were measured using Nicolet ‘‘Nexus’’
FTIR spectrometer at 2 cm-1 resolution.
Results and Discussion.
Fe(II)(TPP) + CO.
Figure 1 represents the IR spectra of the adducts of Fe(TPP) co-condensed with CO/Ar, C18O/Ar
and mixture (CO+ C18O)/Ar at 10 K. The CO stretching bands at 2138 cm-1 (2086 cm-1 for
C18O) and 2149 cm-1 (2097 for C18O cm-1) are due to well-known matrix isolated monomeric
carbon monoxide and CO bound H2O traces respectively [22-25].
4

Fig. 1. FTIR spectra of the co-condensation adducts of Fe(TPP) with CO (a), C18O (b) and
CO+C18O mixture (c) in Ar matrix at 10 K, (Ar/CO=200).
The bands in the spectrum (a) at 1974 cm-1 and 2036 cm-1 are the ν(CO) of mono- and bis-
carbonyl complex of Fe(TPP) and have been assigned previously in Ar matrix using 12CO -13CO
substitution [13]. Similarly, the bands at 1926 cm-1 and 1987 cm-1 in the spectrum (b) should be
assigned to the respective ν(C18O) of the mono- and bis-carbonyl complex Fe(TPP), in excellent
agreement with the 1926.4 cm-1 and 1986.9 cm-1 values, calculated by the diatomic harmonic
oscillator approximation. It is clearly seen in the spectrum (c), that a new band at 2003 cm-1 has
appeared between the bands of Fe(TPP)(CO)2 and Fe(TPP)(C18O)2 at 2036 cm-1 and 1987 cm-1.
Thus, the band at 2003 cm-1 should be undoubtedly assigned to the mixed bis-carbonyl complex
Fe(TPP)(CO)(C18O). A weak sign of this isotope mixed bis-carbonyl complex (due to not fully
enriched C18O) is also seen in the spectrum (b).
Co(II)(TPP) + CO.
The IR spectra of the adducts of Co(II)(TPP) co-condensed with CO/Ar , C18O/Ar and mixture
(CO+ C18O)/Ar are shown in Figure 2.
5

Fig. 2. FTIR spectra of the co-condensation adducts of Co(TPP) with CO (a), C18O (b) and
CO+C18O mixture (c) in Ar matrix at 10 K, (Ar/CO=200).
Similar to Fig. 1, the bands of monomeric CO and CO-H2O are also present in the
spectra. However, in contrast to the Fe(TPP) showing two widely spaced bands of mono- and
bis-coordinated adducts, only a single band persists in the spectrum at 2078 cm-1 (a). This
spectrum completely reproduces the earlier reported data, where the band at 2078 cm-1 has been
assigned to the bis-carbonyl complex Co(TPP)(CO)2 [18]. The band at 2078 cm-1 shifts to 2028
cm-1 upon C18O substitution (calculated value is 2027 cm-1). It is clearly seen that spectrum (c),
obtained by co-condensation of Co(TPP) and (CO+ C18O)/Ar shows only doublet of the pure
isotopic CO adducts at 2078 cm-1 and 2028 cm-1. However, if Co(TPP)(CO)2 exists under given
experimental conditions, three bands should be expected similarly to Fe(TPP). Experiments with
higher concentration of CO/Ar resulted in the same spectral picture. Thus, this result makes the
previous assignment of the band at 2078 cm-1 to the ν(CO) of Co(TPP)(CO)2 questionable.
As it can be seen from Fig. 1, predominantly mono-carbonyls of Fe(TPP) were formed in
low-temperature rigid Ar matrix at 10 K. Similar spectral pictures have also been observed for
iron and ruthenium porphyrins. However, upon stepwise elevation of the temperatures to 20-30
6

K the free monomeric CO diffused and reacted with the five-coordinate complexes to form a six-
coordinate adduct [13, 14].
Figures 3 and 4 show changes in the IR spectra of the Fe- and Co(TPP) co-condensation
adducts with CO isotopic mixture in the Ar matrix as a function of temperature. Annealing of the
Ar matrix leads to an increase of the bis-carbonyl adducts in case of Fe(TPP), while only a small
rise in intensities of the bands at 2078 cm-1 and 2028 cm-1 are seen in the spectra of Co(TPP)
adducts upon the same procedure.
Fig.3. FTIR spectra of co-condensation adducts of Fe(TPP) with CO+C18O in Ar at 10 K-
(a), spectra taken after a stepwise 10K-30K-10K procedure - (b). Ar/CO=200.
Fig.4. FTIR spectra of co-condensation adducts of Fe(TPP) with CO+C18O in Ar at 10 K-
(a), spectra taken after a stepwise 10K-30K-10K procedure - (b). Ar/CO=200.
7

As a possible reason for the existence of a single band in the suspected Co(TPP)(CO)2 (
2078 cm-1) in Ar matrix, bending of the Co-C-O bonds has been suggested [18]. Generally, since
the σ-bonding interaction involves an electron pair in the sp hybridized carbon on CO, the
preferred geometry for CO binding to a metal is linear [26]. The X-ray crystal structures of the
molecular Co(II) porphyrin carbonyl complexes are yet not known, however X-ray data of
Fe(OEP)CO and Fe(OEP)(CO)2 (OEP- octaethyl porphyrin) show, that the Fe-C-O angles are
essentially linear to be 177° and 174° respectively [12]. On the other hand, the X-ray structural
analysis of the bis-carbonyl Ru(TPP)(CO)2 revealed markedly bent Ru-C-O bonds (154°) [27].
Nevertheless, even in this case the ν(CO) of Ru(TPP)(CO)2 and Ru(TPP)(CO) were quiet largely
spaced and measured at 2005 and 1945 cm-1 [10, 28]. We also have detected the ν(CO) of mono-
and bis-carbonyls of Ru(TPP) at 1944 and 2009 cm-1 in the solid sublimed films [29]. Thus, even
the bent coordinated CO ligands can still quite effectively compete for metal dp electrons.
A new approach has recently been applied for investigation of the coordinatively
unsaturated metal porphyrins with the gaseous ligands. It is based on incorporation of the metal
porphyrins into a metal organic frameworks (MOF), which exhibit remarkable stabile
organic−inorganic hybrid porous solids that enable facile diffusion of gas-phase substrates to
interact with immobilized metal porphyrin centers [31-32]. In the very recent work an interaction
of CO gas with Zr-based porphyrinic MOF PCN-224Co(II) (PCN - porous coordination network)
has been reported [33]. In particular, under 1 atm of dry CO single-crystal X-ray diffraction
analysis at 8 K revealed formation of the cobalt porphyrin bis-carbonyl adduct of PCN-
224Co(CO)2, with Co−C−O angles of 180°. This bis-carbonyl complex was also observed by
low temperature by EPR spectroscopy, and warming of the sample showed the conversion of this
species to a monocarbonyl analogue. In addition, the CO binding at the Co(II) center of PCN-
8

224Co was detected by diffuse-reflectance infrared Fourier transform spectroscopy. Upon
cooling the sample to 238 K, a strong ν(CO) stretching vibration appeared at 2086 cm−1. Based
on the temperature at which the spectra were collected, this species was postulated to be a
monocarbonyl adduct. However, the ν(CO) of bis-carbonyl adduct was not reported. The
interaction of CO with the cobalt porphyrin center in this MOF is distinct from molecular
cobalt(II) porphyrin complexes. The slightly higher (relative to the matrix isolated analogue)
ν(CO) stretching vibration and exact of 180° angles Co−C−O measured in MOF are possibly
reflective for the absence of intra- and/or intermolecular interactions between well spatially
separated in the MOF porphyrin molecules. It is well known, that energies of CO stretches are
very sensitive to the structural constraint of model complexes or bulky amino acid residues of
hemes, which can affect geometry and frequencies of Fe–CO group [34].
It should be also noted, that Co(II)(TPP) (compared to Ru- and Fe(II)(TPP)) has an
additional electron in the metal dz2 orbital that may decrease  back-bonding. Indeed, recent
detailed computations of Fe(II) and Co(II) porphyrin mono-carbonyls predict much less
pronounced back-donation from cobalt d orbitals than from iron [35]. At the same time, similarly
to their isoelectronic iron(II) analogues, mono and bis-carbonyls of cobalt(III) porphyrins are
also known. The ν(CO) of [Co(III)(OEP)(CO)]ClO4 and [Co(III)(OEP)(CO)2]ClO4 were
observed at 2110 and 2137 cm-1 respectively [36]. Finally, taking the abovementioned
experimental results together with the literature data, the band at 2078 cm-1 should be re-assigned
to the ν(CO) of monocarbonyl adduct of Co(TPP)(CO) in Ar matrix. Splitting of this band into
two closely spaced bands at 2073 and 2077 cm-1, observed in Kr matrix is probably due to
Co(TPP)(CO) and it's six coordinate complex with H2O, which is a typical impurity in low-
temperature matrix isolation experiments.
9

Conclusions.
The FTIR spectra of co-condensation products of Fe(II)(TPP) and Co(II)(TPP) with CO/Ar,
C18O/Ar and equimolar (CO+ C18O)/Ar at 10 K, are all consistent in supporting formation of the
bis-carbonyl complex of iron porphyrin, but testifies against the existence of the earlier
suggested similar adduct of cobalt porphyrin.
Acknowledgment.
The financial support of SCS RA (Project 18T‐1D242) is gratefully acknowledged.
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O
C
R
R
CO+Ar
10 K
N
N
N
N
R
Fe
R
Fe
R
R
N
R
N
R
N
N
C
O
O
C
R
R
CO+Ar
10 K
N
N
N
R
Co
N
R
Co
R
N
R
N
N
R
N
R
R = Phenyl
Highlights:
 Adducts of Co(II)(TPP) and Fe(II)TPP with CO have been isolated in Ar matrix at 10K.
 The CO, C18O and their equimolar mixture were used.
 FTIR spectra of the adducts were measured.
13