Six-Coordinate Ferrous Nitrosyl Complex FeII(TTP)(PMe3)(NO) (TTP = meso-Tetra-p-tolylporphyrinato Dianion)

Article abstract of DOI:10.1021/acs.inorgchem.6b01744

Low-temperature in situ Fourier transform infrared and UV-vis measurements show that trimethylphosphine (PMe₃) reacts with microporous layers of Fe^II(TTP)(NO) (TTP = meso-tetra-p-tolylporphyrinato dianion; NO = nitric oxide) to form

Full text of DOI:10.1021/acs.inorgchem.6b01744

Communication
pubs.acs.org/IC
Six-Coordinate Ferrous Nitrosyl Complex FeII(TTP)(PMe3)(NO) (TTP =
meso-Tetra‑p‑tolylporphyrinato Dianion)
Tigran S. Kurtikyan,*,† Astghik A. Hovhannisyan,† and Peter C. Ford*,‡
†Molecule Structure Research Centre of the Scientific and Technological Centre of Organic and Pharmaceutical Chemistry NAS,
0014 Yerevan, Armenia
‡Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California 93106-9510, United
States
S
* Supporting Information
phosphine−imidazole complexes of FeII(TPP).11 Regarding
ABSTRACT: Low-temperature in situ Fourier transform
infrared and UV−vis measurements show that trimethyl-
phosphine (PMe3) reacts with microporous layers of
FeII(TTP)(NO) (TTP = meso-tetra-p-tolylporphyrinato
dianion; NO = nitric oxide) to form the previously
unknown six-coordinate complex FeII(TTP)(PMe3)(NO).
Upon warming this compound to room temperature in the
presence of excess phosphine, the NO ligand is completely
replaced by phosphine, resulting in formation of the
bis(trimethylphosphine) complex FeII(TTP)(PMe3)2. Si-
multaneously, the NO released oxidizes free PMe3 to the
corresponding phosphine oxide (OPMe3) with concom-
itant formation of nitrous oxide (N2O).
diatomic ligands, the carbonyl complexes with trans-phosphine
ligands FeII(TPP)(CO)(PR3) and FeII(Me2PPIX)(CO)(PR3)
were obtained in situ by stirring carbon monoxide (CO)-
saturated methylene chloride solutions with the corresponding
bis(phosphine) complexes.12 This reaction is reversible, and the
mixed phosphine−carbonyl complexes were not isolated.
Phosphine−carbonyl complexes have also been characterized
by UV−vis and magnetic circular dichroism spectroscopy for
mutant H93G myoglobin.8b For this protein, the dioxygen
complex with a proximal phosphine was characterized by
Dawson et al. in a low-temperature buffer solution.8b However,
to our knowledge, no mixed phosphine−nitrosyl complexes of
the type FeII(Por)(PR3)(NO) have been characterized with
simple iron porphyrins or with mutant hemoproteins with open
axial sites.
itric oxide (NO) performs important physiological
Our previous studies have shown that metal tetraarylpor-
Nfunctions, many closely tied to its interaction with the phyrinato complexes sublime onto a low-temperature substrate
iron porphyrin center of heme proteins.1 Because most heme
to form amorphous microporous layers. The reaction of these
with NO plus other ligands allows one to prepare a variety of
five- and six-coordinate nitrosyl complexes7,13 and to use in situ
IR and UV−vis spectroscopy to characterize their reactivities
without solvent interference. In the present study, the binding
of trimethylphosphine to the FeII(TTP)(NO) center is
elaborated by vibrational and electronic absorption spectros-
copies. Furthermore, it will be shown that the Fe−N(NO)
bond of the FeII(TTP)(PMe3)(NO) complex formed at low
temperatures is strongly destabilized and that NO readily
dissociates upon warming.
proteins contain an axial electron-donor ligand in the proximal
site, 6-coordinate nitrosyl complexes are of particular interest.
Six-coordinate iron(II) porphyrin nitrosyl complexes with axial
N-donor ligands have been extensively studied to model the
proximal histidine ligation of soluble guanylyl cyclase,
hemoglobin, and myoglobin.2−4 Although S- and O-donor
ligands have received less attention, there have been limited
studies of thioether and ether ligation to ferrous porphyrin
nitrosyls. For example, Yoshimura used EPR to probe the
interaction of FeII(Me2PPIX)(NO) (Me2PPIX = protoporphyr-
in IX dimethyl ester) with various S- and O-donor ligands at
low temperatures.5 Analogous studies by Lehnert et al.6
examined thiophenolate and thioether coordination to a
FeII(TPP)(NO) (TPP = meso-tetraphenylporphyrinato dia-
nion). Quantitative vibrational spectroscopy has been used by
Martirosyan et al.7 to interrogate interactions of ethers and
thioethers with FeII(TTP)(NO) both in low-temperature
layered solids and in a toluene solution. These studies show
that the interactions of the FeII(Por)(NO) moiety (Por =
porphyrinato dianion) with neutral S- and O-donor ligands are
relatively weak.
Exposure of microporous FeII(TTP) layers to excess NO gas
gives an IR spectrum with a very strong band at 1676 cm−1,
which shifts to 1645 cm−1 when 15NO is used. This band was
assigned as the ν(NO) stretching frequency of the {FeNO}7
complex FeII(TTP)(NO) with a bent Fe−NO unit.14 Figure 1
shows Fourier transform infrared (FTIR) spectral changes
observed upon stepwise addition of PMe3 vapors into a cryostat
containing FeII(TTP)(NO) in low-temperature layers. The
ν(NO) band shifts from 1676 to 1634 cm−1 (from 1645 to
1600 cm−1 for 15NO; Figure S1), indicating formation of the
six-coordinate adduct FeII(TTP)(PMe3)(NO) (Scheme 1).
This is in agreement with previous spectroscopic results and
quantum-chemical calculations, showing that the ν(NO)
Although P-donor ligands, such as phosphines, are not
native, they have been used as structural and functional probes
of heme protein active sites.8,9 A number of mixed six-
coordinate ferrous porphyrin complexes containing phosphine
and σ-donor ligands have been described,10 including
Received: July 24, 2016
© XXXX American Chemical Society
A
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Table 1. ν(NO) and Soret Band Values for the Six-
Coordinate Nitrosyl Complexes FeII(TTP)(L)(NO)
b
ν(NO),
Δν,
Soret band,
nm
a
complex
cm−1
cm−1
ref
FeII(TTP)(NO)
1676 (1646)
411
425
7
7
FeII(TTP)(THF)
(NO)
1651 (1626) 25 (20)
1648 (1622) 28 (24)
1636 (1605) 40 (41)
1634 (1601) 42 (45)
FeII(TTP)(THT)
(NO)
FeII(TTP)(Pyrr)
(NO)
FeII(TTP)(PMe3)
(NO)
427
427
437
7
7
this work
a
THF = tetrahydrofuran; THT = tetrahydrothiophene; Pyrr =
Figure 1. FTIR spectral changes in the range of ν(NO) upon the
stepwise addition of PMe3 vapors to the cryostat containing layered
FeII(TTP)(NO) at 120 K and annealed to 180 K.
b
pyrrolidine; PMe3 = trimethylphosphine. The data for the 15NO
isotopomer are given in parentheses.
Scheme 1
frequency in the six-coordinate FeII(Por)(L)(NO) systems
occurs at lower frequencies than that in the five-coordinate
analogues.3a,4a,5,15
Detailed structural,13 spectroscopic, and density functional
theory (DFT) investigations4 have shown that this frequency
lowering is due to a strong σ-trans-interaction between NO and
its trans ligand. DFT calculations show that binding of a N-
donor ligand in the trans position to NO reduces mixing of the
Figure 2. FTIR spectral changes upon warming of the layer,
containing FeII(TTP)(PMe3)(NO) from 180 to 293 K. Three spectra
are shown; the last of these indicates that the ν(NO) band at 1634
cm−1 has completely disappeared. Asterisks indicate bands of
coordinated PMe3.
2
singly occupied π* orbital of NO with the dz orbital of iron due
to a σ-trans-interaction with the proximal N-donor ligand. The
resulting higher electron population in the π*(NO) orbital
leads to corresponding decreases in ν(NO). The change in σ
bonding is also responsible for weakening of the Fe−NO bond
and explains the experimentally observed correlation of the N−
O and Fe−NO bond strengths in the six-coordinate porphyrin
nitrosyls.4c This consideration also operates for the trans-S- and
-O-donor ligands7 and, according to data described in the
present work, for the P-donor ligand as well.
of the five-coordinate nitrosyl species, as has been observed in
the case of weaker electron-donor trans ligands for which
formation of the six-coordinate complexes was completely
reversible.7 For example, warming the six-coordinate
FeII(TTP)(L)(NO) (L = THF or THT) complexes formed
in similar low-temperature porous layers completely restored
the FTIR and UV−vis spectra of FeII(TTP)(NO).
The spectra shown in Figure 1 also show that some five-
coordinate FeII(TTP)(NO) remains in the layer [ν(NO) at
1676 cm−1]. Attempts to completely transform this to six-
coordinate FeII(TTP)(PMe3)(NO) were unsuccessful. Intro-
ducing more PMe3 into the cryostat leads not only to
consumption of the former but also to a decrease of the
intensity of the ν(NO) frequency of the latter species because
of further transformations in the layer (see below).
With strong N-donor ligands,15a and the pattern is somewhat
different.16 Choi and Ryan reported that the excess N-donor
used to form six-coordinate nitrosyl complexes facilitated NO
dissociation and formation of bis-N-donor-ligated iron
porphyrin complexes.16a Lehnert and co-workers have also
noted4a that the tendency toward denitrosylation corresponds
to the strength of the N-donor binding constants, and
computational studies by Heinecke et al.4d point to the
weakening of the Fe−NO bond in model FeII(Por)(PMe3)-
(NO) complexes. Bohle and Hung have also found that
dissociation of NO promoted by N bases depends on the iron
porphyrin substituents and, for some derivatives, is very rapid at
RT.16b In the present case, the P-donor PMe3 is a relatively
strong base (pKa 8.65) and the irreversible NO dissociation
seen here is additionally maintained by the consumption of
released NO in a reaction with PMe3 to form trimethylphos-
phine oxide OPMe3 and N2O (Scheme 2). These products are
evident in the RT IR spectrum recorded after warming the
layers containing FeII(TTP)(NO) and PMe3 (Figure 2) in
which new bands at ∼1170 cm−1 (overlapping with the
As seen in Table 1, coordination of O-, S-, N-, and P-donor
ligands results in the shift of the ν(NO) frequency for
FeII(TTP)(NO) to lower frequency by 25, 28 40 and 42 cm−1,
respectively, a trend that qualitatively parallels the basicities of
these ligands. Yoshimura showed this trend FeII(Me2PPIX)-
(NO) with a series of N-donor ligands,15a and Choi and Ryan
also noted the strong correlation of the ligand pKa and binding
constants for iron porphyrin nitrosyl interactions with N
bases.16a
When a layer containing predominantly FeII(TTP)(PMe3)-
(NO) was warmed from 180 K to room temperature (RT), the
result was the complete disappearance of the ν(NO) band
characteristic of this six-coordinate complex (Figure 2).
However, this process was not accompanied by regeneration
B
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Communication
disappearance of the nitrosyl ν(NO) band, and this is
accompanied by dramatic changes in the UV−vis spectrum.
The final electronic spectrum (Figure 3, dotted line) exhibits
the properties of a hyperporphyrin complex with a red-shifted
Soret band at 459 nm and a near-UV band at 350 nm along
with relative intensity changes in the visible bands. This
spectrum is very similar to that measured by Ohia et al.21 for
the bis(phosphine) complex FeII(TPP)(PBu3)2 in CH2Cl2−
PBu3 solution. Thus, we conclude that the product is
FeII(TTP)(PMe3)2. Furthermore, the interaction of PMe3
with microporous of FeII(TTP) gives the same electronic
spectrum as that seen upon warming of the layers containing
FeII(TTP)(PMe3)(NO), FeII(TTP)(NO) (some), and PMe3
(excess) to RT (Figure 3, dotted line).
In summary, the low-temperature interaction of PMe3 with
layered FeII(TTP)(NO) generates the mixed six-coordinate
FeII(TTP)(PMe3)(NO) complex, which was characterized by
in situ FTIR and UV−vis spectroscopies. Coordination of the
phosphine ligand in the trans position to NO leads to a strong
weakening of the Fe−NO bond and its cleavage even at rather
low temperatures. Presumably, this is a major reason why mixed
phosphine−nitrosyl complexes of iron porphyrins have not
been characterized previously.
Scheme 2
porphyrin band in this range) and in the vicinity of 2220 cm−1
appear. The former can be assigned as the ν(PO) stretching of
OPMe3 and the latter as νa(NNO) of gas-phase N2O.17,18
When 15NO was used, the latter band shifted to 2165 cm−1 in
accordance with this assignment. Oxidation of PPh3 by NO in
solution to form OPPh3 and N2O was shown by Drago and co-
workers many years ago,19 and subsequently mechanistic
studies of this reaction were carried out by Lim et al.20 The
present study indicates that the analogous reaction can take
place in the solid state.
The direct reaction of PMe3 vapors with the porous
FeII(TTP) layers was studied by FTIR spectroscopy in order
to confirm the identification of FeII(TTP)(PMe3)2 as the
product formed upon warming FeII(TTP)(PMe3)(NO) layers
containing excess PMe3 from 180 K to RT. While formation of
the bis-coordinated PMe3 complex is difficult to confirm by
FTIR spectroscopy solely on the behavior of coordinated PMe3
bands, there are prominent changes in the porphyrin bands
that, together with UV−vis spectra (see below), unambiguously
support formation of the bis(phosphine) species (Figure S2).
The intense porphyrin band at 1003 cm−1 shifts to 992 cm−1,
and the porphyrin band at ∼1540 cm−1 sharply grows in
intensity in a manner that is inherent to the six-coordinate
complexes of iron tetraarylporphyrins with basic ligands.
The UV−vis spectrum of the layered FeII(TTP)(NO) also
undergoes changes in the course of reaction with PMe3. These
spectra were recorded on the samples for which the FTIR
spectra indicated formation of FeII(TTP)(NO), followed by
reaction with PMe3 to give a lower-frequency ν(NO). As seen
in Figure 3, the Soret band of FeII(TTP)(NO) at 411 nm (solid
Experimental details for the preparation and reactions of
FeII(TTP) sublimed layers and characterization of the reaction
products by FTIR and electronic spectroscopy are described in
the Supporting Information.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b01744.
Figure S1 representing FTIR spectra of FeII(TTP)-
(PMe3)(NO) and FeII(TTP)(PMe3)(15NO) in the range
of ν(NO), Figure S2 demonstrating the formation of
FeII(TTP)(PMe3)2 upon interaction of PMe3 with
layered FeII(TTP) and experimental procedures (PDF)
AUTHOR INFORMATION
■
Corresponding Authors
*E-mail: kurto@netsys.am.
*E-mail: ford@chem.ucsb.edu.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by SCS RA (Grant 15T-1D172).
Figure 3. UV−vis spectra of layered FeII(TTP)(NO) (solid line) after
the stepwise addition of PMe3 portions into the cryostat at 120 K and
annealing to 180 K to give FeII(TTP)(PMe3)(NO) (dashed line) and
further warming to RT to give FeII(TTP)(PMe3)2 (dotted line).
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