Synthesis of iron(III)-carbonyl complex with variable wavelength range for CO release depending on protonation and deprotonation of axial phosphorous ligands

Article abstract of DOI:10.1016/j.jorganchem.2021.121843

N,C,S-pincer iron(III) carbonyl complexes with two phosphorus ligands—trans-[Fe(L-κ³N,C,S)(CO)(PR₃)₂]PF₆ ([1]PF₆, R = Me; PF₆, R = OEt) are light-inducible CO releasers under ambient conditions. Optical wavelength ranges corresponding to such a CO release depend on the phosphorous ligands. Complex [1]PF₆ responds to light with wavelengths shorter than 500 nm, while the photosensitivity of [2]PF₆ is extended to wavelengths up to 800 nm, reaching the so-called phototherapeutic window. Theoretical and experimental studies suggest that the electron-donating ability of phosphorous ligands is essential to determine the wavelength range corresponding to photo-CO dissociation, although the steric difference in the phosphorous ligands for [1]PF₆ and [2]PF₆ hampers the exclusive evaluation of the electronic effect for CO dissociation from such complexes. A newly synthesized N,C,S-pincer iron(III) carbonyl complex with 1,3,5-triaza-7-phosphaadamantane (PTA) as axial ligands ([3]PF₆) has shown variable photosensitivity for the dissociation of CO in response to protonation of the PTA ligand. Based on this finding, in the present study, [3]PF₆ is used to confirm the photo-CO dissociation mechanism proposed for [1]PF₆ and [2]PF₆ and provide clues to molecular design for adjusting the ranges of optical wavelengths required for CO dissociation from trans-[Fe(PyBPT-κ³N,C,S)(CO)(PR₃)₂]PF₆.

Full text of DOI:10.1016/j.jorganchem.2021.121843

Journal of Organometallic Chemistry 943 (2021) 121843
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis of iron(III)-carbonyl complex with variable wavelength range
for CO release depending on protonation and deprotonation of axial
phosphorous ligands
Seiya Tanaka a, Natsuo Nomuraa, Takanori Nishiokaa, Masakazu Hirotsub,
Hiroshi Nakajimaa,∗
a Division of Molecular Materials Science, Graduate School of Science, Osaka City University, 3-3-138 Sugimoto, Sumiyoshi-ku Osaka, 558–8585, Japan
b Department of Chemistry, Faculty of Science, Kanagawa University, 2946, Tsuchiya, Hiratsuka-shi, Kanagawa, 259-1293, Japan
a r t i c l e i n f o
a b s t r a c t
Article history:
N,C,S-pincer
iron(III)
carbonyl
complexes
with
two
phosphorus
ligands—trans-[Fe(L-
κ3N,C,S)(CO)(PR3)2]PF6 ([1]PF6, R = Me; PF6, R = OEt) are light-inducible CO releasers under ambient
conditions. Optical wavelength ranges corresponding to such a CO release depend on the phosphorous
ligands. Complex [1]PF6 responds to light with wavelengths shorter than 500 nm, while the photosen-
sitivity of [2]PF6 is extended to wavelengths up to 800 nm, reaching the so-called phototherapeutic
window. Theoretical and experimental studies suggest that the electron-donating ability of phosphorous
ligands is essential to determine the wavelength range corresponding to photo-CO dissociation, although
the steric difference in the phosphorous ligands for [1]PF6 and [2]PF6 hampers the exclusive evaluation of
the electronic effect for CO dissociation from such complexes. A newly synthesized N,C,S-pincer iron(III)
carbonyl complex with 1,3,5-triaza-7-phosphaadamantane (PTA) as axial ligands ([3]PF6) has shown
variable photosensitivity for the dissociation of CO in response to protonation of the PTA ligand. Based
on this finding, in the present study, [3]PF6 is used to confirm the photo-CO dissociation mechanism
proposed for [1]PF6 and [2]PF6 and provide clues to molecular design for adjusting the ranges of optical
wavelengths required for CO dissociation from trans-[Fe(PyBPT-κ3N,C,S)(CO)(PR3)2]PF6.
Received 2 March 2021
Revised 2 April 2021
Accepted 12 April 2021
Available online 15 April 2021
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
sure regulation, immune control, and apoptosis through reactions
with some mediators, which suggests a potential for the use of CO
Carbon monoxide (CO) is a colorless and odorless gas that is
mainly produced by the incomplete combustion of hydrocarbon
materials and has been recognized as a toxic molecule [1,2]. The
strong affinity of CO with the iron center of heme leads to the
formation of carboxyhemoglobin (COHb), which inhibits oxygen-
transportation in blood [3]. In cells, CO impairs the function of
cytochrome c oxidase and blocks the aerobic respiratory chain. In
general, toxic symptoms such as headache, shortness of breath and
dizziness occur at the 10 - 30% COHb level in the blood, and fatal
consequences are caused at higher COHb levels (> 50%) [3]. Mean-
while, for decades, studies of CO have focused on its physiological
role in mammalian cells. Heme oxygenase produces endogenous
CO in the heme metabolic pathway. The CO produced acts as a
gasotransmitter like nitric oxide or hydrogen sulfide [4–7], and par-
ticipates in many biological events, such as respiration, blood pres-
as a therapeutic agent. Studies of exogenous CO with animals have
revealed cytoprotective functions, including anti-apoptosis, anti-
inflammation effect, and anti-proliferative actions [8,9]. However,
the concentration of inhaled CO (250 - 500 ppm), which is essen-
tial for activating CO as a therapeutic agent [9], risks toxic effects
due to the lack of specific delivery to target organs. Thus, alter-
native materials for CO inhalation, which are referred to as CO-
releasing molecules CORM, are required for developing the thera-
peutic effects of CO as a medicine [10–12].
Metal carbonyl complexes, which are generally composed of
a transition metal, CO and ancillary ligands, are expected to be
potential CORMs [12–14]. Metal carbonyl complexes that progres-
sively release CO in the course of their hydrolysis allow for CO
administration at a lower concentration than the toxic level in
blood [15–17]. The current goal of metal carbonyl CORMs is spe-
cific CO release without toxic side effects, depending on external
stimuli such as environmental molecules [15], pH [16,17], and light
[12,14,18–20]. Some metal carbonyl CORMs have been studied in
∗
Corresponding author.
E-mail address: hnakajima@osaka-cu.ac.jp (H. Nakajima).
https://doi.org/10.1016/j.jorganchem.2021.121843
0022-328X/© 2021 Elsevier B.V. All rights reserved.

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
ascription of the Fe-CO bond attenuation to the electronic effects
of the phosphorous ligands.
In this study, we synthesized a N,C,S-pincer iron(III) carbonyl
complex [3]PF6 with 1,3,5-triaza-7-phosphaadamantane (PTA [45–
47], Fig. 1b) as axial ligands to elucidate the electronic effect of
phosphorous ligands on CO dissociation in response to near-IR
light irradiation. PTA is a cage-like water-soluble phosphine, which
possesses a protonation site at an amino group with a pKa value
of ca. 6 [47]. Experimental and theoretical studies have shown that
additional protonation at residual amino groups is thermodynam-
ically unfavorable since it changes the orbital hybridization of the
nitrogen atoms and leads to distortion of the cage-like structure
[47]. Meanwhile, previous studies have shown that single protona-
tion of PTA is sufficient to modulate the electron density on the
central metal of the metal-PTA complexes [47]. As the steric effect
of single protonation of PTA on the coordination of the CO ligand
is negligible, the comparative photoreaction of [3]PF6 before and
after protonation can allow for verifying the CO dissociation mech-
anism that distinguishes [1]PF6 and [2]PF6 in terms of sensitivity
to near-IR light without considering the steric aspect of the axial
phosphorous ligands.
Fig. 1. Molecular structures for (a) [1]PF6
- [3]PF6 and (b) 1,3,5-triaza-7-
phosphaadamantane (PTA).
cultured cell lines or animals to evaluate their therapeutic effects
and biological toxicities [10,21,22].
Photo-responsive CORMs (photoCORMs) are attractive materi-
als, which release CO upon photo-irradiation of the molecules
[12,14,18–20]. To achieve site-specific and time-independent ad-
ministration of CO in the living body, photoCORMs should be sta-
ble in aqueous media under dark conditions and quickly release CO
upon the irradiation of light within the photo-therapeutic window
(wavelength range of λ = 600 - 900 nm) [23]. The use of biomet-
als is another criterion for designing metal carbonyl photoCORMs
with high biocompatibility [10,21]. Recently, manganese-carbonyl
photoCORMs have been reported that are designed to acquire bio-
compatibility [24–30]. Iron-carbonyl complexes are another choice
because of the higher permissible amount of iron ions in the
body. Mammals possess iron regulatory systems that respond to an
abundance of iron ions in the body [31,32]. This reduces the risk
of free iron ions being produced after CO release from CORMs [33–
35]. The first reported iron-based photoCORM was CORM-S1, which
consists of an iron(II)-dicarbonyl unit with an ancillary ligand cys-
teamine [36]. This complex achieved CO dissociation in response to
visible light irradiation with an effective wavelength of up to 470
nm. The construction of an iron-based photoCORM responsive to
longer-wavelength light has been achieved by using a combination
of conservative iron(0) and iron(II) carbonyl complexes with near-
IR sensitive materials [37,38]. The exothermic reaction of materials
by near-IR irradiation induces thermal degradation of the carbonyl
complexes [39,40].
2. Material and methods
2.1. Materials
All solvents and reagents were used as received. All solvents
and reagents for synthesis were purchased from Kanto Chemi-
cal Co., Inc., FUJIFILM Wako Pure Chemical Industries, Ltd., Tokyo
Chemical Industry Co., Ltd., Sigma-Aldrich, Nacalai Tesque, Inc.,
BLD Pharmatech Ltd., Ark Pharm, Inc, and Santa Cruz Biotechnol-
ogy, Inc. The [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3] complex, which
is the precursor of complexes 1, 2, and 3, was prepared by a liter-
ature procedure [48].
2.2. Spectroscopy and spectrometry
All spectroscopy and spectrometry were performed under am-
bient conditions unless otherwise noticed. NMR spectra were
recorded by using
a Bruker AVANCE 300 FT-NMR spectrome-
ter (Bruker Japan Co.) at room temperature. IR spectra were
recorded with a JASCO FT/IR-4600 type A (Jasco Co.) equipped
with an attenuated total reflection (ATR) unit. Electronic absorp-
tion spectra were recorded by a HITACHI UH4150 (Hitachi Co.).
EPR spectra were recorded by a Bruker Biospin Elexsys E500
spectrometer (Bruker Japan Co.). Mass spectra were recorded by
an Applied Biosystems Mariner time-of-flight mass spectrometer
(Thermo Fisher Scientific Co.).
In a previous study, we reported iron(III) carbonyl complexes,
trans-[Fe(PyBPT-κ3N,C,S)(CO)(PR3)2]PF6 ([1]PF6, R = Me; [2]PF6,
R = OEt) as photoCORMs, where PyBPT is a doubly deproto-
nated form of 3’-(2’’-pyridyl)-1,1’-biphenyl-2-thiol formed via the
C-S bond cleavage of the dibenzothiophene derivative (Fig. 1a) [41].
Complex [1]PF6 releases CO in response to visible light (λ <
500 nm), and [2]PF6 is sensitive to visible–near-IR light (λ < 800
nm). Experimental and theoretical analyses for the CO dissociation
mechanism from [1]PF6 and [2]PF6 suggest two modes of photo-
CO dissociation in both complexes: dπ-dσ∗ transition by visible
light and dπ-dπ transition by near-IR light. These transitions lead
to the depopulation of the π-bonding orbital of the Fe-CO bond
and destabilize the coordination of CO, although the dπ-dπ tran-
sition is inefficient due to overlapping with strong ligand to metal
charge transfer (LMCT) transition [41]. According to Tolman elec-
tronic parameters [42,43], the electron-donating ability of P(OEt)3
to a metal atom is lower than PMe3. The reduced electron dona-
tion from P(OEt)3 to iron(III) in comparison to PMe3 consequently
attenuates the π-bonding of Fe-CO. This increases the sensitivity
of [2]PF6 to near-IR light for CO dissociation by the inefficient dπ-
dπ transition. The controversial point of the mechanism laid in the
different cone angles of the phosphorous ligands (118° for PMe3
of [1]PF6 and 109° for P(OEt)3 of [2]PF6),[44] which raise different
steric effects on the cis-arranged CO ligand and prevent a simple
2.3. Electrochemistry
Cyclic voltammetry was performed with an ALS600A electro-
chemical analyzer (BAS Co.) under an N2 atmosphere at room tem-
perature in the dark. The working, reference, and counter elec-
trodes were a glassy carbon disk electrode with a diameter of 3
mm, a reference Ag/Ag+ (0.01 M AgNO3) and a platinum wire, re-
spectively. The electrolysis solution of CH2Cl2 contained 1.1 mM
sample complex and 0.1 M nBu4NPF6 as a supporting electrolyte.
The electrode potentials were corrected using the redox potential
of ferrocene/ferrocenium E°’ (Fc/Fc+).
2.4. Synthesis of trans-[FeII(PyBPT-κ3N,C,S)(CO)(PTA)2] (3)
[{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3] (102 mg, 1.9 × 10−4 mol)
and PTA(131 mg, 8.3 × 10−4 mol) were placed in a Schlenk tube
with a Teflon valve and dissolved in dehydrated THF (10 mL). The
2

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
solution was then stirred at 70 °C for 3 h under an N2 atmo-
sphere to obtain an orange suspension. The precipitate was fil-
tered under reduced pressure, and the filtrate was evaporated to
dryness. The orange residue was suspended in ice-cold n-hexane
and collected by filtration. The collected solid was recrystallized in
CH2Cl2/cyclohexane = 1: 10 (v/v) at 0 °C to produce fine crystals,
which were collected by decantation, washed with THF, and dried
in vacuo (137 mg, 2.1 × 10−4 mol, 93%). The product was stored
under ambient conditions until use.
are shown in Figure S12. The selected bond distances and angles
for the complexes are shown in Table S1, and the molecular coor-
dinates are listed in Tables S2-S4. Harmonic vibrational frequen-
cies were calculated for the optimized geometries using the 6-
311G(d,p) level.
Time-dependent DFT calculations for [3]+ and [3H+]+ were per-
formed using the optimized structures at the uB3LYP/6-311+G(d,p)
level. Electronic excitations and the related molecular orbitals are
given in Tables S7 and S5 for [3]+ and Tables S8 and S6 for [3H+]+,
respectively. Selected Mulliken atomic spin densities for [3]+ and
[3H+]+ are shown in Table S9.
A single crystal suitable for X-ray crystallography was obtained
by recrystallization with CH2Cl2/toluene/n-hexane = 1: 1: 4 at 0
2
°C. 1H NMR (300 MHz, C6D6): δ 3.43 (d, 1H, JHH = 15.6 Hz, PTA),
2
2
3.56 (d, 1H, JHH = 15.1 Hz, PTA), 3.70 (d, 1H, JHH = 12.8 Hz, PTA),
2.8. X-ray crystallography
2
3
3.90 (d, 1H, JHH = 12.5 Hz, PTA), 6.04 (ddd, 1H, JHH = 5.7, 7.3
4
3
Hz, JHH = 1.4 Hz, 5-py), 6.74 (t, 1H, JHH = 7.5 Hz, 4-py), 6.89
Diffraction data for
3 was collected by using a Rigaku
3
4
(ddd, 1H, JHH = 7.2, 7.5 Hz, JHH = 1.6 Hz, 4-BPT), 6.97 (ddd, 1H,
AFC11/Saturn 724+ CCD diffractometer with monochromated Mo-
3JHH = 7.2, 7.8 Hz, JHH = 1.6 Hz, 5-BPT), 7.07 (d, 1H, JHH = 7.9
4
3
˚
Kα radiation (λ = 0.710747 A) (Rigaku Co.). The data were pro-
3
Hz, 3-py), 7.21 (tt, 1H, JHH = 7.7 Hz, JPH = 1.3 Hz, 5-BPT), 7.48 (dd,
cessed and corrected for Lorentz and polarization effects using the
CrystalClear software package. The analyses were carried out us-
ing the WinGX software [50]. Absorption corrections were applied
using the MultiScan method. The structures were solved using di-
rect methods (SIR97) [51] and refined by full-matrix least-squares
with F2 using SHELXL-2018/3 [52]. Crystallographic data are sum-
marized in Table S10. Non-hydrogen atoms were refined anisotrop-
ically. Hydrogen atoms were located in a difference Fourier map
and isotopically refined.
´
3
4
3
1H, JHH = 7.7 Hz, JHH = 1.0 Hz, 4-BPT), 7.60 (dd, 1H, JHH = 7.7
´
4
3
Hz, JHH = 1.4 Hz, 6-BPT), 7.87 (d, 1H, JHH = 7.6 Hz, 6-BPT),
´
3
4
8.02 (dd, 1H, JHH = 7.7 Hz, JHH = 1.6 Hz, 3-BPT) 8.51 (d, 1H,
3JHH = 5.6 Hz, 6-py). 31P{1H} NMR (121 MHz, C6D6): δ −32.4. IR
(KBr): νCO = 1908 cm−1. Anal. Calcd for 3 (C30H35FeN7OP2S): C,
54.63; H, 5.36; N, 14.87. Found: C, 54.23; H, 5.41; N, 14.59.
2.5. Synthesis of trans-[FeIII(PyBPT-κ3N,C,S)(CO)(PTA)2]PF6 ([3]PF6)
Tris(4-tolyl)aminium hexafluorophosphate (113 mg, 2.7 × 10−4
mol) was mixed with 3 (100 mg, 1.5 × 10−4 mol) in CH2Cl2 (20
mL) in the dark. The reaction mixture was stirred at room temper-
ature for 1 h, followed by the addition of toluene (10 mL). After
removing the solvent to a minimum volume under reduced pres-
sure, the suspension was filtered to collect a dark greenish-blue
powder, which was rinsed with toluene and dried in vacuo (83 mg,
1.0 × 10−4 mol, 68%). The mass spectrum for the product was con-
sistent with the composition formula of [3]+ (Figure S1). The prod-
uct was stored in the dark under ambient conditions until use.
3. Results and discussion
3.1. Synthesis of N,C,S-pincer iron(II/III) carbonyl complex with PTA
ligand
The synthesis of trans-[Fe(PyBPT-κ3N,C,S)(CO)(PTA)2] (3) was
achieved in analogy with the synthesis of 1 and 2 [41]. Simple re-
placement of PMe3 of 1 with PTA in synthetic procedures provided
3. Complexes 1 - 3 are the one-electron reduced forms and syn-
thetic sources of [1]PF6 - [3]PF6, respectively. The assignment of
the 1H NMR signals for 3 in C6D6 was carried out in comparison
with the spectra obtained for [{Fe(μ-PyBPT-κ3N,C,S)(CO)2}Fe(CO)3]
[48] and intact PTA (Figure S2). Signals at 6.05-8.52 ppm were as-
signed to the protons of PyBPT. The 31P{1H} NMR spectrum for 3
in C6D6 showed a single signal at -32.4 ppm, which was similar
to that observed for PMe3 of 1 [41] and confirmed that the PTA
ligands were bound to the central iron (Figure S3). The single sig-
nal suggested that the coordination spheres of the two PTA ligands
were equivalent in solution [41].
2.6. Photoreaction of [3]PF6 and [3H+]PF6
A 0.24 mM CH2Cl2 solution of the complex placed in a 1 cm2
square optical cell (1 mL) was irradiated with a monochrome
light (λ = 400 - 800 nm) generated by a fluorescent spectrome-
ter (HITACHI F-7000 fluorescence spectrometer, Hitachi Co.) in the
dark. A UV light cut filter was used to remove light diffracted at
a higher order of the incident light from the monochrome light
before irradiating the sample solution. The photon flux density
(/mol•s−1•m−2) at each wavelength adopted in this study was cal-
culated from the optical power density measured on the surface of
the sample cell using a pyranometer (ML-01, EKO Instruments Co.)
with an effective spectral range of 400-1100 nm (Figure S11). UV-
Vis spectra for the reaction solution were recorded every 15 min
to 30 min during irradiation. The photolytic kinetics was traced by
changes in absorbance at 600 and 612 nm for [3]PF6 and [3H+]PF6,
respectively. Photolysis plots were obtained as the average of three
independent experiments and analyzed by the first-order kinetic
equation.
A crystal structure of 3 obtained by X-ray analysis is depicted
in Fig. 2.
Selected bond distances and angles for 3 are listed in Table 1.
Corresponding data for 1 and 2 are reproduced in the same table
for comparison [41]. The overall conformation of 3 was similar to
those of 1 and 2,[41] except for a larger bending of the equato-
rial tridentate ligand at the thiophenol ring. As with cases 1 and
2, the structural variance in the crystal and NMR data implied a
quick flip-flop motion of the equatorial ligand in solution [41]. The
˚
mean Fe-P distances followed the order of 2.243 A (1, PMe3) >
˚
˚
2.223 A (3, PTA) > 2.206 A (2, P(OEt)3), which was reversed be-
tween 3 and 2 in terms of the steric bulk of the ligands estimated
from the Tolman cone angles (118° (1, PMe3) > 109° (2, P(OEt)3)
> 103° (3, PTA)) [44,47]. The more bent shape of the equatorial
ligand observed in the crystal structure of 3 is a possible cause of
larger steric hindrance, which prevents the PTA ligands from ap-
proaching more closely the central iron despite the smaller cone
angle compared to P(OEt)3.
2.7. Computational details
The structures of complexes 3, [3]+, and [3H+]+ were optimized
by DFT calculations using the Gaussian 09 program package [49].
The crystal structure of 3 was used as the initial model of 3, [3]+,
and [3H+]+. The rB3LYP (3) and uB3LYP ([3]+ and [3H+]+) den-
sity functional methods and the 6-311+G(d,p) basis set were used
for the calculations. Optimized structures for 3, [3]+, and [3H+]+
Alternatively, the stronger π-electron acceptance of P(OEt)3
from the central iron in comparison to the other phosphine lig-
3

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
–
Fig. 3. UV-Vis spectra for 3 ( ) and [3]PF6 (—-) dissolved in CH2Cl2.
Table 2
The νCO energies (cm−1) for 3, [3]PF6, and the protonated forms. The corre-
sponding data for complexes 1 and 2 are reproduced for comparison [41].
3
[3]PF6
1992
3H+
[3H+]PF6
2015
1
[1]PF6
1983
2
[2]PF6
2011
Fig. 2. ORTEP drawing of 3 with thermal ellipsoids at the 50% probability level.
Hydrogen atoms are omitted for clarity.
1908
1929
1891
1922
Table 1
Selected bond lengths (A) and angles (deg) for 3. [a]Reproduced from
˚
ref. 41. [b]Interplanar angle 1 was calculated as the dihedral angle be-
tween the least-squares planes of C1−C6 and C7−C12. [c]Interplanar
angle 2 was calculated as the dihedral angle between the least-squares
planes of C7−C12 and C13−C17, N1.
and 408 nm for 1 and 2, respectively). The shoulder of 3 at 530
nm is also assignable to charge transfer transitions (550 and 510
nm for 1 and 2, respectively). Absorption bands characteristic of
the one-electron oxidation product are observed at 600 nm and
904 nm (573, 853 nm for [1]PF6 and 581, 813 nm for [2]PF6, re-
spectively), which are predominantly ascribed to the LMCT transi-
tions [53,54].
Complex
3
1[a]
2[a]
Fe1–S1
2.2944(18)
2.013(5)
1.991(4)
1.785(5)
2.2242(17)
2.2208(16)
1.144(7)
174.6(5)
172.08(12)
175.2(5)
179.40(6)
38.5(5)
2.2777(7)
1.997(2)
1.9799(19)
1.761(2)
2.2511(8)
2.2340(8)
1.161(3)
176.2(2)
175.97(6)
178.46(10)
174.68(3)
14.1(2)
2.2915(10)
2.005(3)
Fe1–C7
Fe1–N1
1.989(3)
The EPR spectrum for [3]PF6 showed an isotropic triplet signal
at giso = 2.043 with a hyperfine coupling constant (Aiso) = 1.73
mT at room temperature (Figure S5). The triple splitting was at-
tributed to an unpaired electron of the central iron equivalently
delocalized to the phosphorous atoms of the two PTA ligands. A
larger Aiso value was obtained for [2]PF6 (giso = 2.038; Aiso = 2.4
mT) [41]. This accords with the theoretical data in a previous re-
port, which suggested that the P-O anti-bonding component in the
phosphite ligand is more favorable than the P-C anti-bonding com-
ponent in the phosphine ligands to accept electrons from the cen-
tral iron through Fe-P π-backbonding [41]. The larger Aiso value of
[3]PF6 compared to [1]PF6 (giso = 2.044; Aiso = 1.4 mT)[41] indi-
cated that PTA is more effective than PMe3 in accepting electrons
from the central iron among the phosphine ligands.
Fe1–C18
1.773(4)
Fe1–P1
2.2193(10)
2.1924(10)
1.156(4)
Fe1–P2
C18–O1
Fe1–C18–O1
N1–Fe1–S1
C7–Fe1–C18
P1–Fe1–P2
Fe1–S1–C1–C6
Interplanar angle 1[b]
Interplanar angle 2[c]
178.2(3)
171.16(8)
178.05(15)
179.27(4)
29.6(3)
28.8(5)
22.46(5)
7.83(4)
27.08(10)
6.26(15)
10.6(5)
ands may be a dominant determiner of the shorter Fe-P bond. The
P-O anti-bonding component in the phosphite ligand was more ef-
fective than the P-C anti-bonding component in phosphine ligands
to accept electrons through Fe-P π-backbonding (vide infra).
Table 2 compares the vibrational energies of the C-O stretching
modes (νCO) of the carbonyl ligands in 3 and [3]PF6 with those in
1, 2 and their oxidized forms. The IR spectra obtained for 3 and
[3]PF6 are shown in Figures S6a and c.
3.2. One-electron oxidation and single protonation of complex 3
The upward shifts in the νCO energies observed for the cor-
responding oxidized forms are attributed to the reduced π-
backbonding to the carbonyl ligands due to removing an elec-
tron from the central iron atoms. The IR data suggested that the
electron-donating ability of the phosphorous ligands immediately
affected the π-backbonding. The orders of the νCO energies for 1
- 3 and for [1]PF6 - [3]PF6 were consistent with the Tolman elec-
tronic parameters for PMe3 (2064 cm−1), P(OEt)3 (2076), and PTA
(2069) [44,55]. One-electron oxidation led to upward shifts of ca.
90 cm−1 regardless of the axial phosphorous ligands.
The one-electron oxidation of 3 to form [3]+, in which the
central iron ion is formally trivalent, was achieved by reaction
with N(4-tolyl)3PF6 in CH2Cl2 in the dark. From UV-Vis spec-
troscopy, 3 showed progressive changes with isosbestic points un-
til an equimolar amount of the oxidant was added (Figure S4). The
purified [3]+ species ([3]PF6) was available by precipitation from
toluene. The mass spectrum of the purified product showed no
changes in the structural components of 3 after the oxidation (Fig-
ure S1). The UV-Vis spectra measured for 3 and [3]PF6 in CH2Cl2
are shown in Fig. 3.
Free PTA accepts a single proton at one of the three amino
groups with a pKa of ca. 6 [47]. Experimental and theoretical stud-
ies have shown that the further protonation of mono-protonated
PTA is thermodynamically unfavorable due to steric disorder in the
amino groups generated by an alteration in the hybridized orbital
The spectral features observed for 3 and [3]PF6 are similar to
those found for 1, 2, and their oxidized forms ([1]PF6 and [2]PF6),
respectively [41]. The charge-transfer bands of 3, including the
metal-to-ligand and intra-ligand were observed at 420 nm (417
4

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
••••
–
Fig. 4. UV-Vis spectra of 3 ( ) dissolved in CH2Cl2; (
) after the addition of
1.1 eq. of CF3SO3H; (– –) after the subsequent addition of an equimolar amount of
Me4NOH.
–
Fig. 6. Cyclic voltammograms of 3 ( ) and 3 in the presence of CF3SO3H (—) mea-
sured in CH2Cl2 containing 0.1
nBu4NPF6. WE, GC; CE, Pt wire; RE, Ag/Ag+
M
;
Scan rate of 0.1 V s−1. The potential was referred to as the redox couple of fer-
rocene/ferrocenium E°’ (Fc/Fc+).
mental data. This indicates that the unpaired electron in the cen-
tral iron is non-equivalently delocalized over the two PTA ligands
due to single protonation on one of the two ligands. Meanwhile,
the total Aiso values for the two phosphorous atoms were found to
be almost the same for [3]PF6 (3.46 mT) and [3H+]PF6 (3.50 mT).
Therefore, the protonation of [3]PF6 did not change the total spin
densities on the phosphorous atoms of the PTA ligands. DFT cal-
culations for [3]+ and [3H+]+ reproduced this result and predicted
an increase in the spin density on the nitrogen atom of the PTA lig-
and in [3H+]PF6 (vide infra). Indeed, the involvement of a nitrogen
atom improved the fitting of the simulation to the experimentally
obtained spectrum for [3H+]PF6 (Figure S7). The protonation raised
the νCO energies of 3 and [3]PF6 by ca. 20 cm−1 (Table 2, Figures
S6b and d). This indicated that the protonation was effective in at-
tenuating the π-backbonding from the central iron atom to the CO
ligand, possibly due to a reduction of the electron-donating abil-
ity of the PTA ligand. As a result, the νCO energy of [3H+]PF6 sur-
passed that of [2]PF6, which exhibits photosensitivity to the near-
IR light for CO dissociation [41].
Fig. 5. Spectral changes of [3]PF6 in CH2Cl2 recorded after the stepwise addition of
••••
0 - 1.0 eqs. (—) and 1.2 - 2.0 eqs. (
). No apparent changes were observed over
the addition of a 1.0 eq. amount of CF3SO3H.
of nitrogen atoms [47]. This is also true for 3 and [3]PF6. When an
equimolar amount of CF3SO3H was added as a proton source, the
UV-Vis spectrum of 3 in CH2Cl2 showed a quick band shift from
432 nm to 390 nm (Fig. 4), and further addition of CF3SO3H led to
no changes in the spectrum.
The cyclic voltammogram obtained for 3 in CH2Cl2 shows ma-
jor anodic and cathodic waves as a reversible couple with a mid-
point potential of -340 mV (vs. Fc/Fc+), corresponding to the one-
electron oxidation of 3 (Fig. 6).
Subsequent addition of Me4NOH restored the band to the orig-
inal wavelength, confirming that the reaction with CF3SO3H was
a reversible protonation reaction. Likewise, [3]PF6 showed spectral
changes, depending on the amount of CF3SO3H added until reach-
ing the equimolar amount of the complex (Figure 5), while addi-
tional CF3SO3H over the equimolar amount did not show produc-
tive changes in the spectrum, except for the gradual diminishment
of the characteristic bands of [3]PF6.
When an equimolar amount of CF3SO3H with 3 was added to
the electrolyte, the redox couple showed a positive shift by 300
mV except for the residual waves at the original potentials. Thus,
the protonation biased the stability of the complex towards the re-
duced form (3) rather than the oxidized form ([3]+). Since a redox
potential is a thermodynamic parameter that depends on the rel-
ative stability of a compound in different oxidation states, the ob-
served potential shift does not directly reflect changes in the elec-
tronic states of the central iron in 3 and [3]+. Nevertheless, given
the IR spectroscopy results, it is plausible to associate the positive
shift in the potential with a decrease in the electron density of the
central iron due to the attenuated electron-donating ability of the
protonated PTA ligand.
The reverse reaction for the protonated [3]PF6 was not observed
because of the immediate decomposition of the complex upon the
addition of Me4NOH. Complexes 1, 2 and their oxidized forms were
insensitive to the protonation under the same reaction conditions
for 3 and [3]PF6, consistent with the assumption that the PTA lig-
and uniquely serves a reactive site for 3 and [3]PF6 with a pro-
ton. Taking these results together, 3 and [3]PF6 react with a pro-
ton at one of six amino groups in the two axial PTA ligands and
form mono-protonated species (3H+ and [3H+]PF6, respectively) as
the unique products of the reaction. Supportive data for the sin-
gle protonation was obtained from EPR spectroscopy with [3]PF6
in the presence of CF3SO3H (Figure S7). The spectral simulation re-
quired different Aiso values (1.45 and 2.05 mT) to fit the experi-
3.3. Photoreaction of 3 and [3]PF6
A previous study demonstrated that [1]PF6 and [2]PF6 undergo
photolysis through the same reaction pathway (Scheme 1) [41].
5

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
Scheme 1. Photolytic processes for [1]PF6 and [2]PF6 with CO dissociation.
Fig. 8. Photolysis kinetics of [3]PF6 by irradiating with the light of different wave-
lengths in CH2Cl2 at room temperature. (●), 400 nm; (ꢀ), 450 nm; (ꢁ), 500 nm;
(ꢂ), 600 nm; (X), in the dark. Each plot shows the average of three independent ex-
perimental results. The photolysis data obtained by light irradiation at wavelengths
of 700 nm and 800 nm are omitted because of overlap with the data obtained in
the dark. The experimental data at each light wavelength were analyzed with first-
order kinetics by tracing the spectral changes at 600 nm (Figure S9).
Fig. 7. Time course of the photolysis of [3]PF6 in CH2Cl2 upon irradiation with light
(λ = 400 nm). The spectra were recorded every 15 min.
The initial step in the photolysis is the dissociation of the CO
ligand, which tentatively forms a penta-coordinated intermediate
without removal or scrambling of ancillary ligands. This is sup-
ported by the fact that the photoreaction in the presence of iso-
cyanide as a substituting ligand results in quantitative recovery
of a hexa-coordinated species without the CO ligand. When the
substituting ligand is absent in the reaction mixture, the penta-
coordinated intermediate rapidly disassembles to an equatorial lig-
Fig. 9. Photolysis kinetics of [3H+]PF6 by irradiating the light of different wave-
lengths in CH2Cl2 at room temperature. (●), 400 nm; (ꢀ), 500 nm; (ꢁ), 600 nm;
(ꢂ), 700 nm; (ꢃ), 800 nm; (X), in the dark. Each plot shows the average of three
independent experimental results. The experimental data at each light wavelength
were analyzed with first-order kinetics by tracing the spectral changes at 612 nm
(Figure S10).
Table 3
ꢀ
and precursor, 4-(2 -pyridyl)dibenzothiophene (PyDBT), free phos-
The rate constants (/10−4 −1) for the photolysis of [3]PF6
s
and [3H+]PF6 irradiated with different wavelengths of
phorous ligands, and iron ion (FeII) under ambient conditions. The
disassembly step was confirmed by 1H and 31P{1H} NMR spec-
troscopy and colorimetric detection of the iron(II) ion [41]. Gas
chromatography was used to determine the stoichiometric amount
of CO dissociated from [1]PF6 and [2]PF6, which excluded the re-
production of some iron-carbonyl compound after the disassembly
step of the ancillary ligands.
light.
Wavelengths of light /nm
[3]PF6
[3H+]PF6
400
450
500
600
700
800
Dark
5.6
1.5 × 10
n.d.
5.8
1.8
3.6 × 10−1
2.6 × 10−1
2.0 × 10−1
2.0 × 10−1
2.0 × 10−1
5.8
The UV-Vis spectral changes observed for [3]PF6 in CH2Cl2 upon
light (λ = 400 nm) irradiation are shown in Fig. 7.
5.8
5.6
1.9
As observed for [1]PF6 and [2]PF6, the characteristic LMCT
bands for [3]PF6 disappear with isosbestic points at 334 and 352
nm; this concomitantly occurs with the emergence of a band at
346 nm, assignable to free PyDBT [41]. Mass spectrometry for the
resultant solution found free PyDBT and the phosphorous monox-
ide of PTA as major photolytic products, which were also con-
firmed by NMR spectroscopy (Figure S8).
traceable via monitoring of the photolytic bleaching of the com-
plex with UV-Vis spectroscopy. Analogous spectral changes were
also observed for [3H+]PF6. The photolysis of [3]PF6 and [3H+]PF6
at different light wavelengths was traced by monitoring the LMCT
bands at 600 nm and 612 nm, respectively (Figs. 8 and 9).
The photolysis rate constants for [3]PF6 and [3H+]PF6 deter-
mined with first-order kinetics under the current experimental
conditions are summarized in Table 3.
Colorimetric detection with phenanthroline revealed the quan-
titative effluence of free FeII for the reaction. The results confirmed
that [3]PF6 underwent photolysis via the same reaction pathway
as [1]PF6 and [2]PF6 and that the kinetics for CO release were
6

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
+
Fig. 10. Oscillator strengths for photo-excitations predicted for (a) [3]+ and (b) [3H+
]
by TD-DFT (TD = 100) at the uB3LYP/6-311+G(d,p) level (bar charts, see Tables S7
and 8). Solid lines show absorption spectra generated from the oscillator strengths.
As the photon flux density of the light source used in this study
(Xenon light equipped in a fluorescent spectrometer) was approxi-
mately constant in the range of 500 nm and 800 nm (Figure S11),
the kinetics data almost reflect the actual activity of [3]PF6 and
[3H+]PF6 on the photolysis by various wavelengths from visible
to near-IR light. The photolysis of [3]PF6 was observed when ir-
radiated with light at wavelengths shorter than 500 nm. The ob-
tained kinetics data was coincident with the photolytic properties
of [1]PF6. Meanwhile, the photolytic activity of [3H+]PF6 was anal-
ogous to that of [2]PF6, which showed an invariable photolysis effi-
ciency in the wavelength range of 500-800 nm, except for reduced
thermal stability in the dark. The steric effect of single protona-
tion on PTA was unlikely to distinguish the photolytic properties of
[3]PF6 and [3H+]PF6. Meanwhile, an apparent difference was found
in the significant upward shift of the νCO energy for [3H+]PF6,
reaching a value comparable to that obtained for [2]PF6 (Table 2).
Thus, the extended photosensitivity of [3H+]PF6 in the near-IR re-
gion should be attributed to the attenuated π-backbonding to CO
due to the reduced electron-donation from the protonated PTA lig-
and to the central iron. The thermal instability of [3H+]PF6 in the
dark implied that the attenuated π-backbonding to CO was no
longer sufficient to stably retain the Fe-CO bond, leading to ther-
mal dissociation of the CO ligand and subsequent disassembly of
the complex. According to IR spectroscopy and the kinetic data,
the current FeIII carbonyl complexes with a νCO energy lower than
2000 cm−1 (1 - 3, [1]PF6, and [3]PF6) showed no photolytic ac-
tivity with near-IR region light, while complexes with a νCO en-
ergy of over ca. 2010 cm−1 acquired near-IR light sensitivity in
the photolysis ([2]PF6, and [3H+]PF6). This is consistent with the
photo-CO dissociation mechanism previously proposed for [1]PF6
and [2]PF6[41], and suggests that the νCO energy is a useful in-
dex for tuning the photolytic sensitivity of the complexes towards
near-IR light. The complex [3]PF6 is a potential platform for dy-
namically turning on and off the photo-CO dissociation by near-
IR light in response to protonation of the PTA ligand. PTA deriva-
tives with slightly enhanced electron-donating ability [45–47] are
potential candidates for a ligand to improve the thermal stability
of [3H+]PF6 while retaining photosensitivity to near-IR light.
tonated PTA ligand shows the little steric effect on the complex.
The Mulliken atomic spin density at the central iron in [3H+]+
(0.600) was calculated to be smaller than that in [3]+ (0.677) (Ta-
ble S9). This was in line with the trend in the ability to donate
electrons to CO ligands, as inferred from the IR data. The sums of
spin densities on the two phosphorous atoms in [3]+ and [3H+]+
were predicted to be 0.108 and 0.115, respectively. The marginal
difference in these values is consistent with the EPR data: the sums
of the Aiso values for phosphorous atoms in [3]+ and [3H+]+ are
almost invariant, irrespective of the protonation of the PTA ligand
(vide supra). The calculation predicted an increase in overall spin
densities on the nitrogen atoms of the PTA ligands in [3H+]+, in-
dicating possible participation of the nitrogen atoms in receiving
electrons from the central iron to reduce the bonding character of
the Fe-CO bond in [3H+]+.
Photo-excitations in [3]+ and [3H+]+ were calculated by using
a time-dependent DFT (TD-DFT) method. The absorption spectra
composed of the predicted photo-excitations are shown in Fig. 10.
The calculation data reasonably reproduced the overall features
of the experimentally obtained spectra for [3]PF6 and [3H+]PF6
(Figure 11).
An apparent discrepancy in the transitions between the experi-
mental and calculated spectra was found in the near-IR region. This
is a possible reflection of the strong charge-transfer character of
the dominant excitations and the fact that their excitation energies
tend to be underestimated in TD-DFT calculations [56]. Dominant
molecular orbitals (MOs) associated with major photo-excitations
of [3]+ and [3H+]+ at the longer wavelengths than 500 nm are
listed in Tables 4 and 5, respectively.
The intense oscillators of [3]+ and [3H+]+ in the near-IR re-
gion (996 and 1164 nm for [3]+ and [3H+]+, respectively) were
attributed to LMCT transitions from orbitals spread over the PTA
or the biphenyl thiolate moiety of the PyBPT ligands to iron dπ
orbitals (Tables S5 and 6). Other prominent oscillators were found
around 600 nm (559 nm and 668 nm for [3]+, 596 nm and 639 nm
for [3H+]+), which were also assigned to LMCT transitions from
the orbitals of the PTA and PyBPT ligands to the iron-centered dπ
and dσ∗ orbitals. In a previous study, the corresponding transi-
tions have been predicted for [1]+ (1030, 568, and 555 nm) and
[2]+ (1087, 617, and 543 nm) [41]. These LMCT transitions did not
contribute to the photo-CO dissociation from the complexes, as the
dominant molecular orbitals concerned with these transitions did
not include the bonding or anti-bonding interactions of the Fe-CO
bond. Meanwhile, as with [1]+ and [2]+, the TD-DFT calculation
for [3]+ and [3H+]+ predicted weak oscillators corresponding to
d-d transitions behind the LMCT transitions in both the 500-600
nm wavelength range and the near-IR region. The ground states
for the d-d transitions showed obvious π-bonding character be-
tween the iron and carbonyl ligand, and the excited states were
3.4. DFT calculations
Quantum chemical calculations with density functional theory
(DFT) at the uB3LYP/6-311+G(d,p) level were performed to confirm
the effect of the protonation on the photosensitivity of [3H+]PF6.
The optimized structure reproduced the crystal structure of 3 (Fig-
ure S12a) and was used to generate cationic moieties of [3]PF6 and
[3H+]PF6 ([3]+ and [3H+]+). No apparent difference was observed
in the structures of [3]+ and [3H+]+ generated by the calculation
(Figures S12b and c), consistent with the assumption that the pro-
7

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
Fig. 11. Comparison of experimental and generated absorption spectra for (a) [3]+ and (b) [3H+
]
+. Solid lines show the experimental data measured in CH2Cl2. Dashed lines
show the spectra reproduced from Fig. 10.
Table 4
The dominant molecular orbitals for [3]+ relevant to major photo-excitations generated by light with wavelengths longer
than 500 nm. The shapes of the MOs are shown in Table S5.
Excitation light wavelength /nm
Oscillation strength
521
0.0048
559
614
0.0019
668
996
1403
0.0029
0.0150
α-HOMO
α-LUMO
LMCT
0.0109
0.0246
β-HOMO
β-LUMO
LMCT
MOs
Ground state
Excited state
β-HOMO-7
β-LUMO
d-d
β-HOMO-8
β-LUMO+3
d-d
β-HOMO-3
β-HOMO-8
Property of excitation
LMCT
d-d
Table 5
The dominant molecular orbitals for [3H+
longer than 500 nm. The shapes of the MOs are shown in Table S6.
+
]
associated with major photo-excitations generated by light with wavelengths
Excitation light wavelength /nm
Oscillation strength
509
0.0035
596
0.0098
639
902
977
1164
0.0071
0.0107
0.0054
0.0192
MOs
Ground state
Excited state
β-HOMO-7
β-LUMO
d-d
α-HOMO-2
α-LUMO
LMCT
β-HOMO-4
β-HOMO-1
β-HOMO
β-HOMO-1
β-LUMO
LMCT
Property of excitation
LMCT
LMCT
LMCT
Excitation light wavelength /nm
Oscillation strength
1439
0.0062
MOs
Ground state
Excited state
β-HOMO-6
β-LUMO
d-d
Property of excitation
found to be oriented perpendicular to the bonding. Thus, the d-
d transitions could depopulate the Fe-CO π-bonding orbital and
destabilize the coordination of CO. However, the photo-absorptions
for the d-d transitions are suppressed due to the overlapping of
the intense absorptions by the LMCT transitions and are, thus, ren-
dered less efficient to induce the photoreaction. The protonation
of the PTA ligand attenuated the π-backbonding from the central
iron atom and induced the partial depopulation of the Fe-CO π-
bonding orbitals. This compensated for the inefficient d-d transi-
tion to promote the destabilization and subsequent dissociation of
the CO ligand in response to light with wavelengths longer than
500 nm. The scenario follows the mechanism previously proposed
for CO dissociation from [1]+ and [2]+, while the steric homology
of [3]+ and [3H+]+ highlights the electronic effect of the phospho-
rous ligands in modulating the photosensitivity of the current iron-
carbonyl complexes to near-IR light.
tonation characterized the photo-CO dissociation from the com-
plex. The results confirmed the photo-CO dissociation mechanism
previously proposed for [1]PF6 and [2]PF6 and provided clues into
a molecular design for dynamically adjusting the ranges of light
wavelengths suitable for CO dissociation from trans-[Fe(PyBPT-
κ3N,C,S)(CO)(PR3)2]PF6.
As expected from the solubility of free PTA in water, [3]PF6 was
soluble in buffer solutions with different pH levels. Although the
thermal instability of [3H+]PF6 in the dark remains to be solved,
[3]PF6 is a potential candidate as a photoCORM that varies the light
wavelength range of CO release depending on the pH of the dis-
solving medium. A preliminary experiment has found that [3]PF6
is reasonably stable in an acidic buffer solution (pH 4) and shows
photo-CO dissociation in response to near-IR light.
Declaration of Competing Interest
4. Conclusion
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared to
influence the work reported in this paper.
In this study, we synthesized an iron(III) carbonyl complex with
axial PTA ligands ([3]PF6) to verify the mechanism of photo-CO dis-
sociation from a series of trans-[Fe(PyBPT-κ3N,C,S)(CO)(PR3)2]PF6
complexes. The complex [3]PF6 showed CO dissociation in re-
sponse to light at wavelengths shorter than 500 nm, whereas sin-
gle protonation at one of the PTA ligands extended the photo-
sensitivity of the complex ([3H+]PF6) to the near-IR region. Spec-
troscopic and theoretical studies indicated that changes in the
electron-donating ability of the PTA ligand in response to the pro-
Acknowledgments
This study was supported by JSPS KAKENHI Grant Number JP
20K05543, and a Grant-in-Aid 2019 from the Koyanagi Research
Foundation.
8

S. Tanaka, N. Nomura, T. Nishioka et al.
Journal of Organometallic Chemistry 943 (2021) 121843
Supplementary materials
[27] R. Mede, M. Klein, R.A. Claus, S. Krieck, S. Quickert, H. Gorls, U. Neugebauer,
M. Schmitt, G. Gessner, S.H. Heinemann, J. Popp, M. Bauer, M. Westerhausen,
CORM-EDE1:
CORM with
a
a
highly water-soluble and nontoxic manganese-based photo-
biogenic ligand sphere, Inorg. Chem. 55 (1) (2016) 104–113,
Supplementary material associated with this article can be
found, in the online version, at doi:10.1016/j.jorganchem.2021.
121843.
doi:10.1021/acs.inorgchem.5b01904.
[28] C. Nagel, S. McLean, R.K. Poole, H. Braunschweig, T. Kramer, U. Schatzschnei-
der, Introducing [Mn(CO)3(tpa-κ3N]+ as a novel photoactivatable CO-releasing
molecule with well-defined iCORM intermediates
- synthesis, spectroscopy,
and antibacterial activity, Dalton Trans 43 (26) (2014) 9986–9997, doi:10.1039/
c3dt51848e.
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