Blue-light induced CO releasing properties of thiourea based manganese(I) carbonyl complexes

Article abstract of DOI:10.1016/j.poly.2017.04.021

A new class of blue-light triggered carbon monoxide releasing molecules based on Mn(CO)₃Br(N,S) ((N,S)?=?N,S-bidentate thiourea ligands) has been reported in this work. The ¹H and ¹³C NMR analysis indicated that the thioureas coordinate to Mn(I) in the thione form. The stability test and the ability to derive CO to DMSO and myoglobin solution upon the exposure to 468?nm light source have been investigated. The benzothiazole-thiourea-CORM released only one CO equivalent, while the pyridine-analogs offered two molecules at the same experimental conditions. Natural bond orbital analysis has been carried out to give details about the hybridization of Mn–C and C[tbnd]O bonds, natural charge, electronic arrangement of metal ion and strength of interaction between Mn(I) and the coordination sites. Both functional, B3LYP and CAM-B3LYP compare well with the experimental findings, where the longest wavelength band is assigned to MLCT.

Full text of DOI:10.1016/j.poly.2017.04.021

Accepted Manuscript
Blue-light induced CO releasing properties of thiourea based manganese car-
bonyl complexes
Ahmed M. Mansour, Alexandra Friedrich
PII:
S0277-5387(17)30290-5
DOI:
http://dx.doi.org/10.1016/j.poly.2017.04.021
POLY 12599
Reference:
To appear in:
Polyhedron
Received Date:
Accepted Date:
19 February 2017
18 April 2017
Please cite this article as: A.M. Mansour, A. Friedrich, Blue-light induced CO releasing properties of thiourea based
manganese carbonyl complexes, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.04.021
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Blue-light induced CO releasing properties of thiourea based
manganese carbonyl complexes
Ahmed M. Mansour, a,b* Alexandra Friedrichb
a
Department of Chemistry, Faculty of Science, Cairo University,
Gamma Street, Giza, Cairo 12613, Egypt
b
Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg,
Am Hubland, D-97074 Würzburg, Germany
*
mansour@sci.cu.edu.eg
*
Corresponding author
1

Abstract
A new class of blue-light triggered carbon monoxide releasing molecules based on Mn(CO) Br(N,S)
3
(N,S) = N,S-bidentate thiourea ligands) has been reported in this work. The H and 13C NMR
1
(
analysis indicated that the thioureas coordinate to Mn(I) in the thione form. The stability test and
the ability to derive CO to DMSO and myoglobin solution upon the exposure to 468 nm light source
have been investigated. The benzothiazole-thiourea-CORM provided only one CO equivalent, while
the pyridine-analogs offered two molecules at the same experimental conditions. Natural bond
orbital analysis has been carried out to give details about the hybridization of Mn-C and C≡O bonds,
natural charges, electronic arrangement of metal ion and strength of interaction between Mn(I) and
the coordination sites. Both functional, B3LYP and CAM-B3LYP compare well with the experimental
finding, where the longest wavelength band is assigned to MLCT.
Keywords: CORMs; Photoactivatable; DFT; Blue-light; Thiourea
2

Introduction
The colorless odorless toxic molecule, carbon monoxide, showed some surprising effects in
the concentration range of 10-250 ppm [1], such as promotion of wound healing processes, the
cytoprotective activity during the inflammation, as well as the signaling properties [2-5]. Several
researchers have been encouraged to undertake initiatives to way by which CO can be
administrated to the humans. Initially, an inhalation system has been introduced to provide 3.0
mg/kg/h of CO to humans. However, the lack of target specificity of CO is a main problem issue [6],
which requires the patient to be exposed to elevated levels of CO to build up a realistic
concentration in the target issue to achieve a desired biological effect. Carbon monoxide releasing
molecules (CORMs) were an alternative way, which act as prodrugs capable of carrying and
releasing controlled equivalents of CO within the cell. The possibility of controlling and targeting
organs and issues without delivering toxic amounts to the whole body are the advantages of using
CORMs. The release of CO from CORMs can be triggered by ligand exchange in the aqueous medium
(
e.g. CORM-3, [RuCl(glycinate)(CO) ]) [7], pH change [8], thermal [9], enzymes [10] and photo-
3
induced [11-17]. The advantages of the photo-activatable process are a well-defined control of the
location, dosage and the timing of the CO release.
Manganese(I) fac-tricarbonyl complexes constitute an important class of photo-CORMs and
several studies on their biological activity and photolysis profiles have been reported [11-17]. The
Schatzschneider group introduced [Mn(tpm)(CO)
3
]PF
6
(tpm: tris(pyrazolyl) methane) [11] as an
example of the photoactivatable CORM. Two CO molecules were released upon the illumination at
365 nm. Another UV-induced CORMs having N,N,N-tridentate ligands such as [Mn(bpea)(CO)
3
]PF (
6
3
bpea: bis(pyrazolyl)methane) [12], [Mn(CO)
3
(tpa-ҡ N)]Br (tpa: tris(2-pyridylmethyl)-amine) [13]
as well as CORM having N,N,O-tridentate 3,3-bis(3,5-dimethylpyrazol-1-yl) propionate (bpzp) [14]
were also synthesized. Owing to the detrimental effects caused by the UV light and that the
penetration depth of light into tissues reaches its maximum value in visible-near IR region, the
synthesis of CORMs capable of releasing CO upon exposure to visible light is the major challenge in
the area of CORMs at the present time. Several strategies have been used to achieve the CO releasing
process in the visible-near IR region. Mascharak and coworkers synthesized several metal carbonyls
bearing pyridine and quinoline ligand frames with MLCT bands in the 535-500 nm range and which
release CO when illuminated with the lower-power (5 mW) visible light [15]. Bengali [16]
(iPr
Ph-DAB)] ( Pr
i
introduced [MnBr(CO)
3
2
2
Ph-DAB = (N,N'-bis(2, 6-di-isopropyl phenyl)-1,4-
diaza1,3-butadiene) ) that liberates CO upon the illumination at 560 nm with t1/2 of 35 min. A series
3

of [MnBr(azpy)(CO) ] (azpy = 2-phenylazopyridine derivatives) capable of releasing CO upon the
3
exposure to red-light (λ ≥ 625 nm) has been synthesized by Zobi et al. [17].
N,S
N,S
H
In this work, three novel manganese(I) carbonyl complexes, [MnBr(CO) (L )] (L = L (1),
3
L4ester (2) and LBenzo (3)) (Scheme 1) bearing N,S-bidentate thiourea derivatives, and capable of rapid
release of CO upon the exposure to 468 nm LED source light have been synthesized and
characterized. For the first time, N,S-bidentate ligands have been used as ancillary ligands in the
field of CORMs. The novelty of these systems is based on the modulation of the Mn-CO bond strength
by changes in the coordination environment of the ancillary ligands that in turn influence the
electron density of the metal center and the position of MLCT band. Although the thiourea motif is a
crucial pharmacophore in drug discovery with interesting biological properties [18-20], it has not
been explored in the context of CORMs so far besides its facile functionalization. The dark stability
test and the photo release properties are reported in DMSO, as an organic solvent, and myoglobin
solution, as the physiological medium in this study. The electronic structure and transitions have
been studied by TD-DFT calculations. The natural charge of Mn(I) ion, electronic arrangement, type
of hybridization and the strength of interaction between Mn(I) and CO molecules have been
obtained by natural bond orbital analysis [21, 22].
Results and discussion
Spectral characterization
Three thiourea ligands were synthesized via the reaction of phenyl isothiocyanate with 2-
amino pyridine (L ), methyl-2-amino-4-pyridine carboxylate (L4ester) and 2-amino benzothiazole
H
(
LBenzo) in either ethanol or pyridine as organic solvent (Scheme 1). The thiourea derivatives were
obtained in a pure and good yield of 69-74%. These ligands were characterized by elemental
1
13
1
1
1
13
H
Benzo
analysis, IR, NMR ( H, C, { H, H} COS90 and { H, C} HSQC). The ligands L [23] and L
[24]
were reported in the literature, while the other derivative (L4ester) could be considered as a new
comer. Thiourea compounds possess NH, C=S and pyridine ring and can associate via H-bond
formation either inter- and/or intramolecular H-bonds [25]. Owing to these H-bond interactions,
H
the IR spectrum of L (Fig. S1†) displays the stretching NH modes at lower wave numbers, 3215 and
-
1
4ester
-1
3
172 cm . The IR active bands observed in the spectrum of L
at 3330, 1712 and 1613 cm are
assigned to ν(NH), ν(C=O) and ν(C=N). Like L4ester derivative, the IR spectrum of the benzothiazole
ligand (LBenzo) shows only one NH band at 3157 cm , while the other cannot be easily identified. The
ν(C=N) mode of LBenzo is observed at 1621 cm . In the high-frequency region of these ligands, the
-1
-1
-
1
absence of ν(S-H) around 2570 cm and the presence of ν(N-H) vibrations indicate the existence of
these derivatives in the thione form in the solid state [26].
4

H
Benzo
H
The NMR analyses [23, 24] of L and L
in DMSO were not given and discussed in details as
1
described here. In the H NMR spectrum of L derivative (Fig. S2†), two singlet signals are observed
at δ 13.73 and 9.66 ppm assigned to two NH groups. The withdrawing effect of -COOCH
3
group
(
L4ester) results in the downfield shift of the pyridine-NH proton (δ 11.11 ppm) compared with the LH
derivative, while the other NH signal is still nearly in the same position, δ 13.54 ppm (Fig. S3†). The
chemical shifts of benzothiazole and phenyl NH in the LBenzo derivative (Fig. S4†) are found at δ
1
2.78 and 10.72 ppm. In the 13C NMR spectra of the thiourea derivatives, the signal of C=S is
observed at ~ δ 178.79 ppm, while the C=O group in the L4ester derivative is found at δ 164.49 ppm
Fig. S3†). Therefore, this class of thiourea derivatives is also survived in the thione form in the
(
solution.
The reaction of the synthesized ligands with manganese(I) pentacarbonyl bromide in acetone
H
4ester
(2) and LBenzo (3))
afforded complexes of the type, fac-[MnBr(CO)
3
(N-S)] (N-S = L (1), L
1
13
1
(
Scheme 1). Their structures were elucidated by elemental analysis, IR, NMR analysis ( H, C, { H,
1
1
13
H} COS90 and { H, C} HSQC) and ESI mass spectrometry (Fig. S5-S8†). However, the unambiguous
proof of the bidentate nature (pyridine-N and C=S group) of our ligands finally came from the single
H
crystal X-ray analysis of complex 1. Compared to free L ligand, the shift of ν(NH) bands to higher
wave numbers, 3264 and 3224 cm-1 excludes the participation of these groups in complex
formation. The liberation of the pyridine N atom from the H-bond effect via the interaction with the
py
-1
Mn(I) ions leads to a non-significant change in the position of the ν(C=N ) band, 1624 cm . Two
-
1
strong stretching ν(C≡O) modes were observed at 2024 (broad) and 1914 cm corresponding to
the fac(CO) arrangement [27]. Unlike complex 1, the IR spectra (Fig. S5†) of 2 and 3 display three
stretching ν(C≡O) bands at (2023, 1936 and 1907) (2) and (2021, 1940 and 1907) (3) cm as well
3
-
1
-
1
as two stretching NH modes at (3235, 3165) and (3205, 3171) cm , respectively. The band at 1624
2) and 1620 cm (3) is assigned to ν(C=N ) and ν(C=NBenzo) modes.
-
1
py
(
The NMR analyses of 1-3 were recorded in acetone-D
signals, with integration of two, are observed in the range of δ 10.61-10.53 ppm (1), and 10.74-
0.64 ppm (2). However, the NH singlet signals are observed clear at δ 11.64 and 10.77 ppm in case
of complex 3. The downfield shift of pyridine-H6 (9.22 (1) and 9.39 ppm (2)) and benzothio-H7
6
(Fig. S6-S8†). Two overlapped singlet
1
13
(
8.80 ppm (3)) confirms the participation of pyridine and benzothiazole rings in chelation. The C
NMR spectrum of 2 (Fig. S7†), as a representative, exhibits six characteristic signals at δ 222.90,
22.50, 219.20, 180.50, 163.85 and 52.82 ppm allocated for three (C≡O) molecules, (C=S), (C=O)
and (CH ). Finally, we can conclude that the thiourea derivatives behave as a bidentate ligand via the
2
3
pyridine N and C=S without the probability of the enolization of -N(H)-C=S- group.
5

Crystal structure
X-ray single crystal diffraction data indicate that CORM 1 crystallizes in monoclinic P2
1
/c
space group. Relevant crystallographic parameters are listed in Table S1†. ORTEP drawing of
complex 1 is shown in Fig. 1 and selected crystallographic data is presented in Table 1. As shown in
Fig. 1, three carbon monoxide molecules [Mn-C(1) = 1.815(3) Å, Mn-C(2) = 1.802(3) Å and Mn-C(3)
=
1.789(4) Å] in the facial arrangement of the octahedral geometry force the N,S-bidentate ligand
(
LH) [Mn-N(1) = 2.083(3) Å and Mn-S(1) = 2.344(2) Å] and Br atom [Mn-Br(1) = 2.560(3) Å] to
occupy the empty positions. The cis-[87.56(6)-93.74(9)°] and trans-[177.2(1) and 177.78(9) °]
angles (Table 1) compare well with the expected values for the regular octahedral stereochemistry
[90, 180°]. The unit cell contains four molecules of CORM 1 (Fig. S9†). In CORM 1, the orientation of
the phenyl rings of two adjacent molecules showing offset face-to-face stacking is exactly parallel
and the distance between the planes spanned by the phenyl rings is about 3.713(6) Å, which is well
within the range of 3.3−3.8 Å accepted for π-π stacking interactions [28] (Fig. S10†). In the
structural packing of CORM 1, two H-bonds of similar lengths exist between each two molecules.
The Br atom of one molecule is involved in two H-bonds with the NH groups of the second molecule
with distances of 2.594(2) Å (Py-NH…Br) and 2.616(2) Å (Ph-NH…Br) (Fig. S11†).
Geometry optimization
The structures of 1-3 were optimized with B3LYP/LANL2DZ method with LANL2DZ ECP
(
effective core potential) starting from the crystal structure of CORM 1. A view of the molecular
structures of 1-3 is shown in Fig. 2. Selected calculated bond lengths and angles are given in Table
S2†. The calculated bond lengths and angles of CORM 1 were compared with those of the crystal
data. A good agreement was found for the Mn-N(1) and Mn-CO bonds, while the calculated Mn-Br(1)
and Mn-S(1) bonds are found to be longer by 0.039 and 0.175 Å. The calculated C-O bond lengths
are also longer than the experimental values by 0.028-0.035 Å. The electronic arrangement of the
Mn(I) ion, and its natural charge have been calculated by natural bond orbital analysis [29, 30]. The
occupancy, hybrid orbitals and orbital contributions of Mn-C and C-O bonds are given in Table S3†.
The natural charge of Mn(I) (-1.37553e) is not affected by the type of the coordinating nitrogen
center. The Mn-C(29) bond in CORM 1, as a representative example, is formed from sp0.84d1.59 hybrid
on Mn atom (mixture of 29.13% s, 24.44% p and 46.42% d) and sp0.51 (33.56% p) on the carbon
atom, and is polarized towards the C(29) atom with about 66.62% of electron density on C(29). The
other Mn-C bonds are nearly formed from the same hybrid orbitals. The strength of interaction
between the Mn(I) ion and the thiourea active coordination sites or carbon monoxide molecules has
2
2
been assigned by the second order interaction energy (E ) [30]. Table 2 lists the calculated E values
6

2
of the Mn-Br, Mn-N, Mn-S and Mn-CO bonds in 1-3. The larger the E value, the more intensive is the
interaction between the metal and coordination sites. The values obtained for 1 and 2 are nearly
identical, and thus the ester group does not introduce any effect on the stability. Compared with 1
and 2, the Mn-N interaction in 3 is stronger. The electron donation from δ(C-O) that is in a trans
*
position to σ(Mn-N) bond towards the antibonding acceptor RY (1)Mn has the highest interaction
value among the three Mn-CO bonds in complexes 1-3.
Photoactivatable CO-release properties
The electronic absorption spectrum of 1 in DMSO shows a broad MLCT band at 398 nm (Fig.
3). When kept in the dark, complex 1 was unstable for 16 h in the aerated DMSO solution because of
the growth of an isosbestic point at 375 nm. Then the same solution was directly illuminated with
-
9
-1
the blue LED lamp (468 nm, λpeak = 455 nm, ∆λ = 25 nm, photon reflux [31] = 1.25×10 Einstein s ).
As shown in Fig. 3, the intensity of the band at 398 nm decreases to reach its minimum value within
40 minutes. CORM 2 displays the MLCT as a shoulder at 407 nm. Like 1, two isosbestic points were
developed at 360 and 326 nm during its storage. The blue-light induces fast changes in the
electronic spectrum of 2 (Fig. 4), and the plateau values were reached after 35 min. with no further
spectral changes upon the extended illumination. Compared with 1 and 2, the λmax of MLCT band of
3
is blue-shifted to 387 nm. CORM 3 shows better stability than 1 and 2. Extended illumination (> 5
min.) of 3 at 468 nm did not induce any spectral changes (Fig. 5). Therefore, CORM 3 exhibits both
long-term stability and fast CO release upon the exposure to the blue-light. Only five minutes are
required to reach the maximum amount of CO release.
In the phosphate-buffered saline (pH = 7.4) containing myoglobin and sodium dithionite, as a
reducing agent, the delivery of CO to this medium was studied upon the illumination of 1-3 at 468
nm. The number of CO equivalents were determined via the monitoring of conversion of myoglobin
into myoglobin-CO species [32, 33]. This assay suffers from some disadvantages such as turbidity,
CORM absorbance and the enhancement of the CO releasing process by the addition of the sodium
dithionite as a reducing agent of the myoglobin solution [34, 35]. As shown in Fig. 6, within 5 min. of
the illumination process, one CO equivalent is released from CORM 1 as verified by the appearance
of two new bands at 540 and 577 nm. Upon the extended illumination (> 5 min.), the intensity of the
new bands increases and another CO molecule liberates within one hour. Similar behavior was
observed by CORM 2 (Fig. S12†), but the rate was faster and 2 CO molecules were released within
20 minutes. In contrast to 1 and 2, one CO equivalent was only liberated from CORM 3 upon the
exposure to the blue-light for one hour (Fig. S13†).
TD-DFT
7

The electronic structure and spectral properties of 1-3 were theoretically studied by time-
dependent density functional theory calculations using two functional, Becke 3-parameter
(
exchange) Lee–Yang–Parr (B3LYP) and CAM-B3LYP with a long-range correction term [22] as well
as LANL2DZ ECP basis set. Before calculating the electronic structures of the complexes, their
geometries should be characterized as local minima through the harmonic frequency analysis. The
selected calculated transitions and their assignments are tabulated in Table S4†. In general, the TD-
DFT spectra of 1-3, calculated by B3LYP or CAM-B3LYP, are characterized by two bands. The
position of the lowest energy band is not affected by the type of functional (B3LYP or CAM-B3LYP).
The UV band, calculated by B3LYP, is red-shifted with respect to that obtained by CAM-B3LYP. The
experimental data agree with the calculated methods in the visible region, whereas the cutoff of
DMSO (240 nm) did give us the chance to compare the UV data. The TD-B3LYP spectrum of 1 (Fig.
S14†) exhibits two main bands at 392 and 280 nm with a group of transitions in the range of 250-
400 nm. The descriptions of frontier MO's and the relocation of the electron density of complex 1
are given in Fig. S15†. The lowest energy band at 392 nm, assigned to HOMO-1→LUMO, agrees with
the band (398 nm) that is experimentally obtained. This transition has a ground state composed of
pyridine π-orbital with a small contribution to d(Mn) character, whereas the excitation state is of
*
*
ligand π -orbital/d(Mn) forming π-π /MLCT transitions. Five electronic transitions are observed in
the electronic TD-B3LYP spectrum of 2 at 409, 390, 337, 326 and 306 nm corresponding to H-2→L,
H-1→L+2, H-3→L, H-4→L and H-5→L (H: HOMO and L: LUMO). As shown in Fig. 7, the MO’s orbitals,
except HOMO and LUMO, are mainly composed of d(Mn) character. HOMO-2 is mainly composed of
͘5 (Mn). Thus, the calculated band at 409 nm, that coincides with the experimental value (407 nm),
v
is MLCT band. The stimulated spectrum of 3, calculated by B3LYP functional, is characterized by
seven transitions at 418, 397, 382, 347, 335, 316, and 305 nm allocated for H→L, H→L+1, H-1→L, H-
2→L, H-2→L+2, H-3→L, and H-4→L transitions. The HOMO-LUMO gap in 3 is 0.19 eV, which is larger
than CORM 1 (0.07 eV) and 2 (0.08 eV) that compares well with the blue-shift of the LMCT band
*
compared with 1 and 2. The LUMO orbital shows predominantly ligand π -character. HOMO-4 to
HOMO-2 results from the d(Mn) character and thus the transitions at 347, 316 and 305 nm are
LMCT. Similarly, the band at 335 nm has both ground and excited states of d(Mn) and thus this
transition could be classified as d-d transition (Table S5†).
Conclusion
For the first time, the N,S-bidentate ligands have been used as ancillary ligands to synthesizes a new
class of photoactivatable fac-[MnBr(CO) L] capable of CO release upon the exposure to 468 nm light
3
source. Three benzothiazole and pyridine based on thiourea have been synthesized to achieve this
8

goal. The H and 13C NMR analysis indicated that the thioureas coordinate to Mn(I) in the thione
form. The unambiguous proof of the bidentate nature of our ligand system came from the single
crystal X-ray structure analysis. The change of the type of the coordinating nitrogen center did not
modulate the nature charge and electronic arrangement of the Mn(I) ion as given by NBO analysis.
1
2
The values given by the second order interaction energy (E ) indicated that the interaction of
benzothiazole-N with Mn(I) is stronger than that of the pyridine-N based ligands. The
benzothiazole-thiourea CORM exhibited both long-term stability and fast CO release. However, the
number of CO equivalents obtained by benzothiazole-CORM was less than that obtained by the
pyridine compounds. The TD-DFT calculations assigned the lowest energy band in the complexes to
MLCT band and the data compares well with our experimental finding.
Experimental section
Synthesis of thiourea ligands:
•
LH [23]. 2-Aminopyridine (1.88 g, 0.02 mol) was dissolved in 50 mL ethanol containing
phenyl isothiocyanate (2.97 g, 0.022 mol). The reaction mixture was stirred overnight under
-
1
argon atmosphere, where a crystalline white compound precipitated. IR(ATR, cm ): 3215
Ar
Ar
Ar
py
(
br, -NH), 3172 (m, -NH), 3102 (w, CH ), 3075 (w, CH ), 3036 (w, CH ), 1620 (w, C=N ),
1
1
594 (s, C=C), 1520, 1468, 1343, 1312, 1266, 1186, 1138, 862, 769, 740. H NMR (DMSO-d
6
,
500.13 MHz, ppm): δ 13.73 (s,1H, phenyl-NH), 9.66 (s,1H, pyridine-NH), 8.19 (d, 1H,
3
pyridine-H6, J = 7.62 Hz), 7.65 (m, 2H, pyridine-H4/5), 7.62 (m, 1H, pyridine-H3), 7.41 (m,
3
13
2
H, phenyl-H2/H6), 7.25 (t, 1H, phenyl-H4, J = 7.44), 6.97 (m, 2H, phenyl-H3/5). C-NMR
(
DMSO-d , 125.75 MHz, ppm): δ 178.79 (C=S), 153.45 (pyridine-C1), 145.58 (pyridine-C3),
6
1
38.91 (phenyl-C1), 138.68 (pyridine-C6), 128.64 (phenyl-C2/6), 126.31 (phenyl-C4),
25.04 (pyridine-C4/5), 118.29, 112.67 (phenyl-C3/5).
1
•
L4ester. Methyl 2-aminopyridine-4-carboxylate (1.52 g, 0.01 mol) was added to a pyridine
solution (50 mL) of phenyl isothiocyanate (2.70 g, 0.02 mol). A yellow solution was
immediately developed and the solution was stirred for 48 h. By adding 200 mL isopropanol
to the reaction mixture, a white precipitate was formed, separated by filtration washed with
-
1
n-hexane and dried under vacuum. Yield: 77% (2.23 g, 0.008 mol). IR(ATR, cm ): 3330 (s,
Ar
py
NH), 3079 (w, CH ), 1712 (s, C=O), 1613 (w, C=N ), 1593 (s, C=C), 1551, 1520, 1438, 1341,
1
1
317, 1166, 1115, 978, 903, 746. H NMR (DMSO-d
6
, 500.13 MHz, ppm): δ 13.54 (s,1H,
3
phenyl-NH), 11.11 (s,1H, pyridine-NH), 8.46 (d, 1H, H6, J = 5.31 Hz), 7.81 (s,1H, pyridine-
3
3
H3), 7.68 (d, 2H, phenyl -H2/6; J = 8.34 Hz), 7.46 (d,1H, pyridine-H5; J = 5.31 Hz), 7.39 (t,
3
3
13
2
H, phenyl-H3/5; J = 7.67 Hz), 7.22 (t,1H, phenyl-H4, J = 7.34 Hz), 3.89 (s, 3H, CH ). C-
3
9

NMR (DMSO-d
46.93 (C6), 139.52 (pyridine-C4), 138.64 (phenyl-C1), 128.48 (phenyl-C3/5), 125.61
phenyl-C4), 124.30 (phenyl-C2/6), 116.37 (pyridine-C5), 112.63 (pyridine-C3), 52.91
CH ). Elemental analysis (%): calcd. for C14 S: C 58.52, H 4.56, N 14.62, S 11.16,
6
, 125.75 MHz, ppm): δ 178.26 (C=S), 164.49 (C=O), 154.14 (pyridine-C2),
1
(
(
3
H
N O
13 3 2
found C 58.40, H 4.48, N 14.58, S 11.06.
•
LBenz. LBenz was prepared by a method different from that given in the literature [24]. It was
synthesized following the same procedure as L4ester except that 2-amino-benzothiazole (1.50
g, 0.01 mol) was replaced with methyl 2-aminopyridine-4-carboxylate. Color: Pale yellow
-
1
py
powder. Yield: 69 % (1.97 g, 0.007 mol). IR(ATR, cm ): 3157 (s, NH), 1621 (m, C=N ), 1515,
1
1
441, 1363, 1258, 1184, 1070, 888, 753, 727. H NMR (DMSO-d
6
, 500.13 MHz, ppm): δ 12.78
3
(
s,1H, phenyl-NH), 10.72 (s,1H, benzothio-NH), 7.84 (d, 1H, benzothio-H7, J = 7.75 Hz), 7.70
3
3
(
d, 2H, phenyl-H2/6, J = 7.75), 7.49 (m, 1H, benzothio-H4), 7.41 (t, 1H, benzothio-H6, J =
3
3
7
.57), 7.34 (t, 2H, phenyl-H3/5, J = 7.37), 7.26 (t, 1H, benzothio-H5, J = 7.62), 7.11 (t, 1H,
phenyl-H4, J = 6.61). 13C-NMR (DMSO-d
3
6
, 125.75 MHz, ppm): δ 177.60 (C=S), 149.59
(
benzothio-C2), 139.46 (phenyl-C1), 136.10 (benzothio-C7a), 129.81 (benzothio-C3a),
1
28.42 (phenyl-C3/5), 126.67 (benzothio-C6), 124.41 (phenyl-C4), 123.63 (benzothio-C5),
23.47 (benzothio-C4), 122.37 (benzothio-C7), 122.9 (phenyl-C2/6). Elemental analysis
1
(
%): calcd. for C14
H
11
N
3
S : C 58.92, H 3.89, N 14.72, S 22.47, found C 59.01, H 3.81, N 14.56, S
2
21.94.
Synthesis of manganese(I) tricarbonyl complexes:
.36 mmol thiourea ligands (L , 82.5 mg; L4ester, 103.4 mg; LBenz, 102.7 mg) and manganese
H
0
pentacarbonyl bromide (100 mg, 0.36 mmol) were dissolved in degassed anhydrous acetone
25 mL) and the reactions were heated to reflux for 2h in the absence of light and in
(
presence of argon atmosphere. The reaction mixtures of 1-3 were left in the dark for slow
evaporation down to 5 mL, whereupon yellow-orange precipitate formed, washed with n-
hexane and then
dried under vacuum for few days.
H
•
[Mn(L )(CO)
3
Br] (1): Color: Yellow-orange powder. Yield: 81% (132 mg, 0.29 mmol).
-
1
Ar
Ar
IR(ATR, cm ): 3264(w, -NH), 3224 (m, -NH), 3162 (w, CH ), 3101 (w, CH ), 2024 (s, C≡O),
py
1
1
914 (s, C≡O), 1624 (w, C=N ), 1593 (s, C=C), 1532, 1367, 1275, 1154, 775, 754, 704. H
NMR (Acetone-d , 500.13 MHz, ppm): 10.61-10.53 (m, 2H, phenyl-&pyridine-NH), 9.22 (d,
6
3
3
1
H, pyridine-H6, J = 4.31 Hz), 8.00 (t, 1H, pyridine-H4, J = 7.95 Hz), 7.53-7.50 (m, 4H,
3
phenyl-H2/4/5/6), 7.39 (m, 2H, pyridine-H5&phenyl-H3), 7.32 (d, 1H, pyridine-H3, J = 7.31
1
0

Hz). 13C-NMR ((CD
)
CO, 125.75 MHz, ppm): δ 181.31 (C=S), 156.90 (pyridine-C6), 153.08
pyridine-C2), 140.92 (pyridine-C4), 137.83 (phenyl-C1), 130.65 (phenyl-C4/5), 128.55
pyridine-C5), 126.31 (phenyl-C2/6), 122.19 (phenyl-C3), 118.15 (pyridine-C3). ESI-MS
3
2
(
(
(
-
-
-
negative mode, acetone): m/z: 527.814 [M+Br] , 447.886 [M-H] , 363.898 [M-H-3CO] .
Elemental analysis (%): calcd. for C15
H
3 3
11BrMnN O S: C 40.20, H 2.47, N 9.38, S 7.15; found: C
39.50, H 2.49, N 9.18, S 7.04.
[
Mn(L4ester)(CO)
3
Br] (2): Color: Orange powder. Yield: 83% (147 mg, 0.29 mmol). IR(ATR,
-
1
Ar
cm ): 3235 (w, -NH), 3165 (w, -NH), 3084 (w, CH ), 2023 (s, C≡O), 1936 (s, C≡O), 1907 (s,
py
C≡O), 1713 (s, C=O), 1624 (w, C=N ), 1593 (m, C=C), 1529, 1484, 1398, 1296, 1235, 1122,
1
7
60, 708. H NMR (Acetone-d
6
, 500.13 MHz, ppm): 10.74-10.64 (m, 2H, phenyl-&pyridine-
3
3
NH), 9.39 (d, 1H, pyridine-H6, J = 5.20 Hz), 7.81 (d, 1H, pyridine-H5, J = 5.28 Hz), 7.78 (s,
1H, pyridine-H3), 7.65-7.53 (m, 4H, phenyl-H2/3/5/6), 7.42 (m, 1H, phenyl-C4), 3.99 (s, 1H,
13
CH
3
). C-NMR ((CD
3
) CO, 125.75 MHz, ppm): δ 222.90 (C≡O), 222.50 (C≡O), 219.20 (C≡O),
2
1
80.50 (C=S), 163.85 (C=O), 157.12 (pyridine-C6), 153.28 (pyridine-C2), 140.63 (pyridine-
C4), 136.85 (phenyl-C1), 129.83 (phenyl-C3/5), 127.98 (phenyl-C4), 125.76 (phenyl-C2/6),
19.54 (pyridine-C5), 116.66 (pyridine-C3), 52.82 (CH ). ESI-MS (negative mode, acetone):
m/z: 503.860 [M-H] , 419.873 [M-H-3CO] . Elemental analysis (%): calcd. for
S: C 40.34, H 2.59, N 8.30, S 6.33; found: C 39.00, H 2.73, N 8.11, S 6.18.
1
3
-
-
C
17
H
13BrMnN O
3 5
[
Mn(LBenz)(CO)
Br] (3): Color: Yellow powder. Yield: 81% (143 mg, 0.28 mmol). IR(ATR, cm-
3
1)
: 3205 (w, -NH), 3171 (w, -NH), 3069 (w, CH ), 3013 (br, CH ), 2021 (s, C≡O), 1940 (s,
Ar
Ar
C≡O), 1907 (s, C≡O), 1620 (w, C=NBenzoS), 1581 (s, C=C), 1513, 1438, 1374, 1247, 1159,
1
1
024, 759, 723. H NMR (Acetone-d
6
, 500.13 MHz, ppm): 11.64 (s, 1H, benzothio-NH), 10.77
3
3
(
s, 1H, phenyl-NH), 8.80 (d, 1H, benzothio-H7, J = 8.59 Hz), 8.02 (d, 1H, benzothio-H4, J =
3
7
.13 Hz), 7.72 (t, 1H, benzothio-H5, J = 7.28 Hz), 7.57 (m, 2H, phenyl-H3/5), 7.54 (m, 3H,
3
13
phenyl-H2/4/6), 7.42 (t, 1H, benzothio-H6, J = 7.33 Hz). C-NMR ((CD
3
) CO, 125.75 MHz,
2
ppm): δ 221.3 (C≡O), 180.10 (C=S), 162.84 (benzothio-C2), 151.16 (benzothio-C7a), 137.57
(
(
(
phenyl-C1), 130.63 (benzothio-C3a), 130.55 (phenyl-C3/5), 128.98 (benzothio-C6), 127.98
benzothio-C5), 126.83 (phenyl-C4), 126.73 (phenyl-C2), 126.68 (phenyl-C6), 126.02
benzothio-C7), 122.93 (benzothio-C4). ESI-MS (negative mode, acetone): m/z: 585.827
-
-
[
M+Br] , 505.899 [M-H] . Elemental analysis (%): calcd. for C17
H
11BrMnN
3
O
3
S : C 40.49, H
2
2.20, N 8.33, S 12.72; found: C 40.21, H 2.47, N 8.13, S 12.63.
Physical measurements
1
1

1
The NMR spectra were recorded on a Brucker-Avance 500 spectrometer ( H, 500.13 MHz,
13
1
C{ H}, 125.77 MHz). Bruker micrOTOF spectrometer was used in negative ion mode with a flow
-
1
rate of 4 µL min to report the electrospray ionization (ESI) mass spectra of the investigated
compounds. Only the most abundant peak of the isotope distribution is given. IR spectra were
recorded in the solid state on a Nicolet 380 FT-IR spectrometer equipped with a smart iFTR
accessory. Elemental microanalysis was determined using an Elementar Vario Micro Cube CHN
analyzer.
Quantification of CO release
The number of CO equivalents were determined by myoglobin assay as previously reported
[27, 33]. Illumination process was carried out with custom-built LED light sources, 468 nm (King-
bright Elec. Co., 5000 mcd, part. no. BL0106-15-299). The ferrioxalate actinometry assay [31] was
used to determine the photo flow of the light source used. The sealed cuvette was also positioned
perpendicular to the LED at a distance of 3 cm, where the illumination was interrupted in regular
intervals to take UV/Vis spectra on an Agilent 8453 diode array spectrophotometer until no more
spectral changes were observed. Dark stability measurements were carried out over 16 h.
X-ray diffraction analysis
A single crystal of 1 was immersed in a film of perfluoropolyether oil, mounted on a glass
fiber and transferred to a stream of cold nitrogen of the diffractometer. Crystallographic data of
complex 1 was collected on a Bruker x8 Apex II diffractometer with multi-layer mirror
monochromated Mo-K radiation (λ = 0.71073 Å) at 100 K. All the diffracted intensities were
α
collected using an APEX2-CCD detector and corrected for Lorentz-polarization and absorption using
the Bruker AXS software. The structure was solved with the SHELXT program using intrinsic
phasing method [36] and refined with a full-matrix least-squares procedure using the SHELXL
program [37]. All non-hydrogen atoms were anisotropically refined. All Hydrogen atoms except H2
were assigned to idealised positions. The coordinate of atom H2 was refined freely. The figures
involving hydrogen-bonds and packing were drawn using Mercury [38]. Crystallographic data have
been deposited with the Cambridge Crystallographic Data Center as supplementary publication no.
CCDC-1526305 for complex 1. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Quantum chemical calculations
The gas-phase geometry optimizations of 1-3 were carried out with the density functional
theory, Becke 3-parameter (exchange) Lee–Yang–Parr (B3LYP) functional [39] and the effective
core potential (ECP) of the Hady and Wadt, LANL2DZ basis set. All the calculations were done with
1
2

Gaussian 03 package [40]. The complexes were characterized as local minima via harmonic
frequency analysis. Time-dependent density functional theory calculations were performed by two
functional, B3LYP and CAM-B3LYP [41], and LANL2DZ ECP basis set as well as the default
polarizable continuum model (PCM) [42] to introduce the effect of DMSO as a solvent. The natural
bond orbital (NBO) and frontier molecular orbital analyses were performed by B3LYP/LANL2DZ
method with LANL2DZ ECP. TD-DFT UV-vis. spectra were visulaized using Gauss view 03 software
[43].
Author contributions
Dr. Alexandra Friedrich’s contribution to the present work was the collection of the X-ray diffraction
data and single-crystal structure analysis of CORM 1.
Acknowledgments
Ahmed M Mansour thanks Prof. Dr. Ulrich Schatzschneider, Julius-Maximilians-Universität
Würzburg, Germany for giving me the access to the analytical facilitates. Ahmed M Mansour thanks
also Alexander von Humboldt foundation and Deutscher Akademischer Austauschdienst (DAAD) for
postdoctoral fellowships. Thanks also for Mr. Wolfgang Obert, from the Electronics Workshop of
Julius-Maximilians-Universität Würzburg, Germany, who constructed the 468 nm LED light source.
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2000.
1
4

O
O
O
O
S
N Br
Mn
S
CO
CO
N
S
N
S
HN
HN
N Br
Mn
CO
CO
HN
HN
N
HN
HN
S
N Br
Mn
CO
CO
CO
HN
HN
HN
HN
HN
HN
S CO
S
S
CO
Benzo
L
H
4ester
L
L
1
2
3
Scheme (1): Molecular structures of the thiourea derivatives and their manganese(I)
photoCORMs.
Fig. 1: Molecular structure of complex 1, as obtained from single-crystal structure analysis
with ellipsoids drawn at the 50% probability level.
1
5

a)
b)
c)
Fig. 2: Local minimum structures of complexes a) 1, b) 2, and c) 3, obtained at the B3LYP/
LANL2DZ level of theory with LANL2DZ ECP.
0
0
1
2
5
1
2
4
7
1
1
before dark
Irrad
1
.4
.2
1
min
min
min
1
0 min
0 min
0 min
0 min
10 min
40 min
0.8
0.6
0.4
0.2
0
330
350
370
390
410
430
450
470
490
Wave length (nm)
Fig. 3: UV/Vis spectral changes of 1 (0.25 mM in DMSO) upon the photolysis at 468 nm for
–140 min after pre-incubation in the dark for 16 h.
0
1
6

2
.8
.6
.4
.2
1
0
0
1
2
before dark
Irrad
1
1
1
1
min
min
10 min
2
5
1
1
2
5 min
5 min
15 min
75 min
05 min
0.8
0.6
0.4
0.2
0
300
350
400
450
500
Wave length (nm)
Fig. 4: UV/Vis spectral changes of 2 (0.25 mM in DMSO) upon the photolysis at 468 nm for
–205 min after pre-incubation in the dark for 16 h.
0
2
1
0
.5
0
0
1
2
5
2
4
7
1
before dark
Irrad
min
min
min
2
0 min
0 min
0 min
00 min
.5
1
.5
0
3
68
388
408
428
448
468
488
Wave length (nm)
1
7

Fig. 5: UV/Vis spectral changes of 3 (0.25 mM in DMSO) upon the photolysis at 468 nm for
0
–100 min after pre-incubation in the dark for 16 h.
1
0
5
1
3
6
min
5 min
0 min
0 min
0
0
0
0
0
0
0
0
0
.9
.8
.7
.6
.5
.4
.3
.2
.1
5
00
520
540
560
580
600
620
Wave length (nm)
Fig. 6: UV/Vis spectral changes in the Q-band region of myoglobin (60 µM in 0.1 PBS at pH
.4) with sodium dithionite (10 mM) and complex 1 (10 µM) under argon atmosphere upon
photolysis (0–60 min) at 468 nm.
7
1
8

1
9

Fig. 7: TD-B3LYP-LANL2DZ ECP calculated electronic absorption transitions of CORM 2
3
(
Isosurface value = 0.02 e/Å )
2
0

Table 1: Selected experimentally determined bond lengths (Å) and angles (°) for complex 1
Bond length (Å) Angles (º)
Mn-C(1)
1.815(3)
1.801(3)
1.789(4)
2.083(3)
2.344(2)
2.560(3)
1.145(4)
1.155(4)
1.152(4)
C(1)-Mn-C(2)
C(1)-Mn-C(3)
C(1)-Mn-N(1)
C(1)-Mn-S(1)
C(1)-Mn-Br(1)
C(2)-Mn-C(3)
C(2)-Mn-Br(1)
C(2)-Mn-S(1)
N(1)-Mn-C(2)
C(3)-Mn-Br(1)
C(3)-Mn-S(1)
N(1)-Mn-C(3)
N(1)-Mn-S(1)
N(1)-Mn-Br(1)
Br(1)-Mn-S(1)
PhN-H…Br
90.3(1)
89.7(1)
177.2(1)
90.35(9)
88.64(9)
88.4(1)
89.91(9)
177.78(9)
91.7(1)
177.62(9)
93.74(9)
92.3(1)
Mn-C(2)
Mn-C(3)
Mn-N(1)
Mn-S(1)
Mn-Br(1)
C(1)-O(1)
C(2)-O(2)
C(3)-O(3)
87.56(6)
89.39(6)
87.97(2)
158.68
PhNH…Br
PhNH…Br
PyNH…Br
PyNH…Br
3.450
2.616
3.410
2.594
PyN-H…Br
162.88
2
Table 2: Second-order interaction energy (E , kcal/mol) between donor and
acceptor orbitals in the studied complexes calculated at B3LYP/LANL2DZ level
of theory with LANL2DZ ECP (selected)
2
E2
Donor→Acceptor
E
Donor→Acceptor
•
1
*
*
σ(C(4)-N(9))→RY (1)Mn
1.80
0.31
1.14
σ(Mn-C(28))→RY (1)Mn
1.04
2.68
0.49
*
*
σ(S(13)-Mn)→RY (1)Mn
σ(Mn-C(29))→RY (1)Mn
*
*
σ(Mn-Br)→RY (1)Mn
σ(Mn-C(30))→RY (1)Mn
2
1

•
2
*
*
σ(C(3)-N(9))→RY (1)Mn
1.71
0.32
1.18
σ(Mn-C(35))→RY (1)Mn
1.01
2.64
0.51
*
*
σ(S(20)-Mn)→RY (1)Mn
σ(Mn-C(36))→RY (1)Mn
*
*
σ(Mn-Br)→RY (1)Mn
σ(Mn-C(37))→RY (1)Mn
•
3
*
*
σ(C(18)-N(29))→RY (1)Mn
3.25
0.12
0.86
σ(Mn-C(32))→RY (1)Mn
0.60
4.48
1.55
*
*
σ(C(3)-S(4))→RY (1)Mn
σ(Mn-Br)→RY (1)Mn
σ(Mn-C(33))→RY (1)Mn
*
*
σ(Mn-C(34))→RY (1)Mn
2
2

0
0
1
2
5
1
2
4
before dark
Irrad
1
.4
.2
1
min
min
min
1
0 min
0 min
0 min
70 min
1
10 min
40 min
1
0.8
0.6
0.4
0.2
0
CO
CO
CO
330
350
370
390
410
430
450
470
490
Wave length (nm)
First example of blue-light triggered carbon monoxide releasing molecules based on
Mn(CO) Br(N,S) and bearing N,S-bidentate ligand.
3
2
3