Wavelength-Dependent Control of the CO Release Kinetics of Manganese(I) Tricarbonyl PhotoCORMs with Benzimidazole Coligands

Article abstract of DOI:10.1002/ejic.201900894

A series of photoactivatable CO-releasing molecules (PhotoCORMs) was prepared from manganese pentacarbonyl bromide and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine ligands (L) bearing different electron-donating and electron-withdrawing groups R = H, 4-CH

Full text of DOI:10.1002/ejic.201900894

DOI: 10.1002/ejic.201900894
Full Paper
CO-Releasing Molecules
Wavelength-Dependent Control of the CO Release Kinetics of
Manganese(I) Tricarbonyl PhotoCORMs with Benzimidazole
Coligands
Ahmed M. Mansour,*[a,b] Christoph Steiger,[c] Christoph Nagel,[b] and Ulrich Schatzschneider[b]
Abstract: A series of photoactivatable CO-releasing molecules rapidly release CO upon illumination at 412–525 nm, depending
(PhotoCORMs) was prepared from manganese pentacarbonyl on the substitution pattern. Its influence on the photophysical
bromide and 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine li- and photochemical properties was systematically explored us-
gands (L) bearing different electron-donating and electron- ing UV/Vis spectroscopy and CO release measurements with a
withdrawing groups R = H, 4-CH3, 4-OCH3, 4-Cl, 4-NO2, 2-, 3-, commercial gas sensor system. In the nitro-substituted com-
and 4-COOCH3 on the phenyl substituent to give octahedral pound, the electronically excited state switched from benzimid-
manganese(I) complexes of the general formula [MnBr(CO)3(L)]. azole- to phenyl-centered, leading to a markedly different pho-
Aerated DMSO solutions of the compounds are stable in the tochemical behavior of this visible-light activated PhotoCORM.
dark for 16 h with no CO release. However, the compounds
Introduction
the whole body. In 2002, Motterlini and co-workers introduced
[Mn2(CO)10] (CORM-1) and [RuCl(μ-Cl)(CO)3]2 (CORM-2) as the
first CORMs.[7] CORM-1 releases one equivalent of CO upon ex-
posure to UV light, but its low water solubility encouraged the
search for more biocompatible metal carbonyl complexes, for
example with amino acid auxiliary ligands such as [RuCl-
(glycinato)(CO)3] (CORM-3).[8] This compound, the most widely
studied CORM to date, immediately releases CO upon dissolu-
tion in human plasma and exhibits beneficial biological proper-
ties such as antimicrobial and anti-inflammatory activity as well
as organ preservation effects.[9] The release of CO from the ruth-
enium coordination sphere is induced by ligand exchange in
aqueous medium,[10] but other trigger mechanisms have also
been explored, such as changes in pH,[11] redox state,[12] and
thermal heating.[13] Furthermore, in so-called enzyme-triggered
CORMs (ET-CORMs), enzymatic bond cleavage in the ligand pe-
riphery induces a series of events finally leading to CO re-
lease.[14] In addition, the use of light as an external trigger has
enabled facile control of the time, location, and dosage of CO
release.[15,16]
Over the last couple of years, the excitation wavelength to
trigger photoactivatable CO-releasing molecules (PhotoCORMs),
has been tuned toward lower energies by proper choice of the
coligand system, to ensure deep penetration depth and mini-
mize photodamage to intervening tissue.[17] Manganese(I) tri-
carbonyl complexes have featured particularly prominent
among such systems.[18–22] A wide range of tridentate ligands
coordinated to the fac-Mn(CO)3 moiety was explored, leading
to [Mn(CO)3(tpm)]PF6 with tpm = tris(pyrazolyl)methane),[18]
[Mn(CO)3(tpa-κ3N)]Br with tpa = tris(2-pyridylmethyl)amine,[19]
and [Mn(bpea)(CO)3]PF6 with bpea = bis(pyrazolyl)methane).[20]
While these compounds require photoactivation at 365 nm to
trigger the CO release, Mascharak and co-workers improved the
In contrast to the well-known general toxicity of high systemic
concentrations of carbon monoxide, lower amounts of CO, ei-
ther enzymatically produced by the activity of heme oxygenase
(HO) enzymes on heme, or released from synthetic CO carrier
molecules, show surprising beneficial biological effects, includ-
ing cytoprotection during inflammation, promotion of wound
healing, and are also involved in vital signaling processes.[1–4]
This has encouraged researchers to explore the administration
of CO to humans for potential therapeutic approaches. Unlike
the other two small molecule bioregulators (SMBs), nitric oxide
and hydrogen sulfide,[5] which interact with a wide range of
intracellular targets, including protein sidechains, the chemistry
of CO is restricted to coordination to transition metal centers in
low oxidation states. As there is a lack of target specificity of
CO gas applied to patients by an inhalative delivery system,[6]
metal-carbonyl complexes are widely explored as carbon mon-
oxide-releasing molecules (CORMs), which act as prodrugs for
the delivery of well-controlled quantities of CO to biological
systems. Advantages of CORMs include the specific targeting of
organs and tissues by tuning of the outer ligand sphere (“drug
sphere”) without delivering potentially toxic amounts of CO to
[a] Department of Chemistry, Faculty of Science, Cairo University,
Gamma Street, Giza, Cairo 12613, Egypt
E-mail: mansour@sci.cu.edu.eg
http://scholar.cu.edu.eg/?q=amansour/
[b] Institut für Anorganische Chemie, Julius-Maximilians-Universität Würzburg,
Am Hubland, 97074 Würzburg, Germany
[c] Institut für Pharmazie und Lebensmittelchemie,
Julius-Maximilians-Universität Würzburg,
Am Hubland, 97074 Würzburg, Germany
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under https://doi.org/10.1002/ejic.201900894.
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photosensitivity of [MnBr(CO)3(pbt)] with pbt = 2-(2-pyridyl)- substituted anilines bearing different functional groups R = H,
benzothiazole) by increasing the conjugation of the ligand 4-CH3, 4-OCH3, 4-Cl, 4-NO2, and 2-, 3-, and 4-COOCH3 in the
framework.[22] These PhotoCORMs release CO upon illumination ortho-, meta-, or para-position of the phenyl ring (Scheme 1).
at 400–550 nm. The properties of manganese(I) CORMs contain- They were chosen to study the influence of the type and posi-
ing other N,N-bidentate ligands with larger conjugated tion of the substituent on the photophysical properties and CO
π-system have also been explored, for example in release kinetics of the title compounds. The reaction of ligands
[MnBr(CO)3(pmtpm)] with pmtpm = 2-pyridyl-N-(2′-methylthio- 1–8 with manganese(I) pentacarbonyl bromide in acetone at
phenyl)methyleneimine) and [MnBr(CO)3(qmtpm)] with reflux in the dark gave the fac-Mn(CO)3 complexes 9–16 in pure
qmtpm
= 2-quinoline-N-(2′-methylthiophenyl)methylene-im- form and moderate to good yield (30–73 %).
ine.[23] Some organic drugs such as nifuroxazide, nitazoxanide,
and tazarotene have also been used to synthesize new Mn(I)
PhotoCORMs, which release CO upon excitation at 468–
525 nm.[24]
An important question in the field is the nature of the CO
release process, which might either follow a concerted or a
step-wise mechanism, as intermediates might have different ab-
sorption properties and release kinetics than the original tri-
carbonyl complex. Kinetic studies on selected compounds by
solution IR and EPR spectroscopy as well as quantum chemical
calculations revealed the intermediate formation of Mn(CO)2
species upon photolysis, followed by facile oxidation to the
Mn(II) and Mn(III) stages in further dark processes.[19,25] Berends
and Kurz[25] as well as Schatzschneider[19] found that only one
CO ligand is initially photolytically released from the Mn(I) coor-
dination sphere, while the remaining CO molecules require an
additional oxidative dark process. Furthermore, in a femtosec-
ond transient absorption UV pump/mid-IR probe spectroscopic
study, one CO molecule was photochemically released on very
short timescales, but a fraction of the molecules excited was
shown to undergo geminate recombination.[26]
In contrast to the excitation wavelength, the CO release ki-
netics are much more difficult to control by compound design.
This is an important bottleneck, as biological effects take place
at different timescales, from seconds to hours, and will require
a range of PhotoCORMs from fast to slow releasers. For a sys-
tematic exploration of the influence of the coligand electron
density on the CO release kinetics, a ligand system which allows
facile introduction of electron-donating and electron-withdraw-
ing functional groups directly in conjugation with the π-system
of the chelating ligand would be highly desirable. Therefore, in
the present work, 1H-benzimidazol-2-ylmethyl-(N-phenyl)amine
is introduced as a novel bidentate ligand to the fac-Mn(CO)3
moiety. Although the benzimidazole motif is a crucial pharma-
cophore in drug discovery with interesting biological properties
such as antiviral, antitumor, antihistaminic, and antimicrobial
activity[27] it has not been extensively explored in the context
of CORMs so far. The influence of the type and position of the
substituent on the phenyl ring on the photophysical properties
of the novel PhotoCORMs was systematically explored and ra-
tionalized with the support of DFT calculations.
Scheme 1. Synthesis of benzimidazole ligands LR 1–8 and their manganese(I)
tricarbonyl complexes 9–16.
The IR spectra of ligands 1–8 are characterized by a sharp
band around 3345–3450 cm–1 assigned to the N–H-stretching
,
mode of the secondary amino group. The stretching vibrations
of the benzimidazole ring could not be identified due to the
presence of strong intermolecular hydrogen bonds between
the benzimidazole sp2 nitrogen atom and the benzimidazole
NH group.[27] The ν(C=O) vibration of the methyl ester group
was found at 1687 (5), 1673 (7), and 1701 cm–1 (8), respectively.
The IR spectra of complexes 9, 10, 12, and 14 (Figure S1) show
three intense ν(C≡O) vibrations at around 2020–2026, 1930, and
1900–1910 cm–1 assigned to the symmetrical and two anti-sym-
metrical stretching modes of the Mn(CO)3 moiety, respectively.
The other complexes 11, 13, 15, and 16 display only two bands
in the 2020–2030 and 1900–1910 cm–1 range, due to the latter
one appearing rather broad. Compared to 1–8, the ν(NH) mode
of the amine linker is shifted to lower wavenumbers upon coor-
dination to Mn(I), while the ν(NH) vibration of the benzimid-
azole cannot be identified due to association of the complexes
in the solid-state forming polymeric structure through
Br···NHbim contacts as determined by X-ray structure analysis
(vide infra). In complex 15, incorporation of the methyl ester
ligand L2–COOCH3 (7), the C=O group is apparently not involved
in the coordination of the Mn(I) center since its stretching mode
Results and Discussion
Synthesis of Ligands and Complexes
Eight benzimidazole-based bidentate N,N-ligands LR 1–8 were appears at a higher wavenumber (1688 cm–1) than in the free
synthesized by reaction of 2-(chloromethyl)benzimidazole with ligand (1673 cm–1).
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In the 1H NMR spectra of ligands 1–8, the benzimidazole NH
resonance appears at 12.20–12.46 ppm but is shifted downfield
to 13.47–13.58 ppm in complexes 9–16.[27] This is attributed to
a change in the electron density on the benzimidazole ring
upon metal coordination to the benzimidazole sp2 nitrogen
atom. The triplet signal of the bridging NH group and the dou-
blet of the methylene group in the free ligands (FigureS2) also
shift downfield upon coordination to the Mn(I) center. In these
metal complexes, the two methylene protons become dia-
stereotopic, giving rise to a pair of doublets between 4.42 and
4.95 ppm (Figure S3).
Table 1. Selected experimental bond lengths (Å) for 9, 10, 12, and 14. For
complex 14, only data for one of two independent molecules with essentially
identical metrics in the unit cell is shown.
9
10
12
14
Mn1–Br
Mn1–C1
Mn1–C2
Mn1–C3
Mn1–N1
Mn1–N2
2.542(4)
1.790(3)
1.811(3)
1.808(2)
2.122(2)
2.037(2)
2.541(4)
1.789(2)
1.809(3)
1.812(2)
2.130(2)
2.037(2)
2.539(3)
1.790(2)
1.806(2)
1.814(2)
2.142(2)
2.034(1)
2.5550(4)
1.803(2)
1.814(2)
1.809(2)
2.141(2)
2.035(2)
Mn1–N2, and Mn1–Br bonds are observed in complexes 9, 10
and 12, while the length of the Mn1–Br bond is notably differ-
ent in the case of 14.
Crystal Structures
Crystals suitable for X-ray structure analysis were obtained by
diffusion of diethyl ether into a solution of the complex in acet-
one for 9, 10, 12, and 14. Relevant crystallographic parameters
Electronic Absorption Spectra and CO-Release Properties
are listed in Table S1 and the molecular structures are shown The electronic absorption spectra of 9–16 in DMSO show a
in Figure 1a–d. Selected bond lengths are presented in Table 1. prominent band centered at around 390 nm with εmax values
In all four compounds structurally characterized, the fac- in the range of 1344–1765 L mol–1 cm–1 (Figure 2 and Table 2).
Mn(CO)3 moiety is coordinated by the bidentate benzimidazole Compound 15, with the methyl ester group in the ortho posi-
ligand and a bromide anion to complete the octahedral coordi- tion, exhibits an additional intense band at 355 nm with εmax
nation sphere of the Mn(I) center.
=
(2193 93) L mol–1 cm–1. The nitro-substituted complex 14 dif-
fers from the others as it exhibits only a moderate absorbance
at 393 nm but additionally shows an intense broad band at
495 nm, which however strongly changes with incubation for
an hour.
The Mn–N1 bond lengths are 2.122(2), 2.130(2), 2.142(2), and
2.141(2) Å for 9, 10, 12 and 14, respectively. This slight variation
is due to the electron-withdrawing effect of the para-substitu-
ent, 4-Cl (12) or 4-NO2 (14), or the extended conjugation effect
of the methyl group (10). As shown in Table 2, the Mn–C bond
Two isosbestic points at 334 and 427 nm are observed dur-
lengths for the equatorial CO groups are also unequal and ing the dark incubation of 14 in DMSO for 1 h. This is attributed
longer than the axial one (1.790(2) Å), except for the nitro com- to exchange of the axial bromide ligand with solvent, as it is
plex, where the difference is very small. Similar lengths for the suppressed by bromide addition in some of the other com-
Figure 1. Molecular structures of (a) [MnBr(CO)3(LH)] 9, (b) [MnBr(CO)3(L4–CH3)] 10, (c) [MnBr(CO)3(L4–Cl)] 12, and (d) [MnBr(CO)3(L4–NO2)] 14, with ellipsoids
drawn at the 50 % probability level. For 14, only one of two independent molecules with essentially identical metrics in the unit cell is shown.
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pearance of a new band at 359 nm, which shifts to 351 nm
upon extended exposure to light (Figure 3). Illumination of
complexes 10 and 11 (Figure S5b), incorporating the electron-
donating methyl and methoxy ligands, at 412 nm had a similar
effect. In both cases, a decrease in the intensity of the main
absorption band at 386 nm with illumination time was ob-
served while a new band at 361 nm and a shoulder at 309 nm
grew in. A plateau value was reached after 75 min of illumina-
tion time with no further spectral changes observed after ex-
tended illumination.
Figure 2. Electronic absorption spectra of complexes 9–16 in DMSO [0.27 m
M
M
(9 and 10); 0.17 mM (11 and 12), 0.24 mM (13), 0.05 mM (14), and 0.38 m
(15 and 16)].
Table 2. Absorbance maxima (λmax, nm) and molar extinction coefficients
(εmax, L mol–1 cm–1) of the lowest energy band in complexes 9–16 in DMSO.
λmax
εmax
9
386
386
386
386
1705 20
1665 13
1558 43
1543 36
1648 62
10
11
12
13
14
15
15
16
390
not determined[a]
Figure 3. UV/Vis spectral changes of 9 (0.27 m
M in DMSO) upon photolysis
355
395
386
2193 93
1344 20
1765 49
at 412 nm with increasing illumination time (0–145 min).
When complexes 9–12 were illuminated at 468 instead of
412 nm, the decrease of the band at 386 nm was faster and
more pronounced than with violet light and only 10–15 min
were required to reach the final state (Figure 4). Furthermore,
the influence of electron-withdrawing groups in the para-,
ortho- and meta-position of the phenyl ring was also studied in
the methyl ester compounds 13, 15, and 16. While their dark
stability was comparable to 9, illumination of a pre-incubated
sample of 13 at 412 nm (Figure S6) gave rise to a two-step
process. The broad band initially observed at 390 nm is first
blue-shifted to 359 nm during the initial 5–10 min and then
significantly increases in intensity up to 25 min of illumination
time without a further shift. Later on, and until the end of the
observation time at 80 min, it decreases again (Figure S6). A
comparison of spectra acquired under 412 nm illumination with
[a] The absorbance of the MLCT band at 495 nm changes with time.
pounds (vide infra). To obtain an insight in the dark stability
and photoinduced reactivity profile of 9–16, the complexes
were first incubated in DMSO in the dark at room temperature
for 16 h and then illuminated with violet or blue light from a
LED source at 412 or 468 nm, respectively. Although the illumi-
nation wavelength at 468 nm does not coincide with the transi-
tion around 390 nm, there is still sufficient absorption at this
wavelength. Compound 14 was also exposed to a green LED
with an emission maximum at 525 nm, in the flank of the main
broad band. In addition, 9–16 were also illuminated at 412 nm
without previous dark incubation. This allows one to elucidate
whether there are additional dark processes such as solvent
displacement of the CO and/or bromide ligands from the metal
coordination sphere.
When kept in the dark, complexes 9–16 were quite stable
for up to 16 h in aerated DMSO solution (Figure S4). Only the
three compounds incorporating the electron-donating methyl
and methoxy substituents (10 and 11) as well as nitro group
exhibited a minor change in absorbance over time. This effect
is suppressed by addition of an excess of tert-butylammonium
bromide (Figure S5a) and therefore supports the assignment of
the spectral changes observed during dark incubation to ex-
change of the axial bromido ligand by solvent in these com-
plexes.
Illumination of complex 9, incorporating the unsubstituted
ligand LH, at 412 nm led to a decrease of the intensity of the
band at 386 nm with time and an intermediate state was
reached after 25 min. Further illumination resulted in the ap-
Figure 4. UV/Vis spectral changes of 9 (0.27 m
468 nm with increasing illumination time (0–45 min).
M in DMSO) upon photolysis at
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and without previous pre-incubation in the dark (Figure S7) in- tion time of 5 min, but then the former band decreases for the
dicates that the photoprocess is faster when the solution is first following 55 min. The UV/Vis spectroscopic data on 9–16 re-
kept in the dark for 16 h. The difference in the photolysis behav- veals the formation of different species during illumination with
ior may be attributed to exchange of the axial bromido ligand unique λmax and εmax values, which also depend on whether
with DMSO solvent molecules. The latter is harder than the the compound is first pre-incubated in the dark or immediately
bromide and thus more labilizing, which leads to a faster CO exposed to light after dissolution. It is reasonable to assume
release (Scheme 2). The photolysis profile of 16 at 412 nm is that the initial dark process is due to exchange of the bromide
the same as observed for complex 13 (Figure S6).
ligand with solvent while the photoinduced processes are sub-
sequent CO release steps, which also depend on the nature of
the axial monodentate ligand (bromide vs. solvent).
Figure 5. UV/Vis spectral changes of 14 (0.05 m
at 525 nm for 0–55 min without pre-incubation.
M in DMSO) upon photolysis
Scheme 2. Illustration of the proposed photoinduced and dark ligand ex-
change processes.
The electronic absorption spectrum of complex 15, featuring
the methyl ester group in the ortho-position of the phenyl ring, Solution IR Spectroscopy
shows a dominant band at 355 nm. In contrast to the other
To obtain more detailed information on the composition of the
species formed during illumination, solution IR experiments
were carried out on 9 and 14 as representative examples.
compounds, a clear isosbestic point was observed at 372 nm
upon illumination at 412 nm (Figure S7). Switching from 412 to
468 nm illumination, the spectral changes observed for 13, 15,
and 16 resemble those obtained at lower wavelength excita-
tion, but the photolysis process is accelerated (Figure S8). While
the 4- and 3-substituted complexes 13 and 16 require 15 and
13 min to reach a plateau level under otherwise identical condi-
tions, the 2-substituted compound 15 reaches this state already
after 3 min of illumination at 468 nm.
Finally, the electronic absorption spectrum of a freshly pre-
pared sample of 14 in DMSO is characterized by two bands at
393 and 492 nm. When this complex, incorporating the elec-
tron-withdrawing nitro-substituted ligand, is further incubated
in the dark for 16 h, two isosbestic points at 334 and 427 nm
are observed (Figure S9) and the band at 492 nm decreases
with time, while the one at 393 nm increases. When the sample
is illuminated at 412 nm after the pre-incubation time, the ab-
sorbance at 492 nm decreases while the intensity at 393 nm
shows an opposite effect (Figure S10). On the other hand, when
a pre-incubated DMSO solution of 14 is exposed to light at
468 nm for 1 min, the intensity of the two bands at 492 and
393 nm both increase (Figure S11). Further illumination, how-
ever, leads to a subsequent decrease of the absorbance at
492 nm, accompanied by a red shift to 506 nm.
A DMSO solution of PhotoCORM 9 (8 mM) was illuminated in a
CaF2 cell at 468 nm for time intervals of 1 min, after which the
light exposure was interrupted to take IR spectra. Before the
start of the illumination, the IR spectrum of 9 shows three
bands at 2021, 1923, and 1906 cm–1, which are assigned to
the symmetrical and antisymmetrical CO stretches of the fac-
Mn(CO)3 moiety. During the illumination, the latter modes dis-
appear while two new bands grow in at 1968 and 1862 cm–1
(Fig. S12), which are assigned to the formation of a dicarbonyl
species. The stability of 14 was investigated by IR spectroscopy.
The IR spectrum of 14 shows three strong bands at 2023, 1927
and 1907 cm–1 (Fig. S13), which slightly shift upon dark incuba-
tion in DMSO. However, this mostly affects the two lower en-
ergy antisymmetrical stretching vibrations while the other
bands remain unchanged. No evidence of the formation of di-
carbonyl species or a fac to mer isomerization process was ob-
served,[19,20] as the formation of a dicarbonyl species would give
rise to only two bands at lower wavenumbers, while the mer-
isomer normally exhibits CO stretching modes at higher values
relative to the fac species.
In contrast to all other compounds discussed in this work,
complex 14 also undergoes pronounced spectral changes upon
CO Release Studies Using a Sensor Chip System
photoactivation at 525 nm (Figure 5). The absorbance of the Due to the complicated photoprocesses occurring in most of
bands at 492 and 393 nm increases during an initial illumina- the compounds and the difficulties associated with the myo-
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globin assay, such as reducing conditions due to the presence 4.3 min),[23] N,N′-bis(2,6-diisopropylphenyl)-1,4-diaza-1,3-buta-
of a large amount of dithionite, the CO scavenging properties diene (λex = 560 nm, DMSO/H2O, t1/2 = 35 min),[28] and 2-phen-
of the heme protein, which might shift the CO release equilib- ylazopyridine derivatives (λex ≥ 625 nm, CH2Cl2, t1/2 = 3.5 h)
rium, and potential spectral overlap of the metal complex and exhibit widely divergent half-life times of CO release.[29]
protein absorbances, CO release from compounds 9–16 dis-
solved in a mixture of water and DMSO (10:1 v/v) was studied
DFT and TDDFT Calculations
with a commercial gas sensor upon illumination at 468 nm (Fig-
ure 6). Under these conditions, all compounds showed rather
similar release profiles and reached a plateau valued after about
30 min of exposure to 468 nm light. In the case of the para-
nitro compound 14, 525 nm photoexcitation was also investi-
gated. The compound initially showed a slow and linear CO
release when triggered at this wavelength, but after 4 h of ex-
tended illumination, also reached a plateau value (Figure 6).
About 0.9–1.8 equivalents of CO are released from 9–16 upon
illumination for 30–40 min at 468 nm. In contrast, at 525 nm
excitation of 14, only 0.9 CO equivalents are detected. This
opens up the interesting prospect of actually controlling the
release kinetics by selection of the excitation wavelength, de-
pending on whether the biological process of interest requires
a fast or slow CO release, without the need to prepare fast and
slow CO releasers separately. Other visible-light inducible Mn(I)
tricarbonyl PhotoCORMs bearing N,N-bidentate ligands such as
To obtain an insight in the structural and electronic features of
the title compounds, in particular those that were not charac-
terized by X-ray structure analysis, the structures of 9–16 were
optimized by DFT using the BP86 functional and a TZVP basis
set and showed a good match of bond parameters and vibra-
tional bond positions with experimental values (Tables S2 and
S3).[19,20,30] For an additional insight in the electronic structure
and assignment of the most relevant UV/Vis transitions, TDDFT
calculations were carried out on 9 and 14 to obtain the 45
lowest-energy spin-allowed singlet excitations (Table 3). While
both compounds 9 and 14 exhibit some closely spaced states
at around 260–300 nm (Table 3 and Table S4), the nature of the
first relevant lowest energy state (state 1 in the case of 9 and
state 3 in the case of 14) is markedly different (Figure 7). In the
unsubstituted parent compound 9, the difference density plot
reveals a shift from the metal to the benzimidazole part of the
bidentate ligand and the transition is thus of MLCT character.
On the other hand, in the case of the nitro-substituted com-
pound 14, the transition also originates from the manganese
center, but leads to a build-up of electron density on the nitro-
phenyl group of the ligand system upon excitation. Thus, by
variation of the substituent on the phenyl ring, the transition is
switched from benzimidazole- to phenyl-centered. This is as-
2-(2-pyridyl) benzothiazole (λex = 400 nm, DMSO/H2O, t1/2
=
Table 3. Energies (in nm), oscillator strength (fosc), main orbital contributions,
and type of transitions involved in the most important singlet excitations for
9 and 14 calculated with TDDFT using the B3LYP functional and a TZVP basis
set.
Compound 9
State[a]
λ
fosc
main transitions[b] type of transition[c]
1
2
361
324
0.0351
0.0139
109→110 (90 %)
108→110 (30 %)
109→111 (45 %)
108→111 (25 %)
109→112 (34 %)
108→114 (54 %)
106→110 (32 %)
109→115 (55 %)
MLCT Mn→bim
MLCT Mn→im/CO
8
290
0.0183
MLCT Mn→phenyl/CO
10
13
273
264
0.0216
0.0127
MLCT Mn→ phenyl/CO
MLCT Mn→bim
Compound 14
State[a]
λ
fosc
main transitions[b] type of transition[c]
3
6
7
653
358
335
299
351
279
0.0161
0.0312
0.0215
0.0217
0.0573
0.2646
118→121 (99 %)
120→122 (92 %)
120→123 (70 %)
119→123 (42 %)
114→121 (66 %)
112→121 (52 %)
MLCT Mn→phenyl
MLCT Mn→bim
MLCT Mn→phenyl/CO
MLCT Mn→phenyl/CO
LLCT Br→phenyl
MLCT Mn→phenyl/CO
LLCT NO2→phenyl/CO
MLCT Mn→phenyl/CO
LLCT NO2→phenyl/CO
16[d]
17[d]
21
22
273
0.1875
112→121 (24 %)
Figure 6. CO release profiles of 9–16 in DMSO/water (1:10 v/v) analyzed with
a commercial CO sensor system under constant illumination with LED light
at 468 or 525 nm (top) during the initial 30 min and (bottom) upon extended
illumination of 14 at 525 nm for up to 4 h. All data are expressed as
mean SD (n = 3).
[a] Only strong transitions with an oscillator strength > 0.01 in the 250–
800 nm range are reported. [b] Only contributions >20 % are listed. [c] Bim =
benzimidazole and im = imidazole. [d] The reversed order is due to state 16
experiencing a smaller solvent shift than most of the others.
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sumed to be the origin of the markedly different photochemical of exposure to 468 nm light. The nitro-substituted compound
behavior of the two compounds.
could also be activated with green light at 525 nm. Under these
conditions, an initial slow and linear increase in CO concentra-
tion was observed. Only after extended illumination for 4 h, a
plateau value was reached for this complex, albeit with a lower
CO release efficiency. In summary, by variation of the peripheral
substituent on the phenyl ring in this series of PhotoCORMs,
green light photoactivation was achieved with a strongly elec-
tron-withdrawing nitro group, and the release kinetics were
controlled by choice of the activation wavelength. This opens
up the opportunity to switch a compound from slow to fast CO
releaser by simple variation of the light source, depending on
the biological process to be investigated.
Experimental Section
General procedures
Reactions were carried out in oven-dried Schlenk glassware under
an atmosphere of pure argon and exclusion of light using degassed
solvents. All the chemicals were purchased from commercial sour-
ces and used as received. Elemental analysis was carried out using
an Elementar Vario Micro Cube CHN analyzer. IR spectra were re-
corded in the solid state on a Nicolet 380 FT-IR spectrometer
equipped with a smart iFTR accessory. UV/Vis spectra were meas-
ured using an Agilent 8453 diode array spectrophotometer. NMR
spectra were recorded on a Bruker Avance 500 spectrometer (1H:
500.13 MHz, 13C{1H}: 125.77 MHz) at ambient temperature. Chemical
shifts δ in ppm indicate a downfield shift relative to TMS and are
referenced relative to the residual proton signal of the solvent.[31]
Coupling constants J are given in Hertz (Hz). Peak multiplicities are
indicated as singlet (s), doublet (d), doublet of doublet (dd), triplet
(t), and multiplet (m), respectively. Electrospray ionization (ESI) mass
spectra were recorded in negative ion mode using a Bruker
micrOTOF spectrometer with a solvent flow rate of 4 μL min–1. Only
the most abundant peak of each isotope distribution is given. The
illumination was carried out with custom-built LED light sources,
Figure 7. TDDFT difference density plots for the most important singlet exci-
tations of 9 and 14 in the 290 to 660 nm range. Isosurface values are plotted
at 0.002, with positive values shown in green and negative ones in red.
Conclusions
Eight manganese(I) carbonyl complexes of the general formula
[MnBr(CO)3(L)] incorporating 1H-benzimidazol-2-ylmethyl-(N-
phenyl)amine derivatives as the chelating ligand L were synthe-
sized and characterized, including X-ray analysis for four of the
compounds. Different electron-withdrawing and electron-
donating substituents in the 2-, 3-, and 4-position of the phenyl
ring were introduced to study their influence on the photophys-
ical properties of the title compounds. All of them do not re-
lease CO in aerated DMSO for up to 16 h upon incubation in
the dark, but the presence of electron-donating substituents 412 nm (King-bright Elec. Co., model YDG-504VC, part no. 181000–
05), 468 nm (King-bright Elec. Co., 5000 m cd, part. no. BL0106–15–
299) and 525 nm (King-bright Elec. Co., 6500 m cd, part. no.
L-34ZGC). Ferrioxalate actinometry was used to determine the pho-
facilitates axial bromide substitution by solvent, which can how-
ever be suppressed by addition of an excess of bromide salt.
The electronic absorbance spectrum of the nitro-substituted
compound features a broad band at 495 nm with a tail extend-
ing to 550 nm while the other complexes show the lowest en-
ergy band at around 385 nm. With the aid of TDDFT calcula-
tions, these differences can be traced by to the nature of the
ton flux of the light sources used.[32] It was found to be 6.69 × 10–10
,
1.25 × 10–9, and 6.69 × 10–11 Einstein s–1 for the 412, 468 and
525 nm LED source light, respectively.
Synthetic procedures
MLCT state, which is from the manganese center to the benz- Synthesis of benzimidazole ligands 1–8. The benzimidazole li-
gands (LH (1), L4–CH3 (2), L4–OCH3 (3), L4–Cl (4), L4–NO2 (6), and
imidazole part of the ligand in the non-substituted compound,
but to the nitrophenyl group in the complex with the strongly
electron-withdrawing functionality. Photoinduced spectral
changes depend on whether the compound is first pre-incu-
bated in the dark or a freshly prepared solution directly illumi-
nated. This may be assigned to axial bromide exchange with
solvent in the pre-incubated samples. Thus, in addition to the
functional groups in the ligand periphery, also the axial ligand
on the metal center has an important influence on the CO re-
L2–COOCH3 (7)) were prepared as previously reported.[27,33] The new
compounds L4–COOCH3 (5) and L3–COOCH3 (8) were synthesized as
follows:
a solution of 2-chloromethyl benzimidazole (1.66 g,
10 mmol) and methyl-4-aminobenzoate (1.51 g, 10 mmol for
L4–COOCH3 (5)) or methyl-3-aminobenzoate (1.51 g, 10 mmol for
L3–COOCH3 (8)) in ethanol (30 mL) was heated to reflux overnight in
the presence of sodium iodide (1.50 g, 10 mmol). The solvent was
then partially removed under reduced pressure and the resulting
precipitate was separated after dilution with water. Compounds 5
lease process. Carbon monoxide release from the title com- and 8 were then recrystallized from a mixture of ethanol and water
(50:50 v/v).
pounds was studied with a sensor chip system at different exci-
tation wavelengths to avoid difficulties associated with the
myoglobin assay. All title complexes showed a rather similar
L4–COOCH3 5. Yellow solid yield 70 % (1.97 g, 7.0 mmol). IR (ATR,
cm–1): ν = 3357 (s, NH), 1687 (s, C=O). 1H NMR (500.13 MHz,
˜
3
release profile and reached plateau values after about 30 min [D6]DMSO, ppm): δ = 12.31 (s, 1H, bim-NH), 7.68 (d, 2H, J = 8.7 Hz,
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phenyl-H3/5), 7.47 (m, 2H, bim-H4/7), 7.16 (t, 1H, 3J = 5.7 Hz, (vs, C≡O), 1597, 1510, 1474, 1434, 1254, 1077. 1H NMR (500.13 MHz,
CH2NH), 7.11 (m, 2H, bim-H5/6), 6.68 (d, 2H, 3J = 8.9 Hz, phenyl-H2/
[D6]DMSO, ppm): δ = 13.54 (s, 1H, bim-NH), 7.80 (m, 1H, bim-H7),
7.69 (m, 1H, bim-H4), 7.52 (m, 1H, Phenyl-H6), 7.40 (m, 2H, bim-H5/
6), 7.15 (m, 1H, phenyl-H2), 6.93–6.99 (m, 3H, CH2NH and phenyl-
H3/5), 4.67–4.51 (m, 2H, CH2NH), 3.07 (s, 3H, OCH3). 13C NMR
(125.75 MHz, [D6]DMSO, ppm): δ = 225.1 (C≡O), 221.4 (C≡O), 220.9
(C≡O), 156.1 (phenyl-C1), 155.9 (bim-C2), 144.2 (phenyl-C4), 141.1
(bim-C7a), 134.7 (bim-C3a), 123.6 (bim-C6), 123.2 (bim-C5), 121.2
(phenyl-C6), 119.5 (phenyl-C2), 116.4 (bim-C7), 114.3 (bim-C3/5),
113.0 (bim-C4), 55.3 (COOCH3), 50.4 (CH2NH), MS (ESI–, acetone):
m/z 385.962 [M – H – 3CO]–, 469.950 [M – H]–, 551.877 [M + Br]–.
C18H15BrMnN3O4: C 45.79, H 3.20, N 8.90; found 45.64, H 3.35, N,
8.64.
3
6), 4.54 (d, 2H, J = 5.8 Hz, CH2NH), 3.71 (s, 3H, COOCH3). 13C NMR
(125.75 MHz, [D6]DMSO, ppm): δ = 166.3 (COOCH3), 152.6 (bim-C2),
152.5 (phenyl-C1), 130.9 (phenyl-C3/5), 121.3–121.6 (bim-C4/5/6/7),
116.6 (phenyl-C4), 111.5 (phenyl-C2/6), 51.3 (CH3), 41.1 (CH2).
C16H15N3O2: C 68.30, H 5.37, N 14.97; found C 66.96, H 5.80, N 15.08.
L3–COOCH3 8. White solid, yield 78 % (2.20 g, 7.8 mmol). IR (ATR, cm–1):
ν = 3371 (s, NH), 1701 (s, C=O). 1H NMR (500.13 MHz, [D6]DMSO,
˜
ppm): δ = 12.29 (s, 1H, bim-NH), 7.47 (m, 2H, bim-H4/7), 7.25 (s, 1H,
phenyl-H2), 7.20–7.15 (m, 2H, phenyl-H5/6), 7.11 (m, 2H, bim-H5,6),
6.89 (d, 1H, 3J = 7.5 Hz, phenyl-H4), 6.65 (t, 1H, CH2NH, 3J = 5.7 Hz),
4.51 (d, 2H, 3J = 5.8 Hz, CH2NH), 3.77 (s, 3H, COOCH3). 13C-NMR
(125.75 MHz, [D6]DMSO, ppm): δ = 166.3 (COOCH3), 153.2 (bim-C2),
148.5 (phenyl-C1), 130.3 (bim-C3a/7a and phenyl-C3), 129.2 (phenyl-
C6), 121.3–121.4 (bim-C4/5/6/7), 117.1 (phenyl-C5), 116.7 (phenyl-
C4), 112.8 (phenyl-C2), 52.0 (COOCH3), 41.6 (CH2NH). C16H15N3O2:
C 68.30, H 5.37, N 14.97; found C 68.17, H 5.43, N 15.10.
[MnBr(CO)3(L4–Cl)] 12. Orange crystals yield 70 % (120 mg,
0.25 mmol). IR (ATR, cm–1): ν = 3212 (w, NH), 2022 (vs, C≡O), 1932
˜
(vs, C≡O), 1903 (vs, C≡O), 1592, 1478, 1436, 1221, 1056. 1H NMR
(500.13 MHz, [D6]DMSO, ppm): δ = 13.58 (s, 1H, bim-NH), 7.78 (m,
2H, bim-C7/phenyl-H3), 7.67 (m, 1H, bim-C4), 7.46 (m, 2H, phenyl-
H2/6), 7.39 (m, 3H, CH2NH, bim-H5/6), 7.18 (m, 2H, and phenyl-H3/
5), 4.67–4.53 (m, 2H, CH2NH). 13C NMR (125.75 MHz, [D6]DMSO,
ppm): δ = 225.1 (C≡O), 220.9 (C≡O), 220.7 (C≡O), 155.8 (bim-C2),
150.6 (phenyl-C1), 141.2 (bim-C7a), 134.7 (bim-C3a), 129.1 (phenyl-
C4), 128.7 (phenyl-C6), 128.2 (phenyl-C2), 123.5 (bim-C6), 123.3
(bim-C5), 121.9 (phenyl-C5), 119.9 (phenyl-C3), 116.3 (bim-C7), 113.0
(bim-C4), 49.5 (CH2NH). MS (ESI–, acetone): m/z 389.928 [M – H –
3CO]–, 473.888 [M – H]–, 555.828 [M + Br]. C17H12BrClMnN3O3:
C 42.84, H 2.54, N 8.82; found 43.44, H 2.95, N, 8.68.
Synthesis of manganese(I) tricarbonyl complexes 9–16. The
benzimidazole ligand (LH, 80 mg; L4–CH3, 85 mg; L4–OCH3, 91 mg;
L4–Cl, 93 mg; L4–COOCH3, 101 mg; L4–NO2, 96 mg; L2–COOCH3, 101 mg;
L3–COOCH3, 101 mg; 0.36 mmol each) and manganese pentacarbonyl
bromide (100 mg, 0.36 mmol) were dissolved under argon in de-
gassed anhydrous acetone (20 mL) and heated to reflux for 3 h
while protected from light by wrapping the apparatus in aluminium
foil. The solution was then left in the dark for slow evaporation
down to 5 mL, whereupon a precipitate formed which was filtered
off, washed with n-hexane and dried under vacuum for 5 d. Crystals
suitable for X-ray structure analysis were obtained by diffusion of
diethyl ether into a solution of the complex in acetone.
[MnBr(CO)3(L4–COOCH3)] 13. Red powder yield 72 % (130 mg,
0.26 mmol). IR (ATR, cm–1): ν = 3202 (m, NH), 2025 (vs, C≡O), 1904
˜
(vs, C≡O), 1697 (m, C=O), 1606, 1432, 1285, 1182, 1114. 1H NMR
(500.13 MHz, [D6]DMSO, ppm): δ = 13.56 (s, 1H, bim-NH), 7.93 (m,
2H, phenyl-H2/6), 7.79 (m, 1H, bim-H7), 7.70 (m, 1H, bim-H4), 7.55
(m, 1H, bim-H3), 7.41 (m, 2H, bim-H5/6), 7.26 (m, 1H, phenyl-H5),
[MnBr(CO)3(LH)] 9. Orange crystals yield 50 % (80 mg, 0.18 mmol).
IR (ATR, cm–1): ν = 3182 (w, NH), 2020 (vs, C≡O), 1928 (vs, C≡O),
˜
1905 (vs, C≡O), 1598, 1477, 1432, 1213, 1053. 1H NMR (500.13 MHz,
[D6]DMSO, ppm): δ = 13.47 (s, 1H, bim-NH), 7.79 (m, 1H, bim-H7),
7.69 (m, 1H, bim-H4), 7.62 (m, 1H, CH2NH), 7.52 (m, 1H, phenyl-C6),
7.39 (m, 2H, bim-H5/6), 7.34 (m,1H, phenyl-H4), 7.18 (m, 2H, phenyl-
H3/5), 7.07–7.16 (m, 1H, phenyl-H2), 4.71–4.56 (m, 2H, CH2NH). 13C
NMR (125.75 MHz, [D6]DMSO, ppm): δ = 226.2 (C≡O), 222.1 (C≡O),
221.8 (C≡O), 157.1 (bim-C2), 152.5 (phenyl-C1), 142.2 (bim-C7a),
135.7 (bim-C3a), 130.1 (phenyl-C4), 125.4 (phenyl-C2), 124.5 (bim-
C6), 124.1 (bim-C5), 121.0 (phenyl-C6), 119.2 (phenyl-C3/5), 117.3
(bim-C7), 113.9 (bim-C4), 50.9 (CH2NH). MS (ESI–, acetone): m/z
355.952 [M – H – 3CO]–, 439.940 [M – H]–, 521.866 [M + Br]–.
C17H13BrMnN3O3: C 46.18, H 2.96, N 9.50; found 46.54, H 2.93, N,
9.52.
3
7.13 (t, 1H, J = 5.8 Hz, CH2NH), 4.57–4.75 (m, 2H, CH2NH), 3.80 (s,
3H, COOCH3), 13C NMR (125.75 MHz, [D6]DMSO, ppm): δ = 225.3
(C≡O), 220.7 (C≡O), 220.6 (C≡O),165.5 (COOCH3), 155.8 (bim-C2),
152.1 (phenyl-C1), 141.2 (bim-C7a), 134.8 (bim-C3a), 130.6 (phenyl-
C6), 130.1 (phenyl-C2), 125.4 (phenyl-C4), 123.6 (bim-C6), 123.3
(bim-C5), 120.2 (phenyl-C3), 118.4 (phenyl-C5), 116.3 (bim-C7), 113.0
(bim-C4), 52.0 (COOCH3), 49.0 (CH2NH). MS (ESI–, acetone): m/z:
413.935 [M – H – 3CO]–, 497.935 [M – H]–. C19H15BrMnN3O5: C 45.62,
H 3.02, N 8.40; found 45.17, H 3.27, N, 8.05.
[MnBr(CO)3(L4–NO2)] 14. Orange powder yield 30 % (53 mg,
0.11 mmol). IR (ATR, cm–1): ν = 3180 (w, NH), 2026 (vs, C≡O), 1950
˜
(vs, C≡O), 1906 (vs, C≡O), 1595, 1526 (s, NO2), 1478, 1342 (s, NO2),
1228. 1H NMR (500.13 MHz, [D6]DMSO, ppm): δ = 13.60 (s, 1H, bim-
NH), 8.39 (s, 1H, CH2NH), 8.25 (m, 2H, phenyl-H3/5), 7.77 (m,1H, bim-
H7), 7.71 (m,1H, bim-H4), 7.41 (m, 2H, bim-H5/6), 7.35 (m, 1H,
phenyl-H6), 7.14 (m, 1H, phenyl-H2), 4.75–4.64 (m, 2H, CH2NH).
13C NMR (125.75 MHz, [D6]DMSO, ppm): δ = 225.5 (C≡O), 220.7
(C≡O), 220.5 (C≡O), 157.6 (phenyl-C4), 155.7 (bim-C2), 154.2 (phenyl-
C1), 141.2 (bim-C7a), 134.8 (bim-C3a), 126.0 (phenyl-C5), 125.3
(phenyl-C3), 123.8 (bim-C6), 123.4 (bim-C5), 121.6 (phenyl-C6), 119.0
(phenyl-C2), 116.3 (bim-C7), 113.1 (bim-C4), 49.0 (CH2NH). MS
(ESI–, acetone): m/z 400.917 [M – H – 3CO]–, 484.904 [M – H]–.
C17H12BrMnN4O5: C 41.91, H 2.48, N 11.50; found 41.79, H 2.61, N,
11.20.
[MnBr(CO)3(L4–CH3)] 10. Yellow powder yield 73 % (120 mg,
0.26 mmol). IR (ATR, cm–1): ν = 3217 (w, NH), 2023 (vs, C≡O), 1929
˜
(vs, C≡O), 1902 (vs, C≡O), 1595, 1476, 1446, 1276, 1064. 1H NMR
(500.13 MHz, [D6]DMSO, ppm): δ = 13.55 (s, 1H, bim-NH), 7.79 (m,
1H, bim-H7), 7.67 (m, 1H, bim-H4), 7.45 (m, 1H, phenyl-H6), 7.35 (m,
2H, bim-H5/6), 7.17 (m, 1H, phenyl-H2), 7.07–7.13 (m, 3H, CH2NH
and phenyl-H3/5), 4.53–4.67 (m, 2H, CH2NH), 2.22 (s, 3H, CH3).
13C-NMR (125.75 MHz, [D6]DMSO, ppm): δ = 225.0 (C≡O), 221.1
(C≡O), 220.7 (C≡O), 156.0 (bim-C2), 149.0 (phenyl-C1), 141.1 (bim-
C7a), 134.7 (bim-C3a), 133.2 (phenyl-C4), 129.5 (phenyl-C6), 129.2
(phenyl-C2), 123.5 (bim-C6), 123.2 (bim-C5), 119.9 (phenyl-C5), 118.1
(phenyl-C3), 116.3 (bim-C7), 112.8 (bim-C4), 49.8 (CH2NH), 20.1
(COOCH3). MS (ESI–, acetone): m/z 369.972 [M – H – 3CO]–, 453.959
[M – H]–, 535.884 [M + Br]–. C18H15BrMnN3O3: C 47.39, H 3.31,
N 9.21; found 47.03, H 3.33, N, 9.22.
[MnBr(CO)3(L2–COOCH3)] 15. Yellow crystals yield 66 % (120 mg,
0.24 mmol). IR (ATR, cm–1): ν = 3173 (w, NH), 2029 (vs, C≡O), 1911
˜
(vs, C≡O), 1688 (m, C=O), 1604, 1493, 1448, 1268, 1096. 1H NMR
(500.13 MHz, [D6]DMSO, ppm): δ = 13.54 (s, 1H, bim-NH), 8.85 (m,
1H, CH2NH), 8.01 (m, 1H, phenyl-H6), 7.80 (s, 1H, bim-H7), 7.70 (s,
[MnBr(CO)3(L4–OCH3)] 11. Orange powder yield 64 % (108 mg,
0.23 mmol). IR (ATR, cm–1): ν = 3194 (w, NH), 2022 (vs, C≡O), 1907
˜
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1H, bim-H4), 7.57 (m, 1H, phenyl-H5), 7.42 (m, 2H, bim-H5/6), 7.11
(m, 1H, phenyl-H3), 6.93 (m, 1H, phenyl-H5), 4.42–4.95 (m, 2H,
CH2NH), 3.96 (s, 3H, COOCH3). 13C NMR (125.75 MHz, [D6]DMSO,
ppm): δ = 225.6 (C≡O), 220.7 (C≡O), 219.9 (C≡O), 167.0 (COOCH3),
The CO release profiles were recorded in a custom-made measure-
ment cell consisting of two DN25 flanges (bottom closed, Rettberg,
Göttingen, Germany) equipped with a XXS CO probe from Dräger
(Lübeck, Germany).[37,38] The probe was connected to an X-am 5000
156.9 (bim-C2), 154.3 (phenyl-C1), 141.3 (bim-C7a), 135.2 (phenyl- gas sensor (Dräger). Of each compound, 0.6 mg were weighted on
C5), 134.7 (bim-C3a), 130.6 (phenyl-C2), 123.7 (bim-C6), 123.2 (bim- a XP 56 microbalance (Mettler Toledo, Columbus, OH), dissolved in
C5), 120.0 (phenyl-C4), 121.1 (phenyl-C3), 116.3 (phenyl-C6), 116.1
(bim-C7), 113.2 (bim-C4), 52.6 (COOCH3), 50.7 (CH2NH). MS (ESI–,
acetone): m/z 413.936 [M – H – 3CO]–, 497.930 [M – H]–.
C19H15BrMnN3O5: C 45.62, H 3.02, N 8.40; found 45.46, H 3.08, N,
8.40.
DMSO (1 mL) and then mixed with water (10 mL). Of this solution,
4 mL were transferred to the measurement cell within 1 h at the
latest and stirred at 140 rpm (Variomag Telesystem, Thermo Scien-
tific, MA). The photoinduced CO release was examined by illumi-
nated at 468 or 525 nm. The light source was placed in a distance of
3.0 cm laterally to the measurement cell. Readouts were corrected
proportionally to the exact mass as well as the molar amount of
each compound.
[MnBr(CO)3(L3–COOCH3)] 16. Orange powder yield 68 % (120 mg,
0.24 mmol). IR (ATR, cm–1): ν = 3201 (m, NH), 2025 (vs, C≡O), 1904
˜
(vs, C≡O), 1696 (m, C=O), 1607, 1426, 1289, 1184, 1058. 1H NMR
(500.13 MHz, [D6]DMSO, ppm): δ = 13.58 (s, 1H, bim-NH), 8.04 (m,
1H, phenyl-H6), 7.94 (m, 1H, phenyl-H4), 7.78 (m, 1H, bim-H7), 7.70
(m, 1H, bim-H4), 7.55 (m, 1H, phenyl-H3), 7.39 (m, 2H, bim-H5/6),
7.26 (m, 1H, phenyl-H2), 7.11 (m, 1H, CH2NH), 4.72–4.59 (m, 2H,
CH2NH), 3.80 (s, 3H, COOCH3). 13C NMR (125.75 MHz, [D6]DMSO,
ppm): δ = 225.1 (C≡O), 220.8 (C≡O), 220.766 (C≡O), 165.1 (COOCH3),
155.8 (bim-C2), 152.2 (phenyl-C1), 141.1 (bim-C7a), 134.7 (bim-C3a),
130.1 (phenyl-C2), 125.4 (phenyl-C4), 123.6 (bim-C6), 123.3 (bim-C5),
120.2 (phenyl-C3), 118.3 (phenyl-C5), 116.3 (bim-C7), 113.0 (bim-C4),
52.1 (COOCH3), 49.1 (CH2NH). MS (ESI–, acetone): m/z 413.942 [M –
H – 3CO]–, 497.930 [M – H]–. C19H15BrMnN3O5: C 45.62, H 3.02, N
8.40; found 45.55, H 3.36, N, 8.31.
Acknowledgments
The stay of A. M. Mansour in Germany was supported by a
German-Egyptian Research Short-Term Scholarship (GERSS)
from the Deutscher Akademischer Austauschdienst (DAAD) and
a Georg Forster Fellowship from the Alexander von Humboldt
Foundation (AvH). Wolfgang Obert from the Electronics Work-
shop of the Institut für Anorganische Chemie, Universität Würz-
burg, is acknowledged for the expert construction of the LED
light sources. Thanks also to Prof. Dr. Lorenz Meinel for access
to the electrochemical CO sensor system.
X-ray crystallography
Keywords: CO-releasing molecules (CORMs) · Manganese
Carbonyl ligands · Benzimidazole · TDDFT
A single crystal of 9, 10, 12, or 14 was immersed in a film of perflu-
oropolyether oil, mounted on a glass fiber and transferred to a
stream of cold nitrogen of the diffractometer. Diffraction data were
collected on a Bruker x8 Apex II diffractometer with multi-layer mir-
ror graphite monochromated Mo-Kα radiation (λ = 0.71073 Å) at
100 K. The final cell constants were obtained from a least square fit
of a subset of a few thousand strong reflections. The program Crys-
Alis CCD was used for data collection. CrysAlis RED and CrysAlis
CCD software were used for cell refinements and data reduction.
The program SADABS was used to account for the absorption.[34]
The OLEX2 program was used for refinement and artwork of the
structure.[35] All the hydrogen atoms were included in the calcu-
lated positions.
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CO release studies using a sensor chip system
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Received: August 20, 2019
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CO-Releasing Molecules
A. M. Mansour,* C. Steiger,
C. Nagel, U. Schatzschneider ....... 1–11
Wavelength-Dependent Control of
the CO Release Kinetics of Manga-
nese(I) Tricarbonyl PhotoCORMs
with Benzimidazole Coligands
The influence of coligand electron imidazole ligands bearing electron-
density on the CO release was system- donating and withdrawing functional
atically explored in a series of manga- groups in direct conjugation with the
nese tricarbonyl complexes with benz- ligand π system.
DOI: 10.1002/ejic.201900894
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© 0000 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim