Towards Cardiolite-Inspired Carbon Monoxide Releasing Molecules - Reactivity of d4, d5 Rhenium and d6 Manganese Carbonyl Complexes with Isocyanide Ligands

Article abstract of DOI:10.1002/ejic.201500756

Carbon monoxide releasing molecules (CORMs) are investigated widely in synthetic and medicinal chemistry owing to the potential therapeutic applications of the CO gas. Organometallic carbonyl complexes are best suited to play the role of CO carriers as they allow the exogenous release of CO under controlled conditions, and the toxicity of the gas can be overcome. With the long-term goal of developing CORMs with similar properties to those of the sesta-methoxyisobutylisonitrile (sesta-mibi) ^99mTc complex (Cardiolite), we have studied the reactivity of isocyanide ligands towards 16- and 17-electron cis-[Re(CO)₂Br₄]^-/2- species and the [Mn(CO)₅Br] complex. Six different isocyanide ancillary ligands (CNR), including mibi, were selected for this study. Their reactions with cis-dicarbonyl Re^III and Re^II complexes were accompanied by two- and one-electron reduction of the metal center and resulted in the formation of stable cis-mer-[Re(CO)₂(CNR)₃Br] species, whereas the same reactions with [Mn(CO)₅Br] gave fac-[Mn(CO)₃(CNR)₂Br] compounds. All of the complexes were fully characterized, and single-crystal X-ray diffraction structure determinations were performed for selected species. In addition, unique monocarbonyl complexes were obtained from the reactions of cis-[Re(CO)₂Br₄]^- (1) with tert-butyl isocyanide and cis-[Re(CO)₂Br₄]^2- (2) with mibi. The species, a heptacoordinate Re^III and a hexacoordinate Re^II complex, respectively, were also characterized structurally. The CO-releasing profiles, the cytotoxic effects against 3T3 fibroblast cells, and the antibacterial properties of the compounds were also investigated. To develop CO-releasing molecules with properties similar to those of the sesta-methoxyisobutylisonitrile (sesta-mibi) ^99mTc complex (Cardiolite), we study the reactivity of isocyanide ligands towards 16-, 17- and 18-electron rhenium and manganese carbonyl complexes. A combined experimental and theoretical approach elucidates the reaction pathway to the species.

Full text of DOI:10.1002/ejic.201500756

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DOI:10.1002/ejic.201500756
Towards Cardiolite-Inspired Carbon Monoxide Releasing
Molecules – Reactivity of d4, d5 Rhenium and d6
Manganese Carbonyl Complexes with Isocyanide Ligands
Emmanuel Kottelat,[a] Valentin Chabert,[a] Aurélien Crochet,[b][‡]
Katharina M. Fromm,[a][‡] and Fabio Zobi*[a]
Keywords: Carbon monoxide / Rhenium / Manganese / Isocyanide ligands
Carbon monoxide releasing molecules (CORMs) are investi-
gated widely in synthetic and medicinal chemistry owing to
the potential therapeutic applications of the CO gas. Organo-
metallic carbonyl complexes are best suited to play the role
of CO carriers as they allow the exogenous release of CO
under controlled conditions, and the toxicity of the gas can
be overcome. With the long-term goal of developing CORMs
with similar properties to those of the sesta-methoxyisobutyl-
isonitrile (sesta-mibi) 99mTc complex (Cardiolite), we have
studied the reactivity of isocyanide ligands towards 16- and
17-electron cis-[Re(CO)2Br4]–/2– species and the [Mn(CO)5Br]
complex. Six different isocyanide ancillary ligands (CNR), in-
cluding mibi, were selected for this study. Their reactions
companied by two- and one-electron reduction of the metal
center and resulted in the formation of stable cis-mer-
[Re(CO)2(CNR)3Br] species, whereas the same reactions with
[Mn(CO)5Br] gave fac-[Mn(CO)3(CNR)2Br] compounds. All
of the complexes were fully characterized, and single-crystal
X-ray diffraction structure determinations were performed
for selected species. In addition, unique monocarbonyl com-
plexes were obtained from the reactions of cis-[Re(CO)2Br4]–
(1) with tert-butyl isocyanide and cis-[Re(CO)2Br4]2– (2) with
mibi. The species, a heptacoordinate ReIII and a hexacoordi-
nate ReII complex, respectively, were also characterized
structurally. The CO-releasing profiles, the cytotoxic effects
against 3T3 fibroblast cells, and the antibacterial properties
with cis-dicarbonyl ReIII and ReII complexes were ac- of the compounds were also investigated.
Introduction
geting of the molecules to local injuries can be achieved
Carbon monoxide (CO) is acknowledged as a fundamen-
tal neurotransmitter in humans and has recently received
attention owing to its documented therapeutic effects. Like
nitric oxide (NO) and hydrogen sulfide (H2S), CO is pro-
duced endogenously in animals. These neurotransmitters
are involved in several cellular, physiological, and patholog-
ical pathways, such as vasodilatation, endothelial injury, or
inflammations.[1–10] The natural targets of CO are heme-
containing proteins. Direct CO inhalation results in the for-
mation of carboxyhemoglobin owing to the high affinity
(ca. 230 times stronger than that of O2) of the gas for the
iron center of the protein.[11,12] The interaction of CO with
hemoglobin constitutes a major barrier to the specific deliv-
ery of CO. To overcome this challenge, CO-releasing mol-
ecules (CORMs) are under investigation.[13–16]
through the modification of the coordination sphere of the
metal ion or by appending CORMs to biomolecules that
accumulate in specific sites or to target-specific receptors.
Romão and co-workers have proposed a conceptual model
for the design of organometallic CORMs[17] in which dif-
ferent components should be considered for the synthesis
of appropriate CORMs: the toxicity of the complex, the
inner coordination sphere for the release of CO in response
to a trigger, and the outer coordination sphere, obtained
through the modification of ancillary ligands to modulate
the pharmaceutical parameters (i.e., the biocompatibility,
water solubility, and tissue targeting).
Rhenium and manganese ions are employed widely as
metal centers for organometallic carbonyl complexes in-
tended as CO carriers.[18–38] The Re-based CORMs tested
to date have shown very low cytotoxicity towards different
cell lines.[37,39] These complexes may be activated towards
CO loss by the aquation of bound ions, and the rate of
this substitution reaction can be modulated by the ancillary
ligands.[37] On the other hand, Mn-based CORMs are acti-
vated by exposure to UV light and are known as photo-
CORMs.[40–43]
In this respect, organometallic carbonyl complexes are
best suited to play the role of CO carriers. Thus, the tar-
[a] Department of Chemistry, University of Fribourg,
Chemin du Musée 9, 1700 Fribourg, Switzerland
E-mail: fabio.zobi@unifr.ch
http://www.chem.unifr.ch/en/research/zobi_group
[b] Fribourg Center for Nanomaterials (Frimat), University of
Fribourg,
Chemin du musée 9, 1700 Fribourg, Switzerland
[‡] Members of the NCCR “Bioinspired Materials”
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201500756.
With the above considerations in mind and with the
long-term aim of developing Cardiolite-type CORMs, we
report herein the reactivity of isocyanide ligands towards
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16- and 17-electron cis-[Re(CO)2Br4]–/2– species and the
To obtain insights into the mechanism of product forma-
[Mn(CO)5Br] complex. Cardiolite or sesta-methoxyiso- tion, the substitution reactions of 1 and 2 were monitored
butylisonitrile (sesta-mibi) 99mTc is a pharmaceutical agent by in situ liquid IR spectroscopy upon the slow addition of
used in imaging and accumulates specifically in the myocar- the ligand (0.5 equiv. every 5 min for 100 min). Representa-
dium.[44,45] There are no reports of the reactivity of carb- tive spectra for the formation of 3, 5, 6, and 8 are shown in
onyl complexes of Re in d4 and d5 electronic configurations Figure 1. With the exception of 3, all reactions gave similar
with the above-mentioned ligands. Previous reports are all wavenumber patterns regardless of the starting precursor.
limited to ReI d6 species. On the other hand, carbonyl man- The peaks of the starting compound in a dichloromethane
ganese chemistry is richer in this respect.[18–20,22–28] For the (DCM) solution appear at ν ≈ 2073 and 2006 cm–1. After
˜
current study, we selected six different isocyanides (CNR), the addition of 1–2 equiv. of the ligand, an intermediate
including mibi; the other ligands were: tert-butyl isocyanide formed with νCO stretching frequencies at ν ≈ 2040 and
˜
(tbu), isopropyl isocyanide (ipp), cyclohexyl isocyanide 1940 cm–1. In this initial phase, the vibration of the free
(chx), (S)-(–)-α-methylbenzyl isocyanide (smb), and 1,1,3,3- ligand could not be observed; therefore, a rapid reaction
tetramethylbutyl isocyanide (tmb). Their reactions with cis- occurs with the cis-[Re(CO)2]3+/2+ core. Indeed, a new sig-
dicarbonyl ReIII and ReII precursors are accompanied by a nal at ν ≈ 2200 cm–1 (νCN stretching) evidenced the binding
˜
two- or one-electron reduction of the metal center to yield of CNR to the rhenium center. After the further addition
stable cis-mer-[Re(CO)2(CNR)3Br], whereas the same reac- of the ligand, both the CO and CNR frequencies suffered
tions with [Mn(CO)5Br] gave fac-[Mn(CO)3(CNR)2Br] a supplementary bathochromic shift to ν ≈ 1960–1880 and
˜
compounds. In addition, unique monocarbonyl species 2200–2125 cm–1, respectively. No further changes were ob-
were obtained from the reactions of cis-[Re(CO)2Br4]– (1)
with tbu and cis-[Re(CO)2Br4]2– (2) with mibi. All com-
plexes were characterized fully [1H NMR spectroscopy, cy-
clic voltammetry (CV), MS, and IR spectroscopy], and sin-
gle-crystal X-ray diffraction structure determinations of se-
lected compounds were performed. The CO-releasing prop-
erties, the antibacterial effects, and the cytotoxic effects of
the compounds against fibroblast cells are also reported.
Results and Discussion
Reactivity of [NEt4][Re(CO)2Br4] and [NEt4]2[Re(CO)2Br4]
with CNR
Before our investigation, reactions involving isocyanides
(CNR) and rhenium carbonyl species were described by the
group of Ko[46] and restricted to ReI d6 complexes. By start-
ing with [Re(CO)5Br] and following different synthetic stra-
tegies, the authors could access fac-[Re(CO)3(CNR)2Br],
fac-[Re(CO)3(CNR)3], and cis-mer-[Re(CO)2(CNR)3Br]
species. In our case, regardless of the starting precursor, we
found that the reactions of 1 (16-electron species) and 2 (17-
electron species) with isocyanides gave stable 18-electron
cis-mer-[Re(CO)2(CNR)3Br] complexes [CNR = tbu (3), ipp
(4), chx (5), smb (6), tmb (7), or mibi (8); see Scheme 1].
Furthermore, a heptacoordinate monocarbonyl species of
formula [ReCO(tbu)3Br3] (3a) was also isolated as a by-
product of the reaction of 1 with tbu.
Figure 1. The evolution of the νCO and νCN bands in the in situ
IR spectra of the reactions of 1 with (A) tert-butyl isocyanide, (B)
cyclohexyl isocyanide, (C) (S)-(–)-α-methylbenzyl isocyanide, and
(D) mibi. The numbers on the spectra indicate the peaks assigned
to the complexes formed (see Scheme 1 and Table 1), whereas the
peaks enclosed within the Ͼ Ͻ symbols refer to the intermediates
formed. Reaction conditions: 25 °C in dry CH2Cl2.
Scheme 1. General synthetic and numbering scheme of cis-mer-
[Re(CO)2(CNR)3Br] complexes derived from 1 and 2.
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served upon the addition of an excess of CNR. The final (CO)5Br] with 3–4 equiv. of CNR gave tricarbonyl MnI
dicarbonyl species, identified as the cis-mer-[Re(CO)2- complexes of the general formula fac-[Mn(CO)3(CNR)2Br]
(CNR)3Br] products, showed more-intense vibrations than in all cases [CNR = tbu (9), ipp (10), chx (11), smb (12),
those of the starting material or the intermediates. Despite tmb (13), or mibi (14); see Scheme 2]. After chromato-
our efforts, the intermediates could not be isolated; how- graphic purification, all of the compounds were isolated in
ever, a computational vibrational analysis of the substitu- high yields as crystalline powders. As expected for octahe-
tion reactions at the DFT level of theory is provided below. dral complexes bearing a fac-[Mn(CO)3]+ core, the isolated
For tbu as the ligand, the formation of 3 was monitored products were air and thermally stable but showed decom-
by liquid IR spectroscopy, and a more complicated signal position upon prolonged photoexposure. All of the species
1
pattern was observed. Two intermediates were observed and gave satisfactory elemental analyses and showed clean H
two distinct final products formed. One of them (ν = NMR spectra and positive-ion ESI mass spectra. Com-
˜
2137 cm–1, νCN stretching; ν = 1973 and 1894 cm–1, νCO plexes 10, 11, and 14 were characterized structurally by X-
˜
vibrations) is the cis-mer-[Re(CO)2(CNR)3Br] product 3, ray crystallography.
whereas the second one was identified as a heptacoordinate
ReIII monocarbonyl species (ν = 2196 cm–1, νCN vibration;
˜
ν = 1991 cm–1, νCO vibration) of formula [Re(CO)-
˜
(CNR)3Br3] (3a) after crystallographic analysis (vide infra,
Figure 2).
Scheme 2. General synthetic and numbering scheme of fac-
[Mn(CO)3(CNR)2Br] complexes derived from [Mn(CO)5Br].
Spectroscopic Characterization of Complexes and
Electrochemical Analysis
The analytical data for 3–14 are summarized in Table 1.
All of the complexes with general formula cis-mer-[Re(CO)2-
(CNR)3Br] exhibit two strong νCO stretching vibrations in
the ν = 1860–1900 and 1950–1970 cm–1 ranges and two
˜
νCN stretches (one strong and one weak) in the ν = 2120–
˜
2160 and 2190–2210 cm–1 ranges. The signals are consistent
with the meridional arrangement of the ligands and the Cs
symmetry of the octahedral complexes. Conversely, the
complexes with the general formula fac-[Mn(CO)3(CNR)2-
Br] are characterized by one strong νCN stretching peak in
Figure 2. Molecular view of 3a. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
the ν = 2170–2211 cm–1 range and three νCO vibrations
˜
All of the Re-based complexes are air and thermally
stable species with good solubility in common organic sol-
vents. Overall, we attribute the formation of stable low-spin
18-electron species to the strong π-acidity of the CNR li-
gands. Complexes 3a, 4, and 8 were characterized structur-
ally by X-ray crystallography (vide infra).
centered at ν ≈ 2030, 1970, and 1920 cm–1. The Re-based
˜
compounds (i.e., 3–8) show strong absorptions in the blue
regions of the UV/Vis spectrum with peaks centered at λ =
220 nm and a band with a lower molar extinction at λ =
Table 1. Analytical data of 3–14.
Solid-state IR [cm–1 [a]
]
λmax [nm][b]
E1/2 [V][c]
Reactivity of [Mn(CO)5Br] with CNR
3
4
5
6
7
8
9
2189, 2121 (νCN), 1956, 1865 (νCO)
2195, 2136 (νCN), 1967, 1898 (νCO)
2201, 2128 (νCN), 1956, 1864 (νCO)
2198, 2124 (νCN), 1959, 1880 (νCO)
2198, 2123 (νCN), 1958, 1878 (νCO)
2211, 2159 (νCN), 1967, 1889 (νCO)
2176 (νCN), 2032, 1967, 1913 (νCO)
314
318
318
314
320
314
415
383
384
384
387
384
+1.16
+1.36
+1.22
+1.28
+1.18
+1.15
–
–
–
–
–
–
Early studies by Treichel and co-workers showed that the
reactions of [Mn(CO)5Br] with CNR give several different
products, depending on the reaction conditions, and that
the product distribution is dependent on the reaction time
and on the ratio of reactants.[47] On the basis of this earlier
work, a mixture of [Mn(CO)5–n(CNR)nBr] (n = 2–4) could
be expected. Later, Treichel also showed that the reactions
of [Mn(CO)5–n(CNR)nBr] complexes with excess CNR
could produce mer- and fac-[Mn(CO)3(CNR)3]+ and cis-
trans-[Mn(CO)2(CNR)4]+ complexes.[48] Under our experi-
10 2180 (νCN), 2033, 1974, 1932 (νCO)
11 2181 (νCN), 2031, 1969, 1922 (νCO)
12 2176 (νCN), 2035, 1979, 1935 (νCO)
13 2171 (νCN), 2031, 1977, 1931 (νCO)
14 2211 (νCN), 2032, 1969, 1922 (νCO)
[a] On spring diamond. [b] In methanol. [c] Relative to ReI to ReII
mental conditions, we found that the reactions of [Mn- redox potential, versus saturated calomel electrode (SCE), in CH3CN.
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314–320 nm (Table 1). For the Mn-based complexes, the around the Re ions and cis conformations in the Mn com-
maximum absorbance was centered at λ = 250 nm, and a pounds, whereas the CO ligands are observed in cis and fac
lower absorptivity band appeared between λ = 383 and arrangements, respectively. Complex 3a was identified as a
387 nm (Table 1). These features are consistent for 10–14, heptacoordinate d4 Re species. Structurally characterized
whereas 9 shows a lower-energy band at λ = 415 nm. The seven-coordinate 18-electron rhenium carbonyl complexes
latter bands are ascribed to the intraligand isocyanide have been reported,[49–51] but, to the best of our knowledge,
πǞπ* transition.[46] Complex 9 shows a lowest energy ab- 3a is the first example of a monocarbonyl ReIII species. In
sorption from the πǞπ* transition, which is due to a 3a, the Re–CO bond length of 1.897(7) Å is not signifi-
higher-energy highest occupied molecular orbital (HOMO) cantly different from the same bond lengths of the other Re
caused by the increased electron density on the tbu ligand species reported herein (see Table 3).
as compared with those of the other isocyanides ligand.
Isocyanide carbonyl rhenium complexes have been pre-
viously reported with the central metal ion bearing three
carbonyl ligands.[46,52] To the best of our knowledge, 4 and
8 are the first examples of octahedral dicarbonyl Re species
X-ray Crystallography
The crystallographic details of 3a, 4, 8, 10, 11, and 14 with isocyanide ligands. The bond lengths in 4 between the
are summarized in Table 2, and selected bond lengths and Re ion and C atoms from the CO ligands are 1.884(11)
angles are listed in Table 3. The molecular structures of the and 1.978(11) Å trans to Br and trans to isocyanide ligands,
Re- and Mn-based complexes are given in Figures 2, 3, 4, respectively, and exemplify the π-accepting/donor abilities
5, 6 and 7, respectively. As expected from the spectroscopic of the two ligands. A similar effect can be seen in the struc-
analyses, the CNR ligands assume mer conformations tural parameters of 8 (more precisely species 8b in Figure 4,
Table 2. Crystallographic data for 3a, 4, 8, 10, 11, and 14.
3a
4
8
10
11
14
Formula
Mw
T [K]
Lattice
Space group
Z
C16H27Br3N3ORe C14H21BrN3O2Re C39H66Br2ClN6O9Re2 C11H14BrMnN2O3 C17H22BrMnN2O3 C15H22BrMnN2O5
703.33
200(2)
monoclinic
P21/n
529.45
200(2)
monoclinic
P21/c
1330.65
200(2)
triclinic
P1
357.09
200(2)
monoclinic
P21/c
437.21
200(2)
triclinic
445.19
200(2)
triclinic
¯
¯
P1
P1
2
4
1
4
2
2
a [Å]
b [Å]
c [Å]
α [°]
β [°]
γ [°]
8.5672(7)
15.9603(11)
8.6213(7)
90
98.474(6)
90
1165.96(16)
2.003
0.0209, 0.0465
13.8904(14)
9.1443(9)
15.0903(15)
90
98.412(8)
90
1896.1(3)
1.855
0.0536, 0.1270
10.605(8)
11.3827(10)
11.3452(9)
95.096(7)
96.793(6)
106.686(6)
1291.74
1.711
6.120(2)
17.954(8)
14.642(5)
90
99.38(3)
90
1587.3 (10)
1.494
0.0425, 0.662
6.2131(8)
10.6591(14)
15.317(2)
97.228(11)
99.344(11)
98.655(11)
977.7 (2)
1.485
5.7667(6)
11.2375(11)
15.4435(15)
89.783(8)
81.639(8)
85.560(8)
987.15 (17)
1.498
V [Å3]
dcalcd [g/cm3]
R1, wR2
0.0394, 0.0995
0.0384, 0.0859
0.1020, 0.2517
Table 3. Selected crystallographic bond lengths [Å] and angles [°] for 3a, 4, 8, 10, 11, and 14.
3a
4
8a
Re1–C1
Re1–C2
Re1–C3
Re1–Br1
Re1–Br2
1.897(7)
2.036(4)
2.039(6)
2.61(4)
Re1–C1
Re1–C2
Re1–C3
Re1–C4
1.884(11)
1.978(11)
2.056(9)
2.099(9)
2.051(9)
2.660(9)
89.5(4)
Re1–C1
Re1–C2
Re1–C3
Re1–C4
Re1–Br1
Re1–Cl1
C1–Re1–C2
C1–Re1–C3
Br1–Re1–Cl1
1.941(3)
2.04(3)
2.11(3)
2.052(3)
2.512(3)
2.453(7)
90.4(9)
178.6(10)
176.64(19)
2.607(7)
Re1–C5
Re1–Br1
C1–Re1–C2
C1–Re1–Br1
C1–Re1–C2
C1–Re1–C3
C1–Re1–Br1
74.57(15)
74.0(2)
125.81(8)
177.4(3)
8b
10
11
14
Re2–C100
Re2–C101
Re2–C102
Re2–C103
Re2–C104
Re2–Br2
C100–Re2–C102 91.1(10)
C100–Re2–C103 178.9(12)
C101–Re2–Br2
1.91(3)
Mn1–C1
Mn1–C2
Mn1–C3
Mn1–C4
1.832(6)
1.817(7)
1.803(6)
1.951(6)
1.946(5)
2.528(11)
92.4(2)
1.826(6)
1.779(7)
1.815(6)
1.942(6)
1.953(5)
2.526(9)
92.4(2)
1.848(6)
1.805(7)
1.847(6)
1.949(6)
1.952(5)
2.53(11)
91.9(3)
2.069(4)
2.062(3)
2.07(3)
2.087(9)
2.623(9)
Mn1–C5
Mn1–Br1
C1–Mn1–C2
C1–Mn1–C5
C2–Mn1–Br1
174.4(2)
178.94(17)
175.4(2)
178.56(16)
177.4(2)
176.87(19)
177.9(8)
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Figure 3. Molecular view of 4. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
Figure 6. Molecular view of 11. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
Figure 4. Molecular view 8a (left) and 8b (right). Ellipsoids are
drawn at 30% probability. Hydrogen atoms are omitted for clarity.
Figure 7. Molecular view of 14. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
unique monocarbonyl hexacoordinate ReII [ReCO(CNR)3-
BrCl] species (8a in Figure 4). The analytical characteriza-
tion of chromatographically purified and isolated powders
of 8 did not show evidence of 8a. Therefore, we assume that
a CO ligand was lost during crystallization in CH2Cl2 and
that the solvent acted as the chloride source. Examples of
tricarbonyl manganese structures with two isocyanide li-
gands have also been reported.[53] Key angles and distances
for structures 10, 11, and 14 are given in Table 3. No signifi-
cant differences in terms of angles and bond lengths were
found for these compounds compared to those described
previously.
Figure 5. Molecular view of 10. Ellipsoids are drawn at 50% prob-
ability. Hydrogen atoms are omitted for clarity.
vide infra). Interestingly, the inverted effect was observed
elsewhere.[52] The solid-state structural analysis of crystals
grown from solutions of 8 gave a unit cell with a 50:50 occu-
pancy of two distinct molecules, namely, the expected
Computational Studies on CO Vibrational Energy by DFT
As outlined in a previous section, the reactions of 1 or 2
[Re(CO)2(CNR)3Br] complex (8b in Figure 4) and the with CNR gave an intermediate after the addition of 1–
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3 equiv. of the ligands before the formation of the final di-
carbonyl product. As those intermediates were not isolated,
we performed DFT calculations to gain insights into the
reaction pathway to the cis-mer-[Re(CO)2(CNR)3Br] spe-
cies. For the calculations, the CNR ligands were reduced to
CNCH3 as the influence of substituents of the α-C atom
was determined to be insignificant on the basis of spectro-
scopic characterization (Table 1). The molecular structures
were first optimized without constraints (DFT level, B3LYP
with LanL2DZ basis set), and then an IR frequency calcu-
lation was performed at the same level of theory. The gas-
phase IR frequencies of 1 were first calculated and then
normalized to the experimental values. The relative shifts
of the νCO IR bands of all other species were then refer-
enced to 1.
All possible intermediates leading from 1 to the generic
cis-mer-[Re(CO)2(CNR)3Br] complex are depicted in
Scheme 3. The intermediates in Scheme 3 are labeled to
convey the binding arrangement of the incoming CNR, and
the numbers indicate the oxidation state of the central
atom. Thus, for example, it3 stands for an intermediate with
one CNR ligand trans to a CO ligand in a Re3+ complex.
Similarly, icc2 indicates an intermediate in which two CNR
ligands are bound in a cis arrangement in a Re2+ complex.
The relative positions of the calculated νCO IR bands were
finally compared to the observed frequencies of the spectra
recorded during the reaction and are plotted in Figure 8.
Figure 8. Comparison between the calculated (colored bars) and
experimental (black spectra) values of the νCO frequencies for
starting material 1, potential intermediates, and the final product
3–8. The labels of the intermediates are shown in Scheme 3.
occurs to form the final cis-mer-[Re(CO)2(CNR)3Br] spe-
cies, complexes bearing two CNR ligands were also calcu-
lated. The calculated frequencies of Re3+ and Re2+ com-
plexes clearly move away from the experimental signals
(green and red bars in Figure 8, B). This indicates that the
binding of a second CNR ligand is accompanied by a sec-
ond one-electron reduction of the metal center and that the
bis-CNR intermediate is most likely a d6 Re species. Indeed,
the calculated frequencies of the Re+ species move closer to
the experimentally observed signals. This is particularly so
for the calculated frequencies for icc1 (Scheme 3, Figure 8,
B). Our findings agree with the work of Coe and Glen-
wright and, thus, owing to a slightly stronger trans effect of
CO over CNR in rhenium complexes, we tentatively assign
the second substitution product to icc1.[54] Overall, our
theoretical analysis seems to point to the conclusion that
the persistent intermediate observed in the reaction of 1
with 0.5–3 equiv. of CNR is a mixture of a mono-CNR d5
Re complex and possibly the bis-CNR d6 Re icc1 species.
Scheme 3. Possible substitution pathways from starting compound
1 to the final general cis-mer-[Re(CO)2(CNR)3Br] complex.
First, the calculations correctly predict the shift to lower
wavenumbers of the νCO IR frequencies as a function of
the reduced charge on the Re atom. The shifts are expected
on the basis of an increased metal π-backbonding donation
to the π* orbital of a CO ligand and a consequent reduction
of the C–O bond order. In all cases, the νCO IR frequencies
of related complexes follow the order Re3+ (green bars in
Figure 8)Ͼ Re2+ (red bars)Ͼ Re+ (blue bars). As shown in
Figure 8 (A and B), the observed CO bands cannot be as-
signed to Re3+ species. This indicates that the binding of a
single CNR ligand is accompanied by a reduction of the
metal center. On the basis of the theoretical frequencies, we
tentatively assigned the intermediate product formed upon
CO-Releasing Properties
The CO-releasing properties of 3–14 were evaluated
the addition of 1 equiv. of CNR to a mixture of ic2 and it2 through the well-known myoglobin (Mb) assay.[37] Given
in Scheme 3 (Figure 8, A and C).
the stability of closed-shell 18-electron rhenium carbonyl
The further addition of 2–3 equiv. of CNR did not pro- complexes, species 3–8 did not release carbon monoxide.
duce experimentally clear shifts that were detectable with The broad Q band corresponding to deoxy-Mb centered at
the in situ IR probe. As further substitution of Br– ligands λ ≈ 561 nm remained unchanged in the presence of 1 equiv.
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of 3–8 over a 2 h monitoring period. Somewhat to our sur- and the effects of 3–8 and 9–14 on 3T3 fibroblast viability
prise, the same result was obtained for 3a (heptacoordinate are shown in Figures 10 and 11, respectively. With the ex-
d4 Re complex) in the same assay. Compounds 9–14 were ception of species 8 bearing the mibi ligands, all of the Re-
similarly tested through the Mb assay. The fac-[Mn(CO)3]+ based complexes were toxic towards 3T3 fibroblast. Similar
species belong to a class of photoactive CORMs, and their results were obtained for the Mn-based complexes. In this
CO-releasing properties were studied by eliciting CO libera- case, two sets of experiments were performed, whereby 9–
tion with 350 nm wavelength radiation. All Mn-based spe- 14 were tested in the absence of light or irradiated before
cies were able to transfer carbon monoxide to Mb. Typical inoculation (Figure 11). Like the cis-mer-[Re(CO)2(CNR)3-
spectral changes of solutions of Mb in the presence of 8 Br] species, the Mn complexes showed a pronounced toxic
and 14 are shown in Figure 9. The time required to reach
CO saturation of Mb was also determined for the Mn spe-
cies and is also shown in Figure 9. We found that the CO
saturation of Mb was slowest for 9 and 14 (ca. twofold in-
crease over the other species). Finally, experiments per-
formed with substoichiometric amounts of 9–14 indicated
that on average (2Ϯ0.33) equiv. of CO are released by the
complexes (data not shown).
Figure 10. Effect on 3T3 fibroblast proliferation for 3–8 after 1
(white bars), 2 (grey bars), and 3 d (black bars): (A) 20 μm of com-
pound; (B) 100 μm of compound.
Figure 9. Typical spectral changes of solutions of Mb in the pres-
ence of rhenium (inset for 8) and manganese carbonyl CNR com-
plexes (14, 350 nm irradiation). The values below the inset refer to
the time required for each Mn-based CORM to induce complete
CO saturation of Mb. Black curve: initial spectrum, red curve: final
spectrum. The data are expressed as the mean Ϯstandard error of
the mean (SEM) for three independent experiments.
Cell Proliferation Assay and Antibacterial Effects of 3–14
To assess the toxicity of our complexes, 3–14 were
studied in a 3T3 fibroblast proliferation assay, and their an-
tibacterial properties were tested against Escherichia coli
bacteria. The biological results are summarized in Table 4,
Table 4. 3T3 fibroblast percentage cell viability and antibacterial ef-
fects of 3–14.
[a]
3T3 CP[a] MIC[b]
Compound
3T3 CP
[%]
MIC[b]
[mg/mL]
[%]
[mg/mL]
3
4
5
6
7
8
0
24.5
0
7.7
0
88.8
Ͼ 1
Ͼ 1
Ͼ 1
Ͼ 1
Ͼ 1
Ͼ 1
9 (9 + hν)[c] 7.9 (7.2)
Ͼ 1 (Ͼ 1)
10 (10 + hν) 75.9 (92.7) Ͼ 1 (Ͼ 1)
11 (11 + hν) 3.3 (0.4)
12 (12 + hν) 3.0 (2.7)
13 (13 + hν) 0 (0)
Ͼ 1 (Ͼ 1)
0.13 (0.26)
Ͼ 1 (Ͼ 1)
Figure 11. Effect on 3T3 mice fibroblast proliferation for 9–14 after
1 (white bars), 2 (grey bars), and 3 d (black bars): (A) 20 μm of
compound in the dark before and after inoculation; (B) 100 μm of
compound in the dark before and after inoculation; (C) 20 μm of
compound exposed to visible light before and after inoculation;
(D) 100 μm of compound exposed to visible light for 30 min before
inoculation.
14 (14 + hν) 60.6 (81.1) Ͼ 1 (Ͼ 1)
[a] CP: cell proliferation after 72 h, 20 μm of compound. [b] MIC:
minimum inhibitory concentration. [c] Compound number (com-
pound number + hν): in the dark and (exposed to visible light for
30 min before inoculation).
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cept for those of 14 which were measured with Cu-Kα radiation (λ
= 1.54186 Å). All measurements were performed at 200 K with
Stoe IPDS-II or IPDS-II T diffractometers equipped with Oxford
Cryosystem open-flow cryostats.[60] The crystals were mounted on
loops, and all geometric and intensity data were taken from these
crystals. The absorption corrections were partially integrated in the
data reduction procedure.[61] The structures were solved and refined
by full-matrix least-squares techniques on F2 with the SHELX-
2014 package.[62] All atoms (except hydrogen atoms) were refined
anisotropically. Hydrogen atoms were introduced as fixed contrib-
effect when incubated at 100 μm whether inoculated in the
dark or previously exposed to light (Table 4). Conversely,
when administered at 20 μm concentration and exposed to
light, species 10 and 14 only slightly reduced cell prolifera-
tion; therefore, the relative inhibition of cell growth was
mitigated by the induction of CO release before inoculation.
The cytotoxicity of the compounds prompted us to study
the antibacterial effects of the molecules. Ruthenium and
manganese CORMs have antimicrobial effects, although
the cellular pathways are still poorly understood.[55–57] For utors if a residual electronic density was observed near their ex-
pected positions.
this study, E. coli was selected, and species 9–14 were tested
in the dark or after light activation. Our antibiograms re-
vealed that none of the complexes (whether bearing Re or
Mn) showed antibacterial properties with the exception of
12. Species 12 showed minimum inhibitory concentrations
(MICs) of 128 μg/mL if administered in the dark and
256 μg/mL if light-activated. Again, as observed in the cell
proliferation assays, CO release seems to mitigate the toxic
CCDC-1402025 (for 3a), -1402026 (for 4), -1402027 (for 8),
-1402028 (for 10), -1402029 (for 11), and -1402030 (for 14) contain
the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
Cyclic voltammetry was performed with the Echem software and
a Princeton Applied Research 263 A instrument. For these mea-
effects of the molecules. Although the MIC value for 12 is surements, 5 mm solutions of the organometallic species were pre-
high with respect to the standard,[58,59] 12 could be used as
pared in 0.5 m tetrabutylammonium hexafluorophosphate as the
electrolyte. The [NEt4][Re(CO)2Br4] (1) and [NEt4]2[Re(CO)2Br4]
a lead structure for further developments.
(2) complexes were prepared according to a reported procedure[36]
in yields of 46 (2.8 g, 6.91 mmol) and 60% (280 mg, 0.69 mmol),
respectively.
Conclusions
DFT Calculations: Geometry optimizations and frequency calcula-
tions for all molecules were performed at the density functional
To develop Cardiolite CORM analogues, we have studied
the reactivity of 16-, 17-, and 18-electron rhenium and man-
ganese carbonyl species with isocyanide ligands. The reac-
tions of CNR with cis-[Re(CO)2Br4]–/2– complexes are ac-
companied by two- and one-electron reductions of the
metal center and result in the formation of cis-mer-[Re(CO)2-
(CNR)3Br] 18-electron species, whereas the same reactions
with [Mn(CO)5Br] gave fac-[Mn(CO)3(CNR)2Br] com-
pounds. The resulting Re complexes were stable and unable
level of theory with the Gaussian03 program package[63] with the
hybrid B3LYP functional[64] in conjunction with the LanL2DZ ba-
sis set.[65–67] Pure basis functions (5d, 7f) were used in all calcula-
tions. The geometries were optimized fully without symmetry re-
strictions. The nature of the stationary points was checked by com-
puting vibrational frequencies to verify true minima. The infrared
spectra were calculated after geometry optimization in the gas
phase (i.e., no solvation was considered).
to release carbon monoxide. Conversely, the Mn species be- Mibi Ligand: Step 1: ZnCl2 (40 g, 293.8 mmol) was dissolved in dry
methanol (27 mL). Then, 2-hydroxyisobutyronitrile (25 g,
293.8 mmol) was added under argon, and the solution was stirred
at 60 °C overnight. At the end of the reaction, the obtained yellow
solution was poured into an ice/water mixture (100 mL), extracted
with diethyl ether (3ϫ100 mL), and dried with MgSO4, and the
solvent was removed to afford 2-methoxyisobutyronitrile as a yel-
low liquid (yield 11.97 g, 120.5 mmol, 41%). Step 2: 2-Methoxyiso-
butyronitrile in diethyl ether (11.97 g, 120.5 mmol) was added
dropwise to a solution of LiAlH4 (5.50 g, 145 mmol) was in dry
diethyl ether (300 mL) under argon. The solution was stirred at
haved as photoCORMs with light activation for CO release
in the UV region of the electromagnetic spectrum. With the
exception of compounds bearing the methoxyisobutylison-
itrile (mibi) ligand, all molecules were toxic in a 3T3 fibro-
blast proliferation assay. None of the reported species
showed high antibacterial effects. Persistent intermediates
were observed in the reactions of 1 or 2 with CNR before
the formation of the final dicarbonyl products. On the basis
of a combined experimental and theoretical analysis, these
intermediates were assigned to a mixture of a mono-CNR reflux overnight. At the end of the reaction, H2O (80 mL) was
d5 Re complexes and a bis-CNR d6 Re species. The former
added dropwise. The grey suspension was filtered, the solid was
washed with diethyl ether, and the aqueous phase was then ex-
might prove to be useful CORMs. Thus, further studies will
tracted with diethyl ether. The organic phases were dried with
be aimed at their isolation and characterization.
MgSO4, and the solvent was removed to give 2-methoxyisobutyl-
amine as a grey liquid (yield 3.77 g, 36.6 mmol, 30%). Step 3: Then,
ethyl formate (2.9 mL, 36.6 mmol) was added dropwise to a solu-
tion of 2-methoxyisobutylamine (3.77 g, 36.6 mmol) in p-tolu-
Experimental Section
Materials: Chemicals and solvents were purchased from standard
sources. Real-time IR spectroscopy analyses were performed with
a Mettler–Toledo react-IR 15 instrument. The UV/Vis spectra were
recorded with a Perkin–Elmer Lambda 40 spectrometer. Solid-state
IR spectra were recorded with a Bruker Tensor 27 instrument. The
mass spectra were recorded with a Bruker esquire HCT spectrome-
ter with methanol as solvent. The crystallographic data of single
crystals were collected with Mo-Kα radiation (λ = 0.71073 Å), ex-
enesulfonic acid (0.017 g), and the mixture was stirred at reflux for
16 h. Then, the solvent was removed under high vacuum, and N-
formyl-2-methoxyisobutylamine was obtained as am orange liquid
(yield 3.2 g, 24.4 mmol, 67%). Step 4: N-Formyl-2-methoxyiso-
butylamine (3.2 g, 24.4 mmol) and trimethylamine (5.01 g,
49.5 mmol) were dissolved in CH2Cl2 (DCM, 36 mL). The mixture
was cooled to –40 °C with an acetone/liquid nitrogen mixture.
Then, diphosgene (1.46 mL) in dry DCM (18 mL) was added drop-
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1
wise to the reaction mixture. The temperature of the mixture was
increased to 0 °C, and the mixture was stirred at 0 °C for 1 h and
then for 30 min at reflux. At the end of the reaction, H2O (20 mL)
was added, and the organic phase was washed with saturated so-
dium hydrogen carbonate and H2O and dried with MgSO4, and
5.77. H NMR (360 MHz, [D6]DMSO): δ = 1.18 (s, 18 H, 6ϫC4-
CH3), 1.19 (s, 9 H, 3ϫC4-CH3), 1.51 (s, 12 H, 4ϫC2-CH3), 1.53
(s, 6 H, 2ϫC2-CH3), 1.60 (s, 4 H, 2ϫCH2), 1.62 (s, 2 H, CH2)
ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 30.8 (CH3), 30.9
(CH3), 31.0 (CH3), 31.17 (CH3), 31.21 (CH3), 50.3, 52.1, 52.8, 53.1,
the solvent was removed to afford methoxyisobutylisonitrile (mibi) 55.1 (CH2), 55.6 (CH2), 160.3 (CNR), 188.6 (CO), 191.1 (CO) ppm.
as a dark orange oil (yield 1.03 g, 9.16 mmol, 74%).
cis-mer-[Re(CO)2(mibi)3Br] (8): Yield 31 mg, 47.6 μmol, 68%. IR:
General Synthesis of cis-mer-[Re(CO)2(CNR)3Br] from 1 or 2: A
solution of 1 or 2 (70 μmol) in DCM (2.5 mL) was placed in a
small flask. Then, 0.5 equiv. of the isocyanide ligand (tbu, ipp, chx,
smb, tmb, or mibi) was added every 5 min up to 10 equiv. of the
ligand. At the end of the reaction, the solvent was removed, and
purification was performed through silica column chromatography
with DCM as the eluent. The purity of the collected compound
was assessed by 1H NMR spectroscopy, elemental analysis (EA),
IR spectroscopy, and MS.
ν = 2211, 2150 (νCN stretching), 1967, 1889 (νCO stretching) cm–1.
˜
ESI-MS (MeOH): m/z (%) 582.2 (100) [M]+. C20H33BrN3O5Re
(661.60): calcd. C 36.31, H 5.04, N 6.35; found C 36.43, H 4.98, N
1
6.27. H NMR (360 MHz, [D6]DMSO): δ = 1.22 (s, 18 H, 6ϫC1-
CH3), 3.17 (s, 9 H, 3ϫOCH3), 3.91 (s, 4 H, 2ϫCH2), 3.93 (s, 2 H,
1ϫCH2) ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 22.10
(CH3), 22.14 (CH3), 22.17 (CH3), 49.4, 51.3 (CH2), 51.5 (CH2),
73.6 (OCH3), 73.8 (OCH3), 139.2 (CNR), 188.4 (CO), 191.5 (CO)
ppm. Single crystals suitable for X-ray diffraction analysis were ob-
tained by layering hexane on top of a methyl tert-butyl ether solu-
tion of the compound.
cis-mer-[Re(CO) (tbu) Br] (3): Yield 30 mg, 52.5 μmol, 75%. IR: ν
˜
2
3
= 2189, 2121 (νCN stretching), 1955, 1865 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 492.1 (100) [M]+. C17H27BrN3O2Re
(571.52): calcd. C 35.72, H 4.77, N 7.35; found C 35.21, H 4.67, N
7.12. 1H NMR {360 MHz, [D6]dimethyl sulfoxide ([D6]DMSO)}: δ
= 1.39 (s, 18 H, 6ϫCH3), 1.41 (s, 9 H, 3ϫCH3) ppm. 13C NMR
Isolation of [ReCO(tbu)3Br3] (3a): Compound 3a was isolated as a
byproduct from the reaction of 1 with tbu. First, a recrystallization
of the reaction mixture was performed directly in DCM/hexane 1:2
to precipitate (Et4N)2Br. The solid was then collected by filtration,
(500 MHz, [D6]DMSO): δ = 29.5 (CH3), 29.9 (CH3), 30.3 (CH3), dried, and recrystallized again in DCM/hexane 1:2 to afford the
50.3, 139.2 (CNR), 188.5 (CO), 191.4 (CO) ppm.
pure complex 3a. We noticed that this compound was removed (i.e.,
decomposed) during the chromatographic column performed to
give the purified species 3. Single crystals suitable for X-ray diffrac-
tion analysis were obtained by layering hexane on top of a DCM
solution of the compound, yield 49 mg, 25.9 μmol, 37%.
cis-mer-[Re(CO) (ipp) Br] (4): Yield 13 mg, 25.2 μmol, 36%. IR: ν
˜
2
3
= 2194, 2135 (νCN stretching), 1967, 1897 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 450.1 (100) [M]+. C14H21BrN3O2Re
(529.44): calcd. C 31.76, H 4.01, N 7.93; found C 31.64, H 3.92, N
7.82. 1H NMR (360 MHz, [D6]DMSO): δ = 1.27 (t, J = 7.1 Hz, 18 General Synthesis of fac-cis-[Mn(CO)3(CNR)2Br]: [Mn(CO)5Br]
H, 6ϫCH3), 4.19–4.23 (m, J = 14.2 Hz, 3 H, 3ϫCH) ppm. 13C (55 mg, 200 μmol) was dissolved in acetone (2.5 mL). Then, the
NMR (500 MHz, [D6]DMSO): δ = 23.0 (CH3), 23.3 (CH3), 48.0,
137.5 (CNR), 188.8 (CO), 192.0 (CO) ppm. Single crystals suitable
for X-ray diffraction analysis were obtained by layering hexane on
top of a DCM solution of the compound.
selected isocyanide ligand (700 μmol, 3.5 equiv.) was added, and
the reaction mixture was stirred at reflux for 1 h. The solvent was
removed, and the solid was purified by silica column chromatog-
raphy with DCM/acetone 30:1 as the eluent. The solvent was re-
moved, and the corresponding fac-cis-[Mn(CO)3L2Br] complex was
obtained as a microcrystalline powder.
cis-mer-[Re(CO) (chx) Br] (5): Yield 20 mg, 30.8 μmol, 44%. IR: ν
˜
2
3
= 2201, 2128 (νCN stretching), 1956, 1864 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 570.1 (100) [M]+. C23H33BrN3O2Re
(649.64): calcd. C 42.52, H 5.13, N 6.47; found C 42.31, H 5.10, N
fac-[Mn(CO) (tbu) Br] (9): Yield 30 mg, 78 μmol, 39%. IR: ν =
˜
3
2
2176 (νCN stretching), 2032, 1967, 1913 (νCO stretching) cm–1.
6.52. 1H NMR (360 MHz, [D6]DMSO): δ = 1.32–1.34 [m, 3 H, ESI-MS (MeOH): m/z (%) = 360.1 (30) [M]+. C13H18BrMnN2O3
3ϫp-CH2, axial (ax.)], 1.43–1.44 [m, 9 H, 6ϫm-CH2, ax., 3ϫp-
CH2, equatorial (eq.)], 1.66–1.75 (m, 18 H, 6ϫm-CH2, eq., 6ϫo-
CH2, ax., 6ϫo-CH2, eq.), 3.62–3.65 (t, J = 3.5 Hz, 3 H, 3ϫCH)
ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 21.2, 21.4, 24.5, 24.6,
31.4, 31.8 (CH2), 53.3 (CH) 138.8 (CN–R), 188.9 (CO), 190.0 (CO)
ppm.
(385.14): calcd. C 40.54, H 4.72, N 7.27; found C 40.78, H 4.68, N
7.19. 1H NMR (360 MHz, [D6]DMSO): δ = 1.41 (s, 18 H, 6ϫCH3)
ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 29.5 (CH3), 31.3
(CH3), 50.0, 150.8 (CNR), 214.4 (CO), 218.9 (CO) ppm.
fac-[Mn(CO) (ipp) Br] (10): Yield 20 mg, 56 μmol, 28%. IR: ν =
˜
3
2
2180 (νCN stretching), 2033, 1974, 1932 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 318.1 (20) [M]+. C11H14BrMnN2O3
(357.09): calcd. C 37.00, H 3.96, N 7.85; found C 36.99, H 4.00, N
7.89. 1H NMR (360 MHz, [D6]DMSO): δ = 1.27–1.29 (d, J =
6.8 Hz, 12 H, 4ϫCH3), 4.19–4.23 (m, J = 7.6 Hz, 2 H, 2ϫCH)
cis-mer-[Re(CO) (smb) Br] (6): Yield 20 mg, 27.3 μmol, 39%. IR: ν
˜
2
3
= 2198, 2124 (νCN stretching), 1959, 1880 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 636.0 (100) [M]+. C29H27BrN3O2Re
(715.66): calcd. C 48.67, H 3.81, N 5.87; found C 48.47, H 3.59, N
5.97. 1H NMR (360 MHz, [D6]DMSO): δ = 1.56–1.59 (d, J = ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 22.8 (CH3), 48.9,
4.2 Hz, 6 H, 2ϫCH3), 1.61–1.63 (d, J = 6.8 Hz, 3 H, CH3), 5.48–
5.52 (m, J = 5.9 Hz, 3 H, 3ϫCH), 7.33–7.56 (m, 15 H, 15ϫbenzyl-
CH) ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 24.7 (CH3), 25.0
(CH3), 25.1 (CH3), 55.0 (CN-CH), 55.2 (CN-CH), 125.3 (benzyl-
CH), 128.2 (benzyl-CH), 128.3 (benzyl-CH), 128.8 (benzyl-CH),
128.9 (benzyl-CH), 138.6 (CNR), 139.3 (CNR), 188.3 (CO), 191.4
(CO) ppm.
152.8 (CNR), 214.3 (CO), 219.7 (CO) ppm. Single crystals suitable
for X-ray diffraction analysis were obtained by layering hexane on
top of a DCM solution of the compound.
fac-cis-[Mn(CO) (chx) Br] (11): Yield 40 mg, 92 μmol, 46%. IR: ν
˜
3
2
= 2177 (νCN stretching), 2032, 1976, 1931 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 438.1 (15) [M]+. C17H22BrMnN2O3
(437.21): calcd. C 46.70, H 5.08, N 6.41; found C 47.78, H 5.57, N
6.45. 1H NMR (360 MHz, [D6]DMSO): δ = 1.30–1.56 (m, 8 H,
4ϫo-CH2, eq.; 4ϫo-CH2, ax.) 1.63–1.91 (m, 12 H, 4ϫm-CH2, eq.,
4ϫm-CH2, ax., 2ϫp-CH2, ax., 2ϫp-CH2, eq.), 4.31–4.37 (t, 2 H,
2ϫCH) ppm. 13C NMR (500 MHz, [D6]DMSO): δ = 21.3, 24.4,
cis-mer-[Re(CO) (tmb) Br] (7): Yield 25 mg, 35 μmol, 50%. IR: ν
˜
2
3
= 2198, 2123 (νCN stretching), 1958, 1878 (νCO stretching) cm–1.
ESI-MS (MeOH): m/z (%) = 660.3 (100) [M]+. C29H51BrN3O2Re
(739.84): calcd. C 47.08, H 6.96, N 5.68; found C 47.28, H 7.11, N
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31.3 (CH2), 54.1 (CH) 153.6 (CNR), 214.2 (CO), 219.6 (CO) ppm. cin at pH 7.4. The assay was performed in 96-well plates with ca.
Single crystals suitable for X-ray diffraction analysis were obtained
by layering hexane on top of a DCM solution of the compound.
1000 cells per well proliferated at 37 °C in the presence of 95% air
and 5% CO2. After 1 d, the twelve compounds were added to the
cells under a dim light to give a final concentration of 20 or 100
μm. After the administration of the compounds, half of the cultures
were exposed to light (neon lamp at a distance of 60 cm) for 30 min,
and the other half were kept in darkness. All of the cultures were
subsequently kept in the dark for the rest of the incubation time.
After 1, 2, and 3 d, a 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltet-
razolium bromide (MTT) cell proliferation assay was performed as
follows: the yellow MTT was added to the cells to give a final
concentration of 4 mm, and the cultures were incubated in the dark
at 37 °C for 4 h. The medium was then removed, and DMSO
(100 μL) was added to each well. After 30 min of incubation at
room temperature in the dark, the optical density at λ = 550 nm
was read with a plate reader.[69]
fac-cis-[Mn(CO)3(smb)2Br] (12): Yield 72 mg, 52.5 μmol, 75%. IR:
ν = 2177 (νCN stretching), 2035, 1982, 1937 (νCO stretching) cm–1.
˜
ESI-MS (MeOH): m/z (%) = 504.0 (20) [M]+. C21H18BrMnN2O3
(481.23): calcd. C 52.41, H 3.78, N 5.82; found C 53.02, H 3.94, N
5.62. 1H NMR (360 MHz, [D6]DMSO): δ = 1.60–1.69 (m, J =
5.8 Hz, 6 H, 2ϫCH3), 5.53–5.58 (m, J = 5.5 Hz, 2 H, 2ϫCH),
7.36–7.50 (m, J = 5.2 Hz, 10 H, 10ϫbenzyl-CH) ppm. 13C NMR
(500 MHz, [D6]DMSO): δ = 24.5 (CH3), 55.0 (CN-CH), 56.0 (CN-
CH), 125.3 (benzyl-CH), 128.4 (benzyl-CH), 129.0 (benzyl-CH),
138.6 (benzyl-CH), 155.6 (CNR), 214.0 (CO), 219.2 (CO) ppm.
fac-cis-[Mn(CO)3(tmb)2Br] (13): Yield 86 mg, 174 μmol, 87%. IR:
ν = 2172 (νCN stretching), 2031, 1977, 1931 (νCO stretching) cm–1.
˜
ESI-MS (MeOH): m/z (%) = 528.2 (20) [M]+. C21H34BrMnN2O3
(497.35): calcd. C 50.71, H 6.90, N 5.63; found C 49.83, H 6.53, N
1
5.39. H NMR (360 MHz, [D6]DMSO): δ = 1.08 (s, 18 H, 6ϫC4-
Acknowledgments
CH3), 1.45 (s, 12 H, 4ϫC2-CH3), 1.70 (s, 4ϫ, 2ϫCH2) ppm. 13C
NMR (500 MHz, [D6]DMSO): δ = 30.6 (CH3), 30.7 (CH3), 30.8
(CH3), 31.4 (CH3), 31.21, 52.6, 60.5 (CH2), 153.8 (CNR), 214.2
(CO), 219.6 (CO) ppm.
Financial support from the Swiss National Science Foundation
(grant number PP00P2_144700) is gratefully acknowledged.
fac-cis-[Mn(CO) (mibi) Br] (14): Yield 36 mg, 80 μmol, 40%. IR: ν
˜
3
2
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Received: July 9, 2015
Published Online: November 9, 2015
Eur. J. Inorg. Chem. 2015, 5628–5638
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