Photosensitive iron(II)-based CO-releasing molecules (CORMs) with vicinal amino and diphenylphosphino substituted chelating ligands

Article abstract of DOI:10.1016/j.jorganchem.2013.02.015

The reactions of [Fe(H₂O)₆] [BF₄] ₂ with aminoethyl-diphenylphosphane and 2-(diphenylphosphino)aniline lead to the formation of trans-[Fe(NC-Me)₂(H₂NCH ₂CH₂PPh₂)₂] [BF₄] ₂ (1a) and trans-[Fe(NC-Me)₂(H₂NC ₆H₄-2-PPh₂)₂] [BF₄] ₂ (1b), respectively. One acetonitrile ligand can be substituted by CO yielding [Fe(CO)(NC-Me)(H₂NCH₂CH₂PPh ₂)₂] [BF₄]₂ (2a, CORM-P1) and [Fe(CO)(NC-Me)(H₂NC₆H₄-2-PPh₂) ₂] [BF₄]₂ (2b, CORM-P2). Upon irradiation with visible light, CO is liberated making especially 2a a promising photo-CORM whereas for 2b a slow and incomplete CO release is observed.

Full text of DOI:10.1016/j.jorganchem.2013.02.015

Journal of Organometallic Chemistry 733 (2013) 63e70
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Photosensitive iron(II)-based CO-releasing molecules (CORMs) with vicinal amino
and diphenylphosphino substituted chelating ligands
,
Taghreed M.A. Jazzazi a, Helmar Görls a, Guido Gessner b, Stefan H. Heinemann b, M. Westerhausen a
*
a Institute of Inorganic and Analytical Chemistry, Friedrich Schiller University Jena, Humboldtstraße 8, 07743 Jena, Germany
b Center for Molecular Biomedicine, Department of Biophysics, Friedrich Schiller University Jena & Jena University Hospital, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
The reactions of [Fe(H2O)6] [BF4]2 with aminoethyl-diphenylphosphane and 2-(diphenylphosphino)an-
iline lead to the formation of trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a) and trans-[Fe(NC-
Me)2(H2NC6H4-2-PPh2)2] [BF4]2 (1b), respectively. One acetonitrile ligand can be substituted by CO
yielding [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2 (2a, CORM-P1) and [Fe(CO)(NC-Me)(H2NC6H4-2-
PPh2)2] [BF4]2 (2b, CORM-P2). Upon irradiation with visible light, CO is liberated making especially 2a
a promising photo-CORM whereas for 2b a slow and incomplete CO release is observed.
Ó 2013 Elsevier B.V. All rights reserved.
Received 27 November 2012
Received in revised form
8 February 2013
Accepted 13 February 2013
Keywords:
CO release
CORM
Iron
Iron carbonyl complexes
MyoglobineCO assay
Photo-CORM
Carbon monoxide, an important signaling molecule [1e3] with a
huge therapeutic potential [4e6], is mostly formed during heme
degradation together with iron(II) ions [7]. However, CO also ex-
presses a well-known toxicity and exhibits poor solubility and lack
of selectivity e all aspects hampering the use of gaseous carbon
monoxide as a therapeutic agent. Therefore, carriers have to be
used for targeting carbon monoxide to cells and tissues. Such car-
riers have to fulfill certain prerequisites such as non-toxicity of
themselves as well as their degradation products after CO release,
adequate solubility and stability in water, and a mode of CO release
that can be triggered by ligand substitution reactions, pH or tem-
perature changes, enzymes, or by visible light. Metal carbonyl
complexes are appropriate carriers for CO [8,9] and represent a
promising class of future pharmaceuticals as CO-releasing mole-
cules (so-called CORMs). Especially metal carbonyl complexes that
liberate CO upon radiation with visible light (photo-CORMs)
recently gained particular interest [10e13]. According to the
importance and the enormous expectations with respect to clinical
benefits several recent review articles summarize the strategies
and medicinal aspects of carbon monoxide treatments [14e16].
In medicinal applications the effects of potentially toxic break-
down products have to be considered. Iron-based CORMs represent
a promising substance class because iron(II) is a non-toxic 3d metal
whose concentration is effectively regulated in biological systems
[17]. Therefore, many investigations focused on iron-based
carbonyl complexes and date back several decades. More than
eighty years ago, Cremer demonstrated that iron complexes with
biogenic cysteine and isocysteine ligands reversibly bind carbon
monoxide [18,19]. The possibility of triggering CO liberation from
iron carbonyl complexes with visible light was discovered very
early, too [20]. The iron carbonyl complexes with chelating cysteine
and cysteamine ligands were used to study the oxidation state of
iron in its enzymes [21,22]. Thereafter, numerous research groups
investigated synthesis, physical behavior, and chemical reactivity of
iron(II) carbonyl complexes containing thiolates with S/N-donor
pairs (see for example Refs. [23e32]). In these complexes the
carbonyl molecules are cis-arranged with the nitrogen donor atoms
in trans-positions. The compound class of iron(II)-based CORMs
also includes thiolates (e.g. Refs. [33,34]), multidentate amines (e.g.
Refs. [35,36]), dithiocarbamates [37], and even
p-bonded unsatu-
rated hydrocarbons (e.g. Refs. [38e41]).
The activity of CORMs is commonly measured by a myoglobin
assay in a buffered solution containing deoxymyoglobin (Mb) with
excess of sodium dithionite. In this test, liberated carbon monoxide
reacts with Mb to myoglobineCO complex (MbeCO) and the
amount of MbeCO formed is quantified spectroscopically. This test
causes challenges when the CORMs show an intense color and,
hence, requires a modified analysis of the spectroscopic data [42].
* Corresponding author. Tel.: þ49 3641 948110; fax: þ49 3641 948102.
E-mail address: m.we@uni-jena.de (M. Westerhausen).
0022-328X/$ e see front matter Ó 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jorganchem.2013.02.015

64
T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
In addition, it was observed that the sulfite species enhance CO
liberation in Ru-based CORM-2 ([Ru(CO)3Cl2]2) and CORM-3
([Ru(CO)3Cl(glycinate)]), which release carbon monoxide via a
ligand substitution mechanism [43].
In the course of our investigations of iron(II)-based CORMs we
compared dicarbonyl-bis(cysteaminato)iron(II) (CORM-S1) and
dicarbonyl-bis(ortho-aminothiophenolato)iron(II) (CORM-S2). In
CORM-S2, a step-wise CO release was observed upon irradiation
with visible light whereas both carbon monoxide molecules were
liberated at the same time from CORM-S1. Here, we intended to
investigate an iron(II)-based photo-CORM with only one CO ligand
coordinated to the metal atom in order to avoid interference be-
tween two CO-releasing steps as obviously detected for CORM-S2.
In addition, an NMR probe should be introduced enabling the
observation not only of CO release but also of the degradation of the
metal complex. This procedure allows direct comparison of the
analytical methodologies and at best validation of the myoglobin
assay for photo-CORMs. Therefore, we investigated complexes of
the type [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2]2þ (CORM-P1) and
[Fe(CO)(NC-Me)(H2NC6H4-2-PPh2)2]2þ (CORM-P2), the counter
anions being tetrafluoroborate, with the 31P nucleus being a very
sensitive NMR probe.
Scheme 1. Synthesis of the CO-releasing molecules 2a (CORM-P1) and 2b (CORM-P2),
starting from [Fe(H2O)6] [BF4]2. Reaction conditions: a) 1. Reaction with aminoethyl-
diphenylphosphane in acetonitrile, 2. Substitution of one acetonitrile by CO; b) 1.
Reaction with 2-diphenylphosphinoaniline in acetonitrile, 2. Substitution of one
acetonitrile by CO.
1. Results and discussion
1.1. Synthesis
The reactions of [Fe(H2O)6] [BF4]2 with aminoethyl-diphenyl-
phosphane and 2-(diphenylphosphino)aniline in acetonitrile sol-
vent yielded the complexes trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2]
[BF4]2 (1a) [44] and trans-[Fe(NC-Me)2(H2NC6H4-2-PPh2)2] [BF4]2
(1b). Exposing these complexes, dissolved in CH2Cl2, to a CO at-
mosphere led to substitution of one acetonitrile molecule by a
carbon monoxide ligand under maintenance of the residual iron(II)
environment, yielding [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2
(2a, CORM-P1) and [Fe(CO)(NC-Me)(H2NC6H4-2-PPh2)2] [BF4]2 (2b,
CORM-P2) according to Scheme 1. Substitution of the second
acetonitrile did neither occur at longer reaction periods nor at
higher carbon monoxide pressure or higher temperature. Therefore,
very pure iron(II) complexes were easily obtained and isolated with
high yields. Iron(II) complexes with two or three carbon monoxide
ligands usually show cis or facial arrangements of these molecules
due to the strong trans-influence of carbon monoxide. Exceptions
are described in mixed cyano/carbonyl iron(II) complexes [34]
which can be explained by the isoelectronic nature of CO and CNꢀ.
In solution solvent-separated ions are observed. The NMR pa-
rameters of the tetrafluoroborate anion of 1a and 1b are very
similar and do not depend on the cation, even though in the crys-
talline state NeH/F bridges are observed.
iron(II) complexes 1a (2163 cmꢀ1) and 1b (2207 cmꢀ1) because CO
is a strong
p-acceptor supporting the p-donor ability of acetonitrile.
1.3. Molecular structures
The cations trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2]2þ [44] (coun-
terion [FeBr4]2ꢀ) and trans-[Fe(NC-Me)2(H2NC6H4-2-PPh2)2]2þ (1b)
(Fig.1, counterions [BF4]ꢀ not displayed for clarity reasons) show very
similar symmetries which deviate only slightly from C2v symmetry.
The iron(II) centers are in distorted octahedral environments with the
acetonitrile ligands trans to each other. The diphenylphosphanyl
groups are cis-arranged as are the amino groups.
The difference between these cations is the more rigid benzo-
backbone of the cation of 1b leading to smaller bites of the
bidentate ligands and, hence, to slightly smaller P1eFe1eN1 and
P2eFe1eN2 bond angles. The structural changes, however, are
insignificant and even the P1eFe1eP2 and N1eFe1eN2 bond an-
gles are very much alike. The nearly linear acetonitrile molecules
show very short CbN bond lengths. Whereas the more flexible
ethylene backbone of trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2]2þ al-
lows to align to steric necessities, the more rigid benzo unit of 1b
suffers more severe distortions. Thus, the P1eC1eC2/6 and P2e
C19eC20/24 angles differ by approximately 10ꢁ due to the bulky
phenyl substituents at P1 and P2. For the small amino groups N1
and N2 the difference between proximal and distal N1eC6eC1/5
and N2eC24eC19/23 angles is much smaller.
1.2. IR investigations
The symmetric and asymmetric stretching frequencies of the
carbon monoxide ligands of CORM-S1 (2014, 1945 cmꢀ1 [31]) and
CORM-S2 (2035,1976 cmꢀ1 [32]) are observed at significantly lower
wavenumbers than those of free carbon monoxide (2134 cmꢀ1
[45]). The iron(II) complexes CORM-P1 (CbN: 2303, CbO:
1985 cmꢀ1) and CORM-P2 (CbN: 2290, CbO: 2001 cmꢀ1) also show
two bands, in this case due to acetonitrile (free gaseous acetonitrile
2268 cmꢀ1) and carbon monoxide stretching vibrations. A shift of
the CbN stretching mode to higher wavenumbers upon coordina-
tion to Lewis acids represents a very common observation for such
Substitution of one acetonitrile ligand by a carbon monoxide
reduces the symmetry of the cation and only the mirror plane
containing the CO and NCeMe ligands is maintained. Fig. 2 shows
[Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2 (2a, CORM-P1) and
Fig. 3 displays the molecular structure of [Fe(CO)(NC-Me)(H2NC6H4-
2-PPh2)2] [BF4]2 (2b, CORM-P2) clarifying also the stabilization by
NeH/F hydrogen bridges to the tetrafluoroborate anions.
complexes [45,46] with acetonitrile acting as a
wavenumbers of the CbN stretching modes of the CORMs 2a and
2b are significantly larger than observed for the bis(acetonitrile)
p
-donor [47]. The
The structural parameters are very similar with respect to the
carbonyl fragment. The CO bond lengths and the FeeC distances
with average values of 114.4 and 176.7 pm, respectively, differ only

T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
65
Fig. 1. Molecular structure and numbering scheme of the cation trans-[Fe(NC-
Me)2(H2NC6H4-2-PPh2)2]2þ of 1b. The ellipsoids represent a probability of 40%. H
atoms with the exception of the amino groups are neglected for clarity reasons.
Selected bond lengths (pm): Fe1eP1 223.72(5), Fe1eP2 222.66(5), Fe1eN1 206.0(2),
Fe1eN2 205.8(2), Fe1eN3 192.0(2), Fe1eN4 191.5(2), N1eC6 145.2(2), N2eC24
145.7(2), N3eC37 114.1(2), N4eC39 114.0(2), P1eC1 182.1(2), P1eC7 182.8(2), P1eC13
182.4(2), P2eC19 181.3(2), P2eC25 182.0(2), P2eC31 183.1(2); angles (deg.): N3eFe1e
N4 175.99(6), P1eFe1eP2 104.62(2), N1eFe1eN2 88.14(6), P1eFe1eN1 83.25(4), P2e
Fe1eN2 84.05(4).
within their estimated standard deviations. Also the FeeCeO an-
gles of 178.6ꢁ for both cations deviate only insignificantly from
linearity. The acetonitrile ligand experiences a significant FeeN
bond elongation due to the trans-positioned CO molecule by
approximately 4 pm as a consequence of a weakened
p-back-
donation of electron density from the metal center to the * orbitals
p
of the nitrile group. This fact is in agreement with the IR spectro-
scopic findings. The effect on the CbN bond lengths is much smaller
and lies within the standard deviations. However, the presence of
the CO ligand also leads to elongation of the FeeP bonds by
approximately 2e3 pm.
Fig. 2. Molecular structure and numbering scheme of the cation [Fe(CO)(NC-
Me)(H2NCH2CH2PPh2)2]2þ (2a, CORM-P1) (top). The ellipsoids represent a probability
of 40%, all C-bound H atoms are omitted for clarity reasons. At the bottom, dimer
formation via
a NeH/F network of [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2 is
The tetrafluoroborate anions form hydrogen bridges to the
amino groups of the cations. In the compounds 1b and 2b (CORM-
P2) the benzo-backbone fixes the nearly parallel alignment of the
NH bonds of the amino groups. The BFꢀ4 -anions are able to form two
intramolecular NeH/F bridges to the same cation. Contrary to this
formation of a contact ion pair due to the flexibility of the ethylene
backbone in CORM-P1 (2a), the tetrafluoroborate anions form Ne
H/F bridges to two different cations leading to the formation of
a dimeric contact ion pair.
represented. Here arbitrary radii were chosen for all atoms. Selected bond lengths
(pm): Fe1eP1 228.84(5), Fe1eP2 228.55(5), Fe1eN1 204.4(2), Fe1eN2 205.9(2), Fe1e
N3 194.7(2), Fe1eC29 176.4(2), C29eO1 114.4(2), N1eC1 148.5(2), N2eC15 149.0(2),
N3eC30 114.1(3), P1eC2 185.0(2), P1eC31 83.4(2), P1eC9 182.5(2), P2eC16 183.3(2),
P2eC17 181.5(2), P2eC23 182.3(2); angles (deg.): N3eFe1eC29 178.11(8), P1eFe1eP2
106.85(2), N1eFe1eN2 86.00(7), P1eFe1eN1 83.32(5), P2eFe1eN2 83.68(5), Fe1e
C29eO1 178.5(2).
decomposition was observed leading to a quantitative conversion
to trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a) and carbon
monoxide within two weeks. Irradiation of a freshly prepared so-
lution of 2a in D3CeCbN led to CO liberation and formation of 1a.
Therefore, initially no free ligand was observed. However, trans-
[Fe(NC-Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a) also degrades during
irradiation but this breakup is slower than the decomposition of
CORM-P1 (2a). The 31P{1H} NMR spectra of this degradation reac-
tion are shown in Fig. 5, the interpretation is displayed in Fig. 6.
Here it becomes obvious that there is an induction period for the
appearance of H2NCH2CH2PPh2 due to the intermediate formation
of 1a. A control experiment verified that a solution of trans-[Fe(NC-
Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a) in [D3]acetonitrile is stable in
the dark but that this complex degrades upon irradiation liberating
free aminoethyl-diphenylphosphane.
1.4. CO release of CORM-P1 and CORM-P2
To infer about the mechanism of CO release we studied the light-
triggered degradation of the CORMs by 31P NMR spectroscopy. For
this purpose CORM-P1 2a and CORM-P2 2b were dissolved in [D6]
dimethylsulfoxide (DMSO) at a final concentration of 23 mM. As
expected, both CORMs showed no degradation in the dark. In
order to quantitatively record the degradation of CORM-P1, the
phosphorus-containing reference compound tetraphenylphospho-
nium chloride [Ph4P Cl:
d
(
31P) ¼ 23.2] was added. From this solution
the degradation was observed in dependency of the irradiation
duration (Fig. 4). The free ligand aminoethyl-diphenylphosphane
was formed during CO release, and no other intermediates were
detected by 31P{1H} NMR spectroscopy.
We further assayed the properties of the compounds in
aqueous solutions. CO release was quantified in a myoglobin-based
assay by spectrophotometric measurement of the conversion of
A different picture resulted from a [D3]acetonitrile solution of
CORM-P1, again at 23 mM. Already in the dark a very slow

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T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
deoxymyoglobin (Mb) into the myoglobineCO complex (MbeCO).
For measurements in the range of 500e600 nm a solution was
prepared that contained equimolar (100 M) amounts of deoxy-
m
myoglobin and either CORM-P1 or CORM-P2. Myoglobin was
reduced with 0.1% sodium dithionite. Both CORMs showed no CO
release in the dark but irradiation with white light for 10 min led to
quantitative (Fig. 7A, left, CORM-P1) or incomplete (Fig. 7A, right,
CORM-P2) carbon monoxide liberation. However, monitoring of the
CO release of CORM-P2 in this wavelength range, which requires a
high concentration, was less accurate because of the self-absorbance
of this complex. We therefore also investigated the light-dependent
CO release at lower concentration (5 mM) in a spectral range covering
the Soret peak (Fig. 7B). The CORMs were irradiated for 10 min in a
buffered solution, which contained 0.1% of sodium dithionite and a
concentration of 10 mM deoxymyoglobin, and the spectra without
and with irradiated CORMs were compared (Fig. 7B for irradiation
with 420-nm light). While stable in the dark, light completely
released CO from CORM-P1. For the benzo derivative CORM-P2,
however, the release was incomplete. A comparable observation
was also valid for a comparison of [(OC)2Fe(SCH2CH2NH2)2] (CORM-
S1) and [(OC)2Fe(SC6H4NH2)2] (CORM-S2) [32].
Kinetics of CO release was monitored by measuring the change
in absorbance at 422 nm. When exposing the samples to light of
420, 470, or 520 nm, CO release followed a single-exponential time
course (Fig. 7C). The corresponding time constants strongly
increased with the wavelength (Fig. 7D). For CORM-P1, total release
of the CO could be obtained for all wavelengths in the chosen
experimental setting, while for CORM-P2 only a partial release was
observed (Fig. 7E).
As it was reported for CORM-2 and CORM-3 that CO release is
strongly facilitated by the reducing agent dithionite [43], we also
performed experiments in which CO release was triggered by light
in the absence of dithionite. Such experiments (Fig. 8) clearly
showed that dithionite itself does not release CO from the com-
pounds studied, which is in agreement with the NMR experiments
shown above.
2. Conclusion
Fig. 3. Molecular structure and numbering scheme of the cation [Fe(CO)(NC-
Me)(H2NC6H4-2-PPh2)2]2þ of 2b (CORM-P2) (top). The ellipsoids represent a proba-
bility of 40%. All C-bound hydrogen atoms are neglected for clarity reasons. At the
bottom, the structure of [Fe(CO)(NC-Me)(H2NC6H4-2-PPh2)2] [BF4]2 (2b, CORM-P2) is
shown, displaying the formation of a contact ion pair via NeH/F bridges. The atoms
are drawn with arbitrary radii. Selected bond lengths (pm): Fe1eP1 225.50(8), Fe1eP2
225.32(8), Fe1eN1 206.3(2), Fe1eN2 206.0(2), Fe1eN3 195.6(2), Fe1eC37 176.9(3),
C37eO1 114.3(4), N1eC6 145.9(4), N2eC24 146.0(4), N3eC38 113.5(4), P1eC1 181.2(3),
P1eC7 181.9(3), P1eC13 181.8(3), P2eC19 181.2(3), P2eC25 182.0(3), P2eC31 182.6(3);
angles (deg.): N3eFe1eC37 178.4(1), P1eFe1eP2 103.79(3), N1eFe1eN2 87.1(1), P1e
Fe1eN1 84.32(7), P2eFe1eN2 84.91(7), Fe1eC37eO1 178.6(3).
Substitution of one acetonitrile ligand in trans-[Fe(NC-Me)2(H2
NCH2CH2PPh2)2] [BF4]2 (1a) and trans-[Fe(NC-Me)2(H2NC6H4-2-
PPh2)2] [BF4]2 (1b) succeeds quantitatively in a carbon monoxide
atmosphere yielding [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2 (2a,
CORM-P1) and [Fe(CO)(NC-Me)(H2NC6H4-2-PPh2)2] [BF4]2 (2b,
CORM-P2), respectively, with a trans-arrangement of the CO and the
acetonitrile ligands. Both CORMs 2a and 2b readily lose the carbon
monoxide ligand upon irradiation with visible light making these
complexes valuable photo-CORMs.
Irradiation of 2a leads to loss of CO followed by complex
degradation and formation of free aminoethyl-diphenylphosphane.
A
31P{1H} NMR study in [D6]DMSO revealed that the iron(II) com-
plex 2a degrades completely after light-induced loss of CO also
simultaneously liberating aminoethyl-diphenylphosphane. If [D3]
acetonitrile is chosen as solvent for this experiment another
degradation pathway is observed. In this solvent, loss of carbon
monoxide upon irradiation with visible light yields trans-[Fe(NC-
Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a). This bis(acetonitrile) iron(II)
complex 1a also degrades upon irradiation but this decomposition
is much slower than the CO release from 2a. Therefore, the following
mechanism can be formulated for the decomposition of 2a in visible
light: upon irradiation of 2a, CO is substituted by another acetoni-
trile ligand yielding 1a. This complex also degrades in visible light
finally forming uncoordinated aminoethyl-diphenylphosphane.
However, this second degradation step is significantly slower than
the light-induced CO release step.
Fig. 4. Time-dependent degradation of 2a (CORM-P1) upon irradiation with visible
light in a 23-mM solution of 2a in [D6]DMSO followed by 31P{1H} NMR spectroscopy.
For quantification reasons Ph4P Cl was added as a reference compound. The continuous
curve is the result of a single-exponential data fit with a time constant of 21 s.

T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
67
Fig. 5. 31P{1H} NMR spectra of the degradation of a 23-mM solution of CORM-P1 (2a) in [D3]acetonitrile during irradiation with visible light. The bottom spectrum shows the
resonance of pure CORM-P1 (t ¼ 0 s, ¼ 61.7). The spectra were recorded at the indicated times. During irradiation with light of 470 nm, the signals of trans-[Fe(NC-
Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a) ( ¼ ꢀ21.0) appear. The resonance of starting 2a disappears and finally, also the acetonitrile
¼ 63.7) and free aminoethyl-diphenylphosphane (
complex 1a vanishes. Only the signal of free aminoethyl-diphenylphosphane can be recognized after 420 s (top spectrum).
d
d
d
Substitution of the ethylene backbone by a benzo unit giving 2b
alters the CO release properties significantly. Iron(II) complex 2b
also liberates carbon monoxide upon irradiation with visible light
but the CO liberation is much slower than observed for 2a and
incomplete. This observation is in agreement with earlier studies of
[(OC)2Fe(SCH2CH2NH2)2] (CORM-S1) and [(OC)2Fe(SC6H4-2-NH2)2]
(CORM-S2); in these thiolate iron(II) complexes the benzo-back-
bone also diminishes the beneficial light-induced CO release
properties.
The myoglobineCO assay is a useful tool in order to follow the
degradation of photo-CORMs. Despite the fact that McLean et al. [43]
concluded that “theclassificationofCORMsonthebasisof theMbeCO
assay must be abandoned”, this statement is mainly based on CORMs
with a ligandexchange triggering the CO release. Herewe couldverify
(Fig. 8) that for these photo-CORMs this simple and fast MbeCO assay
still represents a valuable method to determine the CO liberation. For
the determination of CO release from colored metal carbonyls it is
advantageous to choose a detection wavelength of 422 nm.

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T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
Fig. 6. Light-induced degradation of 2a (circles, CORM-P1) in a 23-mM [D3]acetonitrile
solution. During the first 20 s, CO is substituted by acetonitrile yielding trans-[Fe(NC-
Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a, squares). Thereafter, the resonance of free
aminoethyl-diphenylphosphane (triangles) is detected originating from light-triggered
decomposition of complex 1a. Competing formation of 1a from ligand substitution in
2a and light-induced decomposition of 1a implicate a rather constant concentration of
1a after approximately 60 min. Straight lines connect data points for clarity.
3. Experimental
3.1. Materials and methods
CO release was measured spectrophotometrically based on
absorbance changes upon conversion of deoxymyoglobin (Mb) to
the myoglobineCO complex (MbeCO). Horse skeletal muscle
myoglobin (Sigma) in phosphate buffered saline (PBS) was reduced
to deoxymyoglobin by addition of 0.1% sodium dithionite. Differ-
ence spectra for 100 mM Mb with and without CORMs were initially
measured in the range of 500e600 nm (Fig. 7A). However, because
of the strong self-absorbance of CORM-P2 further experiments
were performed at lower concentration (5 mM CORM and 10 mM
Mb) in a spectral range covering the Soret region (380e450 nm).
Spectra in the absence of CO and in CO-saturated solution were
recorded to calculate the maximal absorbance difference, required
to convert absorbance changes to relative CO release. The time
course of CO release from CORMs was monitored by measuring
absorbance at 422 nm every 10 s. For illumination we used a cold
light source (20 W halogen lamp, Osram GZX4) with a light guide
yielding 2 W output power placed 2 cm above the cuvette. In
addition, a monochromator based on a 150-W xenon lamp (Poly-
chrome V, TILL Photonics, Gräfelfing, Munich, Germany) was used
to generate light of 15 nm bandwidth (at 50% of the intensity) to
stimulate CORM breakdown with a defined wavelength. At the end
of the light guide placed directly above the cuvette output powers
of 12.9 mW at 420 nm, 15.2 mW at 470 nm, and 11.3 mW at 520 nm
were obtained. To test for dependence of CO release from light,
myoglobin, or dithionite, samples were preincubated for 10 min
under different conditions. Spectra in the Soret region were
recorded immediately after addition of myoglobin and sodium
Fig. 7. Light-dependent CO release. (A) Absorption spectra for 100
and CORM-P2 (right) irradiated with white light for 10 min in PBS þ 0.1% sodium
M deoxymyoglobin (Mb) (black line). Absorption spectra for CO-free
(dotted line) and CO-saturated (dashed line) Mb are shown for comparison. Also
indicated is the self-absorbance of CORM-P2, which precludes the use of this assay for
quantitative CO release determination. (B) Absorption spectra in the range of 380e
mM CORM-P1 (left)
dithionite þ 100
m
450 nm for 5
irradiation with 420-nm light. For controls Mb with 5
CO (short dashes), as well as Mb with CORMs before irradiation (grey line) are shown.
(C) Absorbance changes at 422 nm ( A422) as a function of time for the indicated
wavelengths. The arrows indicate addition of 5 M CORM-P1 (left) or CORM-P2 (right).
mM of the indicated CORMs and 10
m
M deoxymyoglobin upon 10-min
mM (long dashes) and 100
mM
D
m
Samples were illuminated 100 s later, as indicated by the horizontal bar. The instan-
taneous increase in absorbance upon addition of CORM-P2 originates from the
absorbance of the CORM. (D, E) The time courses in (C) were fit with single-
exponential functions and the resulting time constants (D) and the maximal absor-
bance changes (E) are shown for the indicated wavelengths. The bars labeled “CO”
show the absorbance change obtained when adding 10
M CO, i.e. they mark the maximal possible release of CO from the CORMs studied.
All data shown are means ꢂ sem for n ¼ 4e5.
mM Mb to a solution containing
5
m
dithionite at their final concentrations of 10
mM and 0.1%,
respectively.
0.275 mmol, 93%). 11B NMR (128.38 MHz, [D6]DMSO):
d
ꢀ1.3. 19F
NMR (188.31 MHz, [D6]DMSO):
CD3CN):
d
ꢀ151.9. 31P{1H} NMR (161.95 MHz,
3.2. Synthesis of trans-[Fe(NC-Me)2(H2NCH2CH2PPh2)2] [BF4]2 (1a)
d
63.7. IR: 3330 (w); 2163 (s); 1604(s); 1312 (w); 873 (vs);
695 (w); 646(s); 607 (vs); 511(w); 495 (m); 480 (m).
This complex was prepared according to a literature procedure
[44]. To a stirring solution of [Fe(H2O)6][BF4]2 (100 mg, 0.296 mmol)
and CH3CN (5 ml) was added 2-(diphenylphosphino)ethylamine
(136 mg, 0.593 mmol) in CH3CN (1 ml) dropwise. The mixture
turned purple immediately and was stirred for 30 min. Then the
solvent was removed in vacuo and the residue was dissolved in 1 ml
of CH2Cl2. The addition of 10 ml of diethyl ether yielded a purple
solid, which was isolated by filtration and dried in vacuum (212 mg,
3.3. Synthesis of [Fe(CO)(NC-Me)(H2NCH2CH2PPh2)2] [BF4]2 (2a,
CORM-P1)
For the synthesis of 2a the isolation of complex 1a is not
required. Therefore, aminoethyl-diphenylphosphane (136 mg,
0.593 mmol) was added dropwise to a stirred solution of [Fe(H2O)6]

T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
69
510 (m); 475 (w). Elemental analysis (C31H35B2F8FeN3OP2,
757.03 g molꢀ1): calc.: C 49.18; H 4.66; N 5.55; found: C 48.93; H
4.35; N 5.35.
3.4. Synthesis of trans-[Fe(NC-Me)2(H2NC6H4-2-PPh2)2] [BF4]2 (1b)
For the synthesis of trans-[Fe(NC-Me)2(H2NC6H4-2-PPh2)2]
[BF4]2 (1b) 2-(diphenylphosphino)aniline (164 mg, 0.593 mmol) in
CH3CN (1 ml), which was prepared as described previously [48],
was added dropwise to a stirred solution of [Fe(H2O)6][BF4]2
(100 mg, 0.296 mmol) in CH3CN (5 ml). The reaction mixture
turned purple immediately and was stirred for an additional hour.
Thereafter, the solvent was removed in vacuo and the residue dis-
solved in 1 ml of CH2Cl2. The addition of 10 ml of diethyl ether
afforded a purple solid, which was isolated by filtration and dried in
vacuum (241 mg, 0.278 mmol, 94%). Recrystallization of a small
portion of this solid from CH3CN/Et2O by using the slow diffusion
method gave violet single crystals suitable for X-ray diffraction
experiments. Physical data: M.p.: 211e213 ꢁC. NMR: 1H NMR
Fig. 8. CO release is not dependent on dithionite. CO release by CORM-P1 and CORM-
P2 during 10-min preincubation without Mb in the presence (“þ”) or absence (“ꢀ”) of
dithionite, with (open bars) or without (grey bars) illumination with white light.
Spectra were taken right after preincubation and subsequent mixing of the samples
with Mb/dithionite. Experimental conditions as in Fig. 7A. Data are means ꢂ sem for
n ¼ 4e5.
[BF4]2 (100 mg, 0.296 mmol) in 5 ml of CH3CN yielding a purple
reaction mixture. After removal of all volatiles in vacuo and disso-
lution of the residue with 10 ml of CH2Cl2, this solution was treated
with gaseous CO for 16 h. During this time a yellow suspension
(400.08 MHz, CD3CN):
8H), 7.53 (t, 8H), 7.73 (m, 2H), 7.84 (d, 2H). 13C{1H} NMR
(150.90 MHz, CD3CN): 3.85 (t), 128.00 (t), 128.77 (t), 129.96 (t),
130.85 (m), 131.67 (s), 132.96 (m), 133.36 (t), 133.66 (s), 134.85 (s),
137.95(s), 151.37 (t). 31P{1H} NMR (161.95 MHz, CD3CN):
68.1 (s).
d 1.96 (s, 6H), 5.94 (s, 4H), 6.87 (t, 8H), 7.33 (t,
d
formed. CORM-P1 was isolated as
a yellow solid (190 mg,
0.251 mmol, 91%) by filtration and the filter cake dried in vacuum.
Recrystallization of a small portion of this powder from a mixture of
CH2Cl2 and DMF/Et2O by using the slow diffusion method yielded
dark orange single crystals suitable for X-ray diffraction experi-
ments. Physical data: M.p.: 202e205 ꢁC. NMR: 1H NMR
d
MS-ESI: 305 [Fe(PPh2C6H4-2-NH2)2]2þ (100); 277 [(PPh2C6H4-2-
NH2)2]2þ (90). IR: 3280 (s); 2207(s); 1607(w); 1566 (m); 1478 (s);
1436 (s); 1094 (vs); 1047 (vs); 820 (w); 756 (s); 694 (s); 575 (m);
520 (vs); 485 (m); 471 (vs); 458 (s); 415 (m); 929 (w); 816 (w); 747
(s); 697 (s); 594 (m); 529 (s); 510 (m); 475 (w). Elemental analysis
(C44H44B2F8FeN6P2, 948.26 g molꢀ1): calc.: C 55.73; H 4.68; N 8.86;
found: C 55.62; H 4.68; N 8.76.
(600.13 MHz, [D6]DMSO):
d 2.18 (s, 3H), 2.92 (broad, 4H), 4.07
(broad, 4H), 4.8 (broad, 4H), 7.04 (d, 8H), 7.36 (t, 8H), 7.57 (t, 4H). 11B
NMR (128.38 MHz, [D6]DMSO):
[D6]DMSO):
d
ꢀ1.3. 13C{1H} NMR (150.90 MHz,
d: 4.22 (broad), 31.02 (m), 41.46 (broad), 129.22 (d),
131.47 (d), 132.11 (d),133.30 (broad), 160.63 (broad), 215.20 (broad).
19F NMR (188.31 MHz, [D6]DMSO):
(161.95 MHz, [D6]DMSO):
d
ꢀ148.7. 31P{1H} NMR
3.5. Synthesis of [Fe(CO)(NC-Me)(H2NC6H4-2-PPh2)2] [BF4]2 (2b,
CORM-P2)
d
61.7 (s). MS-ESI: 257 [Fe(Ph2PC2
H4NH2)2]2þ (100); 229 [(Ph2PC2H4NH2)2]2þ (16); 185 [(Ph2P)2]2þ
(37). IR [Nujol, KBr windows]: 3555 (w); 3307(s); 3269 (s);
3175(m); 3065 (m); 2922 (vs); 2854 (vs); 2303 (m); 1985(vs); 1597
(m); 1460 (s); 1436 (s); 1376(m); 1256 (m); 1094(vs); 1042 (vs);
1011 (s); 996 (s); 929 (w); 816 (w); 747 (s); 697 (s); 594 (m); 529 (s);
For the synthesis of CORM-P2, 1b (241 mg, 0.278 mmol) was
dissolved in 10 ml of CH2Cl2 and stirred under a CO atmosphere for
16 h. The resulting orangeeyellow solution was evaporated to
dryness to leave a yellow powder, which was washed with diethyl
Table 1
Crystal data and refinement details for the X-ray structure determinations of the compounds 1e2b.
Compound
1b
2a
2b
Formula
fw (g molꢀ1
T/ꢁC
[C40H38FeN4P2]2þ, 2[BF4]ꢀ, 2(C2H3N)
948.26
[C31H35FeN3OP2]2þ, 2[BF4]ꢀ
757.03
[C39H35FeN3OP2]2þ, 2[BF4]ꢀ, CH2Cl2
938.04
)
ꢀ140(2)
ꢀ140(2)
ꢀ140(2)
Monoclinic
P 21/n
13.4610(3)
23.0816(4)
13.5722(3)
90
100.427(1)
90
4147.26(15)
4
Crystal system
Space group
Triclinic
Triclinic
ꢀ
P ı
ꢀ
P ı
ꢁ
a/A
11.3798(2)
12.4188(2)
17.8206(3)
74.966(1)
85.178(1)
68.964(1)
2270.05(7)
2
9.8383(1)
11.7972(3)
14.4190(3)
98.138(1)
92.800(1)
95.042(1)
1647.08(6)
2
ꢁ
b/A
ꢁ
c/A
ꢁ
ꢁ
a
/
b
/
ꢁ
g
/
3
ꢁ
V/A
Z
r
(g cmꢀ3
(cmꢀ1
)
1.387
4.75
1.526
6.32
1.502
6.43
m
)
Measured data
14,722
9907
23,878
Data with I > 2
s(I)
9507
6869
7781
Unique data (Rint
)
10,305/0.0197
0.0986
0.0390
7443/0.0151
0.0828
0.0380
9451/0.0390
0.1347
0.0547
wR2 (all data, on F2)a
R1 (I > 2
s
(I))a
sb
1.057
1.070
1.054
ꢁꢀ3
Res. dens./e A
CCDC No.
0.589/ꢀ0.469
0.822/ꢀ0.441
0.843/ꢀ0.979
905061
905062
905063
P
P
P
P
Definition of the R indices: R1 ¼ ð jjFoj ꢀ jFcjjÞ= jFoj; wR2 ¼ f ½wðFo2 ꢀ Fc2Þ2ꢃ= ½wðFo2Þ2ꢃg1=2 with wꢀ1
¼
s2ðFo2Þ þ ðaPÞ2 þ bP; P ¼ ½2Fc2 þ MaxðFo2Þꢃ=3.
a
P
s ¼ f ½wðFo2 ꢀ Fc2Þ2ꢃ=ðNo ꢀ NpÞg1=2
.
b

70
T.M.A. Jazzazi et al. / Journal of Organometallic Chemistry 733 (2013) 63e70
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ether. CORM-P2 was isolated as a yellow powder (218 mg,
0.256 mmol, 92%). Recrystallization of a small portion of this
powder from a solvent mixture of CH2Cl2 and DMF/Et2O using the
slow diffusion method yielded light orange single crystals suitable
for X-ray diffraction experiments. Physical data: M.p.: 216e218 ꢁC.
1H NMR (400.08 MHz, [D6]DMSO):
d
2.04 (s, 3H, CH3), 5.71 (s, 4H,
H2N), 6.92 (broad 8H), 7.39 (dd, 8H), 7.57 (broad, 8H), 7.76 (broad,
2H), 7.94 (broad, 2H). 13C{1H} NMR (100.60 MHz, [D6]DMSO):
3.19
d
(s), 126.36 (m), 127.20 (m), 128.10 (t), 129.67 (s), 130.86 (d), 131.32
(m), 132.50 (d), 132.88 (d), 133.11 (m), 134.99 (d), 162.12 (broad),
206.35 (broad). 31P{1H} (161.95 MHz, CD3CN):
d 58.2 (s). MS-ESI:
305 [Fe(PPh2C6H4-2-NH2)2]2þ (100); 277 [(PPh2C6H4-2-NH2)2]2þ
(90); 201 [(PPh2-NH2)2]2þ (7). IR: 3280 (s); 2290 (m); 2001 (vs);
1590 (w); 1480(s); 1437 (s); 1096 (w); 1056 (vs); 751 (s); 696 (vs);
578 (m); 516 (vs); 464 (vs); 448 (m); 411 (w). Elemental analysis
(C40H37B2Cl2F8FeN3OP2, 938.04 g molꢀ1): calc.: C 51.22; H 3.98; N
4.48; found: C 50.76; H 4.03; N 4.65.
3.6. Structure determinations
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Acknowledgments
We thank Robert Braun for technical assistance, and the German
Research Foundation (DFG, Bonn-Bad Godesberg/Germany) for
generous financial support within the collaborative research group
FOR 1738. T.M.A. Jazzazi is also grateful to the Carl Zeiss Foundation/
Stuttgart (Germany) for a Ph.D. grant. Infrastructure of our institute
was provided by the EU (European Fonds for Regional Develop-
ment, EFRE) and the Friedrich Schiller University Jena.
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Appendix A. Supporting Information available
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Crystallographic data (excluding structure factors) has been
deposited with the Cambridge Crystallographic Data Centre as
supplementary publication CCDC-905061 for 1b, CCDC-905062 for
2a, and CCDC-905063 for 2b. Copies of the data can be obtained free
of charge on application to CCDC, 12 Union Road, Cambridge CB2
1EZ, UK [E-mail: deposit@ccdc.cam.ac.uk].
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Appendix B. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.jorganchem.2013.02.015.
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