Bioorganometallic Tagging of N-acetylhistamine with Fe(CO)₃ Unit

Turkish Journal of Chemistry
Volume 49
Number 6
Article 12
12-25-2025
Bioorganometallic tagging of N-acetylhistamine with an Fe(CO)₃
Fe(CO)
unit: synthesis, X-ray structure, and protonation behavior
SALAH MERNIZ
LOUIZA HIMED
ROFIA DJERRI
BELKIS AKACHAT
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Recommended Citation
MERNIZ, S, HIMED, L, DJERRI, R, & AKACHAT, B (2025). Bioorganometallic tagging of N-acetylhistamine
with an Fe(CO)₃ unit: synthesis, X-ray structure, and protonation behavior. Turkish Journal of Chemistry 49
(6): 821-830. https://doi.org/10.55730/1300-0527.3773
This work is licensed under a Creative Commons Attribution 4.0 International License.
This Research Article is brought to you for free and open access by TÜBİTAK Academic Journals. It has been
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Turkish Journal of Chemistry
Turk J Chem
(2025) 49: 821-830
© TÜBİTAK
doi:10.55730/1300-0527.3773
http://journals.tubitak.gov.tr/chem/
Research Article
Bioorganometallic tagging of N-acetylhistamine with an Fe(CO)₃ unit: synthesis,
X-ray structure, and protonation behavior
1,*
2
2
2
Salah MERNIZ , Louiza HIMED , Rofia DJERRI , Belkis AKACHAT 
1
Institute of Industrial Hygiene and Safety, University of Batna 2, Batna, Algeria
2
Division of Biotechnology and Food Quality (BIOQUAL-INATAA), Department of Food Biotechnology, Institute of Nutrition,
Food and Agro-Food Technologies, University of Freres Mentouri Constantine 1, Constantine, Algeria
Received: 27.08.2025
Accepted/Published Online: 28.10.2025
Final Version: 25.12.2025
Abstract: This study highlights the potential of organometallic carbonyl complexes as selective markers for biomolecules, enabling
sensitive infrared (IR) detection. The regio- and stereoselective coupling of N-acetylhistamine, a histidine analogue, with the
precursor complex 1 [Fe(CO)₃(1,4-η⁵-N-pyridiniocyclohexa-1,3-diene)] BF₄ affords the labeled complex 3 [Fe(CO)₃(1,4-η⁵-Nacetylhistaminocyclohexa-1,3-diene)]. X-ray diffraction (XRD) confirms the exo stereochemistry and reveals a rigid, well-defined
architecture. IR and 1H nuclear magnetic resonance spectroscopic studies combined with IR-monitored acid–base titration demonstrate
the complex’s stability in aqueous media between pH 5 and 8, alongside a modest increase in basicity relative to the free ligand. These
findings establish the Fe(CO)₃ moiety as a robust platform for selective labeling of peptides and proteins, paving the way for targeted
applications in bioorganometallic chemistry and spectroscopic imaging.
Key words: Iron carbonyl complexes, molecular labeling, acid–base analysis, Fourier transform infrared (FTIR) spectroscopy, singlecrystal X-ray diffraction study
1. Introduction
The selective covalent incorporation of transition-metal complexes into biomolecules has emerged as a powerful strategy
for probing and manipulating biological systems at the molecular level [1,2]. Among these, metal carbonyl complexes are
distinguished by their unique infrared (IR) signatures, characterized by intense and well-defined C≡O stretching bands,
which make them exceptional spectroscopic probes for elucidating biomolecular microenvironments [3].
Pioneering work by Jaouen and coworkers demonstrated the covalent tagging of estradiol with bimetallic molybdenum
complexes exhibiting high affinity for the cytosolic estradiol receptor isolated from sheep uterus. The resulting covalent
linkage can be readily monitored by Fourier transform infrared (FTIR) spectroscopy through characteristic carbonyl
vibrations, providing direct insight into ligand–receptor interactions [4,5]. This methodology has since been extended
to other complexes, notably Cr(CO)₃-based systems, confirming the robustness and generality of this approach [6].
Beyond conventional spectroscopy, this concept has inspired the development of innovative immunoassays leveraging
the IR properties of carbonyl complexes, giving rise to the so-called “carbonyl metallo immuno assay.” This technique
offers a nonradioactive, highly sensitive alternative for trace biomarker quantification by exploiting the spectroscopic
detection of carbonyl signals [7]. The versatility of these organometallic probes largely relies on synthetic strategies
enabling their covalent introduction into complex macromolecules. In particular, the regio- and stereoselective addition
of N- and S-nucleophiles to the cationic intermediate [Fe(CO)₃(1,5-η⁵-C₆H₇)]⁺ affords exclusively exoconfigured C₅–Y
(Y = N, S) bonds, ensuring remarkable complex stability in aqueous media and compatibility with delicate biological
systems [8]. Recent advances in molecular imaging have further expanded the applications of these carbonyl complexes
to subcellular visualization. Infrared photothermal induced resonance spectroscopy now enables nanoscale mapping of
metal carbonyl complexes within living cells without fluorescent labeling and offers unprecedented spatial resolution [9].
Such developments open new avenues for in situ studies of metallobiomolecular interactions in complex environments.
Herein, we report the synthesis, reactivity, and single-crystal X-ray diffraction (SCXRD) structural characterization of
a novel complex, 3 [Fe(CO)₃(1,4-η⁵-N-acetylhistaminocyclohexa-1,3-diene)] (Figure 1). This complex was obtained via
covalent coupling between the cationic [Fe(CO)₃(1,4-η⁵-N-pyridiniocyclohexa-1,3-diene)] BF₄ 1 and the biologically
* Correspondence: [email protected]
This work is licensed under a Creative Commons Attribution 4.0 International License.
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MERNIZ et al. / Turk J Chem
Figure 1. Schematic representation of the coupling process
between N-acetylhistamine and the labeling complex 1.
relevant N-acetyl histamine, an analogue of histidine. N-acetyl histamine was selected as a model amine-bearing substrate
due to its dual relevance: it features a primary amine group suitable for nucleophilic substitution, and it is a biologically
active derivative of histamine. This choice provides a representative system for exploring the covalent tagging of bioactive
amines using organometallic Fe(CO)₃ fragments.
We used FTIR spectroscopy to monitor the acid–base behavior of the labeled complex 3, bearing an N-acetyl histamine
ligand (HNhist) sensitive to solution pH.
2. Experimental section
2.1. Instrumentation and analytical methods
IR spectra were recorded on a Bruker VERTEX 70v FTIR spectrometer equipped with an attenuated total reflectance
accessory, ensuring high sensitivity and reproducibility. Spectra were collected over 4000–400 cm⁻¹ with a resolution of
4 cm⁻¹. When needed, KBr pellet preparations were also used. Absorption bands are reported in wavenumbers (cm⁻¹).
1
H Nuclear Magnetic Resonance (1H NMR) spectra were recorded on a Bruker Avance III 400 MHz spectrometer at
room temperature. Chemical shifts (δ) are reported in ppm relative to tetramethylsilane (TMS) as the internal standard.
Coupling constants (J) are reported in hertz (Hz). SCXRD data were collected on a Bruker D8 Venture X-ray diffractometer
equipped with a Photon 100 CMOS area detector, using monochromated Mo Kα radiation (λ = 0.71073 Å). Data collection,
cell refinement, and data reduction were performed using the APEX4 Software package [10]. The structure of complex 3
was solved by direct methods with SHELXT 2018/2 [11] and refined on F² by full-matrix least-squares techniques using
SHELXL-2018/3 [12].
All hydrogen atoms, including the amide hydrogen atom (H3), were placed in geometrically calculated positions
and refined using standard riding models with isotropic displacement parameters. Nonhydrogen atoms were refined
anisotropically. Structural illustrations and packing diagrams were generated using ORTEP-3 [13] and Mercury 4.3.1 [14].
Crystallographic data for compound 3 have been deposited at the Cambridge Crystallographic Data Centre (CCDC)
under deposition number 2421142 (see Supplementary information).
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2.2. Synthetic procedures
All reactions were carried out under an inert argon atmosphere using standard Schlenk line techniques. Anhydrous
solvents were rigorously dried and freshly distilled prior to use according to established procedures. The progress of the
reactions was monitored by thin-layer chromatography and IR spectroscopy. Products were purified by flash column
chromatography on silica gel, followed, when necessary, by recrystallization from appropriate solvents to ensure high
purity and crystallinity. Final compounds were fully characterized by spectroscopic (IR, NMR) and analytical methods,
providing unambiguous structural confirmation.
2.2.1. Synthesis of complex 1 [Fe(CO)₃(1,4-η⁵-N-pyridiniocyclohexa-1,3-diene)] BF₄
Complex 1, [C₁₁H₉NFe(CO)₃][BF₄], was synthesized following a modified literature procedure for pyridiniocyclohexadienyl iron tricarbonyl derivatives [15–17]. The compound was obtained in 70% yield after column
chromatography and recrystallization.
IR (KBr, cm⁻¹): ν(C≡O) = 2056, 1979; ν(B–F) = 1118, 1084, 1030.
¹H NMR (250 MHz, CD₃COCD₃, δ in ppm vs. TMS): 2.00 (m, 1H), 2.90 (m, 1H), 3.35 (m, 2H), 5.55 (m, 1H), 6.00 (m,
2H), 8.18 (t, J = 7.7 Hz, 2H), 8.62 (t, J = 7.8 Hz, 1H), 9.10 (d, J = 5.6 Hz, 2H).
2.2.2. Synthesis of complex 2 [Fe(CO)₃(1,4-η⁵-N-acetylhistaminocyclohexa-1,3-diene)] BF₄
Complex 2 was prepared by reacting complex 1 ([C₁₁H₉NFe(CO)₃][BF₄], 0.77 g, 2.0 mmol) with N-acetylhistamine (0.306
g, 2.0 mmol) in acetone (20 mL) under an inert argon atmosphere at room temperature. After 3 h of stirring, the reaction
mixture was filtered and evaporated under reduced pressure. The residue was dissolved in CH₂Cl₂, washed with water,
dried over MgSO₄, and purified by flash chromatography on silica gel (diethyl ether/petroleum ether, 15:85 v/v), affording
a pale yellow powder—yield: 75% (0.688 g).
IR (KBr, cm⁻¹): ν(N–H) = 3419, 3260; ν(C–H) = 3019, 2855; ν(C≡O) = 2053, 1982; ν (amide I) = 1642; ν(amide II) =
1541; ν(B–F) = 1124, 1084, 1038.
¹H NMR (250 MHz, CD₃COCD₃, δ in ppm vs. TMS): 1.87 (m, 4H, CH₃ and H6), 2.73 (ddd, 1H, J = 15.6, 11.1, 3.7 Hz,
H6′), 3.01 (t, 2H, J = 6.3 Hz, CH₂–imidazole), 3.12 (t, 1H, J = 4.6 Hz, H1), 3.22 (m, 1H, H4), 3.46 (m, 2H, CH₂–NH), 5.19
(dt, 1H, J = 11.2, 2.9 Hz, H5′), 5.87 (t, 1H, J = 5.2 Hz, H3), 5.96 (t, 1H, J = 4.8 Hz, H2), 7.54 (s, 1H, imidazole Hb), 8.99 (s,
1H, imidazole Ha).
The elemental composition of complex 2 was confirmed by carbon–hydrogen–nitrogen (CHN) elemental analysis.
The experimentally determined values (C 40.96%, H 4.15%, N 8.91%) are consistent with the theoretical values calculated
for C₁₆H₁₈BF₄FeN₃O₄ (C 41.86%, H 3.92%, N 9.16%), supporting the proposed molecular formula and confirming the
compound’s homogeneity and purity.
Complex 2 was isolated as a tetrafluoroborate salt, resulting from the reaction of the BF₄-containing cationic precursor
1 with N-acetylhistamine. The formation of the protonated ammonium (histaminio) species occurs spontaneously
during the substitution process, driven by internal proton transfer and the electron-deficient nature of the organometallic
fragment. This salt form is supported by the ¹H NMR spectrum, where the chemical shifts of the NH and imidazole
protons indicate protonation, and confirmed by the elemental analysis.
No external acid was added during the synthesis. The salt form of complex 2 results from an internal proton transfer,
allowing the reaction to proceed under mild, near-neutral, and aprotic conditions that are potentially compatible with
sensitive biomolecular systems.
2.2.3. Synthesis of complex 3 [Fe(CO)₃(1,4-η⁵-N-acetylhistaminocyclohexa-1,3-diene)]
Complex 3 was obtained by treating a solution of the tetrafluoroborate precursor 2 (0.918 g, 2.0 mmol) in acetonitrile (20
mL) with equimolar triethylamine (0.278 mL, 2.0 mmol) under an inert atmosphere. After stirring for 30 min at ambient
temperature, the reaction mixture was filtered and concentrated under reduced pressure. The resulting solid was triturated
with toluene to efficiently remove pyridine byproducts, then dried under vacuum. Recrystallization from an acetone/
pentane mixture furnished complex 3 as a pale yellow powder in 41% yield (0.302 g).
IR (KBr, cm⁻¹): ν(N–H) = 3427, 3255; ν(C–H) = 3054, 2927, 2854; ν(C≡O) = 2047, 1980, 1943; ν(amide I) = 1672;
ν(amide II) = 1566.
¹H NMR (250 MHz, CD₃COCD₃, δ ppm vs. TMS): 1.71 (dt, 1H, J = 11.0, 3.0 Hz, H6), 1.83 (s, 3H, CH₃ amide), 2.57
(m, 3H, H6′ and βCH₂), 3.14 (t, 1H, J = 4.8 Hz, H1), 3.24 (t, 1H, J = 4.8 Hz, H4), 3.36 (m, 2H, CH₂–NH), 4.80 (dt, 1H, J =
10.9, 3.0 Hz, H5′), 5.76 (t, 1H, J = 4.6 Hz, H3), 5.90 (t, 1H, J = 4.6 Hz, H2), 6.85 (s, 1H, Hb), 7.50 (s, 1H, Ha).
CHN elemental analysis was also conducted for complex 3 to assess its bulk purity. The observed values (C 51.60%, H
4.86%, N 11.14%) are in close agreement with the calculated values for C₁₆H₁₇FeN₃O₄ (C 51.78%, H 4.58%, N 11.32%).
These data confirm the high purity and molecular integrity of the neutral complex.
This transformation highlights a straightforward and efficient protocol for accessing the neutral iron tricarbonyl
complex bearing an N-acetylhistaminio ligand, with facile removal of byproducts and a moderate isolated yield.
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2.3. Acid–base titration of complex 3 [Fe(CO)₃(1,4-η⁵-N-acetylhistaminocyclohexa-1,3-diene)]
The acid–base behavior of complex 3 was investigated using FTIR spectroscopy. Aqueous solutions of the complex (5 µL,
0.001 M) were prepared and deposited onto 3 mm KBr pellets, then air-dried prior to analysis. The titrations were carried
out in phosphate buffer (0.2 M) at pH 5.3, 7.0, 7.6, and 7.9, and in carbonate buffer (0.1 M) at pH 8.3, 9.0, and 10.1. FTIR
spectra were recorded in the range 1800–2100 cm⁻¹ to monitor the carbonyl stretching vibrations as a function of pH. These
experiments enabled the assessment of the pH-dependent protonation–deprotonation behavior of the N-acetylhistamine
ligand coordinated to the Fe(CO)₃ unit.
3. Results and discussion
The targeted labeling of N-acetylhistamine was successfully achieved through regio- and stereoselective coupling with
the organometallic precursor [Fe(CO)₃(η⁵-C₆H₇N⁺)]BF₄ 1, a well-known source of the electrophilic [Fe(CO)₃(η⁵-C₆H₇)]⁺
moiety. This reaction proceeds cleanly to furnish the cationic complex [Fe(CO)₃(η⁵-C₆H₇-N-acetylhistaminio)]BF₄ 2 in
75% isolated yield. Notably, the transformation highlights the remarkable compatibility of the η⁵-pyridinium platform
with nucleophilic biorelevant amines, enabling a straightforward entry to labeled histaminic derivatives. Subsequent
treatment of complex 2 with triethylamine induces selective deprotonation of the histaminio moiety, affording the neutral
complex [Fe(CO)₃(η⁵-C₆H₇-N-acetylhistaminio)] 3 in 41% yield. This step further demonstrates the modular reactivity
of the iron tricarbonyl framework and its suitability for tuning electronic and structural properties via simple acid–base
manipulation (Figure 1).
3.1. Spectroscopic studies
3.1.1. Infrared spectroscopy
The IR spectra of complexes 2 and 3, recorded as KBr pellets, exhibit two to three strong absorption bands in the 1900–
2100 cm⁻¹ region, characteristic of terminal carbonyl stretching vibrations (νC≡O) in iron tricarbonyl complexes. These
bands are highly sensitive to the electron density at the metal center and the nature of the coordinated ligands [18‒19].
Complex 3 displays a noticeable shift of the ν(CO) bands to lower wavenumbers compared to complex 2, indicating
a stronger electron-donating effect from the N-acetylhistamine ligand relative to pyridine. The presence of multiple
absorptions around 1100 cm⁻¹ in the spectrum of 2 is consistent with a salt form, whereas 3 appears to be neutral [20].
Relative to the starting material 1, the ν(CO) bands of 3 are red-shifted by 3 to 9 cm⁻¹. This shift is attributed to enhanced
metal-to-CO back-donation, resulting from increased σ-donor strength of the organic ligand [21‒22]. The stronger electron
donation from N-acetylhistamine increases the electron density on the Fe center, thereby reinforcing backbonding into the
π* orbitals of the carbonyl ligands and weakening the C≡O bond, which lowers the vibrational frequency. Recent studies
have confirmed that such IR shifts serve as sensitive probes for electronic interactions in Fe–CO systems, especially when
biologically relevant N-donor ligands are involved [23]. Moreover, DFT-assisted IR analysis provides reliable insight into
the electronic structure and ligand effects in carbonyl complexes [24].
3.1.2. ¹H NMR spectroscopy
The ¹H NMR spectra of complexes 2 (protonated) and 3 (neutral) display diagnostic features reflecting their electronic
environments. A general downfield shift is observed in the signals of complex 2, consistent with the increased electronwithdrawing character of the protonated metal center, which induces deshielding of the ligand protons.
Both complexes exhibit small coupling constants between protons H5′ (equatorial) and H6 (axial), with values of 2.9
Hz and 3.0 Hz, respectively. These weak couplings suggest the formation of a single stereoisomer. They are consistent
with an exo configuration of the N-acetylhistamine ligand at the C5 position of the cyclohexadienyl ring, as commonly
observed in related iron tricarbonyl systems [25]. In complex 3, the olefinic protons of the cyclohexadienyl moiety (H1,
H4, and H2, H3) appear as four well-defined triplets, with coupling constants of 4.8 Hz and 4.6 Hz. This pattern indicates
that the coordination of the N-acetylhistamine ligand does not significantly perturb the π-electronic structure of the
organometallic fragment. The retention of signal symmetry and modest coupling values supports the delocalized nature of
the cyclohexadienyl system upon ligand coordination [26].
3.2. Crystallographic study
A detailed crystallographic analysis of complex 3 is presented in Table 1, including data collection and refinement
parameters. Selected bond lengths and bond angles are compiled in Tables 2 and 3, respectively. A perspective view of the
molecular structure of complex 3, highlighting the coordination environment and atom labeling, is depicted in Figure 2.
SCXRD analysis of complex 3 confirms an exo configuration, where the N-acetylhistamine ligand is covalently bound
to the cyclohexadienyl ring at C(5) via a C–N bond measuring 1.477(4) Å. The iron center adopts a distorted octahedral
geometry, coordinated by three carbonyl ligands and the η⁴-cyclohexa-1,3-diene π-system. The Fe atom is slightly displaced
out of the diene plane, at 0.861 and 1.627 Å from two defined planes, indicative of steric influence from the coordinated
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MERNIZ et al. / Turk J Chem
ligand. The cyclohexadienyl fragment exhibits significant puckering, with a dihedral angle of 39.27° between planes C(1)–
C(2)–C(3)–C(4) and C(1)–C(6)–C(5)–C(4), larger than in unsubstituted analogues [27,28]. The Fe(CO)₃ unit lacks ideal
C₃ᵥ symmetry, showing two C–Fe–C angles near 100°, while the third angle (cis to the C(2)–C(3) bond) is 92.83(17)°,
reflecting perturbation caused by the N-acetylhistamine ligand. The Fe–C(11) bond is slightly shorter at 1.757(4) Å
compared to the average Fe–C bonds (1.785 Å), consistent with enhanced metal-to-CO back-donation facilitated by the
ligand’s electron-donating character [29]. Fe–C–O angles range from 176.1(4)° to 178.5(4)°, showing minor bending
attributed to intermolecular hydrogen bonding. Complex 3 forms three hydrogen bonds (N–H···N and C–H···O) (Table
4) that stabilize a three-dimensional hydrogen-bonded network. The molecular packing consists of parallel dimeric layers
along the a-axis, as projected on the (0 ī 1) plane (Figure 3), comparable to other biologically functionalized Fe tricarbonyl
complexes [30].
Table 1. Crystallographic parameters for complex 3.
Empirical formula
Crystal system
Space group
a (Å)
b (Å)
c (Å)
α (°)
β (°)
γ (°)
V (A3)
Z
dx
F (000)
Reflection read
Unique reflections (R int)
Data/restraints/parameters
Largest diff. peak / hole (eÅ⁻³)
wR2
R1 (all data)
R1 (observed data)
Goof
C16 H17 Fe N3 O4
Triclinic
P1
7.318(5)
10.611(5)
12.402(5)
103.947(5)
106.834(5)
100.363(5)
861.7(8)
2
1.431
384
18722
2913 (0.0463)
2913/0/217
+0.309/–0.317
0.1204
0.0553
0.0389
1.0930
Table 2. Selected interatomic distances (Å) in the structure of complex 3.
Fe(1)–C(11) 1.757(4)
N(2)–C(9) 1.378(4)
Fe(1)–C(12) 1.780(4)
N(3)–C(15) 1.338(4)
Fe(1)–C(10) 1.791(4)
N(3)–C(14) 1.447(4)
Fe(1)–C(2) 2.040(4)
C(4)–C(3) 1.418(4)
Fe(1)–C(3) 2.044(3)
C(4)–C(5) 1.510(4)
Fe(1)–C(1) 2.097(3)
C(9)–C(8) 1.349(4)
Fe(1)–C(4) 2.103(3)
C(9)–C(13) 1.489(4)
N(1)–C(7) 1.343(4)
C(15)–C(16) 1.503(5)
N(1)–C(8) 1.368(4)
O(1)–C(10) 1.138(5)
N(1)–C(5) 1.477(4)
O(2)–C(11) 1.148(4)
N(2)–C(7) 1.317(4)
O(3)–C(12) 1.150(4)
C(1)–C(2) 1.410(5)
C(14)–C(13) 1.507(4)
C(1)–C(6) 1.501(4)
O(4)–C(15) 1.225(4)
C(2)–C(3) 1.394(5)
C(6)–C(5) 1.532(4)
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Table 3. Selected bond angles (°) in the structure of complex 3.
C(11)–Fe(1)–C(12) 100.41(18)
C(11)–Fe(1)–C(10) 101.85(19)
C(12)–Fe(1)–C(10) 92.83(17)
C(11)–Fe(1)–C(2) 130.95(17)
C(12)–Fe(1)–C(2) 91.24(16)
C(10)–Fe(1)–C(2) 125.16(17)
C(11)–Fe(1)–C(3) 134.76(16)
C(12)–Fe(1)–C(3) 120.77(16)
C(10)–Fe(1)–C(3) 94.48(17)
C(2)–Fe(1)–C(3) 39.93(14)
C(11)–Fe(1)–C(1) 91.70(16)
C(12)–Fe(1)–C(1) 93.09(15)
C(10)–Fe(1)–C(1) 163.98(16)
C(2)–Fe(1)–C(1) 39.82(13)
C(3)–Fe(1)–C(1) 69.74(14)
C(11)–Fe(1)–C(4) 96.70(15)
C(12)–Fe(1)–C(4) 160.31(16)
C(10)–Fe(1)–C(4) 93.14(15)
C(2)–Fe(1)–C(4) 70.05(13)
C(3)–Fe(1)–C(4) 39.98(12)
C(1)–Fe(1)–C(4) 76.65(13)
C(7)–N(1)–C(8) 105.8(3)
C(7)–N(1)–C(5)
126.2(3)
C(8)–N(1)–C(5) 127.4(3)
C(7)–N(2)–C(9)
104.8(2)
C(15)–N(3)–C(14) 122.2(3)
C(3)–C(4)–C(5)
120.2(3)
C(3)–C(4)–Fe(1) 67.78(18)
C(5)–C(4)–Fe(1) 108.31(19)
C(8)–C(9)–N(2)
C(8)–C(9)–C(13) 129.5(3)
N(2)–C(9)–C(13) 121.1(3)
C(9)–C(8)–N(1)
C(2)–C(1)–C(6)
107.4(3)
109.3(3)
119.9(3)
C(2)–C(1)–Fe(1) 67.9(2)
C(6)–C(1)–Fe(1) 109.8(2)
C(3)–C(2)–C(1)
115.2(3)
C(3)–C(2)–Fe(1) 70.19(19)
C(1)–C(2)–Fe(1) 72.3(2)
O(4)–C(15)–N(3) 121.6(3)
O(4)–C(15)–C(16) 122.6(3)
N(3)–C(15)–C(16) 115.8(3)
C(2)–C(3)–C(4)
C(2)–C(3)–Fe(1) 69.9(2)
115.4(3)
C(4)–C(3)–Fe(1) 72.25(19)
C(1)–C(6)–C(5)
O(2)–C(11)–Fe(1) 176.1(4)
O(3)–C(12)–Fe(1) 177.7(4)
O(1)–C(10)–Fe(1) 178.5(4)
N(1)–C(5)–C(4)
111.4(2)
N(1)–C(5)–C(6)
C(4)–C(5)–C(6)
110.9(2)
111.2(3)
N(3)–C(14)–C(13) 112.4(3)
N(2)–C(7)–N(1)
110.7(3)
C(9)–C(13)–C(14) 113.9(3)
112.7(3)
Figure 2. ORTEP representation of the molecular structure of complex 3, with
thermal ellipsoids shown at the 50% probability level. Only heteroatoms are
labeled for clarity. Hydrogen atoms are omitted, except for the amide proton.
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MERNIZ et al. / Turk J Chem
Table 4. Intermolecular hydrogen-bonding interactions observed in complex 3.
D–H…A
D–H (Å)
H...A (Å)
D…A (Å)
D–H…A (°)
N(3)–H(2A)…..N(2)
0.859
2.128
2.985
174.31
C(4)–H(4)….O(1)ii
0.979
2.618
3.511
151.67
C(3)–H(3)….O(1)iii
0.980
2.507
3.164
124.29
i
Symmetry codes: I (‒ 1‒ x, ‒ y, 1 ‒ z); ii (1 + x, y, z); iii (‒ x, 1 ‒ y, 2 ‒ z)
Figure 3. Molecular packing of complex 3 viewed along the (a, c) plane, highlighting
the formation of infinite dimeric chains along the a-axis. Dashed lines indicate
intermolecular hydrogen bonding of the N–H···N and C–H···O types.
These structural insights demonstrate that N-acetylhistamine coordination preserves the core organometallic
architecture while introducing subtle geometric and electronic modifications, highlighting its potential in the design of
biorelevant Fe(CO)₃ complexes.
3.3. Acid–base characterization of complex 3 [Fe(CO)₃(1,4-η⁵-N-acetyl histaminocyclohexa-1,3-diene)]
Metal carbonyl complexes bearing functionalized organic ligands such as CO₂H, OH, SH, or NH₂ groups serve as sensitive
molecular probes for monitoring pH-dependent behavior. Protonation or deprotonation of such ligands directly affects the
carbonyl stretching vibrations, observable in the FTIR spectral window of 1800–2100 cm⁻¹. In particular, the symmetric and
asymmetric stretching modes of the CO ligands (υ_sym(CO) and υ_asym(CO)) exhibit distinct frequency shifts, enabling
direct spectroscopic correlation with pH variations in solution [31–36]. Recent advancements in FTIR instrumentation
have significantly improved both sensitivity and resolution, facilitating the simultaneous analysis of multiple carbonylcontaining complexes within mixed aqueous systems [33–35].
In this study, the acid–base behavior of complex 3 was monitored by FTIR spectroscopy (Figure 4) over a pH range of
5–10, using the procedure described in Section 2.3.
At pH 5.3, two ν_C≡O bands were observed at 2058 and 1990 cm⁻¹, corresponding to the protonated form of the
complex. A minor band at 2116 cm⁻¹, attributable to the [Fe(CO)₃(η⁵-C₆H₇)]⁺ cation, indicated partial dissociation of
the N-acetylhistamine ligand under mildly acidic conditions. As the pH increased, the ν_C≡O bands gradually shifted to
lower wavenumbers (2048 and 1979 cm⁻¹), accompanied by the complete disappearance of the 2116 cm⁻¹ band.
The resulting IR spectra showed clear shifts in the carbonyl stretching frequencies as a function of pH (Scheme),
highlighting the spectroscopic sensitivity of the complex to its protonation state. These findings demonstrate the potential
of such functionalized iron tricarbonyl complexes as pH-responsive probes for analytical applications in organometallic
chemistry.
The apparent pKₐ of the complex was estimated to be approximately 8.2 from the IR spectral transition midpoint. This
value is higher than the pKₐ of free N-acetylhistamine (≈ 6.9), suggesting that Fe(CO)₃ coordination increases the basicity
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Figure 4. Acid–base titration of complex 3 monitored by infrared spectroscopy.
Scheme. Acid–base titration profile of complex 3 bearing the N-acetylhistamine ligand (HNhist), illustrating its pH-responsive behavior in solution.
of the ligand, consistent with the increased electron density at the metal center and stabilization through π-backbonding,
a phenomenon frequently observed in iron tricarbonyl complexes with nitrogen-based ligands [37–39].
Overall, complex 3 demonstrates good aqueous stability above pH 5 and remains intact up to slightly basic conditions.
The increased basicity and electronic adaptability make this complex a promising scaffold for pH-responsive applications
in bioorganometallic chemistry, including pH-triggered delivery systems and sensing platforms [40–41].
Although no direct IR data were recorded outside the studied pH range, the known lability of Fe(CO)₃ complexes under
extreme pH conditions suggests that the complex may undergo protonation-induced dissociation at low pH and hydrolytic
degradation at high pH, potentially releasing the N-acetylhistamine ligand and generating iron carbonyl fragments such
as Fe₂(CO)₉ or Fe₃(CO)₁₂. This hypothesis is supported by well-documented base-mediated degradation mechanisms for
iron carbonyl complexes via Hieber-type reactions and hydroxide-induced CO attack [42].
4. Conclusion
The synthesis and characterization of the complex 3 [Fe(CO)₃(1,4-η⁵-N-acetyl histaminocyclohexa-1,3-diene)] demonstrate
the feasibility of selective organometallic tagging on a histidine-analog ligand. Structural analysis confirms a well-defined
exo geometry suitable for directed functionalization. Spectroscopic data and acid–base titration reveal stability in aqueous
solution and a slight increase in basicity compared to free N-acetylhistamine. These findings suggest that Fe(CO)₃ units
can serve as robust platforms for selective biomolecule labeling, paving the way for applications in bioorganometallic
chemistry, particularly for recognition or modification of histidine-containing proteins.
Future developments may focus on enzymatic targeting and the design of IR-active probes for biochemical imaging.
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Acknowledgments
The authors wish to express their sincere gratitude to the Algerian Ministry of Higher Education and Scientific Research
for its continuous support and steadfast commitment to advancing scientific research.
Author contributions
Salah Merniz conceived and designed the research, performed the experimental work, conducted the formal analysis, and
developed the methodology. Louiza Himed contributed equally to the conceptualization, experimental investigation, and
methodological development, and supervised the overall research process. Rofia Djerri provided essential resources and
contributed to supervising the work. Belkis Akachat assisted in the conceptualization, experimental investigation, and
supervision.
All authors have read and approved the final version of the manuscript.
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Supplementary information
Crystallographic data for the structure of compound 3, [Fe(CO)₃(1,4-η⁵-N-acetylhistaminocyclohexa-1,3-diene)], have
been deposited with the CCDC under deposition number CCDC 2421142.
The corresponding CIF file is available free of charge from the CCDC via www.ccdc.cam.ac.uk/structures or by email at
[email protected]
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