Steric congestion at, and proximity to, a ferrous center leads to hydration of α-nitrile substituents forming coordinated carboxamides

Article abstract of DOI:10.1021/ic500096h

The question of the conversion of nitrile groups into amides (nitrile hydration) by action of water in mild and eco-compatible conditions and in the presence of iron is addressed in this article. We come back to the only known example of hydration of a nitrile function into carboxamide by a ferrous [Fe(II)] center in particularly mild conditions and very efficiently and demonstrate that these unusual conditions result from the occurrence of steric stress at the reaction site and formation of a more stable end product. Two bis(cyano-substituted) (tris 2-pyridyl methyl amine) ligands have been prepared, and the structures of the corresponding FeCl₂ complexes are reported, both in the solid state and in solution. These two ligands only differ by the position of the nitrile group on the tripod in the α and β position, respectively, with respect to the pyridine nitrogen. In any case, intramolecular coordination is impossible. Upon action of water, the nitrile groups are hydrated however only if they are located in the α position. The fact that the β-substituted β-(NC)₂TPAFeCl₂ complex is not water sensitive suggests that the reaction proceeds in an intramolecular way at the vicinity of the metal center. In the bis α-substituted α-(NC)₂TPAFeCl₂ complex, both functions are converted in a very clean fashion, pointing out that this complex exhibits ligand flexibility and is not deactivated after the first hydration. At a preparative scale, this reaction allows the one-pot conversion of the bis(cyano-substituted) tripod into a bis(amido-substituted) one in particularly mild conditions with a very good yield. Additionally, the XRD structure of a ferric compound in which the two carboxamido ligands are bound to the metal in a seven-coordinate environment is reported.

Full text of DOI:10.1021/ic500096h

Article
pubs.acs.org/IC
Steric Congestion at, and Proximity to, a Ferrous Center Leads to
Hydration of α‑Nitrile Substituents Forming Coordinated
Carboxamides
Nasser K. Thallaj,†,§ Pierre-Yves Orain, Aurore Thibon, Martina Sandroni, Richard Welter,∥
‡
†,⊥
‡
,†,‡
and Dominique Mandon*
†
Laboratoire de Chimie Biomimet
iment Le Bel, 4 rue Blaise Pascal, CS 90032, F-67081 Strasbourg Cedex, France
́
Laboratoire de Chimie, Electrochimie Moleculaires et Chimie Analytique, UMR 6521, CNRS−Universite de Bretagne Occidentale, 6
́ ́ ́
ique des Metaux de Transition, Institut de Chimie de Strasbourg−UMR 7177, CNRS−Universite
de Strasbourg, Bat
‡
̂
́
Avenue Victor Le Gorgeu, CS 93837, F-29238 Brest Cedex 3, France
§
Faculty of Applied Sciences, Department of Chemistry, Kalamoon University at Deratiah, P.O. Box 222, Deratiah, Syria
∥
Institut de Biologie Molec
́
ulaire des Plantes, UPR CNRS 2357, Universite
Laboratoire CLAC, Institut de Chimie de Strasbourg−UMR 7177, CNRS−Universite
Strasbourg Cedex, France
S Supporting Information
́
de Strasbourg, F-67084 Strasbourg, France
⊥
́
de Strasbourg, 1 rue Blaise Pascal, F-67000
*
ABSTRACT: The question of the conversion of nitrile groups into amides (nitrile
hydration) by action of water in mild and eco-compatible conditions and in the
presence of iron is addressed in this article. We come back to the only known example
of hydration of a nitrile function into carboxamide by a ferrous [Fe(II)] center in
particularly mild conditions and very efficiently and demonstrate that these unusual
conditions result from the occurrence of steric stress at the reaction site and formation
of a more stable end product. Two bis(cyano-substituted) (tris 2-pyridyl methyl amine)
ligands have been prepared, and the structures of the corresponding FeCl complexes
2
are reported, both in the solid state and in solution. These two ligands only differ by the
position of the nitrile group on the tripod in the α and β position, respectively, with
respect to the pyridine nitrogen. In any case, intramolecular coordination is impossible.
Upon action of water, the nitrile groups are hydrated however only if they are located in
the α position. The fact that the β-substituted β-(NC) TPAFeCl complex is not water
2
2
sensitive suggests that the reaction proceeds in an intramolecular way at the vicinity of
the metal center. In the bis α-substituted α-(NC) TPAFeCl complex, both functions are converted in a very clean fashion,
2
2
pointing out that this complex exhibits ligand flexibility and is not deactivated after the first hydration. At a preparative scale, this
reaction allows the one-pot conversion of the bis(cyano-substituted) tripod into a bis(amido-substituted) one in particularly mild
conditions with a very good yield. Additionally, the XRD structure of a ferric compound in which the two carboxamido ligands
are bound to the metal in a seven-coordinate environment is reported.
INTRODUCTION
the biological material. However, the difficulties inherent to the
extensive use of the biological media for synthesis are still
inciting chemists to study more classical synthetic pathways.
A number of metal-containing complexes are able to mediate
■
Conversion of nitrile-containing compounds into amides in
ecologically friendly conditions is a particularly challenging area
in our current economical context.
hydration reactions are generally carried out in basic or acidic
conditions under rigorous control to minimize formation of
side products. Metal-mediated hydration of nitrile groups is an
1
−5
The so-called nitrile
10−15
the hydration of nitriles,
and the factors affecting their
6
reactivity have been reviewed. The reaction conditions
however are not always in line with the current demand in
terms of eco-compatibility. In any case, it seems obvious that
the nitrile has to be activated either by coordination or by
interaction with its own environment, according to three
different possible pathways involving (i) an inner-sphere
mechanism, (ii) an outer-sphere mechanism, or (iii) a
6
interesting alternative way, for it presents some selectivity
since the reaction products (the carboxamide functions) can
coordinate, thus impeding further conversion into carboxylic
acids or other byproducts.
Nature is able to perform such reactions: the nitrile
hydratases constitute a class of enzymes with iron- or cobalt-
4
,7−9
containing active sites.
biomolecules gives rise to a number of studies carried out from
The promising reactivity of these
Received: January 15, 2014
Published: July 15, 2014
©
2014 American Chemical Society
7824
dx.doi.org/10.1021/ic500096h | Inorg. Chem. 2014, 53, 7824−7836

Inorganic Chemistry
Article
second-outer sphere mechanism. ,16−18 From a general and
fundamental point of view the question of nitrile hydration by
first-row and cheap metal centers in mild conditions is of
particular interest and justifies further studies.
8
We then show that the location of the uncoordinated nitrile
groups on the tripod skeleton is crucial: when they are present
at the β position, no reaction occurs. The perfect positioning of
the nitrile groups when located in the α position, within a
particularly flexible tripodal ligand, induces formation of π-
bonded carboxamido ligand. Finally, we report a very easy and
An interesting example of a copper-mediated intramolecular
hydration of a nitrile substituent of a macrocyclic tetraaza ligand
19
was reported more than 30 years ago. In that case it was
proposed that a coordinated hydroxide would attack the nitrile
straightforward method to prepare a bis(NH CO)-substituted
2
tripod from the bis-cyano precursor and the XRD structure of a
ferric compound in which the two carboxamido ligands are
bound to the metal in a seven-coordinate environment.
2
0−22
group which is located nearby:
the reaction product is
stabilized by carboxamide complexation to the metal to afford a
rigid and coordinately saturated complex unable to further
react. Such a reaction results from a close interaction between
two potentially coordinating reactive groups to generate a very
stable end product.
EXPERIMENTAL SECTION
■
General Considerations. Chemicals were purchased from Aldrich
Chemicals and used as received. 2-Cyano-6-bromomethylpyridine was
prepared according to a previously published procedure. Analytical
23
In a preliminary study we reported the first example of
conversion of a nitrile function into an amide group in mild
conditions by an Fe(II) center using the (6-cyano-2-
pyridylmethyl)bis(2-pyridylmethyl)amine ligand. This reaction
was unusual, since weak Lewis acid centers such as Fe(II) are
considered incapable to activate water. In this “outer-sphere
mechanism” the first step was displacement of the coordinated
chloride by water which further reacted with the neighboring
nitrile to finally yield a Fe(II) carboxamido complex which was
isolated and structurally characterized. However, in absolute
terms we could not completely rule out that the nitrile group
might be transiently coordinated to the Fe(II) atom of another
complex in solution in some sort of binuclear complex, thus
activating the nitrile carbon toward nucleophilic attack.
Although seeming unlikely to us, this hypothesis prompted us
to come back to this reaction.
23
anhydrous FeCl was obtained as a white powder by treating iron
2
powder (ACS grade) with hydrochloric acid in the presence of
methanol in an argon atmosphere. Unless otherwise stated, all solvents
used during the metalation reactions and workup were distilled and
7
1
dried according to standard methods. Preparation and handling of all
compounds were performed in an argon atmosphere using the Schlenk
technique following standard procedures. Metalation of α- and β-
(
NC) TPA by FeCl was carried out according to already published
2 2
27
procedures. The purity of the dry dioxygen was 99.999% (grade 5).
Mass spectrometric experiments and elemental analyses were carried
out, respectively, by the Service Commun de Spectromet
and the Service Commun de Microanalyses de l’Institut de Chimie de
Strasbourg (Universite de Strasbourg). The molecular structure of β-
NC) TPAFeCl was resolved by the Service Commun de Diffraction
́
rie de Masse
́
(
2
2
X de la Faculte des Sciences et Techniques de l’UBO, Brest.
1
Physical Methods. H NMR data were recorded in CD CN for
3
We decided to address the question of an intra- vs
intermolecular mechanism and prepared two different bis-
the complexes and CDCl for the ligands at ambient temperature on a
Bruker AC 300 spectrometer at 300.1300 MHz with the residual signal
3
of CD HCN (CHCl ) as a reference for calibration. UV−vis spectra
(
cyano-substituted) ligands, the structure of which only differs
2
3
were recorded on a Varian Cary 05 E UV/vis NIR spectrophotometer
equipped with an Oxford instrument DN1704 cryostat with optically
transparent Schlenk cells. Cyclic voltammetry measurements were
obtained from a PAR 173A potentiostat in a 0.1 M solution of TBACl
by the location of the nitriles on the tripod skeleton, i.e., in the
α or β position at the pyridine site. Both ligands [bis(6-cyano-
2
-pyridylmethyl)(2-pyridylmethyl)amine)], α-(NC) TPA, and
2
[
(
bis(5-cyano-2-pyridylmethyl)(2-pyridylmethyl)amine)], β-
(supporting electrolyte) in acetonitrile using platinum electrodes as
NC) TPA, are displayed in Figure 1.
2
working and counterelectrodes and saturated calomel electrode as the
reference. Potentials were calibrated by addition of ferrocene in the
end of the measurements. Conductivity measurements were carried
out under argon at 25 °C with a CDM 210 Radiometer Copenhagen
Conductivity Meter using a Tacussel CDC745-9 electrode.
Synthesis of 3-Cyano-6-bromomethylpyridine. A 3.000 g
25.4 mmol) amount of 6-methyl-3-carbonitrile pyridine was dissolved
(
in 100 mL of carbon tetrachloride. A 4.980 g (27.9 mmol) amount of
N-bromosuccinimide and 0.060 g (0.25 mmol) of benzoil peroxide
were added, and the reaction mixture was refluxed over 5 h. Solvent
was evaporated, and the residues were chromatographed on silica gel
using methylene chloride as eluent. The desired compound is a yellow
oil, progressively turning red upon standing at room temperature.
Figure 1. Two ligands of the FeCl complexes which this work is
based on.
1
2
Obtained: 2.036 g, i.e., 41% yield. H NMR, CDCl , δ, ppm: 8.79 (S,
3
1
H), 7.95 (dd, 1H), 7.56 (dd, 1H), 4.53 (s, 1H).
Synthesis of α-(NC) TPA. To a solution of 1.40 g (7.1 mmol) of
2
In this article, we compare the structures in the solid state
2-cyano-6-bromomethylpyridine in 100 mL of CH
3
CN were added
0
.40 g (3.7 mmol) of picolylamine and 0.37 g (3.5 mmol) of Na CO .
and in solution of the α-(NC) TPAFeCl2 and β-
2
3
2
The reaction mixture was refluxed over 18 h. Solid residues were
filtered, and the solvent was evaporated to dryness. After water was
added, the mixture was extracted several times with CH Cl , and the
(
NC) TPAFeCl complexes. Their reactivity toward water at
2 2
room temperature is further examined. In both complexes the
nitrile group is not able to coordinate to the metal center. We
first demonstrate that two nitrile groups, when located in the α
position of the bound heteroatom of the ligand, can be
hydrated in a stepwise way by temporary interaction with a
Fe(II) complex. During the reaction, crucial steps have been
carefully studied. In particular, a bis-carboxamido Fe(II)
complex (resulting from nitrile hydration) is characterized.
2
2
combined organic phases were washed with water and then dried over
MgSO . The solvent was evaporated, and the sticky material was
4
extracted several times with pentane. The concentration of the pentane
phases yielded 1.17 g (93%) of a white solid, which was further
recrystallized in pentane. Anal. Calcd for C H N : C, 70.57; H, 4.74;
20
16
6
1
N, 24.69. Found: C, 70.18; H, 4.79; N, 24.23. H NMR, CDCl , δ,
3
ppm: 8.54 (m, α-CH, 2H); 7.80 (dd, β-CH, 2H); 7.79 (dd, β-CH,
7
825
dx.doi.org/10.1021/ic500096h | Inorg. Chem. 2014, 53, 7824−7836

Inorganic Chemistry
Article
2
7
H); 7.66 (td, γ-CH, 1H); 7.56 (dd, γ-CH, 2H); 7.47 (dt, β-CH, 1H);
.17 (m, β-CH, 1H); 3.99 (s, CH , 4H); 3.89 (s, CH , 2H).
Preparation of {[α-(H NCO) TPAFe] -O}Cl . A 100 mg (0.21
2
2
2
4
mmol) amount of α-(NC) TPAFeCl was dissolved in 100 mL of
2
2
2
2
Synthesis of β-(NC) TPA. To a solution of 1.40 g (8.91 mmol) of
CH CN in a Schlenk tube. A 10 μL (0.5 mmol) amount of distilled
2
3
3
0
-cyano-6-bromomethylpyridine in 100 mL of CH CN were added
.480 g (4.46 mmol) of picolylamine and 0.55 g (4.5 mmol) of
water was introduced, and oxygen was bubbled for 15 s. The medium
was stirred 48 h in a stoppered vessel. The solvent was then
concentrated, and diethyl ethyl was added, allowing precipitation of a
3
Na CO . The reaction mixture was refluxed over 16 h. The solvent was
evaporated to dryness, and the residue was treated with aqueous
K CO3 and extracted with methylene chloride. The compound
obtained upon complete evaporation of the solvent was purified by
column chromatography on alumina using acetonitrile as the eluent.
The desired compound is an orange oil. Obtained: 0.755 g, i.e., 50%
yield. Anal. Calcd for C H N : C, 70.57; H, 4.74; N, 24.69. Found:
2
3
brown solid. This brown solid was recrystallized from a wet (5% H
solution of CH CN by diffusion of diethyl ether. A 70 mg amount of
analytically pure compound was obtained. Anal. Calcd for
Fe : C, 46.99; H, 3.94; N, 16.44. Found: C, 47.20;
H, 3.51; N, 16.62. IR, ATR, solid state: ν
O)
2
2
3
C
H
40Cl
N O
2 12 5
40
4
−1
= 1661 cm . UV−vis: λ
CO
−1
−1
−1
=
367 nm (ε = 3553 L mol cm ), λ = 316 nm (ε = 3365 L mol
20
16
6
−
1
−1
−1
1
cm ), and λ = 500 nm (ε = 915 L mol cm ).
Preparation of α-(H NCO) TPA. i. Direct Preparation. A 100 mg
C, 70.31; H, 4.36; N, 24.91. H NMR, CDCl , δ, ppm: 8.77 (dd, α-
3
CH, 2H); 8.53 (dq, α-CH, 1H); 7.90 (dd, β/γ-CH, 2H); 7.70 (dm, β/
γ-CH, 2H); 7.64 (td, γ-CH, 1H); 7.40 (dm, β-CH, 1H); 7.16 (m, β-
CH, 1H); 3.96 (s, CH , 4H); 3.87 (s, CH , 2H).
2
2
(
0.29 mmol) amount of free α-(NC) TPA and 40 mg (0.31 mmol) of
2
anhydrous FeCl were introduced in a Schlenk tube, and 35 mL of dry
2
2
2
and degassed CH CN were added under argon. The medium was
Preparation of α-(NC) TPAFeCl . A 150 mg (0.44 mmol)
3
2
2
stirred overnight under argon. To this medium 30 mL of distilled
water was added under aerobic atmosphere. The medium was stirred
for 2 days. Following, 500 mg (excess, 8.9 mmol) of solid KOH was
added and the medium was vigorously stirred for 5 min. A 100 mL
amount of CH Cl and 50 mL of water were added to the medium,
amount of free α-(NC) TPA was dissolved in a Schlenk tube
containing 20 mL of dry and degassed THF. A 56 mg (0.44 mmol)
amount of anhydrous FeCl was dissolved in a second Schlenk tube
containing 10 mL of dry and degassed THF. The solution of FeCl2
was transferred under argon in the Schlenk containing the ligand, and
the medium was stirred overnight. The solvent was then evaporated to
dryness, and the compound was extracted with dry and degassed
2
2
2
2
and the organic solution was washed with water (4 × 50 mL). The
organic phase was dried over magnesium sulfate and the solvent
concentrated. Addition of pentane afforded 80 mg (73%) of a white
CH CN, filtered under inert atmosphere, and concentrated. Addition
3
+
solid. HR-ESI-MS ([(CONH ) TPA-H] ): m/z calcd for C H N O
2
2
20 21
6
2
of diethyl ether afforded a red solid, which was washed thoroughly
with this solvent, prior to being dried under vacuum. A 165 mg (80%)
amount of α-(NC) TPAFeCl with good analytical and spectroscopic
data could be obtained. Anal. Calcd for C H Cl FeN : C, 51.42; H,
3
3
3
77.172, found 377.170; m/z calcd for C H N O Na 399.154, found
20 20 6 2
1
99.151. H NMR, CDCl , δ, ppm: 8.53 (m, α-CH, 2H); 8.06 (d, β-
3
2
2
CH, 2H); 7.96 (s, NH , 2H); 7.81 (t, γ-CH, 2H); 7.66 (m, β-CH,
2
20
16
2
6
1
H);7.64 (d, β-CH, 2H); 7.51 (d, β-CH, 1H); 7.16 (m, γ-CH, 1H);
.45; N, 17.99. Found: C, 51.21; H, 3.87; N, 18.06. IR, ATR, solid
1
3
−1
5.69 (s, NH , 2H); 3.92 (s, CH , 2H); 3.90 (s, CH , 4H). C NMR,
2 2 2
state: ν
= 2239 cm . UV−vis: λ = 350−550 nm, broad and
−1 −1
CN
CDCl , δ, ppm: 166 (ipso C); 158 (CO); 157 (ipso C); 148 (ipso
3
featureless absorption (ε = 800 L mol cm ) at λ = 462 nm.
Spectroscopic data are detailed in the text and shown in the
Supporting Information.
C); 147 (ipso C); 137 (CH); 135 (CH); 125 (CH); 122 (CH); 121
1
(
CH); 120 (CH); 59 (CH ); 58 (CH ). H NMR analysis of the
2
2
residues of the mother liquor showed that some ligand was still present
Preparation of β-(NC) TPAFeCl . This complex was obtained
2
2
in good amount in the sample.
following the same procedure from 0.126 g (0.37 mmol) of β-
ii. By Decomplexation.
H NCO) TPAFeCl (99 μmol) or {[α-(H NCO) TPAFe] -O}Cl
4
A 50 mg amount of α-
(
NC) TPA and 0.044 g (0.35 mmol) of FeCl . Anal. Calcd for
2
2
(
(
2
2
2
2
2
2
C H Cl FeN : C, 51.42; H, 3.45; N, 17.99. Found: C, 50.98; H, 3.92;
N, 17.71. IR, ATR, solid state: ν
nm (ε = 1600 L mol cm ), 506 nm (shoulder, ε
mol cm ). Spectroscopic data are detailed in the text and shown in
20
16
2
6
49 μmol) was dissolved/suspended in a flask containing 20 mL of a
−1
= 2234 cm . UV−vis: λ = 400
CN
8
0/20 CH CN/H O mixture. A 250 mg (excess, 4.45 mmol) amount
−1
−1
3
2
= 1300 L
506 nm
of solid KOH was added, and the medium was stirred for 5 min. A 50
mL amount of CH Cl and 20 mL of aqueous Na CO were added to
−
1
−1
2
2
2
3
the Supporting Information.
the colorless medium, and the organic solution was washed with water
(2 × 50 mL). The organic phase was dried over magnesium sulfate and
the solvent concentrated. Addition of pentane allowed crystallization
of a white solid whose spectroscopic properties were identical to those
above described. A 33 mg amount of free ligand (90%) could be
obtained.
Hydration of α-(NC) TPAFeCl : Preparation of α-
2
2
H NCO) TPAFeCl . A 100 mg (0.21 mmol) amount of α-
2
2
2
(
NC) TPAFeCl was dissolved in a Schlenk tube containing 80 mL
2 2
of dry and degassed CH CN. A 780 μL (43 mmol) amount of
3
previously degassed distilled water was added. The medium became
bright orange, and the color progressively turned into burgundy red.
Thirty hours after the beginning of the reaction, the medium was
concentrated and a purple microcrystalline solid was obtained upon
slow addition of diethyl ether. It was washed and dried under vacuum.
Yield: 80 mg (76%). Anal. Calcd for C H Cl FeN O : C, 47.74; H,
Action of Water on β-(NC) TPAFeCl . Addition of an excess of
2
2
H O 200−500 equiv on a CD CN solution of the complex resulted in
2
3
no other change than a neglectible downfield shift in the chemical shift
1
of the paramagnetic H NMR spectrum (0.5 < Δ < 1.0 ppm) and a
δ
20
20
2
6
2
very small shift from 400 to 397 nm in the UV−vis spectrum
neglectible variation in the epsilon value). The traces are given in the
4
.01; N, 16.70 Found: C, 47.62; H, 3.97; N, 17.01. HR-ESI-MS
(
+
(
[(CONH ) TPAFeCl-H] ): m/z calcd for C H Cl Fe N O
2 2 20 19 1 1 6 2
Supporting Information.
4
1
4
66.060, found 466.059. IR, ATR, solid state: ν
645 cm . UV−vis: λ = 364 nm (ε = 1300 L mol cm ) and λ =
= 1664 and
CO
X-ray Analysis. Single crystals of α-(NC) TPAFeCl ·CH CN and
2
2
3
−1
−1
−1
{
[β-(H NCO) TPAFe] -O}Cl were mounted on a Nonius Kappa-
2 2 2 4
−1
−1
97 nm (ε = 1660 L mol cm ). Spectroscopic data are detailed in
CCD area detector diffractometer and those of β-(NC) TPAFeCl ·
2
2
the text and shown in the Supporting Information.
Detection and Characterization of α-(NC)(H NCO)TPAFeCl .
To obtain the traces displayed in Figure 4, the same procedure as
indicated above was followed except that the reaction times were 8 and
CH CN on a Xcalibur-2 diffractometer from Oxford Diffraction (Mo
3
2
2
Kα λ = 0.71073 Å for all complexes). Quantitative data were obtained
at 173 K for all complexes. Complete conditions of data collection
(Denzo software) and structure refinements are given in the
Supporting Information. Cell parameters were determined from
reflections taken from one set of 10 frames (1.0° steps in phi
angle), each at 20 s exposure. Structures were solved using direct
18 h, respectively.
HR-ESI-MS characterization was carried out at t = 18 h.
+
(
NC)(H NCO)TPAFeCl in CH CN/H O 80/20: m/z calcd for
2
3
2
+
2
C H ClFeN O 449.060, found 449.06. (NC)(H NCO)TPAFeCl in
CH CN: calcd for C H ClFeN O 449.060, found 449.058. (NC)-
methods (SIR97 and 92) and refined against F using the SHELXL97
20
18
6
2
7
2
3
20 18
6
software. Absorption was not corrected. All non-hydrogen atoms
were refined anisotropically. Hydrogen atoms were generated
according to stereochemistry and refined using a riding model in
SHELXL97.
+
(
4
H NCO)TPAFeCl −H in CH CN: calcd for C H ClFeN O
2
3
20 17
6
48.060, found 448.050. All traces are given in the Supporting
Information.
7
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Inorganic Chemistry
Article
Crystal data for β-(NC) TPAFeCl ·CH CN: red crystals, mono-
2
2
3
Scheme 1. Direct Hydration of α-(NC) TPA into α-
2
a
clinic, space group P2 /c, a = 11.0253(8) Å, b = 8.9504(6) Å, c =
1
(H NCO) TPA
2
2
3
2
5.0265(18) Å, β = 95.868(6), V = 2456.7(3) Å , D
cm , Z = 4. For 6067 unique observed reflections with I > 2σ(I) and
= 1.374 g·
calcd
−3
290 parameters, the discrepancy indices are R = 0.0587 and Rw =
0
.1292.
Crystal data for α-(NC) TPAFeCl ·CH CN: red crystals, mono-
2
2
3
clinic, space group P2 /n, a = 12.4650(4) Å, b = 14.2120(5) Å, c =
1
3
13.3120(5) Å, β = 102.090(2), V = 2305.95(14) Å , D
= 1.464 g·
calcd
−3
cm , Z = 4. For 5219 unique observed reflections with I > 2σ(I) and
2
0
89 parameters, the discrepancy indices are R = 0.0528 and Rw =
.1288.
a
This reaction does not occur with the β-substituted tripod β-
Crystal data for {[α-(H NCO) TPAFe] -O}Cl : brown crystals,
2
2
2
4
(CN) TPA ligand which is fully recovered in the end of the workup.
2
monoclinic, space group P2 /c, a = 11.0360(4) Å, b = 18.3800(8) Å, c
1
3
=
14.3460(5) Å, β = 130.44(3), V = 2217.4(10) Å , D
cm , Z = 2. For 5499 unique observed reflections with I > 2σ(I) and
= 1.531 g·
calcd
−3
free ligand with FeCl . Single crystals could be obtained by slow
2
diffusion of diethyl ether into a solution of complex in
acetonitrile.
3
0
02 parameters, the discrepancy indices are R = 0.0516 and Rw
=
.1078.
Solid-State Structure of α-(NC) TPAFeCl . The molecular
2
2
structure is displayed in Figure 2. At a first glance, this structure
RESULTS
■
looks similar to those already reported within the class of FeCl
2
Syntheses. α-(NC) TPA and β-(NC) TPA were obtained
by reaction of 2 equiv of 2-cyano-6-bromomethylpyridine and
-cyano-6-bromomethylpyridine, respectively, with 1 equiv of
picolylamine according to standard procedures.
substituted ligand is a white powder, whereas the β-substituted
one is a yellow oil. They have been fully characterized and
obtained with good to excellent yields as analytically pure
compound. All data are available in the Supporting Information.
Reactivity versus Water of the Cyano-Substituted
25,29,30
2
2
complexes with moderately hindered TPA tripods,
i.e.,
the metal center lies in a six-coordinate environment, with
distances supporting a high-spin state for the iron.
A more detailed examination evidences a strong elongation
of the metal-to-substituted pyridines distances, with dFeN5 =
3
24,25
The α-
2
.414(3) Å and dFeN3 = 2.325(2) Å. Moreover, a repulsive
interaction between the nitriles and the chloride Cl2 causes a
significant deviation from linearity of the cyano groups with
C19C20N6 = 173.3° and C12C13N4 = 175.0°. Finally, the
ligand skeleton is extremely distorted. The top view of the
diagram does not reflect the flexibility of the ligand. Rather, this
is evidenced by a side view of the structure which reveals a
severe torsion of the equatorial pyridines with the dihedral
angle α = 42.5°, as illustrated in Figure 3. Also, the octahedral
Tripods, α- and β-(CN) TPA. Direct Conversion of α-
2
(
NC) TPA into α-(H NCO) TPA by Hydration Reaction. The
2 2 2
off-white solid thus obtained was complexed to FeCl under
anaerobic conditions following well-known methods,
2
2
3,25−28
and a reddish color developed progressively, accompanied by
precipitation of part of the complex. Twelve hours after the
beginning of the reaction, the reaction vessel was opened and
distilled water was poured into the reaction mixture, affording a
transparent burgundy solution. At this time, there was no
longer a need for keeping the medium under inert atmosphere.
The solution was simply left under stirring for 48 h at room
temperature. Solid potassium hydroxide was finally added to
the solution, resulting in complete bleaching. Five minutes
later, the mixture was treated, and α-(H NCO) TPA was
31,32
distortion parameter
114°.
Solid-State Structure of β-(CN) TPAFeCl . The molecular
indicates significant distortion with Σ
=
2
2
structure is displayed in Figure 4. This is a more classical
structure, the description of which is reported in order to
compare the structural parameters with those of the α-
substituted congener. We shall use this structure to provide a
rationale for the reactivity differences between the two
complexes.
2
2
recovered after precipitation as a white solid with a 73% yield as
an analytically pure compound, without the need for any
chromatographic separation. The product was identified by HR
The coordination polyhedron displays a distorted octahedral
geometry with a six-coordinate iron center. When compared to
the α-(CN) TPAFeCl congener, the first difference is the
2
2
1
13
mass spectroscopy, and H and C NMR data confirmed the
shortening of the equatorial Fe−pyridine distances, with dFeN3
= 2.168(3) Å and dFeN4 = 2.164(3) Å. As a consequence, the
other distances are slightly elongated with dFeN1 = 2.283(3) Å
and dFeN2 = 2.234(3) Å, which however lie in the expected
range for stable six-coordinate complex. Another obvious
difference in this structure by contrast with the above-reported
one is the absence of significant equatorial distortion, as can be
seen from Figure 5. In the present case, the α angle has
decreased to 4.2°. These observations obviously reflect the
absence in this complex of any repulsion between the
coordinated chloride ions and the nitrile functions, those
being now attached to the β position. Indeed, with C11C20N6
= 178.1(5)° and C5C19N5 = 178.6(5)°, the distortion from
linearity of the nitrile groups has become negligible. Finally,
with Σ = 98°, a smaller distortion than that previously found
with the α-substituted ligand is observed.
quality of the sample. All spectroscopic data are available in the
1
Supporting Information. H NMR analysis showed that the
mother solutions still contained some ligand. Conversion is
illustrated in Scheme 1.
Chemical Stability of β-(CN) TPA under the Same
2
Conditions. When the same procedure as that described
above was followed starting from the β-substituted tripod, the
unmodified β-(CN) TPA ligand was quantitatively recovered.
2
In order to understand the hydration reaction and the
reasons for the difference of reactivity between the two ligands,
we carried out a careful structural analysis of the FeCl2
complexes and monitored the hydration reaction by UV−vis
1
and H NMR spectroscopy.
Structural Considerations on the FeCl Complexes
2
with the Two Tripods: Solid-State Structures. Both α-
(
NC) TPAFeCl2 and β-(NC) TPAFeCl complexes were
The main structural differences, especially the bond lengths
and nitrile distortion between the two complexes α-
2
2
2
27
prepared by well-known methods upon treatment of the
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Figure 2. ORTEP diagram of α-(NC) TPAFeCl . Selected distances (Å): Fe−N5 2.414(3), Fe−N1 2.234(2), Fe−N3 2.325(2), Fe−N2 2.189(3).
2
2
(
Left) Top view of a fragment of molecule showing the contacts between the equatorial Cl2 and the distorted nitrile groups.
Figure 3. Top and side view of a fragment of molecule showing the dihedral angle of the equatorial pyridines around the C19−C12 axis.
(
NC) TPAFeCl and β-(NC) TPAFeCl , are summarized in
2 2 2 2
Table 1.
Spectroscopic Studies in Solution of the FeCl2
Complexes with the Two Tripods. Spectroscopic Data
for α-(NC) TPAFeCl . By contrast to most of its parent TPA-
2
2
type complexes which generally display a yellow color, α-
NC) TPAFeCl was found to be a red compound. In solution,
(
2
2
a broad and featureless MLCT absorption in the UV−vis was
observed between 350 and 550 nm (ε = 800 L mol− cm at λ
1
−1
=
462 nm). The molecular conductivity measurement of a 2
2
−1
mM solution yielded Λ = 21 S cm mol , indicating a neutral
electrolytic behavior of the compound in solution at this
concentration.
3
3,34
In cyclic voltammetry, a quasi-reversible
wave assigned to the redox Fe(III)/Fe(II) couple was measured
+
35
at E1/2 = −380 mV vs Fc/Fc (i /i = 0.85). This value lies
pc pa
at the upper limit within the potential range typically
36−39
1
corresponding to five-coordinate compounds.
NMR spectrum of the complex exhibited very broad resonances
at δ = 66 (Δ = 1100 Hz), 56 (Δν1/2 = 400 Hz), 53 (Δν1/2 =
= 400 Hz), and 28 ppm (Δ
and poorly defined signals in the diamagnetic region between
The H
Figure 4. ORTEP diagram of β-(NC) TPAFeCl . Selected distances
2
2
ν1/2
(
2
Å): Fe−N3 2.168(3), Fe−N1 2.283(3), Fe−N4 2.164(3), Fe−N2
1
40 Hz), 40 (Δ
= 650 Hz)
ν1/2
ν1/2
.234(3).
Figure 5. Top and side view of a fragment of molecule showing the dihedral angle of the equatorial pyridines around the C12−C18 axis.
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Table 1. Metal-to-Ligand Bond Lengths (Angstroms) and
elongations and distortion which were observed in the solid
Distortion of the Nitrile Groups (degrees) in α-
state (vide supra). Such a difference between solid state and
solution has already been reported in related complexes.
a
30,37,41
(
NC) TPAFeCl and β-(NC) TPAFeCl
2
2
2
2
By contrast, the solid-state structure of the β-(NC) TPAFeCl
congener is retained in solution, as expected for a moderately
2
2
α-(NC) TPAFeCl
β-(NC) TPAFeCl2
2
2
2
Fe−Cl, trans-amine
Fe−Cl, trans-pyr
Fe−amine
Fe−pyr, trans-Cl
Fe−pyridine
Cl2 2.3217(8)
Cl1 2.3976(10)
N1 2.234(2)
N2 2.189(3)
N3 2.325(3)
N5 2.414(3)
Cl1 2.3216(10)
Cl2 2.4404(11)
N1 2.283(3)
N2 2.234(3)
N4 2.164(3)
N3 2.168(3)
C11C20N6 178.1(5)
C5C19N5 178.6(5)
98
distorted complex. This is shown in Scheme 2.
Experimental Insights into the Hydration Reaction of
α-(NC) TPAFeCl into α-(H NCO) TPAFeCl : UV−Vis and
1
2
2
2
2
2
H NMR Monitoring. At preparative scale when water is
poured in the medium under normal laboratory atmosphere
vide supra) it cannot be ruled out that the α-(NC) TPAFeCl
Fe−pyridine
(
2
2
nitrile distortion
nitrile distortion
Σ (dev. from 90°)
C19C20N6 173.4(4)
C12C13N4 174.9(4)
114
present in the medium is first converted into some ferric
species. To establish whether Fe(II) rather than Fe(III) could
be responsible for the hydration, the reaction was followed
under strictly anaerobic conditions.
a
Σ: octahedral distortion parameter according to refs 31 and 32.
Reactivity of α-(NC) TPAFeCl with water: as shown in
2
2
Figure 6, addition of 200 equiv of degassed H O under argon to
1
0 and 0 ppm. All data were consistent with the presence of a
2
a C = 1.5 × 10− M solution resulted in the progressive raise of
3
3
36
κ coordination mode of the ligand in solution.
Spectroscopic Data for β-(CN) TPAFeCl . This complex also
displayed a red color, due to a broad MLCT absorption
two new absorptions observed in UV−vis at λ = 368 (ε = 1300
2
2
L mol− cm ) and 505 nm (ε = 1660 L mol cm ). The
reaction was considered to be complete over 30 h, the time by
which no further spectroscopic changes could be observed.
Similar time ranges were observed upon work with more dilute
1
−1
−1
−1
between 340 and 550 nm, with a maximum at λ = 400 nm (ε =
600 L mol− cm ). The high molecular extinction coefficient
1
−1
1
27,36
corresponds to coordination of the three pyridyl units.
The
4
−2
(
C = 10− M) or concentrated (C = 10 M) solutions. The
molecular conductivity of a 1.5 mM solution in acetonitrile
yielded Λ = 18 S cm mol , in line with a negligible
dissociation of the complex in these conditions. The cyclic
voltammogram of the complex exhibited an irreversible
Fe(III)/Fe(II) couple at E = −100 mV vs Fc/Fc , in line
with a six-coordinate compound in which the iron center is
ligated to an electro-deficient tripod.
with α-(CN) TPAFeCl is evidenced in the H NMR spectrum
of β-(CN) TPAFeCl . This new complex displays sharp
2
−1
first absorption was assigned to the standard MLCT transition
observed with FeCl complexes. The second one is similar in
2
energy and intensity to the value reported in a ferrous
+
compound with a similar pivaloylamido-substituted
a
TPA2
C(O)NHtBu
tripod, prepared by conventional ways (i.e.,
42
3
0,36−41
from an already existing pivaloylamido-substituted pyridine).
Another difference
1
It has recently been shown that this absorption reflects the
2
2
coordination of the carbonyl groups within a seven-coordinate
2
2
4
2,43
environment.
In the present case, we proposed that the two
absorptions at δ = 51 (Δ
= 70 Hz), 47 (Δ
ν1/2
= 65 Hz),
ν1/2
ν1/2
nitrile groups of the α-(NC) TPA tripod were converted into
4
3
3 (Δ = 140 Hz), 28 (Δ = 44 Hz), and 25 ppm (Δ
=
ν1/2
2
ν1/2
amide functions yielding the α-(H NCO) TPAFeCl complex,
5 Hz) corresponding to the β and γ protons of the
2
2
2
2
7,36
coordinated pyridines. It was shown in the past
that
the carbonyl groups of the ligand being bound to the metal
center.
The reaction was performed on a preparative scale, and
sharp signals (for paramagnetic NMR) of this type are
benchmarks for six coordination within this class of complexes.
Structure of the Complexes in Solution. The correlation
between the spectroscopic properties and the geometry of the
complexes in solution within this class of compounds is now
well established.
Fe(III)/Fe(II) redox potentials (Ea/c/E1/2), the molecular
progressive conversion of α-(NC)1 TPAFeCl into this new
2 2
Fe(II) species was monitored by H NMR. Aliquots of the
reaction mixture were taken and treated (evaporation and
careful washing with diethyl ether followed by vacuum drying)
8, 18, and 30 h following addition of water. The result is shown
in Figure 7. The broad lines of the starting material, typical for
five-coordinate Fe(II) species, were finally converted into clean
and sharp signals, indicating a serious modification in the
coordination geometry at the metal site which remains in the
high-spin Fe(II) state. The intermediate traces showed the
presence of a mixture consisting of the starting material and
some end product, together with an intermediate species
2
7,36
The most significant patterns are the
extinction coefficients of the MLCT bands in UV−vis (ε),
1
and the midintensity width of the signals in H NMR (Δ ).
ν1/2
In the present case these values differ very significantly from
one complex to the other. They are reported in Table 2. Low
E1/2 and ε values and large resonances in NMR are benchmarks
2
7,36
of five-coordinate complexes in solution:
α-(NC) TPAFeCl only. In fact, a five-coordinate geometry for
this complex in solution was expected from the bond
this is the case for
2
2
assigned to the mixed cyano/carboxamido α-(NC)(H NCO)-
2
TPAFeCl complex (vide infra).
2
To get insight into the nature of this intermediate species,
the t = 18 h medium was submitted to mass spectroscopy
analysis. A clean molecular ion was detected at m/z = 449.060.
Table 2. Spectroscopic Data in Solution of Complexes α-
(
NC) TPAFeCl and β-(NC) TPAFeCl
2 2 2 2
We noted that this m/z = 449.60 value may account for either
α-(NC) TPAFeCl
β-(NC) TPAFeCl2
2
2
2
+
λMLCT, nm (ε, L mol−1 cm−1
the monocarboxamido α-(NC)(CONH )TPAFeCl species or
)
350−550 (800 at
340−550 (1600 at
2
+
4
62 nm)
400)
the mono aquo adduct α-(NC) TPAFe(OH )Cl . As this latter
2
2
2
−1
Λ, S cm mol
21
−380 (0.85)
18
hypothesis is unlikely because of an already strong MLCT band
at t = 18 h in UV−vis, we ran an IR spectrum on the t = 18 h
sample. Indeed, the presence of both a nitrile group and a
carboxamido function on the ligand was confirmed by the
observation in the IR spectroscopy of a nitrile and carboxamido
+
E1/2, mV/Fc/Fc (i /i )
irreversible
−100
pc pa
+
E , mV/Fc/Fc
a
1
δ, H NMR, ppm
midintensity width, Hz)
66−28 (1100−140) 51−25 (140−35)
(
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a
Scheme 2. Changes in Geometry of the Metal Center in the Complexes as a Function of the State (solid vs solution)
a
27,37
For α-(NC) TPAFeCl , we propose uncoordination of the nitrile-substituted pyridine for steric reasons.
2
2
Figure 6. UV−vis spectral absorption changes upon addition of 200 equiv of distilled water to a solution of α-(NC) TPAFeCl in acetonitrile: 1
2
2
scan/h, 30 h total time, C = 1.5 mM.
1
Figure 7. H NMR monitoring (CD CN, paramagnetic area) of the conversion of α-(NC) TPAFeCl into α-(H NCO) TPAFeCl : (black) t = 0;
3
2
2
2
2
2
(
green) t = 8 h; (blue) t = 18 h; (red) t = 30 h. The sharp signals of the end product are assigned to the γ,γ′ and β, β′ protons of the pyridines. See
Supporting Information for full assignment and references.
= 2240 cm− and ν
1
= 1662
(H NCO) TPAFeCl ion with m/z = 233.533 and α-
2 2
+2
vibration, respectively, at ν
CN
CO
−1
+
cm . The trace is displayed in the Supporting Information.
In the end, i.e., 30 h after addition of water, the reaction was
stopped and a dark purple solid could be isolated. Its UV−vis
spectrum was identical to that obtained as described above.
HRMS measurements indicated the presence of the α-
(H NCO) TPAFeCl − H at m/z = 466.059 (all MS data
2
2
available in the Supporting Information). Infrared measure-
ments indicated the disappearance of the ν
band of the starting material and the presence of two new
= 2239 cm−
1
CN
1
intense bands at ν
= 1645 and 1664 cm− in line with the
CO
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830

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a
Scheme 3. Hydration Reaction Reported in the Present Study
a
The presence of the intermediate species as the mono-carboxamido compound drawn in square brackets (top line) is proposed on the basis of its
detection by HRMS in the reaction medium. In the bis-carboxamido ferrous species, the iron has been drawn as a seven-coordinate center on the
basis of similar spectroscopic properties to that reported with a parent compound fully characterized in ref .42. Species in brackets (bottom line) are
proposed as short-lived intermediates.
Figure 8. Forty-eight hour conversion of α-(H NCO) TPAFeCl into {[α-(H NCO) TPAFe] -O}Cl in CH CN/H O 80/20, monitored by UV−
2
2
2
2
2
2
4
3
2
vis spectroscopy.
44
presence of a bound amide function. Finally, elemental
analysis was also carried out and confirmed the formulation of
the complex. Taken together, all collected data indicate
formation of the bis-carboxamido α-(H NCO) TPAFeCl
no change in the structure of the ligand. Following unsuccessful
attempts to convert the ligand in a preparative way (vide
supra), this confirmed the inertness of the tripod in β-
(CN) TPAFeCl2 when exposed to water. All data and
2
2
2
2
species, drawn in Scheme 3 as a seven-coordinate complex by
analogy to the above-mentioned Fe(II) complex with the
experimental traces are given in the Supporting Information.
6
Oxidation of α-(H NCO) TPAFeCl : κ Coordination
2
2
2
parent TPA2
C(O)NHtBu
tripod. In view of further reactivity
42
Mode of α-(H NCO) TPA. Reactivity of α-(H NCO) TPAFeCl
2 2 2 2 2
studies, it should be mentioned that this complex is poorly
soluble in acetonitrile and should be studied in solution in a
mixture CH CN/H O 80/20.
with Oxygen. As displayed in Figure 8, bubbling O in a
2
CH CN/H O solution of the complex resulted in the
3
2
−
1
appearance of a new band at λ = 316 nm (ε = 3365 L mol
3
2
−1
Full Stability of β-(NC) TPFeCl when Exposed to
cm ) and an important increase of a signal at λ = 367 nm (ε =
2
2
3553 L mol− cm ). This particular pattern is the distinctive
1
−1
Water. The same experiments were carried out on the β-
2
5,26,38
substituted congener. Addition of 200−500 equiv of H O to a
mark of the presence of a diferric μ-oxo species.
The
2
solution of β-(CN) TPAFeCl in CH CN had negligible effects
third absorption indicating amide coordination decreased in
intensity and was slightly shifted to λ = 500 nm, with ε = 915 L
2
2
3
1
on the UV−vis and H NMR spectra of the complex, indicating
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mol− cm . UV−vis data of the amido-coordinated complexes
1
−1
DISCUSSION
■
are summarized in Table 3.
Nitrile groups are considered as thermodynamically unstable
functions, while being kinetically inert their conversion into
amide requires harsh conditions or activation by metal centers
with or without adjuvants.
5
4
Table 3. Spectroscopic Data in Solution of the Amido-
Coordinated Complexes α-(H NCO) TPAFeCl and {[α-
H NCO) TPAFe] -O}Cl
2 2 2 4
1−6,55
2
2
2
In many cases, nitrile hydration
(
by metal centers involves nucleophilic attack of an hydroxide
1
,6
λMLCT, nm (ε, L mol−1 cm−1
group on a coordinated nitrile. In the iron chemistry of TPA-
type tripods, the hydration of a coordinated nitrile into an
)
α-(H NCO) TPAFeCl
2
368 (1300), 505 (1660)
2
2
53
56
acetamide and acetate has been reported within dinuclear
Fe(III) complexes. This is not unexpected at a ferric center,
since Fe(III) is also found at the active site of nitrile
{
[α-(H NCO) TPAFe] -O}Cl
316 (3365), 367 (3553), 500 (915)
2
2
2
4
4
,5,7,8
hydratase.
In the present case, the hydration reaction is
The reaction occurred slowly and was considered to be
complete over 48 h.
As shown in Scheme 4, it was reproduced at the preparative
scale, and after workup a dark brown compound was isolated
and first characterized by elemental analysis, which was
consistent with the presence of the μ-oxo diferric complex
performed by a ferrous Fe(II) center, which is unusual.
The following lines aim at replacing this reaction in the
context of the chemistry of aminomethylpyridine-containing
complexes and more generally with Fe(II) centers.
Both Complexes Display Relatively High Redox
Potentials. The redox potentials measured in this work both
stand at the upper limit of the reported range for this kind of
(
[α-(H NCO) TPAFe] -O)(Cl) . Its UV−vis spectrum was
2
2
2
4
3
6
identical to that detected in the course of the reaction
monitoring. In infrared spectroscopy, the carbonyl vibration
complexes, those being either five or six coordinate.
Obviously, tripods with electron-withdrawing groups afford
complexes with an increased Lewis acid character. This
indicates that the metal center, although being Fe(II), must
be electron deficient, at least with respect to standard Fe(II)
complexes. It is also likely that in the intermediate steps, i.e.,
−1
was measured at ν
coordination to the metal center.
= 1661 cm , in line with weak
CO
Single crystals of this compound could be obtained by
diffusion of diethyl ether in an acetonitrile solution, and its
ORTEP diagram is displayed in Figure 9.
4
those in which water coordinates, both tripods act as κ ligands:
4
+
The structure of the {[α-(H NCO) TPAFe] -O} ion is
2
2
2
if so, the potentials should not differ by much whether the
ligand is α- or β-substituted. In any case, both complexes
should display an increased propensity to activate a coordinated
water molecule, even in the Fe(II) state.
Coordination of Nitriles to the Fe(II) Center. The
coordination of acetonitrile to Fe(II) centers is known in the
C(O)NHtBu
similar to that of the recently reported [(TPA
Fe)2-
2
4+
O] one, in which the pivaloylamido-substitued tripod was
synthesized by conventional methods. This compound
belongs to the class of ferric complexes in which the metal is
4
2
42,45−51
seven coordinate.
In the present case the nitrogen atoms
from the substituted pyridines together with the oxygen atoms
from the amide are coplanar and define an equatorial plane.
The amine nitrogen atom N2 is displaced 0.38 Å from this
plane. Thus, the geometry is consistent with a slightly distorted
pentagonal bipyramidal structure. The longest distance is that
between the iron atom and the central amine, with dFe−N2 =
chemistry of aminomethylpyridyl-containing ligands. Many
57−60
structures are known,
either in the solid state or in
solution, examples of which are shown in Figure 10. We
recently reported the structure of a bis-aquo Fe(II) complex
59
with a TPA-type tripod. All these complexes are extremely
stable, and as cationic species or/and simply because the
ligands are substituted with electron-withdrawing groups, they
exhibit high redox couples. As a consequence, the metal center
must be electron deficient, and even an Fe(II) center should be
able to activate a coordinated molecule. For instance, we could
envisage that in Figure 10A one acetonitrile might be replaced
by water which would hydrate the second ancillary ligand. This
might also be the case in the Fe(II) complex of the bpmpn
ligand displayed in Figure 10C: one acetonitrile and one water
molecule are ideally placed in cis position to be able to react
2
.360(2) Å, the shortest been the one between the metal and
the pyridine trans to the μ-oxo group, dFe−N1 2.154(2) Å.
The Fe−O3−Fe segment is linear, and the intermetallic
distance dFe−Fe = 3.54 Å lies in the expected range for μ-
2
5,52,53
oxo diferric complexes.
Recovery of the Free α-(H NCO) TPA Tripod by
2
2
D e c o m p l e x a t i o n . T r e a t m e n t o f e i t h e r α -
H NCO) TPAFeCl or {[α-(H NCO) TPAFe] -O}Cl by
(
2
2
2
2
2
2
4
KOH in a mixture of CH CN/H O resulted in immediate
3
2
bleaching. A standard extraction of the medium at neutral pH
by methylene chloride afforded the free tripod in a very clean
way as a white-off solid, whose spectroscopic data were
identical to those indicated above.
61
each with the other. To our knowledge, there is no mention
that any complex of this kind could lead to the corresponding
amides by any treatment.
Scheme 4. Oxidation of α-(H NCO) TPAFeCl Yielding {[α-(H NCO) TPAFe] -O}Cl
2
2
2
2
2
2
4
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4+
Figure 9. ORTEP diagram of the {[α-(H NCO) TPAFe] -O} ion, and coordination skeleton with the main angles. Chloride ions are omitted for
2
2
2
clarity. Selected distances (Å): Fe−O1 2.1616(18), Fe−O2 2.1439(18), Fe−O3 1.7703(5), Fe−N1 2.154(2), Fe−N2 2.360(2), Fe−N3 2.229(2),
Fe−N4 2.212(2), Fe−Fe 3.541(5). This complex has inversion symmetry with O3 occupying the inversion center: bond distances and angles are
identical for the inversion counterpart.
Figure 10. Examples of chemically stable bis-acetonitrile or bis-aquo cations with high redox couples (A and B, see refs 57−60). Precursors are
triflate- or perchlorate-bound complexes. (A and B) Ancillary ligands (NCMe or H O) can bind and exchange each other by a simple control of the
2
ligand concentration. (C) Example of a stable mixed acetonitrile/aquo complex characterized by X-ray diffraction analysis; see ref 61.
Figure 11. Example of a complex with a nitrile-substituted TPA ligand in which the nitrile is ideally positioned to undergo hydration by action of
water. This complex is stable in wet conditions; see ref 41.
Finally, Figure 11 illustrates a case of a binuclear assembly,
where the nitrile is not coordinated and water can bind to the
in excess, replace the ancillary chloride ions. In the present case,
we also work with an excess of water. Considering this, the
coordination of water to the metal center is more than likely.
Numerous examples can be found in the literature where a
water molecule is coordinated to a metal center that, in turn,
facilitates its deprotonation, increasing its nucleophilicity and
4
1
metal: in such a case, the position of the reagents would also
be ideal to promote nitrile hydration. However, even in wet
conditions, the hydration of the nitrile could never be observed.
It thus appears that the only coordination of the reagents to
an Fe(II) center, even with a high redox couple as found in
cationic species, or simply a good positioning in cis position, as
illustrated in Figures 10C and 11, do not define sufficient
conditions to ensure nitrile hydration.
1
,6,62
ability to attack neighboring groups.
We believe that this is
what happens here, even if for stability reasons we could not
detect these aquo-substituted species. We suggest that the
instability of the aquo species results from a fast hydration
reaction due to the perfect positioning of both reagents within
the complex as outlined below and displayed in Figure 12.
Ideal Positioning of the Nitrile Group in α-
(NC) TPAFeCl . From the structure analysis of α-
Coordination of Water to the Fe(II) Center. We recently
showed that the dissociation equilibrium of the chloride ions is
displaced toward uncoordination as better as the coordination
2
5,29,59
29
polyhedron is sterically constrained.
dioxygen,
Acetonitrile or
2
2
2
5,59
− which is a weak ligand−can easily when used
(NC) TPAFeCl in the solid state and in solution the proximity
2 2
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Inorganic Chemistry
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Synthetic Interest: Easy Preparation of α-
H NCO) TPA. The hydration reaction which is described in
(
2
2
the present study is not devoid of interest in itself. The
presence of amide substituents at the secondary coordination
sphere of tripodal ligands may have dramatic effects on the
63−67
reactivity of metal sites.
RNHCO-substituted tripods are
known, but their preparation requires working from picolinic
4
2,68,69
acid in harsh conditions
(by contrast with RCONH-
substituted ones which can be easily obtained from the
commercially available amino-substituted pyridines). CN-
substituted TPA-type tripods are easy to prepare with good
Figure 12. Short water-substituent distance in the crystallographically
+
2
59
characterized [F TPAFe(H O) ] dication according to ref (black
2
2
2
to excellent yields and constitute simple precursors of H NCO-
2
drawing). The light blue adds-on feature the presence of the two nitrile
groups instead of fluorine atoms, within a hypothetical six-coordinate
bis aquo α-(NC) TPAFe(H O) .
substituted ligands.
Coordination Chemistry. As expected, α-
H NCO) TPAFeCl reacts with O to yield a μ-oxo diferric
2 2 2 2
2
2
2
(
species. This reaction, which has been thoroughly described in
2
5,26,37
59,63
the past,
generally involves ligand exchange steps
and
of the nitrile groups with the reaction center is obvious. What
about a hypothetical complex with aquo groups instead of
chloride ions? We need to focus on the intermediate step that
in which water is coordinated instead of chloride and for which
we have no crystal structure. We do, however, have XRD data
proceeds very smoothly in the present case. This reflects a
relative rigidity and excellent stability of the coordination
polyhedron within the ferrous precursor in which the tripod is
5
tightly bound in a κ mode. The structure of the μ-oxo {[α-
4+
+2
59
(H NCO) TPAFe] -O} represents one of the few examples
2 2 2
42,51,70
on the bis-aquo complex [F TPAFe(H O) ] , which show
2
2
2
of seven-coordinate Fe complexes.
by comparison with F TPAFeCl (i) that replacement of a
2
2
chloride ligand by a water molecule provides a more compact
coordination polyhedron and (ii) that short distances between
the coordinated water proton and the substituent can be
measured in the 2.5−2.9 Å range. Thus, coming back to our
present study, the nitrile in the α-position of the pyridine
should be kept even closer to the ancillary ligand within an
aquo complex as shown in Figure 12 than in the chloro one.
ASSOCIATED CONTENT
Supporting Information
All synthetic details, including preparation and characterization
of all compounds mentioned; experimental details for the
hydration reaction; for the three reported structures, crystallo-
graphic files in CIF format have been deposited. This material
is available free of charge via the Internet at http://pubs.acs.org.
■
*
S
CONCLUSION
■
AUTHOR INFORMATION
Corresponding Author
Phone. ++33 (0)298 016 028. Fax: ++33 (0)298 017 001. E-
mail: dominique.mandon@univ-brest.fr.
■
Intramolecular Nitrile Hydration Reaction at an Fe(II)
Site as a Result of Combined Steric and Positioning
Effects. The fact that the β-substituted β-(NC) TPAFeCl is
not water sensitive suggests the involvement of an intra-
molecular hydration reaction of α-(NC) TPAFeCl , the nitrile
being ideally positioned and kept in place by the ligand
skeleton. In the end, the reaction product is stabilized by
carboxamide coordination to the metal, but it is also probably
stabilized relative to the starting material by release of steric
strain. In fact, in the present case, all structural and electronic
ingredients favor the reaction to take place in unexpected
conditions, i.e., by a ferrous Fe(II) center at room temperature.
To our knowledge, the present study together with our
preliminary communication of 2008 represent the only case of a
nitrile hydration with ferrous Fe(II).
*
2
2
Notes
2
2
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
We thank the CNRS, the Universite
■
́
Louis Pasteur, the
Universite
́
de Strasbourg, and the Universite de Bretagne
́
Occidentale in Brest. The Institut de Chimie de Strasbourg
(UMR 7177) and the CEMCA Laboratory in Brest (UMR
6521) are also gratefully acknowledged. We thank all colleagues
from the Service de RMN, Service de Microanalyse, and the
́
Service de Spectrometrie de Masse of the Institut de Chimie de
Figure 13. α-(NC) TPAFeCl complex exhibits flexibility, and by contrast with its β-(NC) TPAFeCl congener, the cyano groups are ideally placed
2
2
2
2
to be hydrated.
7
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Inorganic Chemistry
Article
Strasbourg. The Service de Diffraction des Rayons X and
Service de RMN of the Faculte des Sciences et Techniques at
UBO, Brest, are gratefully acknowledged. We finally thank the
Agence Nationale de la Recherche (ANR CATHYMETOXY)
for financial support.
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H. J. Am. Chem. Soc. 2000, 122, 2512−2522.
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