Bioinspired manganese(ii) complexes with a clickable ligand for immobilisation on a solid support

Article abstract of DOI:10.1039/c3dt53636j

Clickable ligands like N,N′-bis((pyridin-2-yl)methyl)prop-2-yn-1- amine (L¹) and N-((1-methyl-1H-imidazol-2-yl)methyl)-N-(pyridin-2- ylmethyl)prop-2-yn-1-amine (L²) have been used to synthesise a series of manganese(ii) complexes for grafting onto appropriate solid supports. These ligands mimic the 2-His-1-carboxylate facial chelation present in the active site of the manganese-dependent dioxygenase (MndD), while the alkyne side function allows grafting of the ligand onto an azido-functionalised support using click chemistry methodologies. Such synthetic analogues of the MndD crystallise in the solid state as double halide or pseudohalide-bridged dinuclear manganese(ii) complexes of the general formula [Mn₂(μ-X) ₂X₂L₂] [L = L¹ with X = Cl (1), Br (2), and N₃ (3); L = L² with X = N₃ (4)]. Complexes 1-4 are characterised by a weak magnetic exchange interaction between the two high-spin Mn^II ions through the two X^- bridges (J in the range of -0.059 to +5.30 cm^-1, H = -J·S _Mn1·S_Mn2 with S_Mn1 = S_Mn2 = 5/2). A new magneto-structural correlation of superexchange bis(μ _1,1-azido)dimanganese(ii) complexes has been proposed using both structural parameters, the Mn-N-Mn bridging angle and the Mn-N_azido distance. In MeOH-EtOH solution the dimeric species are present together with few percents of mononuclear manganese(ii) complexes as evidenced by electron paramagnetic resonance (EPR) spectroscopy. Grafting the complexes onto mesoporous silica of MCM-41 type stabilises both dimers and monomers in the nanopores of the solid.

Full text of DOI:10.1039/c3dt53636j

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Bioinspired manganese(II) complexes with a
clickable ligand for immobilisation on a solid
support†
Cite this: Dalton Trans., 2014, 43,
9704
Jérémy Chaignon,a,b Salah-Eddine Stiriba,a Francisco Lloret,a Consuelo Yuste,c
Guillaume Pilet,d Laurent Bonneviot,b Belén Albela*b and Isabel Castro*a
Clickable ligands like N,N’-bis((pyridin-2-yl)methyl)prop-2-yn-1-amine (L1) and N-((1-methyl-1H-imida-
zol-2-yl)methyl)-N-(pyridin-2-ylmethyl)prop-2-yn-1-amine (L2) have been used to synthesise a series of
manganese(II) complexes for grafting onto appropriate solid supports. These ligands mimic the 2-His-1-
carboxylate facial chelation present in the active site of the manganese-dependent dioxygenase (MndD),
while the alkyne side function allows grafting of the ligand onto an azido-functionalised support using
“click chemistry” methodologies. Such synthetic analogues of the MndD crystallise in the solid state as
double halide or pseudohalide-bridged dinuclear manganese(II) complexes of the general formula
[Mn2(µ-X)2X2L2] [L = L1 with X = Cl (1), Br (2), and N3 (3); L = L2 with X = N3 (4)]. Complexes 1–4 are charac-
terised by a weak magnetic exchange interaction between the two high-spin MnII ions through the two
X− bridges (J in the range of −0.059 to +5.30 cm−1, H = −J·SMn1·SMn2 with SMn1 = SMn2 = 5/2). A new
magneto-structural correlation of superexchange bis(μ1,1-azido)dimanganese(II) complexes has been proposed
using both structural parameters, the Mn–N–Mn bridging angle and the Mn–Nazido distance. In MeOH–EtOH
solution the dimeric species are present together with few percents of mononuclear manganese(II) complexes
as evidenced by electron paramagnetic resonance (EPR) spectroscopy. Grafting the complexes onto meso-
porous silica of MCM-41 type stabilises both dimers and monomers in the nanopores of the solid.
Received 26th December 2013,
Accepted 17th April 2014
DOI: 10.1039/c3dt53636j
www.rsc.org/dalton
solution with molecular bioinspired complexes. Therefore, it is
important to develop new ligands with side functions allowing
Introduction
A natural evolution of bioinspired coordination chemistry is reactions that are orthogonal to complexation for fixation on a
the immobilisation of metal complexes on solid supports solid support. One possibility is to use the Huisgen azide–
where confinement can be created as in metalloproteins.1–3 alkyne [2 + 3] cycloaddition reaction referred to as “click chem-
Fixation on a support allows not only achieving site isolation istry” by Sharpless and co-workers.4 Indeed, this efficient and
but also controlling site nuclearity. Indeed, for monomeric versatile strategy, developed mainly for the synthesis of elabo-
sites, oligomerisation and degradation are often observed in rate organic molecules (such as inhibitors for pharmaceutical
applications), removes constraining considerations of protec-
tion/deprotection and cross-reactivity.5,6 This synthetic strategy
has been applied to graft molecules onto various substrates
aInstituto de Ciencia Molecular, Universitat de València, C/Catedrático José Beltrán
2, 46980 Paterna, València, Spain. E-mail: isabel.castro@uv.es;
such as organic polymers, carbon nanotubes, oxides and
Fax: +34-963 543 274; Tel: +34-963 544 439
metals for the design of hybrid nanocomposite materials.7–9 In
the case of silica-based supports, either azide or alkyne func-
tions are grafted onto the solid support using the suitable
organosiloxane molecules before carrying out the Huisgen
cycloaddition reaction. In the case of immobilisation of a
metal complex, it is preferred to have the alkyne function in
the ligand and the azide in the tether to avoid complexation of
azide to the metal ion. This modular approach has been used
for example in the grafting of Co and V complexes with Schiff
base ligands, or Pt and Pd organometallic complexes to
develop heterogeneous catalysts.10,11
bLaboratoire de Chimie, Ecole Normale Supérieure de Lyon, University of Lyon,
46 Allée d’Italie, 69364 Lyon cedex 07, France. E-mail: belen.albela@ens-lyon.fr;
Fax: +33-472 72 88 60; Tel: +33-472 72 88 56
cPhysics Department, Universidade de Coimbra, Rua Larga, P-3004-516 Coimbra,
Portugal
dLaboratoire des Multimatériaux et Interfaces, UMR 5615 CNRS-Université Claude
Bernard Lyon 1, 43 boulevard du 11 novembre 1918, 69622 Villeurbanne cedex,
France
†Electronic supplementary information (ESI) available: Crystallographic details
of the complexes, crystal packing figures of 1–4, EPR simulation of 1 and N2
sorption isotherms of the porous materials. CCDC 905065, 911104–911106 for
1–4. For ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c3dt53636j
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strates to the acidic nature of the silica. Yet this latter should
not be taken as a standard reaction yield as results found in
the literature show average yields around 90%.27
The second method is a straightforward one-step protocol
consisting of a double electrophilic substitution involving pro-
pargylamine and 2-picolyl chloride.28 The reaction time of
12 h was prolonged up to 5 days allowing us to retrieve L1 in a
90 to 96% yield, compared to a 32% yield for 12 h. Therefore
this one-step protocol was the appropriate synthetic route to
the L1 ligand (Scheme 2a).
Scheme 1 Representation of ligands L1 and L2. Nitrogen atoms involved
in the coordination of the metal ion are highlighted in red.
Among metalloproteins, iron and manganese oxygenases
are particularly attractive because they can perform oxidation
of organic substrates by activating molecular oxygen.12–19 In
particular, catechol dioxygenases are ring-cleaving enzymes
that oxidise catechol derivatives with a concomitant ring
opening, which is a critical step in the aerobic degradation of
aromatic compounds by bacteria.19 Therefore, these systems
can be used as models to develop new materials for the degra-
dation of aromatic chemicals. For instance, manganese-depen-
dent dioxygenase (MndD) is known to catalyse the extradiol
cleavage of catechol derivatives.15–18,20 The active site of this
metalloprotein contains a mononuclear manganese centre co-
ordinated to two histidines (His) and one glutamate (Glu) or
one aspartate (Asp) in the facial position.21–23 This motif,
called the 2-His-1-carboxylate facial triad, is observed in cate-
chol dioxygenases as well as in other metalloproteins.15,24
In this work a new series of manganese complexes with two
different biomimetic clickable ligands N,N′-bis((pyridin-2-yl)-
methyl)prop-2-yn-1-amine (L1) and N-((1-methyl-1H-imidazol-2-
yl)methyl)-N-(pyridin-2-ylmethyl)prop-2-yn-1-amine (L2) is pres-
ented (Scheme 1). Both ligands can mimic the facial triad
present in the active site of the MndD and possess an alkyne
side function that allows for an efficient grafting onto a solid
support using click chemistry.
L2 is an original ligand that has been synthesized using an
adapted version of the first method, i.e., condensation of an
amine and an aldehyde, namely 2-picolylamine and 1-methyl-
2-imidazolecarboxaldehyde, respectively, followed by
a
reduction29 and an electrophilic substitution by propargyl
bromide (Scheme 2b). The first step provided the secondary
amine in 96% yield. It is interesting to note the absence of
undesired by-products in the second step, in contrast to the
synthesis of L1 that required purification by column chromato-
graphy. In fact, re-dissolution in CH2Cl2 and filtration through
KBr provided the pure ligand L2 in 80% yield.
Synthesis of the Mn(II) complexes
Complexes 1–4 were synthesized in MeOH using a stoichio-
metric amount of the ligand and metal salt (Scheme 3). For
compounds 1 and 2 involving chloride and bromide counter-
anions, respectively, the corresponding manganese salt was
available and directly used for the reaction. In the case of 3
and 4, involving azide anions, manganese nitrate was reacted
with the ligand and 4 equivalents of sodium azide were added
to exchange the nitrate anion by the azide one. Except for 2,
each compound precipitated as light brown powder in MeOH
within 30 min. Those powders were then redissolved in a
minimum amount of MeOH, while the solution containing 2
was used as it is. The crystals were grown using the same tech-
nique in all the cases, i.e., slow diffusion of diethyl ether in a
methanolic solution of the compound to afford crystals within
We report herein the crystal structures and the magnetic
properties of four novel manganese(II) complexes of formulae
[Mn2(L1)2Cl2(µ-Cl)2] (1), [Mn2(L1)2Br2(µ-Br)2] (2), [Mn2(L1)2(N3)2-
(µ1,1-N3)2]·2CH3OH (3), and [Mn2(L2)2(N3)2(µ1,1-N3)2] (4). The
grafting is exemplified by the reaction of 1 with mesoporous
silica (MCM-41 type) modified with azide functions.
Results and discussion
Synthesis of the ligands
The ligands L1 and L2 were synthesised according to experi-
mental procedures reported in the literature. In the case of L1,
two different synthetic methods were explored. The first one is
a two-step protocol starting with the reduction with NaBH4 of
the Schiff base resulting from the condensation of 2-picolyl-
amine with 2-pyridinecarboxaldehyde which is followed by an
electrophilic substitution with propargyl bromide to obtain the
desired L1 ligand.25,26 The yield of the first step was 96% with
a simple work-up whereas the purification by column chrom-
atography on silica in the second step only yielded 11% of the
product likely due to the less affinity of amino-pyridinic sub- Scheme 2 (a) Synthesis of the ligand L1 and (b) synthesis of the ligand L2.
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Scheme 3 Synthesis of the manganese(II) complexes.
a couple of days. Apart from 3, all the crystals were stable in air
at room temperature. Molecules of MeOH found in the struc-
ture of 3 were lost when left at room temperature, damaging
the crystallinity of the solid.
Description of the structures
Complexes 1–4 are double halide or pseudohalide-bridged
neutral dimers of Mn(II) and exhibit the same structural layout
(Fig. 1). They differ in the nature of the donor groups from the
organic ligand counterpart, i.e., amine and pyridines from L1
(1–3), or amine, pyridine and imidazole from L2 (4), as well as
the coordinating anion composing the double bridge between
the metal ions and completing the metal environment, i.e.,
chloride (1), bromide (2), or azide (3 and 4). Selected bond dis-
tances and angles for 1–4 are compiled in Table 1.
Within all complexes, the Mn(II) ions are located in a dis-
torted N3X3 [X = Cl (1), Br (2) and N (3 and 4)] octahedral
environment. In all the complexes, the Mn–N bond lengths
from the tridentate ligands are similar and in good agreement
with similar complexes involving a double bridge with double
chloride,30–32 bromide33 and end-on azide34–37 bridges. It is
important to note that the Mn–N bond length involving the
central amine nitrogen atom of the ligand is usually ∼0.1 Å
longer than the two external ones involving the pyridine and/
or imidazole nitrogen atoms. In the case of 4, involving L2, one
of the Mn–N bond lengths (N2) or the external nitrogen atoms
of the ligand is shorter (∼0.1) and the Mn–N bond length for
the central (N3) one is longer (∼0.1 Å) than those observed for
complexes 1–3 involving L1. The environment of the metal
ions is completed by three X atoms with two of them involved
in the double bridge between the two Mn(II) ions. As expected,
Mn–Cl bond lengths are shorter than the Mn–Br ones. More-
over, Mn–X bond lengths within the double bridge (X = Cl1,
Br1) are longer (∼0.1 Å) than the terminal ones (X = Cl3, Br3).
Fig. 1 Structures of Mn(II) complexes with L1 and L2 ligands 1–4.
In the crystal lattice of 1 and 2, structural packing is built
The distortion of the octahedral environment of the metal by weak hydrogen bonding H⋯X (X = Cl and Br, respectively)
ion is also exemplified by the distribution, far from the ideal interactions leading to a dense 3D network. In the case of 3,
values of 90 and 180°, of the X–Mn–X angles (X = N, Br and Cl) the co-crystallized methanol molecule, through the hydrogen
ranging from 69.72(5)° to 104.193(16)° and from 158.24(3)° to atom of the alcoholic group, forms hydrogen bonds with the
163.59(4)° for 1, from 71.45(8)° to 101.689(15)° and from terminal nitrogen atom of the double end-on azido-bridge.
159.85(5)° to 165.10(6)° for 2, from 72.86(5)° to 105.08(6)° and Finally, for 3 and 4, the crystal packing, in both cases, is
from 154.43(6)° to 163.57(6)° for 3, and from 69.79(4)° to assumed to be formed by weak interactions as van der Waals
103.22(7)° and from 154.83(6)° to 161.98(5)° for 4.
interactions for example.
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At room temperature, χMT for 1 and 2 is ca. 8.75 cm3 mol−1
K. Whereas this value is expected for two magnetically isolated
high-spin MnII ions (SMn = 5/2), the corresponding values for 3
and 4 are somewhat greater (ca. 9.5 cm3 mol−1 K). Upon
cooling, χMT for 1 remains constant until 25.0 K and it further
decreases to 7.70 cm3 mol−1 K at 2.0 K, suggesting the occur-
rence of a very weak antiferromagnetic interaction. In contrast,
χMT for 2–4 continuously increases with the decrease of the
temperature to reach maxima at 5.5 K (ca. 11.3 cm3 mol−1 K)
for 2 and at 10 K (ca. 14.8 cm3 mol−1 K) for 3 and 4. After these
maxima, χMT decreases to 7.5 (2), 13.7 (3) and 13.9 cm3 mol−1
K (4) at 2.0 K. The increase of χMT in the high temperature
region for 2–4 is indicative of the occurrence of a ferro-
magnetic interaction between the paramagnetic Mn(II) ions,
whereas the decrease at low temperatures can be attributed to
Table 1 Selected bond distances (Å) and angles (°)
1
2
3
4
Mn(1)–N(1)
Mn(1)–N(2)
Mn(1)–N(3)
Mn(1)–X(1)a
Mn(1)–X(2)a
Mn(1)–X(1)a
Mn⋯Mn
2.280(4)
2.272(3)
2.388(3)
2.515(2)
2.423(1)
2.574(1)
3.827(1)
97.52(4)
2.264(2)
2.269(2)
2.389(2)
2.6641(5)
2.5740(5)
2.7333(5)
4.0076(5)
95.88(1)
2.255(2)
2.265(2)
2.384(2)
2.225(2)
2.153(2)
2.244(2)
3.533(3)
104.50(6)
2.296(1)
2.192(2)
2.466(2)
2.231(1)
2.131(2)
2.209(1)
3.486(3)
103.47(6)
Mn–X(1)–Mna
a 1: X = Cl; 2: X = Br; 3 and 4: X = N3.
Magnetic properties
Fig. 2 shows the magnetic properties of 1–4 in the form of χMT weak intermolecular antiferromagnetic interactions and/or
versus T plots and M versus H plots, χM and M being the mag- zero-field splitting of the S = 5 ground spin state. However,
netic susceptibility and the magnetisation per two Mn(II) ions because of the large isotropic character of the six-coordinate
respectively, while T and H are the absolute temperature and high-spin Mn(II) ion, the zero-field splitting effects are
the applied magnetic field, respectively.
expected to be negligible.
Fig. 2 Temperature dependence of χMT for 1 (top left), 2 (top right), 3 (bottom left) and 4 (bottom right). The solid line represents the best fit based
on the parameters discussed in the text (Table 2). The inset shows the magnetization curve at 2.0 K and the solid line is the theoretical Brillouin
curve for two magnetically independent spin sextuplets, S = 5/2.
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The expression of the magnetic susceptibility for a diman- occurrence of antiferro- (1) or ferromagnetic (2–4) interactions
ganese(II) unit derived from the Van Vleck’s equation (the spin between the Mn(II) ions.
Hamiltonian being defined as H = −JSMn1·SMn2) is given by
The main magneto-structural parameters for a series of
[MnII(μ-X)2MnII] (X = Cl or Br) dinuclear complexes which were
the subject of previous studies are listed in Table 3. As far as
we know, no theoretical magneto-structural correlation has
been reported for this family of compounds. As one can see
therein, no correlation seems to exist between the structural
parameters (angles or distances) and the magnetic coupling
parameter (J), this circumstance being most likely due to the
low number of compounds, the small variation of the struc-
tural parameters and the experimental errors. However, there
must exist a correlation between the nature and the magnitude
of the magnetic exchange and the structural parameters, the
value of the Mn–X–Mn angle being the most determinant one,
as is known for other metal ions and bridging ligands.41 In
fact, the experimental data from Table 3 predict a change from
ferro- to antiferromagnetic behaviour at an angle of ca. 97° for
the bridgehead chloro atom. The value of this crossover Mn–
Cl–Mn angle is very close to that observed for the
bis(μ-hydroxo)dicopper(II) complexes (ca. 97.5°).42 Compound
eqn (1),38
2Ng2β2 ex þ 5e3x þ 14e6x þ 30e10x þ 55e15x
χM
¼
ð1Þ
kðT ꢀ θÞ 1 þ 3ex þ 5e3x þ 7e6x þ 9e10x þ 11e15x
where x = J/kT and θ is the Weiss constant defined as θ = zJ′S-
(S + 1)/3k with zJ′ accounting for the magnetic interaction
between the z nearest dinuclear units. The other parameters
have their usual meaning. The best-fit parameters obtained
are listed in Table 2.
As indicated above, the decrease of χMT at low temperatures
for 2–4 could be alternatively interpreted by the presence of a
zero-field splitting (D). Having this possibility in mind, the
magnetic data for 2–4 were also analysed through the Hamil-
tonian of eqn (2) using VPMAG.39
H ¼ ꢀJSMn1 ꢁSMn2 þ DðS2z Mn1 þ S2z Mn2 þ 35=6Þ
ð2Þ
2
is the first example of a bis(μ-bromo)dimanganese(II)
The least-squares fit of the experimental data to eqn (2)
gave values for the g and J parameters similar to those
obtained through eqn (1) (Table 2), but the values for the D
parameter [4.9 (2), 1.4 (3) and 1.1 cm−1 (4)] are too high to be
considered real and they are meaningless. In fact, the reported
D values for the high-spin d5 MnII ion in an octahedral
complex studied for its magnetic properties and so, we can
only compare it with the chloro derivatives listed in Table 3.
Concerning the bis(μ1,1-azido)dimanganese(II) complexes (3
and 4), an experimental relationship between the magnetic
coupling constant (J) and the Mn–N–Mn bridging angle (θ),
J (cm−1) = 0.552θ (deg) − 53.8, was proposed.36,43 Table 4
shows the magneto-structural parameters for double end-on
azido-bridged Mn(II) complexes. 3 and 4 follow roughly the
above relationship and they are in good agreement with the
fact that the value of the exchange coupling constant increases
with the Mn–N–Mn bridging angle, which is consistent with
theoretical predictions.41 A careful inspection of the data listed
in Table 4 shows that the values of the coupling parameter, J,
generally increase with an increase in the difference between
the two Mn–N(bridging azide) bond distances (Δd in
Table 4),36,43 which is an increase in the asymmetry of the
azido bridging. In this sense, a better magneto-structural cor-
relation is observed when we use both structural parameters (θ
and Δd). Fig. 3 shows the values of J versus the corresponding
values of (θ + 10Δd) from Table 4. The best linear fit gives the
equation J (cm−1) = 0.553[θ (deg) + 10Δd (Å)] − 54.111, which
is slightly better than J (cm−1) = 0.576θ (deg) − 54.040, where
the influence of Δd has been neglected.
environment are lower than 1 cm−1 40
So, the decrease of the
.
χMT values has to be mainly attributed to intermolecular anti-
ferromagnetic interactions.
Field dependence magnetisation plots of 1–4 at 2.0 K are
shown in the inset of the corresponding plot (Fig. 2). The iso-
thermal magnetisation data for 1 are well below the Brillouin
curve for two magnetically isolated S = 5/2 Mn(II) ions (with no
ZFS), while they are well above for 2–4, in agreement with the
Table 2 Best-fit parameters for 1–4 (see the text)
Compound
g
J/cm−1
zJ′/cm−1
1
2
3
4
1.99(1)
1.99(1)
2.01(1)
2.02(1)
−0.059(1)
+1.04(2)
+5.30(4)
+4.99(3)
0.0
−0.20(1)
−0.060(2)
−0.052(2)
Table 3 Selected magneto-structural parameters for bis(μ-X)dimanganese(II) complexes (X = Cl or Br)
Compounda
dMn–Mn/Å
dMn–Cl/Å
Mn–X–Mn/°
J/cm−1
Ref.
[Mn2(biz)2(μ-Cl)2]Cl2
3.74
3.79
3.9
3.82
4.00
2.57
2.48
2.50
2.54
2.69
93.5
95.7
96.4
97.5
95.9
+0.66
+0.68
+1.1
−0.06
+1.04
32
30
31
t.w.
t.w.
[Mn2(bpea)2Cl2(μ-Cl)2]
[Mn2(mpba)2Cl2(μ-Cl)2]
1
2
a Abbreviations: biz = 2,2′-biimidazoline; bpa = N,N′-bis(2-pyridylmethyl) ethylamine; mpbpa = N-(3-methoxypropyl)-N,N′-bis(pyridin-2-ylmethyl)
amine; L1 = see text; t.w. = this work.
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Table 4 Selected magneto-structural parameters for bis(μ1,1-azido)dimanganese(II) complexesa
Compound
Δd/Å
θ (°)
θ + 10Δd
J/cm−1
Ref.
[Mn2(ttp)2(N3)2(μ1,1-N3)2]
[Mn2(ttp-N3)2(N3)2(μ1,1-N3)2]
[Mn2(ttp-N3)2(N3)2(μ1,1-N3)2]2[Mn(ttp-N3)(N3)3]
[Mn2(terpy)2(N3)2(μ1,1-N3)2]·2H2O
[Mn2(LA)2(μ1,1-N3)2](ClO4)2
[Mn2(LB)2(μ1,1-N3)2](ClO4)2
[Mn2(LC)2(μ1,1-N3)2](ClO4)2
[Mn2(phen)4(μ1,1-N3)2][Co(bpb)(CN)2]2·H2O
[Mn2(phen)4(μ1,1-N3)2][Cr(bpb)(CN)2]2·H2O
[Mn2(phen)4(μ1,1-N3)2][Fe(bpb)(CN)2]2·H2O
[Mn2(2,2′-dpa)2(N3)2(μ1,1-N3)2]
[Mn(tptz)(μ1,1-N3)2]n
0.236
0.252
0.150
0.090
0.030
0.072
0.053
0.002
0.003
0.006
0.059
0.002
0.000
0.032
0.202
0.019
0.020
103.45
103.13
105.29
104.60
102.12
104.29
103.58
102.60
102.55
101.40
103.11
106.16
98.80
105.81
105.65
106.79
105.50
102.42
105.01
104.11
102.62
102.58
101.46
103.70
106.18
98.800
100.82
102.40
104.69
104.16
4.92
4.50
3.84
4.86
1.54
4.09
3.50
2.54
2.20
2.40
2.24
4.06
1.22
0.80
2.62
5.30
4.99
36
36
36
44
43
43
43
45
45
45
34
46
47
48
37
t.w.
t.w.
[Mn(pyz)(μ1,1-N3)2]
[Mn(2-bzpy)(μ1,1-N3)2]n
100.50
100.38
104.50
103.96
[Mn2LD(N3)2(μ1,1-N3)2]
3
4
a Average values. Δd = d(Mn–Nazido) – d(Mn–N′azido). ttp = 4′-p-Tolyl-2,2′:6′,2′’-terpyridine, ttp-N3 = 4′-p-azidomethylphenyl-2,2′:6′,2′’-terpyridine, LA
= [N,N-bis(pyridine-2-yl)benzylidene]ethane-1,2-diamine, LB = [N,N-bis(pyridine-2-yl)-benzylidene]propane-1,3-diamine, LC = [N,N-bis(pyridine-2-
yl)benzylidene]butane-1,4-diamine, bpb2− = 1,2-bis(pyridine-2-carboxamido)benzenate, 2,2′-dpa = 2,2′-dipicolylamine, tptz = 2,4,6-tris(2-pyridyl)-
1,3,5-triazine, pyz = pyrazine, 2-bzpy = 2-benzoylpyridine, LD = 2-(benzimidazol-2-yl)-N-(pyridin-2-ylmethyl)ethanamine.
EPR characterization of the complex in solution
main hyperfine lines (Fig. 4).49 A quantitative study was per-
formed with complex 1 at room temperature in order to deter-
mine the proportion of monomeric and dimeric species
present in the solution. The spin concentration was measured
using manganese(II) perchlorate in methanolic solution at
various concentrations for calibration. Accordingly, only
1–1.5% of this dimeric complex yields EPR active monomeric
species in solution at room temperature.
The molecular complex 1 is conveniently dissolved in MeOH–
EtOH solution. Such a solution exhibits at room temperature a
6 line EPR signal centered at g = 2.001
0.001, typical of
Mn(II). It is assigned to the hyperfine coupling between the
electronic spin (S = 5/2) and the nuclear manganese spin (I =
5/2) for the allowed transitions (ΔmS
simulation, the hyperfine coupling constant A is 97
=
1, ΔmI = 0). Upon
1 G
(Fig. S5†). At low temperatures, a pair of forbidden lines due to
Grafting onto silica
cross terms in the spin Hamiltonian lie between each of the
The above solution was used to graft the manganese complex
in the nanopores of a mesoporous silica. 2D hexagonal LUS
silica (MCM-41 type) prepared using cetyltrimethylammonium
tosylate as the surfactant was chosen as a support.50,51 It pos-
sesses a narrow pore size distribution of 3.8 0.1 nm and a
large mesopore volume of 0.78 0.01 cm3 g−1. Then azide-
functionalised N3–(CH2)3–Si– groups were introduced into the
Fig. 3 Plot of J versus (θ + 10Δd), θ being the average Mn–N–Mn
angles and Δd being the average Mn–N (azido-bridge) bond distances
(see the text), for double end-on azido-bridged Mn(II) complexes (see
Table 3). The solid straight line represents the best linear fit to equation
J = 0.553(θ + 10Δd) − 54.111.
Fig. 4 EPR spectra of 1 dissolved in the mixture MeOH–EtOH 1 : 1 (v/v)
at 116 K and MnL1@SiO2 at 298 K. Microwave frequency: 9.41 GHz,
amplitude modulation: 1 G (top), 5 G (bottom), gain: 60 dB, power:
4 mW (top), 2 mW (bottom).
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solid support using a molecular stencil patterning technique
that allows an homogeneous distribution of the desired func-
Conclusions
tions, which are separated by hydrophobic (CH3)3–Si– A series of dinuclear manganese(II) complexes with two μ-X
groups.3,52–54 The manganese complex 1 was covalently grafted bridges (X = Cl, Br or N3) has been synthesised using two
by using click chemistry. As an illustrative example, the azide- different clickable ligands. These ligands allow both coordi-
functionalized silica (N3@SiO2) was reacted with 1 in a MeOH– nation to a metal ion and also grafting onto an azide-functio-
MeCN mixture in the presence of the tris-(triphenylphosphine) nalized support using the Huisgen alkyne–azide cycloaddition
copper(I) bromide catalyst55 to obtain the hybrid material reaction. Within this series we present the first μ-Br bridged
MnL1@SiO2, with 2.5 wt% manganese. This complex loading Mn(II) dinuclear complex for which a complete magnetic pro-
corresponds to ∼80% yield for the click reaction. Note that the perty study has been performed. It turned to be weakly ferro-
use of such a Cu(I) complex avoids the displacement of Mn(II) magnetic compared to the μ-Cl analogue, which is weakly
from the L1 ligand by Cu(I) during the grafting of the complex. antiferromagnetic. It is also noteworthy that a better magneto-
Indeed, there are no traces of Cu(II) that would form under structural correlation is observed when both structural para-
aerobic conditions, according to the EPR spectrum (Fig. 4). On meters (θ and Δd) are considered for the bis(μ1,1-azido)diman-
the other hand, the signal reveals the presence of Mn(II) as in ganese(II) complexes. Successful stabilisation of the complex
solution. Indeed, the proportion of EPR active species remains mainly in its dimeric form has been achieved by grafting onto
similar to that in solution, consistent with most of the species mesoporous silica. This kind of hybrid nanocomposite material,
being still dimeric. In addition, the signal is broader than in where confinement is at stake, could help by comparison to
solution both at room temperature and at 116 K. Therefore, better understand the catalytic mechanism of MndD.16,23,56 The
this broadening is not due to dynamic effects but due to distri- study of the catalytic properties of these silica-supported
bution of species on the surface. This gives evidence of the manganese(II) complexes is under progress. Alternatively, the
heterogeneity of the environment around the grafted species alkyne function present in the ligand opens up other opportu-
as usually observed in such hybrid materials.2
nities such as using these complexes as building blocks for the
The hexagonal array of the internal pores of the material synthesis of metal–organic frameworks (MOFs).57
was unaltered all through the different steps of the synthesis,
as shown from the X-ray diffraction patterns (Fig. 5). The
decrease of pore volume from 0.78 cm3 g−1 down to 0.48 cm3
Experimental
Synthesis of the ligands
g−1 after grafting confirmed the presence of the metal complex
inside the pores (ESI, Fig. S6†). Concomitantly, the specific
surface area underwent a diminution from 990 m2 g−1 to
340 m2 g−1. This is consistent with partial pore filling due to
internal molecular functionalization of the silica.
N,N′-Bis((pyridin-2-yl)methyl)prop-2-yn-1-amine ligand (L1).
The synthesis was based on a published protocol.28 A mixture
of propargylamine (1.50 g, 27.3 mmol) and potassium carbon-
ate (22.8 g, 163.8 mmol) was stirred in 140 mL of acetonitrile
for 5 min. Then 2-picolyl chloride hydrochloride (9.84 g,
60.0 mmol) dissolved in 140 mL of acetonitrile was added. The
resulting solution was stirred under reflux for 5 days. The solu-
tion was filtered and the filtrate was evaporated under reduced
pressure. The resulting brown oil was dissolved in 100 mL of
distilled water and extracted thrice with dichloromethane. The
organic phases were gathered and dried over sodium sulphate.
The solvent was finally evaporated to afford L1 in 94% yield.
1H NMR (CDCl3, 300 MHz): δ (ppm) 8.55 (ddd, J = 4.80, 1.80,
0.90 Hz, 2H), 7.64 (td, J = 7.50, 1.80 Hz, 2H), 7.50 (d, J =
7.80 Hz, 2H), 7.15 (ddd, J = 7.50, 4.80, 1.20 Hz, 2H), 3.91 (s,
4H), 3.41 (d, J = 2.40 Hz, 2H), 2.29 (t, J = 2.40 Hz, 1H). 13C
NMR (300 MHz, CDCl3): δ (ppm) 158.76, 149.26, 136.47,
123.14, 122.10, 78.32, 73.62, 59.44, 42.54. IR (KBr): 3294 (w),
3062 (w), 3011 (w), 2921 (w), 2839 (w), 2095 (m, CuC) cm−1
.
N-((1-Methyl-1H-imidazol-2-yl)methyl)-N-(pyridin-2-ylmethyl)-
prop-2-yn-1-amine (L2). This ligand was synthesized in two
steps.
N-(1-Methyl-1H-imidazol-2-ylmethyl)pyrid-2-ylmethylamine.29
2-Picolylamine (2.45 g, 22.7 mmol) in 25 mL of methanol was
added dropwise to a solution of 1-methyl-2-imidazolecarboxal-
dehyde (2.5 g, 22.7 mmol) in 50 mL of methanol. The resulting
solution was stirred for 3 h at room temperature. Sodium boro-
Fig. 5 X-ray diffraction patterns (powder) of LUS (mesoporous SiO2
support), N3@SiO2 and MnL1@SiO2 materials.
9710 | Dalton Trans., 2014, 43, 9704–9713
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hydride (0.86 g, 22.7 mmol) was added portionwise and the solved in methanol, and slow diffusion of diethylether into
solution was further stirred for two more hours at room temp- that solution provided crystals suitable for X-ray diffraction.
erature. The solvent was evaporated under reduced pressure
[Mn2(L2)2(N3)2(μ-N3)2] (4). The synthesis is similar to the
and the orange oil thus obtained was redissolved in a mixture synthesis of 3, the only difference being that L2 (24 mg,
of water–methanol (25 mL/50 mL), before being extracted 0.1 mmol) was used instead of L1. Suitable crystals for X-ray
thrice with 50 mL of dichloromethane. The combined organic diffraction were obtained following the same diffusion
phases were washed with 50 mL of water and dried over protocol.
sodium sulphate. The solvent was finally removed to obtain
Synthesis of MnL1@SiO2
the product as an orange oil in 96% yield. 1H NMR (CDCl3,
300 MHz): δ (ppm) 8.54 (ddd, J = 4.80, 1.80, 0.90 Hz, 1H), 7.63 The manganese complex was grafted onto LUS mesoporous
(td, J = 7.50, 1.80 Hz, 1H), 7.31 (d, J = 7.80 Hz, 1H), 7.15 (ddd, silica where azide functions were previously incorporated
J = 7.50, 4.80, 1.20 Hz, 1H), 6.92 (d, J = 1.20 Hz, 1H), 6.80 (d, using the so-called molecular stencil pattering technique
J = 1.20 Hz, 1H), 3.93 (s, 2H), 3.89 (s, 2H), 3.66 (s, 3H).
described elsewhere.53
Synthesis of L2. N-(1-Methyl-1H-imidazol-2-ylmethyl)pyrid-2-
ylmethylamine (3 g, 14.8 mmol) and potassium carbonate
(8.18 g, 59.2 mmol) were dissolved in 40 mL of tetrahydro-
furan. Then propargyl bromide was added dropwise, and the
resulting solution was heated at 50 °C for 18 h. The solvent
was removed under reduced pressure, and the brown oil thus
obtained was dissolved in dichloromethane before being
passed over several pads of potassium carbonate until disap-
pearance of the hydrobromic acid. The dichloromethane was
removed under reduced pressure and the ligand L2 was
obtained in an 80% yield. 1H NMR (CDCl3, 300 MHz): δ (ppm)
8.51 (ddd, J = 4.80, 1.80, 0.90 Hz, 1H), 7.60 (td, J = 7.50, 1.80
Hz, 1H), 7.32 (d, J = 7.80 Hz, 1H), 7.12 (ddd, J = 7.50, 4.80, 1.20
Hz, 1H), 6.88 (d, J = 1.20 Hz, 1H), 6.76 (d, J = 1.20 Hz, 1H), 3.80
(s, 2H), 3.79 (s, 2H), 3.58 (s, 3H), 3.32 (d, J = 2.40 Hz, 2H), 2.27
(t, J = 2.40 Hz, 1H). 13C NMR (300 MHz, CDCl3): δ (ppm)
158.28, 149.26, 144.59, 136.45, 127.27, 123.44, 122.21, 121.58,
78.27, 73.74, 59.29, 49.54, 42.44, 32.84. IR (KBr): 3010 (w),
Mesoporous silica LUS. A solution of cetyltrimethylammo-
nium tosylate (CTATos) (1.96 g, 4.3 mmol) in distilled water
(71 mL) was stirred at 60 °C for 1 h. In the meantime, a
sodium silicate solution (49 mL) was also stirred at 60 °C for
1 h. The silicate solution was added to the surfactant one by
pouring it slowly on the edge of the recipient. After a vigorous
shaking by hand, the resulting white mixture was placed in
two autoclaves and heated in a microwave oven for 10 min at
180 °C. The autoclaves were cooled in an ice bath for 10 min
before filtration and washing with distilled water (about
100 mL). The white solid obtained was dried at 80 °C to obtain
3.1 to 3.4 g of LUS. Elemental analyses: Si (23.63%), C
(30.91%), H (6.31%), N (1.72%), S (0.20%).
N3@SiO2. LUS silica was functionalized with trimethylsilyl
functions (TMS) using
a
reported protocol to afford
TMS@SiO2.53 Then, TMS@LUS (500 mg) was pre-treated at
130 °C under argon for 2 h, and at 130 °C under vacuum for
2 more hours. After cooling under argon down to room temp-
erature, a solution of 3-azidopropyl triethoxysilane (371 mg,
1.5 mmol) dissolved in 5 mL of dry toluene was added. The
suspension was stirred for 1 h, 10 mL of toluene was added,
and the mixture was stirred at 80 °C for 18 h. After filtration
2926 (w, br), 2839 (w), 2100 (m, CuC) cm−1
.
Synthesis of the manganese(II) complexes
[Mn2(L1)2(Cl)2(μ-Cl)2] (1). A solution of L1 (47 mg, and washing with 20 mL of toluene, 20 mL of EtOH 70% and
0.2 mmol) in 2 mL of methanol was slowly added to a solution 20 mL of acetone, the solid was finally dried overnight at
of MnCl2 (40 mg, 0.2 mmol) in 2 mL of methanol. The solu- 80 °C. Elemental analysis (wt%): C: 5.18%, H: 1.80%,
tion was stirred for 1 h, and then the solid was filtered and N: 2.06%.
washed with methanol to obtain 38 mg of the complex in 52%
MnL1@SiO2. N3@SiO2 (300 mg) was added to the mixture
yield. This powder was redissolved in a minimum amount of MeOH–MeCN (60 mL/20 mL) and stirred. After 10 min,
methanol, and pale yellow crystals of 1 were obtained by slow complex 1 (176 mg, 0.5 mmol) was added and the suspension
diffusion of diethylether into this solution.
was stirred for 1 h, before addition of CuBr(PPh3)3 (93 mg,
[Mn2(L1)2(Br)2(μ-Br)2] (2). A solution of L1 (47 mg, 0.10 mmol). Then the mixture was stirred at 60 °C for 6 days
0.2 mmol) in 2 mL of methanol was slowly added to a solution before filtration. The pale brown solid was washed with
of MnBr2·4H2O (57 mg, 0.2 mmol) in 2 mL of methanol. This 200 mL of MeOH to eliminate any catalyst and unreacted
solution was kept under stirring for 1 h. Brown crystals of 2 species left, and dried overnight at 80 °C. Elemental analysis
were obtained by slow diffusion of diethylether into the latter (wt%): C: 6.42%, H: 1.75%, N: 1.44%, Mn: 2.43%, Cl: 0.74%.
solution.
Crystal structure determination and refinement
[Mn2(L1)2(N3)2(μ-N3)2]·2CH3OH (3). A solution of L1 (24 mg,
0.1 mmol) in 2 mL of methanol was slowly added to a solution Single crystal X-ray diffraction data for 2–3 were collected at
of Mn(NO3)2·4H2O (25 mg, 0.1 mmol) in 2 mL of methanol. 293(2) K on a Bruker APEX II diffractometer using graphite-
The solution was stirred for 30 min, and then a solution of monochromated Mo-Kα radiation while data for 1 and 4 were
NaN3 (26 mg, 0.4 mmol) in 3 mL of methanol was added. The collected and treated on a Gemini Oxford Diffractometer at
mixture was further stirred for 1 h, and the solid thus formed 150 K and the related analysis software.58 Data reduction for
was filtered and washed with methanol. The solid was redis- 2–3 were performed with the SMART and SAINT software.59
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The structures were solved by direct methods using the dação para a Ciência e Tecnologia (Project PEst-C/FIS/UI0036/
SHELXS-97 program,60 and refined on F2s by full-matrix least- 2011).
squares with the SHELXL-97 program included in the WINGX
software package.60,61 Absorption correction was applied using
SADABS.62 For complexes 1 and 4, an absorption correction
based on the crystal faces was applied to the datasets (analyti-
Notes and references
cal).63 Structures of 1 and 4 were solved by direct methods
combined with Fourier difference syntheses and refined
against F using reflections with [I/σ(I) > 3] using the CRYSTALS
program.64 All non-hydrogen atoms were refined using aniso-
tropic terms. The hydrogen atoms were placed at the calcu-
lated positions and refined with isotropic parameters as riding
atoms. The hydrogen atoms for the co-crystallized methanol
molecules were located from difference maps for complex 3.
The final geometrical calculations and the graphical manipula-
tions were carried out with PARST95,65 PLATON,66 (for com-
plexes 2–4) and DIAMOND67 programs.
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