Mononuclear complexes of amide-based ligands containing appended functional groups: Role of secondary coordination spheres on catalysis

Article abstract of DOI:10.1039/c4dt02079k

Amide-based ligands H₂L¹, H₂L² and H₂L³ containing thiazole, thiazoline and benzothiazole appended groups have been used to synthesize Zn²⁺ (1 and 3), Cd²⁺ complexes (2 and 4), and a Mn²⁺ complex (5). In all cases, potentially multidentate ligands create a meridional N₃ coordination environment around the M(ii) ion whereas additional sites are occupied by labile nitrate ions in 1-4 and MeOH in 5. Interestingly, metal complexation caused the migration of protons from amidic N-H sites to the appended heterocyclic rings in complexes 1-4. Structural studies show that the protonated heterocyclic rings in these complexes create a hydrogen bond based cavity adjacent to the metal ion. Importantly, binding studies confirm that the substrates are bound within the complex cavity closer to the Lewis acidic metal in all complexes including the oxidation-sensitive Mn ion in complex 5. All complexes have been utilized as the reusable and heterogeneous catalysts for ring-opening reactions of assorted epoxides, cyanation reactions of various aldehydes, and epoxidation reactions of several olefins. This journal is

Full text of DOI:10.1039/c4dt02079k

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Synopsis (Graphic)
Synopsis (Abstract)
Coordination complexes of amide-based ligands with appended heterocyclic rings create a
hydrogen bonding cavity that effectively bind the substrate(s). Such cavity-based complexes
function as reusable and heterogeneous catalysts for the ring-opening reactions of epoxides;
cyanation reactions of aldehydes; and epoxidation reactions of olefins.

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Journal Name
RSCPublishing
ARTICLE
Mononuclear complexes of amide-based ligands
containing appended functional groups: Role of
secondary coordination sphere on catalysis
Cite this: DOI: 10.1039/x0xx00000x
Deepak Bansal,a Gulshan Kumar,a Geeta Hundalb and Rajeev Gupta*a
Received 00th January 2012,
Accepted 00th January 2012
Amideꢀbased ligands H2L1, H2L2 and H2L3 containing thiazole, thiazoline and benzothiazole
appended groups have been used to synthesize Zn2+
(
1
and
). In all cases, potentially multidentate ligands create a meridional N3
coordination environment around the M(II) ion whereas additional sites are occupied by labile
nitrate ion in and MeOH in . Interestingly, metal complexation caused the migration of
3 2 and 4); and a
), Cd2+ complexes (
DOI: 10.1039/x0xx00000x
Mn2+ complex (
5
www.rsc.org/
1
–
4
5
protons from amidic NꢀH sites to the appended heterocyclic rings in complexes. Structural
studies show that the protonated heterocyclic rings in all complexes create a hydrogen bond
based cavity adjacent to metal ion. Importantly, binding studies confirm that the substrates are
bound within the complex cavity closer to the Lewis acidic metal in all complexes including
the oxidationꢀsensitive Mn ion in complex 5. All complexes have been utilized as the reusable
and heterogeneous catalysts for ringꢀopening reactions of assorted epoxides; cyanation
reactions of various aldehydes; and epoxidation reactions of several olefins.
Introduction
In recent years, significant interest has been devoted to develop
metalꢀbased catalysts due of their uniqueness in controlling
reactivity and selectivity of a transformation.1 The motivation for
such metal mediated catalytic transformations is derived from
biological systems which are paramount for the precise control over
the reactivity and selectivity.2 Importantly, biological catalysts
achieve such an extraordinary feat by the modulation of secondary
coordination sphere and often create a substrateꢀspecific cavity for
Scheme 1. Schematic representation for possible activation of substrate
within the cavity as expected from complexes 1 – 5. M = Metal: X = anion
and/or solvent; R = Appended groups; S = Substrate.
In this context, multidentate ligands containing appended
functional groups have shown promising results by showcasing
creation of secondary coordination sphere adjacent to metal site.5
Out of various multidentate ligands, ligands based on pyridineꢀ2,6ꢀ
dicarboxamide scaffold have displayed noteworthy results in recent
time.6,7 Such ligands8 in their deprotonated form generate a pincer
cavity via two Nꢀamidate groups and an anchoring pyridine ring that
strongly hold a metal ion while appended functional groups are
adequately positioned to create secondary coordination sphere.
Fiedler and coꢀworkers9 have used pyridineꢀ2,6ꢀdicaboxamide
ligands appended with pyridine, pyrimidine or quinolone rings to
prepare their Cu(II) complexes. Interestingly, protonation of the
appended heterocyclic rings was observed during the synthesis and
such rings create intramolecular Hꢀbonds to the ligated chloro and/or
aqua ligand. Wright and coꢀworkers10 have also used a similar ligand
the binding and activation of
a
substrate.2 For secondary
coordination sphere, hydrogen bonding (Hꢀbonding) sensitive
functional groups play crucial role in holding and organising the
substrate closer to the metal site.3 However, incorporation of desired
secondary coordination sphere adjacent to a metal site is always a
daunting challenge in synthetic complexes. Thus, continuous efforts
are being made to enhance the catalytic relevance and performance
of metalꢀbased catalysts by incorporating functional groups capable
of fostering weak interactions including hydrogen bonds (Hꢀbonds).4
Conceptually, scheme 1 explains our approach of cavity based
catalyst where central part of a multidentate ligand coordinates a
metal ion whereas appended “R” groups create an artificial cavity
capable of interacting and organising the substrate “S” closer to the
metal center. Such a situation is likely to enhance the interaction of
substrate to metal ion and therefore may assist in its activation and
subsequent transformation.
This journal is © The Royal Society of Chemistry 2013
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DOI: 10.1039/C4DT02079K
to prepare a Ru(II) complex where protonated pyridine rings formed
Hꢀbonds to the ligated chloro ligand.
Results and Discussion
Synthesis and characterisation of complexes 1-5
These examples nicely illustrate that the substituted pyridineꢀ2,6ꢀ
dicarboxamideꢀbased ligands are attractive frameworks for the
incorporation of secondꢀsphere Hꢀbonding interactions into the
transitionꢀmetal complexes. In this work, we have used similar
ligands based on pyridineꢀ2,6ꢀdicarboxamide scaffold; however,
with assorted appended groups. We show that such appended
functional groups in the resultant coordination complexes create an
Hꢀbonding based cavity around the metal ion. Our results suggest
that the presence of cavity adjacent to metal ion assist in holding and
organising the substrate closer to the catalytic site. In particular,
ligands H2L1, H2L2 and H2L3 having thiazole, thiazoline and
benzothiazole appended groups (Figures S1 – S9; ESI) have been
used to synthesize Zn2+ (1 and 3) and Cd2+ complexes (2 and 4); and
This works discusses two types of coordination complexes; 1 – 4
that are based on Zn(II) and Cd(II) ions and 5 which is a Mn(II)
complex. Complexes 1 – 4 were synthesised by treating ligand H2L1
or H2L2 with [M(NO3)2].xH2O without any requirement of a base.
Interestingly, complexation resulted in the migration of protons from
amidic NꢀH sites to the appended heterocyclic rings. Complex 5,
however, was synthesised by treating the deprotonated form of
ligand H2L3 with [Mn(H2O)6](ClO4)2. All complexes were obtained
as pale yellow crystalline material after standard workup and
recrystallization. FTIR spectra of complexes 1, 3 and 4 revealed an
isostructural nature except for the minor differences due to different
appended rings. The νC=O stretches for 1 – 4 were noted at 1612ꢀ1654
cmꢀ1 which were redꢀshifted by 40–60 cmꢀ1 than their respective
ligands and suggest the involvement of deprotonated Namidate in
bonding.11 In addition, νNH stretches for the protonated heterocyclic
rings appeared at 3276 and 3195 cmꢀ1 for 1 and 2 while at 3323 and
3319 cmꢀ1 for 3 and 4, respectively. These NꢀH stretches are
significantly different (ca. 85 – 160 cmꢀ1) when compared to the
original ligands and conclusively prove that the protons have been
reꢀlocated (Figures S10ꢀS13, ESI). The presence of nitrate ion was
established by the observation of νN=O stretches at 1335ꢀ1355, 1268ꢀ
1287, and 1016ꢀ1027 cmꢀ1. Complex 5 exhibits νOꢀMe stretch at 1016
cmꢀ1 indicative of coordinated methanol.11 The conductivity
measurements12 substantiate that the nitrate ions in complexes 1 – 4
are loosely coordinated and are dissociated once these complexes are
dissolved in DMF or DMSO. The absorption spectra of ligands
H2L1, H2L2 and H2L3 exhibit λmax at 303, 285 and 310 nm,
respectively. Complexes 1, 2 and 5 display bathochromically shifted
a
Mn2+ complex (5) (Scheme 2). These wellꢀcharacterized
complexes have been used as the reusable and heterogeneous
catalysts for the ringꢀopening reactions (RORs); cyanation reactions
(CRs); and epoxidation reactions (ERs).
O
O
N
S
S
N
N
Zn(NO3)2.xH2O
Zn
O
O
N
N
H
O
O
N
H
O
N
O
R = S
N
(1)
O
H2L1
O
N
N
S
N
O
S
Cd
NO3
N
O
N
H
Cd(NO3)2.xH2O
OMe
N
H
H
λ
max at 325, 330 and 340 nm, respectively whereas complexes 3 and
O
H
O
N
O
N
4 exhibit spectral features below 300 nm (Figure S14, ESI).
Complexes 1 – 4 have also been characterised by the 1H NMR
spectra and display minor changes in chemical environment when
compared to freeꢀligand spectra (Figures S15 – S18, ESI). Notably,
NMR spectra also established the presence of heterocyclic NꢀH
protons that disappeared on deuterium exchange.
O
O3N
NH
O
R = S
N
Cd
S
N
N
S
H2L1
N
(2)
O
O
R
O
R
N
O
O
NH
HN
N
Crystal Structures
S
S
N
O
N
N
M(NO3)2.xH2O
M
All five complexes; [{L1(H)2}Zn(NO3)2] (1), [{L1(H)2}Cd(NO3)1.5 2
]
O
N
N
H
O
O
O
N
(ꢀꢀOCH3) (2), [{L2(H)2}Zn(NO3)2] (3), [{L2(H)2}Cd(NO3)2] (4) and
[Mn(L3)(CH3OH)3] (5) have been crystallographically characterised.
The molecular structures of complexes 1, 2 and 5 are shown in
figures 1, 2 and 3 whereas crystal structures of 3 and 4 are presented
in figures S19 and S20. The selected bonding parameters are
contained in Tables S1 and S2.
R =
N
H
O
S
H2L2
[Mn(H2O)6](ClO4)2
M = Zn2+ (3); Cd2+(4)
O
N
O
N
N
R =
S
N
N
Mn
S
S
O
O
H
H
N
OH
(5)
H2L3
Scheme 2. Preparative routes for the synthesis of complexes 1-5
.
2 | J. Name., 2012, 00, 1-3
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ion making shorter bonds. In this case also, nitrogen atoms of the
appended heterocyclic rings are protonated to afford thiazolium
moieties. Such thiazolium rings are involved in Hꢀbonding with the
O atoms of monodentate nitrate ions. The heteroatom separations for
the O3nitrate…H5–N5 and O5nitrate…H1–N1 synthons are 2.747 and
2.726 Å, respectively.
(a)
(b)
(a)
(b)
(d)
(c)
Figure 2. (a) Thermal ellipsoidal representation of complex 2; ellipsoids are
drawn at 30% probability level whereas only selected hydrogen atoms are
shown for clarity. (b) Secondary coordination sphere created by the
protonated heterocyclic rings around the nitrate ion in complex 2.
Figure 1. (a) Thermal ellipsoidal representation of complex 1; ellipsoids are
drawn at 30% probability level whereas only selected hydrogen atoms are
shown for clarity. (bꢀd) Secondary coordination sphere created by the
protonated heterocyclic rings around the nitrate ions in complexes 1, 3, and 4.
Crystal structure of 5
Crystal Structures of 1, 3 and 4
Complex 5 displays a similar structure to that of complexes 1, 3, and
4; however, without protonation of the appended heterocyclic rings.
Here Mn2+ ion is present in the pincer part of the ligand coordinated
by two anionic Namide and one neutral Npyridyl atoms while
additionally ligated by three methanol molecules. The MnꢀNamide and
MnꢀNpyridyl bond lengths are comparable whereas Mn…O(H)Me
distances vary between 2.128 – 2.205 Å. The geometry around the
Mn2+ ion can be best described as distorted octahedral with two
methanol molecules occupying the axial positions. Complex 5
displays reversed arrangement of Hꢀbonding interactions than what
observed in complexes 1 – 4. In this case, appended heterocyclic
rings function as the Hꢀbond acceptors whereas coordinated
methanol molecules serve as the Hꢀbond donors. For heterocycleꢀ
N…HꢀOCH3 synthon, the heteroatom separation is 2.653 Å.
The complexes 1, 3 and 4 are isostructural; however with two
different ligands. The crystal structures of these three complexes
show that the metal ion is present in the pincer part of the ligand and
is coordinated to two deprotonated Namide groups and a Npyridyl atom.
The MꢀNamide bond lengths are longer than that of MꢀNpyridyl
distances. In addition, two nitrate ions coordinate the metal center
with highly asymmetrical MꢀOnitrate bond distances. The geometry
around the metal in complexes 1, 3 and 4 can be described as
distorted squareꢀpyramidal (with monodentate nitrate) with a large τ5
distortion parameter of 0.303 and 0.247 for complexes 1 and 3,
respectively.13 The ideal value of τ5 is 0 and 1 for a perfect squareꢀ
pyramidal and trigonalꢀbipyramidal geometry.13 Importantly, both
appended heterocyclic rings in all three complexes are protonated
due to the migration of protons from amidic NꢀH groups to these
rings. The result is the generation of thiazolium and thiazolinium
rings in these complexes. Such a protonation results in the formation
of H – bonds from the protonated rings to the coordinated nitrate
anions. The heterocycleꢀH…ONO2 Hꢀbonding synthons display
O…N heteroatom separations between 2.869 – 2.995 Å. An
interesting consequence of such Hꢀbonding is the orientation of the
coordinated nitrate anion within the complex cavity and may have
noteworthy implications during the catalysis.
Crystal structure of 2†
The crystal structure of complex 2 is quite different from complexes
1, 3 and 4 due to a dinuclear nature of complex and the presence of a
bridging methoxide ion. Every Cd2+ ion displays a sevenꢀcoordinate
geometry where ligand coordinates in a N3 meridional fashion while
one nitrate ion ligates in a bidentate ηꢀ1,2ꢀmode and the second one
is monodentate. Notably, two monomeric halves are bridged by a
methoxide ion with Cd…OMe distance of 2.474 Å. The Cd…ONO2
distances vary between 2.262 – 2.535 Å with monodentate nitrate
Figure 3. Thermal ellipsoidal representation of complex 5; ellipsoids are
drawn at 30% probability level whereas hydrogen atoms are omitted for
clarity.
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Except complex 2, all other complexes display interesting solidꢀ
play subtle yet decisive role in holding the substrate while the
state packing due to the involvement of Onitrate…H–Nring, C– Lewis acidic metal activates the substrate before the attack of
H…Onitrate and Oamide…H–OMeOH intermolecular Hꢀbonding nucleophile. In order to minimize the competition from solvent
interactions. In particular, complexes 1, 3, and 4 form zigꢀzag chains molecules, we decided to use solventꢀfree reaction conditions for
due to the operation of Onitrate…H–Nring Hꢀbonding interactions catalysis. Of course, such reactions provide notable advantages such
whereas complex 5 generates a similar Hꢀbonding motif due to the as efficient separation of products, easy recovery of catalyst, nonꢀ
Oamide…H–OMeOH intramolecular interactions (Figures S21 – S25 and interference of solvent molecules, and high conversion. For a typical
Tables S3 – S7; ESI).
aminolysis reaction, when cyclohexene oxide was stirred with only 2
mol% of complexes 1 – 5 followed by the addition of aniline, a
smooth reaction took place that produced the corresponding βꢀamino
alcohol in high yield. As shown in Table 1, all five catalysts were
equally effective in promoting the reaction between cyclohexene
oxide with aniline (entry 1) as well as with assorted paraꢀsubstituted
anilines containing electronꢀdonating or electronꢀwithdrawing
functional groups (entries 2ꢀ5).
Lewis acidic catalytic applications
One of the objectives in this work was to evaluate the catalytic
efficiency of the synthesized complexes. This was due to a notable
cavity structure and the presence of intramolecular Hꢀbonds in all
five complexes. We envisioned that such structural features may
assist in holding a substrate closer to the Lewis acidic metals in all
five complexes including oxidationꢀsensitive Mn ion in complex 5.
For Lewis acidicꢀbased catalysis, we selected ringꢀopening reactions
(RORs) of epoxides and cyanation reactions (CRs) of aldehydes.
RORs of epoxides with a variety of nucleophiles have attracted
immense research due to the pharmaceutical and commercial
importance of ringꢀopened products.14 In such reactions, activation
of epoxideꢀbased substrate is a prerequisite for catalysis and assorted
chemicals/reagents15 including wellꢀdefined complexes16 have been
used for such an activation. Our complexes with Lewis acidic metals
and weakly coordinated nitrates ions and/or labile MeOH molecules
offered a perfect platform to explore RORs. We anticipated that the
oxygenꢀbased substrates are likely to replace labile nitrate ions16b
and/or MeOH molecules and the complex cavity lined with Hꢀbond
sensitive functional groups may assist in directing the substrate
towards the metal ion. We, therefore, believed that the cavity may
Notably, catalytic reactions did not proceed at all in the absence of
complexes; while the control experiments with metal salts (Zn(NO3)2
or Cd(NO3)2) did not show any trace of product. Furthermore,
reaction immediately ceases when the catalyst is filtered off from the
reaction. These experiments unambiguously confirm the catalytic
role of complexes. Importantly, taking advantage of heterogeneous
nature of reactions, all five complexes can be conveniently recovered
by mere filtration after the completion of the reaction. Such
recovered complexes can be reused without any purification or
regeneration and were tested for five consecutive reactions without
much loss of catalytic efficiency. The stability of all recovered
complexes was analysed with Xꢀray powder diffraction (XRPD)
studies and FTIR spectra. Both these studies showed identical results
to that of freshly prepared samples (Figure S26ꢀS28; ESI) and
convincingly prove that the structural integrity of these complexes
remain intact during the catalysis.
Table 1. Ringꢀopening reactions of cyclohexene oxide (1ꢀ5) and
styrene oxide (6ꢀ10) with aniline and paraꢀsubstituted anilines using
complexes 1 – 5.a
To understand the regioselectivity, styrene oxide was used as a
representative asymmetrical substrate (entries 6 – 10; Table 1).16bꢀe,
Importantly, a perfect regioselectivity was observed with a single
product in all reactions.16bꢀe Out of two possible products due to the
nucleophilic attack either at benzylic carbon or less hindered carbon
atom of the epoxide; nucleophile exclusively attacked at the less
hindered carbon atom. The regioꢀisomer showed the characteristic
molecular ion peak at m/z [M+ꢀ31] due to the loss of –CH2OH
fragment.17a Such an observation conclusively proves the formation
of a single product as the other regioꢀisomer is expected to show the
molecular ion peak [M+ –107] due to the loss of C6H5CHOH
fragment if the nucleophile had attacked the terminal carbon atom of
the epoxide ring.17a Results suggest that the epoxide has approached
the Lewis acidic metal through less hindered side assisted via the
cavity structure.18
Entry
R
%Yieldb
1
2
3
98
4
98
5
94
97, 95c, 94d
97, 96c, 94d
1
2
ꢀH
ꢀC2H5
ꢀOCH3
ꢀF
92
90
80
75
97
90
95
93
93
92
89
93
85
82
70
97
95
90
97
82
90
82
76
63
96
96
92
93
81
85
84
3
80
77
4
70
65
5
ꢀCl
With the success of RORs, all five complexes were further tested
for the CRs. CRs of carbonylꢀbased substrates offer a convenient
route to cyanohydrins which are intermediates for a variety of fine
chemicals and pharmaceuticals.19 Extensive research has been
devoted to develop improved Lewis acid catalysts for the reaction of
cyanide ion with aldehydes.20 In this context, heterogeneous
catalysts incorporating Lewis acidic sites are advantageous.16f,g As
displayed in Table 2, all five complexes were able to catalyse CRs of
assorted aldehydes with trimethylsilylcyanide as the CNꢀ source.
Benzaldehyde and several functionalized analogues provided the
corresponding cyanohydrins in good to excellent yield. We propose
that the catalysis is occurring via the displacement of labile nitrate
98, 96c, 95d
95, 94c, 92d
6
ꢀH
96
92
95
75
97
89
91
80
7
ꢀC2H5
ꢀOCH3
ꢀF
8
9
10
ꢀCl
aConditions: 2 mol% catalyst; 4 h, 30ºC. bYield was determined by GC.
Isolated yield for cthird run and dfifth run with reused catalyst.
4 | J. Name., 2012, 00, 1-3
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ions and/or MeOH molecules by aldehyde before its activation and used as a representative case which on excitation at 320 nm
reaction with cyanide. Importantly, all five complexes functioned as exhibited emission at ca. 460 nm. Importantly, when complex 1 was
the heterogeneous catalysts whereas recyclability experiments titrated with cyclohexene oxide, benzaldehyde and 1ꢀ and 2ꢀ
substantiated that these complexes remain intact after the catalysis. naphthaldehyde; significant changes in the emission intensities were
Finally, no transformation was noted without catalysts while use of noted (Figure 4). While titration with cyclohexene oxide resulted in
metal salt (Zn(NO3)2 or Cd(NO3)2) did not produce any cyanohydrin. the enhancement of emission at ca. 460 nm (Figures S29ꢀS31; ESI);
similar titrations with benzaldehyde (Figure S32; ESI) as well as 1ꢀ
and 2ꢀnaphthaldehyde caused considerable quenching in the
Table 2. Cyanation reactions of assorted aldehydes with (CH3)3SiCN
using complexes 1-5.a
emission intensities (Figures S33 – S37; ESI). It is worth to mention
here that titrations using cyclohexene and naphthalene did not cause
any change in the emission intensity, therefore, strongly suggesting
that the complex cavity is specific for Oꢀbased substrates and such
substrates are bound within the complex cavity and caused
perturbation to the emission intensities.
Entry
R
%Yieldb
1
2
95
3
95
4
96
5
96
The moleꢀratio plot unambiguously showed a stoichiometry of 1:2
between complex 1 and substrates (cyclohexene oxide, 1ꢀ and 2ꢀ
naphthaldehyde). A representative example of titration and fittings
of complex 1 with 2ꢀnaphthaldehyde is shown in Figure 5. The
results confirm that complex 1 binds two equivalents of substrates
which were further proved by the BenesiꢀHildebrand method.21 The
BenesiꢀHildebrand equation is as follows:
90,88c,86d
1
2
3
4
5
6
7
8
9
ꢀC6H5
97
94
99
90
95
98
95
95
95
93
98
97
96
90
93
94
96
94
98
91
95
97
94
96
93
95
97
97
94
96
90
93
95
93
98
89
94
90
94
97
3ꢀMeOC6H4
4ꢀMeOC6H4
3ꢀNO2C6H4
4ꢀNO2C6H4
4ꢀClC6H4
1/[I – I0] = 1/K(Imax – I0)[Substrate]n + 1/[Imax – I0]
1ꢀNaphthyl
2ꢀNaphthyl
9ꢀAnthryl
where I0 is the emission intensity of complex 1, I is the emission
intensity obtained with substrate, Imax is the emission intensity
obtained with excess amount of substrate, K is the association
constant (Mꢀ2), [substrate] is the concentration of substrate added
(M), and n is the substrate equivalents. As shown in Figure 5, the
plot of 1/(I – I0) against 1/[Substrate]2 shows a liner relationship,
aConditions: 5 mol% catalyst; 6 h, 30ºC. bYield was determined by GC.
Isolated yield for cthird run and dfifth run with reused catalyst.
It is noteworthy to mention that the molecular size of the substrate
and the electronic substituents present on substrates have negligible
effect on the product yield. For example, CRs with sterically
demanding aldehydes such as 1ꢀ and 2ꢀnaphthaldehyde as well as 9ꢀ
anthraldehyde resulted in as good yield as noted for benzaldehyde.
The results suggest that all aldehydic substrates have similar access
0.3
1 + Benzaldehyde
0.0
1 + 1-Naphthaldehyde
1 + 2-Naphthaldehyde
1 + Cyclohexene Oxide
-0.3
-0.6
-0.9
Figure 5. Fluorescence titration of complex 1 with 2ꢀnaphthaldehyde in
DMFꢀCHCl3 (1:5). Top inset: change in emission intensity as a function of
moles of 2ꢀnaphthaldehyde. Bottom inset: Linear regression fitting curve for
1:2 binding between complex 1 and 2ꢀnaphthaldehyde.
0
1
2
3
4
Mole Ratio
Figure 4. Plots of relative emission intensities versus amount of substrates
(cyclohexene oxide benzaldehyde ■, 1ꢀnaphthaldehyde ●, and 2ꢀ
naphthaldehyde ) in DMFꢀCHCl3 (1:5).
▼,
indicating association with substrate in a 1:2 stoichiometry, therefore
confirming the 1:2 stoichiometry.22,23 The association constant, K,
was determined from the slope. This method afforded the association
constants (K x 105 Mꢀ2) of 10.5, 9.8, and 9.0 for cyclohexene oxide,
1ꢀ and 2ꢀnaphthaldehyde, respectively. Notably, plots assuming
stoichiometry of 1:1 as well as 1:3 do not show linear relationship of
▲
and affinity to the Lewis acidic metal within the complex cavity.
At this stage, we decided to confirm that the substrate(s) is indeed
bound within the complex cavity. For such studies, complex 1 was
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DOI: 10.1039/C4DT02079K
1/(I–I0) against 1/[Substrate]n, therefore, ruling out both 1:1 and 1:3 MeOH significantly affected the yield of styrene oxide. Further,
but confirming a stoichiometry of 1:2 between complex and solventꢀfree reaction conditions resulted in multiple products
substrate. Importantly, these studies justify that a metal ion situated suggesting a cascade of oxidation reactions. We also attempted to
inside the cleft is able to bind all substrates with similar binding understand the optimal loading of the catalyst in order to efficiently
affinities. These experiments precisely prove our assumption that the carry out the epoxidation reactions. For such a purpose, catalyst
catalysis starts after substrate coordinates to a metal center (Scheme loading of 0.5, 1, 2, and 5 mol% were used for the oxidation of
S1, ESI).
styrene. The product yield of styrene oxide varied in an anticipated
manner in the following order: 0.5 (50) < 1 (60) < 2 (88) < 5 (92)
and therefore we decided to use 4 mol% catalyst loading.
We also attempted to understand if the binding of substrate within
the complex cavity could also be evaluated by the FTIR spectral
studies. Thus, when complex 1 was treated with a CH2Cl2 solution
With these optimized reaction conditions; complex 5 showed
containing benzaldehyde, 1ꢀ or 2ꢀnaphthaldehyde and styrene oxide; catalytic oxidation of alkenes to epoxides within 4 h at room
noticeable spectral differences were observed when compared to temperature (30 ºC). A variety of alkenes were tested; such as paraꢀ
neat samples. For example, νC=O stretch for the cavity bound substituted styrenes (Table 3; entries 1ꢀ5); sterically bulkier alkenes
benzaldehyde or 1/2ꢀnaphthaldehyde was redꢀshifted by ca. 15 cmꢀ1. (Table 3; entries 6ꢀ10); cyclic alkenes (Table 4; entries 1ꢀ5); and
More importantly, νNꢀH stretches of the complex cavity were also longꢀchain alkenes (Table 5; entries 1ꢀ4). For paraꢀsubstituted
shifted implying that substrates are indeed bound within the complex styrenes, facile oxidation to epoxides resulted with aldehydes as the
cavity.
minor side product. Interestingly, for bulkier alkenes (entry 6ꢀ8),
oxidation to epoxide decreases considerably with 9ꢀvinylanthracene
(entry 8) showing the minimum conversion. It appears that the
approach of such substrates towards Mn(II) ion was significantly
lessꢀfavoured due to the presence of hydrophobic vinyl group. It is
important to mention that all aldehydic substrates caused quenching
to the emission intensity of complex 1 and showed a stoichiometry
of 1:2 suggesting that the complex cavity has affinity to bind Oꢀ
based hydrophilic substrates. We, therefore, conclude that the
hydrophilicity and not the size of a substrate control the catalysis
outcome.
Oxidative catalytic applications
Notably, complex 5 contains Mn(II) ion and is coordinated by three
labile MeOH molecules. Mn(II) ion is known to support oxidation
reactions23 and we selected oxidation of assorted olefins as epoxides
constitute an important class of starting materials and
intermediates.24 Epoxidation reactions have been studied in detail;
however, harsh reaction conditions and multiple oxidation products
have limited wider commercial utilization.25 We, therefore, targeted
to use molecular oxygen as an oxidant in presence of
isobutyraldehyde as a sacrificial oxidizing agent.26 In order to
optimize reaction conditions, styrene was selected as a model
For the oxidation of cisꢀ and transꢀstilbene, stilbene oxide was the
predominant product along with a small amount of benzaldehyde
(entries 9ꢀ10). The oxidation of cisꢀstilbene was poor and afforded
only 30% conversion while transꢀstilbene was transformed in
quantitative yield. We believe that cisꢀ to trans isomerisation is one
of the major factors for the observed stereoꢀselectivity; however,
poor oxidation of cisꢀstilbene. Notably, only small amount of
benzaldehyde was produced both in cisꢀ and transꢀstilbene
epoxidation.
substrate. Thus, when
isobutyraldehyde (3 equiv.) was stirred with suspended complex 5 in
a mixture of styrene (1 equiv.) and
Table 3. Epoxidation of assorted olefins using complex 5.a
Table 4. Epoxidation of cyclic olefins using complex 5.a
%Yieldb
% Conversion
O
Entry
Ar / R
A
B
8c
8
8
5
18
11
12
12
4
catalyst
O
+
1
2
3
4
5
6
7
8
9
H
ꢀOMe
ꢀMe
ꢀCl
ꢀC(CH3)3
92c, 90d, 89e
100
98
100
100
96
66
72
66
30
O
90
92
95
78
55
60
54
26
92
H
Substrate
B
A
Entry
R
%Yieldb
% Conversion
1ꢀvinylnaphthalene
2ꢀvinylnaphthalene
9ꢀvinylanthracene
cisꢀstilbene
transꢀstilbene
A
B
1
2
95
5
100
97
73
24c
10
8
100
aConditions: 4 mol% catalyst; MeCN; 4 h, 30ºC. bYield was determined by
GC. cIsolated yield. dSecond and efourth run with reused catalyst.
3
4
5
74
74
25
26
0
99
100
MeCN under an oxygen atmosphere; a smooth reaction occurred to
afford styrene oxide (92%) with a small amount of benzaldehyde
(8%) as the byꢀproduct. Notably, oxidation reaction did not show
any conversion in absence of complex 5 whereas use of THF or
100
100
aConditions: 4 mol% catalyst; MeCN; 4 h; 30 ºC. bYield was determined
by GC. cProduct is cyclohexꢀ2ꢀeneꢀ1,4ꢀdione.
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DOI: 10.1039/C4DT02079K
Complex 5 was also effective for the oxidation of cyclic alkenes
(Table 4). In all cases, quantitative oxidation resulted, however, with
variable proportions of products. Cyclopentene displayed the
maximum amount of desired epoxide (95%) whereas higher ring
analogues exhibited comparatively lower product yield (compare
entries 1 – 4; Table 4). We speculate that higher ring analogues
offered several structural conformations which caused undesirable
oxidation of the ring. Interestingly, placement of methyl group
adjacent to the double bond caused exclusive formation of epoxide
with no sign of any byꢀproduct (entry 5). The oxidation of longꢀ
chain alkenes has been considered a difficult process and generally
requires stronger oxidants.21a Noticeably, complex 5 effectively
oxidizes such alkenes with 90 – 100% conversion (Table 5).
Interestingly, conversion was quantitative in case of 1ꢀhexene and 1ꢀ
decene while other two alkenes afforded ca. 90% conversions. The
reason for this behaviour is probably related to the optimal
orientation of a substrate within the complex cavity in absence of
other limiting parameters.
added dropꢀwise to the aforementioned reaction mixture. The reaction
mixture was finally stirred at 70 °C for 12 h. The reaction mixture was
poured to iceꢀcold water that caused precipitation of crude product which was
filtered, washed with water followed by ethanol and dried. The crude product
was recrystallized from MeOH/CHCl3. Yield: 1.58 g (80 %). Anal. Calc. for
C13H9N5O2S2: C, 47.12; H, 2.74; N, 21.13; S, 19.35. Found: C, 47.52; H,
2.92; N, 20.83; S, 19.41. M.P.: 282ꢀ284 ºC. MS (ESI+, CHCl3, m/z): calcd.
331.0198 for H2L1; found 332.0291 for M + H+; found 354.0110 for M + Na+.
FTIR spectrum (KBr pellet, cmꢀ1): 3109 (NꢀH), 1676 (C=O). 1H NMR
spectrum (400 MHz, CDCl3): δ = 8.47 (d, J=8.1Hz, 2H), 8.18 (t, J=7.3Hz,
1H), 7.49 (d, J=3.6Hz, 2H), 7.03 (d, J=3.6Hz, 2H), 12.30 (broad, NꢀH). 13C
NMR spectrum (100 MHz, CDCl3): 140.01 (C1); 126.03 (C2); 146.97 (C3);
158.48 (C4); 160.84 (C5); 137.66 (C6); 113.91 (C7).
Synthesis of H2L2. This ligand was synthesized in a similar manner as
mentioned for H2L1 with following reagents: pyridineꢀ2,6ꢀdicarboxylic acid
(1.0 g, 0.0059 mol ); 2ꢀaminoꢀ2ꢀthiazoline (1.222 g, 0.0119 mol); and
P(OPh)3 (3.877g, 0.0125 mol). The product was isolated after the removal of
pyridine at reduced pressure followed by triturating the oily remains with
methanol. Yield: 1.30 g (65%). Anal. Calc. for C13H13N5O2S2: C, 46.55; H,
3.91; N, 20.88; S, 19.12. Found: C, 46.24; H, 4.21; N, 20.93; S, 19.25. M.P.:
220ꢀ222 ºC. MS (ESI+, CHCl3, m/z): calcd. 335.0511 for H2L2; found
336.0602 for M + H+; found 358.0421 for M + Na+. FTIR spectrum (KBr,
cmꢀ1): 3219 (NꢀH), 1730, 1623 (C=O). 1H NMR spectrum (400 MHz,
CDCl3): δ = 8.39 (d, J=8.1Hz, 2H), 8.11 (t, J=8.1Hz, 1H), 3.98 (t, J=8.1 Hz,
4H), 3.32 (t, J=8.1Hz, 4H), 11.07 (broad, NꢀH). 13C NMR spectrum (100
MHz, CDCl3): 139.80 (C1); 126.27 (C2); 147.12 (C3); 158.74 (C4); 161.91
(C5); 56.58 (C6); 33.35(C7).
In order to ascertain the stability of complex 5 during the
oxidative catalysis; catalyst 5 was recovered after the reaction and
characterized. The FTIR spectrum of complex 5, recorded after the
catalysis, showed very similar spectral features as that of a pristine
sample. Notably, however, one of the prominent features for the
coordinated methanol molecule (O–C stretch at 1015 cmꢀ1) was in
fact diminished whereas νOꢀH stretches were intensified. We,
therefore, propose that the coordinated methanol molecules are first
replaced by isobutyraldehyde and/or substrate molecules during the
course of catalysis and subsequently by the water molecules after the
catalysis. (Figure S16).
Synthesis of H2L3. This ligand was also prepared with a similar procedure
with following chemicals: pyridineꢀ2,6ꢀdicarboxylic acid (1.0 g, 0.0059 mol);
2ꢀaminobenzothiazole (1.792 g, 0.0119 mol), and P(OPh)3 (3.877g, 0.0125
mol).. Recrystallization was achieved from THF. Yield: 1.95 g (76%). Anal.
Calc. for C21H13N5O2S2: C, 58.45; H, 3.04; N, 16.23; S, 14.86. Found: C,
58.62; H, 3.19; N, 15.95; S, 15.02. M.P.: 288ꢀ290 ºC. MS (ESI+, CHCl3,
m/z): calcd. 431.0511 for H2L3; found 432.0602 for M + H+; found 454.0422
for M + Na+. FTIR spectrum (KBr, cmꢀ1): 3235 (NꢀH), 1695 (C=O). 1H
NMR spectrum (400 MHz, d6ꢀDMSO): δ = 13.60 (s, 1H), 8.48 (d, J=8.1Hz,
2H), 8.36 (t, J=8.1, 7.3Hz, 1H), 8.05 (d, J=16.1Hz, 2H), 7.88 (d, J=8.0Hz,
2H), 7.50 (d, J=8.1Hz, 2H), 7.36 (t, J=7.1Hz, 2H). 13C NMR spectrum (100
MHz, CDCl3): 140.52 (C1); 124.11 (C2); 147.37 (C3); 158.11 (C4); 162.92
(C5); 131.75 (C6); 121.96 (C7); 126.43 (C8); 127.07 (C9); 120.78 (C10);
148.71 (C11).
Table 5. Epoxidation of longꢀchain olefins using complex 5.a
Entry
%Yieldb
% Conversion
Substrate
A
B
1
2
74
26
18
100
91
73
78
3
15
93
Synthesis of [{L1(H)2}Zn(NO3)2] (1). Ligand H2L1 (0.2 g, 0.604 mmol) was
dissolved in 5 mL CHCl3 and layered with a solution of [Zn(H2O)6](NO3)2
(0.179 g, 0.604 mmol) dissolved in 3 mL MeOH. White crystalline
compound appeared within a day. Yield: 0.114 g (71%). Anal. Calc. for
C13H9ZnN7O8S2: C, 29.98; H, 1.74; N, 18.83; S, 12.31. Found: C, 29.64; H,
1.51; N, 18.34; S, 12.08. FTIR spectrum (ZnꢀSe ATR, cmꢀ1): 3276 (NꢀH),
1637 (C=O), 1345, 1282, 1016 (NꢀO). Molar conductivity (DMF ~ 1mM
solution, 298 K): ΛM = 140 ꢁꢀ1 cm2 molꢀ1 (the range for 1:2 electrolyte in
DMF is 130 ꢀ 170). Molar conductivity (DMSO ~ 1mM solution, 298 K): ΛM
= 90 ꢁꢀ1 cm2 molꢀ1. 1H NMR spectrum (400 MHz, d6ꢀDMSO): δ = 13.37 (s,
2H), 8.45 (d, J=8.1Hz, 2H), 8.35 (dd, J=7.3ꢀ8.1Hz, 1H), 7.66 (d, J=3.7Hz,
2H), 7.40 (d, J=2.3Hz, 2H).
4
98
2
100
aConditions: 4 mol% catalyst; MeCN; 4 h; 30ºC. bYield was determined by
GC.
Experimental Section
General procedures
All reagents were purchased from the commercial sources and used without
further purification. The solvents were dried and purified using standard
procedures.27
Synthesis of [{L1(H)2Cd(NO3)2}2(ꢀ-CH3OH)] (2). Complex
synthesised in a similar manner as that of complex 1 by using following
reagents: H2L1 (0.2 g, 0.604 mmol) and [Cd(H2O)6](NO3)2 (0.175 g, 0.604
2 was
Synthesis of H2L1. Pyridineꢀ2,6ꢀdicarboxylic acid (1.0 g, 0.0059 mol) and 2ꢀ
aminothiazole (1.196g, 0.0119 mol) were taken in 25 ml pyridine and
refluxed with stirring for 30 min at 120°C. P(OPh)3 (3.877g, 0.0125 mol) was
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mmol). Yield: 0.125 g (73%). Anal. Calc. for C27H22Cd2N14O17S4: Calc: C, chromatography on silica gel using hexanes/ethyl acetate mixture (5:1) as the
27.77; H, 1.90; N, 16.79; S, 10.98. Found: C, 27.60; H, 1.96; N, 16.64; S, eluent. The products were separated, isolated and analyzed by GC and/or
10.62. FTIR spectrum (ZnꢀSe ATR, cmꢀ1): 3195 (NꢀH), 1619 (C=O), 1355, GCꢀMS techniques. The recovered catalysts were washed with diethyl ether,
1274, 1027 (NꢀO). Molar conductivity (DMF ~ 1mM solution, 298 K): ΛM
=
dried and reused without further purification or regeneration. Moreover, the
150 ꢁꢀ1 cm2 molꢀ1 (the range for 1:2 electrolyte in DMF is 130 ꢀ 170). Molar recovered catalysts were characterized by FTIR spectra as well as by the Xꢀ
1
conductivity (DMSO ~ 1mM solution, 298 K): ΛM = 100 ꢁꢀ1 cm2 molꢀ1. H ray powder diffraction studies. Both these studies showed identical results to
NMR spectrum (400MHz, d6ꢀDMSO): δ = 13.39 (s, 2H), 8.44 (d, J=8.1Hz, those of the pristine samples.
2H), 8.38(dd, J= 7.2ꢀ8.2Hz, 1H), 7.66 (d, J=3.7Hz, 2H), 7.40 (d, J=3.7Hz,
Physical Measurements. The conductivity measurements were carried out
2H).
with a digital conductivity bridge from the Popular Traders, India (model
Synthesis of [{L2(H)2}Zn(NO3)2] (3). Complex 3 was synthesised in a number: PT 825). The elemental analysis data were obtained with an
similar manner as mentioned for complex 1 using the following reagents: Elementar Analysen Systeme GmbH Vario ELꢀIII instrument. The NMR
H2L2 (0.2 g, 0.593 mmol) and [Zn(H2O)6](NO3)2 (0.173 g, 0.593 mmol). spectral measurements were carried out with a Jeol 400 MHz instrument. The
Yield 0.115 g (74%). Anal. Calc. for C13H13ZnN7O8S2.CH3OH.H2O: C, FTIR spectra (ZnꢀSe ATR) were recorded with a Perkin–Elmer Spectrum–
29.25; H, 3.30; N, 17.06; S, 11.16. Found: C, 29.11; H, 3.44; N, 17.18; S, Two spectrometer. The absorption spectra were recorded with a Perkin–
11.32. FTIR spectrum (ZnꢀSe ATR, cmꢀ1): 3323 (NꢀH), 1654 (C=O), 1346, Elmer Lambda 25 spectrophotometer. GC and GC–MS studies were
1287, 1018 (NꢀO). Molar conductivity (DMF ~ 1mM solution, 298 K): ΛM
=
performed with a Perkin Elmer Clarus 580 and Shimadzu QP 2010
145 ꢁꢀ1 cm2 molꢀ1 (the range for 1:2 electrolyte in DMF is 130 ꢀ 170). Molar instrument, respectively, with RTXꢀ5SILꢀMS column. The Xꢀray powder
1
conductivity (DMSO ~ 1mM solution, 298 K): ΛM = 95 ꢁꢀ1 cm2 molꢀ1. H diffraction studies were performed either with an X’Pert Pro from Panalytical
NMR spectrum (400 MHz, d6ꢀDMSO): δ = 10.11 (broad, 2H), 8.15ꢀ8.69 or a Bruker AXS D8 Discover instrument (CuꢀKα radiation, λ = 1.54184 Å).
(broad, 3H), 3.58ꢀ4.15 (broad, 8H).
The samples were ground and subjected to the range of θ = 2–50° with a scan
rate of 1° per minute at room temperature.
Synthesis of [{L2(H)2}Cd(NO3)2] (4). Ligand H2L2 (0.2 g, 0.593 mmol)
dissolved in 5 mL DMA was layered by a solution of [Cd(H2O)6](NO3)2
(0.173 g, 0.593 mmol) dissolved in 8 mL MeOH. This solution was left for
evaporation at room temperature. White crystalline product appeared within
48 h. Yield: 0.116 g (69%). Anal. Calc. for C13H13CdN7O8S2: C, 27.25; H,
1.60; N, 17.27; S, 11.29. Found: C, 26.97; H, 1.68; N, 17.13; S, 11.13. FTIR
spectrum (ZnꢀSe ATR, cmꢀ1): 3319 (NꢀH), 1637 (C=O), 1335, 1268, 1026
(NꢀO). Conductivity (DMF ~ 1mM solution, 298 K): ΛM = 143 ꢁꢀ1 cm2 molꢀ1
(the range for 1:2 electrolyte in DMF is 130 ꢀ 170). Molar conductivity
(DMSO ~ 1mM solution, 298 K): ΛM = 92 ꢁꢀ1 cm2 molꢀ1. 1H NMR spectrum
(400 MHz, d6ꢀDMSO): δ = 10.42 (broad, 2H), 8.26ꢀ8.72 (broad, 3H), 3.71ꢀ
4.02 (broad, 8H).
Crystallography. The intensity data were obtained with a Bruker Kappa
ApexꢀCCD and Oxford XCalibur CCD diffractometer for complexes 1ꢀ4 and
5, respectively, by using graphiteꢀmonochromated MoꢀKα radiation (λ =
0.71073 Å).28,29 For complexes 1 – 4, intensity data were corrected for
Lorentz polarization effects while an empirical absorption correction
(SADABS) was applied.30,31 A multiꢀscan absorption correction was applied
using CrysalisPRO for complex 5.29 The structures were solved by the direct
methods using SIRꢀ9232 and refined by fullꢀmatrix leastꢀsquares refinement
techniques on F2 using SHELXL97.33 The hydrogen atoms were placed into
the calculated positions and included in the last cycles of the refinement. All
calculations were done using WinGX software package.34 The refinement of
complex 2 showed that the asymmetric unit contains two nitrate ions, both of
them were disordered. For the first one (with atoms N6, O3, O4, and O5);
disorder could only be resolved for O5. The second nitrate ion (with atoms
N7, O6, O7, and O8) when refined with site occupancy factor (SOF) 1.00
gave high Ueq values. Thus, these atoms were subsequently refined with
SOF of 0.5 which improved their Ueq values significantly. Both nitrate ions
were refined with restraints over their bond lengths. There was a disordered
methoxide ion sitting on the centre of inversion and bridging two monomeric
halves. This methoxide ion was refined by keeping carbon atom on the centre
of inversion with its SOF as 0.5 and two oxygen atoms (appearing in the
Fourier) as the disordered oxygen atoms of the methoxide ion. These atoms
were refined with part instruction with their SOF and thermal parameters
being refined as free variables. Their combined SOF was refined to be equal
to 0.5, so that there is half molecule of methoxide ion in the asymmetric unit.
The disorder of methoxide oxygen atom could be successfully resolved and it
was refined anisotropically as well. However, hydrogen atoms of methoxide
ion could not be located. The final difference Fourier shows some electron
density around the methoxide ion and also around the metal ion. The latter
may be due to the series termination error. The hydrogen atoms attached to
nitrogen atoms of the thiazole rings were easily located from the difference
Fourier and were included with fixed NꢀH distances (0.88 Å) and Uiso 1.2
times that of the corresponding nitrogen atoms. For complex 5, disordered
oxygen atom (O4) was resolved by fixing it at two positions using part
command. Details of the crystallographic data collection and structural
solution parameter are provided in Table S8. CCDCꢀ 1011221ꢀ1011225
Synthesis of [MnL3](CH3OH)3 (5). Ligand H2L3 (0.1 g, 0.232 mmol) was
dissolved in N2 flushed DMF (5 mL) and treated with NaH (0.012g, 0.487
mmol). The resulting solution was stirred for 10 min at room temperature
followed by the dropꢀwise addition of a solution of [Mn(H2O)6](ClO4)2
(0.083g, 0.232 mmol) dissolved in 3 mL DMF. The reaction mixture was
stirred for 1 h. The solvent was removed under reduced pressure and the oily
remains were treated with CH3CN to afford a yellow solid. Crude compound
thus obtained was washed with diethyl ether and dried. This product was
dissolved in DMF, passed through a pad of celite and the filtrate was layered
with MeOH to afford yellow crystalline product within a day. Yield: 0.09 g
(67 %). Anal. Calc. for C24H23MnN5O5S2: C, 49.65; H, 3.99; N, 12.06; S,
11.05. Found: C, 49.32; H, 4.21; N, 11.52; S, 10.65. FTIR (ZnꢀSe ATR, cmꢀ
1): 3455 (OH), 1612 (C=O), 1016 (OꢀCH3).
General Procedure for Catalytic Reaction: For aminolysis, 1 equiv. of
epoxide was treated with 1.1 equiv. of aniline or substituted aniline in
presence of 2 mol% catalyst. In a typical cyanation reaction, 5 mol% catalyst
was added to a mixture of aldehyde (1 equiv.) and trimethylsilylcyanide (5
equiv.). For both these reaction, the mixture was stirred at ambient conditions
for 4 – 6 h. For epoxidation reactions, alkenes (1 equiv.) were treated with
isobutyraldehyde (3 equiv.) in presence of 4 mol% catalyst 5 in MeCN under
an oxygen atmosphere for 4 h. In all cases, reactions were monitored by thinꢀ
layer chromatography (TLC) and/or gas chromatography (GC). After
mentioned time, catalyst was filtered off and the filtrate was concentrated
under reduced pressure. The crude product was purified using flash column
8 | J. Name., 2012, 00, 1-3
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DOI: 10.1039/C4DT02079K
contain the supplementary crystallographic data for this paper. These data
can be obtained free of charge from The Cambridge Data Center via
www.ccdc.cam.ac.uk/data_request/cif.
Stock, S. A. Maves, D. E. Benson, R. M. Sweet, D. Ringe, G.
A. Petsko and S. G. Sligar, Science, 2000, 287, 1615; (c) N.
Yamamoto, S. Danos, P. D. Bonnitcha, T. W. Failes, E. J. New
and T. W. Hambley, J. Biol. Inorg. Chem., 2008, 13, 861.
Conclusions
4. (a) Y. Lu and J. S. Valentine, Curr. Opin. Struct. Biol., 1997, 7,
495; (b) J. H. Dawson, Science, 1988, 240, 433; (c) P. R. O. De
Montellano, Acc. Chem. Res., 1987, 20, 289.
This work has shown the synthesis and characterization of a
few mononuclear coordination complexes with amideꢀbased
ligands containing appended heterocyclic rings. Interestingly,
metal complexes displayed the migration of amidic NꢀH
protons to the appended heterocyclic rings during the synthesis.
Such protonated heterocyclic rings create a hydrogen bond
based cavity adjacent to metal ion and assist in binding of Oꢀ
based substrates within the complex cavity. The binding studies
confirm that the substrates are bound within the complex cavity
closer to the Lewis acidic metal in complexes 1 – 5 and
5. (a) A. S. Borovik, Acc. Chem. Res., 2005, 38, 54; (b) L. M.
Berreau, Eur. J. Inorg. Chem. 2006, 273; (c) A. Wada, S.
Yamaguchi, K. Jitsukawa, and H. Masuda, Angew. Chem. Int.
Ed. 2005, 44, 5698; (d) J. C. MarequeꢀRivas and R. Prabaharan,
Chem. Commun. 2004, 76.
6. (a) P. J. Donoghue, A. K. Gupta, D. W. Boyce, C. J. Cramer,
and W. B. Tolman, J. Am. Chem. Soc. 2010, 132, 15869; (b) P.
J. Donoghue, J. Tehranchi, C. J. Cramer, R. Sarangi, E. I.
oxidationꢀsensitive Mn ion in complex 5. All complexes have
been utilized as the reusable and heterogeneous catalysts for the
ringꢀopening reactions of assorted epoxides; cyanation
reactions of various aldehydes; and epoxidation reactions of
several olefins. The work presented in this manuscript
illustrates a viable method of designing substrateꢀspecific
cavity reminiscent to biological catalysts.
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RG acknowledges Council of Scientific and Industrial Research
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instrumental facilities and crystallographic data collection. DB
thanks UGC for a SRF fellowship.
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