Synthesis and structural characterization of homochiral coordination polymers with imidazole-based monocarboxylate ligands

Article abstract of DOI:10.1039/c9dt01237k

Chiral Na[(S)-L^R] (R = Me, 1a; ⁱPr, 1b; CH₂ⁱPr, 1c, and (S)-^secBu, 1d) and Na[(R)-L^R] (R = Me, 1a′) compounds were synthesised following standard procedures. New compounds 1d and 1a′ were analytically and spectroscopically characterised. 1a and 1c were structurally identified by single-crystal X-ray diffraction methods as homochiral 2D coordination polymers, {Na(H₂O)[(S)-L^Me]}_n and {Na[(S)-L CH₂ⁱPr]}_n, respectively. Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these coordination polymers: μ₃κ²OκO′ for 1a and μ₄κ²Oκ²O′ for 1c. Enantiomeric species 1a′, {Na(H₂O)[(R)-L^Me]}_n, showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal structure. DFT calculations on the [L^R]^- anions confirmed their coordination capabilities as ditopic linkers. In fact, the reaction of Na[L^R] with several metal salts yielded the following coordination polymers: {Ag[(S)-L^Me]}_n, 2a, {Ag[(R)-L^Me]}_n, 2a′, {Cu[(S)-L^R]₂}_n (R = Me, 3a; ⁱPr, 3b), {Cu[(R)-L^Me]}_n, 3a′, {Zn[(S)-L^R]₂}_n (R = Me, 4a; ⁱPr, 4b; (S)-^secBu, 4d) and {Zn[(R)-L^Me]₂}_n, 4a′. For the known compounds 3a and 4a, this procedure is a new synthetic route that avoided high temperature reaction conditions. New complexes 2, 3a′, b, and 4a′, b, d were characterised by elemental analysis, infrared and XRPD methods and complex 2a by single-crystal X-ray diffraction. This complex is also a two-dimensional coordination polymer in which the [(S)-L^Me]^- anion acts as a μ₄κN,κ²O,κ²O′ bridging ligand. Compounds 1-4a′ are the first examples of homochiral coordination polymers with imidazole-monocarboxylate ligands based on non-natural amino acids. Preliminary studies on the metal-catalysed preparation of chiral α-aminophosphonates were carried out but, unfortunately, no enantioselectivity was observed.

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Synthesis and structural characterization of
homochiral coordination polymers with
Cite this: DOI: 10.1039/c9dt01237k
imidazole-based monocarboxylate ligands†
b
Elena Borrego, a Antonio I. Nicasio, a Eleuterio Álvarez,
*
Francisco Montilla, a José Manuel Córdoba a and Agustín Galindo
*
a
Chiral Na[(S)-LR] (R = Me, 1a; iPr, 1b; CH2 Pr, 1c, and (S)-secBu, 1d) and Na[(R)-LR] (R = Me, 1a’) compounds
were synthesised following standard procedures. New compounds 1d and 1a’ were analytically and spec-
i
troscopically characterised. 1a and 1c were structurally identified by single-crystal X-ray diffraction
i
methods as homochiral 2D coordination polymers, {Na(H2O)[(S)-LMe]}n and {Na[(S)-LCH Pr]}n, respectively.
2
Both (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions displayed unprecedented coordination modes in these
coordination polymers: μ3κ2OκO’ for 1a and μ4κ2Oκ2O’ for 1c. Enantiomeric species 1a’, {Na(H2O)[(R)-
L
Me]}n, showed the same X-ray powder diffractogram (XRPD) as 1a, in agreement with a similar crystal
structure. DFT calculations on the [LR]− anions confirmed their coordination capabilities as ditopic linkers.
In fact, the reaction of Na[LR] with several metal salts yielded the following coordination polymers:
i
{Ag[(S)-LMe]}n, 2a, {Ag[(R)-LMe]}n, 2a’, {Cu[(S)-LR]2}n (R = Me, 3a; Pr, 3b), {Cu[(R)-LMe]}n, 3a’, {Zn[(S)-LR]2}n
(R = Me, 4a; iPr, 4b; (S)-secBu, 4d) and {Zn[(R)-LMe]2}n, 4a’. For the known compounds 3a and 4a, this pro-
cedure is a new synthetic route that avoided high temperature reaction conditions. New complexes 2, 3a’,
b, and 4a’, b, d were characterised by elemental analysis, infrared and XRPD methods and complex 2a by
single-crystal X-ray diffraction. This complex is also a two-dimensional coordination polymer in which the
[(S)-LMe − anion acts as a µ4κN,κ2O,κ2O’ bridging ligand. Compounds 1–4a’ are the first examples of
]
homochiral coordination polymers with imidazole-monocarboxylate ligands based on non-natural amino
acids. Preliminary studies on the metal-catalysed preparation of chiral α-aminophosphonates were carried
out but, unfortunately, no enantioselectivity was observed.
Received 22nd March 2019,
Accepted 15th May 2019
DOI: 10.1039/c9dt01237k
rsc.li/dalton
Following our interest in chiral imidazolium-based dicar-
boxylate ligands,21–23 we decided to explore the use of imid-
Introduction
The interest in chiral imidazole-based monocarboxylate and azole-based monocarboxylate ligands (see Scheme 1) as chiral
imidazolium-based dicarboxylate compounds has increased bridging ligands of metal complexes. Here, we report the syn-
markedly in the past few years.1–3 Their ease of preparation thesis and characterization of several homochiral coordination
makes them useful starting precursors for the synthesis of polymers of sodium, 1, silver, 2, copper, 3, and zinc, 4, with
chiral derivatives, such as ionic liquids,4–6 supported ionic chiral 2alkyl,2-(1H-imidazol-1-yl)acetate linkers ([(S)-LR]− for
i
i
liquids7,8 and NHC–carbene ligands.9–15 Additionally, they R = Me, Pr, CH2 Pr and (S)-secBu and [(R)-LMe]−). 1a, 1c and 2a
behave as suitable linkers in the construction of were structurally identified by single-crystal X-ray diffraction
homochiral coordination polymers16–18 and metal–organic
frameworks.19,20
aDepartamento de Química Inorgánica, Facultad de Química, Universidad de Sevilla,
Aptdo 1203, 41071 Sevilla, Spain. E-mail: galindo@us.es
bInstituto de Investigaciones Químicas, CSIC-Universidad de Sevilla,
Avda. Américo Vespucio 49, 41092 Sevilla, Spain. E-mail: ealvarez@iiq.csic.es
†Electronic supplementary information (ESI) available: DFT optimised struc-
tures and MOs, crystal data, supramolecular packings, comparison of IR spectra
and XRPD patterns, and coordinates of optimised compounds. CCDC 1904848
(1a), 1904849 (1c) and 1904850 (2a). For ESI and crystallographic data in CIF or
Scheme 1 Coordination modes of the 2-(1H-imidazol-1-yl)acetate
other electronic format see DOI: 10.1039/c9dt01237k
moiety.
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methods as two-dimensional coordination polymers {Na(H2O) expected, the structural and MO characteristics of the com-
i
i
i
[(S)-LMe]}n, 1a, {Na[(S)-LCH Pr]}n, 1c, and {Ag[(S)-LMe]}n, 2a, puted enantiomeric species [(R)-LR]− (R = Me, Pr, CH2 Pr and
respectively. The bonding mode of [(S)-LR]− ligands found in (S)-secBu) are completely similar (see Fig. S1 in the ESI†).
1a, μ3κ2O,κO′, and 1c, μ4κN,κ2O,κ2O′, is novel for ligands based
2
Synthesis and characterisation of sodium coordination
on the 2-(1H-imidazol-1-yl)acetate moiety.
polymers
Compounds Na[(S)-LR] (R = Me, 1a; Pr, 1b; CH2 Pr, 1c, and
(S)-secBu, 1d) and Na[(R)-LR] (R = Me, 1a′) were prepared follow-
ing the procedure described by Bao and co-workers (Scheme 2).
1a and 1b are known and well characterised,4 while the prepa-
ration of 1c as an intermediate in the preparation of chiral ionic
liquids has been described but its spectroscopic characteriz-
ation was not detailed.5 1d and 1a′ are new and they were ana-
lytically and spectroscopically characterised. In general, com-
pounds 1 show a broad and intense IR absorption at around
1590 cm−1, due to the antisymmetric COO vibration of the car-
boxylate group, and a second one at ca. 1390 cm−1, which is
assigned to the symmetric υ(COO). The imidazole group of com-
pounds 1 gives a common NMR pattern with three singlets in
1H NMR (ca. at 6.9, 7.2 and 7.7 ppm for C4H, C5H and C2H,
respectively) and three singlets in 13C{1H} NMR (ca. at 120, 128
and 138 ppm for C4H, C5H and C2H, respectively). CH(R)
groups of 1 show the conventional NMR resonances, while the
carboxylate carbon atom appears as a singlet at around
176 ppm in the 13C{1H} NMR (for instance, at 175.0 and
178.6 ppm for 1d and 1a′, respectively).
i
i
Results and discussion
Coordination capabilities of imidazole-based monocarboxylate
ligands
Imidazole-carboxylate ligands based on the 2-(1H-imidazol-1-
yl)acetate moiety (Scheme 1) feature one carboxylate group
plus one nitrogen atom of imidazole as potential donor func-
tionalities. Consequently, the bridging µ-coordination is the
foreseeable bonding mode of this type of ligand. In fact, a
search in the Cambridge Structural Database24 indicates that
this is the most common binding mode, in which the ligand is
capable of bridging two (µ2κN,κO, I in Scheme 1), three
(µ3κN,κO,κO′, II, and µ3κN,κ2O,κO′, III) or four metals
(µ4κN,κ2O,κO′, IV). Additionally, other bonding modes are
found if the 2-(1H-imidazol-3-ium-1-yl)acetate moiety is con-
sidered (see Scheme S1 in the ESI†).
In order to gain further information about the features of
the [(S)-LR]− anions as potential ligands, DFT calculations25 for
i
i
these anions were carried out with R = Me, Pr, CH2 Pr and
(S)-secBu. Fig. 1 depicts the optimised structures of anions and
three selected MOs for each species. The HOMO and
HOMO−1 orbitals are centred on the O atoms and are clearly
identified as the two σ O lone pairs of the carboxylate group.
HOMO−5 is centred on the N atom of the imidazole moiety
and is essentially the σ N lone pair. The coordination mode I
(Scheme 1) is compatible with the use of HOMO and
HOMO−5 orbitals in the bonding to metals, while the three
MOs are involved in the coordination modes II–IV. As
To our knowledge, no structural data has been reported for
chiral Na[LR] compounds. There is only an X-ray structure of a
related non-chiral example of lithium 2-(1H-imidazol-3-ium-1-
yl)acetate26 and that of the chiral imidazole-carboxylic com-
pound [HLCH Ph].27 For this reason, we decided to characterize
2
1a and 1c by single-crystal X-ray crystallography. The selected
structural parameters of 1a and 1c are collected in Table 1.
Fig. 2 shows the asymmetric unit of 1a that consists of the (S)-
2-(1H-imidazol-1-yl)propanoate anion, one sodium cation and
one water molecule. The structure of 1a shows an unpre-
cedented coordination mode for the [(S)-LMe −
] anion in which
it acts as a μ3κ2O,κO′ ligand and the nitrogen atom of imid-
azole is not bonded to the sodium ion (Fig. 2, bottom). The
distances of the sodium-oxygen interactions fall in the
2.33–2.42 Å range and each sodium cation is surrounded by
five oxygen atoms (O1 and O2 from three symmetry related
[(S)-LMe −
anions and O3 from two symmetry related water
]
molecules). Cations are bridged by O atoms of water and car-
boxylate groups, O3 and O1, respectively, and this originates
an arrangement of sodium cations with an Na⋯Na separation
of 3.6428(5) Å and an Na⋯Na⋯Na angle of 139.90(3)°. The
i
i
Fig. 1 Optimised structures of [(S)-LR]− anions (R = Me, Pr, CH2 Pr and
(S)-secBu) and selected MOs (from top to bottom: HOMO, HOMO−1 and
HOMO-5, respectively).
Scheme 2 Synthesis of compounds NaLR.
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Table 1 Selected structural parameters for complexes 1a and 1ca
Distances (Å)
1a
1c
Na–O
Na(1)–O(1)
Na(1)–O(1)#2
Na(1)–O(2)#1
Na(1)–O(3)
Na(1)–O(3)#2
—
2.4019(15)
2.4188(15)
2.3308(15)
2.3909(15)
2.3908(11)
Na(1)–O(1)
2.2637(18)
2.6096(17)
2.2639(17)
2.3727(16)
Na(1)–O(1)#2
Na(1)–O(2)#1
Na(1)–O(2)#2
Na–N
O–C
Na(1)–N(1)
O(1)–C(4)
O(2)–C(4)
2.4193(19)
1.243(3)
1.238(3)
O(1)–C(5)
O(2)–C(5)
1.252(2)
1.256(2)
Angles (°)
O–Na–O
O(2)#1–Na(1)–O(3)#2
O(2)#1–Na(1)–O(1)
O(3)#2–Na(1)–O(1)
O(2)#1–Na(1)–O(3)
O(3)#2–Na(1)–O(3)
O(1)–Na(1)–O(3)
O(2)#1–Na(1)–O(1)#2
O(3)#2–Na(1)–O(1)#2
O(1)–Na(1)–O(1)#2
O(3)–Na(1)–O(1)#2
—
97.73(5)
102.20(6)
101.03(5)
111.65(6)
149.54(4)
81.04(5)
108.54(5)
80.96(5)
148.67(4)
82.18(5)
O(1)–Na(1)–O(2)#1
O(1)–Na(1)–O(2)#2
O(2)#1–Na(1)–O(2)#2
O(1)–Na(1)–O(1)#2
O(2)#1–Na(1)–O(1)#2
O(2)#2–Na(1)–O(1)#2
113.25(6)
90.63(6)
132.72(5)
135.24(6)
84.83(5)
52.27(5)
O–Na–N
C–O–Na
O–C–O
O(1)–Na(1)–N(1)
O(2)#1–Na(1)–N(1)
O(2)#2–Na(1)–N(1)
N(1)–Na(1)–O(1)#2
C(4)-O(1)-Na(1)
C(4)-O(1)-Na(1)#3
C(4)-O(2)-Na(1)#5
C(4)–O(2)–Na(1)#3
O(2)-C(4)-O(1)
105.64(7)
93.54(7)
119.58(7)
114.02(7)
168.40(16)
85.35(12)
145.80(13)
96.50(12)
125.45(16)
C(5)–O(1)–Na(1)
C(5)–O(1)–Na(1)#3
C(5)–O(2)–Na(1)#4
128.48(12)
130.60(12)
130.03(12)
O(1)–C(5)–O(2)
126.10(17)
a Symmetry transformations used to generate equivalent atoms: for 1a: #1 −x + 1, y − 1/2, −z + 3/2; #2 −x + 2, y − 1/2, −z + 3/2; #3 −x + 2, y + 1/2,
−z + 3/2; and #4 −x + 1, y + 1/2, −z + 3/2. For 1c: #1 x, y − 1, z; #2 −x + 1, y − 1/2, −z; #3 −x + 1, y + 1/2, −z; #4 −x, y − 1/2, −z; #5 x, y + 1, z; and #6
−x, y + 1/2, −z.
Fig. 3 Perspective view of the molecular packing of {Na(H2O)[(S)-
Fig. 2 Asymmetric unit of 1a (top) and the unprecedented μ3κ2O,κO’
LMe]}n, 1a, viewed along the crystallographic b-axis.
coordination mode of the [(S)-LMe − anion in {Na(H2O)[(S)-LMe]}n, 1a,
]
with selected bond distances in Å (bottom).
architecture is constructed by this interaction between the
coordination of the second O atom of carboxylate, O2, to a water O3–H bond of one 2D lamellar sheet and the N(2) atom
symmetry related arrangement of sodium cations causes the of the imidazole group of a neighbour 2D polymer (Fig. 3 and
2D distribution of the polymer (Fig. 3). The absence of metal- Fig. S2 in the ESI†).
coordination through the N atom of imidazole is unique (see
The crystal structure of 1c is different from that of 1a. Its
the coordination modes above) and the N lone pair is satu- asymmetric unit shows the (S)-2-(1H-imidazol-1-yl)-4-methyl-
rated by a hydrogen bond with the water O–H bond (O3– pentanoate anion and one sodium cation without the presence
H⋯N2 length of 2.819(2) Å). Actually, the 3D supramolecular of water molecules (Fig. 4, up). In contrast to 1a, the N1 atom
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generated 1a′ structure (see Experimental) and its final
Rietveld plot is given in Fig. S5.† The observed differences
between the diffraction and simulation patterns are caused by
preferential orientation. From these results, we deduced that
the overall 3D structure of 1a′ can be identified as the
{Na(H2O)[(R)-LMe]}n coordination polymer and no further com-
ments are required.
Synthesis and characterisation of silver coordination polymers
The interaction of Na[(S)-LMe], 1a, with silver nitrate affords,
after the appropriate work up, crystals of complex {Ag[(S)-
LMe]}n, 2a. They were stable upon exposure to air, but slow
decomposition was observed after exposure to light. Complex
2a is insoluble in common organic solvents and only sparingly
soluble in water.
A broad IR band centred at around
1640 cm−1, due to the antisymmetric COO vibrations, is indica-
tive of the presence of the coordinated carboxylate group in
this complex, while the absorption at 1394 cm−1 was attributed
to the symmetric COO vibration. A 1 : 1 ratio of silver cation
and (S)-2-(1H-imidazol-1-yl)propanoate ligand was deduced
from the microanalysis, but the polymeric nature of complex
2a was only evidenced after a single-crystal X-ray study. Fig. 6a
displays the asymmetric unit that consists of the (S)-2-(1H-imi-
dazol-1-yl)propanoate anion and the silver ion. Selected struc-
tural parameters of 2a are collected in Table 2. Each cation is
surrounded by three oxygen atoms (O1 and O2 from three sym-
Fig. 4 Asymmetric unit of 1c (top) and the novel μ4κN,κ2O,κ2O’ coordi-
i
2iPr −
nation mode of the [(S)-LCH
]
anion in {Na[(S)-LCH Pr]}n, 1c, with
2
selected bond distances in Å (bottom).
of imidazole is coordinated to the sodium ion (2.419(2) Å) and
i
the overall coordination mode of the [(S)-LCH Pr −
]
anion is
2
novel, μ4κN,κ2O,κ2O′ (Fig. 4, bottom). Besides the N coordi-
nation, each sodium ion is surrounded by four oxygen atoms
i
(O1 and O2 from two symmetry related [(S)-LCH Pr −
]
anions).
2
This produces a zigzag disposition of Na ions (with an Na⋯Na
separation of 3.4140(9) Å and an Na⋯Na⋯Na angle of 120.47
(4)°) and, accordingly, a 2D coordination polymer along the
crystallographic bc plane (Fig. 5). Non-covalent interactions
metry related [(S)-LMe −
anions) and one nitrogen atom of
]
imidazole (N2) with bond distances in the 2.24–2.51 Å range.
Similar distances were found in the related complexes: silver 2-
(1H-imidazol-1-yl)acetate28 and (2-(1H-imidazol-3-ium-1-yl)
acetato)silver nitrate.29 The coordination mode of the [(S)-
i
between the CH2 Pr groups of the homochiral 2D coordination
polymers afford the crystal packing of 1c (Fig. S3, ESI†).
LMe − anion is μ4κN,κ2O,κO′ (type IV in Scheme 1), in which
]
As one might expect, complex 1a′ showed identical spectro-
scopic properties (IR, 1H and 13C{1H} NMR) of its known enan-
tiomer 1a. The only difference comes from the polarimetry
measurements. The observed [α]D value of 1a in water is +27.1,
which is similar to the values determined for related (S)-com-
pounds ([α]D range of +20–27 for 1a–d), while [α]D for 1a′ is
approximately the same as 1a but with the opposite sign
([α]D = −27.5 in water). 1a′ showed identical experimental powder
diffractograms to the simulated one of {Na(H2O)[(S)-LMe]}n, 1a,
from single-crystal X-ray studies (Fig. S4a, ESI†). Rietveld
refinement from the XRPD of 1a′ was performed using the as-
four silver cations are bridged by the ligand (see the selected
bond distances in Fig. 6b). This bonding mode produces a 2D
coordination polymer extended along the crystallographic bc
plane (Fig. 7). The 3D crystal distribution was produced by the
Fig. 6 Asymmetric unit of {Ag[(S)-LMe]}n, 2a (top) and the μ4κN,κ2O,κO’
Fig. 5 Perspective view of the 2D coordination polymer of {Ag[(S)-
L
coordination mode of the [(S)-LMe −
tances in Å (bottom).
]
anion in 2a with selected bond dis-
Me]}n, 2a, viewed along the crystallographic b-axis.
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Table 2 Selected structural parameters for complex 2a a
co-workers by solvothermal reactions and were identified as
metal–organic frameworks by X-ray single-crystal diffraction.20
The preparation of these compounds does not require high
temperature conditions, and 3a and 4a were synthesised by us
at room temperature. Besides this advantage, this method
afforded better yields (ca. 35%) than those reported previously
(ca. 22%). The drawback is that this procedure usually afforded
3a and 4a as solid amorphous materials. The IR spectra of
these compounds perfectly match with those reported by Tian
and co-workers (Fig. S8, ESI†).20 Occasionally, the zinc deriva-
tive 4a was obtained as a microcrystalline solid and the experi-
mental XRPD coincides with the simulated one from single-
crystal X-ray diffraction (Fig. S9, ESI†).20
Additionally, 4a′ was also studied by XRPD. Its formulation
as the coordination polymer {Zn[(R)-LMe]2}n was proposed
on the basis of its identical experimental powder
diffractogram with respect to the simulated one of 4a, from
the single-crystal X-ray determination of Tian and co-workers
(Fig. S10, ESI†).
Distances (Å)
Ag–O
Ag(1)–O(1)
Ag(1)–O(2)#2
Ag(1)–O(1)#3
Ag(1)–N(2)#1
O(1)–C(5)
2.25(2)
2.43(2)
2.51(2)
2.25(2)
1.21(3)
1.31(4)
Ag–N
O–C
O(2)–C(5)
Angles (°)
O–Ag–O
O(1)–Ag(1)–O(2)#2
O(1)–Ag(1)–O(1)#3
O(2)#2–Ag(1)–O(1)#3
N(2)#1–Ag(1)–O(1)
N(2)#1–Ag(1)–O(2)#2
N(2)#1–Ag(1)–O(1)#3
C(5)–O(1)–Ag(1)
C(5)–O(1)–Ag(1)#2
C(5)–O(2)–Ag(1)#3
O(1)–C(5)–O(2)
106.6(9)
117.6(6)
102.1(9)
138.3(9)
86.7(8)
96.9(7)
125(2)
113(2)
143(2)
123(3)
N–Ag–O
C–O–Ag
O–C–O
a Symmetry transformations used to generate equivalent atoms: #1 x, y,
z − 1; #2 −x + 1, y − 1/2, −z; #3 −x + 1, y + 1/2, −z; and #4 x, y, z + 1.
Preliminary studies in the preparation of chiral
α-aminophosphonates
As previously studied by us with related complexes,22 silver
and zinc coordination polymers 2a and 4 were tested as cata-
lysts in the Kabachnik–Fields reaction for the synthesis of
chiral α-aminophosphonates.31–36 The catalytic efficiency in
this reaction was evaluated using the typical one-pot three-
component procedure and selecting 4-methoxybenzaldehyde,
4-methoxyaniline and diethyl phosphonate as the model reac-
tion (Scheme 3).
Table 3 reports selected results from the reactions carried
out in a reactor with a 1 : 1 : 1 : 0.1 ratio of aldehyde : aniline :
phosphonate : catalyst for 24 h. Although the observed conver-
sions are high, the selectivity toward the α-aminophosphonate
species is good only for Zn derivatives, 4, at 60 °C. Silver
complex 2a gave a lower selectivity (55%, entry 1) and poorer
results were observed at room temperature (entry 5). The best
conversion and selectivity were obtained with 4a in methanol
at 60 °C. However, unfortunately, no enantioselectivity was
observed in these reactions (<5% ee). Because of these results,
no further investigation was carried out. As we described
before for related zinc imidazolium-dicarboxylate complexes,22
the low solubility and the ligand dissociation from the coordi-
nation sphere of the zinc ion is the reason for the
Fig. 7 Perspective view of the 2D coordination polymer of 2a viewed
along the crystallographic b-axis.
packing of the 2D lamellar sheets due to non-classical inter-
laminar interaction between the methyl group of the [(S)-LMe −
]
ligand and the silver ion, –CH3⋯Ag+ (Fig. S6, ESI†). This situ-
ation is similar to that found in the crystal structure of the
related chiral coordination polymer, silver (2-(4H-1,2,4-triazol-
4-yl)propanoato) monohydrate.30 The intramolecular contact
–C6⋯Ag+ of ca. 3.55 Å in 2a is slightly longer than the upper
part of the observed 3.13–3.42 Å range.24
As occurred with 1a′, complex 2a′ was identified as the
coordination polymer {Ag[(R)-LMe]}n on the basis of its XRPD.
2a′ showed an identical experimental powder diffractogram to
the simulated one of {Ag[(S)-LMe]}n, 2a, from single-crystal
X-ray studies (Fig. S7, ESI†). Therefore, the crystal packing of
2a′ is similar to that of its enantiomer 2a, as previously
discussed.
Synthesis and characterisation of first-row transition metal
coordination polymers
The treatment of solutions of different metal salts with Na[LR],
1, results in the formation of complexes {Cu[(S)-LR]2}n (R = Me,
i
i
3a; Pr, 3b), {Cu[(R)-LMe]2}n, 3a′, {Zn[(S)-LR]2}n (R = Me, 4a; Pr,
4b; (S)-secBu, 4d) and {Zn[(R)-LMe]2}n, 4a′ after the appropriate
work up. All products, which were isolated as crystalline
solids, are air stable and not soluble in common solvents.
Scheme 3 Model
reaction
for
the
synthesis
of
chiral
Compounds 3a and 4a were previously prepared by Tian and α-aminophosphonates.
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Table 3 Synthesis of α-aminophosphonates using silver(I) 2a and zinc(II) 4 coordination polymers as catalystsa
Entry
Catalyst
Solvent
T (°C)
t (h)
Conversionb
Selectivityb
%eec
1
2
3
4
5
2a
4a
4a
4b
4b
MeOH
H2O
MeOH
MeOH
MeOH
60
60
60
60
25
24
24
24
24
24
92
80
94
79
79
55
70
77
65
26
<5
—
<5
<5
<5
a Reaction conditions: Catalyst 0.05 mmol, 4-methoxybenzaldehyde 0.5 mmol, 4-methoxyaniline 0.5 mmol, diethyl phosphonate 0.5 mmol,
solvent 2.0 mL. b Determined by 1H NMR analysis. c Determined by HPLC.
observed poor enantioselectivities in the Kabachnik–Fields to use, using standard procedures. Chemicals were obtained
reaction.
from commercial sources and used as supplied. Infrared
spectra were recorded on a PerkinElmer FT-IR Spectrum Two
spectrophotometer, using the ATR technique. NMR spectra
were recorded on Bruker AMX-300 or Avance III spectrometers
Conclusions
at the Centro de Investigaciones, Tecnología
e Innovación
Chiral Na[LR] species are widely used as intermediates in the
synthesis of chiral derivatives, carbene ligands and ionic
liquids. However, the exact solid-state nature of this species
(CITIUS) of the University of Sevilla. 1H and 13C{1H} NMR
shifts were referenced to the residual signals of deuterated sol-
vents, while 31P{1H} shifts were referenced to external 85%
phosphoric acid. All data are reported in ppm downfield from
Si(CH3)4. Elemental analyses (C, H, N) and X-ray Powder
Diffraction (XRPD) studies were conducted at CITIUS of the
University of Sevilla on an Elemental LECO CHNS 93 analyser
and on a Bruker D8 advance A25 diffractometer, respectively.
Polarimetry was carried out using a JASCO P-2000 Digital
Polarimeter and the measurements were carried out at room
temperature (concentration of ca. 10 mg mL−1). HPLC chrom-
atography was performed on an Agilent 1260 Infinity instru-
ment with a Chiralpak IA column at a flow rate of 0.5 mL
min−1 with heptane/isopropanol = 9/1 (v/v) and using a UV
detector at 254 nm. High-resolution mass spectra were
recorded using a Q-Exactive Hybrid Quadrupole-Orbitrap Mass
Spectrometer from Thermo Scientific at CITIUS of the
University of Sevilla. The synthesised transition-metal coordi-
nation polymers 2 and 4 are insoluble in common solvents
and cannot be purified by recrystallization. For this reason,
some of the microanalytical data are out of the required stan-
dard accuracy. Figures of 3D coordination polymers were
obtained with the ToposPro program.37
was unknown. Compounds Na[(S)-LR] (R = Me, 1a, and CH2 Pr,
i
1c) have been structurally identified by single-crystal X-ray
diffraction methods as two-dimensional homochiral coordi-
i
nation polymers {Na(H2O)[(S)-LMe]}n and {Na[(S)-LCH Pr]}n,
2
respectively. In both compounds, novel coordination modes
for (S)-2alkyl,2-(1H-imidazol-1-yl)acetate anions, μ3κ2OκO′ for
1a and μ4κ2Oκ2O′ for 1c, have been found. In particular, 1a is a
rare example in which the [(S)-LMe −
anion is bonded to
]
sodium ions only through oxygen atoms without the partici-
pation of the N atom of imidazole. This situation contrasts
with all the reported compounds, where these types of ligands
always behave as ditopic linkers. Their coordination capabili-
ties as ligands have been analysed here by DFT calculations on
the [LR]− anions. Additionally, several coordination polymers
of silver, 2, copper, 3, and zinc, 4, have been prepared by the
reaction of Na[LR] with several metal salts. For known {Cu[(S)-
LMe]2}n, 3a, and {Zn[(S)-LMe]2}n, 4a, this procedure is a new syn-
thetic route that avoided solvothermal reaction conditions.
Enantiomeric species {Na(H2O)[(R)-LMe]}n, 1a′, {Ag[(R)-LMe]}n,
2a′, {Cu[(R)-LMe]}n, 3a′, and {Zn[(R)-LMe]2}n, 4a′, are the first
examples of homochiral coordination polymers with imid-
azole–monocarboxylate ligands based on non-natural amino
acids. They showed crystal structures, according to the X-ray
powder diffractograms, similar to those of complexes 1–4 with
ligands obtained from natural amino acids. Finally, as we pre-
viously observed for related zinc complexes,22 the catalytic per-
formance of complexes 2a and 4 in the synthesis of chiral
α-aminophosphonates is very poor, with ee values lower
than 5%.
Syntheses
i
i
Compounds Na[(S)-LR] (R = Me, 1a; Pr, 1b; CH2 Pr, 1c, and
(S)-secBu, 1d) and Na[(R)-LR] (R = Me, 1a′) were prepared
according to the procedure reported by Bao and coworkers (see
the ESI† for the data of compounds 1a–c).4
Sodium (R)-2-(1H-imidazol-1-yl)propanoate, 1a′. Yield 71%.
IR (cm−1): 3300 (w, br), 3090 (w), 1584 (s), 1513 (m), 1491 (w),
1477 (w), 1449 (m), 1416 (m), 1396 (s) 1374 (s), 1260 (s), 1235
(m), 1114 (m), 1081 (m), 1082 (m), 1032 (m), 920 (m), 888 (w),
867 (w), 810 (s), 753 (s), 678 (s), 660 (s), 539 (w). 1H NMR
(CD3OD, 300 MHz): δ 1.69 (d, 3JHH = 7.2 Hz, 3 H, CH3), 4.74 (q,
3JHH = 7.2 Hz, 1H, CHCH3), 6.94 (s, 1H, C4H), 7.18 (s, 1H, C5H),
Experimental
General considerations
All preparations and other operations were carried out under 7.72 (s, 1H, C2H). 13C{1H} NMR (CD3OD, 75.47 MHz): δ 20.9 (s,
aerobic conditions. Solvents were purified appropriately prior CH3), 60.6 (s, CHCH3), 120.9 (s, C5H). 129.1 (s, C4H), 138.6 (s,
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C2H), 178.6 (s, COO). [α]D21 = −27.5 (H2O). ESI-MS: found m/z 46% yield). IR (cm−1): 3147 (m), 2964 (w), 2876 (w), 1629 (s),
163.0478 for [1a′ + H]+, calculated for C6H8N2NaO2, 163.0483. 1514 (m), 1466 (m), 1384 (vs), 1327 (m) 1290 (m), 1224 (m),
Elemental Anal. Calc. for C6H7N2NaO2: C, 40.01; H, 5.04; N, 1109 (m), 1086 (m) 1028 (w), 981 (w), 954 (w), 918 (w), 837 (w),
15.55. Found: C, 39.72; H, 5.26; N, 15.04%.
755 (m), 665 (w), 536 (w). Elemental Anal. Calc. for
Sodium (2S,3S)-2-(1H-imidazol-1-yl)-3-methylpentanoate, 1d. C12H14CuN4O4·3H2O: C, 36.41; H, 5.09; N, 14.15. Found: C,
Yield 82%. IR (cm−1): 3322 (m br), 2965 (m), 2937 (w), 2876 36.48; H, 4.54; N, 14.61%.
(w), 1595 (vs), 1500 (m), 1460 (m), 1378 (s), 1313 (w), 1285 (w),
Copper (S)-2-(1H-imidazol-1-yl)-3-methylbutanoate, {Cu[(S)-
1224 (m), 1153 (w), 1114 (w), 1078 (m), 1035 (w), 917 (w), 821 LiPr]2}n, 3b. Following a similar procedure, but starting from
(w), 735 (m), 665(m), 635 (m). 1H NMR (CD3OD, 300 MHz): Na[(S)-LiPr] (1b, 0.500 g, 2.63 mmol) and copper(II) nitrate tri-
3
δ 0.86 (t, JHH = 7.2 Hz, 3 H, CH3), 0.96 (m, 2H, CH2CH3), 1.04 hydrated (0.320 g, 1.32 mmol), 3b was obtained as a dark blue
(d, JHH = 6.7 Hz, 3H, CHCH3), 2.19 (m, 1H, CH3CHCH2CH3), solid (0.163 g, 28% yield). IR (cm−1): 3147 (m), 2964 (w), 2876
4.29 (d, JHH = 9.7 Hz, 1H, CH), 6.97 (s, 1H, C4H), 7.29 (s, 1H, (w), 1629 (s), 1514 (m), 1466 (m), 1384 (vs), 1327 (m) 1290 (m),
C5H) 7.81 (s, 1H, C2H). 13C{1H} NMR (CD3OD, 75.47 MHz): 1224 (m), 1109 (m), 1086 (m) 1028 (w), 981 (w), 954 (w), 918
δ 9.89 (s, CH3), 14.99 (s, CH3), 24.6 (s, CH2CH3), 37.80 (s, CH), (w), 837 (w), 755 (m), 665 (w), 536 (w). Elemental Anal. Calc.
69.24 (s, CH), 119.4 (s, C5H), 126.2 (s, C4H), 136.8 (s, C2H) for C16H22CuN4O4·H2O: C, 46.20; H, 5.82; N, 13.47. Found: C,
175.0 (s, COO). [α]2D1 = +20.5 (H2O). ESI-MS: found m/z 45.41; H, 5.45; N, 12.97%.
205.0945 for [1d + H]+, calculated for C9H14N2NaO2, 205.0953.
Zinc (S)-2-(1H-imidazol-1-yl)propanoate, {Zn[(S)-LMe]2}n,
Silver (S)-2-(1H-imidazol-1-yl)propanoate, {Ag[(S)-LMe]}n, 2a. 4a.20 Over an aqueous solution of Na[(S)-LMe] (1a, 0.500 g,
Over an aqueous solution of Na[(S)-LMe] (1a, 0.162 g, 1 mmol), 3.085 mmol), an aqueous solution of zinc(II) chloride (0.210 g,
an aqueous solution of silver(I) nitrate (0.1708 g, 1 mmol) was 1.54 mmol) was added. The resulting mixture was stirred for
added in a light-protected reactor. The resulting mixture 1 h. A small amount of 4a was formed, as a yellow solid, and
(15 mL) was stirred for 1 hour. Then, 5 mL of DMF was added was isolated by filtration. Slow evaporation of the resulting
and the mixture was left in a dark box to avoid photolysis solution afforded yellow crystals of 4a (0.200 g, 38% yield). IR
decomposition. Slow evaporation of this solution resulted in (cm−1): 3147 (w), 3129 (w), 3004 (w), 2976 (w), 1638 (vs), 1514
the formation of uncoloured crystals of compound 2a (w), 1463 (w), 1417 (w), 1388 (s), 1360 (s), 1302 (s), 1267 (s),
(80.9 mg, 32% yield). IR (cm−1): 3122 (w), 3015 (w), 2976 (w) 1231 (s), 1110 (s), 1099 (s) 1078 (m), 1031 (w), 985 (m), 956
1595 (vs), 1506 (m), 1488 (m), 1467 (w), 1413 (w), 1378 (s), 1345 (m), 856 (m), 841 (m), 785(m), 743 (m), 729 (m), 657 (vs), 524
(s), 1335 (s), 1288 (w) 1278 (w), 1256 (vs), 1221 (m), 1113 (s), (m). Elemental Anal. Calc. for: C12H14N4O4Zn: C, 41.94, H,
1085 (s) 1070 (s), 1028 (m), 974 (m), 928 (m), 878 (m), 839 (s) 4.11, N, 16.30. Found: C, 41.51; H, 4.14; N, 15.64%.
765 (m), 753 (s), 731 (m), 667 (s), 652 (s), 643 (m). Elemental
Anal. Calc. for C6H7N2AgO2: C, 29.18; H, 2.86; N, 11.34. Found: Over a solution of Na[(R)-LMe] (1a′, 0.200 g, 1.23 mmol) in
C, 28.55; H, 2.67; N, 11.60%. methanol (5 ml), a solution of zinc(II) acetate dihydrated
Silver (R)-2-(1H-imidazol-1-yl)propanoate, {Ag[(R)-LMe]}n, 2a′. (0.130 g, 0.615 mmol) in methanol (5 ml) was added. The
Following a similar procedure, but starting from Na[(R)-LMe
resulting solution was stirred at 60 °C for 24 h. Solid 4a′ was
Zinc (R)-2-(1H-imidazol-1-yl)propanoate, {Zn[(R)-LMe]2}n, 4a′.
]
(1a′, 0.162 g, 1 mmol) and silver(I) nitrate (0.1708 g, 1 mmol), precipitated from the reaction and was separated by filtration.
2a′ was obtained as an uncoloured crystalline solid (95 mg, From the mother liquor, a second crop of yellow crystals of 4a′
38% yield). IR (cm−1): 3122 (w), 3015 (w), 2976 (w) 1595 (vs), was obtained, which were isolated by filtration (243.5 mg, 40%
1506 (m), 1488 (m), 1467 (w), 1413 (w), 1378 (s), 1345 (s), 1335 yield). IR (cm−1): 3147 (w), 3129 (w), 3004 (w), 2976 (w), 1638
(s), 1288 (w) 1278 (w), 1256 (vs), 1221 (m), 1113 (s), 1085 (s) (vs), 1514 (w), 1463 (w), 1417 (w), 1388 (s), 1360 (s), 1302 (s),
1070 (s), 1028 (m), 974 (m), 928 (m), 878 (m), 839 (s) 765 (m) 1267 (s), 1231 (s), 1110(s), 1099 (s) 1078 (m), 1031 (w), 985 (m),
753 (s), 731 (m), 667 (s), 652 (s), 643 (m).
956 (m), 856 (m), 841 (m), 785(m), 743 (m), 729 (m), 657 (vs),
Copper (S)-2-(1H-imidazol-1-yl)propanoate, {Cu[(S)-LMe]2}n, 524 (m). Elemental Anal. Calc. for: C12H14N4O4Zn·H2O: C,
3a.20 Over an aqueous solution of Na[(S)-LMe] (0.500 g, 39.85; H, 4.46; N, 15.49. Found: C, 39.67; H, 4.32; N, 15.18%.
3.085 mmol), an aqueous solution of copper(II) nitrate trihy-
Zinc (S)-2-(1H-imidazol-1-yl)-3-methylbutanoate, {Zn[(S)-
drated (0.370 g, 1.54 mmol) was added. The resulting mixture LiPr]2}n, 4b. Following the same procedure as 4a, but starting
was stirred for 1 h. The solution was slowly evaporated and a from Na[(S)-LiPr] (500 mg, 2.63 mmol) and zinc(II) chloride
blue precipitate was formed. Crystallization from
a
(0.180 g, 1.31 mmol), 4b was obtained as a yellow solid
1 : 3 mixture of H2O/DMF afforded the compound 3a (0.182 g, (0.114 g, 23% yield). IR (cm−1): 3136 (w), 2967 (m), 1635 (vs),
34% yield). IR (cm−1): 3129 (w), 2993 (w), 2929 (w) 1595 (vs), 1518 (m), 1469 (w), 1384 (s), 1238 (m), 1091 (m), 954 (w), 844
1520 (m), 1456 (m), 1385 (s), 1356 (vs), 1278 (s), 1235 (s), 1110 (w), 754 (m), 731 (w), 659 (m). Elemental Anal. Calc. for:
(s), 1074 (m) 1035 (m), 978 (m), 950 (w), 882 (w), 825 (w), 739 C16H22N4O4Zn·2H2O: C, 44.10; H, 6.01; N, 12.86. Found: C,
(m), 692 (s), 653 (s).
44.64; H, 5.60; N, 12.82%.
Zinc (2S,3S)-2-(1H-imidazol-1-yl)-3-methylpentanoate, {Zn
Copper (R)-2-(1H-imidazol-1-yl)propanoate, {Cu[(R)-LMe]2}n,
3a′. Following a similar procedure, but starting from Na[(R)- [(S)-LsecBu]2}n, 4d. Over an aqueous solution of Na[(S)-LsecBu
]
LMe] (324 mg, 2 mmol) and copper(II) chloride (170 mg, (1d, 0.204 g, 1 mmol), an aqueous solution of zinc(II) chloride
1 mmol), 3a′ was obtained as a blue crystalline solid (0.182 g, (68.1 mg, 0.5 mmol) was added. The resulting mixture was
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stirred for 1 h at room temperature. Compound 4d was formed DFT calculations were performed using the Gaussian 09
as a yellow solid and was isolated by filtration, washed with suite of programmes.46 Coordinates of optimised com-
ethanol and dried in air (68.8 mg, 32% yield). IR (cm−1): 3460 pounds are collected in Table S4 (ESI).†
(w, br), 3136 (w), 2969 (w), 2937 (w), 1634 (vs), 1519 (m), 1380
Catalytic assays
(s), 1267 (s), 1236 (m), 1110 (s), 1085 (s), 952 (m), 848 (w), 744
(vs), 665 (s). Elemental Anal. Calc. for: C18H26N4O4Zn: C,
50.54; H, 6.13; N, 13.10. Found: C, 50.98; H, 6.06; N, 12.58%.
Synthesis of diethyl ((4-methoxyphenyl)((4-methoxyphenyl)
amino)methyl)phosphonate. A 50 mL reactor (vial equipped
with a Young valve and containing a stirrer flea) was charged
with 4-methoxybenzaldehyde (0.5 mmol), 4-methoxyaniline
(0.5 mmol), diethyl phosphonate (0.5 mmol), complexes 2a or
4 (0.05 mmol) and the reaction solvent (2 mL), in the afore-
mentioned order. The reactor was sealed and the mixture was
stirred at 60 °C in a thermostated bath for 24 h. Upon com-
pletion, the resulting mixture was evaporated to dryness
affording a residue that was extracted with dichloromethane
and then filtered. A 1 mL aliquot of the resulting solution was
evaporated to dryness and the residue was analysed by 31P{1H}
NMR. A second 1 mL aliquot was diluted with 10 mL of isopro-
panol and the resulting solution was analysed by chiral HPLC.
X-ray crystallography
A summary of the crystallographic data and structure refine-
ment results for compounds 1a, 1c and 2a is given in Table S1
(ESI).† Crystals of suitable size for X-ray diffraction analysis
were coated with dry perfluoropolyether and mounted on glass
fibers and fixed in a cold nitrogen stream (T = 213 K) on the
goniometer head. Data collection was performed on a Bruker-
Nonius X8Apex-II CCD diffractometer, using monochromatic
radiation λ(Mo Kα) = 0.71073 Å, by means of ω and φ scans
with a width of 0.50 degrees. The data were reduced (SAINT38)
and corrected for absorption effects by the multi-scan method
(SADABS).39 The structures were solved by direct methods
(SIR-200240) and refined against all F2 data by full-matrix least-
squares techniques (SHELXL-2016/641) minimizing w[Fo
−
2
Conflicts of interest
2 2
Fc ] . All non-hydrogen atoms were refined anisotropically. The
hydrogen atoms were included from calculated positions and
refined riding on their respective carbon atoms with isotropic
displacement parameters. The corresponding crystallographic
data were deposited with the Cambridge Crystallographic Data
Centre as supplementary publications, CCDC 1904848 (1a),
1904849 (1c) and 1904850 (2a).†
There are no conflicts to declare.
Acknowledgements
The University of Sevilla (VI Plan Propio) supported this
research. We thank the Centro de Servicios de Informática y
Redes de Comunicaciones (CSIRC), Universidad de Granada,
for providing the computing time and the CITIUS, Universidad
de Sevilla, for the measurements.
X-ray powder diffraction
X-ray powder diffraction (XRPD) patterns were collected on a
Bruker D8 advance A25 diffractometer. The crystal structure
refinements were performed by means of the Rietveld
method42 using the FULLPROF software.43 A Thompson–Cox–
Hastings pseudo-Voigt function convoluted with an axial diver-
gence asymmetry function was chosen to generate the peak
shapes. The following parameters were refined: background
points, zero point, scale factor, pseudo-Voigt parameters of the
peak shape, atomic positions and cell parameters. The simu-
lated Crystallographic Information File of Na[(R)-LMe], 1a′,
was obtained by using the Materials Studio software (BIOVIA
DS) and this CIF was used as the starting model (Fig. S4b,
ESI†). Rietveld adjustment was carried out and the data are
shown in Table S2 (ESI),† while selected results of the structure
refinement of the XRPD sample 1a′ are summarized in
Table S3.†
Notes and references
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