Zinc and Cadmium Complexes of Pyridinemethanol Carboxylates: Metal Carboxylate Zwitterions and Metal–Organic Frameworks

Article abstract of DOI:10.1002/cplu.202000175

The heterofunctional lactone furo[3,4-b]pyridin-5(7H)-one (L₁) undergoes a coordination-induced ring-opening reaction with Zn(NO₃)₂ ? 6H₂O to yield the zwitterionic [Zn(L₁′)₂(H₂O)₂] (1, L₁′=2-(hydroxymethyl)nicotinate) with an uncoordinated carboxylate. The same reaction with Cd(NO₃)₂ ? 4H₂O provides a two-dimensional (2D) network of [Cd(L₁′)₂]_n (3) with the carboxylates coordinated to cadmium(II) propagating the assembly. The corresponding reactions of Zn(NO₃)₂ ? 6H₂O and Cd(NO₃)₂ ? 4H₂O with 2-(hydroxymethyl)isonicotinic acid (HL₂) generated zwitterionic [Zn(L₂)₂(H₂O)₂] (2) and a 2D network [Cd(L₂)₂]_n?nDMF (4, DMF=N,N′-dimethylformamide), respectively. Complexes 1–4 are weakly emissive, giving ligand-centered emissions at 409 nm (1), 412/436 nm (2), 404 nm (3), and 412/436 nm (4) in CHCl₃ solutions upon excitation at 330 nm. This work points to the potential of using ‘hidden’ functionalities widely found in small organic molecules and natural products for the construction of coordination complexes with new functionality and potential applications.

Full text of DOI:10.1002/cplu.202000175

A Multidisciplinary Journal Centering on Chemistry
Accepted Article
Title: Zinc and Cadmium Complexes of Pyridinemethanol
Carboxylates: Metal Carboxylate Zwitterions and Metal–Organic
Frameworks
Authors: Yan Liu, Shi-Xin Lin, Ru-Jie Niu, Quan Liu, Wen-Hua Zhang,
and David J. Young
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10.1002/cplu.202000175
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Zinc and Cadmium Complexes of Pyridinemethanol Carboxylates:
Metal Carboxylate Zwitterions and Metal–Organic Frameworks
Yan Liu,[a] Shi-Xin Lin,[a] Ru-Jie Niu,[a] Quan Liu,*[b] Wen-Hua Zhang,*[a] and David J. Young[c]
Abstract: Hetero-functional lactone furo[3,4-b]pyridin-5(7H)-one
(L1) undergoes a coordination-induced ring-opening reaction with
Zn(NO3)2·6H2O to yield zwitterionic [Zn(L1’)2(H2O)2] (1, L1’ = 2-
(hydroxymethyl)nicotinate) with an uncoordinated carboxylate.
susceptible to structural variation while retaining their chemical
composition when exposed to external stimuli, leading to the
observation of some unusual behaviors such as breathing and
negative gas adsorption.[7]
The same reaction with Cd(NO3)2·4H2O provides
a
two-
Most pristine MOFs are prepared by a one-pot assembly in
solution, and usually involving hydrothermal conditions. This
“black-box” method necessarily precludes the observation of the
potentially interesting meta-stable phases that can significantly
contribute to our understanding of MOF-shaping processes. We
wondered if this hurdle could be at least partially overcome by
employing hetero-functional ligands that permit observation of the
coordination reactions in sequence before the ultimate MOF
formation.
In this regard, we were interested in exploring MOFs
supported by 2-pyridylmethanol carboxylate conjugated
ligands.[3c, 8] Our initial interest in these compounds arises from
earlier observations that 2-pyridinemethanol and its derivatives
are able to support intriguing and diverse homo-/hetero-metallic
cluster species under mild conditions, including Mn2/Ir2,[9]
Ni4/Ni3Gd,[10] Mn4Ca,[11] Ni7/Co7/Co6Na,[12] Mn25,[13] among others.
These clusters, as well as some mononuclear species, have
applications as single-molecular magnets,[10c, 12-13] cluster models
related to biological oxygen-evolving complexes[11] and synthetic
water oxidation electrocatalysts.[9] Our use of 2-pyridylmethanol
carboxylates for isolating MOFs bearing large cluster secondary
building units (SBUs) unexpectedly leads to the isolation of MOFs
with rod-shaped SBUs, similar to those found in MOF-74 and
isoreticular networks.[3c, 8a] More recently, we have also reported
the reaction of a porphyrin-based 2-pyridylmethanol ligand with
Co(II) salt to provide a two-dimensional (2D) MOF wherein only
half of the alcoholic oxygens were used for coordination to the
Co2+ center, leaving the MOF with sufficient reactive 2-
pyridylmethanol moieties on the surface for possible further
reactions.[14]
During our previous studies, we found that metal carboxylate
zwitterionic complexes of Co(II), Ni(II), Cu(II), and Zn(II), were
readily isolable en route to the corresponding MOFs.[8] These
zwitterions formed likely because of the strong chelation of the 2-
pyridylmethanol moiety, coupled with the instability of metal-
carboxylate bonds for these particular metal ions (especially Zn)
in water,[15] as well as the intrinsic driving force to form neutral
molecular species in solution.
In the present work, we have extended the study to isomeric
2-pyridylmethanol carboxylate ligands anticipating synergistic
coordination of the two functionalities to yield new MOF patterns.
We employed the same synthetic methodology as previously
described for 2-(hydroxymethyl) isonicotinate (iso-hmnH, HL2)
and 6-(hydroxymethyl)nicotinate (hmnH) in the attempted
synthesis of 2-(hydroxymethyl)nicotinate (Scheme 1).[8b]
dimensional (2D) network of [Cd(L1’)2]n (3) with the carboxylates
coordinated to Cd2+ propagating the assembly. The
corresponding reactions of Zn(NO3)2·6H2O and Cd(NO3)2·4H2O
with 2-(hydroxymethyl)isonicotinic acid (HL2) generated
zwitterionic [Zn(L2)2(H2O)2] (2) and 2D network [Cd(L2)2]n∙nDMF
(4, DMF = N, N’-dimethylformamide), respectively. Complexes
1−4 are weakly emissive, giving ligand-centered emissions at 409
nm (1), 412/436 nm (2), 404 nm (3), and 412/436 nm (4) in CHCl3
solutions upon excitation at 330 nm. This work points to the
potential of using ‘hidden’ functionalities widely found in small
organic molecules and natural products for the construction of
coordination complexes with new functionality and potential
applications.
Introduction
Metal–organic frameworks (MOFs) as a subset of coordination
chemistry have experienced an explosion of interest over the last
two decades.[1] Prepared by the self-assembly of metal ions and
organic ligands, these unique materials have a variety of
applications, including gas adsorption/separation,[2] analyte
1d, 4]
sensing,[3] catalysis,[1b,
and in biological systems.[5] This
plethora of diverse uses is due to an almost limitless number of
structural patterns by the combination of different metal sources
and ligands. The porosity of some MOFs allows for guest species
to diffuse into the inner space of the material to accomplish
guest/metal/ligand exchange and ligand functionalization, further
enriching the structural library.[6] Some flexible MOFs are also
[a]
[b]
[c]
Y. Liu, S.-X. Lin, R.-J. Niu, Prof. W.-H. Zhang
College of Chemistry, Chemical Engineering and Materials
Science, Soochow University, Suzhou 215123, China
E-mail: whzhang@suda.edu.cn
Prof. Q. Liu
College of Chemistry and Chemical Engineering, Nantong
University, Nantong 226019, China
E-mail: liu.q@ntu.edu.cn
Prof. D. J. Young
College of Engineering, Information Technology & Environment,
Charles Darwin University, Darwin, Northern Territory 0909,
Australia
Supporting information for this article is given via a link at the end
of the document.
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However, a lactone furo[3,4-b]pyridin-5(7H)-one (L1) was formed
by intramolecular esterification. The subsequent reactions of L1
and HL2 with Zn(NO3)2·6H2O yielded zwitterions [Zn(L1’)2(H2O)2]
Zwitterionic complexes 1 and 2, and 2D MOFs 3 and 4 were
stable under aerobic conditions and in common organic solvents,
such as CH2Cl2, CHCl3, MeOH, EtOH, MeCN, and DMF. These
compounds, together with their constituent ligands L1 and HL2
were accordingly characterized by Fourier transform infrared (FT-
IR), microanalysis, electrospray ionization mass spectrometry
(ESI-MS), powder X-ray diffraction (PXRD, Figure S1, Supporting
Information), and thermogravimetric analysis (TGA, Figure S2,
Supporting Information). The experimental PXRD of 1–4 matched
those simulated from the X-ray single-crystal data, indicating the
bulk phase purity of these compounds. TGA curves showed that
MOF 3 is stable until around 350 °C (Figure S2a, Supporting
Information), while the DMF-solvated MOF 4 lost DMF solvate
between 170–315 °C. The total solvent loss of 16.2% (Figure S2b,
Supporting Information) agrees well with the calculated value
from the X-ray structure (14.9%) and microanalysis results. The
structures of L1, HL2, and 1–4 were further authenticated by
single-crystal X-ray crystallographic analysis.
[8a]
(1, L1’ = 2-(hydroxymethyl)nicotinate) and [Zn(L2)2(H2O)2] (2)
featuring un-coordinated carboxylates, while their corresponding
reactions with Cd(NO3)2·4H2O yielded 2D MOFs [Cd(L1’)2]n (3)
and [Cd(L2)2]n∙nDMF (4, DMF = N, N’-dimethylformamide)
featuring coordinated carboxylates that propagate the assembly.
The formation of the zwitterion 1 through the coordination-driven
ring-opening of lactone may point to an opportunity of using
‘hidden’ functionalities found in synthetic or naturally occurring
species for the construction of coordination complexes with new
properties and applications.
Scheme 1. Synthesis of L1 and HL2.
Figure 1. The single molecular structure of L1 (a) and HL2 (b). Colour legend:
C (black), H (light-yellow), N (blue), O (red).
Results and Discussion
We initially targeted the ligand 2-(hydroxymethyl)nicotinic acid
wherein the N, –CH2OH, and –COOH functionalities are aligned
to achieve cooperative coordination and potentially yield MOFs
sustained by new cluster SBUs (Scheme 1). However, the
reduction step provided a white fibrous crystalline solid of lactone
furo[3,4-b]pyridin-5(7H)-one
(L1)
from
intramolecular
esterification in ca 25% yield. Both L1 and HL2 were
spectroscopically characterized and their solid-state structures
verified by single-crystal X-ray crystallography (Figure 1).
Reactions of L1 and HL2 with Zn(NO3)2·6H2O in a DMF/H2O
mixed solvent yielded zwitterions [Zn(L1’)2(H2O)2] (1) and
[Zn(L2)2(H2O)2] (2).[8] By contrast, their similar reactions using
Cd(NO3)2∙4H2O as the metal source provided 2D MOFs of
[Cd(L1’)2]n (3) and [Cd(L2)2]n∙nDMF (4). We reason that the Zn2+
preference for the zwitterionic complex formation and the Cd2+
preference for 2D matrices is due to their different binding
affinities for the ligands relative to H2O. The Zn(II)-carboxylate
bond is susceptible to hydrolysis as observed for many Zn(II)-
based MOFs, such as the iconic MOF-5.[15] On the other hand,
Figure 2. The zwitterionic structure of 1 (a) and 2 (b). Colour legend: Zn
(magenta), C (black), H (light-yellow), N (blue), O (red).
Both ligands L1 (Figure 1a) and HL2 (Figure 1b) crystallize in
the monoclinic space group P21/n and P21/c. Ligand L1 is a
lactone without conventional hydrogen-bonding donors but
stabilized by non-conventional C−H∙∙∙N and C−H∙∙∙O hydrogen-
bonding to give a three-dimensional (3D) network (Figure S3a and
Table S2, Supporting Information). By contrast, ligand HL2 is rich
in hydrogen-bonding donors, such as the alcoholic –OH,
carboxylic –OH, and water –OH donors, as well as the hydrogen-
bonding acceptors such as N, Owater, and carboxylic C=O,
resulting in a 2D hydrogen-bonded network structure (Figure S3b
and Table S2, Supporting Information).
Cd(II)-carboxylate coordination is comparatively more stable in
6g, 8b, 16]
water.[3c,
These small differences in coordination
stabilities, though widely acknowledged, are rarely studied and
compared in a systematic manner for coordination complexes and
MOFs.
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The zwitterionic structures of 1 (Figure 2a) and 2 (Figure
2b)[8a] crystallize in the monoclinic P21/n and orthorhombic Pbca
space groups and are isomeric in the location of the free and
deprotonated carboxylates for charge balance. Pairs of alcoholic
–OH, N and coordinated H2O were mutually trans in both
complexes, resembling those found in trans-Co(iso-
hmnH)2(H2O)2, trans-Ni(iso-hmnH)2(H2O)2, and trans-Zn(3,2,4-
hpbH)2(H2O)2 (3,2,4-hpbH = 3-(2-(hydroxymethyl)pyridin-4-yl)
benzoate).[8b] In 1, the two C–O bond distances of 1.227(3) Å and
1.238(3)
Å
(Table S1, Supporting Information) for the
uncoordinated carboxylates are nearly identical, indicating that
they are conjugated and the complex is zwitterionic. Zwitterionic
1 features a complicated 3D structure as stabilized by various
hydrogen-bonding interactions between alcohol –OH and water –
OH as donors, and the free, uncoordinated –COO as acceptors
(Figure S3c and Table S2, Supporting Information).
Figure 4. Normalized emission spectra of zwitterions 1 (black) and 2 (light pink)
and MOFs 3 (red) and 4 (blue) suspended in CHCl3 at an excitation wavelength
of 330 nm.
MOF 3 crystallizes in the monoclinic space group C2/c. In 3,
each Cd2+ is chelated by a pair of 2-pyridylmethanol moieties in
addition to a pair of monodentate carboxylates in the cis
configuration (Figure 3a), thus defining a distorted octahedron
geometry. Each L1’ ligand, in turn, bridges two Cd centers to form
a 2D grid (Figure 3c).
MOF 4 also crystallizes in the monoclinic space group C2/c.
Each Cd2+ in 4 is chelated by a pair of 2-pyridylmethanol moieties
which is a similar arrangement to that of 3 (Figure 3b). The Cd-O
distance of 2.4779(9) Å and Cd-N distance of 2.3439(10) Å are
similar to those identified in MOF 3 (2.3458(17) Å for Cd-O and
2.3441(18) Å for Cd-N, Table S1, Supporting Information).
However, in 4, the distances between Cd and the two O atoms of
the coordinated carboxylates are 2.5872(9) Å and 2.3393(8) Å,
contrasting with 2.2809(16) Å and 3.0821(16) Å in 3, and
suggesting a chelating coordination of the carboxylate in 4. Thus
each Cd2+ is eight-coordinate and associated with four L2 ligands.
Each L2 ligand, in turn, bridges two Cd2+ centers to give a 2D grid
(Figure 3d) with connectivity resembling that of MOF 3.
The 2D layers of both 3 and 4 are packed in ABAB fashion
supported by the inter-layer hydrogen-bonding between the
alcoholic –OH as the donor and the carboxylate as the acceptor
(Figures S3d and S3e, Supporting Information). While MOF 3 is
non-porous, MOF 4 features voids of 552.2 Å3 per cell which
corresponds to 30.2% of the total cell volume. This void volume is
occupied by DMF solvates as revealed by Platon.15
Crystalline powders of L1, HL2, zwitterions 1 and 2, and
MOFs 3 and 4 were dissolved or suspended in CHCl3 for
photoluminescence measurements. Upon excitation at 330 nm,
L1 and HL2 gave rise to weak emissions at 400 nm (L1) and
412/435 nm (HL2) (Figure S4, Supporting Information). The
emissions of zwitterion 1 (409 nm) and MOF 3 (404 nm) resemble
that of L1, whereas the nearly identical emissions of zwitterion 2
(412/436 nm) and MOF 4 (412/436 nm) resemble that of HL2
(Figure 4), suggesting ligand-centered emissions for all four
coordination compounds, facilitated by the d10 electronic
configurations of Zn2+ and Cd2+ that resists both oxidation and
reduction.[3f, 17]
Figure 3. Views of the full coordination sphere of Cd in MOFs 3 (a) and 4 (b),
as well as their 2D grid structures looking along the crystallographic [102] (3, c)
and c directions (4, d). Colour codes: Cd (dark magenta), C (black), N (blue), O
(red).
Neither MOFs 3 or 4 adsorbs N2 at 77 K (Figures S5a and
S5b, Supporting Information), presumably due to the large kinetic
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carefully added. The residue was extracted with CHCl3 twice and the
extracts combined, washed with brine 3 times and dried over anhydrous
MgSO4. Chromatographic purification on silica using EA/hexane (2:1, v/v)
as eluent yielded 2,3-diethylcarboxypyridine as a light-yellow oil. Yield
size of N2 (3.64 Å). MOF 3 does not adsorb CO2 (Figure S5c,
Supporting Information) while MOF 4 adsorbed 32.9 cm3 g–1 CO2
(1.47 mmol g–1, 6.50 wt% at standard temperature and pressure,
STP, Figure S5d, Supporting Information) which is low compared
to most MOFs, such as [Zn(iso-hmn)] (1.63 mmol g–1; 7.23 wt%
at STP),[8a] and Mn(2,6-ndc) (3.0 mmol g–1; 13.0 wt% at STP; ndc
= 2,6-naphthalenedicarboxylate).[18]
1
30.96 g, 91.8%. H NMR (400 MHz, CDCl3): δ 8.73–8.74 (m, 1H), 8.16–
8.18 (m, 1H), 7.44–7.47 (m, 1H), 4.34–4.47 (m, 4H), 1.34–1.42 (m, 6H).
13C {1H} NMR (101 MHz, CDCl3): δ 167.0, 165.9, 152.3, 151.9, 138.2,
127.0, 125.3, 62.7, 14.7.
Step 2. CaCl2 (0.83 g, 7.5 mmol) was added portion-wise with stirring
to a degassed solution of 2,3-diethylcarboxypyridine (1.67 g, 7.5 mmol)
and NaBH4 (0.19 g, 5.0 mmol) in EtOH (75 mL) at 0 °C. The resulting
mixture was stirred for 2.5 h at the same temperature and quenched with
concentrated H2SO4 (2 drops). The mixture was stirred for 3 h and left
stand overnight. White precipitate of CaSO4 was removed by filtration and
the solvent evaporated. Brine was added and the mixture extracted with
CHCl3, followed by solvent evaporation and column chromatography
purification (EA/hexane, 2:1, v/v) to give light-yellow crystalline fibres of
furo[3,4-b]pyridin-5(7H)-one (L1, 0.25 g, 25%). High quality colourless
block crystals were grown by slow evaporation of L1 in CH2Cl2/EtOH or
CH2Cl2/hexane. 1H NMR (400 MHz, CDCl3): δ 8.85 (d, J = 4.2 Hz, 1H),
8.19 (d, J = 7.7 Hz, 1H), 7.46–7.49 (m, 1H), 5.31 (s, 2H) 13C {1H} NMR
(101 MHz, CDCl3): δ 167.2, 156.0, 139.0, 134.7, 124.6, 120.5, 71.0. Anal.
Calcd. (%) for C7H5NO2: C 62.22, H 3.73, N 10.37; found: C 62.25, H 3.46,
N 10.16. IR (KBr pellet): 3506(w), 3076 (w), 2966 (w), 2941 (w), 2768 (w),
2526 (w), 2447 (w), 2282 (w), 2083 (w), 1992 (w), 1974 (w), 1955 (w), 1875
(w), 1769 (s), 1649 (sh), 1605 (s), 1589 (s), 1475 (s), 1458 (s), 1421 (s),
1371 (s), 1357 (s), 1301 (s), 1266 (s), 1241 (s), 1196 (m), 1184 (s), 1099
(s), 1050 (s), 1001 (s), 963 (sh), 866 (w), 822 (m), 748 (s), 698 (s), 601
Conclusions
In summary, we have prepared a pyridyl-fused lactone and 2-
pyridinemethanol carboxylate ligand, as well as their
corresponding metal zwitterions (for Zn) and 2D MOFs (for Cd).
The variation of the product outcome likely originates from the
different coordination stabilities Zn and Cd toward carboxylate,
relative to H2O. Zwitterion and MOF preparation from a lactone is
an interesting route and may point to some new coordination
chemistry using abundant synthetic and naturally occurring
molecules such as Lamellarin D, Camptothecin, and D-
Glucuronic acid lactone. One possible application might be the
development of drug-based Trojan-horse coordination complexes.
These temporarily altered drug molecules could exhibit different,
and hopefully lower toxicity than their unmodified progenitors. The
coordination bond is often unstable in the complex, and aqueous
environment of a biological system may induce the release of the
therapeutic species in a sustained and more-or-less predictable
manner.
(m), 506 (w), 468 (s), 406 (w) cm–1
.
Synthesis of [Zn(L1’)2(H2O)2] (1)
Zn(NO3)2·6H2O (29.8 mg, 0.1 mmol) in 2 mL H2O was added to L1 (27.0
mg, 0.2 mmol) and the mixture heated at 120 °C for 3 days and then slowly
cooled to r.t. to give a colourless solution, which was filtered and slowly
evaporated to give colourless, platelet crystals. These crystals were
filtered, washed with H2O and dried under vacuum. Yield: (13.0 mg, 32 %
based on Zn). Anal. Calcd. (%) for C14H16N2O8Zn: C 41.45, H 3.98, N 6.91;
found: C 41.89, H 3.73, N 6.89. IR (KBr pellet): 3195(s, br), 2775 (s), 2642
(s), 2576 (sh), 2484 (sh), 1923 (w), 1805 (w), 1611 (s), 1570 (s), 1477 (s),
1421 (s), 1379 (s), 1271 (m), 1240 (m), 1197 (m), 1155 (m), 1112 (m),
1074 (m), 1043 (s), 1006 (sh), 866 (s), 805 (s), 777 (s), 718 (s), 635 (s),
Experimental Section
Ligand HL2 and [Zn(L2)2(H2O)2] (2) were synthesized following our
reported procedures.[8] Zn(NO3)2·6H2O, Cd(NO3)2·4H2O and other
reagents were obtained from commercial sources and used as received.
FT-IR spectra were measured on a Varian 1000 FT-IR spectrometer as
KBr disks (400–4000 cm−1). Elemental analyses for C, H, and N were
performed on a Carlo-Erba CHNO-S microanalyzer. High-resolution mass
spectra (HRMS-ESI) were obtained on a MicroTM or a Micro YA263 Q-
TOF mass spectrometer. Thermogravimetric analyses (TGA) were
performed on a Mettler Toledo Star System at a heating rate of 5 °C min−1
under a nitrogen gas flow in an Al2O3 pan. Powder X-ray diffraction (PXRD)
patterns were recorded on a Bruker D8 GADDS micro-diffractometer
equipped with a VANTEC-2000 area detector using φ rotation. The N2 and
CO2 adsorption isotherms for MOFs 3 and 4 were measured at 77 K (liquid
nitrogen) and room temperature using a BELSORP-max (BEL, Japan).
Single crystals of MOFs 3 and 4 (around 100 mg for each sample) were
ground into fine powders and washed with CHCl3 several times, before
further dynamic vacuum at 100 °C for 12 hours prior to gas adsorption
studies.
571 (m), 489 (w), 451 (w), 438 (w) cm–1
.
Synthesis of [Cd(L1’)2]n (3)
Cd(NO3)2∙4H2O (15.5 mg, 0.05 mmol) and L1 (13.5 mg, 0.1 mmol) were
added to a Pyrex glass tube (10 cm in length and 0.5 cm in diameter)
followed by DMF/H2O (1 mL/2 mL). The tube was sealed and heated in an
oven at 120 °C for three days and gradually cooled to ambient temperature
at a rate of 5 °C min−1. The colourless crystals formed were filtered,
washed with DMF and dried under vacuum. Yield: (8.1 mg, 39 % based on
Cd). Anal. Calcd. (%) for C14H12CdN2O6: C 40.36, H 2.90, N 6.72; found:
C 40.78, H 2.91, N 6.93. IR (KBr pellet): 3432 (br), 3164 (br), 2963 (w),
2929 (w), 1773 (s), 1607 (m), 1590 (m), 1475 (m), 1458 (w), 1400 (s), 1385
(s), 1357 (w), 1301 (w), 1266 (w), 1138 (s), 1058 (s), 1001 (m), 823 (w),
751 (m), 617 (w), 537 (w), 468 (w), 401 (w) cm–1
.
Synthesis of furo[3,4-b]pyridin-5(7H)-one (L1)
Synthesis of [Cd(L2)2]n∙nDMF (4)
Step 1. 2,3-Pyridinedicarboxylic acid (25.25 g, 0.15 mol) was mixed with
p-toluenesulfonic acid monohydrate (60.35 g, 0.32 mol, 2.1 eq) in EtOH (1
L). The solution was refluxed at 100 °C for 5 days and then cooled to r.t.
The solvent was evaporated and saturated Na2CO3 (73.98 g, 0.70 mol)
Cd(NO3)2∙4H2O
(31
mg,
0.1
mmol)
and
ethyl
2-
(hydroxymethyl)isonicotinate (the precursor of HL2, 36 mg, 0.2 mmol) were
dissolved in DMF/H2O (4 mL/4 mL) and heated at 125 °C for 72 h in a
DURAN glass bottle. The light-yellow crystals formed were filtered and
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washed with DMF. Yield: (25.0 mg, 51 % based on Cd). Anal. Calcd. (%)
for C17H19CdN3O7: C 41.69, H 3.91, N 8.58; found: C 41.88, H 3.91, N 7.90.
IR (KBr pellet): 3178 (s), 2923 (s), 2850 (s), 2729 (w), 1735 (w), 1638 (s),
1599 (s), 1559 (s), 1462 (s), 1379 (s), 1329 (s), 1281 (s), 1256 (s), 1215
(w), 1116 (m), 1063 (s), 1018 (m), 898 (w), 882 (m), 818 (s), 787 (s), 737
initially modeled as disordered DMF molecules. However, such modeling
gave high atomic displacement parameters and short contacts between
some of the disordered atoms with the latter prompting A-level alerts in the
Platon checkCIF routine.[21] The solvent contribution was then modeled
using SQUEEZE in the Platon program suite.[22] A total of 157 electrons
were calculated for each cell, which corresponds to 3.92 DMF molecules.
(m), 702 (s), 632 (w), 595 (w), 569 (w), 494 (w), 420 (w) cm–1
.
X-ray crystallography
The correct formula of 4 can thus be written approximately as
Crystallographic measurements were made on a Bruker AXS APEX II
diffractometer using graphite-monochromated Mo Kα (λ = 0.71073 Å).
Absorption and other corrections were applied using SADABS.[19] All
crystal structures were solved by direct methods and refined on F2 by full-
matrix least-squares techniques with the SHELXTL-2016 program.[20] For
HL2, 1, 3 and 4, the hydrogen atoms on the alcoholic O–H were located
from the difference Fourier map with their O–H distances restrained to O–
H = 0.83 Å and thermal parameters constrained to Uiso(H) = 1.2Ueq(O). For
4, some spatially delocalized electron density in the lattice was found and
[Cd(L2)2]n∙nDMF (Z = 4). Crystallographic data for L1, HL2, 1, 3, and 4 have
been deposited in the Cambridge Crystallographic Data Center (CCDC)
as supplementary publication numbers 1851500–1851504. These data
can be obtained free of charge either from the CCDC via
www.ccdc.cam.ac.uk/data_request/cif or from the Supporting Information.
A summary of the key crystallographic data for L1, HL2, 1, 3 and 4 are
listed in Table 1, and selected bond distances and angles for L1, HL2, and
1–4 are listed in Table S1 (Supporting Information).
Table 1. Summary of crystallographic data for L1, HL2, 1, 3, and 4.
Compounds
Formula
FW
L1
HL2
1
3
4
C7H5NO2
135.12
monoclinic
P21/n
C14H16N2O7
324.29
monoclinic
P21/c
C14H16N2O8Zn
405.66
monoclinic
P21/n
C14H12CdN2O6
416.66
monoclinic
C2/c
C14H12CdN2O6
416.66
monoclinic
C2/c
Crystal system
Space group
a (Å)
3.9376(3)
6.8834(4)
22.4069(15)
90.00
7.1153(9)
27.746(3)
7.4756(9)
90.00
8.0129(7)
11.1371(10)
8.6477(7)
90.00
13.6320(9)
8.2693(6)
13.6886(9)
90.00
14.0656(7)
12.5058(6)
10.8846(6)
90.00
b (Å)
c (Å)
α (º)
β (º)
91.641(2)
90.00
106.634(4)
90.00
107.705(2)
90.00
118.2690(10)
90.00
107.1520(10)
90.00
γ (º)
V (Å3)
607.07(7)
4
1414.1(3)
4
735.17(11)
2
1359.04(16)
4
1829.47(16)
4
Z
ρcalc, g cm–3
, mm–1
F(000)
1.478
1.523
1.833
2.036
1.513
0.111
0.124
1.722
1.643
1.221
280
680
416
824
824
Total reflns.
Uniq. reflns.
Reflns. (I ≥ 2σ(I))
Rint
22350
2897
22769
2506
10036
1347
2515
13017
1287
2183
2349
2312
1183
1274
2142
0.0289
91
0.0462
226
0.0233
127
0.0128
108
0.0228
108
Parameters
[a]
R1
0.0519
0.1455
1.098
0.0814
0.0812
1.152
0.0240
0.0592
1.074
0.0192
0.0491
1.191
0.0139
0.0348
1.080
[b]
wR2
GOF[c]
ρmax/ρmin, e Å–3
0.325/–0.285
0.286/–0.382
0.442/–0.412
0.342/–1.092
0.438/–0.449
[a]
[b]
2
2
2
R1 = Σ||Fo|−|Fc||/Σ|Fo|, wR2 = {Σ[ω(Fo −Fc2)2]/Σ[ω(Fo )2]}1/2, and [c] GOF = {Σ[ω(Fo −Fc2)2]/(n−p)}1/2, where n is the number of reflections, and p the
total number of parameters refined.
Conflict of interest
Acknowledgments
The authors declare no conflict of interest.
This work was financially supported by the National Natural
Science Foundation of China (Grant Nos. 21871203 and
21701089). We are grateful to Mr. Ya-Dong Zhu for his
experimental assistance.
Keywords: carboxylates • lactones • metal−organic frameworks
• ring-opening reactions • zwitterions
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Entry for the Table of Contents
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Yan Liu, Shi-Xin Lin, Ru-Jie Niu,
Quan Liu,* Wen-Hua Zhang,*
and David J. Young
Page No. – Page No.
Zinc and Cadmium Complexes of
Pyridinemethanol Carboxylates:
Metal Carboxylate Zwitterions and
Metal–Organic Frameworks
Zn-based zwitterions of [Zn(L1’)2(H2O)2] (1, L1’ = 2-(hydroxymethyl)nicotinate) and
[Zn(L2)2(H2O)2] (2, HL2 = 2-(hydroxymethyl)isonicotinic acid), and Cd-based MOFs of
[Cd(L1’)2]n (3) and [Cd(L2)2]n∙nDMF (4) have been synthesized and characterized.
Complexes 1−4 exhibit ligand-centered emissions at 409 nm (1), 412/436 nm (2),
404 nm (3), and 412/436 nm (4) in CHCl3 solutions upon excitation at 330 nm.
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