Two-step postsynthetic modifications of a dinuclear Zn(II) coordination compound: Investigating the stability of the coordination chromophore

Article abstract of DOI:10.1016/j.ica.2012.03.031

Attempted two-step postsynthetic modifications (Click reaction and recrystallization) of a dinuclear Zn(II) coordination compound Zn ₂(OOC-C₆H₄-N₃) ₄(quinoline)₂ lead to the formation of a mononuclear Zn(II) compound - Zn(OOC-C₆H₄-C₂HN ₃-COOCH₃)₂(quinoline)·DMSO. In this feasibility study, the relatively low stability of the coordination chromophore around Zn(II) in the starting material is evidenced by Hirshfeld surface analysis and results in the transformation of coordination chromophore during the postsynthetic reaction. Diffusion Ordered Spectroscopy NMR experiments confirmed that the intermediate complex Zn(OOC-C₆H₄-C ₂HN₃-COOCH₃)₂ formed by Click reaction exists in the coordinated form in the solution phase despite the loss of the neutral ligand-quinoline. The final product Zn(OOC-C₆H ₄-C₂HN₃-COOCH₃)₂· (quinoline)·DMSO with a mononuclear crystal structure was formed by recrystallizing the intermediate with presence of quinoline.

Full text of DOI:10.1016/j.ica.2012.03.031

Inorganica Chimica Acta 388 (2012) 135–139
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Inorganica Chimica Acta
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Two-step postsynthetic modifications of a dinuclear Zn(II) coordination compound:
Investigating the stability of the coordination chromophore
,2,3
, Shuangbing Han ,1,2,3, Russell Hopson 3, Yanhu Wei 1,3, Brian Moulton 3
⇑
⇑
Zhenbo Ma
Department of Chemistry, Brown University, 324 Brook St., Providence, RI 02912, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Attempted two-step postsynthetic modifications (Click reaction and recrystallization) of a dinuclear
Zn(II) coordination compound Zn2(OOC–C6H4–N3)4(quinoline)2 lead to the formation of a mononuclear
Zn(II) compound – Zn(OOC–C6H4–C2HN3–COOCH3)2(quinoline)ÁDMSO. In this feasibility study, the rela-
tively low stability of the coordination chromophore around Zn(II) in the starting material is evidenced
by Hirshfeld surface analysis and results in the transformation of coordination chromophore during
the postsynthetic reaction. Diffusion Ordered Spectroscopy NMR experiments confirmed that the inter-
mediate complex Zn(OOC–C6H4–C2HN3–COOCH3)2 formed by Click reaction exists in the coordinated
form in the solution phase despite the loss of the neutral ligand-quinoline. The final product Zn(OOC–
C6H4–C2HN3–COOCH3)2Á(quinoline)ÁDMSO with a mononuclear crystal structure was formed by recrys-
tallizing the intermediate with presence of quinoline.
Received 6 February 2012
Received in revised form 10 March 2012
Accepted 14 March 2012
Available online 22 March 2012
Keywords:
Crystal
Postsynthetic
DOSY
Hirshfeld surface
Coordination chromophore
Click reaction
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
tetracarboxylate coordination chromophore with axial quinoline
ligands remains intact throughout the Click reaction [7–9], which
In recent years, materials scientists have explored postsynthetic
modification (PSM) as a powerful approach for functionalizing
coordination complexes, particularly coordination polymers, in or-
der to enhance chemical and physical properties of these materials
[1–3]. Biochemists have also used PSM as a route to facilitate
chemical modifications of metal-binding proteins [4]. More inter-
estingly, covalent reactions on coordinated forms of certain organic
molecules could result in the selective introduction of substituents
on the ligand, while the same reactions on free forms of ligand
molecules cannot do so [5]. A number of advantages of the PSM ap-
proach as described above are often related to the stability of coor-
dination chromophore of the starting material throughout the PSM
reaction process, where the coordination chromophore is defined
as the coordination environment around transition metal centers.
Our group recently reported the postsynthetic modification of a
dinuclear Cu(II) coordination compound with a paddle-wheel
dicopper tetracarboxylate structure and two axial ligand molecules
(quinoline) [6]. In that study, the paddle–wheel dicopper(II)
was evidenced by NMR and ESR results. The dicopper(II) tetracar-
boxylate chromophore has exhibited its robustness under diverse
reaction conditions in a few newly reported cases as well [10,11].
Very recently, Stang and co-workers also reported that a class of
dinuclear Pt(II) coordination complexes decorated with amine or
maleimide groups can be modified via suitable postsynthesis with-
out disrupting prismatic cores of those compounds [12]. With
comparison to Cu(II) and Pt(II), some transition metals such as
Zn(II), are often more ‘‘labile’’ due to weaker affinities between
the transition metal and organic ligands [13,14]. Therefore, we
were interested in investigating PSM of Zn(II) coordination systems
not only for comparison purposes from our previous study and
others, but also for the fact that Zn(II) containing coordination
complexes include some important functional materials [15,16]
and bioactive compounds [17–19] in general. In this context,
Zn2(OOC–C6H4–N3)4(quinoline)2 (complex 1) was chosen as a
proof-of-principle compound to investigate the stability of coordi-
nation chromophores of Zn(II) coordination systems during PSM.
As a follow up to our previous work [6], current study reports
on the attempted two-step PSM of Zn2(OOC–C6H4–N3)4(quino-
line)2 (complex 1) (note: complex 1 is used as the code for the
starting material; 2 is the code for the intermediate compound
after 1st step; 3 is the code for the final product): 1st step – forma-
tion of an intermediate compound 2 with methyl propiolate via
Huisgen 1,3-dipolar cycloaddition (Click reaction); 2nd step – for-
mation of the final product 3 by recrystallization of 2 with
⇑
Corresponding authors. Present address: Ashland Inc., 1361 Alps Rd., Wayne, NJ
07470, USA. Fax: +1 973 628 3759.
E-mail addresses: mazhenbo@ustc.edu (Z. Ma), shuangbinghan@gmail.com (S.
Han).
1
Present address: Department of Chemical and Biological Engineering, Northwest-
ern University, 2145 Sheridan Road, Evanston, IL 60208, USA.
2
These authors contributed equally to this work.
Tel.: +1 401 699 2172.
3
0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.ica.2012.03.031

136
Z. Ma et al. / Inorganica Chimica Acta 388 (2012) 135–139
presence of quinoline. It is worth noting that several examples of
Click reactions to modify metal–organic coordination complexes
or organometallic species have been reported in recent years
[20–23]. Single-crystal XRD, 1D, 2D NMR, and Diffusion Ordered
Spectroscopy (DOSY) [24,25] NMR were used to characterize these
coordination complexes in this work. In current study, DOSY and
Hirshfeld Surface Analyses were particularly used to investigate
the stability of coordination chromophore during PSM.
flask. Sodium ascorbate (25 mg, prepared 1 M solution in water) was
added and copper (II) sulfate pentahydrate (0.1 mmol, 25 mg, pre-
pared in water) was added sequentially while stirring. The mixture
was stirred vigorouslyovernight. Pale yellowsolid product 2 was ob-
tained by filtration and washed with cool water and ether for several
times, and then was dried under vacuum for several hours. Yield:
259 mg, 93.0%. IR (DMSO, 2500–1400 cmÀ1
) 1659.7, 1437.3,
1314.6 cmÀ1 1H NMR (DMSO-d6, 400 MHz): d 9.59, 8.13, 8.07,
.
3.9 ppm. 13C NMR (DMSO-d6, 400 MHz): d 160.98, 140.11, 138.20,
131.45, 127.85, 120.46, 52.49 ppm.
2. Experimental
2.2.1. Recrystallization of 2
2.1. Materials and methods
Zn(OOC–C6H4–C2HN3–COOCH3)2Á(quinoline)ÁDMSO (3): 20 mg
pale yellow zinc complex 2 was added in 14 ml mixture solution
of methanol, quinoline and DMSO (volume ratio 9:3:2). The mix-
ture was heated up to 90 °C until the solid was dissolved. The solu-
tion was allowed to slowly cool down to room temperature. Single
crystal did not form immediately after the solution was cooled
down. Pale yellow single crystals 3 were formed gradually in about
one week. Crystal data: CCDC 832827, ZnC33O9N7SH29, M = 765.06,
Zinc nitrate hexahydrate, solvents, 3-ethynylpridine and methyl
propiolate were purchased from Sigma Aldrich Co. Sodium 4-ethy-
nylbenzote was purchased from VWR international Inc, and 4-azi-
dobenzoic acid was purchased from TCI America. Deuterated
solvents were purchased from Cambridge Isotope Laboratory. All
the materials were used as received, without further purification.
Single-crystal X-ray diffraction data were collected on a BRU-
ꢀ
triclinic, P1, a = 7.8535(16) Å, b = 12.429(3) Å, c = 17.564(4) Å,
KER SMART-APEX CCD diffractometer using Mo K
a radiation
a
= 98.68(3), b = 100.40(3),
c
= 91.27(3). Z = 2, V = 1664.8(6) Å3, R
(k = 0.71073 Å). The structure was solved by direct methods and
refined by full-matrix least-squares refinement with anisotropic
displacement parameters for all non-hydrogen atoms. The hydro-
gen atoms were generated geometrically and included in the
refinement with fixed position and thermal parameters. IR spectra
were recorded on an ATI Mattson Infinity series FTIR instrument.
NMR experiments were recorded on either a Bruker DRX-300 with
a z-gradient BBI probe or Bruker DPX-400 with a z-gradient BBO
probe operating at 300.13 MHz and 400.13 MHz for 1H observe
respectively. DOSY spectra were acquired using the Bruker pulse
program ledbpgp2s for 2, using sinusoidal gradients with durations
between 1.25 and 3.5 ms. The gradient strength was varied be-
tween 2% and 95% in 128 increments for 2. Diffusion times of 70
to 80 ms were used for 2. DOSY spectra were processed using the
Bruker Topspin software with exponential line fitting.
values: (I > 2 (I)) R1 = 0.0659, wR2 = 0.1141; GOF = 0.928. Yield:
12 mg, 43.7 % (based on Zn).
r
3. Results and discussion
The postsynthetic modification of complex 1 in this work in-
cludes 2 steps: (1) the formation of an intermediate (complex 2)
via Click reaction and (2) recrystallization of complex 2 to form
the final product (complex II) with presence of quinoline (anicllary
ligand). The two-step PSM process is schematically shown in Fig. 1.
3.1. NMR characterizations of complexes 1 and 2
The 1H spectra of methyl propiolate, complex 1, and complex 2
are displayed in Fig. 2. Methyl propiolate was successfully ‘clicked’
with all the 4-azidobenzoate moieties on complex 1 to form 1-(4-
carboxyphenyl)-4-methoxycarbonyl-1,2,3-triazole (L) as evi-
denced by the disappearance of the acetylenic proton at
4.57 ppm, the persistence of the ester methyl resonance at
3.9 ppm, and the emergence of the new triazole proton at
ꢀ9.7 ppm. The 1H spectrum of 2 indicates that quinoline ligands
disappeared after the reaction. The loss of quinoline may be partly
related to the existence of Cu in the reaction mixture. All proton
and carbon assignments in Fig. 2 were based on HSQC and HMBC
NMR experiments (see Supporting information).
Single crystal molecular graphics were generated using Accelrys
MS Modeling v4.0. Hirshfeld surface graphics were generated using
CRYSTALEXPLORER 2.1, (2007) University of Western Australia [34].
2.2. Synthesis of the complexes
Note: Standard personal protective equipments including safety
glasses, lab coat, and gloves must be worn. Handle azide reactions
behind a blast shield in the fume hood. Do not combine untreated
aqueous or organic azide waste with other waste.
Zn2(OOC–C6H4–N3)4(quinoline)2
(1):
4-azidobenzoic
acid
(1 mmol, 163 mg) and zinc nitrate hexahydrate (0.5 mmol,
297 mg) were dissolved in 10 ml methanol, then 1 ml quinoline
was added into the solution. The solution was transferred to a
20 ml glass vial which was loosely capped and allowed to stand
for about a week. Colorless prismatic crystals (complex 1) were ob-
Complex 2 in DMSO was investigated by using Diffusion Or-
dered Spectroscopy NMR experiments. DOSY methods are based
on pulsed-field gradient spin-echo NMR experiments. DOSY is an
effective tool to analyze intermediates and to discriminate differ-
ent species in solution [14,26–28]. The aligned triazole proton,
phenyl proton and ester methyl proton resonances shown in 1H
DOSY spectra of 2 (Fig. 3a) indicate all these nuclei are from a sin-
gle species in solution. These two sets of aligned triazole proton,
phenyl proton and ester methyl proton resonances shown in the
DOSY spectra of the mixture of free 1,4-triazole ligand and 2
(Fig. 3b) correspond to free and coordinated ligands, respectively.
tained at the bottom of the vial. Crystal data: CCDC 832826,
ꢀ
Zn2C46O8N14H30
b = 9.4324(19) Å, c = 15.273(3) Å,
= 69.63(3). Z = 1, V = 1110.0(4) Å3,
,
M = 1037.58, Triclinic, P1, a = 8.3381(17) Å,
a
= 80.33(3), b = 87.11(3),
values: (I > 2 (I))
c
R
r
R1 = 0.0950, wR2 = 0.2665; GOF = 0.902. Yield: 164 mg, 63.1%. IR
(DMSO, 2500–1400 cmÀ1): 2122.72, 1700.53, 1618.15, 1601.68,
1568.50, 1501.35, 1460.39 cmÀ1
.
1H NMR (DMSO-d6, 300 MHz): d
These aligned peaks exhibiting a faster diffusion coefficient
8.908 (d, J = 3.12 Hz), 8.37(d, J = 8.16 Hz), 8.04(s), 8.005 (d,
J = 3.66 Hz), 7.96 (d, J = 8.04 Hz), 7.77 (t, J = 7.5 Hz), 7.61(t,
J = 7.5 Hz), 7.54 (q, J = 4.1 Hz), 7.14 (d, J = 8.16 Hz).
(LogD ꢁ À8.15 m2/s) correspond to free ligand (L) and the aligned
peaks exhibiting a slower diffusion coefficient (LogD ꢁ À8.72 m2/
s) correspond to the coordinated ligand (L). The comparison be-
tween DOSY spectra in Fig. 3a and b clearly indicates that the inter-
mediate compound-2 exists in the coordinated form in solution.
Although the exact molecular structure of 2 cannot be derived
Zn(OOC–C6H4–C2HN3–COOCH3)2 (2): Complex
260 mg) and 90 l methyl propiolate (1 mmol) were suspended in
10 ml mixture solution of water and t-butanol (v:v = 1:1) in a round
1 (0.25 mmol,
l

Z. Ma et al. / Inorganica Chimica Acta 388 (2012) 135–139
137
Fig. 1. Schematic illustration of the two-step PSM process.
Fig. 2. 1H spectra of methyl propiolate (top), complex 1 (middle), and complex 2 (bottom).
from our NMR data, it is apparent that coordination chromophore
of complex 1 was disrupted with the loss of quinoline ligand over
Click reaction.
1 crystallises on an inversion center. The four azide groups at
peripheral positions of complex 1 provide reactive sites to further
decorate this coordination compound. The final product – complex
3 after Click reaction and recrystallization processes has a mono-
nuclear crystal structure with Zn(II) surrounded by two carboxyl-
ate groups, one quinoline and one DMSO. The coordination
geometry of 3 can be described as a distorted tetrahedron as well
(Fig. 5). To further identify and understand the coordination chro-
mophore around Zn(II) in the crystal, we have performed Hirshfeld
surface analysis by selecting Zn(II) and its adjacent atoms in crystal
structures for both the starting material (complex 1) and the final
product (complex 3). The Hirshfeld surface [29] defines the volume
of space where the promolecule electron density exceeds that from
all neighboring molecules, and Hirshfeld surface analysis has
3.2. Crystal structures and Hirshfeld surface analyses
The coordination geometry of zinc(II) in complex 1 is a distorted
tetrahedron. Bond length results (Å) around Zn(II) in complex 1 are
as follows: (two closely coordinated Zn–O bonds: Zn1–O1
1.979(6); Zn1–O3 1.938(7)); Zn–O (the carbonyl in the neighboring
structure unit) bond: Zn1–O4 2.023(6); (Zn-quinoline bond) Zn1–
N1 2.030(7). As shown in Fig. 4, complex 1 is a weakly bound dimer
in which the carbonyl from an adjacent structure unit binds to the
zinc (Zn1–O4 bond length 2.023(6) Å). The dimeric unit of complex

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Z. Ma et al. / Inorganica Chimica Acta 388 (2012) 135–139
Fig. 5. The single crystal structure of complex 3 showing a mononuclear structure
unit.
Fig. 3. (a) 1H DOSY spectrum of complex 2 and (b) 1H DOSY spectrum of mixture of
complex 2 and free L–H.
Fig. 6. (a) The Hirshfeld Surface of the coordination chromophore around Zn(II) in
complex 1; (b) The Hirshfeld Surface of the coordination chromophore around Zn(II)
in complex 3.
surface analysis can also provide insightful supporting information
to evaluate the stability of coordination chromophores in this case.
Hirshfeld surface of the coordination chromophore of complex 1
shows two red circles (highlighted in Fig. 6a) which correspond
to close contacts between a Zn(II) center and the carbonyl in the
neighboring structure unit, based on which the dimer of complex
1 is constructed. The relatively faint red color of these two circles
in Fig. 6a implies the weak interaction between neighboring mono-
nuclear Zn(II) structural units. This is also consistent with the rel-
atively long bond length between Zn1 and O4. The Hirshfeld
surface analysis of the coordination chromophore of complex 3
shows no other close contacts around Zn(II) except atoms within
the mononuclear four-coordinated coordination chromophore,
Fig. 6b. Therefore, it is unsurprising that the original coordination
chromophore of complex 1 was not retained throughout the PSM
process. The coordination environment around Zn(II) transformed
from weakly bound dimeric chromophore in complex 1 to a tightly
bound mononuclear chromophore in complex 3.
Fig. 4. The single crystal structure of complex 1 showing a dimeric structure unit.
proved to be a powerful approach to study intermolecular interac-
tions by visualization [30–32]. It is interesting that Hirshfeld

Z. Ma et al. / Inorganica Chimica Acta 388 (2012) 135–139
139
Based on the single crystal structure analysis, the coordination
Appendix A. Supplementary material
chromophore of complex 1 involves anionic carboxylic ligands
and neutral quinoline ligands around Zn(II) centers. It was deter-
mined by NMR spectra that the dinculear Zn(II) compound- com-
plex 1 was not able to retain its coordination chromophore over
Click reaction with the loss of quinoline ligand. And recrystalliza-
tion of the intermediate compound 2 with presence of quinoline
formed a mononuclear Zn(II) compound. Quinoline was used for
recrystallization as an attempt to restore the original coordination
chromophore around Zn(II) in complex 1, because quinoline is an
ancilliary ligand existing in the structure of complex 1. Single crys-
tal structure analysis of the recrystallized product 3 reveals that
the final product possesses a different coordination chromophore
from complex 1. The Hirshfeld surface analysis of complex 1 indi-
cates the weak interaction between a Zn(II) center and the car-
bonyl in the neighboring structure unit, which partly explains
the disruption of coordination chromophore during the PSM
process.
CCDC 832826 and 832827 contain the supplementary crystallo-
graphic data for complexes 1 and 3, respectively. These data can be
obtained free of charge from The Cambridge Crystallographic Data
Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary
data associated with this article can be found, in the online version,
at http://dx.doi.org/10.1016/j.ica.2012.03.031.
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In summary, methyl propiolate was successfully linked to azide
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1,4-regioisomer was formed. With comparisons to Cu2(OOC–
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