Postsynthetic modification of a coordination compound with a paddlewheel motif via click reaction: DOSY and ESR studies

Article abstract of DOI:10.1016/j.inoche.2011.09.043

Postsynthetic strategy based on organic coupling reactions has been explored for the synthesis of novel coordination complexes with larger dimensions. A discrete coordination species with a paddlewheel motif, dicopper(II) tetracarboxylate Cu₂(OOC-C₆H ₄-N₃)₄(quinoline)₂ (SBU1), was covalently modified with methyl propiolate via "Click" reaction to generate a new coordination compound Cu₂(OOC-C₆H ₄-C₂N₃H-COOCH₃)₄? (quinoline)₂ (1). The combination of single-crystal X-ray diffraction, diffusion NMR and ESR has confirmed the success of covalently modifying SBU1 and the retention of the structural integrity after the postsynthetic modification.

Full text of DOI:10.1016/j.inoche.2011.09.043

Inorganic Chemistry Communications 15 (2012) 78–83
Contents lists available at SciVerse ScienceDirect
Inorganic Chemistry Communications
journal homepage: www.elsevier.com/locate/inoche
Postsynthetic modification of a coordination compound with a paddlewheel motif via
click reaction: DOSY and ESR studies
Shuangbing Han a, ,1, Zhenbo Ma a, ,1, Russell Hopson a, Yanhu Wei a, David Budil b,
⁎
⁎⁎
Stefano Gulla b, Brian Moulton a
a
Department of Chemistry, Brown University, 324 Brook St, Providence, RI 02912 USA
Department of Chemistry and Chemical Biology, Northeastern University, 360 Huntington Ave, Boston, MA 02115 USA
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Postsynthetic strategy based on organic coupling reactions has been explored for the synthesis of novel coor-
dination complexes with larger dimensions. A discrete coordination species with a paddlewheel motif, dicop-
per(II) tetracarboxylate Cu2(OOC-C6H4-N3)4(quinoline)2 (SBU1), was covalently modified with methyl
Received 16 August 2011
Accepted 28 September 2011
Available online 4 October 2011
propiolate via “Click” reaction to generate
a new coordination compound Cu2(OOC-C6H4-C2N3H-
COOCH3)4·(quinoline)2 (1). The combination of single-crystal X-ray diffraction, diffusion NMR and ESR has
confirmed the success of covalently modifying SBU1 and the retention of the structural integrity after the
postsynthetic modification.
Keywords:
Postsynthetic
DOSY
© 2011 Elsevier B.V. All rights reserved.
ESR
Crystal
Click chemistry
Paddlewheel
For decades, coordination complexes have received great attention
for their utilitarian properties and interesting structures [1–9]. A variety
of synthetic techniques, such as diffusion [10,11], hydro/solvo-thermal
[12–14], refluxing [15,16], mechanochemistry [17,18] and direct mixing
[19], have been developed to synthesize coordination complexes utiliz-
ing transition metal salts and organic ligands. Ligand-exchange reac-
tions at the metal site have also been explored as an advanced route
to construct supramolecular coordination species from relatively small
coordination subunits [20–22]. Furthermore, postsynthetic modifica-
tion (PSM) is an alternative approach to synthesize new coordination
materials from existing coordination species while generally maintain-
ing some structure features of starting materials such as framework to-
pology and coordination chromophore. In the past few years, PSM of
metal-organic frameworks (MOFs) has proved to be a powerful ap-
proach for functionalizing MOFs to enhance their chemical and physical
properties [23–25]. Several interesting examples that concerned post-
synthetic reactions of metal-organic polyhedra (MOPs) have been
reported very recently as well [26,27]. PSM of discrete coordination
complexes has also become a topical area in supramolecular chemistry
in recent years [28–30].
PSMs of coordination complexes are usually conducted in a het-
erogeneous fashion due to the poor solubility of coordination com-
plexes in conventional organic solvents [25,31]. In practice, the
exploration of PSM on discrete coordination complexes often raises
a number of issues: (1) the stability of coordination chromophore (in-
cluding the coordination geometry of the metal site and the connec-
tion of ancillary ligands, if any) during the reaction; and (2) the
structure determination of the product when it is not possible to ob-
tain a single crystal of suitable size for data collection. In order to ad-
dress such issues, we have demonstrated herein a systematic study of
the covalent postsynthetic modification of a discrete Cu(II) coordina-
tion species - Cu2(OOCC6H4N3)4·(quinoline)2 (SBU1) [note: SBU1 is
only used as the code for the starting material]. SBU1 has a dicopper
tetracarboxylate chromophore (also known as the paddlewheel
motif) with 4 azide groups at the peripheral positions and 2 quinoline
molecules at the axial position. We have covalently modified SBU1
with methyl propilate to form a new coordination compound 1 via
Huisgen 1, 3-dipolar cycloaddition (also known as Click reaction).
Single crystal XRD, (1H, 13C, HSQC, HMBC and 1D DOSY) NMR, and
ESR were used to characterize these coordination species [32].
Postsynthetic modification of SBU1 via Click reaction. Cu2(OOC-
C6H4-N3)4(quinoline)2 (SBU1) [33] has
a dinuclear copper(II)
paddlewheel structure with 4 azide groups at the peripheral posi-
tions and 2 quinoline molecules at the axial positions (Fig. 1a, b).
Two uncoordinated acetonitrile molecules exist in the crystal
structure of SBU1·2CH3CN as guest molecules. These 4 azide groups
at peripheral positions of SBU1 provide potential for further modi-
fying this coordination species. Coupling small organic molecules
⁎
Corresponding author. Tel.: +1 401 699 2172.
⁎⁎ Correspondence to: Z. Ma, ISP, 1361 Alps Rd, Wayne, NJ 07470, USA. Tel.: +1 401
699 2172; fax: +1 973 628 3759.
E-mail addresses: shuangbinghan@gmail.com (S. Han), mazhenbo@ustc.edu
(Z. Ma).
1
These two authors equally contributed to this work.
1387-7003/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.inoche.2011.09.043

S. Han et al. / Inorganic Chemistry Communications 15 (2012) 78–83
79
with discrete coordination complexes provides a simple way to
modify these starting materials in terms of functionality and
dimension.
In the current study, methyl propiolate was covalently linked to
SBU1 via Click reaction. Click reaction was chosen because it works at
mild reaction conditions (e.g. ambient temperature) and in a variety of
solvents [34,35]. It is worth noting that Click reaction has been used
to modify metal-organic coordination complexes or organometallic spe-
cies by a few research groups [28,36–38]. More interestingly, the forma-
tion of 1,4-regioisomer [35,39] offers a ca. 144° angle, which is critical for
generating some specific geometries, such as rhombicosidodecahedron.
Green solid 1 was synthesized by coupling 4 equivalent methyl pro-
piolate with 1 equivalent SBU1 in THF without any catalysts [40], be-
cause certain catalyst, such as sodium ascorbate, might reduce the
copper(II) in SBU1, thereby destroying the integrity of the paddlewheel
structure. The completion of the coupling was monitored by IR spec-
troscopy. Disappearance of the azide peak at ca. 2122 cm−1 indicated
the completion of the reaction.
If the coordination chromophore of SBU1 remains intact during
the Click reaction of the 4-azido-benzoate moieties with methyl
propiolate, we should obtain product 1 depicted in Fig. 2. Various
analytical techniques, including 1H, 13C, DOSY NMR, and ESR,
were used to determine the structure.
NMR characterizations of SBU1 and 1. Based on the 1H, COSY and
HSQC NMR spectra of SBU1 and 1H spectra of free quinoline in DMSO-
d6 (see Supporting Information, SI, Fig. S1-3), the 5 broadened reso-
nances between 7 and 10 ppm observed in the 1H spectrum of SBU1
are assigned to quinoline (Fig. 3, middle). The 1H NMR spectrum of
1 (Fig. 3, bottom) in DMSO-d6 confirmed that methyl propiolate
was successfully Clicked on 4-azidobenzoate in SBU1 to form 1-(4-
carboxyphenyl)-4-methoxycarbonyl-1,2,3-triazole (the 1,4-triazole
ligand is simplified as LH in this paper). Comparisons of the 1H spec-
tra of methyl propiolate (Fig. 3, top), SBU1 and 1 clearly illustrate the
disappearance of the alkyne proton at 4.57 ppm in 1, a downfield shift
Fig. 1. (a) Molecular structure of SBU1 derived from the single-crystal structure;
(b) SBU1 viewed along axial direction.
Fig. 2. Top: 1-(4-carboxyphenyl)-4-methoxycarbonyl-1,2,3-triazole (right) and 1-(4-carboxyphenyl)-5-methoxycarbonyl-1,2,3-triazole (left); Bottom: the proposed structure of 1.
* Theoretically both 1,4-regioisomer LH and 1,5-regioisomer L'H should be present in 1. Only 1,4-regioisomer LH was drawn in the proposed structure for simplicity and the later on
it was proved to be the real structure by experiments.

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S. Han et al. / Inorganic Chemistry Communications 15 (2012) 78–83
Fig. 3. 1H spectra of methyl propiolate (top), SBU1 (middle), and 1 (bottom) in DMSO-d6.
of the ester methyl protons from 3.72 ppm to 3.79 ppm, and the
emergence of a new sharper peak at 9.3 ppm which lies in the
expected range of triazole protons [35,41]. The integral ratio of the
triazole proton peak to the ester methyl peak is 1:3, consistent with
the triazole structure. The integral ratio of the new peak to the best
resolved quinoline proton is 2: 1 which indicates that all 4 azide
groups on SBU1 reacted with methyl propiolate. Noteworthy in the
1H spectra of SBU1 and 1 is the absence of any resonances for the phe-
nyl protons of the benzoate moiety. This should be attributed to the
significant line broadening experienced by these protons due to fast
relaxation caused by interactions of these nuclei with the unpaired
electrons of paramagnetic Cu (II) center [42-45]. Resonances for the
triazole/methyl ester groups could be observed, as the impact of Cu
(II) diminishes with the increase of the distance from the paramag-
netic center. Unexpectedly, the quinoline ligands which are closer to
the paramagnetic Cu (II) center displayed sharper resonances than
those of the benzoate ligands in 1. Diffusion Ordered Spectroscopy
(DOSY) [6,46–49] measurement of 1 indicated that the quinoline li-
gands exchanged with the solvent molecules, dimethyl sulfoxide
(DMSO), because they diffused faster than the triazole resonances in
the DOSY spectrum (Fig. 4a). NMR experiments conducted on free
1,4-triazole ligands synthesized by using Cu(I) catalyst, the triazole li-
gands extracted from 1, and the mixture of 1 and 1,4-triazole ligands
confirmed that the triazole moieties formed in 1 are 1,4 regioisomer
(SI, Fig. S4–7).
this case, quinoline switches between ON and OFF state on molecule 1
and behaves more like a “free” ligand in DMSO rather than a coordinat-
ed one. Free 1,4-triazole ligand was added to the NMR sample of 1 in an
effort to see both free and coordinated triazole ligands. 1H DOSY spec-
trum (see Fig. 4b) of the mixture of free 1,4-triazole ligand and 1
shows two sets of aligned triazole ligand peaks indicating free and coor-
dinated triazole ligands. These aligned peaks exhibiting a faster diffu-
sion coefficient (LogD≈−8.32 m2/s) correspond to free 1,4-triazole
ligands, and the aligned peaks exhibiting a slower diffusion coef-
ficient (LogD≈−8.45 m2/s) correspond to coordinated 1,4-triazole
ligands. The data highlight the excellent utility of diffusion ordered
NMR spectroscopy for determining solution structures of coordination
compounds.
ESR characterizations of SBU1 and 1. The experimental ESR spec-
trum of crystalline SBU1, as shown in Fig. 5a (blue), exhibits feature
characteristic of dinuclear Cu(II) species with three peaks at 400 G
(Hz1
)
(g 1 =16.69), 4700 G(H⊥2)(g⊥ =1.41), and 6000 G(Hz2)
(g 2 =1.10) due to the spin triplet state of the binuclear copper com-
plex [50-54]. The spectrum can be simulated with reasonable agree-
ment assuming large negative exchange energy (J).
In this case, the spin state energy levels may be described as a sin-
glet ground state (S=0) and an excited triplet state (S=1) separated
by an energy difference of 2 J [55,56]. For large J, the ESR spectrum of
the triplet state can be described using an effective total spin S=1
state. Fig. 5 shows the calculated spectrum (red spectrum in Fig. 5a)
and the energy level diagram (Fig. 5b) for the case of large zero field
spitting (−D|Nhν). The parameters used to simulate the spectra are
given in Table 1[57]. A small peak at 3300 G can be attributed to
mononuclear Cu (II). The seven hyperfine lines evident in the z
peaks at 400 G and 6000 G clearly show the interaction between two
Cu(II) (I=3/2) nuclei in the paddlewheel structure. The spectrum is
In the 1H DOSY spectrum of 1 in DMSO-d6 (Fig. 4a), the triazole pro-
ton Ha (9.3 ppm) and the ester methyl proton Hb (3.79 ppm) exhibit the
same diffusion coefficient (LogD≈−8.45 m2/s), which indicates that
they are from the same species 1. All the signals corresponding to quin-
oline protons were aligned and exhibited a faster diffusion coefficient
(Log D≈−8.25 m2/s) due to exchange with the solvent, DMSO. In

S. Han et al. / Inorganic Chemistry Communications 15 (2012) 78–83
81
Fig. 4. (a) 1H DOSY spectrum of 1 in DMSO-d6 and (b) 1H DOSY spectrum of mixture of 1 and free 1,4-regioisomer LH in DMSO-d6. (X axis: ppm and Y axis: LogD).
consistent with two copper ions occupying identical sites in an axially
symmetric crystal field (E≈0).
of the mononuclear and dinuclear peaks is unaffected by Click reac-
tion; however, it is temperature dependent, with the dinuclear Cu
(II) exhibiting relatively less intensity at lower temperatures. This is
consistent with a large negative J value for the dinuclear Cu (II), so
that the triplet state becomes depopulated when the temperature is
reduced below 200 K. Such temperature dependence has previously
been observed in similar dinuclear copper complexes [58].
Fig. 6 compares the X-band ESR spectra of SBU1 and 1 in frozen
DMSO at two different temperatures. In addition to the peaks from
the dinuclear Cu(II) noted above, the spectra exhibit a somewhat
more prominent mononuclear Cu(II) peak at 3300 Gauss. The peaks
in the dinuclear Cu spectra occur at nearly identical magnetic fields,
indicating retention of the dinuclear paddlewheel chromophore of
SBU1 after Click reaction employed to form 1. The relative intensity
Because of this inverse temperature dependence, it is difficult to
quantify the relative amounts of mononuclear and dinuclear Cu from

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S. Han et al. / Inorganic Chemistry Communications 15 (2012) 78–83
Fig. 6. X-band ESR spectra of SBU1 and 1 in frozen DMSO at (a) 160 K and (b) 200 K.
(red: SBU1, blue: 1) Inset shows simulated ESR spectrum of mononuclear Cu(II) plus
dinuclear Cu(II) in a 1:10 ratio.
respectively. SBU1 was proved stable enough to survive the reaction
conditions of Click reaction. Not only was the dicopper(II) tetracarbox-
ylate coordination geometry retained throughout the Click reaction, the
ancillary ligand-quinoline also remained in place in the formula of the
product, which may be related to the weak coordination capacity of
the used solvent-THF. Although the displacement of axial ligands can
occur under certain conditions, there was no evidence of any negative
effect on the integrity of SBU1 structure.
In conclusion, we have successfully carried out the postsynthetic
modification of a dicopper(II) coordination compound with a paddle-
wheel motif and performed structural characterizations using a vari-
ety of characterization techniques. The postsynthetic modification of
coordination compound-SBU1 with a paddle wheel structure has
been successfully achieved via Click reaction. Meanwhile, the dicop-
per tetracarboxylate coordination chromophore of SBU1 remains in-
tact after Click reaction of the 4-azido-benzoate moieties with
methyl propiolate. In addition, the combination of NMR and ESR tech-
niques has proved to be powerful tools to characterize structures of
non-single-crystalline coordination complexes, particularly in the so-
lution phase. Currently, research is being focused on the synthesis of
coordination polymers from discrete coordination complexes via
postsynthetic modifications.
Fig. 5. (a) Crystal X-band spectrum of SBU1 recorded at 160 K (blue) and calculated spec-
trum (red); (b) energy level diagram showing Δms=1 transitions (red) and Δms=2
transitions (gray) for the two canonical orientations B0||x,y (top) B0||z (bottom).
double integration of the spectrum. An upper limit on the amount of
mononuclear present may be estimated by simulating the spectrum as
a superposition of mononuclear and dinuclear spectra that have been
normalized to a doubly integrated intensity of 1. The inset of Fig. 6
shows a simulation of the spectrum assuming mononuclear to dinuclear
ratio of 1:10. This simulation exhibits relative intensities of the mono-
nuclear peak at 3300 G and dinuclear peak at 6000 G that are compara-
ble to those of the 200 K experimental spectrum. Based on the
previously observed temperature dependence of a similar dinuclear
Cu complex [58], the dinuclear intensity at 200 K is still below its max-
imum value, which indicates an upper limit of about 10% on the relative
amount of monomer present for both SBU1 and 1 in DMSO. The results
from the comparisons in Fig. 5 suggest that SBU1 maintains its structur-
al integrity after Click reaction and the structures of SBU1 and 1 in
DMSO are mainly dinuclear copper paddlewheel with trace amount of
mononuclear impurity.
Caution: Standard personal protective equipments including safe-
ty glasses, lab coat, and gloves must be worn when conducting exper-
iments reported in this paper. Handle azide reactions behind a blast
shield in the fume hood. Do not combine untreated organic azide
waste with other waste.
NMR and ESR analyses have confirmed the proposed structure 1 as
depicted in Fig. 2. Click reaction was successfully employed to couple
the 4 azide groups of SBU1 with 4 methyl propiolate molecules
Table 1
ESR parameters of calculated spectrum.
Appendix A. Supplementary material
T(K)
160
g−−
2.06
g⊥
A−− (cm1)
77
A⊥ (cm1)
15
D (cm−1
3670
)
Electronic Supplementary Information (ESI) available: [Synthesis of
the free ligand LH and Figures S1-S7]. CCDC (Cambridge Crystallographic
2.45

S. Han et al. / Inorganic Chemistry Communications 15 (2012) 78–83
83
spectrometer equipped with a Bruker high sensitivity cylindrical resonator. Variable
temperature studies were performed using a Bruker variable temperature unit (ER
4111VT) with liquid nitrogen reservoir and quartz dewar insert. ESR spectra and ener-
gy level diagrams were simulated using the EasySpin19 set of MATLAB program.
[33] Synthesis of Cu2(OOC-C6H4-N3)4(quinoline)2 (SBU1): 4-azidobenzoic acid (10 mmol,
1.631 g) was suspended in 40 ml mixture solution of water and ethanol (volume ratio
1:1). Copper carbonate basic (2.5 mmol, 552.7 mg) was added into the suspension
portion-wise. The suspension was stirred for several hours until a green precipitate
formed. The green powder was obtained by filtration, and then the powder was dis-
solved in about 40 mL acetonitrile in presence of 5 mL quinoline and then divided
into 10 small vials. Green block crystals [Cu2(OOC-C6H4-N3)4(quinoline)2]·2CH3CN
formed in these vials in about 3–4 days with ~80% yield. Crystallographic data for
SBU1·2CH3CN: C50H36Cu2N16O8. CCDC reference number 828956. M=1116.05, TRI-
GONAL, space group: P-1, a=10.852(8) Å, b=9.292(6) Å, c=13.409(9) Å, alpha=
77.048(13), beta=80.273(11), gamma=69.079(11), Z=1, V=1224.9(15) Å [3]. R
values: (IN2σ(I)) R1=0.1368, wR2=0.3620; GOF=1.085. The green crystals,
SBU1·2CH3CN, were collected, dried and then re-dissolved into chloroform. SBU1
was obtained by rotovapping the chloroform solution to remove uncoordinated aceto-
Data Centre) reference number for SBU1·2CH3CN: 828956. Copy of the
data can be obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB2 1EZ (Telefax: +44 1223 336408; e-mail:
deposit@ccdc.cam.ac.uk).
Supplementary data to this article can be found online at doi:10.
1016/j.inoche.2011.09.043.
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[40] Synthesis of Cu2(OOC-C6H4-C2HN3-COOCH3)4·(quinoline)2 (1): 0.25 mmol of SBU1
was added into
a round flask. Then 4 mL THF and 100 μl methyl propiolate
(1.12 mmol) were added to the flask. Reaction mixtures were stirred at room temper-
ature for 9 days. Green solid product 1 was obtained by filtration (washed with
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1738.89 cm−1
1461.52 cm−1
,
.
1636.35 cm−1
,
1609.25 cm−1(sh), 1581.26 cm−1
,
1545.79,
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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 programs, dstebpgp3s for 1 using sinusoidal gradients with durations between
1.25 and 3.5 ms. The gradient strength was varied between 2% and 95% in 16–32 square
spaced increments for 1. Diffusion times of 14 to 20 ms were used for 1. DOSY spectra
were processed using the Bruker Topspin software with exponential line fitting. X-
band ESR Spectra were recorded at X-band (9.5 GHz) using a Bruker EMX ESR