Metal-Diamidobenzoquinone Frameworks via Post-Synthetic Linker Exchange

Article abstract of DOI:10.1021/jacs.9b11952

Metal-organic frameworks with amidic linkers often exhibit exceptional physical properties, but, owing to their strong metal-nitrogen bonds, are exceedingly challenging to isolate through direct synthesis. Here, we report a route to access metal-diamidobenzoquinone frameworks from their dihydroxobenzoquinone counterparts via postsynthetic linker exchange. The parent compounds (Me₂NH₂)₂[M₂L₃] (M = Zn, Mn; H₂L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) undergo linker exchange upon exposure to a solution of monodeprotonated 2,5-diamino-3,6-dibromo-1,4-benzoquinone or 2,5-diamino-3,6-dichloro-1,4-benzoquinone, proceeding through single-crystal-to-single-crystal reactions. The presence of both types of linker in the resulting frameworks is confirmed by a combination of NMR, Raman, and energy-dispersive X-ray (EDX) spectroscopies. Moreover, the extent of linker exchange in the Zn frameworks is quantified using ¹³C NMR spectroscopy, and spatially resolved EDX spectroscopy reveals the two types of linker to be homogeneously distributed within a crystal. Finally, we propose a tentative mechanism of linker exchange based on pK_a measurements, considerations of framework solubility, and powder X-ray diffraction analysis. This work provides the first method to exchange organic linkers with different donor atoms in metal-organic frameworks and in doing so demonstrates exchange between linkers with donor atoms differing in acidity by a remarkable 11 units of pK_a. Together, these results offer a potentially general synthetic strategy toward new materials with exotic metal-linker coordination modes.

Full text of DOI:10.1021/jacs.9b11952

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Article
Metal-Diamidobenzoquinone Frameworks via Post-Synthetic Linker Exchange
Lujia Liu, Liang Li, Michael E. Ziebel, and David Harris
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.9b11952 • Publication Date (Web): 04 Feb 2020
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Metal-Diamidobenzoquinone Frameworks via Post-Synthetic Linker Ex-
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Lujia Liu,†,§,║ Liang Li,†,§ Michael E. Ziebel‡ and T. David Harris*,†,‡
†Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 60208, United States
‡Department of Chemistry, University of California, Berkeley, California 94720, United States
ABSTRACT: Metal-organic frameworks with amidic linkers often exhibit exceptional physical properties, but, owing to their strong metal-
nitrogen bonds, are exceedingly challenging to isolate through direct synthesis. Here, we report a route to access metal-diamidobenzoquinone
frameworks from their dihydroxobenzoquinone counterparts via post-synthetic linker exchange. The parent compounds (Me2NH2)2[M2L3]
(M = Zn, Mn; H2L = 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) undergo linker exchange upon exposure to a solution of mono-deproto-
nated 2,5-diamino-3,6-dibromo-1,4-benzoquinone or 2,5-diamino-3,6-dichloro-1,4-benzoquinone, proceeding through crystal-to-single-crys-
tal reactions. The presence of both types of linker in the resulting frameworks is confirmed by a combination of NMR, Raman, and energy-
13
dispersive X-ray (EDX) spectroscopies. Moreover, the extent of linker exchange in the Zn frameworks is quantified using C NMR spectros-
copy, and spatially-resolved EDX spectroscopy reveals the two types of linker to be homogenously distributed within a crystal. Finally, we pro-
pose a tentative mechanism of linker exchange based on pKa measurements, considerations of framework solubility, and powder X-ray diffrac-
tion analysis. This work provides the first method to exchange organic linkers with different donor atoms in metal-organic frameworks, and in
doing so demonstrates exchange between linkers with donor atoms differing in acidity by a remarkable 11 units of pKa. Together, these results
offer a potentially general synthetic strategy toward new materials with exotic metal-linker coordination modes.
been almost exclusively limited to installing linkers with the same do-
nors as the original linker, and has only been observed for carboxylate-
or N-heterocycle-based linkers.13–27 Indeed, exchange reactions involv-
ing linkers with different coordinating atoms or across several pKa
units remain unreported.
INTRODUCTION
Metal-organic frameworks (MOFs) with amidic linkers have been
shown to exhibit rich redox chemistry and high, modular electrical con-
ductivity.1 – 5 These remarkable properties enable application of the
compounds as thermoelectrics,6 chemical sensors,7 electrocatalysts,8
field-effect transistors,3 and energy storage materials.5,9,10 Nevertheless,
despite this tremendous importance and versatility, the synthesis of
crystalline frameworks with amidic linkers represents a daunting syn-
thetic challenge. Specifically, amino protons are much less acidic than
hydroxo protons,11 and consequently, upon deprotonation, their con-
jugate bases tend to form strong and relatively covalent metal-amide
bonds. Since reversible coordination is critical to the formation of crys-
talline framework materials, very little progress has been made toward
synthesizing compounds with amidic linkers. Indeed, among the small
number of amidic frameworks reported to date, including a recent re-
port of microcrystals of Ni3(hexaiminotriphenylene)2,12 none of them
have been characterized by single-crystal X-ray diffraction.1-5
Inspired by the above examples, we sought to employ post-synthetic
linker exchange as a strategy to install amidic linkers into frameworks.
To illustrate this approach, we chose parent materials based on 2,5-di-
hydroxo-1,4-benzoquinone linkers, which can be readily produced as
–
large single crystals.32 46 Upon standing in hot DMF over the course of
days, these compounds partially dissolve. In principle, this slow disso-
lution process should lend itself to substitution by an exogenous linker
that forms stronger metal-ligand bonds. Specifically, such a process
should provide a route to introduce amidic benzoquinoid linkers into
the solid-state structure.
Herein, we report just such a series of linker exchange reactions. The
parent framework compounds (Me2NH2)2[M2L3] (M = Zn, Mn; H2L
= 2,5-dichloro-3,6-dihydroxo-1,4-benzoquinone) undergo linker ex-
change upon exposure to a solution of deprotonated 2,5-diamino-3,6-
dibromo-1,4-benzoquinone (H2 L′) or 2,5-diamino-3,6-dichloro-1,4-
benzoquinone (H2 L′). These processes occur via single-crystal-to-
As a strategy to surmount this synthetic challenge, one can envision
employing post-synthetic linker exchange, which has emerged as a
powerful tool to synthesize frameworks that are otherwise inaccessible
Br
Cl
–
through direct synthesis. 13 Historically, through linker exchange,
19
single-crystal reactions to afford four structurally-characterized metal-
diamidobenzoquinone framework compounds. The identities of the
linkers in these frameworks were confirmed by NMR, Raman, and en-
ergy-dispersive X-ray (EDX) spectroscopies. To quantify the linker ex-
functionalities that are not tolerant to solvothermal conditions have
–
been successfully incorporated onto framework materials.20 23 For ex-
ample, a myriad of catalytic units embedded on organic linkers have
been introduced post-synthetically to imbue a range of catalytic func-
13
–
tion.24 27 In addition, linker exchange has recently been utilized to con-
change in the Zn frameworks, we developed a facile quantitative C
NMR spectroscopic method using standard pulse sequence and exter-
nal calibration curves. This work provides a method to exchange or-
ganic linkers with different donor atoms and acidities differing by up to
struct heterostructures with multiple domains distributed macroscop-
–
ically within each crystal.28 31 Despite its extraordinary ability to gener-
ate new materials and superstructures, linker exchange has thus far
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Figure 1. (a) Structures and linker exchange schemes from 1 and 4 to give 2, 3, 5, and 6. Structures are viewed along the crystallographic c axis. Cyan and gold
octahedra represent Zn and Mn atoms, respectively; green, brown, red, gray, and white spheres represent Cl, Br, O, C, and H atoms, respectively; (Me2NH2)+ and
(Et4N)+ are omitted for clarity. To aid visualization, L2−, (BrL′)2−, and (ClL′)2− are colored in orange, purple, and green, respectively. Linkers are disordered; only one
representative distribution is shown. (b, c) Optical microscopy images for 1 (b) and 3 (c), highlighting the color change associated with conversion of 1 to 2 and 3.
11 pKa in MOFs, which may lead to new materials with exotic metal-
In an initial attempt to carry out linker exchange, crystals of 1 or 4
Br Cl
linker coordination bonds and unusual physical properties.36,40,44,46–51
were soaked in a DMF solution of H2 L′ or H2 L′. However, after
prolonged reaction time at room temperature or elevated tempera-
ture, no diaminobenzoquinone linkers were incorporated. Instead,
significant dissolution of the crystals was observed. Since values of
RESULTS AND DISCUSSION
The
compounds
(Me2NH2)2[Zn2L3]
(1)
and
Br
Cl
pKa1 for H2L, H2 L′ and H2 L′ were determined to be 4.7, 16.1 and
12.3, respectively, in DMF (see discussion of exchange mechanism
below), we surmise that dimethylamine, generated by DMF hydrol-
ysis, is not sufficiently basic to deprotonate the diaminobenzoqui-
none ligands and that ligand exchange through protonolysis is simi-
larly unfavorable. We therefore prepared DMF solutions of mono-
(Me2NH2)2[Mn2L3] (4) were selected as parent materials from
which to carry out post-synthetic linker exchange. Compound 1was
prepared according to a previously reported procedure,36 whereas 4
was synthesized as brown, hexagonal crystals through the reaction of
Mn(NO3)2 with H2L in DMF at 130 °C for 16 h. Single-crystal X-ray
diffraction analysis indicated that 4 is isostructural to 1 (see Figure
1a), featuring eclipsed, stacked layers of [Mn2L3]2− honeycomb net-
works that are charge balanced with interstitial (Me2NH2)+ counter-
ions located in the hexagonal channels (see Figure S1).
Br
Cl
deprotonated H2 L′ or H2 L′ by addition of one equivalent of
Br
−
(Et4N)OH to give mono-deprotonated linkers (H L′) and
Cl
−
(H L′) . Soaking MOF crystals in these solutions at 75 °C for three
days afforded hexagonal crystals of
Table 1. Summary of linker exchange quantitation, unit cell parameters (Å), selected interatomic distances (Å), and bond angles (°) for compounds 1−6.
2
3
5
6
1 (ref 28)
(M = Zn)
4
(M = Zn;
X = Br)
(M = Zn;
X = Cl)
(M = Mn;
X = Br)
(M = Mn;
X = Cl)
(M = Mn)
% XL′2− (X = Cl, Br)a
58(1) / − / 57
64.8(3) / − / 63
− / 40(3) / 40
− / − / 50
a
c
13.6682(6)
8.8870(5)
13.942(2)
10.262(1)
13.953(1)
10.133(1)
14.017(4)
8.970(2)
14.1160(5)
9.970(1)
14.138(1)
10.142(1)
M1–O/N1
O/N1–C1
C1–C2
O/N1–M1–O/N1′
M1–O/N1–C1
O/N1–C1–C2
C1–C2–Cl'
2.082(3)
1.244(5)
1.384(5)
78.4(2)
114.7(3)
125.3(4)
122.8(5)
2.103(7)
1.28(1)
1.40(1)
76.4(4)
115.9(6)
125.1(9)
122(1)
2.102(8)
1.28(1)
1.40(1)
77.1(4)
116.2(6)
126(1)
122(1)
2.151(6)
1.234(8)
1.395(8)
74.2(2)
117.6(4)
125.4(6)
121.4(8)
2.147(6)
1.24(1)
1.43(1)
74.4(3)
117.6(5)
125.3(7)
121(1)
2.16(1)
1.28(1)
1.40(2)
74.0(5)
118.0(8)
124(1)
118(2)
aAs determined by quantitative 13C NMR spectroscopy / EDX spectroscopy / combustion elemental analysis.
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(Et4N)2[Zn2L1.3BrL′1.7]∙0.57DMF∙0.14Et2O
(2),
The change of the interlayer distance as a consequence of counterion
(Et4N)2[Zn2L1.1ClL′1.9]∙0.64CH2Cl2
(3, see
Figure
1c),
exchange is also reflected by the shifting of the <001> peak toward
lower angles. From 1 to 2 or 3, the <001> peak moved from 9.425° to
8.405° or 8.390°, respectively, corresponding to the change in c from
9.38 Å to 10.51 Å or 10.53 Å. A similar trend was observed from 4 to 5
or 6. Note that the c parameter obtained from the <001> peaks of
PXRD analysis are slightly larger than those obtained from single-crys-
tal diffraction analysis due to differences in temperature between the
two techniques. Nevertheless, the same trends are observed in both
analyses. Importantly, these bulk measurements suggest that the ex-
changed frameworks are all pure, single-phase materials (see Figures
S6–S9), rather than macroscopically-separated crystalline domains
with distinct chemical compositions as have been observed in manga-
nese-benzoquinoid framework crystals with partial metal exchange.40
1
2
3
4
5
6
7
8
(Et4N)2[Mn2L1.8BrL′1.2]∙0.65DMF (5), or (Et4N)2[Mn2L1.5ClL′1.5] (6),
which were all suitable for single-crystal X-ray diffraction analysis. No-
tably, these linker exchanges proceed via single-crystal-to-single-crystal
(SC-SC) reactions. In contrast, addition of Zn2+ or Mn2+ to the
Br
Cl
equimolar solution of (Et4N)OH and H2 L′ (or H2 L′) at room tem-
perature invariably resulted in the immediate precipitation of amor-
phous solids.
Attempts to directly synthesize the amidic MOFs through sol-
vothermal methods without the addition of (Et4N)OH produced no
solid products. In addition, solvothermal synthesis with M2+ (M = Zn,
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X
Mn) and H2 L′ (X = Cl, Br) with the addition of H2L solely produced
X
1 and 4 without incorporation of H2 L′, as determined by Raman spec-
troscopy and 13CNMR spectroscopy on acid-digested samples (see rel-
evant discussions below). These observations eliminate the possibility
of a dissolution-recrystallization mechanism, and they underscore the
necessity of post-synthetic linker exchange to obtain these kinetic
products.
Amine-substituted benzoquinoid molecules are prone to hydrolysis
,
under hydrothermal conditions.52 53 This tendency has been exploited
to deliberately synthesize the 3D framework compounds
(Bu4N)2[M2(dhbq)3] (M = Mn, Fe, Co, Ni, Zn, Cd; H2dhbq = 2,5-di-
hydroxo-1,4-benzoquinone) via in situ hydrolysis.50,51 Nevertheless,
under the linker exchange conditions reported here, hydrolysis was de-
The addition of one equivalent of (Et4N)OH per molecule of linker
in these exchange reactions corresponds to an optimized stoichiometry
based on both pH (see below) and solubility considerations. The solu-
Br
Cl
finitively not observed for H2 L′ or H2 L′, as demonstrated by Raman
spectroscopy, 13CNMR spectroscopy, and combustion elemental anal-
ysis.
X
X
bility of both H2 L′(X = Br, Cl) and (Et4N)2 L′is low in DMF, whereas
the solubility of mono-deprotonated (Et4N)HXL′ can reach 0.15 mM
in DMF. Considering the relative pKa values of (Me2NH2)+ and H2 L′,
X
some protonation of (HXL′)− is expected over the course of the ligand
exchange, with concomitant counterion exchange of (Me2NH2)+ for
(Et4N)+. As such, a high initial concentration of (Et4N)HXL′ is desired.
Lowering the stoichiometry of (Et4N)OH resulted in the incomplete
dissolution of H2 L′and a lower concentration of (Et4N)HXL′, thereby
X
slowing the linker exchange. Addition of additional Et4NOH (between
one and two equivalents) resulted in the precipitation of the linker as
X
insoluble (Et4N)2 L′. Finally, slow addition of excess (Et4N)OH (more
than two equivalents) over the course of the exchange reaction resulted
in framework dissolution and formation of an amorphous precipitate.
The single-crystal X-ray structures of 2, 3, 5, and 6 reveal that the
compounds belong to the hexagonal space group P͞31m. Similar to 1
and 4, these structures feature eclipsed, stacked anionic layers with a
honeycomb topology (see Figure 1a). Both the exchanged (XL′)2− and
the parent linkers L2− occupy the same lattice sites with partial occupan-
cies. During the linker exchange reactions, the (Me2NH2)+ counterions
in 1 and 4 are replaced by (Et4N)+. As discussed above, this counterion
exchange is likely driven by deprotonation of the weakly acidic
X
(Me2NH2)+ by the basic (Et4N)HXL′ to form Me2NH and H2 L′. Fol-
lowing linker exchange, the (Et4N)+ ion resides equidistant between
the two nearest metal ions from two adjacent layers (see Figures S2–
S5). These linker exchange reactions cause an elongation of both a and
c unit cell axes (see Table 1), with more significant changes in 2 and 3
compared to 5 and 6 (see Table 1).
The lengthening of unit cell parameter a conveniently allows the
linker exchange to be followed by powder X-ray diffraction (PXRD).
As shown in Figure 2 (right, expanded views), linker-exchanged 2, 3, 5,
and 6 feature a pronounced shift in 2θ toward lower angles relative to
parent 1 and 4, consistent with the elongation of a. For instance, the
<100> peak moved from 7.445° to 7.295° or 7.265° from 1 to 2 or 3,
respectively, corresponding to elongation of a from 13.70 Å to 13.98 Å
or 14.04 Å. Similarly, from 4 to 5 or 6, the <100> peak moved from
7.280° to 7.145° or 7.115°, respectively, corresponding an elongation of
a from 14.01 Å to 14.27 Å or 14.33 Å. These unit cell elongations are
consistent between single crystal and powder X-ray diffraction data.
Figure 2. PXRD patterns for 1–3 (a), and 4–6 (b). Expanded views of the range
containing <100> peaks are shown on the right to highlight peak shifts upon
linker exchange.
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Figure 3. Raman spectra for crystalline samples of 1–6, following excitation at
473 nm. Vertical bars are guides for the eye.
First, Raman spectroscopy indicates that vibrations associated with
both L2− and (XL′)2− (X = Cl, Br) are present in 2, 3, 5, and 6. As shown
in Figure 3, the bands at 1346–1364 cm−1 (orange) are assigned to an
averaged C–C ring stretch of L2−.36 The corresponding C–C ring vibra-
tions for (BrL′)2− and (ClL′)2− appear at 1369–1373 cm−1 (blue) and
1389 cm−1 (green), respectively. Additionally, the Raman bands at
1602–1612 cm−1 (purple) are assigned to the C–O stretching vibration
of L2−,36 whereas the features at 1532–1544 cm−1 (pink) are assigned to
the collective C–N/C–O stretching vibration for (BrL′)2− and (ClL′)2−.
Upon linker exchange, a new band appears at 561–573 cm−1 (cyan),
which we assign to the O–M–N (M = Zn, Mn) vibrations for (BrL′)2−
and (ClL′)2−. Finally, the O–M–O vibrations for L2− at 586–595 cm−1
(gray) are preserved upon linker exchange.36
13
The linker identities were further corroborated by C NMR spec-
troscopy of acid-digested samples of 1, 2, and 3. As shown in Figure 4,
X
both H2L and H2 L′ are present in solutions of acid-digested 2 and 3.
In contrast, in the case that (BrL′)2− were hydrolyzed during MOF syn-
thesis, one would expect the hydrolyzed linker 2,5-dibromo-3,6-dihy-
Figure 4. (a) Top to bottom: 13C NMR spectra for H2L, 1, H2BrL′, 2, H2ClL′, and
3. Peaks corresponding to (Me2NH2)+, (Et4N)+, DMF, CH2Cl2, and Et2O are
denoted with a–e, respectively. (b, c) 13C NMR external calibration curves for
Br
droxo-1,4-benzoquinone instead of H2 L′ to be present in the solution
of 2, and similarly, only H2L peaks would be present in 3.
mixtures of H2L/H2BrL′ (b) and H2L/H2ClL′ (c). 13C NMR index = H2BrL′ (or
H2ClL′) peak areas / total peak areas of the two ligands. Exchanged linkers are
quantified based on 13C NMR index for 2 and 3.
To quantify the extent of linker exchange in the reactions involving
13
Zn frameworks, we developed a facile quantitative C NMR method
using a standard pulse sequence.54 Since the transverse relaxation time
constant (T2) for the 13C nucleus is highly dependent on chemical en-
vironment and is longer than the 0.4 s relaxation delay defined by the
standard pulse sequence, quantitation of 13C atoms with different
chemical environments is precluded. Although it is possible to extend
the relaxation delay at a cost of significantly prolonged acquisition time,
we decided to instead establish external calibration curves by acquiring
ships were obtained for both of these linker mixtures. Finally, these ex-
ternal calibration curves enable us to quantify linker exchange by calcu-
lating the 13C NMR index from a standard, qualitative 13C NMR spec-
trum of an acid-digested sample of the corresponding compound (Fig-
ures S10-11). As a result of this analysis, the mole fractions of the ex-
Br
13C NMR spectra for a series of solution mixtures of H2L and H2 L′ or
Br
Cl
changed linkers H2 L′ and H2 L′ were determined to be 58(1)% and
64.8(3)% in 2 and 3, respectively, establishing the composition of the
anionic layers as [Zn2L1.3BrL′1.7]2− and [Zn2L1.1ClL′1.9]2−. Additionally,
Cl
H2L and H2 L′. Peaks corresponding to each ligand were integrated,
Br
Cl
and the fractional peak area of H2 L′or H2 L′with respect to total peak
area of both ligands (i.e. the 13C NMR index) was plotted against their
mole fractions. As shown in Figures 4b and 4c, excellent linear relation-
1
quantitation of the occluded solvent molecules by H NMR provides
overall
chemical
formulas
of
and
(Et4N)2[Zn2L1.3BrL′1.7]·0.57DMF·0.14Et2O
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Figure 5. (a−c) Electron microscopy images of 5. (d) Quantitation of (BrL′)2−
based on 24 EDX spectra collected at the areas and points marked in (a−c) as
yellow rectangles and magenta filled circles, respectively. (e, f) EDX line scan
data, showing normalized photon counts for Br and Cl Ka1 emission. Scan di-
rections are marked in (c).
Figure 6. (a) Raman spectrum for 3. (b-d) Raman mapping of a crystal of 3,
highlighting the intensity at 1550 cm−1 for (ClL′)2− (b), 1612 cm−1 for L2− (c), and
the sum intensity at these two wavelengths (d). (e) Optical microscopy image
of this crystal. (f) Line scan data at these two wavelengths with the scan direc-
tion marked in (e).
(Et4N)2[Zn2L1.1ClL′1.9]·0.64CH2Cl2 for 2 and 3, respectively (see Fig-
ures S12 and S13). These chemical formulas are further corroborated
by combustion elemental analysis (see Table 1 and Experimental Sec-
tion).
To probe the spatial distribution of linkers in the parent and ex-
changed frameworks, we performed mapping and line scan experi-
ments using EDX and Raman spectroscopies for 5 and 3, respec-
tively. First, EDX mapping (see Figures S16–20) and line scans (see
Figure 5c, e, f) for 5 indicate a homogenous distribution of Br and Cl
atoms along both the ab plane and the c axis. Similarly, Raman mapping
and line scans for 3 reveal a constant intensity ratio between 1550 cm−1
(C–N for ClL′2−) and 1612 cm−1 (C–O for L2−) across the entire crystal
(see Figure 6).31 These spatially resolved EDX and Raman spectros-
copy studies indicate that both the parent L2− and diamido (BrL′)2− or
(ClL′)2− linkers are homogenously distributed within linker-exchanged
MOF crystals on the mesoscopic to macroscopic length scale, further
precluding sample heterogeneity caused by the diffusion barriers. Since
linker exchange does not proceed further by subjecting linker-ex-
changed crystals to additional linker exchange conditions, these mixed-
linker frameworks likely represent thermodynamically metastable
products.
Owing to the long electronic relaxation time of high-spin Mn2+, a
similar NMR analysis for the Mn-containing compounds is not possi-
ble. Instead, to quantify linker exchange in these compounds, we em-
ployed energy-dispersive X-ray (EDX) spectroscopy and combustion
elemental analysis. For 5, the Cl and Br atoms on L2− and (BrL′)2−, re-
spectively, were used as elemental probes. As shown in Figure 5a–d, ac-
quisition of 24 EDX spectra over three sites gave a value of 40(3)% Br
per total number of halogen atoms, corresponding to a composition of
[Mn2L1.8BrL′1.2]2− for the anionic layer (see Table S2 and Figure S14).
The same analysis performed on two additional independently-pre-
pared batches of 5 gave 40(2)%, and 40(5)% Br, based on 25 and 20
EDX spectra, respectively, both collected over multiple sites (see Ta-
bles S3 and S4). To explore whether more than 40% of the (BrL′)2−
linker could be incorporated through exchange reactions, crystals of 5
were further subjected to linker exchange conditions for a total of 48 h.
Following this treatment, 25 EDX spectra collected over five sites gave
40(2)% of Br (see Table S5), suggesting that 40% exchange represents
a local thermodynamic minimum. The chemical formula for 5 deter-
mined by EDX spectroscopy is consistent with that obtained from
combustion elemental analysis data on an activated, solvent-free sam-
ple (see Figure S15), giving an overall chemical formula of
(Et4N)2[Mn2L1.8BrL′1.2]∙0.65DMF. Considering the agreement be-
tween these two methods in determining the chemical formula for 5,
we obtained a formula for 6 of (Et4N)2[Mn2L1.5ClL′1.5] based on com-
bustion elemental analysis of a similarly activated sample.
To gain some insight into the mechanism of linker exchange, we first
Br
Cl
carried out acid-base titrations of H2L, H2 L′, and H2 L′ in DMF. As
shown in Figure S21, all three linkers exhibit two distinct deprotona-
tion processes at well-isolated pH regions. The corresponding first
deprotonation process can be expressed as follows:
[H2Ligand] ⇌ [H+] + [HLigand−]
Ka1 = [H+][HLigand−]/[H2Ligand]
For weak acids in DMF, the assumption that [H+] = [HLigand−] <<
[H2Ligand] can be made, such that pKa1 values can be calculated ac-
cording to the following expression:
pKa1 = −log([H+]2/[H2Ligand])
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Br
rated varies between framework metal and linker identities. The disso-
ciative mechanism proposed above involves dynamic binding of L2−.
Dissociation of the ligand from one metal ion requires some structural
flexibility of the crystal lattice, which may involve the simultaneous dis-
sociation of multiple ligands across the 2D layer. In contrast, the more
covalent M–(XL′)2− linkage is expected to provide static, irreversible
binding. As the linker exchange proceeds towards saturation, local
strain from linker disorder and the increasing lattice parameters, paired
with the reduced concentration of dynamic L2− linkers, may inhibit the
lattice dynamics that are required to observe complete linker ex-
change. In support of this hypothesis, the observation of high-angle
peak broadening in the PXRD data of the linker-exchanged materials
(see Figure 2) suggests the presence of increased strain of the frame-
work lattice.44 We note that other effects, such as hydrogen bonding in-
teractions involving the protons of (XL′)2−, may also play a role in re-
ducing lattice dynamics and arresting linker exchange in these materi-
als.
This analysis gives pKa1 values of 4.7, 16.1 and 12.3 for H2L, H2 L′,
and H2 L′, respectively, in DMF.
Cl
1
2
3
4
5
6
7
8
Based on these values of pKa1, upon addition of one equivalent
(Et4N)OH to a suspension of H2 L′ (X = Br, Cl) in DMF, the ligand
becomes deprotonated and consequently solubilized to give
(Et4N) (H L′) at a concentration of 0.15 mM. Counterion ex-
X
+
X
−
change is then driven by the deprotonation of (Me2NH2)+:
X
(Me2NH2)+ + Et4N(HXL′) → (Et4N)+ + H2 L′ + Me2NH
While this acid-base reaction will consume a stoichiometric amount of
solubilized (HXL′)−, the large excess of linker relative to pre-exchanged
framework ensures that a high concentration of (HXL′)− is present in
solution and can thus participate in linker exchange.
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To obtain a snapshot of counterion and linker exchange processes,
we halted the reaction from 1 to 3 after 15 hours and collected a PXRD
pattern of the product. As shown in Figure S22, the diffractogram dis-
plays two <001> peaks at 8.375° and 8.465°, corresponding to two dis-
tinct c lattice parameters and thus interlayer distances of 10.55 Å and
10.44 Å. These distances correspond to the full and partially counter-
ion-exchanged phases, respectively, and they match much more closely
with the (Et4N)+ phases than the (Me2NH2)+ phases. This suggests that
counterion exchange is nearly complete after 15 h. Conversely, two
<100> peaks at 7.250° and 7.415° reveal a different scenario. The <100>
peak at 7.250° corresponding to a lattice parameter a of 14.07 Å indi-
cates the presence of a mixed-linker framework phase, whereas the
other <100> peak at 7.415° is consistent with a lattice a parameter of
13.76 Å that corresponds to the parent [Zn2L3]2− framework. From
these data, we deduce that counterion exchange proceeds faster than
does linker exchange, as would be expected for a simple acid-base reac-
tion.
Increased lattice strain is likely to destabilize framework materials
upon linker exchange, and this proposal is supported by the findings
from gas adsorption experiments. According to N2 adsorption and de-
sorption isotherms collected at 77 K (see Figure S23), 1 exhibits a
Brunauer-Emmett-Teller (BET) surface area of 610 m2/g (see Figure
S24). Conversely, compounds 3-6 do not exhibit permanent porosity
upon solvent removal (see SI for activation procedures). In the case of
Zn, this loss of permanent porosity may result from lattice strain asso-
ciated with linker exchange, particularly considering the tendency of
this family of frameworks to undergo structural changes upon desolva-
tion.36 Specifically, lattice strain induced by linker disorder may lead to
more substantial changes in the local coordination environment during
activation. The lack of permanent porosity for all Mn-containing com-
pounds is likely due to the relative flexibility of the Mn coordination
sphere. Note that the related compound (Et4N)2[Mn2L3], was also
shown to exhibit no permanent porosity when activated using a similar
procedure.42
The observation of two distinct a lattice parameters, seemingly cor-
responding to the initial framework and the exchange-saturated frame-
work, suggests the exchange process may be kinetically limited by
linker diffusion through the pores of the framework. We envision that
linker exchange initiates at the exterior of a crystal and gradually prop-
agates toward the center. This mechanism implies that local linker ex-
change rate is much faster than linker diffusion such that phase segre-
gation within a single crystal is anticipated at intermediate levels of ex-
change. Similar diffusion-limited phase segregation has been observed
during the metal exchange process of (Et4N)2[Mn2L3] with Co2+ or
Zn2+.40 The appearance of two <100> reflections could also be sugges-
tive of a cooperative exchange phenomena in which a small amount of
ligand exchange promotes rapid exchange within a single two-dimen-
sional layer. While diffusion effects are more likely to explain the ob-
served data, both effects could be operative in the materials described
here. Further studies of crystallites at intermediate exchange are neces-
sary to better understand the macroscopic exchange mechanism in
these framework materials.
These preliminary mechanistic studies illustrate the potential for ex-
changing linkers with different donor atoms and with very different
pKa values. Further studies of frameworks at different levels of linker
exchange and of the local coordination of the linker-exchanged frame-
works will provide additional insights into the mechanism of linker ex-
change and the origin of incomplete exchange in these frameworks. We
envision this base-assisted linker exchange procedure could be gener-
alized to a broad scope of MOFs and linkers if the following criteria are
met: 1) the new linker should increase the metal–ligand bond strength
to provide an energetic driving force for the reaction; 2) the new linker
should be less acidic than the starting linker to favor irreversible ex-
change under basic conditions; and 3) the framework should possess
dynamic and flexible metal–ligand coordination to enable associative
or dissociative exchange processes.
Regarding the local linker exchange mechanism, we propose a disso-
ciative mechanism (see Figure S23).55 The observation of framework
dissolution upon extended soaking in hot DMF indicates relatively dy-
namic metal–linker coordination at these conditions. Upon dissocia-
tion of L2− at one metal site, the deprotonated N atom of (HXL′)− can
bind irreversibly. Subsequent dissociation of L2− from the second metal
site should lead to preferential formation of a bridging (XL′)2− linker, in
which deprotonation of (HXL′)− by L2− is enabled by the reduced pKa
of (HXL′)− upon coordination to a metal ion.
SUMMARY AND OUTLOOK
The foregoing results report a synthetic method that provides mem-
bers of an exceedingly rare class of amidic MOFs. Specifically, this
method expands the scope of post-synthetic linker exchange to intro-
duce linkers with coordinating atoms of dramatically different basicity.
This synthetic route harnesses the relatively irreversible coordination
bonds, which typically lead to amorphous solids, to synthesize crystals
that are not accessible through direct synthesis. This work may exem-
plify a general synthetic route toward frameworks without reversible
coordination bonds and those with exotic metal-linker coordination
modes. Further, this strategy could provide a new synthetic route to
An explanation for the values of exchange saturation is more chal-
lenging to propose. The maximum amount of (XL′)2− being incorpo-
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(3) Lahiri, N.; Lotfizadeh, N.; Tsuchikawa, R.; Deshpande, V. V.; Louie,
J., Hexaaminobenzene as A Building Block for A Family of 2D Coordination
Polymers. J. Am. Chem. Soc. 2017, 139, 19–22.
frameworks with increased metal–ligand covalency for enhanced elec-
tronic and magnetic properties. We envision that this strategy should
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imine or thiolene for oxolene linkers and amidate or thiocarboxylate for
carboxylate linkers. The increased metal–ligand covalency afforded by
this approach, paired with the single-crystal-to-single-crystal transfor-
mations, may facilitate the growth of large single crystals of highly co-
valent frameworks for electronic transport and magnetic measure-
ments. Future work will aim to explore the physical properties of these
linker-exchanged frameworks and to expand the scope of this strategy
to other families of frameworks, to materials with metal-phosphine and
metal-carbon linkages, and to frameworks which undergo quantitative
linker exchange.
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the ACS Pub-
lications website at DOI: 10.1021/
Experimental details, crystallography table, tables for EDX quantitation,
additional crystal structure figures, PXRD patterns, NMR spectra, SEM
and EDX mapping images (PDF)
Crystallography data, combined (CIF)
AUTHOR INFORMATION
Corresponding Author
*dharris@berkeley.edu
Present Address
║School of Chemical and Physical Sciences, Victoria University of Wel-
lington, P.O. Box 600, Wellington 6012, New Zealand.
Author Contributions
§Lujia Liu and Liang Li contributed equally to this work.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was funded by the Army Research Office (W911NF-14-1-
0168/P00005). This work made use of the IMSERC at Northwestern
University, which has received support from the Soft and Hybrid Nano-
technology Experimental (SHyNE) Resource (NSF ECCS-1542205);
the State of Illinois and International Institute for Nanotechnology
(IIN). This work made use of the EPIC facility of Northwestern Univer-
sity’s NUANCE Center, which has received support from the Soft and
Hybrid Nanotechnology Experimental (SHyNE) Resource (NSF
ECCS-1542205); the MRSEC program (NSF DMR-1720139) at the
Materials Research Center; the International Institute for Nanotechnol-
ogy (IIN); the Keck Foundation; and the State of Illinois, through the
IIN. We thank Prof. J. R. Long for use of his gas adsorption analyzer.
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