The noncoordinating redox reagents co-
baltocene (Cp 2 Co, reductant; Cp stands for
cyclopentadienyl anion) and ferrocenium hexa-
fluorophosphate ([Cp 2 Fe]PF 6 , oxidant) have
been shown to be effective and innocuous
as far as mechanical adsorption on Zr-BTB-
MPCG1 is concerned ( 52 ). We subjected Zr-
BTB-MPCG1 and a large excess of CBPQT4+
rings to one redox cycle using Cp 2 Co and
[Cp 2 Fe]PF 6 (Fig. 3C). After repeated washings
with MeCN to remove unthreaded and phys-
isorbed rings, we isolated Zr-BTB-MPCG1-
CBPQT(+) by centrifugation as a solid whose
composition—Zr 6 (MPCG1)0.4(CBPQT)0.4—
could be obtained (Fig. 3F) by solution-state
NMR spectroscopy after digestion by D 2 SO 4 /
CD 3 SOCD 3. The integrated ratio of CBPQT4+
to MPCG13+was calculated to be 0.90:1.00
(fig. S28), corresponding to a 90% threading
efficiency on the Zr-BTB surface after one redox
cycle. The presence of mechanically adsorbed
CBPQT4+rings on Zr-BTB-MPCG1-CBPQT(+)
was also verified by^13 C MAS SSNMR (fig. S70),
where the chemical shift of 65 ppm corre-
sponds to the carbon resonances from CH 2
and the chemical shifts of 136 and 151 ppm
correspond to the carbon resonances from
BIPY2+units on CBPQT4+rings. Control ex-
periments show (figs. S29 and S30) that there
is no uptake of CBPQT4+rings by Zr-BTB or
Zr-BTB-MPCG1 in the absence of the redox
cycling, ruling out the possibility of phys-
isorption or chemisorption of CBPQT4+rings
on the MOF surfaces after washing.
The porosity of Zr-BTB nanosheets before
and after mechanisorption was estimated
by evaluation of their N 2 isotherms at 77 K
(figs. S79 to S81). The Brunauer-Emmett-Teller
(BET) surface area of the untreated surface was
436 m^2 g–^1 (table S5). After grafting with
MPCG13+, the BET surface area decreased
from 436 to 307 m^2 g–^1 , and the mechanisorp-
tion of the CBPQT4+rings reduced the surface
area further, to 183 m^2 g–^1. No notable change
to the PXRD patterns or TEM images of the
MOFs after grafting or mechanisorption was
observed (Fig. 3B and figs. S77 and S90),
which is indictive of their well-maintained
crystallinity.
This AMP-grafted MOF also exhibits the
capability of desorbing and resorbing rings
without compromising the structural integ-
rity of either the robust MOF or the dynamic
AMPs for the simple reason that the mecha-
nisorption only involves the cleavage and
reformation of noncovalent bonding inter-
actions, for example, mechanical and coor-
dination bonds. Desorption of the threaded
rings was achieved by an acid-base reaction
(figs. S10 and S11), where a strong acid reacts
with the MOF surface at the CG site to gen-
erate weak acid MPCG13+. Mixing Zr-BTB-
MPCG1-CBPQT(+) with 1 M HCl aqueous or
MeCN solutions at room temperature for1218 3 DECEMBER 2021•VOL 374 ISSUE 6572 science.orgSCIENCE
Fig. 3. Redox-driven adsorption and acid-triggered desorption of CBPQT4+rings on Zr-BTB-MPCG1
surface.(A) Graphical representations of the structure of the Zr-BTB layer and the CBPQT4+ring.
The Zr-BTB layer consists of well-organized six-connected Zr 6 clusters with replaceable terminal
BA−/HCOO−and H 2 O/OH−ligands. (B) TEM image of Zr-BTB-MPCG1-CBPQT(+) nanosheets—where
(+) refers to the fully charged states of the CBPQT4+rings and the BIPY2+units on the MOF surfaces.
(C) The adsorption and desorption mechanisms on the surface of Zr-BTB-MPCG1. The Zr-BTB-MPCG1
is prepared by grafting (step 1) MPCG13+onto Zr-BTB nanosheets at room temperature. Initially,
the CBPQT4+rings and the Zr-BTB-MPCG1 repel each other because of strong Coulombic repulsion.
Subsequently, all the BIPY2+units are reduced to their radical cationic states (BIPY•+),leadingtothe
threading (step 2) of a collection of CBPQT2(•+)rings, one onto each MPCG13+end (the pseudo
pumping cassette), on Zr-BTB-MPCG1-CBPQT(•)—where (•) refers to the radical states of CBPQT2(•+)
rings and BIPY•+units—as a result of the formation of trisradical tricationic complexes. After oxidation,
the rings proceed (step 3) onto collecting chains on account of strong Coulombic repulsion between
the charged PY+and BIPY2+units, leading to the formation of Zr-BTB-MPCG1-CBPQT(+). Desorption
(step 4) of the rings is achieved by an acid-base reaction between Zr-BTB-MPCG1-CBPQT(+) and H 3 O+at
the carboxylate gates (CG). Meanwhile, this treatment also releases the MPCG13+and Zr-BTB for
subsequent use. (D) Partial^1 HNMRspectra(600MHz,D 2 SO 4 /CD 3 SOCD 3 , 298 K) of digested
Zr-BTB. (E) Partial^1 H NMR spectra (600 MHz, D 2 SO 4 /CD 3 SOCD 3 , 298 K) of digested Zr-BTB-MPCG1.
(F) Partial^1 H NMR spectra (600 MHz, D 2 SO 4 /CD 3 SOCD 3 , 298 K) of digested Zr-BTB-MPCG1-CBPQT(+).
The ratio between CBPQT4+and MPCG13+was calculated as 0.90:1.00, revealing a 90% threading
efficiency on the Zr-BTB-MPCG1 surface during the redox cycle.
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