Using geometric simulation software 'GASP' to model conformational flexibility in a family of zinc metal-organic frameworks

Article abstract of DOI:10.1039/d1nj01158h

Here, a new tripodal tricarboxylic acid ligand, 4,4′-(4′-(4′-carboxy-[1,1′-biphenyl]-4-yl)-[2,2′:6′,2′′-terpyridine]-5,5′′-diyl)-dibenzoic acid (H3cbt), was synthesised using a three-step convergent strategy. Subsequent reactions with zinc(ii) nitrate hexahydrate yielded three metal-organic frameworks (MOFs). The three MOFs, [Zn(Hcbt)]·4DMF (1), [Zn(Hcbt)]·4DMSO·1.5H2O·DMF (2), and [Zn(Hcbt)]·2DMF·3H2O (3), each adopt flexible interdigitated 2D net topologies. Framework 1 has DMF-filled channels that retain porosity upon desolvation, with a measured BET surface area of 248 m2 g-1. Framework 2 possesses larger DMSO-containing channels that collapse upon desolvation, resulting in near-equivalent porosity values to framework 1. In silico calculations and topological considerations determined using the geometric simulation software GASP dictate that framework 2 can feasibly alter conformation to approximate 1, but cannot perfectly replicate the interdigitated motif. Framework 3 formed when wet solvents were used to synthesise 1. Interestingly, the interdigitated structure of 3 contains a unique carboxylate binding mode that precludes its subsequent adoption by either 1 or 2 upon their exposure to water. This diverse array of structural considerations recommends this MOF family for modelling using GASP. Interrogating frameworks 1-3 using this software provided insights that justified experimentally observed conformational trends, as well as barriers to interconversion between members of this MOF family. In a broader sense, this work demonstrates the wider applicability of GASP software to modelling structural changes within flexible MOF materials.

Full text of DOI:10.1039/d1nj01158h

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Using geometric simulation software ‘GASP’ to
model conformational flexibility in a family of zinc
metal–organic frameworks†
Cite this: DOI: 10.1039/d1nj01158h
a
b
c
d
William J. Gee,
Stephen A. Wells,
Simon J. Teat,
Paul R. Raithby
and
d
Andrew D. Burrows
*
Here,
a
new tripodal tricarboxylic acid ligand, 4,40-(40-(40-carboxy-[1,10-biphenyl]-4-yl)-[2,20:60,200-
terpyridine]-5,500-diyl)-dibenzoic acid (H3cbt), was synthesised using a three-step convergent strategy.
Subsequent reactions with zinc(II) nitrate hexahydrate yielded three metal–organic frameworks (MOFs).
The three MOFs, [Zn(Hcbt)]ꢀ4DMF (1), [Zn(Hcbt)]ꢀ4DMSOꢀ1.5H2OꢀDMF (2), and [Zn(Hcbt)]ꢀ2DMFꢀ3H2O (3),
each adopt flexible interdigitated 2D net topologies. Framework 1 has DMF-filled channels that retain
porosity upon desolvation, with a measured BET surface area of 248 m2
g
ꢁ1. Framework 2 possesses
larger DMSO-containing channels that collapse upon desolvation, resulting in near-equivalent porosity
values to framework 1. In silico calculations and topological considerations determined using the
geometric simulation software GASP dictate that framework 2 can feasibly alter conformation to
approximate 1, but cannot perfectly replicate the interdigitated motif. Framework 3 formed when wet
solvents were used to synthesise 1. Interestingly, the interdigitated structure of 3 contains a unique
carboxylate binding mode that precludes its subsequent adoption by either 1 or 2 upon their exposure
to water. This diverse array of structural considerations recommends this MOF family for modelling using
GASP. Interrogating frameworks 1–3 using this software provided insights that justified experimentally
observed conformational trends, as well as barriers to interconversion between members of this MOF family.
In a broader sense, this work demonstrates the wider applicability of GASP software to modelling structural
changes within flexible MOF materials.
Received 10th March 2021,
Accepted 16th April 2021
DOI: 10.1039/d1nj01158h
rsc.li/njc
nanometre-sized cavities, which would grant chemists, physicists
and material scientists the ability to manipulate separation,
Introduction
Conformational flexibility in metal–organic frameworks storage, and catalytic behaviours of the resulting materials.2
(MOFs), also known as porous coordination polymers (PCPs), Designing such systems requires a combination of organic linker
is one of the preconditions for solid-state switchable behaviours and metal connecter to produce a 1D, 2D, 3D or 4D framework3
that are much coveted by materials scientists.1 Interest in these that is amenable to one of the six classes of movement proposed
systems stems from the desire to control and manipulate by Kitagawa.4 These classes may rely on the addition or loss of
guest molecules to drive changes in conformation,1c,d however in
the absence of interchangeable guests, flexibility can be triggered
by other stimuli, such as temperature, mechanical stress or
photochemistry.1a,5 In such cases, the inorganic component is
typically associated with rigidity, whereas the organic component
imparts flexibility.1 Numerous examples of MOFs exhibiting
conformational flexibility when subjected to a range of stimuli
were detailed in a recent review by Fischer.1a Illustrative examples
a School of Environment and Science, Griffith University, 170 Kessels Road,
Brisbane, QLD 4111, Australia
b Department of Chemical Engineering, University of Bath, Claverton Down, Bath,
BA2 7AY, UK
c Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA, USA
d Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY, UK.
E-mail: a.d.burrows@bath.ac.uk
† Electronic supplementary information (ESI) available: Analysis of the H3cbt
reaction mixture by accurate mass spectrometry; 1H-NMR spectrum of H3cbt; include MIL-88 that showed guest dependent swelling,6 MOF-5
crystal data tables for 1–3; hydrogen-bonding environment about the nonbonding
and HMOF-1 that exhibited thermally-induced structural
changes,7 ZIF-8 for mechanical stress induced deformation,8
Hcbt arm in 1–3; TGA traces for 1 and 2; PXRD analysis for bulk samples of 1 and
2; a comparison of the PXRD pattern of 1 to the predicted pattern of 3; behaviour
and incorporation of photo-responsive linkers within a MOF-5
of 1 immersed in water for 12 hours; PXRD comparisons of activated 1 and 2
analogue that triggered reversible changes to CO2 uptake
capacity.9
relative to solvated forms. CCDC 2062086–2062088. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/d1nj01158h
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Despite the diverse modes of movement available to MOF per million (ppm) and coupling constants (J) in Hertz (Hz).
materials,4 progress in computational modelling of the Signal multiplicity is denoted as singlet (s), doublet (d), triplet (t),
flexibility exhibited by MOFs has been slow. A specialized quartet (q), multiplet (m) or broad (b). High resolution mass
modelling approach of ‘‘template-based geometric simulation’’ spectroscopy was performed using electrospray ionisation (ESI)
has proved very successful in describing flexibility in polyhedral with either positive or negative ionisation.
mineral frameworks such as perovskites and aluminosilicates,
especially zeolites,10 as well as proteins.11 The geometric
Synthesis
simulation approach implements
a simplified, localized All reagents were purchased from commercial sources and used
physical model incorporating only steric exclusion and local as received except for 2-acetyl-5-bromopyridine, which was
bonding. Harmonic penalties are applied to the steric overlap synthesised following a known literature procedure.13
of atoms and to deviations from idealised bonding geometry,
4-(6-Acetylpyridin-3-yl)benzoic acid
represented as an overlapping system of templates. Geometric
simulation for periodic frameworks is implemented in a code
A suspension of 4-carboxyphenylboronic acid (0.896 g,
called ‘‘GASP’’ (Geometric Analysis of Structural Polyhedra). 5.4 mmol), 2-acetyl-5-bromopyridine (1.080 g, 5.4 mmol),
The current version of GASP12 is capable of handling not only K2CO3 (2.0 g, 14.5 mmol) and [Pd(PPh3)4] (0.150 g, 0.13 mmol)
polyhedral but also molecular entities in a framework structure, in a mixture of water and methanol (1 : 2 v/v, 50 mL) was heated
thereby providing compatibility to flexible MOF motifs. In the to reflux overnight under an atmosphere of nitrogen. The
study of zeolites, GASP was principally used to examine the reaction mixture was then cooled to room temperature and
response of frameworks to imposed strain (i.e. a change in unit filtered to remove the catalyst. The basic mixture was acidified
cell parameters), and in particular to determine whether a given by the addition of concentrated HCl, which caused the product
strain could be accommodated by framework flexibility, or to precipitate as an off-white powder. The powder was then
whether distortions of the structural units were inevitable.10b
isolated by filtration (74% yield). 1H-NMR (300 MHz, DMSO-d6):
In this study, the first application of GASP to MOF structures d = 9.11 (s, 1 H, PyH), 8.35 (d, J = 9.0 Hz, 1 H, PyH), 8.07 (m, 3 H,
is demonstrated. We use geometric simulation to explore PyH), 7.96 (d, J = 9.0 Hz, 2 H, PhH), 2.68 (s, 3 H, CH3) ppm.
framework flexibility in a challenging family of interdigitated 13C-NMR (125 MHz, DMSO-d6): 199.33, 167.27, 152.53, 147.81,
2D frameworks, and thus link local bonding phenomena (i.e. 140.48, 138.14, 135.92, 130.44, 128.45, 127.80, 121.73, 26.09 ppm.
the carboxylate-zinc binding modes) to global framework IR (ATR): n~ = 2979 (m), 2546 (m), 1677 (s), 1608 (m), 1587 (m),
properties (e.g. channel closures upon desolvation, and barriers 1558 (m), 1509 (w), 1424 (m), 1362 (m), 1288 (s), 1232 (m), 1189
to interconversion between frameworks upon solvent exchange). (m), 1128 (m), 1096 (m), 1015 (m), 1002 (m), 957 (m), 842 (s), 774
Detailed materials characterisation is also provided for new (s), 710 (s), 607 (m) cmꢁ1. MS [M ꢁ H]ꢁ: 240.0666 found:
ligands and frameworks synthesised during this work.
240.0667, [2M ꢁ H + Na]ꢁ: 503.1224; found: 503.1268.
40-Formyl-[1,10-biphenyl]-4-carboxylic acid
Synthesised analogously to 4-(6-acetylpyridin-3-yl)benzoic acid by sub-
stituting 4-carboxyphenylboronic acid and 4-bromobenzaldehyde
in equimolar amounts (89% yield). 1H-NMR (300 MHz,
Experimental
General considerations
Single-crystal X-ray diffraction measurements for compounds DMSO-d6): d = 10.05 (s, 1 H, CHO), 8.08–7.95 (m, 6 H, PhH),
1–3 were carried out at Station 11.3.1 of The Advanced Light 7.86 (d, J = 9.0 Hz, 2 H, PhH) ppm. 13C-NMR (125 MHz, DMSO-
Source (ALS), Lawrence Berkeley National Laboratory, CA. d6): 192.76, 167.04, 144.66, 142.88, 135.64, 130.66, 130.21,
Powder X-ray diffraction data (PXRDs) for 1 and 2, and their 130.07, 127.73, 127.35 ppm. IR (ATR): n~ = 2949 (m), 2836 (m),
corresponding desolvated activated forms, were collected using 2633 (m), 1709 (s), 1676 (s), 1602 (s), 1601 (s), 1578 (m), 1559
a Bruker AXS D8 Advance diffractometer with Cu Ka radiation (m), 1425 (m), 1403 (m), 1293 (m), 1248 (s), 1219 (s), 1170 (s),
of wavelength 1.5406 Å at 298 K. Samples were placed on a flat 1109 (m), 1006 (m), 875 (w), 824 (s), 786 (s), 710 (s), 658 (m) cmꢁ1
.
plate and measured with a 2y range of 5–401. Simulated X-ray MS [M ꢁ H]ꢁ: 225.0557 found: 225.0537, Na[M ꢁ H]2ꢁ: 473.1012;
powder patterns were generated from single crystal data that found: 473.1017.
were imported into Xrdplot. Infrared spectra were recorded on
a PerkinElmer Spectrum 100 spectrometer equipped with an
ATR sampling accessory. Abbreviations for IR bands are s,
4,40-(40-(40-Carboxy-[1,10-biphenyl]-4-yl)-[2,20:60,200-terpyridine]-
5,500-diyl)dibenzoic acid (H3cbt)
strong; m, medium; w, weak. Elemental analyses (C, H, N) were To a 10 mL microwave vial were added 4-(6-acetylpyridin-3-yl)
performed on a CE-440 elemental analyser (Exeter Analytical). benzoic acid (500 mg, 2.07 mmol), 40-formyl-[1,10-biphenyl]-4-
TGA experiments were carried out on a PerkinElmer TGA 4000 carboxylic acid (234 mg, 1.03 mmol), potassium hydroxide
Thermogravimetric Analyser. The samples were heated from (250 mg, 4.46 mmol) and a 2 : 1 mixture of water and conc.
20 1C to 650 1C at a heating rate of 10 1C minꢁ1, under a flow of aq. ammonia (6 mL). The contents were stirred until dissolved
nitrogen (20 mL minꢁ1). NMR spectra were recorded at and the vial sealed. The vial was subsequently heated in a
300 MHz (1H) or 75 MHz (13C) using the residual solvent signals microwave oven at 100 1C for 30 minutes. After this time, the
as an internal standard. Chemical shifts are reported in parts contents of the vial were poured into 20 mL of conc. aq. NH4Cl
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and the resulting precipitate separated by filtration. The followed by full-matrix least-squares refinement (all data) was
precipitate was then dissolved in hot DMSO (5 mL) and poured performed using SHELXL-9715 using the XSEED package.16 1:
into ethanol (20 mL) to precipitate the product a second time.
C53H42N7O10Zn, M = 1002.30, monoclinic, P21/n, a =
The resulting solid was isolated by filtration. This precipitate 16.5565(17), b = 14.5139(15), c = 21.112(2) Å, b = 104.938(2)1,
was dissolved in hot DMF (5 mL) and any insoluble material V = 4901.8(9) Å3, Z = 4, m = 1.358 mmꢁ1, Ntotal = 72 286,
removed by filtration. After cooling over several hours, the Nindependent = 15 845 [R(int) = 0.0635], wR2 = 0.1845 (all data),
resulting precipitate was isolated by filtration and washed with R1
diethyl ether, yielding an off-white powder (29% yield). 1H-NMR
106H100N8O25S8Zn2, M = 2273.15, monoclinic, P21/c (No. 14),
(300 MHz, DMSO-d6): d = 9.15 (d, J = 3.0 Hz, 2 H, PyH), 8.82 a = 15.823(4), b = 16.228(5), c = 22.530(6) Å, b = 108.133(4)1, V =
(s, 2 H, PyH), 8.78 (d, J = 9.0 Hz, 2 H, PyH), 8.40 (dd, J1 = 3.0 Hz 5498(3) Å3, Z = 2, m = 1.373 mmꢁ1, Ntotal = 25 904, Nindependent
=
0.0754 [13 090 reflections with
I
4
2s(I)]. 2:
C
=
J2 = 9.0 Hz, 2 H, PyH), 8.11–7.99 (m, 6 H, PhH), 7.97 (d, J = 5654 [R(int) = 0.1087], wR2 = 0.3224 (all data), R1 = 0.1583 [3635
9.0 Hz, 4 H, PhH), 7.90 (d, J = 9.0 Hz, 2 H, PhH) ppm. IR (ATR): reflections with I 4 2s(I)]. 3: C48H24N5O11Zn, M = 912.09,
n~ = 3157 (m), 2546 (m), 1704 (s), 1662 (m), 1607 (s), 1539 (m), monoclinic, P21/n (No. 14), a = 17.733(5), b = 11.753(3), c =
1481 (w), 1391 (m), 1361 (m), 1241 (s), 1182 (m), 1106 (m), 1005 22.790(6) Å, b = 96.427(4)1, V = 4720(2) Å3, Z = 4, m =
(m), 1106 (m), 771 (s), 699 (m), 658 (m) cmꢁ1. MS [M ꢁ H]ꢁ: 1.284 mmꢁ1, Ntotal = 38 697, Nindependent = 6786 [R(int) =
668.1827; found: 668.1829.
0.1037], wR2 = 0.3004 (all data), R1 = 0.1249 [5246 reflections
with I 4 2s(I)]. CCDC 2062086–2062088 contain the supple-
mentary crystallographic data for coordination polymers 1–3
[Zn(Hcbt)]ꢀ4DMF (1)
To a glass vial were added solutions of H3cbt (25 mg, described in this paper.†
0.037 mmol) dissolved in hot anhydrous DMF (B4 mL), and
Variata
Zn(NO3)2ꢀ6H2O (11 mg, 0.037 mmol) dissolved in hot anhydrous
DMF (B1 mL). The vial was sealed and heated at 100 1C for (1) Two of the four lattice DMF molecules exhibited rotational
2 days. After this time, the resulting colourless crystalline disorder. One such instance required that the formyl group be
material was isolated by filtration (74% yield). IR (ATR): n~ = modelled over two positions, each possessing half occupancy.
3039 (w), 2928 (w), 1719 (m), 1660 (m), 1602 (m), 1541 (m), The other disordered DMF molecule could not sustain aniso-
1480 (s), 1365 (s), 1240 (m), 1099 (m), 1006 (m), 831 (s), 774 (s), tropic refinement or addition of hydrogen atoms and was
735 (w), 704 (m), 671 (m) cmꢁ1. Elemental analysis calcd (%) for consequently modelled without. The geometry of this second
C53H49N7O10Zn: C 63.07, H 4.89, N 9.71; found: C 63.09, H 5.03, molecule of DMF was fixed using the DFIX and DANG
N 6.47.
restraints. (2) The single lattice DMF molecule was restrained
using the DFIX and DANG commands to maintain proper
geometry and was modelled isotropically. The lattice waters
[Zn(Hcbt)]ꢀ4DMSOꢀ1.5H2OꢀDMF (2)
This material was synthesised analogously to 1 but substituting and a single molecule of DMSO were modelled isotropically and
the solvent used to dissolve Zn(NO3)2ꢀ6H2O from anhydrous without hydrogen atoms to ensure a stable refinement. Within
DMF to wet DMSO (65% yield). IR (ATR): n~ = 3388 (m), 3042 (m), the terpy group C(10) exhibited non-positive definite behaviour
1702 (m), 1604 (s), 1541 (m), 1482 (s), 1356 (s), 1240 (s), that was corrected using the ISOR command. (3) The free
1178 (m), 1119 (w), 1008 (s), 949 (m), 833 (s), 775 (s), 702 (m), 671 carboxylic acid group of the Hcbt ligand was disordered over
(m) cmꢁ1. Elemental analysis calcd (%) for C106H118N8O25S8Zn2: C two positions with ca. one-third and two-thirds occupancy,
55.56, H 5.19, N 4.89; found: C 56.01, H 5.47, N 4.29.
respectively. Two of the disordered atoms, C(42) and O(6A),
were made more isotropic with the ISOR command to prevent
non-positive definite behaviour. The geometry of one of the two
[Zn(Hcbt)]ꢀ2DMFꢀ3H2O (3)
Attempts to synthesise large batches of 3 using DMF spiked DMF molecules was restrained with the DFIX and DANG
with water were unsuccessful, resulting instead in crystalline commands. None of the lattice solvent refined stably with
mixtures that predominantly contained 1 that were inter- anisotropic parameters and were thus refined isotropically.
spersed with small amounts of crystalline 3. Characterisation Similarly, hydrogen atoms were omitted from lattice solvents
of 3 was consequently limited to X-ray diffraction studies to maintain stable refinement.
performed on hand-picked individual crystals. Both 1 and 3
adopted similar colourless plate crystal habits, however crystals
Geometric simulations using GASP
of 3 were typically larger (ca. twice the size of 1) with a higher The GASP (Geometric Simulation of Structural Polyhedra) code
quality appearance, rendering them identifiable. Elemental was recently extended12 to handle molecular entities as well as
analysis calcd (%) for C48H45N5O11Zn: C 61.77, H 4.86, N 7.50. regular polyhedra. The code is applied here to take one MOF
crystal structure as input and to impose a new set of cell
Crystallographic data
parameters, either hypothetical or taken from a second crystal
Crystallographic data was collected on a Bruker Apex2 CCD structure. The geometric simulation engine then seeks to adapt
diffractometer at 150 K using synchrotron radiation (Station the contents of the simulation cell to the new cell parameters
11.3.1, Advanced Light Source, Berkeley). Data were processed while retaining the bonding geometry of the input structure
using the supplied BRUKER software.14 Structure solution, and resolving any steric clashes. The code is freely available to
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researchers and can be downloaded from the Github repository The addition of a range of substitute oxidants to the reaction
at https://github.com/EssayWells/GASP_6. Detailed settings for (e.g. FeCl3, H2O2, KMnO4, ceric ammonium nitrate) all failed to
this investigation were as follows. Element radii were assigned improve the isolated yield of H3cbt.
in Å as C 1.35, N 1.4, H 1.0, Zn 0.6, O 1.35. Bonding of zinc to
H3cbt exhibited limited solubility, dissolving only in hot
coordinating oxygens was set to a fixed length using a bar DMF or DMSO. The proton NMR spectrum of H3cbt showed two
constraint assigned to a length of 1.96 Å, to permit flexibility in groupings of aromatic protons: aryl resonances totalling sixteen
the bonding angles about the Zn centre. All other bonds were protons in the region from 7.85–8.20 ppm, and pyridyl
assigned based on the input geometry and the following cluster resonances totalling eight protons at higher ppm values.
building rules: carbon atoms were bonded to adjacent C, H, O, A characteristic singlet at 8.82 ppm that integrated for two
or N atoms within 1.8 Å. Nitrogen atoms were bonded to protons was attributed to the pyridyl ring situated in the central
adjacent C or H atoms within 1.8 Å. Zinc atoms were bonded hub of the ligand (ESI,† S2). Infrared analysis of H3cbt
to adjacent nitrogen atoms within 2.3 Å. By default, GASP would identified a band at 1704 cmꢁ1 with a well-defined shoulder
identify all adjacent sp2-hybridised atoms as members of a at 1662 cmꢁ1, corresponding to the asymmetric carboxylic acid
single rigid cluster; this would make each three-armed ligand stretch of two non-equivalent carboxylic acid groups.
into a single rigid object. However, in this case rotation of the
Crystallographic analysis
bonds between adjacent aromatic rings must be permitted
based on the crystal structure data. The default behaviour Crystalline samples of zinc frameworks 1–3 were obtained by
was therefore suppressed using the ‘‘nodeloc’’ option, allowing adding H3cbt dissolved in hot anhydrous DMF to an equimolar
variations of the dihedral angles between aromatic rings. Steric solution of hydrated zinc nitrate dissolved in either anhydrous
clashes were identified using a coarse grid of edge length 4 Å. DMF for 1; wet DMSO for 2; or wet DMF for a mixture of 1
containing 3. Once combined, each mixture was subsequently
heated at 100 1C in a sealed vessel for two days. The resulting
Results and discussion
MOF samples all possessed similar 2D network topologies and
Zn(II) coordination environments. In each instance the zinc
This study commenced with the development of a novel ligand
atom adopted five-coordinate geometry localised within the
that draws its inspiration from the known H3bbc (1,3,5-tris(40-
tridentate terpy group of the Hcbt ligand. The remaining two
carboxy[1,10-biphenyl]-4-yl)benzene) scaffold.17 The core of the
coordination sites are filled by carboxylate groups derived from
H3bbc ligand was redesigned to contain a terpyridine group,
neighbouring Hcbt ligands. In this way, every Hcbt ligand
thereby enhancing its chelative affinity towards transition
serves to localise a metal node within its terpy group while
metals. This new terpy-tricarboxylic acid, H3cbt, was accessed
simultaneously projecting two framework struts in the form of
using a convergent synthetic strategy in three steps from
coordinating polyaromatic carboxylate arms. Propagation in this
commercially available starting materials (Scheme 1). This
manner yields a (4,4) sheet, in which the non-coordinating
strategy harnessed Suzuki couplings to furnish the two unique
carboxylic acid arm of Hcbt is arrayed orthogonally to the 2D
carboxyl arms, followed by a Hantzsch pyridine synthesis to
sheet, resulting in regular interdigitation of the sheets above and
generate the tripodal core. While each Suzuki coupling could be
below. Tables of crystal and structure refinement data for 1–3 are
achieved in high yield (474%), the Hantzsch pyridine synthesis
provided as ESI† (S3).
only provided an isolated yield of 29%. This low yield likely
occurs due to the aromatization step wherein the pyridyl group
[Zn(Hcbt)]ꢀ4DMF (1)
forms. The oxidation of the dihydropyridine occurs with
The asymmetric unit of 1 contains a single Hcbt ligand chelating
concomitant reduction of the precursor aldol condensation
a zinc centre in the tridentate manner typical of terpyridine
product, as inferred by mass spectrometry evidence (ESI,† S1).
moieties, coupled with four molecules of DMF in the lattice.
One molecule of DMF acts as a hydrogen bond acceptor to the
carboxylic acid group of the non-coordinating Hcbt arm (ESI,†
S4). Two symmetry generated (1/2 ꢁ x, y ꢁ 1/2, 3/2 ꢁ z; 3/2 ꢁ x,
3/2 + y, 3/2 ꢁ z) Hcbt ligands complete the coordination sphere
of the zinc centre, each coordinating in a kO fashion (Fig. 1, left).
The distance between the Zn(II) centre and the terpy group
ranges from 2.0733(17) Å (Zn(1)–N(2)) to 2.1708(18) Å (Zn(1)–
N(1)), with the central nitrogen atom having closest contact to
the metal centre. The coordinating carboxylate oxygen atoms
have interatomic distances of 1.9641(15) Å (Zn(1)–O(1)) and
1.9563(16) Å (Zn(1)–O(4)). The non-coordinating carboxylate
oxygens, O(2) and O(3), are each ca. 3.0 Å from the metal centre,
precluding any form of covalent interaction. The coordination
Scheme 1 Reagents and conditions: (a) 2 : 1 MeOH : H2O, 4-
geometry of 1 approaches perfect trigonal bipyramidal, with a
t value of 0.92.18 The angle formed between the two coordinating
carboxyphenylboronic acid, [Pd(PPh3)4], K2CO3, reflux, 12 h. (b) 2 : 1 H2O :
aq. NH3, 2 : 1 aldehyde : ketone ratio, 100 1C, 30 min microwave heating.
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bipyramidal geometry (t value: 0.88).18 The central ring of the
terpy group is twisted from planarity by ca. 101, which shortens
the Zn(1)–N(2) bond length to 2.051(9) Å. The Zn–O bond
lengths of 2 are analogous to 1, however the coordination
angles vary considerably. Variation in the angles relative to
the terpy group include the O(1)–Zn(1)–N(2) angle of 133.3(4)1
(compared with 141.95(7)1 in 1), and the O(4)–Zn(1)–N(2) of
128.0(4)1 (122.21(7)1 in 1). Furthermore, the coordinating
carboxylate oxygen atoms in
2 are both approximately
Fig. 1 Left: The zinc(II) coordination environment of 1 showing only the
terpy region of the coordinating Hcbt ligand and coordinating benzoate
groups of neighbouring Hcbt ligands. Ellipsoids are shown with 50%
probability. Right and centre: Two stacked 2D sheets (shown in red and
blue) of [Zn(Hcbt)] in 1 as viewed down the crystallographic b and c axes.
Lattice solvents present within the channels have been removed for clarity.
equidistant in location between N(1) and N(3) of the terpy
group, with (O(1)–Zn(1)–N(1) and O(1)–Zn(1)–N(3)) angles of
99.8(3)1 and 98.5(4)1 respectively. This compares to angles of
95.43(7)1 and 105.48(7)1 observed for 1. The angle formed
between the two coordinating benzoate groups (O(1)–Zn(1)–
O(4)) of 2 is 98.6(4)1. The 2D (4,4) sheets no longer stack in a
manner that produces solvated channels along the crystallo-
graphic c axis (Fig. 2, right), instead pronounced channels
form along the crystallographic b axis to accommodate the
additional solvent molecules. A strong hydrogen bond again
provides an ordering influence on the lattice solvent, located
between the carboxylic acid group and a molecule of DMSO,
with an inter-oxygen distance of 2.626(15) Å.
benzoate groups (O(1)–Zn(1)–O(4)) is 95.47(7)1, with asymmetric
carboxylate coordination relative to the terpy group (O(1)–Zn(1)–
N(2): 141.95(7)1, O(4)–Zn(1)–N(2): 122.21(7)1). Similarly, while
O(4) coordinates approximately equidistant between N(1) and
N(3) of the terpy group, O(1) favours coordination with closer
proximity to N(1), (O(1)–Zn(1)–N(1): 95.43(7)1, O(1)–Zn(1)–N(3):
105.48(7)1). The [Zn(Hcbt)] motif propagates to form 2D (4,4)
sheets that adopt offset layers to accommodate interdigitation of
the non-coordinating carboxylic acid arms through voids made
by the sheet above and below (Fig. 1, right). Solvated channels
exist within the structure to accommodate the four lattice
molecules of DMF. These channels propagate along the crystal-
lographic c axis and are ordered by a strong hydrogen bonding
interaction from the non-coordinating arm of the Hcbt ligand.
The inter-oxygen distance between the donor carboxylic acid
group and the acceptor DMF molecule is 2.636(3) Å.
[Zn(Hcbt)]ꢀ2DMFꢀ3H2O (3)
Altering the synthesis of 1 to make use of wet DMF produced a
mixture of products. The major product remained 1 however
this was now interspersed with small amounts of a new
hydrated form 3. The asymmetric unit of 3 differs from 1 in
that it contained only two molecules of DMF, with the remaining
sites occupied by three molecules of water, one of which showed
positional disorder. While the zinc(II) atom remained five-
coordinate, a striking difference was seen in the carboxylate
coordination mode (Fig. 3, left). One of the carboxylate groups
has pivoted, resulting in substitution of O(1) coordination for
O(2) coordination to the metal centre. The newly formed
Zn(1)–O(2) bond in 3 is slightly longer than the former Zn(1)–
O(1) bond in 1 (1.980(5) Å versus 1.9641(15) Å). The angles
observed about the Zn(II) centre also vary considerably relative
to the parent motif, exemplified by O(2)–Zn(1)–O(4) angle of
104.3(3)1 (former angle: 95.47(7)1 for O(1)–Zn(1)–O(4) in 1), and
carboxylate coordination angles relative to the terpy group of
[Zn(Hcbt)]ꢀ4DMSOꢀ1.5H2OꢀDMF (2)
The addition of wet DMSO to the solvent mixture of 1 resulted
in isolation of a second 2D framework that contained DMSO,
DMF and water within its pores. In addition to a Hcbt ligand
chelating zinc(II) via the terpy group, the asymmetric unit of 2
also contained four molecules of DMSO, one molecule of DMF
and two molecules of water, of which one exhibited half
occupancy. The zinc(II) centre is made five-coordinate by a
pair of carboxylate groups that coordinate analogously to 1
(Fig. 2, left), albeit with greater distortion from trigonal
Fig. 2 Left: The zinc(II) coordination environment in 2 showing only the
terpy region of the coordinating Hcbt ligand and coordinating benzoate
groups of neighbouring Hcbt ligands. Ellipsoids are shown with 50%
probability. Right: Stacked 2D sheets (shown in red and blue) of
[Zn(Hcbt)] in 2 as viewed down both the crystallographic b and c axes.
Lattice solvents present within the channels have been removed for clarity.
Fig. 3 Left: The zinc(II) coordination environment in 3 showing only the
terpy region of the coordinating Hcbt ligand and coordinating benzoate
groups of neighbouring Hcbt ligands. Ellipsoids are shown with 50%
probability. Right: Two stacked 2D sheets (shown in red and blue) of
[Zn(Hcbt)] in 3 as viewed down the crystallographic b and c axes. Solvent
molecules present within the structure have been removed for clarity.
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O(2)–Zn(1)–N(2): 123.9(2)1 and O(4)–Zn(1)–N(2): 131.5(2)1
(formerly: 141.95(7)1 and 122.21(7)1 in 1 respectively). Because
of these changes in geometry, the zinc coordination sphere of 3
deviates considerably from perfect trigonal bipyramidal, with a
t value of 0.77.18 The net influence of these conformational
changes is a twisting effect on the (4,4) sheet motif that restricts
continuity of the solvent channels down both the crystallo-
graphic b and c axes (Fig. 3, right).
Modes of conformational flexibility in 1–3
Multiple modes of flexibility can be identified within structures
1–3 which hold relevance for GASP modelling. Globally, the
structures consist of interdigitated 2D sheets that stack along
the crystallographic c axis. In every layer the non-coordinating
H3cbt arms protrude above or below into the quadrilateral
interstices of adjacent layers. There are no strong interactions
connecting stacked layers, meaning that in the absence of
lattice solvents the layers would experience two degrees of
translational freedom orthogonal to the c axis. In practice,
the presence of lattice solvents restricts this type of motion.
Instead, two other mechanisms of conformational flexibility are
observed within 1–3: (1) hinge-like changes to carboxylate
coordination of the Zn(II) atom that influence both the angle
and relative orientation of the ligand, and (2) distortion of the
Hcbt ligand away from planarity and linearity.
The first mechanism, hinge-like variation of the coordina-
tion angle between carboxylates and the Zn(II) atom, enables
the MOF channels to expand or contract in response to variable
solvent loadings. Accommodating sterically challenging solvents
appears to drive this effect, with substitution of larger DMF
molecules with water in 3 promoting a closed-channel form,
whereas accommodation of sterically challenging DMSO
molecules drives formation of an open-channel form in 2. This
hinge-like effect has been quantified in structures 1–3 by
measuring the angle made between coordinating benzoate ring
centroids in relation to the zinc atom. The observed range varied
from 991 (3) to 1201 (2), as shown in Fig. 4.
Fig. 5 Distortions to the Hcbt ligand observed in structures 1–3. The
distortions take the form of bending of the tripodal arms, and in the case of
2, a 101 distortion from planarity across the terpy group (see also Fig. 4).
In framework 2 planarity of the terpy group is broken by ca. 101
(Fig. 4), which reorients the central Hcbt arm, and by extension,
alters the stacking of interdigitated sheets. Simultaneously one
of the remaining two terminal benzoates also deviates from
linearity by ca. 111. This inherent flexibility of the Hcbt ligand is
one possible explanation for the lower-than-expected porosity
value (vide infra) for the activated open-channel form of 2.
The ability of the ligands to distort provides a means for the
framework to close empty channels as they desolvate.
GASP modelling of flexibility at zinc centres
The X-ray diffraction studies of structures 1–3 identified several
classes of movement that recommend this family of structures
for further analysis using the geometric simulation approach
implemented by the software GASP. In geometric simulation,
the input geometry of a structure is maintained by a system of
templates, while the unit cell parameters are varied. This allows
the mechanisms by which the flexible framework responds to
the imposed strain to be observed. GASP was originally
developed for mineral frameworks composed of polyhedra,
e.g. the aluminosilicates; however a recent extension of the
logic permits it to handle the organic ligands typical of MOF
frameworks as well.12 In this case, GASP allows us to investigate
the significance of the change in carboxylate coordination
mode in structure 3 compared to 1 and 2. Details of the
simulation approach are described in the Methods section,
and all simulation files are provided as ESI.†
The second mode of flexibility quantified by this study is
distortion of the Hcbt ligand away from planarity and linearity.
Fig. 5 shows that framework 3 contains the least distorted Hcbt
ligand in that the tripodal arms of Hcbt remain linear, and the
terpy group planar. In contrast, while framework 1 retains a
planar terpy group, the terminal benzoate/benzoic acid
groups of all three arms bend away from linearity by 8–141.
The flexibility of each structure can be examined by imposing
the cell parameters of a different structure upon it; GASP then
seeks to adapt to the new cell while maintaining the local
bonding geometry defined by the original input structure.
A particularly intriguing result is obtained using structure 3 as
input. In Fig. 6, panel A shows a unit cell of structure 3 viewed
along the crystallographic c axis. Panel B shows a simple scaling
Fig. 4 A demonstration of hinge-like flexibility observed at the zinc
coordination sphere for structures 1–3. Both the angle, orientation and
coordination mode of the Hcbt ligand influences the size of the resulting
channels within the motif. Angles are rounded to the nearest integer.
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Fig. 6 Panel A: One unit cell of framework 3, viewed along the crystallographic c axis. Panel B: Uniform scaling of framework 3 to the cell parameters of
framework 1. Panel C: Geometric simulation of framework 3 with the cell parameters of framework 1. Panel D: One unit cell of framework 1, for
comparison. Panel E: Zinc coordination in framework 3 with uniform scaling. Panel F: Zinc coordination in framework 3 with geometric simulation,
showing rotation of carboxylate unit. Panel G: Unit cell from geometric simulation of framework 1 with cell parameters comparable to framework 3.
Panel H: Zinc coordination in framework 1 with geometric simulation, showing free adaptation of zinc-carboxylate binding. In all panels, the metal,
oxygen and zinc atoms are shown with radii comparable to the values used in the simulations (see Methods), while the radii of carbon atoms are reduced
for clearer viewing.
of the structure, in which the fractional coordinates are left Topology of interdigitation
unchanged but the cell parameters of structure 1 are imposed.
Panel C shows the result of geometric simulation, seeking to
maintain the bonding geometry of structure 3 with the cell
parameters of structure 1. This structure clearly differs from
the simple scaling case, particularly in the arrangement of the
carboxylate oxygen atoms around the zinc centre. This simulated
structure differs in turn from the experimental structure 1 which
is shown for comparison in Panel D. Panel E shows the geometry
Further insights regarding the differences between frameworks
1–3 become evident when the topology of successively stacked
layers along the c axis are considered. Each layer provides a
chequerboard of roughly square interstices, into which the
central arms of Hcbt ligands from the layers above and below
insert. Due to the corrugation of the layers, adjacent interstices
are topologically distinct, which in turn leads to two distinct
possible topologies for interdigitation of the layers.
of the terpy-zinc-carboxylate unit in the simple scaling case (i.e. a
detail from the structure of Panel B). The scaling has clearly
introduced distortions to the geometry of the aromatic rings.
In all three structures the unit cell contains an upper layer
whose zinc atoms lie at z coordinates of approximately 5/6 and
4/6, and a lower layer whose zinc atoms lie at z coordinates of
Panel F shows the geometry of the terpy-zinc-carboxylate unit in
the geometric simulations (i.e. a detail from the structure of
Panel C). It can be inferred that as the structure sought to adapt
while maintaining its bonding geometry, the carboxylate unit
rotates to bring the other oxygen atom into contact with the zinc
centre – a reversal of the hinging motion noted between structures
1 and 3. Due to this rotation, the simulated structure contains
substantial steric clashes, on the order of 0.1 Å, with consequent
smaller distortions to the bonding geometry nearby. Thus, there is
a direct link in framework 3 between changes in the global
geometry, i.e. closure of the channels, and locally in terms of
the carboxylate binding mode. If, by contrast, the cell parameters
of structure 2 are imposed upon structure 1, or vice versa, and the
structures are relaxed in geometric simulations, no such clashes
or changes in carboxylate binding mode are observed. It is
instructive to observe the result when the framework of structure
1 is used as the input, and cell parameters comparable to frame-
work 3 are imposed upon it, as illustrated in Panel G. This
represents a flexion of the channels, making them narrower, as
would be expected if the channels contained smaller solvent
molecules such as water. The carboxylate binding mode in frame-
work 1 affords the structure more conformational freedom: in the
approximately 2/6 and 1/6. By aligning all three structures on
the upper layer the stacking topologies can be observed and
directly compared. In Fig. 7, Panel A shows a 2 ꢂ 2 ꢂ 1 supercell
of the structure, viewed along the [001] direction. The zinc
atoms are coloured by increasing z coordinate in order blue,
green, yellow, orange. Quadrilateral outlines highlight an inter-
stice in the upper layer overlaid by one in the lower layer. Note
particularly the relative position of the uppermost (orange) and
lowest (blue) zincs. The tail of the ligand coordinating the blue
zinc atom inserts into the yellow-highlighted interstice of the
other layer.
Framework 2 can be aligned by a simple translation of the
unit cell contents, bringing the upper layer into an analogous
position and orientation to that of framework 1. The alignment
of the terpy units around the zinc centres provides an
unambiguous definition of the layer orientation; note the
alignment of the three nitrogen atoms coordinating each zinc.
Panel B shows analogous interstices highlighted as per Panel A.
The topology of interdigitation is opposite to that seen in
framework 1. It is now the green, not the blue, zinc centres
that align with the yellow-highlighted interstice in the opposing
layer, meaning that the free arm of Hcbt coordinating the blue
geometric simulation, the structure adapts flexibly to the new cell zinc centre inserts into a different interstice. Thus, although
parameters and no new clashes are generated. frameworks 1 and 2 are similar in terms of their cell parameters
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being a consequence of lesser ligand distortion inherent to
this motif (i.e. Fig. 5). The discrepancy between the number of
DMF molecules observed thermogravimetrically as opposed to
crystallographically for 1 is likely a result of solvent loss during
the overnight air-drying step prior to analysis. Bulk purity of
powder material produced on larger scale was determined by
both elemental analysis and PXRD methods (ESI:† Fig. S6).
The nitrogen values of the elemental analysis were slightly
lower than expected, which we attributed to known issue of
incomplete oxidation affecting heterocyclic nitrogen species
such as H3cbt.19 Interconversion between the two unique
solvates by suspending samples of 1 or 2 in either neat DMF
or DMSO proved unsuccessful, and similarly 3 could not be
accessed by suspending 1 in water (ESI:† Fig. S6). These results
can now be interpreted with respect to the differences in
interdigitation topology and carboxylate binding mode among
the structures as modelled by GASP. The geometric simulations
of flexibility about the zinc centres highlight that framework 1
could adapt flexibly to accommodate water in narrower channels
without needing to alter carboxylate binding mode to that seen
in framework 3, however the differences in interdigitation
topology would necessitate complete delamination and
reformation of the layered stacking for this to occur, which
ultimately precludes interconversion, as was observed experi-
mentally (ESI,† S9).
Fig. 7 In all panels, zinc atoms are colour coded: upper layer, orange (z =
5/6) and yellow (z = 4/6), lower layer, green (z = 2/6) and blue (z = 1/6).
Overlapping interstices are highlighted in the upper layer (yellow transpar-
ency) and the lower layer (blue transparency). Panel A, upper left: framework
1, 221 supercell, viewed along the [001] direction, with the [010] direction
pointing up the page. Panel B, lower left: framework 2, 221 supercell, viewed
as in Panel A. Panel C, right: framework 3, 221 supercell, viewed along the
[001] direction with the [100] direction pointing up the page.
and the geometry of their individual layers, they remain
topologically distinctive, and cannot interconvert without com-
plete delamination of the layers.
Framework 3 is an interesting case in that its upper layer
must undergo both a translation of unit cell contents, and a 901
rotation about the [001] direction to adopt an analogous
position and orientation to that of framework 1. As noted, it
is the orientation of the terpy units about the zincs which
defines the layer orientation. The structure is displayed in
Panel C with analogous interstices highlighted. Note that in
this panel the [100] direction is aligned with the page, whereas
in Panels A and B it is the [010] direction that is aligned to the
page. The topology of interdigitation is clearly the same as seen
in framework 2 (Panel B), and different to that of framework 1
(Panel A); note that it is once again the green, not the blue, zinc
which aligns with the yellow-highlighted interstice of the other
layer. The different shape of the interstices in framework 3, as
compared to 1 and 2, is also visible in this comparison.
Bulk powder samples of 1 and 2 were activated by heating at
150 1C under high vacuum for eight hours. BET surface area
calculations were then performed on these activated samples
that yielded near equivalent values of 248 m2 gꢁ1 for 1, and
269 m2 gꢁ1 for 2 after a nitrogen adsorption–desorption
experiment. The first value is in good agreement witꢁh1 the
predicted value obtained for crystalline 1 of 206 m2
g . By
contrast, the predicted BET surface area for 2 was 537 m2 gꢁ1
,
ca. twice the observed value. PXRD analysis performed on the
activated powders show retention of crystallinity in both
samples (ESI,† S9). Both activated materials show variations in
both intensity and peak position relative to the ‘as synthesised’ 1
and 2, particularly at low 2y angles. We attribute these variations
to changes in the topological structure as the frameworks contort
Bulk characterisation of 1 and 2
Frameworks 1 and 2 could be accessed in sufficient quantities to minimise void space upon activation. Unique patterns of peaks
to facilitate their thorough characterization. Infrared analysis and intensities observed in the PXRD patterns of activated 1 and 2
performed on crystalline samples identified the expected shift support GASP’s assertion that the topologies of interdigitation for
in carbonyl band from 1704 cmꢁ1 (H3cbt) to 1601 cmꢁ1 (1) and these frameworks are non-interchangeable (i.e. Fig. 7). This makes
1604 cmꢁ1 (2) upon deprotonation and coordination to zinc. it impossible for either framework to structurally mimic the other
The carbonyl bands from the non-coordinating carboxylic acid without a complete delamination of the assembled layers.
within the frameworks were observed at 1719 cmꢁ1 for 1, and Modelling from GASP also supports the finding that the two
1702 cmꢁ1 for 2. Thermogravimetric analysis (TGA) of 1 showed frameworks can adopt similar conformations upon activation to
a mass loss of 11.2% in the range of 30 1C to 300 1C corres- minimise void space, explaining the near-identical BET surface
ponding to loss of two molecules of DMF (predicted: 14.5%). area calculations determined for 1 and 2.
Framework decomposition was then observed in two stages
(onset at 330 1C and 510 1C). Framework 2 displayed a two-step
solvent loss profile spanning from 30 1C to 330 1C for a mass
loss of 32.8%, equating to four molecules of DMSO and a single
Conclusions
molecule of DMF (predicted: 33.9%). Framework 2 exhibited a Synthesis of a new terpyridine-functionalised tricarboxylic acid
similar decomposition profile to 1 from 360 1C onwards (ESI,† ligand, H3cbt, has allowed the crystallographic characterisation
S5), with the slight improvement in thermal stability likely of three new zinc(II) MOFs, two of which were further
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Chem. Commun., 2014, 50, 14436–14439; (d) W. J. Gee,
L. E. Hatcher, C. A. Cameron, C. Stubbs, M. R. Warren,
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M. F. Mahon, C. Richardson, V. M. Sebestyen, D. Turski
and M. R. Warren, CrystEngComm, 2017, 19, 5549–5557.
2 (a) M. Eddaoudi, J. Kim, N. Rosi, D. Vodak, J. Wachter,
M. O’Keeffe and O. M. Yaghi, Science, 2002, 295, 469–472;
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characterized by elemental analysis, TGA, and IR spectroscopy.
BET measurements performed on activated powdered samples
of 1 and 2 yielded analogous BET surface areas, each being
consistent with the predicted values of motif 1. Geometric
simulation using the code ‘GASP’ unambiguously demon-
strated that frameworks 1 and 2 are topologically distinct,
and thus unable to interconvert without delamination of their
constituent layers. It also showed that the topology of inter-
digitation of framework 3 matched that of framework 2.
Furthermore, GASP was able to justify structural trends in
global geometry after identifying links between framework
channel size and local features such as carboxylate binding
modes. It primarily achieved this by assessing changes to the
magnitude of steric clashing when simulating variations to the
carboxylate-zinc binding region (i.e. hinge-like motion) while
allowing for rotation of adjacent aromatic rings in the arms of
the Hcbt ligand. The analogous BET surface areas of activated 1
and 2 was also justified using GASP, with modelling showing
the feasibility of conformational rearrangement of activated
framework 2 to approximate 1 based on geometric and topological
grounds. In summary, the insights from geometric simulations
performed on this new family of flexible MOFs provide a
compelling demonstration of GASP’s applicability to challenging
MOF systems. It is hoped that this proof-of-concept
work improves understanding and enhances the modelling of
mechanisms and barriers to conformational change in flexible
MOF motifs.
3 (a) S. R. Batten, S. M. Neville and D. R. Turner, Coordination
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Author contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version
of the manuscript. Geometric simulations were carried out by
SAW and interpreted in discussion with all authors.
6 (a) C. Mellot-Draznieks, C. Serre, S. Surble, N. Audebrand
Conflicts of interest
´
and G. Ferey, J. Am. Chem. Soc., 2005, 127, 16273–16278;
(b) S. Surble, C. Serre, C. Mellot-Draznieks, F. Millange and
G. Ferey, Chem. Commun., 2006, 284–286.
The authors declare no competing financial interests.
´
7 (a) W. Zhou, H. Wu, T. Yildirim, J. R. Simpson and
A. R. Hight Walker, Phys. Rev. B: Condens. Matter Mater.
Phys., 2008, 78, 0541141; (b) L. D. DeVries, P. M. Barron,
E. P. Hurley, C. Hu and W. Choe, J. Am. Chem. Soc., 2011,
133, 14848–14851.
Acknowledgements
We are grateful to the EPSRC for financial support of the
project (EP/K004956/1). We would also like to thank the ALS,
LBNL for the beamtime to perform these measurements. The
Advanced Light Source is supported by the Director, Office of
Science, Office of Basic Energy Sciences, of the U.S. Department
of Energy under Contract No. DE-AC02-05CH11231.
8 K. W. Chapman, G. J. Halder and P. J. Chupas, J. Am. Chem.
Soc., 2009, 131, 17546–17547.
9 J. Park, D. Yuan, K. T. Pham, J.-R. Li, A. Yakovenko and
H.-C. Zhou, J. Am. Chem. Soc., 2012, 134, 99–102.
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Phys. Biol., 2005, 2, S127–S136; (b) S. A. Wells, S. J. Crennell
and M. J. Danson, Proteins: Struct., Funct., Bioinf., 2014, 82,
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