2D porphyrinic metal-organic frameworks featuring rod-shaped secondary building units

Article abstract of DOI:10.3390/molecules26102955

Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with diverse potential applications including gas storage, molecular separation, sensing and catalysis. So-called ‘rod MOFs’, which comprise infinitely extended 1D secondary

Full text of DOI:10.3390/molecules26102955

molecules
Article
2
D Porphyrinic Metal-Organic Frameworks Featuring
Rod-Shaped Secondary Building Units
1
2
1
1
1
, Mathias O. Senge 3
Rory Elliott , Aoife A. Ryan , Aviral Aggarwal , Nianyong Zhu , Friedrich W. Steuber
1
,
and Wolfgang Schmitt *
1
2
3
School of Chemistry & AMBER Centre, Trinity College, University of Dublin, Dublin, Ireland;
relliot@tcd.ie (R.E.); aviralaggarwal23@gmail.com (A.A.); zhun@tcd.ie (N.Z.); steuberf@tcd.ie (F.W.S.)
School of Chemistry, Chair of Organic Chemistry, Trinity Biomedical Science Institute, 152-160 Pearse Street,
Trinity College, The University of Dublin, Dublin, Ireland; ryana16@tcd.ie
Focus Group—Molecular and Interfacial Engineering of Organic Nanosystems, Institute for Advanced
Study (TUM-IAS), Technical University of Munich, Lichtenberg-Str. 2a, 85748 Garching, Germany;
mathias.senge@tum.de
*
Correspondence: schmittw@tcd.ie
Abstract: Metal-organic frameworks (MOFs) encompass a rapidly expanding class of materials with
diverse potential applications including gas storage, molecular separation, sensing and catalysis.
So-called ‘rod MOFs’, which comprise infinitely extended 1D secondary building units (SBUs),
represent an underexplored subclass of MOF. Further, porphyrins are considered privileged ligands
for MOF synthesis due to their tunable redox and photophysical properties. In this study, the CuII
ꢀ
ꢁꢂꢀꢃꢄꢅꢆꢇ
II
complex of 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H2
L
-Cu , where H refers to the
2
ꢀ
ꢁꢂꢃꢄꢅꢆ
ligand’s carboxyl H atoms) is used to prepare two new 2D porphyrinic rod MOFs PROD-1 and
Citation: Elliott, R.; Ryan, A.A.;
Aggarwal, A.; Zhu, N.; Steuber, F.W.;
Senge, M.O.; Schmitt, W. 2D
II
II
PROD-2. Single-crystal X-ray analysis reveals that these frameworks feature 1D Mn - or Co -based
rod-like SBUs that are coordinated by labile solvent molecules and photoactive porphyrin moieties.
Both materials were characterised using infrared (IR) spectroscopy, powder X-ray diffraction (PXRD)
spectroscopy and thermogravimetric analysis (TGA). The structural attributes of PROD-1 and PROD-
Porphyrinic Metal-Organic
Frameworks Featuring Rod-Shaped
Secondary Building Units. Molecules
2
render them promising materials for future photocatalytic investigations.
2
021, 26, 2955. https://doi.org/
Keywords: metal-organic framework; MOF; 2D MOF; 2D materials; rod MOF; Porphyrin MOF;
1
0.3390/molecules26102955
Porphyrinoids; coordination chemistry
Academic Editor: Constantina
Papatriantafyllopoulou
Received: 1 April 2021
Accepted: 12 May 2021
Published: 16 May 2021
1. Introduction
Over the last decades, MOFs have attracted substantial scientific attention [1,2]. Mem-
bers of this emerging class of modular, metallo-supramolecular polymeric materials can be
conceptualised as repeating combinations of inorganic secondary building units (SBUs or
Publisher’s Note: MDPI stays neutral
with regard to jurisdictional claims in
published maps and institutional affil-
iations.
‘
nodes’) that are bridged by multitopic organic ligands (or ‘linkers’) [
gives infinitely extended multidimensional framework structures with long-range order
and high crystallinity [ ]. As MOFs demonstrate chemical tunability and high surface areas,
3]. This arrangement
4
they are versatile materials with the potential to advance technologies to tackle several
substantial scientific challenges [5,6].
To date, tens of thousands of MOFs have been reported in the literature [
7–11]. The vast
majority of these structures, including the archetypal frameworks MOF-5 [Zn O(BDC) ]
4
3
Copyright: © 2021 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
(
BDC = 1,4-benzene dicarboxylate) and HKUST-1 [Cu (BTC) (H O) ] (HKUST = Hong
3 2 2 3
Kong University of Science and Technology, BTC = benzene-1,3,5-tricarboxylate), comprise
discrete SBUs [12 13]. Topologically, these frameworks can be understood by abstracting
,
their sub-components as simple geometric shapes such as triangles, squares, tetrahedra,
etc., which interconnect to yield a network structure [14,15].
The inimitable modularity and surface areas of MOF materials grants them broad
applicability in areas such as gas storage, molecular separation, drug delivery, sensing,
4
.0/).
Molecules 2021, 26, 2955. https://doi.org/10.3390/molecules26102955
https://www.mdpi.com/journal/molecules

Molecules 2021, 26, 2955
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and spintronics [16
–
21]. MOFs can accommodate well-defined, physically separated redox-
active sites, which also makes them attractive compounds for catalytic applications [22–26].
Frameworks featuring open metal sites (non-coordinatively saturated inorganic nodes)
or labile coordinated solvent moieties are particularly promising for catalysis, as these
aspects allow substrates to bind with the metal ions of a MOF’s SBU [27
catalytic processes can occur at redox-active metal ions that are embedded within the
linkers of certain MOFs, for example porphyrin-based MOFs [29 30]. MOFs are also prime
,28]. Alternatively,
,
materials for a myriad of electrochemical technologies such as photovoltaics, fuel cells,
batteries and supercapacitors [31]. However, porous 3D MOFs constructed from redox-
inactive linkers are typically electronically insulating, which limits their suitability for these
applications [32,33].
2
D MOFs or coordination polymers with layered architectures are a subclass of MOF
with distinctive dimensional-dependent characteristics, including exposed surface sites
and good flexibility and mechanical stability [34]. The layered architectures of these MOFs
can give rise to exceptional electronic, optical and magnetic properties [35
–37]. Moreover,
2
D MOFs often exhibit favourable conductivities, as lower dimensional systems limit
charge carrier scattering [38]. Their unique attributes make layered metal-organic materials
promising systems for catalysis, conductive devices, and “smart membranes” [39–41].
Among the various organic ligands used in MOF synthesis, porphyrins and metallopor-
phyrins are advantageous due to their unique photophysical and redox properties [30]. These
tetrapyrrolic macrocycles are ubiquitous in nature, where they execute essential functions for
fundamental biological processes including catalysis, photosynthesis, and gas transport [42,43].
Introducing coordinating carboxylic acid or pyridyl functional groups at a porphyrin’s meso
positions allows a macrocycle to be incorporated within a MOF as a rigid, functional linker [44].
In 1991, Robson and co-workers reported the first porphyrinic MOF, which was prepared in a
II
reaction between [5,10,15,20-tetrakis(4-pyridyl)porphyrinato]palladium(II) (MTPP-Pd ) and
Cd(NO )
·
4H O in a mixture of boiling MeOH and H O. This 3D framework, formulated as
3
2
2
2
II
II
II
[
Cd (MTPP-Pd )(NO ) (H O) ]
·
5H O, features mononuclear Cd nodes that are bridged by
2
3 4
2
4
2
palladium tetrapyridyl porphyrin linkers [45]. Subsequent to the discovery of this prototypal
porphyrinic framework, significantly more porphyrin-based MOFs have been developed with
a wide range of potential applications such as molecular separation, light-harvesting, and
photocatalysis [46–53]. The facile tunability of metalloporphyrins has expedited this effort, as
simply substituting the central metal ion or
β-pyrrole position of a porphyrin yields a linker
that can impart improved functionality to a framework.
In contrast to MOFs constructed from finite SBUs, rod MOFs represent a subclass of
frameworks with infinitely extended 1D nodes containing chains of periodically repeating
metal ions linked by polytopic ligands [54–56]. Rod MOFs afford significant advantages
over other MOFs, including a lower tendency to form interpenetrated networks and a
higher propensity to stabilise open metal sites. Despite this, rod MOFs remain relatively
underexplored in the literature [57,58].
In natural light-harvesting systems, highly ordered supramolecular architectures fun-
nel solar energy towards a reaction centre [59
regular arrays of proximally positioned -conjugated porphyrin ligands in porphyrinic
MOFs can promote long-range charge transport via networks of stacking interac-
tions [61 65]. Hupp et al. recently investigated exciton migration in two porphyrinic
,60]. Analogously to biological systems, the
π
π–π
–
MOFs using fluorescence quenching experiments, revealing that these frameworks fa-
cilitate long range anisotropic energy transfer over up to ca. 45 porphyrin struts [46].
This study suggests that porphyrin-based MOFs may be useful bioinspired materials for
photoelectrochemical applications, for example as thin film electrode coatings within arti-
ficial photosynthetic devices [66–69]. Similarly, light-harvesting porphyrins and related
metal-organic materials with favourable charge transport characteristics can be applied in
dye-sensitized or perovskite solar cells to increase the efficiencies of such energy conversion
devices [70–75].

Molecules 2021, 26, 2955
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II
II
In this study, two novel porphyrinic rod MOFs [Mn (L-Cu )(MeOH) ]
·
DEA
·
MeOH
2
II
II
(
PROD-1, DEA = (N,N-diethylacetamide) and [Co (L-Cu )(DEA)]
·
8MeOH (PROD-2) are
II
reported. These compounds comprise distinct, infinitely extended rod-shaped Mn - or
Co -based SBUs that are connected by porphyrin ligands, giving rise to 2D sheet architec-
II
tures. Both PROD-1 and PROD-2 are characterised using single-crystal X-ray diffraction,
infrared (IR) spectroscopy, powder X-ray diffraction (PXRD) spectroscopy and thermo-
gravimetric analysis (TGA). Ultimately, due to the presence of specific structural motifs in
these frameworks, some potential applications of these materials are discussed.
2. Results and Discussion
2.1. Synthesis of Metalloporphyrin Rod MOFs
The ditopic porphyrin 5,15-bis(4-carboxyphenyl)-10,20-diphenylporphyrin (H L) was
4
selected to synthesise MOFs endowed with the unique electronic and photophysical prop-
erties of porphyrins. This choice was rationalised due to the rich chemical diversity of
MOFs constructed from other, more rudimentary dicarboxylate linear linkers, and because
at the time of writing only two MOFs reported in the Cambridge Structural Database
(CSD, Version 5.41) contain this underexplored porphyrin [76–82]. Additionally, the cen-
tral cavity of H L can accommodate an array of metal ions, which facilitates further
4
tuning of frameworks constructed using this linker [83
light-harvesting [85] and catalytic [86] properties of comparable Cu metalloporphyrin
,
84]. Considering the promising
II
complexes, and the low cost and toxicity of Copper, [5,15-bis(4-carboxyphenyl)-10,20-
II
diphenylporphyrinato]copper(II) (H L-Cu ) was prepared in moderate yield (see experi-
2
mental for details) [87,88].
II
II
II
H L-Cu was used to prepare two 2D MOFs [Mn (L-Cu )(MeOH) ]
·
DEA MeOH
·
2
2
II
II
II
II
(
PROD-1) and [Co (L-Cu )(DEA)]
·
8MeOH (PROD-2) with 1D Mn - or Co -based SBUs,
II
respectively (Figure 1). PROD-1 was prepared by heating H L-Cu and MnCl
in a mixture of DEA and MeOH to 120 C for four days in a Teflon-lined stainless-steel
autoclave. Slowly cooling this reaction mixture to room temperature afforded the formation
·
2H O
2
2
2
◦
of uniform, rod-shaped, crimson crystals of PROD-1. Similarly, PROD-2 was synthesised
II
by heating H L-Cu and CoCl in a mixture of DEA, MeOH and acetic acid (AcOH) under
2
2
solvothermal conditions. After four days, this reaction mixture was slowly cooled to
ambient temperature, resulting in the formation of red, plate-shaped crystals of PROD-2.
Both products PROD-1 and PROD-2 form reproducibly, in good yields and were of suitable
quality for analysis using single-crystal X-ray diffraction.
II
II
2
.2. Crystal Structure of [Mn (L-Cu )(MeOH) ]·DEA·MeOH (PROD-1)
2
The single-crystal X-ray structure of PROD-1 was solved in the triclinic space group
P1. This analysis was hampered due to twinned crystals and weak diffraction. However,
the data allowed us to establish the structure’s connectivity. PROD-1 comprises stacking
II
2
D sheets, containing 1D rod-like Mn -based SBUs that extend infinitely in the direction
of the crystallographic a-axis (Figure 2a). The SBUs of PROD-1 are each linked to two
II 2−
other identical inorganic nodes by ditopic (L-Cu ) ligands (Figure 2b). This connectivity
gives rise to 2D sheets which extend with the crystallographic ac-plane, and in which rod-
shaped SBUs stack in parallel with one another (Figure 2c,d). Neighbouring interconnected
inorganic nodes within the 2D sheets are separated by ca. 22 Å.
II
II 2−
The asymmetric unit of PROD-1 contains one Mn centre, one (L-Cu ) ligand, two
coordinated MeOH solvent moieties, and constitutional DEA and MeOH solvent molecules
which locate between the small channels that extend between the MOF’s 2D layers. The
II
SBU of PROD-1 comprises an infinite chain of octahedrally coordinated Mn centres, each
of which connect to two adjacent metal ions through four bridging carboxylate moieties
II 2−
II
that derive from four (L-Cu ) linkers. The interatomic distance between two Mn centres
within the 1D SBU is ca. 4.6 Å.

Molecules 2021, 26, 2955
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II
Figure 1. H2
L-Cu was used to prepare two 2D porphyrinic rod MOFs, PROD-1 and PROD-2. Insets show the 1D chain SBUs
of PROD-1 and PROD-2. H atoms and solvent molecules are omitted for clarity. Colour scheme: C white, N blue, O red, Cu
II
II
orange, Mn pink, Co violet. Mn and Co coordination environments are represented by blue and red polyhedra, respectively.
Figure 2. View of the structure of PROD-1 in the crystal, showing (
a
) the MOF’s rod-shaped SBU which comprises an
II
infinite 1D chain of octahedrally coordinated Mn ions bridged by the syn–syn coordinating carboxylate functionalities
II 2− II 2−
of (L-Cu ) linkers, (b) the linear metalloporphyrin ligand (L-Cu ) and (c,d) the packing arrangement of PROD-1,
highlighting the MOF’s layered architecture. H atoms and solvent molecules are omitted for clarity. Colour scheme: C
II
white, N blue, O red, Mn pink, Cu orange. Mn coordination environments are shown as blue polyhedra.

Molecules 2021, 26, 2955
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II
The binding environment of the Mn centre Mn(1) within the 1D SBU of PROD-1 is
shown in Figure 3a. The coordination sphere of Mn(1) comprises four O-donors O(1), O(3),
1
1
O(5) and O(6) from four distinct syn–syn bridging, µ2-η :η binding carboxylate function-
alities and two O-donors O(2) and O(4) which derive from two ‘cis’-coordinated MeOH
solvent moieties. The bond distances between Mn(1) and each of the four carboxylate
II
O-donors are within the range 2.15–2.17 Å, whereas the distances from the Mn ion and
the more labile, monodentate MeOH moieties range between 2.21 Å and 2.23 Å. The bond
angles surrounding Mn(1) render its coordination geometry a slightly distorted octahedron,
II
and are consistent with values reported for comparable Mn -carboxylate complexes in the
literature [89,90].
Figure 3.
(
a
) The coordination environment of the MnII ion Mn(1) within the 1D chain SBU of PROD-1, which includes two
) a wireframe representation highlighting stacking interactions
MeOH solvent-derived O-donors O(2) and O(4), and (
b
π–π
which stabilize PROD-1. H atoms have been omitted for clarity. Colour scheme: C white, N blue, O red, Cu orange, Mn
II
pink. Blue polyhedra represent Mn coordination environments. π–π interactions are highlighted using dashed red lines.
An extensive network of
which are shown in Figure 3b. For example, T-shaped intersheet
π
–
π
stacking interactions stabilise this structure, some of
stacking interactions
π–π
II 2−
between the meso-phenyl and tetrapyrrole moieties of (L-Cu ) linkers, and parallel-
displaced intrasheet stacking interactions between adjacent metalloporphyrins are
π–π
highlighted. These supramolecular interactions propagate in parallel with the crystallo-
graphic b-axis and extend the 2D network into a 3D framework. The distances between
π–π interacting moieties in PROD-1 are within the range of 3.4–3.8 Å, which is consistent
with π–π stacking interactions reported in the literature [91,92].
The porphyrin linkers within PROD-1 adopt saddle-shaped configurations and have
staggered meso-functionalities [93]. The bond angles between ‘trans’-coordinated pyrrolic
◦
N-donors deviate from the ideal square planer angle by up to ca. 8 , whilst the dihedral
◦
angles between meso-carboxyaryl and meso-phenyl functional groups are ca. 27(1) and
3(1) , respectively. This configuration facilitates stabilisation of the structure through
◦
4
π–
π
stacking interactions.
II 2−
(
L-Cu ) is deprotonated at both of its carboxylic acid binding sites, giving the linker
II
a charge of
−
2. As one ligand is present per Mn centre in the crystal structure, the overall
charge of PROD-1 is balanced. Bond valence sum analysis (BVSA) calculations confirmed
the oxidation states of all metal ions in PROD-1.
In the crystal structure of PROD-1, 2D sheets pack densely and porphyrin moieties
interdigitate between neighbouring layers. Small, interlayer channels filled with consti-
II
tutional solvent molecules and infinite 1D zig-zag Mn chains extend in parallel with the
crystallographic a-axis. The average distance between two adjacent 2D sheets in PROD-1
3
is ca. 15 Å. The solvent-accessible void volume of PROD-1 was calculated as 250 Å (ac-

Molecules 2021, 26, 2955
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counting for 10.4% of the unit cell volume) using the CCDC-mercury program with a probe
radius of 1.2 Å and a grid spacing of 0.7 Å [94].
II
II
2
.3. Crystal Structure of [Co (L-Cu )(DEA)]·8MeOH (PROD-2)
The single-crystal X-ray structure of PROD-2 was solved in the monoclinic space group
P2/c, revealing that this MOF and PROD-1 are structurally alike, as both of these frame-
II 2−
works feature 2-connected rod-shaped SBUs (Figure 4a) and ditopic (L-Cu ) ligands
(
Figure 4b). Unlike PROD-1, however, the infinite 1D rod-like node of PROD-2 comprises
II
periodically repeating, alternating tetrahedral and octahedral Co centres. Interconnected,
infinitely extended Co chains and porphyrin linkers form 2D layers that extend in parallel
II
with the crystallographic ac-plane and undulate in the crystallographic b-directions due to
the alternating coordination geometries of this MOF’s metal ions. The 2D layers of PROD-2
of stack on top of one another and interdigitate, giving rise to the corrugated sheet structure
shown in Figure 4c.
II
The asymmetric unit of PROD-2 contains two distinct Co centres Co(1) and Co(2)
1
which locate at two-fold rotational axes and have a crystallographic occupancy of
,
2
II 2−
one doubly deprotonated (L-Cu ) ligand and one coordinated DEA solvent molecule.
Disordered constitutional solvent molecules which could not be refined in the crystal
structure of PROD-2 were masked using the Platon-Squeeze routine [95]. The interatomic
distance between Co(1) and Co(2) is 4.5020(14) Å, and the distance between two connected
rod-shaped nodes is ca. 22 Å.
The binding environments of Co(1) and Co(2) within the zig-zag chain SBU of PROD-2
are depicted in Figure 5a. These ions adopt distorted octahedral and distorted tetrahedral
coordination geometries, and are each coordinated by four O-donors from four bridging,
1
1
II
µ
2-
η
:
η
binding carboxylate moieties that link adjacent Co centres into infinite 1D chains.
0
In addition two O-donors, O(5) and O(5 ), derived from two labile, ‘cis’-coordinated DEA
solvent moieties are contained within the coordination sphere of Co(1). The bond distances
II
surrounding the octahedral Co centre Co(1) are within the range of 2.2626(5)–2.3850(5)
II
Å, whilst the bond distances around the tetrahedrally coordinated Co ion Co(2) range
between 1.9484(4) Å and 1.9604(5) Å. The bond distances and angles surrounding Co(1)
and Co(2) are consistent with the values reported for other tetrahedral and octahedral CoII
carboxylate complexes in the literature [96–98].
As in PROD-1, the porphyrin ligand of PROD-2 adopts a saddle-shaped configuration
II 2−
which facilitates intrasheet π–π stacking interactions between neighbouring (L-Cu )
moieties (Figure 5b). The extent to which the linkers of PROD-2 are distorted is marginally
less than those of PROD-1. Dihedral angles between meso-substituted phenyl, and carbox-
◦
yaryl functionalities in PROD-2 vary by up to 33.0 , which promotes intersheet T-shaped
π–
π
interactions between nearby porphyrins. The distances between
π–π stacking moieties
in PROD-2 fall within the range of 3.5–3.8 Å [92].
The porphyrin linker in PROD-2 is doubly deprotonated, and thus has a charge of
−
2.
This charge is balanced by two crystallographically 1 occupied Co ions Co(1) and Co(2),
II
2
giving PROD-2 a net charge of 0. BVSA calculations confirmed the assignment of the +2
oxidation state of Co(1), Co(2) and Cu(1). Undulant interdigitated 2D sheets pack densely
in the crystal structure, giving PROD-2 its characteristic corrugated conformation. Small,
interlayer solvent-accessible channels propagate in parallel with the crystallographic a-axis,
3
constituting a solvent-accessible void volume of 452 Å which corresponds to 8.9% of the
MOF’s unit cell (calculated using CCDC-mercury with a probe radius and grid spacing of
1.2 and 0.7 Å, respectively) [94].

Molecules 2021, 26, 2955
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Figure 4. View of the structure of PROD-2 in the crystal, highlighting (
an infinite chain of alternating tetrahedrally and octahedrally coordinated Co ions, (
a) the framework’s 1D rod-shaped SBU comprising
II
II 2−
b
) the linker (L-Cu ) and (c) the
packing arrangement of PROD-2. Hydrogen atoms and solvent molecules are omitted for the purpose of clarity. Colour
II
scheme: C white, O red, N blue, Cu orange. Red polyhedra represent Co coordination environments.
PROD-1 and PROD-2 crystallize under comparable reaction conditions and share
several structural similarities. Both structures inhabit an intersection in chemical space be-
tween 2D, rod and porphyrin-based MOFs, which confers a constellation of characteristics
making them desirable for various future technologies. For instance, PROD-1 and PROD-2
contain well-ordered arrays of metalloporphyrin linkers and are stabilised by π–π stacking
interactions [99]. This could give rise to light-harvesting or energy transfer properties
that are advantageous for photo- or electrochemical applications including photovoltaics
or photocatalysis [100]. PROD-1 and PROD-2 are particularly promising for catalytic
applications, as their layered architectures, 1D channels and rod-shaped SBUs support
high concentrations of exposed labile coordination solvent sites. Moreover, the presence
II
of photoactive porphyrin linkers that coordinate directly to potentially redox-active Mn
II
and Co centres in PROD-1 and PROD-2 could give rise to photocatalytic activity. Finally,

Molecules 2021, 26, 2955
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the tunable and fully noble metal-free nature of these compounds is advantageous, as it
facilitates facile modifications and lowers the investment costs associated with these MOFs.
II
0
Figure 5.
O(5) and O(5 ), and (
(
a
) Coordination environments of the Co ions within PROD-2 s 1D SBU, featuring two DEA-derived O-donors
0
b) selected
π
–
π
stacking interactions that stabilize PROD-2. H atoms are omitted for clarity. Colour
scheme: C white, N blue, O red, Cu orange, Co violet. Red polyhedra represent Co coordination spheres and dashed red
II
lines show π–π stacking.
2.4. Physicochemical Characterisation of PROD-1 & PROD-2
The IR spectra of PROD-1 (Figure S1) and PROD-2 (Figure S2) exhibit characteristic
II 2−
signals confirming the presence of the porphyrin ligand (L-Cu ) . For example, several
−
1
signals centred around 3000 cm are attributed to aromatic C–H stretching vibrations of
the metalloporphyrin linkers in both MOFs. In addition, bands at ca. 1540 and 1380 cm
are assigned to asymmetric and symmetric stretching vibrational modes, respectively, of
−
1
2
1
1
the
µ -η :η bridging carboxylate moieties of PROD-1 and PROD-2 [101]. Weak signals
−
1
at around 1280 cm can be attributed to C–N stretching vibrations of the MOF’s (L-
II 2−
−1
Cu ) linkers. Furthermore, sharp signals at 1001 and 994 cm may arise due to in-
plane vibrations (ring breathing) of the porphyrin macrocycles of PROD-1 and PROD-2,
−
1
respectively [102]. Finally, bands at ca. 1600 cm can be attributed to C=O stretching
vibrations from the coordinated and constitutional DEA solvent molecules within PROD-1
and PROD-2 [103].
The thermal stabilities of PROD-1 and PROD-2 were investigated using TGA by
◦
heating these compounds from 30–800 C under N . The TGA trace of PROD-1 (Figure S3)
2
◦
reveals that when heated from 25–80 C, the sample undergoes a weight loss of 2.5% which
can be attributed to the loss of one constitutional MeOH solvent molecule (calculated:
3
.1%). It is likely that some solvent of crystallisation was lost from the framework prior
to this analysis. An additional mass loss of 5.2% is observed as the sample is heated
◦
from 80–300 C, which can be accounted for by the loss of a DEA coordination solvent
◦
molecule from PROD-1 (calculated: 6.2%). Further heating of the sample from 300–455 C
was associated with a further mass loss of 11.0%, which results from the loss of one
constitutional DEA solvent molecule from PROD-1 (calculated: 11.2%). Two additional
◦
◦
thermogravimetric steps are observed between 455–540 C and above 540 C, which can
be attributed to the decomposition of the MOF’s organic components and to the formation
of metal oxide species, respectively.
The TGA trace of PROD-2 (Figure S4) shows that the structure degrades in several
◦
distinct thermogravimetric steps. As the sample is heated from 30–105 C a mass loss of
20.6% is observed. This decrease in mass can be attributed to the loss eight constitutional
MeOH solvent molecules from PROD-2 (calculated: 21.5%). When PROD-2 is heated
◦
further from 105–360 C, a weight loss of 9.3% is observed. This weight loss is associated
with the loss of one coordinated DEA molecule from PROD-2 (calculated: 9.7%). Heating

Molecules 2021, 26, 2955
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◦
the sample from 360–500 C is associated with the decomposition of the framework’s
organic ligands. Finally, the formation of metal oxide species occurs as the sample is heated
◦
above 500 C. These TGA experiments reveal that both PROD-1 and PROD-2 are thermally
◦
stable up to ca. 350 C, as the MOF’s porphyrin linkers start to decompose above this
temperature. As the disordered constitutional solvent molecules of PROD-2 could not
be modelled crystallographically, interpretation of these data enabled the assignment of
II
II
this compound’s molecular formula [Co (L-Cu )(DEA)] 8MeOH. Upon desolvation, both
·
PROD-1 and PROD-2, do not take up significant quantities of N .
2
PXRD patterns of PROD-1 (Figure S5) and PROD-2 (Figure S6) were measured and
compared with simulated PXRD patterns that were calculated from the corresponding
compound’s single-crystal X-ray diffraction data to evaluate the phase purity of the pre-
pared samples. The experimentally obtained PXRD pattern of PROD-1 agrees well with its
simulated pattern. Discrepancies for PROD-2 may result from structural changes due to
rapid sample desolvation prior to or during PXRD analysis and crystal orientation effects.
3. Materials and Methods
3.1. Reagents & Analytical Methods
All chemicals and solvents were of reagent grade and used as received without fur-
ther purification unless otherwise stated. Single-crystal X-ray structural analyses were
performed on a Bruker SMART APEX CCD diffractometer with a Cu-K X-ray source
λ = 1.54184 Å). The omega scan method was used to collect either a full sphere or hemi-
α
(
sphere of data for each crystal with a detector to crystal distance of either 5 or 6 cm at
temperatures of 100 or 215 K. Data were collected, processed, and corrected for Lorentz
and polarisation effects using SMART [104] and SAINT-PLUS [105] programs. The struc-
tures were solved using direct methods with the SHELXTL [106] software package. All
non-hydrogen atoms were refined anisotropically. Hydrogen atom positions were assigned
using a riding model with appropriately fixed isotropic thermal parameters. Solvent-
accessible void volumes were calculated using the ‘voids’ tool in Mercury (CCDC) using
a probe radius of 1.2 Å and a grid spacing of 0.7 Å [94]. Crystallographic information
files for PROD-1 and PROD-2 can be obtained free of charge from the Cambridge Crys-
tallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif using the accession
identifiers CCDC-2073036 and CCDC-2065629, respectively. For the structural refinement
of PROD-1, the DFIX constraint was applied to the bonds of an identified coordinated
N,N-diethylacetamide (DEA) molecule due its disorder in the structure. Common C–C,
C–N and C–O bond lengths, characteristic for DEA molecules were applied. Additionally, a
FLAT command, restraints some of the atoms of the DEA molecule to lie in a common plane.
The approach resulted in convergence upon least-square refinements. Further DEA and
MeOH solvent molecules were located in the voids of the structure and their occupancies
were refined to achieve convergence. The occupancies <1 are caused by solvent loss during
mounting and data collection or disorder of the solvent molecules whereby parts of the
disordered positions could not be located. The twin character of the crystals of PROD-2
was noted during the crystallographic data collection. A careful selection of reflection
spots was required prior to indexing and integration. The HKLF5 command was applied
to the refinement. This approach allowed us to solve the structure. Finally, the Platon
twin routine was applied to further resolve the degree of twinning, leading to the reported
quality values. The Platon-Squeeze routine was applied due to the diffuse electron density
that results from highly disordered solvent molecules located in the voids of the structure.
3
The solvent-accessible void volume accounts to 1143 Å and 272 electrons. This electron
contribution stems from solvent molecules, i.e., methanol which was used in the synthesis.
The TGA analysis is consistent with the crystallographic data. Based on this analysis, 8
constitutional MeOH molecules were assigned to the structure.
IR spectra were recorded using a PerkinElmer Spectrum One FT-IR spectrometer
equipped with a universal ATR sampling accessory. Data were collected and processed
using Spectrum v5.0.1 (2002 PerkinElmer Instrument LLC) software. The scan rate was

Molecules 2021, 26, 2955
10 of 16
1
6 scans per second with a resolution of 4 cm 1 in the range 4000–650 cm . Standard
− −1
abbreviations were used to describe signal intensities: s, strong; m, medium; w, weak;
br, broad. Thermogravimetric analysis (TGA) data were collected using a simultaneous
◦
−1
SDT thermal analyser at a heating rate of 5 C min under a N atmosphere (N flow
2
2
−
1
rate = 0.06 L min ). PXRD data were collected at room temperature with a Bruker D2
Phaser diffractometer equipped with a CuK X-ray source. Simulated PXRD patterns were
α
calculated from the single-crystal X-ray data of PROD-1 and PROD-2 using the CCDC-
1
Mercury program (Cambridge, UK) [94]. H NMR spectra were recorded on a Bruker DPX
4
00 spectrometer operating at 400.13 MHz. Samples were analysed in deuterated solvents
that are listed for each spectrum. Standard abbreviations are used for spectral assignments:
s, singlet; d, doublet; t, triplet; m, multiplet; br, broad; J, coupling constant. UV-Vis spectra
◦
were recorded in the range 300–800 nm on a Cary Scan spectrophotometer at 20 C using
disposable cells with a path length of 1 cm.
II
3
.2. Synthesis of the Porphyrin Ligand H L-Cu
2
II
The dicarboxylic acid porphyrin H L-Cu was prepared in four steps, beginning with
2
the synthesis of 5-(4-carbomethoxyphenyl)dipyrromethane, according to adapted proce-
dures described by Lindsey et al. [107] and Meindl et al. [108] Under an inert atmosphere
and while shielding from light, trifluoroacetic acid (TFA, 0.23 mL 0.343 mmol) was added
to a solution of methyl-4-formyl benzoate (5.00 g, 30.4 mmol) in freshly distilled pyrrole
(150 mL, 2.162 mol), before stirring the solution for 3 h. NaOH (3.60 g, 0.090 mol) was
then added, and the reaction was stirred for a further hour. Following this, the reaction
mixture was filtered before concentrating the filtrate by evaporating the solvent under
reduced pressure, while excess pyrrole was recovered for later use. The crude product
obtained was subsequently purified by silica gel column chromatography using a mixture
of hexane, dichloromethane (DCM), ethyl acetate and triethylamine (TEA) in a ratio of
4
5
:2:1:0.05 (vol/vol) as the eluting solvent before washing with cold ethyl acetate yielding
-(4-carbomethoxyphenyl)dipyrromethane as a white powder. Yield: 4.26 g (50%). H
1
NMR (400 MHz, CD CN):
δ
(ppm) = 8.94 (m, 2H, NH), 7.92 (m, 2H, aryl-H), 7.32 (m, 2H,
3
aryl-H), 6.66 (m, 2H, pyrrole-H), 6.02 (m, 2H, pyrrole-H), 5.77 (m, 2H, pyrrole-H), 5.52 (s,
1H, CH), 3.85 (s, 3H, CH3).
Next, under an inert atmosphere and darkness, benzaldehyde (3.6 mL, 35.3 mmol)
was added to a solution of 5-(4-carbomethoxyphenyl)dipyrromethane (9.60 g, 34.5 mmol)
in dry DCM (3.3 L). To this solution, TFA (6.6 mL) was added dropwise over 1 min. The
reaction mixture was then stirred for 3 h before adding p-chloranil (12.48 g, 50.7 mmol)
and stirring overnight. Following this, TEA (6.6 mL) was added to quench the reaction
before removing the solvent under reduced pressure. The crude product was then dry
loaded onto silica and purified using silica gel column chromatography with CH Cl as the
3
eluting solvent. A purple powder consisting of several different methoxy ester-substituted
porphyrins was obtained as the second purple band after 5,10,15,20-tetraphenylporphyrin.
This powder was then dry loaded on to silica and purified using silica gel column chro-
matography with DCM and hexane in a ratio of 2:1 (vol/vol) as the eluent, giving the
desired diester porphyrin in the third purple band. Evaporation of the solvent under
reduced pressure yielded 5,15-bis(4-carbomethoxyphenyl)-10,20-diphenylporphyrin as a
1
purple powder. Yield: 0.6 g (17%). H NMR (400, MHz CDCl ):
δ (ppm) = 8.80 (m, 4H,
3
pyrrole-H), 8.75 (m, 4H, pyrrole-H), 8.45 (d, 4H, benzyl-H, J = 9.2 Hz), 8.28 (d, 4H, benzyl-
H, J = 6.9 Hz), 8.20 (d, 4H, aryl-H, J = 9.2 Hz), 7.78 (m, 6H, aryl-H), 4.12 (s, 6H, CH ), 2.78
−
5
3
−
1
−1
(
br, s, 2H, pyrrole-H). UV-Vis (DMF): λmax/nm (
ε
/L mol cm ) = 415 (1.8
×
10 , Soret
3
3
3
band,
Q band,
π
–
π
π
*), 513 (9.0 × 10 , Q band,
π
–
π
*), 546 (4.0
×
10 , Q band,
π–
π
*), 590 (3.4
×
10 ,
3
–
π
*), 645 (2.9
×
10 , Q band,
π
–
π
*). MS (MALDI-TOF): Found m/z = 730.2417
.
Calculated m/z = 730.2580 for [C H N O ].
48
34
4
4
Following this, 5,15-bis(4-methoxycarbonylphenyl)-10,20-diphenylporphyrin (600 mg,
◦
0
.821 mmol) was dissolved in CHCl (100 mL) and heated to 70 C. Cu(OAc) monohy-
3
2
drate (1.624 g, 8.210 mmol) dissolved in MeOH (10 mL) was then added to the heated

Molecules 2021, 26, 2955
11 of 16
◦
solution and the reaction was stirred at 70 C for 3 h. Upon complete consumption of
the starting material, monitored via TLC analysis (CH Cl : hexanes, 2:1, v/v), the reac-
2
2
tion mixture was washed with saturated NaHCO , water and brine (2
×
100 mL each),
3
extracting with CH Cl . The organic extracts were dried over Na SO , filtered and the sol-
2
2
2
4
vents were removed in vacuo. The red residue {5,15-bis(4-methoxycarbonylphenyl)-10,20-
diphenylporphyrinato}copper(II)} was used without further purification in the ester hy-
drolysis step. (5,15-Bis(4-methoxycarbonylphenyl)-10,20-diphenylporphyrinato)copper(II)
(
5
approximately 600 mg) was dissolved in THF (50 mL) and to this solution KOH (12 g in
◦
0 mL H O) was added. The reaction mixture was heated to 80 C and was stirred at this
2
temperature for 18 h. Upon complete consumption of the starting material, the reaction
mixture was cooled to ambient temperature before acidification to pH 6 using 1M HCl.
The product was extracted using CH Cl and the organic extracts were combined, and the
2
2
solvents were removed in vacuo. The red residue was triturated with CHCl and filtered
3
to give a red solid which was subsequently transferred to a 250 mL RBF, suspended in
CHCl and the solvent was removed in vacuo to yield a red residue (600 mg, 0.785 mmol,
3
◦
9
6%). Mp >300 C; R (CH Cl : hexane: MeOH 1:1:0.02 v/v/v): 0.63; UV-vis (MeOH):
f
2
2
λmax (log ε) = 412 (5.35), 538 (4.02) nm; HRMS (MALDI): m/z = 764.2849 calculated for
C H N O Cu: found; 763.1432.
46
28
4
4
II
II
3
.3. Synthesis of [Mn (L-Cu )(MeOH) ]·DEA·MeOH (PROD-1)
2
II
H L-Cu (15.0 mg, 19.7
in N,N-diethylacetamide (DEA) (11.0 mL) and MeOH (6.0 mL) by sonicating the mixture
µ
mol) and MnCl2 2H O (3.20 mg, 19.8 µmol) were dissolved
·
2
2
for 30 min. The resulting solution was then transferred into a Teflon-lined stainless-steel
◦
autoclave and heated to 120 C for 4 days, before slowly cooling to ambient temperature.
This afforded the formation of crimson, rod-shaped crystals of PROD-1 which were of
suitable quality for analysis using single-crystal X-ray diffraction. Yield: 4.78 mg (24%).
FT-IR: υmax = 2978 (br, w), 1607 (m), 1581 (s), 1536 (m), 1487 (w), 1366 (vs), 1282 (w),
−
1
1210 (w), 1175 (w), 1012 (w), 1001 (s), 831 (w), 796 (s), 697 (s) cm
.
II
II
3
.4. Synthesis of [Co (L-Cu )(DEA)]·8MeOH (PROD-2)
II
H L-Cu (1.2 mg, 1.56
µ
mol) and CoCl (1.5 mg, 11.5
µmol) were dissolved in N,N-
2
2
diethylacetamide (DEA) (1.0 mL) and MeOH (0.5 mL) by sonicating for 30 min. Following
this, a drop of acetic acid was added to the reaction mixture and the solution was transferred
into a small glass vial, which was subsequently placed inside a sealed Teflon-lined stainless-
◦
steel autoclave and heated to 120 C for 4 days. Slow cooling of the reaction mixture to
room temperature over 24 h afforded the formation of red, plate-shaped crystals of PROD-2
which were suitable for single-crystal X-ray diffraction studies. Yield: 0.6 mg (39%). FT-IR:
υmax = 2969 (vbr, m), 1594 (s), 1548 (m), 1373 (s), 1334 (s), 1276 (m), 1204 (w), 1181 (w), 1067
−
1
(
w), 994 (s), 832 (w), 797 (s), 773 (s), 699 (s) cm
.
4. Conclusions
In conclusion, we report two new 2D porphyrinic MOFs with rod-shaped SBUs,
PROD-1 and PROD-2, which are rare examples of frameworks featuring the underexplored
II
linker H2
L
-Cu . Single-crystal X-ray diffraction studies show that the contrasting coordina-
tion geometries found within the MOFs’ 1D metal chain SBUs give rise to either planar or
◦
corrugated sheet configurations. PROD-1 and PROD-2 are thermally stable up to ca. 350 C
and exhibit extensive networks of π–π stacking interactions. The present structures are
intriguing supramolecular substances which span several subclasses including 2D, por-
phyrinic and rod MOFs. Their unique attributes render them hopeful compounds for a
wide range of future applications, and their structural relationship to Nature’s paragon
H O splitting Mn complex in PS II, suggests that PROD-1 and PROD-2 might ultimately
2
find use within bioinspired artificial photosynthetic systems [109].

Molecules 2021, 26, 2955
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Supplementary Materials: Electronic supporting information containing crystallographic tables and
physicochemical characterisation data for PROD-1 and PROD-2, and crystallographic information
files (CIFs) are available online.
Author Contributions: Conceptualisation, W.S.; MOF syntheses and structural characterisations,
R.E. and A.A.; X-ray crystallographic analysis, F.W.S. and N.Z; preparation of porphyrin precursor
ligands, A.A.R. and M.O.S.; writing—original draft preparation, R.E.; writing—review and editing,
R.E., M.O.S. and W.S.; visualisation, R.E.; supervision, W.S.; project administration, W.S.; funding
acquisition, W.S. and M.O.S. All authors have read and agreed to the published version of the
manuscript.
Funding: This research was funded by Science Foundation Ireland (SFI IvP 13/IA/1896) and the
European Research Council (CoG 2014–647719). This work was prepared with the support of
the Technical University of Munich—Institute for Advanced Study through a Hans Fischer Senior
Fellowship.
Institutional Review Board Statement: Not applicable.
Informed Consent Statement: Not applicable.
Data Availability Statement: Crystallographic information files for PROD-1 and PROD-2 can be
obtained free of charge from the Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif using the accession identifiers CCDC-2073036 and CCDC-2065629, respectively.
Conflicts of Interest: The authors declare no conflict of interest.
Sample Availability: Samples of the compounds are not available from the authors.
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