Interpenetration in π-Rich Mixed-Ligand Coordination Polymers

Article abstract of DOI:10.1021/acs.cgd.6b00901

Structural and chemical influences on interpenetration have been investigated through the preparation and structural analysis of a series of seven chiral coordination polymers using a phenylalanine-substituted naphthalenediimide ligand (H₂PheNDI). The reaction of H₂PheNDI with Mn^II or Cd^II and a range of linear dipyridyl-based coligands forms a series of coordination polymers which vary greatly in terms of their topologies and interpenetration while largely retaining a common metallomacrocyclic motif. The metallomacrocyclic motif is found in a tube-like 1D coordination polymer poly-[Cd(bipy)(OH₂)(PheNDI)] (2) and a closely related 2D polythreaded network poly-{[Cd₂(bipy)₂(PheNDI)₂][Cd(bipy)(DMF)_1.5(NO₃)₂(OH₂)_0.5]} (3) which are synthesized as pure phases under slightly different conditions. The longer 1,2-di(4-pyridyl)ethylene (dpe) ligand gives rise to a 2D coordination polymer poly-[Cd₄(DMF)(dpe)₄(OH₂)₂(PheNDI)₄] (5) in which the metallomacrocycles are connected only sideways rather than as a tube. This difference allows for 2-fold 2D → 2D interpenetration, between two crystallographically distinct sheets, whereby the dpe passes through the metallomacrocycle, assisted by face-to-face aromatic interactions. The use of a larger dipyridyl ligand, N,N′-bis(4-pyridyl)naphthalenediimide (4pyNDI), yielded 3D coordination polymers with distorted pcu topologies of the form poly-[M₂(PheNDI)₂(4PyNDI)₂] (M = Cd, 6; Mn, 7) which contain neither the metallomacrocyclic motif nor interpenetration.

Full text of DOI:10.1021/acs.cgd.6b00901

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Article
Interpenetration in #-Rich Mixed-Ligand Coordination Polymers
Stephanie A Boer, and David R. Turner
Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00901 • Publication Date (Web): 26 Sep 2016
Downloaded from http://pubs.acs.org on October 4, 2016
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Interpenetration in πꢀRich MixedꢀLigand
Coordination Polymers
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Stephanie A. Boer and David R. Turner*
School of Chemistry, Monash University, Clayton, VIC 3800, Australia.
ABSTRACT
Structural and chemical influences on interpenetration have been investigated through the
preparation and structural analysis of a series of seven chiral coordination polymers using a
phenylalanineꢀsubstituted naphthalenediimide ligand (H2PheNDI). The reaction of H2PheNDI
with MnII or CdII and a range of linear dipyridylꢀbased coligands, forms a series of
coordination polymers which vary greatly in terms of their topologies and interpenetration
whilst largely retaining a common metallomacrocyclic motif. The metallomacrocyclic motif
is found in a tubeꢀlike 1D coordination polymer polyꢀ[Cd(bipy)(OH2)(PheNDI)] (2) and a
closelyꢀrelated
2D
polythreaded
network
polyꢀ
{[Cd2(bipy)2(PheNDI)2][Cd(bipy)(DMF)1.5(NO3)2(OH2)0.5]} (3) which are synthesised as
pure phases under slightly different conditions. The longer 1,2ꢀdi(4ꢀpyridyl)ethylene (dpe)
ligand gives rise to a 2D coordination polymer polyꢀ[Cd4(DMF)(dpe)4(OH2)2(PheNDI)4] (5)
in which the metallomacrocycles are connected only ‘sideways’ rather than as a tube. This
difference allows for twoꢀfold 2D→2D interpenetration, between two crystallographically
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distinct sheets, whereby the dpe passes through the metallomacrocycle, assisted by faceꢀtoꢀ
face aromatic interactions. The use of a larger dipyridyl ligand, N,Nʹꢀbis(4ꢀpyridyl)ꢀ
naphthalenediimide (4pyNDI), yielded 3D coordination polymers with distorted pcu
topologies of the form polyꢀ[M2(PheNDI)2(4PyNDI)2] (M = Cd, 6; Mn, 7) which contain
neither the metallomacrocyclic motif nor interpenetration.
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INTRODUCTION
The design and synthesis of coordination polymers is a burgeoning field, particularly
investigations into the reproducibility and predictability of supramolecular features within
their structures.1 Coordination polymers are an important area of research due to their many
applications including catalysis,2ꢀ4 gas storage5ꢀ7 and separation.8ꢀ10 The structures of
coordination polymers are influenced by numerous weak interactions, making them very
difficult to predict.11, 12 The ligand size, shape and functionality and the metal ion, as well as
the synthetic conditions including the stoichiometric ratios of reagents, temperature, solvent
and time are also all factors which influence the final structure of a coordination polymer.13
The design of coordination polymers not only involves design and selection of the
components required to construct an isolated framework but also consideration of how the
coordination polymers may pack in the solid state. In general, packing in the solid state by
molecular species and by coordination polymers is in such a way as to maximize packing
efficiency. Being essentially infinite networks, coordination polymers can pack efficiently
through a number of interlocked or interwoven topologies.14,15 The most commonly
encountered mode of entanglement is interpenetration, in which two or more coordination
polymers within a system are interlocked such that they are not chemically bonded together
but cannot be broken without breaking chemical bonds, which can be considered as
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coordination polymers involving polycatenane or polyrotaxane motifs.14,16,17 Other topologies
include polythreading, which can be considered as an extended analogue of a molecular
pseudoꢀrotaxane with a coordination polymer ‘thread’ passing through a series of ‘loops’ of a
separate coordination polymer in the same system. Polyknotting is also possible, otherwise
known as selfꢀpenetration, in which a single coordination polymer net contains rings through
which another component of the same network passes.17,18 The study into interpenetration
and related phenomena, and understanding how these complex selfꢀassembly processes can
be designed and predicted, is of importance because of the influence they have on the
properties of the coordination polymer materials.19,20
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Investigation into supramolecular systems which display reproducible structural motifs is
therefore a significant area of research, as these motifs provide some predictability in
designing coordination polymers. The use of πꢀπ interactions to form reproducible
supramolecular motifs such as catenanes or rotaxanes within coordination polymers is an area
which has been much less explored than other supramolecular interactions.21ꢀ24
Naphthalenediimides are a versatile class of molecules which have been extensively
investigated. NDIs are electron deficient, making them highly valuable in the field of nꢀtype
semiconductors and can also be functionalized to have absorption bands within the entire
visible spectrum.25,26 Functionalization of NDIs with coordinating groups can also lend to
NDIs being valuable ligands for synthesis of discrete and polymeric coordination
compounds.27ꢀ31
Our previous work has concentrated on the synthesis of coordination polymers using a
naphthalenediimide substituted at the imide position with either Lꢀalanine or L
ꢀleucine.32,33
The structures of these compounds showed a reproducible {Cd2L2} metallomacrocyclic motif
in which the planes of the aromatic cores of the two ligands that form the macrocycle are
approximately 7 Å apart, ideal for faceꢀtoꢀface πꢀinteractions of ~3.5 Å with aromatic guests
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inside the metallomacrocycle. Previously reported polyꢀ[Cd2(AlaNDI)2(DMF)4] is a 1D chain
of metallomacrocycles connected by bimetallic nodes.33 The chains are 1D→2D
interpenetrated by polycatenation of the metallomacrocycle motifs driven by πꢀinteractions
between the aromatic cores of the ligands. The bulkier sideꢀchain in polyꢀ
[Cd2(LeuNDI)2(DMA)4] maintained a 1D chain comprising metallomacrocycle motifs, yet
displayed no interpenetration. However, the addition of a 4,4′ꢀbipyridine (bipy) coligand
formed 1D chains of polyꢀ[Cd2(LeuNDI)2(bipy)(DMF)3(OH2)] in which the
metallomacrocycles are bridged by the bipy coligands which then pass through the
metallomacrocycles of a different chain to form a 1D→3D polyrotaxane.32
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It is clear that there is significant influence of the amino acid side chain and the coligand on
the structures of this family of coordination polymers and their ability to form
interpenetrating architectures, necessitating further studies. Herein we report the use of the
phenylalanineꢀsubstituted naphthalenediimide ligand, H2PheNDI, which introduces additional
complexity to the system with the addition of more aromatic functionalization when used
alone or in combination with coligands (Figure 1).
Figure 1. The phenylalanineꢀsubstituted naphthalene diimide ligand (H2PheNDI) and the
dipyridyl ligands utilized in this study.
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EXPERIMENTAL DETAILS
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Materials and physical measurements
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All chemicals and reagents were obtained from commercial sources and used without
purification. 4PyNDI was synthesized following a literature procedure.34 Nuclear magnetic
resonance spectra were collected using a Bruker DRXꢀ400 spectrometer using residual
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1
solvent peaks as reference. 13CꢀNMR spectra were collected at 100 MHz and HꢀNMR
spectra were collected at 400 MHz. Resonances are reported in parts per million (ppm) and
coupling constants, J, are reported in Hertz (Hz). Low resolution mass spectrometry was
performed using a Micromass Platform Electrospray mass spectrometer. Infrared spectra
were obtained using an Agilent Cary 630 diamond attenuated total reflectance (ATR)
spectrometer with MicroLab software used to process the data. Thermogravimetric analysis
(TGA) was conducted using a Mettler TGA/DSC 1 instrument. The temperature was ramped
at 5 °C/min to 400 °C under an N2 supply of 10.0 mL/min and the data were analyzed using
the STARe system. Microanalyses were performed at the Science Centre, London
Metropolitan University, UK. Powder Xꢀray diffraction were collected on a Bruker D8 Focus
diffractometer equipped with CuꢀKα radiation (λ = 1.5418 Å) at room temperature. The
sample was mounted on a zero background silicon single crystal stage. Data were collected in
the angle interval 2θ = 5ꢀ55° with a step size of 0.02°. The collected data were compared to
predicted patterns based on lowꢀtemperature single crystal data. Fluorescence spectra were
run on a Cary Eclipse Fluorescence Spectrophotometer. The crystals were ground in KꢀY
jelly and spread onto 150 µm cover slips and mounted inside the spectrophotometer. The
emission was measured with the samples excited at 380 nm and scanned from 400ꢀ680 nm at
800 V with the excitation and emission slits at 10 mm. The fluorescence was also tested using
a confocal microscope setꢀup previously reported.35 The sample was excited using a 375 nm
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pulsed diode laser driver (PicoQuant, PDL 800ꢀD) and a 375 nm laser head (PicoQuant,
LDHꢀPꢀCꢀ375) at a repetition rate of 10 MHz. The light was focused onto the sample using a
40 x 1.00 N.A. oil immersion objective (Olympus). Light was passed through a 460 nm long
pass filter before being sent to either the APD or the EMCCD camera. Both florescence
methods showed no luminescent properties above those of the blank sample, (ESI Figure
S16).
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Synthesis of H2PheNDI
Naphthaleneꢀ1,4,5,8ꢀtetracarboxylic dianhydride (0.950 g, 3.56 mmol) and Lꢀphenylalanine
(1.29 g, 7.83 mmol) were dissolved in DMF (20 mL) and stirred at 80ꢀ90°C overnight giving
a brown solution which was cooled to room temperature before being poured over ice. The
product was extracted with ethyl acetate (2x100 mL), washed with brine (4x200 mL) and
dried over magnesium sulfate before the solvent was removed under vacuum. The solid was
washed with toluene to give the product as a brown solid. Yield 1.30g, 65%. m.p. 360ꢀ364°C.
Found C, 67.72; H, 4.23; N, 5.20%; C32H22N2O8ꢁ0.25H2O requires C, 67.78; H, 4.00; N,
2
3
4.94%. δH (400 MHz, d6ꢀDMSO), 3.32 (dd, J=14.2, J=9.3 Hz, 2 H, CH(CH2), 3.60 (dd,
3
2
2
2J=13.9, J=5.6 Hz, 2 H, CH(CH2), 5.86 (dd, J=9.5, J=5.6 Hz, 2 H, CH), 7.15 (m, 10H,
CHPhe), 8.64 (s, 4 H, CHNDI), 13.09 (br., 2 H, COOH). δC (100 MHz, d6ꢀDMSO), 34.3, 54.6,
125.6, 126.0, 126.3, 128.1, 128.9, 131.2, 137.8, 161.9, 170.2. υmax /cmꢀ1 2937w, 1706s,
1667s, 1581m, 1452m 1335s, 1248s, 1154m, 835m, 778s. m/z (ESꢀ) 561.1 ([MꢀH]ꢀ, 50%),
517.1 ([Mꢀ CO2ꢀH]ꢀ, 100%).
Synthesis of polyꢀ[Mn(DMF)(HPheNDI)2]·H2O·MeOH (1)
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H2PheNDI (20.0 mg, 35.6 µmol) and MnCl2·4H2O (21.0 mg, 106 µmol) were added to a
solvent mixture of DMF (2 mL), methanol (1 mL) and H2O (1 mL) in a glass vial and
sonicated to dissolve. The reaction mixture was heated to 70 °C in a dry block incubator for
seven days, after which time yellow needles of 1 were formed which were isolated by
vacuum filtration. Yield 5.4 mg, 22.4 %. Found C, 61.92; H, 4.32; N, 6.14%;
C70H56N6O18Mn·H2O·MeOH ([Mn(HPheNDI)(DMF)2]·H2O·MeOH) requires C, 62.05; H,
4.55; N, 6.12 %. υmax /cmꢀ1 3612w, 2937w, 1330m, 1245s, 1185m, 1111m, 1062m, 991m,
931m, 872m, 827m, 746s, 701s. TGA: Onꢀset, 25 °C, mass loss = 3.0 % (calculated 2.9 % for
loss of MeOH and H2O). Bulk phase purity was confirmed by PXRD (ESI Figure S1).
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Synthesis of polyꢀ[Cd(bipy)(OH2)(PheNDI)]·3.5H2O·DMF (2)
H2PheNDI (20.0 mg, 35.6 µmol), Cd(NO3)2·4H2O (43.6 mg, 141 µmol) and 4,4ʹꢀbipyridine
(2.8 mg, 17.8 µmol) were added to a solvent mixture of DMF (2 mL), methanol (1 mL) and
H2O (1 mL) in a glass vial and sonicated to dissolve. The solution was heated at 85 °C in a
dry block incubator for four days, after which time yellow needle crystals of 2 were formed
which were isolated by vacuum filtration. Yield 17.8 mg, 98.3 %. Found C, 54.98; H, 4.31;
N, 7.13%; C42H30N4O9Cd·3.5H2O·DMF ([Cd(PheNDI)(bipy)(H2O)]·3.5H2O·DMF) requires
C, 54.97; H, 4.51; N, 7.12 %. υmax /cmꢀ1 1705m, 1655s, 1577s, 1499w, 1452m, 1413m
1376m, 1331s, 1277m, 1247s, 1219m, 1174m, 1098w, 1068w, 1042w, 988w, 939w, 911w,
872w, 837w, 814m, 783s, 747m, 697m, TGA: Onꢀset, 25 °C, mass loss = 13.3 % (calculated
13.8 % for loss of 1 DMF and 3.5 H2O). Bulk phase purity confirmed by PXRD (ESI Figure
S2). The Xꢀray diffraction data was processed using the SQUEEZE routine of PLATON,
which showed a void space of 328 eꢀ per unit cell, (82 eꢀ per asymmetric unit) corresponding
to 1 DMF and 3.5 H2O molecules per asymmetric unit.
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Synthesis
of
polyꢀ
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{[Cd2(bipy)2(PheNDI)2][Cd(bipy)(DMF)1.5(NO3)2(OH2)0.5]}ꢁH2OꢁDMF (3)
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H2PheNDI (10.0 mg, 17.8 µmol), 4,4ʹꢀbipyridine (1.40 mg, 8.9 µmol) and Cd(NO3)2ꢁ4H2O
(10.9 mg, 35.6 µmol) were added to DMF (3 mL) in a glass vial and sonicated to dissolve.
The reaction mixture was heated at 110 °C in a dry bath incubator for 96 hours, after which
time a red crystalline material of 3 was formed, which was recovered by vacuum filtration.
Yield 0.43 mg, 5%. Found C, 53.42; H, 3.66; N, 8.72%; C97H75N13O26Cd3·H2O·DMF
([Cd2(PheNDI)2(bipy)2][Cd(bipy)(NO3)2 (DMF)(H2O)]·H2O·DMF) requires C, 53.40; H,
3.67; N, 8.72 %. υmax /cmꢀ1 3516w, 2937w, 1709m, 1654m, 1605s, 1574s, 1454m, 1415s,
1335s, 1249s, 1173m, 1113m, 1071m, 815m, 784m, 704m. TGA: Onꢀset, 30 °C; mass loss =
8.7 % (calculated 9.0% for loss of 1 H2O and 1 DMF). Phase purity was confirmed by PXRD
(ESI Figure S3).
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Synthesis of polyꢀ[Mn2(bipy)2(PheNDI)2][Mn(bipy)(DMF)(NO3)2]·0.5DMF·7H2O (4)
H2PheNDI (20.0 mg, 35.6 µmol), 4,4ʹꢀbipyridine (5.6 mg, 17.8 µmol) and Mn(NO3)2·4H2O
(18.0 mg, 71.2 µmol) were added to a solvent mixture of DMF (2 mL) and H2O (1 mL) in a
glass vial and sonicated to dissolve. The reaction mixture was heated to 70 °C in a dry block
incubator for five nights, after which time orange crystals of 4 were formed which were
recovered by filtration. Yield 8.5 mg, 67%. Found C, 55.85; H, 3.70; N, 8.38%;
C97H71N13O23Mn3·0.5DMF·7H2O
([Mn2(PheNDI)2(bipy)2][Mn(bipy)(DMF)(NO3)2]·0.5DMF·7H2O) requires C, 55.95; H, 4.22;
N, 8.95 %. υmax /cmꢀ1 2934w, 1707s, 1666s, 1620s, 1602s, 1572, 1452m, 1412s, 1378m,
1333s, 1300s, 1248s, 1221m, 1173m, 1110m, 1088m, 1068m, 1044m, 991m, 916w, 878m,
853w, 812s, 781s, 752m, 731m, 700m, 678m. TGA Onꢀset, 25 °C; mass loss = 7.0 %
(calculated 7.6 % for loss of 0.5 DMF and 7 H2O). Phase purity is confirmed by PXRD (ESI
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Figure S4). The Xꢀray diffraction data was processed with the SQUEEZE routine of
PLATON, which showed voids of 330 Å3 containing 95eꢀ per asymmetric unit, which would
correspond to 7 H2O molecules which are not modelled in the crystal structure.
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Synthesis of polyꢀ[Cd4(DMF)(dpe)2(PheNDI)4(OH2)2]·10H2O·0.5DMF (5)
H2PheNDI (40.0 mg, 71.2 µmol), Cd(NO3)2ꢁ4H2O (87.2 mg, 142.4 µmol) and 1,2ꢀdi(4ꢀ
pyridyl)ethylene (9.2 mg, 71.2 µmol) were added to a solvent mixture of DMF (4 mL),
MeOH (1.5 mL) and H2O (1.5 mL) in a glass vial and sonicated to dissolve. The reaction
mixture was heated to 85 °C in a dry block incubator for 12 hours, after which time yellow
crystals of 5 were formed, which were recovered by vacuum filtration. Yield 47.7 mg, 40 %.
Found C, 55.27; H, 3.83; N, 5.62%; C155H111N13O35Cd4·10H2O·0.5DMF
([Cd4(PheNDI)4(dpe)4(H2O)2(DMF)]·10H2O·0.5DMF) requires C, 55.58; H, 4.01; N, 5.59%.
υmax /cmꢀ1 3334w, 3028w, 1991w, 1946w, 1771w, 1663s, 1573s, 1409m, 1331s, 1245m,
1174m, 1111w, 988m, 835m, 783s, 697m. TGA: Onꢀset, 25 °C, mass loss = 7.0 %
(calculated 6.4 % for loss of 0.5 DMF and 10 H2O molecules.) Phase purity is confirmed by
PXRD (ESI Fig S5).
Synthesis of polyꢀ[Cd2(PheNDI)2(4PyNDI)2]·6H2O·2DMF·MeOH (6)
H2PheNDI (20.0 mg, 35.6 µmol), Cd(NO3)2·4H2O (43.6 mg, 141 µmol) and N,Nʹꢀbis(4ꢀ
pyridyl)ꢀ1,4,5,8ꢀnaphthalene tetracarboxylic diimide (15.0 mg, 35.6 µmol) were added to a
solvent mixture of DMF (2 mL), MeOH (1 mL) and H2O (1 mL) in a glass vial and sonicated
to dissolve. The reaction mixture was heated at 85 °C in a dry block incubator for 48 hours,
after which time yellow needle crystals of 6 were formed which were recovered by vacuum
filtration. Yield 32.3 mg, 38.7 %. Found C, 57.57; H, 3.42; N, 8.19%;
C112H64N12O24Cd2·6H2O·2DMF·MeOH
([Cd2PheNDI2(4PyNDI)2]·6H2O·2DMF·MeOH)
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7
8
9
requires C, 57.79; H, 3.83; N, 7.93 %. υmax /cmꢀ1 1705m, 1663s, 1577s, 1493w, 1448w,
1407w, 1379w, 1329s, 1245s, 1191m, 1146s, 1118s, 1092.4w, 1064w, 984w, 913w, 865w,
829w, 766s, 699.m, 658m. TGA: Onꢀset, 53 °C, mass loss = 6.3 % (calculated 6.7 % for loss
of 2 DMF). Phase purity is confirmed by PXRD (Figure S6). The structure was processed
with the SQUEEZE routine of PLATON, showing voids of 435 Å3 containing 138eꢀ per
asymmetric unit, which corresponds to 6 H2O and 1DMF molecules more than the DMF and
MeOH already modelled in the crystal structure (6H2O, 2DMF and 1 MeOH in total). The
MeOH and H2O within the structure was likely lost before the TGA and microanalysis was
conducted.
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Synthesis of Polyꢀ[Mn2(PheNDI)2(4PyNDI)2]·3DMF·2MeOH·H2O (7)
H2PheNDI (10 mg, 18 µmol), Mn(NO3)2·4H2O (9.0 mg, 36 µmol) and N,Nʹꢀbis(4ꢀpyridyl)ꢀ
1,4,5,8ꢀnaphthalene tetracarboxylic diimide (7.5 mg, 18 µmol) were added to a solvent
mixture of DMF (2 mL), methanol (1 mL) and H2O (1 mL) in a glass vial and sonicated to
dissolve. The reaction mixture was heated to 85 °C in a dry block incubator for 24 hours,
after which time yellow crystals of 7 formed which were recovered by vacuum filtration.
Yield
8.0
mg,
38%.
Found
C,
60.14;
H,
3.46;
N,
8.75%,
[Mn2(PheNDI)2(4PyNDI)2]·3DMF·2MeOH·1H2O, requires C, 60.61; H, 3.93; N, 8.62%. υmax
/cmꢀ1 3389w, 1659m, 1577s, 1439m, 1346s, 1241s, 1181m, 1096m, 1014w, 962w, 865w,
775m, 660s. TGA: Onꢀset 50 °C; mass loss = 15.1 % (calculated 14.9 % for loss of 3 DMF, 2
MeOH and 1 H2O). Phase purity is confirmed by PXRD (Figure S7).
XꢀRAY CRYSTALLOGRAPHY
Data collection for all compounds was performed at the Australian Synchrotron on the
MX1 or MX2 beamline operating at 17.4 keV (λ = 0.7108)35 with data collections conducted
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using BluIce control software.37 Data indexing and integration was conducted using the XDS
program.38 All structures were solved using direct methods with SHELXS
or the dualꢀ
39
6
7
8
40
space method with SHELXT and refined by leastꢀsquares procedures with SHELXLꢀ2014
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41
within OLEXꢀ2.42 Nonꢀhydrogen atoms were refined with anisotropic displacement
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parameters (with the exception of the nonꢀmetal atoms in 5, further information given in the
ESI). Hydrogen atoms were placed in calculated positions and refined using a riding model
with isotropic displacement parameters 1.2 or 1.5 times the isotropic equivalent of their
carrier atoms. Where possible, the hydrogen atoms on oxygen were located from the Fourier
difference map and refined with appropriate restraints and isotropic parameters 1.5 times the
isotopic equivalent of their carrier atoms. Most structures had difficulty in refinement due to
disorder, poor diffraction or solvent accessible voids (requiring SQUEEZE),43 and specific
refinement details are given in the supplementary information. The crystallographic and
refinement data are presented in Table 1.
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Compound
reference
1
2
3
4
5
6
7
5
6
Empirical
formula
C71H60MnN6O C42H30CdN4O9 C101.5H80.5Cd C98.5H74.5Mn C155H111Cd4
C115H71Cd2N13 C60H44MnN7O
3N14.5O25
3N13.5O23.5
N13O35
O25
20
14.5
7
8
Formula
weight
1372.19
847.1
2240.50
1988.03
3165.16
2259.64
1149.96
9
Crystal system Monoclinic
Orthorhombic
18.928(4)
20.344(4)
11.804(2)
90
Monoclinic
31.575(6)
11.694(2)
16.024(3)
90
Monoclinic
11.574(2)
27.811(6)
15.961(3)
90
Triclinic
14.833(3)
15.630(3)
16.029(3)
90.55(3)
106.95(3)
98.30(3)
3512.2(14)
100(2)
Orthorhombic
13.852(3)
19.665(4)
39.965(8)
90
Orthorhombic
14.030(3)
19.359(4)
19.915(4)
90
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45
46
47
48
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60
a/Å
8.3310(17)
25.564(5)
14.599(3)
90
b/Å
c/Å
α/°
β/°
96.14(3)
90
90
118.40(3)
90
93.86(3)
90
90
90
γ/°
90
90
90
Volume/Å3
3091.4(11)
100(2)
4545.4(16)
100(2)
5205(2)
100(2)
5125.9(18)
100(2)
10886(5)
100(2)
5409.0(19)
100(2)
Temperature
/K
Space group
Z
P21
2
P212121
4
C2
P21
P1
P212121
4
P21212
4
2
1
1
Reflections
measured
96739
57565
62455
132448
34160
92530
92183
Independent
reflections
18060
0.0566
10824
9083
24382
10997
25817
12900
Rint
0.0618
0.0599
0.0907
0.0696
0.1106
0.0667
0.1013
0.0969
0.0413
0.0961
0.0826
0.0648
Final
R1 0.0404
values (I>=2σ
(I))
Final wR(F2) 0.1065
values (I>=2σ
(I))
0.1638
0.0630
0.1662
0.1918
0.0861
0.2088
0.1644
0.0809
0.1738
0.2545
0.1118
0.2702
0.2479
0.1008
0.2561
0.1691
0.0892
0.1893
Final
values
data)
R1 0.0412
(all
Final wR(F2) 0.1071
values
data)
(all
Flack
parameter
0.018(3)
1484835
0.059(9)
1484836
0.000(3)
1484837
0.044(5)
1484838
ꢀ0.24(3)
1484839
0.019(5)
1484840
0.020(6)
1484841
CCDC
Number
Table 1. Crystallographic and refinement data for compounds 1 – 7.
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RESULTS AND DISCUSSION
5
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8
Our previous work using H2AlaNDI and H2LeuNDI showed that both ligands were capable
of forming an M2L2 metallomacrocyclic motif when incorporated into coordination
polymers.32,33 In the case of polyꢀ[Cd(AlaNDI)], catenation of these rings occurs, whereas
this is not the case for the more sterically encumbered polyꢀ[Cd(LeuNDI)] material. The
phenylalanineꢀcontaining H2PheNDI is similarly bulky, with the added ‘complication’ of
phenyl rings that can compete against NDI···NDI interactions, and therefore the structure of a
PheNDI coordination polymer in the absence of any coligands was sought.
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The reaction of H2PheNDI with MnCl2 at 70 °C in a solvent mixture of DMF, MeOH and
H2O (2:1:1) yielded yellow crystals that were structurally characterized to reveal the
coordination polymer polyꢀ[Mn(DMF)2(HPheNDI)2]·MeOH·H2O (1). Whereas our
previously reported coordination polymers contain fully deprotonated amino acid NDI
ligands, only one of the carboxylic acids of each ligand in 1 is deprotonated. The MnII ion has
a distorted octahedral geometry in which the equatorial sites are occupied by two
monodentate carboxylate groups and two monodentate carboxylic acid groups, and the axial
sites are occupied by two DMF ligands (Figure 2). Each carboxylic acid group forms a
hydrogen bond with a carboxylate group coordinated to the same metal ion. The HPheNDI
ligands bridge between two metal nodes, forming a (4,4) sheet (Figure 2). The combination
of the πꢀrich naphthalene core and the phenyl side chain of the ligand leads to πꢀπ interactions
within the structure. There are two nearꢀparallel faceꢀtoꢀface πꢀinteractions between a phenyl
group of an HPheNDI ligand with the naphthalene core of a neighboring ligand within the
windows of the 2D sheets (closest C···C distances = 3.376(3) and 3.4355(9) Å). Two unique
nonꢀparallel πꢀinteractions exist between adjacent sheets (closest C···C distances = 3.2789(5)
and 3.3959(7) Å, interꢀplanar angles 32.148(7) and 33.197(9) °, respectively). Despite
repeated attempts, coordination polymers of the form [M(PheNDI)] could not be isolated.
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(a)
(b)
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Figure 2. (a) The metal coordination environment in the structure of 1, showing hydrogen
bonding as red dashed lines. (b) The 2D sheet of 1 showing πꢀπ interactions within the sheet
between the NDI core and the phenyl side chain as blue dashed lines (right). Hydrogen atoms
are omitted for clarity.
The influence of the addition of pyridyl coligands to the synthesis of coordination polymers
with carboxylate ligands has been explored in the literature as well as in our own work with
NDI ligands, and has shown to lead to a wide range of topologies and entanglement which
would not be possible in the absence of coligands.32,33,44ꢀ48 The synthesis of coordination
polymers utilizing PheNDI was therefore explored using a series of linear pyridyl coligands.
It is known from our earlier work that dipyridyl ligands are able to thread through bisꢀNDI
metallomacrocycles, therefore H2PheNDI, Cd(NO3)2·4H2O and 4,4’ꢀbipyridine (bipy) were
reacted at 85 °C in a solvent mixture of DMF, MeOH and H2O (2:1:1). The reaction yielded
crystals which were analyzed by Xꢀray crystallography to reveal a coordination polymer of
the formula polyꢀ[Cd(bipy)(OH2)(PheNDI)] (2). The CdII ion adopts a distorted pentagonal
bipyramidal geometry, with two chelating carboxylate groups from PheNDI ligands and an
aqua ligand occupying the equatorial sites and the nitrogen donor atoms of two bipy ligands
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occupying the axial sites (Figure 3). The PheNDI ligands adopt a “Uꢀshaped” geometry and
form {Cd2(PheNDI)2} metallomacrocycles similar to those previously reported.32,33 The
{Cd2(PheNDI)2} metallomacrocycles are bridged into a ladderꢀlike 1D chain by the bipy
ligands forming an apparent ‘tube’ of metallomacrocycles that propagates parallel to the cꢀ
axis. The plane of the naphthalene core of the PheNDI ligand is approximately perpendicular
(84°) to the plane of the bipy ligands, which allows πꢀπ stacking between these two ligands of
adjacent chains (closest C···C distance = 3.353(11) Å) (Figure 3). In addition to this faceꢀtoꢀ
face πꢀinteraction, the bipyridine is also nestled between the flanking phenyl side chains with
CH···π interactions from the bipy to the phenyl rings (CH···centroid distances = 2.7216(4)
and 2.9890(4) Å).
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Although the planes of the NDIs within the metallomacrocycles in 2 are 7.621(3) Å apart, a
suitable distance for πꢀπ interactions with an aromatic guest to form a polyrotaxane or
polycatenane, the metallomacrocycle remains ‘empty’. The structure of 2 exhibits 24.6 %
void volume (as calculated using Mercury),49 corresponding to the channels through the
metallomacrocycles in which no solvent was able to be modelled from the diffraction data.
Based on previous results of coordination polymers involving NDI metallomacrocycles, it
was expected that the bipy ligand may thread through the metallomacrocycle, given the
potential for favorable πꢀπ interactions. However, the configuration of the phenyl side chains
on the PheNDI ligand seems to generate too much steric bulk for two NDIs to approach
closely to each other, as demonstrated by the excellent size fit of the bipy ligands between the
phenyl rings. In a similar manner to the previously reported 1D→3D polyrotaxane, polyꢀ
[Cd2(LeuNDI)2(bipy)(DMF)3(H2O)], in which the isobutyl side chain of the LeuNDI ligand
was too bulky to allow polycatenation of the NDI metallomacrocycles and there were πꢀ
interactions between the NDI and the bipy, the structure of 2 demonstrates that the addition of
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bipy to an NDI ligand with a bulky side chain will lead to were πꢀinteractions between the
bipy and NDI ligands.
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(a)
(b)
(c)
Figure 3. The {Cd2(PheNDI)2} metallomacrocycle motif in the structure of 2, showing the
coordinated bipy ligands and the aqua ligand (left). The 1D chain of 2 (center). The packing
of the 1D chains by πꢀπ stacking between the bipy ligands of one chain with the NDIs of the
adjacent chains (right, aqua ligands are omitted for clarity).
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The influence of synthetic conditions on the structure of coordination polymers is
demonstrated by the reaction of H2PheNDI, Cd(NO3)2 and bipy at 120 °C in DMF, which
7
8
yielded
crystals
of
the
formula
polyꢀ
9
{[Cd2(bipy)2(PheNDI)2][Cd(bipy)(DMF)1.5(NO3)2(OH2)0.5]}ꢁDMF (3). The structure of 3
involves two different entangled coordination polymers, namely a 2D sheet, polyꢀ
[Cd2(bipy)2(PheNDI)2] and a 1D chain, polyꢀ[Cd(bipy)(DMF)1.5(NO3)2(OH2)0.5] (Figure 4).
The 2D sheet is structurally similar to the 1D ‘tubes’ in the structure of 2 and can be seen as
tubes that are connected sideꢀbyꢀside through bimetallic nodes. The {Cd2(PheNDI)2}
metallomacrocycles bridge between bimetallic nodes which have one carboxylate chelating to
each metal and two carboxylates that bridge the metals in a µꢀ1κO,2κOʹ coordination mode,
thereby forming a continuous chain of metallomacrocycles similar to those previously
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reported
with AlaNDI in polyꢀ[Cd2(AlaNDI)2(DMF)4] and LeuNDI in polyꢀ
[Cd2(LeuNDI)2(DMA)4].32,33 The CdII ion in the 2D sheet adopts a distorted octahedral
geometry with the bridging and chelating carboxylates in the equatorial sites and the bipy
ligands in the axial sites. Axial coordination of the bipy ligands on each CdII of the bimetallic
node allows formation of ‘double pillars’ between the bimetallic nodes to bridge the
metallomacrocycle chains into a (4,4) sheet (Figure 4).
The CdII ion in the 1D chain adopts a distorted octahedral geometry in which the axial sites
are occupied by the nitrogen donor atoms of bipy, two of the equatorial sites are occupied by
disordered DMF (75%) and H2O (25%) molecules, and the other two equatorial sites are
occupied by nitrate anions. The sheets of 2 cannot interpenetrate because the double pillars of
bipy ligands cannot fit within the metallomacrocycles, as single bipy ligands have been
shown
to
do
in
previously
reported
Instead,
1D→3D
the
polyrotaxane,
1D chains
polyꢀ
[Cd2(LeuNDI)2(bipy)(DMF)3(H2O)].32
of
[Cd(bipy)(DMF)1.5(NO3)2(OH2)0.5] thread through the metallomacrocycles, with nearꢀparallel
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πꢀπ interactions between the bipy of the chains and PheNDI ligands of the
metallomacrocycles (closest C···C distance = 3.42(2) Å) to form a polythreaded assembly or
polyꢀpseudoꢀrotaxane (Figure 4). Despite 2 and 3 involving the same chemical components,
the structures of these compounds are vastly different, due to the difference in reaction
conditions and reagent ratios, although interestingly the H2PheNDI:bipy ratio remains
constant in both reactions and both products are obtained as pure phases (see supplementary
information).
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The use of MnII in place of CdII yields an analogous structure to that of 3, highlighting the
relative consistency with which the metallomacrocycle forms. The reaction of H2PheNDI,
Mn(NO3)2 and bipy at 70 °C in a DMF/H2O mixture (2:1) yielded orange crystals of the
formula polyꢀ{[Mn2(PheNDI)2(bipy)2][Mn(bipy)(DMF)(NO3)2]} (4). The structure of 4 is
largely similar to that of 3, with 2D sheets of {Mn2(PheNDI)2} metallomacrocyclic chains
with bimetallic nodes bridged by double pillars of bipy ligands. Through these sheets pass 1D
[Mn(bipy)(DMF)(NO3)2] chains with πꢀπ interactions between the PheNDI ligands and the
bipy ligands (closest C···C distance = 3.369(10) Å). There are however subtle differences
between the structures of 3 and 4 due to the change in metal ion. The CdII in the 1D chains in
the structure of 3 have two coordinated nitrate ions and two disordered DMF/H2O
coordination sites, whereas the 1D chains in the structure of 4 involve two coordinated nitrate
ions and only a single, wellꢀordered DMF ligand, presumably due to the smaller radius of the
metal ion. The less symmetrical coordination environment of the metal ion in the 1D chain in
4 compared to that in 3 means that the overall structure of 4 is of lower symmetry,
crystallizing in P21 as opposed to C2 for 3. The influence of the metal on the material is also
observed through the thermal stabilities of the materials, shown by TGA. Both 3 and 4 show
similar amounts of mass loss due to loss of solvent, however they show ligand degradation at
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different temperatures. The beginning of ligand degradation in 3 occurs at 290 °C, while 4
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shows slightly less thermal stability, beginning to degrade at 260 °C.
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(a)
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(b)
(c)
Figure 4. (a) The [Cd2(PheNDI)2(bipy)2] 2D sheets in the structure of 3. (b) The
[Cd(bipy)(NO3)2(DMF)(H2O)] 1D chain in the structure of 3. (c) The polythreaded sheets of
3 showing the 2D sheets in red and the 1D chains in blue. Hydrogen atoms and phenyl rings
are omitted for clarity.
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Following on from the results obtained with bipy, an alternative ligand, 1,2ꢀdi(4ꢀ
pyridyl)ethylene (dpe) was employed in order to investigate the influence of a different, and
longer, coligand on the structure of coordination polymers with PheNDI. The reaction of
H2PheNDI, Cd(NO3)2 and dpe in a solvent mixture of DMF, MeOH and H2O (2:1:1) at 85 °C
yielded orange crystals which were structurally analyzed to show a coordination polymer of
the formula polyꢀ[Cd4(DMF)(dpe)4(OH2)2(PheNDI)4], 5. The asymmetric unit contains four
PheNDI ligands, four dpe ligands, four CdII ions, two aqua ligands, and one DMF ligand
(Figure 5). The four CdII ions form two crystallographically unique bimetallic nodes in the
asymmetric unit of the structure, each of which is part of a crystallographically unique 2D
sheet. Similarly to the structures of 3 and 4, the structure of 5 also involves {Cd2(PheNDI)2}
metallomacrocycles coordinated to these bimetallic nodes forming continuous chains, which
are then bridged by the dpe ligands to form (4,4) sheets. However, unlike the structures of 3
and 4, the four metal centers making up the two bimetallic nodes in 5 all have different
coordination spheres (Figure 5a). Cd(1) has a distorted pentagonal bipyramidal coordination
geometry. The equatorial sites of Cd(1) are occupied by five oxygen donor atoms from three
different carboxylate groups; one carboxylate solely chelates to Cd(1), another carboxylate
bridges between Cd(1) and Cd(2) in a µꢀ1κO,2κOʹ coordination mode and the third chelates
to Cd(1) whilst also bridging to Cd(2) in an overall µꢀ1κO,2κ2O,Oʹ coordination mode. The
axial sites of Cd(1) are occupied by a dpe ligand and a DMF ligand. Cd(2) has a distorted
trigonal bipyramidal geometry. The equatorial sites of Cd(2) are occupied by a monodentate
carboxylate group and the aforementioned two bridging carboxylate groups. Similarly to
Cd(1), the axial sites of Cd(2) are occupied by a dpe ligand and a solvent molecule, in this
case water. In the second bimetallic node, the coordination environment of Cd(3) is distorted
octahedral with the equatorial sites occupied by one chelating carboxylate group and two
carboxylate groups which bridge between the metals of the bimetallic node in a µꢀ1κO,2κOʹ
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coordination mode; the axial sites are occupied by a dpe ligand and an aqua ligand. Cd(4)
adopts a distorted square pyramidal geometry in which the basal sites are occupied by
carboxylate groups in the same coordination modes as Cd(3), and the apical site is occupied
by a dpe ligand.
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In a similar manner to the structures of 3 and 4, the {Cd2(PheNDI)2} metallomacrocycles in
the structure of 5 are connected through the bimetallic nodes to form 1D chains, and these
chains are bridged by coligands to form (4,4) sheets (Figure 5b. Note, this topological
description refers to an isolated sheet, ignoring interpenetration). In the structure of 5
however, the dpe ligands form single pillars between the nodes in contrast to the double
pillars of bipy in the structures of 3 and 4. The change from double pillars to single pillars
leads to a significant change in the interactions between polymers. The 2D sheets in the
structure of 5 are 2D→2D parallel interpenetrated by virtue of the single pillars of dpe
passing through the {Cd2(PheNDI)2} metallomacrocycles, with πꢀπ interactions between the
dpe and the PheNDI ligands giving a polyrotaxane motif (Figure 5c).15 The interpenetration
of 5 confirms that the double pillars of bipy present in the structures of 3 and 4 prevented
interpenetration between the sheets and that single, linear aromatic guests can pass through
the metallomacrocycles as intended, in a similar manner to the polyrotaxane polyꢀ
[Cd2(LeuNDI)2(bipy)(DMF)3(H2O)] reported previously.
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(b)
(c)
Figure 5. (a) The coordination environments of the two crystallographically unique
bimetallic nodes in the structure of 5. (b) One of the two crystallographically unique 2D
sheets in the structure of 5 (involving the Cd(1) and Cd(2) bimetallic node). (c) The 2D→2D
interpenetration of the two unique sheets. Hydrogen atoms and phenyl side chains are omitted
for clarity.
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The use of
a
wider dipyridyl ligand, N,Nʹꢀbis(4ꢀpyridyl)ꢀ1,4,5,8ꢀnaphthalene
5
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8
tetracarboxylic diimide (4PyNDI) was also investigated. 4PyNDI has a naphthalene core
group and so is a much more πꢀrich system than bipy or dpe, although is sterically more
demanding. The reaction of H2PheNDI, Cd(NO3)2 and 4PyNDI at 85 °C in a solvent mixture
of DMF, MeOH and H2O (2:1:1) yielded yellow crystals which were structurally
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characterized
to
reveal
a
coordination
polymer
of
formula
polyꢀ
[Cd2(PheNDI)2(4PyNDI)2]·6H2O·2DMF·MeOH (6). The asymmetric unit of 6 contains two
CdII metal centers, two PheNDI ligands, two 4PyNDI ligands and one nonꢀcoordinated DMF
(other lattice solvent molecules were assigned by analytical methods, see experimental
section). The structure of 6 contains bimetallic nodes in a similar manner to those reported in
the structures of compounds 1 and 3ꢀ5. The two CdII ions in the structure of 6 adopt different
coordination environments, very similar to Cd(1)/(2) in the structure of 5. Cd(1) has a
distorted octahedral geometry with the equatorial sites occupied by three different
carboxylate groups; one carboxylate solely chelating to Cd(1), one carboxylate bridging
between Cd(1) and Cd(2) in a µꢀ1κO,2κOʹ coordination mode and the third bridging between
Cd(1) and Cd(2) whilst also chelating to Cd(2) in an overall µꢀ1κO,2κ2O, Oʹ coordination
mode. The axial sites of Cd(1) are occupied by the nitrogen donor atoms of two 4PyNDI
ligands. Cd(2) has a distorted pentagonal bipyramidal coordination geometry in which the
equatorial sites are occupied by three different carboxylate groups, one solely chelating to
Cd(2) and two bridging carboxylates as described above. The axial sites of Cd(2) are
occupied by two 4PyNDI ligands. The 4PyNDI coligands that occupy the axial sites of each
metal in the bimetallic node form double pillars of the coligand in a similar manner to that in
the structures of 3 and 4.
Unlike the structures of 3, 4 and 5, the structure of 6 does not involve {Cd2(PheNDI)2}
metallomacrocycles, instead the PheNDI ligands form (4,4) sheets with bimetallic nodes
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(Figure 6a). The double pillars of 4PyNDI ligands bridge between the bimetallic nodes to link
these sheets into a 3D network which is a distorted pcu topology (Figure 6). Due to the
presence of two different NDIꢀbased ligands the structure of 6 is heavily influenced by πꢀπ
interactions. Within the (4,4) sheets there is a single πꢀπ interaction between the cores of two
PheNDI ligands across the 4ꢀmembered ring (closest C···C distance = 3.34(2) Å). The close
approach of two PheNDI ligands is allowed as they adopt a different conformation to those in
all other structures. In the structure of 6 the phenyl groups are oriented away from the central
core whilst in 2ꢀ5 the phenyl groups are positioned directly over the core thereby sterically
inhibiting interaction with another NDI group. In the structure of 1 one phenyl group is over
the core, and the other is facing outwards. The phenyl groups in the structure of 6 are not
involved in any faceꢀtoꢀface interactions, but do act as acceptors to CH··· π interactions from
NDIs. The two 4PyNDI ligands in the double pillars are involved in faceꢀtoꢀface πꢀπ
interactions with each other (closest C···C distance = 3.54(3) Å).
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(b)
(c)
Figure 6. (a) The (4,4) sheet formed by PheNDI ligands and bimetallic CdII nodes in the
structure of 6, with the faceꢀtoꢀface πꢀinteractions between the ligands shown as blue dotted
lines (phenyl groups and hydrogen atoms are omitted for clarity). The 3D network of 6 with
the {Cd(PheNDI)} sheets going into the page in molecular form (b) and topologically (c)
showing the centroid of the bimetallic node in purple, PheNDI ligands in red and double
ladders of 4PyNDI in blue.
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The ligand combination of PheNDI and 4PyNDI was also conducted with Mn(NO3)2 under
identical reaction conditions to those under which 6 was formed. The reaction yielded orange
crystals of the formula polyꢀ[Mn2(PheNDI)2(4PyNDI)2]·2DMF·2MeOH·H2O, 7. The
structure of 7 is almost identical to the structure of 6 with some differences induced by the
incorporation of a smaller metal ion. The asymmetric unit of the structure of 7 is half that of 6
and contains one MnII ion, one PheNDI ligand and one 4PyNDI coligand, as well as nonꢀ
coordinated solvent molecules. The MnII adopts a distorted octahedral geometry in which the
equatorial sites are occupied by carboxylate groups; one carboxylate is chelating solely to
Mn(1) and two carboxylates are bridging between Mn(1) and its symmetry equivalent in a µꢀ
1κO,2κOʹ coordination mode. The structures of 6 and 7 are modelled in different space
groups, P212121 and P21212, respectively, and the unit cell of 7 is half that of 6 (along the cꢀ
axis). Although the overall connectivity and topology of the structures of 6 and 7 are identical
the structural difference can be understood by the slightly different metal coordination
geometries, with the symmetry of the node broken in 6 by the bridging carboxylates (Figure
7). The comparison of 6 and 7 show a structural tolerance with a change in metal ion having
no influence on the coordination polymers which were synthesized with the same ligands
under identical synthetic conditions.
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Figure 7. The difference in the coordination environment within the bimetallic nodes in the
structures of 6 (left) and 7 (right) leads to a change in space group despite the overall
structure remaining the same.
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The series of coordination polymers obtained within this study shows an interesting trend
in thermal stabilities based on their dimensionality. The H2PheNDI ligand shows onset of
decomposition at 170 °C, before showing rapid mass loss at 300 °C. Thermal stability is
significantly increased by incorporation of the ligand into the coordination polymers. The
increase in dimensionality and interpenetration of the structures of the coordination polymers
appears to lead to higher degradation temperatures. The material consisting of 1D chains, 2,
shows degradation at 230 °C, while that comprising 2D sheets, 1, shows pronounced
degradation at 255 °C. The 1D + 2D → 2D polythreaded compounds, 3 and 4, are slightly
more thermally robust, degrading at 290 °C and 260 °C, respectively. The 2D + 2D → 2D
interpenetrated coordination polymer, 5, shows significantly more thermal stability than the
lower dimensionality or nonꢀinterpenetrated compounds, degrading at 380 °C. The 3D
coordination polymers, 6 and 7 are also much more thermally stable than those of lower
dimensionality, with the exception of 5, both showing degradation at 370 °C. It appears
therefore that within this system higher dimensionality or interpenetration will increase the
thermal stabilities of the coordination polymers (ESI Figure S15).
Given the presence of naphthalenediimides within the coordination frameworks, the
materials were tested for any solidꢀstate luminescence activity, although none was observed
(see supporting information).
CONCLUSIONS
Control of interpenetration in coordination polymers which incorporate a number of
different aromatic groups is subject to a variety of structural considerations. There exists
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potential for a number of different interactions to form, complicating the ability to predict
structure. The reported series of compounds demonstrates that the structures of coordination
polymers containing NDI ligands are heavily influenced by πꢀπ interactions and show
similarities with the previously reported alanine and leucine analogues.
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The common metallomacrocyclic motif of bisꢀamino acid NDIs, which has previously been
reported as recurring in structures of AlaNDI and LeuNDI, occurs in all of the coordination
polymers involving PheNDI alongside either 4,4’ꢀbipyridine or di(4ꢀpyridyl)ethene. These
results further demonstrate the reproducibility of this cyclic motif and its potential in
‘engineered’ systems. The alteration of synthetic conditions, typically solvent adjustment
which influences the primary coordination sphere (i.e. affecting the ability of the dipyridyl
coligand to coordinate), or the change in coligand was sufficient to change the dimensionality
of the coordination polymer and/or the presence of aromatic ‘guests’ inside the metallocycle
(i.e. interpenetration or polythreading). In the structures of 2 and 3/4, the solvation influences
the dimensionality (by capping the metals in 2, yielding a 1D polymer, but not in 3/4,
yielding a 2D polymer with more coꢀligand coordinating rather than water). Compound 5 has
significant structural similarity to 3/4, and to reported AlaNDI structures, with continuous
chains of metallocycles joined by bimetallic nodes.
The use of a larger coligand, di(4ꢀpyridyl)naphthalenediimide, yields two near identical 3D
coordination polymers (6 and 7) in which the metallocyclic motif is not present. The
difference in metal ions between these two compounds gives slightly different metal
coordination environments and therefore a different space group, however the overall
structure remains essentially unchanged.
The utilization of robust metallocyclic motifs in pseudoꢀdesigned interpenetration has clear
merit, although the vagaries of metal coordination environments still impart some degree of
unpredictability. The nature of the amino acid sideꢀchain in these instances appears to play a
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role in terms of competing intermolecular interactions and steric influences (which can
prevent catenation), particularly in the case of phenylalanine where significant competing π
interaction may form. Further investigations of the role of side chains in influencing the
behavior of the metallocyclic motif are underway.
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