Hydrophilic microporous lanthanide-organic frameworks based on 4,4′-biphenyldiacetate: Synthesis, crystal structures and sorption properties

Accepted Manuscript

Hydrophilic microporous lanthanide-organic frameworks based on 4,4'-biphe-

nyldiacetate: Synthesis, crystal structures and sorption properties

Gamall Makhloufi, Biju Francis, Janina Dechnik, Alexander Strzelczyk,

Christoph Janiak

PII:

S0277-5387(17)30075-X

http://dx.doi.org/10.1016/j.poly.2017.01.038

POLY 12442

DOI:

Reference:

Polyhedron

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Received Date:

Revised Date:

Accepted Date:

11 November 2016

20 January 2017

21 January 2017

Please cite this article as: G. Makhloufi, B. Francis, J. Dechnik, A. Strzelczyk, C. Janiak, Hydrophilic microporous

lanthanide-organic frameworks based on 4,4'-biphenyldiacetate: Synthesis, crystal structures and sorption

properties, Polyhedron (2017), doi: http://dx.doi.org/10.1016/j.poly.2017.01.038

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Hydrophilic microporous lanthanide-organic frameworks based on 4,4'-biphenyldiacetate:

Synthesis, crystal structures and sorption properties

Gamall Makhloufi^a, Biju Francis^a,b, Janina Dechnik^a, Alexander Strzelczyk^a, Christoph Janiak*^,a

^aInstitut für Anorganische Chemie und Strukturchemie, Heinrich-Heine Universität Düsseldorf,

Universitätsstr. 1, D-40225 Düsseldorf, Germany. Email: janiak@hhu.de

^bPermanent address: CSIR-Network of Institutes for Solar Energy, National Institute for

Interdisciplinary Science & Technology (NIIST), Thiruvananthapuram-695 019, India

* Corresponding author:

C. Janiak

Tel.: +49 211 81 12286; fax: +49 211 81 12287. E-mail address: janiak@hhu.de

Keywords: Metal-organic frameworks, MOFs; lanthanide organic frameworks, LOFs;

4,4'-biphenyldiacetic acid; gas adsorption; stability

1

Abstract

Fifteen microporous coordination polymers of symmetrical and flexible 4,4'-biphenyldiacetate

(bpda^2–) with lanthanide(III) cations (Ln³⁺) of formula {[Ln₂(bpda)₃(H₂O)]·2H₂O}_n(Ln = La (1), Ce

(2), Pr (3), Nd (4), Sm (5), Eu (6), Gd (7), Tb (8), Dy (9), Ho (10), Er (11), Tm (12), Yb (13), Lu (14),

Y (15)), have been synthesized and characterized. Single crystal X-ray diffraction (for 1, 3 and 4) in

combination with powder X-ray diffraction showed isostructural compounds with an infinite

secondary building unit (SBU). In this SBU two crystallographically independent lanthanide ions

are connected by three carboxylate groups in chelating-bridging mode to zig-zag Ln-carboxylate

chains. These chains are assembled by the biphenyl part of the ligand to a 3D network with water

molecules in channels with rectangular cross-sections of ~1.6 x 3 Å². Thermogravimetric analysis

indicates complete removal of the water molecules at 150 °C and thermal stability up to 380 °C.

Stability studies on compound 1 show no degeneration in crystallinity after refluxing in water or

different organic solvents. The dehydrated compound 1 exhibited no N₂or H₂uptake at 77 K and

no CH₄or CO₂uptake at 273 K up to 1 bar. High-pressure (17 bar, 273 K) CO₂uptake of

dehydrated [Ln₂(bpda)₃]_ncorresponded to the formation of {[Ln₂(bpda)₃]·CO₂}_nwith about 40% of

the adsorbed CO₂retained upon desorption. Water vapor sorption of activated and hygroscopic

[Ln₂(bpda)₃]_nreformed {[Ln₂(bpda)₃(H₂O)]·2H₂O}_n. 4,4'-Biphenyldiacetic acid (H₂bpda) shows blue

emission, its europium 6 and terbium 8 compounds show red and weak green luminescence under

UV light irradiation, with the bpda^2–ligand acting as an antenna ligand for the europium(III) ions.

1.

Introduction

Metal-organic frameworks (MOFs) are porous organic-inorganic hybrid materials built from metal

ions or clusters with bridging organic multidentate ligands. Different from amorphous silica gel and

active carbons, MOFs feature ordered structures with open frameworks of high crystallinity [1]. This

makes them attractive for potential applications such as gas storage [2] and separation [3],

catalysis [4] and water adsorption for heat transformation [5]. A special group of MOFs are the so

called LOFs (lanthanide-organic frameworks), which combine the luminescence properties of

lanthanide(III) ions with organic linkers resulting in promising potential light-emitting materials or

luminescent probes [6] and attract thereby growing attention over the last years [7,8]. Lanthanide

materials often exhibit sharp and intense luminescence emission lines, as a metal-based

luminescence from the mostly trivalent ions [7,9] due to transitions between 4f-states (e.g. red for

Eu³⁺, green for Tb³⁺) [10]. In LOFs, the organic ligands not only act as joints between the metals

but also as antenna (sensitizers) for the rare earth metals [11]. Compared to MOFs with transition

metal ions the construction of novel LOFs is more challenging due to their high coordination

numbers (up to 12) and more flexible coordination geometries. The prediction of topologies has

made progress in recent years in the area of molecular crystal engineering [12] but it is still very

2

demanding to predict network geometries [13]. LOFs often show high stability towards water [14] or

thermal stress [15].

Recently MOFs are investigated in connection with sorption-based environmentally friendly heating

and cooling technologies [16,17,18,19,20,21,22]. In this technology, a working fluid, typically water,

is driven to evaporation by an activated porous material. The heat of evaporation of the working

fluid then generates useful cold in a cooling case and heat of adsorption, which is released to the

environment, or used when the device is operated as a heat pump. For regeneration of the porous

adsorbent, heat from an external source such as a solar collector or waste heat is applied to

desorb the working fluid. The vapor of the working fluid is ad/desorbed reversibly. The sorption

material should be hydrophilic and water stable. For less water vapor stable porous materials a

working fluid like methanol may be used [17,23,24]. The hydrophilicity of lanthanide salts and the

potentially sizeable amount of labile water molecules in the coordination sphere of large lanthanide

cations prompted us to check on the water vapor sorption of lanthanide-organic frameworks.

In this work, we present a structural, stability and luminescence study of lanthanide(III)

coordination polymers based on the easy to synthesize organic linker 4,4'-biphenyldiacetic acid

(H₂bpda) [25] (Scheme 1a), which has been reported only once for transition metal coordination

polymers [26]. Thereby we identify a novel infinite lanthanide-carboxylate chain topology. Although

the coordination properties in the dinuclear lanthanum-carboxylate repeating unit are known [27],

the infinite elongation, with solely three carboxylate groups in chelating-bridging mode

(Scheme 1b) is a novel variant of carboxylate bridging motifs between Ln atoms.

Scheme 1 Structure of (a) lanthanide (Ln)-4,4’ biphenyldiacetate (bpda) showing the coordination

mode (µ-κO,O':O') and (b) dinuclear repeating unit as part of infinte zig-zag chain with the Ln

3

coordination numbers and coordination modes as well as carboxylate bridging mode observed in

compounds 1-15.

2.

Experimental Section

2.1.

Materials and methods

The chemicals used were obtained from commercial sources. No further purification has been

carried out. Doubly de-ionized water was used. The H₂bpda ligand has been synthesized from 4,4’-

bis(chloromethyl)-1,1’-biphenyl according to a previously reported [28] but modified procedure

(details in Supp. Info.). Elemental (CHN) analyses were performed on a Perkin Elmer CHN 2400

1

Analyzer. Electron impact (EI) mass spectra were obtained on a Finnigan MAT TSQ 7000. H-

NMR spectra were recorded with a Bruker Avance DRX-200 at 290 K. FT-IR spectra were

measured in ATR-mode (Platinum ATR-QL, Diamond) on a Bruker TENSOR 37 IR spectrometer in

the range of 4000-600 cm^-1. The intensity of absorption has been described with the terms strong

(s), medium (m) and weak (w). Solid-state fluorescence spectra (2D) were obtained with a

FluoroMax spectrometer from Horiba at 293 K. The samples were ground to a powder and pressed

to fit in the sample holder. The reflection angle was set to be 60°. Thermogravimetric analyses

(TGA) were done with a Netzsch TG 209 F3 Tarsus in the range 25 to 600 °C, equipped with Al-

crucible and applying a heating rate of 1 K·min^-1. Powder X-ray diffraction (PXRD) patterns were

collected on a Bruker D2 Phaser powder diffractometer equipped with a flat silicon, low

background sample holder using Cu-K_αradiation (λ = 1.5418 Å) with scan speed of 0.2 s/step and

a step size of 0.02°(2θ) at 290 K in the range of 2θ = 5-70°. The samples were ground in an agate

mortar in air till fine granularity. Simulated P-RD patterns were calculated with CCDC Mecury 3.6

program using the single-crystal data of compound 1. The isostructural character of 1-15 and the

phase purity of the bulk products were confirmed by comparing the experimental powder patterns

with the simulated powder pattern from the single-crystal dataset of compound 1. Experimental and

simulated power patters were always found in good agreement with each other (see Figure S5-S20

for compounds 1-15 in Supp. Info.). A HITACHI S-4800 FE SEM instrument was used for acquiring

scanning electron microscopy (SEM) images of gold-coated samples under vacuum. Nitrogen

physisorption isotherms (0-1 bar) for BET surface determination were measured with a

Quantachrome Nova 4200e at 77 K after activation by degassing at 473 K and 10^-3mbar for three

hours. Adsorption isotherms for H₂(77 K) and for CO₂and CH₄(273 K) up to 1 bar were measured

on a Micromeritics ASAP 2020 gas sorption analyzer. High-pressure adsorption properties for

carbon dioxide were recorded with a RUBOTHERM IsoSORB STATIC up to 17 bar and for H₂

high-pressure adsorption a Quantachrome iSorbHP1 was used up to 80 bar. All gases were of

ultra-pure grades (99.999 %). Solvent vapor adsorption properties were measured with a

Quantachrome Autosorb iQ MP automated gas sorption analyzer at 293 K using vapor from

solvents for analytical purposes grades (min. 99.5 % purity).

4

2.2.

Syntheses

For the syntheses, IR and CHN analytical details of compounds 2 and 5-15 see Supp.

Info.

2.2.1. {[La₂(bpda)₃(H₂O)]·2H₂O}_n(1)

A colorless solution of La(NO₃)₃·6H₂O (195 mg, 0.45 mmol, 2 eq) in water (5.5 mL) was

added to a yellow-orange solution of H₂bpda (182 mg, 0.67 mmol, 3 eq) in dimethylacetamide

(5.5 mL). The mixture became turbid and was transferred to a Pyrex^®tube and sealed with a

silicon cap. The tube was placed in a preheated oven at 90 °C for 3 d. After cooling to room

temperature the solvent was decanted off, the solid washed three times with dimethylacetamide

(5 mL) and three times with water (5 mL). Tiny colorless crystals of compound 1 were obtained

after drying at 50 °C overnight in vacuum at 50 mbar. Yield: 215 mg, 85 %. IR (ATR, cm^–1): 1981

(w), 1902 (w), 1659 (w), 1541 (s), 1527 (s), 1499 (m), 1415 (s), 1392 (s), 1284 (m), 1200 (w), 1153

(w), 1130 (w), 1024 (w), 1006 (w), 951 (w), 937 (w), 871 (m), 856 (w), 802 (m), 782 (w), 746 (m),

739 (m), 696 (m), 652 (m), 629 (m), 614 (w), 581 (w), 561 (w). Elemental analysis, calcd. (%) for

C₄₈H₃₈O₁₃La₂·2H₂O (1136.67 g/mol) C 50.72, H 3.72; for C₄₈H₃₈O₁₃La₂·1.5H₂O (1127.66 g/mol) C

51.13, H 3.66; found C 51.19, H 3.68. Thus, the observed values on the dried sample correspond

to compound 1 containing only about one and a half lattice water molecules per formula unit

instead of two as found from single crystal X-ray structure refinement. The phase purity of the bulk

product was confirmed by positively matching the experimental powder pattern and the simulated

powder patterns of compound 1 (Figure S5, Supp. Info.).

Compounds 3 and 4 were prepared following the synthetic procedure for 1 using two equivalents of

the Ln salt (0.45 mmol) and three equivalents of H₂bpda (182 mg, 0.67 mmol).

2.2.2. {[Pr₂(bpda)₃(H₂O)]·2H₂O}_n(3)

Tiny colorless crystals were obtained from Pr(NO₃)₃·5H₂O (188 mg). Yield: 227 mg, 89 %.

IR (ATR, cm^–1): 1980 (w), 1903 (w), 1664 (w), 1574 (m), 1525 (s), 1499 (m), 1416 (s), 1393 (s),

1286 (s), 1200 (w), 1154 (w), 1131 (w), 1023 (w), 1006 (w), 952 (m), 939 (w), 871 (w), 856 (w), 802

(m), 782 (w), 747 (m), 739 (m), 696 (m), 653 (m), 629 (w), 615 (w), 581 (w), 561 (w). Elemental

analysis, calcd. (%) for C₄₈H₃₈O₁₃Pr₂·2H₂O (1140.67 g/mol) C 50.54, H 3.71; for C₄₈H₃₈O₁₃Pr₂·1H₂O

(1122.66 g/mol) C 51.35, H 3.59; found: C 51.42, H 3.57.

2.2.3. {[Nd₂(bpda)₃(H₂O)]·2H₂O}_n(4)

Tiny colorless crystals were obtained from Nd(NO₃)₃·6H₂O (197 mg). Yield: 227 mg, 89 %.

IR (ATR, cm^–1): 1982 (w), 1961 (w), 1901 (w), 1701 (w), 1657 (w), 1578 (m), 1526 (s), 1499 (m),

5

1415 (s), 1394 (s), 1334 (w), 1286 (s), 1200 (w), 1154 (w), 1131 (w), 1109 (w), 1024 (w), 1006 (w),

952 (w), 939 (w), 871 (w), 857 (w), 802 (m), 782 (w), 747 (m), 697 (m), 653 (m), 629 (w), 616 (w),

582 (w), 562 (w). Elemental analysis, calcd. (%) for C₄₈H₃₈O₁₃Nd₂·2H₂O (1147.33 g/mol) C 50.25,

H 3.69; for C₄₈H₃₈O₁₃Nd₂·1H₂O (1129.32 g/mol) C 51.05, H 3.57; found: C 51.19, H 3.59.

2.3.

X-ray crystallography

Suitable single crystals placed in viscous oil were carefully selected under a polarizing microscope

and mounted in air onto a nylon loop. Accurate unit cell parameters were determined by a least-

squares fit of 2θ values and intensity data were measured on a Bruker APEX DUO with APEX II

CCD area detector equipped with microfocus sealed tube, Mo-Kα radiation (λ = 0.71073 Å) and

multilayer mirror monochromator at 100 K. The intensities were corrected for empirical absorption

based on multi-scan technique using the SADABS program [29]. The structures were solved by

direct methods and refined by full-matrix least-squares fitting on F2 by SHELX 97 [30]. All non-

hydrogen atoms were refined with anisotropic thermal parameters. Hydrogen atoms were placed in

calculated positions with fixed isotropic thermal parameters. Aromatic hydrogen atoms (CH =

0.95 Å) and CH₂(CH = 0.99 Å) were refined using a riding model (AFIX 43, AFIX 23, respectively)

with U_iso(H) = 1.2U_eq(C). The unique hydrogen atom on the solvent water molecule (oxygen atom

on the two-fold axis) which is coordinated to La1 was found and refined using DFIX O-H bond

length restraints with U_iso(H) = 1.2U_eq(O). The hydrogen atoms of the crystal water molecules could

not be found. Crystallographic data are summarized in Table 1. For the X-ray structure analyses

we picked the largest crystals we could locate in the compounds. Even these were of very small

size (cf. Table 1). It is known that very small crystals diffract weaker than larger crystals, resulting

in lower data quality and subsequent problems during refinement [31]. Some of the checkcif alerts

are due to the small crystal size, e.g., poor data/parameter ratio.

6

Table 1 Crystallographic data and structural refinements for compounds 1, 3 and 4.

Compound

Data set

1

3

4

bpda-La

1485624

C₄₈H₃₈O₁₅La₂

1132.60

0.08 x 0.01 x

0.01

bpda-Pr

bpda-Nd

1485626

C₄₈H₃₈O₁₅Nd₂

1142.25

CCDC no.

1485625

C₄₈H₃₈O₁₅Pr₂

1136.60

0.05 x 0.002 x

0.002

Empirical formula

M/g mol⁻¹

Crystal size/mm³

0.01 x 0.002 x

0.002

Exposure time (s)

Temperature/K

30

120

150(2)

100(2)

θ range/°(completeness)

2.22-21.18

(99.5%)

31; 10; 14

2.17-20.31

(98.3%)

2.55-18.37

(98.6%)

h; k; l range

35; 11; 16

-33,32; 11; -

15,14

Crystal system

Space group

Monoclinic

C 2/c (no. 15)

30.346(3)

10.0323(10)

14.3326(14)

97.694(6)

4324.1(7)

4

Monoclinic

C 2/c (no. 15)

30.231(5)

9.875(2)

Monoclinic

C 2/c (no. 15)

30.174(4)

9.9839(15)

14.099(2)

97.353(6)

4212.5(11)

4

a/Å

b/Å

c/Å

14.191(2)

97.128(10)

4203.7(14)

4

β/°

V/Å³

Z

D_calc/g cm⁻³

1.740

1.796

1.801

µ (Mo Kα)/mm⁻¹

2.022

2.366

2.513

F(000)

2240

2256

2260

Max./min. transmission

Reflections collected

Independent reflect. (R_int)

Data/restraints/parameters

Max./min. ∆ρ/e Å^−3a

R₁/wR₂[I>2σ(I)]^b

R₁/wR₂(all data)^b

Goodness-of-fit on F^2c

^aLargest difference peak and hole.

0.7447/0.6872

31453

0.7451/0.6512

16838

0.7449/0.6848

18788

2539 (0.1036)

2539/2/300

0.623/-0.538

0.0343/0.0726

0.0545/0.0818

1.034

3521 (0.1257)

3521/2/299

0.845/-0.829

0.534/0.0876

0.1083/0.1357

1.049

3092 (0.1249)

3092/2/298

0.641/-0.620

0.0492/0.0832

0.1007/0.1102

0.998

b

2

2 2

²)²]]^1/2.

∑[w(F

− F_c) ]/

∑(||F − |F ∑|F

∑[w(F

R₁= [

|

_c||)/

_o|]; wR₂= [

o

2 2

^cGoodness-of-fit = [∑[w(F_o²− F_c) ]/(n − p)]^1/2.

7

3.

Results and Discussion

3.1.

Synthesis

The tris(4,4'-biphenyldiacetato)dilanthanum(III) compounds have been obtained in high yields as

microcrystalline powders through solvothermal syntheses from two equivalents of lanthanide(III)

nitrates or chlorides with three equivalents of 4,4'-biphenyldiacetic acid in a 1:1 (v:v) mixture of

water and dimethylacetamide. All compounds are visually identical tiny needle-shaped crystals,

which appear yellowish in bulk material, except for the holmium and erbium compounds which are

orange and pink, respectively. The reproducibility of the syntheses was tested by repeated

preparations also with different stoichiometric ratios. Scanning electron microscopy (SEM) images

of the bulk material show about 20 µm large hedge-hog-like spheres, consisting of radially-grown

needle shaped crystals (Figure S1, Supp. Info.). The IR spectra of the compounds are similar

(Figure S23-S37, Supp. Info.). The absence of characteristic carboxyl bands at 907, 1247, 1422

and 1720 cm^-1for H₂bpda indicates the complete deprotonation and metal coordination of bpda^2–.

The strong absorption bands between 1280 and 1550 cm^–1indicate coordinated carboxylate

groups [32]. The compounds contain water as aqua ligands and crystal water as was evident from

elemental analysis and X-ray structure determination. Drying at 50 °C overnight in vacuum at

50 mbar removes part of the crystal water as evidenced from the difference of the X-ray

crystallographically found water and the CH analysis.

3.2.

Crystal structure

The lanthanum, praseodymium and neodymium compounds 1, 3 and 4, respectively, were

structurally characterized using single-crystal X-ray analyses. For the remaining compounds, the

single crystals were too small and the isostructural nature of all compounds was established by

powder X-ray diffraction analyses (Figure 1). From single-crystal X-ray analysis the isostructural

compounds are formulated empirically as 3D coordination polymers of formula

{[Ln₂(bpda)₃(H₂O)]·2H₂O}_n(Ln = La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu, Y; bpda =

4,4'-biphenyldiacetate dianion). The structure of compound 1 is described here in detail

(compounds 3 and 4 are isostructural).

8

Figure 1 As-synthesized powder X-ray diffractograms for 1-15 in comparison to illustrate the

isostructural character of these compounds.

The asymmetric unit consists of two half-occupied crystallographically independent lanthanum(III)

cations, one and a half symmetry independent bpda-ligands, a half-occupied aqua ligand (with O7

Figure 2a) and one crystal water molecule. Atom La1 sits on a 2-fold rotation axis and La2 on an

inversion center. The "half"-ligand with donor atoms O5 and O6 sits on the special position of an

inversion center between the C₆rings of the biphenyl system. This gives a formula unit of two

lanthanum(III) cations with three bpda anions plus an aqua ligand and two crystal water molecules.

The adjacent lanthanum(III) ions are connected by three bridging oxygen atoms from three

carboxyl groups with a La-La distance of 4.013 Å. Each of these three carboxyl groups is chelating

or ƞ²-bound to one of the La atoms and bridging or ƞ¹-bound towards the other La atom through

one of the carboxyl oxygen atoms.

The two lanthanum(III) cations differ in their coordination numbers and coordination spheres (cf.

Scheme 1). La1 shows coordination number nine by the coordination of eight oxygen donors from

six carboxylate groups and one oxygen atom from a water molecule (Figure 2b). Two of the six

carboxylate groups on La1 are chelating (ƞ²) and four are coordinated through a bridging oxygen

atom (ƞ¹). The coordination polyhedron of La1 is unsymmetrical and cannot be classified

topologically (Figure 2c). La2 shows elongated square-bipyramidal coordination (Figure 2d) by the

coordination of ten oxygen donors from six carboxylate groups (Figure 3b). Four of the six

carboxylate groups on La2 are chelating (ƞ²) and two are coordinated through a bridging oxygen

atom (ƞ¹) (see also Scheme 1b). The La-O (carboxylate) bond distances range from 2.475(4) to

2.763(6) Å and the La-OH₂(aqua ligand) bond is 2.413(11) Å. These values are in good

agreement with previously reported nine- or ten-coordinated lanthanum(III) complexes with oxygen

donor ligands [33]. Selected bond distances are presented in Table 2.

9

(a)

(d)

(b)

(c)

(d)

Figure 2 (a) Extended asymmetric unit in 1 with (b-d) coordination environment around the

crystallographically independent lanthanum(III) atoms (50% thermal ellipsoids, except for O7 with

−x+1, y, −z+3/2; ii

20%). Water of crystallization is not shown for clarity. Symmetry codes: i =

=

−z+1.5; iii −x+1, −y+1, −z+1; v −x+1/2, −y+1/2, −z+1; vi

−x+1/2, y+1/2,

= x+1/2, y+1/2, z; iv =

=

−

−1/2, z; vii −x+3/2, −y+1/2, −z+1; viii

−1/2, z. Selected bond distances and

= x+1/2, y

x 1/2, y

=

angles are given in Table S4 in Supp. Info. For bond lengths, see Table 2 (for bond angles see

Table S4 in Supp. Info.).

Table 2 Selected bond distances for [La₂(bpda)₃(H₂O)]·2H₂O ^a

La1—O1

La1—O2

La1—O4ⁱⁱ

La1—O5

La1—O7

2.575(4)

2.602(5)

2.499(6)

2.477(5)

2.413(11)

La2—O1

La2—O3ⁱⁱⁱ

La2—O4ⁱⁱⁱ

La2—O5

La2—O6

2.475(4)

2.554(6)

2.641(5)

2.763(6)

2.514(5)

^aSelected bond angles are given in Table S4 in Supp. Info.. Symmetry transformations: ii = -x+1/2,

y+1/2, -z+1.5; iii = x+1/2, y+1/2, z.

Overall the bpda ligand bridges between four La atoms with bis-tridentate chelating-bridging (ƞ²:ƞ¹)

coordination behavior in trans-configuration (of the carboxylate groups) towards the four

coordinated La atoms (cf. Scheme 1b). In the only previous structural investigation of metal-bpda

compounds [26] the ligand shows trans-configuration, but coordination of the individual carboxyl

groups differ from solvent hydrogen bonding up to penta-dentate behavior towards transition

metals in different coordination environments (cf. Scheme 2).

Scheme 2 Known coordination modes of bpda with different metal atoms [26].

The lanthanum atoms and the chelating-bridging carboxyl groups of bpda form infinite zig-

zag chains along the crystallographic

c direction, with La1 atoms occupying the vertex

positions (Figure S2a, Supp. Info.). The aqua ligand and the crystal water molecules

connect adjacent chains along the

Info.).

b direction by hydrogen bonding (Figure S2b, Supp.

Along the a direction, the bpda linkers connect each chain to four surrounding chains giving a 3D

coordination network (Figure 3). In this compound each zig-zag chain can be seen as an infinite

one-dimensional SBU. The sole chelating-bridging (ƞ²:ƞ¹) coordination of the carboxylate groups to

lanthanide atoms in this chain shows a novel variant of known Ln-carboxylate bridging motifs. The

(ƞ²,ƞ¹) coordination was known in the infinite one-dimensional SBU of La with the o-toluic acetate

ligand [27,34] which, however, additionally include an (ƞ¹:ƞ¹) bridging-mode for the carboxylates,

depicted between La1 and La3 in Figure S3 (Supp. Info.) [34b].

11

(a)

(b)

Figure 3 (a) and (b) chain connectivity along the a and b axis in the 3D coordination network of 1

(crystal water molecules are not shown in the 3D network)

The unit cell packing of 1 can be seen as a typical separation [35] of the hydrophilic lanthanum-

carboxylate zig-zag chains with the aqua ligand on La1 and the crystal water molecules and the

hydrophobic biphenyl rings (sandwiched between the hydrophilic layers) (Figure 3). The

lanthanum-carboxylate zig-zag chains with the aqua ligand on La1 and the crystal water molecules

are arranged in layers parallel to the bc plane. So is the biphenyl moiety of the bpda linker.

Hydrophilic and hydrophobic bc layer then alternate along a (Figure 3).

The crystal water molecules and aqua ligands reside in channels along the c direction (Figure S2b,

Supp. Info., Fig. 3b). Removal of the water molecules and aqua ligands should give rise to

channels with rectangular cross-sections of approximately 1.6×3 Å²(taking into account the van

der Waals radii of surrounding atoms) depicted in Figure S4 (Supp. Info.). Elemental (CH) analyses

indicated that 1.5 to 2 crystal water molecules (out of 2) were already removed after drying at

50 °C overnight in vacuum at 50 mbar The coordinated and crystal water molecules can be fully

removed through heating the sample at 150 °C and ambient pressure (Figure 4 for TGA) with

retention of crystallinity and network structure (Figure 5).

3.3.

Stability and Analysis

Prior to thermogravimetric analysis (TGA) the samples were dried at 50 °C overnight in vacuum at

50 mbar, followed by storing at ambient conditions under air. The TGA profiles of all compounds,

1-15, exhibited a weight loss of 3.6-5.3 % in the first step (25-150 °C), which corresponds to the

loss of one to three water molecules per formula unit (Figure 4, Figure S38-S52, Supp. Info.). This

is in good agreement with the crystal structure, which indicates the presence of one terminal

coordinated water molecule and two lattice water molecules. Complete dehydration could be

achieved by drying sample of 1 at 200 °C and 10^-3mbar for three hours. Such an activated sample

(activ. in Fig. 4) was then immediately subjected to TG analysis and showed essentially no weight

loss up to ~380 °C. When the fully dehydrated sample was stored under ambient air with 60-70 %

humidity, the water uptake could be followed by the mass gain with time. After 10 min already

1 mol water/mol MOF was adsorbed again. The subsequent water uptake occurs slowly

(Figure S54, Supp. Info.).

Figure 4 Thermogravimetric analysis (TGA) curves of compound 1, 3, 4 and an activated sample

of 1 (activ.). (For detailed mass difference assignments see Figure S38-S52, Supp. Info.)

In the TG analyses, after the water removal a slight weight loss was observed up to ~380 °C. Then

an abrupt weight loss above 380 °C corresponds to the decomposition of the compounds, as

exemplarily depicted in Figure 4 for compound 1, 3 and 4. No attempt was made to identify the

decomposition products. The remaining mass percent at 600 °C indicates the existence of

miscellaneous decomposition products. It is known that the final decomposition of lanthanide

organic compounds to Ln₂O₃occur at higher temperatures up to 1200 °C [36].

The lanthanum compound 1 was used as an example for the stability studies against water and

other solvents and degassing of the {[Ln₂(bpda)₃}_ncompounds (Figure 5). The powder X-ray

diffraction (PXRD) pattern for the sample heated (activated) at 200 °C for three hours under high

vacuum (10^-3mbar) is identical to that of the as synthesized sample, which indicates that the loss

of the lattice water and coordinated water molecules does not lead to an observable phase

transformation (Figure 5, details in Figure S20, Supp. Info.). The crystalline compounds 1-15 are

insoluble in water or common organic solvents such as dimethylacetamide, ethanol or

dimethylformamide. Refluxing a sample of 1 for two hours in different organic solvents and neutral

water did not show any variations or just slightly broader reflections in the powder pattern

(Figure 5, Figure S21 and S22, Supp. Info.). Also after water sorption of the activated sample the

PXRD matches the simulation for {[La₂(bpda)₃(H₂O)]·2H₂O}_n(1).

3.4.

Gas and vapor adsorption

13

The gas sorption properties were explored exemplarily for the lanthanum-compound 1. The crystal

structure indicates that the cross sections of the water-depleted channels (~1.6 x 3 Å²) may be too

small for most gases and solvent molecules. Yet, through the flexible ligand there might be the

possibility of increasing pore size akin to the 'breathing' effect of MIL-53 which exists in a large-

pore and water-filled small pore structure [37]. Also, the [Ni₂(4,4'-bipy)₃(NO₃)₄]_n· 2EtOH network

has only very small channels with dimension of ca. 1.1 Å x 1.1 Å [38,39]. The two ethanol guest

molecules per formula unit are rather located in cavities ("zero-dimensional closed space") [38].

These ethanol molecules are hydrogen-bonded to the nitrate ligands and are removed upon

heating to 100 °C and reversibly re-adsorbed again by exposure of the activated network to

ethanol vapor. It is suggested that the passage of ethanol molecules through the small 1.1 Å x

1.1 Å channel windows proceeds by a temporary expansion of the window size. N₂adsorption

measurement on [Ni₂(4,4'-bipy)₃(NO₃)₄] at 77 K showed that under these conditions N₂is not

adsorbed [40].

Removal of the coordinated and crystal water molecules from {[Ln₂(bpda)₃(H₂O)]·2H₂O}_ncould give

a porous framework with coordinatively unsaturated and Lewis-acidic lanthanide ions. The

absence of changes in the powder XRD pattern after removing the coordinated and lattice water

molecules (Figure 5) indicates framework stability. So, we examined the nitrogen, methane,

hydrogen and carbon dioxide adsorption properties in the pressure range up to 1 bar. For testing of

pore dilatation, we also examined the high-pressure adsorption properties of hydrogen (100 bar)

and carbon dioxide (17 bar). Before the sorption measurements a preparative-scale sample of

compound 1 was heated at 200 °C under vacuum for three hours, causing complete dehydration

(cf. TG analysis of 1 that indicated complete dehydration at 150 °C and ambient pressure, "activ."

in Figure 4).

Figure 5 Powder X-ray diffractograms (PXRDs) for compound 1 as synthesized (a.s.), after

refluxing in water for two hours (reflux), after degassing at 200 °C in high vacuum for three hours

(activated) and after water adsorption experiment (sorption) in comparison with the simulated

14

diffractogram (sim) which based on the X-ray data refinement of compound 1 (for details see

Figure S20-S22, Supp. Info.).

The adsorption measurements of nitrogen (at 77 K), methane and carbon dioxide (at 273 K) up to

1 bar did not show any appreciable gas uptake. Hence, no BET surface area could be derived from

very low N₂adsorption. Also in subsequent low- and high-pressure H₂adsorption measurements

(at 77 K) we did not observe a gas uptake.

At the cryogenic temperature of 77 K, diffusion of N₂and similar sized molecules into small

micropores is very slow. In other words, with slow thermal motion at 77 K the N₂molecules will

statistically only seldom approach the small pore aperture with the correct orientation for

penetration, that is, in line with its molecule axis. Diffusion limitations at this temperature influence

adsorption in micropores (particularly in pores smaller than 7 Å diameter). Such materials would

require time-consuming N₂adsorption measurements and may still have under-equilibration of

measured adsorption isotherms, which will give lower than real surface areas. Such errors can

usually be avoided by using CO₂adsorption analysis at 273 K. The saturation pressure of CO₂at

0 °C is very high (~26141 Torr), so that low relative pressure measurements for the micropore

analysis are achieved in the range of moderate absolute pressures (1–760 Torr). At 273 K CO₂

molecules can more easily access small micropores than N₂at 77 K [41]. The kinetic diameters of

the used gases were 3.64 Å for N₂, 3.3 Å for CO₂, 2.9 Å for H₂and 3.8 Å for CH₄. For activated 1 it

still required higher CO₂pressure to enforce a significant CO₂uptake. The carbon dioxide high

pressure adsorption measurement up to 17 bar gave a gas uptake of 39 mg CO₂per gram

compound 1 (corresponding to 1 mol CO₂per formula unit of [La₂(bpda)₃], that is,

{[Ln₂(bpda)₃]·CO₂}_n) (Figure 6). In the degassing process to 0 bar over 40 % of the adsorbed

carbon dioxide was retained (16 mg) in 1, corresponding to 0.4 mol CO₂per formula unit of

[La₂(bpda)₃]) (Figure 6). We suggest that the high affinity of the coordinatively unsaturated open

metal site on La1 (to which a water molecule was coordinated) to oxygen atoms leads to

chemisorption, that is, coordination of carbon dioxide molecules to every second La1 atom.

Thereby, we also explain the low carbon dioxide uptake up to 1 bar (at 273 K). The CO₂molecules

can block the pore mouths by chemisorption to La1 and thereby prevent a further access of carbon

dioxide molecules at low pressure.

15

Figure 6 High-pressure CO₂sorption isotherms for [La₂(bpda)₃] (activated 1) at 20 °C.

Figure 7 Methanol and water sorption isotherm for [La₂(bpda)₃] (activated 1) at 20 °C. For

calculation of the uptake to gram/gram the value in cm³/g (STP) is divided by 22,414 cm³/mol (the

molar gas volume at 0 °C and 1 atm = 1.013 bar) and multiplied by the molar mass of the

adsorptive (32.04 g/mol for MeOH and 18.02 g/mol for water). This means division of the cm³/g

value by 700 cm³/g for MeOH and 1243 cm³/g for water.

The same retention effect could be observed in solvent vapor ad- and desorption measurements.

For methanol and water vapor, activated 1 shows an uptake of 20 cm³/g MeOH (28.5 mg/g at p/p₀

= 0.9) and 63 cm³/g H₂O (51 mg/g at p/p₀= 0.9), corresponding to ~1 mol MeOH and 3 mol H₂O

per formula unit of [La₂(bpda)₃] (M = 1100 g/mol), respectively (Figure 7). Desorption at 10^–2mbar

and 293 K. retains over 80 % of the adsorbed methanol molecules and 50 % of the adsorbed water

molecules remain in [La₂(bpda)₃]. Thus, high vacuum and heating is required to remove the

coordinated molecules, analogous to the activation step at 200 °C.

16

The small methanol uptake relative to water is due to the larger molecule size. The methanol

molecules will coordinate to the unsaturated La1 site with its methyl group occupying the in-

between space (the space previously taken by the crystal water molecule).

3.5.

Luminescence properties

The solid-state luminescence spectra of the free ligand, H₂bpda and visible-light emitting

europium- and terbium-compounds, 6 and 8, were recorded at room temperature and are depicted

in Figure 8a. H₂bpda exhibited a broad emission band with the peak maximum at around 434 nm,

when excited at 365 nm, which imparts a blue color (Figure 8b). The molar absorptivity values of

Ln³⁺absorption transitions are very low (< 100 L mol⁻¹cm⁻¹). The europium compound 6 exhibited

the emission band of the ligand and the characteristic emission peaks of europium(III) ion (at 579,

592, 616, 651 and 698 nm) when excited at the ligand excitation band (λ_ex= 370 nm). This

indicates that the sensitization of the Eu³⁺ions via the excited states of the ligand are efficient and

that the bpda^2–ligand acts as an antenna ligand for europium(III) ions. The most intense transition

5

7

is observed at 616 nm, corresponding to the hypersensitive D₀→ F₂transition, which is

responsible for the red color of compound 6, under UV irradiation (Figure 8b). The peaks at 579,

592, 651 and 698 nm result from the deactivation of the excited ⁵D₀state to the ground ⁷F₀, ⁷F₁, ⁷F₃

and ⁷F₄levels, respectively (see Fig. S55 in Supp. Info. for assignment).

A close match between the energy of the ligand triplet state and the energy of the receiving 4f level

of the lanthanide ion is not desirable, because energy back transfer from the lanthanide ion to the

5

triplet state of the ligand can occur. The main resonance level, D₀, of Eu³⁺ion is situated at 579

nm (17 250 cm^–1), which may be energetically compatible for energy transfer from blue or green

emitting ligands like H₂bpda. Whereas, the resonance level of Tb³⁺ion is situated at 489 nm (20

430 cm^–1), so the back energy transfer from the metal to ligand may be present in this case [42].

When excited at the ligand excitation band (λ_ex= 370 nm), the emission peaks of terbium(III) ion in

compound 8 are found to overlap with the strong ligand emission band, implying that the ligand

bpda^2–is a poor sensitizer for terbium(III) ions. The peaks at 488, 544, 584 and 621 nm arise due

5

7

to the radiative deactivation of the excited D₄state to the corresponding/different ground F_J(J =

6, 5, 4 and 3, respectively) levels of terbium(III) ions (see Fig. S56 in Supp. Info. for assignment).

The peak at 544 nm (⁵D₄→ ⁷F₅) gives the green color to compound 8 (Figure 8b).

17

Figure 8 (a) Solid-state room temperature emission spectra of compound 6 (red) and compound 8

(green) along with the spectrum of the free ligand H₂bpda (blue). Spectra with peak assignments

for 6 and 8 are given in Figure S55 and Figure S56 in Supp. Info. (b) Images of the compounds

under UV-irradiation red for 6, green for 8 and blue for H₂bpda.

We have decided not to examine magnetic properties, because it is known that bridging carboxyl

groups mediate only a very weak magnetic exchange interaction between lanthanide(III) ions and

then the lanthanide(III) ions display essentially single-ion behavior [43].

4.

Conclusion

We have presented fifteen novel isostructural 3D lanthanide metal-organic frameworks

[Ln₂(bpda)₃(H₂O)·2H₂O] with 4,4'-biphenyldiacetate (bpda^2–)linker. Despite its flexibility, the linker

shows only one coordination motif towards Ln³⁺. The triply chelating-bridging (ƞ²:ƞ¹) coordination of

three carboxylate groups between two lanthanide atoms gives a zig-zag chain as an infinite one-

dimensional SBU. The triply-bridging (ƞ²,ƞ¹) coordination of three carboxylate groups is a variant of

previous infinite one-dimensional Ln-carboxylate SBUs which also featured simple bridging (ƞ¹:ƞ¹)

carboxylate groups. The 3D metal-ligand arrangement gives a channel structure with coordinated

aqua ligands and lattice water molecules in these channels. The water molecules in these

channels can be reversibly removed in vacuum without observing a phase transition by powder X-

ray diffraction. The dehydrated material [La₂(bpda)₃] (activated 1) is hygroscopic and shows a

water uptake of about 2.5 mol/mol MOF under ambient air (about 60-70 % humidity). Complete

water uptake to 3.0 mol/mol MOF as found in the crystal structures could be achieved at 100 %

humidity during water sorption experiments. The small channel cross-section of ~1.6 x 3 Å²

18

prevents gas adsorption at low pressures up to 1 bar. CO₂adsorption was enforced in activated 1

at pressures up to 17 bar to a composition of {[La₂(bpda)₃]·CO₂}_nand about 40 % of the adsorbed

CO₂was retained upon desorption. Furthermore the framework is robust against different solvents

and thermal stress.

Acknowledgements

The work was supported in part by BMBF project OptiMat 03SF0492C. We thank Dr. Christian

Reichenbach from Quantachrome GmbH Leipzig for the high pressure hydrogen adsorption

measurement of compound 1.

Appendix A. Supplementary material

CCDC 1485624-1485626 contain the supplementary crystallographic data for compounds 1, 3 and

4. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html,

or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK;

fax: (+44) 1223-336-033; or e-mail: deposit@ccdc.cam.ac.uk.. Supplementary data associated with

this article (Synthesis of 4,4'-biphenyldiacetic acid, additional images of crystal structure, powder

X-ray patterns, IR-spectra, TGA curves, gas and vapor sorption isotherms, fluorescence spectra

and crystallographic details of compounds 1, 3 and 4) can be found, in the online version, at

http://dx.doi.org/10.1016/j.poly.2016.XXXXX.

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23

Ultramicroporous hygroscopic lanthanide organic frameworks show high-pressure CO₂uptake with

40% CO₂retention upon desorption.

Article Doi

Hydrophilic microporous lanthanide-organic frameworks based on 4,4′-biphenyldiacetate: Synthesis, crystal structures and sorption properties

DOI: 10.1016/j.poly.2017.01.038

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Full text of DOI:10.1016/j.poly.2017.01.038

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