Thermal transformation of a layered multifunctional network into a metal-organic framework based on a polymeric organic linker

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pubs.acs.org/JACS

Thermal Transformation of a Layered Multifunctional Network into a

MetalÀOrganic Framework Based on a Polymeric Organic Linker

Patrícia Silva,^†Fabiana Vieira,^†Ana C. Gomes,^†Duarte Ananias,^†Josꢀe A. Fernandes,^†Soﬁa M. Bruno,^†

Rosꢀario Soares,^‡Anabela A. Valente,^†Jo~ao Rocha,^†and Filipe A. Almeida Paz*^,†

^†Department of Chemistry and ^‡Central Analytical Laboratory, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

S Supporting Information

b

ABSTRACT: The preparation of layered [La(H₃nmp)] as micro-

crystalline powders from optimized microwave-assisted synthesis or

dynamic hydrothermal synthesis (i.e., with constant rotation of the

autoclaves) from the reaction of nitrilotris(methylenephosphonic acid)

(H₆nmp) with LaCl₃7H₂O is reported. Thermogravimetry in con-

3

junction with thermodiﬀractometry showed that the material under-

goes a microcrystal-to-microcrystal phase transformation above 300 °C,

being transformed into either a three-dimensional or a two-dimensional

network (two models are proposed based on dislocation of molecular

units) formulated as [La(L)] (where L^3À= [À(PO₃CH₂)₂(NH)-

(CH₂PO₂)O_1/2À]_n^3nÀ). The two crystal structures were solved from

ab initio methods based on powder X-ray diﬀraction data in conjunc-

tion with structural information derived from ¹³C and ³¹P solid-state

NMR, electron microscopy (SEM and EDS mapping), FT-IR spectroscopy, thermodiﬀractometry, and photoluminescence studies.

It is shown that upon heating the coordinated H₃nmp^3Àanionic organic ligand undergoes a polymerization (condensation) reaction

to form in situ a novel and unprecedented one-dimensional polymeric organic ligand. The lanthanum oxide layers act, thus,

simultaneously as insulating and templating two-dimensional scaﬀolds. A rationalization of the various steps involved in these

transformations is provided for the two models. Photoluminescent materials, isotypical with both the as-prepared

([(La_0.95Eu_0.05)(H₃nmp)] and [(La_0.95Tb_0.05)(H₃nmp)]) and the calcined ([(La_0.95Eu_0.05)(L)]) compounds and containing

stoichiometric amounts of optically active lanthanide centers, have been prepared and their photoluminescent properties studied in

detail. The lifetimes of Eu³⁺vary between 2.04 ( 0.01 and 2.31 ( 0.01 ms (considering both ambient and low-temperature studies).

[La(H₃nmp)] is shown to be an eﬀective heterogeneous catalyst in the ring opening of styrene oxide with methanol or ethanol,

producing 2-methoxy-2-phenylethanol or 2-ethoxy-2-phenylethanol, respectively, in quantitative yields in the temperature range

40À70 °C. The material exhibits excellent regioselectivity to the β-alkoxy alcohol products even in the presence of water. Catalyst

recycling and leaching tests performed for [La(H₃nmp)] conﬁrm the heterogeneous nature of the catalytic reaction. Catalytic

activity may be attributed to structural defect sites. This assumption is somewhat supported by the much higher catalytic activity of

[La(L)] in comparison to [La(H₃nmp)].

In the past few years, our research group has been particularly

interested in the use of tripodal ﬂexible organic molecules to

construct multidimensional coordination polymers. Our strategy

has been focused on the simultaneous replacement of carboxylic

acid (CA) groups by phosphonic acid (PA) moieties and

transition metal centers by rare-earth cations. Phosphonate

moieties are interesting chelating moieties for the construction

of MOFs¹²because while the three tetrahedral oxygen atoms

mimic the building units of zeolites (thus providing various

anchoring sites for a wide range of metals) they also induce the

formation of networks with high thermal and mechanical robust-

ness, which is an important prerequisite for possible applications

(for example, catalytic activity at high temperatures).¹³Our aim

1. INTRODUCTION

MetalÀOrganic Frameworks (MOFs) and, in a more wide

sense, coordination polymers remain nowadays one of the most

studied topics in synthetic chemistry.¹The ﬁeld evolved con-

siderably from the initial quest to design and isolate novel

frameworks with unusual (and exotic) structural architectures²

into the ﬁeld of materials chemistry, sometimes with direct

industrial applications.³Indeed, the unique combination into

the same crystalline hybrid network of metal centers and organic

molecules promotes the formation of compounds with interest-

ing properties and structural features. For example, MOFs can

now be employed as eﬀective heterogeneous catalysts,⁴in the

selective adsorption of gases,⁵as contrasting agents,⁶and in the

fabrication of membranes⁷or thin ﬁlms⁸and exhibit interesting

photoluminescent properties,⁹enantioselective separation of

molecules,¹⁰and magnetism.¹¹

Received: June 7, 2011

Published: August 04, 2011

r

2011 American Chemical Society

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is to design more (mechanically and thermally) robust networks

containing photoluminescent probes,¹⁴ultimately envisaging the

potential fabrication of novel devices.¹⁵In this context, we have

investigated several families of structures, all layered and based

on: nitrilotriacetic acid (3 CA groups),¹⁶N-(phosphonomethyl)-

iminodiacetic acid (2 CA and 1 PA groups),¹⁷(carboxymethyl)-

iminodi(methylphosphonic acid) (1 CA and 2 PA),¹⁸and also

nitrilotris(methylenephosphonic acid) (H₆nmp; 3 PA groups).¹⁹

Surprisingly, the two latter organic molecules have been to date

scarcely employed in the construction of multidimensional net-

works, either with d- or f-block elements.^19,20

Scheme 1. T_mP_nt_pDiagram for the Optimization of the

Microwave-Assisted Hydrothermal Synthesis of

[La(H₃nmp)] (1)^a

Following our recent motivation to explore faster and more

eﬃcient synthetic approaches with higher yields and smaller

chemical waste, such as microwave heating,²¹in the present

manuscript we report the isolation, full structural characteriza-

tion, and heterogeneous catalytic activity of a novel layered

material: [La(H₃nmp)]. We further demonstrate that the com-

pound undergoes a microcrystal-to-microcrystal transformation

(2Df3D or 2Df2D) with temperature, yielding an unprece-

dented compound with empirical formula [La(L)] (where L^3À

=

[À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ). Noteworthy, this

latter compound is obtained by the in situ condensation of

residues of H₆nmp, leading to the formation of a one-dimen-

sional (i.e., inﬁnite) organic ligand. To the best of our knowledge,

this type of ligand transformation, which also leads to a (poorly)

crystalline material, has never been reported to date. Optical

centers, in particular Eu³⁺, have been engineered into the frame-

works of [La(H₃nmp)] and [La(L)] to produce photolumines-

cent materials with lifetimes above 2 ms. We further explore the

use of [La(H₃nmp)] and [La(L)] as a heterogeneous catalysts in

the ring opening of styrene oxide. Epoxide ring-opening reac-

tions with alcohols are an important transformation in the

synthesis of β-alkoxy alcohols, which can be employed as

valuable organic solvents, natural products, or intermediates, in

organic and inorganic synthesis.²²Noteworthy, early ring-open-

ing reactions resulted in polymerization and low selectivity

because of the required high temperatures. We show in this

work that both the layered and the MOF materials are highly

regioselective catalysts in the conversion of styrene oxide into

2-methoxy-2-phenylethanol (or 2-ethoxy-2-phenylethanol) by

epoxide ring opening with methanol (or ethanol) under rela-

tively mild reaction conditions. It is further shown that the

catalytic reaction is heterogeneous in nature, and the active

sites may be structural defects. The MOF compound [La(L)]

revealed excellent regioselectivity, similar to [La(H₃nmp)], and

the catalytic activity was much higher.

^aLegend: solid blue circle, phase-pure 1;shadedcircle, mixtureof1with other

phases, which include [La(H₃nmp)] 1.5H₂O or other unknown materials.

3

variables to obtain (i) phase-pure and (ii) highly crystalline materials

while (iii) exhibiting uniform crystal habit and size distribution

had to be performed (results in the following subsection).

As for related systems, these materials are likely to be isolated

as microcrystalline powders. La³⁺was therefore selected as the

pivot metallic node because: (i) it promotes diamagnetic net-

works which can be studied using solid-state NMR techniques

(to derive information on the composition of the asymmetric

unit) and (ii) it is of great usefulness to prepare a nonemitting (in

the visible region) hybrid matrix which can be doped with

stoichiometric amounts of emitting lanthanide centers such as

Tb³⁺and Eu³⁺. In addition, small percentages of these optically

active metal centers can also boost the observed photolumines-

cent properties by reducing self-quenching eﬀects.

2.1.1. Microwave-Assisted Hydrothermal Synthesis: Optimi-

zation. The optimization process covered a wide range of

experimental conditions (see Materials and Methods for details).

Several compounds have been prepared (Scheme 1) and

inspected using, mainly, a combination of powder X-ray diffrac-

tion and electron microscopy (SEM imaging and EDS analyses).

Phase-pure [La(H₃nmp)] (1) can be isolated over a wide

range of experimental conditions. At 120 °C, mixtures of 1 with

other phases, either known or unknown, are mainly observed.

Considering that mild and faster experimental conditions are

usually preferred, 140 °C arises as the optimal minimum

temperature to isolate 1, independently of either the power of

the irradiation or the time of reaction employed.

2. RESULTS AND DISCUSSION

2.1. Structural Design and Synthesis Strategy. Our main

2.1.2. Crystal Morphology. Crystal morphology and average

particle size vary slightly according to the selected reaction

conditions. Crystals of 1 exhibit similar platelike morphology

for a wide range of conditions with individual crystallites being

stacked into different types of aggregates, such as flower- and

bone-like ones (Figure 1a). Crystals are, thus, very regular thin

(in the nanometer range) plates which grow uniformly without

the presence of precipitates (or other defects) in the matrix. This

was clearly observed using TEM studies (Figure 1b). Because of

the sensitivity of 1 toward the electron beam (most likely due to

the presence of the organic component), more detailed high-

resolution TEM studies could not be performed.

objective focuses on the preparation of compounds topologically

similar to the layered [La(H₃nmp)] 1.5H₂O network¹⁹while

3

using a faster, simpler, and also environmentally more friendly

synthetic procedure, thus envisaging future potential applications

in the fabrication of functional materials. Microwave-assisted

heating can address some of these requirements.²¹Because the

experimental conditions cannot be directly transposed from

conventional hydrothermal synthesis into microwave-assisted

synthesis (even though a relatively identical composition for

the reactive system was indeed used—see dedicated Materials

and Methods), a systematic variation of the experimental

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Figure 2. Powder X-ray diﬀraction patterns and SEM images of the bulk

materials with stoichiometric amounts of Eu³⁺and Tb³⁺lanthanide

centers and prepared using either microwave-assisted synthesis or

dynamic hydrothermal synthesis.

pronounced eﬀect on particle dimension as noticeable in Figure 1d

when comparing samples 1ÀT₁₄₀P₅₀t₁and 1ÀT₁₆₅P₅₀t₁.

2.1.3. Dynamic Hydrothermal Synthesis of [La(H₃nmp)].

Under comparable synthetic conditions (i.e., temperature and

chemical composition of the reactive mixtures), [La(H₃nmp)]

(1) cannot be prepared using typical static hydrothermal synthesis:

a mixture of [La(H₃nmp)] 1.5H₂O and a novel, poorly crystal-

3

line and unidentified phase is always isolated. We envisaged that

stirring during the synthesis (as it occurs in the microwave-assisted

method) could be a key experimental factor for the successful

preparation of 1. By employing experimental conditions rela-

tively similar to those optimized for the microwave-assisted

synthesis, 1 was isolated in just 48 h of reaction when the reaction

vessel was kept under constant rotation at 165 °C. Noteworthy,

the optimal frequency of rotation was found to be 25 rpm.

2.1.4. Engineering Optical Centers into the Materials. Opti-

cally active compounds containing stoichiometric amounts of

Eu³⁺or Tb³⁺were isolated as microcrystalline [(La_0.95Eu_0.05)-

(H₃nmp)] and [(La_0.95Tb_0.05)(H₃nmp)] bulk materials, with

only small differences in crystallite size and morphology being

observed (see SEM pictures in Figure 2). As registered for the parent

[La(H₃nmp)] (1) material, photoluminescent [(La_0.95Eu_0.05)(L)]

(where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ) can

also be readily prepared by calcination of the mixed-lanthanide

[(La_0.95Eu_0.05)(H₃nmp)] compound (Figure 2, top). EDS mapping

studies (Figures S1ÀS5 in the Supporting Information in the ESI)

clearly show that: (i) Eu³⁺and Tb³⁺centers were incorporated

Figure 1. (a) SEM image of [La(H₃nmp)] (1ÀT₁₄₀P₅₀t₁) showing the

two main diﬀerent types of crystallite aggregates isolated directly from

the reaction vessel. (b) TEM images (bright ﬁeld) of 1ÀT₁₄₀P₅₀t₁

depicting the nanosized thickness and homogeneity of the crystallites.

Eﬀect on the crystallite morphology and size when varying (c) the

reaction time and (d) the temperature.

On average, the increase of the reaction time of the microwave-

assisted synthesis is concomitant with an average increase of the

particle size, even though smaller crystals can also be isolated for

longer periods of irradiation (see, for example, the SEM picture

of the material 1-T₁₄₀P₅₀t₁₀, Figure 1c below). This eﬀect can

also be promoted by the fact that long microwave irradiation periods

are also known to lead to the partial destruction of the prepared

compounds, as reported for other MOF-type materials.²³

Distinct irradiation powers do not seem to produce signiﬁcant

changes in either the size or the shape of the crystallites (data not

shown). Conversely, an increase of the temperature of reaction has a

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Figure 3. Final Rietveld plot (powder X-ray diﬀraction data) of [La(H₃nmp)] (1). Observed data points are indicated as red circles, and the best-ﬁt

proﬁle (upper trace) and the diﬀerence pattern (lower trace) are drawn as solid black and blue lines, respectively. Green vertical bars indicate the angular

positions of the allowed Bragg reﬂections. Reﬁnement details are given in Table 1. Inset: Crystal packing of 1 viewed in perspective along the [100]

direction of the unit cell.

into the prepared networks; (ii) the distribution of these elements is

completely random inside the crystallites; (iii) the inclusion of these

optical centers occurs in an identical fashion using either microwave-

assisted synthesis or dynamic hydrothermal synthesis.

highly distorted tricapped trigonal prism as depicted in Figure 4b.

The LaÀO bond lengths were found in the 2.487(12)À2.932-

(11) Å range (Table 2). We note that the latter, relatively long

bond length is directly associated with the P1 phosphonate group

which is deeply embedded inside the lanthanum oxide layer,

establishing physical connections between four adjacent La³⁺

cations (see Figures S11a and S11b in the Supporting Information

for detailed views). This type of behavior has already been reported

2.2. Crystal Structure Elucidation and Description of

[La(H₃nmp)] (1). Materials produced using either microwave-

assisted hydrothermal synthesis or the dynamic hydrothermal

approach are systematically isolated as microcrystalline powders,

which prevented a priori a straightforward structural elucidation from

single-crystal X-ray diffraction studies. As in our previous study

for [Pr(H₃nmp)] 1.5H₂O but with a slightly distinct coordina-

3

tion mode: while in 1 the P1 phosphonate group is O,O-chelated

to two La³⁺cations, in the Pr³⁺-containing material this type of

chelation only occurs to one neighboring metal center; this also

seems to be the structural reason for the existence of a nine-

coordination geometry in the present material, contrasting with

the eight-coordinated dodecahedral environment of our previous

on the [La(H₃nmp)] 1.5H₂O and [Ln(H₂cmp)(H₂O)] [Ln³⁺

=

3

Y³⁺, La³⁺, Pr³⁺, Nd³⁺, Sm³⁺, Eu³⁺, Gd³⁺, Tb³⁺, Dy³⁺, Ho³⁺,

and Er³⁺; H₅cmp = N-(carboxymethyl)iminodi(methylphosphonic

acid)] materials,^18bsolid-state NMR studies provided fundamental

data on the composition of the asymmetric unit: the ³¹P MAS

spectrum of 1 shows the presence of three well-resolved isotropic

resonances in the À8 to 14 ppm spectral region (Figure S7 in the

Supporting Information); the ¹³C{¹H} CP MAS spectrum further

revealed the existence of six resonances, grouped into three pairs of

peaks separated by ca. 126À133 Hz (within experimental error;

Figure S6 in the Supporting Information), which is consistent with

the existence of spin multiplets arising from ¹J_C,Pcoupling (usually in

the 5À90 Hz range).²⁴In summary, this technique clearly supports

the existence of a single crystallographically unique H_6Àxnmp^Àx

residue in the asymmetric unit. Additionally, and based on the per-

formed photoluminescence studies (see dedicated section below), the

material should also contain a single independent lanthanide center.

The crystal structure of [La(H₃nmp)] (1) was unveiled from

ab initio X-ray powder studies (Figure 3). The material crystallizes in

the noncentrosymmetric orthorhombic Pca2₁space group

(Table 1), with the composition of the asymmetric unit being

in good agreement with the assumptions highlighted above. The

single La³⁺center is nine-coordinated, {LaO₉}, to a total of seven

phosphonate groups arising from four symmetry-related H₃nmp^3À

anionic ligands, with the coordination polyhedron resembling a

structure.¹⁹While in [Pr(H₃nmp)] 1.5H₂O the layer arrange-

3

ment was mainly based on the presence of a 1D chain of Pr³⁺

polyhedra, in 1 the distribution of the metallic centers within the

layer is signiﬁcantly more uniform (as shown in Figure 4a) with the

intermetallic La La distances varying only between ca. 4.59 and

3 3 3

4.82 Å (for reference, in [Pr(H₃nmp)] 1.5H₂O the intralayer

3

Pr Pr distances varied instead between ca. 4.24 and 5.89 Å).

Another striking distinction between 1 and [Pr(H₃nmp)]

3 3 3

3

1.5H₂O concerns the connectivity among adjacent lanthanide

polyhedra: in the latter compound layers are based on 1D chains

formed by edge-shared Pr³⁺polyhedra, further interconnected

via a phosphonate group to yield the layer; in 1, each La³⁺

polyhedron is connected instead to another four via corner

sharing, leading to a signiﬁcantly more dense structure (Figure 4a).

This connectivity in 1 seems to be the main driving force for the

existence of an almost planar and inﬁnite lanthanum oxide layer

(inset in Figure 3), while in [Pr(H₃nmp)] 1.5H₂O the phos-

3

phonate bridge induces ﬂexibility, ultimately promoting a kink in

the layer and an empty space within the crystal structure ﬁlled

with uncoordinated water molecules.

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Table 1. X-ray Data Collection, Crystal Data, and Structure Reﬁnement Details for [La(H₃nmp)] (1) and the Two Models for

3nÀ

[La(L)] (2) (where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

)

1

2 (model I)

2 (model II)

Unit Cell

formula

formula weight

crystal system

space group

a/Å

C₃H₉LaNO₉P₃

434.93

C₃H₇LaNO₈P₃

416.92

orthorhombic

Pca2₁

orthorhombic

Pca2₁

orthorhombic

Pca2₁

9.1762(3)

11.7369(4)

9.8340(3)

1059.12(6)

4

9.7660(6)

11.3438(3)

8.7307(5)

967.21(8)

4

8.7247(5)

11.3493(3)

9.7728(6)

967.69(8)

4

b/Å

c/Å

volume/Å³

Z

D_c/g cm^À3

2.728

2.863

Proﬁle Parameters

proﬁle function

pseudo-Voigt

ThompsonÀCoxÀHastings

pseudo-Voigt

ThompsonÀCoxÀHastings

pseudo-Voigt

Caglioti law parameters (for 1)

U = 0.115(6)

V = À0.079(3)

W = 0.0263(4)

Y00 = 3.48(6);

Y00 = 3.44(5);

spherical harmonics for size broadening

Y20 = À1.7(1);

Y22+ = À0.8(1);

Y40 = À0.9(1);

Y42+ = À1.84(8);

Y44+ = 0.71(9)

0.0183(4) and 0.0036(1)

À

Y20 = 0.4(1);

Y22+ = À0.85(8);

Y40 = À0.6(1);

Y42+ = À0.09(7);

Y44+ = 0.20(7)

0.0162(3) and 0.0038(1)

À

orthorhombic crystal system (for 2)

asymmetry parameters (up to 20° 2θ)

zero shift [2θ°]

0.0343(7) and 0.0218(2)

À0.0159(8)

Reﬁnement Details

no. of independent reﬂections

no. of global reﬁned parameters

no. of proﬁle reﬁned parameters

no. of intensity-dependent reﬁned parameters

653

1

516

0

512

0

9

13

52

13

52

56

Reliability Factors for All Nonexcluded Data Points with Bragg Contribution (Conventional—Not Corrected for Background)

R_p

7.15

9.78

2.38

17.5

5.14

6.81

6.52

1.09

4.33

5.61

6.51

0.74

R_wp

R_exp

χ²

Structure Reliability Factors

R_Bragg

R_F

9.58

14.7

7.90

20.0

6.93

19.0

Indexing Figures-of-Merit

M(n)

F(n)

44.3 (n = 20)

66.3 (n = 20)

14.3 (n = 19)

19.0 (n = 19)

14.3 (n = 19)

19.0 (n = 19)

The {LaO₉} coordination polyhedron of 1 is water-free (as

(Figure 5). Thermogravimetric analysis in conjunction with

thermodiffractometry clearly shows that the overall crystalline

structure of the material remains intact up to ca. 280 °C. A small

continuous weight drop registered between ca. 200 and 250 °C is

mainly attributed to physisorbed and chemisorbed water mol-

ecules arising from local structural defects because, as described

above, 1 does not contain any water molecules in its crystal

structure composition. Between 250 and 400 °C the material

undergoes a crystalline phase transition with the first reflection

(corresponding to the (010) plane) being shifted to higher 2θ

values while getting significantly broader. This shift points to a

reduction of the unit cell dimensions, in particular along the

[010] direction, which further indicates a closer proximity

observed in [Pr(H₃nmp)] 1.5H₂O), and the material itself does

3

not contain water. Consequently, the interlayer bridges based on

OÀH O hydrogen bonds reported for [Pr(H₃nmp)]

3 3 3

3

1.5H₂O are completely absent in 1. The protonated phospho-

nate groups are instead engaged in strong hydrogen bonding

interactions which cover the outer shell of the two-dimensional

layers as depicted in Figure 4a. Individual layers pack in a parallel

fashion along the [010] direction of the unit cell, leading to the

crystal structure depicted as an inset in Figure 3.

2.3. Thermogravimetry and Thermodiffractometry. The

thermal behavior of the bulk layered [La(H₃nmp)] (1) material

was investigated between ambient temperature and ca. 800 °C

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Table 2. Bond Lengths (in Å) and Angles (in Degrees) for the

La³⁺Coordination Environment in [La(H₃nmp)] (1)^a

La1ÀO1ⁱ

2.487(12)

2.706(12)

2.501(15)

2.671(15)

2.518(11)

La1ÀO3ⁱⁱⁱ

La1ÀO4ⁱⁱⁱ

La1ÀO7

La1ÀO8ⁱ

2.932(11)

2.634(8)

La1ÀO1ⁱⁱ

La1ÀO2ⁱⁱⁱ

2.503(11)

2.601(11)

La1ÀO2ⁱⁱ

La1ÀO3

O1ⁱⁱÀLa1ÀO1ⁱ

O1ⁱÀLa1ÀO2ⁱⁱ

O1ⁱÀLa1ÀO2ⁱⁱⁱ

O1ⁱⁱÀLa1ÀO2ⁱⁱ

O1ⁱⁱÀLa1ÀO2ⁱⁱⁱ

O1ⁱÀLa1ÀO3

O1ⁱÀLa1ÀO3ⁱⁱⁱ

O1ⁱⁱÀLa1ÀO3ⁱⁱⁱ

O1ⁱⁱÀLa1ÀO3

O1ⁱÀLa1ÀO4ⁱⁱⁱ

O1ⁱⁱÀLa1ÀO4ⁱⁱⁱ

O1ⁱÀLa1ÀO7

O1ⁱⁱÀLa1ÀO7

O1ⁱÀLa1ÀO8ⁱ

O1ⁱⁱÀLa1ÀO8ⁱ

O2ⁱⁱⁱÀLa1ÀO2ⁱⁱ

O2ⁱⁱÀLa1ÀO3

O2ⁱⁱÀLa1ÀO3ⁱⁱⁱ

128.6(5)

O2ⁱⁱⁱÀLa1ÀO3

O2ⁱⁱⁱÀLa1ÀO3ⁱⁱⁱ

O2ⁱⁱÀLa1ÀO4ⁱⁱⁱ

O2ⁱⁱⁱÀLa1ÀO4ⁱⁱⁱ

O2ⁱⁱÀLa1ÀO7

O2ⁱⁱⁱÀLa1ÀO7

O2ⁱⁱÀLa1ÀO8ⁱ

O2ⁱⁱⁱÀLa1ÀO8ⁱ

O3ÀLa1ÀO3ⁱⁱⁱ

O3ÀLa1ÀO4ⁱⁱⁱ

O3ⁱⁱⁱÀLa1ÀO4ⁱⁱⁱ

O3ÀLa1ÀO7

71.4(6)

54.5(5)

70.5(6)

77.4(5)

142.8(5)

67.2(6)

78.9(5)

132.60(4)

122.3(5)

112.2(5)

76.5(5)

81.7(5)

61.3(5)

78.3(5)

123.8(5)

135.4(5)

149.2(5)

73.3(5)

76.1(6)

113.0(6)

55.1(6)

116.7(6)

151.4(5)

59.1(5)

158.9(5)

62.1(5)

96.1(5)

82.8(5)

74.9(5)

136.9(5)

79.3(5)

76.9(4)

147.5(5)

116.4(6)

119.9(6)

O3ⁱⁱⁱÀLa1ÀO7

O3ÀLa1ÀO8ⁱ

O3ⁱⁱⁱÀLa1ÀO8ⁱ

O4ⁱⁱⁱÀLa1ÀO7

O4ⁱⁱⁱÀLa1ÀO8ⁱ

O7ÀLa1ÀO8ⁱ

^aSymmetry transformations used to generate equivalent atoms: (i) 1/2

À x, y, 1/2 + z; (ii) 1 À x, 1 À y, 1/2 + z; (iii) À1/2 + x, Ày, z.

temperature range 25À800 °C as depicted in Figure S9 (in the

Supporting Information). Despite the several (and discernible)

consecutive weight losses, it is not possible to unequivocally

identify which molecular entities are being released at one given

temperature. It is, however, possible to observe that the residue

reminiscent at ca. 800 °C agrees well with the stoichiometric

formation of the inorganic residue LaP₃O₈(by assuming a change

of the oxidationstate of one phosphorus from+5 to +3). This result

indicates the complete transformation of the material into an

inorganic phase, which is in agreement with the TGA results

previously discussed for 1.

Figure 4. (a) Schematic representation of a portion of the two-

dimensional network of [La(H₃nmp)] emphasizing the connectivity

of the H₃nmp^3Àorganic residues to the lanthanum oxide core. Intralayer

hydrogen bonding interactions are represented as blue-ﬁlled dashed

lines (symmetry codes have been omitted for clarity): N1ÀH1C O3

3 3 3

with d_D_{3 3 3}_A= 2.823(13) Å and <(DHA) of ca. 128°; O5ÀH5A O9

with d_D_{3 3 3}_A= 2.328(15) Å and <(DHA) of ca. 126°; O6ÀH6A O5

3 3 3

with d_D_{3 3 3}= 2.720(15) Å and <(DHA) of ca. 170°. (b) Highly

A

distorted {LaO₉} tricapped trigonal prismatic coordination environ-

ment of La³⁺. For selected bond lengths (in Å) and angles (in degrees),

see Table 2. Symmetry transformations used to generate equivalent atoms:

(i) 1/2 À x, y, 1/2 + z; (ii) 1 À x, 1 À y, 1/2 + z; (iii) À1/2 + x, Ày, z.

2.4. Crystal Structure Elucidation and Description of

[La(L)] (2). The crystalline material isolated in the ca.

300À400 °C temperature range was studied using ³¹P solid-

state NMR (Figure S8 in the Supporting Information). Three

sharp resonances peaking at ca. À7.1, À3.3, and 10.5 ppm remain

indicative of the presence of three distinct crystallographic sites

for phosphorus, coherent with the previous results for

[La(H₃nmp)] (1), still supporting the existence of a single

crystallographically independent H_6Àxnmp^Àxresidue in the

asymmetric unit of the newly formed compound. The observed

chemical shifts are slightly distinct from those registered for 1

(ca. À3.6, 3.2, and 8.4 ppm; Figure S7 in the Supporting

Information), indicating that the local environments of the phos-

phorus atoms were modified in the process. We further note the

presence of a very broad resonance centered at about À26.4

ppm, typical of purely inorganic compounds,²⁵with its large full-

width-at-half-maximum (fwhm) indicating an amorphous phase.

Photoluminescence studies (see below) also indicate the exis-

tence of such phase, which already seems to exist in the parent

[La(H₃nmp)] (1) compound even though it passes undetected

using most of the characterization techniques. This secondary

between lanthanum oxide layers. Indeed, in the 250À400 °C

temperature window the compound loses approximately 4.2% of

total weight, which agrees well with the release of one water

molecule per formula unit (calculated of about 4.1%). We note

that this occurrence is coherent with the observed structural

transformation described in the following subsection. Above

400 °C additional weight losses are attributed to the full

decomposition of the organic component, with thermodiffracto-

metry indicating the existence of an almost amorphous com-

pound. At 680 °C the residue was identified (PDF4-release 2010

database) as a crystalline mixture composed of monazite (LaPO₄,

ICDD 00-032-0493) and lanthanum catenatriphosphate

(LaP₃O₉, ICDD 01-084-1635).

A representative portion of the [La(L)] (2) material (where

L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ) was iso-

lated by calcining the parent compound 1 at 340 °C for ca. 3

h. As already suggested by the previous studies, the slow heating

of 2 promotes a continuous release of mass over the wide

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Figure 5. Thermogram and variable-temperature powder X-ray diﬀraction studies of [La(H₃nmp)].

inorganic phase seems to contain a small amount of protons

because the resonance is significantly less visible in the ³¹P{¹H}

CP MAS spectrum (Figure S8 in the Supporting Information).

As in the parent material 1, and despite the considerable

decrease in overall crystallinity, structural models for the struc-

ture of the calcined material were derived from powder X-ray dif-

fraction data (Figure 6). The material was formulated as [La(L)]

(2) (where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ),

which depending on the model (I or II) could be either a 2D or a

3D network. Electron microscopy imaging shows that the overall

crystal morphology is not signiﬁcantly aﬀected by the thermal

treatment (see, for example, Figure 2 at the top and the inset in

Figure 6). We thus infer that the observed reduction of crystal-

linity must then occur within the crystallites, as observed from

the TEM studies at the nanolevel depicted in Figure S22 in the

Supporting Information: the crystals contain holes and nano-

domains with distinct contrast. The most remarkable struc-

tural feature of 2, indicated by either of the derived models,

concerns the existence of a one-dimensional inﬁnite organic

ligand (see the following section for additional details) either

decorating or pillaring the lanthanum oxide layers reminiscent

from the parent compound (Figure 7). In model I, this

polymeric organic linker runs parallel to the [100] crystal-

lographic direction, in a wavelike fashion with a period of the

length of the a-axis of the unit cell. Conversely, in model II the

same polymeric organic linker is parallel to the [001] direction

(period of the c-axis).

coordinated to a total of six phosphonate moieties due to the

rupture of one LaÀO bond to form the polymeric ligand (details

in subsection 2.7). The resulting highly distorted square anti-

prismatic (model I) or bicapped trigonal prismatic (model II)

coordination environments have, in average, signiﬁcantly longer

LaÀO bond lengths when compared to 1: found in the 2.647-

(9)À2.823(9) Å and 2.543(10)À2.657(9) Å ranges (Tables 3

and 4). While the P1 phosphonate group remains strongly O,O-

chelated to two neighboring La³⁺cations as in 1, the two

remaining phosphonate moieties modify considerably their

environment in the two models: in model I, while one group

establishes a new μ₂-bridge between adjacent cations, the other

does not interact with any metal center being instead involved in

the formation of the pyrophosphonate bridge (P P distance of

3 3 3

about 2.95 Å); in model II, both groups are interacting with an

adjacent La³⁺cation as depicted in Figure S13 in the Supporting

Information. This transformation induces, for the two models,

small changes in the lanthanum oxide layer, with adjacent

{LaO₈} polyhedra being edge-shared which ultimately leads to

closer intermetallic distances as depicted in Figure 8.

2.5. Vibrational Spectroscopy. Vibrational FT-IR spectros-

copy studies support, on the one hand, the highlighted structural

features of [La(H₃nmp)] (1) and, on the other, that it is highly

sensitive to the structural transformation that leads to the

formation of [La(L)] (2).²⁶Figure 9 focuses on the ca.

1500À700 spectral region of the FT-IR spectra of 1 and 2,

including assignments for each main observed band. We note

that the most relevant differences were clearly observed in this

region, which corresponds to that where the various vibrational

modes of PÀO groups appear. A representation of the progres-

sive modification of selected spectral regions with increasing

temperature (i.e., when going from compound 1 to 2 by thermal

Independently of the model for 2, upon thermal treatment the

coordination environment of the crystallographically indepen-

dent La³⁺center changes considerably as depicted in Figure 8.

On one hand, the coordination number is reduced to eight in

both models, {LaO₈}, because the metal center is now only

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Figure 6. Final Rietveld plot (powder X-ray diﬀraction data) for the two models derived for the [La(L)] material (where L^3À= [À(PO₃CH₂)₂-

(NH)(CH₂PO₂)O_1/2À]_n^3nÀ). Observed data points are indicated as red circles, and the best-ﬁt proﬁle (upper trace) and the diﬀerence pattern (lower

trace) are drawn as solid black and blue lines, respectively. Green vertical bars indicate the angular positions of the allowed Bragg reﬂections. Reﬁnement

details are given in Table 1. Inset: SEM picture of the bulk material and a pictorial representation of the average crystallite shape as calculated from the

Rietveld reﬁnement (image created using GFourier²⁹from the FullProf.2k³⁷software suite).

treatment) is provided as Supporting Information (Figure S10).

Ligand condensation above 300 °C leads to the release of water

molecules with this process being clearly observed in the FT-IR

spectra with a relative increase of the broad band centered at

about 3400 cm^À1(see Figure S10, Supporting Information).

Thermodiffractometry further shows that at 400 °C the thermal

decomposition of 2 is almost complete. Concomitantly, the

FT-IR spectrum is typical of an amorphous compound as also

evidenced by the corresponding powder X-ray diffraction pattern

(Figure S10, top in the Supporting Information).

attributed to its displacement to ca. 2251 cm^À1upon deuteration

(data not shown). Conversely, the large broad bands peaking at

ca. 2950, 2925, 2873, and 2855 cm^À1(two sets) are, thus,

assigned to υ_sym+asym(CÀH). In the 1500À1300 cm^À1region,

a number of very weak bands are observed and can be assigned to

υ(CÀH) modes typical of PÀCH₂groups. The POÀH stretching

modes were observed at ca. 2746 and 2615 cm^À1as very faint and

broad bands that also completely disappear upon deuteration.

In the aforementioned 1500À700 cm^À1spectral region, the

two central and very intense bands (found at ca. 1113 and

1074 cm^À1) are attributed to the υ(CÀN) stretching vibrational

modes typical of tertiary amines. Upon calcination these bands

shift slightly to ca. 1103 and 1079 cm^À1, respectively, thus

The typical symmetric and asymmetric υ(CÀH) and υ(NÀH)

stretching vibrational modes appear in the 3100À2800 cm^À1

region. The latter mode is found at ca. 3037 cm^À1, unequivocally

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Figure 7. Perspective views of selective portions of the two structural models of 2 emphasizing the undulated 1D polymeric anionic ligand

[À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ. In model I, the polymer runs parallel to the [100] direction of the unit cell, pillaring adjacent lanthanum

oxide layers. In model II the polymer is instead placed on top of the lanthanum oxide layers.

Table 3. Selected Bond Lengths (in Å) and Angles (In

Degrees) for the La³⁺Coordination Environment

Table 4. Selected Bond Lengths (in Å) and Angles (In

Degrees) for the La³⁺Coordination Environment

in Model I of [La(L)] (2) (Where L^3À

=

in Model II of [La(L)] (2) (Where L^3À

=

3nÀ a

[À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

)

[À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

)

La(1)ÀO(1)

2.712(9)

2.823(9)

2.708(9)

2.799(9)

La(1)ÀO(3)ⁱ

2.683(9)

2.671(9)

2.647(9)

2.714(9)

La1ÀO1

2.595(10)

2.576(10)

2.621(10)

2.634(10)

56.9(4)

La1ÀO3ⁱⁱ

2.605(10)

2.543(10)

2.657(9)

La(1)ÀO(1)ⁱ

La(1)ÀO(3)ⁱⁱⁱ

La1ÀO1ⁱ

La1ÀO3ⁱⁱⁱ

La(1)ÀO(2)ⁱⁱ

La(1)ÀO(2)ⁱⁱⁱ

O1ÀLa1ÀO1ⁱ

O1ÀLa1ÀO2ⁱⁱ

O1ÀLa1ÀO2ⁱⁱⁱ

O1ⁱÀLa1ÀO2ⁱⁱ

O1ⁱÀLa1ÀO2ⁱⁱⁱ

O1ÀLa1ÀO3ⁱ

O1ÀLa1ÀO3ⁱⁱⁱ

O1ⁱÀLa1ÀO3ⁱ

O1ⁱÀLa1ÀO3ⁱⁱⁱ

O1ÀLa1ÀO7

O1ÀLa1ÀO7ⁱⁱ

O1ⁱÀLa1ÀO7

O1ⁱÀLa1ÀO7ⁱⁱ

O2ⁱⁱÀLa1ÀO2ⁱⁱⁱ

La(1)ÀO(7)

La1ÀO2

La1ÀO6

La(1)ÀO(7)ⁱⁱ

La1ÀO2ⁱⁱ

La1ÀO8ⁱⁱⁱ

2.586(10)

140.5(4)

O2ⁱⁱÀLa1ÀO3ⁱ

O2ⁱⁱÀLa1ÀO3ⁱⁱⁱ

O2ⁱⁱⁱÀLa1ÀO3ⁱ

O2ⁱⁱⁱÀLa1ÀO3ⁱⁱⁱ

O2ⁱⁱÀLa1ÀO7

O2ⁱⁱÀLa1ÀO7ⁱⁱ

O2ⁱⁱⁱÀLa1ÀO7

O2ⁱⁱⁱÀLa1ÀO7ⁱⁱ

O3ⁱÀLa1ÀO3ⁱⁱⁱ

O3ⁱÀLa1ÀO7

O3ⁱÀLa1ÀO7ⁱⁱ

O3ⁱⁱⁱÀLa1ÀO7

O3ⁱⁱⁱÀLa1ÀO7ⁱⁱ

O7ⁱⁱÀLa1ÀO7

105.4(3)

O1ÀLa1ÀO2

O1ÀLa1ÀO6

O1ÀLa1ÀO2ⁱⁱ

O1ÀLa1ÀO3ⁱⁱ

O1ÀLa1ÀO3ⁱⁱⁱ

O1ÀLa1ÀO8ⁱⁱⁱ

O1ⁱÀLa1ÀO1

O1ⁱÀLa1ÀO2

O1ⁱÀLa1ÀO6

O1ⁱÀLa1ÀO2ⁱⁱ

O1ⁱÀLa1ÀO3ⁱⁱ

O1ⁱÀLa1ÀO3ⁱⁱⁱ

O1ⁱÀLa1ÀO8ⁱⁱⁱ

O2ÀLa1ÀO6

O2ÀLa1ÀO3ⁱⁱ

O2ÀLa1ÀO3ⁱⁱⁱ

O2ÀLa1ÀO8ⁱⁱⁱ

O2ⁱⁱÀLa1ÀO2

O2ⁱⁱÀLa1ÀO6

O2ⁱⁱÀLa1ÀO3ⁱⁱ

O2ⁱⁱÀLa1ÀO3ⁱⁱⁱ

O2ⁱⁱÀLa1ÀO8ⁱⁱⁱ

O3ⁱⁱÀLa1ÀO6

O3ⁱⁱÀLa1ÀO3ⁱⁱⁱ

O3ⁱⁱÀLa1ÀO8ⁱⁱⁱ

O3ⁱⁱⁱÀLa1ÀO6

O3ⁱⁱⁱÀLa1ÀO8ⁱⁱⁱ

O6ÀLa1ÀO8ⁱⁱⁱ

118.9(5)

143.4(4)

55.5(3)

55.3(3)

136.8(4)

92.5(3)

106.4(3)

51.7(3)

103.5(4)

84.8(4)

79.9(3)

64.9(3)

138.1(4)

145.3(3)

95.6(3)

100.7(4)

52.5(3)

76.6(5)

86.1(3)

137.0(4)

67.3(4)

113.3(4)

61.6(3)

168.0(4)

168.3(4)

60.9(3)

126.1(3)

71.3(5)

120.1(6)

131.4(6)

137.5(5)

66.0(5)

55.7(4)

69.3(4)

90.9(5)

87.4(5)

118.3(5)

80.6(5)

94.5(6)

74.3(5)

156.8(6)

111.3(5)

158.6(6)

63.4(4)

118.6(6)

128.5(6)

76.9(5)

117.2(6)

118.3(6)

62.8(4)

148.0(6)

74.5(5)

71.8(5)

^aSymmetry transformations used to generate equivalent atoms:

(i) À1/2 + x, Ày, z; (ii) 1/2 À x, y, À1/2 + z; (iii) 1 À x, Ày, À1/2 + z.

(i) 1/2 + x, Ày, z; (ii) 1.5 À x, y, 1/2 + z; (iii) 1 À x, Ày, 1/2 + z.

pointing to a small structural modiﬁcation for this section of the

molecule. The υ(PÀC) stretching vibrational modes, found in

the ca. 790À680 cm^À1region, are also markedly present in both

spectra. Remarkably, and as clearly depicted in Figure 9, these

bands which are directly associated with the central portion of the

ligand become signiﬁcantly narrower upon phase transition. This

feature indicates the formation of a more rigid, constrained

structure, coherent with the proposed 2D-to-3D or 2D-to-2D

transition and with the results derived from the photolumines-

cence studies.

A number of modiﬁcations are also notably visible in the

regions attributed to the υ(PdO) (from ca. 1250 to 1150 cm^À1

)

and υ(PÀO) (from ca. 1050 to 900 cm^À1) stretching vibra-

tional modes (Figure 9). It is worth noting the simultaneous

appearance of a new band centered at about 929 cm^À1and the

increase of the number of bands in the 1250À1150 cm^À1

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Figure 8. Highly distorted {LaO₈} square antiprismatic (model I) and bicapped trigonal prismatic (model II) coordination environments modeled for

La³⁺in compound 2. For selected bond lengths (in Å) and angles (in degrees) see Tables 3 and 4. On the side there are also depicted portions of the

compact two-dimensional lanthanum oxide layers for the two models of 2, depicting the edge-sharing connectivity between adjacent {LaO₈} polyhedra.

Symmetry transformations are used to generate equivalent atoms. Model I: (i) À1/2 + x, Ày, z; (ii) 1/2 À x, y, À1/2 + z; (iii) 1 À x, Ày, À1/2 + z.

Model II: (i) 1/2 + x, Ày, z; (ii) 1.5 À x, y, 1/2 + z; (iii) 1 À x, Ày, À1/2 + z.

region. These modiﬁcations are in line with the formation of

pyrophosphonates.²⁶

2.6. Look Into the Structural Transformation of

ÀCH₂PO₃H₂moiety, which becomes free and able to react

with neighboring moieties.

Model I. In one approach, the ÀCH₂PO₃H₂group may

migrate to the interlayer space and approach the lanthanum

oxide layer immediately above. Because of bond constraints, this

process must necessarily be accompanied by a rotation of the

entire organic molecule (as depicted in Figure S11 in the

Supporting Information), ultimately promoting a modification

of the coordination fashion of the P3 phosphonate group (step 3

in Figure S11b in the Supporting Information): one oxygen atom

establishes a new μ₂bridge between adjacent La³⁺cations, with

the newly formed free oxygen being available to react with the

migrating ÀCH₂PO₃H₂group from the adjacent layer. We note

that this type of rotation associated with phosphonate groups

within crystalline hybrid frameworks, promoting distinct coordi-

nation fashions, has already been reported in the past using

single-crystal X-ray diffraction studies.²⁷The condensation reac-

tion between groups leads to the release of one water molecule

(as observed from the TGA measurements) and the concomitant

formation of the one-dimensional organic ligand based on a

pyrophosphonate connection. A pictorial representation of this

transformation in the form of a storyboard-like cartoon (Figure

S12) and a movie of the transformation are provided as Support-

ing Information.

[La(H₃nmp)] into [La(L)]. The transformation of layered

[La(H₃nmp)] (1) into either the 3D or 2D networks [La(L)]

(2) (where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

)

3nÀ

occurs in a microcrystal-to-microcrystal type fashion. In both

models the lanthanum oxide layers remain approximately in the

same location, and the transformation occurs mainly in the

interlayer space involving individual blocks of the organic ligand.

The connectivity of the P1 phosphonate group (Figures S11b

and S11d in the Supporting Information) does not change

significantly during this transformation: the group remains

strongly chelated to four adjacent La³⁺centers via six LaÀO

bonds. Therefore, structural modification must be solely driven

by the inherent structural freedom associated with the individual

P2 and P3 moieties which decorate the surfaces of the lanthanum

oxide layer.

From the crystal structure of 1, one can observe that the

diprotonated ÀCH₂PO₃H₂moiety is the most weakly bonded to

the layer: (i) on one hand, this tetrahedral group is only bound to

a single La³⁺cation; (ii) on the other, this single LaÀO bond is

the longest (and, thus, the weakest) among all belonging to the

phosphonate groups which protrude into the interlayer space

[La1ÀO4 of 2.634(8) Å]. It is, thus, feasible to assume that

the externally driven increase in temperature can easily promote

the rupture of this bond. Indeed, independently of the model

assumed, the formation of the polymeric L^3Àligand starts always

with modiﬁcations in the connectivity of this diprotonated

Model II. The second model is less straightforward to derive as

it requires an inversion of the a- and c-axes of the parent unit cell

of compound 1. This change in unit cell parameters ultimately

allows a smaller migration of the ÀCH₂PO₃H₂group, which only

rotates on top of the lanthanum oxide layer approaching the P3

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Figure 10. Excitation spectra of (bottom) [(La_0.95Eu_0.05)(H₃nmp)]

and (top) [(La_0.95Eu_0.05)(L)] recorded at ambient temperature (black

line) and at 11 K (red line), while monitoring the emission at 618 nm.

The intensity is only comparable for the variation of the temperature for

each sample, not between diﬀerent samples.

3nÀ

the [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

ligand in 2. The

scaﬀold of these arrays is essentially maintained by strong and

highly directional PÀO^ÀHOÀP hydrogen bonding inter-

3 3 3

actions, which may be envisioned as the supramolecular

precursors of potential pyrophosphonate bonds (PÀOÀP).

The thermogravimetric studies of the compound with proto-

nated 1,7-phenanthroline reported by Sharma and Clearﬁeld²⁸

reveal a total loss of about four water molecules (in two stages)

per formula unit up to 260 °C. Because their structure only

contains two water molecules of crystallization, we now (while

comparing with our material) infer that the remaining loss most

certainly arises from the formation of pyrophosphonates from

Figure 9. Comparison between the FT-IR spectral features of

[La(H₃nmp)] (1) with [La(L)] (2) in the 1500À650 cm^À1region.

phosphonate group of an adjacent organic moiety (located on the

same side of the inorganic layer, see Figure S13 in the Supporting

Information). As in the previous model, the approaching groups

condensate, leading to the release of one water molecule. The

pictorial representation of this alternative transformation is given

in Figure S14 (in the Supporting Information), and the corre-

sponding movie is also provided as an electronic MOV file.

Noteworthy, despite this smaller structural modification, the

modeled crystal structure could only be obtained using the same

battery of constraints as for model I, even though the reliability

factors summarized in Table 1 are slightly better.

the PÀO^ÀHOÀP interactions: during the ﬁrst loss (up to

3 3 3

150 °C) crystallinity is maintained; heating up the compound

(between 150 and 260 °C) the following weight loss leads to a

completely amorphous compound which could not be charac-

terized. Because 2 is still a crystalline material and the transfor-

mation occurs at a higher temperature (above ca. 300 °C), we

infer that the lanthanum oxide layers which conﬁne the H₃nmp^3À

residues act as insulating coordinating inorganic blocks templat-

ing, on the one hand, the condensation reaction and, on the other,

retaining some degree of structural crystallinity.

This crystalline structural transformation has a strong impact on

the unit cell metrics of 2, even though the overall symmetry

remains intact (both materials crystallize in the Pca2₁space group;

see Table 1). Surprisingly, using as example model I (which does

not require the inversion of the unit cell axes), along the b-axis

(which corresponds to the interlayer distance, see Figures 3 and 6)

the unit cell only shrinks by ca. 3.3%. It is across the lanthanum

oxide layers, in particular parallel to the c-axis (see Figure 4a), that

the La³⁺metallic centers become signiﬁcantly closer, going from

4.819(2) to 4.365(7) Å (corresponding to a shrinkage of about

11.2% of the cell axis). This occurs because the local rearrange-

ment of the lanthanum oxide layers described above promotes a

coalescence of the La³⁺polyhedra, which are corner-shared in 1

(Figure 4a) and become edge-shared in 2 due to a newly formed

μ₂-bridge (Figure S11d in the Supporting Information).

To be able to isolate crystalline 2 with a higher degree of

crystallinity, the structural transformations rationalized in the

previous paragraphs must necessarily occur in a single step to

avoid overall connection mistakes. This does not seem to be the

case as the isolated material is poorly crystalline. The pattern is

dominated by the (010) reﬂection arising from the lanthanum

oxide layer (Figure 7), with the very weak and broad reﬂections

observed above ca. 25° 2θ being indicative of a poorly organized

interlayer structure ﬁlled with defects. Because the overall crystal-

lite morphology is not signiﬁcantly modiﬁed, we then infer that

calcination leads instead to the formation of nanosized domains

within the crystallites. This assumption is supported by two facts.

First, microstructural analysis performed using powder X-ray

diﬀraction indicates, for either of the proposed structural models,

an average crystallite size of ca. 255 nm (with standard deviation

of 85 nm). The shape of this average crystallite (as produced by

The research group of Clearﬁeld has reported supramolecular

arrays in which H_6Àxnmp^xÀresidues self-assemble in the solid state

with heterocyclic amines,²⁸leading to the formation of honeycomb-

type or one-dimensional chains. Noteworthy, these organic crystals

exhibit a distribution of H_6Àxnmp^xÀresidues resembling that of

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Scheme 2. Reaction of Styrene Oxide with an Alcohol

(Methanol or Ethanol) in the Presence of the Catalyst

[La(H₃nmp)] to Give the Respective 2-Alkoxyalcohol

Products

Table 5. Ring Opening of Styrene Oxide with Methanol^a

temperature

time

(h)

conversion^b

(%)

selectivity^c

(%)

catalyst

none

(°C)

40

55

72

1

8

100

Figure 11. Emission spectra of (bottom) [(La_0.95Eu_0.05)(H₃nmp)] and

(top) [(La_0.95Eu_0.05)(L), with excitation at 393 and 394 nm, respec-

tively, recorded at ambient temperature (black line) and at 11 K (red

line). The intensity is only comparable for the variation of the tempera-

ture for each sample, not between diﬀerent samples. The inset depicts

the characteristic red emission of Eu³⁺when [(La_0.95Eu_0.05)(H₃nmp)] is

irradiated with a laboratory UV lamp (wavelength of 254 nm).

H₆nmp

100

93^d

LaCl₃7H₂O

24

71

3

1

24/48/72

40/77/100

76/100^b

100/100/100

100/100

1

24/48

^aReaction conditions: PhEtO (0.83 mmol), catalyst (34.7 mg, 0.08

mmol of La³⁺), alcohol (2 mL). ^bConversion of PhEtO. ^cSelectivity to

MeOPhEtOH. ^dA chlorophenylethyl alcohol product and 2-methox-

yphenyl ethanol were formed in ca. 6% and 1% selectivity.

GFourier²⁹) is depicted in Figure 6 as an inset next to the SEM

picture. Second, TEM images of the calcined material 2

clearly show a modiﬁcation of the crystallites, which evidence

the presence of (nanosized) domains with diﬀerent contrast

and holes in the crystal (see Figure S22 in the Supporting

Information).

2.7. Photoluminescent Properties. As described in section

2.1.4, the doped [(La_0.95Eu_0.05)(H₃nmp)] (1ÀLaEu) and

[(La_0.95Tb_0.05)(H₃nmp)] (1ÀLaTb) compounds could be read-

ily prepared, and the materials emit the characteristic red (inset in

Figure 11) and green (inset in Figure S16 in the Supporting

Information) colors when irradiated under UV light. The photo-

luminescent properties detailed below will be focused in the

1ÀLaEu material and the dense 3D/2D network obtained by its

calcination, [(La_0.95Eu_0.05)(L)] (2ÀLaEu). The excitation and

emission spectra of 1ÀLaTb are provided in the Supporting

Information (see Figures S15 and S16, respectively).

The excitation spectra of 1ÀLaEu and 2ÀLaEu, recorded at

ambient temperature and 11 K (Figure 10), display a series of

sharp lines typical of the Eu³⁺intra-4f⁶transitions, namely:

⁷F_0,1f⁵D_4À0, ⁵L₆, ⁵G_2À6, ⁵H_3À7, and ⁵F_1À5. On the high-energy

region, the spectra exhibit at 11 K an additional and well-deﬁned

broad band attributed to ligand-to-Eu³⁺charge transfer (CT),

peaking at 246.0 and 243.9 nm for 1ÀLaEu and 2ÀLaEu,

respectively. The corresponding energies (5.04 and 5.08 eV)

and fwhm (0.24 and 0.23 eV) of the CT bands are in the range of

the values reported for related compounds containing Eu³⁺.³⁰As

discussed in the following paragraphs, the extra broad peaks at

274 and 282 nm of 1ÀLaEu and the broad UV band from ca. 270

to 350 nm of 2ÀLaEu are mostly due to the presence of a very

small amount of an unidentiﬁed impurity phase which also

contains Eu³⁺.

septet. Except for the ⁵D₀f⁷F₁lines, which have a predominant

magnetic-dipole character independent of the Eu³⁺crystallo-

graphic site, the observed transitions are mainly of electric-dipole

nature. The local-ﬁeld splitting of the ⁷F_1,2levels into three and a

maximum of ﬁve Stark components, respectively, and the pre-

dominance of the ⁵D₀f⁷F₂transition in relation to the ⁵D₀f⁷F₁

one indicate the presence of a single low-symmetry Eu³⁺

environment for both 1ÀLaEu and 2ÀLaEu. This result is in

good agreement with the crystallographic evidence described

above. As mentioned above, 1ÀLaEu and 2ÀLaEu exhibit a

number of structural similarities such as the local Eu³⁺symmetry

and multiplicity. However, their emission properties allow us to

infer some important distinctions between the two compounds:

(i) 1ÀLaEu has sharper emission lines, particularly at 11 K,

characteristic of a compound with higher crystallinity; (ii) the

integrated intensity ratios between ambient temperature and 11

K (II_RT/II_11K), 1.79 and 1.34 for 1ÀLaEu and 2ÀLaEu,

respectively, indicate a more ﬂexible structure (a more tempera-

ture-dependent photoluminescence) for the former material;

(iii) the integrated intensity ratio R = ⁵D₀f⁷F₂/⁵D₀f⁷F₁, also

known as asymmetry ratio and calculated as 2.50 and 3.25 for

1ÀLaEu and 2ÀLaEu, respectively, indicates a higher asymme-

try of the Eu³⁺coordination polyhedron for the latter material

(please note: the lower the R-value, the higher the site symmetry

of the Eu³⁺), in good agreement with the X-ray diﬀraction

studies.

Although the emission spectra of the two compounds can be

attributed to the presence of a single Eu³⁺site with low

symmetry, a detailed analysis allows the detection of additional

Eu³⁺signals, which, on the present situation, are attributed to a

very small amount of an impurity phase. For example, Figure S17

in the Supporting Information shows a magniﬁcation of the

Figure 11 shows the emission spectra of 1ÀLaEu and

2ÀLaEu excited at 393 and 394 nm, respectively. The sharp

lines are assigned to transitions between the ﬁrst excited non-

degenerate ⁵D₀state and the ⁷F_0À4levels of the fundamental Eu³⁺

region of the nondegenerated D₀f⁷F₀transition, and Figure

5

S18 (also in the Supporting Information) compares, at 11 K, the

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former emission spectra with those collected when exciting on the

selected unattributed UV bands. These inorganic impurities, also

detected by solid-state NMR (Figure S8 in the Supporting Informa-

tion), electron microscopy, and powder X-ray diﬀraction studies

(Figure S21 in the Supporting Information), become slightly more

evident in the photoluminescence studies at either low tempera-

ture with selected excitation or ambient temperature with the very

Table 6. Ring Opening of Styrene Oxide with Ethanol^a

temperature

time

(h)

conversion^b

(%)

selectivity^c

(%)

catalyst

(°C)

none

55

70

72

4

100

1

24/48/72

23/60/100

77/100^b

100/100/100

100/100

24/48

5

sensitive D₀f⁷F₀transition. We must stress that attempts to

^aReaction conditions: PhEtO (0.83 mmol), catalyst (34.7 mg, 0.08 mmol of

remove or, at least, diminish the amount of these impurities proved

to be unsuccessful. In addition, due to their presence in a very small

amount, their identiﬁcation was not possible (only two reﬂec-

tions were observed in the powder X-ray diﬀraction pattern).

La³⁺), alcohol (2 mL). ^bConversion of PhEtO. ^cSelectivity to EtOPhEtOH.

5

The D₀lifetimes of Eu³⁺of 1ÀLaEu and 2ÀLaEu were

determined by monitoring the emission decay curves within the

⁵D₀f⁷F₂transition (618 nm) at ambient temperature and 11 K,

using an excitation at ca. 394 nm (Figures S19 and S20 in the

Supporting Information). The decay curves were well-ﬁtted by

single exponential functions, yielding lifetimes for 1ÀLaEu of

2.09 ( 0.01 and 2.31 ( 0.01 ms, at ambient temperature and 11

K, and 2.04 ( 0.01 and 2.08 ( 0.01 ms, at ambient temperature

and 11 K, respectively, for 2ÀLaEu. Changing the monitoring

and/or excitation wavelength can result in the detection of more

complex exponential curves (not shown) due to the detection of

the aforementioned secondary inorganic phases. As discussed

above, 2ÀLaEu shows a smaller dependence of the Eu³⁺lifetime

with the temperature, which is a typical behavior of a rigid struc-

ture.³¹Furthermore, the shorter lifetime values for this material can

only be justiﬁed by its lower crystallinity and possible presence of

local defects. These two conclusions agree well with the assumptions

highlighted above in the description of the crystal structures.

2.8. Heterogeneous Catalysis. The [La(H₃nmp)] (1) material

revealed itself to be a promising catalyst in the ring-opening

reaction of styrene oxide (PhEtO) with methanol or ethanol

(Scheme 2). The reaction of PhEtO with methanol in the

presence of 1 at 40 °C gives 2-methoxy-2-phenylethanol

(MeOPhEtOH) in quantitative yields within 72 h reaction

(Table 5); in the absence of a catalyst the reaction under similar

conditions was very sluggish, and only 8% conversion was

reached in the same period of time. These results indicate

excellent regioselectivity to the β-alkoxy alcohol, which may be

formed by nucleophilic attack of the alcohol at the phenyl-

substituted carbon atom.³²The reaction of 2-phenylethanol with

methanol in the presence of 1 does not take place, suggesting that

the mechanism does not involve the condensation of a β-

hydroxyl intermediate with the alcohol. According to the litera-

ture, the MOF catalyst may thus coordinate with PhEtO by way

of an acidÀbase interaction to produce an increase in the

electrophilic nature (partial positive charge) of the more sub-

stituted carbon of the substrate (polarization of the CÀO bond),

which in turn is attacked by the alcohol to yield the β-alkoxy

alcohol product.^32aDespite the catalytic activity of 1 being lower

than that reported for the Fe- or Cu-MOFs used as catalysts in

the same reaction at 40 °C (more than 92% conversion at 1 h

reaction),³³the selectivity to MeOPhEtOH is always 100%.

Indeed, for the transition metal MOFs, selectivity toward the

same product at more than 92% conversion reached, at best, ca.

95%.³³Similar excellent regioselectivities to the β-alkoxy alcohol

product have also been reported for lanthanide-containing

heterodimetallic coordination polymers in the alcoholysis ring-

opening reaction of PhEtO.^32b

Figure 12. Recycling tests on the use of [La(H₃nmp)] (1) as a

heterogeneous catalyst in the ring-opening reaction of styrene oxide

with methanol at 40 °C. Experimental data points are connected by a

dotted line for indicative purposes only.

for the catalytic alcoholysis reactions), no ring opening of PhEtO

in the presence of 1 is observed (40 °C, 72 h). These results contrast

with that reported in the literature for decatungstocerate(IV)

used as catalyst (0.04 M) in the same reaction, which also

exhibited activity in the hydrolysis of the epoxide to the cor-

responding diol.³⁴Therefore, 1 possesses alcoholysis but no

hydrolysis activity.

For comparative purposes, the reactants H₆nmp and LaCl₃

3

7H₂O were also tested as catalysts in the methanolysis of PhEtO,

at 40 °C, using molar ratios of (H₆nmp or La³⁺)/PhEtO/

methanol equivalent to those for 1 (Table 5). When LaCl₃

3

7H₂O was employed as the (Lewis acid) catalyst, MeOPhEtOH

is formed in 93% selectivity at 71% conversion, reached at 24 h

reaction; a chlorophenylethyl alcohol product and 2-methoxy-

phenyl ethanol were formed with ca. 6% and 1% selectivity,

respectively. High catalytic activity and regioselectivity to the β-

alkoxy alcohol product has been reported by Berkessel et al.³⁵for

rare-earth triﬂates used as homogeneous catalysts in the alcoho-

lysis of PhEtO under mild reaction conditions. A catalytic

amount of H₆nmp gives MeOPhEtOH in quantitative yields

after only 1 h of reaction. These results indicate that the synthetic

precursors are more active than 1 itself, possibly due to the low

amount of accessible active sites in the latter case. It is worth

mentioning that the synthetic precursor H₆nmp dissolves com-

pletely in the reaction medium and is, thus, a homogeneous

catalyst which requires signiﬁcantly more demanding separation

and puriﬁcation processes than for the heterogeneous catalyst 1. On

the other hand, the synthetic precursor LaCl₃7H₂O dissolves

When a mixture of water/acetonitrile (1:3) is used instead of

methanol (total volume of the reaction mixture was the same as

3

completely in the initial stage of the reaction, and eventually a

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suspension is formed. The homo/heterogeneous nature of the

ﬁltering the used catalysts through a 0.20 μm nylon GVS

membrane (after 4 h reaction, at 40 °C) and leaving the reaction

solution under stirring for a further 20 h. The increment in

conversion in the period of time 4À24 h was negligible, indicat-

ing 1 is a truly heterogeneous catalyst (Figure 12).

catalytic reaction for the case of LaCl₃7H₂O was investigated by

3

performing a separate experiment which consisted of ﬁltering the

solid (white in color) through a 0.20 μm nylon GVS membrane,

after 4 h reaction, at 40 °C, and leaving the reaction solution

under agitation a further 20 h. The increment in conversion in

the interval of time 4À24 h was 38% which is similar to that

observed for the experiment without ﬁltration (41%). Hence,

The [La(L)] (2) material was tested as a catalyst in the

reaction of PhEtO with methanol at 55 °C, under conditions

similar to those used for 1. The only reaction product was

MeOPhEtOH obtained in quantitative yield within 4 h, indicat-

ing that 2 is much more active than 1. The crystalline structure of

2 was preserved during the catalytic reaction (ascertained by

powder X-ray diﬀraction). We note that the catalytic activity of 2

is somewhat comparable and product selectivity superior to that

reported in the literature for transition metalÀorganic

frameworks.³²As discussed above, 2 is a less crystalline material

than 1 and therefore may possess a greater amount of local defect

sites, which could explain the considerably faster reaction of

PhEtO in the case of 2. Indeed, TEM images of 2 clearly depict a

signiﬁcant increase of widespread structural defects of the

individual crystallites (see Figure S22 in the Supporting

Information).

although the reaction of PhEtO is much faster for LaCl₃7H₂O

3

than for 1, in the former case the catalytic reaction is less selective

and homogeneous in nature.

The rate of conversion of PhEtO into MeOPhEtOH (in

quantitative yields without aﬀecting product selectivity) in the

presence of 1 could be increased by increasing the reaction

temperature from 40 to 55 °C (Table 5).

The catalytic performance of 1 was further investigated in the

reaction of PhEtO with ethanol. The sole product was 2-ethoxy-

2-phenylethanol (EtOPhEtOH), which was obtained in quanti-

tative yields within 72 h of reaction (Table 6). We note that

without the catalyst the PhEtO conversion was negligible. A

temperature increase from 55 to 70 °C enhances considerably

the rate of ethanolysis of PhEtO. The reaction of PhEtO is slower

in ethanol than in methanol (Tables 5 and 6), which may be

attributed to the inherent steric hindrance of each alcohol. No

secondary reactions of ethanol were detected, indicating an

excellent productive consumption of the alcohol in the ring-

opening reaction. To check if the ring opening of PhEtO via

hydrolysis can be a competitive reaction, a separate catalytic test

was performed using PhEtO/H₂O/ethanol molar ratios of

1:2:19, at 70 °C. The presence of excess water relative to the

substrate did not inﬂuence the product selectivity (EtOPhEtOH

was always the only reaction product), but catalytic activity

decreased; conversion at 24 h was 42% compared to 77% without

adding water (Table 6). As discussed above, in section 2.4, water

may bind to defect sites of 1 which may be responsible for the

catalytic reaction. Accordingly, excess water may inhibit the

catalytic activity of 1, explaining the decrease in reaction rate

observed when water was added to the reaction mixture.

The recyclability of catalyst 1 was investigated in ﬁve con-

secutive batch runs of the reaction of PhEtO with methanol at

40 °C. Prior to reuse, the solid catalyst was separated by

centrifugation, washed with n-hexane, and dried at ambient

temperature overnight. In all runs, MeOPhEtOH was always

the sole product. An increase in the reaction rate was clearly

observed from the ﬁrst to the second run; the catalytic activity

remained roughly constant in subsequent runs (Figure 12). The

structural integrity of the regenerated catalyst was inspected

using in tandem powder X-ray diﬀraction, vibrational spectros-

copy, and electron microscopy studies (Figure S23 in the

Supporting Information). No signiﬁcant modiﬁcations were

detected in the structure of the material even though a small

reduction of the overall crystallinity was observed. After the

catalytic process, the bulk catalyst was isolated as very small

particles characterized by an undeﬁned morphology, which most

certainly results from the cracking of the starting crystallites of 1.

This crystal fragmentation explains, on the one hand, the

observed reduction in crystallinity and, on the other, the positive

eﬀect on the catalytic performance in subsequent catalytic runs

by way of an increase of the number of accessible active sites

which may be defects.

3. CONCLUDING REMARKS

In the present study, we have shown that by using diﬀerent

synthetic conditions, based on microwave heating or a dynamic

hydrothermal synthesis (i.e., with the reaction vessels spinning at

a constant rate during the entire period of reaction), the lanthanide/

nitrilotris(methylenephosphonic acid) system can lead to the

preparation of a whole new layered material: [La(H₃nmp)] (1).

Independently of the employed synthetic method, it was found

that this compound could only be isolated when the lanthanide

oxide layer is based on La³⁺; other lanthanide centers promote

the formation of mixtures of which ([La(H₃nmp)] 1.5H₂O) is,

3

in most of the cases, one of the main constituents. Photolumi-

nescent materials could be, however, successfully engineered by

the selective inclusion of stoichiometric amounts (5%) of

optically active Eu³⁺and Tb³⁺centers in the reactive mixtures.

In opposition to that reported for [La(H₃nmp)] 1.5H₂O, 1

3

does not have any solvent molecules incorporated into the

network, and, upon heating, the structure is transformed into a

dense novel 3D or a layered 2D material formulated as [La(L)]

(2) (where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n^3nÀ).

Because the two compounds could only be prepared as micro-

crystalline powders, structural solution was not trivial and had to

be ultimately based on powder X-ray diﬀraction studies (in

conjunction with data from FT-IR spectroscopy, ¹³C and ³¹P

solid-state NMR, and themogravimetry). We have further shown

(using X-ray powder data) that the thermal transformation of 1

into 2 was accompanied by the creation of nanosized domains

within the crystallites.

The present work shows that small structural changes in the

overall connectivity of the organic ligand can promote important

diﬀerences in the overall thermal behavior and stability of the

materials. In the previous material reported by us, heating-

induced dehydration of the network (which was partially rever-

sible when the temperature was decreased) was immediately

followed by thermal decomposition. The layered compound

herein reported suﬀers instead, between 300 and 400 °C, a

structural transformation into a novel structure, which remains

unaltered when the system is cooled to ambient temperature.

This occurs because the H₃nmp^3Àanionic ligand undergoes an

To get a better insight into the hetero/homogeneous nature of

the catalytic reaction, a leaching test was performed for 1 by

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in situ polymerization (condensation) into a novel polymeric

anionic ligand which was formulated as [À(PO₃CH₂)₂(NH)-

(CH₂PO₂)O_1/2À]_n^3nÀ. It was shown that this polymerization

can occur either on top of the lanthanide oxide layers or across

adjacent layers, promoting either 2D or 3D materials with the

same crystal symmetry; due to the poor crystallinity of the ﬁnal

compound, powder X-ray diﬀraction could not unequivocally

show which process occurs preferentially. To the best of our

knowledge, this type of structural change, mostly focused in the

organic ligand, is clearly unprecedented. Prior work from the

research group of Clearﬁeld on organic crystals further shows

that the lanthanum oxide layers of 1 act as simultaneously

insulating and templating scaﬀolds having, thus, a strong inﬂu-

ence on the formation of this unprecedented one-dimensional

polymeric ligand. The transformation process was further

accompanied using FT-IR and solid-state NMR studies, and we

have devised a schematic model representation based on the

modeled structures to illustrate the dynamics and transformation

of the system when the temperature is increased.

We demonstrated that lanthanide cations can be included and

uniformly distributed among the La³⁺matrix (as shown by

electron microscopy studies), promoting the isolation of red-

and green-emitting materials ([(La_0.95Eu_0.05)(H₃nmp)] and

[(La_0.95Tb_0.05)(H₃nmp)], respectively) when irradiated upon

UV light. The photoluminescent properties of layered

[(La_0.95Eu_0.05)(H₃nmp)] and [(La_0.95Eu_0.05)(L)] materials have

shown the presence of a single crystallographic lanthanide center

with lifetimes ranging from 2.04 ( 0.01 to 2.31 ( 0.01 ms for

both materials (at ambient and lower temperature). These

studies were also particularly informative regarding the robust-

ness of the isolated compounds: while the layered material

yielded sharp emission lines and a higher II_RT/II_11Kratio of

integrated intensities, which indicates the presence of a highly

crystalline material with some structural ﬂexibility, for the calcined

structure the integrated intensity ratio, R = ⁵D₀f⁷F₂/⁵D₀f⁷F₁,

points to a highly distorted Eu³⁺coordination environment.

[La(H₃nmp)] is an eﬀective heterogeneous catalyst in the

ring-opening reaction of styrene oxide with methanol and

ethanol producing, respectively, 2-methoxy-2-phenylethanol or

2-ethoxy-2-phenylethanol. Outstanding features of its catalytic

performance may be pointed out as:

while it exhibited similar excellent regioselectivity toward the β-

alkoxy product and much higher catalytic activity than the parent

compound.

Currently, we are investigating the possibility to isolate new

materials using the same synthetic procedures herein employed

(based on microwave heating or on dynamic hydrothermal

synthesis) but containing distinct lanthanide centers. The main

goal is also clearly focused on the reduction of crystallite size.

From the current study, we envisage that nanosized crystallites of

these materials will contain a signiﬁcantly higher concentration of

local defects which, along with the better access of substrate

molecules due to the higher external speciﬁc surface area, will

improve the heterogeneous catalytic performances. In addition,

nanocrystallites may also be employed as the active chemical

photoluminescent entities in the fabrication of composite

materials.

4. MATERIALS AND METHODS

4.1. General Instrumentation. SEM (Scanning Electron Micro-

scopy) images and EDS data were collected using either a Hitachi

S-4100 field emission gun tungsten filament instrument working at 25

kV or a scanning electron microscope SU-70 working at 4 kV. Samples

were prepared by deposition on aluminum sample holders followed by

either (i) carbon coating or (ii) Au/Pd coating. The cathodic deposition

of the 60% Au/40% Pd metallic alloy was performed using a Polaron

Equipment Limited SEM Coating E5000. SEM mapping images and

STEM of 1 were recorded using the microscope SU-70 working at 15 or

30 kV, respectively.

TEM (Transmission Electron Microscopy) images were acquired in a

HR-TEM Hitachi H9000na. A suspension of 1 in ethanol was sonicated

for 180 s. One drop of this suspension was then placed on a carbon-

coated copper grid and left in open air to dry. TEM analysis reveals that 1

is extremely sensitive to the electron beam, and under normal condi-

tions, crystallites are destroyed after a few seconds of intense irradiation.

To minimize damage, a minimum electron voltage was employed for

TEM measurements (200 kV), and the magniﬁcation was kept as low as

possible.

Thermogravimetric analyses (TGA) were carried out using a

Shimadzu TGA 50, from room temperature to ca. 800 °C, with a heating

rate of 5 °C/min, under a continuous stream of air with a ﬂow rate of

20 mL/min.

Fourier Transform Infrared (FT-IR) spectra (in the range

4000À500 cm^À1) were recorded as KBr pellets (2 mg of sample was

mixed in a mortar with 200 mg of KBr) using a Mattson 7000 galaxy

series spectrometer equipped with a DTGS CsI detector or using a

Bruker Tensor 27 spectrometer by averaging 200 scans at a maximum

(i) excellent regioselectivity toward the β-alkoxy product,

formed in quantitative yields, under quite mild reaction

conditions (ca. atmospheric pressure, less than 70 °C,

without cosolvents);

(ii) it possesses alcoholysis activity but no hydrolysis activity,

and thus, competitive epoxide ring-opening reactions in

the presence of water do not take place;

(iii) the reaction rate may be enhanced by increasing the reaction

temperature, without aﬀecting product selectivity;

(iv) it may be recycled without the need for regeneration

treatments (only simple washing and drying at ambient

temperature) and no drops in catalytic activity and

product selectivity;

resolution of 2 cm^À1

.

Elemental analyses for carbon, nitrogen, and hydrogen were carried

out on an LECO CHNS À 932 equipment. Samples were combusted at

1000 °C for 3 min with helium used as the purge gas.

Routine powder X-ray diﬀraction (PXRD) data for all synthesized

materials were collected at ambient temperature on a X’Pert MPD

Philips diﬀractometer, equipped with an X’Celerator detector and a

ﬂat-plate sample holder in a BraggÀBrentano para-focusing optics

conﬁguration (40 kV, 50 mA). Intensity data were collected by the

step-counting method (step 0.04°), in continuous mode, in the ca. 5 e

2θ e 50° range.

(v) the catalytic reaction is heterogeneous in nature.

It is postulated that the active sites are structural defect sites based

on the following results: the reused catalyst is somewhat less

crystalline and more active than the fresh catalyst; water which

may coordinate to defect sites has an inhibitory role, decreasing the

catalytic activity. On the other hand, the material [La(L)] (formed

by the thermal treatment of 1) is a less crystalline material than 1,

¹³C CP MAS and ³¹P spectra were recorded at 9.4 T on a Bruker

Avance 400 wide-bore spectrometer (DSX model) on a 4 mm BL cross-

polarization magic-angle spinning (CPMAS) VTN probe at 100.6 and

161.9 MHz, respectively. For the ¹³C CP MAS spectra, the Hart-

13C

1H

mannÀHahn (HH) “sideband” matching condition ν₁

= ν₁

+

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ARTICLE

nν_R(n = ( 1) was carefully matched by calibrating the ¹H and the ¹³C rf

ﬁeld strengths: recycle delay, 5s; contact time, 2 ms; ν_R= 12 kHz. For the

³¹P spectra, a 90° single pulse excitation of 2.5 μs was employed: recycle

delay, 60s; ν_R= 10 kHz. For the ³¹P CP MAS spectra, a 90° single pulse

excitation of 2.5 μs was employed: recycle delay, 5s; contact time, 2 ms;

ν_R= 10 kHz. Chemical shifts are quoted in parts per million with respect

ca. 3 and 5. The following experimental conditions were systematically

varied to optimize the synthetic conditions:

(i) temperature, 120 and 165 °C;

(ii) speed of rotation, 25 and 50 rpm;

(iii) reaction time, 48 and 72 h;

(iv) chemical composition of the reactive mixtures (ligand/metal/

water molar ratios), 1:1:325 and 1:1:650 (total volume of distilled

water of ca. 10 mL);

to TMS for the ¹³C nuclei and to an 85% H₃PO₄solution for the ³¹

nucleus.

P

(v) reaction vessel internal volume, 24 and 42 mL.

4.2. Reagents. Chemicals were readily available from commercial

sources and were used as received without further purification: lantha-

After reacting, the vessels were allowed to cool slowly to ambient

temperature. Products were recovered by vacuum ﬁltration, washed with

copious amounts of distilled water, and air-dried, and their crystalline

structure was investigated using powder X-ray diﬀraction. For several

combinations of the conditions highlighted above, we found that the

isolated products were systematically obtained as physical mixtures of

the desired material alongside with other known and unknown products.

The overall optimal conditions to isolate [La(H₃nmp)] (1) as a phase-

pure compound were found to be 165 °C and 48 h of reaction with a

speed of rotation of about 25 rpm.

nide (III) chloride hydrates (LnCl₃xH₂O, Ln³⁺= La³⁺, Eu³⁺and Tb³⁺;

3

at least 99% of purity, Sigma-Aldrich); nitrilotris(methylenephosphonic

acid) (H₆nmp, N(CH₂PO₃H₂)₃, 97%, Fluka); n-hexane puriss p.a.

(Sigma-Aldrich); styrene oxide (Fluka, g97%), ethanol (Panreac,

Absolute), methanol (Sigma-Aldrich, 99%).

4.3. Microwave-Assisted Hydrothermal Synthesis. 4.3.1.

Typical Preparation of [La(H₃nmp)]. A suspension containing H₆nmp

(0.153 g, 0.512 mmol) and LaCl₃7H₂O (0.190 g, 0.512 mmol) in ca.

3

6.0 mL of distilled water (molar ratios of about 1:1:650) was stirred

thoroughly in open air (room temperature) for five minutes. The

resulting suspension, with a pH varying between ca. 3 and 5, was

transferred to a 10 mL IntelliVent reactor which was placed inside a

CEM Focused Microwave Synthesis System Discover S-Class equip-

ment. Reactions took place with constant magnetic stirring (controlled

by the microwave equipment) and by monitoring the temperature and

pressure inside the vessels. A constant flow of air (ca. 10 bar of pressure)

assured a close control of the temperature inside the vessel. After

reacting, a white suspension was obtained, and the final product was

recovered by vacuum filtration, followed by washing with copious

amounts of distilled water, and then air-dried overnight.

4.5. Structural Determination from Powder X-ray Diffrac-

tion. Conventional powder X-ray diffraction data suitable for structural

refinements were collected at ambient temperature on a X’Pert MPD

Philips diffractometer (Cu K_R1,2X-radiation, λ₁= 1.540598 Å and λ₂=

1.544426 Å), equipped with an X’Celerator detector and a flat-plate

sample holder in a BraggÀBrentano para-focusing optics configuration

(40 kV, 50 mA). Intensity data were collected by the step-counting

method (step 0.01°; 500 or 2500 s per step for the as-prepared

and calcined materials, respectively), in continuous mode, in the ca.

4 e 2θ e 110° range.

The collected powder X-ray diﬀraction pattern for [La(H₃nmp)] (1) and

3nÀ

[La(L)] (2) (where L^3À= [À(PO₃CH₂)₂(NH)(CH₂PO₂)O_1/2À]_n

)

CHN elemental composition. Calcd (in %): C, 8.28; H, 2.09; N, 3.22

(C/N = 2.57). Found (in %): C, 7.90; H, 2.12; N, 3.09 (C/N = 2.56).

Thermogravimetric analysis (TGA) data (weight losses in %) and

derivative thermogravimetric peaks (DTG; in italics inside the

parentheses): 280À400 °C À4.1% (366 °C), 400À650 °C À4.8%

(508 °C), 650À800 °C À4.1% (768 °C).

were indexed by means of the routines provided with the software

program DICVOL04³⁶and by employing the ﬁrst 20 well-resolved

reﬂections (located using the derivative-based peak search algorithm

provided with Fullprof.2k)³⁷and a ﬁxed absolute error on each line of

0.03° 2θ. Initial unit cell metrics were obtained with reasonable ﬁgures-

of-merit (see Table 1 for the M³⁸and F³⁹values).

4.3.2. Optimization. The microwave-assisted synthesis conditions to

prepare [La(H₃nmp)] (1) were investigated by systematically varying

the following experimental parameters:

Analysis of the systematic absences was performed using

CHECKCELL⁴⁰which identiﬁed the orthorhombic Pca2₁space group

as the most suitable. Structure solution was carried out using the direct

methods of SIRPOW, included in the software package eXpo2004

(Version 2.1).⁴¹After subtraction of the background using polynomial

functions in selected angular intervals, the intensity of each individual

reﬂection was extracted by employing Pearson proﬁle functions. This

strategy allowed the location of most non-hydrogen atoms composing

the asymmetric unit, namely, the lanthanide metallic center and the

phosphorus atoms. Despite that the powder X-ray diﬀraction pattern of

1 has a high signal-to-noise ratio, the location of the remaining atoms of

the organic molecule proved to be diﬃcult. This was particularly

challenging for the calcined material 2. Initial crystallographic positions

were ultimately calculated in eXpo2004 by employing a battery of heavy

geometrical constraints for the organic moiety.

(i) temperature (T) from 120 to 165 °C;

(ii) power (P) from 50 to 100 W;

(iii) reaction time (t) between 1 and 10 min of microwave

irradiation.

For simplicity, each sample is identiﬁed as T_mP_nt_p, as described in the

optimization diagram, with the subscripts indicating each set of speci-

ﬁcally employed experimental conditions. The set of the prepared

materials is depicted in Scheme 1.

4.3.3. Preparation of Mixed-Lanthanide Materials. Mixed-lantha-

nide compounds having 5% of Eu³⁺or Tb³⁺(randomly dispersed in the

La³⁺matrix) were obtained using an experimental procedure similar to

that described above while adjusting the stoichiometric amounts of the

lanthanide chloride salts to the desired amounts.

4.4. Dynamic Hydrothermal Synthesis. Dynamic hydrother-

mal syntheses, i.e., with continuous rotation of the vessels during the

period of the reaction, have been performed using Teflon-lined auto-

claves purchased from Parr Instruments (internal volume of

ca. 24 mL) and a custom-modified oven with the internal temperature

controlled by an Eurotherm thermostat system.

Reaction mixtures with chemical compositions identical to those

described for the microwave-assisted synthesis were stirred thoroughly

in open air for a few minutes and then transferred into adapted Teﬂon-

lined Parr Instruments reaction vessels, which were then placed inside a

preheated oven. The pH of the initial reactive mixtures varied between

Rietveld structural reﬁnement⁴²was performed with FullProf.2k³⁷by

applying ﬁxed background points throughout the entire angular range

determined by the linear interpolation between consecutive (and

manually selected) breakpoints in the powder pattern. For material 1,

typical pseudo-Voigt proﬁle functions, along with two asymmetry

correction parameters, were selected to generate the line shapes of the

simulated diﬀraction peaks. The powder proﬁle of compound 2 could

not be adequately simulated by using the same peak shape proﬁle as that

used for 1. Instead, an instrument resolution ﬁle has been employed in

conjunction with ThompsonÀCoxÀHastings pseudo-Voigt proﬁles to

generate the line shapes of individual simulated diﬀraction peaks. This

allowed the modeling of each reﬂection by assuming an anisotropic

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Journal of the American Chemical Society

ARTICLE

broadening due to crystallite size eﬀects. Ylm’s up to the fourth order

(spherical harmonics) have been employed and reﬁned during Rietveld

stages which converged to the values summarized in Table 1. The full-

width-at-half-maximum (fwhm) of the Gaussian (H_G) and Lorentzian

(H_L) components were reﬁned to G = 0.0151(4) and L = 3.48(6). For

both materials, the angular dependence of the individual reﬂections was

also taken into account by employing a Caglioti function correction.⁴³

The collected powder pattern of compound 2 was found to exhibit

strong preferential orientation along the [010] vector. A modiﬁed

March’s correction function was employed producing reﬁned G1 values

of about 0.579(1) (for model I) or 0.654(2) (for model II), which are

coherent with the platy crystal habit observed for the material (see SEM

inset picture in Figure 6).

600 nm for the spectral distribution of the lamp intensity using a

photodiode reference detector. Emission spectra were corrected for the

spectral response of the monochromators and the detector using typical

correction spectra provided by the manufacturer. Time-resolved mea-

surements were carried out using a 1934D3 phosphorimeter coupled to

the Fluorolog-3, and a XeÀHg flash lamp (6 μs/pulse half width and

20À30 μs tail) was used as the excitation source. Measurements at 11 K

were performed using a He closed-cycle cryostat.

4.8. Heterogeneous Catalysis. A 5 mL borosilicate batch reactor

equipped with a magnetic stirrer and a valve for sampling was charged

with catalyst 1 (34.7 mg, ca. 0.08 mmol of La³⁺), alcohol (2 mL), and

epoxide (0.83 mmol). The reaction was carried out under atmospheric

air and immersed in an external thermostatted oil bath. Screening tests

showed that 1 exhibited poor catalytic activity in the oxidative dehy-

drogenation of alcohols (may take place over basic sites), the epoxidation of

olefins (may take place over Lewis acid sites), and the isomerization of

R-pinene (may take place over Lewis acid sites), at 55 °C (see Table S1

in the Supporting Information). The evolution of the catalytic reactions was

monitored using a Varian 3900 GC equipped with a capillary column

(SPB-5, 20 m Â 0.25 mm) and a flame ionization detector (using

1-octene as an external standard). Reaction products were identified by

GC-MS (Trace GC 2000 Series (Thermo Quest CE Instruments) - DSQ II

(Thermo Scientific)), using He as the carrier gas.

Overall structural reﬁnements for the two structures derived from

eXpo2004 were performed in consecutive stages to avoid instability and

divergence. Zero shift (whenever pertinent), scale factor, parameters

related to peak shape, and unit cell parameters were consecutively added

as fully reﬁneable variables upon previous full convergence of the

remaining parameters to their optimal values. Fractional atomic coordi-

nates for all non-hydrogen atoms were ultimately allowed to reﬁne in

conjunction with weighted soft distance constraints solely applied to the

H₃nmp^3Àresidue (NÀC, CÀC, CÀP, PÀO, and O O bond lengths

3 3 3

and internuclear distances, respectively). This approach, when applied

to the as-synthesized [La(H₃nmp)] material, has the advantage of

ensuring a ﬁnal chemically feasible geometry for this organic moiety,

while it allows the rare-earth environment to reﬁne freely. For the

calcined compound, further restraints associated with the LaÀO bond

lengths were employed. For both structures, each family of atom type has

been reﬁned using common reﬁneable isotropic parameters. No correc-

tion was made for absorption eﬀects. After full convergence of the

model, the hydrogen atoms associated with the ÀCH₂À, ÀNH, and

ÀPOH moieties were placed in the structural model in calculated

positions. The ﬁnal Rietveld reﬁnements were performed with the

fractional atomic coordinates of the hydrogen atoms ﬁxed (to avoid

divergence), ultimately converging to the proﬁle and reliability factors

summarized in Table 1.

’ ASSOCIATED CONTENT

S

Supporting Information.

Additional crystallographic

b

information and studies: crystal structures in CIF format and

videos depicting the two models for the in situ condensation

reaction which produces the inﬁnite one-dimensional organic

ligand. Electron microscopy studies: EDS mapping and quanti-

ﬁcation of the various elements constituting the mixed-lantha-

nide materials; TEM studies of the calcined material 2. Solid-

state NMR studies: ¹³C{¹H} and ³¹P{¹H} CP MAS spectra of

[La(H₃nmp)] (1) and [La(L)] (2). Thermogravimetric studies

on compound 2. Transformation of 1 into 2 accompanied by FT-

IR spectroscopy. Additional photoluminescence data. Structural

characterization of the reused catalyst 1 in the catalytic experi-

ments reported in the manuscript. Table of all catalytic reactions

studied for 1 which did not produce feasible conversions. This

material is available free of charge via the Internet at http://pubs.

acs.org.

Table 1 collects the X-ray data collection, crystal data, and ﬁnal

Rietveld reﬁnement details (proﬁle and reliability factors) for the

[La(H₃nmp)] (1) and the two models of the [La(L)] (2) materials.

The ﬁnal Rietveld plots for 1 and 2 (both models) are supplied in

Figures 3 and 6, respectively.

Crystallographic data (excluding structure factors) for the two

structures reported in this paper have been deposited with the Cam-

bridge Crystallographic Data Centre as supplementary publication nos.

CCDC-818078, -818079, and -836614 (for 1 and model I and II of 2,

respectively). Copies of the data can be obtained free of charge on

application to CCDC, 12 Union Road, Cambridge CB2 2EZ, U.K. (fax:

(+44) 1223 336033; e-mail: deposit@ccdc.cam.ac.uk).

’ AUTHOR INFORMATION

Corresponding Author

ﬁlipe.paz@ua.pt

4.6. Variable-Temperature Powder X-ray Diffraction. Vari-

able-temperature powder X-ray diffraction data for [La(H₃nmp)] were

collected on an X’Pert MPD Philips diffractometer (Cu K_RX-radiation,

λ = 1.54060 Å), equipped with an X’Celerator detector, a curved

graphite-monochromated radiation, a flat-plate sample holder in a

BraggÀBrentano para-focusing optics configuration (40 kV, 50 mA),

and a high-temperature Anton Parr HKL 16 chamber controlled by an

Anton Parr 100 TCU unit. Intensity data were collected in the step mode

(0.04°, 15 s per step) in the range ca. 5 e 2θ e 35°. Data were collected

between 30 and 800 °C.

’ ACKNOWLEDGMENT

We are grateful to Fundac-~ao para a Ci^encia e a Tecnologia

(FCT, Portugal) for their ﬁnancial support through the R&D

project PTDC/QUI-QUI/098098/2008, for the doctoral

research grant No. SFRH/BD/46601/2008 (to PS), and for the

postdoctoral research grants Nos. SFRH/BPD/63736/2009 (to

JAF) and SFRH/BPD/46473/2008 (to SMB). We further wish

to thank FCT for their BII research grants (to FV) programme.

4.7. Photoluminescence. Photoluminescence measurements

were recorded on a Fluorolog-3 model FL3-2T with a double excitation

spectrometer (Triax 320), fitted with a 1200 grooves/mm grating blazed

at 330 nm, and a single emission spectrometer (Triax 320), fitted with a

1200 grooves/mm grating blazed at 500 nm, coupled to an R928

photomultiplier. Excitation spectra were corrected from 240 to

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Thermal transformation of a layered multifunctional network into a metal-organic framework based on a polymeric organic linker

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