doi.org/10.26434/chemrxiv.8241887.v2

Investigating the Effect of Positional Isomerism on the Assembly ofZirconium Phosphonates Based on Tritopic LinkersMarco Taddei, Stephen J. I. Shearan, Anna Donnadio, Mario Casciola, Riccardo Vivani, FerdinandoCostantino

Submitted date: 30/07/2019 • Posted date: 30/07/2019Licence: CC BY-NC-ND 4.0Citation information: Taddei, Marco; Shearan, Stephen J. I.; Donnadio, Anna; Casciola, Mario; Vivani,Riccardo; Costantino, Ferdinando (2019): Investigating the Effect of Positional Isomerism on the Assembly ofZirconium Phosphonates Based on Tritopic Linkers. ChemRxiv. Preprint.

We report on the use of a novel tritopic phosphonic linker,2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine, for the synthesis of a layered zirconium phosphonate,named UPG-2. Comparison with the structure of the permanently porous UPG-1, based on the related linker2,4,6-tris[4-(phosphonomethyl)phenyl]-1,3,5-triazine, reveals that positional isomerism disrupts the porousarchitecture in UPG-2 by preventing the formation of infinitely extended chains connected throughZr-O-P-O-Zr bonds. The presence of free, acidic P-OH groups and an extended network of hydrogen bondsmakes UPG-2 a good proton conductor, reaching values as high as 5.7x10-4 S cm-1.

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Received 00th January 20xx,

Accepted 00th January 20xx

DOI: 10.1039/x0xx00000x

Investigating the effect of positional isomerism on the assembly of zirconium phosphonates based on tritopic linkers

Marco Taddei,a,*Stephen J. I. Shearan,a Anna Donnadio,b Mario Casciola,c Riccardo Vivani,b Ferdinando Costantinoc

We report on the use of a novel tritopic phosphonic linker,

2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine, for the

synthesis of a layered zirconium phosphonate, named UPG-2.

Comparison with the structure of the permanently porous

UPG-1, based on the related linker 2,4,6-tris[4-

(phosphonomethyl)phenyl]-1,3,5-triazine, reveals that

positional isomerism disrupts the porous architecture in UPG-

2 by preventing the formation of infinitely extended chains

connected through Zr-O-P-O-Zr bonds. The presence of free,

acidic P-OH groups and an extended network of hydrogen

bonds makes UPG-2 a good proton conductor, reaching values

as high as 5.7x10-4 S cm-1.

The quest for porous metal phosphonates is a niche research

area that was first explored in the early 90s1 and has recently

gained renewed momentum,2 thanks to some novel synthetic

approaches that have granted access to ordered and porous

compounds. Use of phosphonate monoester linkers, which

feature similar coordination geometry to carboxylates, has

been explored in a systematic fashion, leading to several

microporous compounds.3-6 Post-synthetic hydrolysis of the

phosphonate ester groups has also been demonstrated, with

benefits to the CO2 affinity of the framework.7, 8An alternative

approach based on a similar rationale involves phosphinates as

carboxylic acid analogues, affording a series of Fe-based

compounds with isoreticular structures and high porosity.9 One

peculiar and intriguing feature of these materials is that

functionalization of the framework can be accomplished by

tuning the size and nature of the pending group attached to the

P atom. Introduction of ancillary ligands, e.g. bipyridines or

dicarboxylates, which can act as additional bridging units

connecting different inorganic struts, has also been successful

in generating porous materials based on either phosphonic or

phosphinic linkers.10-13 Shifting the focus to the reaction

medium, ionic liquids have recently been found to promote the

formation of secondary building units, similar to those observed

in carboxylate-based metal-organic frameworks (MOFs),

yielding single crystals of three new zirconium phosphonates

with open framework structures.14 The use of linkers having

specific geometrical features, not compatible with the

formation of the dense inorganic layers typical of metal

phosphonates, is another strategy of election for inducing

porosity in this family of materials.15-17 Several reports have

appeared over the last few years, involving tritopic,18-22

tetrahedral14, 23-25 and tetratopic square linkers,26-28 which

prove that tuning the geometry and symmetry of the linker is

indeed an effective way to access open framework

architectures, reaching surface areas above 1000 m2 g-1. Besides

porosity, the most striking characteristic of these phosphonate-

based MOFs is their exceptional chemical stability, which often

exceeds that of the most stable classical carboxylate-based

counterparts. This can be attributed to the strong affinity of the

phosphonic group for metal ions, which makes metal

phosphonates very insoluble and resistant to hydrolysis, even in

aggressive conditions.29, 30 According to the hard and soft acids

and bases (HSAB) theory,31 combination of hard acids, such as

Zr4+, Ti4+, Al3+, Cr3+, and hard oxygenated bases, such as

carboxylate or phosphonate groups, leads to formation of

strong metal-oxygen bonds.32 The higher stability of metal

phosphonates derives from the higher charge and the increased

number of donor atoms of the -PO3H2 group.29

We recently reported on the synthesis of the first crystalline and

microporous zirconium phosphonate, UPG-1,19 which is based

on the 2,4,6-tris[4-(phosphonomethyl)phenyl]-1,3,5-triazine

linker (hereafter H6pttbmp, scheme 1). UPG-1 features two

types of monodimensional channels with diameter of about 5 Å

and 10 Å and is permanently porous, with a BET surface area of

410 m2 g-1 and a pore volume of 0.2 cm3 g-1 (Figure S1). In an

effort to gain deeper understanding of the structure directing

function of the linker, we have focused on the influence of

positional isomerism on the assembly of the crystal structure.

To this purpose, we have prepared the novel 2,4,6-tris[3-

(phosphonomethyl)phenyl]-1,3,5-triazine linker (hereafter

a. Energy Safety Research Institute, Swansea University, Fabian Way, Swansea, SA1 8EN, United Kingdom. Email: marco.taddei@swansea.ac.uk

b. Dipartimento di Scienze Farmaceutiche, University of Perugia, Via del Liceo 1, 06123 Perugia, Italy

c. Dipartimento di Chimica Biologia e Biotecnologia, University of Perugia, Via Elce di Sotto 8, 06123 Perugia, Italy

Electronic Supplementary Information (ESI) available: [Synthetic procedures; organic compounds characterisation (1H NMR, 13C NMR, HSQC, MS, FTIR); structure solution and refinement details; additional structural figures; additional PXRD patterns]. See DOI: 10.1039/x0xx00000x

2

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H6mttbmp, scheme 1), where the phosphonomethyl moiety is

in meta position with respect to the central triazine core, rather

than in para position as in H6pttbmp.

Scheme 1. Molecular formulae of the H6pttbmp and H6mttbmp linkers.

H6mttbmp was synthesised following a very similar three-step

route to that previously developed for H6pttbmp19 (see ESI for

details and full characterisation of intermediate and final

products, Figures S2-15). H6mttbmp was employed as a linker

for the synthesis of a new microcrystalline zirconium

phosphonate, named UPG-2, whose structure was determined

and refined using powder X-ray diffraction (PXRD) data (see ESI

for details on structure solution and refinement, Table S1,

Figure S16). The crystal structure of UPG-2 is shown in Figure 1.

Figure 1. View of the crystal structure of a single layer of UPG-2 along the b axis

(a) and perpendicular to the bc plane (b). The carbon atoms belonging to the linker

molecule lying lower along the a axis are coloured in orange for the sake of clarity.

Colour code: Zr, pink; P, green; N, blue; O, red; C, grey and orange.

UPG-2 crystallizes in the triclinic space group P-1, with lattice

parameters a = 8.8147(4) Å, b = 9.6066(5) Å, c = 16.338(1) Å, α

= 88.426(4) °, β = 83.914(4) °, γ = 85.607(4) °. The asymmetric

unit consists of one Zr atom sitting on an inversion centre

(special position with multiplicity = 1), one H4mttbmp2-

fragment and two water molecules (each with occupancy of

0.75). The Zr atom is octahedrally coordinated by six oxygen

atoms belonging to six different phosphonate groups (three of

which are crystallographically independent and three

generated by symmetry). Each H4mttbmp2- molecule is

coordinated to three different Zr atoms through monodentate

phosphonate groups. The resolution of PXRD data is not

sufficient to locate hydrogen atoms, but, based on

electroneutrality requirements, we can deduce that four of six

P-O groups coordinated to Zr are negatively charged P-O-,

whereas two of them are neutral P=O. The presence of purely

monodentate phosphonate groups is, to the best of our

knowledge, observed here for the first time in any known

zirconium phosphonate. This feature leads to lack of connection

among ZrO6 octahedra, which remain isolated from each other

(the shortest Zr-Zr distance is 8.82 Å). The connection of

isolated ZrO6 octahedra and H4mttbmp2- units gives rise to a

layered structure, with layers about 9 Å thick lying in the bc

plane. Each layer is connected to adjacent ones through a

network of hydrogen bonds extending along the b axis that

involves free P-O groups and two water molecules sitting in the

interlayer space (Figure S17). Detailed views of the network of

hydrogen bonds involving each phosphonate group and each

water molecule are provided in Figures S18-22. In addition, a

system of π-π stacking interactions extending along the a axis

exists among the aromatic rings of H4mttbmp2-, further

contributing to efficient stacking of layers (Figure S23).

Given the similar linkers used for the synthesis of UPG-1 and

UPG-2, detailed analysis and comparison of their structural

features is in order. The two compounds were prepared in very

similar reaction conditions: same temperature (80 °C), same

Zr/linker/HF ratio (1:1:50), same concentration of metal and

linker (0.018 M). Therefore, the differences in the resulting

crystal structures can purely be attributed to the influence of

the geometrical arrangement of the linkers. UPG-1 and UPG-2

display identical chemical composition (if extraframework

water molecules are not considered), with the same Zr/linker

ratio (1:2) and the linkers in the same protonation state (four

protons of the original six are retained after reaction with the

metal). Both linkers display a cis-trans-trans configuration of the

phosphonate groups, with respect to the aromatic backbone,

however the intramolecular P-P distances are significantly

different: 13.0, 14.7 and 14.9 Å in UPG-1; 6.8, 12.4 and 13.8 Å in

UPG-2 (Figure 2). This suggests that H4mttbmp2- can adopt a

more “compact” conformation than H4pttbmp2-. A simple

optimisation of the molecular structure of the linker H6mttbmp

using the Merck molecular force field 94 (MMFF94),33 as

implemented in the software Avogadro,34 reveals that the

lowest energy configuration achievable displays two

intramolecular hydrogen bonds between two phosphonic

groups (Figure S24). The possible presence of these non-

covalent interactions also in reaction conditions could explain

why the linker H6mttbmp prefers the more compact

conformation. As a result, the connectivity between the organic

linkers and the metal atoms in UPG-1 and UPG-2 is remarkably

different: both H4pttbmp2- and H4mttbmp2- are coordinated to

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three Zr atoms, but the former linker displays one bidentate,

one monodentate and one non-coordinated phosphonate

group (Figure 2a), whereas the latter linker displays three

monodentate phosphonate groups (Figure 2b). The presence of

bidentate phosphonate groups in UPG-1 is ultimately crucial to

afford connection of adjacent Zr atoms along the c axis direction

and formation of infinite inorganic building units (IBUs) (Figure

S25). These 1D IBUs are connected in the remaining two

dimensions by the organic linkers, resulting in the formation of

a 3D framework (Figure S1). As previously discussed, the lack of

polydentate phosphonate groups prevents formation of

extended IBUs in UPG-2 and, as a consequence, the structure

cannot extend in the third dimension, giving rise to a 2D layered

motif.

Figure 2. Linker conformation and connectivity in UPG-1 (a) and UPG-2 (b). Color code:

Zr, pink; P, green; N, blue; O, red; C, gray.

The thermogravimetric curve of UPG-2, measured under air, is

shown in Figure 3. The first weight loss, occurring at

temperature lower than 130 °C, is attributed to the desorption

of three water molecules per formula unit (calculated: 4.0%;

observed: 4.0%). The compound is then stable until about 280

°C, when another 5.0% weight loss is observed. This loss could

be due to some degree of condensation of the many free P-O

groups present in the structure. Similar behaviour was observed

for UPG-1.19 Decomposition of the organic part of the structure

takes place above 480 °C. The total weight loss at 1200 °C is

73.4%. Zirconium phosphonates with P/Zr ratio ≥ 2 usually

thermally decompose to ZrP2O7.35, 36 This product is also the

only crystalline phase observed in the PXRD pattern of the

decomposition residue of UPG-2 after TGA (Figure S26),

allowing to calculate a weight loss of 80.0%, larger than the

observed one. Since UPG-2 features an unusually high P/Zr ratio

of 6, it is possible that part of the phosphonate groups is not

completely decomposed at 1200 °C and a mixture of ZrP2O7 and

other, amorphous phosphorus containing residues is formed.

Figure 3.Thermogravimetric curve of UPG-2.

The proton conductivity of pellets of UPG-2 was measured at

100 °C, under controlled relative humidity (RH). Figure 4 shows

that the conductivity increases by a factor of ca. 7, from 8.5x10-

5 to 5.7x10-4 S cm-1, with increasing RH from 40 to 95%. The

value at 95% RH is in line with those previously observed for

other Zr phosphonates.37 To get insight into the physical origin

of the proton transport, the hydration of UPG-2 was

determined under the same conditions used to measure the

conductivity. At 100 °C and 95% RH, UPG-2 takes up 6.2 water

molecules per unit formula (about one water molecule per –PO3

group), while 0.8 water molecules are lost after lowering RH to

40%. This loss appears to be too small to account for the large

conductivity changes observed in this RH range. Therefore, it

may be inferred that the pellet conductivity originates mainly

from surface/intergrain proton transport and is affected by the

hydration of the microcrystal surface, which is expected to be

more susceptible than bulk hydration to RH changes. The

excellent stability of UPG-2 in measurement conditions is

proved by the PXRD pattern of the ground pellet, which is

practically identical to that of the as synthesized material

(Figure S27).

Figure 4. Proton conductivity of UPG-2 at 100 °C as a function of relative humidity.

Conclusions

In this work, we have investigated the effect of positional

isomerism in tritopic phosphonic linkers on the assembly of the

crystal structure of the relative Zr4+ derivatives. We found that

4

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combination of Zr4+ and H6mttbmp, having -CH2-PO3H2 groups

in meta position with respect to the central triazine core,

affords a compound with unprecedented 2D layered structure

featuring isolated Zr octahedra (UPG-2), as opposed to the 3D

open framework obtained when H6pttbmp, having -CH2-PO3H2

groups in para position with respect to the central triazine core

(UPG-1). The main structure-driving factor seems to be the

ability of H6mttbmp to adopt a conformation where the

intramolecular P-P distances are significantly shorter than in

H6pttbmp, thus preventing formation of extended inorganic

building units and development of the crystal structure in the

third dimension. This is attributed to the presence of

intramolecular hydrogen bonding interactions between

phosphonic groups in the free linker. Thanks to the large

number of hydrogen bonds involving phosphonic groups and

water molecules, UPG-2 is a good proton conductor, reaching

conductivity as high as 5.7x10-4 S cm-1 at 100 °C and 95% relative

humidity. These results add to the existing body of knowledge

concerning the crystal engineering of Zr phosphonates and can

help in designing new phosphonic linkers with specific

geometrical features to induce formation of structural

arrangements with the desired dimensionality.

Conflicts of interest

There are no conflicts to declare.

Acknowledgement

The authors acknowledge the European Union’s Horizon 2020

research and innovation programme under the Marie

Skłodowska-Curie grant agreement No 663830 (M.T.) and the

National Mass Spectrometry Facility (NMSF) at Swansea

University.

Notes and references

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5

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TABLE OF CONTENTS

Combination of the novel linker 2,4,6-tris[3-

(phosphonomethyl)phenyl]-1,3,5-triazine and Zr(IV)

afforded a layered compound that lacks extended

inorganic connectivity and displays good proton

conductivity.

download fileview on ChemRxivTaddei_Manuscript_ChemRxiv_v2.pdf (542.03 KiB)

Investigating the effect of positional isomerism on the

assembly of zirconium phosphonates based on tritopic

linkers

Marco Taddei,1,* Stephen J. I. Shearan,1 Anna Donnadio,2 Mario Casciola,3 Riccardo Vivani,2 Ferdinando

Costantino3

1Energy Safety Research Institute, Swansea University, Fabian Way, Swansea, SA1 8EN, United

Kingdom 2Dipartimento di Scienze Farmaceutiche, University of Perugia, Via del Liceo 1, 06123 Perugia, Italy 3 Dipartimento di Chimica Biologia e Biotecnologia, University of Perugia, Via Elce di Sotto 8, 06123

Perugia, Italy

Email: marco.taddei@swansea.ac.uk

ELECTRONIC SUPPLEMENTARY INFORMATION

Figure S1. Crystal structure of UPG-1 seen along the c axis. Colour code: Zr, pink; P, green; N, blue; O,

red; C, grey. Solvent molecules were removed for the sake of clarity.

Experimental Section

Chemicals.

Zirconium oxide chloride octahydrate was obtained from Merck Millipore. 3-

bromomethylbenzonitrile,trifluoromethanesulfonic acid and chlorotrimethylsilane were obtained

from Fluorochem. Triethylphosphite was obtained from VWR. Potassium iodide, concentrated

ammonium hydroxide, isopropanol, hexane, diethyl ether, hydrofluoric acid and anhydrous

acetonitrile were obtained from Sigma-Aldrich. Methanol was obtained from Carlo Erba Reagenti. All

chemicals were used as received, with no further purification.

Analytical methods.

Nuclear magnetic resonance (NMR): 1H and 31P NMR spectra were recorded on a Bruker AV-500

instrument. All chemical shifts are reported in ppm and coupling constants are reported in Hz.

Mass spectrometry (MS): MS data for mttbmBr was acquired by atmospheric pressure chemical

ionisation (APCI) via an atmospheric solids analysis probe (ASAP) on a Waters Xevo G2-S instrument.

A small amount of solid sample was transferred to the tip of a glass capillary, which was then placed

within the ASAP source and inserted into the instrument. The vaporizer temperature was increased

to 450°C, at which point ions were observed and acquired. Data was processed using vendor MassLynx

software. MS data for Et6mttbmp was acquired by positive mode nano-electrospray ionisation (nESI)

via and Advion TriVersa NanoMate on a Thermo Fisher Scientific LTQ Orbitrap. The sample was

prepared by dissolving in dichloromethane (DCM; HPLC grade, Fisher Scientific) and diluting into a

solution of ammonium acetate (NH4OAc; Sigma-Aldrich) in methanol (MeOH; HPLC grade, Fisher

Scientific). An aliquot was loaded into the NanoMate microtiter plate well and infused into the

Orbitrap instrument for acquisition. The Orbitrap tube lens voltage was +150V. Data was processed

using vendor Xcalibur software. MS data for H6mttbmp mass spectrometry data was acquired by

negative mode nano-electrospray ionisation (nESI) via and Advion TriVersa NanoMate on a Thermo

Fisher Scientific LTQ Orbitrap. The sample was prepared by dissolving in a 1:1 (v:v) mixture of

water:methanol (H2O:MeOH; HPLC grade, Fisher Scientific) or 1:1 (v:v) mixture of water:acetonitrile

(H2O:MeCN; HPLC grade, Fisher Scientific), respectively, and diluting into a solution of diethyl amine

(DEA; Sigma-Aldrich) in MeOH. An aliquot was loaded into the NanoMate microtiter plate well and

infused into the Orbitrap instrument for acquisition. The Orbitrap tube lens voltage was −100V. Data

was processed using vendor Xcalibur software.

Attenuated-total reflectance infrared (ATR-IR) spectroscopy: a Thermo Scientific Nicolet iS10 FT-IR

Spectrometer was used to collect the ATR-IR spectra of all samples. Spectra were recorded in the 650

– 4000 cm-1 region with 16 scans.

Elemental analysis: The zirconium and phosphorus contents were obtained by inductively coupled

plasma optical emission spectroscopy using a Varian Liberty Series II instrument working in axial

geometry, after mineralization of the sample with concentrated hydrofluoric acid. The carbon,

hydrogen, and nitrogen contents were obtained with an EA 1108 CHN Fisons instrument.

Thermogravimetric analysis (TGA): TGA was performed using a Netzsch STA490C thermoanalyser

under a 20 mL min−1 air flux with a heating rate of 5 °C min−1.

Powder X-ray diffraction (PXRD): the PXRD pattern for structure solution and Rietveld refinement was

collected in the 4-80 °2θ range, with 0.017 ° step size and 150 s step-1 counting time, using the Cu Kα

radiation on a PANalytical X’PERT PRO diffractometer, PW3050 goniometer, equipped with an

X’Celerator detector. The long fine focus (LFF) ceramic tube operated at 40 kV and 40 mA.

Proton conductivity and water uptake: Conductivity measurements were carried out on pellets of

pressed powder by impedance spectroscopy as described elsewhere.1 The pellet conductivity was

determined at 100 °C and decreasing relative humidity (RH) from 95% to 40%. All of the conductivity

values here reported refer to measurements carried out after the conductivity had reached a constant

value for at least 2 hours. Water uptake at 100 °C, and RH of 40% and 100%, was determined by means

of a cell, having the same size and shape as the conductivity cell, where the pellet holder was replaced

by a glass container hosting the sample.1 After one day equilibration at the desired RH value, the water

content was calculated on the basis of the weight change after sample drying at 120 °C, by taking into

account the amount of water trapped in the sample container at the temperature and RH of the

experiment.

Synthesis of 2,4,6-tris[3’-(bromomethyl)phenyl]-1,3,5-triazine (mttbmBr).

3-bromomethylbenzonitrile (5 g, 6.38 mmol) was placed in a 100 mL Schlenk flask and kept in a water-

ice bath. Trifluoromethanesulfonic acid (5 mL) was progressively added under N2. After the addition,

the flask was closed, and the mixture was warmed to room temperature and stirred for 20 h. Then,

the mixture was poured in ice and neutralised with concentrated ammonium hydroxide. The solid

phase was collected by filtration and washed with water and ethanol. The white solid was dried in a

hot air oven at 60 °C. 4.95 g of product was recovered (Yield: 99%).

1H NMR (CDCl3, 500 MHz): = 8.74 – 8.61 (m, 6H, aromatic), 7.61 (m, 3H, aromatic), 7.52 (m, 3H,

aromatic), 4.62 (s, 6H, Ph-CH2-Br) ppm (Figure S2).

The low solubility of mttbmBr in the deuterated solvents we have available in our laboratory (CDCl3,

CD3OD, d6-DMSO) prevented us from obtaining well resolved 13C NMR and HSQC spectra.

Mass spectrometry: 585.9136 m/z [M+H]+ (Figure S3).

Attenuated total reflectance infrared (ATR-IR) analysis also clearly shows that the 3-

bromomethylbenzonitrile starting material and the mttbmBr product produce very different spectra

(Figure S6).

Figure S2. 1H-NMR spectrum of mttbmBr.

Figure S3. Mass spectrum of mttbmBr (bottom) and theoretical isotope pattern for the [M+H]+ species

(bottom).

Figure S4. ATR-IR spectra of 3-bromomethylbenzonitrile (black) and mttbmBr (red). The band at 2227

cm-1 in the spectrum of 3-bromomethylbenzonitrile is assigned to the stretching of the nitrile group.

The band has a weird shape because it falls in the same region as the asymmetric stretching of CO2,

which is accounted for in the background spectrum. This band completely disappears in the spectrum

of mttbmBr, while a new band at 1518 cm-1 appears, which is assigned to the stretching of the C=N

double bonds in the triazine ring.

Synthesis of hexaethyl [1,3,5-triazine-2,4,6-triyltris(3,1-phenylenemethylene)]tris(phosphonate)

(Et6mttbmp).

mttbmBr (4.64 g, 8.0 mmol) and triethylphosphite (8.2 mL, 47.8 mmol) were introduced in a 100 mL

round bottom flask. The mixture was heated under reflux for 4.5 h under a N2 atmosphere. After

cooling, a viscous liquid was obtained, which was stirred overnight in 200 mL of hexane under a N2

atmosphere, forming a white solid. The white solid was filtered and dried in a desiccator under

vacuum. 4.45 g of product was recovered (Yield: 74%).

1H NMR (CDCl3, 500 MHz): = 8.74 – 8.63 (m, 6H, aromatic), 7.62 (m, 3H, aromatic), 7.56 (t, J = 7.6 Hz,

3H, aromatic), 4.10 (dq, J = 8.0, 7.1 Hz, 12H, O-CH2), 3.37 (d, J = 21.6 Hz, 6H, Ph-CH2-P), 1.30 (t, J = 7.1

Hz, 18H, -CH3) ppm (Figure S5).

13C NMR (CDCl3, 500 MHz): = 171.47, 136.46, 133.99, 132.32, 130.17, 128.94, 127.75, 62.40, 34.82

(d), 16.38 ppm. (Figures S6-7).

31P NMR (CDCl3, 500 MHz): = 26.09 (m) ppm (Figure S8).

Mass spectrometry: 760.2656 m/z [M+H]+; 777.2942 [M+NH4]+ (Figure S9).

Figure S5. 1H-NMR spectrum of Et6mttbmP: aromatic protons region (top) and aliphatic protons region

(bottom). For the sake of consistency, the labelling of H atoms in Figure S2 is transferred here and

additional labels are used for the ethyl groups’ protons.

Figure S6. 13C-NMR spectrum of Et6mttbmP. For the sake of consistency, the labelling of H atoms in

Figure S5 has been translated to the corresponding C atoms.

Figure S7. HSQC spectrum of Et6mttbmP: aromatic signals region (top) and aliphatic signals region

(bottom). Labelling is the same as in Figures S5-6.

Figure S8. 31P-NMR spectrum of Et6mttbmP.

Figure S9. Mass spectrum of Et6mttbmp (top) and theoretical isotope profiles for the [M+H]+ (centre)

and the [M+NH4]+ species (bottom).

Synthesis of 2,4,6-tris[3-(phosphonomethyl)phenyl]-1,3,5-triazine (H6mttbmp).

Et6mttbmp (3.0 g, 4.0 mmol) was dissolved in anhydrous acetonitrile (50 mL) inside a 250 mL round

bottom flask under N2 flow. Then, chlorotrimethylsilane (4.5 mL, 36.0 mmol) and potassium iodide

(5.94 g, 36.0 mmol) were added. The mixture was heated to reflux under a N2 atmosphere for 3.5 h.

Then, the mixture was filtered to remove potassium chloride, the mother liquor was dried in a rotary

evaporator to remove acetonitrile and the excess chlorotrimethylsilane and water was added. After

hydrolysis of the trimethylsilylester, the mixture was washed with diethyl ether to remove the

trimethylsilanol side product. The water phase was recovered and dried in a rotary evaporator. The

white solid was washed in isopropanol, filtered and dried in a hot air oven at 60 °C. 1.35 g of product

was recovered (Yield: 58%).

1H NMR (D2O + K2CO3, 500 MHz): δ = 8.37 (m, 3H, aromatic), 8.31 (dq, J = 7.7, 1.6 Hz, 3H aromatic),

7.56 (dq, J = 7.7, 1.7 Hz, 3H, aromatic), 7.47 (t, J = 7.7 Hz, 3H, aromatic), 2.92 (d, J = 19.5 Hz, 6H, Ph-

CH2-P) ppm. (Figure S10)

13C NMR (D2O + K2CO3, 500 MHz): = 172.66, 139.45, 135.03, 134.26, 130.12, 128.72, 126.03, 37.39

(d) ppm. (Figures S11-12).

31P NMR (D2O + K2CO3, 500 MHz): = 17.30 (t, J = 19.3 Hz) ppm. (Figure S13)

Mass spectrometry: 590.0656 m/z [M-H]- + [2M-2H]2- (Figure S9); 294.5290 m/z [M-2H]2-; 305.5198

m/z [M-3H+Na]2-. (Figure S14)

Figure S10. 1H-NMR spectrum of H6mttbmP: aromatics proton region (top) and methylene protons

region (bottom).

Figure S11. 13C-NMR spectrum of H6mttbmP. For the sake of consistency, the labelling of H atoms in

Figure S10 has been translated to the corresponding C atoms.

Figure S12. HSQC spectrum of H6mttbmP: aromatic signals region (top) and methylene signals region

(bottom). Labelling is the same as in Figures S10-11.

Figure S13. 31P-NMR spectrum of H6mttbmP.

Figure S14. Mass spectrum of H6mttbmP in the 589.5-593.5 m/z region (top) and theoretical isotope

profiles for the [M-H]- (centre) and the dimeric [2M-2H]2- species (bottom).

Figure S15. Mass spectrum of H6mttbmP in the 293-310 m/z region (top) and theoretical isotope

profiles for the [M-2H]2- (centre) and the [M-3H+Na]2- species (bottom).

Synthesis of Zr(H4mttbmp)23H2O (UPG-2).

ZrOCl28H2O (64 mg, 0.2 mmol) was dissolved in HF 2.9 M (3.5 mL, 10 mmol) in a plastic bottle. The

mixture was diluted with water (8.0 mL), then H6mttbmp (118 mg, 0.2 mmol) was added. The bottle

was closed and kept in an oven at 80 °C for seven days. The white solid was then filtered under

vacuum, washed with water and methanol, and dried at room temperature. 108 mg of product was

recovered (Yield: 80%, based on H6mttbmp).

Analysis: Calcd for C48H50N6O21P6Zr: Zr = 6.9%,P = 14.1%, C = 43.5%; H = 3.8%, N = 6.3%; Found: Zr =

7.1%, P = 12.9%, C = 46.3%; H = 4.5%, N = 6.1%.

Structure Determination and Refinement.

The crystal structure of UPG-2 was solved ab initio from PXRD data. Indexing was performed using the

TREOR program,2 using the positions of 49 peaks in the 5-30 °2θ range and finding a triclinic cell with

M(20) = 26 and no unindexed lines. The structural model was determined using the real space global

optimization methods implemented in the FOX program:3 one mttbmp fragment and one ZrO6

octahedron, placed in the inversion center, were input, according to the observed P/Zr ratio of 6. Trial

structures were generated using the “Parallel Tempering” algorithm4 implemented in FOX, using the

following antibump distances: P-Zr = 3 Å, P-P = 3 Å, C-Zr = 4 Å, C-O = 2.5 Å, O-O = 2.5 Å, O-N = 4 Å, P-N

= 4 Å. Rietveld refinement of the structural model was performed using the GSAS program.5 First, zero-

shift, unit cell, background, and profile-shape parameters were refined. A corrected pseudo-Voigt

profile function (six terms) with two terms for the correction of asymmetry at the low-angle region

was used. Then, atomic coordinates were refined by restraining the bond distances to the following

values: Zr-O = 2.00(5) Å, P-O = 1.55(5) Å, P-C = 1.80(5) Å, aromatic C-C = 1.39(5) Å, and C-N = 1.32(5)

Å. The statistical weight of these restraints was decreased as the refinement proceeded. The position

of water molecules was determined using difference Fourier maps, which showed two peaks of

residual electron density in the interlayer space. The occupancy of these water molecules was

arbitrarily set to 0.75, in agreement with the observed weight loss from TGA, discussed herein. Finally,

atomic displacement parameters were refined by constraining them to have the same value. At the

end of the refinement, the shifts in all parameters were less than their standard deviations.

Table S1 lists the crystal data and refinement details. Figure S16 shows the final Rietveld and difference

plots.

Table S1. Structural data and refinement details for UPG-2.

empirical formula C48H50N6O21P6Zr

formula weight 1323

crystal system triclinic

space group P-1

a/Å 8.8147(4)

b/Å 9.6066(5)

c/Å 16.338(1)

α/° 88.426(4)

β/° 83.914(4)

γ/° 85.607(4)

volume/Å3 1371.4(2)

Z 1

calculated density/g cm-3 1.59

data range/°2θ 4-80

wavelength/Å 1.54056

n. of data points 4470

n. of reflections 1626

n. of parameters 151

n. of restraints 136

Rpa 0.0395

Rwpb 0.0511

RF2c 0.05960

GOFd 2.91

a Rp = |Io-Ic | / Io; bRwp = [ w(Io-Ic) 2 / wIo2 ]1/2; cRF2 = |Fo

2 - Fc2 | / |Fo

2 |; d GOF = [ w(Io-Ic)2 /

(No - Nvar)]1/2

Figure S16. Final Rietveld plot for UPG-2, reporting the observed pattern (red symbols), the calculated

pattern (green line), and their difference (pink line). Black markers at the bottom indicate the

calculated positions of peaks.

Figure S17. View of the hydrogen bonding network existing between the layers of UPG-2. Zr octahedra

belonging to adjacent layers are coloured different for the sake of clarity. Colour code: Zr, pink/light

blue; P, green; O, red; C, grey. Hydrogen bonds are represented as red dashed lines.

Figure S18. View of the hydrogen bonding interactions involving the phosphonate group centred

around P26. Colour code: Zr, pink; P, green; O, red; C, grey. Hydrogen bonds are represented as red

dashed lines.

Figure S19. View of the hydrogen bonding interactions involving the phosphonate group centred

around P31. Colour code: Zr, pink; P, green; O, red; C, grey. Hydrogen bonds are represented as red

dashed lines.

Figure S20. View of the hydrogen bonding interactions involving the phosphonate group centred

around P36. Colour code: Zr, pink; P, green; O, red; C, grey. Hydrogen bonds are represented as red

dashed lines.

Figure S21. View of the hydrogen bonding interactions involving Ow1. Colour code: P, green; O, red.

Hydrogen bonds are represented as red dashed lines.

Figure S22. View of the hydrogen bonding interactions involving Ow2. Colour code: P, green; O, red.

Hydrogen bonds are represented as red dashed lines.

Figure S23. Space filling model view of the system of π-π stacking interactions extending along the a

axis. Colour code: N, blue; C, grey.

Figure S24. Optimised conformations of the H6mttbmp linker and their relative energy, normalised to

the energy of the least stable conformer (a). Colour code: P, yellow; O, red; N, blue; C, grey; H, white.

Hydrogen bonds are represented as red dashed lines.

Figure S25. Comparison of the 1D IBU found in UPG-1 (left) and the 0D IBU in UPG-2 (right). Colour

code: Zr, pink; P, green; O, red; C, grey.

Figure S26. Calculated PXRD pattern for ZrP2O7 (COD 1010464) (black) and PXRD pattern of the TGA

residue for UPG-2 (red).

Figure S27. PXRD patterns of UPG-2 as synthesised (black) and after the conductivity measurement

(red).

REFERENCES

1. M. Taddei, A. Donnadio, F. Costantino, R. Vivani and M. Casciola, Inorg. Chem., 2013, 52, 12131-12139.

2. P.-E. Werner, L. Eriksson and M. Westdahl, J. Appl. Crystallogr., 1985, 18, 367-370. 3. V. Favre-Nicolin and R. Cerny, J. Appl. Crystallogr., 2002, 35, 734-743. 4. M. Falcioni and M. W. Deem, J. Chem. Phys., 1999, 110, 1754-1766. 5. A. C. Larson and R. B. V. Dreele, "General Structure Analysis System (GSAS)" Los Alamos

National Laboratory Report LAUR 86-748, 2000.

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