Insertion of Guest Molecules into a Mixed Ligand 

Metal−Organic Framework via Single-Crystal-to-SingleCrystal Guest Exchange

by Lily Giri, Rose Pesce-Rodriguez, Shashi P Karna,

and Nirupam J Trivedi

ARL-TR-7004 July 2014

Approved for public release; distribution unlimited.
NOTICES

Disclaimers

The findings in this report are not to be construed as an official Department of the Army position unless 

so designated by other authorized documents.

Citation of manufacturer’s or trade names does not constitute an official endorsement or approval of the 

use thereof.

Destroy this report when it is no longer needed. Do not return it to the originator.
Army Research Laboratory

Aberdeen Proving Ground, MD 21005-5069

ARL-TR-7004 July 2014

Insertion of Guest Molecules into a Mixed Ligand 

Metal−Organic Framework via Single-Crystal-to-SingleCrystal Guest Exchange

Lily Giri, Rose Pesce-Rodriguez, Shashi P Karna, 

and Nirupam J Trivedi

Weapons and Materials Research Directorate, ARL

Approved for public release; distribution unlimited.
ii

REPORT DOCUMENTATION PAGE Form Approved

OMB No. 0704-0188

Public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, gathering and maintaining the 

data needed, and completing and reviewing the collection information. Send comments regarding this burden estimate or any other aspect of this collection of information, including suggestions for reducing the 

burden, to Department of Defense, Washington Headquarters Services, Directorate for Information Operations and Reports (0704-0188), 1215 Jefferson Davis Highway, Suite 1204, Arlington, VA 22202-4302.

Respondents should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a currently valid 

OMB control number.

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ADDRESS.

1. REPORT DATE (DD-MM-YYYY)

July 2014

2. REPORT TYPE

Final

3. DATES COVERED (From - To)

June 2013–December 2013

4. TITLE AND SUBTITLE

Insertion of Guest Molecules into a Mixed Ligand Metal−Organic Framework via 

Single-Crystal-to-Single-Crystal Guest Exchange

5a. CONTRACT NUMBER

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

Lily Giri, Rose Pesce-Rodriguez, Shashi P Karna, and Nirupam J Trivedi

5d. PROJECT NUMBER

5e. TASK NUMBER

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

U.S. Army Research Laboratory

ATTN: RDRL-WML-B

Aberdeen Proving Ground, MD 21005-5069

8. PERFORMING ORGANIZATION

REPORT NUMBER

ARL-TR-7004

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES) 10. SPONSOR/MONITOR’S ACRONYM(S)

11. SPONSOR/MONITOR'S REPORT

 NUMBER(S)

12. DISTRIBUTION/AVAILABILITY STATEMENT

Approved for public release; distribution unlimited.

13. SUPPLEMENTARY NOTES

14. ABSTRACT

Metal–organic frameworks (MOFs) are constructed from metal ions or metal ion clusters and bridging organic linkers, which 

provide diverse structural topologies with porous architectures. They have potential to be applied to molecular sieving, storage, 

ion exchange, heterogeneous catalysis, sensor technology, and optoelectronics. The robust open framework of the MOF can 

accommodate various guest molecules. The nanoporous lattices of MOFs have a high degree of chemical versatility and an 

immensely rich host–guest chemistry that include reversible guest and ion exchange and heterogeneous catalysis. In this work, 

we have successfully introduced two small guest molecules, viz. 3-methoxypropionitrile and nitrobenzene, in a cage of host 

MOF Zn2NDC2DPNI-DMF. The complete exchange of the reagent solvent for the new guests has been verified by gas 

chromatography-mass spectrometry (GC-MS), powder x-ray diffraction (PXRD), thermogravimentric analysis (TGA), and 

Fourier transform infrared (FTIR).

15. SUBJECT TERMS

Metal organic frame work, Porous structure, Inclusion of guest molecules

16. SECURITY CLASSIFICATION OF:

17. LIMITATION

 OF 

 ABSTRACT

UU

18. NUMBER

 OF 

 PAGES

22

19a. NAME OF RESPONSIBLE PERSON

Rose Pesce-Rodriguez

A. Report

Unclassified

b. ABSTRACT

Unclassified

c. THIS PAGE

Unclassified

19b. TELEPHONE NUMBER (Include area code)

(410) 306-1877

Standard Form 298 (Rev. 8/98)

Prescribed by ANSI Std. Z39.18
iii

Contents

List of Figures iv

Acknowledgments v

1. Introduction and Background 1

2. Experiment 1

2.1 Materials..........................................................................................................................1

2.2 Experimental ...................................................................................................................2

2.2.1 Solvent Exchange Method...................................................................................2

2.2.2 Vapor Diffusion Method .....................................................................................2

2.3 Characterizations.............................................................................................................3

3. Results and Discussion 3

3.1 Guest Molecule: 3-methoxypropionitrile (3MPN)..........................................................3

3.2 Guest Molecule: Nitrobenzene (NB)...............................................................................8

4. Summary and Conclusions 11

5. References 12

List of Symbols, Abbreviations, and Acronyms 13

Distribution List 14
iv

List of Figures

Fig. 1 (a) Crystal structure of the host MOF. The yellow polyhedra represent the Zn ions: 

carbon, gray; oxygen, red; and nitrogen, blue. (b) 3MPN, (c) NB, (d) NDC, and 

(e) DPNI d and e from Bae et al.4(used with permission) .......................................................2

Fig. 2 Total ion chromatogram for neat MOF ...............................................................................4

Fig. 3 Mass spectrum for 3.35-min peak in Fig. 1 (top) with library search match (bottom) .......5

Fig. 4 Total ion chromatogram for guest-host complex ................................................................5

Fig. 5 Mass spectrum for 3.41-min peak in Fig. 1 (top) with library search match (bottom). .....6

Fig. 6 PXRD pattern of the host MOF and the MOF + 3MPN......................................................6

Fig. 7 TGA curve of neat MOF and the MOF + 3MPN complex .................................................7

Fig. 8 FTIR spectrum of the host MOF and the MOF + 3MPN (spectra normalized to largest 

peak)...........................................................................................................................................8

Fig. 9 Total ion chromatogram for guest-host complex ................................................................8

Fig. 10 Mass spectrum for 5.2 min peak in Fig. 1 (top) with library search match (bottom)........9

Fig. 11 PXRD pattern of the host MOF and the MOF + NB.........................................................9

Fig. 12 TGA curve of neat MOF and the MOF + NB complex...................................................10

Fig. 13 FTIR spectrum of the host MOF and the MOF + NB (spectra normalized to largest 

peak).........................................................................................................................................11
v

Acknowledgments

The authors wish to acknowledge and thank Dr Andrew P Nelson Jr of Naval Air Weapons 

Station, China Lake, CA, for supplying the metal–organic framework (MOF) and Shaan Sharma 

for his preliminary work on metal organic frameworks while at the US Army Research 

Laboratory (ARL) in the 2012 Summer Intern Program. This research was supported by an 

appointment to the Research Participation Program at ARL administered by the Oak Ridge 

Institute for Science and Education through an interagency agreement between the US 

Department of Energy and ARL.
vi

INTENTIONALLY LEFT BLANK.
1

1. Introduction and Background

Metal–organic framework (MOF) materials have attracted a great deal of attention during the last 

few years.

1,2 These materials are formed by coordination bonds between metal ions and organic 

molecular ligands, leading to robust open framework structures that can accommodate various 

guest molecules. Organic ligands in the MOFs make the structure flexible and different from 

traditional microporous materials like zeolites, which are fully inorganic and thus lack synthetic 

flexibility. During the synthesis of MOFs, solvent molecules get trapped in the pores and can be 

removed via desolvation. The resulting empty framework may maintain structural integrity, 

yielding a porous MOF material that has a large apparent surface area. The original solvent or 

other guest molecules can then be adsorbed into this porous structure.

MOFs have a number of useful properties such as large internal surface areas (some as high as 

103–105 m

2

/g), high thermal stability (400–500 °C), ultra low densities, and the availability of 

uniformly structured cavities of molecular dimensions that make them attractive candidate for a 

wide range of chemical and physical applications such as hydrogen and methane storage,

3

chemical separations,

4 selective chemical catalysis,5

photonics,

6 and drug delivery.7 Here we 

present our work on the study of MOFs as thermally stable hosts for small molecule organic 

materials. We have successfully introduced two molecules, viz. 3-methoxypropionitrile (3MPN) 

inside the cage of a MOF and verified the inclusion by using a variety of analytical techniques,

such as powder x-ray diffraction (PXRD), thermo-gravimetric analysis (TGA), Fourier transform 

infrared (FTIR), and gas chromatography-mass spectrometry (GC-MS). 

2. Experiment 

2.1 Materials

Commercial reagents were purchased from Sigma-Aldrich (ACS grade) and used as received. In 

this work, we used Zn2NDC2DPNI-DMF, as our inert MOF host, where NDC is the dianion of 

1,6-napthalene dicarboxylic acid, DPNI is N,N-di-(4-pyridyl)1,4,5,8-napthalenetetracarboxydiimide, and DMF is N, N dimethylformamide, which is employed as a reaction medium during 

the synthesis and is subsequently displaced by the guest molecule. This MOF is constructed with 

zinc (Zn) atoms as metal centers, and NDC and DPNI molecules as ligands, as shown in Fig. 1a.

Our first guest molecule is 3MPN and the second molecule is nitrobenzene (NB). The structures 

of these two guest molecules are shown in Fig. 1b and Fig. 1c.
2

Fig. 1 (a) Crystal structure of the host MOF. The yellow polyhedra represent the Zn ions: carbon, 

gray; oxygen, red; and nitrogen, blue. (b) 3MPN, (c) NB, (d) NDC, and (e) DPNI d and e from 

Bae et al.

4

(used with permission)

2.2 Experimental

Distilled de-ionized water was used for all solution preparations. All glassware was cleaned in 

the order: tap water and dish detergent, purified water, ethanol, and nitrogen gas to dry. Two 

different methods were used to include the guest molecules inside the cage of the host MOF. For 

our first guest molecule (3MPN), we used the solvent exchange method and, for the second 

molecule (NB), the vapor diffusion method was used.

2.2.1 Solvent Exchange Method

In a typical synthesis, 10 mg of the yellow MOF is mixed with 5 mL of 3MPN, resulting in a 

clear yellow liquid. The solution is stirred at room temperature for 3 weeks and then centrifuged. 

The powder left is quickly rinsed in acetone, allowed to dry, and then characterized by GC-MS, 

FTIR, TGA, and PXRD. The nitrile end of the 3MPN molecule has an affinity for metals, so in 

suspending these substances in a solution, we hypothesize that the 3MPN displaces the DMF 

inhabiting the framework.

2.2.2 Vapor Diffusion Method

In this method, the inclusion of the guest molecule NB was done by simply putting the MOF 

under NB atmosphere. Here, 1 mL of NB (guest) was placed in an open vial (2 mL) and 1 mg of 

the yellow MOF (host) was placed in an open DSC pan. Both the pan and the small vial 

a b c

d e
3

containing NB were placed in a larger vial (20 mL) and sealed. The whole system was placed in 

a heater base to maintain a temperature of ~35 °C. 

2.3 Characterizations

Analysis of the MOF and the complexes with the MOF and the guest molecules was performed 

using an Agilent GC-MS (Model 6890N GC and Model 5973N MSD) system fitted with a 

Pyroprobe 2000 (CDS Analytical, Oxford, PA). The GC column used was a HP-5 capillary 

column (0.25 mm × 30 m, 0.25-mm film). The injector temperature was 200 °C and the 

Pyroprobe interface temperature was 175 °C. The GC oven temperature program was as follows: 

50 °C isothermal for 1 min, 50–250 °C at 40 °C/min, and 250 °C isothermal for 1 min. The 

Pyroprobe was programmed to give a 20-s desorption pulse at 175 °C (heating rate 1000 °C/s). 

The sample was held within the coil of the Pyroprobe by first placing it in a quartz tube 

containing a small plug of glass wool and then inserting the entire tube into the coil.

All FTIR spectra were obtained using a ThermoScientific (formerly Nicolet) Nexus 870 FTIR 

spectrometer fitted with a Thermo Nicolet OmniSampler (attenuated total reflectance [ATR]) 

accessory. A deuterated triglycine sulfate (DTGS) and potassium bromide (KBr) beam splitter 

was used. For each spectrum, 64 scans were collected at a resolution of 4 cm–1.

A TGA (TA Instruments Q500) operating under Universal Analysis software (TA Instruments) 

was used for the TGA measurements. All samples were run in open aluminum pans. The heating 

rate was 10 °C/min, and analyses were run under a nitrogen flow of 50 mL/min.

PXRD results of the samples were recorded with a Rigaku-miniflex II diffractometer using Cu 

Kα radiation (λ = 1.541 Å) in –2 geometry and with a position-sensitive detector. The 

diffractometer was operated at 30 kV with 15 mA and the data were collected in the 2 range of 

5–40° with a step size of 0.02° per 1.2 s. 

3. Results and Discussion

3.1 Guest Molecule: 3-methoxypropionitrile (3MPN)

D-GC-MS analysis was performed to identify and confirm the presence of the guest molecule 

inside the MOF complex. The total ion chromatogram of the neat MOF is shown in Fig. 2, which 

shows the presence of a peak at 3.35 min. The mass spectrum of this peak, as well as the library 

search match, is shown in Fig. 3. Based on this finding, we conclude that the starting host MOF

has DMF inside the MOF cage. After the inclusion of the guest molecule 3MPN, D-GC-MS 

analysis was performed to re-identify the constituents inside the host MOF (Fig. 4). While the 

total ion chromatogram of the complex shows a peak at nearly the same retention time observed 

for the neat MOF, the associated mass spectrum of the peak reveals that the peak is from 3MPN 
4

(Fig. 5). The similar retention times for DMF and 3MPN are reasonable considering their 

respective boiling point (i.e., 153 and 165 °C). Based on D-GC-MS results, we conclude that the 

DMF inside the MOF cage has been completely displaced by the guest molecule 3MPN.

PXRD is a powerful technique to follow guest inclusion by performing a detailed analysis of the 

changes in the relative intensities of the low-index diffraction peaks, which are more or less 

sensitive to the cavity filling. In general, the overall intensity of the host XRD reflections will 

decrease upon inclusion of guest molecules due to the enhancement of the X-ray scattering 

contrast.

8

Fig. 6 shows both the PXRD of the original host MOF and the MOF with the guest 

molecule 3MPN. As expected, the overall intensity of the host MOF with 3MPN as guest has 

decreased. Drastic changes in intensity of individual peaks in the XRD pattern can result from 

changes of distribution of electron density within the unit cell of the MOF. The electron density 

distribution is dependent upon the extent of filling of pores in the MOF with guest molecules and 

on the nature of the guest molecules. Esken et al.9showed that in MOF-5, the intensity ratio of 

the reflections at especially 2= 6.9° and 9.7° will change upon loading the guest molecule. In 

this work, the relative intensity of the two low angle peaks at 2= 6.8° and 7.7° changed 

drastically, consistent with the inclusion of the guest molecule. PXRD of the MOF with 3MPN 

displays many additional reflections, which originate from the ordering of the guest molecules in 

the MOF with new reflections observed at 2 = 10°–20°, with the most prominent new reflection 

at 2 = 19°.

Fig. 2 Total ion chromatogram for neat MOF
5

Fig. 3 Mass spectrum for 3.35-min peak in Fig. 1 (top) with library search match (bottom)

Fig. 4 Total ion chromatogram for guest-host complex
6

Fig. 5 Mass spectrum for 3.41-min peak in Fig. 1 (top) with library 

search match (bottom).

Fig. 6 PXRD pattern of the host MOF and the MOF + 3MPN

The temperature-dependent guest release and thermo-stability of the complex were explored by 

TGA. The TG curve shown in Fig. 7 shows a weight loss of 10% in the temperature range of 

25–140 °C, which corresponds to the loss of guest molecule adsorbed on the exterior of the cage 

of the host MOF, and a slow weight loss of 20% between 140–400 °C, which corresponds to the 
7

loss of the guest molecule 3MPN that was inside the MOF cage. The weight loss beyond 400 °C 

is attributed to the decomposition of the framework structure. It should be noticed that the release 

of the guest molecule by heating spans a broad temperature range up to 400 °C, which implies 

that the loss of 3MPN molecules from the cavities of the host MOF grid networks is a prolonged 

process.

Fig. 7 TGA curve of neat MOF and the MOF + 3MPN complex

The FTIR spectrum of the host MOF and the complex (MOF + 3MPN) gave further evidence of 

the inclusion of the guest molecule inside the MOF cage (Fig. 8). The library spectrum of the 

guest molecule is taken from the Bio-Rad “Know it all” informatics system analytical addition 

infrared (IR) spectra database. The spectra of both 3MPN and the complex exhibits the vibration 

of the nitrile groups (C≡N) at 2250.8 cm−1, proving the inclusion of the guest molecule inside the 

MOF cage. The complete removal of DMF molecules is indicated by the disappearance of the 

characteristic C=O band of DMF at 1660 cm−1. The FTIR spectrum of the complex shows a 

new peak at 1638 cm–1, which is not present in either the host or the guest molecule. The peaks at 

1100.3 and 1115.9 cm–1 of the MOF spectra merges to a single peak at 1112.3 cm–1in the 

complex spectra. An additional vibrational band at 1000.5 cm–1is due to the inclusion of the 

guest molecule since this band is also present in the spectrum of the guest molecule but not in

that of the host MOF.
8

Fig. 8 FTIR spectrum of the host MOF and the MOF + 3MPN (spectra normalized to largest peak)

3.2 Guest Molecule: Nitrobenzene (NB)

The total ion chromatogram of the neat MOF shows the presence of DMF (Fig. 2 and Fig. 3)

while that of the complex with the second guest molecule NB shows no trace of DMF. A new 

peak for NB was observed at 5.2 min, as shown in Fig. 9. The mass spectrum for this peak along 

with the library match for NB is shown in Fig. 10. Based on this finding, we can conclude that in 

the complex, the DMF molecule inside the MOF cage has been displaced by the guest molecule 

NB. 

Fig. 9 Total ion chromatogram for guest-host complex
9

Fig. 10 Mass spectrum for 5.2 min peak in Fig. 1 (top) with library search match (bottom)

PXRD of both the original host MOF and the complex with the guest molecule NB is shown in 

Fig. 11. As expected, the overall intensity of the complex has decreased and relative intensity of 

the two low angle peaks at 2= 7.9° and 8.8° has changed proving the inclusion of the guest 

molecule. PXRD of the complex displays some additional reflections, which originate from the 

ordering of the guest molecules in the MOF with new reflections observed at 2= 9–20°, with 

the most prominent new reflection at 2 = 17.9°.

Fig. 11 PXRD pattern of the host MOF and the MOF + NB
10

TGA was performed to find the temperature-dependent guest release and thermo-stability of the 

host-guest complex. The TG curve of neat MOF and the MOF complex with the NB guest is 

shown in Fig. 12. The complex shows approximately 15 wt-% loss between 25–125 °C at one 

rate and another 15 wt-% loss between 125–400 °C. The MOF structure is observed to 

decompose starting at about 400 °C. We can infer that the first weight loss is from the outside 

guest molecule, and that the next weight loss is due the guest molecule inside the MOF cage, 

which took a longer time and a higher temperature to be released.

Fig. 12 TGA curve of neat MOF and the MOF + NB complex

The host MOF and the host-guest complex were also characterized by FTIR spectroscopy 

(Fig. 13). The complete removal of DMF molecules is indicated by the disappearance of the 

characteristic C=O band of DMF at 1660 cm−1. The strong band at 1523 cm−1 is characteristic 

for the asymmetric (NO2) stretching of NB as well as two new aromatic NB bands at 1478 and 

1021 cm−1 were also clearly visible that were not present in the MOF. The band at 851 cm−1 is 

also coming from the C-N stretch of the guest molecule NB.
11

Fig. 13 FTIR spectrum of the host MOF and the MOF + NB (spectra normalized to largest peak)

4. Summary and Conclusions

We have successfully trapped two small molecules, viz. 3MPN and NB, inside the cage of a 

mixed ligand MOF, Zn2NDC2DPNI .DMF. The complete exchange of the reagent solvent for the 

new guests has been verified by GC-MS, PXRD, TGA, and FTIR. 
12

5. References

1. Collins DJ, Hong CZ. Hydrogen storage in metal–organic frameworks. J Materials Chem.

2007;17:3154–3160.

2. Férey G. Hybrid Porous solids: past, present, future. Chem Soc Rev. 2008;37(1):191–214.

3. Rowsell JLC, Yaghi OM. Strategies for hydrogen storage in metal-organic frameworks.

Angewandte Chemie Intl Ed. 2005;44:4670–4679.

4. Bae YS, Farha OK, Spokoyny AM, Mirkin CA, Hupp JT, Snurr RQ. Carborane-based metalorganic frameworks as highly selective sorbents for CO2 over methane. Chem Comm.

2008:4135–4137.

5. Cho SH, Ma BQ, Nguyen ST, Hupp JT, Albrecht-Schmitt TE. A metal-organic framework 

material that functions as an enantioselective catalyst for olefin epoxidation. Chem Comm.

2006:2563–2565.

6. Evans OR, Lin W. Crystal engineering of nonlinear optical materials based on 

interpenetrated diamondoid coordination networks. Chem Mat. 2001;13:2705–2712.

7. Horcajada P, Serre C, Vallet-Regı ́ M, Sebban M, Taulelle F, Fe ́rey G. Metal-organic 

frameworks as efficient materials for drug delivery. Angewandte Chemie Intl Ed.

2006;45:5974–5978.

8. Sauer J, Marlow F, Spliethoff B, Schüth F. Rare earth oxide coating of the walls of SBA-15. 

Chem Mat. 2002;14:217–224.

9. Esken D, Zhang X, Lebedev OI, Schröder F, Fischera RA. Pd@MOF-5:Limitations of gasphase infiltration and solution impregnation of [Zn4O(bdc)3] (MOF-5) with metal–organic 

palladium precursors for loading with Pd nanoparticles. J Mat Chem. 2009;19:1314–1319.
13

List of Symbols, Abbreviations, and Acronyms

3MPN 3-methoxypropionitrile

ARL US Army Research Laboratory 

ATR attenuated total reflectance 

DMF dimethylformamide

DTGS deuterated triglycine sulfate 

FTIR Fourier transform infrared

GC-MS gas chromatography-mass spectrometry

IR infrared

KBr potassium bromide 

MOF metal–organic framework

NB nitrobenzene

PXRD powder x-ray diffraction

TGA thermo-gravimetric analysis

Zn zinc
14

1 DEFENSE TECH INFO CTR

(PDF) ATTN DTIC OCA

2 US ARMY RSRCH LABORATORY

(PDF) ATTN IMAL HRA MAIL & RECORDS MGMT

ATTN RDRL CIO LL TECHL LIB

1 GOVT PRNTG OFC

(PDF) ATTN A MALHOTRA

4 US ARMY RSRCH LAB

(PDF) ATTN RDRL WM S KARNA

ATTN RDRL WML B R PESCE-RODRIGUEZ

ATTN RDRL WMM A L GIRI

ATTN RDRL WML B N TRIVEDI

1 US ARMY RSRCH LAB

(PDF) ATTN RDRL D T RUSSELL