Using Variable Temperature and Low Pressure Diffuse Reflectance UV-Vis-NIR Spectroscopy to Study the Reactivity of Ni2+ in an Air Sensitive Metal Organic Framework (MOFs)

Metal organic frameworks (MOFs) are porous coordination polymers that contain inorganic nodes and organic linkers in a crystalline arrangement. These materials usually include transition-metal ions that are capable of taking in UV-Vis- NIR radiation.

Spectra of such radiations provide a better understanding of the geometry surrounding the absorbing ions and also provide insight into the physical and chemical properties of the atoms enclosing them.

Given the fact that these two features control the ions’ inherent reactivity and also these properties change during the course of a reaction, electronic spectroscopy proved to be a suitable tool for differentiating MOFs with reactive metal nodes.

Electronic Spectroscopy

The electronic spectroscopy of MOFs relies on studying the diffuse reflectance of incident light dispersed inside their microporous crystalline lattices. While the Harrick Praying Mantis^TM accessory is suitable for standard measurements of air-stable MOF powders, it is not appropriate for tracking the behavior of reactive metal sites in MOFs.

Owing to the unstable nature of the metal ion, samples that are considered as good candidates for reactivity analyses are often sensitive to air. Moreover, to analyze the samples’ behavior it is important to control the pressure and temperature around the sample in situ and the sample also has to be dosed with substrates for reactivity studies.

This article demonstrates the application of the Harrick Low Temperature Reaction Chamber accessory for the Praying Mantis^TM enabled spectroscopic characterization of an extraordinary Ni²⁺ species within a MOF, as well as the in situ monitoring of its reactivity with molecules in different environmental settings.

For the analysis, a new MOF was prepared as NiZn₃O(O₂C-C₆H₄-CO₂) ₃ and known as Ni-MOF-5. This can be observed as a Ni²⁺-doped analog of the MOF-5 material, where NiZn₃O replaces each octahedral Zn₄O cluster.

Using diffuse reflectance UV-Vis spectroscopy, the tetrahedral geometry and all-oxygen environment around Ni²⁺ was differentiated. Here, the material had to be maintained in an inert atmosphere.

Additionally, the reactivity of this material with tiny molecules was also examined because Ni²⁺ in this geometry can possibly house two extra molecules in its coordination sphere.

Diffuse reflectance of the material as and when it reacts with the tiny molecules may provide a better understanding of the electronic properties and geometrical distortions of Ni²⁺ in this extraordinary atomic environment.

The Harrick Low Temperature Reaction Chamber accessory rendered an inert atmosphere for determining the diffuse reflectance of Ni-MOF-5. It also supported measurements under increased temperature and reduced pressure for tracking the sequential loss of N,N-dimethylformamide or DMF molecules from the Ni²⁺ site.

Experimental Framework

A UV-Vis-NIR spectrometer that is commercially available in the market was utilized to perform all the measurements in its double-beam mode along with the Harrick Praying Mantis™ (Figure 1) and Low Temperature Reaction Chamber fitted with SiO₂ windows (Figure 2).

Figure 1. The Praying Mantis Diffuse Reflection Accessory.

Figure 2. The Praying Mantis Low Temperature Chamber.

All data were obtained in % reflectance and then changed to the Kubelka-Munk function. Next, BaSO₄ was loaded into the reaction chamber and positioned into the Praying Mantis^TM so as to ascertain a baseline spectrum ranging between 2000 and 200nm. The information collected at ambient pressure and temperature was later subtracted from all following experimental traces.

Under an inert atmosphere of a N₂-filled glovebox, Ni-MOF-5 was loaded into the reaction chamber. This sample was earlier immersed in a DMF bath to obtain a DMF-adduct of Ni-MOF-5.

Then, the reaction chamber was connected to the Praying Mantis^TM and linked to circulating water via the ports. In order to control the sample’s temperature, the reaction chamber was linked to the Harrick Automatic Temperature Controller™ via a K-type thermocouple.

Under ambient conditions, the spectrum of this sample was obtained followed by another collection subsequent to introducing dynamic vacuum via the valve. This valve in turn was joined to the sample holder.

The temperature was increased in 10°C increments to 200°C and the spectra was collected after the temperature stabilized following each step. Once the sample enclosure reaches 200°C temperature, spectra was collected every 3 minutes until all the data stopped evolving.

Results and Discussion

Figure 3 shows the ensuing set of spectra in KubelkaMunk units sans any normalization. The original trace obtained under dynamic vacuum at room temperature is illustrated in yellow. The ensuing data progresses from yellow to orange to red and finally ends in blue.

Figure 3. Diffuse reflectance spectra of a DMF-adduct of Ni-MOF-5 collected in situ while heating the sample under reduced pressure. The initial trace is shown in yellow and progresses through orange, red, and terminates in blue.

The data evolution indicated that the Ni²⁺ site in the DMF-adduct changed under increased temperature and vacuum. The previous spectra matched six and five-coordinate Ni²⁺, whilst the final blue trace corresponded with the predicted absorption profile of tetrahedral Ni¹⁺.

Given that DMF can possibly adhere to the vacant coordination sites of Ni¹⁺ in Ni-MOF-5, the supplementary atoms around Ni²⁺ probably arise from the pendant DMF molecules. The tight isosbestic point close to 700nm indicated that the Ni²⁺experienced a neat conversion from six to five to four coordinate geometry owing to the following loss of two DMF molecules at each metal site.

Conclusion

The above date provides a better understanding about the conversion of a reactive metal site in a MOF. The Harrick Low Temperature Reaction Chamber accessory helped in obtaining diffuse reflectance spectra of the DMF adduct of Ni-MOF-5 in situ as the Ni²⁺ subsequently lost DMF molecules from its coordination sphere.

The final trace captures Ni²⁺ in an extraordinary false-tetrahedral environmental condition that would be not be possible to study sans an inert atmosphere. Thus, the Harrick accessory helped in performing a study, which showed that MOFs is suitable for reactivity studies as well as coordination chemistry of inorganic samples that are too complex to obtain as molecules.

About Harrick Scientific Products, Inc.

Since its beginnings in 1969, Harrick Scientific has advanced the frontiers of optical spectroscopy through its innovations to transmission, internal reflection, external reflection, diffuse reflection, and emission spectroscopy. The president and founder of the corporation, Dr. N. J. Harrick, pioneered internal reflection spectroscopy and became the principal developer of this technique.

Harrick Scientific offers a large selection of standard and custom-built accessories for IR and UV-VIS spectrometers. Many of these attachments were originally forerunners in their field and their contemporary versions are considered industry standards. Harrick Scientific continues to introduce innovative new products. In addition to these state-of-the-art accessories, Harrick Scientific supplies a complete line of optical elements, including windows, ATR plates, prisms, and hemispheres.

This information has been sourced, reviewed and adapted from materials provided by Harrick Scientific Products, Inc.

For more information on this source, please visit Harrick Scientific Products, Inc.