The
development of efficient sorbent materials for capturing harmful
gases, in particular CO2, is one of the most important
research topics due to climate change concerns. In this regard, metal–organic
frameworks (MOFs) incorporating open metal sites (OMSs) have emerged
as promising materials. In this article, we propose the spin crossover
as an experimentally feasible strategy for controllable gas molecule
capturing on the OMS-containing MOFs. To this end, we have selected
the neutral and cationic forms of M2(BTC)4 with
M = Ni and Cu as prototypes of the paddlewheel secondary building
unit of HKUST-1 and DUT-8, respectively. The interplay between magnetic
properties and the adsorption behaviors of M2(BTC)4 toward six small gas molecules (CH4, H2, N2, CO2, CO, and H2S) is investigated
using first-principle calculations. It is found that the adsorption
of gas molecules on the low spin (LS) state of copper and high spin
(HS) state of nickel is thermodynamically more favored. Whereas the
differences between the adsorption enthalpies on the LS and HS states
for cupric systems are not significant, in the case of nickel centers,
the differences can reach more than 150 kJ mol–1. Our results indicate that all gas molecules are strongly adsorbed
at the HS state of nickel centers and change from being chemisorbed
to being physisorbed at the LS state. Taking inspiration from these
results, we introduce Ni2(BTC)4 paddlewheels
as controllable CO2 capture materials via spin crossover
at the Ni center. To our knowledge, this is the first study that suggests
the spin transition as a promising approach for CO2 capture/release
technology.
Metal–organic frameworks incorporating mixed-metal sites (MM-MOFs) have emerged as promising candidates in the development of sensing platforms for the detection of paramagnetic species.
The new planar tetracoordinate carbon (ptC) compounds have received significant research attention in recent years. The present study is devoted to investigating the structural, electronic, and magnetic features of one-dimensional chains and two-dimensional sheets composed of CAl(CH) building blocks. All possible condensations were studied, and the stabilities of different ptC assemblies were compared. Several properties such as energy gap, dipole polarizability, electronic excitation energies, and nucleus chemical shift were computed for chains up to 7 and sheets up to 16 units. A systematic analysis was performed to assess the impact of condensation pattern and number of units on the calculated properties. Topological analysis of density and electron localization functions reveals that Al-C bonds in the considered ptCs have mixed covalent/ionic character with larger ionic contribution. It is found that the electronic spectra of the condensed ptCs exhibit red shift toward larger wavelengths when compared to the CAl(CH) building block. The amount of red shift enhances with increasing number of units. We show that the stability trend, predicted by electronic and magnetic descriptors, are in qualitative agreement with the thermodynamic stability obtained through Gibbs free energy change of condensation reaction.
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