Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, India.
Department of Chemistry, Indian Institute of Science Education and Research, Kolkata, Mohanpur, Nadia, 741246, India.
Chemphyschem. 2023 Feb 14;24(4):e202200257. doi: 10.1002/cphc.202200257. Epub 2022 Nov 24.
Herein, we report a detailed periodic DFT investigation of Mn(II)-based [(Mn Cl) (BTT) ] (BTT =1,3,5-benzenetristetrazolate) metal-organic framework (MOF) to explore various hydrogen binding pockets, nature of MOF…H interactions, magnetic coupling and, H uptake capacity. Earlier experiments found an uptake capacity of 6.9 wt % of H with the heat of adsorption estimated to be ∼10 kJ/mol, which is one among the highest for any MOFs reported. Our calculations unveil different binding sites with computed binding energy varying from -6 to -15 kJ/mol. The binding of H at the Mn site is found to be the strongest (site I), with H found to bind Mn ion in a η fashion with a distance of 2.27 Å and binding energy of -15.4 kJ/mol. The bonding analysis performed using NBO and AIM reveal a strong donation of σ (H ) to the d orbital of the Mn ion responsible for such large binding energy. The other binding pockets, such as -Cl (site II) and BTT ligands (site III and IV) were found to be weaker, with the binding energy decreasing in the order I>II>III>IV. The average binding energy computed for these four sites put together is 9.6 kJ/mol, which is in excellent agreement with the experimental value of ∼10 kJ/mol. We have expanded our calculations to compute binding energy for multiple sites simultaneously, and in this model, the binding energy per site was found to decrease as we increased the number of H molecules suggesting electronic and steric factors controlling the overall uptake capacity. The calculated adsorption isotherm using the GCMC method reproduces the experimental observations. Further, the magnetic coupling computed for the unbound MOF reveals moderate ferromagnetic and strong antiferromagnetic coupling within the tetrameric {Mn } unit leading to a three-up-one-down spin configuration as the ground state. These were then coupled ferromagnetically to other tetrameric units in the MOF network. The magnetic coupling was found to alter only marginally upon gas binding, suggesting that both exchange interaction and the spin-states are unlikely to play a role in the H uptake. This is contrary to the O uptake studied lately, where strong dependence on exchange-coupling/spin state was witnessed, suggesting exchange-coupling/magnetic field dependent binding as a viable route for gas separation.
在此，我们报告了对基于 Mn(II)的 [(Mn Cl) (BTT) ]（BTT=1,3,5-苯三氮唑）金属有机骨架（MOF）的详细周期密度泛函理论（DFT）研究，以探索各种氢键口袋、MOF…H 相互作用的性质、磁耦合和 H 吸收能力。早期的实验发现该 MOF 的 H 吸收容量为 6.9wt%，吸附热估计约为 10kJ/mol，这是报道的任何 MOF 中最高的之一。我们的计算揭示了不同的结合位点，计算得到的结合能从-6 到-15kJ/mol 不等。发现 H 在 Mn 位点的结合最强（位点 I），H 以 η 方式与 Mn 离子结合，距离为 2.27Å，结合能为-15.4kJ/mol。使用 NBO 和 AIM 进行的键合分析表明，H 向 Mn 离子的 d 轨道强烈捐赠 σ（H），这是导致如此大的结合能的原因。其他结合口袋，如-Cl（位点 II）和 BTT 配体（位点 III 和 IV）较弱，结合能依次降低 I>II>III>IV。这四个位点的平均结合能计算为 9.6kJ/mol，与实验值约 10kJ/mol 非常吻合。我们已经扩展了我们的计算，以同时计算多个位点的结合能，在该模型中，随着 H 分子数量的增加，每个位点的结合能降低，表明电子和空间因素控制着整体吸收能力。使用 GCMC 方法计算的吸附等温线再现了实验观察结果。此外，对未结合 MOF 计算的磁耦合显示出四聚体{Mn}单元内的中等铁磁性和强反铁磁性耦合，导致作为基态的三上一下自旋构型。然后，这些自旋构型在 MOF 网络中与其他四聚体单元铁磁耦合。气体结合后，磁耦合仅略有变化，这表明交换相互作用和自旋态不太可能在 H 吸收中起作用。这与最近研究的 O 吸收相反，在 O 吸收中观察到强烈依赖于交换耦合/自旋态，这表明交换耦合/磁场依赖的结合是气体分离的一种可行途径。
