Among various applications that are proposed for {3d–4f} clusters, magnetic refrigeration based on the principle of the magnetocaloric effect (MCE) is gaining attention in recent years due to the substantially large MCE values reported for these types of molecules. While various factors play a role in controlling the MCE values, understanding the structural parameters that control the magnetic exchange play a vital role in the development of novel molecules possessing attractive MCE characteristics. In this regard, theoretical tools based on density functional methods are indispensable. In this work, we have employed density functional methods to study the magnetic properties of six {MnIIIGdIII} clusters. This comprises a trinuclear complex {MnIII 2GdIII}, [Mn2GdO­(Piv)2­(dmem)2(NO3)3] (dmem = 2-{[2-(dimethylamino)­ethyl]­methylamino}­ethanol) (1), along with four tetranuclear {MnIII 2GdIII 2} complexes, [Mn2Gd2O2­(Piv)8­(HO2CCMe3)2­(MeOH)2] (Piv = 2,2-dimethylpropanoic acid) (2), [Mn2Gd2O2(Piv)8­(HO2CCMe3)4] (3), [Mn2Gd2(OH)2­(O2CPh)4­(NO3)2­(teaH)2] (tea = triethanolamine) (4), and [Mn2Gd2­(O)­(Piv)2­(hep)4­(NO3)4] (hep = 2-(2-hydroxyethyl)­pyridine) (5), and a single-chain compound containing the {MnIII 2GdIII 2} core, [Mn2Ln2­(OH)­(OMe)­(hmp)4­(NO3)4­(O3SC6H4CH3)2] n (hmp = 2-hydroxymethylpyridine) (6). Here we have evaluated the exchange interactions between MnIII and GdIII ions and MnIII···MnIII ions in trinuclear as well as tetranuclear complexes. Our DFT-computed exchange interaction (J) values reproduce the experimental susceptibility data well, offering confidence in the estimated J values. Our calculations yield a diverse set of J values among these complexes ranging from weak ferromagnetic to moderate antiferromagnetic {MnIII···GdIII} coupling. Using orbital overlap and NBO analysis, we have explored the mechanism of magnetic coupling and deciphered the origin of diverse J values noted among these complexes. Particularly, the importance of Jahn–Teller axes of the MnIII ions and its orientation with respect to the nature of coupling is established using the qualitative mechanism derived. The {MnIII···MnIII} coupling in all complexes are estimated to be antiferromagnetic, and the consequence of this on the {MnIII···GdIII} J values and how this influences the ground-state S values are discussed in detail. Further, we have developed magneto–structural correlations to evaluate the importance of structural parameters that control the {MnIII···GdIII} coupling. Our results reveal that Mn–O–Gd bond angles and Mn–O–Gd–O dihedral angles hold the key to the sign and magnitude of the {MnIII···GdIII} J values. Further on, utilizing the computed J values, we have estimated the MCE values for these complexes and offer insight into how these two factors are correlated. To this end, our study reveals that the incorporation of anisotropic MnIII ions in the cluster aggregation could lead to respectable MCE values if a suitable ligand design that offers a way to control the direction of the Jahn–Teller axes of MnIII ions...

Ab initio calculations performed in two three-coordinate complexes [Mn{N(SiMe3)2}3] (1) and [K(18-crown-6)(Et2O)2][Cr{N(SiMe3)2}3] (2) reveal record-high magnetic anisotropy with the D values -64 cm -1 and -15 cm -1 respectively, enlisting d 4 ion back in the race for single-ion magnets. For the first time, a detailed spin-vibrational analysis was performed in 1 and 2 that suggests a dominant under barrier relaxation due to flexible coordination sphere around the metal ion offering design clues for low coordinate transition metal SIMs. ASSOCIATED CONTENTThe following files are available free of charge.

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The coordinatively unsaturated chromium(II)‐based Cr3[(Cr4Cl)3(BTT)8]2 (Cr−BTT; BTT3−=1,3,5‐benzenetristetrazolate) metal–organic framework (MOF) has been shown to exhibit exceptional selectivity towards adsorption of O2 over N2/H2. Using periodic density functional theory (DFT) calculations, we attempted to decipher the origin of this puzzling selectivity. By computing and analyzing the magnetic exchange coupling, binding energies, the partial density of states (pDOS), and adsorption isotherms for the pristine and gas‐bound MOFs [(Cr4(X)4Cl)3(BTT)8]3− (X=O2, N2, and H2), we unequivocally established the role of spin states and spin coupling in controlling the gas selectivity. The computed geometries and gas adsorption isotherms are consistent with the earlier experiments. The binding of O2 to the MOF follows an electron‐transfer mechanism resulting in a CrIII superoxo species (O2.−) with a very strong antiferromagnetic coupling between the two centers, whereas N2/H2 are found to weakly interact with the metal center and hence only slightly perturb the associated coupling constants. Although the gas‐bound and unbound MOFs have an S=0 ground state (GS), the nature of spin the configurations and the associated magnetic exchanges are dramatically different. The binding energy and the number of oxygen molecules that can favorably bind to the Cr center were found to vary with respect to the spin state, with a significant energy margin (47.6 kJ mol−1). This study offers a hitherto unknown strategy of using spin state/spin couplings to control gas adsorption selectivity in MOFs.

More than 47,000 articles have been published in the area of Metal-Organic Framework since its seminal discovery in 1995, exemplifying the intense research carried out in this short span of time. Among other applications, gas adsorption and storage are perceived as central to the MOFs research, and more than 10,000 MOFs structures are reported to date to utilize them for various gas storage/separation applications. Molecular modeling, particularly based on density functional theory, played a key role in (i) understanding the nature of interactions between the gas and the MOFs geometry (ii) establishing various binding pockets and relative binding energies, and (iii) offering design clues to improve the gas uptake capacity of existing MOF architectures. In this review, we have looked at various MOFs that are studied thoroughly using DFT/ periodic DFT (pDFT) methods for CO 2 , H 2 , O 2 , and CH 4 gases to provide a birds-eye-view on how various exchange-correlation functionals perform in estimating the binding energy for various gases and how factors such as nature of the (i) metal ion, (ii) linkers, (iii) ligand, (iv) spin state and (v) spin-couplings play a role in this process with selected examples. While there is still room for improvement, the rewards offered by the molecular modelling of MOFs were already substantial that we advocate experimental and theoretical studies to go hand-in-hand to undercut the trial-and-error approach that is often perceived in the selection of MOFs and gas partners in this area.