Single-Molecule Magnets (SMMs) are coordination compounds that exhibit slow relaxation of magnetization of molecular origin. In the case when SMMs contain only one paramagnetic metal center we distinguish the group of so-called Single-Ion Magnets (SMMs) [1]. In SIMs, the occurrence of slow relaxation of magnetization is closely related to the existence of non-negligible magnetic anisotropy on the magnetic center of the molecule. Since the magnetic anisotropy is strongly influenced by the topology and strength of the applied ligand field it could be expected that significant prolongation of coordination bonds or occurrence of non-covalent interactions involving the metal center may have a fundamental impact on resulting magnetic properties.

In 2016, we reported on static and dynamic magnetic properties of compound [Co(dpt)(NCS)₂], (dpt = 1,7-diamino-4-azaheptane). Two non-covalent interactions between the Co(II) atoms and π electrons of NCS^- ligands from the neighboring complex molecule (d(C···NC_centroid) = 3.55 Å) caused a mediation of ferromagnetic exchange interaction within the centrosymmetric dimer and also the dynamic magnetic properties were affected markedly [2]. This inspired us to investigate in greater detail the magnetic properties of Co(II) compounds having some of their metal-ligand bonds at distances longer than typical coordination bonds.

Semicordination bond can be considered as a non-covalent analogue of the coordination bond, which occurs when a weak attractive non-covalent interaction between an electrophilic region (associated with a metal center) and a nucleophilic region (associated with a nonmetal atom in another or in the same molecular entity) is formed [3,4]. In typical semicoordination bonds, the distances between the metal atoms and electron-donating groups are significantly longer than the sum of their covalent radii but shorter than the sum of van der Waals radii, the interactions are dominantly of electrostatic character and topology of electron density between the particular atoms exhibits bond path and critical point [4].

In line with the above-mentioned considerations, we chose to investigate three different series of mononuclear Co(II) compounds: (a) [Co(2NH₂-R^₁-py)₂(R^₂COO)₂], where R^₁ = H, 3/4/5-CH₃, R^₂ = CH₃, C₆H₅, t-Bu, the carboxylate ligand form Co-O bonds with lengths of 2.0 – 3.1 Å, (b) [Co(bq)(NO₃)₂(ROH)], where bq is 2,2'-biquinoline and ROH are various alcohol ligands, one of the nitrate ligands forms the Co-O bond with lengths of 2.5 – 3.3 Å, (c) [Co(R-pymep)₂], where H-R-pymep are various derivatives of 2-{(E)-[(pyridin-2-yl)imino]methyl}phenol, two Co-N bonds with lengths between 2.5 and 2.7 Å. We studied these compounds by a combination of experimental (X-ray diffraction, magnetometry, HF-EPR) and theoretical (DFT, CASSCF, Electronic localization function, non-covalent interaction index, and QTAIM) methods. In this talk, we report on the character of semicoordination in these compounds and the relationship between the structure and observed magnetic properties.

[1] Craig, G.A., Murrie, M. (2015). Chem. Soc. Rev. 44, 2135-2147.

[2] Nemec, I. et al. (2016). Dalton Trans. 31, 12479–12482.

[3] Ananyev, I.V. et al. (2020). Acta Cryst. B. 76, 436-449

[4] Efimenko, Z. M. et al. (2020). Inorg. Chem. 59, 2316–2327.

C-H and Si-H bond activation by metal-hydrogen bonding (agostic interactions) plays a central role in catalytic processes [1]. These processes are directly dependent on metal-hydrogen bond energies. The versatility of the coordination modes of the heavy metals allows wide structure and topology variations of the complexes. Therefore, it is of major importance to accurately describe these chemical bonds.

One important drawback is the difficulty of deriving accurate and precise hydrogen atom positions by any kind of experiment. Neutron-diffraction experiments would be the only reliable source of such information, but there is a lack of available accurate X-H bond distances with X being a transition metal from neutron diffraction. Therefore, it would be desirable to determine both the elongation of the C-H and Si-H bonds in agostic interactions and the metal-hydrogen bonding parameters from standard X-ray diffraction experiments. In this context, the capabilities of Hirshfeld Atom Refinement [2] to obtain precise and accurate C-H/Si-H…X bond parameters (with X=transition metal) are tested.

Experimental and theoretical charge densities of agostic interactions involving transition metal compounds have been determined and analyzed in the past [3]. Here, we use a combination of HAR with subsequent X-ray constrained wavefunction fitting [4] and purely theoretical calculations on the accurate HAR and neutron geometries to analyze the related chemical bonding beyond a charge-density analysis. We use three different test systems: Figure 1a shows Si-H…Cu/Ag interactions enforced through the ligands used by proximity constraints. We discuss whether there are signatures of agostic interactions in these systems with closed-shell (d¹⁰) coinage metal atoms. [5] Figure 1b shows a system where the proximity enforcing ligands have caused an oxidative addition reaction so that the hydrogen atom is now more closely bonded to the transition metal in a Rh-H…Si interaction. We analyze again to which extent (inverse) agostic interactions are present in this system. [6] Our findings will be referenced against classical C-H…Ti agostic interactions found in titanium amides (Figure 1c).[7]

Figure 1. Compounds (a) 1·MCl (M = Cu, Ag). Metal hydrides (b) RhH, (c) Titanium amide compounds.

[1] Bäckvall, J. E. (2002). J. Organomet. Chem. 652(1-2), 105-111.

[2] Jayatilaka, D., Dittrich, B. (2008). Acta Cryst. A, 64, 383-393.

[3] (a) Scherer, W., Wolstenholme, D. J., Herz, V., Eickerling, G., Bruck, A., Benndorf, P., Roesky, P. W. (2010). Angew. Chem., Int. Ed., 49, 2242-2246. (b) Hauf, C.; Barquera-Lozada, J. E.; Meixner, P.; Eickerling, G.; Altmannshofer, S.; Stalke, D.; Zell, T.; Schmidt, D.; Radius, U.; Scherer, W. (2013). Z. Anorg. Allg. Chem., 639, 1996-2004.

[4] (a) Jayatilaka, D. (1998). Phys. Rev. Lett. 80, 798-801. (b) Jayatilaka, D., Grimwood, D. J. (2001). Acta Cryst. A, 57, 76-86.

[5] Hupf, E., Malaspina, L. A., Holsten, S., Kleemiss, F., Edwards, A. J., Price, J. R., Kozich, V., Heyne, K., Mebs, S., Grabowsky, S., Beckmann, J. (2019). Inorg. Chem., 58 (24), 16372-16378.

[6] Holsten, S., Malaspina, L.A., Mebs, S., Hupf, E., Grabowsky, S., Beckmann, J. (2021) Organometalics. Under revision.

[7] Adler, C., Bekurdts, A., Haase, D., Saak, W., Schmidtmann, M., & Beckhaus, R. (2014). Eur. J. Inorg. Chem., 8, 1289-1302.

Single-ion magnets (SIMs) are a class of metal-organic coordination complexes with the intriguing ability to sustain a magnetic moment after the removal of a magnetizing field [1]. This ability originates in orbital angular momentum of unpaired electrons, introducing magnetic anisotropy that increases the magnetic relaxation time of the SIM magnetic moment. Magnetic anisotropy is therefore a key property in the search for new and improved SIMs, and it is imperative to be able to measure magnetic anisotropy experimentally.

In 2002, it was shown that polarized neutron diffraction from single crystals (PND) could be used to obtain information on so called ionic site-susceptibilities [2], which are tensor quantities that show the response of the magnetic moment of the ion to an external magnetic field. Site susceptibilities give direct access to the magnetic anisotropy of a compound, and we have earlier used this technique to measure the magnetic anisotropy of both lanthanide and transition metal SIMs in single crystals [3, 4]. Importantly, the technique was recently extended for application to powder samples [5].

Utilizing this exciting development, we have performed polarized neutron powder diffraction (pPND) on the SIM CoCl₂(tmtu)₂, tmtu=tetramethylthiourea (1). The compound shows zero-field splitting and slow relaxation of its magnetic moment [6], both requirements for a SIM. With pPND we obtain the orientation of the magnetic anisotropy with respect to the molecular structure (Fig. 1), and follow its dependence on magnetic field strength, directly from a powder sample. Comparison with PND measured on a single crystal of the Br-analogue CoBr₂(tmtu)₂ (2) shows that the powder and single crystal techniques give comparable results.

In this contribution, I will discuss the site susceptibility model, the application of the model to both single crystal and powder data and the magneto-structural correlations that we obtain from these measurements. The extension of the technique to powders, and the dramatic reduction in data acquisition times that it entails, means that compounds can be studied, for which the growth of suitably sized crystals for single crystal neutron diffraction is unattainable. This opens the possibility for magnetic anisotropy studies on a much wider range of molecular magnetic compounds under a larger range of experimental conditions.

Metal-Organic Frameworks (MOFs) are a class of crystalline organic-inorganic hybrid materials derived from metal nodes and organic linkers, that exhibit features like high surface area, well-defined pore, tunable structures and their properties [1]. Use of redox-active metal nodes or organic linkers, stable radical based ligands can introduce a special feature like conductivity, electrocatalyst, electrochromic behavior in MOFs apart from their conventional uses such as gas storage, gas separation, etc. This idea is impeded mainly due to the insulating nature of organic linkers and the instability of the framework to the redox process. This hindered the study of electroactive MOFs until the last decade. Recent advancement in this field has directed a surge of interest in understanding their mechanism of charge transfer. MOFs are a unique platform to investigate the charge transfer mechanism where the corresponding metal ions or organic linkers are well defined in a highly crystalline rigid system. Charge transfer is directed by either the through-space or through-bond approach [2]. The through-bond mixed-valance charge transfer has been well explored whereas, through-space intervalency in MOF is rare [3].

We have synthesized a new Cobalt (II) based metal-organic framework using redox-active organic linker, N,N′-di(4-pyridyl)thiazolo-[5,4-d]thiazole (DPTTZ). The framework exhibits through-space intervalence charge transfer (IVCT) arise from cofacially arranged DPTTZ linkers (Figure 1). The IVCT is elucidated computationally using time-dependent density functional theory (TD-DFT) methods. The computational study also exploits the distance-dependent through-space intervalence charge transfer (IVCT) in this system.

Here, I will present experimental observation of through-space intervalence charge transfer (IVCT) using redox-active organic linkers in the metal-organic framework and its computational understanding using TD-DFT. This interrogation of charge transfer mechanism and electrical conductivity in MOF provides a better understanding of conducting materials.

[1] Furukawa, H., Cordova, K. E., O'Keeffe, M., Yaghi, O. M. Science 2013, 341, 1230444.

[2] Sun, L.; Campbell, M. G.; Dincă, M. Angew. Chem. Int. Ed. 2016, 55, 3566.

[3] Hua, C. et al. J. Am. Chem. Soc. 2018, 140, 6622.

[4] Nath, A. et al. (Manuscript under preparation)

Molecular framework materials can combine the functional properties typical of the traditional inorganic solid state, such as magnetism, with the remarkable tunability and flexibility that arises from the incorporation of molecular components. They therefore offer the opportunity to discover unusual behaviour that arises from the coupling of these properties.

We have recently shown that thiocyanate (NCS–) based frameworks are a fruitful ground for the study of these novel properties, as thiocyanate can both facilitate strong magnetic coupling (TCW>100K, [1]) and create intense optically absorption in the visible region [2]. Despite this, the rich chemistry of metal thiocyanates remains unexplored compared to the equivalent metal formates, azides or hypophosphites.

Our investigations of the functional properties of metal thiocyanate frameworks began with the simplest examples: the layered binary thiocyanates, M(NCS)2. We demonstrated, through powder neutron diffraction studies, that this M(NCS)2 family possesses a wide variety of interesting magnetic phases. As part of this investigation we synthesised three new binary materials, M = Cu, Mn, Fe; and demonstrated that their magnetic interactions are significantly stronger than the previously studied exemplars (M=Co,Ni) increasing |TCW| by a factor of four.[1] Our results also uncovered that Cu(NCS)2 is a good example of a quasi-1D quantum Heisenberg antiferromagnet which a significantly reduced ordered moment in its ordered state.[3]

We have also investigated the family of CsM(NCS)3 materials which adopt the 'post-perovskite' structure.[4] The post-perovskite structure type is so-called as it occurs at pressures beyond the stability field of conventional perovskites (e.g. MgSiO3, CaIrO3, NaMnF3), but these molecular post-perovskites readily form in standard solution chemistry. We find that this family of materials shows significantly reduced ordering temperatures and adopt non-collinear magnetic structures that give rise to considerable magnetic hysteresis.[5]

[1] E. Bassey et al., Inorg. Chem. (2020).
[2] M. Cliffe et al., Chem. Sci., 10, 793 (2019).
[3] M. Cliffe et al., Phys. Rev. B, 97, 144421 (2018).
[4] M. Fleck, Acta Crystallogr. C60, i63 (2004).
[5] M. Geers et al., in prep.

Single-molecule magnets (SMMs) are molecular compounds possessing a magnetic bistability of their ground state, allowing them to maintain the direction of induced magnetization for a significant amount of time, after having first applied an external magnetic field [1]. Understanding the driving force behind good single-molecule magnet properties and developing improved rational synthesis design of them go hand in hand. This has been demonstrated in recent years, with record-breaking magnetic properties found in SMMs that utilizes a single Dy(III) centre in a highly axial ligand field [2-3]. A compound designed with this in mind is the pentacoordinate [Dy(Mes*O)₂(THF)₂Br]3THF (Mes*: 2,4,6-tri-tert-buylphenyl, THF: Tetrahydrofuran, DyBrTHF), Figure 1 (left). In a recent study on this compound, the molecular environment was found to be critical for the magnetic properties [4].

One way of systematically changing the molecular environment is through induced hydrostatic pressure. The resulting structural changes can then be probed using X-ray diffraction (XRD), by utilizing a diamond-anvil cell (DAC). We have performed high-pressure single-crystal XRD at several pressure points up until 2.9(2) GPa, and analysed the ensuing structures. Looking at the first coordination sphere, we can investigate how the applied pressure alters the molecular environment of Dy, Figure 1 (middle). At the last two pressure points, a slight drop is noted for some of the Dy-O bonds.

The magnetic properties of SMMs are closely tied to their electronic structure, which can change when undergoing external pressure, as investigated earlier in our group [5]. This information can be accessed through theoretical ab initio calculations, done here using CASSCF+NEVPT2 in ORCA. The found NEVPT2 energies of the Kramers doublets at varying pressure reveal a significant change in the energy levels, Figure 1 (right), perhaps due to the pressure-induced alteration of the ligand field.