Square‐Planar Ruthenium(II) Complexes: Control of Spin State by Pincer Ligand Functionalization

Functionalization of the PNP pincer ligand backbone allows for a comparison of the dialkyl amido, vinyl alkyl amido, and divinyl amido ruthenium(II) pincer complex series [RuCl{N(CH₂CH₂PtBu₂)₂}], [RuCl{N(CHCHPtBu₂)(CH₂CH₂PtBu₂)}], and [RuCl{N(CHCHPtBu₂)₂}], in which the ruthenium(II) ions are in the extremely rare square‐planar coordination geometry. Whereas the dialkylamido complex adopts an electronic singlet (S=0) ground state and energetically low‐lying triplet (S=1) state, the vinyl alkyl amido and the divinyl amido complexes exhibit unusual triplet (S=1) ground states as confirmed by experimental and computational examination. However, essentially non‐magnetic ground states arise for the two intermediate‐spin complexes owing to unusually large zero‐field splitting (D>+200 cm^?1). The change in ground state electronic configuration is attributed to tailored pincer ligand‐to‐metal π‐donation within the PNP ligand series.     

Redox Switches for Single‐Molecule Magnet Activity: An Ab Initio Insight

A dinuclear Co^II complex ( 1 ) featuring unprecedented anodic and cathodic switches for single‐molecule magnet (SMM) activity has been recently investigated (J. Am. Chem. Soc. 2013 , 135, 14670). The presence of sandwiched radicals in different oxidation states of this compound mediates magnetic coupling between the high‐spin (S=3/2) cobalt ions, which gives rise to SMM activity in both the oxidized ([ 1 (OEt₂)]⁺) and reduced ([ 1 ]^?) states. This feature represents the first example of a SMM exhibiting fully reversible, dual ON/OFF switchability. Here we apply ab initio and broken‐symmetry DFT calculations to elucidate the mechanisms responsible for magnetic properties and magnetization blocking in these compounds. It is found that due to the strong delocalization of the magnetic molecular orbital, there is a strong antiferromagnetic interaction between the radical and cobalt ions. The lack of high axiality of the cobalt centres explains why these compounds possess slow relaxation of magnetization only in an applied dc magnetic field.     

Trends in trigonal prismatic Ln-[1]ferrocenophane complexes and discovery of a Ho3+ single-molecule magnet

Lanthanide metallocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to transition metal ions. Herein, we present a systematic study of the structural and magnetic properties of the ferrocenophanes, [LnFc₃(THF)₂Li₂]⁻, of the late trivalent lanthanide ions (Ln = Gd (1), Ho (2), Er (3), Tm (4), Yb (5), Lu (6)). One major structural trend within this class of complexes is the increasing diferrocenyl (Fc²⁻) average twist angle with decreasing ionic radius (r_ion) of the central Ln ion, resulting in the largest average Fc²⁻ twist angles for the Lu³⁺ compound 6. Such high sensitivity of the twist angle to changes in r_ion is unique to the here presented ferrocenophane complexes and likely due to the large trigonal plane separation enforced by the ligand (>3.2 Å). This geometry also allows the non-Kramers ion Ho³⁺ to exhibit slow magnetic relaxation in the absence of applied dc fields, rendering compound 2 a rare example of a Ho-based single-molecule magnet (SMM) with barriers to magnetization reversal (U) of 110–131 cm⁻¹. In contrast, compounds featuring Ln ions with prolate electron density (3–5) don''t show slow magnetization dynamics under the same conditions. The observed trends in magnetic properties of 2–5 are supported by state-of-the-art ab initio calculations. Finally, the magneto-structural relationship of the trigonal prismatic Ho-[1]ferrocenophane motif was further investigated by axial ligand (THF in 2) exchange to yield [HoFc₃(THF*)₂Li₂]⁻ (2-THF*) and [HoFc₃(py)₂Li₂]⁻ (2-py) motifs. We find that larger average Fc²⁻ twist angles (in 2-THF* and 2-py as compared to in 2) result in faster magnetic relaxation times at a given temperature.

Lanthanide ferrocenophanes are an intriguing class of organometallic complexes that feature rare six-coordinate trigonal prismatic coordination environments of 4f elements with close intramolecular proximity to iron ions.     

Magnetic Properties of a Terbium–[1]Ferrocenophane Complex: Analogies between Lanthanide–Ferrocenophane and Lanthanide–Bis‐phthalocyanine Complexes

A rare example of an organometallic terbium single‐ion magnet is reported. A Tb³⁺–[1]ferrocenophane complex displays a larger barrier to magnetization reversal than its isostructural Dy³⁺ analogue, which is reminiscent of trends observed for lanthanide–bis‐phthalocyanine complexes. Detailed ab initio calculations support the experimental observations and suggest a significantly larger ground‐state stabilization for the non‐Kramers ion Tb³⁺ in the Tb complex than for the Kramers‐ion Dy³⁺ in the Dy complex.     

Axial Elongation of Mononuclear Lanthanide Metallocenophanes: Magnetic Properties of Dysprosium‐ and Terbium‐[1]Ruthenocenophane Complexes

We report the first f‐block‐ruthenocenophane complexes 1 (Dy) and 2 (Tb) and provide a comparative discussion of their magnetic structure with respect to earlier reported ferrocenophane analogues. While axial elongation of the rare trigonal‐prismatic geometry stabilizes the magnetic ground state in the case of Dy³⁺ and results in a larger barrier to magnetization reversal (U), a decrease in U is observed for the case of Tb³⁺.     

Axial Elongation of Mononuclear Lanthanide Metallocenophanes: Magnetic Properties of Dysprosium- and Terbium-[1]Ruthenocenophane Complexes

We report the first f-block-ruthenocenophane complexes 1 (Dy) and 2 (Tb) and provide a comparative discussion of their magnetic structure with respect to earlier reported ferrocenophane analogues. While axial elongation of the rare trigonal-prismatic geometry stabilizes the magnetic ground state in the case of Dy³⁺ and results in a larger barrier to magnetization reversal (U), a decrease in U is observed for the case of Tb³⁺.     

Electrostatics by Brownian dynamics: solving the Poisson equation near dielectric interfaces

The isomorphism between electrostatics and diffusion is discussed and utilized to develop a Brownian dynamics algorithm for solving the Poisson equation near dielectric interfaces. The electrostatic potential behaves as if carried by noninteracting, randomly moving pseudo-particles whose residence time in a given region of space is proportional to the electrostatic potential there. By applying random numbers from the exact solution for diffusion near a planar discontinuity, the Brownian motion of these particles can be propagated for large time steps, independent of spatial grids or artificial boundary conditions. The applicability of the Brownian algorithm is demonstrated in simple illustrative calculations.     

Holmium(iii) molecular nanomagnets for optical thermometry exploring the luminescence re-absorption effect

Coordination complexes of lanthanide(3+) ions can combine Single-Molecule Magnetism (SMM) with thermally modulated luminescence applicable in optical thermometry. We report an innovative approach towards high performance SMM-based optical thermometers which explores tunable anisotropy and the luminescence re-absorption effect of Ho^III complexes. Our concept is shown in dinuclear cyanido-bridged molecules, {[Ho^III(4-pyridone)₄(H₂O)₂][M^III(CN)₆]}·nH₂O (M = Co, 1; Rh, 2; Ir, 3) and their magnetically diluted analogues, {[Ho^III_xY^III_1–x(4-pyridone)₄(H₂O)₂][M^III(CN)₆]}·nH₂O (M = Co, x = 0.11, 1@Y; Rh, x = 0.12, 2@Y; Ir, x = 0.10, 3@Y). They are built of pentagonal bipyramidal Ho^III complexes revealing the zero-dc-field SMM effect. Experimental studies and the ab initio calculations indicate an Orbach magnetic relaxation with energy barriers varying from 89.8 to 86.7 and 78.7 cm⁻¹ K for 1, 2, and 3, respectively. 1–3 also differ in the strength of quantum tunnelling of magnetization which is suppressed by hyperfine interactions, and, further, by the magnetic dilution. The Y^III-based dilution governs the optical properties as 1–3 exhibit poor emission due to the dominant re-absorption from Ho^III while 1@Y–3@Y show room-temperature blue emission of 4-pyridone. Within ligand emission bands, the sharp re-absorption lines of the Ho^III electronic transitions were observed. Their strong thermal variation was used in achieving highly sensitive ratiometric optical thermometers whose good performance ranges, lying between 25 and 205 K, are adjustable by using hexacyanidometallates. This work shows that Ho^III complexes are great prerequisites for advanced opto-magnetic systems linking slow magnetic relaxation with unique optical thermometry exploiting a luminescence re-absorption phenomenon.

Ho^III complexes bearing organic luminophores and inorganic metalloligands are an effective tool for achieving the unique conjunction of single-molecule magnetism and thermometric luminescence re-absorption phenomenon.