Synthesis, Characterization, and the Effect of Lewis Bases on the Nuclearity of Iron Alkoxide Complexes.

Inspired by the potential of alkoxides as weak-field ligands and their ability to bridge, we report herein a series of high-spin iron complexes supported by a bis-alkoxide framework PhDbf. A diiron complex [Fe2(PhDbf)2] (1a) is obtained upon metalation of the ligand, whereas addition of substituted pyridines affords five-coordinate mononuclear iron complexes [(R-Py)2Fe(PhDbf)] (2a-4a, R = H, p-tBu, p-CF3). The potential for nuclearity control of the metal complexes via auxiliary ligands is highlighted by the formation of asymmetric diiron species [(p-CF3-Py)Fe2(PhDbf)2] (5a) and [(m-CF3-Py)Fe2(PhDbf)2] (6a) with trifluoromethyl substituted pyridines, while electron-rich pyridines only produced monomeric species. Electronic properties analysis via UV-vis, electron paramagnetic resonance, 57Fe Mössbauer spectroscopy, and time-dependent density functional theory, along with redox capabilities of these complexes are reported to illustrate the effect of nuclearity on reactivity and the potential of these complexes to access higher oxidation states relevant in oxidative chemistry. Species 1a-5a, [(THF)2Fe(PhDbf)][PF6] (7), [PyFe(PhDbf)Cl] (2b), and [Py2Fe(PhDbf)][PF6] (2c) were characterized via SCXRD. Indirect evidence for the formation of dimeric Fe(III) species (1b, 5b, and 6b) is discussed.

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+ .

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.

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.

Hard Single-Molecule Magnet Behavior by a Linear Trinuclear Lanthanide-[1]Metallocenophane Complex.

A synthetic protocol was developed that involves the transmetalation of a mono-dysprosium-[1]ferrocenophane complex with DyX3 (X = Cl- or I-) to afford [Dy3Fc6Li2(THF)2]-, featuring a rare linear arrangement of magnetically anisotropic Dy3+ ions. The close spatial inter-lanthanide proximity, in combination with µ2-bridging sp2-hybridized CCp groups, enforces significant magnetic coupling and results in hard single-molecule magnet (SMM) behavior, with an effective barrier to magnetization reversal of up to 268 cm-1. Our results highlight the versatility of lanthanide metallocenophane architectures toward the development of novel multinuclear SMM frameworks.

A comparative study of magnetization dynamics in dinuclear dysprosium complexes featuring bridging chloride or trifluoromethanesulfonate ligands.

We utilized a rigid ligand platform PyCp22- (PyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) to isolate dinuclear Dy3+ complexes [(PyCp2)Dy-(µ-O2SOCF3)]2 (1) and [(PyCp2)Dy-(µ-Cl)]2 (3) as well as the mononuclear complex (PyCp2)Dy(OSO2CF3)(thf) (2). Compounds 1 and 2 are the first examples of organometallic Dy3+ complexes featuring triflate binding. The isolation of compounds 1 and 3 allows us to comparatively evaluate the effects of the bridging anions on the magnetization dynamics of the dinuclear systems. Our investigations show that although the exchange coupling interactions differ for 1 and 3, the dynamic magnetic properties are dominated by relaxation via the first excited state Kramers doublet of the individual Dy sites. Compounds 1 and 3 exhibit barriers to magnetization reversal (Ueff = 49 cm-1) that can be favorably compared to those of the previously reported examples of [Cp2Dy(µ-Cl)]2 (Ueff = 26 cm-1) and [Cp2Dy(thf)(µ-Cl)]2 (Ueff = 34 cm-1).

Slow Magnetic Relaxation in a Lanthanide-[1]Metallocenophane Complex.

The first example of a lanthanide metallocenophane complex has been isolated as [Li(THF)4][DyFc3Li2(THF)2] (1). The molecular structure of complex 1 differs dramatically from those of main group and transition metal ferrocenophane complexes and features a distorted trigonal prismatic geometry around the Dy(III) ion and close intramolecular Dy···Fe distances. Furthermore, complex 1 exhibits all characteristics of a soft single-molecule magnet.