Unsupported Lanthanide-Transition Metal Bonds: Ionic vs Polar Covalent?

Lanthanide-transition metal complexes continue to be of interest, not only because of their synthetic challenge but also of their promising magnetic properties. Computational work examining the chemical bonding between lanthanides and transition metals in PyCp2Ln-TMCp(CO)2 (DyPyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) reveals strong Ln-TM dative bonds. Gas-phase optimized geometries are in good agreement with experimental structures at the density functional theory (DFT) level with large-core pseudopotentials. From La to Lu, there is a small increase in the bond dissociation energy, as well as a decrease in Ln-Fe bond lengths. Energy decomposition analyses attribute this trend to an increase in the electrostatic contribution from the decreasing bond length and a modest increase in the orbital contribution. The natural bond orbital analysis clearly indicates that 3d6 "lone pairs" in the [FeCp(CO)2]- fragment act as a Lewis bases donating nearly 0.5 electron to Ln virtual orbitals of mainly d character. The interfragment bonding was also quantified by the quantum theory of atoms in molecules, which indicates that the Ln-Fe bond is more covalent than the Ca-Fe bond in the hypothetical CpCa-FeCp(CO)2 but less covalent than the Zn-Fe bond in the hypothetical CpZn-FeCp(CO)2. Further comparisons suggest that to the [PyCp2Ln]+ cation the [FeCp(CO)2]- anion appears much like a halide. Overall, these Ln-TM dative bonds appear to have strong electrostatic contributions as well as significant orbital mixing and dispersion contributions.

Kinetic versus thermodynamic metalation enables synthesis of isostructural homo- and heterometallic trinuclear clusters.

Temperature-dependent metalation of the new hexadentate ligand (tris(5-(pyridin-2-yl)-1H-pyrrol-2-yl)methane; H3TPM) enables the selective synthesis of both mononuclear (i.e. Na(THF)4[Fe(TPM)], kinetic product) and trinuclear (i.e. Fe3(TPM)2, thermodynamic product) complexes. Exposure of Na(THF)4[Fe(TPM)] to FeCl2 or ZnCl2 triggers cluster expansion to generate homo- or heterometallic trinuclear complexes, respectively. The developed approach enables systematic variation of ion content in isostructural metal clusters via programmed assembly.

Towards understanding of lanthanide-transition metal bonding: investigations of the first Ce-Fe bonded complex.

The syntheses, structural, and magnetic characterization of three new organometallic Ce complexes stabilized by PyCp22- (PyCp22- = [2,6-(CH2C5H3)2C5H3N]2-) are reported. Complex 1 provides the first example of a crystallographically characterized unsupported Ce-Fe bond in a molecular compound. Results from IR spectroscopy and computational analyses suggest weaker Fe → Ce electron-donation than in a previously reported Dy-Fe bonded species.

Structure and Magnetization Dynamics of Dy-Fe and Dy-Ru Bonded Complexes.

We present an investigation of isostructural complexes that feature unsupported direct bonds between a formally trivalent lanthanide ion (Dy3+ ) and either a first-row (Fe) or a second-row (Ru) transition metal (TM) ion. The sterically rigid, yet not too bulky ligand PyCp22- (PyCp22- =[2,6-(CH2 C5 H3 )2 C5 H3 N]2- ) facilitates the isolation and characterization of PyCp2 Dy-FeCp(CO)2 (1; d(Dy-Fe)=2.884(2) Å) and PyCp2 Dy-RuCp(CO)2 (2; d(Dy-Ru)=2.9508(5) Å). Computational and spectroscopic studies suggest strong TM→Dy bonding interactions. Both complexes exhibit field-induced slow magnetic relaxation with effectively identical energy barriers to magnetization reversal. However, in going from Dy-Fe to Dy-Ru bonding, we observed faster magnetic relaxation at a given temperature and larger direct and Raman coefficients, which could be due to differences in the bonding and/or spin-phonon coupling contributions to magnetic relaxation.

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).