Strong Ferromagnetic Exchange Coupling and Single-Molecule Magnetism in MoS43--Bridged Dilanthanide Complexes.

We report the synthesis and characterization of the trinuclear 4d-4f compounds [Co(C5Me5)2][(C5Me5)2Ln(µ-S)2Mo(µ-S)2Ln(C5Me5)2], 1-Ln (Ln = Y, Gd, Tb, Dy), containing the highly polarizable MoS43- bridging unit. UV-Vis-NIR diffuse reflectance spectra and DFT calculations of 1-Ln reveal a low-energy metal-to-metal charge transfer transition assigned to charge transfer from the singly occupied 4dz2 orbital of MoV to the empty 5d orbitals of the lanthanides (4d in the case of 1-Y), mediated by sulfur-based 3p orbitals. Electron paramagnetic resonance spectra collected for 1-Y in a tetrahydrofuran solution show large 89Y hyperfine coupling constants of A⊥ = 23 MHz and A|| = 26 MHz, indicating the presence of significant yttrium-localized unpaired electron density. Magnetic susceptibility data support similar electron delocalization and ferromagnetic Ln-Mo exchange for 1-Gd, 1-Tb, and 1-Dy. This ferromagnetic exchange gives rise to an S = 15/2 ground state for 1-Gd and one of the largest magnetic exchange constants involving GdIII observed to date, with JGd-Mo = +16.1(2) cm-1. Additional characterization of 1-Tb and 1-Dy by ac magnetic susceptibility measurements reveals that both compounds exhibit slow magnetic relaxation. Although a Raman magnetic relaxation process is dominant for both 1-Tb and 1-Dy, an extracted thermal relaxation barrier of Ueff = 68 cm-1 for 1-Dy is the largest yet reported for a complex containing a paramagnetic 4d metal center. Together, these results provide a potentially generalizable route to enhanced nd-4f magnetic exchange, revealing opportunities for the design of new nd-4f single-molecule magnets and bulk magnetic materials.

NH3 and (NH2)1- as ligands in yttrium metallocene chemistry.

(C5Me5)2Y(C3H5)(THF) reacts with ammonia in THF to form the first crystallographically-characterized rare-earth metal complex with a terminal (NH2)1- ligand, (C5Me5)2Y(NH2)(THF), 1. Complex 1 can be protonated with [HNEt3][BPh4] to form [(C5Me5)2Y(NH3)(THF)][BPh4], 2. Both complexes were characterized by X-ray crystallography which allowed a comparison of the Y-NH2 bond length, 2.226(2) Å, with an Y-NH3 bond length, 2.476(2) Å, in analogous coordination environments. Reactions of 2 with NaN3 and acetone produced the known complexes [(C5Me5)2YN3]x and [(C5Me5)2Y(OCMe2)2][BPh4], respectively, suggesting that the ammonia moiety in 2 functions as a displaceable solvate like THF.

Slow Magnetic Relaxation in a Dysprosium Ammonia Metallocene Complex.

We report the serendipitous discovery and magnetic characterization of a dysprosium bis(ammonia) metallocene complex, [(C5Me5)2Dy(NH3)2](BPh4) (1), isolated in the course of performing a well-established synthesis of the unsolvated cationic complex [(C5Me5)2Dy][(µ-Ph)2BPh2]. While side reactivity studies suggest that this bis(ammonia) species owes its initial incidence to impurities in the DyCl3(H2O)x starting material, we were able to independently prepare 1 and its tetrahydrofuran (THF) derivative, [(C5Me5)2Dy(NH3)(THF)](BPh4) (2), from the reaction of [(C5Me5)2Dy][(µ-Ph)2BPh2] with ammonia in THF. The low-symmetry complex 1 exhibits slow magnetic relaxation under zero applied direct-current (dc) field to temperatures as high as 46 K and notably exhibits an effective barrier to magnetic relaxation that is more than 150% greater than that previously reported for the [(C5Me5)2Ln][(µ-Ph)2BPh2] precursor. On the basis of fitting of the temperature-dependent relaxation data, magnetic relaxation is found to occur via Orbach, Raman, and quantum-tunneling relaxation processes, and the latter process can be suppressed by the application of a 1400 Oe dc field. Field-cooled and zero-field-cooled dc magnetic susceptibility measurements reveal a divergence at 4 K indicative of magnetic blocking, and magnetic hysteresis was observed up to 5.2 K. These results illustrate the surprises and advantages that the lanthanides continue to offer for synthetic chemists and magnetochemists alike.

Structural and spectroscopic characterization of iron(II), cobalt(II), and nickel(II) ortho-dihalophenolate complexes: insights into metal-halogen secondary bonding.

Metal complexes incorporating the tris(3,5-diphenylpyrazolyl)borate ligand (Tp(Ph2)) and ortho-dihalophenolates were synthesized and characterized in order to explore metal-halogen secondary bonding in biorelevant model complexes. The complexes Tp(Ph2)ML were synthesized and structurally characterized, where M was Fe(II), Co(II), or Ni(II) and L was either 2,6-dichloro- or 2,6-dibromophenolate. All six complexes exhibited metal-halogen secondary bonds in the solid state, with distances ranging from 2.56 Å for the Tp(Ph2)Ni(2,6-dichlorophenolate) complex to 2.88 Å for the Tp(Ph2)Fe(2,6-dibromophenolate) complex. Variable temperature NMR spectra of the Tp(Ph2)Co(2,6-dichlorophenolate) and Tp(Ph2)Ni(2,6-dichlorophenolate) complexes showed that rotation of the phenolate, which requires loss of the secondary bond, has an activation barrier of ~30 and ~37 kJ/mol, respectively. Density functional theory calculations support the presence of a barrier for disruption of the metal-halogen interaction during rotation of the phenolate. On the other hand, calculations using the spectroscopically calibrated angular overlap method suggest essentially no contribution of the halogen to the ligand-field splitting. Overall, these results provide the first quantitative measure of the strength of a metal-halogen secondary bond and demonstrate that it is a weak noncovalent interaction comparable in strength to a hydrogen bond. These results provide insight into the origin of the specificity of the enzyme 2,6-dichlorohydroquinone 1,2-dioxygenase (PcpA), which is specific for ortho-dihalohydroquinone substrates and phenol inhibitors.