Coordination of 2,2‘-Bipyridyl and 1,10-Phenanthroline to Substituted Ytterbocenes:  An Experimental Investigation of Spin Coupling in Lanthanide Complexes

Abstract

Addition of 2,2‘-bipyridyl to diamagnetic (Me5C5)2Yb(OEt2) gives the brown adduct (Me5C5)2Yb(bipy). The solution 1H NMR and electronic absorption spectra show that the bipyridyl complex is paramagnetic, containing a bipyridyl radical anion, which can also be detected in the solid-state infrared spectrum and by the single-crystal X-ray crystallographic analysis. However, the measured magnetic moment, which varies from less than 1 μB at 5 K to 2.5 μB at 300 K, is higher than expected for (Me5C5)2YbII(bipy0) and less than expected for (Me5C5)2YbIII(bipy-). An electron exchange model for spin coupling between Yb(III), with electron configuration 4f13, and the single unpaired electron in the bipyridyl radical anion is presented, based on comparison with the iodide salt [(Me5C5)2YbIII(bipy0)]+[I]-. Comparing the magnetic susceptibility of (Me5C5)2Yb(phen) with its iodide salt shows similar behavior with phenanthroline as ligand. The extent of paramagnetism and therefore the exchange coupling is changed by the nature of the substituents on the cyclopentadienide rings; electron-withdrawing SiMe3 groups favor Yb(II), while electron-donating alkyl groups stabilize the Yb(III) species. The molecular structures of many of the compounds have been determined in the solid state, and the bond distances and angles are consistent with the interpretation of the magnetism. The ring substituents, and therefore the different magnetic environments about the ytterbium center, also influence the rate of intermolecular exchange of the heterocyclic base ligands in solution; when the ligand is reduced, it exchanges more slowly than in the diamagnetic compounds.

Reversible Sigma C–C Bond Formation Between Phenanthroline Ligands Activated by (C5Me5)2Yb

“…The 3,8-Me 2 phen adduct is somewhat more soluble in toluene, but all of the neutral adducts are much less soluble than the 2,2'-bipyridine adducts described in earlier papers. 4,5,14,28 The solid-state and solution-state physical properties of the adducts are quite different and these properties are described in the separate sections that follow. The neutral phen adduct may also be sublimed at 190 °C under reduced pressure to yield dark purple crystalline material.…”

“…14 The spectra are available in SI. Morris and co-workers have given a detailed analysis of the solution spectra from 400 nm to 2500 nm of [Cp* 2 Yb(phen)] 0,+ .…”

Application of the Hubbard Model to Cp*2Yb(bipy), a Model System for Strong Exchange Coupling in Lanthanide Systems

“…In particular, Cp* 2 Yb(bipy), 3, where Cp* is pentamethylcyclopentadienyl and bipy is 2,2'-bipyridine, displays, as does 1, strong exchange coupling between a ligand-based radical and a trivalent Ln center. [14][15][16][17] Previous studies have shown that 3 is multiconfigurational. 15 16 Computational modeling showed that the multiconfigurational ground state is due to mixing of low-lying excited states into the ground state.…”

Coordination of 1,4-Diazabutadiene Ligands to Decamethylytterbocene: Additional Examples of Spin Coupling in Ytterbocene Complexes

“…[1][2][3] However, the magnetic moments in these complexes are lower than expected, which implies that the spin carriers are coupled, that is, the spin on the [Cp' 2 Yb(III, 4f 13 )] fragment is coupled with the spin on the bipy-X radical anion (S= ½) fragment. Although the spin carriers are antiferromagnetically coupled, the mechanism by which they couple is not clear; the coupling mechanism is important since exchange coupling between f-electrons with paired electrons or with other f-electrons is generally weak.…”

“…The sigma donor orbitals on each nitrogen atom of bipy and dad(H)-R transform as a 1 +b 2 (in C 2v symmetry). The LUMO is of b 1 symmetry (in C 2v symmetry) in each case as illustrated in A and B. The energy of the LUMO is measured by the reduction potential of the neutral ligands.…”