Reversible
Sigma C–C Bond Formation Between
Phenanthroline Ligands Activated by (C<sub>5</sub>Me<sub>5</sub>)<sub>2</sub>Yb
- Publication date
- Publisher
Abstract
The electronic structure and associated
magnetic properties of
the 1,10-phenanthroline adducts of Cp*<sub>2</sub>Yb are dramatically
different from those of the 2,2′-bipyridine adducts. The monomeric
phenanthroline adducts are ground state triplets that are based upon
trivalent Yb(III), f<sup>13</sup>, and (phen<sup>•–</sup> ) that are only weakly exchange coupled, which is in contrast to
the bipyridine adducts whose ground states are multiconfigurational,
open-shell singlets in which ytterbium is intermediate valent (J. Am. Chem. Soc 2009, 131, 6480; J. Am. Chem. Soc 2010, 132, 17537). The origin
of these different physical properties is traced to the number and
symmetry of the LUMO and LUMO+1 of the heterocyclic diimine ligands.
The bipy<sup>•–</sup> has only one π*<sub>1</sub> orbital of b<sub>1</sub> symmetry of accessible energy, but phen<sup>•–</sup> has two π* orbitals of b<sub>1</sub> and a<sub>2</sub> symmetry that are energetically accessible. The
carbon p<sub>π</sub>-orbitals have different nodal properties
and coefficients and their energies, and therefore their populations
change depending on the position and number of methyl substitutions
on the ring. A chemical ramification of the change in electronic structure
is that Cp*<sub>2</sub>Yb(phen) is a dimer when crystallized from
toluene solution, but a monomer when sublimed at 180–190 °C.
When 3,8-Me<sub>2</sub>phenanthroline is used, the adduct Cp*<sub>2</sub>Yb(3,8-Me<sub>2</sub>phen) exists in the solution in a dimer–monomer
equilibrium in which Δ<i>G</i> is near zero. The adducts
with 3-Me, 4-Me, 5-Me, 3,8-Me<sub>2</sub>, and 5,6-Me<sub>2</sub>-phenanthroline
are isolated and characterized by solid state X-ray crystallography,
magnetic susceptibility and L<sub>III</sub>-edge XANES spectroscopy
as a function of temperature and variable-temperature <sup>1</sup>H NMR spectroscopy