Reversible sigma C-C bond formation between phenanthroline ligands activated by (C₅Me₅)₂Yb

The electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp*₂Yb 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), f¹³, and (phen^•-) 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^•- has only one π*₁ orbital of b₁ symmetry of accessible energy, but phen^•- has two π* orbitals of b₁ and a₂ symmetry that are energetically accessible. The carbon p_π-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*₂Yb(phen) is a dimer when crystallized from toluene solution, but a monomer when sublimed at 180-190 °C. When 3,8-Me ₂phenanthroline is used, the adduct Cp*₂Yb(3,8- Me₂phen) exists in the solution in a dimer-monomer equilibrium in which δG is near zero. The adducts with 3-Me, 4-Me, 5-Me, 3,8-Me ₂, and 5,6-Me₂-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and L_III-edge XANES spectroscopy as a function of temperature and variable-temperature ¹H NMR spectroscopy.