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 pπ-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-Me2phenanthroline 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-Me2, and 5,6-Me2-phenanthroline are isolated and characterized by solid state X-ray crystallography, magnetic susceptibility and LIII-edge XANES spectroscopy as a function of temperature and variable-temperature 1H NMR spectroscopy.