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

International audienceThe electronic structure and associated magnetic properties of the 1,10-phenanthroline adducts of Cp* 2 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 13 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 amine ligands. The bipy·-has only one π* 1 orbital of b 1 symmetry of accessible energy but phen·-has two π* orbitals of b 1 and a 2 symmetry that are energetically accessible. The carbon pπ-orbitals have different nodal properties and coefficients and their energies and therefore 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* 2 Yb(phen) is a dimer, when crystallized from toluene solution, but a monomer when sublimed at 180-190 °C. When 3,8-Me 2 phenanthroline is used, the adduct Cp* 2 Yb(3,8-Me 2 phen) exist 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-Me 2-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 1 H NMR spectroscopy