Carbon–Hydrogen Bond Breaking and Making in the Open-Shell Singlet Molecule Cp*₂Yb(4,7-Me₂phen)

The adducts formed between the 4,7-Me₂-, 3,4,7,8-Me₄-, and 3,4,5,6,7,8-Me₆-phenanthroline ligands and Cp*₂Yb are shown to have open-shell singlet ground states by magnetic susceptibility and L_III-edge XANES spectroscopy. Variable-temperature XANES data show that two singlet states are occupied in each adduct that are fit to a Boltzmann distribution for which Δ<i>H</i> = 5.75 kJ mol^–1 for the 4,7-Me₂phen adduct. The results of a CASSCF calculation for the 4,7-Me₂phen adduct indicates that three open-shell singlet states, SS1–SS3, lie 0.44, 0.06. and 0.02 eV, respectively, below the triplet state. These results are in dramatic contrast to those acquired for the phenanthroline and 5,6-Me₂phen adducts, which are ground state triplets (J. Am. Chem. Soc. 2014, 136, 8626). A model that accounts for these differences is traced to the relative energies of the LUMO and LUMO+1 orbitals that depend on the position the methyl group occupies in the phenanthroline ligand. The model also accounts for the difference in reactivities of Cp*₂Yb­(3,8-Me₂phen) and Cp*₂Yb­(4,7-Me₂phen); the former forms a σ C–C bond between C(4)­C(4′), and the latter undergoes C–H bond cleavage at the methyl group on C(4) and leads to two products that cocrystallize: Cp*₂Yb­(4-(<i>CH</i>₂),7-Mephen), which has lost a hydrogen atom, and Cp*₂Yb­(4,7-Me₂-<i>4H</i>-phen), which has gained a hydrogen atom