摘要:
The adducts formed between the 4,7-Me-2-, 3,4,7,8-Me-4-, and 3,4,5,6,7,8-Me-6-phenanthroline ligands and Cp*Yb-2 are shown to have open-shell singlet ground states by magnetic susceptibility and LIII-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 Delta H = 5.75 kJ mol(-1) for the 4,7-Me(2)phen adduct. The results of a CASSCF calculation for the 4,7-Me(2)phen adduct indicates that three open-shell singlet states, SS1SS3, 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-Me(2)phen 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*Yb-2(3,8-Me(2)phen) and Cp*Yb-2(4,7-Me(2)phen); the former forms a s CC bond between C(4)C(4'), and the latter undergoes CH bond cleavage at the methyl group on C(4) and leads to two products that cocrystallize: Cp*Yb-2(4-(CH2),7-Mephen), which has lost a hydrogen atom, and Cp*Yb-2(4,7-Me-2-4H-phen), which has gained a hydrogen atom.