Thermal Dihydrogen Elimination from Cp☆2Yb(4,5-diazafluorene)

International audience; The reaction of 4,5-diazafluorene with Cp*2Yb(OEt2), where Cp* is pentamethylcyclopentadienyl, affords the isolable adduct Cp*2Yb(4,5-diazafluorene) (1), which slowly eliminates H2 to form Cp*2Yb(4,5-diazafluorenyl) (2); the net reaction is therefore 1 → 2 + H*. The ytterbium atom in 1 is shown to be intermediate valent by variable-temperature LIII-edge X-ray absorption near-edge (XANES) spectroscopy, consistent with its low effective magnetic moment (μeff). The experimental studies are supported by complete active space self-consistent field (CASSCF) calculations, showing that two open-shell singlets lie below the triplet state. The two open-shell singlets are calculated to be multiconfigurational and closely spaced, in agreement with the observed temperature dependence of the XANES and χ data, which are fit to a Boltzmann distribution. A mechanism for dihydrogen formation is proposed on the basis of kinetic and labeling studies to involve the bimetallic complex (Cp*2Yb)2(4,5-diazafluorenyl)2, in which the heterocyclic amine ligands are joined by a carbon-carbon bond at C(9)-C(9′).