Selective Reaction Based on the Linked Diamido Ligands of Dinuclear Lanthanide Complexes

The dinuclear ytterbium pyridyl diamido complexes [Cp2Yb(THF)]2[μ-η1:η2-(NH)2(C5H3N-2,6)] (1a) and [Cp2Yb(THF)]2[μ-η1:η2-(NH)2(C5H3N-2,3)] (1b) are easily prepared by protonolysis of Cp3Yb with 0.5 equiv of the
corresponding diaminopyridine in accepted yields, respectively. Treatment of 1a with 2 equiv of dicyclohexylcarbodiimide (CyNCNCy) in THF at low temperature leads to the isolation of the formal double N−H addition
product (Cp2Yb)2[μ-η2:η2-(CyN(CyNH)CN)2(C5H3N-2,6)] (2) in 42% yield. Compound 2 is unstable to heat and
slowly isomerized to the mixed neutral/dianionic diguanidinate complex (Cp2Yb)2[μ-η2:η2-(CyNH)2CN(C5H3N-2,6)NC(NCy)2](THF) (3) at room temperature. Similarly, treatment of 1b with 2 equiv of CyNCNCy gives the addition/
isomerization product (Cp2Yb)2[μ-η2:η2:η1-(CyNH)2CN(C5H3N-2,3)NC(NCy)2] (4). Moreover, the reaction of various
ytterbium aryl diamido complexes (prepared in situ from [Cp2YbMe]2 and aryldiamine, respectively) with CyNCNCy affords the corresponding addition products (Cp2Yb)2[μ-η2:η2-{CyN(CyNH)CN}2(C6H4-1,4)] (5), (Cp2Yb)2[μ-η2:η2-{CyN(CyNH)CN}2(C6H4-1,3)](6), and (Cp2Yb)2[μ-η2:η2-{CyN(CyNH)CN}2(C13H8-2,7)] (7), respectively. In
contrast to pyridyl-bridged bis(guanidinate monoanion) complexes, aryl-bridged bis(guanidinate monoanion) complexes
5−7 are stable even with prolonged heating at 110 °C. All the results not only demonstrate that the presence of
the pyridyl bridge can impart the diamido complexes with a unique reactivity and initiate the unexpected reaction
sequence but also indicate evidently that the number and distribution of negative charges of the diguanidinate
ligand is tunable from double monoanionic units to mixed neutral/dianionic isomers. All the complexes are characterized
by elemental analysis and IR spectroscopies. The structures of complexes 1a, 3, 5, 6, and 7 are also determined
through X-ray single-crystal diffraction analysis.