Synthesis and Structural Characterization of Sm(II) and Yb(II) Complexes Containing Sterically Demanding, Chelating Secondary Phosphide Ligands

Metathesis between [{(Me3Si)2CH}(C6H4-2-OMe)P]K and SmI2(THF)2 in THF yields [{([Me3Si]2CH)(C6H4-2-OMe)P}2Sm(DME)(THF)] (1), after recrystallization. A similar reaction between [{(Me3Si)2CH}(C6H3-2-OMe-3-Me)P]K and SmI2(THF)2 yields [{([Me3Si]2CH)(C6H3-2-OMe-3-Me)P}2Sm(DME)]·Et2O (2), while reaction
between [{(Me3Si)2CH}(C6H4-2-CH2NMe2)P]K and either SmI2(THF)2 or YbI2 yields the five-coordinate complex
[{([Me3Si]2CH)(C6H4-2-CH2NMe2)P}2Sm(THF)] (3) or the solvent-free complex [{([Me3Si]2CH)(C6H4-2-CH2NMe2)P}2Yb] (4), respectively. X-ray crystallography shows that complex 2 adopts a distorted cis octahedral
geometry, while complex 1 adopts a distorted pentagonal bipyramidal geometry (1, triclinic, P1̄, a = 11.0625(9)
Å, b = 15.924(6) Å, c = 17.2104(14) Å, α = 72.327(2)°, β = 83.934(2)°, γ = 79.556(2)°, Z = 2; 2, monoclinic,
P21, a = 13.176(4) Å, b = 13.080(4) Å, c = 14.546(4) Å, β = 95.363(6)°, Z = 2). Complex 3 crystallizes as
monomers with a square pyramidal geometry at Sm and exhibits short contacts between Sm and the ipso-carbon
atoms of the ligands (3, monoclinic, C2/c, a = 14.9880(17) Å, b = 13.0528(15) Å, c = 24.330(3) Å, β = 104.507(2)°, Z = 4). Whereas preliminary X-ray crystallographic data for 4 indicate a monomeric structure in the solid
state, variable-temperature 1H, 13C{1H}, 31P{1H}, and 171Yb NMR spectroscopies suggest that 4 undergoes an
unusual dynamic process in solution, which is ascribed to a monomer−dimer equilibrium in which exchange of
the bridging and terminal phosphide groups may be frozen out at low temperature.