Synthesis and Structural Characterization of the First Lanthanide(II) Cuboidal Cluster. Unexpected C−O Activation by an Unchanged Ytterbium(II) Center

The two new sterically demanding secondary phosphines (C6H4-2-OMe)RPH (1a) and (C6H3-2-OMe-3-Me)RPH (1b) [R = CH(SiMe3)2] are synthesized by the reaction of RPCl2 with 1
equiv of LiC6H4-2-OMe or LiC6H3-2-OMe-3-Me, respectively, followed by in situ reduction
with LiAlH4. Metalation of 1 with BunLi, followed by metathesis with KOBut, yields the
potassium salts K{PR(C6H4-2-OMe)} (2a) and K{PR(C6H3-2-OMe-3-Me)} (2b). The reaction
of K{PR(C6H3-2-OMe-3-Me)} with YbI2 yields the diphosphide complex [YbII{PR(C6H3-2-OMe-3-Me)}2(THF)2] (3), in which the phosphides act as P,O-chelating ligands. In contrast,
reaction of K{PR(C6H4-2-OMe)} with YbI2 under similar conditions yields the unexpected
alkoxophosphide complex [YbII{PR(C6H4-2-O)}(THF)]4·4Et2O (4), via an unusual ligand
cleavage reaction involving the transfer of a methyl group from oxygen to phosphorus.
Compounds 1−4 have been characterized by elemental analyses and multinuclear NMR
spectroscopy and compounds 3 and 4 by X-ray crystallography. Compound 3 is monomeric
in the solid state, with a distorted all-trans-octahedral geometry about the Yb center;
compound 4 crystallizes as an unprecedented tetrameric cluster containing a Yb4O4 cuboidal
core. Multi-element NMR spectroscopy suggests that 4 maintains an oligomeric structure
in THF solution. The P-containing side product formed in the synthesis of 4 has been isolated
and identified as (Me)PR(C6H4-2-OMe) by comparison of its NMR and mass spectra with
those of an authentic sample.