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 (BuLi)-Li-n, 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 [Yb-II{PR(C6H3-2OMe-2-OMe)}(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 [Yb-II{PR(C6H4-2-O)}(THF)](4). 4Et(2)O (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.