Structure and Reactivity of Polynuclear Divalent Lanthanide Disiloxanediolate Complexes.

Trinuclear molecular complexes of europium (II) and ytterbium(II) [Ln 3 {(Ph 2 SiO) 2 O} 3 (THF) 6 ], 1-Ln 3 L 3 (Ln = Eu and Yb), supported by the dianionic tetraphenyl disiloxanediolate ligand, were synthesized via protonolysis of the [Ln{N(SiMe 3 ) 2 } 2 (THF) 2 ] complexes. In contrast, the reaction of [Sm{N(SiMe 3 ) 2 } 2 (THF) 2 ] with the (Ph 2 SiOH) 2 O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III) complex [Sm 3 {(Ph 2 SiO) 2 O} 3 {N(SiMe 3 ) 2 }(THF) 4 ], 2-Sm 3 L 3 , which was crystallographically characterized. The Eu(II) complex 1-Eu 3 L 3 displays weak ferromagnetic coupling between the Eu(II) metal centers ( J = 0.1035 cm -1 ). The addition of 3 equiv of (Ph 2 SiOK) 2 O to 1-Eu 3 L 3 resulted in the formation of the polynuclear Eu(II) dimer of dimers [K 4 Eu 2 {(Ph 2 SiO) 2 O} 4 (Et 2 O) 2 ] 2 , 3-Eu 2 L 4 . Complexes 1-Ln 3 L 3 (Ln = Eu and Yb) are stable in solution at room temperature, while 3-Eu 2 L 4 shows higher reactivity and rapidly decomposes to give the mixed-valent Eu(II)/Eu(III) species [K 3 Eu 2 {(Ph 2 SiO) 2 O} 4 ], 4-Eu 2 L 4 . Complex 1-Yb 3 L 3 affects the slow reductive disproportionation of carbon dioxide, but 1-Eu 3 L 3 does not display any reactivity toward CO 2 . However, the presence of one additional (Ph 2 SiO - ) 2 O per Eu(II) metal center in 3-Eu 2 L 4 increases dramatically the reductive ability of the Eu(II) metal centers, affording the first example of carbon dioxide activation by an isolated divalent europium complex. The reduction of CO 2 by 3-Eu 2 L 4 is immediate, and carbonate is formed selectively after the addition of a stoichiometric amount of CO 2.