Trinuclear molecular complexes of
europium (II) and ytterbium(II)
[Ln3{(Ph2SiO)2O}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(SiMe3)2}2(THF)2] complexes. In contrast, the reaction of [Sm{N(SiMe3)2}2(THF)2] with the (Ph2SiOH)2O ligand led to the isolation of the mixed-valent Sm(II)/Sm(III)
complex [Sm3{(Ph2SiO)2O}3{N(SiMe3)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 (Ph2SiOK)2O to 1-Eu
3
L
3
resulted in the formation of the polynuclear Eu(II) dimer
of dimers [K4Eu2{(Ph2SiO)2O}4(Et2O)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
2L4
shows higher reactivity and rapidly decomposes to
give the mixed-valent Eu(II)/Eu(III) species [K3Eu2{(Ph2SiO)2O}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 CO2. However, the presence of one additional (Ph2SiO–)2O 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 CO2 by 3-Eu
2
L
4
is immediate, and carbonate is
formed selectively after the addition of a stoichiometric amount of
CO2.