Divalent Tetra- and Penta-phenylcyclopentadienyl Europium and Samarium Sandwich and Half-Sandwich Complexes: Synthesis, Characterization, and Remarkable Luminescence Properties

The synthesis of the bulky divalent
(polyphenylcyclopentadienyl)­lanthanoid
sandwich complexes [Ln­(C5Ph5)2] (Ln
= Sm, Eu) and [Ln­(C5Ph4H)2(solv)]
(Ln = Sm, solv = thf; Ln = Eu, solv = dme)], from redox-transmetalation/protolysis
(RTP) reactions, has been achieved. An analogous reaction with Yb
afforded the solvent-separated ion pair [Yb­(dme)4]­[C5Ph4H]2. In addition, rare divalent samarium
halide complexes [Sm­(C5Ph5)­(μ-Br)­(thf)2]2 and [Sm­(C5Ph4H)­I­(thf)3], were also prepared, either by RTP or ligand rearrangement.
X-ray studies showed that the [Ln­(C5Ph5)2] complexes adopt highly symmetrical sandwich structures,
whereas the [Ln­(C5Ph4H)2(solv)] complexes
have open sandwich structures. The unexpected, but limited, solubility
of the [Ln­(C5Ph5)2] complexes allowed
for variable-temperature NMR spectra of [Sm­(C5Ph5)2] to be obtained. Detailed 1D and 2D NMR studies were
conducted on [Sm­(C5Ph4H)2(thf)] to
ascertain its structure in donor and nondonor solvents. During the
course of these studies, the mixed tetraarylcyclopentadienyl sandwich
complex [Sm­{C5(2,5-Ph)2­(3,4-p-tol2)­H}2(thf)] was also prepared in order
to fully assign the spectrum of [Sm­(C5Ph4H)2(thf)]. The europium sandwich complexes [Eu­(C5Ph5)2] and [Eu­(C5Ph4H)2(dme)] exhibit remarkable luminescence properties with high quantum
yields (45% and 41%, respectively) coupled with long emission
lifetimes (approximately 800 and 1300 ns, respectively) in toluene.