Physical Properties of Superbulky Lanthanide Metallocenes: Synthesis and Extraordinary Luminescence of [EuII(CpBIG)2] (CpBIG=(4-nBu-C6H4)5-Cyclopentadienyl)

The superbulky deca-aryleuropocene [Eu(Cp-BIG)(2)], Cp-BIG=(4-nBu-C6H4)(5)-cyclopentadienyl, was prepared by reaction of [Eu(dmat)(2)(thf)(2)], DMAT=2-Me2N--Me3Si-benzyl, with two equivalents of (CpH)-H-BIG. Recrystallizyation from cold hexane gave the product with a surprisingly bright and efficient orange emission (45% quantum yield). The crystal structure is isomorphic to those of [M(Cp-BIG)(2)] (M=Sm, Yb, Ca, Ba) and shows the typical distortions that arise from (CpCpBIG)-Cp-BIG attraction as well as excessively large displacement parameter for the heavy Eu atom (U-eq=0.075). In order to gain information on the true oxidation state of the central metal in superbulky metallocenes [M(Cp-BIG)(2)] (M=Sm, Eu, Yb), several physical analyses have been applied. Temperature-dependent magnetic susceptibility data of [Yb(Cp-BIG)(2)] show diamagnetism, indicating stable divalent ytterbium. Temperature-dependent Eu-151 Mossbauer effect spectroscopic examination of [Eu(Cp-BIG)(2)] was examined over the temperature range 93-215K and the hyperfine and dynamical properties of the Eu-II species are discussed in detail. The mean square amplitude of vibration of the Eu atom as a function of temperature was determined and compared to the value extracted from the single-crystal X-ray data at 203K. The large difference in these two values was ascribed to the presence of static disorder and/or the presence of low-frequency torsional and librational modes in [Eu(Cp-BIG)(2)]. X-ray absorbance near edge spectroscopy (XANES) showed that all three [Ln(Cp-BIG)(2)] (Ln=Sm, Eu, Yb) compounds are divalent. The XANES white-line spectra are at 8.3, 7.3, and 7.8eV, for Sm, Eu, and Yb, respectively, lower than the Ln(2)O(3) standards. No XANES temperature dependence was found from room temperature to 100K. XANES also showed that the [Ln(Cp-BIG)(2)] complexes had less trivalent impurity than a [EuI2(thf)(x)] standard. The complex [Eu(Cp-BIG)(2)] shows already at room temperature strong orange photoluminescence (quantum yield: 45%): excitation at 412nm (24270cm(-1)) gives a symmetrical single band in the emission spectrum at 606nm ((max)=16495cm(-1), FWHM: 2090cm(-1), Stokes-shift: 2140cm(-1)), which is assigned to a 4f(6)5d(1)4f(7) transition of Eu-II. These remarkable values compare well to those for Eu-II-doped ionic host lattices and are likely caused by the rigidity of the [Eu(Cp-BIG)(2)] complex. Sharp emission signals, typical for Eu-III, are not visible.