Three samarium(II) crown ether complexes,
[Sm(15-crown-5)2]I2 (1), [Sm(15-crown-5)2]I2·CH3CN (2), and
[Sm(benzo-15-crown-5)2]I2 (3),
have been prepared via the
reaction of SmI2 with the corresponding crown ether in
either THF or acetonitrile in good to moderate yields. The compounds
have been characterized by single crystal X-ray diffraction and a
variety of spectroscopic techniques. In all cases, the Sm(II) centers
are sandwiched between two crown ether molecules and are bound by
the five etheric oxygen atoms from each crown ether to yield 10-coordinate
environments. Despite the higher symmetry crystal class of 1 (R3c), the samarium center resides
on a general position, whereas in 2 and 3 (both in P21/c) the
metal centers lie upon inversion centers. Moreover, the complexes
in 2 and 3 are approximated well by D
5d
symmetry. The molecule in 1, however, is distorted from idealized D
5d
symmetry, and the crown ethers are
more puckered than observed in 2 and 3.
All three complexes luminesce in the NIR at low temperatures. However,
the nature of the luminescence differs between the three compounds. 1 exhibits broadband photoluminescence at 20 °C but at
low temperatures transitions to narrow peaks. 2 only
exhibits nonradiative decay at 20 °C and at low temperatures
retains a mixture of broadband and fine transitions. Finally, 3 displays broadband luminescence regardless of temperature.
Spin–orbit (SO) CASSCF calculations reveal that the outer-sphere
iodide anions influence whether broadband luminescence from 5d → 4f or fine 4f → 4f transitions occur through the alteration
of symmetry around the metal centers and the nature of the excited
states as a function of temperature.