Cyanomethylene-bis(phosphonate)-Based Lanthanide Complexes: Structural, Photophysical, and Magnetic Investigations

The syntheses, structural investigations,
magnetic and photophysical
properties of a series of 10 lanthanide mononuclear complexes, containing
the heteroditopic ligand cyanomethylene-bis­(5,5-dimethyl-2-oxo-1,3,2λ5-dioxa-phosphorinane) (L), are described. The
crystallographic analyses indicate two structural types: in the first
one, [LnIII(L)3­(H2O)2]·H2O (Ln = La, Pr, Nd), the metal
ions are eight-coordinated within a square antiprism geometry, while
the second one, [LnIII(L)3­(H2O)]·8H2O (Ln = Sm, Eu, Gd, Tb, Dy, Ho, Er),
contains seven-coordinated LnIII ions within distorted
monocapped trigonal prisms. Intermolecular hydrogen bonding between
nitrogen atoms of the cyano groups, crystallization, and coordination
water molecules leads to the formation of extended supramolecular
networks. Solid-state photophysical investigations demonstrate that
EuIII and TbIII complexes possess intense luminescence
with relatively long excited-state lifetimes of 530 and 1370 μs,
respectively, while PrIII, DyIII, and HoIII complexes have weak intensity luminescence characterized
by short lifetimes ranging between a few nanoseconds to microseconds.
The magnetic properties for PrIII, GdIII, TbIII, DyIII, and HoIII complexes are in
agreement with isolated LnIII ions in the solid state,
as suggested by the single-crystal X-ray analyses. Alternating current
(ac) susceptibility measurements up to 10 kHz reveal that only the
HoIII complex shows a frequency-dependent ac response,
with a relaxation mode clearly observed at 1.85 K around 4500 Hz.