Synthetic strategies to yield molecular complexes of
high-valent lanthanides, other than the ubiquitous Ce<sup>4+</sup> ion, are
exceptionally rare, and thorough, detailed characterization in these systems is
limited by complex lifetime and reaction and isolation conditions. The
synthesis of high-symmetry complexes in high purity with significant lifetimes
in solution and solid-state are essential for determining the role of
ligand-field splitting, multiconfigurational behavior, and covalency in
governing the reactivity and physical properties of these potentially technologically
transformative tetravalent ions. We report the synthesis and physical
characterization of an <i>S</i><sub>4</sub> symmetric, four-coordinate tetravalent
terbium complex, [Tb(NP(1,2-bis-<i><sup>t</sup></i>Bu-diamidoethane)(NEt<sub>2</sub>))<sub>4</sub>]
(where Et is ethyl and <i>t</i>Bu is <i>tert</i>-butyl). The ligand field in
this complex is weak and the metal-ligand bonds sufficiently covalent so that
the tetravalent terbium ion is stable and accessible via a mild oxidant from
the anionic, trivalent, terbium precursor, [(Et<sub>2</sub>O)K][Tb(NP(1,2-bis-<i><sup>t</sup></i>Bu-diamidoethane)(NEt<sub>2</sub>))<sub>4</sub>].
The significant stability of the tetravalent complex enables its thorough
characterization. The step-wise development of the supporting ligand points to
key ligand control elements for further extending the known tetravalent lanthanide
ions in molecular complexes. Magnetic susceptibility, electron paramagnetic
resonance (EPR) spectroscopy, X-ray absorption near-edge spectroscopy (XAS),
and density functional theory studies indicate a <i>4f<sup>7</sup></i> ground
state for [Tb(NP(1,2-bis-<i><sup>t</sup></i>Bu-diamidoethane)(NEt<sub>2</sub>))<sub>4</sub>]
with considerable zero-field splitting: demonstrating that magnetic,
tetravalent lanthanide ions engage in covalent metal-ligand bonds. This result
has significant implications for the use of tetravalent lanthanide ions in
magnetic applications since the observed zero-field splitting is intermediate
between that observed for the trivalent lanthanides and for the transition
metals. The similarity of the multiconfigurational behavior in the ground state
of [Tb(NP(1,2-bis-<i><sup>t</sup></i>Bu-diamidoethane)(NEt<sub>2</sub>))<sub>4</sub>]
(measured by Tb L<sub>3</sub>-edge XAS) to that observed in TbO<sub>2</sub>
implicates ligand control of
multiconfigurational behavior as a key component of the stability of the
complex.
Design, Isolation, and Spectroscopic Analysis of a Tetravalent Terbium Complex
