posted on 2019-08-09, 16:33authored byNatalie
T. Rice, Ivan A. Popov, Dominic R. Russo, John Bacsa, Enrique R. Batista, Ping Yang, Joshua Telser, Henry S. La Pierre
Synthetic
strategies to yield molecular complexes of high-valent
lanthanides, other than the ubiquitous Ce4+ 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 the solid state is 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 S4 symmetric, four-coordinate tetravalent terbium
complex, [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] (where Et is ethyl and tBu is tert-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,
[(Et2O)K][Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4]. The significant
stability of the tetravalent complex enables its thorough characterization.
The stepwise 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
(XANES), and density functional theory studies indicate a 4f7 ground state for [Tb(NP(1,2-bis-tBu-diamidoethane)(NEt2))4] 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-tBu-diamidoethane)(NEt2))4] (measured
by Tb L3-edge XAS) to that observed in TbO2 implicates
ligand control of multiconfigurational behavior as a key component
of the stability of the complex.