Inelastic Neutron Scattering Measurement of the Ground State Tunneling Gap in Tb and Ho Analogues of a Dy Field-Induced Single-Molecule Magnet

Recent
advances in single-molecule magnet (SMM) research
have placed
great value on interpretation of inelastic neutron scattering (INS)
data for rare earth (RE)-containing SMMs. Here, we present the synthesis
of several rare earth complexes where combined magnetic and INS studies
have been performed, supported by ab initio calculations.
The reaction of rare earth nitrate salts with 2,2′-bipyridine
(2,2′-bpy) and tetrahalocatecholate (X4Cat2–, X = Br, Cl) ligands in methanol (MeOH) afforded two new families
of compounds [RE­(2,2′-bpy)2(X4Cat)­(X4CatH)­(MeOH)] (X = Br and RE = Y, Eu, Gd, Tb, Dy, Ho, Yb for 1-RE; X = Cl and RE = Y, Tb, Dy, Ho, and Yb for 2-RE). Addition of triethylamine (Et3N) to the reaction mixture
delivered Et3NH­[RE­(2,2′-bpy)2(Br4Cat)2] (3-RE, RE = Er and Yb). Interestingly,
cerium behaves differently to the rest of the series, generating (2,2’-bpyH)2[Ce­(Br4Cat)3(2,2′-bpy)] (4-Ce) with tetravalent Ce­(IV) in contrast to the trivalent
metal ions in 1–3. The static magnetic
properties of 1-RE (RE = Gd, Tb, Dy and Ho) were investigated
in conjunction with INS measurements on 1-Y, 1-Tb, and 1-Ho to probe their ground state properties and
any crystal field excitations. To facilitate interpretation of the
INS spectra and provide insight into the magnetic behavior, ab initio calculations were performed using the single-crystal
X-ray diffraction structural data of 1-RE (RE = Tb, Dy
and Ho). The ab initio calculations indicate ground
doublets dominated by the maximal angular momentum projection states
of Kramers type for 1-Dy and Ising type for 1-Tb and 1-Ho. Dynamic magnetic susceptibility measurements
indicate that 1-Dy exhibits slow magnetic relaxation
in the presence of a small applied magnetic field mainly through Raman
pathways. Inelastic neutron scattering spectra exhibit distinct transitions
corresponding to crystal field-induced tunneling gaps between the
pseudo-doublet ground state components for 1-Tb and 1-Ho, which is one of the first direct experimental measurements
with INS of such tunneling transitions in a molecular nanomagnet.
The power of high-resolution INS is demonstrated with evidence of
two distinct tunneling gaps measurable for the two crystallographically
unique Tb coordination environments observed in the single crystal
X-ray structure.