Dual Structure of a Vanadyl-Based Molecular Qubit Containing a Bis(β-diketonato) Ligand

We designed [VO­(bdhb)] (1′) as a
new electronic
qubit containing an oxovanadium­(IV) ion (S = 1/2)
embraced by a single bis­(β-diketonato) ligand [H2bdhb = 1,3-bis­(3,5-dioxo-1-hexyl)­benzene]. The synthesis afforded
three different crystal phases, all of which unexpectedly contain
dimers with formula [(VO)2(bdhb)2] (1). A trigonal form (1h) with a honeycomb structure and 46% of solvent-accessible voids
quantitatively transforms over time into a monoclinic solvatomorph 1m and minor amounts of a triclinic
solventless phase (1a).
In a static magnetic field, 1h and 1m have detectably
slow magnetic relaxation at low temperatures through quantum tunneling
and Raman mechanisms. Angle-resolved electron paramagnetic resonance
(EPR) spectra on single crystals revealed signatures of low-dimensional
magnetic behavior, which is solvatomorph-dependent, being the closest
interdimer V···V separations (6.7–7.5 Å)
much shorter than intramolecular V···V distances (11.9–12.1
Å). According to 1H diffusion ordered spectroscopy
(DOSY) and EPR experiments, the complex adopts the desired monomeric
structure in organic solution and its geometry was inferred from density
functional theory (DFT) calculations. Spin relaxation measurements
in a frozen toluene-d8/CD2Cl2 matrix yielded Tm values reaching
13 μs at 10 K, and coherent spin manipulations were demonstrated
by Rabi nutation experiments at 70 K. The neutral quasi-macrocyclic
structure, featuring nuclear spin-free donors and additional possibilities
for chemical functionalization, makes 1′ a new
convenient spin-coherent building block in quantum technologies.