Heavy atom effect in oxovanadium(IV) quantum bits

Heterobimetallic paddlewheel complexes containing paramagnetic (S = 1/2) vanadyl ion and a diamagnetic Group 10 divalent metal ion have recently joined the pool of molecular spin systems displaying respectable coherence times (Tm). We describe the effect of replacing M = Pt with Pd in monothiocarboxylate complexes [MVO(SOCR)4] (R = Me, Ph), which serve as neutral, high-symmetry (C4) quantum bits. In this series, the vanadium(IV) ion is surrounded only by O atoms, and weakly coupled nuclear spins are confined to the R protons and to the naturally occurring fraction of 105Pd (I = 5/2, 22%) and 195Pt isotopes (I = 1/2, 34%). In the crystalline state, the complexes form staggered (R = Me) or square (R = Ph) dimers with short M···M or M···S contacts, respectively. Staggered dimers position the two vanadium(IV) ions 8.9-9.0 Å apart, but nevertheless show significant antiferromagnetic interactions with a singlet-triplet gap of 5 cm1. The slightly more effective coupling observed in the Pd derivative is reproduced by DFT calculations. In organic solvents, these dimers dissociate into stable monomers, as shown by 1H DOSY NMR and EPR spectroscopy. They undergo a quasi-reversible one-electron reduction, more favourable in the Pd derivatives (for the same R) and in the thiobenzoate complexes (for the same M). The X-band EPR spectra of frozen CD2Cl2/toluene-d8 solutions show the eight-line hyperfine pattern characteristic of the 51V nucleus (I = 7/2, 100%), with virtually identical spin Hamiltonian parameters across the series. The superhyperfine VM interaction, which leads to a further twofold line splitting in the Pt derivatives, is unresolved in the Pd analogues. DFT calculations attribute this mainly to the drastically decreased hyperfine coupling. At 10 K, Tm values for the Pd derivatives (4-6 s) are slightly but systematically lower than for the Pt counterparts (6-11 s), the longer values being in both cases observed with the Ph substituents. The results show that the proximal heavy metal ion, beside acting as an important structural element, significantly affects the electronic and magnetic properties of these heterobimetallic complexes, but has only a limited impact on their quantum coherence.