  In the present work, we investigate the quantum thermal entanglement in
molecular magnets composed of dimers of spin $S$, using an Entanglement Witness
built from measurements of magnetic susceptibility. An entanglement
temperature, $T_{e}$, is then obtained for some values of spin $S$. From this,
it is shown that $T_{e}$ is proportional to the intradimer exchange interaction
$J$ and that entanglement appears only for antiferromagnetic coupling. The
results are compared to experiments carried on three isostructural materials:
KNaMSi$_{4}$O$_{10}$ (M$=$Mn, Fe or Cu).
