  Geometric frustration inhibits magnetic systems from ordering, opening a
window to unconventional phases of matter. The paradigmatic frustrated lattice
in three dimensions to host a spin liquid is the pyrochlore, although there
remain few experimental compounds thought to realize such a state. Here we go
beyond the pyrochlore via molecular design in the metal-azolate framework
[Mn(II)(ta)$_2$], which realizes a closely related centred pyrochlore lattice
of Mn-spins with $S=5/2$. Despite a Curie-Weiss temperature of $-21$ K
indicating the energy scale of magnetic interactions, [Mn(II)(ta)$_2$] orders
at only 430 mK, putting it firmly in the category of highly frustrated magnets.
Comparing magnetization and specific heat measurements to numerical results for
a minimal Heisenberg model, we predict that this material displays distinct
features of a classical spin liquid with a structure factor reflecting Coulomb
physics in the presence of charges.
