Specific Heat Studies of a New 2D S $=$ 1/2 Heisenberg Antiferromagnet, [Cu(pz)$_{2}$(2-OHpy)$_{2}$](PF$_{6})_{2}$

We report on the zero-field specific heat (0.3 -- 300 K) of a highly two-dimensional Heisenberg, S $=$ 1/2 antiferromagnet (2D QHAF), [Cu(pz)$_{2}$(2-OHpy)$_{2}$](PF$_{6})_{2}$ (pz $=$ pyrazine and 2-OHpy $=$ 2-hydroxypyridine). The copper atoms and pyrazine molecules form square layers of pyrazine-bridged copper(II) ions with the pyridone molecules normal to the layers, providing exceptional spacing between layers. The magnetic specific heat of this compound corresponds to an exchange strength J $=$ 6.6 K, in excellent agreement from the value deduced from susceptibility experiments. The N\'{e}el temperature of this compound has been found to be 1.37 K by muon spin relaxation .[1] The critical ratio T$_{N}$/J $=$ 1.37/6.6 $\approx_{\, }$2.1 is the lowest value known for a molecular-based 2D QHAF and indicates a remarkable degree of isolation between the magnetic planes. [1] T. Lancaster, S. Blundell et al, private communication.