[Zr₆(₃-O)₄(₃-OH)_{4](1-adamantanecarboxylate)12]: a model for extrinsic "defect-engineerable" porosity}

The molecular zirconium-oxo cluster with 1-adamantanecarboxylate of ideal formula [Zr6O4(OH)(4)(AdCA)(12)] (AdCA=1-adamantanecarboxylate) has a near spherical shape with a hydrophobic "shell" of bulky rigid adamantyl groups. The use of formic acid as a modulator and a large excess of the HAdCA were found to be crucial for its reproducible synthesis and the formation of & SIM;100-150 & mu;m well-formed rhombohedral single crystals. Crystallization in the presence of formic acid leads, however, to ligand-substitution defects (1-2 out of 12) which were quantified by thermogravimetry and NMR. A defect-engineering with a higher defect ratio in such oxo clusters is a promising approach aiming at extrinsic porosity.