Isoreticular expansion of metal-organic frameworks with triangular and square building units and the lowest calculated density for porous crystals.

The concept and occurrence of isoreticular (same topology) series of metal-organic frameworks (MOFs) is reviewed. We describe the preparation, characterization, and crystal structures of three new MOFs that are isoreticular expansions of known materials with the tbo (Cu(3)(4,4',4''-(benzene-1,3,5-triyl-tris(benzene-4,1-diyl))tribenzoate)(2), MOF-399) and pto topologies (Cu(3)(4,4',4''-(benzene-1,3,5-triyl-tribenzoate)(2), MOF-143; Cu(3)(4,4',4''-(triazine-2,4,6-triyl-tris(benzene-4,1-diyl))tribenzoate)(2), MOF-388). One of these materials (MOF-399) has a unit cell volume 17 times larger than that of the first reported material isoreticular to it, and has the highest porosity (94%) and lowest density (0.126 g cm(-3)) of any MOFs reported to date.