Two- and Three-Dimensional Cadmium−Organic Frameworks with Trimesic Acid and 4,4‘-Trimethylenedipyridine

Three novel cadmium−organic frameworks built-up from 1,3,5-benzenetricarboxylate anions (HXBTCx-3) and
4,4‘-trimethylenedipyridine (TMD) have been hydrothermally synthesized, and characterized using single-crystal
X-ray diffraction, thermoanalytical measurements, elemental analysis, and IR and Raman spectroscopies:
[Cd(HBTC)(TMD)2]·8.5H2O (I), [Cd(HBTC)(TMD)(H2O)]·4.5H2O (II), and [Cd2(BTC)(TMD)2(NO3)]·3H2O (III), with
structures I and II being isolated as a mixture of crystals. Structure I contains an undulating infinite two-dimensional
[Cd(HBTC)(TMD)2] framework, with a (4,4) topology and rectangular pores, ca. 3.4 × 11.0 Å in cross-section,
distributed in a herringbone manner. The crystal structure of I is obtained by parallel packing of this 2D framework
in an [ABAB···] fashion. Compound II has a porous 3D diamondoid framework with channels running in several
directions of the unit cell, which allows 2-fold interpenetration to occur. The most prominent channels are distributed
in a brick-wall fashion along the c axis and have a cross-section of ca. 3.2 × 13.2 Å. Structure III can be seen as
the three-dimensional assembly of binuclear secondary building units (SBU), which leads to a compact, neutral,
and coordinatively bonded eight-connected framework, [Cd2(BTC)(TMD)2(NO3)], exhibiting an unusual 36422 topology.
The increased flexibility of the TMD ligands (brought about by the three methylene groups between the two 4-pyridyl
rings) can lead, for the same reactive system, to a large variety of crystal architectures.