Synthesis and Solid-State Behavior of Host-Guest Carbonyl Rh(I) Polymers Assembled through Bidentate Phosphine Ligands

The self-assembly of Rh(I) carbonyl and bis(diphenylphosphino)hexane (dpphex) ligands yields polymers with the general formula [RhCl(CO)(dpphex)] ∞, obtained in two crystalline polymorphs characterized by one-dimensional networks segregated in layers to produce channels occupied by guest molecules (solvent). We report here simple methods to grow single crystals of these species, whereas all previous attempts using the same class of ligands and a variety of metal moieties were unsuccessful. The length of the alkyl chains is very important to produce suitable cavities of a host-guest system. The chain flexibility allows the solids to undergo crystal-to-crystal transformations upon the removal of the guest molecules and produce a new framework without cavities. However, without special precaution, the crystallinity appears to be fairly reduced, and it is impossible to revert the process preserving a single crystal and test the porosity properties of the material. On the other hand, at high temperatures, a recrystallization phenomenon is observed just before the melting point. The use of complementary techniques, such as polarizing microscopy, X-ray diffraction, and differential scanning calorimetry, provides some insights into the mechanics and the dynamics of the solid-solid transformations, both at the microscopic and molecular levels.