Abstract
A phenomenological study is carried out on a complex two-component diffusion-reacting system in gel, that is, the Cd-1,3-bis(4-pyridyl)propane (Cd-bpp) coordination polymer. The latter can exist in three solid forms, which exploit a 1:1 correspondence among the Cd/bpp ratio, the crystal structure and the crystal morphology (1/2: bipyramids; 2/3: needles; 1/3: plates). The aim was to clarify the role of key physicochemical variables (reactant concentrations, composition of the solvent and density of the transport medium) in determining the chemical nature and the morphology of the final crystallization products. The gel method was tested in a variety of different crystallization configurations, including single and double diffusion techniques. The density of the gel primarily affects the morphology of the synthesized crystals, with denser media favouring the needle-like 2/3 Cd-bpp species and diluted ones the 1/2 Cd-bpp bipyramidal one. However, higher densities of the gel are generally associated to strained crystals. The solvent composition is also important, as for example the 1/2 Cd-bpp bipyramids require at least a minimum amount of ethanol to appear. We demonstrated that in gel the strict “equality” stoichiometric criteria for metal-to-ligand ratios can be sometimes eluded, as non–equilibrium concentrations can be locally attained. In this respect, the crystallization geometry was proven to act as a key tool to influence the crystallization output, as it determines the direction and magnitude of the concentration gradients. Finally, the use of U tubes to perform one-pot screenings of a large part of the crystallization space is discussed.
Highlights
Understanding the outcome of crystallizing organic and metal-organic compounds is still a largely unresolved mystery of synthetic chemistry [1]
We extend the experimental study of CdCl2 :bpp (bpp = 1,3-bis(4-pyridyl)propane, Scheme 1) coordination polymers (CPs)
To understand the role of ethanol in determining the phase diagram, we have performed a crystallization screening analogous to those described in Section 3.2.1 by using water as solvent for both components and switching the position of the two reactants, i.e., metal salt solution layered over a gelified solution of the ligand
Summary
Understanding the outcome of crystallizing organic and metal-organic compounds is still a largely unresolved mystery of synthetic chemistry [1]. This method offers several advantages with respect to classical experiments in solution, as in gel all gradients can be finely tuned [22] Before this method can be extensively applied, it is the diffusion processes are slowed down and fluctuations are hampered, meaning that concentration mandatory to explore its potentiality in the reproducible design of known crystal phases, and to check gradients can be finely tuned [22]. Before this method can be extensively applied, it is its effect on the quality of the synthesized crystals.
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