In the Bottlebrush Garden: The Structural Aspects of Coordination Polymer Phases formed in Lanthanide Extraction with Alkyl Phosphoric Acids

Coordination
polymers (CPs) of metal ions are central to a large
variety of applications, such as catalysis and separations. These
polymers frequently occur as amorphous solids that segregate from
solution. The structural aspects of this segregation remain elusive
due to the dearth of the spectroscopic techniques and computational
approaches suitable for probing such systems. Therefore, there is
a lacking of understanding of how the molecular building blocks give
rise to the mesoscale architectures that characterize CP materials.
In this study we revisit a CP phase formed in the extraction of trivalent
lanthanide ions by diesters of the phosphoric acid, such as the bis­(2-ethylhexyl)­phosphoric
acid (HDEHP). This is a well-known system with practical importance
in strategic metals refining and nuclear fuel reprocessing. A CP phase,
referred to as a “third phase”, has been known to form
in these systems for half a century, yet the structure of the amorphous
solid is still a point of contention, illustrating the difficulties
faced in characterizing such materials. In this study, we follow a
deductive approach to solving the molecular structure of amorphous
CP phases, using semiempirical calculations to set up an array of
physically plausible models and then deploying a suite of experimental
techniques, including optical, magnetic resonance, and X-ray spectroscopies,
to consecutively eliminate all but one model. We demonstrate that
the “third phase” consists of hexagonally packed linear
chains in which the lanthanide ions are connected by three O–P–O
bridges, with the modifying groups protruding outward, as in a bottlebrush.
The tendency to yield linear polynuclear oligomers that is apparent
in this system may also be present in other systems yielding the “third
phase”, demonstrating how molecular geometry directs polymeric
assembly in hybrid materials. We show that the packing of bridging
molecules is central to directing the structure of CP phases and that
by manipulating the steric requirements of ancillary groups one can
control the structure of the assembly.