Copper Phosphonatoethanesulfonates: Temperature Dependent in Situ Energy Dispersive X‑ray Diffraction Study and Influence of the pH on the Crystal Structures

The system Cu2+/H2O3P–C2H4–SO3H/NaOH was
investigated
using in situ energy dispersive X-ray diffraction (EDXRD) to study
the formation and temperature induced phase transformation of previously
described copper phosphonosulfonates. Thus, the formation of [Cu2(O3P–C2H4–SO3)­(OH)­(H2O)]·3H2O (4) at 90 °C is shown to proceed via a previously unknown intermediate
[Cu2(O3P–C2H4–SO3)­(OH)­(H2O)]·4H2O (6), which could be structurally characterized from high resolution
powder diffraction data. Increase of the reaction temperature to 150
°C led to a rapid phase transformation to [Cu2(O3P–C2H4–SO3)­(OH)­(H2O)]·H2O (1), which was also studied
by in situ EDXRD. The comparison of the structures of 1, 4, and 6 allowed us to establish a possible
reaction mechanism. In addition to the in situ crystallization studies,
microwave assisted heating for the synthesis of the copper phosphonosulfonates
was employed, which allowed the growth of larger crystals of [NaCu­(O3P–C2H4–SO3)­(H2O)2] (5) suitable for single crystal
X-ray diffraction. Through the combination of force field calculations
and Rietveld refinement we were able to determine the crystal structure
of [Cu1.5(O3P–C2H4–SO3)] 2H2O (3) and thus
structurally characterize all compounds known up to now in this well
investigated system. With the additional structural data we are now
able to describe the influence of the pH on the structure formation.