Polymorphism in P,P-ferrocenophanes: insights from an NMR crystallographic approach
Polymorphism phenomena in P,P-ferrocenophanes were studied by 31P and 13C solid-state NMR spectroscopy and suitable DFT calculations. “No-bond” indirect 31P⋯31P spin–spin coupling constants serve as a rather sensitive tool for the characterization of such systems, particularly since this NMR observable strongly depends on intermolecular P⋯P distances and mutual orientations of the phosphorus lone pairs. Indeed, the structure of a previously unknown pseudo-polymorphism of a P,P-ferrocenophane was determined via the emerging tool kit of “NMR crystallography”, where structural inputs and constraints determined by modern solid-state NMR techniques, aided by DFT calculations, are used for faster and more reliable structure solution or refinement of X-ray powder diffraction patterns. Based on this approach it is demonstrated that the observed pseudo polymorphism is related to reversible incorporation of dichloromethane in the crystal structures.