Polymorphism in P,P-[3]ferrocenophanes: insights from an NMR crystallographic approach

Polymorphism phenomena in P,P-[3]ferrocenophanes were studied by P-31 and C-13 solid-state NMR spectroscopy and suitable DFT calculations. No-bond indirect P-31 center dot center dot center dot P-31 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 center dot center dot center dot P distances and mutual orientations of the phosphorus lone pairs. Indeed, the structure of a previously unknown pseudo-polymorphism of a P,P-[3]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.