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

Polymorphism phenomena in P,P-[3]ferrocenophanes were studied by (31)P and (13)C solid-state NMR spectroscopy and suitable DFT calculations. "No-bond" indirect (31)P(31)P 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 PP 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.