SPANphos: Trans-spanning diphosphines as cis chelating ligands!

Several SPANphos ligands based on a spirobichroman backbone, introduced as a putative trans ligand, form compounds of the type [Rh(nbd)(SPANphos)]BF₄ (1–6) in which both norbornadiene and SPANphos act as cis chelating ligands. The cyclooctadiene rhodium chloride derivatives form bimetallic complexes. Crystal structures for several of these compounds and free ligands are reported. Semiemperical AM1 and DFT calculations show that spirobichroman can assume several conformations, some of which are suitable for the formation of cis chelating SPANphos. All calculations on SPANphos complexes of PdCl₂, PtCl₂ and Rh(CO)Cl show that the trans complex is more stable by 4-10 kcal mol^-1. The cis conformation in 1–6 is enforced by the cis chelating norbornadiene ligand.