Synthesis, characterization and crystal structures of rhenium(V) complexeswith diphosphines

Two cationic complexes containing a Re-v(O)(2)(+) core have been prepared b y substitution on the precursor [Re(O)(2)(PPh3)(2)I]. Careful control of re action conditions allowed the synthesis of [Re(O)(2)L-2]I (L = bis(1,2-diph enylylphosphino)ethane (dppe), bis(1,3-diphenylphosphino)propane (dppp)) co mpounds in high yield. Complexes were characterized by UV-Vis and IR spectr ophotometry, elemental analyses, FAB, H-1, C-13{H-1} and P-31{H-1} NMR spec tra and high-performance liquid chromatography. Four compounds have also be en characterized by single-crystal X-ray analyses. Cations consist of a six -coordinated rhenium atom bonded to two tr ans-oxo groups. The phosphines l ie on the equatorial plane, almost perpendicular to the Re-v(O)(2)(+) core. The average value for the Re-O bond is 1.77 Angstrom, similar to that in o ther Re(V) dioxo complexes. Diphosphine ligands form five- and six-membered rings in which lambda delta conformation is adopted. The unexpected [Re(O) (OCH,)(dppp)(CI)2] CH,OH has been additionally characterized from X-ray dat a. It is also an octahedral complex with a methoxy ligand trans to an oxo g roup. Species are very stable towards redox processes and can be protonated in acidic media (70:30 dioxane:water, v/v) to yield [Re(OH)(2)L-2](3+). Th e protonation constants of the diphosphine ligands are also obtained for th e first time. (C) 1999 Elsevier Science S.A. All rights reserved.