Model compounds for polymeric redox-switchable hemilabile ligands

The syntheses and characterization of two new tetradentate hemilabile ligands 1,2-bis (2-diphenylphosphinoethoxy) benzene (5)and 2,2'-bis( 2-diphenylphosphinoethoxy)-1,1'-binaphthalene (10) are reported. Ligands 5 and 10 were synthesized as models to test the suitability of specific phosphinoether coordination environments for complexing Rh(I) in high surface area thiophene-based, redox-active polymeric systems. Ligands 5 and 10 react with the product formed from the reaction between (bicycle[2.2.1] hepta-2,5-diene) rhodium(I) chloride dimer and AgBF4 to form [eta(2)-(1,2-bis (2-diphenylphosphinoethoxy) benzene) eta(4)-norbornadiene rhodium(I)] tetrafluoroborate (6) and [ eta(2)(2,2'-bis( 2-diphenylphosphinoethoxy) -1,1'-binaphthalene) eta(4)-norbornadiene rhodium(I)] tetrafluoroborate (II), respectively. Complexes 6 and 11 react with H-2 in CD2Cl2 to form the two new square-planar cis-phosphine, cis-ether Rh(I) complexes 7 and 12, respectively. Compound 7, which could be characterized on the basis of its P-31 NMR spectrum, is extremely reactive and decomposes in CD2Cl2. In THF compounds 6 and 11 react with H-2 to form the dihydride, bis-THF adducts 8 and 16, respectively, which upon removal of solvent form 7 and 12, respectively. Compound 12 is a stable, isolable complex that reacts with acetonitrile to form a cis-phosphine, cis-acetonitrile adduct 15. Removal of solvent from 15 leads to the quantitative reformation of 12. Compound 12 does not react to a detectable extent with gross excesses of benzene or even thiophene, demonstrating the suitability of this ligand environment for implementation into a thiophene-based polymeric system. Compound 12 does catalyze the hydrogenation of cyclohexene to form cyclohexane, and mechanistic implications of such a transformation are discussed.