Formation of a chelate bis(phosphino)[3]ferrocenophane ligand and its use in palladium-catalyzed alternating CO/ethene copolymerization

Treatment of 1,1'-diacetylferrocene (1) with dimethylamine, catalyzed by titanium tetrachloride, proceeded in a Mannich-type C-C coupling reaction to yield the unsaturated [3]ferrocenophane 2. Subsequent catalytic hydrogenation gave the saturated [3]ferrocenophane derivative [(C5H4CHMeCH2CH(NMe2)C5H4)Fe] (trans-3). Directed Cp metalation of trans-3 was carried out by treatment with n-BuLi, followed by reaction with ClPPh2 to yield the ortho-phosphorylated P,N-[3]ferrocenophane chelate ligand 4. Its treatment with HPPh2 in acetic acid resulted in a clean substitution of the -NMe2 group by -PPh2 with overall stereochemical retention to yield the P,P-[3]ferrocenophane ligand 5. The stereochemical pathway of such substitution reactions at the [3]ferrocenophane framework, which proceed cleanly by a double-inversion route with participation of the metal center, was modeled by computational chemistry and its reactive cationic intermediate 14 characterized by a DFT calculation. Reaction of 4 with (cod)PdCl2 gave the chelate complex 6. The analogous reaction of 5 with (cod)PdCl2 furnished the P,P-[3]ferrocenophane-PdCl2 chelate complex 7. Both the complexes 6 and 7 and the ligand 5 were characterized by X-ray diffraction. Treatment of 7 with AgBF4 in dichloromethane or alternatively of 5 with palladium acetate, followed by BF3 addition, gave very active catalyst systems for alternating carbon monoxide/ethene copolymerization.