Polymer Rings and Chains Consisting of Doubly Silyl-Bridged Metallocenes

With the formation of novel organometallic macromolecules in mind, the polycondensation of transition metal ions and bridged cyclopentadienyl ligands was studied. To this end solvated salts MX2 (M=Fe, Ni, and Cr) were treated with a ligand that consisted of two doubly silyl‐bridged cyclopentadienyl anions. For M=Fe and diluted solutions a series of rings ○i was obtained that consisted of a minimum of six (○6) and up to 17 (○17) ferrocene moieties in the backbone. They were separated partly by medium pressure liquid chromatography. The macrocycles were established by high‐resolution MALDI‐TOF mass spectroscopy which also yielded the molecular weight, the polydispersion, and the mean ring size, χn, of the mixture of reaction products. When the reaction temperature was decreased from 25 °C to −20 °C, χn increased from 8.1 to 10.8. Ferrocene‐containing chains, Λj, with 2≤j≤12 were obtained in addition to rings in the presence of water; the terminal groups were cyclopentadiene moieties. The reaction of two ferrocene‐fused cyclopentadienyl anions with [FeCl2(thf)1.5] gave chains consisting of exclusively uneven numbers of ferrocenes. For M=Ni and Cr the formation of doubly silyl‐bridged nickelocenes and chromocenes was proven by NMR spectroscopy. MALDI‐TOF mass spectroscopy showed nickelocene‐containing chains accompanied by some rings. For M=Fe the H,H‐DQF COSY spectra established the structure of ○7, ○8, and ○9. The oxidation of the ferrocene‐containing ring ○7 with I2, NOPF6, and AgPF6 gave ionic species [○7]n+ which suffered from low stability. The ring‐closing reaction is discussed, and the relative abundance of the various rings is related to MNDO calculations. The polycondensation of two doubly silyl‐bridged cyclopentadienyl anions with FeCl2, NiCl2, and CrCl2 yields rings and chains. Depending on the conditions, the most abundant ring contains seven or ten (shown here) ferrocenes.