The linking of the cyclopentadienyl rings of ferrocene by a short ansa [n] bridge gives rise to a broad class of strained organometallic rings, which incorporate the ferrocenyl fragment and a large number of possible bridging components. The resulting [n]ferrocenophanes exhibit fascinating structures and reactivity, and interesting comparisons can be drawn between these species and the familiar strained cyclic organic molecules. Providing the bridging moiety is sufficiently short as to induce strain, [n]ferrocenophanes have the propensity to undergo ring-opening reactions, in many cases to yield high-molecular-weight polyferrocenes with a range of interesting properties. This review aims to summarize the recent advances in the field, focusing on the preparation, structural characterization, electronic structure, and unusual reactivity of strained ferrocenophanes.

General rightsThis document is made available in accordance with publisher policies. Please cite only the published version using the reference above. AbstractInteractions between metal ions and ligands in metal-containing polymers involve two bonding extremes: persistent covalent bonding, where the polymers are essentially static in nature, or labile coordination bonding, which leads to dynamic supramolecular materials.Main chain polymetallocenes based on ferrocene and cobaltocene fall into the former category due to the presence of strong metal-cyclopentadienyl bonds. Herein we describe a main chain polynickelocene formed by ring-opening polymerization of a moderately strained[3]nickelocenophane monomer, that can be switched between static and dynamic states as a result of the relatively weak Ni-cyclopentadienyl ligand interactions. This is illustrated by the observation that, at low concentration or at elevated temperature in a coordinating or polar solvent, depolymerization of the polynickelocene occurs. A study of this dynamic polymermonomer equilibrium by 1 H NMR spectroscopy allowed for determination of the associated thermodynamic parameters. Microrheology data, however, indicated that under similar conditions the polynickelocene is considered to be static on the shorter, rheological timescale.2

We report the synthesis, reactivity studies, and ring-opening polymerization of a tricarba[3]nickelocenophane. The resulting green polynickelocene (5) possesses a -(CH2)3- spacer between the nickelocene units and is shown to be of high molecular weight. SQUID magnetometry measurements indicate that 5 is a macromolecular material with an S = 1 repeat unit.

In order to gain insight into the mechanism for the thermal ring-opening polymerization of strained dicarba [2]ferrocenophanes, the thermal reactivity of selected examples of these species with different substitution patterns has been explored. When heated at 300 C dicarba[2]ferrocenophanes meso/rac-[Fe(h 5 -C 5 H 4 ) 2 (CHPh) 2 ] (meso/rac-7) and meso-[Fe(h 5 -C 5 H 4 ) 2 (CHCy) 2 ] (meso-13) were found to isomerize or to undergo disproportionation, respectively. These processes are apparently general for dicarba [2]ferrocenophanes with one or more non-hydrogen substituents at each carbon atom in the dicarba bridge and both appear to involve homolytic cleavage of the C-C bond in the bridge as a key step. In striking contrast, derivatives containing either one or no non-hydrogen substituents on the bridge such as {Fe 17) undergo thermal ringopening polymerization (ROP) under similar conditions (300 C, 1 h). Thus, thermolysis of 15 yielded polyferrocenylethylene {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph)CH 2 ]} n (16a) with a broad molecular weight distribution (M w ¼ 13,760, PDI ¼ 3.27). Analysis of 16a by MALDI-TOF mass spectrometry suggested that the material was macrocyclic. Thermal treatment of linear polyferrocenylethylenes {Fe[h 5 -C 5 H 4 ] 2 [CH(Ph) CH 2 ]} n with narrow molecular weight distributions (prepared by photocontrolled ROP) at 300 C confirmed that the macrocycles detected form directly, and not as a result of depolymerization.Copolymerizations of 15 with 17 and of 15 with the deuterated species [Fe(h 5 -C 5 H 4 ) 2 (CD 2 ) 2 ] (d 4 -17) were conducted in order to probe the bond cleavage mechanism. Comparative NMR spectroscopic analysis of the resulting copolymers 18 and d 4 -18, respectively, and of homopolymer 16a, indicated that thermal ROP does not occur via a homolytic C-C bridge cleavage mechanism. A series of thermolysis experiments were conducted with MgCp 2 (Cp ¼ h 5 -C 5 H 5 ) at 300 C, which resulted in the isolation of ring-opened species formed from 15 and 17, and indicated that the Fe-Cp bonds can be cleaved under the thermal ROP conditions employed. The studies indicated that a chain growth process that involves heterolytic Fe-Cp bond cleavage in the monomers is the most probable mechanism for the thermal ROP of dicarba[2]ferrocenophanes.

Sila[1]ferrocenophanes bearing bulky nitrogen- and silicon-based substituents at the ansa bridge have been prepared by nucleophilic substitution of chloride at silicon in Fe(η5-C5H4)2­SiClH (3) by Li[N(SiMe3)2] and K[Si(SiMe3)3]. The resulting compounds, Fe(η5-C5H4)2­Si[N­(SiMe3)2]H (4) and Fe(η5-C5H4)2­Si[Si­(SiMe3)3]H (5), have been fully characterized, and their ring-opening polymerization (ROP) chemistry has been explored. Upon heating for 3 h, 4 yields the polymer [(η5-C5H4)­Fe(η5-C5H4)Si­{N(SiMe3)2}H] n (M n = 38 000 Da, PDI = 7.5), but 5 does not react due to the larger steric bulk of the substituent on silicon. ROP initiated by Na[C5H5] under photolytic conditions involving Fe−Cp bond cleavage occurred for both 4 and 5, to afford (η5-C5H5)­Fe(η5-C5H4)­[SiRH(η5-C5H4)­Fe(η5-C5H4)] n−1­SiRH(η1-C5H5) (R = N(SiMe3)2: n = 20 and 60; R = Si(SiMe3)3: n = 20 and 50), but anionic ROP initiated with nBuLi was unsuccessful, presumably due to the close proximity of the bulky group to the site of nucleophilic attack. The polymeric materials were fully characterized, including the elucidation of atactic microstructures by NMR spectroscopy.