Main-chain organometallic polymers comprising redox-active iron(II) centers connected by ditopic N-heterocyclic carbenes

Main-chain organometallic polymers were synthesized from bimetallic iron(II) complexes containing a ditopic N-heterocyclic carbene (NHC) ligand [(cp)(CO)LFe(NHCNHC)Fe(cp)(CO)L]X₂ (where NHCNHC represents a bridging dicarbene ligand, L = I⁻ or CO). Addition of a diimine ligand such as pyrazine or 4,4′-bipyridine, interconnected these bimetallic complexes and gave the corresponding co-polymers containing iron centers that are alternately linked by a dicarbene and a diimine ligand. Diimine coordination depended on the wingtip groups at the carbene ligands and was accomplished either by photolytic activation of a carbonyl ligand from the cationic [Fe(cp)(NHC)(CO)₂]⁺ precursor (alkyl wingtips) or by AgBF₄-mediated halide abstraction from the neutral complex [FeI(cp)(NHC)(CO)] (mesityl wingtips). Remarkably, the polymeric materials were substantially more stable than the related bimetallic model complexes. Electrochemical analyses indicated metal–metal interactions in the pyrazine-containing polymers, whereas in 4,4′-bipyridine-linked systems the metal centers were electronically decoupled.