Electrically conductive [Fe4S4]-based organometallic polymers

Abstract

Tailoring the molecular components of hybrid organic-inorganic materials enables precise control over their electronic properties. Designing electrically conductive coordination materials, e.g. metal-organic frameworks (MOFs), has relied on single-metal nodes because the metal-oxo clusters present in the vast majority of MOFs are not suitable for electrical conduction due to their localized electron orbitals. Therefore, the development of metal-cluster nodes with delocalized bonding would greatly expand the structural and electrochemical tunability of conductive materials. Whereas the cuboidal [Fe₄S₄] cluster is a ubiquitous cofactor for electron transport in biological systems, few electrically conductive artificial materials employ the [Fe₄S₄] cluster as a building unit due to the lack of suitable bridging linkers. In this work, we bridge the [Fe₄S₄] clusters with ditopic N-heterocyclic carbene (NHC) linkers through charge-delocalized Fe-C bonds that enhance electronic communication between the clusters. [Fe₄S₄Cl₂(ditopic NHC)] exhibits a high electrical conductivity of 1 mS cm^-1 at 25 °C, surpassing the conductivity of related but less covalent materials. These results highlight that synthetic control over individual bonds is critical to the design of long-range behavior in semiconductors.

Fig. 1. Schematic representation of the [Fe₄S₄] and NHC molecules and materials related to this work. (a) [Fe₄S₄] clusters in the biological system. Crystal structure of respiratory enzyme I containing [Fe₄S₄]. (b) Biomimetic, discrete [Fe₄S₄] cluster with NHC capping ligands. (c) Electrically conductive mononuclear metal–NHC polymers. (d) Formation of [Fe₄S₄]–NHC polymer (1) via organometallic bond bridging.

Fig. 2. Structural characterizations on 1. (a) FT-IR spectra of 1 (red) and Me–BBI–I (black) under N₂. (b) SEM-EDX mapping images of 1 for Fe (blue) and S (green). (c) PDF profiles of 1 and the simulated profile from the model structure. The intra-cluster atom–atom correlations, (i) directly bonding Fe–S, (ii) Fe–Fe, and (iii) diagonal Fe–S, are displayed, respectively.

Fig. 3. Electronic properties of 1. (a) DR and solution-state UV-vis spectra of 1 (red) and (PPh₄)₂(Fe₄S₄Cl₄) in DMF (black). Inset: Tauc plot of 1. (b) LSV profiles of 1 (red) and (PPh₄)₂(Fe₄S₄Cl₄) (black) under N₂ at 25 °C. (c) Plots of the electronic conductivity for [Fe₄S₄]-based materials and molecules. The conductivity was obtained by the pressed-pellet two-probe methods at 25 °C. (d) Calculated SOMOs of the NHC (left) and thiol (right) model structures displayed at isosurface values of 0.02 and 0.05 respectively.

Fig. 4. Temperature-dependent electronic response of 1. (a) VT DR-UV-vis of 1 under vacuum. Inset: the plot of the intensities at 232 nm as a function of temperature. (b) VT electrical conductivity of 1 by four-contact probe, pressed-pellet method. Inset: fitting of the data to the Mott 3D VRH model.