Inorganic-organic hybrid Cu-dipyridyl semiconducting polymers based on the redox-active cluster [SFe₃(CO)₉]²⁻: filling the gap in iron carbonyl chalcogenide polymers

The construction of sulfur-incorporated cluster-based coordination polymers was limited and underexplored due to the lack of efficient synthetic routes. Herein, we report facile mechanochemical ways toward a new series of SFe₃(CO)₉-based dipyridyl-Cu polymers by three-component reactions of [Et₄N]₂[SFe₃(CO)₉] ([Et₄N]₂[1]) and [Cu(MeCN)₄][BF₄] with conjugated or conjugation-interrupted dipyridyl ligands, 1,2-bis(4-pyridyl)ethylene (bpee), 1,2-bis(4-pyridyl)ethane (bpea), 4,4′-dipyridyl (dpy), or 1,3-bis(4-pyridyl)propane (bpp), respectively. X-ray analysis showed that bpee-containing 2D polymers demonstrated unique SFe₃(CO)₉ cluster-armed and cluster-one-armed coordination modes via the hypervalent μ₅-S atom. These S-Fe-Cu polymers could undergo flexible structural transformations with the change of cluster bonding modes by grinding with stoichiometric amounts of dipyridyls or 1/[Cu(MeCN)₄]⁺. They exhibited semiconducting behaviors with low energy gaps of 1.55-1.79 eV and good electrical conductivities of 3.26 × 10⁻⁸-1.48 × 10⁻⁶ S cm⁻¹, tuned by the SFe₃(CO)₉ cluster bonding modes accompanied by secondary interactions in the solid state. The electron transport efficiency of these polymers was further elucidated by solid-state packing, X-ray photoelectron spectroscopy (XPS), X-ray absorption near-edge spectroscopy (XANES), density of states (DOS), and crystal orbital Hamilton population (COHP) analysis. Finally, the solid-state electrochemistry of these polymers demonstrated redox-active behaviors with cathodically-shifted patterns compared to that of [Et₄N]₂[1], showing that their efficient electron communication was effectively enhanced by introducing 1 and dipyridyls as hybrid ligands into Cu⁺-containing networks.