Research paper Cyclic tetranuclear iron-carbonyl complex containing thiobisbenzenethiolate ligands: Synthesis and structural characterization Shakeena L. Johnson a , Nikolay N. Gerasimchuk b , Charles A. Mebi a,⇑ a Department of Physical Sciences, Arkansas Tech University, 1701 N. Boulder Ave, Russellville, AR 72801, USA b Missouri State University, Department of Chemistry, 901 S. National Ave, Springfield, MO 65897 USA article info Article history: Received 9 February 2018 Received in revised form 28 March 2018 Accepted 28 March 2018 Available online 29 March 2018 Keywords: Iron clusters Hydrogenase model Iron carbonyls Metallacycle Macromolecule abstract A new macromolecular double-butterfly Fe/S carbonyl cluster, [Fe 2 (CO) 6 ] 2 (l-4,4 0 -thiobisbenzenethio- lato) 2 (1), a model for the active site of [Fe-Fe] hydrogenases, has been prepared and characterized by spectroscopic methods and X-ray crystallography. The molecular structure of 1 contains two units of S 2 Fe 2 (CO) 6 covalently linked to two thiobisbenzene groups to form a 26-membered metallocycle with the benzene substituents on sulfur adopting an ae stereochemistry. The Fe-Fe bond in 1 is 2.5046(5) Å which is close to that of the enzyme and related complexes. Crystals of 1 obtained from dichlormethane solution of [Fe 2 (CO) 6 ] 2 (l-4,4 0 -thiobisbenzenethiolato) 2 contain two dichloromethane molecules trapped in the cavity of 1. Infrared spectrum of 1 has peaks at 2071, 2030, and 1972 cm 1 charateristic of the pres- ence of the Fe 2 (CO) 6 core. Spectroscopic evidence suggest significant electronic interaction between the thiobisbenzene moieties and the S 2 Fe 2 (CO) 6 units in 1. Ó 2018 Elsevier B.V. All rights reserved. 1. Introduction [Fe-Fe] hydrogenase enzymes, identified in microbes, catalyze the reduction of protons as a terminal electron acceptor to yield molecular hydrogen. Crystallographic characterization of the active site of the enzyme revealed the structure shown in Chart 1 (A). The active site is called H-cluster and contains a butterfly Fe2S2 group linked to a chain of Fe4S4 clusters. The Fe2S2 unit is the catalytic center and the Fe4S4 clusters constitute the electron transport chain [1–5]. Synthetic butterfly-shaped Fe/S carbonyl clusters (Chart 1B and C) are of great interest as mimics of [Fe-Fe] hydrogenases. These clusters are structurally similar to the diiron center of the active site of the enzyme. It is hoped that studies of these synthetic assemblies by our group [6–13] and others [14–28], will help guide the design of new materials for hydrogen production. An interesting group of hydrogenase models is cyclic double- butterfly Fe/S carbonyl complexes depicted in Chart 2(A). These cyclic double-butterfly Fe/S carbonyl compounds are polynuclear iron-carbonyl clusters containing two diiron centers linked by two equivalent dithiolate ligands. A search of the literature reveals that the reported cyclic dou- ble-butterfly Fe/S carbonyl complexes contain aliphatic chains as linkers; alkanes [29–31], or functionalized carbon chains contain- ing oxygen [32,33], nitrogen [31,34], sulfur [35], hydroxyl group [36], and organotin groups [37]). In this study, we present the syn- thesis, spectroscopic characterization, and molecular structure of a new cyclic tetranuclear iron-carbonyl complex, [Fe 2 (CO) 6 ] 2 (m-4,4 0 - thiobisbenzenethiolato) 2 (1, Chart 2B). Compound 1 is a cyclic dou- ble-butterfly Fe/S carbonyl complex with the Fe/S units directly linked to two equivalent aromatic thiolate (thiobisbenzenethio- late) ligands. The macrocyclic cluster was constructed using thio- bisbenzenethiolate ligands to examine the mutual influence between the redox-active diiron centers and the bridging arenethi- olate moieties. Non-innocent arenethiolate ligands provide p-elec- tronic systems which influence the stability and electrochemical properties of the [Fe 2 (CO) 6 ] core through electron delocalization [7–14]. 2. Experimental 2.1. General methods The chemicals, Fe 3 (CO) 12 and thiobisbenzenethiol, and organic solvents were obtained from commercial sources and used without further purification. The elemental composition of the complex on https://doi.org/10.1016/j.ica.2018.03.043 0020-1693/Ó 2018 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: cmebi@atu.edu (C.A. Mebi). Inorganica Chimica Acta 477 (2018) 306–311 Contents lists available at ScienceDirect Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica