Synthesis and Mechanism of Formation of Hydride–Sulfide Complexes of Iron

Abstract

Iron-sulfide complexes with hydride ligands provide an experimental precedent for spectroscopically detected hydride species on the iron-sulfur FeMoco of nitrogenase. In this contribution, we expand upon our recent synthesis of the first iron sulfide hydride complex from an iron hydride and a sodium thiolate (Arnet, N. A.; Dugan, T. R.; Menges, F. S.; Mercado, B. Q.; Brennessel, W. W.; Bill, E.; Johnson, M. A.; Holland, P. L., J. Am. Chem. Soc. 2015, 137, 13220–13223). First, we describe the isolation of an analogous iron sulfide hydride with a smaller diketiminate supporting ligand, which benefits from easier preparation of the hydride precursor and easier isolation of the product. Second, we describe mechanistic studies on the C-S bond cleavage through which the iron sulfide hydride product is formed. In a key experiment, use of cyclopropylmethanethiolate as the sulfur precursor leads to products from cyclopropane ring opening, implicating an alkyl radical as an intermediate. Combined with the results of isotopic labeling studies, the data are consistent with a mechanism in which homolytic C-S bond cleavage is followed by rebound of the alkyl radical to abstract a hydrogen atom from the iron to give the observed alkane and iron-sulfide products.

Metal–Sulfur Compounds in N2 Reduction and Nitrogenase-Related Chemistry

“…323 This C−S bond cleavage reaction was suggested to proceed through the generation of a short-lived alkyl radical which abstracts a hydrogen atom from Fe, based on a radical-clock experiment using a cyclopropyl-methanethiolate anion. 324 The hydride ligand in 117 was found to function as a base to deprotonate a terminal alkyne (mtolyl)CCH to furnish a terminal acetylide complex or as a hydride donor to insert into CO 2 to give a bridging formate complex. 323 As anticipated from the coordinative unsaturation in 114, its Fe centers were found to accommodate various N-donor ligands, e.g., hydrazines, ammonia (NH 3 ), and NCMe.…”

“…It should be noted that the Fe I state is unusual in iron–sulfur cluster chemistry and only 116 and [{(PhBP 3 )­Fe} 2 (μ-S)] 2– (PhBP 3 = PhB­(CH 2 PPh 2 ) 3 ) are unambiguously identified examples of sulfide-bridged Fe I 2 complexes, with the excception of NO complexes for which assignment depends on the formalism of the NO ligands. − A relevant Fe II 2 complex with one bridging sulfide and one bridging hydride, [Na]­[(L 1 Fe) 2 (μ-S)­(μ-H)] ( 117 ), was synthesized from the reaction of a hydride complex [L 1 FeH] 2 with sodium dodecanethiolate (NaSC 12 H 25 ), where homolytic cleavage of a C–S bond took place to generate the bridging sulfide . This C–S bond cleavage reaction was suggested to proceed through the generation of a short-lived alkyl radical which abstracts a hydrogen atom from Fe, based on a radical-clock experiment using a cyclopropyl-methanethiolate anion . The hydride ligand in 117 was found to function as a base to deprotonate a terminal alkyne ( m -tolyl)­CCH to furnish a terminal acetylide complex or as a hydride donor to insert into CO 2 to give a bridging formate complex …”

Synthesis and Structure of Thiolate‐Bridged Diiron and Dicobalt Complexes Supported by Modified β‐Diketiminate Ligands

“…Treatment of complex 1 or 2 with strong reductant KC 8 at room temperature facilely generated thiolatebridged diiron complex 3 or dicobalt complex 4 in high yields. Similar to other iron-sulfur clusters supported by β-diketiminate ligands, [16] complexes 3 and 4 are also both paramagnetic species. In their 1 H NMR spectra, a series of obviously broadened signals appear in an unusual range from À 10 to 15 ppm.…”

Reversible Alkyl-Group Migration between Iron and Sulfur in [Fe4S4] Clusters

“…Given that oxidation of [ 1–MP ] + triggers alkyl-group migration from Fe to S, we were interested to learn if the reverse reactionalkyl-group migration from S to Fewould occur upon reduction of [ 2 ] 2+ (Figure ). Such a finding would build on prior work that has established the cleavage of S–C bonds at Fe–thiolate complexes, − though in these examples the organic groups were lost as radicals instead of recombining with the complex to form an Fe–C bond. In the present study, we observed that treatment of [ 2 ]­[PF 6 ] 2 with 1 equiv Cp 2 Co in o -DFB at −35 °C for 5 min resulted in a color change to dark brown.…”