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Iron Carbide-Sulfide Carbonyl Clusters.

ABSTRACT: Described is the preparation of the first iron carbide-sulfides. The cluster [Fe6C(CO)15(SO2)]2- ([2]2-), which is generated quantitatively from [Fe6C(CO)16]2- ([1]2-), was O-methylated to give the sulfinite [2Me]-. Demethoxylation of [2Me]- with BF3 gave the face-capped octahedral cluster Fe6C(CO)15(SO) (3). In solution, 3 spontaneously converted to the sulfide Fe6C(CO)16(S) (4), an edge-fused double cluster with Fe5C and Fe3S subunits. Although 4 undergoes 1e- reduction reversibly, 2e- reduction (or base hydrolysis) of 4 gives closo-[Fe6C(CO)14(S)]2- ([5]2-). The synthetic entries into the Fe6CS x manifold may underpin the preparation of active-site analogues of the FeMoco and FeVco cofactors.

SUBMITTER: Liu L

PROVIDER: S-EPMC6602809 | biostudies-literature | 2019 Jul

REPOSITORIES: biostudies-literature

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Iron Carbide-Sulfide Carbonyl Clusters.

Liu Liang L Rauchfuss Thomas B TB Woods Toby J TJ

Inorganic chemistry 20190611 13

Described is the preparation of the first iron carbide-sulfides. The cluster [Fe6C(CO)15(SO2)]2- ([2]2-), which is generated quantitatively from [Fe6C(CO)16]2- ([1]2-), was O-methylated to give the sulfinite [2Me]-. Demethoxylation of [2Me]- with BF3 gave the face-capped octahedral cluster Fe6C(CO)15(SO) (3). In solution, 3 spontaneously conver ...[more]

PMID: 31184487

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Project description:The present study employs a suite of spectroscopic techniques to evaluate the electronic and bonding characteristics of the interstitial carbide in a set of iron-carbonyl-carbide clusters, one of which is substituted with a molybdenum atom. The M6C and M5C clusters are the dianions (Et4N)2[Fe6(μ6-C)(μ2-CO)2(CO)14] (1), [K(benzo-18-crown-6)]2[Fe5(μ5-C)(μ2-CO)1(CO)13] (2), and [K(benzo-18-crown-6)]2[Fe5Mo(μ6-C)(μ2-CO)2(CO)15] (3). Because 1 and 2 have the same overall cluster charge (2-) but different numbers of iron sites (1: 6 sites → 2: 5 sites), the metal atoms of 2 are formally oxidized compared to those in 1. Despite this, Mössbauer studies indicate that the iron sites in 2 possess significantly greater electron density (lower spectroscopic oxidation state) compared with those in 1. Iron K-edge X-ray absorption and valence-to-core X-ray emission spectroscopy measurements, paired with density functional theory spectral calculations, revealed the presence of significant metal-to-metal and carbide 2p-based character in the filled valence and low-lying unfilled electronic manifolds. In all of the above experiments, the presence of the molybdenum atom in 3 (Fe5Mo) results in somewhat unremarkable spectroscopic properties that are essentially a "hybrid" of 1 (Fe6) and 2 (Fe5). The overall electronic portrait that emerges illustrates that the central inorganic carbide ligand is essential for distributing charge and maximizing electronic communication throughout the cluster. It is evident that the carbide coordination environment is quite flexible and adaptive: it can drastically modify the covalency of individual Fe-C bonds based on local structural changes and redox manipulation of the clusters. In light of these findings, our data and calculations suggest a potential role for the central carbon atom in FeMoco, which likely performs a similar function in order to maintain cluster integrity through multiple redox and ligand binding events.

Project description:The syntheses, interconversions, and spectroscopic properties of a set of iron carbonyl clusters containing an interstitial carbide are reported. This includes the low temperature X-ray structures of the six-iron clusters (Y)2[Fe6(?6-C)(?2-CO)4(CO)12] (1a-c; where Y = NMe4, NEt4, PPh4); the five-iron cluster [Fe5(?5-C)(CO)15] (3); and the novel formulation of the five-iron cluster (NMe4)2[Fe5(?5-C)(?2-CO)(CO)13] (4). Also included in this set is the novel charge-neutral cluster, [Fe6(?6-C)(CO)18] (2), for which we were unable to obtain a crystallographic structure. As synthetic proof for the identity of 2, we performed a closed loop of interconversions within a family of crystallographically defined species (1, 3, and 4): [Fe6]2- ? [Fe6]0 ? [Fe5]0 ? [Fe5]2- ? [Fe6]2-. The structural, spectroscopic, and electronic properties of this "missing link" cluster 2 were investigated by IR, Raman, XPS, and Mössbauer spectroscopies-as well as by DFT calculations. A single ?CO feature (1965 cm-1) in the IR spectrum of 2, as well as a prominent Raman feature (?symm = 1550 cm-1), are consistent with the presence of terminal carbonyls and a {(?6-C)Fe6} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p3/2 feature (707.4 eV) as compared to that of 1 (705.5 eV), consistent with the two-electron oxidation induced by treatment of 1 with two equivalents of [Fc](PF6) under CO atmosphere (for the two added CO ligands). DFT calculations indicate two axial and four equatorial Fe sites in 1, all of which have the same or similar oxidation states, for example, two Fe(0) and four Fe(+0.5). These assignments are supported by Mössbauer spectra for 1, which exhibit two closely spaced quadrupole doublets with ? = 0.076 and 0.064 mm s-1. The high-field Mössbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with ? = -0.18, -0.11, and +0.41 mm s-1. This indicates three pairs of chemically equivalent Fe sites. The first two pairs arise from irons of a similar oxidation state, while the last pair arises from irons in a different oxidation state, indicating a mixed-valent cluster. Variable field Mössbauer spectra for 2 were simulated assuming these two groups and a diamagnetic ground state. Taken together, the Mössbauer results and DFT calculations for 2 indicate two axial Fe(II) sites and four equatorial sites of lower valence, probably Fe(0). In the DFT optimized pentagonal bipyramidal structure for 2, the Fe(II)-Ccarbide distances are compressed (?1.84 Å), while the Fe(0)-Ccarbide distances are elongated (?2.05 Å). Analysis of the formulations for 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered in the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent intracluster disproportionation of Fe oxidation states is concluded to arise from changes in bonding to the central carbide. A similar phenomenon may be promoted by the central carbide of the FeMoco cluster of nitrogenase, which may in turn stimulate N2 reduction.

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Iron Carbide-Sulfide Carbonyl Clusters.

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Iron Carbide-Sulfide Carbonyl Clusters.

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