Abstract
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.
Highlights
Small-molecule 3d−5d metal clusters that encapsulate light atoms have been of interest to synthetic and catalytic chemistry since their discovery in 1962.1 Metal−carbonyl and metalcarbonyl-carbide clusters have been utilized as catalysts in carbon monoxide oxidation, decarbonylation, and hydrogenation reactions
While the spectroscopic interpretation of iron-carbonyl clusters has historically been dominated by the influence of redoxadaptive π back-bonding interactions between iron and the surrounding CO ligand manifold, we have shown the significant influence of the interstitial carbide and its ability to significantly alter electronic and structural environments.[51−53] In these iron-carbonyl carbide clusters, the extreme σ- and πdonating carbido character takes precedence over these backbonding effects, and the C4− moiety becomes the most dominant force in modulating the cluster’s overall electronic structure
This study evaluated the electronic and bonding flexibility of the interstitial carbon atom in a series of M6C and M5C ironcarbonyl carbide clusters
Summary
Small-molecule 3d−5d metal clusters that encapsulate light atoms have been of interest to synthetic and catalytic chemistry since their discovery in 1962.1 Metal−carbonyl and metalcarbonyl-carbide clusters (metal = Ir, Rh, Os, Re, Fe) have been utilized as catalysts in carbon monoxide oxidation, decarbonylation, and hydrogenation reactions. Most notable are the 5 and 6 atom clusters of iron, in square pyramidal and octahedral geometries, respectively, which allow for almost modular substitution with Rh and Co atoms.[1,15−17] Recently, Rose and co-workers reported the crystal structure of a substituted six Fe octahedron, in which one axial iron is replaced with molybdenum.[18−20] Rauchfuss et al recently published the first report of a synthetic multinuclear iron-carbonyl cluster containing both a totally inorganic carbide and sulfide ligand.[21].
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