Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation

Abstract

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(μ₆-C)(μ₂-CO)₄(CO)₁₂] (1a-c; where Y = NMe₄, NEt₄, PPh₄); the five-iron cluster [Fe₅(μ₅-C)(CO)₁₅] (3); and the novel formulation of the five-iron cluster (NMe₄)₂[Fe₅(μ₅-C)(μ₂-CO)(CO)₁₃] (4). Also included in this set is the novel charge-neutral cluster, [Fe₆(μ₆-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 ν_CO feature (1965 cm^-1) in the IR spectrum of 2, as well as a prominent Raman feature (ν_symm = 1550 cm^-1), are consistent with the presence of terminal carbonyls and a {(μ₆-C)Fe₆} arrangement of iron centers around the central carbide. The XPS of 2 exhibits a higher energy Fe 2p_3/2 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 δ = 0.076 and 0.064 mm s^-1. The high-field Mössbauer spectrum of 2 (4.2 K) exhibits three prominent quadrupole doublets with δ = -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)-C_carbide distances are compressed (∼1.84 Å), while the Fe(0)-C_carbide 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.

Figure 1.

Molecular structures (isotropic view) of the anions in the crystal structures of 1a−c (Y = NMe₄, NEt₄, PPh₄; from left to right, respectively); counter-cations and solvents are omitted for clarity. Full thermal ellipsoid plots are shown in Figures S23−25.

Figure 2.

Thermal ellipsoid plots (30% probability) of the molecular structures of the [Fe₅]⁰ cluster 3 (left) and the [Fe₅]²⁻ cluster 4 (right); counter-cations and solvents are omitted for clarity. Full thermal ellipsoid plots are shown in Figures S26 and S27.

Figure 3.

Solid-state IR spectra of 1a and 2. The feature at 1730 cm⁻¹ in the spectrum of 1a indicates bridging CO ligands.

Figure 4.

Solid-state Raman spectra of 1a, 2, 3, and 4; λ_ex = 532 nm.

Figure 5.

XP spectra for 1a and 2 in the Fe 2p region (left) and Fe 3p region (right). Experimental conditions: solid samples, rt, 1 × 10⁻⁹ Torr, Al Kα X-ray source.

Figure 6.

Mössbauer spectra of 1a (A) and 2 (B) in frozen THF at 5 K and 0.05 T. Solid red lines are composite simulations using parameters given in Table 5. The paramagnetic impurity in cluster 2 (blue line in B) constitutes 16% of the iron in the sample.

Figure 7.

Variable field 4.2 K Mössbauer spectra for a frozen solution of 2 in toluene. Fields were applied perpendicular to the γ radiation. Red lines are composite simulations assuming parameters given in Table 5 and described in the text. The gold and green lines are simulations for the equatorial and axial irons, respectively.

Figure 8.

Molecular structure of the DFT-optimized structure for 1_DFT: (left) complete view of the structure including CO ligands; (right) truncated view of the iron-carbide structure to highlight the [Fe₆(C)]²⁻ core.

Figure 9.

Molecular structure of the DFT-optimized structure for [Fe₆(μ₆-C)(CO)₁₈] (2_DFT): (left) complete view of the structure including CO ligands; (right) truncated view of the iron-carbide structure to highlight the [Fe₆(C)] core.

Scheme 1.

Synthesis of the Starting [Fe₆]²⁻ Cluster 1 (Top Left), Its Neutral Congener [Fe₆]⁰ (2, Top Right), and the Closed Synthetic Loop Including the Crystallographically Defined Five-Iron Clusters [Fe₅]ⁿ (n = 0 or 2−), Shown at Bottom

Scheme 2.

General Formulation and Electron-Counting for Metal−Carbido Clusters of the Closo and Nido Variety Synthesized Herein and Predicted by This Work