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

The, interconversions, and spectroscopic properties of a set iron carbonyl clusters containing an interstitial carbide are reported. includes the low temperature X-ray structures of the six-iron (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 (NMe₄)₂[Fe₅(μ₅-C)­(μ₂-CO)­(CO)₁₃] (4). Also in this set is the novel charge-neutral cluster, [Fe₆(μ₆-C)­(CO)₁₈] (2), which we were unable to obtain a crystallographic structure. As proof for the identity of 2, we performed a loop of interconversions within a family of crystallographically 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 IR, Raman, XPS, and Mössbauer spectroscopiesas well by DFT calculations. A single νCO feature (1965–¹) in the IR spectrum of 2, as well a prominent Raman feature (νsymm = 1550 cm–¹), are consistent with the presence of terminal carbonyls a {(μ₆-C)­Fe₆} arrangement of iron around the central carbide. The XPS of 2 exhibits higher energy Fe 2p3/2 feature (707.4 eV) as compared that of 1 (705.5 eV), consistent with the two-electron induced by treatment of 1 with two equivalents [Fc]­(PF₆) under CO atmosphere (for the two added CO). DFT calculations indicate two axial and four equatorial sites in 1, all of which have the same or similar states, for example, two Fe(0) and four Fe­(+0.5). These 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 of 2 (4.2 K) exhibits three prominent quadrupole with δ = −0.18, −0.11, and +0.41 mm s⁻¹. This indicates three pairs of chemically equivalent sites. The first two pairs arise from irons of a similar oxidation, while the last pair arises from irons in a different oxidation, indicating a mixed-valent cluster. Variable field Mössbauer for 2 were simulated assuming these two groups a diamagnetic ground state. Taken together, the Mössbauer and DFT calculations for 2 indicate two axial­(II) sites and four equatorial sites of lower valence, probably(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 elongated (∼2.05 Å). Analysis of the formulations 1 (closo-square bipyramid) and 2 (nido-pentagonal bipyramid) is considered the context of the textbook electron-counting rules of 14n+2 and 14n+4 for closo and nido clusters, respectively. This redox-dependent disproportionation of Fe oxidation states is concluded arise from changes in bonding to the central carbide. A similar may be promoted by the central carbide of the FeMoco cluster nitrogenase, which may in turn stimulate N₂ reduction.