Gd13Fe10C13: Indications of Fe–Fe Multiple Bonding Emerging from Chemical Frustration

Flux Synthesis of a Metal Carbide Hydride Using Anthracene As a Reactant

La₁₅(FeC₆)₄H was synthesized from the reaction of iron and anthracene in La/Ni eutectic flux. Anthracene was the source of both the carbon and hydrogen in the product. The structure of this metal carbide hydride features hydride ions in tetrahedral interstitial sites surrounded by lanthanum ions, which was confirmed by single-crystal neutron diffraction studies. The trigonal planar FeC₆ units in which the central iron atom is coordinated by three ethylenide groups are similar to those found in La_3.67FeC₆, a previously reported compound that is formed in the absence of a hydride source. Magnetic susceptibility data confirm that the iron sites do not have magnetic moments. Density of states calculations indicate that La₁₅(FeC₆)₄H is metallic and is stabilized by the incorporation of hydride anions.

Pr62Fe21M16C32 Versus Pr21Fe8M7'C12 (M = Si, P; M' = Si, Ge, Sn): Competing Intermetallic Carbides Grown from a Pr/Ni Flux

Reactions of silicon, carbon, and iron in a low-melting flux mixture of praseodymium and nickel produced two competing intermetallic compounds. Pr₆₂Fe₂₁Si₁₆C₃₂ has a new structure type in tetragonal space group P4/ mmm ( a = 15.584(2) Å, c = 11.330(1) Å, Z = 1) that features trigonal planar FeC₃ units that share corners to form a framework of cylindrical channels encompassing a network of silicon-centered praseodymium clusters. Slight variation of reactant ratio and heating profile produced Pr₂₁Fe₈Si₇C₁₂ instead; this compound has the previously reported cubic La₂₁Fe₈Sn₇C₁₂ structure type. Identical Pr/Si clusters and FeC₃ subunit motifs are found in both structure types. In addition to reactant ratio and heating profile, size effects play a role in determining which structure forms. Replacing silicon with smaller phosphorus atoms produces only the tetragonal structure; replacement with larger elements (M = Ge, Sn) yields only cubic Pr₂₁Fe₈M₇C₁₂. Magnetic susceptibility measurements on single crystals of Pr₆₂Fe₂₁Si₁₆C₃₂ indicate antiferromagnetic ordering of the Pr moments below 17 K and no magnetic moment on iron atoms. The behavior of Pr₂₁Fe₈Si₇C₁₂ is more complex, revealing magnetic contributions from both Pr and Fe atoms and possible spin frustration.

π-Conjugation in Gd13Fe10C13 and its oxycarbide: unexpected connections between complex carbides and simple organic molecules

Carbometalates are a diverse family of solid state structures formed from transition metal (TM)-carbon polyanionic frameworks whose charges are balanced by rare earth (RE) cations. Remarkable structural features, such as transition metal clusters, are often encountered in these phases, and a pressing challenge is to explain how such features emerge from the competing interaction types (RE-TM, TM-TM, TM-C, etc.) in these systems. In this Article, we describe a joint experimental and theoretical investigation of two compounds, Gd13Fe10C13 and its oxycarbide Gd13Fe10C(13-x)O(x) (x ≈ 1), which add a new dimension to the structural chemistry of carbometalates: π-conjugation through both TM-C and TM-TM multiple bonds. The crystal structures of both compounds are built from layers of Fe-centered Gd prisms stacked along c and surrounded by an Fe-C network, and differ chiefly in the stacking sequence of these layers. The phases' identical local structures have two types of Fe environment: trigonal planar FeC3 sites and H-shaped Fe2C4 sites, with unusually short Fe-Fe and Fe-C bonds. (57)Fe Mössbauer spectroscopy and DFT-calibrated Hückel calculations on Gd13Fe10C13 build a picture of covalent Fe-C σ bonds and conjugated π systems for which Lewis structures can be drawn. Using the reversed approximation Molecular Orbital approach, we can draw isolobal analogies between the Fe centers of this compound and molecular TM complexes: 18-electron configurations could be achieved through σ and π bonds with 18 electrons/Fe for the FeC3 site and 18-n (n = 2 for an Fe═Fe double bond) electrons/Fe for the Fe2C4 site. In this way, the vision of a unified bonding scheme of carbometalates and organometallics proffered by earlier studies is realized in a visual manner, directly from the 1-electron wave functions of the Hückel model. The bonding analysis predicts that Gd13Fe10C13 is one electron/formula unit short of an ideal electron count, explaining the tendency of the system toward a small degree of oxygen substitution. Analogies between the π bonding in Gd13Fe10C13 and that of the allyl anion help rationalize the presence of trigonal planar Fe and linear C units in the structure. The isolobal analogy between Gd13Fe10C13 and an 18-electron coordination complex is expected to apply to carbometalates as a whole, and will be extended to other examples in our future work.