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

Title Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.
Authors S. Kuppuswamy; J.D. Wofford; C. Joseph; Z.L. Xie; A.K. Ali; V.M. Lynch; P.A. Lindahl; M.J. Rose
Journal Inorg Chem
DOI 10.1021/acs.inorgchem.7b00741
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(?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.

Citation S. Kuppuswamy; J.D. Wofford; C. Joseph; Z.L. Xie; A.K. Ali; V.M. Lynch; P.A. Lindahl; M.J. Rose.Structures, Interconversions, and Spectroscopy of Iron Carbonyl Clusters with an Interstitial Carbide: Localized Metal Center Reduction by Overall Cluster Oxidation.. Inorg Chem. 2017;56(10):59986012. doi:10.1021/acs.inorgchem.7b00741

Related Elements

Iron

See more Iron products. Iron (atomic symbol: Fe, atomic number: 26) is a Block D, Group 8, Period 4 element with an atomic weight of 55.845. The number of electrons in each of Iron's shells is 2, 8, 14, 2 and its electron configuration is [Ar] 3d6 4s2. Iron Bohr ModelThe iron atom has a radius of 126 pm and a Van der Waals radius of 194 pm. Iron was discovered by humans before 5000 BC. In its elemental form, iron has a lustrous grayish metallic appearance. Iron is the fourth most common element in the Earth's crust and the most common element by mass forming the earth as a whole. Iron is rarely found as a free element, since it tends to oxidize easily; it is usually found in minerals such as magnetite, hematite, goethite, limonite, or siderite.Elemental Iron Though pure iron is typically soft, the addition of carbon creates the alloy known as steel, which is significantly stronger.

Related Forms & Applications