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Low Valent Iron Isocyanide Complexes: Multinuclear Mimics of Metal Surfaces and Mononuclear Analogues of Classical Organometallic Systems


This dissertation details the utilization of anionic, nucleophilic iron complexes supported by m-terphenyl isocyanide and carbonyl ligands to address longstanding questions in organometallic and inorganic chemistry. Chapter 1 offers a brief account of the development of low valent transition-metal chemistry with carbon monoxide (CO) and isocyanides in mononuclear and multinuclear systems. Metal carbonyl clusters gained popularity as molecular surrogates for reactive sites on heterogeneous catalyst surfaces, and some successes and shortcomings of this cluster-surface analogy are revisited in Chapter 2. The synthesis and reactivity of the tetra-iron nitrido cluster [Fe(µ4-N)(CO)8(CNArMes2)4]– (ArMes2 = 2,6-(2,4,6-Me3C6H2)2C6H3) is contrasted with the less electron-rich all-carbonyl congener [Fe(µ4-N)(CO)12]–. Ligand substitution is shown to impart nucleophilicity to the interstitial nitride, and this characteristic enables rational cluster expansion with main-group and transition-metal ions to yield unsaturated sites. The resulting clusters were found to display surface-like reactivity through coordination-sphere-dependent atom rearrangement and metal-metal cooperativity.

The remaining three chapters stem from K2[Fe(CO)2(CNArTripp2)2] (ArTripp2 = 2,6-(2,4,6-(i-Pr)3C6H2)2C6H3). In Chapter 3, this metalate is used to generate Fe(BF)(CO)2(CNArTripp2)2, the first stable terminal fluoroborylene complex. Importantly, the isoelectronic species Fe(N2)(CO)2(CNArTripp2)2 and Fe(CO)3(CNArTripp2)2 are also described allowing for the direct comparison of neutral 10 valence-electron ligands. Single-crystal X-ray diffraction, nuclear magnetic resonance, infrared, and Mössbauer spectroscopic studies demonstrate that the terminal BF ligand possesses particularly strong -donor and -acceptor properties in accord with theoretical predictions. Density functional theory and electron-density topology calculations support this conclusion. The reactivity of Fe(BF)(CO)2(CNArTripp2)2 is discussed in Chapter 4. Like all terminal borylene ligands, coordinated BF is shown to be electrophilic at boron, forming Lewis acid-base adducts with various nucleophiles. However, the fluoroborylene ligand can be derivatized further than other borylenes and converted stepwise into aminoborylene and iminoboryl moieties. Additionally, BF can be transformed directly to the oxoboryl anion [BO]–. The last chapter presents efforts toward an analogue of the unsaturated binary metal carbonyl Fe(CO)4. Unusual solvent binding and bond activations suggest that Fe(N2)(CO)2(CNArTripp2)2 may indeed serve as a masked functional analogue of Fe(CO)4.

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