Reactivity Studies on [Cp′FeI]₂: Monomeric Amido, Phenoxo, and Alkyl Complexes

A series of monomeric mono­(cyclopentadienyl) iron amido, phenoxo, and alkyl complexes were synthesized, and their structure and reactivity are presented. The iron­(II) centers in these 14VE one-legged piano stool complexes are high spin (<i>S</i> = 2) in solid state and solution independent of solvent. The silylamide compound [Cp′FeN­(SiMe₃)₂] (<b>2a</b>, Cp′ = 1,2,4-(Me₃C)₃C₅H₂) is an excellent starting material for the reaction with more acidic substrates such as phenols. Sterically encumbered phenols 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H, Me, and <i>t</i>Bu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (<b>4-R</b>) are initially formed. Rearrangement of <b>4-R</b> to the diamagnetic oxocyclohexadienyl complex [Cp′Fe­(η⁵-OC₆H₂R′₂R″)] (<b>5-R</b>) is observed for 2,6-(Me₃C)₂(4-R)­C₆H₂OH (R = H and Me) and the Gibbs free enthalpy of activation (Δ<i>G</i>^⧧) was determined. In contrast this rearrangement is inhibited when the 4-position is blocked by a <i>t</i>Bu group. Removing the steric bulk from the 2,6-positions leads to the formation of a μ-phenoxo dimer, [Cp′Fe­(μ-OC₆H₃<i>t</i>Bu₂-3,5)]₂ (<b>5</b>). Density functional theory (DFT) was used to further elucidate the structure–reactivity relationship in these molecules. The one-legged piano stool anilido complex [Cp′Fe­(NHC₆H₂<i>t</i>Bu₃-2,4,6)] (<b>7</b>) is not accessible via acid–base reaction between <b>2a</b> and H₂NC₆H₂<i>t</i>Bu₃-2,4,6, but can be prepared by conventional salt metathesis reaction from [Cp′FeI]₂ and [Li­(NHC₆H₂<i>t</i>Bu₃-2,4,6)­(OEt₂)]₂. In contrast, reaction of <b>2a</b> with Ph₂NH yields the bimetallic [Cp′Fe­(<i>N,C</i>-κ¹,η⁵-C₆H₅NPh)­Fe­(<i>N</i>-κ¹-NPh₂)­Cp′] (<b>8</b>) which combines two iron centers in the same oxidation state (+2), but different spin-states (<i>S</i> = 0 and <i>S</i> = 2) which is reflected in very different Cp­(cent)–Fe distances of 1.68 and 2.04 Å, respectively. A monomeric iron alkyl half-sandwich complex [Cp′FeCH­(SiMe₃)₂] (<b>9</b>) was prepared that exhibits no reactivity toward H₂, C₂H₄ or N₂O. This behavior might be rationalized by a spin-state induced reaction barrier. However, <b>9</b> reacts in the presence of CO to the iron acyl-complex [Cp′Fe­(CO)₂(C­(O)­CH­(SiMe₃)₂)] (<b>10</b>) and with a CO/H₂ mixture [Cp′Fe­(CO)₂]₂ (<b>11</b>) and CH₂(SiMe₃)₂ are formed