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 (S = 2) in solid state and solution independent of solvent. The silylamide compound [Cp′FeN­(SiMe3)2] (2a, 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 tBu) were investigated. In all cases monomeric iron phenoxo half-sandwich complexes [Cp′FeOR′] (4-R) are initially formed. Rearrangement of 4-R to the diamagnetic oxocyclohexadienyl complex [Cp′Fe­(η5-OC6H2R′2R″)] (5-R) is observed for 2,6-(Me3C)2(4-R)­C6H2OH (R = H and Me) and the Gibbs free enthalpy of activation (ΔG⧧) was determined. In contrast this rearrangement is inhibited when the 4-position is blocked by a tBu group. Removing the steric bulk from the 2,6-positions leads to the formation of a μ-phenoxo dimer, [Cp′Fe­(μ-OC6H3tBu2-3,5)]2 (5). 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(NHC6H2tBu3-2,4,6)] (7) is not accessible via acid–base reaction between 2a and H2NC6H2tBu3-2,4,6, but can be prepared by conventional salt metathesis reaction from [Cp′FeI]2 and [Li­(NHC6H2tBu3-2,4,6)­(OEt2)]2. In contrast, reaction of 2a with Ph2NH yields the bimetallic [Cp′Fe­(N,C-κ1,η5-C6H5NPh)­Fe­(N-κ1-NPh2)­Cp′] (8) which combines two iron centers in the same oxidation state (+2), but different spin-states (S = 0 and S = 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] (9) was prepared that exhibits no reactivity toward H2, C2H4 or N2O. This behavior might be rationalized by a spin-state induced reaction barrier. However, 9 reacts in the presence of CO to the iron acyl-complex [Cp′Fe­(CO)2(C­(O)­CH­(SiMe3)2)] (10) and with a CO/H2 mixture [Cp′Fe­(CO)2]2 (11) and CH2(SiMe3)2 are formed.