Borylene-based functionalization of iron-alkynyl-σ-complexes and stepwise reversible metal-boryl-to-borirene transformation: Synthesis, characterization, and density functional theory studies

Thermally induced chemoselective borylene transfer from [(OC)₅Mo = BN(SiMe₃)₂] (2a) to the carbon-carbon triple bond of an iron dicarbonyl alkynyl complex [(η⁵-C₅Me ₅)Fe(CO)₂C≡CPh] (3) led to the isolation of an iron aminoborirene complex [(η⁵-C₅Me₅)(OC) ₂Fe{μ-BN(SiMe₃)₂C = C}Ph] (4) in satisfactory yield. Room temperature photolysis of 4 resulted in an unprecedented rearrangement and a concurrent decarbonylation, affording the novel C₂ side-on coordinated iron boryl complex [(η⁵- C₅Me₅)(OC)FeBN(SiMe₃)₂(η ²-CC)Ph] (5). Carbonylation of 5 under CO atmosphere at ambient temperature yielded [(η⁵-C₅Me₅)(OC) ₂FeBN(SiMe₃)₂CCPh] (6), which is an isomer of 4. Decarbonylation of 6 at 80 °C led to 5, which could be upon introduction of CO gas further converted into 4 under same conditions. Reaction of 5 with PMe₃ at 80 °C yielded the phosphane complex [(η⁵- C₅Me₅)(OC)(PMe₃)Fe{μ-BN(SiMe ₃)₂C = C}Ph] (7). All above-mentioned iron complexes 4-7 were isolated as air and moisture sensitive crystalline solids in good yields and have been fully characterized in solution and by X-ray crystallography. Quantum chemical calculations using density functional theory (DFT) have been carried out to understand the mechanisms of the experimentally observed reactions and to analyze the bonding situation in the molecules 4-7.