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-C5Me5)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(η2-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(SiMe3)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.