Multisite Amino-Allylidene Ligands from Thermal CO Elimination in Diiron Complexes and Catalytic Activity in Hydroboration Reactions

The new diiron(I) complexes [Fe2Cp2(mu-CO){mu-k3C,kN-C(R1)C(R2)C(CN)NMe(R)}], 3 a-o (R=alkyl or 4-C6H4OMe; R1=alkyl, aryl, ferrocenyl (Fc), thiophenyl, CO2Me or SiMe3; R2=H, Me or CO2Me) were synthesized in moderate to high yields from the thermal decarbonylation of the bis-carbonyl precursors 2 a-o, and were structurally characterized by IR and NMR spectroscopy, and single crystal X-ray diffraction in four cases. Electrochemical behavior of one complex was also investigated. Complexes 3 a-o comprise a highly functionalized, multisite amino-(cyano)allylidene ligand, including metal coordination of a tertiary amine group. Selected complexes displayed negligible to moderate catalytic activity in CO2/propylene oxide coupling, working under ambient conditions. Additionally, they were investigated as catalysts for the conversion of benzaldehyde to the corresponding borate ester 6 a, using pinacolborane (HBpin) as the borylating agent. Most complexes achieved good conversions at room temperature with 1 % catalyst loading, and data highlight the significant influence of the multisite ligand substituents on the catalytic performance. Notably, complex 3 m (featuring R=4-methoxyphenyl, R1=Fc, R2=H) displayed the highest activity and effectively catalyzed the hydroboration of various aldehydes and ketones. A plausible mechanistic cycle involves metal coordination of the carbonyl substrate, its activation being possibly facilitated by intramolecular interactions.