A rotational isomerization of diiron complexes of ferrole type involved in the catalytic hydroboration of aldehydes under mild conditions

Abstract

In this work, two new diiron(I) complexes of ferrole or ferracyclopentadiene type, [Fe2(CO)6{RC2(CH2)4C2R}] (R = H, 1; CH2CH3, 2) were synthesized by reacting the [Fe2(CO)9] with diynes (1,7-octadiyne and 3,9-dodecanediyne). The ferrole-type complexes were fully characterized, wherein the absolute structure of 2 was determined by single-crystal X-ray diffraction analysis. Interestingly, a rotational isomerization of the diiron(I) complexes (1 and 2) between the sawhorse conformer and the non-sawhorse conformer is verified, originated from the switch of CO coordination. Additionally, the sawhorse conformer with the terminal CO is dominant in solution due to the tricarbonyl iron units undergo a drastic geometrical turnstile rotation, whereas the non-sawhorse conformer with a semi-bridging CO is rigid in a solid state. Particularly, the isomerism of the diiron(I) complexes of ferrole type leads to a unique catalytic activity for hydroboration of aldehydes with pinacolborane (HBpin). Notably, twelve aldehydes (3a-3l) were exampled to be converted quantitatively into the corresponding organoboronates (4a-4l) mediated by complex 2 under mild conditions. Mechanistically, the 1H NMR and FTIR spectroscopic results proved that the ferrole-type complex 2 bound with the aldehyde to activate the carbonyl of the substrate rather than to cleave the H-B bond of pinacolborane. Collectively, a plausible mechanism for the catalytic hydroboration is proposed, with the rotational isomerization of the diiron(I) complexes involved in.