π-Complexes Derived from Non-classical Diboriranes: Side-on vs. End-on Carbonylative Ring Expansion

Area of Science:

• Organometallic Chemistry
• Boron Chemistry
• Coordination Chemistry

Background:

• Diboriranes, B2C analogues of cyclopropanes, exhibit non-classical Hückel-aromatic structures with bridging moieties.
• The coordination chemistry of diboriranes to transition metals, particularly their cyclic π-systems, remains largely unexplored.
• Understanding the reactivity of these unique boron-containing rings is crucial for expanding synthetic methodologies.

Purpose of the Study:

• To investigate the coordination of non-classical diboriranes to transition metal fragments.
• To explore the carbonylative ring expansion reactions of diboriranes upon complexation.
• To elucidate the influence of the bridging moiety on the reaction pathway and product formation.

Main Methods:

• Synthesis and characterization of non-classical diboriranes (cyclo-μ-RB2Dur2CPh, R=H, SnMe3).
• Complexation of diboriranes with iron carbonyl fragments (Fe(CO)3).
• Spectroscopic analysis (NMR, X-ray crystallography) to determine the structures of reaction products.

Main Results:

• Successful complexation of H-bridged and SnMe3-bridged diboriranes to Fe(CO)3 fragments.
• The H-bridged diborirane undergoes side-on ring expansion to a five-membered B2C2O ring via formal hydroboration of CO.
• The SnMe3-bridged diborirane exhibits end-on CO insertion into the B-B bond, leading to a different ring expansion product.
• Products are characterized as nido and closo clusters according to Wade-Mingos rules.

Conclusions:

• Transition metal coordination enables novel carbonylative ring expansion of diboriranes.
• The nature of the bridging substituent dictates the regioselectivity and mechanism of CO insertion.
• This work expands the known reactivity of diboriranes and introduces new boron-containing metallacycles.