Metalized Borylene in Boron-Gold Carbonyl Complexes: Infrared Spectra and Theoretical Calculations.

Borylenes (:B-R), built on a single B-R bond between boron and another nonmetallic atom or group, are a heated subject of special interest due to their intriguing transition-metal-mimicking reactivity, but the relative lack of understanding for the electronic structure and chemical bonding of transition metal borides leads to lingering neglect of metalized borylenes (:B-M) based on covalent B-M bonding. Here we use infrared photodissociation spectroscopy in combination with density functional calculations to study the geometric structure and chemical bonding of boron-gold carbonyl complex cations. The structure and bonding analyses demonstrated that the BAu(CO)3+ and BAu2(CO)4+ complexes can be described as bis-carbonyl-trapped borylene adducts. While the metal-rich BAu3(CO)4+ complex represents an unusual multicenter-bond-stabilized borylene cation with excellent σ-acidity and π-backbonding capability for CO activation, featuring Cs symmetry with a quasi-T-shaped BAu3+ core. It is manifested that BAu3+ presents greater amphoteric reactivity and improved stability compared to BAu1,2+ due to the presence of the three-center-two-electron Au-B-Au bond. This study discloses a conceptually new platform for accessing reactive metalized borylenes by exploiting the boron-mediated multicenter-bond stabilization strategy and using more bench-stable and ubiquitous metal carbonyl fragments as starting materials, thus providing a broader opportunity for the design of novel chemical structures and catalysts.