Mechanism and Roles of Carbonate in Photoinduced Dual Gold-Catalyzed Cyclization <i>gem</i> -Diborylation of Aryl Iodides: Theoretical Investigations

Homobimetallic gold(I) compounds exhibit unique properties in photocatalysis. [Au₂(μ-dppm)₂](OTf)₂ (denoted as [Au-Au](OTf)₂) demonstrates excellent catalytic activity in the carbocyclization/<i>gem</i>-diborylation cascade of aryl iodides, but the photocatalytic mechanism and electronic structure of the active catalyst remain elusive. This study investigates the detailed catalytic mechanism of photoinduced energy transfer reactions, using density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations. Computational results reveal that the coordination of the carbonate ion (CO₃^2-) ligand to [Au₂(μ-dppm)₂]²⁺ ([Au-Au]²⁺) not only shortens the Au-Au bond length, which enhances aurophilic interactions, but also distinctly influences the dinuclear gold architecture. The two structures formed between [Au-Au](OTf)₂ and Na₂CO₃, [Au-Au]CO₃-I (axis coordination) and [Au-Au]CO₃-II (bond coordination), induce a bathochromic shift in the absorption spectrum. This shift matches the emission spectrum of blue LEDs, enabling the efficient sensitization of triplet-state aryl iodides. Sensitization of the aryl iodide substrate by [Au-Au]CO₃-I and [Au-Au]CO₃-II via an energy transfer (EnT) process from their high triplet-state energies leads to homolytic C-I bond cleavage, which enables the radical carbocyclization/<i>gem</i>-diborylation cascade. This study unveils the crucial role of carbonate in modulating the photophysical properties and reactivity of dinuclear gold catalysts, providing important insight into the design of efficient gold-based photocatalysts.