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The entrance channel of bimolecular reactions sometimes involves the formation of outer complexes as weakly bound, fleeting intermediates. Here, we characterize such an outer complex in a system that models the bimolecular, C-O bond–forming reaction of a phosphine oxide Lewis base with a carbenium Lewis acid. Crystallographic studies show that the C-O distance in the outer form exceeds that of the final or inner adduct by 1.1 angstroms. As the system samples the two forms of the complex, which correspond to minima on the corresponding potential energy surface, the C-O linkage switches from a secondary interaction in the outer complex to a dative bond in the inner complex. This phenomenon is harnessed as a functional feature to stabilize xanthylium-based photoredox catalysts. 

We describe a novel gold chloride complex supported by an ambiphilic phosphine/xanthylium ligand in which the AuCl moiety interacts with the π + surface of the xanthylium unit as indicated by structural studies. Energy decomposition analyses carried out on a model system indicates the prevalence of non-covalent interactions in which the electrostatic and dispersion terms cumulatively dominate. The presence of these AuCl–π + interactions correlates with the high catalytic activity of this complex in the cyclisation of 2-(phenylethynyl)phenylboronic acid, N -propargyl- t -butylamide, and 2-allyl-2-(2-propynyl)malonate. Comparison with the significantly less active acridinium and the 9-oxa-10-boraanthracene analogues reinforces this conclusion. 

Abstract With the discovery of late transition metal platforms that support clean photoreductive halogen eliminations, we now describe an indazol‐3‐ylidene gold trichloride complex ([7]⁺) decorated at the 4‐position by a xanthylium unit. This orange complex features a low energy band in the visible part of the spectrum, assigned to the charge transfer excitation of the indazol‐3‐ylidene/xanthylium donor/acceptor dyad. Green‐light irradiation of this complex in the presence of a chlorine trap elicits the clean photoelimination of chlorine radicals, producing the corresponding gold(I) complex. This visible‐light‐induced photoreduction is very efficient, reaching quantum yields close to 10 %. A neutral analog of [7]⁺featuring an anthryl group rather than a xanthylium unit proved to be perfectly photostable, supporting the importance of the xanthylium‐based photoredox unit present in [7]⁺. 

Abstract Our efforts in the chemistry of gold complexes featuring ambiphilic phosphine‐carbenium L/Z‐type ligand have led us to consider the reduction of the carbenium moiety as a means to modulate the gold–carbenium interaction present in these complexes. Here, it was shown that the one‐electron reduction of [(o‐Ph₂P(C₆H₄)Acr)AuCl]⁺(Acr=9‐N‐methylacridinium) produces a neutral stable radical, the structure of which showed a marked increase in the Au–Acr distance. Related structural changes were observed for the phosphine oxide analogue [(o‐Ph₂P(O)(C₆H₄)Acr]⁺, the reduction of which interfered with the P=O→carbenium interaction. These structural effects, driven by a reduction‐induced change in the electronic and electrostatic characteristics of the compounds, showed that the charge and accepting properties of the carbenium unit can be modulated. These results highlight the redox‐noninnocence of carbenium Z‐type ligand, a feature that can be exploited to induce specific conformational changes.