Unraveling the Triplet-State-Enabled Phosphine Ligand Isomerization and Orthometalation in Dirhodium Complexes...

Dirhodium(II,II) paddlewheel complexes are central to modern catalysis, yet their rigid bimetallic core restricts ligand coordination to axial sites, severely constraining opportunities for new reactivity and selectivity. Orthometalated complexes with an equatorial coordination mode have been known for a long time, but catalytic applications remain limited owing to harsh synthesis conditions. Herein, we present a comprehensive theoretical framework using DFT and TD-DFT that establishes a predictive model for overcoming this structural rigidity via photochemical activation. Our computational investigations reveal that photoexcitation of dirhodium tetracarboxylates in the presence of phosphines, xantphos and SPhos, and (S)-BINAP drives axial-to-equatorial isomerization followed by orthometalation. We compare the photoexcited pathway to ground-state thermal pathways to understand key differences in reactivity. Molecular orbital analyses reveal that photoexcitation unlocks a triplet-state pathway where α-SOMO and β-SOMO predominantly lie on the antibonding Rh-Rh and Rh-O bonds, effectively lowering the isomerization barrier. Activation Strain Analysis (ASA) elucidates ligand-dependent divergence in orthometalation, explaining why flexible scaffolds like xantphos undergo double orthometalation while rigid chiral ligands like (S)-BINAP are restricted to equatorial geometry. Computationally predicted reactivity is validated by preliminary ³¹P{¹H} NMR experiments. These insights provide a rational blueprint for accessing diverse dirhodium architectures under milder conditions.