Diborane Reductions of CO2 and CS2 Mediated by Dicopper μ-Boryl Complexes of a Robust Bis(Phosphino)-1,8-Napthyridine Ligand

A symmetrical dinucleating 1,8-naphthyridine ligand featuring phosphino side arms linked through fluorene-9,9-diyl moieties (PNNPFlu) was synthesized and used to obtain the cationic organodicopper complexes 2, [(PNNPFlu)Cu2(µ‐Ph)][NTf2]; [NTf2] = bis(trifluoromethane)sulfonimide, and 6, [(PNNPFlu)Cu2(µ‐CCPh)][NTf2], as well as the µ‐tert-butoxide 3, [(PNNPFlu)Cu2(µ‐OtBu)][NTf2]. Complex 3 reacted with diboranes to afford dicopper µ‐boryl species (4, with µ‐Bcat; cat = catecholate and 5, with µ‐Bpin; pin = pinacolate). Complexes 4 and 5 are more reactive in C–H bond activations of terminal alkynes and toward activations of CO2 and CS2, in comparison to dicopper µ‐boryl complexes supported by a 1,8-naphthyridine-based ligand with two di(pyridyl) side arms. The molecular structures (determined by X-ray crystallography) and DFT analysis indicate that the higher reactivity of 4 and 5 relate to changes in the coordination sphere of copper, rather than to perturbations on the Cu–B bonding interactions. Addition of xylyl isocyanide (CNXyl) to 4 gave 7, [(PNNPFlu)Cu2(µ‐Bcat)(CNXyl)][NTf2], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 4 and 5 with CO2 yielded the corresponding dicopper borate complexes (8, [(PNNPFlu)Cu2(µ‐OBcat)][NTf2]; 9, [(PNNPFlu)Cu2(µ‐OBpin)][NTf2]), with 4 demonstrating catalytic CO2 reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS2 provided the insertion products 10, {[(PNNPFlu)Cu2]2[µ‐S2C(Bcat)2]}[NTf2]2 and 11, [(PNNPFlu)Cu2(µ,κ2‐S2CBpin)][NTf2], respectively. These insertion products feature Cu–S–C–B linkages analogous to those of proposed, analogous CO2 insertion intermediates.