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

A dinucleating 1,8-naphthyridine ligand featuring fluorene-9,9-diyl-linked phosphino side arms (PNNP^Flu) was synthesized and used to obtain the cationic dicopper complexes 2, [(PNNP^Flu)Cu₂(μ-Ph)][NTf₂]; [NTf₂] = bis(trifluoromethane)sulfonimide, 6, [(PNNP^Flu)Cu₂(μ-CCPh)][NTf₂], and 3, [(PNNP^Flu)Cu₂(μ-O^tBu)][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) that are more reactive in C(sp)-H bond activations and toward activations of CO₂ and CS₂, compared to dicopper μ-boryl complexes supported by a 1,8-naphthyridine-based ligand with di(pyridyl) side arms. The solid-state structures and DFT analysis indicate that the higher reactivities 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, [(PNNP^Flu)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, [(PNNP^Flu)Cu₂(μ-OBcat)][NTf₂]; 9, [(PNNP^Flu)Cu₂(μ-OBpin)][NTf₂]), with 4 demonstrating catalytic reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS₂ provided insertion products 10, {[(PNNP^Flu)Cu₂]₂[μ-S₂C(Bcat)₂]}[NTf₂]₂, and 11, [(PNNP^Flu)Cu₂(μ,κ²-S₂CBpin)][NTf₂], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO₂ insertion intermediate.