Diborane Reductions of CO 2 and CS 2 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 )Cu 2 (μ-Ph)][NTf 2 ]; [NTf 2 ] = bis(trifluoromethane)sulfonimide, 6 , [(PNNP Flu )Cu 2 (μ-CCPh)][NTf 2 ], and 3 , [(PNNP Flu )Cu 2 (μ-O t Bu)][NTf 2 ]. 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 CO 2 and CS 2 , 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 )Cu 2 (μ-Bcat)(CNXyl)][NTf 2 ], demonstrating that the lower coordination number at copper is chemically significant. Reactions of 4 and 5 with CO 2 yielded the corresponding dicopper borate complexes ( 8 , [(PNNP Flu )Cu 2 (μ-OBcat)][NTf 2 ]; 9 , [(PNNP Flu )Cu 2 (μ-OBpin)][NTf 2 ]), with 4 demonstrating catalytic reduction in the presence of excess diborane. Related reactions of 4 and 5 with CS 2 provided insertion products 10 , {[(PNNP Flu )Cu 2 ] 2 [μ-S 2 C(Bcat) 2 ]}[NTf 2 ] 2 , and 11 , [(PNNP Flu )Cu 2 (μ,κ 2 -S 2 CBpin)][NTf 2 ], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO 2 insertion intermediate.