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

Abstract

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.