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 <b>2</b>, [(PNNP^Flu)Cu₂(μ-Ph)][NTf₂]; [NTf₂] = bis(trifluoromethane)sulfonimide, <b>6</b>, [(PNNP^Flu)Cu₂(μ-CCPh)][NTf₂], and <b>3</b>, [(PNNP^Flu)Cu₂(μ-O^<i>t</i>Bu)][NTf₂]. Complex <b>3</b> reacted with diboranes to afford dicopper μ-boryl species (<b>4</b>, with μ-Bcat; cat = catecholate and <b>5</b>, 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 <b>4</b> and <b>5</b> 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 <b>4</b> gave <b>7</b>, [(PNNP^Flu)Cu₂(μ-Bcat)(CNXyl)][NTf₂], demonstrating that the lower coordination number at copper is chemically significant. Reactions of <b>4</b> and <b>5</b> with CO₂ yielded the corresponding dicopper borate complexes (<b>8</b>, [(PNNP^Flu)Cu₂(μ-OBcat)][NTf₂]; <b>9</b>, [(PNNP^Flu)Cu₂(μ-OBpin)][NTf₂]), with <b>4</b> demonstrating catalytic reduction in the presence of excess diborane. Related reactions of <b>4</b> and <b>5</b> with CS₂ provided insertion products <b>10</b>, {[(PNNP^Flu)Cu₂]₂[μ-S₂C(Bcat)₂]}[NTf₂]₂, and <b>11</b>, [(PNNP^Flu)Cu₂(μ,κ²-S₂CBpin)][NTf₂], respectively. These products feature Cu-S-C-B linkages analogous to those of proposed CO₂ insertion intermediate.