Low-Coordinate W–W Quadruple Bond as an Alkene Surrogate: Divergent [2 + 2 + 2] and [2 + 2 + 1] Cycloadditions with Alkynes

Abstract Although quadruply bonded metal–metal complexes constitute a large and well-established family, their cycloaddition chemistry remains underdeveloped because the quadruple bond is typically confined within a tetragonal, highly ligand-protected coordination environment. Herein, we demonstrate that a low-coordinate amidinate-supported W–W quadruple bond in W2(μ-Cl)(μ-κ2-Cl2Li(THF))[μ-κ2-HC(N-2,6-iPr2C6H3)2]2 behaves as an alkene surrogate in cycloaddition chemistry. Depending on ligand coordination, this W–W quadruple bond engages alkynes in either formal [2 + 2 + 2] or [2 + 2 + 1] pathways. In the absence of ancillary donor ligands, direct reaction with two alkynes proceeds through tail-to-tail [2 + 2 + 2] cycloaddition to afford C4W2 metallacycles, W2Cl2[μ-η2:η2-1,4-R2C4H2][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R = C6H5, 2-MeC6H4, 4-MeOC6H4, and n-C3H7), and a corresponding [2 + 2] intermediate, W2Cl2[μ-η2-2-MeC6H4CCH][μ-κ2-HC(N-2,6-iPr2C6H3)2]2, could be isolated and structurally characterized. In contrast, coordination of σ-donating ligands to the W–W core diverts the reaction pathway by stabilizing vinylidene intermediates, W2Cl2(C5H5N)[μ-η1:η2-CC(H)R1][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 (R1 = 2-MeC6H4, tBu, and SiMe3), which subsequently react with a second alkyne (R2CCH) to furnish C3W2 ring products of the type W2Cl2[μ-η2:η2-1-R2-4-R1H–C4H][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 bearing an exocyclic alkene fragment. In both [2 + 2 + 2] and [2 + 2 + 1] manifolds, while the initial cycloadducts adopt nonplanar conformations, sequential reduction populates the W–W δ manifold and drives planarization via δ-π conjugation between the W2 unit and the organic fragment. Ultimately, the C4W2 systems are transformed into aromatic bimetallic metallabenzenes, [W2(μ-κ2-1,4-R2C4H2)(μ-κ2-HC(N-2,6-iPr2C6H3)2)2] (R = C6H5, 2-MeC6H4, 3-MeC6H4, 4-MeC6H4, 4-MeOC6H4, 4-FC6H4, and n-C3H7), whereas the C3W2 system evolves into a bimetallic fulvene analogue, W2Cl[μ-κ2-1-C6H5-4-tBuH-C4H][μ-κ2-HC(N-2,6-iPr2C6H3)2]2 and W2(μ-H)[μ-κ2-1-C6H5-4-tBuH-C4H][μ-κ3-HC(N-2,6-iPr2C6H3)(N-2-iPr-6-(C3H6)C6H3)][μ-κ2-HC(N-2,6-iPr2C6H3)2]. Taken together, these results expand the scope of cycloaddition partners to include a low-coordinate W–W quadruple bond.