Isomerism and dynamic behavior of bridging phosphaalkynes bound to a dicopper complex

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu(2)(μ-η(1):η(1)-MeCN)DPFN](NTf(2))(2), was treated with phosphaalkynes (RC[triple bond, length as m-dash]P, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η(2):η(2) binding coordination modes. X-ray crystallography revealed that these unusual “tilted” structures exist in two isomeric forms (R “up” vs. R “sideways”), depending on the steric profile of the phosphaalkyne's alkyl group (R = Me, Ad, or (t)Bu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and (t)Bu (up). With intermediate steric bulk (R = Ad), the energy difference between the two geometries is small enough that both are observed in solution, and NMR spectroscopy and computations indicate that the solid-state structure corresponds to the minor isomer observed in solution. Meanwhile, treatment of [Cu(2)(μ-η(1):η(1)-MeCN)DPFN](NTf(2))(2) with 2-butyne affords [Cu(2)(μ-η(2):η(2)-(MeC[triple bond, length as m-dash]CMe))DPFN](NTf(2))(2): its similar ligand geometry demonstrates that the tilted μ-η(2):η(2) binding mode is not limited to phosphaalkynes but reflects a more general trend, which can be rationalized via an NBO analysis showing maximization of π-backbonding.