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

Abstract

A dicopper complex featuring a symmetrically bridging nitrile ligand and supported by a binucleating naphthyridine-based ligand, [Cu₂(μ-η¹:η¹-MeCN)DPFN](NTf₂)₂, was treated with phosphaalkynes (RCP, isoelectronic analogues of nitriles) to yield dicopper complexes that exhibit phosphaalkynes in rare μ-η²:η² 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 ^tBu). Only one isomer is observed in both solution and the solid state for R = Me (sideways) and ^tBu (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 [Cu₂(μ-η¹:η¹-MeCN)DPFN](NTf₂)₂ with 2-butyne affords [Cu₂(μ-η²:η²-(MeCCMe))DPFN](NTf₂)₂: its similar ligand geometry demonstrates that the tilted μ-η²:η² 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.