Structures and properties of copper alkene complexes. preorganization effects and the binding of different isomers of cyclododecatriene to copper triflate

The single-crystal X-ray structures of the copper 1,5,9-cyclododecatriene complexes (cis,cis,trans-C ₁₂H ₁₈)Cu(OTf), cct-1, (cis,trans,trans-C ₁₂H ₁₈)Cu(OTf), ctt-1, and (trans,trans,trans-C ₁₂H ₁₈)Cu(OTf), ttt-1, have been determined. In all three compounds, the triflate ligand and all three C=C double bonds of the triene are bound to the copper center, which adopts a distorted tetrahedral coordination geometry. The Cu-C distances are 2.190-2.217Å and the Cu-O distances are 2.096-2.135Å. The complex ctt-1 is a nonelectrolyte in nitromethane. The ^lH and ¹³C NMR spectra of ctt-1 and ttt-1 show that the two faces of the trienes, which are inequivalent in the solid state structure, are equivalent in solution even at - 70 °C, suggesting that these molecules undergo a dynamic exchange process that is rapid on the NMR time scale (the two faces of cct-C ₁₂H ₁₈ are inequivalent even for the free triene, and so dynamic processes in its copper complex are not so easily observed). Most likely, the dynamic processes in ctt-1 and ttt-1 involve movement of the triflate ligand between one "side" of the molecule and the other by means of a dissociative process, although dissociation of one or more double bonds of the alkene from the copper center, followed by an intramolecular rearrangement, cannot be ruled out. ⁶³Cu NMR chemical shifts are also reported. Competition studies in toluene show that the free energy of ctt-1 is about 1 kcal mol ^-1 lower (i.e., more stable) than expected from the relative energies of the free trienes. This stabilization can be attributed to a preorganization effect.