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,transC12H18)Cu(OTf), cct-1, (cis,trans,trans-C12H18)Cu(OTf), ctt-1, and (trans,trans,trans-C12H18)Cu(OTf), ttt1, have been determined. In all three compounds, the triflate ligand and all three CdC double bonds of the triene are bound to the copper center, which adopts a distorted tetrahedral coordination geometry. The Cu-C distances are 2.190-2.217 Å and the Cu-O distances are 2.096-2.135 Å. The complex ctt-1 is a nonelectrolyte in nitromethane. The 1H 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-C12H18 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.