Dicopper(I) trefoil knots: Topological and structural effects on the demetalation rates and mechanism

The cyanide-assisted demetalation kinetics of four molecular dicopper(I) trefoil knots, related face-to-face constitutional isomers and of a double-stranded helical knot's precursor, have been investigated by single wavelength and multiwavelength absorption spectrophotometry in order to gain insight into the particular properties induced by interlacing four 1,10-phenanthroline chelating units. The nonentangled face-to-face and helical dinuclear complexes are demetalated according to a two consecutive, bimolecular rate-limiting step mechanism. In each case, both coordination sites are structurally equivalent and dissociate in an almost statistical way, reflecting weak interactions between them. In contrast, the knotted topology is responsible for a strong coupling as evidenced by the one and two rate-limiting step processes found for the methylene- and phenylene-bridged knots, respectively. This result has been rationalized in terms of molecular rearrangements of the knotted structure following the release of the first copper(I) cation. Hence, the coordination sphere becomes either more accessible to the entering cyanide anion and the mononuclear intermediate dissociates in a fast second step or highly protecting, leading to an inert mononuclear species in the case of the phenylene-bridged knot. The 9 orders of magnitude span of the second-order demetalation rate constants reflects the versatile structural parameters around the copper(I) center. They are primarily affected by the steric hindrance of the substituents in the 2,9-positions of the phenanthroline nuclei and by the topology of the complex. Compared with their face-to-face analogues, the methylene-bridged knots are about 100 times more inert as a result of the helicoidal arrangement of the internal spacer in the core of the molecule. Furthermore, the kinetic parameters an sensitive to even more subtle structural variations, like the length of the internal and external spacers.