The 1 H NMR spectrum of the isolated unknown product (Figure ) bears the hallmarks of a crowded interlocked structure; many proton environments attributable to the axle and macrocycle components are shifted to lower ppm relative to their non-interlocked counterparts (Figure and Figure ). In keeping with the mass ion observed by MS, the solid obtained was deep-red, consistent with presence of bipyridine-ligated Cu I ions, and the 1 H spectrum contains two sets of resonances attributable to the macrocycle component, the sum of which co-integrate with signals attributable to the axle in a 2 : 1 ratio. Furthermore, many well-dispersed signals attributable to the macrocycle ethylene glycol moieties were observed, suggesting that the faces of the two macrocycles are well-differentiated, in contrast with non-interlocked macrocycle 1, [2]rotaxane 4 (Figure ) and [Cu(4H)]PF6. Ultimately, the mystery product was identified unambiguously by single crystal x-ray diffraction (SCXRD) analysis as [3]rotaxane-based complex [Cu a Cu b (5)]PF6 in which the two Cu I centres are distinguishable with one metal ion bound in a "normal" tetrahedral neutral 4N chelate and the other identifiable as a Cu I -triazolide (Figure ). The isolation of [Cu a Cu b (5)]PF6 is unexpected for several reasons. Firstly, although both the 4N chelate and Cu(I)-triazolide 10 coordination motifs have been identified previously in interlocked molecules, the former most famously in Sauvage's ground-breaking passive template synthesis of catenanes but also in the solid-state structure of [Cu(4H)]PF6 (Figure ), the reactive organometallic moiety is expected to be rapidly decomposed in the presence of strong acids such as TFA , and both Cu centers are expected to be rapidly removed in the presence of cyanide anions. Secondly, the space filling representation of [Cu a Cu b (5)]PF6 highlights that the structure is exceptionally congested, even compared with [Cu(4H)]PF6, because the axle component is extremely short; it is very surprising that two macrocycles can be accommodated on this axle, let alone that such a congested structure would be a major product, raising the question of how it is formed. )]PF6 suggests it is formed via a bimetallic pathway in which two macrocycle-bound Cu I ions are involved in the mechanical bond forming step. Indeed, [3]rotaxane products have previously been observed in the AT-CuAAC reactions of larger macrocycles . Enticingly, this suggests that [Cu a Cu b (5)]PF6 represents the first direct observation of a trapped bimetallic intermediate of this important "click" reaction, for the discovery of which Sharpless and Meldal recently received the Nobel Prize alongside Bertozzi 32 ; although the CuAAC reaction has been proposed, based on kinetic measurements , to be bimetallic in Cu I , which was supported by subsequent indirect mechanistic observations , as well as through computational modelling , to our knowledge no direct observation of such a species has been reported. We note that heating demetallated ligand 5H (vide infra) under the reaction conditions did not lead to [Cu a Cu b (5)]PF6, consistent with the proposal that this species is a trapped intermediate of the reaction, rather than a thermodynamically stable product of coordination and C-H metalation of the free ligand.