Unexpected Synthesis of a Bimetallic [3]Rotaxane Complex. We have previously reported the active template CuAAC (AT-CuAAC) reaction between macrocycle 1 , azide 2 and ethynyl pyridine 3 to give the corresponding rotaxane in good yield after demetallation of the initial product, [Cu(4H)]PF6, with KCN (Figure , conditions i.). However, when the synthesis was repeated at higher temperature (conditions ii.) , we identified a minor biproduct in addition to [Cu(4H)]PF6 by H NMR analysis prior to demetallation. Mass spectrometry (MS) suggested that the unknown product had a mass ion consistent with a structure in which one equivalent of the axle was associated with two equivalents of the macrocycle and two equivalents of Cu (m/z = 1643.7). However, the unknown species persisted when the crude product mixture was treated with either KCN or TFA , conditions that rapidly demetallate [Cu(4H)]PF6 and other interlocked metal complexes. Intrigued by these observations, we varied the reaction conditions to increase the proportion of the mystery compound, which ultimately allowed it to be isolated as the major product of the reaction (Figure , conditions iii.).