Synthesis of [2]rotaxane intermediates. The cobalt alkyne complex 1 (Fig. ) was prepared as reported previously . Standard bromination conditions (e.g. NBS/AgNO3) failed to convert 1 to the bromoalkyne 2, probably because they oxidize the Co2(CO)4dppm group. Fortunately, this transformation can be achieved by treating 1 with carbon tetrabromide in the presence of potassium carbonate and 18-crown-6 in THF/methanol. Bromoalkyne 2 is not stable as a solid at room temperature, but it can be handled as a solution. Two phenanthroline-based macrocycles Ma and Mb were chosen for this study . The larger macrocycle Ma often gives higher yields of polyyne rotaxanes in active template coupling reactions , whereas the smaller cavity of Mb is expected to protect the threaded polyyne more effectively. Active metal template Cadiot-Chodkiewicz coupling of a mixture of 1 and 2 in the presence of the macrocycles gave the symmetric [2]rotaxanes 3•Ma/Mb (Fig. ). Single crystal X-ray diffraction studies