Recently, we have developed logical approaches to a family of phosphorus-nitrogen-based macrocycles of the type [{P(m-NR)} 2 (m-Y)] n . [1,2] The tetrameric, nitrogen-bridged macrocycle [{P(m-NtBu)} 2 (m-NH)] 4 (Scheme 1 a) is obtained by the condensation of the dimers [ClP(m-NtBu)] 2 and [(NH 2 )P-(m-NtBu)] 2 in the presence of Et 3 N.[3] However, the pentameric homologue is produced exclusively if this reaction is undertaken in the presence of I À ions, resulting in the hostguest complex [{P(m-NtBu)} 2 (m-NH)] 5 I À (Scheme 1 b).[4] The toroidal structures of the tetramer and pentamer and the presence of endo NÀH functionalities combine the appearance and host-guest characteristics of organic macrocycles such as calixarenes and porphyrins.Less progress has been made in attempts to extend this class of new ligands to macrocycles containing other bridging elements, owing largely to the inability to generalize the synthetic methods used for N-bridged species. Indeed, the only representative of this type reported to date is the oxygenbridged dimer [{P(m-N(2-py)} 2 (m-O)] 2 (2-py = 2-pyridyl), obtained by hydrolysis of [ClP{m-N(2-py)}] 2 in the presence of CuCl and pyridine.[5] However, we reported a key synthetic step in the potential extension of this methodology to a broader range of macrocycles by showing that oligomerization can be effected between P 2 N 2 units by the reaction of a deprotonated P(H) = O group with a P À Cl bond (Scheme 2). [6] This reaction can be rationalized by a change from a Pcentered to an O-centered nucleophile. In this sense, the reaction can be seen to involve an intermediate possessing "masked" functionality. However, it should be noted in this regard that the ambiguous nature of species containing [R 2 P = E] À anions (E = O, S, Se, NR, etc.) is well established by previous structural and theoretical studies, as is the fact that the negative charge normally lies primarily over to the side of E. [7] We report herein the success of this approach by showing that the reaction of the dimer [(Cl)(Se=)P(m-NtBu)] 2 (1) with excess sodium metal in toluene at reflux gives the macrocyclic, selenium-bridged hexamer [(Se = )P(m-NtBu) 2 P(m-Se)] 6 (2) as a crystalline product in good yield (45 %) by formal head-to-tail cyclization of the intermediate anion. The formation of the P-Se-P bridge again stems from the ambidentate nature of the intermediate anion (Scheme 3). Evidence of the resulting presence of alternate P III and P V centers within the backbone of 2 is seen in the roomtemperature 31 P{ 1 H} NMR spectrum in toluene, which shows P III (d, d = 206.8 ppm) and P V centers (d, d = 54.4 ppm) that are coupled to each other with a coupling constant that is typical of a P-(m-NR)-P bridge ( 2 J31 P-31 P = 44 Hz). [8] This spectrum can be compared to that of the precursor 1, which shows two singlet resonances in the P V region only, for the cis (d = 22.1 ppm, major) and trans (d = 22.7 ppm, minor) isomers (together with 77 Se satellites attributed to the AA'X isotopomer containing one 77 Se atom...
