The synthesis and characterization of a new threefold symmetric hemilabile phosphino-alkylthioether ligand are described. This ligand can be used in combination with Rh(I) and Ir(I) precursor complexes to prepare 40-membered macrocyles with threefold symmetry via the Weak-Link Approach. Synthesis and characterization of two such structures are reported along with a single crystal x-ray diffraction analysis of one of the key intermediates in the case of Rh(I). This is a demonstration of the viability of the Weak-Link Approach for preparing structures other than bimetallic macrocycles and suggests that it could be generalized for a wide range of higher symmetry structures through appropriate hemilabile ligand design. R ecent developments in the field of supramolecular chemistry have resulted in the establishment of several new synthetic strategies for forming multimetallic macrocyclic architectures (1-6). Our group has introduced and focused on developing one of these strategies, which is referred to as the Weak-Link Approach (Scheme 1). This strategy allows one to prepare bimetallic macrocycles from simple transition metal and ligand precursors (7-12), and it is based on reactions between flexible symmetrical hemilable ligands (13-16) and transition metal precursors. Typically, macrocyclic intermediates that contain both strong (S-M) and weak (W-M) ligand-metal bonds (Scheme 1) are initially formed in very high and usually quantitative yield. For the systems studied thus far, ligands that consist of flexible phosphinoalkyl ether (7), thioether (12), or amino (11) bidentate ligand functionalities connected with rigid aromatic spacers (R) have been used. Once condensed, macrocyclic intermediates are formed, the weak ligand-metal bonds in these intermediates can be cleaved by means of simple ligand substitution reactions to afford open macrocyclic ring structures. Alternatively, both condensed and opened macrocycles can be used to construct more complex three-dimensional architectures such as cylinders (8) and trilayer metallocyclophanes (9).Although the generality of the weak-link methodology has been demonstrated with respect to twofold symmetric hemilabile ligands and transition metals, the application of this synthetic approach to the preparation of macrocyclic structures with more than two metals has not been demonstrated. Herein, we report the first application of the Weak-Link Approach to the formation of trimetallic (17-20) rhodium and iridium macrocycles from a flexible threefold symmetric hemilabile ligand.
Methods and MaterialsGeneral Procedures. All reactions were performed under a nitrogen atmosphere in reagent grade solvents by using standard Schlenk or dry-box techniques (21). All other solvents were purified by published methods (22,23). Chemicals were purchased from Aldrich, unless otherwise mentioned, or prepared by literature methods, as referenced below.