Intramolecular C-H Activation Directed Self-Assembly of an Organoplatinum(II) Molecular Square Shu-Bin Zhao, Rui-Yao Wang, and Suning Wang* Department of Chemistry, Queen’s UniVersity, Kingston, Ontario K7L 3N6, Canada Received January 14, 2007; E-mail: wangs@chem.queensu.ca Pd II or Pt II nanoscopic macrocycles and cages are of current interest because of their great potential in guest-host chemistry and catalysis. 1-4 The commonly used strategy for assembling Pd II or Pt II macrocycles or cages is the use of “linker” ligands such as polypyridines or polyphosphines via ligand substitution reactions. For example, Stang and co-workers have explored extensively the syntheses of many molecular polygons and polyhedra by using preassembled Pd II /Pt II acceptor units and appropriate linkers. 3 Fujita and co-workers have demonstrated the use of highly effective guest- templated self-assembly methods to achieve elegant Pd II /Pt II nano- cages. 4 The majority of previously reported Pd II or Pt II molecular assemblies are ionic, but several examples of neutral Pt II molecular macrocycles are known. 5 One attractive but hardly explored approach is the use of a chelate ligand capable of undergoing chelate ring opening and the subsequent intramolecular C-H activation, hence creating a new binding site, as an internal linker for the formation of Pt II macrocycles and cages. Ring opening process without C-H activation for polymerization of ferrocene derivatives has been successfully and extensively explored by Manners and co-workers. 6 The feasibility of using ring opening and C-H activation processes for assembling polymeric organoplatinum compounds was dem- onstrated by Young and co-workers. 7 They observed that, upon heating, the 2,2′-bipy ligand in Pt(2,2′-bipy)Ph 2 undergoes a “roll- over” C3 C-H cyclometalation, resulting in the elimination of benzene, and the successive self-assembly of insoluble Pt polymers via the freed N donor site and the Pt acceptor site. 7 However, due to the geometry of 2,2′-bipy, it is not possible to use it and the derivatives to achieve cyclic Pt(II) structures without using external linkers. Although intramolecular cyclometalation involving C-H bond activation is a fairly common phenomenon among Pd or Pt compounds, 8 using this process in a controlled manner to construct molecular macrocycles and cages has hardly been explored. We report herein a rare example of spontaneous self-assembly by a mononuclear complex Pt(NPA)(CH 3 ) 2 (1), NPA ) N-(2′-pyridyl)- 7-azaindole, 9 into an organoplatinum Pt 4 macrocycle Pt 4 (N,C,N- NPA) 4 (CH 3 ) 4 (2) via a roll-over cyclometalation driven self- assembly process at ambient temperature. Complex 1 was prepared by the reaction of [PtMe 2 (μ-SMe 2 )] 2 10 with NPA in THF at -10 °C. 1 is stable in the solid state at ambient temperature or in solution below 5 °C. It is characterized by NMR spectroscopy in CD 2 Cl 2 at -20 °C. In either THF, CH 2 Cl 2 , or benzene solution, 1 undergoes clean transformation to produce 2 at ambient temperature, which has been fully characterized by NMR, elemental analysis, and X-ray diffraction. As shown by the diagrams in Figure 1, 2 has a highly rigid cyclic Pt 4 structure. Through an anionic “N,C,N” tridentate mode, the NPA ligand exhibits a dual role both as a chelating ligand and as a bridging ligand. The four Pt subunits in 2 are related by an approximate S 4 axis. The four Pt atoms are not coplanar with a butterfly shape. The two diagonal Pt‚‚‚Pt distances are 8.34 and 8.85 Å, while the four edge Pt‚‚‚Pt distances are 6.14, 6.26, 6.15, and 6.25 Å. To minimize the steric interactions between neighboring py groups, the 7-azaindolyl (azain) N atom shows large distortion from linear coordination to the Pt center. The average deviation from linearity as defined by the angle between the Pt-N (azain) vector and the corresponding N‚‚‚C (para) vector is ∼30°. Accordingly, the Pt-N (azain) distances, 2.135 Å on average, are much longer than those of Pt-N (py), 2.108 Å on average. The py ring and the azain ring in each NPA ligand are out of coplanarity by a dihedral angle of ∼13.5° on average. Interestingly, as shown in Figure 1, the internal cavity in 2 has a distinct tetrahedral shape, defined by the four py groups. Our earlier investigation on Zn(NPA)(O 2 CR) 2 complexes 9 es- tablished that the NPA ligand is a poor N,N-chelating ligand and has a strong tendency to dissociate from the metal center in solution due to the chelate ring strain and the interactions between the ortho- hydrogen atoms on the py ring and the azain ring in the complex. We believe that the same poor stability of the N,N-chelate mode of NPA in 1 drives the facile roll-over cyclometalation process. Indeed, the crystal structure of the analogue compound Pt(NPA)- Ph 2 (3) (Figure 2) shows that the py and the azain ring have a dihedral angle of 22.4° and a short separation distance of 2.02 Å between the two ortho-hydrogen atoms, much less than the sum of van der Waals radii (2.40 Å). 11 In addition, the formation of a non- strained five-membered N,C-chelate ring in the product may also facilitate the 1 to 2 transformation. Figure 1. Left: the molecular structure of 2 with 50% ellipsoids. Right: space-filling drawing showing the internal cage formed by the four pyridyl groups. Figure 2. Left: the molecular structure of 3. Right: the molecular structure of Pt(N,C,N-NPA)(CH3)(CH3CN), E-MeCN. Both structures are shown with 50% thermal ellipsoids. Published on Web 02/27/2007 3092 9 J. AM. CHEM. SOC. 2007, 129, 3092-3093 10.1021/ja0702770 CCC: $37.00 © 2007 American Chemical Society