A rigid trans - spanning dinitrile ligand ( trans - spanning ligand / nitrile / bidentate ligand )

A rigid dinitrile ligand was synthesized from two xanthene units condensed to a naphthalene-1,4,5,8diimide spacer. The rigidity and C shape of the ligand gave exclusively trans complexes with Pd(II), Ag(I), and Au(I). Evidence for complexation, coordination geometry, and stoichiometry was provided by a combination of 1H NMR, 19F NMR, and IR spectroscopy. The AuBF4 and PdCl2 complexes were shown to have a 1:1 (metal-to-ligand) stoichiometry and the AgBF4 complex was shown to have a 1:2 stoichiometry in solution. The preorganization of the dinitrile ligand resulted in complexes much more stable than their monodentate counterparts. Bidentate ligands that bind in a trans or rectilinear fashion are very rare (1, 2). The difficulties can be seen in Fig. 1. A framework of sufficient size, rigidity, and convergency is necessary to constrain the chelating functionality in order to eschew adjacent cis sites in favor of more distant trans sites. Only a few trans-spanning ligands have been reported, Transphos being the most cited example (3, 4). Previous trans ligands have all chelated metals through nitrogen (5) or phosphorus (6, 7) atoms of tetrahedral geometry and possess a more open U shape (Fig. la). In contrast, functional groups such as nitriles take on a linear arrangement at nitrogen and necessitate a larger scaffold and more convergent C shape to achieve the same geometries (Fig. lb). Accordingly, multidentate nitriles are commonly oligomeric and polymeric structures and have been successfully used to assemble expansive threedimensional arrays (8, 9). Chelating diand trinitriles are also known, but they invariably bind to the same face of the metal and not to opposing faces (10-13). The recent synthesis of a new family of large rigid molecular frameworks presented us with an irresistible opportunity and unique responsibility to study a trans-dinitrile ligand (14). To our knowledge, 1 is the first example of a trans-chelating dinitrile ligand.