Trans-spanning acetylenic bispyridine ligands: synthesis and structural characterization of novel organic and organometallic pseudodehydroannulenes

The synthesis of a novel ferrocene based pincer ligand C5H5-Fe[C5H3(--C5H4N)2] is reported. This pincer ligand was prepared by the Pd-catalyzed reaction of 1,2-diethynylferrocene with 2-iodopyridine. The complex is a trans-spanning ligand and analogous to 1,2-bis(2-pyridinylethynyl)-4,5-dimethoxybenzene. A series of trans-coordinated 1:1 or 2:1 complexes (empirical formulae C51H37ClCuF3N4O7S and C23H18Cl4N2O2Pd) that form from the ligands and (CH3CN)2PdCl2 or Cu(OTf)2 were obtained. These complexes form 13-membered rings. Structurally these complexes resemble a hexadehydro[14]annulene in which two benzene groups are replaced by pyridine rings and one alkyne group is replaced by the co-ordinated metal. All of the trans-spanning complexes were characterized by single crystal X-ray diffraction. Their molecular and solid-state structures are discussed. The Pd-complexes are of interest as potential high activity catalysts for Sonogashira-type couplings of alkynes to aryl halides. The 1,2-(bis-2′-pyridylethyn-1′-yl)ferrocene, was crystallized with PdCl2 to obtain a novel organometallic trans-spanning complex of the formula C24H16Cl2FeN2Pd that was crystallographically characterized.