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A series of pyridine complexes are prepared of the general form TpW(NO)(PMe3)(pyr) where pyr is either pyridine or a substituted pyridine. Depending on substitution pattern, the pyridine can be either N- or eta2-coordinated, and the role of the pyridine substituents and metal oxidation state in determining this equilibrium is explored. For eta2-pyridine complexes, the substituent pattern and solubility characteristics also determine the ratio of coordination diastereomers. Rates of both intra- and interfacial linkage isomerizations are explored along with the pyridine rotational barrier. This study is supported by DFT calculations and X-ray data and includes characterization of both eta2-pyridine and eta2-pyridinium complexes.

Nucleophiles capable of four-electron donation (e.g., anilines, enolates) add to C2 of the N-acetylpyridinium (NAPy) ligand of TpW(NO)(PMe3)(NAPy)(OTf) and induce a Zincke−König-like ring scission to generate 3,4-coordinated acetamidotrienes or 1-azatrienes with 3Z,5E stereochemistry. This ring opening is driven by the enhanced π-acidity of the cyanine, relative to its purported dihydropyridine predecessor, which stabilizes the π-base {TpW(NO)(PMe3)}. X-ray data of the indoline-derived analogue (2) indicates that metal coordination disrupts the donor−acceptor (i.e., amide−iminium) conjugation in the cyanine ligand. Photolysis of 2 in the presence of indoline liberates a further derivatized cyanine.

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The complex TpW(NO)(PMe3)(DMP), where DMP is 2,6-dimethoxypyridine, features an η2-coordinated pyridine ligand that chemically resembles an electron-rich 2-azadiene. As such, it readily reacts with a full range of dienophiles including examples of both alkenes and alkynes to generate azabicyclo[2.2.2]octadiene complexes. Often, one diastereomer spontaneously precipitates from the reaction solution. The bicyclic ligand in these complexes can in some cases be directly removed from the metal by the action of an oxidant (e.g., CAN) and in other cases can be chemically modified prior to its removal. Crystal structures of five tungsten cycloadduct complexes are included. A related complex of 2,6-lutidine is found to react with the nitrile dienophile ethyl cyanoformate to form a new dihapto-coordinated pyridine complex that is derived from a purported diaza[2.2.2]octatriene intermediate.