Tautomerization of Pyridine and 2‑Substituted Pyridines to Pyridylidene Ligands by the Iridium(I)–Diene Complex Tp^Me2Ir(η⁴‑CH₂C(Me)C(Me)CH₂)

The complex Tp^Me2Ir­(η⁴-CH₂C­(Me)­C­(Me)CH₂) (<b>3</b>; Tp^Me2 = hydrotris­(3,5<b>-</b>dimethylpyrazolyl)­borate) reacts with pyridines NC₅H₄-2-R (R = H, Me, SiMe₃, F, OMe, NMe₂, C­(O)­Me) in cyclohexane, with formation of Ir­(III) products whose natures depend strongly on the reaction conditions and on the R substituent. The simplest case is for R = NMe₂, C­(O)­Me, where κ²:σ²-but-2-enediyl N–H pyridylidenes, i.e. the result of the metal-promoted tautomerization of the pyridines, are the only species obtained from 60 to 150 °C. For R = Me, F the N-bonded adducts Tp^Me2Ir­(κ²-CH₂C­(Me)C­(Me)­CH₂)­(NC₅H₄-2-R) are formed at 60 °C but, under harsher conditions (120–150 °C), the observed products are, exclusively and respectively, the N–H pyridylidene and a bicyclic carbene compound derived from the formal, trans-stereospecific transfer of the N–H hydrogen of the corresponding (not observed) pyridylidene onto one of the carbons of the CC double bond of the but-2-enediyl moiety, the other experiencing C–N formation. For R = OMe, the N adduct formed at 60 °C transforms, at higher temperatures, into a mixture of the N–H pyridylidene and the bicyclic carbene, with no further evolution. More complex behavior is observed for the rest of the pyridines studied. Thus, when R = SiMe₃, in addition to the expected N–H pyridylidene, two isomeric N–H pyridylidenes containing a κ²:σ²-but-1-enediyl coligand are also formed under kinetic control (60 °C) but with both cleanly transforming into the former compound at higher temperatures. Finally, for R = H only the N adduct is formed under kinetic control at 25 °C but this species transforms almost completely into a mixture of the N–H pyridylidene and two epimeric, N–C bicyclic carbenes after prolonged heating at 150 °C. A detailed study of the temperature-dependent behavior of <b>3</b> in C₆H₆ has also been undertaken, revealing the interesting deuteration of its CH₂ termini by C₆D<sub>6