Unusual 1‐Alkyne Dimerization/Hydrogenation Sequences Catalyzed by [Ir(H) 2 (NCCH 3 ) 3 (P‐ i ‐Pr 3 )]BF 4 : Evidence for Homogeneous‐Like Mechanism in Imidazolium Salts

The reaction of complex [Ir(H) 2 (NCCH 3 ) 3 (P‐ i ‐Pr 3 )]BF 4 ( 1 ) with excess of 1‐alkynes such as t ‐BuC≡CH and PhC≡CH gave the butadiene compounds Ir[η 4 ‐(R) 2 C 4 H 4 ](NCCH 3 ) 2 (P‐ i ‐Pr 3 )}BF 4 (R= t ‐Bu, 5 ; R=Ph, 6 ). Compound 5 was obtained as a single isomer containing a 1,3‐disubstituted butadiene ligand, whereas 6 was formed as a 7:1 mixture of isomers containing 1,3‐ and 1,4‐disubstituted butadienes, respectively. Spectroscopic observations showed alkenyl hydride reaction intermediates, consistent with a double insertion/C‐C coupling sequence. Complexes 5 and 6 were found to react with dihydrogen to give 1 and alkenes resulting from the partial hydrogenation of the butadiene moieties. This dimerization/hydrogenation sequence has been found to be the major reaction of t ‐BuC≡CH under conditions of homogeneous hydrogenation whereas that of PhC≡CH produced styrene and ethylbenzene as major products. Similar selectivity was observed for these hydrogenations using organic/ionic liquid biphasic conditions with toluene/BMIM⋅BF 4 , suggesting reaction mechanisms similar to those operating under homogeneous conditions. This conclusion is also supported by the spectroscopic observation of alkenyl hydride intermediates during the formation of 6 in BMIM⋅BF 4 as solvent.