C−H Bond Activation of Thiophenes by Ir Complexes of the Hydrotris(3,5-dimethylpyrazolyl)borate Ligand, TpMe2

Abstract

The bis(ethylene) derivative TpMe2Ir(C2H4)2 (1) reacts with thiophene, 2-methylthiophene, and 3-methylthiophene with formation of TpMe2Ir(2-thienyl)2(SC4H3R) (R = H, 4; R = Me, 7, 8) species in which the two thienyl moieties are the result of the α-C−H bond activation of the thiophenes. PMe3 and CO adducts of formulation TpMe2Ir(2-thienyl)2(L) are readily synthesized from the corresponding S-bonded thiophene derivatives. The structure of the CO complex TpMe2Ir(2-SC4H3)2(CO) (6) has been confirmed by a single-crystal X-ray analysis. NMR spectroscopic studies are in accord with these adducts existing in solution as mixtures of three rotameric species that arise from restricted rotation around the Ir−thienyl bonds. Hydrogenation of complexes 4, 7, and 8 under relatively mild conditions gives the corresponding TpMe2IrH2(SC4H3R) dihydrides. These last mentioned species experience interesting Ir−H and C−H(thienyl) deuteration exchanges with C6D6 as well as a complex decomposition reaction that gives mainly dimeric species with an unusual C,S-μ2-η1-η1 bridging ligand. Mechanistic proposals for these two processes are presented. Monothienyl Ir(III) species are also accessible; for example the complex TpMe2IrH(2-SC4H3)(SC4H4) (19) can be obtained from TpMe2IrH2(C2H4) and thiophene. Finally, the thermal activation of SC4H4, 2- and 3-SC4H3Me, and 2,5-SC4H2Me2 by the 2,3-dimethylbutadiene−Ir(I) complex TpMe2(η4-CH2C(Me)C(Me)CH2) has been investigated. Thiophene and the monomethylthiophenes give the complexes 4, 7, and 8, whereas for 2,5-SC4H2Me2, the reaction takes a different course and affords a hydride-thienyl derivative along with the E and Z isomers of a thienyl-substituted olefin.