Reactivity of the Ruthenium Hydride Complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] (HB(mt)3 = hydrotris(methimazolyl)borate): Catalytic Dimerization of 1-Alkynes and Proton Transfer Reactions

Abstract

The complex [Ru{κ3-H,S,S-HB(mt)3}H(PPh3)2] (1, HB(mt)3 = hydrotris(methimazolyl)borate; mt = N-methyl-2-mercaptoimidazolyl) containing a three-center two-electron Ru−H−B bonding interaction has been prepared by reaction of [RuHCl(PPh3)3] with Na[HB(mt)3] in MeOH at 40 °C. This compound, which occurs in solution as a mixture of two stereoisomers (1a and 1b), has been spectroscopically and structurally characterized. 1 reacts over a two-day period with CH2Cl2/MeOH at room temperature, furnishing the methimazolate complex [RuH(mt)(PPh3)3] (2). The latter compound exists in solution as a 55:45 mixture of stereoisomers, and they were structurally characterized. Both 1 and 2 are catalyst precursors for the dimerization of 1-alkynes (toluene, 85 °C, 2% catalyst load) to mixtures of Z, E, and gem-stereoisomers of the corresponding enynes. Compound 1 is much more active and selective than 2. In the presence of benzoic acid, the activity of the catalyst decreases, but the stereoselectivity toward the (Z)-enyne isomer increases. NMR monitoring of the reaction of 1 with PhCOOH in CD2Cl2 in the temperature range −60 to 0 °C has allowed the identification of what seems to be a dihydrogen-bonded complex between the stereoisomer 1b and PhCOOH. This species exhibits a very short minimum longitudinal relaxation time of 5.9 ms. Protonation of 1 with HBF4·OEt2 in CD2Cl2 at −60 °C yields the metastable dihydrogen complex [Ru{κ3-H,S,S-HB(mt)3}(H2)(PPh3)2][BF4] (3a), which loses H2 as the temperature increases, furnishing the complex [Ru{κ3-H,S,S-HB(mt)3}(PPh3)2][BF4], which is most likely coordinatively unsaturated. Likewise, the protonation of 2 with HBF4·OEt2 in CD2Cl2 at −60 °C generates the dihydrogen complex [Ru(mt)(H2)(PPh3)3][BF4] (4) as a 5:1 mixture of two stereoisomers.