C−C versus C−H Bond Activation of Alkynes by Early Second-Row Transition Metal Atoms

Abstract

The reactions of Y (a2D), Zr (a3F), Nb (a6D), Mo (a7S), and electronically excited-state Mo* (a5S) with propyne (methylacetylene) and 2-butyne (1,2-dimethylacetylene) were investigated using crossed molecular beams. For all of the metals studied, reactions with propyne led to H2 elimination, forming MC3H2. For Y + propyne, C-C bond cleavage forming YCCH + CH3 also was observed, with an energetic threshold in good agreement with an earlier determination of D0(Y-CCH). For Y + 2-butyne, three reactive channels were observed: YC4H4 + H2, YC3H3 + CH3, and YC3H2 + CH4. The C-C bond cleavage products accounted for 21 and 27% of the total products at Ecoll = 69 and 116 kJ/mol, respectively. For Zr and Nb reactions with 2-butyne, competition between H2 and CH4 elimination was observed, with C-C bond cleavage accounting for 12 and 4% of the total product signal at Ecoll = 71 kJ/mol, respectively. For reactions of Mo and Mo* with 2-butyne, only H2 elimination was observed. The similarity between reactions involving two isomeric species, propyne and allene, suggests that H atom migration is facile in these systems.

C–C versus C–H bond activation of propyne by Y: A DFT study

“…4, the rate-determining step has an activation energy barrier of 40.7 kJ/mol (TS8) and the formation of YCH 2 + CH 3 was calculated to be endothermic by 45.2 kJ/mol. Based on the previously determined D 0 (Y-CCH), Davis et al estimated the formation of YCH 2 + CH 3 to be endothermic by around 60 kJ/mol and this value is consistent with the observation that the YCH 2 signal appeared at E coll P 63 kJ/mol [18]. Apparently, our calculations only slightly underestimated this threshold energy for this reaction and our calculations also suggest that this reaction may have close kinetic and thermodynamic barriers.…”

“…Base on the above mechanism, now we can understand why the reaction of Y with allene, H 2 C@C@CH 2 , also produced YCCH + CH 3 [18]. For this reaction, once the initial p-complex forms, one hydrogen atom will migrates to Y, leading to an intermediate same as IM2.…”

“…As we mentioned above, Davis et al have proposed two possible mechanisms for the elimination of CH 3 in the reaction of Y with propyne [18]. However, the proposed mechanisms can not explain the formation of YC 2 H + CH 3 in the reaction of Y with allene.…”

“…Very recently, Davis et al surveyed the reactions of Y( 2 D), Zr( 3 F), Nb( 6 D) and Mo( 7 S, 5 S) with propyne and 2-butyne [18]. For all the metals studied, H 2 -elimination products MC 3 H 2 were observed.…”

“…Another interesting topic is weather the C-C activation involves insertion of Y into C-C bond. Davis et al proposed two possible mechanisms [18]: (1) the formation of a Y-propyne complex followed by a loss of CH 3 via directly a barrierless dissociation; (2) insertion of sp-sp 3 C-C bond forming H-C"C-Y-CH 3 intermediate, followed by loss of CH 3 . However, in their experiment, the reaction of Y with allene, H 2 C@C@CH 2 also produced YC 2 H and CH 3 .…”