Competition between C−C and C−H Activation in Reactions of Neutral Yttrium Atoms with Cyclopropane and Propene

Abstract

Branching ratios between C−C and C−H bond activation were measured for reactions of ground-state Y (a2D, s2d) atoms with two C3H6 isomers (cyclopropane and propene) in crossed molecular beams. For both isomers, C−C bond activation led to formation of YCH2 + C2H4, whereas C−H activation led to YC3H4 + H2 and YH2 + C3H4. The angular and velocity distributions for all three product channels and for nonreactive collisions were measured at several collision energies (E coll). For Y + cyclopropane, the branching ratio for YCH2 + C2H4 increased relative to YC3H4 + H2 with increasing E coll, this C−C activation channel becoming dominant at E coll ≥ 19 kcal/mol. For the propene reaction, φYCH 2 /φYC 3 H 4 also increased with E coll, reaching 0.75:1.00 at E c oll = 28.8 kcal/mol. For both C3H6 reactants, formation of YH2 + C3H4 was observed as a minor channel at the highest collision energies. Experimental results and Rice−Ramsperger−Kassel−Marcus (RRKM) modeling indicate that for propene reactions all three channels involve initial formation of π-association complexes that undergo insertion into one of the sp3-hybridized β-C−H bonds in the methyl substituent. Decay of the yttrium allyl hydride intermediate by β-H migration leads to the two C−H activation products, YC3H4 + H2 and YH2 + C3H4. We propose that the YCH2 + C2H4 channel involves reverse β-H migration forming the same strongly bound metallacyclobutane intermediate formed in the Y + cyclopropane reaction.

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

“…In the past decade, crossed molecular beam (CMBs), kinetic techniques and theoretical calculations have been carried out to study the reactions of transition metal atoms with small hydrocarbons in gas phase [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. Experimental studies indicate that the reactions of ground state Y( 2 D), Zr( 3 F), Nb( 6 D) and Mo * with hydrocarbons generally yield the C-H insertion products or dehydrogenation products [7][8][9][10], whereas C-C bond activation has only been observed in select cases, such as the reactions of Y with cyclopropane and propene [3,13,15].…”

“…Experimental studies indicate that the reactions of ground state Y( 2 D), Zr( 3 F), Nb( 6 D) and Mo * with hydrocarbons generally yield the C-H insertion products or dehydrogenation products [7][8][9][10], whereas C-C bond activation has only been observed in select cases, such as the reactions of Y with cyclopropane and propene [3,13,15]. The factors which lead to strong preference for C-H insertion have elucidated [13].…”

“…In contrast, C-C bonds are highly directional, resulting in less favorable multicenter bonding and larger potential energy barriers [13]. An interesting issue is that theoretical calculations indicated that the C-C bonds activation of cyclopropane and propene do not involve C-C bond insertion processes [15]. For example, in the reaction of Y with propene [15], the C-C bond activation products YCH 2 + C 2 H 4 were observed, but as proposed by Davis et al, this reaction does not involve insertion of Y into the sp 3 -sp 2 C-C bond but involves pcomplex formation followed by b-H (methyl hydrogen) migration to Y and then migration to vinylic carbon atom to form a fourmembered ring intermediate, namely metallacyclobutane, which decays to give YCH 2 + C 2 H 4 without any transition states.…”

A DFT study on the mechanism of the gas phase reaction of ground-state Y (4d15s2,2D) with 2-butyne

“…In the past decade, a number of experimental and theoretical studies have been carried out to investigate the reactivity of neutral transition metal atoms, such as Y, Zr, Nb and Mo, toward small hydrocarbons in gas phase [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16]. In these studies, the groundstate Y ( 2 D), Zr( 3 F), Nb ( 6 D) and Mo ( 7 S) were found to be able to activate CAH bonds in hydrocarbons containing three or less carbon atoms, generally yielding dehydrogenation products [7][8][9][10], whereas CAC bond activation products were only observed in selected cases, such as the reactions of propene and cyclopropene [3,13,15].…”

“…In these studies, the groundstate Y ( 2 D), Zr( 3 F), Nb ( 6 D) and Mo ( 7 S) were found to be able to activate CAH bonds in hydrocarbons containing three or less carbon atoms, generally yielding dehydrogenation products [7][8][9][10], whereas CAC bond activation products were only observed in selected cases, such as the reactions of propene and cyclopropene [3,13,15]. Previous theoretical studies [13,16,17] on the reactions of Y and Zr with ethene and ethyne proposed that the formation of the dehydrogenation products involves initial insertion of metal atom into a CAH bond, followed by concerted or stepwise H-migration, leading to elimination of H 2 .…”