TY - JOUR
T1 - 1,3-metal-carbon bonding and alkyne metathesis
T2 - DFT investigations on model complexes of group 4, 5, and 6 transition metals
AU - Suresh, Cherumuttathu H.
AU - Frenking, Gernot
PY - 2012/10/22
Y1 - 2012/10/22
N2 - The formation of metallacyclobutadienes (MCBs) from chloro-ligated alkylidyne complexes of group 4, 5, and 6 transition metals (MCl n(C 3H 3)) has been studied at the BP86/def2-TZVPP level. All the MCBs showed M-C β distances (∼2.1 Å) very close to M-C α distances (1.8-2.0 Å), suggesting a bonding interaction between the metal and the β-carbon (1,3-MC bond). Energy decomposition analysis using C 2v symmetric structures revealed that a b 2 orbital composed of mainly metal d π and C β p π overlap and an agostic a 1 orbital contributed to the orbital interaction of the 1,3-MC bond. The bond order of the 1,3-MC bond is a minimum of 0.26 for M = Cr and a maximum of 0.43 for M = Ta. Further, all the MCBs showed a characteristic δ orbital interaction through an a 2 orbital, which contributed to the double-bond character of M-C α bonds (bond order 1.27-1.44). Although the formation of b 2 and a 2 orbitals increased the M-C interactions, they significantly reduced the π interactions within the C 3H 3 fragment (C-C bond order 1.09-1.18). 1,3-MC bonding suggested a planar tetracoordinate configuration for C β, as the C α-C β bonds possessed largely formal C sp2-C sp2 single-bond character. Electron density analysis showed a "catastrophic" character of the 1,3-MC bond. In groups 4 and 5, MCBs were more stable than the isomeric η 3- structures (metallatetrahedranes). A mechanistic study on the reaction between acetylene and alkylidyne complex MCl nCH showed that a nearly barrierless and exothermic pathway exists for MCB formation (exothermic value 75-102 kcal/mol for groups 4 and 5; 6-27 kcal/mol for group 6). The rich metathesis chemistry associated with Mo and W is attributed mainly to the moderate activation energy required for the alkyne disproportionation step of metathesis. A mechanistic possibility other than Katz's is also proposed for alkyne metathesis that showed that the 1,3-MC bonded MCB complex can act as a metathesis catalyst by reacting with alkyne to form a bicyclic intermediate and subsequently disproportionating to yield the alkyne and the MCB. For this mechanism to be effective, rearrangement of the bicyclic intermediate to a more stable cyclopentadienyl complex has to be prevented.
AB - The formation of metallacyclobutadienes (MCBs) from chloro-ligated alkylidyne complexes of group 4, 5, and 6 transition metals (MCl n(C 3H 3)) has been studied at the BP86/def2-TZVPP level. All the MCBs showed M-C β distances (∼2.1 Å) very close to M-C α distances (1.8-2.0 Å), suggesting a bonding interaction between the metal and the β-carbon (1,3-MC bond). Energy decomposition analysis using C 2v symmetric structures revealed that a b 2 orbital composed of mainly metal d π and C β p π overlap and an agostic a 1 orbital contributed to the orbital interaction of the 1,3-MC bond. The bond order of the 1,3-MC bond is a minimum of 0.26 for M = Cr and a maximum of 0.43 for M = Ta. Further, all the MCBs showed a characteristic δ orbital interaction through an a 2 orbital, which contributed to the double-bond character of M-C α bonds (bond order 1.27-1.44). Although the formation of b 2 and a 2 orbitals increased the M-C interactions, they significantly reduced the π interactions within the C 3H 3 fragment (C-C bond order 1.09-1.18). 1,3-MC bonding suggested a planar tetracoordinate configuration for C β, as the C α-C β bonds possessed largely formal C sp2-C sp2 single-bond character. Electron density analysis showed a "catastrophic" character of the 1,3-MC bond. In groups 4 and 5, MCBs were more stable than the isomeric η 3- structures (metallatetrahedranes). A mechanistic study on the reaction between acetylene and alkylidyne complex MCl nCH showed that a nearly barrierless and exothermic pathway exists for MCB formation (exothermic value 75-102 kcal/mol for groups 4 and 5; 6-27 kcal/mol for group 6). The rich metathesis chemistry associated with Mo and W is attributed mainly to the moderate activation energy required for the alkyne disproportionation step of metathesis. A mechanistic possibility other than Katz's is also proposed for alkyne metathesis that showed that the 1,3-MC bonded MCB complex can act as a metathesis catalyst by reacting with alkyne to form a bicyclic intermediate and subsequently disproportionating to yield the alkyne and the MCB. For this mechanism to be effective, rearrangement of the bicyclic intermediate to a more stable cyclopentadienyl complex has to be prevented.
UR - http://www.scopus.com/inward/record.url?scp=84867777332&partnerID=8YFLogxK
U2 - 10.1021/om3007097
DO - 10.1021/om3007097
M3 - 文章
AN - SCOPUS:84867777332
SN - 0276-7333
VL - 31
SP - 7171
EP - 7180
JO - Organometallics
JF - Organometallics
IS - 20
ER -