Theoretical Study of Cp₂Zr-, (MeO)₂Zr-, and M(PH₃)-Mediated Coupling Reactions of Acetylenes (M = Ni, Pt). Significant Differences between Early- and Late-Transition-Metal Complexes

Abstract

Theoretical calculations with DFT, MP2 to MP4(SDQ), and CCSD(T) methods clearly display that the Cp 2 Zr-mediated coupling reaction of two acetylene molecules easily takes place through a nonsymmetrical transition state with nearly no barrier and significantly large exothermicity but the M(PH 3 )-mediated reaction (M ) Ni, Pt) takes place through a symmetrical transition state with either a considerably large activation barrier for M ) Pt or a moderately large activation barrier for M ) Ni. The charge-transfer (CT) interaction between the d orbital of the transition-metal center and the π*-π* bonding couple of two acetylene molecules plays an important role for the C-C bond formation in the M(PH 3 )mediated coupling reaction, which needs a symmetrical transition state structure. On the other hand, a CT interaction between the d π -π* back-donation orbital of Cp 2 Zr(C 2 H 2 ) and the π* orbital of the second acetylene molecule is strongly formed in the Cp 2 Zr-mediated coupling reaction, which leads to a nonsymmetrical transition state structure. These differences are reasonably interpreted by the d π -π* back-donation of Cp 2 Zr(C 2 H 2 ) being much stronger than that of M(PH 3 )(C 2 H 2 ). This is because the early-transition-metal element has d orbitals at higher energy than does the late-transition-metal element. The Ni(PH 3 )mediated coupling reaction takes place with a smaller activation barrier than does the Pt-(PH 3 )-mediated reaction. This result is interpreted by noting that the 3d transition-metal element has d orbitals at higher energy than does the 5d transition-metal element. Also, (MeO) 2 Zr-mediated coupling reaction proceeds with nearly no barrier, in spite of the fact that the d orbital is at lower energy in (MeO) 2 Zr(C 2 H 2 ) than that in Cp 2 Zr(C 2 H 2 ). This is because the MeO group is flexible and gives rise to little steric repulsion between ligands and substrates.

Time‐dependent density functional theory study on hydrogen‐bonded intramolecular charge‐transfer excited state of 4‐dimethylamino‐benzonitrile in methanol

“…Much more works concentrate on the determining the structures of the LE and ICT states,4–9 while the precise nature of the difference between the properties of the electronically excited state in protic and aprotic solvents is little understood. Hydrogen bonding in protic solvents can lead to complicated interactions, and thus may significantly influence the ICT state for DMABN in the protic solvents 10–65. Kowk et al have demonstrated and monitored the formation of a hydrogen‐bonded ICT state for the photoexcited DMABN in the protic solvent methanol (MeOH) using the time‐resolved infrared (TRIR) absorption spectra 61–63.…”

“…However, some weak interactions in many molecular systems have been described by DFT or TDDFT methods with comparable accuracy to the higher level. Sakaki et al have extensively investigated the intramolecular and intermolecular weak interactions as well as the bonding nature of the transition metal‐silylene complexes with DFT and other high‐level methods 51–56. Sobolewski and Domcke have perfectly studied the intramolecular and intermolecular hydrogen bonding in both the ground state and electronically excited states using the DFT and TDDFT methods 33, 40–45…”

Cβ–Cβ Bond Fission of Metallacyclopentadiene over a Low-Valent Ditantalum Scaffold

“…Metallacyclopentadiene complexes of transition metals have been extensively investigated as key intermediates in various catalytic reactions, such as cyclotrimerization of three alkynes and co-cyclotrimerization of two alkynes with nitriles and heterocumulenes, for the production of aromatic and heteroaromatic compounds. − In addition, application as synthetically useful reagents for some stoichiometric reactions is demonstrated for the reaction with sulfur sources, main group element halides, and halogen sources for constructing five-membered heterocycles, 1,4-dihalo-1,3-butadienes, diene-based polymers, and so on. − Metallacyclopentadienes of late transition metals can be generated by concerted oxidative cyclization of two alkynes at low-valent metal centers, while the formation of metallacyclopentadiene of early transition metals proceeds in a stepwise mechanism through the initial generation of η 2 -alkyne complexes followed by insertion of the second alkyne . Notably, retro-oxidative cyclization via C β –C β bond fission is observed for some early transition metal complexes upon introducing bulky substituents at the α-position of the metallacyclopentadienes, producing the isomerized metallacyclopentadiene with bulky substituents at the β-position via the η 2 -alkyne complex …”

Time‐dependent density functional theory study on the hydrogen bonding‐induced twisted intramolecular charge‐transfer excited states of 2‐(4′‐N,N‐dimethylaminophenyl)imidazo[4,5‐b]pyridine

“…Furthermore, some weak interactions in many molecular systems have been described by DFT or TDDFT methods with comparable accuracy to the higher level 74–76. Sakaki and coworkers have excellently investigated the intramolecular and intermolecular weak interactions as well as the bonding nature of the transition metal‐silylene complexes with DFT and other high‐level methods 77–82…”