Dissociation Of Phosphine Ligand Research Articles

DFT Mechanistic Study on Alkene Hydrogenation Catalysis of Iron Metallaboratrane: Characteristic Features of Iron Species

The variable coordination geometries, multiple spin states, and high density of states of first row transition metals offer a new frontier in the catalytic chemistry. A DFT study has been performed in order to unveil these characteristic features of iron metallaboratrane complex in alkene hydrogenation. A detailed spin-state analysis reveals there exist two minimum energy crossing points in the formation of (TPB)(μ-H)Fe(H) 3triplet from (TPB)Fe(N2) 1triplet. In the catalytic cycle, 3triplet at triplet state plays a role of active species, and the hydrogenation at triplet state is more favorable than that at singlet state. The dissociation of phosphine arm of TPB ligand from Fe center occurs easily in the triplet state, because the antibonding dσ is singly occupied in 3triplet. However, a nondissociative pathway without any phosphine ligand dissociation is not likely to occur. The product is formed via the σ-bond metathesis between a dihydrogen molecule and a Fe-styryl moiety. The usual direct reductive el...

Palladium-catalyzed regioselective and stereo-invertive ring-opening borylation of 2-arylaziridines with bis(pinacolato)diboron: experimental and computational studies.

A palladium catalyzed regioselective borylative ring opening reaction of 2-arylaziridines to give β-amino-β-arylethylborates was developed. The reaction reported herein represents the first example of ring-opening borylation of non-vinylic aziridines and direct borylative C(sp3)-N bond cleavage of neutral organic substrates. NMR studies and density functional theory (DFT) calculations suggested that the active intermediate for the reaction is a PdL2 complex [L = P(t-Bu)2Me]. The multi-component artificial force-induced reaction method (MC-AFIR) located the transition states for the regioselectivity-determining aziridine ring opening that proceeds in an SN2 fashion, and explained the selectivity of the reaction. The full catalytic cycle consists of a selectivity-determining aziridine ring opening (oxidative addition), a proton transfer, phosphine ligand dissociation from the catalyst, boron-boron bond cleavage, and reductive elimination. Water is important to the drive the transmetalation step. The calculated overall mechanism and selectivity are consistent with the experimental results.

Mechanism and Catalytic Impact of Ir–Ta Heterobimetallic and Ir–P Transition Metal/Main Group Interactions on Alkene Hydrogenation

Transition metal heterobimetallic catalysts provide an alternative to classic transition metal ligand catalyst design. The resurgence in popularity of heterobimetallic complexes prompted our use of density functional theory to examine the mechanism and reactivity of alkene hydrogenation catalyzed by the transition metal heterobimetallic complex Cp2Ta(CH2)2Ir(CO)(PPh3) and the transition metal/main group complex Ph2P(CH2)2Ir(CO)(PPh3). Calculations indicate that the Ir–Ta and Ir–P catalysts operate by different mechanisms. For the Ir–Ta complex, initial H2 oxidative addition to the Ir metal center followed by reductive elimination of an Ir–H and μ-CH2 bridge transforms the starting heterobimetallic complex into an active Ir–H catalyst. This catalyst precursor transformation occurs because the cationic Cp2Ta group provides a low activation barrier for reductive elimination. This transformation does not occur for the Ir–P catalyst because the reductive elimination activation barrier is significantly higher in energy. The active heterobimetallic Ir–H likely catalyzes multiple turnovers of alkene hydrogenation before reforming the original heterobimetallic Ir–Ta complex. The Ir–H catalytic cycle involves a series of classic organometallic reaction steps: alkene migratory insertion, H2 oxidative addition, and reductive elimination. In the Ir–P mechanism, the Ph2P(CH2)2 group remains as a spectator ligand throughout the active catalytic cycle. The Ir–P catalytic cycle involves H2 oxidative addition, phosphine ligand dissociation, ethylene migratory insertion, and reductive elimination.

A DFT computational study of phosphine ligand dissociation versus hemilability in a Grubbs-type precatalyst containing a bidentate ligand during alkene metathesis

Using density functional theory, the metathesis reaction of 1-octene in the presence of a Grubbs-type ruthenium alkylidene complex bearing a chelating pyridinyl alcoholate ligand, [RuCl(L)(O∧ N)( = CHPh)] (L = H2IMes or PCy3, O∧ N = 1-(2′-pyridinyl)cyclohexan-1-olate)), was investigated. The complete geometry optimisation and activation energy of various activation steps in the dissociative mechanism were performed at the GGA-PW91/DNP level of theory using Accelrys Materials Studio® 4.0. Two possible precatalyst initiations were explored, i.e. the dissociation of the labile N-atom of the O∧ N-ligand as well as the dissociation of ligand L, due to the belief that ruthenium-catalysed metathesis reactions proceed through 14-electron intermediates. The formation of the catalytically active heptylidene species is kinetically favoured for both the first- and second-generation chelating complexes. The computational results are in agreement with the experimental results obtained with NMR for the second-generation system. The computational results suggest that both phosphine ligand dissociation and hemilability may play a role in the metathesis reaction with the first-generation system.

Mechanistic study on dehydrogenation of methanol with [RuCl 2(PR 3) 3]-type catalyst in homogeneous solutions

Catalytic dehydrogenation of methanol has been investigated in homogeneous solutions with a series of Ru(II) complexes, [RuCl 2(P( p-C 6H 4X) 3) 3] (X = H (1), Me (2), F (3), OMe (4)) and [RuCl 2(PMePh 2) 3] (5). In the gas phase, hydrogen was formed selectively (> 99.5%), and formaldehyde, methylal (formaldehyde dimethyl acetal) and methyl formate were found in the liquid phase with satisfactory stoichiometry to the formed hydrogen. The reaction was retarded by the extra addition of free phosphine, suggesting the presence of pre-equilibrium dissociation of phosphine ligand. Kinetic analyses from this viewpoint explained well the dependence of rate on the concentration of catalyst (saturation curve). The order of evaluated pre-equilibrium constant (1 ≈ 2 > 5) is in accord with the general idea that the dissociation of phosphine ligand is controlled principally by steric bulk of ligands. The order of rate (3 > 1 > 2 > 4) for 1–4, possessing the same cone angle of phosphine ligand, correlated clearly with basicity of phosphines. The results are interpreted in terms of the mechanism of rate-determining β-hydrogen abstraction in the Ru-OCH 3 intermediate.

FACILE INTERMOLECULAR PHOSPHINE LIGAND EXCHANGE REACTIONS BETWEEN SQUARE PLANAR IRIDIUM AND PLATINUM CENTERS

Abstract Reactions between square planar complexes, trans-Ir(CO)(R)L2 (R = Cl, Me; L = PPh3, P(p-tolyl)3, PMePh2), trans-Pt(Cl)(H)L2 (L′ = PPh3, P(p-tolyl)3, PEt3), cis-Pt(Cl)2L′2 and trans-Pt(Cl)2(PEt3)2, result in intermolecular exchange of phosphine ligands. Exchange between two iridium complexes is facile; exchanges between iridium and platinum complexes or between two platinum complexes occur with half-lives of hours. The reactions are followed by 31P NMR. The reactions are best interpreted as phosphine ligand dissociation with dissociation from iridium occurring more readily than from platinum. The observed phosphine dependence on the rate, L = P(p-tolyl)3 > PPh3 > PEt3 is consistent with a dissociative process. The dissociated phosphine then causes an associative substitution with the other complexes. Associative substitution on square planar iridium(I) complexes are known to occur much more readily than on square planar platinum(II) complexes.

Trans-cis isomerization of dimethylbis(diethylphenylphosphine)palladium(II) catalyzed by methylmagnesium compounds

Dimethylbis(diethylphenylphosphine)palladium(II) complex was found to undergo a facile trans- cis isomerization reaction in tetrahydrofuran containing methylmagnesium compounds such as Me 2Mg and MeMgBr. Kinetic and isotopic studies on the isomerization process suggest the occurrence of methylmagnesium-catalyzed isomerization through an intermolecular methyl exchange process between methylpalladium and methylmagnesium compounds involving a partial dissociation of phosphine ligand.

The photodecomposition of platinacycloalkanes in solution: II. 1,4-butanediylplatinum(II) and (IV) compounds

The products of the photolysis of a number of platinacyclopentanes in solution at 25°C under a variety of conditions have been determined. With [I 2P tCH 2CH 2CH 2C H 2(L 2)] (L = PMe 2Ph, PPh 3) in CH 2Cl 2, CH 2Br 2 and (CH 3) 2SO the hydrocarbon products are exclusively ethylene and but-1-ene. Formation of the latter through a 1,3-hydrogen shift is preceded by phosphine ligand dissociation. The photolysis of [ICH 3P tCH 2CH 2C H 2(L 2)] gave methane, ethylene, but-1-ene and n-pentane together with a little n-butane, the methane being formed from internal hydrogen abstraction by the CH 3 group in the excited reactant molecule. Photodecomposition of the platinum(II) compounds [P tCH 2CH 2CH 2C H 2(L 2)] (L = (PMe 2Ph) 2, (PPh 3) 2, Ph 2PCH 2CH 2PPh 2) gave ethylene, but-1-ene, pent-1-ene (with the halogenated solvents) and with some systems appreciable yields of n-butane, the latter being the results of internal abstraction of two hydrogen atoms by the C 4H 8 moiety. The formation of pentene is probably preceeded by the addition of CH 2Cl 2 or CH 2Br 2 to the excited reactant molecule. Addition of diphenylphosphine promotes the production of n-butane.