Insights into the intramolecular acetate-mediated formation of ruthenium vinylidene complexes: a ligand-assisted proton shuttle (LAPS) mechanism

Abstract

The ruthenium bis-acetate complex Ru(κ(2)-OAc)(2)(PPh(3))(2) reacts with HC≡CPh to afford the vinylidene-containing species Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2). An experimental study has demonstrated that this reaction occurs under very mild conditions, with significant conversion being observed at 255 K. At lower temperatures, evidence for a transient metallo-enol ester species Ru(κ(1)-OAc)(OC{Me}O-C=CHPh)(PPh(3))(2) was obtained. A comprehensive theoretical study to probe the nature of the alkyne/vinylidene tautomerisation has been undertaken using Density Functional Theory. Calculations based on a number of isomers of the model system Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHMe)(PH(3))(2) demonstrate that both the η(2)(CC) alkyne complex Ru(κ(1)-OAc)(κ(2)-OAc)(η(2)-HC≡CMe)(PH(3))(2) and the C-H agostic σ-complex Ru(κ(1)-OAc)(κ(2)-OAc)(η(2){CH}-HC≡CMe)(PH(3))(2) are minima on the potential energy surface. The lowest energy pathway for the formation of the vinylidene complex involves the intramolecular deprotonation of the σ-complex by an acetate ligand followed by reprotonation of the subsequently formed alkynyl ligand. This process is thus termed a Ligand-Assisted Proton Shuttle (LAPS). Calculations performed on the full experimental system Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2) reinforce the notion that lowest energy pathway involves the deprotonation/reprotonation of the alkyne by an acetate ligand. Inclusion of the full ligand substituents in the calculations are necessary to reproduce the experimental observation of Ru(κ(1)-OAc)(κ(2)-OAc)(=C=CHPh)(PPh(3))(2) as the thermodynamic product.

DFT Study on the Mechanism of Formic Acid Decomposition by a Well-Defined Bifunctional Cyclometalated Iridium(III) Catalyst: Self-Assisted Concerted Dehydrogenation via Long-Range Intermolecular Hydrogen Migration

“…Such a concerted H 2 -formation process with the assistance of a HCOOH molecule is remarkably different from the step-by-step one shown in Figure 1. Similar examples of ligand-assisted proton migrations in the area of organometallic chemistry, have been reported in several recent publications, 40,[44][45][46] including the so called CMD mechanism by Fagnou et al, 40 AMLA mechanism by…”

“…Macgregor and Davies, 44 LAPS mechanism by Lynam and Slattery, 45 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 without the involvement of the γ-NH functionality and also to the situation where the γ-NH functionality is substituted by an O atom. Figure 3 shows the calculated results for the rate determining state along the pathway where the γ-NH functionality acts as a spectator, when the HCOOH protonates the hydride to form H 2 .…”

Ruthenium carboxylate complexes as easily prepared and efficient catalysts for the synthesis of β-oxopropyl esters

“…A 31 P{ 1 H} NMR spectrum of the reaction mixture exhibited two very broad resonances in the region d 66, at a chemical shift similar to that observed for pure 1a and for cis-[Ru (k 2 -O 2 CPh) 2 (PPh 3 ) 2 ], 1b (q.v.). Related complexes possessing mutually trans-phosphine ligand typically exhibit resonances in the 31 P{ 1 H} spectrum between d 40 and 30 [16,17]. We therefore concluded that rapid exchange of acetate and benzoate ligands was occurring, even at room temperature.…”

“…These products are thought to arise via intramolecular attack of a coordinated carboxylate ligand onto the a-carbon of a vinylidene ligand (Scheme 6b) [22,42]. In related studies, we have demonstrated that the acetate ligand in complexes of type 3 may undergo facile nucleophilic attack at the a-carbon of the vinylidene ligand to give isolable metallo-enol esters 11 (Scheme 6c) [17]. Indeed, in stoichiometric studies we have shown that protonation of 11 with acetic acid affords 7a and 10a [43].…”

“…Treatment of 1a with propargyl alcohols HC^CC(OH)RR 1 (4a R ¼ R 1 ¼ Ph; 4b R ¼ R 1 ¼ Me) afforded long-lived hydroxyl-substituted vinylidene complexes [Ru(k 2 -O 2 CMe)(k 1 -O 2 CMe)(]C]CHC{OH)RR 1 }(PPh 3 ) 2 ], (5a R ¼ R 1 0 ¼ Ph; 5b R ¼ R 1 ¼ Me). This reaction occurs under extremely mild conditions and the key step in alkyne/vinylidene tautomerisation appears to be the ability of an acetate ligand to participate in a ligand-assisted proton shuttle (LAPS) mechanism (Scheme 2) [17]. Here the acetate initially acts as an intramolecular base to deprotonate the coordinated alkyne and in a second step reprotonation occurs to afford the final vinylidene complex.…”