Thermal and Electrochemical C−X Activation (X = Cl, Br, I) by the Strong Lewis Acid Pd₃(dppm)₃(CO)²⁺ Cluster and Its Catalytic Applications

Abstract

The stoichiometric and catalytic activations of alkyl halides and acid chlorides by the unsatured Pd(3)(dppm)(3)(CO)(2+) cluster (Pd(3)(2+)) are investigated in detail. A series of alkyl halides (R-X; R = t-Bu, Et, Pr, Bu, allyl; X = Cl, Br, I) react slowly with Pd(3)(2+) to form the corresponding Pd(3)(X)(+) adduct and "R(+)". This activation can proceed much faster if it is electrochemically induced via the formation of the paramagnetic species Pd(3)(+). The latter is the first confidently identified paramagnetic Pd cluster. The kinetic constants extracted from the evolution of the UV-vis spectra for the thermal activation, as well as the amount of electricity to bring the activation to completion for the electrochemically induced reactions, correlate the relative C-X bond strength and the steric factors. The highly reactive "R(+)" species has been trapped using phenol to afford the corresponding ether. On the other hand, the acid chlorides react rapidly with Pd(3)(2+) where no induction is necessary. The analysis of the cyclic voltammograms (CV) establishes that a dissociative mechanism operates (RCOCl --> RCO(+) + Cl(-); R = t-Bu, Ph) prior to Cl(-) scavenging by the Pd(3)(2+) species. For the other acid chlorides (R = n-C(6)H(13), Me(2)CH, Et, Me, Pr), a second associative process (Pd(3)(2+) + RCOCl --> Pd(3)(2+.....)Cl(CO)(R)) is seen. Addition of Cu(NCMe)(4)(+) or Ag(+) leads to the abstraction of Cl(-) from Pd(3)(Cl)(+) to form Pd(3)(2+) and the insoluble MCl materials (M = Cu, Ag) allowing to regenerate the starting unsaturated cluster, where the precipitation of MX drives the reaction. By using a copper anode, the quasi-quantitative catalytic generation of the acylium ion ("RCO(+)") operates cleanly and rapidly. The trapping of "RCO(+)" with PF(6)(-) or BF(4)(-) leads to the corresponding acid fluorides and, with an alcohol (R'OH), to the corresponding ester catalytically, under mild conditions. Attempts were made to trap the key intermediates "Pd(3)(Cl)(+)...M(+)" (M(+) = Cu(+), Ag(+)), which was successfully performed for Pd(3)(ClAg)(2+), as characterized by (31)P NMR, IR, and FAB mass spectrometry. During the course of this investigation, the rare case of PF(6)(-) hydrolysis has been observed, where the product PF(2)O(2)(-) anion is observed in the complex Pd(3)(PF(2)O(2))(+), where the substrate is well-located inside the cavity formed by the dppm-Ph groups above the unsatured face of the Pd(3)(2+) center. This work shows that Pd(3)(2+) is a stronger Lewis acid in CH(2)Cl(2) and THF than AlCl(3), Ag(+), Cu(+), and Tl(+).

New Insights into the Stoichiometric and Catalytic Reactivity of Unsaturated Pd3(dppm)3COn+ Clusters (n = 0, 1) Towards Halocarbons – First Evidence for Inorganic By‐Products

“…As reported earlier, [11,14,19] Pd 3 + can be quantitatively obtained from a 1-electron reduction of the corresponding dicationic compound. Its RDE voltammogram is depicted in Figure 1 (trace a) and exhibits two closely spaced waves for oxidation and reduction labelled AЈ 1 + can be seen (δ = -6.4 ppm).…”

“…[14] However, whereas the reactivity of Pd 3 2+ towards RX has been fully investigated under both stoichiometric and catalytic conditions, the corresponding reduced to demonstrate the intermediacy of the radical Bz · . The catalytic generation of "Bz · " was performed using two methods, i.e.…”

“…On the other hand, the second proposal is more likely to occur based upon the well-established mechanism reported for the Pd 3 2+ cluster [Pd 3 2+ + RX Ǟ Pd 3 ···(XR) 2+ Ǟ Pd 3 (X) + + "R + "]. [11][12][13][14] Going back to the Pd 3 + /MeI reaction, the absence of observed organics is due to their high volatility and, based on the observation made for the Pd 3 + /BzBr reaction, one can suggest that these are Me 2 and MeH.…”

A novel binuclear palladium complex with benzothiazole-2-thiolate: Synthesis, crystal structure and interaction with DNA

“…So the structure and activity of palladium complexes has drawn broad attention. In the previous study on the structure of palladium complexes, not only involved covalent bond, but also intramolecularly or intermolecular weak interaction, such as PdÁ Á ÁPd [3][4][5][6][7][8][9], pÁ Á Áp stack [10], hydrogen bonds [11,12] and so on. In the aspect of anticancer activity, some palladium complexes show significant cytotoxic activity, for example, Pd(L) 2 (L = 3-(1-aminoethyl)-4-hydroxychromen-2-one) has strong inhibition on two human leukemia cell lines, HL-60 and NALM-6 [13].…”

Unexpected Reaction of the Unsaturated Cluster Host and Catalyst [Pd3(μ3‐CO)(dppm)3]2+ with the Hydroxide Ion: Spectroscopic and Kinetic Evidence of an Inner‐Sphere Mechanism

“…[12] Related to this high affinity for halides, the cluster is also reactive towards the carbon-halogen bond, which led to important new catalytic processes in organic electrosynthesis. [13][14][15] . From electron paramagnetic resonance analysis of the reaction medium using the spin-trap agent 5,5-dimethyl-1-pyrroline-N-oxide (DMPO), the 2-tetrahydrofuryl or methyl radicals, deriving from the tetrahydrofuran (THF) or dimethyl sulfoxide (DMSO) solvent, respectively, were detected.…”