Structure of Complexes Formed by Dissolution of Palladium Diacetate in Methanol and Chloroform. In Situ NMR Study

Abstract

The behavior of palladium diacetate cyclic trimer [Pd(OAc)(2)](3) (1) upon its dissolution in methanol and wet chloroform was studied by (1)H and (13)C NMR including 2D-HSQC and 2D-DOSY techniques. Upon dissolution, trimer 1 reacts with methanol and is completely transformed first into the methoxo complex Pd(3)(μ-OMe)(OAc)(5) (2), which already at -18 °C undergoes a slow exchange of second bridging acetate ligand between the same palladium atoms to form the symmetric dimethoxo complex Pd(3)(μ-OMe)(2)(OAc)(4), the maximum relative concentration of which reaches 20-30 mol % of initial loading trimer 1. Along with the dimethoxo complex, both soluble and insoluble polynuclear palladium clusters are gradually formed at -18 °C, and their total amount reaches up to 60% of the starting Pd(2+) loading. The increase of temperature to 27 °C results in the reduction of palladium(II) to Pd metal by methanol, which is oxidized and transformed into formaldehyde hemiacetal and methyl formate. Upon dissolution in wet chloroform, trimer 1 is reversibly hydrolyzed to the hydroxo complex Pd(3)(μ-OH)(OAc)(5) (10) in ratio 1/10 ≈ 3/1. The temperature decrease and addition of acetic acid shift the equilibrium in this system toward trimer 1, and addition of water shifts it in the opposite direction. Addition of methanol to the equilibrium mixture of 1 and 10 results in the fast exchange of bridging acetate in trimer 1 by the μ-OMe group. Substitution of the μ-OH ligand by μ-OMe in 10 occurs in parallel but more slowly. Complex 2 formed in both cases is more stable in chloroform than in methanol.

Understanding Palladium Acetate from a User Perspective

“…As mentioned in the introduction, 1 is known to react with trace amounts of water to form the hydrolysis product, 4 , which can further complicate its isolation and characterization. This observation is supported by our own experiences in this area as well as the characterization studies by Cotton in 2005, and more recently by Stolyarov, Fairlamb, Bedford, and Nosova …”

“…Compound 1 has also been shown to react readily with water or alcohols where one acetate group is substituted by a solvent molecule ( 4 and 5, Figure ) while retaining its trimeric structure . While in the presence of water, equilibrium between species 1 and 4 in solution is observed; in MeOH, 1 will react irreversibly to form complex 5 . Therefore, as Bedford et al.…”

Palladium‐Catalyzed Asymmetric Quaternary Stereocenter Formation

“…With 7 as ligand, a jump in conversion from 22 to 51 % was observed when acetone was substituted for methanol as the solvent (Table 1, entries 5 and 6), and a high ee value of 92 % was maintained. This result, together with the observation of Pd black when the reaction was performed in methanol, led us to assume that the reduction of Pd II to Pd 0 by MeOH was the cause of the poor conversions 13…”

Facile Hydrolysis and Alcoholysis of Palladium Acetate

“…Cotton and co‐workers were the first to notice that the trimer reacts reversibly with water to generate a new species that, on the basis of 1 H NMR spectroscopic analysis, they assigned as the trimetallic complex 2 (Scheme ),5 in which water coordinates to one of the palladium atoms, converting a μ 2 ‐acetate ligand into a κ 1 ‐acetate on an adjacent palladium center 6. Subsequently, Nosova and co‐workers postulated, again based solely on NMR studies, that this species was actually the μ 2 ‐hydroxy complex [Pd 3 (μ 2 ‐OH)(OAc) 5 ] ( 1 ) 7. Intrigued by this apparent dichotomy, we examined the reaction of palladium acetate with a large excess of water (10 equiv per Pd) in acetone.…”