Monovalent copper as a potential catalyst for formation of acetaldehyde via the migration of methyl radicals to the coordinated carbonyl in the complex (CO)Cu^II-CH₃⁺

Cu⁺_aq forms stable complexes with carbon monoxide in aqueous solutions. Furthermore it reacts very fast with aliphatic radicals. The reaction of Cu(CO)⁺_naq with methyl radicals, *CH₃, was studied using the pulse-radiolysis technique. The results point out that methyl radicals react with Cu(CO)⁺_aq to form an unstable intermediate with a Cu^II-C σ bond identified as (CO)Cu^II-CH₃⁺, k=(1.1 ± 0.2) × 10⁹ M^-1 s^-1. This intermediate has a strong LMCT charge transfer band (λ_max=385 nm, ε_max=2500 M^-1 cm^-1) which is similar to the absorption bands of other transient complexes with Cu^II-alkyl σ bonds. The coordinated carbon monoxide in (CO) Cu^II-CH₃⁺ inserts into the copper-carbon bond (or rather the coordinated methyl migrates to the coordinated carbon monoxide ligand) at a rate of (3.0 ± 0.8) × 10² s^-1 to form the copper-acetyl complex, (CO)_mCu^II-C(CH₃)=O⁺ (λ_max=480 nm, ε_max=2100 M^-1 cm^-1). The rate of formation of (CO)Cu^II-CH₃⁺ and of the insertion reaction are pH independent. The complex (CO)_mCu^II-C(CH₃)=O⁺ is also unstable and decomposes heterolytically to yield acetaldehyde and Cu²⁺_aq as the final stable products. This reaction is slightly pH dependent. The same reactivity pattern has been observed for the Cu(CO)⁺_naq complexes (n=2 or 3). The results clearly point out that CO remains coordinated to transient complexes of the type Cu^II-alkyl.