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+

摘要

Cu-aq(+) forms stable complexes with carbon monoxide in aqueous solutions. Furthermore it reacts very fast with aliphatic radicals, The reaction of Cu(CO)(maq)(+), 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 (CuC)-C-II sigma band identified as (CO)Cu-II-CH3+, k = (1.1 +/- 0.2) x 10(9) M-1 s(-1). This intermediate has a strong LMCT charge transfer band (lambda(max) = 385 nm, epsilon(max) = 2500 M-1 cm(-1)) which is similar to the absorption bands of other transient complexes with Cu-II-alkyl sigma 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) x 10(2) s(-1) to form the copper-acetyl complex, (CO)(m)Cu-II-C(CH3) = O+ (lambda(max) = 480 nm, epsilon(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)(m)Cu-II-C(CH3) = O+ is also unstable and decomposes heterolytically to yield acetaldehyde and Cu-aq(2+) as the final stable products, This reaction is slightly pH dependent. The same reactivity pattern has been observed for the Cu(CO)(maq)(+) complexes (n = 2 or 3). The results clearly point out that CO remains coordinated to transient complexes of the type Cu-II-alkyl, (C) 1998 Elsevier Science S.A. [References: 36]