Anion Effects in Oxidative Aliphatic Carbon–Carbon Bond Cleavage Reactions of Cu(II) Chlorodiketonate Complexes

Abstract

Aliphatic oxidative carbon-carbon bond cleavage reactions involving Cu(II) catalysts and O2 as the terminal oxidant are of significant current interest. However, little is currently known regarding how the nature of the Cu(II) catalyst, including the anions present, influence the reaction with O2. In previous work, we found that exposure of the Cu(II) chlorodiketonate complex [(6-Ph2TPA)Cu(PhC(O)CClC(O)Ph)]ClO4 (1) to O2 results in oxidative aliphatic carbon-carbon bond cleavage within the diketonate unit, leading to the formation of benzoic acid, benzoic anhydride, benzil, and 1,3-diphenylpropanedione as organic products. Kinetic studies of this reaction revealed a slow induction phase followed by a rapid decay of the absorption features of 1. Notably, the induction phase is not present when the reaction is performed in the presence of a catalytic amount of chloride anion. In the studies presented herein, a combination of spectroscopic (UV-vis, EPR) and density functional theory (DFT) methods have been used to examine the chloride and benzoate ion binding properties of 1 under anaerobic conditions. These studies provide evidence that each anion coordinates in an axial position of the Cu(II) center. DFT studies reveal that the presence of the anion in the Cu(II) coordination sphere decreases the barrier for O2 activation and the formation of a Cu(II)-peroxo species. Notably, the chloride anion more effectively lowers the barrier associated with O-O bond cleavage. Thus, the nature of the anion plays an important role in determining the rate of reaction of the diketonate complex with O2. The same type of anion effects were observed in the O2 reactivity of the simple Cu(II)-bipyridine complex [(bpy)Cu(PhC(O)C(Cl)C(O)Ph)ClO4] (3).

Reaction mechanism between Cu(II)-enolate complex and O2 as a test case for methodology used in DFT computational studies

“…Crystal structure of S Cl is not available, yet UV-vis and EPR spectroscopic data together with the DFT optimized structure suggest that chloride anion coordinates Cu in a complex with distorted square pyramidal geometry. The composition of the final reaction mixture is the same as the one obtained in an independent reaction between NaOCl, d i p h e n y l p r o p a n e t r i o n e ( t r i k e t o n e ) , a n d [ ( 6 -Ph 2 TPA)Cu(CH 3 CN)](ClO 4 ) 2 , which strongly indicates that triketone and hypochlorite are formed as reactive intermediates in the initial stage of the reaction [5].…”

“…Concerning the qualitative aspects, all of the enthalpy profiles computed with different models reproduce the catalytic effect introduced by the chloride ligand, as the barriers connected with TS1 Cl and TS2 Cl are markedly lower than those Scheme 2 Alternative reactions of P Cl [5] corresponding to TS1 and TS2. Similarly, for all models the first elementary step, i.e., S → P, is more exothermic when Cl is bound in the first coordination shell of Cu(II), whereas T Cl_homo is the least stable of all products.…”

“…The results taken from the previous study are used as a reference and are labeled as B3LYP-D3/BS3//B3LYP-D2/ BS1 [5]. In previous work, although the TPSSh functional was used, it was only to compute electronic energies for geometries of stationary points that were optimized at the B3LYP-D2/BS1 level; such energy profiles were very similar to those obtained with B3LYP only.…”

“…To assess to what extent such particular decisions may affect conclusions one draws from computational results, we did a small yet systematic case study for a synthetic mononuclear copper(II)-chloroenolate complex, for which congruent experimental and computational results are already available [4,5]. This system has several particularly attractive features concerning testing variations of computational methodology.…”

“…First, the O-O bond may be cleaved homolytically via TS2 Cl_homo . Second, instead of oxidizing Cl2, one of the phenyl appendages of the 6-Ph 2 TPA ligand may be oxidized to Scheme 1 Mechanisms for the initial stage of oxidation of S or S Cl formulated on the basis of a joined experimental and computational study [5] arene oxide (T cl_Ph ). In the original study, both of these processes involved barriers higher than that connected with TS2 Cl ; however, the latter reaction was also found for the parent compound P, and in this case TS2 Ph was lower in energy than TS2.…”