Lisha Maa,
Qiancheng Zhanga,
Lin Cheng*a,
Zhijian Wu*b and
Jucai Yanga
aKey Laboratory of Industrial Catalysis of the Inner Mongolia Autonomous Region, Inner Mongolia University of Technology, Huhehot 010051, China. E-mail: lcheng1983@aliyun.com
bState Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China. E-mail: zjwu@ciac.ac.cn; Fax: +86 431 85698041
First published on 26th June 2014
Density functional theory (DFT) calculations have been performed to investigate the catalytic mechanism for the oxidation of veratryl alcohol to veratraldehyde by Cu–phen (phen = 1,10-phenanthroline) catalyst. The catalytic cycle consists of alcohol oxidation and O2 reduction. For the alcohol oxidation, both mononuclear mechanism (path A) and binuclear mechanism (path B) are proposed. Our calculations show that path B is preferred over path A. Namely, for the Cu–phen (phen = 1,10-phenanthroline) catalytic system, the mechanism is the binuclear mechanism, which is consistent with the experimental suggestion. For the O2 reduction, two possible paths are proposed as well, which are (1) “path I” in which CuI is oxidized by O2 via a binuclear mechanism, and (2) “path II” in which CuI is oxidized by O2 via a mononuclear mechanism. According to our calculations, path I is favored both in thermodynamics and kinetics.
For 13, the positive spin density of ρ = 0.59 on Cu(1), ρ = 0.13 on phen(1), ρ = 0.11 on O(2)H, and ρ = 0.13 on the substrate benzyl alcohol are accompanied by a spin density of opposite sign of ρ = −0.56 on Cu(2), ρ = −0.13 on phen(2), ρ = −0.15 on O(3)H, and ρ = −0.14 on O(4)H. According to the spin density distribution, the complex 13 possesses{[Cu(1)II(phen)(1)(O(2)H)(OCH2Ph)][Cu(2)II(phen)(2)(O(3)H)(O(4)H)]}character. For the product of the H atom abstraction (1r4), all the spin densities of the atoms in 1r4 are zero. 1r4 is best formulated as {[Cu(1)I(phen)(1)(O(2)H)(OCHPh)][Cu(2)I(phen)(2)(O(4)H)]}. In comparison with 13 and 14, the above change indicates that the oxidation state of Cu for both Cu(1) and Cu(2) center is reduced from CuII to CuI, accompanied by the formation of H2O and the product PhCHO, which is consistent with the experimental observation.
To further investigate the H atom abstraction, the electron transfer process during 13 → 1TS3–4 is discussed. In 1TS3–4, the spin density on the Cu(2) center is −0.56, indicating that Cu(2) center is in the CuII oxidation state. This means that the homolytic cleavage of the Cu(2)–O(3)H bond does not occur in 1TS3–4. This conclusion could be supported by the Cu(2)–O(3)H bond length change (13 → 1TS3–4: 1.83 Å → 1.89 Å). The bond length change for the similar homolytic cleavage of the Cu–OH bond should be about 0.26 Å.18 Therefore, the homolytic cleavage of the Cu(2)–O(3)H bond did not happen in 1TS3–4. Namely, the spin density located on O(3)H should not be affected by the Cu(2)–O(3)H bond in 1TS3–4. However, for the O(3)H group, the spin density changes from −0.15 to −0.05 in the process of 13 → 1TS3–4, indicating that a very small fraction of α-spin electron has been migrated to the O(3)H species. This fraction of the α-spin electron comes from the homolytic cleavage of Cα–H bond of –OCH2Ph. The corresponding β-spin electron generating from homolytic cleavage of Cα–H bond migrates to the substrate, which should result in the decrease of the α-spin density on the substrate. However, it is interesting to mention that the α-spin density on the substrate increases from +0.16 to +0.55, indicating some α-spin densities migrate to the substrate. In addition, the spin density on Cu(1) center decreases from +0.59 to +0.3 (13 → 1TS3–4), indicating that a small fraction of β-spin electron transferring to the Cu(1) center. The α-spin densities migrating to the substrate and the β-spin electron migrating to the Cu(1) center could trace to a fraction of the homolytic cleavage of the Cu(1)–OCHPh bond. This could give a fraction of β-spin electron to the Cu(1) atom and α-spin electron to the substrate radical, which results in the decrease of the α-spin density on Cu(1) center (+0.59 → +0.3), coupled with the increase of the α-spin density on OCHPh (+0.13 → +0.55). In a word, in the process of 13 → 1TS3–4, two bonds were partially homolytic cleavage (partially homolytic cleavage of Cu(1)–OCHPh bond and minor homolytic cleavage of the Cα–H bond of the –OCH2Ph group). This conclusion is further supported by the molecular orbitals for 13 → 1TS3–4 (Scheme 2). In the process of 13 → 1TS3–4, the increase of a small fraction of the α-spin electron on substrate and a new small fraction of the β-spin electron distributed on Cu(1) indicates the partial homolytic cleavage of the Cu(1)–OCHPh bond. Moreover, a small fraction of the α-spin electron on O(3)H and a new small fraction of the β-spin electron located on substrate implies the partial homolytic cleavage of the Cα–H bond of the –OCH2Ph group. Thus, only Cu(1)–OCHPh bond and Cα–H bond are partially homolytic cleavage in the process of 13 → 1TS3–4.
In summary, the 1r4 → 35 process is exothermic by 26.8 kcal mol−1 and the energy barrier for path I (about 3 kcal mol−1) is observed (22.3 kcal mol−1 in path II). Therefore, for the O2 oxidation reaction, path I is the preferred pathway.
Model | O(1)⋯H | C⋯H | Cu(1)–O(2) | Cu(2)–O(1) |
---|---|---|---|---|
Benzyl alcohol | 1.19 Å | 1.45 Å | 1.86 Å | 1.91 Å |
Veratryl alcohol | 1.20 Å | 1.42 Å | 1.87 Å | 1.91 Å |
Footnote |
† Electronic supplementary information (ESI) available: Table S1 contains the cartesian coordinates of all the structures considered in this work from the B3LYP optimized geometries. See DOI: 10.1039/c4ra02896a |
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