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Naked d -orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3 + 2] cycloaddition

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ABSTRACT

Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz2 orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz2 orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.

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Electron density distribution maps of I.(a) 3D isosurface static deformation density of I; surfaces drawn at +0.2 e Å−3 in green and at −0.2 e Å−3 in orange, (b) static model map on the O(3)–Ni–O(4) plane; contours drawn at 0.05 e Å−3 interval in blue (positive), red (negative) and black (zero) lines, Laplacian distribution of total EDD (c) on the O(1)–Ni–O(2) plane, (d) on the O(3)–Ni–O(4) plane; the blue and red lines denote negative and positive Laplacian contours, respectively. The contours are drawn at ±2 × 10n, ±4 × 10n, ±8 × 10n (where n=0, 1, 2) e Å−5. Bond path (BP) and bond critical points (BCPs) are depicted as orange lines and black dots, respectively, in c,d.
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f3: Electron density distribution maps of I.(a) 3D isosurface static deformation density of I; surfaces drawn at +0.2 e Å−3 in green and at −0.2 e Å−3 in orange, (b) static model map on the O(3)–Ni–O(4) plane; contours drawn at 0.05 e Å−3 interval in blue (positive), red (negative) and black (zero) lines, Laplacian distribution of total EDD (c) on the O(1)–Ni–O(2) plane, (d) on the O(3)–Ni–O(4) plane; the blue and red lines denote negative and positive Laplacian contours, respectively. The contours are drawn at ±2 × 10n, ±4 × 10n, ±8 × 10n (where n=0, 1, 2) e Å−5. Bond path (BP) and bond critical points (BCPs) are depicted as orange lines and black dots, respectively, in c,d.

Mentions: To characterize the bonding mode around the distorted Ni(II) in I, we performed EDD analysis47 using single-crystal X-ray diffraction data (Fig. 3, Supplementary Data 1). A 3D plot of the static deformation density of I highlights its valence electron density topology (Fig. 3a). The distribution of d-orbitals along the coordination axes of the Ni(II) centre were found as electron-deficient regions. With respect to ligands, the lone pairs of oxygen [O(1), O(2) and O(3)] and nitrogen [N(1) and N(2)] atoms are directed to the electron-deficient regions at Ni(II), showing the conventional character of coordination bonds. In sharp contrast, the lone pair of O(4) is directed towards the electron-rich region at Ni(II). The 2D static model map on the O(3)–Ni–O(4) plane also demonstrates the different bonding character between the Ni–O(3) and the Ni–O(4) bonds (Fig. 3b). We can also discuss the unique bonding nature of Ni–O(4) with maps of Laplacian distribution [∇2ρ(r)] of total EDD along bond paths by comparing the ∇2ρ(r) distribution in the O(1)–Ni–O(2) plane (Fig. 3c) and the O(3)–Ni–O(4) plane (Fig. 3d)484950. The valence shell charge concentration (VSCC) region at the Ni(II) centre are located between the bond paths of the Ni–O(1) and the Ni–O(2) bonds on the O(1)–Ni–O(2) plane (Fig. 3c). The VSCCs on O(1) and O(2) expand towards the charge-depletion regions around the Ni(II) centre along the bond paths. In contrast, the Ni–O(4) bond path goes through inside the VSCC region at O(4) and the VSCC region at the Ni(II) centre on the O(3)–Ni–O(4) plane (Fig. 3d). On the other hand, the Ni–O(3) bond shows similar features to the Ni–O(1) and Ni–O(2) bonds. The results described in Fig. 3 represent experimental evidence that a weaker orbital interaction between Ni(II) and O(4) is involved, while electrons are donated from lone pairs in the orbitals on other N and O atoms to the unfilled d-orbital on Ni(II) (Supplementary Figs 6–8). The density at the bond critical point (BCP) of the Ni(II)–O(4) bond [0.260(2) e Å−3] is remarkably lower than at the other coordination bonds: Ni(II)–O(1); 0.431(2), Ni(II)–O(2); 0.480(2), Ni(II)–O(3); 0.494(2), Ni(II)–N(1); 0.559(3), and Ni(II)–N(2); 0.550(3) e Å−3. Thus, the dissymmetric, distorted octahedral Ni(II)–diamine–acetates I possessing an elongated Ni(II)–oxygen bond has d8 18-electronic configuration with a weak electrostatic interaction with O(4) at the pseudoapical position. The density at the BCP of the N(1)–H⋯O(5) is 0.084(10) e Å−3, which fits reasonably with the topological properties [d(H⋯O): 2.200 Å, d(N⋯O): 3.155(2) Å, α(N–H⋯O): 159.20°] of the H-bonding51.


Naked d -orbital in a centrochiral Ni(II) complex as a catalyst for asymmetric [3 + 2] cycloaddition
Electron density distribution maps of I.(a) 3D isosurface static deformation density of I; surfaces drawn at +0.2 e Å−3 in green and at −0.2 e Å−3 in orange, (b) static model map on the O(3)–Ni–O(4) plane; contours drawn at 0.05 e Å−3 interval in blue (positive), red (negative) and black (zero) lines, Laplacian distribution of total EDD (c) on the O(1)–Ni–O(2) plane, (d) on the O(3)–Ni–O(4) plane; the blue and red lines denote negative and positive Laplacian contours, respectively. The contours are drawn at ±2 × 10n, ±4 × 10n, ±8 × 10n (where n=0, 1, 2) e Å−5. Bond path (BP) and bond critical points (BCPs) are depicted as orange lines and black dots, respectively, in c,d.
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Related In: Results  -  Collection

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f3: Electron density distribution maps of I.(a) 3D isosurface static deformation density of I; surfaces drawn at +0.2 e Å−3 in green and at −0.2 e Å−3 in orange, (b) static model map on the O(3)–Ni–O(4) plane; contours drawn at 0.05 e Å−3 interval in blue (positive), red (negative) and black (zero) lines, Laplacian distribution of total EDD (c) on the O(1)–Ni–O(2) plane, (d) on the O(3)–Ni–O(4) plane; the blue and red lines denote negative and positive Laplacian contours, respectively. The contours are drawn at ±2 × 10n, ±4 × 10n, ±8 × 10n (where n=0, 1, 2) e Å−5. Bond path (BP) and bond critical points (BCPs) are depicted as orange lines and black dots, respectively, in c,d.
Mentions: To characterize the bonding mode around the distorted Ni(II) in I, we performed EDD analysis47 using single-crystal X-ray diffraction data (Fig. 3, Supplementary Data 1). A 3D plot of the static deformation density of I highlights its valence electron density topology (Fig. 3a). The distribution of d-orbitals along the coordination axes of the Ni(II) centre were found as electron-deficient regions. With respect to ligands, the lone pairs of oxygen [O(1), O(2) and O(3)] and nitrogen [N(1) and N(2)] atoms are directed to the electron-deficient regions at Ni(II), showing the conventional character of coordination bonds. In sharp contrast, the lone pair of O(4) is directed towards the electron-rich region at Ni(II). The 2D static model map on the O(3)–Ni–O(4) plane also demonstrates the different bonding character between the Ni–O(3) and the Ni–O(4) bonds (Fig. 3b). We can also discuss the unique bonding nature of Ni–O(4) with maps of Laplacian distribution [∇2ρ(r)] of total EDD along bond paths by comparing the ∇2ρ(r) distribution in the O(1)–Ni–O(2) plane (Fig. 3c) and the O(3)–Ni–O(4) plane (Fig. 3d)484950. The valence shell charge concentration (VSCC) region at the Ni(II) centre are located between the bond paths of the Ni–O(1) and the Ni–O(2) bonds on the O(1)–Ni–O(2) plane (Fig. 3c). The VSCCs on O(1) and O(2) expand towards the charge-depletion regions around the Ni(II) centre along the bond paths. In contrast, the Ni–O(4) bond path goes through inside the VSCC region at O(4) and the VSCC region at the Ni(II) centre on the O(3)–Ni–O(4) plane (Fig. 3d). On the other hand, the Ni–O(3) bond shows similar features to the Ni–O(1) and Ni–O(2) bonds. The results described in Fig. 3 represent experimental evidence that a weaker orbital interaction between Ni(II) and O(4) is involved, while electrons are donated from lone pairs in the orbitals on other N and O atoms to the unfilled d-orbital on Ni(II) (Supplementary Figs 6–8). The density at the bond critical point (BCP) of the Ni(II)–O(4) bond [0.260(2) e Å−3] is remarkably lower than at the other coordination bonds: Ni(II)–O(1); 0.431(2), Ni(II)–O(2); 0.480(2), Ni(II)–O(3); 0.494(2), Ni(II)–N(1); 0.559(3), and Ni(II)–N(2); 0.550(3) e Å−3. Thus, the dissymmetric, distorted octahedral Ni(II)–diamine–acetates I possessing an elongated Ni(II)–oxygen bond has d8 18-electronic configuration with a weak electrostatic interaction with O(4) at the pseudoapical position. The density at the BCP of the N(1)–H⋯O(5) is 0.084(10) e Å−3, which fits reasonably with the topological properties [d(H⋯O): 2.200 Å, d(N⋯O): 3.155(2) Å, α(N–H⋯O): 159.20°] of the H-bonding51.

View Article: PubMed Central - PubMed

ABSTRACT

Chiral metal catalysts have been widely applied to asymmetric transformations. However, the electronic structure of the catalyst and how it contributes to the activation of the substrate is seldom investigated. Here, we report an empirical approach for providing insights into the catalytic activation process in the distorted Ni(II)-catalysed asymmetric [3+2] cycloaddition of α-ketoesters. We quantitatively characterize the bonding nature of the catalyst by means of electron density distribution analysis, showing that the distortion around the Ni(II) centre makes the dz2 orbital partially ‘naked', wherein the labile acetate ligand is coordinated with electrostatic interaction. The electron-deficient dz2 orbital and the acetate act together to deprotonate the α-ketoester, generating the (Λ)-Ni(II)–enolate. The solid and solution state analyses, together with theoretical calculations, strongly link the electronic structure of the centrochiral octahedral Ni(II) complex and its catalytic activity, depicting a cooperative mechanism of enolate binding and outer sphere hydrogen-bonding activation.

No MeSH data available.


Related in: MedlinePlus