Halogen Exchange and Scrambling between C−X and M−X‘ Bonds in Copper, Nickel, and Cobalt Complexes of 6,6‘-bis(bromo/ chloromethyl)-2,2‘-bipyridine. Structural, Electrochemical, and Photochemical Studies

Abstract

The synthesis, reactivities, spectroscopic, electrochemical, and structural studies of copper(I), copper(II), nickel(II), and cobalt(II) complexes of 6,6'-bis(bromomethyl)-2,2'-bipyridine (bpy-Br2) and 6,6'-bis(chloromethyl)-2,2'-bipyridine (bpy-Cl2) have been reported. The copper(I) complex [CuI(bpy-Br2)2](ClO4) (1) has been obtained in two crystallographic modifications, in which the coordination geometry of the metal center has the D2d symmetry. The reaction between CuCl2.2H2O and bpy-Br2 has been followed spectrophotometrically at 45 degrees C over a period of 7 h, and a mechanism for the intramolecular halogen exchange and scrambling in the initially formed compound [CuII(bpy-Br2)Cl2] (5) has been proposed. Depending upon the reaction conditions, several halogen-exchanged products, namely [CuII(bpy-Br1.86Cl0.14)(Cl1.89Br0.11)] (2), [CuII(bpy-Br1.81Cl0.19)(Cl1.70Br0.30)(H2O)] (3), and [CuII(bpy-Br0.63Cl1.37)(Cl0.54Br1.46)] (4), have been isolated in crystalline form. The reaction between bpy-Cl2 and CuCl2.2H2O provides [CuII(bpy-Cl2)Cl2] (7) and [CuII(bpy-Cl2)Cl2(H2O)] (8), whereas CoCl2.6H2O and NiCl2.6H20 on reaction with bpy-Br2 under boiling condition produce [CoII(bpy-Br0.5Cl1.5)(ClBr)] (11) and [NiII(bpy-Br0.46Cl1.54)(Cl0.73Br1.27)(H2O)] (12), respectively. The X-ray structures determined for the 4-coordinate compounds 2, 4, and 7 show flattened tetrahedral geometry for the metal center with the D2 symmetry. Both 5-coordinate compounds 3 and 12 have square pyramidal geometry, and whereas the nickel(II) complex 12 has near-perfect geometry (tau = 0.015), considerable distortion is observed for the copper(II) complex 3 (tau = 0.25). Complexes [CuII(bpy-Cl2)Br2] (6) and [CuII(bpy-Br2)Br2] under boiling condition undergo photoreduction to produce the dimeric copper(I) complexes [{CuI(bpy-Cl1.30Br0.70)(mu-Br)}2](9) and [{CuI(bpy-Br2)(mu-Br)}2] (10), respectively. The fact that the photoreduction of [CuII(bpy-Cl2)Br2] (6) and [CuII(bpy-Br2)Br2] do not take place in absence of light has been established by spectrophotometric measurements. The crystal structures of 9 and 10 have been determined. The electrochemical behavior of all the copper complexes 1-10 has been studied in acetonitrile and dichloromethane. The E1/2 values for the CuI/CuII redox couples show strong solvent dependence and for a given system the E1/2 value is more positive in dichloromethane relative to that in acetonitrile. For the compounds [CuII(bpy-Br2-xClx)(Cl2-yBry)] (x = 0-2, y = 0-2), the E1/2 values become more positive with the increase of y value.

Pd–Carbene Migratory Insertion: Application to the Synthesis of Trifluoromethylated Alkenes and Dienes

“…The UV/vis absorption profiles of the copper(0)‐ L3 complex before and after oxidation are shown in Figure 3. Adsorption bands at less than 400 nm in the UV region were attributed to π–π* transitions of L3 26. In the red solution, two well‐defined peaks were detected at 432 and 970 nm (Figure 3, left).…”

“…The green solution showed a broad and weak peak at 650 nm and a strong peak at 975 nm. The absorption at 650 nm is believed to originate from ligand‐to‐metal charge transfer (LMCT), which is characteristic of copper(II) 26. The latter absorption may be assigned to the transition of d xz or d yz →d, which indicates a trigonal bipyramidal geometry 27.…”

Design Principles for High H2 Storage Using Chelation of Abundant Transition Metals in Covalent Organic Frameworks for 0–700 bar at 298 K

“…Some of the ligands have been synthesized with Tm chelation. − The initial experimental geometry was used to optimized the structure. Some of the ligands have been synthesized previously, and in some cases, the chelation has been reported with Ni­(II), Cu­(II), Pt­(II), and Pd­(II) . The initial experimental geometry was used when available to chelate other Tm.…”

Asymmetric Boron Conjugate Additions to Enones in Water Catalyzed by Copper(0)

“…Adsorption bands at less than 400 nm in the UV region were attributed to p-p* transitions of L3. [26] In the red solution, two well-defined peaks were detected at 432 and 970 nm (Figure 3, left). The peak at 432 nm may be ascribed to the interband electronic transitions because of the discrete energy levels.…”

“…The absorption at 650 nm is believed to originate from ligandto-metal charge transfer (LMCT), which is characteristic of copper(II). [26] The latter absorption may be assigned to the transition of d xz or d yz !d x 2 Ày 2, which indicates a trigonal bipyramidal geometry. [27] Although we can distinguish catalysis by either copper(0) or copper(I) through ESI-MS and UV/vis analysis, the identification of the red solution as copper(II) was not possible.…”