Tuning a Metal's Oxidation State: The Potential of Clathrochelate Systems

Abstract

"Oxidation state" is one of most fundamental concepts in chemistry. The stabilization of previously unknown or unusual oxidation states of metal ions allows us to obtain results that are important both for theoretical inorganic chemistry and for the design of new molecular electronic devices, electrochromic materials, and electron-storage systems.Complexes with a metal ion encapsulated in a threedimensional macropolycyclic ligand cavity have undoubted promise in stabilizing unusual oxidation states of such caged ions. This is due to the fact that an encapsulated and coordinatively saturated metal ion is completely isolated from environmental factors (in particular, effects of solvation and additional coordination that exclude occurrence of redox processes by bridge mechanisms). The size of a three-dimensional ligand cavity may be controlled by changing the nature of the capping and chelating fragments of the macropolycyclic framework, and donor atoms (groups) that are optimal for stabilization of a desired electronic configuration of this ion may be chosen. [1] Varying the substituents on a clathrochelate framework is thought to be a new and unique opportunity to stabilize unusual oxidation states of an encapsulated metal ion: although the apical substituents in the capping fragments of the same type affect the electronic characteristics of ligand donor atoms (and, therefore, the encapsulated metal ion) to a very minor extent, [2] the influence of electron-donating (electron-withdrawing) substituents in chelate (ribbed) fragments with a system of conjugated p-bonds may prove to be crucial. [3] We suppose that the electron-withdrawing substituents in such fragments of a clathrochelate ligand stabilize the lowest oxidation states of an encapsulated metal ion. A new approach to the synthesis of cobalt(i) complexes uses a macrobicyclic hexachloride ligand with six strongly electronwithdrawing substituents in chelating p-conjugated a-dioximate fragments. The template condensation of dichloroglyoxime with the corresponding boron-containing Lewis acid on a Co 2+ ion matrix leads to formation of the brown n-butylborane-capped hexachloride cobalt(ii) clathrochelate [Co-{(Cl 2 Gm) 3 (Bn-C 4 H 9 ) 2 }] (1) in high yield (Gm = glyoxime).We were unable to oxidize, either chemically or electrochemically, the clathrochelate 1 to the corresponding cobalt-(iii) complex (the process is irreversible and is accompanied by destruction of the clathrochelate structure), but 1 easily undergoes a chemical reduction in acetonitrile solution with silver in the presence of tetraalkylammonium or tetra(dimethylamido)phosphonium halides to produce the dark-blue complex 1 À in high yield (Scheme 1). Analogous reductions were also carried out for phenylboronic and fluoroboronic hexachloride cobalt(ii) clathrochelates. The resulting tetraalkylammonium and tetra(dimethylamido)phosphonium salts of clathrochelate anions with an encapsulated Co + ion proved to be air-stable as solid for several months.The X-ray crystallography data for 1 and [(NMe 2 ...

The Trigonal Prism in Coordination Chemistry

“…Such distortions have been already reported in the experimental structure of d 7 -cobalt(II) compounds. [95] In short, metals with high-spin d 4 and low-spin d 7 electron configurations show a similar Jahn-Teller behavior to that of the previously studied d 9 -copper(II) compounds. [28] The last model complexes optimized were [Fe(sepulchrate)] 3 + and [Fe(sepulchrate)] 2 + in both their high-and lowspin states, since several Fe II complexes with sepulchrate ligands have been found to be either diamagnetic or high spin in the solid state, but to present spin equilibria in solution.…”

A New Series of Cobalt and Iron Clathrochelates with Perfluorinated Ribbed Substituents

“…The CV data (see below) suggested the stability of the Co(I) intermediates in the CV time scale, we attempted to obtain a chemically reduced cobalt(I) clathrochelate using the synthetic approach described earlier. 4 The reduction of Co((C 6 F 5 ) 2 Gm) 3 (BC 6 H 5 ) 2 ( 3 ) by an excess of powder silver in the presence of tetramethylammonium chloride (in combination with which the metallic silver is known to form a strong reducing system 16 ) in acetonitrile led to the formation of a dark-blue solution with the intensive coloration caused by the clathrochelate anion [Co((C 6 F 5 ) 2 Gm) 3 (BC 6 H 5 ) 2 ] − . This anion was isolated as a salt with the bulky tetramethylammonium cation, a navy blue solid product ((CH 3 ) 4 N)[Co I ((C 6 F 5 ) 2 Gm) 3 (BC 6 H 5 ) 2 ] ( 7 ), that was air-stable for several months but rapidly oxidized in a solution.…”

Synthesis, X-Ray Structure and (Spectro)electrochemical Study of an Electrochromic Iron(II) Chlathrochelate with Tuned Redox Properties

“…cobalt(I)/cobalt(II) or cobalt(II)/cobalt(III) pairs}. [17][18][19][20][21][22] Another pathway of transformations of these cage metal complexes is based on the ligand-centered processes, such as chemical or electrochemical redox reactions of heterocyclic ribbed fragments, annulated to the quasiaromatic macrobicyclic framework. Recently [23] we reported the synthesis and reactivity of an iron(II) clathrochelate FeBd 2 (QnGm) (BF) 2 (where Bd 2-and Gm are a-benzildioxime dianion and glyoxime residue, respectively) carrying an annulated ribbed quinoxaline (Qn) fragment.…”