Electrochemical studies, chromium complexes

The very air-sensitive compounds [Mo(terpy)2] (purple) and [W(terpy)2] (green) have been described by a number of workers 44,174). The chromium compound may be prepared by the reaction of [Cr(CO)g] 44), [Cr(CN)g] 42,47), [Cr(CO)3(C6H6)] 47), or [Cr(bipy)3] 44) with two equivalents of terpy. The molybdenum 44,174) and tungsten 44) compounds have been prepared from [M(CO)g] in a similar manner. Electrochemical studies on [Mo(terpy)2] indicate three one-electron oxidations 8). Photosubstitution reactions of [M(CO)g] in the presence of limited amounts of terpy lead to the formation of [M(CO)4(terpy)] (M = Cr, Mo, or W) 205). It is unlikely that these compounds are seven-coordinate, and they may well provide examples of a bidentate terpy. [Mo(CO)3(terpy)] may be prepared by the reaction of terpy with [Mo(CO)3(mesitylene)], but attempts to prepare the other group VI complexes of this stoichiometry lead to the formation of [M(CO)4(terpy)] 205). 

The oxidation of substituted dicarbonyl complexes of chromium(I) featuring the bidentate ligands dppm and dppe has been examined. Chromium(O) starting materials, Cr(CO)2(dppm)2 and Cr(CO)2(dppe)2, both with cis carbonyl configurations, were oxidized to trans cations [Cr(CO)2(dppm)2]+ and [Cr(CO)2(dppe)2]" chemical and electrochemical methods were employed. Chemical oxidation of the dppm compound proved more facile than that of the dppe analogue this had been predicted on the basis of electrochemical studies. The dicarbonyl hydride, [Cr (CO)2(dppm)2H]+, was isolated from the oxidation reaction of [Cr(CO)2(dppm)2] with O2/HCIO4, while the same reaction conditions oxidized the neutral dppe complex to the cation. 

Another series of octahedral chromium complexes, the electrochemical behavior of which has been studied, is represented by the isocyanides shown in Scheme 7-5. 

Cuadrado and co-workers have reported the synthesis of chromium-containing organosilicon dendrimers.334 These dendrimers, 282, have chromium tricarbonyl units incorporated pendent to the terminal aromatic rings.334 Synthesized via the reaction of the silane dendrimer precursor with chromium hexacarbonyl, complete complexation was not possible due to steric hindrance at higher generations. Electrochemical studies showed that the oxidation of the chromium atoms occurred reversibly in the absence of a nucleophilic species and that the chromium tricarbonyl units behaved as isolated redox centers. 

The resolution of tris(catecholato)chromate(III) has been achieved by crystallization with L-[Co(en)3]3+ the diastereomeric salt isolated contained the L-[Cr(cat)3]3 ion.793 Comparison of the properties of this anion with the chromium(III) enterobactin complex suggested that the natural product stereospeeifically forms the L-cis complex with chromium(III) (190). The tris(catecholate) complex K3[Cr(Cat)3]-5H20 crystallizes in space group C2/c with a = 20.796, 6 = 15.847 and c = 12.273 A and jS = 91.84° the chelate rings are planar.794 Electrochemical and spectroscopic studies of this complex have also been undertaken.795 Recent molecular orbital calculations796 on quinone complexes are consistent with the ligand-centred redox chemistry generally proposed for these systems.788... 

The solvolysis of hexaaquachromium(HI) in DMSO proceeds via the series of complexes [Cr(DMS0)n(H20)6 ]3+ (n = 1 to 6).945,946 The anation and solvolysis of several chromium(III) complexes in DMSO have been studied 947,948 in general the reactions proceed by /d mechanisms. A direct electrochemical synthesis of [Cr(DMSO)6][BF4]3 has been reported.949 A comprehensive review of metal ion complexation by DMSO contains an extensive section on chromium(III).950... 

The formations of chloro,1087,1088 bromo1089 and iodo1090 complexes have been studied. Recently, electrochemical methods have been used to measure equilibrium constants for the formation of chloro (0.086), bromo (1.1 x 10 3) and iodo (1.1 x 1(T5) complexes of chromium(III).1091... 

The electrochemical oxidations of chromium and tungsten tricarbonyl complexes, respectively, were studied. 

The ferrocenylcarbene complexes shown in Scheme 7-3 were the first octahedral chromium fragments studied from an electrochemical viewpoint [32]. 

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