Dinuclear chromium complexes

It has recently been shown that oxidation of Cr+2 by such one-electron oxidizing agents as Fe(III) and Cu(II), yields Cr(H20) fs or one of its substitution products, whereas oxidation with such two electron oxidizing agents as HCIO and H202 yields a dinuclear chromium complex, probably (H20)5Cr—0—Cr(H20) 4. This difference in behavior... 

Cothermolysis of dinuclear chromium complexes [ CpxCr(CO)n 2] [n=2, (Cr=Cr) 3, (Cr-Cr)] with white phosphorus has been used by Goh and coworkers to generate a family of derivatives incorporating P2, P3 and P5 ligands (Scheme 7) [58, 96]. Remarkably, the paramagnetic dichromium triple decker derivative [ Cp Cr 2(jLZ,/f-P5)] (72) has been the first cyclo-P5 complex characterized by X-ray crystallography [94b]. 

Related reactions have also been performed starting directly from M(CO)6 precursors, via decar bony lation (UV irradation) of the corresponding intermediate [M =C(0Li)C=CCR20Li] and subsequent treatment with COCI2 [43, 90, 93]. However, these reactions are not always straightforward and, in some cases, different types of products derived from subsequent cyclization or addition reactions have been obtained. As an example, reaction of the intermediate chromium complex obtained from Cr(CO)6 and [C=CCMe20] with MeCOCl led to the bicyclic dinuclear allenylidene-carbene complex 3 (see Fig. 3) [94]. 

Finally, it should be noted that this chemistry may have biological relevance. Several metalloenzymes are believed to contain more than one metal ion bound at the active site. One relevant example is the glucose tolerance factor (GTF) which is important for the metabolic degradation of glucose (398-401). GTF is a low-molecular-weight protein which contains chromium(III). Its structure is not known, but it has been suggested that the active site contains a dinuclear chromium(III) complex (401). The fact that hydroxo-bridged dinuclear chromium(III) complexes exhibit reactions which are often very fast compared with those observed for the parent mononuclear species seems to support such a proposal. 

Dinuclear carbonyl complexes, with copper, 2, 187-188 Dinuclear chromium(III) complexes, with halides, alkyls, aryls, 5, 317... 

Dinuclear chromium(IV) complexes, examples, 5, 350 Dinuclear chromium(V) complexes, examples, 5, 350... 

Diphosphaferrocene, with chromium carbonyls, 5, 220 l,l -Diphospha[2]ferrocenophane, synthesis, 6, 210 Diphosphazanes, in dinuclear Ru complexes, 6, 674 l,l -Diphosphetanylferrocenes, preparation, 6, 200-201 Diphosphine borane complex, polypyrrole support for,... 

Excited States of Mononuclear and Dinuclear Chromium till) Complexes... 

D. Paramagnetic Mono- and Dinuclear Pentacarbonyl Complexes of Chromium... 

Fig. 29 Series of dinuclear chromium(III)-salen complexes synthesized by Vagin et al.

A bidirectional benzannulation strategy allows the extension of an existing biaryl skeleton. The BINOL-derived bis-carbene complex 69, accessible through a sequence of regioselective double ortfio-lithiation/Fischer carbene complex synthesis, undergoes bidirectional benzannulation to give the dinuclear biphenanthrene complex 70. The optical induction exerted by the binaphthyl core is, however, only moderate a mixture of C2- and Cj-symmetrical bis(phenanthrohydroquinone) bis-chromium complexes is formed, in which the C2-... 

The complexes [Cr(H20)jCH2X]2+ (X = Cl, Br, or OCH3) were found to react with mercuric nitrate [Eq. (8)] (38,65). This reaction is believed to involve a binuclear displacement of Cr by attack of Hg on the carbon atom. In contrast, the reaction of [Cr(H20)JCH2I]2+ with Hg2+ involves an abstraction of I- by Hg2+ [Eq. (9)]. The reaction of [Cr(H20)5CH2I]2+ with Cr2+ as shown in Eq. (10) has also been reported (66). This reaction is believed to proceed via a carbon-bridged dinuclear chromium intermediate. 

Dinuclear niobium sulfido and selenido dithiophosphates, Nb2Q4[S2P(OR)2]4,75,76 (Q = S, Se R = Et) (also xanthates and dithiocarbamates) have been prepared but no crystal structure was reported. Optically active chromium complexes, Cr[S2P(OR)2]3 derived from Z)-borneol and L-menthol, have been described.77... 

The properties of the PPN cation have also facilitated the crystallization of salts of the dinuclear metal carbonyl anions, [M2(CO)10]2 (M = Cr, Mo, W), and permitted a crystal-structure determination7 of these remarkable complexes. The dinuclear chromium decacarbonyl anion was first prepared by Behrens and Vogl8 by the reaction of Cr(CO)6 with NaBH4 in liquid NH3 at 40°. More recently Hayter9 prepared dinuclear decacarbonyl anions of all three Group VI metals by a photochemical process and isolated them as the tetraethylammonium salts. A convenient synthesis involving isolation of the PPN salts of these anions is described in Sec. D. 

The most uncommon molecular structure known so far to produce lyotropic mesophases is that of the chiral, dinuclear chromium(II) complex of phenanthroline and tartaric acid, (323). The complex does not possess an amphiphilic structure, i.e., no polar headgroup nor aliphatic tail, does not belong to the columnar type, and does not show thermotropic properties. Thus, the phenanthroline ligands must play the role of the hydrophobic part, while the central tartrate link must act as the hydrophilic one. A room-temperature, lyotropic mesophase was induced at low concentration in water (0.006 mol dm ). Moreover, in the concentration range of 0.06 to 0.1 mol dm the mesophase was still observed and remained stable up to 100 °C. The self-aggregation of the complex into a birefringent mesophase was proved by the use of several techniques including polarized optical microscopy, viscosity, and electrical conductivity measurements, as well as Na... 

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