Oxide-bridged dinuclear complexes

Structural Parameters for Oxide-Bridged Dinuclear Complexes... 

Diazidobiphenyl is transformed by diiron nonacarbonyl into a cyclic urea together with a small amount of the bridged dinuclear complex 142 the latter is a minor product but it can be converted oxidatively into benzocinnoline (Scheme 165).191... 

It has long been known (93) that cobalt(II) complexes of phthalocyanines interact with molecular oxygen. The water-soluble tetrasulfonato derivative of the parent phthalocyanine selectively and catalytically oxidizes 2,6-di-tert-butylphenol to the benzoquinone and the dipheno-quinone in both homogeneous solution (94) and when polymer-supported (95). The active intermediate in the catalytic cycle is proposed to be the (as expected) mononuclear dioxygen complex of the cobalt-tetrasulfonatophthalocyanine system (92). It has been proposed that the formation of a peroxo-bridged dinuclear complex is responsible for the deactivation of the cobalt(II)-tetrasulfonatophthalocyanine system, since such a dinuclear system would be unable to further bind and activate dioxygen (96). Such deactivation results, ultimately, in loss of the catalyst and low turnover ratios. 

Oxidation of cobalt(II) with 02 or H202 may also yield di- or trihydroxo-bridged dinuclear complexes, some of which have already been mentioned in Section IV,A. Other examples may be found in the literature (248- 250). 

Excited states responsible for the photodisproportionation of p-oxo-bridged dinuclear complexes (Por)M,u-0-M,l(Por), where M = Fe or Mn, are believed to be of LMCT nature. The transfer of an electron 02p - M3d leads to the reduction of this central atom and decomposition of the binuclear entity. The second central atom as a part of an oxo compound will be in a higher oxidation state, the whole process is then a disproportionation. In this sense the photochemical decomposition of the complex (TPP)Fe111 (p-0-ll)Fe,n(TPP) is illustrative [249-253]. 

Fig. 3. Basic structures of quadruply bridged dinuclear complexes for the oxidation states of +2, + 3, and + 2.5.

Figure 2. Simplified qualitative molecular orbital description of the n-type interaction of oxide-bridged di- and tri-nuclear complexes (only the relevant orbitals are shown), (i) Linear oxo-bridged dirmdcar complexes, (ii) Bent oxo-bridged dinuclear complexes (with additional caiboxylate bridges), (iii) Oxo-centered tiinuclear complexes, (iv) a- and n-type interaction of edge-shared dinuclear complexes.

Hydroxo-bridged dinuclear complexes, [Pt(P-P)(/u-OH)]2 (where P-P = DIOP, BINAP, NORPHOS, MeDUPHOS, and BPPM) as precursors have been used for the enantioselective Baeyer-Villiger oxidation of ketones (290). The platinum(O) catalyst precursor, Pt(MeDUPHOSXfrans-stylbene), catalyzes the asymmetric hydrophosphination of activated olefins (291). 

Chlorodicarbonyl(tetraphenylcyclobutadiene)cobalt loses carbon monoxide as well as chlorine when stirred with trifluoromethylthiosilver, to give the sulphur-bridged dinuclear complex [Ph4C4Co(CO)(S CF3)]2, contrasting with the formation of the mononuclear iron complex C5H5Fe(CO)2(S-CF3). Cobalt(ii) salts form complex ions [Co(HHIB)2] with hexafluoro-hydroxyisobutyric acid, which are not oxidized to cobalt(iii) under the reaction conditions. ... 

Superoxo-bridged dinuclear complexes of rhodium(ni) with pyridine or y-picoline ligands (L), [(HaO)L4Rh Oa RhL4(HaO)] +, have been prepared, in which acid-base equilibria involving the complex water molecules have been demonstrated. The corresponding dichloro-complex has been shown to be a strong oxidant, the reaction... 

Gold in the oxidation state +1 also tends to form dinuclear complexes with bridging amidinate ligands. A typical example is Au2[HC(NC6H3Me2-2,6)2]2 (cf. Section Oxidative addition of iodomethane to the dinuclear gold(I)... 

Oxidative-addition reactions have been widely studied with bridged dinuclear metal complexes [1,2,5, 32]. Earlier work with the ylides and sulfur bonded ligands... 

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