Copper complexes hydrogen ligands

A side-on p,-Tq2 Tq2-peroxo dicopper(II) complex. A very important development in copper-dioxygen chemistry occurred in 1989 with the report by Kitajima et al. [10,108] that another Cu202 species could be prepared and structurally characterized by using copper complexes with a substituted anionic tris(pyrazolyl)borate ligand. This intensely purple compound, Cu[HB(3,5-iPr2pz)3] 2(02) (5), was prepared either by reaction of Cu[HB(3,5-iPr2pz)3] (4) with 02 or by careful addition of aqueous hydrogen peroxide to the p-dihydroxo... 

Moreover, we believe that the azo form helps in stabilizing several of the reactive copper complexes involved in this catalytic cycle such as the hydroxy copper complex 17. Thus, we surmise that this novel catalytic, aerobic oxidation procedure for alcohols into carbonyl derivatives proceeds via a dehydrogenation mechanism and relies on the effective role of hydrazine or azo compounds as hydrogen shuttles and stabilizing ligands for the various copper complexes (20). 

This hydrogen transfer is essentially an intramolecular acid-base reaction. The hydrogen of the coordinated alcohol function is acidified by coordination to the copper center whilst the hydrazine ligand possesses basic properties. The elimination of the hydrazine substituent is irreversible under these neutral conditions. Indeed, in the absence of excess base, DBADH2 is unable to displace the alkoxide ligand from the copper complex G. 

Some metal- (especially copper) complexes catalyse the dismutation of superoxide at rates that compare favourably with catalysis by superoxide dismutase. One could therefore argue that the presence of such complexes in vivo might be beneficial. There are, however, additional considerations (1) such metal complexes may also reduce hydrogen peroxide, which could result in the formation of hydroxyl radicals, and (2) it is extremely likely that the metal will be displaced from its ligands (even when those ligands are present in excess), and becomes bound to a biomolecule, thereby becoming less active as a superoxide dismutase mimic. As an example, copper binds well to DNA and catalyses the formation of hydroxyl radicals in the presence of hydrogen peroxide and ascorbate [30],... 

The crystal structures of the free ligand (47a) and that of its copper(II) complex were reported.18 In both cases the terminal /3-diketone residue is present in the keto tautomeric form. The /3-ketoimine fragment of the free ligand is planar and is stabilized by an internal hydrogen bond. The crystal structure of the copper complex shows the metal coordinated in the inner N202 site.18... 

Cu+ forms strong complexes with ligands such as CN- and I- so that the CuVCu potential becomes negative and copper metal will react with acids to form hydrogen these ligands also stabilize the Cu1 state against disproportionation. 

Complexes of metal + ligand + protein or DNA can also catalyze the Diels Alder cycloaddition or oxidations with hydrogen peroxide. Copper complexes bound to DNA catalyzed the Diels-Alder cycloaddition with up to 99% ee [15, 16], Cu(phthalocyanine) complexed to serum albumin also catalyzed the enantioselective (98% ee) Diels-Alder reaction, but only with very high catalyst loading (10 mol%) and only with pyridine-bearing dienophiles (presumably to complex the copper) [17]. Achiral Cr(III) complexes or Mn(Schiff-base) complexes inserted into the active site of apomyoglobin variants catalyzed the sulfoxidation of thio-anisole with up to 13 and 51% ee, respectively [18, 19]. A copper phenanthroline complex attached to the adipocyte lipid-binding protein catalyzed the enantioselective hydrolysis of esters and amides [20]. 

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