Dicopper -peroxo complex

Using 1,4,8,11-tetraazacyclotetradecane, the structure of complex (800) (distorted trigonal planar Cu-Cu 6.739 A) was determined. Reactivity with 02 was investigated to demonstrate the formation of trans-l,2-peroxo species.585 As part of their work with copper(I) complexes with 02, the structure of a dicopper(I) complex ((801) distorted tetrahedral 7.04 A), supported by macrocyclic ligand environment, was reported by Comba and co-workers. Tolman and co-workers structurally characterized a three-coordinate copper(I)-phenoxide complex (802) (planar T-shaped) that models the reduced form of GO.587 The copper(I) analogue [Cu(L)][CF3-SO3]-0.43MeOI I (803) of a copper(II) complex (534) was also reported to demonstrate the role of ligand framework conformability in CV /Cu1 redox potentials.434 Wilson and co-workers... 

Reversible Binding and Activation of Dioxygen and the Reactivity of Peroxo and Hydroperoxo Dicopper(II) Complexes... 

At present, we do not completely understand why only some of these very similar m-xylyl dicopper(I) complexes systems described above undergo ligand oxygenation reactions. However, based on the results outlined above, we can speculate on a number of aspects of this 02-activation process. Our studies implicate the presence of a copper-dioxygen (peroxo dicopper(II)) adduct as an intermediate in the oxygenation reaction and more recent kinetic studies (51) further support this conclusion. This adduct then either directly or via some further intermediate undergoes an electrophilic attack of the arene. The unique nature of this very fast reaction 2->3, and the observed inability to intercept the active... 

A related explanation has to do with the stability of the peroxo dicopper(II) intermediate, since it will either attack the substrate or decompose the kinetics of formation of the intermediate relative to those of the ensuing decomposition reactions will thus be important. Nelson (53) and Sorrell (90) have both described systems that undergo a Cu 02 = 4 1 reaction stoichiometry for dicopper(I) complexes where they propose that degradation of the peroxo dicopperfll) intermediate proceeds by the fast bimolecular two-electron transfer fiom a second dicopper(I) molecule to the putative peroxo-dicopper(II) intermediate to give an aggregated oxo-copper(II) product. [The latter may form hydroxo-Cu(II) species in the presence of protic solvents]. 

Figure 7 Reversible binding of 02 to 2 to give structurally characterized species 3, a trans- L-1,2-peroxo dicopper(TI) complex.

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... 

Figure 8 Reactions generating complex 5, a structurally characterized g-r 2 r 2-peroxo dicopper(II) complex.

Sorrell [114] has prepared ligands similar to that found in 8, but possessing pyrazole or mixed pyrazole/pyridine ligands. The corresponding phenoxo-bridged dicopper(I) complexes appear to bind 02 in the same fashion these 02 complexes all exhibit a characteristic purple color with strong 500-510 and 610-630 nm bands that are presumed also to be peroxo-to-Cu(II) LMCT transitions. 

Figure 11 Reversible 02 and CO binding to dicopper(I) complex 13 (n = 3-5). Complexes 15 possess p,-r 2 r 2-peroxo coordination.

Figure 12 Formation of a peroxo dicopper(H) complex 22 containing a simple imidazole ligand. The preformed Cu202 moiety can be transferred by substitution of L donors by tnipa, giving known complex 3.

Figure 13 Reactivity comparisons of peroxo-dicopper(II) complexes 3, 8, and 15.

A further extension of m-xylyl dicopper complexation and xylyl hydroxy la-tion is seen in a Schiff-base macrocycle [175], When dicopper(I) complex 38 is reacted with 02, one of the two arene rings is hydroxylated, again producing a phenoxo-bridged dicopper(II) species a peroxo-dicopper(II) intermediate is suggested as the actual oxidant. 

The properties of Kitajima s p-p2 Ti2-pcroxo dicopper(II) complex lead to the conclusion that this is the likely structure in oxyhemocyanin and oxytyrosi-nase this is perhaps the most important contribution from this type of model chemistry. A distorted or closely related peroxo-dicopper(II) species appears to be involved in aromatic hydroxylation proceeding in a well-characterized tyrosinase model system. 

A trans-n- 1,2-Peroxo Dicopper(II) Complex. Our own efforts have resulted in the structural and spectroscopic characterization of five types of copper-dioxygen complexes (6), distinguished on the basis of the ligands used for their synthesis and on their distinctive structures or physical properties. Thus, the manner in which hemocyanin binds 02 is not the only one possible, and it is of considerable interest to deduce the structures, along with associated spectroscopy and reactivity of a variety of types. Dioxygen can bind to dinuclear transition metals in a variety of structural modes, shown in Figure 2. As mentioned, mode C is present in oxy-Hc and Kitajima s model complex (Scheme 1), whereas we have structural and spectroscopic evidence for types A (30-32), B (33-35), and F (36-38) for peroxo 022- binding, and mode D (39, 40) in the case of hydroperoxo (OOH ) complexes. 

The notion of an electrophilic attack mediated by the peroxo group in [Cu2(R-XYL-H)(02)]2+ (7) is also in accord with studies on reactivity comparisons of three classes of peroxo-dicopper(II) complexes, including [ (TMPA)Cu 2(02)]2+ (3) and [Cu2(Nn)(02)]2+ (5, n = 4) (Figure 5) (48). We found that the ii-rfirf-022 ) group in [Cu2(N4)(02)]2+ (5) behaves as a nonbasic or electrophilic peroxo ligand, in contrast to the basic or nucleophilic behavior of the peroxo group in 3, which possesses end-on coordination. For example, in reactions with H+, C02, and PPh3,... 

Figure 5. Summary of reactivity comparisons of end-on vs. side-on bound peroxo-dicopper(II) complexes (48,).

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