Synthetic Models for Type III Copper

In 1988, the author s group reported the formation of a fx-peroxo dicopper complex by low-temperature treatment of the p-oxo di- 

Interaction diagram for the frontier orbitals of [(NH3)3Cu]2(02)2. Two (NH 1)3Cu2 fragments are combined and have a staggered (D3h) geometry, and the [(NH3)3Cu]2+ composite is mixed with O2-. 

Both p-1,2 and p-p2 172 peroxo structures can account for the occurrence of two intense bands observed at 350 and 580 nm for oxy-Hc, because two distinct LMCT bands due to Of n and 07r are predicted to split significantly. Detailed theoretical analyses of the Of — Cu(II) LMCT bands have been reported by Ross and Solomon (254). According to the analyses, the 7r -7r split in p-172 tj2 is larger than that of cis-p-1,2, suggesting that the p-172 172 structure is more likely as a model for oxy-Hc. The low 0-0 vibrational stretch of oxy-Hc was also interpreted in terms of the p-172 tj2 structure based on MO calculations (254), which indicated that Of o- can act as a 7r acid. Hence, the back-donation of the copper to weaken the 0-0 bond of the peroxide ion may result in an unusually low 0-0 frequency in the p-172 Tj2 peroxo complex. In contrast, no clear account was addressed for the low 0-0 frequency of oxy-Hc, assuming the cis-p-1,2 structure. 

As mentioned above, both cis-p-1,2 and p-rj2 172 remain vital candidates as the coordination mode of the peroxide in oxy-Hc and oxy-Tyr. The p-172 172 structure does seem, however, more likely because its particular structure gives a simpler and more straightforward account of all the unusual features of oxy-Hc. In order to distinguish these two possibilities conclusively, the synthesis and characterization of a cis-p-1,2 peroxo complex with a hydroxide bridge presents a serious challenge for inorganic chemists. On the other hand, a single-crystal X-ray analysis for oxy-Hc is currently underway and may provide a definitive answer (234). 

Reaction Aspects of p-Peroxo Dinuclear Copper Complexes Relevant to Tyrosinase Catalysis 

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