BIOINORGANIC CHEMISTRY A Short Course Second Edition

CYTOCHROME c OX IDASE 451

ligands provide (a) steric hindrance to stabilize the binuclear copper ion MV
core and (b) a less positively charged overall complex than the neutral octa-
azacryptand ligands discussed above. In spite of their quite different nature,
both types of ligands result in model complexes with similar physical proper-
ties. X - ray crystallography of the carboxylate - bridged complexes reveals
square pyramidal geometry for each copper ion with Cu – Cu distances between
2.40 and 2.42 Å. EPR spectroscopy shows that the complexes exhibit the
seven - line signals demonstrating fully delocalized copper ion electronic
structure.
Next in the discussion of Cu A site model complexes, Borovik and co - workers
have prepared mixed - valence dicopper complexes using the tripodal ligand
tris [( N′ - tert - butylureayl) - N - ethyl)]amine.^165 The ligand has three N - ethylene -
N′ - tert - butyl urea arms attached to an apical nitrogen atom. (See Figure 7.48A .)
This ligand links two copper ions through two mono - deprotonated urea ligands
producing a completely delocalized Cu 1.5 · · · Cu 1.5 core with two μ - 1,3 - ( κ N : κ O) –
ureate bridges and copper centers separated by 2.39 Å. The ligand produces a
pseudomacrocyclic ligand around the copper ions through intramolecular
hydrogen bonds formed from urea groups on adjacent ligands. Spectroscopic

Figure 7.48 Two cytochrome c oxidase Cu A center model compounds: (A) delocalized
Cu1.5 · · · Cu 1.5 core with two μ - 1,3 - ( κ N: κ O) – ureate bridges as reported in reference 165 ;
and (B) a dithiolate - bridged mixed - valence binuclear copper ion complex as reported
in reference 166.

HN

O

HN

R = But

N N

+Na S

N N

N S N

SCu

Cu1.5 1.5

Cu1.5 Cu1.5

N O

O N

Bu NH

t

HN But

N

R

R

N

R

R

MeOH

B

A

A tris[(N'-tert-butylureayl)-N-ethyl)]amine ligand reacted with two Cu ions to form Cu1.5.. .Cu1.5 complex as

3

NaLiPrdacoS

OTf− = triflate anion, O 3 SCF 3 -

+OTf−

(LiPrdacoSCu) 2 (O 3 SCF 3 )

A dithiolate ligand reacted with two Cu ions to form Cu1.5.. .Cu1.5 complex as described in reference 166.

2 Cu(OTf) 2

described in reference 165.