452 IRON-CONTAINING PROTEINS AND ENZYMES
properties are similar to those found for (a) the other model complexes
discussed and (b) the CcO enzyme ’ s Cu A center: (1) UV – visible (568 nm,
1400 M − 1 c m − 1 , 446 nm, 1600 M − 1 c m − 1 ) and more intense near - IR (705 nm,
4000 M − 1 c m − 1 ) bands and (2) EPR seven - line hyperfi ne coupling from the fully
delocalized mixed - valent copper species.
Tolman and co - workers reported the fi rst example of a dithiolate - bridged
mixed - valence binuclear copper ion complex in 1996.^166 (See Figure 7.48B .)
X - ray crystallography of this model compound revealed a planar MV [Cu 2 ( μ -
SR) 2 ] + core with a Cu – Cu distance of 2.92 Å , considerably longer than that
found in the CcO enzyme Cu A center (2.43 – 2.58 Å ) or in the other model
complexes discussed above (2.39 – 2.42 Å ). The complex involved the use of a
newly synthesized ligand, NaL iPrdacoS , whose structure is shown in Figure 7.48B.
The reaction shows three moles of NaL iPrdacoS reacting with two moles of
Cu(OTf) 2 to produce the mixed - valent product (L iPrdacoS Cu) 2 (O 3 SCF 3 ) — one
mole of NaL iPrdacoS acting as a reducing agent to produce the desired Cu 1.5 · · · Cu 1.5
product. EPR data for (L iPrdacoS Cu) 2 (O 3 SCF 3 ) exhibit the expected seven - line
hyperfi ne splitting for the MV product. The UV – visible spectrum of
(LiPrdacoS Cu) 2 (O 3 SCF 3 ) shows absorbances that are red - shifted to longer wave-
lengths than those for the CcO Cu A center; and the near - IR band for
(LiPrdacoS Cu) 2 (O 3 SCF 3 ) occurs at 1466 nm ( ε = 1200 M − 1 c m − 1 ), a much lower
energy than that for Cu A (800 nm ( ε = 1600 M − 1 c m − 1 )). Further comparisons
between CcO ’ s Cu A center and the (L iPrdacoS Cu) 2 (O 3 SCF 3 ) complex have been
carried out using absorption, magnetic circular dichroism (MCD), and X - ray
absorption (XAS) spectroscopies. Molecular orbital (MO) - splitting diagrams
and detailed descriptions of the relationship between these and observed spec-
troscopic results for CcO ’ s Cu A and mixed - valent [Cu 2 ( μ - SR) 2 ] + complexes
have been published.^167 For [Cu 2 ( μ - SR) 2 ] + these researchers defi ned ψ and ψ
molecular orbitals derived from delocalization of electron density between the
two copper centers. The doubly occupied ψ molecular orbital arises from
overlap of two Cudxy orbitals to form a πg antibonding interaction, while the
ψ singly occupied HOMO arises from the same Cu dxy orbitals to form a πu -
bonding interaction. Figure 6 of reference 167a illustrates the complete molec-
ular orbital diagram for coupling two C 2v CuS 2 fragments. The ψ → ψ transition,
occurring at 13,000 cm − 1 for CcO ’ s Cu A center is assigned to the class III, mixed
valence Cu – Cu overlap because a large change in Cu – Cu separation accom-
panies the excitation in the near - IR resonance Raman spectrum. Further analy-
sis of the 13,000 cm − 1 band (that converts to an ∼ 800 - nm band in UV – visible
and MCD spectra) assigns the transition to Cu – S plus Cu – Cu orbital overlaps.
In the [Cu 2 ( μ - SR) 2 ] + model, the ψ → ψ * transition occurs at ∼ 5600 cm − 1 (trans-
lating to the UV – visible or MCD absorbances at ∼ 1450 – 1600 nm), a much
lower energy. This observation led to the conclusion that the valence delocal-
ization in the model complex (L iPrdacoS Cu) 2 (O 3 SCF 3 ) — where the Cu – Cu dis-
tance is equal to 2.92 Å — is mediated by the bridging thiolate ligands only,
whereas in CcO ’ s Cu A center the valence delocalization arises not only through
the bridging thiolates but also by direct Cu – Cu overlap, given the Cu – Cu dis-