DISTORTION OF THE Cu2O2CORE
The VBCI model has been employed for the interpretation of the optical absorption and CD spectra of of peroxo bridged CuIIdimers like oxy-Hemocyanin and corresponding small-molecule analogs (Figure 3).15–17,24–26 Hemocyanin (Hc), the oxygen-transport protein of mollusks and arthropods, contains a binuclear Cu(I) active site that reversibly binds dioxygen as peroxide in a side-on bridging (–2:2) geometry (Scheme 1(A)).27 The highest occupied molecular orbitals of peroxide are a doubly degenerate * set which split in energy upon bonding to a metal center: a -bonding orbital within the plane of the metal–peroxo bond, *, (cf. Scheme 1(A)) and a -bonding orbital vertical to this plane, *v. As - is much stronger than -bonding, the *
orbital is much lower in energy than the *v orbital, and the *!dx2y2 CT transition is expected to be at much higher energy than the*v!dx2y2transition. From overlap considerations with respect todx2y2, the intensity of the*will also be considerably higher than that of the*v
transition. In agreement with this qualitative picture, the two intense bands in the optical absorption spectrum of oxy–Hc at 17,200 cm1("=1,000 M1cm1) and 29,000 cm1("=20,000 M1cm1) have been assigned to the ED allowed transitions to the *vand *CT states, respectively, and the positive feature in the CD spectrum near 480 nm (21,000 cm1;"=1 M1cm1) as the MD allowed transition to thegcomponent of the*vCT state (Figure 3).
546 Valence Bond Configuration Interaction Model
O Cu Cu
Cu Cu
O
O O α
α
α α
(a) (b)
π∗
πσ v
∗
Scheme 1
In order to reproduce these transition energies with the VBCI model, parameters derived from DFT calculations and spectroscopy were used.28Initially, the active site of oxy-Hc was modeled using a simplified [(NH3)4Cu2(O2)]2þplanar core (symmetryD2h), and the relative energies of the MO triplet states were determined with SCF-X-SW transition-state calculations.29 From these values the zeroth-order energies for the * CT () and the *v CT states (v) as well as the magnitude of the in-plane transfer element (hd)were calculated. The zeroth-order energy for the MMCT state was assumed to be equal to the Mott–Hubbard energyU, which for CuIIdimers was found by PES to be 6.5 eV (cf.Section 2.42.5).30In planar D2hsymmetry, however, the out- of-plane *v orbital has no overlap with the Cu dx2y2 orbitals, and hence the transfer matrix element (hd)v between *v and dx2y2 vanishes; consequently, no splitting of the *v states is predicted in this symmetry. In order to reproduce the experimentally observed splitting of these states, the actual dimer geometry has to be considered. Inclusion of the transaxial ligands lowers the symmetry toC2h, and due to the pyramidal coordination the quantization axis of each copper center is tilted with respect to the Cu2O2plane (inScheme 1(A)). This leads to a nonzero value of the matrix element (hd)v. An estimate of its magnitude was obtained from the value of (hd)
and the relative, experimentally determined oscillator strengths of the*and*vabsorption bands.
Figure 3 Optical absorption and CD spectrum of oxy-Hemocyanin and the model complex [Cu2(N3PY2)O2]2þ. While the spectrum of oxy-Hc is typical for -2:2Cuperoxo systems with an almost planar Cu2O2 unit, the model complex has a bent side-on peroxo bridged (butterfly) structure (see text;
adapted from ref.31).
Using these parameters, the VBCI energy level scheme shown on the left side of Figure 4 was obtained. Importantly,
the Ag component of the *! CuIICT transition (electric dipole forbidden) was calculated to be2,000 cm1above the electric-dipole allowedBucomponent (observed experimentally at 345 nm), and
the splitting of the two components of the*v!CuIItransition was calculated to be3,000 cm1 with the CD-active band (Ag component) being above the electric-dipole allowed band (Bucomponent), in agreement with the experimental observation.
More recently, the above treatment was also applied to Cu2O2 cores that exhibit a butterfly structure, i.e., bent around the O–O axis (Scheme 1(b)).31As compared to the planar Cu2O2cores the absorption spectra of these systems exhibit one additional intense band in the 420–490 nm region (cf.Figure 3). Resonance Raman Spectroscopy shows that this feature is one component of the *! CuII transition which becomes allowed in the lower symmetry (C2v) of the bent geometry. To apply the VBCI model to the C2v butterfly core, values for , (hd), v and (hd)vderived from DFT triplet CT transition-state calculations were employed. In analogy to the C2hcase (vide supra), a non-vanishing value of (hd)vresults from a nonplanar geometry (Scheme 1(B)). Theand (hd)parameters obtained are smaller than those calculated for the planarC2h
model, reflecting the loss of interaction between the peroxide*and the Cudx2y2orbitals in the butterfly core. Due to the greater interaction between the peroxide*vand the Cudx2y2orbitals, on the other hand, the magnitude of the zeroth-order energy vand the transfer matrix element (hd)vare larger than those of the planar core. Diagonalization of the VBCI matrices using these parameters gives the energies for the *and*vCT states as shown inFigure 4, right, with the following results:
The A1 component of the * CT is now calculated to be approx. 900 cm1 below the B1
component, in contrast to the results obtained for oxy–Hc (C2h), where theAgcomponent of the*CT was calculated to be approximately 2,000 cm1abovetheBucomponent. This order of *CT state components is in agreement with the experiment.
1BuMMCT
1BuCT
1BuCT
1A1MMCT 1B1MMCT
1B1CT
1B1CT 1A1CT 1A1CT 1Ag DCT at
1AgCT 1Ag CT
1Ag CS
1A1 GS
1A1 DCT at 1AgMMCT
140,000cm–1 119,200cm–1
MMCT π*σ CT
π*σ CT π*σ CT
∇
∇ ∇
σ
v v
U 80,000
60,000
40,000
20,000
–20,000 Energy (cm–1)
0
π*v CT σ ∇ MMCT U
Figure 4 VBCI diagram for a side-on peroxide-bridged copper dimer in planar (left) and butterfly (right) geometries. Unperturbed singlet states are shown for both planar and butterfly geometries (far left and far right, respectively) and after CI is introduced between the GS, CT, and higher energy CT states (MMCT and DCT, DCT omitted from diagram for clarity) (center left, planar; center right, butterfly) (reproduced by
permission of the American Chemical Society fromJ. Am. Chem. Soc.1999,121, 1299).
548 Valence Bond Configuration Interaction Model
The splitting between the two components of the*vCT state is calculated to be4,000 cm1, compared with the splitting of2,900 cm1calculated for the planar C2hcore. This is in good agreement with the spectroscopic data which show the splitting of the *v states to be approximately 5,000–6,000 cm1, compared with the 3,000 cm1splitting in oxy–Hc.
Thus, the VBCI model accounts for the spectroscopic properties of CuIIperoxo systems with both planar and bent –2:2 Cu2O2 cores and correctly reproduces the spectroscopic–structural correlations observed experimentally. It shows that the * interaction produces the super- exchange pathway for AF coupling in the GS and provides an experimentally calibrated VBCI estimate of2JGS.