A R T I C L E S
Henson et al.
Table 1. Absorption Maxima for the Dioxygen Bound R-MePY2 (in CH2Cl2) and R-TMPA Series (Et2O)
H
MeO
Me2N
λ
(nm)
energy
(cm-1
ꢀ
λ
(nm)
energy
ꢀ
λ
(nm)
energy
ꢀ
-1
-1
R )
)
(cm-1 M-1
)
(cm
)
(cm-1 M-1
)
(cm
)
(cm-1 M-1
)
MePY2
356
410
530
654
28 100
24 400
18 900
15 300
14 800
2900
570
356
410
535
650
28 100
24 400
18 700
15 400
20 000
3500
1000
750
360
∼410a
515
27 800
∼24 400
19 400
21 000
1400
730
460
650
15 400
TMPA
520
19 200
∼14 000
520
19 200
∼15 000
523
19 100
∼11 000
a This peak exists as a shoulder on the 360 nm absorption peak.
The samples were prepared by dissolving the Cu(I) amine precursor
in solvent in 5 mm NMR tubes and oxygenating at -80 °C. Typical
rR solution sample concentrations were in the range of 2-5 mM of
Cu (1-2.5 mM of dimer) to minimize self-absorption. The samples
were cooled to 77 K in a quartz liquid N2 EPR finger dewar (Wilmad)
and hand spun during scan collection to minimize sample decomposi-
tion. Isotopic substitution was achieved by oxygenation with 18O2
(ICON, 99% labeled).
Computational Details. Normal coordinate analysis (NCA) was
performed on a simplified [(N2Cu)2O2] complex by using QCPE
program 576 by M. R. Peterson and D. F. McIntosh, which involves
solution of the secular equation |FG - λE| ) 033,34 by the diagonal-
ization procedure of Miyazawa.35 The calculations were based on a
general valence force field. Force constants were refined with the
nonlinear optimization routine of the simplex algorithm according to
Nelder and Mead.36 Input data sets can be found and additional NCA
details can be found in the Supporting Information.
DFT calculations were performed with the program package Gauss-
ian 9837 using the BP86 functional38 (modified to include 38% Hartree-
Fock exchange) and the LANL2DZ basis set.39-42 A geometry optimized
[{(NH3)3CuII}2(O2)]2+ structure in the broken symmetry and delocalized
singlet spin states (the delocalized singlet was calculated to directly
evaluate the effect of N-donor strength on the peroxide σ*u orbital;
this mixed orbital is dominated by π*σ interaction in the broken
symmetry calculation) was used as a control. Replacement of the
histidine ligands by ammonia ligands is supported by previous
studies.25,26 Increased N-donor strength was modeled by shortening the
equatorial Cu-N bonds by 0.05 Å and performing a single-point
calculation. Input data sets are given in the Supporting Information.
Figure 1. Resonance Raman spectra (77 K) of the [{(R-TMPA)CuII}2
(O2)]2+ series in Et2O for a 514 nm excitation. R ) H is the green spectrum,
R ) MeO is the red spectrum, and R ) Me2N is the blue spectrum. See
Figure S2 for an expanded view of the Cu-Nax region. Inset: Resonance
Raman (77 K) 16O2 (solid line)/18O2(dashed line) isotopic comparison of
[{(R-TMPA)CuII}2(O2)]2+ (R ) MeO, Me2N). The solvent was Et2O, and
λex ) 514 nm. R ) MeO is the red spectrum, and R ) Me2N is the blue
spectrum. Note: 18O2 spectra contain a small amount of isotopic 16O2.
at ν ) 822 cm-1 shifts to 779 cm-1 16,18∆ ) -43 cm-1), while
(
the peak at ν ) 557 cm-1 shifts to 533 cm-1 16,18∆ ) -24
(
cm-1). Similarly, a shift is observed in the more intense peak
in the trans µ-1,2 end-on dimer [{(Me2N-TMPA)CuII}2(O2)]2+
spectrum (Figure 1, inset): ν ) 812 cm-1 16,18∆ ) -43 cm-1).
(
However, the 16O2 [{(Me2N-TMPA)CuII}2(O2)]2+ spectrum
shows a doublet of peaks at 545 and 557 cm-1 arising from a
Fermi resonance (Figure 1), as this splitting is not present in
the 18O2 spectrum at 524 cm-1. Averaging of the two peaks in
the 16O2 spectrum (av ) 551 cm-1) gives an isotope shift of
-27 cm-1 for this feature (Figure 1, inset). These data are
consistent with results on the end-on peroxo-dicopper(II)
complex using the unsubstituted TMPA ligand: νÃ-à ) 827
Results and Analysis
R-TMPA. The resonance Raman (rR) spectrum of 16O2
bound [{(MeO-TMPA)CuII}2(O2)]2+ in diethyl ether (Et2O) for
λex ) 514.5 nm shows two main peaks which display isotope
shifts upon 18O2 substitution (Figure 1, inset). The intense peak
(33) Wilson, E. B., Jr.; Decius, J. C.; Cross, P. C. Molecular Vibrations; Dover
Publications: New York, 1980.
cm-1 16,18∆ ) -44 cm-1); νCu-O ) 561 cm-1 16,18∆ ) -26
( (
(34) Woodward, L. A. Introduction to the Theory of Molecular Vibrations and
Vibrational Spectroscopy; Clarendon Press: Oxford, 1972.
cm-1).29 Thus, the intense peaks at 827-812 cm-1 are assigned
as the νO-O stretches, while the 551-561 cm-1 bands cor-
respond to the symmetric combination of the Cu-O stretches.
[Note that the asymmetric combination of the Cu-O stretches
is also observed in the H-TMPA rR spectrum: ν ) 525 cm-1
(35) Miyasawa, T. J. Chem. Phys. 1958, 29, 246.
(36) Nelder, J. A.; Mead, R. Comput. J. 1965, 7, 308.
(37) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A.; Strattmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Oritz,
J. V.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.;
Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng,
C. Y.; Nanayakkara, A.; Gonzalez, C.; Challacombe, M.; Gill, P. M. W.;
Johnson, B. G.; Chen, W.; Wong, W. M.; Andres, J. L.; Head-Gordon,
M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.7; Gaussian,
Inc.: Pittsburgh, PA, 1998.
(
16,18∆ ) -19 cm-1) and ν ) 511 cm-1 16,18∆ ) -20 cm-1),
(
consistent with previous normal coordinate predictions for
TMPA (see Supporting Information Figure S1).]29
The Cu-Nax stretch (nitrogen trans to the axial peroxo
O-ligand in the trigonal bipyramidal coordination geometry) was
seen at 429 cm-1 for [{(H-TMPA)CuII}2(O2)]2+ (Table 2).29
Similar stretches are observed for the [{(MeO-TMPA)CuII}2
(O2)]2+ and [{(Me2N-TMPA)CuII}2(O2)]2+ complexes (Figure
1). After the deconvolution of the Fermi resonance,33 the pure
(38) Perdew, J. P. Phys. ReV. B: Condens. Matter 1986, 33, 8822.
(39) Dunning, T. H. J.; Hay, P. J. In Modern Theoretical Chemistry; Schaefer,
H. F., III, Ed.; Plenum: New York, 1976; Vol. 3, p 1.
(40) Hay, P. J.; Wadt, W. R. J. J. Chem. Phys. 1985, 82, 270.
(41) Hay, P. J.; Wadt, W. R. J. J. Chem. Phys. 1985, 82, 299.
(42) Wadt, W. R. J.; Hay, P. J. J. Chem. Phys. 1985, 82, 284.
Cu-Nax modes are estimated to be at 427 and 426 cm-1
,
9
5188 J. AM. CHEM. SOC. VOL. 125, NO. 17, 2003