Copper/α-ketocarboxylate chemistry with supporting peralkylated diamines: Reactivity of copper(I) complexes and dicopper-oxygen intermediates

Article abstract of DOI:10.1021/ic100032n

To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N-di-tert-butyl-N,Ndimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by X-ray crystallography and spectroscopy in solution, and their reactivity with O₂ at -80 °C examined. Oxidative decarboxylation of the α-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene, cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates involving displacement of the α-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I) moiety to yield copper-oxygen species that subsequently induce decarboxylation. To test these hypotheses, spectroscopic and kinetic studies of the reactions of Bu₄NBF with preformed μ-n²:n²-peroxodlcopper(II) and/or bls(μ-oxo) dicopper(III) complexes supported by L or N,N,N,N-tetramethylpropylenediamine were performed. In an illustration of a new mode of reactivity for such dicopper-oxygen cores, decarboxylation of the added α-ketocarboxylate was observed and the intermediacy of a carboxylate-brldged μ-n²:n ²-peroxodicopper(II) complex was implicated.

Full text of DOI:10.1021/ic100032n

Inorg. Chem. 2010, 49, 3531–3539 3531
DOI: 10.1021/ic100032n
Copper/r-Ketocarboxylate Chemistry With Supporting Peralkylated Diamines:
Reactivity of Copper(I) Complexes and Dicopper-Oxygen Intermediates
Aalo K. Gupta and William B. Tolman*
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant St. SE,
Minneapolis, Minnesota 55455
Received January 6, 2010
To further understand copper-promoted oxidation reactions, the Cu(I) complexes LCuX (L = N,N0-di-tert-butyl-N,N0-
dimethylethylenediamine; X = benzoylformate (BF) or p-nitro-benzoylformate) were synthesized, fully characterized by
X-ray crystallography and spectroscopy in solution, and their reactivity with O2 at -80 °C examined. Oxidative
decarboxylation of the R-ketocarboxylate ligand was observed, but only to a significant extent when cyclohexene,
cyclooctene, or acetonitrile was present. Spectroscopic and conductivity data are consistent with mechanistic postulates
involving displacement of the R-ketocarboxylate by the additives to a small extent, followed by oxygenation of the LCu(I)
moiety to yield copper-oxygen species that subsequently induce decarboxylation. To test these hypotheses,
spectroscopic and kinetic studies of the reactions of Bu4NBF with preformed μ-η2:η2-peroxodicopper(II) and/or
bis(μ-oxo)dicopper(III) complexes supported by L or N,N,N0,N0-tetramethylpropylenediamine were performed. In an
illustration of a new mode of reactivity for such dicopper-oxygen cores, decarboxylation of the added R-ketocarboxylate
was observed and the intermediacy of a carboxylate-bridged μ-η2:η2-peroxodicopper(II) complex was implicated.
Introduction
Cu(I) complexes with O2. Recent interest has focused on the
isolation of monocopper-oxo species, [Cu2þ;O•-
T
In efforts to understand copper-promoted oxidation reac-
tions important in biology and industry, extensive research
has aimed to isolate, characterize, and probe the reactivity of
copper-oxygen intermediates.1-6 Particularly well-studied
examples of such intermediates include those with bis(μ-
oxo)dicopper(III), μ-peroxodicopper(II), and monocopper-
superoxo/peroxo cores, which typically are prepared through
low temperature reactions of N-donor ligand-supported
Cu3þdO], which have been studied by theory,7 postulated
as reactive intermediates in enzymes7a-d,8 and some oxida-
tion reactions,9 yet only identified experimentally in the gas
phase.7e In one attempt to access such species we decided to
explore a route analogous to one that yields related Fe4þdO
moieties,10 but where M = Cu instead of Fe (Figure 1).11
In this route that has been postulated to be followed by
several known iron-containing metalloenzymes and model
complexes, an R-ketocarboxylate co-ligand is oxidatively
decarboxylated via O2 activation and nucleophilic attack
of the ketocarbonyl carbon. Our previous efforts to extend
this chemistry to M = Cu involved reactions of O2 with Cu(I)
complexes of R-ketocarboxylates supported by bidentate
pyridyl-imine ligands.11 Oxidative decarboxylation ensued,
resulting in the formation of the respective carboxylic acid
*To whom correspondence should be addressed. E-mail: wtolman@
umn.edu.
(1) Mirica, L. M.; Ottenwaelder, X.; Stack, T. D. P. Chem. Rev. 2004, 104,
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A. A. N.; Karlin, K. D. J. Am. Chem. Soc. 2007, 129, 6998–6999. (c) Capdevielle,
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€
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Chem. B 2004, 108, 14407–14416. (f) Gherman, B. F.; Tolman, W. B.; Cramer,
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r
2010 American Chemical Society
Published on Web 03/10/2010
pubs.acs.org/IC

3532 Inorganic Chemistry, Vol. 49, No. 7, 2010
Gupta and Tolman
evacuating, and thawing. Tetrahydrofuran (THF) was dried
over Na/benzophenone and distilled under vacuum. CH2Cl2
and CD2Cl2 were dried over CaH2 and distilled under vacuum.
Et2O and pentanes were passed through solvent purification
columns (Glass Contour, Laguna, CA). All metal complexes
were prepared and stored in a Vacuum Atmospheres inert
atmosphere glovebox under dry nitrogen or were manipulated
under argon or dry nitrogen using standard Schlenk tech-
niques. [LCu(CH3CN)]OTf (L=Me4pda12 or tBu2Me2eda13),
Cu4Mes4,14 Bu4NBF (BF = benzoylformate),15 and TlBF16
were prepared by literature methods, and Bu4NBA (BA =
benzoate) was purchased from Aldrich.
Physical Methods. NMR spectra were recorded on either
Varian VI-300, VXR-300, or VI-500 spectrometers at room
temperature. Chemical shifts (δ) for 1H and 13C NMR spectra
were referenced to residual protium in the deuterated solvent
(1H NMR experiments) or the characteristic solvent resonances
of the solvent nuclei (13C NMR experiments). Variable tem-
Figure 1. Proposed mechanism for the metal mediated oxidative dec-
arboxylation of an R-ketocarboxylate utilizing O2 (M = Fe, n = 2, or
M = Cu, n = 1; S = substrate; R = alkyl or aryl group).
1
perature H NMR spectra were obtained on a Varian VI-300
spectrometer fitted with a liquid nitrogen cryostat. UV-vis
spectra were collected on a HP8453 (190-1000 nm) diode array
spectrophotometer. Low temperature UV-vis experiments were
performed using an Unisoku low temperature UV-vis cell
holder. UV-vis spectra that had drifting baselines because of
minor frosting caused by the low-temperature device were
corrected when necessary by subtracting the average of a region
with no absorbance fromthe entirespectrum. Resonance Raman
spectra were recorded on an Acton 506 spectrometer using a
Princeton Instruments LN/CCD-11100-PB/UBAR detector and
ST-1385 controller interfaced with Winspec software. The spec-
tra were obtained at -196 °C using backscattering geometry.
Excitation at 406.7 nm was provided by a Spectra-Physics
BeamLok 2060-KR-V Krypton ion laser. Raman shifts were
externally referenced to liquid indene and internally referenced
to solvent (CH2Cl2). Elemental analysis was performed by
Robertson Microlit Lab (Madison, NJ). Electrospray ionization
mass spectrometry (ESI-MS) was performed on a Bruker Bio-
TOF II instrument. All GC-MS experiments were conducted on
an Agilent Technologies 7890A GC system and 5975C VL MSD.
The GC column was a HP-5 ms with dimensions 30 m ꢀ 0.250
mm. The standard method used for all runs involved an initial
oven temperature of 60 °C (held for 4 min) followed by a 20 °C/
min ramp to 230 °C that was held for 15 min. Infrared spectra
were collected on a Nicolet Avatar 370 FT-IR equipped with an
attenuated total reflectance attachment, using a CaF2 solution
cell (International Crystal Laboratories) or a Smart OMNI-
Sampler for solid samples.
Figure 2. Ligands and abbreviations used in this work.
and hydroxylation of a ligand arene substituent. These
findings, in conjunction with theoretical calculations,
were interpreted to indicate the intermediacy of a highly
reactive monocopper-oxo species that was “trapped” in
intramolecular fashion by the proximal arene ring of the
supporting pyridyl-imine ligand through a mechanism
similar to that proposed for Fe/R-ketocarboxylate chem-
istry (Figure 1).11
Herein, we report the results of efforts to broadenthe scope
of the initial studies by examining the effect of using more
electron-donating diamine supporting ligands lacking arene
substituents (Me4pda and tBu2Me2eda; Figure 2).12,13 We
envisioned that such ligands might enable the isolation of a
monocopper-oxo species and/or oxidation of an exogenous
substrate. Instead, we found that while Cu(I)-R-ketocarbo-
xylates could be isolated using tBu2Me2eda, decarboxylation
upon treatment with O2 was insignificant unless alkenes or
acetonitrile was present. Subsequent mechanistic studies
indicated the involvement of μ-η2:η2-peroxodicopper(II)
and/or bis(μ-oxo)dicopper(III) species. These species were
independently shown to induce decarboxylation of exogen-
ous R-ketocarboxylate, thus demonstrating a new type of
reactivity for such cores.
Electrical conductivity measurements were performed in
CH2Cl2 using a Fischer Scientific Accumet Portable AP65
model conductivity bridge with a cell constant of 1.0 cm-1
.
Anaerobically prepared solutions of (tBu2Me2eda)Cu(BF) in
CH2Cl2 (5 mL, 0.1-0.6 mM) were loaded into a 10 mL Schlenk
flask, sealed with a glass stopper, and cooled to -78 °C in a dry
ice/acetone bath under positive argon pressure. Measurements
were taken by removal of the glass stopper under an argon purge
and insertion of the probe. Values were acquired after ∼5-10 s
to compensate for solution warming from the probe. Samples
involving additives were prepared in an identical manner with
additives added to stock solutions of the Cu(I) complex followed
by serial dilutions. The equivalent conductance Λe was calcu-
lated from the conductance measurement and plotted versus the
square root of the concentration. Extrapolation of the linear
portion of the plot allowed determination of the equivalent
conductance at infinite dilution, Λ0. A plot of (Λo - Λe) versus
Experimental Section
General Considerations. All solvents and reagents were ob-
tained from commercial sources unless otherwise noted. All
solvents were thoroughly degassed via three cycles of freezing,
(14) Tsuda, T.; Yazawa, T.; Watanabe, K.; Fujii, T.; Saegusa, T. J. Org.
Chem. 1981, 46, 192–194.
(15) Hayoz, P.; Ilg, S. WO Patent 2006067061, 2006, 92
(16) Friese, S. J.; Kucera, B. E.; Young, V. G.; Que, L., Jr.; Tolman, W. B.
Inorg. Chem. 2008, 47, 1324–1331.
(12) Mahadevan, V.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack,
T. D. P. J. Am. Chem. Soc. 1999, 121, 5583–5584.
(13) Mahadevan, V.; Henson, M. J.; Solomon, E. I.; Stack, T. D. P. J. Am.
Chem. Soc. 2000, 122, 10249–10250.

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3533
the square root of the concentration (Onsager plot) was then
constructed.17,18
Global fitting of UV-vis data was performed using Olis
GlobalWorks software; for complete details see the Supporting
Information.
some experiments, 1-5 equiv of cyclooctene or cyclohexene
were preloaded into the cuvette prior to cooling and oxygena-
tion. Samples preloaded with substrates were subjected to GC-
MS analysis. These solutions were gradually warmed to room
temperature, solvent was removed under vacuum, and the
resulting residues were washed with 0.1 M HCl (∼5 mL). The
aqueous layers were extracted with CH2Cl2 (3 ꢀ 5 mL), and the
extracts dried with Na2SO4, filtered through Celite, and ana-
lyzed by GC-MS. Observed peaks were compared to commer-
cially available oxidized variants of the original alkene
(cyclohexene oxide, 2-cyclohexene-1-one, 2-cyclohexene-1-ol,
and cyclooctene oxide).
Reaction of [L2Cu2(μ-η2:η2-O2)](OTf)2/[L2Cu2(μ-O)2](OTf)2
(L = tBu2Me2eda) Mixture with Bu4NBF. An anaerobically pre-
pared CH2Cl2 solution (0.1 mM) of [(tBu2Me2eda)Cu(CH3-
CN)]OTf was cooled to -80 °C under argon in a septum sealed
quartz cuvette. Dry O2 was bubbled through the solution, and
monitoring by UV-vis spectroscopy revealed the growth of
absorption bands (λmax = 365 and 450 nm) indicative of the for-
mation of a mixture of [L2Cu2(μ-η2:η2-O2)](OTf)2/[L2Cu2(μ-
O)2](OTf)2 (L = tBu2Me2eda).28 Once growth of the bands
ceased, the solution was purged with Ar (10 min) and a degassed
solution of Bu4NBF in dry CH2Cl2 (3 mM, prepared in a
glovebox) was added via syringe to the stirred reaction solution,
which was monitored by UV-vis spectroscopy. For subsequent
GC-MS quantification, samples were allowed to warm to room
temperature. Solvent was removed in vacuo, and the residue
washed with 0.1 M HCl (∼5 mL). The aqueous layer was
extracted with CH2Cl2 (3 ꢀ 5 mL) to isolate the Cu-free organic
products. The solvent was again removed in vacuo, and the
residue redissolved in acetone. K2CO3 (5 equiv per added
TBABF) and MeI (5 equiv per added TBABF) were added,
and the solution was allowed to stir for 2 h under reflux. The
acetone was removed in vacuo, the residue was extracted with
toluene (∼2 mL), and the solution was filtered through a silica
plug. The filtrate was diluted to a known volume and analyzed
by GC-MS, using a standard curve prepared with commercially
available methylbenzoate for comparison.
(tBu2Me2eda)Cu(X) (X = BF or nitro-BF). In a typical pro-
cedure, Cu4Mes4 (41.5 mg, 0.055 mmol) and benzoylformic acid
(34.1 mg, 0.22 mmol) were combined in CH2Cl2 (5 mL) and
stirred for 20 min to give a cloudy yellow solution. tBu2Me2eda
(45.5 mg, 0.22 mmol) in CH2Cl2 (1 mL) was added to afford a
pale yellow solution. The solvent was removed under vacuum to
yield a yellow oil, which was redissolved in a small amount of
CH2Cl2 (∼ 0.5 mL). Pentanes (∼ 5 mL) was added to precipitate
a bright yellow solid, which was collected by filtration and
washed with pentanes (3 ꢀ 5 mL) (77.4 mg, 82% for X = BF;
56 mg, 65% for X = nitro-BF). Crystals suitable for X-ray crys-
tallography were obtained by slow diffusion of pentanes into a
concentrated CH2Cl2 solution. For X = BF: 1H NMR (CDCl3,
300 MHz): δ 7.92 (d, J = 9 Hz, 2H), 7.53 (t, J = 9 Hz, 1H), 7.42
(t, J = 6.9 Hz, 2H), 2.42 (s, 6H), 2.64 (b), 1.29 (s, 18H) ppm.
13C{1H} NMR (75.0 MHz, CD2Cl2): δ 195.23, 170.94, 134.76,
133.64, 130.02, 58.02, 49.28, 36.46, 26.62 ppm. Anal. Calcd for
C20H33CuN2O3: C, 58.16; H, 8.05; N, 6.78. Found: C, 58.21; H,
8.24; N, 6.58. IR (Neat): 1197, 1224, 1393, 1448, 1477, 1617,
1683 cm-1. UV-vis [λmax, nm (ε, M-1 cm-1) in CH2Cl2]: 362
(465). For X = nitro-BF: 1H NMR (CD2Cl2, 300 MHz): δ 8.19
(dd, J = 8.1, 30.3 Hz, 4H), 2.44 (s, 6H), 2.64 (b), 1.29 (s, 18H)
ppm. 13C{1H} NMR (75.0 MHz, CD2Cl2): δ 193.19, 169.53,
150.79, 139.64, 131.13, 124.12, 58.09, 49.23, 36.51, 26.64 ppm.
Anal. Calcd for C20H33CuN2O3: C, 52.44; H, 7.04; N,
9.17. Found: C, 51.85; H, 6.75; N, 8.79. IR (Neat): 1210, 1348,
1348, 1389, 1473, 1529, 1628, 1695 cm-1. UV-vis [λmax, nm
(ε, M-1 cm-1) in CH2Cl2]: 470 (162).
Attempted Synthesis of (Me4pda)Cu(BF). Cu4Mes4 (25 mg,
0.56 mmol) and benzoylformic acid (20 mg 0.14 mmol) were
combined in THF (10 mL). The solution was allowed to stir
for 20 min to give a cloudy yellow solution. Me4pda (0.022 mL,
0.14 mM) was added via syringe resulting in a golden clear
solution. Upon standing (∼10-15 min) or reduction in volume,
the solution changed to a bright green supernatant with an
orange solid deposit typical of a disproportionation. Crystals
suitable for X-ray crystallography were obtained by allowing
the filtered supernatant to sit at room temperature for several
days, and were found to be (Me4pda)Cu(BF)2. Complete char-
acterization of this complex was performed for an indepen-
dently synthesized sample (see below).
Reactions of [(Me2pda)2Cu2(μ-O)2](OTf)2 with Bu4NY (Y=BF
or BA). These reactions were performed using the same proce-
dure as described above, except using a 0.2 mM solution of
[(Me4pda)Cu(CH3CN)]OTf and involving the intermediacy of
[L2Cu2(μ-O)2](OTf)2, as indicated by an intense absorption
feature with λmax = 397 nm. Samples for analysis by resonance
Raman spectroscopy were prepared using a 20 mM solution
(0.8 mL) of [(Me4pda)Cu(CH3CN)]OTf. The solutions were
cooled under an argon purge to -78 °C by submerging the flask
in a dry ice/acetone bath. Dry O2 was bubbled through the
solution forming the characteristic yellow brown color of the
bis(μ-oxo)dicopper(III) core. After 30 min, O2 flow was discon-
tinued, and the flask was purged for 20 min with argon to remove
excess O2. The solution was transferred via a precooled pipet to a
NMR tube charged with 60 equiv of Bu4NBF under an argon
purge in a dry ice/acetone bath. This reaction was allowed to sit
at -78 °C for 5 s and then rapidly frozen in liquid nitrogen for
resonance Raman spectroscopic analysis. For the reaction with
Bu4NBA, a portion (58 mg, 1.6 mmol) was dissolved in 0.1 mL of
CH2Cl2 and loaded into a syringe in the glovebox. This solution
was injected into the Schlenk flask containing the solution of the
bis(oxo)dicopper complex and allowed to mix for 5 min before
transferring to a precooled (-78 °C) NMR tube. The mixture
was then subsequently frozen in liquid N2 for resonance Raman
spectroscopic analysis.
(Me4pda)Cu(BF)2. CuCl2 (124 mg, 0.9 mmol) was suspended
in CH2Cl2 (5 mL) and Me4pda (0.128 mL, 0.77 mmol) was added
via syringe. After stirring for 2 h, the solution was filtered and
TlBF (541.6 mg, 1.5 mmol) was added and the mixture was
allowed to stir overnight. The mixture was filtered and solvent
was removed under vacuum from the bright blue-green filtrate
to leave the product as a blue-green solid (328 mg, 87%).
Crystals suitable for X-ray crystallography were obtained al-
lowing a concentrated CH2Cl2 solution to stand at -20 °C.
Anal. Calcd for C23H28CuN2O6 0.25CH2Cl2: C, 54.41; H, 5.12;
3
N, 5.46. Found: C, 54.58; H, 5.12; N, 5.31. IR (Neat): 1232,
)
1405, 1477, 1591, 1685 cm-1. UV-vis [λmax, nm (ε, M-1 cm-1
in CH2Cl2]: 740 (141).
Low Temperature Oxygenations of (tBu2Me2eda)Cu(X) (X =
BF or nitro-BF). Anaerobically prepared CH2Cl2 solutions
(0.7 mM) of (tBu2Me2eda)Cu(X) (X = BF or nitro-BF) were
cooled to -80 °C under argon in a septum sealed quartz cuvette.
Dry O2 was bubbled through the solution with monitoring
by UV-vis spectroscopy (data for X = BF shown in Figure 6;
X = nitro-BF shown in Supporting Information, Figure S1). In
Results and Discussion
Synthesis and Characterization of LCu(r-ketocarboxy-
late) Complexes. Cu(I) complexes supported both by
a peralkylated diamine ligand (L = tBu2Me2eda) and
an R-ketocarboxylate (BF or nitro-BF) were synthesized
(17) Geary, W. J. Coord. Chem. Rev. 1971, 7, 81–122.
(18) Casella, L.; Ibers, J. A. Inorg. Chem. 1981, 20, 2438–2448.

3534 Inorganic Chemistry, Vol. 49, No. 7, 2010
Gupta and Tolman
Figure 3. Representation of the X-ray structure of (tBu2Me2)Cu(BF)
with all nonhydrogen atoms shown as 50% thermal ellipsoids. Selected
˚
interatomic distances (A) and angles (deg): Cu1-O1, 1.9341(17); Cu1-O2,
2.8096(17); Cu1-N1, 2.0455(19); Cu1-N2, 2.1615(19); N1-Cu1-N2,
87.75(8); O1-Cu1-N1, 150.72(7); O1-Cu1-N2, 121.38(7).
Figure 4. Representation of the X-ray structure of (tBu2Me2eda)Cu-
(nitro-BF) with nonhydrogenatoms shown as 50%thermal ellipsoids and
tBu2Me2eda carbonatoms omittedfor clarity. The monodentate(top) and
bidentate (bottom) R-ketocarboxylate bound forms are shown. Selected
by first reacting Cu4Mes4 with the free R-ketocarboxylic
acid. Subsequent addition of tBu2Me2eda led to the for-
mation of the complexes (tBu2Me2eda)Cu(X) (X = BF or
nitro-BF), which were isolated as air sensitive solids and
characterized thoroughly, including by X-ray crystallo-
graphy.
˚
interatomic distances (A) and angles (deg): [Monodentate] Cu1-O1,
1.942(5); Cu1-O2, 2.981(6); Cu1-N1, 2.094(4); Cu1-N2, 2.087(4);
O1-Cu-N1, 138.30(19)°; O1-Cu-N2, 119.1(5)°; N1-Cu-N2,
88.07(14)°; [Bidentate] Cu1-O1, 2.028(18); Cu1-O2, 2.344(16); Cu1-N1,
2.094(4); Cu1-N2, 2.087(4); N1-Cu-N2, 88.07(14); O1-Cu-N1,
139.1(5); O1-Cu-N2, 119.1(5); O2-Cu-N1, 124.9(5); O2-Cu-N2,
132.6(5).
The complex (tBu2Me2eda)Cu(BF) contains a three
coordinate Cu(I) center in a planar geometry (sum of
donor-Cu-donor angles = 359.9°) with the R-ketocar-
boxylate (BF) bound in monodentate fashion via O1
(Figure 3). A relatively insignificant interaction between
Cu1 and O2 is suggested by a long Cu1-O2 distance of
ordination has been reported for simple carboxylate
complexes of copper,20 to the best of our knowledge such
a binding mode in an R-ketocarboxylate complex has not
been reported for complexes of any metals.16,21-23 The
existence of both monodentate and bidentate carboxylate
binding modes within the same crystal implies similar
stabilities for these structures, as noted in previously
reported computations on analogous compounds.11
Attempts to isolate Cu(I) complexes of BF or nitro-BF
utilizing Me4pda as supporting ligand where unsuccessful
because of problems with disproportionation. Thus,
upon addition of Me4pda to solutions of CuBF (i.e., from
mixing Cu4Mes4 and benzoylformic acid) an initial
change from cloudy yellow to clear gold was observed,
but the solutions soon turned bright green and deposited
an insoluble dark solid (presumably Cu metal). From
these solutions a small amount of crystals of the Cu(II)
complex (Me4pda)Cu(BF)2 were isolated and characteri-
zed by X-ray crystallography (Figure 5). More substan-
tive samples of the complex were independently synthe-
sized and fully characterized (unit cell analysis, elemental
analysis, FTIR, and UV-vis spectrosocpy) by reacting
(Me4pda)CuCl2 with 2 equiv of TlBF. As shown in
˚
2.81 A. The keto-carboxylate torsion angle, defined by
O1-C-C-O3 or O2-C-C-O3, is 104°. This value is
closer to that of an ideal perpendicular orientation (90°)
than one in which the ketocarbonyl is coplanar with the
carboxylate functionality (180°). Overall, the BF binding
geometry is similar to that previously reported for Cu(I)-
R-ketocarboxylate complexes supported by pyridyl-imine
ligands.11
In the X-ray crystal structure of (tBu2Me2eda)Cu-
(nitro-BF) (Figure 4), a highly disordered O2 atom was
modeled by splitting the R-ketocarboxylate ligand over
two positions, one with monodentate (minor, 25%
occupancy) and the other with bidentate carboxylate
coordination (75% occupancy). The former, monoden-
tate geometry is similar to that in the parent BF complex
(Figure 3), with Cu1-O1 and -O2 distances of 1.942(5)
˚
˚
A and 2.981(6) A, respectively, and a planar coordination
geometry (sum of donor-Cu-donor angles = 356.2°). In
the latter, bidentate structure, the Cu-O distances are
˚
˚
more similar (2.028(18) A and 2.344(16) A), with an
overall copper coordination geometry best described as
distorted tetrahedral (τ4 = 0.63, where τ4 = 0 corre-
sponds to a square planar and τ4 = 1 corresponds to a
tetrahedral geometry).19 While bidentate carboxylate co-
(22) Selected examples of complexes of Fe(II): Reference 16 and (a)
Mehn, M. P.; Fujisawa, K.; Hegg, E. L.; Que, L., Jr. J. Am .Chem. Soc.
2003, 125, 7828–7848. (b) Paine, T. K.; Zheng, H.; Que, L., Jr. Inorg .Chem.
2005, 44, 474–476. (c) Chiou, Y.-M.; Que, L., Jr. J. Am. Chem. Soc. 1992, 114,
7567–7568. (d) Hegg, E. L.; Ho, R. Y. N.; Que, L., Jr. J. Am. Chem. Soc. 1999,
121, 1972–1973. (e) Hikichi, S.; Ogihara, T.; Fujisawa, K.; Kitajima, N.; Akita,
M.; Moro-oka, Y. Inorg. Chem. 1997, 36, 4539–4547.
(23) Selected examples of complexes of other metal ions other than Cu or
Fe: (a) Law, G.-L.; Wong, K.-L.; Lau, K.-K.; Tam, H.-L.; Cheah, K.-W.;
Wong, W.-T. Eur. J. Inorg. Chem. 2007, 5419–5425. (b) Muller, R.; Hubner, E.;
Burzlaff, N. Eur. J. Inorg. Chem. 2004, 2151–2159. (c) Ruman, T.; Ciunik, Z.;
Szklanny, E.; Wolowiec, S. Polyhedron 2002, 21, 2743–2753. (d) Tekeste, T.;
Vahrenkamp, H. Eur. J. Inorg. Chem. 2006, 5158–5164. (e) Teoh, S.-G.; Looi,
E.-S.; Teo, S.-B.; Ng, S. W. J. Organomet. Chem. 1996, 509, 57–61. (f) Sharutin,
V. V.; Sharutina, O. K.; Bondar', E. A.; Pakusina, A. P.; Krivolapov, D. B.;
Gubaidullin, A. T.; Litvinov, I. A. Russ. J. Gen. Chem. 2002, 72, 245. (g)
Krogmann, K. Z. Anorg .Allg. Chem. 1968, 358, 67–81. (h) Ng, S. W. Z.
Kristallogr.-New Cryst. Struct. 1997, 212, 279–281.
(19) Yang, L.; Powell, D.; Houser, R. Dalton Trans. 2007, 955–964.
(20) For a pertinent example, see: Reyes-Ortega, Y.; Alcantara-Flores,
J. L.; Hernandez-Galindo, M. d. C.; Gutierrez-Perez, R.; Ramirez-Rosales, D.;
Bernes, S.; Cabrera-Vivas, B. M.; Duran-Hernandez, A.; Zamorano-Ulloa, R.
J. Mol. Struct. 2006, 788, 145–151.
(21) Selected examples of complexes of Cu(II): (a) Arnaud, C.; Faure, R.;
Loiseleur, H. Acta. Crystallogr., Sect. C: Cryst. Struct. Commun. 1986, 42, 814–
816. (b) Nakasa, K.; Nakagawa, H.; Kani, Y.; Tsuchimoto, M.; Ohba, S. Acta
Crystallogr., Sect. C: Cryst. Struct. Commun. 1999, 55, 513–517. (c) Ovcharenko,
V. I.; Vostrikova, K. E.; Podoplelov, A. V.; Romanenko, G. V.; Ikorskii, V. N.;
Reznikov, V. A. Polyhedron 1997, 16, 1279–1289. (d) Kaizer, J.; Csonka, R.; Speier,
G.; Giorgi, M.; Reglier, M. J. Mol. Catal. A: Chem. 2005, 236, 12–17. (e) Lippai, I.;
Speier, G.; Huttner, G.; Zsolnai, L. Chem. Commun. 1997, 741–742. (f) Zheng, H.;
Que, L., Jr. Inorg. Chim. Acta 1997, 263, 301–307.

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3535
In addition to the aryl peaks, two singlets assignable
to methyl and t-butyl groups on the tBu2Me2eda ligand
for both complexes were observed at 2.4 and 1.3 ppm,
respectively. A well resolved peak or set of peaks was not
observed for the ethyl bridge in the tBu2Me2eda ligand,
although a broad baseline feature centered at ∼2.64 ppm
was observed in the expected region for these protons in
both samples. Upon cooling to -60 °C this broad,
undefined feature converted to relatively sharp features
at 3.06 and 2.07 ppm that integrated to two protons each
and were modeled as an A2X2 spin system (Supporting
Information, Figures S2 and S4). We attribute this beha-
vior to conformational fluxionality of the chelate ring
that is sufficiently slowed at low temperature to result in
the observation of distinct peaks for the diastereotopic
ethylene hydrogens.
All Cu(I) and Cu(II) complexes were further characteri-
zed by Fourier transform infrared spectroscopy (FTIR)
in both the solid and the solution state. Data were also
acquired for the respective free R-ketocarboxylate salt
or free acid for comparison (Supporting Information,
Figures S5-S8). All complexes containing an R-ketocar-
boxylate displayed two peaks between 1600 and 1800
cm-1 attributable to carboxylate and/or ketone C-O
vibrations (e.g., vsym), as well as a complex set of peaks
between 1300 and 1500 cm-1 that resemble those typically
assigned to vsym vibrations from the carboxylate group.25
The complex (tBu2Me2eda)Cu(nitro-BF) also displayed a
feature at ∼1530 cm-1 that was not observed in samples
with BF, suggesting that this feature is due to a N-O
vibration from the nitro functionality.26 Overall, the data
are consistent with the solid state structures determined
by X-ray crystallography.
Oxygenation of (tBu2Me2eda)Cu(X) (X = BF or nitro-
BF). Room temperature reactions of (tBu2Me2eda)Cu(X)
(X = BF or nitro-BF) in CH2Cl2 with excess O2 led to a
rapid change of the initially pale yellow solutions to bright
green. ESI-MS analysis of these solutions indicated a peak
assignable only to free diamine [tBu2Me2eda þ Hþ]þ.
Attempts to crystallize any compounds out of the reaction
mixture were unsuccessful and led to green oils. After a
mild acidic workup and extraction to remove copper and
isolate organic products, 1H NMR spectroscopic analysis
revealed <1% decarboxylation (conversion of BF to
benzoic acid, BA) with the only observed peaks being
assigned to the benzoylformic acid and tBu2Me2eda. The
extent of decarboxylation increased slightly (to ∼5%)
when the oxygenation was performed at -80 °C and the
solution subsequently allowed to warm to room tempera-
ture. Monitoring of the reaction at -80 °C by UV-vis
spectroscopy revealed the formation of a transient feature
at λmax ∼ 373 nm for X = BF and λmax ∼ 371 nm for X =
nitro-BF (Figure 6 and Supporting Information, Figure
S1, respectively). The growth and decay of these features
could be fit to a simple bi-exponential expression, consis-
tent with first-order rate laws for both processes
(presumably, the growth of the intermediate is pseudo-
first order with respect to excess O2). For X = BF, the
feature reached a maximum intensity at ∼14 min with rate
Figure 5. Representation of the X-ray structure of (Me4pda) Cu(BF)2
with all nonhydrogen atoms shown as 50% thermal ellipsoids. Selected
interatomic distances (A) and angles (deg): Cu(1)-N(1), 2.035(2); Cu-
(1)-N(2), 2.031(2); Cu(1)-O(1), 2.014(2) Cu(1)-O(4), 1.9966(19); N-
(2)-Cu(1)-N(1), 97.47(8); O(4)-Cu(1)-O(1), 88.50(7).
˚
Figure 5, both R-ketocarboxylates are bound by carboxy-
late O-atoms to a copper ion displaying a distorted square
planar geometry (τ4 = 0.36).19 The atoms O2 and O5
display an elongated, but still significant, interaction
˚
with the Cu(II) center (∼2.5 A). The Cu1-O1 and Cu1-
O4 distances observed in this complex are longer than
those observed for a previously reported Cu(II)BF com-
plex supported by a tris(methylpyridyl)amine ligand
(Cu-O=1.932(2) A).21f Both coordinatedR-ketocarboxy-
˚
lates have torsion angles (107.12° and 93.25°) that indicate
significant deviation of the carboxylate and keto-carbonyl
from coplanarity.
In addition to determining their structures by X-ray
crystallography, all of the copper-R-ketocarboxylate
complexes were characterized by UV-vis and 1H NMR
spectroscopy in solution and by FTIR spectroscopy as
solids and in solution. For (tBu2Me2eda)Cu(X) (X = BF
and nitro-BF), the lowest energy features in UV-vis
spectra are weak absorptions with λmax = 362 nm (ε ∼
465 M-1 cm-1) and λmax = 470 nm (ε ∼ 162 M-1 cm-1),
respectively. Drawing analogies to what is known for
Fe(II)-R-ketocarboxylate complexes,22,24 the lack of in-
tense absorptions in the visible region for these Cu(I)
compounds is consistent with monodentate and/or bi-
dentate coordination of the R-ketocarboxylate via the
carboxylate group. Coordination via both the carboxy-
late and ketone units to Fe(II) results in intense absorp-
tions in the 500-700 nm region that have been assigned as
Fe(II) f BF charge transfer bands, their low energy being
a result of the coplanarity and π conjugation between the
R-ketocarboxylate moiety and the aryl ring.24 The ab-
sence of such features in the Cu(I) complexes argues
against such a ketocarboxylate binding mode in solution
and is in agreement with the solid-state structures deli-
neated by X-ray crystallography.
1H NMR spectra of (tBu2Me2eda)Cu(X) (X = BF or
nitro-BF) displayed peaks between 7 and 9 ppm with
chemical shifts and splitting patterns consistent with
assignment as the aryl protons of the respective R-keto-
carboxylate ligand (Supporting Information, Figures
S2-S3). The peak pattern is consistent with either reten-
tion of a single binding mode or rapid fluxionality
between two or more binding geometries in solution.
(25) Deacon, G. B.; Phillips, R. J. Coord. Chem. Rev. 1980, 33, 227–250.
€
(26) Pretsch, E.; Buhlmann, P.; Affolter, C. Structure Determination of
(24) Chiou, Y.-M.; Que, L., Jr. J. Am .Chem. Soc. 1995, 117, 3999–4013.
Organic Compounds: Tables of Spectral Data; Springer: New York, 2003.

3536 Inorganic Chemistry, Vol. 49, No. 7, 2010
Gupta and Tolman
Figure 7. Onsager plot of [N(Bu)4PF6] compared to (tBu2Me2eda)Cu-
(BF) in CH2Cl2 at -78 °C. The best fit line for [N(Bu)4PF6] data has a
slope of 8.31 versus that for (tBu2Me2eda)Cu(BF) with a slope of 0.91.
Slopes for (tBu2Me2eda)Cu(BF) þ 5 equiv of S (S = Additive): cyclohex-
ene = 0.87; cyclooctene = 0.99; acetonitrile = 0.88.
Figure 6. UV-vis spectroscopic data for the reaction of (tBu2Me2eda)-
Cu(BF) in CH2Cl2 (0.67 mM) (blue spectrum) with O2 at -80 °C with the
feature at 373 nm having an approximate ε ∼ 2,300 M-1 cm-1. The inset
displays the time trace for the formation and decay of the intermediate
(red spectrum) with data monitored at 373 nm and fit to a bi-exponential
equation [At = A1 þ A2exp(-k2t) - A3exp(-k1t), k1 = 0.0034 s-1 and
k2 = 0.00039 s-1; R = 0.999].
starting Cu(I) complex, (tBu2Me2eda)Cu(BF), is neutral,
displacement of the anionic R-ketocarboxylate ligand
by an alkene or acetonitrile would result in the forma-
tion of a salt that would behave as a 1:1 electrolyte.
The conductivity of solutions of (tBu2Me2eda)Cu(BF)
both with and without additives and of an ideal 1:1
electrolyte ([Bu4N]PF6) over various concentrations (c)
are compared in the Onsager plots shown in Figure 7. A
linear fit with a similar slope to that for ([Bu4N]PF6)
would indicate the displacement of the BF ligand to
form a 1:1 electrolyte. However, the Onsager plot for
(tBu2Me2eda)Cu(BF) in the absence and presence of
additives has a small, invariant slope (∼0.9) that is
significantly smaller than that for [Bu4N]PF6 (8.3), sug-
gesting that the additives do not displace the R-ketocar-
boxylate from the copper atom to a significant degree. On
the other hand, the data do not rule out an equilibrium
between species with bound and unbound R-ketocarboxy-
late with only a small, experimentally unobservable
amount of the latter (i.e., small Keq).
constants for growth and decay being k1 = 0.0034 s-1 and
k2 = 0.00039 s-1, respectively. Both processes were faster
for X = nitro-BF, as the feature reached a maximum
intensity at ∼6.7 min with k1 = 0.0062 s-1 and k2 =
0.0010 s-1. Unfortunately, attempts to identify these inter-
mediates through resonance Raman spectroscopy were
unsuccessful, as peaks that could be attributed to oxygen-
containing species could not be identified (λex = 406.7 nm,
CH2Cl2, 20 mM, -196 °C).
Reasoning that the small amount of decarboxyla-
tion seen in the -80 °C reactions implicated the inter-
mediacy of an oxidizing intermediate that might be
trapped, we performed the oxygenation using solutions
of (tBu2Me2eda)Cu(BF) containing excess cyclohexene
or cis-cyclooctene (5 equiv). GC-MS analysis of the orga-
nic products after removing copper via an acidic workup
did not show any appreciable amounts of oxidized sub-
strates such as epoxides, alcohols, enones, or coupled
1
Taking into account both the H NMR spectroscopy
and conductivity data, two possible reaction pathways
can be envisioned to explain the enhanced decarboxyla-
tion in the presence of additives “S” (Figure 8). Both
presume rapid equilibration between mono- and biden-
tate carboxylate binding modes in solution for the start-
ing LCuBF complexes; adoption of just one of these
modes in solution is also consistent with the available
data. One pathway involves the binding of an additive to
the Cu(I)-complex with the R-ketocarboxylate ligand
remaining bound (path A). This binding of S would
somehow prime the complex to react with O2 and facil-
itate decarboxylation relative to the complex lacking S.
A second possible route (path B) involves displacement of
the R-ketocarboxylate by S to a small extent in an
unfavorable equilibrium process, generating a small
amount of a cation/anion pair. Such a displacement is
consistent with the relatively low basicity of BF; benzoyl-
formic acid has a lower pKa of 1.21 than that of benzoic
acid (pKa = 4.20).27 The cationic Cu(I) fragment sup-
ported by tBu2Me2eda is known to be highly reactive
with O2, ultimately yielding bis(μ-oxo)dicopper(III) and
μ-η2:η2-peroxodicopper(II) cores that can be observed by
spectroscopy at low temperatures.13 According to this
1
products. However, H NMR spectroscopic analysis re-
vealed a significant increase in the extent of decarboxyla-
tion when cyclohexene (11.5%) and cyclooctene (16%)
were present. A similar increase in decarboxylation yield
(12%) also was observed when the oxygenation was con-
ducted in the presence of acetonitrile (5 equiv). Taken
together, these results suggest that added alkene or ace-
tonitrile coordinates to the initial copper(I)-ketocarboxy-
late complex, and that this ligand binding somehow
facilitates the decarboxylation reaction.
1
To test this notion, H NMR spectra of either (tBu2-
Me2eda)Cu(BF) or (tBu2Me2eda)Cu(nitro-BF) with 5 equiv
of cyclohexene at room temperature or -60 °C were
acquired. The data did not indicate any significant inter-
action between the alkene and the metal complex; the
peaks assigned to the BF and tBu2Me2eda ligands, and
the added cyclohexene remained unaffected (Supporting
Information, Figures S9-S10). We conclude from these
results that if alkene binding occurs, the equilibrium
constant for such a reaction must be small.
To further examine if the R-ketocarboxylate remains
bound to the Cu-center in the presence of the additives
cyclohexene, cyclooctene, and acetonitrile, conductivity
measurements were explored. We reasoned that while the
(27) Scherrer, R. A.; Donovan, S. F. Anal. Chem. 2009, 81, 2768–2778.

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3537
Figure 8. Proposed mechanisms for decarboxylation of Cu(I)-R-keto-
carboxylate complexes in the presence of additives “S” (R = aryl group).
Figure 9. UV-vis spectra of the reaction of the mixture of bis(μ-
oxo)dicopper(III) and μ-η2:η2-peroxodicopper(II) complexes resulting
from oxygenation of [(tBu2Me2eda)Cu(NCCH3)]OTf (0.1 mM) (blue)
with TBABF (10 equiv) at -80 °C in CH2Cl2, with intermediate spectra
shown every 6s. The insetshowsthetimetraceforthe decay ofthe μ-η2:η2-
peroxodicopper(II) (red) and bis(μ-oxo)dicopper(III) (blue) cores at 369
and 455 nm, respectively. The datawere fit to exponential equations [At =
A1 þ A2exp(-kt)] (see text for k values).
hypothetical pathway, these species, or a 1:1 Cu/O2
precursor (not shown), would then react with the dis-
placed R-ketocarboxylate to generate the observed
benzoate. To our knowledge, however, there is no pre-
cedent for decarboxylation of an R-ketocarboxylate by a
copper-oxygen species such as a bis(oxo)- or peroxodi-
copper complex. Thus, we decided to test the viability of
path B by independently generating such species and
exploring their reactions with added R-ketocarboxylate
anions.
Reactivity of [Cu2O2]2þ with Exogenous BF. Following
the published procedure,13 we oxygenated [(tBu2Me2-
eda)Cu(NCCH3)]OTf in CH2Cl2 at -80 °C and observed
both bis(μ-oxo)dicopper(III) and μ-η2:η2-peroxodicop-
per(II) by UV-vis spectroscopy (Figure 9). Thus, the ab-
sorption band with λmax = 455 matches that reported
for [(tBu2Me2eda)2Cu2(μ-O)2](OTf)2 (λmax = 455 nm;
ε = 14,000 M-1 cm-1), while the feature with λmax
=
369 matches that for [(tBu2Me2eda)2Cu2(μ-η2:η2-O2)]-
(OTf)2 (λmax=369 nm; ε=15,000 M-1 cm-1).12,13 Also,
the ratio of the two species matches that reported for
this particular counterion and solvent. After purging
with argon to remove unreacted O2 (which did not affect
the UV-vis spectrum), Bu4NBF (10 equiv) was added.
Both oxygenated species decayed rapidly (Figure 9), but
at slightly different rates. Fitting the decay at the λmax
values of 369 and 455 nm to simple exponential functions
as appropriate for pseudo-first order processes yielded
k = 0.055 s-1 and 0.067 s-1, respectively. It is not obvious
how to interpret these slightly different rate constants in
view of the rapid equilibration between the bis(μ-oxo)-
dicopper(III) and μ-η2:η2-peroxodicopper(II) isomers
under these conditions (CH2Cl2, OTf- counterion); either
or both species could be reacting with the added R-keto-
carboxylate.13 The yield of decarboxylation was deter-
mined by GC-MS analysis of the organic fraction remain-
ing after warming the reaction solution to room temp-
erature, extracting the copper ions with aqueous base,
and performing GC/MS analysis of the organic fraction
after methylation with MeI/K2CO3 (i.e., determination of
the amounts of methyl esters of BF and BA; see Experi-
mental Section); we found it to be 19% based on the
amount of BA produced relative to the total amount of
[Cu2O2]2þ species reacting. In sum, either or both of the
Figure 10. UV-vis spectra of the reaction of [(Me4pda)2Cu2(O)2]-
(OTf)2 in CH2Cl2 (0.1 mM) at -80 °C (blue) with 60 equiv of Bu4NBF
to form a transient species (green) and then the final product (red), with
intermediate spectra at 0.5 s intervals shown in gray.
dicopper-oxygen complexes react with an R-ketocarbo-
xylate and induce its decarboxylation (as hypothesized
for path B, Figure 8), but the facile isomerization between
these cores complicates more detailed analysis of the
reaction pathway.
To simplify matters and enable deeper mechanistic
evaluation, we turned to a system supported by a biden-
tate diamine, Me4pda, that only yields a bis(μ-oxo)-
dicopper(III) complex and no discernible μ-η2:η2-peroxo-
dicopper(II) isomer.12 As described previously, bubbling
dry O2 through a solution of [(Me4pda)Cu(NCCH3)]OTf
in CH2Cl2 at -80 °C results in the formation of intense
UV-vis absorption features with λmax = 297 and 397 nm
corresponding to a bis(μ-oxo)dicopper(III) complex
(Figure 10, blue spectrum).12 Addition of Bu4NBF
(1-60 equiv) caused the feature at λmax = 397 nm to rap-
idly bleach concomitant with generation of an intermedi-
ate spectrum characterized by an absorption at λmax
=
352 nm and weaker shoulder ∼420 nm (Figure 10, green
spectrum). This spectrum subsequently decayed to yield a
spectrum with a feature at λmax = 364 nm (Figure 10, red
spectrum). Approximate first order rate constants for

3538 Inorganic Chemistry, Vol. 49, No. 7, 2010
Gupta and Tolman
Figure 11. Yields of decarboxylation for the reaction of [(Me4pda)2-
Cu2(O)2](OTf)2 with various amountsofBu4NBF. The dataare plotted as
averages of 3 replicate determinations with standard deviation indicated
as error bars. The yields were determined by GC-MS analysis of methyl-
benzoate (see Experimental Section) and are reported relative to the
amount of starting complex, [(Me4pda)2Cu2(O)2](OTf)2.
Figure 12. Resonance Raman spectra of [(Me4pda)2Cu2(O)2](OTf)2
(20 mM in CH2Cl2) after addition of (a) Bu4NBF (60 equiv) or (b)
Bu4NBA (10 equiv), for samples prepared from 16O2 (solid) or 18O2
(dotted). Conditions: λex = 406.7 nm, -196 °C. Solvent peaks are marked
with an asterisk.
formation and decay of the intermediate were obtained
through global fitting of the UV-vis spectroscopic data,
although the fast rate of formation of the intermediate
in these batch experiments limits the accuracy of the
analysis (see Supporting Information, Figure S11 for
details). For the reaction with 60 equiv of Bu4NBF, values
of kformation = (5-7) ꢀ 10-1 s-1 and kdecay = (4-7) ꢀ
10-1 s-1 were found, with the latter decreasing slightly
to 1 ꢀ 10-2 s-1 when 2 equiv of Bu4NBF was used.
The decarboxylation yield after warming of the solu-
tion was evaluated by GC-MS analysis of organic frac-
tions after workup (see Experimental Section). The
results as a function of equivalents of Bu4NBF added
are plotted in Figure 11; the decarboxylation yield in-
creases to a maximum of ∼ 30% at about 5 equiv of
Bu4NBF added. Importantly, the data confirm the viabi-
lity of path B (Figure 8), wherein a dicopper-oxygen
complex induces the decarboxylation of exogenous BF.
Resonance Raman spectroscopy was used in an effort
to identify the intermediate species (Figure 10, green
spectrum) observed in the reaction of [(Me4pda)2Cu2-
(O)2](OTf)2 with Bu4NBF. A sample for such analysis
was prepared by mixing a solution of [(Me4pda)2Cu2-
(16O)2](OTf)2 (20 mM in CH2Cl2) with a Bu4NBF (60
equiv), followed within a few seconds by rapid freezing
by immersion into liquid N2. Excitation at 406.7 nm
yielded a Raman spectrum (Figure 12a) with non-solvent
features at 609 (not shown) and 764 cm-1. Because of
difficulties trapping the short-lived intermediate, we were
unable to obtain reliable data with 18O-labeled material,
so unambiguous identification of the Raman features was
not possible. We speculate that the 609 cm-1 peak is a
core breathing mode of the bis(μ-oxo)dicopper(III) start-
ing material28 and that the feature at 764 cm-1 is an O-O
stretching vibration of a μ-η2:η2-peroxodicopper(II) core
formed upon coordination of the BF anion. Precedent for
this notion comes from observations of isomerization of
bis(μ-oxo)dicopper(III) cores to μ-η2:η2-peroxodicopper-
(II) units from interactions with basic anions or other
substrates.29,30
Figure 13. UV-vis spectroscopic changes for the reaction of [(Me4-
pda)2Cu2O2]OTf2 in CH2Cl2 (0.1 mM; blue) at -80 °C with Bu4NBA
(10 equiv), with spectra displayed every 4 s.
To test this idea experimentally, we treated [(Me4pda)2-
Cu2(O)2](OTf)2 with Bu4NBA (BA = benzoate), which
we reasoned would be capable of binding to the dicopper
core in a manner akin to BF, but because of the absence of
the R-ketocarbonyl moiety would not be susceptible to
decarboxylation and thus would yield a more stable
and readily characterized species. Addition of Bu4NBA
(10 equiv) to a solution of [(Me4pda)2Cu2(O)2](OTf)2
(0.1 mM in CH2Cl2) at -80 °C resulted in a rapid
decay of the feature at λmax = 397 nm to a new species
with UV-vis features at λmax = 352, 413, and 587 nm
(Figure 13) similar to those of the intermediate observed
in the reaction with Bu4NBF.31 Excitation at 406.7 nm led
to enhancement of a feature in the Raman spectrum at
764 cm-1 (Figure 12b) identical to that observed for the
transient intermediate produced from Bu4NBF
(Figure 12a), but in the case of the product of the reaction
with Bu4NBA data for the 18O isotopomer was obtained
(Δ18O = 33 cm-1). Taken together, the UV-vis, kinetic,
and resonance Raman data for the reactions of
[(Me4pda)2Cu2(O)2](OTf)2 with Bu4NBA or Bu4NBF
support carboxylate binding that induces isomerization
(28) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon,
E. I. J. Am. Chem. Soc. 1999, 121, 10332–10345.
(29) Funahashi, Y.; Nishikawa, T.; Wasada-Tsutsui, Y.; Kajita, Y.;
Yamaguchi, S.; Arii, H.; Ozawa, T.; Jitsukawa, K.; Tosha, T.; Hirota, S.;
Kitagawa, T.; Masuda, H. J. Am. Chem. Soc. 2008, 130, 16444–16445.
(30) Ottenwaelder, X.; Rudd, D. J.; Corbett, M. C.; Hodgson, K. O.;
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(31) In contrast to this result, the analogous reaction of an equilibrium
mixture of [(tBu2Me2eda)2Cu2(μ-O)2](OTf)2 and [(tBu2Me2eda)2Cu2(μ-
η2:η2-O2)](OTf)2 with Bu4NBA resulted in the decay of both observed
features. It is also noteworthy that oxygenation of a premixed solution of
[(tBu2Me2eda)Cu(NCCH3)]OTf þ Bu4NBA (1-10 equiv) at -80 °C did not
result in the formation of any observable intermediates.

Article
Inorganic Chemistry, Vol. 49, No. 7, 2010 3539
dicopper(II) species are likely candidates for the responsible
reactant.
In support of this idea, an independently generated mix-
ture of bis(μ-oxo)dicopper(III) and/or μ-η2:η2-peroxodicop-
per(II) complexes supported by tBu2Me2eda was found to
decarboxylate added Bu4NBF. Similarly, a bis(μ-oxo)dicop-
per(III) complex supported by Me4pda also effected the
decarboxylation of exogenous Bu4NBF. In this instance,
an intermediate was identified, and on the basis of UV-vis,
kinetic, and resonance Raman spectroscopy and parallel
experiments using Bu4NBA, we hypothesize that it features
a μ-η2:η2-peroxodicopper(II) core bridged by the carboxylate
unit (BA or BF). Taken together, the data support the
hypothesis that dicopper-oxygen species are capable of
inducing decarboxylation of R-ketocarboxylates, thus pro-
viding supporting evidence for the mechanism proposed
(Figure 8) for decarboxylation of (tBu2Me2eda)Cu(BF).
Such a pathway differs from that likely for the Cu(I)-R-
ketocarboxylate complexes supported by pyridyl-imine li-
gands, which because of their poorer electron-donating
capabilities relative to peralkylated diamines do not appear
to be capable of supporting the generation of dicopper-
oxygen complexes.11 More broadly, our findings suggest that
metal-R-ketocarboxylate reactivity10 may involve multiple
pathways, including ones whereby the R-ketocarboxylate is
displaced prior to formation of a metal-oxygen reagent
capable of inducing decarboxylation.
Figure 14. Proposed isomerization of a bis(μ-oxo)dicopper(III) unit to
a μ-η2:η2-peroxodicopper(II) core upon binding of BF or BA (N = N-
donor atoms of Me4pda).
of the bis(μ-oxo)dicopper(III) unit to a μ-η2:η2-peroxo-
dicopper(II) core. We speculate on the basis of the
spectroscopic data and precedent30 that the latter species
has the structure drawn in Figure 14. In the case of BA
this core is stable at low temperature, but for the case of
BF it reacts further to yield BA as a result of decarboxy-
lation.
Summary and Conclusions
While disproportionation plagued attempts to synthesize a
Cu(I)-R-ketocarboxylate complex supported by Me4pda,
such complexes were successfully isolated and fully charac-
terized using tBu2Me2eda as auxiliary ligand. The data
support coordination via the carboxylate, but not the ketone
functional group of the R-ketocarboxylate to the Cu(I) center
in the complexes of BF or nitro-BF. Unlike previously
reported Cu(I)-R-ketocarboxylate complexes with pyridyl-
imine ligands,11 oxygenation of the complexes with tBu2-
Me2eda did not result in significant decarboxylation, except
when alkenes or acetonitrile were present. We hypothesize
that these additives coordinate to (tBu2Me2eda)Cu(BF)
and that the binding enhances the decarboxylation reac-
tion, but the equilibrium constant for binding must be
small to explain the NMR spectroscopy and conductivity
data. A plausible mechanism (Figure 8) involves generation
of a small amount of a reactive copper-oxygen species that
reacts with the R-ketocarboxylate. On the basis of precedent
for Cu(I)/O2 reactivity supported by tBu2Me2eda,13 we
suggest that bis(μ-oxo)dicopper(III) and/or μ-η2:η2-peroxo-
Acknowledgment. We thank the University of Minne-
sota Department of Chemistry (Graham N. Gleysteen
Fellowship to A.K.G) and the NIH (GM47365 to W.B.
T.) for financial support of this work and L. Que, Jr., for
access to the resonance Raman spectroscopy facility.
Supporting Information Available: X-ray crystallographic
data in cif format with parameters listed in Table S1; UV-vis,
FTIR, variable temperature 1H NMR spectra and global fitting
details shown in Figures S1-S11. This material is available free
of charge via the Internet at http://pubs.acs.org.