Heterobimetallic dioxygen activation: Synthesis and reactivity of mixed Cu-Pd and Cu-Pt bis(μ-oxo) complexes

Article abstract of DOI:10.1021/ja071744g

Heterobimetallic CuPd and CuPt bis(μ-oxo) complexes have been prepared by the reaction of (PPh₃)₂MO₂ (M = Pd, Pt) with LCu(I) precursors (L = β-diketiminate and di- and triamine ligands) and characterized by low-temperature UV-vis, resonance Raman, and ¹H and ³¹P{¹H} NMR spectroscopy in conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh₃, and in one case this was shown to occur by an intramolecular process through crossover experiments using double-labeling (oxo and phosphine). The reactivity of one of the complexes, L^Me2Cu(μ-O)₂Pt(PPh ₃)₂ (L^Me2 = β-diketiminate), with a variety of reagents including CO₂, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate, [NH₄][PF₆], and dihydroanthracene, was compared to that of homometallic Pt₂ and Cu₂ counterparts. Unlike typical [CU₂(μ-O) ₂]²⁺ cores which have electrophilic oxo groups, the oxo groups in the [Cu(μ-O)₂Pt]⁺ core behave as bases and nucleophiles, similar to previously described Pt₂ compounds. In addition, however, the [Cu(μ-O)₂Pt]⁺ core is capable of oxidatively coupling 2,4-di-terf-butylphenol and 2,4-di-tert-butylphenolate. Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics of the Cu₂ and CuM (M = Pd, Pt) cores showed that the latter are more basic and form stronger O-H bonds.

Full text of DOI:10.1021/ja071744g

Published on Web 06/06/2007
Heterobimetallic Dioxygen Activation: Synthesis and
Reactivity of Mixed Cu-Pd and Cu-Pt Bis(µ-oxo) Complexes
John T. York,† Antoni Llobet,‡ Christopher J. Cramer,*,† and William B. Tolman*,†
Contribution from the Department of Chemistry, Center for Metals in Biocatalysis, and
Supercomputer Institute, UniVersity of Minnesota, 207 Pleasant Street SE, Minneapolis,
Minnesota 55455, Institute of Chemical Research of Catalonia (ICIQ), AV. Pa¨ısos Catalans, 16.
43007 Tarragona, Spain, and Departament de Qu´ımica, UniVersitat Auto`noma de Barcelona,
E-08193 Barcelona, Spain
Received March 12, 2007; E-mail: cramer@chem.umn.edu; tolman@chem.umn.edu
Abstract: Heterobimetallic CuPd and CuPt bis(µ-oxo) complexes have been prepared by the reaction of
(PPh3)2MO2 (M ) Pd, Pt) with LCu(I) precursors (L ) â-diketiminate and di- and triamine ligands) and
1
characterized by low-temperature UV-vis, resonance Raman, and H and 31P{1H} NMR spectroscopy in
conjunction with DFT calculations. The complexes decompose upon warming to yield OPPh3, and in one
case this was shown to occur by an intramolecular process through crossover experiments using double-
labeling (oxo and phosphine). The reactivity of one of the complexes, LMe2Cu(µ-O)2Pt(PPh3)2 (LMe2
)
â-diketiminate), with a variety of reagents including CO2, 2,4-di-tert-butylphenol, 2,4-di-tert-butylphenolate,
[NH4][PF6], and dihydroanthracene, was compared to that of homometallic Pt2 and Cu2 counterparts. Unlike
typical [Cu2(µ-O)2]2+ cores which have electrophilic oxo groups, the oxo groups in the [Cu(µ-O)2Pt]+ core
behave as bases and nucleophiles, similar to previously described Pt2 compounds. In addition, however,
the [Cu(µ-O)2Pt]+ core is capable of oxidatively coupling 2,4-di-tert-butylphenol and 2,4-di-tert-butylphenolate.
Theoretical evaluation of the electron affinities, basicities, and H-atom transfer kinetics and thermodynamics
of the Cu2 and CuM (M ) Pd, Pt) cores showed that the latter are more basic and form stronger O-H
bonds.
1. Introduction
mononuclear Cu(I) complexes supported by N-donor ligands,
[Cu2(µ-O)2]2+ complexes have been shown to contain Cu(III)
ions11 bound to oxo groups that exhibit electrophilic character.2c
While the reactivity of these complexes depends in often subtle
ways on the nature of their supporting N-donor ligands,
counterions, and/or reaction conditions, they generally share a
propensity to abstract hydrogen atoms from ligand substituents12
In efforts to gain a mechanistic understanding of oxidation
catalysis, an important objective is to characterize metal-oxygen
complexes relevant to postulated reaction intermediates.1 Ho-
mometallic bis(µ-oxo) complexes with [M2(µ-O)2]n+ cores are
important examples, having been studied extensively for M )
Cu,2 Mn,3 Ni,4 Co,4a,b Fe,2a,5 and Pt,6 in large part because of
their possible involvement in biological and/or biomimetic
oxidations. For example, the [Cu2(µ-O)2]2+ core has been
considered as the active oxidant in phenol hydroxylation by
tyrosinase,7 as well as in methane hydroxylations by particulate
methane monooxygenase8,9 and a heterogeneous zeolite-based
system.10 Typically prepared via the reaction of O2 with
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Am. Chem. Soc. 1998, 120, 10567-10568. (b) Hikichi, S.; Yoshizawa, M.;
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† University of Minnesota.
‡ Institute of Chemical Research of Catalonia. Universitat Auto`noma de
Barcelona.
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7990
J. AM. CHEM. SOC. 2007, 129, 7990-7999
10.1021/ja071744g CCC: $37.00 © 2007 American Chemical Society

Mixed Cu−Pd and Cu−Pt Bis(µ-oxo) Complexes
A R T I C L E S
or exogeneous reagents such as phenols.13,14 Oxo transfers to
thioethers15 and phosphines16 also have been observed. The oxo
groups in [M2(µ-O)2] cores with M ) Ni, Co, and Fe also exhibit
electrophilic character,2a,4,5 but a profoundly different reactivity
has been found for M ) Pt.6 Countercations (e.g., Li+) associate
with oxo groups bridging Pt(II) in the solid state, and these oxo
groups act as nucleophiles and Brønsted bases.17
report an extension of this strategy for the synthesis of novel
heterobimetallic CuPd and CuPt complexes from the reaction
of copper(I) complexes supported by N-donor ligands of variable
types with the metal peroxo species (PPh3)2MO2 (M ) Pd, Pt).
The thermally unstable products were shown to contain [M(µ-
O)2Cu]1+ cores on the basis of UV-vis and resonance Raman
spectroscopic data, with further structural insights provided by
NMR spectroscopy and DFT calculations. Comparison of the
reactivity of a member of the class, LCu(µ-O)2Pt(PPh3)2 (L )
â-diketiminate), with that of a homometallic dicopper counter-
part revealed significant differences, particularly with respect
to the course of their reactions with phenols, phenolates, and
weak acids. These differences have been examined via theoreti-
cal evaluation of the thermodynamic and kinetic parameters
associated with electron, proton, and/or H-atom transfer reac-
tions of the Cu2 and CuPt cores supported by a range of ligand
sets. Preliminary aspects of this work have been communi-
cated.24
Mixed metal or heterobimetallic oxygen intermediates are less
common than homometallic species, yet they are of great interest
due to the possible “synergistic” effect of two different metal
ions acting together. An illustrative example is the activation
of dioxygen by the cytochrome c oxidase Cu-Fe(heme) site
and relevant synthetic models.18 Heterometallic species are
prevalent in heterogeneous catalysis19 and have been shown to
be useful for homogeneous catalytic oxidations.20 Of particular
relevance in the current context are Cu-Pd intermediates
postulated in Wacker-type oxidations.21 Examples such as these
support the notion that two metals might work together to
activate O2 in such a way that the resulting oxo moiety exhibits
reactivity different from that observed using either of the metals
individually.
2. Experimental Section
2.1. General Considerations. All solvents and reagents were
obtained from commercial sources and used as received unless noted
otherwise. The solvents tetrahydrofuran (THF), toluene, diethyl ether
(Et2O), pentane, and dichloromethane (CH2Cl2) were degassed and
passed through a solvent purification system (Glass Contour, Laguna
CA) before use. All metal complexes were prepared and stored in a
Vacuum Atmospheres inert atmosphere glovebox under a dry nitrogen
atmosphere or were manipulated using standard inert atmosphere
vacuum and Schlenk techniques. Labeled dioxygen (18O2, 99%) was
obtained from Cambridge Isotopes, Inc. The complexes Pt(PPh3)4,28
Pd(P(p-tolyl)3)3,29 (PPh3)2PdO2,24 (PPh3)2PtO2,30 [(Me4chd)Cu(MeCN)]-
PF6,31 [(Me4pda)Cu(MeCN)]OTf,32 [LMe2Cu]2,33 and [(iPr3tacn)Cu-
Because of the possibility of obtaining intractable reaction
mixtures, the procedure typical for preparing homobimetallic
metal-oxygen intermediates (e.g., [M2(µ-O)2]n+ species) whereby
a monomeric precursor is treated with O2 generally cannot be
used to cleanly isolate heterobimetallic products (for exceptions,
see ref 18a). An alternate route involves the reaction of a
preformed mononuclear metal-dioxygen complex with a second
reduced metal compound, in the absence of free O2. This route
has been successfully used to prepare dimetal complexes with
different supporting ligands on each metal (e.g., with Cu22 and
Ni23), as well as heterobimetallic oxygen-containing species with
CuNi,24 CuGe,25 PtMo,26 and PtGe27 combinations. Herein we
34
(MeCN)]SbF6 were synthesized according to published procedures.
The NMR solvents CDCl3 and C6D6 were stirred over CaH2 and vacuum
distilled prior to use.
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2.2. Physical Methods. NMR spectra were recorded on a VXR-
300 or VI-300 spectrometer. Chemical shifts (δ) for 1H and 13C NMR
spectra are reported versus tetramethylsilane and were referenced to
residual protium in the deuterated solvent. 31P{1H} NMR spectra are
referenced to an external H3PO4 standard (85%). Mass spectra were
obtained on a Bruker Biotof II instrument. UV-vis spectra were
recorded on an HP8453 (190-1100 nm) diode array spectrophotometer
equipped with a Unisoku low-temperature device. X-band EPR spectra
were recorded on a Bruker E-500 spectrometer with an Oxford
Instruments EPR-10 liquid helium cryostat (4-20K, 9.61 GHz).
Quantization of EPR signal intensity was accomplished by comparing
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2657. (d) Anandhi, U.; Sharp, P. R. Inorg. Chem. 2004, 43, 6780-6785.
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104, 1077-1134. (b) Collman, J. P.; Boulatov, R.; Sunderland, C. J.; Fu,
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M. M. Organometallics 2002, 21, 5373-5381. (b) Litz, K. E.; Banaszak-
Holl, M. M.; Kampf, J. W.; Carpenter, G. B. Inorg. Chem. 1998, 37, 6461-
6469.
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2000.
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407-455. (b) Nesterova, D. S.; Volodymyr N. Kokozay; Dyakonenko,
V. V.; Shishkin, O. V.; Jezierska, J.; Ozarowski, A.; Kirillov,
A. M.; Kopylovich, M. N.; Pombeiro, A. J. L. Chem. Commun. 2006,
4605-4607.
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W. B. J. Am. Chem. Soc. 2005, 127, 13752-13753.
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P. J. Am. Chem. Soc. 1999, 121, 5583-5584.
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C. G. Inorg. Chem. 2004, 43, 3324-3326.
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B. Inorg. Chem. 2002, 41, 6307-6321.
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9
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A R T I C L E S
York et al.
the integration with that of H(Me2LiPr2)CuCl.35 Resonance Raman
spectra were collected on an Acton AM-506 spectrometer using a
Princeton Instruments LN/CCD-11100-PB/UVAR detector and ST-1385
controller interfaced with Winspec software. A Spectra-Physics Beam-
Lok 2065-7S Ar laser provided excitation at 457.9 nm. The spectra
were obtained at -196 °C using a backscattering geometry. Samples
were frozen in an NMR tube submerged in liquid N2 or frozen in a
copper cup attached to a coldfinger Dewar filled with liquid nitrogen.
Raman shifts were externally referenced to liquid indene. IR spectra
were obtained using a ThermoNicolet Avatar 370 FT-IR equipped with
an ATR attachment.
2.3. (PPh3)2Pt18O2. A method adapted from the reported procedure
for (PPh3)2Pt16O2 was used.30A 25 mL Schlenk flask was charged with
a suspension of Pt(PPh3)4 (200 mg, 0.16 mmol) in 10 mL of Et2O. The
suspension was frozen at -196 °C, the headspace was evacuated, and
18O2 was transferred from a glass bulb into the flask. Warming to
ambient temperature and stirring for 2 h resulted in the deposition of
a tan solid, which was collected, washed with Et2O (3 × 5 mL), and
dried under reduced pressure (92 mg, 77%). FT-IR (νO-O): 776 cm-1
(∆18O2 ) 48 cm-1).
solutions of (PPh3)2MO2 (M ) Pd, Pt) in CH2Cl2 (2 mL, 10 mM) at
-80 °C in a 10 mL Schlenk flask. The resulting solution was stirred
for 30 min and transferred by micropipet to a copper cup attached to
a liquid N2 coldfinger Dewar for collection of the resonance Raman
spectra.
2.7. Reactivity of (LMe2Cu)2(µ-O)2 with Exogenous Substrates.
Anaerobic solutions of [LMe2Cu]2 in THF or toluene (2 mL, 0.1 mM
for UV-vis; 3 mL, 5 mM for preparatory scale) were cooled to -80 °C,
and dry O2 was bubbled for 5 min. The solutions were then stirred at
-80 °C for 30 min to ensure complete formation of (LMe2Cu)2(µ-O)2.
Excess O2 was removed by purging with argon in the UV-vis
experiments and by 6-10 vacuum/purge cycles with argon in the
preparatory scale experiments. Reactivity in the UV-vis experiments
was followed by monitoring the decay of the absorption feature at
422 nm associated with the [Cu2(µ-O)2]2+ core. In the following, the
number of equivalents of substrate are relative to the amount of the
[Cu2(µ-O)2]2+ core.
2.7.1. [NH4][PF6], PPh3, Thioanisol, 1-Decene, 2,4-Di-tert-bu-
tylphenol, Sodium 2,4-Di-tert-butylphenolate, 9,10-Dihydroan-
thracene, and CO2 at -80 °C by UV-vis. The substrates (10 equiv)
were added in 0.2 mL of solvent by syringe (for CO2, the dry gas was
bubbled for 10 s). No change in the absorption feature at 422 nm was
observed after several hours for any of the substrates.
2.4. (P(p-tolyl)3)2Pd16O2. A 50 mL Schlenk flask was charged with
a suspension of Pd(P(p-tolyl)3)3 (200 mg, 0.2 mmol) in 20 mL of Et2O.
Bubbling of dry O2 into the solution for 10 min resulted in the
precipitation of the product as a pale blue solid, which was collected,
washed with Et2O (3 × 5 mL), and dried under reduced pressure (91
2.7.2. 2,4-Di-tert-butylphenol at -40 °C. UV-vis. 2,4-Di-tert-
butylphenol (10 equiv) was added by syringe in 0.2 mL of solvent to
a solution of (LMe2Cu)2(µ-O)2 in THF at -40 °C. The absorption feature
at 422 nm decayed gradually over the course of several hours (t1/2 ∼4
h). Preparatory Scale. The substrate (2 equiv) was added by syringe
in 0.2 mL of solvent to a solution of (LMe2Cu)2(µ-O)2 in THF at -80
°C. The solution was then warmed to -40 °C, where it was stirred for
12 h. The solution was warmed to room temperature, and the solvent
was removed under reduced pressure. The resulting brown residue was
dissolved in 5 mL of CH2Cl2 and stirred with 5 mL of aqueous
ammonium hydroxide solution. The organic fraction was collected, and
the aqueous solution was extracted with a further 2 × 5 mL of CH2-
Cl2. The organic fractions were combined, dried over Na2SO4, and
filtered through a plug of neutral Alumina, which was washed with a
further 5 mL of CH2Cl2. The solvent was removed under reduced
pressure, and 2 equiv of 1,3,5-trimethoxybenzene were added as an
internal standard. The solution was dissolved in CDCl3 and characterized
1
mg, 61%). FT-IR (νO-O): 882 cm-1. H NMR (CDCl3): δ 7.22 (m,
12H), 6.97 (d, 12H), 2.31 (s, 18H) ppm. 13C{1H} NMR (CDCl3): δ
140.2, 133.9 (t), 129.0 (t), 21.5 ppm. 31P{1H} NMR (CDCl3): δ 31.7
ppm. Anal. Calcd for C42H42O2P2Pd: C, 67.52; H, 5.67. Found: C,
67.22; H, 5.97.
2.5. (P(p-tolyl)3)2Pd18O2. A 25 mL Schlenk flask was charged with
a suspension of Pd(P(p-tolyl)3)3 (200 mg, 0.2 mmol) in 10 mL of Et2O.
The suspension was frozen at -196 °C, the headspace was evacuated,
and 18O2 was transferred from a glass bulb into the flask. Warming to
ambient temperature and stirring for 20 min resulted in the deposition
of a light blue solid, which was collected, washed with Et2O (3 × 5
mL), and dried under reduced pressure (84 mg, 56%). FT-IR (νO-O):
835 cm-1 (∆18O2 ) 47 cm-1).
2.6. Characterization of [Cu(µ-O)2M]+ Complexes. 2.6.1. UV-
vis Spectroscopy. Anaerobic solutions of (PPh3)2MO2 (M ) Pd, Pt)
in CH2Cl2, toluene, or THF (0.2 mM, 2 mL) in a UV-vis cuvette were
cooled to -80 °C in the UV-vis cryostat. The copper(I) reagent (1
equiv) in 0.2 mL of solvent was injected by syringe into the stirred
cold metal peroxo solution. The progress of intermediate formation
and decay was followed by monitoring the main absorption feature at
∼450 nm associated with the [Cu(µ-O)2M]+ cores.
1
by GC/MS and H NMR. The yield of the coupled phenolic product
was 60% (1.2 equiv of phenol coupled, 0.8 equiv of recovered
unreacted).
2.8. Decomposition Reactions of [M(µ-O)2Cu]+ Complexes.
Anaerobic solutions of (PPh3)2MO2 (M ) Pd, Pt) in CH2Cl2, toluene,
or THF (3 mL, 10 mM) were cooled to -80 °C, and 1 equiv of the
copper(I) complex in 0.2 mL of solvent was added by syringe to
form the [M(µ-O)2Cu]+ complex. The solutions were allowed to
decompose and/or were warmed to room temperature, and the solvent
was removed under reduced pressure. The resulting brown residue was
dissolved in 5 mL of CH2Cl2 and stirred with 5 mL of aqueous
ammonium hydroxide solution. The organic fraction was collected, and
the aqueous solution was extracted with a further 2 × 5 mL CH2Cl2.
The organic fractions were combined, dried over Na2SO4, and filtered
through a plug of neutral alumina, which was washed with a further 5
mL of CH2Cl2. The solvent was removed under reduced pressure, and
2 equiv of triphenylphosphate per [M(µ-O)2Cu]+ core (31P{1H}, δ )
-18 ppm) were added as an internal standard for quantification of the
phosphorus containing products. The resulting mixture was dissolved
in C6D6 or CDCl3 for characterization by 1H and 31P{1H} NMR
spectroscopy.
2.6.2. EPR Spectroscopy. Anaerobic solutions of (PPh3)2MO2 (M
) Pd, Pt) in CH2Cl2, toluene, or THF(2 mL, 1.0 mM) in a 10 mL
Schlenk flask were cooled to -80 °C in a MeOH/liquid N2 bath. The
desired copper(I) reagent (1 equiv) in 0.2 mL of solvent was then added
by syringe, and the resulting solution was transferred Via a precooled
cannula to a precooled EPR tube, which was immediately frozen at
-196 °C.
2.6.3. NMR and Resonance Raman Spectroscopy. An NMR
tube was purged with argon gas through an 8” needle for approxi-
mately 15 min. The tube was charged with an anaerobic solution of
(PPh3)2MO2 (M ) Pd, Pt) in CH2Cl2, toluene, or THF (0.5 mL, 10
mM) by syringe and cooled to -80 °C by submersion in a MeOH/
liquid N2 bath while maintaining the argon purge. The desired
copper(I) reagent (1 equiv) in 0.1 mL solvent was then added rapidly
by syringe. The NMR tubes were maintained at -80 °C for the
collection of NMR spectra and were frozen at -196 °C for the
collection of resonance Raman data. For L ) iPr3tacn, 1 equiv of the
copper(I) complex in 0.2 mL CH2Cl2 was added by syringe to anaerobic
2.9. Crossover Experiments. Anaerobic solutions of (PPh3)2Pd18O2
(3.0 mg, 0.0045 mmol) and (P(p-tolyl)3)2Pd16O2 (3.4 mg, 0.0045 mmol)
were added sequentially Via syringe to a -80 °C solution of [LMe2Cu]2
(3.4 mg, 0.0045 mmol) in 3 mL of toluene. The solution immediately
became deep yellow-brown, indicating formation of the [Cu(µ-O)2Pd]+
(35) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270-7271.
9
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Mixed Cu−Pd and Cu−Pt Bis(µ-oxo) Complexes
A R T I C L E S
core.36 The solution was stirred at low temperature for 1 h and then
allowed to decompose by warming to room temperature. Analysis of
the product mixture by ESI-MS revealed the phosphine oxidation
products in the ratios 18OPPh3(∼80%):16OPPh3(∼20%) and 16OP(p-
tolyl)3(100%):18OP(p-tolyl)3(0%). An analogous experiment using
(PPh3)2Pd16O2 and (P(p-tolyl)3)2Pd18O2 revealed the formation of
products in the ratios 18OPPh3(0%)/16OPPh3(100%) and 16OP(p-tolyl)3-
(32%)/18OP(p-tolyl)3(68%). Control experiments involving characteriza-
tion of the decomposition products of LMe2Cu(µ-18O)2Pd(PPh3)2 and
LMe2Cu(µ-18O)2Pd(P(p-tolyl3)2 revealed the formation of ∼80% 18OPPh3
and ∼68% 18OP(p-tolyl)3, respectively, with adventitious 16O2 or water
possibly resulting in the 16O-incorporation in the phosphine oxide
products.
solution immediately lightened from deep orange-brown to pale
yellow-brown. The solution was stirred for an additional 30 min
and warmed to room temperature, and the solvent was removed
under reduced pressure to give a brown residue. The resulting
brown residue was dissolved in 5 mL of CH2Cl2 and stirred
with 5 mL of aqueous ammonium hydroxide solution. The organic
fraction was collected, and the aqueous solution was extracted
with a further 2 × 5 mL of CH2Cl2. The organic fractions were
combined, dried over Na2SO4, and filtered through a plug of
neutral alumina, which was washed with a further 5 mL of CH2Cl2.
The solvent was removed under reduced pressure, and 2 equiv of 1,3,5-
trimethoxybenzene were added as an internal standard. The solution
1
was dissolved in CDCl3 and characterized by GC/MS and H NMR
2.10. Reactivity of LMe2Cu(µ-O)2Pt(PPh3)2 with Exogenous Sub-
strates. Anaerobic solutions of (PPh3)2PtO2 in THF (2 mL, 0.2 mM
for UV-vis; 3 mL, 5 mM for preparatory scale) were cooled to
-80 °C and 0.5 equiv of [LMe2Cu]2 in 0.2 mL THF was injected by
syringe into the cold platinum peroxo solution. The solutions were
maintained at -80 °C for 30 min to ensure complete formation of the
intermediate before substrate addition. In the following, the number of
equivalents of substrate are relative to the amount of the [Cu(µ-O)2Pt]1+
core.
2.10.1. [NH4][PF6], UV-vis Scale. [NH4][PF6] (2 equiv) was added
in 0.1 mL THF by syringe to a -80 °C solution of LMe2Cu(µ-O)2Pt-
(PPh3)2, at which time immediate bleaching of the absorption feature
at 455 nm in the UV-vis spectrum was observed. Preparatory Scale.
[NH4][PF6] (2 equiv) was added in 0.5 mL of THF by syringe to a
-80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2, at which time the solution
immediately lightened from deep orange-brown to pale green-brown.
The solution was stirred for an additional 30 min and warmed to room
temperature, and the solvent was removed under reduced pressure to
giveabrownresidue.ESI-MS(m/z): Calculatedfor[PtCuP2N2C57H57O2]+,
1121.2840; Found, 1121.2883.
2.10.2. 2,4-Di-tert-butylphenol, UV-vis Scale. 2,4-Di-tert-bu-
tylphenol (2 equiv) was added in 0.1 mL of THF by syringe to a
-80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2, at which time the immediate
bleaching of the absorption feature at 455 nm in the UV-vis spectrum
was observed. Preparatory Scale. 2,4-Di-tert-butylphenol (1 or 2
equiv) was added in 0.5 mL of THF by syringe to a -80 °C solution
of LMe2Cu(µ-O)2Pt(PPh3)2, at which time the solution immediately
lightened from deep orange-brown to pale yellow-brown. The solution
was stirred for an additional 30 min and warmed to room temperature,
and the solvent was removed under reduced pressure to give a brown
residue. The resulting brown residue was dissolved in 5 mL of CH2Cl2
and stirred with 5 mL of aqueous ammonium hydroxide solution. The
organic fraction was collected, and the aqueous solution was extracted
with a further 2 × 5 mL CH2Cl2. The organic fractions were combined,
dried over Na2SO4, and filtered through a plug of neutral alumina, which
was washed with a further 5 mL of CH2Cl2. The solvent was removed
under reduced pressure, and 2 equiv of 1,3,5-trimethoxybenzene were
added as an internal standard. The solution was dissolved in CDCl3
and characterized by GC/MS and 1H NMR. The yield of coupled
phenolic product was ∼80% for the addition of 1 equiv of phenol and
46% for the addition of 2 equiv (i.e., ∼1 equiv of substrate coupled in
each case, the other left unreacted).
spectroscopy. The yield of coupled phenolic product was 56% for the
addition of 1 equiv of phenolate and 33% for 2 equiv of phenolate
added (i.e., ∼0.6 equiv of substrate coupled, 1.4 equiv of recovered
unreacted).
2.10.4. PPh3, 1-Decene, Thioanisol, and 9,10-Dihydroanthracene.
The substrates (10 equiv) in 0.2 mL of THF were added by syringe to
a -80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2. No change in the
absorption spectrum was observed for any substrate.
2.10.5. With CO2, UV-vis. Dry CO2 gas was bubbled through a
-80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2 in THF for 5 s, upon which
time the immediate formation of an absorption feature at 550 nm was
observed. Preparatory Scale. Dry CO2 gas was bubbled for 5 s
through a -80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2, prepared from
(PPh3)2PtO2 (20 mg, 0.026 mmol) and [LMe2Cu]2 (10 mg, 0.013 mmol)
in THF (5 mL), during which time the solution turned deep
purple. Over the course of several minutes, the solution then lightened
to pale brown. The solution was warmed to room temperature, and the
solvent was removed under reduced pressure to give a brown residue.
The residue was washed on a frit with benzene (5 mL) to give a tan
solid (12 mg, 58%). FT-IR (νCdO ) 1680 cm-1); 31P{1H} NMR
(CDCl3): δ 7.15 ppm (JPtP ) 3697 Hz); 1H NMR (CDCl3): δ 7.3 ppm
(m); ESI-MS (m/z): Calculated for [PtP2C37H31O3]+, 780.1394; Found,
780.1412.
2.11. Theory. Geometry optimizations were performed with Density
Functional Theory (DFT) using the B98 functional37 (a 10-parameter
hybrid functional designed by Becke) as implemented in the Gaussian03
suite of ab initio quantum chemistry programs.38 The 6-31G(d) and
6-311G(d,p) basis sets were used for H and C atoms and N, O, and P
atoms, respectively. For Cu, Pd, and Pt, relativistic Stuttgart effective
core potentials and associated basis functions were used.39 This
combination of functional and basis sets gave bond distances and angles
consistent with analogous model systems and chemical intuition for
all of the compounds described in this work. For a particular case for
which an X-ray crystal structure is available, [CuIII(H33m)]2+ (H33m
is the macrocyclic ligand 3,7,11-triaza-bicyclo[11.3.1]heptadeca-1(16),-
13(17),14-triene), bond distances and angles computed at the
B98 level show excellent agreement with the experimental structure
(Supporting Information). In addition,
a recent comparison of
34 density functionals40 found the B98 functional to be one of the
two best functionals for the prediction of a variety of molecular
properties, including geometries, electron affinities, and hydrogen-atom
transfer barriers, all three of which are particularly important to the
present study. Partial atomic charges were computed by the
methods of Lo¨wdin41 and Mulliken.42 Vibrational frequencies were
determined analytically in order to confirm the computed stationary
2.10.3. Sodium 2,4-Di-tert-butylphenolate, UV-vis Scale. Sodium
2,4-di-tert-butylphenolate (2 equiv) was added in 0.1 mL of THF by
syringe to a -80 °C solution of [LMe2Cu(µ-O)2Pt(PPh3)2, at which time
the immediate bleaching of the absorption feature at 455 nm in the
UV-vis spectrum was observed. Preparatory Scale. Sodium 2,4-di-
tert-butylphenolate (2 equiv) was added in 0.5 mL of THF by syringe
to a -80 °C solution of LMe2Cu(µ-O)2Pt(PPh3)2, at which time the
(37) (a) Becke, A. D. J. Chem. Phys. 1997, 107, 8554-8560. (b) Schmider, H.
L.; Becke, A. D. J. Chem. Phys. 1998, 108, 9624-9631.
(38) Frisch, M. J. et al. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford,
CT, 2004.
(39) Dolg, M.; Wedig, U.; Stoll, H.; Preuss, H. J. Chem. Phys. 1987, 86, 866-
872.
(36) Addition of 0.5 equiv of [LMe2Cu]2 to a THF solution of (P(p-tolyl)3)2PdO2
at -80 °C resulted in the appearance of a UV-vis feature identical to that
observed for [LMe2Cu(µ-O)2Pd(PPh3)2], confirming the formation of the
analogous species LMe2Cu(µ-O)2Pd(P(p-tolyl)3)2.
(40) Riley, K. E.; Op’t Holt, B. T.; Merz, K. M. J. Chem. Theory Comput.
2007, 3, 407-433.
(41) Lowdin, P.-O. J. Chem. Phys. 1950, 18, 365-375.
(42) Mulliken, R. S. J. Chem. Phys. 1955, 23, 1833-1840.
9
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A R T I C L E S
York et al.
Table 1. Selected Spectroscopic Data for Heterobimetallic
Bis(µ-oxo) Complexes
UV−vis
complex
λmax
(
ꢀ a
)
Raman shift (18O)b
c
LMe2Cu(µ-O)2Pd(PPh3)2
448 (∼5900)
450 (∼5600)
472 (∼2800)
483 (∼2700)
463 (∼3500)
457 (∼3600)
458 (∼2500)
462 (∼3000)
660 (631)
628, 613 (594)
610 (580)
d
LMe2Cu(µ-O)2Pt(PPh3)2
[(Me4pda)Cu(µ-O)2Pd(PPh3)2]OTfe
[(Me4pda)Cu(µ-O)2Pt(PPh3)2]OTfe
595 (569)
e
[(Me4chd)Cu(µ-O)2Pd(PPh3)2]PF6
640, 616 (600)
647, 616 (603)
630/f
e
[(Me4chd)Cu(µ-O)2Pt(PPh3)2]PF6
e
[(iPr3tacn)Cu(µ-O)2Pd(PPh3)2]SbF6
e
[(iPr3tacn)Cu(µ-O)2Pt(PPh3)2]SbF6
628 (601, 585)
a Measured in solution at -80 °C. Units: λmax ) nm, ꢀ ) M-1 cm-1
per complex. b Measured as frozen solutions at -196 °C with λex ) 457.9
nm. Units: cm-1 c Solvent ) THF. d Solvent ) 4:1 v/v CH2Cl2/THF.
.
e Solvent ) CH2Cl2. f 18O-isotope data not collected.
Figure 1. UV-vis spectrum (-80 °C) and resonance Raman spectra
(inset, -196 °C, λex ) 457.9 nm as noted by arrow in the UV-vis
spectrum; solid line ) 16O, dashed line ) 18O) of LMe2Cu(µ-O)2Pt-
(PPh3)2 in 4:1 v/v CH2Cl2/THF. The peak at ∼700 cm-1 is due to
CH2Cl2.
Scheme 1. Synthesis of Heterobimetallic Bis(µ-oxo) Complexes
but at a significantly decreased rate (∼1 h). A representive
spectrum for the case where L ) LMe2 and M ) Pt is shown in
Figure 1, with others provided in the Supporting Information
(Figures S1-S6). In one case, L ) LMe2 and M ) Pd, a
spectrophotometric titration was performed, which showed that
the maximum aborbance was reached at a Cu/Pd complex ratio
of 1:1 (Figure S7).
Although we are currently unable to assign the low energy
shoulder in the absorption spectra, the similarity of the feature
at λmax ) 450-480 nm to one typical of [Cu2(µ-O)2]2+
cores2a,b,46 suggests an analogous O f Cu(III) LMCT assign-
ment for an [M(µ-O)2Cu]+ core. For the complexes with L )
Me4pda, Me4chd, and iPr3tacn, the intensity of this feature (ꢀ
∼ 2500-3600 M-1 cm-1) is significantly lower than that in
the analogous [L2Cu2(µ-O)2]2+ complexes (ꢀ ∼5000-10 000
M-1 cm-1 per Cu), which may reflect decreased covalency in
the Cu-O bonding within the [M(µ-O)2Cu]+ units. Consistent
with the O f Cu(III) LMCT assignment, laser excitation into
this band enables observation of O-isotope sensitive features
in Raman spectra (Figure 1, Table 1, and Figures S1-S6 in the
Supporting Information). Strong peaks are observed in the
∼600-650 cm-1 region, which shift by ∼25-30 cm-1 upon
18O substitution (using (PPh3)2M18O2 as reagent). We assign
these peaks to a [M(µ-O)2Cu]+ core vibrational mode(s) on the
basis of analogy to the well-studied [Cu2(µ-O)2]2+ systems, for
which similar Raman shifts and O-isotope sensitivities were
reported.46,47 In a few cases (cf. Figure 1), two peaks in the
spectrum of the species comprising one O-isotope shift to a
single peak for the other O-isotopomer. Such phenomena were
seen previously in [Cu2(µ-O)2]2+ 47 and [GeCu(µ-O)2]+ 25
complexes and were ascribed to a Fermi doublet that collapses
upon isotope substitution, suggesting a similar interpretation of
the data for the [M(µ-O)2Cu]+ cores (M ) Pd or Pt). No isotope
sensitive vibrations are apparent in the 750-900 cm-1 region,
arguing against the presence of peroxo species.48
points as either minima or transition-state structures and were also
used in the computation of zero-point energy, enthalpy, and entropy
under the standard rigid-rotor, harmonic-oscillator, ideal-gas approxima-
tion.43
Single-point solvation free energies in dichloromethane as solvent
were calculated using the Polarized Continuum Model (PCM).44 The
PCM cavity was constructed from united atom radii (UA0).45
3. Results and Discussion
3.1. Synthesis and Characterization of [LCu(µ-O)2M-
(PPh3)2]n+ (M ) Pd, Pt). Addition of 1 equiv of [LCuI-
(MeCN)]+ (L ) Me4pda, Me4chd, iPr3tacn) or 0.5 equiv of
[LMe2Cu]2 to a solution (0.2 mM) of (PPh3)2MO2 (M ) Pd, Pt)
in CH2Cl2, toluene, or THF at -80 °C yielded heterobimetallic
bis(µ-oxo) complexes [LCu(µ-O)2M(PPh3)2]n+ (Scheme 1),
which were identified on the basis of UV-vis, resonance
Raman, and, in selected cases, 1H and/or 31P{1H} NMR
spectroscopy. The products are thermally sensitive, as indicated
by bleaching of their UV-vis absorption features upon warming
to ambient temperature, and attempts to isolate them as
crystalline solids have not been successful. For L ) LMe2, Me4-
pda, and Me4chd, addition of the copper(I) reagent resulted in
an immediate color change from nearly colorless to deep yellow-
to orange-brown, with concurrent formation of a strong absorp-
tion feature in the UV-vis spectrum at 450-480 nm (ꢀ ∼2500-
6000 M-1 cm-1) and a weaker shoulder at ∼550-650 nm
(Table 1). The reaction proceeded similarly for L ) iPr3tacn
Solutions of the heterobimetallic reaction products were found
to be EPR silent (X-band, ∼2.5K), consistent with the diamag-
netic ground state expected for the M(II)-Cu(III) core.49 Low
(43) Cramer C. J. Essentials of Computational Chemistry. Theories and
Models, 2nd ed.; John Wiley & Sons: Chichister, U.K., 2004; pp 299-
(46) Henson, M. J.; Mukherjee, P.; Root, D. E.; Stack, T. D. P.; Solomon, E. I.
J. Am. Chem. Soc. 1999, 121, 10332-10345.
(47) Holland, P. L.; Cramer, C. J.; Wilkinson, E. C.; Mahapatra, S.; Rodgers,
K. R.; Itoh, S.; Taki, M.; Fukuzumi, S.; Que, L., Jr.; Tolman, W. B. J. Am.
Chem. Soc. 2000, 122, 792-802.
306.
(44) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. ReV. 2005, 105, 2999-3093.
(45) Barone, V.; Improta, R.; Rega, N. Theor. Chem. Acc. 2004, 111, 237-
245.
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Mixed Cu−Pd and Cu−Pt Bis(µ-oxo) Complexes
A R T I C L E S
Figure 2. Key bond lengths (Å) for structures 1-3 (roman text) optimized at the B98 level (legend inset). The same bond lengths for the one-electron
reduced species 4-6 (italic text) are also provided.
temperature (-80 °C, THF) 1H and/or 31P{1H} NMR data were
acquired for two representative cases, LMe2Cu(µ-O)2Pd(PPh3)2
and LMe2Cu(µ-O)2Pt(PPh3)2. The spectra support the presence
of a single diamagnetic reaction product. For example, in the
31P{1H} NMR spectrum of LMe2Cu(µ-O)2Pd(PPh3)2, a sharp
resonance at 26.8 ppm is observed, distinct from that for (PPh3)2-
PdO2 (34.2 ppm) and the decomposition product OPPh3 (see
below; 24.7 ppm). Similarly, the 31P{1H} NMR spectrum of
LMe2Cu(µ-O)2Pt(PPh3)2 contains a single resonance at 12.4 ppm
(JP-Pt ) 3280 Hz, Figure S8) which differs significantly from
that of the starting complex (PPh3)2PtO2 (15.6 ppm, JPtP ) 4061
Hz). Consistent with the structural assignment of a bis(µ-oxo)
core, the 31P NMR resonance is similar to that observed in the
These data are consistent with those presented in Table 1, where
the absolute rhomb breathing frequencies and 18O-isotope shifts
of the heterobimetallic compounds are similar to those of
characterized homodicopper analogues.46,47 After averaging over
Fermi doublet peak positions, the computed isotope shifts for
the CuPd and CuPt compounds are in essentially quantitative
agreement with experiment, and absolute values of the frequen-
cies are reasonable given the simplicity of the ligands compared
to the experimentally characterized systems (which themselves
show substantial influence from the various ligands).
3.2. Thermal Decomposition Reactions. The [M(µ-O)2Cu]+
complexes decompose upon warming to room temperature as
indicated by bleaching of the solution color and decay of the
associated UV-vis absorption features. The metal-containing
products of the decay process have not been identified, although
EPR spectra of the decomposed solutions reveal variable
amounts of Cu(II) species (g ∼2 signal, ∼10-56% by integra-
tion). 31P{1H} NMR spectra of the residue remaining after
removal of solvent from the decomposed solutions revealed
OPPh3 as the sole observable reaction product in all cases, and
in the cases where M ) Pd quantitative yields were observed
by integration versus an added internal standard (e.g., 2 equiv.
OPPh3 per heterobimetallic complex). In the cases where M )
Pt, variable yields much less than quantitative (<20%) were
observed.
related complexes [(PPh3)2Pt]2(µ-O)2‚LiBF4 (7.8 ppm, JPtP
)
3286 Hz) and [(PPh3)2Pt]2(µ-O)2 (8.5 ppm, JPtP ) 3183 Hz).17a
Further information on the structures of the heterobimetallic
bis(µ-oxo) complexes was obtained from quantum chemical
calculations. Systems 1-3, shown in Figure 2, were optimized
at the B98 level of density functional theory, and their
vibrational frequencies were computed for comparison to the
experimental Raman data. Selected geometrical data for 1-3
are provided in Figure 2, and the complete set of optimized
geometries is provided in the Supporting Information. Com-
pound 1 has D2h symmetry, with the Cu(III) centers coordinated
in a square-planar fashion. Swapping Pd or Pt for Cu, by virtue
of the reduced charge on the group 10 metals, involves
replacement of the negatively charged diketiminate ligand with
phosphine ligands, as in the experimentally characterized mixed-
metal analogues. This introduces asymmetry in the core;
compared to Cu-O, the Pd-O and Pt-O bonds in compounds
2 and 3 are about 0.2 Å longer. The CuPd and CuPt distances
in 2 and 3 are 0.125 and 0.159 Å longer than the CuCu distance
in 1. Nevertheless, in spite of differences in the second metal
and the variation of diketiminate vs phosphine ligands, there is
overall a substantial similarity between the various cores.
Computed vibrational frequencies indicate that the rhomb
Mechanistic information was obtained from studies of the
decomposition of LMe2Cu(µ-O)2Pd(PPh3)2. Monitoring of the
reaction by 31P{1H} NMR at -40 °C showed clean conversion
of the complex to the product OPPh3, with no other peaks
apparent at intermediate reaction times. These data rule out the
buildup of measurable quantities of diamagnetic intermediates
during the reaction. Decomposition of the 18O-isotopomer
derived from (PPh3)2Pd18O2 revealed ∼80% incorporation of
the isotopically labeled oxygen in the OPPh3 product by ESI-
MS, confirming that the O-atom incorporated into the phosphine
is derived from the bis(µ-oxo) core. In order to probe whether
the phosphine oxidation involves inter- or intramolecular O-atom
transfer, crossover experiments were performed. In these studies,
the heterobimetallic bis(µ-oxo) complexes derived from (PPh3)2-
Pd18O2 and (P(p-tolyl)3)2Pd16O2 were mixed in a 1:1 ratio and
then allowed to decompose by warming to ambient temperature.
Solvent was removed and the residue was analyzed by ESI-
MS, which showed the predominant products to be 18OPPh3
and 16OP(p-tolyl)3; no 18OP(p-tolyl)3 and only a small amount
of 16OPPh3 (∼20%, possibly derived from adventitious exchange
“breathing” modes46,47 for 1-3 are 635, 597, and 603 cm-1
,
with 18O isotope shifts of 48, 30, and 30 cm-1, respectively.
(48) Additional unassigned peaks in the Raman spectrum of the complexes with
L ) iPr3tacn were observed (Figure S6) at 572 cm-1 and 505 cm-1, which
shifted upon 18O-substitution to 541 cm-1 and 477 cm-1, respectively (∆18O
) 31 cm-1 and 28 cm-1). With the data currently available, we cannot
discern whether these are due to an additional species or to other low
frequency modes for the [MCu(µ-O)2]+ core for this case.
(49) For the system with the ligand Me4pda, significant amounts (∼15%) of
Cu(II) decomposition products were observed in the EPR samples due to
the extreme thermal sensitivity of the [Cu(µ-O)2M]+ complex.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7995

A R T I C L E S
York et al.
or DMSO),17 but those in [Cu2(µ-O)2]2+ cores are not typically
reactive with strong protic acids like HBF4‚Et2O and instead
typically act as electrophiles.2 Consistent with these previous
observations, addition of the weak acid [NH4][PF6] to (LMe2
-
Cu)2(µ-O)2 at -80 °C also resulted in no change in its UV-
vis spectral features. In contrast, the similar reaction with
LMe2Cu(µ-O)2Pt(PPh3)2 resulted in instantaneous bleaching of
the solution color and disappearance of the absorption feature
at 455 nm in the UV-vis spectrum. While efforts to isolate
and thoroughly characterize the reaction product(s) upon warm-
ing have not been successful, high-resolution ESI-MS analysis
of the crude reaction residue revealed a major peak at m/z )
1121.2883 with the appropriate isotope pattern for the doubly
protonated and one-electron reduced product [LMe2Cu(µ-OH)2Pt-
(PPh3)2]+ (predicted m/z ) 1121.2838; Figure S9). Thus, the
oxo groups in LMe2Cu(µ-O)2Pt(PPh3)2 appear to act as bases
similar to [(PPh3)2Pt)]2(µ-O)2.
3.3.2. CO2. Bubbling of CO2 through a -80 °C solution of
LMe2Cu(µ-O)2Pt(PPh3)2 resulted in the instantaneous formation
of a deep purple solution having a strong absorption feature at
530 nm (ꢀ ∼5000 M-1 cm-1; Figure S10), which decayed
rapidly to give a light brown solution. Removal of the solvent
followed by washing with benzene resulted in the isolation of
a tan solid (58% yield), the identity of which was confirmed as
the known carbonato complex (PPh3)2Pt(CO3) on the basis of
Figure 3. Comparative summary of the reactivity of homo- and hetero-
bimetallic complexes with exogenous substrates. See the text for details of
the reaction conditions, yields, and characterization results.
with air and/or water) were observed. The complementary result
was obtained when (PPh3)2Pd16O2 and (P(p-tolyl)3)2Pd18O2 were
used. The data clearly show that O-atom transfer occurs via an
intramolecular pathway and rule out mechanisms involving
dissociation/association of phosphine ligands.
1
FT-IR (νCdO ) 1680 cm-1), H NMR, 31P NMR (δ ) 7.15
ppm, JPtP ) 3697 Hz), and high-resolution ESI-MS data (m/z
) 780.1412 [M + 1]+).54 A similar reaction was reported for
[(PPh3)2Pt)]2(µ-O)2,17a consistent with analogous nucleophilic
attack by the bridging oxo units on the carbon of CO2. No
reaction was observed upon treatment of (LMe2Cu)2(µ-O)2 with
CO2 at -80 °C, and CO2 fixation has not been seen for other
bis(µ-oxo)dicopper systems. Thus, the oxo groups in the
heterobimetallic complex can act as nucleophiles, reactivity that
is similar to that of the diplatinum analogue but is in contrast
to that typical of dicopper species.
Monitoring the progress of the decomposition reactions of
the various heterobimetallic bis(µ-oxo) complexes by UV-vis
spectroscopy revealed irreproducible and/or complicated kinetics
that were not amenable to analysis. Despite these difficulties
encountered in obtaining and quantitatively interpreting kinetic
data, some qualitative observations point to heterometal and
ligand effects on the stability of the heterobimetallic [Cu(µ-
O)2M]+ cores. For instance, in general, the species with M )
Pt are significantly more stable than those with M ) Pd, as
expected on the basis of their relative metal-ligand bond
strengths50 and the kinetic behavior typical for their complexes.51
This stability difference is illustrated by observation of an
essentially complete decay of [(Me4pda)Cu(µ-O)2Pd(PPh3)2]+
in less than 10 min at -80 °C versus several hours under
identical conditions for the Pt analogue. The species with L )
Me4chd are more stable than those with L ) Me4pda, as
indicated by a decomposition time for [(Me4chd)Cu(µ-O)2Pd-
(PPh3)2]+ of ∼1 d at -80 °C and no visible signs of
decomposition of the Pt analogue after several days under the
same conditions.
3.3.3. 2,4-Di-tert-butylphenol, 2,4-di-tert-butylphenolate,
and 9,10-dihydroanthracene. Most bis(µ-oxo)dicopper com-
plexes rapidly react at low temperature (-80 °C) with phenols
to give products resulting from H-atom abstraction (e.g., to yield
a bis(µ-hydroxo)dicopper(II) complex and the coupled diphenol,
often in quantitative yields).2 However, at -80 °C no change
in the UV-vis spectrum of (LMe2Cu)2(µ-O)2 was observed upon
addition of 10 equiv of 2,4-di-tert-butylphenol, even after several
hours. After allowing the mixture to stir for 24 h at -80 °C
1
followed by warming and analysis by GC/MS and H NMR
spectroscopy, no coupled phenolic product was seen. When the
phenol was added at -40 °C, gradual decay of the bis(µ-oxo)
UV-vis feature at 422 nm was observed (t1/2 ∼4 h). Analysis
of a solution maintained at -40 °C for 12 h and then warmed
3.3. Reactivity with Exogenous Substrates. In order to
assess the impact of metal substitution on the reactivity of the
bridging oxo ligands, we compared the reactivity of the known
1
52
17
species (LMe2Cu)2(µ-O)2 and [(PPh3)2Pt]2(µ-O)2 with the
heterobimetallic counterpart LMe2Cu(µ-O)2Pt(PPh3)2 (Figure 3).
While comparison to [(PPh3)2Pt]2(µ-O)2 is facilitated by previous
reports of aspects of its reactivity,17a,c only observation of the
decomposition of (LMe2Cu)2(µ-O)2 to (LMe2Cu)2(µ-OH)2 has
been described,53 necessitating parallel studies of its reaction
chemistry.
to room temperature by GC/MS and H NMR spectroscopy
revealed the main oxidation product to be the coupled diphenol
(∼60% yield). No reaction was observed with sodium 2,4-di-
tert-butylphenolate or 9,10-dihydroanthracene, which contains
relatively weak C-H bonds. The sluggish reactivity with 2,4-
(51) For example, ligand substitution rates for analogous Pd(II) and Pt(II)
complexes vary by ∼105. See: Rund, J. V. Inorg. Chem. 1974, 13, 738-
740.
(52) Spencer, D. J. E.; Reynolds, A. M.; Holland, P. L.; Jazdzewski, B. A.;
Duboc-Toia, C.; Pape, L. L.; Yokota, S.; Tachi, Y.; Itoh, S.; Tolman, W.
B. Inorg. Chem. 2002, 41, 6307-6321.
(53) Dai, X.; Warren, T. H. Chem. Commun. 2001, 1998-1999.
(54) Scherer, O. J.; Jungmann, H.; Hussong, K. J. Organomet. Chem. 1983,
247, c1-c4.
3.3.1. [NH4][PF6]. The oxo groups in [(PPh3)2Pt)]2(µ-O)2 are
readily protonated even by weak acids such as H2O (pKa of
protonated form approximated as between 18 and 25 in THF
(50) Skinner, H. A.; Connor, J. A. Pure Appl. Chem. 1985, 57, 79-88.
9
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Mixed Cu−Pd and Cu−Pt Bis(µ-oxo) Complexes
A R T I C L E S
Table 2. Key Computed Thermodynamic Properties of 1-3
di-tert-butylphenol is distinct from what has been observed for
other [Cu2(µ-O2)]2+ cores, such as those supported by di- or
triamine ligands.2,55 The decreased rate of H-atom abstraction
could be a result of inhibition of substrate approach by the bulky
â-diketiminate or, perhaps more likely, of decreased oxidizing
power due to its strong electron donating properties. Consistent
with the latter notion, addition of other electron-rich substrates
including PPh3, thioanisole, and 1-decene at -80 °C resulted
in no observed reaction or oxidation products upon warming.
In contrast to what was observed for (LMe2Cu)2(µ-O)2,
addition of 1 or 2 equiv of 2,4-di-tert-butylphenol to a -80 °C
solution of LMe2Cu(µ-O)2Pt(PPh3)2 resulted in instantaneous
bleaching of the solution color to pale yellow-brown and decay
of the 450 nm UV-vis feature. Analysis of the reaction products
upon warming revealed the oxidative coupling of 80% of the
starting phenol with the addition of 1 equiv of phenol and
coupling of 46% of the starting phenol when 2 equiv of phenol
were added (i.e., approximately 1 equiv of phenol coupled in
each case). Similar bleaching of the solution occurred upon
addition of sodium 2,4-di-tert-butylphenolate, with formation
of the oxidatively coupled diphenol product being observed upon
warming, although at a lower yield than that for the phenol (56%
coupled for 1 equiv of phenolate added, 33% coupled for 2 equiv
of phenolate added). However, no reaction was observed with
9,10-dihydroanthracene or with the electron-rich substrates
thioanisole and 1-decene.
b
,
c
∆
HBDE
°
,
∆ °,
Gq
a
compound
ESSCE
°
, V
pKa
kcal/mol
kcal/mol
1
2
3
-1.99
-2.19
-1.86
9.5
15.7
14.0
81.7
88.1
86.2
17.4
17.8
18.9
a Refers to conjugate acids 7-9. b 0 K bond dissociation enthalpy for
10-12. c 298 K free energy of activation for H atom abstraction from 1,4-
cyclohexadiene.
roughly 0.1 Å, consistent with Cu being the metal site at which
reduction takes place when a choice is available. Thus, in spite
of the electron-rich character of the anionic â-diketiminate
ligand, it appears to be the Cu site that is preferentially reduced
in the mixed-metal complexes with Pd(II) and Pt(II), consistent
with the rarity of oxidation state +I for these metals in
complexes lacking a metal-metal bond.56
Absolute one-electron reduction potentials (E° ) in CH2Cl2
abs
solution were calculated from the appropriate thermodynamic
cycle,57 adding differential solvation effects to the gas-phase
electron affinity including thermal contributions.58,59 Continuum
solvation free energies in CH2Cl2 for the different species were
calculated using the PCM approach and are included in the
Supporting Information. Absolute reduction potentials were then
converted to reduction potentials relative to the standard calomel
electrode (SSCE) by the following procedure. The standard
reduction potential for [CuIII(H33m)]2+ (where H33m is the
macrocyclic ligand 3,7,11-triaza-bicyclo[11.3.1]heptadeca-1(16),-
13(17),14-triene) has been reported, and its III/II redox couple
is well behaved.60 Following the above computational protocol,
the absolute reduction potential of this compound was computed
(as noted in the Computational Methods section, the choice of
the B98 functional was motivated in part by its good perfor-
mance in reproducing the experimental structure of this com-
pound), and the difference between the absolute value and that
relative to the SSCE was used as a constant to place all other
computed values on the SSCE scale. Table 2 contains these
Together, these results demonstrate that, unlike (LMe2Cu)2-
(µ-O)2 and notwithstanding the basic character of its bridging
oxo ligands, the [M(µ-O)2Cu]+ core of LMe2Cu(µ-O)2Pt(PPh3)2
is competent at one-electron oxidations and/or H-atom abstrac-
tions. Considering that (a) LMe2Cu(µ-O)2Pt(PPh3)2 is protonated
by NH4 (vide supra), (b) the oxo groups in LMe2Cu(µ-O)2Pt-
+
(PPh3)2 are nucleophilic, as shown by the reaction with CO2 to
yield (PPh3)2Pt(CO3), and (c) phenols are weakly acidic similar
to the case of NH4+, we believe it is likely that the reaction of
2,4-di-tert-butylphenol occurs via initial proton transfer, rather
than H-atom transfer. In this scenario, oxidation to give the
coupled diphenol involves subsequent reaction of the protonated
complex, which would be expected on the basis of charge
considerations alone to be a stronger oxidant than the bis(µ-
oxo) precursor.
3.4. Theoretical Characterization of Structures and Re-
activities. To better understand those factors affecting the
reactivity observed for the mixed-metal oxo species, a series of
calculations were undertaken on the computational models
introduced above. In particular, reduction potentials, proton
affinities, and transition-state structures for prototypical H-atom
transfer reactions were examined.
3.4.1. Reduction Potentials. The potential for complexes 1-3
to accept an electron and form the corresponding anionic
complexes 4-6 was investigated in the gas phase and also in
solution. Cartesian coordinates for the reduced structures are
provided in the Supporting Information, and the most relevant
geometrical parameters are displayed in Figure 2 where they
may be conveniently compared to those of their oxidized
counterparts 1-3. In each case, corresponding structures are
similar in the sense that both the [M(µ-O)2Cu]+ core and the
â-diketiminate ligand continue to lie in the same plane. The
most notable change is the elongation of all Cu-O bonds by
calculated E°
values for the three redox couples involving
SSCE
complexes 1-3 and their corresponding reduced species 4-6,
together with additional thermodynamic data discussed below.
Table 2 indicates that the reduction potentials for the Pd and
Pt complexes are within 200 mV on either side of that for 1,
suggesting that the redox active site is indeed Cu (as inferred
above on the basis of geometric changes, and as further
supported by population analysis which is not discussed here).
The distinctly negative reduction potentials are consistent with
the strongly electron-donating nature of the diketiminate ligand
supporting Cu.
3.4.2. Proton Affinities. The pKa values for the conjugate
acid forms of 1-3 (i.e., 7-9) were computed in the gas phase
and also in a hypothetical aqueous solution modeling water as
a continuum solvent. Cartesian coordinates for the protonated
structures (complexes 7-9, respectively) are provided in the
(56) Cotton, F. A.; Wilkinson, G.; Murillo, C. A.; Bochmann, M. AdVanced
Inorganic Chemistry, 6th ed.; John Wiley & Sons: New York, 1999; pp
1063-1084.
(57) Gherman, B. F.; Tolman, W. B.; Cramer, C. J. J. Comput. Chem. 2006,
27, 1950-1961.
(58) Lewis, A.; Bumpus, J. A.; Truhlar, D. G.; Cramer, C. J. J. Chem. Educ.
2004, 81, 596.
(59) Cramer C. J. Essentials of Computational Chemistry. Theories and Models,
2nd ed.; John Wiley & Sons: Chichister, U.K., 2004; pp 379-381.
(60) Xifra, R.; Ribas, X.; Llobet, A.; Poater, A.; Duran, M.; Sola´, M.; Stack, T.
D. P.; Benet-Buchholz, J.; Donnadieu, B.; Parella, T. Chem.sEur. J. 2005,
11, 5146-5156.
(55) Mahadevan, V.; DuBois, J. L.; Hedman, B.; Hodgson, K. O.; Stack, T. D.
P. J. Am. Chem. Soc. 1999, 121, 5583-5584.
9
J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007 7997

A R T I C L E S
York et al.
Figure 4. Key bond lengths (Å) for structures 7-9 (roman text) optimized at the B98 level (see Figure 2 for legend). The same bond lengths for the
one-electron reduced species 10-12 (italic text) are also provided.
Supporting Information, while the most relevant geometrical
parameters are displayed in Figure 4. In all cases protonation
produces a slight bending distortion of the Cu2O2 and MCuO2
cores and also distorts the planarity of the â-diketiminate ligand.
The protonation also increases the M-N and M-OH bond
distances on the side of the protonated oxygen atom, and the
coordination geometry about Cu is distorted from square planar
to a D2d-like flattened tetrahedral arrangement. Lo¨wdin partial
atomic charges indicate that the positive charge of the proton
is distributed over both metal centers and that a significant
portion is delocalized onto the â-diketiminate ligand.
significantly reduced reactivity of (LMe2Cu)2(µ-O)2 with 2,4-
di-tert-butylphenol, compared to that of LMe2Cu(µ-O)2Pt(PPh3)2.
The other data in Table 2 suggest that the dominant cause of
the observed reactivity difference between the homometallic and
heterobimetallic cores is the higher proton affinity of the latter.
The Pd- and Pt-containing species have similar ∆H°BDE values,
consistent with their similar net gas-phase electron and proton
affinities.
3.4.4. Transition-State Structures for C-H Bond Activa-
tion. In order to evaluate the relative abilities of complexes 1-3
to abstract a H atom from an external substrate and to understand
the mechanistic character of this process, transition-state (TS)
structures were calculated for the reactions of 1-3 with 1,4-
cyclohexadiene (C6H8; CHD). The gas-phase Cartesian coor-
dinates for these transition-state (TS) structures (species 13-
15) are provided in the Supporting Information, and the most
relevant geometric parameters are included in Figure 5. The
structure of the diamond core in each case is slightly distorted
and is intermediate between the initial and final reduced
complexes. In the TS structures derived from the mixed metal
complexes there is a stereochemical distinction with respect to
the orientation of the reacting CHD molecule, which can
partially eclipse either the Cu-diketiminate or the Pd or Pt
fragments. The energetically preferred structures are the ones
shown in Figure 4, with the CHD moiety directed toward the
diketiminate; the alternative structures rotated approximately
180° about the C - - H - - O axis are 0.6 and 3.8 kcal/mol higher
in free energy for 14 and 15, respectively. The most interesting
feature of the TS structures is the H atom in flight between an
oxo bridging atom and an sp3 carbon atom of CHD. The
geometries of the copper-containing portions of 13-15 are
remarkably similar to one another, and differences between Pd
and Pt are also small. With respect to the H atom in flight, 13
has a slightly later transition state (shorter forming OH bond
and longer breaking CH bond) than is found in either of the
other two structures, consistent with this reaction being the least
exergonic (-14.5 kcal/mol compared to -19.9 and -18.4 for
Pd and Pt, respectively), but the bond length differences are
not very large.
By combining59gas-phase protonation enthalpies with dif-
ferential aqueous solvation free energies computed using the
PCM continuum solvent model (and the experimental solvation
free energy for the proton61), pKa values for 7-9 were computed.
Compound 1 is significantly less basic than the mixed-metal
cases, consistent with the experimental results of the reactions
of (LMe2Cu)2(µ-O)2 and LMe2Cu(µ-O)2Pt(PPh3)2 with [NH4]-
[PF6], as well as the hypothesis that the reaction of the Cu-Pt
complex with 2,4-di-tert-butylphenol proceeds by an initial
protonation event. This reduced basicity is even larger in the
gas phase than in solution: the gas-phase protonation free energy
for 1 is -236.3 kcal/mol compared to about -258 kcal/mol for
the Pd and Pt mixed-metal species.
3.4.3. Hydrogen-Atom Affinities. Having considered indi-
vidually one-electron reduced and protonated species, we next
examined the µ-hydroxo species 10-12 generated by the sum
of these two processes applied to 1-3, respectively. Figure 4
contains key geometrical data for these species, where they may
be compared to those for their oxidized analogues (full
geometries are provided in the Supporting Information). Inspec-
tion of the geometries indicates that M-O and M-N distances
for M ) Cu are very similar to those in the one-electron reduced
species 4-6, while those about M ) Pd or Pt are very similar
to those in the protonated species 7-9. Thus, transfer of an H
atom appears to reduce Cu(III) to Cu(II) and leave the H atom
with protonic character.
Interestingly, although cationic 7 was predicted to have Cs
symmetry, the reduced species 10 prefers to break symmetry
to a mixed valence Cu(III)/Cu(II) species. This C1 radical is
about 5 kcal/mol lower in energy than a structure forced to
remain in the Cs point group. In every case, then, the Cu-OH
bond to the Cu(II) atom is about 2.08 Å (Figure 3).
The 298 K free energies of activation for H-atom abstraction
from CHD are included in Table 2. There is only a very small
variation (1.5 kcal/mol) over all three systems. The absolute
free energies of activation are rather high, considering the very
Gas-phase O-H bond dissociation enthalpies (∆H°BDE) were
calculated for 10-12 and are listed in Table 2. The smallest
(61) (a) Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen,
M. H.; Earhart, A. D.; Coe, J. V.; Tuttle, T. R. J. Phys. Chem. A 1998,
102, 7787-7794. (b) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. J. Phys.
Chem. B 2006, 110, 16066-16081.
∆H° value is associated with the bis-diketiminate homodi-
BDE
copper species, consistent with the experimental finding of the
9
7998 J. AM. CHEM. SOC. VOL. 129, NO. 25, 2007

Mixed Cu−Pd and Cu−Pt Bis(µ-oxo) Complexes
A R T I C L E S
Figure 5. Key bond lengths (Å) for structures 13-15 optimized at the B98 level (see Figure 2 for legend).
4. Conclusions
weak nature of the CH bond in CHD (298 K BDE of about 77
kcal/mol)62 and the similarly weak OH bonds in 7-9. For
stronger CH bonds, e.g., H-CH2Ph (89.8 kcal/mol), H-CH2-
CN (91.0 kcal/mol), H-C6H11 (95.5 kcal/mol), H-CH3 (104.9
kcal/mol), or H-C6H5 (113.1 kcal/mol),63 one would expect
free energies of activation to be substantially increased.
A careful examination of the TS structures in Figure 4,
particularly their comparison to structures 7-9 and 10-12 in
Figure 3, suggests that the H-atom abstraction process is
precisely that, i.e., a process involving roughly simultaneous
electron and proton transfer. Thus, the TS geometries in Figure
4 have key bond lengths, particularly about the Cu center that
is being reduced, roughly intermediate between protonated and
hydrogenated derivatives of 1-3. The case is slightly less clear
for 1, because desymmetrization of the two copper centers is
not very advanced in the TS structure 13 but is well represented
in the structures for 14 and 15.
Karlin et al.64 studied the oxidation of several substrates by
a peroxo Cu2O2 complex and found that the mechanism with
easily oxidized substrates was a two-step electron-transfer (ET)/
proton-transfer (PT) process, while with less easily oxidized
substrates a concerted ETPT mechanism was observed. The
importance of the redox potential for experimental H-atom
abstraction by Fe65 and Mn66,67 complexes has also been
emphasized. Thus, highly mismatched electron affinities lead
to a two-step ET/PT mechanism, highly mismatched proton
affinities lead to a two-step PT/ET mechanism, and in the
absence of large mismatches a concerted mechanism dominates.
The C-H activation cases studied theoretically here appear to
fall into the final category, which is consistent with prior work
on C-H activation by bis(µ-oxo)dicopper species.12 It is worth
noting that the mechanism of phenol oxidation by the [M(µ-
O)2Cu]+ cores appears to be rather different from those
presumed for the dicopper systems, insofar as proton transfer
likely precedes oxidative coupling.
A new class of heterobimetallic CuPd and CuPt complexes
has been prepared by the reaction of (PPh3)2MO2 (M ) Pd, Pt)
with Cu(I) precursors, and the products have been identified as
bis(µ-oxo) species on the basis of spectroscopic data and DFT
calculations. The thermally unstable complexes decompose to
yield OPPh3, and in one instance this was shown to be an
intramolecular process from the results of crossover experiments
using doubly labeled reagents. Studies of the reactivity of LMe2
-
Cu(µ-O)2Pt(PPh3)2 with CO2 and [NH4][PF6] gave results
consistent with basic, nucleophilic oxo groups in the complex,
in distinct contrast to what is typical for bis(µ-oxo)dicopper
compounds2 but similar to what is known for the [Pt2(µ-O)2]
core.6 In addition, oxidative coupling of 2,4-di-tert-butylphenol
or 2,4-di-tert-butylphenolate by LMe2Cu(µ-O)2Pt(PPh3)2 dem-
onstrated that the mixed-metal core is also a competent one-
electron oxidant, with a significantly increased reactivity
compared to the homodicopper analogue (LMe2Cu)2(µ-O)2.
Assessment of the electron affinities, basicities, and H-atom
transfer kinetics and thermodynamics of Cu2 and CuM bis(µ-
oxo) species by theoretical methods provided insight into the
bases for the observed reactivity differences. Notably, reduction
occurs at the Cu(III) ion in the heterobimetallic systems, which
are more basic than the Cu2 counterpart and form stronger O-H
bonds. Despite their high proton affinities, all of the systems
exhibit low electron affinities which dominate their predicted
sluggish C-H bond activation reactivity. In sum, the results
reported herein, in conjunction with others reported previously,
show that replacement of a Cu(III) ion in a [Cu2(µ-O)2]2+ core
with a different metal ion has important electronic structural
and reactivity consequences. These results thus motivate further
studies of heterobimetallic systems with different metal com-
binations.
(62) McMillen, D. F.; Golden, D. M. Annu. ReV. Phys. Chem. 1982, 33, 493-
532.
Acknowledgment. We thank the NIH (GM47365 to W.B.T.),
the NSF (CHE-0610183 to C.J.C.), the MEC (CSD2006-0003
to A.L.), and the University of Minnesota (Doctoral Dissertation
fellowship to J.T.Y.) for financial support of this work.
(63) (a) Lide, D. R. Handbook of Chemistry and Physics, 85th ed.; CRC Press:
Boca Raton, FL, 2004-2005; pp 9-65 to 9-68. (b) Blanksby, S. J.; Ellison,
G. B. Acc. Chem. Res. 2003, 36, 255-263.
(64) Shearer, J.; Zhang, C. X.; Zakharov, L. N.; Rheingold, A. L.; Karlin, K.
D. J. Am. Chem. Soc. 2005, 127, 5469-5483.
(65) (a) Jonas, R. T.; Stack, T. D. P. Am. Chem. Soc. 1997, 119, 8566. (b)
Goldsmith, C. R.; Jonas, R. T.; Stack, T. D. P. J. Am. Chem. Soc. 2002,
124, 83.
Supporting Information Available: Spectroscopic data and
results of calculations. Also, full citation for ref 38. This material
is available free of charge via the Internet at http://pubs.acs.org.
(66) (a) Baldwin, M. J.; Pecoraro, V. L. J. Am. Chem. Soc. 1996, 118, 11325.
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