Oxidative dissolution of copper and zinc metal in carbon dioxide with tert-butyl peracetate and a β-diketone chelating agent

Article abstract of DOI:10.1021/ic049765o

A series of β-diketone ligands, R¹COCH₂COR ² [tmhdH (R¹ = R² = C(CH₃) ₃); tfacH (R¹ = CF₃; R² = CH ₃); hfacH (R¹ = R² = CF₃)], in combination with tert-butyl peracetate (t-BuPA), have been investigated as etchant solutions for dissolution of copper metal into carbon dioxide solvent. Copper removal in CO₂ increases in the order tfacH < tmhdH < hfacH. A study of the reactions of the hfacH/t-BuPA etchant solution with metallic copper and zinc was conducted in three solvents: scCO₂ (supercrical CO₂); hexanes; CD₂Cl₂. The etchant solution/metallic zinc reaction produced a diamagnetic Zn(II) complex, which allowed NMR identification of the t-BuPA decomposition products as tert-butyl alcohol and acetic acid. Gravimetric analysis of the amount of zinc consumed, together with NMR studies, confirmed the 1:1:2 Zn:t-BuPA:hfacH reaction stoichiometry, showing t-BuPA to be an overall two-electron oxidant for Zn(0). The metal-containing products of the copper and zinc reactions were characterized by elemental analysis, IR spectroscopy, and, as appropriate, NMR spectroscopy and single-crystal X-ray diffraction [trans-M(hfac) ₂-(H₂O)(CH₃CO₂H) (1, M = Cu; 2, M = Zn)]. On the basis of the experimental results, a working model of the oxidative dissolution reaction is proposed, which delineates the key chemical variables in the etching reaction. These t-BuPA/hfacH etchant solutions may find application in a CO₂-based chemical mechanical planarization (CMP) process.

Full text of DOI:10.1021/ic049765o

Inorg. Chem. 2005, 44, 316−324
Oxidative Dissolution of Copper and Zinc Metal in Carbon Dioxide with
tert-Butyl Peracetate and a -Diketone Chelating Agent
â
Pamela M. Visintin,† Carol A. Bessel,*,‡ Peter S. White,† Cynthia K. Schauer,*,† and
Joseph M. DeSimone*,†,§
Department of Chemistry, UniVersity of North Carolina at Chapel Hill,
Chapel Hill, North Carolina 27599-3290, Department of Chemical Engineering,
North Carolina State UniVersity, Raleigh, North Carolina 27695-7565, and Department of
Chemistry, VillanoVa UniVersity, 800 Lancaster AVenue, VillanoVa, PennsylVania 19085
Received February 22, 2004
A series of
(R1 R2
â
-diketone ligands, R1COCH2COR2 [tmhdH (R1
) ) ) ) CH3); hfacH
R2 C(CH3)3); tfacH (R1 CF3; R2
)
) CF3)], in combination with tert-butyl peracetate (t-BuPA), have been investigated as etchant solutions
for dissolution of copper metal into carbon dioxide solvent. Copper removal in CO2 increases in the order tfacH <
tmhdH < hfacH. A study of the reactions of the hfacH/t-BuPA etchant solution with metallic copper and zinc was
conducted in three solvents: scCO2 (supercrical CO2); hexanes; CD2Cl2. The etchant solution/metallic zinc reaction
produced a diamagnetic Zn(II) complex, which allowed NMR identification of the t-BuPA decomposition products
as tert-butyl alcohol and acetic acid. Gravimetric analysis of the amount of zinc consumed, together with NMR
studies, confirmed the 1:1:2 Zn:t-BuPA:hfacH reaction stoichiometry, showing t-BuPA to be an overall two-electron
oxidant for Zn(0). The metal-containing products of the copper and zinc reactions were characterized by elemental
analysis, IR spectroscopy, and, as appropriate, NMR spectroscopy and single-crystal X-ray diffraction [trans-M(hfac)2-
(H2O)(CH3CO2H) (1, M
) Cu; 2, M ) Zn)]. On the basis of the experimental results, a working model of the
oxidative dissolution reaction is proposed, which delineates the key chemical variables in the etching reaction.
These t-BuPA/hfacH etchant solutions may find application in a CO2-based chemical mechanical planarization
(CMP) process.
Introduction
environmental difficulties. As feature sizes approach line
widths below 150 nm and aspect ratios greater than 3, the
high surface tension of water makes it difficult to completely
remove aqueous solvents from the porous low-k interlayer
dielectric materials, leading to higher than desired dielectric
constants. Furthermore, porous materials do not have suf-
ficient mechanical strength to withstand the destructive forces
associated with high surface tension liquids such as water,4
resulting in image collapse of the structures.
Research focused on enabling a liquid and/or supercritical
(sc) CO2 technology platform for microelectronics applica-
tions is responding to needs for solvent-compatible wafer
processing and more environmentally benign replacements
for organic and aqueous solvents.5 Condensed CO2 has
desirable characteristics for CMP processing, including high
surface wetting, extremely low viscosity, low surface tension,
and a low dielectric constant. Additionally, condensed CO2
The fabrication of copper interconnect devices for logic
circuits in the microelectronics industry requires a funda-
mental understanding of copper chemical mechanical pla-
narization (CMP) technologies.1-3 CMP is a process which
uses a slurry containing chemicals and abrasive particles to
oxidize, chelate, and remove the copper metal overburden
while the polishing pad mechanically removes the surface
materials until a flat surface is achieved. Current CMP
processes use water as the solvent, leading to technical and
* Authors to whom correspondence should be addressed. E-mail:
carol.bessel@villanova.edu (C.A.B.); schauer@unc.edu (C.K.S.);
desimone@unc.edu (J.M.D.).
† University of North Carolina at Chapel Hill.
‡ Villanova University.
§ North Carolina State University.
(1) Steigerwald, J. M.; Murarka, S. P.; Gutmann, R. J. Chemical
Mechanical Planarization of Microelectronic Materials; John Wiley
& Sons: New York, 1997.
(2) Singh, R. K.; Bajaj, R. MRS Bull. 2002, 27, 743.
(3) Hanazono, M.; Amanokura, J.; Kamigata, Y. MRS Bull. 2002, 27, 772.
(4) DeSimone, J. M. Science 2002, 297, 799.
(5) Weibel, G. L.; Ober, C. K. Microelectron. Eng. 2003, 65, 145.
316 Inorganic Chemistry, Vol. 44, No. 2, 2005
10.1021/ic049765o CCC: $30.25
© 2005 American Chemical Society
Published on Web 12/16/2004

OxidatiWe Dissolution of Cu and Zn Metal in CO2
(tmhdH), tert-butyl peracetate (t-BuPA, 75 wt % in aliphatic
hydrocarbons), Cu(hfac)2‚H2O, and Cu2(O2CCH3)4‚2H2O were
purchased from Aldrich and were used as received. Unless otherwise
noted, all other solvents were purchased in reagent grade from
commercial suppliers and used without further purification. Carbon
dioxide (SFC/SFE grade) was supplied by Air Products. Caution!
Although we haVe experienced no difficulty with t-BuPA, this
solution should be treated as potentially explosiVe and handled with
care.
Characterization Methods. Melting points were determined
with an Electrothermal melting point apparatus. NMR spectra were
recorded at room temperature on a Bruker Avance 400 MHz
spectrometer. Chemical shifts are reported in parts per million (δ).
1H and 13C{1H} NMR chemical shifts are referenced to the residual
1H and 13C{1H} signals of the deuterated solvents. 19F NMR
resonances are reported using C6F6 (δ ) -164.9 from CFCl3) as
the internal reference. Infrared spectra were recorded using KBr
pellets on a Bio-Rad FTS-7 spectrometer, in the 4000-500 cm-1
range. UV-visible spectroscopy was performed using a Perkin-
Elmer Lambda 40 spectrophotometer in the range 190-860 nm.
Elemental analyses were performed by Atlantic Microlab, Inc.
(Norcross, GA).
Dissolution of Copper Metal in scCO2. A 10-mL high-pressure
316 stainless steel sapphire-windowed view cell was employed for
each reaction. The t-BuPA (0.07 mL, 3.5 × 10-4 mol), a â-diketone
chelating agent (6.9 × 10-4 mol), and a copper coupon (0.022 g,
3.5 × 10-4 mol, contained in a glass holder) were placed in the
cell. The cell was charged with CO2 (214 ( 3 bar, 40 ( 2 °C), and
the mixture was stirred for 20 h. The cell was then vented, and the
coupon was rinsed with hexanes, dried, and weighed. The number
of monolayers (m) of copper removed was calculated from the
weight loss, area of the coupon × 2 (for a two-sided coupon), and
the atomic radius of copper (r).13 The average dissolution rate was
then calculated using the total reaction time (t) (eq 2).
is a very attractive solvent choice from an environmental
perspective because it is readily available, nontoxic, non-
flammable, and has the potential to be recycled. Recently,
we reported that copper metal can be oxidized and chelated
in CO2 media using ethyl peroxydicarbonate (EPDC) as an
oxidant with a â-diketone chelating agent (eq 1).6 This
observation, which builds upon extensive studies of vapor
phase etching of copper metal and oxidized copper species
with â-diketone chelating agents,7-11 has opened the pos-
sibility of a CO2-based CMP technology for copper. The CO2
solution reaction medium and the soluble oxidant allow these
etching reactions to be conducted at low temperatures relative
to the vapor phase reactions.
Cu(0) EPDC, â-diketone8 bis(â-diketonato)copper(II) (1)
The oxidant, EPDC, has several inherent limitations,
including the lack of its availability from commercial sources,
its highly toxic starting material (ethyl chloroformate), and
its very high reactivity. This study examines reaction of a
series of â-diketone ligands, R1COCH2COR2 [tmhdH (R1 )
R2 ) C(CH3)3); tfacH (R1 ) CF3; R2 ) CH3); hfacH (R1 )
R2 ) CF3)], in combination with tert-butyl peracetate (t-
BuPA) as etchant solutions, for application in copper CMP
in scCO2. The oxidant, t-BuPA, was selected for this study
due to its commercial availability, CO2 solubility, efficiency,
safe handling ability, and cost-effectiveness.
The reaction between metallic copper and zinc with the
hfacH/t-BuPA etchant solution is examined in detail to
identify the peroxy ester decomposition products and metal
products of the reaction. The fact that the Zn(II) product is
diamagnetic allows use of NMR spectroscopy to determine
the reaction stoichiometry and to identify the t-BuPA
decomposition products for the zinc reaction. The effect of
solvent on the etching reaction is assessed by comparing
reactions performed in conventional organic solvents (hex-
mr
t
rate )
(2)
Reaction of Metallic Copper with t-BuPA and hfacH in
Hexanes. To a suspension of copper powder (0.095 g, 1.5 × 10-3
mol) in 10 mL of hexanes at 40 °C under nitrogen were added
t-BuPA (0.45 mL, 2.3 × 10-3 mol) and hfacH (0.53 mL, 3.8 ×
10-3 mol). The reaction mixture was stirred for 4.5 d, yielding a
homogeneous dark green solution. The solution was cooled, and
on slow evaporation of the solvent, green crystals of trans-Cu-
(hfac)2(H2O)(CH3CO2H) (1) suitable for X-ray diffraction measure-
ments were formed. The crystalline solid was filtered, washed with
pentane (5 mL), and dried under vacuum (0.69 g, 84% yield based
on copper metal): mp 107-112 °C; IR (KBr) 3568 (m) ν(H2O),
1718 (w) νAcOH(CdO), 1646 (s) ν(C-O), 1617 (m) ν(C-C), 1563
(m) ν(C-O) + δhfac(CH), 1537 (m), 1470 (s), 1357 (m) ν(C-C)
+ ν(CF3), 1258 (vs) ν(CF3), 1223 (s) νAcOH(C-O) + δAcOH(OH),
1208 (sh), 1150 (vs) âhfac(CH), 1109 (m), 809(s) γhfac(CH) + ν-
(C-CF3), 772 (w) and 748 (m) ν(C-CF3), 681 (s), 599 (s), 532
(m) cm-1. Anal. Calcd for C12H8O7F12Cu: C, 25.93; H, 1.45; F,
41.03. Found: C, 25.42; H, 1.42; F, 41.61. The difference between
the calculated and observed elemental analysis is likely due to an
impurity of Cu(hfac)2(H2O). Attempts to separate these complexes
by recrystallization were unsuccessful.
anes and CD2Cl2) with those conducted in scCO2.12
A
working model of the oxidative dissolution reaction is
proposed, which delineates the key chemical variables in the
etching reaction.
Experimental Section
Materials and General Procedures. Copper powder (Alfa
Aesar, APS 0.2-0.3 µm, 99.9%) was stored and handled in an
argon-filled Vacuum Atmospheres glovebox. Copper coupons
(99.999%, 0.1 mm thick), zinc coupons (99.999%, 0.1 mm
thick), 1,1,1,5,5,5-hexafluoroacetylacetone (hfacH), 1,1,1-trifluoro-
2,4-pentanedione (tfacH), 2,2,6,6-tetramethyl-3,5-heptanedione
(6) Bessel, C. A.; Denison, G. M.; DeSimone, J. M.; DeYoung, J.; Gross,
S.; Schauer, C. K.; Visintin, P. M. J. Am. Chem. Soc. 2003, 125, 4980.
(7) Sekiguchi, A.; Kobayashi, A.; Koide, T.; Okada, O.; Hosokawa, N.
Jpn. J. Appl. Phys., Part 1 2000, 39, 6478.
(8) Farkas, J.; Chi, K. M.; Hampden-Smith, M. J.; Kodas, T. T.; Dubois,
L. H. Mater. Sci. Eng., B 1993, 17, 93.
(9) Jain, A.; Kodas, T. T.; Hampden-Smith, M. J. Thin Solid Films 1995,
269, 51.
(10) George, M. A.; Hess, D. W.; Beck, S. E.; Ivankovits, J. C.; Bohling,
D. A.; Lane, A. P. J. Electrochem. Soc. 1995.
Reaction of Metallic Zinc with t-BuPA and hfacH in Hexanes.
A zinc coupon (0.098 g, 1.5 × 10-3 mol) was added to t-BuPA
(11) Steger, R.; Masel, R. Thin Solid Films 1999, 342, 221.
(12) Although this reaction does occur in liquid CO2, the set of conditions
used in this study are in the scCO2 regime (see ref 6).
(13) Chemistry: WebElements Periodic Table: Professional Edition:
Copper: radii; http://www.webelements.com/webelements/elements/
text/Cu/radii.html (accessed Oct 2003).
Inorganic Chemistry, Vol. 44, No. 2, 2005 317

Visintin et al.
(0.45 mL, 2.3 × 10-3 mol) and hfacH (0.53 mL, 3.8 × 10-3 mol)
in 10 mL of hexanes at 40 °C under nitrogen. The coupon began
to dissolve within 30 min and was completely dissolved within 2
h. The pale yellow solution was stirred for 24 h and cooled. Upon
slow evaporation of the solvent, white crystals of trans-Zn(hfac)2-
(H2O)(CH3CO2H) (2) suitable for X-ray diffraction measurements
were formed. The crystals were isolated by filtration, washed with
hexanes (3 × 5 mL) and pentane (3 × 5 mL), and dried in vacuo
(0.40 g, 50% yield based on zinc metal). Elemental analysis was
fit to a 1:1 ratio of complexes trans-Zn(hfac)2(H2O)(CH3CO2H)
Table 1. Crystallographic Data for 1 and 2
1
2
formula
C12H8F12O7Cu
555.72
green
monoclinic
P21/n (No. 14)
0.30 × 0.25 × 0.15
13.8024(6)
20.8587(8)
14.2894(5)
90
114.76
90
3735.8(3)
8
1.976
C12H8F12O7Zn
557.57
mol wt (g/mol)
cryst color
cryst system
space group
cryst dimens (mm)
a (Å)
white
orthorhombic
Cmca (No. 64)
0.35 × 0.10 × 0.10
21.4274(24)
7.4996(8)
23.8542(25)
90
b (Å)
c (Å)
1
R (deg)
(2) and Zn(hfac)2‚2H2O. H NMR spectroscopy further supports
â (deg)
90
90
this 1:1 ratio. Attempts to separate these complexes by recrystal-
lization were unsuccessful: mp 128-133 °C; IR (KBr) 3482 (br)
ν(H2O), 1733 (w) νAcOH(CdO), 1646 (vs) ν(C-O), 1615 (m) ν-
(C- C), 1566 (s) ν(C-O) + δhfac(CH), 1540 (s), 1489 (vs), 1459
γ (deg)
V (Å3)
3833.3(7)
8
Z
F
calc (g/cm3)
1.922
1.43
35 112
1752
0.085
0.128
3.4731
µ (mm-1
)
1.321
26 246
9035
0.1032
0.2024
1.044
(s), 1349 (m) ν(C-C) + ν(CF3), 1262 (vs) ν(CF3), 1225 (vs) νAcOH
-
rflcns collcd
rflcns (unique)
R (all data)a
(C-O) + δAcOH(OH), 1202 (sh), 1148 (vs) âhfac(CH), 1097 (m),
808 (vs) γhfac(CH) + ν(C-CF3), 773 (m) and 745 (s) ν(C-CF3),
670 (vs), 591 (vs), 527 (s) cm-1; 1H NMR (400 MHz, acetone-d6)
δ 1.95 (s, 3H, CH3), 5.93 (s, 4H, CH); 13C{1H} NMR (400 MHz,
acetone-d6) δ 20.6 (s, CH3), 88.6 (s, CH), 118.5 (q, J ) 285.9 Hz,
CF3), 173.0 (s, C)O), 178.8 (q, J ) 33.7 Hz, CO); 19F NMR (400
MHz, acetone-d6, δ vs C6F6) δ -77.7 (s, CF3). Anal. Calcd for
C12H8O7F12Zn + C10H6O6F12Zn (1:1): C, 24.62; H, 1.32; F, 42.49.
Found: C, 24.26; H, 1.31; F, 42.32.
R
w (all data)b
GOFc
a R ) Σ||Fo| - |Fc||/Σ|Fo|. b Rw ) [Σ(w(|Fo - Fc|)2)/Σ(w|Fo|)2)]1/2
.
c GOF
) [Σ(w(|Fo - Fc|)2)/(no. of reflections - no. of parameters)]1/2
.
was then removed, and the product, Cu(hfac)2‚H2O, was dried under
vacuum: IR (KBr) 3564 (br) ν(H2O), 1643 (s) ν(C-O), 1611 (m)
ν(C-C), 1562 (s) ν(C-O) + δ(CH), 1534 (m), 1474 (s), 1359
(w) ν(C-C) + ν(CF3), 1256 (vs) ν(CF3), 1218 (s), 1147 (vs) â-
(CH), 1109 (m), 810(s) γ(CH) + ν(C-CF3), 750 (m) ν(C-CF3),
679 (s), 597 (s), 526 (m).
The above reaction was also conducted in scCO2: Cu2(O2-
CCH3)4‚2H2O (3) (1.2 × 10-4 mol), contained in a glass holder,
and hfacH (2.9 × 10-4 mol) were placed in a sapphire-windowed
2.5-mL high-pressure view cell. The cell was then charged with
scCO2 (214 ( 3 bar, 40 ( 2 °C). A green solution, indicative of
Cu(hfac)2‚H2O, resulted immediately. The reaction, which was
monitored by UV-visible spectroscopy, went to completion in less
than 1 min: UV-visible λmax 675 nm.
X-ray Structure Determinations. X-ray crystallographic data
for 1 and 2 were collected at -100 °C on a Bruker SMART 1K
CCD diffractometer with Mo KR radiation (λ ) 0.710 73 Å), using
the ω-scan mode. The structures were solved by direct methods
and refined by a full-matrix least-squares method. Calculations for
1 were performed using the Bruker SHELXTL program package,14
and computations for 2 were performed using the NRCVAX suite
of programs.15 Analysis of the results of 1 with MISSYM indicated
the presence of noncrystallographic translational symmetry. Close
examination of intensity data from several samples indicated that
the doubled unit cell was indeed the correct one. The pseudosym-
metry gave rise to higher than normal correlation factors in the
final refinement steps. Details of the crystal data and intensity
collections for 1 and 2 are summarized in Table 1, while selected
bond lengths and angles are tabulated in Tables 3 for 1 and 4 for
2. Complete tables of bond lengths and angles are given in Tables
S1-S3 (Supporting Information).
Reaction of Copper with t-BuPA. To a suspension of copper
powder (0.095 g, 1.5 × 10-3 mol) in 10 mL of absolute EtOH at
40 °C under nitrogen was added t-BuPA (0.45 mL, 2.3 × 10-3
mol). A dark blue/turquoise mixture resulted immediately. The
reaction mixture was stirred for approximately 4.5 d, and then the
excess copper solid was filtered off. The solvent was then removed,
and the crude product was washed with hexanes (3 × 5 mL) and
pentane (2 × 5 mL) and then dried under vacuum to afford Cu2(O2-
CCH3)4‚2H2O (3) (0.082 g, 28% yield based on copper metal).
Crystals suitable for X-ray diffraction measurements were grown
by vapor diffusion of toluene into the solution of 3 in absolute
EtOH: mp 234-237 °C (dec); IR (KBr) 3473 (s) ν(H2O), 3371
(s) ν(H2O), 1602 (vs) νas(COO), 1422 (vs) νs(COO), 1355 (m) δs-
(CH3), 1052 (m) F(CH3), 943 (vw) ν(C-CH3), 691 (s) δs(COO),
628 (s) F(COO), 526 (w) F(H2O) cm-1. Anal. Calcd for C8H16O10-
Cu2: C, 24.07; H, 4.03; O, 40.07. Found: C, 24.67; H, 4.31; O,
40.44.
The above procedure was conducted using a copper coupon (96.3
mg) to confirm the stoichiometry of the reaction on the basis of
the moles of copper consumed/mol of dinuclear 3 produced. The
remaining copper coupon was washed with EtOH and hexanes,
dried under vacuum, and weighed. Approximately 41.3 mg (0.650
mmol) of copper was consumed and 129 mg (0.325 mmol) of 3
was produced. The molar ratio of Cu(0):3 indicates a 1:2 (Cu:t-
BuPA) stoichiometry in the EtOH reaction.
The above procedure using copper powder was also conducted
using hexanes as a solvent rather than absolute EtOH. After 24 h
of stirring, no reaction was apparent for either the solution or the
suspended copper solid. The solid was then filtered out and washed
twice with hexanes and finally with EtOH. The EtOH wash resulted
in a blue/turquoise solution. Upon removal of solvent, a trace of
blue solid resulted, which was identified as 3 by IR spectroscopy.
Reaction of Cu2(O2CCH3)4‚2H2O (3) with hfacH in Hexanes
or scCO2. To a suspension of Cu2(O2CCH3)4‚2H2O (3) (0.095 g,
1.5 × 10-3 mol) in 10 mL of hexanes at 40 °C under nitrogen was
added hfacH (0.53 mL, 3.8 × 10-3 mol). A green mixture resulted
immediately. This reaction mixture was stirred for approximately
20 h, which allowed the reaction to go to completion. The solvent
Blank NMR Studies. All experiments were performed at 40 °C
under nitrogen. A copper or zinc coupon (1 equiv) combined with
only t-BuPA (0.1 equiv) in CD2Cl2 resulted in no peroxy ester
decomposition within 24 h as observed by NMR spectroscopy.
(14) SHELXTL PC, version 4.2/360; Bruker Analytical X-ray Instruments,
Inc.: Madison, WI, 1994.
(15) Gabe, E. J.; Le Page, Y.; Charland, J. P.; Lee, F. L.; White, P. S. J.
Appl. Crystallogr. 1989, 22, 384.
318 Inorganic Chemistry, Vol. 44, No. 2, 2005

OxidatiWe Dissolution of Cu and Zn Metal in CO2
Table 2. Reaction of Cu(0) and t-BuPA (or EPDC) with a Series of â-Diketone Ligands
t-BuPA oxidant
ligand
pKa1
Cu(0) removed
(%)
monolayers of
Cu(0) removed
avg dissolution
rate (nm/min)
EPDC oxidant avg
dissolution rate (nm/min)
19,a
ligand
hfacH
tmhdH
tfacH
6.0
15.9
8.8
14
4
1
6.9 × 104
2.0 × 104
5.7 × 103
3.9
1.1
0.3
10.7
3.9
3.3
a pKa values measured in dioxane (75%)-water mixture at 30 °C.
In a second blank experiment, a stirred solution of t-BuPA (1
equiv) and hfacH (3 equiv) in CD2Cl2 (or hexane-d14) was
monitored by 1H NMR spectroscopy. The originally colorless
solution turned bright orange (λmax ) 388 and 446 nm) over time,
suggesting a conjugated system. The hfacH OH proton shifted from
approximately 12.29 ppm (t ) 0 min) to 11.00 ppm (t ) 35 h). 1H
and 19F NMR spectroscopies indicated that the hfacH (98% at t )
0 min) slightly hydrolyzed to 1,1,1,5,5,5-hexafluoro-4,4-dihydroxy-
2-pentanone (2%) and 1,1,1,5,5,5-hexafluoro-2,2,4,4-pentanetetrol
(9%) within 35 h.16 No resonances for peroxy ester decomposition
products were observed.
Copper Metal with t-BuPA and hfacH NMR Studies. A
solution of t-BuPA (0.03 mL, 2 × 10-4 mol) and hfacH (0.07 mL,
5 × 10-4 mol) was stirred in 2 mL of CD2Cl2 (or hexane-d14) at
40 °C under nitrogen. The copper coupon (0.095, 1.5 × 10-3 mol)
was then added to the colorless solution. An aliquot was im-
mediately transferred to the NMR spectrometer, and the 1H NMR
spectrum was acquired at room temperature (t ) 0 min). This
aliquot was immediately returned to the reaction mixture. The
copper reaction was monitored every 5 h for 33.5 h.
Zinc Metal with t-BuPA and hfacH NMR Studies. A zinc
coupon was treated with t-BuPA and hfacH in CD2Cl2 at 40 °C
under nitrogen. The same procedure and amounts of reagents were
used as described above for copper. The zinc reaction was
monitored every 20 min for 280 min. The peroxy ester completely
decomposed after 220 min. This reaction mixture was then stirred
for approximately 3 d, and the white solid was collected by
filtration, washed with hexanes and pentane, and dried under
vacuum. Product 2 and Zn(hfac)2‚2H2O were confirmed by 1H
NMR spectroscopy.
stirred for 19 h. The coupon was thoroughly washed with hexanes,
air-dried, and transferred into a flask containing hfacH (0.05 mL)
and CD2Cl2. This mixture was stirred at room temperature for 1 h.
1
The H NMR spectrum of a solution aliquot showed tert-butyl
alcohol, acetone, and acetic acid in a ratio of 1:2:3. When an
analogous reaction was conducted in a glovebox, acetone was still
observed as a decomposition product.
Results and Discussion
Dissolution of Copper Metal in scCO2. Copper coupons
were treated with the oxidant, t-BuPA, and a â-diketone
chelating agent in scCO2 (40 °C, 214 bar) for 20 h. The
originally colorless t-BuPA/â-diketone solution in scCO2
transforms to a green, homogeneous solution after only a
few minutes. The reaction was allowed to proceed for 20 h
to obtain accurate Cu(0) weight differences.
Table 2 shows the effect of ligand variations on copper
removal. Copper removal increases in the order tfacH <
tmhdH < hfacH. A similar trend in copper removal is
observed when EPDC is used as the oxidant;6 however, the
average dissolution rate is significantly greater in the EPDC
reactions (Table 2). This rate difference is attributed to the
higher reactivity of EPDC (t1/2 ) 28 h at 40 °C) compared
to t-BuPA (t1/2 ) 145 h at 40 °C).17,18 Although hfacH is the
poorest ligand among those investigated, it is the strongest
acid due to the presence of two electron-withdrawing CF3
groups19 and yields the most soluble copper complex in
CO2.20,21 The role that the ligand properties and the solubility
of the resulting metal complexes play in the reaction is
discussed below (working model of the oxidative dissolution
reaction). Due to its high reactivity, hfacH was employed
for all subsequent studies.
Metallic Copper Oxidation and Chelation with t-BuPA
and hfacH: Synthesis and Characterization of Resulting
Copper Complex. In preparative scale reactions, copper
powder is used as the metal source instead of a copper
coupon to reduce the overall reaction time and to allow the
reactions to go to completion. Treatment of copper powder
with t-BuPA and hfacH at 40 °C in hexanes affords a
homogeneous green solution. Green crystals of trans-Cu-
(hfac)2(H2O)(CH3CO2H) (1) (Scheme 1) are obtained from
slow evaporation of the original reaction mixture. The water
An analogous reaction was performed using a 10-mL high-
pressure view cell with scCO2 (40 °C, 214 bar) as the solvent. The
t-BuPA (0.45 mL, 2.3 × 10-3 mol), hfacH (0.53 mL, 3.8 × 10-3
mol), and a zinc coupon (0.098 g, 1.5 × 10-3 mol, contained in a
glass holder) were placed in the cell. The cell was charged with
CO2 (214 ( 3 bar, 40 ( 2 °C). The originally colorless solution
resulted in a pale yellow, homogeneous solution (except for the
zinc coupon) within 5 h. After 6 d of stirring, the cell was vented
1
into a vial containing CD2Cl2, and the H NMR spectrum was
immediately taken, confirming the presence of unreacted t-BuPA,
tert-butyl alcohol, and acetic acid. Analysis of the NMR spectrum
showed 31% (7.0 × 10-4 mol) of t-BuPA and 36% (1.4 × 10-3
mol) of hfacH were consumed. These percentages were calculated
from the integral areas of t-BuPA, tert-butyl alcohol, and hfacH
normalized to the total amount of t-BuPA and hfacH initially added.
The remaining zinc coupon was washed with MeOH and acetone,
dried, and weighed. Approximately 46% of Zn(0) (6.9 × 10-4 mol)
dissolved in the t-BuPA/hfacH etchant solution. These data confirm
the 1:1:2 (Zn:t-BuPA:hfacH) stoichiometry of the reaction.
t-BuPA Decomposition by Zinc. A zinc coupon (0.098 g, 1.5
× 10-3 mol) was treated with t-BuPA (0.45 mL, 2.3 × 10-3 mol)
in 10 mL of hexanes at 40 °C under argon. This mixture was gently
(17) Brandrup, J.; Immergut, E. H. Polymer Handbook, 3rd ed.; Wiley &
Sons: New York, 1989.
(18) The half-life of t-BuPA at 40 °C was determined by extrapolation of
the following data: t1/2 ) 13 h at 100 °C, 2 h at 115 °C, and 0.3 h at
130 °C.
(19) Yokoi, H.; Kishi, T. Chem. Lett. 1973, 7, 749.
(20) Lagalante, A. F.; Hansen, B. N.; Bruno, T. J.; Sievers, R. E. Inorg.
Chem. 1995, 34, 5781.
(16) Aygen, S.; Van Eldik, R. Chem. Ber. 1989, 122, 315.
(21) Lin, Y.; Wai, C. M. Anal. Chem. 1994, 66, 1971.
Inorganic Chemistry, Vol. 44, No. 2, 2005 319

Visintin et al.
Scheme 1. Reaction of M(0) with t-BuPA and HfacH (M ) Cu; M
) Zn; Y ) H2O)
Figure 1. ORTEP diagram of 1 with thermal ellipsoids drawn at the 50%
probability level. Only one of the two crystallographically independent
molecules is shown.
Table 3. Selected Bond Lengths (Å) and Angles (deg) for 1
Cu1-O23
Cu1-O6
Cu1-O10
Cu1-O19
Cu1-O1
Cu1-O2
O2-C3
1.948(3)
1.953(3)
1.957(3)
1.962(3)
2.318(4)
2.382(4)
1.201(6)
1.331(6)
1.488(6)
1.393(6)
1.365(6)
C20-C21
C21-C22
C7-C11
C9-C15
C20-C24
C22-C28
C9-O10
C7-O6
1.390(5)
1.391(6)
1.541(6)
1.548(6)
1.535(6)
1.539(6)
1.268(5)
1.242(5)
1.268(5)
1.254(4)
C3-O4
C3-C5
C22-O23
C20-O19
C7-C8
C8-C9
O10-Cu1-O19
O1-Cu1-O2
O23-Cu1-O6
O6-Cu1-O2
O19-Cu1-O2
O23-Cu1-O1
O23-Cu1-O10
O6-Cu1-O19
175.9(1)
O10-Cu1-O1
O10-Cu1-O2
O6-Cu1-O10
O23-Cu1-O19
O19-Cu1-O1
O6-Cu1-O1
O2-Cu1-O23
C3-O2-Cu1
90.5(1)
91.3(1)
92.5(1)
92.6(1)
93.5(1)
94.4(1)
96.1(1)
133.1(3)
177.1(1)
179.0(1)
83.2(1)
84.7(1)
86.3(1)
86.8(1)
88.0(1)
Figure 2. ORTEP diagram of 2 with thermal ellipsoids drawn at the 50%
probability level.
one of the water hydrogen atoms (H1) are hydrogen-bonded
to four neighboring complexes.
ligand arises either from adventitious water in the solvent
or from protonation of the initial copper oxide layer with
hfacH. The product formulation of 1 was established by a
single-crystal X-ray structure determination.
The solid-state IR spectrum of 1 has the expected bands
for the coordinated aqua and hfac- ligand on the basis of
the data for Cu(hfac)2‚H2O.25 A sharp band is observed at
1718 cm-1, which is assigned to the CdO stretch for the
acetic acid ligand. This band is shifted 63 cm-1 to lower
energy from the ν(CdO) stretch in uncoordinated acetic acid
(1781 cm-1),26-28 indicative of metal coordination.27 The
CH2Cl2 solution IR spectrum of 1 shows two CdO stretches
at 1718 and 1764 cm-1, which are assigned to coordinated
and uncoordinated acetic acid, respectively, indicating that
the acetic acid ligand partially dissociates in CH2Cl2 solution.
Metallic Zinc Oxidation and Chelation with t-BuPA
and hfacH: Synthesis and Characterization of Resulting
Zinc Complexes. The analogous zinc reaction proceeds
much more rapidly than the copper reaction. The zinc coupon
completely dissolved within 2 h after treatment with t-BuPA
and hfacH in hexanes (Scheme 1), resulting in a homoge-
neous pale yellow solution. X-ray-quality white crystals of
trans-Zn(hfac)2(H2O)(CH3CO2H) (2) precipitate upon cooling
the original reaction mixture.
An ORTEP diagram of 1 is depicted in Figure 1. Complex
1 crystallizes with two crystallographically independent
isostructural molecules in the asymmetric unit. Metric data
are presented for one of the molecules. The coordination
environment around Cu1 is pseudooctahedral with the water
and acetic acid ligands occupying trans positions (Table 3).
The metal-ligand bond distances and angles correspond well
with those in the trans-[Cu(hfac)2(H2O)2]‚H2O complex.22
The Cu1-O(hfac-) bond distances average 1.955(6) Å, while
the Cu1-O1(water, 2.318(4) Å) and Cu1-O2(acetic acid,
2.382(4) Å) distances are significantly elongated due to a
Jahn-Teller distortion. The carbonyl group of the acetic acid
ligand is coordinated to copper, as indicated by the bond
distances for the acetic acid moiety (C3-O2 ) 1.201(6) Å
and C3-O4 ) 1.331(6) Å). The CdO distance in 1 is similar
to the reported CdO distances in uncoordinated acetic acid
(1.24(2) Å)23 and in [Ni(CH3CO2H)6][BF4]2 (1.217(5) Å).24
An extensive hydrogen-bonding network is present in the
solid-state structure of 1. The acetic acid hydroxyl H and O
atoms (H4 and O4), the hfac- O atoms (O19 and O6), and
An ORTEP view of 2 is given in Figure 2. The zinc
complex 2 adopts a pseudooctahedral geometry similar to
(25) Morris, M. L.; Moshier, R. W.; Sievers, R. E. Inorg. Chem. 1963, 2,
411.
(22) Maverick, A. W.; Fronczek, F. R.; Maverick, E. F.; Billodeaux, D.
R.; Cygan, Z. T.; Isovitsch, R. A. Inorg. Chem. 2002, 41, 6488.
(23) Jones, R. E.; Templeton, D. H. Acta Crystallogr. 1958, 11, 484.
(24) Cramer, R. E.; Van Doorne, W.; Dubois, R. Inorg. Chem. 1975, 14,
2462.
(26) The Aldrich Library of FT-IR Spectra, 1st ed.; Pouchert, C. J., Ed.;
Aldrich Chemical Co.: Milwaukee, WI, 1985.
(27) Patten, K. O., Jr.; Andrews, L. J. Phys. Chem. 1986, 90, 1073.
(28) Hasan, M. A.; Zaki, M. I.; Pasupulety, L. Appl. Catal., A 2003, 243,
81.
320 Inorganic Chemistry, Vol. 44, No. 2, 2005

OxidatiWe Dissolution of Cu and Zn Metal in CO2
Table 4. Selected Bond Lengths (Å) and Angles (deg) for 2
ligands on the basis of the data for Zn(hfac)2‚2H2O.25 A band
at 1733 cm-1 is observed for ν(CdO) of the coordinated
acetic acid ligand. This stretch is 15 cm-1 higher in energy
than the corresponding stretch for 1.
Zn1-O1
Zn1-O2
Zn1-O13
Zn1-O8
O8-C10
C10-O11
C10-C12
2.06(1)
2.06(1)
2.06(1)
2.34(1)
1.20(1)
1.30(1)
1.51(1)
C3-C14
C15-C14
C3-C4
1.38(1)
1.39(1)
1.52(1)
1.53(1)
1.25(1)
1.25(1)
C15-C16
C3-O2
Spectroscopic Investigation of the Decomposition Prod-
ucts of t-BuPA on Reaction with Metallic Copper or Zinc
and hfacH. The metallic zinc reaction appears to proceed
similarly to the copper reaction on the basis of the structures
of the oxidized metal complexes, 1 and 2. NMR spectroscopy
was used as a tool to monitor the peroxy ester decomposition
reaction. In control reactions of t-BuPA and hfacH in CD2-
Cl2 (or hexane-d14) at 40 °C, no resonances are observed
for peroxy ester decomposition products after 35 h, indicating
that the hfacH alone does not promote t-BuPA decomposi-
tion. Further, in reactions of copper or zinc metal with only
t-BuPA in CD2Cl2 at 40 °C, no resonances are observed for
peroxy ester decomposition products after 24 h, confirming
the requirement of both hfacH and t-BuPA to carry out the
oxidative dissolution reaction.
In the NMR experiments to study the oxidative dissolution
reactions, an excess of metal reagent is employed to enhance
the reaction rate. For the copper reaction in CD2Cl2 at 40
°C, as the reaction proceeds, dissolution of paramagnetic Cu-
(hfac)2 results in a severe broadening and shifting of the
resonances for t-BuPA and the decomposition products,
limiting the utility of NMR spectroscopy. An experiment
conducted in hexanes-d14 shows the same reaction products
resonances, indicating that the reaction products are inde-
pendent of the solvent chosen here.
Time-dependent 1H NMR spectra for the zinc reaction in
CD2Cl2 are more informative (Figure 3). Signals A and B
are assigned to the tert-butyl and methyl hydrogens of
t-BuPA, respectively. After 20 min, product signals C-E
appear in the spectrum, and the reaction is complete at 220
min. The product chemical shifts are assigned to tert-butyl
alcohol (C, HOC(CH3)3; D, HOC(CH3)3,) and acetic acid
(E, CH3). Equimolar amounts of tert-butyl alcohol and acetic
acid are formed, as illustrated by the normalized integrations
(Figure 4). The Zn(hfac)2 product precipitates from solution
as a white solid as the reaction proceeds, and therefore, no
resonances are observed for the zinc product. Since acetic
acid appears in the correct molar ratio in the supernatant,
the solid that precipitates from CD2Cl2 must not have acetic
acid coordinated. The expected resonances are observed in
the 13C NMR spectrum of the reaction mixture (Figure S3),
and assignments are made on the basis of the 1H-13C COSY
NMR spectrum (Figure S4). Only one resonance is observed
in the 19F NMR spectrum of this solution at t g 220 min,
implying that the enolate ion did not decompose.
To confirm that the t-BuPA products are identical for
reactions conducted in scCO2, the reaction of zinc with the
etchant solution was conducted in scCO2. After reacting for
6 days, the remaining zinc coupon was washed and weighed,
and an 1H NMR (CD2Cl2) of the reaction mixture was taken
to evaluate the extent of the reaction on the basis of the
amount of zinc consumed. Signals were observed for
unreacted t-BuPA, its decomposition products (tert-butyl
alcohol and acetic acid), and free hfacH (Figure S5). The
C15-O13
O1-Zn1-O8
168.3(3)
O8-Zn1-O13
O1-Zn1-O2
O1-Zn1-O13
O2-Zn1-O2a
Zn1-O8-C10
92.2(1)
93.1(1)
96.2(1)
96.3(1)
138.4(6)
O2-Zn1-O13
O2-Zn1-O8
169.8(1)
79.2(1)
87.4(1)
87.4(1)
O13-Zn1-O13a
O2-Zn1-O13a
the copper complex 1. A crystallographic mirror plane
containing Zn1 and the trans acetic acid and aqua ligands
bisects the molecule. The hfac- ligands are domed out of
the equatorial plane toward the acetic acid ligand (Table 4).
The Zn1-O(hfac-) (2.06(1) Å) and Zn1-O1(water) (2.06-
(1) Å) bond distances are similar to each other and are
significantly shorter than Zn1-O8(acetic acid, 2.34(1) Å).
The carbonyl group of the acetic acid is coordinated to zinc
like for 1, as indicated by the metric parameters of the acetic
acid ligand (C10-O11 ) 1.30(1) Å; O8-C10 ) 1.20(1)
Å). The Zn-ligand bond distances correspond well with the
trans-Zn(hfac)2‚2H2O29 and Zn(hfac)2‚2H2O‚diglyme30 com-
plexes. The average Zn1-O(hfac-) bond distance in 2 is
nearly 0.11 Å longer than for the copper complex, while the
M-O(water) and M-O(acetic acid) are both shorter in 2
than in 1 (by 0.26 and 0.04 Å, respectively). An extensive
hydrogen-bonding network is also present in the solid-state
structure of 2. The water hydrogen atoms, the hfac- O atoms
(O13 and O13a), and the acetic acid hydroxyl H and carbonyl
O atoms are hydrogen-bonded to three adjacent complexes.
The isolated zinc product is contaminated with Zn(hfac)2‚
2H2O, and elemental analysis of the solid fits to a 1:1 mixture
of 2 and Zn(hfac)2‚2H2O. Solid 2 (+Zn(hfac)2‚2H2O) was
characterized by 1H and 13C NMR spectroscopy in acetone-
1
d6. The H NMR spectrum of 2 (+Zn(hfac)2‚2H2O) shows
two singlets at δ 5.93 and 1.95 ppm, corresponding to the
protons from hfac- and the methyl group of the acetic acid,
respectively. A 1H-13C COSY NMR spectrum confirms that
the resonance at 1.95 ppm corresponds to the methyl protons
and not the water protons (Figure S1). The integrated ratio
of acetic acid (CH3):hfac (C-H) of 3:4 is consistent with
the elemental analysis results. No resonances are observed
for the coordinated water molecule or the acidic proton of
acetic acid, presumably due to chemical exchange.31 It should
be noted that acetone is expected to be a better ligand than
acetic acid and would likely displace acetic acid from the
coordination sphere of zinc. The 13C{1H} NMR spectrum
(Figure S2) shows resonances for acetic acid and the
coordinated hfac-.
The solid-state IR spectrum of 2 (+Zn(hfac)2‚2H2O) has
the expected bands for the coordinated aqua and hfac-
(29) Adams, R. P.; Allen, H. C., Jr.; Rychlewska, U.; Hodgson, D. J. Inorg.
Chim. Acta 1986, 119, 67.
(30) Gulino, A.; Castelli, F.; Dapporto, P.; Rossi, P.; Fragala`, I. Chem.
Mater. 2000, 12, 548.
(31) Chattoraj, S. C.; Cupka, A. G., Jr.; Sievers, R. E. J. Inorg. Nucl. Chem.
1966, 28, 1937.
Inorganic Chemistry, Vol. 44, No. 2, 2005 321

Visintin et al.
Table 5. Comparison of the Solvent Polarities, Dielectric Constants,
and Densities32
dipole
moment (D)
dielectric
constant
density
(g/mL)
solvent
CO2
0
1.52a
0.852b
hexane
methylene chloride
0
1.60
1.89 (20 °C)
9.08 (20 °C)
0.656 (25 °C)
1.31 (25 °C)
a Value refers to scCO2 (40 °C, 215 bar).47 b Value refers to scCO2 (40
°C, 214 bar).48
medium, the zinc coupon completely dissolves within 2 h;
however, when scCO2 is used as the solvent, only 46% of
the zinc coupon dissolves after 6 days of reaction, indicating
that the overall reaction rate is significantly slower in scCO2
than hexanes. These solvents differ in polarities, dielectric
constants, and densities (Table 5),32 but the origin of the rate
differences with solvent for this reaction is not understood
in detail.
Oxidation of Metallic Copper or Zinc with t-BuPA. The
observation of a blue solution upon EtOH washing of copper
powder treated with t-BuPA in hexanes prompted additional
experiments to identify the reaction products in the absence
of added hfacH. Reaction between copper powder and a
hexanes solution of t-BuPA in EtOH at 40 °C yields a blue
solution. A blue solid was isolated after removal of solvent,
which was identified as the well-known dicopper(II) tetra-
acetate dihydrate complex, Cu2(O2CCH3)4‚2H2O, by a single-
crystal X-ray determination,33,34 IR spectroscopy,35 and
elemental analysis. Quantification of the amount of copper
consumed relative to 3 produced shows that all of the copper
dissolved ends up as 3 within experimental error. This result
combined with the fact that 4 equiv of acetate is available
to form 3 implies that the t-BuPA acts as a one-electron
oxidant for copper in EtOH solution (eq 3). The t-BuO•
product, which is required for a balanced reaction, apparently
does not play a role in oxidation of copper in EtOH solvent.
Solid 3 reacts immediately with hfacH in hexanes or scCO2
to form Cu(hfac)2‚H2O(soln) and acetic acid (eq 4), sug-
gesting that the exchange of coordinated acetate for hfac-
would be favorable under the etching reaction conditions.
1
Figure 3. Selected time-dependent H NMR spectra during the reaction
of zinc metal (1 equiv) with t-BuPA (0.10 equiv) and hfacH (0.33 equiv)
in CD2Cl2. Benzene was used as the calibration standard. The hfacH signals
are not shown.
Cu(s) + 2t-BuO-OAc + H2O EtOH8
1
•
Cu (O CCH ) ‚2H O
/
(soln) + 2t-BuO (3)
2
2
3 4
2
2
3
1/2Cu2(O2CCH3)4‚2H2O(s) + 2hfacH f
Cu(hfac)2‚H2O(soln) + 2CH3CO2H (4)
Figure 4. Plot of integral areas normalized to one hydrogen for t-BuPA
(([) C(CH3)3; (0) CH3)), tert-butyl alcohol ((4) C(CH3)3; (b) OH ), and
acetic acid ((*) CH3) over time for the reaction of zinc metal (1 equiv)
with t-BuPA (0.10 equiv) and hfacH (0.33 equiv) in CD2Cl2 at 40 °C.
As mentioned above, no soluble products are observed by
NMR spectroscopy upon reaction of t-BuPA with copper
and zinc metal in CD2Cl2 solution. To determine if there
are t-BuPA decomposition products associated with the metal
surface in hexanes, a two-step reaction was conducted for
reaction stoichiometry was confirmed as 1:1:2 (Zn:t-BuPA:
hfacH), demonstrating that t-BuPA acts as an overall two
electron oxidant for zinc.
Solvent Effects. The reactions of copper and zinc metal
with the etchant solution yield a similar product distribution
in scCO2, hexanes, and CD2Cl2. For the hexanes and scCO2
reactions of metallic zinc with t-BuPA and hfacH, the same
reagent concentrations were used, so the results can be
directly compared. When hexanes is used as the reaction
(32) CRC Handbook of Chemistry and Physics, 74th ed.; Lide, D. R., Ed.;
CRC Press: Boca Raton, FL, 1993.
(33) Van Niekerk, J. N.; Schoening, F. R. L. Acta Crystallogr. 1953, 6,
227.
(34) Brown, G. M.; Chidambaram, R. Acta Crystallogr., Sect. B 1973, 29,
2393.
(35) Mathey, Y.; Greig, D. R.; Shriver, D. F. Inorg. Chem. 1982, 21, 3409.
322 Inorganic Chemistry, Vol. 44, No. 2, 2005

OxidatiWe Dissolution of Cu and Zn Metal in CO2
zinc to enable NMR spectroscopy to be used as a charac-
terization tool. A zinc coupon was first treated with t-BuPA
in hexanes, thoroughly washed with hexanes, air-dried, and
then transferred to an hfacH/CD2Cl2 solution and stirred for
1 h. 1H NMR spectroscopy of a solution aliquot showed tert-
butyl alcohol, acetone, and acetic acid in a 1:2:3 molar ratio.
An experiment performed in a glovebox, avoiding exposure
to air, also showed acetone as a decomposition product. It
is proposed that both acetone and tert-butyl alcohol are
produced from a surface-bound t-BuO species. This proposal
is supported by the fact that the sum of the amounts of tert-
butyl alcohol and acetone observed is equal to the amount
of acetic acid. The initial oxide coating on the surface present
in this experiment would provide different reaction sites to
the t-BuO• radical than in the one-pot reaction and may
explain why different products are observed in the two-step
reaction. Acetone most likely arises from â-scission of
t-BuO• radical, which is presumably released from the
surface upon reaction with hfacH.36-39 This experiment
provides evidence that even though no soluble products are
formed upon reaction of zinc with t-BuPA, t-BuPA does react
with the zinc surface in the absence of added hfacH and that
hfacH plays a role in removing the t-BuPA reaction products
from the surface.
nearly 1 V more driving force for the reduction reaction than
copper,41 consistent with the more rapid reaction of zinc. In
nonpolar scCO2 or hexanes media, the anionic reduction
products would be associated with the metal surface.
(2) Stoichiometry of the Oxidation Reaction and
Working Model. In the reaction of zinc with the t-BuPA/
hfacH etchant solution, NMR studies establish that t-BuPA
cleanly decomposes to tert-butyl alcohol and acetic acid
under an inert atmosphere. The product distribution after
partial etching of zinc confirms that t-BuPA acts as an overall
two-electron oxidant for Zn(0) in scCO2 solvent.
A possible working model for the oxidative dissolution
of copper or zinc metal by reaction with the t-BuPA/hfacH
etchant solution that takes into account these features and
the known mode of reaction of t-BuPA is given in eqs 7
and 8. At the outset of the reaction, the metal coupons will
have a surface layer of oxide,1,6 which passivates the surface.
Given that the etching reaction proceeds without a pretreat-
ment to remove the oxide surface, it is likely that hfacH is
effective in removing this passivating layer under the reaction
conditions. The scheme begins with an unpassivated metal
surface, written as Mn(s). In hexanes and CO2 media, we
propose that the t-BuPA is reduced at the surface of the metal
to form acetate and t-butoxide bound to the metal surface
(eq 7). The very low dielectric constants of the hexanes and
scCO2 media (Table 5) dictate that these redox reactions must
occur at the surface of the metal. The proposal that the
t-BuPA cleavage products associate with the metal surface
is supported by the fact that (1) the reaction of copper or
zinc with t-BuPA in the absence of added hfacH is very
limited and no peroxy ester decomposition products are
observed in solution by NMR spectroscopy, (2) pretreatment
of a Zn(0) coupon with t-BuPA followed by reaction with
hfacH shows soluble products derived from t-BuPA cleavage,
and (3) EtOH washing of a Cu(0) coupon treated with
t-BuPA in hexanes yields soluble Cu2(O2CCH3)4‚2H2O.
Working Model for the Oxidative Dissolution Reaction.
On the basis of the above experiments, several features
emerge that are relevant to the mechanism of the CMP
reaction, which are discussed below.
(1) Reductive Cleavage of Peroxy Esters. The peroxy
ester, t-BuPA, is thermally stable, with a half-life of 13 h at
100 °C.17 Accordingly, thermal decomposition of t-BuPA is
not expected to be an important reaction pathway under the
conditions of our oxidative dissolution reactions. In addition,
NMR experiments establish that the decomposition of
t-BuPA is not promoted by hfacH, the other reagent in the
reaction. Therefore, it is most likely that the decomposition
of t-BuPA is initiated by the metal(0) source acting as a
reducing agent for t-BuPA. There is precedence for reductive
initiation of the decomposition of peroxy esters. Cu(I) salts
are homogeneous catalysts for the decomposition of peroxy
esters.39 These reactions proceed by a one-electron transfer
from Cu(I) to the peroxy ester, yielding Cu(II), acetate, and
t-BuO•. A reductive cleavage mode of reaction for peroxy
esters is also supported by recent electrochemical studies.40
The electrochemical data in DMF solution for t-BuOOC-
(O)t-Bu are consistent with a concerted electron transfer/
O-O bond cleavage reaction to yield acetate and t-BuO•
(eq 5). The t-BuO• is reduced in a step after the cleavage
reaction (eq 6).
Mn(s) + t-BuOOAc f Mn2+(OAc)-(O-t-Bu)-(s) (7)
Mn2+(OAc)-(O-t-Bu)-(s) + 2hfacH f
Mn-1(s) + M(hfac)2(soln) + AcOH + t-BuOH (8)
For the metal coupon reactions, metal dissolution requires
both t-BuPA and hfacH. In addition to removing the initial
oxide layer, hfacH has a critical second role in protonating
the surface-bound acetate and t-butoxide ligands to produce
acetic acid and tert-butyl alcohol (eq 8), thereby facilitating
their desorption from the surface. The pKa of the â-diketone
and the coordinating ability of the conjugate ion will dictate
the thermodynamic driving force for the removal of the
surface bound species. While we have not experimentally
observed surface bound M(hfac) intermediates, on the basis
of gas-phase studies,7-11,42-46 it is reasonable to propose that
t-BuOOC(O)t-Bu + e- f t-BuCO2- + t-BuO• (5)
t-BuO• + e- f t-BuO-
(6)
For the two metal reducing agents employed in this study,
on the basis of the aqueous reduction potentials, zinc provides
(39) Kochi, J. K.; Mains, H. E. J. Org. Chem. 1965, 30, 1862.
(40) Antonello, S.; Formaggio, F.; Moretto, A.; Toniolo, C.; Maran, F. J.
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Inorganic Chemistry, Vol. 44, No. 2, 2005 323

Visintin et al.
eq 8 proceeds via exchange of the surface-bound M(OAc)
and M(O-t-Bu) species to produce surface-bound M(hfac).
To ultimately extract a M(II) ion from the surface, it is
necessary for two hfac- ligands to coordinate to a single
M(II) ion and for the M(hfac)2 complex, produced at the
metal surface, to dissolve into solution.
oxidized copper surface atoms to produce Cu(hfac)2, which
desorbs from the surface at temperatures greater than 250
K. The removal of the oxidized metal from the surface by
coordination to hfac- ligands is a critical reaction component
for both the solution oxidative dissolution reaction and the
gas-phase etching reactions.
The ability to remove the M(hfac)2 product from the metal
surface into solution could influence the rate of metal
dissolution. The etching ability of the various chelating
ligands toward copper metal in scCO2 (Table 2) correlates
well with the solubility of the copper(II) complexes in scCO2
[we have chosen to compare similar reaction conditions, i.e.,
P ) 207 bar, T ) 40 °C, d ) 0.847 g/mL, and mole fraction
solubilities Cu(tfac)2 (4.2 × 10-4), Cu(tmhd)2 (5.8 × 10-4),
and Cu(hfac)2(H2O) (2.4 × 10-3)].20 The complex with two
hfac- ligands is significantly more soluble than the others,
in part due to the higher fluorine content for the ligand, which
makes the complex more CO2-philic.20,21
(3) Relation to Gas-Phase Metal Deposition/Etching
Reactions. The heterogeneous solution oxidation reactions
reported herein are similar to redox transmetalation chemical
vapor deposition reactions that have been extensively
studied.44-46 In these reactions, M(hfac)2 acts as an oxidant
for copper surfaces, and the net reaction involves the
simultaneous deposition of M and etching of copper (eq 9).
The hfac- ligands originally associated with the metal
complex that acts as an oxidant are transferred to the copper
surface. In a subsequent step, the hfac- ligands combine with
M(hfac)2 + Cun(s) f MCun-1(s) + Cu(hfac)2
(9)
Conclusions. These studies provide an understanding of
the chemical reactions and products in a potential CO2-based
CMP process during which copper is removed in the presence
of t-BuPA and hfacH. This work lays a foundation for
converting the current aqueous/organic-based CMP technol-
ogy into a “dry” CO2-based process, by demonstrating that
the t-BuPA/hfacH mixture is an effective etchant solution
for both copper and zinc metals in nonpolar media. During
the course of the oxidative dissolution reaction, no significant
decomposition of hfacH is observed, and the peroxy ester
cleanly undergoes reductive O-O bond cleavage to produce
acetic acid and tert-butyl alcohol. These factors suggest that
post-CMP cleaning steps may be minimized with the choice
of this oxidant system. Further, the CO2 solvent, the M(hfac)2
product, and the organic products (acetic acid and tert-butyl
alcohol) in this reaction should be recyclable, thereby
reducing the environmental impact of the nonaqueous
process. The generality of this reaction for dissolution of
other metals and metal oxides in nonpolar solvents is
currently under investigation.
Acknowledgment. We acknowledge the Kenan Center
for the Utilization of CO2 in Manufacturing and NSF-STC
ROA No. 537494 for financial support. This work made use
of STC shared experimental facilities supported by the
National Science Foundation under Agreement No. CHE-
9876674. We thank the reviewers for their insightful com-
ments.
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D.; Muenchausen, J. Mater. Chem. 1992, 2, 893.
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Standard Reference Database Number 69; http://webbook.nist.gov.
Supporting Information Available: Tables S1-S3 and Figures
S1-S4 (PDF) and X-ray crystallographic files (CIF). This material
is available free of charge via the Internet at http://pubs.acs.org.
IC049765O
324 Inorganic Chemistry, Vol. 44, No. 2, 2005