Structures of anhydrous and hydrated copper(II) hexafluoroacetylacetonate

Article abstract of DOI:10.1021/ic020448w

Crystal structure analyses are reported for anhydrous copper(II) hexafluoroacetylacetonate (Cu(hfac)₂) and for two of its hydrates. The anhydrous compound (Cu(hfac)₂, 1: P1; at 100 K, a = 5.428(1), b = 5.849(1), c = 11.516(3) A; α = 81.47(2), β = 74.57(2), γ = 86.96(2)°; Z = 1) contains centrosymmetric square-planar complexes with close intermolecular Cu···F contacts. The geometry of the complex is similar to that previously reported for Cu(hfac)₂· toluene. The monoaquo compound (Cu(hfac)₂(H₂O), 2: P2₁/c; at 100 K, a = 10.8300(8), b = 6.5400(6), c = 21.551(3) A; β = 90.282(8)°; Z = 4) consists of square-pyramidal molecules with apical H₂O ligands, and close-lying F atoms in the sixth coordination sites. The major difference between this structure and the two other polymorphs previously reported is the nature and direction of hydrogen bonds. The yellow-green solid formed from Cu(hfac)₂ with excess H₂O is identified as the trihydrate. In crystalline form it is the previously unreported [trans-Cu(hfac)₂-(H₂O)₂] ·H₂O (3: P1; at 150 K, a = 8.3899(3), b = 9.6011(3), c = 11.4852(4) A; α = 72.397(2), β = 79.161(2), γ = 87.843(2)°; Z = 2). There is no conclusive evidence in favor of any solid with the composition Cu(hfac)₂· 2H₂O.

Full text of DOI:10.1021/ic020448w

Inorg. Chem. 2002, 41, 6488−6492
Structures of Anhydrous and Hydrated Copper(II)
Hexafluoroacetylacetonate
,†
,†
,‡
Andrew W. Maverick,* Frank R. Fronczek,* Emily F. Maverick,* Damon R. Billodeaux,†
Zuzanna T. Cygan,† and Ralph A. Isovitsch†,§
Department of Chemistry, Louisiana State UniVersity, Baton Rouge, Louisiana 70803-1804, and
Department of Chemistry, UniVersity of California, Los Angeles, California 90095-1569
Received July 11, 2002
Crystal structure analyses are reported for anhydrous copper(II) hexafluoroacetylacetonate (Cu(hfac)2) and for two
of its hydrates. The anhydrous compound (Cu(hfac)2, 1: P1h; at 100 K, a ) 5.428(1), b ) 5.849(1), c ) 11.516(3)
Å; R ) 81.47(2), â ) 74.57(2), γ ) 86.96(2)°; Z ) 1) contains centrosymmetric square-planar complexes with
close intermolecular Cu‚‚‚F contacts. The geometry of the complex is similar to that previously reported for Cu(hfac)2‚
toluene. The monoaquo compound (Cu(hfac)2(H O), 2: P21/c; at 100 K, a ) 10.8300(8), b ) 6.5400(6), c )
2
21.551(3) Å; â ) 90.282(8)°; Z ) 4) consists of square-pyramidal molecules with apical H O ligands, and close-
2
lying F atoms in the sixth coordination sites. The major difference between this structure and the two other polymorphs
previously reported is the nature and direction of hydrogen bonds. The yellow-green solid formed from Cu(hfac)2
with excess H O is identified as the trihydrate. In crystalline form it is the previously unreported [trans-Cu(hfac)2-
2
(H O)2]‚H O (3: P1h; at 150 K, a ) 8.3899(3), b ) 9.6011(3), c ) 11.4852(4) Å; R ) 72.397(2), â ) 79.161(2),
2
2
γ ) 87.843(2)°; Z ) 2). There is no conclusive evidence in favor of any solid with the composition Cu(hfac)2‚
2H O.
2
Introduction
the solid phases in the Cu(hfac)2-H2O system. Reaction of
hfac- with Cu2+ in water produces a yellow-green precipi-
tate,6 which has never been clearly characterized but was
believed to be the dihydrate.7 This material loses water
readily in air to form the blue-green monohydrate, whose
composition was first demonstrated conclusively by Funck
and Ortolano.7 Both hydrates can be dehydrated completely
to dark blue-violet anhydrous Cu(hfac)2. X-ray analyses of
two polymorphs of Cu(hfac)2(H2O),8,9 and the toluene solvate
Fluorinated â-diketonate complexes of metals have been
studied extensively in recent years: their relatively high
volatility makes them attractive as precursors for chemical
vapor deposition of metals1 and oxides.2 The high affinity
of copper(II) hexafluoroacetylacetonate (Cu(hfac)2) and
related compounds for donor ligands has been utilized in
thermodynamic studies of adduct formation,3 and in the
construction of discrete4 and polymeric4b,4c,5 supramolecular
systems.
Cu(hfac)2 is normally obtained commercially or synthe-
sized in hydrated form. However, the state of hydration of
commercial material is uncertain: of six companies offering
the compound for sale, five list it as Cu(hfac)2‚xH2O and
one as Cu(hfac)2‚2H2O. This uncertainty is probably attribut-
able to confusion in the early literature over the identity of
(2) See, for example: Jones, A. C. Chem. Vap. Deposition 1998, 4, 169-
179. Neumayer, D. A.; Belot, J. A.; Feezel, R. L.; Reedy, C.; Stern,
C. L.; Marks, T. J.; Liable-Sands, L. M.; Rheingold, A. L. Inorg. Chem.
1998, 37, 5625-5633. Malandrino, G.; Benelli, C.; Castelli, F.;
Fragala`, I. L. Chem. Mater. 1998, 10, 3434-3444. Zhang, J. M.;
Wessels, B. W.; Richeson, D. S.; Marks, T. J.; DeGroot, D. C.;
Kannewurf, C. R. J. Appl. Phys. 1991, 69, 2743-2745.
(3) McMillin, D. R.; Drago, R. S.; Nusz, J. A. J. Am. Chem. Soc. 1976,
98, 3120-3126; and references therein.
(4) (a) Soldatov, D. V.; Ripmeester, J. A. Chem. Mater. 2000, 12, 1827-
1839. (b) Tabellion, F. M.; Seidel, S. R.; Arif, A. M.; Stang, P. J. J.
Am. Chem. Soc. 2001, 123, 7740-7741. (c) Belford, R. C. E.; Fenton,
D. E.; Truter, M. R. J. Chem. Soc., Dalton Trans. 1972, 2208-2213.
(5) Horikoshi, R.; Mochida, T.; Moriyama, H. Inorg. Chem. 2001, 40,
2430-2433. Dong, Y.-B.; Smith, M. D.; Layland, R. C.; zur Loye,
H.-C. Inorg. Chem. 1999, 38, 5027-5033.
* To whom correspondence should be addressed. E-mail:
maverick@lsu.edu (A.W.M.).
† Louisiana State University.
‡ UCLA.
§ Present address: Department of Chemistry, Xavier University of
Louisiana, New Orleans, LA 70125.
(1) Chemistry of Metal CVD; Kodas, T. T., Hampden-Smith, M. J., Eds.;
VCH: Weinheim, 1994; Chapters 4 and 5.
(6) Bertrand, J. A.; Kaplan, R. I. Inorg. Chem. 1966, 5, 489-491.
(7) Funck, L. L.; Ortolano, T. R. Inorg. Chem. 1968, 7, 567-573.
6488 Inorganic Chemistry, Vol. 41, No. 24, 2002
10.1021/ic020448w CCC: $22.00 © 2002 American Chemical Society
Published on Web 11/06/2002

Structures of Anhydrous and Hydrated Cu(hfac)2
Table 1. Data Collection and Refinement Parametersa
of the anhydrous compound (Cu(hfac)2‚toluene),10 have been
published. However, the ansolvous compound has not been
crystallographically characterized, and the yellow-green
“dihydrate” has never been identified conclusively. We now
report the crystal structures of ansolvous Cu(hfac)2 (1) and
a new polymorph of the monohydrate Cu(hfac)2(H2O) (2).
We have also found that the yellow-green material isolated
from aqueous solution6 is actually the trihydrate, and it can
be crystallized as [trans-Cu(hfac)2(H2O)2]‚H2O (3).
[trans-Cu(hfac)2-
Cu(hfac)2(H2O) (2) (H2O)2]‚H2O (3)
Cu(hfac)2 (1)
formula
color
fw
C
10H2CuF12O4
C10H4CuF12O5
blue-green
495.67
C10H8CuF12O7
yellow-green
531.70
dark blue-violet
477.65
space group
a/Å
b/Å
P1h, No. 2
5.428(1)
5.849(1)
11.516(3)
81.47(2)
74.57(2)
86.96(2)
348.5(2)
2.258
P21/c, No. 14
10.8300(8)
6.5400(6)
21.551(3)
90
90.282(8)
90
1526.4(4)
2.157
P1h, No. 2
8.3899(3)
9.6011(3)
11.4852(4)
72.397(2)
79.161(2)
87.843(2)
865.91(5)
2.039
c/Å
R/deg
â/deg
γ/deg
V/Å3
Experimental Section
Fx/g cm-3
Z
Cu(hfac)2(H2O) (hfacH ) 1,1,1,5,5,5-hexafluoro-2,4-pentane-
dione) was either obtained commercially (Gelest or Strem) or
synthesized by the method of Bertrand and Kaplan6 (reaction of
hfacH, sodium acetate, and copper(II) nitrate in water, followed
by drying of the precipitate in air). This material was converted to
the anhydrous compound on storage in a vacuum desiccator over
P2O5 or concentrated H2SO4. X-ray quality crystals were obtained
by the following methods:
1
1.72
100 ( 1
4
1.59
100 ( 1
2
1.42
150 ( 1
µx/mm-1
temp/K
λ/Å
0.710 73 (Mo KR) 0.710 73 (Mo KR) 0.710 73 (Mo KR)
32.6
θ
max/deg
35.0
35.0
unique/obsd/nvarb 2535/2255/128
5727/5298/253
0.67-0.73
0.072
0.124
4.0042, 0.0195
7434/3878/293
0.55-0.87
0.087
0.116
0, 0.0545
transm coeff
0.65-0.68
0.046
R(F) (all data)c
Rw(F2) (all data)c 0.105
c
w1, w2
0.2007, 0.0585
Cu(hfac)2 (1). The powdery anhydrous material, prepared as
above, was sublimed under vacuum to give dark blue-violet crystals.
Cu(hfac)2(H2O) (2). Blue-green crystals were obtained over a
period of several months by sublimation in a closed container at
room temperature and atmospheric pressure.
a In Tables 1-3, estimated standard deviations in the least significant
digits of the values are given in parentheses. b Observed data have I > 2σ(I).
c R(F) ) ∑||Fo| - |Fc||/∑|Fo|; Rw(F2) ) (∑w(Fo - Fc )2/∑w(Fo )2)1/2; w
2
2
2
2
2
2
) 1/(σ2(Fo ) + w1P + (w2P)2); P ) (Fo + 2Fc )/3.
Results and Discussion
[trans-Cu(hfac)2(H2O)2]‚H2O (3). The method of Bertrand and
Kaplan6 was modified as follows: A solution of CuSO4‚5H2O (0.315
g, 1.26 mmol) in 3 mL of H2O was layered on a solution prepared
from hfacH (0.525 g, 2.52 mmol) and NaOAc (0.23 g, 2.8 mmol)
in ca. 0.5 mL of H2O. Yellow-green platelike crystals were obtained
over a period of 2-3 days. Crystals can also be obtained by
substituting Cu(NO3)2(aq) for CuSO4(aq) in the above method, or
by layering Cu(OAc)2(aq) on hfacH(aq). Diffraction patterns were
examined for several crystals from each type of experiment; these
all showed the unit-cell parameters of 3.
Material with the same composition was prepared in powder form
starting from solid Cu(hfac)2(H2O) by placing it in an atmosphere
saturated with water vapor. The blue-green monohydrate turned
yellow-green, reaching constant weight after 2 days. The observed
weight gains in two such experiments were 7.36% and 7.49%;
the calculated weight gain is 7.27% for the formation of
Cu(hfac)2‚3H2O.
Chemistry of the Cu(hfac)2-H2O System. Hexafluoro-
acetylacetone and its copper(II) complex were first reported
by Henne et al. in 1947.11 The blue-green (or “grass-green”)
solid that is stable under normal laboratory conditions was
recognized by early investigators to contain water,12 but its
state of hydration was not well established until the late
1960s. Walker and Li,13 and Bertrand and Kaplan,6 first
reported the anhydrous material, prepared by dehydration
over P2O5 or H2SO4. Attempts in these studies to establish
the composition of the hydrated material by microanalysis
and by weight gain in the presence of H2O were conflicting
or ambiguous. However, Funck and Ortolano’s electronic
spectral data7 clearly identified the three important species
in the system: (a) the anhydrous material, which is
stable in dry organic solvents; (b) blue-green Cu(hfac)2(H2O);
and (c) the yellow-green material, which (like dilute
aqueous solutions of Cu(hfac)2) was inferred to contain
Cu(hfac)2(H2O)2 molecules.
Since that time, X-ray analyses of two polymorphs of blue-
green Cu(hfac)2(H2O) have been published.8,9 Pinkas et al.
reported the structure of the anhydrous compound in the form
of its toluene solvate;10 however, they were unable to mount
crystals of the ansolvous compound without damage. No
studies of the composition of the yellow-green solid have
appeared between Funck and Ortolano’s 1968 report and the
present work.
Both powdered and crystalline 3 begin to lose water (with a color
change from yellow-green to blue-green) within a few minutes in
normal laboratory air.
Crystal-Structure Analyses. Crystals were mounted on glass
fibers with the use of mineral oil, and immediately cooled in the
N2 gas stream of the diffractometer for data collection. For 1 and
2, the diffractometer was an Enraf-Nonius CAD4 instrument fitted
with a graphite monochromator, and the θ-2θ scan method was
used. The intensities were corrected for absorption using ψ scan
data. A Nonius KappaCCD instrument was used for 3. Data
collection and refinement parameters for 1, 2, and 3 are presented
in Table 1. Details of structure solution and refinement, modeling
of disordered groups, and thermal-motion analysis are in the
Supporting Information.
Structure of Anhydrous Cu(hfac)2 (1). This compound
consists of centrosymmetric Cu(hfac)2 molecules; see the
(8) Pinkas, J.; Huffman, J. C.; Baxter, D. V.; Chisholm, M. H.; Caulton,
K. G. Chem. Mater. 1995, 7, 1589-1596.
(11) Henne, A. L.; Newman, M. S.; Quill, L. L.; Staniforth, R. A. J. Am.
Chem. Soc. 1947, 69, 1819-1820.
(12) Belford, R. L.; Martell, A. E.; Calvin, M. J. Inorg. Nucl. Chem. 1956,
(9) Jain, A.; Kodas, T. T.; Corbitt, T. S.; Hampden-Smith, M. J. Chem.
Mater. 1996, 8, 1119-1127.
(10) Pinkas, J.; Huffman, J. C.; Bollinger, J. C.; Streib, W. E.; Baxter, D.
V.; Chisholm, M. H.; Caulton, K. G. Inorg. Chem. 1997, 36, 2930-
2937.
2, 11-31.
(13) Walker, W. R.; Li, N. C. J. Inorg. Nucl. Chem. 1965, 27, 2255-
2261.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6489

Maverick et al.
Figure 1. ORTEP drawing of Cu(hfac)2 molecule in 1, with displacement
ellipsoids at the 50% probability level. Dashed lines indicate close
intermolecular contacts between Cu and F3 in adjacent molecules (2.709-
(2) Å). (A packing diagram for this structure is available in the Supporting
Information.)
Figure 2. ORTEP drawing of three Cu(hfac)2(H2O) molecules in 2. Close-
lying atoms F6 in neighboring molecules are shown; all other F atoms are
omitted for clarity. Dashed lines indicate hydrogen bonds and close
Cu‚‚‚F6′′′ contacts (2.895(2) Å). H5B and H5C (occupancy 0.5) are shaded;
H5B is disordered across the center of symmetry and is shown in one of its
two equivalent positions.
Table 2. Selected Interatomic Distances (Å) and Angles (deg)
Cu(hfac)2 (1)
Cu-O1
1.914(2)
Cu-O2
1.924(2)
Cu‚‚‚F3′
2.709(2)
O1-Cu-O2 93.24(10)
Cu(hfac)2(H2 O) (2)
Cu-O1
Cu-O2
1.9545(17) Cu-O3
1.9370(18) Cu-O4
1.9411(18) Cu-O5
1.9410(18) Cu‚‚‚F6′
2.260(2)
2.895(2)
The water H atoms in 2 are disordered over three positions.
The water molecule is hydrogen-bonded to hfac and water
O atoms in two neighboring complexes (H5A‚‚‚O1′, 2.24;
H5B‚‚‚O5′′, 2.05; H5C‚‚‚O4′, 2.19 Å). The O atoms are in
a square-pyramidal arrangement, but the environment about
Cu is nearly octahedral due to a close intermolecular
Cu‚‚‚F contact; see Figure 2. Similar close Cu‚‚‚F contacts
are present in the other two Cu(hfac)2(H2O) structures
(see Table 3), and also in two other “five-coordinate”
Cu(hfac)2 adducts: [Cu(hfac)2]2(µ-pyrazine), 3.069 Å;15
[Cu(hfac)2]3(µ-5-methylpyrimidine)2, 2.962 Å.16
O1-Cu-O2 91.88(7)
O2-Cu-O3 86.51(8)
O3-Cu-O4 92.16(8)
O1-Cu-O3 165.87(8) O2-Cu-O4 170.11(8) O3-Cu-O5 103.22(9)
O1-Cu-O4 87.02(7)
O1-Cu-O5 90.88(8)
O2-Cu-O5 100.47(8) O4-Cu-O5 89.38(8)
[trans-Cu(hfac)2(H2O)2]‚H2O (3)
Cu-O1
Cu-O2
1.9492(13) Cu-O3
1.9632(13) Cu-O4
1.9517(13) Cu-O5
1.9770(14) Cu-O6
2.2916(17)
2.3109(17)
O1-Cu-O2 93.28(6)
O2-Cu-O3 85.93(6)
O3-Cu-O5 89.41(6)
O1-Cu-O3 178.25(6) O2-Cu-O4 177.12(6) O3-Cu-O6 94.99(6)
O1-Cu-O4 88.22(6)
O1-Cu-O5 89.10(6)
O1-Cu-O6 86.56(6)
O2-Cu-O5 94.81(7)
O2-Cu-O6 90.24(6)
O3-Cu-O4 92.64(6)
O4-Cu-O5 87.67(7)
O4-Cu-O6 87.39(7)
O5-Cu-O6 173.52(6)
To summarize, Cu(hfac)2(H2O) is now known to crystallize
in three different polymorphs, all of which have similar
molecular geometry and close intermolecular contacts. Other
packings of the molecules might well produce similarly stable
structures.
Structure of [trans-Cu(hfac)2(H2O)2]‚H2O (3). These
crystals consist of approximately centrosymmetric trans-Cu-
(hfac)2(H2O)2 molecules (Figure 3), each with a water
molecule of solvation. All atoms are in general positions. In
3, the Cu-O(hfac) distances are approximately 0.02 Å longer
(weighted average 1.960 Å), and the Cu-OH2 distances
approximately 0.07 Å longer, than in the Cu(hfac)2(H2O)
polymorphs; see Table 3.
Hydrogen bonds are also present in the structure of 3. The
water of hydration (O7) lies approximately in the Cu(hfac)2
plane, so that its hydrogen atoms make close approaches to
O3, F6, and F8 (2.04, 2.27, and 2.63 Å, respectively). The
coordinated water molecules (O5 and O6) are also hydrogen-
bonded to O atoms in two adjacent asymmetric units; see
the Supporting Information for details.
ORTEP drawing in Figure 1. Bond distances and angles (see
data for 1 in Table 2 and comparison with other structures
in Table 3) are similar to those determined via gas-phase
electron diffraction by Thomas et al.,14 and to those obtained
by Pinkas et al.10 for crystalline Cu(hfac)2‚toluene. The most
significant new interaction in 1 is between the Cu atom and
F3 in two neighboring molecules; the intermolecular
Cu‚‚‚F3 distance is 2.709(2) Å (see Figure 1 and a packing
drawing in the Supporting Information). In all three of these
anhydrous Cu(hfac)2 structures, the Cu, O, and C atoms are
coplanar (maximum deviation in 1, 0.066(2) Å).
Structure of the New Polymorph of Cu(hfac)2(H2O) (2).
The structure of the copper complex in 2 (see Figure 2) is
similar to those obtained by Pinkas et al.8 and Jain et al.9
(Our unit-cell parameters are very close to those of Jain et
al.,9 but the structures are significantly different; see detailed
discussion in the Supporting Information.) The Cu atom in
2 is displaced 0.2034(3) Å out of the plane of the four hfac
O atoms toward the coordinated water molecule. The
hydrogen bonding of the water molecule differs in the three
polymorphs, as does the Cu-OH2 distance; in addition, all
three Cu(hfac)2(H2O) structures show larger Cu-O(hfac)
distances (weighted average 1.942 Å) than the anhydrous
compound (1.917 Å); see Table 3.
Other M(hfac)2(H2O)2 Structures. A search of the
Cambridge Structural Database17 for structures similar to 3
(15) Belford, R. C. E.; Fenton, D. E.; Truter, M. R. J. Chem. Soc., Dalton
Trans. 1974, 17-24. Romero, R. R.; Porter, L. C. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1993, C49, 1487-1489.
(16) Kogane, T.; Kobayashi, K.; Ishii, M.; Hirota, R.; Nakahara, M. J.
Chem. Soc., Dalton Trans. 1994, 13-18.
(14) Thomas, B. G.; Morris, M. L.; Hilderbrandt, R. L. J. Mol. Struct. 1976,
35, 241-252.
6490 Inorganic Chemistry, Vol. 41, No. 24, 2002

Structures of Anhydrous and Hydrated Cu(hfac)2
Table 3. Comparison of Interatomic Distances (Å) in the Cu(hfac)2-H2O System
compd
Cu-O(hfac)
Cu-OH2
Cu‚‚‚F
Cu(hfac)2 (1)a
1.914(2), 1.924(2)
1.919(8)
n/ab
n/a
n/a
2.260(2)
2.204(3)
2.709(2)
n/a
n/a
2.895(2)
2.817
3.106
n/a
Cu(hfac)2(g)c
Cu(hfac)2‚toluened
Cu(hfac)2(H2O) (2)a
Cu(hfac)2(H2O)e
1.908(7), 1.914(6)
1.937(2)-1.954(2)
1.929(2)-1.947(2)
1.932(4)-1.947(4)
1.9492(13)-1.9770(14)
Cu(hfac)2(H2O)f
2.221(6)
2.2916(17), 2.3109(17)
[trans-Cu(hfac)2(H2O)2]‚H2O (3)a
a This work. b Not applicable (no such contact is found in this structure). c Gas-phase electron diffraction; ref 14. d Reference 10. e Reference 8. f Reference
9.
Another Cu(hfac)2 adduct has been reported with a high
melting point (205 °C): Cu(hfac)2(NH3).22 The unit-cell
parameters for the NH3 complex reported by Pinkas et al.22
are considerably closer to those for the “dihydrate” of Kidd
et al.21 than are those for the other M(hfac)2(H2O)2 crystals.
Furthermore, in contrast to the usual preparative methods
for hydrated Cu(hfac)2, Kidd et al. used aqueous ammonia.
Pinkas et al.22 prepared the NH3 complex by using NH3(g)
in toluene solution, but Anker et al.23 showed that it can also
be made with aqueous NH3. For these reasons, we believe it
is likely that the high-melting compound studied by Kidd et
al. was actually Cu(hfac)2(NH3). Thus, there is no conclusive
evidence for any solid with the composition Cu(hfac)2(H2O)2;
Figure 3. ORTEP drawing of trans-Cu(hfac)2(H2O)2 molecule and water
of hydration (O7) in 3, with displacement ellipsoids at the 40% probability
level. One orientation of the disordered CF3 group at C10 is shown.
the previously described yellow-green phase actually contains
three water molecules per Cu(hfac)2 unit.
Relationship to Chemical Vapor Deposition (CVD). The
structures of these compounds provide insights into inter-
molecular forces and bond distances that may affect their
volatility and reactivity.
identified two others with the same [trans-M(hfac)2(H2O)2]‚
H2O formula (M ) Zn,18 Mn19). The Zn and Mn compounds
are isomorphous with each other, but they are clearly
different from 3. In the Zn18 and Mn19 structures, the hfac
ligands are bent substantially out of the equatorial plane; in
contrast, in 3, the Cu(hfac)2 moiety is nearly planar (maxi-
mum deviation 0.158(2) Å, excluding F atoms).
Two isomorphous cis-M(hfac)2(H2O)2 structures (M )
Zn,18 Ni20) have also been reported. The unit-cell parameters
for these compounds are similar to those reported by Kidd
et al. for “Cu(hfac)2‚2H2O”.21 Thus, at first, we thought the
“yellow-green” material of Funck and Ortolano7 and the
“blue-green” material of Kidd et al.21 might both be cis-
Cu(hfac)2(H2O)2. However, the yellow-green 3 as prepared
in our laboratory includes a third water molecule, contains
trans complexes, and forms crystals with unit-cell volume
about 53% of that of the material of Kidd et al. We did not
observe any materials with unit-cell parameters similar to
those of Kidd et al. Also, the melting point of their Cu
compound was 207 °C. In contrast, 3 loses water rapidly on
heating, turning blue-green, and then melts at ca. 130 °C,
close to the published melting point for Cu(hfac)2(H2O)
(133-136 °C7). Thus, 3 is clearly different from any of the
previously described M(hfac)2(H2O)2-containing phases.
Cu(hfac)2 is more volatile than Cu(hfac)2(H2O).24 This
behavior might be expected if the anhydrous compound is
relatively nonpolar and Cu(hfac)2(H2O) is capable of forming
intermolecular hydrogen bonds. However, the difference in
volatility is small enough that both compounds are suf-
ficiently volatile for use as CVD precursors.25 The X-ray
analyses of Cu(hfac)2 (1) and Cu(hfac)2(H2O) suggest two
reasons for this small difference in volatility. First, although
anhydrous Cu(hfac)2 is nonpolar, it also possesses significant
intermolecular attractions, primarily in the form of the short
Cu‚‚‚F contacts. Second, in Cu(hfac)2(H2O), the hydrogen-
bond-accepting ability of the O atoms is weakened by the
electronegative F atoms; thus, water may not provide as great
a stabilizing effect in the solid as expected.
Structural changes in Cu(hfac)2 on binding of H2O may
also be related to the effects of water on the reactivity of
Cu(hfac)2 as a precursor for CVD of Cu metal. Normally,
H2 is used as a reducing carrier gas in these systems. Awaya
(22) Pinkas, J.; Huffman, J. C.; Chisholm, M. H.; Caulton, K. G. Inorg.
Chem. 1995, 34, 5314-5318.
(23) Anker, M. W.; Moffett, J. H.; Moxon, N. T.; Gregson, A. K. Aust. J.
Chem. 1981, 34, 2455-2459.
(17) Allen, F. H.; Kennard, O.; Taylor, R. Acc. Chem. Res. 1983, 16, 146-
153.
(18) Adams, R. P.; Allen, H. C., Jr.; Rychlewska, U.; Hodgson, D. J. Inorg.
Chim. Acta 1986, 119, 67-74.
(19) Dickman, M. H. Acta Crystallogr., Sect. C: Cryst. Struct. Commun.
1997, C53, 402-404.
(20) Romero, R. R.; Cervantes-Lee, F.; Porter, L. C. Acta Crystallogr.,
Sect. C: Cryst. Struct. Commun. 1992, C48, 993-995.
(21) Kidd, M. R.; Sager, R. S.; Watson, W. H. Inorg. Chem. 1967, 6, 946-
951.
(24) Borgharkar, N. S.; Griffin, G. L.; James, A.; Maverick, A. W. Thin
Solid Films 1998, 320, 86-94.
(25) For measurements of the vapor pressure of 1, see: Wolf, W. R.;
Sievers, R. E.; Brown, G. H. Inorg. Chem. 1972, 11, 1995-2002 (solid,
ca. 80-100 °C; liquid, ca. 100-130 °C); Temple, D.; Reisman, A. J.
Electrochem. Soc. 1989, 136, 3525-3529 (liquid, 97-120 °C). The
vapor pressure of Cu(hfac)2 is ca. 3 mmHg at 88 °C, and the
evaporation rate data of ref 24 suggest that the value for Cu(hfac)2-
(H2O) at the same temperature is about 2.3 mmHg.
Inorganic Chemistry, Vol. 41, No. 24, 2002 6491

Maverick et al.
and Arita26 showed early on that the Cu deposition rate
(under H2, with metallic substrates) increases substantially
as the amount of added water increases. Pinkas et al.8
attributed this increase to the ability of H2O to assist in
dissociation of hfacH. Kinetic studies in the absence of water
by Borgharkar and Griffin27 suggested that the rate-limiting
step is the reaction of adsorbed H atoms and hfac groups to
produce hfacH:
trans-Cu(hfac)2(H2O)2 (as found in 3) is less directly
relevant to CVD, because even at room temperature it is
stable only under very high partial pressures of water vapor.
However, if a species such as Cu(hfac)2(H2O) adsorbs (see
above) by coordination of a surface O atom to Cu, the Cu
atom will be effectively six-coordinate, and the Cu-O(hfac)
bonds may be lengthened and weakened further, leading to
still greater reactivity in Cu CVD.
Conclusions. The high affinity of Cu(hfac)2 for axial
donors is evident in all three of the present structures: hfac
F atoms occupy coordination sites if water molecules are
not available. Also, the structures of the three polymorphs
of Cu(hfac)2(H2O) suggest that hydrogen bonding and close
Cu‚‚‚F interactions can be achieved in a variety of packing
arrangements. Finally, the yellow-green compound 3 that
forms in the presence of excess water is [trans-Cu(hfac)2-
(H2O)2]‚H2O, not the dihydrate as previously believed.
H(ads) + hfac(ads) f hfacH(g)
(1)
Two mechanisms could account for the enhanced deposi-
tion rates in the presence of water:8,26
(a) Water can dissociate to H(ads) and OH(ads) on the
surface, increasing [H(ads)] and thereby raising the rate of
reaction 1; or (b) water can coordinate to an adsorbed Cu
complex (e.g., making Cu(hfac)2(H2O)(ads)), which can be
followed by intramolecular proton transfer to produce hfacH,
effectively bypassing reaction 1.
Acknowledgment. We thank the National Science Foun-
dation (CTS-9612157) for support of this research, and the
Louisiana Board of Regents (LEQSF(1996-97)-ENH-TR-10
and LEQSF(1999-2000)-ENH-TR-13), for improvements to
the LSU X-ray Crystallography Facility.
If the reaction occurs by mechanism b, then differences
in Cu-O(hfac) bond distances between Cu(hfac)2 and its
water adducts may be important. These bonds are longer in
Cu(hfac)2(H2O) than in Cu(hfac)2 (and they are longer still
in Cu(hfac)2(H2O)2), meaning that dissociation of hfac could
be more favorable when water is present.
Supporting Information Available: Details of X-ray analyses
(5 pages, and CIF files). This information is available free of charge
via the Internet at http://pubs.acs.org.
(26) See, for example: Awaya, N.; Arita, Y. Jpn. J. Appl. Phys., Part 1
1993, 32, 3915-3919.
(27) Borgharkar, N. S.; Griffin, G. L. J. Electrochem. Soc. 1998, 145, 347-
352.
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