Syntheses, Structures, and Characterization of Nickel(II) Stibines: Steric and Electronic Rationale for Metal Deposition

Article abstract of DOI:10.1021/acs.inorgchem.8b01565

Reactions of the homoleptic and heteroleptic antimony ligands SbⁱPr₃, SbⁱPr₂Ph, SbMe₂Ph, and SbMePh₂ with NiI₂ generate rare Ni^II stibine complexes in either square planar or trigonal bipyramidal (TBP) geometries, depending on the steric size of the ligands. Tolman electronic parameters were calculated (DFT) for each antimony ligand to provide a tabulated resource for the relative strengths of simple antimony ligands. The electronic absorbance spectra of the square planar complexes exhibit characteristic bands [λ_max ≈ 560 nm (17 900 cm^-1), ? ≈ 4330 M^-1 cm^-1] at lower energies compared to the reported phosphine complexes, indicating the weak donor strength of the stibine ligands and resultant low-energy ligand field d→d transitions. The square planar complex Ni(I)₂(SbⁱPr₃)₂ reacts with CO to form the TBP complex Ni(I)₂(SbⁱPr₃)₂(CO). Lastly, the complexes were investigated for nickel metal deposition on Si|Cu(100 nm) substrates. The complexes with the strongest donating ligand, SbⁱPr₃, deposited the purest layer of NiCu alloy according to the balanced reaction Ni(I)₂(Sb^IIIiPr₃)₂ → Ni⁰ + Sb^V(ⁱPr₃)I₂; the iodinated Sb^V byproduct was unambiguously detected in the supernatant by ¹H NMR and mass spectrometry. Complexes with weaker ligands (poor I₂ acceptors/scavengers) resulted undesired deposition of iodine and CuI on the surface. This work thus serves as a guide for the design and synthesis of 3d metal complexes with neutral, heavy main-group donors that are useful for metal deposition applications.

Full text of DOI:10.1021/acs.inorgchem.8b01565

Article
pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Syntheses, Structures, and Characterization of Nickel(II) Stibines:
Steric and Electronic Rationale for Metal Deposition
William V. Taylor, Zhu-Lin Xie, Nicholas I. Cool, Sofia A. Shubert, and Michael J. Rose*
Department of Chemistry, The University of Texas at Austin, Welch Hall, 105 East 24th Street, Austin, Texas 78712, United States
S
* Supporting Information
ABSTRACT: Reactions of the homoleptic and heteroleptic antimony ligands SbiPr3, SbiPr2Ph, SbMe2Ph, and SbMePh2 with
NiI2 generate rare NiII stibine complexes in either square planar or trigonal bipyramidal (TBP) geometries, depending on the
steric size of the ligands. Tolman electronic parameters were calculated (DFT) for each antimony ligand to provide a tabulated
resource for the relative strengths of simple antimony ligands. The electronic absorbance spectra of the square planar complexes
exhibit characteristic bands [λmax ≈ 560 nm (17 900 cm−1), ε ≈ 4330 M−1 cm−1] at lower energies compared to the reported
phosphine complexes, indicating the weak donor strength of the stibine ligands and resultant low-energy ligand field d→d
transitions. The square planar complex Ni(I)2(SbiPr3)2 reacts with CO to form the TBP complex Ni(I)2(SbiPr3)2(CO). Lastly,
the complexes were investigated for nickel metal deposition on Si|Cu(100 nm) substrates. The complexes with the strongest
donating ligand, SbiPr3, deposited the purest layer of NiCu alloy according to the balanced reaction Ni(I)2(SbIIIiPr3)2 → Ni0 +
SbV(iPr3)I2; the iodinated SbV byproduct was unambiguously detected in the supernatant by 1H NMR and mass spectrometry.
Complexes with weaker ligands (poor I2 acceptors/scavengers) resulted undesired deposition of iodine and CuI on the surface.
This work thus serves as a guide for the design and synthesis of 3d metal complexes with neutral, heavy main-group donors that
are useful for metal deposition applications.
Levason and Reid in the early 2000s showcased the first example
of a nickel(II) halide stibine complex that was structurally
characterized, namely [Ni{1,2-C6H4(CH2SbMe2)2}2(I)]-
(ClO4).17 An allyl nickel(II) bromide complex containing
INTRODUCTION
■
Nickel phosphine compounds have been studied for their
catalytic properties for decades, and they are associated with a
variety of catalytic reactions such as hydrogen production,1
hydrocyanation,2 and oligomerization.3 In an effort to better
comprehend and optimize catalysis, phosphine ligands and their
corresponding organic substituents have been studied for both
steric and electronic effects.4,5 Concerning heavier phosphorus
group ligands, there have been studies on the relative steric and
electronic effects in stand-alone antimony molecules,6,7 but
there are few extensive studies regarding the steric, electronic,
magnetic, and spectroscopic properties of antimony−nickel (or
other 3d metal) complexes.8−10 Such studies would provide a
base of understanding for the design and synthesis of functional
antimony-based complexes. Indeed, antimony ligands have
proven to be effective donors to metal centers under certain
conditions,11,12 and productive catalysts that utilize antimony
ligands have already been reported.13−16
́
triphenylstibine was synthesized in 2009 by Jimenez-Tenorio,
Valerga, and Mereiter for use in the oligomerization of
styrenes.18 In 2014, Gabbaı published several intriguing
̈
nickel(II) structures stabilized with tripodal antimony−
phosphorus ligands (SbP3), which showcased the diverse
functionality of antimony to behave as an L-, X-, or Z-type
ligand.19 Relatedly, there are reports of several nickel−antimony
dimeric structures20,21 and clusters.22−25 One likely reason for
the general lack of nickel(II)−antimony compounds is the
prevalent use of commercially available triphenylantimony as a
ligand.26 Antimony is a weak σ-donor compared with
phosphorus or arsenic, and phenyl groups further weaken the
donor strength due to their electron-withdrawing nature.27
However, it is plausible that electron-donating groups, such as
alkyl substituents, would increase the σ-donating capability of
Although a plethora of nickel−phosphine complexes have
been synthesized, there are very few reported nickel−antimony
complexes and even fewer (three) nickel(II)−antimony
complexes with associated crystal structures. Elegant work by
Received: June 6, 2018
© XXXX American Chemical Society
A
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Article
antimony,28 thus affording novel metal complexes that are
otherwise inaccessible with triphenylantimony or related aryl
ligands. Thus, a fundamental study on electron-donating alkyl
substituents on a series of antimony−nickel complexes would be
beneficial to transition metal chemists presently working (or
who might consider working) with stibines, and this would be a
valuable contribution to the literature.
Magnesium turnings (Acros), iodine crystals (Acros), 2-chloropropane
(Sigma-Aldrich), nickel iodide (Strem Chemicals), fluorobenzene
(Oakwood Chemical), and CO gas (Praxair) were purchased and used
as received without further purification.
Ligand Syntheses. Safety Precaution. Alkyl-substituted antimony
ligands are volatile and air-sensitive and can be pyrophoric: they must be
handled with caution under completely inert atmosphere. The corresponding
metal complexes are also highly air-sensitive and often pyrophoric and must
be handled with similar care.
In addition to studying the electronic effects of antimony
ligands, it is equally critical to examine the effects of sterics in
stibine−metal complexes. Most five-coordinate nickel−stibine
complexes exhibit a geometric preference toward trigonal
bipyramidal;9,19 among the few four-coordinate nickel−stibine
complexes in literature, only one is square planar.29 Thus, no
systematic study to date has been performed to examine how the
sterics and electronics of the ligand substituents affect the final
geometries of the metal complexes. In contrast, extensive studies
of varying phosphine ligands on nickel complexes have resulted
in many successful instances of catalytic function and improve-
ment.30−32 Additionally, the Tolman electronic parameter has
long been used to determine the donor strength of ligands.
However, no such study has been compiled for tertiary antimony
ligands.33 It is thus important to conduct a thorough analysis of
possible ligands that could be utilized for future applications.
From initial observations of decomposition of our complexes
when heated, we were interested to investigate deposition
properties of such metalated compounds. Deposition of thin
filmswhether singular metals or alloysis critical in the
industries of semiconductor fabrication and electronic materials.
Typical deposition methods (physical, thermal, or chemical
vapor) are inefficient and/or expensive due to their non-
specificity to the substrate and energy-intensive procedures
(high temperature, vacuum, etc.). Electrodeposition is a
promising avenue that can be more reagent-efficient than
others, but the field is dominated by deposition from aqueous
media, which leads to undesired oxidation of the substrate or
deposition of mixtures of metals and metal oxides.34,35 Similarly,
the utility of electrodeposition is eliminated when the target
substrate is not electrically conductive (bulk silicon), or parts of
the substrate are not in direct electrical contact.36 Deposition
under milder conditions from organic solvents could be valuable
and reveal more information on the utility and reactivity of
antimony-based metal complexes.
Triisopropylantimony (SbiPr3). Magnesium turnings (2.46 g, 101.25
mmol) were activated with iodine (0.01 g, 0.04 mmol) and heated for
30 min under vacuum. Dry Et2O (125 mL) was added to the flask, and
1,2-dibromoethane (0.22 g, 1.16 mmol) was subsequently added. Next,
2-chloropropane (5.88 g, 75 mmol) was added to the flask at 0 °C, and
the mixture was stirred overnight. The solution was filtered and
antimony trichloride (3.42 g, 15 mmol) was added to the flask at 0 °C.
The solution was refluxed for 3 h and then quenched with degassed
water (70 mL). Under completely inert atmosphere, the organic layer
was separated, dried over sodium sulfate, and filtered. The solvent was
removed in vacuo at 0 °C to afford the product as a colorless oil (3.31 g).
Yield: 88% yield. 1H NMR (CDCl3): δ 1.31 (d, J = 7.3 Hz, 18H, −CH3),
1.85 (hept, J = 7.3 Hz, 3H, −CH). 13C NMR (CDCl3): δ 17.0, 22.5.
Diisopropylphenylantimony (SbiPr2Ph). Trichloroantimony (5.64
g, 24.7 mmol) and triphenylantimony (4.36 g, 12.4 mmol) were stirred
together solvent-free for 3 h to afford dichlorophenylantimony (10 g,
37.06 mmol) in quantitative yield. Separately, the isopropylmagnesium
chloride Grignard solution was generated in situ in the same manner as
above with magnesium turnings (2.70 g, 111 mmol) and 2-
chloropropane (7.27 g, 92.6 mmol). The SbCl2Ph was then dissolved
in Et2O (90 mL) and added dropwise to the filtered Grignard solution,
and the reaction was refluxed for 3 h. The solution was then quenched
with degassed water (50 mL). Under completely inert atmosphere, the
organic layer was separated, dried over sodium sulfate, and then filtered.
The solvent was removed in vacuo to afford a colorless oil. A fractional
distillation was performed, which first distilled trace amounts of SbiPr3
(75 °C, 0.01 mmHg), followed by the desired product (120 °C, 0.01
1
mmHg) as a colorless oil (4.7 g, 16.48 mmol). Yield: 45%. H NMR
(CDCl3): δ 1.32 (d, J = 7.2 Hz, 6H, −CH3), 1.22 (d, J = 7.3 Hz, 6H,
−CH3), 2.08 (hept, J = 7.4 Hz, 2H, −CH), 7.51 (mult 2H, aromatic
CH), 7.32 (mult 3H, aromatic CH). 13C NMR (CDCl3): δ 20.1, 21.9,
22.1, 128.3, 128.5, 136.5, 136.6.
Dimethylphenylantimony (SbMe2Ph). The methylmagnesium
iodide Grignard solution was generated in situ in the same manner as
above with magnesium turnings (4.08 g, 168 mmol) and iodomethane
(19.7 g, 139 mmol). Trichloroantimony (8.45 g, 37.1 mmol) and
triphenylantimony (6.55 g, 18.5 mmol) were stirred together (no
solvent) for 3 h to afford dichlorophenylantimony (15.0 g, 55.6 mmol)
in quantitative yield. The SbCl2Ph was dissolved in Et2O (125 mL) and
added dropwise to the filtered Grignard solution, and the reaction was
refluxed for 3 h. The solution was then quenched with degassed water
(70 mL). Under completely inert atmosphere, the organic layer was
separated, dried over sodium sulfate, and filtered. The solvent was
removed in vacuo to produce a slightly yellow oil. A distillation was
performed to afford the desired product (50 °C, 0.01 mmHg) as a
Herein, we report the synthesis, characterization, and
structural elucidation of a series of nickel iodide complexes
i
derived from homoleptic and heteroleptic SbR3 (R = Pr, Me,
Ph) ligands. DFT was used to calculate the Tolman donor
strength for each antimony ligand in the series. Additionally, the
compounds are tested for the deposition of nickel metal on to
silicon−copper wafers. This work lays fundamental groundwork
regarding the steric and electronic effects of antimony ligands
with late 3d metal ions, and the utility and structure−activity
relationship are developed.
1
colorless oil (8.9 g). Yield: 70%. H NMR (CDCl3): δ 0.98 (s 6H,
−CH3), 7.55 (mult 2H, aromatic CH), 7.32 (mult 3H, aromatic CH).
13C NMR (CDCl3): δ −1.24, 128.3, 128.6, 135.0, 137.5.
Diphenylmethylantimony (SbPh2Me). Methylmagnesium iodide
Grignard solution was generated in situ in the same manner as above
with magnesium turnings (0.77 g, 32 mmol) and iodomethane (3.80 g,
26.9 mmol). Trichloroantimony (1.13 g, 5.33 mmol) and triphenyl-
antimony (3.76 g, 10.7 mmol) were stirred together solvent-free for 3 h
to afford diphenylchloroantimony (5.0 g, 16 mmol) in quantitative
yield. The SbPh2Cl was dissolved in Et2O (75 mL) and added dropwise
to the filtered Grignard solution, and the reaction was refluxed for 3 h.
The yellow solution was then quenched with degassed water (50 mL).
Under completely inert atmosphere, the organic layer was separated,
dried over sodium sulfate, and then filtered. The solvent was removed in
vacuo to produce a slightly yellow oil. A fractional distillation was
performed, which first distilled SbMe2Ph (50 °C, 0.01 mmHg),
EXPERIMENTAL SECTION
■
Reagents and General Procedures. All reactions were conducted
under a dry dinitrogen atmosphere with a Schlenk line (ligand
syntheses) or under a dry argon atmosphere in a drybox (metalations
and depositions). Dry solvents were HPLC-grade and purified over
alumina using a Pure Process Technology solvent purification system.
Deuterated solvents were purchased from Cambridge Laboratories and
used without further purification; the standard freeze−pump−thaw
technique was used to degas deuterated solvents as necessary. The
starting materials trichloroantimony and triphenylantimony were
purchased from Strem Chemicals and used without further purification.
B
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followed by the desired product (80 °C, 0.01 mmHg) as a colorless oil
measurements were obtained using a 500 MHz Bruker AVANCE III
NMR (supported by NSF grant 1 S10 OD021508-01); CDCl3 was
referenced to 7.26 and 77.2 ppm for 1H and 13C NMR spectra,
respectively. Infrared spectra were obtained under air-free conditions in
the glovebox on a Bruker Alpha Fourier transform infrared (FTIR)
spectrometer equipped with a diamond ATR crystal. UV/vis spectra
were measured under air free conditions using an Agilent Technologies
Cary 60 UV/vis spectrometer. All UV/vis measurements were
performed in dry pentane using an air free cuvette. SEM images and
EDX spectra were collected on a FEI Quanta 650 ESEM equipped with
a Bruker EDX system. Powder XRD were collected using a Rigaku R-
Axis Spider diffractometer operating with Cu Kα radiation (λ = 1.5418
Å). Data were collected in the range of 40−93° 2θ. ICP-OES spectra
were collected with a Varian 710-ES optical emission spectrometer.
X-ray Data Collection. For complexes 1, 2, and 5, the X-ray
diffraction data were collected on a Rigaku AFC12 diffractometer with a
Saturn 724+ CCD using a Bruker AXS Apex II detector and a graphite
monochromator with Mo Kα radiation (λ = 0.71073 Å). Low
temperatures were maintained using an Oxford Cryostream low-
temperature device. Data reduction was performed using the Rigaku
Crystal Clear version 1.40.37 Structures were solved by direct methods
using SHELXT38 and refined by full-matrix least-squares on F2 with
anisotropic displacement parameters for the non-H atoms using
SHELXL-2014/7.39 Structure analysis was performed using the
programs PLATON40 and WinGX.41 For complexes 3 and 4, the X-
ray diffraction data were collected at −173 °C on a Nonius Kappa CCD
diffractometer using a Bruker AXS Apex II detector and a graphite
monochromator with Mo Kα radiation (λ = 0.71073 Å). Reduced
temperatures were maintained by use of an Oxford Cryosystems 700
low-temperature device. Data reduction was performed using SAINT
V8.27B.42 The structure was solved by direct methods using
SHELXT38 and refined by full-matrix least-squares on F2 with
anisotropic displacement parameters for the non-H atoms using
SHELXL-2014/7.39 Structure analysis was aided by use of the programs
PLATON40 and WinGX.41
1
(2.0 g, 6.9 mmol). Yield: 43%. H NMR (CDCl3): δ 1.20 (s 3H,
−CH3), 7.50 (mult 4H, aromatic CH), 7.30 (mult 6H, aromatic CH).
13C NMR (CDCl3): δ = 0.62, 128.5, 128.8, 135.6, 138.1.
Synthesis of the Metal Complexes. Ni(I)2(SbiPr3)2 (1). Nickel
iodide (0.62 g, 2.0 mmol) was added to SbiPr3 (1.0 g, 4.0 mmol) in
fluorobenzene at −20 °C under an inert argon atmosphere. The
reaction was allowed to warm to room temperature and stirred
overnight. The violet solution was filtered through Celite, and the
solvent was removed in vacuo to afford a violet solid (0.80 g). Yield:
34%. The solid was redissolved in pentane and placed in a freezer at −20
°C. This yielded violet crystals suitable for X-ray diffraction. IR
spectrum (cm−1): 2929m, 2847s, 1450m, 1379w, 1361m, 1193s, 1144s,
1085w, 987m, 866w, 499s. 1H NMR (CDCl3): δ 1.69 (d, J = 6.5 Hz,
18H, −CH3), 2.92 (mult 3H, −CH). 13C NMR (CDCl3): δ 22.4, 28.6.
Ni(I)2(SbiPr2Ph)2 (2). Nickel iodide (0.27 g, 0.86 mmol) was added to
SbiPr2Ph (0.50 g, 1.8 mmol) in fluorobenzene at −20 °C under an inert
argon atmosphere. The reaction was allowed to warm to room
temperature and stirred overnight. The violet solution was filtered
through Celite, and the solvent was removed in vacuo to yield a violet
oil. The violet oil was redissolved in pentane and placed in a freezer at
−20 °C. This yielded violet crystals suitable for X-ray diffraction (0.15
g). Yield: 19%. IR spectrum (cm−1): 2948m, 2845s, 1572w, 1452m,
1423m, 1378w, 1358w, 1193m, 1143m, 988s, 867w, 720s, 688s, 499s,
1
448s. H NMR (CDCl3): δ 1.57 (d 12H, −CH3), 2.89 (mult 2H,
−CH), 7.30−7.60 (mult 5H, aromatic CH). 13C NMR (CDCl3): δ
21.6, 22.2, 22.5, 129.7 (br, 2 resonances), 139.2 (br, 2 resonances).
Ni(I)2(SbMe2Ph)3 (3). Nickel iodide (0.34 g, 1.1 mmol) was added to
SbMe2Ph (1.0 g, 3.2 mmol) in fluorobenzene at −20 °C under an inert
argon atmosphere. The reaction was allowed to warm to room
temperature and stirred overnight. The violet solution was filtered
through Celite, and the solvent was removed in vacuo. The violet oil was
redissolved in pentane and placed in a freezer at −20 °C. This afforded
1
violet crystals suitable for X-ray diffraction (0.25 g). Yield: 23%. H
NMR (CDCl3): δ 1.34 (s 6H, −CH3), 7.57 (mult 2H, aromatic CH),
7.32 (mult 3H, aromatic CH). 13C NMR (CDCl3): δ 1.28, 128.8, 129.4,
134.8, 135.5.
DFT Calculations. For the four putative Ni tricarbonyl complexes
used to computationally evaluate the Tolman electronic parameter
i.e., (iPr3Sb)Ni(CO)3, (iPr2PhSb)Ni(CO)3, (Me2PhSb)Ni(CO)3, and
(MePh2Sb)Ni(CO)3geometry optimizations were performed using
the Firefly software package43 with the functional of mPW1PW91; the
basis set of 6-31G(d) was used for H, C, and Ni; and TZP for Sb. IR
calculations were accomplished using the same functional and basis
sets. All IR calculations showed no imaginary frequencies. Graphical
manipulations of the predicted spectra were performed with Chem-
Craft,44 and structures were visualized with MacMolPlt.45
Ni(I)2(SbMePh2)3 (4). Nickel iodide (0.11 g, 0.34 mmol) was added
to SbMePh2 (0.30 g, 1.0 mmol) in fluorobenzene at −20 °C under an
inert argon atmosphere. The reaction was allowed to warm to room
temperature and stirred overnight. The violet solution was filtered
through Celite, and the solvent was removed in vacuo. The violet oil was
redissolved in pentane and placed in a freezer at −20 °C. This yielded
1
violet crystals suitable for X-ray diffraction (0.04 g). Yield: 30%. H
NMR (CDCl3): δ 1.23 (s 3H, −CH3), 7.33 (mult 4H, aromatic CH),
7.07 (mult 6H, aromatic CH). 13C NMR (CDCl3): δ 3.47, 128.8, 129.5,
135.8, 136.5.
RESULTS AND DISCUSSION
Ni(I)2(SbiPr3)2CO (5). Complex 1 (0.10 g, 0.12 mmol) was dissolved
in Et2O in a Schlenk tube. The headspace was filled with CO gas, and
the tube was then sealed and allowed to stir for 3 h. The solvent was
removed in vacuo, yielding a violet solid (0.06 g). Yield: 60%. The solid
was redissolved in pentane inside an argon glovebox and placed in a
freezer at −20 °C, which yielded crystals suitable for X-ray diffraction.
IR spectrum (cm−1): 2931m, 2848m, 1963s, 1447m, 1379w, 1360w,
1194s, 1143s, 1082w, 989m, 917w, 866w, 498m, 445w. 1H NMR
(CDCl3): δ 1.59 (d 18H, −CH3), 2.87 (mult 3H, −CH). 13C NMR
(CDCl3): δ 22.7, 27.0, 190.0.
■
Synthesis and Rationale. Three of the ligands used in this
work (SbiPr3, SbMe2Ph, SbMePh2) have been synthesized
previously,46,47 while the heteroleptic SbiPr2Ph is reported here
for the first time. In general, these syntheses utilize Grignard
reagents to displace antimony chloride bonds with the desired
alkyl substituents. The heteroleptic ligands require a fractional
distillation purification, while the homoleptic ligands are
analytically pure following an aqueous wash and can be used
without further purification. These ligands have been used in
several metalations;48−50 however, metal complexes of these
ligands with nickel(II) have not been reported. Initially,
metalations were performed in non-coordinating solvents to
promote stable Sb−Ni bonding and to prevent competition for
solvent binding at the metal site. The reactions also proceeded
when performed in the weakly coordinating solvent, THF, but
could only be crystallized as solvent-free, antimony-bound
complexes from non-coordinating solvents (e.g., FPh, pentane).
In contrast, the reaction of NiI2 with SbPh3 did not proceed in
Et2O, FPh or toluene. However, a reaction did occur in THF,
yielding a dark yellow solution. But removal of solvent afforded a
General Deposition Procedure. Copper-coated silicon wafers
(100 nm PVD Cu on 750 μm Si substrate) were provided by Lam
Research and used as the deposition substrates for these experiments.
The wafers were cut into a 1 cm × 1 cm square pieces and washed with
acetone prior to deposition. The metal complex (0.08 g, 0.09 mmol)
was dissolved in 15 mL of fluorobenzene and placed in a pressure vessel
with the Si|Cu wafer. The solution was heated to 80 °C until the violet
color of the solution had fully disappeared (2−4 days). The colorless
solution was removed, along with a non-adherent black precipitate. The
wafer was washed with fluorobenzene and pentane and then removed
from the glovebox for analysis.
Physical Measurements. The 1H NMR measurements were
obtained using a 400 MHz Varian spectrometer, and the 13C NMR
C
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turquoise solid that was insoluble in any non-coordinating
solvents, unlike all cases with alkyl-substituted antimony ligands.
A summary of the synthetic preparations of the ligands and
metal complexes is shown in Schemes 1 and 2, respectively.
Scheme 1. Preparation of Ligands
Figure 1. ORTEP diagrams (50% thermal ellipsoids) for Ni(I)2-
(SbiPr3)2 (1, left) and Ni(I)2(SbiPr2Ph)2 (2, right).
as [V(DIPP)4{Li(THF)}2] (DIPP = diisopropylphenolate) and
(Ph3P)2Ni(o-Tol)(ISQ-Me) (ISQ = 4,6-di-tert-butyl-N-(2,6-
dimethylphenyl)-o-iminobenzosemiquinonate.52,53 However,
complex 1 co-crystallizes with another molecule of Ni(I)2-
(SbiPr3)2 in the asymmetric unit cell, which is nearly entirely
planar (Supporting Information, Figure S19). When averaging
the bond distances and bond angles for these two molecules, the
aforementioned structural deviations between 1 and 2 (2.50(1)
vs 2.487(2) Å and 177(3) vs 179.7(2)° for Ni−Sb bond distance
and Sb−Ni−Sb bond angles, respectively) become moot.
Ni(I)2(SbMe2Ph)3 (3) and Ni(I)2(SbMePh2)3 (4). Figure 2
shows the crystal structures of Ni(I)2(SbMe2Ph)3 (3) and
Scheme 2. Preparation of Metal Complexes 1−5
Figure 2. ORTEP diagrams (30% thermal ellipsoids) for Ni(I)2-
(SbMe2Ph)3 (3, left) and Ni(I)2(SbMePh2)3 (4, right).
Ni(I)2(SbMePh2)3 (4). Ni(I)2(SbMe2Ph)3 has a trigonal
bipyramidal (TBP) geometry with a Ni−Sb average bond
length of 2.44(1) Å. The average Sbax−Ni−Sbax bond angle is
171(1)°, and the average Sbeq−Ni−Sbax is 94.9(9)°. Interest-
ingly, the TBP geometry accommodates the three smaller
SbMe2Ph ligands (cone angle = 119°) around the nickel(II)
center, thus expanding the coordination number from four to
five (square planar to TBP) due to the decreased steric
constraints as compared with the larger isopropyl-based ligands.
Binding of five total ligands allows access to the presumably
more stable 18-electron species. In a similar vein, the reaction
between nickel(II) iodide and diphenylmethylantimony (cone
angle = 137°) yielded another TBP complex, 4. The average
Ni−Sb bond length for complex 4 was determined to be 2.45(1)
Å. The Sbax−Ni−Sbax bond angle is 174.1(1)°, and the average
Sbeq−Ni−Sbax bond angle is 92.4(6)°, affording a slightly
distorted TBP geometry similar to 3. As with the comparison
between the two isopropyl-based complexes, closer inspection
of the averaged Ni−Sb bond lengths (2.44(1) vs 2.45(1) Å) and
bond angles (171(1) vs 174.1(1)° for Sbax−Ni−Sbax and
X-ray Structures and Steric Ligand Substituent Effects.
Ni(I)2(SbiPr3)2 (1) and Ni(I)2(SbiPr2Ph)2 (2). Figure 1 shows the
crystal structures of Ni(I)2(SbiPr3)2 (1) and Ni(I)2(SbiPr2Ph)2
(2). Complex 1 has slightly distorted square planar geometry,
and it has an average Ni−Sb bond length of 2.50(1) Å and a Sb−
Ni−Sb bond angle of 177(3)°. The ligand cone angle for SbiPr3
was calculated to be 155°.51
Complex 2 exhibits a more ideal square planar geometry than
1, with a Sb−Ni−Sb bond angle of 179.7(2)°. The Ni−Sb
average bond distance is 2.487(2) Å. The ligand cone angle for
diisopropylphenylantimony was calculated to be 147°. Complex
1 is slightly distorted, likely due to the increased steric bulk
imposed by the SbiPr3 ligand (155°) compared to the smaller
SbiPr2Ph ligand (147°). This phenomenon has been shown in
several cases of square planar complexes with bulky ligands, such
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94.9(9) vs 92.4(6)° for Sbeq−Ni−Sbax) between 3 and 4 reveal
they are structurally similar.
The relationships between ligand cone angle, complex
geometry, and Ni−Sb bond length are summarized in Table 1.
Table 1. Comparison between Ligand Cone Angle, Metal-to-
Ligand Bond Length, and Metal Complex Geometry
ligand cone
angle (deg)
PBV
(%)
Ni−Sb bond
complex
geometry
ligand
length (Å)
SbiPr3
SbiPr2Ph
SbMePh2
155
147
137
27.5
26.9
25.8
2.50(1)
2.487(2)
2.45(1)
square planar
square planar
trigonal
bipyramidal
Figure 3. ORTEP diagram (50% thermal ellipsoids) for Ni(I)2-
(SbiPr3)2CO (5).
SbMe2Ph
119
24.0
2.44(1)
trigonal
bipyramidal
An additional parameter that can be used to classify these ligands
is “percent buried volume” (PBV), which has been suggested to
be a more accurate representation of steric hindrance of organic
substituents in metal coordination complexes.5 PBV is defined as
the fraction of a sphere originating from the metal (first
coordination sphere) that a given ligand occupies.54 Although
originally developed for NHC ligands, it has recently been used
to quantify steric hindrance in bulky phosphine ligands and has
scaled well with Tolman cone angles.55,56 The PBVs were
calculated using the online web application SambVca developed
by Cavallo et al.57 As evident in Table 1, the increase in ligand
cone angle is directly correlated to the PBV. The average Ni−Sb
bond length appears to be split into two groups: isopropyl- and
methyl-based ligands. The presence or number of phenyl
substituents bound to the central antimony atom has no
empirical effect on the Ni−Sb bond length, once standard
deviations are accounted for. However, the isopropyl-based
ligands with a Ni−Sb bond length of ∼2.50 Å adopt a square
planar geometry to account for the larger size of the ligands. The
methyl-based ligands with an average Ni−Sb bond length of
∼2.45 Å conform to TBP geometry. There appears to exist a
“cutoff” in both the Ni−Sb average bond length (>2.45 Å) and
the ligand cone angle parameters (>137°) at which these
nickel−antimony complexes revert to square planar geometry
from TBP, although further investigation of stibine ligands of
varying sizes between SbMePh2 and SbiPr2Ph must be
performed to reinforce this claim. Whether the ligand has
electron-withdrawing or electron-donating organic substituents
appears to have little effect on the bond lengths or geometries in
the complexes: it is exclusively a steric effect.
Figure 4. IR spectra of 1 (top) and 5 (bottom) showing prominent CO
peak (1966 cm−1).
(Figures S24 and S25), and the one CO peak is persistent in
solution.
DFT Calculations and Tolman Electronic Parameters.
To further understand the electronic contributions of the
antimony ligands to the geometry and properties of the
complexes, DFT calculations were performed to determine
their relative donor strengths, in a manner similar to the original
Tolman electronic parameter developed for phosphine
ligands.33 We simulated the structures of four model complexes
with general formula Ni(L)(CO)3 (L = SbiPr3, SbiPr2Ph,
SbMe2Ph, SbMePh2) and calculated the carbonyl stretching
frequencies for each complex (Table 2 and Table S2). The most
representative feature (the A1 carbonyl stretch) for each
corresponding complex is listed in Table 2.
On this basis, the electronic donor properties of our ligands
can be quantified, with SbiPr3 representing the most strongly
donating ligand (2061 cm−1) and SbMe2Ph being the weakest
ligand (2070 cm−1). This is somewhat expected, as the
corresponding phosphine analogues follow a similar trend
both experimentally and computationally.33,61 The isopropyl
groups are more electron-donating than the corresponding
Ni(I)2(SbiPr3)2CO (5). Although nickel(II) can only coordinate
two larger antimony ligands (SbiPr3, SbiPr2Ph) in square planar
geometry, these complexes can expand to the TBP geometry by
accommodating a fifth smaller ligand, such as CO. This has been
observed with nickel phosphine complexes,58−60 as well, and
shows thatdespite its weak σ-donating strengththe CO
ligand will complete the coordination sphere without displacing
either antimony ligand. Subjecting the square planar complex 1
to a CO atmosphere for a short amount of time affords a violet
solid. The complex is TBP geometry. Figure 3 shows the crystal
structure of Ni(I)2(SbiPr3)2CO (5), the result of the reaction of
complex 1 with CO(g).
The IR spectrum of complex 5 is shown alongside the IR
spectrum of 1 in Figure 4, and it exhibits one strong CO feature
at 1966 cm−1. Other than the presence of metal-bound CO
feature, the IR of 5 is nearly identical to the IR spectrum of 1.
Solution IR spectra were also collected for complexes 1 and 5
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Table 2. DFT-Calculated A1 Carbonyl Stretching
Frequencies for Ni(L)(CO)3 (L = SbiPr3, SbiPr2Ph, SbMe2Ph,
a
SbMePh2) Determined Using the Functional mPW1PW91
Sb ligand
ν(CO) (cm−1
)
SbiPr3
2060.8
2063.9
2069.9
2067.5
SbiPr2Ph
SbMe2Ph
SbMePh2
a
The basis set TZP was used on Sb, and 6-31G on the remaining
elements.
methyl groups, and the phenyl groups serve as electron-
withdrawing substituents. In that regard, it was expected that
SbMe2Ph would be a stronger donor than SbMePh2; however,
the opposite result was calculated by DFT. In related
(experimental) phosphine work by Dartiguenave with identical
R-group substituents,59 the ν(CO) values were obtained for the
TBP series of complexes analogous to those studied
experimentally hereinnamely of general formula Ni(I)2-
(PR1R2 )(CO), where PR1R2 = PMe2Ph, PMePh2. Notably,
our DFT-predicted substituent trend held true: the PMe2Ph
ν(CO) value was observed at 2022 cm−1, while the PMePh2
ν(CO) feature was observed at slightly lower energy, 2018 cm−1.
However, the 2−4 cm−1 difference between the EMe2Ph and
EMePh2 (E = P, Sb) cases is too small to definitively declare one
ligand stronger or weaker. Inspection of the A1 stretching
frequencies for the analogous phosphine complexes with general
structure Ni(CO)3L revealed the following values: PiPr3 = 2059
1.4 cm−1, PiPr2Ph = 2066 1.7 cm−1, PMe2Ph = 2067 2.7
cm−1, PMePh2 = 2068 1.7 cm−1.62 Thus, in conjunction with
existing tabulated values,33,63 the collective data suggest that in
the case of both antimony and phosphorus bound to nickel−
carbonyl moieties, the methyl substituent is electronically
equivalent to phenyl. These results can serve as a useful
reference for the design of future antimony-based ligands in
transition metal complexes.
Electronic Absorbance Spectra. The three unique
coordination geometries (square planar, TBP-Sb3, and TBP-
Sb2CO) of the complexes were analyzed for their electronic
absorption spectra to further characterize these complexes. Low-
spin TBP Ni(II) complexes such as [NiI{1,2-C6H4-
(CH2SbMe2)2}2](ClO4) and [Ni(TAP)Br]+ (TAP = tris[3-
dimethylarsinopropyl]phosphine) typically display character-
istic absorption features near 17 000 and 22 000 cm−1,17,64 while
related square planar Ni(II) compounds such as Ni(PEt3)2Br2
and Ni(2,5-DMP)Br2 (2,5-DMP = 2,5-dimethylpyrazine)
exhibit a single intense band within the broad range of
16 000−22 000 cm−1, as well as a second band between
23 000 and 30 000 cm−1.65 The UV/vis absorption spectrum
of 1 in pentane (Figure 5) exhibits an intense band at 17 860
cm−1 (ε = 4330 M−1 cm−1) and a less intense band at 26 180
cm−1 (ε = 1750 M−1 cm−1). We assign the 17 860 cm−1
absorption as the [b2g(dxy)→b1g(dx2−y2)] transition and the
26 180 cm−1 absorption as the [a1g(dz2) → b1g(dx2−y2)]
transition.
Figure 5. UV/vis absorption spectrum of a solution of 1 in pentane at
298 K.
than the lower energy absorption, which encompasses the higher
energies of the visible spectrum.65 We assign the ligand field
transitions as 1A2g←1A1g (17 860 cm−1) and 1B1g←1A1g (26 180
cm−1). These energies are at the low end of the range for the two
bands as previously described, which is indicative of the ligand
field strength;65 the presence of relatively weak SbR3 and I−
ligands is thus responsible for the lower energies of the
absorption bands
Figure 6 shows the UV/vis absorption spectra of 3 (TBP, no
CO) and 5 (TBP, with CO) in pentane. Complex 3 exhibits an
intense band at 19 120 cm−1 (ε = 3 135 M−1 cm−1) and a weaker
band at 26 390 cm−1 (ε = 2350 M−1 cm−1). From Lever, this is
very typical of a low-spin Ni(II) d8 system.65 The ground state
configuration of the diamagnetic TBP complexes is (e″)4(e′)4.
Thus, the two excitations correspond to the [(e″)4(e′)4→
(e″)4(e′)3(a1′)] transition (19 120 cm−1) and the [(e″)4(e′)4→
(e″)3(e′)4(a1′)] transition (26 390 cm−1). We assign these
transitions, respectively, as 1E′←1A′1 (19 120 cm−1) and
1E″←1A′1 (26 390 cm−1). Complex 5 displays an intense band
at 17 760 cm−1 (ε = 3200 M−1 cm−1) and a less intense band at
26 180 cm−1 (ε = 2230 M−1 cm−1). As with complex 3, we assign
the larger absorption feature to the [(e″)4(e′)4→
(e″)4(e′)3(a1′)] transition and the less intense band to the
[(e″)4(e′)4→(e″)3(e′)4(a1′)] transition. Similarly, these tran-
sitions are assigned as 1E′←1A′1 (17 760 cm−1) and 1E″←1A′1
(26 180 cm−1).
From a comparison of the UV/vis absorption spectra for 3 and
5 to known high-spin d8 complexes, it is reasonably evident that 3
and 5 are low-spin. High-spin d8 complexes exhibit multiple
absorptions in the red region of the visible region with low molar
intensities (ε ≈ 20−50 M−1 cm−1), as well as a more intense
blue-shifted band (ε ≈ 1000 M−1 cm−1). These features are quite
distinct from four-coordinate and low-spin five-coordinate
nickel complexes, further supporting the low-spin state for all
of the complexes herein.65,67,68
It is noted that NIR absorption spectra were also obtained for
each complex to detect the presence of lower energy transitions.
However, no metal-based absorption features were observed
below 10 000 cm−1 (compared with pentane solutions of ligand
only). This is not entirely unexpected, as the large crystal field
splitting between the HOMO and LUMO in both square planar
and TBP complexeseven with heavy atom ligationshould
be greater than 10 000 cm−1.65
2
2
Although the violet color of the complex is not uncommon
among square planar derivatives of nickel(II), the corresponding
phosphine complexes with general formula Ni(PR3)2(X)2 are
generally red in color.66 The violet colored solution in the
present case is explained by the relatively low intensity of the
absorption feature at 26 180 cm−1. The higher energy band in
Ni(II) square planar complexes is typically a more intense band
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The SEM image reveals a homogeneous layer that consists of
nickel and copper, per the EDX spectrum. It is important to note
that the surface is devoid of iodine/iodide, which could be
expected from the iodide in the starting complex. The surface
was analyzed with powder XRD (Figure 7, right), which revealed
significant presence of iodine or CuI and shows four main
features. The 2θ values for these features reside in between the
textbook 2θ values for Cu and Ni metal, suggesting an alloy
composition. The surface composition was further probed with
ICP-OES, which found the mol ratio of Ni:Cu as 3.25:1, or as
the alloy Ni0.77Cu0.23
.
The Si|CuNi wafer resulting from the deposition of 1 was
subjected to cross-sectional SEM analysis to provide the surface
thickness and composition (Figure 8). The crystals are
Figure 6. UV/vis absorption spectra of pentane solutions of 3 (top) and
5 (bottom) obtained at 298 K.
Figure 8. SEM images (top) and EDX mapping images for Cu (bottom
left) and Ni (bottom right) for the cross-sectional view of a sliced wafer
after the deposition reaction of Ni(I)2(SbiPr3)2 (1). Top right image
and bottom images were obtained from the same area on the wafer.
Nickel Metal Deposition. Empirical Depositions. We
tested our metal complexes for nickel deposition on silicon
wafers coated with 100 nm of copper. We were motivated to
attempt these reactions after observing loss of violet color and
the presence of a fine black powder when our complexes were
subjected to heating. The complexes were heated at 80 °C for 3
days in a pressure vessel containing the acetone-washed
substrate. The SEM image and EDX scan results for the
deposition of 1 (L = SbiPr3) can be seen in Figure 7.
composed of both Cu and Ni and range from 10 to 20 μm
thick. These data corroborate the powder XRD and ICP-OES
regarding the alloyed nature of the deposited metal.
Complexes 2, 3, and 5 (L = SbiPr2Ph, SbMe2Ph, and SbiPr3 +
CO, respectively) were also tested for nickel deposition under
identical conditions. While some extent of nickel deposition was
observed as with 1, a large amount of iodine was also deposited
(SEM, EDX for 2 and 3, Figure 9; for 5, Figure S26). The surface
Figure 7. SEM image (left), EDX spectrum (center), and powder XRD spectrum (right) for the deposition of Ni(I)2(SbiPr3)2 (1) onto a Si|Cu wafer.
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Figure 9. SEM image (top left) and EDX spectrum (top right) of the
deposition results from Ni(I)2(SbiPr2Ph)2 (2) on to a Cu−Si wafer.
SEM image (bottom left), EDX spectrum (bottom right) of the
deposition results from Ni(I)2(SbMe2Ph)3 (3).
composition of these iodinated surfaces was probed via powder
XRD, and it was determined that the crystalline units were
copper iodide (Figure S27).
Deposition Mechanism. In attempts to remove iodine
deposition when using 2, 3, and 5, we used Tl(BArF ) to
4
Figure 10. EDX spectra of the thallium-treated deposition reactions of
Ni(I)2(SbiPr3)2 (1) (bottom), Ni(I)2(SbMe2Ph)3 (3) (middle), and
Ni(I)2(SbiPr3)2(CO) (5) (top).
abstract the iodide ions, followed by the standard deposition
protocol (80 °C, 3 days). Figure 10 shows the EDX spectra of
the deposition results from the thallium treated reactions of
complexes 1, 3, and 5.
Scheme 3. Proposed Mechanism for Nickel Metal Deposition
from 1
It was found that the thallium treatment moderately increased
the amount of nickel deposited in the case of 3. However, the
iodine remained evident in the EDX spectrum. Interestingly, for
1 and 5 the amount of nickel deposited decreased when
compared with the non-thallium-treated reactions. In the case of
1, the EDX presence of iodine was unexpected in the thallium-
treated reaction as there was no iodine observed in the
deposition of 1 without thallium treatment. The SEM image
of the surface of complex 1 + Tl shows the presence of CuI which
then renders the results similar to those for 2, 3, and 5 (Figure
S28). Although unexpected from a chemical perspective, this
result does reveal evidence for a potential mechanism of
deposition (Scheme 3).
1
which is soluble in fluorobenzene. The H NMR for the
deposition product does indeed show the presence of Sb-
(iPr)3(I)2, which was compared to a sample of independently
synthesized Sb(iPr)3(I)2 [synthesized from the addition of
ethereal iodine to a solution of SbiPr3 under nitrogen] (see
Figures S29 and S30). The CI mass spectra for both the
deposition product and the synthesized Sb(iPr)3(I)2 are nearly
identical (Figure 11) and correspond to the theoretical
ionization pattern of [Sb(iPr)3(I)]+ (Figure S31).
It is critical to note that the deposition results accurately
reflect the computational work used to determine the Tolman
electronic parameter of each of the ligands. The strongest σ-
donating ligand, SbiPr3, was best able to scavenge I2 from
solution [→Sb(iPr3)(I)2] and prevent the formation of CuI, thus
allowing for the cleanest deposition of nickel metal (ultimately
as NiCu alloy) on the Si|Cu wafers. In contrast, the weakest
donating and therefore least reactive I2 acceptor (SbMe2Ph)
showed minimal efficacy in removing I−/I2, and the resulting
surface exhibited a significant extent of CuI. It can be inferred
from our data that in order to exclusively deposit nickel metal
desirable for any industrial or semiconductor deposition
We postulate that at higher temperatures, the ligand(s)
dissociate from the Ni2+ center, lowering the reduction potential
and rendering the Ni2+ susceptible to reduction by the bound I−
ions. The resulting Ni0 is suitable for nucleation and deposition;
the stoichiometric and oxidized byproduct (I2) then reacts with
the I2 acceptor SbIII(iPr3) (strongest σ donor), thus forming
SbV(iPr3)(I)2. This scheme appears plausible, as antimony has
been shown to be an excellent Lewis acid, especially in the case
of halides.69,70 For example, Gabbaı has demonstrated that
̈
fluoride will preferably bind to a stibine ligand over a metal
center in both Pt and Au complexes.71,72 The balanced reaction
and proposed mechanism for iodine scavenging by SbiPr3 are
delineated in Scheme 3. However, the exact order and synergism
between the elementary steps is not addressed in the present
work.
To test this hypothesis, we analyzed the filtrate after a
deposition procedure using 1 for the presence of Sb(iPr)3(I)2,
H
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a CO ligandhinder the deposition process. The use of even
more strongly donating antimony ligands, such as Sb(tBu)3 or
Sb(Cy)3, bound to nickel(II) may result in cleaner depositions,
milder conditions, and shorter reaction times for deposition on
the Si|Cu wafers.
CONCLUSION
The findings of this work are summarized as follows:
■
(1) A range of synthetic, alkyl-substituted antimony ligands of
the homoleptic (R = iPr, Me) and heteroleptic (R = iPr,
Me, Ph) variety stably bind NiI2 in non-coordinating
solvents to form complexes that are square planar (two Sb
ligands) or trigonal bipyramidal (three Sb ligands).
(2) The steric size of these antimony R-groupsas
determined by “classical” Tolman cone angle (119−
155°), and the newly calculated percent buried volume
(PBV = 24.0−27.5%)is an effective determinant of the
resulting geometry of the complexes (trigonal bipyramidal
<26% PBV [140°] < square planar).
(3) Deposition of a clean NiCu alloy using mild conditions in
organic solvent on a Si|Cu substrate is possible with a
nickel−antimony complex that has strong enough σ-
donating ligands (i.e., SbiPr3) to scavenge I−/I2 from
solution. The use of weaker σ-donating ligands (e.g.,
SbMe2Ph) promotes undesired deposition of iodine-
containing species (CuI).
(4) More broadly, this work refutes the dogma that antimony
can be categorically described as a poor or ineffective σ
donor for divalent 3d metals, and this report illustrates
that judicious selection of alkyl substituents can greatly
enhance the practical binding ability of antimony(III)
ligands.
ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b01565.
■
S
1H and 13C NMR spectra of ligands and metal complexes;
X-ray diffraction details; full crystal structures of
complexes 1−5; crystallographic data and refinement
parameters of complexes 1−5; DFT experimental details;
theoretical bond distances and CO stretching frequencies
for simulated complexes; solution phase IR spectra; nickel
deposition SEM images; and powder XRD spectra,
including Figures S1−S31 and Tables S1 and S2 (PDF)
Accession Codes
CCDC 1590146−1590150 contain the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge via www.ccdc.cam.ac.uk/data_request/cif, or by email-
ing data_request@ccdc.cam.ac.uk, or by contacting The Cam-
bridge Crystallographic Data Centre, 12 Union Road, Cam-
bridge CB2 1EZ, UK; fax: +44 1223 336033.
Figure 11. CI mass spectrum of the filtrate following nickel metal
deposition from 1 (A) and of the independently synthesized
Sb(iPr3)(I)2 (B). The main feature at 377/379 m/z is [Sb(iPr3)I]+,
which results from the loss of I− during the ionization and injection
process. The lower spectra in both images are the expanded views in the
m/z region of interest.
AUTHOR INFORMATION
Corresponding Author
*E-mail: mrose@cm.utexas.edu.
■
ORCID
Michael J. Rose: 0000-0002-6960-6639
processa strongly donating antimony ligand bound to the
Notes
nickel is required. Any extraneous factorssuch as Tl(BArF ) or
The authors declare no competing financial interest.
4
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Examples of Neutral and Cationic Indenyl Nickel(II) Complexes
Bearing Arsine or Stibine Ligands: Highly Active Catalysts for the
Oligomerisation of Styrene. Dalton Trans. 2015, 44, 17015−17019.
(17) Levason, W.; Matthews, M. L.; Reid, G.; Webster, M. Synthesis
and Characterisation of Transition Metal Halide Complexes of the
Xylyl-Distibine, 1,2-Bis(dimethylstibanylmethyl)benzene. Dalton
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ACKNOWLEDGMENTS
■
We acknowledge financial support from the Robert A. Welch
Foundation (F-1822), Research Corporation for Scientific
Advancement (Cottrell Scholar Award, RCSA 23640), the
American Chemical Society (PRF 53542-DN13), and the UT
Austin College of Natural Sciences. We gratefully acknowledge
Lam Research for providing the copper-coated silicon
substrates.
́
(18) Jimenez-Tenorio, M.; Puerta, M. C.; Salcedo, I.; Valerga, P.; de
los Ríos, I.; Mereiter, K. Oligomerization of Styrenes Mediated by
Cationic Allyl Nickel Complexes Containing Triphenylstibine or
Triphenylarsine. Dalton Trans. 2009, 1842−1852.
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