Synthesis and electrochemistry of late transition metal complexes containing 1,1′-bis(dicyclohexylphosphino)ferrocene (dcpf). The X-ray structure of [PdCl2(dcpf)] and Buchwald-Hartwig catalysis using [PdCl2(bisphosphinometallocene)] precursors

Article abstract of DOI:10.1016/j.jorganchem.2006.08.030

The electrochemistry of 1,1′-bis(dicyclohexylylphosphino)ferrocene (dcpf) was examined in methylene chloride with tetrabutylammonium hexafluorophosphate or tetrabutylammonium tetrakis(pentafluorophenyl)borate as the supporting electrolyte. The oxidation of dcpf is complicated by a follow-up reaction. Seven new complexes containing dcpf and one new compound containing 1,1′-bis(di-tert-butylphosphino)ferrocene (dtbpf) were prepared and characterized. The new complexes were analyzed by cyclic voltammetry and the oxidation of these complexes occurred at a more positive potential than the free ligand. In addition, the X-ray structure of [PdCl₂(dcpf)] was determined and compared to other palladium complexes containing bisphosphinometallocene ligands. Five different palladium complexes containing bisphosphinometallocene ligands were examined as catalyst precursors in Buchwald-Hartwig catalysis.

Full text of DOI:10.1016/j.jorganchem.2006.08.030

Journal of Organometallic Chemistry 691 (2006) 4890–4900
www.elsevier.com/locate/jorganchem
Synthesis and electrochemistry of late transition metal complexes
containing 1,10-bis(dicyclohexylphosphino)ferrocene (dcpf). The
X-ray structure of [PdCl2(dcpf)] and Buchwald–Hartwig catalysis
using [PdCl2(bisphosphinometallocene)] precursors
a
b,2
Laura E. Hagopian , Alison N. Campbell a,1, James A. Golen
,
b
a,
*
Arnold L. Rheingold , Chip Nataro
a
Department of Chemistry, Lafayette College, Hugel Science Center, Easton, PA 18042, USA
Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, CA 92093, USA
b
Received 3 July 2006; received in revised form 14 August 2006; accepted 15 August 2006
Available online 22 August 2006
Abstract
The electrochemistry of 1,10-bis(dicyclohexylylphosphino)ferrocene (dcpf) was examined in methylene chloride with tetrabutylam-
monium hexafluorophosphate or tetrabutylammonium tetrakis(pentafluorophenyl)borate as the supporting electrolyte. The oxidation
of dcpf is complicated by a follow-up reaction. Seven new complexes containing dcpf and one new compound containing 1,10-bis(di-
tert-butylphosphino)ferrocene (dtbpf) were prepared and characterized. The new complexes were analyzed by cyclic voltammetry and
the oxidation of these complexes occurred at a more positive potential than the free ligand. In addition, the X-ray structure of
[PdCl2(dcpf)] was determined and compared to other palladium complexes containing bisphosphinometallocene ligands. Five different
palladium complexes containing bisphosphinometallocene ligands were examined as catalyst precursors in Buchwald–Hartwig
catalysis.
ꢀ 2006 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry; Cyclic voltammetry; Crystal structure; 1,10-Bis(dicyclohexylphosphino)ferrocene; 1,10-Bis(diphenylphosphino)ferrocene;
Buchwald–Hartwig catalysis
1. Introduction
twig reaction, in which aryl amines (or aryl ethers) are
formed using a palladium (0) catalyst containing chelating
ligands, is of particular interest. The use of chelating
ligands has been shown to improve the synthesis of arylam-
ines by inhibiting competing reactions [3].
Bidentate phosphines, in particular those with a metallo-
cene backbone, are one type of chelating ligands that have
been investigated as ligands in Buchwald–Hartwig catalysis
due to their unique steric and electronic properties. The
most widely studied of these chelating phosphines is 1,10-
bis(diphenylphosphino)ferrocene (dppf) which has been
shown to have numerous advantages as compared to 1,2-
bis(diphenylphosphino)ethane (dppe) and 1,3-bis(diph-
enylphosphino)propane (dppp), which contain alkyl, rather
than metallocene, backbones. For example, dppe and dppp
Palladium catalysts have become an important means of
facilitating the formation of a variety of different bonds,
including carbon–nitrogen [1–4], carbon–carbon [5–11],
carbon–oxygen [12–14] and carbon–sulfur bonds [15,16].
Carbon–nitrogen bond formation via the Buchwald–Har-
*
Corresponding author. Fax: +1 610 330 5714.
E-mail address: nataroc@lafayette.edu (C. Nataro).
Present address: Department of Chemistry, The University of North
1
Carolina at Chapel Hill, Chapel Hill, NC 27599, USA.
2
Permanent address: Department of Chemistry and Biochemistry,
University of Massachusetts, Dartmouth, North Dartmouth, MA 02747,
USA.
0022-328X/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2006.08.030

L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
4891
R
R
Examining a variety of palladium catalyst precursors
containing 1,10-bis(phosphino)metallocene ligands will
result in a better understanding of how the sterics and elec-
tronics of these ligands affect catalytic efficiency. There is a
limited understanding of how the steric and electronic
properties of dcpf differ from those of other 1,10-bis(diph-
osphino)metallocene ligands. To better understand the
electronic nature of dcpf, the electrochemistry of dcpf
and several new compounds containing dcpf was investi-
gated. The X-ray structure of [PdCl2(dcpf)] is presented
and compared to analogous 1,10-bis(phosphino)metallo-
cene palladium dichloride complexes in order to examine
the steric nature of dcpf. Finally, catalysis of the Buch-
wald–Hartwig reaction using five different [PdCl2(PÆP)]
(PÆP = dppf, dppr, dippf, dcpf and dtbpf) complexes as cat-
alyst precursors, three different aryl bromides and four dif-
ferent amines was examined.
Pd Catalyst
+
H2NR'
R'
Br
N
H
R = Bu, NMe2
R' = Bu, i-Bu, Ph
Scheme 1. Model reactions studied by Hamann and Hartwig.
undergo decomposition reactions and produce no aryl-
amine products when used as ligands in a Buchwald–Har-
twig coupling reaction [8]. However, using dppf as a ligand
leads to intermediates which readily produce arylamines in
the Buchwald–Hartwig reaction [17]. In fact, [PdCl2(dppf)]
has been reported to be one of the most efficient catalyst
precursors for coupling primary amines and aryl halides
[18]. Mechanistic studies of the Buchwald–Hartwig reac-
tion have suggested that the steric nature, specifically the
P–Pd–P bite angle, and electronic nature of the ligands play
a role in the efficiency of the catalyst [2] (Scheme 1).
Although there have been several catalytic studies
employing dppf, few have examined how changing the sub-
stituents on the phosphorus atoms of 1,10-bis(phos-
phino)metallocene affects the catalytic activity. The study
employing the largest variety of 1,10-bis(phosphino)metal-
locene ligands focused on dppf derivatives in which there
were different substituents added to the aromatic ring
[2]. Numerous other 1,10-bis(diorganophosphino)metallo-
cene ligands and dichloropalladium complexes containing
these ligands are known. The most commonly used deri-
vatives of dppf are 1,10-bis(diphenylphosphino)ruthenocene
(dppr), 1,10-bis(diisopropylphosphino)ferrocene (dippf),
1,10-bis(dicyclohexylphosphino)ferrocene (dcpf) and 1,10-
bis(di-tert-butylphosphino)ferrocene (dtbpf). Palladium
dichloride compounds of several 1,10-bis(phosphino)metal-
locene ligands have been effectively used as catalyst precur-
sors in a variety of catalytic systems such as the synthesis of
oxindoles by amide a-arylation [19] and the arylation of
ketones and malonates [20].
Of the previously mentioned 1,10-bis(phosphino)metal-
locene ligands, dcpf and dtbpf have received the least atten-
tion. Most of the research focusing on dcpf involves Pd(II)
derivatives used for catalysis [2–4]. The only study that
directly compared dcpf and dppf containing catalyst pre-
cursors was for the alkoxycarbonylation of aryl chlorides
and dcpf was found to be a better ligand [2]. Studies using
dtbpf as a ligand have also been centered on palladium cat-
alysts [4–15]. The bulkiness of dtbpf was proposed to be the
key to its reactivity in catalytic applications [6,7,13–16].
For example, in the synthesis of oxindoles by amide a-ary-
lation, the catalyst containing dtbpf was found to have sim-
ilar catalytic abilities as the catalyst containing dcpf [4]. It
was also determined that dtbpf was a better ligand than
dppf in the reductive elimination of diaryl ethers [13], but
it was a poorer ligand than dppf for hydrogenation reac-
tions [16].
2. Experimental
2.1. General procedures
All reactions were completed under an argon atmo-
sphere using Schlenk methods. Reagent-grade isopropanol,
chloroform, ethanol, and methanol were used without
additional purification. Tetrahydrofuran (THF) and
diethyl ether (Et2O) were distilled under a nitrogen atmo-
sphere and refluxed over potassium benzophenone ketyl.
Methylene chloride (CH2Cl2) and hexanes were distilled
over CaH2 under a nitrogen atmosphere. Electrochemistry
was performed using HPLC grade CH2Cl2 that was
distilled over CaH2 under an argon atmosphere. NMR
spectra were obtained on a JEOL Eclipse 400 FT-NMR
1
in CDCl3. TMS was used as an internal reference for H
NMR while 85% H3PO4 served as an external standard
for 31P{1H} NMR. Analyses were carried out by
Quantitative Technologies, Inc. UV–Vis experiments were
conducted on a Beckman DU-65 spectrophotometer.
Ferrocene, decamethylferrocene, ruthenocene, chloro-
dicyclohexylphosphine, bis(benzonitrile)dichloropalladium,
dppf, dippf and dtbpf were purchased from Strem. Ferro-
cene was sublimed prior to use. NiCl2 Æ 6H2O, PtCl2-
(CH3CN)2, ZnCl2, CdCl2, HgCl2, tetrabutylammonium
hexafluorophosphate ([NBu4][PF6]), cyclohexylamine,
hexylamine, aniline, 2,6-dimethylaniline, bromobenzene, 4-
bromotoluene, 4-bromotrifluorotoluene, potassium tert-
butoxide, and decane were purchased from Aldrich. The
[NBu4][PF6] was dried under vacuum with heating prior to
use. The following compounds were prepared according to
the literature procedures: dppr [21], dcpf [22], [PdCl2(dppf)]
[23], [PdCl2(dppr)] [21], [PdCl2(dippf)] [24], and
[PdCl2(dtbpf)] [25]. HAuCl4 Æ H2O was purchased from
Fisher. Li[B(C6F5)4] Æ (OEt2)2.5 was obtained from Boulder
Scientific and metathesized to ([NBu4]+ [B(C6F5)4]ꢀ) in
accordance with the literature [26].

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L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
2.2. Synthesis
to yield 4 as a yellow solid (0.0594 g, 18%). Anal. Calc. for
C34H52Cl2FeP2Au2: C, 39.14; H, 5.02. Found: 39.40,
1
2.2.1. [NiCl2dcpf)] (1)
5.24%. 31P{1H} (CDCl3): d 41.2 (s). H NMR (CDCl3): d
A solution of dcpf (0.0984 g, 0.170 mmol) in 30 mL of iso-
propanol was heated in a warm water bath (ꢁ85 ꢁC). A solu-
tion of NiCl2 Æ 6H2O (0.0404 g, 0.170 mmol) in 15 mL of
isopropanol/methanol (2:1 v/v) was stirred for 15 min and
transferred by filter cannula into the dcpf solution. The
resulting mixture was refluxed for 3 h and then filtered.
The precipitate was washed with 10 mL of Et2O, then
10 mL of CH2Cl2, yielding a gray-green solid. A second crop
of product was obtained by cooling the initial filtrate in the
freezer. These crystals were added to the first crop of product
to give a total of 0.0609 g (51%) of 1. Anal. Calc. for C34H52-
Cl2FeNiP2 Æ 1/4CH2Cl2: C, 56.40; H, 7.26. Found: 56.78,
7.33%. UV–Vis (CH2Cl2, k nm, e L/(cm mol)): 607 (264).
4.72 (s, 4H, C5H4), 4.43 (s, 4H, C5H4), 2.7–0.9 (m, 44H, Cy).
2.2.5. [ZnCl2(dcpf)] (5)
A solution of dcpf (0.0668 g, 0.115 mmol) in 12 mL of
isopropanol was heated in a warm water bath (ꢁ85 ꢁC).
A solution of ZnCl2 (0.0161 g, 0.118 mmol) in 2.3 mL of
an isopropanol/methanol mixture (2:1 v/v) was added.
The resulting solution was refluxed for 2 h and cooled over-
night in the freezer. The solution was filtered, and the
resulting solid was washed with Et2O and dried to isolate
5 as a pale orange solid (0.0321 g, 39%). Anal. Calc. for
C34H52Cl2FeP2Zn: C, 57.13; H, 7.31. Found: 56.87,
1
7.38%. 31P{1H} (CDCl3): d ꢀ15.15 (s). H NMR (CDCl3):
d 4.46 (br s, 8H, C5H4), 2.3–1.2 (m, 44H, Cy).
2.2.2. [PdCl2(dcpf)] (2)
PdCl2(C6H5CN)2 (0.1335 g, 0.3481 mmol) and dcpf
(0.1992 g, 0.3443 mmol) were dissolved in 10 mL of CH2Cl2
and stirred for 2 h. The solution was initially dark brown
but, after stirring, turned a lighter brown, and a precipitate
appeared. Et2O (20 mL) was added and the reaction mix-
ture was stirred for 5 min. The solution was subsequently
filtered, and the resulting liquid was concentrated, layered
with Et2O, and cooled in the freezer, yielding red crystals
of 2 (0.1424 g, 55%). Anal. Calc. for C34H52Cl2FeP2Pd: C,
54.02; H, 6.93. Found: 54.05, 6.93%. 31P{1H} (CDCl3): d
2.2.6. [CdCl2(dcpf)] (6)
A solution of dcpf (0.0578 g, 0.0999 mmol) in 4 mL of
isopropanol was heated in a warm water bath (ꢁ85 ꢁC). A
solution of CdCl2 (0.0183 g, 0.0998 mmol) in 6 mL of an
isopropanol/methanol mixture (2:1 v/v) was added to the
dcpf solution. The resulting solution was refluxed for 2 h,
filtered, and the resulting solid was washed with Et2O, yield-
ing 6 as a pale orange solid (0.0377 g, 50%). Anal. Calc. for
C34H52Cl2FeP2Cd: C, 53.60; H, 6.88. Found: 53.33, 6.88%.
31P{1H} (CDCl3): d 2.48 (s, JP–Cd = 1450 Hz, JP–Cd
=
1
1
1
1
56.6 (s). H NMR (CDCl3): d 4.54 (s, 4H, C5H4), 4.46 (s,
1380 Hz). H NMR (CDCl3): d 4.53 (s, 4H, C5H4), 4.34
4H, C5H4), 2.7–1.0 (m, 44H, Cy).
(s, 4H, C5H4), 2.3–1.2 (m, 44H, Cy).
2.2.3. [PtCl2(dcpf)] (3)
2.2.7. Mercury compounds
PtCl2(CH3CN)2(0.0206 g, 0.0588 mmol) and dcpf
(0.0347 g, 0.0600 mmol) were dissolved in 3 mL of CH2Cl2
and stirred for 1 h. The resulting solution was concentrated
to one third its original volume, layered with Et2O, and
cooled in the freezer for three days to form crystals. The solu-
tion was subsequently filtered, and the crystals were washed
with 5 mL Et2O, giving 3 as a yellow solid (0.0147 g, 30%).
2.2.7.1. [HgCl2(dcpf)] (7a). HgCl2 (0.0478 g, 0.176
mmol) and dcpf (0.1018 g, 0.176 mmol) were added to
150 mL of refluxing ethanol and refluxed for an additional
5 min. Upon cooling to room temperature, the solution
was concentrated to approximately 30 mL and filtered to
give a yellow solid. The solid was recrystallized by dissolv-
ing in CH2Cl2, layering with Et2O, and cooling in the free-
zer. The crystals were collected by filtration and washed
with Et2O. The combined filtrates were dried in vacuo
and the resulting solid was recrystallized from a mixture
of CH2Cl2 and Et2O to give a second crop of crystals which
was isolated by filtration. This second batch of crystals was
washed twice with Et2O and combined with the first batch
to give 7a as a dark yellow solid (0.0857 g, 57%). Anal.
Anal. Calc. for C34H52Cl2FeP2Pt Æ Et2O: C, 49.68; H, 6.80.
1
Found: 50.08, 6.56%. 31P{1H} (CDCl3): d 23.0 (s, JP–Pt
=
3800 Hz). 1H NMR (CDCl3): d 4.44 (s, 4H, C5H4), 4.43 (s,
4H, C5H4), 2.7–1.0 (m, 44H, Cy).
2.2.4. [Au2Cl2(dcpf)] (4)
HAuCl4 Æ H2O (0.224 g, 0.626 mmol) was dissolved in a
mixture of 1.5 mL of water and 8.0 mL of methanol, and
Calc. for C34H52Cl2FeP2Hg: C, 48.04; H, 6.17. Found:
1
the solution was cooled to 0 ꢁC.
A
solution of
47.89, 6.31%. 31P{1H} (CDCl3): d 30.98 (s, JP–Hg
=
1
S(CH2CH2OH)2 (0.8 mL, 8 mmol) in 1.5 mL of methanol
was added, and the resulting mixture was stirred for
15 min. dcpf (0.2610 g, 0.4511 mmol) was dissolved in a min-
imal amount of chloroform and then added to the mixture.
The reaction was allowed to stir for 6 h while slowly warming
to room temperature. Methanol (25 mL) was added, and the
resulting solution was concentrated to approximately
12 mL. Et2O (20 mL) was then added, and the solution was
filtered to give a solid. This solid was washed with methanol
4130 Hz). H NMR (CDCl3): d 4.56 (s, 4H, C5H4), 4.41
(s, 4H, C5H4), 2.5–1.2 (m, 44H, Cy).
2.2.7.2. [HgCl2(dtbpf)] (7b). Prepared by a method anal-
ogous to the preparation of 7a, 7b was isolated as an orange
solid (0.0864 g, 66%). Anal. Calc. for C26H44Cl2FeP2Hg: C,
41.86; H, 5.95. Found: 41.78, 5.94%. 31P{1H} (CDCl3): d
1
1
61.42 (s, JP–Hg = 4260 Hz). H NMR (CDCl3): d 4.63 (s,
4H, C5H4), 4.59 (s, 4H, C5H4), 1.63 (dd, 36H, CH3).

L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
4893
2.3. X-ray crystallography
2.4. Electrochemistry
Crystals of 2a Æ 2CH2Cl2 were formed by the diffusion of
Et2O into a solution of 2 in CH2Cl2. A yellow-orange col-
ored crystal, approximately 0.20 · 0.20 · 0.05 mm3 in size,
was mounted on a glass fiber with epoxy and cooled to
ꢀ55 ꢁC. Diffraction intensity data were collected with a Sie-
mens/Bruker Smart Apex CCD diffractometer using
Mo Ka radiation at ꢀ55 ꢁC. Additional crystal, data col-
lection, and refinement parameters are given in Table 1.
The structure was solved using direct methods, completed
by subsequent difference Fourier syntheses, and refined
by full matrix least-squares procedures on F2. Sources of
software and scattering factors are contained in the SHEL-
XTL (5.10) program package (G. M. Sheldrick, Bruker
AXS, Madison, WI) [27]. SADABS absorption corrections
were applied to all data. All non-hydrogen atoms (except
C13, C14 and C16) were refined with anisotropic displace-
ment coefficients and hydrogen atoms were treated as ide-
alized contributions. Three of the C atoms (C13, C14,
and C16) of one cyclohexyl ring were disordered over
two positions at 51.67% occupancy. No hydrogen atoms
were placed on this ring. One of the CH2Cl2 solvent mole-
cules was disordered and on a center of symmetry. It was
treated by using the Platon Program Squeeze (found for
The oxidative electrochemistry of all compounds was
examined in CH2Cl2 under an argon atmosphere at ambi-
ent temperature (22 1 ꢁC). The supporting electrolyte
was 0.1 M [NBu4][PF6] and the concentration of the ana-
lyte was 1.0 mM. Additional electrochemical studies of
dcpf were carried out in 0.50 M [NBu4][B(C6F5)4], with
analyte concentrations of 0.50 mM, 1.0 mM, and
2.0 mM. Data were collected at ꢀ10, 0, 10, and 20
0.1 ꢁC and scan rates ranging between 25 and 500 mV/s.
For all experiments, the counter electrode was a platinum
wire, and the reference electrode was non-aqueous silver/
silver chloride separated from the solution by a fine glass
frit. The working electrode was 1.5 mm diameter glassy
carbon which was polished with 1 lm diamond paste,
rinsed with acetone, polished with 0.25 lm diamond paste,
and rinsed with CH2Cl2 prior to use. The potentials were
indirectly referenced to ferrocene by the addition of decam-
ethylferrocene at the end of the experiment [29]. The cyclic
voltammagrams were obtained using a Princeton Applied
Research 263-A potentiostat, and the resulting data were
collected and analyzed with Power Suite. All simula-
tions were carried out by DIGISIM (Bioanalytical Sys-
tems) [30].
3
3
3
˚
˚
˚
the unit cell 113.3 e/A , calculated 96 e/A and 118 e/A
if the missing H atoms included) [28]. There were two resid-
3
3
˚
˚
ual peaks greater than 1 e/A , Q1 1.06 e/A and Q2 1.03
2.5. Catalytic studies
3
˚
˚
e/A were found respectively at 0.837 and 0.872 A of Pd1.
2.5.1. Amination of aryl halides
Table 1
In a typical reaction, 5 mol% of the catalyst, 15 mol% of
the corresponding free ligand and potassium tert-butoxide
(19.2 mg, 0.171 mmol) were placed in a screw-cap NMR
tube. The NMR tube was brought into a glovebox where
degassed THF (0.50 mL) and aryl bromide (0.137 mmol)
were added. The tube was sealed with a cap containing a
PTFE septum and was removed from the glovebox. The
appropriate amine (0.171 mmol) was added via syringe
through the septum and the reaction mixture was heated
to 100 ꢁC for 3 h. The reaction was then allowed to cool
to room temperature. THF was added to the reaction mix-
ture, bringing the total volume to 1.5 mL and the resulting
solution was mixed thoroughly. After precipitate settled,
two 0.5 mL aliquots of the mixture were transferred to
screw-cap vials with TFE septa. Decane (2.0 lL) was added
to each duplicate as an internal standard. The samples were
then analyzed by GC/MS. All GC/MS analyses were car-
ried out on a Fisons Instruments 8030 gas chromatograph
equipped with a Fisons MD800 mass spectral analyzer.
The gas-chromatograph employed an Alltech ECONO-
CAP Faster Fatty Acid Phase column, with phase SE-54,
and dimensions 30 m · 0.25 mm ID · 0.25 lm. Lab-Base
2 data system for DOS was used for data analysis. The area
of each peak was computer integrated. A ratio of peak area
to concentration of analyte was established for both the
decane and the reaction product, and this was used to
determine the product yield for each reaction.
Crystal data and structure analysis results of 2
PdCl2(dcpf) Æ 2 CH2Cl2
Empirical formula
Formula weight
Crystal system
Space group
C36H56Cl6FeP2Pd
925.70
Triclinic
ꢀ
P1
˚
a (A)
9.8245(5)
9.8374(5)
˚
b (A)
˚
c A
20.4247(11)
97.8070(10)
97.8700(10)
90.8210(10)
1936.19(17)
2
0.20 · 0.20 · 0.05
Yellow-orange
0.71073
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
Z
Crystal size (mm)
Crystal color
radiation; k (A)
Temperature, K
h Range (ꢁ)
Data collected
˚
218(2)
2.03–28.33
h
ꢀ12 to 11
ꢀ13 to 13
ꢀ26 to 23
13,916
8693
Multiscan/SADABS
k
l
No. of data collected
No. of unique data
Absorption correction
Final R indices (observed data)
R1
0.0475
0.1189
1.040
wR2
Goodness of fit

4894
L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
2.5.2. Isolation of arylamine product
which oxidation occurs (E 0) is 0.02 V relative to FcH0/+
.
The product of the reaction of aniline with bromotolu-
ene was isolated and purified in order to confirm its iden-
tity. The reaction of aniline with bromotoluene was
carried out as described above with [PdCl2(dppf)] used as
the catalyst precursor. The reaction was allowed to cool
to room temperature and the solvent was removed. The
remaining oil was dissolved in minimal diethyl ether and
applied to a short silica column. The column was washed
with a mixture of hexanes and diethyl ether (10:1) and
the purified desired product was eluted with a 5:1 mixture
of hexanes and diethyl ether. The identity of the product
This is similar to the potentials at which oxidation of the
closely related dippf (0.05 V vs. FcH0/+) [31] and dtbpf
(0.06 V vs. FcH0/+) [32] occur and is significantly less posi-
tive than the potential at which oxidation of dppf occurs
[23]. The electrochemical parameter, EL, is defined as
1/2E0 (FeIII/FeII) (vs. NHE) for symmetric ferrocenes; EL
was determined to be 0.34 V for dcpf [33]. The EL value
can be used to estimate the Hammett parameter for deriv-
atives of ferrocene with the equation EL = 0.45rp + 0.36.
Using this data, the rp value for the –PCy2 group can be
estimated as ꢀ0.04.
1
was confirmed by H NMR and GC/MS.
The reversibility of the dcpf wave was found to be lar-
gely dependent on temperature, scan rate, and concentra-
tion (Fig. 1). The chemical reversibility parameter (ir/if)
for dcpf was calculated at a variety of temperatures and
concentrations (Table 2) [34,35]. At slow scan rates, the
oxidized form of dcpf appears to undergo a reaction which
is proposed to be an EC dimerization [36] similar to that of
dppf [37] and dippf [31] (Scheme 2). Using the chemical
reversibility parameter, the second order rate constant of
an EC dimerization reaction (kd) was calculated at various
temperatures [35].
3. Results and discussion
Electrochemical studies of dcpf were carried out at a
variety of temperatures and concentrations. The cyclic vol-
tammagrams of dcpf were analyzed and the potential at
Table 2
Reversibility and second-order rate constants for the oxidation of dcpf
ꢀ10 ꢁC
0 ꢁC
10 ꢁC
20 ꢁC
Reversibility (ir/if)
0.50 mM
1.0 mM
0.58
0.56
0.55
0.54
0.51
0.50
0.53
0.49
0.47
0.51
0.45
0.43
2.0 mM
Rate constant [102 kd (Mꢀ1 sꢀ1)]
Experimental
Simulated
2.0
4.3
3.6
6.6
4.9
10
6.6
14
+
..
..
R2P
R2P
-e
Fe (II)
Fe (III)
+e
..
..
PR2
PR2
+
.
R2P
Dimer
?
Fe (II)
..
PR2
Fig. 1. Cyclic voltammagrams of 1.0 mM dcpf in CH2Cl2 with 0.05 M
[NBu4][B(C6F5)4] supporting electrolyte. Scans were run at 293 K at scan
rates of 25 mV/s (top) and 100 mV/s (bottom).
Scheme 2. Proposed dimerization reaction of 1,10-bis(diphosphino)-
ferrocenes.

L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
4895
The dimerization reaction was also simulated using
DIGISIM, and theoretical cyclic voltammagrams were
compared to the experimental data (Fig. 2). An Arrhenius
relationship (Fig. 3) was used to determine the activation
parameters from the rate constants. The experimental val-
ues for DHꢀ and DSꢀ were 22 1 kJ/mol and ꢀ220 10 J/
K mol, respectively, while the simulated values for DHꢀ
and DSꢀ were 23 1 kJ/mol and ꢀ220 10 J/K mol.
When compared to dppf and dippf, the experimental and
theoretical DSꢀ values for the EC dimerization are identi-
cal. However, the enthalpies of dimerization are not equiv-
alent; dcpf has a lower DHꢀ than dppf and a higher DHꢀ
than dippf [31,37]. The difference between the DHꢀ of
dimerization for dppf and dcpf can be attributed to the
phenyl groups of dppf which can participate in delocaliza-
tion of the proposed phosphorus radical, making the cation
less likely to dimerize. The difference in the DHꢀ of dimer-
ization for dippf and dcpf is likely due to the difference
in the steric bulk of the groups; the cone angle of PCy3 is
170ꢁ while that of P(i-Pr)3 is 160ꢁ [38].
A total of eight new compounds containing either dcpf
or dtbpf were synthesized and characterized. In most cases,
both the dcpf and dtbpf [32] compounds have been pre-
pared, however, after repeated attempts, the Zn, and Cd
derivatives of dtbpf could not be isolated. This is likely
due to steric effects; the bulkier tert-butyl groups of dtbpf
prevent coordination of dtbpf to the relatively small Zn
or Cd centers. In addition, it is likely that the bulk of the
dtbpf ligand plays a role in the synthetic procedures; it
was necessary to stir and/or reflux the reactions for prepar-
ing the dtbpf complexes for twice as long as their dcpf
analogues.
Of all the compounds synthesized, only compound 1
was paramagnetic. The magnetic moment, lB, was deter-
mined to be 3.0 by the Evans Method [39]. This value is
similar to the values for the corresponding dppf [37] and
dippf [31] compounds and is in the range expected for
Ni(II) complexes in pseudo-tetrahedreal geometries [40].
To further characterize 1, the visible spectrum was
obtained. A 0.0010 M solution of 1 in CH2Cl2 was scanned
between 400 and 800 nm, and a peak was found at 607 nm.
The kmax and extinction coefficient are similar to those seen
in the spectra of the dppf [37] and dippf [31] analogues.
The remaining compounds, 2–7, were diamagnetic and
1
could easily be characterized by 31P{1H} and H NMR.
With the exception of the zinc compound, the coordination
of the bidentate phosphine to a metal center resulted in a
downfield shift in the 31P{1H} NMR. In addition, for the
Zn, Cd, and Hg dcpf compounds, the 31P{1H} signal
shifted downfield going down the group. Also noteworthy
are the coupling constants observed in the 31P{1H} NMR
for 3, 6, and 7 which clearly show that the ligand is coordi-
nated to metals with NMR active isotopes. In general, the
spectral data for 2–7 are similar to those reported for the
dippf analogues [31].
A crystal of 2 was obtained, and its structure was deter-
mined by X-ray crystallography (Fig. 4). Selected bond
angles and lengths are given in Table 3. This is the
first X-ray structure of a compound containing dcpf to
Fig. 2. Experimental (–) and simulated (d) cyclic voltammagrams for
1.0 mM dcpf in methylene chloride at 273 K with a scan rate of 100 mV/s.
7.5
7
6.5
6
5.5
5
4.5
4
0.00335 0.0034 0.00345 0.0035 0.00355 0.0036 0.00365 0.0037 0.00375 0.0038 0.00385
1/T (K)
Fig. 3. Arrhenius plot of experimental (r) and simulated (j) rate data for dcpf. For the experimental data, the slope was ꢀ2984, the intercept was 16.6,
and R2 was 0.9369. For the simulated data, the slope was ꢀ3068, the intercept was 17.7, and R2 was 0.9995.

4896
L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
Table 4
be determined. The dcpf ligand adopts a synclinal stag-
gered arrangement about the Pd atom; this is the most
common arrangement for coordinated dppf [41]. Pd adopts
a pseudo-square planar geometry with the two P atoms cis,
similar to dppf and dippf analogues [24,25]. In addition,
the P–Pd–P angle of 102.45ꢁ is 4.47ꢁ larger than the corre-
sponding angle of the dppf analogue [25] and 2.43ꢁ larger
than the corresponding angle of the dppr analogue [37]
(Table 4); this difference can be attributed to the steric hin-
drance of the bulky cyclohexyl groups [42]. However, it is
somewhat surprising that the corresponding angle in the
dippf analogue is 1.14ꢁ larger than the angle in 2 [31].
PCy3 has a larger cone angle than P(i-Pr)3 [38], so it was
anticipated that the P–Pd–P angle would be larger for
PdCl2(dcpf) (2). The difference may be attributed to pack-
ing of the molecules in the crystal.
P–Pd–P bite angle for various [PdCl2(bisphosphinometallocene)]
complexes
Compound
Bite angle
Reference
[PdCl2(dppf)]
[PdCl2(dppr)]
[PdCl2(dppo)] a
[PdCl2(dcpf)]
[PdCl2(dippf)]
[PdCl2(dmpf)]b
a
97.98(4)
100.02(17)
100.82(9)
102.45(3)
103.59(4)
99.30c
[25]
[37]
[47]
This work
[24]
[48]
dppo = 1,10-bis(diphenylphosphino)osmocene.
dmpf = 1,10-bis(dimethylphosphino)ferrocene.
b
c
Average of two crystallographically independent molecules.
The oxidative electrochemistry of compounds 1–7 was
also investigated, and, upon coordination, oxidation of
the iron center shifts to more positive potentials (Table
5). A similar trend has been noted for compounds contain-
ing dppf, dippf and dtbpf ligands [31,32,37]. The data show
that potentials at which oxidation of derivatives of dcpf
and dtbpf are similar to their dippf analogues but less posi-
tive than the dppf analogues [31,32,37]. This difference can
be attributed to the more donating nature of alkyl groups
as compared to aryl groups [43]. In addition, the data dem-
onstrate that when dcpf binds to a 3d metal, the oxidation
occurs at a less positive potential than when it binds to a 4d
or 5d metal. Although a similar trend is observed for dippf,
an explanation for this observation is not apparent at this
time [31].
In addition, the oxidation for most of the complexes
containing dcpf was chemically and electrochemically
reversible. For dcpf, coordination to a metal center occu-
pies the lone pair on the phosphorus atom and therefore
prevents dimerization. The oxidation of compounds 1, 5,
and 6 was on chemically reversible at higher scan rates,
as were the oxidations of the dppf, dippf and dtbpf ana-
logues of 1 and the dppf and dippf analogues of 6
[31,32,37]. It has been proposed that the bidentate phos-
phines bind weakly to the metal centers in these com-
pounds and form highly reactive cations upon oxidation
Fig. 4. ORTEP diagram of [PdCl2(dcpf)].
Table 3
˚
Selected bond lengths (A) and angles (ꢁ) for compound 2
Table 5
Oxidation potentials and chemical reversibility parameters (at 100 mV/s
scan rate) for dcpf, dtbpf, and compounds 1–7
Bond lengths
Average P–Pd
Average Fe–C
2.287
2.035
ꢀ0.114
Average Pd–Cl
P–P
2.353
3.566
Compound
E0 (V) vs. FcH0/+
iopx=irped
a
Average dp
dcpf
1
0.02a
0.30b
0.45
0.84
Bond angles
P–Pd–P
102.45(3)
169.11(3)
86.36(3)
178.32
Cl–Pd–Cl
P(1)–Pd–Cl(2)
P(2)–Pd–Cl(2)
P–Fe–P
87.49(3)
84.53(3)
170.51(4)
61.88
2
0.47
0.95
P(1)–Pd–Cl(1)
P(2)–Pd–Cl(1)
3
0.46
0.94
4
0.52
0.95
0.89
0.80
0.42, 0.89
0.98
0.33b
b
Xa–Fe–Xb
sc
5
21.27
hd
2.25
6
7a
0.42b
0.46c, 0.62b
0.06a
a
Deviation of the P atom from the Cp plane; the negative value here
means that the P is farther from the Fe.
dtbpf [28]
7b
0.43c, 0.57b
0.00, 0.83
b
Centroid–Fe–centroid.
Torsion angle CA–XA–XB–CB where C is the carbon bonded to P and
c
a
At 20 ꢁC.
b
X is the centroid.
Chemically and electrochemically reversible at higher scan rates.
Chemically irreversible.
d
c
The dihedral angle between the two Cp rings.

L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
4897
[23,31]. While the oxidation of 5 was chemically reversible
at higher scan rates, the oxidation of its dppf and dippf
analogues was chemically irreversible [23,31]. This incon-
gruity can be attributed to the greater electron-donating
ability of cyclohexyl groups as compared to phenyl and iso-
propyl groups [43]. It is possible that the more r-donating
cyclohexyl groups make the phosphorus atoms more elec-
tron rich, which leads to stronger metal–phosphorus
bonds. These stronger bonds are less likely to break and
lead to side products. Finally, compounds 7a and7b were
unique because they had more than one wave. Compound
7a produced two distinct, chemically reversible waves at
higher scan rates due to the oxidation of both mercury
and iron. The dppf analogue of 7 displayed two chemically
irreversible waves, suggesting that the rP ! M bond of the
dppf derivative breaks more easily and leads to the forma-
tion of side products [23]. However, the proposed stronger
P–M bond in compound 7a prevents side product forma-
tion. Oxidation of 7b produced three waves at scan rates
below 100 mV/s. At higher scan rates, there were only
two waves which suggests that the third wave observed at
low scan rates likely resulted from a side reaction. Of the
two waves observed at higher scan rates, one was chemi-
cally reversible at high scan rates and the other was chem-
ically and electrochemically reversible.
With the electrochemical and structural data in hand,
the complexes [PdCl2(P Æ P)] (P Æ P = dppf, dppr, dippf,
dcpf and dtbpf) were used as catalyst precursors in the
Buchwald–Hartwig coupling reaction. The general reaction
for the studied system is shown in Scheme 3 and the condi-
tions employed were similar to a previous study [18]. All
five of the palladium dichloride catalysts led to the forma-
tion of the arylamine product for each aryl bromide and
primary amine combination. The GC yields for each com-
bination of catalyst, amine, and aryl bromide were calcu-
lated and are reported in Table 6. The yields were
comparable to those previously described for the amina-
tion of aryl bromides by palladium catalysts containing
bidentate phosphine ligands [2]. Each catalyst contained
a bidentate phosphine with a unique combination of steric
and electronic properties which affected the rate of the
reaction. The catalytic mechanism for arylamine formation
can help to explain the differences in product yield based on
varying sterics and electronics.
A recent reexamination of the mechanism for the cata-
lytic coupling of aryl bromides and primary amines has
been carried out by Buchwald and Hartwig [1]. In this reex-
amination, a palladium (0) species, Pd(BINAP)2, was
determined to be present in the solution but not an active
part of the catalytic cycle [1]. A 31P{1H} NMR of the
[PdCl2(dppf)] catalyzed coupling of 4-bromotoluene to
cyclohexylamine displayed a major resonance at 25.1 ppm
(THF-d8). The 31P{1H} NMR shift of Pd(dppf)2 was
reported to be 24.1 ppm (in C6D6) [44] indicating that
Pd(dppf)2 is present in the reaction mixture. In the pro-
posed mechanism, the active state of the catalyst, Pd(dppf),
is in equilibrium with the inactive Pd(dppf)2 [1]. The first
step in the catalytic cycle involves the oxidative addition
of the aryl bromide to Pd(dppf) followed by substitution
of the bromide by the deprotonated amine. This generates
an amido aryl palladium (II) intermediate. The formation
KOtBu
R
R
P
P
R'NH2
+
R'
PdCl2(P P)
THF, 100˚C
Br
N
H
R = H, Me, CF3
R' = cyclohexyl, hexyl, phenyl
P = dppf, dppr, dippf, dcpf, dtbpf
P
Scheme 3. General reaction schemes for catalytic reactions.
Table 6
GC yields (%) for all catalytic reactions run in THF at 100 ꢁC for 3 h
Bromide
Catalyst precursor
Primary amine
Cyclohexylamine
Hexylamine
Aniline
2,6-Dimethylaniline
Bromotoluene
PdCl2(dppf)
PdCl2(dppr)
PdCl2(dippf)
PdCl2(dcpf)
PdCl2(dtbpf)
60
35
39
11
39
70
35
31
21
27
33
57
49
18
21
14
37
30
21
15
Bromobenzene
PdCl2(dppf)
PdCl2(dppr)
PdCl2(dippf)
PdCl2(dcpf)
PdCl2(dtbpf)
67
25
39
2
36
18
50
34
39
18
34
34
24
3
18
34
40
<1
7
14
Bromotrifluorotoluene
PdCl2(dppf)
PdCl2(dppr)
PdCl2(dippf)
PdCl2(dcpf)
PdCl2(dtbpf)
21
37
4
13
2
22
23
24
27
27
4
9
9
8
22
2
5
5
<1
1

4898
L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
of the product is the result of reductive elimination of aryl-
amine from the amido aryl palladium (II) intermediate.
Several other processes that compete with reductive elim-
ination and lead to undesired byproducts have been reported
[2]. The formation of arene and imine products due to b-H
elimination has frequently been observed in systems using
monodentate phosphine ligands [4]. However, it has also
been demonstrated, based on mechanistic data, that the
use of bidentate phosphine ligands inhibits b-H elimination,
thus inhibiting the formation of undesired arene byproduct
[17]. The presence of arene was very infrequent in the reac-
tions carried out in this study, likely due to the inhibition
of b-H elimination by the bidentate phosphines. Another
byproduct reported in the literature was diarylamine [2].
However, this was not observed in any of the reactions car-
ried out in this study. The main byproduct observed in this
study was the coupling of the aryl groups of the aryl bromide
to form the biphenyl analogue of the starting material.
Because the catalytic mechanism involves both oxidative
and reductive processes, the electronic properties of the cat-
alyst are expected to play a critical role in determining the
efficiency of the catalyst and rate of catalysis. An electron-
rich catalyst will favor the oxidative addition step in the
catalytic mechanism while an electron-poor catalyst will
favor the reductive elimination step. In order to explore
the electronic properties of the catalysts, oxidative electro-
chemistry data and titration calorimetry data were exam-
ined. Based on the electrochemistry, the alkyl groups of
dippf, dcpf, and dtbpf make the catalysts containing these
ligands more electron-rich than the catalysts containing
dppf and dppr [31,32,37]. Additionally, based on heat of
protonation values, dppr has been previously determined
to be slightly more basic than dppf [45].
In addition to electronics, steric effects were also
expected to influence the catalytic system. Each catalyst
has a unique bite angle (P–Pd–P) that affects the addition
of substrate and elimination of product. A large bite angle
may make oxidative addition more difficult because the
substrate being added will have less room to fit onto the
palladium center. However, a large bite angle may encour-
age reductive elimination to relieve steric strain on the cat-
alyst. The sterics of the catalysts were compared by
examining P–Pd–P bite angles from X-ray crystallographic
data which has been reported for all of the palladium cat-
alysts except [PdCl2(dtbpf)]. [PdCl2(dppf)] has the smallest
bite angle [5], [PdCl2(dippf)] [31] and [PdCl2(dcpf)] have
larger bite angles and [PdCl2(dppr)] [37] falls in between.
Although the bite angle of [PdCl2(dtbpf)] has not been
determined, it is anticipated to be similar to or slightly lar-
ger than the bite angles of [PdCl2(dippf)] and [PdCl2(dcpf)].
Therefore, [PdCl2(dippf)], [PdCl2(dcpf)] and [PdCl2(dtbpf)]
are the more electron rich catalyst precursors and are also
the catalyst precursors with the largest bite angles.
[PdCl2(dppf)] and [PdCl2(dppr)] are less electron rich than
the complexes with ligands containing alkyl groups and
have smaller bite angles than the analogous alkyl substi-
tuted complexes.
In addition to altering the catalysts, substrates with dif-
ferent electronic and steric properties should also influence
the catalytic process. Substrates were chosen to selectively
vary the steric environment and electronics of the system
and to probe the effects of these changes on the catalytic
efficiency. To study electronic effects, three aryl bromides
with different substituents in the four-position were
selected. Based on the Hammet parameters, the groups in
the four-position were electronically neutral (H, 0.0), elec-
tron donating (CH3, ꢀ0.17) and electron withdrawing
(CF3, 0.54) [46]. The use of four different primary amines
also contributed to both the electronic and steric perturba-
tions of the system. Electronically, cyclohexylamine and
hexylamine are similar. However, the bulky cyclohexyl ring
of cyclohexylamine makes it more sterically hindered.
Compared to both cyclohexylamine and hexylamine, ani-
line and 2,6-dimethylaniline are both less electron rich. In
addition, the methyl groups in the two and six positions
of 2,6-dimethylaniline provide an opportunity to examine
the effect of sterics on the formation of arylamines.
The complex interplay between steric and electronic
properties uniquely influenced each catalytic reaction.
The electronic effects can be examined by comparing the
electronic nature of the substrates and the ligands. Based
on the yields, [PdCl2(dppf)] seems to be the best catalyst
precursor for alkyl amines and the electron rich aryl bro-
mides. This suggests that the best catalyst for electron rich
substrates has the least electron rich ligand. In addition,
when electron-poor substrates are used, all of the catalysts
examined show poor efficiency. Because one of the first
steps in the proposed catalytic mechanism is oxidative
addition, it is not surprising that the presence of an elec-
tron-withdrawing group is detrimental to the rate of the
reaction. In the catalytic reactions with bromotrifluorotol-
uene, the catalysts with alkyl groups are either similar or
superior to dppf and dppr. A similar trend can be observed
in the reactions with the less electron rich aniline and 2,6-
dimethylaniline.
Sterics effects also play a role in the efficiency of cataly-
sis. When comparing electronically similar amines, the cat-
alysts with smaller bite angles were more efficient with less
bulky amines. For example, when comparing cyclohexyl-
amine and hexylamine, [PdCl2(dppf)], the catalyst precur-
sor with the smallest bite angle, gave higher yields when
hexylamine, the less hindered of the two amines, was the
amine substrate. The same is true when comparing aniline
and 2,6-dimethylaniline. The reaction with the less hin-
dered substrate, aniline, catalyzed by [PdCl2(dppf)] gave a
higher yield than when the substrate was 2,6-dimethylani-
line. This was not unexpected, as the catalysts with larger
bite angles have less room on the palladium center for
the amine to add. If the amine is bulkier, the addition will
be more difficult.
The complex interplay of steric and electronic factors is
best observed in the catalysts with alkyl groups on the
phosphorus atoms. The tert-butyl groups of dtbpf are the
most donating and the bulkiest of the alkyl-containing

L.E. Hagopian et al. / Journal of Organometallic Chemistry 691 (2006) 4890–4900
4899
phosphine ligands studied. Rates with the bulkier amines,
cyclohexylamine and 2,6-dimethylaniline, are significantly
lower compared to the less bulky amines when
[PdCl2(dtbpf)] was used as the catalyst precursor. However,
when using bromotrifluorotoluene and aniline as the sub-
strates, the most efficient catalyst precursor is [PdCl2-
(dtbpf)]. The steric and electronic nature of the substrates
clearly influences which catalyst will be most efficient for
the system.
336033; e-mail: deposit@ccdc.cam.ac.uk or www:http://
www.ccdc.cam.ac.uk).
Acknowledgements
L.E.H., A.N.C. and C.N. thank the donors of the Petro-
leum Research Fund, administered by the American Chem-
ical Society, for partial funding of this research, the Kresge
Foundation for the purchase of the JEOL NMR, the Aca-
demic Research Committee at Lafayette College for fund-
ing EXCEL scholars.
4. Summary
Electrochemical studies of dcpf were carried out and the
oxidation is proposed to follow an EC dimerization mech-
anism. Seven compounds containing dcpf and one com-
pound with dtbpf were synthesized and characterized.
The X-ray structure of [PdCl2(dcpf)] was determined, and
it showed the pseudo-square planar geometry predicted
for d8 metals. The remaining compounds were character-
ized by NMR and UV–Vis spectroscopy. Almost all of
the compounds showed a downfield shift in the 31P{1H}
NMR when coordinated to a metal. The oxidative electro-
chemistry of the dcpf and dtbpf compounds is also
reported. Upon coordination, the potential at which oxida-
tion of dcpf and dtbpf occurs is more positive. In addition,
the oxidation of compounds containing the 3d metals
occurred at less positive potentials than those containing
4d and 5d metals.
The use of [PdCl2(dppf)], [PdCl2(dppr)], [PdCl2(dippf)],
[PdCl2(dcpf)] and [PdCl2(dtbpf)] in the formation of aro-
matic carbon–nitrogen bonds by the Buchwald–Hartwig
reaction led to the desired arylamine product. Differences
in the electronic and steric properties of the bidentate phos-
phine ligands on the palladium catalyst affected the effi-
ciency of the catalytic reaction. [PdCl2(dppf)] appears to
be the best catalyst precursor unless the substrates are elec-
tron poor. The presence of an electron withdrawing group
on the aryl bromide led to poor efficiency for all of the cat-
alysts studied. However, catalysts containing alkyl groups
rather than aryl groups, as in the case of [PdCl2(dippf)],
[PdCl2(dcpf)] and [PdCl2(dtbpf)], were slightly more effi-
cient when coupling electron-poor substrates. When com-
paring electronically similar amines, catalysts with
smaller bite angles were more efficient with less bulky
amines. The steric and electronic properties of the sub-
strates play a critical role in determining which catalyst will
be most effective for a given coupling.
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4
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