Electrochemistry and complexation of Josiphos ligands

Article abstract of DOI:10.1016/j.jorganchem.2007.02.039

The oxidative electrochemistry of 11 chiral bis-phosphinoferrocene ligands, all within the Josiphos class of ligands, was examined in methylene chloride. The oxidation of these ligands displays multiple waves of varying chemical reversibility. Palladium(II) and platinum(II) complexes with the general formula [MCl₂(P-P)] (M = Pd or Pt; P-P = Josiphos) were prepared, characterized by NMR and cyclic voltammetry. The electrochemistry simplifies greatly upon coordination of the Josiphos ligands. The X-ray structures of a palladium(II) and platinum(II) complex of the same Josiphos ligand are reported.

Full text of DOI:10.1016/j.jorganchem.2007.02.039

Journal of Organometallic Chemistry 692 (2007) 2365–2374
www.elsevier.com/locate/jorganchem
Electrochemistry and complexation of Josiphos ligands
1
Brenna L. Ghent, Sarah L. Martinak, Lauren A. Sites, James A. Golen ,
*
Arnold L. Rheingold, Chip Nataro
Department of Chemistry, Lafayette College, Easton, PA 18042, United States
Department of Chemistry and Biochemistry, University of California–San Diego, La Jolla, CA 92093, United States
Received 1 December 2006; received in revised form 7 February 2007; accepted 8 February 2007
Available online 6 March 2007
Abstract
The oxidative electrochemistry of 11 chiral bis-phosphinoferrocene ligands, all within the Josiphos class of ligands, was examined in
methylene chloride. The oxidation of these ligands displays multiple waves of varying chemical reversibility. Palladium(II) and plati-
num(II) complexes with the general formula [MCl2(P–P)] (M = Pd or Pt; P–P = Josiphos) were prepared, characterized by NMR and
cyclic voltammetry. The electrochemistry simplifies greatly upon coordination of the Josiphos ligands. The X-ray structures of a palla-
dium(II) and platinum(II) complex of the same Josiphos ligand are reported.
ꢀ 2007 Elsevier B.V. All rights reserved.
Keywords: Electrochemistry; Cyclic voltammetry; Crystal structures; Josiphos ligands
1. Introduction
is considered to be important to the effectiveness of the cat-
alyst [3,4]. Typically, chiral ferrocenylphosphine ligands
have a –PR2 group bonded directly to one of the C5 rings
of the ferrocene and a chiral functional group bonded to
the same C5 ring but alpha to the –PR2 group. The chiral
group, typically a –C*HR1R2R3 group, can have various
types of functional groups that change the electronic and/
or steric properties of the ligand. It is thought that this
functional group is what interacts with the reactants to
force the formation of a chiral product [3,4]. A more recent
development of the chiral ferrocenylphosphine ligands is a
diphosphine ligand where the second phosphine is part of
the chiral functional group that is bonded to the metallo-
cene. An example of these types of ligands is Josiphos
(Fig. 1). Josiphos ligands provide chiral catalysts that dis-
play two types of chirality, planar and central [5,6]. The
two types of chirality have been found to be advantageous
in a number of catalytic applications such as asymmetric
allylic aminations and the asymmetric reduction of b-b-
disubstituted enoates [5].
Asymmetric catalysis has become a growing field of
study as the demand for more enantiomerically pure com-
pounds arises. The major demand for these types of com-
pounds is in the pharmaceutical industry because of the
specificity needed for effective drugs [1]. Asymmetric catal-
ysis requires that a chiral catalyst be used in order to trans-
fer its chirality to the substrate. An effective asymmetric
catalyst will quickly produce a chiral product in good yield
with high enantiomeric purity of the desired enantiomer [2].
Compounds containing chiral metallocene ligands, in
particular chiral ferrocenylphosphine ligands, are one class
of asymmetric catalysts that has received significant atten-
tion in recent years due to the versatility of these catalysts
in a variety of applications [3,4]. The ridged planar chirality
enforced by the ferrocene backbone of ferrocenylphosphine
*
Corresponding author. Fax: +1 610 330 5714.
E-mail address: nataroc@lafayette.edu (C. Nataro).
Permanent address: Department of Chemistry and Biochemistry,
While complexes containing Josiphos ligands have pro-
ven to be effective in a number of catalytic applications the
reasons why they are effective are not completely under-
1
University of Massachusetts, Dartmouth, North Dartmouth, MA 02747,
United States.
0022-328X/$ - see front matter ꢀ 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2007.02.039

2366
B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
(1) R = Ph, R' = tBu
NMR. TMS (d = 0.00 ppm) was used as an internal stan-
dard for the H NMR acquisitions. An external standard,
1
(2) R = Cy, R' = tBu
R'
(3) R = 4-C6H4CF3, R' = tBu
P
85% H3PO4, was used as the reference value for the
31P{1H} NMR data. Elemental analysis was performed
by Quantitative Technologies, Inc. Ferrocene, decamethyl-
ferrocene and the Josiphos ligands were purchased from
Strem. Ferrocene was purified by sublimation prior to
use. Bis(benzonitrile)dichloropalladium(II) and cis-bis(ace-
tonitrile)dichloroplatinum(II), were purchased from
Aldrich. Compounds 1PdCl2 [31], 2PdCl2 [43], 5PdCl2
[44], 9PdCl2 [45] and 5PtCl2 [46] were prepared according
to literature procedures. Tetrabutylammonium hexafluro-
phosphate ([NBu4][PF6]) was purchased from Aldrich and
dried in vacuo prior to use. Tetrabutylammonium tetra-
(4) R = Cy, R' = Cy
(5) R = Ph, R' = Cy
(6) R = 3,5-Me2-4-OMeC6H2, R' = Cy
(7) R = Ph, R' = 3,5-C6H3Me2
(8) R = furanyl, R' = 3,5-C6H3Me2
(9) R = 3,5-C6H3(CF3)2, R' = Cy
(10) R = 3,5-C6H3(CF3)2, R' = 3,5-C6H3Me2
(11) R = Cy, R' = Ph
R'
P
R
R
Fe
Fig. 1. Josiphos ligands.
stood. Several ideas as to the ligand characteristics that are
the most important to the effectiveness of the catalyst have
been proposed [3,4]. Unlike many other metallocene con-
taining diphosphines, such as 1,10-bis(diphenylphosph-
ino)ferrocene (dppf), the phosphines of the Josiphos are
attached to the same C5 ring of the ferrocene backbone.
The process by which they are synthesized is straightfor-
ward allowing for great variety in the types of R groups
that can bonded to the phosphorus atoms [7,8]. Because
of its versatility, these phosphines have been complexed
with various metals and studied as catalysts in a variety
of asymmetric reactions [2,5–41]. The varying R groups
provide Josiphos complexes with a steric bulk that can
be either a hindrance or an advantage. The steric hin-
drance of Josiphos was seen as an advantage in the palla-
dium catalyzed amination of aryl tosylates due to the
increase in the rate of the reaction, however, the reason
for this increased rate is unknown [40]. In contrast, the
catalytic hydrogenation of 3-alkylidenelactams using Josi-
phos containing catalysts gave very low yields and enan-
tiomeric purity of the desired product due to the bulk of
Josiphos [34].
While most of the work using Josiphos ligands has
focused on catalytic properties, very little work has been
done in terms of investigating the electronic nature of Josi-
phos ligands. This work reports the electrochemistry of 11
Josiphos ligands. The synthesis of the Pd(II) and Pt(II)
dichloride complexes, which were characterized by NMR
and cyclic voltammetry, is also reported. X-ray crystal
structures were also obtained for a palladium and platinum
complex and provide greater insight into the bonding of
these ligands.
kis(pentafluorophenyl)borate
([NBu4][B(C6F5)4])
was
metathesized from Li[B(C6F5)4] (Boulder Scientific Co.)
according to the literature procedure [47].
2.2. Electrochemistry
All of the experiments were performed at ambient tem-
perature (22 1 ꢁC) under an Ar atmosphere using
10.0 mL of a 0.1 M solution of [NBu4][PF6] in CH2Cl2.
The working electrode was a 1.5 mm glassy carbon elec-
trode with a platinum wire as the auxiliary electrode and
an Ag/AgCl (non-aqueous) reference electrode separated
from the solution by a fine glass frit. The analyte concen-
tration was 1.0 mM and the cyclic votammetric data was
collected using a Princeton Applied Research 263-A poten-
tiostat at scan rates of 100–1000 mV/s at 100 mV/s inter-
vals. Depending on the potentials at which oxidation of
the analyte occurred, either ferrocene or decamethylferro-
cene was added at the end of the experiment to give a solu-
tion that was 1.0 mM in the standard [48].
Using a CH Instruments Model 630B Electrochemical
Analyzer, bulk electrolysis experiments were performed
under an atmosphere of argon. Platinum mesh baskets
were used as the working and auxiliary electrodes and the
electrodes were placed in compartments separated by a fine
glass frit. In the same compartment as the working elec-
trode was the Ag/AgCl reference electrode, but it was also
separated by a fine glass frit. Cyclic voltammograms were
obtained before and after the bulk electrolysis using a
1.5 mm glassy carbon working electrode. Linear scan vol-
tammograms were obtained with a 10 lm glassy carbon
working electrode at scan rates of 10 and 25 mV/s before
and after the bulk electrolysis. Experiments were conducted
in CH2Cl2 using either 0.1 M [NBu4][PF6] or 0.05 M
[NBu4][B(C6F5)4]. The concentration of the analyte was
2.0 mM and the temperature was 22 1 ꢁC.
2. Experimental
2.1. General procedures
Standard Schlenk techniques under an argon atmo-
sphere were used in the preparative reactions. Dichloro-
methane (CH2Cl2), diethyl ether (Et2O), and hexanes
were purified under Ar using a Solv-tek purification system
similar to one previously described [42]. Acetonitrile
(MeCN) was distilled over CaH2 under Ar. NMR spectra
were obtained in CDCl3 using a JEOL Eclipse 400 FT-
2.3. X-ray crystallography
Crystallographic data are contained in Table 1. Crystals
of 2PdCl2 and 2PtCl2 were mounted on nylon loops. Laue
symmetry and systematic absences in the data indicated a
monoclinic setting for 2PdCl2 and trigonal for 2PtCl2. In

B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
2367
compound, yields and 31P{1H} NMR data are presented
in Table 2. The H NMR and elemental analysis data for
Table 1
1
Crystal data and structure refinement for X-ray structures
[2PdCl2] Æ 0.67Et2O
[2PtCl2] Æ 0.67Et2O
all new compounds are presented below. Crystals of
2PdCl2 were prepared by dissolving the solid in 2.0 mL
of CH2Cl2 and then layered the solution with 20.0 mL of
Et2O. The solution was left in the freezer for 5 days and
a gel containing crystals of 2PdCl2 formed. The gel was
allowed to thaw; the solvent was decanted which yielded
crystals suitable for X-ray analysis.
Formula C104H176Cl6Fe3O2P6Pd3 C34.667H58.667Cl2FeO0.667P2Pt
Formula weight 2343.72
869.96
Trigonal
P3121
15.1502(6)
151502(6)
27.332(2)
90
90
120
5432.9(5)
6
1.505
0.30 · 0.20 · 0.20
Orange
100(2)
0.71073
4.513
Crystal system
Space group
Monoclinic
P21
˚
a (A)
18.1892(11)
16.8713(11)
19.3683(12)
90
111.3910(10)
90
5534.2(6)
2
1.406
˚
b (A)
˚
c (A)
Compound 3PdCl2: 1H NMR (CDCl3): d (ppm) 8.61 (m,
2H, –C6H4CF3), 7.9–7.5 (m, 6H, –C6H4CF3), 4.70 (br, s,
1H, C5H3), 4.50 (br, s, 1H, C5H3), 4.21 (m, 1H, CHMe),
a (ꢁ)
b (ꢁ)
c (ꢁ)
3
˚
V (A )
3
3.97 (s, 5H, Cp), 3.89 (br, s, 1H, C5H3), 2.11 (dd, JH–H
Z
Dcalc
3
3
and JH–P = 10.1 and 7.1 Hz, 3H, C–CH3), 1.64 (d, JH–P
t
3
t
Crystal size (mm) 0.30 · 0.30 · 0.10
Crystal color Orange
Temperature (K) 100(2)
= 13.6 Hz, 9H, Bu), 1.45 (d, JH–P = 15.0 Hz, 9H, Bu).
Anal. Calc. for C34H38Cl2F6FeP2Pd: C, 47.72; H, 4.48.
Found: C, 48.07; H, 4.34%.
˚
Radiation; k (A) 0.71073
1
Compound 4PdCl2: H NMR (CDCl3): d (ppm) 4.84
Absorption
1.137
(br, s, 1H, C5H3), 4.54 (br, s, 1H, C5H3), 4.51 (br, s, 1H,
CHMe), 4.26 (s, 5H, Cp), 3.67 (br, s, 1H, C5H3), 2.33–
0.77 (m, 47H, C–CH3 and –Cy). Anal. Calc. for
C36H56Cl2FeP2Pd Æ 0.25CH2Cl2: C, 54.07; H, 7.07. Found:
C, 54.30; H, 7.43%.
coefficient
F(000)
h Range (ꢁ)
Data collected
2448
2.56–27.49
2472
2.69–27.50
h
k
l
ꢀ23 to 23
ꢀ21 to 21
ꢀ25 to 24
ꢀ19 to 19
ꢀ19 to 19
ꢀ35 to 35
46487
Compound 6PdCl2: 1H NMR (CDCl3): d (ppm) 7.92 (d,
3
3JH–P = 12.8 Hz, 2H, –C6H2Me2OMe); 7.17 (d, JH–P
=
Number of data 49038
collected
Number of
unique data
Absorption
correction
Final R indices (obs. data)
R1
wR2
12.1 Hz, 2H, –C6H2Me2OMe); 4.56 (br, s, 1H, C5H3),
4.36 (br, s, 1H, C5H3), 4.21 (br, s, 1H, C5H3), 3.79 (s,
3H, –OCH3); 3.69 (s, 5H, Cp), 3.47 (m, 1H, CHMe), 2.39
(s, 6H, –CH3), 2.21 (s, 6H, –CH3), 1.8–1.2 (m, 22H, –
Cy); 0.91 (m, 3H, C–CH3). Anal. Calc. for C42H56-
Cl2FeO2P2Pd: C, 56.81; H, 6.36. Found: C, 56.55; H,
6.39%.
22065
7859
SADABS
SADABS
0.0375
0.0803
1.048
0.0229
0.0543
1.095
Goodness-of-fit
Compound 7PdCl2: 1H NMR (CDCl3): d (ppm) 8.1–7.0
(m, 16H, –C6H5 and –C6H3Me2), 4.42 (s, 1H, C5H3), 4.36
(s, 1H, C5H3), 4.01 (s, 1H, C5H3), 3.64 (s, 5H, Cp), 3.61
(m, 1H, CH–Me), 2.42 (s, 6H, –CH3), 2.18 (s, 6H, –
both cases, chiral space groups were indicated and these
choices were confirmed, along with the correct enantiomer,
by refinement of the Flack parameter, which yielded values
near zero for both. The structures were solved by direct
methods and refined using anisotropic thermal parameters
for the non-hydrogen atoms. Hydrogen atoms were ideal-
ized. In both cases, the asymmetric unit consists of a 3:2
ratio of complex to the recrystallization solvent, diethyle-
ther. For 2PtCl2, a model for the badly disordered solvent
was created using SQUEEZE [49]; for 2PdCl2, the solvent
was ordered and treated normally.
3
3
CH3), 1.00 (dd, JH–H and JH–P = 10.14 and 7.3 Hz, 3H,
C–CH3). Anal. Calc. for C40H40Cl2FeP2Pd: C, 58.89; H,
4.94. Found: C, 59.20; H, 5.02%.
Compound 8PdCl2: 1H NMR (CDCl3): d (ppm) 7.9–7.2
(m, 6H, –C6H3Me2); 7.01 (s, 2H, furanyl), 6.66 (m, 2H,
furanyl), 6.56 (m, 2H, furanyl), 4.25 (br, 2H, C5H3), 4.09
(s, 1H, C5H3), 3.89 (s, 5H, Cp), 3.62 (m, 1H, CHMe),
2.41 (s, 6H, –CH3), 2.22 (s, 6H, –CH3), 1.17 (m, 3H,
CH–CH3). Anal. Calc. for C36H36Cl2FeO2P2Pd: C, 54.33;
H, 4.56. Found: C, 54.50; H, 4.73%.
1
Compound 10PdCl2: H NMR (CDCl3): d (ppm) 8.5–
2.4. Synthesis
7.0 (m, 12H, –C6H3Me2 and –C6H3(CF3)2); 4.82 (br, s,
1H, C5H3), 4.58 (br, s, 1H, C5H3), 4.07 (br, s, 1H, C5H3),
3.82 (m, 1H, CHMe), 3.62 (s, 5H, Cp), 2.40 (s, 6H, –
CH3), 2.18 (s, 6H, –CH3), 1.22 (m, 3H, C–CH3). Anal.
Calc. for C44H36Cl2F12FeP2Pd: C, 48.58; H, 3.34. Found:
C, 48.24; H, 3.58%.
2.4.1. Palladium complexes
Equal molar amounts (approximately 0.05 mmol) of the
desired Josiphos ligand and (C6H5CH2CN)2PdCl2 were
dissolved in 5.0 mL of CH2Cl2 and stirred for 24 h. The
solution was then filtered using a cannula and the solvent
was removed in vacuo. The solid was washed with three
5.0 mL portions of Et2O and the resulting solid was dried
in vacuo, yielding the desired complex. The colors of the
1
Compound 11PdCl2: H NMR (CDCl3): d (ppm) 7.8–
7.3 (m, 10H, –C6H5); 4.70 (br, s, 1H, C5H3), 4.35 (br, s,
1H, C5H3), 4.23 (br, s, 1H, C5H3), 3.48 (s, 5H, Cp), 3.42
(m, 1H, CHMe), 2.4–1.1 (m, 25H, C–CH3 and –Cy).

2368
B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
Table 2
Data for(1–11)PdCl2
31P{1H} NMR data
31Pa
% Yield
Color
2JP–P
31Pa
2JP–P
b
b
3PdCl2
4PdCl2
6PdCl2
7PdCl2
8PdCl2
10PdCl2
11PdCl2
24.8
33.1
80.9
47.9
60.3
49.0
65.6
s
s
113.4
91.4
20.7
s
s
32
20
50
58
33
48
63
Red
Light orange
Light brown
Yellow
Orange
Red-brown
Orange
6.94
s
s
10.4
s
6.94
s
s
10.4
s
16.4
ꢀ13.4
15.9
34.1
a
In ppm.
In Hz.
b
3
C36H44Cl2FeP2Pd Æ 0.5CH2Cl2: C, 53.84; H, 5.57. Found:
C, 53.67; H, 5.63%.
s, 1H, C5H3), 3.30 (m, 1H, CHMe), 2.08 (dd, JH–H and
3JH–P = 10.4 and 7.1 Hz, 3H, C–CH3), 1.58 (d, JH–P
=
3
t
3
t
16.5 Hz, 9H, Bu), 1.41 (d, JH–P = 14.3 Hz, 9H, Bu).
Anal. Calc. for C32H52Cl2FeP2Pt: C, 47.54; H, 4.99.
Found: C, 47.91; H, 4.84%.
2.4.2. Platinum complexes
Equal molar amounts (approximately 0.05 mmol) of
the desired Josiphos ligand and (CH3CN)2PtCl2were dis-
solved in 5.0 mL of CH2Cl2 and stirred for 1 h. The solu-
tion was filtered via cannula and the volume was reduced
to approximately 0.5 mL. Et2O (10.0 mL) was then added
causing the product to precipitate. The solution was fil-
tered using a cannula and the solid was washed with
three 5.0 mL portions of Et2O. The remaining solid was
dried in vacuo, yielding the desired complex. Data for
the characterization of these platinum complexes is found
Compound 2PtCl2: 1H NMR (CDCl3): d (ppm) 4.88
(br, s, 1H, C5H3), 4.54 (br, s, 1H, C5H3), 4.50 (br, s,
1H, C5H3), 4.28 (s, 5H, Cp), 3.88 (m, 1H, CHMe), 2.00
3
3
(dd, JH–H and JH–P = 10.4 and 7.2 Hz, 3H, C–CH3),
1.63 (d, JH–P = 13.2 Hz, 9H, tBu), 1.24 (d, JH–
3
3
t
P = 14.3 Hz, 9H, Bu), 2.56–0.87 (m, 22H, –Cy). Anal.
Calc. for C32H52Cl2FeP2Pt: C, 46.84; H, 6.39. Found:
C, 46.62; H, 6.53%.
Compound 3PtCl2: 1H NMR (CDCl3): d (ppm) 8.60 (m,
2H, –C6H4CF3), 7.9–7.6 (m, 6H, –C6H4CF3), 4.69 (br, s,
1H, C5H3), 4.47 (br, s, 1H, C5H3), 4.21 (m, 1H, CHMe),
1
in Table 3. The elemental analysis and H NMR data for
the new complexes are presented below. Crystals of
2PtCl2 were prepared by dissolving the complex in
approximately 0.5 mL of CH2Cl2 in a vial. The vial was
then placed in a second vial containing Et2O. The outer
vial was capped and place in a refrigerator. The Et2O
slowly diffused into the CH2Cl2 solution over 5 days
resulting in the formation of a gel. The gel was pierced
with a needle and after 48 h crystals suitable for X-ray
analysis formed.
3
3.97 (s, 5H, Cp), 3.85 (br, s, 1H, C5H3), 2.11 (dd, JH–H
and JH–P = 10.2 and 7.3 Hz, 3H, C–CH3), 1.61 (d, JH–
P = 12.8 Hz, 9H, Bu), 1.42 (d, JH–P = 14.6 Hz, 9H, Bu).
Anal. Calc. for C34H38Cl2F6FeP2Pt Æ 0.5CH2Cl2: C, 41.99;
H, 3.98. Found: C, 41.70; H, 3.63%.
3
3
t
3
t
Compound 4PtCl2: 1H NMR (CDCl3): d (ppm) 4.80 (br,
s, 1H, C5H3), 4.50 (br, s, 1H, C5H3), 4.45 (br, s, 1H,
CHMe), 4.25 (s, 5H, Cp), 3.64 (br, s, 1H, C5H3), 2.3–0.8
(m, 47H, C–CH3 and –Cy). Anal. Calc. for
C36H56Cl2FeP2Pt Æ 0.25 CH2Cl2: C, 48.71; H, 6.37. Found:
C, 48.34; H, 6.22%.
Compound 1PtCl2: 1H NMR (CDCl3): d (ppm) 8.48 (m,
2H, –C6H5), 7.6–7.3 (m, 8H, –C6H5), 4.60 (br, s, 1H,
C5H3), 4.38 (br, s, 1H, C5H3), 3.97 (s, 5H, Cp), 3.85 (br,
Table 3
Data for (4-11)PtCl2
31P{1H} NMR data
% Yield
Color
31Pa
2JP–P
1JP–Pt
31Pa
2.35
2JP–P
1JP–Pt
b
b
b
b
1PtCl2
2PtCl2
3PtCl2
4PtCl2
6PtCl2
7PtCl2
8PtCl2
9PtCl2
10PtCl2
11PtCl2
a
76.7
75.8
77.1
54.6
45.8
24.5
32.1
46.3
25.2
38.3
13.9
13.8
17.3
s
13.9
24.3
20.7
17.3
17.3
17.3
3800
3830
3760
3680
3580
3630
3580
3430
3470
3760
13.9
13.9
17.3
s
13.9
24.3
20.8
17.4
17.3
17.3
3600
3530
3630
3520
3600
3500
3640
3570
3500
3410
37
37
20
69
62
36
97
77
93
60
Orange
Orange
5.43
3.40
8.82
Yellow-orange
Light orange
Orange
Light orange
Dark yellow
Orange
ꢀ1.59
ꢀ2.71
ꢀ32.4
2.98
ꢀ0.37
Light orange
Orange
11.1
In ppm.
b
In Hz.

B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
2369
Compound 6PtCl2: 1H NMR (CDCl3): d (ppm) 7.90 (d,
The oxidative electrochemistry of compounds 1–11 was
investigated by cyclic voltammetry. Most of the Josiphos
ligands displayed three oxidative waves (Table 4); the one
at least positive potential is chemically and electrochemi-
cally irreversible, the intermediate wave displays varying
amounts of chemical reversibility and the wave at the most
positive potential is chemically and electrochemically
reversible (Fig. 2). Ligands 2 and 4 display an additional
wave that is partially chemically reversible. Ligands 2 and
4 are the only ligands that do not have aryl groups off
either phosphine which may account for this additional
wave. Ligands 9 and 10, the only ligands that have 3,5-
C6H3(CF3)2 groups, do not display the intermediate wave
mentioned previously. There has been one previous report
of the anodic electrochemistry of 11 in which three waves
are noted, but the nature of these waves was not examined
in detail [50].
3
3JH–P = 12.8 Hz, 2H, –C6H2Me2OMe); 7.15 (d, JH–P
=
12.1 Hz, 2H, –C6H2Me2OMe); 4.53 (br, s, 1H, C5H3),
4.33 (br, s, 1H, C5H3), 4.18 (br, s, 1H, C5H3), 3.77 (s,
3H, –OCH3); 3.54 (s, 5H, Cp), 3.45 (m, 1H, CHMe), 2.42
(s, 6H, –CH3), 2.24 (s, 6H, –CH3), 1.8–1.2 (m, 22H, –
Cy); 0.94 (m, 3H, C–CH3). Anal. Calc. for
C42H56Cl2FeO2P2Pt: C, 51.65; H, 5.78. Found: C, 51.51;
H, 5.96%.
1
Compound 7PtCl2: H NMR (CDCl3): d (ppm) 8.1–7.0
(m, 16H, –C6H5 and –C6H3Me2), 4.37 (s, 1H, C5H3), 4.32
(s, 1H, C5H3), 3.97 (s, 1H, C5H3), 3.77 (m, 1H, CH–Me),
3.57 (s, 5H, Cp), 2.44 (s, 6H, –CH3), 2.19 (s, 6H, –CH3),
0.91 (m, 3H, C–CH3). Anal. Calc. for C40H40Cl2FeP2Pt:
C, 53.11; H, 4.46. Found: C, 52.90; H, 4.83%.
1
Compound 8PtCl2: H NMR (CDCl3): d (ppm) 7.9–7.2
(m, 6H, –C6H3Me2); 6.99 (s, 2H, furanyl), 6.63 (m, 2H,
furanyl), 6.53 (m, 2H, furanyl), 4.24 (br, 2H, C5H3), 4.09
(s, 1H, C5H3), 3.75 (s, 5H, Cp), 3.62 (m, 1H, CHMe),
2.43 (s, 6H, –CH3), 2.24 (s, 6H, –CH3), 2.17 (m, 3H,
CH–CH3). Anal. Calc. for C36H36Cl2FeO2P2Pt: C, 48.89;
H, 4.10. Found: C, 48.75; H, 3.97%.
Bulk anodic electrolysis, linear scan voltammetry (LSV)
and cyclic voltammetry (CV) of 1 was performed in
CH2Cl2 using either [NBu4][PF6] or [NBu4][B(C6F5)4] as
1
Compound 9PtCl2: H NMR (CDCl3): d (ppm) 8.6–7.8
Table 4
(m, 6H, –C6H3(CF3)2); 4.78 (br, s, 1H, C5H3), 4.53 (br, s,
1H, C5H3), 4.23 (br, s, 1H, C5H3), 3.63 (s, 5H, Cp), 3.47
(m, 1H, CHMe), 2.5–1.2 (m, 22H, –Cy), 0.87 (m, 3H, C–
CH3). Anal. Calc. for C40H40Cl2F12FeP2Pt Æ 0.5CH2Cl2:
C, 41.40; H, 3.52. Found: C, 41.70; H, 3.63%.
Electrochemistry of Josiphos ligands and complexes in a solution of 0.1 M
[NBu4][PF6] in CH2Cl2
Compound Eopx (V vs.
Fc0/+
Eopx (V vs.
Eopx (V vs.
Fc0/+
E0 (V vs.
Fc0/+
)
)
Fc0/+
)
)
1
ꢀ0.01
ꢀ0.02
0.33
0.43
0.35
0.33
0.26
0.33
0.31
0.41
0.40
0.41
0.25
0.33
0.32
0.34
0.43
0.44
0.32
0.30
0.29
0.31
0.33
0.32
0.31
0.30
0.31
0.47
0.56
1a
Compound 10PtCl2: 1H NMR (CDCl3): d (ppm) 8.5–7.0
(m, 12H, –C6H3Me2 and –C6H3(CF3)2); 4.74 (br, s, 1H,
C5H3), 4.54 (br, s, 1H, C5H3), 3.99 (br, s, 1H, C5H3),
3.82 (m, 1H, CHMe), 3.58 (s, 5H, Cp), 2.41 (s, 6H, –
CH3), 2.17 (s, 6H, –CH3), 1.16 (m, 3H, C–CH3). Anal.
Calc. for C36H44Cl2FeP2Pt: C, 50.25; H, 5.15. Found: C,
50.13; H, 5.06%.
1PdCl2
1PtCl2
2
ꢀ0.10
0.14
ꢀ0.12
0.03
0.01
0.10
0.08
0.25
0.27
0.00
0.39
0.51
0.50
0.68
0.46
0.45
0.47
0.47
0.47
0.45
0.65
2PdCl2
2PtCl2
3
3PdCl2
3PtCl2
4
Compound 11PtCl2: 1H NMR (CDCl3): d (ppm) 7.9–7.2
(m, 10H, –C6H5); 4.64 (br, s, 1H, C5H3), 4.31 (br, s, 1H,
C5H3), 4.24 (br, s, 1H, C5H3), 4.23 (s, 5H, Cp), 3.47 (m,
1H, CHMe), 2.4–1.1 (m, 25H, C–CH3 and –Cy). Anal.
Calc. for C44H36Cl2F12FeP2Pt Æ CH2Cl2: C, 42.85; H,
3.04. Found: C, 42.70; H, 3.22%.
0.42
4PdCl2
4PtCl2
5
5PdCl2
5PtCl2
6
6PdCl2
6PtCl2
7
3. Results and discussion
7PdCl2
7PtCl2
8
The new complexes reported in this study were prepared
using equal molar amounts of the desired Josiphos ligand
and the appropriate MCl2(nitrile)2 (M = Pd or Pt). The
reactions were performed in methylene chloride and the
products precipitated upon addition of ether. The products
8PdCl2
8PtCl2
9
1
were characterized by H and 31P{1H} NMR. Since the
9PdCl2
9PtCl2
10
0.48
0.49
phosphorus atoms are not equivalent, two peaks were
2
observed in the 31P{1H} NMR. The JP–P coupling con-
10PdCl2
10PtCl2
11
0.51
0.49
0.44
0.33
0.33
stants were typically in the range of 10–20 Hz, although
in a number of the compounds, coupling was either not
observed or unresolved. For the species containing plati-
11PdCl2
11PtCl2
a
1
num, the JP–Pt coupling constants were approximately
In a 0.05 M solution of [NBu4][B(C6F5)4] in CH2Cl2.
3500 Hz.

2370
B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
based on a cyclic voltammogram of the bulk solution,
T = 22 + 1 ꢁC). After the bulk electrolysis, the solution
was examined by CV. Sweeping the potential positive of
that used for the electrolysis showed the two waves at more
positive potentials (Fig. 4) however no electroactive fea-
tures were observed on the negative potential sweep. The
oxidative LSV also displayed two features (Fig. 3) in the
presence of either supporting electrolyte.
The second bulk electrolysis, for the wave marked B
(Fig. 2), results in the transfer of one electron ([NBu4][PF6]:
napp = 1.00 eꢀ, Eapp = 1.12 V vs. Ag/AgCl based on a cyc-
lic voltammogram of the bulk solution, T = 22 + 1 ꢁC;
[NBu4][B(C6F5)4]: napp = 0.98 eꢀ, Eapp = 1.54 V vs. Ag/
AgCl based on a cyclic voltammogram of the bulk solu-
tion, T = 22 + 1 ꢁC). The CV and LSV after the second
bulk oxidation in the presence of PFꢀ6 display one electro-
active species which suggests that a reaction has occurred.
The CV and LSV after the second oxidation in the presence
Fig. 2. Cyclic voltammetry scan of the oxidation of 1.0 mM 1 in CH2Cl2/
0.10 M [NBu4][PF6] at 100 mV/s.
of BðC6F5Þꢀ display two waves indicating the presence of
4
two electroactive species (Fig. 3).
the supporting electrolyte. Prior to bulk electrolysis, the
LSV of 1 in solutions containing either supporting electro-
lyte displayed multiple features (Fig. 3). The diffusion coef-
ficient of 1 was determined to be 1.9 · 10ꢀ5 in [NBu4][PF6]
and 2.1 · 10ꢀ5 in [NBu4][B(C6F5)4]. Bulk electrolysis at a
potential positive of the wave labeled A (Fig. 2) gave
one-electron transferred ([NBu4][PF6]: napp = 0.94 eꢀ,
Eapp = 0.75 V vs. Ag/AgCl based on a cyclic voltammo-
gram of the bulk solution, T = 22 + 1 ꢁC; [NBu4]-
[B(C6F5)4]: napp = 0.96 eꢀ, Eapp = 1.15 V vs. Ag/AgCl
The wave marked C (Fig. 2) was a one-electron oxida-
tion ([NBu4][PF6]: napp = 0.94 eꢀ, Eapp = 1.55 V vs. Ag/
AgCl based on a cyclic voltammogram of the bulk solu-
tion, T = 22 + 1 ꢁC; [NBu4][B(C6F5)4]: napp = 0.98 eꢀ,
Eapp = 1.80 V vs. Ag/AgCl based on a cyclic voltammo-
gram of the bulk solution, T = 22 + 1 ꢁC). In PFꢀ6 , the
LSV displays no electroactive species after the third bulk
oxidation, while in BðC6F5Þꢀ there is one feature (Fig. 3).
4
In addition, the CV in PFꢀ6 displays one reversible wave
Fig. 3. Linear scan voltammery scans of the oxidation of 2.0 mM 1 in
CH2Cl2/0.050 M [NBu4][B(C6F5)4] at 25 mV/s prior to bulk electrolysis
Fig. 4. Cyclic voltammery scans of the oxidation of 2.0 mM 1 in CH2Cl2/
0.050 M [NBu4][B(C6F5)4] at 100 mV/s prior to bulk electrolysis (bottom),
after the first bulk electrolysis (second from bottom), after the second bulk
electrolysis (second from top) and after the third bulk electrolysis (top).
(
(
), after the first bulk electrolysis ( ), after the second bulk electrolysis
) and after the third bulk electrolysis ( ).

B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
2371
+
+2
tBu
tBu
tBu
Ph
tBu
tBu
Ph
tBu
tBu
Ph
P
P
O
P
O
P
O
tBu
Ph
P
P
P
P
- e-
[O]
- e-
- e-
Ph
Ph
Ph
Fe(II)
Ph
+ e-
Fe(II)
+ e-
Fe(II)
Fe(III)
Scheme 1. Proposed mechanism for the oxidation of 1 on the CV timescale.
while the CV in BðC6F5Þꢀ displays a reversible wave and a
oxidation of the phosphine oxide is reversible on the CV
time scale, however, in the presence of PFꢀ6 and on the bulk
electrolysis time scale, the oxide is converted to a difluo-
4
chemically reversible wave (Fig. 4).
Oxidation of 1 in the presence of either supporting elec-
trolyte likely follows an ECEE mechanism on the CV time-
scale (Scheme 1). The product after the first oxidation and
chemical reaction is likely the phosphine oxide formed at
the phosphorus bound to the C5 ring. In comparing the
potentials of 1–3, the first wave (A) is very sensitive to
the groups on the phosphorus bound directly to the C5
ring, which also suggests that the initial oxidation occurs
at that site. A 31P{1H} spectrum taken after the initial bulk
electrolysis in either supporting electrolyte shows a small
downfield shift (from 54 to approximately 50 ppm) for
the –P(tBu)2 group and a much more significant shift (from
ꢀ25 to approximately 30 ppm) for the –PPh2 group. The
31P shift for the –PPh2 group in 1 and the product of the
first bulk electrolysis are reminiscent to those seen for dppf,
ꢀ17 ppm [51,52], and the corresponding phosphine oxide,
1,10-bis(diphenylphoshineoxide)ferrocene (dppfO2), which
occurs at 28.3 ppm [53].
ride. Although BðC6F5Þꢀ is well known for its ability to sta-
4
bilize highly reactive species [54,58], the CV in the presence
of BðC6F5Þꢀ (Fig. 4) indicates chemical irreversibility of the
4
second wave, suggesting that while more stable in the pres-
ence of this supporting electrolyte, the product of this oxi-
dation will undergo a reaction. On the bulk electrolysis
time scale, the oxidation of 1 likely follows an ECECE
mechanism in which the second chemical step is dependent
on the nature of the supporting electrolyte.
The product of the third bulk electrolysis was not iso-
lated and its structure is unknown. In both cases, the color
of the solution changed to green upon completing the third
bulk oxidation. The 31P{1H} spectrum of the PFꢀ6 contain-
ing solution is identical to the spectrum after the second
bulk electrolysis. The spectrum of the solution containing
BðC6F5Þꢀ displays two singlets at 72 and 47 ppm. The sig-
4
nificant color change and the small shift in the 31P spectra
suggest that the third oxidation occurs at the iron center.
Further studies to isolate the oxidation products of these
steps and investigate the low temperature stability of the
oxidation products are underway.
The product of the second bulk electrolysis could not be
isolated and the exact composition remains unknown. In
solutions of either supporting electrolyte, the product of
the oxidation appears to be stable on the CV time scale
based on the reversibility of the wave. However, the solu-
tions appear quite different after the bulk electrolysis; the
color of the solution containing PFꢀ6 did not change signif-
The new PdCl2 and PtCl2 complexes of 1–4 and 6–11
were prepared by the reaction of the phosphines with the
appropriate dichlorobisnitrile metal species. The products
were characterized by NMR and, in general, the 31P signals
of the complexes were shifted downfield as compared to the
free phosphines. In most of the compounds, phosphorus-
phosphorus coupling was seen suggesting that both phos-
phorus atoms were bonded to the Pd(II) or Pt(II). For
the PtCl2 complexes, 195Pt satellites were observed indicat-
ing that both phosphorus atoms are bonded directly to the
Pt.
icantly while the solution containing BðC6F5Þꢀ was yellow-
4
green.
The NMR data suggests that on the bulk electrolysis
time scale the product of the second oxidation is less stable
in the presence of PFꢀ6 than BðC6F5Þꢀ. The 31P{1H} spec-
4
trum of the bulk solution containing PFꢀ6 displayed a sin-
glet at 51 ppm and a triplet (or overlapping doublet of
doublets) at ꢀ15 ppm (J = 974 Hz). In the 31P{1H} spec-
trum of the solution containing BðC6F5Þꢀ there were two
The structure of [2PdCl2] (Fig. 5) was determined by X-
ray diffraction. There are three distinct molecules of
[2PdCl2] in the unit cell; average bond lengths and angles
for the three crystallographically independent molecules
4
singlets, one at 62 ppm and the other at 47 ppm. Similar
instability of oxidized species in the presence of PFꢀ6 but
not BðC6F5Þꢀ have been noted for other systems [54,55].
4
The upfield shift of the peak in the 31P NMR spectrum
and the large coupling constant suggest that the phospho-
rus bound to the C5 ring could be bonded to two fluorine
atoms. The 31P NMR spectrum of PPh3F2 shows a triplet
at ꢀ56.1 ppm with a 1JP–F of 666 Hz [56]. The electrochem-
ical preparation of R3PF2 compounds from R3P@O in the
presence of HF has been reported [57]. We propose that the
are presented in Table 5. The P1–Pd distance is 0.036 A
˚
longer than the P2–Pd distance. In the asymmetric com-
plexes, [2PdBrPh] and [2PdIPh], the differences were
˚
0.131 and 0.1151 A respectively; the difference in P–Pd
bond lengths was attributes to the stronger trans-influence
of phenyl as compared to the halide [29,43]. Somewhat sur-
prisingly, the P–Pd distances are not significantly different

2372
B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
Fig. 5. ORTEP diagram of 2PdCl2.
Fig. 6. ORTEP diagram of 2PtCl2.
Table 5
˚
Average selected bond lengths (ꢁ) and distances (A) for 2PdCl2
different Group 10 metals that have the same ancillary
ligands, in this case chloride. The P–Pt–P bite angle is
slightly larger than the corresponding angle in the palla-
dium analogue (Table 6). A similar trend has been noted
in Pd(dppf)Cl2 and Pt(dppf)Cl2 [59,60]. As in the palladium
analogue, the platinum in 2PtCl2 is distorted from square
planar geometry (Fig. 6). The angle between P1–Pt–P2
plane and the Cl1–Pt–Cl2 plane is 14.92ꢁ, which is signifi-
cantly less than that found for the palladium analogue.
In addition, the Me group of the stereocenter is pointing
away from the platinum atom.
The oxidative electrochemistry of the Josiphos ligands
simplified upon coordination to either Pd or Pt; only one
chemically and electrochemically reversible oxidation was
present (Fig. 7). Bulk anodic electrolysis confirmed that
the oxidation of 1PdCl2 and 1PtCl2 are one-electron pro-
cesses. For 1PdCl2 the color changed from red to green-yel-
low upon oxidation (napp = 1.0 eꢀ, Eapp = 1.05 V vs. Ag/
AgCl based on a cyclic voltammogram of the bulk solution,
T = 22 1 ꢁC) while 2PtCl2 changed from yellow-orange
Bond lengths
Pd–P1
2.312
2.365
1.883
ꢀ0.306
Pd–P2
2.276
2.354
1.810
ꢀ0.135
Pd–Cl1
P1–Cchiral
Pd–Cl2
P2–CCp
Avg. dC
a
b
Avg. dP
Bond angles
P–Pd–P
97.03
92.41
167.88
173.85
Cl–Pd–Cl
P2–Pd–Cl2
P2–Pd–Cl1
hd
86.48
87.74
160.53
7.48
P1–Pd–Cl1
P1–Pd–Cl2
c
Xa–Fe–Xb
a
Deviation of the P2 atom from the C5 plane, a positive value means the
P2 is closer to the Fe.
b
Deviation of the Cchiral atom from the C5 plane, a positive value means
the P is closer to the Fe.
c
Centroid–Fe–centroid.
The dihedral angle between the two C5 rings.
d
2
˚
(0.00085 A) in the related symmetric complex, [2Pd(g -
trans-stilbene)] [43]. The geometry about the palladium
can be described as distorted square planar. The P–Pd–P
angle is greater than 90ꢁ while the Cl–Pd–Cl angle is less
than 90ꢁ and the average angle between the planes defined
by P1–Pd–P2 and Cl1–Pd–Cl2 is 21.23ꢁ. The P–Pd–P bite
angle in [2PdCl2] is approximately 1ꢁ smaller than that of
[2PdBrPh] which is the smallest bite angle of 2 previously
reported [29]. Looking down the P1–Cchiral bond, the Me
group on the chiral carbon is anti- to the palladium.
The structure of [2PtCl2] was also determined, making it
the second structure of a platinum Josiphos complex
reported (Fig. 6). The P–Pt bond lengths in [2PtCl2] are
Table 6
˚
Average selected bond lengths (ꢁ) and distances (A) for 2PtCl2
Bond lengths
Pt–P1
2.2632(9)
2.3696(9)
1.886(4)
ꢀ0.337
Pt–P2
2.2870(10)
2.3674(9)
1.813(4)
ꢀ0.069
Pt–Cl1
P1–Cchiral
Pt–Cl2
P2–CCp
Avg. dC
a
b
dP
Bond angles
P–Pt–P
97.33(3)
96.95(3)
163.30(3)
173.56
Cl–Pt–Cl
P2–Pt–Cl2
P2–Pt–Cl1
hd
83.54(3)
93.80(3)
172.11(3)
7.97
P1–Pt–Cl1
P1–Pt–Cl2
˚
approximately 0.03 A longer than the P–Pt distances in
the other reported Josiphos platinum structure, [5PtCl2]
[47]. In addition, the P–Pt–P bite angle in [2PtCl2] is
approximately 2ꢁ larger than that found for the analogous
complex with 5. These differences are likely due to the more
sterically demanding tert-butyl groups in 2.
c
Xa–Fe–Xb
a
Deviation of the P2 atom from the C5 plane, a positive value means the
P2 is closer to the Fe.
b
Deviation of the Cchiral atom from the C5 plane, a positive value means
the P is closer to the Fe.
c
Centroid–Fe–centroid.
The dihedral angle between the two C5 rings.
The structures of [2PdCl2] and [2PtCl2] provide the first
opportunity to compare one Josiphos ligand bound to two
d

B.L. Ghent et al. / Journal of Organometallic Chemistry 692 (2007) 2365–2374
2373
Fig. 7. Cyclic voltammetric scan of the oxidation of 1.0 mM 1PdCl2 in CH2Cl2/0.10 M [NBu4][PF6] at 100 mV/s.
to green (napp = 1.0 eꢀ, Eapp = 1.00 V vs. Ag/AgCl based on
cyclic voltammogram of the bulk solution,
tures of 2PdCl2 and 2PtCl2 were determined and the pal-
ladium and platinum atoms are distorted from square-
planar geometry.
a
T = 22 1 ꢁC). The potentials at which oxidation of the
palladium complexes occur are within 0.02 V of the analo-
gous platinum complexes. The potentials are sensitive to
changing the groups on the Josiphos ligands, in particular
the groups on the phosphorus bound directly to the C5 ring;
comparing the complexes of ligands 4, 5, and 11 provides
the best example of this. The potentials at which oxidation
of the palladium and platinum complexes of 4 and 11, in
which the only difference is cyclohexyl groups at the phos-
phorus off the chiral carbon, 4, vs. phenyl, 11 are quite sim-
ilar. However, there is an approximately 0.1 V difference
between complexes of 4, in which the change is cyclohexyl
groups on the phosphorus bound to the C5 rings, vs. com-
plexes 5, in which there are phenyl groups. The anticipated
effect of electron withdrawing fluorinated-groups making
the potential at which oxidation occurs more positive can
be seen by comparing complexes of 1 vs. 3, 5 vs. 9, and 7
vs. 10.
Acknowledgments
B.L.G., S.L.M., L.A.S., and C.N. thank the donors of
the Petroleum Research Fund, administered by the Amer-
ican Chemical Society, for partial funding of this research,
the Kresge Foundation for the purchase of the JEOL
NMR and the Academic Research Committee at Lafayette
College for funding EXCEL scholars. C.N. thanks Profes-
sor R.J. LeSuer of Chicago State University for helpful dis-
cussion in preparing this manuscript.
Appendix A. Supplementary material
CCDC 627003 and 627004 contain the supplementary
crystallographic data for 2PdCl2 and 2PtCl2. These data
can be obtained free of charge via http://www.ccdc.cam.
ac.uk/conts/retrieving.html, or from the Cambridge
Crystallographic Data Centre, 12 Union Road, Cambridge
CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: depos-
it@ccdc.cam.ac.uk. Supplementary data associated with
this article can be found, in the online version, at
doi:10.1016/j.jorganchem.2007.02.039.
4. Summary
The oxidative electrochemistry of Josiphos ligands is
complicated, displaying multiple waves in the CV. The
oxidation is proposed to follow an ECEE mechanism
on the CV timescale. The first oxidation is followed by
a fast chemical reaction leading to the formation of the
corresponding phosphine oxide. Evidence from the 31P
NMR indicates that the phosphine oxide forms on the
phosphorus bound directly to the C5 ring. The electro-
chemical process at the intermediate potential is likely
due to oxidation of the phosphine oxide, possibly at a
phenyl ring [61]. The final wave is likely due to oxidation
of the iron center. Upon coordination to Pd(II) and
Pt(II), the oxidative electrochemistry of Josiphos ligands
simplify, giving rise to one reversible wave. The potentials
at which oxidation of the Josiphos containing compounds
occur are sensitive to the varying R groups. The struc-
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