Synthesis, crystal structure and comparative electrochemistry of metallocenyldiphenylphosphines of ruthenocene, osmocene, ferrocene and cobaltocenium hexafluorophosphate

Article abstract of DOI:10.1016/j.jorganchem.2013.12.027

The metallocenyldiphenylphosphines [M(C₅H₅)(C ₅H₄PPh₂)] with M = Fe(II) (ferrocenyl = Fc), 1, Ru(II) (ruthenocenyl = Rc), 2, Os(II) (osmocenyl = Oc), 3, and Co(III) ⁺PF₆^- (cobaltocenium hexafluorophosphate = [Cc][PF₆]), 4, were synthesized and the crystal structure of RcPPh₂, 2, (Z = 4, monoclinic, space group P2₁/c) was determined. The differences in reactivity of each metallocenyl derivative were such that 1 could be obtained from a Friedel Crafts reaction between ferrocene and PPh₂Cl in the presence of AlCl₃ as catalyst. Both the ruthenocene and osmocene derivatives 2 and 3 were obtained by reacting the monolithiated metallocene precursor with PPh₂Cl. However, monolithiation of ruthenocene had to be achieved via a stoichiometric amount of ^tBuLi. For osmocene, monolithiation was achieved by a 20% excess of ⁿBuLi. This was evidenced by the failure to isolate any bisphosphine, Oc(PPh₂)₂, during workup. Complex 4 could not be obtained via phosphination of free cobaltocenium hexafluorophosphate. Phosphine derivatisation of free cyclopentadiene prior to complexation with Co ^III was required to form [CcPPh₂][PF₆], 4. The electrochemistry of all phosphines was studied by voltammetric techniques in CH₂Cl₂/0.1 mol dm^-3 [N(ⁿBu) ₄][B(C₆F₅)₄]. A reversible one-electron transfer process for the ferrocenyl group of 1 was observed at 0.078 V vs. FcH/FcH⁺. The osmocenyl and ruthenocenyl derivatives exhibited irreversible metallocenyl oxidations at 0.355 and 0.476 V respectively. The cobaltocenium complex, 4, exhibited two reversible one-electron transfer reductions to liberate first a neutral Co^II cobaltocene species at -1.062 V and then an anionic Co^I cobaltocene species at -2.122 V. A single electrochemical irreversible, one-electron oxidation at the phosphorus centre which forms a quickly-decomposing phosphorus radical cation, Mc⁺Ph₂P⁺, was also observed at E_pa > 0.754 V. The newly-formed Mc⁺Ph₂P ⁺ species or its chemical decomposition products can be oxidized at E_pa > 1.090 V vs. FcH/FcH⁺.

Full text of DOI:10.1016/j.jorganchem.2013.12.027

Journal of Organometallic Chemistry 754 (2014) 80e87
Contents lists available at ScienceDirect
Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
Synthesis, crystal structure and comparative electrochemistry
of metallocenyldiphenylphosphines of ruthenocene, osmocene,
ferrocene and cobaltocenium hexafluorophosphate
*
Eleanor Fourie, J. Marthinus Janse van Rensburg, Jannie C. Swarts
Department of Chemistry, University of the Free State, PO Box 339, Bloemfontein 9300, South Africa
a r t i c l e i n f o
a b s t r a c t
Article history:
The metallocenyldiphenylphosphines [M(C5H5)(C5H4PPh2)] with M ¼ Fe(II) (ferrocenyl ¼ Fc), 1, Ru(II)
(ruthenocenyl
¼
Rc), 2, Os(II) (osmocenyl
¼
Oc), 3, and Co(III)þPFꢀ6 (cobaltocenium
Received 6 September 2013
Received in revised form
10 December 2013
hexafluorophosphate ¼ [Cc][PF6]), 4, were synthesized and the crystal structure of RcPPh2, 2, (Z ¼ 4,
monoclinic, space group P21/c) was determined. The differences in reactivity of each metallocenyl de-
rivative were such that 1 could be obtained from a Friedel Crafts reaction between ferrocene and PPh2Cl
in the presence of AlCl3 as catalyst. Both the ruthenocene and osmocene derivatives 2 and 3 were ob-
tained by reacting the monolithiated metallocene precursor with PPh2Cl. However, monolithiation of
Accepted 13 December 2013
Keywords:
Ferrocene
Ruthenocene
t
ruthenocene had to be achieved via a stoichiometric amount of BuLi. For osmocene, monolithiation was
achieved by a 20% excess of nBuLi. This was evidenced by the failure to isolate any bisphosphine,
Oc(PPh2)2, during workup. Complex 4 could not be obtained via phosphination of free cobaltocenium
hexafluorophosphate. Phosphine derivatisation of free cyclopentadiene prior to complexation with CoIII
was required to form [CcPPh2][PF6], 4. The electrochemistry of all phosphines was studied by voltam-
metric techniques in CH2Cl2/0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4]. A reversible one-electron transfer process
for the ferrocenyl group of 1 was observed at 0.078 V vs. FcH/FcHþ. The osmocenyl and ruthenocenyl
derivatives exhibited irreversible metallocenyl oxidations at 0.355 and 0.476 V respectively. The cobal-
tocenium complex, 4, exhibited two reversible one-electron transfer reductions to liberate first a neutral
CoII cobaltocene species at ꢀ1.062 V and then an anionic CoI cobaltocene species at ꢀ2.122 V. A single
electrochemical irreversible, one-electron oxþidation at the phosphorus centre which forms a quickly-
decomposing phoþsphorus radical cation, Mc Ph2Pꢁþ, was also observed at Epa > 0.754 V. The newly-
formed McþPh2Pꢁ species or its chemical decomposition products can be oxidized at Epa > 1.090 V
vs. FcH/FcHþ.
Osmocene
Cobaltocenium hexafluorophosphate
Diphenyl phosphine
Electrochemistry
Ó 2014 Published by Elsevier B.V.
1. Introduction
rate of b-diketonato substitution with phenanthroline in complexes
of the type [Rh(FcCOCHCOR)(cod)] [5] where R ¼ Fc, Ph, CH3 or CF3.
Ferrocenes are frequently the substance of choice to employ in
energy [6] and electron-transfer processes because of their
reversible one-electron redox behaviour [7], ease of chemical
modification [8] and high thermal stability [9]. The cytotoxic
properties of ferrocene derivatives [10] are also related to the redox
potentials of the ferrocenyl group [11].
The electrochemistry of ruthenocene differs from that of ferro-
cene in that oxidation of the ruthenocenyl group to the unstable 17
electron monomeric ruthenocenium species can only be achieved
in solvents that have no propensity towards solvation, such as
CH2Cl2, while simultaneously using electrolyte systems such as
[N(nBu)4][B{C6H3(CF3)2}4] or [N(nBu)4][B(C6F5)4] [12]. These elec-
trolytes do not form ion pares of the type Rcþ.ꢀ (electrolyte
anion). The ruthenocenium cation, Rcþ, spontaneously dimerizes
Incorporating metallocenes as part of a phosphine ligand creates
very useful ligands for catalytic processes [1]. The value of phos-
phines as ligand components stem from their unique and very
specific geometries, as well as their electronic properties [2]. The
ferrocenyl (Fc), ruthenocenyl (Rc) and osmocenyl (Oc) groups are all
strongly electron donating substituents that increase the electron
density of most of the species that it is bound to substantially [3].
This property leads to a substantial increase in the rate of oxidative
addition of methyl iodide to ferrocene-containing
b-diketonato
complexes [Rh(FcCOCHCOR)(CO)(PPh3)] [4], but to a decrease in the
* Corresponding author. Tel.: þ27 (0)51 4012781; fax: þ27 (0)51 4446384.
E-mail address: swartsjc@ufs.ac.za (J.C. Swarts).
0022-328X/$ e see front matter Ó 2014 Published by Elsevier B.V.
http://dx.doi.org/10.1016/j.jorganchem.2013.12.027

E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
81
into two separate dimers, one of which dominates at room tem-
perature. The other dominates at low (ꢀ40 ꢂC) temperature [13].
Although the organic chemistry of ruthenocene parallels that of
ferrocene, this metallocene does not undergo chemical modifica-
tion, e.g. acetylation reactions, as readily as ferrocene [14].
The chemical modification of cobaltocene is difficult to achieve
and functionalization usually takes place on free cyclopentadienyl
rings prior to coordination with cobalt [15]. Cobaltocene itself
containing a Co(II) nucleus is very air sensitive but the cobaltoce-
nium cation bearing a Co(III) centre is stable in air [16]. Geiger [17]
reported two reversible electrochemical one-electron reductions
for the cobaltocenium cation, the first involving the CoII/III couple
at ꢀ0.94 V vs. FcH/FcHþ, and the second a CoI/II couple at ꢀ1.88 V.
Being cationic, the cobaltocenium group is expected to be very
electron-withdrawing.
The chemical oxidation of osmocene [18] parallels the above
described electrochemical oxidation of ruthenocene [13]. Taube
isolated both dimers of oxidized osmocene and reported their
crystal structures [18]. Osmocene must also form these dimers
electrochemically, but on CV time-scale they were not yet
identified.
In contrast to the wealth of knowledge available on metal-
locenylbisphosphines [1,2,19,20], only ferrocene- and ruthenocene-
containing monophosphines are known [21]. The mono, di and tri
ferrocenylated monophosphines FcPPh2, Fc2PPh and Fc3P have
been synthesized under Friedel Crafts conditions in the presence of
AlCl3 via a stepwise process [22]. They are all readily air-oxidized in
the presence of aluminium trichloride. However, similar to tri-
phenylphosphine, FcPPh2 and Fc2PPh do show appreciable stability
towards air oxidation when isolated and stored in the solid state
hexafluorophosphate [CcþPPh2][PFꢀ6 ], 4. We also report a comparison
of the electrochemistry of these complexes and describe the crystal
structure of 2. By changing the metallocene metal centre, the phos-
phine cone-angle as well as the chemical stability of the catalyst may
be influenced [20].
2. Experimental
2.1. General information
Solid reagents (Aldrich and Strem) were used without any
further purification. Organic solvents were dried and distilled
directly prior to use where specified. Doubly distilled water was
used. Cyclopentadiene was prepared by cracking of dicyclopenta-
diene as described before [28]. Column chromatography was per-
formed on Kieselgel 60 (Merck, grain size 0.040e0.063 nm) using
hexane:diethyl ether (1:1) as mobile phase. [N(nBu)4][B(C6F5)4] was
synthesized utilizing a published procedure [29]. 1H NMR spectra at
20 ꢂC were recorded on a Bruker Advance DPX 300 NMR spec-
trometer at 300 MHz with chemical shifts presented as
d values
referenced to SiMe4 at 0.00 ppm utilizing CDCl3 as solvent. The
CDCl3 was made acid free by passing it through basic alumina
immediately before use. Elemental analysis was conducted by the
Analytical Chemistry Section of the Chemistry Department of the
UFS on a Leco TruSpec Micro instrument.
2.2. Synthesis
2.2.1. Ferrocenyldiphenylphosphine, FcPPh2, 1
The previously published procedure by Sollott et al. was fol-
lowed [22]. The workup was modified to decant the solution
through filter paper after cooling. The remaining solids were
washed with hot n-heptane (20 ml) and added to the n-heptane
filtrate thereby removing all unreacted ferrocene. The remaining
solids were washed with hot water (20 ml) in portions, followed by
hot toluene (80 ml) in portions. The combined toluene solution was
evaporated. The product was purified by repeated recrystallization
from ethanol to yield 1.44 g (36%) of 1, m.p. ¼ 122 ꢂC. Elemental
analysis (%): calc. for C22H19FeP (370.2): C, 71.4; H, 5.2; found: C,
[23]. The
s-donor ability of these ligands was found to increase
with the increasing number of ferrocenyl groups. Electrochemi-
cally, FcPPh2 [24,25] shows two irreversible oxidation peaks at
approximately Epa ¼ 0.48 V (for ferrocenyl) and 1.5 V (for phos-
phorus) vs SCE, as well as a reduction peak at approximately
Epc ¼ 0.5 V. The first (ferrocenyl) oxidation process becomes
reversible when the positive scan direction is reversed at 0.8 V,
before the phosphorus oxidation process at Epa ¼ 1.5 V can take
place. Under such conditions the reduction peak at Epa ¼ 0.5 V also
disappears. These results were explained as the oxidation of the
iron-centre of the ferrocenyl group at 0.48 V, followed by a chem-
ical step [25]. When the free electron pair of phosphorus is involved
in a chemical bond by either coordination to a Lewis acid or
chemical oxidation to FcP(O)Ph2, only one reversible ferrocene-
based redox wave is observed, linking the chemical step to the
phosphorus atom [25]. FcPPh2, Fc2PPh and Fc3P were also more
recently reinvestigated by Barrière and Geiger [26]. They estab-
lished that medium effects play a significant role in the electro-
chemistry of these compounds. Their studies benefitted from
utilizing CH2Cl2 and [N(nBu)4][B(C6F5)4] as solvent and electrolyte
system because CH2Cl2 limit solvation processes while [N(nBu)4]
[B(C6F5)4] does not engage in ion pair formations as described
above. Interestingly, in this regard, the electrochemistry of
Fc(PPh2)2 was found to be dependent on solvent and electrolyte
used in that [N(nBu)4][ClO4] reacted with oxidation products [27].
Geiger and co-workers [26] also observed irreversible ferrocenyl
redox couples and some follow-up chemical phosphorus activity.
They also showed by occupying the free electron pair of the phos-
phine in a chemical bond as in the phosphine chalcogenides
Fc2PhP]O and Fc3P]Se, reversible and resolved ferrocenyl redox
couples may be obtained [26].
71.1; H, 5.0. 1H NMR (CDCl3,
d, ppm): 4.10 (s, 5H, C5H5); 4.13 (t, 2H,
0.5ꢃ C5H4, 3JHH 1.84 Hz), 4.40 (t, 2H, 0.5ꢃ C5H4, 3JHH 1.84 Hz); 7.37
(m, 10H, 2ꢃ C6H5). 31P{1H} NMR (CDCl3,
d
, ppm): ꢀ17 (s, P). 13C{1H}
3
NMR (CDCl3,
d
, ppm): 69 (s, C5H5); 71 (d, C5H4, JPC 4 Hz); 73 (d,
C5H4, 2JPC 15 Hz); 76 (d, Cq/C5H4, 1JPC 6 Hz); 128 (d, C6H5, 3JPC 7 Hz);
128 (s, C6H5); 134 (d, C6H5, 2JPC 19 Hz); 140 (d, Cq/C6H5, 1JPC 9 Hz).
2.2.2. Ruthenocenyldiphenylphosphine, RcPPh2, 2
Ruthenocene (1 g, 4.3 mmol) was dissolved in dry THF (15 ml)
and the system degassed under Ar for 30 min. The solution was
cooled to ꢀ78 ꢂC and t-butyl lithium (2.55 ml, 4.3 mmol, 1 eq.
Warning: t-butyl lithium combusts spontaneously upon air expo-
sure) was added dropwise to the solution under Ar. The solution
was allowed to warm to room temperature and stirred for 2 h. It
was again cooled to ꢀ78 ꢂC and chlorodiphenylphosphine (2.34 ml,
13.0 mmol, 3 eq.) was slowly added under Ar. The solution was
allowed to warm to room temperature and stirred for a further 2
days. The reaction mixture was quenched with saturated sodium
bicarbonate and extracted with CH2Cl2. The product was separated
by column chromatography, (Rf ¼ 0.81) to yield 0.34 g (19%) of 2,
m.p. ¼ 127 ꢂC. Recrystallisation of 2 from CH2Cl2 and n-hexane gave
crystallographic quality crystals. Elemental analysis (%): calc. for
Withthiswork we highlightdifferentsynthetic routes towardsthe
monometallocenylphosphines, FcPPh2, 1, ruthenocenyldiphenyl
phosphine, RcPPh2, 2, osmocenyldiphenylphosphine, OcPPh2, 3, and
the positively charged compound diphenylphosphinocobaltocenium
C
22H19RuP (415.4): C, 63.6; H, 4.6; found: C, 63.5; H, 4.6. 1H NMR
3
(CDCl3,
d
, ppm): 4.47 (s, 5H, C5H5); 4.50 (t, 2H, 0.5ꢃ C5H4, JHH
3
1.61 Hz), 4.73 (t, 2H, 0.5ꢃ C5H4, JHH 1.65 Hz); 7.36 (m, 10H, 2ꢃ
C6H5). 31P{1H} NMR (CDCl3,
d
, ppm): ꢀ16 (s, P). 13C{1H} NMR (CDCl3,

82
E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
ꢀ
d
, ppm): 72 (s, C5H5); 73 (d, C5H4, 3JPC 3 Hz); 75 (d, C5H4, 2JPC 16 Hz);
with fixed CeH distances, for aromatic a CeH of 0.93 A (CH) [Uiso
80 (d, Cq/C5H4, 1JPC 9 Hz); 128 (d, C6H5, 3JPC 7 Hz); 128 (s, C6H5); 133
(H) ¼ 1.2 Ueq] and for methyl a CeH of 0.96 A (CH) [Uiso
ꢀ
2
(d, C6H5, JPC 19 Hz); 139 (d, Cq/C6H5, 1JPC 10 Hz).
(H) ¼ 1.5 Ueq]. Molecular diagrams were drawn using the software
package DIAMOND [36].
2.2.3. Osmocenyldiphenylphosphine, OcPPh2, 3
OcPPh2 was prepared from osmocene (0.3 g, 0.94 mmol) by
following the same procedure as in the preparation of RcPPh2, but
by using dry ether (6 ml) instead of THF and by adding n-butyl
lithium (0.6 ml, 1.1 mmol, 1.2 eq.) rather than t-butyl lithium.
Chlorodiphenylphosphine (1.0 ml, 5.7 mmol, 5 eq.) was added and
the product was separated by column chromatography, (Rf ¼ 0.79)
to yield 0.062 g (13%) of 3, m.p. ¼ 141 ꢂC. Elemental analysis (%):
calc. for C22H19OsP (504.6): C, 52.4; H, 3.8; found: C, 52.2; H, 3.7. 1H
2.4. Electrochemistry
Measurements on ca. 1.0 mmol dmꢀ3 solutions of the complexes
in dry air free dichloromethane containing 0.10 mol dmꢀ3 tetra-
butylammonium tetrakis(pentafluorophenyl)borate, [N(nBu)4]
[B(C6F5)4], as supporting electrolyte were conducted under a
blanket of purified argon at 25 ꢂC utilizing a BAS 100 B/W elec-
trochemical workstation interfaced with a personal computer. A
three electrode cell, which utilized a Pt auxiliary electrode, a glassy
carbon working electrode (surface area 0.0707 cm2) and an in-
house constructed Ag/Agþ (a silver wire, 0.01 mol dmꢀ3 AgNO3)
reference electrode with vycor tip was used. Successive experi-
ments under the same experimental conditions showed that all
formal reduction and oxidation potentials were reproducible
within 5 mV. Results are referenced against ferrocene, utilizing
decamethyl ferrocene (Fc*) as internal standard. To achieve this,
each experiment was first performed in the absence of ferrocene
and decamethyl ferrocene, and then repeated in the presence of
<1 mmol dmꢀ3 decamethyl ferrocene. A separate experiment
containing only ferrocene and decamethyl ferrocene was also per-
formed. Data was then manipulated on a spreadsheet to set the
formal reduction potentials of the FcH/FcHþ couple to 0 V. Under
our conditions the Fc*/Fc*þ couple was at ꢀ607 mV vs. FcH/FcHþ,
while the FcH/FcHþ couple was at 220 mV vs. Ag/Agþ. In CH3CN/
0.1 mol dmꢀ3 [N(nBu)4][PF6], the Fc*/Fc*þ couple was at ꢀ510 mV
vs. FcH/FcHþ [37].
NMR (CDCl33, , ppm): 4.65 (m, 7H, C5H5, 0.5ꢃ C5H4); 4.91 (t, 2H,
d
0.5ꢃ C5H4, JHH 1.26 Hz), 7.38 (m, 10H, 2ꢃ C6H5). 31P{1H} NMR
(CDCl3,
d
, ppm): ꢀ14 (s, P). 13C{1H} NMR (CDCl3,
d, ppm): 66 (s,
C5H5); 66 (d, C5H4, 3JPC 3 Hz); 68 (d, C5H4, 2JPC 16 Hz); 73 (d, Cq/C5H4,
1JPC 8 Hz); 128 (d, C6H5, 3JPC 7 Hz); 128 (s, C6H5); 133 (d, C6H5, 2JPC
19 Hz); 140 (d, Cq/C6H5, 1JPC 9 Hz).
2.2.4. Diphenylphosphinocobaltocenium hexafluorophosphate,
[CcPPh2][PF6], 4
The procedure published by Rudie et al. [20b] was modified by
reducing the phosphine substituents from two to one, as follows:
To a slurry of finely cut sodium wire (0.56 g, 24.4 mmol,1 eq.) in THF
(200 ml) at 0 ꢂC and under N2, freshly cracked cyclopentadiene [28]
(2 ml, 24.4 mmol, 1 eq.) was slowly added, and the mixture stirred
until all the sodium was consumed. The pink solution was cooled
to ꢀ78 ꢂC and chlorodiphenylphosphine (4.4 ml, 24.4 mmol, 1 eq.)
was slowly added. The solution was allowed to warm to room
temperature and stirred for 30 min. It was again cooled to ꢀ78 ꢂC
and again cyclopentadiene (2 ml, 24.4 mmol, 1 eq.) was added,
followed by n-butyl lithium (24.3 ml, 48.7 mmol, 2 eq.). After
15 min of stirring, cobalt (II) bromide (5.32 g, 24.4 mmol) was
added under a counter stream of N2. The mixture was allowed to
warm to room temperature and stirred for 16 h. Acetic acid (1.4 ml,
24.4 mmol, 1 eq.) was added and the mixture stirred in an open
vessel for 15 min. The mixture was filtered, NH4PF6 was added to
the filtrate and the solvent evaporated. The brown sludge was
washed with warm n-hexane and from this, 7.3 g (58%) of 4 was
crystallized from warm hexane, m.p. ¼ 207 ꢂC. Elemental analysis
(%): calc. for C22H19CoP2F6 (518.3): C, 51.0; H, 3.7; found: C, 50.7; H,
3. Results and discussion
3.1. Synthesis
The different reactivity of each parent metallocene necessitated
different reaction conditions, or in the case of 4, a completely
different synthetic route to obtain each of the different metal-
locenylphosphines (Scheme 1). FcPPh2 (1) was synthesized by
Friedel Crafts reaction of ferrocene and chlorodiphenylphosphine,
in the presence of AlCl3 [22,23]. In our hands slightly better yields
in shorter reaction times were obtained utilizing n-heptane (36%)
3.6. 1H NMR (CDCl3,
d
, ppm): 5.73 (s, 5H, C5H5); 5.91 (t, 2H, 0.5ꢃ
3
3
C5H4, JHH 1.83 Hz), 6.13 (t, 2H, 0.5ꢃ C5H4, JHH 1.68 Hz); 7.69 (m,
10H, 2ꢃ C6H5). 31P{1H} NMR (CDCl3,
d, ppm): 21 (s, P); 144 (m, PF6,
JPF 712 Hz). 13C{1H} NMR (CDCl3,
d, ppm): 86 (s, C5H5); 88 (d, C5H4,
2JPC 10 Hz); 88 (d, C5H4, 3JPC 8 Hz); 129 (d, C6H5, 2JPC 12.4 Hz); 131 (d,
C6H5, 3JPC 10 Hz); 133 (s, C6H5); Cq/C6H5 and Cq/C5H4 not observed).
2.3. Single crystal X-ray crystallography
Crystals of RcPPh2, 2, were obtained by slow evaporation of the
solvents chloroform and n-hexane (1:1 mixture). A colourless sin-
gle crystal, of dimensions 0.27 ꢃ 0.12 ꢃ 0.08 mm3, was selected and
used for data collection on a Bruker X8 ApexII 4K Kappa CCD
diffractometer [30] with an exposure time of 10 s/frame collecting a
total of 567 frames with a frame width of 0.5% covering up to
q
¼ 28.28 ꢂto accomplish a 99.8% completeness. Frame integration
and data reduction were performed using the software packages
SAINT-Plus and XPREP [31]. Multi-scan absorption correction was
performed on the data using SADABS [32]. The structure was solved
by the direct methods software SIR97 [33] and refinement done
with the WinGX [34] software package that includes SHELXL [35].
The non-hydrogen atoms were refined anisotropically. All H-atoms
were positioned geometrically and refined using the riding model
Scheme 1. Synthesis of metallocenylphosphines 1 (FcPPh2), 2 (RcPPh2), 3 (OcPPh2)
and 4, [CcPPh2][PF6].

E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
83
as solvent due to its higher boiling point (98 ꢂC) compared to n-
hexane as solvent (b.p. ¼ 69 ꢂC, with 31% yield). Purification of 1
required repeated recrystallization from ethanol. For RcPPh2 (2)
and OcPPh2 (3), the metallocene first had to be lithiated, followed
by reaction with chlorodiphenylphosphine. After column chroma-
tography, low yields were obtained (19% and 13% respectively), but
unreacted metallocene could be recovered. For 2, tBuLi has to be the
lithiating reagent because nBuLi leads to di-lithiated products, even
in stoichiometric ratios. For osmocene, monolithiation was ach-
ieved utilizing nBuLi as inferred from the isolation of pure 3, while
no Oc(PPh2)2could be isolated. Compound 4 required a different
synthetic approach, since it is not possible to lithiated the oxidised
form of cobaltocene. For 1, 2 and 3, the Fe, Ru and Os metal centres
are all in the þ2 oxidation state which results in a neutral metal-
locene. Complex 4 exists as a cationic species, due to the cobalt
centre being in the þ3 oxidation state. To obtain 4, cyclopentadiene
was first derivatized to cyclopentadienyldiphenylphosphine,
Scheme 1. After addition of an equal amount of unsubstituted
cyclopentadiene, complexation with CoBr2 led to a mixture of
cobaltocenium, diphenylphosphinocobaltocenium, 4, and 1,10-
Complex 2 exhibits a distorted pyramidal geometry around the
P atom with C(11)ePeC(21) being the smallest at 100.70(10)ꢂ. The
C(1)eP bond from the ruthenocene group to the phosphorus atom
ꢀ
ꢀ
is 1.8129(3) A and 0.022 A shorter than the distance between the
phosphorus atom and the C-atom of the phenyl groups bonded to
it. This indicates a stronger bond between the ruthenocene group
and the P atom compared to bonds to the phenyl rings due to the
better electron-donating capabilities of the ruthenocenyl group. A
convenient measure of the electron-donating capability of groups
are Gordy scale group electronegativities, cR, with cR¼Rc ¼ 1.99
(most electron-donating) and cR¼Ph 2.21 (relatively more electron
withdrawing) [38]. The same bond length tendency was also
observed in the crystal structure of FcPPh2, 1. The corresponding
distance between the phosphorus atom and the ferrocenyl group in
ꢀ
1 was 1.810(1) A [39] (cR¼Fc) ¼ 1.87 [38] .
The average CeC bond length and bond angle in the phenyl rings
ꢂ
ꢀ
are 1.384 A and 120.3 respectively and compares well with liter-
ature values [20a,39,40]. The average CeC bond distance in the
ꢀ
ꢀ
aromatic P-substituted cyclopentadienyl ring (1.427(4) A) is 0.021 A
longer than the average CeC bonds in the unsubstituted cyclo-
ꢀ
bis(diphenylphosphino)cobaltocenium
hexafluorophosphate
pentadienyl ring (1.406(5) A). This can be ascribed to donation of
[(dppc)þ(PF6)ꢀ] [20b]. These three products could conveniently be
electron density from the ruthenocenyl group towards the P atom
which decreases the PeCRc bond length but increases the CeC bond
lengths in the ring. For FcPPh2 (1) [39] and other metallocene de-
rivatives [41], the difference in cyclopentadienyl CeC bond lengths
separated from each other by recrystallization in n-hexane.
3.2. Single crystal X-ray structure of 2
ꢀ
was found to be 0.01 A. The cyclopentadienyl rings of 2 were found
to deviate 7.50ꢂ from the eclipsed conformation. The dihedral angle
between the planes formed by the cyclopentadienyl rings is
approximately 0.6ꢂ. They are separated by a centroid-to-centroid
Crystallographic quality crystals of RcPPh2, 2, were obtained
from CH2Cl2 and n-hexane by slow evaporation. The compound
crystallized in the monoclinic space group P21/c. Fig. 1 shows the
molecular structure of 2 hꢂ ighlighting atom labelling. Selected bond
ꢀ
distance of 3.627 A, which falls between the values for free ruth-
enocene (3.68 A) [20a], and 1,1’-bis(diphenylphosphino)ruth-
ꢀ
ꢀ
lengths (A) and angles ( ) are summarized in the caption. Crystal
ꢀ
enocene, Rc(PPh2)2 (3.606 A) [42].
data and refinement parameters are summarized in Table 1.
3.3. Electrochemistry
The cyclic voltammetry of free ferrocene, ruthenocene, osmo-
cene and cobaltocenium hexafluorophosphate has been well
Table 1
Crystal data and structural refinement summary for RcPPh2 2.
Empirical formula
C
22H19PRu
q
range for data
2.41e28.00
0.9251 and 0.7761
50,798
collection/deg
Max. and min.
transmission
Reflections
Molecular weight 415.41
Temperature/K
100(2)
collected
Independent
reflections
ꢀ
Wavelength/A
0.71073
Monoclinic
P2(1)/c
4221 [R(int)
¼ 0.0360]
100.0%
Crystal system
Space group
Unit cell
Completeness
to
q
¼ 28.00ꢂ
Absorption
correction
None
a ¼ 14.2986(7)
b ¼ 10.4474(5)
c ¼ 11.6886(6)
1745.30(15)
Index ranges
ꢀ18 ꢄ h ꢄ 18
ꢀ13 ꢄ k ꢄ 13
ꢀ15 ꢄ l ꢄ 15
Full-matrix least-
squares on F2
4221/0/217
ꢀ
dimensions/A
3
ꢀ
Volume/A
Refinement
method
Z
4
Data/restraints/
parameters
Goodness-
of-fit on F2
Density
1.581
1.072
(calculated)/
Fig. 1. Molecular structure of RcPPh2, 2, showing atom labelling. Displacement ellip-
Mg/m3
Absorption
coefficient
F(000)
ꢂ
ꢀ
soids are drawn at the 50% probability level. Selected bond distances (A) and angles ( )
are: C(1)eP 1.812(3), C(11)eP 1.832(2), C(21)eP 1.836(2), C(1)eC(2) 1.438(3), C(2)e
C(3) 1.418(4), C(3)eC(4) 1.417(4), C(4)eC(5) 1.421(3), C(1)eC(5) 1.441(3), C(11)eC(12)
1.387(3), C(11)eC(16) 1.393(3), C(21)eC(22) 1.391(3), C(21)eC(26) 1.376(3); C(1)ePe
C(11) 102.35(10), C(1)ePeC(21) 102.46(11), C(11)ePeC(21) 100.70(10), C(11)eC(12)e
C(13) 120.9(2), C(11)eC(16)eC(15) 120.7(2), C(12)eC(11)eC(16) 118.2(2), C(21)e
C(22)eC(23) 120.6(3) C(21)eC(26)eC(25) 120.6(2), C(22)eC(21)eC(26) 118.3(2).
0.989 mmꢀ1
840
Final R indices
[I > 2s(I)]
R1 ¼ 0.0275, wR2
¼ 0.0710
R indices (all data) R1 ¼ 0.0320, wR2
¼ 0.0741
Crystal size/mm3 0.27 ꢃ 0.12 ꢃ 0.08 Largest diff. peak 1.520 and ꢀ0.498
ꢀꢀ3
and hole/e A

84
E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
neutral CoII species at Eo0 ¼ ꢀ2.37 V. In acetonitrile containing
[N(nBu)4][PF6] this peak is detected at ꢀ2.46 V. The [CcH]þ/[CcH]0
electrochemical reversible redox process is at Eo0 ¼ ꢀ1.324 V vs FcH/
FcHþ. The electrochemical reversible decamethyl ferrocene, Fc*,
and ferrocene couples are under the conditions of the CV in Fig. 2 at
Eo0 ¼ ꢀ601 mV and 0 mV respectively, although when Fc* and Fc
o
þ
0
were the only analytes in solution, E (Fc*) ¼ ꢀ608 mV vs FcH/FcH
[37]. Osmocene and ruthenocene showed Eo0 ¼ 389 mV and 523 mV
vs FcH/FcHþ respectively for the [OcH]þ/0 and [RcH]þ/0 couples [44].
When ruthenocene is scanned in the presence of only decamethyl
ferrocene,
additional
weak
cathodic
peaks
at
approximately ꢀ300 mV and 100 mV are observed at T ¼ 250 K or
298 K respectively and are associated with two separate ruth-
enocenium dimers [13]. In all cases,
(100 mV sꢀ1) scan rate except the [CcH]0/ꢀ couple which showed
Ep ¼ 123 mV under the conditions of the scan shown 2nd from the
top in Fig. 2.
The electrochemistry of phosphines 1e4 has been studied in
detail. Results at a scan rate of 100 mV sꢀ1 are summarised in
Table while selected CV’s are shown in Figs. and 3.
Supplementary data contains electrochemical data at scan rates
DEp < 88 mV at a slow
D
2
2
100, 200, 300, 400 and 500 mV sꢀ1
.
The electrochemistry of [CcþPPh2][PF6], 4, in CH2Cl2/[N(nBu)4]
[B(C6F5)4], like free cobaltocene, showed two reversible one-
electron transfer waves, Figs. 2 and 3. The wave labelled Ccꢀ/0 at
Epc ¼ ꢀ2.176 (Eo0 ¼ ꢀ2.122 V,
D
Ep ¼ 108 mV at 100 mV sꢀ1) rep-
resents the reduction of the CoII core of cobaltocene to a CoI species
and showed less electrochemical reversibility than the Ccꢀ/0 couple
of 4 (Table 2). Chemical reversibility of the Ccꢀ/0 couple was good
with ipa/ipc ¼ 0.89. Electrochemical and chemical reversibility is
characterised by
D
E ¼ 59 mV and ipa/ipc ¼ 1 respectively as
described elsewhere [45]. The second peak labelled Cc0/þ is much
more ideal in appearance. The electrochemically and chemically
reversible reduction of Co3þ to Co2þ was observed at Eo0 ¼ ꢀ1.062 V,
Fig. 2. Cyclic voltammograms of 1 mmol dmꢀ3 solutions of pure PPh3, as well as a
mixture of all metallocenes used in this study in the same solution, and compounds 1,
2,
3
and 4, in dichloromethane containing 0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4] at
with
D
Ep ¼ 86 mV and ipa/ipc approaching 1 at slower scan rates.
100 mV sꢀ1 on a glassy-carbon working electrode and at 25 ꢂC. Linear sweep vol-
tammetric curves at 2 mV sꢀ1 of phosphines 2, 3 and 4 (in acetonitrile) are shown
below the CV of the compound. The insert above the CV of 4 shows phosphine
oxidation peaks P1 and P2 in acetonitrile containing 0.1 mol dmꢀ3 [N(nBu)4][PF6] at
100 mV sꢀ1. For compounds 1, 2, and 3, the dotted lines indicate scans reversed at
various potentials. The peak labelled Fc* is that of decamethyl ferrocene, which was
used throughout as an internal standard.
Incorporation of the PPh2 substituent as part of the compound
structure in phosphine 4 caused the formal reduction potentials of
the cobaltocenium fragment to be shifted to more positive values
by
D
Eo0
¼
D
Eo04 ꢀ
D
Eo0
¼ 254 mV and 263 mV relative to free
free Cc
cobaltocenium hexafluorophosphate for the CoII/I and CoIII/II redox
couples respectively. This implies that the PPh2 substituent
decreased the electron density on the Co1þ, Co2þ and Co3þ centres
of 4. The phosphorus oxidation of 4 falls outside of the potential
window of CH2Cl2 as solvent, Figs. 2 and 3. For this reason, the
electrochemistry of 4 was repeated in CH3CN/[N(nBu)4][PF6]. This
allows observation of the phosphorus oxidation at the solvent po-
tential limit, Fig. 3.
documented [13,17,43]. For comparative purposes the CV of these
four metallocenes mixed together in the same CH2Cl2 solution
containing 0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4] is shown in Fig. 2.
Relative to FcH/FcHþ, the very reactive cobaltocenium anion,
[CcH]ꢀ undergoes poorly resolved but reversible oxidation to the
Table 2
Cyclic voltammetric data at 100 mV sꢀ1 (potentials vs. FcH/FcHþ) of ca. 1 mmol dmꢀ3 solutions of 1e4 in CH2Cl2 containing 0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4] supporting
a
electrolyte at 25 ꢂC.
Compound
Wave
Epa (V)
Eo (V)
0
DEp (mV)
ipa
(
mA)
ipc/ipa
Wave
Epa (V)
ipa
(
m
A)
Wave
Epa (V)
ipa (mA)
4: [CcþPPh2]b
4: [CcþPPh2]b
1: FcPPh2
3: OcPPh2
2: RcPPh2
Ccꢀ/0
Cc0/þ
Fc0/þ
Oc0/þ
Rc0/þ
ꢀ2.176
ꢀ1.105
0.117
0.355
0.476
ꢀ2.122
ꢀ1.062
0.078
0.224
0.362
108
87
76
262
229
0.54
0.50
0.55
0.70
0.17
0.89
0.99
1.136d
1.128
0.893
0.754
0.70d
0.25e
0.36
P2
1.370d
1.441
1.090
1.160
2.43d
0.56e
0.86
c,d
c,d
P1
0.99
P1
P1
P1
P2
P2
P2
0.31f
<0.15f
0.07e
0.18e
a
Supplementary data provide details for 1 and 4 in CH3CN/0.1 mol dmꢀ3 [N(nBu)4][PF6].
b
c
Counter anion is PF6ꢀ
.
Wave P1 represents oxidation of McþPh2P:, wave P2 represents electrochemical oxidation of either McþPh2Pꢁþ or its chemically decomposed product.
No peaks observed in CH2Cl2/0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4], data provided are for experiments in CH3CN/0.1 mol dmꢀ3 [N(nBu)4][PF6].
d
e
Peak currents of waves P1 and P2 are smaller than that of wave Fc0/þ because of the irreversible nature and broadness of the peaks. It follows that the size of ipa for waves P1
and P2 has no analytical meaning as the size of ipa for the Fc0/þ couple has. The LSV did confirm the correct number of electrons was transferred at wave P1. The same applies to
waves P1 and P2 of 2.
f
Peak cathodic currents were very weak for these electrochemical reversible processes, probably because the fast rate of destruction of the unstable 17 electron Rcþ and Ocþ
species in the presence of the phosphine group.

E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
85
Scheme 2. Schematic representation of electrochemical reactions of metallocene-
containing phosphines FcPPh2 1, RcPPh2 2, OcPPh2 and [CcPPh2)][PF6] 4. Wave P2 in-
volves either electrochemical oxidation of MIII Cp(C5H4PꢁþPh2]þ or of its chemical
decomposition product.
waves at 1.128 V and 1.441 V (peaks labelled P1 and P2) due to
oxidation at the phosphorus centre, Figs. 2 and 3 [24e26]. Wave P1
appears as a low current-intensity shoulder on P2. Linear sweep
voltammetry (LSV) shows the number of electrons transferred
during oxidations at waves Fc0/þ and P1 are equal (Fig. 2), namely
one. This result is consistent with peak P1 representing a one-
electron oxidation of the phosphorus lone-pair of (FcþPh2P:) to
þ
þ
liberate the radical cation FcþPh2Pꢁ . Either FcþPh2Pꢁ or its
chemically decomposed products undergoes further oxidation at
peak P2 (1.441 V). Neither of the two phosphine peaks shows any
electrochemical reversibility.
The cathodic waves Fcred1 and Fcred2 associated with ferrocenyl
group reduction (Figs. 2 and 3) provide additional insight to the
electrochemical profile of 4. When the scan direction is reversed at
potentials small enough to exclude wave P1, the Fc0/þ couple show
electrochemical reversibility with Eo0 ¼ 0.078 V vs. FcH/FcHþ,
Fig. 3. Effect of CH2Cl2/0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4] (CV at 500 mV sꢀ1) or CH3CN/
0.1 mol dmꢀ3 [N(nBu)4][PF6] (CV’s at 100, 200, 300, 400 and 500 mV sꢀ1) as solvent and
electrolyte on redox processes of 1 mmol dmꢀ3 solutions of 1, FcPPh2, and 4, [CcPPh2]
[PF6], on a glassy-carbon working electrode and at 25 ꢂC. The top CV is that of pure
PPh3 in CH2Cl2/0.1 mol dmꢀ3 [N(nBu)4][B(C6F5)4] at 100 mV sꢀ1. The peak labelled Fc* is
that of decamethyl ferrocene, which was used throughout as an internal standard.
DEp ¼ Epa ꢀ Epc ¼ Epa ꢀ EFc,red1 ¼ 116 ꢀ 40 ¼ 76 and ipc/ipa ¼ 0.99.
However, at fast scan rates (ꢅ300 mV sꢀ1), and if the reversal po-
tential is large enough to include wave P1, wave EFc,red1 is not any
more observable. It has been replaced by the ferrocenium reduction
wave Fcred2 at ca. 0.476 V (Figs. 2 and 3). At slow (100 mV sꢀ1) scan
rates utilizing potential ranges wide enough to include wave P1,
both reduction waves Fcred2 and Fcred1, are observed. Cathodic wave
In CH3CN, the cobalt reduction peaks exhibit similar DEp and ipa/
ipc values to that in CH2Cl2 (Supplementary data). Two phosphorus
peaks, P1 and P2 are clearly visible. Peak P1 at 1.136 V appears as a
shoulder on peak P2 at 1.370 V at 100 mV sꢀ1, see Figs. 2 and 3. Peak
P1 is interpreted as a one electron oxidation of the phosphine
functional group at the free electron pair; the LSV of this peak
shows it to involve the same number of electrons as the two cobalt
waves, Fig. 2. The second phosphorus-related oxidation is inter-
Fcred2 leads to
DEp ¼ Epa ꢀ EFc,red2 ¼ 116 ꢀ 476 ¼ ꢀ360 mV. Cathodic
wave Fcred2 is therefore not assigned to reduction of the ferroce-
nium group of FcþPPh2. It probably belongs to the reduction of the
ferrocenium group of a species that was generated during (or
directly after) phosphorus oxidation at peak P1. This new species
survives long enough to detect ferrocenium reduction at wave
Fcred2. The electronic properties of this new species causes the
ferrocenium group to be much more electron deficient than in
FcþPPh2 and resulted in ferrocenium reduction to move from
Epc ¼ 40 mV (Fcred1) to 0.476 V (wave Fcred2). In the wide scan, at
slow scan rates (100 mV sꢀ1), the origin of reduction peak Fcred1 can
be attributed to the diffusion of yet unoxidised molecules of 1 from
the bulk of the solution to the electrode surface (i.e. which have not
undergone phosphine oxidation, but which have undergone fer-
rocenyl oxidation) while the applied potential decreased from ca.
0.7 V to ca. 0.04 V. Incorporation of the ferrocenyl group as part of
the phosphine ligand caused the formal oxidation potential of this
metallocene to be observed at a potential 78 mV more positive than
free ferrocene, which implies the PPh2 group withdraws electron
þ
preted to involve electrochemical oxidation of the CcþPh2Pꢁ
radical cation product, Scheme 2, or of a chemically decomposed
form of this species. Eo0 of the [CcPPh2]ꢀ/0 couple (at far negative
potentials) was within ca. 100 mV of each other in the two solvent
systems, while for the [CcPPh2]0/þ couple, Eo0 shifted by 33 mV
from ꢀ1.062 V (CH2Cl2) to ꢀ1.095 V (CH3CN).
The phosphorus related peaks P1 and P2 were not observable in
CH2Cl2/[N(nBu)4][B(C6F5)4], but they were detected in CH3CN/
[N(nBu)4][PF6] at 1.136 and 1.370 V respectively.
The CV’s of ferrocenyldiphenylphosphine in CH2Cl2/[N(nBu)4]
[B(C6F5)4] exhibited, similar to what was observed during previous
studies, an anodic wave for the oxidation of the ferrocenyl FeII
centre at Epa ¼ 0.117 V followed by two irreversible oxidation

86
E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
density from the ferrocenyl group. Electrochemical reactions
summarizing the above redox processes are shown in Scheme 2.
The electrochemistry of 1 was also studied in CH3CN and
[N(nBu)4][PF6] as solvent and supporting electrolyte system. Peaks
P1 and P2 were more clearly identifiable, as shown in Fig. 3. The
same redox processes as in the case of CH2Cl2 as solvent was
observed but all redox processes were slightly shifted to different
potentials compared to the CH2Cl2 result. Ferrocenyl oxidation was
observed at Epa ¼ 0.139 V (anodic peak of wave Fc0/þ, at
100 mV sꢀ1), followed by an irreversible oxidation wave at 1.052 V,
wave P1, due to phosphorus oxidation, as well as an oxidation wave
at 1.436 V (wave P2). When the switching potential was such that
phosphorus oxidation at wave P1 does not commence, the ferro-
cenyl oxidation is chemically reversible with Eo0 ¼ 0.105 V,
1 and 4, the metallocenes acts as electron-donating groups relative
to PPh2. In donating electron density to the PPh2 group, the redox
potential of 1 and 4 is increased.
For 2 and 3, wave P1 is, as in the case of FcPPh2, associated with
oxidation on the phosphorus centre, and peak P2 with oxidation of
oxidised products. The overall reaction sequence is summarized in
Scheme 2. The LSV indicates the number of electrons transferred
during metallocene oxidation (at waves Rc0/þ and Oc0/þ) and
phosphine oxidation at wave P1 are equal and one.
4. Conclusions
The synthesis of the metallocene-containing monophosphine
series FcPPh2 (1), RcPPh2 (2), OcPPh2 (3) and [CcþPPh2][PF6] (4)
from PPh2Cl highlighted differences in reactivity of each metal-
locene. Ferrocene could react with PPh2Cl by a Friedel Crafts pro-
cedure in the presence of AlCl3 to liberate 1. Ruthenocene required
D
E ¼ 67 mV and ipc/ipa ¼ 0.99 at 100 mV sꢀ1 scan rate. This is about
30 mV larger than that observed in CH2Cl2. The source of peak Fcred1
in the wide scan to include waves P1 and P2 at slow scan rates is
again due to diffusion of 1 from the bulk of the solution while the
potential decreased from ca. 0.700 V to peak Fcred1. The reduction
wave Fcred2 was observed at the higher potential of 0.189 V (rather
than the expected 0.071 V of wave Fcred1). Observation of this wave
is again due to the reduction of the ferrocenium group of a species
that was generated during (or directly after) phosphorus oxidation
at peak P1. Reversing the scan before peak P2 commences caused no
change in the observation of wave Fcred2 which indicates that peak
P2 does not influence the reduction reaction associated with peak
Fcred2. The redox processes of 1 in CH3CN are thus also consistent
with the electron-transfer reactions shown in Scheme 2 although
each redox process occurred at lower potentials than that observed
in CH2Cl2/[N(nBu)4][B(C6F5)4].
t
lithiation with BuLi to prevent formation of di-lithiated ferrocene
and subsequently also Rc(PPh2)2. RcLi was then reacted with PPh2Cl
to obtain 2. For osmocene, monolithiation was achieved with a 1.2
equivalent addition of nBuLi. Subsequent reaction with PPh2Cl
liberated pure 3 and no Oc(PPh2)2 could be isolated. For the syn-
thesis of 4, phosphination of cationic cobaltocenium hexa-
fluorophosphate proved to be impossible. Derivatisation of free
cyclopentadiene prior to complexation with CoIII to form [CcþPPh2]
[PF6], 4, was needed. The ruthenocene complex 2 crystallizes in the
monoclinic space group P21/c and the Cp rings deviated 7.5ꢂ from
the eclipsed conformation. A detailed study of the electrochemistry
of these mono metallocenyldiphenylphosphines showed the
traditional metallocenyl0/þ couple in the following order: Ccþ (4,
o
0
RcPPh2, 2, and OcPPh2, 3, show very similar CV’s compared to 1
(Fig. 2), but they differ in terms of metal reversibility during elec-
trochemical processes. Both show three irreversible oxidation
peaks. Oxidation of the metallocene centre was observed at waves
Oc0/þ and Rc0/þ respectively and does not show electrochemical or
chemical reversibility. The lack of pronounced Rcþ or Ocþ reduction
peaks is probably due to the instability of the 17eꢀ ruthenocenium
and osmocenium centres. No dimeric ruthenocene species, as re-
ported before [13] for free ruthenocene could be observed in the
reduction waves of 2 and 3. It is very likely that the two metal-
locenium species Rcþ and Ocþ are destroyed faster than Fcþ by
internal electron transfer from the free electron pair on the phos-
phine group, just as the oxidised RcHþ cation radical of free ruth-
enocene is destroyed by any nucleophilic or electron donating
species [13]. The phosphine-bound osmocenyl group underwent
oxidation (Fig. 2) at a potential of Epa ¼ 0.355 V. This is 73 mV
smaller (more negative) than that of free osmocene in CH2Cl2 at
100 mV sꢀ1. The phosphine bound ruthenocenyl group was
oxidized at an Epa value of 127 mV more negative than free ruth-
enocene. These lowering in potentials imply that in the case of 2
and 3, the PPh2 group donates electron density to the ruthenocenyl
and osmocenyl groups, directly opposite to what was found for the
iron and cobalt derivatives, 1 and 4. This tendency is following the
trend set in ease of metal oxidation from the MII to MIII state. The Co
and Fe complexes 4 and 1 have the lowest redox potentials
at ꢀ1.062 and 0.078 V respectively. The Os and Ru complexes 3 and
2 exhibited these redox processes at much larger potentials: 0.224
and 0.362 V vs. FcH/FcHþ respectively, Table 1. Because the
measured potentials are also a function of the group electronega-
tivity of the metallocenes (i.e. its capability to donate or withdraw
electrons to or from a molecule) [38,45a], it is clear that in the case
of 2 and 3, which exhibits larger redox potentials, the metallocenyl
groups could act as electron-withdrawing agents relative to the
PPh2 functionality. This electron donation from PPh2 to metal-
locenyl would lower the metallocenyl redox potential. In the case of
reversible, Eo0 ¼ ꢀ1.062 V) < Fc (1, reversible, E ¼ 0.078 V) < Oc (3,
irreversible, Epa ¼ 0.355 V) < Rc (2, irreversible, Epa ¼ 0.476 V). For 1
and 4, the metallocenyl Eo0 was larger than that of the parent free
metallocene, which indicated the PPh2 group in FcPPh2 and
[CcþPPh2][PF6] removes electron density from these metallocenyl
groups. In contrast, in OcPPh2 and RcPPh2, the PPh2 behaved as an
electron-donating group which increased the electron density of
the Rc and Oc groups. This lowered the oxidation potentials of 2 and
3 relative to the parent metallocene. [CcþPPh2][PF6] also showed a
quasi-reversible Ccꢀ/0 couple at ꢀ2.122 V vs FcH/FcHþ. This po-
tential is 254 mV more positive than that observed for the parent
metallocene, [Ccþ][PF6]. Finally, two separate irreversible phos-
phorus oxidation peaks, P1 and P2 were identified. The first, at
Epa > 0.754 V, corresponds to a one-electron oxidation of the free
electron-pair on the phosphorus atom to generate the radical
cation McþPh2Pꢁþ. The second at Epa > 1.090 V relates to electro-
chemical oxidation of the radical cation and/or its chemical decay
products.
Acknowledgements
The authors acknowledge the Central Research fund of the UFS
and the NRF under grant 81829 for financial support.
Appendix A. Supplementary material
CCDC 953336 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Appendix B. Supplementary data
Supplementary data related to this article can be found at http://
dx.doi.org/10.1016/j.jorganchem.2013.12.027.

E. Fourie et al. / Journal of Organometallic Chemistry 754 (2014) 80e87
87
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