The intriguing substitution behavior of CO with bidentate phosphine ligands induced by a gem-dialkyl effect

Article abstract of DOI:10.1016/j.jorganchem.2010.12.024

The reaction of the complexes [FeCpX(CO)₂] (X = Cl, Br, I) into either [FeCp(CO)(PP)]X or [FeCpX(PP)] (PP = a bidentate diphosphine ligand) is shown to be highly dependent of the phosphine ligand used. Diphosphine ligands that form stable chelates favor formation of the neutral complex, whereas diphosphine ligands that form less stable chelates favor formation of the cationic complex. Thus, with the use of dppdmp (= 1,3-bis(diphenylphosphino)-2, 2-dimethylpropane) the [FeCpX(PP)] complexes (X = Cl, Br, I) are selectively formed, induced by a gem-dialkyl effect. Apart from the bidentate phosphine ligand, the halide ion present in the iron complex has a significant influence on the course of the substitution reaction.

Full text of DOI:10.1016/j.jorganchem.2010.12.024

Journal of Organometallic Chemistry 696 (2011) 1899e1903
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Journal of Organometallic Chemistry
journal homepage: www.elsevier.com/locate/jorganchem
The intriguing substitution behavior of CO with bidentate phosphine ligands
induced by a gem-dialkyl effect
Jimmy A. van Rijn a, Eric Gouré a, Maxime A. Siegler b, Anthony L. Spek b, Eite Drent a,
,
Elisabeth Bouwman a
*
a Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, 2300 RA Leiden, The Netherlands
b Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands
a r t i c l e i n f o
a b s t r a c t
Article history:
The reaction of the complexes [FeCpX(CO)2] (X ¼ Cl, Br, I) into either [FeCp(CO)(PP)]X or [FeCpX(PP)]
(PP ¼ a bidentate diphosphine ligand) is shown to be highly dependent of the phosphine ligand used.
Diphosphine ligands that form stable chelates favor formation of the neutral complex, whereas
diphosphine ligands that form less stable chelates favor formation of the cationic complex. Thus, with the
use of dppdmp (¼ 1,3-bis(diphenylphosphino)-2,2-dimethylpropane) the [FeCpX(PP)] complexes (X ¼ Cl,
Br, I) are selectively formed, induced by a gem-dialkyl effect. Apart from the bidentate phosphine ligand,
the halide ion present in the iron complex has a significant influence on the course of the substitution
reaction.
Received 16 July 2010
Received in revised form
3 December 2010
Accepted 20 December 2010
Keywords:
Gem-dialkyl effect
Iron
Ó 2011 Elsevier B.V. All rights reserved.
Phosphine
Bidentate
Carbon monoxide
1. Introduction
second step, where the [FeCpX(PP)] complex is formed, ferrocene is
generally co-produced, decreasing the yield of the reaction.
We therefore investigated another synthetic route, using a
different starting material. An alternative was found in the
commonly used and commercially available [Fe(I)Cp(CO)2]2 dimer.
It has been reported that from a similar starting material, [Fe(I)Cp0
RutheniumeCp complexes (Cp ¼
h
5-C5H5) with bidentate phos-
phine ligands have been employed in the catalytic allylation of
phenolswith allyl alcoholastheallylatingagent[1,2]. Althoughthese
complexes are veryactive, ruthenium remains a relatively scarce and
thus costly metal. Our goal is to synthesize similar complexes, but
with the use of the cheapest of the group 8 metals, namely iron.
The synthetic route leading to the ruthenium catalysts starts
with the reduction of RuCl3 in ethanol in the presence of triphe-
nylphosphine and cyclopentadiene to form [Ru(II)CpCl(PPh3)2] [3].
This route, however, is not applicable for iron(III) chloride, since
reduction to Fe(II) with ethanol does not occur. Different synthetic
procedures for [FeCpX(PP)] (X ¼ halide anion; PP ¼ two mono-
dentate or one bidentate phosphine ligand) have been reported,
mostly starting from anhydrous FeX2 [4]. The FeX2 is initially con-
verted to FeX2(PP) by reaction with monodentate [5] or bidentate
phosphine ligands [4]; subsequent reaction with an alkali Cp salt
yields the desired compound [FeCpX(PP)] [6]. These routes have the
major disadvantage that hygroscopic anhydrous FeX2 salts are not
easy to handle and need to be dried thoroughly before use. In the
(CO)2]2 (Cp0 ¼
h
5-C5H4CH(Ph)Me) after a oxidation reaction with
the halogen the [Fe(II)Cp0X(CO)2] complexes are obtained in high
yields [7]. When a bidentate phosphine ligand is added to such a
complex, the first CO ligand is easily displaced, but the displace-
ment of the second carbonyl group is more cumbersome; photo-
chemical activation is needed to remove the remaining carbonyl
from the complex to obtain the compound [FeCpX(dppe)] [8,9]. The
substitution of CO with phosphine ligands has also been reported
via the in situ oxidation of CO to CO2 with a trialkylamine-N-oxide
[10], but the presence of such an oxidizing agent in the reaction
limits the choice of the phosphine ligands.
In this paper, a route is described in which both carbonyl ligands
are displaced in a very simple reaction, without the need for
photochemical activation. Although it turned out that the resulting
iron complexes are not active as allylation catalysts, an interesting
substitution behavior was observed. The resulting complexes have
been used previously and the improved synthesis might be useful
in extensions of such studies [11,12]. Herein we report the synthesis
of a number of new ironeCp complexes with several diphosphine
* Corresponding author. Tel.: þ31 71 527 4550; fax: þ31 71 527 4761.
E-mail address: bouwman@chem.leidenuniv.nl (E. Bouwman).
0022-328X/$ e see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jorganchem.2010.12.024

1900
J.A. van Rijn et al. / Journal of Organometallic Chemistry 696 (2011) 1899e1903
ligands; the phenyl-substituted diphosphine ligands used in these
investigations are shown in Fig. 1 [1,2].
ArH), 7.37e7.30 (m, 12H, ArH), 7.36e7.24 (m, 4H, ArH), 4.35 (s, 5H,
Cp), 2.78e2.50 (m, 2H, CH), 1.58e1.54 (m, 2H, CH), 1.02 (s, 3H, CH3),
0.27 (s, 3H, CH3). 31P{1H} NMR (CDCl3):
d 53.1 (s).
[FeCp(CO)(dppdmp)]Cl was obtained as a yellowish solid in
2. Experimental
a yield of 75 mg, 0.12 mmol (60%). Anal. Calcd for C35H35ClFeOP2: C,
67.27; H, 5.65. Found: C, 67.44; H, 5.91. 1H NMR (CDCl3):
d 7.76e7.72
2.1. General remarks
(m, 4H, ArH), 7.72e7.53 (m, 6H, ArH), 7.51e7.30 (m, 6H, ArH),
7.22e7.17 (m, 4H, ArH), 4.88 (s, 5H, Cp), 3.07e2.71 (m, 4H, CH2), 1.11
All reactions were performed under an argon atmosphere using
standard Schlenk techniques. Solvents were dried and distilled by
standard procedures and stored under argon. [FeCp(CO)2]2, dppm,
dppe, dppp and dppb were purchased from Acros Organics and
used as received. The ligand dppdmp was synthesized following
a literature procedure [2]. C, H determinations were performed on
a PerkineElmer 2400 Series II analyzer. 1H NMR spectra (300 MHz)
and 31P{1H} NMR spectra (121.4 MHz) were recorded on a Bruker
DPX-300 spectrometer. Chemical shifts are reported in ppm. Proton
chemical shifts are relative to TMS, and phosphorus chemical shifts
are relative to 85% aqueous H3PO4. The spectra were taken at room
temperature.
(s, 3H, CH3), 0.27 (s, 3H, CH3). 31P{1H} NMR (CDCl3):
d 54.1 (s). IR
(
nmax/cmꢀ1): 1932 (CO stretch).
[FeCpBr(dppe)] was obtained as a black solid in a yield of 87 mg,
0.15 mmol (74%). Anal. Calcd for C31H29BrFeP2: C, 62.13; H, 4.88.
Found: C, 62.49; H, 5.10. 1H NMR (CDCl3):
8.00e7.98 (m, 4H, ArH),
d
7.58e7.44 (m, 6H, ArH), 7.33e7.27 (m, 6H, ArH), 7.24e7.12 (m, 4H,
ArH), 4.16 (s, 5H, Cp), 2.52e2.36 (m, 4H, CH2). 31P{1H} NMR (CDCl3):
d
95.5 (s).
[FeCp(CO)(dppe)]Br was obtained as a yellow solid in a yield of
23 mg, 0.04 mmol (18%). Anal. Calcd for C32H29BrFeOP2: C, 61.27; H,
4.66. Found: C, 62.88; H, 5.31. 1H NMR (CDCl3):
d
7.75e7.61 (m, 4H,
ArH), 7.60e7.57 (m, 8H, ArH), 7.56e7.27 (m, 4H, ArH), 7.24e7.17 (m,
4H, ArH), 4.91 (s, 5H, Cp), 3.31e3.02 (m, 2H, CH2), 3.01e2.75 (m, 2H,
2.2. Synthetic procedures and characterization
CH2). 31P{1H} NMR (CDCl3):
stretch).
d
93.5 (s). IR (nmax/cmꢀ1): 1980 (CO
Synthesis of [FeCpX(CO)2]: The dimeric compound [FeCp(CO)2]2
was reacted according to a literature procedure [7], to obtain
[FeCpCl(CO)2], [FeCpBr(CO)2] and [FeCpI(CO)2] of which the char-
acterizations corresponded with those reported in literature [13].
General procedure for the formation of [FeCp(CO)(PP)]X and
[FeCpX(PP)]: A solution of [FeCpX(CO)2] (X ¼ Cl, Br, I) (0.2 mmol)
and the bidentate phosphine ligand (0.2 mmol) in 5 ml toluene was
refluxed for 1 h. The solution was cooled to room temperature and
filtered to yield 2 as the residue. The filtrate was evaporated to
dryness and product 3 was precipitated with hexane. Elemental
analyses of the complexes [FeCp(CO)(dppe)]I and [FeCp(CO)(dppp)]
I have been reported [14]. Crystals suitable for X-ray diffraction
were formed by slow diffusion of hexane into a solution of the
compound in dichloromethane.
[FeCpBr(dppp)] was obtained as a black solid in a yield of 37 mg,
0.10 mmol (52%). Anal. Calcd for C32H31BrFeP2: C, 62.65; H, 5.09.
Found: C, 62.76; H, 5.54. 1H NMR (CDCl3):
d 7.94e7.88 (m, 4H, ArH),
7.44e7.24 (m, 8H, ArH), 7.24e7.19 (m, 8H, ArH), 7.24e7.17 (m, 4H,
ArH), 4.98 (s, 5H, Cp), 2.45e2.35 (m, 4H, CH2), 2.19e2.15 (m, 2H,
CH2). 31P{1H} NMR (CDCl3):
d 53.1 (s).
[FeCp(CO)(dppp)]Br was obtained as a yellow solid in a yield of
34 mg, 0.09 mmol (47%). Anal. Calcd for C33H31BrFeOP2: C, 61.80; H,
4.87. Found: C, 61.70; H, 4.61. 1H NMR (CDCl3):
d 7.72e7.28 (m, 16H,
ArH), 7.27e7.09 (m, 4H, ArH), 4.84 (s, 5H, Cp), 3.01e2.75 (m, 2H,
CH2), 2.75e2.64 (m, 1H, CH), 2.35e2.15 (m, 2H, CH2), 1.85e1.61 (m,
1H, CH). 31P{1H} NMR (CDCl3):
stretch).
d
52.7 (s). IR (nmax/cmꢀ1): 1960 (CO
[FeCpBr(dppdmp)] was obtained as a black solid in a yield of
[FeCp(CO)(dppe)]Cl was obtained as a yellowish solid in a yield of
93 mg, 0.16 mmol (80%). Anal. Calcd for C32H29ClFeOP2: C, 65.95; H,
124 mg, 0.19 mmol (97%). Anal. Calcd for C34H35FeBrP2: C, 63.67; H,
5.50. Found: C, 63.55; H, 5.17. 1H NMR (CDCl3):
d 7.80e7.41 (m, 4H,
5.02. Found: C, 66.12; H, 4.88. 1H NMR (CDCl3):
d 7.77e7.70 (m, 4H,
ArH), 7.35e7.18 (m, 16H, ArH), 4.54 (s, 5H, Cp), 3.01e2.88 (m, 2H,
ArH), 7.70e7.50 (m, 6H, ArH), 7.49e7.29 (m, 6H, ArH), 7.20e7.16 (m,
CH), 1.58e1.10 (m, 2H, CH), 1.03 (s, 3H, CH3), 0.28 (s, 3H, CH3). 31P
4H, ArH), 4.85 (s, 5H, Cp), 3.07e2.78 (m, 4H, CH2). 31P{1H} NMR
{1H} NMR (CDCl3):
d 50.1 (s).
(CDCl3):
d
93.2 (s). IR (nmax/cmꢀ1): 1960 (CO stretch).
[FeCpI(dppe)] was obtained as a black solid in a yield of 95 mg,
[FeCp(CO)(dppp)]Cl was obtained as a yellow solid in a yield of
0.15 mmol (74%). Anal. Calcd for C31H29FeIP2: C, 57.61; H, 4.52.
92 mg, 0.15 mmol (77%). Anal. Calcd for C33H31ClFeOP2: C, 66.41; H,
Found: C, 57.84; H, 4.57. 1H NMR (CDCl3):
d 8.01e7.99 (m, 4H, ArH),
5.23. Found: C, 65.44; H, 5.35. 1H NMR (CDCl3):
d 7.79e7.72 (m, 4H,
7.46e7.35 (m, 6H, ArH), 7.32e7.19 (m, 6H, ArH), 7.13e7.10 (m, 4H,
ArH), 7.71e7.52 (m, 6H, ArH), 7.49e7.32 (m, 6H, ArH), 7.25e7.16 (m,
ArH), 4.20 (s, 5H, Cp), 2.71e2.59 (m, 4H, CH2). 31P{1H} NMR (CDCl3):
4H, ArH), 4.91 (s, 5H, Cp), 3.12e2.64 (m, 6H, CH2). 31P{1H} NMR
d
54.0 (s). IR (nmax/cmꢀ1): 1971 (CO stretch).
d
97.0 (s).
(CDCl3):
[FeCp(CO)(dppe)]I was obtained as a yellow solid in a yield of
[FeCpCl(dppdmp)] was obtained as a black solid in a yield of
20 mg, 0.03 mmol (15%). 1H NMR (CDCl3):
d 7.69e7.58 (m, 4H, ArH),
45 mg, 0.08 mmol (38%). Anal. Calcd for C34H35ClFeP2: C, 68.42; H,
5.91. Found: C, 68.98; H, 5.81. 1H NMR (CDCl3):
d 7.71e7.68 (m, 4H,
7.56e7.7.31 (m, 12H, ArH), 7.31e7.26 (m, 4H, ArH), 4.86 (s, 5H, Cp),
3.05e2.98 (m, 2H, CH2), 2.84e2.77 (m, 2H, CH2). 31P{1H} NMR
(CDCl3):
d
92.8 (s). IR (nmax/cmꢀ1): 1984 (CO stretch).
[FeCpI(dppp)] was obtained as a dark green solid in a yield of
56 mg, 0.09 mmol (47%). Anal. Calcd for C32H31FeIP2: C, 58.21; H,
4.73. Found: C, 58.18; H, 4.64. 1H NMR (CDCl3):
d 7.93e7.92 (m, 4H,
ArH), 7.40e7.35 (m, 8H, ArH), 7.26e7.18 (m, 8H, ArH), 4.06 (s, 5H,
Cp), 2.65e2.57 (m, 2H, CH), 2.52e2.2.48 (m, 2H, CH), 2.27e2.17 (m,
2H, CH). 31P{1H} NMR (CDCl3):
d
55.4 (s).
[FeCp(CO)(dppp)]I was obtained as a yellow solid in a yield of
37 mg, 0.05 mmol (27%). 1H NMR (CDCl3):
7.58e7.7.54 (m, 9H,
d
ArH), 7.51e7.39 (m, 8H, ArH), 7.25e7.16 (m, 3H, ArH), 4.81 (s, 5H,
Cp), 2.95e2.79 (m, 2H, CH2), 2.75e2.50 (m, 1H, CH), 2.21e2.12 (m,
2H, CH2), 1.74e1.71 (m, 1H, CH). 31P{1H} NMR (CDCl3):
d 52.6 (s). IR
Fig. 1. Bidentate phosphine ligand used in substitution studies of Fe(II)Cp(CO)-
complexes.
(
nmax/cmꢀ1): 1956 (CO stretch).

J.A. van Rijn et al. / Journal of Organometallic Chemistry 696 (2011) 1899e1903
1901
Table 1
[FeCpI(dppdmp)] was obtained as a dark red solid in a yield of
Yields of FeCp(PP)-complexes after reaction of [FeCpX(CO)2] with bidentate phos-
136 mg, 0.19 mmol (99%). Anal. Calcd for C34H35FeIP2: C, 59.33; H,
phine ligandsa
5.12. Found: C, 58.50; H, 5.41. 1H NMR (CDCl3):
d 7.86e7.81 (m, 4H,
ArH), 7.56e7.27 (m, 12H, ArH), 7.23e7.13 (m, 4H, ArH), 4.69 (s, 5H,
Cp), 3.18e3.03 (m, 2H, CH), 1.40e1.21(m, 2H, CH), 1.14 (s, 3H, CH3),
0.27 (s, 3H, CH3). 31P{1H} NMR (CDCl3):
d 51.6 (s).
.
In situ preparation and detection of [FeCpI(CO)(dppm)]: The
complex has been reported previously in literature [15,16]. The 31P
NMR spectrum has not been reported thus far. 31P{1H} NMR
Entry Ligand
Halide X in
[FeCpX(CO)2] 2 (%)
Yield [FeCp(CO)(PP)]X Yield [FeCpX(PP)]
(CDCl3):
d
63.3 (d, J ¼ 49 Hz, PeFe), ꢀ25.1 (d, J ¼ 49 Hz, P free).
3 (%)
1
2
3
4
5
6
7
8
dppe
dppp
dppdmp Cl
dppe
dppp
dppdmp Br
dppe
dppp
dppdmp
dppm
dppb
Cl
Cl
80
77
60
18
47
0
15
27
0
0
0
2.3. X-ray crystal structure determination
38
74
52
97
74
47
100
0
Br
Br
C32H31FeIP2, Fw ¼ 660.26, triclinic, P1 (No. 1), Z ¼ 2, a ¼ 9.1220
(11), b ¼ 9.1282(9), c ¼ 17.9920(15) Å,
a
¼ 91.931(3), ¼ 104.416(3),
b
g
¼ 106.824(4)ꢁ, V ¼ 1379.8(2) Å3, Dx ¼ 1.5892(2) g/cm3. X-ray data
I
I
I
I
I
were collected (MoKa, 190 K) on a Nonius Kappa CCD diffractom-
eter with rotating anode under the control of the COLLECT software
[17]. The diffraction images were integrated with the program
PIXEL15. The structure was solved with DIRDIF08 [18], corrected for
absorption with TWINABS [19] and refined with SHELXL97 [20].
Geometry calculations, checking for higher symmetry and valida-
tion were performed with the PLATON program [21]. Convergence
was reached at R ¼ 0.0404 (8460 reflections), wR2 ¼ 0.1002 (9435
reflections), S ¼ 1.029, Flack x ¼ 0.08 (2). The central part of the two
molecules is disordered in ratios of 0.554(2):0.446 and 0.783
(2):0.217 respectively and refined with a disorder model. In addi-
tion, about 10% non-merohedral twinning was taken into account
with an HKLF 5 type refinement. There are no residual density
maxima in the final electron density difference map beyond the
range ꢀ0.33 and 0.54 e/Å3.
9
10b
11b
5
3
0
a
Reagents and conditions: ratio [FeCpX(CO)2]/PP ¼ 1/1, toluene, 110 ꢁC, 1 h;
isolated yields.
b
[FeCpI(PP)(CO)] with monodentate binding of the PP ligand was detected in
solution by NMR spectroscopy (see text).
petroleum ether. This compound was identified as [FeCpCl
(dppdmp)] with 1H and 31P NMR spectroscopy, elemental analysis
and IR spectroscopy. The presence or absence of CO ligands was
clearly confirmed by means of IR spectroscopy.
In the reaction of [FeCpBr(CO)2] with bidentate phosphine
ligands, again both 2 and 3 are formed, but in different amounts
compared to the chloride analogs. In the presence of dppe (entry 4),
compound 2 is formed, but the main product is compound 3. From
the reaction with dppp (entry 5), both 2 and 3 were isolated,
however, the amount of 2 was considerably higher than when dppe
was used. In the reaction with dppdmp (entry 6), 2 is not formed
and 3 is obtained in high yield.
For [FeCpI(CO)2], the reaction with dppe (entry 7) yields mainly
3 and only a small amount of 2 is obtained. The reaction with dppp
(entry 8) is, as for the bromide analog, less selective and gives
a slightly increased amount of 2 compared to dppe. The use of
dppdmp again very selectively yields 3 and formation of 2 is not
observed.
The reactions with [FeCpI(CO)2] are thus most selective for
formation of 3 and with this complex the reactivity of other
bidentate phosphine ligands was investigated. The one-carbon-
bridged ligand dppm was reacted with [FeCpI(CO)2] and after 1 h
reaction time, a small amount of 2 precipitated from the reaction
mixture, but the main product present in solution is a compound
with the bidentate phosphine ligand coordinated in a monodentate
fashion, as was clearly indicated by 31P NMR spectroscopy (Fig. 2).
Such a compound has been reported in literature and is even used
in reactions [15,23]. After longer reaction times (6 h), 2 is selectively
formed in 67% yield. For the C4-bridged ligand dppb, a similar
behavior was observed and 2 was obtained in 74% yield after 6 h.
3. Results and discussion
3.1. Synthesis
The [Fe(II)CpX(CO)2] complexes (X ¼ Cl, Br, I) were synthesized
following a literature procedure [7]. Although in the original
procedure
h
5-C5H4CH(Ph)Me was used, the reaction is also very
effective with [FeCp(CO)2]2 as starting material. The reaction of
[FeCpX(CO)2] with a selection of bidentate phosphine ligands has
been investigated, yielding the cationic compounds [FeCp(CO)(PP)]
X 2 and the neutral compounds [FeCpX(PP)] 3, as described below.
An overview of the results is listed in Table 1.
The chloride complex was reacted with three different bidentate
phosphine ligands (entries 1e3). The reaction with the ligand dppe
(entry 1) selectively yields the cationic product [FeCp(CO)(dppe)]Cl 2.
The bidentate phosphine ligand displaces one of the carbonyl ligands
and the chloride anion from the coordination sphere, forming the
yellow complex [FeCp(CO)(dppe)]Cl, a reaction that has been repor-
ted earlier [22]. Dinuclear complexes with bridging phosphine
ligands, as described in literature, are not observed after the reaction
[15,16]. Also the reaction with the bidentate phosphine ligand dppp
(entry 2), leads to the precipitation of the cationic complex [FeCp
(dppp)(CO)]Cl, and
a colorless solution remains. These ionic
compounds are stable as a solid and in solution and are not reactive in
normal ambient light.
3.2. Crystal structure
Surprisingly, when the gem-dialkyl substituted ligand dppdmp
is reacted with [FeCpCl(CO)2] (entry 3), different reactivity pattern
is observed. As for the reaction described in entries 1 and 2,
a yellow precipitate of the cationic compound 2 is formed after 1 h;
however, the amount of product formed is relatively low. Even
more striking is that the remaining solution is very darkly colored.
After filtration of the yellow product and concentration of this
darkly colored solution, a black solid precipitated upon addition of
The molecular structure of [FeCpI(dppp)] is shown in Fig. 3.
Selected bond distances and angles are listed in Table 2. The unit
cell contains two crystallographically independent molecules. The
central parts of both molecules are disordered in different ratios.
Restraints were applied to handle the disorder in the refinement
(see details in the experimental section and CIF). The molecules
pack in layers parallel with the ab-plane.

1902
J.A. van Rijn et al. / Journal of Organometallic Chemistry 696 (2011) 1899e1903
Table 2
Selected bond lengths (Å) and angles (ꢁ) for the complex [FeCpI
(dppp)].
Compound
[FeCpI(dppp)]
Bond distances (Å)
Fe(1)eI(2)
Fe(1)eP(3)
2.663(2)
2.204(2)
2.194(3)
1.702(5)
Fe(1)eP(7)
Fe(1)eCpa
Angles (ꢁ)
P(3)eFe(1)eP(7)
P(3)eFe(1)eI(2)
P(7)eFe(1)eI(2)
CpaeFe(1)eI(2)
CpaeFe(1)eP(3)
CpaeFe(1)eP(7)
91.02(15)
92.21(12)
92.47(11)
119.3(2)
126.8(2)
125.5(2)
a
Cp refers to the center of the cyclopentadienyl ring.
Fig. 2. 31P NMR spectrum of [FeCpI(dppm)(CO)], in which the bidentate ligand is
thus forming ionic compound 2 or neutral complex 3 respectively.
Indeed, for the ligands dppm and dppb that form relatively unstable
chelates with the Fe(II) center the intermediate monodentate coordi-
nation of the ligand (in species IM) is even detected using 31P NMR
spectroscopy; only a low yield of the ionic compound 2 is obtained.
The loss of a CO ligand is irreversible, but the halide counter ion
may dissociate reversibly. However, under the reaction conditions
used, it can be excluded that compound 3 may be formed via
a consecutive reaction from 2. Since the ionic compound 2 is hardly
soluble in toluene it readily precipitates and thus is not available for
further reaction. Longer reaction times did not result in an increase
of the yield of 3 and heating of the ionic complex 2 under reaction
conditions does not result in formation of 3. A thermodynamic vs
kinetic product development thus seems not to be involved.
The selectivity toward the neutral compound 3 increases going
from chloride to bromide to iodide, indicating an increasing affinity
of the Fe(II) center for binding to the softer halide ions over the CO
ligand. The low spin Fe(II) center is a relatively soft acid according
to the HSAB definitions and thus favors iodide coordination.
The ligands dppe and dppp with a rather flexible di- or tri-
methylenebackbone tend tofavor formation of the cationic complex
coordinated in
a
monodentate fashion. PeFe
¼
coordinated phosphine;
P ¼ uncoordinated phosphine; * ¼ free dppm; ꢂ ¼ oxidized dppm.
The distances of the iron center to the cyclopentadienyl and
iodide anion in the structure of [FeCpI(dppp)] (1.702(5) and 2.663
(2), respectively) are very similar to those in the structure of [FeCpI
(dppe)] (1.707(3) and 2.643(1)) [24]. Also the bond distances Fe(1)e
P(3) and Fe(1)eP(7) are comparable to those in related iron
complexes [24,25]. The bite angle P(3)eFe(1)eP(7) (91.02(11)ꢁ)
reflects the incorporation of a C3-ligand backbone and is an inter-
mediate distance between the 86.22(3)ꢁ reported for [FeCpI(dppe)]
with a C2-bridging group [24] and the 99.1(3)ꢁ in [FeCpI(diop)]
(diop
¼
2,3-O-isopropylidene-2,3-dihydroxy-1,4-bis(diphenyl-
phosphino)butane) with a C4-bridging group [25].
3.3. Substitution mechanism
The substitution behavior of the [Fe(II)CpX(CO)2] complexes
appears to be dependent on the halide present and on the phos-
phine ligand used in the reaction. Generally, CO ligands are not
easily displaced and activation with light or an oxidant such as
trimethylamine-N-oxide is necessary. It has been reported that
substitution of the first CO ligand in iron(II) carbonyl halide
complexes with a monodentate phosphine ligand proceeds rapidly,
especially for chloride and bromide complexes [26,27]. For the
bidentate ligands used in this study the outcome of the competition
between the halide anion and neutral CO ligand displacement
clearly is dependent on the ligands’ properties.
Substitution of two ligands in the starting iron compound with
a bidentate ligand containing a flexible backbone most likely proceeds
step-wise. Initially, one of the carbonyl ligands is displaced (Scheme 1).
Then either the halide ion or the other carbonyl ligand has to dissociate,
Scheme 1. Proposed pathway for the formation of 2 via intermediate IM (sequential
reaction) and formation of 3 (concerted reaction) as a function of the chelating effect of
the bidentate phosphine ligand.
PP ¼ bidentate phosphine ligand
Fig. 3. Displacement ellipsoid plot of [FeCpI(dppp)], with the adopted atom labeling.
Displacement ellipsoids are drawn at the 50% probability level. Only the main disorder
form of one of the two independent molecules is shown. Hydrogen atoms are omitted
for clarity.

J.A. van Rijn et al. / Journal of Organometallic Chemistry 696 (2011) 1899e1903
1903
2, unless starting materials with bromide or iodide anion are used.
Appendix A. Supplementary material
The use of the ligand dppdmp with the more rigid 2,20-dimethyl-
propane backbone makes synthesis of compound 3 possible, even in
the presence of the relatively weakly coordinating chloride anion.
It has been reported previously that diphosphine ligands with
gem-dialkyl substituted backbones form very stable chelates [2].
The electron pairs of the phosphine donors are both directed
toward the metal center due to entropic restrictions enforced by
the ligand backbone. It is thus likely that coordination of the rigid
ligand dppdmp proceeds via a concerted displacement of two
ligands. From the results in Table 1 it may be concluded that
a chloride anion has a similar binding strength as a CO ligand under
the reaction conditions used, as the reaction with the rigid ligand
dppdmp results in nearly equal amounts of 2 and 3. For the ligands
dppe and dppp the relatively lower flexibility of dppe is reflected in
the slightly lower amounts of 2 formed in all cases.
CCDC (CSD number 771873) contains the supplementary crys-
tallographic data. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
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In conclusion, we have found an accessible route toward [FeCpX
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This work was supported by the Technology Foundation STW
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