Reaction of ferrocenium ion with secondary phosphines: replacement of cyclopentadienyl ligand rather than its C—H functionalization

Article abstract of DOI:10.1007/s11172-019-2565-5

The reactions of ferrocenium salts with secondary phosphines Ph₂PH, Cy2PH, and Et2PH proceed as replacement of the cyclopentadienyl ring to afford half-sandwich complexes [C5H₅Fe(R2PH)3](X) (X = PF6, BF₄) rather than ferrocenylphosphonium salts [C5H₅FeC5H4— (PHR₂)](X).

Full text of DOI:10.1007/s11172-019-2565-5

Russian Chemical Bulletin, International Edition, Vol. 68, No. 7, pp. 1380—1383, July, 2019
1380
Reaction of ferrocenium ion with secondary phosphines:
replacement of cyclopentadienyl ligand rather than its C—H functionalization
A. A. Chamkin, V. V. Krivykh, N. A. Shtel´tser, K. I. Utegenov, F. M. Dolgushin, and N. A. Ustynyuk
A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences,
28 ul. Vavilova, 119991 Moscow, Russian Federation.
Fax: +7 (499) 135 5085. E-mail: ustynyuk@ineos.ac.ru
The reactions of ferrocenium salts with secondary phosphines Ph2PH, Cy2PH, and Et2PH
proceed as replacement of the cyclopentadienyl ring to afford half-sandwich complexes
[C5H5Fe(R2PH)3](X) (X = PF6, BF4) rather than ferrocenylphosphonium salts [C5H5FeC5H4—
(PHR2)](X).
Key words: ferrocenium, secondary phosphines, replacement of cyclopentadienyl ligand.
Ferrocenium radical cation (hereinafter, ferrocenium)
can react with nucleophilic reagents; however, such reac-
tions are poorly studied. They usually result in the reduc-
tion of ferrocenium to ferrocene or complete replacement
of its cyclopentadienyl ligands and destruction of metal-
locene structure. The last path is realized in the reaction
of ferrocenium with Cl–, Br–, and SCN– anions and
neutral bases, such as phen, bipy, DMF, HMPA, and
DMSO.1,2 Another possible reaction pathway is nucleo-
philic substitution of the hydrogen atom in the cyclopenta-
dienyl ring under the action of a nucleophile; however,
only one example of such reaction was known until now,
viz., azolation of ferrocenium ion with azolyl anions.3
Recently, we have discovered the reaction of ferrocenium
with tertiary phosphines PR3 (see Ref. 4) and aminophos-
phines R3–nP(NEt2)n (n = 1—3; R = Ph, Alk) (see Ref. 5),
which proceeds as a ring hydrogen substitution to afford
an equimolar mixture of ferrocenylphosphonium salts
[C5H5Fe(C5H4PRR´Rʺ)](PF6) and ferrocene (Scheme 1).
These reactions were carried out in a flask and an
electrochemical cell and studied theoretically by DFT.
Based on the experimental and calculation data, a mech-
anism including three steps was proposed for this reaction:
(a) the nucleophilic addition of phosphine to the cyclo-
pentadienyl ring of ferrocenium to form a radical cation
adduct [CpFe(4-C5H5—PR3)]+• (A); (b) the oxidation
of adduct A by the starting ferrocenium to form ferrocene
and a dicationic iron complex [CpFe(C—HFe,4-
C5H5—PR3)]2+ (B) where one Csp3—H bond is involved
in an agostic interaction with the iron atom; and (c) the
final deprotonation of the agostic Csp3—H bond in complex
B by the starting phosphine.
In order to more precisely assess the synthetic poten-
tial of these phenomenologically new reactions, in the
present work we studied the reaction of ferrocenium with
secondary phosphines Ph2PH, Cy2PH (Cy is cyclohexyl),
and Et2PH. If these reactions proceed according to Scheme 1,
one can expect the formation of ferrocenylphosphonium
salts [CpFeC5H4-PHR2]+X– followed by their deproton-
ation into phosphinoferrocene CpFeC5H4—PR2.
Scheme 1
Results and Discussion
The reaction of ferrocenium salts 1+• with secondary
phosphines 2 proceeds at room temperature in dichloro-
methane to afford half-sandwich complexes [C5H5Fe-
(PR2H)3](X) (X = PF6, BF4) (3) and ferrocene as the main
products (Scheme 2).* During the reaction, the color of
solution changes from blue to orange.
We assume that the reaction proceeds according to the
stoichiometry as shown in Scheme 2. The used ratio of
ferrocenium to secondary phosphine = 1 : 2 is the optimum
* Counter ions are not shown in Scheme 2.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 7, pp. 1380—1383, July, 2019.
1066-5285/19/6807-1380 © 2019 Springer Science+Business Media, Inc.

Ferrocenium with secondary phosphines
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1381
Scheme 2
R = Ph (a), Cy (b), Et (c). Yield (%): R = 38 (3a), 42 (3b), 16 (3c).
one, since the yields of half-sandwich complexes 3 decrease
with a decrease in the concentration of phosphine and, at
a higher content of phosphine, the reaction rate increases,
but the yields of compounds 3 remained almost unchanged.
The yields given in the present work were calculated based
on the stoichiometry shown in Scheme 2.
The reaction rate strongly depends on the electron-
donating ability and steric hindrance of phosphine. For
example, the reaction with Ph2PH (2a) taken in two-fold
excess requires one day for completion, while the reaction
with Et2PH (2c) proceeds very fast (2 min) at the same
ratio of reagents. At the same time, the reaction with
a two-fold excess of bulkier Cy2PH (2b) having a close
basicity takes 5 h.
only in the case of reaction between ferrocenium and 2a.
In remaining cases, ferrocene cannot be isolated in pure
state due to the formation of mixtures with organophos-
phorus by-products, which are difficult to separate because
their chromatographic mobilities on a sorbent are close to
that of ferrocene. We observed the same situation with
isolation of pure ferrocene in the reactions of ferrocenium
ion with aminophosphines.5
Complexes 3 can be isolated with ease by fractional
extraction, which allowed us to obtain them in the indi-
vidual state and to characterize. These compounds are
quite stable in air, except for compound 3c whose 1H NMR
spectrum displays a paramagnetic broadening of signals.
The structure of the half-sandwich complex 3b was con-
firmed by X-ray diffraction (Fig. 1, Table 1).
Besides the half-sandwich complexes 3, ferrocene
easily identified by 1H NMR spectroscopy is produced in
the reaction, but its precise amount (87%) was determined
Complex 3b is the only example of structurally char-
acterized complex with three secondary phosphines co-
C(2)
C(1)
C(3)
C(5)
C(6)
C(24)
C(4)
P(2)
P(1)
C(12)
C(18)
H(2)
H(1)
P(3)
H(3)
C(30)
C(36)
Fig. 1. Molecular structure of complex 3b (atoms are represented as thermal ellipsoids at the 50% probability). The hydrogen atoms
of the Cp ring and Cy groups, the [PF6]– ion, and solvate molecules are omitted for clarity.

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Chamkin et al.
/deg
Table 1. Selected bond lengths (d) and angles () in complex 3d
Bond
d/Å
Bond
d/Å
Angle
Fe—Cp(centroid)
Fe(1)—C(Cp)
Fe(1)—P(1)
1.730(2)
2.105(3)—2.115(3)
2.2164(8)
Fe(1)—P(3)
P(1)—H(1)
P(2)—H(2)
P(3)—H(3)
2.2378(8)
1.27(3)
1.28(3)
1.32(3)
P(1)—Fe(1)—P(2)
P(1)—Fe(1)—P(3)
P(2)—Fe(1)—P(3)
89.06(3)
94.37(3)
92.92(3)
Fe(1)—P(2)
2.2246(8)
ordinated to the CpFe+ moiety. The complex has a piano-
stool geometry with typical P—Fe—P bond angles close
to 90. The Fe—P bond lengths in complex 3b (2.226 Å)
are close to those in the analogous half-sandwich com-
plexes with tertiary phosphines PR3 (for example, 2.214 Å
for R = Me)6 and elongated compared to Fe—P bonds
(2.171 Å) in the primary phosphine complex
[CpFe(H2PC3H5)3](PF6).7 The distinctive feature of
complex 3b consists in different orientation of substituents
at phosphorus atoms relative to the CpFe+ moiety. For
example, the hydrogen atoms at P(1) and P(2) are in the
transoid orientation relative to the center of the Cp ring
and the hydrogen atom at P(3) is in the gauche orientation
(the center(Cp)—Fe—P—H torsion angles are equal to
176, 152, and 72, respectively).
ligand in 19e iron arenecyclopentadienyl complexes
[CpFe(Arene)]• to form structurally close 18e half-
sandwich complexes [CpFe(PR3)3]+ also proceeds dis-
sociatively as a stepwise equilibrium of corresponding 19e
and 17e compounds.8
Thus, the reaction of ferrocenium with secondary
phosphines proceeds along a pathway other than that with
tertiary phosphines and aminophosphines to result in re-
placement of the cyclopentadienyl ligand with three phos-
phine molecules rather than substitution of the hydrogen
atom in the cyclopentadienyl ring. The reasons for different
proceeding of these two reactions require further studies.
Experimental
We assume that cyclopentadienephosphonium salts
4a—c are produced in the reactions; however, they are not
stable and, therefore, we failed to characterize them or
their decomposition products.
All reactions and manipulations were performed in argon
atmosphere using standard Schlenk technique in water- and air-
free solvents (dichloromethane, diethyl ether). Organic solvents
were purified according to standard procedures and distilled
immediately prior to use.
The discovered reaction is the first example of forma-
tion of half-sandwich 18e complexes of the type
[CpFe(PR3)3]+ by replacement of the cyclopentadienyl
ligand in 17e sandwich 1+•. We assume that the initial step
of the reaction is the nucleophilic addition of phosphine
to the Cp ring of ferrocenium to form the 17e radical
cation adduct [C5H5Fe(4-C5H5—PHR2)]+• (A) (see
Scheme 1, a), i.e., the same as for tertiary phosphines and
aminophosphines.4,5 At later steps, adduct A undergoes
substitution of the secondary phosphine for the cyclo-
pentadiene ligand (4-C5H5—+PHR2) and a redox reac-
tion with the starting ferrocenium to afford product 3 and
ferrocene.
In special experiments, we showed that primary phos-
phines RPH2 (R = Ph, Cy) does not react with ferroce-
nium 1+• in dichloromethane at room temperature, which
is likely due to their insufficient nucleophilicity. Earlier,7
analogous complexes [CpFe(PHR2)3]+ and [CpFe(PH2R)3]+
were obtained not only for secondary phosphines, but
also for primary ones by the photochemical substitution
of phosphine for the arene ligand in the 18e complex
[C5H5FeC6H6]+Х–. The difference in the outcomes
of reactions of primary phosphines with 1+• and
[C5H5FeC6H6]+X– can be explained by the fact that, in
the latter case, the reaction proceeds in a dissociative
manner and the nucleophilicity of phosphine has no defin-
ing value. Substitution of tertiary phosphines for an arene
Ferrocenium salts9 and phosphines Ph2PH (see Ref. 10) and
Et2PH (see Ref. 11) were prepared according to known pro-
cedures; Cy2PH (Sigma Aldrich) was distilled in vacuo at
104—106 C (2.5 Torr). 1H, 13C, and 31P NMR spectra were
recorded in deuterated acetone on a Bruker AMX 400 instrument
at room temperature.
Elemental analysis was carried out on a Carlo-Erba 1106
elemental analyzer at the Laboratory of Microanalysis of the
A. N. Nesmeyanov Institute of Organoelement Compounds of
the Russian Academy of Sciences.
X-ray diffraction study. Single crystals of 3b suitable for X-ray
diffraction study were obtained by slow recrystallization
from an Et2O—CH2Cl2 mixture. Crystals (C41H74FeP4F6•
•0.85(C4H10O)•0.15(CH2Cl2), M = 936.47) are monoclinic,
space group P21/n, at 120 K a = 10.3931(4), b = 23.0086(8),
c = 20.0526(7) Å,  = 96.2791(7), V = 4766.4(3) Å3, Z = 4,
dcalc = 1.305 g cm3, (Mo-K) = 5.23 cm–1. The intensities of
11522 unique reflections were measured on a Bruker SMART
APEX II diffractometer12 equipped with an area detector (graph-
ite monochromator, (Mo-K) = 0.71073 Å, -scanning tech-
nique, 2max = 56). The structure was solved by direct methods
and refined by the full-matrix least-squares method based on
F2
with anisotropic displacement parameters for all non-
hkl
hydrogen atoms. The PH hydrogen atoms of the cation were
determined from the Fourier synthesis and refined isotropically;
remaining hydrogen atoms were positioned geometrically and
refined using a riding model. The final R factors R1 = 0.0580
(based on Fhkl for 8794 reflections with I > 2(I)), wR2 = 0.1622
(based on F2hkl for all unique reflections). All calculations were

Ferrocenium with secondary phosphines
Russ. Chem. Bull., Int. Ed., Vol. 68, No. 7, July, 2019
1383
performed using the SHELXL program package.13 The full tables
of atomic coordinates, bond lengths, bond angles, and aniso-
tropic thermal parameters were deposited at the Cambridge
Crystallographic Data Centre (CCDC 1895805).
Reaction of ferrocenium salts with secondary phosphines
(general procedure). Appropriate phosphine was added to a stirred
solution of ferrocenium salt in dichloromethane (15 mL). The
mixture was stirred at room temperature until the blue color of
ferrocenium completely disappeared.
The resulting orange solution was evaporated. The residue
was dissolved in a minimum amount of dichloromethane and
poured into cold diethyl ether (50 mL) with vigorous stirring.
After 5 min stirring, the mixture was allowed to stand. The orange
ethereal solution of ferrocene was decanted and the residue was
washed with diethyl ether (in 10 mL portions) until the extract
was decolorized. The residue was dried, applied in dichlorometh-
ane onto a chromatographic column with silica gel (115 cm),
and eluted with dichloromethane—acetone (5 : 1), with a bright-
orange fraction being collected. The eluate was evaporated to
dryness. The residue was slowly reprecipitated from dichloro-
methane with diethyl ether and dried.
1.98 (br.s, 12 H, CH2); 4.50 (d, 3 H, PH, J = 333.7 Hz); 4.69
(s, 5 H, C5H5). 31P{1H} NMR, : –144.25 (sept, PF6, J = 711 Hz);
50.04 (s).
This work was financially supported by the Ministry of
Science and Higher Education of the Russian Federation.
The contribution of the Center for Molecule Composition
Studies of the Nesmeyanov Institute of Organoelement
Compounds of the Russian Academy of Sciences is grate-
fully acknowledged.
References
1. R. Prins, A. R. Korswagen, A. G. T. G. Kortbeek, J. Organomet.
Chem., 1972, 39, 335.
2. A. N. Nesmeyanov, E. G. Perevalova, L. P. Yur´eva, Bull.
Acad. Sci. USSR, Div. Chem. Sci., 1968, 17, 2282.
3. V. N. Babin, Yu. A. Belousov, T. A. Belousova, Yu. A. Borisov,
V. V. Gumenyuk, Yu. S. Nekrasov, Russ. Chem. Bull., 2011,
60, 2081.
Tris(diphenylphosphine)cyclopentadienyliron tetrafluorobor-
ate, [C5H5Fe(Ph2PH)3](BF4) (3a). Complex 3a (36 mg, 38%)
as a yellow-orange powder and ferrocene (20 mg, 87%) were
obtained from ferrocenium tetrafluoroborate (67 mg, 0.246 mmol,
1 equiv.) and Ph2PH (171 μL, 0.982 mmol, 4 equiv.) after stirring
for 24 h. 1H NMR, : 4.97 (s, 5 H, C5H5); 6.57 (d, 3 H, PH,
J = 370 Hz); 7.10—7.65 (m, 30 H, Ph). 31P{1H} NMR, : 56.2 (s).
Found (%): C, 64.10; H, 5.10. C41H38BF4FeP3. Calculated (%):
C, 64.26; H, 5.00.
Tris(dicyclohexylphosphine)cyclopentadienyliron hexafluoro-
phosphate, [C5H5Fe(Cy2PH)3](PF6) (3b) was obtained from
ferrocenium hexafluorophosphate (180 mg, 0.544 mmol, 1 equiv.)
and Cy2PH (440 μL, 2.175 mmol, 4 equiv.) in yield of 98 mg
(42%) as a yellow-orange powder. The reaction takes 5 h. 1H NMR,
: 1.24—1.45, 1.64—2.17 (m, 66 H, Cy); 4.50 (d, 3 H, PH,
J = 335.2 Hz); 4.81 (d, 5 H, C5H5, J = 1.4 Hz). 13C{1H} NMR,
: 26.36 (s, Cy); 28.26 (m, Cy); 28.75 (br.s, Cy); 30.66 (s, Cy);
36.66 (s, Cy); 42.16 (m, Cy); 76.20 (s, C5H5). 31P{1H} NMR, :
–144.25 (sept, PF6, J = 711 Hz); 66.81 (s). Found (%): C, 56.85;
H, 8.88. C41H74F6FeP4. Calculated (%): C, 57.21; H, 8.67.
Tris(diethylphosphine)cyclopentadienyliron hexafluorophos-
phate, [C5H5Fe(Et2PH)3](PF6) (3c) was obtained from ferroce-
nium hexafluorophosphate (166 mg, 0.502 mmol, 1 equiv.) and
Et2PH (116 μL, 1.003 mmol, 2 equiv.) in yield of 22 mg (16%)
as a yellow-orange powder. 1H NMR, : 1.29 (br.s, 18 H, CH3);
4. A. A. Chamkin, V. V. Krivykh, O. M. Nikitin, A. A. Kreindlin,
N. A. Shteltser, F. M. Dolgushin, O. I. Artyushin, N. S.
Ikonnikov, Yu. A. Borisov, Yu. A. Belousov, N. A. Ustynyuk,
Eur. J. Inorg. Chem., 2018, 2018, 4494.
5. A. A. Chamkin, V. V. Krivykh, N. A. Shtel´tser, O. V.
Semeikin, F. M. Dolgushin, N. A. Ustynyuk, Russ. Chem.
Bull., 2019, 68, 532.
6. D. E. Herbert, M. Tanabe, S. C. Bourke, A. J. Lough,
I. Manners, J. Am. Chem. Soc., 2008, 130, 4166.
7. P. G. Edwards, K. M. A. Malik, Li-ling Ooi, A. J. Price,
Dalton Trans., 2006, 433.
8. J. Ruiz, M. Lacoste, D. Astruc, J. Am. Chem. Soc., 1990,
112, 5471.
9. N. G. Connelly, W. E. Geiger, Chem. Rev., 1996, 96, 877.
10. V. D. Bianco, S. Doronzo, Inorg. Syn., 1976, 16, 161.
11. K. Issleib, A. Tzschach, Chem. Ber., 1959, 92, 704.
12. APEX II software package, Bruker AXS Inc., 5465 East Cheryl
Parkway, Madison, WI 5317, 2005.
13. G. M. Sheldrick, Crystal structure refinement with SHELXL,
Acta Crystallogr., 2015, C71, 3.
Received February 8, 2019;
in revised form March 25, 2019;
accepted April 14, 2019