Synthesis and redox properties of crowded triarylphosphines possessing ferrocenyl groups

Article abstract of DOI:10.1021/ic051038l

Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6- diisopropylphenyl)_n(2,4,6-triisopropylphenyl)_3-nP (5a, n = 1; 5b, n = 2; 5c, n = 3)] were synthesized by the reaction of the corresponding arylcopper(I) reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by ¹H, ¹³C, and ³¹P NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded ³¹P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are 110.8° and 1.854 A, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers. The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at 293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the ferrocenyl groups in the periphery of the molecules are oxidized at first.

Full text of DOI:10.1021/ic051038l

Inorg. Chem. 2006, 45, 992−998
Synthesis and Redox Properties of Crowded Triarylphosphines
Possessing Ferrocenyl Groups
Katsuhide Sutoh, Shigeru Sasaki,* and Masaaki Yoshifuji
Department of Chemistry, Graduate School of Science, Tohoku UniVersity,
Aoba, Sendai 980-8578, Japan
Received June 24, 2005
Crowded triarylphosphines possessing ferrocenyl groups [(4-ferrocenyl-2,6-diisopropylphenyl)n(2,4,6-triisopropylphen-
yl)3 nP (5a, n 1; 5b, n 2; 5c, n 3)] were synthesized by the reaction of the corresponding arylcopper(I)
)
)
)
-
reagents with the diarylchlorophosphines. Structures of the triarylphosphines were studied by 1H, 13C, and 31P
NMR spectroscopies, and the characteristic patterns of the proton signals of the 2,6-isopropyl groups and upfielded
31P chemical shifts suggest structural similarities of the triarylphosphine moiety to the previously reported tris-
(2,4,6-triisopropylphenyl)phosphine (2). X-ray crystallography of 5c also revealed that the structure around the
phosphorus is similar to that of 1, where the average bond angle and length around the phosphorus atom are
110.8° and 1.854 Å, respectively. According to the electrochemical measurements, phosphines 5a, 5b, and 5c are
reversibly oxidized in two, three, and four steps, respectively, which suggests significant electronic interaction among
the triarylphosphine and the ferrocene redox centers as well as weak interaction among the ferrocene redox centers.
The EPR spectra obtained at cryogenic temperature after oxidation of phosphines 5a, 5b, and 5c are superpositions
of those for the cation radicals of the crowded triarylphosphine and ferricinium. The solution spectra obtained at
293 K, which consist of two lines typical of the cation radical of the crowded triarylphosphines, become weaker as
the number of the ferrocenyl groups increases and the cation radical of 5c does not show EPR signals. These
findings suggest not only instability of the tetra(cation radical) of 5c but also the course of oxidation where the
ferrocenyl groups in the periphery of the molecules are oxidized at first.
Introduction
(Scheme 1). Structures of the crowded triarylphosphines and
the related compounds have been extensively studied because
the structural change around the phosphorus atom and the
dynamic behavior of the substituents are worth much
attention.5 The structure around the phosphorus atom is
closely related to the redox properties, and the low oxidation
potential and stability of the cation radicals can be explained
Construction of multistep reversible redox systems has
attracted considerable interest from the fundamental to
practical viewpoints. Since the key structures applicable as
stable redox centers are still limited, development of novel
redox centers is actively explored. Although nitrogen com-
pounds such as triarylamines1 have long been accepted as
the redox centers of functional molecules, their heavier main
group counterparts have not attracted such attention because
most of them are generally unstable in oxidized/reduced as
well as neutral forms. Recently we and others reported
construction of multistep redox systems carrying phosphorus
redox centers such as sterically protected diphosphenes2 and
phosphaethenes.3 Crowded triarylphosphines such as tri-
mesitylphosphine (1)4 are known to be reversibly oxidized
to the stable cation radicals at low potential and are expected
to work as the redox centers in multistep redox systems
(1) Recent examples: (a) Selby, T. D.; Blackstock, S. C. J. Am. Chem.
Soc. 1998, 120, 12155-12156. (b) Selby, T. D.; Blackstock, S. C. J.
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Oda, M.; Murai, H.; Baumgarten, M. J. Am. Chem. Soc. 1996, 118,
3047-3048. (d) Wienk, M. M.; Janssen, R. A. J. Chem. Commun.
1996, 267-268. (e) Stickley, K. R.; Selby, T. D.; Blackstock, S. C. J.
Org. Chem. 1997, 62, 448-449. (f) Yano, M.; Sato, K.; Shiomi, D.;
Ichimura, A.; Abe, K.; Takui, T.; Itoh, K. Tetrahedron Lett. 1996,
51, 9207-9210. (g) Sato, K.; Yano, M.; Furuichi, M.; Shiomi, D.;
Takui, T.; Abe, K.; Itoh, K.; Higuchi, A.; Katsuma, K.; Shirota, Y. J.
Am. Chem. Soc. 1997, 119, 6607-6613. (h) Shirota, Y. J. Mater.
Chem. 2000, 10, 1-25. (i) Louie, J.; Hartwig, J. F.; Fry, A. J. J. Am.
Chem. Soc. 1997, 119, 11695-11696. (j) Lambert, C.; No¨ll, G. J.
Am. Chem. Soc. 1999, 121, 8434-8442. (k) Low, P. J.; Paterson, M.
A. J.: Puschmann, H.; Goeta, A. E.; Howard, J. A. K.; Lambert, C.;
Cherryman, J. C.; Tackley, D. R.; Leeming, S.; Brown, B. Chem.s
Eur. J. 2004, 10, 83-91.
* To whom correspondence should be addressed. Telephone: +81-22-
795-6561. Fax: +81-22-795-6561. E-mail: sasaki@mail.tains.tohoku.ac.jp.
992 Inorganic Chemistry, Vol. 45, No. 3, 2006
10.1021/ic051038l CCC: $33.50
© 2006 American Chemical Society
Published on Web 12/21/2005

Crowded Triarylphosphines Possessing Ferrocenyl Groups
Scheme 1
Scheme 2. Synthesis of Crowded Triarylphosphines Carrying
Ferrocenyl Groups
by thermodynamic stabilization by the structural change and
kinetic stabilization by the bulky substituents.6 Although 1
has been regarded as one of the most typical crowded
triarylphosphines, more crowded triarylphosphines, which
have larger bond angles and lengths around the phosphorus
atom, have been synthesized recently. Tris(9-anthryl)phos-
phine7 has a unique structure and optical properties arising
from the anthryl groups that can be modulated by simple
derivatization.8 We synthesized tris(2,4,6-triisopropylphenyl)-
phosphine (2), which is oxidized at lower potential to a more
stable cation radical than 1, and demonstrated the structure-
properties relationship among several crowded triarylphos-
phines.9 On the other hand, we have employed triarylphos-
phine moieties similar to 1 as the redox center, but model
compounds, such as aminophosphinobenzene 3, proved to
be irreversible redox systems at room temperature probably
due to insufficient stability of the phosphorus redox center.10
Therefore, we employed more crowded triarylphosphine
moieties similar to 2 as the phosphorus redox sites, and
biphenyl 4, which was synthesized as the first attempt, was
shown to be a considerably stable two-step redox system.11
(2) (a) Tsuji, K.; Sasaki, S.; Yoshifuji, M. Tetrahedron Lett. 1999, 40,
3203-3206. (b) Sasaki, S.; Aoki, H.; Sutoh, K.; Hakiri, S.; Tsuji, K.;
Yoshifuji, M. HelV. Chim. Acta 2002, 85, 3842-3847. (c) Pietschnig,
R.; Niecke, E. Organometallics 1996, 15, 891-893. (d) Nagahora,
N.; Sasamori, T.; Takeda, N.; Tokitoh, N. Chem.sEur. J. 2004, 10,
6146-6151. (e) Dutan, C.; Shah, S.; Smith, R. C.; Choua, S.; Berclaz,
T.; Geoffroy, M.; Protasiewicz, J. D. Inorg. Chem. 2003, 42, 6241-
6251.
(3) (a) Sasaki, S.; Murakami, F.; Yoshifuji, M. Angew. Chem., Int. Ed.
1999, 38, 340-343. (b) Murakami, F.; Sasaki, S.; Yoshifuji, M. Angew.
Chem., Int. Ed. 2002, 41, 2574-2576. (c) Kawasaki, S.; Nakamura,
A.; Toyota, K.; Yoshifuji, M. Organometallics 2005, 24, 2983-2987.
(4) (a) Blount, J. F.; Maryanoff, C. A.; Mislow, K. Tetrahedron Lett. 1975,
48, 913-916. (b) Stepanov, B. I.; Karpova, E. N.; Bokanov, A. I. J.
Gen. Chem. USSR 1969, 39, 1514.
Herein, we report the synthesis and structure of tri-
arylphosphines 5a, 5b, and 5c carrying ferrocenyl groups.
To investigate intramolecular interaction of the phosphorus
redox center with the donors, ferrocenes have been chosen
as the second redox sites. Ferrocenes are also reversibly
oxidized to stable radicals (ferriciniums) at low potential,
and the synthesis and various intramolecular interactions of
the large number of the compounds carrying ferrocenyl and
other redox sites have been extensively studied.12 The
ferrocenes connected to phosphines13 and amines14 have also
attracted considerable attention, although the group 15
elements act as linkers rather than redox sites in many cases.
(5) Bellamy, A. J.; Gould, R. O.; Walkinshaw, M. D. J. Chem. Soc., Perkin
Trans. 2 1981, 1099-1104.
(6) (a) Culcasi, M.; Berchadsky, Y.; Gronchi, G.; Tordo, P. J. Org. Chem.
1991, 56, 3537-3542. (b) Palau, C.; Berchadsky, Y.; Chalier, F.; Finet,
J.-P.; Gronchi, G.; Tordo, P. J. Phys. Chem. 1995, 99, 158-163. (c)
Boere´, R. T.; Zhang, Y. J. Organomet. Chem. 2005, 690, 2651-2657.
(7) (a) Wesemann, J.; Jones, P. G.; Schomburg, D.; Heuer, L.; Schmutzler,
R. Chem. Ber. 1992, 125, 2187-2197. (b) Mingos, D. M. P.; Mu¨ller,
T. E. J. Organomet. Chem. 1995, 500, 251-259. (c) Mu¨ller, T. E.;
Green, J. C.; Mingos, D. M. P.; McPartlin, C. M.; Whittingham, C.;
Williams, D. J.; Woodroffe, T. M. J. Organomet. Chem. 1998, 551,
313-330.
(9) Sasaki, S.; Sutoh, K.; Murakami, F.; Yoshifuji, M. J. Am. Chem. Soc.
2002, 124, 14830-14831.
(10) Sasaki, S.; Murakami, F.; Yoshifuji, M. Tetrahedron Lett. 1997, 38,
7095-7098.
(11) Sasaki, S.; Murakami, F.; Murakami, M.; Watanabe, M.; Kato, K.;
Sutoh, K.; Yoshifuji, M. J. Organomet. Chem. 2005, 690, 2664-2672.
(12) Nishihara, H. AdV. Inorg. Chem. 2002, 53, 41-86, and references
therein.
(13) Recent example: Barrie`re, F.; Kirss, R. U.; Geiger, W. E. Organo-
metallics 2005, 24, 48-52.
(8) (a) Yamaguchi, S.; Akiyama, S.; Tamao, K. J. Organomet. Chem.
2002, 646, 277-281. (b) Yamaguchi, S.; Akiyama, S.; Tamao, K. J.
Organomet. Chem. 2002, 652, 3-9.
Inorganic Chemistry, Vol. 45, No. 3, 2006 993

Sutoh et al.
Figure 1. ORTEP drawings of 5c with 50% thermal ellipsoids. Solvents
and hydrogen atoms are omitted for clarity. (a) Top view; (b) side view.
Selected bond lengths (Å) and angles (deg): P1-C1, 1.852(2); P1-C23,
1.853(2); P1-C45, 1.857(2); C1-P1-C23, 110.90(10); C1-P1-C45,
113.36(9); C23-P1-C45, 108.23(9).
(Ferrocenylaryl)phosphines 5a, 5b, and 5c are not only
regarded to be model compounds for practical molecules such
as multistep redox systems or functional polyradicals, but
they are also expected to reveal properties of the newly
developed crowded triarylphosphine structure.
Results and Discussion
Synthesis and Structure. The crowded triarylphosphine
moieties analogous to 2 have been synthesized by reaction
of an arylcopper(I) reagent with phosphorus chlorides.9,11 (4-
Ferrocenylaryl)phosphines 5a, 5b, and 5c were similarly
synthesized by using the arylcopper(I) reagent or diarylchlo-
rophosphine carrying 4-ferrocenyl-2,6-diisopropylphenyl
groups (Scheme 2). The ferrocenes carrying (diphenylphos-
phino)phenyl groups have been synthesized by a palladium-
catalyzed Suzuki coupling of the boronic acid of ferrocene
with (bromophenyl)diphenylphosphine oxides, followed by
reduction with trichlorosilane.15 We admit the analogous way
is worth pursuing, but construction of the crowded tri-
arylphosphine moiety in the final step seems better for the
synthesis of a series of the (ferrocenylaryl)phosphines 5.
4-Ferrocenylbromobenzene 6 was synthesized by palladium-
catalyzed cross coupling of ferrocenylzinc chloride16 with
2-bromo-1,3-diisopropyl-5-iodobenzene.11 Lithiation of 6
followed by reaction with phosphorus trichloride gave
diarylchlorophosphine 7. Bromobenzene 6 was converted to
Figure 2. Cyclic voltammograms of (a) 5a, (b) 5b, and (c) 5c at 293 K.
Conditions: glassy carbon, Pt wire, and Ag/0.01 M AgNO3/0.1 M
n-BuNClO4/CH3CN as working, counter, and reference electrodes, respec-
tively [E1/2(ferrocene/ferricinium) ) 0.18 V]. Scan rate ) 30 mV s-1. A
substrate (ca. 10-4 M) was dissolved in dichloromethane with 0.1 M n-Bu4-
NClO4 as a supporting electrolyte.
the corresponding arylcopper(I) reagent and allowed to react
with chlorobis(2,4,6-triisopropylphenyl)phosphine11,17 and 7
to afford (4-ferrocenyl-2,6-diisopropylphenyl)bis(2,4,6-tri-
isopropylphenyl)phosphine (5a) and tris(4-ferrocenyl-2,6-
diisopropylphenyl)phosphine (5c), respectively. On the other
hand, reaction of the arylcopper(I) reagent prepared from
2,4,6-triisopropylbromobenzene with 7 afforded bis(4-fer-
rocenyl-2,6-diisopropylphenyl)(2,4,6-triisopropylphenyl)-
phosphine (5b). Phosphines 5a, 5b, and 5c were isolated as
air-stable solids and displayed an orange color typical of
ferrocenyl compounds. 31P NMR (162 MHz, CDCl3) signals
are observed in a high-field area (δ -51.6 (5a), -50.9 (5b),
-51.0 (5c)) typical of crowded triarylphosphines such as 3
(δ -52.4).9 The 1H and 13C NMR spectra of 5c at 294 K are
rather simple despite the crowded structure. The 4-ferrocenyl-
2,6-diisopropylphenyl group gives only one aromatic, one
methine, and two methyl signals as if the aromatic substituent
(14) (a) Britton, W. E.; Kashyap, R.; El-Hashash, M.; El-Kady, M.
Organometallics 1986, 5, 1029-1031. (b) Lu, S.; Strelets, V. V.; Ryan,
M. F.; Pietro, W. J.; Lever, A. B. P. Inorg. Chem. 1996, 35, 1013-
1023.
(15) Stead, R.; Xiao, J. Lett. Org. Chem. 2004, 1, 148-150.
(16) (a) Iyoda, M.; Kondo, T.; Okabe, T.; Matsuyama, H.; Sasaki, S.;
Kuwatani, Y. Chem. Lett. 1997, 35-36. (b) Iyoda, M.; Okabe, T.;
Kondo, T.; Sasaki, S.; Matsuyama, H.; Kuwatani, Y.; Katada, M.
Chem. Lett. 1997, 103-104. (c) Siemeling, U.; Neumann, B.;
Stammler, H.-G.; Kuhnert, O. Polyhedron 1999, 18, 1815-1819.
(17) (a) Freeman, S.; Harger, M. J. J. Chem. Soc., Perkin Trans. 1 1987,
1399-1406. (b) Brauer, D. J.; Bitterer, F.; Do¨rrenbach, F.; Hessler,
G.; Stelzer, O.; Kru¨ger, C.; Lutz, F. Z. Naturforsch., B: Chem. Sci.
1996, 51, 1183-1196.
994 Inorganic Chemistry, Vol. 45, No. 3, 2006

Crowded Triarylphosphines Possessing Ferrocenyl Groups
Table 1. Oxidation Potentials of (Ferrocenylaryl)phosphines and Related Compounds Obtained by Cyclic Voltammetrya
1E1/2/V
2E1/2/V
3E1/2/V
4E1/2/V
∆E(2E1/2- E1/2)/V
∆E(3E1/2- E1/2)/V
∆E(4E1/2- E1/2)/V
1
2
3
compd
5a
5b
5c
2
0.16
0.16
0.16
0.16
0.20
0.35
0.23
0.23
0.19
0.07
0.07
0.44
0.23
0.21
0
0.57
0.34
6
a Versus Ag/Ag+ in dichloromethane with 0.10 M n-Bu4NClO4 [E1/2(ferrocene/ferricinium) ) 0.18 V], scan rate ) 30 mV s-1, and T ) 293 K.
Scheme 3. Oxidation of 5c
except for the ferrocenyl groups is C2 symmetrical relative
to the P-C bond. The 4-ferrocenyl-2,6-diisopropylphenyl
and 2,4,6-triisopropylphenyl groups of phosphines 5a and
5b, where one of the three aryl groups is different, are also
observed as C2 symmetrical similar to 5c, but the two 2,4,6-
triisopropylphenyl and 4-ferrocenyl-2,6-diisopropylphenyl
groups of 5a and 5b are observed as if they are two different
C2 symmetrical aryl groups. These phenomena suggest
averaging due to rapid inversion of the phosphorus atom and
slow enantiomerization of the helicity of the propeller
composed of the three aromatic rings. Similar averaging in
the NMR spectra is also observed for 2.9 UV-vis spectra
of 5a, 5b, and 5c have absorption characteristic of both the
ferrocenes (λmax 442 (5a), 443 (5b), 450sh nm (5c)) and the
crowded triarylphosphines (λmax 344 (5a), 351 (5b), 357 nm
(5c)). Ferrocenylarene 6 also displays the corresponding
absorption responsible for an orange color at a similar
wavelength (λmax 448 nm). On the other hand, introduction
of the ferrocenyl substituents leads to a red shift of the
absorption corresponding to triarylphosphine as compared
with 2 (λmax 327 nm).9 The molecular structure of 5c was
further investigated by X-ray crystallography (Figure 1). The
bond lengths and angles around the phosphorus reflect the
crowded structure, and the average C-P bond length and
angle (1.854 Å, 110.8°) are similar to those of 2 (1.845 Å,
111.5°).9
the third one occurs on the phosphorus atom so as to
minimize electrostatic repulsion between the positive charges.
Phosphine 5c shows an unresolved large redox wave
followed by a reversible one, which can be interpreted to be
a three-step reversible redox process with the second
2,3
oxidation as a two-electron process (1E1/2 ) 0.16 V,
E
1/2
4
) 0.23 V, E1/2 ) 0.57 V) from comparison with the
Osteryoung square wave voltammogram. The first and
second oxidation waves of phosphine 5c can be attributed
to the oxidation of the ferrocenyl groups, and the last
oxidation wave can be attributed to the oxidation of the
phosphine unit similarly to 5b (Scheme 3). The difference
between the first and second oxidation potentials (∆E ) 0.07
V), which reflects electrostatic repulsion among the ferri-
ciniums across the triarylphosphine framework, is similar
to that of 5b. On the other hand, the difference between the
second and third oxidation potentials becomes reasonably
larger (∆E ) 0.34 V) than that for 5b, since it reflects
repulsion between the positive charges on the three ferrocenyl
groups and the phosphorus.
Redox Properties. Redox properties of 5a, 5b, and 5c
were studied by cyclic voltammetry (Figure 2), and the
oxidation potentials are summarized in Table 1. Phosphine
5a shows two-step reversible redox waves (1E1/2 ) 0.16, 2E1/2
) 0.35 V vs Ag/Ag+) with the difference of the oxidation
2
1
potentials (∆E ) E1/2 - E1/2) as 0.19 V. Since 2 (E1/2
)
0.16 V)9 and 6 (E1/2 ) 0.20 V) are oxidized at similar
potentials, the considerably large ∆E suggests substantial
electronic interaction between the phosphorus and the iron
redox centers. Although the assignment of the first and the
second oxidation waves is not clear, the cyclic voltammo-
grams of 5b and 5c and EPR study indirectly indicate that
the first oxidation takes place on the ferrocenyl group (see
below).
EPR Study of Cation Radicals. To observe the oxidation
products of (ferrocenylaryl)phosphines 5a, 5b, and 5c
directly, the chemical oxidation was studied by EPR spec-
troscopy. Since X-band EPR spectra of ferricinium cations
are generally observed only at cryogenic temperature due to
rapid relaxation,18 the samples were cooled to cryogenic
temperature just after contact with the oxidant.
Oxidation of ferrocenylarylphosphines 5a, 5b, and 5c with
silver(I) perchlorate in dichloromethane gave purple solu-
tions, and the EPR spectra of the frozen solutions at 5 K
and those of the fluid solution at 293 K are shown in Figure
3. The EPR spectra at 5 K are interpreted to be a superposi-
tion of the signals of the phosphorus cation radical on those
of ferricinium, which suggests small dipole-dipole interac-
The cyclic voltammogram of phosphine 5b consists of
three reversible redox waves (1E1/2 ) 0.16, 2E1/2 ) 0.23, 3E1/2
) 0.44 V). Since the second wave is severely overlapped
with the first one, oxidation potentials are further confirmed
by Osteryoung square wave voltammetry. As compared with
the ∆E of 5a, a smaller difference between the first and
second oxidation potentials (∆E ) 0.07 V) and a larger
difference between the second and the third ones (∆E ) 0.21
V) suggest the redox process of 5b, where the first and the
second oxidations take place on the ferrocenyl group and
(18) Prins, R.; Reinders, F. J. J. Am. Chem. Soc. 1969, 91, 4929-4931.
Inorganic Chemistry, Vol. 45, No. 3, 2006 995

Sutoh et al.
Figure 3. EPR spectra obtained after oxidation of 5 with AgClO4 in dichloromethane: 5a at (a) 5 and (b) 293 K; 5b at (c) 5 and (d) 293 K; 5c at (e) 5
and (f) 293 K.
Table 2. EPR Parameters of Cation Radicals of 5a, 5b, and 5c, and Related Compoundsa
solution
frozen solution
phosphorus cation radical
ferricinium
compd
g
a(31P)/mT
g|
g
a|(31P)/mT
a (31P)/mT
g|
g
5a
5b
5c
2
2.015
2.015
23.1
23.8
2.003
2.003
2.005
2.002
2.009
2.0015
2.021
2.009
40.7
40.0
38.6
41.7
10.2
10.4
10.5
13.0
4.10
4.10
4.09
1.59
1.59
1.59
2.007
23.7
6
4.05
4.36
1.50
1.30
ferriciniumb
a Measured in dichloromethane. b Reference 18.
tion between the radical centers. The anisotropic hyperfine
coupling constants with 31P nucleus, which are summarized
in Table 2, are similar to those of 2 reflecting the structural
similarities. The anisotropic g values of the ferricinium
moieties are close to those corresponding to 6 rather than
ferricinium.18 The signals of the ferricinium around g ) 4.1
and 1.6 broaden as temperature is raised and disappear at
60 K; on the other hand, signal intensities of the phosphorus
cation radical follow a Curie law from 5 to 130 K. The EPR
spectrum of the cation radical of 5a in solution is regarded
as a doublet due to the hyperfine coupling with 31P nucleus
(a(31P) ) 23.1 mT at g ) 2.015), which is similar to that of
2•+. However, as the number of ferrocenyl substituents
increases, the EPR signals become weaker, and the cation
radical of 5c does not show any signals at 293 K. As shown
by the cyclic voltammograms, the phosphorus cation radical
centers of tri(cation radical) 5b3(•+) and tetra(cation radical)
5c4(•+) are thermodynamically destabilized by electrostatic
repulsion with positive charges on the ferrocenyl groups and
become more reactive than that of 2•+. Thus, the phosphorus
cation radical centers are lost due to decomposition or back
electron transfer at high temperature in solution.
tramolecular interaction between the crowded triarylphos-
phine moiety and the donors. The triarylphosphine moieties
similar to 2 as well as ferrocenyl groups serve as reversible
redox sites, and the whole molecules become multistep
reversible redox systems. Considerable electronic interaction
between the phosphorus and the ferrocenyl redox sites and
weak interaction among the ferrocenyl redox sites contribute
construction of the multistep redox systems. Although the
EPR study supports formation of both the phosphorus-
centered cation radicals and the ferricinium cations, exchange
and dipole-dipole interactions are too small to be detected.
The crowded triarylphosphine moieties employed in this
work turned out to contribute formation of considerably
stable redox systems; however, the phosphorus redox sites
are not stable enough for tri- and tetra(cation radicals).
Further investigation is in progress on intramolecular interac-
tion between the crowded triarylphosphine moieties and
various functional sites such as acceptors and radicals as well
as development of more stable phosphorus redox centers.
Experimental Section
1
General. H, 13C, and 31P NMR spectra were measured on a
Bruker AV400 spectrometer. 1H and 13C NMR chemical shifts are
expressed as δ downfield from external tetramethylsilane. The
chemical shifts were calibrated to the residual proton of the
deuterated solvents (δ 7.25 for chloroform-d, δ 7.20 for benzene-
d6, δ 5.32 for dichloromethane-d2) or the carbon of the deuterated
solvent (δ 77.0 for chloroform-d, δ 128.5 for benzene-d6). 31P NMR
chemical shifts are expressed as δ downfield from external 85%
H3PO4. Mass spectra were measured on a Hitachi M-2500S with
Conclusion
Crowded triarylphosphines possessing one, two, or three
4-ferrocenyl-2,6-diisopropylphenyl substituents were syn-
thesized by the reaction of the corresponding arylcopper(I)
reagent with the phosphorus chlorides. These (ferrocenyl-
aryl)phosphines are good model compounds to study in-
996 Inorganic Chemistry, Vol. 45, No. 3, 2006

Crowded Triarylphosphines Possessing Ferrocenyl Groups
electron impact (EI) ionization at 70 eV or a JEOL HX-110 with
fast atom bombardment (FAB) ionization using m-nitrobenzyl
alcohol matrix. Fourier transform ion cyclotron resonance (FT-ICR)
mass spectra were measured on a Bruker APEX III with electron
spray ionization (ESI). Melting points were measured on a
Yanagimoto MP-J3 apparatus without correction. UV-vis and IR
spectra were measured on a Hitachi U-3210 and Horiba FT-300
spectrometers, respectively. Microanalyses were performed at the
Instrumental Analysis Center for Chemistry, Graduate School of
Science, Tohoku University. Merck silica gel 60 and Sumitomo
basic alumina (KCG-30) were used for column chromatography.
All reactions were carried out under argon. Tetrahydrofuran was
distilled from sodium diphenylketyl under argon just prior to use.
(CH3)2), 1.44 (d, 12H, J ) 6.54 Hz, o-CH(CH3)2), 1.23 (d, 12H, J
) 6.90 Hz, p-CH(CH3)2), 1.02 (d, 6H, J ) 6.54 Hz, o-CH(CH3)2),
0.95 (d, 12H, J ) 6.01 Hz, o-CH(CH3)2); 13C NMR (101 MHz,
C6D6) δ 153.64 (d, JPC ) 18.15 Hz, Ar-o), 153.54 (d, JPC ) 18.16
Hz, Tip-o), 149.93 (s, Tip-p), 140.38 (s, Ar-p), 133.23 (d, JPC
)
23.95 Hz, Ar-ipso), 132.60 (d, JPC ) 23.71 Hz, Tip-ipso), 122.54-
122.50 (d x 2, Tip-m), 122.28 (d, JPC ) 4.39 Hz, Ar-m), 85.58 (s,
C5H4), 70.03 (s, C5H5), 69.31 (s, C5H4), 66.95 (s, C5H4), 66.81 (s,
C5H4), 34.45 (s, p-CH(CH3)2), 32.50 (d, JPC ) 17.96 Hz, o-CH-
(CH3)2), 32.44 (d, JPC ) 17.41 Hz, o-CH(CH3)2), 25.04 (s, o-CH-
(CH3)2), 25.00 (s, o-CH(CH3)2), 24.91 (s, o-CH(CH3)2), 24.10 (s,
p-CH(CH3)2), 24.00 (s, p-CH(CH3)2), 23.61 (s, o-CH(CH3)2), 23.44
(s, o-CH(CH3)2), 23.28 (s, o-CH(CH3)2); 31P NMR (162 MHz, C6D6)
δ -51.6 (s); IR (KBr) 2966, 2929, 2868, 1633, 1601, 1547, 1462,
1419, 1383, 1360, 1308, 1238, 1161, 1128, 1103, 1063, 1028, 1003,
933, 879, 816, 787, 648, 525, and 505 cm-1; UV-vis (CH2Cl2)
Synthesis. 4-Ferrocenyl-2,6-diisopropylbromobenzene (6). To
a solution of ferrocene (380 mg, 2.04 mmol) in tetrahydrofuran
(15 mL) was added tert-butyllithium (2.71 mL, 1.51 M in pentane)
at -78 °C. The reaction mixture was stirred for 15 min and warmed
to 0 °C in 30 min. Anhydrous zinc chloride (794 mg, 5.83 mmol)
dried over a flame in vacuo was added through a solid-dropping
tube at 0 °C, and the mixture was stirred for 30 min at 0 °C. A
suspension of 2,6-diisopropyl-4-iodobromobenzene (500 mg, 1.36
mmol) and bis(triphenylphosphine)palladium(II) dichloride (143 mg,
204 mmol) in tetrahydrofuran (13 mL) was added into the orange
suspension of ferrocenylzinc chloride at 0 °C, and the mixture was
stirred for 12 h and then allowed to warm to room temperature.
The reaction mixture was concentrated in vacuo and purified by
column chromatography (SiO2, hexane) and gel permeation chro-
matography (JAIGEL 1H + 2H/chloroform) to afford 6 (171 mg,
λmax (log ꢀ) 442 (2.93), 344 nm (4.36); FABMS m/z (rel intensity)
783 (M+ + 1; 92), 662 (M+ - Fe - Cp + 1; 2), 579 (M+ - Tip;
5), 437 (M+ - FcAr; 41), 395 (M+ - FcAr - i-Pr + 1; 4), 380
(M+ - FcAr - i-Pr - CH3 + 1; 50), 346 (FcAr + 1; 100); FT-
ICR-MS (ESI, positive) m/z found 782.4643. Calcd for C52H71-
PFe+: M+, 782.4637. Found: C, 78.88; H, 9.01%. Calcd for
C52H71PFe‚0.5C2H5OH: C, 78.98; H, 9.25%.
Chlorobis(4-ferrocenyl-2,6-diisopropylphenyl)phosphine (7).
To a solution of 6 (157 mg, 0.37 mmol) in tetrahydrofuran (8 mL)
was added butyllithium (0.24 mL, 1.56 M in hexane) at -78 °C.
The mixture was stirred for 30 min, and phosphorus trichloride
(0.02 mL, 0.18 mmol) was added at -78 °C. The mixture was
stirred for 6 h at -78 °C, warmed to room temperature, concentrated
in vacuo, and purified by column chromatography (SiO2, hexane,
AcOEt:hexane ) 1:12) to afford 7 (120 mg, 0.16 mmol, 85%). 7:
1
0.40 mmol, 29%). 6: Orange solid, mp 100-102 °C; H NMR
(400 MHz, CDCl3, 293 K) δ 7.24 (s, 2H, arom), 4.62 (t, 2H, J )
1.62 Hz, C5H4), 4.32 (t, 2H, J ) 1.59 Hz, C5H4), 4.06 (s, 5H, C5H5),
3.52 (2H, sept, J ) 6.82 Hz, o-CH(CH3)2), 1.30 (d, 12H, J ) 6.85
Hz, o-CH(CH3)2); 13C NMR (101 MHz, CDCl3, 293 K) δ 147.33
(s, o-arom), 138.28 (s, p-arom), 123.89 (s, ipso-arom), 122.39 (s,
m-arom), 85.84 (s, C5H4), 69.59 (s, C5H5), 68.82 (s, C5H4), 66.87
(s, C5H4), 33.41 (s, o-CH(CH3)2), 23.13 (s, o-CH(CH3)2); IR (KBr)
2966, 2927, 2902, 2866, 1595, 1460, 1417, 1383, 1360, 1327, 1124,
1105, 1068, 1018, 881, 818, and 501 cm-1; UV-vis (CH2Cl2) λmax
(log ꢀ) 448 (2.58), 283 nm (4.12); LRMS (EI, 70 eV) m/z (rel
intensity) 426 (M+ + 2; 95), 424 (M+; 100), 380 (M+ - CH(CH3)2;
28), 330 (M+ - Br - CH3; 13), 314 (M+ - Br - 2 × CH3; 9);
HRMS (EI, 70 eV) m/z found 424.0484. Calcd for C22H25BrFe:
M, 424.0489. Found: C, 62.63; H, 6.05%. Calcd for C22H25BrFe:
C, 62.15; H, 5.93%.
1
Orange oil; H NMR (400 MHz, C6D6) δ 7.45 (d, 4H, J ) 3.08
Hz, arom), 4.58 (t, 4H, J ) 1.34 Hz, C5H4), 4.19 (t, 4H, J ) 1.86
Hz, C5H4), 4.16 (m, 4H, o-CH(CH3)2), 3.99 (s, 10H, C5H5), 1.26
(d, 12H, J ) 6.69 Hz, o-CH(CH3)2), 1.23 (d, 12H, J ) 6.67 Hz,
o-CH(CH3)2); 31P NMR (162 MHz, C6D6) δ 88.4 (s); FT-ICR-MS
(ESI, positive) m/z found 756.2038. Calcd for C44H50PClFe2+: M+,
756.2032.
Bis(4-ferrocenyl-2,6-diisopropylphenyl)(2,4,6-triisopropylphe-
nyl)phosphine (5b). To a solution of 2,4,6-triisopropylbromoben-
zene (158 mg, 0.56 mmol) in tetrahydrofuran (7 mL) was added
butyllithium (0.35 mL, 1.58 M in hexane) at -78 °C. The reaction
mixture was stirred for 20 min, and CuCl (55.2 mg, 0.56 mmol)
was added through a solid-dropping tube at -78 °C. The mixture
was stirred for 45 min, warmed to room temperature, stirred for 1
h, and then cooled to -78 °C. A solution of 7 (106 mg, 0.14 mmol)
in tetrahydrofuran (8 mL) was added to the dark-green suspension
of the arylcopper(I) reagent at -78 °C, and the mixture was stirred
for 2 h at -78 °C, refluxed for 12 h, cooled to room temperature,
concentrated in vacuo, and purified by column chromatography
(Al2O3, hexane, AcOEt:hexane ) 1:11) to afford 5b (80 mg, 0.086
(4-Ferrocenyl-2,6-diisopropylphenyl)bis(2,4,6-triisopropyl-
phenyl)phosphine (5a). To a solution of 6 (210 mg, 0.49 mmol)
in tetrahydrofuran (8 mL) was added butyllithium (0.47 mL, 1.57
M in hexane) at -78 °C. The mixture was stirred for 30 min, and
CuCl (49.0 mg, 0.49 mmol) was added through a solid-dropping
tube at -78 °C. The mixture was stirred for 30 min at -78 °C,
warmed to room temperature, stirred for 1 h, and then cooled to
-78 °C. A solution of chlorobis(2,4,6-triisopropylphenyl)phosphine
(234 mg, 0.49 mmol) in tetrahydrofuran (6 mL) was added into
the dark-green suspension of the arylcopper(I) reagent at -78 °C,
and the mixture was stirred for 2 h at -78 °C, then refluxed for 12
h, and cooled to room temperature. The reaction mixture was
concentrated in vacuo and purified by column chromatography
(Al2O3, hexane) to afford 5a (130 mg, 0.16 mmol, 34%). 5a:
Orange solid, mp 180-183 °C; 1H NMR (400 MHz, C6D6) δ 7.35
(d, 2H, J ) 3.13 Hz, arom), 7.05 (d, 4H, J ) 3.17 Hz, Tip-m),
4.63 (d, 2H, J ) 1.51 Hz, C5H4), 4.19 (t, 2H, J ) 1.77 Hz, C5H4),
4.00 (s, 5H, C5H5), 3.96-3.80 (m, 6H, o-CH(CH3)2), 2.79 (sept,
2H, J ) 6.54 Hz, p-CH(CH3)2), 1.45 (d, 6H, J ) 6.32 Hz, o-CH-
1
mmol, 62%). 5b: Orange solid, mp 175-178 °C (dec); H NMR
(400 MHz, C6D6) δ 7.46 (d, 4H, J ) 3.18 Hz, arom), 7.16 (d, 2H,
J ) 3.24 Hz, Tip-m), 4.64 (d, 4H, J ) 1.77 Hz, C5H4), 4.19 (t, 4H,
J ) 1.79 Hz, C5H4), 4.01 (s, 10H, C5H5), 3.98-3.78 (m, 6H,
o-CH(CH3)2), 2.80 (sept, 1H, J ) 6.87 Hz, p-CH(CH3)2), 1.47 (d,
12H, J ) 6.76 Hz, o-CH(CH3)2), 1.45 (d, 6H, J ) 6.68 Hz, o-CH-
(CH3)2), 1.24 (d, 6H, J ) 6.91 Hz, p-CH(CH3)2), 1.06 (d, 6H, J )
5.80 Hz, o-CH(CH3)2), 1.04 (d, 6H, J ) 6.02 Hz, o-CH(CH3)2),
0.97 (d, 6H, J ) 6.63 Hz, o-CH(CH3)2); 13C NMR (101 MHz, C6D6)
δ 153.59 (d, JPC ) 18.17 Hz, o-arom), 153.56 (d, JPC ) 18.46 Hz,
o-arom), 153.48 (d, JPC ) 18.28 Hz, o-arom), 140.51 (s, Tip-p),
132.96 (d, JPC ) 23.58 Hz, Ar-ipso), 132.34 (d, JPC ) 23.40 Hz,
Inorganic Chemistry, Vol. 45, No. 3, 2006 997

Sutoh et al.
Tip-ipso), 122.57 (d, JPC ) 4.31 Hz, Tip-m), 122.32 (d, JPC ) 3.82
Hz, Ar-m), 85.52 (s, C5H4), 70.05 (s, C5H5), 69.36 (s, C5H4), 66.99
anisotropically refined. The hydrogen atoms were included but not
refined. The structure solution, refinement, and graphical repre-
sentation were carried out using the teXsan package.21 Crystal data
of 5c‚0.5C6H14: C69H82Fe3P, M ) 1109.92, orange platelet grown
from hexane, crystal dimensions 0.45 × 0.45 × 0.10 mm3, triclinic,
space group P1h (no. 2), a ) 14.3594(6), b ) 20.6361(7), c )
10.6119(5) Å, R ) 102.842(1), â ) 109.590(2), γ ) 90.417(3)°,
V ) 2876.9(2) Å3, Dc ) 1.281 g cm-3, µ ) 0.814 mm-1, T )
90(1) K, Z ) 2, F(000) ) 1178.00. Of 25 230 reflections measured
(2θmax ) 51.0°), 10 086 were observed [I > 0.0σ(I)] (Rint ) 0.043).
Variable parameters 659. Goodness of fit S ) 1.28 for observed
reflections, and R1 ) 0.038 for I > 2.0σ(I), R ) 0.048, Rw ) 0.056
for all reflections. The maximum and minimum peaks on the final
(s, C5H4), 66.82 (s, C5H4), 34.46 (s, p-CH(CH3)2), 32.53 (d, JPC
)
18.06 Hz, Tip-o-CH(CH3)2), 32.48 (d, JPC ) 17.26 Hz, Ar-o-CH-
(CH3)2), 25.07 (s, o-CH(CH3)2), 25.00 (s, o-CH(CH3)2), 24.94 (s,
o-CH(CH3)2), 24.11 (s, p-CH(CH3)2), 24.00 (s, p-CH(CH3)2), 23.65
(s, o-CH(CH3)2), 23.51 (s, o-CH(CH3)2), 23.31 (s, o-CH(CH3)2);
31P NMR (162 MHz, C6D6) δ -50.9 (s); IR (KBr) 2966, 2929,
2906, 2887, 2864, 2825, 2806, 2763, 1600, 1462, 1383, 1104, 1028,
816, and 507 cm-1; UV-vis (CH2Cl2) λmax (log ꢀ) 443 (3.34), 351
nm (4.47); FABMS m/z (rel intensity) 924 (M+; 100), 804 (M+
-
Fe - Cp; 6), 721 (M+ - Tip; 2), 579 (M+ - FcAr; 13), 330 (M+
- FcAr - Tip - CH3; 22); FT-ICR-MS (ESI, positive) m/z found
924.4153. Calcd for C59H73PFe2+: M+, 924.4143. Found: C, 76.29;
H, 8.14%. Calcd for C59H73PFe2: C, 76.62; H, 7.96%.
difference Fourier map corresponded to 0.89 and -0.71 e Å-3
,
respectively. CCDC 261625.
Cyclic Voltammetry. Cyclic voltammetry was performed on a
BAS CV-50W controller with glassy carbon, Pt wire, and Ag/0.01
M AgNO3/0.1 M n-BuNClO4/CH3CN as working, counter, and
reference electrodes, respectively [E1/2(ferrocene/ferricinium) ) 0.18
V]. A substrate ca. 10-4 M was dissolved in dichloromethane with
0.1 M n-Bu4NClO4 as a supporting electrolyte, and the solution
was degassed by bubbling with nitrogen gas.
EPR Measurement. X-band EPR spectra were measured on a
Bruker ESP300E spectrometer. Samples were dissolved in dichlo-
romethane, which was distilled over calcium hydride, degassed by
freeze-and-thaw cycles, and transferred to a sample by bulb-to-
bulb distillation. Oxidation was carried out in a H-shaped sealed
tube. An Oxford ESR900 continuous flow liquid helium cryostat
was used for the variable-temperature measurement at cryogenic
temperature.
Tris(4-ferrocenyl-2,6-diisopropylphenyl)phosphine (5c). To a
solution of 6 (89.6 mg, 0.21 mmol) in tetrahydrofuran (6 mL) was
added butyllithium (0.16 mL, 1.58 M in hexane) at -78 °C. The
reaction mixture was stirred for 30 min, CuCl (23 mg, 0.23 mmol)
was added through a solid-dropping tube at -78 °C, and the mixture
was stirred for 1 h at -78 °C. A solution of 7 (79.8 mg, 0.10 mmol)
in tetrahydrofuran (5 mL) was added into a dark-green suspension
of the arylcopper(I) reagent at -78 °C, and the mixture was stirred
for 30 min at -78 °C, refluxed for 12 h, and cooled to room
temperature. The reaction mixture was concentrated in vacuo and
purified by column chromatography (Al2O3, hexane, AcOEt:hexane
) 1:12) to afford 5c (25 mg, 0.023 mmol, 23%). 5c: Orange solid,
mp 191-191 °C (dec); 1H NMR (400 MHz, C6D6) δ 7.49 (d, 6H,
J ) 3.19 Hz, arom), 4.66 (d, 6H, J ) 1.56 Hz, C5H4), 4.20 (t, 6H,
J ) 1.66 Hz, C5H4), 4.02 (s, 15H, C5H5), 3.92 (m, 6H, o-CH(CH3)2),
1.49 (d, 18H, J ) 6.69 Hz, o-CH(CH3)2), 1.09 (d, 18H, J ) 6.63
Acknowledgment. The authors are grateful for a Grant-
in-Aid for Scientific Research from the Ministry of Educa-
tion, Culture, Sports, Science and Technology (Nos. 15550025,
17310063, and 1330409) for financial support and thank the
Instrumental Analysis Center for Chemistry, Graduate School
of Science, Tohoku University, for measurement of mass
spectra and elemental analyses. This work was partially
carried out in the Advanced Instrumental Laboratory for
Graduate Research of Department of Chemistry, Graduate
School of Science, Tohoku University.
Hz, o-CH(CH3)2); 13C NMR (101 MHz, C6D6) δ 153.54 (d, JPC
)
18.34 Hz, o-arom), 140.65 (s, p-arom), 132.72 (d, JPC ) 23.02 Hz,
ipso-arom), 122.38 (d, JPC ) 4.39 Hz, m-arom), 85.50 (s, C5H4),
70.07 (s, C5H5), 69.39 (s, C5H4), 67.03 (s, C5H4), 66.84 (s, C5H4),
32.53 (d, J ) 17.58 Hz, o-CH(CH3)2), 25.04 (s, o-CH(CH3)2), 23.55
(s, o-CH(CH3)2); 31P NMR (162 MHz, C6D6) δ -51.0 (s); IR (KBr)
2966, 2941, 2929, 2906, 2889, 2866, 1601, 1543, 1460, 1417, 1385,
1103, 1065, 1026, 1003, 881, 816, 791, and 507 cm-1; UV-vis
(CH2Cl2) λmax (log ꢀ) 450sh (3.34), 357 nm (4.39); FABMS m/z
(rel intensity) 1066 (M+; 100), 946 (M+ - Fe - Cp; 7), 721 (M+
- FcAr; 20), 330 (M+ - FcAr - i-Pr; 67); FT-ICR-MS (ESI,
+
positive) m/z found 1066.3659. Calcd for C66H75PFe3
:
M+,
Supporting Information Available: Osteryoung square
wave voltammograms of 5b and 5c, EPR spectrum of the cation
radical of 6, and crystallographic information files (CIF) of 5c.
This material is available free of charge via the Internet at
http://pubs.acs.org.
1066.3649. Found: C, 72.92; H, 7.03%. Calcd for C66H75-
PFe3‚2C2H5OH: C, 72.54; H, 7.57%.
X-ray Crystallography. Single-crystal data were collected using
a Rigaku RAXIS-IV imaging plate area detector with graphite
monochromated Mo KR radiation (λ ) 0.710 69 Å). A total of 51
5.00° oscillation images were collected, each being exposed for
25.0 min. The structure was solved by direct methods (SIR92),19
expanded using Fourier techniques (DIRDIF94),20 and refined by
full matrix least squares on F. The non-hydrogen atoms were
IC051038L
(20) Beuskens, P. T.; Admiraal, G.; Beuskens, G.; Bosman, W. P.; de
Gelder, R.; Israel, R.; Smits, J. M. M.. The DIRDIF-94 program
system; Technical Report of the Crystallography Laboratory; University
of Nijmegen: Nijmegen, The Netherlands, 1994.
(21) teXsan. Single-Crystal Structure Analysis Software, version 1.9;
Molecular Structure Corporation: The Woodlands, TX, Rigaku Cor-
poration, Tokyo, Japan, 1998.
(19) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, M.; Giacovazzo,
C.; Guagliardi, A.; Polidori, G. J. Appl. Crystallogr. 1994, 27, 435.
998 Inorganic Chemistry, Vol. 45, No. 3, 2006