Synthesis of a stable 1,2-bis(ferrocenyl)diphosphene

Article abstract of DOI:10.1039/c2cc33277a

The synthesis and characterization of a stable 1,2-bis(ferrocenyl) diphosphene, wherein a PP π-bond connects two ferrocenyl units will be reported. This represents an unprecedented example for a d-π electron system containing a heavier pnictogen π-spacer group. Stabilization of the highly reactive PP π-bond was achieved by steric protection using two bulky ferrocenyl moieties.

Full text of DOI:10.1039/c2cc33277a

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Communication
Synthesis of a Stable 1,2-Bis(ferrocenyl)diphosphene
Takahiro Sasamori,*a Michiyasu Sakagami,a Masatoshi Niwa,a Heisuke Sakai,b Yukio Furukawa,b and
Norihiro Tokitoh*a
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
electron systems containing phosphorus, this prompted us to
50 investigate the chemistry of the corresponding phosphorus-
containing d-π electron systems.7 The simplest conceivable
model compound for 1,2-bis(ferrocenyl)diphosphenes is Fc–
P=P–Fc, the phosphorus analogue of Fc–N=N–Fc,1b,4 The
limited steric demand of the ferrocenyl groups should
55 however offer only insufficient protection for the reactive P=P
bond.8 As a solution of this synthetic challenge,9 we would
like to report the synthesis of a custom-tailored ferrocenyl
unit with an increased ability for steric protection, and its
subsequent implementation in the synthesis of 1,2-
60 bis(ferrocenyl)diphosphene 1 as a stable crystalline compound.
The synthesis and characterization of
a
stable 1,2-
bis(ferrocenyl)diphosphene, wherein a P=P π-bond connects
two ferrocenyl units will be reported. This represents an
unprecedented example for a d-π electron system containing
10 a heavier pnictogen π-spacer group. Stabilization of the
highly reactive P=P π-bond was achieved by steric protection
using two bulky ferrocenyl moieties.
The sustained interest in d-π electron systems of bimetallic
complexes with π-bond spacers, i.e., M–(π-conjugated
15 spacer)–M (M = transition metal), is due to their unusual
electronic, optical, and magnetic properties.1 Moreover, they
can serve as models for compounds with mixed-valence states
and for molecular wires, since the high energy level of the
occupied orbitals and the small HOMO-LUMO gap facilitates
20 electron delocalization over the d and π orbitals. In such d-π
electron systems, ferrocene is frequently used as the d-
electron moiety because (i) ferrocene exhibits a stable redox
behavior, (ii) the Cp ligands of ferrocene effectively correlate
the d-orbitals (Fe) with the external π-electron system and (iii)
25 organic π-electron systems can be easily attached onto the Cp
rings by a variety of established synthetic tools such as
metallations or cross-coupling reactions.2 Some d-π electron
systems with two ferrocenyl groups bridged by a π-bond
1) LDA
2) ZnCl2
3) DmpI,
cat. [Pd(PPh3)4]
1) NaI
2) (CF3CO)2O
Fe
O
Fe
Fe
Dmp
Dmp
S
S
S
THF
–78 ˚C
3 h
acetone
0 ˚C
0.5 h
O
Ph
Ph
Ph
2
3
(88%)
(91%)
1) LDA
2) ZnCl2
3) DmpI,
cat. [Pd(PPh3)4]
1) t-BuLi
2) I2
Fe
Fe
mCPBA
O
Dmp
CH2Cl2
0 ˚C
Et2O
–78 ˚C
THF
–78 ˚C
Dmp
Dmp
S
S
O
3 h
(90%)
Ph
Ph
3 h
(70%)
3 h
(82%)
moiety, e.g., Ph(Fc)C=C(Fc)Ph3 or Fc–N=N–Fc4 (Fc
=
4
5
30 ferrocenyl) have been reported and they show the unique
properties of multi-step redox systems. A still prevailing
limitation for the π-electron systems in these compounds is
the restriction to the 2nd row of the p-block elements, even
though the chemistry of π-electron systems containing heavier
35 main group elements such as disilenes (R2Si=SiR2) and
diphosphenes (RP=PR) has recently been the subject of
substantial progress. Although they are known to be highly
reactive, the isolation of compounds containing π-bonds of
heavier main group elements is possible at ambient
40 temperature, when the π-bond is sufficiently stabilized by
1) n-BuLi
2) PCl3
LiAlH4
Fe
Fe
Fe
Dmp
Dmp
PCl2
Dmp
PH2
Et2O
–78 ˚C
1 h
THF
r.t.
5 h
Dmp
Dmp
Dmp
I
6
7
8
(92%)
(72%)
(= Fc*I)
(= Fc*PCl2)
(= Fc*PH2)
Dmp = 3,5-dimethylphenyl
Scheme 1 Synthesis of bulky ferrocenylphosphines 7 and 8.
As a suitable ferrocenyl precursor, we identified 2,5-Dmp2-
1-iodoferrocene (6, Fc*I, Dmp = 3,5-dimethylphenyl) bearing
65 two Dmp groups in 2 and 5-positions of iodoferrocene
(Scheme 1).10 Sulfoxide 2 was obtained from a Negishi-cross-
coupling reaction of ferrocenylphenylsulfoxide11 with DmpI.12
Reduction of 2 with NaI and (CF3CO)2O afforded sulfide 3,13
which was oxidized with mCPBA to give sulfoxide 4, a
70 structural isomer of 2. The inversion of the sulfoxide at this
point is the key step for the introduction of another Dmp
group to give sulfoxide 5, which was achived by a second
sterically
demanding
substituents.5 Still,
systematic
investigations into the synthesis and properties of d-π electron
systems containing heavier main group element π-spacers
remain scarce to date. Recent reports on phosphorus-based π-
45 electron systems proposed an immense potential for these
compounds, not only with respect to applications in functional
materials but also from a fundamental perspective.6 Combined
with the recent developments in the field of extended π-
Negishi-cross-coupling reaction between
4
and DmpI.
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Treatment of sulfoxide 5 with n-BuLi and I2 resulted in the 45 chemical shifts previously reported for stable diaryl- and
formation of 6,12b which was subjected to sequential treatment
with n-BuLi and PCl3 to form Fc*PCl2 (7), which in turn was
reduced with LiAlH4 to give primary phosphine Fc*PH2 (8).14
The targeted 1,2-bis(ferrocenyl)diphosphene 1 was obtained
ferrocenyl-diphosphenes,7,9,17 corroborating the preservation
of the P=P double bond in 1 in solution. The UV/vis spectra
of 1 in THF or hexane showed three characteristic absorption
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5
bands at λmax = 351 (ε 9,000), 448 (1,400), 556 (3,000) nm
as a stable, purple, crystalline solid in 40% yield from the 50 (THF) or λmax = 349 (6,100), 446 (990), 546 (2,000) nm
reaction of
deprotonation
dehydrochlorination using DBU (Scheme 2).
7
with Fc*PHLi (9), prepared from the
(hexane). Previous reports on stable diaryldiphosphenes, e.g.,
Mes*P=PMes* (Mes* = 2,4,6-tri-t-butylphenyl)20 assigned the
two absorptions at shorter wavelengths with λmax = ca. 350
and ca. 450 nm to symmetry-allowed π-π* and symmetry-
55 forbidden n-π* transitions of the P=P chromophore. The
absorption of 1 at longer wavelengths (ca. 550 nm) should
accordingly be attributed to d-π* electron transitions from
filled d-orbitals at the Fe center to the π* orbital of the P=P π-
spacer. This is supported by the solvochromacy of this
60 electron transition (Δλmax = 10 nm) as well as by the accurate
reproducibility of the λmax values from theoretical calculations,
which suggest HOMO(Fe,d)–LUMO(P=P,π*) and HOMO-
3(Fe,d)–LUMO(P=P,π*) electron transitions at 553 and 542
nm, respectively.21
of
8
with
n-BuLi,
followed
by
Fe
1) n-BuLi (1.0 eq.)
DBU
(1.0 eq.)
Dmp
2) Fc*PCl2 (7, 1.0 eq.)
Dmp
Fc*PH2
8
Dmp
P
P
THF, –78 ˚C, 10 min.
THF
–78 ˚C
2 h
Dmp
Fe
(40%)
1
10
Scheme 2 Synthesis of 1,2-bis(ferrocenyl)diphosphene 1.
Molecule A Molecule B
Fe
P
65
The cyclic voltammograms of
1 (Fig.2) showed one
Fe
reversible redox couple in the reduction region at E1/2 = –2.03
P
P
P
V
(all potentials referenced vs. FcH/FcH+), which is
significantly lower than that of FcN=NFc (–2.32 V).1b The
lower reduction potential should be a direct consequence of
Fe
Fe
70 the lower LUMO level of the P=P π−bond relative to that of
the corresponding N=N π-bond.22 The reduction potential of 1
is also considerably lower than that of the previously reported
diaryldiphosphenes, e.g., Mes*P=PMes* (ca. –2.4 V),23-25
which can probably be attributed to the effective π-
Fig. 1 Molecular structure of diphosphene 1 with thermal displacement
ellipsoids at 50% probability. Two crystallographically independent
15 molecules (A and B) are contained per unit cell. All hydrogen atoms as
well as one molecule of hexane have been omitted for clarity.
The X-ray diffraction (XRD) analysis of single crystals of 1 75 conjugation in 1. The oxidation region showed three oxidation
revealed two crystallographically independent molecules with
similar structural parameters per unit cell (Fig. 1).10,15 Both
20 P=P moieties exhibit E-configuration with C–P=P–C angles of
ca. 180º. The P=P bond lengths of 2.0312(8) and 2.0379(8) Å
waves at Epa = +0.69, +1.03, and +1.75 V. Upon reversing the
sweep at E<1 V, the first wave was found to be reversibe (Epc
= +0.41 V), showing a first oxidation potential at E1/2 = +0.55
V. The two subsequent oxidation steps at Epa = +1.03 and
are shorter than typical P–P single bonds (ca. 2.2 Å)16 and 80 +1.75 V were found to be irreversible, suggesting facile
similar to those of previously reported diphosphenes.5,17 The
dihedral angles between the P–P–C(Cp) and the Cp planes are
25 ca. 42-45º. Stable metallocenyldiphosphenes such as
TbtP=PMc (Mc = ferrocenyl or ruthenocenyl, Tbt = 2,4,6-
decomposition of the transient dicationic and/or tricationic
species. By comparison with previously reported
ferrocenyldiphosphenes sucha as Tbt–P=P–Fc,7b the first two
oxidation potentials of 1 (Epa = +0.69, +1.03 V) should be
[CH(SiMe3)2]3-C6H2) show in contrast to that an almost 85 assigned to the oxidation processes of the ferrocenyl moieties,
coplanar alignment of the Cp–P=P moieties.7b,9f,9g The
relatively moderate dihedral angles of ca. 45º (i.e. not
30 perpendicular) in 1 do not impose significant restrictions on
the π-conjugative interaction. The solid state Raman spectrum
whereas the third (Epa = +1.75 V) should be assigned to that of
the P=P moiety. We would like to point out that the oxidation
processes of both ferrocenyl moieties were observed in two
separate steps with ΔE = 0.34 V, which is considerably larger
of 1 shows one strong Raman shift at 621 cm–1 for the νPP 90 than the separation in Ph(Fc)C=C(Fc)Ph3c (ΔE = 0.18 V) or
vibration. This vibrational frequency is consistent with those
for previously reported diphosphenes (ca. 610-650 cm–1)17 and
35 supported by theoretical calculations (647 cm–1).18 Combined
XRD analysis and Raman spectroscopy therefore suggests a
Tip(Fc)Si=Si(Fc)Tip9a (ΔE
=
0.19 V). The increased
separation suggests an increase in electronic communication
through the P=P π-electron moiety and thus leads to the
conclusion that the P=P double bond is highly effective π-
degree of P=P double bond character for 1 in the solid state 95 electron spacer for bimetallic d-π electron systems.
that is comparable to that in previously reported diphosphenes.
The 1H and 31P NMR spectra suggested a C2h symmetric
40 structure of 1 in C6D6 solution at room temperature.10 One
singlet signal was observed for the methyl protons of the Dmp
In summary, we have shown here the synthesis of the first
stable 1,2-bis(ferrocenyl)diphosphene by taking advantage of
a bulky ferrocenyl group. The assessment of its structural
parameters and physical properties allowed the conclusion
groups, indicative of facile rotation around the C(Cp)– 100 that the P=P π-bond is stable both in the solid state and
C(Dmp) bonds. The 31P NMR spectra of 1 (C6D6) showed one
solution and can act as an efficient π-electron spacer to couple
resonance for the P nuclei at 482 ppm. This is consistent with
the d-electrons of two ferrocenyl groups.
2 | Journal Name, [year], [vol], 00–00
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(“private communication”). All synthetic attempts to isolate
(a)
(b)
–2.28
FcP=PFc by us and other groups have so far been unsuccessful,
suggesting the instability of FcP=PFc under ambient conditions: (a)
V. Naseri, R. J. Less, R. E. Mulvey, M. McPartlin and D. S. Wright,
50µA
10µA
1.75
1.03
60
65
70
75
Chem. Commun., 2010, 46, 5000. (b) C. Spang, F. T. Edelmann, M.
0.69
View Online
Noltemeyer and H. W. Roesky, Chem. Ber., 1989, 122, 1247.
For reports on related d-π electron systems containing heavier main
group elements, see: (a) A. Yuasa, T. Sasamori, Y. Hosoi, Y.
Furukawa and N. Tokitoh, Bull. Chem. Soc. Jpn., 2009, 82, 793. (b)
T. Sasamori, H. Miyamoto, H. Sakai, Y. Furukawa and N. Tokitoh,
Organometallics, 2012, 31, 3904. (c) C. Moser, M. Nieger and R.
Pietschnig, Organometallics, 2006, 25, 2667. (d) C. Moser, A.
Orthaber, M. Nieger, F. Belaj and R. Pietschnig, Dalton
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Nieger and K. Airola, J. Organomet. Chem., 1997, 529, 127. (f) N.
Nagahora, T. Sasamori, Y. Watanabe, Y. Furukawa and N. Tokitoh,
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Kaneko and N. Tokitoh, New J. Chem., 2010, 34, 1560.
9
0.41
–1.77
0.5 1.0 1.5 2.0
E (V) vs. FcH/FcH+
–3.0 –2.5 –2.0 –1.5
E (V) vs. FcH/FcH+
Fig. 2 Cyclic voltammograms of 1 (2.0 mM, 0.1 M n-Bu4NBF4): (a)
Oxidation region, –78 ºC, in CH2Cl2 (b) Reduction region, r. t., in THF.
This work was partially supported by Grants-in-Aid for
Scientific Research (B) (No. 22350017) and Young Scientist
(A) (No. 23685010) from the Ministry of Education, Culture,
Sports, Science and Technology, Japan. The authors would
like to thank Mr. Toshiaki Noda and Ms. Hideko Natsume
(Nagoya Univ.) for the manufacturing of custom-tailored
5
10 Detailed experimental procedures as well as spectral and
crystallographic data for the discussed new compounds can be
found in the supporting information section.
11 (a) P. Diter, O. Samuel, S. Taudien and H. B. Kagan, Tetrahedron
Asym., 1994, 5, 549. (b) P. Diter, S. Taudien, O. Samuel and H. B.
Kagan, J. Org. Chem., 1994, 59, 370.
10 glassware used during the electrochemical measurements at
low temperatures.
80 12 (a) H. K. Cotton, F. F. Huerta and J. E. Backvall, Eur. J. Org. Chem.,
2003, 2756. (b) R. J. Kloetzing and P. Knochel, Tetrahedron Asym.,
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Notes and references
13 B. F. Bonini, M. Fochi, M. Comes-Franchini, A. Ricci, L. Thijs and
B. Zwanenburg, Tetrahedron Asym., 2003, 14, 3321.
a Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto,
611-0011, Japan. Fax: +81-774-38-3200; Tel: +81-774-38-3209; E-
15 mail: sasamori@boc.kuicr.kyoto-u.ac.jp, tokitoh@boc.kuicr.kyoto-u.ac.jp
b Department of Chemistry and Biochemistry, School of Advanced Science
and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo
169-8555, Japan.
85 14 Probably as a result of steric congestion, primary phosphine Fc*PH2
is airstable enough to be handled under atmospheric conditions. For
other stable primary phosphines, see: B. Stewart, A. Harriman and L.
J. Higham, Organometallics, 2011, 30, 5338, and references therein.
15 The crystallographic data for 1 can be obtained free of charge from
The Cambridge Crystallographic Data Centre (CCDC-879016) via
www.ccdc.cam.ac.uk/data_request/cif.
16 For example, the P–P bond length in Ph2P–PPh2 is 2.217(1) Å. A.
Dashti-Mommertz and B. Neumüller, Z. Anorg. Allg. Chem., 1999,
625, 954.
95 17 (a) L. Weber, Chem. Rev., 1992, 92, 1839. (b) T. Sasamori and N.
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18 B3PW91/6-31G(d) (C, H), 6-31+G(2d) (P), DZVP (Fe).
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see: M. Yoshifuji, I. Shima, N. Inamoto, K. Hirotsu and T. Higuchi,
J. Am. Chem. Soc., 1981, 103, 4587.
21 TD-B3PW91/6-311G(2d,p) (C, H), 6-311+G(3df) (P), DZPV
(Fe)//B3PW91/6-31G(d) (C, H), 6-31+G(2d) (P), DZVP (Fe).
22 K. Ito and S. Nagase, Chem. Phys. Lett., 1986, 126, 531.
† Electronic Supplementary Information (ESI) available: [Experimental
20 procedures and spectroscopic data for all new compounds, as well as X-
90
ray crystallographic data for
DOI: 10.1039/b000000x/
1
in PDF format.]. See
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reductive dehalogenation of FcPCl2. Unfortunately, we were unable
to obtain synthetic and/or characterization details for this reference
8
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