Syntheses, structures, and properties of the first stable 1,1′-bis(diphosphenyl)ferrocenes

Article abstract of DOI:10.1246/cl.2006.220

Kinetically stabilized 1,1′-bis(diphosphenyl)ferrocenes have been synthesized by taking advantage of extremely bulky substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt) and 2,6- bis[bis(trimethylsilyl)rnethyl]-4-[tris(trimethylsilyl)-methyl]ph

Full text of DOI:10.1246/cl.2006.220

220
Chemistry Letters Vol.35, No.2 (2006)
Syntheses, Structures, and Properties of the First Stable 1,10-Bis(diphosphenyl)ferrocenes
Noriyoshi Nagahora, Takahiro Sasamori, and Norihiro Tokitohꢀ
Institute for Chemical Research, Kyoto University, Gokasho, Uji, Kyoto 611-0011
(Received December 9, 2005; CL-051513; E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp)
Kinetically stabilized 1,10-bis(diphosphenyl)ferrocenes
have been synthesized by taking advantage of extremely bulky
substituents, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt)
and
2,6-bis[bis(trimethylsilyl)methyl]-4-[tris(trimethylsilyl)-
methyl]phenyl (Bbt) groups, and fully characterized by the
spectroscopic and X-ray crystallographic analyses. The electro-
chemical behavior showed well-defined two reversible one-
electron reduction couples.
Figure 1. ORTEP drawings of 1a (left) and 1b (right) with ther-
mal ellipsoid plots (50% probability). The CH(SiMe3)2 and
C(SiMe3)3 groups and hydrogen atoms ꢁare omitted for clarity.
ꢁ
Selected bond lengths [A] and angles [ ] of 1a/1b: P(1)–P(2)
Remarkable progress has been made in the chemistry of
double-bond compounds containing heavier group 15 elements,
especially in the field of diphosphenes1 and their heavier conge-
ners.2 It is well-known that diphosphenes, which have relatively
low lying ꢀꢀ orbitals, undergo one-electron reduction to give
their anion-radical species.3 We have also reported the synthesis
of novel doubly bonded systems between heavier group 15 ele-
ments by taking advantage of efficient steric protection groups,
Tbt and Bbt.4 Furthermore, the redox properties of BbtE=EBbt
(E ¼ P, Sb, and Bi) have been systematically elucidated based
on the measurement of cyclic voltammetry and DFT calcula-
tions.4i Yoshifuji et al. synthesized the first bis(diphosphene) de-
rivative, the voltammogram of which exhibited one reduction
wave and indicating no electronic interaction between the intra-
molecular diphosphene units.5 Recently, 1,4-bis(diphosphenyl)-
benzene derivatives, in which the P=P ꢀ electron systems are
extended to those of the conjugated aromatic rings, have been
synthesized by Protasiewicz and co-workers, and they have
disclosed the unique electrochemical behavior of the newly
obtained ꢀ-conjugated diphosphenes.6
2.015(3)/2.0398(18), P(1)–C(1) 1.860(4)/1.854(4), P(2)–C(28/
31) 1.797(9)/1.807(6), C(1)–P(1)–P(2) 98.5(2)/103.25(15),
C(28/31)–P(2)–P(1) 102.1(5)/101.9(3).
and Bbt groups.
Treatment of 1,10-bis(dichlorophosphino)ferrocene9 with
two molar amounts of LiP(H)Ar (Ar ¼ Tbt or Bbt) followed
by the double-dehydrochlorination reaction using 1,8-diazabicy-
clo[5.4.0]undec-7-ene (DBU) afforded 1,10-bis(diphosphenyl)-
ferrocenes 1a and 1b in 68 and 62% isolated yields, respectively
(Scheme 1).10
The 31P NMR spectra of 1a and 1b in C6D6 showed a diag-
nostic pair of doublets at 497.3 and 488.9 ppm (1a) and 496.9
1
and 491.1 ppm (1b), respectively, with JPP ¼ 550 Hz (both 1a
and 1b), which are characteristic of unsymmetric E-diphos-
phenes. The observed coupling constants of 1a and 1b were
somewhat smaller than those of the reported diphosphenes
having two different bulky aryl groups,1a indicating their charac-
teristic polarized resonance structures similar to the case of the
reported ferrocenyldiphosphenes.4h,8
On the other hand, organometallic architectures incorporat-
ing metal atoms in organic ꢀ-frameworks have given rise to a
great deal of interest from the viewpoint of the elaboration of
molecular materials.7 In 1996, Niecke et al. have reported the
first synthesis of the ferrocenyldiphosphene consisted of two re-
dox active sites.8 Recently, we have also reported the structural
characterization of a new ferrocenyldiphosphene bearing a Tbt
group and revealed its unique electrochemical behavior.4h As
an extension of the studies on novel d–ꢀ systems containing
heavier group 15 elements, we report here the syntheses, struc-
tural characterization, and properties of the first 1,10-bis(diphos-
phenyl)ferrocenes, 1a and 1b, kinetically stabilized by the Tbt
The molecular structures of 1a and 1b were determined by
the X-ray crystallographic analyses (Figure 1).11 The conforma-
tions of 1a and 1b in the solid states are curiously different to
each other in spite of the similarity between Tbt and Bbt groups.
ꢀ
That is, 1a (triclinic, P1) has a crystallographic centrosymmetry
on the Fe atom, though 1b (monoclinic, C2=c) shows a two-fold
rotation axis through the Fe atom. The torsion angle between the
centoroid(Cp)–P(2) moieties of 1b (ꢁ in the Figure 1) exhibits
48.0ꢁ, while that of 1a is inherently 180ꢁ. Both 1a and 1b
show the E-conformations with the C–P–P–C torsion angles of
179.3(4) and 176.0(2)ꢁ, respectively, and their C–P–P–C planes
are almost coplanar with the adjacent Cp ring, suggesting a con-
jugative interaction between the ꢀ-electrons of the diphosphene
units with those of the Cp rings of 1a and 1b. Their P=P bond
ꢁ
ꢁ
lengths [2.015(3) A for 1a and 2.0398(18) A for 1b] are consid-
erably shorter than the typical P–P single-bond lengths12 and are
within the range of those for the previously reported diaryldi-
ꢁ
phosphenes (1.985–2.049 A),
1a,4f
revealing the concrete P=P
double-bond character of 1a and 1b in the solid state. Interest-
ingly, the characteristic P=P vibrational frequencies of 1a/1b
were found to be active for both Raman (ꢂ ¼ 612=611 cmꢂ1
)
Scheme 1. (a) n-BuLi, Et2O, ꢂ40 ꢁC; (b) 1,10-bis(dichloro-
and IR (ꢂ ¼ 614=615 cmꢂ1) spectra in the solid state due to
phosphino)ferrocene, C6H6, rt; (c) DBU, rt.
Copyright Ó 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.2 (2006)
221
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phenyl).13
Soc., Dalton Trans. 1985, 383. d) J. Escudie, C. Couret, H.
The UV–vis spectra of 1a/1b in C6H6 showed three
absorption maxima at 384/389 (" 7300/7400), 480/485 (sh, "
1800/1300), and 539/553 nm (" 2200/2200).10 The first one
(ꢃmax ¼ 384=389 nm), which should be assigned to the ꢀ–ꢀꢀ
electron transitions due to the large ", was within the range of
those for the reported diaryldiphosphenes (277–418 nm),1a,4f
while it was found to show a hypsochromic shift as compared
with those for the 1,4-bis(diphosphenyl)benzenes (398 and
422 nm).6 The second one (ꢃmax ¼ 480=485 nm) was assignable
to the n–ꢀꢀ electron transitions for the diphosphene units in con-
sideration of the ꢃmax values of those for the diaryldiphosphenes
(437–532 nm)1a,4f and 1,4-bis(diphosphenyl)benzenes (476 and
481 nm).6 The third one (ꢃmax ¼ 539=553 nm) should be attrib-
uted to the MLCT band due to the electron transitions from d
orbitals of the iron atom to the ꢀꢀ orbital of the P=P moiety,
indicating a subtle bathochromic shift as compared with those
for the ferrocenyldiphosphenes ArP=PC5H4FeC5H5 (Ar ¼
Mesꢀ; 515 nm,8 Ar ¼ Tbt; 542 nm4h). In addition, assignment
of the observed absorption maxima for 1a/1b are reasonably
supported by theoretical calculations for the excited states of
the model molecule.13
3
a) B. Cꢀetinkaya, A. Hudson, M. F. Lappert, H. Goldwhite,
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The redox behavior of 1a and 1b has been furnished by
cyclic voltammetry.10 In the THF solution, two reversible
one-electron redox waves due to the intramolecular two redox
centers was observed at ꢂ1:84 and ꢂ2:19 V (1a) and ꢂ1:78
and ꢂ2:13 V (1b) vs Ag/Agþ, respectively. In both cases of
5
6
M. Yoshifuji, N. Shinohara, K. Toyota, Tetrahedron Lett. 1996,
37, 7815.
a) S. Shah, T. Concolio, A. L. Rheingold, J. D. Protasiewicz,
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S. Choua, T. Berclaz, M. Geoffroy, J. D. Protasiewicz, Inorg.
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1a and 1b, the difference between the half-potentials (ꢂE1=2
¼
0:35 V) means a comproportionation constant of Kc ¼ 8:2 ꢃ
105 for the mixed-valence state, indicating an effective electron-
ic interaction through the central ferrocene unit to some extent
similar to the case of the previously reported 1,4-bis(diphos-
phenyl)benzene [ꢂE1=2 ¼ 0:34 V and Kc ¼ 5:6 ꢃ 105 in THF–
(n-Bu)4NBF4].6
7
8
9
R. Pietschnig, E. Niecke, Organometallics 1996, 15, 891.
I. E. Nifant’ev, A. A. Boricenko, L. F. Manzhukova, E. E.
Nifant’ev, Phosphorus, Sulfur Silicon 1992, 68, 99.
10 Experimental procedures, the analytical, spectral, and voltammet-
ric data for 1a and 1b are in the Supporting Information.
In summary, we have succeeded in the syntheses of the
first stable 1,10-bis(diphosphenyl)ferrocenes 1a and 1b by taking
advantage of the effective steric protection groups, Tbt and Bbt.
The solid-state structures and the unique properties of 1a and 1b
indicate that these novel d–ꢀ systems containing P=P units may
be good candidates for molecular electronics and give helpful in-
formation to understand the electronic communications between
diphosphene units. Further investigation on the properties and
the reactivity of 1a and 1b are currently in progress.
11 Crystallographic data for 1a; C64H126FeP4Si12, MW ¼ 1412:46,
ꢀ
ꢁ
ꢁ
triclinic, P1 (# 2), a ¼ 10:834ð3Þ A, b ¼ 10:640ð3Þ A, c ¼
ꢁ
19:972ð6Þ A,
ꢄ ¼ 75:001ð10Þꢁ,
ꢅ ¼ 87:417ð12Þꢁ,
ꢆ ¼
71:851ð9Þ , V ¼ 2111:7ð11Þ A , Z ¼ 1, ꢇcalcd ¼ 1:111 g cmꢂ3
,
ꢁ
ꢁ 3
2ꢁmax ¼ 50:0ꢁ, 13923/7272 measured/independent reflections,
668 refined parameters, R1 ðwR2Þ ¼ 0:079 (0.163) [I > 2ꢈðIÞ],
R1 ðwR2Þ ¼ 0:131 (0.197) (for all data), T ¼ 103ð2Þ K, GOF ¼
1:096. Crystallographic data for 1b; C70H142FeP4Si14, MW ¼
ꢁ
1556:83, monoclinic, C2=c (# 15), a ¼ 22:9804ð5Þ A, b ¼
11:6477ð3Þ A,
c ¼ 34:7151ð8Þ A,
ꢅ ¼ 96:8190ð9Þꢁ,
V ¼
ꢁ
ꢁ
ꢁ 3
9226:4ð4Þ A , Z ¼ 4, ꢇcalcd ¼ 1:121 g cmꢂ3
,
2ꢁmax ¼ 50:0ꢁ,
This work was partially supported by Grant-in-Aid for
Scientific Research (Nos. 12CE2005, 14078213, 15655011,
16750033, and 17GS0207) and the 21st Century COE on
Kyoto University Alliance for Chemistry from the Ministry of
Education, Culture, Sports, Science and Technology, Japan.
We are grateful to Prof. Yukio Furukawa, Waseda University,
for the measurement of FT-Raman spectra.
36323/8118 measured/independent reflections, 510 refined pa-
rameters, R1 ðwR2Þ ¼ 0:077 (0.210) [I > 2ꢈðIÞ], R1 ðwR2Þ ¼
0:100 (0.228) (for all data), T ¼ 103ð2Þ K, GOF ¼ 1:049. Crystal-
lographic data reported in this manuscript have been deposited
with Cambridge Crystallographic Data Centre as supplementary
publication nos. CCDC-287252 (1a) and -287251 (1b). Copies
of the data can be obtained free of charge via www.ccdc.cam.
ac.uk/conts/retrieving.html (or from the Cambridge Crystallo-
graphic Data Centre, 12, Union Road, Cambridge, CB2 1EZ,
UK; fax: +44 1223 336033; or deposit@ccdc.cam.ac.uk).
12 M. Baudler, K. Glinka, Chem. Rev. 1993, 93, 1623, and references
cited therein.
References and Notes
1
For reviews see: a) L. Weber, Chem. Rev. 1992, 92, 1839. b) N.
Tokitoh, J. Organomet. Chem. 2000, 611, 217.
´
2
a) C. Couret, J. Escudie, Y. Madaule, H. Ranaivonjatovo, J.-G.
13 Details of the theoretical calculations of the model molecule,
(DmpP=PC5H4)2Fe, are shown in the Supporting Information.
Wolf, Tetrahedron Lett. 1983, 24, 2769. b) A. H. Cowley, J. G.
Lasch, N. C. Norman, M. Pakulski, J. Am. Chem. Soc. 1983,
Published on the web (Advance View) January 21, 2006; DOI 10.1246/cl.2006.220