Preparation and properties of 1,2-diferrocenyl-substituted 3,4-diphosphinidenecyclobutenes and their complexes: The first application of a diphosphinidenecyclobutene unit as a linker of functional moieties

Article abstract of DOI:10.1021/om0500772

Compounds with two ferrocenyl units connected with 3,4- diphosphinidenecyclobutene (DPCB) linkers at the 1,2-positions (1) and their transition metal chelate complexes (2) were prepared, and the spectroscopic and electrochemical properties were studied. The compounds were examined by cyclic voltammetry, and ΔE_1/2 values (the separation between successive Fe²⁺/Fe³⁺ oxidation potentials) were determined. Being fully conjugated with DPCB linkers, those compounds demonstrated electrochemical communication: the Cr(CO)₄ complex of 1 (2Cr) showed a strong interaction (ΔE_1/2 = 350 mV), whereas PdCl₂ and PtCl₂ complexes of 1 (2Pd and 2Pt) displayed a decreased interaction compared to that of 1.

Full text of DOI:10.1021/om0500772

Organometallics 2005, 24, 2983-2987
2983
Preparation and Properties of
1,2-Diferrocenyl-Substituted
3,4-Diphosphinidenecyclobutenes and Their Complexes:
The First Application of a Diphosphinidenecyclobutene
Unit as a Linker of Functional Moieties
Subaru Kawasaki, Akitake Nakamura, Kozo Toyota, and Masaaki Yoshifuji*
Department of Chemistry, Graduate School of Science, Tohoku University, Aoba, Sendai
980-8578, Japan
Received February 2, 2005
Compounds with two ferrocenyl units connected with 3,4-diphosphinidenecyclobutene
(DPCB) linkers at the 1,2-positions (1) and their transition metal chelate complexes (2) were
prepared, and the spectroscopic and electrochemical properties were studied. The compounds
were examined by cyclic voltammetry, and ∆E1/2 values (the separation between successive
Fe2+/Fe3+ oxidation potentials) were determined. Being fully conjugated with DPCB linkers,
those compounds demonstrated electrochemical communication: the Cr(CO)4 complex of 1
(2Cr) showed a strong interaction (∆E1/2 ) 350 mV), whereas PdCl2 and PtCl2 complexes of
1 (2Pd and 2Pt) displayed a decreased interaction compared to that of 1.
Chart 1. Compounds 1 and 2
Introduction
3,4-Diphosphinidenecyclobutenes (DPCB)1 possess
unique structures containing sp2 phosphorus atoms and
work as bidentate ligands to transition metals. We have
studied the structures and the coordination nature of
DPCB derivatives.1c In addition, we have applied DPCB
ligands to Pd-, Pt-, and Cu-catalyzed reactions.2 Al-
though the DPCB molecules are bidentate ligands with
conjugated flat structures, there has been no report on
the application utilizing the characteristic nature of
their π-systems. Taking the unique character of the
DPCB molecule into account, a molecular device with
the DPCB moiety is expected to possess interesting
physical properties of DPCB derivatives.
Among the possible approaches to molecular devices,
studies on the construction of multiredox systems and
the understanding of electronic communication are
interesting subjects for developing new electronic ma-
terials and devices.3 One approach to construction of
multiredox systems is to utilize molecules that contain
redox center(s) and conjugated linkers.4 For example,
ferrocenes connected via alkene or benzene linkers are
promising. Since the preparation of the Creutz-Taube
ion,5 studies of molecules where two or more metal
centers are linked by a bridging ligand with pronounced
electronic interactions have been of interest.
* To whom correspondence should be addressed. E-mail: yoshifj@
mail.tains.tohoku.ac.jp.
Here we report the syntheses and properties of a 1,2-
diferrocenyl-substituted DPCB derivative (1) and its
transition metal complexes 2 (Chart 1) as the first
attempted application of DPCB as a linker of functional
moieties. The DPCB 1 exhibited two reversible oxidation
waves in cyclic voltammetry, indicating electronic in-
teraction between the two iron centers. The transition
metal complexes 2 showed a shift of the oxidation waves,
(1) (a) Yoshifuji, M.; Toyota, K.; Murayama, M.; Yoshimura, H.;
Okamoto, A.; Hirotsu, K.; Nagase, S. Chem. Lett. 1990, 2195. (b)
Yamada, N.; Abe, K.; Toyota, K.; Yoshifuji, M. Org. Lett. 2002, 4, 569.
(c) Yoshifuji, M. J. Synth. Org. Chem. Jpn. 2003, 61, 1116. See also:
(d) Appel, R.; Winkhaus, V.; Knoch, F. Chem. Ber. 1987, 120, 243. (e)
Ma¨rkl, G.; Hennig, R. Liebigs Ann. 1996, 2059.
(2) (a) Ozawa, F.; Kawagishi, S.; Ishiyama, T.; Yoshifuji, M. Orga-
nometallics 2004, 23, 1325. (b) Ozawa, F.; Ishiyama, T.; Yamamoto,
S.; Kawagishi, S.; Murakami, H.; Yoshifuji, M. Organometallics 2004,
23, 1698. (c) Minami, T.; Okamoto, H.; Ikeda, S.; Tanaka, R.; Ozawa,
F.; Yoshifuji, M. Angew. Chem., Int. Ed. 2001, 40, 4501. (d) Ozawa,
F.; Okamoto, H.; Kawagishi, S.; Yamamoto, S.; Minami, T.; Yoshifuji,
M. J. Am. Chem. Soc. 2002, 124, 10968. (e) Gajare, A. S.; Toyota, K.;
Yoshifuji, M.; Ozawa, F. J. Org. Chem. 2004, 69, 6504. (f) Gajare, A.
S.; Toyota, K.; Yoshifuji, M.; Ozawa, F. Chem. Commun. 2004, 1994.
(g) Gajare, A. S.; Jensen, R. S.; Toyota, K.; Yoshifuji, M.; Ozawa, F.
Synlett 2005, 144. (h) Toyota, K.; Masaki, K.; Abe, T.; Yoshifuji, M.
Chem. Lett. 1995, 221. (i) Ozawa, F.; Yoshifuji, M. C. R. Chim. 2004,
7, 747.
(3) (a) Paul, F.; Lainte, C. Coord. Chem. Rev. 1998, 178-180, 431.
(b) Wosnick, J. H.; Swager, T. M. Curr. Opin. Chem. Biol. 2000, 4,
715. (c) Tour, J. M.; Rawlett, A. M.; Kozaki, M.; Yao, Y.; Jagesssar, R.
C.; Dirk, S. M.; Price, D. W.; Reed, D. W.; Reed, M. A.; Zhou, C.-W.;
Chen, J.; Wang, W.; Campbell, I. Chem. Eur. J. 2001, 7, 5118. (d)
Cahen, D.; Hodes, G. Adv. Mater. 2002, 14, 789. (e) Holliday, B. J.;
Swager, T. M. Chem. Commun. 2005, 23.
10.1021/om0500772 CCC: $30.25 © 2005 American Chemical Society
Publication on Web 05/05/2005

2984 Organometallics, Vol. 24, No. 12, 2005
Kawasaki et al.
Table 1. 31P NMR (162 MHz) Data for 1 and 2
Scheme 1. Synthesis of 1a
1
2Cr
2Mo
2W
2Pd
2Pt
a
δP
154.2
170.0
153.4
132.9b
134.9
111.4c
1
a Measured in CDCl3. b Satellite, JPW ) 256.6 Hz. c Satellite,
1JPPt ) 4486.0 Hz.
Although the reaction of 1 with PtCl2(PhCN)2 was slow
in THF at room temperature, heating of the mixture at
50 °C for 5 h led to the formation of platinum complex
2Pt (52% yield). Complexes of group 6 transition metals,
2Cr, 2Mo, and 2W, were also prepared, using (norbor-
nadiene)M(CO)4 (M ) Cr, Mo, W).
2. Spectroscopic Properties. The 31P NMR data for
1 and 2 are listed in Table 1. Platinum complex 2Pt
appeared at δP 111.4 with characteristic satellite sig-
nals, and tungsten complex 2W appeared at δP 132.9
with satellite signals. It should be noted that the heavier
metal causes a shift to a higher field in the case of group
6 and 10 metal complexes of other DPCB derivatives.9
The 1H NMR spectra of 1 and 2 showed some
interesting features (see Supporting Information, Fig-
ures S1-S6, S7a-d, and S8a-g). In the case of 1, the
o-tert-butyl protons (δ 1.60-1.65) and the protons in the
Cp ring (δ 4.0-4.3) adjacent to the DPCB appeared as
broad signals in CDCl3 at room temperature (Figure S1).
In contrast, the corresponding signals of complexes 2
appeared as sharp signals, especially in the cases of 2Pd
(Figure S5) and 2Pt (Figure S6), in CDCl3 at room
temperature. In the cases of the 1H NMR spectra of 2Cr,
2Mo, and 2W (Figures S2-S4), relatively sharp signals,
compared with those of 1, were observed.
a Reagent and conditions: (a) n-BuLi (2 equiv), THF, -78
°C. (b) Mes*P(H)Cl, THF, rt. (c) n-BuLi (1 equiv), THF, -78
°C, 10 min; then 1,2-dibromoethane (0.5 equiv), -78 °C, 10
min; rt, 1 h. (d) PhMe, reflux, 30 min. Fc ) ferrocenyl.
Scheme 2. Synthesis of Transition Metal
Complexes of 1a
To clarify the reasons for the broadening of the
signals, variable-temperature 1H NMR studies of 1 and
2Pd were carried out (Figures S7a-d and S8a-g), and
we found that conformational change of the bulky
ferrocenyl substituents and the cyclobutene ring in the
NMR time scale caused the broadening of the signals
at room temperature.10 These signals of 1 become sharp
as the temperature increases in toluene-d8 (Figure S7a-
d). At room temperature in toluene-d8, signals due to
protons of the Cp ring adjacent to the DPCB are unclear,
while the o-tert-butyl protons appear as a broad signal
(Figure S7a). At 80 °C, the o-tert-butyl protons appear
as a sharp singlet (δ 1.69), and the Cp ring protons
a MLnL′m ) (nor-C7H8)M(CO)4 (for 2Cr, 2Mo, and 2W);
MLnL′m ) PdCl2(MeCN)2 (for 2Pd); MLnL′m ) PtCl2(PhCN)2
(for 2Pt).
indicating that electronic interactions between the two
iron centers might be controlled. The control of redox
activities using transition metals will attract interest
in inorganic and organic materials chemistry.6
(4) (a) Barlow, S.; O’Hare, D. Chem. Rev. 1997, 97, 637. (b) Ward,
M. D. Chem. Soc. Rev. 1995, 121. (c) Demadis, K. D.; Hartshorn, C.
M.; Meyer, T. J. Chem. Rev. 2001, 101, 2655. (d) Launay, J.-P. Chem.
Soc. Rev. 2001, 30, 386. (e) Xu, G.-L.; Zou, G.; Ni, Y.-H.; DeRosa, M.
C.; Crutchley, R. J.; Ren, T. J. Am. Chem. Soc. 2003, 125, 10057. (f)
Cotton, F. A.; Donahue, J. P.; Murillo, C. A. J. Am. Chem. Soc. 2003,
125, 5436. (g) Yip, J. H. K.; Wu, J.; Wong, K.-Y.; Yeung, K.-W.; Vittal,
J. J. Organometallics 2002, 21, 1612. (h) Xu, G.-L.; Jablonski, C. G.;
Ren, T. J. Organomet. Chem. 2003, 683, 388. (i) Xu, G.-L.; DeRosa, M.
C.; Crutchley, R. J.; Ren, T. J. Am. Chem. Soc. 2004, 126, 3728.
(5) For example: (a) Creuz, C. Prog. Inorg. Chem. 1983, 30, 1, and
references therein. (b) Brunschwig, B. S.; Creutz, C.; Sutin, N. Chem.
Soc. Rev. 2002, 31, 168. (c) Richardson, D. E.; Taube, H. Inorg. Chem.
1981, 20, 1278.
Results and Discussion
1. Preparations of Diferrocenyldiphosphinidene-
cyclobutene and Its Complexes. Scheme 1 shows the
synthesis of 1: Ferrocenylethynyllithium was generated
in situ from dibromoalkene 3 by Corey-Fuchs reaction7
using 2 equiv of n-BuLi and was allowed to react with
Mes*P(H)Cl8 (Mes* ) 2,4,6-tert-Bu3C6H2) to give the
corresponding (ferrocenylethynyl)phosphine 4. Depro-
tonation of 4 with n-BuLi in THF at -78 °C followed
by treatment with 1,2-dibromoethane and refluxing in
toluene gave the desired diphosphinidenecyclobutene 1,
via intermediates 5 and 6, in 64% yield based on 4.
Transition metal complexes of 1 were then prepared
as shown in Scheme 2. When 1 was allowed to react
with PdCl2(MeCN)2 in THF at room temperature, the
reaction proceeded to give 2Pd almost quantitatively.
(6) (a) Beer, P. D.; Gale, P. A.; Chen, G. Z. Coord. Chem. Rev. 1999,
186, 3. (b) Sprigings, T. G.; Hall, C. D. Organometallics 2001, 20, 2560.
(7) Corey, E. J.; Fuchs, L. Tetrahedron Lett. 1972, 3769. For a
preparative application to ethynylferrocene, see: Luo, S.-J.; Liu, Y.-
H.; Liu, C.-M.; Liang, Y.-M.; Ma, Y.-X. Synth. Commun. 2000, 30, 1569.
(8) Cowley, A. H.; Kilduff, J. E.; Norman, N. C.; Pakulski, M.;
Atwood, J. L.; Hunter, W. E. J. Am. Chem. Soc. 1983, 105, 4845.
(9) (a) Toyota, K.; Tashiro, K.; Yoshifuji, M. Chem. Lett. 1991, 2079.
(b) Nakamura, A.; Kawasaki, S.; Toyota, K.; Yoshifuji, M. Chem. Lett.
2004, 33, 1570.
(10) Attempted crystallographic analysis of compounds 1 and 2
failed, because of lack of suitable single crystals for X-ray analysis.

Diphosphinidenecyclobutene Linkers
Organometallics, Vol. 24, No. 12, 2005 2985
Table 2. CV Data for 1 and 2a
Chart 2. Schematic Conformation of 1
E1/2(2) or E1/2(3) or
compound E1/2(1)/V
Ep/Vb
Ep/Vb
∆E1/2/Vc
Kc
1
0.23
0.21
0.24
0.20
0.45
0.41
0.48
0.56
0.25
0.35
2.0 × 104
2Cr
2Mo
2W
2Pd
2Pt
(0.96)
(0.97)
(0.89)
1.0 × 106
(0.63)
(0.66)
0.64
0.19
0.21
1.8 × 103
4.1 × 103
0.62
(1.37)
a Cyclic voltammograms were recorded in CH2Cl2 containing 0.1
M n-Bu4NClO4. b Data in parentheses are irreversible waves (Ep).
c ∆E1/2 ) E1/2(2) - E1/2(1).
Chart 3. Schematic Chelation Effect on DPCB
Complexes
Chart 4. Compounds 7-10
adjacent to the DPCB become detectable (δ 4.0-4.3)
(Figure S7d).
complex formation (573 nm for 2Pd, 556 nm for 2Pt,
532 nm for 2Mo, 543 nm for 2W). This tendency is also
valid in the case of 2Cr: Although the corresponding
absorption band of 2Cr appeared as a shoulder (λmax
was unclear), comparison of the spectrum of 2Cr with
that of 1 indicated the red-shift for 2Cr.
In the case of 2Pd, these signals become broad with
decreasing temperature in CD2Cl2 (Figure S8a-g). At
-40 °C, both the o-tert-butyl protons and the m-Mes*
protons appear as broad split signals (Figure S8d). At
-80 °C, these signals appear as sharp inequivalent
signals [Figure S8f, δ 1.53 and 1.77 (o-tert-Bu), δ 7.56
and 7.72 (m-Mes*)]. At this temperature, the Cp ring
protons adjacent to the DPCB appear as four inequiva-
lent signals at δ 2.26, 4.15, 4.56, and 5.17.
3. Electrochemical Properties. The cyclic voltam-
mogram of 1 in CH2Cl2 (1 mM, n-Bu4NClO4, 30 mV/s)
exhibited two reversible oxidation waves at 0.23 and
0.48 V (Table 2). The ∆E1/2 values (250 mV) reflect a
considerable electronic interaction between the iron
centers through the DPCB linker. This interaction is
larger than that of (E)- or (Z)-1,2-bis(1′-ethyl-1-ferroce-
nyl)-1,2-dimethylethylene, 7 (Chart 4, ∆E1/2 ) 150 mV,
in MeCN with 0.1 M n-Bu4NBF4).12,13 Chelation of
metals may perturb the interaction between the two
ferrocenyl groups. We thus investigated electrochemical
properties of 2. The cyclic voltammogram of 2Cr in CH2-
Cl2 (1 mM, n-Bu4NClO4, 30 mV/s) exhibited two revers-
ible oxidation waves (0.21 and 0.56 V) and one irrevers-
ible oxidation wave (0.96 V). The reversible waves
presumably correspond to oxidation at the two ferroce-
nyl centers, by comparison with the results of 1, while
the irreversible wave is attributable to the Cr0/1+ oxida-
tion,14 although some factors such as medium effects
should be taken into account.4i,15 The results indicate
that chelation of Cr affects the second oxidation poten-
These changes in the 1H NMR spectra are explicable
as follows. In the case of 1, one bulky ferrocenyl group
is located above the cyclobutene ring and the other is
located below the cyclobutene ring due to steric conges-
tion between the ferrocenyl groups and o-tert-butyl
groups of the Mes* groups (Chart 2), similarly to the
case of the 1,2-diphenyldiphosphinidenecyclobutene
derivative1d in the crystal, and conformational exchange
occurs slowly in solution at room temperature on the
NMR time scale. When the temperature increases, the
positional exchange occurs faster and this makes the
signals sharp, as shown in Figure S7b-d.
In the case of the complex 2Pd, the steric congestion
between the ferrocenyl substituents and the o-tert-butyl
groups is released by complex formation, because che-
lation makes the C(3)dP(1)-C(ipso) and C(4)dP(2)-
C(ipso) angles larger (Chart 3) and the Mes* group
moves away from the ferrocenyl group.11 Consequently,
those signals become sharp. It is worth noting that these
dynamics changes caused by the metal chelation on the
NMR time scale were first observed in our DPCB
systems.
(11) This type of changes in angles were observed in our previous
crystallographic studies of some DPCB ligands and their complexes.
For example, in the case of seven-membered-ring-fused DPCB deriva-
tive, the CdP-C angles change from 99.6(2)° and 100.4(2)° (av 100.0°)
to 114.1(4)° and 118.2(4)° (av 116.2°) (ref 1b).
(12) Dong, T.-Y.; Ke, T.-J.; Peng, S.-M.; Yeh, S.-K. Inorg. Chem.
1989, 28, 2103.
(13) Strictly speaking, comparison of the ∆E1/2 values should be
carefully made taking the difference of CV conditions, such as solvent
and electrolyte, into account.
In addition, a charge delocalization to the planar
DPCB moiety was indicated by UV-vis spectra: absor-
bance of 1 at 523 nm shifted to a longer wavelength by

2986 Organometallics, Vol. 24, No. 12, 2005
Kawasaki et al.
tial E1/2(2) rather than the first oxidation potential
E1/2(1) (probably, 2Cr2+ is less stable than 12+, while
stabilities of 2Cr+ and 1+ are similar). Consequently,
the electronic interaction of 2Cr becomes very large
[∆E1/2 ) 350 mV; ∆E1/2 ) E1/2(2) - E1/2(1)] and compa-
rable to those of biferrocene 8 (∆E1/2 ) 350 mV, in CH2-
Cl2 with 0.2 M n-Bu4NBF4) and bisacetylene-bridged
ferrocenophane 9 (∆E1/2 ) 355 mV, in CH2Cl2 with 0.2
M n-Bu4NBF4).13,16
This tendency is also observed in the cases of 2Mo
and 2W, although the second oxidation potential ap-
peared to be irreversible. The cyclic voltammogram of
2Mo exhibited one reversible oxidation wave (0.24 V)
and two irreversible waves (0.63 and 0.97 V). Similarly,
CV for 2W exhibited one reversible oxidation wave (0.20
V) and two irreversible waves (0.66 and 0.89 V). Again,
the group 6 neutral M(0) metal in the M(CO)4 moiety
affects the second oxidation potential rather than the
first oxidation potential.
On the other hand, chelation of group 10 M(II) metal
in the MCl2 moiety affected both E1/2(1) and E1/2(2)
oxidation potentials: the cyclic voltammogram of 2Pd
and 2Pt exhibited two reversible oxidation waves (0.45
and 0.64 V for 2Pd; 0.41 and 0.62 V for 2Pt) and one
irreversible reduction wave attributable to the Pd or Pt
center (-1.06 and -1.47 V, respectively). As the DPCB-
group 10 metal(II) complexes are likely to be more
electron deficient than the DPCB-group 6 metal(0)
complexes, the first oxidation of the ferrocenyl units of
2Pd and 2Pt will be more difficult than that of the
DPCB-group 6 metal carbonyl complexes.
The effect of the metal center on the second oxidation
wave in both the group 6 metal complexes and the group
10 metal complexes is explicable as follows: a positive
charge generated by oxidation of the ferrocenyl unit
delocalizes to the cyclobutene ring and the metalla-
cycles. This delocalization onto the metallacycles makes
the second oxidation reluctant.
the chromium(0) metal and the system is stabilized by
the contribution of Cr(I). On the other hand, in the case
of palladium and platinum complex cations 2Pd+ and
2Pt+, the delocalization of positive charge extends onto
the formal M2+ metal, which has already been oxidized
due to Lewis acidity. This delocalization leads to desta-
bilization of the oxidized species. These phenomena are
consistent with UV-vis spectral data mentioned above,
which suggest charge delocalization onto the planar
DPCB moiety.
Conclusion
In conclusion, several noteworthy features have been
found in this study: (1) the DPCB skeleton was applied
as a linker of metal centers for the first time, and strong
interactions between the iron centers via electronic
delocalization to the cyclobutene ring was indicated. (2)
Chelation of a transition metal affected the interactions
between the iron centers in the oxidation process.
Further studies of ferrocenyl and other metallocenyl
group substituted DPCB derivatives and the mixed
valence state of 1+ are underway in our laboratory.
Experimental Section
General Considerations. Melting points were measured
on a Yanagimoto MP-J3 micro melting point apparatus and
are uncorrected. NMR spectra were recorded on a Bruker
AVANCE-400 or a Bruker AM-600 spectrometer. IR spectra
were obtained on a Horiba FT-300 spectrometer. MS data were
taken on either a JEOL HX-110 or a Hitachi M-2500S
spectrometer. Cyclic voltammograms were recorded on a BAS-
CV-50W voltammetric analyzer under nitrogen. Elemental
analyses were performed at Instrumental Analysis Center for
Chemistry, Graduate School of Science, Tohoku University.
Reactions were performed under an argon atmosphere, unless
otherwise specified. Starting materials except for Mes*PH2
were obtained from commercial suppliers and used without
purification.
The ∆E1/2 values found for the redox processes reflect
a large degree of electronic interaction between the iron
centers through the DPCB moiety. The equilibrium
constant, Kc,5b for the comproportionation reaction
shown in eq 1 is given by eq 2, where FcDPCB is
Preparation of 1,2-Diferrocenyl-3,4-diphosphinidenecy-
clobutene (1). A mixture of (2,4,6-tri-tert-butylphenyl)phos-
phine (486.6 mg, 1.748 mmol) and 2,2′-azobisisobutyronitrile
(AIBN) (17.2 mg, 0.105 mmol) in carbon tetrachloride (5.3 mL)
was refluxed for 4 h and concentrated. THF (4.0 mL) was
added to the residual material, chloro(2,4,6-tri-tert-butylphe-
nyl)phosphine. To a solution of 3 (629.0 mg, 1.748 mmol) in
THF (4.0 mL) was added 3.50 mol of n-butyllithium (1.59 M
solution in hexane) at -78 °C. The resulting solution was
stirred for 1 h and allowed to warm to room temperature. After
the solution was stirred for 1 h, it was added to a THF solution
of the chlorophosphine at room temperature. The mixture was
stirred for 1 h, concentrated, and purified by silica gel column
chromatography to give 505.5 mg (1.039 mmol, 60% yield) of
4. To a solution of 4 (71.3 mg, 0.147 mmol) in THF (0.50 mL)
was added 0.150 mmol of n-butyllithium (1.58 M solution in
diferrocenyldiphosphinidenecyclobutene 1. In many cases,
5c
∆E° can be estimated from ∆E1/2
.
[FcDPCB] + [FcDPCB2+] h 2[FcDPCB+] (1)
Kc ) [FcDPCB+]2/[FcDPCB][FcDPCB2+] )
exp[∆E°(F/RT)] (2)
where F is the Faraday constant, R is the gas constant,
and T is the temperature, in K.
The calculated Kc values were as follows, assuming
∆E° = ∆E1/2: 1+ (2.0 × 104); 2Cr+ (1.0 × 106); 2Pd+
(1.8 × 103); 2Pt+ (4.1 × 103). As the Kc values exhibit
the stabilities of the oxidized species, the chromium
complex stabilizes the oxidized species, while palladium
and platinum complexes destabilize the oxidized species
(i.e., 2Pd+ is less stable than 2Cr+, for example).
This tendency is explainable also by effects of the
metal center: one-electron oxidation of the ferrocenyl
group generates a cation, in which positive charge
delocalizes into the cyclobutene ring. In the case of
2Cr+, delocalization of the positive charge extends onto
(14) Although the CV of the Cr(CO)4 complex of 3,4-bis(2,4,6-tri-
tert-butylphenyl)phosphinidene-1,2-bis(trimethylsilyl)cyclobutene9a ex-
hibited only one reversible oxidation wave at 0.53 V under similar
conditions, the second oxidation potential of 2Cr (0.56 V) seems to
correspond to the ferrocenyl oxidation, because the third potential (0.97
V, irreversible) is too large as a ferrocenyl oxidation wave.
(15) (a) Barrie`re, F.; Camire, N.; Geiger, W. E.; Mueller-Westerhoff,
U. T.; Sanders, R. J. Am. Chem. Soc. 2002, 124, 7262. (b) Neyhart, G.
A.; Hupp, J. T.; Curtis, J. C.; Timpson, C. J.; Meyer, T. J. J. Am. Chem.
Soc. 1996, 118, 3724. (c) Gło¨ckle, M.; Kaim, W. Angew. Chem., Int.
Ed. 1999, 38, 3072. (d) Hill, M. G.; Lamanna, W. M.; Mann, K. R. Inorg.
Chem. 1991, 30, 4687. (e) LeSuer, R. J.; Geiger, W. E. Angew. Chem.,
Int. Ed. 2000, 39, 248.
(16) Levanda, C.; Bechgaard, K.; Cowan, D. O. J. Org. Chem. 1976,
41, 2700.

Diphosphinidenecyclobutene Linkers
Organometallics, Vol. 24, No. 12, 2005 2987
hexane) at -78 °C. The reaction mixture was stirred for 10
min, treated with 1,2-dibromoethane (0.0696 mmol) at -78
°C, and stirred for 10 min. The resulting mixture was then
allowed to warm to room temperature and stirred for 1 h. The
volatile materials were removed under reduced pressure.
Toluene (0.5 mL) was then added to the residue. The mixture
was refluxed for 30 min, concentrated, and purified by silica
gel column chromatography to give 45.8 mg (0.0472 mmol, 64%
yield) of 1. Purple solid (melting point was unclear because it
gradually decomposed above 100 °C): 1H NMR (400 MHz,
CDCl3) δ 1.45 (18H, s, Me), 1.63 (36H, br, Me), 4.00 (10H, s,
ferrocene), 4.09 (4H, br, ferrocene), 4.24 (4H, br, ferrocene),
7.48 (4H, br, m-arom.); 13C{1H } NMR (100 MHz, CDCl3) δ
32.0 (s, Me), 33.6 (br, Me), 35.5 (s, CMe3), 38.7 (s, CMe3), 69.9
(s, ferrocene), 122.3 (br, m-Mes*), 137.0 (pseudo t, ipso-Mes*,
JPC ) 27.4 Hz), 150.5 (s, p-Mes*), 154.7 (pseudo t, PdC-C,
JPC ) 4.3 Hz), 155.6 (br, o-Mes*), 177.3 (dd, PdC, JPC ) 18.1,
10.2 Hz); 31P{1H} NMR (162 MHz, CDCl3) δ 154.2; IR (KBr)
ν/cm-1 2959, 2904, 2879, 1473, 1257, 1101, 1024, 814; UV-
vis (hexane) 344 (log ꢀ 4.36), 523 nm (3.52); HRMS (70 eV, EI)
found m/z 970.4131, calcd for C60H76Fe2P2 970.4116. Anal.
Calcd for C61H78Fe2P2Cl2 (1 + CH2Cl2): C, 69.39; H, 7.45.
Found: C, 69.20; H, 7.49.
Preparation of 1,2-Diferrocenyl-3,4-diphosphinidenecy-
clobutene-Transition Metal Complexes (2). 2Cr. A mix-
ture of 1 (61.5 mg, 0.0634 mmol) and (norbornadiene)Cr(CO)4
(33.0 mg, 0.129 mmol) was dissolved in THF (2.0 mL) and was
stirred at room temperature until disappearance of the start-
ing DPCB (by TLC) was observed. The residue was purified
by column chromatography (elution with hexane/ethyl acetate
mixtures) to give 2Cr almost quantitatively. Black solid
(melting point was unclear because it gradually decomposed
above 100 °C): 1H NMR (600 MHz, CDCl3) δ 1.47 (18H, s, Me),
1.71 (36H, s, Me), 4.00 (10H, s, ferrocene), 4.16 (4H, s,
ferrocene), 7.60 (4H, d, m-arom.); 13C{1H } NMR (150 MHz,
CDCl3) δ 31.5 (s, Me), 33.9 (s, Me), 35.3 (s, CMe3), 38.8 (s,
CMe3), 68.9 (br, ferrocene), 69.6 (s, ferrocene), 70.2 (s, fer-
rocene), 74.3 (s, ferrocene), 122.4 (s, m-Mes*), 130.7 (d, ipso-
Mes*, JPC ) 6.0 Hz), 152.3 (s, p-Mes*), 153.0 (dd, PdC-C, JPC
) 54.3, 34.7 Hz), 157.8 (s, o-Mes*), 177.6 (dd, PdC, JPC ) 30.2,
22.6 Hz), 219.9 (t, CO, JPC ) 18.1 Hz), 228.2 (d, CO, JPC ) 6.0
Hz); 31P{1H } NMR (162 MHz, CDCl3) δ 170.0 (br); IR (KBr) ν
/cm-1 2962, 2910, 2871, 2011, 1919, 1886, 1587, 1473, 1398,
1252, 819, 671, 633, 482; UV-vis (CH2Cl2) 329 (log ꢀ 4.46),
456 nm (4.23); HRMS (70 eV, EI) found m/z 1022.3533, calcd
for C60H76CrFe2P2 [M+ - 4CO] 1022.3520.
2Mo. A mixture of 1 (11.0 mg, 0.0113 mmol) and (norbor-
nadiene)Mo(CO)4 (5.1 mg, 0.017 mmol) was dissolved in THF
(0.5 mL) and was stirred at room temperature until disap-
pearance of the starting DPCB was observed by TLC. The
residue was purified by column chromatography (elution with
hexane/ethyl acetate mixtures) to give 2Mo almost quantita-
tively. Black solid (melting point was unclear because it
gradually decomposed above 100 °C): 1H NMR (600 MHz,
CDCl3) δ 1.47 (18H, s, Me), 1.72 (36H, s, Me), 4.01 (10H, s,
ferrocene), 4.17 (6H, s, ferrocene), 7.60 (4H, d, m-arom.);
13C{1H} NMR (150 MHz, CDCl3) δ 31.5 (s, Me), 34.0 (s, Me),
35.3 (s, CMe3), 38.9 (s, CMe3), 69.1 (s, ferrocene), 69.6 (s,
ferrocene), 70.3 (s, ferrocene), 74.2 (s, ferrocene), 122.5 (s,
m-Mes*), 130.4 (s, ipso-Mes*), 152.3 (s, p-Mes*), 153.3 (dd, Pd
C-C, JPC ) 54.3, 33.2 Hz), 157.5 (s, o-Mes*), 177.8 (dd, PdC,
JPC ) 27.2, 22.4 Hz), 209.3 (t, CO, JPC ) 12.1 Hz), 217.1 (dd,
CO, JPC ) 37.7, 7.5 Hz); 31P{1H} NMR (162 MHz, CDCl3) δ
153.4; IR (KBr) ν /cm-1 2962, 2021, 1921, 1886, 1261, 1097,
1026, 806; UV-vis (CH2Cl2) 328 (log ꢀ 4.31), 430 (4.19), 532
nm (3.80); HRMS (70 eV, EI) found m/z 1068.3225, calcd for
C60H76Fe2MoP2 [M+ - 4CO] 1068.3204.
mixtures) to give 2W (82.3 mg, 0.650 mmol, 72% yield). Black
solid (melting point was unclear because it gradually decom-
posed above 100 °C): 1H NMR (600 MHz, CDCl3) δ 1.48 (18H,
s, Me), 1.72 (36H, s, Me), 4.01 (10H, s, ferrocene), 4.18 (4H, s,
ferrocene), 7.61 (4H, d, m-arom.); 13C{1H} NMR (150 MHz,
CDCl3) δ 31.5 (s, Me), 34.1 (s, Me), 35.3 (s, CMe3), 39.0 (s,
CMe3), 69.0 (br, ferrocene), 69.6 (s, ferrocene), 70.4 (s, fer-
rocene), 74.0 (s, ferrocene), 122.6 (s, m-Mes*), 129.4 (s, ipso-
Mes*), 152.1 (dd, PdC-C, JPC ) 57.3, 34.7 Hz), 152.5 (s,
p-Mes*), 157.8 (s, o-Mes*), 180.1 (dd, PdC, JPC ) 36.2, 22.6
Hz), 203.5 (t, CO, JPC ) 10.6 Hz), 207.4 (m, CO); 31P{1H} NMR
(162 MHz, CDCl3) δ 132.9 (satellite, 1JPW ) 256.6 Hz); IR (KBr)
ν/cm-1 2960, 2870, 2017, 1913, 1886, 1585, 1471, 1398, 1363,
1261, 1105, 1007, 881, 818, 629, 596, 499, 455; UV-vis (CH2-
Cl2) 328 (log ꢀ 4.46), 431 (4.37), 543 nm (3.99); HRMS (70 eV,
EI) found m/z 1154.3627, calcd for C60H76Fe2P2W [M+ - 4CO]
1154.3649.
2Pd. A mixture of 1 (10.7 mg, 0.0110 mmol) and Pd-
(MeCN)2Cl2 (3.0 mg, 0.0116 mmol) was dissolved in THF (0.5
mL) and was stirred at room temperature until TLC-observed
disappearance of the starting DPCB. The residue was purified
by column chromatography (elution with hexane/ethyl acetate
mixtures) to give 2Pd almost quantitatively. Black solid
(melting point was unclear because it gradually decomposed
above 100 °C): 1H NMR (600 MHz, CDCl3) δ 1.41 (18H, s, Me),
1.74 (36H, s, Me), 3.73 (4H, s, ferrocene), 4.06 (10H, s,
ferrocene), 4.33 (4H, s, ferrocene), 7.62 (4H, s, m-arom.);
13C{1H} NMR (150 MHz, CDCl3) δ 31.3 (s, Me), 34.8 (br, Me),
35.5 (s, CMe3), 39.5 (s, CMe3), 68.0 (s, ferrocene), 69.4 (s,
ferrocene), 70.2 (s, ferrocene), 71.9 (s, ferrocene), 72.7 (s,
ferrocene), 122.3 (t, ipso-Mes*, JPC ) 7.5 Hz), 123.8 (t, m-Mes*,
JPC ) 4.5 Hz), 152.3 (m, PdC-C), 155.0 (s, p-Mes*), 157.4 (s,
o-Mes*), 166.3 (dd, PdC, JPC ) 49.8, 39.2 Hz); 31P{1H} NMR
(162 MHz, CDCl3) δ 134.9; IR (KBr) ν/cm-1 2954, 2923, 2906,
1591, 1475, 1267, 821, 644, 459; UV-vis (CH2Cl2) 315 (log ꢀ
4.46), 370 (4.58), 573 nm (3.84); HRMS (70 eV, EI) found m/z
1076.3190, calcd for C60H76Fe2P2Pd [M+ - 2Cl] 1076.3183.
2Pt. A mixture of 1 (11.0 mg, 0.0133 mmol) and Pt-
(PhCN)2Cl2 (10.0 mg, 0.0212 mmol) was dissolved in THF (1.0
mL) and was stirred at 50 °C. The residue was purified by
column chromatography (elution with hexane/ethyl acetate
mixtures) to give 2Pt (8.6 mg, 0.070 mmol, 52% yield). Black
solid (melting point was unclear because it gradually decom-
posed above 100 °C): 1H NMR (600 MHz, CDCl3) δ 1.41 (18H,
s, Me), 1.76 (36H, s, Me), 3.87 (4H, br, ferrocene), 4.05 (10H,
s, ferrocene), 4.28 (4H, s, ferrocene), 7.65 (4H, d, m-arom.);
13C{1H} NMR (150 MHz, CDCl3) δ 31.2 (s, Me), 34.6 (s, Me),
35.4 (s, CMe3), 39.5 (s, CMe3), 69.0 (s, ferrocene), 69.9 (s,
ferrocene), 71.3 (s, ferrocene), 73.0 (s, ferrocene), 119.8 (d, ipso-
Mes*, JPC ) 33.2 Hz), 123.8 (pseudo t, m-Mes*), 150.5 (dd, Pd
C-C, JPC ) 60.4, 30.2 Hz), 154.8 (s, p-Mes*), 157.8 (s, o-Mes*),
163.9 (dd, PdC, JPC ) 81.5, 16.6 Hz); 31P{1H} NMR (162 MHz,
1
CDCl3) δ 111.4 (satellite, JPPt ) 4486.0 Hz); IR (KBr) ν/cm-1
2962, 1593, 1475, 1265, 1099, 1053, 1026, 816, 646, 461; UV-
vis (CH2Cl2) 264 (log ꢀ 4.53), 379 (4.43), 556 nm (3.93); HRMS
(70 eV, EI) found m/z 1063.3635, calcd for C60H74Fe2P2Pt [M+
- 2H - 2Cl] 1163.3623.
Acknowledgment. This work was supported in part
by the Grants-in-Aid for Scientific Research (Nos.
13304049, 14044012, 15036206, and 16033207) from the
Ministry of Education, Culture, Sports, Science and
Technology, Japan.
Supporting Information Available: 1H NMR spectra of
1 and 2 at 600 MHz at room temperature and variable-
2W. A mixture of 1 (87.4 mg, 0.0900 mmol) and (norborna-
diene)W(CO)4 (69.9 mg, 0.180 mmol) was dissolved in THF (2.0
mL) and was stirred at 50 °C. The residue was purified by
column chromatography (elution with hexane/ethyl acetate
1
temperature H NMR spectra of 1 and 2Pd. This material is
available free of charge via the Internet at http://pubs.acs.org.
OM0500772