Synthesis, structures and properties of biferrocenyl- and ruthenocenyl-substituted diphosphenes

Article abstract of DOI:10.1039/c0nj00062k

Biferrocenyl and ruthenocenyl diphosphenes were synthesized in a stable crystalline form as new members of a family of d-π systems containing heavier main group elements. Their structures and redox behavior were revealed using X-ray crystallographic analysis and cyclic voltammetry.

Full text of DOI:10.1039/c0nj00062k

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LETTER
www.rsc.org/njc | New Journal of Chemistry
Synthesis, structures and properties of biferrocenyl- and
ruthenocenyl-substituted diphospheneswz
Takahiro Sasamori,* Akimi Hori, Yoshikazu Kaneko and Norihiro Tokitoh*
Received (in Montpellier, France) 26th January 2010, Accepted 6th April 2010
DOI: 10.1039/c0nj00062k
Biferrocenyl and ruthenocenyl diphosphenes were synthesized in
a stable crystalline form as new members of a family of
d-p systems containing heavier main group elements. Their
structures and redox behavior were revealed using X-ray
crystallographic analysis and cyclic voltammetry.
reflecting the difference in electronic properties between Fe
and Ru in these unique d-p electron systems.
Dilithiation of dibromobiferrocene 412 with t-BuLi followed
by the addition of an excess amount of ClP(O)(OEt)2
afforded a mixture of phosphinobiferrocenes 5 and 6, which
were separated using GPLC (CHCl3) (Scheme 1). The treatment
of 5 with Me3SiCl/LiAlH4, giving the corresponding primary
phosphine 8, followed by chlorination with PCl5, afforded
biferrocenyldichlorophosphine 10. Ruthenocenyldichloro-
phosphine 11 was prepared in a similar way to 10.13
Multiply-bonded systems between heavier group 15 elements,
such as diphosphenes (RPQPR), are interesting species as
unique and novel p electron systems.1 Although such multiple
bond compounds between heavier main group elements are
generally highly reactive and very difficult to isolate under
ambient conditions, it has been demonstrated that such reactive
p bond systems can be isolable when they are kinetically
stabilized by taking advantage of bulky substituents. For
example, a series of heavier congeners of azo compounds
(REQER, EQP, As, Sb, Bi) bearing bulky substituents have
been synthesized and isolated as stable compounds,2 since the
first isolation of a stable diphosphene, Mes*PQPMes*
(Mes* = 2,4,6-tri-t-butylphenyl).3 Recently, there has been much
interest in the creation of functionalized p electron-conjugated
systems containing such unique p electron systems of heavier
main group elements.4–6 From this viewpoint, much attention
has been focused on the synthesis of novel d-p electron systems
containing heavier main group elements and transition
metals,6,7 since initiative works with Mes*PQPFc (Fc =
ferrocenyl).6f We have already reported the synthesis of stable
ferrocenyl-substituted diphosphenes TbtPQPFc8 (3) and
(ArPQP)2fc (Ar = Tbt or Bbt, Tbt = 2,4,6-[CH(SiMe3)2]3–
C6H2, Bbt = 2,6-[CH(SiMe3)2]2-4-[C(SiMe3)3]–C6H2, fc =
1,10-ferrocenylidene),9,10 and stable metallocenyl-substituted
disilenes (E)-Tip(Fc)SiQSi(Fc)Tip and (E)-Tip(Rc)SiQ
Si(Rc)Tip (Tip = 2,4,6-triisopropylphenyl, Rc = ruthenocenyl),11
and revealed their unique electrochemical properties. These
were isolated as stable crystalline compounds due to the steric
protection afforded by the Tbt and Tip groups. In this Letter,
we present the synthesis of biferrocenyldiphosphene 1 as a
more extended d-p electron system than ferrocenyldiphosphene 3
and ruthenocenyldiphosphene 2 as a heavier analogue of 3,
The treatment of a benzene solution of biferrocenyldichloro-
phosphine 10 with an ether solution of TbtPHLi8a,9 at r.t.
afforded an orange suspension, which may contain the possible
intermediates of a diastereomer mixture of Fc–fc–P(Cl)–P(H)–Tbt
(12) [as judged by the two sets of AX signals in the 31P NMR
spectrum: d = 109, ꢀ56.1 (J = 186 Hz) and d = 93.2, ꢀ52.6
(J = 230 Hz)]. Biferrocenyldiphosphene 1 was obtained as a
purple crystalline compound by the addition of DBU as a base
to the reaction mixture (Scheme 2). Similarly, the reaction of
ruthenocenyldichlorophosphine 11 with TbtPHLi, followed by
a treatment with DBU, afforded ruthenocenyldiphosphene 2
as stable deep-red crystals. The intermediacy of the diastereomer
mixture of chlorodiphosphanes, 13, was confirmed by 31P NMR
spectroscopy [as judged by the two sets of AX signals in the
31P NMR spectrum: d = 98.0, ꢀ68.5 (J = 170 Hz) and
d = 87.8, ꢀ54.1 (J = 250 Hz)], as in the case of the synthesis
of 1.
The 31P NMR spectra of metallocenyldiphosphenes 1 and 2
in C6D6 showed AX signals at dP = 501.6, 477.1 (J = 552 Hz)
for 1 and dP = 492.8, 473.5 (J = 553 Hz) for 2 in the low-field
region characteristic of diphosphenes, respectively. The
1
characteristic dP and JPP values of 1 and 2 were found to be
similar to those of previously reported stable diaryldiphosphenes1a
1
(e.g., Mes*PQPMes: dP = 540.4, 467.6, JPP = 573.7 Hz)14
and ferrocenyldiphosphenes (e.g. Mes*PQPFc:6f dP = 471.2,
442.6, 1JPP = 532 Hz and TbtPQPFc (3):8a dP = 501.7, 479.5,
1JPP = 546 Hz), showing the PQP double bond character of 1
Institute for Chemical Research, Kyoto University, Gokasho, Uji,
Kyoto 611-0011, Japan. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp,
sasamori@boc.kuicr.kyoto-u.ac.jp;
Fax: +81 774-38-3209; Tel: +81 774-38-3200
w Dedicated to the late Professor Pascal Le Floch. This article is part
of a themed issue on Main Group chemistry.
z Electronic supplementary information (ESI) available: Synthetic
procedures and spectral data of biferrocenyldichlorophosphine 10
and ruthenocenyldichlorophosphine 11. CCDC reference numbers
762039 and 762040. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0nj00062k
Scheme 1 The synthesis of phosphinated biferrocene.
ꢁc
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Fig. 1 The structures of (a) 1 and (b) 2. Displacement ellipsoids are
drawn at the 50% level. Hydrogen atoms are omitted for clarity.
case of 3. In the UV-vis spectra of 1 and 2 in hexane,
characteristic absorptions were observed at lmax(e) = 375
(9.4 ꢂ 104), 467 (2.1 ꢂ 103) and 557 (3.4 ꢂ 103) nm for 1,
and lmax(e) = 356 (5.6 ꢂ 103), 420 (6.0 ꢂ 103) and 473
(2.9 ꢂ 103) nm for 2. The absorption at the longest lmax is
assignable to the MLCT transitions, as in the case of
Mes*PQPFc [lmax = 515 nm at ꢀ78 1C]6f and 3 [lmax(e) =
542 nm (9.0 ꢂ 102)].8a,9b Although the lmax value of 1 is similar
to that of 3, the corresponding absorption coefficients (e) for 1
are much larger than those for 3, suggesting the effectively
Scheme 2 The synthesis of biferrocenyldiphosphene 1 and ruthenocenyl-
diphosphene 2.
and 2 in solution and the negligible electronic effect of the
metallocenyl moieties towards the PQP moiety.
The molecular structures of 1 and 2 were revealed by X-ray
crystallographic analysis (Fig. 1 and Table 1). Both diphos-
phenes 1 and 2 exhibit an E-configuration with C–PQP–C
torsion angles of ca. 1801. It should be noted that the P–C(Cp)
bond lengths of both 1 and 2 are slightly shorter than the
P–C(Tbt) bond lengths, respectively, suggesting an effective
interaction between the PQP and Cp units, as compared to
those between the PQP and aryl (Tbt) moieties.16 The angle
between the PQP–C(Cp) plane and the Cp ring of the
metallocenyl moiety (defined as y1) in 1 is small (8.81),
indicating the conjugative interaction of the p electrons
between the PQP and metallocenyl moieties.17 In addition,
the angle between the two Cp rings of the two ferrocenyl units
(defined as y2) of 1 is only 5.01, showing the high coplanarity of
the biferrocene units. On the other hand, y1 of ruthenocenyl-
diphosphene 2 is 22.31, which is somewhat larger than those of
1 and ferrocenyldiphosphene 3 (5.71).8a The PQP bond
lengths of 1 and 2 are 2.0384(9) (1) and 2.0361(8) (2) A, which
are slightly longer than those of TbtPQPFc [2.021(2) A]7a and
TbtPQP–fc–PQPTbt [2.015(3) A],8 indicating a slightly more
effective interaction between the p electrons of the PQP unit
and the Cp ring of the biferrocenyl or ruthenocenyl moieties in
1 and 2.17,18
extended delocalization of
p electrons. On the other
hand, ruthenocenyldiphosphene 2, colored deep-red, showed
characteristic MLCT absorptions at shorter wavelengths than
those of 1 and 3. Although preliminary TDDFT calculations19
suggested that several types of MLCT transition would be
included in the observed absorption, the assignment of the
transitions is likely to be difficult due to the complication of
electron transitions. Since ruthenocene is colorless, in contrast
to ferrocene (colored orange), the difference in the electronic
spectra between ruthenocenyldiphosphene 2 and ferrocenyl-
diphosphene 3 should reflect the electronic features of the
metallocenes.
The redox behavior of 1 and 2 was observed by cyclic and
differential pulse voltammetry (Table 2). In the oxidation
region, 1 exhibited reversible redox couples at E1/2 = ꢀ0.07 V
(vs. FcH/FcH+), suggesting that 1 would undergo ready
oxidation compared to ferrocene and ferrocenyldiphosphene
37a (E1/2 = +0.08 V (vs. FcH/FcH+), probably due to the
much extended d-p conjugation in 1+. On the other hand, 2
showed an irreversible oxidation wave at Epa = +0.65 V
(vs. FcH/FcH+), reflecting the electrochemical properties of
ruthenocene, which has been reported to show an irreversible
or quasi-reversible oxidation wave under various conditions.20
Thus, the oxidative features of metallocenyldiphosphenes 1
Metallocenyldiphosphenes 1 and 2 are strongly colored
purple and deep-red in solution, respectively, similar to the
Table 1 Observed and calculateda structural parameters of 1, 2, and 3
PQP/A
P–C(Cp)/A
P–C(Tbt)/A
P–P–C(Cp) (1)
P–P–C(Tbt) (1)
1obs
1calc
2obs
2calc
3obs
3calc
2.0384(9)
2.044
2.0361(8)
2.045
2.0285(15)
2.043
1.801(2)
1.808
1.805(2)
1.809
1.805(4)
1.807
1.849(2)
1.853
1.843(2)
1.852
1.848(3)
1.850
102.55(9)
103.34
100.97(8)
102.29
101.64(16)
102.85
98.94(8)
102.14
99.55(7)
102.29
103.15(12)
103.37
a
a
a
a
B3LYP/6-311+G(3d) for P; 6-31G(d) for Si, C, H; DZVP for Fe, Ru.15
ꢁc
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Table 2 Redox potentials (V vs. FcH/FcH+) of 1, 2 and 3
0.45 mmol) was added to the solution. The reaction mixture
immediately turned into a deep-purple suspension. After
stirring for 5 min, the solvent of the reaction mixture was
removed under reduced pressure. The reaction mixture was
dissolved in n-hexane and then filtered through Celites. The
filtrate was concentrated and precipitated from n-hexane to
give biferrocenyldiphosphene 1 (112 mg, 0.11 mmol, 38%) as
purple crystals.
1a
2a
3
1st
2nd
1st
1st
2nd
Oxidation
Epa
Epc
+0.06
ꢀ0.21
ꢀ0.07
+0.68
—
—
+0.65
—
—
+0.17b
ꢀ0.11b
+0.08b
+0.42b
—
—
E1/2
Reduction
Epc
Epa
ꢀ2.35
ꢀ2.09
ꢀ2.22
ꢀ2.37
ꢀ2.01
ꢀ2.19
ꢀ2.29a
ꢀ2.13a
ꢀ2.21a
1. Purple crystals, m.p. 142 1C (decomp.). dH (300 MHz,
C6D6, Me4Si): 6.82 (br s, 1H), 6.71 (br s, 1H), 4.77 (pseudo-t,
E1/2
a
b
3JHH = 1.82 Hz, 2H), 4.35 (pseudo-t, JHH = 1.82 Hz,
0.1 M (n-Bu)4NBF4 in THF. 0.1 M (n-Bu)4NBF4 in CH2Cl2.
3
3
2H), 4.30 (pseudo-t, JHH = 1.82 Hz, 2H), 4.27 (pseudo-t,
3
3JHH = 1.82 Hz, 2H), 4.22 (pseudo-t, JHH = 1.82 Hz,
3
2H), 4.04 (pseudo-t, JHH = 1.82 Hz, 2H), 3.94 (s, 5H), 2.73
and 2 seem to be affected by the electrochemical properties of
the metallocene unit. In the reductive region, both 1 and 2
showed reversible redox couples at E1/2 = ꢀ2.22 (1) and ꢀ2.19
(2) (vs. FcH/FcH+), respectively, which are redox potentials
similar to those of ferrocenyldiphosphene 3 (E1/2 = ꢀ2.21 V)7a
and TbtPQPTbt (E1/2 = ꢀ2.21 V),21 suggesting that the
reduction potentials of 1 and 2 are strongly affected by the
PQP double bond character. It can be concluded that
the biferrocenyl- and ruthenocenyl-diphosphenes can work as
multi-redox systems, while the metallocenyl moieties take part
with difficulty in the conjugative interaction for anionic species of
1 and 2.
(br s, 1H), 2.66 (br s, 1H), 1.55 (s, 1H), 0.28 (s, 36H), 0.23
(s, 18H). dC {1H} (75 MHz, C6D6, Me4Si): 146.7 (C),
144.2 (C), 133.6 (C, d, JPC = 85 Hz), 128.5 (C), 127.3
1
1
(CH), 122.7 (CH), 86.6 (C), 85.3 (C, dd, JPC = 58 Hz,
2JPC = 16 Hz), 82.8 (C), 75.6 (CH), 73.4 (CH), 70.1 (CH),
69.7 (CH), 68.5 (CH), 68.1 (CH), 66.8 (CH), 31.3 (CH), 30.7
(CH), 1.42 (CH3), 1.00 (CH3). dP (120 MHz, C6D6, H3PO4):
1
501.6, 477.1 (AX system, JPP = 552 Hz). HRMS (FAB)
m/z calc. for C47H77Fe2P2Si6 983.2819, found 983.2798
([M + H]+).
In summary, biferrocenyldiphosphene 1 and ruthenocenyl-
diphosphene 2 were synthesized and characterized as novel d-p
electron systems containing a PQP bond. It was demonstrated
that 1 exhibits a unique multi-step redox behavior, indicating a
possible application of p electron systems between heavier
main group elements toward electrochemical materials.
Further investigations on the properties of metallocenyl-
diphosphenes are currently in progress.
Synthesis of ruthenocenyldiphosphene 2
To a solution of ruthenocenyldichlorophosphine (11; 400 mg,
1.20 mmol) in benzene (120 mL) was added an Et2O solution
of TbtPHLi, which was prepared by the treatment of
TbtPH2 (700 mg, 1.20 mmol) with n-BuLi (1.5 M in hexane,
0.80 mL, 1.20 mmol) in Et2O (15 mL) at ꢀ78 1C. The reaction
mixture was stirred for 1 h to give a dark-orange suspension.
After removal of the solvent under reduced pressure, the
resulting reaction mixture was dissolved in benzene and then
filtrated through Celites. The solvent of the filtrate was
removed under reduced pressure to afford a dark-orange
solid of a diastereomer mixture of chlorodiphosphanes 13.
The obtained crude mixture, including 13, was dissolved
in benzene (40 mL), and then DBU (180 mL, 1.20 mmol)
was added to the solution. The reaction mixture immediately
This work was partially supported by a Grant-in-Aid
for Creative Scientific Research (no. 17GS0207), Science
Research on Priority Areas (no. 20036024, ‘‘Synergy of
Elements’’) and the Global COE Program (B09, ‘‘Integrated
Materials Science’’, Kyoto University) from the Ministry
of Education, Culture, Sports, Science and Technology
(MEXT), Japan.
turned into
a deep-red suspension. After stirring for
20 min, the solvent of the reaction mixture was removed under
reduced pressure. The reaction mixture was dissolved in
n-hexane and then filtered through Celites. The filtrate
was concentrated and precipitated from n-hexane to give
ruthenocenyldiphosphene 2 (500 mg, 0.57 mmol, 56%) as
deep-red crystals.
Experimental
Synthesis of biferrocenyldiphosphene 1
To a solution of biferrocenyldichlorophosphine (10; 190 mg,
0.40 mmol) in benzene (5 mL) was added an Et2O solution of
TbtPHLi, which was freshly prepared by the treatment of
TbtPH2 (187 mg, 0.32 mmol) with n-BuLi (1.5 M in hexane,
0.23 mL, 0.35 mmol) in Et2O (18 mL) at ꢀ78 1C. The reaction
mixture was stirred for 0.5 h to give an orange suspension.
After removal of the solvent under reduced pressure, the
resulting reaction mixture was dissolved in benzene and then
filtered through Celites. The solvent of the filtrate was
removed under reduced pressure to afford a light-orange solid
of a diastereomer mixture of chlorodiphosphane 12 (411 mg,
0.40 mmol, quant.). Chlorodiphosphane 12 (295 mg, 0.29 mmol)
was dissolved in benzene (10 mL), and then DBU (69 mg,
2. Deep-red crystals, m.p. 99 1C (decomp.). dH (300 MHz,
C6D6, Me4Si): 6.78 (br s, 1H), 6.68 (br s, 1H), 5.23
(m, 2H), 4.62 (m, 2H), 4.52 (s, 5H), 2.63 (br s, 1H), 2.56
(br s, 1H), 1.52 (s, 1H), 0.24 (s, 36H), 0.21 (s, 18H). dP
(120 MHz, C6D6, H3PO4): 492.8, 473.5 (AX system,
1JPP
= 553 Hz). HRMS (APPI-TOF) m/z calc. for
C37H69P2RuSi6 845.2537, found 845.2574 ([M + H]+).
Elemental analysis calc. for C37H68P2RuSi6: C 52.62, H 8.12;
found C 52.64, H 8.38%.
ꢁc
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X-Ray crystallographic data for 1 and 2z
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7 In the case of metallocenyldiphosphenes, the MLCT observed
through UV-vis spectra would be one piece of experimental
evidence for the electronic correlation between the d electrons of
the metal and the p electrons of the PQP moiety. Thus, d-p
electron systems containing heavier main group elements mean
compounds containing a p-bond between heavier main group
elements connected with a transition metal moiety, such as a
metallocene unit, through an organic p-conjugated system, where
there is an electronic correlation between the d electrons and the p
electrons, should conceivably exist.
1 (C47H76Fe2P2Si6): M = 983.26, T = 103(2) K, triclinic, P-1
(no. 2), a = 12.3334(3), b = 12.9518(4), c = 17.4667(5) A,
a = 107.7383(10), b = 97.2601(10), g = 93.3283(16)1,
V = 2622.37(13) A3, Z = 2, Dc = 1.245 g cmꢀ3, m =
0.782 mmꢀ1, l = 0.71069 A, 2ymax = 51.0, 23 442 measured
reflections, 9703 independent reflections (Rint = 0.0462),
532 refined parameters, GOF = 1.014, R1 = 0.0365 and wR2
= 0.0612 [I > 2s(I)], R1 = 0.0688 and wR2 = 0.0669 [for all
data], largest differential peak and hole 0.462 and ꢀ0.377 e Aꢀ3
.
2 (C37H68P2RuSi6): M = 844.46, T = 103(2) K, triclinic, P-1
(no. 2), a = 9.6151(1), b = 10.6365(2), c = 24.8520(4) A,
a = 97.9195(7), b = 96.8663(6), g = 110.1667(8)1, V =
2324.61(6) A3, Z = 2, Dc = 1.206 g cmꢀ3, m = 0.584 mmꢀ1
,
l = 0.71069 A, 2ymax = 51.0, 20 542 measured reflections,
8550 independent reflections (Rint = 0.0244), 433 refined
parameters, GOF = 1.052, R1 = 0.0297 and wR2 = 0.0760
[I > 2s(I)], R1 = 0.0332 and wR2 = 0.0784 [for all data],
largest differential peak and hole 0.838 and ꢀ0.741 e Aꢀ3
.
The intensity data were collected on a Rigaku/MSC Mercury
CCD diffractometer. The structure was solved by direct methods
(SHELXS-97) and refined by full-matrix least-squares procedures
on F2 for all reflections (SHELXL-97).
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10 Independently, Pietschnig et al. have reported stable 1,10-
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16 The P–C(Cp) bond length is shorter than the P–C(Tbt) bond
length. This could not be due to steric reasons because the
theoretically optimized structure of the less hindered models
DmpPQPFc (Dmp = 2,6-dimethylphenyl) and DmpPQPRc
showed shorter P–C(Cp) bonds than the P–C(Dmp) bonds,
indicating the electronic interaction between the Cp and the
PQP moieties.
17 Simple calculations for the model compound Me–PQP–Fc suggest
that the rotational barrier around the P–Cp axis is less than
15 kcal molꢀ1 (see the ESIz). Thus, the metallocenyl moiety would
rapidly rotate along the P–Cp axis. The observed y values would be
affected by packing forces in the crystal, and it should be discussed
only in the crystalline state.
18 The y1 and y2 values of the optimized structure of 1 are 17.9 and
11.341, respectively, which are slightly larger those actually observed
for 1. Similarly, the y1 value of the optimized structure of 2 is 29.01,
which is also slightly larger that actually observed for 2. Although the
structural parameters calculated for 1 and 2 are to some extent in
good agreement with those observed, packing forces in the crystalline
ꢁc
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state would not be negligible in these cases. Further investigation to
elucidate the reason for the slight difference between the observed and
calculated structural parameters is in progress.
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19 TD-B3LYP/6-311+G(3d) for P; 6-31G(d) for Si, C, H; DZVP for
Fe, Ru.
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ꢁc
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2010
1564 | New J. Chem., 2010, 34, 1560–1564