Synthesis, spectroscopic properties, and reactivity of ferrocenyl(2,4,6-tri-tert-butylphenyl)diphosphene

Article abstract of DOI:10.1021/om950384f

A ferrocenyl-substituted diphosphene has been synthesized and characterized, which seems to represent a borderline case concerning the dimerization of such species. Thus, the tetraphosphetane formed by dimerization readily undergoes cycloreversion to the initial diphosphene upon heating in xylene solution.

Full text of DOI:10.1021/om950384f

Organometallics 1996, 15, 891-893
891
Syn th esis, Sp ectr oscop ic P r op er ties, a n d Rea ctivity of
F er r ocen yl(2,4,6-tr i-ter t-bu tylp h en yl)d ip h osp h en e
Rudolf Pietschnig and Edgar Niecke*
Institut fu¨r Anorganische Chemie, Gerhard-Domagk-Strasse 2,
D-53121 Bonn, Federal Republic of Germany
Received May 24, 1995X
Summary: A ferrocenyl-substituted diphosphene has
been synthesized and characterized, which seems to
represent a borderline case concerning the dimerization
of such species. Thus, the tetraphosphetane formed by
dimerization readily undergoes cycloreversion to the
initial diphosphene upon heating in xylene solution.
(1) dimerizes slowly at room temperature, forming the
cyclotetraphosphane (2), which upon heating in xylene
readily undergoes cycloreversion to the original diphos-
phene (1). Furthermore, this process is completely
reversible and strictly stereospecific.
The ferrocenyl-substituted diphosphene (1) is syn-
thesized from ferrocenyldichlorophosphane and lithium
(2,4,6-tri-tert-butylphenyl)(trimethylsilyl)phosphanide at
-78 °C. The compound shows a characteristic intensive
red-violet color due to a long-wavelength absorption at
515 nm. In comparison to other diphosphenes (355-
484 nm)1 this is a remarkable bathochromic shift,
indicating considerable electronic interaction between
the ferrocenyl group and the PdP double bond.
In tr od u ction
Tetraphosphetanes and other cyclopolyphosphanes
are common byproducts in the synthesis of diphos-
phenes.1 Even the first diphosphene reported, the
historically interesting “phosphobenzene”,2 turned out
to be a mixture of several different cyclopolyphos-
phanes,3 Since the reactions of polyphosphanes often
lead to the products expected in the formal reaction of
the corresponding diphosphene or phosphinidene,4 sev-
eral attempts have been made to generate these com-
pounds from the easily available polyphosphanes.5 Up
to now only few cases are known in which diphosphenes
have been generated by cycloreversion of tetraphosphe-
tanes.6 In all of these cases the tetraphosphetane was
formed by initial cycloaddition of an unsymmetrical
diphosphene, whereas the products of the following
cycloreversion were symmetrical diphosphenes, at least
one of which proved to be stable against further cy-
cloaddition reactions (diphosphene metathesis). The
ferrocenyl substituent selected for our investigations has
been characterized by its remarkable ability to stabilize
a positive charge on an adjacent atom.7 We consider
this to be a very useful feature concerning the cyclor-
eversion of tetraphosphetanes to diphosphenes under
mild conditions, as shall be pointed out later.
This hypothesis is corroborated by the NMR data.
Accordingly, the 31P NMR spectrum of 1 shows an AX
system in the low-field range typical for trans-diphos-
phenes with δ(PA) 471.2 ppm and δ(PB) 442.6 ppm. In
1
contrast, the coupling constant J PP ) 532 Hz is rather
small for trans-diphosphenes in general but is in good
accordance with E-configurated systems bearing at least
one strong π-donating substituent, as for instance in
aminodiphosphenes.9 The reduced coupling constant
can be explained by assuming a reduced PsP bond
order, as depicted in formulation 1b (Scheme 2), which
has been confirmed for certain aminodiphosphenes by
X-ray structure analysis.10
Evidence that formulation 1b is of considerable
1
importance can be found in the H NMR spectrum of 1,
since the R-protons of the substituted cyclopentadienyl
ring are chemically not equivalent. While the signals
due to the â-protons of the same ring are detected
indistinguishably at 4.29 ppm (2H) as a multiplet, the
two R-protons are observed at different values, namely
4.76 ppm (1H) for the downfield proton and 4.65 ppm
(1H) for the upfield one.11
Resu lts a n d Discu ssion
Although the electronic properties of the ferrocenyl
substituent alone are insufficient to stabilize a mono-
meric diphosphene,8 we succeeded, by combining both
steric and electronic features, in generating a diphos-
phene (1) that seems to represent a borderline case
concerning dimerization. The ferrocenyldiphosphene
Since the steric demand of the ferrocenyl moiety is
comparable to that of the phenyl group12 and the
PhPdPMes* (Mes* ) 2,4,6-tri-tert-butylphenyl) substi-
tuted diphosphene can not be isolated in the monomeric
form,13 diphosphene 1 also dimerizes slowly when
exposed to daylight and ambient temperature, taking
weeks, depending on temperature and concentration.14
Remarkably, only one of the various possible isomers
X Abstract published in Advance ACS Abstracts, December 15, 1995.
(1) Weber, L. Chem. Rev. 1992, 92, 1839.
(2) Ko¨hler, H.; Michaelis, A. Ber. Dtsch. Chem. Ges. 1877, 10, 807.
(3) Kuchen, W.; Buchwald, H. Chem. Ber. 1958, 91, 2296.
(4) Mathey, F. In Multiple Bonds and Low Coordination in Phos-
phorus Chemistry; Regitz, M., Scherer, O. J ., Eds.; Thieme: Stuttgart,
Germany, 1990; p 33.
(5) (a) Fluck, E.; Issleib, K. Z. Naturforsch. 1966, 21B, 736. (b)
Gru¨tzmacher, H. F. Chem. Ber. 1969, 102, 3230.
(6) (a) Niecke, E.; Nieger, M.; Kramer, B. Angew. Chem. 1989, 101,
217. (b) Altmeyer, O.; Niecke, E.; Nieger, M. Angew. Chem. 1991, 103,
1158.
(9) Yoshifuji, M. In Multiple Bonds and Low Coordination in
Phosphorus Chemistry; Regitz, M., Scherer, O. J ., Eds.; Thieme:
Stuttgart, Germany, 1990; p 321.
(10) (a) Markovski, L. N.; Romanenko, V. D.; Ruban, A. V. Chemistry
of Acyclic Compounds of Two-coordinated Phosphorus; Naukova
Dumka: Kiev, Ukraine, 1988; p 199. (b) Busch, T.; Schoeller, W. W.;
Niecke, E.; Nieger, M.; Westermann, H. Inorg. Chem. 1989, 28, 4334.
(11) Coalescence of the two protons cannot be observed up to a
temperature of 120 °C.
(7) Dannenberg, J . J .; Levenberg, M. K.; Richardson, J . H. Tetra-
hedron 1973, 29, 1575.
(8) Spang, C.; Edelmann, F. T.; Noltemeyer, M.; Roesky, H. W.
Chem. Ber. 1989, 122, 1247.
(12) Falk, H.; Lehner, H.; Schlo¨gl, K. J . Organomet. Chem. 1973,
55, 191.
(13) Inamoto, N.; Shibayama, K.; Yoshifuji, M. J . Am. Chem. Soc.
1983, 105, 2495.
0276-7333/96/2315-0891$12.00/0 © 1996 American Chemical Society

892 Organometallics, Vol. 15, No. 2, 1996
Notes
Sch em e 1a
served values. A further decision between B, C, and D
is not possible merely on the basis of the NMR data.
However, due to possible steric interaction an arrange-
ment as shown in B seems to be highly unlikely.
Therefore, we presume tetraphosphetane 2 to have a
structure like C or D, in which the bulky Mes* substit-
uents are oriented trans to each other.
a
Legend: Fc ) ferrocenyl; Mes* ) 2,4,6-tri-tert-butylphe-
If the observed stereospecifity is not a consequence
of the concerted course of reaction but of the relative
stability of the resulting products, structure C would
be the most probable one. But it should be pointed out
that in all known cases where cycloreversion occurs, one
of the two substituents is a π-donor. As recently
proposed by Bickelhaupt et al.,16 a thermal dimerization
of diphosphenes is preceded by initial formation of an
intermediate cis isomer. Similarly, structure D repre-
sents the adduct of this intermediate. Therefore, from
the mechanistic point of view, structure D is the most
likely one. The corresponding cycloreversion, which is
the retro reaction, should also proceed via the genera-
tion of an initial cis diphosphene, followed by subse-
quent cis-trans isomerization. This process should,
according to formulation 1b (Scheme 2), be facilitated
by an adjacent π-donor substituent.
nyl.
Sch em e 2
Ch a r t 1
Chemically, the ferrocenyl-substituted diphosphene
1 behaves like other diphosphenes, forming the expected
[2 + 4] cycloadduct 3 with cyclopentadiene (Scheme 3).
While at high temperature predominant formation of
one of the two possible diastereomers 3a or 3b cannot
be observed, at ambient temperature only one of them
is formed.
Treatment of 1 with methanol or water resultssas
common for diphosphenes9s in cleavage of the P-P
bond. As indicated by the polarity expressed by formu-
lations 1a ,b (Scheme 2), Mes*PH2 and Fc-P(OR)2 are
the sole products of this reaction. Products of the
opposite polarity, as for instance the well-known Fc-
PH2, cannot be detected.
is formed, showing an AA′BB′ spin system. At a
temperature of -30 °C and in the absence of light, the
ferrocenyl-substituted diphosphene remains stable in
the monomeric form. When it is heated in boiling xylene
to approximatly 140 °C, the tetraphosphetane 2 under-
goes spontaneous cycloreversion, yielding diphosphene
1 stereospecifically in the E configuration. Irradiation
of solid 2 with UV light leads to the same product. In
neither case is Mes*PdPMes* formed.
Stabilization of diphosphene 1 can be achieved by
complexation to Pt(0) (Scheme 3). The resulting com-
plex 4 is stable at room temperature (δP (121.5 MHz,
1
toluene): PA 25.5, PB 24.8, PC 26.4, PX -16.1; J AX
)
2
2
2
-424.0 Hz, J AB ) 52.2 Hz, J AC ) 4.0 Hz, J XB ) 3.0
2
2
Hz, J CX ) 36.0 Hz, J BC ) 8.0 Hz). In contrast, the
presence of Fe2(CO)9 even seems to accelerate the
formation of tetraphosphetane 2 and does not lead to
the desired monomeric diphosphene complex.
Due to the fact that an AA′BB′ spin system is
observed for tetraphosphetane 2, only a few relative
arrangements of substituents are possible (Chart 1).
Simulated and measured 31P NMR spectra of 2 are in
good accordance, using as a set of coupling parameters
J AA′/J BB′ ) 127.6/(156.8 Hz and J AB/J AB′ ) (150.9/-
64.8 Hz. Assuming an arrangement of substituents as
shown in A, the coupling constants AA′ and BB′ would
Exp er im en ta l Section
Gen er a l Con sid er a tion s. All steps were carried out under
a dry argon atmosphere using common Schlenk techniques.
Solvents were dried and freshly distilled from Na/K alloy
before use. NMR spectra were recorded on a Bruker AMX 300.
31P NMR spectra were recorded using 85% H3PO4 as external
standard and 1H NMR spectra with tetramethylsilane as
external standard.
2
be of the type J PP, which is in contradiction to the
1
observed values revealing a J PP coupling.15 Hence, a
“head to tail” dimer can be excluded. Considering a
“head to head” dimer showing the observed spin system,
the substituent adjacent to A is arranged either cis (B)
or trans (C,D) in reference to A′. Since in these cases
P r ep a r a tion of 1. (E)-1-Ferrocenyl-2-(2,4,6-tri-tert-bu-
tylphenyl)diphosphene was synthesized from (2,4,6-tri-tert-
butylphenyl)(trimethylsilyl)phosphane (Mes*PHTms)17 and
the coupling constants AA′ and BB′ are of the type 1J PP
,
these structures are in good agreement with the ob-
(14) As measured by 31P NMR spectroscopicly the dimerization is
second order in 1. The half-lifetime at a concentration of 2.3 × 10-2
mol/L turned out to be approximately 4 days.
(15) Albrand, J . P.; Cogne, A.; Robert, J . B. J . Am. Chem. Soc. 1978,
100, 2600.
(16) Bickelhaupt, F.; Goede, S. J .; de Kanter, F. J . J .; Komen, C. M.
D. J . Chem. Soc., Perkin Trans. 1993, 807.
(17) Romanenko, V. P.; Ruban, A. V.; Iksanova, S. V.; Polyachenko,
L. K.; Markovski, L. N. Phosphorus Sulfur Silicon Relat. Elem. 1985,
22, 365.

Notes
Organometallics, Vol. 15, No. 2, 1996 893
Sch em e 3
2
dichloroferrocenylphosphane (FcPCl2).18 To a solution of Mes*-
PHTms (1.75 g, 5.0 mmol) in diethyl ether (20 mL) was added
a solution of n-butyllithium (3.4 mL, 5.5 mmol) in hexane at
-78 °C. After being stirred for 30 min at -78 °C, the reaction
mixture was stirred at room temperature for 1.5 h. The
resulting mixture was slowly added to a cooled (-78 °C)
solution of FcPCl2 (1.44 g, 5.0 mmol) in diethyl ether (100 mL).
After the reaction mixture was warmed to room temperature,
the solvent was evaporated under vacuum. The residue was
extracted with pentane and the LiCl filtered. After evapora-
tion of the pentane under vacuum 1 was obtained as a deep
red-violet solid (yield 2.02 g, 82%). On recrystallization from
toluene at room temperature, 2 is obtained as a yellow solid
(mp 98 °C dec).
(150.9 Hz, and J AB′ ) -64.8 Hz. δH (300 MHz, CDCl3): 7.21
(4H, m, Mes*), 4.43 (4H, m, HR-Fc), 4.37 (4H, m, Hâ-Fc), 4.21
(10H, m, Cp′), 1.65 (36H, m, ortho-C(CH3)3), 1.49 (18H, m,
para-C(CH3)3). MS (FAB): 983.4 (20%), M+ - 1; 799 (17%),
M+ - FcH; 739.2 (15%), M+ - Mes*; 708.2 (40%) M+ - Mes* -
PH; 492.2 (42%), M+/2; 217.0 (100%) FcPH+.
2-F er r ocen yl-3-(2,4,6-tr i-ter t-bu tylp h en yl)-2,3-d ip h os-
p h a n or bor n -5-en e (3) is synthesized from 1 and cyclopen-
tadiene. To a solution of 1 (0.3 g, 1 mmol) in pentane (15 mL)
was added an excess of cyclopentadiene (CpH) (1 mL) at room
temperature. After the mixture was stirred for 1.5 h, the
solvent and excess cyclopentadiene were removed under
vacuum, yielding 0.46 g (82.4%) of an orange oil. Satisfactory
analysis could not be performed, since the product contained
traces of dicyclopentadiene, which could not be removed
without decomposition of the product. 3a : δP (121.5 MHz,
toluene) 35.9 (1P, d, 1J PP ) 274.7 Hz), -9.5 (1P, d, 1J PP ) 274.7
1
1. δP (121.5 MHz, pentane): 471.2 (1P, d, J PP ) 532 Hz,
P-Fc) and 442.6 (1P, d, 1J PP ) 532 Hz, P-Mes*). δH (300 MHz,
C6D6) 7.61 (2H, s, Mes*), 4.76 (1H, s, HR-Fc), 4.65 (1H, s, HR-
Fc), 4.29 (2H, s, Hâ-Fc), 4.08 (5H, s, Cp′-Fc), 1.90 (18H, s,
ortho-C(CH3)3), 1.40 (9H, s, para-C(CH3)3). δC (75.5 MHz,
CDCl3) 68.9 (s, Cp′), 68.7 (m, CR-Fc), 69.5 (m, CR′-Fc), 69.9
(m, Câ-Fc), 31.4 (s, para-C(CH3)3), 34.9 (s, para-C(CH3)3), 31.6
(d, 4J CP ) 16.4 Hz, ortho-C(CH3)3), 39.8 (d, 3J CP ) 4.0 Hz, ortho-
C(CH3)3), 149.5, 149.6, 149.9, and 123.2 (m, Mes*). MS (EI,
70 eV, 180 °C): M+ 492.1790 (found), 492.179 08 (calculated),
62.2%; 435 (54%), M+ - t-Bu; 277 (39%), Mes*PH+; 217 (100%),
FcPH+; 186 (62%), FcH+; 121 (18%), CpFe+; 57 (40%), Fe/
butyl+. UV-vis (pentane): 515 nm (ꢀ ) 1065 L/(mol cm), 447
nm (ꢀ ) 648 L/(mol cm), 357 nm (ꢀ ) 3518 L/(mol cm), 228 nm
(ꢀ ) 12 454 L/(mol cm), 209 nm (ꢀ ) 13 190 L/(mol cm).
2. Anal. Calcd for C56H76Fe2P4: C, 68.30; H, 7.78. Found:
C, 68.11; H, 7.93. δP (121.5 MHz, toluene): -32.0 (2P, m),
Hz) (isomer 3b, obtained in mixture with 3a when heated
without solvent δP (121.5 MHz, toluene) 35.7 (1P, d, J PP
1
)
1
279.8 Hz), -7.6 (1P, d, J PP ) 279.8 Hz)); δH (300 MHz, C6D6)
4.3 (5H, s, Cp′), 4.4 (4H, m, CR/â), 1.7 (s, o-t-Bu), 1.5 (s, p-t-
Bu), 2.8 (m), 3.1 (m), 5.6 (m), 5.7 (m), 1.2 (d, J HP ) 7.1 Hz), 1.7
(d, J HP ) 9.3 Hz), 7.6 (d, J HP ) 2.1 Hz); δC (75.5 MHz, C6D6)
69.9 (s, Cp′), 70.0 (d, 1J CP ) 31.1 Hz, Cipso), 70.4 (d, 2J CP ) 24.7
4
Hz, CR), 70.9 (s, Câ), 33.0 (d, J CP ) 7.5 Hz, o-t-Bu), 32.2 (s,
p-t-Bu), 35.3 (s, Cq-p-t-Bu), 148.3 (d, 1J CP ) 45.3 Hz, Cipso), 138.2
(d, 2J CP ) 11.8 Hz, Cortho), 137.5 (d, 3J CP ) 25.1 Hz, Cmeta), 136.1
4
1
(d, J CP ) 2.3 Hz, Cpara), 136.7 (d, J CP ) 43.9 Hz), 132.3 (d,
1J CP ) 56.2 Hz), 47.6 (d, J CP ) 26.1 Hz), 45.2 (d, J CP ) 31.1
Hz), 53.9 (s, CH2). MS (24 eV, 80 °C): 556 (0.5%), M+ - 2;
492 (1.1%), M+ - Cp; 484 (6.3%), M+ - C2H2; 277 (47.3%),
Mes*PH+; 251 (6.5%), M+ - Mes*P; 217 (18.5%), FcPH+; 186
(24.0%), FcH+; 66 (100%), Cp+; 57 (65.1%), Fe/butyl+.
2
2
1
1
1
-42.9 (2P, m) J AA′ ) 127.6 Hz, J BB′ ) (156.8 Hz, J AB
)
(18) Boricenko, A. A.; Manzhukova, L. F.; Ninfant′ev, E. E.;
Ninfant’ev, E. I. Phosphorus Sulfur Silicon Relat. Elem. 1992, 68, 99.
OM950384F