Synthesis of a rhodium carbonyl phosphaalkenyl-phosphido complex: A phosphorus congener of schiff base Type N,N′-Chelating monoanionic ligands

Article abstract of DOI:10.1021/om500065n

A monoanionic, bidentate phosphaalkenyl-phosphido ligand was designed, synthesized, and used for the complexation of a rhodium carbonyl fragment. The characterization of the latter revealed delocalized π electrons on the phosphaalkenyl-phosphide moiety. T

Full text of DOI:10.1021/om500065n

Communication
pubs.acs.org/Organometallics
Synthesis of a Rhodium Carbonyl Phosphaalkenyl−Phosphido
Complex: A Phosphorus Congener of Schiff Base Type N,N′-Chelating
Monoanionic Ligands
Teruyuki Matsumoto, Takahiro Sasamori,* Hideaki Miyake, and Norihiro Tokitoh*
Institute for Chemical Research, Kyoto University, Gokasho Uji, Kyoto 611-0011, Japan
S
* Supporting Information
ABSTRACT: A monoanionic, bidentate phosphaalkenyl−phosphi-
do ligand was designed, synthesized, and used for the complexation
of a rhodium carbonyl fragment. The characterization of the latter
revealed delocalized π electrons on the phosphaalkenyl−phosphide
moiety. The catalytic activity of this rhodium carbonyl complex has
been demonstrated in a hydrosilylation reaction.
chiff base type N,N′-chelating monoanionic ligands, i.e. β-
diketiminate ligands 1 and anilido-imine ligands 2 (Scheme
1), have been successfully employed for the stabilization of
ligands with a low-coordinated phosphorus atom.7 We have
also developed the phosphorus analogues of anilido−imine
ligands and reported the synthesis of their rhodium carbonyl
complexes, which exhibited a strong trans influence due to the
sp2 hybridization of the phosphorus atom.8 Despite their
prospective rewards, P,P′-chelating monoanionic ligands have
not yet been intensively explored. Only one example of an
attempted synthesis of a P,P′-chelating monoanionic ligand was
reported by Ionkin et al.,9 who postulated the generation of a
phosphorus analogue of a β-diketiminato ligand, which
allegedly underwent ready intramolecular cyclization after its
formation. In this paper, we report the synthesis, complexation,
and some catalytic activity of the monoanionic, P,P′-chelating,
phosphaalkenyl−phosphido ligand 3. The design is based on
the steric congestion of the low-coordinated phosphorus atoms
in order to avoid potential intramolecular cyclization. As a
suitable protecting group for the reactive PC double bond,
the 2,4,6-tri-tert-butylphenyl (Mes*) group was chosen,10 since
such bulky substituents on the phosphorus atoms are expected
to protect not only the PC double bond but also the
coordinated metal center.
S
Scheme 1
many transition metals and main-group elements in unusual
oxidation and coordination states.1 Monoanionic N,N′-
chelating ligands have accordingly attracted much interest as
they offer not only offer fundamental insights into coordination
chemistry but also desirable practical applications, e.g., in
catalytic olefin polymerization reactions, where they can act as
supporting ligands.2 On the other hand, the coordination
chemistry of low-coordinated organophosphorus compounds
has recently drawn a great deal of attention in its own right,
mainly due to their unique electronic features.3 These low-
coordinated organophosphorus compounds exhibit low-lying
π*-orbital levels with both strong electron-accepting character
and trans influence,4 and they have been identified as exciting
research targets and promising ligands for transition-metal-
mediated catalysis. However, the coordination chemistry of
these low-coordinated sp2-hybridized organophosphorus com-
pounds is still less developed in comparison to their sp3-
hybridized phosphine analogues, mostly as a result of the
extremely high reactivity of the former with respect to the
latter.
As a suitable precursor for ligand 3, the corresponding
protonated phosphaalkenyl−phosphine 4 was prepared
(Scheme 2). A phospha-Peterson reaction between 511 and
Mes*P(Li)SiMe3 was used for the transformation of a carbonyl
group into a phosphaethenyl moiety.12 Despite the reactive P
C double bond present in 4, it was obtained as pale yellow
crystals, which are stable under ambient atmospheric
conditions. Phosphaalkenyl−phosphine 4 was fully character-
ized by solution spectroscopy and in the solid state by X-ray
crystallographic analyses.13 The 31P NMR spectrum of 4
showed two sets of doublets at −62.7 ppm (sp3-hybridized P)
On the basis of these considerations, phosphorus analogues
of N,N′-chelating monoanionic ligands bearing sp2-hybridized
phosphorus atoms have become attractive research targets in
coordination chemistry. Mathey5 and Mindiola6 have inde-
pendently reported the synthesis of monoanionic P,N-chelating
4
and at 265.2 ppm (sp2-hybridized P) with a JPP coupling
Received: January 21, 2014
Published: March 7, 2014
© 2014 American Chemical Society
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Communication
spectrum, however, 6 showed only one ddd signal, due to the
carbonyl groups at δC 192.1 ppm (2JCP = 32.7 and 38.2 Hz,
1JCRh = 66.0 Hz). Even an NMR measurement of 6 at −100 °C
in toluene-d8 was unable to resolve this signal and resulted only
in broadening. The integration of this signal relative to an
external standard indicated that the signal should correspond to
two overlapping CO moieties. Furthermore, we found that a
C6D6 solution of 6 was inert toward ethylene gas bubbling,
suggesting that no dissociation equilibrium of CO from the
rhodium center is present. Thus, we concluded that in solution
both CO moieties should easily exchange with each other even
at low temperature.17 The variable-temperature (room temper-
ature to −100 °C) UV/vis spectra of 6 in hexane did not show
any temperature dependence (λmax 539 (ε 3400), 450 (3400),
368 (11000) nm). In the IR spectrum, the carbonyl stretching
frequencies of 6 were observed at νCO 2030 and 1973 cm−1
(KBr), which are slightly shifted in comparison to those of the
related compounds bearing imino-phosphido ligands (νCO 1989
and 2046 cm−1).8
The solid-state structure of crystalline rhodium phosphaal-
kenyl−phosphide 6 is shown in Figure 1. Monoanionic
phosphaalkenyl−phosphido ligand 3 is bound to the rhodium
center in a P,P′-chelating fashion. The central six-membered
Rh−P−C−C−C−P ring shows a quasi-planar alignment, and
the internal angle sum is approximately 718°, indicative of a
delocalization of the π electrons over the ring skeleton. The
angle sum around Rh is 366°, showing a slightly distorted
square planar structure. A bond length for P1−C1 of 1.774(3)
Å represents an intermediate value between typical P−C single
bonds (1.79−1.83 Å)18 and PC double bonds (1.60−1.70
Å).15 While the bond angle sum around P2 (ca. 360°) reflects a
planar geometry consistent with sp2 hybridization, the bond
angle sum around P1 (351.2°) is indicative of slight
pyramidalization. The theoretically optimized structure of
6OPT exhibits structural parameters which are in agreement
with the experimentally observed values (Figure 2).
However, optimized geometric parameters of a less hindered
model compound with Me substituents (6Me) are also in good
agreement with the experimentally observed values (Figure 2),
suggesting that the structural features of 6 should not be
perturbed by the steric hindrance of the bulky Mes* groups.
The observed pyramidalized geometries should be character-
istic for the presence of phosphorus, because the observed
structures of P,N-analogues of 7 and 88 and the theoretically
optimized structure of the N,N′ analogue of 6NNOPT exhibit
planar geometries around the N atoms (Figure 2). In spite of
the slightly pyramidalized geometry of the P1 atom, the P1−C1
bond length of 1.774(3) Å is shorter than the corresponding
P−C bond length of 1.845(2) Å in protonated species 4,
indicating a substantial contribution of canonical structure 3B
(Figure 1). In addition, the C1−C2 and C2−C3 bond lengths
of 6 are 1.437(3) and 1.428(3) Å, respectively, suggesting that
3 exhibits an electronic resonance structure between 3A and 3B
(Figure 1) caused by the coordination toward the Rh moiety.
The calculated HOMO of 6 shows π-conjugated character of
the P−C6H4−CP moiety, and especially the π orbitals on the
P1−C1 and C2−C3 bonds indicate a conjugative contribution
(Figure 3). Nevertheless, the P2−C3 bond length of 1.677(3)
Å is comparable to the corresponding PC bond length of
1.678(2) Å in 4, indicating a decreased electronic effect toward
the P2−C3 moiety as a result of the complexation with the
rhodium atom. In the case of the N,P analogues 7 and 8
(Figure 2), the trans influence of the P moiety should be,
Scheme 2
1
constant of 37 Hz. The H NMR spectrum of 4 exhibited a
characteristic doublet for the P−H moiety at 6.51 ppm (1JPH
=
236 Hz), which is slightly shifted to low field in comparison to
the signals for diarylphosphines (ca. 5 ppm).14 The doublet
arising from the PCH moiety was observed at 9.08 ppm
(2JPH = 25 Hz), which is slightly shifted to low field relative to
(E)-Mes*PCHPh at 8.12 ppm (2JPH = 25.3 Hz).15
Attempted deprotonation reactions of 4 with n-BuLi, LDA,
NaH, and KH resulted in the formation of complicated
mixtures, from which it was difficult to isolate or use the
lithiated species 3 as an appropriate precursor for complexation
reactions. Therefore, we decided to use protonated ligand 4
itself for the reaction with a transition-metal halide complex
1
under basic conditions. When 4 was treated with /2 equiv of
[RhCl(CO)2]2 in the presence of triethylamine at room
temperature for 3 h, rhodium phosphaalkenyl−phosphide 6
was obtained quantitatively in the form of air- and moisture-
stable reddish purple crystals. The 31P NMR spectrum of 6
showed two sets of doublets of doublets at 117.0 ppm (2JPP
=
1
49 Hz, JPRh = 127 Hz) for P1 and at 172.0 ppm (2JPP = 49
Hz,1JPRh = 160 Hz) for P2 (Figure 1). The signal for P1 is
shifted to low field in comparison to that of [(dippe)Rh-
(PPh2)(PHPh2)] (dippe = 1,2-bis(diisopropylphosphino)-
ethane; δP(PPh2) −47.2 to −46.3).16 In the 13C NMR
Figure 1. Molecular structure of rhodium phosphaalkenyl−phosphide
6. Displacement ellipsoids were drawn at the 50% probability level.
Hydrogen atoms, benzene, and hexane are omitted for clarity. Selected
bond lengths (Å) and angles (deg): P1−Rh, 2.2661(7); P2−Rh,
2.2473(7); Rh−C4, 1.921(3); Rh−C5, 1.892(3); C4−O1, 1.101(3);
C5−O2, 1.133(3); P1−C1, 1.774(3); P2−C3, 1.677(3); C1−C2,
1.437(3); C2−C3, 1.428(3); P1−Rh−P2, 87.44(3); C4−Rh−C5,
93.08(13); Rh−P1−C1, 126.40(9); Rh−P2−C3, 125.89(10); P1−
C1−C2, 123.8(2); C1−C2−C3, 124.8(2); C2−C3−P2, 129.9(2).
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Communication
Scheme 3
phosphorus congener of N,N′-chelating monoanionic ligands,
and its rhodium carbonyl complex 6. The unique properties
derived from the low-coordinated sp2-hybridized phosphorus
atom have been revealed on the basis of spectroscopic and X-
ray crystallographic analyses. The phosphaalkenyl−phosphido
ligand 3 should be a very promising candidate for a supporting
capacity in catalytic applications. Further investigations into the
chemical reactivity and catalytic reactivity of metal complexes
are currently in progress and will be reported in due course.
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, and CIF files giving experimental details and
chemical data of the newly obtained compounds, crystallo-
graphic data for 4 and 6, and details of the theoretical
calculations. This material is available free of charge via the
Internet at http://pubs.acs.org.
Figure 2. Experimentally observed and theoretically optimized
structural parameters for 6 and related compounds.
AUTHOR INFORMATION
Corresponding Author
*E-mail for T.S.: sasamori@boc.kuicr.kyoto-u.ac.jp.
■
Notes
The authors declare no competing financial interest.
Figure 3. Graphic representation of the HOMO of 6 from different
angles (this figure is shown in color in the Supporting Information).
ACKNOWLEDGMENTS
■
This paper is dedicated to Professor Renji Okazaki on the
occasion of his 77th birthday. This work was partially supported
by Grants-in-Aid for Scientific Research (B) (No. 22350017),
Young Scientist (A) (No. 23685010), Scientific Research on
Innovative Areas, “New Polymeric Materials Based on Element-
Blocks” [#2401] (No. 25102519), Scientific Research on
Innovative Areas, “Stimuli-responsive Chemical Species for
the Creation of Functional Molecules” [#2408] (No.
24109013), and MEXT Project of Integrated Research on
Chemical Synthesis from Ministry of Education, Culture,
Sports, Science and Technology (MEXT) of Japan.
irrespective of sp3 or sp2 hybridization, stronger in comparison
to that of the N moiety. Indeed, longer Rh−CO bond lengths
for the CO ligands in positions trans to the P atom were
observed. In the case of the P,P′ ligand 6, the trans influences of
both P moieties are almost identical. As previously concluded,
the lack of a distinct sp3-/sp2-hybridized pattern in 6 should
result from comparable contributions of the canonical forms 3A
and 3B, which is reflected in similar Rh−CO bond lengths.
These bond lengths are moreover comparable to those in trans
positions of the P atoms in 7 and 8. The strong trans influence
of both P sites in 6 should also be responsible for the dynamic
CO exchange in solution, as observed in the NMR spectra.
As a preliminary investigation into the catalytic activity of 6,
we examined a 1,4-hydrosilylation reaction. Even though 6
bears CO substituents on the rhodium atom,19 we expected 6
to work as a catalyst for the reaction of 2-cyclohexen-1-one with
triethylsilane. The treatment of 2-cyclohexen-1-one with 3
equiv of triethylsilane in the presence of 0.5 mol % of 6 in
benzene under reflux conditions for 4 h afforded the
corresponding silylenolate 9 in an almost quantitative yield
(Scheme 3), thus demonstrating the ability of 6 to work as a
hydrosilylation catalyst.20
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(20) Complex 6 was found to be inert toward 2-cyclohexen-1-one or
triethylsilane in C6D6 at room temperature. Although heating of the
solution of 6 and 2-cyclohexen-1-one at 60 °C for 15 h resulted in no
change, that of the solution of 6 and triethylsilane under the same
conditions afforded a complicated mixture, indicating the 4-hydro-
silylation catalyzed by 6 could be initiated by the reaction of 6 with
triethylsilane. However, the detailed reaction mechanism is unclear at
present.
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