Homometallic rare-earth metal phosphinidene clusters: Synthesis and reactivity

Article abstract of DOI:10.1002/anie.201307422

Two new trinuclear μ₃-bridged rare-earth metal phosphinidene complexes, [{L(Ln)(μ-Me)}₃(μ₃-Me)(μ₃- PPh)] (L=[PhC(NC₆H₄iPr₂-2,6)₂] ^-, Ln=Y (2 a), Lu (2 b)), were synthesized through methane elimination of the corresponding carbene precursors with phenylphosphine. Heating a toluene solution of 2 at 120 °C leads to an unprecedented ortho C-H bond activation of the PhP ligand to form the bridged phosphinidene/phenyl complexes. Reactions of 2 with ketones, thione, or isothiocyanate show clear phospha-Wittig chemistry, giving the corresponding organic phosphinidenation products and oxide (sulfide) complexes. Reaction of 2 with CS₂ leads to the formation of novel trinuclear rare-earth metal thione dianion clusters, for which a possible pathway was determined by DFT calculation. A rare gathering: Two new trinuclear μ₃-bridged rare-earth metal phosphinidene complexes were synthesized by treatment of the corresponding carbene precursors with phenylphosphine; some new transformation patterns of phosphinidenes are revealed. A possible pathway for reaction of these phosphinidene complexes with CS₂ was determined by DFT calculations. Copyright

Full text of DOI:10.1002/anie.201307422

Angewandte
Chemie
DOI: 10.1002/anie.201307422
Rare-Earth Complexes
Homometallic Rare-Earth Metal Phosphinidene Clusters: Synthesis
and Reactivity**
Kai Wang, Gen Luo, Jianquan Hong, Xigeng Zhou,* Linhong Weng, Yi Luo,* and Lixin Zhang*
Abstract: Two new trinuclear m3-bridged rare-earth metal
phosphinidene complexes, [{L(Ln)(m-Me)}3(m3-Me)(m3-PPh)]
(L = [PhC(NC6H4iPr2-2,6)2]ꢀ, Ln = Y (2a), Lu (2b)), were
synthesized through methane elimination of the corresponding
carbene precursors with phenylphosphine. Heating a toluene
[{L(Ln)(m2-PAr)(thf)x}2] (Ln = Sc, Nd, Lu; x = 0, 1, 3; L =
[N(2-PiPr2-5-MeC6H3)2], C5Me5, b-diimino, I; Ar= 2,4,6-
Me3C6H2, 2,6-iPr2C6H3).[7] Very recently, two rare-earth
metal phosphinidene complexes stabilized through alkali
metal halides, [(pnp)Sc(m-PC6H3Mes2-2,6)(m-Br)Li(dme)]
(pnp = N(2-PiPr2)2-4-MeC6H3)2, Mes = 2,4,6-Me3C6H2)[8] and
[IY(m6-P){Y(thf)(m3-PC6H3iPr2-2,6)}4(m-I)4K(toluene)],[9]
were disclosed. Reactivity studies on these mono- or dinu-
clear rare-earth metal phosphinidene complexes showed their
great potential to serve as a source of phosphorus for
constructing phosphorus-containing higher organic mole-
cules.[7,8] Therefore, development of a diverse variety of
phosphinidene complexes of rare-earth metals and new
methods for their construction would be highly desirable.
The polynuclear frameworks show a strong influence on
the reactivities of methylidene[10] and imido[11] rare-earth-
metal complexes. For example, trinuclear rare earth methyl/
methylidenes react easily with various primary amines to
generate the corresponding imido complexes,[11c] however,
[(bipm)YI(thf)2] (bipm = C(PPh2NSiMe3)2) and [(pnp)Sc(m3-
CH2)(m2-Me)2{Al(Me)2}2], react with 2,6-iPr2C6H3NH2 to
form only the anilides.[10b,f] Thus, with the aim of establishing
how the multimetallic cooperative effect imparts to the
phosphinidene methyl complexes an enhanced reactivity
analogous to that already observed in the case of methylidene
and imido ligands, we decided to extend our study to
phosphinidene analogues. Herein, we report the synthesis of
the first trinuclear rare-earth metal phosphinidene complexes
featuring methyl and amidinate ligands, and their unique
reactivity patterns.
ꢀ
solution of 2 at 1208C leads to an unprecedented ortho C H
bond activation of the PhP ligand to form the bridged
phosphinidene/phenyl complexes. Reactions of 2 with ketones,
thione, or isothiocyanate show clear phospha-Wittig chemistry,
giving the corresponding organic phosphinidenation products
and oxide (sulfide) complexes. Reaction of 2 with CS2 leads to
the formation of novel trinuclear rare-earth metal thione
dianion clusters, for which a possible pathway was determined
by DFT calculation.
M
etal phosphinidene complexes have received increasing
attention, not only because of their rich chemistry, but also
their potential utility as intermediates or catalysts in the
preparation of phosphorus compounds, organometallic deriv-
atives, and new materials.[1–3] It has been shown that variation
of the metals bonded to the phosphinidene ligands could lead
to specific changes in structure and catalytic activity, as well as
chemical and physical properties.[4]
Various kinds of transition metal phosphinidene com-
plexes with different coordinating modes and reactivities have
been documented.[1–6] However, only a few phosphinidene
complexes of rare-earth metals, the largest subgroup in the
periodic table, have been known so far, owing both to the
mismatch in the orbital energy of the electropositive rare-
earth metals and the electronegative phosphinidene ligand,
and the difficulty in stabilizing low-coordination lanthanide
systems having terminal metal–ligand multiply bonded scaf-
folds.[7–9] Recently, Kiplinger and Chen reported binuclear m2-
bridged phosphinidene complexes of rare-earth metals
The homometallic trinuclear phosphinidene complexes
[{L(Ln)(m2-Me)}3(m3-Me)(m3-PPh)]
(L = [PhC(NC6H4iPr2-
2,6)2]ꢀ; Ln = Y(2a), Lu(2b)) were synthesized in high yield
by a methane elimination reaction of [{L(Ln)(m2-Me)}3(m3-
Me)(m3-CH2)][10e] with 1 equiv of PhPH2 (Scheme 1). How-
ever, attempts to synthesize the corresponding scandium
complex by this method were unsuccessful even with pro-
longed heating at 808C, wherein most of starting materials
was recovered.
[*] K. Wang, Dr. J. Hong, Prof. Dr. X. Zhou, Prof. Dr. L. Weng,
Assoc. Prof. Dr. L. Zhang
Department of Chemistry, Fudan University
Shanghai 200433 (China)
E-mail: lixinzh@fudan.edu.cn
G. Luo, Prof. Dr. Y. Luo
State Key Laboratory of Fine Chemicals, School of Pharmaceutical
Science and Technology, Dalian University of Technology
Dalian 116024 (China)
[**] This work was supported by the NSFC, 973 Program
(2012CB821604), the Ministry of Education of China. The authors
also thank the Network and Information Center of Dalian University
of Technology for computational resources.
Supporting information, including full synthetic, spectroscopic, and
structural and computational details, for this article is available on
the WWW under http://dx.doi.org/10.1002/anie.201307422.
Scheme 1. Synthesis of phosphinidene complexes 2.
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Scheme 2. Formation of complexes 3.
(JYP = 25.9 Hz) in the 31P NMR spectrum of 3a was observed,
which indicates the existence of a m2-P bridge. Crystal
structures of 3 (Figure 1; see also Figures S33 and S34)[15]
reveal that the phosphorus atom coordinates to two metal
atoms, whereas the ortho carbon atom of the phenyl ring is
bonded to three metals. The average Lu–P(m2) distance of
2.646 ꢀ is slightly longer than the observed value in
[{(pnp)Lu(m-PC6H2Me3-2,4,6)}2].[7a] The average Ln–C(m3-
C6H4P) distance (2.565 ꢀ (3a), 2.530 ꢀ (3b)) is shorter than
the average Ln–C1(m3-Me) distance (2.682 ꢀ (3a), 2.618 ꢀ
(3b)), but is much longer than the average Ln–C(m2-CH2)
distance (2.401 ꢀ (1a), 2.376 ꢀ (1b)) in [L{(Ln)(m2-Me)}3(m3-
Me)(m3-CH2)].[10e] The phenyl ring on P1 is nearly perpendic-
ular to the Ln3 plane, displaying a dihedral angle of 84.38 (3a)
or 82.48 (3b).
In contrast to imido analogues, in which the imido group is
inert to many unsaturated substrates such as ketones and
thioketones,[11c] complexes 2 show high reactivity toward
these substrates (Scheme 3). The reactions of 2 with aceto-
Figure 1. ORTEP structures of 2b and 3a. Thermal ellipsoids set at
30% probability. Isopropyl groups of benzamidinate ligands and
hydrogen atoms are omitted for clarity.
The 31P NMR spectrum of complex 2a shows a quartet of
peaks at d = 138.79 ppm, thus indicating a m3-P coordination
mode, which was further confirmed by the X-ray crystal
diffraction analysis of 2 (Figure 1; see also the Supporting
Information, Figures S31 and S32).[15] Characteristically, com-
pounds 2 have two distinctive Ln–P distances. In 2a, the Y1–
P1 (2.714 ꢀ) and Y2–P1 (2.732 ꢀ) bond lengths are compa-
rable to the values found in [{L(Ln)(m-PAr)}2]-type com-
plexes,[7] whereas the Y3–P1 (2.943 ꢀ) bond length falls in the
=
Scheme 3. Reactions of 2 with substrates containing C X (X=O, S)
bonds.
!
range of the Ln3+ DP donating bond lengths for neutral
phosphorus ligands when the difference in metal radii is taken
into consideration.[12] Similarly, the longest Lu P bond in 2b
ꢀ
ꢀ
is 0.241 ꢀ longer than the other two Lu P bonds (avg. of
phenone and benzophenone gave the corresponding lantha-
nide oxo complexes 4 in 82–90% yields. The structures of 4
were determined by X-ray single-crystal diffraction analysis
ꢀ
2.661 ꢀ). Furthermore, the P C bond displays a bent angle
(59.18 for 2a, 49.38 for 2b) to the Ln3 plane, which is different
from the observation in analogous transition-metal com-
plexes, where P C bonds are approximately perpendicular to
(Figure S35).[15] The resulting bulky phosphaalkene PhP
=
CPh2 (4-P2) was confirmed by 31P NMR spectroscopy (in
ꢀ
the metal-based plane.[6]
C6D6: s, d = 233.2 ppm; Figure S15) and GC-MS.[13] Attempts
=
Complexes 2 are stable at room temperature. But unusual
phosphinidene deprotonated products 3 were generated in
high yields (91% for 3a, 93% for 3b; Scheme 2), when
a toluene solution of 2 was heated at 1208C for 2 days. The
liberated CH4 was monitored by 1H NMR spectroscopy of the
in situ reaction, which clearly showed a sharp signal of
methane at 0.16 ppm. A triplet resonance at d = 262.48 ppm
to isolate [PhP CMePh] (4-P1) have been unsuccessful owing
=
to its instability. 2 reacted with (p-Me2NC6H4)2C S, generat-
ing m3-sulfide complexes 5 (Figures S36 and S37) and (p-
Me2NC6H4)2C PPh (5-P1; 31P NMR: s, d = 201.3 ppm; Figur-
=
es S18–20). Complexes 5 can also be obtained from the
reaction of 2 with phenylisothiocyanate. However, the newly
= =
formed PhP C NPh (5-P2) is not stable under the conditions
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involved, and immediately transforms to a complicated
mixture that may result from symmetrical and unsymmetrical
dimerization.[14] These results indicate that 2 can act as
a source of the phosphinidene moiety, which may be
exchanged for an oxo or a sulfido group.
The treatment of 2 with one equivalent of CS2 in toluene
led to the occurrence of an unprecedented reaction, giving
unusual thione-dianion-ligated rare-earth sulfide complexes 6
(Scheme 4). This reaction differs from that of the m2-
phosphinidene Sc complex [{LSc(m-PAr)}2] wherein only
Scheme 4. Reaction of 2 with CS2.
ꢀ
CS2 insertion into the Sc P bond occurs to give a stable
[{LSc(m-S2CPAr)}2] species.[7d]
Ln metal center no longer exists in solution. Hydrolysis of 6
afforded 1-(methylphenylphosphino)-ethanethiol (6-P),
which was observed by GC-MS analysis (Figure S26).
Compounds 6 represent rare examples of thione dianion
complexes. This one-pot sequence of transformations leading
In the structures of 6, the P atom is bonded to three
carbon atoms, the Ln–P distances of 3.098 ꢀ (6a) and 2.992 ꢀ
(6b) are slightly longer than the typical Ln–P donating bond
length (Figure 2).[12,15] In the 31P NMR spectrum of complex
6a, only a singlet signal was observed at d = 9.5 ppm in C6D6,
which indicates that the weak interaction between the P and
ꢀ
ꢀ
to the formations of two C P bonds and one C C bond,
provides a straightforward method for the three-component
synthesis of phosphinothiols. To elucidate the formation
mechanism of 6, the reaction was further investigated by
theoretical calculations (Figures S40–42 and Tables S14–17).
As shown in Figure 3, the reaction starts with the insertion of
ꢀ
CS2 into the Ln P bond of 2a via the transition state TSAB
,
with an energy barrier of 16.1 kcalmolꢀ1 to give intermediate
B. B subsequently undergoes changes in the coordination of
a S atom from m1- in B to m2- in C, and of the P atom from m2-
ꢀ
in B to m1- in C. The cleavage of the C S bond of the more
stable C occurs easily via the transition state TSCD, with only
a 0.6 kcalmolꢀ1 energy barrier, leading to the phosphathio-
= =
S atom. The computed 31P NMR chemical shifts of PhP C S
ketene species [PhP C S] and intermediate 5a with a m3-
= =
(ꢀ36.6 ppm; Table S18; at the MP2/6-311 ++ G(3df,3pd)
level of theory), is close to the observed signal at
ꢀ29.4 ppm (d, JYP = 38.3 Hz; Figure S30), when a mixture of
2b and CS2 in toluene was assayed by 31P NMR spectroscopy
Figure 2. ORTEP structures of 6a. Thermal ellipsoids set at 30%
probability. Isopropyl groups of benzamidinate ligands and hydrogen
atoms are omitted for clarity.
= =
at ꢀ308C. The newly formed [PhP C S] then attacks the m3-
Figure 3. Computed energy profile (Gibbs free energies in kcalmolꢀ1) for the formation of 6a by the reaction of 2a with CS2 at the M06 L/SDD&6-
31G**//ONIOM(TPSSTPSS/LanL2DZ&6-31G*:HF/LanL2MB) level of theory.
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ꢀ
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Received: August 23, 2013
Revised: October 15, 2013
Published online: December 5, 2013
ꢀ
Keywords: C H activation · CS2 activation ·
density functional calculations · phosphinidenes · rare earths
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