C-H activation motivated by N,N′-diisopropylcarbodiimide within a lutetium complex stabilized by an amino-phosphine ligand

Article abstract of DOI:10.1039/b710039f

A lutetium bis(alkyl) complex stabilized by a flexible amino-phosphine ligand LLu(CH₂Si(CH₃)₃)₂(THF) (L = (2,6-C₆H₃(CH₃)₂)NCH(C ₆H₅)CH₂P(C₆H₅) ₂) was prepared which upon insertion of N,N′- diisopropylcarbodiimide led to C-H activation via metalation of the ligand aryl methyl followed by reduction of the CN double bond. The Royal Society of Chemistry.

Full text of DOI:10.1039/b710039f

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C–H activation motivated by N,Nꢀ-diisopropylcarbodiimide within a lutetium
complex stabilized by an amino–phosphine ligand†‡
Bo Liu,a,b Yi Yang,a,b Dongmei Cui,*a Tao Tang,a Xuesi Chena and Xiabin Jinga
Received 2nd July 2007, Accepted 6th July 2007
First published as an Advance Article on the web 25th July 2007
DOI: 10.1039/b710039f
A lutetium bis(alkyl) complex stabilized by a flexible amino–
phosphine ligand LLu(CH2Si(CH3)3)2(THF) (L = (2,6-C6H3-
(CH3)2)NCH(C6H5)CH2P(C6H5)2) was prepared which upon
insertion of N,Nꢀ-diisopropylcarbodiimide led to C–H acti-
vation via metalation of the ligand aryl methyl followed by
=
reduction of the C N double bond.
Rare earth metal alkyl complexes have shown an upsurge in
research interest for their rich and unique chemistry, such as
Scheme 1
hydrogenolysis,1 amination,2 alkynide,2a,3 carbon dioxide fixation,4
within the NMR timescale. The solid-state structure of 1 was
and catalysis of the polymerization of olefins5 and polar
characterized by X-ray analysis to be a monomer, adopting
monomers,6 among which those supported by phosphide lig-
trigonal-bipyramidal geometry around the metal center (Fig. 1).§
ands are less common but have received considerable attention
Atoms N(1), C(29), C(33) and Lu(1) are equatorial with Lu(1)
recently.7 The impetus for phosphorus chelating complexes centr-
˚
lying 0.2491 A above the plane. While atoms P(1) and O(1) locate
ers around their potential for catalytic activity toward non-polar
monomers8 and new stoichiometric chemistry via the introduction
of “large” and “soft” phosporus donors. Our group has focused
on amino–phosphine ligands and successfully isolated several
Group 3 metal bis(alkyl) complexes, LrLu(CH2Si(CH3)3)2(THF)
(Lr = (2,6-C6H3(CH3)2)NCH2C6H4P(C6H5)2), where Lr is a rigid
bidentate amino–phosphine ligand. These complexes are highly
active initiators for the ring-opening polymerization of lactide.2b
Herein we report a lutetium bis(alkyl) complex attached by
a flexible amino–phosphine ligand LLu(CH2Si(CH3)3)2(THF)
(L = (2,6-C6H3(CH3)2)NCH(C6H5)CH2P(C6H5)2) and its unique
reactivity with carbodiimide and C–H activation within this
complex. Although C–H activation via metalation of ligand aryl
substituents2b,7e,9 or motivated upon CO insertion10 has been
reported, we present for the first time, as far as we aware, C–H
activitation via metalation of aryl methyl motivated by insertion of
carbodiimide; in addition, the newly formed metal–carbon bond
at the axial positions. The two alkyl species arrange in cis-positions.
˚
The Lu–C bond lengths (av. 2.331 A) and C(29)–Lu(1)–C(33) bond
angle (114.9(2)◦) are comparable to the corresponding values in
the literature.7c,8a,11 Complex 1 represents the first example of rare
earth metal bis(alkyl)s stabilized by this flexible amino–phosphine
=
is extremely active, enough to reduce the C N double bond of
carbodiimide. This is proved further to depend significantly on
the ligand framework.
Treatment of a toluene solution of Lu(CH2Si(CH3)3)3(THF)2
with HL for 2.5 h at room temperature afforded complex 1
in 65% yield (Scheme 1). The 1H NMR spectrum shows a
singlet at −0.24 ppm assigned to the methylene protons of two
metal alkyls which are equivalent owing to their fluxional nature
aState Key Laboratory of Polymer Physics and Chemistry, Changchun
Institute of Applied Chemistry, Chinese Academy of Science, 5625 Renmin
street, Changchun, 130022, China. E-mail: dmcui@ciac.ji.cn; Fax: +86 431
8526 2773; Tel: +86 431 8526 2773
Fig. 1 Molecular structure of 1 (hydrogen atoms omitted for clarity;
thermal ellipsoids drawn◦ to 50% probability level). Selected bond dis-
˚
tances (A) and angles ( ): Lu(1)–N(1) 2.244(5), Lu(1)–O(1) 2.269(4),
bGraduate School of Chinese Academy of Sciences, Beijing, 100039, China
Lu(1)–C(29) 2.327(6), Lu(1)–C(33) 2.355(6), Lu(1)–P(1) 2.9161(17);
N(1)–Lu(1)–O(1) 102.84(17), N(1)–Lu(1)–C(29) 112.2(2), O(1)–Lu(1)–
C(29) 91.7(2), N(1)–Lu(1)–C(33) 129.4(2), O(1)–Lu(1)–C(33) 93.4(2),
C(29)–Lu(1)–C(33) 114.9(2), N(1)–Lu(1)–P(1) 70.48(12), O(1)–Lu(1)–P(1)
172.60(12), C(29)–Lu(1)–P(1) 88.12(16), C(33)–Lu(1)–P(1) 93.42(17).
† CCDC reference numbers 641712 and 641713. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b710039f
‡ Electronic supplementary information (ESI) available: Experimental
section, 1H NMR, 13C NMR 1H–1H COSY and 1H–13C HMQC and their
assignment of complexes 2 and 4. See DOI: 10.1039/b710039f
4252 | Dalton Trans., 2007, 4252–4254
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ligand although their transition-metal counterparts12 have been
extensively reported and have their unique chemistry studied.
Addition of an equimolar amount of N,Nꢀ-diisopropyl-
carbodiimide (DIPCDI) to a hexane suspension of 1 started the
reaction immediately, and the precipitate disappeared gradually.
Concentration of the reaction solution under reduced pressure
followed by cooling at −30 ◦C for several days, gave light yellow
crystals of complex 2 (40%). The 1H NMR spectrum of 2‡
is clean but complicated. The resonances of the metal alkyl
species LuCH2Si(CH3)3 at upfield region −0.24 ppm disappear;
meanwhile, a downfield singlet at 1.34 ppm shows up, which
could be assigned to the methylene protons CH2Si(CH3)3 of the
amidinate moiety;13 one aryl methyl of the ligand fragment gives
a singlet at 2.45 ppm similar to that in 1, whereas, the two discrete
˚
length of 1.301(12) A for C(44)–N(5) suggests a double bond
nature.16
Thus, the synthetic pathway can be depicted as shown in
Scheme 2. Nucleophilic attack of metal alkyl to the carbon
atom of DIPCDI results in an insertion intermediate A of
a mono(alkyl) bearing amidinato moiety. C–H activation via
metalation of the aryl methyl group of the amino–phosphine
moiety by the metal alkyl species in A leads to elimination of
tetramethylsilane and formation of a Lu–C linkage, generating a
heteroleptic intermediate B bearing mixed amidinato/dianionic
P,N,C fragments. Insertion of DIPCDI into the newly formed
=
Lu–C bond of B results in reduction of the C N double bond
to form a dianionic P,N,N tridentate chelate accompanied by
cleavage of Lu–C and formation of Lu–N bonds, to afford 2.
This is different from the formation of C–H activation products
reported previously that are disproportionation derivatives formed
during isolation of the corresponding bis(alkyl)s.2b,7c,9c In our case,
the bis(alkyl) complex is stable, and C–H activation takes place
only upon insertion of DIPCDI.
AB spins at 2.03 and 2.17 ppm (J
the diastereotopic methylene protons derived from another aryl
methyl of the ligand fragment with the loss of one proton.
= 12 Hz) are assignable to
H, H
The molecular structure of 2 was eventually determined by
X-ray diffraction analysis to be a heteroleptic monomer (Fig. 2).§
The Lu ion is coordinated by a monoanionic amidinate ligand
and a dianionic P,N,N tridentate moiety. The molecular core
adopts distorted square-bipyramidal geometry with P(1) and N(4)
locating at the apexes and the N(1), N(2), N(3) and O(1) atoms
˚
defining the base. The bond distances Lu(1)–N averaging 2.275 A
are within the reasonsble range for a Lu–N single bond.2a,11,14
˚
The C(29)–N(2) bond length (1.348(11) A) is close to C(29)–
˚
N(3) (1.339(11) A). Both are between the values for C–N single
=
bond and C N double bond. This indicates that the p-electrons
delocalize within N–C–N of the amidinate unit, consistent with
2
an g coordination mode.15 In contrast, a 1.372(11) A bond
˚
length assigns C(44)–N(4) to be a single bond, meanwhile a bond
Scheme 2
To confirm that formation of 2 is via a mixed alkyl/amidinate
intermediate A, we tried to isolate it but failed. Monitoring the
reaction by NMR techniques still can not provide information
for the transient existence of A. We have reported previously
a P,N-bidentate ligand stabilized lutetium bis(alkyl) complex
32b that was employed to react with DIPCDI, anticipated to
afford a stable analogue of A because it bears a more rigid
phosphine–amido moiety. The reaction between 3 and 1 equiv.
of DIPCDI took place smoothly at room temperature over
night. Fortunately, the mono-DIPCDI insertion product, lutetium
mixed alkyl/amidinate complex 4, was isolated from the above
mixture cooled at −30 ◦C for several days (Scheme 3). Although
Fig. 2 Molecular structure of 2 (hydrogen atoms omitted for clarity;
thermal ellipsoids drawn to 50% probability level). Selected bond distances
◦
˚
(A) and angles ( ): Lu(1)–N(4) 2.222(8), Lu(1)–N(3) 2.246(7), Lu(1)–N(1)
2.304(7), Lu(1)–N(2) 2.329(7), Lu(1)–O 2.3816, Lu(1)–P(1) 3.033(3);
C(44)–N(4) 1.372(11); C(44)–N(5) 1.301(12); N(4)–Lu(1)–N(3) 99.8(3),
N(4)–Lu(1)–N(1) 102.6(3), N(3)–Lu(1)–N(1) 108.1(3), N(4)–Lu(1)–N(2)
105.7(3), N(3)–Lu(1)–N(2) 58.8(3), N(1)–Lu(1)–N(2) 150.4(3),
N(4)–Lu(1)–O 86.0(3), N(3)–Lu(1)–O 157.3(2), N(1)–Lu(1)–O 91.7(2),
N(2)–Lu(1)–O 98.6(2), N(4)–Lu(1)–P(1) 165.8(2), N(3)–Lu(1)–P(1)
92.9(2), N(1)–Lu(1)–P(1) 66.92(18), N(2)–Lu(1)–P(1) 86.41(19),
O–Lu(1)–P(1) 84.81(17).
1
there is no X-ray crystallographic proof, the H NMR spectrum
is clear enough to confirm the molecular structure of 4‡ in
solution. The two singlets at upfield region −0.34 ppm and
downfield region 1.96 ppm are assignable to the metal alkyl
2b
species LuCH2Si(CH3)3 and CH2Si(CH3)3 of the newly formed
amidinate, respectively. Correspondingly, the silylmethyl protons
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no. of unique reflections = 9546, Rint = 0.0409, GOF = 1.059, R = 0.0655,
Rw = 0.1726, R (all data) = 0.0864, Rw (all data) = 0.1884.
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exhibit two singlets at 0.19 and 0.39 ppm compared to one
singlet in 3. The typical resonance for the isopropyl methine
of DIPCDI is found at 3.57 ppm. The integrated intensities of
the related resonances indicate firmly one alkyl moiety and one
amidinate ligand in 4. Thus, we have demonstrated indirectly
that the lutetium bis(alkyl) complex 1 bearing a flexible P,N
bidentate ligand reacts with carbodiimide to form an unstable
mixed alkyl/amidinato intermediate A in analogy to 4, which
facilitates C–H activation of the ortho methyl proton of the
phosphine–amido ligand via metalation with release of alkane.
This is significantly attributed to the different flexibility of the
ligand backbone of complex 1 from 3.
In summary, we have prepared and characterized a new lutetium
bis(alkyl) complex stabilized by a flexible amino–phosphine lig-
and, which exhibits novel reactivity: C–H activation of the ligand
promoted by N,Nꢀ-diisopropylcarbodiimide. We also demon-
strated that backbone-flexibility of ligand plays a significant role
in this process. The polymerization catalytic activity of these
complexes toward some monomers is in process.
We are grateful for financial support from Jilin Provincial
Science and Technology Bureau for project No. 20050555; The
National Natural Science Foundation of China for project No.
20571072 and 20674081; The Ministry of Science and Technol-
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Scientists Program” of CAS.
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Notes and references
§ Crystal data for 1·(C6H6): C46H63NOSi2PLu, M = 908.09, monoclinic,
˚
˚
˚
C2/c, a = 41.074(3) A, b = 11.8955(10) A, c = 19.6868(16) A,
◦
b = 107.9790(10) , V = 9149.2(13) A , Z = 8, Dc = 3.218 g cm−3
,
3
˚
l = 17.617 cm−1, no. of reflections collected = 23425, no. of unique
reflections = 8379, Rint = 0.0719, GOF = 1.003, R = 0.0521, Rw = 0.0965,
R (all data) = 0.0883, Rw (all data) = 0.1078. For 2: C50H71N5OSiPLu, M =
¯
˚
˚
˚
992.15, triclinic, P1, a = 11.4181(11) A, b = 12.6687(12) A, c = 19.722(2) A,
◦
◦
◦
3
˚
a = 106.954(2) , b = 102.899(2) , c = 91.243(2) , V = 2648.5(4) A , Z =
2, Dc = 3.300 g cm−3, l = 18.067 cm−1, no. of reflections collected = 14262,
4254 | Dalton Trans., 2007, 4252–4254
This journal is
The Royal Society of Chemistry 2007
©