Rare earth metal boryl complexes: Synthesis, structure, and insertion of a carbodiimide and carbon monoxide

Article abstract of DOI:10.1002/anie.201101107

Among the first of their kind: Scandium boryl complex 1 and its Gd analogue were prepared by reaction of a boryl lithium compound with [Ln(CH ₂SiMe₃)₂(THF)_x][BPh₄] (Ln=Sc, Gd). Reaction of 1 with N,N′-diisopropylcarbodiimide gave bis(amidinate) complex 2, which affords 3 by double CO insertion into the Sc-B bond, nucleophilic addition of an amidinate, and migration of the SiMe ₃ to the resulting ketene moiety (see scheme). Copyright

Full text of DOI:10.1002/anie.201101107

Communications
DOI: 10.1002/anie.201101107
Boryl Complexes
Rare Earth Metal Boryl Complexes: Synthesis, Structure, and Insertion
of a Carbodiimide and Carbon Monoxide**
Shihui Li, Jianhua Cheng, Yanhui Chen, Masayoshi Nishiura, and Zhaomin Hou*
The chemistry of transition metal complexes bearing boryl
ligands (R2BÀ) has fascinated scientists in organometallic and
synthetic chemistry over the past two decades, because of
their important roles in functionalization of organic substrates
by borylation processes and their interesting chemistry in
their own right.[1–5] Since transition metal boryl complexes
were first postulated in 1963[2a] and structurally characterized
in 1990,[2b,c] numerous metal boryl complexes have been
Scheme 1. Synthesis of rare earth metal boryl dialkyl complexes.
reported. However, most research in this area has focused on
complexes of late and middle transition metals.[1,2] In contrast,
early transition metal complexes bearing boryl ligands have
been much less extensively studied, because of difficulties in
synthesis.[3,4] In particular, a Group 3 or f-block metal boryl
complex has not been reported previously, as far as we are
aware.[6]
For many years, we have been pursuing the fundamental
chemistry and synthetic applications of rare earth metal
complexes ligated by various electron-rich donor ligands
containing C, N, O, P, and so on.[7,8] We were intrigued by the
electron-deficient nature of boryl moieties,[1,5] which moti-
vated us to explore the chemistry of rare earth metal
complexes bearing boryl ligands. Herein, we report the
synthesis, structure, and some reactions of the first rare
earth metal boryl complexes.
complex 2-Gd was obtained in 67% yield as pale yellow
crystals.
1
Diamagnetic Sc complex 2-Sc showed well-resolved H
and 13C NMR spectra in [D8]toluene, while paramagnetic Gd
complex 2-Gd did not give informative 1H or 13C NMR
signals. The resonances of the methylene and methyl groups
of the CH2SiMe3 units in 2-Sc overlapped around d =
0.12 ppm in the 1H NMR spectrum, but showed distinct
singlets at d = 59.4 and 3.6 ppm, respectively, in the 13C NMR
spectrum. The methyl protons of the isopropyl groups of the
boryl ligand appeared as two doublets (3JH,H = 6.9 Hz) at d =
1.24 and 1.40 ppm, and the methine protons gave one septet at
d = 3.52 ppm (3JH,H = 6.9 Hz) in the 1H NMR spectrum,
À
À
suggesting that rotations around the N Ar and iPr Ar
bonds in the boryl ligand are restricted. The vinyl protons in
the diazaborole ring showed a singlet at d = 6.25 ppm. The
11B NMR spectra of 2-Sc and 2-Gd showed broad signals at
d = 35.5 ppm and 29.2 ppm, respectively, which are shifted to
high field compared to lithium boryl compound 1 (d =
45.4 ppm),[5a,d] and are in sharp contrast with those found in
scandium borohydride complexes such as [(C5Me4-C6H4-o-
NMe2)Sc(BH4)2] (d = À19.7 ppm)[10a] and [(C5H5)2Sc(BH4)]
(d = À17.7 ppm).[10b] The 11B NMR signal of scandium boryl
complex 2-Sc is close to that of the titanium boryl complex
[(L)Ti(OiPr)3] (L = C26H36N2B; d = 38.2 ppm).[4a]
Our initial attempt to obtain a rare earth boryl complex
through deprotonation[7,8] of a hydroborane compound with
rare earth trialkyl complexes [Ln(CH2SiMe3)3(THF)2] (Ln =
Sc, Gd) was not successful because of the weak acidity of the
À
B H group. We then treated boryl lithium compound 1, which
was reported previously by Nozaki, Yamashita, and co-
workers,[5] with rare earth metal alkyl ion-pair complexes
[Ln(CH2SiMe3)2(THF)x][BPh4], reported by Okuda et al.[8k,9]
Reaction of [Sc(CH2SiMe3)2(THF)3][BPh4] with 1 equiv of 1
in THF proceeded smoothly at room temperature to afford
corresponding boryl-ligated scandium dialkyl complex 2-Sc in
78% yield as deep purple crystals after recrystallization from
hexane (Scheme 1). In a similar manner, gadolinium boryl
An X-ray diffraction study revealed that the Sc atom in
2-Sc is coordinated by one boryl, two alkyl, and one THF
ligands in a distorted tetrahedral fashion (Figure 1, left). In
contrast, the Gd atom in 2-Gd is bonded to one boryl, two
alkyl, and two THF ligands in a slightly distorted square-
based pyramidal geometry, in which the boron atom B1
occupies the apex position and the oxygen atoms (O1, O2) of
the two THF ligands and the methylene carbon atoms (C1,
C2) of the two CH2SiMe3 groups form the square-based plane
(Figure 1, right). That the gadolinium complex 2-Gd bears an
additional THF ligand compared to 2-Sc is apparently due to
its larger ionic radius. No rare earth metal–boron s-bond
lengths are available for comparison in the literature, but the
[*] Dr. S. Li, Dr. J. Cheng, Dr. Y. Chen, Dr. M. Nishiura, Prof. Dr. Z. Hou
Organometallic Chemistry Laboratory and Advanced Catalyst
Research Team, RIKEN Advanced Science Institute
2-1 Hirosawa, Wako, Saitama 351-0198 (Japan)
Fax: (+81)48-462-4665
E-mail: houz@riken.jp
[**] This work was partly supported by a Grant-in-aid for Scientific
Research (S) (No. 21225004) from the Ministry of Education,
Culture, Sports, Science and Technology of Japan and the Key
Project of International Cooperation of NSFC (20920102030). We
thank Dr. H. Koshino for assistance with 11B NMR measurements.
À
Sc B bond length in 2-Sc (2.433(12) ꢀ) is comparable to the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201100893.
À
Gd B bond length in 2-Gd (2.699(4) ꢀ), when the difference
6360
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2011, 50, 6360 –6363

natively, addition of 1 equiv of the carbodiimide com-
pound to a solution of 3 in C6D6 gave 4 quantitatively in
1
5 h, as monitored by H NMR spectroscopy (Scheme 2).
An X-ray crystallographic study revealed that 3
adopts a distorted pyramidal geometry around the
scandium center, with the B1 atom occupying the apex
and the four nitrogen atoms N3–N6 residing in the basal
À
plane (Figure 2). The Sc B bond length in 3 (2.508(3) ꢀ)
is about 0.075 ꢀ longer than that in 2-Sc, and reflects
increasing steric encumbrance around the metal center in
1
3. The H NMR spectrum of 3 in C6D6 showed a sharp
Figure 1. ORTEP diagrams of 2-Sc (left) and 2-Gd (right) with thermal
ellipsoids at 30% probability. Hydrogen atoms have been omitted for clarity.
singlet at d = 1.94 ppm assignable to the methylene
protons of the CH2SiMe3 groups in the amidinate units.
The 11B NMR spectrum of 3 in C6D6 showed a broad peak
at d = 36.3 ppm, which is close to that of 2-Sc.
À
À
À
Selected bond lengths [ꢀ]: 2-Sc: Sc1 B1 2.433(12), Sc1 O1 2.116(6), Sc1
À
À
À
C1 2.151(10), Sc1 C5 2.170(11). 2-Gd: Gd1 B1 2.699(4), Gd1 O1 2.386(3),
À
À
À
Gd1 O2 2.399(3), Gd1 C1 2.423(5), Gd1 C5 2.446(5).
in ionic radii between Sc and Gd and the difference in
coordination numbers between 2-Sc and 2-Gd are taken into
account.[11] These Ln B bond lengths in 2-Sc and 2-Gd are
À
respectively much shorter than the sum of the covalent radii
[12]
À
À
(Sc B: 2.54 ꢀ, Gd B: 2.80 ꢀ) and much longer than the
Ln···B interatomic distances in the borohydride complexes
(Sc···B: 2.341 ꢀ,[10a] Gd···B: 2.587 ꢀ[10c]). The Ln C bond
À
lengths in 2-Sc (av 2.161 ꢀ) and 2-Gd (av 2.435 ꢀ) are
À
comparable with the corresponding Ln CH2SiMe3 bond
lengths in related complexes.[8,13]
The reaction of 2-Sc with 2 equiv of N,N’-diisopropylcar-
bodiimide in [D8]toluene at room temperature took place
rapidly, and the starting materials disappeared within 30 min,
as monitored by 1H NMR. Bis(amidinate)-ligated boryl com-
plex 3, formed through carbodiimide insertion into the two
Figure 2. ORTEP diagram of 3 with thermal ellipsoids at 30% proba-
bility. Hydrogen atoms have been omitted for clarity. Selected bond
À
À
À
lengths [ꢀ]: Sc1 B1 2.508(3), Sc1 N3 2.147(2), Sc1 N4 2.206(2),
À
À
Sc1 N5 2.196(2), Sc1 N6 2.154(2).
[7d,e,8c,g–i]
À
Sc CH2SiMe3 bonds in 2-Sc,
was isolated as a pale
yellow microcrystalline solid in 30% yield from a concen-
trated solution of the reaction mixture in THF/hexane. A
certain amount of tris(amidinate) complex 4, formed by
The X-ray structure of 4 is shown in Figure 3. The Sc atom
is bonded to six N atoms in a distorted octahedral fashion. The
À
À
À
carbodiimide insertion into both Sc C bonds and the Sc B
bond, was also observed in the mother liquor. When 3 equiv
of N,N’-diisopropylcarbodiimide reacted with 2-Sc, complex 4
was obtained in 75% yield as pale yellow crystals. Alter-
Sc N bond lengths in 4 (av 2.221 ꢀ) are slightly longer than
those in 3 (av 2.176 ꢀ). The 11B NMR signal of 4 in C6D6
appeared at d = 22.2 ppm, which is shifted to higher field
compared with those of 2-Sc and 3 owing to transfer of the
boryl ligand from the metal center to the carbodiimide
moiety.[5a,d]
Figure 3. ORTEP diagram of 4 with thermal ellipsoids at 30% proba-
bility. Hydrogen atoms have been omitted for clarity. Selected bond
À
À
À
lengths [ꢀ]: B1 C28 1.619(4), Sc1 N3 2.213(2), Sc1 N4 2.224(2),
À
À
À
À
Scheme 2. Reactions of scandium boryl complexes with carbodiimide
and carbon monoxide.
Sc1 N5 2.221(2), Sc1 N6 2.220(2), Sc1 N7 2.237(2), Sc1 N8
2.210(2).
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www.angewandte.org
6361

Communications
When a solution of 3 in benzene was stirred under an
could be assigned to the carbon atom bonded to the boryl
group, because it was broadened, presumably due to 13C–11B
coupling. These results strongly support that both carbon
atoms in the OCCO unit originate from external CO
molecules.
atmosphere of CO at room temperature for 16 h, complex 5
was obtained as a pale yellow crystalline solid in 85% yield
(Scheme 2).[14] An X-ray diffraction study established that 5
À
=
À
contains a trans-O C(B) C(N) O moiety with one oxygen
atom bonded to the scandium atom and the other to an SiMe3
group (Figure 4). One of the two carbon atoms is bonded to a
A possible pathway for formation of 5 is given in
À
Scheme 3. Insertion of one molecule of CO into the Sc B
bond in 3 would give an h2-CO(boryl) species such as A,
which on reaction with another molecule of CO could afford a
ketene-like intermediate such as B.[15] Nucleophilic attack of
an amidinate unit on the ketene unit, accompanied by
migration of the SiMe3 group from the amidinate unit to the
oxygen atom of the ketene species, would yield 5.
Figure 4. ORTEP diagram of 5 with thermal ellipsoids at 30% proba-
bility. The isopropyl groups of the boryl and amidinate ligands and
hydrogen atoms have been omitted for clarity. Selected bond
À
À
À
lengths [ꢀ]: Sc1 O1 1.961(1), Sc1 N3 2.383(2), Sc1 N4 2.081(2),
À
À
À
À
Sc1 N5 2.143(2), Sc1 N6 2.153(2), B1 C27 1.570(3), O1 C27
À
À
À
À
1.377(2), C27 C28 1.347(3), O2 C28 1.381(2), Si1 O2 1.670(1), N3
À
À
À
C28 1.454(2), N3 C35 1.478(3), N4 C35 1.351(3), C35 C36 1.331(3),
À
À
N5 C40 1.337(3), N6 C40 1.340(3).
boryl unit and the other to a nitrogen atom of a former
amidinate unit, which has now lost its SiMe3 group and
become a 1-aminovinylamido unit chelating the Sc atom. The
other amidinate group remains chelating the Sc center. The
OCCO unit is planar with a maximum deviation of 0.012 ꢀ.
The scandium and boron atoms lie slightly above (0.524 ꢀ)
and below (0.160 ꢀ) this plane, respectively. In agreement,
Scheme 3. Possible mechanism of formation of complex 5.
À
the C27 C28 bond length is 1.347(3) ꢀ and can be assigned to
In summary, we have demonstrated for the first time that
rare earth metal boryl dialkyl complexes such as 2-Sc and
2-Gd can be easily obtained by reaction of a lithium boryl salt
with dialkyl rare earth tetraphenylborate ion-pair com-
pounds. Preliminary reactivity studies have shown that the
=
À
a C C double bond. The Sc1 O1 distance in 5 (1.961 (1) ꢀ) is
À
significantly shorter than the Sc O(THF) distance in 2-Sc
(2.116(6) ꢀ), and could be regarded as a Sc O covalent bond.
Reflecting the presence of a terminal Sc amido covalent
bond, the Sc1 N4 bond length (2.081(2) ꢀ) is much shorter
than the Sc1 N3 amino coordination bond (2.383(2) ꢀ) and
also significantly shorter than the Sc N amidinate bonds
(Sc1 N5 2.143(2), Sc1 N6 2.153(2) ꢀ). The B C distance of 5
(1.570(3) ꢀ) is shorter than that of 4 (1.619(4) ꢀ).
Complex 5 was further characterized by NMR spectros-
copy, including DEPT and C–H COSY experiments. The
methyl protons of the CH2SiMe3 and OSiMe3 groups showed
two singlets at d = 0.069 and 0.436 ppm, respectively, in the
1H NMR spectrum in C6D6 at room temperature. The
terminal vinyl methylene protons gave two sharp singlets at
d = 3.47 and 3.62 ppm. In the 13C NMR spectrum, the
terminal vinyl methylene carbon atom showed a singlet at
d = 69.66 ppm. The 11B NMR spectrum of 5 showed a singlet
at d = 25.49 ppm, which is comparable with that of 4.
À
À
À
À
Ln B bonds in 2-Sc or 3 can undergo insertion reactions with
À
carbodiimide and carbon monoxide to give new boron-
containing rare earth metal complexes such as 4 and 5.
Further studies on the synthesis of other rare earth metal
boryl complexes and on the reactivity of this new class of
complexes are in progress.[16,17]
À
À
À
À
Received: February 14, 2011
Published online: May 5, 2011
Please note: Changes have been made to this manuscript since its
publication in Angewandte Chemie Early View. The Editor.
Keywords: boron · boryl ligands · insertion · lanthanides ·
.
scandium
Formation of the OCCO unit in 5 was also confirmed by
reaction of 13CO with 3. The 13C NMR spectrum of the
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6362 www.angewandte.org
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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[16] The analogous yttrium, erbium, and lutetium boryl complexes
were also obtained in a similar way, see Supporting Information
for details.
[17] Note added in proof May 24, 2011: After submission of this
work, the groups of Aldridge, Mountford, Jones, and Kaltsoyan-
nis reported the synthesis and structural characterization of the
Sc, Y, and Lu boryl complexes. See: L. M. A. Saleh, K. H.
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Angew. Chem. Int. Ed. 2011, 50, 6360 –6363
ꢀ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
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