Intramolecular C-H bond activation induced by a scandium terminal imido complex

Article abstract of DOI:10.1039/c2cc33179a

A CpPN-based scandium terminal imido complex was isolated, which could induce the intramolecular C-H bond activation of a phenyl group even at room temperature.

Full text of DOI:10.1039/c2cc33179a

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Intramolecular C–H bond activation induced by a scandium terminal
imido complexw
Zhongbao Jian,ab Weifeng Rong,ab Zehuai Mou,ab Yupeng Pan,ab Hongyan Xieab and
Dongmei Cui*ac
Received 3rd May 2012, Accepted 8th June 2012
DOI: 10.1039/c2cc33179a
A CpPN-based scandium terminal imido complex was isolated,
which could induce the intramolecular C–H bond activation of a
phenyl group even at room temperature.
complex based on a Cp ligand has been disclosed to date. On
the other hand, highly reactive terminal metal imido species
have been known to activate intermolecular C–H bonds of
alkane, alkene, arene and pyridine via 1,2-addition across the
MQNR functionality.2a,b,e–i,7,8c,9 However, intramolecular
C–H bond activation reactions induced by terminal metal
imido species are scarce, as far as we are aware.3j,10 In this
contribution, we uncover the first Cp-based scandium terminal
imido complex, which can induce the intramolecular C–H
bond activation of a phenyl group even at room temperature.
The CpPN-type scandium dialkyl complex 1 was synthesized
as reported recently by us.11 Slow addition of 1 equiv. of ArNH2
(Ar = 2,6-iPr2C6H3) to a mixed solution of hexane and toluene
of 1 resulted in the formation of a pale yellow scandium anilide/
alkyl complex (Z5:k1-C5H4–PPh2QN–Ar)Sc(HNAr)(CH2SiMe3)
(2) in a 58% yield (Scheme 1). 2 has good solubility in aromatic
For the last two decades, transition metal complexes containing
terminal imido ligands have gathered an upsurge in research
interest owing to their versatile applications involving in NR
group transfer and catalytic processes.1 To date, numerous
terminal imido complexes based on Group 4,2 5,3a–d 6,3e,f 7,3g
8,3h–j 9,3k 103l and the actinides4 have been isolated and their
reactivity has been extensively explored. Absent from the
above list are the Group 3 rare-earth elements (Sc, Y and
the lanthanides). This may be attributed to the mismatch of
orbital energy between the d0 rare-earth metal ions and the
imido groups,5 which gives the high polarization of this type of
bond that prohibits the formation of a terminal imido LnQN
moiety. Therefore, rare-earth metal imido complexes usually
adopt the more stable bi- or multi-nuclear geometry possessing
bridging NR2À moieties, instead of terminal LnQN species.6
The stalemate was broken in 2008 by Mindiola and co-workers
with the discovery of evidence for the transient existence of a
terminal imidoscandium species attached to a PNP pincer ligand.7
Subsequently, landmark scandium terminal imido complexes
stabilized by tridentate (NNN) or tetradentate (NNNN)
b-diketiminato ligands were isolated and structurally characterized
by Chen and co-workers.8 These findings revealed the crucial role of
a well-designed ligand on the synthesis of a terminal imido complex,
thus greatly stimulating the renaissance of rare-earth metal imido
chemistry. In contrast to the discovery of non-cyclopentadienyl
(Cp) imido complexes,7,8 however, no scandium terminal imido
1
solvents such as benzene and toluene. The H NMR spectrum
of 2 shows a sharp singlet at d = 5.51 ppm attributed to the
NH proton.7,8a,b The Sc–CH2SiMe3 methylene protons give a
resonance at d = 0.11 ppm. Notably, in the 31P NMR spectrum
of 2, a sharp singlet appears at d = 10.78 ppm close to that in 1
(d = 11.11 ppm).11 The solid-state structure of 2 was further
determined by X-ray diffraction analysis (Fig. 1). The
CpPN ligand bonds to the scandium center in a Z5:k1 mode.
a State Key Laboratory of Polymer Physics and Chemistry,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, 5625 Renmin street, Changchun 130022, China.
E-mail: dmcui@ciac.jl.cn; Fax: +86 431 85262774;
Tel: +86 431 85262773
b Graduate School of the Chinese Academy of Sciences,
Beijing 100039, China
c State Key Laboratory of Rare Earth Resources Utilization,
Changchun Institute of Applied Chemistry, Chinese Academy of
Sciences, Changchun, China
w Electronic supplementary information (ESI) available: Experimental
section, NMR and crystallographic data. CCDC 877472 (2) and
877473 (4). For ESI and crystallographic data in CIF see DOI:
10.1039/c2cc33179a
Scheme 1 Synthesis of the scandium complexes 2–4.
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Fig. 2 1H NMR spectra of complexes 2–4 determined in C6D6 at 25 1C.
When the toluene solution of 3 was stirred at room
temperature for one day, we unexpectedly found that the red
solution became colorless. Monitoring the toluene-d8 solution
of 3 revealed that 3 is stable at À30 1C for one week, but
it completely converts to the anilide complex 4 at room
temperature within 24 h (Scheme 1). 4 was collected from
toluene and hexane at room temperature as a colorless crystalline
solid (54%), which exhibits a sharp singlet at d = À5.59 ppm
in the 31P NMR spectrum. In the 1H NMR spectrum of 4,
the anilide proton NH appears at d = 5.88 ppm in C6D6, or at
d = 5.44 ppm in THF-d8; the four new signals at d = 6.02, 6.84,
8.05 and 8.46 ppm in C6D6, or at d = 6.29, 6.55, 7.67 and 7.82 ppm
in THF-d8 can be assigned to the newly generated Sc–C6H4P
Fig. 1 The X-ray structure of 2 with 40% probability of thermal ellipsoids.
Hydrogen atoms are partly omitted for clarity. Selected bond distances (A)
and angles (deg): Sc1–CpCent 2.246, Sc1–N1 2.209(2), Sc1–N2 2.036(3),
Sc1–C42 2.207(4), Sc1ÁÁÁH2 2.432, Sc1–N2–C30 147.7(3), CpCent–Sc1–N1
95.9. (CpCent is the centroid of the cyclopentadienyl ring).
The Sc1–CpCent bond distance (2.246 A) is comparable to that
in 1 (2.260 A).11 The Sc1–N2 bond length (2.036(3) A) from
the anilide group is in good agreement with the analogous
Sc–Nanilide bond lengths in (PNP)Sc(HNAr)(CH3) (2.03 A),
(NNN)Sc(HNAr)(CH3) (2.047(3) A) and (NNNN)Sc(HNAr)-
(CH2SiMe3) (2.054(3) A).7,8a,b
groups,12 which gives the resonance at d = 192.12 ppm (2JPC
=
To obtain the scandium terminal imido complex, the scandium
anilide/alkyl complex 2 was firstly heated in toluene-d8 at 50 1C
for 24 h, but the anticipated proton abstraction reaction did not
take place. According to the previous reports that addition of
external Lewis base can promote the proton abstraction,7,8a
ArNHC (NHC = N-heterocyclic carbene), pyridine or 2-dimethyl-
aminopyridine was therefore added to the reaction system, how-
ever, the reaction still did not happen. Delightfully for us, when
adding 4-dimethylaminopyridine (DMAP) to a toluene solution of
2, the color of the solution changed from yellow to red immediately,
which is a typical feature of the formation of scandium
terminal imido species.8a Monitoring of the reaction in
toluene-d8 by 1H NMR spectrum technique showed that, in the
presence of DMAP, complex 2 converted to the corresponding
terminal imido complex 3 within 20 min at room temperature
or 8 h at À30 1C (Scheme 1). 3 could be easily isolated from a
mixture of toluene/THF at À30 1C as a red crystalline solid
in a 62% yield. In contrast to the good solubility of 2, 3 is
soluble in THF, but sparingly soluble in toluene and benzene.
Notably, the 31P NMR spectrum of 3 gives the resonance at
d = À7.25 ppm shifting upfield remarkably compared with
that of 2 (d = 10.78 ppm). Particularly, in the 1H NMR
spectrum of 3, the signals of NH proton and Sc–CH2SiMe3
group unambiguously disappear (Figs 2 and S2w), meanwhile
the signals of the two coordinated DMAP molecules show up
and those of isopropyl groups also change correspondingly.
The solid-state structure of 3 was characterized by X-ray
diffraction, but unfortunately the crystallographic data are
not satisfactory.
43.5 Hz) in the 13C NMR spectrum, which is typical for the
metalated aromatic carbon (Sc–C6H4P).7,12 X-ray diffraction
analysis further confirmed the molecular structure of 4 as
[Z5-C5H4–P(Z1-C6H4)PhQN–Ar]Sc(NHAr)(DMAP)2 (Fig. 3),
Fig. 3 The X-ray structure of 4 with 40% probability of thermal
ellipsoids. Hydrogen atoms are partly omitted for clarity. Selected
bond distances (A) and angles (deg): Sc1–CpCent 2.219,
Sc1–C32
2.321(2),
Sc1–N6
2.129(2),
Sc1–N1
2.354(2),
Sc1–N3 2.343(2), Sc1–N6–C20 148.3(2).
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a product of the intramolecular C–H bond activation of phenyl
group induced by the highly reactive ScQN fragment. The
bond distance of Sc1–C32 (2.321(2) A) is slightly longer than
that of Sc–C(Ph) (2.275(2) A),7 but the distance of Sc1–N6
bond (2.129(2) A) is clearly longer than those of Sc–Nanilide
bond (1.986(2) A–2.054(3) A).7,8a,b By contrast, addition of a
large excess of external benzene, Ph3P or pyridine to the
toluene-d8 solution of terminal imido complex 3 at room
temperature did not result in the intermolecular activation of
C–H bonds. To our knowledge, this is the first example of
intramolecular C–H bond activation explicitly induced by a
rare-earth metal terminal imido species.
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In summary, we have isolated the CpPN-based scandium
terminal imido complex at room or lower temperature. This
imidoscandium complex can induce the intramolecular C–H
bond activation of a phenyl group at room temperature to
form the scandium anilide complex, but is incapable of
promoting the intermolecular activation of C–H bonds, such
as those of benzene, pyridine and Ph3P, at room temperature.
This work was partly supported by The National Natural
Science Foundation of China for project Nos. 20934006,
51073148 and 51021003 and The Ministry of Science and
Technology of China for project No. 2011DFR50650. We
are grateful to Prof. Ninghai Hu for crystal analysis.
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