Non-Pincer-Type Mononuclear Scandium Alkylidene Complexes: Synthesis, Bonding, and Reactivity

Article abstract of DOI:10.1002/chem.201504725

The first non-pincer-type mononuclear scandium alkylidene complexes were synthesized and structurally characterized. These complexes exhibited short Sc-C bond lengths and even one of the shortest reported to date (2.1134(18) ?). The multiple character of

Full text of DOI:10.1002/chem.201504725

DOI: 10.1002/chem.201504725
Communication
&
Organometallic Chemistry
Non-Pincer-Type Mononuclear Scandium Alkylidene Complexes:
Synthesis, Bonding, and Reactivity
[
a]
[a]
[b]
[b]
[a]
Chen Wang, Jiliang Zhou, Xuefei Zhao, Laurent Maron,* Xuebing Leng, and
[a]
Yaofeng Chen*
Dedicated to Professor Xuelong Hou on the occasion of his 60th birthday
2À
phosphinoyl) methandiide dianion [C(PPh NSiMe ) ] into the
2
3 2
rare-earth metal chemistry and synthesized a pincer-type sa-
Abstract: The first non-pincer-type mononuclear scandi-
um alkylidene complexes were synthesized and structural-
ly characterized. These complexes exhibited short ScÀC
bond lengths and even one of the shortest reported to
date (2.1134(18) Š). The multiple character of the ScÀC
bond was highlighted by a DFT calculation. This was con-
firmed by experimental reactivity study where the com-
plex underwent [2+1] cycloaddition with elemental seleni-
um and [2+2] cycloaddition with imine. DFT calculation
also revealed a strong nucleophilic behavior of the alkyli-
dene complex that was experimentally demonstrated by
the CÀH bond activation of phenylacetylene.
[
5]
marium carbene complex [Sc{C(PPh NSiMe ) }(NCy )(THF)].
2
3 2
2
Later, Liddle, Le Floch, MØzailles, and Nief made important con-
[
6–8]
tributions to this field:
Liddle and Le Floch synthesized
other pincer-type rare-earth metal carbene complexes with the
[
7]
bis(iminophosphinoyl) methandiide dianion and Le Floch,
MØzailles, and Nief developed the di-lithium bis(thiophosphi-
noyl) methandiide Li [C(PPh S) ] and synthesized pincer-type
2
2
2
[
8]
rare-earth metal carbene complexes based on this dianion.
Introducing the pincer-type structure with two hypervalent
phosphorous substituents stabilizes the mononuclear rare-
earth metal alkylidene (or carbene) complexes, but simultane-
[
9]
ously reduces their reactivity. Reactivity studies of these com-
plexes were mainly focused on ketones and the highly reactive
heteroallenes, such as isocyanide, isothiocyanide, and carbodi-
[
10]
Mononuclear transition metal alkylidene (or carbene) com-
plexes have attracted intense attention and been extensively
imide. How to balance stabilization and reactivity of mono-
nuclear rare-earth metal alkylidene (or carbene) complexes re-
mains a big challenge. Herein, we report the synthesis, bond-
ing analysis (DFT), and reactivity (cycloaddition and CÀH bond
activation) of the first non-pincer-type mononuclear scandium
alkylidene complexes.
[1]
studied in past decades. Such transition metal complexes are
not only of fundamental interest for coordination chemistry,
but are also known to have important applications, especially
in the area of synthetic chemistry. Therefore, a great number
of mononuclear transition metal alkylidene (or carbene) com-
plexes have been synthesized. One exception is those with
rare-earth metal ions. Due to HOMO/LUMO orbital energies
Scandium
methyl
chlorides
[L1Sc(Me)Cl]
(L1=
À
[MeC(NDIPP)CHC(Me)(NCH CH N(iPr) )] , DIPP=2,6-(iPr) C H )
2
2
2
2
6
3
À
and [L2Sc(Me)Cl] (L2=[MeC(NDIPP)CHC(Me)(NCH CH N(Me) )] )
2
2
2
0
[11a]
mismatch between the d rare-earth metal ions and the alkyli-
were
treated
with
11b] 1
Li[CH(SiMe )PPh S]
or
3
2
[
dene (or carbene) groups, the formation of mononuclear rare-
Li[CH(SiPh )PPh S](THF).
H NMR spectral monitoring of the
reactions in C D showed the formation of new complexes 1–4
3
2
[
2]
earth metal alkylidene (or carbene) complexes is unfavorable.
6
6
The alkylidene (or carbene) groups have a strong tendency to
bind more than one rare-earth metal ion or one rare-earth
with concomitant appearance of CH (d=0.15 ppm) at room
4
temperature. The scaled-up reactions in toluene provided the
complexes 1–4 in 58–87% yields. The complexes 1–4 were
characterized by NMR spectroscopy and elemental analysis;
the solid-state structures of 1–3 were determined by crystallo-
graphic methods. The complexes 1–4 are mononuclear scandi-
um alkylidene complexes [LSc{C(SiR’ )PPh S}] (1: L=L1, R’=
[3,4]
metal ion and two other metal ions.
For the purpose of sta-
bilization of mononuclear rare-earth metal alkylidene (or car-
bene) complexes, Cavell and co-workers introduced a bis(imino-
[
a] C. Wang, Dr. J. Zhou, Dr. X. Leng, Prof. Dr. Y. Chen
State Key Laboratory of Organometallic Chemistry
Shanghai Institute of Organic Chemistry
Chinese Academy of Sciences
3
2
Me; 2: L=L1, R’=Ph; 3: L=L2, R’=Me; 4: L=L2, R’=Ph) as
shown in Scheme 1.
The molecular structure of 1 is shown in Figure 1, while
those of 2 and 3 are given in the Supporting Information (Fig-
3
45 Lingling Road, Shanghai 200032 (P. R. China)
E-mail: yaofchen@mail.sioc.ac.cn
À
À
[
b] X. Zhao, Prof. Dr. L. Maron
LPCNO, CNRS & INSA
ures S1 and S2). In 1, 2, and 3, the [L1] or [L2] ligand coordi-
nates to the scandium center through three nitrogen atoms;
UniversitØ Paul Sabatier
2À
2À
the [C(SiMe )PPh S] or [C(SiPh )PPh S] dianion coordinates
3
2
3
2
1
35 Avenue de Rangueil, 31077 Toulouse (France)
to the scandium center through carbon and S atoms. The ScÀ
C bond lengths are 2.125(2), 2.159(4), and 2.1134(18) Š, respec-
tively. It is noteworthy that the ScÀC bond lengths in the less
E-mail: laurent.maron@irsamc.ups-tlse.fr
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201504725.
Chem. Eur. J. 2016, 22, 1258 – 1261
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
ing the propensity of the latter to deal with such a complex.
The ScÀC bonding situation was scrutinized by means of natu-
ral bonding orbital (NBO) calculations, natural population anal-
ysis (NPA) and Wiberg bond index determination on the opti-
mized structure. The analyses were previously used by Liddle
[
7a]
[10a]
et al. or MØzailles et al.
to propose a multiple character of
their supported YÀC or ScÀC bond. NPA indicates that the
charge on scandium is +1.4, whereas the charge on carbon is
À1.9. The latter clearly indicates that the carbon is strongly nu-
cleophilic, in line with a Schrock-type character. The NBO anal-
Scheme 1. Synthesis of complexes 1–4.
ysis reveals at the first order a strongly polarized ScÀC s bond
2
(
11% 3d(Sc)–89% sp (C)) and a pure p lone pair on the carbon
atom. At the second order NBO, the latter lone pair interacts
strongly with an empty 3d orbital of the metal center in a p
À1
fashion (82.1 kcalmol ). This donor–acceptor interaction is
very strong and much stronger than the one reported by MØ-
À1
zailles et al. (39 kcalmol reported and computed with our
À1 [10a]
method compared to 19.7 kcalmol ).
This difference
matches perfectly the difference in computed bond lengths of
the ScÀC bond (2.10 vs. 2.19). Finally, the Wiberg bond index
(
WBI) of the ScÀC bond is 0.73 (0.60 in MØzailles’ complex), in
line with a polarized s bond and a donor–acceptor p interac-
tion. Therefore, the complex 3 can be described as a scandium
alkylidene complex with Sc=C double-bond character (no sig-
nificant ylidic character could be found). The correctness of
this bonding description is definitively demonstrated by the
experimental reactivity study of complex 1.
Figure 1. Molecular structure of complex 1 (ball and stick representation).
DIPP isopropyl groups and hydrogen atoms were omitted for clarity. Select-
ed bond distances [Š] and angles [8]: ScÀN1 2.2418(16), ScÀN2 2.1463(17),
ScÀN3 2.5648(17), ScÀC26 2.125(2), ScÀS 2.5520(7), Sc-C26-Si 132.47(11), Sc-
C26-P 95.73(9), Si-C26-P 130.27(12), SC26=358.5.
Indeed, as expected for a Sc=C double bond, complex 1 un-
dergoes a [2+2] cycloaddition reaction with benzylidene-
methanamine at 508C to give complex 5 in 70% yield, as
shown in Scheme 2. This is reminiscent of the reaction pattern
of the early-transition-metal alkylidene complexes (or the
Schrock carbene complexes). Complex 5 was characterized by
NMR spectroscopy, elemental analysis, and single crystal X-ray
diffraction. In 5, the newly formed [C(CPhNMe)(Si-
bulky complexes 1 and 3, 2.125(2) and 2.1134(18) Š, are much
shorter than those in the pincer-type scandium carbene
[
10a]
complexes
[Sc{C(PPh S) }(bpy)Cl]
(2.207(3) Š)
and
2
2
[8c]
[
Sc{C(PPh S) }{CH(PPh S) }THF] (2.204(4) Š), and those in the
2
2
2
2
scandium m-methylidene complexes [((NN)Sc) (m -CH )(m -
2
2
2
2
À
PDIPP)] (NN=[MeC(NDIPP)CHC(NDIPP)Me] ) (2.213 Š in the
[
4h]
average)
and [(PNP)Sc(m -CH )(m -CH ) {Al(CH ) } ] (PNP=
3
À
2
3
3 2
3 2 2
[4d]
[
N{2-P(iPr) -4-methylphenyl} ] ) (2.317(2) Š).
To the best of
2
2
our knowledge, the ScÀC bond length of 2.1134(18) Š in 3 is
[12]
the shortest ScÀC bond length observed to date. As expect-
ed, the carbon atom of alkylidene group displays a trigonal
planar geometry with SC of 358.58 in 1, 359.08 in 2 and 358.88
1
3
1
in 3. Complexes 1–4 are also featured in their C{ H} NMR
spectra. The alkylidene carbon signals appear at d=117.0, 92.9,
1
12.0, and 92.3 ppm in C D for 1–4, respectively, which are
6 6
greatly downshifted in comparison with those of the alkyl
[
13]
carbon in Li[CH(SiMe )PPh S] (11.7 ppm in C D )
or
2
3
2
6
6
Li[CH(SiPh )PPh S](THF) (8.4 ppm in [D ]THF), in line with a sp
3
2
8
carbon.
To better understand the nature of the ScÀC bond in these
scandium alkylidene complexes and to compare with previous-
ly reported pincer-type ones, DFT (B3PW91) calculations were
carried out (see the Supporting Information for computational
details) on complex 3. The optimized geometry of complex 3
is in excellent agreement with the experimental structure. In
particular, the short ScÀC bond distance of 2.11 Š is very well
reproduced by the computational method (2.10 Š), highlight-
Scheme 2. Reactions of complex 1 with imine, selenium, and phenylacety-
lene.
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Communication
Figure 3. Molecular structure of complex 6 (ball and stick representation).
DIPP isopropyl groups and hydrogen atoms were omitted for clarity. Select-
ed bond distances [Š] and angles [8]: Sc1ÀN1 2.1572(19), Sc1ÀN2 2.1070(19),
Sc1ÀC26 2.312(2), Sc1ÀSe1 2.5362(7), Sc1ÀS1 2.5104(7), C26ÀSe1 1.990(2),
Sc1-Se1-C26 60.01(7), Sc1-C26-Se1 71.80(7), Sc1-C26-P1 91.48(10), Se1-C26-
Si1 113.20(12), Si1-C26-P1 130.18(13).
Figure 2. Molecular structure of complex 5 (ball and stick representation).
DIPP isopropyl groups and hydrogen atoms were omitted for clarity. Select-
ed bond distances [Š] and angles [8]: ScÀN1 2.202(4), ScÀN2 2.105(4), ScÀN4
1
1
1
.969(4), ScÀC26 2.423(5), ScÀS 2.5585(17), C42ÀN4 1.452(7), C26ÀC42
.583(7), Sc-C26-C42 81.4(3), Sc-C26-P 91.3(2), C42-C26-Si 108.7(3), Si-C26-P
24.4(3).
2À
Me )PPh S] dianion coordinates to the scandium center
3
2
through nitrogen, carbon, and sulfur atoms (Figure 2). The ScÀ
C bond length of 2.423(5) Š is significantly longer than that
observed for 1 (2.125(2) Š), and the carbon atom involved in
the ScÀC bond displays a tetrahedron geometry, demonstrat-
2
3
ing a transformation of the sp carbon in 1 into the sp carbon
in 5. The C42ÀN4 bond (1.452(7) Š) is consistent with a single
bond, and the ScÀN4 bond (1.969(4) Š) is in accordance with
the anionic nature of the N4 atom in 5. Due to the increase of
steric congestion caused by the replacement of [C(Si-
2
À
2À
Me )PPh S] with the [C(CPhNMe)(SiMe )PPh S] dianion, the
Figure 4. Molecular structure of complex 7 (ball and stick representation).
DIPP isopropyl groups and hydrogen atoms (except the H26 atom) were
omitted for clarity. Selected bond distances [Š] and angles [8]: ScÀN1
3
2
3
2
amino side arm of the tridentate nitrogen ligand (L1) is no
longer coordinated to the scandium center. More interestingly,
complex 1 reacts with elemental selenium at room tempera-
ture to give a [2+1] cycloaddition product 6 in 79% yield. Al-
though there are reports on the reactions of organolantha-
2
.165(2), ScÀN2 2.069(2), ScÀC26 2.408(3), ScÀC42 2.226(3), ScÀS 2.5815(9),
C42ÀC43 1.216(4), Sc-C26-Si 120.99(13), Sc-C26-P 93.44(11), Si-C26-P
127.17(16), Sc-C42-C43 175.7(2).
[14]
nides with elemental selenium to form LnÀSe bonds, the ob-
served [2+1] cycloaddition is unique and complex 6 is the first
rare-earth metal complex containing a Ln-Se-C three-mem-
bered ring. The isolation of 6 also supports our previous as-
sumption that the reactions of scandium terminal imido com-
plexes with selenium to give the CÀH bond selenation prod-
88% yield, which resembles the reactivity of scandium terminal
[
16]
imido complex. The single crystals of 7 (see Figure 4) were
[
17]
obtained and analyzed by the X-ray diffraction method. The
isolation and crystallographic characterization of 7 allows the
structural comparison between the scandium alkylidene com-
plex 1 and the corresponding scandium alkyl complex 7. It was
observed that the ScÀC bond length in 1 (2.125(2) Š) is 0.28 Š
shorter than that in 7 (2.408(3) Š); such difference in bond
length is even larger than that observed between the scandi-
um terminal imido complex and the scandium anilido complex
[
15]
ucts might proceed via a [2+1] cycloaddition intermediate.
Complex 6 was also characterized by single-crystal X-ray dif-
fraction. There are two crystallographically independent mole-
cules in the unit cell of 6; the structure of molecule one is
shown in the Figure 3. The distances from selenium to scandi-
um and carbon are 2.5362(5) and 1.990(2) Š (or 2.5408(5) and
[
18]
(0.16 Š difference). The sum of Sc-C26-Si, Sc-C26-P, and Si-
1
6
.990(2)), respectively; the Sc-Se-C angle of 60.01(7)8 (or
0.58(7)8) is small. As expected, the ScÀC bond length in 6
C26-P angles in 1 (358.58) is 178 larger than that in 7 (341.6). In
13
1
the C{ H} NMR spectra, the alkylidene carbon signal for 1 (d=
117.0 ppm) is much downshifted in comparison with the alkyl
carbon for 7 (d=27.2 ppm).
In summary, the non-pincer-type mononuclear scandium al-
kylidene complexes 1–4 were synthesized from reactions of
the b-diketiminato-based tridentate-ligand-supported scandi-
(
2.312(2) or 2.334(2) Š) is longer than that in 1, and the carbon
atom involved in the ScÀC bond displays tetrahedral geome-
try.
Complex 1 rapidly undergoes CÀH bond activation with
phenylacetylene at room temperature to provide complex 7 in
Chem. Eur. J. 2016, 22, 1258 – 1261
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ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Communication
um
methyl
chlorides
with
Li[CH(SiMe )PPh S]
or
Zhou, T. F. Li, L. Maron, X. B. Leng, Y. F. Chen, Organometallics 2015, 34,
3
2
4
70–476.
5] K. Aparna, M. Ferguson, R. G. Cavell, J. Am. Chem. Soc. 2000, 122, 726–
27.
Li[CH(SiPh )PPh S]. X-ray diffraction analysis showed very short
3
2
[
ScÀC bond lengths, ranging from 2.1134(18) to 2.159(4) Š. The
multiple bond character as well as the strong nucleophilic be-
havior of the alkylidene group was underlined by NBO and
NPA calculations but also demonstrated experimentally. Be-
sides the cycloaddition reactions with imine and elemental se-
lenium, the mononuclear scandium alkylidene complex is also
able to activate the CÀH bond of phenylacetylene. Further
work to explore and enlarge the reactivity scope of these and
other non-pincer-type mononuclear rare-earth alkylidene com-
plexes is currently ongoing.
7
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[
This work was supported by the State Key Basic Research &
Development Program (Grant No. 2012CB821600), the National
Natural Science Foundation of China (Grant Nos. 21132002,
2
[
10] a) M. Fustier-Boutignon, X. F. Le Goff, P. Le Floch, N. MØzailles, J. Am.
Chem. Soc. 2010, 132, 13108–13110; b) D. P. Mills, L. Soutar, W. Lewis,
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2
1325210, and 21421091), and the Chinese Academy of Scien-
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The Humboldt foundation, the Chinese Scholarship Council
and the Chinese Academy of Science are also acknowledged
as well as CalMip for computing time.
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[
12] There is a report of ScÀC bond length of 2.115(9) Š for the ion pair
[
Cp’ScCH SiMe (THF) ][B(C ] (Cp’=C Me PhOMe), but the alkyl group
2
3
2
6
F
5
)
4
5
4
of the molecule is highly disordered, see: a) X. F. Li, M. Nishiura, L. H.
Hu, K. Mori, Z. M. Hou, J. Am. Chem. Soc. 2009, 131, 13870–13882; the
other shortest ScÀC bond length reported is 2.133(3) Š for an ion pair
Keywords: alkylidene · cycloaddition · density functional
calculations · multiple bonds · scandium
[{tBuC(NDIPP)CHC(NDIPP)tBu}ScCH
2
SiMe
2
CH
2
SiMe
3
][CH
3
B(C
6
F
5
3
) ],
see:
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