Scandium terminal imido complex induced C-H bond selenation and formation of an Sc-Se bond

Article abstract of DOI:10.1039/c0cc03212c

The reaction between scandium terminal imido complexes and elemental selenium showed an unprecedented C-H bond selenation and the formation of an Sc-Se bond. The Royal Society of Chemistry 2011.

Full text of DOI:10.1039/c0cc03212c

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Scandium terminal imido complex induced C–H bond selenation
and formation of an Sc–Se bondw
Erli Lu,a Jiaxiang Chu,a Yaofeng Chen,*a Maxim V. Borzovb and Guangyu Lia
Received 12th August 2010, Accepted 13th October 2010
DOI: 10.1039/c0cc03212c
The reaction between scandium terminal imido complexes and
elemental selenium showed an unprecedented C–H bond selena-
tion and the formation of an Sc–Se bond.
temperature during 3.5 h. The reaction was then scaled up with
toluene as a solvent and the complex 2 was isolated as
brownish yellow crystals (Scheme 1). Complex was
2
characterized by H, 13C and 77Se NMR, elemental analysis
and X-ray crystallography, indicating that 2 is a scandium
anilide selenolate, in which one sp2 carbon at the ortho-
position of the DMAP ligand was selenolated. The Sc(III)
center adopts a pseudo-octahedral geometry with three
nitrogen atoms of the tridentate nitrogen ligand and a
selenium atom of the 2-selenolated DMAP forming the
equatorial plane, and a nitrogen atom of the anilide ligand
and a nitrogen atom of the 2-selenolated DMAP occupying the
apical positions (Fig. 1). The anilide ligand coordinates to the
metal ion with the Sc–Nanilide distance being 2.074(4) A, which
is close to that in a scandium anilide alkyl complex (2.047(3) A)
and is significantly longer than the Sc–Nimido distance (1.881(5) A)
in the scandium terminal imido complex 1.11 The 2-selenolated
DMAP serves as a monoanionic bidentate ligand with the Se
atom at cis-position to the anilide ligand and the N atom in
trans-position to the anilide ligand. The Sc–Se distance is
2.7654(11) A. To the best of our knowledge, complex 2 is the
first X-ray characterized scandium selenolate.12 The
Sc–NDMAP distance is 2.270(4) A, which is very close to that
in 1 [2.271(5) A].
1
Alkyl, amide and phosphide complexes of rare-earth metals
have been extensively studied during the last three decades.
These complexes exhibit diverse structural properties and rich
reactivities.1–3 However, chemistry of their counterparts, the
terminal alkylidene, imido and phosphinidene complexes,
which contain typical rare-earth metal main-group element
double bonds, remain unexplored. The absence of rare-earth
metal terminal alkylidene, imido and phosphinidene complexes
is mainly due to the relative mismatch in LUMO/HOMO
orbital energies between the d0 rare-earth metal ions and the
alkylidene (imido or phosphinidene) groups.4 These complexes
once formed easily assemble into more stable m or mn (n = 3, 4)
bridged bimetallic or multi-metallic species. Thus, up to 2009,
only a few examples of bridged alkylidene,5 imido6 and
phosphinidene7 complexes, phosphorus-stabilized ‘‘pincer’’
type alkylidene complexes,8 or imidazolin-2-iminato
complexes9 of rare-earth metals have been reported.
Meanwhile, the increased Lewis acidity of rare-earth metal
ions compared to that of group 4, 5 and 6 metal ions and highly
active LnQE (Ln = rare-earth metal; E = C, N, P) double
bond should lead to fascinating reactivities. The b-
diketiminato rare-earth metal complexes have attracted a
growing attention in the last decade.10 Recently, we had
The formation of 2 is intriguing. As the terminal imido
complex 1 is thermostable even at 70 1C for 24 h, it is unlikely
that 2 is formed via a DMAP Hortho abstraction to generate an
anilide pyridyl complex and followed by an insertion of
selenium into the Sc–C bond of the anilide pyridyl complex.
We propose that the formation of 2 might proceed via a [2 + 1]
cycloaddition intermediate, where one selenium atom adds
onto the ScQN bond of the terminal imido complex 1 to
form a species containing a Sc–N–Se three-membered ring
(Scheme 2). This intermediate is highly unstable due to the
instability of the [N–Se]2À functional group13 and the strain in
the Sc–N–Se three-membered ring and subsequent
rearrangements involving N–Se bond cleavage, the transfer
of Hortho of DMAP to the N atom and Cortho of DMAP to the
Se atom lead to the final product 2. It is also noteworthy that
the experiment showed DMAP alone does not react with
developed
a
type of b-diketiminato-based tridentate
ligands10d and synthesized the first terminal imido complex
of a rare-earth metal, [MeC(NAr)CHC(Me)(NCH2CH2NMe2)
(DMAP)ScQNAr] (DMAP = 4-dimethylaminopyridine, Ar =
2,6-iPr2C6H3) (1), by employing one of these ligands.11 Herein,
we report the reactivities of scandium terminal imido complexes
towards elemental selenium, which induce an C–H bond
selenation and the formation of an Sc–Se bond.
1H NMR spectral monitoring of the reaction of 1 with one
equivalent of elemental selenium in C6D6 showed that 1 was
nearly completely converted into a new complex 2 at ambient
a State Key Laboratory of Organometallic Chemistry, Shanghai
Institute of Organic Chemistry, Chinese Academy of Sciences,
354 Fenglin Road, Shanghai 200032, China.
E-mail: yaofchen@mail.sioc.ac.cn; Fax: +86-21-64166128
b Key Laboratory of Synthetic and Natural Chemistry of the Ministry
of Education, College of Chemistry and Material Science,
the North-West University of Xi’an, Taibai Bei avenue 229,
Xi’an 710069, Shaanxi Province, China
w Electronic supplementary information (ESI) available: Synthesis
details and characterization data for 2–6, molecular structures of 3
and 4, and CIF files giving X-ray crystallographic data for 2–6. CCDC
785930, 785932–785934. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c0cc03212c
Scheme 1 Synthesis of the complex 2.
c
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Fig. 1 Molecular structure of 2 with thermal ellipsoids at 30%
probability level. Isopropyl groups at the Ar substituents, solvent
molecule, minor component of the disordered C6, C7, C31 and C32
atoms, and hydrogen atoms (except of the anilide hydrogen atom) are
omitted for clarity. Selected bond distances (A) and angles (1): Sc–N6
2.074(4), Sc–Se 2.7654(11), Sc–N4 2.270(4), Sc–N1 2.213(4), Sc–N2
2.178(4), Sc–N3 2.427(4); Sc–N6–C40 156.1(4).
Fig. 2 Molecular structure of 5 with thermal ellipsoids at 30%
probability level. Isopropyl groups at the Ar substitutents and
hydrogen atoms are omitted for clarity. Selected bond distances (A)
and angles (1): Sc–N5 1.8591(18), Sc–N1 2.2398(19), Sc–N2 2.2087(19),
Sc–N3 2.3731(19), Sc–N4 2.369(2); Sc–N5–C36 167.90(17).
(Fig. S1 and S2). The structural data reveal that the ligand L
serves as a monoanionic tridentate ligand and the –NMe2 group
of the ligand does not coordinate to the scandium ion in neither
3 nor 4. In the synthesis of our previously reported scandium
terminal imido complex 1 from its anilide alkyl precursor, the
presence of external Lewis base (DMAP) is essential. Unlike in
the case of the formation of 1, heating the scandium anilide alkyl
complex 4 alone in hexane at 50 1C for three days gives a new
scandium terminal imido complex 5 in 48% yield. The solid-
state structure of 5 was established by X-ray crystallography
(Fig. 2). The metal center in 5 is in a distorted square pyramid
coordination environment. Of interest, four nitrogen atoms of L
form the basal plane and a nitrogen atom of the imido ligand
occupies the apical position. L serves as a monoanionic ligand,
and the distances from the metal center to four nitrogen atoms
of L are 2.2398(19), 2.2087(19), 2.3731(19) and 2.369(2) A,
respectively. The Sc–Nimido distance [1.8591(18) A] is much
shorter than the Sc–Nanilide distance [2.054(3) A] in the
corresponding scandium anilide alkyl complex 4. The
Sc–Nimido–C angle [167.90(17)1] is significantly larger than the
Scheme 2 Proposed [2 + 1] cycloaddition intermediate.
elemental selenium either at ambient temperature or even at
50 1C.
It is important to elucidate the reaction pattern of the
scandium terminal imido complex with elemental selenium in
the absence of DMAP ligand. To this end, a tetradentate
nitrogen ligand precursor (LH) was designed and synthesized
(Scheme 3). The alkane elimination reaction between LH and
Sc(CH2SiMe3)3(thf)2 in hexane afforded a scandium dialkyl
complex 3 in 73% yield. The protonolysis of 3 with 2,6-
diisopropylaniline in toluene gave a scandium anilide alkyl
complex 4 in 44% yield. Single crystals of 3 and 4 were
obtained and characterized by X-ray diffraction studies
and the molecular structures of 3 and 4 are given in ESIw
Sc–Nanilide–C angle in
4 [147.8(2)1]. The complex 5 is
thermostable at room temperature for several days and very
slowly decomposes at 50 1C to generate a new complex
{MeC[ArN]CHC[Me][NCH2CH2N(Me)]Sc(NHAr)}2
along
with the release of N,N-dimethyl vinylamine via C–N bond
cleavage.14
Reaction of 5 with one equivalent of elemental selenium in
C6D6 was monitored by 1H NMR, which indicated that 5 was
nearly completely converted into a new complex 6 at ambient
temperature after 12 h. The reaction was scaled up (toluene as
a solvent) and complex 6 was isolated as a yellow solid
(Scheme 4). Complex 6 was characterized by 1H, 13C and
Scheme 3 Synthesis of complexes 3, 4 and 5.
744 Chem. Commun., 2011, 47, 743–745
Scheme 4 Synthesis of the complex 6.
c
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molecule and hydrogen atoms (except of the anilide hydrogen atom)
are omitted for clarity. Selected bond distances (A) and angles (1):
Sc–N5 2.051(6), Sc–Se 2.7360(15), Sc–N1 2.260(5), Sc–N2 2.208(5),
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This work was supported by the National Natural Science
Foundation of China (Grant Nos. 20872164, 20821002 and
21072209) and Chinese Academy of Sciences.
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c
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Chem. Commun., 2011, 47, 743–745 745