A double addition of Ln-H to a carbon-carbon triple bond and competitive oxidation of ytterbium(II) and hydrido centers

Article abstract of DOI:10.1002/anie.201108662

Addition of two Ln-H bonds of an Yb^II hydrido complex supported by bulky amidinate ligand to a Ci-C bond lead to the formation of 1,2-dianionic bibenzyl fragment (see picture). Both Yb^II and hydrido centers are oxidized under the reaction conditions. The resulting Yb^II- ·⁶-arene interaction is surprisingly robust: the arene cannot be replaced from the metal coordination sphere when treated with Lewis bases. Copyright

Full text of DOI:10.1002/anie.201108662

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DOI: 10.1002/anie.201108662
Hydrides of Lanthanides
À
A Double Addition of Ln H to a Carbon–Carbon Triple Bond and
Competitive Oxidation of Ytterbium(II) and Hydrido Centers**
Ivan V. Basalov, Dmitry M. Lyubov, Georgy K. Fukin, Andrei S. Shavyrin, and
Alexander A. Trifonov*
The unique reactivity and catalytic activity of organolantha-
nide hydrido complexes in various transformations of unsa-
turated substrates has given a powerful incentive to develop-
ment this chemistry.[1] Although rare-earth hydrides have
been known since the 1980s,[2] until very recently they were
represented exclusively by LnIII derivatives. The first struc-
turally defined LnII hydrido complex supported by bulky
hydrotris(pyrazolyl)borate ligand was published by Takats
et al. in 1999.[3] The second example of a YbII hydride involves
a complex coordinated by a DIPP-nacnac ligand.[4] Despite
the fact that some classes of stoichiometric reactions of
[{(TptBu,Me)Yb(m-H)}2][5] and the catalytic activity of [{(DIPP-
nacnac)YbH(thf)}2] in 1,1-diphenylethylene hydrosilylation[4]
have been described, the reactivity of LnII hydrido species still
remains poorly investigated. Indeed, LnII hydrido complexes
containing two reaction centers are of great interest for
investigation of reactivity patterns. The presence of a low-
valent YbII ion and hydrido ligand provides a basis for a rich
1 with PhSiH3 in hexane (208C, 12 h) allowed the synthesis of
the hydrido complex 2 in 88% yield (Scheme 1). The reaction
afforded PhSiH2N(SiMe3)2 as a byproduct, which was isolated
in 76% yield. The blackish-violet crystals of 2 are highly air-
and moisture-sensitive crystals that are poorly soluble in
aliphatic and aromatic hydrocarbons.
Scheme 1. Synthesis of hydride complex 2.
The X-ray diffraction study revealed that complex 2, in
a similar fashion to the previously reported [{(TptBu,Me)Yb(m-
H)}2][3] and [{(DIPP-nacnac) YbH(thf)}2],[4] adopts a dimeric
À
redox chemistry of these complexes. The Yb H bond can
undergo insertions of multiple bonds and s-bond metathesis
reactions with reagents containing acidic X H bonds. More-
structure owing to the two hydrido ligands m2-bridging two
À
II
À
À
over the Yb ion can coordinate Lewis bases, while the basic
Yb centers (Figure 1). The Yb H bond lengths and Yb Yb
distance in 2 (2.14(4), 2.18(5), and 3.3553(4) ꢀ, respectively)
are close to the values previously reported for the dimeric
YbII hydrido complexes.[3,4]
À
M H bond can form complexes with Lewis acids. Formerly
we synthesized new classes of rare-earth (III) hydrido and
alkyl hydrido species containing guanidinate, amidopyridi-
nate, amidomethylpyridinate, and linked amidinate ligands.
These compounds demonstrated high catalytic activities in
olefin polymerization and hydrosilylation.[6] Herein we pres-
ent the synthesis and the structure of the novel YbII hydrido
complex [{[tBuC(NC6H3-2,6-iPr2)2]Yb(m-H)}2], supported by
a bulky amidinate ligand, and new types of reactions observed
The amidinate ligands in 2 chelate the YbII ions by k1-
amide and h6-arene interactions, leading to a coordination
mode that is different from the k2-N,N’-chelating mode
detected in the parent complex 1. The k1-N,h6-arene-chelating
coordination mode was previously observed in the YbII
complex with the bulky guanidinate ligand [{[Cy2NC-
À
for hydrido species: oxidation reactions and double Ln H
ꢀ
addition to a C C bond.
Amido complex [{tBuC(NC6H3-2,6-iPr2)2}YbN(SiMe3)2-
(thf)] (1)[7] was used as a precursor for the synthesis of related
hydrido species (for the molecular structure of 1, see the
Supporting Information). The s-bond metathesis reaction of
[*] I. V. Basalov, Dr. D. M. Lyubov, Dr. G. K. Fukin, Dr. A. S. Shavyrin,
Prof. Dr. A. A. Trifonov
G. A. Razuvaev Institute of Organometallic Chemistry of Russian
Academy of Sciences
Tropinina 49, GSP-445, 603950 Nizhny Novgorod (Russia)
E-mail: trif@iomc.ras.ru
[**] This work was supported by the Russian Foundation for Basic
Research (Grant No 11-03-97030-p), Grant of President of R.F. for
young scientists MK-4908.2012.3.
Figure 1. ORTEP of 2 with ellipsoids set at 30% probability; non-
hydride hydrogen atoms and iPr substituents are omitted for clarity.
Selected bond lengths [ꢀ]: Yb1–H1 2.14(4), Yb1–H1a 2.18(5), Yb1–N2
2.329(3), Yb1–Arcentroid 2.410(5), Yb1–Yb1a 3.3553(4), N1–C1 1.330(5),
N2–C1 1.370(5).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201108662.
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(NAr2)]Yb(m-I)}2].[8] The Yb N and Yb Ar centroid distan-
ces in 2 (2.329(3), 2.420(4) ꢀ) are close to the related values in
[{[Cy2NC(NAr2)]Yb(m-I)}2] (2.360(3), 2.424(4) ꢀ).[8] Despite
the presence of THF in the reaction mixture and the highly
oxophilic nature of the Yb ion, 2 crystallizes as THF-free
species, whereas in [{[Cy2NC(NAr2)]Yb(m-I)}2][8] the Yb-h6-
arene interaction is labile and the arene ligand can be easily
replaced by a THF molecule in the metal coordination sphere.
The chemical shift of the signal for the hydrido ligands for
2 (7.74 ppm) is shifted noticeably upfield compared to those
for the related YbII derivatives in an N,N-coordination
environment: [{(TptBu,Me)YbH}2] (10.5 ppm)[3] and [{(DIPP-
nacnac)YbH(thf)}2] (9.92 ppm).[4] This can be explained by
electron density donation from aromatic p system of the 2,6-
diisopropylphenyl ring to the ytterbium atom. Unfortunately
low solubility of 2 in C6D8 did not allow to record 13C and
171Yb spectra of 2.
À
À
Scheme 2. Formation of bis(dithiocarbamato)amidinato ytterbium(III)
complex 4.
To evaluate the strength of the Yb–h6-arene interaction by
means of competitive coordination of arene and Lewis base to
the metal center as well as to obtain monomeric hydrido
species the reactions of 2 with various Lewis bases were
carried out. No reaction was detected in the case of TMEDA,
DPPE, or DPPM. Similar to [{(TptBu,Me)YbH}2], addition of
a ten-fold molar excess of THF to the solution of 2 in C6D6
gave no evidence for the dimer dissociation or THF coordi-
nation (ca. 24 h). However, unlike [{(TptBu,Me)YbH}2] 2 is
stable in THF solution for a short time (ca. 0.5 h) at ambient
temperature. Dissolution of 2 in THF and subsequent
recrystallization of the reaction product from toluene allowed
2 to be recovered in quantitative yield. However when 2 was
kept in a THF–hexane mixture over several days, the
bis(amidinate) YbII complex [{tBuC(NC6H3-2,6iPr2)2}Yb] (3)
was isolated from the reaction mixture.[7] If complex 3 is the
result of the ligand redistribution reaction, [YbH2(thf)n][9] can
be suggested as the second product, however it was not found
in the reaction mixture. The results of the reactions of 2 with
Lewis bases allow the conclusion that Yb–h6-arene interac-
tion is quite robust.
Figure 2. ORTEP of 4 with ellipsoids set at 30% probability; hydrogen
atoms and iPr substituents are omitted for clarity. Selected bond
lengths [ꢀ]: Yb1–N1 2.294(4), Yb1–N2 2.295(4), N1–C1 1.344(5), N2–
C1 1.350(5), Yb1–S4 2.6946(12), Yb1–S2 2.6980(11), Yb1–S3
2.6989(10), Yb1–S1 2.7004(12).
ratio. Obviously, formation of 4 is the result of the ligand
redistribution reaction, and [{tBuC(NC6H3-2,6iPr2)2}YbH2]n
could be suggested as the second reaction product. At
present, isolation and identification of the reaction byprod-
ucts are in progress.
An X-ray study of complex 4 revealed the expected k2-
N,N’chelating coordination mode of the amidinate ligand.
The redox reactions of YbII hydrido complexes has to date
remained unexplored. This prompted us to carry out a series
of reactions with one-electron oxidants (I2, AgBF4, AgBPh4,
N,N’-dimethylthiuramdisulfide) with the aim of the selective
oxidation of the metal ion and the synthesis of cationic or
mixed ligand YbIII hydrido species. This study was also
directed towards an estimation of comparative reductive
properties of hydrido and YbII ions. The reactions of 2 with I2,
AgBF4, and AgBPh4 turned out to be not selective; they
resulted in hydrogen evolution (resulting from oxidation of
the hydrido anion) and the formation of intractable mixtures
of products containing both YbII and YbIII species.[10] In
contrast, the reaction with the milder oxidant N,N’-dime-
thylthiuramdisulfide occured without H2 evolution, but
regardless the reagents ratio the bis(dithiocarbamato) amidi-
nato YbIII complex [{tBuC(NC6H3-2,6iPr2)2}Yb(S2CNMe2)2]
(4) was afforded in 36% yield (Scheme 2, Figure 2). The value
of the effective magnetic moment of 4 at 293 K is 4.30 mB and
corresponds to YbIII. The fact that no remaining 2 was found
in the reaction mixture indicates that at the initial stage it
reacts with N,N’-dimethylthiuramdisulfide in a 1:1 molar
The lengths of Yb-S and Yb N bonds in 4 are indicative of the
À
trivalent state of the ytterbium ion.[11]
ꢀ
À
C C bond insertions into a Yb H bond were described
for [{(TptBu,Me)Yb(m-H)}2].[5] The reaction of 2 with two moles
ꢀ
of PhC CPh was carried out in benzene at 508C. Regardless
the reagent ratio, the reaction affords a product involving the
À
ꢀ
addition of two Yb H bonds to a PhC CPh molecule, namely
[{[tBuC(NC6H3-2,6iPr2)2]Yb}2(m-h4:h4-PhCHCHPh)]
(5;
Scheme 3).
The X-ray study of 5 showed that the complex adopts
a dimeric structure in which two YbII centers are linked by
Scheme 3. Synthesis of 5.
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a m-bridging 1,2-dianionic PhCHCHPh fragment (Figure 3).
Each Yb ion is bound to two benzylic carbon atoms
(2.6218(16) and 2.6355(17) ꢀ) and ipso (2.66242(16) ꢀ) and
ortho carbon atoms (2.8568(18) ꢀ) of the phenyl ring, thus
in 2 turned out to be surprisingly robust, as arene cannot be
replaced from the metal coordination sphere when treated
with Lewis bases. Obviously k1-N,h6-arene coordination of
amidinate and guanidinate ligands bearing bulky aryl groups
by nitrogen atoms is characteristic for YbII derivatives, and
the interconversion of k1-N,h6-arene and k2-N,N’ coordination
modes can be influenced by modification of steric and
coordination saturation of the metal. Work directed towards
synthesis of YbIII hydrido species coordinated by the bulky
amidinate ligand is currently in progress.
Experimental Section
Synthesis of [{[tBuC(NC6H3-2,6-iPr2)2]Yb(m-H)}2] (2): PhSiH3
(0.125 g, 1.15 mmol) was added to
a solution of 1 (0.953 g,
1.15 mmol) in hexane (20 mL) at room temperature and the reaction
mixture was left overnight. The mother liquor was separated from
blackish-violet crystals by decantation. The crystals of 2 were washed
with hexane and dried for 20 min in vacuum. Compound 2 was
isolated in 88% yield (0.603 g). 1H NMR (400 MHz, C6D6, 293 K):
d = 0.91 (s, 18H, CH3 tBu), 1.22–1.37 (complex m, 48H, CH3 iPr),
3.24–3.55 (complex m, 8H, CH iPr), 6.93–7.10 (complex m, together
Figure 3. ORTEP of 5 with ellipsoids set at 30% probability; hydrogen
atoms and iPr substituents are omitted for clarity. Selected bond
lengths [ꢀ]: Yb1–N2 2.4075(14), N1–C1 1.316(2), N2–C1 1.359(2),
Yb1–C30a 2.6218(16), Yb1–C30 2.6355(17), C30–C30a 1.482(3), Yb1–
Arcentroid 2.416(5).
1
12H, CH Ar), 7.74 ppm (s, 2H, satellites 171Yb, JYbH = 460 Hz). IR
(KBr, Nujol): n˜ = 1660 (w), 1620 (s), 1585 (m), 1565 (w), 1410 (m),
1320 (m), 1255 (m), 1230 (m), 1210 (m), 1160 (s), 1060 (s), 930 (s), 845
(m), 800 (m), 765 (s), 670 (w), 630 (w), 510 cmÀ1 (s); elemental
analysis calcd (%) for C58H88N4Yb2 (1187.45 gmolÀ1): C 58.67, H 7.47,
Yb 29.15; found: C 58.95, H 7.69, Yb 29.04. The volatiles were
removed from the mother liquor at room temperature in vacuum
and the vacuum distillation of the resulted viscous liquid afforded
PhSiH2N(SiMe3)2 as colorless liquid (0.235 g, 76%).1H NMR
(400 MHz, C6D6, 293 K): d = 0.27 (s, 18H, SiMe3), 5.22 (s, 2H, SiH2,
with 29Si satellites, 1JSiH = 205 Hz), 7.16–7.25 (complex m, together
3H, o-CH and p-CH Ph), 7.67 (d, 3JHH = 5.0 Hz, 2H, m-CH Ph) ppm;
13C{1H} NMR (100 MHz, C6D6, 293 K): d = 3.4 (SiMe3), 128.1 (m-CH
Ph), 129.7 (ipso-C Ph), 133.8 (o-CH Ph), 135.7 (p-CH Ph) ppm;
29Si{1H} NMR (79.5 MHz, C6D6, 293 K): d = À36.2 (SiH2Ph), 5.8
(SiMe3) ppm. Elemental analysis calcd (%) for C12H25NSi3
(267.59 gmolÀ1): C 53.86, H 9.42, N 5.23; found: C 53.70, H 9.48,
N 4.99. MS (EI): m/z = 267.1 [M+].
resulting in h4-coordination.[12] The bond lengths between Yb
atoms and benzylic carbon atoms (2.6218(16), 2.6355(17) ꢀ)
in 2 are slightly shorter than those in YbII benzylic derivatives
[(Me3SiC6H4NMe2)2Yb(thf)2].[13] The length of the bond of
the former alkyne carbon atoms (1.482(3) ꢀ) is close to that
of a single bond.[14]
Magnetic measurements showed that 5 is diamagnetic,
thus indicating a divalent state for the ytterbium atom.
Complex 5 is stable in the crystalline state and can be kept at
room temperature without decomposition; nevertheless, all
our attempts to record the NMR spectra of 5 failed.
Solubilization of brownish-black crystals of 5 in C6D6 or
[D12]cyclohexane results in immediate decoloration of the
solution and formation of an off-white precipitate. The
1H NMR spectrum of this solution clearly shows the presence
of bibenzyl and NH protons of the parent amidine. The
absence of deuterium in bibenzyl implies that the amidinate
ligand can be the only source for hydrogen abstraction for its
formation.
Received: December 8, 2011
Revised: January 13, 2012
Published online: February 28, 2012
Keywords: alkynes · hydrides · insertion reactions · lanthanides ·
.
structure elucidation
ꢀ
Surprisingly, 2 does not react with Me3SiC CSiMe3 and
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À
ꢀ
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[7] For the synthesis and characterization of 1 and 3–5, see the
Supporting Information.
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