Chameleon behavior of a newly synthesized scandium nitrilimine derivative

Article abstract of DOI:10.1021/ja403387v

The synthesis, structural characterization, and reactivity of the first example of a scandium -substituted nitrilimine are presented. This unique complex exhibits high thermal stability but shows a rich reactivity toward a variety of unsaturated substrates, including aldehyde, ketone, nitrile, and allene derivatives. The versatility of the complex was further highlighted by density functional theory mechanistic studies.

Full text of DOI:10.1021/ja403387v

Communication
pubs.acs.org/JACS
Chameleon Behavior of a Newly Synthesized Scandium Nitrilimine
Derivative
Jiaxiang Chu,† Christos E. Kefalidis,‡ Laurent Maron,*,‡ Xuebing Leng,† and Yaofeng Chen*,†
†State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Lingling Road, Shanghai 200032, People’s Republic of China
‡LPCNO, CNRS & INSA, Universite
́
Paul Sabatier, 135 Avenue de Rangueil, Toulouse 31077, France
S
* Supporting Information
Scheme 2. Synthesis of Scandium Nitrilimine 1
ABSTRACT: The synthesis, structural characterization,
and reactivity of the first example of a scandium
-substituted nitrilimine are presented. This unique
complex exhibits high thermal stability but shows a rich
reactivity toward a variety of unsaturated substrates,
including aldehyde, ketone, nitrile, and allene derivatives.
The versatility of the complex was further highlighted by
density functional theory mechanistic studies.
of a scandium-substituted nitrilimine. This unique complex
shows a rich reactivity to a variety of unsaturated substrates.
Herein we report the synthesis, crystal structure, and reactivity
of this scandium nitrilimine derivative as well as a mechanistic
study with different substrates performed using dispersion-
corrected density functional theory (DFT-D).
Reaction of the scandium terminal imido complex [LSc
N(DIPP)] (L = [MeC(N(DIPP))CHC(Me)(NCH2CH2N-
(Me)CH2CH2NMe2]−, DIPP = 2,6-(iPr)2C6H3)13b with
(trimethylsilyl)diazomethane, (Me3Si)(H)CN2, in toluene at
room temperature gave scandium nitrilimine complex 1 in 72%
yield (Scheme 2). Complex 1 was characterized by NMR
spectroscopy, elemental analysis, and single-crystal X-ray
diffraction (Figure 1). During the reaction of the imido
complex with (Me3Si)(H)CN2 (Scheme 2), proton transfer
from (Me3Si)(H)CN2 to the imido group yields a
iazoalkanes (R1)(R2)CN2 (A in Scheme 1) are important
D
reagents in organic synthesis, and they are widely used as
1,3-dipoles for the synthesis of carbon−nitrogen heterocycles1
and as facile source of carbenes.2 The main reaction of
hydrogen-substituted diazoalkanes (R1)(H)CN2 with metal
complexes, apart from N2 loss to give metal carbene complexes,
is deprotonation leading to metal complexes containing
[(R1)CN2]− anions, which can coordinate to the metal centers
through the carbon atom (C) or the terminal nitrogen atom
(D). Metal complexes with N-bonded [(R1)CN2]− ligands are
Scheme 1. Resonance Structures of Diazoalkane (A) and
Nitrilimine (B) and the Corresponding Metal-Substituted
Analogues C and D
of interest as a type of metal-substituted nitrilimines (B), which
are important nitrogen-containing 1,3-dipoles in addition to
diazoalkanes.1,3 Nitrilimines substituted with main-group
metals have been known for a long time, and a considerable
number of these complexes have been synthesized, including
Li,4 Al,5 Ga,6 and Ge7 derivatives. However, to the best of our
knowledge, there are no examples of transition-metal-
substituted nitrilimines. On the other hand, C-bonded
[(R1)CN2]− anions have been reported in a variety of
transition-metal complexes, such as Pd,8 Rh/Os,9 Ni,10 Au,11
and rare-earth-metal complexes.12 Recently, we synthesized
scandium terminal imido complexes and studied their
reactivity.13 During these studies, we obtained the first example
Figure 1. Molecular structure of 1 with thermal ellipsoids at the 30%
probability level. The DIPP isopropyl groups and the H atoms have
been removed for clarity.
Received: April 9, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/ja403387v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
[Me3SiCN2]− anion, which coordinates to the Sc center
through its terminal nitrogen atom. A DFT-D study showed
that the activation barrier for the proton transfer reaction is low
(13.6 kcal mol−1) and that the reaction is thermodynamically
favorable (32.6 kcal mol−1) [see Figure S20 in the Supporting
Information (SI)]. It is noteworthy that the [(R1)CN2]− anion
has a strong tendency to bind two metal ions through the
terminal nitrogen atom or through the terminal nitrogen atom
and the carbon atom. Most of the reported main-group-metal-
substituted nitrilimines exist as bimetallic or multimetallic
species, and no example of crystallographically characterized
mononuclear main-group-metal-substituted nitrilimine has
been reported.14 It is also noteworthy that reactions of
[(C5Me5)2Sm][(μ-Ph)2BPh2] with Li[Me3SiCN2] and
[(C5Me5)2Sm(μ-H)]2 with (Me3Si)(H)CN2 give the bimetallic
isocyanotrimethylsilylamide complexes {(C5Me5)2Sm[(μ-N-
(SiMe3)NC]}2 as a result of 1,3-silyl migration during the
reactions.15 The existence and thermal stability of monometallic
complex 1 can be attributed to the presence of the bulky
ligands L− and [NH(DIPP)]−. In 1, the NNC unit of the
[Me3SiCN2]− ligand is almost linear with an N6−N7−C48
angle of 176.5(5)°. The Sc−N6−N7 and N7−C48−Si angles
are 149.5(3)o and 157.4(5)o, respectively. The C48−N7 bond
[1.199(6) Å] is longer than a typical C−N triple bond (1.14 Å),
and the N6−N7 bond [1.188(5) Å] is consistent with a double
bond.16 These structural data indicate that the NNC unit of the
[Me3SiCN2]− ligand in 1 adopts an allenic structure rather than
a propargylic one.17 The Sc−Nnitrilimine bond [2.075(3) Å] is
similar in length to the Sc−Namido single bond [2.088(4) Å] and
much longer than the Sc−Nimido double bond in the scandium
terminal imido complex [1.859(2) Å].13b In contrast to organic
nitrilimines,3 1 is thermally stable and shows less than 5%
decomposition over 12 h in C6D6 at 75 °C.
Scheme 3. Reactions of 1 with Unsaturated Substrates
diazomethane with but-3-yn-2-one, which yields a pyrazole
compound via a 1,3-cycloaddition to the CC triple bond of
but-3-yn-2-one, the reaction of 1 with this substrate gave
scandium alkoxide complex 3 in 89% yield (similar to the
reactivity with benzaldehyde). The structural features of 3 are
similar to those of 2, and the C−N and N−N bond lengths of
the 1-diazo-2-methyl-1-(trimethylsilyl)but-3-yn-2-ol ligand in 3
are 1.311(4) and 1.138(3) Å, respectively (see the SI).
Complexes 2 and 3 offer the possibility of further reactivity
at the diazo moiety.
Lithium nitrilimines have been extensively used as reagents in
organic synthesis.18 However, they are multimetallic species
and coexist with their lithium diazoalkane isomers in solution,
and therefore, a deeper understanding of their reactivity is
difficult to eluicidate.4 Bertrand and co-workers synthesized a
mononuclear aluminum nitrilimine and found that it reacts with
a 1,3-dipolarophile (Lawesson’s reagent) to give a [3+2]
addition product, but its reactivity toward other substrates was
not reported.5 The high thermal stability of 1 allowed a detailed
investigation of its reactivity. Complex 1 is very reactive with a
variety of unsaturated substrates and displays interesting
reaction patterns (Scheme 3). The reaction of 1 with 1 equiv
of benzaldehyde in toluene at −35 °C for 2 h gave scandium
alkoxide complex 2 in 68% yield. The molecular structure of 2
is shown in the SI. After coordination of the substrate, the
carbon atom of the nitrilimine ligand in 1 undergoes
nucleophilic attack at the carbon atom of benzaldehyde,
yielding a 2-diazo-1-phenyl-2-(trimethylsilyl)ethoxide ligand,
which leads to cleavage of the Sc−N σ bond with subsequent
formation of a Sc−O bond. The C−N [1.289(4) Å] and N−N
[1.145(4) Å] bonds in the 2-diazo-1-phenyl-2-(trimethylsilyl)-
ethoxide ligand in 2 are consistent with double bonds, showing
a typical structural feature of diazoalkanes.19 It should be noted
that (trimethylsilyl)diazomethane does not react with benzal-
dehyde in C6D6 at room temperature or even at 75 °C, as
Complex 1 can also react with several other unsaturated
1
substrates such as allene derivatives and nitriles. H NMR
monitoring of the reaction of 1 with phenylallene in C6D6 at 75
°C showed that 1 was almost fully converted into the new
complex 4 in 2 days. A subsequent scaled-up reaction in
toluene provided 4 in 59% isolated yield. 4 is a scandium 1,2-
diazolato complex, as shown in Scheme 3, and its molecular
structure is shown in the SI. The formation of 4 implies a [3+2]
cycloaddition followed by a 1,3-hydrogen shift. Complex 1
reacted rapidly with benzonitrile. The reaction was complete
within 15 min at room temperature and afforded scandium
1,2,3-triazolato complex 5 in 87% yield. It is worth mentioning
that reactions of organic nitrilimines with benzonitrile give
1,2,4-triazoles4 while that of diazomethane with cyanic bromide
gives 1,2,3-triazole.21 Therefore, the reaction of 1 with
benzonitrile is similar to those of diazoalkanes rather than
those of organic nitrilimines. In 5, the 1,2,3-triazolato ligand
coordinates to the Sc center through two nitrogen atoms with
two nonequivalent Sc−N bonds [the Sc−N6 and Sc−N7 bond
lengths are 2.180(2) and 2.281(2) Å, respectively] (Figure 2),
which is consistent with the different electronic donating
abilities of these two nitrogen atoms.
20
1
monitored by H NMR spectroscopy. The aforementioned
reaction is also different from those of organic nitrilimines with
benzaldehyde, which give 1,3,4-oxadiazolines.1a The reaction of
1 with the unsaturated ketone but-3-yn-2-one was subsequently
investigated. In contrast with the reaction of (trimethylsilyl)-
The reaction of 1 with diphenylcarbodiimide (heteroallene)
was also rapid, and scandium 3-amino-1,2,4-triazolato complex
6 was obtained in 84% yield. Unlike the reactions of 1 with
B
dx.doi.org/10.1021/ja403387v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
SI). In the case of diphenylcarbodiimide, two plausible
pathways were investigated (Figure 3). The first leads to the
experimentally observed complex 6 and the second to the 1,2,3-
triazolato complex 6′. In the first case, the reaction proceeds
through insertion of the CN double bond of diphenylcarbo-
diimide into the Sc−Nnitrilimine bond of 1 to form the
metallacyclic intermediate int2. The activation barrier for this
step is 11.3 kcal mol−1, and the step is exoergic by 18.3 kcal
mol−1. The reaction continues via TSint2−6 corresponding to
heterocyclic ring closure with an activation barrier of only 6.6
kcal mol−1. Overall, the reaction is strongly exoergic by 58.2
kcal mol−1, which is the driving force for this transformation.
The computed low barrier, especially in view of the well-known
problem of entropy overestimation by calculations, is in line
with a rapid reaction. We also examined a second possibility in
which, instead of the insertion, nucleophilic attack of the
diphenylcarbodiimide at the nitrilimine carbon occurs. The
activation barrier required for this step is higher than that for
the insertion step (13.4 kcal mol−1 free energy difference). Also,
the intermediate int2′ is less stable by 33.1 kcal mol−1 relative
to int2. In this pathway, the hypothetical 1,2,3-triazolato
complex 6′ is formed after heterocyclic ring closure. Moreover,
the formation of 6′ is exergonic by only 28.5 kcal mol−1,
making this pathway also less thermodynamically favored than
the first one. The products arising from other regioisomeric
insertions were found to be less favored than int2 by 58 kcal
mol−1, ruling out the possibility that these complexes would be
formed. For benzonitrile, the reaction is proposed to proceed
through an inner-sphere 1,3-dipolar addition transition state to
allow the formation of 5. This reaction is predicted to be
kinetically accessible (ΔG⧧ = 13.3 kcal mol−1) and
thermodynamically strongly exergonic (ΔG0 = −42.3 kcal
mol−1). This reaction is controlled by electrostatic repulsion
between the incoming substrate and the nitrilimine ligand (see
the SI for details of the mechanism).
Figure 2. Molecular structures of 5 and 6 with thermal ellipsoids at the
30% probability level. Trimethylsilyl methyl groups, DIPP isopropyl
groups, and H atoms have been removed for clarity.
benzaldehyde and benzonitrile, nucleophilic attack of the
terminal nitrogen atom of the nitrilimine ligand at the carbon
atom of diphenylcarbodiimide occurs. This is similar to the
reactions of organic nitrilimines but different from those of
diazoalkanes.1a In 6, the 3-amino-1,2,4-triazolato ligand
coordinates to the Sc center through one amino nitrogen
atom and one nitrogen atom of triazolato ring, and the distance
from the Sc center to the amino nitrogen atom [2.252(2) Å] is
shorter than that to the nitrogen atom of triazolato ring
[2.309(2) Å] (Figure 2). The reaction pattern of 1 with phenyl
isothiocyanate was very similar to that with diphenylcarbodi-
imide, giving scandium 2-amino-1,3,4-thiadiazolato complex 7
in 85% yield. The reaction of 1 with methyl diazophenylacetate
provided scandium tetrazolato derivative 8 in 93% yield.
Apparently, the 1-methoxy-2-phenyl-2-(5-trimethylsilyl-2H-tet-
razol-2-yl)ethenoxide ligand of 8 is formed through [3+2]
cycloaddition of the nitrilimine ligand of 1 to the NN double
bond of methyl diazophenylacetate followed by an electronic
rearrangement. The molecular structures of 7 and 8 were both
determined by single-crystal X-ray diffraction (see the SI).
To gain further insight into the reactivity of 1, DFT-D
calculations were carried out at the B3PW91-D2 level of
theory.22 The reactions with diphenylcarbodiimide and
benzonitrile were chosen as representative examples (see the
In summary, the synthesis, structural characterization, and
reactivity of the first example of a scandium-substituted
nitrilimine have been presented. The high thermal stability of
this scandium derivative allowed a detailed analysis of its
reactivity toward several unsaturated substrates, including
aldehyde, ketone, nitrile, and allene derivatives.23 In the two
Figure 3. Gibbs free energy profile for the formation of 6. In the structures shown, the ligands of Sc have been removed for clarity.
C
dx.doi.org/10.1021/ja403387v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX

Journal of the American Chemical Society
Communication
(9) (a) Deydier, E.; Menu, M. J.; Dartiguenave, M.; Dartiguenave, Y.;
Simard, M.; Beauchamp, A. L.; Brewer, J. C.; Gray, H. B.
Organometallics 1996, 15, 1166. (b) Gallop, M. A.; Jones, T. C.;
Rickard, C. E. F.; Roper, W. R. J. Chem. Soc., Chem. Commun. 1984,
1002.
latter cases, the versatility of the nitrilimine ligand coordinated
to the scandium center was demonstrated by analogy with
organic nitrilimines. Indeed, with the nitrile substrate, the
nitrilimine ligand reacted more like a diazoalkane, whereas with
the heteroallene, the reactivity was closer to that of an organic
nitrilimine. This versatility was investigated by computing the
reaction pathways for the diphenylcarbodiimine and benzo-
nitrile substrates using DFT. It has been shown that the
formation of the 1,2,4-triazolato ring is both thermodynamically
and kinetically driven. In the case of benzonitrile, the difference
in the barrier is associated with repulsion between the
negatively charged carbon and nitrogen atoms in the 1,3-
dipolar addition transition state. To date, the present work is
the only joint experimental/theoretical study that deals with a
transition-metal nitrilimine complex, and it exhibits the unique
and rich chemistry of such a complex.
(10) Konig, H.; Menu, M. J.; Dartiguenave, M.; Dartiguenave, Y.;
̈
Klein, H. F. J. Am. Chem. Soc. 1990, 112, 5351.
(11) Tsui, E. Y.; Muller, P.; Sadighi, J. P. Angew. Chem., Int. Ed. 2008,
̈
47, 8937.
(12) Siebald, H.; Dartiguenave, M.; Dartiguenave, Y. J. Organomet.
Chem. 1992, 438, 83.
(13) (a) Lu, E. L.; Li, Y. X.; Chen, Y. F. Chem. Commun. 2010, 46,
4469. (b) Lu, E. L.; Chu, J. X.; Chen, Y. F.; Borzov, M. V.; Li, G. Y.
Chem. Commun. 2011, 47, 743. (c) Chu, J. X.; Lu, E. L.; Liu, Z. X.;
Chen, Y. F.; Leng, X. B.; Song, H. B. Angew. Chem., Int. Ed. 2011, 50,
7677. (d) Lu, E. L.; Zhou, Q. H.; Li, Y. X.; Chu, J. X.; Chen, Y. F.;
Leng, X. B.; Sun, J. Chem. Commun. 2012, 48, 3403.
(14) Bertrand and co-workers synthesized a mononuclear aluminum
nitrilimine that is an oil (see ref 5).
(15) Evans, W. J.; Montalvo, E.; Champagne, T. M.; Ziller, J. W.;
DiPasquale, A. G.; Rheingold, A. L. J. Am. Chem. Soc. 2008, 130, 16.
(16) Allen, F. H.; Kennard, O.; Watson, D. G.; Brammer, L.; Orpen,
A. G.; Taylor, R. J. Chem. Soc., Perkin Trans. 2 1987, S1.
(17) Wong, M. W.; Wentrup, C. J. Am. Chem. Soc. 1993, 115, 7743.
(18) Shioiri, T.; Aoyama, T. In Advances in The Use of Synthons in
Organic Chemistry; Dondoni, A., Ed.; JAI Press: London, 1993; Vol. 1,
pp 51−101.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental and computational details and a zip file
containing CIFs for 1−8. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
laurent.maron@irsamc.ups-tlse.fr; yaofchen@mail.sioc.ac.cn
■
(19) Zollinger, H. In Diazo Chemistry II; VCH: Weinheim, Germany,
1995.
(20) (Trimethylsilyl)diazomethane reacts with benzaldehyde in the
presence of triethylamine to give a complicated mixture of two ketones
and two epoxides. See: Hashimoto, N.; Aoyama, T.; Shioiri, T.
Heterocycles 1981, 15, 975.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
(21) (a) Pedersen, C. Acta Chem. Scand. 1959, 13, 888. (b)
Diazomethane does not react with benzonitrile. See: Lappert, M. F.;
Poland, J. S. J. Chem. Soc. C 1971, 3910.
■
This work was supported by the National Natural Science
Foundation of China (Grants 21072209, 21132002, and
21121062), the State Key Basic Research and Development
Program (Grant 2012CB821600), and the Chinese Academy of
Sciences. L.M. is a member of the Institut Universitaire de
France. CALMIP and CINES are acknowledged for a
genereous grant of computing time. The Humboldt Foundation
is also acknowledged.
(22) See the SI for computational details.
1
(23) H NMR monitoring of the reactions in C6D6 showed that
(trimethylsilyl)diazomethane does not react with phenylallene and
benzonitrile at room temperature (RT) or at 75 °C. (Trimethylsilyl)
diazomethane also does not react with methyl diazophenylacetate at
RT, but it gives a complicated mixture of products at 75 °C. Finally, it
can react very slowly with diphenylcarbodiimide or phenyl
isothiocyanate at room temperature to give complicated product
mixtures.
REFERENCES
■
(1) (a) Huisgen, R. Angew. Chem., Int. Ed. Engl. 1963, 2, 565.
(b) Kobayashi, S.; Jørgensen, K. A. In Cycloaddition Reactions in
Organic Synthesis; Wiley-VCH: Weinheim, Germany, 2001. (c) Sharp,
J. T. Chem. Heterocycl. Compd. 2002, 59, 473.
(2) (a) Doyle, M. P. In Reactive Intermediate Chemistry; Moss, R. A.,
Platz, M. S., Jones, M., Eds.; Wiley-Interscience: New York, 2004.
(b) Doyle, M. P.; Duffy, R.; Ratnikov, M.; Zhou, L. Chem. Rev. 2010,
110, 704.
(3) Bertrand, G.; Wentrup, C. Angew. Chem., Int. Ed. Engl. 1994, 33,
527.
(4) (a) Boche, G.; Harms, K.; Marsch, M.; Schubert, F. Chem. Ber.
1994, 127, 2193. (b) Armstrong, D. R.; Davies, R. P.; Haigh, R.;
Hendy, M. A.; Raithby, P. R.; Snaith, R.; Wheatley, A. E. H. Eur. J.
Inorg. Chem. 2003, 3363.
(5) Emig, N.; Gabbaï, F. P.; Krautscheid, H.; Rea
Angew. Chem., Int. Ed. 1998, 37, 989.
́
u, R.; Bertrand, G.
(6) Uhl, W.; Hannemann, F.; Saak, W.; Wartchow, R. Eur. J. Inorg.
Chem. 1999, 771.
(7) Leue, C.; Reau, R.; Neumann, B.; Stammler, H. G.; Jutzi, P.;
́
Bertrand, G. Organometallics 1994, 13, 436.
(8) (a) Murahashi, S. I.; Kitani, Y.; Hosokawa, T.; Miki, K.; Kasai, N.
J. Chem. Soc., Chem. Commun. 1979, 450. (b) Murahashi, S. I.; Kitani,
Y.; Uno, T.; Hosokawa, T.; Miki, K.; Yonezawa, T.; Kasai, N.
Organometallics 1986, 5, 356.
D
dx.doi.org/10.1021/ja403387v | J. Am. Chem. Soc. XXXX, XXX, XXX−XXX