Nitrile-nitrile C-C coupling at group 4 metallocenes to form 1-metalla-2,5-diaza-cyclopenta-2,4-dienes: Synthesis and reactivity

Article abstract of DOI:10.1002/anie.201303748

The C-C coupling of aryl nitriles at Group 4 metallocenes leads to unusual ring-strained 1-metalla-2,5-diaza-cyclopenta-2,4-dienes. The structural, energetic, and chemical properties of these complexes are described. The reactions of these compounds towar

Full text of DOI:10.1002/anie.201303748

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Angewandte
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DOI: 10.1002/anie.201303748
À
C C Coupling
À
Nitrile–Nitrile C C Coupling at Group 4 Metallocenes to Form 1-
Metalla-2,5-diaza-cyclopenta-2,4-dienes: Synthesis and Reactivity**
Lisanne Becker, Perdita Arndt, Haijun Jiao, Anke Spannenberg, and Uwe Rosenthal*
4À
TiIV.[11] Enediimido ligands [N C(R) C(R) N] (B) are
obtained in four-electron reductive coupling of nitriles at
TiII compounds.[12]
À
=
À
During the last decades, the synthesis and reactivity of small
metallacycles of Group 4 metallocenes have been of great
interest. The focus lies on all-carbon three- and five-mem-
bered cycles, namely metallacyclopropenes, 1-metallacyclo-
pent-3-ynes (metallacyclopentynes), 1-metalla-cyclopenta-
2,3,4-trienes (metallacyclocumulenes), and 1-metalla-cyclo-
penta-2,3-dienes (metallacycloallenes).[1]
Another possibility for a nitrile–nitrile coupling product is
a complex with a five-membered bridge (as in C), which was
established in the reaction of [Sr(dpp-bian)(thf)4] (dpp-bian =
1,2-bis[(2,6)-diisopropylphenyl)imino]acenaphthene)
with
Metallacyclopropenes are prepared by reduction of
metallocene dichlorides in the presence of the alkynes.[2] In
many cases, an excess of the alkyne yields the 1-metal-
lacyclopenta-2,4-dienes by a coupling of two alkynes.[2] Owing
to these results with all-carbon species, we switched to
heterometallacycles[3] and investigated the formation of the
corresponding 1-metalla-2,5-diaza-cyclopenta-2,4-dienes by
a coupling of two nitriles at Group 4 metallocenes. Nitriles
are one of the most common and versatile building blocks and
have attracted considerable attention for the development of
new synthetic strategies.
acetonitrile.[13] In contrast to the aforementioned coupling
reaction, the coordination of the first nitrile was followed by
a deprotonation, yielding a keteneiminate. Subsequently,
a new bond formed between the a-carbon atom of the second
CH3CN and the b-carbon atom of the keteneiminate unit.
Furthermore, four-, five-, and six-membered metallacycles
are possible products of nitrile–nitrile coupling reactions.
These are mostly generated by an additional interaction with
ligands or stabilizing transformations. Four-membered cycles
À
of type D as result of an N C coupling of nitriles were
obtained for an yttrium[14] and a titanium[15] complex.
The reactions of (CH3)3M (M = Al, Ga, In) with acetoni-
trile lead by trimerization to the six-membered cycles (E).[16]
Such a complex can also be obtained by a dimerization of
nitriles.[17] The first step in these reactions is always an
Only few nitrile–nitrile coupling reactions were observed
before, producing several kinds of complexes (Scheme 1).
Bridging ligands of type A, as a result of such a coupling, were
À
À
insertion of the N C moiety into the M C bond. The
À
resulting C C coupling with a second nitrile does therefore
not always take place between the nitriles. Such six-mem-
bered cycles as formal products of a nitrile–nitrile coupling
were established with a tin[17b] and a scandium[17c] complex.
=
The six-membered cycles (F) with X N(R) can be either
formed in stepwise coupling reactions of three nitriles[18] or of
[Cp*Ti{MeC(NiPr)2}{NNCPh2}] with two nitriles (Cp* = pen-
tamethylcyclopentadienyl).[19] Additionally, the conversions
=
of the intermediates [Cp*2Zr X] with two nitriles yields the
complexes of type F with X = O, S.[20]
Five-membered cycles of type G as a result of a nitrile–
nitrile coupling are very rare, but they are known for
tungsten[7b,21] and rhenium.[22] These special structures are
stabilized by an additional coordination of a metal to one of
the N atoms and an H atom as substituent at the other N
atom. Such a stabilization by H-transfer was already observed
for alkyne–nitrile coupling products at Group 4 metallocene
complexes with 1-metalla-2-aza-cyclopenta-2,4-dienes as
intermediates.[23]
All of these selected examples A–G clearly demonstrate
how sophisticated the coupling reactions of nitriles are. It
strongly depends on the metal, the ligands, and the structure
of the nitrile whether the first step of the reaction is
a coordination, a reduction, a deprotonation, or an insertion.
Scheme 1. Products of nitrile–nitrile coupling reactions.
observed for metals such as molybdenum,[4] tantalum,[5]
niobium,[6] tungsten,[7] zirconium,[8] germanium,[9] and iri-
dium.[10] Similar products are possible for titanium; this is
always accompanied by an oxidation of the metal from TiIII to
[*] Dipl.-Chem. L. Becker, Dr. P. Arndt, Dr. H. Jiao, Dr. A. Spannenberg,
Prof. Dr. U. Rosenthal
Leibniz-Institut fꢀr Katalyse e.V. an der Universitꢁt Rostock
Albert-Einstein-Strasse 29a, 18059 Rostock (Germany)
E-mail: uwe.rosenthal@catalysis.de
[**] Support by the Deutsche Forschungsgemeinschaft (RO 1269/8-1) is
acknowledged.
À
The desired C C coupling takes place in the second step and
therefore yields so many different products. Nonetheless, the
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201303748.
stable 1-metalla-2,5-diaza-cyclopenta-2,4-dienes (H) as
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a result of a nitrile–nitrile coupling are rare and to the best of
our knowledge there are only two hints for this reaction. Such
compounds were briefly mentioned for aluminum[24] and
zirconium[25] but without X-ray crystal structure analysis of
the products and only characterization by elemental analysis
and IR and NMR spectroscopy. The zirconium compound
reacted with hydrogen to the bis(iminyl) complex [Cp*2Zr-
=
(N CHpTol)2] (most likely via [Cp*2ZrH2] and a free nitrile).
Additionally, a mixture of two different pure 1-zircona-2,5-
diaza-cyclopenta-2,4-dienes gave a near-statistical mixture of
the cross-coupled metallacycles by an exchange of the nitriles,
indicating the reversibility of the coupling reaction. None of
these results were published and only briefly cited in
a paper.[17c]
Figure 1. Molecular structure of 2a-Ti in the solid state. Hydrogen
atoms are omitted for clarity; ellipsoids are set to 30% probability.
Selected bond lengths [ꢂ] and angles [8]: C1–C2 1.540(2), C1–N1
1.274(2), C2–N2 1.276(2), Ti1–N1 2.006(1), Ti1–N2 2.006(1); C1-N1-
Ti1 115.54(9), C2-N2-Ti1 115.60(9), N1-C1-C2 114.28(11), N2-C2-C1
114.10(11).
À
The lack of information regarding the nitrile–nitrile C C
coupling to 1-metalla-2,5-diaza-cyclopenta-2,4-dienes moti-
vated us to investigate the reactions of nitriles with Group 4
metallocene complexes in detail. Very recently, our group
published first results in this direction with the mono- and
diphenylated acetonitriles PhCH2CN and Ph2CHCN, which
led (depending on the metals and Cp ligands) to different
products.[15] All of these investigations about elemental steps
of substrate coupling are important for synthetic and catalytic
applications of nitriles. Herein, we report on the synthesis,
structural and energetic properties, and the reaction behavior
of the complexes 2a-Ti–2c-Ti and 2a-Zr–2c-Zr. These were
obtained by reactions of [Cp*2M(h2-Me3SiC2SiMe3)] (1-Ti:
M = Ti; 1-Zr: M = Zr) with two equivalents of the appropri-
ate aryl nitriles. The analogous complexes with Cp (Cp =
cyclopentadienyl) instead of Cp* do not form similar products
with these substrates.[26]
Table 1: Comparison of selected bond lengths [ꢂ] and angles [8] of the
diazacyclopentadiene complex 2a-Ti and the diazacyclopentene complex
[28]
À
À
=
À
À
Cp2Ti( N(H) C(Ph) C(Ph) N(H) ).
2a-Ti[a]
[Cp2Ti(N(H)C(Ph)C(Ph)N(H))]
À
N M
2.006(1)
2.006(1)
1.274(2)
1.276(2)
1.540(2)
115.54(9)
115.60(9)
2.036(2)
2.005(2)
1.360(3)
1.357(3)
1.400(4)
108.9(2)
111.6(2)
À
C N
À
C1 C2
C-N-M
toluene solution of [Cp*2Ti(h2-
compound is folded at the N1 N2 axis by 32.18, whereas it is
nearly planar in 2a-Ti (angle between the planes defined by
Ti1, N1, N2 and N1, C1, C2, N2: 0.818).
À
Treatment of
a
Me3SiC2SiMe3)] (1-Ti)[27] with two equivalents of nitriles at
À
elevated temperatures led to a nitrile–nitrile C C coupling
and the formation of the 1-titana-2,5-diaza-cyclopenta-2,4-
For the similar reaction of the zirconium complex
Cp*2Zr(h2-Me3SiC2SiMe3) (1-Zr),[29] we found, in contrast to
1-Ti, a coordination of the nitriles at room temperature in the
first step (Scheme 3, 3-Zr), and no further reaction with
dienes 2-Ti (Scheme 2), which were isolated as red crystals in
Scheme 2. Synthesis of 1-titana-2,5-diaza-cyclopenta-2,4-dienes
(*=yield determined by NMR spectroscopy).
Scheme 3. Synthesis of 1-zircona-2,5-diaza-cyclopenta-2,4-dienes.
high yields. For 2a-Ti and 2b-Ti, crystals suitable for X-ray
a second equivalent of the nitriles was observed. Warming the
crystal structure analysis could be obtained. One example is
solution of the coordination products 3-Zr for several days led
À
À
À
shown in Figure 1 (2a-Ti) with C N double bonds of C1 N1
to the elimination of the alkyne and a nitrile–nitrile C C
À
À
1.274(2), C2 N2 1.276(2) ꢀ, and a C1 C2 single bond of
1.540(2) ꢀ in the metallacycle.
coupling yielded the desired products 2-Zr (Scheme 3). The
described reactions show, as often observed, the preference of
a dissociative reaction by titanium and of an associative
pathway by zirconium complexes, which is determined by the
size of the metals.[1d]
A comparison of the main structural parameters in the
diazacyclopentadiene complex 2a-Ti and the diazacyclopen-
tene complex [Cp2Ti( N(H) C(Ph) C(Ph) N(H) )] is given
in Table 1, and reveals significant differences in the geometry
of the metallacycle.[28] The five-membered cycle in the latter
À
À
=
À
À
Yellow crystals were obtained in high yields for 2a-Zr,
which are isostructural to those of 2-Ti. Additionally, the
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conversion of 3-Zr to 2-Zr was monitored by 1H NMR
spectroscopy. Such a reaction behavior of 1-Zr was recently
described by us in its reaction with Ph2CHCN. Owing to the
acidic proton neighboring to the nitrile function, the second
step was a H shift instead of the remarkable coupling that we
describe here.[15] The complexes 3-Zr were isolated as dark
blue crystals and for 3a-Zr, which were suitable for X-ray
crystal-structure analysis (Figure 2), showing the same struc-
tural motif as the already described [Cp*2Zr(h2-
Me3SiC2SiMe3)(NCCHPh2)].[15]
Scheme 4. Calculated reaction path from 1-Ti/1-Zr to 2a-Ti/2a-Zr.
Investigating the reactions of 2a-Ti and 2a-Zr with
CH3CN shows that in addition to 2a-Ti and 2a-Zr, two new
complexes are formed: the 1-metalla-2,5-diaza-cyclopenta-
2,4-dienes with two substituents R = CH3 (2e-Ti and 2e-Zr),
and the mixed compounds with one R = CH3 and the other
R = Ph (2d-Ti and 2d-Zr; Scheme 5).
Figure 2. Molecular structure of 3a-Zr in the solid state. Hydrogen
atoms and the second molecule of the asymmetric unit are omitted for
clarity; ellipsoids are set to 30% probability. Selected bond lengths [ꢂ]
and angles [8], corresponding values of the second molecule in the
asymmetric unit are given in square brackets: C1–C2 1.314(3)
[1.306(3)], Zr1–N1 2.269(2) [2.281(2)], N1–C9 1.152(3) [1.143(3)]; C2-
C1-Zr1 73.80(12) [73.87(12)], N1-C9-C10 178.7(2) [176.5(2)].
To understand the reaction behavior of 1-Ti and 1-Zr with
nitriles, we have computed the energetic changes during the
reactions. In our calculations, we have used the real-size
molecules 1-Ti and 1-Zr as well as PhCN and CH3CN at the
BP86 level with the TZVP basis set for non-metal elements
and the effective core potential LANL2DZ basis set for Ti
and Zr. It is firstly noted that the computed structural
parameters for 1-Ti and 1-Zr are in excellent agreement with
those determined by X-ray crystal structure analysis. The
computational details are given in the Supporting Informa-
tion.
Scheme 5. Exchange of the nitriles.
Indeed, our calculations show that the free energy change
is only À1.5 kcalmolÀ1 from 2a-Ti to 2d-Ti, and À2.2 kcal
molÀ1 from 2d-Ti to 2e-Ti. The corresponding free energy
change from 2a-Zr to 2d-Zr is À1.4 kcalmolÀ1, and that from
2d-Zr to 2e-Zr is À0.9 kcalmolÀ1. These rather small free
energy changes from R = Ph to CH3 have their steric origin
between the two R substituents on the one hand, and reveal
their possible equilibrium as found experimentally on the
other.
For the substitution of the alkyne ligand in 1-Ti and 1-Zr
by two PhCN molecules to [Cp*2Ti(NCPh)2] and [Cp*2Zr-
(NCPh)2] (Scheme 4), the computed reaction energy is highly
exergonic by 20.9 and 18.2 kcalmolÀ1, respectively, indicating
À
their thermodynamic probability. For the C C coupling of
The coupling reaction of CH3CN with 1-Ti and 1-Zr was
also computed, and structure optimization led directly to 2e-
Ti and 2e-Zr without any barriers, but the experiment did not
yield the expected products. Therefore, the nitrile-exchange
reaction is important for the possible isolation of these
compounds. In some aspects this resembles the Group 4 1-
metallacyclopenta-2,4-dienes, which can undergo ring open-
ing reactions to the bis(alkyne) complexes and subsequently
eliminate one alkyne.[22a,30]
Furthermore, the reaction behavior of the very unusual 1-
metalla-2,5-diaza-cyclopenta-2,4-dienes was investigated in
the reactions with H2, CO2, and HCl. The reaction of the
zirconium complex 2a-Zr with H2 at room temperature,
giving compound 4a-Zr and free PhCN, was investigated by
two nitrile units in [Cp*2M(NCPh)2], we have located the
authentic transition states [(Cp*2M(NCPh)2]-TS), and the
computed free energy barrier is 10.3 for M = Ti and 9.6 kcal
molÀ1 for M = Zr. The formation of 2a-Ti (À1.6) and 2a-Zr
(À3.6 kcalmolÀ1) is exergonic. These very low exergonic
energy shows the thermodynamic probability of the reverse
reaction from 2a-M back to [Cp*2M(NCPh)2] (the barrier of
the reverse reaction is 11.9 and 12.2 kcalmolÀ1 for 2a-Ti and
2a-Zr, respectively), and also a possible equilibrium between
2a-M and [Cp*2M(NCPh)2]. The total reaction free energy
from 1-Ti to 2a-Ti is À22.5 and from 1-Zr to 2a-Zr
À21.8 kcalmolÀ1.
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metallacycles and their reaction behavior. This shows that the
À
new C C bond between the nitriles is relatively labile and can
be easily cleaved. This leads to the release of one nitrile and
its replacement by different co-substrates, giving a high
potential for synthetic strategies which are currently under
investigation.
Scheme 6. Reaction of 2a-Ti and 2a-Zr with H2.
Received: May 2, 2013
Revised: July 25, 2013
Published online: September 11, 2013
1H- and 13C NMR spectroscopy (Scheme 6). Further warming
of these products led to the formation of 5a-Zr by insertion of
Keywords: DFT calculations · metallacycles · metallocenes ·
nitrile–nitrile coupling
À
PhCN. Insertion reactions of nitriles into M H bonds are well
.
known.[17c,31] In contrast to this, 2a-Ti did not react with H2 at
room temperature. Warming the mixture to 658C for three
days yielded the desired product 4a-Ti in quantitative yield. A
nitrile insertion does not occur as a result of further heating.
An exchange of one nitrile is also observed in the reaction
of 2a-Ti with CO2. The formation of 6a-Ti (Scheme 7) is the
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Scheme 7. Reaction of 2a-Ti with CO2.
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solution (Scheme 4). In contrast, the corresponding zirconium
compound 2a-Zr forms complex mixtures in the reaction with
CO2 at room temperature. The coupling reaction of CO2 with
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In comparison to these exchange reactions, the release of
a diimine can be accomplished in the reaction with HCl.
Unfortunately, these diimines are not stable at room temper-
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be isolated (Scheme 8).
À
In summary, we were able to accomplish a remarkable C
C coupling between two nitriles to form unusual ring-strained
1-metalla-2,5-diazacyclopenta-2,4-dienes. In the case of the
reactions of [Cp*2Zr(h2-Me3SiC2SiMe3)] (1-Zr), we could
isolate 3-Zr as an intermediate as well. We investigated the
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Scheme 8. Reaction of 2a-Ti and 2a-Zr with HCl.
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À
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