Hydrogen activation by an intramolecular boron lewis acid/zirconocene pair

Article abstract of DOI:10.1002/anie.201202218

Let's split: Reaction of the zirconium complex A with Piers' borane [HB(C₆F₅)₂] yields the unusual borylalkyne zirconocene complex B which reacts with dihydrogen, activating it to give the doubly hydrido bridged alkyne zirconium complex C. Copyright

Full text of DOI:10.1002/anie.201202218

Angewandte
Chemie
DOI: 10.1002/anie.201202218
H2 Activation
Hydrogen Activation by an Intramolecular Boron Lewis Acid/
Zirconocene Pair**
Santhosh Kumar Podiyanachari, Roland Frçhlich, Constantin G. Daniliuc, Jeffrey L. Petersen,
Christian Mꢀck-Lichtenfeld, Gerald Kehr, and Gerhard Erker*
Small-molecule activation is a very important feature in
stoichiometric synthetic chemistry as well as in catalysis. The
cleavage and activation of dihydrogen is a typical example.[1]
In many molecular systems hydrogen activation is carried out
by oxidative addition of the H2 molecule to a single transition-
metal center, the Wilkinson catalyst being a typical example.[2]
Alternatively transition-metal centers can split dihydrogen
heterolytically with the assistance of a proton accepting main-
group-element Lewis base, variants of the Noyori catalysts
being typical examples.[3] We have now prepared a system
featuring an inverse pair of functionalities, namely a bifunc-
Scheme 1. Preparation of complexes 2 and 4; Cp=C5H5.
tional Group 4 metallocene/borane system where the main-
group-element component may serve as a hydride acceptor.
Herein we describe the synthesis of this new zirconocene/
RB(C6F5)2 pair and its reaction with dihydrogen under mild
conditions.
We first treated the aldimido zirconocene complex 1[4]
with trimethylsilylethynyl lithium. Product 2 (see Scheme 1)
was isolated in over 90% yield as a yellow solid. It was
characterized by X-ray diffraction (see the Supporting
Information) and shown to contain a close to linear “met-
alla-2-aza allene” type unit.
Subsequent treatment of compound 2 with one molar
equivalent of “Piersꢀ borane” [HB(C6F5)2][5] gave complex 4
(isolated in over 80% yield; see Scheme 1 and Figure 1).
Apparently, compound 4 is the product of a 1,1-hydrobora-
tion reaction[6,7] of HB(C6F5)2 with the pendant trimethylsilyl
acetylide s-ligand. This reaction is typically initiated by s-
alkynyl ligand transfer from zirconium to the boron Lewis
acid.[7] Hydride shift from boron to carbon followed by
coordination of the imino nitrogen atom to the boron Lewis
acid then straightforwardly leads to 4.
We then investigated the formally related, but in some
essential aspects decidedly different reaction of the s-alkenyl/
Figure 1. Molecular structure of compound 4. Selected bond
lengths [ꢀ] and angles [8]: Zr1–C1 2.165(4), Zr1–C2 2.513(4), Zr1–N1
2.277(3), C1–C2 1.338(5), B1–C2 1.693(5), Si1–C1 1.872(4), N1–C3
1.269(4), B1–N1 1.578(5); C1-Zr1-N1 101.8(1), B1-N1-Zr1 93.2(2), C1-
Zr1-C2 32.1(1), N1-Zr1-C2 69.7(1), C2-C1-Si1 121.5(3), C2-C1-Zr1
88.4(2), B1-C2-Zr1 82.5(2).[19]
[*] Dr. S. K. Podiyanachari, Dr. R. Frçhlich,[+] Dr. C. G. Daniliuc,[+]
Dr. C. Mꢁck-Lichtenfeld,[++] Dr. G. Kehr, Prof. Dr. G. Erker
Organisch-Chemisches Institut, Westfꢂlische Wilhelms-Universitꢂt
Correnstrasse 40, 48149 Mꢁnster (Germany)
s-alkynyl zirconocene complex 5 with HB(C6F5)2.[8] Complex
5 was prepared by hydrozirconation of tert-butylacetylene[9]
ꢀ
followed by treatment with the Li-C C-SiMe3 reagent (see
E-mail: erker@uniso-muenster.de
Prof. Dr. J. L. Petersen[+]
Bennett Department of Chemistry, West Virginia University
P.O. Box 6045, Morgantown, WV 26506 (USA)
Scheme 2). It was then treated with one molar equivalent of
HB(C6F5)2 in benzene at room temperature (12 h). Workup
gave the product 7 as a dark red solid in 86% yield. The X-ray
crystal structure analysis revealed that one equivalent of tert-
butylethene had apparently been eliminated during the
reaction leaving the substituted 2-boryl-1-silylacetylene zir-
conocene complex 7 behind (see Figure 2). Both the acety-
lenic carbon atoms are strongly bonded to zirconium (Zr1–
C1: 2.275(2), Zr1–C2: 2.390(2), C1–C2: 1.281(3) ꢁ). The C1–
[+] X-ray crystal structure analyses
++] Computational chemistry
[
[**] Financial support from the Deutsche Forschungsgemeinschaft is
gratefully acknowledged.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201202218.
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C6F5 signal at d = ꢁ167.1 ppm (2F, 233 K; the spectra are
shown in the Supporting Information). The d = ꢁ167.1 ppm
resonance is an averaged 19F NMR signal of the bridging Zr-
F-C(Ar) with the remaining non-bridged o-F resonance of the
one o-C6F5 ring. Consequently, this signal starts to decoalesce
upon further lowering the temperature without reaching
a fully resolved 19F spectrum (temperature limit 193 K).
Finally, the pair of m-F atoms of the two C6F5 rings at boron
give rise to a pair of signals at d = ꢁ158.7, ꢁ163.2 ppm (each
2F, 193 K).
We can safely assume that the initial step of the reaction of
5 with HB(C6F5)2 is again acetylide transfer from zirconium to
boron[7] (to generate 6). In contrast to the nitrogen case
described above (see Scheme 1) the intermediate 6 appa-
rently disfavors entering into the 1,1-hydroboration route but
chooses stabilization by hydride transfer and reductive
coupling to eventually yield 7 and tert-butylethene (see
Scheme 2).
Scheme 2. Synthesis of compound 7.
Compound 7 reacted rapidly with dihydrogen at ambient
conditions in benzene solution (1.5 bar H2 pressure, RT,
15 min) to give the H2 addition product 9 (see Scheme 3). The
Scheme 3. Preparation of the hydrogen-addition product 9.
hydrido zirconocene/hydrido borate complex 9 was isolated
as a pale yellow solid in over 80% yield. The X-ray crystal
structure analysis (Figure 3) shows the presence of
Figure 2. A view of the molecular structure of compound 7 featuring
a weak Zr···F contact. Selected bond lengths [ꢀ] and angles [8]: Zr1–C1
2.275(2), Zr1–C2 2.390(2), Si1–C1 1.857(2), Zr1–F36 2.512(1), C1–C2
1.281(3), C2–B1 1.455(3); C1-Zr1-C2 31.7(6), C1-Zr1-F36 121.5(5), B1-
C2-Zr1 94.6(1).[19]
+
ꢁ
ꢀ
a [Cp2ZrH ] cation moiety to which a [HB(C6F5)2 ]-C C-
SiMe3 anion is p-coordinated. The resulting structure features
a C1–C2 bond length of 1.265(5) ꢁ (there are two chemically
equivalent crystallographically independent molecules in the
Si1 vector is bent away from the zirconocene unit (angle C2-
C1-Si1: 136.3(2)8). Most remarkable is that the C2–B1 vector
is slightly leaning toward the zirconocene moiety (angle C1-
C2-B1: 162.61(2)8). A close inspection of the coordination
sphere of the central zirconium atom revealed a weak
interaction with a fluorine atom of one of the C6F5 groups
at boron Zr1–F36: 2.512(1) ꢁ.[10] The Zr1···B1 separation is
2.896(2) ꢁ.
In solution complex 7 features 1H and 13C NMR signals of
the Cp2Zr core at d = 5.42 (s, 10H) ppm and d = 110.7 ppm,
respectively. The acetylene ligand 13C NMR resonances occur
ꢀ
ꢀ
at d = 173.9 ( C[Si]) and d = 178.6 ppm ([B]C ). Compound 7
shows a 11B NMR resonance at d = 15.5 ppm and the 29Si
NMR signal of the -SiMe3 group at d = ꢁ6.0 ppm (with
a
corresponding 1H NMR singlet at d = 0.16 ppm). In
[D8]THF solution the spectrum of compound 7 features
three sharp 19F NMR signals at d = ꢁ130.2 (4F, ortho), ꢁ160.9
(2F, para), ꢁ166.3 ppm (4F, meta) for the C6F5 substituents at
boron. In the non-coordinating solvent [D8]toluene we
observed decoalescence of the averaged o-C6F5 19F NMR
resonance at d = ꢁ145.2 ppm (4F, 363 K) to a broad “normal”
o-C6F5 signal at d = ꢁ126.7 ppm (2F, 233 K), and a broad o-
Figure 3. A projection of the molecular structure of the zwitterionic
hydrogen-addition product 9. Selected bond lengths [ꢀ] and angles [8]:
Zr1–C1 2.516(4), Zr1–C2 2.327(3), Zr1–H1 1.86(4), Zr1–H01 2.04(3),
B1–C2 1.554(5), B1–H01 1.24(3), C1–C2 1.265(5), C1–H1 1.54(4); C1-
Zr1-C2 29.9(1), C1-Zr1-H1 37.5(1), C2-Zr1-H01 60.3(8), C2-C1-Si1
143.2(3), C2-C1-Zr1 66.7(2), Si1-C1-Zr1 149.4(2).[19]
2
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Chemie
crystal; the values of molecule A are given here) which is
slightly shorter than found in its precursor 7 (see above). In 9
the C1–Si1 vector is markedly pointing away from the
zirconocene unit (angle C2-C1-Si1: 143.2(3)8) and there
seems to be a weak interaction of the zirconium-hydride
with C1. The C2–B1 vector is slightly oriented toward the
metallocene (angle C1-C2-B1: 169.8(3)8) and the hydride at B
is oriented toward the Zr atom; it is probably unsymmetri-
cally bridging. Complex 9 shows a pronouncedly unsym-
metrical coordination of the substituted acetylene ligand to
the [Cp2ZrH+] moiety with a short Zr1–C2 bond (2.327(3) ꢁ)
and a markedly longer Zr1–C1 linkage (2.516 (4) ꢁ), as is
often observed in related alkene or alkyne units coordinated
to alkyl zirconocene cations.[11]
In solution complex 9 shows a 1H NMR Cp singlet at d =
4.94 (13C: d = 104.1 ppm). The B-H-Zr hydrogen atom was
detected at d = ꢁ2.29 ppm, the Zr-H-C hydrogen atom at d =
Scheme 4. Mechanistic pathways for the formation of zwitterionic
compound 9.
2
3.80 ppm (d, JHH = 5.5 Hz).[12,13] We observe the large chem-
ical shift difference of the alkyne ligand carbon NMR signals
1
ꢀ
ꢀ
([Si]C : d = 79.8 ppm ( JCH ꢂ 44 Hz); C[B]: d = 151.4 ppm)
that is typical for the unsymmetrical bonding situation of an
acetylene ligand p-coordinated to
a
[Cp2ZrR+] cation
moiety.[11] We also detect a single set of three 19F NMR
signals at d = ꢁ129.5 (o), ꢁ157.2 (p), and ꢁ162.9 ppm (m)
with a small dp,m shift difference [11B NMR: d = ꢁ22.8 ppm (d,
1JBH ꢂ 60 Hz)].
Our study shows that the borane HB(C6F5)2 readily and
selectively abstracts a s-acetylide ligand from a zirconocene
complex. The resulting zwitterionic intermediate then has
several competing choices for stabilization. In the imido
zirconocene case (3) the rarely observed 1,1-hydroboration
reaction is preferred, whereas in the alkenyl zirconocene
analogue (6) hydride transfer to zirconium followed by
reductive coupling prevails to yield the unique product 7.
Although p-alkene and p-alkyne zirconocenes are known to
sometimes exhibit a pronounced metallacyclopropene s-
complex character,[14] the system 7 formally may be regarded
as a Cp2ZrII derivative. Two pathways for the reaction of 7
with dihydrogen can thus be formulated. Pathway (b) in
Scheme 4 describes a “conventional stepwise route” where
both the zirconocene and the borane functionalities act
separately in two consecutive steps, namely oxidative H–H
addition to zirconium (to give 11) followed by hydride
abstraction by the borane to generate 8.
Figure 4. Calculated transition-state structure of the 7+H2!8 reac-
tion.
molꢁ1, DH°(220 K) = 0.1 kcalmolꢁ1) of the activation. Rear-
rangement of the zwitterionic intermediate 8 to its isomer 9b
is exothermic again by ꢁ4.7 kcalmolꢁ1 (at 298 K). Interest-
ingly, our DFT optimizations in the gas phase identified two
isomers of 9: Besides the structure found in the solid state
(9b), a second, slightly more stable isomer exhibits a long H1–
Zr1 and a shorter C1–H1 bond (9a) [see the Supporting
Information].
DFT calculations (TPSS-D3/def2-TZVP)[15] have been
performed on the intermediates and the transition structure
of H2 cleavage in Scheme 4. We have tried to identify
a product (11) of oxidative H2 addition to the zirconocene
complex 7 (pathway (b)) without success. However, path-
way (a) in Scheme 4 has been found to be a facile and
exothermic process (Figure 4 and Table 1): Side-on coordina-
tion of H2 at the metal center is enthalpically favored by
ꢁ6.1 kcalmolꢁ1 at 298 K. Coordination of H2 at the side of the
boron atom (10a) is preferred over the alternative coordina-
tion at the silyl-substituted side (10b, ꢁ0.8 kcalmolꢁ1). The
transition structure of H2 cleavage (see Figure 4) does not
require extensive geometrical changes of 10a except the
Table 1: DFT calculated (TPSS-D3/def2-TZVP) relative energies (in
kcalmolꢁ1) of intermediates and the H2 cleavage transition structure in
the formation of compound 9 from 7.[a]
Structure
DE
DH(298 K)[b]
DH(220 K)[b]
10a
10b
TS(10a-8)
8
ꢁ9.8
ꢁ6.1
ꢁ0.8
ꢁ5.9
ꢁ0.7
ꢁ4.9
ꢁ8.8
ꢁ6.1
ꢁ5.8
ꢁ12.3
ꢁ17.6
ꢁ17.0
ꢁ8.4
ꢁ8.1
9a
ꢁ13.3
ꢁ13.1
ꢁ12.9
ꢁ12.8
9b (=“9”)
ꢁ
elongation of the H H bond to 1.043 ꢁ, accompanied by an
[a] Referenced to the sum of energies of complex 7 and H2. [b] Includes
ZPVE, Hvib, Hrot, Htrans at the given temperature and 1 atm.
almost vanishing enthalpic barrier (DH°(298 K) = 0 kcal
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Dierker, R. Frçhlich, G. Kehr, G. Erker, J. Organomet. Chem.
2011, 696, 1184 – 1188.
It seems that the synergistic oxidative addition path-
way (a)[16] involving both the formally low-oxidation state
zirconium and the borane Lewis acid in the actual step of
heterolytic cleavage of the H–H molecule leading to 8 (and
subsequently 9) is favored over the conventional stepwise
mechanism pathway (b).
It is interesting to note that the heterolytic splitting of
dihydrogen by a variety of “frustrated Lewis pairs”,[17] such as
the recently reported very reactive hydrogen activating
system 12 (see Scheme 5,)[18] resembles the synergistic path-
way (a) in Scheme 4 conceptually. It seems that currently
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Received: March 20, 2012
Revised: May 22, 2012
Published online: && &&, &&&&
Keywords: boron · frustrated Lewis pairs · hydrogen activation ·
.
oxidative addition · zirconium
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4
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Angew. Chem. Int. Ed. 2012, 51, 1 – 6
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Chemie
Communications
H2 Activation
S. K. Podiyanachari, R. Frçhlich,
C. G. Daniliuc, J. L. Petersen,
C. Mꢁck-Lichtenfeld, G. Kehr,
G. Erker*
&&&& — &&&&
Let’s split: Reaction of the zirconium
complex A with Piers’ borane [HB(C6F5)2]
yields the unusual borylalkyne zircono-
cene complex B which reacts with dihy-
drogen, activating it to give the doubly
hydrido bridged alkyne zirconium com-
plex C.
Hydrogen Activation by an Intramolecular
Boron Lewis Acid/Zirconocene Pair
H2-Aktivierung
S. K. Podiyanachari, R. Frçhlich,
C. G. Daniliuc, J. L. Petersen,
C. Mꢁck-Lichtenfeld, G. Kehr,
G. Erker*
&&&& — &&&&
Wasserstoffspaltung: Die Reaktion des
Zirconiumkomplexes A mit dem Piers-
Boran [HB(C6F5)2] liefert den ungewçhn-
lichen Borylalkin-Zirconocenkomplex B,
der Diwasserstoff aktivieren kann und
dabei den doppelt Hydrido-verbrꢁckten
Alkinzirconiumkomplex C ergibt.
Hydrogen Activation by an Intramolecular
Boron Lewis Acid/Zirconocene Pair
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ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5
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.
Angewandte
Communications
Communications
H2 Activation
S. K. Podiyanachari, R. Frçhlich,
C. G. Daniliuc, J. L. Petersen,
C. Mꢁck-Lichtenfeld, G. Kehr,
G. Erker*
&&&& — &&&&
Let’s split: Reaction of the zirconium
complex A with Piers’ borane [HB(C6F5)2]
yields the unusual borylalkyne zircono-
cene complex B which reacts with dihy-
drogen, activating it to give the doubly
hydrido bridged alkyne zirconium com-
plex C.
Hydrogen Activation by an Intramolecular
Boron Lewis Acid/Zirconocene Pair
6
www.angewandte.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
These are not the final page numbers!