Preparation of iniidazolin-2-iminato molybdenum and tungsten benzylidyne complexes: A new pathway to highly active alkyne metathesis catalysts

Article abstract of DOI:10.1002/chem.201000597

The reaction of [PhC=MBr₃(dme)] (dme = 1,2-dimethoxyethane) with the hexafluoro-fert-butoxides LiJ. or KX [X = OC(CF₃)₂Me] afforded the benzylidyne complexes [PhC=MX₃(dme)] (2a: M = W, 2b: M = Mo), which further

Full text of DOI:10.1002/chem.201000597

DOI: 10.1002/chem.201000597
Preparation of Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne
Complexes: A New Pathway to Highly Active Alkyne Metathesis Catalysts
Birte Haberlag,[a] Xian Wu,[a] Kai Brandhorst,[b] Jçrg Grunenberg,[b]
Constantin G. Daniliuc,[a] Peter G. Jones,[a] and Matthias Tamm*[a]
ꢀ
Abstract: The reaction of [PhC MBr3-
Complexes 3a and 3b are able to effi-
ciently catalyse alkyne cross metathesis
of various 3-pentynyl benzyl ethers 5
and benzoic esters 7 at room tempera-
ture, to afford 2-butyne and the corre-
sponding diethers 6 and diesters 8. The
tungsten complex 3a proved to be a su-
perior catalyst for ring-closing alkyne
metathesis, and the [10]cyclophanes 10
and 12 were synthesised in high yield
from 1,3-bis(3-pentynyloxymethyl)ben-
zene (9) and bis(3-pentynyl) phthalate
(11), respectively. The molecular struc-
tures of compounds 2a, 2b, 3a, 3b, 4,
and 12 were determined by single-crys-
tal X-ray diffraction. DFT calculations
have been carried out for catalyst sys-
tems based on the imidazolin-2-iminato
tungsten and molybdenum complexes
3a and 3b by choosing the alkyne
metathesis of 2-butyne as the model re-
action; the studies revealed a lower ac-
tivation barrier for the tungsten
system.
A
(dme=1,2-dimethoxyethane)
with the hexafluoro-tert-butoxides LiX
or KX [X=OC(CF3)2Me] afforded the
benzylidyne complexes [PhC MX3-
(dme)] (2a: M=W, 2b: M=Mo),
which further reacted with the lithium
reagent Li
(ImtBuN), generated with
ACHTUNGTRENNUNG
ꢀ
ACHTUNGTRENNUNG
ACHTUNGTRENNUNG
MeLi from 1,3-di-tert-butylimidazolin-
2-imine (ImtBuNH), to form the imida-
ꢀ
zolin-2-iminato complexes [PhC MX2-
(ImtBuN)] (3a: M=W, 3b: M=Mo).
ꢀ
The propylidyne complex [EtC MoX2-
Keywords: alkynes · imines · meta-
thesis · molybdenum · tungsten
ACHTUNGTRENNUNG
ment of 3b with an excess of 3-hexyne.
ꢀ
Introduction
G
(OCCMe3)3];[3] 3) molybdenum-
ACHTUNGTRENNUNG
To date, there are only a limited number of catalysts known
that efficiently promote the metathesis of alkynes, despite
the great potential of catalytic alkyne metathesis as a syn-
thetic tool for the preparation of natural products and
highly unsaturated conjugated polymers.[1] The catalyst sys-
tems at hand include: 1) the Mortreux system, consisting of
[Mo(CO)6] and phenol additives;[2] 2) Schrock-type tungsten
alkylidyne complexes, such as the prototypical neopentyl-
(tBu)Ar}3];[4] and, more recently, 4) molybdenum nitride
ACHTUNGTRENNUNG
complexes with Ph3SiO ligands developed by Fꢀrstner.[5] As
a variation of Schrock-type alkylidyne complexes, we have
introduced
imidazolin-2-iminato
tungsten
idyne complexes such as 1,[6] which display high
catalytic activity in alkyne cross metathesis (ACM), ring-
closing alkyne metathesis (RCAM) and ring-opening alkyne
metathesis polymerisation (ROMP), even at ambient tem-
perature and low catalyst loadings (Scheme 1).[6,7] According
[a] Dipl.-Chem. B. Haberlag, X. Wu, Dr. C. G. Daniliuc,
Prof. Dr. P. G. Jones, Prof. Dr. M. Tamm
Institut fꢀr Anorganische und Analytische Chemie
Technische Universitꢁt Carolo-Wilhelmina zu Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Fax : (+49)531-391-5309
E-mail: m.tamm@tu-bs.de
[b] Dr. K. Brandhorst, Priv.-Doz. Dr. J. Grunenberg
Institut fꢀr Organische Chemie
Technische Universitꢁt Carolo-Wilhelmina zu Braunschweig
Hagenring 30, 38106 Braunschweig (Germany)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201000597.
Scheme 1. Catalytic alkyne metathesis with an imidazolin-2-iminato tung-
sten alkylidyne complex.
8868
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8868 – 8877

FULL PAPER
to theoretical calculations,[6a,c,d] the excellent catalytic perfor-
mance seems to rely on the combination of electron-with-
drawing alkoxides, such as hexafluoro-tert-butoxide, with
strongly electron-donating imidazolin-2-iminato ligands,[8] to
provide a subtle balance between the stability and activity
of the resulting tungsten alkylidyne complexes.
Our best and most reliable pre-catalyst, the neopentyl-
(ImtBuN)] (1)
ACHTUNGTRENNUNGidyne complex [Me3CC W{(OCACHUTNTRGEGN(NNU CF3)2Me)}2ACTHUNGTRENNGUN
ꢀ
(ImtBuN=1,3-di-tert-butylimidazolin-2-iminato) was synthes-
ised following the classical high-oxidation-state route,[6a,d]
starting from tungsten(VI) chloride, which requires the use
of six equivalents of neopentyl magnesium chloride to
ꢀ
afford [Me3CC WACHTUNGTRENNUNG(CH2CMe3)3] by a twofold a-deprotona-
tion.[3] Subsequently, three more neopentyl equivalents are
sacrificed by treatment with ethereal HCl to produce the tri-
ꢀ
chloride
A
[Me3CC WCl
(dme)]
(dme=1,2-dimethoxy-
ethane)[3b] as the key intermediate for further introduction
of the alkoxide and imidazolin-2-iminato ligands
(Scheme 2). For us, this protocol proved rather difficult in
terms of up-scaling, which was attempted in order to supply
larger quantities of the alkyne metathesis pre-catalyst. Con-
sequently, we aimed to develop a low-oxidation-state route
starting from [W(CO)6], as Mayr et al. used this starting ma-
ꢀ
terial for the synthesis of the benzylidyne complex [PhC
WBr
3A
quent preparation of the benzylidyne analogue of 1. Natu-
rally, both systems should exhibit similar catalytic activities
in alkyne metathesis because the original alkylidyne moiety
is lost during the initiation step. It is therefore the scope of
this contribution to give an account of the preparation and
structural characterisation of the imidazolin-2-iminato tung-
ꢀ
(ImtBuN)]
sten benzylidyne complex [PhC M{OCACTHNUTRGENN(UG CF3)2Me}2ACHUTNGTRENNGUN
(M=W, 3a) together with an investigation of its catalytic
performance. In addition, the corresponding molybdenum
complex 3b (M=Mo) is reported, and its catalytic activity is
compared to that of its tungsten congener by means of ex-
perimental and theoretical methods.
Results and Discussion
Preparation of imidazolin-2-iminato tungsten benzylidyne
complexes: As described by Mayr,[9a,b] the dark green tung-
Scheme 2. High- and low-oxidation-state routes to tungsten and molybde-
num alkylidyne complexes.
ꢀ
sten tribromo complex [PhC WBr
ised conveniently from [W(CO)6] by reaction with PhLi fol-
lowed by addition of [NMe4]Br, to afford the acyl complex
(dme)] can be synthes-
ꢀ
temperature affords the trialkoxide [PhC W{OC
A
[NMe4][(CO)5W
(COPh)].[10] This stable intermediate is
N
treated with oxalyl bromide in CH2Cl2 at À788C, resulting
78% yield by recrystallisation from diisopropyl ether
(iPr2O) at À358C.
in the formation of the Fischer carbyne complex trans-
[9,11]
[PhC W(CO)4Br].
Subsequently, decarbonylation and
The 13C NMR spectrum of 2a shows a characteristic low-
field resonance at 278.5 ppm attributable to the alkylidyne
carbon atom, which is slightly upfield from the resonance
observed for the corresponding neopentylidyne complex
(294.7 ppm),[12] and falls in the range typically observed for
ꢀ
oxidation can be achieved by reaction with Br2 at À788C in
ꢀ
the presence of 1,2-dimethoxyethane (dme), to give [PhC
WBr
(dme)] in approximately 60–70% overall yield. Simi-
larly to the preparation of 1,[6a] the fluorinated alkoxides are
1
best introduced at this stage, and the reaction of [PhC
alkylidyne complexes.[1g] Two broad H NMR resonances at
ꢀ
WBr
3A
3.06 and 3.45 ppm are observed for the CH2 and CH3 groups
tert-butoxide, Li[OC
N
of the dme ligand, and the 19F NMR spectrum exhibits only
Chem. Eur. J. 2010, 16, 8868 – 8877
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8869

M. Tamm et al.
one singlet at À76.8 ppm, indicating dynamic behaviour of
2a at room temperature on the NMR timescale. The crystal
structure of 2a was determined by X-ray diffraction analy-
sis; Figure 1 shows an ORTEP diagram of 2a, confirming
exhibit two quartets each, at 125.7 and 126.2 ppm (1JCF
=
285 Hz), and at À78.2 and À76.4 ppm (4JFF =18 Hz), respec-
tively, for the diastereotopic CF3 groups. 3a crystallised in
the orthorhombic space group P212121 with three independ-
ent molecules in the asymmetric unit; an ORTEP diagram
of molecule 1 is shown in Figure 2, confirming the formation
Figure 1. ORTEP diagram of 2a with thermal displacement parameters
À
drawn at 50% probability. Selected bond lengths [ꢃ] and angles [8]: W
Figure 2. ORTEP diagram of one of the three independent molecules 3a
with thermal displacement parameters drawn at 50% probability. Select-
ed bond lengths [ꢃ] and angles [8] in molecule1/molecule2/molecule3:
À
À
À
À
C1 1.761(3), W O1 1.9554(19), W O2 1.912(2), W O3 1.970(2), W O4
À
2.3675(19), W O5 2.217(2); O2-W-O1 95.45(8), O2-W-O3 95.26(8), O2-
W-O5 156.08(7), O1-W-O5 79.51(8), O3-W-O5 79.63(8), C1-W-O4
172.46(11), O2-W-O4 82.47(7), O1-W-O4 76.19(7), O3-W-O4 78.19(7),
O5-W-O4 73.62(7), C2-C1-W 178.3(2).
À
À
À
À
À
W C1 1.759(3), W’ C1’ 1.766(3), W’’ C1’’ 1.765(3), W N1 1.841(2), W’
À
À
À
À
N1’ 1.842(2), W’’ N1’’ 1.845(2), W O1 1.914(2), W’ O1’ 1.924(2), W’’
À
À
À
O1’’ 1.930(2), W O2 1.930(2), W’ O2’ 1.923(2), W’’ O2’’ 1.916(2); C2-
C1-W 175.36(2), C2’-C1’-W’ 173.6(2), C2’’-C1’’-W’’ 172.7(2), C1-W-N1
107.89(10), C1’-W’-N1’ 107.62(10), C1’’-W’’-N1’’ 107.85(10), C8-N1-W
159.44(18), C8’-N1’-W’ 156.22(18), C8’’-N1’’-W’’ 157.55(19).
the formation of a monomeric tungsten alkylidyne complex
that is stabilised by coordination of a chelating dme ligand.
The three alkoxides adopt a meridional arrangement around
the tungsten atom, giving a strongly distorted octahedral co-
ordination sphere. In fact, the structure is much better de-
scribed as a square pyramid, with the carbyne carbon atom
C1 in the apical position and the oxygen atom O4 capping
the basal plane; this is consistent with the observations that
the coordinated dme ligand features two distinctly different
W O bond lengths (W O4=2.368(2) ꢃ and W O5=
2.217(2) ꢃ) and that the C1-W-O angles containing the four
equatorial oxygen atoms O1–O3 and O5 range from
98.88(11)8 to 105.04(11)8, thus deviating significantly from
the rectangular orientation expected for a regular octahe-
of a monomeric tungsten benzylidyne complex with a slight-
À
ly distorted tetrahedral geometry. The W C distances in the
three independent molecules are 1.759(3), 1.766(3), and
1.765(3) ꢃ, respectively, effectively identical to the values
found for 1 (1.768(3) and 1.764(3) ꢃ for two independent
molecules).[6a] The W-C-C axes are close to linearity
(175.36(19), 173.6(2), 172.7(2)8), whereas considerably
smaller W-N-C angles are observed (159.44(18), 156.22(18),
157.56(19)8). Two of the three molecules are closely similar
(r.m.s. deviation 0.69 ꢃ); the third differs in the orientation
À
À
À
of the CMeACHTNUGTRNEUGN(CF3)2 group at O2.
À
dron. The W C1 distance of 1.761(3) ꢃ and the W-C1-C2
angle of 178.3(2)8 are very similar to the values found for
the corresponding neopentylidyne complex.[6c]
Preparation of imidazolin-2-iminato molybdenum benzyli-
dyne complexes: Because molybdenum complexes are com-
monly expected to exhibit higher catalytic activities in olefin
metathesis than their tungsten congeners,[1b,i,13] we aimed to
prepare molybdenum alkylidyne complexes containing the
same combination of electron-withdrawing alkoxides and
strongly electron-donating imidazolin-2-iminato ligands as
described for the tungsten complexes 1 and 3a. A high-oxi-
dation-state route would involve the preparation of
The reaction of 2a with the lithium reagent (ImtBuN)Li,
obtained from the reaction of 1,3-di-tert-butylimidazolin-2-
imine (ImtBuNH) with methyl lithium, effects the substitu-
tion of one hexafluoro-tert-butoxide ligand together with the
dme solvate molecule, and the formation of the imidazolin-
2-iminato benzylidyne complex 3a as an orange crystalline
solid in satisfactory yield (70%). Again, the 13C NMR reso-
nance for the carbyne carbon atom in the benzylidyne com-
plex 3a is observed at higher field (270.6 ppm) compared to
that of its neopentylidyne congener 1 (285.6 ppm).[6a] In
agreement with the formation of a configurationally stable
Cs-symmetric complex, the 13C and 19F NMR spectra of 3a
trialkoxy
ACHTUNGTRENUNmNG olybdenum(VI) alkylidyne complexes from
[MoO2Cl2], as described by Schrock in 1985.[14] However,
ꢀ
the key intermediate [Me3CC MoACHTNUTRGNEU(NG CH2CMe3)3] was synthes-
ised only in low yield (35%),[15] and therefore we immedi-
ately focused on adapting the low-oxidation-state route, out-
8870
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8868 – 8877

Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes
FULL PAPER
lined above for tungsten, to the corresponding molybdenum
benzylidyne complexes. Starting from [Mo(CO)6], the anion-
ic acyl pentacarbonyl molybdenum complex [NMe4]
(1H, 13C, 19F) of 3b are almost identical to those observed
for its tungsten congener 3a, with the only exception being
the marked downfield shift for the carbyne carbon atom
(287.0 ppm). Similarly to the 2a/2b pair, 3a and 3b are iso-
typic, and, accordingly, 3b crystallised in the orthorhombic
space group P212121 with three independent molecules in the
asymmetric unit; an ORTEP diagram of molecule 1 is
shown in Figure 4. Naturally, the structural parameters are
[(CO)5MoACHTUNGTRENNUNG(COPh)] is obtained after addition of phenyl lithi-
um and [NMe4]Br.[10]
Treatment with oxalyl bromide at À788C affords the in-
ꢀ
termediate
Fischer
carbyne
complex
trans-
G
Mo(CO)4Br], which further reacts with bromine in the pres-
ꢀ
À
ence of dme to give the tribromo complex [PhC MoBr3-
A
very similar to those observed for 3a, with the Mo C distan-
ces of 1.743(2), 1.742(2) and 1.742(2) ꢃ being slightly short-
(dme)] as a brown crystalline solid in 60% overall yield.[9a]
À
Subsequently, the use of potassium hexafluoro-tert-butoxide,
K[OC(CF3)2Me], is required to achieve clean substitution
and formation of the trialkoxy benzylidyne complex [PhC
Mo{OC(CF3)2Me}3A(dme)] (2b). It should be noted that the
er than the W C distances in 3a (vide supra).
ACHTUNGTRENNUNG
ꢀ
N
CHTUNGTRENNUNG
latter complex had already been isolated by Schrock from
the alkyne metathesis reaction between the neopentylidyne
ꢀ
complex [Me3CC Mo{OC
A
(CF3)2Me}
E
acetylene.[14a] The 13C NMR resonance for the carbyne
carbon atom in 2b is observed at lower field (294.6 ppm) in
comparison with 2a, whereas all other chemical shifts are
almost identical. Red single crystals were isolated from
iPr2O solution at À358C, and the molecular structure of 2b
was determined by X-ray diffraction analysis (Figure 3).
Figure 4. ORTEP diagram of one of the three independent molecules 3b
with thermal displacement parameters drawn at 50% probability. Select-
ed bond lengths [ꢃ] and angles [8] in molecule1/molecule2/molecule3:
À
À
À
À
Mo C1 1.743(2), Mo’ C1’ 1.742(2), Mo’’ C1’’ 1.742(2), Mo N1
À
À
À
1.8397(18), Mo’ N1’ 1.8444(18), Mo’’ N1’’ 1.8351(18), Mo O1
À
À
À
1.9311(15), Mo’ O1’ 1.9353(15), Mo’’ O1’’ 1.9311(15), Mo O2
À
À
1.9448(14), Mo’ O2’ 1.9376(15), Mo’’ O2’’ 1.9379(15); C2-C1-Mo
176.10(18), C2’-C1’-Mo’ 174.40(19), C2’’-C1’’-Mo’’ 174.02(18), C1-Mo-N1
106.62(9), C1’-Mo’-N1’ 106.05(9), C1’’-Mo’’-N1’’ 106.37(9), C8-N1-Mo
157.55(16), C8’-N1’-Mo’ 153.73(16), C8’’-N1’’-Mo’’ 155.57(16).
To investigate the reactivity of 3b towards alkynes, a
hexane solution of 3b was treated with a tenfold excess of
Figure 3. ORTEP diagram of 2b with thermal displacement parameters
À
drawn at 50% probability. Selected bond lengths [ꢃ] and angles [8]: Mo
ꢀ
3-hexyne (EtC CEt) at room temperature. The solution was
À
À
À
C1 1.7555(17), Mo O1 1.9599(11), Mo O2 1.9148(11), Mo O3
subsequently cooled to À358C to afford orange crystals,
which were subjected to X-ray diffraction analysis. The re-
sulting molecular structure is shown in Figure 5, revealing
that the propylidyne complex 4 has formed (Scheme 3).
Complex 4 exhibits a slightly distorted tetrahedral geome-
À
À
1.9697(11), Mo O4 2.378(11), Mo O5 2.2487(11); O2-Mo-O1 96.11(5),
O2-Mo-O3 95.75(5), O2-Mo-O5 156.72(4), O1-Mo-O5 79.12(5), O3-Mo-
O5 79.68(4), C1-Mo-O4 172.01(6), O2-Mo-O4 83.49(4), O1-Mo-O4
76.65(4), O3-Mo-O4 78.84(4), O5-Mo-O4 73.24(4), C2-C1-Mo 178.19(14).
ꢀ
try; the Mo C1 triple bond (1.739(4) ꢃ) is shorter, and the
À
À
À
À
Complex 2b is isotypic to its tungsten congener 2a, and, ac-
cordingly, the geometries around the metal atoms in 2a and
Mo C1 C2 (171.8(3)8) and Mo N1 C4 angles (149.9(3)8)
are smaller than observed for the benzylidyne complexes 2b
and 3b. It should be noted that similar reactions employing
the tungsten complexes 1 (or 3a) lead to the clean forma-
À
2b are very similar (vide supra). The Mo C1 distance
(1.756(2) ꢃ) is marginally shorter than the corresponding
À
W C1 distance (1.761(3) ꢃ), which is in agreement with the
tion of a metallacyclobutadiene complex with a {WACHTUNGTRENNUNG(C3Et3)}
almost identical ionic radii reported for MoVI and WVI.[16]
Finally, the imidazolin-2-iminato ligand can be introduced
by substitution of one hexafluoro-tert-butoxide ligand to-
gether with the dme ligand (Scheme 2), affording 3b as
orange crystals in 60% yield. Again, the spectroscopic data
moiety,[6a] from which the corresponding tungsten propyli-
dyne complex can be obtained by cycloreversion.[6c] In con-
trast, we were unable to isolate a stable metallacyclobuta-
diene intermediate from the reaction of 3b with 3-hexyne,
and this failure is in line with the fact that the formation of
Chem. Eur. J. 2010, 16, 8868 – 8877
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8871

M. Tamm et al.
Figure 5. ORTEP diagram of 4 with thermal displacement parameters
À
drawn at 50% probability; the O1 CMe
two positions, only one position is shown. Selected bond lengths [ꢃ] and
G
Scheme 4. Cross metathesis of the 3-pentynyl ether 5 and esters 7; reac-
tion conditions: toluene (8 mL), n(substrate)=0.5 mmol, nACTHNUTRGNEUG(N catalyst)=
À
À
angles [8]: Mo C1 1.739(4), Mo N1 1.844(3), Mo O1 1.950(6), Mo O2
1.949(2); C2-C1-Mo 171.8(3), C1-Mo-N1 105.83(16), C4-N1-Mo 149.9(3).
5 mmol (1 mol%), T=298 K, pꢁ200 mbar, t=1 h; 5, 7a, catalyst=3a or
3b; 7b–e, catalyst=3a; yield: 8a 97%, 8b,c 98%, 8d 94%, 8e 17%.
Scheme 3. Reaction of 3b with an excess of 3-hexyne.
molybdenacyclobutadienes has rarely been observed, and
that such species have only been detected by NMR stud-
Figure 6. Conversion versus time diagram for the cross metathesis of 5;
reaction conditions: toluene (4 mL), n(substrate)=0.5 mmol, n-
AHCTUNGERTG(NNUN catalyst)=5 mmol (1 mol%), T=298 K, reduced pressure. Samples were
collected in a stream of argon in less thnn 3 min. The conversion was
monitored by gas chromatography.
ies.[14,17] Instead, only the propylidyne complex [EtC Mo-
ꢀ
(NImtBu
){OCMe
G
from the hexane solution.
Catalytic alkyne metathesis with tungsten and molybdenum
alkylidyne complexes: To test the complexes 3a and 3b for
preparative alkyne cross metathesis (ACM), 3-pentynyl
ether 5 was chosen as a model substrate (Scheme 4). In a
typical experiment, a toluene solution of 5 and the catalyst
(1 mol%) was stirred for 60 min at room temperature under
vacuum-driven conditions to remove 2-butyne continuously.
Within 10 min intervals, the solvent was completely evapo-
rated, and samples for gas chromatographic analysis were
taken after re-addition of the original solvent volume
(4 mL). The resulting conversion versus time diagram is
shown in Figure 6, and illustrates that both catalysts are
active at room temperature and are able to accomplish full
conversion within 60 min, despite the observation of a faster
initiation rate for the tungsten complex 3a. After 60 min,
the dimer 6 could be isolated in over 90% yield by filtration
through alumina and evaporation of the solvent.
and the diester 8a was isolated in 97% (3a) and 95% yield
(3b). Additionally, the tungsten complex 3a was used for
the cross metathesis of various 3-pentynyl benzoates 7 with
various substituents in the 4-position of the phenyl ring. The
resulting diesters 8 were isolated in more than 90% yield
for X=H, Cl, OMe, and SMe, indicating a promising func-
tional group tolerance for this catalyst system. In contrast,
only 17% of the nitro compound 8e could be obtained in
the presence of 1 mol% catalyst loading.[18] Future studies
are necessary to uncover the full range of functional groups
that are compatible with our catalyst system.
We have recently demonstrated that the neopentylidyne
complex 1 exhibits high catalytic activity in the ring-closing
alkyne metathesis (RCAM) of a,w-diynes such as o-, m- and
p-di(3-pentynyloxymethyl)benzenes. Depending on the sub-
stitution pattern, different selectivities towards the forma-
tion of monomeric [10]cyclophanes or dimeric [10.10]cyclo-
phanes were observed, which could be attributed to the rela-
Similar results were obtained by using the corresponding
3-pentynyl ester 7a as the substrate[4g] for catalytic ACM,
8872
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8868 – 8877

Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes
FULL PAPER
tive thermodynamic stability of the emerging cycloalkyne-
s.[7a] In case of the meta-diyne 9, exclusive formation of the
[10]-meta-cyclophane 10 was observed, and, consequently,
this substrate was used for studying the performance of
complexes 3a and 3b in catalytic RCAM reactions
(Scheme 5). A 4.5 mm solution of 7 and the catalyst 3a or
Figure 7. ORTEP diagram of 12 with thermal displacement parameters
À
drawn at 50% probability. Selected bond lengths [ꢃ] and angles [8]: O1
À
À
C1 1.2074(18), O3 C8 1.2025(17), C4 C5 1.189(2); O2-C1-C14
111.75(11), O4-C8-C9 110.01(11).
temperature, theoretical calculations using density function-
al theory (DFT) were performed. Similarly to our previous
studies,[6a,c] the symmetric metathesis of 2-butyne (MeC
CMe) was chosen as the model reaction (Scheme 6). Based
ꢀ
Scheme 5. RCAM of 1,3-bis(3-pentynyloxymethyl)benzene 9 and bis(3-
pentynyl)phthalate 11; reaction conditions: toluene (4.5 mm), 2 mol%
catalyst (3a or 3b), T=298 K, p=300 mbar, t=2 h.
Scheme 6. Symmetric metathesis of 2-butyne (R=OCMeACHTNUGTRNEG(UN CF3)2; L=
ImtBuN).
3b (2 mol%) in toluene was stirred for 2 h at room temper-
ature under reduced pressure to remove 2-butyne continu-
ously. Filtration through alumina and evaporation of the sol-
vent afforded 10 in 86% isolated yield by using the tungsten
catalyst 3a, whereas only 47% could be obtained for the
molybdenum system 3b, indicating that the latter seems to
be a less active catalyst for alkyne metathesis at room tem-
perature than 3a.
The di(3-pentynyl) phthalate 11 was chosen as a second
substrate for RCAM, as theoretical calculations indicate a
strong preference for the formation of the [10]-ortho-cyclo-
phane 12.[19] Under the same conditions as described for 9,
the RCAM of 11 in the presence of catalytic amounts of 3a
or 3b (2 mol%) afforded the cycloalkyne 12 in excellent
yield (98%) in the case of 3a, whereas, once again, a signifi-
cantly lower yield (20%) was obtained for 3b (Scheme 5).
12 was fully characterised by means of spectroscopic meth-
ods,[4d,m] and a single crystal of 12 was additionally subjected
to X-ray diffraction analysis. The resulting molecular struc-
ture is shown in Figure 7, which confirms unequivocally the
formation of a monomeric cycloalkyne, and the existence of
on the conventional [2+2] cycloaddition–cycloreversion
mechanism,[21,22] the first three relevant stationary points
(catalyst plus 2-butyne, transition state TS, and metallacyclo-
butadiene intermediate IN) of the rate-determining alkyne
ꢀ
metathesis step were characterised for the [MeC WR2L]
ꢀ
and [MeC MoR2L] model systems (R=OCMe
A
ImtBuN). It should be noted that a full catalytic cycle addi-
tionally involves the interconversion between two metallacy-
clobutadiene intermediates. However, since this rearrange-
ment has a significantly lower barrier, no attempts were
made to locate this second transition state. Table 1 summa-
rises the calculated B3LYP energies and enthalpies. For
comparison of these gas-phase computations with our exper-
imental results, it should be noted that, for the associative
mechanism at work, entropic effects are expected to be
smaller in the condensed phase, which should afford consid-
erably lower free activation barriers.
ꢀ
an unstrained 12-membered ring. The C4 C5 triple bond
Table 1. Relative B3LYP energies and enthalpies (in kcalmolÀ1) for the
reaction of selected catalysts with 2-butyne (Scheme 6).[a]
length is 1.189(2) ꢃ, and the tilted conformation of the two
carboxylic groups is similar to those found for other organic
phthalates.[20]
TS[b]
IN[b]
DH298
Catalyst
DE°0
DH°298
DG°298
DE0
DG298
3a
3b
M: W
M: Mo
9.78
15.24
8.99
14.49
26.05
30.10
À2.47
À3.54
13.67
19.98
Theoretical investigation of alkyne metathesis with tungsten
and molybdenum alkylidyne complexes: To study the rela-
tive catalytic activities of molybdenum and tungsten imida-
zolin-2-iminato benzylidyne complexes 3a and 3b at room
5.39
4.54
[a] For details, see the Experimental Section. [b] DE0: relative energies at
0 K, DH298: enthalpies at 298 K, DG298: Gibbs free energies at 298 K; all
values are given with respect to the isolated catalyst and 2-butyne.
Chem. Eur. J. 2010, 16, 8868 – 8877
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8873

M. Tamm et al.
GC-2010, GC-MS on a JOEL AccuTOF, and MS on a Finnigan MAT 95
The molybdenum system exhibits a free energy barrier
(DG2°98) of 30.10 kcalmolÀ1, which is 4.05 kcalmolÀ1 higher
than that calculated for the tungsten system (26.05 kcal
molÀ1) (see Table 1). Assuming a similar frequency factor in
the Arrhenius equation for both reactions, this difference
(DDG°298) results in a decrease of the rate constant for the
Mo system, which is in line with the observed inferior cata-
lytic behaviour of 3b as an RCAM catalyst (vide supra).
The entropic contributions to DG for the W and Mo systems
are computed to be 17.06 and 15.61 kcalmolÀ1, respectively,
for the activation barrier (DG°298), and 17.21 and 15.44 kcal
molÀ1, respectively, for the formation of the metallacyclobu-
tadiene intermediates (DG2o98). Accordingly, the differences
in DG°298 and DG2o98 are largely enthalpic in nature. The for-
mation of the metallacyclobutadiene intermediate IN is exo-
thermic for the tungsten system (DHo298 =À3.54 kcalmolÀ1),
whereas the corresponding reaction with the Mo catalyst
proceeds endothermically (DHo298 =+4.54 kcalmolÀ1), which
might be one reason for our inability to isolate a stable mo-
lybdenacyclobutadiene from the reaction of 3b with 3-
hexyne (vide supra).
(EI) and Finnigan MAT 95 (ESI). The compounds [PhC WBr
(dme)],[9b]
ꢀ
[NMe4][Mo(CO)5COC6H5],[10] and LiOCCH
3A
R
pared according to the literature procedures.[6a] The preparation of 1,3-
bis(3-pentynyloxymethyl)benzene (9) was previously reported.[7a] The
procedure for the synthesis of the compounds 5, 7a–e, and 11 can be
found in the Supporting Information.
ꢀ
Synthesis and characterisation of [PhC
The deep green tungsten benzylidyne complex [PhC WBr
W
{OCMe
E
E
(400 mg, 0.664 mmol) was added slowly to the lithium alkoxide Li-
[OCCH3A(CF3)2] (374 mg, 1.989 mmol), suspended in diethyl ether
A
CHTUNGTRENNUNG
(30 mL). The suspension turned brown during the addition, and was
stirred for 16 h at ambient temperature, then concentrated and filtered.
After removal of the diethyl ether, diisopropyl ether (8 mL) was added
and the brown suspension was filtered again. Orange crystals were ob-
tained after keeping the iPr2O solution at À358C for several hours.
Yield: 470 mg (78%); 1H NMR (400.1 MHz, C6D6, 258C): d=1.82 (s,
9H; OCCH
6.61–6.71 (m, 1H; p-CH), 6.87 (m, 2H; m-CH), 7.19 ppm (m, 2H; o-
CH); 13C NMR (100.6 MHz, C6D6, 258C): d=18.8 (s, OCCH
3A(CF3)2),
68.2 (s, OCH2CH2O), 74.9 (s, OCH3), 86.9 (s, OCCH3A(CF3)2), 126.3 (q,
3ACTHGNURTENN(UNG CF3)2, 3.06 (br, 4H; OCH2CH2O), 3.45 (br, 6H; OCH3),
CHTUNGTRENNUNG
CTHUNGTRENNUNG
CF3, JCF =289 Hz), 127.5 (s, CH), 127.9 (s, CH), 128.6 (s, CH), 133.8 (i-
C), 278.5 ppm (PhC W); 19F NMR (188.3 MHz, C6D6, 258C): d=
ꢀ
À76.8 ppm (s, CF3); elemental analysis calcd (%) for C23H24F18O5W: C
30.48, H 2.67; found: C 30.49, H 2.80.
ꢀ
Synthesis and characterisation of [PhC
Li
(NImtBu) (44.5 mg, 0.221 mmol) was dissolved in toluene (5 mL) and
added to [PhC WACTHUNTGNREN{NUG OCMeACHTUNTRGNEG(UN CF3)2}3AHCTUGNRTNENG(U DME)] (200 mg, 0.221 mmol) dissolved
(NImtBu
WACHTUNTGERUNNGN )ACHUTTNGRENN{GU OCMeACHTUTGNRENNUG(CF3)2}2] (3a):
AHCTUNGTRENNUNG
ꢀ
Conclusion
in toluene (5 mL). The mixture was stirred at ambient temperature for
15 min, and then concentrated and filtered. The solvent was removed and
the product was extracted with hexane (5 mL). Orange crystals were ob-
tained after keeping the hexane solution at À358C for several hours.
Yield: 128 mg (70%); 1H NMR (300.1 MHz, C6D6, 258C): d=1.27 (s
A low-oxidation-state route starting from [W(CO)6] and
[Mo(CO)6] has been developed to provide convenient and
economic access to tungsten and molybdenum imidazolin-2-
iminato alkylidyne complexes, which have been designed to
catalyse alkyne metathesis reactions efficiently. The result-
ing W and Mo benzylidyne complexes are both active pre-
catalysts for the cross metathesis of aliphatic ether- and
ester-functionalised alkynes even at room temperature.
However, the tungsten system tends to show a superior per-
formance, particularly in ring-closing alkyne metathesis; this
is in line with DFT studies, which predict a higher barrier
for the molybdenum-based catalyst system. A similar trend
has previously been derived for symmetric model complexes
ꢀ
18H; NC
6.78 (m, 1H; p-CH), 6.93–7.02 (m, 2H; m-CH), 7.12–7.22 ppm (m, 2H;
o-CH); 13C NMR (75.5 MHz, C6D6, 258C): d=19.9 (s, OCCH
3A(CF3)2),
28.5 (s, NC(CH3)3), 57.4 (s, NC(CH3)3), 83.3 (s, OCCH3A(CF3)2), 110.3 (s,
ACHUTTGNRENUN(G CH3)3), 1.94 (s, 6H; OCCH3CAHTUNTGERN(NUGN CF3)2), 5.92 (s, 2H; NCH), 6.68–
CHTUNGTRENNUNG
A
R
CHTUNGTRENNUNG
NC=CN), 125.7 (q, JCF =285 Hz, CF3), 126.2 (q, JCF =285 Hz, CF3), 127.9
(s, CH), 129.7 (s, CH), 131.8 (s, CH), 148.1 (s, i-C), 158.5 (s, NCN),
270.6 ppm (s, PhC W); 19F NMR (376.4 MHz, C6D6, 258C): d=À78.2 (q,
ꢀ
4JFF =18 Hz, 6F; CF3), À76.4 ppm (q, 4JFF =18 Hz, 6F; CF3); elemental
analysis calcd (%) for C26H31F12N3O2W: C 37.65, H 3.77, N 5.07; found: C
37.13, H 3.84, N 5.14.
ꢀ
Synthesis and characterisation of [PhC MoBr
N
(À788C) of oxalyl bromide in CH2Cl2 was added to a cold solution of
[NMe4][Mo(CO)5COC6H5] (4,25 g, 10 mmol) in CH2Cl2. After stirring the
suspension for 15 min at À788C, it was warmed to À408C and then im-
mediately cooled back to À788C. The reaction mixture was filtered
through Celite at À788C. Then ten equivalents of dme and a cold solu-
tion of Br2 in CH2Cl2 (0.18 mL, 10 mmol) were added to the filtrate at
À788C and stirred for 45 min. Finally, the solvent was removed in an ice-
water bath. The resulting brown solid was taken up in CH2Cl2 (20 mL)
and precipitated with pentane three times. The final CH2Cl2 solution was
filtered, and after precipitation with pentane a brown crystalline solid
was obtained. Yield: 3.06 g (60%); 1H NMR (200.1 MHz, C6D6, 258C):
of the type [MeC MACTHNUTRGNEUNG
(OMe)3] (M=Mo, W).[22] However, ad-
ditional calculations are required to fully uncover the key
factors that control the efficiency of d0 alkyne metathesis
catalysts.[23]
Experimental Section
d=2.93
(br,
2H;
MeOCH2-CH2OMe),
3.10
(br,
5H;
General procedures: All operations were performed in a glove box
(MBraun 200B, argon atmosphere) or on a Schlenk line using Schlenk
techniques. All solvents were purified and dried by a solvent purification
system from MBraun or using standard methods, and stored over a mo-
lecular sieve (4 ꢃ). The 1H, 13C, and 19F NMR measurements were per-
formed on Bruker DPX 200 (200 MHz) and Bruker DRX 400 (400 MHz)
devices. The chemical shifts are expressed in parts per million (ppm)
MeOCH2CH2OCH3), 3.55 (s, 3H; CH3OCH2CH2OMe), 6.64 (m, 1H;
CH), 6.98 (m, 2H; CH), 7.52 ppm (m, 2H; CH); elemental analysis calcd
(%) for C11H15Br3O2Mo: C 25.66, H 2.94; found: C 25.59, H 3.62.
ꢀ
Synthesis and characterisation of [PhC Mo
The brown molybdenum benzylidyne complex [PhC MoBr
(500 mg, 0.97 mmol) was added to the potassium alkoxide KOCCH3-
AHCTUNGTRENNG(NU CF3)2 (663 mg, 3.01 mmol) suspended in diethyl ether (20 mL). The sus-
A
(CF3)2}
(dme)] (2b):
(dme)]
1
using tetramethylsilane (TMS) (for H and 13C) and CFCl3 (for 19F) as in-
pension was stirred for 16 h at RT, and the resulting suspension was fil-
tered and extracted with small amounts of Et2O. After removal of the
solvent, the brown solid was recrystallised from iPr2O (5 mL) at À358C
as orange crystals. Yield: 700 mg (88%); 1H NMR (400.1 MHz, C6D6,
ternal standards. Coupling constants (J) are reported in Hertz (Hz). Split-
ting patterns are indicated as s (singlet), d (doublet), t (triplet), q (quar-
tet), m (multiplet) and br (broad). Elemental analysis (C, H, N) was per-
formed by combustion and gas chromatographical analysis using an Ele-
mentar varioMICRO. GC analysis was performed on a SHIMADZU
258C): d=1.81 (s, 9H; OCCH3ACHTUNTGREN(UNNG CF3)2), 3.05 (s, 4H; OCH2CH2O), 3.25 (s,
8874
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim Chem. Eur. J. 2010, 16, 8868 – 8877

Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes
FULL PAPER
6H; OCH3), 6.74–6.78 (m, 1H; p-CH), 6.91–6.99 (m, 2H; m-CH), 7.09–
7.16 ppm (m, 2H; o-CH); 13C NMR (100.6 MHz, C6D6, 258C): d=18.7 (s,
OCCH3ACHTUNGTRENNUNG(CF3)2), 63.4 (s, OCH3), 71.3 (s, OCH2CH2O), 83.7 (s, OCCH3-
(m, 4H; ArH); 13C NMR (50.3 MHz, CDCl3, 258C): d=19.3 (s, CH2),
ꢀ
63.2 (s, OCH2), 77.5 (s, C C), 128.4 (s, i-C), 128.7 (s, Ar-C), 131.0 (s, Ar-
C), 133.0 (s, Ar-C), 139.5 (C-Cl), 165.4 ppm (s, C=O); elemental analysis
ACHTUNGTRENNUNG(CF3)2), 126.1 (q, JCF =290 Hz, CF3), 127.6 (s, CH), 129.2 (s, CH), 129.9
calcd (%) for C20H22O2: C 61.40, H 4.12; found: C 61.18, H 4.21.
(s, CH), 143.5 (s, i-C), 294.6 ppm (s, PhC Mo); 19F NMR (188.3 MHz,
C6D6, 258C): d=À76.7 ppm (s, CF3); elemental analysis calcd (%) for
C23H24F18O5Mo: C 33.76, H 2.96; found: C 33.19, H 3.20.
ꢀ
Data for 8c: White crystalline solid, yield: 94 mg (98%); 1H NMR
3
ꢀ
(200.1 MHz, CDCl3, 258C): d=2.54 (t, JHH =6.9 Hz, 4H; C CCH2), 3.75
(s, 6H; OCH3) 4.27 (t, 3JHH =6.6 Hz, 4H; OCH2), 6.77–6.85 (m, 4H;
ArH), 7.87–7.94 ppm (m, 4H; ArH); 13C NMR (50.3 MHz, CDCl3,
The potassium salt KOCCH3ACHTUNRGTNEUNG(CF3)2 was synthesised in advance by treat-
ing the corresponding alcohol (3 g; 16.48 mmol) with KH (661 mg;
16.48 mmol) in Et2O (30 mL). The reaction mixture was stirred for 4 h at
RT. Filtration and evaporation of the solvent gave the salt as a white
powder.
ꢀ
258C): d=18.7 (s, CH2), 54.7 (s, OCH3), 62.1 (s, OCH2), 76.9 (s, C C),
112.9 (s, i-C), 121.8 (s, Ar-C), 130.9 (s, Ar-C), 162.7 (s, Ar-C), 165.4 ppm
(s, C=O); elemental analysis calcd (%) for C20H22O2: C 69.1, H 5.8;
found: C 69.12, H 5.93.
ꢀ
Synthesis and characterisation of [PhC Mo
(NImtBu
)
{OCMe
(CF3)2}2]
Data for 8d: White solid, yield: 97 mg (94%); 1H NMR (200.1 MHz,
(3b): A suspension of (ImtBuN)Li (25 mg, 0.122 mmol) in toluene (2 mL)
CDCl3, 258C): d=2.43 (s, 6H; SCH3), 2.55 (t, 3JHH =6.9 Hz, 4H; C
ꢀ
was added to
a solution of the molybdenum benzylidyne complex
3
CCH2), 4.29 (t, JHH =6.6 Hz, 4H; OCH2), 7.11–7.19 (m, 4H; ArH), 7.81–
(100 mg, 0.122 mmol) dissolved in toluene (5 mL). After stirring for 1 h
at 408C the solution was filtered and the solvent removed. The resulting
solid was dissolved in iPr2O (4 mL) and the solution filtered. After keep-
ing the solution at À358C for several hours orange crystals were ob-
tained. Yield: 55 mg (60%); 1H NMR (400.1 MHz, C6D6, 258C): d=1.27
7.91 ppm (m, 4H; ArH); 13C NMR (50.3 MHz, CDCl3, 258C): d=14.1 (s,
ꢀ
CH3), 18.7 (s, CH2), 62.3 (s, OCH2), 76.9 (s, C C), 124.2 (s, i-C), 125.5 (s,
Ar-C), 129.2 (s, Ar-C), 144.9 (s, Ar-C), 165.4 ppm (s, C=O); elemental
analysis calcd (%) for C20H22O4S2: C 63.74, H 5.35, S 15.47; found: C
63.82, H 5.48, S 14.90.
(s, 18H; NCACHTUNGTRENNUNG(CH3)3), 1.89 (s, 6H; OCAHCUTNRTGEG(NUNN CH3)ACHTNUGRTNE(NUGN CF3)2), 5.96 (s, 2H; NCH),
Data for 8e: White solid, yield: 17% based on GC analysis, as no full
conversion was achieved; 1H NMR (200.1 MHz, CDCl3, 258C): d=2.57
6.81–6.84 (m, 1H; p-CH), 7.00–7.04 (m, 2H; m-CH), 7.10–7.16 ppm (m,
2H; o-CH); 13C NMR 100.6 MHz, C6D6, 258C): d=20.0 (s, OCCH3-
(t, 3JHH =7.1 Hz, 4H; C CCH2), 4.35 (t, 3JHH =6.8 Hz, 4H; OCH2), 8.09–
ꢀ
ACHTUNGTRENNUNG(CF3)2), 28.6 (s, NCACHTUNGTRENNUNG(CH3)3), 57.6 (s, NCACHTNUGTREN(UNGN CH3)3), 110.8 (s, NC=CN), 123.5
8.24 ppm (m, 8H; ArH); 13C NMR (50.3 MHz, CDCl3, 258C): d=18.6 (s,
(q, JCF =288 Hz, CF3), 126.3 (q, JCF =288 Hz, CF3); 127.6 (s, CH), 127.9
(s, CH), 128.2 (s, CH), 128.9 (s, CH), 146.6 (s, i-C), 158.0 (s, NCN),
ꢀ
CH2), 63.4 (s, OCH2), 76.8 (s, C C), 122.8 (s, i-C), 130.0 (s, Ar-C), 134.6
(s, Ar-C), 149.9 (s, Ar-C), 163.7 ppm (s, C=O); ESI: m/z calcd for
C20H16N2O8 +Na: 435.08044; found: 435.07988.
287.0 ppm (s, PhC Mo); 19F NMR (376.4 MHz, C6D6, 258C): d=À78.3
ꢀ
(q, 4JFF =18 Hz, 6F; CF3), À76.5 ppm (q, 4JFF =18 Hz, 6F; CF3); elemen-
tal analysis calcd (%) for C26H31F12MoN3O2: C 42.12, H 4.21, N 5.67;
found: C 42.01, H 4.20, N 5.52.
General procedure for ring-closing alkyne metathesis: In a glove box, a
250 mL Schlenk tube was charged with a solution of the substrate
(0.55 mmol) and the catalyst 3a (11 mmmol, 2 mol%) in toluene
(120 mL). The Schlenk tube was taken out of the glove box and connect-
ed to a Schlenk line. Reduced pressure (300 mbar) was applied to
remove 2-butyne continuously. Under these conditions, the volume of the
toluene solution remained almost constant. After 2 h the solution was fil-
tered through alumina in order to remove the catalyst, and elution with
Et2O afforded the cyclisation product as colourless crystals after evapora-
tion of the solvent.
Comparison of catalytic activities: In order to compare the catalytic ac-
tivity of the tungsten and molybdenum benzylidyne complexes 3a and
3b, the homodimerisation metathesis reaction of 3-pentynyloxymethyl-
benzene (5) was performed. In a glove box, a 50 mL Schlenk tube was
charged with a solution of 5 (0.5 mmol) and the catalyst 3a or 3b
(5 mmol, 1 mol%) in toluene (4 mL). The Schlenk tube was taken out of
the glove box and connected to a Schlenk line. At 10 min intervals, the
solvent (together with 2-butyne) was completely evaporated under re-
duced pressure. With disconnection from the vacuum line for approxi-
mately three minutes, samples for gas chromatographic analysis were iso-
lated after re-addition of the original solvent volume (4 mL), and filtered
through alumina to remove the catalyst. The first sample was taken di-
rectly after the addition of the catalyst to 5 (no vacuum).
Data for 10: Yield: 102 mg (86%); 1H NMR (200.1 MHz, CDCl3, 258C):
d=2.45 (t, 3JHH =5.2 Hz, 4H; C CCH2), 3.65 (t, 3JHH =4.8 Hz, 4H;
ꢀ
OCH2), 4.65 (s, 4H; OCH2), 7.03 (m, 2H; ArH), 7.27 (m, 1H; ArH),
8.30 ppm (m, 1H; ArH); 13C NMR (50.3 MHz, CDCl3, 258C): d=20.9 (s,
ꢀ
CH2), 68.9 (s, OCH2), 71.1 (s, OCH2), 79.5 (s, C C), 124.5 (s, Ar-C), 125.0
(s, Ar-C), 127.6 (s, Ar-C), 139.7 ppm (s, i-C); elemental analysis calcd
General procedure for alkyne cross metathesis: In a glove box, a 50 mL
Schlenk tube was charged with a solution of the substrate (0.5 mmol) and
the catalyst 3a (5 mmol, 1 mol%) in toluene (8 mL). The Schlenk tube
was taken out of the glove box and connected to a Schlenk line. Reduced
pressure (ꢁ200 mbar) was applied, and the solvent (together with 2-
butyne) was slowly evaporated over a period of one hour. The residue
was re-dissolved in toluene and filtered through alumina to remove the
catalyst. Evaporation of the solvent afforded the dimerisation product.
(%) for C14H16O2: C 77.75, H 7.46; found: C 77.66, H 7.70.
Data for 12: Yield: 134 mg (98%); 1H NMR (200.1 MHz, CDCl3, 258C):
d=2.60 (t, J=5.6 Hz, 4H; C CCH2), 4.51 (t, 3JHH =5.4 Hz, 4H; OCH2),
ꢀ
7.55–7.64 (m, 2H; ArH), 7.71–7.80 ppm (2H; m, ArH); 13C NMR
ꢀ
(50.3 MHz, CDCl3, 258C): d=19.8 (s, CH2), 62.7 (s, OCH2), 78.9 (s, C
C), 128.4 (s, Ar-C), 130.9 (s, Ar-C), 133.2 (s, i-C), 167.8 ppm (s, C=O); el-
emental analysis calcd (%) for C14H12O4: C 68.85, H 4.95; found: C
68.47, H 4.94.
Data for 6: Colourless oil, yield: 73 mg (98%); 1H NMR (200.1 MHz,
3
CDCl3, 258C): d=2.40 (t, 3JHH =7.2 Hz, 4H; C CCH2), 3.48 (t, JHH
=
ꢀ
Single-crystal X-ray structure determination: Data were recorded on an
Oxford Diffraction Xcalibur diffractometer at low temperature by using
monochromated MoKa radiation (l=0.71073 ꢃ). Absorption corrections
(except for 12) were performed on the basis of multi-scans. Structures
were refined anisotropically using the program SHELXL-97. Hydrogen
atoms were included using rigid methyl groups or a riding model.
6.8 Hz, 4H; OCH2), 4.46 (s, 4H; OCH2), 7.18–7.29 ppm (m, 10H; ArH);
13C NMR (50.3 MHz, CDCl3, 258C): d=20.2 (s, CH2), 68.7 (s, OCH2),
ꢀ
72.9 (s, OCH2), 77.9 (s, C C), 127.5 (s, Ar-C), 127.6 (s, Ar-C), 128.3 (s,
Ar-C), 138.2 ppm (s, i-C); elemental analysis calcd (%) for C20H22O2: C
81.60, H 7.53; found: C 81.10, H 7.55.
Data for 8a: White crystalline solid, yield: 78 mg (97%); 1H NMR
3
ꢀ
(200.1 MHz, CDCl3, 258C): d=2.57 (t, JHH =7.0 Hz, 4H; C CCH2), 4.31
Special features: The Flack parameters for the noncentrosymmetric struc-
tures of 3a and 3b were refined to À0.015(2) and À0.009(11), respective-
ly. These isotypic compounds crystallised by chance with opposite polari-
ty. In compound 4, one of the hexafluoro-tert-butoxide groups is disor-
dered over two positions; the atoms of the minor component were re-
fined isotropically. For compound 12, the anomalous dispersion is negligi-
ble; Friedel opposite reflections were therefore merged and the Flack
parameter is meaningless. Cystallographic data are given in Table 2 for
structures 2a, 2b, 3a, 3b, 4, and 12.
(t, 3JHH =6.8 Hz, 4H; OCH2), 7.29–7.52 (m, 8H; ArH), 7.97–8.06 ppm
(m, 2H; ArH); 13C NMR (50.3 MHz, CDCl3, 258C): d=19.4 (s, CH2),
ꢀ
63.0 (s, OCH2), 77.5 (s, C C), 128.3 (s, Ar-C), 129.6 (s, Ar-C), 130.1 (s, i-
C), 133.0 (s, Ar-C), 166.3 ppm (s, C=O); elemental analysis calcd (%) for
C20H22O2: C 74.52, H 5.63; found: C 74.42, H 5.66.
Data for 8b: White crystalline solid, yield: 97 mg (98%); 1H NMR
3
ꢀ
(200.1 MHz, CDCl3, 258C): d=2.56 (t, JHH =7.0 Hz, 4H; C CCH2), 4.30
(t, 3JHH =6.8 Hz, 4H; OCH2), 7.27–7.36 (m, 4H; ArH), 7.85–7.93 ppm
Chem. Eur. J. 2010, 16, 8868 – 8877
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8875

M. Tamm et al.
Table 2. Crystallographic data.
2a
2b
3a
3b
4
12
formula
Mr
T [K]
crystal system
space group
a [ꢀ]
b [ꢀ]
c [ꢀ]
C23H24F18O5W
C23H24F18O5Mo
818.36
100(2)
C26H31F12N3O2W
829.39
100(2)
orthorhombic
P212121
11.285
20.8290(2)
39.0341(4)
90
C26H31F12N3O2Mo
741.48
100(2)
orthorhombic
P212121
11.1850(6)
20.9088(8)
39.0969(8)
90
C22H31F12N3O2Mo
693.44
100(2)
orthorhombic
Pbca
21.7351(3)
11.9124(2)
22.0931(3)
90
C11H12O4
244.24
100(2)
monoclinic
P21
8.4753(4)
7.6964(4)
9.6504(4)
90
109.167(4)
90
594.59(5)
2
906.27
100(2)
triclinic
triclinic
¯
¯
P1
P1
10.5640(4)
11.0411(6)
12.4480(6)
86.572(4)
84.574(4)
84.859(4)
1437.66(12)
2
10.5062(4)
11.0573(6)
12.4533(6)
86.616(4)
84.441(4)
84.975(4)
1432.50(12)
2
a [8]
b [8]
90
90
90
90
90
90
g [8]
V [ꢃ3]
9175.58(13)
12
9143.4(6)
12
5720.28(15)
8
Z
crystal size [mm]
2q range [8]
reflections collected
independent reflections
goodness of fit on F2
0.09ꢄ0.03ꢄ0.03
2.41–27.10
39663
6321
0.974
0.21ꢄ0.14ꢄ0.13
3.20–28.28
29059
7064
1.038
0.15ꢄ0.13ꢄ0.12
4.34–28.28
189717
22595
0.958
0.39ꢄ0.13ꢄ0.08
2.10–26.37
174309
18652
0.971
0.26ꢄ0.06ꢄ0.04
2.07–26.37
121294
5846
0.893
0.28ꢄ0.28ꢄ0.17
2.23–27.10
13317
1413
1.109
F
G
876
812
4872
4488
2800
256
]
2.094
1.897
1.801
1.616
1.610
1.364
]
4.169
0.610
3.879
0.533
0.561
0.100
R(Fo), [I>2s(I)]
0.0233
0.0410
1.174/À0.630
0.0236
0.0650
0.758/À0.408
0.0182
0.0334
0.958/À0.789
0.0243
0.0478
0.454/À0.809
0.0403
0.0943
0.967/À0.740
0.0239
0.0614
0.170/À0.174
Rw (F2O)
D1 [eꢃÀ3
]
CCDC-768402 (2a), 768403 (2b), 768404 (3a), 768405 (3b), 768406 (4),
and 768407 (12) contain the supplementary crystallographic data for this
paper. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.
2307–2320; i) R. R. Schrock, A. H. Hoveyda, Angew. Chem. 2003,
115, 4740–4782; Angew. Chem. Int. Ed. 2003, 42, 4592–4633; j) A.
Fꢀrstner in Handbook of Metathesis, Vol. 2 (Eds.: R. H. Grubbs),
Wiley-VCH, Weinheim, 2003, p. 432.
Electronic structure calculations: The B3LYP hybrid functional[24,25] as
implemented in the Gaussian 03[26] set of programs was employed for all
calculations. All atoms except molybdenum and tungsten were described
by a standard triple zeta all-electron basis set augmented with one set of
[2] a) A. Mortreux, M. Blanchard, J. Chem. Soc. Chem. Commun. 1974,
786–787; b) A. Mortreux, N. Dy, M. Blanchard, J. Mol. Catal. 1975,
1, 101–109; c) A. Mortreux, F. Petit, M. Blanchard, Tetrahedron
Lett. 1978, 19, 4967–4968; d) A. Bencheick, M. Petit, A. Mortreux,
F. Petit, J. Mol. Catal. 1982, 15, 93–101; e) A. Mortreux, J. C. Del-
grange, M. Blanchard, B. Lubochinsky, J. Mol. Catal. 1977, 2, 73–82;
f) A. Mortreux, F. Petit, M. Blanchard, J. Mol. Catal. 1980, 8, 97–
106; g) N. Kaneta, K. Hikichi, M. Mori, Chem. Lett. 1995, 1055–
1066; h) W. Zhang, J. S. Moore, Angew. Chem. 2006, 118, 4524–
4548; Angew. Chem. Int. Ed. 2006, 45, 4416–4439; i) D. Zhao, J. S.
Moore, Chem. Commun. 2003, 807–818; j) G. Brizius, N. G. Pschir-
er, W. Steffen, K. Stitzer, H.-C. zur Loye, U. H. F. Bunz, J. Am.
Chem. Soc. 2000, 122, 12435–12440; k) P.-H. Ge, W. Fu, W. A. Herr-
mann, E. Herdtweck, C. Campana, R. D. Adams, U. H. F. Bunz,
Angew. Chem. 2000, 112, 3753–3756; Angew. Chem. Int. Ed. 2000,
39, 3607–3610; l) S. Hçger, Angew. Chem. 2005, 117, 3872–3875;
Angew. Chem. Int. Ed. 2005, 44, 3806–3808; m) O. S. Miljanic,
K. P. C. Vollhardt, G. D. Whitener, Synlett 2003, 29; n) C. A. Johnso-
n II, Y. Lu, M. M. Haley, Org. Lett. 2007, 9, 3725–3728; o) A. Fꢀrst-
ner, O. Guth, A. Rumbo, G. Seidel, J. Am. Chem. Soc. 1999, 121,
11108–11113.
polarisation functions (6–311GACHTUNTRGNEU(GN d,p)). For molybdenum and tungsten, re-
spectively, a double-zeta basis set optimised for use with effective core
potentials (ECP) in combination with the corresponding Stuttgart ECP
was employed,[27] and an additional polarisation function was added
(x(f)=1.043 for molybdenum and x(f)=0.823 for tungsten).[28] All geom-
etry optimisations were performed without any imposed symmetry con-
straints. After the relevant stationary points were localised on the energy
surface, they were further characterised as minima or transition states by
normal mode analysis based on the analytical energy second derivatives.
The self-consistent field iteration was terminated after reaching the con-
vergence criterion of 1ꢄ10À7 hartree. The root mean square of the inter-
nal forces is always below 0.0003 hartreebohrÀ1. Enthalpic and entropic
contributions were calculated by statistical thermodynamics as imple-
mented in the Gaussian 03 set of programs.[26] Cartesian coordinates of
all calculated structures can be found in the Supporting Information.
[3] a) J. Sancho, R. R. Schrock, J. Mol. Catal. 1982, 15, 75–79; b) R. R.
Schrock, D. N. Clark, J. Sancho, J. H. Wengrovius, S. M. Rocklage,
S. F. Pederson, Organometallics 1982, 1, 1645–1651; c) J. H. Wengro-
vius, J. Sancho, R. R. Schrock, J. Am. Chem. Soc. 1981, 103, 3932–
3934; d) A. Fꢀrstner, G. Seidel, Angew. Chem. 1998, 110, 1758–
1760; Angew. Chem. Int. Ed. 1998, 37, 1734–1736; e) K. Grela, J. Ig-
natowska, Org. Lett. 2002, 4, 3747–3749; f) D. Song, G. Blond, A.
Fꢀrstner, Tetrahedron 2003, 59, 6899–6904; g) A. Fꢀrstner, G.
Mꢀller, J. Organomet. Chem. 2000, 606, 75–78.
[4] a) C. E. Laplaza, A. L. Odom, W. M. Davis, C. C. Cummins, J. D.
Protasiewicz, J. Am. Chem. Soc. 1995, 117, 4999–5000; b) C. E. Lap-
laza, C. C. Cummins, Science 1995, 268, 861–863; c) C. E. Laplaza,
A. R. Johnson, C. C. Cummins, J. Am. Chem. Soc. 1996, 118, 709–
710; d) A. Fꢀrstner, C. Mathes, C. W. Lehmann, J. Am. Chem. Soc.
1999, 121, 9453–9454; e) A. Fꢀrstner, C. Mathes, C. W. Lehmann,
Acknowledgements
This work was supported by the Deutsche Forschungsgemeinschaft
(DFG) through grant Ta 189/6-2.
[1] For reviews on alkyne metathesis, see: a) W. Zhang, J. S. Moore,
Adv. Synth. Catal. 2007, 349, 93–120; b) R. R. Schrock, C. Czekelius,
Adv. Synth. Catal. 2007, 349, 55–77; c) P. Van de Weghe, P. Bisseret,
N. Blanchard, J. Eustache, J. Organomet. Chem. 2006, 691, 5078–
5108; d) A. Mortreux, O. Coutelier, J. Mol. Catal. A 2006, 254, 96–
104; e) R. R. Schrock, Chem. Commun. 2005, 2773–2777; f) U. H. F.
Bunz, Science 2005, 308, 216–217; g) R. R. Schrock, Chem. Rev.
2002, 102, 145–179; h) A. Fꢀrstner, P. Davis, Chem. Commun. 2005,
8876
www.chemeurj.org
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Chem. Eur. J. 2010, 16, 8868 – 8877

Imidazolin-2-iminato Molybdenum and Tungsten Benzylidyne Complexes
FULL PAPER
Chem. Eur. J. 2001, 7, 5299–5317; f) Y.-C. Tsai, P. L. Diaconescu,
C. C. Cummins, Organometallics 2000, 19, 5260–5262; g) J. M.
Blackwell, J. S. Figueroa, F. H. Stephens, C. C. Cummins, Organome-
tallics 2003, 22, 3351–3353; h) W. Zhang, S. Kraft, J. S. Moore,
Berlin, 1998; i) S. Monsaert, A. Lozano Vila, R. Drozdzak, P. Van
Der Voort, F. Verpoort, Chem. Soc. Rev. 2009, 38, 3360–3372; j) C.
Samojłowicz, M. Bieniek, K. Grela, Chem. Rev. 2009, 109, 3708–
3742.
Chem. Commun. 2003, 832–833; i) W. Zhang, S. Kraft, J. S. Moore,
J. Am. Chem. Soc. 2004, 126, 329–335; j) W. Zhang, J. S. Moore, J.
Am. Chem. Soc. 2005, 127, 11863–11870; k) W. Zhang, J. S. Moore,
J. Am. Chem. Soc. 2004, 126, 12796; l) W. Zhang, M. Brombosz, J. L.
Mendoza, J. S. Moore, J. Org. Chem. 2005, 70, 10198–10201;
m) H. M. Cho, H. Weissmann, J. S. Moore, J. Org. Chem. 2008, 73,
4256–4258; n) H. Weissman, K. N. Plunkett, J. S. Moore, Angew.
Chem. 2006, 118, 599–602; Angew. Chem. Int. Ed. 2006, 45, 585–
588.
[14] a) D. N. Clark, R. R. Schrock, J. Am. Chem. Soc. 1978, 100, 6774–
6776; b) L. G. McCullough, R. R. Schrock, J. C. Dewan, J. C. Murd-
zek, J. Am. Chem. Soc. 1985, 107, 5987–5998; c) L. G. McCullough,
R. R. Schrock, J. Am. Chem. Soc. 1984, 106, 4067–4068.
[15] It should be noted, however, that Cummins has provided an im-
proved protocol for the preparation of non-fluorinated trialkoxymo-
lybdenum(VI) alkylidyne complexes, see ref. [4f]; furthermore John-
son has published the synthesis of fluorinated trialkoxymolybde-
num(VI) alkylidyne complexes from nitrides by metathesis with al-
kynes, see: R. L. Gdula, M. J. A. Johnson, J. Am. Chem. Soc. 2006,
128, 9614–9615.
[16] R. D. Shannon, Acta Crystallogr. Sect. A 1976, 32, 751–767.
[17] a) R. R. Schrock, J. Y. Jamieson, J. P. Araujo, P. J. Bonitatebus, A.
Sinha, L. P. H. Lopez, J. Organomet. Chem. 2003, 684, 56–67;
b) R. R. Schrock, Polyhedron 1995, 14, 3177–3195.
[18] Using 2 mol% of 3a, 33% of 8e can be obtained.
[19] K. Brandhorst, J. Grunenberg, M. Tamm, unpublished results; see
ref. [7a] for representative calculations.
[20] a) G. Bocelli, M. F. Grenier-Loustalot, J. Crystallogr. Spectrosc. Res.
1983, 13, 19–29; b) V. Kçnigstein, W. Tochtermann, E.-M. Peters, K.
Peters, H. G. von Schnering, Tetrahedron Lett. 1987, 28, 3483–3486;
c) M. Balog, I. Grosu, G. Plꢅ, Y. Ramondenc, E. Condamine, R. A.
Varga, J. Org. Chem. 2004, 69, 1337–1345.
[21] T. J. Katz, J. McGinnis, J. Am. Chem. Soc. 1975, 97, 1592–1594.
[22] J. Zhu, G. Jia, Z. Lin, Organometallics 2006, 25, 1812–1819.
[23] A. Poater, X. Solans-Monfort, E. Clot, C. CopÞret, O. Eisenstein, J.
Am. Chem. Soc. 2007, 129, 8207–8216.
[5] a) M. Bindl, R. Stade, E. K. Heilmann, A. Picot, R. Goddard, A.
Fꢀrstner, J. Am. Chem. Soc. 2009, 131, 9468–9470; b) M. Bindl, L.
Jean, J. Herrmann, R. Mꢀller, A. Fꢀrstner, Chem. Eur. J. 2009, 15,
12310–12319.
[6] a) S. Beer, C. G. Hrib, P. G. Jones, K. Brandhorst, J. Grunenberg, M.
Tamm, Angew. Chem. 2007, 119, 9047–9051; Angew. Chem. Int. Ed.
2007, 46, 8890–8894; b) M. Tamm, S. Beer, T. Bannenberg, EP
07112300.4; c) S. Beer, K. Brandhorst, C. G. Hrib, X. Wu, B. Haber-
lag, J. Grunenberg, P. G. Jones, M. Tamm, Organometallics 2009, 28,
1534–1545; d) M. Tamm, X. Wu, Chim. Oggi 2010, 28, 10–13.
[7] a) S. Beer, K. Brandhorst, J. Grunenberg, C. G. Hrib, P. G. Jones, M.
Tamm, Org. Lett. 2008, 10, 981–984; b) S. Lysenko, B. Haberlag, X.
Wu, M. Tamm, Macromol. Symp. 2010, in press.
[8] a) M. Tamm, D. Petrovic, S. Randoll, S. Beer, T. Bannenberg, P. G.
Jones, J. Grunenberg, Org. Biomol. Chem. 2007, 5, 523–530; b) M.
Tamm, S. Randoll, T. Bannenberg, E. Herdtweck, Chem. Commun.
2004, 876–877; c) M. Tamm, S. Beer, E. Herdtweck, Z. Naturforsch.
B 2004, 59, 1497–1504; d) M. Tamm, S. Randoll, E. Herdtweck, N.
Kleigrewe, G. Kehr, G. Erker, B. Rieger, Dalton Trans. 2006, 459–
467; e) T. K. Panda, S. Randoll, C. G. Hrib, P. G. Jones, T. Bannen-
berg, M. Tamm, Chem. Commun. 2007, 5007–5009; f) S. H. Stelzig,
M. Tamm, R. M. Waymouth, J. Polym. Sci. Polym. Chem. Ed. 2008,
46, 6064–6070.
[24] A. D. Becke, Phys. Rev. B 1988, 38, 3098–3100.
[25] C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785–789.
[26] Gaussian 03, Revision E.01, M. J. Frisch, G. W. Trucks, H. B. Schle-
gel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Montgom-
AHCTUNGTREGeNNUN ry, Jr., T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S.
[9] a) A. Mayr, G. A. McDermott, J. Am. Chem. Soc. 1986, 108, 548–
549; b) G. A. McDermott, A. M. Dorries, A. M. Mayr, Organometal-
lics 1987, 6, 925–931; c) A. Mayr, G. A. McDermott, A. M. Dorries,
Organometallics 1985, 4, 608–610; d) M. A. Stevenson, M. D. Hop-
kins, Organometallics 1997, 16, 3572–3573.
[10] E. O. Fischer, A. Maasbçl, Chem. Ber. 1967, 100, 2445–2456.
[11] a) S. T. Diver, A. J. Giessert, Chem. Rev. 2004, 104, 1317–1382;
b) E. O. Fischer, G. Kreis, Chem. Ber. 1976, 109, 1673–1683; c) E. O.
Fischer, N. Q. Dao, W. R. Wagner, Angew. Chem. 1978, 90, 51–51;
Angew. Chem. Int. Ed. Engl. 1978, 17, 50–51; d) E. O. Fischer, U.
Schubert, J. Organomet. Chem. 1975, 100, 59–81.
Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani,
N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K.
Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda,
O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian,
J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E.
Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W.
Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J.
Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C.
Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari,
J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cio-
slowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaro-
mi, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham, C. Y. Peng,
A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W.
Chen, M. W. Wong, C. Gonzalez, J. A. Pople, Gaussian Inc., Wall-
ingford CT, 2004.
[12] J. H. Freudenberger, R. R. Schrock, M. R. Churchill, A. L. Rhein-
gold, J. W. Ziller, Organometallics 1984, 3, 1563–1573.
[13] a) T. M. Trnka, R. H. Grubbs, Acc. Chem. Res. 2001, 34, 18–29;
b) A. Deiters, S. F. Martin, Chem. Rev. 2004, 104, 2199–2238; c) S. J.
Connon, S. Blechert, Angew. Chem. 2003, 115, 1944–1968; Angew.
Chem. Int. Ed. 2003, 42, 1900–1923; d) A. Fꢀrstner, Angew. Chem.
2000, 112, 3140–3172; Angew. Chem. Int. Ed. 2000, 39, 3012–3043;
e) M. R. Buchmeiser, Chem. Rev. 2000, 100, 1565–1604; f) M. R.
Buchmeiser, Adv. Polym. Sci. 2005, 176, 89–119; k) M. R. Buchme-
iser, Monatsh. Chem. 2003, 134, 327–342; g) Handbook of Metathe-
sis, Vol. 1–3 (Eds.: R. H. Grubbs), Wiley-VCH, Weinheim, 2003;
h) A. Fꢀrstner, Alkene Metathesis in Organic Synthesis, Springer,
[27] D. Andrae, U. Hꢁußermann, M. Dolg, H. Stoll, H. Preuß, Theor.
Chim. Acta 1990, 77, 123–141.
[28] A. W. Ehlers, M. Bohme, S. Dapprich, A. Gobbi, A. Hollwarth, V.
Jonas, K. F. Kohler, R. Stegmann, A. Veldkamp, G. Frenking, Chem.
Phys. Lett. 1993, 208, 111–114.
Received: March 7, 2010
Published online: June 22, 2010
Chem. Eur. J. 2010, 16, 8868 – 8877
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
8877