Intermolecular hydrogen-fluorine interaction in dimolybdenum triply bonded complexes modified by fluorinated formamidine ligands for the construction of 2D- and 3D-networks

Article abstract of DOI:10.1002/ejic.201001357

The formamidines ArNHC(H)=N-Ar [Ar = Ph (2a), 4-F-Ph (2b), 3,5-F ₂Ph (2c) and 2,6-F₂Ph (2d) and R = 2,3,5-F₃Ph (2e), 3,4,5-F₃Ph (2f), F₅Ph (2g) and 4-CF₃Ph (2h)] were synthesized and the influence of the introduction of a fluorine or a trifluoromethyl group into the aryl unit on the solid-state structures was investigated. On comparing the experimental data, only marginal differences in the geometrical and electronic features of the diverse substituted species were detected. DFT calculations and X-ray crystallography of 2d-2g revealed that the E-syn-configuration corresponded to the thermodynamically most stable motif of all of the examined formamidines. However, in their solid-state, these ligands showed a range of H...F interactions, which varied depending on the number and position of the fluorine atoms on the aryl group and thus led to interesting solid-state structures. Moreover, compounds 2 were used for the synthesis of the new heteroleptic dimolybdenum triply-bonded complexes, Mo ₂[(2a-2c; 2e-f)-H]₂(OtBu)₄ (3a-3c; 3e-3f). X-ray crystallography of complexes 3c and 3f revealed two different isomers in the solid state: in trans-3c the two formamidines are in one plane, while in cis-3c and cis-3f they are next to each other. The DFT calculations showed only a small distinction in energy between the configurations, which led us to assume that the different configurations were induced by the crystal packing. The specific H...F interactions provided by the different formamidines led to a two-dimensional arrangement for trans-3c and a three-dimensional network for cis-3c and cis-3f.

Full text of DOI:10.1002/ejic.201001357

FULL PAPER
DOI: 10.1002/ejic.201001357
Intermolecular Hydrogen–Fluorine Interaction in Dimolybdenum Triply Bonded
Complexes Modified by Fluorinated Formamidine Ligands for the Construction
of 2D- and 3D-Networks
Sebastian Krackl,[a,b] Shigeyoshi Inoue,[b] Matthias Driess,[a,b] and Stephan Enthaler*[a]
Keywords: Molybdenum / Fluorine / Fluorinated ligands / N ligands / Heteroleptic complexes / Multiple bonds / Crystal
engineering / Hydrogen-fluorine interactions
The formamidines ArNHC(H)=N–Ar [Ar = Ph (2a), 4-F-Ph
(2b), 3,5-F2Ph (2c) and 2,6-F2Ph (2d) and R = 2,3,5-F3Ph (2e),
3,4,5-F3Ph (2f), F5Ph (2g) and 4-CF3Ph (2h)] were synthesized
and the influence of the introduction of a fluorine or a tri-
fluoromethyl group into the aryl unit on the solid-state struc-
tures was investigated. On comparing the experimental data,
only marginal differences in the geometrical and electronic
features of the diverse substituted species were detected.
the aryl group and thus led to interesting solid-state struc-
tures. Moreover, compounds 2 were used for the synthesis of
the new heteroleptic dimolybdenum triply-bonded com-
plexes, Mo2[(2a–2c; 2e–f)–H]2(OtBu)4 (3a–3c; 3e–3f). X-ray
crystallography of complexes 3c and 3f revealed two dif-
ferent isomers in the solid state: in trans-3c the two for-
mamidines are in one plane, while in cis-3c and cis-3f they
are next to each other. The DFT calculations showed only a
DFT calculations and X-ray crystallography of 2d–2g re- small distinction in energy between the configurations,
vealed that the E-syn-configuration corresponded to the ther-
modynamically most stable motif of all of the examined for-
mamidines. However, in their solid-state, these ligands
showed a range of H···F interactions, which varied de-
pending on the number and position of the fluorine atoms on
which led us to assume that the different configurations were
induced by the crystal packing. The specific H···F interac-
tions provided by the different formamidines led to a two-
dimensional arrangement for trans-3c and a three-dimen-
sional network for cis-3c and cis-3f.
C–F···πF and C–F···metal interactions, that significantly al-
ter the intra- and intermolecular interactions in the crys-
tal.[1,3,4] During the last decades complexes have been re-
ported that cover several types of fluorine interactions.[5–7]
For example, Cotton et al. reported on the coordination of
fluorinated N,NЈ-bis(phenyl)formamidines ligands to chro-
mium.[8,9] The obtained complexes consist of the structural
motif Cr2L4, which means that the four formamidine li-
gands are bonded to the quadruply bonded metal centre.
The structural information showed a Cr···F interaction for
the ortho-fluorine on the phenyl ring of the ligand. So far,
to the best of our knowledge, the equivalent metal–metal
bonded systems for the higher homologues of chromium,
namely molybdenum and tungsten, have not been synthe-
sized. Thus, we were interested in studying the coordination
of these types of ligands towards these metals. The coordi-
nation of non-fluorine substituted formamidine to quadru-
ply-bonded molybdenum complexes has been demon-
strated, while neutral, triply-bonded dimolybdenum ana-
logues have not been reported as yet. Undoubtedly, triply-
bonded molybdenum complexes are of interest because of
their applicability as precursor for heterogeneous materi-
als[10] and as precursor for heterobimetallic compounds,[11]
which makes them profitable systems for further applica-
tions. Recently, some of us synthesized monodentate
MoϵMo alkoxides,[11,12] which exhibit the general motif of
Introduction
The replacement of hydrogen by the sterically similar
fluorine atom in organic molecules has a tremendous effect
on the properties of the obtained material.[1] This is demon-
strated by the extensive range of applications of these mate-
rials, for example, bulk chemicals, pharmaceuticals, agro-
chemicals, polymers, solvents and as key intermediates in
organic syntheses.[2] Furthermore, the exchange of carbon-
bonded hydrogen with fluorine in the ligand scaffold in or-
ganometallic complexes usually leads to alterations in the
physical and chemical properties of the respective com-
pound. The high electronegativity and the low polarizability
of the fluorine atom are the main reasons for the modifica-
tions. In addition, the influence of this substitution is ob-
served in the study of the solid-state structures. Fluorine
has the capacity to form various intermolecular interac-
tions, especially phenyl-perfluorophenyl-, C–F···H, F···F,
[a] Technische Universität Berlin, Department of Chemistry,Clus-
ter of Excellence “Unifying Concepts in Catalysis”,
Straße des 17. Juni 135/C2, 10623 Berlin, Germany
Fax: +49-30314 29732
E-mail: stephan.enthaler@tu-berlin.de
[b] Institute of Chemistry: Metalorganics and Inorganic Materials,
Technische Universität Berlin,
Straße des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201001357.
Eur. J. Inorg. Chem. 2011, 2103–2111
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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S. Krackl, S. Inoue, M. Driess, S. Enthaler
an “ethane-like“ structure with a staggered ligand confor- Table 1. Characterization of formamidines 2.
FULL PAPER
mation and unequally oriented ligands with different
Mo···O distances at each of the molybdenum atoms.[13]
Based on these types of complexes we were interested in the
coordination abilities of the formamidine ligands. In this
work, fluorinated formamidine ligands will be studied with 2d
regards to the fluorine interactions and the consequent for-
L
1H NMR [ppm][a] 13C NMR [ppm][b] IR [cm–1][c] λ [cm–1][d]
2a
2b
2c
8.24
8.07
8.03
7.94
8.14
7.89
8.28
8.21
149.5
150.2
149.1
154.3
149.8
150.2
–
1679
1671
1676
1669
1672
1677
1678
1672
281.5
279.5
294.5
268.0
292.0
271.0
264.5
300.8
2e
2f
2g
2h
mation of 2D- and 3D-networks in the crystal.
159.0
[a] The chemical shift (1H NMR) for RN=C(H)NHR was
measured in [D1]chloroform at 25 °C. [b] The chemical shift (13C
NMR) for RN=C(H)NHR was measured in [D1]chloroform at
25 °C. [c] KBr. [d] All of the measurements were carried out in
CH3CN at 25 °C.
Results and Discussion
The formamidines 2 were synthesized by following the
reported procedures. Two equivalents of the corresponding
aniline were treated with triethyl orthoformate under re-
fluxing conditions (Scheme 1). The crude products were
purified by recrystallization from n-hexane to obtain 2 in
fair to good yields. Notably, in all of the cases the E-isomer
selectively formed, which is in accordance with the litera-
ture.[14]
Scheme 2. The synthesis of the complexes Mo2[(2a–2c; 2e–2f)–
H]2(OtBu)4 (3a–3c; 3e–3f).
Table 2. The calculated and observed geometrical details for 2.
2a
1.284 1.283 1.283 1.284 1.283 1.283 1.285 1.283
1.283[a]
1.286 1.298 1.280 1.311 –
1.369 1.369 1.369 1.371 1.375 1.369 1.374 1.369
1.342[a]
1.344 1.331 1.351 1.331 –
2b
2c
2d
2e
2f
2g
2h
N1–C1 [Å]
Observed
N2–C1 [Å]
Observed
–
–
–
–
N1–C1–N2 [°] 119.8 119.8 119.5 118.3 117.8 119.5 118.0 120.5
Observed 120.5 122.2 121.8 121.0 –
[a] The values for 2b were taken from ref.[14]
–
122.6[a]
–
Scheme 1. The synthesis of the substituted N,NЈ-bis(phenyl)form-
amidines (2).
In addition, we studied the influence of the various sub-
stituents in comparison to the parent structure 2a, and in 2e and 2g, all of which exhibit a fluorine atom in the ortho-
particular the effect of the fluorine substitution. The typical position. The aryl groups in these compounds were sym-
analytical parameters are given in Table 1. In order to de- metrically oriented to each other (see Figure 1).
tect geometrical and electronic changes, the formamidine
Short intramolecular Ar–F···H–C(=NAr)NHAr dis-
proton was applied as a sensor. A significant influence on tances for compounds 2d, 2e and 2g were observed (2.356–
the 1H NMR chemical shift was found when the number of 2.502 Å), which most probably favours this geometry since
fluoro substituents on the aryl unit was increased, however, for 2f the aryl groups are twisted towards each other at an
no effect was observed when a para-trifluoromethyl group angle of nearly 90°. In their solid-state, these compounds
was embedded. In general, only a marginal influence was show a variety of dimensional networks that were mediated
seen when the formamidine carbon was used as the sensor, by the H···F interactions, which include the short Ar–
since all of the 13C NMR spectroscopic values are in the F···Hpara–Ar, Ar–F···H–C(=NAr)NHAr and Ar–F···F–Ar
range of 149 to 151 ppm. However, a significant shift of distances as well as the stacking interactions between the
159.0 ppm was measured for ligand 2h.
adjacent aryl units (for a detailed description, see Support-
In order to get insight into the geometrical structure of ing Information).
the formamidines (Scheme 2), DFT-calculations at the
At this point we were interested in attaching the studied
RB3LYP/6-31++G(d,p) level were performed. The E-syn- formamidines to transition metals in order to design metal-
configuration corresponded to the most thermodynamically containing dimensional networks. We decided to coordinate
stable motif (Table 2), which is in agreement with the exper- the fluorine substituted formamidines to the dimolybdenum
imental observations. This was further confirmed by single- triple bond by means of a direct protolysis reaction of 2
crystal X-ray crystallography, since in all of the cases the E- with Mo2(OtBu)6.[15] The starting material Mo2(OtBu)6 was
syn-isomer was obtained.
easily accessible by means of salt metathesis from
[16]
In addition, a different orientation of the aryl groups was Mo2Cl6(dme)2 (dme = dimethoxyethane). A ratio of 2:1
observed in the molecular structures for the compounds 2d, [ligand: Mo2(OtBu)6] was chosen, which should theoretic-
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Fluorinated Formamidine Dimolybdenum Complexes
signal. The peaks that correspond to the proton at the car-
bon atom in the formamidine functionality are significantly
shifted to the lower field as compared to those of the non-
shifted signal for the uncoordinated species (Table 3).
Table 3. Selected NMR data (δ values, ppm)[a] for 3a–3c and 3e–
3f.
–N=C(H)N–1H NMR –OC(CH3)3 H
–ArH5–nFn H
NMR
1
1
L
13C NMR NMR 13C NMR
3a 8.83
3b 8.88
3c 8.97
3e 9.13
3f 8.87
169.6
176.3
176.0
179.1
175.9
1.32
1.40
1.43
1.44
1.46
33.0
32.2
32.1
32.8
32.3
6.13–6.32
6.11–6.32
6.00–6.28
6.07–6.36
6.13–6.29
[a] The chemical shifts (1H and 13C NMR spectroscopy) were mea-
sured in [D1]chloroform at 25 °C.
In contrast, the signal corresponding to the tert-butoxy
protons shifted to a higher field compared to that of the
homoleptic analogue, Mo2(OtBu)6. This phenomenon has
been observed in other heteroleptic complexes[17] and is
probably caused by the change in orientation of the OtBu
groups, which consequently results in a different magnetic
environment due to the magnetic anisotropy of the dimo-
lybdenum triple bond.[11] The strong shift to a lower field
of the methine proton (N=CH–N) can also be attributed to
the diamagnetic anisotropy, since it is placed over the centre
of the triple bond and thus strongly deshielded.[12] In both
1
the H NMR and the 13C NMR spectra we observed only
one set of broad signals for both of the aryl ligands on
the same formamidine. This implied a symmetrical bonding
contribution for both of the nitrogen atoms. The assumed
ligand distribution in the complexes was confirmed by the
matching integral ratios for the corresponding proton sig-
nals. Since solvent molecules are readily lost from the
Figure 1. The molecular structures for 2d–2g. The thermal ellip-
soids are drawn at the 50% probability level. The solvent molecules
for 2g (toluene) are omitted for clarity.
ally lead to a partial substitution of the alkoxy ligands, in recrystallized material, it was crushed and dried in vacuo at
order to investigate the coordination preference of the di- elevated temperatures and, thus, no residual signals for the
molybdenum centre and to retain the solubility of the com- solvent molecules (neither toluene nor dichloromethane)
plexes in nonpolar solvents. The addition of 2a–2c and 2e– were observed in the spectra.
2f to Mo2(OtBu)6 resulted in a partial protolysis and af-
Further evidence was provided by the single-crystal X-
forded the complexes Mo2[(2a–2c; 2e–2f)–H]2(OtBu)4 (3a– ray diffraction data. Suitable single crystals for compounds
3c; 3e–3f) (Scheme 2). To the best of our knowledge, 3 are 3c and 3f were obtained from toluene/dichloromethane
the first neutral, triply-bonded dimolybdenum complexes (DCM). Compound 3c crystallizes in the monoclinic space
that bear a formamidine ligand.
The complexes formed by the reaction of formamidines molecule was found in the unit cell (Figure 2).
2d and 2g with Mo2(OtBu)6 could not be obtained in an The Mo–Mo distance for trans-3c·toluene (2.253 Å) is
group, P21/c with Z = 2, and one cocrystallized toluene
analytically pure form (mismatching elemental analysis), al- slightly elongated compared to that of other MoϵMo com-
1
though the H NMR spectra appeared to be fairly clean, plexes. This is probably due to the substitution with a bridg-
and thus these complexes are not discussed. Furthermore, ing ligand that enforces the eclipsed conformation as re-
the reaction of 2h with Mo2(OtBu)6 led to a complex mix- ported for other compounds (Table 4).[15] The Mo–N bond
ture of products that did not include the desired triply– distances are statistically identical with 2.175 and 2.183 Å
bonded complex. Changing the reaction conditions has not and are in the expected range. Each formamidine consti-
resulted in a suitable outcome for this reaction as yet. The tutes a planar five-membered ring with the molybdenum
complexes 3a–3c and 3e–3f are green–brown powders that atoms. The Mo–Mo–N1 and Mo–Mo–N2 bond angles of
are extraordinarily sensitive towards oxygen and moisture. 90.69 and 91.01°, respectively, are close to each other and
They decomposed without subliming between 80 and both formamidine ligands are opposite to each other in one
100 °C and did not give rise to the appropriate mass spec- plane.
1
tra. In the H NMR spectra successful coordination was
The Mo–O bond distances are 1.901 and 1.931 Å for
easily followed by the disappearance of the N–H proton both of the molybdenum atoms and the alkoxide ligands
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S. Krackl, S. Inoue, M. Driess, S. Enthaler
FULL PAPER
Mo1–O distances are statistically identical (1.896 and
1.899 Å). In order to understand the reason for the two
different configurations observed for 3, we decided to per-
form DFT calculations at the B3LYP level. For both com-
pounds the calculations showed an almost equal stability
for both confirmations, with a slightly higher stability for
the trans-configuration, with 1.59 kcalmol–1 for 3c and
1.29 kcalmol–1 for 3f. Simulating the molecular orbitals for
the cis-configuration did not reveal any delocalization of
the electron density over the ligand or the triple bond in
both cases. This means that, most probably, packing effects,
and thus the specific H···F interactions formed by the re-
spective ligand as well as the nature of the cocrystallized
solvent, are responsible for the change in configuration,
since we assumed fast ligand scrambling with the involve-
ment of a rapid change from terminal to bridging modes for
the alkoxy ligands in solution, as reported for other triply-
bonded dimolybdenum complexes.[18] Further proof for this
assumption was provided by the solvent exchange experi-
ments. Therefore, we recrystallized complex 3c in a 1:1 mix-
ture of cyclopentane/DCM instead of the 1:1 mixture of
toluene/DCM applied in the synthesis. After cooling to
–20 °C for two weeks, we obtained crystals that significantly
differed in their crystal morphology. The complementary
structure of 3c, namely, the cis-configuration, was observed
by means of single-crystal X-ray diffraction measurements
(Figure 4) and cyclopentane replaced the cocrystallized tol-
uene.
Figure 2. The ORTEP presentation of complex trans-3c. The hy-
drogen atoms and the solvent molecules (toluene) are omitted for
clarity.
Table 4. Selected bond lengths [Å] and angles [°] for trans-3c, cis-
3c and cis-3f.
trans-3c·toluene[a]
cis-
cis-3f·DCM
3c·cyclopentane
Mo1–Mo2
Mo1–N1
Mo1–N3
Mo2–N2
Mo2–N4
Mo1–O1
Mo1–O2
Mo2–O3
2.253(4)
2.175(3)
2.175(3)
2.183(3)
2.183(3)
1.901(3)
1.931(2)
1.901(3)
1.931(2)
112.2(7)
101.1(7)
112.2(7)
101.1(7)
90.6(8)
2.2503(5)
2.176(4)
2.221(4)
2.232(4)
2.194(4)
1.912(3)
1.891(3)
1.883(4)
1.908(3)
101.3(1)
110.8(1)
110.9(1)
100.6(1)
92.5(1)
2.256(1)
2.174(6)
2.247(7)
2.243(6)
2.176(7)
1.896(5)
1.899(5)
1.864(6)
1.910(5)
110.3(2)
101.3(2)
112.0(2)
102.2(2)
92.8(2)
Mo2–O4
Mo1–Mo2–O1
Mo1–Mo2–O2
Mo1–Mo2–O3
Mo1–Mo2–O4
Mo1–Mo2–N1
Mo1–Mo2–N2
Mo1–Mo2–N3
Mo1–Mo2–N4
N1–Mo1–O1
91.0(8)
90.6(8)
91.0(8)
90.1(1)
87.6(1)
87.6(1)
92.6(1)
150.2(1)
88.1(2)
87.8(2)
93.0(2)
151.3(2)
[a] For trans-3c Mo2=Mo1Ј, O1=O3, O2=O4, N1=N3, N2=N4.
are oriented differently in space: one points towards the
Mo–Mo triple bond and the other one points away from
the Mo–Mo triple bond.
Figure 3. The ORTEP presentation of complex cis-3f. The hydro-
gen atoms, the solvent molecules (DCM) and the equal ligands are
omitted for clarity.
In contrast, we obtained a different structural motif from
the crystallization of compound cis-3f under similar condi-
tions. Compound 3f crystallizes in the triclinic space group,
¯
P1, with Z = 2. The Mo–Mo distances for trans-3c and cis-
The cis-isomers of 3f and 3c show very similar structural
3f are in a similar range, with a bond length of 2.256 Å for features, namely, similar Mo–Mo, Mo–N and Mo–O bond
the latter (Figure 3). However, in the molecular structure of lengths and the resulting bond angles, and thus only a small
complex cis-3f no planar ring between the formamidine and amount of change is induced in the molecule by varying the
the triple bond is constituted and the Mo–N bond distances fluorine substitution pattern of the aryl ligand (Table 4).
for the same ligand differ strongly (average 2.174 and The latter results support the assumption that there is a
2.245 Å), which indicates a weaker Lewis base donation of rapid equilibrium between the cis/trans-configurations in
the ligand nonbonding nitrogen. The two OtBu ligands at- solution and that the observed isomers in the solid state are
tached to the same molybdenum atom also have a different determined by the crystal packing. However, more detailed
orientation in space. Intriguingly, the Mo2–O bond dis- experimental and theoretical investigations on this subject
tances differ from each other (1.864 and 1.910 Å), but the are in progress.
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Fluorinated Formamidine Dimolybdenum Complexes
No short Ar–F···H–C(NAr)2 contacts are present for cis-
3f, as observed in the crystal structure of the free ligand 2f.
However, the H···F interactions are present between the
Ar–fluorine atoms and the protons at the OtBu groups
(average 2.675 Å) and the stacking interactions are present
between the adjacent molecules with an aryl···aryl average
distance of 3.429 Å (Figure 6). These interactions result in
the construction of a three-dimensional network. For the
solid-state structure of cis-3c we obtained a similar picture
as for cis-3f, namely, a three-dimensional network. How-
ever, in this case, no stacking interactions and only short
H···F–Ar interactions were observed. Unfortunately,
recrystallization of the synthesized complexes 3a, 3b and 3e
did not afford crystals that were suitable for single-crystal
X-ray diffraction measurements, and thus, the potential
Mo···F interactions could not be observed. Attempts con-
cerning this matter are still in progress.
Figure 4. The ORTEP presentation of complex cis-3c. The hydro-
gen atoms, the solvent molecules (cyclopentane) and the equal li-
gands are omitted for clarity.
Further investigation into the solid-state structures of the
obtained complexes showed that the formamidines are an
excellent tool for the design of transition metal-containing
dimensional networks that are mediated by H···F interac-
tions. In the solid-state structure of trans-3c, short Ar–
F···H–C(NAr)2 interactions are present (2.403 Å) with a
F···H–C bond angle of 153.99°, longer Ar–F···H–Ar inter-
actions of 2.540 and 2.625 Å are also present with F···H–C
bond angles of 132.48 and 121.81°, respectively (Figure 5).
Figure 6. A cut-out along the ac plane of the crystal structure for
complex cis-3f. The solvent molecules (DCM) are omitted for clar-
ity. The blue lines represent the H···F contacts between the Ar–
fluorine atoms and the protons at the OtBu groups (average
2.675 Å) and the Ar–F···Ar–F distance (average 3.429 Å) between
the parallel-stacked aryl substituents. The cocrystallized DCM mo-
lecules are accommodated in the voids of the network, which has a
short Ar–F···H2CCl2 distance that is in the range of 2.575–2.655 Å.
Figure 5. A cut-out along the bc-plane of the crystal structure for
complex trans-3c. The solvent molecules (toluene) are omitted for
clarity. The blue lines represent the Ar–F···H–C(NAr)2 interactions
(2.403 Å) and the F···H–C bond angle of 153.99°, as well as the
longer Ar–F···H–Ar interactions (2.540 and 2.625 Å) with the
F···H–C bond angles of 132.48 and 121.81°.
Conclusions
In summary we have demonstrated the application and
usefulness of fluorine-substituted formamidines for the con-
These F···H interactions led to a planar arrangement of struction of dimensional networks. Firstly the formamid-
the subunits in the bc plane without short contacts between ines 2a–2h were synthesized and investigated in detail con-
the different planes. The OtBu ligands are sited above and cerning the effect of fluorine and trifluoromethyl substitu-
beneath these planes and are in separate, adjacent planes tion on the properties of the ligand and its influence on
from each other. The cocrystallized toluene is found in the their corresponding solid-state structures. Although the ex-
interspaces between the layers.
perimental data only showed marginal differences in the
The solid-state structure of cis-3f varies strongly from the geometrical and electronic features between the diverse sub-
described network of trans-3c and shows a similar appear- stituted species, these compounds showed manifold H···F
ance to that of the noncoordinated ligand 2f (see Support- interactions, which specifically altered the solid-state struc-
ing Information).
ture of the corresponding compound and led to 2D- and
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S. Krackl, S. Inoue, M. Driess, S. Enthaler
FULL PAPER
N,NЈ-Bisphenylformamidine (2a):[20,21] Yield 76% (3.7 g); m.p.
139 °C (crystallized from n-hexane). 1H NMR (CDCl3): δ = 8.90
(br., 1 H, NH), 8.24 (s, 1 H, N=CHN), 7.04–7.37 (m, 10 H, Ar)
ppm. 13C NMR (CDCl3): δ = 149.5 (N=CHN), 145.2, 129.3, 123.3,
3D-networks with different appearances. Having synthe-
sized these fluorinated formamidines, we questioned
whether this specificity could be applied in order to design
the solid-state structures of the corresponding metal com-
plexes. The formamidine ligands were coordinated to a tri-
ply-bonded dimolybdenum complex and, depending on the
position of the fluorine and the applied solvent, different
crystal morphologies and different configurations for the
resulting complexes, Mo2[(2a–2c; 2e–2f)–H]2(OtBu)4, were
observed. For instance, trans-3c displayed a planar configu-
ration for the attached formamidines and the molecular
subunits constructed a 2D-arrangement by means of the
H···F interactions, while in the corresponding cis-isomer
(cis-3c or cis-3f) the ligands were oriented next to each
other and the specific interactions lead to the formation of
a three-dimensional network.
119.1 ppm. IR (KBr): ν = 3050 (br.), 3052 (w), 2923 (w), 1679 (s),
˜
1660 (s), 1601 (w), 1583 (s), 1488 (s), 1450 (w), 1321 (m), 1209 (m),
1171 (w), 987 (w), 900 (w), 766 (m), 754 (m), 695 (m) cm–1. ESI-
MS: m/z (%) = 197 (100) [M+]. HRMS: calcd. for C13H12N2+H:
197.10733; found 197.10687. UV/Vis (CH3CN, 25 °C)
281.5 nm.
λ =
N,NЈ-Bis(4-fluorophenyl)formamidine (2b):[8,22–25] Yield 63%
(3.7 g); m.p. 142–144 °C (crystallized from n-hexane). 1H NMR
(CDCl3): δ = 9.65 (br., 1 H, NH), 8.07 (s, 1 H, N=CHN), 6.97–
7.10 (m, 8 H, Ar) ppm. 13C NMR (CDCl3): δ = 161.8, 157.0, 150.2
(N=CHN), 141.2, 120.5, 120.4, 116.2, 115.8 ppm. 19F NMR
(CDCl ): δ = –120.0 (m) ppm. IR (KBr): ν = 3428 (w), 2928 (w),
˜
3
2862 (w), 1671 (s), 1603 (w), 1502 (s), 1380 (m), 1313 (m), 1202
(m), 999 (w), 825 (m), 750 (w), 501 (w) cm–1. ESI-MS: m/z (%) =
236 (100), 233 (13) [M+], 227 (41), 214 (24), 159 (23), 149 (48), 133
(26). HRMS: calcd. for C13H10F2N2+H: 233.08848; found
233.08671. UV/Vis (CH3CN, 25 °C) λ = 279.5 nm.
Experimental Section
N,NЈ-Bis(3,5-difluorophenyl)formamidine (2c): Yield 51% (3.4 g);
General: All of the manipulations with oxygen- and moisture-sensi-
tive compounds were performed under dinitrogen by using the
standard Schlenk technique. 1H, 19F and 13C NMR spectra were
recorded with a Bruker AFM 200 spectrometer (1H: 200.13 MHz;
13C: 50.32 MHz; 19F: 188.31 MHz) and by using the proton signals
of the deuterated solvents as the reference. The single-crystal X-ray
diffraction measurements were recorded with an Oxford Diffrac-
tion Xcalibur S Saphire spectrometer. The IR spectra were re-
corded either with a Nicolet Series II Magna-IR-System 750 FTR-
IR instrument or with a Perkin–Elmer Spectrum 100 FTIR instru-
ment. The electron ionisation mass spectra (EI-MS) were recorded
with a Finnigan MAT95S instrument. The melting points (m.p.)
were determined with a BSGT Apotec II capillary-tube apparatus
and are uncorrected. The UV/Vis spectra were recorded with a Per-
kin–Elmer Lambda 20 spectrometer at ambient temperature. The
elemental analyses were performed with a Perkin–Elmer Series II
CHNS/O Analyzer 2400 instrument.
1
m.p. 182 °C. H NMR (CDCl3): δ = 8.03 (s, 1 H, N=CHN), 6.46–
6.73 (m, 6 H, Ar) ppm. 13C NMR (CDCl3): δ = 165.6, 165.5, 163.2,
163.1, 149.1, 118.3, 103.4, 99.2, 98.9, 98.7 ppm. 19F NMR (CDCl3):
δ = –108.6 (m) ppm. IR (KBr): ν = 3095 (m), 1676 (s), 1614 (s),
˜
1513 (m), 1478 (m), 1453 (m), 1380 (m), 1356 (m), 1322 (m), 1284
(m), 1224 (m), 1161 (m), 1135 (s), 1121 (s), 1034 (m), 986 (s), 869
(m), 855 (m), 838 (m), 788 (w), 748 (w), 676 (m), 649 (w), 594 (w),
565 (w), 531 (w), 510 (w) cm–1. ESI-MS: m/z (%) = 269 (100) [M+].
HRMS: calcd. for C13H8F4N2+H: 269.06964; found 269.04294.
UV/Vis (CH3CN, 25 °C) λ = 294.5 nm.
N,NЈ-Bis(2,6-difluorophenyl)formamidine (2d): Yield 89% (5.9 g);
m.p. 151 °C (crystallized from n-hexane). 1H NMR (CDCl3): δ =
8.44 (br., 1 H, NH), 7.94 (s, 1 H, N=CHN), 6.85–7.10 (m, 6 H,
Ar) ppm. 13C NMR (CDCl3): δ = 157.7, 157.6, 154.44, 154.39,
152.8, 152.7, 123.4, 123.2, 123.0, 121.9, 112.0, 111.9, 111.7,
111.6 ppm. 19F NMR (CDCl ): δ = –123.9 (s) ppm. IR (KBr): ν =
˜
3
2884 (w), 1669 (s), 1613 (w), 1492 (m), 1466 (m), 1313 (m), 1270
Single-Crystal X-ray Structure Determinations: The single crystals
were mounted on a glass capillary in perfluorinated oil and
measured in a cold steam of N2. The data for 2d–2g, trans-3c, cis-
3c and cis-3f were collected with an Oxford Diffraction Xcalibur S
Saphire at 150 K (Mo-Kα radiation, λ = 0.71073 Å, ω-scan). The
structures were solved by direct methods. The refinements were car-
ried out with the SHELXL-97 package.[19] All of the thermal dis-
placement parameters were refined anistropically for non-hydrogen
atoms and the positions of the hydrogen atoms were calculated and
considered isotropically according to a riding model. All of the
refinements were carried out by full-matrix least-squares refine-
ment on F2. The crystallographic data for the seven compounds
are summarized in Table 5.
(m), 1240 (w), 1206 (m), 993 (m), 777 (m), 738 (w), 711 (w) cm–1.
ESI-MS: m/z (%)
=
269 (100) [M+]. HRMS: calcd. for
C13H8F4N2+H: 269.06964; found 269.05377. UV/Vis (CH3CN,
25 °C) λ = 268.0 nm.
´
N,N-Bis(2,3,5-trifluorophenyl)formamidine (2e): Yield 71% (5.4 g),
m.p. 124 °C (crystallized from n-hexane). 1H NMR (CDCl3): δ =
8.14 (s, 1 H, N=CHN), 6.73–6.60 (m, 4 H, Ar) ppm. 13C NMR
(CDCl3): δ = 159.4, 156.9, 152.3, 149.8, 149.1, 103.4, 100.4 ppm.
19F NMR (CDCl ): δ = –114.1, –133.3, –159.4 ppm. IR (KBr): ν
˜
3
= 2974 (w), 1672 (s), 1609 (s), 1514 (m), 1492 (w), 1417 (w), 1315
(s), 1213 (m), 1156 (m), 1107 (s), 1069 (s), 988 (w), 848 (w), 832
(m), 777 (w), 731 (w), 673 (w), 643 (m), 619 (w), 591 (w), 553 (w),
510 (w) cm–1. ESI-MS: m/z (%) = 305 (100) [M+], 236 (40), 146
(39). HRMS: calcd. for C13H6F6N2+H: 305.05079; found
305.04923. UV/Vis (CH3CN, 25 °C) λ = 292.0 nm.
CCDC-793747 (for trans-3c), -793752 (for cis-3f) and -808037 (for
cis-3c) contain the supplementary crystallographic data for this pa-
per. These data can be obtained free of charge from The Cambridge
Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_
request/cif.
´
N,N-Bis(3,4,5-trifluorophenyl)formamidine (2f): Yield 38% (2.9 g),
m.p. 142 °C (crystallized from n-hexane). 1H NMR (CDCl3): δ =
7.89 (s, 1 H, N=CHN), 6.61–6.92 (m, 4 H, Ar) ppm. 13C NMR
(CD3CN): δ = 161.8, 157.0, 150.2 (N=CHN), 141.3, 120.5, 120.4,
116.2, 115.8 ppm. 19F NMR (CDCl3): δ = –132.8 (s), –166.4, –165.9
General Synthesis of N,NЈ-Bis(phenyl)formamidines: A mixture of
the corresponding aniline (50 mmol) and triethyl orthoformate
(25 mmol) was heated under reflux for 12 h. All of the volatiles
were removed in vacuo and the residue was washed with n-hexane
to yield a white powder.
(m) ppm. IR (KBr): ν = 3105 (w), 3010 (w), 2917 (w), 1677 (s),
˜
1619 (s), 1519 (s), 1437 (m), 1398 (w), 1380 (m), 1341 (m), 1280 (s),
2108
www.eurjic.org
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Inorg. Chem. 2011, 2103–2111

Fluorinated Formamidine Dimolybdenum Complexes
Table 5. The data collection and the refinement parameters for 2d–2g, trans-3c, cis-3c and cis-3f.
2d
2e
2f
2g·toluene
Empirical formula
Formula weight [g/mol]
Temperature [K]
Space group
a [Å]
b [Å]
c [Å]
C13H8F4N2
268.21
150(2)
C13H6F6N2
304.20
150(2)
C13H6F6N2
304.20
C20H10F10N2
468.30
150(2)
150(2)
¯
P21/c
P21/c
P1
P21/n
7.9581(6)
14.2415(9)
10.1617(9)
90
98.700(8)
90
1138.43(15)
4
1.565
11.3272(6)
25.5824(14)
8.2359(4)
90
99.683(5)
90
2352.6(2)
8
1.718
6.9917(6)
7.6522(6)
11.4522(7)
83.865(6)
84.241(6)
84.642(7)
604.01(8)
2
12.2830(8)
6.8687(6)
22.2265(17)
90
92.510(6)
90
1873.4(2)
4
1.660
α [°]
β [°]
γ [°]
Volume [Å3]
Z
Dcald. [g/cm3]
F(000)
1.673
304
544
1216
936
Crystal size [mm3]
θ range [°]
Index ranges
0.13ϫ0.09ϫ0.08
3.36 to 24.99
–8 Յ h Յ 9
–16 Յ k Յ 16
–12 Յ l Յ 10
8285
0.17ϫ0.12ϫ0.11
3.44 to 25.00
–13 Յ h Յ 13
–27 Յ k Յ 30
–6 Յ l Յ 9
9331
0.16ϫ0.15ϫ0.15
3.38 to 25.00
–7 Յ h Յ 8
–9 Յ k Յ 9
–13 Յ l Յ 13
4159
0.12ϫ0.11ϫ0.09
3.49 to 25.00°
–14 Յ h Յ 14
–8 Յ k Յ 8
–24 Յ l Յ 26
12802
Reflections collected
Independent reflections
Rint
1996
0.0360
4131
0.0433
2124
0.0231
3290
0.0254
Completeness to θ = 25.00° [%]
Rel. transmission factors
Parameters
99.8
99.8
99.8
99.8
0.9889 and 0.9847
172
0.949
R1 = 0.0343
wR2 = 0.0757
R1 = 0.0539
wR2 = 0.0811
0.9815 and 0.9716
399
0.876
R1 = 0.0470
wR2 = 0.0783
R1 = 0.1085
wR2 = 0.0926
1.000 and 0.9912
194
0.933
R1 = 0.0343
wR2 = 0.0685
R1 = 0.0582
wR2 = 0.0732
0.9850 and 0.9800
290
1.118
R1 = 0.0464
wR2 = 0.1083
R1 = 0.0613
wR2 = 0.1148
GOF
Final R indices [IϾ2σ(I)][a,b]
R indices (all data)[a,b]
trans-3c·toluene
cis-3f·DCM
cis-3c·cyclopentane
Empirical formula
Formula weight [g/mol]
Temperature [K]
Space group
a [Å]
b [Å]
c [Å]
C49H58F8Mo2N4O4
1203.01
150(2)
C43H48Cl2F12Mo2N4O4 C47H60F8Mo2N4O4
1175.63
150(2)
P1
1088.87
150(2)
CC
¯
P21/c
11.1268(2)
20.6347(4)
13.1366(3)
90
110.784(2)
90
2819.86(10)
2
1.417
10.7695(11)
11.0482(13)
21.810(2)
92.493(9)
103.398(9)
100.087(9)
2475.8(5)
2
10.43190(10)
42.3604(4)
23.5800(2)
90
102.6260(10)
90
10168.01(16)
8
1.423
α [°]
β [°]
γ [°]
Volume [Å3]
Z
Dcald. [g/cm3]
F(000)
1.577
1236
1184
4464
Crystal size [mm3]
θ range [°]
Index ranges
0.23ϫ0.20ϫ0.12
3.35 to 25.00
–13 Յ h Յ 13
17 Յ k Յ 24
–15 Յ l Յ 15
21034
0.26ϫ0.17ϫ0.07
3.31 to 25.00
–12 Յ h Յ 11
12 Յ k Յ 13
–25 Յ l Յ 25
18573
0.18ϫ0.16ϫ0.16
3.28 to 25.00
–12 Յ h Յ 12
–50 Յ k Յ 49
–28 Յ l Յ 23
37855
Reflections collected
Independent reflections
Rint
4960
0.0185
8705
0.0829
14796
0.0205
Completeness to θ = 25.00° [%]
Rel. transmission factors
Parameters
99.8
99.7
99.7
0.9404 and 0.8901
340
1.063
R1 = 0.0357
wR2 = 0.0921
R1 = 0.0398
wR2 = 0.0941
0.9524 and 0.8383
616
0.946
R1 = 0.0692
wR2 = 0.1268
R1 = 0.1385
wR2 = 0.1452
1.00000 and 0.95080
1197
1.025
R1 = 0.0307
wR2 = 0.0817
R1 = 0.0347
wR2 = 0.0830
GOF
Final R indices [IϾ2σ(I)][a,b]
R indices (all data)[a,b]
[a] R1 = ∑||Fo| – |Fc||/∑|Fo|. [b] ωR2 = {∑ [w(Fo – Fc ) ]/∑[w(Fo2)2]}1/2
.
2
2 2
1233 (s), 1044 (s), 983 (m), 874 (m), 853 (m), 841 (m), 824 (w), 785 304 (8) [M+], 288 (23), 236 (100), 226 (49), 220 (21), 214 (24), 159
(m), 712 (w), 689 (w), 642 (w), 585 (w) cm–1. ESI-MS: m/z (%) = (21), 149 (30), 117 (35). HRMS: calcd. for C13H6F6N2+H:
Eur. J. Inorg. Chem. 2011, 2103–2111
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.eurjic.org
2109

S. Krackl, S. Inoue, M. Driess, S. Enthaler
FULL PAPER
305.05079; found 305.05016. UV/Vis (CH3CN, 25 °C)
λ
=
4 H, Ar), 6.07–6.21 (m, 4 H, Ar), 1.44 (s, 36 H, tBu) ppm. 13C
NMR (CDCl3): δ = 179.1, 158.4, 155.9, 152.1, 149.1, 106.0, 99.5,
80.9, 32.8 ppm. 19F NMR (CDCl3): δ = –114.5 (m), –132.0 (m),
–156.2 (m) ppm.
271.0 nm.
´
N,N-Bis(2,3,4,5,6-pentafluorophenyl)formamidine (2g): Yield 74%
(5.6 g); m.p. 162 °C (crystallized from n-hexane). 1H NMR
(CDCl3): δ = 8.28 (s, 1 H, N=CHN) ppm. 19F NMR (CDCl3): δ =
Mo2[2f–H]2(OtBu)4 (3f): Compound 2f (193 mg, 0.64 mmol) af-
forded 3f (248 mg, 0.23 mmol) in 72% yield. The obtained single
crystals were suitable for single-crystal X-ray diffraction measure-
ments. C42H46F12Mo2N4O4 (1090.7): calcd. C 46.25, H 4.25, N
–152.1 (br), –160.8 (br), –162.6 (br) ppm. IR (KBr): ν = 2861 (w),
˜
1678 (s), 1640 (m), 1514 (s), 1462 (m), 1380 (w), 1322 (m), 1288
(m), 1170 (w), 1035 (m), 978 (s), 783 (w), 607 (w), 564 (w), 493 (w)
cm–1. ESI-MS: m/z (%) = 377 (100) [M+], 236 (21), 219 (27).
HRMS: calcd. for C13H2F10N2+H: 377.01311; found 305.01062.
UV/Vis (CH3CN, 25 °C) λ = 264.5 nm.
1
5.14; found C 46.11, H 4.20, N 5.32. H NMR (CDCl3): δ = 8.87
(s, 2 H, N=CHN), 6.13–6.29 (m, 8 H, Ar), 1.46 (s, 36 H, tBu) ppm.
13C NMR (CDCl3): δ = 175.9, 169.9, 152.6, 144.7, 107.5, 82.4,
32.3 ppm. 19F NMR (CDCl3): δ = –132.0 (m), –163.9 (m) ppm.
´
N,N-Bis(p-trifluoromethylphenyl)formamidine (2h):[26–31] Yield 59%
1
Supporting Information (see footnote on the first page of this arti-
cle): The information on the H···F interactions for compounds 2
and the details of the DFT calculations.
(4.9 g), m.p. 166 °C. H NMR (CDCl3): δ = 8.42 (br. s, 1 H, NH),
8.21 (s, 1 H, N=CHN), 7.60 (d, J = 17.8 Hz, 4 H, C6H2), 7.16 (d,
J = 16.3 Hz, 4 H, C6H2) ppm. 13C NMR (CDCl3): δ = 162.9, 159.0,
126.8, 126.8, 126.7, 126.6, 119.5, 118.9, 118.1, 114.1 ppm. 19F
NMR (CDCl ): δ = –61.8 ppm. IR (KBr): ν = 2975 (m), 1672 (s),
˜
3
Acknowledgments
1609 (s), 1514 (m), 1492 (m), 1418 (m), 1315 (s), 1236 (m), 1214
(m), 1180 (m), 1157 (m), 1107 (s), 1069 (s), 1011 (m), 988 (m), 946
(w), 849 (m), 833 (m), 778 (w), 731 (w), 674 (w), 643 (w), 620 (w),
591 (w), 553 (w), 511 (w) cm–1. ESI-MS: m/z (%) = 333 (100) [M+],
236 (23), 146 (20). HRMS: calcd. for C15H10F6N2+H: 333.08209;
found 333.08174. UV/Vis (CH3CN, 25 °C) λ = 300.75 nm.
Financial support by the Technical University of Berlin (Cluster of
Excellence, Unifying Concepts in Catalysis, EXC 314/1; www.unicat.
tu-berlin.de), funded by the Deutsche Forschungsgemeinschaft
(DFG) is gratefully acknowledged. S. K. thanks the Fonds der
Chemischen Industrie for a Kekulé scholarship and the Berlin In-
ternational Graduate School of Natural Sciences and Engineering
(BIG-NSE) for ideational support. S. I. thanks the Japan Society
for the Promotion of Science (JSPS) for financial support of his
work.
Synthesis of Mo2(L–H)2(OtBu)4 (3a–3c and 3e–3f) with L = 2a–
2c and 2e–2f, respectively: Mo2(OtBu)6 (200 mg, 0.32 mmol) was
dissolved in pentane (10 mL) and cooled to –20 °C. The corre-
sponding formamidine, 2a–2f, (2:1 molar ratio) was dissolved in
DCM and added dropwise to the solution whereupon the colour
of the solution slowly turned from bright orange to dark red-
brown. After the solution was stirred for three hours, the solution
was filtered and all of the volatiles where removed in vacuo. The
obtained green-brown solid was dissolved in a mixture of toluene/
DCM (1:1) and recrystallized at –20 °C. The obtained crystals were
thoroughly crushed and dried in vacuo at elevated temperatures in
order to completely remove the cocrystallized solvent for the ele-
mental analysis and the NMR spectroscopic measurements.
[1] K. Reichenbächer, H. I. Süss, J. Hulliger, Chem. Soc. Rev. 2005,
34, 22–30.
[2] “Fluorine in the Life Science Industry”, P. Maienfisch (Ed.),
Chimia 2004, 58, 92–162.
[3] G. W. Coates, A. R. Dunn, L. M. Hennling, J. W. Ziller, E. B.
Lobkosky, R. H. Grubbs, J. Am. Chem. Soc. 1998, 120, 3641–
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L. R. Falvello, R. Llusar, Organometallics 1988, 7, 2279–2285.
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2249–2264.
Mo2[2a–H]2(OtBu)4 (3a): Compound 2a (124 mg, 0.64 mmol) af-
forded 3a (230 mg, 0.26 mmol) in 82% yield. C42H58Mo2N4O4
(874.8): calcd. C 57.66, H 6.68, N 6.40; found C 57.54, H 6.75, N
6.44. 1H NMR (CDCl3): δ = 8.83 (s, 2 H, N =CHN), 6.13–6.32
(m, 20 H, Ar), 1.32 (s, 36 H, tBu) ppm. 13C NMR (CDCl3): δ =
169.6, 145.1, 129.4, 124.4, 123.0, 83.0, 33.0 ppm.
[8] F. A. Cotton, C. A. Murillo, I. Pascual, Inorg. Chem. 1999, 38,
2182–2187.
Mo2[2b–H]2(OtBu)4 (3b): Compound 2b (179 mg, 0.64 mmol) af-
forded 3b (176 mg, 0.19 mmol) in 59% yield. C42H54F4Mo2N4O4
(946.8): calcd. C 53.28, H 5.75, N 5.92; found C 52.97, H 5.74, N
6.01. 1H NMR (CDCl3): δ = 8.88 (s, 2 H, N=CHN), 6.11–6.32 (m,
16 H, Ar), 1.40 (s, 36 H, tBu) ppm. 13C NMR (CDCl3): δ = 176.3,
162.3, 146.0, 124.2, 116.0, 81.06, 32.2 ppm. 19F NMR (CDCl3): δ
= –118.8 (m) ppm.
[9] F. A. Cotton, T. Ren, J. Am. Chem. Soc. 1992, 114, 2237–2242.
[10] K. L. Fujdala, T. D. Tilley, Chem. Mater. 2004, 16, 1035–1047.
[11] J.-G. Ma, Y. Aksu, L. J. Gregoriades, J. Sauer, M. Driess, Dal-
ton Trans. 2010, 39, 103–106.
[12] S. Krackl, J.-G. Ma, Y. Aksu, M. Driess, Eur. J. Inorg. Chem.
2011, DOI: 10.1002/ejic.201001236.
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tween Metal Atoms, 3rd ed., Springer Science and Business Me-
dia, Inc., New York City, 2005.
[14] H. Komber, H.-H. Limbach, F. Böhme, C. Kunert, J. Am.
Chem. Soc. 2002, 124, 11955–11963.
Mo2[2c–H]2(OtBu)4 (3c): Compound 2c (172 mg, 0.64 mmol) af-
forded 3c (176 mg, 0.17 mmol) in 53% yield. The obtained single
crystals were suitable for single-crystal X-ray diffraction measure-
ments. C42H50F8Mo2N4O4 (1018.7): calcd. C 49.52, H 4.95, N 5.50;
[15] T. M. Gilbert, C. B. Bauer, A. H. Bond, R. D. Rogers, Polyhe-
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31, 3438–3444.
1
found C 49.93, H 5.01, N 5.71. H NMR (CDCl3): δ = 8.97 (s, 2
H, N=CHN), 6.00–6.28 (m, 12 H, Ar), 1.43 (s, 36 H, tBu) ppm.
13C NMR (CDCl3): δ = 176.0, 164.2, 150.2, 107.2, 100.1, 82.4,
32.1 ppm. 19F NMR (CDCl3): δ = –108.3 (m) ppm.
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2011, DOI: 10.1002/cctc.201100007.
Mo2[2e–H]2(OtBu)4 (3e): Compound 2e (195 mg, 0.64 mmol) af-
forded 3e (237 mg, 0.22 mmol) in 69% yield. C42H46F12Mo2N4O4
(1090.7): calcd. C 46.25, H 4.25, N 5.14; found C 46.86, H 4.33, N
5.22. 1H NMR (CDCl3): δ = 9.12 (s, 2 H, N=CHN), 6.27–6.36 (m,
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Received: December 27, 2010
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