The activation of sulfur hexafluoride at highly reduced low-coordinate nickel dinitrogen complexes

Article abstract of DOI:10.1002/anie.201308270

The greenhouse gas sulfur hexafluoride is the common standard example in the literature of a very inert inorganic small molecule that is even stable against O₂ in an electric discharge. However, a reduced β-diketiminate nickel species proved to be capable of converting SF ₆ into sulfide and fluoride compounds at ambient standard conditions. The fluoride product complex features an unprecedented [NiF]⁺ unit, where the Ni atom is only three-coordinate, while the sulfide product exhibits a rare almost linear [Ni(μ-S)Ni]²⁺ moiety. The reaction was monitored applying ¹H NMR, IR and EPR spectroscopic techniques resulting in the identification of an intermediate nickel complex that gave insight into the mechanism of the eight-electron reduction of SF₆. SF₆ taken into pieces: Reduced β-diketiminate nickel species are capable of converting SF₆, which is used as an inert gaseous dielectric medium for high-voltage switches, into sulfide and fluoride compounds. The fluoride product complex features an unprecedented [NiF] ⁺ unit, where the Ni atom is only three-coordinate. The low-coordinate sulfide product exhibits an almost linear [Ni(μ-S)Ni] ²⁺ moiety.

Full text of DOI:10.1002/anie.201308270

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DOI: 10.1002/anie.201308270
S-F Activation
The Activation of Sulfur Hexafluoride at Highly Reduced Low-
Coordinate Nickel Dinitrogen Complexes**
Patrick Holze, Bettina Horn, Christian Limberg,* Corinna Matlachowski, and Stefan Mebs
Abstract: The greenhouse gas sulfur hexafluoride is the
common standard example in the literature of a very inert
inorganic small molecule that is even stable against O2 in an
electric discharge. However, a reduced b-diketiminate nickel
species proved to be capable of converting SF6 into sulfide and
fluoride compounds at ambient standard conditions. The
fluoride product complex features an unprecedented [NiF]+
unit, where the Ni atom is only three-coordinate, while the
sulfide product exhibits a rare almost linear [Ni(m-S)Ni]2+
Scheme 1. The dinitrogen nickel complexes [(LtBuNiI)2(m-h1:h1-N2)] (I)
and K2[(LtBuNiI)2(m-h1:h1-N22À)] (II) (LtBu =[HC(CtBuNC6H3(iPr)2)2]À).
1
moiety. The reaction was monitored applying H NMR, IR
and EPR spectroscopic techniques resulting in the identifica-
tion of an intermediate nickel complex that gave insight into the
mechanism of the eight-electron reduction of SF6.
functionalized.[7] Herein, we show that the same nickel
compounds are even capable of converting SF6 under
standard ambient conditions. Compound I features a weakly
activated N2 ligand coordinated by two NiI centers in an end-
on bridging mode. In II, the N2 ligand is further activated by
two additional electrons in the system. Previous work has
shown that the reaction of II with carbon monoxide gives the
dinuclear Ni0 compound K2[LtBuNi0CO]2.[7c] Thus, the N2
ligand within II can be readily displaced, despite its formal
diazene character, yielding in the formation of a [LtBuNi0]
species.
As II is diamagnetic, we set out to investigate its reaction
towards SF6 with the aid of 1H NMR spectroscopy (Figure 1).
Exposure to SF6 leads to the complete conversion of II within
10 minutes, giving rise to a signal set that is characteristic for
the paramagnetic NiI complex I;[8] concomitantly, potassium
S
ulfur hexafluoride is an extremely inert molecular gas.[1]
Accordingly, it is used as a gaseous dielectric medium for
high-voltage circuit breakers and as a protecting gas for the
casting of magnesium. Also, its long lifetime in the atmos-
phere is most likely the reason why SF6 is the most potent
greenhouse gas known.[2] Owing to its vast stability towards
both nucleophilic and electrophilic attack, reports on its
successful chemical activation are scarce and commonly
involve harsh conditions, for example elevated pressures
and temperatures or strong reducing agents such as elemental
alkali metals.[1,3–5]
The conversion of SF6 in the coordination sphere of
transition metals has to date only been described by Ernst and
co-workers: Low-valent complexes were reacted with SF6 to
give mixtures of different metal fluorides. The fate of the
sulfur remained unclear; in case of late transition metals, the
formation of hazardous sulfur fluoride compounds such as
S2F10 and SF4 was discussed.[6]
In previous studies, we have evaluated the behavior of
low-coordinate
b-diketiminate
nickel
complexes
[(LtBuNiI)2(m-h1:h1-N2)] (I) and K2[(LtBuNiI)2(m-h1:h1-N22À)]
(II; Scheme 1) towards small molecules. In fact, dinitrogen,
dihydrogen, and carbon oxides could be activated and/or
[*] Dipl.-Chem. P. Holze, Dipl.-Chem. B. Horn, Prof. Dr. C. Limberg,
Dipl.-Chem. C. Matlachowski, Dr. S. Mebs
Humboldt-Universitꢀt zu Berlin, Institut fꢁr Chemie
Brook-Taylor-Strasse 2, 12489 Berlin (Germany)
E-mail: christian.limberg@chemie.hu-berlin.de
Homepage: http://www.chemie.hu-berlin.de/aglimberg/
[**] We are grateful to the Solvay Fluor GmbH for the kind provision of
SF6 and to the Cluster of Excellence “Unifying Concepts in Catalysis”
funded by the Deutsche Forschungsgemeinschaft (DFG) and the
Humboldt-Universitꢀt zu Berlin for financial support. F. Schax is
thanked for the X-ray analysis of 1.
Figure 1. 1H NMR spectra before and after the reaction of II with SF6.
a) diamagnetic starting material K2[(LtBuNiI)2(m-h1:h1-N22À)] (II); b) after
one day, the paramagnetic compounds, [LtBuNiIIF] (1) and [(LtBuNiII)2(m-
S)] (2) are observed in a 2:1 ratio; # [D6]benzene; § TMS;
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201308270.
* impurities; yield in 1: 75%, 2: 60% (in reference to TMS).
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fluoride is formed.[9] Within one day, the resonances of I
vanish in favor of two paramagnetically shifted signal sets for
two NiII complexes that we assign to [LtBuNiIIF] (1) and
[(LtBuNiII)2(m-S)] (2) on the basis of the findings described
below. Species 1 and 2 formed in an approximate 2:1 ratio.
No sulfur- or fluorine-containing species other than SF6
was observed while monitoring the reaction with the aid of
19F NMR and gas-phase IR spectroscopy. Focusing on the
band corresponding to the n3 vibrational mode of SF6, time-
dependent measurements revealed a rapid decrease of the SF6
concentration (Figure 2). Owing to diffusion restrictions in
Figure 3. Molecular structure of 1. H atoms are omitted for clarity.[10]
an X-ray analysis. Its molecular structure is depicted in
Figure 3. The nickel atom in 1 is trigonal planar and
coordinated by the N atoms and the F atom (sum of bond
angles 359.99(9)8). To the best of our knowledge, 1 is the first
three-coordinate nickel fluoride complex that has been
structurally characterized.
Attempts to access the sulfide complex 2 in pure form
through an independent route have been unsuccessful. This
can be rationalized by the lability that should be expected for
2 considering on the one hand the close proximity of the
opposite bulky b-diketiminate ligands and on the other hand
the high electron density a single chalcogenide atom experi-
ences between two d8 metal centers (note that [(LtBuNiII)2(m-
O)] is not known yet). However, subsequent to the reaction of
II with SF6 in n-hexane, single crystals of 2 could be picked
from the crude solid reaction product that were suitable for an
X-ray analysis, and its molecular structure is illustrated in
Figure 4.
Figure 2. Gas-phase IR spectroscopy: Time dependent change of the
intensity of the n3 band of SF6 relative to its initial intensity in course
of the reaction of II with stoichiometric amounts of SF6 in benzene
(solid line); in a reference experiment, the same set-up with II replaced
by unreactive [LtBuNiIIBr] was used (dashed line).
the gas cell used, the reaction is decelerated somewhat in
comparison to the 1H NMR experiments. Consistent with the
NMR spectroscopic results obtained for II, species I was
found to also react with SF6, which again yielded 1 and 2 as
products.
Upon exposure of I or II to more than five equivalents of
SF6, the relative yield in 1 increases at the cost of the yield in 2
and accordingly in metal-bound sulfur. This suggests the
concomitant formation of either elemental sulfur or gaseous
sulfur fluorides like SF4 or S2F10. However, no evidence of the
formation of either compound was obtained through ele-
mental analysis, by GC-MS and EI-MS, or applying 19F NMR
and EPR spectroscopy.
Figure 4. Molecular structure of 2. H atoms and one molecule of
n-hexane are omitted for clarity.[10]
In 2, the two Ni centers are coordinated by the S atom and
the N atoms in trigonal planar fashion (sum of bond angles
360.00(9)8 and 359.98(10)8, respectively). Most probably
because of the strained character of the molecule, the torsion
angle of the two opposite C3N2Ni planes is close to 908 (N1-
N2-N3-N4 86.26(10)8). Species 2 exhibits a rare, almost-linear
1
To confirm the H NMR assignments for 1 and 2, their
independent synthesis was pursued. While the preparation of
1 appears straight forward setting out from the corresponding
bromide homologue through halide exchange, it eluded
synthesis in attempts using common nucleophilic fluorinating
reagents like AgF or Me3SnF. Ultimately, the metathesis
reaction of [LtBuNiIIBr] with [Me4N]F was successful, when it
was carried out in dichlormethane, leading to 1 in up to 61%
yield. Indeed, comparison of the 1H NMR spectrum of 1 with
the spectrum observed after the reaction of II with SF6
revealed identical chemical shifts. During the reaction of
a concentrated benzene solution of I with an excess of SF6,
1 precipitated in form of single crystals that were suitable for
[NiII(m-S)NiII]2+ unit.[11] The Ni S distances in 2 are compa-
À
rable to those of previously reported tetrahedral NiII com-
plexes with a linear central arrangement.[11g,h]
Altogether, the findings indicate a reaction sequence as
depicted in Scheme 2. Two equivalents of II react quickly with
SF6 to yield four equivalents of potassium fluoride, two
equivalents of I, and an unknown SFx compound (x < 6),
which is not coordinatively bound to a nickel center.
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indicating that potential intermediate [PhSFx] species (x < 5)
are more reactive than [SFx] suggested as an intermediate in
Scheme 2.
Unequivocal identification of 3 again required the devel-
opment of an independent synthesis. Compound 3 was
isolated after the salt metathesis reaction of [LtBuNiIIBr]
with KSPh in 81% yield. Single crystals suitable for an X-ray
analysis could be obtained by cooling a saturated n-hexane
solution of 3 to À308C. The structure of 3 is displayed in
Figure 5. In 3 the coordination sphere of the three-coordinate
Scheme 2. Reaction of II with SF6: Initially, I, KF, and a SFx (x<6)
species are formed. Subsequently, I reacts with SFx to give the final
products 1 and 2 in a 2:1 ratio.
Subsequently, I reacts further with SFx and unconsumed SF6,
respectively, to give 1 and 2 in a 2:1 ratio. As SFx is more
reactive than SF6, a parallel reaction with unconsumed SF6 is
noncompetitive if its concentration is low. Attempts to
identify the SFx species were unsuccessful. The formation of
sulfur-based radicals or fluorine radicals is unlikely as these
should be reduced by I instantaneously. Indeed, no radicals
were detected by EPR measurements. Also, the formation of
soluble diamagnetic compounds such as SF4 or S2F10 can be
ruled out as they were not detected in gas-phase IR
spectroscopic experiments and/or 19F NMR experiments
that we conducted at temperatures between À808C and
508C. A realistic option is the intermediate formation of
[SF5]À salts, which have been found as products of the
reduction of SF6 using organic reducing agents: Their
insolubility in benzene would explain their elusiveness in
19F NMR spectroscopic investigations, and on the other hand
they readily decompose after isolation in vacuo to give SF4
and KF, so that only the latter remains for detection.[4b]
To further support our results concerning the reaction of
II with SF6, SF5Ph was employed as an easy-to-handle SF6
derivative. Indeed, the reaction of II with SF5Ph proceeded
analogously: Initially I and KF were formed, and ultimately
1 as well as the complex [LtBuNiIISPh] (3) were detected as
products in a 2:1 ratio (Scheme 3). The reaction rate is higher,
Figure 5. Molecular structure of 3. H atoms are omitted for clarity.[10]
NiII center can best be described as T-shaped planar (sum of
À
bond angles 359.84(7)8). The Ni S distance is comparable to
values reported for other NiII arylthiolate complexes.[12]
Sulfur hexafluoride is renowned for its rather inert
behavior under extreme conditions. Research focusing on its
activation is valuable in two respects: a) to determine the
actual limits of its stability, and b) to find ways for its
conversion, first as SF6 is the most efficient greenhouse gas,
and second as in the long term this may open up non-toxic
paths for SF5 functionalization. Still, reports on successful SF6
activation at transition-metal centers are scarce.[6,13] Herein
we showed that SF6 (as well as SF5Ph) can be converted within
the coordination sphere of a reduced nickel atom bound by b-
diketiminato ligands. Additionally, we have successfully
identified the resulting fluoride and sulfide products, which
are very interesting complexes in their own rights: 1 is the first
structurally characterized three-coordinate nickel fluoride
complex, while 2 is a rare example of a transition-metal
complex exhibiting an almost linear [Ni(m-S)Ni]2+ arrange-
ment. Currently, studies concerning the reactivity of 1 towards
silanes and other fluorine accepting compounds are under
way. Once a convenient, SF6-free synthesis of 2 has also been
developed, its chemical behavior towards small molecules like
dioxygen or dihydrogen will be studied.
Received: September 20, 2013
Published online: January 30, 2014
Keywords: coordination chemistry · fluoride · N ligands ·
small-molecule activation · sulfur
Scheme 3. The reaction of II with SF5Ph: After the formation of I, KF,
and a PhSFx species (x<5), 1 and 3 are generated in a 2:1 ratio.
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